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McG.RAW-HILL   PUBLICATIONS   IN  THE 

ZOOLOGICAL    SCIENCES 
A.  FRANKUN  SIIULL,  CONSULTING  KDITOB 


HUMAN  BIOLOGY 


The  quality  of  the  materials  used  in  the 

manufacture  of  this  book  is  governed  by 

continued  postwar  shortages. 


SELECTED  TITLES  FROM 

McGRAW-HILL  PUBLICATIONS  IN  THE 
ZOOLOGICAL  SCIENCES 

A.  FRANKLIN  SHULL,  Consulting  Editor 

Baitsell  •  HUMAN  BIOLOGY 

Brcland  •  MANUAL  OF  COMPARATIVE  ANATOMY 

Burlingame  -  HEREDITY  AND  SOCIAL  PROBLEMS 

Chapman  •  ANIMAL  KCOLOGY 

Clausen  -  ENTOMOPHAGOUS  INSECTS 

Frost  •  GENERAL  ENTOMOLOGY 

Goldschmidt  -  PHYSIOLOGICAL  GENETICS 

Graham  -  FOREST  ENTOMOLOGY 

Haupt  •  FUNDAMENTALS  OF  BIOLOGY 

Hyman  -  THE  INVERTEBRATES:  PROTOZOA  THROUGH  CTENOPHORA 

Johannsen  and  Butt  -  EMBRYOLOGY  OF  INSECTS  AND  MYRIAPODS 

Metcalf  and  Flint  •  INSECT  LIFE 

Mitchell  •  GENERAL  PHYSIOLOGY 

Mitchell  and  Taylor  •  LABORATORY  MANUAL  OF  GENERAL  PHYSI- 
OLOGY 

Pearse  •  ANIMAL  ECOLOGY 

Reed  and  Young  -  LABORATORY  STUDIES  IN  ZOOLOGY 
Riley  and  Johannsen  •  MEDICAL  ENTOMOLOGY 
Rogers  -  TEXTBOOK  OF  COMPARATIVE  PHYSIOLOGY 

LABORATORY  OUTLINES  IN  COMPARATIVE  PHYSIOLOGY 
Senning  •  LABORATORY  STUDIES  IN  COMPARATIVE  ANATOMY 
Skull  -  EVOLUTION 

HEREDITY 

PRINCIPLES  OF  ANIMAL  BIOLOGY 

Skull,  LaRue,  and  Ruthven  •  PRINCIPLES  OF  ANIMAL  BIOLOGY 
Simpson  and  Roe  •  QUANTITATIVE  ZOOLOGY 
Snodgrass  -  PRINCIPLES  OF  INSECT  MORPHOLOGY 
Storer  -  GENERAL  ZOOLOGY 

LABORATORY  MANUAL  FOR  GENERAL  ZOOLOGY 
Van  Cleave  -  INVERTEBRATE  ZOOLOGY 
Welch  -  LIMNOLOGY 
Wieman  -  GENERAL  ZOOLOGY 

AN  INTRODUCTION  TO  VERTEBRATE  EMBRYOLOGY 
Wolcott  •  ANIMAL  BIOLOGY 

There  are  also  the  related  series  of  McGraw-Hill  Publications  in 
the  Botanical  Sciences,  of  which  Edmund  W.  Sinnott  is  Consulting 
Editor,  and  in  the  Agricultural  Sciences,  of  which  Leon  J.  Cole  is 
Consulting  Editor. 


HUMAN  BIOLOGY 


BY 

GEORGE  ALFRED  BAITSELL 

C>)l(/(i+(j  /Vo/c.s-.s'or  ot  Violoyy 

Fellow  of  CdUnnin  College, 

Yale  University 


FIRST  EDITION 
NINTH  IAIPKESSION 


McGRAW-HILL  BOOK  COMPANY,  INC. 

N'EW    YORK    AND    LONDON 
1940 


COPYRIGHT,  1940,  BY  THE 
McGR A \v-HiLL,  BOOK  COMPANY,  INC. 


PRINTED    Itf   THE    UNITED    STATES    OF   AMERICA 

All  rights  reserved.   This  book,  or 

parts  thereof,  man  not  be  reproduced 

in  any  form  without  permission  of 

the  publishers. 


MARTIN  J.  POLLAK,  INC.  •  PRINTERS  •  NEW  YORK 


PREFACE 

" Human  Biology"  endeavors  to  present  the  pertinent  facts  of 
biology  from  the  vantage  ground  of  the  most  interesting  and  impor- 
tant organism  ill  the  world  of  life,  namely,  man.  Accordingly,  the 
study  of  human  biology  involves  a  great  deal  more  than  human  anat- 
omy and  physiology;  it  is  essentially  a  humanizing  of  general  biology 
in  that  attention  is  centered  primarily  on  human  structure  and  func- 
tion rather  than  on  the  characteristics  of  types  selected  from  the  lower 
organisms. 

At  least  two  major  factors  have  influenced  the  author  to  devote 
the  time  and  energy  and  to  submit  to  the  trials  and  tribulations  neces- 
sarily associated  with  writing  and  publishing  a  college  textbook  in 
biology.  First  in  importance  has  been  the  increasing  realization, 
year  by  year,  that  the  great  majority  of  students  beginning  work  in 
college  biology  were,  inherently,  far  more  interested  in  acquiring 
knowledge  about  the  human  organism  than  they  were  concerning  any 
other  living  species.  Student  interest  in  any  subject  is  naturally 
expected  to  lead  to  increased  endeavor.  Nevertheless,  the  author  has 
frequently  been  surprised  at  the  efforts  voluntarily  assumed  by  inter- 
ested students  in  collecting  the  available  information  relevant  to 
some  structural  or  functional  feature  of  man.  Of  first-rate  importance 
in  this  connection  is  the  fact  that  scientific  *data  dealing  with  mam- 
malian physiology  andr  anatomy  are  available  in  abundance,  possibly 
to  a  greater  degree  than  elsewhere  in  the  biological  field.  Further- 
more, this  body  of  scientific  knowledge,  particularly  when  associated 
with  the  functional  aspects  of  man,  is  being  augmented  continuously 
from  the  results  obtained  by  many  investigators  in  the  best  labora- 
tories of  this  and  other  countripe. 

Second,  the  author  has  been  impressed  with  the  necessity  of  supply- 
ing new  and  vital  material  at  an  advanced  level  for  the  basic  courses  in 
college  biology.  Biological  knowledge  possessed  by  the  students  now 
entering  college  is  undoubtedly  greatly  superior  both  in  quantity  and 
quality  to  that  of  their  predecessors.  By  this  is  meant  that  a  larger 
percentage  of  students  take  a  laboratory  course  in  biology  before 
entering  college  and  that  the  material  presented  in  these  courses  is 
much  more  extensive  than  in  earlier  years.  Any  college  instructor 


vi  PREFACE 

who  takes  the  trouble  to  examine  the  contents  of  various  excellent  and 
widely  used  biology  texts  for  secondary  schools  and  representative 
student  notebooks  covering  the  year's  work  in  these  courses  will  cer- 
tainly be  convinced  that  careful  consideration  must  be  given  to  the 
content  of  college  courses  in  biology  so  that  the  students '  interest  may 
not  be  dulled  and  their  time  wasted  by  the  repetitious  study  of  labora- 
tory types  which  have  been  carefully  considered  in  an  earlier  course. 
Particularly  is  this  condition  important  to  the  great  majority  of  col- 
lege students  electing  biology,  for  their  majoi;  scholastic  interests  lie 
elsewhere  and  they  will,  therefore,  take  only  one  year  in  the  biolog- 
ical field. 

The  central  problem  is  evident:  Shall  the  incoming  students  be 
reintroduced  at  college  levels  to  a  series  of  more  or  less  standardized 
biological  types,  most  of  which  they  feel — rightly  or  wrongly — are 
well  known  to  them  from  previous  study,  or  shall  the  college  course  be 
built,  for  the  most  part,  around  materials  previously  untouched?  It 
seems  evident  that  a  biology  course  in  which  primary  consideration  is 
focused  upon  the  organization  and  activities  of  human  protoplasm 
offers  new  and  superior  possibilities  for  the  presentation  of  highly 
important  material  and  for  increasing  student  interest  in  the  biological 
field.  If  the  human  biology  material  is  presented  from  a  comparative 
standpoint,  the  student  will  learn  not  only  the  biology  of  man  but  also 
biology  in  its  broader  aspects,  for  man  is  a  part  of,  not  apart  from,  the 
world  of  life. 

One  example  may  be  noted :  The  study  of  human  nutrition  cannot  be 
completed  until  the  photosynthetic  processes  of  the  green  plants  and 
the  decay  processes  of  thp  colorless  plants  are  brought  into  the  picture. 
The  fact  that  the  nutrition  of  every  type  of  organism  depends  upon 
enzyme  action  gives  opportunity  for  extended  consideration  of  these 
organic  catalysts  which  are  involved  in  every  vital  process.  And  the 
same  condition  obtains  with  the  other  basic  phenomena  associated 
with  the  living  state  for,  as  is  generally  recognized,  organisms  perform 
the  same  vital  functions  in  essentially  the  same  way.  They  eat,  grow, 
respire,  secrete,  excrete,  react,  and  reproduce  as  a  result  of  the  activi- 
ties of  the  associated  cellular  units  of  which  they  are  composed. 
Accordingly,  it  seems  evident  that  to  "Know  thyself"  is  not  only  an 
important  and  interesting  discipline,  but  it  may  also  be  excellent 
biology. 

In  an  endeavor  to  widen  the  scope  of  the  book,  so  that  the  interested 
student  may  have  abundant  material  to  pursue  important  fields  of 
interest  at  advanced  levels,  an  Appendix  has  been  supplied  containing 
direct  quotations  from  the  publications  of  various  authorities.  It  is 


PREFACE  vii 

hoped  that  this  material  will  prove  to  be  highly  stimulating  to  instruc- 
tor and  student  and,  at  the  same  time,  provide  reference  to  a  note- 
worthy list  of  books  for  additional  collateral  reading.  Original 
material  by  the  author  has  also  been  included  in  the  Appendix  when 
it  was  felt  that  its  content  tended  to  mar  the  continuity  and  appro- 
priate level  of  the  main  text. 


GEORGE  A.  BAITSELL. 


OSBORN  ZOOLOGICAL  LABORATORY, 

YALE  UNIVERSITY, 
NEW  HAVEN,  CONN. 

May,  1940. 


ACKNOWLEDGMENTS 

The  author  finds  it  difficult  to  express  in  any  adequate  manner  his 
indebtedness  to  his  colleagues  in  the  Department  of  Zoology  and  to 
many  others  for  their  help  in  the  preparation  of  this  manuscript.  In 
particular,  mention  should  be  made  of  the  assistance  of  the  following 
staff  members  for  reading  various  portions  of  the  manuscript  and  for 
contributing  many  valuable  constructive  suggestions  during  the  past 
three  years  while  the  material  has  been  used  in  temporary  form: 
Drs.  S.  C.  Ball,  T.  C.  Barnes,  E.  J.  Boell,  G.  E.  Hutchinson,  D.  Merri- 
man,  J.  S.  Nicholas,  D.  F.  Poulson,  T.  K.  Ruebush,  and  L.  L.  Woodruff. 
Once  more,  great  credit  is  due  to  Prof.  L.  L.  Woodruff,  who  has  been 
interested  enough  to  read  the  proof  of  the  entire  manuscript  and  has 
offered  valuable  aid  in  many  ways,  including  the  generous  permission 
to  use  important  material,  both  figures  and  text,  from  the  "  Foun- 
dations of  Biology "  (Maemillan)  and  "The  Development  of  the 
Sciences"  (Yale  University  Press). 

The  original  illustrations  are  almost  entirely  the  work  of  Mr. 
Armin  Hemberger,  artist  in  the  Department  of  Pathology,  Yale 
School  of  Medicine.  An  examination  of  his  drawings  in  this  book  will 
quickly  reveal  the  author's  indebtedness  to  him.  In  the  development 
of  the  drawings  of  the  various  organ  systems,  Mr.  Hemberger  has  had 
the  advantage  of  helpful  criticism  and  suggestions  from  his  student, 
Miss  Jean  B.  Herrman,  and  from  the  following  members  of  the  Medical 
School  faculty:  Drs.  Harold  S.  Burr  and  Leon  S.  Stone,  of  the  Depart- 
ment of  Anatomy,  and  Drs.  Clyde  Deming  and  Harlan  Perrins,  of  the 
Department  of  Clinical  Medicine. 

Sincere  thanks  are  due  to  the  members  of  the  technical  staff  of  the 
Osborn  Zoological  Laboratory,  particularly  Miss  Lisbeth  Krause,  who 
has  redrawn  a  considerable  number  of  figures,  Misses  Elinor  Rungee 
and  Elizabeth  Gelback,  Mrs.  R.  H.  Hamilton,  and  F.  W.  Countryman, 
Yale,  1942,  on  whom  has  fallen  the  burden  of  a  great  deal  of  secre- 
tarial work  in  connection  with  the  preparation  of  the  manuscript,  the 
reading  of  proof,  and  the  development  of  the  index. 

It  is  a  real  pleasure  to  the  author  to  acknowledge  the  hearty 
cooperation  of  the  following  authors  and  publishers  who  have  gener- 
ously granted  permission  for  reproducing  copyrighted  illustrations 
and  textual  material ;  in  all  instances  the  source  of  material  is  given  in 
the  text  or  in  the  legend  of  the  figure : 


X  ACKNOWLEDGMENTS 

George  Allen  &  Unwin,  Ltd. : "  Human  Heredity/'  by  Baur,  Fischer, 
and  Lenz. 

American  Book  Company:  "Biology,"  by  Hunter,  Walter,  and 
Hunter. 

D.  Appleton-Century  Company,  Inc.:  "Outlines  of  Evolutionary 
Biology,"  by  Dendy. 

P.  Blakiston's  Son  &  Company,  Inc.:  "Animal  Biology,"  by  Lane; 
"Human  Anatomy,"  by  Morris;  "Comparative  Anatomy,"  by  Neal 
and  Rand. 

Ginn  and  Company:  "The  Human  Mechanism,"  by  Hough, 
Sedgwick,  and  Waddcll,  2d  edition  revised. 

Harcourt,  Brace  &  Company,  Inc. : "  Exploring  Biology,"  by  Smith. 

Harper  &  Brothers:  "Elements  of  Biology,"  by  Buchanan;  "Ani- 
mal Biology,"  by  Guyer; " Science  in  Health  and  Disease,"  by  Haggard; 
"Story  of  Living  Things,"  by  Singer. 

Harvard  University  Press:  "Genetics  and  Eugenics,"  by  Castle. 

Henry  Holt  &  Company,  Inc.:  "The  Human  Body,"  by  Martin; 
"General  Biology,"  by  Sedgwick  and  Wilson;  "History  of  the  Human 
Body,"  by  Wilder. 

Lea  &  Febiger:  "Principles  of  Hematology,"  by  Haden. 

W.  W.  Norton  &  Company,  Inc.:  "Tides  of  Life,"  by  Hoskins; 
"Genetics,"  by  Jennings;  "Scientific  Basis  of  Evolution,"  by  Morgan; 
"Physiological  Basis  of  Personality,"  by  Stockard. 

Prentice-Hall,  Inc.:  "Ascaris,"  by  Goldschmidt. 

W.  B.  Saunders  Company:  "Developmental  Anatomy,"  by 
Arey;  "Introduction  to  Human  Physiology,"  by  Crandall;  "Funda- 
mentals of  Bacteriology,"  by  Frobisher;  "Elementary  Bacteriology," 
by  Greaves;  "Textbook  of  Physiology,"  by  Howell;  "Textbook  of 
Pathology,"  by  MacCallum;  "Textbook  of  Histology,"  by  Maximow- 
Bloom. 

Charles  C.  Thomas,  Publisher:  "Selected  Readings  in  the  History 
of  Physiology,"  by  Fulton. 

John  Wiley  &  Sons:  "Animal  Parasites  and  Human  Disease,"  by 
Chandler;  "Outlines  of  Biochemistry,"  by  Gortner. 

The  Williams  &  Wilkins  Company:  "To  Remind,"  by  Hardy; 
"The  Harvey  Lectures,"  Series  32,  1937,  Harvey  Society  of  New 
York;  "The  Kahn  Test,"  by  Kahn;  "Blood  Groups,"  by  Snyder. 

Yale  University  Press:  "Evolution  of  Earth  and  Man,"  edited  by 
the  author;  "The  Development  of  the  Sciences,"  edited  by  Dr.  L.  L. 
Woodruff;  "Science  in  Progress,"  edited  by  the  author. 

The  Chemical  Foundation:  "Chemistry  and  Medicine,"  edited  by 
Dr.  J.  Stieglitz. 


ACKNOWLEDGMENTS  xi 

Special  mention  should  be  made  of  the  courtesies  shown  by  The 
Macmillan  Company  in  permitting  the  use  of  considerable  material 
from  their  publications,  including  that  from  the  author's  "  Manual  of 
Biology"  and  " Manual  of  Animal  Biology/7  both  of  which  they 
publish.  Also,  for  permission  to  reproduce  a  number  of  figures  from 
their  publications  which  are  designated  in  the  legend  by  the  author's 
name  only.  These  are  Figs.  35,  46,  53,  62-66,  81,  86,  88,  91,  107,  108, 
133,  148,  174,  190-194,  200,  204,  212,  242,  246,  251,  and  260.  These 
have  been  reproduced  from  the  following  Macmillan  publications: 
" College  Zoology,"  by  Hegner;  "Lessons  in  Elementary  Physiology," 
by  Huxley-Barcroft;  "Textbook  of  Anatomy  and  Physiology,"  by 
Kimbcr,  Gray,  and  Stackpole;  "General  Biology,"  by  Mavor;  "Food 
Products,"  by  Sherman;  "Biology  of  Vertebrates,"  by  Walter; 
"Genetics,"  by  Walter;  "The  Human  Skeleton,"  by  Walter;  "Vita- 
min B,  and  Its  Use  in  Medicine,"  by  Williams  and  Spies,  "The  Cell 
in  Development  and  Heredity,"  by  Wilson;  "Foundations  of  Biology,? 
by  Woodruff;  "Animal  Biology,"  by  Woodruff. 

Plate  II,  Cell  Types,  drawn  by  Dr.  J.  Manson  Valentine  for  the 
Weber  Charts,  is  reproduced  by  permission  of  the  New  York  Scien- 
tific Supply  Company  and  of  Bruce  M.  Mills,  administrator  of  the 
Weber  estate. 

The  author  is  greatly  indebted  to  Prof.  G.  E.  Hutchinson  for  an 
original  article  on  the  "Biological  Elements"  and  to  Dr.  Grace  E. 
Pickford  for  a  noteworthy  treatment  of  the  "Enzymes." 

Special  mention  should  also  be  made  for  permission  received  for 
the  use  of  textual  and  illustrative  material  from  various  publications 
of  the  McGraw-Hill  Book  Company.  These  illustrations  are  desig- 
nated in  the  legend  by  the  author's  name  only,  and  comprise  all  those 
not  otherwise  credited.  This  material  has  been  taken  from  the  follow- 
ing McGraw-Hill  publications:  "Fundamentals  of  Biology,"  by  Haupt; 
"An  Introduction  to  Botany,"  by  Haupt;  "Microbiology,"  by  Lut- 
man;  "General  Physiology,"  by  Mitchell;  "Laboratory  Studies  in 
Zoology,"  by  Reed  arid  Young;  "Textbook  of  Comparative  Physiol- 
ogy," by  Rogers;  "Protoplasm,"  by  Seifriz;  "Introduction  to  Cytol- 
ogy," by  Sharp;  "Evolution,"  by  Shull;  "Heredity,"  by  Shull; 
"Principles  of  Animal  Biology,"  by  Shull;  "Botany:  Principles  and 
Problems,"  by  Sinnott ;  "Principles  of  Genetics,"  by  Sinnott  and  Dunn; 
"An  Orientation  in  Science,"  by  Watkeys  and  Associates;  "General 
Zoology,"  by  Wieman;  "An  Introduction  to  Vertebrate  Embryology," 
by  Wieman;  "Animal  Biology,"  by  Wolcott. 


CONTENTS 

PAGE 
PREFACE v 

ACKNOWLEDGMENTS ix 

CHAPTER 

I.  STRUCTURE  AND  FUNCTION  IN  THE  WORLD  OF  LIFE 3 

Nature  of  Protoplasm 3 

Protoplasmic  Activities 8 

II.  THE  ORGANIZATION  OF  THE  HUMAN  BODY 19 

Cell  Structure 20 

Human  Tissues 23 

Organs  and  Organ  Systems 32 

The  Body  Plan 34 

III.  THE  BIOLOGY  OF  NUTRITION 41 

Structural  Features  Associated  with  Nutrition 41 

Functional  Features  Associated  with  Nutrition 55 

Photosynthesis 66 

TV.  THE  BIOLOGY  OF  RESPIRATION 73 

Structural  Features  Associated  with  Respiration 74 

The  Respiratory  System  of  Man 76 

Breathing 81 

Functional  Features  Associated  with  Respiration 85 

V.  THE  BIOLOGY  OF  SECRETION 93 

Structural  Features  Associated  with  Secretion 93 

Functional  Features  Associated  with  Secretion 96 

The  Liver 97 

Endocrine  Glands 102 

VI.  THE  BIOLOGY  OF  EXCRETION .119 

Excretion  in  the  Skin 119 

Excretion  in  the  Lungs 121 

Excretion  in  the  Liver 121 

Excretion  in  the  Kidneys 122 

VTI.  THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM 133 

Structural  Features  Associated  with  the  Vascular  System 135 

Course  of  the  Circulation  in  the  Body 147 

Functional  Features  Associated  with  the  Vascular  System 152 

Transportation  of  Materials  in  the  Blood 158 

Uniformity  and  Variation  in  the  Blood 161 

xiii 


xiv  CONTENTS 

CHAPTER  PAGE 

Blood  Coagulation 163 

The  Spleen 167 

VIII.  BIOLOGY  OF  THE  MUSCULAR  SYSTEM 169 

Structural  Features  Associated  with  Movement 170 

Functional  Features  Associated  with  Movement 178 

JX.  BIOLOGY  OF  THE  SKELETAL  SYSTEM 189 

Structural  Features  Associated  with  the  Skeletal  System 189 

Exoskeleton 189 

Endoskeleton 192 

Functional  Features  Associated  with  the  Skeletal  System 209 

X.  BIOLOGY  OF  THE  NERVOUS  SYSTEM  (I) 215 

Structural  Features  Associated  with  the  Nervous  System 216 

Sense  Organs 218 

Peripheral  Nervous  System 239 

Autonomic  Nervous  System 243 

XI.  BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II) 249 

Central  Nervous  System 240 

The  Spinal  Cord 252 

The  Brain 257 

Functional  Features  Associated  with  the  Nervous  System 268 

XII.  THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION 281 

Types  of  Reproduction 28.3 

Development  of  the  Frog 29 1 

Development  of  the  Chick 301 

Mammalian  Development 308 

Human  Reproduction 314 

XIII.  THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II) 325 

Mitosis 327 

Chromosome  structure 332 

Germ  Cell  Formation 341 

Fertilization 349 

XIV.  THE  BIOLOGY  OF  INHERITANCE 354 

The  Particulate  Nature  of  Inheritance 355 

Mendelian  Inheritance 356 

Multiple  Factors '.    .  372 

Linkage 377 

Mutations 389 

XV.  HUMAN  HEREDITY 397 

Inherited  Characteristics 400 

Galton  and  the  Principles  of  Biometry 406 

Human  Hybridization 413 

Eugenics:  Negative  and  Positive 419 


CONTENTS  xv 

CHAPTER  '  PAGE 

XVI.  THE  WEB  OF  LIFE 428 

Auto  trophic  Organisms 429 

Heterotrophic  Organisms 435 

Enzymes 437 

The  Biotic  Environment 447 

XVII.  BIOLOGY  OF  DISEASE 467 

Noninfectious  Diseases 468 

Immunity 469 

Immunology:  Uses  and  Techniques 473 

Epidemiology 485 

Types  of  Cellular  Response 488 

APPENDIX 495 

INDEX 589 


HUMAN  BIOLOGY 


CELL  WALL 
PLASMA  MEMBRANE 

NUCLEAR  MEMBRANE- 
CHROMATJAl 

AJUCLEOLUS 
KARYOLYMPfl 

VAC  U  OLE 
CHOMDRIOSOME 


PLASTIDS 

GOLGf  APPARATUS 

CENTR1OLE 

CENTROSPHERE 

CYTOPLASM 

PLASTJD  DIVIDING 

METAPLASM 


GENTROSOME 


PLATE  I. — Diagrams  showing  the  cells  as  14-sided  figures  (totrakaidecahedra).  A, 
three  cells  to  show  idealized  arrangement  in  tissues  of  plants  and  animals  as  described 
by  F.  T.  Lewis  (Proceedings  of  the  American  Academy  of  Arts  and  Sciences,  vol.  68,  1933) ; 
J5,  one  of  the  cells  sectioned  to  show  internal  structure;  C,  detailed  study  of  section  from 
B  as  observed  in  a  microscopic  preparation  (cf.  pages  20-23). 


CHAPTER  I 
STRUCTURE  AND  FUNCTION  IN  THE  WORLD  OF  LIFE 

Biology  is  concerned  with  life  phenomena  of  every  kind  and  nature. 
Man  recognizes  life  in  the  innumerable  plant  and  animal  organisms 
that  are  abundantly  present  in  practically  every  niche  of  this  earth, 
whether  on  land,  sea,  or  air.  And  each  of  us,  as  a  conscious  human 
being,  recognizes  an  inherent  living  principle  that  is  linked  with  the 
surrounding  world  of  life.  Life;  is  known  to  us  only  in  the  focm  of 
completely  organized  units  which  we  term  individuals.  Also  each 
individual  in  this  world  of  life,  whether  small  or  large,  simple  or  com- 
plex, plant  or  animal,  is  characterized  by  a  number  of  fundamental 
structural  and  functional  features  that  serve  to  distinguish  it  from 
nonliving  organization  and  also  to  identify  it  as  a  distinct  type  of 
living  organism.  This  unity  of  design  and  behavior,  underlying  all 
life,  is  due  to  the  fact  that  the  building  material  utilized  in  every  type 
of  living  organism  consists  of  a  basic  substance,  technically  designated 
as  protoplasm,  which  is  the  vehicle  for  all  life  phenomena.  Proto- 
plasm, wherever  found,  exhibits  certain  unique  structural  and  func- 
tional features  that  may  now  be  indicated. 

NATURE  OF  PROTOPLASM 

Microscopic  observations  on  protoplasm  in  living  units  show  that  it 
varies  considerably  in  its  physical  state.  It  may  appear  at  one  time 
as  a  rather  thick,  slow-flowing  liquid,  such  as  might  be  described  by 
the  words  "sirupy  consistency"  (the  sol  condition),  and  again  as  a 
more  or  less  rigid,  gelatinous  substance  (the  gel  condition).  This 
variation  in  its  physical  state,  together  with  other  identifying  char- 
acters, gives  clear  evidence  that  protoplasm  is  a  colloid.  The  colloidal 
state  is  not  a  unique  feature  of  living  matter,  for  many  nonliving  col- 
loids are  known,  both  inorganic  and  organic.  Colloids  are  character- 
ized structurally  by  the  presence  of  innumerable,  exceedingly  minute 
particles  dispersed  through  a  continuous  medium.  The  dispersed 
materials  are  too  small  to  be  seen  even  under  the  highest  magnifica- 
tions. Colloids  are  heterogeneous,  rather  than  homogeneous,  sys- 
tems. In  the  protoplasmic  colloid  the  continuous  medium  is  liquid, 

3 


HUMAN  BIOLOGY 


but  in  other  colloids  it  may  be  a  solid  or  a  gas,  as  in  the  case  of  a  cloud, 
which  is  a  colloid  formed  £rom  a  liquid  and  a  gas,  the  latter  serving  as 
the  continuous  medium,  with  the  minute  water  droplets  dispersed 


Cream 


Butter 


Fia.  1. — Diagrams  illustrating  the  difference  between  cream  (sol  state)  in  which  water 
is  the  continuous  medium,  with  dispersed  oil  droplets;  and  butter  (gel  state)  in  which  oil 
is  the  continuous  medium.  (Buchanan,  "Elements  of  Biology,"  Harper  &  Brothers.) 

throughout.  Variation  in  the  relations  between  the  particles  and  the 
medium  in  which  they  are  dispersed  results  in  the  reversible  sol-gel 
states  present  in  colloids.  A  common  example  of  this  phenomenon  is 
found  in  the  behavior  of  the  fat  globules  in  liquid  cream,  in  which  they 
are  dispersed  through  a  liquid  medium.  In  butter,  however,  the 


A  B  C  D 

FIG.  2. — Diagrams  showing  variations  in  the  microscopic  structure  of  a  colloidal 
emulsion.  A,  alveolar  appearance  when  suspended  droplets  are  widely  separated  by  the 
continuous  phase;  B  and  C,  reticular  appearance  present  in  continuous  phase  when 
droplets  are  close  together;  D,  droplets  now  form  the  continuous  phase  following  close 
apposition  and  coalescence.  (Sharp.) 


globules  of  fat  are  consolidated  to  form  a  continuous  solid  medium 
through  which  liquid  particles  are  dispersed.1     (Figs.  1,  2.) 

From  the  functional  standpoint,  biologists  are  in  general  agreement 
that  the  colloidal  state  is  essential  to  protoplasmic  activity  in  that  it 
permits  the  continuous  energy  transformations  invariably  associated 

1  Additional  material  on  Colloids  will  be  found  in  the  Appendix.  It  will  be 
found  helpful  to  consult  the  Appendix  frequently. 


STRUCTURE  AND  FUNCTION  IN  THE  WORLD  OF  LIFE 


with  every  form  of  life.  So  long  as  there  is  life,  there  is  activity.  And 
activity,  in  this  instance,  does  not  mean  simply  movement  but  rather 
the  continuous  activity  associated  with  the  operation  of  the  essential 
life  functions,  as  will  be  discussed  in  the  following  chapters.  Presum- 
ably, vital  activities  are  dependent  upon  the  fact  that  the  protoplasmic 
colloid  permits  an  unparalleled  distribution  of  surface  activity  between 
dispersed  particles  and  the  medium,  with  the  end  result  that  potential 
molecular  energy  present  in  the  nutritive  materials  is  made  available 
for  maintaining  the  life  processes.  And,  of  course,  it  should  be  made 
clear  that  the  energy  thus  released  in  the  organism  is  the  radiant 
energy  of  the  sun  previously  stored  in  the  foodstuffs  by  photosynthesis 
in  green  plants.  The  living  organism  can  in  no  way  manufacture 
energy;  it  can  only  indirectly  utilize  the  radiant  energy  received  from 
the  sun  for  the  maintenance  of  life 
activities. 

As  might  be  expected  from  the 
structural  variations  occurring  in 
protoplasm  during  life  as  the  result 
of  the  reversible  sol-gel  phenomena, 
the  appearance  of  the  protoplasmic 
material  is  by  no  means  uniform 
when  it  is  preserved  and  prepared 
for  intensive  microscopic  study  by 
sectioning  and  staining.  Accord- 
ingly, various  concepts  of  the  architecture  of  protoplasm  are 
current  in  biological  literature.  As  a  matter  of  fact,  one  can 
speak  only  in  very  general  terms  on  this  subject,  for  our  present 
knowledge  concerning  the  ultimate  structural  organization  of 
protoplasm  is  limited.  This  is  due  primarily  to  the  fact  that 
the  basic  organization  pattern  of  protoplasm  is  so  minute  that  it  is 
ultrarnicroscopic — far  beyond  the  highest  magnifying  powers  of  the 
microscope.  Furthermore,  scientific  investigation  along  this  line  is 
definitely  limited  by  the  fact  that  protoplasm  cannot  be  subjected  to 
intensive  analysis  by  any  known  method  without  destroying  the  pri- 
mary object  of  the  research,  namely,  the  unique,  dynamic  life  principle. 
The  dead  organic  material,  which  was  formerly  living  protoplasm,  can, 
of  course,  be  subjected  to  many  intensive  types  of  analysis,  but  such 
studies  have  so  far  failed  to  reveal  the  deeply  hidden  structural  secrets 
that  appear  to  be  essential  to  the  maintenance  of  life  itself.  (Figs. 
3  to  5.) 

Chemical  analyses  of  protoplasmic  material  show  that  it  contains  a 
very  high  percentage  of  water,  frequently  more  than  90  per  cent  by 


FIG.  3. — Diagram  illustrating  tho 
microscopic  appearance  of  an  emulsion 
as  often  seen  in  protoplasm.  (Skull.) 


6 


HUMAN  BIOLOGY 


weight,    with   various    complex    compounds   in   solution.1     And    so, 
according  to  one  noted  authority,  tjie  living  organism  is  to  be  regarded 


FIG.  4. — Living  human  protoplasm  as  it  appears  in  a  fresh,  unstained  preparation 
under  high  magnification.  Specimen  from  epithelial  cells  in  the  lining  of  the  mouth. 
(Buchanan, "  Elements  of  Biology ,"  Harper  &  Brothers.) 

as  "  essentially  an  aqueous  solution  in  which  are  spread  out  colloidal 
substances  of  great  complexity. "  These  complex  constituent  com- 
pounds present  in  protoplasm  fall 
into  three  great  groups  of  organic 
compounds  designated  as  the 
carbohydrates,  fats,  and  proteins. 
The  first  two  of  these  contain  only 
three  elements,  namely,  carbon, 
hydrogen,  and  oxygen  in  widely 
varying  proportions;  but  the  pro- 
teins usually  contain  a  wide  range 
of  common  elements,  in  addition  to 
the  three  just  named.  However, 
the  chemical  analyses  show  that, 
when  the  compounds  present  in 
protoplasm  are  broken  down  into 
their  constituent  elements,  over  99 
per  cent  of  the  material  is  derived 
from  the  following  eight  elements :  carbon,  oxygen,  hydrogen,  nitrogen, 
sulphur,  calcium,  phosphorus,  and  potassium.  Also  present  are  minute 

1  Extreme  limit  in  water  content  is  apparently  reached  in  the  jellyfish,  in  certain 
species  of  which  it  is  stated  that  the  constituent  tissues  may  contain  as  much  as 
96  per  cent  of  water. 


Fio.  5. — Fibrillar  structure  of  proto- 
plasm as  seen  in  a  permanent  preparation 
of  a  nerve  cell.  Highly  magnified. 
(Seifriz,  after  Tschernjachiwsky.} 


STRUCTURE  AND  FUNCTION  IN  THE  WORLD  OF  LIFE  7 

amounts  of  iron,  chlorine,  copper,  sodium,  magnesium,  and  probably 
many  others.  It  is  by  no  means  certain,  however,  that  all  the  elements 
present  are  actually  bound  up  in  the  protoplasmic  molecule.1 

It  is  noteworthy  that  there  is  nothing  rare  or  peculiar  about  these 
constituent  elements  which  are  present  in  protoplasm;  they  are  of 
common  occurrence.  Gold,  silver,  platinum,  and  other  rare  elements 
are  conspicuously  absent.  It  is  apparent,  therefore,  that  the  life 
qualities  characteristic  of  protoplasm  do  not  depend  upon  an  assem- 
blage of  rare  or  unknown  materials  but  rather  on  a  unique  and  highly 
intricate  arrangement  of  various  common,  widely  distributed  ele- 
ments. Furthermore,  it  will  be  shown  later  that  cycles  of  elements 
are  present  in  nature,  as  the  result  of  which  the  constituent  materials 
pass  repeatedly  from  the  nonliving  into  the  world  of  life  and  then  back 
again  to  the  lifeless  state. 

Cellular  Organization. — Whatever  may  prove  to  be  the  ultimate 
arrangement  of  materials  in  the  protoplasmic  fabric,  it  is  ( always 
revealed  at  the  level  of  microscopic  visibility 
in  the  form  of  definite  entities,  the  cells; 
that  is,  a  distinctive  plan  of  organization 
pervades  the  life  substance  which  is  indi- 
cated by  the  term  cellular  organization. 
This  means,  in  a  word,  that  the  common 
denominator  of  vital  architecture  is  a  tiny 
bit  of  protoplasm  known  to  the  biologist  as 
the  Cell.  Cells  are  protoplasmic  building 
blocks  which,  associated  in  incredible  num- 
bers, constitute  the  basic  materials,  tissues,  \^_^>^—  CELL  WALL 

r       i       j  i          •        i          T       ji  •      •!•  FIG.   6. — A  primitive  urii- 

of  plants  and  animals.  In  the  primitive  cellular  grecn  pplant|  Chla_ 
forms  of  plants  and  animals,  many  species  mydomonas.  Chlorophyll 

/,  i  i  •    i       i  i  j  •  •  present  in  the  crescent-shaped 

are  found  in   which   the    entire    organism     chioropiast.    Highly    magni- 

COnsistS      of        a      single      microscopic      Cell,      ned.      (Sinnott,      after     Goro- 

unassociated  with  other  cellular  units  and 

completely  equipped  for  maintaining  all  the  life  functions.     These  are 

known  as  the  unicellular  organisms.     (Figs.  6-8;  Plate  II,  page  18. )2 

The  bodies  of  higher  organisms,  including  man  and  all  the  familiar 
forms  of  plants  and  animals,  are  multicellular.  They  are  composed  of 
an  almost  inconceivable  number  of  cells.  The  examination  of  a  bit  of 
any  plant  or  animal  tissue  under  the  microscope  will  quickly  give 

1  Consult  the  section  on  Biological  Elements  in  the  Appendix  for  additional 
material. 

2  Consult  the  following  sections  in  the  Appendix  for  additional  material: 
Protozoa,  Amoeba,  Pararnecium. 


8  HUMAN  BIOLOGY 

visible  evidence  of  the  established  fact  that  protoplasm  does  not  occur 
as  a  homogeneous  substance  but  rather  as  a  mosaic  composed  of  associ- 
ated cellular  units.  And  of  even  greater  significance  is  the  fact  that 
each  cell  is  an  independent  unit  of  life — &  theater,  as  it  were,  in  which 
the  complete  drama  of  essential  life  activities  is  continuously  being 
enacted.  For  the  functional  activities  inseparably  associated  with 
the  living  state  are  housed  in  the  cells  as  the  ultimate  units  of  function 
as  well  as  of  structure.  Someone  has  said  that  it  is  as  if  each  of  the 
tiny  structural  elements  in  a  motor  possessed  a  microscopic  apparatus 
to  duplicate  the  functions  of  the  complete  motor.  (Fig.  10.) 

PROTOPLASMIC  ACTIVITIES 

In  the  preceding  pages  it  has  been  shown  that  a  basic  structural 
unity  exists  throughout  the  world  of  life.     There  is  a  common  living 


PSEUDOPODIUM- 

{FOOD   VACUOLE 


CONTRACTILE    VACUOLC 

NUCLEUS 

FIG.  7. — A  primitive  unicellular  animal,  Amoeba.  Essentially  a  microscopic  bit  of 
naked  protoplasm  which  flows  in  various  directions  to  form  temporary  projections, 
pseudopodia.  Highly  magnified.  (Buchanan,  "Elements  of  Biology"  Harper  & 
Brothers.) 

substrate,  protoplasm,  and  a  common  design  for  building,  cellular 
organization.  But  possibly  even  more  impressive  to  the  biologist  is 
the  functional  unity  that  pervades — is  essential  to — all  life,  whether  it 
be  the  most  primitive  form  of  microscopic  plant  or  animal  or  man  him- 
self. That  is  to  say,  various  essential  life  processes  are  continually  in 
operation  in  the  protoplasm  of  every  living  cell;  and  the  organism  as  a 
whole  is  striving  to  provide  the  wherewithal,  food,  so  that  these  vital 
activities  may  continue  without  cessation. 

It  is  noteworthy  that  the  principle  that  we  call  life,  though  extra- 
ordinarily abundant  and  operating  in  each  one  of  us,  is  very  difficult, 
if  not  impossible,  to  define.  We  conceive  of  life  as  a  unique  temporary 
state  of  matter,  and  there  is  nothing  with  which  it  can  be  compared. 
Accordingly,  attempted  definitions  of  life  do  not  really  define;  they 
only  describe  certain  outstanding  characteristics.  Thus  it  has  been 
said  that  life  is  "the  capacity  of  an  animal  or  plant  for  self-preserva- 


STRUCTURE  AND  FUNCTION  IN  THE  WORLD  OF  LIFE          9 

tion  and  growth,  the  cessation  of  which  means  death. "  Or  again  that 
it  is  "a  series  of  definite  and  successive  changes,  both  of  structure  and 
composition,  which  take  place  within  an  individual  without  destroying 
its  identity. "  One  of  the  most  successful  of  the  attempts  to  define 
life  is  that  framed  by  'the  famous  biologist  and  philosopher  of  the 
nineteenth  century,  Herbert  Spencer,  who  defined  life  as  "the  con- 
tinuous adjustment  of  internal  relations  to  external  relations/' 

METABOLISM 

This  definition  of  Spencer's,  it  will  be  noted,  stresses  the  dynamic 
or  functional  aspect  of  life  with  its  continuous  interplay  between 
organism  and  environment,  and  truly  this  is  a  basic  characteristic 
that  immediately  sets  a  living  organism  apart  from  the  nonliving 


CONTRACTILE  VACUOLE 


MICRONUCLEUS 
MACRONUCLEUS 


FOOD  VACUOLE 


FIG.  8. — The  highly  specialized  unicellular  organism,  Paramecium,  as  observed 
under 'the  microscope.  Some  of  the  details  are  seen  only  in  stained  preparations.  Note 
the  even  coat  of  cilia.  Food  vacuoles  are  moved  through  the  cytoplasm  by  cyclosis. 
(Buchanan,  "  Elements  of  Biology ,"  Harper  &  Brothers.) 

world.  For  the  living  cells  of  all  organisms  continually  admit  essential 
materials  from  the  environment  and  at  the  same  time  release  substances 
that  are  no  longer  of  value.  A  complex  life  chemistry  is  involved  in 
these  intracellular  reactions — so  complex  in  fact  that,  as  yet,  scarcely 
any  of  the  processes  are  fully  known.  Certain  basic  facts  relative  to 
the  chemistry  of  the  life  functions  are,  however,  definitely  established. 
Thus  it  is  universally  recognized  that  two  essentially  antagonistic 
processes  are  always  involved,  namely,  a  constructive,  nutritive 
phase,  andbolism,  in  which  necessary  materials  are  taken  into  the 
cell  and  assimilated  and  so  made  available  for  the  protoplasmic  repair, 
growth,  and  reproduction;  secondly,  a  destructive  phase,  katabolism, 
which  is  marked  by  a  disruption  of  the  complex  compounds,  present  in 
or  forming  a  part  of  the  life  stuffs,  with  the, constant  release  of  energy 
for  the  maintenance  of  life  functions.  It  is  obvious  that  if  the  sum 
total  of  the  anabolic  processes  exceeds  that  of  the  katabolic  processes, 


10  HUMAN  BIOLOGY 

there  will  be  a  surplus,  and  an  increase  in  the  size  of  the  cell  or  growth 
will  result.  The  opposite  of  this  condition,  indicated  by  an  excess  of 
the  katabolic  reactions,  must  finally  terminate  in  the  death  of  the  cell. 

Anabolism  and  katabolism,  together,  include  all  the  chemical 
activities  essential  to  life  and  are  usually  grouped  under  the  inclusive 
term  metabolism.  Metabolism  means  change  and  aptly  describes  the 
living  state,  with  its  continuous  building  up  of  the  materials  essential 
to  growth  and  repair  and  its  synchronous  destruction  of  the  energy- 
containing  compounds  in  order  that  vital  activities  may  be  main- 
tained. Yet  through  a  lifetime  of  change,  the  protoplasm  in  each 
individual  cell  maintains  an  essentially  uniform  organization  and 
exhibits  a  characteristic  rhythmicity  of  function.  Since  the  metabolic 
activities  include  all  the  chemical  changes  characteristic  of  life,  they 
involve  the  essential  life  processes  of  nutrition,  respiration,  secretion, 
excretion,  growth,  reproduction,  movement,  and  adaptation;  all  of 
which,  in  the  final  analysis,  center  in  the  cell  as  the  basic  structural 
and  functional  unit  of  life. 

Energy  Relations. — Intracellular  chemical  activities  acquire  their 
paramount  importance  from  the  fact  that  the  maintenance  of  the 
essential  life  processes  requires  the  continuous  expenditure  of  energy.1 
This  essential  supply  of  energy  is  made  available  by  oxidative  processes, 
involving  the  assistance  of  enzymes  or  ferments,  in  the  cytoplasm  of 
each  cell  which  result  in  the  dissociation  of  various  relatively  unstable 
carbon  compounds  and  the  formation  of  simpler  compounds  of  rela- 
tively low  energy  content  which  are  given  off  from  the  cells  as  excre- 
tions. The  oxidative  phenomena  essential  to  the  release  of  energy 
require  a  continuous  supply  of  free  oxygen  from  the  environment  and 
the  synchronous  removal  of  the  resulting  carbon  dioxide  from  the 
cells.  This  energy  release  and  the  associated  interchange  of  oxygen 
and  carbon  dioxide  constitute  one  of  the  most  important  of  the  meta- 
bolic processes,  namely,  respiration — a  function  that  persists  in  every 
cell  throughout  life  and  is,  at  all  times,  a  true  measure  of  the  extent  of 
the  life  processes. 

The  destructive,  energy-yielding,  chemical  activities  in  the  cyto- 
plasm are  crudely  comparable  to  the  methods  used  to  secure  energy 
to  run  an  automobile  in  which  gasoline,  a  complex  carbon  compound 
with  a  high  potential  energy  content,  is  vaporized  and  mixed  with 
oxygen  of  the  air.  A  very  unstable  mixture  is  thus  obtained.  An 
electric  spark  is  used  to  upset  the  chemical  equilibrium;  oxidation  of 
the  gasoline  occurs  with  explosive  violence;  and  a  considerable  propor- 
tion of  the  stored  potential  energy  of  the  gasoline  is  released  as  active, 

1  Consult  the  section  on  Energy  in  the  Appendix  for  additional  material. 


STRUCTURE  AND  FUNCTION  IN  THE  WORLD  OF  LIFE         11 

or  kinetic,  energy  in  the  cylinders.  In  part,  this  kinetic  energy  is 
utilized  in  the  work  performed  in  moving  the  car,  arid  some  is  dis- 
sipated as  heat.  At  the  same  time,  various  simpler  and  more  stable 
compounds  with  reduced  energy  content,  comparable  to  the  cellular 
excretions,  are  formed  which  are  released  from  the  engine  through  the 
exhaust. 

And  so  it  is  apparent  that  the  living  organism  is  not  able  to  per- 
form work  without  using  energy  any  more  than  is  a  mechanical  engine. 
The  perpetual-motion  machine,  whether  animate  or  inanimate,  is  a 
myth.  All  require  suitable  carbon  compounds  as  fuel  from  which  the 
potential  energy  can  be  released  by  oxidative  processes.  Both  the 
steam  engine  and  the  gasoline  motor  are  able  to  secure  the  energy  from 
carbon-containing  fuels  which  cannot  be  utilized  in  the  living  organ- 
ism, but  the  principle  is  the  same.  It  is  important  to  note,  however, 
that  the  living  organism  can  do  more  than  merely  utilize  foods  for 
energy  requirements.  It  can  retain  suitable  portions  for  repair,  for 
growth,  and  for  reproduction — facilities  entirely  lacking  in  the  mechan- 
ical engine. 

Foodstuffs. — Three  great  classes  of  complex  carbon  compounds  are 
available  for  use  as  food  by  man  and  other  living  organisms.  These 
are  carbohydrates,  which  include  the  various  sugars  and  starches; 
fats,  which  include  a  wide  variety  of  edible  oils  and  fats;  and  proteins, 
which  include  an  almost  infinite  variety  of  plant  and  animal  tissues. 
The  proteins  are  essential  for  cell  nutrition  because  they  always  con- 
tain nitrogen  and  various  other  elements  necessary  to  the  repair  and 
construction  of  protoplasm.  The  carbohydrates  and  fats  contain  only 
carbon,  hydrogen,  and  oxygen  and  are  utilized  as  a  source  of  energy 
which  may  be  used  as  needed  to  "keep  the  home  fires  burning/'  and 
the  remainder  stored  away  for  supplying  later  requirements.  Also  the 
cells  must  be  supplied  with  water,  oxygen  for  respiration,  various 
inorganic  compounds,  such  as  table  salt,  and  minute  but  constant 
quantities  of  Certain  organic  compounds,  the  vitamins,  the  exact 
chemical  nature  of  most  of  which,  as  well  as  their  functions  in  cell 
metabolism,  is  the  subject  of  extended  studies  at  the  present  time. 

Photosynthesis. — The  nutritive  requirements  just  indicated  are 
basically  the  same  in  both  plant  and  animal  cells;  all  are  dependent 
upon  the  release  and  utilization  of  the  potential  energy  stored  in  the 
foodstuffs.  Inquiry  as  to  the  method  of  formation  of  suitable  food- 
stuffs, essential  to  the  maintenance  of  life,  leads  to  a  consideration  of 
an  all-important  life  function  that  occurs  in  green  plant  cells.  This 
process,  technically  designated  as  photosynthesis,  is  superimposed  upon 
the  underlying  metabolism  of  the  plant  cells  and  is  based  upon  the 


12  HUMAN  BIOLOGY 

presence  of  a  unique  green  pigment,  chlorophyll,  which  originates  in 
the  cytoplasm  of  the  plant  cells.  Chlorophyll  makes  it  possible  to 
utilize  the  radiant  energy  of  sunlight  for  the  formation,  or  synthesis, 
of  complex  carbon  compounds  suitable  for  food  from  the  simple 
inorganic  materials  abundantly  present  in  the  immediate  environment 
of  the  plant.  Photosynthesis  is  fundamental  for  all  life  because,  in 
the  final  analysis,  it  is  the  method  by  which  the  plant  and  animal  foods 
are  universally  formed.  Man  and  other  animals  get  their  food  from 
plants,  directly  or  indirectly.  In  the  latter  case,  the  carnivorous  types 
utilize  the  tissues  of  plant-eating,  or  herbivorous,  animals.  Further- 
more, and  also  of  paramount  importance,  the  photosynthetic  proc- 
esses of  green  plants  release  free  oxygen  into  the  air,  which  is  essential 
to  the  animal  respiratory  processes  by  which  the  foods  are  utilized  in 
the  tissues  of  the  body. 

Cycle  of  Elements. — Not  all  plants  are  constructive  food-forming 
types.  There  is  an  extremely  abundant  and  diverse  series  of  plant 
organisms,  the  so-called  colorless  plants,  or  Fungi,  that  are  not  equipped 
with  the  essential  food-synthesizing  chlorophyll  of  green  plants  and 
hence  find  it  necessary  to  satisfy  their  nutritive  requirements  in  essen- 
tially the  same  way  as  animals;  that  is,  they  require  organic  compounds 
of  high  complexity  which  trace  their  origin  back  to  the  photosynthetic 
processes.  The  fungi  include  such  widely  separated  plant  types  as 
bacteria,  yeasts,  molds,  mildews,  mushrooms,  smuts,  and  rusts.  These 
include  many  species  of  parasitic  organisms  which  attack  the  living 
tissues  of  man  and  his  valuable  plant  and  animal  associates,  thus  pro- 
ducing a  great  many  of  our  worst  diseases.  But  of  primary  impor- 
tance for  our  present  consideration  is  the  fact  that  the  colorless  plants 
are  responsible  for  maintaining  the  cycle  of  elements  in  nature.  They 
are  really  essential  to  the  continued  existence  of  life  on  this  earth,  for, 
through  the  various  oxidative  and  decay  processes  incited  by  them  and 
associated  with  supplying  their  own  nutritive  requirements,  they 
release  the  essential  elements  and  compounds  locked  in  the  tissues  of 
dead  plants  and  animals  and  in  their  wastes  given  off  during  life  and 
thus  make  these  materials  once  more  available  for  photosynthetic 
food  formation  by  the  green  plants.  This,  in  essence,  is  the  cycle  of 
elements  in  nature  which  will  be  discussed  in  a  later  chapter. 

GROWTH  AND  REPRODUCTION 

When  the  food  supply  is  plentiful  enough  for  the  living  cells  of  any 
organism  to  secure  the  essential  foodstuffs  in  such  amounts  that  the 
continuous  katabolic  wastes  are  more  than  met,  an  increase  in  size,  or 
growth,  is  possible.  Growth  in  a  living  organism,  often  referred  to  as 


STRUCTURE  AND  FUNCTION  IN  THE  WORLD  OF  LIFE        13 

intussusceptive,  or  interstitial,  growth,  depends  upon  the  ability  of  the 
individual  cells  to  secure  and  assimilate  suitable  materials  from  the 
environment,  together  with  the  transformation  and  intercalation  of 
these  substances  into  the  protoplasmic  complex.  Thus,  in  the  cells, 
additional  living  material  is  built  from  nonliving  substances.  This 
process  of  growth  in  the  living  organism  is- generally  regarded  as  being 
of  a  different  nature  from  that  observed  in  the  growth  of  crystals  in 
saturated  solutions.  In  the  latter  case,  the  increase  of  crystal  size 
occurs  through  the  external  deposition  of  additional  material  secured 
from  the  surrounding  saturated  solution. 

Cell  Reproduction. — The  size  of  all  types  of  cell  is  quite  definitely 
limited  by  inherent  restrictive  factors  so  that  growth  is  brought  to  a 


r*  ** 

FIG.  9. — Cell  division  (mitosis)  of  the  fertilized  egg  of  the  parasitic  round  worm, 
A scaris,  to  form  two  daughter  cells.  The  details  of  this  process  are  considered  in  a  later 
chapter.  A,  early  stage  (metaphase)  with  chromatin  in  center;  B,  separation  of 
chroinatin  (anaphase)  and,  C,  division  between  two  daughter  cells  (telophase).  X  1000. 
(Haupt.) 

stop  when  a  certain  size  has  been  attained.  At  this  point,  provided 
the  dominance  of  the  anabolic  processes  continues,  another  char- 
acteristic function  of  protoplasm  appears,  namely,  reproduction, 
during  which  two  daughter  cells  are  formed  by  the  splitting  or  division 
of  the  full-sized  parent  cell  in  half.  Cell  reproduction  normally  takes 
place  following  a  complicated  process  known  as  mitosis,  which  involves 
profound  nuclear  changes.  These  result  in  the  correct  quantitative 
and  qualitative  division  of  the  chromatin  material  in  the  nucleus  of 
the  dividing  cell  and  its  equal  distribution  to  the  two  daughter  cells. 
Since  it  is  established  that  the  chromatin  is  the  chief  vehicle  for  the 
transmission  of  hereditary  characters,  the  necessity  for  an  accurate 
division  of  the  material  is  evident.  Cell  division  is  an  exquisitely 
beautiful  and  exact  process,  the  underlying  mechanism  of  which  has 
not  as  yet  been  fully  revealed.  On  its  normal  functioning  in  every 
cell,  during  all  the  stages  of  embryonic  development  and  throughout 


14  HUMAN  BIOLOGY 

life,  depends  the  structural  and  functional  integrity  of  every  tissue 
and  organ  of  the  body  and,  in  addition,  the  transfer  of  the  specific 
characters  to  the  next  generation.  (Fig.  9.) 

Each  qf  the  two  daughter  cells,  when  first  formed  by  cell  division, 
is  one-half  the  volume  of  the  parent  cell.  Under  ttormal  conditions 
with  suitable  nutritive  materials  available,  rapid  growth  occurs,  and 
in  a  comparatively  short  time  each  of  the  half-sized  daughter  cells 
will  have  attained  full  size.  And  so  we  arrive  at  this  fact,  which  is  of 
the  highest  importance;  namely,  it  is  by  repeated  cycles  of  growth  and 
cell  division  that  man  and  all  multlcellular  organisms  gradually  attain 
the  adult  condition.  Each  individual  organism  begins  existence  as  a 
single  cell,  the  fertilized  egg,  which  is  capable  of  continued  growth  and 
division  into  two,  four,  eight,  and,  finally,  unknown  millions  and 
millions  of  associated  and  differentiated  cells  which  constitute  t he- 
complete  organism.  It  is  estimated  that  the  cubic  inch  or  so  of  living 
material  present  in  the  master  tissue  of  all,  the  cortex  of  the  human 
brain,  contains  more  than  nine  billion  cells.  It  is  a  staggering  thought 
that  all  of  these  brain  cells  and  the  countless  others  present  in  the 
entire  body  of  the  human  organism  have  arisen  during  development 
from  the  repeated  division  of  a  single  microscopic  cell,  the  fertilized 
egg,  which  is  the  basic  life  unit — the  starting  point  of  every  living 
organism.  (Fig.  13). 

IRRITABILITY  AND  ADAPTATION 

Inasmuch  as  continuous  metabolic  activity  is  essential  to  the 
maintenance  of  life,  it  follows  that  the  living  organism  must  have  at 
all  times  an  environment  that  supplies  the  necessary  materials  arid 
also  provides  suitable  conditions  of  temperature  and  moisture.  If  a 
particular  environment  is  too  hot  or  too  cold  or  too  dry  for  a  certain 
type  of  organism,  metabolism  may  be  hindered  or  entirely  stopped. 
If  water,  oxygen,  and  the  complex  foodstuffs  are  not  present  in  ade- 
quate supply,  life  cannot  long  persist.  Of  fundamental  importance, 
therefore,  since  it  is  really  responsible  for  all  the  life  activities,  is  the 
omnipresent  function  of  irritability  which  enables  protoplasm  to 
receive,  interpret,  and  respond  to  stimuli  from  both  its  external  and 
internal  environment.  Protoplasm  is  irritable,  sensitive  material  and 
is  therefore  affected  by  the  stimuli  that  impinge  upon  it.  And,  within 
certain  limits,  protoplasm  can  do  something  about  the  stimuli  that  are 
continually  making  themselves  felt.  Irritability  results  in  adaptation 
that  involves  continuous  temporary  adjustments  between  the  organ- 
ism and  its  environment.  If  the  organism  finds  that  the  environment 
is  unsuitable,  another  is  sought,  or  protective  measures  invoked  if 


STRUCTURE  AND  FUNCTION  IN  THE  WORLD  OF  LIFE        15 

possible;  if  it  is  hungry,  it  feeds;  if  more  oxygen  is  needed,  the  intake 
is  increased.  .  • 

The  environmental  conditions  are  seen  by  the  physiologist  as 
definite  fields  of  force  resulting  from  the  energy  relations  of  the  light, 
heat,  temperature,,  electrical,  chemical,  and  other  phenomena  that 
affect  the  organism.  The  oriented  reactions  of  the  organism  in  these 
fields  of  force  are  known  as  tropistic  reactions,  or  tropisms.  The 
response  of  the  organism  may  be  positive,  negative,  or  neutral.  Tro- 
pisms are  very  clearly  in  evidence  in  the  lower  types,  such  as  the 
free-swimming  protozoa.  A  broader  phase  of  the  adaptation  problem 
is  associated  with  the  origin  of  the  permanent  adaptations  that  organ- 
isms exhibit  on  every  hand.  Thus  fish  are  permanently  adapted  for 
an  aquatic  life  and  cannot  secure  the  essential  oxygen  elsewhere,  and 
the  reverse  condition  is  characteristic  of  £he  air-breathing  types. 
Temporary  adjustments  cannot  be  made  to  overcome  this  permanent 
"built-in"  adaptation  to  a  particular  environment. 

Adaptation  is  seen,  therefore,  to  be  dependent  upon  the  fact  that 
protoplasm  possesses  a  certain  degree  of  plasticity;  it  can  make  adjust- 
ments to  environmental  changes  that  do  not  transgress  the  outer 
limits.  Everything  in  life  involves  the  function  of  irritability  and 
the  adaptive  response.  Furthermore,  the  latter  does  not  appear  to  be 
haphazard  in  the  higher  types  possessing  a  nervous  system.  There  is 
coordinated  control  of  the  complete  organism,  extending  down  to 
cellular  levels.  Coordination  is  of  supreme  necessity  in  unifying  the 
activities  of  all  the  structures  responsible  for  the  essential  life  functions 
and,  through  the  amazing  development  of  the  central  nervous  system, 
leads  to  the  very  pinnacle  of  life  phenomena  in  the  human  mental 
processes. 

MOVEMENT 

Even  a  superficial  examination  of  the  various  activities  associated 
with  protoplasm,  as  indicated  in  the  preceding  pages,  shows  that  ^he 
function  of  movement  is  inseparably  bound  up  with  most  of  them. 
In  fact,  visible  spontaneous  movement  is  one  of  the  most  characteristic 
and  readily  recognizable  activities  associated  with  the  living  state, 
particularly  in  the  higher  animal  types.  The  microscopic  study  of 
living  cells,  both  plant  and  animal,  gives  additional  evidence  of  the 
universality  of  protoplasmic  movement  in  revealing  a  regular  intra- 
ceUular  flow,  or  streaming,  of  the  cytoplasm,  which  is  undoubtedly 
ti^d  up  with  the  maintenance  of  the  other  life  processes.  This  is  the 
phenomenon  of  cyclosis  which,  in  a  favorable  type  of  cell  such  as  the 
unicellular  Paramecium,  is  seen  to  be  essential  both  for  the  distribu- 


16 


HUMAN  BIOLOGY 


tion  of  nutritive  materials  and  for  the  elimination  of  waste  substances. 
(Fig.  8.) 

In  addition  to  the  intracellular  cyclosis,  Paramecium  and  many 
other  unicellular  organisms  have  filaments  of  cytoplasm  projecting 
through  the  cell  boundary  into  the  surrounding  medium.  Through 
a  beautifully  coordinated  oar-like  beating  movement  of  these  cilia, 
the  ciliated  animal  is  able  to  move  about  in  search  of  food  or  of  more 
favorable  environmental  conditions.  And  cells,  with  essentially  the 
same  type  of  ciliary  action,  line  various  ducts  and  cavities  of  the 

higher  organisms  and  serve  to  move 
various  materials.  The  need  for  a  motor 
tissue  to  supply  the  host  of  movements 
required  in  the  multicellular  animals  for 
locomotion  and  other  activities  has  been 
met  by  the  development  of  the  con- 
tractile muscle  tissue,  which  is  one  of  the 
most  highly  developed  of  all  the  tissues 
and  also  one  of  the  most  widely  distrib- 
uted. If  the  muscle  tissue  of  a  verte- 
brate were  removed,  together  with  the. 
accessory  bones,  tendons,  and  nerve 
tissue,  all  of  the  organ  systems  would  be 
dismantled,  and  only  a  relatively  small 
amount  of  unorganized  cellular  material 
would  remain.  Coordinated  muscular 
movement  is  essential  throughout  thb 
vertebrate  organism.  (Figs.  10,  17,  21.) 
And  so  biologists  find  in  protoplasm 
a  unique  material  of  the  highest  com- 
plexity which  exhibits,  in  man  and  all  organisms,  various  definite 
characteristics  in  its  structural  organization  and  also  in  the 
associated  functional  features,  which  may  be  summarized  as  follows: 
All  the  vital  processes  center  in  the  cell  as  the  fundamental  unit  of 
structure  and  function,  arid  all  are  bound  up  with  the  fact  that  the 
living  state  requires  the  constant  expenditure  of  energy  to  keep  it  going. 
The  energy  for  all  types  of  life  is  originally  received  from  the  sun  by 
means  of  the  photosynthetic  activity  of  the  green  plants  and  then 
stored  as  potential  chemical  energy  in  complex  carbon  compounds. 
The  latter  also  contain  the  elements  necessary  for  the  repair  and 
growth  that  involve  the  construction  of  new  protoplasm.  There  is  a 
cycle  of  elements  in  nature  by  which  the  materials  used  in  the  living 


FIG.  10. — The  primitive  multi- 
cellular  organism,  Volvox. 
Barely  visible  to  the  naked  eye 
as  a  spherical  colony.  Micro- 
scopic examination  shows  that 
the  colony  consists  of  several 
thousand  chlorophyll-bearing 
cells,  each  with  two  flagella. 
Asexual  reproduction  by  daugh- 
ter-colonies (three  are  shown) ; 
sexual  reproduction  by  eggs  and 
sperm  (not  shown).  (Sinnott, 
after  Cohn.) 


STRUCTURE  AND  FUNCTION  IN  THE  WORLD  OF  LIFE        17 

organism  are  returned  to  the  inorganic  world  and  thus  made  available 
once  more  for  food  formation. 

In  the  following  chapters  an  endeavor  will  be  made  to  show  how 
the  basic  biological  features,  just  noted,  which  pervade  all  types  of  life, 
are  exhibited  in  the  human  organism.  Thus  it  is  hoped  that  a  clear 
conception  may  be  gained  of  the  materials  and  methods  used  by  Man 
— a  dominant  and  highly  developed  type  of  life — in  the  solution  of  the 
fundamental  problems  associated  with  the  maintenance  of  the  living 
state  and  the  propagation  of  his  kind. 


I';6/S;''^ 

•SSfr' ' •  ^'fe^'Iv- I C-^  ;  j-fy  >' ; ,  "^ 

^W1 1,  J 


PLATE  II.— Cell  types. 


:ella  from 


Drawings  of  a  considerable  variety  of  plant  and   animal 

microscopic  observations,  as  follows:  1,  Amoeba,  a  primitive  unicellular  animal;  2,  Radiolaria 
(Thalaasicolla) ,  a  relative  of  Amoeba;  3,  Protococcua  (2  cells),  a  primitive  plant  cell;  4,  Diatom 
(Pinnidarifi) ,  a  unicellular  marine  plant;  5,  yeast  plant;  6,  Anthrax  bacillus;  7,  Spirilla,  a  type  of 
bacteria  cell;  8,  pollen  of  lily;  9,  cells  from  leaf  (Castania),  with  chlorophyll;  10,  root-hair  cells  from 
corn  root;  11,  pith  cells  from  corn  stem;  12,  embryonic  tissue  of  chick  (mesenchyme);  13,  egg  and 
sperm  of  cat  (sperm  proportions  large);  14,  blood  of  frog,  showing  three  red  blood  corpuscles  (ery- 
throcytes)  and  one  amoeboid  blood  corpuscle  (leucocyte);  15,  ciliated  epithelium  from  intestine  of 
clam;  16,  stratified  epithelium  from  skin  of  frog;  17,  liver  cella  of  cow  during  digestion;  18,  cardiac 
muscle  fibers;  19,  nerve  cell  from  rabbit;  20,  visual  cells  from  human  retina  (rod  and  cone);  21, 
auditory  cells  from  organ  of  Corti  (guinea  pig);  22,  cartilage  cells  from  frog;  23,  bone  cell.  Abbrevi- 
ations: a.,  axon;  a.*.,  air  apace;  b.f/.,  basal  granules;  b.s.,  bone  substance;  ca.,  canaliculi;  c.6.,  cell 
body;  c.br.,  cell-bridge;  c.c.,  central  capsule;  ch.,  chloroplast;  co.v.,  contractile  vacuole;  c.v.,  central 
yacuole;  c.w.,  cell-wall;  cy.,  cytoplasm;  d.,  dendrites;  ex.,  epidermal  cells;  ect.,  ectoplasm;  en.,  endo- 
plasm;/.p.,  food  particle ;/.v.,  food  vacuole;  y.n.,  generative  nucleus;  A.,  head  of  sperm;  i.a.,  ingested 
algae;  i.f.s.,  ingested  foreign  substance;  I.,  lacuna;  l.ch.,  lobed  chloroplast;  m.,  matrix;  me.,  meta- 
plasm;  n.,  neuron;  nu.,  nucleus;  nus.,  nucleolus;  p.n.,  pollen  tufx-  nucleus;  ps.,  ixseudopodium ; 
r.h.,  root-hair;  «.,  spore;  s.v.,  secretory  vacuole;  s.cy.,  specialized  cytoplasm;  t.,  tail  of  sperm;  t.8., 
transverse  striation;  va.,  vacuole;  v.m.t  vitelline  membrane;  y.(j.,  yolk  globule;  y.n.,  yolk  nucleus. 
Drawn  by  J.  M.  Valentine,  Ph.D.,  for  Weber's  Biological  Chart  "  Cell-Types,"  Mew  York  Scientific 
Supply  Co.,  New  York.) 


CHAPTER  II 
THE  ORGANIZATION  OF  THE  HUMAN  BODY 

The  human  body  exhibits  the  characteristic  cellular  organization 
that  is  everywhere  apparent  in  the  living  world.  Examined  micro- 
scopically, bits  of  the  various  body  materials,  such  as  brain,  bone, 
muscle,  liver,  or  skin,  reveal  the  presence  of  the  constituent  cells. 
The  number  of  these  microscopic  life  units  in  even  a  small  amount  of 
tissue  r3aches  almost  incredible  figures,  ylt  has  been  estimated  that  in 
the  body  of  the  child  at  birth  there  are  approximately  26  trillion  cells, 
with  a  total  weight  1,500  million  times  that  of  the  original  egg  cell/ 
Both  the  number  of  constituent  cells  and  the  weight  are  still  further 
increased  before  maturity  is  reached.  All  these  cells  are  derived  by 
the  repeated  splitting  of  a  single  cell,  the  fertilized  egg,  which  marks 
the  starting  point  of  each  individual  life.  When  one  considers  the 
number  of  cellular  divisions,  beginning  with  the  fertilized  egg,  that 
are  essential  to  produce  the  26  trillion  cells  of  the  body,  the  number 
does  not  seem  so  difficult  of  attainment;  for  if  a  cell  divides  forty-six 
times,  and  each  of  the  resulting  cells  continues  to  divide  regularly 
after  each  division,  the  number,  represented  by  245,  would  be  reached. 
(Fig.  13.) 

A  survey  of  the  world  of  life  shows  that  in  the  more  primi- 
tive multicellular  plants  and  animals  many  examples  can  be  found  of 
colony  formation  in  which,  as  the  name  suggests,  the  organism  consists 
of  a  number  of  associated  cells,  all  of  the  same  design  and  with  each 
cell  functioning  as  an  independent  unit.  In  the  higher  multicellular 
types  of  life,  a  dependent  type  of  cell  association  is  found  in  which  the 
activities  of  the  individual  cells  are  much  more  restricted  than  in  a 
colony.  This  restriction  is  due  to  the  fact  that  cellular  differentiation 
has  entered  into  the  picture  and  become  of  great  importance.  Cellular 
differentiation  in  an  organism  means  that  groups  of  cells  are  structur- 
ally modified  to  perform  some  function  essential  to  the  organism  as  a 
whole,  such  as  digestion,  respiration,  or  movement.  Thus  there  is  a 
segregation  of  function,  a  division  of  labor,  between  the  various  groups 
of  differentiated  cells.  For  instance,  certain  cells  lining  portions  of 
the  alimentary  canal  are  responsible  for  the  digestion  and  absorption 
of  food,'  and  they  are  structurally  differentiated  for  these  functions. 
Such  nutritive  cells  would  not  find  it  possible  to  function  in  vision. 

19 


20  HUMAN  BIOLOGY 

nor  would  the  visual  cells  of  the  eye  be  of  any  use  in  the  alimentary 
canal.  Each  cellular  type  is  a  specialist  in  its  field.  And  so  it  is  clear 
that  structural  differentiation  among  groups  oT  cells  leads  to  functional 
division  of  labor  in  which  certain  cells  perform  specific  functions  for 
the  organism  as  a  whole  and,  in  return,  are  cosharers  in  tfie  benefits 
derived  from  the  activities  of  the  other  specialized  cellular  groups. 
It  is  to  be  remembered,  however,  that  every  cell,  no  matter  how  highly 
differentiated  it  may  be  for  its  particular  service  to  the  organism,  must 
be  able  to  maintain  the  independent  intracellular  life  functions  essen- 
tial to  its  own  existence.  (Figs.  10,  11.) 

Cellular  differentiation  is  responsible  for  the  construction  of  special 
building  materials,  the  tissues,  as  seen  in  nerve  tissue,  skin  tissue,  or 


IRRITABILITY 


SUPPORT 


TRANSPOFtTAT/ON 

FIG.  11. — Diagram  showing  the  possibilities  of  cellular  differentiation  inherent  in  the 
fertilized  egg  cell  of  a  higher  organism.      (After  Rogers;  slightly  modified.} 

muscle  tissue.  Thus  the  metazoan  body  is  composed  of  distinct  kinds 
of  living  tissue  which  have  characteristic  structural  and  functional 
features.  The  cells  of  muscle  tissue,  the  cells  of  nerve  tissue,  the  cells 
of  the  blood,  and  various  other  kinds,  though  basically  related  in  that 
they  are  all  descendants  of  the  fertilized  egg,  show  marked  diversity 
in  their  characteristic  features.  Furthermore,  the  tissues  are  not  tho 
final  stage  of  organization  in  living  architecture,  for  they  are  com- 
bined into  larger  structural  and  functional  units,  the  organs,  as  shown, 
for  example,  in  the  heart,  liver,  or  brain.  And,  finally,  the  organs 
concerned  with  a  particular  function  are  associated  into  the  major 
organ  systems  that  together  make  up  the  complete  functional  organ- 
ism. Consideration  may  now  be  given  to  the  structural  features  of 
cells.  (Fig.  12.) 

CELL  STRUCTURE 

A  typical  cell  consists  of  a  microscopic  globule  of  protoplasm  differ- 
entiated into  cytoplasm,  which  forms  the  main  mass  of  the  cell  body, 


THE  ORGANIZATION  OF  THE  HUMAN  BODY 


21 


and  a  very  much  smaller  spherical  body,  ih^jmcl^s,  usually  situated 
near  the  center  of  the t cytoplasm.  The  cytoplasm  is  always  enclosed 
by  some  type  of  a  limiting  membrane  that  forms  the  cell  boundary. 

Cells    are    known,    for    example,    in    the  ,  . 

primitive  unicellular  animal  Amoeba,  in 
which  this  limiting  membrane  consists 
merely  of  a  slightly  modified  region  of  the 
peripheral  cytoplasm,  the  plasma  mem- 
brane. Usually,  however,  in  addition  to 
the  plasma  membrane,  an  outer  cell  wall 
of  varying  thickness  is  present,  formed  as 
a  nonliving  secretion  by  the  peripheral 
cytoplasm,  so  that  the  cell  is  doubly 
enclosed.  In  any  case,  the  limiting  mem- 
branes of  all  cells  are  of  such  a  nature  as 
to  permit  a  continuous  interchange  of 
materials  between  the  living  cell  and  its 
environment,  which  is  essential  to  life. 

Cells     present     a     galaxy     of    shapes    111 
.  .  ,.    ,.    .  „ 

the  infinite  variety  of  living  forms  in 
the  plant  and  animal  kingdoms.  Almost  every  conceivable  shape 
can  be  found,  ranging  from  spherical  egg  cells  to  free-swimming 
male  gametes.  Possibly  we  are  inclined  to  regard  the  egg  cells  as 
typical  in  shape,  but  it  might  be  questioned  if  they  are  any  more 


FIG.  12. — Muscle  fibers  from 
the  cardiac  tissue  of  the  verte- 
brate heart.  Highly  magnified. 
?iu.,  nucleus;  t.s.,  transverse 
striations.  (Weber,  Valentine.} 


wn, 


FIG.  13. — Egg  cell  and  sperm  of  the  domestic  cat.  The  sperm  are  magnified  more 
highly,  /i.,  head  of  sperm;  mi.,  nucleus;  nus.,  nuclcolus;  £.,  tail  of  sperm;  v.m,,  vitelline 
membrane;  y.g.,  yolk  granules;  y.n.,  yolk  nucleus.  (Weber,  Valentine.} 

typical  than  the  active  sperm  that  fertilizes  them  (Fig.  13).  However, 
recent  researches  show  that  the  cells  present  in  the  tissues  of  plants 
and  animals  are  typically  14-sided  figures  (tetrakaidecahedra).  It  has 


22 


HUMAN  BIOLOGY 


also  been  shown,  in  some  instances,  that  the  embryonic  cell  is  an  eight- 
sided  figure,  that  is,  one  with  six  sides  plus  the  top  and  bottom.  The 
mature  14-sided  cell  body  is  attained  from  the  eight-sided  cell  as  a 
result  of  the  pressure  of  adjacent  cells  that  divides  each  of  the  six  sides 
into  two  parts,  thus  giving  cells  with  12  sides,  in  addition  to  the  top 
and  bottom,  to  make  a  total  of  14  sides.  (Plate  I,  page  2;  Fig.  14.) 
Cytoplasm. — The  cytoplasm  contains  a  number  of  differentiated 

bodies  of  distinctive  design  and 
function,  notably  plastids,  chondri- 
osomes,  and  Golgi  bodies,  all  of 
which  undoubtedly  have  essential 
assignments  associated  with  the  life 
functions  of  the  cells.  Almost  all 
of  these  tiny  units  present  in  the 
cytoplasm  are  still  the  subjects  of 
extended  research  in  an  endeavor  to 
find  the  answers  to  the  numerous 

problems  involved  in  cell  function.  The  plastids  are  of  particular 
interest  to  the  biologist  because  of  the  fact  that  a  certain  type  of  plas- 
tid,  the  chloroplast,  found  in  the  green  plant  cells,  contains  the  basic 
food-forming  substance,  chlorophyll,  which,  as  shown  above,  is  really 
responsible  for  the  formation  of  all  plant  and  animal  food.  Another 
tiny  body,  the  centrosome,  which  is  very  active  during  cell  division  or 
reproduction,  is  a  characteristic  feature  of  animal  cells.  It  lies  in  the 
cytoplasm  close  to  the  nuclear  wall. 


FIG.  14. — Idealized  arrangement  of 
14-sided  cells  (tetrakaidecahedra)  in 
tissues.  (After  F.  T.  Lewi*.) 


CELL  WALL 
PLASM  A- MEMBRANE 

NUCLEAR  MEMBRANE 
CHROMATIAJ 

MUCLEOLUS 
KARYOLYMPH 

VACUOLE 
CHOMDRIOSOME 


PLASTIDS 

GOLGI  APPARATUS 

CENTRIOLE  1 


-"-CENTROSPHEREj 
CYTOPLASM 


GEMTROSOME 


PLASTID  DIVIDING 
M  ETA  PLASM 


FIG.  15. — Diagram  illustrating  the  cytoplasmic  and  nuclear  elements  of  a  cell  as  described 
on  pages  22-23.     Highly  magnified.     Cf.  also  Plate  II. 

In  addition  to  the  various  cytoplasmic  inclusions,  as  just  noted,  a 
more  or  less  heterogeneous  assemblage  of  nonliving  materials,  com- 
monly referred  to  as  metaplasm,  are  present.  The  metaplasm  varies 
greatly  in  character  and  amount  in  different  types  of  cell  and  in  the 
same  cell  at  different  times.  It  consists  chiefly  of  waste  products  and 
reserve  food  materials  and  may  be  in  the  form  of  fat  droplets,  crystals, 
water,  cell  sap,  etc.  The  liquids  are  typically  present  in  tiny  cavities 


THE  ORGANIZATION  OF  THE  HUMAN  BODY  23 

or  vacuoles.  Mature  plant  cells,  particularly,  often  possess  a  large 
central  cavity,  the  cell  vacuoJe,  which  is  filled  with  the  cell  sap  con- 
taining materials  essential  to  the  nutrition  of  the  cell.  (Fig.  15.) 

Nucleus. — Of  paramount  importance  in  the  cell  is  the  nucleus. 
This  minute  intracellular  body  is  generally  regarded  as  the  main 
directing  agency  of  the  cell  and  also  as  the  portion  primarily  respon- 
sible for  the  control  of  inheritance.  The  nucleus,  surrounded  by  p, 
special  nuclear  membrane,  consists  of  highly  differentiated  protoplasm, 
in  which,  in  properly  prepared  material,  a  delicate  fifcrillar  network,  the 
reticulum,  appears  to  permeate  a  homogeneous  fluid  ground  substance, 
the  karyolymph.  The  most  important  feature  of  the  nuclear  proto- 
plasm is,  however,  the  chromatin  present  in  the  reticulum;1  the 
chromatin  derives  its  name  from  the  fact  that,  during  mitosis,  it 
stains  heavily  with  certain  dyes  used  by  histologists  in  preparing  micro- 
scopic preparations.2  It  is  known  to  be  the  chief  vehicle  for  the  trans- 
mission of  hereditary  characters  from  one  generation  to  the  next,  and, 
with  such  a  major  assignment  in  the  economy  of  life,  its  dominating 
position  is  evident.  The  complicated  methods  of  cell  division,  or 
mitosis,  involved  in  carrying  out  this  transmission  will  be  considered 
in  the  later  chapters  on  Reproduction  and  Heredity.  It  may  be  said 
that  the  appearance  of  chromatin  varies  in  accordance  with  the 
cellular  condition.  In  a  resting  cell,  it  is  more  or  less  granular; 
whereas  in  a  cell  preparing  to  divide,  the  chromatin  condenses  to  form 
a  definite  number  of  chromosomes,  which  are  of  characteristic  size 
and  shape  in  a  particular  species  and  readily  studied  when  properly 
stained.  Also  frequently  present  in  the  nucleus  is  a  minute  spherical 
nucleolus,  the  function  of  which  is  quite  obscure.  (Figs.  9,  15.) 

HUMAN  TISSUES 

Man  as  a  representative  vertebrate  is  a  very  highly  specialized 
organism  with  a  wide  variety  of  body  tissues,  which  arise  compara- 
tively early  in  development  as  the  cells  become  differentiated. 
Differentiation  is  a  gradual  process  and  begins  essentially  with  the 
formation  in  the  early  embryo  of  three  primary  tissues,  ectoderm,  meso- 
derm,  and  endoderm,  which  together  are  known  as  the  primary  germ 
layers.  Later  specialization  among  the  cells  of  the  three  germ  layers 
results  in  the  development  of  the  diversified  tissues  present  in  the  adult 
body.  The  study  of  the  vertebrate  organism  reveals  the  presence  of 
five  basic  types  of  tissue  which  are  almost  omnipresent  throughout  the 
body  structures.  Each  of  these,  as  will  be  seen  later,  is  variously 

1  Reticulum  is  also  designated  as  linin. 

2  Consult  the  section  on  Histology  in  the  Appendix  for  additional  material. 


24  HUMAN  BIOLOGY 

subdivided  into  more  specialized  tissue  types.  The  constituent  tissues 
are  (1)  epithelial  tissue,  which  forms  $  covering  material  over  the 
exposed  surfaces  of  all  organs,  whether  internal  or  external,  and  also 
functions  in  various  other  ways  as  noted  just  below;  (2)  connective 
tissue,  which  forms  the  framework  of  the  body  and  each  of  its  parts, 
giving  support  and  protection  to  the  delicate  cells  and  tissues  and 
forming  the  levers  by  means  of  which  many  of  the  muscular  movements 
are  translated  into  action;  (3)  vascular  tissue,  which  is  responsible  for 
the  transportation  of  a  host  of  essential  materials  to  all  the  cells  of  the 
body  and  also  for  carrying  the  cellular  wastes  to  the  excretory  organs; 
(4)  muscle  tissue,  which  is  specialized  for  contraction  and  responsible 
for  the  continuous  and  almost  innumerable  movements  associated 
with  the  functioning  of  the  typical  animal ;  and,  finally,  (5)  nerve  tissue, 
which  is  the  supreme  coordinating  and  directing  agency  of  the  body 
and,  with  its  associated  sensory  tissues  and  conducting  paths,  con- 
stitutes a  tissue  system  of  the  greatest  complexity  and  supreme  impor- 
tance. With  reference  to  their  origin,  it  is  found  that  the  muscular 
and  vascular  tissues  are  definitely  mesodermal,  that  nerve  tissue  is 
ectodermal,  whereas  all  three  germ  layers  make  their  contribution  to 
the  epithelial  and  connective  tissues.  Mention  should  also  be  made 
of  (6)  the  germinal  tissue,  which  is  localized  in  the  goiiads  and  serves 
primarily  for  the  propagation  of  the  species  rather  than  for  the  needs 
of  the  individual. 

Brief  consideration  may  now  be  given  to  tho  fundamental  struc- 
tural and  functional  features  of  the  five  body  tissues  that  are  inti- 
mately associated  in  the  various  organs  and  organ  systems  to  make 
the  complete  functioning  organism.  In  the  later  chapters  dealing  with 
the  various  organ  systems,  additional  consideration  is  given  to  each 
of  these  tissues  and  associated  organs. 

EPITHELIAL  TISSUE 

A  considerable  variety  of  epithelial  tissues  covers  the  numerous 
exposed  body  surfaces,  both  internal  and  external,  as  well  as  the  lin- 
ings of  cavities  and  ducts.  Thus  the  outer  tissue  of  the  skin  or 
epidermis,  which  is  in  constant  contact  with  the  external  environment, 
consists  of  several  layers  of  epithelial  cells.  The  cells  are  considerably 
modified  according  to  their  position.  The  outermost  layers  are  built 
up  of  exceedingly  thin,  flattened  cells,  lying  in  close  contact  to  form 
a  tile-like  surface,  the  so-called  squamous  epithelium.  Next  in  order, 
below,  are  thicker  cells,  the  shape  of  which  is  well  described  by  the 
term  cuboidal  epithelium.  Finally,  there  is  a  columnar  epithelium,  in 
which  the  cells  are  found  to  be  elongated,  more  or  less  tubular  units. 


THE  ORGANIZATION  OF  THE  HUMAN  BODY 


25 


The  type  of  epithelium  just  described,  which  consists  of  a  number  of 
cellular  layers  with  a  gradual  change  in  shape,  is  commonly  termed 
stratified  epithelium  and  may  be  found  covering  various  body  struc- 
tures. Other  important  covering  types  of  epithelium  are  found  in  the 


FIG.  10. — Stratified  epithelium 
from  the  skin  of  the  frog.  c.br., 
cell  bridges;  col.,  columnar  cells; 
cu.,  cuboidal  cells;  sq.,  squamous 
cells  of  outer  surface.  (Weber,  Val- 
entine.) 


V-'1  ''!•"'  "''*'T"V'l 


FIG.  17.— Ciliated  epithelial  cells 
from  the  intestinal  lining  of  a  clam. 
b.g.,  basal  granules;  cil.,  cilia;  nu., 
nucleus;  s.va.,  secretory  vacuole;  va., 
cell  vacuole.  Highly  magnified. 
(Weber,  Valentine.) 


peritoneal  epithelium  lining  the  abdominal  cavity  and  in  the  epithe- 
lium of  the  alimentary  canal  which  covers  it  inside  and  out.  The 
epithelium  lining  the  alimentary  canal,  endodermal  in  origin,  is  of  par- 
ticular importance  because  it  consists  of  the  nutritive  cells  of  the  body 
which  are  essential  to  the  digestion  and  absorption  of  the  foodstuffs. 
(Fig.  16.) 

The  cells  of  another  important  type  of  epithelium  bear  cilia  ori 
the  exposed  surfaces  and  accordingly  give  rise  to  ciliated  epithelium. 
This  type  of  epithelial  tissue  is  found  in  the  lining  of  various  ducts 
and  tubular  structures  in  the  body  where  movement  of  the  enclosed 
liquids  and  other  substances  is  required.  This  is  accomplished  by  the 
coordinated  action  of  the  beating  cilia.  Examples  of  ciliated  epithe- 
lium may  be  found  in  the  lining  of  the  nasal  and  throat  cavities,  in 
the  oviducts,  and  in  ducts  of  the  kidneys.  (Fig.  17.) 

Many  types  of  gland,  which  manufacture  and  secrete  important 
substances,  are  developed  from  specialized  epithelial  cells  and  so  give 
rise  to  glandular  epithelium.  Such  glands  may  be  unicellular  glands 
or  goblet  cells,  each  of  which  is  formed  from  a  single  secreting  cell, 
as  found  in  the  epithelium  lining  various  regions  of  the  alimentary 
canal  (page  52).  It  will  be  found,  however,  that  most  of  the  glands 
present  in  the  body  are  multicellular.  Common  examples  of  these 
are  seen  in  the  sweat  glands  of  the  skin  and  also  in  the  sebaceous  glands 


26 


HUMAN  BIOLOGY 


MUC.' 


FIG.  18.-— Glandular 
epithelium.  Two  goblet 
cells  are  shown  from  the 
nutritive  epithelium  of  a 
vertebrate,  with  the  ex- 
trusion of  the  mucous 
secretion.  Highly  mag- 
nified, muc.,  mucus  in 
glandular  portion  of  gob- 
let cell;  nu.,  nucleus  of 
typical  epithelial  cell. 
(Wolcott.) 


of  the  hair,  which  secrete  an  oily  substance.  Noteworthy  are  the 
paired  mammary  glands  of  the  mammalian  female,  which  secrete  an 
abundant  supply  of  milk  to  nourish  the  newly 
born  offspring.  In  certain  domesticated  ani- 
mals, notably  the  goat  and  cow,  the  mammary 
glands  are  particularly  well  developed,  and  the 
milk  that  they  synthesize  is  highly  desirable 
for  human  consumption.  (Fig.  18.) 

Finally,  the  surface  epithelial  cells  of  tho 
body  are,  in  numerous  instances,  modified  to 
form  peripheral  sense  organs,  which  function  in 
association  with  the  nervous  system.  Such 
epithelial  cells  form  a  very  important  type  of 
ectodermal  epithelium,  the  sensory  or  nervous 
epithelium,  examples  of  which  are  to  be  found 
widely  distributed  in  the  epidermis  of  the  skin, 
where  they  respond  to  cold,  heat,  pressure,  etc.  The  sense  of  taste 
and  the  amazingly  keen  olfactory  sense  are  also  due  to  the  activities 
of  sensory  epithelial  cells. 

CONNECTIVE  AND  SUPPORTING  TISSUES 

Multicellular  animals  require  a  considerable  variety  of  connective 
and  supporting  tissues,  widely  distributed  throughout  every  region 
of  the  body,  for  the  support  and  protection  of  the  various  organs. 
These  tissues  are  commonly  divided  into  the  exoskeletal  arid  the  endo- 
skeletal  types  and  attain  their  highest  development  in  the  vertebrate 
animals. 

The  outer,  or  exoskeletal,  tissues  form  a  more  or  less  complete 
protective  covering  over  the  underlying  soft  tissues.  In  the  verte- 
brates, the  exoskeleton  develops  primarily  by  the  transformation  of 
skin  tissues.  In  certain  vertebrates  such  as  the  turtle,  the  exoskeleton 
forms  a  protective  covering  over  practically  the  entire  body.  In  the 
majority  of  vertebrates,  however,  the  exoskeletal  structures,  repre- 
sented by  hair,  feathers,  nails,  claws,  or  scales,  are  more  or  less  restricted 
in  their  coverage  and  may  even  be  entirely  lacking,  as  in  the  frog. 

The  endoskeletal  structures  are  internal,  comprising  many  types 
adapted  to  widely  varying  needs  and  culminating  in  bone  tissue, 
which  is  regarded  as  the  highest  development  of  the  endoskeleton. 
In  general,  the  endoskeletal  tissues  contain  a  relatively  large  amount 
of  collagenous  ground  substance,  or  matrix,  which  is  intercellular  in 
position;  that  is,  it  lies  between  the  cells,  not  enclosed  by  the  cell  walls 
In  some  of  the  connective  tissues,  the  ground  substance  constitutes  by 


THE  ORGANIZATION  OF  THE  HUMAN  BODY 


27 


far  the  larger  amount  of  material.  It  begins  to  develop  very  early 
in  the  embryo  as  a  secreted,  nonliving  substance  which  in  time  becomes 
variously  modified  as  may  be  required  for  a  particular  type  of  tissue. 
For  example,  the  ground  substance  may  remain  largely  unchanged,  as 
in  some  of  the  less  differentiated  types  of  connective  tissue;  it  may 
become  transformed  into  a  dense  fibrillar  material,  as  in  the  tendons 
that  connect  muscles  with  bones;  or  it  may  become  heavily  infiltrated 
with  inorganic  lime  salts,  chiefly  calcium  carbonate,  and  form  the  main 
substance  of  the  hard  bone  tissues.  (Fig.  19.) 


Havers! an  cancrf 


^Fiberbundfe 


Connective 
tissue  cor- 
pusclc 


B 


Cartilage 
cell 


FIG.  19. — Various  typos  of  vertebrate  connective  and  supporting  tissues;  somewhat 
diagrammatic.  A,  bone;  B,  white  fibrous  tissue;  C,  fatty  (adipose)  tissue;  D,  tendon 
largely  composed  of  bundles  of  white  fibers;  E,  cartilage.  Highly  magnified.  (Wolcott.') 

Five  basic  types  of  endoskeletal  tissue  are  usually  "recognized  by 
the  histologists.  The  essential  features  of  each  may  be  briefly  indi- 
cated at  this  time  and  then  left  for  further  consideration  in  the  later 
chapter  on  the  Skeletal  System. 

White  fibrous  tissue  is  widely  distributed  throughout  the  vertebrate 
body  and  accordingly  may  be  obtained  from  almost  any  region  as, 
for  example,  in  the  skin,  permeating  and  surrounding  the  muscles, 
nerves,  and  various  organs  of  the  body.  Tendons,  which  connect 
muscles  with  bones,  are  almost  entirely  white  fibrous  tissue.  (Fig.  195.) 

Elastic  tissue  is  found  chiefly  in  the  walls  of  the  blood  vessels  but 
also  in  various  other  places  where  give  and  take  is  essential,  notably 
in  the  ligaments,  which  are  highly  elastic.  Elastic  tissue  is  constructed 


28  HUMAN  BIOLOGY 

in  such  a  way  that  it  will  stretch  and  then  return  to  the  original  con- 
dition when  the  tension  is  released.  This  is  the  reverse  condition 
from  that  found  in  the  white  fibrous  tendons,  which  lack  elasticity  and 
therefore  transmit  the  full  strength  of  the  muscle  contraction  to  the 
attached  bones.  (Fig.  19£,  D.) 

Fatty,  or  adipose,  tissue  is  generally  regarded  as  a  modified  type  of 
fibrous  tissue  in  which  the  cells  have  become  enlarged  and  adapted  for 
the  storage  of  fat  resulting  from  an  excess  of  nutritive  materials.  It 
is  rather  widely  distributed  throughout  the  body.  (Fig.  19C.) 

Cartilage  is  a  highly  developed  connective  tissue  which  is  par- 
ticularly abundant  in  the  vertebrate  embryo.  In  the  lower  types  of 
vertebrates,  cartilage  remains  throughout  life  as  a  permanent  skeletal 
framework,  but,  in  the  higher  types,  it  is  largely  replaced  by  bone 
tissue.  Even  in  the  highest  animals,  various  cartilaginous  areas 
remain  unchanged,  as  in  the  joints,  between  the  separate  vertebrae  of 
the  backbone,  in  the  nose,  and  in  the  outer  ear.  Cartilage  is  character- 
ized by  the  presence  of  an  exceptionally  large  amount  of  a  transparent 
collagenous  ground  substance  possessing  considerable  elasticity  and 
great  strength.  (Fig.  1QE.) 

Bone,  for  the  most  part,  is  first  laid  down  in  the  embryo  as  cartilage. 
Such  bones  are  known  as  cartilage  bones  in  contradistinction  to  the 
much  less  common  membrane  bones  in  which  the  bone  tissue  is  formed 
by  the  gradual  ossification  of  soft  fibrous  tissue  membranes  rather 
than  by  the  cartilage  transformation.  The  general  structure  of  bone 
is  highly  intricate,  with  the  ossified  matrix  arranged  in  concentric 
layers  and  containing  numerous  widely  branching  cavities  in  which 
the  bone  cells  lie.  It  is  covered  on  the  outside  by  a  soft  connective 
tissue  sheath,  the  periosteum,  which  is  continuous  with  the  tendons. 
Bones  typically  contain  a  central  cavity  filled  with  a  soft,  highly 
vascularized  tissue,  the  bone  marrow,  which  has  a  very  important 
function  in  association  with  the  vascular  system  in  blood  cell  formation. 
The  human  bony  skeleton  comprises  some  200  separate  bones  and  is 
of  major  importance  in  various  functional  activities  of  the  organism. 
(Fig.  19 A.) 

VASCULAR  TISSUE 

Permeating  every  nook  and  cranny  of  the  human  body  is  the 
vascular  tissue  which  functions  as  a  continuous  transportation  system. 
Vascular  tissue  does  not  occur  in  the  relatively  simple  animals  with  a 
tiny  body  and  low  degree  of  tissue  differentiation.  It  is  first  present 
in  what  we  may  term  the  "  earth  worm  stage  of  development "  and  is  of 
increasing  prominence  throughout  the  vertebrate  series,  where  it 


THE  ORGANIZATION  OF  THE  tiUMAN  BODY 


29 


constitutes  one  of  the  most  complex  and  prominent  of  all  the  organ 
systems.  Vascular  tissue  may  possibly  best  be  thought  of  as  con- 
sisting of  (1)  a  specialized  type  of  epithelium,  the  eridothelium,  which 
lines  every  type  of  blood  vessel  throughout  the  organism;  and  (2)  a 
liquid  tissue,  the  blood,  the  only  example  to  be  found  in  the  body  in 
which  the  intercellular  ground  substance  is  fluid  in  nature  and  permits 
the  blood  cells  to  float  freely  in  it. 

The  liquid  portion  of  the  blood,  the  plasma,  is  not  regarded  as  living 
material.  It  is  colloidal  in  nature  and  a  very  heterogeneous  mixture — 
a  temporary  storehouse,  as  it  were,  for  the  multitudinous  nutritive 
requirements  of  all  the  body  cells  and  for  their  secretions  and  excre- 
tions as  well.  Under  the  proper  con- 
ditions, the  blood  plasma  clots  to  form 
a  permanent  gel;  the  latter  is  essential 
in  the  stoppage  of  blood  flow,  as  in  an 
injury.  Blood  coagulation  is  also 
probably  linked  up  with  the  formation 
of  the  connective  tissue  ground  sub- 
stance. In  fact,  vascular  tissue  is  com- 
monly regarded  as  one  of  the  various 
types  of  connective  tissues. 

The  living  cellular  elements  of  the 
vascular  tissue  consist  of  the  blood  cells 
circulating  in  the  plasma  and  the 
endothelial  cells  lining  the  blood  vessels. 
The  latter  are  believed  to  function  in 
the  formation  and  secretion  of  the  blood 
plasma  and  also,  to  some  extent,  in  the 
formation  of  the  specialized  blood  cells. 
To  the  formation  of  the  latter,  the  bone  marrow  and  spleen  tissue 
also  make  important  contributions.  The  complete  picture  of  the 
functional  and  structural  attributes  of  the  blood  may  be  deferred  for 
later  consideration  as  an  organ  system.  (Fig.  20.) 


nu.. 


Fio.  20. — Blood  cells  of  the 
frog:  three  nucleated  red  blood 
corpuscles  (erythrocytes)  and  one 
amoeboid  white  cell  (leucocyte). 
Highly  magnified,  i.f.a.,  ingested 
foreign  substances;  nu.t  nucleus. 
(Weber,  Valentine.) 


MUSCLE  TISSUE 

Muscle  tissue,  since  it  is  the  source  of  power  for  all  bodily  move- 
ments, is  necessarily  very  widely  distributed  throughout  the  body. 
It  may  be  divided  into  three  distinct  types:  smooth,  striated,  and 
cardiac.  Of  these,  only  the  striated  mutfcle  tissue  is  voluntary,  that  is, 
under  the  direct  control  of  the  will.  The  smooth  muscle  tissue  is  under 
the  involuntary  control  of  the  autonomic  nervous  system,  whereas  the 


30 


HUMAN 


cardiac  type,  found  only  in  the  heart,  possesses  an  inherent  power  of 
rhythmic  contraction  which,  however,  is  subject  to  general  regulation 
by  nerve  impulses.  (Figs.  12,  21.) 

The  smooth,  or  involuntary,  muscle  tissue  is  regarded  as  the 
simplest  type  structurally.  It  is  widely  distributed,  forming  the 
muscular  layers  in  the  walls  of  a  number  of  important  organs,  such  as 
those  of  the  alimentary  canal,  blood  vessels,  and  urinary  bladder.  A 
microscopic  examination  shows  that  it  consists  of  long  pointed  cells, 
each  with  a  prominent  nucleus  elongated  in  the  same  direction  as  the 
cell  body.  The  cells  are  frequently  branched  at  the  ends.  Also  their 
length  varies  considerably.  For  example,  in  the  walls  of  the  blood 
vessels  they  are  typically  short  and  thick,  whereas  in  the  walls  of  the 
bladder  they  tend  to  be  long  and  thin.  The  cytoplasm  shows  a  fine, 

longitudinal  striation  which  is  very 
different  from  the  marked  transverse 
striation  characteristic  of  the  striated 
muscle  tissue.  The  smooth  muscle 
cells  lie  close  together  and  are  essen- 
tially embedded  in  an  intercellular 
matrix  of  connective  tissue  elements. 
So  they  are  closely  held  together  and 
work  as  a  unit  in  the  muscular  con- 
traction. The  rate  of  contraction  of 
smooth  muscle  is  much  slower  than 
that  of  striated  muscle  tissue,  but 
the  movements  can  be  continued 
almost  indefinitely  without  tiring. 
(Fig.  21J3.) 

Striated,  or  voluntary,  muscle  tissue  is  largely  localized  in  the 
muscles  of  the  body  wall  and  those  of  the  arms  and  legs,  where  we 
find  it  divided  into  definite  contractile  units,  the  muscles  proper. 
Altogether,  there  are  some  374  voluntary  muscles  in  the  body.  Muscle 
contraction  is  translated  into  bodily  movement  through  attachment 
to  the  connective  tissues.  Thus  each  muscle  is  enclosed  in  a  connec- 
tive tissue  sheath  which  continues  beyond  the  ends  of  the  muscles  as 
a  tendon.  The  latter  is  attached  directly  to  a  bone  which  may  serve 
either  as  a  lever  for  movements  or  as  an  anchor.  Muscle  tissue  does 
work  only  when  it  contracts  and  the  pull  is  translated  to  the  moveable 
bone.  Movements  in  opposite  directions  require  that  the  muscles 
work  in  pairs.  The  members  of  a  pair  of  muscles  are  so  mounted  that 
the  contraction  of  one  muscle  causes  movement  in  the  opposite  direc- 
tion from  that  of  the  other.  For  example,  adductor  muscles  on  con- 


Contrac+ile  fibril 

A  B 

FIG.  21. — Vertebrate  muscle  tissue. 
A,  portion  of  striated  muscle  fiber;  B, 
three  isolated  cells  of  smooth  muscle 
tissue.  Highly  magnified.  (Wolcott.) 


THE  ORGANIZATION  OF  THE  HUMAN  BODY  31 

traction  draw  the  leg  backward  toward  the  long  axis  of  the  body,  and 
the  opposed  abductor  muscles  draw  the  leg  anteriorly.  (Fig.  87.) 

The  structural  and  functional  units  of  striated  muscle  tissue  are 
the  microscopic  muscle  fibers  which  are  associated  in  great  numbers  to 
form  the  various  muscles  of  the  body.  Each  fiber  is  enclosed  in  a 
delicate  connective  tissue  membrane  and  contains  several  nuclei.  It 
is  believed  that  each  muscle  fiber  represents  a  greatly  modified  single 
cell,  in  which  the  original  nucleus  has  divided  several  times  without 
corresponding  divisions  of  the  cell  body.  The  cytoplasm  of  the  striated 
fibers  exhibits  both  transverse  and  longitudinal  striations,  the  former 
being  much  the  more  prominent.  When  contraction  occurs,  the 
alternating  transverse  bands  become  broader  as  the  length  of  the  fiber 
decreases.  (Fig.  21  A.) 

Cardiac  muscle  tissue,  which  is  localized  in  the  walls  of  the  heart, 
is  regarded  as  a  distinct  type  of  muscle  tissue  though  showing  struc- 
tural relationships  to  both  smooth  and  striated  tissues.  Thus  there  is 
a  distinct  transverse  striation,  as  in  voluntary  muscle;  but  on  the 
other  hand,  the  cardiac  cells,  though  .connected  by  cytoplasmic  strands, 
retain  their  individuality  much  as  in  the  unstriated  tissue.  (Fig.  12.) 

NERVE  TISSUE 

The  beginnings  of  the  specialized  irritable  nerve  tissue  in  the 
animal  body  are  found  in  simpler  animals  than  those  in  which  vascular 
tissue  is  first  noted.  Thus,  in  the  tiny  fresh-water  polyp  Hydra, 
differentiation  among  the  outer  ectodermal  cells  produces  branching 
nerve  cells  with  long  processes  which  are  receptive  to  stimuli  and  which 
also  stimulate  contractile  cells  in  the  body  wall  to  coordinated  move- 
ment. But  increasingly,  in  the  higher  animal  types,  the  highly 
differentiated  cells  of  the  nerve  tissue  are  grouped  together  to  form 
the  most  complex  tissue  system  of  the  body  and  one  that  is  possibly 
even  more  widely  distributed  than  the  elements  of  the  vascular  system. 
The  cellular  unit  of  nerve  tissue  is  the  neuron,  which  is  always  ecto- 
dermal in  origin  but  develops  into  a  variety  of  types  essential  to  the 
various  positions  and  functions  assigned  to  them.  Three  main  groups 
of  neurons  are  recognized  as  follows:  the  sensory  neurons,  which  are 
located  in  the  skin  and  the  various  external  and  internal  sense  organs 
and  serve  as  outposts  for  the  reception  of  the  infinitude  of  stimuli 
projected  upon  the  organism;  the  motor  neurons,  which  are  concerned 
with  stimulating  the  proper  muscle  elements ;  and  the  adjuster  neurons 
(association  or  integrative)  in  the  brain  and  spinal  cord,  which  mediate 
between  the  sensory  and  motor  neurons  to  bring  about  integrated 


32  HUMAN  BIOLOGY 

responses.  Certain  of  the  neurons  of  the  brain  cortex,  primarily  con- 
cerned with  the  higher  mental  processes,  possibly  constitute  a  fourth 
type  of  neuron.  Structurally  the  cell  bodies  of  every  type  of  neuron 
are  characterized  by  cytoplasmic  processes  of  varying  length  and 
branching  over  which  the  nerve  impulses  travel.  (Fig.  22.) 


FIG.  22. — Nerve  cell  (neuron)   from  the  central  nervous  system  of  u  rabbit.      Highly 
magnified,     a.,  axon;  d.t  dendritc;  nu.,  nucleus;  mts.,  nucleolus.      (Weber,   Valentine.) 

ORGANS  AND  ORGAN  SYSTEMS 

The  next  step  in  the  organization  of  the  animal  body  beyond  that  of 
tissue  development  is  the  permanent  association  of  various  tissues  to 
form  definite  structural  and  functional  units,  the  organs.  Examples 
of  organs  are  to  be  seen  in  such  commonly  recognized  parts  of  the  body 
as  the  eye,  heart,  and  stomach.  An  examination  of  any  organ  reveals 
the  fact  that  it  is  built  up  not  of  one  tissue  alone  but  of  several— a 
mosaic,  as  it  were,  in  which  each  of  the  associated  tissues  has  its 
characteristic  assignments.  However,  it  is  also  clear  that  in  most 
organs  one  of  the  associated  tissues  is  essential  for  the  particular 
function  assigned  to  that  organ  in  the  economy  of  the  organism, 
whereas  the  other  associated  tissues  of  such  an  organ  are  accessory. 
Thus,  for  example,  in  the  alimentary  canal,  the  functional  activities 
center  in  the  nutritive  epithelium  which  forms  the  inner  lining  and  is 
responsible  for  the  digestion  and  absorption  of  food.  Associated  with 
this  essential  nutritive  epithelium  to  form  the  complete  organ  arc  othei* 


THE  ORGANIZATION  OF  THE  HUMAN  BODY  33 

accessory  tissues,  namely,  muscle  tissues  in  the  wall  which  function 
in  moving  the  food  materials  through  the  alimentary  canal  by  peri- 
staltic contractions ;  vascular  tissue  which  receives  the  digested  foods 
and  transports  them  to  all  regions  of  the  body;  nerve  tissue  which 
controls  and  coordinates  all  the  constituent  tissues  of  the  nutritive 
system;  and,  finally,  the  connective  tissue  elements  which  bind  all  the 
functionally  associated  tissues  into  a  structural  unity.  (Fig.  32.) 

Another  good  example  of  the  association  of  diverse  tissues  in  an 
organ  may  be  found  in  the  eye,  in  which  the  essential  functional  tissue 
is  a  very  complex,  inner  layer  of  nerve  tissue,  the  retina,  on  which  the 
light  rays  impinge  and  act  as  a  stimulus.  In  addition,  the  eye  contains 
various  accessory  tissues  including  nerve  fibers  for  conducting  the 
stimuli  received  in  the  retina  to  the  brain :  the  transparent  lens,  which 
focuses  the  light  rays  on  the  retina;  the  connective  tissues  forming  the 
protective  layers  of  the  eyeball  and  permeating  the  retina  itself  to 
hold  the  functional  units  together;  the  muscular  tissue  responsible  for 
the  movements  of  the  eyeball  and  also  governing  the  amount  of  light 
admitted  to  the  interior  of  the  eye.  Finally,  all  of  the  eye  tissues  are 
permeated  with  tiny  branches  of  the  vascular  system.  (Plate  XIII, 
p.  229.) 

One  final  stage  in  the  organization  of  the  well-developed  animal 
body  is  found  in  the  organ  systems  in  which  the  organs  associated  with 
a  particular  function  are  grouped  together  for  the  performance  of 
the  essential  functions  of  the  organism.  Thus,  in  man,  we  have  the 
nutritive  system,  the  respiratory  system,  the  secretory  system,  the 
excretory  system,  the  vascular  system,  the  motor  or  muscular  system, 
the  skeletal  system,  the  nervous  system,  and  the  reproductive  system, 
all  of  which  represent  complete  functioning  units  of  the  organism. 
Each  of  these  organ  systems  will  be  found  to  consist  of  an  assemblage 
of  integrating  structural  units,  or  organs.  Thus,  in  the  vascular 
system,  the  heart  is  an  organ  of  first  importance  functioning  as  a 
powerful  pump,  but  the  complete  vascular  system  includes  not  only 
the  heart  but  also  the  blood,  the  blood-forming  tissues,  and  literally 
miles  of  tubes  of  different  sizes  through  which  the  liquid  blood  is 
supplied  to  all  the  tissues.  And  so  it  is  with  all  the  organ  systems. 

In  summarizing,  it  is  seen  that  the  human  organism  begins  its 
individual  existence  as  a  single  microscopic  cell  and,  in  time,  becomes 
a  multicellular  unit  with  many  billions  of  cells.  And  as  the  cells  are 
increased  in  number,  so  are  they  increasingly  set  apart  by  differentia- 
tion to  form*the  basic  tissues  of  the  body,  and  these,  in  turn,  are  further 
differentiated  to  form  more  and  more  specific  types.  But  the  tissues 
do  not  remain  separated  functionally;  they  group  together  to  form 


34 


HUMAN  BIOLOGY 


definite  functional  organs,  which  are  linked  to  form  the  organ  systems ; 
the  sum  total  of  which  comprises  the  complete  organism. 

THE  BODY  PLAN 

Having  considered  the  general  organization  of  the  living  materials 
in  the  human  organism,  extending  from  cellular  levels  to  organ  systems, 
attention  may  next  be  centered  on  the  complete  structural  plan  of  the 
human  body.  It  is  at  once  evident  to  the  biologist  that  there  is  an 
underlying  relationship  to  certain  features  first  apparent  in  the  animal 
world  in  the  earthworm  type  of  organism;  that  is  to  say,  the  human 
body  does  not  exhibit  a  startling  array  of  new  anatomical  features  but 


Mesonephros 
,Notochord 


Pharynx 


Coehm 


Gill 
Slits 


Urinary 
bladder 


..^r—  .       .  C/oaccr 

1  Bite  duct    I        / 
Liver  I  Intestine 

Heart'   Stomach  Spleen 

Oviduct 

FIG.  23. — Body  plan  of  a  typical  lower  vertebrate,  female,  as  seen  in  a  median 
longitudinal  section.  Diagrammatic.  (Wolcolt,  after  Wiedersheim.  Redrawn  'with 
modifications.) 

rather  modifications  and  further  development  of  structural  plans 
which  the  comparative  anatomist  has  seen  incompletely  expressed  in 
various  of  the  lower  types  of  animal  life.  Of  particular  importance 
are  certain  zoological  landmarks  shown  at  the  earthworm  level. 
These  include  the  three-layered  or  triploblastic  condition  of  the  body 
in  which  the  tissues  are  derived  during  development  from  three  pri- 
mary germ  layers :  ectoderm,  endoderm,  and  mesoderm ;  the  coelomate 
type  of  structure  in  which  the  body  plan  is  best  described  as  a  tube 
within  a  tube,  the  outer  tube  forming  the  body  wall  and  the  inner,  the 
alimentary  canal;  bilateral  symmetry  which  is  basically  two-sided — 
right  and  left — with  the  organs,  as  a  rule,  developed  in  pairs  and 
lying  to  the  right  and  left  of  a  median  line  so  that  only  one  plane 
divides  the  body  into  symmetrical  halves;  the  segmented  body  with  the 


THE  ORGANIZATION  OF  THE  HUMAN  BODY  35 

segments  arranged  in  a  linear  series;  and,  finally,  the  grouping  of 
tissues  to  form  highly  specialized  organs  and  organ  systems. 

To  these  important  landmarks  in  animal  organization  have  been 
added,  in  the  body  of  man,  several  other  structural  features  that  are 
characteristic  of  the  vertebrates  in  general.  There  is,  in  the  first 
place,  an  internal  framework  of  supporting  tissues,  the  endoskeleton, 
which  reaches  its  climax  in  the  formation  of  bone  tissue.  One  of  the 
principal  parts  of  the  bony  endoskeleton  is  the  vertical  axis  or  back- 
bone (vertebral  column)  built  up  of  several  segments  (vertebrae)  and 
with  two  pairs  of  jointed  appendages  attached  to  it.  Then  there  is  a 
dorsal,  tubular  nerve  cord  which  receives  protection  and  support 
from  the  vertebral  column  that  encloses  it.  Finally,  there  is  a  four- 
chambered  heart,  lying  ventrally.  (Fig.  23.) 

The  human  body  is  divided  into  two  major  divisions,  the  head 
and  trunk,  which  are  connected  by  a  lesser  division,  the  neck.  The 
head  consists  of  the  facial  portion,  with  terminal  accommodations  for 
the  nutritive,  respiratory,  and  sensory  functions;  and  of  the  skull,  or 
cranium,  which  is  essentially  a  brain  case.  Comparatively  speaking, 
the  human  head  is  noteworthy  for  a  great  increase  in  the  size  of  the 
cranium  and  a  corresponding  reduction  in  the  facial  portion.  Thus 
the  horse  has  a  facial  portion  several  times  the  size  of  the  human  face, 
whereas  tho  brain  case  is  smaller  than  that  of  man.  Vertebrate 
capitalization  reaches  a  climax  in  man  through  the  very  extensive 
development  of  the  forebrain  and  of  the  bony  skull,  which  provides  a 
complete  rigid  armor  of  great  strength  to  protect  the  extraordinarily 
delicate  brain  tissues.  (Fig.  102.) 

The  trunk  region  is  naturally  divided  into  the  anterior  chest  region, 
or  thorax,  and  the  posterior  abdominal  cavity.  Internally  these  two 
regions  are  separated  in  man  and  the  higher  vertebrates  by  a  mem- 
branous sheet  of  tissue,  the  diaphragm.  The  thorax,  containing  the 
lungs  and  heart,  is  given  over  almost  entirely  to  the  respiratory  and 
vascular  functions.  Considerable  reinforcement  of  the  body  wall  is 
attained  in  the  thorax  by  the  12  pairs  of  encircling  ribs,  which  provide 
a  great  deal  of  protection  to  the  comparatively  delicate  and  vitally 
important  tissues  of  the  heart  and  lungs.  It  may  be  noted  that  the 
ribs  are  also  important  in  the  respiratory  movements  that  occur  through 
the  action  of  the  attached  muscles.  The  abdominal  cavity  contains 
the  entire  length  of  the  alimentary  canal  except  the  small  portion,  the 
esophagus,  which  connects  the  throat  region  with  the  stomach.  The 
esophagus  passes  upward  through  the  diaphragm,  above  the  stomach, 
then  continues  anteriorly  through  the  thorax  to  the  throat.  Several 
other  important  organs  are  present  in  the  abdominal  cavity,  notably 


36  HUMAN  BIOLOGY 

the  liver,  pancreas,  spleen,  and  kidney,  together  with  the  major 
elements  of  the  autonomic  nervous  system  which  is  the  controlling 
agent  for  the  entire  group.  Bony  tissue  is  lacking  in  the  wall  of  the 
abdomen.  The  latter  consists  merely  of  the  skin  attached  to  the  under- 
lying muscles  by  the  subcutaneous  fibrous  connective  tissue,  the 
various  layers  forming  a  resistant  but  not  too  rigid  retaining  wall  for 
the  enclosed  organs.  (Plate  III,  page  40  and  Plate  X,  page  168.) 

Attached  to  the  trunk,  or,  rather,  to  the  vertebral  column  in  the 
trunk  region,  are  two  pairs  of  jointed,  five-fingered  appendages,  the 
arms  and  legs,  which  are  homologous  with  the  appendages  present 
throughout  the  vertebrate  series.  Although  the  vertebrate  append- 
ages are  basically  organs  for  locomotion,  the  erect  posture  of  man, 
with  bipedal  instead  of  quadrupedal  locomotion,  has  left  the  fore- 
limbs  free  for  the  performance  of  a  myriad  of  important  duties  to  which 
they  are  remarkably  adapted.  Nowhere  in  nature  is  a  more  adaptable 
structural  unit  to  be  found.  When  the  human  hand  is  compared  with 
the  hoofed  appendages  of  the  horse  and  ox,  the  tremendous  advantages 
that  have  accrued  to  man  through  the  possession  of  his  amazingly 
versatile  hands  become  at  once  apparent.  But  even  so,  the  quad- 
rupedal type  of  locomotion  would  have  largely  nullified  the  uses  of 
even  so  extraordinary  a  structure  as  the  human  hand.  And  it  is  also 
evident  that  the  erect  posture  of  man  is  of  prime  importance  in  that, 
it  has  increased  his  outlook,  as  is  indicated  in  the  admonition  to 
"keep  head  erect  and  look  things  straight  in  the  face  like  a  man." 
An  erect  body  posture  is  not  easy  to  maintain  and  requires  coordinated 
control  of  numerous  muscles  under  the  continuous  supervision  of  the 
nervous  system.  Maintenance  of  an  erect  position  and  the  associated 
bipedal  locomotion  are  learned  by  intensive  effort  in  early  life  and  then 
become  an  automatic  function  which,  under  normal  conditions,  is 
controlled  involuntarily. 

Head,  neck,  trunk,  and  limbs— these  constitute  the  prominent 
external  structural  divisions  of  the  human  body.  But,  as  already 
shown  in  the  earlier  pages  of  this  chapter,  the  external  characteristics 
give  essentially  no  idea  of  the  complexities  present  internally  in  the 
association  of  cells,  tissues,  and  organs  responsible  for  the  inherent 
functional  phenomena  essential  to  life. 

THE  SKIN 

Finally  it  will  be  worth  while  to  focus  attention  upon  a  remarkable 
material,  the  skin,  which  forms  the  covering  over  practically  the  entire 
body  and  which  possesses  noteworthy  properties  essential  to  the  under- 
lying body  structures.  But  the  skin  is  far  more  than  a  resistant, 


THE  ORGANIZATION  OF  THE  HUMAN  BODY 


37 


covering  material,  for  it  functions  also  in  connection  with  temperature 
control,  excretion,  and  as  an  efficient  sensory  organ  equipped  for  the 
detection  of  a  wide  variety  of  environmental  stimuli,  so  that  the 
organism  is  able  to  keep  in  touch  and  adapt  to  the  external  conditions. 
Possibly  above  all  else,  the  vertebrate  skin  stands  as  a  tremendously 
effective  barrier  between  the  body  and  all  sorts  of  destructive  parasites 
which  otherwise  would  invade  the  body  tissues  and  produce  disease. 
Very  few  disease-producing  organisms  are  known  which  arc  able  to 
penetrate  the  unbroken  skin  of  the  human  body.  (Fig.  24.) 

Examined  microscopically,  the  skin  is  found  to  be  divided  into  two 
main  portions:  an  outer  epidermis  and  an  underlying  dermis,  or  corium. 

SEBACEOUS 

GLAND 


EPIDERMIS 


MALPJGHIAN 
LAYER 


ERECTOR  MUSCLE 
OF  THE  HAIR 


DERMIS 
(CORIUM) 


!WEAT  GLANP 
BLOOD  VESSEL 

-FAT  IN 

SUBCUTANEOUS 
TISSUE 

"NERVE 

FIG.   24.— -Mammalian   skin,   as  seen  in   a   vertical   section.     Diagrammatic.     Highly 
magnified.     (Redrawn  from  Wolcott.} 

Consider,  first,  the  epidermis;  it  consists  essentially  of  numerous 
layers  of  epithelial  cells  which  adhere  tenaciously  to  each  other, 
though  with  a  minimum  of  intercellular  material,  to  form  a  resistant 
material  suitable  for  contact  with  the  environment.  Continued 
examination  of  the  epidermis  reveals  a  rather  clear  differentiation  into 
a  relatively  thick,  horny  outer  region,  below  which  is  a  thinner  Mal- 
pighian  region  joining  with  the  upper  boundary  of  the  dermis.  The 
cells  in  the  outer  layers  of  the  epidermis  are  very  much  flattened  and 
are  mostly  clead  anucleate  bodies  characterized  by  heavy  cell  walls, 
particularly  on  the  palms  of  the  hands  and  the  soles  of  the  feet  where 
they  are  subject  to  almost  continuous  friction.  The  dead  epidermal 
cells  are  constantly  being  shed,  and  new  ones  supplied  from  the  under- 
lying cells  in  the  Malpighian  region. 


38  HUMAN  BIOLOGY 

Tiny  openings  of  the  sweat  glands  perforate  the  epidermis  in 
practically  all  regions  of  the  body.  It  is  estimated  that  a  total  of 
around  2)^  millions  of  these  openings  occur  in  the  entire  body  surface. 
They  are  most  abundant  in  the  palms  and  soles,  where  there  are  pos- 
sibly 2,500  per  square  inch,  and  are  fewest  in  number  on  the  back  of 
the  body  where  an  average  of  around  400  to  the  square  inch  is  esti- 
mated. The  body  of  a  sweat  gland,  the  secreting  portion,  lies  deep  in 
the  dermis  and  is  seen  as  a  tiny  sac-like  structure  surrounded  by  a  net 
of  blood  capillaries.  Thus  the  gland  consists  essentially  of  a  fine  tube, 
coiled  at  one  end,  and  with  the  opposite  end  continuing  as  a  straight 
tube  to  the  external  opening  at  the  surface  of  the  skin.  The  amount  of 
liquid  removed  from  the  blood  and  released  by  the  sweat  glands  is 
considerable,  but  both  the  quantity  and  composition  of  sweat  varies 
a  great  deal  under  different  conditions  of  temperature  and  activity,  as 
will  be  seen  later  in  considering  the  question  of  temperature  control. 

In  contrast  to  the  simplicity  of  the  outer  epidermis  is  the  relative 
complexity  of  the  underlying  corium  with  its  intimate  association  of 
vascular  and  nervous  tissues,  together  with  very  numerous  hair 
follicles  in  which  the  hairs  are  developed  and  nourished.  Microscopic 
examination  of  a  transverse  section  through  the  skin  shows  that  the 
upper  dermal  boundary,  lying  in  contact  with  the  epidermis,  is  not 
smooth  and  regular  in  appearance  but  is  raised  in  mound-like  bodies 
which  project  into  the  lower  layers  of  the  Malpighian  colls.  Some  of 
these  dermal  projections  are  occupied  by  networks  of  capillaries;  others 
contain  groups  of  sensory  cells  of  the  nervous  system  arranged  to  form 
tactile  corpuscles.  As  a  result,  the  dermis  is  highly  vascular  and  also 
very  sensitive  to  environmental  stimuli.  The  main  body  of  the  dermis 
consists  of  a  dense  network  of  connective  tissue  fibers  in  which  the 
various  other  tissues  are  embedded. 

Forming  an  almost  complete  covering  over  the  skin  surface  of  the 
typical  mammalian  type  is  a  coating  of  hair.  In  man,  the  hair  forms 
a  dense  covering  on  the  head,  a  thin  covering  on  most  other  regions  of 
the  body  and  may  be  entirely  lacking  as  on  the  palmar  surfaces  of  the 
hands.  Further  consideration  of  the  hair  may  be  deferred  until  the 
later  chapter  (IX)  on  the  Skeletal  System.  A  difference  in  the  charac- 
ter of  the  skin  covering  is  clearly  apparent  on  the  lips  and  in  associa- 
tion with  other  openings.  Such  differences,  however,  are  more 
superficial  than  basic  in  character,  for  the  microscopic  examination 
of  the  mucous  membranes,  as  these  tissues  are  called,  shows  that  the 
general  plan  of  the  skin  tissues  with  outer  epidermis  and  underlying 
corium  remains  essentially  unaltered.  But  neither  of  these  regions 
are  so  strongly  developed  in  the  mucous  membranes,  and  accordingly 


THE  ORGANIZATION  OF  THE  HUMAN  BODY  39 

they  are  somewhat  less  resistant.  In  certain  instances,  as  in  the  lips 
of  the  Caucasian  peoples,  the  typically  opaque  outer  epidermal  layers 
are  quite  transparent,  arid  so  the  underlying,  highly  vascularized 
dermal  layer  is  revealed  by  the  characteristic  blood-red  color.  The 
mucous  membrane  does  not  stop  at  the  entrance  of,  the  mouth  but 
continues  throughout  the  length  of  the  alimentary  canal  as  the  epi- 
thelial lining  (mucosa)  which  becomes  specialized  for  the  nutritive 
processes.  Thus  all  of  the  tissues  and  organs  of  the  body  may  be 
thought  of  as  lying  between  the  external  covering  of  skin  and  the  inner 
mucous  membrane  which  lines  the  alimentary  canal. 

Lying  underneath  the  skin  and  gradually  merging  with  the  con- 
nective tissue  fibers  of  the  corium  is  the  layer  of  subcutaneous  con- 
nective tissue  which  binds  the  skin  to  the  underlying  muscle  tissues  of 
the  body  wall.  Subcutaneous  tissue  consists  largely  of  bundles  of 
white  fibrous  tissue.  They  are  continuous  both  with  the  connective 
tissue  of  the  corium  above  and  with  that  which  penetrates  the  deeper 
tying  tissues.  The  connective  tissue  elements  of  the  subcutaneous 
tissues  are  loosely  arranged  with  plenty  of  space  for  nerves  and  blood 
vessels,  and  this  condition  also  permits  the  skin  considerable  freedom 
of  movement.  In  certain  regions,  however,  notably  in  the  soles  of 
the  feet,  the  skin  is  more  firmly  attached,  and  here  it  will  be  found 
that  the  subcutaneous  tissues  are  heavier  and  more  compact.  One 
of  the  characteristic  features  of  the  subcutaneous  layer  is  its  ability  to 
store  reserve  fat  in  modified  connective  tissue  cells.  This  condition 
is  particularly  evident  in  the  subcutaneous  tissue  of  the  ventral  abdomi- 
nal region  and  often  results  in  a  marked  accumulation  of  fatty  tissue. 

Removal  of  the  skin  and  the  subcutaneous  tissue  from  almost  any 
portion  of  the  body  reveals  an  essentially  unbroken  layer  of  muscle 
tissue.  Connective  tissue  elements  permeate  through  the  muscles 
and  separate  them  into  definite  units  as  has  already  been  indicated 
(page  27).  Thus  the  connective  tissues  bind  together  the  skin, 
muscles,  bony  skeletal  structures,  and  associated  vascular  and  nerve 
components  in  an  essential  structural  unity.  Perhaps  this  condition 
is  most  clearly  evident  in  a  definite  motor  unit  such  as  the  leg,  but 
it  is  no  less  a  fact  in  other  organs  of  the  body. 

With  the  general  plan  of  the  body  in  mind,  as  indicated  in  the 
previous  pages,  the  way  is  cleared  for  a  study  of  the  various  organ 
systems,  essential  to  the  maintenance  of  life  in  the  individual.  In  the 
present  volume,  consideration  is  given  to  the  nine  primary  organ 
systems  responsible  for  the  functions  of  nutrition,  respiration,  secre- 
'Aon,  excretion,  transportation,  contraction,  support,  irritability,  and 
reproduction.. 


NASAL  SEPTUM 


LIVER 

GALL  BLADDER 

HEPATIC  DUCT 

AND  PANCREATIC  DUCT 

OPENING  INTO  THE 

DUODENUM 


PAROTID  GLAND 
SUBM  AXILLARY  GLAND 
SUBL1NGUAL  GLAND 

LUNG 
ESOPHAGUS 

DIAPHRAGM 
STOMACH 
SPLEEN 
PANCREAS 

TRANSVERSE  COLON 


ASCENDING  COLON  - — 


CAECUM 
APPENDIX 


DESCENDING  COLON 
RECTUM 


HARD  PALATE 
SOFT  PALATE 

UVULA 
PAPILLA 

3  MOLARS 

2  PREMQLARS 

1  CANINE 

2  INCISORS 


PAROTID  DUCT  ORIFICE 

TONSIL 

TONGUE 


SUBMAXILLARYAND 
SUBL1NGUAL  DUCT  ORIFICE 


B 


PLATE  III. — The  nutritive  system  of  man.     Somewhat  idealized.     A,  digestive  tract; 
B,  mouth  cavity;  C,  region  of  the  pharynx;  D,  salivary  glands,  left  side. 


CHAPTER  III 
THE  BIOLOGY  OF  NUTRITION 

Man  as  a  living  organism  requires  a  constant  supply  of  energy  in 
order  that  the  " wheels  of  life"  may  be  kept  continuously  revolving. 
Materials  must  be  available  also  for  the  repair  and  for  the  growth  of 
the  bodily  structures.  Both  of  these  requirements  must  be  supplied 
from  the  food  that  is  taken  into  the  body.  Furthermore,  we  know 
that  the  mere  ingestion  of  food  materials  will  not  suffice,  because  they 
cannot  be  utilized  until  properly  prepared.  They  must  undergo 
chemical  change — the  process  of  digestion— before  they  can  be 
absorbed  and  assimilated  by  the  cellular  units  that  make  up  the  organ- 
ism. Digestion,  absorption,  assimilation—these  are  the  nutritive  proc- 
esses that  form  the  basis  of  our  discussion  in  the  present  chapter. 

STRUCTURAL  FEATURES  ASSOCIATED  WITH  NUTRITION 

From  the  comparative  standpoint,  the  foundations  of  the  highest 
types  of  nutritive  apparatus,  as  found  in  n\an,  are  to  be  seen  in  the 
primitive  multicellular  hydra  with  its  permanent  diploblastic  con- 
dition in  which  an  outer  layer  of  ectoderm  forms  the  body  covering 
and  an  inner  layer  of  endoderm  lines  the  simple  enteric  cavity  where 
the  food  is  digested.  For,  in  the  embryonic  condition  of  the  higher 
animal  types,  a  hydra-like  stage  occurs  in  which  the  newly  formed 
endoderm  is  permanently  assigned  to.  the  nutritive  function.  This 
two-layered  stage  is  quickly  followed  in  the  embryo  by  the  permanent 
three-layered,  or  triploblastic,  stage  in  which  the  important  mesoderm 
layer  develops  between  the  ectoderm  and  endoderm,  but  the  addition 
of  the  mesoderm  does  not  affect  in  any  way  the  position  or  function 
of  the  endoderm;  the  latter  forming  the  nutritive  epithelium  which 
remains  throughout  life  as  the  essential  functional  layer  lining  the 
alimentary  canal  throughout  its  length.  (Fig.  25.) 

The  enteric  cavity  of  the  primitive  hydra  is  essentially  a  blind 
sac  with  a  single  opening  which  serves  for  the  ingestion  of  food  and 
for  the  egestion  of  the  refuse.  But  the  more  advanced  earthworm 
type  has  an  anterior  mouth  opening  and  a  posterior  anal  opening. 
Thus  the  nutritive  apparatus,  with  its  endodermal  lining,  becomes  a 
tubular  structure  extending  the  length  of  the  body  through  which  the 

41 


42 


HUMAN  BIOLOGY 


food  is  propelled  by  peristaltic  contractions  of  the  muscular  wall. 
The  growth  of  the  mesoderm  layer  during  development  separates  the 
ectoderm  and  endoderm  of  the  body  wall.  Finally,  the  mesoderm 
divides  into  an  outer  and  an  inner  layer  to  form  the  permanent  body 
plan  which  may  be  described  as  a  tube  within  a  tube;  the  inner  tube 
consists  of  the  alimentary  canal  with  endodermal  lining  and  meso- 
derm outside,  and  the  outer  tube,  with  ectodermal  covering  and 
mesoderm  inside  forms  the  body  wall.  The  body  cavity,  or  coelom — 
an  important  landmark  in  animal  structure — lies  between  the  two 

TENTACLE 
NEMA  TOCYST  ^  M  ^         MOUTH 

ENTERIC  CAVITY 


BUD 
FLAGELLATED  CELL 

ECTODERM 
MESOGLEA 

ENDODERM 


ENDODERM 
OF  BASAL  DISH 


FIG.  25. — Longitudinal  section  through  the  metazoan,1  Hydra,  to  show  the  primitive 
nutritive,  or  enteric,  cavity.      (After  Kepner  and  Miller,  redrawn.) 

tubes.  This  type  of  alimentary  canal  remains  as  the  permanent  type 
in  all  the  higher  animals,  and  the  variations  in  the  nutritive  mecha- 
nisms that  are  found  in  the  various  animal  groups  are  primarily  associ- 
ated with  adaptive  features  essential  to  particular  food  requirements 
rather  than  to  a  change  in  the  basic  plan.  (Fig.  26.) 

Typically,  the  vertebrate  alimentary  canal,  as  in  man,  is  separable 
into  seven  primary  regions  which,  beginning  anteriorly,  are  as  follows: 
the  mouth,  or  buccal  cavity;  the  throat,  or  pharynx;  the  gullet,  or 
esophagus;  the  stomach;  the  small  intestine;  the  large  intestine;  and 
the  rectum.  In  addition,  there  are  a  number  of  associated  glands 

1  Consult  the  Appendix:  Metazoa. 


THE  BIOLOGY  OF  NUTRITION 


43 


which  give  off  their  secretions  into  the  alimentary  canal  through  the 
attached  ducts.     (Plate  III  A,  page  40.) 

MOUTH 

The  mouth,  as  the  specialized  anterior  end  of  the  alimentary  canal, 
is,  of  course,  primarily  concerned  with  the  intake  of  food,  but  it  shows 
wide  variation  in  the  various  vertebrates,  from  a  comparatively 
undifferentiated  cavity  with  a  slit-like  opening  to  a  highly  developed 
masticating,  tasting,  digestive  organ  with  oratorical  proclivities  as  seen 
in  man.  Geographically  speaking,  the  human  mouth  cavity  may  be 
said  to  be  bounded  in  front  by  the  flexible  muscular  lips;  laterally  by 
the  cheeks;  dorsally  by  the  immovable,  bony,  hard  palate;  below  by 

TYPHLOSOLE  ~ 

BODY  WALL 


SEGMENT 


tNTEST/NE         COELOM 

FIO.  26. — Transverse  section  of  the  earthworm,  illustrating  the  "tube  within  a  tube." 
Diagrammatic.      (Buchanan,  "Elements  of  Biology,"  Harper  &  Brothers.) 

the  movable  lower  jaw  with  its  bony  framework,  attached  soft  tissues, 
and  median  tongue;  and  posteriorly  by  the  throat  region  with  which  it 
merges.  In  the  throat  region,  the  hard  palate  is  replaced  by  the  soft 
palate,  supported  on  each  side  by  a  pair  of  muscular  pillars  and  pro- 
longed into  a  fleshy  teat-like  median  structure,  the  uvula.  The  soft 
palate,  pillars,  and  uvula  converge  when  food  is  being  swallowed  and 
thus  form  a  fleshy  partition  between  the  mouth  and  throat  cavities. 
(Fig.  27.) 

The  most  prominent  structures  in  the  mouth  are  the  teeth  and 
tongue.  The  consideration  of  the  former  may  well  be  deferred  until 
the  skeletal  system  is  described.  It  is  sufficient  to  note  at  this  point 
that  the  teeth  are  efficient  tearing  and  grinding  organs  which  have  the 
ability,  if  properly  used,  to  break  up  the  solid  food  masses  to  such  a 
degree  that  digestive  juices  can  begin  their  action  without  delay. 
Proper  mastication  is  particularly  important  in  the  case  of  plant  tissues 


44 


HUMAN  BIOLOGY 


because  the  abundant  cellulose  material  is  very  resistant  to  the  diges- 
tive juices.  But  the  complete  process  of  mastication  is  not  wholly  a 
function  of  the  teeth;  both  the  tongue  and  the  cheek  aid  in  manipu- 
lating the  food  mass  and  in  keeping  it  between  the  grinding  apparatus. 
The  body  of  the  tongue  is  composed  of  striated  muscle  tissue,  with 
the  fibers  running  in  all  three  planes  and  with  intermingled  connective 
tissue  elements.  It  is  a  highly  flexible  structure  admirably  adapted 
for  aiding  in  speech ;  in  fact,  it  is  essential  for  the  production  of  various 
letter  sounds.  But  the  tongue  is  really  not  responsible  for  speech  and 
is,  therefore,  not  the  " unruly  member"  of  the  body,  as  often  desig- 
nated. Sounds  associated  with  the  talking  function  have  their  origin 
primarily  in  the  vibrating  vocal  cords  of  the  voice  box,  or  larynx,  and 
are  a  by-product,  so  to  speak,  of  ttic  mechanism  for  breathing. 


HARD  PALATE 
SOFT  PALATE 

UVULA 
PAPILLA 

3  MOLARS 

2  PREMOLARS 

1  CANINE 

2  INCISORS 


PAROTID  DUCT  ORIFICE 

TONSIL 

TONGUE 


SUBMAXILLARY  AND 
SUBLINGUAL  DUCT  ORIFICE 


FIG.  27. — The  mouth,  or  oral,  cavity  of  man.     The  upper  and  lower  jaws  are  separated 
more  than  normal  to  show  all  the  structures. 

Thp  tongue  is  covered  by  a  well-formed  mucous  membrane  which  is 
smooth  underneath  but  notably  rough  on  the  upper  surface.  Con- 
siderable areas  of  the  upper  surface,  particularly  toward  the  back  of 
the  mouth,  are  covered  with  a  highly  modified  mucous  membrane, 
which  is  essentially  a  sensory  epithelium  associated  with  the  sense  of 
taste.  It  presents  a  surface  studded  with  numerous  slightly  elevated 
circular  areas,  the  papillae,  containing  many  tiny  barrel-shaped  pits, 
the  taste  buds.  The  latter  are  present  on  the  sides  of  the  papillae 
rather  than  on  the  upper  surface.  In  each  of  the  taste  buds  is  a  group 
of  sensory  neurons,  with  associated  supporting  cells.  These  neurons 
are  the  essential  taste  cells  and  are  connected  by  tiny  nerve  fibers  to 
the  nervous  system.  (Fig.  116.) 

It  is  clear  that  many  so-called  tastes  are  due  to  a  combination  of 
taste  and  smell.  This  fact  is  quickly  evident  when  one  has  a  head 
cold  and  the  olfactory  sense  is  greatly  reduced.  Under  such  con- 


THE  BIOLOGY  OF  NUTRITION  45 

ditions,  the  sense  of  taste  disappears  to  a  large  extent  though  the 
neurons  of  the  taste  buds  are  not  directly  affected.  It  is  generally 
recognized  that  only  four  primary  tastes  really  exist,  namely,  sweet, 
bitter,  sour  (acid),  and  salt.  All  the  very  numerous  other  gustatory 
sensations  appear  to  be  combinations  of  taste  and  smell. 

Opening  into  the  mouth  cavity  are  numerous  small  buccal  glands 
which  are  widely  distributed  in  the  lining  membranes,  but  the  char- 
acteristic and  abundant  mouth  fluid,  saliva,  is  largely  the  product  of 
three  pairs  of  glands  of  considerable  size,  namely,  the  parotid,  sub- 
maxillary,  and  sublingual  glands.  The  ducts  of  the  two  pairs  last 
named  open  in  the  floor  of  the  mouth,  almost  directly  below  the  tip 
of  the  tongue,  and  the  bodies  of  these  glands  are  situated  laterally  on 
each  side  and  toward  the  back  of  the  mouth  cavity.  The  pair  of 
parotid  glands  is  the  largest  of  these  so-called 
salirary  glands,  and  each  lies  embedded  in  the 
cheek  tissues  in  front  and  to  some  extent  below 
the  tip  of  the  ear.  The  ducts  of  the  parotids 
continue  forward  along  the  upper  jaw  and  open 
on  each  side,  opposite  the  second  molar.  (Fig. 

'        t  t  PAROTID  GLAND 

Saliva,     the    composite    product    of    the  SUBMAXILLARY  GLAND 

salivary  glands,  is  a  liquid  substance  with  a  SUBLINGUAL  GLAND 

i            ,      •   i-        v                   .    •                   T,        i         ,  FIG.  28. — Human  saii- 

charactenstic  slimy  or  stringy  quality  duo  to  vary  glands<    Drawing 

the  presence  of  a  proteinaceous  lubricant,  shows  glands  on  left  side 
mucin.  Saliva  consists  principally  of  water  °nnl£g>  °7penings  indicated 
with  some  mucin  and  other  protein  substances, 

inorganic  salts,  and,  particularly,  a  digestive  enzyme,  ptyalin,1  in 
solution.  Ptyalin  acts  specifically  on  the  carbohydrates  and  begins 
their  digestion  in  the  mouth  as  the  food  is  being  chewed. 

THROAT 

In  the  journey  of  the  food  materials  through  the  alimentary  canal, 
the  next  station  is  the  throat  region,  or  pharynx.  This  may  be 
regarded  as  an  important  transfer  point,  or  junction,  used  jointly  by 
the*  nutritive  and  respiratory  systems.  The  food  materials  scarcely 
delay  at  all  in  their  passage  through  the  throat,  and  no  additional 
nutritive  processes  occur.  It  is  simply  a  question  of  making  the  right 
connection  so  that  the  food  will  reach  the  stomach  by  way  of  the 
gullet  and  thus  prevent  even  the  tiniest  particle  from  being  wrongly 
directed  so  that  it  invades  the  windpipe,  or  trachea,  leading  to  the 
lungs.  (Figs.  29,  30.)  ' 

1  Ptyalin  is  also  known  as  salivary  amylase. 


46 


HUMAN  BIOLOGY 


Itogether,  mere  are  seven  separate  openings  into  the  throat. 
These  consist  of  two  openings  of  the  nasal  cavities ;  two  openings  of  the 
Eustachian  tubes  which  lead  to  the  middle-ear  region;  an  opening 
(glottis)  into  the  trachea;  an  opening  into  the  gullet;  and,  finally,  an 
opening  from  the  mouth  to  the  throat.  It  is  easy  to  see  how  an 
infection  in  the  throat  region  may  spread  widely  through  the  body. 
Particularly  susceptible  to  throat  infections  are  the  nasal  region,  the 
trachea  of  the  respiratory  system  leading  to  the  lungs,  and  even  the 
cavity  of  the  middle  ear,  the  infection  coming  through  the  Eustachian 
tubes.  It  is  also  apparent  that  the  food  materials  passing  through  the 
throat  en  route  to  the  gullet  must  be  accurately  directed. 

The  mass  of  chewed  food,  or  bolus,  resting  upon  the  back  part  of 
the  tongue,  is  pushed  into  the  throat  region  by  coordinated  tongue 
movements,  the  soft  palate  with  the  attached  uvula  being  elevated 

during  this  process.  Projecting  up- 
ward, or  anteriorly,  from  the  aperture 
of  the  ventrally  situated  trachea  is  the 
flexible  epiglottis,  cartilaginous  in 
nature.  During  the  passage  of  the  air 
in  or  out  of  the  lungs,  the  epiglottis 
stands  erect,  leaving  the  tracheal  open- 
ing unobstructed,  but  when  food  passes 
from  the  mouth  to  the  throat,  the 
erect  epiglottis  is  pushed  back  and  do>vn 
by  the  oncoming  mass,  thus  closing  the  respiratory  passage  through 
the  trachea  and  permitting  the  food  to  pass  over  the  "  epiglottis 
bridge "  and  into  the  opening  of  the  gullet  lying  dorsally.  At  the 
same  time,  the  tissues  of  the  soft  palate  act  in  such  a  way  as  to  prevent 
any  of  the  food  from  passing  up  into  the  openings  of  the  nasal  cavities 
or  back  into  the  mouth. 

This  complicated  arrangement  for  the  directed  movements  of  the 
food  masses  through  the  common  throat  passage  functions  efficiently 
unless,  unfortunately,  as  not  infrequently  happens,  a  person  starts  to 
laugh  or  to  say  something  as  the  food  is  passing  over  the  epiglottis. 
If  this  happens,  the  epiglottis  is  forced  up  by  the  pressure  of*  the 
outgoing  air,  and  some  food  particles  enter  the  respiratory  passage. 
Immediately  a  violent  contraction  of  the  respiratory  muscles  is  incited, 
and  air  under  high  pressure  is  forced  from  the  lungs  and  through  the 
trachea  in  an  endeavor  to  expel  the  foreign  particles  without  delay. 
Almost  invariably  this  succeeds,  but  instances  are  not  unknown  where 
a  person  has  choked  to  death  before  the  obstruction  could  be  removed. 
Swallowing  is  not  a  haphazard  process  but  a  carefully  planned 


PHARYNX 
ESOPHAGUS 

TRACHEA 
NASAL  SEPTUM 

Fio.  29. — Illustrating  the  vari- 
ous openings  into  the  throat,  or 
pharynx,  of  man. 


THE  BIOLOGY  OF  NUTRITION  47 

series   of   events   which    function    perfectly    unless    interrupted    by 
carelessness. 

THE  ESOPHAGUS,  OR  GULLET 

The  esophagus,  which  connects  the  throat  with  the  stomach,  is  a 
muscular  walled  tube  structurally  specialized  for  the  rapid  conveyance 
of  food  to  the  stomach.  Although  glandular  tissues  are  present  in 
certain  regions,  no  digestive  enzymes  are  secreted.  The  esophagus 
is  about  15  in.  long  in  man  and  very  distensible,  but  wide  variation 
occurs  in  different  animal  types  depending  upon  the  length  of  the  neck. 
Thus,  in  a  long-necked  giraffe,  the  esophagus  is  several  feet  in  lengtfr. 
In  birds,  the  structure  of  the  esophagus  is  complicated  by  the  develop- 
ment of  a  storage  sac,  or  crop,  in  which  the  unchcwed  foods  are 
temporarily  stored  before  passing  on  to  the  stomach  (gizzard)  which 
is  equipped  for  grinding.  (Fig.  30.) 

It  is  important  to  note  that  the  muscular  tissue  in  the  walls  of  the 
esophagus,  as  in  the  remainder  of  the  alimentary  canal,  is  very  largely 
smooth,  or  involuntary.  Swallowing  is  a  voluntary  act;  but  when  the 
food  is  finally  and  carefully  placed  in  the  opening  of  the  esophagus,  it 
passes  from  voluntary  control,  and  involuntary  peristalsis  is  then 
responsible  for  the  movements  of  the  food  through  the  remainder  of 
the  journey.  Peristalsis,  under  normal  conditions,  is  a  slow-traveling 
wave  of  contraction  which  moves  posteriorly,  reducing  the  diameter 
of  the  alimentary  canal  as  it  goes  and  thus  forcing  the  contents  of  the 
intestine  along  ahead  of  it.  A  more  rapid  and  powerful  peristaltic 
action  may  be  seen  in  the  esophagus  of  a  horse  when  drinking  from  a 
low-lying  trough  or  brook.  Even  in  man,  peristalsis  may  be  speeded 
up,  or  the  direction  of  contraction  may  be  reversed/  as  when  the 
stomach  gets  "upset,"  and  the  contents  are  regurgitated  through  the 
mouth. 

THE  STOMACH 

The  human  stomach  is  roughly  conical  in  shape  with  distensible 
muscular  walls  lined  by  the  essential  nutritive  epithelium,  or  mucosa. 
It  has  a  capacity  of  approximately  2  qt.  at  its  minimum  size  but  is 
capable  of  considerable  temporary  enlargement.  The  stomach  lies 
in  the  abdominal  cavity,  just  below  the  diaphragm,  in  close  proximity 
to  the  liver.  It  is  situated  well  to  the  left  of  the  median  line  with  its 
long  axis  more  or  less  transverse  to  that  of  the  body.  The  larger 
portion  of  the  stomach,  or  what  might  be  termed  the  base  of  the  cone, 
points  upward  and  to  the  left  and  is  almost  in  contact  with  the  under 
surface  of  the  diaphragm.  This  region  is  designated  as  the  cardiac 


48 


HUMAN  BIOLOGY 


portion,  01  fundus.  IJere  the  esophagus  opens  into  the  fundus  through 
its  upper  surface.  Esophageal  muscle  fibers  extend  for  some  distance 
into  the  wall  of  the  stomach  before  losing  their  identity.  From  the 
fundus,  the  stomach  curves  markedly  toward  the  right  side  of  the  body 
and  gradually  tapers  down  to  the  tip  of  the  cone  where  connection  is 
made  with  the  small  intestine.  This  is  the  pyloric  portion  (pylorus) 
and  it  terminates  in  a  specialized  ring  of  tissue,  the  pyloric  valve,  which 
guards  the  entrance  to  the  small  intestine.  (Fig.  30.) 


TRACHEA 


LIVER 

GALL  BLADDER 

HEPATIC  bUCT 

AND  PANCREATIC  DUCT 

OPENING  INTO  THE 

DUODENUM 

ASCENDING  COLON 


CAECUM 
APPENDIX 


LUNG 
ESOPHAGUS 

DIAPHRAGM 
STOMACH 
SPLEEN 
PANCREAS 

TRANSVERSE  COLON 
JEJUNUM 

ILEUM 

DESCENDING  COLON 
RECTUM  ' 


FIG.   30.- 


-The  human  digestive  tracf  as  described  on  pages  45  to  55. 
idealized. 


Somewhat 


Commonly  regarded  as  the  most  important  organ  of  the  digestive 
system,  the  stomach,  as  a  matter  of  fact,  is  not  so  important  for  the 
essential  digestive  processes  as  it  is  for  its  services  as  a  mixing  and 
homogenizing  organ  for  the  diversified  foodstuffs  that  pour  into  it 
after  their  rapid  descent  by  the  peristaltic  elevator  through  the 
esophagus.  The  motility  of  the  stomach  walls  is  very  great,  as  is  well 
shown  by  X-ray  pictures  taken  during  the  digestive  processes. 
Repeated  waves  of  peristalsis  move  from  the  cardiac  portion,  where 
the  food  is  received  from  the  esophagus,  toward  the  pylorus.  Separate 


THE  BIOLOGY  OF  NUTRITION  49 

waves  of  peristalsis  originate  in  the  pylorus  walls  and  move  towards 
the  pyloric  valve.  As  many  as  three  distinct  waves  of  contraction 
may  be  noted  moving  over  the  stomach  walls  at  the  height  of  the 
process.  The  pyloric  valve,  guarding  the  entrance  into  the  intestine, 
remains  closed  until  the  food  is  thoroughly  churned.  Then  it  gradu- 
ally relaxes  in  response  to  the  repeated  contractions  coming  from  the 
pylorus  and  permits  small  amounts  of  the  food  mass,  now  a  liquid, 
chyme,  to  move  into  the  intestinal  regions  for  the  final  stages  of 
digestion  and  absorption. 

The  mucosa  forming  the  lining  of  the  stomach  is  highly  glandular. 
It  is  estimated  that  as  many  as  35  million  microscopic  gastric  glands 
pour  their  secretions  into  the  stomach  cavity,  and  the  total  amount  of 
daily  secretion  varies  around  iy2  qt.  Several  types  of  gastric  gland  are 
recognized  by  the  histologists,  but  essentially  all  of  them  may  be 
said  to  be  simple  tubular  structures  which  penetrate  the  mucosa 
perpendicularly  to  the  surface,  and  so  their  secretion  passes  directly 
into  the  stomach  cavity  through  these  tiny  openings.  Three  or  more 
types  of  secretory  cell  may  occur  in  these  glands.  Gastric  juice,  the 
composite  product  of  the  gastric  glands,  is  a  clear  fluid,  mostly  water, 
but  decidedly  acid  since  it  contains  about  0.4  per  cent  hydrochloric 
acid.  At  least  two  important  digestive  enzymes  are  present  in  the 
gastric  juice:  pepsin  for  digestive  action  on  the  proteins,  and  rennin 
which  coagulates  the  proteins  of  milk.  The  determination  of  the 
exact  origin  of  the  two  gastric  enzyniOvS  and  also  of  the  hydrochloric 
acid  secreted  in  the  stomach  from  among  the  three  types  of  secretory 
cells  has  proved  to  be  a  difficult  problem.  And  it  is  still  a  mystery 
how  the  mucosal  secretory  cells  are  able  to  form  hydrochloric  acid 
from  the  materials  brought  to  them  by  the  alkaline  blood  plasma. 
(Fig.  32.) 

THE  SMALL  INTESTINE 

The  small  intestine  of  man  is  a  greatly  coiled  tube,  about  1J^  in.  in 
diameter  and  some  20  ft.  in  length.  It  joins  the  pyloric  region  of  the 
stomach  through  the  pyloric  valve,  a  little  to  the  right  of  the  median 
body  line  and  about  midway  between  the  ventral  and  dorsal  surfaces 
of  the  abdominal  cavity.  The  portion  of  the  small  intestine  attached 
to  the  stomach  is  known  as  the  duodenum,  and  it  continues  about 
12  in.  and  is  succeeded  by  the  jejunum.  The  latter  has  a  length  of 
about  9  ft.,  and  then  comes  the  ileum  which  makes  up  the  remainder  of 
the  small  intestine  and  extends  about  10  ft.  to  its  connection  with 
the  large  intestine.  The  junction  between  the  small  and  large  intes- 
tines takes  place  through  the  ileocaecal  valve  which  is  situated  in  the 


50 


HUMAN  BIOLOGY 


lower  right-hand  corner  of  the  abdominal  cavity,   just  above  the 
pouch-like  caecum.     (Fig.  30.) 

Mention  should  be  made  at  this  time  of  the  supporting  mem- 
branes, or  mesenteries,  which  hold  the  stomach,  intestines,  and  other 
viscera  in  place  and  also  completely  cover  them  with  a  delicate  tissue, 
the  serosa,  so  that  they  do  not  really  lie  exposed  in  the  abdominal 
cavity.  Essentially,  the  mesenteries  are  continuous  with  the  meso- 
dermal  epithelium,  the  peritoneum,  which  lines  the  body  wall  of  the 
abdominal  cavity.  This  peritoneal  lining  is  reflected  from  the  dorsal 


LYMPH  NODE 


LYMPHATICS 


SMALL  INTESTINE 


FIG.  31. — Portion  of  the  small  intestine  of  man,  showing  attached  mesentery  with  its 
abundant  vascular  and  nerve  supply.  Arteries,  dark;  veins,  light.  (Haggard,  ''Science 
of  Health  and  Disease"  Harper  &  Brothers.) 

wall  along  the  median  line  and  encloses  the  alimentary  tract  in  such  a 
way  as  to  hold  the  various  parts  in  definite  positions.  The  omentum  is 
the  largest  of  the  mesenteries  associated  with  the  alimentary  canal  and 
hangs  suspended  from  the  stomach  as  a  curtain-like  membrane,  ventral 
to  the  intestines.  A  similar  material,  the  pleura,  lines  the  thorax  and 
is  reflected  over  the  lungs  and  heart.  (Figs.  31,  32.) 

A  microscopical  examination  of  a  prepared  section  through  the  wall 
of  the  small  intestine  shows  that  it  is  composed  of  several  distinct 
tissue  layers.  This  same  condition  obtains  with  considerable  uni- 
formity throughout  the  entire  length  of  the  alimentary  canal — such 
modifications  as  occur  being  for  the  most  part  found  in  the  nutritive 
epithelium  and  in  accordance  with  the  functional  demands  of  the 


THE  BIOLOGY  OF  NUTRITION 


51 


various  regions.  The  first  layer  to  be  noted  externally  is  the  thin 
layer  of  serosa  which,  as  just  noted,  is  a  membranous  tissue  directly 
continuous  with  the  peritoneum  and  the  mesenteries.  Below  the 
serosa  come  twb  distinct  layers  of  smooth  muscle  tissue.  In  the  outer 
one  of  these,  the  fibers  run  longitudinally,  that  is,  lengthwise  of  the 
intestine,  whereas  the  fibers  of  the  inner  muscle  layer  run  in  a  circular 
fashion  around  the  intestine.  It  is  the  progressive  contraction  of  the 
circular  layer  that  is  essential  to  peristalsis.  Within  the  muscular 
tissue  is  a  layer  of  loosely  arranged  connective  tissue,  the  submucosa, 
which  is  plentifully  supplied  with  vascular  and  nervous  elements. 


GLANP  (PANCRE 
MESENTERY_ 


MUSCULARIS 
MUCOSAE 

GLAND 


CIRCULAR 
MUSCLE  LAYER 
LONGITUDINAL- 
MUSCLE  LAYER 


DUCT 


NERVE  PLEXUS 
(MEISSWER) 

NERVE  PLEXUS 
(AUETIBACH) 


FIG.  32. — Diagram  of  a  transverse  section  through  the  vertebrate  intestine,  duodenal 
region.  Only  two  of  the  intestinal  glands  are  shown.  (Redrawn  from  Maximow-Bloom, 
"Histology"  W.  B.  Saunders  Company.) 

Finally,  we  reach  the  essential  functional  lining  layer,  the  nutritive 
epithelium,  or  mucosa.     (Fig.  32.) 

The  nutritive  epithelium  of  the  small  intestine  follows  the  same 
general  pattern  as  it  does  in  the  stomach,  as  described  above,  with  the 
added  fact  that  the  mucosa  of  the  intestine  is  more  highly  differentiated 
in  keeping  with  its  increased  digestive  and  absorptive  functions.  In 
the  first  place,  the  lining  mucosa  does  not  present  a  smooth  surface 
but  is  characterized  by  irregular  circular  folds  which  project  a  con- 
siderable distance  into  the  lumen  of  the  intestine.  These  folds  greatly 
increase  the  area  of  effective  mucosal  surface  and  are  such  as  would  be 
formed  if  the  mucosa  lining  the  intestine  were  too  long  for  the  other 
intestinal  layers  that  enclose  it,  and  so,  instead  of  fitting  smoothly 


52 


HUMAN  BIOLOGY 


FIG.  33. — Section  of  the  small 
intestine  of  man,  showing  the  circular 
folds  in  the  mucosa.  (Buchanan, 
"Elements  of  Biology"  Harper  & 
Brothers.) 


inside  when  pushed  into  place,  it  is  thrown  into  irregular  projections. 
(Fig.  33.) 

Covering  the  surface  of  the  projecting  folds  of  the  mucosa,  as  well 
as  in  the  spaces  between  the  folds,  are  microscopic,  finger-like  pro- 
jections, the  villi,  which  function  primarily  in  the  absorption  of  the 

digested  foodstuffs.  Internally,  the 
villi  contain  connective  tissue  elements 
and  an  abundant  network  of  capillaries 
through  which  the  absorbed  food  is 
carried  from  the  intestinal  region  to 
the  outlying  districts.  Also  present  in 
each  villus  is  a  thin-walled  lymph 
vessel,  the  lacteal,  which  connects  with 
a  special  division  of  the  vascular  sys- 
tem, known  as  the  lymphatic  system. 
The  lac  teals  are  largely  concerned 
with  the  absorption  of  fats.  The 
mucosa  on  the  outer  surface  of  the 
villi,  that  is,  the  surface  in  contact 
with  the  food,  consists  of  secretory 
and  absorptive  cells,  the  latter  for  removing  the  food  from  the  canal 
after  it  has  been  digested.  Both  these  types  of  cell  represent  modified 
mucosal  cells.  The  absorptive  cells  are  long  columnar  cells  with  a 
granular  cytoplasm.  Microscopic  examination  does  not  reveal  any 
noteworthy  structural  characteristics  adapt- 
ing them  for  their  absorptive  function. 
Nevertheless,  these  cells  continually  absorb 
large  quantities  of  the  digested  foodstuffs 
from  the  alimentary  canal  and  transfer 
them  to  the  blood  stream.  (Fig.  34.) 

Intestinal  Secretions. — The  secretory 
cells  of  the  villi  are  the  unicellular  goblet 
cells,  so  named  because  of  their  shape. 
Each  goblet  cell  has  an  oval-shaped  vacuole 
lying  in  the  cytoplasm  in  which  liquid 
mucus  is  constantly  formed  and  then 
secreted  into  the  digestive  cavity.  The 
secretion  of  the  goblet  cells  apparently  does  not  contain  digestive 
enzymes,  but  it  forms  a  protective  covering  over  the  mucosal 
tissues.  The  digestive  enzymes  appearing  in  the  intestinal  juice  are 
secreted  by  other  gland  cells  present  in  distinct  tubular  glands,  the 
glands  of  Lieberktihn,  which  are  essentially  similar  in  structure  to 


FIG.  34. — Diagram  show- 
ing the  lining  of  small  intestine 
with  projecting  villi.  Highly 
magnified.  G,  intestinal  gland 
opening  near  base  of  a  villus 
(V).  (Wieman.) 


THE  BIOLOGY  OF  NUTRITION  53 

those  previously  noted  in  the  walls  of  the  stomach.  The  glands  of 
Lieberkiihn  are  embedded  in  the  intestinal  wall  between  the  bases  of 
the  villi  and  contain  at  least  two  distinct  types  of  secretory  cell  which 
are  responsible  for  the  digestive  enzymes  present  in  the  intestinal 
juices.  The  latter  are  formed  in  great  abundance,  possibly  as  much  as 
3  qt.  per  day,  and  contain  several  important  digestive  enzymes. 
These  include  the  erepsiri  group  and  the  enterokinases  which  are 
concerned  with  protein  changes;  lipase  for  the  digestion  of  fats;  and 
maltase,  lactase,  and  sucrase  which  act  upon  the  carbohydrates. 

But  the  intestinal  secretion  is  not  the  only  enzyme-containing 
fluid  in  the  small  intestine,  nor,  in  fact,  is  it  the  most  important. 
That  distinction  belongs  to  the  pancreatic  juice  which  is  poured  into 
the  pyloric  end  of  the  duodenum  through  the  pancreatic  duct.  Secre- 
tions from  the  liver  also  enter  the  intestine  near  by.  The  structural 
and  functional  features  of  pancreas  and  liver  are  fully  considered  in  the 
later  chapter  dealing  with  Secretion,  but  it  will  be  helpful  at  this  point 
to  indicate  the  nature  of  the  products  of  these  organs  so  far  as  they  are 
concerned  with  digestion.  The  secretion  of  the  pancreas  is  a  clear 
fluid,  markedly  alkaline  in  nature  with  a  pH  of  about  9.1  It  contains 
three  enzymes  associated  with  the  digestion  of  every  type  of  foodstuff, 
including  trypsin  for  the  proteins,  amylase  for  the  starches,  and 
lipase  for  the  fats.  The  liquid  received  from  the  liver  is  a  hetero- 
geneous mixture,  termed  bile,  and  varying  in  color  from  golden  yellow 
to  dark  green.  It  is  usually  somewhat  alkaline  and  contains  lipoids, 
various  pigments  (notably  bilirubin  which  results  from  the  destruction 
of  old  red  blood  corpuscles),  and  bile  salts  which  are  used  in  connection 
with  the  digestion  of  fats. 

Thus  the  complete  intestinal  fluid,  with  the  combined  contributions 
from  the  mucosa  of  intestine  and  from  both  the  pancreas  and  liver, 
contains  all  the  substances  essential  for  the  completion  of  digestion 
of  all  types  of  foodstuff. 

THE  LARGE  INTESTINE 

The  large  intestine,  or  colon,  starts  with  the  caecum,  which  is 
situated  in  the  lower  right-hand  corner  of  the  abdominal  cavity. 
Just  anterior  to  the  caecum  is  the  ileocaccal  valve  guarding  the 
aperture  of  the  ileum  which  perforates  the  wall  of  the  large  intestine 
at  this  point.  The  location  of  this  opening  from  the  ileum  into  the 

1  The  symbol  pH  is  commonly  used  to  indicate  the  hydrogen  ion  concentration 
in  solutions.,  A  pH  of  7.0  is  a  neutral  solution.  Values  of  pH  below  7.0  indicate 
increasing  acidity,  whereas  values  above  7.0  indicate  increasing  alkalinity.  Con- 
sult the  Appendix,  Hydrogen  Ion  Concentration,  for  additional  information. 


54  HUMAN  BIOLOGY 

colon  does  not  seem  to  be  ideal  because  it  leaves  a  blind  sac,  the 
caecum,  lying  posterior  to  it  into  which  some  of  the  intestinal  con- 
tents, largely  indigestible,  from  the  small  intestine  may  be  sidetracked. 
Projecting  posteriorly  from  the  lower  end  of  the  caecum  is  the  small 
tubular  vermiform  appendix,  which  not  infrequently  has  to  be  removed 
surgically  following  infection  and  inflammation.  (Fig.  30.) 

The  entire  colon  is  about  5  ft.  long  and  somewhat  larger  in  diameter 
than  the  small  intestine,  approximating  2J^  in.  It  is  divided  into 
three  main  regions  known,  respectively,  as  the  ascending  colon,  the 
transverse  colon,  and  the  descending  colon.  The  ascending  colon  begins 
with  the  ileocaecal  valve  and  extends  anteriorly  along  the  right  side 
of  the  abdominal  cavity  until  it  approaches  the  diaphragm.  Here  it 
turns  abruptly  to  the  left  and,  as  the  transverse  colon,  crosses  to  the 
left  side  of  the  abdominal  cavity,  lying  close  to  the  ventral  body  wall 
and  almost  directly  above  (ventral)  the  pyloric  region  of  the  stomach. 
Another  abrupt  turn  posteriorly  marks  the  beginning  of  the  descending 
colon,  which  continues  along  the  left  side  of  the  abdominal  cavity  until 
it  almost  reaches  the  posterior  border  whore  it  turns  dorsally  and 
merges  into  the  rectum.  The  latter  continues  to  the  external  opening, 
the  anus.  The  rectal  and  anal  regions  exhibit  various  structural 
features  associated  with  the  egestion  of  the  fecal  material.  (Fig.  30.) 

The  microscopic  anatomy  of  the  large  intestine  reveals  the  char- 
acteristic five-layered  type  of  wall  described  above  for  the  small 
intestine,  but  certain  differences  are  to  be  noted.  Thus  the  wall  of 
the  large  intestine  is  seen  to  have  irregular  constrictions  giving  it  a 
peculiar  puckered  appearance  externally.  Internally,  these  con- 
strictions result  in  the  formation  of  lateral  cavities  which  are  partially 
cut  off  from  the  main  central  intestinal  cavity.  This  structural 
arrangement  appears  to  be  a  measure  to  increase  the  absorptive  sur- 
faces. It  is  due  to  the  presence  of  longitudinal  muscle  fibers  in  the 
intestinal  wall  which  are  shorter  than  the  associated  tissues.  The 
fibers  consist  of  three  longitudinal  cables  grouped  in  a  median  dorsal 
line  and  visible  as  a  distinct  ridge. 

The  mucosa  lining  the  large  intestine,  unlike  that  in  the  small 
intestine,  is  essentially  smooth,  and  projecting  villi  are  lacking. 
However,  absorptive  cells  line  the  comparatively  large  mucous  glands 
which  are  present  in  great  numbers  in  the  mucosa.  The  function  of 
absorption  in  the  colon  is  very  important,  but  it  is  largely  concerned 
with  the  removal  of  excess  water  from  the  intestinal  contents.  The 
latter  enter  the  large  intestine  as  a  rather  thin  liquid  and  finally  leave 
it,  after  the  water  has  been  gradually  absorbed,  as  solid  fecal  material. 
JThe  total  secretion  per  day  of  the  mucosal  glands  of  stomach,  intes- 


THE  BIOLOGY  OF  NUTRITION  55 

tine,  liver,  and  pancreas  total  approximately  4  qt.,  most  of  which  is 
water,  and  this  amount,  together  with  that  lost  through  other  channels, 
must  be  recovered  or  the  deficiency  supplied  by  drinking  more  liquid. 

FUNCTIONAL  FEATURES  ASSOCIATED  WITH  NUTRITION 

With  the  main  structural  features  of  the  "  tubular  chemical 
laboratory"  in  mind,  as  just  indicated  in  the  previous  section,  consider- 
ation may  now  be  given  to  the  important  functional  features  of 
nutrition  in  an  endeavor  to  see  how  the  ingested  food  materials  are 
chemically  changed  during  digestion  and  thereby  made  ready  for  the 
absorption  into  the  body,  transportation  through  the  body,  and  final 
assimilation  by  the  individual  cellular  units. 

FOODSTUFFS 

If  we  define  a  food  as  "any  substance  that,  when  ingested  in  the 
proper  amount,  is  absorbed  from  the  gastro-intestinal  tract  and  con- 
tributes to  the  maintenance  of  the  normal  state  of  the  body,"  a  con- 
siderable variety  of  organic  compounds  will  be  included  belonging  to 
the  carbohydrates,  fats,  proteins,  and  vitamins:  also  various  inorganic 
compounds  and  elements,  notably  salt  and  water — the  latter  being 
by  far  the  most  abundant  of  all  the  body  materials.  However,  when 
one  considers  the  infinite  variety  of  inorganic  and  organic  compounds 
known  to  the  chemist,  built  of  the  same  common  elements  as  those 
which  are  used  in  the  construction  of  the  body  tissues,  it  is  apparent 
that  comparatively  few  are  suitable  for  use  as  food  by  the  human 
organism.  One  basic  reason  for  this  condition  is  undoubtedly  the  fact 
that  the  human  digestive  system  is  equipped  with  a  limited  number  of 
digestive  enzymes,  and  these  are  adapted  for  the  digestion  of  relatively 
few  substances. 

Carbohydrates. — The  carbohydrates  are  compounds  of  carbon, 
hydrogen,  and  oxygen  and  include  three  related  groups  of  compounds, 
namely,  the  simple  sugars  (monosaccharides),  commonly  represented 
by  glucose  and  fructose,  with  the  molecular  formula  CcH^OeJ  a  more 
complex  group  of  sugars,  the  disaccharides,  represented  by  sucrose 
(cane  sugar),  maltose,  and  lactose,  with  the  formula  C^H^Ou;  and 
the  most  complex  carbohydrate  group  of  all,  the  polysaccharides, 
represented  by  starch,  cellulose,  arid  glycogen  (animal  starch)  with 
the  formula  (C6HioO5)^.  Of  these  three  groups,  only  the  mono- 
saccharides are  absorbed  from  the  digestive  tract  unchanged  and  so  are 
ready  to  be  assimilated  at  once  by  the  body  cells  and  oxidized  as 
necessary  to  supply  energy  requirements.  As  will  be  shown  below, 
digestion  of  any  of  the  other  carbohydrates  reduces  them  also  to  the 


56  HUMAN  BIOLOGY 

monosaccharide  type.  Thus  during  digestion  each  disaccharide  mole- 
cule is  transformed  into  two  molecules  of  the  monosaccharide.  A 
digestible  polysaccharide,  such  as  starch,  requires  more  extensive 
digestive  action,  but  the  end  result  is  the  same,  that  is,  the  production 
of  a  simple  sugar  which  can  be  absorbed.  Unfortunately,  it  would 
seem,  the  human  digestive  apparatus  is  unable  to  digest  one  of  the 
most  noteworthy  of  the  polysaccharides,  cellulose,  the  most  abundant 
of  all  plant  materials  (p.  510). 

Fats. — The  fats,  or  hydrocarbons,  belong  to  a  large  and  rather 
heterogeneous  group  of  organic  compounds,  known  as  the  lipoids. 
The  grouping  of  the  lipoids  is  on  the  basis  of  their  solubility  character- 
istics rather  than  their  inherent  chemical  nature.  Thus  all  of  them 
are  soluble  in  alcohol  or  ether.  Also  lipoids  have  a  peculiar  reaction 
to  the  skin;  they  feel  greasy  to  the  touch.  Various  kinds  of  lipoid  are 
found  in  all  the  body  tissues  and  are  believed  to  be  important  con- 
stituents of  the  protoplasmic  molecule.  Some  of  the  important 
lipoids  are  the  fatty  acids,  the  fats  and  oils,  the  sterols,  and  the 
phospholipins.  Of  these,  the  fatty  acids  form  the  most  important 
group  and  may  be  regarded  as  the  building  stones  of  the  fats  just  as 
the  amino  acids  are  the  building  stones  of  the  proteins  (page  70). 
Fats  are  formed  by  the  union  of  a  fatty  acid  and  glycerine.  Both  the 
fatty  acids  and  true  fats  are  compounds  of  carbon,  hydrogen,  and 
oxygen,  but  the  proportion  of  oxygen  is  much  less  than  it  is  in  the 
carbohydrates.  This  is  particularly  true  of  the  fatty  acids,  which 
typically  contain  only  a  very  few  atoms  of  oxygen.  Thus  in  stearic 
acid,  with  the  molecular  formula  of  CisHsoC^,  it  is  seen  that  only  two 
atoms  of  oxygen  are  present  in  the  entire  molecule.  Inasmuch  as 
both  the  fats  and  carbohydrates  are  oxidized  in  the  body  to  yield 
energy,  and  since  the  end  results  of  the  oxidative  processes  is  the  same 
with  both  compounds,  namely,  the  formation  of  carbon  dioxide  and 
water,  it  follows  that  more  oxygen  is  needed  for  the  oxidative  processes 
when  fats  are  burned  in  the  tissues. 

The  body  tissues  store  up  fats  whenever  an  excess  supply  of 
nutritive  materials  is  taken  into  the  body.  By  the  proper  chemical 
changes,  either  carbohydrates  or  proteins  may  be  converted  into  fat 
for  storage  and  accumulated  in  the  subcutaneous  tissues  over  the  body 
or  in  association  with  various  organs,  notably  around  the  kidneys. 

Proteins. — By  far  the  most  complex  and  diverse  group  of  the 
organic  compounds  is  the  proteins.  They  are  characterized  by  the 
presence  of  nitrogen  and  sulphur  in  addition  to  the  carbon,  hydrogen, 
and  oxygen  of  the  carbohydrates  and  fats.  Proteins  usually  contain 
a  number  of  other  elements,  notably  phosphorus,  calcium,  and 


THE  BIOLOGY  OF  NUTRITION  57 

magnesium.  A  typical  protein  contains  about  50  per  cent  carbon, 
25  per  cent  oxygen,  16  per  cent  nitrogen,  7  per  cent  hydrogen,  with  the 
remainder  consisting  of  sulphur,  phosphorus,  and  various  other 
elements.  Proteins  are  formed  by  the  union  of  the  somewhat  simpler 
amino  acids,  some  two  dozen  of  which  are  known  (page  70).  The 
amino  acids  are  characterized  by  a  particular  grouping  of  nitrogen 
arid  hydrogen  in  the  molecule  of  the  acid  indicated  by  the  symbols  NH2 
(amino  group).  When  excess  proteins  are  eaten,  as  frequently  occurs 
in  the  average  diet,  the  liver  cells  are  able  to  remove  the  NH2  group — 
the  process  of  deaminization — from  the  excess  amino  acids,  resulting 
from  the  protein  digestion.  The  final  result  of  deaminization  is  the 
formation,  from  the  excess  amino  acids,  of  a  carbohydrate  that  can  be 
used  to  supply  the  energy  requirements  of  the  body  or  converted  into 
fat  for  storage  if  the  energy  requirements  are  supplied,  just  as  any 
other  carbohydrate.  The  exact  composition  of  the  proteins,  which 
build  the  cytoplasm  of  the  different  types  of  cells  of  the  body,  is  highly 
variable  due  to  the  fact  that  each  cell  selects  the  proper  amino  acids 
and  other  substances  from  the  blood  stream  for  the  construction  of 
its  own  particular  protein  or  proteins. 

The  proteins  include  the  following  groups  of  compounds:  (1) 
simple  proteins,  which  are  broken  down  during  digestion  into  amino 
acids  and  their  derivatives  and  include  most  of  the  common  protein 
foods  from  both  animal  and  plant  tissues,  such  as  the  albumins,  the 
globulins,  and  the  glutelins;  (2)  the  conjugated  proteins,  such  as 
nucleoproteiris,  glycoproteins,  phosphoproteins,  and  hemoglobins,  all 
of  which  contain  a  protein  molecule  in  combination  with  some  other 
substance  (thus  in  the  respiratory  pigment  of  the  red  blood  cells, 
hemoglobin,  the  protein  molecule  is  united  with  hcmatin);  (3)  the 
derived  proteins,  which  represent  the  result  of  chemical  changes  in  the 
protein  molecule  following  enzyme  action  as  in  digestion,  such,  for 
example,  as  the  proteoses  and  peptones  formed  in  digestion. 

VITAMINS 

The  fundamental  importance  of  certain  accessory  substances  in  the 
diet  of  man  and  various  other  animals  has  been  increasingly  recognized 
since  1912  when  Hopkins1  established  the  basic  fact  that  other  organic 
compounds  besides  carbohydrates,  fats,  and  proteins  were  required  for 
an  adequate  diet.  These  substances,  commonly  designated  as 
vitamins,  comprise  a  heterogeneous  group  of  carbon  compounds.  In 
the  earlier  periods  it  was  supposed  that  they  constituted  a  closely 
related  group  of  essential  amino  acids,  hence  the  term  "vitamine," 

1  Consult  the  Appendix:  Hopkins. 


58  HUMAN  BIOLOGY 

given  at  that  time  (later  changed  to  vitamin),  was  chosen  to  indicate 
that  they  were  " vital  amines."  The  identification  and  synthesis  of 
various  vitamins  in  the  last  decade  has  shown  that  the  original  belief 
was  erroneous.  It  is  probable,  therefore,  that  the  term  vitamin  will 
in  time  disappear,  and  the  chemical  name  for  each  of  these  essential 
nutritive  substances  will  be  used.  In  the  meantime,  for  convenience, 
an  alphabetical  terminology  is  used:  vitamins  A,  B,  C,  D,  E,  and  K. 

At  all  events,  adequate  amounts  of  the  various  chemical  compounds 
which  are  now  grouped  together  as  vitamins  are  essential  to  normal 
animal  nutrition.  The  amounts  needed  are  almost  infinitesimal  in 
comparison  with  the  total  intake  of  the  body,  but  supplying  these 
requirements  means  all  the  difference  between  the  maintenance  of  the 
normal  functioning  of  the  body  and  the  gradual  development  of  serious 
pathological  conditions,  grouped  under  the  phrase :  nutritive  deficiency 
diseases. 

Vitamins  are  technically  defined  as  "indispensable  organic  sub- 
stances which  the  organism,  lacking  the  ability  to  synthesize,  must 
obtain  from  dietary  sources."  Three  primary  characteristics  roughly 
serve  to  differentiate  the  vitamins.  Thus  some  are  soluble  in  fats, 
whereas  others  are  soluble  in  water;  they  may  be  resistant  to  heat 
(thermostable)  or  destroyed  by  heat  (thermolabile) ;  and  they  may  or 
may  not  be  inactivated  by  oxygenation.  But,  on  the  other  hand,  the 
vitamins  as  a  group  show  certain  important  characteristics  in  common. 
Thus,  in  performing  their  various  functions  in  the  body,  all  of  the 
vitamins  act  as  specific  compounds,  which  enter  into  the  chemical 
reactions,  and  not  as  catalysts,  as  were  the  enzymes,  a  fact  noted  in 
the  following  discussion  of  digestion.  Furthermore,  it  can  be  said  that 
all  the  vitamins  are  highly  specific  in  their  activity  and  amazingly 
potent.  Finally,  with  the  exception  of  vitamin  D,  none  of  them  is 
synthesized  in  the  human  body.  Brief  consideration  may  now  be 
given  to  the  various  vitamins  as  at  present  identified.1 

Vitamin  A  is  a  fat  soluble  substance  which  is  widely  distributed  in 
plant  tissues,  particularly  those  which  contain  the  yellow  pigment 
carotene  (carrots,  squash,  sweet  potato,  etc.),  and  is  also  abundantly 
stored  in  various  animal  fats,  such  as  egg  yolk,  butter,  and  cod-liver 
oil.  Animal  tissues  are  able  to  transform  carotene  (CsoHse)  from  the 
plant  into  vitamin  A  (C2oH3oO).  It  has  not  been  artificially  syn- 
thesized. This  vitamin  is  primarily  a  growth-promoting  substance 
and  also  has  a  basic  effect  upon  the  epithelial  tissues  generally. 

1  Highly  recommended  for  presenting  a  general  survey  of  the  latest  develop- 
ments in  the  vitamin  field  is  the  Weston-Levine  Vitamin  Chart,  published  and 
distributed  by  Dr.  R.  E.  Remington,  280  Calhoun  St.,  Charleston,  S.C. 


THE  BIOLOGY  OF  NUTRITION 


59 


Night  blindness,  due  to  a  lack  of  visual  purple  in  the  retina,  is  also 
associated  with  vitamin  A  deficiency  and  may  be  relieved  by  adequate 
supplies  of  carotene,  from  which  the  essential  substances  may  be 
synthesized. 

Vitamin  B  is  a  water-soluble  vitamin  of  highly  complex  molecular 
structure.  The  other  vitamins,  so  far  identified,  consist  of  the  three 
elements  carbon,  hydrogen,  and  oxygen,  but  the  vitamin  B  complex 
also  contains  nitrogen,  sulphur,  and  chlorine.  Vitamin  B  was  the 
first  one  of  these  substances  to  be  discovered.  This  was  due  to  the 


VITAMIN    B   IN  YEAST.    RICE  POLISH,    LIVER     ETC 

ANTINEimiTIC    AND   GROWTH   PROMOTING 

SUPPOSfO   SINGLE    ENTITY 

1697-  1919 


THERMOSTABLE 
COMPONENT 

DISTINGUISHED  1926 


THERMOLABILE 
COMPONENT 

STRON&UY  ADSORBED 


NICOTINIC  ACIO 
BLACK  TONGUE 
AND  PELLAGRA 
CURED  1937 


a,  (THIAMIN) 

ISOLATED    1920 
SYNTHESIZED  1936 


RAT  ACRODYNIA 

FACTOR 

RAT  DERMATITIS 
ISOLATED  1930 


FIG.  35. — Diagram  illustrating  the  components  of  the  vitamin  B-complex.  Black 
face  lettering  indicates  components  which  have  been  isolated  in  a  pure  state;  loops  in  the 
strands  indicate  discrepancies  in  physiological  properties  of  crude  extracts.  (Williams 
and  Spies.) 

work  of  Eijkman,  a  Dutch  investigator,  working  in  Java  in  the  closing 
years  of  the  last  century.  He  found  that  the  bran  coats,  or  hulls,  of 
cereals  contained  a  substance  essential  to  animal  life.  In  its  absence, 
degeneration  of  nerve  tissue  occurred  and  the  development  of  a 
paralytic  disease  known  as  beriberi.  Vitamin  B  also  stimulates  the 
general  metabolic  activities.  The  vitamin  B-complex  has  since  been 
found  to  be  present,  to  some  extent,  in  a  wide  variety  of  plant  tissues 
as  well  as  in  milk,  oysters,  and  lean  pork,  but  its  greatest  concentration 
is  in  bran,  the  wheat  embryo,  and  yeast.  The  complexity  of  the 
vitamin  B-complex  and  the  amount  of  work  yet  to  be  done  before  a 
full  understanding  is  reached  are  well  shown  by  the  diagram  of 


60  HUMAN  BIOLOGY 

Williams,  who  succeeded  in  the  artificial  synthesis  of  Bi  (thiamin 
chloride),  one  of  the  most  important  fracti9ns.  Another  compound 
present,  identified  in  1937  as  nicotinic  acid,  is  the  antipellagra  factor. 
Pellagra  has  long  been  known  as  one  of  the  most  serious  of  the  defi- 
ciency diseases.  (Fig.  35.) 

Vitamin  C,  now  known  as  ascorbic  acid,  is  a  relatively  simple  water- 
soluble  compound  widely  distributed  in  raw  fruits  and  vegetables, 
particularly  citrus  fruits.  It  is  inactivated  by  the  oxygenation  that 
occurs  during  cooking.  It  was  isolated  in  1932  from  lemon  juice  and 
also  from  Hungarian  red  peppers,  and  its  chemical  nature  determined. 
A  year  later,  it  was  artificially  synthesized.  Ascorbic  acid  is  essential 
for  the  intracellular  oxidative  processes  and  for  the  maintenance  of 
normal  conditions  in  the  connective  tissues  of  the  body,  notably 
bones  and  teeth.  The  deficiency  disease,  known  as  scurvy,  results  in 
widespread  tissue  degeneration. 

Vitamin  D,  now  known  as  calciferol,  is  a  fat-soluble  compound 
which  is  synthesized  from  fats  in  the  animal  body  under  the  influence 
of  direct  sunlight  on  the  skin.  It  is  naturally  present  in  egg  yolk, 
salmon,  and,  particularly  in  the  liver  oils  of  various  fish,  notably  cod 
and  halibut.  Calciferol  is  resistant  to  chemical  action  and  may  be 
heated  or  subjected  to  oxygenation  without  injury.  Irradiation  of 
yeast,  milk,  and  various  plant  and  animal  fats  with  ultraviolet  light 
will  produce  calciferol.  This  substance  was  the  first  of  the  vitamins 
to  be  produced  artificially.  This  was  accomplished  in  1927  by 
irradiation  of  a  plant  substance,  ergosterol.1  The  specific  nutritive 
deficiency  disease  due  to  a  lack  of  calciferol  is  rickets,  which  is  char- 
acterized by  an  upset  in  the  calcium  and  phosphorus  metabolism  and 
a  failure  to  develop  normal  bone  tissue. 

Vitamin  E,  now  known  as  tocopherol,  is  a  fat-soluble  compound, 
essential  for  the  maintenance  of  the  normal  reproductive  activities. 
It  is  not  affected  by  heating  but  is  inactivated  by  oxygenation.  So 
far  as  known  at  present,  its  normal  distribution  is  rather  closely 
restricted  in  plant  tissues.  It  is  abundantly  present  in  the  oil  obtained 
from  the  wheat  embryo  and  also  in  lettuce  and  water  cress.  The 
normal  diversified  diet  provides  adequate  supplies  of  tocopherol. 
Dietary  deficiencies  of  vitamin  E,  culminating  in  sterility,  are  known 
only  in  laboratory  animals,  notably  in  rats,  that  have  been  kept  on  an 
experimental  diet.  Tocopherol  was  artificially  synthesized  in  1938, 
and  so  sufficient  time  has  not  yet  elapsed  to  obtain  the  results  from 
later  experiments. 

1  Consult  the  Appendix:  Sterols. 


THE  BIOLOGY  OF  NUTRITION  61 

Vitamin  K  belongs  to  a  class  of  compounds  known  as  the  naptho- 
quinones.  It  is  a  £a1/-soluble  compound  which  apparently  is  essential 
to  blood  clotting.  In  its  absence,  hemorrhagic  conditions  develop  in 
the  body.  The  isolation  of  vitamin  K  was  announced  only  a  few 
months  ago  when  the  compound  was  obtained  from  spinach  and 
alfalfa  and  also  from  decomposed  fish  meal.  In  the  latter  source, 
the  presence  of  vitamin  K  is  believed  to  be  due  to  the  chemical  activities 
of  decay.  Two  varieties  of  vitamin  K  are  recognized  under  the  names 
Ki  and  K2. 

As  was  recently  stated:1 

A  review  of  the  past  decade  clearly  demonstrates  that  the  discovery  of 
each  new  vitamin  has  gone  hand  in  hand  with  increased  purification  of  ingre- 
dients in  experimental  diets.  We  do  not  know  how  extensive  the  list  of 
vitamin  factors  may  be  when  the  biochemist  can  express  every  component  of 
his  experimental  diet  by  indisputable  structural  formulae.  By  this  time,  the 
term  " vitamin"  will  have  long  since  served  its  purpose  and  these  substances 
will  be  labeled  with  more  specific  chemical  names  such  as  we  now  apply  to  the 
indispensable  amino  acids  and  other  constituents  of  diet. 

DIGESTION  AND  ABSORPTION 

The  problem  confronting  the  animal  organism,  in  making  use  of  the 
varied  substances  brought  into  the  alimentary  canal,  is  to  get  them 
into  a  condition  that  will  permit  their  absorption  by  the  mucosal  cells. 
It  must  be  remembered  that  every  type  of  cell,  including  the  absorptive 
cells  of  the  mucosa,  are  completely  enclosed  in  a  definite  membrane 
of  a  semipermcable  nature.  Basically,  this  means  that  the  open- 
ings through  the  membrane  are  so  limited  in  size  that  only  very  small 
molecules  can  pass  through  them.  Such  a  molecule,  for  example, 
is  the  water  molecule  formed  by  the  union  of  two  atoms  of  hydro- 
gen and  one  atom  of  oxygen  or  the  molecule  of  table  salt,  sodium 
chloride,  composed  of  one  atom  of  sodium  and  one  atom  of  chlorine. 
Substances  with  molecules  the  size  of  the  water  or  salt  molecule, 
or  even  somewhat  larger,  pass  readily  through  the  membranes  of  the 
absorptive  cells  and  so  do  not  have  to  be  changed  or  digested  in  the 
alimentary  canal— they  are  absorbed  as  they  are. 

Molecular  Size. — Now,  the  size  of  a  molecule  naturally  depends 
upon  the  number  of  particles  or  atoms  that  are  associated  to  form  that 
particular  substance — the  more  atoms  associated  the  larger  the 
molecule.  An  idea  as  to  the  size  of  the  openings  in  the  membranes  of 
the  absorptive  cells  can  be  obtained  by  comparing  the  size  of  a  molecule 
of  cane  'sugar,  or  sucrose,  with  the  monosaccharide  molecule.  The 

1  Mason,  "Science  in  Progress,"  p.  156. 


62  HUMAN  BIOLOGY 

chemical  formula  of  glucose,  it  will  be  remembered,  is  CeH^Oe,  and 
that  of  sucrose  is  C^H-^On.  The  former  is  absorbed  unchanged  by 
digestion  ;  tho  latter  must  undergo  digestive  action,  which  results  hu  the 
formation  of  two  monosaccharide  molecules  from  each  molecule  of  cano 
sugar.  Since  the  molecules  of  cane  sugar  are  so  small  that  it  would 
take  more  than  50  million  of  them  placed  side  by  side  to  cover  an  inch 
in  length  and  they  have  to  be  split  in  half  before  they  can  pass  through 
the  cell  walls,  it  is  apparent  that  the  openings  in  the  semipermeablo 
membranes  of  the  absorptive  mucosal  cells  are  very  small  indeed. 
(Fig.  36.) 

Hydrolysis.  —  It  can  be  said,  then,  that  digestion  is  essentially  a 
chemical  process  by  which  the  too  large  molecules  of  the  foodstuffs 
are  reduced  to  molecules  of  the  proper  size.  Biologists  have  long  been 
aware  of  the  nature  of  the  digestive  processes  that  accomplish  this 
molecule  splitting.  It  is  hydrolysis,  a  term  that  means  a  loosening 
or  change  by  the  action  of  water.  And  this  is  just  what  happens,  for, 

o   O      o     O    o      O     o    O    o     O 


Z3  p? 
oo       oooooooo 

FIG.  36.  —  Illustrating  diffusion  through  a  semipermeable  membrane  as  in  tho  lining 
of  the  intestine.  The  large  circles  represent  sucrose  molecules  which  cannot  pass 
through  without  digestion;  the  small  circles,  water  molecules  which  are  small  enough 
to  pass  through  unchanged.  (Seifriz.) 

during  digestion,  water  is  added  to  the  complex  molecular  associations 
found  in  the  foodstuffs,  and  the  result  is  the  disassociation  of  the 
molecules.  Hydrolysis  is  based  on  enzyme  action  as  described  in  the 
following  paragraphs.  As  an  example  of  this  essential  process,  let  us 
again  consider  the  relations  between  cane  sugar  (sucrose,  C^H^On) 
and  glucose  (C6Hi206).  In  digestion,  the  hydrolytic  action  adds 
one  molecule  of  water  to  each  molecule  of  sucrose,  thus  CijH^On  + 
H2O  =  Ci2H24Oi2;  the  latter  does  not  exist  as  a  compound  but  instead 
there  are  two  separate  molecules  of  the  monsaccharide,  or  2C6Hi2O6. 
One  of  these  is  glucose,  but  the  other  has  a  different  arrangement  of 
the  associated  atoms  and  is  known  as  fructose,  but  both  have  the  same 
number  of  atoms,  and  both  can  be  absorbed  from  the  digestive  tract.1 

ENZYMES 

It  is  important  at  this  point  to  consider  the  essential  substances, 
the  digestive  enzymes  or  ferments,  that  incite  the  hydrolytic  changes 
associated  with  digestion,  for  chemists  know  very  well  that  merely 
adding  water  to  either  cane  sugar  or  starch  will  not  cause  hydrolysis 
and  change  the  molecular  structure  of  these  substances.  The  answer 

1  Consult  the  Appendix:  Chemical  Equations. 


THE  BIOLOGY  OF  NUTRITION  63 

is  enzyme  action.  Enzymes,  as  found  in  the  digestive  tract,  are  com- 
plex organic  compounds  produced  by  protoplasmic  action  within  the 
mucosal  cells.  They  are  able  to  cause  chemical  changes  in  substances 
without  being  changed  themselves.  Chemists  term  such  substances 
catalysts,  and  various  inorganic  substances  are  known  that  have  this 
property.  Enzymes,  however,  are  organic  catalysts  of  such  an 
intricate  molecular  pattern  that  comparatively  little  is  known  about 
their  chemical  composition,  although  recent  work  on  the  proteolytic 
enzyme  pepsin  gives  evidence  of  a  protein  nature. 

Enzymes  are  highly  specific  in  the  substances  that  they  disrupt, 
but  the  basic  chemical  reactions  appear  to  be  either  hydrolytic  in 
nature  in  which  water  is  added  or  taken  away  from  a  particular  sub- 
stance, as  just  noted,  or  processes  that  cause  an  increase  or  decrease 
in  the  amount  of  oxygen  or  hydrogen  present.  Enzymes  may  be 
divided  into  the  extracellular  group  which  are  secreted  by  the  cells 
and  do  their  work  outside  the  cell  body,  as  in  the  digestive  enzymes 
which  the  mucosal  cells  secrete  into  the  digestive  cavity;  and  the 
intracellular  enzymes,  which  are  formed  and  remain  within  the  cells. 
Every  cell  must  have  its  complement  of  intracellular  enzymes,  some  of 
which  break  down,  whereas  others  build  up,  the  materials  needed  for 
the  continuous  chemistry  of  life.1 

Digestive  Enzymes. — In  the  earlier  discussion  of  the  structure  of 
the  nutritive  system  in  man,  the  localized  secretion  of  enzymes  in 
various  regions  of  the  tract  has  been  briefly  indicated  (page  52). 
Taking  the  enzymes  now  in  order  of  appearance,  as  they  say  of  the 
actors  on  the  theater  program,  further  consideration  may  be  given  to 
their  functional  activities. 

Salivary  and  Gastric  Enzymes. — The  enzyme  ptyalin  is  secreted  by 
the  salivary  glands  in  the  mouth  and  at  once  begins  the  digestion  of  the 
starch  present.  Given  sufficient  time,  it  will  change  the  starch  to 
maltose,  a  disaccharide  product.  Maltose  cannot  be  absorbed  but 
must  undergo  further  enzyme  action  in  the  small  intestine  before  the 
absorbable  glucose  stage  is  reached.  Ptyalin  is  effective  only  in  a 
neutral  or  slightly  alkaline  medium,  as  in  the  saliva,  and  is  inactivated 
when  the  food  reaches  the  stomach  where  an  acid  condition  obtains. 

Pepsin2  is  secreted  by  the  gastric  glands  in  the  mucosa  of  the  cardiac 
portion  of  the  stomach  as  an  inactive  substance,  pepsinogcn.  When 
it  comes  into  contact  with  the  acid  gastric  juice,  it  is  changed  to  the 
active  protein-splitting  enzyme  pepsin,  which  is  effective  only  in  the 

1  The  basic  importance  of  enzyme  action  in  all  the  life  processes  makes  addi- 
tional consideration  advisable.     Reference  should  be  made  to  Chap.  XVI. 

2  Consult  the  Appendix:  Beaumont,  p.  501. 


64  HUMAN  BIOLOGY 

markedly  acid  condition  of  the  stomach.  The  action  of  pepsin  is 
specific  for  proteins  and  results  in  the  formation  of  the  derived  protein 
substances,  proteose  and  peptone,  which  are  incapable  of  absorption 
by  the  gastric  mucosa.  The  first  product  of  protein  digestion  is  a 
proteose,  and  most  of  the  proteins  remain  in  this  stage  during  digestion 
in  the  stomach,  though  a  slight  amount  of  the  proteose  reaches  the 
peptone  stage.  At  this  point,  gastric  digestion  stops,  and  the  food  is 
passed  in  the  form  of  liquid  chyme  into  the  small  intestine  for  the 
final  stages. 

Rennin  is  also  secreted  by  the  gastric  glands  in  an  active  form. 
This  enzyme  has  a  coagulating  action  on  the  milk  proteins  with  the 
result  that  the  liquid  condition  is  changed  into  a  soft  curd,  or  casein, 
which  can  be  more  readily  attacked  by  the  pepsin  and  other  protein 
enzymes.  Since  milk  is  the  one  food  of  early  life,  the  importance  of 
rennin  is  correspondingly  great  at  this  period.  The  same  process  of 
the  coagulation  of  milk  proteins  by  rennin  is  used  in  the  cheese  industry. 

Pancreatic  Enzymes. — Trypsin1  is  secreted  by  certain  cells  of  the 
pancreas  in  an  inactive  form,  trypsinogen.  It  is  activated  after  reach- 
ing the  intestine  by  a  coenzyme,  enterokinase,  secreted  by  the  cells  of 
the  small  intestine.  It  is  known  that  the  secretion  of  trypsinogen  in 
the  pancreas  is  incited  by  a  hormone,  secretin,  thrown  into  the  blood 
stream  by  duodenal  mucosal  cells.  The  latter  arc  brought  into  activ- 
ity by  the  influence  of  the  chyme  from  the  stomach  with  its  high 
acidity.  Trypsin  is  one  of  the  most  important  of  all  the  digestive 
enzymes.  It  disrupts  the  proteoses  and  peptones  present  in  the  chyme, 
.as  well  as  any  unchanged  protein  foodstuffs  into  the  constituent 
absorbable  amino  acids.  In  association  with  erepsin,  noted  below, 
protein  digestion  is  finally  completed.  The  complete  pancreatic  juice 
is  markedly  alkaline,  the  bile  weakly  so;  together  they  quickly  change 
the  acid  chyme  into  an  alkaline  fluid,  a  condition  which  is  essential  for 
the  pancreatic  enzymes. 

Amylase,  another  important  enzyme  of  the  pancreatic  juice,  is 
concerned  with  the  digestion  of  starch,  one  of  the  most  important  of 
the  carbohydrates.  Thus  it  has  the  same  function  as  ptyalin  in  the 
saliva,  but  it  has  the  opportunity  for  much  more  thorough  work  as  the 
food  is  under  its  influence  for  a  much  longer  period.  Comparatively 
little  of  the  starch  present  in  the  food  is  changed  in  the  mouth,  and  the 
amylase  converts  all  that  is  unchanged  into  the  intermediate  disac- 
charide  product,  maltose,  which  is  later  changed  into  the  simple 
sugars  by  the  enzyme,  maltase,  present  in  the  intestinal  juice. 

1  It  is  established  that,  in  addition  to  trypsin  proper,  there  are  two  other 
pancreatic  enzymes  in  the  trypsin-complex. 


THE  BIOLOGY  OF  NUTRITION  65 

Lipase,  the  only  fat-splitting  enzyme  of  the  digestive  juices,  is  the 
third  member  of  the  pancreatic  enzyme  triad.  It  is  effective  on  all  of 
the  nutritive  fats  and  oils  and,  through  hydrolytic  action,  splits  the  fat 
molecule  into  a  fatty  acid  and  glycerol,  both  of  which  pass  through 
the  intestinal  mucosa  into  the  blood  stream.  It  is  recognized  that  the 
effective  action  of  lipase  in  the  intestinal  tract  is  greatly  aided  by  the 
so-called  bile  salts  from  the  liver,  which  emulsify  the  fats.  The  tiny 
fat  droplets  of  the  fat  emulsion  can  be  brought  into  more  effective 
contact  with  lipase.  Also  the  bile  salts  aid  in  the  absorption  of  the 
digested  fats  by  forming  a  temporary,  water-soluble  compound  which 
readily  passes  through  the  cell  walls  and  into  the  blood  stream.  The 
bile  salts,  thus  removed  from  the  intestine  in  association  with  the  fat 
products,  are  later  removed  from  the  blood  stream  by  the  liver  cells 
and  again  secreted  into  the  intestine — the  so-called  circulation  of  the 
bile  salts. 

Intestinal  Enzymes. — Erepsin  is  an  important  enzyme,  or  rather  a 
group  of  enzymes,  concerned  like  trypsin  with  the  final  stages  of  pro- 
tein digestion.  It  is  a  product  of  the  mucosal  glands  in  the  duodenum 
and  ileum.  It  is  effective  only  in  the  highly  alkaline  contents  of  the 
small  intestine,  which  are  largely  due,  as  noted,  to  the  pancreatic 
juice.  The  complete  action  of  erepsin  and  trypsin  transforms  all  the 
digestible  protein  material  into  the  constituent  amino  acids  which  are 
absorbed  and  rapidly  transported  to  the  body  cells  by  the  blood  stream. 

Enterokinase  is  not  directly  concerned  with  digestion  but  acts  as 
a  coenzyme  with  the  inactive  pancreatic  ei^byme  trypsinogcn  to  form 
active  trypsin,  as  indicated  above  (page  64).  0 

Lipase,  the  fat  splitting  enzyme,  has  two  sources,  being  secreted 
by  the  duodenal  mucosa  as  well  as  by  the  pancreatic  cells  as  just 
noted. 

Maltase,  sucrase,  and  lactase  are  three  important  carbohydrate 
enzymes — all  products  of  the  intestinal  glands.  They  are  able  to 
split  the  molecule  of  the  particular  disaccharide  for  which  they  are 
adapted  into  two  monosaccharide  molecules.  As  we  know,  all  the 
disaccharides  have  the  same  formula,  C^H^On,  but  the  molecular 
arrangements  are  different,  so  that  a  specific  enzyme  is  necessary  for 
the  digestion  of  each  one.  The  three  disaccharides  to  be  digested 
are  maltose  which,  it  will  be  remembered,  is  an  intermediate  product 
following  the  action  of  either  ptyalic  or  intestinal  amylase  on  the 
starch  molecule;  sucrose,  or  cane  sugar,  which  is  commonly  present  in 
the  plant  foods;  and  lactose,  or  milk  sugar,  which  is  the  carbohydrate 
present  in  milk  and  a  product  of  the  mammary  glands  of  the  mammalian 
female.  The  enzymes  maltase,  sucrase,  and  lactase  are  so  named 


66  HUMAN  BIOLOGY 

because  they  are  specific  for  the  digestion  of  the  three  correspondingly 
named  sugars;  the  end  result  in  all  cases  is  the  splitting  of  the  disac- 
charide  molecules  into  two  molecules  of  a  moriosaccharide,  as  indi- 
cated above  (page  62).  The  products  of  carbohydrate  digestion 
pass  from  the  stomach  to  the  liver  where,  by  a  reverse  process  involv- 
ing the  removal  of  water,  the  glucose  molecules  may  be  changed  back 
by  the  hepatic  cells  into  still  another  disaccharide,  glycogen  or  animal 
starch,  which  is  temporarily  stored  in  the  liver  until  needed  for  fuel 
by  the  body  tissues.  When  this  need  arises,  enzymes  in  the  liver  cells 
hydrolyze  the  glycogen  and  so  form  glucose  again,  which  is  secreted 
into  the  blood  stream  and  supplied  to  the  body  cells  for  their  energy 
requirements. 

Synthesizing  Enzymes. — It  may  be  well  at  this  point  to  link  up 
the  enzyme  actions  associated  with  the  digestive  processes  with  those 
in  which  the  large  molecular  units  are  reestablished  within  the  living 
cellular  units.  This  is  the  process  of  assimilation,  that  is,  the  actual 
incorporation  of  the  new  materials  into  the  life  stream.  It  can  be 
said  in  a  word  that  the  intracellular  synthesis  is  exactly  the  reverse  of 
digestion,  water  being  removed  as  the  molecules  are  joined  to  form 
the  more  complex  compounds.  In  either  case  specific  enzymes  are 
responsible.  Those  concerned  with  digestion  are  able  to  add  water 
and  thus  split  the  large,  complex  molecules,  of  the  foodstuffs  whereas 
the  synthesizing  enzymes  are  able  to  remove  water  and  join  the  mole- 
cules together  in  more  complicated  association. 

Obviously,  the  dinintegrative  enzyme  actions  essential  to  digestion 
are  not  to  be  regarded  as  of  more  importance  than  the  synthetic  reac- 
tions by  which  the  digested  materials — glucose,  fats,  and  amino  acids, 
together  with  the  inorganic  substances — are  constantly  being  syn- 
thesized within  each  cell  to  form  the  specific  materials  essential  for 
upkeep,  growth,  and  fuel.  No  matter  what  specialized  functions  the 
various  types  of  cell  may  have  in  the  body,  each  one  must  be  able  to 
construct  its  own  type  of  protoplasm  by  means  of  the  particular 
intracellular  synthesizing  enzymes  that  are  characteristic  of  it.  Thus 
even  the  mucosal  cells  of  the  alimentary  tract,  which  secrete  digestive 
enzymes  for  extracellular  use,  are  at  the  same  time  maintaining  and 
utilizing  their  own  intracellular  equipment  of  enzymes  for  synthesizing 
and  disrupting  organic  cellular  compounds  as  occasion  demands. 

PHOTOSYNTHESIS 

In  connection  with  the  synthetic  enzyme  actions,  it  will  be  prof- 
itable to  reexamine  in  some  detail  the  photosynthetic  processes  in 
green  plants  that  were  briefly  indicated  in  the  opening  chapter  (page 


THE  BIOLOGY  OF  NUTRITION 


67 


11).  Photosynthesis  is  based  upon  the  unique  ability  of  chlorophyll1 
to  utilize  the  energy  of  sunlight  for  synthesizing  carbon  compounds 
of  such  a  nature  that  they  can  be  utilized  as  food  by  living  organisms. 
The  substances  taken  into  the  plant  cell  for  the  photosynthetic  reac- 
tions consist  of  carbon  dioxide  (CCh),  which  is  a  gas  composed  of  the 

nlight 


FIG.  37. — Diagram  illustrating  the  process  of  food  formation,  or  photosynthesis 
in  the  green  leaf  as  described  on  pages  66-68.  (Smith,  "Exploring  Biology,"  Harcourt, 
Brace  &  Company,  Inc.) 

elements  carbon  (C)  and  oxygen  (0)  combined  in  the  proportion  of  one 
carbon  atom  to  two  oxygen  atoms,  and  water  (H2O),  which  is  com- 
posed of  two  atoms  of  hydrogen  and  one  atom  of  oxygen.  Essentially, 
chlorophyll  is  able  to  make  use  of  the  radiant  energy  of  the  sun  to 
separate  the  carbon  from  the  oxygen  in  the  carbon  dioxide  molecules 
and  to  combine  the  carbon  thus  secured  with  the  hydrogen  and  oxygen 
of  the  water  to  form  a  carbohydrate,  a  simple  sugar,  glucose,  having 
1  Consult  the  Appendix:  Chlorophyll. 


68 


HUMAN  BIOLOGY 


the  chemical  formula  CeH^Oe,  which  is  the  first  food  product  of  photo- 
synthesis. At  least  one  synthetic  enzyme,  chlorophyllase,  is  associated 
with  the  chlorophyll,  but  the  exact  relations  are  obscure.  At  all 
events  the  chlorophyll-enzyme  complex  is  responsible  for  the  union 
of  the  carbon  dioxide  arid  water  since  it  is  not  possible  to  get  the  reac- 
tion in  the  laboratory  in  the  presence  of 
sunlight  alone.  (Fig.  37.) 

In  photosynthesis,  six  molecules  of  carbon 
dioxide  are  combined  with  six  molecules  of 
water  to  form  one  molecule  of  sugar.  When 
this  is  done,  six  molecules  of  oxygen  remain. 
The  essential  facts  of  photosynthesis,  as  the 
chemist  sees  them,  are  expressed  in  the 
following  chemical  equation: 

6CO2  +  6H2O  =  C6H12O6  +  602. 

The  oxygen  separated  from  the  carbon  diox- 
ide molecule  and  not  utilized  in  photosyn- 
thesis passes  off  into  the  atmosphere  as  free 
oxygen.  Oxygen  is  an  active  element  and 
tends  to  combine  very  quickly  with  other 
elements  when  it  comes  into  contact  with 
them.  Practically  the  only  available  supply 
of  free  oxygen  for  respiration  is  that  liberated 
through  photosynthesis.  Thus  it  is  apparent 
that  living  organisms  depend  upon  photosyn- 
thesis not  only  for  their  food  but  also  for  the 
essential  oxygen  which  alone  makes  it  pos- 
sible for  the  complex  foods  to  be  utilized. 
(Fig.  38.) 

The  basic  food  compound  formed  by 
photosynthesis,  as  just  noted,  is  a  simple 
sugar,  glucose,.  It  is  present  in  animal  cells 
as  well  as  plant  cells.  Thus  in  the  human 
organism  it  is  glucose  that  is  finally  formed 
from  the  digestion  of  starch  and  all  types  of  carbohydrate.  And 
glucose  is  the  primary  carbohydrate  carried  in  the  blood  stream  to  the 
cells  of  the  body  where  it  is  oxidized  as  necessary,  thus  releasing  the 
energy  previously  stored  in  photosynthesis.  Glucose,  having  been 
formed  in  the  green  plant  cell,  (1)  may  be  oxidized  at  once  to  secure 
energy;  or  (2)  it  may  be  changed  to  other  more  complex  carbohydrates 
and  stored  for  later  use;  or  (3)  the  proportion  of  oxygen  present  in 


FIG.  38. — Diagram  of  an 
experiment  illustrating  re- 
lease of  oxygen  during  pho- 
tosynthesis in  Elodea,  a 
common  water  plant.  The 
bubbles  are  passing  up  from 
the  cut  end  of  the  stem  and 
displacing  water  in  the  test 
tube.  (Haupt.) 


THE  BIOLOGY  OF  NUTRITION  69 

either  the  sugar  or  the  starch  may  be  decreased,  and  thereby  the  sub- 
stance changed  from  a  carbohydrate  to  a  fat  which  may  also  be  stored 
for  later  utilization  as  necessary;  or,  finally,  (4)  the  plant  cells  have  the 
power  to  add  additional  elements,  essential  to  the  living  tissues,  to  the 
carbohydrate  molecule  thus  forming  the  proteins  which,  in  turn,  are 
built  into  protoplasm. 

It  has  just  been  noted  that  various  possibilities  await  the  glucose 
after  being  synthesized.  Let*  us  first  consider  its  transformation  into 
other  carbohydrates.  As  the  first  step,  the  monosaccharide  may  be 
changed  by  the  plant  cells  into  cane  sugar,  that  is,  the  disaccharide, 
sucrose  (Ci2H22On).  Synthesizing  plant  enzymes  accomplish  this  by 
the  removal  of  one  molecule  of  water  from  each  two  molecules  of  glu- 
cose. Thus:  2C6Hi206  — H2O  =  Ci2H22On.  When  the  cane  sugar 
is  taken  into  the  animal  digestive  tract,  it  will  be  acted  upon  by  the 
specific  digestive  enzyme,  sucrase;  a  molecule  of  water  is  added  to 
each  sucrose  molecule,  which  brings  back  the  original  condition  with 
two  molecules  of  glucose  (page  62). 

But  the  glucose  of  the  green  plant  cell  may  be  changed  for  storage 
into  a  still  higher  type  of  carbohydrate,  the  polysaccharides,  notably 
cellulose  and  starch.  When  a  polysaccharide  is  formed,  the  synthetic 
enzyme  action  results  in  the  removal  of  one  molecule  of  water  from 
each  molecule  of  glucose.  Thus:  C6Hi206  —  H20  =  C6Hi005.  But 
a  polysaccharide  consists  of  many  of  these  anhydrous  glucose  residues 
joined  together,  and  so  the  formula  is  written  (CeHioOs)*.  Further- 
more, the  ultramicroscopic  starch  molecules  may  be  combined  to  form 
starch  grains  which  are  of  microscopic  visibility.  The  starch  grains 
of  a  particular  species  are  always  of  characteristic  size  and  shape.1 

From  the  standpoint  of  plant  structure,  the  most  important  carbo- 
hydrate is  the  polysaccharide,  cellulose.  This  carbohydrate  has  the 
same  composition  as  starch;  that  is,  it  consists  of  associated  glucose 
residues,  (CoHioOs)*,  but  the  arrangement  of  the  atoms  is  different 
and  results  in  a  much  more  resistant  substance,  which  is  utilized  as  a 
plant-building  material.  Plant  cells  are  characterized  by  prominent 
cellulose  cell  walls  which  are  formed  iis  a  secretion  around  the  living 
protoplasm.  As  the  cells  get  older,  more  and  more  cellulose  accumu- 
lates, and  so  this  material  largely  forms  the  structural  basis  of  plant 
tissues.  Woody  tissue,  for  example,  is  predominantly  cellulose  and 
associated  compounds.  Plant  cells,  as  a  rule,  are  not  long-lived,  but, 
before  the  protoplasmic  activities  cease,  considerable  cellulose  is  laid 
down,  and  these  cellulose-encased  cellular  units  remain  as  the  perma- 
nent structural  units  of  the  woody  tissues.  Consideration  of  the 

1  Consult  the  Appendix:  Starch. 


70  HUMAN  BIOLOGY 

many  uses  that  man  finds  for  cellulose,  for  example,  in  the  building, 
textile,  and  chemical  industries,  makes  evident  the  commercial  impor- 
tance of  this  most  abundant  of  all  carbohydrates.  The  use  of  cellulose 
as  a  foodstuff,  however,  is  limited  by  the  fact  that  the  digestive  enzymes 
of  many  organisms,  including  man,  are  not  able  to  digest  this  resistant 
material.1 

In  cellulose,  the  linkage  of  the  glucose  residues  is  so  rigid  that  the 
digestive  action  of  the  human  tract  is  not  strong  enough  to  separate 
them,  though  cellulose  digestion  is  accomplished  by  the  herbivorous 
animals.  In  the  woody  tissue  of  trees,  which  is  also  largely  cellulose, 
the  molecules  are  even  more  strongly  attached  to  each  other  so  that 
woody  tissue  cannot  be  utilized  as  food  even  by  the  plant  caters. 
Certain  wood-eating  insects,  the  termites,  manage  to  utilize  woody 
tissue,  but  they  do  it  by  maintaining  in  their  digestive  tract  a  tre- 
mendous staff  of  one-celled  animals,  the  flagellates,  which  possess 
enzymes  sufficiently  powerful  to  produce  hydrolysis  of  woody  tissue. 
(Figs.  231,  232.) 

The  essentials  of  the  story  are  the  same  in  the  fats  and  proteins  as 
in  the  carbohydrates.  Fats  are  built  up  either  in  the  plant  cells  or 
in  the  animal  cells  by  a  combination  of  fatty  acids  and  glycerol,  and 
water  is  removed  during  the  synthesis.  In  the  digestion  of  a  fat  the 
restoration  of  the  water  by  an  enzyme  action  splits  it  into  the  original 
fatty  acids  and  glycerol.  Finally,  plant  cells  have  the  power  to  com- 
bine other  essential  elements  of  the  protoplasmic  assemblage  with  the 
carbohydrates  produced  during  photosynthesis  and  thus  to  form  the 
proteins  which  are,  in  turn,  built  into  the  protoplasm.  It  is  ttot  cer- 
tainly known  that  protein  formation  is  directly  associated  with  the 
photosynthetic  processes  that  produce  the  carbohydrates.  Evidence 
exists  that  protein  formation  may  occur  in  any  plant  tissue  and 
in  the  absence  of  light,  but,  nevertheless,  the  chlorophyll-bearing 
cells  in  the  leaves  appear  to  be  the  most  active  centers  of  protein 
formation. 

Proteins  are  not  formed  directly  from  the  carbohydrate  molecule, 
but  the  less  complex  amino  acids  are  first  built  up,  the  simplest  of 
which  contain  only  nitrogen  in  addition  to  the  carbon,  hydrogen,  and 
oxygen.  Altogether,  around  two  dozen  amino  acids  are  known,  but 
from  these  relatively  few  "amino  acid  building  stones'7  it  is  possible 
to  construct  an  almost  infinite  number  of  proteins  in  the  cells  of  the 
plant  and  animal  tissues.  Thus  from  20  known  amino  acids,  it  has 
been  calculated  that  at  least  2,432,902,008,176,640,000  different  com- 
pounds could  be  formed  without  even  varying  the  proportion  of  the 

1  Consult  the  Appendix:  Cellulose. 


THE  BIOLOGY  OF  NUTRITION  71 

different  amino  acids  in  a  single  protein.1  Furthermore,  it  must  be 
emphasized  that  protoplasm,  as  the  "vehicle  of  vital  manifestations/' 
consists  not  of  one  but  of  various  proteins,  which,  in  turn,  are  associated 
with  representatives  of  the  carbohydrates  and  fats,  the  entire  proto- 
plasmic setup  being  so  extraordinarily  complicated  and  capable  of 
such  great  variation  as  to  make  possible  the  almost  infinite  variety  of 
organisms  present  in  the  living  world  today. 

In  the  digestion  of  proteins,  the  proteolytic  enzymes  (pepsin, 
trypsin,  erepsin)  are  able  to  bring  about  the  introduction  of  water  and 
thereby  split  the  protein  molecules  step  by  stop  into  the  amino  acids 
(page  57).  Thus  the  complicated  chemical  processes  associated  with 
the  digestion  of  the  proteins  finally  yield  amino  acids,  and  the  latter 
are  resynthesized  into  proteins  by  each  cell  individually. 

Synthesis  of  protein  and  other  substances  in  the  cell  from  the 
materials  rigidly  selected  from  the  environment  results  in  the  forma- 
tion of  proteins  that  are  "custom-tailored"  to  supply  the  needs  of 
each  individual  cell  and  not  necessarily  a  duplication  of  the  proteins  in 
the  foodstuffs  originally  taken  into  the  digestive  tract.  The  various 
plant  and  animal  proteins  that  are  eaten  represent  the  synthetic 
activities  of  the  cells  of  some  other  plant  or  animal  organism.  All 
proteins  built  into  the  living  protoplasm  are,  of  course,  within  the 
cells — life  exists  only  within  the  cells — but  less  complex  proteins  are 
found  in  the  blood  plasma,  the  ground  substance  of  the  connective 
tissues,  etc.  These  proteins  outside  the  cells  represent  cell  secretions, 
that  is,  materials  that  have  been  manufactured  within  the  cells  and 
then  secreted. 

In  summarizing,  the  essentials  of  the  nutritive  processes  can  be 
stated  very  simply;  the  battery  of  enzymes  present  in  the  alimentary 
tract  is  able  to  hydrolyze  the  specific  foodstuffs  for  which  they  are 
adapted — the  end  result  being  that  all  suitable  types  of  carbohydrates 
are  finally  converted  into  the  simple  sugars,  that  all  the  fats  are  con- 
verted into  fatty  acids  and  glycerol,  and  that  all  the  proteins  are 
changed  into  amino  acids.  These  constitute  the  organic  substances 
that  are  absorbed  from  the  alimentary  tract  and  transported  to  the 
body  cells  and  from  which  the  cells  make  their  own  individual  selec- 
tions for  intracellular  reactions,  as  noted  above.  Added  to  the 
absorbed  nutrients  are  the  soluble  vitamins  and  simple  inorganic  sub- 
stances, notably  water  and  table  salt,  together  with  a  wide  assortment 
of  mineral  elements  combined  in  some  way  with  the  organic  substances ; 
all  of  which  are  apparently  unchanged  by  the  digestive  actions. 

1  The  possible  number  of  proteins  that  can  be  formed  by  combinations  of 
amino  acids  may  perhaps  be  visualized  by  considering  the  number  of  words  that 
can  be  formed  from  the  26  letters  of  the  alphabet. 


EPIGLOTTIS 
HY01D  BONE 


PULMONARY  ARTERYO 

BRONCHUS 
PULMONARY  VEIN  (LEFT] 


VOCAL  CORD 
THYROID  CARTILAGE-4 


PULMONARY  ARTERY(RIGHT)T 
PULMONARY  VEIN  (RIGHT) 


PULMONARY  ARTERY(LEH) 
PULMONARY  VEIN  (LEFT) 

I 

HEART 


PLEURA  (PARIETAL) 


INTERCOSTAL  MUSCLE 
ESOPHAGUS 


VENACAVAUNF.) 


ABDOMINAL  AORTA 


PULMONARY  ARTERY  (RIGHT 
PULMONARY  VEIN  (RIGHT) 


BRANCH  OF  PULMONARY  VEIN 
BRANCH  OF  PULMONARY  ARTERY- 


ALVEOLAR  DUCT 


PULMONARY  ARTERY(LEFT) 
PULMONARY  VEIN  (LEFT) 


PLATE  IV. — Human  lungs  and  associated  structures.  A,  relationship  between  lungs 
and  vascular  system;  B,  relationship  between  lungs  and  trachea;  C,  longitudinal  section 
of  anterior  end  of  trachea;  !>.  left  lung  and  bronchi;  E,  infundibula;  F,  microscopic 
section  of  lung  tissue. 


CHAPTER  IV 
THE  BIOLOGY  OF  RESPIRATION 

/ 

We  have  seen  that  the  elements  essential  to  the  living  organism  are 
assembled  and  combined  in  the  green  plant  cells  and  that,  through 
the  formation  of  organic  compounds  from  inorganic  materials,  the 
radiant  energy  of  the  sun  is  made  available  for  the  metabolic  activities 
following  intracellular  oxidation — the  process  of  respiration.  The 
changes  associated  with  digestion,  considered  in  the  previous  chapter, 
do  not  disturb  the  energy  relations  of  the  foodstuffs;  the  nutrient 
materials  are  merely  changed  so  that  they  can  be  absorbed  and 
assimilated  by  the  living  cells.  Respiration,  by  which  the  potential 
energy  of  the  stored  compounds  is  transformed  into  active,  or  kinetic, 
energy,  occurs  in  each  cell.  Required  for  this  process  is  free  oxygen 
which  combines,  under  the  influence  of  intracellular  enzymes,  with  the 
accumulated  materials  arid  disrupts  them  through  complex  oxidative 
processes. 

Certain  microscopic  fungi,  the  anaerobic  bacteria,  do  not  utilize 
the  free  oxygen  of  the  air  but  secure  this  element  by  breaking  down 
suitable  oxygen-containing  compounds  through  enzyme  action.  The 
overwhelming  majority  of  plants  and  animals,  however,  are  aerobic; 
that  is,  they  must  have  free  oxygen  which  they  secure  either  from  the 
air  or  from  fresh  or  salt  water,  all  of  which  normally  contain  a  suffi- 
cient amount  of  oxygen  uncombined  with  other  substances.  The 
oxygen  in  solution  can  easily  be  driven  out  by  boiling  the  water. 
Then  no  aerobic  organism  could  live  in  it,  for  none  of  them  are  able  to 
destroy  the  water  molecules  and  thus  secure  oxygen. 

In  the  microscopic  unicellular  forms  of  life,  the  interchanges  of 
gases  associated  with  respiration  appear  to  be  comparatively  simple. 
The  organism  finds  sufficient  free  oxygen  dissolved  in  the  surrounding 
liquid  environment  which  it  utilizes  as  needed  and  releases  the  sub- 
sequently formed  carbon  dioxide  in  the  same  fashion.  In  the  highly 
developed  multicellular  forms  of  life,  the  basic  processes  associated 
with  respiration  are  identical  with  those  of  the  unicellular  types  of 
life,  but  the  mechanism  necessary  to  get  the  oxygen  into  the  body  and 
to  convey  it  to  each  of  the  constituent  cells  greatly  obscures  the 
respiratory  picture.  In  fact,  respiration  in  man  is  commonly  thought 

73, 


74 


HUMAN  BIOLOGY 


of  as  being  identical  with  the  process  of  breathing.  The  latter  func- 
tion, however,  is  merely  a  method  of  getting  the  oxygen  into  the  body. 
Breathing  is  essential  to  respiration,  but  it  is  not  respiration.  Some 
authorities  speak  of  external  respiration,  meaning  the  act  of  breathing 
as  distinguished  from  internal  respiration  which  is  used  to  indicate  the 
cellular  intake  and  utilization  of  oxygen,  the  basic  feature  of  respira- 
tion. Physiologists  do  not  know  just  how  respiration  is  accomplished 
in  the  cell;  they  know  only  the  start  (oxygen  taken  in),  and  the 
result  (energy  released),  with  the  elimination  of  the  end  products. 
Accordingly  this  chapter  is  from  necessity  mainly  a  description  of  the 
structural  and  functional  features  of  the  respiratory  mechanism  in  man 
by  which  oxygen  is  secured  and  the  resulting  compounds,  which  are 
of  no  use  to  the  cell,  excreted. 

STRUCTURAL  FEATURES  ASSOCIATED  WITH  RESPIRATION 

In  the  aquatic  animals,  a  variety  of  structures  are  utilized  to  secure 
the  necessary  oxygen  from  the  environment.     In  all  of  these  organisms, 


GILL  ARCH 


ARTERY. 


ARTERY 


VEIN 


(a)  (b) 

FIG.  39.— a,  diagram  illustrating  the  general  gill  structure  of  a  fish;  b,  portion  of  the 
gill,  highly  magnified.  Arrows  show  the  course  of  the  blood  stream  in  its  passage 
through  the  gill  tissues.  (Goldschmidt,  "  Aacaris,"  Prentice-Hall,  Inc.) 

however,  the  essential  element  in  the  oxygen-collecting  equipment 
proves  to  be  a  tissue  abundantly  supplied  with  blood  vessels  and 
covered  on  the  surface  in  contact  with  water  by  specialized  epithelial 
cells,  which  are  able  to  take  up  the  oxygen  from  the  water  and  to  turn 
it  over  to  the  special  cells  in  the  blood  stream  for  transportation  to 
the  cells  of  the  body.  Such,  essentially,  are  the  gills  of  fish  and  similar 
organs  in  other  water-living  forms.  (Fig.  39.) 

In  the  air-breathing  animals,  the  oxygen-collecting  apparatus  is  of 
the  same  basic  design  as  in  the  water-living  animals.     Again  there  is 


THE  BIOLOGY  OF  RESPIRATION 


75 


a  special  tissue  abundantly  supplied  with  blood  vessels  and  covered 
on  the  outer  surface  with  specialized  epithelial  cells,  but  the  latter,  in 
the  air-breathers,  are  adapted  to  take  oxygen  from  the  air  instead  of 
from  water.  The  reason  why  a  fish  cannot  live  out  of  water  and  a  man 
cannot  live  in  water  rests  directly  upon  the  inability  of  the  epithelial 
cells  of  these  organisms  to  extract  oxygen  from  the  strange  environ- 
ment in  which  they  suddenly  find  themselves.  In  man  and  air- 
breathers  generally,  the  lungs  serve  as  the 
oxygen-collecting  apparatus.  But  in  one 
very  large  and  important  animal  group,  the 
insects,  air  tubes  are  not  concentrated  to 
form  lungs  but  ramify  all  through  the  body 
tissues  and  open  to  the  exterior  on  the  sides 
of  the  body.  Air  is  carried  to  the  body  cells 
through  these  traeheal  tubes,  arid  the  body 
cells  pick  up  the  oxygen  directly  from  them 
so  that  an  intermediate  vascular  system  is 
not  needed  for  its  conveyance.  (Fig.  40.) 

The  Lungs  and  Trachea. — In  the  sim- 
plest condition,  found  in  the  more  primitive 
air-breathing  vertebrates  such  as  the  adult 
frog,  the  lungs  consist  of  a  pair  of  small 
distensible  sacs.  Each  of  these  lung  sacs  is 
connected  with  an  air-conducting  tube,  the 
trachea,  which  opens  into  a  common  laryn- 
gotracheal  cavity.  The  latter  lies  under- 
neath the  floor  of  the  mouth,  near  the  pos- 
terior end,  and  opens  into  the  mouth  cavity 
through  an  elevated  circular  glottis  at  the 
base  of  the  tongue.  The  walls  of  this  com- 
paratively simple  type  of  lung  are  more  or  less 
contractile  and  highly  vascularized.  The 
lining  of  the  lung  is  thrown  into  folds  that 
project  in  the  cavity  to  such  an  extent  that 
numerous  tiny,  partially  closed  air  cavities  are  formed.  Air  passing 
through  the  glottis  reaches  the  lungs  and  comes  into  contact  with  the 
functional  respiratory  cells  that  line  it.  These  absorb  the  free  oxygen 
from  the  air  and  turn  it  over  to  the  blood  stream  and  at  the  same  time 
remove  the  carbon  dioxide  for  excretion.  Even  more  important  than 
the  lungs  in  the  respiratory  activities  of  the  frog  is  the  skin,  for  the  lat- 
ter is  responsible  for  the  greater  proportion  of  the  respiratory  exchange. 
Through  living  in  the  water,  the  surface  of  the  f rog's  skin  is  constantly 


FIG.  40. — Diagram  illus- 
trating the  tracheae  in  an 
insect  which  carry  air  directly 
to  the  tissues.  I,  longi- 
tudinal trachea;  o,  external 
opening  (spiracle.)  (Gold- 
Schmidt  ,  "A  scaris , ' '  Pren- 
tice-Hall, Inc.) 


76 


HUMAN  BIOLOGY 


wet.  In  this  condition,  the  skin,  with  its  abundant  supply  of  blood 
vessels,  readily  permits  the  inflow  of  oxygen  and  release  of  carbon  diox- 
ide. The  result  is  that  the  blood  returning  to  the  body-tissues  from 
the  skin  is  as  well  oxygenated  as  is  that  from  the  lungs.  The  capacity 
of  the  frog's  lungs  is  not  sufficient  to  maintain  adequate  oxygenation  of 
the  blood.  Accordingly  the  animal  will  die  if  the  skin  becomes  dry, 
^thereby  preventing  the  normal  respiratory  interchange.  (Fig.  41.) 


C  ^2^^%"Posferhr 

A  Tubu/es       D  E 

FIG.  41. — Diagrams  illustrating  lung  structure  in  various  vertebrates.  The  higher 
types  are  characterized  by  an  increasing  amount  of  lung  surface  exposed  to  the  air. 
A,  necturus,  no  alveoli;  B,  frog;  C,  lizard;  D,  bird;  E,  mammal,  with  branching  bronchi. 
(Wolcott,  after  Locy  and  Larsdl.) 

THE  RESPIRATORY  SYSTEM  OF  MAN 

In  the  higher  vertebrate  animals  such  as  man,  which  are  not 
adapted  for  aquatic  life,  the  skin  has  almost  en tirely  losi  its  respira- 
tory function,  and  a  corresponding  development  of  the  lungs  has 
necessarily  occurred,  since  they  must  bear  the  -entire-.  hurdeaajf  jbhe 
respiratory  exchange  with  the  environment.  The  position  of  the 
Itlfigs  in  the  thoracic  cavity  has  already  been  noted,  as  has  also  that 
of  the  trachea,  or  windpipe,  connecting  them  to  the  throat.  Both  the 
nasal  passages  and  the  mouth  may  be  used  for  the  intake  and  release 
of  the  respiratory  gases.  It  is  apparent  that  breathing  through  the 
nose  is  the  correct  method,  for  the  epithelium  lining  the  large  areas  of 
the  nasal  cavities  contain  mucus-secreting  cells.  The  secretion  from 
these  -cells  covers  the  nasal  epithelium  and  aids  in  removing  dust 
particles  from  the  incoming  air  stream.  Also  the  temperature  of  the 
incoming  air  is  rapidly  brought  to  body  temperature  as  it  moves 
through  the  nasal  passages,  which  thus  serve  as  an  efficient  air-con- 
ditioning apparatus.  Unfortunately,  the  nasal  passages  are  subject  to 
partial  or  complete  stoppage  following  head  infections,  and  then  the 
mouth  opening  must  be  utilized  to  maintain  the  constant  air  supply. 


THE  BIOLOGY  OF  RESPIRATION 


77 


EPIGLOTTIS 
HYO1D  BONE 


VOCAL  CORD 
THYROID  CARTILAGE 

CRICOID  CARTILAGE 
TRACHEA 


FIG.  42. — Longitudinal  sec- 
tion through  the  anterior  end 
of  the  human  trachea. 


Whether  taken  in  through  the  nose  or  mouth,  the  air  current  passes 
swiftly  to  the  common  throat  region,  enters  the  trachea  through  the 
upraised  epiglottis,  finds  its  way  through  the  glottis,  and  reaches  the 
lungs.  (Figs.  30,  42.) 

The  trachea  is  a  cartilaginous  tube  with  rings  of  heavier  material 
encircling  the  wall  at  regular  intervals  so  that  it  does  not  collapse. 
Considerable  areas  of  the  trachea  are  lined  with  ciliated  epithelial 
cells,  the  effective  beats  of  which  are  di- 
rected away  from  the  lungs.  The  ciliary 
action  is  effective  in  removing  foreign 
materials  from  the  respiratory  tract.  The 
anterior  end  of  the  trachea  is  modified  to 
form  a  complicated  box-like  larynx,  or 
Adam's  apple.  The  epiglottis  is  attached 
anterior  to  the  larynx,  with  the  glottis, 
through  which  the  air  passes  in  and  out  of 
the  trachea,  lying  just  below.  The  larynx 
is  essentially  an  apparatus  for  the  produc- 
tion of  sound  and  has  nothing  to  do  with  respiration.  It  is  so  placed 
and  constructed  that  advantage  can  be  taken  of  the  outgoing  air  cur- 
rents to  initiate  vibrations  of  the  taut  vocal  cords  which  form  the 
boundaries  of  the  glottis.  Sounds  of  varying  pitch  are  thus  produced. 
The  use  of  the  larynx  in  speech  is  considered  in  more  detail  below. 
The  Lungs.— Shortly  before  reaching  the  lungs,  the  trachea 

divides  to  form  right  and  left 
branches,  the  bronchi,  each  of  which 
enters  the  lung  on  its  side  and  then 
proceeds  to  form  smaller  and  smaller 
subdivisions,  the  bronchioles.  The 
bronchioles  terminate  in  tiny,  en- 
closed air-sac  structures,  shaped 
somewhat  like  a  bunch  of  grapes, 
the  infundibula,  which  approximate 
1/30  m-  in  diameter.  The  inner 
lining  of  the  infundibula  is  arranged 
in  such  a  way  that  incredible  num- 
bers of  the  basic  units  for  respiratory  exchange,  the  alveoli,  are 
formed.  The  alveoli  are  essentially  closed  cavities  or  air  sacs,  each 
one  of  which  has  a  direct  connection  to  the  trachea  through  the 
attached  bronchiole;  the  latter  in  turn  communicating  with  the  bron- 
chi, trachea,  and  then  the  exterior.  The  walls  of  the  alveoli  contain 
supporting  tissues,  particularly  elastic  tissue  which  permits  considerable 


TRACHEA 

PULMONARY  ARTERY(LEFT; 

BRONCHUS 
PULMONARY  VEIN  (LEFT) 

LEFT  LUNG 


FIG.  4.3.™Ilhistrating  the  general 
structure  of  the  human  lung  and  its 
connection  with  the  trachea,  as  seen 
from  the  ventral  surface. 


78  HUMAN  BIOLOGY 

expansion.  But  functionally  of  greatest  importance  is  the  respiratory 
epithelial  tissue  which  forms  a  lining,  one  cell  thick,  throughout 
every  alveolus.  It  permits  the  exchange  of  oxygen  and  carbon  dioxide 
between  the  contained  alveolar  air  and  the  blood  stream  flowing 
through  the  tiny  vessels  that  abundantly  permeate  the  alveolar  walls 
just  beneath  the  lining  epithelium.  Thus  air  passing  through  the 
trachea,  bronchi,  bronchioles,  and  infundibula  finally  reaches  the 
alveoli  and  here  comes  into  contact  with  the  moist  lining  epithelium 
through  which  the  respiratory  exchange  takes  place.  Since  the  lungs 
develop  in  the  early  embryo  as  outgrowths  from  the  primitive  endo- 
dermal-lined  gut,  it  is  clear  that  the  functional  respiratory  epithelium 
lining  the  lungs  is  of  endodermal  origin  just  as  is  the  mucosa  of  the 
alimentary  tract.  (Figs.  43,  44.) 


BRONCHIOLE 


BRANCH  OF  PULMONARY  VEIN 
BRANCH  OF  PULMONARY  ARTERY' 

ALVEOLAR  DUCT 


ALVEOLUS 
ALVEOLAR  SAC 


FIG.  44. — Illustrating  the  microscopic  structure  of  lung  tissue.  Right,  termination 
of  the  bronchioles  to  form  alveolar  sacs  which  are  grouped  as  infundibula;  left,  section  of 
lung  tissue  as  seen  under  the  microscope. 

The  lungs  are  covered  with,  and  the  thorax  is  lined  by,  the  pleura, 
a  thin  epithelial  membrane  developed  from  the  mesoderm.  It  is  of 
the  same  nature  as  the  peritoneal  lining  of  the  abdominal  cavity, 
which,  it  will  be  remembered,  is  reflected  over  the  viscera  and  forms 
the  mesenteries  supporting  the  intestines.  When  the  lungs  are  fully 
inflated,  their  pleural  covering  is  in  contact  with  the  pleural  lining 
of  the  thoracic  cavity;  but  under  conditions  of  partial  inflation,  a 
space,  the  pleural  cavity,  lies  between  the  two  layers.  Infection  of  the 
pleural  membranes,  which  is  not  uncommon  following  pneumonia,  is 
known  as  pleurisy.  (Plate  IVA,  page  72.) 

The  respiratory  epithelium,  lining  the  innumerable  air  sacs  of  the 
lungs,  is  a  very  thin  layer,  but  it  covers  a  great  surface  area.  It  has 
been  computed  that  the  total  area  of  the  lung  cavities  is  some  sixty 
times  that  of  the  body  surface.  Also  the  total  air  capacity  of  the  lungs 
is  far  in  excess  of  the  actual  needs  of  the  body.  The  average  person 
when  resting  does  not  use  more  than  5  per  cent  of  the  total  lung  capac- 


THE  BIOLOGY  OF  RESPIRATION 


79 


ity.  Under  conditions  of  severe  muscular  activity,  the  gaseou? 
exchange  through  the  lungs  is  greatly  increased,  but,,  even  so,  the 
normal  lung  capacity  is  always  more  than  ample.  Many  persons  are 
living  active  lives  whose  lung  capacity  has  been  reduced  by  half  as 
the  result  of  the  destruction  of  lung  tissue  following  tuberculosis, 
(Fig.  44.) 

Lung  Capacity. — The  vital  capacity  of  the  lungs  is  the  maximum 
amount  of  air  that  can  be  exhaled  following  the  maximum  inspiration, 
and  it  amounts  to  about  240  cu.  in.,  or  3,700  cc.,  in  the  average  medium- 
sized  person.  Residual  air  is  the  air  remaining  in  the  lungs  that  cannot 
be  expelled.  It  amounts  to  another  1,000  or  1,200  cc.,  so  that  the 
maximum  air  capacity  of  the  lungs  is  not  far  from  5,000  cc.  But 
these  maximum  figures  arc  possibly  not  so  important  as  the  ones 
associated  with  normal  breathing.  The  latter  condition  involves  the 
movement  in  or  out  of  the  lungs  of  approximately  500  cc.  of  tidal  air. 
An  additional  1,600  cc.  of  complemented  air  can  be  drawn  into  the 
lungs,  or  an  additional  1,600  cc.  of  supplemental  air  can  be  expelled 
if  desired.  Tidal  air  plus  complements!  air  plus  supplemental  air 
totals  about  3,700  cc.,  which  is  the  vital  capacity  as  defined  above. 
And  the  vital  capacity  plus  the  residual  air  makes  the  maximum 
capacity  of  the  lungs  about  4,800  to  5,000  cc.,  as  stated  earlier. 

Respiratory  Gases. — It  is  clear  from  the  data  just  given  that  the 
500  cc.  of  tidal  air,  which  is  all  that  is  moved  in  ordinary  breathing,  is 
less  than  one-seventh  of  the  vital  capacity.  The  supplemental  and 
residual  air  are  essentially  stationary.  Furthermore,  of  the  500  cc. 
of  tidal  air  brought  into  the  lungs  at  each  normal  inspiration,  only 
about  350  cc.  of  new  air  enters,  because  of  the  fact  that  some  150  cc. 
of  tidal  air  was  in  the  bronchi,  trachea,  and  throat  when  the  inspiration 
stopped,  and  this  old  air,  therefore,  goes  back  into  the  lungs  when  the 
tide  turns.  The  point  to  emphasize  in  all  this  is  that  relatively  little 
air  is  brought  into  the  lungs  from  the  outside  at  each  breath;  a  mixture 
of  old  air  and  new  air  is  always  present;  and  accordingly  no  radical 
change  in  the  alveolar  air  takes  place  when  inspiration  occurs.  The 
amount  of  air  taken  in  at  each  inspiration  is  sufficient  to  keep  the 
oxygen  and  carbon  dioxide  content  at  the  proper  levels  so  that 
the  blood  is  always  adequately  aerated.  The  oxygen-carbon  dioxide 


Air 

Nitrogen, 
per  cent 

Oxygen, 
per  cent 

Carbon  dioxide, 
per  cent 

Inspired  

79 

20  96 

0  04 

Expired  

79 

16  62 

4.38 

80  HUMAN  BIOLOGY 

relationships  are  shown  by  the  table  on  page  79  giving  the  gaseous 
content  of  the  inspired  and  expired  air. 

Thus  it  is  shown  that  the  expired  air  drawn  from  the  lungs  contains 
a  much  higher  percentage  of  oxygen  than  of  carbon  dioxide — in  fact, 
nearly  four  times  as  much.  The  expired  air  contains  certain  other 
substances  picked  up  in  lungs  in  addition  to  the  carbon  dioxide, 
notably  water  vapor,  which  amounts  to  some  250  cc.  per  day  (J^  pt.), 
and  a  slight  amount  of  organic  excretions.  And,  of  course,  the  tem- 
perature of  the  air  leaving  the  body  is  that  of  the  body  and  not  that 
of  the  atmosphere  (page  127). 

An  important  factor  in  maintaining  an  adequate  supply  of  oxygen 
appears  to  be  the  relative  amount  of  carbon  dioxide  present.  Thus, 
if  air  is  inhaled  that  contains  around  5  per  cent  of  carbon  dioxide  instead 
of  the  normal  0.04  per  cent,  the  breathing  movements  are  markedly 
increased  as  the  percentage  of  carbon  dioxide  gradually  increases  in 
•the  lungs.  On  the  other  hand,  if  one  voluntarily  resorts  'to  forced, 
heavy  breathing  of  air  containing  the  normal  amounts  of  oxygen  and 
carbon  dioxide,  the  percentage  of  carbon  dioxide  in  the  alveolar  air  will 
soon  be  decreased.  In  correspondence  with  the  decrease  in  the  carbon 
dioxide,  the  respiratory  movements  will  be  involuntarily  reduced  or 
suspended  entirely — the  condition  of  acapnia — until  the  percentage  of 
carbon  dioxide  again  reaches  the  normal  level. 

In  addition  to  the  nitrogen,  oxygen,  and  carbon  dioxide  given  in  the 
foregoing  table,  atmospheric  air  contains  about  1  per  cent  of  a  mixture 
of  inert  gases,  notably  argon  and  neon.  Adding  this  to  the  nitrogen, 
which  is  also  inert  and  goes  in  and  out  of  the  lungs  with  essentially  no 
change  in  volume,  it  is  found  that  79  per  cent,  or  nearly  four-fifths, 
of  all  the  gases  in  the  air  we  breathe  has  no^part  in  the  chemical 
activities  of  respiration.  The  inert  gases  do,  however,  serve  to  dilute 
the  oxygen,  and  this  function  is  important,  since  pure  oxygen  is  a 
destructive  agent  to  the  tissues. 

Normal  breathing  occurs  about  every  4  seconds,  or  15  times  per 
minute,  but  the  rate  is  subject  to  considerable  variation  in  different 
persons  and  in  the  same  person  under  different  conditions.  As  noted, 
500  cc.  of  tidal  air  is  inhaled  at  each  breath  so  that  each  minute 
7,500  cc.,  or  7.5  liters,  nearly  8  qt.,  of  air  is  taken  into  the  lungs  and 
the  same  amount  removed.  However,  only  5.4  liters  of  this  is  new  air, 
as  noted  above.  It  is  commonly  stated  that  every  minute  of  the  day, 
under  normal  conditions,  some  250  cc.  of  oxygen  is  removed  from  the 
inspired  air,  and  slightly  less  than  this  amount  of  carbon  dioxide  added 
to  the  air  leaving  the  lungs.  The  oxygen  acquired  in  this  fashion  is 
the  amount  required  by  the  cells  of  the  body  for  the  essential  metabolic 


THE  BIOLOGY  OF  RESPIRATION  81 

processes.     Computing  this  on  the  24-hour  basis,  the  oxygen  need  is 
found  to  be  360  liters,  or  between  12  and  13  cu.  ft. 

BREATHING 

Respiratory  Movements.  —  The  movements  associated  with  breath- 
ing result  from  the  coordinated  coritr&l  by  the  nervous  system  of  a 
number  of  diverse  muscular  elements,  situated  in  the  walls  and  in  the 
floor  of  the  chest  cavity,  which  exert  their  pull  upon  the  bony  ribs. 
The  ribs  are  attached  dorsally  to  the  spinal  column  in  a  manner  that 
permits  of  considerable  freedom  of  movement.  Ventrally,  the  anterior 
10  pairs  of  ribs  are  attached  to  the  unjointed  cartilaginous  sternum, 
whereas  the  last  two  pairs  are  attached  only  to  the  spinal  column.  The 
curvature  of  the  ribs  and  their  mode  of  attachment  are  such  that  when 
the  intercostal  muscles  lying  between  the  ribs  contract,  the  ribs  are 
drawn  up  (toward  the  head)  and 

out  (toward  the  ventral  body  wall).  ,     /  ^ 

This  results  in  markedly  increas-      *"'"  *   '          /*****»* 
ing  the  size  of  the  chest  cavity  in 
which  the  lungs  are  situated.     The 
size  of  the  chest  may  also  be  in- 
creased by  the  contraction  of  the  * 

muscle  fibers  in   the   diaphragm,  * 

which  forms  the  floor  of  the  thorax.     .     Fm-  ,45;~Diaf  a™   grating   en- 

largement  of  the  ohest  during  inspiration 
In  the  relaxed  Condition,  the   dia-      by    movements    of    the    ribs.      (Watkeys, 


phragm  is  somewhat  U-shaped, 
with  the  bottom  of  the  U  turned  upwards  toward  the  lungs.  When 
contraction  occurs,  the  U  is  greatly  flattened,  and  thus  the  diaphragm 
is  pulled  posteriorly  (that  is,  away  from  the  lungs  and  against  the 
abdominal  organs),  and  this,  of  course,  results  in  an  enlargement  of  the 
chest  cavity.  (Fig.  45;  Plate  IV  A,  page  72.) 

Those  who  ha  ^e  not  studied  the  matter,  generally  have  the  impres- 
sion that  air  is  sucked  into  the  lungs  and  that  the  chest  expands  to 
accommodate  the  air  that  has  been  taken  in.  This  is  getting  the  cart 
before  the  horse  because  what  happens  when  air  is  inhaled  is  that  the 
size  of  the  chest  is  first  increased  by  the  contractions  of  the  intercostal 
and  diaphragm  muscles  as  described  in  the  preceding  paragraph.  The 
walls  of  the  chest  cavity  are  airtight;  and  when  the  cavity  is  enlarged 
and  a  partial  vacuum  thereby  created,  the  outside  air,  impelled  by  the 
atmospheric  pressure  of  15  Ib.  per  square  inch,  rushes  into  the  region 
of  lowered  pressure,  and  the  lungs  are  expanded  to  fill  the  additional 
space.  This  action  is  nicely  demonstrated  by  a  model  in  which  a  glass 
bell  jar  is  used  to  illustrate  the  chest  wall.  The  open  end  of  the  bell  jar 


82 


HUMAN  BIOLOGY 


is  closed  by  a  sheet  of  flexible  rubber  which  represents  the  diaphragm. 
The  lungs  and  windpipe  are  represented  in  the  model  by  a  rubber 
sack  tied  to  the  end  of  a  glass  tube.  The  rubber  lungs  and  a  portion 
of  the  glass  windpipe  are  inserted  through  the  top  of  the  bell  jar  and 
sealed  airtight.  Now  if  the  air  capacity  of  the  bell  jar  is  increased  by 
exerting  a  pull  on  the  rubber  diaphragm,  air  will  rush  through  the 
open  tube  into  the  "  rubber  lungs,"  and  they  will  expand  in  accordance 
with  the  lowered  pressure  in  the  belj  jar.  (Fig.  46.) 

Accordingly,  it  is  evident  that  the  intake  of  air  into  the  lungs  is 
essentially  dependent  upon  rhythmic  muscular  contractions  which 


FIG.  46. — Diagram  illustrating  the  inflation  of  the  lungs  as  described  on  page  81. 
In  this  experiment  a  pair  of  mammalian  lungs  have  been  used,  but  the  use  of  a  rubber 
sack,  such  as  a  toy  balloon,  is  simpler.  (Woodruff,  after  Tigerstedt;  redrawn.} 

result  in  the  enlargement  of  the  chest  cavity.  Less  evident  is  the 
muscular  action  in  expiration,  which  appears  to  be  more  of  a  passive 
process  associated  with  muscular  relaxation  and  the  natural  tendency 
of  the  expanded  alveoli  in  the  lungs  to  return  to  normaLsize.  In  the 
lowering  of  the  ..ribs,  J^e^gr&vity  pull  undoubtedly  has  someJ.njSiience. 
The  elevation  of  the  diaphragm  that  occurs  in  expiration  is  associated 
both  with  the  relaxation  of  the  muscle  fibers  and  also  with  a  con- 
traction of  another  set  of  muscles  in  the  abdominal  wall.  The  con- 
traction of  the  latter  compresses  the  abdominal  viscera  and  pushes 
them  upward  against  the  under  surface  of  the  diaphragm  and  thus 
accelerates  its  return  to  the  original  U-shape.  In  heavy  breathing 
following  increased  physical  exertion,  both  the  inspiration  and  expira- 
tion are  aided  by  the  contraction  of  additional  muscles  in  the  ribs  and 
body  wall  that  come'into  play  when  necessary. 


THE  BIOLOGY  OF  RESPIRATION  83 

Certain  common  variations  of  ordinary  breathing  are  noteworthy. 
One  of  these  is  coughing,  which  is  a  violent  expiratory  effort  resulting 
from  various  types  of  respiratory  irritation.  The  cough  is  preceded 
by  a  heavy  inspiration.  The  glottis  is  then  closed;  the  expiratory 
muscles,  chiefly  in  the  abdominal  wall,  contract  and  force  the  air 
through  the  glottis  and  out  the  mouth.  Sneezing  is  essentially  the 
same  as  coughing,  except  that  the  passage  through  the  mouth  is  closed 
by  the  soft  palate,  and  the  outrushing  air  is  forced  to  escape  through 
the  nasal  passages.  Another  all  too  common  respiratory  irregularity 
is  the  hiccough.  This  is  due  to  the  closing  of  the  glottis  during  a 
sudden  inspiration.  The  trouble  apparently  originates  in  the  dia- 
phragm which  contracts  irregularly  and  thus  brings  on  the  sudden 
inspiration.  The  incoming  air,  hitting  the  closed  glottis,  causes  the 
characteristic  sound. 

%  Control  of  Breathing.— Breathing  is  normally  under  involuntary 
control.  It  keeps  going  whether  we  think  about  it  or  not.  But  if  it 
is  desired,  breathing  can  be  increased,  reduced,  or  even  temporarily 
stopped.  Thus  voluntary  control  is  possible  within  certain  limits. 
However,  when  ,a  certain  stage  has  been  reached,  as  .in  holding  one's 
breath,  the  involuntary  mechanism  again  takes  control,  and  breathing 
is  resumed.  The  respiratory  center,  which  normally  governs  the 
rhythmic  muscular  contractions  associated  with  breathing,  is  situated 
in  a  portion  of  the  hindbrain,  known  as ^tfajm^dutta-  The  nature  of 
the  respiratory  center  itself  is  in  "doubt.  Some  hold  that  it  is  entirely 
automatic  in  its  action,  a  " robot,"  so  to  speak,  influenced  when  neces- 
sary by  changing  conditions  but  continuously  forming  and  discharging 
the  impulses  that  pass  over  the  nerve  fibers  and  cause  the  muscular 
movements.  On  the  other  hand,  the  respiratory  center  may  be  merely 
i  reflex  center  that  simply  relays  impulses  received  from  higher  centers 
bo  the  respiratory  muscles.  At  any  rate,  it  is  certain  that  the  condition 
3f  the  blood,  particularly  the  carbon  dioxide  content,  as  well  as  various 
external  influences  are  effective  in  modifying  the  impulses  from  the 
respiratory  center. 

The  determining  factor  in  the  behavior  of  the  respiratory  center 
appears  to  be  the  amount  of  carbon  dioxide  released  into  the  blood 
by  the  body  tissues,  and  this  amount,  of  course,  is  in  direct  ratio  to  the 
cellular  *  activities,  particularly  those  associated  with  movement. 
The  carbon  dioxide  picked  up  by  the  blood  stream  is  not  carried  as 
such,  for  it  immediately  combines  with  the  water  in  the  plasma  to 
Form  carbonic  acid,  thus:  CO2  +  H^O  =  CH2O3.  Carbonic  acid  tends 
bo  lower  the  normal  alkalinity  of  the  blood  plasma,  and  this  condition 
affects  the  respiratory  center.  The  latter,  in  turn,  stimulates  the 


84  HUMAN  BIOLOGY 

respiratory  muscles  to  greater  activity  in  an  endeavor  to  keep  the 
blood  gases  at  normal  levels  by  increasing  the  rate  of  breathing.  On 
the  other  hand,  if  the  alkalinity  of  the  blood  tends  to  rise  above  normal 
as  the  result  of  superaeration  of  the  blood  in  the  lungs,  impulses  flowing 
to  the  muscles  from  the  respiratory  center  will  be  lessened,  and  breath- 
ing activities  will  be  greated  reduced  or  even  entirely  suspended  until 
the  normal  levels  are  attained.  More  attention  will  be  given  to  the 
respiratory  gases  in  the  blood  plasma  in  the  later  chapter  dealing  with 
the  vascular  system. 

The  Voice. — Breathing  is  greatly  modified  when  the  expiratory  air 
currents  are  used  to  vibrate  the  vocal  cords  in  the  larynx  arid  thus 
produce  sounds,  as  in  talking  or  singing.     In  such  cases,  breathing  is 
voluntarily  controlled  so  that  expiration  is  prolonged.     The  outgoing 
stream  of  air  is  then  modified  as  necessary  for  the  vibration  of  the 
vocal  cords.     And  so  the  human  voice,  as  well  as  the  vocal  sounds  of 
other  vertebrates,  is  the  sound  produced  J>y  vibrations  of  the  stretched 
membranes.     The  pitch  of  the  voice  depends  upon  the  amount  of 
stretching.     If  the  vocal  cords  are  drawn  tight,  they  will  vibrate 
rapidly  and  produce  a  tone  of  high  pitch,  whereas  the  reverse  condition 
will  produce  lower  tones.     The  range  of  the  pitch,  which  is  important 
for  singing,  is  dependent  upon  the  amount  of  tension  that  can  be  placed 
upon  the  vocal  cords  by  manipulation  of  the  laryngeal  cartilages,  but 
the  quality  of  the  tone  produced  in  speaking  or  singing  is  determined 
by  a  number  of  factors  including  the  essential  character  of  the  cords 
themselves  and  the  resonance  of  the  throat  region.     Given  wide  range 
of  tone  and  normal  resonance,  the  singer  will  still  be  decidedly  lacking 
in  artistic  accomplishment  if  unable  to  secure  any  desired  pitch  accu- 
rately and  instantly.     This  is  dependent  upon  the  ability  to  adjust 
the  cartilages  of  the  larynx  through  muscular  contraction  so  that  just 
the  right  amount  of  tension  will  be  placed  upon  the  vocal  cords. 
(Fig.  42.) 

Speech  represents  definite  modulations  of  the  voice  sounds  issuing 
from  the  larynx,  in  order  to  produce  the  established  letter  sounds  of  a 
particular  language.  The  modulation  of  the  laryngeal  voice  is  due  to 
the  actions  of  muscles  in  the  throat,  tongue,  and  lips.  When  once 
learned,  the  actions  become  essentially  automatic,  or  reflex.  The 
rather  common  impression  that  the  tongue  alone  is  responsible  for 
speech  is  known  to  be  erroneous,  because,  in  cases  where  the  tongue 
has  been  accidentally  removed,  the  individual  is  able  to  produce  most 
of  the  letter  sounds  in  fairly  intelligible  fashion,  but  certain  sounds  in 
which  the  tip  of  the  tongue  is  needed,  as  in  the  th  sound,  are  defective. 
It  is  interesting  to  analyse  the  position  of  the  tongue  and  lips  and  also 


THE  BIOLOGY  OF  RESPIRATION  85 

the  control  of  the  air  stream  in  pronouncing  the  various  letters  of  our 
language.  Thus  the  vowel  sounds  A,  E,  /,  0,  U  may  all  be  produced 
by  a  continuous  expiration  through  the  open  mouth,  but  each  vowel 
requires  certain  adjustments  of  the  lips  to  make  the  different  letter 
sounds.  The  same  air  movement  is  found  in  8,  Z,  F,  /,  F,  etc.,  but 
individual  modifications  in  the  mouth  cavity  are  produced  which 
involve  both  the  tongue  and  the  lips.  M  and  N  can  be  produced 
only  by  completely  blocking  the  air  passage  through  the  mouth  cavity 
and  thus  forcing  the  air  through  the  nasal  passages.  Temporarily 
blocking  both  the  mouth  and  the  nasal  passages  by  the  lips  or  tongue 
results  in  explosive  sounds  necessary  for  such  letter  sounds  as  B,  P, 
T,  D,  K,  and  G. 

FUNCTIONAL  FEATURES  ASSOCIATED  WITH  RESPIRATION 

The  process  of  respiration,  in  which  oxygen  is  received  into  the 
cells  and  carbon  dioxide  released  from  them,  is,  as  we  already  know, 
a  basic  phenomenon  of  life  which  is  universally  present  in  every  type 
of  living  cell.  It  is  a  continuous  feature  of  the  energy  traffoc  between 
the  organism  and  the  environment  made  necessary  by  the  fact  that 
energy  is  required  to  maintain  the  life  functions.  These  may  be  sum- 
marized as  (1)  metabolic,  including  the  chemical  activities  necessary 
for  enzyme  digestion,  for  the  synthesis  of  the  protoplasmic  material, 
and  for  the  liberation  of  energy  with  heat  production;  (2)  muscular 
activity;  (3)  nerve  activity;  and  (4)  secretory  activity.  About  80  per 
cent  of  the  energy  released  in  the  body  is  utilized  in  the  maintenance  of 
body  temperature.  Life  cannot  exist  without  sufficient  energy  to 
maintain  these  vital  activities,  and  energy  cannot  be  secured  except 
by  oxidizing  the  organic  materials  in  each  individual  cell.  A  muscle 
cell  maintains  its  respiratory  rate  at  a  level  sufficiently  high  to  supply 
its  own  intracellular  needs  and  to  contribute  its  share  to  the  work 
performed  when  the  muscle  of  which  it  is  a  part  contracts  in  response 
to  a  nerve  stimulus.  Likewise  a  nerve  cell  maintains  its  respiratory 
rate  for  individual  needs  and  to  contribute  toward  the  maintenance  of 
nerve  function  in  the  organism.  So  it  is  with  every  type  of  cell  in  the 
body.  Finally  the  combined  needs  of  all  the  cells  are  summated  in 
the  respiratory  icquirements  of  the  individual. 

BASAL  METABOLIC  RATE 

It  is  possible  for  the  physiologist  to  determine  the  amount  of  oxygen 
intake  and,  carbon  dioxide  output  necessary  to  maintain  the  metabolic 
activities  of  the  body  under  varying  conditions.  The  minimum  rate 
at  which  the  life  processes  can  operate  is  known  as  the  basal  metabolic 


86 


HUMAN  BIOLOGY 


rale  (B.M.R.),  and  its  determination  is  of  considerable  interest  to  the 
student  of  life  activities  but  of  particular  importance  to  the  clinician 
in  the  diagnosis  of  certain  diseases,  notably  those  associated  with  the 
thyroid  gland.  In  order  for  the  life  functions  to  be  measured  at  the 
minimum,  or  basal,  rate,  the  individual  must  lie  quietly,  and  voluntary 
muscle  movements  must  be  restricted  so  far  as  possible.  Also  no  food 
is  eaten  for  12  hours  previously  so  that  the  body  is  expending  no  energy 
in  the  digestive,  assimilative,  and  synthetic  processes.  Thus  the 
energy-liberating  processes  of  the  cells  are  at  a  minimum  and  sufficient 
only  to  maintain  essential  involuntary  muscular  movements  associated 


Water^L       \ 
Pump    V       J 


Water  s" 
Thermometer 


Thermometer     Pump 

FIG.  47. — Diagram   of  calorimeter  for  measuring  the  basal  metabolic  rate  of  man, 
(Watkeys,  Daggs;  after  Murlin  and  Burton.} 

with  breathing,  circulation  of  the  blood,  etc.,  and  to  produce  heat 
enough  to  maintain  the  normal  body  temperature.  The  latter  requires 
by  far  the  greater  energy  supplies  and  is  taken  as  the  measurement  of 
the  basal  metabolic  rate.  This  rate  varies  with  age  and  in  accordance 
with  the  surface  area,  and  has  been  found  to  be  in  the  neighborhood 
of  40  calories1 — of  heat  per  hour  for  each  square  meter  of  surface  in  a 
normal  young  adult. 

There  are  two  ways  of  determining  the  basal  metabolic  rate.  In 
the  first  method,  a  person  is  placed  in  a  calorimeter.  This  is  an  insu- 

1  The  term  calorie,  as  commonly  used  in  physiology,  is  defined  as  the  amount 
of  heat  that  will  raise  the  temperature  of  1  kg.  of  water  1°C.  (15  to  16°).  This  is 
known  as  the  large  calorie  and  is  often  capitalized  as  Cal.  The  small  calorie  (cal.) 
is  one-thousandth  of  the  large  calorie,  that  is,  the  amount  of  heat  necessary  to 
raise  the  temperature  of  1  g.  of  water  from  15  to  16°C.  Consult  the  Appendix: 
Calorie;  Measurements. 


THE  BIOLOGY  OF  RESPIRATION 


87 


lated  chamber  designedly  large  enough  to  admit  the  entire  body  of  the 
experimental  animal,  which  may  include  almost  any  size  from  a  mouse 
to  an  elephant.  The  walls  of  the  calorimeter  contain  water  coils,  and 
it  is  so  equipped  that  normal  breathing  may  occur.  Heat  is  dissipated 
from  the  body  of  the  individual  through  the  lungs  and  skin  and  is  meas- 
ured by  the  increase  in  the  temperature  of  the  water  in  the  walls  of 
the  calorimeter  together  with  that  of  the  expired  air.  A  second  method 
for  determining  the  basal  metabolic  rate  is  by  measuring  the  oxygen 


FIG.  48. — Diagram  of  calorimeter  designed  by  Benedict  for  measuring  the  basal 
metabolism  of  an  elephant,  a,  pipe  for  admission  of  outdoor  air;  b,  pipe  for  air  passing 
from  calorimeter  to  be  analyzed;  c,  pipe  connecting  blowers  with  meter  (M) ;  R,  instru- 
ment for  indicating  rate  of  ventilation;  F,F,  rubber  bags  for  collection  of  samples  of  air 
coming  from  calorimeter;  T\T«,  dry  bulb  thermometers;  7Ta,  wet  bulb  thermometer;  d, 
discharge  pipe  for  air  passing  from  calorimeter  through  meter  to  the  exterior;  e,  pipe  con- 
necting with  small  blower  (/)  for  forcing  portion  of  air  discharged  from  meter  through 
pipe  (g)  into  boxes  enclosing  sampling  bags  (F).  (Bent-diet,  "Science  in  Progress"  Yale 
University  Press.} 

intake,  inasmuch  as  the  latter  is  always  in  direct  ratio  to  the  amount  of 
materials  oxidized  and,  therefore,  the  amount  of  energy  released. 
Another  factor  that  must  be  taken  into  consideration  with  this  method 
is,  however,  whether  carbohydrates,  fats,  or  proteins  are  being  oxi- 
dized, for  each  requires  a  different  amount  of  oxygen,  and  each  yields 
correspondingly  varying  amounts  of  heat  energy.  Thus  1  g.  of  glucose, 
when  completely  oxidized,  produces  4.1  calories  of  heat.  The  oxida- 
tion of  the  same  amount  of  a  fat  will  produce  9.3  calories,  but  in  so 


88 


HUMAN  BIOLOGY 


doing  only  about  50  per  cent  more  oxygen  will  be  used  than  with 
glucose.  In  a  word,  it  is  clear  that,  when  fat  is  being  oxidized  in  the 
body,  a  given  rate  of  oxygen  intake  shows  a  higher  rate  of  heat  pro- 
duction than' when  a  carbohydrate  is  oxidized.  (Figs.  47  to  49.) 

Respiratory  Quotient. — It  is  possible  to  determine  whether  carbo- 
hydrate, fat,  or  protein  is  being  oxidized  by  the  relationship  between 
the  volume  of  oxygen  intake  and  volume  of  carbon  dioxide  eliminated. 
The  relationship  shown  by  dividing  the  latter  (carbon  dioxide  elimi- 
nated) by  the  former  (oxygen  taken  in)  is  known  as  the  respiratory 
quotient  (R.Q.).  When  a  carbohydrate  is  burned,  the  volume  of 
oxygen  required  is  equal  to  the  volume  of  carbon  dioxide  formed. 


Os                  Na                       W        <*>2 

Inlet               OPoor                 Absorbers  Absorber 

A£    1 
H20 

O2Ricw 

Mouths- 
Piece  *~ 

H20  1 

m  Wafer 
(to  mo  is 

Valves 

ten  air 

1 

\C02 
\N2 
\O2Poor 

Spirometer 

°°2\ 

Air  Pump 

O2Poor 

f^\ 

-    >              ^  v_y  —  ^   ji  j 

HzO  Absorbers 

FIG.  49. — Diagram  illustrating  apparatus  for  determining  the  basal  metabolic  rate 
by  measuring  the  oxygen  intake.  The  patient  breathes  through  the  mouthpiece  (left). 
The  oxygen  intake  is  measured  when  admitted  to  the  system  (inlet).  The  carbon 
dioxide  output  is  measured  by  weighing  the  CO 2  and  H2O  absorbers.  (Watkeys,  Daggs.) 

This  is  seen  from  the  equation  C6Hi2O6  +  6O2  =  6CO2  +  6H2O,  show- 
ing that  the  oxidation  of  sugar  requires  six  molecules  of  oxygen,  and 
releases  six  molecules  of  carbon  dioxide;  accordingly  the  respiratory  quo- 
tient is  1 .0.  When  a  fat  is  burned,  the  amount  of  carbon  dioxide  released 
is  less  than  the  oxygen  taken  in.  Thus  each  molecule  of  tristearin,  with 
the  formula  CsiHnoOe,  requires  81.5  molecules  of  oxygen  for  complete 
oxidation,  and  only  57  molecules  of  carbon  dioxide  are  released,  which, 
divided  by  the  oxygen  consumption,  gives  a  respiratory  quotient  of 
0.70.  The  respiratory  quotient,  when  protein  is  oxidized,  is  around 
0.80. 

Thus  the  respiratory  quotient  of  a  person  with  a  high  proportion 
of  carbohydrate  in  reserve  for  oxidation  will  approach  1.0,  whereas  the 
inclusion  of  fat  will  reduce  the  respiratory  quotient  to  lower  levels. 
Inasmuch  as  the  digestion,  assimilation,  and  oxidation  of  carbohydrates 
are  normally  completed  within  a  few  hours  after  being  received  in  the 


THE  BIOLOGY  OF  RESPIRATION  89 

alimentary  canal,  it  is  clear  that  the  respiratory  quotient  of  a  person 
who  has  been  deprived  of  food  for  some  time  will  fall  below  the  carbo- 
hydrate levels  as  the  stored  fats  are  increasingly  oxidized  in  ord^r  to 
maintain  the  metabolic  activities.  It  is  found  that  the  absence  of  food 
for  12  hours  gives  a  respiratory  quotient  of  about  0.74.  In  cases  of 
severe  fasting,  which  results  in  the  depletion  of  the  fat  reserves  as  well 
as  the  carbohydrate,  the  respiratory  quotient  tends  to  approach  0.80 
as  the  proteins  of  the  cells  are  increasingly  sacrificed  on  the  altar  of 
oxidation. 

NORMAL  METABOLIC  RATE 

It  is  apparent  that  life  could  not  be  maintained  very  long  at  the 
basal  rate,  for  a  continual  decrease  in  weight  occurs  as  the  energy 
requirements  are  supplied  at  the  expense  of  the  stored  materials,  and 
so,  when  a  certain  stage  of  starvation  is  reached,  the  activities  neces- 
sarily cease.  A  more  important  question  to  the  average  person  is  the 
determination  of  the  rate  of  metabolism  reached  in  an  active  individual 
pursuing  his  daily  routine  and  maintaining  normal  weight.  This  rate 
will  be  found  to  vary  widely  in  different  individuals  depending  upon 
their  age,  temperature  conditions,  and  amount  of.  muscular  activity 
associated  with  their  duties.  The  highest  metabolic  rate  per  pound 
)f  body  weight  occurs  in  the  early  years,  when  the  child  is  not  only 
rery  active  all  day  long,  but  also  new  tissues  are  being  formed  con- 
tinually. The  lowest  metabolic  rate  will  be  found  in  an  inactive  aged 
individual  with  a  routine  in  which  lengthy  periods  are  devoted  to 
resting  in  bed  and  sleeping.  The  one-year-old  child  requires  the 
release  of  about  45  calories  each  day  per  pound  of  body  weight,  but 
the  octogenarian,  with  his  eighty  or  more  years,  requires  about  one- 
fourth  of  this  caloric  output,  unless  indulging  in  unusual  muscular 
activity.  The  average  man,  weighing  about  150  lb.,  requires  about 
17  or  18  calories  per  pound  of  body  weight,  or  from  2,500  to  3,000  per 
day,  when  engaged  in  ordinary  activities,  which  is  about  10  per  cent 
more  than  is  required  by  a  woman  under  comparable  conditions. 
Under  conditions  of  hard  physical  labor  the  rate  is  more  than  doubled, 
so  that  7,000  to  8,000  calories  may  be  required  daily. 

If  the  adult  body  weight  is  to  be  maintained,  the  foodstuffs  eaten 
should  supply  enough  calories  to  approximate  the  daily  expenditure 
without  oxidation  of  reserve  materials.  With  too  little  food  intake, 
the  body  weight  will  gradually  be  reduced;  with  too  much  food,  there 
will  be  a  tendency  in  the  average  individual  to  store  up  the  excess 
materials  in  the  form  of  fat.  Fat  accumulation,  though,  varies  greatly 
in  different  individuals,  and,  occasionally,  heavy  eaters  remain  at 


90  HUMAN  BIOLOGY 

about  the  same  weight  over  a  period  of  years.  In  general,  the  appetite 
is  the  judge  of  the  amount  of  food  to  be  eaten;  but  unfortunately  it  is  a 
fickle  guide  in  many  instances  and  tricks  the  individual  into  eating 
more  than  is  necessary  to  supply  the  maximum  metabolic  require- 
ments. Since  the  chemist  can  determine  the  exact  number  of  calories 
that  the  various  foods  yield  when  they  are  consumed  in  the  body,  and 
the  physiologist  can  determine  the  amount  of  calories  required  by  the 
individual,  it  is  possible  to  fix  an  adequate  diet  for  each  individual  with 
great  accuracy.  If  the  diet  is  well  balanced,  it  will  not  only  satisfy 
the  energy  requirements  but  will  also  supply  enough  proteins  to  replace 
the  nitrogen  and  other  essential  elements  present  in  the  broken-down 
tissues  as  well  as  those  expended  in  the  formation  of  various  secretions, 
epidermal  cells,  hair,  nails,  etc.  And,  finally,  it  is  essential  that  the 
vitamin  requirements  be  met  (page  57) . 

HEMOGLOBIN,  THE  RESPIRATORY  PIGMENT 

The  cells  of  the  body  have  a  contract  with  the  vascular  system  to 
transport  the  essential  materials  to  them  from  the  collecting  organs. 
Of  first  importance  in  this  connection  for  oxygen  transport  is  the 
respiratory  pigment,  hemoglobin,  found  in  the  blood  of  vertebrates. 
Other  respiratory  pigments  having  the  same  function  and  essentially 
the  same  composition  are  present  in  various  invertebrate  animals. 
These  various  respiratory  pigments  are  all  adapted  for  the  transporta- 
tion of  oxygen  to  the  cells.  The  jved^x&te  Jiemo^^ 
in  highly;, jdi£erentiated...c.ellay-  the  red  blood  corpuscles,  whergas~Jim 
respiratory  pigments  of  the  inver^ebratje&^j^ia^soiutioii*,  (Fig.  67,) 
"""""Hemoglobin  is  an  exceedingly  complicated  protein  compound  with  a 
high  molecular  weight  in  which  the  protein,  globin,  is  combined  with 
a  heme  pigment.  The  latter,  in  turn,  consists  of  the  element  iron 
united  to  the  pigment  portion,  porphyrin,  which  is  commonly  found  in 
various  plant  and  animal  pigments.  In  the  oxidized  form,  as  in  the 
blood,  the  heme  pigment  is  known  as  hematin.  Hematin,  although 
constituting  less  than  5  per  cent  of  the  hemoglobin  molecule,  is 
certainly  the  portion  of  the  molecule  that  has  an  affinity  for  oxygen. 
The  belief  is  that  this  affinity  is  largely. due  to  the  presence  of  iron. 
Witness  the  readiness  with  which  iron  rusts  as  a  result  of  the  union 
with  oxygen  in  the  air.  At  any  rate,  hemoglobin  is  a  great  oxygen 
carrier.  Experiments  show  that  blood  plasma  with  no  red  corpuscles 
cannot  absorb  more  than  0.38  per  cent  of  oxygen  but  that  whole  blood, 
containing  the  red  corpuscles  with  hemoglobin,  will  absorb  about 


THE  BIOLOGY  OF  RESPIRATION  91 

sixty  times  as  much  oxygen,  or  more  than  20  parts  of  oxygen  in  100 
parts  of  blood.  The  chemical  basis  of  oxygen  transportation  appears 
to  be  the  ability  of  the  hemoglobin  to  form  with  oxygen  a  definite,  but 
unstable,  compound,  oxyhemoglobin.  This  new  compound  is  more 
brilliantly  red  than  hemoglobin  and  is  characteristic  of  arterial  blood, 
which  has  just  received  its  full  complement  of  oxygen  during  the 
passage  through  the  lungs. 

Oxyhemoglobin.  —  When  oxyhemoglobin  reaches  the  tissue  cells 
throughout  the  body,  it  is  changed  to  hemoglobin,  and  the  oxygen 
released  for  entrance  into  the  cell  cytoplasm.  Itjis  L 


U^s^  is  present  in  the 

cells,  which  is  oxidized  by  the  incoming  moleciTRiF^xygen  and  pre- 
sumably acts  in  bringing  about  the  utilization  of  the  oxygen  in  the 
cytoplasm.  Specific  enzymes  are  also  present.  The  corpuscles,  with 
the  hemoglobin  molecule  restored,  return  to  the  lungs  for  a  new  supply 
of  oxygen.  The  unstable  nature  of  oxyhemoglobin,  which  is  essential 
for  the  release  of  oxygen  to  the  cells,  is  due  Jx>  the  relatively  weak 
affinity  existing  between  oxygen  and  hemoglobin. 

This  condition  has  its  inherent  dangers  when  some  gas  with  a 
greater  affinity  for  hemoglobin,  notably  carbon  monoxide  (CO), 
reaches  the  lungs,  because  a  relatively  stable  hemoglobin  -carbon 
monoxide  compound  will  be  formed  to  the  exclusion  of  the  oxyhemo- 
globin.- In  fact,  in  a  mixture  of  equal  parts  of  oxygen  and  carbon 
monoxide,  the  hemoglobin  will  take  250  parts  of  the  latter  to  one  of 
the  oxygen.  Accordingly,  when  air  containing  carbon  monoxide  is 
breathed  (as  may  happen  in  a  closed  garage  when  the  car  is  running 
or  at  night  in  the  home  when  the  carbon  monoxide  is  released  into  the 
air  by  a  faulty  furnace)  the  red  blood  corpuscles  will  very  soon  be 
carrying  large  loads  of  the  stable  hemoglobin-ciirbon-monoxide  com- 
pound and  very  little  of  the  essential  oxyhemoglobin. 

Of  great  interest  is  the  fact  that  the  chemical  composition  of 
hemoglobin  is  closely  related  to  that  of  chlorophyll,  which  is  responsible 
for  the  synthesis  of  the  organic  compounds  in  green  plants.  In  the 
chlorophyll  molecule,  magnesium  is  present  in  place  of  the  iron  that  is 
essential  to  the  hemoglobin  molecule.  Functionally,  it  will  be  remem- 
bered that  chlorophyll  releases  free  oxygen  to  the  atmosphere  during 
photosynthesis,  whereas  hemoglobin  collects  oxygen  and  carries  it  to 
the  tissues.  Chlorophyll,  though  present  only  in  green  plants,  is 
indispcnable  as  the  agent  for  food  synthesis  essential  to  all  types  of 
life.  Hemoglobin  is  not  of  so  great  biological  importance,  for  it  has' 
no  relationship  to  the  plant  world  or  to  the  lower  types  of  animal  life. 


ITUITARY 


PARATHYROID 
THYROID 


LIVER 
p  >ANCREAS 
ADRENAL 


TESTIS 


LUNG 


KIDNEY 


BLADDER 


Ar/T?//?  Hernberger 

PLATE  V. — Diagram  to  show  the  positions  of  the  important  endocrine  glands  (stippled) 

in  the  human  male. 


CHAPTER  V 
THE  BIOLOGY  OF  SECRETION 

Increasingly  during  the  recent  years,  the  underlying  importance 
of  the  secretory  processes  in  the  living  organism  has  been  brought  home 
to  the  biologist.  At  present,  it  appears  that  essentially  all  the  life 
functions  in  the  highly  developed  human  organism  are  either  based 
upon  or  closely  associated  with  the  process  of  secretion.  Secretion 
seems  to  be  a  fitting  and  normal  process  for  the  digestion  of  food,  but 
the  uninitiated  find  it  difficult  to  realize  that  various  types  of  hormonal 
secretions  are  also  responsible  for  the  control  of  the  general  metabolic 
activities,  including  carbohydrate  utilization  in  the  muscles,  growth, 
and  reproduction.  And  recently  it  has  become  evident  that  the 
stimulus  to  muscle  contraction  is  by  a  secretion  rather  than  by  a 
direct  impulse  from  the  nervous  system.  Possibly  it  is  not  over- 
emphasizing the  situation  to  state  that  every  cell  in  the  organism 
secretes  substances  that  make  the  internal  environment  more  suitable 
for  the  other  associated  cells,  that  every  cell  gives  to  and  every  cell 
partakes  of  innumerable  body  secretions. 

STRUCTURAL  FEATURES  ASSOCIATED  WITH  SECRETION 

Secretions,  as  generally  recognized,  are  synthesized  in  the  cytoplasm 
of  epithelial  cells  differentiated  for  that  purpose.  Such  cells  are 
known  as  secretory  or  gland  cells,  and,  necessarily,  they  are  very  widely 
distributed  throughout  the  body.  In  the  previous  chapters,  they  have 
been  encountered  in  the  skin,  alimentary  canal,  pancreas,  liver, 
trachea,  etc.  Secretory  cells,  either  singly  or  associated  in  con- 
siderable numbers,  constitute  a  gland.  In  the  human  organism, 
practically  the  only  type  of  unicellular  gland  is  the  widely  distributed 
goblet  cell  of  the  alimentary  tract  and  associated  structures  which  was 
previously  described  (page  52).  Goblet  cells  manufacture  the  secre- 
tion mucigen,  which  is  passed  to  the  exterior  through  a  tiny  opening  in 
the  cell  wall  near  the  center  of  the  free  surface.  Milcigenls  chemically 
changed  after  secretion  to  form  a  protein  substance,  mucin.  The 
latter  in  combination  with  water  forms  mucus,  which  is  an  important 
surface-protecting  and  lubricating  material  throughout  the  length  of 
the  alimentary  canal,  beginning  with  the  nasal  cavities. 

93 


94  HUMAN  BIOLOGY 

The  multicellular  glands  comprise  a  variety  of  types  both  struc- 
turally and  functionally.  The  simplest  type  is  found  in  a  flat  epithelial 
surface  in  which  the  undifferentiated  epithelial  cells  in  a  restricted  area 
are  replaced  by  secretory  epithelial  cells,  thus  forming  a  multicellular 
gland.  Such  a  condition  is  found,  for  example,  in  regions  of  the 
stomach  mucosa.  Increasing  differentiation  of  the  glandular  area 
occurs  in  the  larger  multicellular  glands,  evidenced  by  the  invagination 
of  the  secretory  cells  to  form  a  depression,  or  pit,  below  the  surface 
in  the  underlying  connective  tissues.  In  the  simplest  example  of  this, 


B  C 

FIG.  50. — Various  types  of  glands  with  ducts  (exocrine).  Diagrammatic.  A, 
simple;  B,  simple  tubular;  C,  coiled  tubular  gland;  D  and  E,  two  types  of  compound 
glands.  (Wolcott.) 

the  secretory  epithelium  forms  a  closed  sac,  microscopic  in  size,  with  a 
Central  cavity  for  the  storage  of  the  secreted  materials  and  a  duct 
leading  to  the  surface.  Such  sac-like  glands  are  walled  off  from  the 
surrounding  tissues  by  a  basement  membrane  which  is  a  type  of 
connective  tissue.  They  may  be  said  to  consist  of  the  nonsecretory 
portion,  or  duct,  and  the  secretory  portion  with  the  functional  glan- 
dular cells.  Glandular  tissue  must  have  an  abundant  blood  supply, 
for  the  blood  is  the  source  of  all  their  raw  materials,  and  so  it  is  found 
that  the  connective  tissues  immediately  surrounding  a  glandular  area 
contain  dense  capillary  networks.  In  a  sweat  gland  the  secretory 
portion  consists  of  a  tightly  coiled,  tubular  body  surrounded  by  a 
capillary  network.  (Fig.  50A,  By  C.) 

From  the  simple  sac-like  gland  with  one  secreting  cavity,  as  just 
described,  the  larger  and  more  differentiated  types  of  glands  are 
derived  by  subdivisions  of  the  original  cavity  to  form  one  or  more 


THE  BIOLOGY  OF  SECRETION 


95 


Fatty  tissue* 


Nfpple 
openings 

Miikducfs 


•Muscle 


Ribs 


additional  connected  cavities,  each  lined  by  outgrowths  of  the  glan- 
dular epithelium  from  the  original  cavity.  These  connected  cavities 
are  all  closed  sacs  except  for  the  opening  into  the  common  duct,  and 
thus  they  form  a  compound  secretory  unit.  A  still  larger  and  more 
complicated  compound  gland  is  formed  by  the  association  of  several 
secretory  units,  or  lobules,  so  that  the  several  ducts  unite  to  form  one 
large  surface  opening  which  carries  the  products  of  several  secreting 
areas.  Such  a  compound  gland  may  be  thought  of  as  tree-like 
in  structure  with  the  groups  of 
leaves  representing  the  secreting 
areas  and  the  twigs,  branches,  and 
main  trunk  as  being  cpmparable  to 
the  ducts.  A  still  further  elabora- 
tion of  glandular  structure  is  to  be 
noted  in  the  mammary  glands  of  the 
mammal  female  in  which  the  sur- 
face opening,  the  nipple,  has  from 
15  to  25  openings,  each  carrying  the 
secretion,  milk,  from  an  individual 
lobe.  Each  lobe  represents  a 
grouping  of  the  lobules  of  the  com- 
pound glands.  (Figs.  501),  E;  51.) 

The  glands  of  the  body  may 
be  separated  into  (1)  the  exocrine 
type,  as  just  described,  in  which  each  gland  gives  off  its  secretions 
through  a  duct  opening  at  the  epithelial  surface,  and  (2)  the  endocrine 
type  in  which  each  gland  has  lost  its  connection  with  the  epithelial 
surface  and  the  duct  is  lacking.  The  endocrine  glands,  therefore, 
give  off  their  secretions  directly  into  the  blood  stream  from  which  they 
are  also  constantly  receiving  their  raw  materials.  Structurally,  the 
endocrine  glands  are  resolvable  into  two  basic  types:  one  in  which  the 
body  of  the  gland  consists  of  a  group  of  separate  sac-like  secreting 
areas,  separated  from  each  other  and  entirely  enclosed  by  connective 
tissue  elements  with  abundant  vascular  tissues.  In  the  other  type  of 
endocrine  gland,  the  functional  epithelium  forms  a  single  compact 
secretory  unit  which  is  permeated  throughout  by  the  capillary  network. 
(Fig.  52.) 

A  number  of  important  glands  are  both  exocrine  and  endocrine 
and  accordingly  are  known  as  mixed  glands.  Examples  are  found  in 
the  pancreas,  liver,  and  testis.  In  the  pancreas  and  testis,  distinct 
types  of  cells  are  associated  with  the  two  types  of  glandular  activities. 
In  the  liver,  however,  the  histologists  have  been  able  to  demonstrate 


Ampul  fa* 


•Secretory  gfandt 


FIG.  51. — Vertical  section  through 
the  mammary  gland.  Diagrammatic. 
(Sherbon.) 


HUMAN  BIOLOGY 


only  one  basic  type  of  secreting  cell.  •  In  general,  the  exocrine  glands 
are  associated  with  particular  organ  systems  and  are  best  considered 
in  connection  with  such  systems  as  has  been  done  previously  in  the 
discussion  of  nutrition.  The  endocrine  glands  and  the  mixed  glands 
are  much  more  individualistic,  so  to  speak,  and  will  be  discussed  as 
independent  units. 


a  6 

FIG.  52. — Illustrating  structure  of  endocrine  glands,  o,  type,  such  as  the  thyroid 
and  ovary,  in  which  the  secreting  areas  shown  in  heavy  black  lines  are  surrounded  by 
connective  tissue  and  blood  vessels;  b,  type  represented  by  the  adrenals,  pituitary,  etc., 
in  which  the  glandular  epithelium  is  permeated  by  blood  vessels,  shown  in  white. 
(Maximow-Bloom,  "Histology,"  W.  B.  Saunders  Company.} 

FUNCTIONAL  FEATURES  ASSOCIATED  WITH  SECRETION 

The  process  of  secretion  should  be  clearly  distinguished  from  that 
of  excretion  which  is  concerned  with  the  formation  and  elimination  of 
the  cellular  waste  products.  The  chief  excretions  of  the  body  are 
carbon  dioxide,  urea,  and  water,  which  result  from  katabolic  activities^ 
in  all  the  cells.  Secretion  .is  concerned  with  the  intracelltilar  syn- 
thesis of  special  substances  which  serve  distinct  functions  in  the 
organism  and  comprise  a  great  variety  of  substances.  In  certain 
instances,  however,  the  distinction  between  a  secretion  and  an  excre- 
tion is  not  clear.  Thus  sweat  contains  excretory  material  and  is 
therefore  designated  as  an  excretion,  but  it  may  also  be  regarded  as  a 
secretion  of  the  glands  of  the  skin  because  it  serves  a  definite  function 
in  connection  with  the  control  of  the  body  temperature,  Or  again, 
carbon  dioxide  given  off  by  every  cell  in  the  body  is  unquestionably  an 
excretion,  'tad  yet  it  serves  a  very  important  and  definite  function  in 
increasing  the  acidity  of  the  blood  and  thereby  influencing  the  respira- 
tory center  as  was  shown  in  the  previous  chapter  (page  80).  It  is 
possible  that  many  of  the  secretions  may  contain  or  be  built  around 


THE  BIOLOGY  OF  SECRETION  9? 

excretory  products  of  the  cells,  with  the  result  that  certain  waste 
products  of  one  type  of  cell  can  be  utilized  as  a  secretion  by  some  other 
type.  "One  man's  meat  may  be  another  man's  poison." 

The  basic  secrets  associated  with  the  manufacture  of  cell  secretions 
lie  deeply  hidden  in  the  metabolic  activities  of  the  particular  cells 
concerned.  About  all  that  can  be  said  is  that  the  secretory  cells  have 
the  power  to  take  in  the  essential  materials  from  the  blood  stream 
and  synthesize  a  particular  essential  secretion.  The  latter  may  be  of 
comparatively  simple  composition,  as  in  urea,  or  so  complex  that  the 
molecular  structure  still  remains  unknown.  Every  function  of  the 
body  appears  to  be  dependent  upon  one  or  more  special  secretions. 
But  superimposed  upon  the  secretions  from  the  exocrine  glands,  'which 
are  concerned  with  some  particular  function  of  the  body,  are  the  secre- 
tions of  the  endocrine  glands,  known  as  internal  secretions,  or  hormones, 
which  aid  in  the  regulation,  control,  and  coordination  of  all  the  bodily 
functions  and  thus,  in  association  with  the  nervous  system,  unify  the 
life  activities  of  the  complete  organism.  This  is  a  large  order,  and  all 
the  details  are  not  yet  known,  but  the  broad  outlines  of  the  picture 
will  be  revealed  in  the  following  descriptions  of  the  various  endocrine 
glands  and  their  hormones. 

The  term  hormone1  was  first  used  about  thirty  years  ago  in  connec- 
tion with  the  discovery  of  secretin  in  the  digestive  tract.  It  is 
derived  from  a  Greek  word  meaning  to  excite,  and  this  is  essentially 
what  many  hormones  do,  as  has  already  been  noted  in  the  action  of 
secretin  in  stimulating  the  pancreas  (page  64),  but  some  of  the  more 
recently  discovered  hormones  are  known  to  inhibit  a  certain  function 
instead  of  increasing  it.  A  hormone,  then,  may  be  said  to  be  a  specific 
substance  given  off  by  an  endocrine  gland  and  carried  by  the  blood  to 
some  other  organ  where  it  produces  a  specialized  type  of  reaction. 
Such  a  definition  excludes  certain  endocrine  secretions,  notably  the 
secretion  of  glucose  into  ^he  blood  by  the  liver,  because  glucose  is 
universally  used  by  all  the  cells.  In  general,  the  responses  to  the 
hormones,  that  is,  chemical  regulation,  arc  slow,  cumulative  ones, 
which  stretch  over  considerable  periods  of  time,  whereas  the  response 
to  nerve  control  is  very  rapid.  Exceptions  are  to  be  found,  however, 
as  in  the  adrenal  secretion. 

THE  LIVER 

Unlike  the  endocrine  glands,  the  general  importance  and  special 
activities  of  the  liver  as  an  exocrine-endocrine  gland  have  long  been 
known.  The  liver  is  the  largest  gland  in  the  body  and  also  one  of  the 

1  Consult  Appendix:  Hormones. 


HUMAN  BIOLOGY 


Radiating  capillary 
network 


Intralobular  or 
central  vein 


most  versatile,  with  various  essential  functions  closely  linked  to  it  and 
depending  upon  its  normal  activities.  Its  removal  from  an  experi- 
mental animal  invariably  causes  death  in  a  very  short  time.  Along 
with  the  pancreas,  thyroid,  and  parathyroid  glands,  the  liver  develops 
as  an  outgrowth  from  the  endodermal  tissue  of  the  primitive  gut  so 
that  its  functional  tissues  are  of  endodermal  origin.  Starting  as  a 
simple  outgrowth,  the  liver  gradually  develops  into  a  large  compound 
gland  with  a  weight  in  the  human  adult  of  from  50  to  possibly  65  oz. 
A  deep  cleft  partially  divides  the  liver  into  right  and  left  lobes,  the 
right  lobe  being  considerably  larger. 

The  glandular  hepatic  tissue  throughout  the  liver  is  separated  into 
lobules  of  varying,  but  typically  polygonal,  shapes  which  are  about 
the  same  diameter  as  a  pin  and  roughly  three  times  this  in  length. 
It  is  impossible  to  dissect  out  the  individual  liver  lobules,  however, 

because  they  are  intimately 
bound  together  by  the  surround- 
ing connective  tissues  arid  the 
vascular  and  conducting  units. 
When  a  transverse  section  of  a 
liver  lobule  is  examined  micro- 
scopically, the  secreting  or  he- 
patic cells  will  be  seen  to  be 
arranged  in  strands  or  cords 
which  run  radially  from  the  ccn- 
transverse  ter  of  a  lobule  to  the  periphery, 
like  the  spokes  in  a  wheel.  Be- 
tween these  hepatic  spokes  aro 
irregular  blood  spaces,  the  sinusoids,  which  connect  at  the  periphery 
with  the  incoming  blood  and  at  the  center  with  the  outgoing  blood 
of  the  central  vein.  The  latter  continues  centrally  through  the  length 
of  each  lobule  and  in  a  transverse  section  is  seen  as  the  hub  of  the 
wheel.  (Figs.  53,  54.) 

The  liver  is  unique  in  that  it  has  a  double  blood  supply :  one  source 
through  the  hepatic  artery  and  the  other  through  the  portal  vein.  The 
hepatic  artery  brings  in  a  relatively  small  supply  of  arterial  blood  to 
the  liver  which,  for  the  most  part,  supplies  the  connective  tissues, 
whereas  the  large  portal  vein  continually  brings  a  large  amount  of 
blood  from  the  alimentary  tract,  carrying  the  absorbed  foodstuffs  to 
the  hepatic  cells.  It  is  the  blood  from  the  portal  vein  that  flows 
through  the  open  sinusoids  of  the  lobules,  in  direct  contact  with  the 
hepatic  cells,  thus  permitting  the  latter  to  remove  nutritive  materials 
for  chemical  conversion  and  storage  or  to  add  secreted  materials 
directly  to  the  blood. 


FIG.     53. — Diagram     of     a 
tion  of  a  hepatic  lobule. 
nified.      (Kimber,  Gray,  and  Stackpole.) 


THE  BIOLOGY  OF  SECRETION  99 

But  the  liver  is  not  merely  an  endocrine  gland  secreting  materials 
directly  into  the  blood  stream,  for  it  also  has  a  complete  system  of 
ducts  ramifying  through  every  lobule  and  carrying  an  exocrine 
secretion,  bile,  to  a  storage  chamber,  the  gall  bladder,  from  which  it  is 
ejected  as  needed  into  the  duodenum.  The  ultimate  units  in  the 
bile-collecting  apparatus  are  the  bile  canaliculi,  which  form  a  tubular 
network  throughout  the  lobules  and  actually  tap  every  hepatic  cell  for 
its  contribution  of  bile  sap  which  ultimately  reaches  the  gall  bladder. 
The  minute  canaliculi  unite  to  form  larger  ducts  which  lie  between  the 
lobules,  and  all  these  are  finally  consolidated  to  form  the  right  and  left 
hepatic  ducts  which  come  from  the  corresponding  lobes  of  which 
the  liver  is  composed.  The  right  and  left  hepatic  ducts  unite  as  the 
common  hepatic  duct.  The  latter  joins  the  cystic  duct  running  to  the 
gall  bladder  and,  finally,  continues  as  the  common  bile  duct  to  an 
opening  through  the  wall  of  the  duodenum.  The  arrangement  of  the 
ducts  may  be  thought  of  as  Y-shaped,  with  the  hepatic  and  cystic 
ducts  forming  the  two  upper  spreading  branches  of  the  Y,  and  the 
common  bile  duct  soon  as  the  supporting  upright.  Bile  collected  in 
the  liver  passes  through  the  hepatic  duct  to  the  junction  with  the  cystic 
duct  and  then  through  the  latter  to  the  gall  bladder.  When  bile  is 
secreted,  it  passes  from  the  gall  bladder  into  the  cystic  duct  and  then 
through  the  bilo  duct  to  the  duodenum.  (Fig.  30.) 

The  gall  bladder  is  a  pear-shaped  sac,  holding  some  2  fl.  oz.  of  bile. 
It  is  about  4  in.  long  by  !;):(  in.  in  diameter  and,  with  the  attached 
cystic  duct,  is  shaped  somewhat  like  a  partially  inflated  toy  balloon. 
The  wall  consists  of  muscular  and  connective  tissue  layers  with 
a  mucosa  lining  which  shows  considerable  folding.  The  mucosa  cells 
are  highly  absorptive  in  function  and  remove  as  much  as  50  per  cent 
of  the  water  from  the  liquid  bile  received  from  the  liver,  thus  concen- 
trating the  essential  bile  salts  for  use  in  the  intestine  when  necessary 
for  fat  digestion.  The  release  of  bile  into  the  intestine  from  the  gall 
bladder  is  intermittent  and  in  response  to  the  action  of  a  duodenal 
hormone,  cholecystokinin,  which  causes  a  contraction  of  the  muscular 
tissue  in  the  wall  of  the  gall  bladder  (page  102). 

Functional. — The  liver  is  an  important  nutritive  organ,  for,  as 
noted  in  the  chapter  011  Nutrition,  the  bile  is  concerned  with  the 
digestion  and  absorption  of  fats  through  the  action  of  the  bile  salts. 
Even  more  important  in  respect  to  the  nutritive  functions  of  the  liver 
is  the  control  of  the  carbohydrate  metabolism  which  it  exercises 
through  the  formation  of  glycogen  from  glucose  and  its  temporary  stor- 
age, the  reconversion  of  glycogen  into  glucose,  and  the  secretion  of  the 
latter  into  the  blood  as  needed  to  maintain  the  fuel  requirements  of 
the  cells.  Also  of  great-  importance  is  the  ability  of  the  hepatic  cells 


100  HUMAN  BIOLOGY 

to  convert  excess  amino  acids  from  the  digested  proteins  into  an 
oxidizable  carbohydrate  by  the  deaminization  processes  (page  57). 
Finally,  excess  supplies  of  vitamins  are  stored  in  the  liver,  so  that  they 
are  constantly  available  for  nutritive  requirements  of  the  cells.1 

The  liver  is  an  important  excretory  organ,  for  it  is  able  to  transform 
the  various  end-products  of  protein  metabolism,  thrown  into  the 
blood  stream  by  every  type  of  cell  in  the  body,  into  urea  CO(NH2)2 
which  can  be  excreted  by  the  kidneys.  Again  the  liver,  in  association 
with  the  spleen  and  bone  marrow,  acts  as  an  excretory  organ  in  the 
daily  destruction  of  millions  of  worn-out  red  blood  cells.  The  com- 
plete story  of  their  dismantling  is  not  known,  but  it  is  certain  that  the 
hemoglobin  in  the  discarded  red  cells  is  changed  to  the  dark-colored 
bilirubin  which  gives  bile  its  characteristic  color  and  finally  leaves  the 
body  through  the  intestine.  The  valuable  iron  compounds,  associated 
with  the  heme  pigment  in  hemoglobin,  are  retained  in  the  body  and 
used  in  the  formation  of  new  hemoglobin. 

The  liver  is  an  important  vascular  organ  for,  as  just  noted,  it  rids 
the  blood  of  the  old  corpuscles  and  conserves  the  essential  materials 
of  the  hemoglobin.  In  addition,  more  interchanges  of  materials 
en  route  to  and  from  the  blood  occur  in  the  liver  than  in  any  other 
organ.  In  part,  these  interchanges  are  concerned  with  maintaining 
body  fluids  at  proper  levels.  It  is  estimated  that  there  i^  more  blood 
in  the  liver  than  in  any  other  organ  with  the  possible  exception  of  the 
muscles.  Finally,  the  liver  prepares  the  material,  fibrinogen,  which  is 
essential  to  blood  clotting  (pages  163,  167). 

Among  the  most  important  functions  of  the  liver  is  the  protection 
of  the  body  against  poisonous  substances  and  invasions  of  living 
parasitic  bacteria  from  the  alimentary  tract.  The  chemistry  of 
digestion  is  highly  involved;  the  compounds  formed  during  the 
process,  particularly  the  partially  digested  proteins,  are  dangerous  if 
received  by  the  blood  stream.  Also,  at  times,  foods  may  be  eaten  that 
are  not  in  the  proper  state  of  preservation,  and  some  of  the  con- 
taminated material  may  get  through  the  intestinal  mucosa  and  into 
the  portal  vein.  The  liver  stands  as  a  barrier  against  the  distribution 

1  "There  is  no  evidence  of  specialization  in  the  mammalian  liver — indeed  the 
evidence  is  definitely  against  it.  Any  or  every  cell  seems  capable  of  synthesizing 
glycogen  from  sugar  or  from  lactic  acid,  of  solving  the  chemical  conundrum: — 
how  to  pass  directly  from  carbohydrates  to  fats  and  back  or  proteins  to  fats,  of 
dealing  with  metallic  poisons,  of  controlling  the  chemical  cycle  of  haemoglobin,  of 
synthesizing  uric  acid,  so  on  and  so  on.  Has  the  biologist  any  picture  even  of  the 
vaguest  kind,  of  how  so  diverse  a  chemical  factory  can  operate  in  a  fluid  mass,  say 
10~8  cubic  millimetres  in  volume?"  "To  Remind — A  Biological  Essay,"  by  Sir 
William  Hardy,  Williams  &  Wilkins  Company. 


THE  BIOLOGY  OF  SECRETION 


101 


to  the  body  tissues  of  any  and  all  unsuitable  compounds  that  may  be  in 
the  blood  stream  and,  usually,  is  able  to  remove  and  destroy  such 
substances  before  damage  is  done.  In  performing  this  function,  the 
liver  is  really  doing  little  more  than  it  does  in  treating  the  nitrogenous 
wastes  of  the  body  cells  and  converting  them  to  urea.  But  the  liver 


to 


FIG.  54. — Drawing  of  a  section  of  rabbit  liver  which  has  been  injected  intravenously 
with  India  ink.  The  figure  illustrates  a  cell  of  Kupffer  (d),  gorged  with  ink  particles, 
lying  in  the  lumen  of  a  sinusoid  between  the  liver  cells  (Lc).  Transition  of  the  Kupffer 
cells  from  the  resting  state  (a)  to  the  active  state  (d)  are  shown  in  b  and  c.  Elc,  leuco- 
cyte; Ere,  erythrbcyte  or  red  cell.  Highly  magnified.  (Maximow-Bloom,  "Histology," 
W.  B.  Saunders  Company.) 


also  may  be  called  upon  to  destroy  living  organisms,  for  the  digestive 
cavities  contain  many  bacteria,  and,  in  rare  instances,  some  of  these 
parasites  may  get  through  the  mucosa  and  into  the  blood  stream. 
When  the  invaders  reach  the  liver,  they  are  eaten  and  destroyed  by  a 
particular  type  of  amoeboid  cell,  the  Kupffer  cell,  which  is  anchored 
in  the  liver  sinusoids  and  give  close  inspection  to  all  the  materials 
present  in  the  slow-moving  blood  stream.  (Fig.  54.) 


102  HUMAN  BIOLOGY 

ENDOCRINE  GLANDS 

It  is  possible  to  classify  the  endocrine  glands  in  various  ways, 
but  perhaps  the  best  arrangement  for  our  purpose  is  to  group  them  in 
accordance  with  the  functions  of  the  hormones  that  are  produced. 
On  this  basis,  three  main  divisions  of  the  endocrines  may  be  recognized 
as  follows:  (1)  hormones  concerned  with  the  regulation  of  digestive 
functions,  (2)  hormones  concerned  with  the  regulation  of  metabolism, 
(3)  hormones  concerned  with  the  general  control  of  body  functions. 

HORMONES  CONCERNED  WITH  THE  REGULATION  OF  DIGESTION 

Intestinal  Mucosa. — The  fact  has  been  recognized  for  some  time 
that  some  hormones  are  associated  with  the  normal  digestive  processes 
in  man.  These  are  gastrin,  secretin,  and  cholecystokinin,  all  three  of 
which  are  secreted  by  the  mucosal  cells  of  the  alimentary  tract. 
Gastrin  is  secreted  by  the  mucosa  in  the  pyloric  region  of  the  stomach, 
whereas  the  other  two  are  formed  by  the  duodenal  mucosa.  More 
than  thirty  years  ago  it  was  found  that,  when  mucosal  tissue,  secured 
from  the  lining  of  the  stomach  or  duodenum,  was  ground  up,  an  active 
fluid  substance  could  be  obtained  from  the  material  which,  when 
injected  into  the  blood  stream,  would  incite  secretory  activity  of  diges- 
tive enzymes.  Some  authorities  believe  that  the  stimulating  sub- 
stances that  cause  the  flow  of  gastric  juices  are  liberated  by  certain 
of  the  ingested  foods  rather  than  by  a  hormonal  secretion  of  the 
mucosal  cells.  Such  foods  arc  termed  secretagogues.1 

Secretin  is  formed  by  the  duodenal  mucosa  cells  and,  when  liberated 
into  the  blood  stream,  causes  an  active  flow  of  pancreatic  juico  into  the 
intestine.  The  flow  of  pancreatic  juice  is  always  exactly  timed  to 
follow  the  arrival  of  chyme  from  the  stomach.  The  acid  condition  of 
the  latter  when  it  reaches  the  duodenal  mucosa  acts  as  an  inciter  for 
the  hormonal  activity  of  the  mucosal  cells.  Accordingly  the  complete 
cycle  of  events  includes  the  stimulation  of  the  mucosa  cells  by  the  acid 
chyme,  the  liberation  of  the  hormone,  secretin,  into  the4 blood  stream, 
the  stimulation  of  the  pancreatic  cells  by  the  secretin  received  from 
the  blood,  and,  finally,  the  flow  of  pancreatic  juice  into  the  intestine. 
Coincident  with  the  flow  of  gastric  juice  is  the  flow  of  bile  from  the 
liver.  Until  recently,  it  was  supposed  that  secretin  was  also  respon- 
sible for  inciting  the  bile  flow.  It  is  now  believed,  however,  that  the 
latter  is  due  to  another  mucosal  hormone,  cholecystokinin,  also 
released  by  the  duodenal  mucosa  following  stimulation  by  the  acid 
chyme.  Cholecystokinin  received  from  the  blood  causes  a  con- 

1  Consult  Appendix:  Secretagogues. 


THE  BIOLOGY  OF  SECRETION 


103 


traction  of  the  muscle  tissue  in  the  wall  of  the  gall  bladder.  When 
the  entire  story  is  known,  it  will  probably  be  found  that  still  other 
hormonal  actions  are  involved  in  the  regulation  of  the  digestive 
processes.1 

HORMONES  CONCERNED  WITH  THE  REGULATION 
OF  METABOLISM 

The  Pancreas. — The  important  position  that  the  pancreas  occu- 
pies in  the  function  of  digestion  has  been  indicated  in  the  chapter 
on  Nutrition;  and  in  the  paragraph  just  preceding,  it  has  been  shown 


FIG.  55. — Section  of  pancreas,  highly  magnified,  showing  an  Island  of  Langerhans 
(/)  which  releases  its  secretion  directly  into  the  blood  vessel  (J3).     This  endocrine  region 
is  surrounded  by  the  exocrine  glandular  alveolae  (A")  which  secrete  into  the  pancreatic  , 
duct.     (Wieman,  after  SWhr.) 

that  the  pancreas  is  influenced  in  the  secretion  of  the  pancreatic 
juice  by  a  hormone  from  the  intestinal  tract.  The  present  final  con- 
sideration of  this  remarkable  organ  has  to  do  with  its  function  as  an 
endocrine  gland,  for  included  in  its  tissues  are  the  islands  of  Langerhans 
which  are  responsible  for  the  synthesis  and  secretion  into  the  blood- 
stream  of  the  hormone  insulin  which  is  essential  to  the  regulation  of 
carbohydrate  metabolism  of  the  body.  The  islands  of  Langerhans 
develop  as  bud-like  outgrowths  from  the  ducts  of  the  glands  that 
secrete  the  pancreatic  juice,  but  they  soon  lose  all  connection  with  the 
ducts  and  form  independent  units  which  secrete  their  hormone  directly 
into  the  surrounding  capillaries.  Insulin  has  been  referred  to  as  the 
carbohydrate  hormone.  Essentially,  it  is  regarded  as  the  "spark  plug" 
of  carbohydrate  metabolism  which  is  necessary  to  bring  about  the 
chemical, union  or  oxidation  of  glucose  and  oxygen  and  the  release  of 
the  potential  chemical  energy.  In  addition,  it  appears  that  insulin 
1  Consult  Appendix:  Cholecystokinin. 


104  HUMAN  BIOLOGY 

is  necessary  for  the  accumulation  of  the  carbohydrate  glycogen  in  the 
liver.  (Fig.  55.) 

An  insulin  deficiency  in  the  body  is  due  to  a  functional  failure 
of  the  cells  in  the  islands  of  Langerhans  and  is  first  marked  clinically 
by  the  appearance  of  sugar  in  the  urine,  the  condition  known  as 
glycosuria,  or  diabetes.  Diabetes  results  from  a  partial  cessation  of 
the  oxidative  processes  throughout  the  body  tissues,  particularly  in  the 
muscles,  so  that  the  amount  of  sugar  in  the  blood  is  greatly  increased. 
Associated  with  this  is  the  almost  complete  depletion  of  the  glycogen 
in  the  liver.  The  continuous  demand  for  fuel  to  maintain  the  life 
processes  and  the  unavailability  of  glucose  in  the  absence  of  insulin  soon 
cause  the  destruction  of  other  nutritive  cell  substances  in  the  cells. 
In  particular,  the  utilization  of  the  fat  reserves  results  in  the  formation 
of  poisonous  substances,  and  acidosis  develops.  The  latter,  if  not 
checked,  leads  to  coma  and  death.  It  appears  that  the  supplies  of 
oxygen  to  the  tissues  are  not  sufficient  for  the  complete  oxidation  of 
fats  when  comparatively  large  amounts  of  the  latter  are  oxidized, 
and  the  poisonous  compounds,  ketones,  are  the  result  of  incomplete 
combustion. 

It  was  long  recognized  that  the  onset  of  diabetes  was  due  to  a 
diseased  condition  of  the  islands  of  Langerhans  before  it  was  possible 
to  isolate  the  insulin  from  pancreatic  tissue  of  cattle  and  other 
domesticated  animals  and  to  use  it  in  the  treatment  of  the  human 
disease.  The  stumbling  block  in  the  isolation  of  insulin  was  pri- 
marily due  to  the  fact  that  it  is  rapidly  destroyed  by  trypsin,  also 
secreted  by  the  pancreas.  This  was  finally  circumvented  by  a  special 
technique  devised  after  years  of  research,  and  since  then  it  has  been 
possible  to  secure  large  quantities  of  pure  insulin  from  the  pancreatic 
tissues  of  animals  slaughtered  for  food.  The  purified  and  crystallized 
insulin,  thus  obtained,  contains  25,000  units  per  gram  for  the  treat- 
ment of  diabetes.  In  moderately  severe  cases  of  insulin  deficiency, 
from  20  to  40  units  of  insulin  per  day  is  required.  Unfortunately 
insulin  cannot  be  taken  by  way  of  the  digestive  tract,  because  of  the 
destructive  action  of  trypsin  and  other  proteolytic  enzymes,  but  a 
solution  must  be  injected  under  the  skin  and  gradually  absorbed  into 
the  blood  stream.  The  insulin  treatment  for  diabetes  was  first  used  in 
January,  1922;  but  in  the  intervening  years,  its  use  has  become  world- 
wide— the  only  remedy  for  millions  suffering  from  insulin  deficiency. 

The  Thyroid. — The  human  thyroid1  gland  consists  of  a  pair  of 
ovoid  bodies  lying  on  each  side  of  the  anterior  end  of  the  trachea, 

1  Consult  Appendix :  Thyroid. 


THE  BIOLOGY  OF  SECRETION 


105 


cartilage 

.pyramid    lobe 
of  thyroid, 

.parathyro  \  cL 


closely  embracing  the  larynx.  The  paired  glands  are  covered  by  a  con- 
nective tissue  capsule  and  connected  by  the  isthmus — a  strip  of 
glandular  tissue  crossing  the  ventral  surface  of  the  trachea  just  below 
the  larynx.  In  the  adult,  the  size  of  the  thyroid  varies  considerably, 
with  an  approximate  normal  weight  of  about  1  oz.  It  is  first  seen 
in  the  embryo  as  an  unpaired  tubular  structure  which  pushes  out  from 
the  endodermal  wall  in  the  hind  part  of  the  mouth  region.  Histo- 
logically,  the  mature  thyroid  tissue  is  found  to  consist  of  a  great  many 
individual  secreting  units,  separated  from  each  other  and  all  held 
together  by  the  surrounding  connective  tissues  which  contain  a  very 
abundant  blood  supply.  Each  secreting  unit,  or  follicle,  is  a  tiny 
closed  sac  lined  by  the  functional 
epithelium  consisting  of  secreting 
cells.  (Fig.  56.) 

The  thyroid  follicles  are  nor- 
mally filled  with  a  secreted  jelly- 
like  material,  the  colloid  sub- 
stance, distinguished  from  all 
other  compounds  in  the  body  by 
the  fact  that  it  contains  a  rich 
supply  of  iodine.  Colloid  sub- 
stance contains  the  reserve  supply 
of  the  thyroid  hormone,  thyrox- 
ine,  the  active  principle  of  the 
gland.  The  complete  hormone 
consists  of  a  protein,  globulin,  in 
association  with  the  active  thyrox- 
ine.  The  latter  was  completely 
analyzed  almost  twenty-five  years 
ago  and  found  to  have  the  formula 
Ci5HnO4NI4.  The  distinctive 
feature,  as  noted,  is  that  it  contains  a  large  amount  of  iodine.  Thy- 
roxine  is  now  synthesized  in  the  laboratory,  and  the  artificial  product 
possesses  all  the  characteristic  properties  of  that  naturally  formed  in 
the  body,  as  tested  by  experimental  animals. 

Functionally,  the  thyroid  hormone  has  a  powerful  effect  in  regulat- 
ing the  general  metabolism  of  the  body.  A  continuous  supply  of  it  is 
required  at  all  times  for  normal  functioning.  The  actual  amount 
required,  however,  is  amazingly  small,  due  to  its  great  potency;  a 
characteristic  that  holds  for  all  the  hormones.  It  has  been  estimated 
that  the  amount  of  thyroxine  circulating  in  the  blood  at  any  one 
time  is  about  %  grain  (about  ^750  °Z0-  Variations  in  either  direction 


US 

..left  lobe^of 
tnyroid  glanct 
ro  icC 


vleveet  fom  f^ont* 

FIG.  56. — Drawing  of  the  anterior  end 
of  tho  trachea,  illustrating  the  position  of 
the  thyroid  and  parathyroid  glands  in  man. 
Somewhat  diagrammatic.  (Hunter, 
Walter,  and  Hunter,  "Biology"  American 
Book  Company.) 


106  HUMAN  BIOLOGY 

from  the  normal  amount  will  produce  serious  functional  disturbances 
as  will  be  indicated  below.  It  is  probable  that  thyroxine  is  largely 
concerned  with  carbohydrate  utilization,  as  is  insulin,  but  it  appar- 
ently has  a  much  broader  base  of  action  in  the  maintenance  of  essential 
environmental  conditions  for  the  body  cells  through  the  control  of  the 
composition  of  the  tissue  fluids,  which  must  contain  the  proper  sub- 
stances and  be  free  from  excess  waste  products.  These  conditions 
are  kept  at  the  normal  levels  by  nerve  control,  but  the  latter,  in  turn, 
is  undoubtedly  affected  by  the  thyroid  hormone.  Endocrine  dis- 
turbances develop  in  the  body  when  there  is  too  much  or  too  little 
thyroxine.  Thus  well-marked  clinical  symptoms  appear  when  a 
deficiency  (hypothyroidism)  occurs  or  when  the  level  is  above  normal 
(hyperthyroidism) . 

Hypothyroidism  indicates  an  insufficient  supply  of  the  thyroid 
hormone.  The  usual  cause  of  this  condition  is  a  lack  of  iodine  in  the 
food  supply,  and  this  prevents  the  synthesis  of  thyroxine.  Apparently 
in  an  effort  to  collect  more  iodine,  the  thyroid  frequently  enlarges  to 
form  a  goiter  which  protrudes  in  the  neck  region.  In  time,  a  mass  of 
tissue  weighing  several  pounds  may  develop.  Usually  this  so-called 
colloid  type  of  goiter  produces  no  ill  effects  except  as  a  detriment  to  the 
personal  appearance.  Under  other  conditions,  however,  the  over- 
growth may  invade  the  chest  region  and  interfere  with  the  respiratory 
activities.  Hypothyroidism  and  goiter  development  are  primarily 
due  to  a  lack  of  iodine  in  the  soil.  Iodine  is  plentiful  in  the  sea  and  in 
the  soil  of  coastal  regions,  but  various  inland  regions  the  world  over 
show  a  more  or  less  marked  iodine  deficiency  with  resulting  pathologi- 
cal conditions  appearing  among  the  inhabitants  and  their  domestic 
animals. 

A  deficiency  in  the  thyroid  hormone  results  in  a  marked  lowering 
of  the  basal  metabolic  rate  (page  85).  This  is  due  to  an  inability  of 
the  cells  to  maintain  the  normal  oxidativc  rate.  Even  with  reduced 
intake  of  food — generally  the  appetite  of  the  sufferer  is  poor — a 
noticeable  increase  in  fat  storage  occurs  in  the  hypothyroid  individual. 
The  temperature  of  the  body  falls  in  correspondence  with  the  reduced 
oxidation,  and  the  patient  feels  chilly.  If  the  thyroxine  deficiency 
persists,  the  skin  becomes  thick,  rough,  and  puffy  with  a  peculiar 
consistency.  But  worst  of  all,  hypothyroidism  causes  a  steady 
deterioration  of  the  nervous  functions  and  may  'result  in  a  complete 
breakdown  of  the  higher  mental  processes.  The  brief  outline  just 
given  summarizes  the  results  of  thyroid  deficiency  in  the  adult,  a 
condition  known  as  myxedema,  which  is  somewhat  different  from 
the  cretinism  that  develops  in  children  from  the  same  cause. 


THE  BIOLOGY  OF  SECRETION  107 

Children  born  in  regions  where  the  iodine  deficiency  is  such  that 
mothers  have  had  an  insufficient  supply  of  thyroid  hormone  during 
pregnancy  are  often  misshapen  at  birth,  with  bloated  face,  thick  pro- 
truding tongue,  pot-bellied  abdomen,  and  abnormal  mental  develop- 
ment. They  are  known  as  cretins.  The  same  cretinous  condition 
may  also  develop  in  children  after  birth  if  the  food  supplied  does  not 
contain  the  necessary  amount  of  iodine.  Until  comparatively  recently, 
such  unfortunates  were  doomed  to  as  sad  an  existence  as  could  be 
imagined,  but  the  discovery  that  supplying  thyroid  gland  substance  of 
some  animal  in  the  food  would  almost  miraculously  relieve  the  con- 
dition—and this  applies  to  adult  myxedema  as  well — flooded  the 
cretinous  stage  with  a  new  life  light.  As  Osier  says: 

Not  the  magic  wand  of  Prospero  or  the  brave  kiss  of  the  daughter  of 
Hippocrates  ever  effected  such  a  change  as  that  which  we  are  now  enabled  to 
make  in  these  unfortunate  victims,  doomed  heretofore  to  live  in  hopeless 
imbecility,  an  unspeakable  affliction  to  their  parents.  .  .  . 

Hyperthyroidism  is  usually  due  to  an  abnormal  activity  of  the 
thyroid  gland  and  the  consequent  production  and  distribution  through- 
out the  body  of  an  excess  amount  of  the  hormone.  The  same  hyper- 
thyroid  condition,  however,  obtains  when  too  much  thyroid  material 
is  taken  into  the  body  through  the  digestive  tract,  as  sometimes 
happens  when  an  overeiithusiastic  "reducer"  unwisely  uses  the 
hormone  in  an  attempt  to  reduce  the  body  weight.  The  latter  cause 
of  hyperthyroidism  is  easily  remedied  by  changing  the  diet,  but  the 
first  cause  can  be  relieved  only  by  the  removal  of  a  portion  of  the  over- 
active  gland.  The  underlying  causes  of  thyroid  superactivity  are  not 
apparent.  Dietary  factors  apparently  are  not  responsible  as  in 
hy  pothy  roidism . 

The  reaction  of  the  body  mechanism  to  an  excess  of  thyroid 
hormone  is  essentially  the  reverse  of  that  when  hy  pothy  roidism  occurs. 
Thus  a  decided  increase  in  the  basal  metabolism  occurs,  which  means 
more  oxidation  and  increased  body  temperature.  More  nitrogen  is 
eliminated  through  the  urine  which  indicates  that  the  protein  metab- 
olism is  also  affected  to  some  extent.  The  circulation  of  blood  if? 
stepped  up,  and  the  nervous  system  is  definitely  more  sensitive.  The 
person  tends  to  be  active,  irritable,  at  high  tension.  Possibly  the 
general  effect  in  the  body  may  be  compared  to  the  action  of  the  motor 
when  the  driver  steps  on  the  accelerator.  These  preliminary  symp- 
toms, characteristic  of  hyperthyroidism,  herald  the  approach  of  the 
serious  disease  condition  known  as  exophthalmic  goiter  which,  in  addi- 
tion to  an  exaggeration  of  the  symptoms  just  indicated,  is  further 


108  HUMAN  BIOLOGY 

marked  by  protruding  eyes,  dangerous  overstimulation  of  the  heart 
muscle,  and,  finally  marked  nervous  disorders. 

In  summary,  Hoskins  has  well  said  with  reference  to  the  general 
effects  of  the  thyroid  gland : 

This  much  we  know.  We  are  what  we  are  in  no  small  measure  by  virtue 
of  our  thyroid  glands.  Our  development  before  birth  and  through  infancy 
depends  upon  their  functional  integrity.  The  hurdles  of  puberty  are  taken 
with  their  aid.  A  pinch  too  little  of  thyroid  spells  idiocy.  A  pinch  too  much 
spells  raving  delirium.  By  its  very  mobility  the  thyroid  plays  a  major  role 
in  keeping  us  attuned  to  our  environment.  Nature  has  done  much  with  the 
thyroid  hormone.1 

The  Parathyroid  Glands. — The  parathyroids  in  the  human  organ- 
ism usually  consist  of  four  oval-shaped  bodies,  each  about  the  size  of 
a  pea.  Both  in  their  development  and  later  location  in  the  body,  they 
are  in  close  association  with  the  thyroid  glands,  as  indicated  by  the 
name  given  to  them.  Functionally,  however,  it  is  rather  surprising 
to  find  that  the  parathyroids  have  little  or  nothing  in  common  with 
the  thyroid  apparatus,  and  in  that  way  the  name  is  misleading.  The 
parathyroids  are  so  small  and  lie  in  such  close  apposition  to  the 
thyroids,  or  even  actually  embedded  in  the  thyroid  tissue,  that  for  long 
years  they  were  regarded  as  misplaced  bits  of  thyroid  tissue. 
(Fig.  56.) 

Surgeons  were  the  first  to  recognize  that  functionally  the  para- 
thyroids were  entirely  distinct  from  the  thyroids,  for  they  found  that, 
in  the  surgical  removal  of  portions  of  the  thyroid  to  relieve  disease 
conditions  associated  with  hyperactivity  of  that  gland,  the  parathyroids 
would,  not  infrequently,  be  injured  or  even  removed.  When  this 
occurred,  distressing  symptoms  unassociated  with  normal  thyroid 
removal  quickly  made  their  appearance,  and  in  cases  where  great 
damage  had  been  done  to  the  parathyroids,  the  patient  always  failed 
to  survive.  When  the  nature  and  importance  of  the  parathyroids 
were  finally  recognized,  great  care  was  taken  in  thyroid  operations  to 
see  that  they  were  left  undisturbed. 

The  parathyroid  hormone  coming  from  these  tiny  endocrine  glands 
is  almost  infinitesimal  in  amount,  and  accordingly  it  has  never  been 
subjected  to  chemical  analysis,  though  an  active  substance  has  been 
isolated  which  is  believed  to  be  the  hormone.  Inasmuch  as  the  hor- 
mone is  digested  in  the  alimentary  tract,  it  apparently  has  a  protein 
nature  as  in  the  case  of  insulin.  It  has,  however,  been  established  by 
experiments  on  various  animals  that  the  parathyroid  hormone  is 
primarily  concerned  with  the  control  of  the  calcium  metabolism  of  the 

i  HOSKINS,  "The  Tides  of  Life,"  W.  W.  Norton  &  Company,  Inc. 


THE  BIOLOGY  OF  SECRETION  109 

body.  This  common  element  is  an  important  constituent  of  our  bones 
and  teeth,  but,  in  addition  to  this,  the  normal  blood  always  contains  a 
small  amount  of  calcium,  about  0.01  per  cent.  Small  as  is  the  amount 
of  calcium  in  the  blood,  it  is  absolutely  essential  that  it  be  present  at 
all  times,  and  the  maintenance  of  the  proper  levels  is  the  function 
assigned  to  the  parathyroid  hormone.  This  is  apparently  accom- 
plished, for  the  most  part,  by  aiding  in  the  absorption  of  calcium  from 
the  alimentary  tract  when  more  is  needed,  also  by  aiding  in  the 
elimination  of  calcium  from  the  body  when  the  level  is  too  high. 
Furthermore,  it  can  be  shown  experimentally  that  an  excess  of  para- 
thyroid extract  will  cause  a  depletion  of  the  bone  calcium. 

•  Authorities  are  in  general  agreement  that  calcium  is  necessary  to 
keep  the  nerve  and  muscle  tissues  at  the  proper  degree  of  irritability. 
Essentially  the  action  of  calcium  appears  to  be  that  of  a  sedative,  for 
when  the  calcium  supply  falls  below  normal,  greatly  increased  irrita- 
bility is  at  once  noted.  The  muscles  contract  spasmodically  and  more 
and  more  violently,  until  the  organism  is  in  a  state  of  tetany.  This 
muscular  activity  apparently  is  due  to  the  increased  irritability  of  the 
motor  nerves,  for  the  latter  are  further  stimulated  by  the  contracting 
muscles  so  that  a  vicious  circle  is  soon  set  up.  Some  authorities  hold 
that  the  maintenance  of  the  proper  calcium-phosphorus  ratio  in  the 
body  is  also  tied  up  in  the  general  parathyroid  complex. 

Tetany  is  a  condition  in  which  the  muscles  become  rigid,  as  in 
convulsions,  with  stiffened  limbs  and  clamped  jaws.  In  the  experi- 
mental animals,  tetany  rapidly  follows  removal  of  the  parathyroids 
and  ends  fatally  unless  parathyroid  extract  is  supplied.  The  latter 
seems  almost  miraculous  in  the  relief  it  brings,  but  this,  of  course,  is 
only  temporary  if  the  glands  have  been  completely  destroyed.  The 
injection  of  calcium  salts  into  the  blood  stream  has  essentially  the 
same  effect  as  the  administration  of  parathyroid  extract.  There 
seems  to  be  no  question,  therefore,  of  the  basic  relationship  between 
the  parathyroid  gland  and  calcium  metabolism,  though  probably  the 
whole  story  is  not  yet  known. 

It  was  noted  above  in  the  discussion  of  the  thyroid  that  both 
hypothyroidism  and  hyperthyroidism  are  not  uncommon.  This  is 
not  the  case  with  the  parathyroid,  for  it  apparently  gives  very  little 
trouble  except  when  it  is  accidentally  disturbed  in  surgical  operations 
associated  with  thyroid  removal. 

The  Adrenal  Glands. — The  paired  adrenal1  glands  are  always  found 
in  close  relationship  with  the  kidneys  and  were,  therefore,  long  thought 
to  be  linked  with  the  kidneys  functionally  as  well  as  anatomically. 
This  proved  not  to  be  the  case,  and  no  functional  reason  seems  to  exist 

1  Consult  Appendix:  Adrenal. 


110  HUMAN  BIOLOGY 

for  their  position  in  close  contact  to  the  anterior  end  of  each  kidney. 
Each  adrenal  gland  weighs  slightly  more  than  ^  oz.,  is  flattened  and 
roughly  triangular  in  shape.  If  the  gland  is  sectioned  and  the  cut 
surface  examined,  it  will  be  found  to  consist  of  two  distinct  regions :  an 
outer  thicker  shell,  the  cortex,  light  yellowish  in  color;  and  an  inner, 
brownish-red  portion,  the  medulla,  making  up  the  rest  of  the  gland. 
The  cortex  and  medulla  have  diff erent  origins  in  the  embryo,  the  former 
coming  from  the  mesoderm  of  the  body  cavity,  whereas  the  latter  is 
derived 'from  the  ectoderm  in  close  association  with  the  autonomic 
nervous  system.  (Plate  F,  page  92.) 

The  cells  of  the  adrenal  cortex  are  arranged  in  three  poorly  defined 
layers,  or  zones,  and  are  characterized  by  the  presence  in  the  cytoplasm 
of  various  lipoids.  The  microscopic  structure  of  the  medulla  is  quite 
different  from  the  cortex  and  resembles  somewhat  the  condition  in  the 
liver  lobule,  described  in  this  chapter,  in  that  groups  or  cords  of  cells 
are  separated  by  the  tiny  channels  or  sinusoids  through  which  the 
blood  flows  (page  101).  Thus  the  cells  are  in  close  contact  with  the 
blood — in  fact,  the  colls  may  be  said  to  form  the  banks  of  the  stream — 
which  makes  it  very  easy  for  materials  to  be  received  or  given  off. 
The  cells  of  the  medulla  are  closely  associated  also  with  the  cells  of  the 
autonomic  nervous  system,  which  are  distributed  generally  through 
this  region.  The  adrenal  gland  is  marked  by  an  extraordinarily  rich 
blood  supply.  It  has  been  estimated  that  six  times  its  own  weight  of 
blood  passes  through  the  adrenal  every  minute,  which  probably  makes 
it  one  of  the  most  highly  vascularized  of  all  the  tissues  of  the  body. 

Just  as  the  two  portions  of  the  adrenal  show  mark6d  structural 
differences,  so  do  they  also  exhibit  characteristic  functional  differences, 
for  they  produce  separate  hormones.  The  hormone  from  the  cortex 
of  the  adrenal,  known  as  cortin,  has  been  isolated  for  only  a  few  years, 
and  its  chemical  composition,  as  well  as  its  function  or  functions  in 
the  normal  animal,  remain  largely  undisclosed.  That  cortin  is  vitally 
important  no  one  can  question,  for,  when  the  cortex  is  removed  from 
an  experimental  animal  or  when  it  is  destroyed  by  disease  in  man 
(Addison's  disease),  the  life  functions  of  the  organism  cannot  long  be 
maintained.  Possibly,  of  course,  the  adrenal  cortex  may  be  essential 
for  the  removal  of  some  poisonous  body  waste,  but  there  seems  to  be 
an  entire  lack  of  evidence  of  such  a  function.  And  on  the  other  hand, 
the  fact  is  established  that  the  lives  of  experimental  animals  with  the 
Cortex  removed  and  human  beings  with  cortex  destroyed  by  disease 
can  be  prolonged  by  the  injection  of  cortin.  Animals  with  the  cortex 
removed  show  a  marked  drop  in  the  basal  metabolic  rate  and  a  dis- 
turbed carbohydrate  metabolism.  In  association  with  these  patho- 


THE  BIOLOGY  OF  SECRETION  111 

logical  symptoms,  the  temperature  control  is  disturbed  and  the  kidneys 
fail  to  function  adequately.  All  of  these  functional  abnormalities 
develop  from  the  lack  of  cortin,  but  they  throw  very  little  light  on  the 
functions  of  cortin  under  normal  conditions.  Like  insulin,  this  hor- 
mone is  rapidly  destroyed  by  the  digestive  enzymes,  and,  in  addition, 
cortin  is  difficult  to  isolate.  The  yield  is  small  by  any  method  yet 
devised.  Another  hormone,  adrenosterone,  with  characteristics  similar 
to  the  mate  hormone,  has  recently  been  obtained  from  the  adrenal 
cortex. 

The  hormone  from  the  medulla  of  the.  adrenal  gland,  variously 
known  as  epinephrine,  adrenaline,  and  adrenin,  has  been  isolated, 
chemically  analyzed,  and  widely  used  in  medicine  for  over  thirty  years. 
Compared  to  many  other  of  the  nitrogenous  compounds  associated 
with  the  living  organism,  it  is  relatively  simple  in  its  chemical  structure, 
with  the  formula  CoH^OaN.  The  use  of  this  substance  in  medicine, 
where  it  has  been  found  to  be  of  particular  value  in  the  alleviation  of 
asthma  and  hay  fever  and  in  stopping  hemorrhage  by  inducing  con- 
traction of  blood  vessels,  is  very  different  from  its  normal  use  in  the 
body,  which  is  best  described  by  the  term  emergency  hormone.  Adrenin 
may  be  said  to  make  better  fighters  of  us  all  and  to  be  essential  when 
all  the  organs  of  the  body  must  be  operating  at  their  highest  rate. 

It  has  been  seen  above  that  the  tissues  of  the  medulla  develop  and 
remain  in  partnership  with  elements  of  the  autonomic  nervous  system. 
The  autonomic  system  controls  the  vital  organs  of  the  body;  and,  when 
necessary,  its  call  to  action  is  reinforced  by  the  adrenin  poured  into 
the  blood,  thus  reaching  all  the  organs  of  the  body  in  less  time  than  it 
takes  to  tell  it.  As  a  result,  the  heart  beats  faster,  the  liver  releases 
additional  carbohydrate  for  fuel,  and  increased  oxygen  intake  occurs. 
At  the  same  time,  the  blood  vessels  supplying  the  skin  and  digestive 
organs  contract  so  that  more  blood  laden  with  essential  materials  can 
be  sent  to  the  muscles.  In  short,  under  the  stimulus  of  adrenin  all 
possible  is  done  by  the  body  tissues1  in  order  to  permit  the  maximum 
amount  of  carbohydrate  metabolism  in  the  muscles,  with  a  corre- 
sponding release  of  energy  made  available  for  a  foot  race,  a  ball  game, 
or  the  more  serious  duties  with  which  everyone  is  confronted  from  time 
to  time. 

HORMONES  CONCERNED  WITH  THE  GENERAL  CONTROL 
OF  BODY  FUNCTIONS 

The  Pituitary  Gland. — The  small  unpaired  gland,  known  as  the 
pituitary1  or  hypophysis,  is  attached  to  the  underside  of  the  brain  by  a 
1  Consult  Appendix:  Pituitary. 


112  HUMAN  BIOLOGY 

short  stalk.  An  inexperienced  person  dissecting  the  brain  of  even  a 
good-sized  animal  would  probably  never  notice  the  pituitary  and  so 
would  remove  the  brain  and  leave  the  pituitary,  with  a  tiny  bit  of  its 
stalk  lying  snugly  in  a  little  bony  cavity  of  the  skull  which  is  built 
around  it.  But,  nevertheless,  the  pituitary  was  early  discovered  and 
long  thought  to  be  a  gland  for  the  secretion  of  the  fluid  mucus  used 
to  lubricate  the  throat  surfaces.  The  human  pituitary  is  about  the 
size  of  a  large  pea  and  consists  of  an  anterior  lobe  and  a  posterior 
lobe  with  an  intermediate  area  lying  between  them.  The  origin  of 
these  two  portions  of  the  pituitary  is  diverse.  Thus  the  anterior 

lobe  develops  very  early  in  the  em- 
bryo as  ,a  tiny  outpocketing  from  the 
roof  of  the  mouth  cavity.  This 
ectodermal  sac  then  proceeds  to  grow 
anteriorly  toward  the  brain  until  it 
meets  a  tiny  body  of  neYve  tissue,  the 
iiifundibulum,  projecting  from  the 
floor  of  the  brain.  The  infundibu- 
lunl  becomes  the  posterior  lobe  of 
the  mature  pituitary.  (Fig.  57.) 
FIG.  57.— Section  through  the  The  pituitary  and  the  liver  are 

adult     human     pituitary.     Diagram-  ,    .    ,         ,,          .  .  . ., 

matic.  (Redrawn from  Hoskins,  "The  certainly  the  two  most  versatile 
Tides  of  Lifer  w.  W.  Norton  &  Com-  glands  in  the  body  (page  99).  It 
pany,  nc.)  might  be  difficult  to  determine  which 

one  should  be  awarded  the  prize  for  association  with  the  most  func- 
tional activities.  But  the  liver,  with  its  enormous  size,  looks  the  part, 
whereas  the  insignificant  pituitary  gives  no  structural  indication  of  its 
importance.  Furthermore,  the  relationship  of  the  pituitary  with  other 
organs  is  not  as  a  minor  agent,  for  it  is  the  actual  controlling  power — 
the  generalissimo,  if  you  will — directing  the  activities  of  various 
glands,  of  numerous  major  functions.  This  pituitary  control  is  accom- 
plished through  various  specific  hormones — at  least  eight  separate  ones 
are  believed  to  be  produced  by  this  bit  of  glandular  tissue — most  of 
which  are  formed  in  the  anterior  lobe.  The  microscopic  examination 
of  tissue  from  the  anterior  lobe  shows  much  the  same  cSilular  arrange- 
ments as  in  the  medulla  of  the  adrenal;  that  of  the  posterior  lobe 
appears  to  be  almost  barren  of  secreting  cells.  Altogether,  there  are 
not  nearly  enough  differentiated  cell  types  to  account  for  the  variety 
of  hormones  produced  by  the  pituitary. 

Possibly  the  best  conception  of  the  pituitary  in  the  limited  space 
available  may  be  gained  by  presenting  a  short  summary  of  the  hor- 
mone actions  as  known  at  present.  Such  a  summary  will  undoubtedly 


THE  BIOLOGY  OF  SECRETION  113 

need  considerable  revision  as  the  years  go  by,  for  the  pituitary  research 
field  is  one  of  the  most  active  known  to  biology,  and  major  problems 
are  still  unsolved.  However,  this  same  statement  holds  true,  in 
general,  for  the  entire  field  of  endocrinology. 

Pituitary  hormones  are  able  to  control  the  growth  processes  in  the 
body  as  a  whole,  as  well  as  that  of  certain  organs.  It  has  long  been 
recognized  that  an  excess,  during  the  formative  years,  of  anterior  lobe 
hormone  from  an  enlarged  pituitary  gland  results  in  gigantism. 
Instances  are  recorded  where  such  individuals  have  attained  a  height 
of  around  9  ft.  Apparently  one  of  the  best  examples  of  gigantism  ever 
recorded  died  recently  in  Illinois.  The  young  man  was  only  about 
twenty-one  years  of  age  at  the  time  of  his  death,  but  had  reached  a  height 
of  nearly  8^2  ft.  and  a  weight  of  around  400  Ibs. 

Where  the  excess  of  the  pituitary  hormone  is  available  during  the 
developmental  period,  it  results  in  a  symmetrical  overgrowth  of  the 
body  tissues  generally.  A  somewhat  different  result  is  evident  when 
the  enlargement  of  the  gland  occurs  in  later  life,  after  adult  size  has 
been  reached,  for  then  a  fatal  disease,  acromegaly,  develops — -a  term 
that  literally  means  "big  extremities"  and  clearly  describes  the 
situation.  Acromegaly  is  characterized  by  a  gradual  enlargement  of 
the  bones  of  the  hands,  feet,  and  face.  It  is  only  slightly  noticeable 
in  the  early  stages  but  gradually  results  in  the  production  of  a  grotesque 
caricature  of  the  earlier  normal  condition.  Both  the  post-mortem 
examinations  and  the  results  from  experimental  animals  confirm  the 
belief  that  acromegaly  is  due  to  an  excess  of  a  pituitary  hormone. 

On  the  other  hand,  a  deficiency  of  the  pituitary  and  other  hormones 
results  in  a  type  of  dwarfism  that  may  be  regarded  as  the  reverse  of 
gigantism.  Such  an  individual  appears  to  be  essentially  a  miniature 
of  the  normal  adult  with  a  symmetrical  development  throughout  the 
body,  generally  pleasing  appearance,  and  intellectually  capable.  Such 
midgets  are  to  be  distinguished  from  achondroplastic  dwarfs  in  which  a 
large  head  and  features  are  placed  upon  a  child-sized  body.  The 
developmental  basis  of  the  midget  type  is  not  known.  Pituitary  insuf- 
ficiency is  also  believed  to  be  responsible  for  the  so-called  "Frohlich's 
syndrome"  in  which  a  marked  condition  of  obesity  develops  in  child- 
hood or  early  maturity.  Dickens  evidently  described  such  a  case  in 
the  F^t  Boy  of  "Pickwick  Papers."  (Fig.  58.) 

Pituitary  hormones  from  the  anterior  lobe  exercise  definite  control 
over  the  activities  of  certain  other  important  endocrine  glands.  This 
fact  has  been  established  in  the  cases  of  the  thyroid,  adrenal,  and  sex 
glands.  For  instance,  experiments  on  rats  have  shown  that  the 
removal  of  the  anterior  lobe  of  the  pituitary  results  in  marked  degenera- 


114 


HUMAN  BIOLOGY 


tive  changes  in  the  secreting  cells  of  the  thyroid  together  with  the 
practical  destruction  of  the  adrenal  cortex.  Closely  associated  is  the 
control  over  the  sex  glands,  both  testes  and  ovaries,  exercised  through 
one  or  two  specific  hormones  (Prolan  A  and  B).  A  wide  array  of 
experimental  results  show  that  the  gonads  and  associated  structures 
of  various  mammals  are  stimulated  in  a  number  of  ways.  Thus  it  has 
become  laboratory  routine  to  stimulate  egg  production  in  the  amphibia 


FIG.  58. — Illustrating  the  body  form  of  a  normal  adult  man  (center)  as  compared 
with  an  achondroplastic  dwarf  (left)  and  the  small,  graceful  form  of  a  true  midget 
(right).  (Redrawn  from  Stockard,  "Physical  Basis  of  Personality"  W.  W.  Norton  & 
Company,  Inc.) 

out  of  season  by  pituitary  extract.1  In  most  cases,  the  pituitary 
hormones  work  indirectly  by  stimulating  the  production  of  sex  hor- 
mones in  the  testis  or  ovaiy,  and  the  sex  hormones  thus  produced  incite 
the  changes  in  the  sexual  apparatus  as  a  whole. 

Closely  associated  with  the  reproductive  function  is  milk  formation, 
or  lactation,  in  the  mammalian  female.  An  active  pituitary  principle, 

1  The  following  advertisement  recently  distributed  by  the  General  Biological 
Supply  House  is  pertinent  in  this  connection:  "Once  upon  a  time  teachers  could 
obtain  living  frog  eggs  for  only  a  week  or  so  in  the  early  spring,  but  such  specimens 
are  now  available  throughout  the  entire  school  year.  And  you  can,  if  you  wish, 
perform  the  entire  experiment  in  your  own  laboratory.  Our  Frog  Pituitary  Set 
includes  two  living  male  grassfrogs,  one  living  female  grassfrog,  one  unit  of  frog 
pituitary  suspension  and  simple  but  detailed  directions  for  producing  laboratory- 
induced  eggs/' 


THE  BIOLOGY  OF  SECRETION  115 

prolactin,  has  been  isolated  which  stimulates  this  activity.  Finally, 
there  seems  to  be  quite  general  agreement  that  pituitary  hormones  exer- 
cise final  control  over  the  utilization  of  carbohydrates,  fats,  and 
proteins  in  the  body.  Thus,  in  carbohydrate  utilization,  abnormal 
activity  of  the  pituitary  will  result  in  an  increase  of  blood  sugar  and  its 
elimination  .through  the  kidneys  just  as  does  the  lack  of  insulin. 
Reciprocally,  pituitary  hormone  deficiency  results  in  a  fall  of  blood 
sugar  below  normal  levels.  And  in  the  case  of  protein  utilization,  an 
excess  of  pituitary  extract  apparently  results  in  the  storage  of  greatly 
increased  amounts  of  proteins. 

At  least  two  separate  hormones  are  believed  to  be  produced  by  the 
posterior  lobe  of  the  pituitary.  They  have  not  been  isolated,  and  the 
results  are  obtained  by  the  use  of  glandular  extracts.  The  hormones 
of  the  posterior  lobe  are  primarily  associated  with  the  contraction  of 
smooth  muscle  tissues,  particularly  in  the  walls  of  the  blood  vessels, 
and  this  results  in  an  increase  of  the  blood  pressure.  Another  distinct 
pituitary  function  is  associated  with  greatly  increased  urine  secretion. 
This  appears  to  be  associated  with  a  decrease  in  water  absorption  by 
the  cells  of  the  body.  In  other  words,  tlie  direct  effect  of  this  hormone 
is  on  the  body  cells  rather  than  the  kidney. 

The  Gonads. — The  detailed  consideration  of  the  gonads  may  well 
be  deferred  until  the  function  of  reproduction  is  described  in  a  later 
chapter;  but  at  this  point,  mention  should  be  made  of  the  hormonal 
activities  of  these  organs  which  have  very  important  functions  in 
human  structure  and  physiology.  The  male  gonads,  or  testes,  include, 
in  addition  to  the  tissues  concerned  with  the  production  of  the  male 
sperm,  an  endocrine  tissue  situated  in  the  areas  between  the  sperm- 
producing  tissues.  This  glandular,  or  interstitial  tissue  consists  of 
secreting  cells  which  are  distinct  from  the  germinal  tissue.  The 
castration  of  male  domestic  animals  and  even  of  man  himself  (eunuchs) 
has  been  practiced  from  early  times  and  has  always  been  followed  by  a 
modification  of  the  secondary  sex  characteristics,  that  is,  the  distinctive 
structural  features  associated  with  the  two  sexes.  In  castrated  males, 
if  done  when  young,  the  results  are :  increased  body  size,  a  wide  distribu- 
tion of  subcutaneous  fat  which  alters  the  body  shape,  arid  a  modifi- 
cation of  normal  behavior.  These  changes  result  from  the  lack  of  the 
male  sex  hormone,  testosterone,  secreted  by  the  interstitial  tissue. 
It  has  recently  been  possible  to  isolate  this  male  hormone,  and  in  1934 
it  was  artificially  synthesized  in  the  laboratory,1  The  injection  of 
testosterone  in  a  castrated  animal  will  gradually  induce  the  formation 

1  The  isolation  of  the  male  hormone  was  accomplished  by  a  Swiss  investigator, 
Ruszicka,  who  received  the  Nobel  Prize  for  this  work  in  the  fall  of  1939. 


116 


HUMAN  BIOLOGY 


of  the  normal  male  secondary  sex  characteristics. l  It  is  well  established 
that  the  final  control  of  the  sex  hormone  secretion  is  normally  a  func- 
tion of  the  pituitary  hormone.  (Fig.  59.) 

The  female  gonads,  or  ovaries,  are  known  to  secrete  at  least  two 
sex  hormones,  but  they  are  not  produced  by  endocrine  tissues  lying 
between  the  reproductive  tissues  as  just  noted  in  the  case  of  the  testis. 

/o     pSpc     8 


w  Spt         p8pt 

FIQ.  59. — Section  through  the  mammalian  testis  (rat) .  Highly  magnified  (  X  250) 
to  show  the  interstitial  tissue  (ic)  which  secretes  the  male  hormone  (testosterone). 
Note  that  the  interstitial  tissue  lies  between  the  seminiferous  tubules.  Various  stages 
in  spermatogenesis  are  shown  as  described  in  Chapter  XIII.  (Maximow-Bloom, 
"Histology,"  W.  B.  Saunders  Company.) 

As  the  ovarian  eggs  mature,  each  is  enclosed  by  a  liquid-filled  vesicle, 
the  Graafian  follicle.  One  of  the  female  sex  hormones,  estrone,  is 
produced  by  the  associated  follicular  cells.  It  is  clear  that  this 
hormone  is  not  responsible  for  the  development  of  the  secondary  female 
characteristics  but  is  concerned  with  the  preparation  of  the  uterus  for 
the  implantation  of  the  fertilized  egg.  Apparently  estrone  is  first 
secreted  at  the  time  of  puberty,  under  the  stimulation  of  the  pituitary 
hormone.  The  other  ovarian  hormone,  progesterone,  is  also  formed 
1  Consult  Appendix:  Sexual  Characteristics. 


THE  BIOLOGY  OF  SECRETION  117 

in  the  follicles  but  by  an  endocrine  tissue,  the  corpus  luteum,  which  is 
not  formed  until  after  the  eggs  have  been  released.  Progesterone, 
secreted  by  the  corpus  luteum,  acts  on  the  muscular  uterine  walls  and 
on  the  entire  genital  apparatus  as  well.  It  prevents,  the  normal 
monthly  cyclical  changes  if  the  egg  is  fertilized  and  a  pregnancy 
develops.  In  case  the  egg  is  not  fertilized,  the  corpus  luteum  soon 
begins  to  degenerate,  loses  its  endocrine  function,  and  the  ovarian 

—  CORPUS  LUTEUM  (FRESH) 
CORPUS  LUTEUM  (OLD) 


GRMFIAN 
FOLLICLE 

EGG 

FIG.  60. — Section  through  human  ovary  to  show  the  corpus  luteum,  an  endocrine 
tissue.  Graafian  follicles,  with  eggs,  in  various  stages  of  development  are  also  jndicated. 
Diagrammatic.  (Redrawn  front  ShulL) 

cycle  is  resumed  (Fig.  60).  Progesterone  may  thus  be  said  to  be 
particularly  concerned  with  pregnancy.  A  hormone  similar  to  pro- 
gesterone in  its  function  is  also  produced  by  the  placenta  during 
pregnancy.  It  has  also  been  shown  that  the  chemical  composition  of 
the  male  hormone,  testosterone,  and  the  female  hormone,  estrone,  are 
very  close  and  that  these  hormones  will  function,  to  a  certain  extent, 
interchangeably  between  the  two  soxes  when  injected  in  the  opposite 
sex.  Consideration  of  the  reproductive  processes  in  a  later  chapter 
will  give  further  opportunity  for  a  discussion  of  the  reproductive  cycle. 


KIDNEY 


VENA  CAVA 
URETER 


URETERAL  ORIFICE 


KIDNEY 


BLADDER 


URETHRA 


CORTEX 


MEDULLA 


PAPILLA 


CORTICAL 
COLUMN 


PYRAMID 
(MEDULLA) 

CAPSULE 


RENAL  ARTERY 
RENAL  VEIN 
PELVIS 

URETER 

ARCUATE  VEIN 
ARCUATE  ARTERY 


B 

PLATE  VI. — Excretory  system  in  man.  A,  general  arrangement  of  the  kidneys, 
associated  ducts,  and  blood  vessels  (cf.  Plate  V,  page  92  for  relationships  of  kidneys  to 
other  organs).  B,  drawing  of  a  longitudinal  section  of  the  kidney  to  show  arrangement 
of  tissues. 


CHAPTER  VI 
THE  BIOLOGY  OF  EXCRETION 

The  chemical  activities  associated  with  the  life  metabolism  of  the 
cells  in  the  body  continually  produce  waste  products  that,  for  the  most 
part,  are  not  only  useless  but  actually  harmful  to  the  continued  exist- 
ence of  the  cells  themselves.  Continuous  removal  of  these  wastes 
from  the  individual  cells  and  their  elimination  from  the  body  is,  there- 
fore, essential  to  the  maintenance  of  the  life  processes.  This  activity 
constitutes  the  function  of  excretion.  It  is  well  to  distinguish  between 
excretion  and  egestion;  the  latter  function  being  concerned  with  the 
elimination  of  materials  that  have  not  been  associated  with  the  proto- 
plasm of  the  organism,  in  particular,  the  egestion  of  indigestible  refuse 
from  the  alimentary  canal. 

Excretion  requires  part-time  service  from  several  organ  systems 
in  addition  to  full-time  service  from  the  kidneys,  which  are  commonly 
regarded  as  the  basic  excretory  organs  of  the  body.  Thus  the  impor- 
tant relations  of  the  skin,  of  the  lungs,  and  of  the  liver  to  excretion 
have  been  indicated  in  previous  chapters.  Later  it  will  be  apparent 
that  the  vascular  system,  as  the  transporting  unit  for  the  excretory 
products,  is  also  an  essential  part  of  the  complete  picture.  All  of  these 
organs  working  together  are  able  to  relieve  the  body  cells  of  the  useless 
end  products  of  iritracellular  life  chemistry,  consisting  of  carbon 
dioxide,  water,  inorganic  salts,  bilirubin,  and  various  nitrogenous 
compound's,  notably  urea. 

EXCRETION  AND  THE  SKIN 

In  its  capacity  as  an  excretory  organ,  the  skin  of  man  is  concerned 
with  the  elimination  of  sweat  which,  secreted  by  sweat  glands,  leaves 
the  body  through  the  pores  opening  at  the  skin  surface  (page  37). 
A  chemical  analysis  of  sweat  shows  that  it  consists  of  from  98  to  99  per 
cent  water,  with  slight  amounts  of  carbon  dioxide  and  nitrogenous 
wastes  in  solution.  The  total  amount  of  perspiration  given  off  each 
day  is  subject  to  wide  variation,  for,  as  pi'eviously  indicated,  it  is 
dependent  upon  the  amount  of  work  performed,  the*  amount  of  water 
absorbed  from  the  alimentary  tract,  and  the  temperature  of  the  external 
environment.  Strictly  speaking,  only  the  first  of  these,  that  is,  the 
amount  of  work  performed,  is  primarily  concerned  with  excretion,  but 

119 


120 


HUMAN  BIOLOGY 


the  other  two,  particularly  the  environmental  temperature,  are  impor- 
tant in  the  complete  picture  of  skin  activity.  Energy  to  do  work 
comes,  as  we  have  seen,  from  the  breakdown  of  glucose  into  carbon 
dioxide  and  water.  Very  small  quantities  of  carbon  dioxide  and 
varying  amounts  of  water  are  eliminated  by  way  of  the  skin.  The 
total  of  these  two  compounds  to  be  eliminated  varies  directly  with 
the  amount  of  sugar  oxidized.  Of  these  three  factors,  the  external 
temperature  is  probably  the  most  decisive  in  determining  the  sweat 

OPENING  OF 
SWEAT  GLAND 


EPIDERMIS 


NERVE  END/NGS 
AFFECTED  BY  TEMPERATURE 


SWEAT 
GLAND 


VASODILATORS  TO 

CUTANEOUS  VESSELS 
(EFFERENT) 

'NERVE  TO  SWEAT  GLAND 
(EFFERENT) 


FIG.  61. — Diagram  illustrating  temperature  regulation  through  nervous  control  of 
the  sweat  glands  in  the  skin  as  described  on  page  119.  (Redrawn  from  Hough,  Sedgwick, 
and  Waddell  "The  Human  Mechanism,"  Ginn  and  Company.) 

output,  because  this  liquid  is  essential  to  the  maintenance  of  the  normal 
body  temperature.  Thus  a  person  who  is  moderately  active  on  a  cold 
day  will  notice  very  little  perspiration,  but,  when  the  temperature  is 
high,  the  perspiration  will  be  very  abundant,  and  its  continual  evapora- 
tion will  aid  in  keeping  the  body  temperature  at  the  normal  level. 
When  the  excess  water  in  the  body  fluids  is  not  needed  for  temperature 
control,  it  is  eliminated  through  the  kidneys.  Thus,  if  the  water 
intake  and  body  activity  remain  uniform,  essentially  the,  same  amount 
of  water  will  be  given  off  from  the  body,  but  the  proportion  of  the 
water  that  is  excreted  through  the  skin  and  through  the  kidneys  will 


THE  BIOLOGY  OF  EXCRETION  121 

show  considerable  variation  depending  upon  environmental  conditions. 
(Fig.  61.) 

It  is  clear  that  the  amount  of  sweat  secreted  depends  upon  the 
amount  of  blood  that  is  permitted  to  flow  through  the  capillaries 
surrounding  the  millions  of  sweat  glands.  This  blood  flow  is  under 
the  control  of  the  autonomic  nervous  system  which,  in  an  endeavor  to 
conserve  the  body  heat,  constricts  the  blood  vessels  in  the  skin  by 
inciting  a  contraction  of  the  muscle  tissue  in  the  walls  of  the  vessels. 
Under  such  conditions,  comparatively  little  blood  comes  into  contact 
with  the  sweat  glands,  and  the  materials  removed  are  correspondingly 
less.  Even  under  low  temperature  conditions,  if  a  person  suddenly 
engages  in  hard  physical  labor,  the  heat  produced  in  the  muscle  tissues 
through  the  oxidation  processes  will  tend  to  raise  the  body  temperature. 
As  a  result  the  flow  of  blood  to  the  skin  will  be  augmented,  secretion  of 
sweat  will  be  correspondingly  increased,  and  the  evaporation  of  the 
latter  at  the  body  surface  will  cool  both  the  skin  tissues  and  the  blood 
flowing  through  them.  Thus  excretion  through  the  skin  is  seen  to  be 
directly  tied  up  with  an  essential  body  function;  in  the  wisdom  of  the 
body  a  waste  product  on  its  way  out  is  used  to  render  an  important 
service.'  Another  instance  of  this  has  already  been  noted  in  the  use  of 
carbon  dioxide  to  influence  the  respiratory  center  (page  80). 

EXCRETION  AND  THE  LUNGS 

The  aeration  of  the  lungs  by  breathing  supplies  oxygen  to  the  cells 
and  at  the  same  time  removes  the  waste  carbon  dioxide  and  a  relatively 
small  amount  of  water  from  the  blood.  Breathing  at  the  normal  rate, 
12  to  13  cu.  ft.  of  carbon  dioxide  leave  the  body  every  24  hours  through 
the  lungs,  together  with  some  250  cc.  of  water  in  the  form  of  vapor. 
Very  minute,  but  often  very  noticeable,  amounts  of  organic  substances 
are  also  eliminated  from  the  body  by  the  outgoing  air  current.  Ade- 
quate consideration  of  the  functional  activities  of  the  lungs  has  already 
been  given  in  the  earlier  chapter  on  Respiration  (page  77). 

EXCRETION  AND  THE  LIVER 

The  association  of  the  liver  with  excretion,  as  already  noted  in  the 
previous  chapter,  is  of  primary  importance.  Thus  the  evidence  indi- 
cates that  the  destruction  of  the  discarded  red  corpuscles  from  the 
blood  is  to  some  extent  a  function  of  the  Kupffer  cells  which  take  up 
an  abode  in  the  sinusoids  of  the  liver  where  they  are  in  a  position  to 
inspect  the  cellular  elements  floating  slowly  by  in  the  blood  stream. 
It  is  also  evident  that  the  spleen  and  the  bone  marrow  share  in  the 
dismantling  of  the  red  cells;  but  at  all  events,  the  hepatic  cells  are 


122  HUMAN  BIOLOGY 

responsible  for  the  formation  and  excretion  of  the  unique  excretory 
product,  bilirubin,  from  the  hemoglobin  (page  100).  But  of  even  more 
importance  is  the  fact  that  the  liver  cells  collect  the  end  products  of 
protein  metabolism  from  the  blood  stream  and  convert  them  into 
urea,  which  is  excreted  through  the  kidneys.  It  is  interesting  to  note 
that  the  liver  carries  the  bilirubin  through  its  own  bile  ducts  and 
deposits  it  in  the  intestine,  but  the  urea  is  secreted  into  the  blood 
stream  for  conveyance  to  the  kidneys. 

Since  protein  metabolism  is  essential  for  life,  a  great  deal  of 
attention  has  been  given  to  the  exceedingly  complex  chemistry 
associated  with  protein  utilization  in,  and  elimination  from,  the  body. 
Even  so,  the  story  is  still  far  from  complete.  Somewhat  less  than 
90  per  cent  of  the  total  nitrogen  excreted  leaves  the  body  as  urea. 
The  remainder  of  the  nitrogenous  end  products,  containing  some  10 
to  15  per  cent  of  the  nitrogen  released  daily,  is  divided  into  several 
groups  of  compounds,  notably  uric  acid,  creatinine,  hippuric  acid,  etc., 
some  of  which  are  well  known,  others  essentially  unknown.  It  can  be 
said,  however,  that  the  synthesis  of  all  the  urea  and  of  most  of  the 
other  nitrogenous  excretions  is  accomplished  by  the  liver  cells,  working 
in  association  with  various  specific  enzymes.  To  give  one  example, 
arginine,  a  split  product  of  protein,  is  hydrolized  in  the  liver  by  the 
action  of  the  enzyme,  arginase,  to  form  urea  and  ornithinc;  the  latter, 
still  containing  nitrogen,  is  subject  to  further  disruptive  enzyme 
actions  in  the  liver,  until  the  final  excretion  product,  urea,  is  formed. 

The  important  deaminization  process  in  the  liver  should  be  con- 
sidered in  connection  with  the  formation  of  urea.  It  will  be  remem- 
bered from  the  previous  discussion  that  when  an  excess  of  amino  acids 
occurs  in  the  blood,  the  liver  splits  off  the  NH2  fraction  and  converts 
the  remainder  of  the  amino  acid  molecule  into  the  carbohydrate, 
glycogen,  which  may  be  used  for  fuel  (page  57).  The  NH2  group 
split  off  from  the  amino  acid  is  further  changed  to  ammonia  (NH3) 
and  united  with  carbon  dioxide  to  form  urea  and  water,  as  shown  by 
the  equation:  2NH3  +  CO2  =  CO(NH2)2  +  H2O. 

EXCRETION  AND  THE  KIDNEYS 

The  establishment  of  the  triploblastic  condition  in  animal  organi- 
zation marks  the  advent  of  a  specialized  excretory  system  which  func- 
tions in  the  elimination  of  the  nitrogenous  cellular  wastes  from  the 
organism.  Possibly  this  new  system  is  seen  to  best  advantage  in  the 
development  of  paired  segmental  excretory  tubes,  the  nephridia,  in 
the  earthworm.  The  nephridia  lie  in  the  coelomic  cavity  near  the  ven- 
tral body  wall,  through  which  they  open  to  form  a  connection  between 


THE  BIOLOGY  OF  EXCRETION  123 

the  coelomic  cavity  and  the  exterior.  For  the  most  part,  the  cellular 
wastes  of  the  earthworm  accumulate  in  the  coelomic  fluid  which  con- 
tinuously bathes  the  tissues.  The  nephridia  are  so  constructed  that 
these  wastes  may  be  drawn  into  the  lumen  and  carried  to  the  exterior. 
Provision  is  also  made  for  the  collection  of  wastes  from  the  blood 
stream  through  the  vascularization  of  the  nephridial  walls.  (Fig.  86.) 

From  a  structural  standpoint,  the  vertebrate  excretory  system  can 
be  regarded  as  an  assemblage  of  great  numbers  of  nephridia-like 
tubules  to  form  a  pair  of  definite  excretory  organs,  the  kidneys,  which 
open  by  special  ducts  to  the  exterior.  The  study  of  the  vertebrate 
excretory  system  is  very  interesting  to  the  comparative  anatomist 
because  of  the  many  homologies  that  are  evident  in  the  different 
groups.  Three  types  of  vertebrate  kidneys  are  recognized,  namely, 
the  pronephros,  mesoncphros,  and  metaiiephros,  the  latter  type  being 
found  in  man  and  the  higher  vertebrates.  Stated  in  essence,  it  may 
be  said  that  the  functional  tubules  of  the  pronephros  open  into  the 
coelomic  cavity;  the  meson ephric  tubules  generally  lose  their  con- 
nection with  the  coelom  and  develop  one  with  the  vascular  system ;  and 
the  metanephric  tubules  are  connected  solely  with  the  vascular  system. 

Human  Kidneys. — The  human  kidneys  consist  of  a  pair  of  brown- 
ish-colored, bean-shaped  bodies.  They  measure  some  4  in.  in  length 
and  about  half  that  in  breadth  and  lie  well  forward  in  the  abdominal 
cavity,  in  contact  with  the  dorsal  body  wall,  one  on  each  side  of  the 
mid-line  as  indicated  by  the  near-by  vertebral  column.  The  long 
axis  of  each  kidney  lies  in  an  antoroposterior  direction,  that  is,  parallel 
to  the  vertebral  column,  with  the  concave  surface  (hilus)  turned 
inwards  toward  the  median  backbone.  Like  the  other  organs  in  the 
body  cavity,  the  kidneys  are  enclosed  in  transparent  capsules  of 
peritoneal  tissue.  (Plate  VIA.) 

When  cut  in  half  lengthwise,  three  distinct  areas  of  kidney  tissue 
are  noted.  Outermost  is  a  dense,  faintly  striated  area,  the  cortex, 
which  encloses  a  lighter  colored  and  more  extensive  area,  the  medulla. 
Projections  of  the  cortex  toward  the  center  serve  to  segregate  the 
medullary  tissues  into  cone-shaped  pyramids.  Each  pyramid  is 
oriented  with  its  base  in  contact  with  the  cortical  tissue.  Inwardly 
each  pyramid  terminates  in  a  pointed  secreting  portion,  the  papilla, 
which  opens  into  a  third  region,  the  pelvis.  Connecting  with  the 
pelvis  of  each  kidney  is  a  large  excretory  duct,  the  ureter,  which 
carries  the  urine  to  the  bladder.  The  latter  is  a  muscular-walled 
reservoir,  for  the  temporary  storage  of  urine,  secreted  by  the  kidneys 
and  brought  to  it  by  the  ureters.  An  unpaired  duct,  the  urethra, 
leads  from  the  bladder  to  the  external  opening.  The  ureters,  bladder, 


124 


HUMAN  BIOLOGY 


JUNCTIONAL 
TUBULE 

DISTAL  CONVOLUTED 
TUBULE 

PROXIMAL 

CONVOLUTED 

TUBULE 


COLLECTING 
TUBULE  FOR 
NUMEROUS  RENAL 
TUBULES 


DESCENDING 
LIMB 


ASCENDING 
LIMB 


LOOP  OF  HENLE 


1NTERLOBULAR  ARTERY 
1NTERLOBULAR  VEIN 

EFFERENT  ARTERY 
AFFERENT  ARTERY 
GLOMERULUSl 


BOWMAN'S     f     BODY 

CAPSULE         J 


ARCUATE  ARTERY 
(FROM  RENAL  ARTERY) 

ARCUATE  VEIN 
(FROM  RENAL  VEIN) 


ARTERIOLAE 
RECTAE  SPURIAE 


VENULAE 
RECTAE 


COLLECTING  TUBULE 

TO  PELVIS  OF  KIDNEY 

PLATE  VII. — Diagram  of  a  portion  of  the  kidney  tissue,  highly  magnified,  illustrating 
the  detailed  arrangement  of  the  functional  collecting  tubules  and  associated  blood 
vessels. 


THE  BIOLOGY  OF  EXCRETION  125 

and  urethra  do  not  function  in  the  production  of  urine;  they  are  solely 
concerned  with  its  orderly  elimination  from  the  body.  (Plate  VLB.) 

Histology  of  Kidney  Tissue. — The  functional  elements,  which  com- 
pose the  kidney  tissue,  are  microscopic  in  size  and  consist  of  an 
enormous  number  of  coiled  renal  tubules,  each  of  which,  originating 
in  the  cortex,  follows  a  tortuous  route  before  finally  joining  a  common 
collecting  tubule  running  to  an  opening  in  the  papilla  at  the  tip  of  one 
of  the  pyramids.  Each  renal  tubule  begins  in  the  cortex  with  an 
enlarged  terminal  portion,  the  Malpighian  body.  The  latter  is 
roughly  spherical  in  shape,  has  a  double  epithelial  wall,  Bowman's 
capsule,  and  contains  a  minute  knot-like  assemblage  of  thin-walled 
capillaries,  the  glomerulus,  through  which  the  blood  continuously  flows. 
The  inner  epithelial  layer  of  the  capsule  is  closely  applied  to  the 
capillary  walls.  Wastes  collected  from  the  blood  stream  during  its 
passage  through  the  glomerulus  diffuse  through  the  capillary  walls  and 
epithelial  lining  and  so  enter  the  lumen  of  the  renal  tubule,  en  route  to 
the  pelvis.  (Plate  VII.) 

But  the  Malpighian  body  with  the  glomerulus  is  not  the  only 
important  functional  part  of  a  renal  tubule.  It  has  long  been  recog- 
nized that  the  renal  tubule  itself,  particularly  the  convoluted  proximal 
portion  which  terminates  in  the  Malpighian  body,  is  not  merely  a 
duct  for  the  passage  of  the  waste  fluids  but  is  active  in  forming  the 
finished  waste  product,  urine,  secreted  by  the  kidneys.  Each  renal 
tubule  measures  about  %  in.  in  length;  but  with  a  diameter  of  only 
0.0023  in.,  it  makes  so  many  turns  and  twists  between  the  proximal 
portion  in  the  cortex  of  the  kidney  and  the  distal  end  (junctional 
tubule)  opening  through  a  papilla  into  the  pelvis  that  it  is  very  difficult, 
if  not  impossible,  to  isolate  a  single  tubule  for  direct  experimentation. 
The  walls  of  the  proximal  portions  of  the  tubules  contain  a  dense 
capillary  network.  (Plate  VII.) 

The  circulation  of  the  blood  in  the  kidneys  is  as  follows:  blood 
reaches  the  kidney  from  the  large  renal  artery.  The  latter  subdivides 
to  form  the  arcuate  and  interlobular  vessels  and,  finally,  forms  the  tiny 
afferent  arteries  which  enter  the  glomerulus  and  give  rise  to  the  capil- 
lary network.  After  the  blood  has  passed  into  and  through  the 
glomerular  capillaries,  it  flows  into  the  connecting  efferent  vessel. 
The  latter,  shortly  after  leaving  the  glomerulus,  forms  a  capillary 
network  in  the  walls  of  the  proximal  convoluted  tubule.  Thus  the 
blood,  which  has  just  passed  through  the  glomerulus  and  been  freed 
from  the  wastes,  passes  next  to  the  epithelial  cells  in  the  walls  of  the 
renal  tubules.  Blood  passing  through  the  kidney  is  conducted  away 
from  the  tubules  by  a  series  of  veins  (venulae  rectae,  interlobular,  and 


126  HUMAN  BIOLOGY 

arcuate)  leading  to  the  renal  vein.  This  circulation  of  the  blood 
through  both  the  glomeruli  and  the  proximal  portion  of  the  tubules  in 
Succession  is  very  important  because  it  permits  the  reabsorption  of 
certain  constituents  of  the  glomcrular  wastes,  as  will  be  seen  in  the 
following  section  when  the  functional  features  of  the  kidney  are 
considered. 

Kidney  Functions. — -From  the  blood  flowing  through  the  kidney 
tissues  about  50  oz.,  or,  roughly,  21 2  pt.,  of  the  composite  waste 
product  urine  is  removed  each  day  and  passed  from  the  body.  The 
amount  of  urine  excreted,  however,  is  subject  to  wide  variation,  from 
a  minimum  amount  of  approximately  1  pt.  to  as  much  as  5  pt.  As 
shown  above,  this  variation  in  urine  secretion  is  associated  with 
temperature  control:  the  more  water  leaving  the  body  through  the 
sweat  glands  the  less  will  leave  in  the  urine  (page  120).  Under 
uniform  temperature  conditions  the  amount  of  urine  excreted  can  be 
markedly  increased  by  drinking  more  water,  particularly  if  it  contains 
certain  soluble  substances,  diuretics,  which  tend  to  increase  kidney 
activity.  Examples  of  diuretics  are  found  in  various  salts  arid  such 
substances  as  digitalis,  caffeine,  and  urea.  (Fig.  62.) 

A  chemical  analysis  of  human  urine  shows  that  it  typically  con- 
sists of  approximately  96  per  cent  water;  1.8  per  cent  inorganic  salts, 
pigments,  and  obscure  nitrogen  compounds;  and  2.2  per  cent  urea, 
this  last  substance  being  the  principal  end-product  of  the  complicated 
protein  metabolism  that  began  with  the  intake  of  the  nitrogen-con- 
taining foods.  All  of  these  solids  are  carried  in  solution  in  the  water 
so  that  the  specific  gravity  of  urine  (1.020)  is  slightly  above  that  of 
pure  water.  We  have  noted  in  the  previous  chapter  that  the  chemical 
changes  necessary  to  convert  the  protein  wastes  of  the  cells  into  urea 
and  certain  other  less  known  nitrogenous  compounds  occur  in  the 
liver,  the  kidney  acting  only  in  the  removal  of  urea  from  the  blood 
(page  100) .  Urea  itself  is  a  soluble  crystalline  substance  whose  chemical 
nature  was  determined  over  a  century  ago.  It  was  the  first  organic 
substance  to  be  synthesized  in  the  laboratory.  This  was  accomplished 
in  1828  by  Wohler  from  an  inorganic  compound  containing  the  ele- 
ments carbon,  hydrogen,  oxygen,  and  nitrogen  of  which  urea  is 
composed. 

It  is  comparatively  easy  to  analyze  urine  and  find  the  percentages 
of  urea,  water,  and  other  substances  present  in  this  complex  waste 
product,  but  the  problem  of  determining  how  it  "got  that  way"  in 
the  kidneys  has  proved  to  be  very  difficult,  and  it  is  still  the  subject 
of  controversy  and  extensive  investigation  by  the  research  workers 
in  this  field.  The  questions  at  issue  center  primarily  around  the 


THE  BlOWdY  OF  EXCRETION  127 


SOURCES  OF  Loss  AND  GAIN  TO  THE  -BLOOD. 

A.  SOURCES  OF  Loss: 

I.  Loss  of  Matter. 

1.  The  lungs:  carbonic  acid  and  water  (fairly  constant). 

2.  The  kidneys:  urea,  water,  salines  (fairly  constant). 

3.  The  skin:  water,  salines  (fairly  constant). 

4.  The  tissues:  constructive  material  (variable  especially 

in  the  case  of  those  tissues  whose  activity  is  inter- 
mittent, such  as  the  muscles,  many  secreting 
glands,  &c.),  water,  &c.,  to  form  lymph. 

II.  Loss  of  Heat. 

1.  The  skin. 

2.  The  lungs. 

3.  The  excretions  by  the  kidney  and  the  alimentary 

canal. 

B.  SOURCES  OF  GAIN:— 

I.  Gain  of  Matter. 

1.  The  lungs:  oxygen  (fairly  constant). 

2.  The  alimentary  canal:  food  (variable). 

3.  The  tissues:  products  of  their  activity,  waste  matters 

(always  going  on  but  varying  according  to  the 
activity  of  the  several  tissues). 

4.  The  lymphatics:  lymph  (always  going  on  but  varying 

according  to  the  activity  of  the  several  tissues) . 

II.  Gain  of  Heat. 

1.  The  tissues  generally,  especially  the  more  active  ones, 

such  as  the  muscles. 

2.  The  blood  itself,  probably  to  a  very  small  extent. 


FIG.  62.  -Table  illustrating  the  sources  of  loss  and  gain  to  the  blood.  "One  must  be 
careful  not  to  confuse  the  losses  and  gains  of  the  blood  with  the  losses  and  gains  of  the 
body  as  a  whole.  The.  two  differ  in  much  the  same  way  as  the  internal  commerce  of  a 
country  differs  from  its  import  and  export  trade."  (Huxley — Barer  oft.} 


128  HUMAN  BIOLOGY 

determination  of  the  specific  functions  of  the  Malpighian  bodies  in 
comparison  with  those  of  the  vascularized  renal  tubules.  Does  the 
glomerulus  in  a  Malpighian  body  act  merely  as  a  mechanical  filter  in 
removing  waste  substances  from  the  blood?  Or  is  there  a  selective 
action  so  that  only  the  excess  water  and  salts  are  removed  from  the 
blood  in  the  glomeruli,  the  other  wastes  found  in  the  urine,  including 
urea,  being  added  by  the  secretory  activities  of  the  epithelial  cells  in 
the  walls  of  the  renal  tubules? 

Very  ingenious  and  difficult  experiments  have  made  it  possible  to 
remove  for  analysis  some  of  the  fluid  given  off  in  the  microscopic 
glomeruli  of  the  frog's  kidney.  Such  analyses  show  that  the  glomeru- 
lar  fluid  in  the  frog  is  basically  the  same  as  urine,  with  all  constituent 
substances  present  but  in  a  very  diluted  form.  For  example,  it  is 
found  that  the  concentration  of  urea  in  urine  is  about  one  hundred 
times  greater  than  it  is  in  the  glomerular  fluid.  Glucose,  however, 
is  present  in  the  glomerular  fluid  but  is  entirely  absent  from  normal 
urine,  and  also  various  inorganic  salts,  notably  sodium  chloride, 
present  in  the  glomerular  fluids  do  not  show  a  constant  relationship 
to  the  amounts  present  in  the  urine.  These  so-called  threshold 
substances  are  considered  below. 

It  seems  clear  from  these  experiments  that,  with  the  exception  of 
glucose,  the  glomeruli  filter  off  a  liquid  from  the  blood  that  may  be 
regarded  as  essentially  a  very  dilute  urine.  As  a  matter  of  fact,  the 
evidence  indicates  that  about  the  only  substances  carried  in  solution  in 
the  blood  that  are  not  permitted  to  pass  through  the  walls  of  the 
glomeruli  are  the  normal  blood  proteins.  This  is  a  remarkable  feature 
because  it  can  be  shown  that  when  foreign  proteins  are  present  in  the 
blood,  they  will  be  quickly  excreted  by  the  kidneys.  Finally,  experi- 
ments show  that  the  amount  of  fluid  removed  from  the  blood  by  the 
glomeruli  varies  directly  with  the  blood  pressure  and  with  the  amount 
of  blood  going  through  the  kidney,  which  indicates  that,  to  some 
extent  at  least,  the  glomeruli  act  as  mechanical  filters. 

The  glomerular  fluid  passes  from  each  glomerulus  to  the  lumen  of 
the  proximal  portion  of  the  renal  tubule,  the  walls  of  which,  it  will  be 
remembered,  are  highly  vascularized  and  the  vessels  so  connected  that 
the  blood  leaving  the  glomeruli  comes  next  to  the  tubules.  This 
arrangement  makes  it  possible  for  the  epithelial  cells  lining  the  tubules 
to  absorb  water  and  salts  from  the  glomerular  fluid  and  to  return  them 
to  the  blood  stream  in  such  quantities  as  will  keep  the  blood  plasma 
at  normal  levels,  also  under  normal  conditions  to  absorb  all  the  glucose 
and  return  it  to  the  blood  stream.  Evidence  exists  also,  in  certain 
cases,  of  a  secretory  activity  in  the  tubules.  If  such  action  occurs,  it 


THE  BIOLOGY  OF  EXCRETION  129 

means  that  the  cells  of  the  tubules  absorb  waste  substances  directly 
from  the  blood  stream  and  add  them  to  the  glomerular  fluid  for 
excretion.  Probably  both  absorption  and  secretion  of  materials  in  the 
glomerular  fluid  can  take  place  in  the  renal  tubules,  but  the  concensus 
of  opinion  is  that  the  main  function  of  the  tubules  is  absorption. 

The  fact  that  the  kidneys  can  excrete  urine,  which  may  vary  in 
the  percentage  of  water  and  solids  present,  enables  them  to  act  as  a 
regulator  of  body  fluids.  When  perspiration  is  abundant,  the  amount 
of  water  excreted  from  the  body  by  the  kidneys  is  reduced.  This  is 
due  to  the  fact  that  the  cells  in  the  kidney  tubules  absorb  water  from 
the  dilute  glomerular  fluid  and  return  it  to  the  blood  stream  for  elimi- 
nation through  the  sweat  glands  of  the  skin  as  an  aid  in  maintaining  a 
uniform  body  temperature.  If  the  external  temperature  and  the  mus- 
cular activity  combined  should  be  sufficient  to  endanger  the  normal 
water  content  of  blood  plasma  through  excessive  perspiration,  the 
kidneys  will  return  a  large  percentage  of  water  back  to  the  blood,  and 
the  small  amount  of  urine  excreted  will  be  correspondingly  concen- 
trated. Tf,  in  spite  of  profuse  perspiration,  the  body  temperature 
rises  even  slightly  above  normal  levels  under  conditions  of  excessive 
heat  and  activity,  perspiration  may  suddenly  decrease.  If  this  occurs,  the 
body  temperature  will  quickly  rise,  and  the  victim  will  be  prostrated 
by  the  heat,  the  so-called  "sunstroke"  and  a  serious  condition. 

Not  only  is  the  normal  water-plasma  relationship  of  the  blood 
maintained  by  the  selective  absorption  of  materials  from  the  glomerular 
fluid  through  the  action  of  the  kidney  tubules,  but  also  the  salt  reserve 
in  the  blood,  particularly  sodium  chloride,  is  controlled  in  the  same 
way.  Excessive  amounts  of  salt  in  the  diet  will  be  absorbed  by  the 
blood  stream  and  then  quickly  eliminated  through  the  kidneys. 
However,  the  exact  amount  of  salt  appearing  in  the  urine  at  any  time 
will  depend  upon  the  needs  of  the  blood  plasma.  Thus,  to  consider 
perspiration  once  more,  it  is  found  that  the  amount  of  salt  released 
from  the  body  in  sweat  may  be  very  large  if  perspiration  is  profuse. 
In  such  conditions,  the  cells  of  the  kidney  tubules  endeavor  to  absorb 
a  sufficient  amount  of  salt  from  the  glomerular  fluid  to  replace  that 
lost  from  the  blood  by  perspiration.  Authorities  have  recently  called 
attention  to  the  serious  depletion  of  salt  in  the  body  as  the  result  of 
excessive  perspiration.  They  have  advised  factory  employees  work- 
ing under  high-temperature  conditions  that  additional  supplies  of  salt 
should  be  taken  with  the  food  or  with  the  drinking  water.  (Fig.  61.) 

Finally,  the  glomerular  filtrate  contains  an  appreciable  amount  of 
glucose  in  solution.  None  of  this  appears  in  the  urine  excreted  from 
normal  kidneys;  it  is  all  absorbed  by  the  tubules  and  returned  to  the 


130  HUMAN  BIOLOGY 

blood  stream.  In  diabetes,  the  amount  of  sugar  in  the  blood  is  greatly 
increased,  with  the  result  that  the  glomerular  fluid  has  an  Abnormal 
sugar  content.  In  such  cases,  the  tubules  do  not  or  cannot  reabsorb 
all  of  the  sugar  found  in  the  glomerular  fluid  and  return  it  to  the  blood 
stream,  and  so  some  of  it  is  excreted  in  the  urine.  The  excess  of  sugar 
in  the  blood  of  the  diabetic  will  intensify  his  thirst  so  that  abnormal 
amounts  of  water  will  be  drunk  which  will,  in  turn,  cause  excessive 
urination.  Also,  some  of  the  excess  fluids  may  gradually  accumulate 
in  the  tissues,  thus  producing  an  edematous  condition  (page  104). 

But  the  functional  kidney  cannot  be  regarded  solely  as  a  filtering 
and  absorbing  organ,  for  it  is  well  known  that  important  chemical 
reactions,  both  synthetic  and  analytic,  occur  in  its  tissues.  One  of 
the  most  important  of  these  reactions  is  concerned  with  the  splitting 
of  urea  to  form  ammonia  and  carbon  dioxide  when  the  acidity  of  the 
urine  tends  to  become  abnormally  high.  Such  a  condition  may  occur 
when  an  excess  of  animal  tissues  is  eaten  or  when  certain  inorganic 
acids  are  taken  into  the  alimentary  tract.  The  ammonia  thus  formed 
from  urea  is  combined  with  the  acid  to  form  a  salt,  and  the  excessive 
acidity  of  the  urine  thereby  reduced.  The  reverse  of  this  process,  if 
it  occurs,  would  increase  the  acidity  of  the  urine  when  it  tended  to  be 
too  alkaline.  The  latter  condition  is  possible  when  there  is  a  great 
excess  of  plant  foods  in  the  diet.  Since,  however,  urine  may  vary 
from  a  markedly  acid  condition  (pH  4.82)  to  an  alkaline  one  (pH  7.45) 
with  a  normal  slightly  acid  condition  (pH  6)  it  is  necessary  only  that 
the  chemical  reactions  in  the  kidney  cells,  as  just  described,  be  performed 
when  the  normally  wide  limits  in  either  direction  are  passed.  The 
upshot  of  the  whole  matter  is,  of  course,  as  emphasized  above,  that 
varying  the  acidity  of  the  secreted  urine  permits  the  maintenance  of 
an  essentially  constant  (pH)  levol  in  the  blood  plasma  and  in  the  body 
tissues. 

Another  demonstrated  case  of  synthesis  by  the  kidney  cells  is  the 
formation  of  hippuric  acid,  which  is  a  prominent  constituent  of  the 
urine  of  herbivorous  animals  and  also  of  man  when  the  diet  consists 
largely  of  plant  tissues.  The  latter  contain  a  considerable  quantity  of 
benzoic  acid,  which  is  absorbed  from  the  digestive  tract  and  must  be 
excreted.  The  kidney  cells  are  able  to  combine  the  benzoic  acid  with 
glycine,  one  of  the  amino  acids,  and  thus  form  hippuric  acid  which  is 
secreted  in  the  urine. 

The  complex  and  somewhat  variable  liquid  product  of  the  kidneys, 
urine — collected  in  very  minute  amounts  by  the  individual  renal 
tubules — leaves  each  kidney  at  the  rate  of  about  one  drop  per  minute 
and  flows  through  the  ureter  to  the  bladder.  It  is  propelled  through 


THE  BIOLOGY  OF  EXCRETION  131 

the  ureter  partly  by  gravity,  especially  when  standing,  and  partly  by 
peristalsis  in  the  walls  of  the  ureter.  The  peristaltic  waves  are  stated 
to  occur  every  10  to  20  seconds  and  to  travel  toward  the  bladder  at  a 
rate  in  excess  of  }^  in.  per  minute.  Since  the  ureter  in  man  is  in 
excess  of  1  ft.  long,  it  probably  takes  from  15  to  20  minutes  for  a  drop 
of  urine  to  travel  this  distance.  It  is  possible  for  the  urologist  to 
insert  an  apparatus  with  an  electric  bulb  up  the  urethra  and  thus  to 
illuminate  the  interior  of  the  bladder  to  determine  the  normality  of  the 
urine  secretion  from  each  kidney.  (Plate  VI-4.) 

The  urine  is  retained  in  the  bladder  until  a  considerable  amount, 
about  J-2  pt.,  has  accumulated.  This  retention  is  accomplished  by  a 
muscular  valve,  the  internal  sphincter  on  the  urethra,  which  is  nor- 
mally in  a  state  of  contraction  and  so  prevents  the  release  of  the  urine 
to  the  exterior  until  desired.  The  continual  accumulation  of  the  urine 
in  the  bladder  sets  up  a  nerve  stimulation  when  it  reaches  a  certain 
point,  giving  the  sensation  of  fullness.  Considerable  latitude  is 
allowed  in  responding  to  the  sensation.  If  urination  is  voluntarily 
delayed,  the  bladder  muscles  relax  somewhat,  and  no  further  sensations 
are  noted  until  considerable  more  urine  has  accumulated,  when  the 
sensation  is  again  set  up  with  added  force.  Urination  (micturition) 
is  accomplished  by  the  contraction  of  the  muscular  tissue  in  the  bladder 
walls  synchronously  with  the  relaxation  of  the  internal  sphincter.  Also, 
additional  pressure  on  the  bladder  is  caused  by  the  contraction  of  the 
muscles  in  the  abdominal  wall,  with  the  glottis  closed.  Urination, 
though  essentially  automatic  or  reflex  in  nature,  is  also  under  volun- 
tary control,  as  evidenced  by  the  fact  that  the  process  may  be  delayed, 
stopped,  or  started. 


INNOMINATE  ARTERY. 
LEFT  CAROTID  ARTERY. 

RIGHT  PULMONARY  ARTER 

AORTA 
RIGHT  PULMONARY  VEIN 

SUPERIOR  VENA  CAVA 
CORONARY  ORIFICE 
PULMONARY  VALVE 
TRICUSP1D  VALVE 

RIGHT  AURICLE 
INFERIOR  VENA  CAVA 


LEFT  SUBCLAVIAN  ARTERY 

•ARCH  OF  AORTA 
PULMONARY  ARTERY 

•LEFT  PULMONARY  VEIN 
LEFT  AURICLE 

BICUSPID  VALVE  (MITRAL) 

•AORTIC  VALVE 
LEFT  VENTRICLE 

RIGHT  VENTRICLE 
•ABDOMINAL  AORTA 


LEFT  AURICLE 
BICUSPID  VALVE 


CHORDAE  TENDINEAE- 


PAPILLARY  MUSCLE 


HEPATIC  VEIN 

LIVER 
INFERIOR  VENA  CAVA 

HEPATIC  ARTERY. 
PORTAL  VEIN 
GASTRODUODENAL  ARTERY 

SUPERIOR  MESENTERIC  VEIN 
HEAD  OF  PANCREAS 

DUODENUM 


VALVES  IN  VEIN, 

E 


ASCENDING  COLON  — 

ILEUM 


BLADDER 


AORTA 

CORONARY  ORIFICE 
AORTIC  VALVE 

LEFT  VENTRICLE 


DIAPHRAGM 
STOMACH 

SPLEEN 

COELIAC  ARTERY 
'SPLENIC  ARTERY  AND  VEIN 
"PANCREAS 

SUPERIOR  MESENTERIC  ARTERY 
KIDNEY 

INFERIOR  MESENTERIC  VEIN 
ABDOMINAL  AORTA 
INFERIOR  MESENTERIC  ARTERY 

DESCENDING  COLON 


•RECTUM 


PLATE  VIII. — Drawings  illustrating  various  structures  associated  with  the  human 
vascular  system.  Semidiagrammatic.  A,  internal  structure  of  the  heart,  ventral  view; 
B,  chief  vessels  of  the  abdominal  viscera;  Ct  bicuspid  valve;  D,  coronary  orifice  in  aorta; 
E,  valves  in  vein. 


CHAPTER  VII 
THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM 

It  is  evident  from  the  previous  chapters  that  the  essential  functions 
of  nutrition,  respiration,  secretion,  and  excretion  in  the  highly  devel- 
oped human  organism  are  dependent  upon  the  vascular  system  for  the 
transportation  of  an  amazing  array  of  substances.  But  even  more 
important  than  this  is  the  basic  fact  that  every  cell  in  the  body  is 
dependent  upon  the  vascular  system  for  bringing  to  it  a  continuous 
supply  of  essential  materials  and  for  removing  the  cellular  wastes. 
The  vascular  system  is  equipped  to  render  this  universal  transportation 


FIG.  63. — The  Giant  Kelp,  a  marine  Alga.     (Woodruff,  after  Ganong.) 

service  in  the  body  through  an  infinitude  of  tubular  vessels,  large,  small, 
microscopic;  associated  open  spaces  in  the  tissues;  a  liquid  river  of  life, 
the  blood;  and  an  efficient  automatic  pump,  the  heart,  which  forces' 
the  blood  through  the  designated  channels. 

In  the  less  highly  differentiated  organisms  the  problems  of  trans- 
portation are  solved  in  a  much  simpler  fashion  by  the  direct  transfer 
of  materials  from  cell  to  cell.  An  aquatic  environment  is  of  great 
assistance  in  these  forms  of  life,  for,  when  the  outer  surface  of  a  cell  is 
in  contact  with  the  water,  the  interchange  of  respiratory  gase&  and 
liquid  wastes  is  greatly  aided.  Thus  in  the  Algae,  which  includes  a 
large  group  of  comparatively  simple  water  plants  in  which  the  cellular 
differentiation  is  not  marked,  plant  bodies  of  varying  size  reach  their 
climax  in  the  enormous  marine  kelps.  The  kelps  may  attain  a  length 
of  200  ft.,  with  no  special  provision  for  the  transportation  of  materials 
throughout  the  plant  body.  Furthermore,  plants  like  the  mosses, 

133 


134 


HUMAN  BIOLOGY 


which  live  outside  the  water  and  have  no  conducting  tissues,  are  always 
very  small  in  size  because  they  are  unable  to  conduct  essential  materials 
from  the  soil  more  than  a  short  distance  above  the  surface  of  the 
ground.  The  higher  types  of  soil-living  plant,  such  as  the  ferns  and 
seed  plants,  which  have  developed  specialized  vascular  tissues,  grow 
to  considerable  heights  because  of  their  ability  to  transport  essential 
materials  from  the  roots  and  leaves  to  all  regions  of  the  plant  body  as 
required.  (Figs.  63  to  65.) 

In  animals,  cellular  differentiation  is,  generally 
speaking,  more  advanced  than  in  plants.  Even  so, 
in  the  smaller,  aquatic  forms  with  a  minimum  of 


FIG.  64. 

FIG.  64. — The  plant  body  of  a  typical  moss. 
FIG.  65.-— The  plant  body  of  a  common  fern. 


FIG.  65. 

X  1.      (Woodruff,  after  Ganong.) 
X  Ko-      (Woodruff,  after  Ganong.) 


cellular  differentiation,  the  cells  of  the  organism  are  able  to 
supply  their  needs  without  the  aid  of  a  vascular  system.  Somo 
of  the  Coelentcrates  attain  considerable  size,  as,  for  example,  certain 
jellyfish,  but  the  low  degree  of  cellular  differentiation  and  the  aquatic 
environment  solve  the  transportation  problems  unaided  by  any  special 
vascular  tissues.  On  the  other  hand,  much  smaller  aquatic  animals, 
as  in  various  species  of  tropical  fish,  require  a  differentiated  vascular 
system  because  the  highly  specialized  cells  of  these  organisms  are  not 
able  to  look  after  the  transference  of  materials  from  cell  to  cell.  (Fig. 
66.) 

Variations  in  the  degree  of  development  of  the  vascular  systems 
of  the  land-dwelling  animals  are  closely  associated  with  the  need  for 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM  135 

oxygen  transportation.  Thus  the  vascular  system  of  the  relatively 
simple  earthworm  is  found  to  be  very  complete  and  highly  organized, 
whereas  in  the  much  more  advanced  insect  group  the  opposite  is  true. 
This  latter  condition  is  directly  associated  with  the  ability  of  the 
insects  to  transport  oxygen  to  the  cells  by  a  unique  system  of  air  tubes. 
The  latter  form  a  remarkably  complete  network  throughout  the  tissues, 
and  thus  the  oxygen  reaches  the  cells  without  the  intervention  of  the 
blood  stream.  Accordingly  the  insect  vascular  system,  relieved  of  the 
basic  duty  of  carrying  a  constant  supply  of  oxygen  to  the  cells,  does 
not  attain  the  high  estate  characteristic  of  less 
differentiated  animal  types  in  which  this  duty  is 
paramount.  (Fig.  40.) 

STRUCTURAL  FEATURES  ASSOCIATED  WITH  THE 
VASCULAR  SYSTEM 

The  vertebrate  system  consists  of  (1)  a  network  of 
tubular  vessels  associated  with  (2)  open  channels  and 
tissue  spaces  through  which  (3)  a  liquid  medium,  the 
blood,  circulates  throughout  the  body  by  the  action  of 
the  pump-like  heart.  The  vessels  of  the  body  are  FlG- .  ??*T~^ 

.  .  .  marine   jellyfish, 

divided  into  arteries,  which  carry  blood  away  from  the     some  species  of 
heart;  veins,  which  carry  blood  to  the  heart;  capillaries,     which   attain   a 

'  }  J  >       t  9       diameter  of  three 

which  connect  the  arteries  and  veins  in  the  tissues  feet  or  more, 
through  an  elaborate  system  of  microscopic  tubes;  (Hcgner.) 
and  the  heart,  which  is  a  highly  modified  blood  vessel  adapted  for 
pumping  blood.  Blood  leaving  the  heart  must  make  a  complete 
circuit  involving  a  connected  system  of  arteries,  capillaries,  and  veins 
before  it  again  reaches  the  heart.  In  addition  to  the  closed  tubular 
system,  there  is  an  open  vascular  system  consisting  of  various-sized 
spaces,  or  sinuses,  in  the  tissues,  together  with  definite  lymph  vessels 
having  extremely  thin  walls,  through  which  a  fluid  derivative  of  the 
blood,  the  lymph,  slowly  moves.  Lymph  is  the  circulating  medium 
that  comes  into  contact  with  the  individual  cells  so  that  this  tissue 
fluid  stands  as  the  final  agent  in  the  actual  transfer  of  materials  to  and 
from  the  cells.  However,  in  the  sinusoids  of  the  liver  and  certain 
other  organs  the  blood,  rather  than  the  lymph,  is  in  direct  contact 
with  the  individual  cells.  (Fig.  54.) 

BLOOD 

Blood  Plasma. — Blood  may  be  regarded  as  a  liquid  tissue,  that  is, 
a  tissue  in  which  the  intercellular  material  is  liquid  rather  than  solid. 
In  the  other  tissues  of  the  body,  the  intercellular  material  is  more  or 


136 


HUMAN  BIOLOGY 


less  solid  and  therefore  holds  the  cells  rigidly  in  place.  The  liquid 
intercellular  material  of  blood,  the  blood  plasma,  is  a  highly  complex 
medium  and  marvelously  adapted  for  the  transportation  of  the  essen- 
tial materials.  Since  the  blood  plasma  is  continually  receiving  an 
almost  infinite  variety  of  materials  from  the  cells,  its  exact  chemical 
composition  varies  continuously,  and  so  an  exact  analysis  can  never 
be  obtained.  In  addition,  blood  plasma  is  equipped  with  a  very 
important  mechanism  that  results  in  the  coagulation,  or  clotting,  of 
blood  when  necessary  to  stop  bleeding.  Finally,  floating  in  the 
plasma  are  enormous  numbers  of  very  highly  specialized  blood  cells, 
the  red  and  white  corpuscles,  which  are  important  agents  in  various 
blood  functions  as  will  be  indicated  later. 

Blood  plasma,  since  it  has  a  number  of  solids  dissolved  in  it,  is  not 
only  figuratively  but  literally  thicker  than  water,  with  a  specific  gravity 
around  1.005.  Of  the  total  weight  of  the  body,  it  is  estimated  that 
from  5  to  7.5  per  cent  consists  of  blood.  In  the  adult  weighing  150  to 
160  lb.,  this  would  mean  in  the  neighborhood  of  6  qt.  of  blood.  Of  this 
total,  it  is  believed  that  approximately  one-fourth  is  normally  present 
in  the  lungs,  major  blood  vessels,  and  heart;  one-fourth  in  the  liver; 
one-fourth  in  the  voluntary  muscles;  and  the  remainder  in  the  various 
other  organs  and  tissues  of  the  body.  If  normal  blood  is  centrifuged, 
it  will  be  found  that  the  cellular  elements  will  be  separated  from  the 
plasma  and  thrown  to  the  bottom  of  the  centrifuge  tube.  This  shows, 
of  course,  that  they  are  heavier  than  the  plasma.  If  the  relative 
amounts  of  plasma  and  blood  cells  are  determined  after  centrifuging, 
it  will  be  found  that  the  plasma  constitutes  about  54  per  cent,  and  the 
cells  46  per  cent  of  the  whole  blood. 

Blood  plasma  consists  of  about  92  per  cent  water  and  8  per  cent 
solids  in  solution;  and,  in  addition  in  every  100  parts  of  plasma  is 
dissolved  between  60  and  70  parts  of  the  gases,  oxygen,  carbon  dioxide, 
and  nitrogen.  The  exact  amount  of  the  dissolved  gases  varies  in 
different  animals  and  in  different  individuals,  and  also  a  marked 
difference  exists  between  the  venous  blood  returned  to  the  lungs  from 
the  tissues  and  the  arterial  blood  sent  from  the  lungs  after  aeration. 
It  should  be  understood  that,  generally  speaking,  the  blood  gases  are 
carried  in  chemical  compounds  (page  160).  An  approximate  compar- 


Blood 

Oxygen, 
per  cent 

Carbon  dioxide, 
per  cent 

Nitrogen, 
per  cent 

Arterial  

19.4 

49.7 

1.6 

Venous  

14.0 

54.6 

1.6 

THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM  137 

ison  between  arterial  and  venous  blood  gases  is  given  in  the  table 
on  page  136. 

The  inert  nitrogen,  it  will  be  noted,  goes  through  the  tissues 
unchanged,  but  the  oxygen  is  decreased  and  the  carbon  dioxide 
increased  in  the  plasma  as  the  blood  moves  through  the  body  tissues. 
This  is,  of  course,  due  to  the  demands  of  the  cells  for  respiratory 
processes,  as  has  been  noted  in  a  previous  chapter  (page  79). 

The  complexity  of  blood  plasma  is  due  in  part  to  the  many  products 
associated  with  cellular  activities  that  it  carries  in  solution  and  in 
part  to  certain  characteristic  compounds  of  its  own.  These  latter 
consist  of  three  soluble  blood  proteins  designated  as  fibrinogen,  serum- 
globulin,  and  serum-albumin  which,  together,  total  around  7  per  cent 
of  the  solids.  Fibrinogen  is  directly  concerned  with  the  coagulation  of 
blood.  During  this  process,  fibrinogen  is  changed  from  a  soluble  to 
an  insoluble  protein  which  precipitates  in  the  plasma  as  needle-shaped 
crystals  (Fig.  82).  When  the  plasma  clots,  a  liquid,  the  blood  serum, 
is  gradually  squeezed  out;  it  will  not  clot  again,  as  the  fibrinogen  has 
been  used  up.  The  serum-globulin  and  serum-albumin  are  typical 
natural  proteins  and,  like  the  proteins  in  general,  so  complex  that  the 
molecular  structure  has  never  been  established.  The  chemical  analysis 
of  serum-albumin  shows  that  it  consists  of  approximately  53  per  cent 
carbon,  7  per  cent  hydrogen,  16  per  cent  nitrogen,  2  per  cent  sulphur, 
and  22  per  cent  oxygen,  with  the  formula  given  as  CysHm^oSC^. 
The  function  of  fibrinogen,  as  noted,  has  been  fully  established,  but  the 
functions  of  the  other  two  blood  proteins  are  still  obscure.  For  one 
thing,  it  is  certain  that  they  are  responsible  for  the  osmotic  pressure 
that  tends  to  draw  water  into  the  blood  vessels.  Their  great  impor- 
tance is  indicated  experimentally  by  the  fact  that,  when  the  normal 
amounts  of  blood  proteins  present  in  the  blood  plasma  are  reduced 
experimentally,  a  rapid  restoration  to  normal  takes  place  as  quickly 
as  possible. 

Cells  of  the  Blood. — The  blood  corpuscles  comprise  two  main 
types  of  living  cells;  the  red  corpuscles,  or  ery throcy tes ;  and  the  white 
corpuscles,  or  leucocytes.  The  former  are  so  named  because  they 
contain  the  red  respiratory  pigment  hemoglobin,  which  is  necessary 
for  the  transportation  of  oxygen  from  the  lungs  to  the  tissue  cells,  as 
described  in  the  earlier  section  on  Respiration.  The  human  erythro- 
cytes  may  be  described  as  tiny,  biconcave  discs  with  a  diameter  of 
about  0.0003  in.  and  not  over  one-fourth  of  this  in  thickness.  The 
mature  ery  throcy  te  does  not  show  a  differentiation  into  nucleus  and 
cytoplasm.  From  the  dimensions  given,  it  can  be  calculated  that 
more  than  10  million  of  the  red  cells  can  be  placed  side  by  side  in  1  sq. 


138  HUMAN  BIOLOGY 

in.  of  space.  In  blood  freshly  drawn  from  the  vessels,  there  is  a 
tendency  for  the  corpuscles  to  adhere  to  each  other  to  form  long  rolls 
(rouleaux)  like  stacks  of  coins.  Microscopic  observations  on  corpus- 
cles flowing  through  tiny  capillaries,  for  example,  in  the  web  of  the 
frog's  foot,  show  that  the  cytoplasmic  body  of  the  erythrocyte  is  soft 
and  flexible,  so  that  the  normal  shape  is  easily  modified  when  necessary 
in  order  to  pass  through  a  tiny  capillary.  Blood  contains  an  enormous 
number  of  erythrocytes  at  all  times.  In  the  male  there  are  normally 
about  5,000,000  per  cubic  millimeter  of  blood  and  about  10  per  cent 
less  or  4,500,000  per  cubic  millimeter,  in  the  female.  (Fig.  67.) 

The  erythrocyte  consists  of  a  ground  substance,  or  stroma,  in 
which  the  hemoglobin  is  suspended;  the  latter  probably  comprises 
about  35  per  cent  of  the  total  weight.  The  hemoglobin  may  be 
separated  from  the  stroma  and  drawn  into  the  plasma  by  hemolytic 


E 
E' 

FIG.  67.—  Sketches  showing  various  typos  of  blood  colls.  E,  rod  blood  cell  (ery- 
throcyte); E'  ,  stacking  of  red  blood  cells  (rouleaux);  A,  B,  and  C,  types  of  white  blood 
cells  (leucocytes).  (Watkeys, 


action,  and  then  the  stroma  is  soon  as  a  colorless  body.  Hemolysis 
occurs  when  the  membranes  of  the  red  cells  are  ruptured  by  mechanical 
means,  such  as  freezing  or  thawing,  or  when  they  are  placed  in  a 
hypotonic  liquid,  such  as  distilled  water.  The  latter  gradually  pene- 
trates the  membrane  and  diffuses  through  the  cytoplasm.  Finally, 
the  membrane  ruptures  from  the  increased  internal  pressure,  and  then 
the  enclosed  hemoglobin  flows  out  into  the  surrounding  liquid.  If  red 
cells  are  placed  in  a  hypertonic  medium,  the  reverse  process  will  occur 
with  a  movement  of  liquid  from  the  cell  into  the  surrounding  medium. 
In  this  condition  the  cell  membrane  will  become  wrinkled  (crenation) 
as  more  and  more  of  the  fluids  leave  the  cell  body.  If  the  corpuscles 
are  put  into  an  isotonic  fluid,  that  is,  one  that  has  the  same  osmotic 
pressure  as  normal  blood  plasma,  no  abnormal  loss  of  materials  occurs 
through  the  cell  wall,  and  the  cells  will  remain  unchanged.  It  is 
sometimes  necessary,  following  accident  or  disease,  to  inject  isotonic 
fluids  into  the  tissues  or  even  directly  into  the  blood  stream.  Two 
important  isotonic  fluids  are  0.9  per  cent  sodium  chloride  and  5  per 
'cent  dextrosg. 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM 


139 


The  leucocytes  are  colorless  cells  and,  for  the  most  part,  amoeboid. 
It  is  clear  that  one  of  their  chief  functions  is  to  ingest,  or  phagocytize, 
invading  parasites  and  thus  control  infection.  Many  other  functions 
have  been  suggested,  but  the  evidence  is  not  conclusive.  The  normal 
number  of  leucocytes  in  the  blood  is  much  less  than  the  number  of 
red  cells,  and  it  is  subject  to  wider  variation.  The  number  commonly 
found  varies  from  6,000  to  10,000  per  cubic  millimeter.  Thus  only 
one  leucocyte  is  present  in  the  blood  for  some  500  to  800  red  cells. 
Morphological  studies  on  the  leucocytes,  based  largely  on  staining 
reactions,  show  that  there  are  several  different  types.  For  our  pur- 
poses, we  may  recognize  two  main  groups ;  the  nongranular  leucocytes 
and  the  granular  leucocytes.  The  latter  comprise  about  75  per  cent 
of  all  leucocytes  and  are  characterized  by  a  granular  cytoplasm  and  a 
nucleus  consisting  of  several  distinct  lobes.  They  are  very  important 
in  the  control  of  invading  microorganisms  because  they  are  actively 
phagocytic.  Mention  should  also  be  made  of  another  cellular  element, 
the  blood  platelets,  which  are  known  to 
be  an  important  factor  in  the  coagulation 
of  blood.  The  platelets  are  extraor- 
dinarily minute,  measuring  only  about 
0.00012  in.  in  diameter,  but  they  are 
considerably  more  numerous  than  the 
white  cells,  approximately  250,000  per 
cubic  millimeter. 

Blood  Counts. — For  the  diagnosis  of 
various  diseases,  the  blood  count  gives 
important  evidence.  This  is  accom- 
plished by  securing  a  small  amount  of 
blood  from  the  patient,  diluting  it  with 
isotonic  salt  solution,  and  then  placing  a 
measured  amount  of  the  diluted  blood  in 
a  counting  chamber  for  microscopic 
observation.  The  counting  chamber  is 
divided  into  numerous  standard  units  so 
that,  by  counting  the  number  of  blood  cells  in  several  units  and  averag- 
ing the  separate  counts,  a  fairly  accurate  determination  of  the  number 
can  be  made.  Thus,  in,the  case  of  anemia,  the  number  of  red  cells  may 
be  abnormally  low,  or  the  abnormal  condition  may  be  due  to  a  lack  of 
hemoglobin  in  the  individual  cells.  Both  conditions  can  be  determined 
with  considerable  accuracy.  On  the  other  hand,  the  number  of  red 
cells  may  be  greatly  increased  without  harm.  People  who  live  in  high 
altitudes  normally  have  increased  numbers  of  the  red  cells  to  compen- 


FIG.  68. — Diagram  illustrat- 
ing methods  used  in  blood  counts. 
A  special  ruled  slide  for  micro- 
scopic observation  is  used.  Cells 
are  counted  in  the  sixteen  squares 
enclosed  within  the  triple  lines: 
an  area  which  represents  1 /200th 
cubic  millimeters  of  blood. 
(Smith, "  Exploring  Biology,'1  Har- 
court,  Brace  &  Company,  Inc.) 


140 


HUMAN  BIOLOGY 


sate  for  the  reduced  oxygen  pressure.  A  temporary  increase  in  the 
count  usually  follows  vigorous  exercise.  Concerned  with  the  increase  in 
the  red  cells  is  the  red  marrow  of  the  bones  in  which  they  are  developed 
and  the  spleen  where  they  are  temporarily  stored  until  needed.  In  the 
case  of  severe  bleeding  following  injury,  tremendous  quantities  of 
these  oxygen  carriers  may  be  quickly  supplied  from  the  reserves  in 
the  spleen.  (Fig.  68;  page  212.) 

The  white  blood  cell  count  is  very  important  in  diagnosing  the 
development  of  an  infection  in  the  body  tissues.  If  the  infection  is 
severe,  the  increase  in  the  white  blood  cells,  as  revealed  in  successive 
counts  extending  over  several  hours,  may  be  very  marked,  and,  in 
general,  the  severity  of  an  infection  can  be  gaged  by  the  variations  in 
the  white  blood  cell  count  from  normal  levels.  Under  certain  condi- 
tions, it  may  be  necessary  to  make  a  differential  blood  count  in  order 
to  determine  the  relative  numbers  of  the  various  types  of  white  blood 
cell  present. 

BLOOD  CHANNELS 

*  The  Heart. — With  the  nature  of  the  circulating  fluid  in  mind, 
attention  may  next  be  directed  to  the  main  features  of  the  tubular 


U^AI  vw  ancC,. 
mccmraal 

FIG.  69. — Diagrams  illustrating  internal  structure  of  "various  types  of  vertebrate 
hearts  from  the  two-chambered  (fish)  to  the  four-chambered  (bird  and  mammal),  a, 
auricle;  sv,  sinus  veiiosus;  v,  ventricle.  (Hunter,  Walter,  and  Hunter,  "Biology,"  Amer- 
ican Book  Company.) 

channels  through  which  blood  passes  in  its  continuous  and  rapid 
circulation  through  the  body.  The  center  of  this  distributing  mecha- 
nism is  a  highly  modified  blood  vessel,  the  heart,  which  from  the  earliest 
stages  of  embryonic  development  to  the  last  moment  of  life  maintains 
a  continuous  rhythmic  beat  and  thus  forces  the  essential  life  fluid  to 
all  parts  of  the  body  so  that  every  cell  may  carry  on  the  essential 
interchange  of  materials. 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM  141 

The  heart  in  the  various  classes  of  vertebrates  shows  considerable 
anatomical  variation.  In  the  lowest  fish-like  forms,  it  is  essentially 
a  portion  of  the  venous  system  consisting  of  a  receiving  chamber,  the 
auricle,  and  a  pumping  chamber,  the  ventricle.  Blood  pours  into  the 
auricle  from  all  the  body  tissues  and  is  passed  into  the  ventricle.  The 
contraction  of  the  latter  drives  the  blood  with  sufficient  power  to 
force  it  through  the  gills  for  oxygenation,  then  through  the  body  tissues 
and,  finally,  to  return  it  once  more  to  the  auricle.  In  a  somewhat 
higher  air-breathing  vertebrate,  like  the  frog,  the  heart  seems  to  be 
in  a  transitional,  and  possibly  rather  unsatisfactory,  stage  with  three 
chambers:  right  auricle,  left  auricle,  and  common  ventricle.  There 
is  the  possibility  in  the  three-chambered  heart  of  mixing,  in  the  ventri- 
cle, the  venous  body  blood  and  the  freshly  oxygenated  blood  returned 
to  the  left  auricle  from  the  lungs  and  skin.  As  a  matter  of  fact,  the 
internal  structural  arrangements  in  the  frog's  heart  reduce  the  possi- 
bility of  mixing  arterial  and  venous  blood  to  a  minimum.  (Fig.  69.) 

Anatomically  the  human  heart  does  not  bulk  very  large  in  the  total 
weight  of  the  body.  Nor  is  it  very  impressive  when  it  is  dissected 
and  found  to  consist  largely  of  what  appears  to  be  a  rather  primitive 
type  of  muscular  tissue,  the  cardiac  tissue,  associated  with  some 
flabby  looking  valves  and  a  minimum  of  connective  tissue  elements 
for  support.  Further  dissection,  however,  reveals  the  presence  of  an 
elaborate,  but  obscure,  neuromuscular  apparatus  for  general  control 
of  the  cardiac  tissues.  The  apparent  simplicity  of  heart  structure 
gives  no  indication  of  the  amazing  functional  ability  that  this  organ 
possesses,  for  it  is  one  of  the  most  noteworthy  and  efficient  organs  to  be 
found  in  the  entire  range  of  protoplasmic  organization.  The  ability 
to  contract  rhythmically  and  continuously — even  a  moment's  cessa- 
tion spells  unconsciousness — throughout  the  life  span  is  an  inherent 
property  of  the  cardiac  muscle  tissue.  Rhythmic  contraction  is  ini- 
tiated in  the  early  embryo  without  the  aid  of  the  nervous  system,  and  it 
remains  basically  independent  throughout  life,  though  the  rate  of  con- 
traction may  be  varied  within  certain  limits  in  accordance  with  bodily 
needs  and  conditions.  (Fig.  79.) 

In  the  four-chambered  heart,  present  in  man  and  the  higher 
vertebrates,  are  two  blood-receiving  chambers  (right  and  left  auricles) 
and  two  blood-dispatching  chambers  (right  and  left  ventricles)  with 
a  complete  separation  between  the  right  and  left  sides.  Thus  the 
blood,  to  get  from  the  left  side  of  the  heart  to  the  right  side,  must 
leave  by  way  of  the  aorta  from  the  left  ventricle,  pass  through  the 
capillary  network  in  some  body  tissue,  and  finally  return  to  the  right 
auricle  through  the  venous  system.  In  passing  from  the  right  to  the 


142 


HUMAN  BIOLOGY 


left  side  of  the  heart,  the  blood  must  leave  by  way  of  the  pulmonary 
artery  from  the  right  ventricle,  pass  through  the  capillaries  in  the 
lungs,  and  return  to  the  left  auricle  through  the  pulmonary  veins. 
(Fig.  70.) 

Possibly  one  of  the  most  striking  examples  of  widely  different  hearts 
may  be  found  in  comparing  the  heart  of  the  elephant  with  that  of  the 
tiny  hummingbird.  The  heart  of  the  elephant  weighs  48^  lb.,  and 
beats  only  a  few  times  per  minute,  whereas  that  of  the  humming  bird 
weighs  0.01  oz.  and  beats  at  the  amazing  rate  of  some  2,000  times  per 
minute.  Large  or  small,  fast  or  slow,  the  heart  never  deviates  from 
its  sole  function — driving  blood  through  the  branches  of  the  vascular 
system,  near  and  far,  so  that  the  needs  of  the  individual  cells  may  be 
supplied  from  the  blood  stream. 


THYROID  CARTILAGE 


CRICOID  CARTILAGE. 
TRACHEA 


•EPIGLOTTIS 
HYOID  BONE 


VENA  CAVA  (SUR 
PULMONARY  ARTERY(RIGHT) 
PULMONARY  VEIN  (RIGHT) 

PLEURA  (PARIETAL) 
PLEURA  (VISCERAL) 

DIAPHRAGM 
VENA  CAVA  (INF.) 


AORTA 


PULMONARY  ARTERY  (  LEFT) 
PULMONARY  VEIN  (LEFT) 

I 

HEART 


INTERCOSTAL  MUSCLE 
ESOPHAGUS 


ABDOMINAL  AORTA 


FIG.  70.  —  Drawing  showing  the  general  position  and  structure  of  the  heart  and  the  course 
of  the  blood  in  the  main  vessels. 

The  human  heart  is  conical  in  shape  and  about  the  same  size  as 
the  closed  fist.  The  average  measurements,  therefore,  are  around 
4.75  in.  long  by  3.5  in.  wide.  The  four  chambers  are  about  equal  in 
their  liquid  capacity  which  amounts  to  about  5  cu.  in.  The  heart 
lies  between  the  right  and  left  lungs  so  that  it  is  not  seen,  when  the 
ventral  wall  of  the  chest  is  removed,  until  the  lungs  are  pulled  aside. 
In  its  normal  position,  the  base  of  the  heart,  to  which  the  large  con- 
necting vessels  are  attached,  lies  roughly  beneath  the  median  sternum 
of  the  chest  wall,  and  the  apex  of  the  heart  is  situated  below  and  well 
to  the  left  side  of  the  thorax,  ending  between  the  fifth  and  sixth  ribs. 
The  relative  positions  (but  not  the  relative  sizes)  of  the  large  blood 
vessels  which  connect  the  heart  at  the  base,  as  seen  from  a  ventral 
view,  may  be  visualized  by  comparing  them  with  the  digits  of  the 
right  'hand,  partially  closed,  palm  down.  In  this  comparison  of 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM 


143 


position,  the  thumb  and  first  finger  are  comparable  to  the  inferior  and 
superior  venae  cavac,  respectively;  the  second  finger,  to  the  aorta;  the 
third  finger,  to  the  pulmohary  artery;  and  the  fourth  finger  to  the 
pulmonary  veins  returning  the  blood  to  the  left  auricle.  (Fig.  71.) 

Histologically,  the  heart  is  largely  composed  of  special  cardiac 
muscle  tissue.  But  there  is  a  basic;  three-layered  arrangement  of  the 
heart  tissues,  as  in  the  case  in  the  arteries  and  veins.  The  lining,  or 
endocardium,  of  the  heart  is  a  thin  layer  of  endothelium  which  is 
continuous  with  the  lining  of  the  other  blood  vessels,  as  noted  below. 
External  to  the  endocardium  is  the  middle  layer,  or  myocardium. 
The  latter  is  by  far  the  thickest  layer  in  the  heart  wall  and  consists 


INNOMINATE  ARTERY. 
LEFT  CAROTID  ARTERY- 


RIGHT  PULMONARY  ARTERY- 
AORTA 
RIGHT  PULMONARY  VEIN 

SUPERIOR  VENA  CAVA 
CORONARY  ORIFICE 
PULMONARY  VALVE 
TRICUSP1D  VALVE 

RIGHT  AURICLE 
INFERIOR  VENA  CAVA 


LEFT  SUBCLAVIAN  ART, 

•ARCH  OF  AORTA     • 
PULMONARY  ARTERY 

LEFT  PULMONARY  VEIN 
LEFT  AURICLE 

BICUSPID  VALVE 
AORTIC  VALVE 

LEFT  VENTRICLE 


RIGHT  VENTRICLE 


•ABDOMINAL  AORTA 


FIG.  71. — -Drawing  illustrating  the  internal  anatomy  of  the  human  heart.  Arrows 
indicate  the  direction  of  the  blood  flow  into,  through,  and  out  of  the  various  chambers 
of  the  heart. 

of  cardiac  muscle  tissue  with  a  minimum  amount  of  connective  tissue 
for  the  support  and  attachment  of  muscle  fibers.  The  third  layer,  the 
epicardium,  encloses  the  heart.  The  epicardium  is  really  double- 
walled;  the  inner  layer,  lying  next  to  the  myocardium,  is  separated 
from  the  outer  layer  by  the  pericardial  space  which  contains  pcri- 
cardial  fluid.  (Fig.  12.) 

Between  the  auricle  and  the  ventricle  on  each  side  of  the  heart  is 
an  important  valve.  On  the  right  side  of  the  heart,  the  valve  is 
known  as  the  tricuspid  and,  on  the  left  side,  as  the  bicuspid.  Both 
valves  have  essentially  the  same  construction  and  consist  of  a  sheet  of 
connective  tissue,  covered  above  and  below  with  endocardium,  and 
lying  between  the  auricle  and  ventricle  at  right  angles  to  the  longi- 
tudinal axis  of  the  heart  wall.  This  transverse  sheet  of  valvular  tissue 
may  be  thought  of  as  being  perforated  in  the  center  so  that  the  flaps 
hang  down  into  the  ventricle  when  the  valve  is  open.  The  tip  of  each 


144 


HUMAN  BIOLOGY 


flap  is  attached  to  ligamentous  threads  which  are  connected  with  the 
ventricular  wall.  When  the  auricle  contracts,  the  flaps  are  relaxed  and 
hang  down  into  the  cavity  of  the  ventricle,  thus  leaving  a  central 
opening  for  the  passage  of  blood  from  auricle  into  ventricle;  when 
the  ventricle  contracts,  the  flaps  are  pushed  up  toward  the  auricle  by 
the  blood  pressure  until  the  opening  is  closed.  They  are  kept  in  the 
proper  position  to  close  the  opening  by  the  attached  ligaments.  (Figs. 
72,  77.) 

Arteries. — The  arteries  are  strong,  muscular  walled  tubes  with  a 
great  deal  of  elasticity.  The  latter  quality  exists  largely  as  a  result 
of  an  abundant  supply  of  elastic  tissue  intermingled  in  the  cardiac 
tissue  of  the  walls  (page  27).  The  arteries  need  to  be  strong  because 
they  are  continually  under  pressure  from  the  blood  which  is  forced 

LEFT  AURICLE 
BICUSPID  VALVE 


FIG.  72- 


CHORDAETENDINEAE 


PAPILLARY  MUSCLE 


-Drawing  illustrating  the  finer  structure  of  the  heart  valves. 

and  77. 


Cf.  Figs.  71 


out  by  the  contraction  of  the  ventricles,  and  they  need  to  be  elastic 
to  compensate  in  some  degree  for  the  additional  blood  forced  into 
them  by  the  heart  action,  so  that  a  uniform  blood  pressure  may  be 
maintained.  Expanding  with  the  heart  contraction,  the  arteries 
gradually  contract  during  diastole.  The  result  is  that  an  essentially 
uniform  flow  of  blood  is  maintained  instead  of  a  decidedly  irregular 
one.  Even  so,  it  is  always  possible  to  detect  the  additional  blood  in 
an  artery,  following  each  ventricular  contraction,  by  the  wave  of 
expansion  that,  beginning  in  the  aorta,  rapidly  moves  along  the  arterial 
network.  This  arterial  expansion  is  spoken  of  as  the  pulse;  and  when 
the  artery  is  located  near  the  surface,  as  in  the  wrist,  it  can  be  easily 
seen  or  felt.  The  pulse  is  strongest  in  the  aorta  and  gets  correspond- 
ingly weaker  as  the  blood  moves  into  the  smaller  peripheral  arteries. 
In  the  capillaries  and  veins  no  pulse  can  be  detected.  (Fig.  73.) 

Arteries  may  be  roughly  separated  into  three  groups  on  the  basis 
of  their  size,  as  large,  medium,  and  small.  Each  of  these  groups  will 
reveal  certain  structural  characteristics  when  examined  microscopi- 
cally, but  they  will  all  show  a  basic  three-layered  arrangement  of  the 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM 


145 


tissues  in  the  arterial  wall.     Always  forming  the  lining  of  the  arteries, 
as  well  as  every  type  of  blood  vessel,  is  the  thin  endothelium,  which  is  a 


.Connec//Ve 


specialized  type  of  mesodermal  epithe- 
lium characteristic  of  the  vascular  sys- 
tem. The  endothelium  is  separated  by 
elastic  tissue  elements  from  the  much 
thicker  middle  layer,  which  is  composed 
of  smooth  muscle  fibers  intermingled 
with  elastic  tissue  elements.  The 
muscle  tissue  of  the  middle  layer  is 
markedly  decreased  in  the  large  vessels 
like  the  aorta  and  replaced  by  addi- 
tional elastic  tissue  to  permit  more 
expansion  when  large  amounts  of  blood 
are  received  from  the  heart.  The  third 
and  outermost  layer  of  the  arterial  wall 
consists  largely  of  connective  tissue, 
both  fibrous  and  elastic  elements  being 
present.  The  size,  strength,  and  elas- 
ticity of  the  arteries  decrease  as  the 
distance  from  the  heart  increases.  Ac- 
cordingly, the  two  main  arteries  are  the 
pulmonary  artery,  which  carries  all  the 
blood  from  the  right  ventricle  of 
the  heart  to  the  lungs,  and  the  aorta,  which  carries  all  the  blood 
from  the  left  ventricle  and  distributes  it  by  connecting  arteries  to 
every  part  of  the  body  except  the  lungs.  The  aorta,  in  particular, 
should  be  regarded  as  the  "No.  1"  blood  vessel 
of  the  body,  since  its  walls  are  the  strongest  and 
most  elastic  in  order  to  handle  the  blood  pumped 
from  the  powerful  left  ventricle. 

Veins. — Comparing  arteries  with  veins  of 
comparable  size  will  show  that  the  latter,  though 
built  on  the  same  three-layered  plan,  are  not  so 
strongly  built  as  arteries  and  have  relatively  less 
muscle  and  elastic  tissue  but  more  fibrous  con- 
nective tissue.  Another  noteworthy  difference  is 
found  in  the  valves,  formed  as  tiny  flaps  in  the 
walls  of  the  veins,  which  prevent  the  backflow  of 
blood.  The  lighter  construction  of  the  veins 
appears  reasonable  from  two  standpoints.  In  the  first  place, 
the  veins  receive  the  blood  at  low  pressure  in  a  regular  slow- 


A  capillary 


FIQ.  73. — The  structure  of  the 
walls  of  an  artery,  vein,  and  capil- 
lary.    Diagrammatic.     (Hunter, 
Walter,    and    Hunter,     "Biology" 
American  Book  Company.) 


VALVES  IH  VEIN 

FIG.  74. — Internal 
structure  of  the  vein 
fco  show  the  valves. 


146 


HUMAN  BIOLOGY 


flowing  stream  after  it  has  passed  through  the  capillary  network. 
Consequently  the  venous  walls  do  not  need  to  be  so  heavy  or  so  elastic 
as  the  arteries  so  long  as  they  are  large  enough  to  accommodate  the 
incoming  blood.  Also,  the  veins  do  not  need  the  well-developed 
muscle  layer  because  they  have  nothing  to  do  with  the  regulation  of 
the  amount  of  blood  flowing  to  a  particular  organ  or  region  of  the 
body.  This  is  controlled  by  elements  of  the  nervous  system  working 
through  the  muscle  tissue  of  the  arterial  walls.  As  the  veins  approach 
the  heart,  they  increase  in  size  and  strength,  just  as  do  the  arteries,  in 
accordance  with  the  amount  of  blood  to  be  carried.  (Figs.  73,  74.) 

The  chief  veins  of  the  body  include  the  superior  vena  cava,  through 
which  the  blood  from  the  head  region  is  returned  to  the  heart,  and 
the  inferior  vena  cava,  which  performs  the  same  function  for  the  blood 
returning  from  all  the  other  regions  of  the  body  except  the  lungs. 
The  pulmonary  veins  return  the  aerated  blood  from  the  lungs  to  the 
left  auricle.  Another  very  important  vessel  is  the  portal  vein  through 
which  all  the  blood  from  the  alimentary  tract  is  carried  to  the  liver. 
This  blood,  after  passing  through  the  liver,  is  received  by  the  hepatic 
vein  for  transfer  to  the  inferior  vena  cava  noted  above.  (Figs.  75,  81.) 


HEPATIC  VEIN 

LIVER 
INFERIOR  VENA  CAVA 

HEPATfC  ARTERY. 
PORTAL  VEIN 
GASTRODUODENAL  ARTERY 

SUPERIOR MESENTERIC  VEIN 
HEAD  OF  PANCREAS 

DUODENUM 


DIAPHRAGM 
•STOMACH      , 

SPLEEN 

COELIAC  ARTERY 
'SPLENIC  ARTERY  AND  VEIN 
"PANCREAS 

SUPERIOR  MESENTERIC  ARTERY 
KIDNEY 

INFERIOR  MESENTERIC  VEIN 
ABDOMINAL  AORTA 
INFERIOR  MESENTERIC  ARTERY 

DESCENDING  COLON 


•RECTUM 


ASCENDING  COLON 

ILEUM 


BLADDER  — j 


FIG.  75. — Drawing  to  show  the  main  blood  vessels  of  the  abdominal  viscera.  Note 
particularly  the  portal  vein  which  carries  the  blood  from  the  alimentary  canal  to  the 
liver  (cf.  Fig.  81  A).  Arteries  are  stippled;  veins,  light. 

Capillaries. — The  capillaries  are  the  ultimate  microscopic  units  of 
the  closed  vascular  system  permeating  all  the  tissues  of  the  body. 
The  capillary  walls  are  very  thin,  and  the  blood  plasma  exudes  through 
them,  thus  coming  into  actual  contact  with  the  cells  of  the  body. 
The  walls  consist  only  of  endothelium — one-cell  layer  in  thickness. 
The  endothelial  cells  are  flattened,  irregular  in  shape,  and  firmly 
fastened  together  to  form  a  tiny  tube.  They  are  too  small  to  be 
visible  to  the  naked  eye;  some  1,500  of  the  larger  capillaries  could  be 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM  147 

placed  side  by  side  in  a  space  1  in.  wide,  and  almost  double  this  number 
of  the  smallest  ones.  The  latter  are  so  narrow  in  diameter  that  the 
erythrocytes  press  against  the  wall  as  they  are  slowly  moved  along,  in 
single  file,  by  the  blood  current.  As  a  matter  of  fact,  the  terminal 
portions  of  the  arteries  and  veins,  which  connect  with  the  capillaries, 
are  not  much  larger  or  more  complex  in  their  structure  than  the  capil- 
laries themselves.  In  general,  every  type  of  blood  vessel,  no  matter 
how  small,  has  some  muscular  and  connective  tissue  elements,  the 
latter  definitely  separating  them  from  the  surrounding  tissue  cells. 
Exceptions  to  this  generalization  are  found  in  the  open  channels  of 
the  sinusoids,  as  previously  described  in  the  liver,  and  also  in  the 
lymphatic  capillaries  which  may  be  considered  next.  (Fig.  73.) 

Lymphatic  Vessels.— Supplementary  to  the  closed  vascular  system, 
with  its  highly  developed  arteries,  veins,  and  capillaries  and  com- 
paratively rapidly  circulating  fluid  medium,  is  the  lymphatic  system 
with  lymph  vessels,  lymph  capillaries,  and  tissue  spaces  permeating 
the  tissues  and  often  paralleling  the  blood  capillaries.  The  circulating 
fluid,  lymph,  though  somewhat  variable  in  composition,  may  be 
regarded  as  being  essentially  the  same  as  blood  plasma  containing 
leucocytes  but  no  red  cells.  In  fact,  lymph  has  its  origin  primarily 
in  the  liquid  blood  plasma  which  exudes  through  the  capillary  walls, 
and  it  is  later  collected  from  all  the  tissues  and  returned  to  the  blood 
stream,  chiefly  through  the  thin-walled,  but  comparatively  large, 
thoracic  duct  which  ppeiis  into  the  venous  system,  anterior  to  the 
heart.  There  is  no  definite  propelling  mechanism  in  the  lymphatic 
system  comparable  to  the  heart,  and  the  flow  of  lymph  from  the  tissues 
is  irregular,  slow,  and  dependent  to  a  large  extent  upon  the  muscular 
activity  of  the  body  as  a  whole.  The  finest  lymph  vessels  are  com- 
parable to  the  capillaries  in  size,  but  the  diameter  of  the  lumen  is  not 
constant,  and  the  lymph  capillaries  may  end  blindly  in  the  tissues,  as 
seen  in  the  lacteals  of  the  villi  in  the  small  intestine.  (Figs.  81,  83; 
page  166.) 

COURSE  OF  THE  CIRCULATION  IN  THE  BODY 

We  shall  now  assemble  the  various  parts  of  the  vascular  system 
into  a  connected  system  and  trace  the  main  routes  of  the  blood  in 
making  a  complete  circuit  from  the  left  auricle  of  the  heart  through  the 
tissues  and  back  to  the  same  chamber.  The  complete  separation  of 
the  right  and  left  sides  of  the  heart  establishes  two  primary  routes: 
the  pulmonary  circulation  and  the  systemic  circulation.  In  the 
pulmonary  circulation,  no  variation  occurs  in  the  route.  The  venous 
blood  received  from  all  over  the  body  through  the  superior  and  inferior 


148 


HUMAN  BIOLOGY 


PARIETAL  AND 
TEMPORAL  ARTERY 

OCCIPITAL  ARTERY 

INTERNAL  JUGULAR  VEIN 
COMMON  CAROTID  ARTERY 
INNOMINATE  ARTERY. 
INNOMINATE  VEIN 
VENACAVA(SUR) 

HEART-COROWARY  ARTERY 
HEPATIC  VEIN 
COELI  AC  ARTERY 
PORTAL  VEIN 
HENAL  VEIN 


VENA  CAVA  (INF.) 


CEPHALIC  VEIN 
RADIAL  ARTERY 

ULNAR  ARTER 


FEMORAL  ARTERY- 
FEMORAL  VEIN 
SAPHENOUS  VEIN 


TIBIAL  ARTERY  (ANT.) 

PERONEAL  ARTERY 
TIBIAL  ARTERY(POST) 

SAPHENOUS  VEIN 


FRONTAL  AND 
TEMPORAL  ARTERY 


EXTERNAL  MAXILLARY  ARTERY 

SUBCLAVIAN  VEIN  (LEFT) 
IBCLAVIAN  ARTERY(LEFT) 
ARCH  OF  AORTA 

PULMONARY  ARTERY 
PULMONARY  VEIN 
CEPHALIC  VEIN 
HEART-CORONARY  ARTERY 
BRACHIAL  ARTERY 
BASILIC  VEIN 
RENAL  ARTERY 
MESENTERIC  ARTERY  (SUP.) 
MESENTERIC  VEIN  (SUR) 

•MESENTERIC  ARTERYllNF.) 

AORTA 

COMMON  ILIAC  ARTERY 
COMMON  ILIAC  VEIN 

CEPHALIC  VEIN 
RADIAL  ARTERY 

ULNAR  ARTERY 


FEMORAL  ARTERY 
FEMORAL  VEIN 
SAPHENOUS  VEIN 


TIBIAL  ARTERY(ANI) 

PERONEAL  ARTERY 
TIBIAL  ARTERY  (POST) 

SAPHENOUS  VEIN 


Pi  ATE  IX. — Drawing  illustrating  routes  of  blood  through  the  chief  arteries  and  veins  as 
described  on  pages  147  to  152.     Arteries  are  stippled;  veins,  light. 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM  149 

venae  cavae  passes  into  the  right  auricle,  then  through  the  tricuspid 
valve  into  the  right  ventricle.  Leaving  the  latter,  it  is  conveyed 
through  the  pulmonary  artery  to  the  lungs  where  it  passes  through  the 
capillaries,  increasing  the  oxygen  content  and  releasing  the  carbon 
dioxide  as  it  does  so.  Leaving  the  lungs,  the  blood  returns  to  the  left 
auricle  through  the  pulmonary  veins,  thus  completing  the  pulmonary 
circulation.  (Plate  IX.) 

The  systemic  circulation  is  concerned  with  the  circulation  through- 
out the  body  of  the  oxygenated  blood  received  in  the  left  auricle  from 
the  lungs.  The  first  stage  in  the  journey  is  the  rapid  transfer  to  the 
left  ventricle  through  the  mitral  valve  and  then  the  departure  of  the 
arterial  blood  from  the  heart  by  way  of  the  single  large  aorta,  under 
the  pressure  of  the  ventricular  systole.  At  the  conclusion  of  each 
systole,  as  the  left  ventricle  begins  to  relax,  there  is  a  strong  tendency 
for  the  blood,  under  pressure  in  the  aorta,  to  flow  back  into  the  ven- 
tricle. This  backflow  is  prevented  by  the  quick  action  of  the  semi- 
lunar  valves  present  in  the  lining  of  the  aorta,  close  to  its  origin  in  the 
left  ventricle.  The  semilunar  valves  are  seen  as  soft,  crescent-shaped 
bags  projecting  from  the  lining  of  the  aorta.  As  the  blood  starts  to 
flow  toward  the  ventricle,  each  of  the  valves  instantly  fills  to  capacity 
and  bulges  into  the  lumen  of  the  aorta  where  contact  is  made  with  the 
other  valves  in  the  same  condition,  thus  completely  closing  the  aorta. 

All  blood  leaving  the  left  ventricle  passes  through  the  aorta;  but 
since  the  latter  supplies  all  the  systemic  arteries  of  the  body,  many 
arteries  branch  off  from  it.  Immediately  after  leaving  the  heart,  the 
aorta  curves  dorsally  and  to  the  left  to  form  the  fl-shaped  aortic  arch. 
Referring  again  to  the  previous  analogy  between  the  right  hand  and  the 
heart,  the  second  finger,  curved  loosely  toward  the  palm,  will  give  a 
general  idea  as  to  the  shape  of  the  aortic  arch.  Three  important  arter- 
ies, carrying  blood  anteriorly,  branch  from  the  aortic  aroh.  These 
are  the  innominate,  left  common  carotid,  and  left  subclavian.  A 
comparable  position  on  the  finger  would  be  just  proximal  to  the  first 
joint  for  the  innominate  and  distal  to  the  second  joint  for  the  other  two 
arteries.  Shortly,  the  innominate  divides  to  form  the  right  subclavian 
artery  and  the  right  common  carotid.  Thus  are  formed  a  pair  of 
subclavians  running  to  arm  and  shoulder  and  a  pair  of  carotids,  which 
carry  arterial  blood  to  the  head. 

Just  beyond  the  aortic  arch,  the  aorta  reaches  the  dorsal  body  wall 
and  continues  posteriorly,  in  a  median  line,  as  the  dorsal  aorta  until  it 
nears  the  posterior  end  of  the  abdominal  cavity.  Here  it  divides  to 
form  a  pair  of  iliac  arteries  which  by  means  of  numerous  branches 
supply  the  leg  tissues.  Between  the  aortic  arch  at  the  heart  and  the 


150 


HUMAN  BIOLOGY 


iliac  division  in  the  abdomen,  various  important  arteries  leave  the 
aorta  to  supply  the  alimentary  canal,  the  mesenteries,  the  liver,  and 
the  kidneys.  The  arterial  supply  to  the  liver  through  the  hepatic 
artery  is  comparatively  small,  since  most  of  the  blood  supply  to  the 
liver  comes  from  the  various  regions  of  the  alimentary  canal  by  way 
of  the  portal  vein.  (Fig.  75.) 

It  is  clear  from  the  description  just  given  that  several  systemic 
routes  are  open  to  the  blood  leaving  the  heart  by  the  aorta.  Thus 
blood  may  be  diverted  from  the  aorta  almost  at  once  and  proceed 
anteriorly  through  the  arteries  that  supply  the  tissues  of  the  head  or 
those  of  the  arms  and  shoulder.  Or  it  may  continue  posteriorly  in 
the  aorta  and  be  diverted  into  an  artery  supplying  the  alimentary 
canal  from  which  it  will  continue  through  the  portal  vein  to  the  liver. 


AORTA 

CORONARY  ORIFICE 
AORTIC  VALVE 

LEFT  VENTRICLE 


FIG.  76. — Opening  of  the  coronary  arteries  in  the  aorta. 

Again,  the  blood  may  go  to  the  liver  directly  through  the  hepatic 
artery  or  to  the  kidneys  via  the  renal  arteries  or,  finally,  to  any  portion 
of  the  legs.  No  matter  what  route  is  taken,  the  circulating  blood 
finally  reaches  the  right  auricle  through  the  superior  vena  cava  if  it 
flowed  anteriorly  and  through  the  inferior  vena  cava  if  it  went  pos- 
teriorly. [Plate  IX.) 

It  has  been  emphasized  that  the  only  way  for  blood  to  get  from  the 
right  side  of  the  four-chambered  heart  to  the  left  is  to  traverse  either 
the  pulmonary  or  the  systemic  routes.  There  is  one  exception  to  this 
statement,  and  that  is  found  in  the  route  open  to  the  blood  which 
supplies  the  tissues  of  the  heart  itself.  It  is  obvious  that  the  con- 
tinuously active  cardiac  tissues  must  be  well  supplied  with  vascular 
tissues  to  cariy  nutritive  materials  and  to  remove  the  cellular  wastes. 
The  arterial  supply  of  the  heart  comes  from  a  pair  of  coronary  arteries 
which  branch  off  from  the  aorta  just  distal  to  the  semilunar  valves 
and  form  an  intricate  network  of  capillaries  in  the  muscle  and  valvular 
tissues.  The  venous  blood  is  collected  from  the  capillary  network 
of  the  heart  by  various  veins  which  open  directly  into  the  right  auricle 
without  passing  into  the  vena  cava.  Any  arterial  obstruction  that 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM 


151 


FIG.  77. — Illustrating  the  high  vascularization  in  the  human  heart  valves.  Semi- 
diagrammatic  from  injected  specimens.  (Courtesy  of  Dean  S.  Bayne- Jones.  Repro- 
duced from  original  drawing  by  Max  Broedcl.) 


152  HUMAN  BIOLOGY 

blocks  the  blood  supply  to  the  heart  through  the  coronary  arteries  is  a 
very  serious  matter  and  accompanied  by  extraordinary  pain.  This 
condition  is  known  as  angina  pectoris.  If  a  large  heart  vessel  is 
blocked,  the  heart  will  soon  cease  to  function.  If  the  block  occurs  in  a 
small  artery,  partial  heart  function  may  continue,  but  the  heart  will 
be  permanently  impaired  by  the  degeneration  of  the  muscle  fibers  in 
the  area  normally  supplied  by  the  blocked  vessels.  (Figs.  76,  77.) 

FUNCTIONAL  FEATURES  ASSOCIATED  WITH  THE  VASCULAR  SYSTEM 

With  the  important  structural  units  of  the  vascular  system  in  mind, 
it  is  next  in  order  to  consider  the  important  questions  associated  with 
the  circulation  of  the  blood  through  them.  In  so  doing,  the  function- 
ing of  the  heart,  as  the  dynamic  center  of  the  entire  system,  supplies  a 
logical  beginning  for  the  discussion. 

HEART  FUNCTION 

Blood  flows  through  the  connected  tubular  network  of  the  vascular 
system  because  it  is  under  pressure.  Blood  pressure  is  the  result  of  a 
rhythmic  pumping  action  of  the  heart  which  involves  an  effective 
coordination  of  the  muscular  walls  and  the  valves  of  the  various  cham- 
bers. The  complete  cycle  of  heart  action  during  which  blood  is 
received  and  then  forced  out  under  pressure  is  known  as  the  cardiac 
cycle  or,  commonly,  as  the  heartbeat.  The  human  heart,  under  normal 
conditions,  beats  from  72  to  75  times  per  minute,  which,  if  the  latter 
figure  is  taken,  means  that  the  complete  cardiac  cycle  occupies  less 
than  0.8  second.  In  this  tiny  unit  of  time,  a  regular  exact  sequence  of 
events  must  occur,  involving  a  period  of  relaxation,  during  which  blood 
is  admitted  to  a  particular  chamber;  a  period  of  contraction,  during 
which  the  blood  is  forced  into  the  arterial  vessels;  and,  finally,  a  rest 
period,  which  permits  the  active  muscle  fibers  to  recuperate. 

Cardiac  Cycle. — Consideration  of  the  cardiac  cycle  may  begin 
with  the  contraction  (systole)  of  the  right  and  left  auricles.  This 
occurs  almost  simultaneously  in  the  two  chambers  and  forces  the  blood 
into  the  corresponding  ventricles.  Auricular  systole  takes  about  0.1 
second  and  is  immediately  followed  by  the  ventricular  systole  which 
drives  the  blood  from  the  right  ventricle  to  the  lungs  and  from  the  left 
ventricle  to  the  systemic  circulation.  Ventricular  systole  occupies 
about  0.3  second.  This  leaves  0.4  second,  or  half  of  each  complete 
cardiac  cycle,  for  a  relaxation  (diastole)  and  rest  period  (diastasis)  of 
the  cardiac  tissue.  No  other  type  of  muscle  tissue  has  the  ability  to 
carry  on  the  continuous  activities  of  the  heart  with  so  little  time  for 
recuperation  between  contractions.  When  one  considers  the  com- 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM 


153 


plicated  chemical  reactions  necessary  for  nourishment  and  for  supply- 
ing energy  for  muscle  contraction,  the  more  difficult  it  is  to  understand 
the  basic  features  of  heart  function.  This  difficulty  is  still  further 
increased  where  even  greater  rhythmic  activity  is  maintained,  as  in 
the  heart  of  the  hummingbird  which  is  stated  to  maintain  a  rate  of 
2,000  beats  per  minute.  (Fig.  78.) 

The  movements  of  the  heart  tissues  and  valves  produce  charac- 
teristic sounds  during  systole  which  can  readily  be  detected  by  the 
stethoscope.  This  instrument  was  invented  over  a  century  ago  and 
enables  the  trained  ear  of  the  physician  to  recognize  abnormal  heart 
action.  The  first  heart  sound  is  given  off  at  the  beginning  of  the 
ventricular  systole  and  has  its  origin  partly  in  the  closure  of  the  valves 
between  auricles  and  ventricles  and  partly  in  the  contraction  of  the 


Z  N     3  | 

FIG.  78. — Diagram  of  the  cardiac  cycle  in  a  dog's  heart,  as  described  on  page  152. 
First  depression  in  curve  at  1  indicates  auricular  systole;  upstroke,  terminating  at  2 
indicates  emptying  of  heart  during  ventricular  systole;  downstroke,  terminating  at 
4,  indicates  filling  of  heart  during  first  part  of  diastole;  4-5,  rest  period  (diastasis). 
(Howell,  "Physiology  "  W.  B.  Saunders  Company.  After  Hirschf  elder.) 

ventricular  tissues.  The  sound  is  best  described  as  a  prolonged, 
low-pitched  "lubb."  The  termination  of  the  ventricular  systole  is 
indicated  by  a  short,  high-pitched,  almost  metallic  "dup"  which  is 
clearly  due  to  the  closure  of  the  semiluiiar  valves  in  the  aorta  at  the 
end  of  the  ventricular  systole. 

The  origin  and  the  control  of  the  heart  beat  have  been  subjects  of 
wonder  and  experiment  from  very  early  times  in  an  endeavor  to  isolate 
the  essential  functional  features,  which,  to  the  ancients,  were  a  question 
of  vital  spirits.  Much  has  been  learned  about  the  structure  of  the 
controlling  mechanism  of  the  heart  but  very  little  a?  to  the  whys  and 
wherefores,  though  certain  facts  stand  out.  Thus  it  can  be  shown 
experimentally  by  culturing  embryonic  heart  tissue  or  by  operating 
on  a  frog  embryo  at  a  very  early  stage  in  development  that  rhythmic 
contraction  of  heart  muscle  begins  independently  of  impulses  from 
nerve  tissue.  And  by  cutting  the  nerves  running  to  the  heart  in  a 


154 


HUMAN  BIOLOGY 


mature  experimental  animal,  it  is  possible;  to  demonstrate  that  the 
heart  will  continue  to  beat,  though  the  rate  may  be  altered  by  nerve 
impulses.  Even  more,  the  heart  of  a  frog  and  various  other  animals 
may  be  entirely  removed  from  the  body;  but  if  they  are  kept  under  the 
proper  conditions  and  supplied  with  fluids  of  the  right  chemical  com- 
position, rhythmic  contraction  will  continue  until  the  cardiac  tissue  is 
weakened  by  lack  of  nutrition  and  accumulated  wastes.  It  is  clearly 
demonstrated,  then,  that  the  heart  muscle  has  an  inherent  rhythmicity. 
Increased  knowledge  of  heart  function,  based  on  many  years  of  experi- 
mentation, has  not  lessened  the  admiration  that  has  from  time  imme- 
morial been  centered  in  this  most  extraordinary  organ  of  life. 

Neuromuscular    Apparatus. — Careful    histological    work    on    the 
vertebrate  heart  has  revealed  the  presence  of  a  complicated  neuro- 

muscular  apparatus  which  is  re- 
sponsible for  the  formation  and  the 
conduction  of  the  rhythmic  impulses 
that  incite  the  contraction  of  the 
cardiac  tissues.  It  has  long  been 
observed  that  the  wave  of  contrac- 
tion in  the  heart  muscle  begins  in 
the  right  auricle  and  rapidly  spreads 
to  the  other  regions  of  the  heart.  A 
tiny  bit  of  special  tissue,  the  sino- 
auricular  node  (S.A.  node),  embed- 
ded in  the  wall  of  the  right  auricle 
between  the  openings  of  the  venae 
cavae,  is  the  dynamic  center  of 
heart  action.  In  some  unknown 
way,  rhythmic  stimuli,  capable  of 
inciting  cardiac  contraction,  develop 
in  it  and  spread  rapidly  through  the 
auricular  tissue,  causing  them  to 
contract.  Each  stimulus  is  accompanied  by  an  electric  action  current 
of  sufficient  force  to  be  detected  by  electrical  instruments.  The 
stimuli  reach  the  base  of  the  auricles,  and  here,  in  the  dividing  wall, 
or  septum,  between  the  right  and  left  auricles,  they  stimulate 
another  unit  of  the  neuromuscular  apparatus,  the  tiny  auriculo- 
ventricular  node  (A.V.  node),  from  which  a  bundle  of  fibers,  the 
auriculo-ventricular  bundle  (A.V.  bundle)  continues  into  the  mus- 
cular wall  between  the  two  ventricles.  The  auriculo-ventricular 
bundle  soon  divides  into  a  right  and  a  left  branch,  and  these 
subdivide  to  form  a  fibrous  network  throughout  the  walls  of 


FIG.  79. — Photograph  of  a  model  of 
the  neuromuscular  apparatus  which  is 
embedded  in  the  tissues  of  the  heart,  as 
described  on  page  154.  (Crandall, 
"Human  Physiology"  W.  B.  Saunders 
Company.) 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM  155 

the  two  ventricles.  The  auriculo-ventricular  node  and  associated 
auriculo-ventricular  bundle  carry  the  impulses  to  the  ventricles.  If 
the  auriculo-ventricular  bundle  is  cut  in  an  experimental  animal  before 
it  reaches  the  ventricles,  the  latter  do  not  contract  rhythmically. 
Other  experiments  involving  disturbances  of  the  normal  neuromuscular 
apparatus  show  beyond  doubt  that  the  secret  of  coordinated  rhythmic 
heart  action  is  closely  associated  with  this  mechanism.  The  general 
regulation  of  the  heart  beat  is  a  function  of  the  central  nervous  system, 
but  consideration  of  this  fact  may  be  deferred  for  later  consideration. 
(Fig.  79.) 

Blood  Pressure. — The  heart  moves  a  lot  of  blood  against  pressure. 
As  a  result,  it  does  a  great  deal  of  work  each  day.  And  it  never  finds  it 
necessary  to  take  any  vacation  from  the  routine  of  the  cardiac  cycle 
during  a  lifetime  of  high  endeavor.  It  is  difficult  to  ascertain  the 
exact  amount  of  blood  forced  from  the  human  heart  at  each  systole, 
but  it  is  in  the  neighborhood  of  4  oz.  Beating  at  the  rate  of  seventy 
times  per  minute,  this  would  mean  that  the  amazing  total  of  27,000  Ib. 
of  blood  is  forced  out  of  the  heart  every  day  under  normal  conditions. 
This  amount  is  automatically  stepped  up  when  the  conditions  require 
to  possibly  ten  times  the  normal  output.  Thus,  it  is  estimated  that 
the  heart  of  an  athlete  during  the  rowing  of  a  strenuous  race  may 
pump  out  15  gal.  of  blood  per  minute,  which  is  more  than  six  times 
the  normal  delivery. 

But  the  blood  is  not  merely  pumped  out  of  the  heart  into  an  open 
vessel.  It  is  pumped  into  closed  vessels  against  considerable  pressure. 
The  difference  is  that  existing  between  a  common  well  pump  delivering 
water  into  an  open  bucket  and  a  fire  engine  pumping  water  at  high 
pressure  into  the  fire  hose.  The  pressure  of  the  blood  in  the  aorta  of 
man  is  sufficient  to  force  the  blood  almost  to  the  top  of  a  vertical  tube 
7  ft.  high.  In  pumping  the  blood  against  this  pressure,  the  heart  is 
doing  work  equivalent  to  carrying  a  weight  of  1,100  Ib.  up  7  ft.  every 
hour  of  the  day  and  night  throughout  life  and  the  accomplishment  of 
189,000  foot-pounds  of  work  per  day.  Add  to  this  the  fact  that  in 
active  individuals  the  output  of  blood  from  the  heart  is  greatly 
increased  from  time  to  time,  as  activity  increases,  and  it  is  apparent 
that  the  usual  statement  of  300,000  foot-pounds  of  work  per  day  by  the 
heart  is  not  too  high. 

It  is  obvious  that  the  blood  pressure  must  be  constantly  maintained 
at  a  sufficiently  high  level  in  the  arteries  to  force  the  blood  uniformly 
to  all  the  outlying  regions  of  the' body,  without  respect  to  their  posi- 
tion. It  is  possible  to  determine  this  level  of  blood  pressure  in  experi- 
mental animals  by  inserting,  into  one  of  the  large  arteries,  a  glass 


156 


HUMAN  BIOLOGY 


cannula  connected  by  rubber  tubing  to  a  manometer.  The  latter  is  a 
U-shaped  glass  tube  partially  filled  with  mercury  and  designed  to  show 
the  blood  pressure  by  the  amount  of  displacement  of  the  mercury.  It 
is  more  convenient  to  use  a  U-shaped  tube  for  a  manometer  than  a 
straight  vertical  tube,  and  it  is  also  better  to  use  mercury  in  the  tube 

rather   than   water,   because  mercury  is 
much  heavier.     The  blood  pressure,  there- 
fore, cannot  lift  it  so  high,  and  the  U-- 
shaped   manometer   tube   can   be   much 
shorter.     When   the   proper   connections 
are  made,  the  arterial  blood  will  quickly 
flow  through  the  cannula  and  tubing  and 
then  against  the  mercury  column  in  the 
connected  arm  of  the  U.     The  level  of  the 
mercury  in  this  arm  of  the  manometer  will 
be  depressed  and  correspondingly  elevated 
in  the  other  arm  in  accordance  with  the 
amount  of  blood  pressure.     The  pressure 
can  be  recorded  in  linear  fashion  as  so 
many  millimeters  of  mercury 
(mm.   Hg.)   by  the  increased 
length  of  the  mercury  column 
in  the  unconnected  arm  of  the 
manometer.     (Fig.  80.) 

It  was  early  recognized 
that  blood  pressure  was  an 
important  factor  in  the  diag- 
nosis of  human  disease,  and 
consequently  it  was  necessary 
to  devise  a  convenient  and  easy  method  of  obtaining  it  that  did  not 
involve  opening  an  artery.  This  was  accomplished  about  fifty  years 
ago  when  the  sphygmomanometer  was  devised.  The  principle  of  this 
instrument  is  the  same  as  that  of  the  manometer,  but,  instead  of  insert- 
ing a  cannula  in  a  blood  vessel,  an  inflatable  band  of  rubberized  cloth 
is  wrapped  around  the  patient's  arm,  just  above  the  elbow.  When  this 
band  is  inflated  by  pumping  air  into  it,  pressure  is  applied  to  the  arm ; 
and  when  this  external  pressure  equals  that  of  the  blood  in  the  artery, 
the  flow  of  the  blood  is  stopped  at  the  pressure  area.  The  cessa- 
tion of  flow  can  be  determined  by  the  absence  of  the  pulse  in  the 
brachial  vessel  below  the  inflated  band.  The  use  of  the  stethoscope 
just  below  the  band  will  give  the  critical  point  with  accuracy.  In 
determining  blood  pressure  by  this  method,  the  air  pressure  around 


FIG.  80. — Blood  pressure  manometer  as 
described  on  page  156.  C,  cannula  for  insertion 
in  the  artery;  Af,  U-shaped  tube  with  mercury; 
P,  writing  point  for  tracing  the  record. 
(Mitchell.) 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM  157 

die  arm  is  increased  until  the  sounds  in  the  vessel  have  entirely 
stopped.  Then  the  air  pressure  is  released  gradually  until  the  first 
sounds  of  the  returning  pulse  are  heard.  The  air  pressure  at  this 
point,  read  as  millimeters  of  mercury  from  the  air  gage  which  is  also 
attached  to  the  pump,  is  taken  as  the  systolic  pressure.  Now  with 
this  point  determined,  if  the  pressure  around  the  arm  is  gradually 
reduced  still  further,  a  point  will  be  reached  where  the  diastolic  sound, 
which  marks  the  low  point  of  arterial  pressure,  can  be  detected.  In 
the  young  adult,  the  normal  systolic  pressure  is  found  to  be  from 
120  to  125  mm.  Hg,  and  the  diastolic  pressure  about  70  mm.  Hg.  In 
general,  in  women  of  comparable  age,  the  pressure  is  about  10  mm. 
Hg  less. 

The  difference  of  about  50  mm.  Hg  between  systolic  and  diastolic 
pressure,  as  just  obtained,  gives  a  definite  indication  of  the  elasticity 
of  the  arterial  walls.  This  factor  of  arterial  elasticity  tends  to  decrease 
in  the  older  individuals  as  the  arterial  walls  become  more  and  more 
rigid.  Accordingly,  the  ability  of  the  arteries  of  the  older  person  to 
compensate  by  expansion  for  the  increase  in  the  amount  of  blood  at 
each  systole  is  reduced.  Correspondingly  the  blood  pressure  through- 
out the  entire  arterial  system  is  increased.  This  is  known  as  hyper- 
tension. A  certain  amount  of  increase  in  blood  pressure  is  the  normal 
accompaniment  of  age,  but,  under  certain  disease  conditions,  marked 
increases  occur  in  which  the  systolic  pressure  may  rise  over  200  mm. 
Hg.  Such  conditions  are  bad  for  the  heart  as  it  has  to  do  extra  work 
in  pumping  the  blood  into  the  arteries  against  the  increased  pressure, 
and  the  extra  pressure  may  also  result  in  the  rupture  of  an  important 
but  thin-walled  vessel,  as  in  the  brain — the  condition  of  apoplexy. 

The  blood  pressure  in  the  circulatory  system  is  greatest  in  the 
aorta  near  the  heart  which  receives  the  full  force  of  the  blood  from  the 
left  ventricle  at  each  systole.  The  pressure  in  the  pulmonary  artery 
is  much  less,  in  fact,  slightly  more  than  a  third  of  the  aorta.  In  all 
cases,  the  blood  pressure  is  reduced  as  the  arteries  get  farther  from  the 
heart  and  subdivide  into  smaller  and  smaller  branches  in  the  tissues. 
In  the  human  capillaries,  the  pressure  is  reduced  to  about  22  mm.  Hg; 
and  in  the  connecting  veins  leading  back  to  the  heart,  the  pressure  con- 
tinues to  decrease  until  this  organ  is  reached.  The  flow  of  the  venous 
blood  into  the  heart  is  aided  by  a  negative  pressure  in  the  thorax  which 
may  amount  to  as  much  as  —8  mm.  Hg  during  an  inspiration. 

Blood  pressure  as  a  whole  is  dependent,  under  normal  conditions, 
upon  two  factors :  the  amount  of  blood  pumped  out  by  the  heart  and 
the  peripheral  resistance.  Both  of  these  are  subject  to  wide  variation 
in  the  normal  activities  of  an  individual.  Just  as  soon  as  an  increase 


158  HUMAN  BIOLOGY 

occurs  in  muscular  activity,  the  call  for  increased  aeration  of  blood 
brings  about  an  increase  of  heart  activity,  arid  more  blood  will  be  forced 
through  the  vessels  which  will  tend  to  raise  the  blood  pressure.  The 
peripheral  resistance  can  be  altered  through  nervous  and  muscular 
action  in  the  walls  of  the  vessels  which  results  in  increasing  or  decreas- 
ing the  size  of  the  vessels  in  a  particular  organ  or  region  of  the  body; 
the  blood  pressure  will  be  increased  or  decreased  accordingly.  Under 
abnormal  conditions,  the  blood  pressure  may  be  affected  by  other 
factors.  Thus,  as  noted  above,  a  decrease  in  the  elasticity  of  the 
arterial  walls  increases  the  blood  pressure.  Again,  when  large  quanti- 
ties of  blood  are  lost  by  bleeding,  the  blood  pressure  falls  until  the 
liquids  are  restored.  Certain  diseases  are  known  that  reduce  the 
viscosity  of  the  blood.  This  also  tends  to  reduce  the  blood  pressure 
because  the  thinned  blood  will  flow  more  easily  through  the  vessels 
and  reduce  the  peripheral  resistance. 

TRANSPORTATION  OF  MATERIALS  IN  THE  BLOOD 

Nutritive  Materials. — The  products  of  digestion  that  have  been 
absorbed  from  the  digestive  tract  and  transferred  to  the  vascular 
system  are  carried  in  solution  in  the  blood  plasma.  As  already  noted, 
the  numerous  smaller  vessels  permeating  the  walls  of  the  alimentary 
tract  finally  unite  to  form  the  large  portal  vein  through  which  all  the 
blood  from  this  region  is  carried  to  the  liver.  The  various  activities 
of  the  liver  in  the  chemical  treatment  of  the  carbohydrates  and  proteins 
have  been  previously  noted.  The  transportation  of  the  digested  fats 
is  very  largely  through  the  lymphatic  vessels  rather  than  the  portal 
vein.  It  will  be  remembered  that  each  villus  in  the  intestinal  wall 
contains  a  lymphatic  capillary,  or  lacteal,  which  ends  blindly  near  the 
tip  of  the  villus.  The  best  evidence  is  that  the  fatty  acids  and  glycerol, 
resulting  from  the  digestion  of  the  fats,  are  absorbed  in  combination 
with  the  bile  salts  by  the  mucosal  cells.  The  bile  salts  are  split  off 
from  the  fatty  products  in  the  absorptive  cells  and  soon  reach  the  liver 
through  the  portal  vein  where  they  are  reabsorbed.  The  fatty  acids 
and  glycerol  are  recombined  in  the  cytoplasm  of  the  absorptive  cells 
to  form  fats,  and  the  latter,  for  the  most  part,  pass  into  the  lacteals  and 
then  the  larger  lymphatic  vessels,  finally  passing  into  the  general 
circulation  through  the  thoracic  duct.  When  fats  are  stained  with  a 
fat  stain,  such  as  Sudan  III,  and  then  fed  to  experimental  animals,  it 
is  possible  to  trace  the  course  of  the  stained  fat  into  the  blood  stream 
by  way  of  the  thoracic  duct.  Once  in  the  blood  stream,  the  fats  are 
carried  in  the  plasma.  (Fig.  81.) 

Respiratory  Gases. — Consideration  has  already  been  given  to  the 
main  features  associated  with  the  transportation  of  oxygen  and  carbon 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM 


159 


dioxide  by  the  blood  stream.  The  transportation  arrangement  exist- 
ing between  oxygen  and  hemoglobin  is  well  established,  but  there  are 
several  difficult  problems  concerned  with  the  return  of  carbon  dioxide 
from  the  tissues  to  the  lungs.  The  general  conception  has  been  that 
the  blood  plasma  is  responsible  for  the  transfer  of  the  carbon  dioxide. 
This  is  true  only  in  part.  The  gas  analyses  of  blood  show  that  around 


L  EFT  JUGULAR  VEIN 


LEFT  SUBCLAVIAN  VEIN 
SUPERIOR  VENA  CAVA 


PORTAL  VEIN 
FROM  SPLEEN 


THORACIC  DUCT 
LIVER 

VEIN 

RECEPTACULUM  CHYLI 
^ 

FROM  STOMACH 


LACTEALS  OR 
LYMPH  VESSELS 
IN  LUMBAR  REGIONS 


INTESTINE 


B 


FIG.  81. — A,  diagram  of  paths  of  absorbed  food  from  the  digestive  tract  to  the  liver,  via 
portal  vein;  B,  chief  lymphatic  vessels  in  man.      Of.  Fig.  83.      (Woodruff.) 

55  cc.  of  carbon  dioxide  is  present  in  100  cc.  of  venous  blood.  Of 
this  amount,  it  is  believed  that  not  more  than  2.5  cc.  can  be  carried  in 
the  plasma  without  chemical  change.  An  additional  20  cc.  is  carried 
in  the  plasma  either  as  carbonic  acid  (H2CO3)  or  sodium  bicarbonate 
(NaHCO3).  The  method  used  for  the  transportation  of  the  remainder 
of  the  carbon  dioxide,  approximating  60  per  cent  of  the  total,  presents 
difficult  problems. 

It  now  appears,  however,  that  the  principal  factor  in  carbon  dioxide 
transportation  is  the  ability  of  the  hemoglobin  in  the  red  cells  to  vary 


160  HUMAN  BIOLOGY 

its  acidity  in  relation  to  the  amount  of  oxygen  it  is  carrying.  Oxy- 
hemoglobin,  formed  in  the  lungs  where  there  is  an  abundance  of 
oxygen,  is  more  acid  than  is  hemoglobin  with  the  reduced  amount  of 
oxygen.  In  the  lungs  the  newly  formed  oxyhemoglobin  combines 
with  potassium  in  the  red  cells  to  form  potassium  hemoglobinate, 
and  the  amount  formed  of  this  compound  is  directly  proportional  to 
the  increased  acidity  of  the  oxyhemoglobin.  When  the  acidity  of  the 
hemoglobin  is  reduced  in  the  tissues  as  the  oxygen  is  taken  up,  some  of 
the  potassium  is  released,  and  the  carbon  dioxide  received  from  the 
tissues  enters  the  blood  cells  and  combines  with  potassium  to  form 
potassium  bicarbonate.  In  the  lungs,  when  the  more  strongly  acid 
oxyhemoglobin  is  again  formed  in  the  red  cells,  the  potassium  bicar- 
bonate breaks  down,  and  the  carbon  dioxide  is  released  for  elimination. 
Also  involved  in  this  picture  of  carbon  dioxide  transportation  is 
the  so-called  chloride  shift1  between  the  plasma  and  the  red  cells 
(page  91). 

The  numerous  reactions  associated  with  the  transportation  of  one 
of  the  main  excretions  of  the  cells,  carbon  dioxide,  has  just  been  dis- 
cussed. The  nitrogenous  wastes  and  inorganic  salts  liberated  by  the 
dismantling  of  the  proteins  in  the  cells  are  carried  in  solution  in 
the  plasma.  The  latter  are  kept  at  constant  levels  by  the  action  of  the 
kidney  cells.  The  nitrogenous  wastes  are  first  absorbed  by  the  hepatic 
cells  of  the  liver,  converted  into  urea,  and  then  turned  over  to  the 
blood  plasma  once  more  for  transfer  to  the  kidneys.  In  the  collec- 
tion of  nitrogenous  wastes,  the  slow-moving  stream  of  lymph  or  tissue 
fluid  that  bathes  the  cells  is  the  primary  agent. 

The  internal  secretions  of  the  various  endocrine  glands  are  poured 
directly  into  the  blood  stream  as  was  seen  in  the  previous  chapter. 
So  far  as  is  known,  all  of  the  internal  secretions  are  carried  in  solution 
in  the  blood  plasma. 

UNIFORMITY  AND  VARIATION  IN  THE  BLOOD 

The  ability  to  maintain  an  essentially  uniform  condition  necessary 
for  cellular  activity,  in  spite  of  the  variety  of  materials  poured  into 
it  by  every  cell  of  the  body,  is  one  of  the  outstanding  features  of  this 
liquid  tissue.  Concerned  in  the  maintenance  of  this  uniformity  in 
the  blood  are  several  associated  organs  which  have  been  previously 
noted  but  may  well  be  brought  together  at  this  point.  Thus  the 
respiratory  center  in  the  nervous  system  endeavors  to  keep  the  carbon 
dioxide  content  of  the  blood  within  the  prescribed  limits  by  accelerating 
the  rate  of  breathing  when  it  gets  too  high  and  retarding  the  rate  when 

1  Consult  Appendix :  Chloride  Shift. 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM  161 

the  opposite  condition  occurs.  The  liver  looks  after  the  carbo}  tydrate 
levels  in  the  blood  by  converting  the  stored  glycogen  into  glucose 
and  secreting  it  as  needed  to  maintain  a  level  of  approximately  0.1  per 
cent  in  the  blood,  which  is  available  for  fuel  by  the  cells.  Another 
important  action  of  the  liver,  which  promotes  blood  uniformity,  is  in 
the  maintenance  of  the  normal  fluid  balance  by  immediately  removing 
excess  fluids  absorbed  from  the  digestive  tract  and  later  releasing  them 
as  needed.  In  this  connection,  the  action  of  the  kidneys  is  also  impor- 
tant both  for  the  fluid  balance  and  for  keeping  the  salts  of  the  blood  at 
the  proper  levels. 

Buffering. — But  in  addition  to  all  these  cooperating  organs  the 
blood  plasma  is  equipped  with  its  own  apparatus  to  insure  blood 
uniformity  with  respect  to  the  important  acid-alkaline  equilibrium. 
This  process,  known  as  buffering,  enables  the  blood  to  maintain  the 
normal  slightly  alkaline  condition  (pH  7.3  to  7.5)1  except  under  serious 
disease  conditions  as  might  result,  for  example,  from  continued 
diabetes.  When  an  acid  is  added  to  pure  water,  which  has  no  buffering 
power,  the  neutral  condition  of  the  water  is  changed  at  once  to  an  acid 
condition,  and  the  amount  of  acidity  increases  in  direct  ratio  to  the 
amount  of  acid  that  is  added.  If  an  alkali  were  added  to  water,  the 
alkalinity  would  be  increased  in  the  same  way.  In  a  buffered  liquid, 
such  as  the  blood,  the  addition  of  an  acid  or  an  alkali  does  not  change 
the  previous  acidity  or  alkalinity  of  the  liquid;  that  is,  the  pH  remains 
the  same  up  to  a  certain  point  because  the  buffered  Iiqui4  contains 
substances,  buffers,  which  combine  with  the  added  acid  or  alkali  and 
thus  neutralize  their  effect.  This  action  in  the  blood  or  other  buffered 
liquids  will  continue  until  all  the  combining  substances  or  buffers  are 
used  up,  after  which  the  addition  of  an  acid  or  an  alkali  will,  of  course, 
immediately  change  the  acid-base  equilibrium  of  the  liquid. 

Blood  contains  substances  that  buffer  it  against  both  acids  and 
alkalies,  but  it  is  more  strongly  buffered  against  acids.  The  buffers 
in  the  blood  include  the  proteins  in  the  plasma  and,  in  particular,  the 
hemoglobin  in  the  red  cells  and  various  salts.  The  extent  of  buffering 
in  the  blood  against  acids  depends  upon  the  alkaline  reserve,  and  this 
is  indicated  by  the  ability  of  the  blood  to  combine  with  carbon  dioxide. 
This  is  due  to  the  fact  that  the  latter,  in  entering  the  blood,  unites 
with  water  to  form  carbonic  acid.  The  amount  of  carbonic  acid  that 
can  be  neutralized  by  the  blood  without  changing  its  normal  alkalinity 
is  a  true  indication  of  its  protection  against  an  acid  condition.  The 
normal  carbon  dioxide  combining  power  of  the  blood  has  been  found  to 
be  around  60  volumes  per  100  cc.  of  blood.  Any  increase  in  the  acids 
received  by  the  blood  tends  towards  acidosis,  because  the  alkaline 

1  Consult  Appendix:  Hydrogen  Ion  Concentration. 


162  HUMAN  BIOLOGY 

reserve  is  reduced  by  their  neutralization.  Normally  this  reduction  of 
the  alkaline  .reserve  can  be  compensated  for  by  an  increased  elimination 
of  carbon  dioxide  through  the  lungs,  but  sometimes,  as  in  the  case  of 
severe  diabetes,  the  acids  received  by  the  blood  are  increased  to  such 
an  extent  that  the  alkaline  reserves  of  the  blood  are  depleted  in  spite 
of  all  that  can  be  done.  When  this  happens,  the  normal  alkalinity 
of  the  blood  is  disturbed  to  such  an  extent  that  it  may  even  become 
slightly  acid  with  very  serious  consequences. 

Variable  Factors. — But  the  provisions  for  functional  diversity,  or 
variation,  in  the  vascular  system  as  a  whole  are  as  impressive  as  those 
concerned  with  uniformity  of  the  blood,  for  they  enable  the  organism 
to  adapt  itself  to  the  changing  conditions  of  the  environment— both 
external  and  internal.  The  vascular  system  is  one  of  the  most  adapt- 
able units  in  the  entire  body.  One  or  two  important  examples  may  bo 
cited.  Consider  first  the  response  of  the  heart  when  heavy  muscular 
activity  is  begun.  Almost  at  once  a  largely  increased  amount  of 
blood  is  pumped  to  the  tissues.  Under  very  severe  conditions,  it  is 
believed  that  the  heart  may  increase  its  output  to  as  much  as  ten  times 
the  normal  rate.  Associated  with  this  activity  is  an  increased  amount 
of  blood  but,  more  particularly,  an  increased  rate  of  flow  through  the 
vessels  due  to  the  higher  pressure. 

Due  to  the  fact  that  the  supply  of  blood  in  the  body  is  not  sufficient 
to  maintain  the  metabolic  functions  in  all  the  organs  at  a  maximum 
rate  of  operation  simultaneously,  adequate  supplies  of  blood  must  be 
given  to  those  organs  where  it  will  do  the  most  good  for  the  organism 
as  a  whole  at  a  particular  time.  One  of  the  best  and  most  common 
examples  of  this  fact  is  in  connection  with  nutrition,  for  it  is  found 
that  after  a  hearty  meal  blood  is  collected  from  all  over  the  body  and 
sent  to  the  vessels  that  supply  the  tissues  of  the  alimentary  tract. 
This  additional  blood  enables  the  secreting  cells  of  the  mucosa  to  be 
fully  supplied  with  all  their  requirements  while  they  are  working  at  a 
maximum  rate  to  furnish  digestive  enzymes.  It  also  provides  for  the 
rapid  transportation  of  the  absorbed  food  to  the  liver  and  then  to  all 
the  cells  of  the  body.  The  extra  blood  supply  to  the  nutritive  organs 
at  such  times  is  collected  from  all  over  the  body  but  possibly  moro 
particularly  from  the  skin.  This  has  a  tendency  to  make  the  person 
feel  chilly;  so  it  is  that  a  warm  quiet  corner  for  a  time  after  a  meal  is 
particularly  enjoyable.  Possibly  even  more  striking  is  the  marshaling 
of  the  blood  away  from  the  skin  and  into  the  vital  organs  and  muscles 
when  the  emergency  hormone  adrenine  is  thrown  into  the  blood  as 
previously  described  (page  111). 

Regulation  of  many  kinds  is  occurring  continuously  in  the  vascular 
system.  The  varying  conditions  of  the  blood— the  amount  of  carbon 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM  163 

dioxide,  the  presence  of  a  little  excess  of  certain  secretions,  muscular 
activity,  and  even  the  position  of  the  body — are  all  effective  in  altering 
the  blood  flow.  Associated  in  all  this  regulation  is  an  amazing  system 
of  checks  and  balances,  which  are,  to  some  extent,  integral  parts  of  the, 
vascular  system  and  also,  to  some  extent,  elements  of  the  nervous 
system  located  in  the  vascular  tissues.  The  relations  existing  between 
the  vascular  and  nervous  tissues  are  complex  and  intimate,  as  will  be 
seen  in  the  later  chapter  devoted  to  the  Nervous  System.  It  may  be 
stated  here,  however,  that  the  wall  of  every  type  of  blood  vessel,  from 
the  smallest  capillary  to  the  largest  vein  or  artery,  contains  a  network 
of  nerve  tissue  through  which  the  conditions  in  the  vascular  system  are 
ascertained  and  regulated.  The  abundance  and  complexity  of  nerve 
elements  in  the  vascular  system  undoubtedly  reach  a  climax  in  the 
tissues  of  the  heart  itself,  because  the  regulation  of  the  heart  beat  is  of 
basic  importance  to  every  cell  in  the  body. 

Two  other  features  of  the  blood  stream,  which  render  service  in  the 
protection  of  the  organism  when  certain  emergencies  arise,  are  note- 
worthy. These  are  the  ability  of  the  blood  to  prevent  excessive  loss 
by  bleeding,  provided  the  injury  is  not  too  serious,  and,  secondly,  its 
very  great  power  to  control  infection  when  a  parasitic  microorganism 
secures  a  foothold  in  the  body  tissues. 

BLOOD  COAGULATION 

Every  drop  of  the  6  qt.  of  blood  in  the  body  is  rightly  regarded  as  a 
precious  material  that  must  be  conserved  whenever  possible.  There 
was  a  time  not  so  long  ago  when  physicians  did  riot  recognize  this  fact, 
and  bleeding  for  various  ailments,  with  consequent  wastage  of  blood, 
was  a  common  practice.  The  only  thing  that  saved  the  patient  fol- 
lowing such  treatments  was  the  amazing  ability  of  the  vascular  tissues 
to  regenerate  the  blood  plasma  and  cells.  This  ability  is  so  marked 
and  the  regeneration  processes  so  rapid  that  a  normal  individual  can 
lose  a  considerable  amount  of  blood  without  experiencing  more  than  a 
temporary  weakness.  But  the  organism  endeavors  not  to  lose  blood; 
and  as  soon  as  a  vessel  is  injured  and  bleeding  starts,  the  clotting 
mechanism  in  the  plasma  goes  into  action,  arid  a  series  of  complex 
chemical  reactions  begins  which  soon  results  in  changing  the  blood 
plasma  issuing  from  the  wound  from  a  free-flowing  liquid  to  a  rather 
firm  jelly-like  mass  which  completely  occludes  the  break  in  the  wall  of . 
the  blood  vessel,  unless  the  latter  is  too  large,  with  a  consequent  rapid 
flow  of  blood  from  it.  The  blood  clot  continues  to  increase  in  strength, 
and  in  a  comparatively  short  time  is  about  as  firm  as  the  original  tissue. 

The  blood  clot  represents  the  culmination  of  a  complicated  series  of 


164 


HUMAN  BIOLOGY 


chemical  reactions  in  the  plasma,  the  complete  details  of  which  are  not 
entirely  known  at  the  present  time.  It  is  widely  accepted,  however, 
that  a  protein  substance,  cephalin,  appearing  in  the  blood  only  when 
cell  injury  occurs  (particularly  to  the  blood  platelets)  acts  to  neutralize 

(1)  an  unidentified  blood  substance.     When   this  neutralization  is 
accomplished  by  the  cephalin,  a  chemical  union  occurs  between  cal- 
cium salts  and  prothrombin,  both  in  solution  in  the  plasma,  to  form 

(2)  a  new  compound,  thrombin.     When  thrombin  is  synthesized,  it 
reacts  with  (3)  one  of  the  serum  proteins,  fibrinogen,  and  this  com- 
bination results  in  the  formation  of  (4)  fibrin.     Fibrin  is  an  insoluble 
protein  material  that  is  precipitated  out  of  the  plasma  as  fine  needle- 

like  crystals.  These  quickly 
increase  in  size  to  form  long 
intertwining  filaments  in  such 
great  numbers  that  the  liquid 
plasma  is  soon  changed  to  a  gel. 
Examined  under  a  microscope, 
tne  blood  clot  shows  the  fibrin 
filaments  present  in  great  abun- 
dance  throughout.  (Fig.  82.) 
Blood  drawn  from  the  ves- 
sels  of  an  experimental  animal 

and  allowed  to  stand  in  a  COn- 

x    •  *ii     i    ±  •         *  •       ± 

tamer  will  clot  in  a  few  minutes. 
If  the  blood  is  stirred  during  clotting,  the  fibrin  filaments  unite  to  form 
long  fibers  which  may  be  removed  together  with  the  enmeshed  blood 
cells  as  a  fibrous  mass,  leaving  behind  a  noncoaguable  blood  fluid,  or 
serum.  If  the  blood  is  left  to  clot  undisturbed,  the  network  of  fibrin 
filaments  will  undergo  spontaneous  contraction  after  a  few  hours,  and 
the  non-coaguable  liquid  serum  will  be  pressed  out  of  the  fibrinousmass. 
The  fibrin  filaments  form  a  temporary  union  of  the  injured  tissues  in  a 
wound  and  are  responsible,  partially  or  entirely,  for  the  permanent 
scar  tissue  that  develops  in  the  site  of  the  wound.  After  the  clot  is 
formed,  uninjured  connective  tissue  cells  from  near-by  regions  move 
into  the  wound  area  and  begin  to  divide  and  form  new  tissue.  Leuco- 
cytes from  the  blood  stream  also  congregate  in  great  numbers  and  prey 
on  invading  bacteria,  if  any  have  got  into  the  wound,  in  an  endeavor 
to  prevent  infection.  The  leucocytes  also  clear  up  cell  debris  that  has 
accumulated  from  the  injured  and  destroyed  cells.  (Fig.  252.) 

Various  methods  are  now  established  by  which  the  normal  process 
of  blood  coagulation  can  be  accelerated  or  retarded  or  even  entirely 
prevented.  Some  of  these  methods  have  been  discovered  very 
recently.  Thus  during  the  past  few  months  it  has  been  established 


high    magnification.     Note    consolidation    of 
fibrin  needles  to  form  long  fibrils. 

* 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM  165 

that  a  hitherto  unknown  factor,  designated  as  vitamin  K,  is  required 
for  normal  coagulation,  as  previously  stated  (page  61).  Contrari- 
wise, various  laboratory  methods  for  the  prevention  of  clotting  have 
been  in  use  for  some  years.  Basically,  these  methods  have  depended 
upon  the  introduction  into  the  drawn  blood  of  a  substance,  such  as 
sodium  or  potassium  oxalate,  that  would  precipitate  the  calcium  in  the 
blood  and  thus  prevent  fibrin  formation.  Also,  it  has  long  been 
observed  that  the  bloodsucking  leeches  and  other  similar  pests  secrete 
a  substance  (hirudin)  from  the  salivary  glands  that  prevents  coagula- 
tion of  the  host  blood  at  the  wound  site  while  the  parasite  is  gorging 
itself.  Of  particular  importance,  however,  as  an  anticoagulant  is  a 
substance,  heparin,  which  is  now  secured  in  considerable  quantities 
from  the  liver,  lung,  and  various  other  tissues  of  cattle  as  a  result  of 
several  years'  research.  It  is  believed  that  heparin  will  be  of  great 
value  in  medicine  in  the  prevention  and  treatment  of  blood  clots,  par- 
ticularly in  the  coronary  vessels  of  the  heart. 

A  well-known  hereditary  disease,  hemophilia,  is  characterized 
by  a  complete  or  partial  failure  of  the  blood  to  coagulate.  This 
failure  is  apparently  due  to  a  defect  in  the  chemical  reactions.  Indi- 
viduals thus  afflicted  are  known  as  bleeders,  because  even  a  slight 
wound  may  result  in  a  fatal  hemorrhage.  The  basis  for  the  hereditary 
transmission  of  this  defect  is  indicated  in  a  later  chapter  (page  383). 
It  is  possible,  though  not  as  yet  established,  that  the  hereditary 
deficiency  may  be  associated  with  a  lack  of  vitamin  K  rather  than 
with  plasma  elements. 

CONTROL,  OF  INFECTION 

The  control  of  infection,  which  always  results  from  the  invasion 
of  a  disease-producing  organism,  is  largely  a  function  of  the  vascular 
system.  This  may  bo  accomplished  in  two  ways.  In  the  first  place, 
there  are  certain  chemical  substances,  known  as  antibodies,  which  ap- 
pear in  the  plasma  of  the  blood  following  an  infection.  Their  exact 
nature  is  unknown,  but  it  is  evident  that  they  are  able  to  render 
the  environment  of  the  host  unsuitable  for  the  invading  organisms. 
Antibodies  are  produced,  presumably  by  the  cells  of  the  host,  and 
given  off  into  the  blood  stream.  Another  method  used  in  the  con- 
trol of  infection  is  through  the  leucocytes  of  the  blood,  which,  *as 
noted  above,  arc  amoeba-like  in  structure  and  phagocytic  in  their 
nutrition.  At  the  time  of  an  infection,  the  numbers  of  leucocytes 
are  greatly  increased,  and  they  congregate  at  the  focus  of  infection  and 
ingest  the  invading  microorganisms.  They  are  aided  in  this  work 
by  a  particular  type  of  antibody,  the  opsonins.  Further  considera- 
tion of  infectious  disease  is  given  in  Chap.  XVII. 


166 


HUMAN  BIOLOGY 


LYMPHATICS 


Of  course,  it  is  the  lymph  that  actually  bathes  the  body  cells  and 
therefore  comes  into  immediate  contact  with  the  infected  areas  and 

the  invading  organism.  The 
lymph,  supplied  as  it  is  with 
leucocytes  and  antibodies,  is  a 
first  line  of  defense  against  in- 
vasion. But  the  tissue  fluids  in 
flowing  through  an  infected  area 
may  become  contaminated  with 
some  of  the  parasites  or  with 
substances  harmful  to  the  tissues 
of  the  host.  Accordingly,  lymph 
returning  from  the  tissues  must 
not  be  permitted  to  enter  tl  3 
blood  stream  for  general  circula- 
tion until  it  has  passed  through 
filtering  stations  equipped  to  re- 
move the  foreign  materials  or  to 
render  them  harmless.  This  is 
the  function  of  the  lymph  nodes 
which  are  present  at  many 
strategic  points  along  the  routes 
of  the  lymphatic  vessels.  A 
typical  lymph  node  may  be  de- 
scribed as  a  small  body,  com- 
parable to  a  bean  in  size  and 
shape,  con  sis  ling  essentially  of  a 
special  lymphoid  tissue.  It  is 
encapsulated  with  connective 
tissue  and  connected  with  affer- 
ent and  efferent  lymph  vessels. 
The  lymph  slowly  filters  through 
channels  in  the  lymphoid  tissue,  and  the  cells  of  the  nodes  are  normally 
able  to  render  the  lymph  harmless  before  passing  it  on  to  the  blood 
stream.  When  the  presence  of  a  harmful  foreign  substance  is  detected, 
the  activity  of  the  nodes  increases;  they  become  enlarged  and  more  or 
less  painful,  as  in  the  case  of  an  infected  throat  or  finger.  (Fig.  83.) 

THE  SPLEEN 

The  spleen  is  situated  in  the  upper  left-hand  region  of  the  abdom- 
inal cavity,  below  (dorsal)  the  cardiac  portion  of  the  stomach.  It  is 
the  largest  lymphatic  gland  in  the  body  and  is  abundantly  supplied 


FIG.  83. — Illustrating  the  lymphatic 
vessels  of  arm  and  chest.  These  join  the 
thoracic  duct  (Fig.  SI  A).  The  chest 
muscle  is  drawn  away  to  expose  the  lymph 
nodes  of  the  arm  pit.  (Haggard,  "Science 
of  Health  and  Disease,"  Harper  &  Brothers.} 


THE  BIOLOGY  OF  THE  VASCULAR  SYSTEM  167 

with  blood  through  the  splenic  artery  and  drained  by  the  splenic 
vein.  As  a  lymphatic  organ,  the  spleen  is  unique  in  that  it  is  directly 
connected  with  the  blood  vascular  system  rather  than  with  the  lymph 
channels.  Histologists  have  shown  that  the  lymphoid  tissue  of  the 
spleen  may  be  differentiated  into  a  so-called  white  pulp,  which  appears 
to  be  primarily  associated  with  arterial  vessels,  and  a  red  pulp  through 
which  the  blood  passes  before  reaching  the  venous  connections.  No 
direct  connections  exist  between  artery  and  vein  through  a  closed 
capillary  system.  In  fact,  the  exact  course  of  the  blood  in  the  splenic 
tissues  is  still  in  doubt.  Intermingled  throughout  the  functional 
splenic  tissues  are  connective  tissue  elements,  together  with  well- 
defined  blood  vessels  and  possibly  muscle  elements.  (Fig.  75.) 

Functionally,  the  spleen  remains  the  great  mystery  organ  of  the 
body.  Suggestions  as  to  its  function  are  numerous  and  varied.  But 
whatever  its  services,  none  of  the  functions  is  absolutely  essential  to 
the  adult  organism,  for  the  spleen  may  be  removed  from  experimental 
animals  without  essentially  affecting  the  vital  functions.  However, 
certain  established  facts  do  indicate  important  splenic  functions  in  the 
normal  vertebrate  organism,  all  of  which  are  apparently  taken  over  by 
the  other  organs  if  the  spleen  is  removed.  In  the  first  place,  the  spleen 
is  clearly  associated  with  the  vascular  system  in  a  number  of  ways, 
notably  in  the  destruction  of  senile  erythrocytes  and  the  subsequent 
formation  of  bilirubin,  a  function  that,  according  to  most  authorities, 
is  shared  by  the  cells  of  the  liver  and  of  the  bone  marrow.  The  spleen 
is  notably  rich  in  iron  compounds,  and  it  must  be  that  the  splenic  cells 
save  and  store  the  essential  iron  from  the  disrupted  hemoglobin 
molecule  and  make  it  available  for  hemoglobin  synthesis  when  needed. 
There  seems  to  be  little  question  that  the  spleen  serves  as  a  reservoir 
for  reserve  stocks  of  normal  erythrocytes  to  be  released  as  needed  fol- 
lowing serious  hemorrhage  or  other  abnormal  demands.  Again,  it 
appears  clear  that  the  spleen  is  continually  forming  leucocytes  for  the 
blood  stream  and  that,  in  the  human  embryo,  during  fetal  life,  it  also 
forms  red  cells  in  great  quantities.  The  nature  of  the  reaction  in 
nutrition  is  not  understood  but  is  clearly  indicated  by  the  fact  that  a 
marked  increase  in  the  size  of  the  spleen  occurs  after  each  meal,  the 
maximum  size  being  reached  some  5  hours  lat^r.  Finally,  considerable 
data  are  available  that  give  strong  evidence  for  an  important  function 
of  the  spleen  in  the  defense  of  the  body  following  various  infections, 
particularly  of  the  blood,  through  the  formation  of  antibodies  and 
phagocytic  cells. 


FRONTAL 
TEMPORAL 

.MASSETER 
Ci:t.it>0-MAST01D 
STERNOHY01D 
TRAPE2IUS 
OMOHYOID" 


DELTOID 
PECTORAtlS  MAJOR 

TRICEPS  - 
BICEPS 

SEftfcATUS  MAGNUS 
BRACH1AL 
RECTUS 

EXTERNAL  OBLIQUE 
BRACHJORADIAL 

LONG  RADIAL  EXTENSOR 
EXTENSOR  CARPI 

RADIAL  BREVIS 

EXTENSOR  OF  FINGERS 
ILIAC 


PSOAS 

PYR  AMI  PALIS 
PECTINEUS 


ADDUCTOR  LONGUS 
SARTORIUS 
RECTUS  FEMORIS 
GRACILIS 


VASTUS  LATERALIS 
VASTUS  MEDIALIS 


GASTROCNEMIUS 
ANTERIOR  TIB1AL 


SOLEUS 
LONG  PERONEAL- 


LONG  EXTENSOR-TOES 
LONG  EXTENSOR:GREAT  TOE 


OCCIPITAL 

STERNO  CLEJDO-MASTOID 
SPLENIUS 


TRAPEZIUS 

DELTOID 
INFRASPINATUS 
TERES  MINOR 
TERES  MAJOR 
RHOMBOID 

TRICEPS 
BRACHIAL 
LATISSIMUS  DORSI 
BRACHIORAD1AL 
LONG  RADIAL  EXTENSOR 
EXTERNAL  OBLIQUE 
PALMARIS  LONGUS 

FLEXOR  CARPI  RADIALIS 
FLEXOR  CARPI  ULNARIS 
•GLUTEUS  MEDIUS 

GLUTEUS  MAXIMUS 
ADDUCTOR  MAGNUS 

BICEPS 

VASTUS  EXTERNUS 

SEMITENDINOUS 

SEMIMEMBRANOUS 

SEMIMEMBRANOUS 


GASTROCNEMIUS 


SOLEUS 


ACHILLES' TENDON 
PERONEUS  LONGUS 
PERONEUS  BREVIS 


PLATE  X. — The  chief  muscles  of  the  human  body  as  seen  from  the  ventral  and  dorsal 

aspects. 


CHAPTER  VIII 
BIOLOGY  OF  THE  MUSCULAR  SYSTEM 

It  should  be  evident  from  the  study  of  the  various  organ  systems 
as  given  in  the  previous  chapters  that  there  is  a  widespread  demand 
in  the  animal  body  for  contractile  tissues  to  perform  the  many  types 
of  movements  essential  to  the  various  functions.  The  primary  func- 
tions of  nutrition,  respiration,  and  transportation  are  all  closely  bound 
up  with  the  activities  of  muscle  tissue. 
But  even  so  it  should  be  remembered  that 
the  muscular  system,  in  supplying  motor 
service  to  the  various  other  organ  systems  of 
the  body,  is  by  these  same  acts  helping  to 
supply  its  own  needs,  for  each  muscle  cell 
must  continually  receive  essential  materials 
and  be  relieved  of  the  metabolic  wastes, 
services  supplied  by  the  other  systems. 
And  so  it  is  with  all  the  tissues  and  organs 
in  the  body;  they  give  to  all  and  receive 
from  all. 

Muscle  tissue,  as  the  essential  unit  in 
voluntary  bodily  motion,  has  its  own  partic- 
ular function  in  the  organism  in  addition  to 
the  general  assistance  it  renders  to  the  other 
organ  systems.  This  special  function  of  the 
muscular  system,  voluntary  movement,  is 
continually  evident  in  the  motions  of  the 
appendages  in  doing  the  almost  numberless 
things  that  fall  to  their  lot,  including 
locomotion,  which  involves  the  movement  of 
the  entire  organism  from  place  to  place.  Locomotion  of  some 
kind  is  essential  for  the  nutrition  of  most  animal  types.  Voluntary 
movement,  then,  as  the  distinct  function  of  the  muscular  system, 
may  well  be  distinguished  from  involuntary  movement  in  which 
special  varieties  of  muscle  tissue  are  built  into  the  functional  units  of 
other  organ  systems  to  supply  the  particular  type  of  movement  essen- 
tial to  the  various  functions. 

169 


FIG.  84. — Group  of  uni- 
cellular Vorticellae,  each 
attached  to  the  surface  of 
a  water  plant  by  a  contrac- 
tile filament  containing 
myonemo  fibers.  (Redrawn 
from  Sedgwick  and  Wilson, 
"  General  Biology"  Henry 
Holt  &  Company,  Inc.) 


170 


HUMAN  BIOLOGY 


STRUCTURAL  FEATURES  ASSOCIATED  WITH  MOVEMENT 

Contractile  elements  are  found  even  in  the  one-celled  organisms. 
Thus,  the  familiar  bell-shaped  protozoan  Vorticella  supplies  a  particu- 
larly good  example  of  a  contractile  element  in  the  so-called  myoneme 
fibers  which  originate  in  the  wall  of  the  bell-shaped  body  and  converge 
to  form^  a  contractile  filament  in  the  handle,  or  stalk,  of  the  bell,  by 

which  the  animal  is  attached. 
Any  disturbance  causes  the  tiny 
Vorticella  to  contract  into  a 
spherical  body,  with  the  attached 
filament  drawn  up  like  a  tightly 
coiled  spiral  spring.  In  the  pri- 
mitive multicellular  organisms, 
such  as  the  Coelenterates,  con- 
tractile elements  are  present  in 
many  of  the  cells.  Hydra  serves 
again  as  a  good  example,  for  the 
ectoderm  that  covers  the  outer 
surface  and  forms  a  large  part  of 
the  body  wall,  consists  chiefly  of 
epitheliomuscular  cells  which  show 
marked  power  of  contraction. 
Muscle  tissue  in  higher  animals 
is  built  up  by  the  union  of  many 
cellular  elements  to  form  definite 
units  which  function  only  in  con- 
traction. It  develops  from  the 
mcsoderm  and  accordingly  is 
found  only  in  the  triploblastic 
animals.  It  is  well  shown  in  the 
body  wall  and  alimentary  canal 
of  the  earthworm.  And  one  of 
the  most  noteworthy  types  of 
muscle  tissue,  possessing  amazing  functional  ability,  is  found  in  the 
wing  muscles  of  various  insects.  Another  type  of  wing  muscle  which 
is  comparable  functionally  is  found  in  a  vertebrate,  the  hummingbird 
(page  142).  Histological  examination  of  invertebrate  and  vertebrate 
muscle  tissues  show  essentially  the  same  structural  pattern.  (Figs. 
84  to  86.) 

At  noted  in  the  earlier  chapter  on  the  Organization  of  the  Body, 
three  types  of  muscle  tissue  are  recognized  in  the  vertebrate  organism: 


B 


FIG.  85. — External  views  of  the  primi- 
tive metazoan,  Hydra.  A,  expanded;  B, 
contracted.  (Haupt.) 


BIOLOGY  OF  THE  MUSCULAR  SYSTEM 


171 


smooth  (involuntary),  striated  (voluntary),  and  cardiac  (striated  and 
involuntary).  Our  present  discussion  will  deal  with  muscles  as  an 
independent  organ  system,  the  muscular  system,  and  accordingly  is 
largely  concerned  with  striated  or  voluntary  muscle  tissue,  which  forms 
many  definite  motor  organs,  the  separate  muscles  of  the  body:  each 
with  a  particular  function,  such,  for  example,  as  the  prominent  leg 
muscles  or  the  biceps  muscle  of  the  arm.  Before  doing  this,  however, 


Dorsal  vessel 
Chloragogen  cells 
Endoderm 
Muscle 
Peritoneum, 

Typhlosole 


Cuticle 

!       Ectoderm 

'      Circular  muscle 


longitudinal  muscle 
'eritoneum 


Coelom 


«i    i.  -A-  '  '    /Ventral      \  Ventral     AEnterOn 

Nephndmm  /      vessel         \  nerve  cord      anceron 


esse 

Lateral  vessel          Subneural  vessel 

FIG.  86.  —  Transverse  section  thnmph  the  body  of  an  earthworm,  illustrating  the 
general  arrangement  of  structures  in  a  liiplohlastic  animal  and  the  presence  of  muscle 
tissue  in  the  wall  of  the  alimentary  canal  and  the  body  wall.  Diagrammatic.  (Hegner.) 

it  will  be  well  to  summarize  the  main  characteristics  of  smooth  muscle 
tissue. 

SMOOTH,  OR  INVOLUNTARY,  MUSCLE 

Though  unstriated  muscle  is  loss  advanced  in  its  cellular  develop- 
ment than  is  striated  muscle,  it  rates  high  in  its  functional  importance. 
The  destruction  of  even  a  small  amount  of  smooth  muscle  tissue  in  an 
organ  system,  such  as  the  nutritive  system,  will  tend  to  disrupt  all  the 
essential  functional  activities.  On  the  other  hand,  considerable  areas 
of  striated  tissue  can  be  destroyed,  as  in  the  amputation  of  one  or  more 
limbs,  without  modifying  the  vital  activities  of  the  organism.  Speak- 
ing generally,  smooth  muscle  consists  of  separate  sheets  of  tissue  which 


'172  HUMAN  BIOLOGY 

are  built  up  by  the  association  of  great  numbers  of  independent  con- 
tractile cells.  These  muscular  sheets  act  as  individual  structural  and 
functional  units;  that  is,  they  are  independently  innervated,  and  the 
constituent  cells  in  a  certain  area  respond  as  a  coordinated  unit  in 
producing  contraction.  When  the  smooth  muscle  tissue  forms  a 
definite  band  of  contractile  tissue  surrounding  an  open  cavity,  as  is 
usually  the  case,  it  will  be  found  that  the  muscular  band  is  composed 
of  a  number  of  associated  sheets  of  contractile  tissue  closely  aligned  to 
each  other.  ^  However,  many  instances  occur  where  smooth  muscle 
tissue  does  not  form  definite  contractile  sheets  or  bands  but  instead 
appears  as  a  localized  contractile  area  containing  a  few  muscle  cells 
and  surrounded  by  connective  tissue  elements,  as,  for  example,  in 
certain  blood  vessels.  (Figs.  21,  73.) 

Throughout  the  body  in  whatever  location  and  condition  found, 
smooth  muscle  tissue  is  involuntarily  controlled  through  the  autonomic 
nervous  system.  Smooth  muscle  tissue  does  not  react  so  quickly  to 
nerve  stimuli  as  does  striated  muscle,  and  it  is  therefore  particularly 
adapted  for  the  comparatively  slow  wave  of  contraction  that  is  charac- 
teristic of  intestinal  peristalsis.  It  also  has  great  ability  to  remain 
contracted  or  to  stay  in  tone,  as  the  physiologists  say,  for  long  periods 
of  time.  This  characteristic  is  very  important  in  many  of  the  organs 
where  it  is  used. 

Smooth  muscle  tissue  is  widely  distributed  throughout  the  body. 
To  get  the  matter  clearly  in  mind,  a  number  of  the  more  important 
locations  of  the  unvStriated  muscle  tissue  may  be  listed.  In  the  corium 
of  the  skin,  smooth  muscle  forms  minute  cylindrical  units  which  are 
connected  to  the  hair  follicles  so  that  the  hairs  stand  on  end  when 
the  muscles  contract,  and,  in  addition,  it  is  widely  but  irregularly 
scattered  throughout  the  skin.  In  the  alimentary  tract,  smooth  mus- 
cle tissue  forms  definite  layers,  both  longitudinal  and  transverse. 
Except  for  a  short  distance  near  the  anterior  and  posterior  terminal 
portions,  these  layers  extend  throughout  the  entire  length  of  the 
alimentary  tract  and  are  responsible,  as  already  noted,  for  the  very 
considerable  motility  exhibited  in  the  peristaltic  actions.  Further- 
more, the  ducts  of  the  associated  nutritive  glands  as  well  as  those  of 
all  the  glands  in  the  body  contain  their  quota  of  smooth  muscle  tissue. 
The  ducts  of  the  urinary  system  and  the  bladder  consist  almost  entirely 
of  unstriated  muscle  tissue.  In  a  later  chapter  on  Reproduction, 
the  abundance  of  smooth  muscle  tissue  will  be  shown  in  the  reproduc- 
tive system,  particularly  in  the  uterus  of  the  mammalian  female. 
Finally,  it  must  be  noted  that  the  walls  of  the  various  types  of  bJood 
vessel,  with  the  exception  of  the  heart,  which  has  its  own  type  of 


BIOLOGY  OF  THE  MUSCULAR  SYSTEM 


173 


muscle  tissue,  contain  varying  quantities  of  smooth  muscle  tissue  in 
correspondence  with  the  functional  demands. 

STRIATED  OR  VOLUNTARY  MUSCLE 

The  374  muscles  comprising  this  organ  system  in  man  are  all 
connected  with  the  nervous  system  in  such  a  way  as  to  be  under 
voluntary  control.  So  far  as  these  muscles  are  concerned,  we  can 
move  the  jaw,  wink  an  eye,  pitch  a  ball,  move  about  a  room,  or  start 
on  a  journey  around  the  world  by  simply  willing  to  do  so.  Striated 


Fio.  87. — Illustrating  the  pair  of  opposed  muscles  (biceps  and  triceps)  which 
c&use  movements  in  the  forearm.  (Haggard,  "Science  of  Health  and  Disease,"  Harper 
&  Brothers.) 

muscles  are  sometimes  referred  to  as  the  skeletal  muscles,  because  they 
are  attached  to  bones  and  other  skeletal  elements  that  form  the 
framework  of  the  body  and  its  appendages.  One  of  the  muscle 
attachments,  the  origin,  is  usually  fixed  and  immovable,  whereas  the 
other  attachment,  the  insertion,  is  to  a  freely  movable  bone  which 
thus  serves  as  a  lever  to  translate  muscle  contraction  into  bodily 
movement.  (Plate  X;  Fig.  97.) 

Muscle  tissue  exerts  power  to  do  work  only  when  it  contracts;  the 
relaxation  of  muscles  following  contraction  has  no  power  to  produce 
bodily  movement.  This  being  so,  it  is  evident  that  to  move  any  part 
of  the  body  in  opposite  directions  requires  two  separate  muscle  units. 
These  mast  be  mounted  in  such  a  way  that  they  pull  in  opposite  direc- 


174  HUMAN  BIOLOGY 

tions  on  the  movable  bone  to  which  they  are  attached.  Thus  the 
forearm  is  elevated  by  the  contraction  of  the  large  biceps  muscle 
(flexor)  lying  ventrally  above  the  elbow  joint;  it  is  lowered  by  the 
contraction  of  the  triceps  muscle  (extensor)  which  is  also  situated 
above  the  elbow  joint,  but  on  the  dorsal  side.  In  the  same  way  there 
are  adductor  muscles  which  draw  the  limbs  backward  toward  the  long 
axis  of  the  body,  and  abductor  muscles  which  work  opposite  to  the 
adductors  and  draw  the  limbs  anteriorly;  or  the  levators  which  elevate 
some  part  of  the  body,  such  as  the  lower  jaw,  when  they  contract; 
and  the  depressors  which  pull  in  the  opposite  direction.  (Fig.  87.) 

Since  voluntary  muscles  are  in  pairs  in  order  that  movement  may 
occur  in  opposite  directions,  it  follows  that  one  member  of  the  paired 
muscles  must  always  relax  synchronously  with  the  contraction  of  the 
other  if  movement  is  to  be  produced  in  the  attached  part.  If  both 
muscles  began  to  contract  at  the  same  time,  they  would  pull  against 
each  other,  and  no  movement  would  result.  The  synchronous  con- 
traction and  expansion  of  the  paired,  but  independent,  muscle  units 
involves  a  nicety  of  control  by  the  nervous  system  that  is  not  generally 
appreciated. 

Types  of  Muscles. — The  voluntary  muscles  that  compose  the 
human  muscular  system  may  be  divided  into  (1)  the  .segmental  muscles 
which  are  associated  with  the  head  (eye  and  tongue  only),  the  trunk, 
and  the  appendages;  and  (2)  the  superficial  skin,  or  integumental, 
muscles  which  are  almost  exclusively  located  in  and  around  the  facial 
region,  just  under  the  skin,  and  are  responsible  for  a  wide  variety  of 
facial  expressions.  If  you  are  pleased  as  you  read  this,  certain  of  these 
integumental  muscles  are  responding  to  your  mental  state  in  a  way 
that  a  person  nqar  by  can  interpret  by  noting  the  expression  on  your 
face.  The  study  of  muscle  development  in  a  vertebrate  embryo  shows 
that  it  is  basically  segmental.  In  the  lower  forms,  this  definite 
segmental  arrangement  of  the  body  muscles  is  very  evident,  as  can 
be  seen,  for  example,  by  removing  the  skin  from  a  salamander.  In 
adult  man  and  the  higher  vertebrates,  the  segmental  muscles  are 
plainly  evident  only  in  the  chest  region,  under  the  arms,  where  they 
are  associated  with  the  ribs.  Outside  of  this  region  the  underlying 
segmental  muscles  are  covered  over  by  very  large  appendicular  mus- 
cles associated  with  the  arms  and  legs.  The  appendicular  muscles 
are  derived  in  the  embryo  from  segmental  muscles;  but  in  the  later 
development,  they  lose  all  evidence  of  their  segmental  character. 
The  same  origin  is  true  of  the  integumental  muscles  in  the  skin,  though 
it  was  formerly  held  that  they  had  an  independent,  nonsegmental 
origin  in  the  corium  of  the  skin.  (Fig.  88.) 


BIOLOGY  OF  THE  MUSCULAR  SYSTEM 


175 


The  voluntary  muscles  show  great  variation  in  length  and  size 
ranging  from  the  tiny  muscles  associated  with  eye  movements  to  the 
large  sartorius  of  the  leg  which  has  its  origin  in  the  connective  tissues 


FIQ.  88. — Diagram  illustrating  the  scgrnental  arrangement  of  the  muscle  rudiments 
In  the  human  embryo.  Roman  numerals  refer  to  the  cranial  nerves  (page  242).  Seg- 
mental  myotomes  iri  the  various  regions  of  the  body  are  numbered  (A,  B,  C,  Nl,  N2, 
N3,  JV4,  1,  2,  3,  4  etc.).  (Walter,  After  Cunningham.) 

of  the  body  wall  above  the  hip  joint,  and  its  insertion  to  the  tibia  below 
the  knee,  thus  giving  it  a  length  between  2  and  3  ft.  In  spite  of  the 
great  variation  in  the  size  of  muscles,  the  structural  features  are 
essentially  the  same  throughout  the  body.  The  larger  muscles  simply 
contain  a  greater  number  of  functional  muscle  fibers.  Each  independ- 
snt  voluntary  muscle,  large  or  small,  is  enclosed  in  a  sheath  of  fibrous 
connective  tissue,  the  fascia,  which  is  considerably  longer  than  the 
muscle  it  encloses.  The  projecting  ends  of  the  sheath,  strengthened 
by  additional  connective  tissue  elements,  converge  to  form  tendons 
which  are  attached  to  bone  or  other  connective  tissue  elements  at  the 
3rigin  and  insertion  of  the  muscle,  as  noted  above.  (Fig.  89.) 

Motion. — Muscle  contraction  is  translated  into  bodily  motion 
by  the  pull  exerted  on  the  bone  levers  through  the  unstretchable 
tendons.  Tracing  the  various  structures  in  reverse  order,  then,  we 
tiave  bone,  tendon,  fascia,  and,  finally  the  muscle  which,  as  the  funda- 
mental contractile  tissue,  gives  the  original  pull.  The  dissection  of  a 
muscle  unit  shows  that  it  is  divided  into  a  varying  number  of  com- 
partments, or  fasciculi,  each  enclosed  by  an  inner  layer  of  connective 


176  HUMAN  BIOLOGY 

tissue,  the  perimysium,  which  is,  in  fact,  a  continuation  of  the  outer 
fascia.  Within  the  fasciculi  are  muscle  bundles  containing  great 
numbers  of  microscopic  muscle  fibers  separated  from  each  other  by  a 


'  • 


.  •  :  -       •          lP^ 


FIG.  89. — Connections  between  a  voluntary  muscle,  tendons,  and  bones.     x%  muscle 
fiber.     (Maximow-Bloom,  "Histology,"  W.  B.  Saunders  Company.) 

still  further  extension  of  connective  tissue,  the  endomysium.  When 
the  fibers  are  carefully  examined  under  the  microscope,  it  is  found  that 
each  one  is  enclosed  in  a  very  delicate  connective  tissue  sheath,  the 
sarcolemma,  which  represents  the  final  subdivision  of  the  connective 
tissue  elements.  (Figs.  89,  90a,  6.) 

Histology  of  Striated  Muscle. — It  will  be  well  at  this  point  to 
consider  the  finer  structure  of  the  functional  muscle  fibers  in  consider- 
able detail.  Each  muscle  fiber  is  regarded  as  essentially  a  single 
multinucleate  cell,  highly  variable  in  size  and  in  the  number  of  nuclei 
it  contains.  These  thread-like  cell  fibers  usually  measure  around  0.04 
in.  in  length,  but  observations  have  been  made  in  which  a  length  of  an 
inch  or  more  were  noted.  The  diameter  of  the  muscle  fiber  is  usually 
stated  to  be  about  one-tenth  of  its  length,  but  here  again  considerable 
variation  has  been  found.  But  the  individual  muscle  fibers,  with  their 
cellophane-like  wrapping  of  sarcolemma,  are  not  the  ultimate  micro- 
scopic units  in  the  muscle  complex,  for  each  fiber  consists  of  great 
numbers  of  myofibrils  of  the  same  length  as  the  muscle  fiber  but  which, 
at  most,  are  probably  not  more  than  0.00004  in.  in  diameter  and  may 
even  narrow  down  beyond  the  limits  of  microscopic  vision.  In  fact, 
evidence  exists  that  the  myofibrils  are  repeatedly  subdivided  to  form 
elongated,  ultramicroscopic  units  finally  reaching  the  molecular  level. 
The  striated  muscle  fiber  is  characterized  by  distinct  longitudinal 
and  transverse  striations.  The  longitudinal  striations  are  due  to  the 
myofibrils  packed  closely  side  by  side  like  thousands  of  tiny  sticks  of 
candy  in  a  glass  jar.  The  glass  wall  of  the  jar  would  correspond  to 
the  sarcolemma,  or  cell  wall.  The  transverse  striations  are  due  to 
the  stripings  on  the  individual  myofibrils  which,  instead  of  going  around 
and  around  spirally,  as  in  sticks  of  candy,  go  directly  across  eao& 


BIOLOGY  OF  THE  MUSCULAR  SYSTEM 


177 


fibril  at  right  angles  to  the  long  axis.  Muscle  fibers  contain  another 
important  structural  unit,  the  sarcoplasm,  which  appears  as  a  trans- 
parent, scmiliquid  substance  surrounding  the  myofibrils  and  enclosed 
by  the  sarcolemma.  Sarcoplasm  might  be  compared  to  a  sirup  that 
had  been  added  to  the  jar  filled  with  sticks  of  candy  so  as  completely 
to  fill  all  the  interstices  between  the  sticks.  To  complete  the  crude 
analogy,  some  disc-shaped  cinnamon  drops,  representing  the  nuclei 
of  the  muscle  fiber,  might  be  added,  which  would  lie  irregularly 
scattered  in  the  sugar  sirup  (sarcoplasm)  close  to  the  glass  wall 
(sarcolemma).  (Fig.  90c.) 


MYO  FIBRILS 


NUCLEUS 


ob  c 

FIG.  90. — Structure  of  striated  muscle.  Diagrammatic,  a,  portion  of  muscle  with 
numerous  bundles  of  fibers  (fasciculi) ;  b,  portion  of  a,  showing  a  single  bundle  of  muscle 
fibers;  c,  portion,  of  7>,  highly  magnified  to  show  a  single  muscle  fiber  composed  of  great 
numbers  of  myofibrils  (sarcolemma  not  shown).  (Goldschmidt,  "  Ascaris,"  Prentice- 
Hall,  Inc.) 

The  fact  that  each  transverse  striation  continues  across  the  fiber 

as  a  regular  unbroken  line  ( )  rather  than  as  a  series  of  irregular 

segments  (  —  ~~_~~~~—  —  ~~~_~),  corresponding  to  the  markings  on 
individual  myofibrils,  is  due  to  the  fact  that  the  striations  on  the  thou- 
sands of  myofibrils  in  a  muscle  fiber  are  equispaced  and  in  the  same  rela- 
tive position ;  that  is,  to  refer  again  to  our  previous  analogy  with  the  candy 
sticks,  the  stripes  all  match  when  the  sticks  are  lined  up  beside  each 
other.  Careful  study  of  the  transverse  myofibril  striations  under  the 
highest  powers  of  the  microscope  does  not  answer  all  the  questions 
about  their  structural  details,  but  it  can  be  seen  that  several  varieties 
of  transverse  markings  occur  in  a  regular  linear  order.  In  properly 


178  HUMAN  BIOLOGY 

prepared  material,  alternate  light  and  dark  areas  (bands  or  discs)  arc 
clearly  seen  under  the  microscope. 

The  dark  bands  seem  to  be  uniform  throughout,  but  in  the  center 
of  each  light  band  is  a  fine  granular  line  (Krause's  membrane),  which 
divides  it  into  halves.  The  light  band  is  commonly  designated  as  the 
/-band;  the  dark  band,  as  the  Q-band;  and  the  granular  line,  as  the 
X-band.1  Without  going  into  further  details,  it  may  be  said  on  the 
basis  of  studies  made  with  polarized  light  that  the  material  ( molecules) 
composing  the  Q-band  is  arranged  in  definite  directions  (aniso tropic), 
whereas  that  of  the  /-band  is  not  oriented  (isotropic)  and  so  appears 
light  in  color  because  it  reflects  light  equally  in  all  directions.  Begin- 
ning with  the  X-band  and  proceeding  along  the  fiber,  the  following 
arrangement  of  bands  occurs;  K — /  (light) — Q  (dark) — /  (light) — K — 
/  (light),  etc.,  continuously  repeated  throughout  the  entire  length  of 
the  fiber.  Thus  the  segment  of  a  myofibril  from  K  to  K  includes  the 
central  dark  Q-band  in  contact  on  each  side  with  the  light  /-band. 
This  linear  arrangement  of  transverse  bands  is,  of  course,  due  to  the 
striation  of  the  constituent  myofibrils.  On  this  basis,  the  fiber  is  seen 
to  consist  of  a  series  of  K  to  K  segments  which  are  termed  sarcomeres. 
(Fig.  90c.) 

The  sarcomeres  appear  to  be  the  basic  functional  units  in  volun- 
tary muscle  tissue,  for  close  microscopic  examination  of  contracted 
and  relaxed  fibers  show  that  these  linear  units  are  shorter  and  wider 
in  a  contracted  muscle  fiber  than  they  are  in  a  relaxed  fiber.  When 
muscle  tissue  contracts,  a  shortening  occurs  of  the  individual  fibers 
with  their  constituent  myofibrils;  and  this,  in  turn,  is  reflected  in  the 
contraction  of  the  sarcomeres.  Thus  it  is  evident  that  the  sarcoplasm, 
in  which  the  fibrils  are  embedded,  is  not  directly  concerned  with  the 
phenomenon  of  contraction;  it  is  probably  a  nutrient  material  com- 
parable, in  a  sense,  to  the  blood  plasma  in  its  functional  relationships 
to  the  living  myofibrils,  the  basic  functional  units  of  the  muscle  fiber. 

FUNCTIONAL  FEATURES  ASSOCIATED  WITH  THE  MUSCULAR  SYSTEM 

Movement  in  the  various  parts  of  our  body  is  so  common  arid 
universal  that  there  is  a  great  tendency  to  overlook  the  various  complex 
phenomena  associated  with  it.  Although  movement  is  primarily  a 
function  of  the  muscles;  that  is,  they  are  the  one  tissue  in  the  body 
that  is  differentiated  for  the  function  of  contraction,  voluntary  motion 
is  really  a  product  of  three  organ  systems;  the  muscular,  the  support- 
ing, and  the  nervous.  The  impulses  that  incite  contraction  come  from 

1  Terminology  used  by  various  authorities  for  the  structural  elements  of  striated 
muscle  fibrillae  varies  considerably. 


BIOLOGY  OF  THE  MUSCULAR  SYSTEM 


179 


the  nervous  system,  and  the  pull  of  the  muscle,  as  we  have  seen,  is 
transmitted  through  the  attached  connective  tissue  tendons  to  the 
bones  which  serve  as  movable  levers  and  as  fixed  anchors.  Our 
concern  in  the  present  chapter  is  confined  to  the  phenomenon  of 
muscle  contraction.  The  contributions  of  the  skeletal  and  nervous 
systems  to  bodily  movement  will  be  considered  in  the  following 
chapters. 

Muscle -nerve  Preparation. — Contractility  of  muscle  tissue  will 
occur  independently  of  any  connection  with  another  organ  system  or 
even  of  the  body  itself.  As  an  example  of  this,  the  study  of  a  muscle- 


Fio.  91. — Illustrating  the  method  for  securing  a  graphic  record  of  muscle  contraction 
by  means  of  the  kymograph,  as  described  on  page  180.  A,  muscle,  with  attached  nerve 
(right) ;  B,  writing  lever  with  hinge  (Cr)  and  counterweight  (D) ;  E,  revolving  drum  of 
kymograph  with  smoked  paper  attached,  on  which  the  record  of  contraction  is  made; 
F,  time  record;  G,  fan  for  regulating  kymograph  speed.  (Kimber,  Gray,  and  Stackpole.) 

nerve  preparation  is  of  value.  Such  a  preparation  is  made  by  care- 
fully removing  a  muscle,  together  with  the  attached  nerve  that 
innervates  it,  from  an  anesthetized  animal.  A  very  good  muscle  for 
this  work  is  the  large  gastrocnemius  muscle  from  the  hind  leg  of  a  frog. 
The  tendon  or  bone  attached  to  one  end  of  the  muscle  is  first  fastened 
to  an  immovable  structure,  corresponding  to  the  origin  of  the  muscle 
in  the  body,  and  the  other  end,  or  insertion,  is  fastened  to  a  movable 
lever.  The  muscle  and  attached  nerve  are  kept  from  drying,  which 
would  quickly  cause  the  death  of  these  tissues,  by  the  application  of 
an  isotonic  salt  solution.  When  so  treated,  an  excised  frog  muscle 
and  nerve  can  be  kept  alive  and  in  good  shape  for  experimental  work 


180  HUMAN  BIOLOGY 

for  some  hours.  Since  no  blood  supply  is  available  for  the  excised 
muscle,  it  means  that  the  muscle  tissues  have  enough  nutritive  mate- 
rials and  oxygen  stored  in  reserve  to  last  several  hours.  Gradually, 
as  these  are  used  up  and  the  wastes  also  accumulate,  the  tissues  will 
die.  (Fig.  91  A.) 

The  muscle  may  now  be  directly  stimulated  by  the  use  of  various 
agents,  such  as  contact,  heat,  chemicals,  and  electric  current.  So  long 
as  the  muscle  remains  in  good  condition,  it  will  respond  to  these 
various  types  of  stimulus  and  contract  essentially  as  it  would  in  the 
body.  Or  the  muscle  may  be  stimulated  indirectly  through  the 
attached  nerve.  When  the  end  of  the  nerve  or  any  portion  of  it  is 
subjected  to  the  same  irritants,  which  were  used  with  the  muscle 
directly,  a  nerve  impulse  will  develop  and  be  transmitted  into  the 
muscle  in  the  same  way,  apparently,  as  if  both  nerve  and  muscle  were 
normally  situated  in  the  body  and  the  stimulus  for  voluntary  con- 
traction had  originated  in  the  brain.  By  varying  the  experimental 
conditions  of  the  muscle-nerve  preparation,  a  great  deal  can  be  learned 
about  muscle  and  nerve  function.  Thus  the  experimenter  can  deter- 
mine how  long  it  takes  the  muscle  to  respond  after  the  stimulus  is 
given;  how  long  it  takes  it  to  relax  after  contraction;  how  much  work 
it  is  capable  of  doing  in  lifting  a  weight  at  a  certain  rate  per  minute; 
what  the  conditions  are  associated  with  fatigue;  the  minimum  stimulus 
necessary  to  cause  a  muscle  to  contract;  the  effect  of  placing  muscle 
tissue  in  various  gases;  etc.  It  will  also  be  relatively  easy,  with  the 
proper  apparatus,  to  determine  the  speed  of  the  impulse  passing 
through  the  nerve  tissue  on  the  way  to  the  muscle  and  the  varying 
conditions  that  will  incite  an  impulse. 

STUDY  OF  MUSCLE  CONTRACTION 

Two  very  useful  instruments  employed  by  the  physiologists  in  the 
studies  of  muscle  function  are  the  induction  coil  and  the  kymograph. 
The  induction  coil  is  used  because,  of  all  the  artificial  stimuli  available, 
the  electric  current  is  best  suited,  and  the  induction  coil  permits  the 
operator  easily  to  control  both  the  strength  of  the  electric  stimulus 
and  the  exact  times  at  which  they  are  given.  The  kymograph  makes 
possible  a  permanent  graphic  recording  of  muscle  action  which  can  be 
studied  after  the  experiment.  The  kymograph  consists  essentially  of 
a  clockwork  motor  attached  so  as  to  revolve  a  vertical  shaft  at  a 
uniform  rate.  A  removable  metal  drum  is  fastened  on  the  shaft  to 
revolve  with  it.  A  strip  of  smoked  kymograph  paper  of  the  same 
width  as  the  side  of  the  drum  is  wrapped  around  the  drum  and  attached 
to  it  by  pasting  the  ends.  When  the  motor  is  started,  the  vertical 


BIOLOGY  OF  THE  MUSCULAR  SYSTEM  181 

shaft,  together  with  the  attached  drum  and  covering  of  smoked 
kymograph  paper,  is  revolved  at  a  specified  number  of  revolutions 
per  minute  which  may  be  varied  according  to  the  needs  of  the  experi- 
ment. (Figs.  91,  92.) 

In  recording  muscle  action,  a  writing  point  is  fastened  to  the  end  of 
the  movable  muscle  lever  of  the  muscle-nerve  preparation  and  placed 
in  contact  with  the  smoked  paper  on  the  drum  in  such  a  way  that, 
when  the  lever  moves,  a  visible  line  will  be  scratched  through  the 
smoke  film  on  the  kymograph  paper.  If,  now,  the  muscle  is  stimu- 
lated and  caused  to  contract,  a  line  will  be  scratched  on  the  paper,  the 
height  of  which  will  record  the  amount  that  the  muscle  contracted. 
If,  however,  the  drum  is  revolving  as  the  muscle  contracts,  a  curve 
will  be  drawn  on  the  drum  instead  of  a  straight  line.  Since  the  drum 


FIG.  92. — Induction  coil  for  muscle  stimulus,  patterned  after  the  original  duBois- 
Reymond  model.  Connections  with  battery  are  made  at  P'  and  p" .  Stimulus  may  be 
strengthened  by  decreasing  the  distance  between  the  secondary  coil  (B)  and  the  primary 
coil  (A).  S,  slide  with  graduated  scale.  (Howdl,  "Textbook  of  Physiology,"  W.  B. 
tiaunders  Company.) 

is  moving  at  a  regular  measured  rate,  the  length  of  the  ascending  curve 
will  indicate  the  time  elapsed  during  contraction,  and  the  height  of  the 
curve  from  a  base  line  the  extent  of  contraction.  As  the  muscle 
relaxes,  the  lever  will  form  a  curve  sloping  down  to  the  base  line.  The 
time  interval  can  be  graphically  recorded  on  the  drum  in  association 
with  the  muscle  curve  by  a  separate  writing  lever  attached  to  a  special 
clock  which  raises  the  lever,  for  example,  every  hundredth  of  a  second 
and  thus  marks  the  smoked  paper.  When  the  complete  experimental 
record  is  obtained,  the  smoked  paper  is  carefully  removed  from  the 
kymograph  and  dipped  into  a  fixing  varnish  which  quickly  dries  and 
thus  makes  a  permanent  record.  (Fig.  91J57,  F]  93.) 

The  data  accumulated  from  this  type  of  experiment  show  that 
muscles  in  different  animals  and  in  different  regions  of  the  same  animal 
show  considerable  variation  in  the  rapidity  and  strength  of  contrac- 
tion. Witness  the  muscle  of  the  insect  wing,  which  requires  only  0.003 
second  to  contract  in  response  to  a  stimulus,  whereas  the  gastrocnemius 
of  the  frog  requires  0.1  second  for  the  operation.  And  much  slower 


182  HUMAN  BIOLOGY 

still  is  the  involuntary  muscle  tissue  of  the  vertebrate,  which  requires 
several  seconds  to  respond  to  a  stimulus.  The  elapsed  time,  between 
the  giving  of  the  stimulus  and  the  beginning  of  contraction  in  a  muscle, 
is  known  as  the  latent  period.  This,  in  the  gastrocnemius  of  the  frog, 
is  about  0.01  second.  Then  comes  the  period  of  contraction  which 
requires  about  0.04  second  and,  finally,  the  period  of  relaxation  which 
is  about  0.05  second.  The  latent  period,  contraction,  and  relaxation 
include  the  complete  cycle  of  contraction  phenomena,  and  together 
comprise  the  muscle  twitch  as  distinguished  from  sustained  contraction. 
Muscle  Fatigue. — When  a  muscle  is  given  a  series  of  separate 
electric  stimuli,  it  will  respond  until  it  gets  tired,  or  fatigued,  provided 
the  stimuli  do  not  come  too  frequently  and  so  prevent  the  muscle  from 


FIG.  93. — Kymograph  record  showing  development  of  tetanus  due  to  rapid  stimuli, 
as  described  on  page  183.  Vertical  lines  represent  single  contractions.  These  disap- 
pear as  rate  of  stimuli  increases.  (Howell,  "  Textbook  of  Physiology,"  W.  B.  Saunders 
Company.) 

completing  the  relaxation  period.  Fatigue,  which  is  marked  by  a  loss 
of  irritability,  gradually  develops,  due,  apparently,  to  the  accumulation 
of  wastes  in  the  muscle  tissue.  It  occurs  in  every  muscle  when  sub- 
jected to  maximum  work  for  considerable  periods.  In  the  muscle- 
nerve  preparations,  it  will  be  found  that  the  response  of  the  tiring 
muscle  becomes  less  and  less  until,  finally,  it  will  not  respond  at  all 
to  a  stimulus.  Normal  irritability  will  be  restored  after  a  period  of  rest. 
Muscle  fatigue  under  normal  conditions  in  the  body  is  undoubtedly 
of  great  value  in  preventing  a  complete  breakdown  and  destruction 
of  tissue  by  over-use. 

A  fresh  muscle  will  respond  to  a  large  number  of  stimuli,  properly 
spaced  so  that  the  muscle  has  time  to  relax  between  shocks.  Suppose 
now  that  the  period  between  successive  stimuli  is  decreased  to  such  an 
extent  that  the  muscle  does  not  have  sufficient  time  to  relax  before 
receiving  another  shock.  This  will  quickly  result  in  tetanus,  a  coridi- 


BIOLOGY  OF  THE  MUSCULAR  SYSTEM 


183 


tion  characterized  by  a  contraction  that  is  maintained  until  the  muscle 
becomes  fatigued.  When  the  period  between  the  stimuli  is  gradually 
shortened,  the  kymograph  curves  of  muscle  contraction  will  show  a 
corresponding  decrease  in  the  amount  of  relaxation  with  the  onset  of 
tetanus,  until  finally,  when  stimuli  are  received  at  the  rate  of  20  to  30 
per  second,  the  muscle  will  remain  fully  contracted,  and  the  kymograph 
record  will  exhibit  a  straight  line  at  the  point  of  maximum  contraction. 
Even  so,  each  stimulus  received  undoubtedly  has  its  individual  effect 
in  keeping  the  muscle  contracted  and  in  preventing  relaxation,  for,  if 
the  time  interval  between  stimuli  is  lengthened  slightly,  the  individual 
stimuli  are  again  evident  in  the  record  as  the  muscle  relaxes  slightly 
between  stimuli.  (Fig.  93.) 

Ergograph. — For  testing  and  recording  fatigue  in  human  muscle 
tissue,  a  measuring  instrument,  the  ergograph,  has  long  been  used.     In 


FIG.  94. — Ergograph  apparatus  used  for  recording  the  work  done  by  finger  muscies 
in  repeatedly  lifting  a  weight,  as  described  on  page  183.  (Haggard,  "Science  of  Health 
and  Disease,"  Harper  &  Brothers.) 

using  it,  the  hand  is  fastened,  palm  up,  on  a  board  placed  on  a  table  of 
convenient  height.  Then  a  leather  band  is  fastened  around  the  large 
middle  finger,  distal  to  the  second  joint  so  that  the  finger  can  be  flexed. 
A  string  with  an  attached  weight  at  one  end  is  fastened  to  the  finger 
band  by  the  other  and  then  run  over  a  pulley  fixed  at  the  end  of  the 
table.  Thus  the  apparatus  is  so  arranged  that,  when  the  finger  is 
flexed  toward  the  palm,  the  pull  is  transmitted  by  the  string,  and  the 
weight  is  raised.  A  writing  lever  is  attached  to  the  string  in  such  a 
way  that  each  contraction  is  recorded  on  the  kymograph.  By  flexing 
the  finger  at  different  rates  and  also  by  changing  the  weight,  variation 


184 


HUMAN  BIOLOGY 


can  be  introduced  in  the  experiment.  If  the  weight  is  raised  at  short 
intervals,  it  will  be  found  that  the  finger  muscles  soon  become  fatigued, 
and  the  amount  of  contraction  is  correspondingly  decreased.  If  the 
rapid  rate  of  flexion  is  continued,  a  state  of  complete  fatigue  is  soon 
reached  in  which  it  is  impossible  for  the  experimenter  to  flex  the  finger 
at  all.  From  this  state  of  complete  fatigue  it  will  require  2  hours  for 
the  normal  condition  of  the  finger  muscles  to  be  completely  restored 
so  that  the  same  amount  of  work  can  be  performed  again.  With  a 
rest  of  about  10  seconds  between  contractions  and  the  use  of  a  proper- 
sized  weight,  it  will  usually  be  found  that  the  flexure  of  the  finger  can 
be  continued  indefinitely  without  fatigue.  It  is  also  found  that  the 


ABC 

FIG.  95. — Records  secured  from  Ergograph  (Fig.  94).  A,  shows  gradual  develop- 
ment of  fatigue  in  finger  muscles  when  weight  is  lifted  sixty  times  a  minute;  B,  con- 
tinuation of  weight  lifting,  as  in  A,  after  a  short  rest  with  increasing  fatigue.  C,  Rapid 
fatigue  resulting  from  an  increase  in  the  rate  to  200  times  per  minute.  (Haggard, 
"Science  of  Health  and  Disease,11  Harper  &  Brothers.) 

muscle  activity  may  vary  somewhat  in  relation  to  the  physical  condi- 
tion of  the  individual.     (Figs.  94,  95.) 

Tonus. — An  important  characteristic  of  muscle  function,  which  is 
closely  linked  with  tetanus,  is  known  as  tonus  and  may  be  described 
as  a  continuously  maintained  partial  contraction.  Tonus  is  a  very 
important  feature  of  the  involuntary  muscles  of  the  body,  as,  for 
example,  in  the  sphincter  valves  of  the  pylorus  and  urethra,  but  it  is 
also  generally  found  and  prominent  in  the  voluntary  muscles.  Thus 
the  maintenance  of  the  upright  position  in  man  is  associated  with 
tonus  in  certain  of  the  leg  and  skeletal  muscles.  Muscle  tonus  is 
caused  by  the  nervous  system  which  sends  stimuli  continually  into 
the  muscles  concerned.  In  so  doing,  the  nerve  tissue  responds  to 
impulses  that  it  receives  from  various  tissues  in  the  peripheral  regions. 
And  so  in  the  maintenance  of  erect  posture,  proprioceptive  impulses 
(page  271)  arising  from  sensory  areas  in  the  muscles  and  associated 
tissues  of  the  leg  and  trunk  regions  pass  into  the  central  nervous  sys- 


BIOLOGY  OF  THE  MUSCULAR  SYSTEM 


185 


tern  and  result  in  efferent  impulses  which,  in  turn,  maintain  the  tonus 
of  the  muscles  concerned. 

Muscle  Efficiency. — It  is  possible,  with  the  proper  apparatus,  to 
determine  the  efficiency  of  muscle  tissue  as  a  mechanism,  just  as  the 
engineer  can  determine  the  efficiency  of  the  steam  or  gasoline  engine 
by  calculating  the  amount  of  work  done  with  a  certain  amount  of  fuel. 
This  is  accomplished  by  using  a  calorimeter  large  enough  to  provide 
comfortable  living  quarters  for  a  person  during  considerable  periods 
(page  86).  The  calorimeter  is  also  equipped  with  a  machine,  known 
as  the  crgometer,  for  accurately  measuring  the  amount  of  work  done. 


FIG.  96. — Illustrating  the  Ergometer  mounted  inside  of  a  calorimeter,  as  seen 
from  above.  A,  B,  C,  D,  insulation;  E,  food  aperture  tube;  H,  ingoing  water  for 
absorbing  heat;  G,  outgoing  water;  V,  ventilating  air  current.  (Howell,  "Textbook  of 
Physiology,'1  W.  B.  Saundcrs  Company.  After  Atwater  and  Benedict.) 

The  ergometer,  designed  for  this  work,  is  a  stationary  bicycle  with  the 
rear  wheel  so  equipped  that  the  amount  of  work  done  in  pedaling  can 
be  measured  and  recorded.  The  calories  required  per  day  by  the 
subject  of  the  experiment  to  maintain  the  vital  activities  when  he  is 
resting  and  eating  normally  are  first  ascertained  over  a  period  of  several 
days.  In  an  important  series  of  experiments,  this  was  found  to  be 
2,397  calories  per  day.  With  this  amount  determined,  a  measured 
amount  of  work  was  done  by  pedalling  the  bicycle-ergometer.  Under 
these  conditions  it  was  found  that  the  amount  of  energy  required 
per  day  increased  to  5,120  calories  and  that  the  mechanical  work 
done  in  pedaling  the  bicycle  proved  to  be  the  equivalent  of  546 
calories.  From  this  series  of  experiments  it  is  clear  that  2,723  calories 
(5,120  -  2,397  =  2,723)  were  used  in  doing  546  calories  of  work. 
Dividing  the  latter  figure  by  the  calories  used,  it  is  found  that  the 


186  HUMAN  BIOLOGY 

efficiency  of  the  body  as  a  muscle-machine  amounts  to  slightly  over  20 
per  cent  (20.51  per  cent)  or,  in  other  words,  1  calorie  out  of  every  5 
taken  into  the  body  is  available  for  work;  the  other  4  are  used  in 
supplying  the  energy  necessary  to  maintain  the  essential  life  functions. 
Many  other  experiments,  essentially  similar  in  nature,  using  man  and 
various  other  experimental  animals  have  given  results  showing  from 
around  25  per  cent  (arm  muscles)  to  some  33  per  cent  (leg  muscles) 
efficiency.  The  rate  of  20  to  25  per  cent,  as  determined  for  the 
complete  muscular  system  of  man,  is  somewhat  higher  than  is  found 
in  locomotives  but  less  than  can  be  obtained  from  the  operation  of 
modern  steam  or  internal  combustion  engines  under  optimum  condi- 
tions, as  in  a  power  plant.  (Fig.  96.) 

Basis  of  Contraction. — Inasmuch  as  it  is  possible  to  remove  a 
muscle  from  the  animal  body  and  study  the  function  of  contractility 
under,  widely  varying  environmental  conditions  which  can  be  supplied, 
it  might  be  thought  that  the  determination  of  the  essential  phenomena 
associated  with  muscle  contraction  Avould  be  comparatively  easy. 
Exactly  the  opposite  condition  obtains,  and  the  great  amount  of 
experimental  work  that  has  been  performed  upon  the  phenomena 
associated  with  the  contraction  of  muscle  tissue  has  thus  far  failed 
to  give  definite  answers  to  the  major  problems  involved.  The  physio- 
logist knows  that  the  energy  required  for  muscle  contraction  is  ulti- 
mately supplied  by  the  oxidation  of  a  carbohydrate;  ho  knows  that 
carbon  dioxide  is  released  and  that  a  certain  amount  of  heat  is  evolved. 
Also  the  respiratory  quotient  of  about  1.0,  which  is  obtained  when  the 
carbohydrate  is  utilized  in  the  body  (page  88),  indicates  a  close 
relationship  between  oxidation  and  muscle  activity.  Nevertheless, 
conclusive  evidence  exists  that  oxidation  is  not  primarily  responsible 
for  contraction,  for  the  latter  can  occur  in  the  absence  of  free  oxygen. 
Also  it  can  be  shown  that  the  respiratory  apparatus  of  a  person  running 
a  race  requiring  a  maximum  amount  of  muscular  work  for  a  compara- 
tively long  period  cannot  possibly  supply  enough  oxygen  to  account 
for  the  work  being  done  by  the  muscles.  In  such  circumstances,  an 
oxygen  deficit  is  built  up  in  the  active  muscle  tissues  which  is  gradually 
paid  off  later  when  the  muscles  are  at  rest  and  oxygen,  in  excess  of  their 
requirements,  can  be  supplied.  The  belief  is,  therefore,  that  the 
energy  required  for  muscle  contraction  is  released  by  chemical  reac- 
tions other  than  those  directly  associated  with  oxidation  and  that  the 
latter  process  is  concerned  with  building  up  reserve  substances  in  the 
muscle  tissue  that  are  not  directly  concerned  with  muscle  contraction. 

Chemistry  of  Muscle  Contraction. — Although  comparatively  little 
is  known  with  certainty  about  the  chemical  changes  involved  in  the 


BIOLOGY  OF  THE  MUSCULAR  SYSTEM  187 

complete  cycle  of  the  contraction-restoration-contraction  phenomena 
in  muscle  tissue,  enough  evidence  is  at  hand  to  show  that  the  reactions 
concerned  in  muscle  chemistry  are  highly  involved.  The  concensus  of 
opinion  at  present  indicates  that  the  basic  reaction  in  muscle  tissue, 
which  releases  energy  for  contraction,  is  the  splitting  of  an  unstable 
nitrogenous  compound,  phosphagen,  into  phosphoric  acid  and  creatine. 
A  relatively  small  amount  of  phosphagen  is  normally  present  in  muscle 
tissue  (around  0.05  per  cent).  The  energy  for  the  resyn thesis  of  the 
essential  phosphagen,  following  contraction,  comes  indirectly  from  the 
oxidation  of  glucose  in  the  muscle  cells.  Apparently  the  glucose 
absorbed  from  the  blood  stream  is  converted  into  glycogen  by  the 
muscle  cells  and  stored  as  a  reserve  fuel  supply.  The  glycogen  can  be 
changed  to  glucose  when  needed,  and  the  latter,  in  turn,  changed  to 
lactic  acid.  The  formation  of  lactic  acid  from  glucose  apparently 
releases  the  energy  necessary  for  the  resynthesis  of  phosphagen.  Here 
again  the  process  is  not  direct  but  through  the  formation  of  inter- 
mediate compounds.  Finally,  the  oxidation  of  a  certain  amount  of 
lactic  acid  occurs  forming  carbon  dioxide  and  water,  by  which  sufficient 
energy  is  released  to  maintain  the  complete  cycle  of  reactions.  Arid,  of 
course,  it  is  known  that  the  hormone  insulin  is  essential  to  muscle 
chemistry  (page  104). 

Just  how  the  energy  released  by  the  chemical  reactions  in  muscle 
fiber  is  applied  to  the  ultramicroscopic  units  of  the  myofibrils  so  that 
contraction  is  induced  is  entirely  unknown,  though  many  theories 
have  been  advanced  from  the  earliest  times  up  to  the  present.  The 
original  idea  of  Galen,  held  for  many  centuries,  has  long  since  been 
abandoned;  this  was  that  "animal  spirits,"  compounded  with  air  in  the 
brain,  flowed  into  the  muscles  through  the  connecting  nerves  and  for- 
cibly distended  them.  The  same  fate  has  overtaken  the  much  later 
belief,  well-established  about  fifty  years  ago,  that  the  muscles  were 
essentially  heat  engines  in  which  chemical  energy  was  converted  into 
heat  and  that  the  latter  acted  directly  on  the  muscle  units.  Calcula- 
tions showed  that  it  would  require  a  temperature  of  about  285°F. 
in  the  muscle  tissues  at  the  beginning  of  contraction  if  they  acted  as 
heat  engines,  which  is  obviously  impossible.  It  is  evident  that  there 
must  be  some  arrangement  in  muscle  tissue  whereby  chemical  energy 
can  be  directly  converted  into  movement.  In  other  words,  heat  has 
nothing  to  do  with  contraction;  it  is  a  by-product  of  the  chemical 
reactions.  Whatever  the  methods  used  to  translate  chemical  energy 
into  muscle  contraction,  the  actual  changes  in  the  myofibrils  must 
involve  some  sort  of  reversible  gelation  phenomenon  which  has  its 
foundation  in  molecular  changes  in  the  individual  myofibrils. 


SPHENOID 
PARIETA1 


TEMPORAL 
OCCIPITAL 
VOMER 


CLAVICLE 


I — MALAR 


SELLA  TURC7CA 


FRONTAL 
ETHMOID 
FRONTAL  SINUS 
NASAL 
TURB1NATES 
MAXILLA 
MANDIBLE 
HYOID  — 


•7TH.CERV1CALVERTEBRA- 
ACROMION 

T CORACOID  PROCESS 

SCAPULA 
STERNUM 
HUMERUS 
7TH.RJB 

•12TH.THORACIC 
I2TH.RIB 

ULNA 

•RADIUS 

ILIUM 

5TH.LUMBAR  VERTEBRA 

•SACRUM  •- 


COCCYX 

PUBIC 

ISCHIUM 
CARPAL  BONES 

•METACARPALS 
PHALANGES 


TEMPORAL 
PARIETAL 

.OCCIPITAL 

IENOID  SINUS 
FORAMEN  MAGNUM 

CERVICAL 
VERTEBRA-7 


THORACIC 
VERTEBRA-12 


VERTEBRAL  CANAL 

LUMBAR 
VERTEBRA-5 


SACRAL  VERTEBRA 
CAUDAL  VERTEBRA 


PARIETAL 
OCCIPITAL 


ATLAS 
AXIS 


PATELLA 
SCAPULA 


TRANSVERSE 
PROCESS 

SPINOUS 
PROCESS 


TARSAL  BONES 

TATARSALS 
BALANCES 


A  B 

XI. — The  human  skeleton.  A,  complete,  anterior  view;  B,  axial,  posterior 
view;  C,  axial,  side  view,  with  the  skull  sectioned  to  show  internal  structures.  End 
\iews  of  three  vertebrae  are  also  shown. 


CHAPTER  IX 
BIOLOGY  OF  THE  SKELETAL  SYSTEM 

Another  of  the  five  basic  tissues  of  the  body,  as  briefly  indicated  in 
the  second  chapter,  is  found  in  the  connective,  or  supporting,  tissues. 
The  connective-tissue  elements,  like  those  of  the  vascular  and  mus- 
cular systems,  are  almost  universally  distributed  through  every  type 
of  body  structure,  even  down  to  the  individual  cells  which  are  held 
together  by  an  intercellular  cement,  as  is  well  shown  in  unstriated 
muscle  tissue.  A  multicellular  organism  cannot  maintain  structural 
integrity  without  some  sort  of  cement  substance  to  hold  the  cells  in 
position,  and  this  material  may  be  regarded  as  the  forerunner  of  the 
various  specialized  types  of  endoskeletal  tissue  which  develop  later. 
But  the  connective  tissues,  unlike  the  vascular  and  muscular  systems, 
are  relatively  inert.  They  contain  comparatively  few  living  cells  and 
much  nonliving  intercellular  material,  so  that,  in  general,  these  tissues 
need  very  little  assistance  from  the  other  tissue  systems  of  the  body. 
Nevertheless,  the  connective  tissues  have  their  independent  structural 
forms  and  essential  functions  and  comprise,  therefore,  one  of  the  major 
organ  systems,  the  skeletal  system,  in  the  commonwealth  of  the  verte- 
brate body  as  well  as  being  heavy  contributors  to  the  structural  ele- 
ments of  the  other  organ  systems  of  the  body. 

STRUCTURAL  FEATURES  ASSOCIATED  WITH  THE  SKELETAL  SYSTEM 

As  previously  indicated,  the  skeletal  system  includes  various  exter- 
nal elements  which,  together,  comprise  the  exoskeleton  and  a  wide 
variety  of  internal  connective  and  supporting  tissues  grouped  as  the 

endoskeleton  (page  26). 

•* 

EXOSKELETON 

The  exoskeleton  is  probably  seen  to  best  advantage  among  the 
invertebrate  animals,  and  one  thinks  at  once  of  the  calcareous  covering 
of  the  island-building  corals,  the  shell  of  the  clam  and  oyster,  or  the 
rigid  chitinous  material  that  completely  encloses  the  soft  body  tissues 
of  the  crab,  lobster,  insect,  and  their  many  relatives  included  in  the 
great  arthropod  group.  The  materials  used  in  these  outer  protecting 
structures  vary  greatly,  ranging  from  a  comparatively  simple  inor- 
ganic limestone,  as  in  the  corals  and  mollusks,  just  noted,  to  much  more 

189 


190 


HUMAN  BIOLOGY 


complex  protein  materials,  such  as  chitin  in  the  arthropods  and  keratin 
in  the  epidermal  plates  of  the  turtle,  and  in  the  hair  and  nails  of  the 
mammals.  But  this  can  be  said  of  all  the  exoskeletal  materials;  they 
are  nonliving  and  formed  in  many  instances  as  noncellular  secretions 
of  the  underlying  living  cells.  This  condition  is  well  shown  in  the 
chitinous  shell  of  the  crayfish.  However,  some  of  the  vertebrate  ani- 
mals are  not  far  behind  the  invertebrates  when  it  comes  to  exoskeletal 
structures.  The  fish  covered  with  scales;  the  reptiles  encased  in 

various  types  of  scaly  armor  or  even 
by  a  complete  dermal  shell,  as  in  the 
turtle;  the  feathers  of  birds;  the  hairy 
coat,  partial  or  complete,  of  the 
mammals — all  these,  together  with 
such  others  as  nails,  claws,  and  teeth, 
are  examples  of  the  exoskeletal  struc- 
tures associated  with  vertebrate  ani- 
mals. (Fig.  97.) 

Hair. — Consideration  may  now 
be  given  to  the  most  prominent 
development  of  the  mammalian 
exoskeleton,  hair,  which,  as  we  know, 
is  formed  in  the  tissues  of  the  skin 
(page  37).  Each  hair  develops  in  a 
separate  hair  follicle  consisting  of 
epidermal  and  connective  tissue 
elements  and  forming  an  elongated 
sac-like  structure.  The  bottom  of 
the  hair  follicle  lies  deeply  embedded 
in  the  dermis  of  the  skin,  while  the  top,  with  the  projecting  hair,  is  at 
the  body  surface.  Epidermal  cells  penetrate  the  dermis,  become  sur- 
rounded by  connective  tissue  elements  to  form  the  follicle,  and  then 
give  off  the  cells  that  form  the  body  of  the  hair.  The  latter  consists 
of  a  great  number  of  keratinized  epidermal  cells  which  gradually  lo^e 
their  characteristic  structural  features  and  become  molded  as  it  were, 
into  the  body  of  the  hair.  They  are  so  closely  applied  to  each  other 
that  it  is  impossible  to  make  out  the  cellular  outlines  of  the  constituent 
cells,  even  when  a  hair  is  subjected  to  microscopic  examination.  A 
hair  is  first  evident  as  a  tiny  projection  below  the  skin  surface  and 
continues  to  elongate  indefinitely  by  the  formation  of  additional  cells 
at  the  root  which  is  in  contact  with  the  hair  papilla — a  dermal  struc- 
ture at  the  base  of  the  hair  follicle  for  the  nourishment  of  the  hair  cells. 
(Fig.  98.) 


A 

FIG.  97. — Diagrams  illustrating  the 
attachment  of  muscles  in  leg  of  insect 
and  man.  A,  insect  leg  with  muscles 
attached  to  exoskeleton;  B,  leg  of  man 
with  muscles  attached  to  bony  en  do- 
skeleton.  /,  femur;  m,  muscle;  o,  ori- 
gin of  muscle;  i,  insertion  of  muscle; 
ta,  tendon  of  Achilles;  ti,  tibia.  (Shull. 
A,  after  Berlese;  B,  after  Hesse  and 
Doflein.} 


BIOLOGY  OF  THE  SKELETAL  SYSTEM 


191 


The  follicles  are  not  permanent  structures.  They  continue  to  pro- 
duce the  cells  that  form  the  hairs  for  a  time  and  then  become  inactive, 
but  usually  not  until  cells  have  been  budded  off  to  form  new  hair 
follicles  in  close  proximity  to  the  old  ones.  Sebaceous  glands  are 
attached  to  the  hair  follicles  and  secrete  an  oil  which  covers  the  outer 
surface  of  the  hair.  Smooth  muscle  fibers,  present  in  the  dermis,  are 
attached  to  the  hair  follicles,  and  the  muscles  of  each  hair  are  inner- 
vated by  separate  nerve  fibers.  Thus,  hairs  do  "stand  on  end"  when 


,  -  .-.,',;* 


, 

.     ,  :    .    .   ,     '.',\ 

'       '   ;    ;,  ;-J 

'  .       •  ...'.-  'X.U 

•     ,  •     .•;    •  /„:  ;^y^>;;ai 

FIG.  98.— Vertical  soction  through  human  skin  showing  microscopic  structure  of  hau' 
and  hair  follicle.      (Maximow-Bloom,  "Histology"  W.  B.  Saunders  Company.) 

\ 

certain  conditions  cause  a  contraction  of  the  attached  muscles  through 
nerve  impulses. 

Examined  microscopically,  a  hair  is  found  to  consist  of  two  regions. 
It  is  covered  externally  by  a  layer  of  very  thin  cells,  irregular  in  outline, 
which  form  a  tile-like  covering.  The  main  body,  or  cortical  portion, 
of  the  hair  consists  of  dense,  horny  material,  the  keratin,1  which  has 
developed  through  the  transformation  of  the  constituent  epithelial 
cells.  Keratin  is  an  important  protein  substance,  widely  distributed 
in  exoskeletal  structures.  The  amount  and  color  of  the  pigment 
present  in  the  keratin  of  the  hair  cortex  varies  greatly,  and  also  air 

1  Consult  Appendix:  Keratin. 


192  HUMAN  BIOLOGY 

spaces  are  not  uncommon  toward  the  center.  Hair  color  depends 
upon  the  amount  and  quality  of  the  pigment  present  and  its  relation 
to  the  transparent  air  spaces.  In  hairs  with  a  heavier  body,  such  as 
those  of  the  beard,  a  definite  central  area,  the  medulla,  is  usually 
noted  throughout  the  length  of  the  hair.  This  region  is  characterized 
by  an  irregular  cellular  arrangement  and  the  presence  of  large  air 
spaces.  (Fig.  98.) 

Closely  related  to  the  hair  in  development  and  structure  are  the 
nails  of  the  fingers  and  toes,  as  well  as  the  claws  of  lower  types  of 
mammals.  Nails  are  formed  from  adhering  keratinized  epithelial 
cells  of  essentially  the  same  nature  as  those  which  form  hairs.  The 
living  tissue,  which  is  continually  forming  and  giving  off  these  cells, 
lies  in  a  fold  at  the  root  of  each  nail  and  also  underneath  the  nail 
where  it  forms  the  nail  bed.  The  nail  is  bounded  on  each  side  by  the 
nail  groove.  Receiving  additions  in  length  by  the  additions  of  cells 
at  the  base  and  in  thickness  by  those  added  underneath,  the  nail  is 
gradually  pushed  forward  and  projects  beyond  the  nail'bed  at  the  tip 
of  each  digit.  This  process,  unlike  that  of  the  hair  follicle,  is  continu- 
ous throughout  life,  and  the  nail  can  grow  indefinitely  in  length  if  left 
undisturbed. 

ENDOSKELETON 

The  bony  endoskeletal  system  is  a  unique  feature  of  the  verte- 
brates. Invertebrate  animals,  like  the  insect,  must  attach  their 
developed  muscles  to  the  inner  surfaces  of  the  nonliving  exoskeleton 
and  detach  them  periodically  when  the  shell  is  shed  during  moulting. 
The  permanent  endoskelcton  of  the  vertebrate  serves  largely  as  an 
inner  supporting  material,  and  the  soft  tissues  can  thus  develop  on 
the  outside  of  the  endoskeletori.  It  may  also  develop  outside  the  soft 
tissues  and  serve  for  protection,  a  relationship  best  shown  in  the  brain 
and  skull.  Though  the  endoskeletal  tissues  are  relatively  inactive  as 
compared  with  the  other  body  tissues,  they  are  basically  living  tissues. 
And,  by  way  of  exception,  it  should  be  noted  that  a  very  active  tissue 
is  supplied  in  bone  marrow. 

Though  wide  variety  exists  in  the  types  of  tissue  associated 
together  in  the  endoskeletal  system,  yet,  fundamentally,  all  of  them 
are  united  in  the  possession  to  a  greater  or  less  degree  of  a  characteristic 
structural  material,  collagen,  lying  between  the  connective  tissue 
cells,  which  is  formed  as  an  intercellular  secretion.  Collagen  is  pro- 
teinaceous,  typically  fibrillar,  and  comparable  in  its  wide  distribution 
in  the  vertebrate  body  (but  not  in  its  chemical  composition)  to  the 
ubiquitous  carbohydrate,  cellulose,  of  the  plant  world.  The  endo- 


BIOLOGY  OF  THE  SKELETAL  SYSTEM  193 

skeletal  tissue  system  may  appropriately  be  grouped  under  the  term 
collagenous.  Lying  embedded  in  the  collagenous  ground  substance 
of  the  connective  and  supporting  tissues  arc  the  cells  that  appear  to 
be  primarily  responsible  for  the  synthesis  and  secretion  of  collagen. 
These  cells,  constituting  the  so-called  fibroblasts,  arc  typically  seen  in 
developing  connective  tissues  as  elongated  spindle-shaped  bodies,  but 
both  the  structural  and  functional  characteristics  of  the  fibroblasts 
are  subject  to  wide  variation  in  the  widely  divergent  types  of  endo- 
skeletal  tissues.  (Figs.  19,  99.) 


FIG.  99. — Bundles  of  collagenous  fibers  in  ground  substance  of  white  fibrous  tissue. 

Photomicrograph. 

The  embryologist  studying  the  origin  of  the  endoskelctal  system 
in  the  embryo  sees  a  unity  throughout  the  various  types  of  tissue  as 
they  gradually  differentiate  during  development.  The  endoskeleton 
begins  with  a  gelatinous  type  of  embryonic  connective  tissue  filling 
various  cavities  in  the  embryo  between  the  germ  layers.  It  continues 
with  the  formation  of  fibrous  tissues  in  which  distinctive  bundles  of 
collagenous  fibers  are  abundantly  present  in  the  ground  substance,  as 
in  white  fibrous  tissue  or  tendons.  Later,  cartilage  appears  in  various 
regions,  with  a  more  rigid  gel-condition  of  the  intercellular  material 
which  may  be  either  fibrous  or  homogenous  in  nature.  Finally,  bone 
tissue  is  formed  largely  by  the  transformation  of  the  cartilage  through 
the  deposition  of  inorganic  salts,  notably  calcium  and  phosphorus. 
Certain  specialized  types  of  connective  tissue,  as  found  in  elastic  and 


194 


HUMAN  BIOLOGY 


adipose  tissue,  contain  a  considerable  amount  of  collagen  but  seem  to 
be  outside  the  main  routes  of  endoskeletal  development  that  culminate 
in  the  formation  of  bone.  Detailed  consideration  may  now  be  given 
to  the  structural  and  functional  features  of  bone  as  the  culmination  of 


SYNOPSIS  OF  SKELETAL  DIVISIONS 

( Brain  case 

Cranium -(Olfactory  capsules 
( Auditory  capsules 


Axial 


Skull 


^  i    (  Upper  jaw 

Visceral   1T    •        • 
.    ,  ,       <  Lower  jaw 
skeleton  ' 


( Hyoid  and  larynx 
Vertebral  column  (including  ribs  when  present) 


Skeleton^ 


Shoulder 


Appendicular 


Girdle 


Scapula,  suprascapula,  cora- 
coid,  procoracoid,  epicora- 
coid,  clavicle,  episternum, 
omosternum,  sternum,  and 
xiphisternum 


Free  limb 


Hip 


Arm 


Leg 


Jlliurn 
Ischium 
Pubis 


Humerus 
Radio-ulna 
Carpal  s 
Metacarpals 
Phalanges 


Femur 

Tibio-fibula 

Tarsals 

Metatarsals 

Phalanges 


FIG.  100. — The  main  division*  of  the  bony  vertebrate  skeleton.     (Reed  and  Young.} 

the  endoskeletal  system  in  man  and  all  the  vertebrates,  with  the  excep- 
tion of  the  cartilaginous  fish. 

BONY  SKELETON 

The  bony  skeleton  shows  wide  variation  in  the  different  classes  of 
vertebrates  in  accordance  with  the  size  of  the  body  and  the  particular 


BIOLOGY  OF  THE  SKELETAL  SYSTEM  195 

environment  for  which  a  certain  group  is  adapted.  Thus  the  fish  and 
other  aquatic  vertebrates  are  able  to  move  freely  in  the  water  by  a 
rhythmic  back-and-forth  movement  of  the  body  and  tail,  but  this  type 
of  locomotor  apparatus  is  of  no  use  for  air-living  birds  or  for  a  bipedal 
and  living  vertebrate.  Nevertheless,  the  basic  resemblances  of  the 
vertebrate  skeleton  are  much  more  apparent  than  the  variations, 
which  are  of  relatively  minor  importance.  Thus  all  vertebrate 

Axial  skeleton 

Vertebral  column: 

Vertebrae 24 

Sacrum 1 

Coccyx 1 

Skull: 

Cranium 8 

Facial  portion 14 

Neck  and  chest  regions: 

Hyoid 1 

Sternum 1 

Ribs 24 

Appendicular  skeleton 

Forelimbs: 

Hands 28 

Wrists 26 

Arms 6 

Shoulder  girdles 4 

Hindlimbs: 

Feet 28 

Ankles 24 

Legs 6 

Kneecaps 2 

Pelvic  girdles 2 

Total 200 

skeletons  are  found  to  consist  of  two  basic  divisions:  the  axial  skeleton, 
consisting  of  the  skull  and  vertebral  column;  and  the  appcndicular 
skeleton,  consisting  of  the  fore  and  hind  limbs  and  their  respective 
girdles,  which  connect  the  limbs  to  the  axial  skeleton.  Furthermore, 
throughout  the  classes  of  vertebrate  animals  it  is  possible  to  homologize 
the  bones  associated  with  a  particular  region  or  organ  in  one  animal 
with  those  present  in  widely  varying  animal  types.  Comparative 
anatomists  have  long  regarded  the  vertebrate  skeleton  as  one  of  the 
most  favorable  organ  systems  for  the  study  of  homologies. l  (Plate  XI ; 
Fig.  100.) 

The  number  of  separate  bones  in  the  skeleton  is  subject  to  wide 
variation  in  the  different  vertebrate  classes.  The  variation  ispartic- 

1  Consult  Appendix:  Comparative  Anatomy. 


196  HUMAN  BIOLOGY 

ularly  evident  in  the  tail  region  and  in  the  bones  of  the  hands  and 
feet.  Many  vertebrates  of  high  development  have  a  long  caudal 
appendage,  the  tail.  Thus,  the  cat,  for  example,  has  22  vertebral 
bones  in  the  tail.  In  the  birds,  the  forelimbs  are  modified  as  wings 
instead  of  hands,  and  the  number  of  bones  in  a  wing  has  been  some- 
what reduced  as  compared  with  the  typical  pentadactyl  appendage 
of  man.  The  reduction  of  bones  in  the  fore  and  hind  limbs  is  even 
more  evident  in  certain  of  the  hoofed  mammals  (Ungulata).  The 
horse,  for  example,  retains  only  one  functional  digit  on  each  of  the 
four  limbs.  The  number  of  bones  in  the  adult  human  skeleton  is 
usually  given  as  200,  but  6  more  bones  may  be  added  by  including  the 
3  tiny  ossicles  in  each  ear.  The  distribution  of  bones  in  the  skeleton 
of  man  is  summarized  on  page  195. 

AXIAL  SKELETON 

Vertebral  Column. — One  of  the  characteristics  of  the  great  phylum 
Chordata,  to  which  man  and  the  other  vertebrate  animals  belong,  is 
a  longitudinal  cylindrical  rod,  the  notochord,  situated  near  the  dorsal 
surface  of  the  body  and  continuing  throughout  the  entire  length. 
The  notochord  thus  serves  as  a  primary  supporting  axis.  In  the 
lowest  vertebrates,  the  Cyclostomata,  the  notochord  retains  its 
original  characteristics;  but  in  the  higher  vertebrate  classes  with  a 
bony  skeleton,  the  notochord  is  replaced  by  a  vsegmented  vertebral 
column  built  up  of  individual  vertebrae.  In  the  more  primitive  types, 
each  vertebra  consists  of  a  solid  disc  of  bony  tissue,  the  centrum,  and 
these,  placed  end  to  end  and  held  in  place  by  ligaments  of  connective 
tissue,  compose  the  segmented,  rod-like  vertebral  columns  for  the 
general  support  of  the  body  and  the  attachment  of  muscles.  (Fig. 
23.) 

The  vertebral  column,  seen  in  man  and  most  of  the  vertebrates, 
has  developed  additional  bony  structures  which  are  arranged  to  supply 
great  protection  to  an  essential  portion  of  the  nervous  system,  the 
spinal  cord.  The  most  important  of  the  new  structures  of  the  verte- 
brae consists  of  a  bony  neural  arch  which  develops  dorsally  from  each 
centrum.  The  cavity  between  the  arch  and  the  centrum  is  the  verte- 
bral canal,  a  well-protected  area  in  which  the  spinal  cord  lies.  Pro- 
jecting from  each  neural  arch  are  the  dorsal  and  lateral  articulating 
processes  which  are  of  great  importance  in  the  articulation  of  separate 
vertebrae  with  each  other  in  forming  the  vertebral  column  and  also 
in  their  availability  for  the  attachment  of  muscles.  Thus  the  back- 
bone, or  vertebral  column,  is  composed  of  independent  articulated 
units,  the  vertebrae,  each  of  which  is  essential  in  the  support  of  the 


BIOLOGY  OF  THE  SKELETAL  SYSTEM 


197 


body  and  each  also  contributes,  by  means  of  the  neural  arch,  a  portion 
of  the  common  vertebral  canal  for  the  protection  of  the  spinal  cord. 
The  vertebral  column,  as  a  whole,  gives  firm  support  to  the  major 
axis  of  the  body  and  at  the  same  time  permits  considerable  freedom 
of  movement.  (Plate  XI;  Fig.  101.) 


SELLA  TURCICA 


FRONTAL 
ETHMOID 
FRONTAL  SINUS 

TURBINATES 
MAXILLA 
MANDIBLE 
HYOID 


7TH.CERVICAL  VERTEBRA 


TEMPORAL 


PARIETAL 

.OCCIPITAL 

SPHENOID  SINUS 
MAGNUM 

CERVICAL 
VERTEBRA-7 


THORACIC 
VERTEBRA-12 


12TH.THORACIC  VERTEBRA 


5TH.  LUMBAR  VERTEBRA- 
SACRUM    

COCCYX  


VERTEBRAL  CANAL 

LUMBAR 
VERTEBRA-5 


SACRAL  VERTEBRA 
CAUDAL  VERTEBRA 


Fia.  101. — Skull  and  vertebral  column  of  man,  side  view.      Knd  views  of  three  vertebra 
are  shown  at  the  right.     Skull  is  sectioned  to  show  interior  structure. 

Five  regions  are  recognized  in  the  vertebral  column  of  the  higher 
vertebrates,  which,  beginning  at  the  anterior  end,  are  known  as  the 
neck  region  (cervical),  chest  region  (thoracic),  abdominal  region  (lum- 
bar), pelvic  region  (sacral),  and  tail  region  (caudal).  In  the  backbone 
of  the  human  adult  there  are  26  vertebrae  divided  as  follows :  7  cervical ; 
12  thoracic,  with  a  pair  of  ribs  attached  to  each;  5  lumbar;  1  sacral; 
and  1  caudal.  In  the  early  stages  of  development,  33  separate  verte- 
brae are  present.  The  vertebral  reduction  in  the  adult  is  due  to  a 
fusion  of  five  sacral  vertebrae  to  form  one  sacral  (sacrum),  and  a  fusion 
of  four  caudal  vertebrae  to  form  one  caudal  (coccyx). 


198 


HUMAN  BIOLOGY 


Skull. — The  vertebrate  skull  is  an  extraordinarily  complex  assem- 
bly of  bone  units  designed  to  offer  adequate  protection  to  the  most 
delicate  organ  of  the  body,  the  brain.  The  skull  consists  primarily 
of  (1)  a  brain  case,  or  cranium,  enclosing  the  brain  proper  and  also 
providing  places  of  refuge  for  the  important  sense  organs  of  the  body 
— eyes,  nose,  ears — and  (2)  a  facial  portion  built  around  the  mouth 
and  provided  with  a  masticating  apparatus  which  involves  an  intricate 
assemblage  of  bony  parts  and  attached  muscles.  Studied  compara- 

fronfopar/efa/, 

sphenethmoid 
-nasal 


occipital- 
masto  id  process 
inferior  max/7 /ary 


-superior 
maxillary 


FIG.    102.— Comparison  of  frog  skull  and  human  skull.     Siclo  views, 
reduced  to  comparable  size.      (Wieman.) 


Human   skull 


tively,  the  skulls  of  the  different  vertebrate  groups  reveal  considerable 
variety  in  the  size  relations  existing  between  the  cranial  and  facial 
portions.  The  skull  of  a  lower  vertebrate,  as  in  the  frog,  shows  a 
comparatively  large  facial  portion  and  a  vety  small  cranium.  The 
opposite  condition,  with  a  greatly  reduced  facial  portion  and  a  large 
cranium  to  provide  adequate  quarters  for  the  enlarged  brain,  is  seen 
to  best  advantage  in  the  human  skull.  (Fig.  102.) 

The  22  bones  present  in  the  adult  human  skull  represent  a  consider- 
able fusion  of  the  bones  originally  present  in  the  embryonic  skull.  The 
arrangement  of  the  eight  cranial  bones  forming  the  cranium  may  now 
be  considered,  beginning  with  the  large  unpaired  occipital  bone  which 
forms  the  floor  of  the  cranium.  It  is  characterized  by  a  large  central 


BIOLOGY  OF  THE  SKELETAL  SYSTEM  199 

opening,  the  foramen  magnum,  through  which  the  spinal  cord  enters 
the  cranium  and  connects  with  the  brain.  The  occipital  bone  is  con- 
tinued well  up  on  the  back  of  the  skull  where  it  joins  a  pair  of  large 
parietal  bones  which  form  the  roof  of  the  skull.  Forming  the  ear 
region  on  each  side  of  the  head  is  a  temporal  bone.  It  is  bounded 
posteriorly  by  the  occipital,  dorsally  by  the  parietal,  and  anteriorly 
by  the  sphenoid;  the  latter,  together  with  the  small  ethmoid,  form  the 
floor  of  the  cranium  anterior  to  the  occipital  region.  The  frontal 
bone,  continuing  anteriorly  from  the  parietal,  forms  the  forehead  and, 
laterally,  at  about  the  level  of  the  eye,  joins  with  the  sphenoid  and 
ethmoid.  (Fig.  101.) 

In  the  facial  portion  of  the  skull  the  14  constituent  bones,  well- 
covered  with  the  muscles  of  the  jaw  and  face,  occur  in  pairs,  with  two 
exceptions.  The  exceptions  are  the  single  vomer,  which  separates  the 
right  and  left  nostrils,  and  the  lower  jaw,  or  mandible.  The  remaining 
six  pairs  of  facial  bones  include,  first,  the  pair  of  maxillae  which  form 
the  upper  jaw.  The  two  members  of  the  pair  meet  in  the  mid-line 
of  the  face  and  continue  posteriorly  as  the  hard  palate,  which  forms 
the  roof  of  the  mouth  and  separates  the  mouth  cavity  from  the  nasal 
cavity  just  above.  Also  included  are  (a)  the  palatine  bones  which  join 
with  the  maxillae  posteriorly  and  continue  the  bony  wall  into  the 
throat  region;  (b)  the  cheek  bones  (malar)  which  join  with  a  maxilla 
bone  on  each  side  of  the  head,  dorsally  with  the  frontal  and,  posteriorly, 
with  a  projection  of  the  temporal  to  form  the  zygomatic  arch  which 
articulates  with  lower  jaw  in  the  ear  region;  and,  finally,  (c)  a  pair  each 
of  nasal,  lachrymal,  and  turbinate  bones  which  together  form  the  roof 
and  side  walls  of  the  nose. 

Teeth. — Strictly  speaking,  the  vertebrate  teeth  are  exoskeletal 
rather  than  endoskeletal  structures,  but  their  close  association  with  the 
bony  jaws  makes  it  advisable  to  consider  them  in  connection  with 
the  skull.  Teeth  are  typically  composed  of  (1)  a  thin  outer  covering, 
the  enamel,  which  develops  as  a  secretion  from  the  invaginated 
epithelial  cells  of  the  mouth  cavity  lying  over  the  jaws;  and  (2) 
dentine  which  develops  from  the  underlying  mcsodermal  tissue.  The 
latter  forms  by  far  the  greater  amount  of  material  in  a  tooth.  In  the 
center  of  the  dentine  area  is  a  pulp  cavity  with  vascular  and  nerve 
elements.  Teeth  in  the  various  vertebrate  classes  are  subject  to  con- 
siderable variation  in  number,  shape,  and  function.  In  man  and  the 
higher  vertebrates,  a  considerable  portion  of  each  tooth  is  embedded 
in  a  deep  pit,  or  alveolus,  in  the  jaw  bone  and  firmly  fastened  by  a 
bone-like  cement  substance.  The  portion  of  the  tooth  within  an 
alveolus  is  known  as  the  root;  and  the  exposed,  enamel-covered  portion 


200  HUMAN  BIOLOGY 

is  termed  the  crown.  Four  types  of  tooth  are  recognized  in  the  human 
jaw,  which,  beginning  anteriorly,  are  designated  as  the  incisors, 
canines,  premolars,  and  molars.  (Fig.  27.) 

Commonly  two  sets  of  teeth  are  formed.  The  first  set,  or  milk 
teeth,  are  replaced  in  the  early  years  of  childhood  by  the  permanent 
teeth  which  begin  to  develop  in  the  jaws  before  the  milk  teeth  are  lost 
and  erupt  shortly  afterward.  The  number  of  milk  teeth  varies  some- 
what, with  a  normal  of  20,  while  in  the  permanent  set  there  are  32 
teeth,  divided  so  that  each  half  of  each  jaw  contains  a  total  of  eight 
teeth,  including  two  incisors,  one  canine,  two  premolars,  and  three 
molars.  The  third  molars  or  wisdom  teeth  are  frequently  delayed  in 
their  appearance  or  fail  entirely  to  erupt,  with  consequent  abnormal 
conditions.  In  order  to  state  concisely  the  number  of  teeth  in  the 
various  animals,  a  dental  formula  is  made  use  of,  in  which  the  letters 
/,  C,  P,  and  M  are  used  to  indicate  the  incisors,  canines,  premolars, 
and  molars,  respectively,  and  the  numbers  of  each  of  these  teeth  in 
half  of  the  upper  and  lower  j  aws  are  shown  by  figures  above  and  below 
a  division  line.  Thus  the  dental  formula  of  man  is  shown  as  /%, 
CM,  P%,  M%  =  32. 

Neck  and  Chest. — Each  of  the  24  ribs  may  be  described  as  a 
slender  bone  gracefully  curved  to  form  the  circular  chest  wall.  All  of 
the  ribs  are  attached  dorsally  to  the  transverse  process  of  the  corre- 
sponding thoracic  vertebra.  The  anterior  seven  pairs  of  ribs  are 
attached  vent  rally  by  means  of  a  short  strip  of  cartilage  to  a  median 
unpaired  dagger-shaped  bone,  the  sternum,  which  forms  the  "key- 
stone" of  the  thoracic  arch  surrounding  the  chest  cavity.  The 
eighth,  ninth,  and  tenth  pairs  of  ribs  are  also  supplied  ventrally  with 
a  terminal  strip  of  cartilage,  but  the  latter,  in  these  ribs,  is  not  con- 
nected directly  with  the  sternum  but  with  the  cartilage  of  the  rib  just 
anterior.  The  ends  of  the  eleventh  and  twelfth  pairs  of  ribs  have  no 
ventral  attachment  but  end  freely  in  the  muscle  tissue  of  the  body 
wall.  In  the  neck  region,  a  U-shaped  hyoid  bone  lying  just  anterior 
to  the  larynx  is  without  great  functional  significance  in  man,  but  great 
interest  is  associated  with  it  from  the  standpoint  of  comparative 
anatomy  because  of  its  condition  in  the  lower  vertebrates.  (Plate 
XLi,  B.) 

APPENDICULAR  SKELETON 

The  appendicular  skeleton  of  man  consists  of  the  paired  penta- 
dactyl  fore  limbs  and  hind  limbs  and  the  girdles  by  which  these 
appendages  are  attached  to  the  axial  skeleton.  (Plate  XI.) 


BIOLOGY  OF  THE  SKELETAL  SYSTEM  201 

Forelimb  and  Pe.ctoral  Girdle.^-The  arm  bones  consist,  first,  of  a 
large  humerus  which  articulates  proximally  with  the  shoulder  girdle. 
The  distal  end  of  the  humerus  is  in  contact  at  the  elbow  with  two  bones, 
the  radius  and  the  ulna,  both  of  which  continue  *  to  the  wrist 
(carpus).  The  wrist  consists  of  an  aggregation  of  eight  small  carpal 
bones,  followed  by  the  hand  proper  with  its  quota  of  19  bones.  Five  of 
the  hand  bones  (metacarpals),  which  are  articulated  with  the  bones  of 
the  wrist,  are  long  and  slender  and  form  the  undivided  main  portion 
of  the  hand.  Finally,  the  five  digits  contain  a  total  of  14  phalanges, 
with  three  for  each  of  the  fingers  but  only  two  for  the  thumb,  which 
has  one  less  joint.  The  shoulder,  or  pectoral  girdle  of  man  has  only 
two  bones.  There  is,  first,  the  flat,  dorsal  shoulder  blade,  or  scapula, 
which  is  firmly  anchored  in  the  shoulder  muscles  but  not  actually 
fused  with  the  axial  skeleton.  The  scapula  is  joined  at  the  shoulder 
by  the  collar  bone  (clavicle).  The  latter  is  slender,  rod-shaped  and 
extends  anteriorly  to  the  head  of  the  dagger-shaped  sternum.  Articu- 
lation between  arm  and  girdle  is  by  a  special  cavity,  the  glenoid  fossa, 
into  which  the  head  of  the  humerus  fits  to  form  the  shoulder  joint. 
(Plate  XI A.) 

Hindlimb  and  Pelvic  Girdle. — The  bony  structure  of  the  leg  is 
very  close  to  that  of  the  arm,  but  the  names  given  to  bones  are  dif- 
ferent. There  is,  first,  the  large  thigh  bone,  or  femur  which  corre- 
sponds to  the  humerus  of  the  arm  and  articulates  at  the  knee  with  the 
tibia  arid  fibula.  The  latter  are  homologous  with  the  radius  and 
ulna.  An  additional  bone,  the  kneecap  or  patella,  gives  added  protec- 
tion to  the  important  knee  joint.  In  the  ankle  (tarsus)  arc  only  seven 
bones,  one  of  which,  the  calcaneum,  is  very  large  and  forms  the  heel 
proper.  The  other  tarsal  bones  are  closely  assembled,  so  that  only  a 
slight  amount  of  movement  is  possible,  and  fastened  with  ligaments 
in  such  a  manner  that  the  inner  portion  of  a  normal  foot  is  elevated 
to  form  the  arch.  The  flat-footed  condition  represents  a  harmful 
breakdown  of  the  normal  tarsal  assembly.  The  five  metatarsal  bones 
form  the  body  of  the  foot  and  extend  to  the  base  of  the  toes.  As  in 
the  hand,  there  is  a  total  of  14  phalanges  in  the  digits,  the  big  toe 
lacking  one  joint  and  the  corresponding  phalangeal  bone.  Compared 
with  the  fingers,  the  toes  show  a  considerable  reduction  in  length  and 
in  functional  adaptability.  And  the  big  toe  is  not  opposable  to  the 
other  digits  as  is  the  thumb  with  the  four  fingers  of  the  hand.  Each 
pelvic  girdle  consists  of  a  single  bone  (os  innominatum)  which  repre- 
sents a  fusion  of  three  embryonic  bones.  The  pelvic  girdle  and  the 
axial  skeleton  are  firmly  united  by  bone  tissue  in  the  sacral  region  of 
the  vertebral  column.  A  deep  cavity,  the  acetabulum,  is  present  in  the 


202 


HUMAN  BIOLOGY 


girdle,  and  this  receives  the  proximal  end  of  the  femur,  thus  forming 
the  ball-and-socket  hip  joint  noted  below.     (Plate  XL4..) 

Variation  in  the  Appendages  of  Vertebrates. — The  paired  fins  of 
the  bony  fish  do  not  appear  at  first  glance  to  show  much  resemblance 
to  the  pentadactyl  appendage  but  nevertheless  are  regarded  as  the 
original  type  from  which  the  others  have  arisen.  The  greatest 
amount  of  variation  between  the  legs  and  arms  of  vertebrates  .is 
found  in  the  bones  of  the  wrist  and  hand  and  those  of  ankle  and 
foot.  The  three  large  b  mes,  which  form,  the  main  axis  of  an  append- 
age, are  relatively  constant  in  their  structure  throughout  the 
vertebrate  series.  Thus  even  in  a  highly  modified  appendage,  as 


FIG.  103. — Comparison  of  the  bones  in  the  forelimbs  of  man,  dog,  horse,  and  bird. 
Dotted  lines  connect  the  homologous  bones.  Appendages  arc  not  drawn  to  scale. 
(Watkeya,  Stern.) 

found  in  the  forelimb  of  the  birds,  the  humerus,  radius,  and  ulna 
remain  essentially  typical  in  their  structure,  but  the  bones  of  the  wrist 
and  hand  region  show  marked  structural  modifications  as  well  as  a 
reduction  in  numbers,  changes  that  better  adapt  them  to  the  needs  of 
a  wing  designed  for  aerial  locomotion.  (Fig.  103.) 

In  the  hoofed  mammals  (Ungulata),  both  the  fore-  and  the  hind- 
limbs  have  departed  more  widely  from  the  pentadactyl  types  than 
have  those  of  most  other  vertebrates.  Thus,  for  example,  in  the 
forelimb  of  the  horse,  the  humerus  is  about  the  only  bone  that  does 
not  show  wide  departure  from  the  typical  condition.  Only  a  small 
portion  of  the  ulna  remains,  and  it  is  fused  with  the  radius.  Digits 
I  and  V  have  entirely  disappeared.  Remnants  of  digits  II  and  IV  are 
present  as  vestigial  structures,  the  splint  bones.  Digit  III,  essentially 
complete  in  its  bony  elements,  is  the  only  functional  digit  in  both  the 
fore-  and  hindlimbs,  so  that  in  locomotion  the  horse  uses  only  the  tip 
of  the  third  digit,  completely  covered  with  the  hoof.  In  the  aquatic 


BIOLOGY  OF  THE  SKELETAL  SYSTEM  203 

mammal,  the  whale,  both  the  hind  limbs  and  the  girdles  have  almost 
completely  disappeared  and  are  not  functional,  whereas  the  furelimbs 
have  become  modified  for  locomotion  in  water.  Even  more  complete 
reduction  in  limb  structure  is  seen  in  snakes,  where  a  complete  dis- 
appearance of  all  functional  appendagcmhas  occurred  in  practically 
all  species.  The  appendages  of  the  primjles,  as  shown  in  the  descrip- 
tion of  the  human  appendages  above,  have  remained  true  to  the  typical 
pentadactyl  type,  and  only  minor  modifications  and  fusion  of  certain 
girdle  bones  and  those  in  the  arms  and  feet  are  in  evidence.  And  so 
the  vertebrate  appendages  are  adapted  for  "all  walks  of  life." 

Joints. — The  human  skeleton  shows  various  methods  for  the 
articulation  of  the  200  separate  bones  of  which  it  is  composed.  In  some 
places,  as,  for  example,  in  the  cranium,  the  bones  are  so  rigidly  articu- 
lated by  toothed  edges,  which  fit  into  corresponding  depressions,  that 
no  movement  between  them  is  possible.  In  the  vertebral  column,  a 
certain  amount  of  flexibility  is  introduced  between  the  separate 
vertebrae  by  the  smoot'i  articulating  sin-races  present  on  the  bony 
processes  of  the  neural  arches  which  are  nicely  fitted  to  each  other. 
In  addition,  pads  of  elastic  cartilage  are  situated  between  the  centra 
of  the  apposed  vertebrae,  which  can  be  compressed  in  response  to 
the  bending  movements  of  the  trunk  region.  (Fig.  101.) 

We  now  come  to  the  articulating  surfaces  of  bones  which  are 
definitely  associated  with  movement  of  the  appendages.  Such  articu- 
lations are  known  as  joints,  and  several  distinct  types  may  be  indicated. 
The  least  differentiated  type  of  joint  and  one  that  affords  compara- 
tively little  opportunity  for  movement  is  the  gliding  joint  found  in 
the  bones  of  the  wrist  and  ankle.  The  articulating  bones  arc  fastened 
so  closely  by  connective  tissue  ligaments  that  only  a  slight  gliding 
movement  is  permitted.  Much  better  developed  are  the  hinge  joints 
of  the  fingers,  toes,  and  knees,  which  permit  a  wide  latitude  of  back- 
and-forth  movement  in  the  same  plane,  like  the  swinging  of  a  door. 
The  very  important  hinge  joint  in  the  knee  is  protected  by  an  addi- 
tional bony  element,  the  kneecap  (patella).  A  modification  of  the 
hinge  joint  which  permits  rotation  of  the  hand,  in  addition  to  the 
back-and-forth  movement,  is  found  in  the  pivot  joint  at  the  elbow. 
The  radius  and  ulna  are  so  articulated  with  the  humerus  at  the  elbow 
that,  when  the  palm  of  the  hand  is  turned  up,  the  distal  end  of  the 
radius  revolves  around  the  ulna,  thus  following  the  thumb  as  it  changes 
position.  (Fig.  87.) 

The  ball-and-socket  type  of  joint  permits  the  greatest  freedom  of 
movement.  It  is  best  shown  in  the  large  hip  joints  which  are  able  to 
support  the  weight  of  the  body  and  also  a  considerable  additional 


204 


HUMAN  BIOLOGY 


weight  when  necessary;  at  the  same  time  the  hip  joints  permit  the 
varying  leg  movements  essential  to  locomotion.  In  the  hip  joint  is 
an  airtight  fit  between  the  almost  spherical  head  of  the  femur  and  the 
deep  socket,  acetabulum,  in  the  pelvic  girdle.  The  fit  is  so  perfect 
that,  even  with  all  the  competing  ligaments  removed,  the  femur  will 
be  held  in  place  by  the  atidHpheric  pressure.  Ball-and-socket  joints 
of  lesser  degree  are  found  in  the  articulation  of  the  humerus  with  the 
shoulder  girdle  and  also  between  the  metacarpal  bone  of  the  thumb 


FIG.  104. — Section  through  the  shoulder  joint  of  man.     (Haggard,  u  Science  of  Health 
and  Disease,"  Harper  &  Brothers.) 

and  the  wrist.  This  ball-and-socket  thumb  joint  permits  this  impor- 
tant digit  to  be  placed  in  opposition  to  the  ends  of  the  other  digits. 
This  movement  is  not  present  in  the  less  adaptable  foot,  where  the  big 
toe  has  only  a  hinge,  rather  than  a  ball-and-socket,  joint.  The  articu- 
lating surfaces  of  the  hinge  and  ball-and-socket  joints,  in  which  bone 
movements  are  extensive,  are  covered  with  a  layer  of  smooth  hyaline 
cartilage.  In  addition,  the  cartilaginous  surfaces  which  are  in  contact 
are  in  turn  covered  by  a  very  thin  synovial  membrane  supplied  with 
secreting  cells  that  continually  secrete  a  synovial  fluid  for  joint 
lubrication.  (Fig.  104.) 

Finally,  mention  should  be  made  of  a  modified  pivot  joint,  forming 
the  connection  between  the  head  and  the  vertebral  column,  which  per- 


BIOLOGY  OF  THE  SKELETAL  SYSTEM  205 

mits  wide  latitude  in  head  movements.  The  occipital  bone  at  the  base 
of  the  head  bears  two  smooth  articulating  surfaces,  the  occipital  con- 
dyles,  near  the  foramen  magnum.  The  first  vertebra  (atlas)  articulates 
with  these  to  give  back-and-forth  movements.  In  rotary  head  move- 
ments, the  atlas  is  aided  by  the  special  odontoid  process  of  the  second 
vertebra  (axis)  which  extends  upward  through  the  atlas  and  functions 
as  a  pivot  or  the  rotary  movements  of  the  head  and  atlas. 

Development  of  Bone. — Bones  develop  as  a  result  of  the  deposition 
of  inorganic  salts  in  the  original  soft  skeletal  elements.  Two  types  of 
bones  are  recognized.  First  there  are  membrane  bones  that  are  formed 
by  the  gradual  transformation  and  hardening  of  soft  connective  tissue 
membranes  as  typically  form  the  basis  for  the  membrane  bones  of  the 
skull.  Outside  of  the  skull  bones,  however,  practically  all  of  the  bones 
in  the  human  skeleton  arise  as  modifications  of  cartilage  and  are, 
therefore,  known  as  cartilage  bones.  It  sounds  simple  enough  to  say 
that  connective  tissue  membranes  and  cartilage  are  transformed  into 
bone,  but,  as  a  matter  of  fact,  the  chemical  and  structural  changes 
necessary  for  such  a  transformation  are  very  complex  and  not  fully 
explained  as  yet.  Two  types  of  bone-forming  cell  are  involved.  One 
type,  the  osteoblasts,  is  able  to  absorb  the  needed  inorganic  salts  from 
the  blood  stream  and  *)  use  them  in  the  building  of  bone  tissue.  The 
other  type  of  bone  cell,  the  osteoclasts,  is  charged  with  the  duty  of 
remodeling  the  original  connective  tissue  model  to  conform  to  the  new 
bone-tissue  requirements.  This  remodeling  involves  the  actual 
destruction  of  certain  portions  of  the  original  cartilage,  presumably  by 
the  use  of  specific  enzymes.  In  this  way,  the  central  cavity  of  bone, 
which  contains  the  highly  vascularized  bone  marrow,  is  formed. 

It  must  be  remembered,  too,  that  the  tiny  bones,  as  first  laid  down 
in  the  embryo,  must  increase  in  size  to  correspond  to  the  general  body 
growth,  and  this  growth  of  the  bones  must  continue  until  maturity 
is  reached.  Increase  in  the  length  of  a  bone  occurs  principally  at  each 
end,  where  new  cartilage  is  continually  being  formed  and,  then, 
gradually  ossified.  Growth  also  occurs  at  the  bone  surface  through  the 
action  of  an  outer  connective  tissue  covering,  the  periosteum,  which 
supplies  bone-forming  cells  and  also  the  materials  for  the  formation 
of  new  bone  tissue.  The  periosteum  continues,  when  necessary,  to 
function  in  this  manner  throughout  life,  as  in  the  case  of  bone  fractures. 
The  general  size  and  shape  of  a  bone,  as  well  as  its  microscopic  struc- 
ture, reveal  a  high  degree  of  adaptation  to  the  exact  functional  needs 
of  the  region  in  which  it  is  found.  This  is  not  merely  a  passive  relation, 
for  bones  continue  to  change  all  through  life  in  accordance  with  the 
structural  needs  that  develop. 


206  HUMAN  BIOLOGY 

The  examination  of  a  typical  bone,  such  as  the  long  thigh  bone 
(femur)  of  the  leg,  shows  that  it  consists  of  a  main  portion,  or  shaft 
(diaphysis),  with  a  terminal  enlargement  at  each  end  which  forms  the 
articulating  surfaces,  or  joints,  with  the  connecting  bones.  The  joints 
as  noted,  are  covered  with  a  smooth  cartilage  which  affords  the  best 
possible  type  of  all  the  tissues  for  articulating  surfaces.  The  cartilage 
at  the  joints  never  becomes  ossified.  The  entire  bone,  with  the  excep- 
tion of  the  articulating  surfaces,  is  covered  by  a  closely  applied  sheet  of 
specialized  connective  tissue,  the  periosteum,  which  was  mentioned 
above  in  connection  with  bone  formation  and  regeneration.  It  can  be 
shown  that  bunches  of  the  connective  tissue  fibrils  from  the  periosteum 
penetrate  deeply  into  the  underlying  bone  tissue.  This  penetration 
of  various  functional  tissues  by  an  outer  layer  of  connective  tissue  is 
characteristic  also  of  the  muscles  and  nerves.  (Fig.  89.) 

If  the  femur  is  split  in  halves  lengthwise,  it  will  be  found  that 
the  bone  is  hollow  throughout  almost  its  entire  length  and;  therefore, 
the  bony  tissue  really  consists  of  a  relatively  thin  outer  shell  surround- 
ing a  central  cavity.  This  is  the  marrow  cavity,  filled  with  the  soft 
pinkish  bone  marrow  and  abundantly  supplied  with  blood  vessels  and 
nerves.  Bone  marrow  functions  not  as  an  endoskeletal  tissue  but  as  a 
vascular  tissue  primarily  concerned  with  the  formation  of  red  blood 
cells.  The  marrow  cavity  is  also  lined  by  a  layer  of  connective  tissue, 
the  endosteum.  An  examination  of  the  cut  surface  of  the  bone  tissue 
will  show,  even  with  the  naked  eye,  that  there  is  a  differentiation  into 
a  compact  bone  tissue  forming  the  walls  of  the  shaft  and  a  so-called 
spongy  bone  tissue  at  each  end.  In  the  spongy  tissue,  the  bone  fibers 
can  be  seen  extending  in  various  directions,  crossing  and  supporting 
each  other.  Careful  study  of  the  arrangement  of  the  bony  fibers  in 
the  spongy  bone  tissue  shows  very  clearly  that  they  are  arranged 
according  to  the  best  engineering  principles  to  give  the  utmost  strength 
with  the  least  use  of  material.  These-  same  excellent  engineering 
features  apply  to  the  bone  as  a  whole.  (Kg.  104.) 

Histology  of  Bone. — The  microscopic  examination  of  bone  tissue 
reveals  an  extraordinarily  intricate  arrangement  of  the  intercellular 
matrix  forming  the  mass  of  bone  tissue,  with  embedded  bone  cells  and 
a  puzzling  array  of  interconnecting  channels  of  various  kinds  and 
sizes.  The  complexity  of  mature  bone  is  particularly  striking  when 
it  is  compared  with  the  very  simple  arrangement  of  the  cartilage 
elements  from  which  it  develops.  In  order  to  study  bone  tissue  under 
the  microscope,  it  is  necessary  to  take  small  fragments  and  carefully 
grind  them  down  by  hand  until  they  are  thin  enough  to  be  transparent. 
After  grinding,  the  bone  fragments  can  be  mounted  on  a  slide  for 


BIOLOGY  OF  THE  SKELETAL  SYSTEM 


207 


detailed  examination.  Under  the  low  power  of  the  microscope,  con- 
siderable structural  differentiation  in  the  bone  tissue  is  apparent. 
The  general  picture  of  bone  structure,  as  revealed  under  the  microscope, 
reminds  one  somewhat  of  the  cyclonic  areas  in  a  weather  map  with 
concentric  lines  curving  around  the  low-pressure  areas.  (Fig.  105.) 

The  curved  areas  in  the  bone  tissue  are  the  lamellae  and  consist 
of  concentric  layers,  or  plates,  of  bone  tissue.  In  the  center  of  each  of 
these  areas  of  bony  tissue,  as  seen  in  a  transverse  section,  is  the  circular 
opening  of  the  Haversian  canal,  measuring  about  0.002  in.  in  diameter. 


LACUNA    CONTAINING 
BONE  CELLS 


LAMELLAE 


HAVERSIAN    CANAL 

CONTAINING    BLOOD 

VESSELS 


FIG.  105. — Microscopic  structure  of  bone.      (Buchanan,  "Elements  of  Biolofjy,"  Harper 

&  Brothers.) 

For  the  most  part,  the  Haversian  canals  run  lengthwise  of  the  bone, 
but  connecting  canals,  extending  transversely,  are  also  found,  some 
of  which  continue  to  the  surface  of  the  bone  tissue  and  open  under- 
neath the  periosteal  covering.  Altogether  the  Haversian  canals  form 
a  ramifying  tubular  network  throughout  the  bone  tissue  in  which  the 
blood  vessels  extending  from  the  periosteum  can  enter  and  supply 
all  regions  of  the  bones. 

The  examination  of  bone  under  a  higher  magnification  reveals 
additional  important  structural  elements.  Many  tiny  cavities 
(lacunae)  are  revealed,  each  containing  a  living  bone  cell  (osteoblast). 
Extending  from  each  lacuna  are  many  exceedingly  minute  channels 
which  pursue  irregular  winding  courses  through  the  bone  tissue  and 
open  either  directly  into  one  of  the  large  Haversian  canals  or  into  other 
canaliculi  that  so  open.  Thus  blood  plasma,  exuded  from  the  tiny 
vessels  in  the  Haversian  canals,  is  carried  by  the  connecting  canaliculi 
to  all  the  bone  cells.  It  is  estimated  that  areas  of  bone  tissue  exceed- 


208 


HUMAN  BIOLOGY 


ing  0.00004  in.  in  diameter  are  not  found  without  being  supplied  with 
blood  by  a  canaliculus.  Directly  under  the  periosteum,  the  lamellae 
and  accompanying  canals  encircle  the  bones  to  form  rings  of  bony 
tissue  rather  than  longitudinal  elements. 

It  is  apparent  from  the  description  just  given  that  bone  tissue  is  a 
living  tissue  with  many  cells  and  a  network  of  large  and  small  channels 
for  the  distribution  of  the  essential  materials.  By  placing  a  piece  of 
bone  in  a  weak  acid  for  a  time,  it  is  possible  to  remove  the  calcareous 
bone  materials  and  leave  the  organic  collagenous  material  which  always 


A  B 

FIG.  106. — Comparison  of  bone  which  has  been  soaked  in  acid  (A)  and  bone  which  has 

been  burned  (B). 

forms  the  underlying  framework.  A  bone  treated  with  acid  is  flexible 
and  can  be  tied  in  a  knot  without  breaking.  On  the  other  hand,  if 
a  bone  is  burned,  the  organic  materials  will  be  destroyed  and  the 
inorganic  materials  will  remain  in  their  original  shape  if  undisturbed. 
Burned  bone  is  very  fragile  and  will  crumble  to  dust  when  handled. 
(Fig.  106.) 

Nowhere  is  the  living  character  of  the  bony  endoskeleton  so  clearly 
revealed  as  in  the  highly  important  bone  marrow,  which  serves  as  a 
specialized  tissue,  largely  concerned  with  the  formation  of  the  red  cells 
of  the  blood.  Bone  marrow  also  functions  in  fat  storage  and  is  usually 
separated  into  the  red  marrow,  in  which  the  blood  cells  are  formed,  and 
yellow  or  fatty  marrow  in  which  fat  accumulates.  No  clear  histolog- 
ical  difference  exist  between  the  two  types,  and  a  shift  from  one  to  the 
other  may  occur.  Histologically,  bone  marrow  consists  of  a  con- 
nective tissue  ground  substance,  or  stroma,  containing  quantities  of 
cells  specialized  for  red  cell  formation,  as  well  as  fat-storage  cells. 
Penetrating  throughout  the  marrow  tissue  are  open  blood  channels, 
or  sinusoids,  such  as  were  previously  noted  in  the  liver.  Since  the 


BIOLOGY  OF  THE  SKELETAL  SYSTEM  209 

sinusoids  lack  a  definite  wall  as  present  in  capillaries,  newly  formed, 
nonmotile  red  cells  are  able  to  pass  directly  into  the  circulating  blood 
as  the  latter  slowly  moves  through  the  sinusoids  of  the  bone  marrow. 
Small  blood  vessels  enter  and  leave  the  bone  by  way  of  the  periosteum 
and  Haversian  canals,  but  the  blood  supply  of  the  bone  marrow  is 
received  directly  by  large  vessels  which  penetrate  the  bone  tissue  by 
definite  openings,  the  foramina. 

FUNCTIONAL  FEATURES  ASSOCIATED  WITH  THE  SKELETAL  SYSTEM 

The  skeletal  system  is  commonly  thought  of  as  being  wholly  con- 
cerned with  the  functions  of  protection,  support,  and  muscle  leverage, 
but  at  least  two  other  functional  features  of  this  system  are  inseparably 
associated,  namely,  the  formation  of  the  blood  cells  in  the  bone  marrow 
and  the  storage  of  calcium  by  the  bone  tissue. 
These  five  functions  may  now  be  considered  in 
the  order  named. 

Protection  and  Support. — Protection  of  the 
delicate  tissues  and  organs  is  primarily  a  func- 
tion of  the  exoskeletal  elements.     This  is  par-  > 
ticularly  evident  in  such  invertebrates  as  the     the^t^^ 
clam  and  the  insect.     The  function  of  support     functions  of  protection  and 

f         ,i        i      i        ..  .-i  •  support     in     the     skeletal 

for  the  body  tissues  appears  as  the  major  sy*tpem<  In  the  higher 
function  of  the  vertebrate  endoskeleton.  animals  (right)  the  sup- 

mi_  i  11  i  •  i    j.'       i      i  x  port  becomes  increasingly 

Thus,  broadly  speaking,  relatively  less  protec-     important.    (Waiter.) 
tion  is  offered  to  the  living  tissues  of  the  higher 

animals  than  to  the  lower  types.  In  man,  the  greatly  reduced  exo- 
skeletal elements  offer  a  minimum  of  protection,  the  living  tissues 
being  mostly  covered  by  a  few  layers  of  epithelial  cells.  However, 
the  structural  development  and  functional  importance  of  the  central 
nervous  system  require  that  it  be  well  protected,  and  this  has  been 
accomplished  by  the  bony  endoskeleton  as  was  indicated  above  in  the 
study  of  the  skull  and  vertebral  column  (page  196).  And  so  the  endo- 
skeleton, primarily  functioning  for  support,  has  also  been  assigned  the 
protection  of  the  central  nervous  system,  the  most  important  and 
delicate  structure  in  the  vertebrate  body.  (Fig.  107.) 

Movement. — The  adaptation  of  the  bones  and  associated  connec- 
tive tissues  in  the  various  vertebrate  types,  so  as  to  serve  with  the 
contractile  muscle  tissues  in  providing  for  the  numerous  essential 
body  movements  and  for  efficient  locomotion  in  water,  in  air,  or  on 
land,  has  reached  great  heights  in  the  higher  vertebrates  but  nowhere 
to  so  marked  a  degree  as  in  the  human  skeleton.  This  skeletal  superi- 
ority is  largely  centered  in  the  human  arm  and  hand,  in  which  the  com- 
bination of  skeletal*  contractile,  and  nerve  elements  has  made  an 


210  HUMAN  BIOLOGY 

unrivaled  living  tool  capable  of  coping  with  the  myriads  of  duties  that 
man  finds  necessary.  It  is  also  apparent  in  the  skeletal  changes  in 
legs  and  bony  girdles  that  make  two4egged,  bipedal,  locomotion 
possible.  A  difference  exists  in  the  skeletal  requirements  for  move- 
ment in  an  organ  that  remains  stationary  as  compared  with  the  require- 
ments in  which  a  part  of  the  body  is  moved  to  a  new  location.  Thus 
the  involuntary  muscles  present  in  the  ducts  of  glands,  the  alimentary 
canal,  and  the  walls  of  blood  vessels  require  only  flexible  fibrous  tissues 
for  support  and  leverage  and  for  binding  them  into  compact  functional 
units  adapted  for  the  stationary  movements  associated  with  these 
organs.  The  voluntary  muscles,  on  the  other  hand,  move  a  finger, 
a  foot,  an  arm,  a  leg,  or  the  entire  body  to  new  positions,  and  the 
movements  may  be  very  rapid.  Necessarily  associated  with  these 
voluntary  contractile  tissues  are  the  hard  bones  and  also  the  mediating 
flexible  fibrous  tissues,  the  complete  skeletal  assembly  being  con- 
tinuous from  the  sarcolemma  of  the  individual  muscle  fibers  to  the 
immobile  bone  at  the  muscle  origin  and  the  movable  bone  at  the  muscle 
insertion  which  serves  as  a  lever  for  the  muscle  pull. 

Bones  and  Levers. — The  physicist  recognizes  three  types  of  rigid 
rods  or  levers  that  are  available  for  leverage.  These  mechanical  aids 
are  commonly  known  as  levers  of  the  first,  second,  and  third  class. 
The  classification  depends  upon  the  relative  positions  of  three  points 
on  the  lever:  (1)  the  fixed  point  of  support  or  attachment  (fulcrum), 
(2)  the  point  at  which  force  is  applied  to  move  the  lever,  and  (3)  the 
point  where  work  is  accomplished.  In  levers  of  the  first  class,  the 
fulcrum  is  situated  between  the  other  two  points,  as  seen,  for  example, 
in  a  pair  of  scissors. 

Two  examples  may  be  selected  to  show  the  use  of  bones  in  the  body 
as  levers  of  the  first  class.  Thus,  when  the  foot  is  lifted  and  the  toes 
tapped  on  the  floor,  the  ankle  joint  is  the  fulcrum;  the  pull  of  the  large 
gastocnemius  muscle  in  the  calf  of  the  leg  is  transmitted  to  the  heel 
bone  (calcaneum)  back  of  the  fulcrum;  and  the  point  of  resistance, 
where  the  weight  is  moved,  is  the  portion  of  the  foot  anterior  to  the 
ankle  joint  fulcrum.  Another  example  is  found  in  the  movements  of 
the  head  in  which  the  atlas  serves  as  a  fulcrum,  lying  between  the 
insertion  of  the  muscle  to  the  occipital  bone  above  and  the  origin 
below  (page  188).  In  levers  of  the  second  class,  the  fulcrum  is  at  one 
end  of  the  lever,  the  force  is  applied  at  the  opposite  end,  and  the  weight 
is  between  the  other  two  points,  a  condition  that  is  well  shown  in  a  nut- 
cracker. An  instance  in  which  bones  are  used  as  levers  of  the  second 
class  is  seen  when  the  weight  of  the  body  is  raised  on  the  toes.  In 
this  case,  the  fulcrum  is  at  the  solid  surface  on  which  the  toes  are 
supported;  the  work  is  accomplished  at  the  ankle  joints  where  the 


BIOLOGY  OF  THE  SKELETAL  SYSTEM  211 

body  weight  is  supported;  and  the  force  is  applied  dorsally  at  the 
calcaneum  by  the  contraction  of  the  gastrocnemius  muscle  as  in 
the  first  example.  In  levers  of  the  third  class,  which  sacrifice  power  for 
speed,  the  fulcrum  is  at  one  end  of  the  lever,  and  the  weight  at  the 
opposite  end  so  that  the  force  is  applied  between  fulcrum  and  weight. 
The  author  notes  as  he  writes  that  this  is  the  method  used  in  mounting 
the  type  bars  of  the  typewriter,  which  are  designed  to  move  a  small 
weight  rapidly.  Examples  in  which  bones  are  used  as  levers  of  the 
third  class  are  well  shown  in  the  arms  where  the  pull  of  the  biceps 
muscle  transmitted  to  the  insertion  below  the  elbow  joint  elevates  the 


*\        F      *         r      *;       P-;  r     />•      /?/ 

Fia.  108. — Illustrating  the  uses  of  bones  in  the  three  kinds  of  levers  as  described  on 
page  210.  /?,  resistance  or  weight  moved;  F,  fulcrum;  P,  point  at  which  power  is  applied 
from  muscle  contraction.  {Redrawn  from  Huxley-Barcroft.) 

forearm.  The  same  arrangement  is  also  seen  in  the  extension  of  the 
leg  by  the  pull  of  the  ventral  thigh  muscles  which  is  transmitted 
through  the  kneecap  to  the  point  of  insertion  below  the  knee.  (Figs. 
87,  108.) 

Bipedal  Locomotion. — Generally  speaking,  voluntary  movements 
in  animals  are  associated  with  locomotion,  and,  in  the  various  groups, 
the  locomotor  organs  present  wide  variation  in  structural  design  in 
order  to  function  efficiently  in  water,  in  air,  or  on  land.  It  would 
seem  that  the  two  most  difficult  conditions  to  surmount  in  animal 
locomotion  are  flight  through  the  air  and  bipedal  locomotion  on  land 
by  man,  birds,  and  to  some  extent  by  a  few  of  the  higher  primates. 
Bipedal  locomotion  involves  the  maintenance  of  the  trunk  and  head 
of  the  body  in  an  upright  position,  the  shifting  of  the  entire  weight 
to  the  hind  limbs,  thus  freeing  the  forelimbs  for  other  duties.  It  has 
been  indicated  in  the  previous  chapter  that  the  delicate  balance 
essential  to  the  maintenance  of  the  upright  position  requires  that  cer- 
tain muscles  be  kept  in  tone  in  agreement  with  the  proprioceptive 
impulses  received  by  the  central  nervous  system  from  the  outlying 
regions  (page  184).  More  attention  will  be  given  to  the  nerve  control 
involved  in  the  erect  posture  in  the  following  chapter.  At  present 
we  are  concerned  with  the  complex  harmony  of  action  between  the 


212  HUMAN  BIOLOGY 

muscle  and  bone  elements  in  human  locomotion,  a  function  that  is 
learned  after  much  effort  in  early  childhood. 

Walking  involves  many  muscles  of  the  trunk  and  legs.  When  one 
starts  to  walk,  the  body  is  inclined  or  really  begins  to  fall  forward, 
which  brings  the  center  of  gravity  beyond  the  feet.  Then  one  of  the 
legs,  say,  the  left,  is  flexed  at  the  knee  joint.  This  raises  the  left  foot 
from  the  floor,  and  it  is  quickly  thrust  forward  to  support  the  falling 
body.  Synchronously  with  the  forward  movement  of  the  left  leg,  the 
gastrocnemius  muscle  in  the  calf  of  the  right  leg  contracts  and  elevates 
the  body  by  the  pull  on  the  heel  bone,  as  noted  in  the  previous  para- 
graph. Then  the  left  foot  is  firmly  planted  in  its  new  position.  At 


FIG.    109. — Walking.     Both  FIG.    110. — Running.     Both    feet    off 

feet  on  the  ground.  the  ground. 

this  moment  both  feet  are  on  the  ground.  When  both  feet  are  in  the 
air  at  this  position,  the  person  is  running  rather  than  walking.  The 
weight  of  the  body  is  now  shifted  to  the  left  leg,  and  this  leaves 
the  right  leg  free  to  swing  forward,  like  a  pendulum,  to  a  new  position  in 
advance  of  the  left  leg.  When  the  right  leg  is  placed  in  position,  the 
weight  of  the  body  is,  shifted  to  it,  and  the  pull  of  the  muscles  is  now 
on  the  heel  of  the  left  leg.  The  elevation  and  flexion  of  the  latter 
enable  it  to  swing  forward  in  a  pendulum-like  motion  and  to  come  to 
rest  once  more  in  advance  of  the  right  leg.  And  so  the  coordinated 
alternate  leg  motions  continue.  (Figs.  109,  110.) 

Blood-cell  Formation. — With  regard  to  the  formation  of  blood  cells 
by  the  bone  marrow,  the  concensus  of  opinion  at  the  present  time  seems 
to  be  that  the  tissues  of  the  bone  marrow  always  contain  large  numbers 
of  a  special  amoeboid  type  of  cell,  the  hemocytoblast,  which  is  regarded 
as  the  basic  type,  the  mother  cell,  so  to  speak,  of  red  cells  and  also 
of  the  various  types  of  granular  leucocytes.  As  to  how  the  differ- 
entiative  processes  are  carried  out  through  the  many  intervening 
cellular  stages,  very  little  information  is  available.  The  end  result, 
namely,  the  production  of  an  abundance  of  red  cells  and  granular 


BIOLOGY  OF  THE  SKELETAL  SYSTEM 


213 


leucocytes  by  the  blood-forming,  or  myeloid,  tissues  of  the  bone 
marrow,  is  an  established  fact.  Apparently  the  spleen  is  the  only 
other  tissue  in  the  body  that  is  equipped  to  play  any  part  in  the 
development  of  the  blood  cells.  (Fig.  Ill;  page  166.) 

Mineral  Reserves. — It  is  only  in  recent  years  that  the  function  of 
the  bones  as  a  storehouse  from  which  the  calcium  content  of  the 
blood  may  be  kept  at  normal  levels  has  been  realized,  though  it  was 
earlier  recognized  that  regular  calcium  deposition  was  imperative  for 
normal  bone  development  in  children.  Abnormalities  in  the  calcium 
metabolism  of  the  developing  bone  tissue  were  first  found  to  be 
responsible  for  rickets.  And  then  it  was  shown  that  a  deficiency  of 


BONE  MARROW 
VARIES  IN! 

I  Capacity  to  respond 

2.  State  of  activity 

J  Degree  of  maturation 
of  myeloid  cells 

4.  Injury  from  toxemia 


TISSUES 

VARY  IN: 

I.  hjeed  to  combat  injury 
(infection  or  toxemia) 

Z.  Speed  of  withdrawal  of 
qranulocytes  from 
blood  stream 

3.  Degree  of  toxemia 


LEUCOCYTES  ENRQUTE 
FROM  MARROW  TO  TISSUE: 

I.  Always  reflect  the  need 
of  tissues  plus  capacity 
of  marrouj  to  respond 

t  May  reflect  condition  of 
marrow  if  cells  are  free- 
ly released 

J  btey  reflect  need  of  tissues 
if  marrow  can  freely  re- 
spond 

FIG.  111. — Illustrating  the  formation  of  blood  cells  in  the  bone  marrow  (left)  and 
their  utilization  in  the  body  tissues  (right).  (Haden,  "Principles  of  Hematology," 
Lea  cfc  Fehiger.) 

vitamin  D  was  at  the  basis  of  the  abnormal  calcium  reaction  in  the 
bone  tissues  (page  60).  The  basic  importance  of  free  calcium  in 
the  blood  plasma,  however,  was  not  realized  until  much  later,  when  the 
experimental  studies  on  the  parathyroid  gland  showed  that  the  hor- 
mone secreted  by  this  gland  was  an  essential  factor  in  the  maintenance 
of  the  normal  amount  of  calcium  in  the  blood  (page  109).  Further- 
more, as  noted  previously,  the  absence  of  calcium  from  the  blood  very 
quickly  produced  other  disastrous  effects  in  the  organism,  primarily 
associated  with  increased  muscle-nerve  irritability,  that,  if  not  reme- 
died at  once  by  supplying  calcium,  were  invariably  fatal.  Linked  in 
the  complete  picture  of  blood  calcium  is  the  little-understood  relation- 
ship between  phosphorus  and  calcium.  In  some  unknown  way,  these 
two  elements  work  together  to  maintain  normal  conditions  in  the 
blood  stream  and  are  apparently  deposited  in  the  bone  or  removed 
from  it  as  the  conditions  demand  in  order  to  maintain  the  normal 
levels  in  the  blood  stream. 


OCCIPITAL- 


FRONTAL 

1NFRAORB1TAL 

MANDIBULAR 

BRACH1AL 
PLEXUS 


DORSAL  ROO' 
SPINAL  GANGLION 


SKIN 


TO  BRAIN 


PHRENIC 
VAGUS 

INTERCOSTAL 
RADIAL 
MEDIAN 
ULNAR 


FEMORAL 
SACRAL  PLEXUS 

SCIATIC 


SENSORY  NEURON 
BIPOLAR 


PERONEAL 
TIBIAL 


MOTOR  NEURON 
MULTI  POLAR 


PHRENIC 


CELT  AC  PLEXUS-1 .. 

MESENTERIC  PLEXUSCSUP.) 

SYMPATHETIC  TRUNK 
MESENTERIC  PLEXUS(lNF.) 
PERONEAL 
TIBIAL 


A  B 

PLATE  XII. — Nervous  system  in  man.  A,  illustrating  the  general  plan,  with  the 
spinal  cord  and  the  chief  nerve  routes  to  the  arms  and  legs;  B,  the  chief  components  of 
the  autonomic  system;  C,  elements  of  the  reflex  arc;  D,  bipolar  neuron;  J5?f  multipolar 
neuron.  Diagrammatic. 


CHAPTER  X 
BIOLOGY  OF  THE  NERVOUS  SYSTEM  (I) 

The  nervous  system  as  the  master  organ  system  of  the  body 
dominates  the  entire  organism.  It  receives  and  interprets  external 
stimuli  which  make  known  the  environmental  factors  and  also  internal 
stimuli  which  reveal  the  conditions  of  the  various  organs.  In  order 
to  do  this,  there  are  located  at  strategic  points  various  types  of  sensory 
cells  which  are  capable  of  receiving  the  particular  stimuli,  and  the 
latter  incite  nerve  impulses  which  travel  into  the  central  nervous 
system  over  the  cable-like  conducting  nerve  fibers.  The  central 
nervous  system  receives  the  impulses  from  the  peripheral  sensory  areas, 
interprets  the  data,  and  then  incites  outgoing  impulses  which  are 
transmitted  over  a  separate  set  of  nerve  fibers  to  the  motor  tissues 
and  cause  coordinated  action  of  these  effector  organs.  Thus  the  basic 
function  of  irritability,  which  is  common  to  all  living  matter,  is  largely 
assigned,  in  the  multicellular  animals,  to  the  nervous  system,  and  the 
extent  of  the  assignment  increases  in  the  higher  types  in  correspondence 
with  the  augmented  tissue  differentiation.  In  addition,  the  higher 
functions  of  consciousness,  memory,  intelligence,  and  volitional  think- 
ing come  into  more  and  more  prominence  and,  finally,  reach  their  full 
flowering  in  the  human  brain. 

In  order  to  receive  the  sensory  impulses  and  to  control  and  coordi- 
nate the  activities  of  the  muscle  tissues  and,  as  a  matter  of  fact,  all 
the  tissues  of  the  body  as  well,  the  nervous  system  must  have  direct 
connections  through  definite  nerve  fibers  with  all  of  the  cooperating 
organ  systems,  with  all  of  the  constituent  tissues,  and,  in  many 
instances,  as  in  the  outlying  sensory  structures,  with  the  individual 
cells.  In  a  highly  differentiated  animal,  it  is  easy  to  see  that  the  sum 
total  of  all  these  nerve  elements  supplying  every  type  of  body  structure 
necessitates  the  presence  of  an  exceedingly  intricate  organ  system— 
far  beyond  anything  encountered  in  the  previous  studies  of  the  other 
organ  systems. 

STRUCTURAL  FEATURES  ASSOCIATED  WITH  THE  NERVOUS  SYSTEM 

From  the  comparative  standpoint,  specialized  nerve  cells,  which 
mark  the  beginning  of  nerve  tissue  in  the  multicellular  animals,  are 

215 


216 


HUMAN  BIOLOGY 


FIG.  112. — Nerve  cells  in  the  body  wall 
of  Hydra.  The  long  fibrils  in  the  stippled 
background  represent  contractile  elements 
from  the  epitheliomuscular  cells.  (Shult, 
after  Schneider,  slightly  modified.} 


first  encountered  in  hydra  and  the  related  coelenterate  animals.  In 
these  lowly  metazoa,  the  nerve  cells  are  not  associated  to  form  a 
definite  tissue  but  appear  as  separate  branched  cells  or  as  a  diffuse 
nerve  net,  connecting  directly  with  near-by  contractile  elements  in  the 

body  wall.  Each  nerve  cell  is  thus 
an  independent  functional  unit  for 
the  reception  and  transmission  of 
stimuli.  A  more  advanced  but 
still  comparatively  simple  animal 
type,  as  seen  in  the  earthworm, 
reveals  important  advances  in  the 
structure  of  the  nerve  elements. 
A  definite  nerve  tissue  is  built  up 
of  associated  cells  and  fibers  with  a 
differentiation  into  a  central  and 
a  peripheral  division,  so  that  the 
principle  of  a  central  adjuster  mechanism  to  mediate  between  the 
incoming  sensory  impulses  from  the  receptors  and  the  outgoing  motor 
impulses  to  the  effectors  is  early  introduced  into  the  animal  organism,  a 
development  that,  it  may  be  said,  persists  throughout  the  higher  types 
up  to  man  and  becomes  of  increasing  importance.  (Figs.  112,  113.) 

It  will  not  be  necessary  to 
pursue  the  field  of  comparative 
neurology  to  greater  lengths  at 
present,  except  to  indicate  one 
or  two  basic  structural  differences 
between  the  invertebrate  and 
vertebrate  nervous  systems. 
Thus,  in  the  invertebrates,  the 
central  nervous  system  is  seen 
to  be  extended  throughout  the 
length  of  the  body  as  a  ventral 
nerve  cord,  lying  in  the  body 
cavity  close  to  the  body  wall.  In 
the  vertebrate  animals,  the  central  nerve  cord  is  dorsal  in  position,  has 
a  central  cavity,  and,  in  all  except  the  most  primitive  vertebrates,  is 
enclosed  by  the  bony  neural  arches  of  the  vertebral  column  as  revealed 
earlier  (page  196). 

Very  early  in  its  development,  the  vertebrate  embryo  begins  to 
form  a  nervous  system  which  in  a  short  time  enables  it  to  keep  in 
touch  with  the  environment  and  to  perform  essentially  adaptive 
responses.  The  first  indications  of  the  embryonic  nervous  system 


•SEGMENTAL 
GANGLIA 


FIG.  11 3. —Nerve  tissue  in  the  anterior 
end  of  the  earthworm.  Shown  are:  the 
ventral  nerve  cord  with  segrnental  ganglia 
lying  under  the  alimentary  canal;  the  sub- 
pharyngeal  ganglia  where  the  nerve  cord 
divides  to  form  a  nerve  collar  which  en- 
circles the  pharynx  and  bears  the  dorsal 
brain.  (Buchanan,  "Elements  of  Biology,'1 
Harper  &  Brothers.) 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (I) 


217 


are  seen  in  a  thickening  and  later  infolding  of  the  outer  ectoderm  on 
the  dorsal  body  surface.  In  this  way,  which  will  be  described  in  more 
detail  later  when  the  problems  of  embryology  are  considered,  a  hollow 
ectodermal  tube  is  formed,  extending  the  length  of  the  body.  As 
the  ectoderm  grows  over  the  invaginated  region,  the  tube  comes  to 
lie  just  beneath  and  entirely  separated  from  the  outer  ectoderm  from 
which  it  was  formed  shortly  before.  The  walls  of  this  primitive 
nerve  tube,  which  is  the  forerunner  of  the  nervous  system,  increase  in 
thickness,  and  the  anterior  end  soon  undergoes  radical  structural 
changes  which  result  in  a  definite  delimination  of  the  forebrain,  mid- 
brain,  and  hindbrain  and  the  further  modification  and  subdivision  of 


sc 


D  E  F 

FIG.  114. — Early  development  of  the  nervous  system  in  the  amphibian  embryo 
(Amblystoma).  A,  B,  (7,  successive  stages  in  the  closure  of  the  medullary  folds  to  form 
the  neural  tubo;  D,  E,  Fy  sections  through  A,  B,  and  C  showing  the  formation  of  the  tube 
as  observed  under  the  microscope;  FB,  forebrain;  MB,  midbrain;  HB,  hindbrain;  SC, 
spinal  cord.  (Wieman.) 

these  parts  to  form  the  complete  master  unit  of  the  vertebrate  body, 
the  brain.  (Fig.  114.) 

From  the  nerve  cells  (neurons)  of  this  newly  formed  neural  tube, 
microscopic  cell  processes,  the  nerve  fibers,  rapidly  develop  and  soon 
connect  all  parts  and  tissues  of  the  developing  embryo.  These  fibers 
are  primarily  concerned  with  the  conduction  of  the  sensory  nerve 
impulses  coming  in  from  the  peripheral  regions  and  the  motor  impulses 
going  out  to  the  contractile  tissues.  And  so  the  embryo  is  soon  in 
possession  of  a  functional  nervous  system,  comparatively  simple  at 
first  but  increasing  in  complexity  as  the  rapidly  differentiating  tissues 
of  the  embryo  continue  to  make  new  demands.  • 

As  finally  constituted,  the  nervous  system  of  man  and  other  higher 
vertebrates  can  be  separated  for  convenience  in  description  into  four 
major  structural  divisions  as  follows:  (1)  sense  organs,  or  receptors, 
which  are  the  outlying  units  specialized  for  the  reception  of  external 
and  internal  stimuli;  (2)  peripheral  nervous  system  with  its  network 


218  HUMAN  BIOLOGY 

of  nerve  fibers  which  innervate  all  regions  of  the  body  and  function 
in  conduction,  involving  the  transmission  of  nerve  impulses  to  and 
from  all  parts  of  the  body;  (3)  autonomic  nervous  system,  essentially 
a  part  of  the  peripheral  nervous  system  in  that  it  conducts  impulses  to 
and  from  the  body  structures  that  it  innervates,  but,  as  will  be  seen 
later,  it  is  also  associated  with  the  involuntary  control  of  various 
important  organ  systems;  (4)  central  nervous  system  which  serves  as 
the  integrative  and  controlling  unit  of  the  nervous  system—from 
the  structural  standpoint,  its  complexity  is  unrivaled.  In  our  present 
consideration,  it  seems  logical  to  begin  with  the  outlying  sensory 
elements,  then  pass  to  the  conducting  elements  of  the  peripheral 
system,  and,  finally,  conclude  with  the  master  units  of  the  central 
nervous  system.  (Plate  XII.) 

SENSE  ORGANS 

It  is  customary  to  recognize  five  primary  senses  in  the  animal 
organism,  namely,  touch  (including  pressure,  temperature,  and  pain), 
taste,  smell,  sight,  and  hearing.  To  these  should  be  added  the  sense 
of  position,  or  equilibrium,  and  those  internal  sensory  phenomena,  such 
as  hunger  and  thirst,  which  enable  the  organism  to  determine  its  own 
internal  conditions.  The  various  sense  organs,  associated  with  the 
primary  senses,  function  only  as  extremely  sensitive  receptors.  Stim- 
uli arise  in  them  that  result  in  the  transmission  of  nerve  impulses  to 
the  central  nervous  system,  and  the  latter  interprets  the  stimuli  and 
determines  the  action  to  be  taken. 

Sense  organs  throughout  the  body,  generally  speaking,  are  com- 
posed of  essential  and  accessory  parts.  The  essential  element  consists 
of  the  specialized  sensory  cells  or  terminal  arborizations  which  arc 
capable  of  responding  to  a  certain  type  or  types  of  stimulus  that 
impinge  upon  them  in  their  particular  peripheral  location.  Also 
generally  present  in  the  various  sense  organs  are  accessory  structures 
that  aid  in  bringing  the  stimuli  to  the  sensory  neurons.  For  example, 
the  lens  of  the  eye  is  an  accessory  structure  which  focuses  the  light 
rays  on  the  sensitive  nerve  cells  of  the  retina,  the  latter  con- 
stituting the  essential  part  of  the  eye.  It  should  be  kept  in  mind 
that  the  sense  organs  of  the  body  are  internal  (interoceptivc)  as  well  as 
external  (exteroccptive).  Thus,  the  sensations  of  hunger  and  thirst 
have  their  origin  in  interoceptive  sense  organs  scattered  through  the 
tissues  of  the  body.  The  structural  features  of  the  internal  sensory 
apparatus  are  not  apparent,  but  those  of  the  various  external  sense 
organs  are  well-known.  Finally,  sense  organs  are  general  or  specific 
in  their  reactions  to  stimuli.  In  the  first  case,  as  seen  in  the  skin,  a 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (/) 


219 


FIG.  115. — Tactile  corpuscle 
in  the  skin  of  man.  Highly 
magnified.  (Mitchell.) 


variety  of  sensory  elements  is  subject  to  influence  by  different  types 
of  stimuli.  In  the  specific  type  of  sense  organ,  as  seem  in  the  eye,  the 
neurons  are  capable  of  responding  to  one  type  of  stimulus  only — that 
received  from  the  light  rays.  (Figs.  98, 
126.) 

Skin  Sensations. — Sensory  elements 
located  in  the  skin  respond  to  a  consider- 
able variety  of  environmental  stimuli, 
notably  pressure  (touch),  temperature 
(heat  or  cold),  and  pain.  Scattered 
through  the  skin  just  under  the  epidermis 
are  numerous  tactile  corpuscles,  each 
covered  by  a  connective  tissue  sheath 
which  contains  the  essential  network  of 
delicate  nerve  fibers  and  their  endings. 
The  tactile  organs  are  sensitive  to  pressure 
or  touch,  as  we  commonly  say.  They  are 
more  highly  differentiated  and  more 
abundant  in  the  skin  covering  the  tips  of 
the  fingers  and  toes  than  in  other  regions 
of  the  body.  By  testing  very  small  areas  of  the  skin  surface  with  a 
fairly  stiff  hair  mounted  on  a  handle,  it  is  possible  to  determine  the 
distribution  of  these  tactile  areas.  It  will  be  found  that,  in  the  middle 
of  the  back,  points  separated  by  less  than  2.7  in.  (68  mm.)  will  be  felt  as 
a  single  stimulus.  Points  less  than  half  this  distance  apart  can  be  felt  as 
separate  stimuli  on  the  back  of  the  hand,  whereas  on  the  tips  of  the 
fingers,  areas  around  0.1  in.  in  diameter  are  felt  as  separate  points  of 
stimulation.  Each  hair  follicle  has  sensory  nerve  endings  that  are 
very  sensitive  to  contact,  and  less  pressure  is  required  to  stimulate 
them.  Even  the  slightest  manipulation  of  a  single  hair  is  noted. 
(Figs.  24,  98.) 

Temperature  sensations  and  pain  are  received  by  separate  nerve 
elements  in  the  skin  so  that,  altogether,  four  different  sensory  areas 
are  found  (pressure,  heat,  cold,  and  pain).  There  is  some  doubt  as  to 
whether  or  not  pain  should  be  regarded  as  a  separate  sensation.  It 
is  possible  that  excessive  stimulation  of  any  sensory  nerve  will  give  the 
sensation  of  pain.  With  the  exception  of  the  tactile  corpuscles,  noted 
above,  sensory  nerves  in  the  skin  do  not  terminate  as  definite  sensory 
bodies,  but  each  splits  into  a  branching  group  of  fibrillae  (arborization) 
which  lie  between  the  epithelial  cells.  (Fig.  115.) 

The  nerve  elements  in  the  living  tissues  of  the  body  are  sensitive 
to  chemical  agents,  and,  as  we  have  seen  earlier,  slight  changes  in  the 


220  HUMAN  BIOLOGY 

chemical  composition  of  the  blood,  due  to  variations  in  the  carbon 
dioxide  content,  are  detected  and  result  in  definite  reactions  to  main- 
tain the  proper  balance.  The  neurons  associated  with  the  sensations 
of  taste  and  smell  are,  however,  specialized  for  the  reception  of  external 
chemical  stimuli.  In  the  epithelium  of  the  tongue  and  nasal  cavities, 
these  chemical  receptors  are  grouped  to  form  highly  developed  sense 
organs.  Attention  was  given  to  the  tasto  buds  of  the  tongue  epithe- 
lium in  an  earlier  chapter  (page  44).  Accordingly,  attention  may  be 
given  at  once  to  the  sense  of  smell  (olfactory  sense)  localized  in  certain 
regions  of  the  nasal  epithelium.  (Figs.  27,  116.) 

Olfactory  Sense. — A  microscopic  study  of  the  olfactory  epithelium 
shows  an  underlying  layer  of  connective  tissue  bearing  the  sensory 
epithelial  cells.  The  latter  contain  three  cellular  types:  olfactory, 
basal,  and  interstitial.  The  olfactory  colls  are  tho  functional  sensorv 


FIG.  116. — Section  of  taste  bud  of  tongue.     Highly  magnified.      (Mitchell,  after  Ranvier.} 

neurons,  situated  between  the  interstitial  cells.  Each  sensory  neuron 
is  an  elongated  cell,  lying  below  and  at  right  angles  to  the  surface 
epithelium.  The  proximal  ends  of  the  olfactory  cells,  lying  next  to 
the  connective  tissue  layer,  connect  with  a  very  fine  nerve  fiber  which 
is  continuous  with  the  conducting  elements  of  the  olfactory  nerve 
extending  to  the  central  nervous  system.  The  opposite  end  of  each 
olfactory  cell  lies  at  the  outer  surface  of  the  epithelium  and  splits  into 
a  terminal  group  of  projecting  olfactory  cilia  which  are  sensitive  to 
chemical  stimuli  or  odors  present  in  the  incoming  air.  The  entire 
surface  of  the  olfactory  epithelium,  bearing  the  exposed  cilia,  is  kept 
continually  moistened  by  secretions  of  the  olfactory  glands.  The 
latter  lie  in  the  basal  connective  tissue  layer  and  open  through  separate 
ducts  at  the  surface.  This  secretion,  of  course,  comes  into  direct 
contact  with  the  chemical  substances  in  the  air  currents  and  with  the 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (I) 


221 


cilia.     It  probably  acts  as  a  solvent  for  the  volatile  substances  detected 
by  the  sensory  cells.     (Fig.  117.) 

The  sense  of  smell  is  unbelievably  acute.  It  is  known  that  a  strong 
odor,  such  as  that  of  vanillin,  can  be  detected  in  a  dilution  of  1  part  in 
10  million,  which  is  a  far  more  delicate  reaction  than  can  be  performed 
by  the  chemist.  It  is  also  certain  that  the  olfactory  sense  of  other 
animals,  notably  dogs,  is  even  more  acute  than  that  of  man.  One  of 
the  best  examples  is  the  ability  of  the  bloodhound  to  follow  a  trail. 
On  the  other  hand,  the  olfactory  sense  is  unreliable  in  some  respects 
and  is  easily  fatigued  by  a  particular  odor  so  that  it  may  become 
entirely  insensitive  to  it  while  still  sensitive  to  other  odors. 

It  has  been  stated  in  an  earlier  section  that 
four  primary  tastes  are  recognized  and  that  the 
flavors  of  foods  and  other  substances  are  usually 
complex  reactions  primarily  associated  with  the 
sense  of  smell  (page  45).  This  can  be  proved  by 
noting  the  absence  of  taste  when  the  olfactory 
sense  is  not  functioning.  A  great  deal  of  work 
has  been  done  in  an  attempt  to  classify  the 
primary  odors  on  the  same  basis  as  primary 
tastes.  Various  schemes  have  been  proposed  in 
which  from  three  to  nine  primary  odors  were 
recognized.  Recent  work  favors  the  establish- 
ment of  four  primary  odors,  thus  bringing  the 
olfactory  sense  into  line  with  the  other  cutaneous 
sensations,  namely,  pressure,  heat,  cold,  and  pain 
in  the  skin;  and  sweet,  sour,  salt,  and  bitter  in  the  chemical 
receptors  of  the  tongue  associated  with  taste.  The  four  primary 
odors  as  now  recognized  arc  termed  fragrant  (flowery),  acid 
(vinegary),  burnt  (tarry),  and  caprylic  (rancid).  By  using  the  numer- 
als from  0  to  9  to  represent  the  complete  absence  and  the  increasing 
concentration  of  the  primary  odors  present  in  a  certain  substance,  the 
perfumers  have  been  able  to  standardize  perfumes  by  designating 
them  with  numbers.  Thus  by  this  system  the  odor  of  oil  of  winter- 
green  is  described  by  the  number  8442  which  means  that  it  is  strongly 
fragrant  (8/),  with  moderate  amounts  of  the  acid  and  burnt  units 
(4a,  46),  and  a  slight  amount  of  caprylic  (2c).  Damask  rose  is  desig- 
nated numerically  as  6523,  and  ethyl  alcohol  as  5301.  Experts  in  this 
field  are  able  to  get  a  conception  of  the  odor  of  a  new  perfume  by  the 
number  assigned. 

The  Sense  of  Hearing  and  Position. — The  ear  is  a  highly  developed 
sense  organ  which  contains  receptors  for  two  types  of  stimuli :  those  of 


FIG.  117. — Section 
of  olfactory  epithe- 
lium. With  types  of 
cells  as  described  on 
page  220.  Diagram- 
matic. (Wieman.) 


222  HUMAN  BIOLOGY 

sound  and  those  of  position,  or  equilibrium.  The  human  ear  consists 
of  three  basic  structural  and  functional  units.  There  is,  first,  the 
visible  outer  ear  which  functions  as  an  accessory  structure  for  collecting 
the  sound  waves  and  passing  them  on  to  the  middle  ear.  The  middle 
ear  is  also  an  accessory  structure  and  contains  the  elements  adapted 
for  sound  transference  from  the  outer  car  to  the  inner  ear.  In  the 
inner  ear  are  additional  accessory  structures  and  the  essential  auditory 
neurons  which  are  the  receptors  for  the  incoming  sound  vibrations. 
(Plate  XIII A,  page  229.) 

The  external  ear  consists  of  a  cartilaginous  pinna,  variable  in  size 
and  shape,  with  a  central  passage  (auditory  canal)  continuing  to  the 
middle  ear  like  the  neck  of  a  funnel.  The  inner  end  of  the  auditory 
canal  is  closed  by  a  vibrating  drum  or  tympanic  membrane.  The 
distal  portion  of  the  auditory  canal  connecting  with  the  external 
pinna  is  cartilaginous,  but  the  inner  portion  is  bony.  The  canal  is 
uniformly  lined  with  skin  containing  special  areas  which  secrete  a  pro- 
tective wax.  Entrance  to  the  auditory  canal  is  guarded  by  numerous 
projecting  stiff  hairs.  The  tympanic  membrane,  which  marks  the 
inner  boundary  of  tho  external  ear,  is  so  constructed  that  impinging 
sound  waves  cause  corresponding  vibrations. 

The  middle  car  is  an  irregular-shaped  cavity  located  in  the  sub- 
stance of  the  temporal  bone.  Laterally,  that  is,  toward  the  outer  ear, 
the  cavity  is  terminated  by  the  tympanic  membrane.  In  the  opposite 
direction,  approach  is  made  to  the  structures  of  the  inner  ear.  Dor- 
sally,  the  middle  ear  cavity  ends  in  irregular  air  spaces  in  the  temporal 
bone.  Opening  into  the  cavity  is  an  air  tube  (Kustachian  tube)  which 
connects  with  the  throat  region  and  serves  to  keep  the  air  pressure 
equalized  on  the  tympanic  membrane.  When  the  Eustachian  tube  is 
partially  or  entirely  clogged  as  the  result  of  an  infection,  the  air  pres- 
sure is  gradually  reduced  in  the  cavity  of  the  middle  ear  by  continued 
swallowing,  and  the  tympanic  membrane  is  pushed  inward  by  the 
external  air  pressure.  This  condition  results  in  lessened  vibrating 
efficiency.  * 

Functionally  important  in  the  middle  ear  are  three  tiny  bones,  the 
auditory  ossicles,  which  are  responsible  for  receiving  the  vibrations 
from  the  tympanic  membrane  and  carrying  them  across  the  cavity  of 
the  middle  ear  to  that  of  the  inner  ear.  Joined  to  the  inner  surface  of 
the  tympanic  membrane  is  the  first  of  these  bones,  the  malleus  (ham- 
mer) which,  in  turn,  articulates  with  the  incus  (anvil).  The  third 
member  of  this  auditory  bone-bridge  is  the  stapes  (stirrup).  It 
articulates  with  the  incus  laterally  and  then  continues  to  the  tiny 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (I) 


223 


so-called  oval  window  (fenestra  ovalis)  which  marks  the  beginning 
of  the  inner  ear.  The  fenestra  ovalis  and  another  similar  opening, 
the  fenestra  rotunda,  lying  just  below,  are  tiny  openings  in  the  bone 
tissue  leading  from  the  middle  ear  to  the  inner  car.  Each  is  completely 
shut  off  from  the  middle  ear  by  a  membrane.  The  two  ends  of  the 
U-shaped  stapes  are,  attached  to  the  fenestra  ovalis  membrane,  which 
clearly  indicates  that  the  auditory  vibrations  are  transmitted  to  the 
inner  ear  at  this  point.  (Plate  XIII A,  B.) 

The  inner  ear  (labyrinth)  is  also  situated  in 
the  temporal  bone  just  beyond  the  middle  ear 
cavity.  It  is  a  structure  of  extreme  complexity 
in  the  human  adult.  The  labyrinth  consists  of 
an  outer  covering  of  unusually  hard  bone  tissue 
(bony  labyrinth)  molded,  so  to  speak,  to 
conform  to  the  shape  of  the  delicate  membra- 
nous labyrinth  that  it  encloses.  Between  the 
membranous  and  the  bony  labyrinths  is  a  space 
filled  with  a  fluid,  the  perilyinph.  The  latter 
apparently  serves  as  a  medium  for  the  transfer 
of  vibrations  received  from  the  auditory  ossicles 
at  the  oval  window  to  the  second  fluid,  the 
endolymph,  which  fills  the  interior  of  the 
membranous  labyrinth.  The  endolymph  actu- 
ally bathes  the  sensory  cells  present  in  the 
membranous  labyrinth  and,  in  some  way, 
manages  to  convey  the  qualities  of  the  original 
sound  waves  impinging  upon  the  eardrums. 
The  essential  functional  elements  of  the  ear  are, 
as  just  indicated,  within  the  membranous 
labyrinth.  (Plate  X1IIC.) 

The  membranous  labyrinth  is  first  seen  in 
the  embryo  as  a  tiny  depression  in  the  outer 
ectoderm  on  each  side  of  the  head  region, 
depressed  area  is  entirely  cut  off  from  the  outer  surface  and  forms 
a  tiny  closed  vesicle.  Then  the  latter  differentiates  into  an  upper 
vestibular  portion  (utriculus)  and  a  lower  cochlear  portion  (sac- 
culus).  From  the  utriculus,  as  development  proceeds,  three  semicircu- 
lar canals  are  formed  which  function  in  connection  with  equilibrium; 
and,  from  the  sacculus,  the  coiled  cochlea,  in  which  the  auditory 
sense  is  localized,  gradually  takes  its  permanent  structure.  Both  the 
semicircular  canals  and  the  cochlea  are  innervated  by  separate  brari- 


118. — Membra- 
nous labyrinth  of  human 
embryo  (<M  cm.).  Cf 
cochlea;  S,  sacculus;  U, 
utriculus  with  semicir- 
cular canals;  CN,  coch- 
lear branch  of  auditory 
nerve ;  VN,  vestibular 
branch  of  auditory  nerve ; 
ED,  endolymphatic  duct. 
(Wieman,  after  Streeter.) 

A   little   later   the 


224  HUMAN  BIOLOGY 

ches  of  the  auditory  nerve  that  comes  from  the  central  nervous  system. 
The  interior  cavities  of  the  labyrinth  are  continuous  and  filled  with 
the  common  endolymph.  (Fig.  118.) 

Semicircular  Canals. — Considering,  first,  the  structural  arrange- 
ment of  the  semicircular  canals  that  develop  from  and  remain  con- 
tinuous with  the  utriculus,  it  is  important  to  note  t^at  the  canals  lie 
in  three  different  planes  at  approximately  right  angles  to  each  other. 
Each  canal  may  be  described  as  a  tubular  horseshoe-shaped  structure, 
both  ends  of  which  are  welded  to  the  utriculus.  One  end  of  each  canal, 
near  its  union  with  the  utriculus,  is  enlarged  to  form  a  circular  ridge,  or 
ampulla,  in  which  a  sensory  structure  (crista  acustica)  is  located.  The 


G       h  g    f 

FIG.  119. — Diagram  showing  sensory  areas  (black)  in  tho  semicircular  canals  and 
cochlea  of  the  left  ear.  a,  6,  c,  superior,  lateral,  and  posterior  semicircular  canals  with 
ampullae;  d,  macula  utriculi;  e,  macula  sacculi;  /,  organ  of  Corti,  k,  saccua  endolymph- 
aticus.  (Maximow-Bloom,  "Histology,"  W»  B.  Saunders  Company.  From  Shaffer, 
after  von  Abner,  slightly  modified.) 

latter,  when  the  ampulla  is  opened,  is  seen  as  an  elevated  ridge  occupy- 
ing about  one-third  of  the  canal  cavity.  Covering  tho  top  of  the  ridge 
and  extending  for  a  distance  on  each  side  arc  ciliated  sensory  neurons 
with  associated  supporting  cells.  The  cilia  do  not  project  directly 
into  the  endolymph  but  into  a  covering  layer  of  gelatinous  material 
perforated  with  tiny  canals,  through  which  the  endolymph  reaches 
the  cilia.  Histologists  have  found  it  impossible  to  make  satisfactory 
microscopic  preparations  of  the  crista  of  the  mammalian  ear,  and 
certain  details  both  of  structure  and  function  are  still  unknown. 

It  has  long  been  known  from  experimental  data  that  the  sensory 
cells  in  the  semicircular  canals  are  influenced  by  changes  in  the  body 
position  and  that,  in  some  way,  the  stimuli  received  by  the  central 
nervous  system  from  these  stimulated  sense  organs  are  used  as  a  basis 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (I)  225 

for  the  coordinated  muscle  control  associated  with  the  erect  posture 
of  man.  Presumably  the  more  or  less  continuous  impulses  from  the 
sensory  cells  of  the  semicircular  canals  are  integrated  with  the  pro- 
prioceptive  impulses  in  determining  the  necessary  course  of  muscle 
action.  It  is  assumed  that  variations  in  the  endolymph  pressure, 
corresponding  to  the  head  movements,  stimulate  the  canal  receptors, 
but  absolute  proof  of  this  and  of  other  suggested  theories  of  semicircu- 
lar canal  functions  is  still  lacking.  The  problem  is  further  compli- 
cated by  the  fact  that  a  highly  developed  sensory  apparatus  (macula 
acustica),  essentially  of  the  same  nature  as  the  crista  of  the  canals  but 
containing  otoliths,  is  a  permanent  feature  of  the  sacculus  and  of  the 
utriculus.  It  is  generally  believed  that  the  maculae  also  share  in  the 
equilibratory  function  of  the  ear,  but  again  proof  is  lacking.  (Fig.  119.) 

Cochlea. — The  essential  auditory  organ,  the  cochlea,  as  found  in 
man  and  the  higher  vertebrates,  is  one  of  the  most  complex  of  all  the 
organs  in  the  body  and  least  understood  from  a  functional  standpoint. 
The  cochlea  is  not  present  at  all  in  the  lower  vertebrates,  where  the  ear 
is  primarily  an  organ  of  equilibration,  but  it  assumes  increasing  struc- 
tural and  functional  importance  in  the  higher  groups.  The  forerunner 
of  the  cochlea  is  seen  in  the  lower  vertebrates  as  a  tiny  teat-like  pro- 
jection (lagena)  of  the  sacculus.  The  mature  cochlea  of  the  human 
ear  viewed  externally  is  a  coiled  bony  structure,  fashioned  like  a  snail 
shell  with  two  and  one-half  turns  around  the  central  axis.  This  coiled 
cochlcar  tube  has  the  greatest  diameter  proximally  where  it  connects 
with  the  sacculus  and  gradually  narrows  down  distally  to  a  blind 
ending  in  the  center  of  the  coil.  All  in  all,  it  is  about  lj^  in.  long, 
around  0.1  in.  in  diameter,  and  filled  with  the  fluid  perilymph.  (Fig. 
119.) 

Contained  in  the  outer  bony  cochlea  is  an  inner  membranous 
cochlear  canal  (scala  media),  which  contains  the  endolymph  and  the 
sensory  cells  grouped  in  the  spiral  organ  of  Corti.  This  cochlear  canal 
is  considerably  smaller  than  the  osseous  cochlea  that  encloses  it,  but 
of  course  it  follows  the  snail-like  curvature  of  the  latter.  If,  now,  a 
transverse  section  of  the  cochlea  is  examined,  it  will  be  seen  that  the 
enclosed  cochlear  canal  is  not  circular  but  roughly  three-sided,  or  tri- 
hedral, in  outline.  The  apex  of  the  trihedral  figure  projects  about  two- 
thirds  of  the  distance  across  the  cavity  of  the  enclosing  bony  cochlea. 
From  the  wall  of  the  latter,  however,  a  bony  projecting  shelf  (spiral 
lamina)  extends  out  the  remaining  distance  so  that  a  transverse  section 
of  the  cochlear  canal  shows  a  complete  partition  that  divides  the  bony 
cochlea  into  an  upper  portion  (scala  vestibuli)  and  a  lower  portion  (scala 
tympani).  The  latter  is  considerably  larger  than  the  scala  vestibuli 


226 


HUMAN  BIOLOGY 


because  the  membranous  cochlear  canal  (scala  media),  noted  above, 
lies  entirely  above  the  partition  and  occupies  about  one-half  the  space 
that  would  otherwise  be  open  to  the  scala  vestibuli.  (Fig.  120.) 

We  have  just  seen  that  one  side  of  the  membranous  cochlear  canal 
is  continuous  at  the  apex  with  a  projecting  shelf  or  spiral  lamina  of  the 
bony  cochlea;  thus  a  division  is  formed  between  the  scala  media  and 
scala  tympani.  The  division  between  scala  media  and  scala  vestibuli 
is  a  thin  undifferentiated  membrane  which  constitutes  a  second  side 
of  the  cochlear  canal.  The  base,  or  third  side,  of  the  latter  is  in 
contact  with  the  inner  wall  of  the  cochlea.  The  arrangement  of 


SCALA  VEST/BULI 


COCHLEAR  CANAL 
(SCALA  MEDIA) 


SPIRAL  LAMINA 


SENSORY  CELL 

AUDITORy\(°urERHAIRC£LL>  BASILAR 
NERVE     \  MEMBRANE 

TUNNEL  OF  CORTI 


SCALA    TYMPAMI 


FIG.  120. — Diagrammatic  section  through  tho  cochlea  to  show  a  portion  of  the 
organ  of  Corti,  as  described  on  page  227  C/.  Plato  XIII  D.  (Buchanan,  "Elements  of 
Biology,"  Harper  &  Brothers.) 

chambers  in  a  transverse  section  of  the  cochlear  canal  may  be  visual- 
ized by  taking  a  pair  of  dividers,  separating  the  points  3  or  4  in.,  and 
then  placing  them  flat  on  the  desk,  with  the  hinge  at  the  left  and  the 
separated  points  at  the  right.  The  hinge  at  the  left  represents  the  apex 
of  the  cochlear  canal  at  the  point  where  it  joins  the  spiral  lamina. 
Now  place  the  back  edge  of  a  book  in  contact  with  the  points.  The 
three-sided  area  formed  by  the  arms  of  the  dividers  and  the  edge  of 
the  book  represents  the  cavity  of  the  cochlear  canal,  or  scala  media, 
which  is  filled  with  endolymph.  The  upper  arm  of  the  dividers  sepa- 
rates the  scala  media  from  the  scala  vestibuli,  and  the  lower  arm  does 
the  same  for  the  scala  tympani.  It  also  bears  on  its  upper  surface, 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (I)  227 

that  is,  within  the  scala  media,  the  highly  differentiated  auditory 
elements  comprising  the  organ  of  Corti. 

Organ  of  Corti. — Many  pages  could  be  devoted  to  a  description  of 
the  known  structural  and  functional  features  of  the  organ  of  Corti. 
Even  so,  at  the  present  time,  it  would  not  be  possible  to  describe  it 
completely,  because  certain  of  the  details  have  never  been  success- 
fully worked  out.  For  the  inherent  delicate  nature  of  the  tissues 
associated  with  the  organ  of  Corti,  together  with  its  very  inaccessible 
position,  embedded  as  it  is  in  the  recesses  of  the  temporal  bone  and 
enclosed  by  the  bony  tissues  of  the  cochlea  and  also  by  the  mem- 
branous wall  of  the  cochlear  canal,  have  so  far  made  it  impossible  to 
fathom  all  of  its  structural  complexities.  Its  general  plan,  however, 
is  well  known.  In  the  first  place,  it  must  be  remembered  that  both 
the  cochlea  and  the  enclosed  cochlear  canal  are  coiled,  spiral  struc- 
tures, and,  therefore,  the  organ  of  Corti,  lying  within  the  cochlear 
canal,  has  essentially  the  same  length  and  spiral  pattern  of  the  enclos- 
ing structures.  In  a  transverse  section  of  the  cochlea,  the  organ  of 
Corti  appears  as  a  ridge  on  the  innor  surface  of  the  wall  that  separates 
the  scala  media  from  the  scala  tympani,  as  just  noted  in  the  preceding 
paragraph.  The  tectorial  membrane,  projecting  from  the  spiral 
lamina,  is  in  close  contact  with  a  part  of  the  upper  surface  of  the 
cochlear  apparatus.  The  portion  of  the  wall  bearing  the  complex 
sensory  mechanism  of  the  organ  of  Corti  is  known  as  the  basilar  mem- 
brane. The  latter  has  an  unusual  and  complex  arrangement  of 
radiating  connective  tissues  that  permits  vibration.  Some  authorities 
have  held  that  the  basilar  membrane  is  the  primary  functional  unit  of 
the  ear.  (Fig.  120.) 

Five  main  types  of  sensory  cell  are  supported  by  the  basilar  mem- 
brane. These  are  the  inner  and  outer  tunnel  cells,  the  inner  and 
outer  hair  cells,  and  the  supporting  cells.  The  tunnel  cells  are  peculiar 
in  shape,  each  with  a  wide  base  in  contact  with  the  basilar  membrane 
and  a  narrow  elongated  body.  The  bases  of  the  inner  tunnel  cells 
are  separated  some  distance  from  those  of  the  outer  tunnel  cells,  but 
distally  they  bend  toward  each  other  so  that  the  distal  ends  of  inner 
and  outer  tunnel  cells  are  in  contact  at  the  upper  surface  of  the  organ 
of  Corti.  The  roughly  triangular  space  thus  formed  between  the 
cells  is  the  tunnel  of  Corti. 

The  arrangement,  just  described,  may  be  visualized  by  placing  the 
right  hand  palm  down  on  the  table.  Now  slightly  flex  the  thumb  and 
fingers,  thus  raising  the  palm  from  the  table  so  that  the  hand  will  be 
supported  by  the  tips  of  the  thumb  and  fingers.  In  this  position,  the 
surface  of  the  table  under  the  hand  will  represent  the  basilar  mem- 
brane; the  forefinger,  one  of  the  inner  tunnel  cells;  the  thumb,  one 


228  HUMAN  BIOLOGY 

of  the  outer  tunnel  cells;  and  the  space  between  forefinger  and  thumb, 
the  tunnel  of  Corti.  It  is  estimated  that  there  are  about  5,600  of  the 
inner  tunnel  cells  and  3,850  of  the  outer  tunnel  cells,  each  type  arranged 
side  by  side  in  a  single  row  and  the  two  rows  separated  proximally  by 
the  width  of  the  tunnel.  The  visualization  of  a  spiral-shaped  hand 
with  this  number  of  fingers  and  thumbs  may  possibly  convey  the  idea. 

In  close  association  with  the  inner  and  outer  tunnel  cells  are  the 
inner  and  outer  hair  cells.  These  are  the  specific  sensory  cells  through 
which  connection  is  made  with  the  nervous  system.  The  hair  cells 
are  small,  fairly  typical  in  shape,  and  are  arranged  in  regular  rows  on 
each  side  of  the  tunnel  cells.  They  are  not  in  contact  with  the  basilar 
membrane  but  hang  down  suspended  by  one  end,  as  it  were,  from  the 
upper  surface  of  the  sensory  epithelium.  Projecting  into  the  endo- 
lymph  from  the  attached  upper  end  of  each  of  these  cells  are  some  40 
to  60  sensory  hairs.  It  is  estimated  that  there  are  around  20,000  of  the 
hair  cells.  Referring  once  more  to  the  analogy  with  the  hand  as 
stated  in  the  preceding  paragraph,  we  may  let  the  knuckles  represent 
the  line  of  fusion  of  the  inner  and  outer  tunnel  cells,  and  the  back  of  the 
hand  the  upper  surface  of  the  sensory  epithelium,  bathed  by  the 
endolymph.  Thus  the  ends  of  the  hair  cells,  together  with  numerous 
associated  supporting  cells,  form  a  cellular  mosaic,  marking  the  upper 
boundary  of  the  organ  of  Corti,  and  the  hairs  on  the  back  of  the  hand 
represent  the  position  of  the  sensory  hairs  projecting  from  the  hair 
cells  into  the  endolymph.  To  complete  the  analogy,  the  hand,  posi- 
tion as  given,  may  be  visualized  as  immersed  in  water.  In  this  case, 
the  portion  of  the  container  on  which  the  hand  rests  will  represent  the 
basilar  membrane,  and  the  water  will  represent  the  endolymph  that 
fills  the  tunnel  of  Corti,  covers  the  upper  surface  of  the  hair  cells  with 
the  sensory  hairs,  and  completely  fills  the  cochlear  canal.  (Fig.  120.) 

Ear  Function. — Various  theories  have  been  proposed  to  account  for 
jbhe  ability  of  the  human  ear  to  detect  and  analyze  the  wide  variety  of 
sounds  that  reach  it.  There  is  no  question  about  certain  features 
concerned  with  the  transmission  of  sound  waves  through  the  external 
and  middle  ear  to  the  perilymph  and  endolymph  of  the  inner  ear, 
but  no  adequate  explanation  has  been  made  of  the  incredible  ability  of 
the  organ  of  Corti  to  distinguish  among  the  tremendous  variety  of 
sounds  in  such  a  way  that  corresponding  distinctive  nerve  impulses 
can  be  sent  to  the  brain  for  interpretation.  Briefly,  the  organ  of 
Corti  transforms  sound  waves  into  a  tremendous  variety  of  distinctive 
nerve  impulses  which  can  be  interpreted  by  the  brain.  When  one 
considers  that  the  human  ear  is  able  to  distinguish  sounds  that  vary 
in  strength  or  loudness,  in  pitch,  and  in  quality,  the  problem  of  inner 
ear  function  becomes  highly  complicated. 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (I) 

h 

-SEMICIRCULAR  CANAL(SUP.)     BONY  LABYRINTH  £*» 

SEMICIRCULAR  CANAL  (POST.)  ^    A  %rt^ 
SEMICIRCULAR  CANAL (LAT.)       SCALA  VESTIBULI 
FENESTRA  OVALIS  VESTIBULAR 

VESTIBULUM         MEMBRANE 
COCHLEA  SPIRAL  LAMINA 

BASILAR  MEMBRANE 
ORGAN  OF  CORTI 
SCALA  TYMPANI 


229 


INCUS  (ANVIL) 
STAPES  (STIRRUP) 


P 

MALLEUS  (HAMMER) 


EXTERNAL  EAR,- 


EXTERNAL  AUDITORY  MEATUS" 


VESTIBULAR  NERVE)  AUDITORY  NERVE 
•COCHLEAR  NERVE  / 

-INTERNAL  CAROTID  ARTERY 
EUSTACHIAN  TUBE 


EAR 


DRUM    TYMPANIC  CAVITY 

A 


CILIARY  BODY 
CONJUNCTIVA 
CILIARY  PROCESSES 
CHAMBER  (POST.) 
CHAMBER  (ANT.) 
CRYSTALLINE  LENS 

PUP1 L 

CORNEA 

IRIS  ~— •—•""•"""'"••-" — — — 

LIGAMENT  OF  LENS 


RECTUS  MUSCLE  (SUP.) 

IVESSELS  OF  RETINA 

^-- 

OPTIC  NERVE 
FATTY  TISSUE 

"INFERIOR  RECTUS  MUSCLE 


RETINA 


SCLERA 
[ — CHOROID 


PLATE  XIII. — Sonso  organs  of  man.  A,  section  showing  the  general  structure  of  the 
ear;  B,  auditory  ossicles;  C,  internal  oar;  D,  section  showing  internal  structure  of  the 
cochlea;  E,  vertical  section  through  the  eye  to  show  internal  structure.  Diagrammatic. 


230  HUMAN  BIOLOGY 

Detectable  variations  in  pitch  alone  range  from  very  low-pitched 
sounds  with  about  20  vibrations  per  second  up  to  very  high  notes  with 
some  40,000  vibrations,  and  all  of  these  tones  may  vary  in  intensity 
and  be  associated  with  overtones  that  give  a  distinctive  quality.  It 
has  been  suggested  that  the  basilar  membrane  acts  as  a  vibrating 
membrane  in  which  certain  regions  are  responsive  to  particular  types 
of  incoming  waves.  Other  authorities  suggest  that  it  acts  like  the 
vibrating  disc  in  a  telephone  receiver  which  vibrates  as  a  unit  for 
every  type  of  sound  wave.  It  was  long  held  that  the  peculiar  tunnel 
cells  (rods  of  Corti),  described  above,  acted  essentially  as  tuning 
forks,  sympathetic  to  certain  types  of  vibration,  but  there  are  far  too 
few  of  them  to  account  for  the  tremendous  variety  of  sounds  received 
by  the  ear.  All  in  all  the  problems  associated  with  our  highly  devel- 
oped sense  of  hearing  are  far  from  solved.  Of  the  end  result,  there  can 
be  no  doubt;  of  the  methods  used  in  the  organ  of  Corti  to  obtain  the 
results,  there  is  uncertainty. 

The  Sense  of  Sight. — The  eyes  are  special  sense  organs  devoted 
exclusively  to  the  function  of  sight,  or  vision.  The  eye  is  essentially 
an  optical  instrument  of  the  camera  type,  so  arranged  that  light  can 
be  focused  upon  a  sensory  tissue,  the  retina,  which  is  capable  of 
determining  the  intensity  and  the  wave  lengths  of  the  incoming  light 
rays. 

Eyeball. — The  main  structural  unit  is  the  eyeball,  which,  in  the 
human  eye,  is  almost  spherical  in  shape  and  approximately  1  in.  in 
diameter  (Fig.  121).  Well-developed  accessory  structures  are  neces- 
sary for  the  functioning  of  the  essential  sensory  neurons  in  the  retina, 
some  of  which  lie  outside  the  eyeball  and  some  lie  inside.  The  external 
accessory  structures  include  the  attached  muscles  which  move  the 
eyeball;  the  eyelids,  with  a  special  epithelial  lining,  the  conjunctiva, 
which  also  covers  the  exposed  anterior  surface  of  the  eyeball;  arid  the 
glandular  lachrymal  apparatus  which  secretes  a  cleansing  and  lubri- 
cating fluid.  The  internal  accessory  structures  include  the  iris  dia- 
phragm which  regulates  the  amount  of  light  admitted  through  the 
anterior  opening,  or  pupil;  the  lens,  directly  back  of  the  pupil,  which  is 
responsible  for  focusing  the  light  rays  on  the  retina;  the  important 
and  highly  specialized  muscle  tissue  which  is  associated  with  the 
necessary  structural  changes  in  the  iris  and  lens;  and,  finally,  the 
transparent,  semifluid  humors  which  fill  the  interior  cavities  of 
the  eyeball.  (Plato  XIII#.) 

The  eyeball  is  enclosed  externally  by  a  strong  connective  tissue 
sheath,  or  sclera  (tunica  fibrosa),  which  forms  a  continuous  covering, 
except  for  a  small  circular  area  in  the  back  of  the  eye  where  it  is 
pierced  by  the  optic  nerve  running  from  the  retina  to  the  brain. 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (I)  231 

The  tissue  of  the  sclera  is  opaque,  except  for  a  small  area  directly  in 
the  front  of  the  eye  where  it  forms  the  transparent,  window-like 
cornea  through  which  the  light  rays  pass  on  their  way  to  the  lens. 
Lying  within  the  sclera  is  the  choroid  layer.  It  is  very  vascular, 
deeply  pigmented  and,  in  the  front  of  the  eye,  forms  the  colored,  con- 
tractile iris  with  a  circular  opening,  the  pupil,  in  the  center.  The  iris 
contains  muscular  elements  and,  in  its  functional  aspect,  may  be 
compared  to  the  adjustable  iris  diaphragm  of  the  camera  which  can  be 
regulated  as  desired  according  to  the  intensity  of  the  light.  When  the 
light  is  dim,  the  radiating  muscles  in  the  iris  contract,  thus  enlarging 
the  pupillary  opening  through  which  light  passes  to  the  interior  of  the 
eye.  In  a  bright  light,  the  circular  muscles  of  the  iris  involuntarily 
contract,  and  this  results  in  a  constriction  of  the  pupil  so  that  a 
reduced  amount  of  light  is  admitted  to  the  sensitive  eye  tissues.  The 
third  and  innermost  layer  of  the  eye  is  the  retina,  characterized  by  the 
presence  of  unique  functional  visual  elements  (rods  and  cones)  adapted 
for  receiving  the  photic  stimuli  and  passing  the  resulting  impulses  into 
the  nervous  system  for  transmission  to  the  brain  via  the  optic  nerve. 
The  retina  lines  the  posterior  portion  of  the  eyeball  but  does  not  form 
a  complete  anterior  layer  as  do  the  sclera  and  choroid. 

The  eye  lies  in  a  deep-seated  skull  cavity,  the  socket,  or  orbit, 
well-protected  by  the  surrounding  bony  tissues.  The  exposed  anterior 
portion  of  the  eyeball  is  covered  by  a  special  type  of  epithelium,  the 
conjunctiva,  arranged  in  two  layers;  the  inner  layer,  thin  and  trans- 
parent, covers  the  exposed  portion  of  the  eye  and  is  directly  attached 
to  the  cornea  and  near-by  regions  of  the  sclera.  Peripherally,  this 
layer  merges  into  the  outer  conjunct! val  layer  which  forms  the  lining 
of  both  eyelids.  At  the  edges  of  the  lids,  in  the  region  of  the  eyelashes, 
the  conjunctiva  becomes  continuous  with  the  skin  tissues.  The  eye- 
lids themselves  contain  muscle  elements,  both  smooth  and  striated, 
together  with  connective  tissues  and  numerous  glands.  The  latter 
open  on  the  edges  of  the  lids  in  the  region  where  the  conjunctiva  and 
skin  merge.  (Plate  XIII#.) 

Lachrymal  Glands. — The  almost  continuous  movements  of  the 
eyeball  and  the  eyelids  during  the  day  require  that  the  conjunct! val 
surfaces  be  well  supplied  with  a  moistening,  lubricating,  and  cleansing 
medium.  To  furnish  this  medium  is  the  function  of  the  lachrymal 
glands  which  pour  their  secretion  through  the  lachrymal  ducts;  the 
latter  open  through  the  conjunctiva  near  the  outer  edge  of  the  upper 
eyelid.  The  lachrymal  gland  of  each  eye  is  roughly  almond-shaped 
and  is  lodged  between  the  eyeball  and  the  bony  socket,  above  and  well 
to  the  outer  edge.  The  lachrymal  secretion  flows  downward  over 
the  exposed  eye  surfaces  and  is  continuously  drained  off  through  the 


232 


HUMAN  BIOLOGY 


openings  of  the  tear  ducts,  situated  close  to  the  inner  junction  of  the 
upper  and  lower  lids.  The  two  ducts  of  each  eye,  one  from  the  upper 
lid  and  one  from  the  lower,  unite  to  form  the  lachrymal  sac  which 
opens  into  the  nasal  cavity.  An  excess  of  the  lachrymal  secretion,  as 
in  laughing  or  crying,  causes  the  tears  to  overflow  and  roll  down  the 
cheeks. 

Voluntary  movements  of  the  eyeball  are  brought  about  by  three 
pairs  of  muscles  which  are  inserted  in  the  sclera  and  anchored  to  the 
surrounding  bony  tissues.  These  paired  muscles  include  (1)  the 
superior  and  inferior  recti,  attached  above  and  below,  respectively, 
by  means  of  which  the  eye  is  rolled  up  and  down;  (2)  the  anterior  and 

SUPERIOR    OBLIQUE 

SUPERIOR   RECTUS 

INTERNAL   RECTUS 


INFERIOR    RECTUS 
EXTERNAL    RECTUS 

INFERIOR   OBLIQUE 

FIG.  121. — Dissection  to  show  the  eyeball  and  the  arrangement  of  the  muscles  responsi- 
ble for  eye  movements.     (Buchanan,  "Elements  of  Biology  "  Harper  &  Brothers.) 

posterior  recti,  attached  medially  and  laterally,  through  which  the  eye 
is  revolved  to  the  right  or  left;  (3)  the  superior  and  inferior  oblique, 
attached  above  and  below  in  such  a  fashion  that  oblique  movements  of 
the  eyeball  are  possible.  A  pair  of  strong  median  and  lateral  ligaments 
attached  to  the  sclera  near  the  junctions  of  the  lids  extends  into  the 
bony  tissue  of  the  nasal  region  and  laterally  into  the  bone  at  the  outer 
edge  of  the  orbit  and  limits  the  eye  movements  and  keeps  the  eyeball 
securely  lodged  in  its  orbit.  (Fig.  121.) 

With  the  general  external  structure  of  the  eye  in  mind,  attention 
may  be  directed  to  its  important  internal  structural  and  functional 
features.  Observations  on  a  mammalian  eye,  which  has  been  cut  in 
half  in  a  vertical  plane,  shows  a  division  into  a  comparatively  large 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (7) 


233 


vitreous  chamber,  lying  posteriorly,  and  a  small  anterior  chamber. 
The  anterior  chamber  extends  from  the  cornea  posteriorly  to  the  iris. 
Between  the  iris  and  the  anterior  surface  of  the  lens,  an  even  smaller 
chamber  communicates  with  the  anterior  chamber  through  the  opening 
of  the  pupil.  These  two  chambers  contain  the  aqueous  humor,  a 
transparent  watery  fluid  secreted  by  certain  of  the  surrounding  tissues. 
The  aqueous  humor  is  continuously  replaced,  as  necessary,  to  restore 
that  lost  by  the  drainage.  Posterior  to  the  lens  is  the  large  vitreous 
chamber,  noted  above,  which  is  mostly  lined  by  the  retina.  It  is  filled 
with  the  vitreous  humor,  a  permanent,  transparent,  noncellular 
material  of  gelatinous  consistency.  (Plate  XIII£f.) 

Accomodation. — The  incoming  light  rays  pass  first  through  the 
pupillary  opening  and  then  through  the  lens  which  is  directly  behind. 


ch 


FIG.  122. — Illustrating  the  mechanism  of  accommodation,  as  described  on  page  233. 
To  the  right,  thickened  lens,  accommodation  for  a  near  object;  to  the  left,  thin  lens, 
accommodation  for  a  distant  object,  ch,  ciliary  process;  i,  iris;  si,  suspensory  ligament; 
rf,  ciliary  muscle.  (Martin,  "Human  Body,"  Henry  Holt  &  Company,  Inc.} 

The  latter  is  constructed  of  highly  modified  epithelial  cells,  each  drawn 
out  into  a  transparent  ribbon-like  stucture,  and  all  closely  joined  and 
held  in  position  by  an  intercellular  cement  substance.  The  lens  is  a 
biconvex,  crystalline  body  so  constructed  that  it  can  change  its  shape 
— the  function  of  accommodation — which  is  necessary  in  order  to 
focus  the  light  waves  from  near  and  distant  objects  on  the  retina.  The 
lens  is  enclosed  by  a  substantial  membrane,  or  lens  capsule,  closely 
applied  to  the  outer  lens  epithelium.  The  lens  capsule  is  continuous, 
all  the  way  around,  with  a  tendon-like  ligament  attached  to  the 
ciliary  muscles.  The  latter  form  a  ridge  extending  toward  the  lens 
from  the  choroid  layer  at  the  base  of  the  iris.  Also  these  muscles  are 
so  arranged  that,  when  they  contract,  tension  is  lessened  on  the  liga- 
ment attached  to  the  lens  capsule,  and  the  highly  elastic  lens  is  thereby 
permitted  to  assume  a  more  spherical  shape.  The  increased  curvature 
makes  the  lens  stronger,  that  is,  capable  of  bending  the  light  rays  from 
near  objects,  as  in  reading,  so  as  to  bring  them  to  a  focus  on  the 


234  HUMAN  BIOLOGY 

retina.  When  the  ciliary  muscles  relax,  the  tension  on  the  lens  capsule 
is  increased,  and  the  lens  is  flattened  and  thus  adapted  for  focusing 
the  images  of  distant  objects  on  the  retina.  And  so  the  lens  is  able  to 
accommodate  for  near  and  distant  vision  by  altering  the  curvature. 
The  same  result  is  attained  in  a  camera  by  adjusting  the  distance 
between  the  lens  and  the  sensitive  film  surface.  (Fig.  122.) 

In  a  considerable  number  of  individuals,  the  accommodation  of 
the  eye  is  not  perfect.  The  lenses  may  be  consistently  too  strong  and 
bring  the  objects  to  a  focus  in  front  of  the  retina.  This  is  nearsighted- 
ness,  and  the  individual  compensates  by  moving  the  object  nearer  to 
the  eye.  Farsightedness  is  the  opposite  condition  in  which  the  lenses 

are  not  strong  enough,  and  consequently 
the  rays  are  focused  back  of  the  retina. 
The  farsighted  individual  endeavors  to 
compensate  by  holding  the  book  or  other 
object  farther  from  his  eyes.  The  faulty 


v  —  -""'^  accommodation  is  not  always  due  to  an 
abnormal  lens;  it  may  be  due  to  the  fact 
that  the  distance  between  the  lens  and  the 
retina  is  abnormally  long  or  short.  A  third 
error  in  refraction,  astigmatism,  is  due  to 
FIG.  i23.-~lliustrating  the  irroguiar  curvature  of  the  leng.  The  oculist 

paths  of  parallel  light  rays  in  CT 

a    normal    eye    (A),    short-    is  able  to  measure  the  optical  condition  of 
sighted  eye   (B),  far-sighted     thc  and  to   prescribe   artificial   lenses 

eye    (C).      (Martin,       Human  ^  ^  . 

Body"  Henry  Holt  &  Com-    that   will    correct    the    deficiencies    of    tho 


Inc.)  regular  lenses.     (Fig.  123.) 

Retina.  —  The  retina  is  ono  of  the  most  complex  tissues  in  the  entire 
body,  and  therefore  it  may  be  well  to  begin  our  description  with  its 
development  in  the  embryo.  Such  a  study  will  show  that  the  retina 
is  the  first  structure  of  the  eye  to  develop  and,  also,  that  it  is  formed 
from  the  ectoderm  of  the  brain  region.  Later,  the  developing  retina 
is  doubly  enclosed  by  the  choroid  and  the  sclerotic  layers,  both  of 
which  are  mesodermal  in  origin.  The  retina  grows  out  laterally  from 
the  newly  formed  neural  tube,  in  the  region  of  the  developing  fore- 
brain,  until  it  comes  into  contact  with  the  epithelial  cells  covering  the 
body  surface.  The  epithelial  cells,  which  are  thus  brought  into  con- 
tact with  a  developing  retina,  gradually  become  transformed  into  the 
lens. 

At  first,  the  retinal  outgrowth  is  seen  as  a  hollow,  club-shaped 
group  of  cells,  but  the  shape  of  the  distal  end  is  soon  changed  by  a 
secondary  invagination  and  develops  into  a  double-walled,  cup- 
shaped  structure  which  becomes  the  retina  proper.  This  develop- 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (I) 


235 


mental  process  can  be  visualized  by  holding  a  spherical  toy  balloon, 
lightly  inflated,  in  the  palm  of  one  hand  and  pressing  against  it  with 
the  closed  fist  of  the  other  hand.  In  this  position,  the  fist  represents 
the  crystalline  lens  at  the  lateral  surface  of  the  head.  The  balloon, 
now  double-walled  and  partially  enclosing  the  lens,  represents  the 
retina,  which  consists  essentially  of  an  inner  sensory  layer  (repre- 
sented by  the  balloon  wall  surrounding  the  fist)  and  an  outer  pig- 
mental layer  (represented  by  the  outer  balloon  wall  in  contact  with 


Optic 
vesicle 


'Pigment  layer 

1  Nervous  layer 


Chorioid  fissure 

Optic  stalk 


Lens  pit 


Chorioid 

fissure 

B 


FIG.  124. — Illustrating  tho  development  of  the  retina  in.  the  human  eye  as  an  out- 
growth from  the  forebraiii,  described  on  page  234.  A,  from  4.5  mm.  embryo;  B,  from 
5.5  mm.  embryo;  C,  from  7.5  rnm.  embryo.  (Arey,  "Developmental  Anatomy,"  W.  B. 
Saunders  Company.} 

the  palm  of  the  other  hand),  which  is  soon  to  be  covered  by  the  choroid 
layer.  The  optic  nerve,  which  connects  the  visual  cells  of  the  retina 
with  the  sensory  area  in  the  brain,  is  gradually  fashioned  out  of  the 
proximal  portion  of  the  original  outgrowth  from  the  forebrain.  (Fig. 
124.) 

Our  next  concern  is  with  the  mature  retina  which  gradually 
assumes  an  extraordinary  complexity  of  the  constituent  tissues  as 
noted  above.  The  general  pattern  of  the  retinal  tissues  follows  to  a 
considerable  extent  the  basic  pattern  of  the  brain  tissue  from  which  ;t 
developed.  The  microscopic  study  of  retinal  tissue  is  most  profitable 


236 


HUMAN  BIOLOGY 


when  a  vertical  section  is  examined,  taken  at  right  angles  to  the  upper 
surface.  Such  a  preparation  shows  that  the  retina  consists  of  no  less 
than  10  distinct  layers.  The  outermost  of  these  is  the  pigmented, 
nonsensory  layer,  which  is  in  contact  and  fused  with  the  choroid  layer. 
The  pigmented  layer  is  relatively  thin  and  consists  primarily  of  pig- 
mented epithelial  cells.  All  of  the  remaining  nine  layers  of  retina 
develop  from  the  original  inner  layer  of  the  retinal  outgrowth  and 
form  a  relatively  thick  sensory  tissue  (about  0.0125  in.)  built  up  of 


FIG.  125. — Diagram  showing  cellular  structure  in  the  human  retina.  Highly 
magnified.  II  to  III,  sensory  layer;  IV  to  VII,  middle  bipolar  layer;  and  VII  to  X, 
inner  ganglionic  layer,  as  described  011  page  236.  (Howell,  "Textbook  of  Physiology," 
W.  B.  Saunders  Company.  After  Greef,  slightly  modified.) 

visual  cells,  neurons,  and  cell  processes,  altogether  so  intricate  in  their 
varied  structural  types  and  general  arrangement  that  it  will  not  be 
profitable  to  attempt  a  detailed  description  of  the  separate  layers  in 
the  present  discussion.  (Fig.  125.) 

The  basic  structural  condition  may  perhaps  be  understood  by 
recognizing  three  divisions  in  the  functional  retina,  namely,  (1)  the 
outer  photoreceptor,  (2)  the  middle  bipolar,  and  (3)  the  inner  ganglionic. 
In  the  photoreceptor  division,  which  lies  in  contact  with  the  outer 
pigmented  layer,  are  the  specialized  visual  elements.  These  cellular 
elements  are  found  to  consist  of  two  types:  the  rods  and  the  cones,  both 
of  which  are  elongated  cells  with  unique  structural  and  functional 
features.  They  lie  perpendicular  to  the  curved  surface  of  the  eyeball, 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (I) 


237 


regularly  arranged,  with  the  distal  ends  in  contact  with  the  pigmented 
layer.  Processes  from  the  pigmented  cells  project  inward  for  a  con- 
siderable distance  between  the  visual  cells  and  thus  partially  separate 
them.  The  rods,  as  the  name  indicates,  are  slender,  tubular  struc- 
tures. They  contain  a  complex  substance,  visual  purple,  which  is 
known  to  be  chemically  changed  by  light  waves.  The  inner  or  prox- 
imal end  of  each  of  these  cells  tapers  down  to  a  very  fine  proto- 
plasmic filament,  or  rod  fiber,  in  which  the  nucleus  is  located.  The 
rod  fibers  extend  toward  the  middle  of  the  retina  where  they  connect 
with  processes  from  the  bipolar  neurons.  The  cones  lack  the  visual 
purple  but  are  essentially  similar  to  the  rods  in  their  - 
structure,  except  that  the  body  of  the  cell  is  heavier, 
shorter,  more  club-shaped,  and  the  nucleus-containing 
cone  fiber  is  considerably  thicker.  There  is  no 
apparent  reason  why  this  should  be  so,  and,  as  a 
matter  of  fact,  certain  types  of  cones  are  thin, 
elongated  bodies  which  are  similar  in  appearance  to 
the  rods.  (Fig.  126.) 

The  middle  region  of  the  retina  contains  the  so- 
called  bipolar  cells.  These  are  elongated  nerve  cells 
adapted  for  receiving  the  impulses  from  the  terminal 
fibers  of  the  rods  and  cones.  The  impulses  are 
received  from  the  visual  cells  at  the  fiber  end  by 
union,  or  synapse,  then  transferred  to  the  neurons  of 
the  ganglionic  division  of  the  retina.  The  cell  proc- 
esses of  the  ganglionic  neurons  extend  to  the  inner 
surface  of  the  retina  and  converge  in  great  numbers 
to  form  the  large  optic  nerve.  Thus  it  is  seen  that 
the  light  rays  entering  from  the  front  of  the  eye 
penetrate  the  ganglionic  and  bipolar  divisions  of  the 
retina  without  stimulating  any  of  the  constituent  cells,  before  finally 
reaching  the  distal  ends  of  the  rods  and  cones  that  are  sensitive  to 
them.  And  the  resulting  impulses  are  carried  in  the  reverse  direction 
through  the  neurons  to  reach  the  elements  of  the  optic  nerve  over 
which  they  are  carried  to  the  brain  for  interpretation.  (Fig.  125.) 

There  is  considerable  variation  in  the  sensory  quality  of  the  retina, 
depending  upon  the  distribution  of  the  rods  and  cones.  Toward  the 
front  of  the  eye,  the  numbers  of  rods  and  cones  are  greatly  reduced,  and, 
in  the  extreme  periphery  of  the  retina,  they  are  entirely  absent.  A 
definite  blind  spot  is  found  in  the  comparatively  small  central  area 
where  the  optic  nerve  terminates  at  the  inner  surface  of  the  retina. 
The  area  of  most  acute  vision  in  the  retina  is  found  in  the  tiny  fovea 


FIG.  126.— Re- 
ceptor cells  from 
human  retina. 
Rod  cell,  left; 
cone,  right. 
Highly  magnified. 
nu.,  nucleus  in  rod 
and  cone  fiber. 
(Weber-  Valentine.) 


238  HUMAN  BIOLOGY 

centralis,  about  \{\  in.  in  diameter,  which  lies  directly  back  of  the  lens 
in  the  so-called  optical  axis  of  the  eye.  In  the  fovea,  the  thickness  of 
the  retina  is  greatly  reduced  by  the  complete  omission  of  the  bipolar 
and  ganglionic  layers  so  that  the  visual  cells  are  almost  directly  exposed 
to  the  incoming  light  rays.  Furthermore,  only  cones  are  present  in 
this  region.  An  object  to  be  seen  clearly  must  be  in  the  line  of  direct 
vision,  or  in  the  optical  axis,  so  that  the  image  will  be  focused  on  the 
fovea,  as  just  noted.  Involuntarily,  the  eyes  are  continually  being 
moved  so  as  to  bring  an  object  in  the  field  of  acute  vision.  Light  rays 
falling  upon  the  more  peripheral  regions  of  the  retina  are  seen  only 
dimly.  This  is  due  primarily  to  the  reduced  numbers  of  visual  cells, 
which  correspondingly  decrease  the  resolving  power  of  this  region. 

Light  Rays. — Certain  important  facts  relative  to  the  physical 
characteristics  of  the  visible  light  rays  should  be  noted.  We  may 
regard  light  as  being  produced  by  minute  but  very  rapid  vibrations 
which  have  their  source  in  an  object  heated  to  a  high  temperature. 
Thus  the  metal  filament  in  an  electric  light  bulb,  when  heated  to  a 
sufficiently  high  temperature  by  the  passage  of  an  electric  current, 
gives  off  visible  light  waves.  In  such  a  case,  the  potential  energy  of  the 
coal  or  other  fuel  burned  in  the  powerhouse  or  that  of  water  power  is 
converted  into  energy-bearing  light  waves,  and  the  latter  stimulate  the 
visual  elements  in  the  eye.  In  nature,  however,  the  source  of  the  light 
energy  is,  as  we  well  know,  the  sun.  As  a  matter  of  fact,  the  sun  is  the 
source  of  the  electric  light  energy,  for  the  energy  both  in  the  fuel 
burned  or  the  water  used  in  order  to  generate  the  electric  current  came 
originally  from  the  sun. 

The  physicist,  by  means  of  the  spectroscope,  can  analyze  light  and 
thus  determine  its  nature.  When,  for  example,  sunlight  is  allowed  to 
pass  through  the  glass  prism  of  a  spectroscope,  it  emerges  as  a  multi- 
colored spectrum  with  visible  bands  of  red,  orange,  yellow,  green,  blue, 
and  violet,  the  so-called  spectral  colors.  These  colors  are  commonly 
seen  in  the  rainbow,  which  results  from  the  dispersal  of  the  rays  of 
sunlight  by  passing  through  raindrops.  The  latter  act  as  does  the 
prism  in  the  spectroscope  of  the  physicist.  Thus  it  is  clear  that  white 
light  is  a  mixture  of  different  colors.  Furthermore,  it  is  known  that 
the  different  colors  are  due  to  variations  in  the  length  of  the  light 
waves.  In  the  visible  spectrum,  the  latter  range  from  an  extreme 
length  of  760  millionths  of  a  millimeter  in  the  case  of  red  light  to 
390  millionths  at  the  violet  end  of  the  spectrum.  Other  energy- 
containing  rays  lie  above  and  below  the  visible  spectrum.  Thus, 
above  the  red  rays  are  longer,  invisible  infrared  rays;  below  the  violet 
are  the  ultraviolet,  both  of  which,  though  ineffective  in  stimulating 


BIOLOGY  OF  THE  -NERVOUS  SYSTEM  (I)  239 

the  visual  cells  of  the  retina  of  the  eye,  can  be  detected  by  the  proper 
physical  instruments.  (Fig.  229.) 

From  the  foregoing,  it  is  evident  that  the  retina  of  the  eye  is 
essentially  a  tissue  capable  of  responding  to  light  rays  of  certain 
lengths.  But  how  the  rods  and  cones  do  this  is  entirely  beyond  our 
present  knowledge.  It  is  clear  that  the  peculiar  compound  present  in 
the  rods,  visual  purple,  is  in  some  important  way  concerned  in  this 
reaction,  but  it  is  also  certain  that  this  substance  is  not  the  primary 
element  of  vision,  for,  in  the  fovea,  the  region  of  most  acute  vision, 
only  cones  are  present,  and  they  lack  the  visual  purple.  Also  it  is 
known  that  visual  purple  is  entirely  lacking  in  the  retinal  tissues  of 
some  animals,  such  as  the  pigeon  and  bat,  which,  as  flying 
animals,  undoubtedly  have  acute  vision.  About  all  that  can  be  said 
with  certainty  is  that  the  essential  visual  elements  are  located  in  the 
protoplasm  of  the  rods  and  cones.  (Fig.  126.) 

Color  Vision. — It  has  been  noted  above  that  white  light  represents 
a  mixture  of  various  colors  of  the  spectrum,  but  wo  also  find  that 
various  combinations  of  two  colors  will  give  the  same  visual  sensation 
as  the  mixture  of  all  the  colors.  These  white-producing  pairs  of  colors 
are  known  as  complementary  colors,  examples  of  which  are  seen  in 
certain  shades  of  red  arid  blue,  yellow  and  blue,  etc.  Since  two  com- 
plementary colors  will  produce  white  when  added  together,  it  follows 
that,  when  one  of  the  complementary  colors  is  removed,  the  visual 
color  sensation  changes  from  white  to  that  of  the  other  member  of  the 
pair.  This  can  be  well  shown  in  the  phenomenon  involving  retinal 
fatigue.  Thus  if  one  looks  intently  for  about  one-half  minute  at  a 
brilliant  red  color  and  then  quickly  transfers  his  vision  to  a  white 
surface,  a  brilliantly  colored  area  will  be  seen  on  the  white  surface,  but 
the  latter  will  be  bluish-green  in  color,  which  is  the  complement  of  red. 
This  is  due  to  the  fact  that  the  retinal  elements  responding  to  the  red 
color  have  become  fatigued  and  do  not  respond,  thus  giving  white 
minus  the  red,  which  is  the  green  complementary.  Furthermore,  the 
physicist  recognizes  three  fundamental  colors  (red,  green,  and  blue) 
by  the  proper  mixture  of  which  white  and  all  other  colors  can  be 
produced.  This  fact  has  long  been  linked  up  with  important  theories 
of  color  vision  which  assume  that  the  visual  cells  respond  to  the  three 
primary  colors  and  that  the  multiplicity  of  color  sensations  is  due  to 
variations  in  the  strength  of  the  stimuli  coming  from  the  primary  colors. 

PERIPHERAL  NERVOUS  SYSTEM 

Elements  of  the  nervous  system  necessarily  innervate  every  tissue 
of  the  body.  The  previous  study  of  the  sensoiy  tissues  has  shown 


240  HUMAN  BIOLOGY 

that  the  invasion  of  the  sense  organs  by  the  peripheral  nerves  is 
for  the  purpose  of  receiving  impulses,  developed  in  accordance  with 
the  external  and  internal  environmental  stimuli  and  transmitting  the 
impulses  to  the  central  nervous  system  for  interpretation.  The  inner- 
vation  of  the  effector  tissues,  including  the  muscles  and  glands,  is  to 
make  possible  the  coordinated  control  of  their  activities  by  the  central 
nervous  system.  Thus  the  essential  function  of  the  peripheral  nerves 
is  the  conduction  of  nerve  impulses  to  and  from  the  master  tissues  of 
the  central  nervous  system.  In  order  to  do  this,  the  highly  differenti- 
ated animal  organism  possesses  an  extraordinarily  complex  system  of 
nerves,  containing  many  individual  nerve  fibers,  from  the  early 
developmental  stages.  It  has  long  been  recognized  that  nerves  typi- 
cally contain  two  types  of  nerve  fibers :  the  sensory,  or  afferent,  fibers 
over  which  impulses  pass  into  the  central  nervous  system;  and  the 
motor,  or  efferent,  fibers  over  which  impulses  travel  peripherally  from 
the  central  nervous  system  to  the  outlying  effectors.  This  condition 
clearly  indicates  that  the  individual  nerve  fiber  is  a  one-way  track  for 
nerve  impulses.  (Figs.  127,  133.) 

Neuron  Concept. — But  the  nerve  fibers  are  not  to  be  regarded  as 
the  basic  structural  and  functional  units  of  the  peripheral  system,  for 
histologists  studying  the  microscopic  anatomy  of  nerve  tissue  were 
able,  more  than  fifty  years  ago,  to  establish  the  important  fact  that  a 
nerve  fiber  is  simply  a  greatly  elongated  protoplasmic  process  of  a 
neuron.  This  so-called  neuron  concept  means,  in  essence,  that  the 
nervous  system,  like  the  other  systems  of  the  body,  is  composed  of 
cells,  the  neurons.  The  neurons,  as  a  group,  are  characterized  by  a 
very  high  degree  of  complexity  and  diversity,  both  structurally  and 
functionally,  marked  instances  of  which  will  be  seen  from  the  later 
description.  One  unifying  structural  feature,  however,  with  which 
we  are  particularly  concerned  in  our  present  discussion  of  the  peripheral 
system  is  the  fact  that  the  cell  bodies  of  the  neurons  are  extended  to 
form  one  or,  usually,  more  projections  for  the  conduction  of  impulses; 
these  projections  are  the  nerve  fibers.  (Plate  XIID,  E.) 

Furthermore,  the  neurons  are  not  indiscriminately  scattered  along 
the  peripheral  nerve  fibers  as  they  ramify  throughout  the  body  tissues 
but  are  concentrated  in  the  central  nervous  system  or  in  the  near-by 
nerve  centers,  the  ganglia.  This  means,  of  course,  that  the  cell  proc- 
esses or  nerve  fibers  of  some  of  these  centrally  situated  neurons  must 
be  several  feet  in  length,  as,  for  example,  to  take  an  extreme  case,  the 
fibers  that  have  their  origin  in  the  neurons  in  the  spinal  cord  of  the  cen- 
tral nervous  system  and  extend  to  the  tips  of  the  fingers  or  toes.  It 
must  be  remembered,  however,  that  the  nerve  fibers  that  originally 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (I)  241 

connected  the  central  nervous  system  and  the  peripheral  parts  of  the 
body  in  the  very  early  embryo  were  not  very  long  and  that  a  gradual 
increase  in  length  occurred  as  the  embryo  increased  in  size.  Finally, 
it  has  already  been  indicated  that  a  nerve  fiber  carries  impulses  in  one 
direction  only.  Since  the  nerve  fibers  are  cell  processes,  this  meant* 
that  there  are  two  types  of  the  latter:  the  dendrites,  which  carry 
impulses  centripetally,  that  is,  toward  the  cell  body;  and  the  axons, 
which  carry  impulses  peripherally,  that  is,  away  from  the  cell  body. 
Microscopic  study  has  not  revealed  any  structural  differences  between 
the  axon  fibers  and  dendrite  fibers. 

Nerves  and  Nerve  Fibers. — The  functional  conducting  elements,  or 
nerves,  are  seen  as  glistening  white  cords  extending  everywhere 
throughout  the  body  structures.  Some  of  the  nerves  are  tiny  fila- 
ments, microscopic  in  size ;  others,  as  in  the  nerve  trunks  of  the  leg  and 
arm,  may  be  comparable  in  diameter  with  that  of  the  little  finger. 
Since  all  of  these  nerves  have  their  origin  in  the  brain  or  spinal  cord  of 
the  central  nervous  system  and  near-by  ganglia,  they  tend  to  increase 
in  size  as  they  approach  the  central  nervous  system  and  to  divide  into 
smaller  and  smaller  units  peripherally  in  accordance  with  the  fibers 
given  off,  or  received  en  route  to  innervate  various  regions.  Care 
must  be  taken  to  distinguish  between  the  nerve  and  the  nerve  fiber. 
The  latter,  as  noted,  is  a  cell  process,  sometimes  greatly  elongated  but 
always  microscopic  in  diameter,  whereas  the  nerve  is  an  aggregation 
of  a  great  many  tiny  individual  nerve  fibers,  insulated  from  each  other 
and  compactly  bound  together  by  connective  tissue  elements.  We 
have  already  seen  an  example  of  this  type  of  tissue  construction  in  the 
building  of  a  large  muscle  from  microscopic  muscle  fibrillae  (page  177). 
And  the  telephone  technicians  make  use  of  the  same  method  in  building 
the  large,  lead-covered  telephone  cables  which  may  contain  thousands 
of  individual  copper  wires,  each  insulated  by  a  special  covering  from 
all  the  others.  The  variation  in  the  size  of  the  nerves  is  due  primarily 
to  the  number  of  associated  fibers.  (Plate  XII A.) 

When  a  transverse  section  of  a  typical  nerve  is  examined  micro- 
scopically, it  will  be  found  that  it  is  completely  covered  by  a  connective 
tissue  sheath,  the  epineurium.  Inside  the  sheath,  considerable  num- 
bers of  the  tiny  nerve  fibers  are  grouped  ni  bundles,  and  each  of  the 
latter  is  surrounded  by  the  perineurium  which  is  a  continuation  of 
the  connective  tissue  elements  of  the  epineurium.  Further  study 
shows  that  the  individual  fibers  are  separated  from  each  other  and 
more  or  less  surrounded  by  a  still  further  continuation  of  the  connective 
tissues  to  form  the  endoneurium.  In  a  fresh  nerve,  it  is  possible  to 
separate  the  individual  nerve  fibers  and  note  their  plan  of  construction. 


242  HUMAN  BIOLOGY 

It  is  found  under  such  conditions  that  a  delicate  sheath,  the  neuri- 
lemma,  surrounds  each  individual  fiber  and  is  in  contact  with  the 
endoneurium.  Lying  under  the  ncurilemma  is  a  comparatively  thick 
layer  of  a  fatty  substance,  the  myelin  sheath.  The  latter  is  inter- 
rupted at  rather  regular  intervals  along  the  nerve  fiber,  and  the  neuri- 
lemma  at  these  points  (nodes  of  Ranvier)  is  in  contact  with  the  tiny 
central  process  of  a  neuron — either  axon  or  deridrite.  Each  axon  (or 
dendrite)  is  thus  separated  from  all  the  other  axons  of  a  nerve  by  the 
myelin  sheath,  the  neurilemma,  and  by  the  connective  tissue  elements 
indicated  above.  The  axon  and  dendrite  differ  functionally,  as  noted 
above,  but  structurally  are  not  distinguishable.  As  a  matter  of 
fact,  the  axons  and  deiidrites  are  not  the  ultimate  conducting  elements 
of  a  nerve  fiber.  This  function  is  believed  to  be  localized  in  cyto- 
plasmic  fibrils  (neurofibrils)  that  apparently  originate  in  the  body  of 
the  neurons,  near  the  nucleus,  and  extend  peripherally  into  the  axons 
and  dendrites.  In  certain  types  of  nerve  fiber,  the  myelin  sheath  is 
lacking.  Such  nerves  are  known  as  the  nonmyelinated  nerves  and  are 
characteristic  of  the  autonomic  system,  as  distinguished  from  the 
myelinated  types  of  spinal  nerves  just  described.  (Fig.  131.) 

Cranial  and  Spinal  Nerves. — From  the  extreme  anterior  end  of  the 
brain  to  the  posterior  tip  of  the  spinal  cord,  a  total  of  43  pairs  of  cranial 
and  spinal  nerves  leave  the  central  nervous  system  and  extend  to  the 
outlying  tissues;  12  pairs  of  these  are  cranial;  and  31  pairs  are  spinal 
nerves.  The  majority  of  the  cranial  nerves1  have  departed  quite 
widely  from  the  typical  condition  of  the  peripheral  nerves,  which  is  to 
be  seen  in  the  spinal  nerves.  Thus  the  spinal  nerves  are  given  off  from 
the  spinal  cord  at  regular  intervals;  that  is,  they  are  segmeiitally 
arranged.  Furthermore,  each  spinal  nerve  arises  in  the  spinal  cord 
by  a  dorsal  (posterior)  root  and  a  ventral  (anterior)  root.  A  short 
distance  from  the  cord,  the  nerve  fibers  of  the  anterior  and  posterior 
roots  unite  and  are  bound  together  by  the  common  connective  tissue 
coverings,  thus  forming  a  single  large  nerve  composed,  therefore,  of 
both  dorsal  and  ventral  fibers.  (Plate  XIII A ;  Plate  XI VD,  page  248.) 

Early  experimental  work  on  the  spinal  nerves  established  beyond 
all  doubt  the  fact  that  there  is  a  functional  differentiation  between  the 
posterior  and  anterior  fibers.  The  posterior  fibers  are  afferent  sensory 
fibers,  which  bring  the  impulses  to  the  central  nervous  system  from 
the  peripheral  sense  organs;  the  anterior  fibers  are  efferent  motor 
fibers,  which  transmit  the  impulses  from  the  central  nervous  system 
to  the  peripheral  effectors  (muscles  and  glands).  And  so  it  is  evident 
that  each  spinal  jierve  contains  both  sensory  and  motor  fibers.  This 

1  Consult  Appendix :  Cranial  Nerves. 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (I)  243 

mixed  type  of  nerve  is  not  always  found  in  the  cranial  nerves,  for  some 
of  them,  as  will  be  seen  from  the  description  below,  contain  either 
sensory  fibers  or  motor  fibers  but  not  both  (page  519).  The  later 
description  of  the  central  nervous  system  will  give  opportunity  for 
consideration  of  the  associations  between  the  spinal  nerves  and  spinal 
cord. 

The  determination  of  the  fact  that  the  spinal  nerves  contain  both 
sensory  and  motor  fibers  led  to  the  establishment  of  the  conception  of 
the  reflex  arc  which  involves  a  peripheral  sense  organ  (receptor),  an 
afferent  sensory  nerve  fiber,  the  central  nervous  system  (adjuster), 
an  efferent  motor  nerve  fiber,  and  the  peripheral  effector,  usually 
muscle  tissue.  Thus,  when  the  tip  of  DORSAL  ROOT  TO  BRAIN 
the  finger  inadvertently  touches  a  hot  I 

surface,  it  is  immediately  jerked  away   SPfNAL  GANGLIONI 

by  a  reflex  action  which  involves  no 

,.          r,    .       i         •  SKIN 

conscious    action.     It   is    clear   in    a    —^-=^ 


reflex  action  that  the  sensory  cells  in 

the  skin  covering  the  finger  tip  are  first 

affected.     The    resulting    impulse    is 

transmitted    to    the   central   nervous  ($glP—  MUSCLE     VENTRAL  ROOT 

system  over  the  afferent  fibers  of  the  +.    F'0-  127/T7D;affraiV  shf°™ng  sec- 

J  tion  through  the  left  side  of  the  spinal 

root.      The     Central     nerVOUS  cord  with  the  dorsal  afferent  root  and 


system  acts  as  an  integrating  mocha-     thc  moto<*  effe!'en.f'  root  of  *  9spinal 

~  .  .  .  nerve  as  described  on  page  242. 

nism   (adjuster),  a  function   that  in- 

volves both  the  interpretation  of  the  incoming  sensory  impulses  and 
the  relaying  of  the  outgoing  impulses  over  the  efferent  fibers  of  the 
proper  motor  roots,  the  ones  that  innervate  tho  arm  muscles.  The 
contraction  of  the  latter  completes  the  drama.  (Fig.  127.) 

THE  ATJTONOMIC  NERVOUS  SYSTEM 

The  autonomic  nervous  system  is  essentially  a  part  of  the  peripheral 
nervous  system,  but  the  distribution  of  the  constituent  nerves  is  largely 
limited  to  the  important  visceral  organs  present  in  the  thoracic  and 
abdominal  cavities,  and  the  impulses  that  pass  over  the  autonomic 
nerves  from  the  central  nervous  system  are  beyond  our  conscious 
control.  It  is  sometimes  referred  to  as  the  involuntary  nervous  system 
because  of  this  condition.  Since  the  autonomic  system  apparently 
exercises  a  complete  involuntary  control  over  the  most  important 
organs  in  the  body,  it  is  usually  ranked  as  one  of  the  four  major  divi- 
sions of  the  complete  nervous  system.  Basically,  however,  it  appears 
clear  that  it  is  not  really  a  controlling  unit  but  a  conducting  unit  which 
receives  efferent  motor  impulses  from  the  central  nervous  system  and 


244 


HUMAN  BIOLOGY 


conveys  them  over  a  network  of  the  utmost  intricacy  to  the  proper 
effectors— the  involuntary  muscle  elements  in  the  visceral  organs. 
There  appears  to  be  no  evidence  that  the  impulses  that  exercise 
involuntary  control  over  the  important  organs  originate  in  the  auto- 
nomic  system  itself.  It  is  evident,  therefore,  that  the  autonomic 
system  is  essentially  a  part  of  the  peripheral  system.  The  autonomic 
system  reaches  its  highest  development  in  man  and  the  mammals 
generally,  where  it  is  found  to  consist  of  two  distinct  conducting  units, 
namely,  the  sympathetic  division  and  the  parasympathetic  division. 
(Fig.  128.) 


VAGUS 


PHRENIC 

CELIAC  PLEXUS- 

MESENTER1C  PLEXUS  (SUP.) 

SYMPATHETIC  TRUNK 
MESENTERIC  PLEXUS(lNF.) 


FIG.  128. — The  general  plan  of  the  autonomic  nervous  system  in  man.     Cf.  Fig.  129. 


Sympathetic  Division. — The  sympathetic  division  consists  essen- 
tially of  a  pair  of  delicate  nerve  cords  which  lie  exposed  in  the  thoracic 
and  abdominal  cavities,  one  on  each  side  of  the  vertebral  column,  close 
to  the  dorsal  body  wall.  Examination  of  these  sympathetic  cords 
show  that  each  consists  of  a  rather  loosely  connected  chain  of  segmental 
bead-like  nerve  centers,  the  sympathetic  ganglia.  Each  sympathetic 
chain  starts  anteriorly  in  the  neck  region  and  extends  to  the  posterior 
part  of  the  abdominal  cavity.  Each  of  the  constituent  sympathetic 
ganglia  receives  one  or  more  branches,  containing  efferent  motor  fibers, 
from  the  nearest  spinal  nerve  shortly  after  the  latter  leaves  the  spinal 
cord.  These  connecting  efferent  fibers,  which  come  from  the  spinal 
nerves  and  extend  to  the  sympathetic  ganglia,  are  known  as  the 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (I) 


245 


preganglionic  fibers.  Another  group  of  sympathetic  fibers,  mostly 
nonmyelinated,  have  their  origin  in  the  sympathetic  ganglia  where  they 
synapse  with  the  preganglionic  fibers  and  then  extend  to  the  tissues  to 
be  innervated.  This  latter  group  constitutes  the  postganglionic  fibers. 
(Fig.  129.) 

The  postganglionic  fibers  may  proceed  directly  from  the  sym- 
pathetic ganglia  to  the  peripheral  tissues,  as  just  indicated,  or  the 


Cervteal 


7horacic<    i 


Lumbar  \   3^ , 


Sacra/ 


Solid  Lines  ~  Parasympoithetic 
Dotted  L ines  -  Sympathetic 

w 

FIG.  129. — Scheme  illustrating  the  distribution  of  the  autonomic  nervous  system  of  man 
and  the  important  plexuses  (Celiac  Ganglion,  etc.,  etc.)      (Watkeys,  Daggs.) 

postganglionic  fibers  from  a  number  of  ganglia  may  be  associated  to 
form  an  intermediate  relay  station,  or  nerve  plexus,  which  is  connected 
to  the  effectors  by  a  third  fiber  group.  There  are  several  plexuses  of 
varying  size  and  importance  in  the  sympathetic  division,  but  the  largest 
is  the  celiac  ganglion,  or  solar  plexus,  from  which  fibers  innervate  the 
important  abdominal  organis,  including  the  stomach,  intestines,  liver, 
pancreas,  and  certain  parts  of  the  urogenital  system. 


246  HUMAN  BIOLOGY 

Parasympathetic  Division. — The  parasympathetic  division  of  the 
autonomic  nervous  system  consists  of  efferent  fibers  which  arise  in 
the  extreme  anterior  and  posterior  portions  of  the  central  nervous 
system.  The  anterior  parasympathetic  fibers  are  given  off  from  certain 
neuronic  areas  lying  in  the  midbrain  and  the  hindbrain,  whereas  the 
posterior  parasympathetic  fibers  have  their  origin  in  the  sacral  portion 
of  the  spinal  cord.  The  parasympathetic  fibers  extend  directly  from 
their  points  of  origin  in  the  central  nervous  system  to  the  tissues  that 
they  innervate,  and  there  they  form  synapses  with  the  postganglionic 
fibers  of  the  sympathetic  division.  In  general,  intermediate  ganglia  are 
not  present  as  in  the  sympathetic  elements.  (Fig.  129.) 

The  basic  effect  of  the  mammalian  autonomic  system  is  to  supply 
the  involuntary  and  cardiac  muscles  and  also  the  glands  with  a  dual 
control  through  the  double  innervation  by  sympathetic  and  para- 
sympathetic fibers.  Altogether,  the  elements  of  the  autonomic 
system  innervate  and  exercise  involuntary  control  over  the  smooth 
muscle  tissue  present  in  all  the  organs  and  organ  systems  of  the 
vertebrate  body,  including,  in  addition  to  those  already  noted  for  tho 
sympathetic  division,  such  diverse  structures  as  the  pupil  of  the  eye, 
the  lungs,  the  heart,  vascular  system,  arid  all  types  of  gland,  among 
which,  of  course,  are  those  of  the  endocrine  system.  In  addition  it  is 
known,  in  certain  instances  at  least,  that  the  autonomic  system  is  also 
equipped  with  sensoiy  fibers  capable  of  transmitting  afferent  impulses 
from  the  innervated  effectors,  as  in  the  case  of  pain  stimuli,  to  the 
central  nervous  system.  The  pain  stimuli  from  the  viscera  are  not 
definitely  localized  as  a  rule  and  may  be  referred  to  other  regions  of  the 
body. 

An  additional  fact  of  great  functional  importance — associated  with 
the  dual  control  of  the  sympathetic  and  parasympathetic  divisions — 
is  based  on  experimental  studies  which  have  shown  that  the  sym- 
pathetic and  parasympathetic  divisions  of  the  autonomic  system  are 
essentially  antagonistic  in  their  control  of  various  organs.  Thus,  to 
take  a  well-known  example,  the  stimulation  of  the  sympathetic  fibers 
that  innervate  the  muscle  tissue  of  the  heart  and  also  of  the  blood 
vessels  results  in  an  increased  activity  of  the  heart  and  a  contraction  of 
the  muscle  tissue  in  the  walls  of  the  blood  vessels.  This  causes  an 
increase  in  the  blood  pressure.  Contrariwise,  stimulation  of  the 
parasympathetic  fibers  innervating  these  vascular  tissues  results  in  a 
slowing  down  of  the  heart  action,  a  dilation  of  the  blood  vessels,  and  a 
consequent  fall  in  blood  pressure.  That  the  action  of  the  sympathetic 
and  parasympathetic  divisions  may  be  reversed  in  other  regions  can  be 
seen,  for  example,  when  the  sympathetic  fibers  innervating  the 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (/)  247 

bronchi  of  the  lungs  are  stimulated,  for,  in  this  organ,  the  sympathetic; 
impulses  cause  a  relaxation  of  the  muscle  fibers  rather  than  a  con- 
traction as  noted  above  in  the  vascular  system,  while  the  stimulation 
of  the  parasympathetic  fibers  innervating  these  tissues  causes  a  con- 
traction of  the  muscle  tissue,  which,  again,  is  the  reverse  of  its  action 
in  the  vascular  system. 

It  is  clear,  therefore,  from  the  experimental  evidence,  that  the 
action  of  the  sympathetic  and  parasympathetic  divisions  in  a  particular 
organ  is  antagonistic  and,  furthermore,  that  their  action  is  not  uniform 
in  the  various  innervated  organs.  Although  a  great  deal  remains 
unknown  about  the  functioning  of  the  autonomic  system  with  its 
tremendously  complex  interrelationships,  the  data  are  more  than 
sufficient  to  show  that  it  is  of  supreme  functional  importance  in  main- 
taining tireless,  efficient,  and  involuntary  control  over  an  almost 
infinite  number  of  details  associated  with  the  proper  and  continuous 
functioning  of  the  important  organ  systems. 


I     OLFACTORY 


H    OPTIC 

ID    OCULOMOTOR 
N   TROCHLEAR 


V  TRIGEMINAL 

VI  ABDUCENS 
m   FACIAL 

TCDI  AUDITORY 

IX  GLOSSOPrtARYNGEAL 

X  VAGUS  

XI  SPINAL  ACCESSORY 
XE  HYPOGLOSSAL     — - 


CEREBRAL  ARTERY 
FRONTAL  LOBE 
COMMUNICATING  ARTERY 

INTERNAL  CAROTID  ARTERY 
PITUITARY  BODY 
TEMPORAL  LOBE 

BASILAR  ARTERY 
PONS  VAROL1I 

CEREBELLAR  ARTERY 
MEDULLA  OBLONGATA 
CEREBELLUM 

CEREBELLAR  ARTERY 


XH  HYPOGLOSSAL - 

CERVICAL- 
2    3 


MEDULLA  OBLONGATA 


1.  OCCIPITAL  LOBE 

2.  PARIETAL  LOBE 

3.  CENTRAL  FISSURE  (ROLANDO) 

4.  FRONTAL  LOBE 

5.  CEREBELLUM 

6.  SPINAL  CORD 

7.  MEDULLA  OBLONGATA 
8.PONS  VAROLII 

9.  LATERAL  FISSURE  (SYLVIUS) 

10.  TEMPORAL  LOBE 


I.  FRONTAL  LOBE  13 

2  CORPUS  CALLOSUM 
3.  MASSA  INTERMEDIA 
4  THALAMUS  (THIRD  VENTRICLE) 
.  CHORO1D  PLEXUS 

6.  CORPORA  QUADRIGEM1NA 

7.  FOURTH  VENTRICLE 

8.  OCCIPITAL  LOBE 

9.  OPTIC  CHIASMA 

10.  PITUITARY  BODY 

II.  PONS  VAROLII 

12.  MEDULLA  OBLONGATA 

13.  CENTRAL  CANAL 

14.  CEREBELLUM 


SACRAL  — 


COCCYGEAL- 


PLATE  XIV. — Central  nervous  system  of  man.  A,  general  structure  as  seen  from 
the  under  surface;  B,  right  side  of  the  brain;  C,  median  sagittal  section  showing  the  cut 
surface  of  the  right  half  of  the  brain;  D,  spinal  cord  with  the  roots  of  the  paired  spinal 
nerves. 


CHAPTER  XI 

BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II) 
THE  CENTRAL  NERVOUS  SYSTEM 

At  long  last  our  consideration  of  the  various  units  of  the  human 
nervous  system  brings  us  to  the  central  nervous  system,  consisting 
of  the  brain  and  spinal  cord,  which  is  the  supreme  structural  and  func- 
tional unit  not  only  of  the  complete  nervous  system  but  of  the  entire 
body  as  well.  It  is  clear  that  the  sensory,  peripheral,  and  autonomic 
divisions  of  the  nervous  system  are  of  functional  importance  only 
because  of  their  association  with  the  central  nervous  system.  Every 
sensation  received  by  the  various  specialized  sense  organs  from  the 
internal  and  external  environment  is  sent  to  the  central  nervous 
system  for  interpretation  and  for  the  determination  of  the  proper 
action  to  be  taken  by  the  organism.  The  nervous  impulses  that  speed 
continuously  over  the  network  of  peripheral  and  autonomic  nerves 
are  directed  either  to  (afferent)  or  from  (efferent)  the  brain  and  spinal 
cord. 

We  have  seen  earlier  that  the  vertebrate  nervous  system  starts  in 
the  embryo  by  the  formation  of  a  thin-walled  ectodermal  tube  which 
gradually  becomes  differentiated  to  form  the  exceedingly  intricate 
adult  nervous  system.  The  numerous  neuronic  descendants  of  the 
pioneer  ectodermal  nerve  cells  become  the  highly  differentiated  neurons 
of  the  adult  brain  and  spinal  cord.  These  pioneers  never  leave  the 
chosen  land,  and,  protected  and  obeyed  by  the  lesser  members  of  the 
body,  their  decisions  constitute  the  law  of  the  bodily  domain.  Thus 
we  have  the  primary  structural  and  functional  fact  that  the  central 
nervous  system  is  the  cellular  unit  of  the  entire  nervous  system,  for 
the  neurons  lie  Actually  within  the  brain  and  the  spinal  cord  or  in  the 
closely  associated  sensory  ganglia  which  are  situated  on  each  of  the 
sensory  nerve  roots  close  to  the  spinal  cord.  Structurally,  it  is  possible 
to  divide  the  neurons  into  two  main  cell  types,  namely,  the  bipolar  type 
and  the  multipolar  type.  Present  in  both  types  of  neuron  and,  there- 
fore, a  characteristic  feature  of  the  neuronic  type  of  cell  structure  is  the 
prolongation  of  the  cell  cytoplasm  into  two  or  more  processes,  the 
nerve  fibers,  which  are  the  specialized  agehts  for  conduction,  as  ha* 
previously  been  seen  in  the  study  of  the  peripheral  system.  From 

240 


250 


HUMAN  BIOLOGY 


the  functional  standpoint,  the  neuronic  cell  processes,  as  shown  above, 
are  divided  into  the  dendrites,  which  carry  impulses  into  the  cell  body, 
and  the  axons,  which  carry  impulses  from  the  cell  body  to  the  periph- 
eral regions  (page  240) .  The  bipolar  neuron 
possesses  one  dendrite  and  one  axon,  while 
the  multipolar  neuron  typically  possesses 
numerous  dendrites  arid  one  axon;  the  latter 
may  give  off  numerous  branches.  (Fig.  130.) 
The  bipolar  neurons  are  present  in  the 
sensory  ganglia  of  the  cranial  and  spinal 
nerves  and  also  in,  the  retina.  Thus  they 
show  considerable  structural  diversity,  but 
it  is  clear  that  they  are  fundamentally 
bipolar,  each  neuron  having  a  separate 
dendrite  and  axon.  In  the  fully  formed 
bipolar  cell,  the  axon  and  dendrite  are 
usually  fused  together  near  the  cell  body  so 
that  the  cell  appears  to  be  unipolar.  If, 
however,  the  single  axon-dendrite  process  is 
traced  some  distance  from  the  cell  body,  it 
will  be  found  to  split  to  form  a  T-shaped 
process  with  the  pear-shaped  cell  body 
attached  to  the  base  of  the  T. 
The  multipolar  neuron  is  the  typical  motor  neuron  present  in  the 
spinal  cord  and  in  the  cortex  of  the  brain.  As  seen  in  the  spinal  cord, 
the  multipolar  neuron  has  a  large  cell  body  with  numerous  short 
dendrites  and  a  long  myelinated  axon.  The  axon  usually  gives  off 
some  tiny  branches  (collaterals)  near  the  cell  body  and  then  continues 
out  of  the  cord,  along  with  other  associated  fibers,  to  form  an  efferent 
division  of  a  spinal  nerve.  In  the  motor  areas  in  the  cortex  of  the  brain 
the  multipolar  cells,  reaching  their  climax  in  the  large  pyramidal  cells, 
are  subject  to  wide  structural  variation  characterized  in  part  by  pro- 
fuse branching  of  the  axons  and  the  dendrites.  It  is  presumed,  but 
not  proved,  that  the  primary  functional  differentiation  between 
rdendrite  and  axon  in  direction  of  impulse  persists  throughout  the 
nervous  system.  Since  several  billion  neurons  are  present  in  the 
central  nervous  system  of  man,  it  is  readily  seen  that  the  actual 
tracing  of  an  almost  infinite  number  of  microscopic  nerve  fibers  is  an 
impossible  task.  (Fig.  131.) 

Neuron  Histology. — The  protoplasm  of  the  neuron  is  largely 
lacking  in  structural  features  at  the  level  of  microscopic  visibility 
*Urtt  give  any  indication  of  its  functional  ability.  The  nucleus  is  large 


FIG.  130. — Diagrams  of 
bipolar  (above)  and  multipolar 
(below)  neurones.  Highly 
magnified.  Cf.  Fig.  131. 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II) 


251 


Cell 
body 


Nucleus 


Dendrifcs 


Co//c*fertxl 


Neurilemma 


and  well  formed  and  contains  one  or  more  prominent  nucleoli  but,  *on 
the  whole,  is  quite  unexceptional  in  appearance.  The  cytoplasm,  how- 
ever, is  at  times  fibrillated  to  form  numerous  very  fine  intracellular 
fibers,  the  neurofibrils,  which  per- 
meate essentially  all  regions  of  the 
cytoplasm  but  particularly  in  the 
region  around  the  nucleus.  Toward 
the  periphery  of  the  cell  the  neuro- 
fibrils tend  to  be  in  parallel  lines, 
thus  forming  bundles  of  neurofibrils 
which  extend  out  into  the  axori  and 
dendrite.  Apparently  there  is  little 
doubt  that  these  fibrillar  elements 
are  directly  concerned  with  the  trans- 
mission of  the  nervous  impulses  to 
and  from  the  cell  bodies  of  the  neu- 
rons. Another  cytoplasmic  feature 
is  noted  in  the  presence  of  the 
chromophil  substance,  so  termed  be- 
cause of  its  reaction  to  the  stains. 
In  most  of  the  neurons,  the  chromo- 
phil substance  is  widely  distributed 
throughout  the  cell  cytoplasm  and 
out  into  the  dendrites,  but  it  is  not 
present  in  the  axons.  Possibly  the 
chromophil  substance  is  stored  nutri- 
tive material,  but  no  proof  exists  of 
either  its  nature  or  function.  Possi- 
bly even  more  obscure  in  its  func- 
tional characters,  but  prominent 
structurally,  is  the  net-like  reticular 
apparatus  (Golgi  body)  lying  close 
to  the  nucleus.  (Fig.  131.) 

One  of  the  most  important  fea- 
tures of  the  neurons  is  the  terminal 
branching,  or  arborization  in  the  region  where  the  connection  (synapse) 
with  the  nerve  fibers  of  another  cell  is  made,  or  where  the  cell  processes 
end  in  the  tissues  of  the  peripheral  effectors.  Considering,  first,  the 
synapses  through  which  a  nervous  impulse  is  passed  from  the  axon  of 
one  neuron  to  the  dendrite  of  another,  the  best  evidence  is  that  the 
processes  of  the  synapsing  cells  are  not  actually  fused  with  each  other 
but  are  only  in  close  apposition.  It  can  be  shown  in  many  instances 


Nucleus  of 
neur/Jemma 


Motor 

end 

plcrfe 

Muscle 
fiber 


FIG.  131. — Diagram  of  a  typical 
multipolar  motor  neuron  with  a 
rnedullated  axon.  (Wolcott.) 


252 


HUMAN  BIOLOGY 


CERVICAL —( 


that  the  region  of  the  synapse  is  plentifully  supplied  with  vascular 
elements,  a  condition  indicating  that  it  must  be  a  region  of  high 
metabolic  activity.  Again  it  should  be  pointed  out  that  the  enormous 
number  of  neurons,  each  with  several  synapses 
of  microscopic  dimensions,  constitute  a  system 
of  incomparable  complexity  in  which  it  is  possi- 
ble only  to  get  an  idea  of  the  main  outlines  and 
never  to  trace  out  the  impulse  routes  and 
synapses  of  the  individual  cells. 

THE  SPINAL  CORD 


THORACIC- 


The  spinal  cord  may  be  said  to  extend  from 
the  foramen  magnum  at  the  base  of  the  skull, 
where  it  merges  into  the  brain,  to  near  the  pos- 
terior end  of  the  vertebral  column,  where  it 
shades  off  into  a  fine  terminal  filament  (filum 
terminale),  the  latter  continuing  into  the  sacral 
region  of  the  vertebral  column.  The  spinal  cord 
proper  is  thus  some  17  or  18  in.  long,  about  % 
in.  in  diameter,  and  weighs  less  than  2  oz.  It  is 
fairly  cylindrical  in  shape  and  with  two  enlarged 
areas,  one  of  which  is  in  the  shoulder  region 
where  the  spinal  nerves  arise  that  innervate  the 
arms,  whereas  the  other  is  in  the  sacral  region 
where  the  spinal  nerves  innervating  the  legs  leave 
the  cord.  It  has  already  been  noted  in  the 
description  of  the  peripheral  system  that  31  pairs 
of  spinal  nerves,  each  with  a  posterior  sensory 
root  and  an  anterior  motor  root,  leave  the  cord 
at  essentially  regular  intervals  throughout  its 
length,  corresponding  to  the  segmentation  of  the 
vertebral  column  that  encloses  it.  (Fig.  132.) 
We  may  now  trace  the  roots  of  a  spinal  nerve  into  the  interior  of 
the  cord  and  thereby  get  a  conception  of  the  internal  arrangements. 
This  can  only  be  accomplished  by  the  microscopic  examination  of  a 
transverse  section  of  a  suitable  spinal  cord.  The  study  of  a  section  of 
the  cord  shows  it  to  be  roughly  circular  in  outline  but  with  a  con- 
siderable anteroposterior  flattening.  A  rather  deep  indentation  occurs 
on  the  ventral  surface  (anterior  fissure),  and  a  shallow  indentation  on 
the  dorsal  surface  (posterior  fissure).  If  these  fissures  were  extended, 
they  would  meet  in  the  center  of  the  cord  and  thus  divide  it  into 


LUMBAR    — 

SACRAL  — 

COCCYGEAL- 

FIG.  132. — Human 
spinal  cord  with  the 
roots  of  the  paired 
spinal  nerves.  Com- 
pare with  the  spinal 
cord  shown  in  Plate 
12  A.  Diagrammatic. 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II) 


253 


symmetrical  right  and  left  halves.  The  transverse  section  of  the  cord 
reveals  also  a  clear  division  of  the  tissues  into  an  inner  portion,  the 
gray  matter,  which  is  enclosed  by  an  outer  portion  of  white  matter. 
The  gray  matter  is  roughly  H-shaped,  with  the  crossbar  of  the  H 
pierced  in  the  center  by  the  tiny  central  canal,  which  is  thus  in  the 
exact  center  of  the  cord.  The  central  canal  is  the  minute  remnant  of 
the  original  large  central  cavity  of  the  thin-walled  embryonic  neural 


Cortex 


BRAIN—, 


Motor  fiber 
fiber 


Dorsal 


Ventral  root 


Synapse 


SPINAL  NERVE 


Sensory  nerve 

/fiber  ending  in 

sense  cell 


/Motor  nerve 

fiber  ending 

in  muscle 


FIG.  133. — Illustrating  the  origin  of  the  spinal  nerves  and  the  sensory  and  motor  nerve 
paths  in  the  spinal  cord,  leading  to  the  cortex  of  the  brain.     (Woodruff.) 

tube.     The  white  matter  of  the  cord  completely  encloses  the  inner 
H-shaped  mass  of  gray  matter.     (Fig.  133.) 

Spinal  Cord  Histology. — Further  study  of  the  spinal  cord  reveals 
the  important  fact  that  the  gray  matter  is  cellular  and  composed  of 
neurons,  whereas  the  white  matter  consists  entirely  of  myelinated 
nerve  fibers  (axons  and  dendrites)  extending  up  and  down  and  across 
the  cord.  Furthermore,  it  is  seen  that  the  roots  of  each  spinal  nerve 


254  HUMAN  BIOLOGY 

are  directly  connected  to  the  inner  gray  matter  of  the  cord,  the  afferent 
sensory  roots  joining  the  dorsal  (posterior)  ends  of  the  uprights  of  the 
H  and  the  efferent  motor  roots  joining  the  ventral  (anterior)  ends. 
The  ventral  horns  are  larger  and  project  more  deeply  into  the  white 
matter,  and  in  them  are  the  large  efferent  multi polar  motor  neurons, 
the  axons  of  which  converge  segmentally  to  form  the  ventral  roots 
of  the  spinal  nerves.  In  the  smaller  dorsal  horns  of  the  gray  matter  lie 
the  multipolar  association  neurons  with  their  numerous  dendrites  and 
axons.  Through  the  dendrites  of  the  association  neurons,  the  afferent 
sensory  impulses  are  received  in  the  cord  by  means  of  synapses  with 
the  axon  fibers  of  the  sensory  roots.  These  sensory  impulses  are  then 
transferred  to  the  dendrites  of  the  motor  neurons  at  the  same  level  of 
the  cord  or,  more  frequently,  at  a  different  level.  In  the  latter  case, 
the  sensory  impulse  received  in  the  cord  must  be  passed  up  or  down  the 
conducting  fiber  tracts  until  the  synapse  is  made  with  the  dendrites 
of  the  motor  neurons  innervating  the  desired  muscles.  And  it  may 
also  be  necessary  that  the  incoming  sensory  impulses  be  conducted  to 
the  brain  before  the  proper  adjustment  with  the  motor  system  can  be 
determined. 

Fiber  Tracts. — One  of  the  notably  difficult  and  important  problems 
in  studying  nerve  function  has  been  the  localization  of  conduction  in 
the  spinal  cord  to  particular  fiber  bundles  in  the  white  matter. 
Though  it  is  impossible  to  trace  single  nerve  fibers  in  the  white  matter 
to  individual  neurons  in  the  gray  matter,  it  has  nevertheless  been  found 
possible  to  identify  bundles  of  fibers  (fiber  tracts)  which  connect  with 
definite  neuronic  areas.  This  was  accomplished  experimentally  by 
sectioning  the  spinal  cord  in  various  experimental  animals  and  thus 
separating  certain  neuronic  areas  of  the  brain  and  spinal  cord  from 
the  connected  nerve  fibers.  When  this  is  done,  it  has  been  found 
possible  sometime  later  to  trace  the  paths  of  degeneration  of  the 
nerve  fibers  up  and  down  the  cord  by  microscopic  study  of  prepared 
material.  This  results  from  the  fact  that,  when  the  neurons  are  sepa- 
rated from  the  fibers,  it  invariably  follows  that  the  fibers  soon  degen- 
erate. The  fibers  (which,  after  all,  are  only  cell  processes)  are  unable 
to  nourish  themselves  or  to  maintain  independent  life  activities  when 
separated  from  the  cell  bodies.  Studies  on  degenerating  areas  of 
nerve  fibers,  following  separation  from  the  neurons,  has  consistently 
revealed  the  presence  of  definite  fiber  tracts  in  the  white  matter  of  the 
injured  cord.  The  fiber  tracts  in  the  spinal  cord  can  be  grouped  into 
(1)  the  ascending  fiber  tracts,  which  carry  impulses  up  the  cord 
toward  the  brain  from  the  peripheral  regions,  and  (2)  ttye  descending 
.fiber  tracts,  which  carry  impulses  down  the  cord,  away  from  the  brain. 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II) 


255 


In  essence,  of  course,  the  ascending  tracts  carry  sensory  impulses  to 
the  brain,  and  the  descending  tracts  carry  motor  impulses  from  the 
brain  or  from  posterior  regions  of  the  central  nervous  system  to  the 
effectors.  (Fig.  134.) 

Afferent  Fiber  Tracts. — The  main  ascending  afferent  fiber  tracts  in 
the  spinal  cord  include  (1)  those  which  carry  the  proprioceptive 
impulses  from  the  muscles  of  the  body  to  the  motor  areas  in  the  cerebral 
cortex  of  the  forebrain.  These  fiber  tracts  occupy  a  considerable  area 
in  the  posterior  central  portion  of  the  cord.  They  lie  above  the 
" connecting  bar"  and  between  the  "uprights"  of  the  H-shaped  area 
of  gray  matter,  continuing  to  the  periphery  of  the  cord  on  each  side  of 


Fasciculus  cuneatus 


Fasciculus  gracilis 


Lateral  cortico-^. 
spinal  tract 


Fasciculi 


Dorsal  spinocere- 
bellar  tract 


Ventral  cortico-.- * 

spinal  tract  \ 

FIG.  134. — Diagram  of  a  transverse  section  of  the  spinal  cord  to  show  the  general 
arrangement  of  the  white  and  gray  matter  (heavy  black  area)  and  the  localization  of 
some  of  the  important  fiber  tracts.  Fiber  tracts  in  the  fasciculus  cuneatus  and  fasciculus 
gracilis  carry  the  afferent  conscious  proprioceptive  impulses,  whereas  the  dorsal  spino- 
cerebdlar  tracts  carry  the  afferent  unconscious  proprioceptive  impulses.  The  crossed 
pyramidal  fibers  (efferent)  are  located  in  the  lateral  corticospinal  tracts,  and  the  direct 
pyramidal  fibers  are  in  the  ventral  cortico-spinal  tracts.  Cf.  Fig.  133,  (Crandall, 
"Human  Physiology,1"  W.  B.  Saunders  Company.  After  von  Geheuchtvn.) 

the  dorsal  fissure.  These  tracts  carry  the  impulses  associated  with 
conscious  proprioception.  When  they  are  destroyed,  the  individual 
loses  the  muscle  sense  that  enables  him  to  know  the  position  of  legs  or 
arms  without  looking  at  them.  No  other  route  exists  for  the  afferent 
sensory  impulses  arising  in  the  muscles,  joints,  and  tendons  to  reach  the 
interpretive  areas  in  the  brain. 

Other  ascending  spinal  fiber  tracts  include  (2)  those  extending 
from  the  body  muscles  to  the  cerebellum  of  the  hindbrain.  These 
tracts  lie  on  the  right  and  left  sides  of  the  cord,  near  the  periphery  and 
ventral  to  the  tracts  just  noted  above  in  (1).  They  transmit  the 
unconscious  proprioceptive  impulses  to  the  cerebellum,  by  means  of 
which  equilibrium  is  constantly,  but  unconsciously,  maintained. 
Finally,  the  ascending  fiber  tracts  in  the  cord  include  (3)  those  which 


256  HUMAN  BIOLOGY 

carry  the  impulses  from  the  sensory  areas  in  the  skin,  associated  with 
the  various  skin  sensations.  In  one  group  of  these  paired  tracts, 
afferent  impulses  associated  with  the  sensations  of  pain,  heat,  and  cold 
pass  upward,  and,  in  the  other  group,  impulses  associated  with  the 
tactile  senses  appear  to  be  segregated.  These  are  relatively  small 
tracts  located  ventrally  in  the  cord;  the  tactile  fiber  tracts  lying  close 
to  and  on  each  side  of,  the  ventral  fissure,  whereas  the  other  pair  of 
sensory  tracts  are  situated  just  below  the  cerebellar  tract  indicated  in 
item  (2)  above.  The  afferent  impulses  from  the  important  sense 
organs  located  in  the  heg,d  enter  the  brain  directly  over  the  corre- 
sponding spinal  nerves  without  association  with  spinal  cord  elements. 

It  should  be  understood  that  the  sensory  impulses  coming  into  the 
cord  through  the  posterior  roots  of  the  spinal  nerves  are  not  necessarily 
transmitted  to  the  brain  in  all  cases.  For  example,  the  afferent 
impulses  associated  with  relatively  simple  reflexes  may  be  transmitted 
up  or  down  the  cord  to  the  proper  level  for  synapse  with  a  particular 
group  of  motor  neurons.  Here  they  are  routed  over  the  synapsing 
association  neurons  of  the  cord  and  then  over  the  axons  of  the  motor 
neurons  to  the  effectors.  Thus,  as  in  a  frog,  the  brain  may  be  entirely 
destroyed,  but  the  reflex  activity  for  the  posterior  portion  of  the  body 
will  remain  intact,  so  that  irritation  of  the  sensory  elements  in  the 
skin  will  produce  reflex  actions  in  the  limbs. 

Efferent  Fiber  Tracts, — The  descending,  or  efferent,  fiber  tracts  of 
the  cord  have  their  origin,  for  the  most  part,  in  certain  neurons  located 
in  the  cerebral  cortex  of  the  brain  and,  accordingly,  are  known  as  the 
corticospinal  pathways.  All  along  the  cord,  the  terminal  arborizations 
of  the  axons  of  these  tracts  synapse  with  the  dendrites  of  the  motor 
neurons  in  the  anterior  horns  of  the  gray  matter.  Passing  posteriorly 
from  the  brain  cortex,  the  corticospinal  fibers  reach  the  posterior 
portion,  or  medulla,  of  the  hindbrain.  In  this  region,  about  three- 
fourths  of  the  descending  fibers  pass  over  to  the  opposite  side  of  the 
cord  (decussation) ;  that  is,  the  fibers  coming  from  the  right  side  of  the 
cortex  find  their  way  to  the  left  side  of  the  cord  and  vice  versa.  (Fig. 
133.) 

These  decussating  fibers  of  the  corticospinal  pathways  form  the 
crossed  pyramidal  tracts.  They  are  situated  in  the  cord  between  the 
areas  of  the  ascending  cerebellar  tracts  and  the  outer  boundaries  of 
the  gray  matter,  to  the  right  and  left  of  the  ventral  fissure.  The 
remaining  one-fourth  of  the  corticospinal  fibers  do  not  cross  over  in  the 
medulla  but  enter  the  spinal  cord  on  the  side  corresponding  with  their 
origin  and  accordingly  constitute  the  uncrossed,  or  direct  pyramidal 
tracts.  As  a  matter  of  fact,  however,  the  fibers  of  the  direct  pyramidal 
tracts  also  cross  to  the  opposite  side  of  the  cord  but  not  until  they 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II)  257 

reach  the  various  levels  where  synapse  is  made  with  the  motor  neurons. 
Consequently,  it  appears  that  all  corticospinal  fibers  innervate  the 
opposite  side  of  the  body  from  that  of  their  cerebral  origin.  The 
significance  of  the  crossing  of  the  nerve  fibers,  which  is  a  basic  feature 
of  the  vertebrate  nervous  system,  is  not  apparent.  (Fig.  133.) 

Over  the  crossed  and  the  direct  pyramidal  tracts  of  the  spinal  cord 
flow  the  efferent  nervous  impulses  from  the  brain  cortex  which  initiate 
voluntary  movement  throughout  the  body.  Unconscious  (involun- 
tary) control  of  the  muscle  tissues,  associated  with  muscle  tonus, 
equilibrium  (in  response  to  afferent  impulses  received  from  the  semi- 
circular canals  and  proprioceptive  impulses),  and  various  other  vital 
activities,  notably  gland  secretion,  are  the  functions  of  two  groups  of 
neurons  situated  in  the  midbrain  and  in  the  hindbrain  respectively — 
the  latter  group  being  situated  near  the  vestibular  division  of  the 
auditory  nerve  which  innervates  the  semicircular  canals. 

The  fiber  tracts  of  the  important  pathways  for  involuntary  con- 
trol, although  not  so  large  as  those  previously  mentioned,  h^ve  been 
clearly  located.  The  cerebellar  tracts  are  situated  on  each  side  of 
the  spinal  cord  in  close  proximity  to  the  outer  anterior  border  of  the 
crossed  pyramidal  tracts,  whereas  the  fiber  tracts  from  the  midbrain 
lie  at  the  anterior  edge  of  the  cord  between  the  ascending  sensory  fiber 
tracts.  The  size  of  the  fiber  tracts  decreases  as  they  extend  pos- 
teriorly in  the  cord,  for,  as  would  be  expected,  the  maximum  number 
of  afferent  and  efferent  fibers  in  each  tract  are  brought  together  just 
posterior  to  the  medulla.  (Fig.  134.) 

THE  BRAIN 

The  spinal  cord  passes  through  the  foramen  magnum  at  the  base 
of  the  skull  and  at  once  merges  into  the  medulla  of  the  hindbrain. 
The  brain  as  a  whole  may  be  regarded  as  consisting  of  two  main 
regions.  There  is,  first,  the  primitive  underlying  brain  stem  which 
appears  essentially  as  an  enlarged  and  differentiated  projection  of 
the  spinal  cord,  ending  anteriorly  with  the  midbrain  and  posteriorly 
with  the  medulla  of  the  hindbrain.  The  brain  stem  is  essentially  a 
bottle-neck  through  which  must  pass  the  millions  of  afferent  and 
efferent  fibers  connecting  the  brain  neurons  with  the  body  as  a  whole. 
On  reaching  the  forebrain,  the  fibers  are  concentrated  into  a  small  area, 
r>r  internal  capsule,  of  the  forebrain  from  which  they  spread  out  (corona 
radiata)  into  the  comparatively  large  spaces  (external  capsule)  of  the 
cerebrum.  Secondly,  a  much  larger  superstructure  lies  above  the 
brain  stem  which  consists  of  the  forebrain  (cerebrum)  and  the  cere- 
bellum of  the  hindbrain.  The  forebrain  has  been  aptly  described  as 
"the  flowering  of  the  brain  stem."  Using  botanical  terminology,  the 


258 


HUMAN  BIOLOGY 


spinal  cord  may  be  thought  of  as  a  tree-like  structure,  from  which 
the  branching  spinal  nerves  emerge  for  connection  with  every  body 
structure.  (Fig.  135.) 

It  was  previously  stated  that  the  brain  and  the  spinal  cord  develop 
through  the  differentiation  of  a  thin-walled  ectodermal  tube  during 
early  development.  The  amazing  structural  and  functional  changes 


2    3 


1.  OCCIPITAL  LOBE 

2.  PARIETAL  LOBE 

3.  CENTRAL  FISSURE  (ROLANDO) 

4.  FRONTAL  LOBE 

5.  CEREBELLUM 

6.  SPINAL  CORD 

7.  MEDULLA  OBLONGATA 

8.  PONS  VAROLII 

9.  LATERAL  FISSURE  (SYLVIUS) 

10.  TEMPORAL  LOBE 


FIG.  135. — Drawings  of  the  human  brain, 
sagittal  section,  showing  the  out 


1.  FRONTAL  LOBE  13 

2.  CORPUS  CALLOSUM 

3.  MASSA  INTERMEDIA 

4.  THALAMUS  (THIRD  VENTRICLE) 

5.  CHOROID  PLEXUS 

6.  CORPORA  QUADRIGEMINA 

7.  FOURTH  VENTRICLE 

8.  OCCIPITAL  LOBE 

9.  OPTIC  CHIASMA 

10.  PITUITARY  BODY 

11.  PONS  VAROLII 

12.  MEDULLA  OBLONGATA 

13.  CENTRAL  CANAL 

14.  CEREBELLUM 

B 

A,  external  view  of  the  right  side;  J5,  median 
surface  of  the  right  half  of  the  brain. 


that  occur  before  the  adult  stage  is  reached  are  due,  first,  to  increases 
in  the  number  and  the  degree  of  differentiation  of  the  neurons  of  the 
primitive  neural  tube  and,  second,  to  the  development  of  the  proto- 
plasmic processes  (axons  and  dcndrites)  from  the  cell  bodies.  These 
make  the  proper  connections  with  those  other  neurons  and  gradually 
become  associated  in  the  fiber  tracts  of  the  brain  and  spinal  cord  and 
also  in  the  cranial  and  spinal  nerves  with  their  receptor  and  effector 
connections.  These  neuronic  associations,  involving  ever  increasing 
complexity,  culminate  in  the  human  brain. 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II) 


259 


Divisions  of  the  Brain. — In  the  human  brain  the  primary  divisions 
into  forebrain,  midbrain,  and  hindbrain  are  apparent  as  in  the  brain  of 
vertebrates  generally,  but  such  a  marked  structural  and  functional 
development  of  the  cerebral  hemispheres  in  the  forebrain  has  occurred 
that  the  other  two  primary  brain  divisions  are  greatly  overshadowed. 


'-..          Mesencephalon 
055-J^-     (raid-brain) 


Myelencephalon 
(medulla  oblongafea) 


Para  sacralis  or 
*conus  medullaris 


„  .Prosencephalon 
(fore-brain) 


Cerebrum 


-•Cerebellum  ^ 


/  Metencephalon 


Spinal  cord 


FIG.  130. — The  main  divisions  of  the  human  central  nervous  system  shown  sepa- 
rately. Left  side.  Diagrammatic.  (Morris,  "Human  Anatomy"  P.  Blakiston's  Son 
tfc  Comimny,  Inc.) 

Thus  the  complete  human  brain  of  an  average  sized  male  adult  weighs 
in  the  neighborhood  of  50  oz.  Of  the  total  weight,  approximately 
nine-tenths  is  due  to  the  cerebral  hemispheres.  The  remaining  5  oz. 
are  divided  between  the  midbrain  and  hindbrain,  with  the  latter  much 
the  larger,  so  that  the  midbrain  is  very  small  indeed.  In  viewing 
the  brain  from  the  upper,  or  dorsal,  surface  only  the  paired  cerebral 
hemispheres  are  seen,  for  they  are  large  enough  completely  to  cover 
over  the  other  portions.  This  overgrowth  of  the  forebrain  is  par- 


260  HUMAN  BIOLOGY 

ticularly  striking  when  one  compares  the  brain  of  man  with  the  brain 
of  a  lower  vertebrate,  such  as  the  frog,  in  which  a  dorsal  view  reveals 
the  three  primary  divisions  of  the  brain  arranged  in  a  linear  fashion 
and  merging  posteriorly  into  the  spinal  cord.  (Fig.  136.) 

From  the  functional  standpoint,  the  neurologist  finds  that  the  two 
main  divisions  of  the  human  brain  are  the  forebrain  and  the  hindbrain, 
with  the  midbrain  functioning  in  the  main  as  a  fiber  tract  and  appar- 
ently without  major  independent  integrating  functions  of  its  own. 
The  midbrain  is  seen  in  gross  structure  as  a  short  section  of  the  under- 
lying brain  stem  situated  between  the  fore-  and  hindbrain.  On  the 
under  surface  of  the  midbrain  region  are  two  rounded,  pillar-like 
prominences,  the  crura  cerebri,  a  term  that  literally  means  the  "legs 
of  the  cerebral  hemispheres, "  and  so  named  because  the  latter  seem 
to  rest  upon  them.  As  a  matter  of  fact,  the  crura  cerebri  are  fiber 
tracts  with  connections  extending  to  the  forebrain.  The  dorsal 
surface  of  the  midbrain  shows  four  rounded  projections,  the  corpora 
quadrigemina,  which  are  also  important  neurological  landmarks 
because  of  their  fiber  tracts. 

Hindbrain. — The  hindbrain  consists  of  three  main  portions:  the 
medulla  oblongata,  the  pons  (pons  Varolii),  and  the  cerebellum.  Of 
these,  the  medulla  is  the  most  posterior  part  of  the  brain  stem  and, 
as  stated  above,  joins  the  anterior  end  of  the  spinal  cord.  Viewed  from 
the  ventral  surface,  the  medulla  appears  as  an  enlarged  portion  of  the 
spinal  cord.  Viewed  dorsally,  it  is  found  to  possess  a  triangular  cavity, 
the  fourth  ventricle,  which  is  continuous  with  the  central  canal  of  the 
spinal  cord.  The  ventricle  is  covered  by  a  thin  membrane.  The 
lateral  walls  and  floor  of  the  medulla  contain  important  fiber  tracts  arid, 
in  addition,  contain  neuronic  areas  which  exercise  involuntary  control 
over  such  vital  functions  as  respiration,  heart  action,  blood  pressure, 
and  various  complex  reflex  actions  of  a  more  difficult  nature  than  those 
handled  by  the  ^eurons  in  the  spinal  cord.  Partially  covering  the 
anterior  portion  of  the  medulla,  as  seen  from  the  ventral  side,  and 
extending  along  the  brain  stem  to  the  midbrain  is  the  pons,  which 
consists  almost  entirely  of  two  main  groups  of  nerve  fibers,  designated 
as  the  longitudinal  tracts  and  the  transverse  tracts.  The  latter  appear 
from  the  ventral  surface  as  a  ring-like  structure  surrounding  the  brain 
stem  and  connecting  dorsally  through  the  middle  peduncle  with  the 
cerebellum  lying  above  the  brain  stem.  (Figs.  135#;  136.) 

Thus  the  transverse  fiber  tracts  of  the  pons  make  possible  a  close 
functional  association  of  the  three  divisions  of  the  hindbrain  (medulla, 
pons,  and  cerebellum).  Of  these,  the  cerebellum  is  the  largest  and 
possibly  the  most  important  unit.  It  is  a  trilobed  structure  with  right, 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II) 


261 


left,  and  median  portions,  held  in  position  and  connected  with  the 
other  elements  of  the  nervous  system  by  three  fiber  tracts,  one  of 
which,  the  middle  peduncle,  consists  of  the  transverse  fibers  from  the 
pons.  The  superior  peduncle  extends  directly  anteriorly  into  the 
cerebrum,  while  the  inferior  peduncle  runs  posteriorly,  connecting 
with  certain  fiber  tracts  of  the  spinal  cord.  (Fig.  140.) 

The  cerebellum  is  a  very  important  brain  center  for  the  control  of 
personally  acquired  reflexes  and  for  general  supervision  over  muscular 
movements,  particularly  those  associated  with  equilibrium.  Many 
of  the  afferent  fibers  from  the  sensory  structures  of  the  body  form 
synapses  in  the  cerebellum,  and  it  is  apparent  that  it  has  a  wide  range 


I     OLFACTORY 


n  OPTIC 

SO.    OCULOMOTOR 
BT   TROCHLEAR 
V    TRIGEMINAL 
YI    ABDUCENS 


FACIAL 


SOU  AUDITORY 

DC    GLOSSOPHARYNGEAL 

X  VAGUS  • 

XI  SPINAL  ACCESSORY 
Xtt  HYPOGLOSSAL     — 


CEREBRAL  ARTERY 
FRONTAL  LOBE 
COMMUNICATING  ARTERY 

INTERNAL  CAROTID  ARTERY 
PITUITARY  BODY 
TEMPORAL  LOBE 

BASILAR  ARTERY 
PONS  VAROLII 

CEREBELLAR  ARTERY 
MEDULLA  OBLONGATA 
CEREBELLUM 
CEREBELLAR  ARTERV 


Fia.  137. — The  human  brain  viewed  from  the  under  surface.     Roots  of  the  cranial 

nerves  shown  to  the  left. 

of  influence  on  both  sensory  and  motor  impulses  passing  to  and  from 
the  forebrain.  No  conclusive  evidence  appears  to  exist,  however, 
that  the  cerebellum  acts  otherwise  than  in  a  subsidiary  manner  to  the 
higher  nerve  centers  of  the  forebrain. 

Before  passing  to  a  consideration  of  the  forebrain,  it  will  be  well  to 
indicate  the  important  relationships  between  the  hindbrain  and  the 
12  pairs  of  cranial  nerves,  for  all  buttthe  first  two  pairs  of  cranial  nerves 
emerge  from  the  hindbrain.  All  of  the  cranial  nerves  have  their  real 
origin  in  definite  neuronic  areas1  which  are  located,  with  the  exception 
of  the  first  pair  (olfactory),  in  the  midbrain  and  hindbrain.  The 
neuronic  areas,  located  deeply  in  the  brain  tissue,  may  be  some  distance 
from  the  point  where  the  nerves  emerge  from  the  brain.2  (Fig.  137.) 

1  Neurologists  commonly  refer  to  the  neuronic  areas  as  nuclei,  but  the  use  of 
the  term  nuclei  in  this  connection  is  somewhat  confusing  to  the  biology  student. 
The  term  neuronic  area  is  therefore  preferable,  indicating  an  area  in  which  a  great 
many  of  the  nucleated  cell  bodies  of  the  neurons  are  concentrated. 

2  Consult  Appendix:  Cranial  Nerves. 


262  HUMAN  BIOLOGY 

Forebrain. — The  forebrain,  or  cerebrum,  greatly  overshadows  all 
other  parts  of  the  brain  in  its  functional  aspects  and  is  clearly  the 
superorgan  of  the  human  nervous  system.  The  cerebrum  consists 
primarily  of  a  pair  of  cerebral  hemispheres,  with  which  three  lesser 
structures  are  associated,  namely,  the  olfactory  lobes,  the  thalami,  and 
the  corpora  striata.  The  olfactory  lobes,  as  the  name  indicates,  are 
concerned  with  the  sense  of  smell.  In  some  of  the  lower  vertebrates 
they  are  very  highly  developed  and  constitute  the  largest  part  of  the 
forebrain.  In  man,  they  are  of  comparatively  small  size,  lying  under- 
neath and  near  the  anterior  border  of  each  cerebral  hemisphere.  Just 
below  this  position  is  the  sensitive  area  of  the  nasal  epithelium,  sepa- 
rated from  the  brain  cavity  by  the  bony  perforated  cribiform  plate. 
Through  the  latter  great  numbers  of  minute  sensory  nerve  fibers  from 
the  olfactory  lobes  project  into  the  nasal  cavity  and  innervate  the 
nasal  epithelium  (page  220). 

The  thalami  (optic  thalami)  consist  of  a  pair  (one  for  each  cerebral 
hemisphere)  of  neuronic  areas  deeply  embedded  in  the  tissue  of  each 
cerebral  hemisphere,  close  to  the  median  line  and  near  the  midbrain 
region  of  the  brain  stem.  As  will  be  seen  later,  the  thalami  are  impor- 
tant association  centers  for  synapses  with  afferent  fibers  connecting 
with  the  forebrain — particularly  with  respect  to  the  so-called  body- 
sense  (pain,  warmth).  Another  pair  of  neuronic  areas  present  in  the 
forebrain  are  the  corpora  striata  which  are  situated  in  close  proximity, 
lateral  and  anterior  to  the  thalami.  The  corpora  striata  are  concerned 
as  association  areas  for  synapses  with  efferent  fibers  from  the  forebrain 
controlling  muscle  action. 

Cerebral  Cortex, — An  examination  of  the  outer  tissue  layer,  or 
cortex,  of  the  cerebral  hemispheres  shows  an  uneven  or  convoluted 
surface  due  to  elevations  (gyri)  and  depressions  (sulci)  which  are  quite 
uniformly  distributed  over  the  cerebral  areas.  Four  main  regions  of 
the  cortex  can  be  noted,  namely,  the  frontal,  parietal,  occipital,  and 
temporal,  corresponding  in  position  to  the  areas  of  the  skull  as  pre- 
viously given  (page  197).  In  each  cerebral  hemisphere  the  boundaries 
of  the  regions  just  noted  are  more  or  less  distinctly  indicated  by  con- 
spicuous depressions  (fissures)  which  are  deeper  than  the  sulci. 
Particularly  prominent  is  the  fissure  of  Rolando  which  marks  the 
boundary  between  the  frontal  and  parietal  lobes  and  constitutes  an  im- 
portant landmark  for  the  study  of  functional  localization  in  the  cortex. 
It  should  be  said  in  this  connection  that  the  results  obtained  from 
numerous  experimental  studies  on  cerebral  function  show  beyond  a 
doubt  that  the  neurons  concerned  with  the  control  of  a  particular 
function  are  localized  in  definite  cortical  areas  of  the  cerebral  hemi- 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II)  263 

spheres.  It  is  possible  to  indicate ; only  a  few  of  the  many  important 
areas  that  have  been  identified.  In  each  hemisphere,  the  area  of  the 
frontal  lobe  lying  just  anterior  to  the  fissure  of  Rolando  is  known  to  be 
definitely  associated  with  the  control  of  voluntary  muscles  in  various 
regions  of  the  body.  Thus  the  muscles  of  the  leg,  body,  and  arms  are 
controlled  by  neurons  in  the  cortex  lying  along  the  anterior  margin  of 
the  fissure  of  Rolando,  whereas  neurons  along  the  posterior  margin  of 
this  fissure  in  the  parietal  lobe  serve  as  interpreters  of  the  afferent 
sensory  impulses  which  give  the  tactile  sense.  In  the  parietal  lobes, 
posterior  to  the  fissure  of  Rolando,  other  neuronic  areas  are  associated 
with  facial  control  and  speech.  The  auditory  center  of  the  cortex  is 
localized  near  by  in  the  anterior  portion  of  each  temporal  lobe,  and 
the  incoming  visual  impulses  from  the  retina  are  received  by  cortical 

..Central  Fissure  (Rolando) 
Parietal  Lobe 


Occipital 

Lobe  Y.r "\  ^S^Jf*,    TK^ronM 

lobe 


-Temporal  Lobe 

Obtongata 

FIG.  138. — Human  brain.     View  of  right  side  showing  localization  of  various  important 
areas  in  the  cortex.      (Watkeys,  Dogga.) 

neurons  located  in  the  posterior  portion  of  the  occipital  lobes.     (Fig. 
138.) 

Cerebral  Fiber  Tracts. — It  is  apparent  that  the  segregation  of  certain 
cerebral  functions  in  definite  and  corresponding  areas  of  the  cerebral 
hemispheres  means  that  the  entire  brain  is  not  ordinarily  involved  in 
the  interpretation  of  the  incoming  impulses  or  in  the  transmission  of 
the  efferent  impulses.  Thus,  to  take  one  example,  afferent  impulses 
from  the  sensory  areas  associated  with  the  voluntary  muscles  of  the 
arms  and  legs  on  the  right  side  of  the  body  are  primarily  received  by 
neurons  in  the  posterior  portion  of  the  frontal  lobe  of  the  left  cerebral 
hemisphere.  But  some  of  the  incoming  impulses  from  the  peripheral 
regions  may  be  so  important  as  to  require  additional  consideration, 
and  this  will  necessarily  involve  other  neuronic  areas  of  the  cerebrum. 
Accordingly,  it  is  found  that  the  various  localized  neuronic  areas  in 
the  cerebral  hemispheres  are  interconnected  by  special  association 
fiber  tracts  composed  of  bundles  of  nerve  fibers  through  which  the 


264 


HUMAN  BIOLOGY 


impulses  from  one  cerebral  region  are  transferred  to  other  regions  as 
required.  In  the  forebrain,  separate  groups  of  the  association  fiber 
tracts  are  established  for  communication  between  the  right  and  left 
cerebral  hemispheres  and  also  for  communication  between  the  various 
neuronic  areas  in  each  of  the  hemispheres.  This  condition  is  easily 
understandable  when  one  thinks  of  the  telephone  central  of  a  large 
city  containing  a  number  of  separate  functional  units,  the  exchanges, 
each  of  which  is  connected  to  all  the  other  exchanges  in  the  central 
office  so  that  messages  may  be  transmitted  back  and  forth  throughout 
the  entire  switchboard.  (Fig.  139.) 

In  addition  to  the  association  fiber  tracts,  Another  very  important 
and  even  more  widely  distributed  system  of  fiber  tracts,  the  projection 


fr&ntal 
ctngcc 


occipital* 
area 


temporal  area 

FIG.  139. — Schematic  section  of  the  human  brain  showing  the  association  fiber 
tracts.  A,  between  adjacent  areas;  B,  connecting  frontal  arid  occipital  areas;  C,  D, 
connecting  frontal  and  temporal  areas;  E,  connecting  occipital  and  temporal  areas. 
The  corpus  callosum  also  contains  association  fibers  connecting  the  cortex  of  the  right 
and  left  cerebral  hemispheres.  C N,  caudate  nucleus;  OT,  thalamus.  (Hunter,  Walter, 
and  Hunter,  "Biology,"  American  Book  Company.  Modified  from  Starr.) 

fibers,  connects  important  regions  of  the  forebrain,  midbrain,  hind- 
brain,  and  spinal  cord  into  a  unified  whole.  Some  of  the  more  impor- 
tant tracts  of  the  projection  fibers  may  now  be  briefly  indicated. 
(1)  In  the  first  place,  there  is  present  in  each  cerebral  hemisphere  a 
cerebro-cortico-pontal  tract  which  originates  from  the  neurons  in  the 
cortex  of  each  frontal  lobe  and  extends  posteriorly  to  the  pons  of  the 
hindbrain  where  the  constituent  axon  fibers  synapse  so  that  the  efferent 
impulses  from  the  cerebrum  reach  the  cerebellum  through  the  fiber 
tract  of  the  middle  peduncle  (page  260).  This  tract  carries  efferent 
impulses,  which,  since  they  originate  in  the  frontal  neurons,  are  pre- 
sumed to  be  of  a  very  high  order.  (2)  The  pyramidal  tracts  originate 
in  the  so-called  pyramidal  neurons  of  the  cortex  of  the  frontal  lobes 
near  the  fissure  of  Rolando,  pass  ventrally  through  the  corpora  striata, 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II) 


265 


and  then  form  the  crossed  and  uncrossed  fiber  tracts  of  the  spinal 
cord  (page  256).  The  crossing  of  the  pyramidal  tracts  occurs  in  the 
medulla  of  the  hind  brain.  The  pyramidal  tracts  transmit  efferent 
impulses  for  the  control  of  voluntary  motion.  (Fig.  140.) 

(3)  The  cutaneous  sensory  projection  tracts,  carrying  afferent 
impulses,  originate  in  the  neurons  situated  in  the  anterior  portion 
of  each  parietal  lobe  and  extend  to  the  optic  thalami  where  the  main 
sensory  fiber  tracts  of  the  spinal  cord  terminate  anteriorly.  It  should 


SSSt* 


FIG.  140. — Schematic  section  of  the  human  brain  showing  the  projection  fiber  tracts 
connecting  the  cerebrum  and  other  parts  of  the  brain  and  spinal  cord.  A,  tracts  con- 
necting cortex  of  frontal  lobe  to  the  pons  varolii  arid  thence  to  the  cerebellum  via  the 
transverse  fibers  (middle  peduncle)  at  G',  B,  pyramidal  motor  tracts;  C,  sensory  tracts; 
Z),  visual  tracts;  E,  auditory  tracts;  F,  projection  fibers  (anterior  peduncle)  connecting 
cerebellum  and  anterior  portions  of  the  brain;  G,  transverse  fibers  of  pons  varolii;  H, 
projection  fibers  (posterior  peduncle)  connecting  cerebellum  and  the  spinal  cord;  J, 
projection  fibers  between  auditory  nucleus  and  the  brain;  Kt  crossing  over  (decussation) 
of  pyramidal  motor  tracts  in  the  brain;  Vtt  fourth  ventricle.  Roman  numerals  refer  to 
cranial  nerves.  (Hunter,  Walter,  and  Hunter,  "Biology,"  American  Book  Company. 
Modified  from  Starr.) 

also  be  remembered  that  a  crossing  of  these  sensory  fibers  occurs  in  the 
hindbrain,  just  anterior  to  the  decussation  of  the  pyramidal  tracts. 
(4)  The  visual  tracts  originate  from  neurons  lying  in  the  posterior  part 
of  the  occipital  lobes  and  extend  posteriorly  to  special  neuronic  areas 
in  the  midbrain  where  synapse  is  made  with  the  afferent  fibers  of  the 
optic  nerves,  carrying  impulses  from  the  retina.  It  is  probable  that 
each  visual  cell  in  the  retina  has  a  direct  connection  through  this 
projection  path  to  one  or  more  cerebral  neurons  where  the  interpreta- 
tion of  the  visual  images  takes  place.  (5)  The  auditory  tracts  originate 


266  HUMAN  BIOLOGY 

in  the  neurons  of  the  temporal  lobes  of  the  cerebrum  from  which  each 
extends  to  neuronic  areas  located  in  the  midbrain.  Here  synapse 
occurs  with  the  fibers  of  the  auditory  nerve  transmitting  auditory 
impulses  from  the  hindbrain.  The  routes  of  the  afferent  impulses 
from  the  cochlea  and  semicircular  canals  are  difficult  to  trace  through 
the  intricate  fiber  tracts  of  the  hindbrain  and  midbrain,  but  the  course 
of  the  auditory  tracts  from  the  temporal  lobes  t6  the  midbrain  is  well 
established.  (6)  Finally,  the  projection  tracts  include  the  superior, 
middle,  and  inferior  peduncles  of  the  cerebellum,  concerned  with  the 
transmission  of  impulses  to  the  forebrain,  to  the  midbrain,  and  to  the 
pons  and  medulla  of  the  hindbrain,  as  has  been  indicated  previously 
(page  261).  (Fig.  140.) 

Histology  of  the  Cortex. — In  the  previous  study  of  the  spinal  cord, 
it  was  noted  that  the  gray  matter,  composed  essentially  of  the  motor 
and  association  neurons,  formed  an  H-shaped  body  in  the  interior  of 
the  cord  which  was  enclosed  by  a  layer  of  white  matter  consisting  of  the 
fiber  tracts.  This  same  arrangement  of  the  white  and  gray  matter 
persists  in  the  medulla,  pons,  and  midbrain,  but  it  is  reversed  in  the 
cerebellum  of  the  hindbrain  and  in  the  cerebral  hemispheres  of  the 
forebrain,  in  both  of  which  the  gray  matter,  containing  the  neurons, 
forms  the  outer  cortex,  while  the  fibrous  white  matter  is  enclosed  in 
the  interior.  It  is  this  cortical  gray  matter  in  the  cerebral  hemispheres 
and  the  cerebellum  that  constitutes  the  dominant,  controlling,  Integra- 
tive,  and  interpretive  tissue  of  the  body,  the  seat  of  consciousness  and 
intelligence.  Histological  studies  of  cortical  tissue  reveal  essentially 
the  same  general  arrangement  of  tissues  throughout.  Forming  the 
thin  outer  covering  is  the  molecular  layer,  about  0.01  in.  thick,  consist- 
ing largely  of  interlacing  nerve  fibers  but  also  containing  numerous 
comparatively  small  neurons.  Below  the  molecular  layer  are  several 
nuclear  layers  composed  of  various  specialized  types  of  neurons.  As 
might  be  expected,  the  vascularization  of  the  gray  matter  is  much 
greater  than  that  of  the  white  matter. 

The  cortex  of  the  cerebral  hemispheres,  which  is  somewhat  more 
highly  differentiated  than  that  of  the  cerebellum,  may  be  subdivided 
into  four  or  even  more  rather  well-defined  layers  in  correspondence 
with  the  various  types  of  neuron  that  are  present.  Characteristic 
structural  features  are  evident  in  the  neurons  from  different  regions  of 
the  cortex.  Thus  in  the  cortex  of  the  cerebral  motor  area  of  the  frontal 
lobes  are  found  the  multipolar  pyramidal  neurons,  the  axon  fibers  of 
which  form  the  crossed  and  uncrossed  pyramidal  tracts.  The 
pyramidal  neurons  are  the  largest  neurons  in  the  entire  nervous  system 
and  almost  reach  naked-eye  visibility.  On  the  other  hand,  in  the 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II)  267 

visual  area  of  the  cortex  the  pyramidal  type  of  neuron  is  almost  entirely 
absent.  It  is  estimated  that  between  13  and  14  billion  neurons  are 
present  in  the  cerebral  cortex  of  the  human  brain,  which,  with  the 
associated  dendrites  and  axons,  make  a  complexity  of  organization 
which  defies  complete  analysis.  A  further  complication  is  found  in 
the  specialized  supporting  tissue  (neuroglia)  which  is  widely  distributed 
throughout  the  neurons  and  fiber  tracts  of  the  central  nervous  system. 
The  neuroglia  (literally,  nerve  glue)  cells  are  of  various  types  but  all  are 
characterized  by  the  presence  of  cellular  processes  of  varying  number 
and  complexity  which  are  intermingled  with  the  true  nerve  fibers  in 
many  cases. 

Brain  Ventricles. — The  central  cavity  of  the  embryonic  neural 
tube  is  comparatively  large  at  first  but  during  development  is  gradually 
reduced  in  size  as  the  walls  are  thickened  by  the  increased  number  of 
neurons.  Remnants  of  the  original  cavity  persist  as  (1)  the  tiny 
central  canal  in  the  spinal  cord  and  (2)  the  much  larger  ventricles  of  the 
brain.  The  posterior  ventricle  is  in  the  medulla  of  the  hindbrain  where 
the  vertebral  canal  enlarges  to  form  the  fourth  ventricle.  Proceeding 
anteriorly  from  the  fourth  ventricle,  the  central  cavity  narrows  again 
to  form  the  aqueduct  of  Sylvius,  which  extends  to  the  midbrain  region; 
here  it  forms  the  third  ventricle.  Anterior  to  the  third  ventricle,  the 
brain  cavity  divides  into  a  pair  of  lateral  ventricles  which  extend  into 
and  throughout  the  length  of  the  cerebral  hemispheres.  (Fig.  1355.) 

It  is  thus  evident  that  the  central  canal  of  the  spinal  cord  together 
with  the  ventricles  of  the  brain  form  a  continuous  central  cavity 
throughout  the  length  of  the  central  nervous  system.  The  vertebral 
canal  of  the  spinal  cord  is  very  small  and  apparently  without  definite 
function,  but  the  brain  ventricles  serve  an  important  function  in  the 
collection  and  distribution  of  the  cerebrospinal  fluid  which  fills  the 
central  cavities  and  surrounds  the  entire  central  nervous  system,  thus 
affording  protection  from  mechanical  shocks  and  also  as  a  medium 
from  which  nutrient  materials  are  obtained  for  the  neurons. 

Meninges. — In  considering  the  cerebrospinal  fluid,  attention  must 
be  given  to  the  three  membranes  (meninges)  that  enclose  the  central 
nervous  system.  The  outermost  membrane  (dura  mater)  completely 
lines  the  bony  tissue  of  the  skull  and,  to  a  lesser  degree,  the  vertebral 
column.  The  middle  meninges  is  the  arachnoid,  and  within  it  is  the 
innermost  one,  the  pia  mater,  lying  in  close  contact  with  the  nerve 
tissues  of  the  brain  and  cord;  in  fact,  the  pia  mater  follows  the  con- 
voluted surface  of  the  cerebral  cortex,  while  the  arachnoid  stretches 
across  from  "peak  to  peak"  of  the  sulci.  Between  the  arachnoid  and 
pia  mater  is  the  subarachnoid  space  filled  with  cerebrospinal  fluid. 


268  HUMAN  BIOLOGY 

The  latter  thus  forms  a  liquid  layer  completely  surrounding  the  central 
nervous  system. 

The  ccrebrospinal  fluid  is  a  product  of  the  blood  and  is  received 
from  the  vascular  system  through  thin-walled,  high-vascularized  areas 
(choroid  plexuses)  present  in  the  walls  of  the  brain  ventricles,  par- 
ticularly in  the  lateral  ventricles  of  the  cerebral  hemispheres.  From 
the  lateral  ventricle  of  each  hemisphere,  the  cerebrospinal  fluid  flows 
posteriorly,  passing  through  the  third,  and  finally  reaching  the  fourth 
ventricle.  Here  it  slowly  flows  through  three  tiny  openings  in  the  thin 
ventricular  covering  and  then  into  the  extended  spaces  of  the  sub- 
arachnoid  cavity.  The  route  of  the  cerebrospinal  fluid  in  returning 
to  the  blood  stream  has  not  been  fully  determined.  The  pia  mater 
serves  not  only  as  an  intimate  membranous  covering  of  the  central 
nervous  system  but  also  as  a  highly  vascularized  tissue  in  which  the 
blood  vessels  supplying  the  nerve  elements  of  the  cord  and  brain  have 
their  origin. 

FUNCTIONAL  FEATURES  ASSOCIATED  WITH  THE  NERVOUS  SYSTEM 

From  the  functional  standpoint,  the  human  nervous  system  is 
primarily  receptive,  conductive,  and  integrative.  We  have  seen  that 
the  function  of  reception  is  localized  in  the  many  and  varied  types  of 
peripheral  sense  organ,  capable  of  receiving  the  continuous  and  multi- 
tudinous internal  and  external  stimuli.  Conduction  of  nerve  impulses 
is  usually  regarded  as  being  primarily  the  function  of  the  peripheral 
nerve  fibers,  and  rightly  so.  It  must  be  remembered,  however,  that 
the  nerve  fibers  are  not  independent  elements  of  the  nervous  system 
but  merely  the  processes  of  the  neurons  located  either  in  the  central 
nervous  system  or  in  near-by  ganglia.  Furthermore,  conduction  is 
also  an  essential  function  of  the  fiber  tracts  extending  through  the 
brain  and  spinal  cord.  The  function  of  integration,  which  involves 
the  coordination  of  all  the  body  structures  so  as  to  unify  the  entire 
organism,  is  of  paramount  importance  and  is  exclusively  a  function  of 
the  central  nervous  system.  Ramifications  of  the  integration  func- 
tion involve  the  higher  functions  of  intelligence,  consciousness,  mem- 
ory, volition,  and  sensation.  Consideration  may  now  be  given  to 
these  primary  functions  of  the  nervous  system — reception,  conduction, 
and  integration — in  the  order  named. 

RECEPTION 

Concerned  with  the  reception  of  stimuli  are  two  groups  of  receptors, 
namely,  those  specialized  for  external  stimuli,  which  comprise  the 
exteroceptive  system,  and  those  influenced  by  the  stimuli  arising 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II)  269 

internally,  which  comprise  the  inter6ceptive  and  proprioceptive  sys- 
tems. The  receptors  of  the  exteroceptive  system  are  located  near  the 
body  surface  where  they  are  in  a  position  to  be  stimulated  by 
the  various  environmental  stimuli  that  impinge  upon  them,  whereas  the 
receptors  of  the  other  two  sensory  systems  are  located  at  strategic 
points  throughout  the  body  where  they  receive  the  internal  stimuli 
and  thus  furnish  information  relative  to  the  condition  of  the  various 
tissues  and  organs  and  of  the  needs  of  the  body  as  a  whole. 

It  is  at  once  apparent  that  the  so-called  primary  senses  of  the 
body,  namely,  touch,  taste,  smell,  temperature,  sight,  and  hearing, 
are  all  components  of  the  exteroceptive  system  with  the  receptors 
located  peripherally  where  they  are  influenced  by  the  particular  types 
of  external  stimulus  for  which  they  are  adapted.  Furthermore  these 
receptors  may  be  divided  into  those  equipped  to  receive  stimuli  from 
a  distance,  as  in  hearing  and  seeing  in  which  the  incoming  sound  and 
light  waves  reach  the  sensory  tissues  from  varying  distances,  and  into 
the  receptors  which  are  stimulated  only  by  actual  contact  with  certain 
substances,  as  in  touch,  taste,  and  smell.  It  might  be  thought  that 
the  olfactory  receptors  belonged  to  the  distance  receptors;  but  as  a 
matter  of  fact,  the  olfactory  epithelium  is  stimulated  only  when  tiny 
particles  of  a  volatile  material  are  brought  into  actual  contact  with  the 
olfactory  cells.  It  appears  probable  that  the  sensation  of  pain  arises 
from  specialized  sensory  cells  which  are  more  or  less  widely  distributed 
among  the  various  types  of  receptors  and  are  affected  whenever  a  par- 
ticular stimulus  reaches  excessive  strength.  Adequate  consideration 
has  been  given  to  the  exteroceptive  sense  organs  in  the  first  section  of 
the  previous  chapter  (page  218),  but  the  interoceptive  system  remains 
for  brief  discussion  at  this  time. 

The  receptors  of  the  interoceptive  system  are  associated  with  less 
generally  recognized  sensations,  notably  hunger,  thirst,  equilibrium, 
and  one  that  may  be  referred  to  as  the  muscle  sense.  The  receptors 
associated  with  equilibrium  and  muscle  sense  are  conveniently  grouped 
as  the  proprioceptors.  •  There  seems  to  be  no  reason  to  regard  the 
interoceptive  receptors  as  essentially  different  in  nature  from  those 
responsible  for  the  reception  of  external  stimuli.  However,  it  is  clear 
that  the  structural  elements  of  the  interoceptive  receptors  are  very 
simple  compared  with  those  of  the  exteroceptive  receptors  in  which, 
in  most  cases,  the  stimuli  are  first  received  by  highly  specialized  sen- 
sory cells,  as  in  the  retina,  and  then  released  to  the  afferent  sensory 
fibers  for  transmission  to  the  central  nervous  system.  An  examination 
of  the  visceral,  muscular,  and  supporting  elements  of  the  body  does 
not  reveal  the  presence  of  definite  sense  organs  with  sensory  cells,  and 


270  HUMAN  BIOLOGY 

so  it  is  evident  that  the  internal  stimuli  are  directly  received  by  the 
dendrites  of  the  sensory  nerve  fibers  at  their  highly  developed  terminal 
arborizations.  The  latter  are  abundantly  distributed  and  in  intimate 
contact  with  the  various  body  tissues.  In  the  voluntary  muscles  and 
skeletal  elements  of  the  body  the  sensory  fibers  are  elements  of  the 
spinal  nerves,  but  throughout  the  viscera  of  the  body,  in  which  involun- 
tary muscle  tissue  forms  the  effector  units,  the  sensory  fibers  are 
associated  with  the  autonomic  system.  The  three  basic  interoceptive 
sensations  of  hunger,  thirst,  and  proprioception  may  now  be  considered. 

Hunger. — The  sensation  of  hunger  appears  to  be  due  primarily  to 
involuntary  muscular  contractions  in  the  wall  of  the  stomach  in  the 
absence  of  the  normal  intake  of  food.  As  the  result  of  the  hunger 
contractions  the  sensory  nerve  fibers  in  the  stomach  tissues  are  stimu- 
lated, and  a  discharge  of  sensory  impulses  reaching  the  central  nervous 
system  causes  a  distinctly  unpleasant,  even  painful,  sensation  which  is 
interpreted  as  a  demand  by  the  nutritive  tissues  for  food.  However, 
the  sensation  of  hunger  appears  to  arise  primarily  from  an  empty 
stomach,  which  incites  a  distinct  type  of  muscular  contraction,  rather 
than  from  an  actual  demand  from  the  tissues  for  nutritive  materials. 
Thus,  when  one  fasts  for  several  days,  it  is  found  that  the  hunger  sensa- 
tions disappear  after  a  time.  If  they  were  primarily  associated  with 
the  body  tissues,  it  would  be  expected  that  the  sensations  would 
increase  with  the  continued  failure  to  supply  additional  nutritive 
materials.  It  seems  evident  also  that  the  unpleasant  hunger  sensa- 
tion is  not  directly  associated  with  the  distinctly  pleasant  mental 
phenomenon  which  we  term  appetite.  The  latter  involves  the  mem- 
ory of  pleasant  tastes,  odors,  and  companionship  around  the  festive 
board  and  seems  to  be  linked  up  with  a  general  feeling  of  well-being. 
There  in,  however,  an  indirect  connection  between  hunger  and  appetite 
in  that  the  sight  or  smell  of  food,  when  one  is  hungry,  will  incite  various 
activities  associated  with  eating,  such  as  a  flow  from  the  salivary 
glands,  whereas,  if  one  is  surfeited  with  food,  the  presence  of  additional 
food  brings  no  response. 

Thirst. — It  has  been  shown,  in  an  earlier  chapter,  how  necessary 
water  is  to  animal  metabolism.  Any  failure  of  the  water  supply  so 
that  the  fluid  reserves  in  the  tissues  begin  to  decrease  results  in  a  very 
definite  and  early  warning,  the  sensation  of  thirst.  This  sensation 
apparently  is  not  localized  in  the  stomach  but  in  the  pharynx.  Accord- 
ingly, the  intake  of  dry  or  salty  food  or  hot  dry  air  very  quickly  results 
in  a  dry  feeling  in  the  throat,  which  is  immediately  associated  with 
the  sensation  of  thirst.  At  first,  the  thirst  sensation  is  only  a  gentle 
warning  which  may  be  satisfied  with  a  slight  amount  of  water.  When, 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II)  271 

however,  the  supply  of  water  is  lacking  for  a  longer  time,  the  demand 
is  increasingly  insistent  and  soon  becomes  one  of  the  most  powerful 
and  painful  of  all  the  sensations,  with  all  the  sensory  elements  of  the 
body  gradually  stimulated  by  the  demands  of  the  tissues  for  water. 

Proprioception. — Proprioception  is  primarily  concerned  with  coor- 
dinated control  of  voluntary  muscles  and  with  the  muscular  sense  that 
makes  one  aware  of  the  position  of  a  particular  muscle  or  group  of 
muscles,  as  in  the  leg  or  arm,  without  looking  at  them.  When  the 
proprioceptive  apparatus  is  destroyed  in  any  region,  sensation  is  lost. 
The  sensory  impulses  arising  in  the  proprioceptors  originate  in  specific 
dendritic  arborizations  of  the  sensory  fibers,  abundantly  distributed 
through  the  voluntary  muscle  tissues  and,  also,  in  the  attached  ten- 
dons. Also  bound  up  in  the  complicated  proprioceptive  association  is 
the  function  of  equilibrium  which  has  its  primary  interpretive  area  in 
the  cerebellum  of  the  hindbrain.  The  cerebellum  also  receives  by  way 
of  the  auditory  nerve  the  afferent  impulses  originating  in  the  sensory 
cells  of  the  semicircular  canals,  which,  as  we  have  seen,  are  the  organs 
of  equilibration.  The  exact  role  of  the  cerebellum  in  maintaining 
equilibrium  is  difficult  to  determine  with  exactness,  but  that  it  is  of 
prime  importance  is  established  by  the  fact  that  the  complete  or  partial 
destruction  of  the  cerebellum  decidedly  mars  not  only  the  normal  pic- 
ture of  equilibrium  but  also  the  essential  integrated  operation  of 
practically  all  types  of  voluntary  muscle  movement.  Again,  the 
maintenance  of  normal  muscle  tonus  is  also  bound  up  in  the  proprio- 
ceptive phenomena.  On  the  whole,  the  cerebellum  apparently  should 
be  regarded  as  the  general  interpretive  center  of  the  afferent  impulses 
from  the  proprioceptors  and  the  organs  of  equilibrium.  Out  of  the 
complete  picture  thus  obtained  by  the  cerebellum,  suitable  efferent 
impulses  are  released  which  govern  tonus,  equilibrium,  and  coordinated 
voluntary  movements,  particularly  those  associated  with  locomotion 
and  other  complex  movements  that  have  been  gradually  learned. 

CONDUCTION 

From  the  external  and  internal  receptors  of  the  body,  scattered  far 
and  wide,  widely  variable  in  design  and  almost  innumerable,  sensory 
impulses  are  continuously  being  received  and  conducted  over  the 
afferent  nerve  fibers  to  the  central  nervous  system,  and,  from  the  lat- 
ter, efferent  impulses  are  released  for  transmission  over  the  motor  and 
autonomic  nerves  to  the  effector  units  of  muscle  and  glandular  tissue. 
And,  as  we  have  already  seen,  the  central  nervous  system  contains 
important  fiber  tracts  over  which  countless  nerve  impulses  continu- 
ously pass  from  one  end  to  the  other.  It  is  apparent  that  in  the 


272  HUMAN  BIOLOGY 

absence  of  conduction  the  receptive  and  integrative  functions  would 
be  without  effect.  The  underlying  phenomena  responsible  for  the 
conduction  of  nerve  impulses  are,  to  a  considerable  extent,  bound  up 
with  secrets  of  the  living  state  which,  as  yet,  are  undisclosed.  Never- 
theless numerous  important  facts  pertaining  to  conduction,  which  were 
established  by  years  of  research,  are  now  recorded,  and  a  few  of  the 
most  important  of  these  may  now  be  considered. 

In  the  first  place,  the  conduction  of  impulses  over  the  nerve  fiber  is 
unqestionably  a  vital  process  in  which  oxygen  is  used,  carbon  dioxide 
is  released,  and  a  slight  rise  in  temperature  occurs,  as  in  other  cellular 
activities.  Accordingly,  conduction  must  be  basically  a  process  in 
which  potential  chemical  energy  present  in  the  protoplasmic  com- 
pounds of  the  neurons  concerned  is  released  by  oxidation.  The  chem- 
ical changes  associated  with  the  movement  of  a  nerve  impulse  along 
the  fiber  has  been  compared  to  the  burning  of  the  explosive  in  a  fuse 
as  it  proceeds  in  regular  fashion  from  the  lighted  end.  However,  it  is 
certain  that,  when  the  explosive  compound  has  undergone  the  chem- 
ical changes  associated  with  combustion,  the  original  substance  can- 
not automatically  be  restored;  whereas  the  chemical  changes  in  the 
cytoplasm  of  a  nerve  fiber,  when  the  impulse  is  transmitted,  are  only 
temporary,  so  that  another  impulse  may  be  transmitted  almost 
immediately. 

It  has  long  been  established  that  the  conduction  of  nerve  impulses 
is  accompanied  by  electrical  phenomena.  This  can  be  shown  by  plac- 
ing the  electrodes  of  a  galvanometer  of  the  proper  sensitivity  in  con- 
tact with  an  active  living  nerve  fiber  and,  then,  artificially  stimulating 
the  latter  at  some  point  beyond  the  electrodes  so  that  nerve  impulses 
will  pass  along  the  fiber.  When  the  advancing  impulse  reaches  the 
first  electrode,  an  electric  current,  known  as  the  action  current,  will  be 
registered  moving  toward  this  spot  from  the  second  electrode.  As 
the  impulse  passes  to  the  portion  of  the  nerve  lying  between  the  two 
electrodes,  no  current  is  detected;  but  when  the  impulse  reaches  the 
point  of  the  nerve  where  the  second  electrode  is  attached,  a  current  is 
detected  flowing  from  the  region  of  the  first  electrode  toward  the 
second;  that  is,  the  direction  of  the  current  is  the  reverse  of  that  first 
indicated.  Since  the  action  current  flows  toward  the  region  of  the 
fiber  over  which  the  impulse  is  passing,  it  is  evident  that  the  latter 
temporarily  reduces  the  electric  potential  of  successive  points  of  the 
fiber  (that  is,  they  become  negative)  as  it  moves  along.  If  no  impulse 
is  passing  over  the  fiber,  the  galvanometer  shows  an  absence  of  current, 
which,  of  course,  means  that  all  regions  of  the  fiber  have  the  same  elec- 
tric potential.  The  action  current  is  distinct  from  the  nerve  impulse 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II)  273 

but  is  induced  by  the  conduction  of  the  latter  along  the  fiber.     (Fig. 
141A,  B.) 

In  the  earlier  chapter  dealing  with  the  Muscular  System,  a  descrip- 
tion was  given  of  the  muscle-nerve  preparation  in  which  a  voluntary 
muscle  and  the  attached  nerve  were  removed  from  an  experimental 
animal  and  used  for  the  study  of  muscle  contraction  (page  179). 
Such  a  preparation  is  also  of  great  value,  as  was  there  indicated,  for 
the  study  of  conduction  in  nerve  fibers.  It  is  possible,  in  the  first 
place,  to  ascertain  the  effects  on  nerve  fibers  of  various  types  of  stimu- 
lus, such  as  electrical,  thermal,  chemical,  and  mechanical,  all  of  which 
will  stimulate  nerve  tissue  and  produce  nerve  impulses.  In  general, 


FIG.  141A. — Illustrating  electrical  phenomena  associated  with  the  passage  of  a  nerve 
impulse,  as  described  on  page  272.  a,  electrodes  for  applying  stimulus  to  nerve;  6,  c, 
electrodes  connecting  with  galvanometer  G.  (Howell,  "Physiology  "  W.  B.  Saunders 
Company.) 


FIG.  141B. — Schematic  diagram  illustrating  the  changes  in  the  electric  potential 
during  the  passage  of  a  nerve  impulse.  (Buchanan,  "Elements  oj  Biology"  Harper  & 
Brothers.  Slightly  modified  after  Gerard.} 

it  has  been  found  that  the  electric  current  offers  the  best  type  of  artifi- 
cial stimulus,  since  it  can  be  controlled  accurately  with  respect  to  the 
area  of  stimulation,  the  strength  of  stimulus,  and  the  time  during 
which  it  acts.  It  should  be  noted  that  the  continuous  flow  of  an  elec- 
tric current  through  a  nerve  does  not  stimulate  it  unless  the  current  is 
of  considerable  strength,  in  which  case  it  is  probably  injurious  to  the 
nerve  tissue.  On  the  other  hand,  nerves  are  readily  stimulated  by 
interruptions  in  the  flow  of  an  electric  current.  Thus  nerve  stimulation 
occurs  at  the  instant  when  the  switch  is  closed  and  the  electric  current 
begins  to  flow  (make-shock)  and  again  when  the  flow  of  the  current  is 
stopped  by  opening  the  switch  (break-shock).  The  physiologist  using 
the  proper  electrical  stimuli  can  study  the  nature  and  characteristics 


274  HUMAN  BIOLOGY 

of  the  artificially  incited  nerve  impulses,  both  in  excised  nerves  and 
also  in  those  in  their  normal  location  in  the  body.  In  the  latter  case, 
electrodes  placed  on  the  skin  may  be  used  to  stimulate  particular  nerves 
and  muscles.  (Figs.  91,  92.) 

Speed  of  Nerve  Impulse. — Experimental  studies  on  nerve  conduc- 
tion show  that  the  speed  of  the  impulse  is  subject  to  wide  variation. 
In  the  first  place,  the  rate  is  much  slower  in  a  cold-blooded  animal, 
like  the  frog,  than  it  is  in  a  warm-blooded  organism.  Thus  the  maxi- 
mum speed  of  the  nerve  impulse  in  the  frog  is  in  the  neighborhood  of 
140  ft.  per  second,  while  in  man  the  impulse  may  attain  a  speed  more 
than  twice  as  rapid,  or  almost  300  ft.  per  second.  Other  experiments 
have  shown  that  the  conduction  of  the  nerve  impulse  under  identical 
conditions  is  more  rapid  in  myelinated  than  in  nonmyelinated  nerves 
and  that  the  rate  also  varies  in  accordance  with  the  size  of  the  nerve; 
in  the  larger  nerves,  with  the  correspondingly  greater  number  of  nerve 
fibers,  the  impulse  is  conducted  more  rapidly  than  in  the  smaller  ones. 
The  figure,  given  just  above,  is  for  conduction  in  a  large  medulla  ted 
nerve.  The  measurements  taken  on  small  nonmyelinated  nerves  show 
an  impulse  rate  of  less  than  5  ft.  per  second  in  some  cases.  Again  the 
speed  of  the  nerve  impulse  may  be  progressively  reduced  by  decreasing 
the  temperature  of  the  nerve  fiber  until,  at  a  point  a  little  above  freez- 
ing, conduction  is  entirely  stopped.  With  all  conditions  equal,  no 
detectable  difference  exists  between  the  transmission  phenomena  of 
the  sensory  fibers  carrying  afferent  impulses  -toward  the  central 
nervous  system  and  those  occurring  in  the  motor  fibers  in  which 
efferent  impulses  are  conducted  toward  the  peripheral  effectors. 

In  the  earlier  discussion  of  muscle  function,  it  was  noted  that  the 
repeated » contraction  of  a  muscle,  without  adequate  rest  periods 
between,  involved  muscle  fatigue  which  considerably  altered  and, 
finally  entirely  prevented,  contraction  (page  184).  The  evidence  for 
fatigue  in  the  nerve  fiber  following  repeated  conduction  of  impulses 
over  considerable  periods,  even  up  to  several  hours,  is  not  clear.  In 
fact,  the  results  obtained  by  various  investigators  have  shown  that  a 
nerve  fiber  is  competent  to  receive  stimulation  and  to  conduct  the 
resulting  impulses  over  long  periods.  Slight  fatigue  immediately 
following  conduction,  as  shown  in  recent  experimental  results,  possibly 
decreases  conduction  somewhat.  Apparently  protecting  the  nerve 
against  too  rapid  onset  of  conduction  is  a  refractory  period  following  a 
stimulus,  during  which  a  complete  loss  of  excitability  occurs  and, 
accordingly,  another  stimulus  cannot  be  received  for  conduction.  The 
refractory  period  lasts  from  0.002  to  0.003  second,  after  which  the 
nerve  fiber  can  again  be  stimulated.  It  is  evident  that  the  refractory 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II)  275 

period,  brief  as  it  is,  is  sufficient  to  permit  the  recuperation  of  the 
conducting  elements  in  the  nerve  fiber.  The  question  as  to  the  nature 
of  the  activity  at  the  terminal  arborization  of  a  nerve  fiber,  which 
incites  activity  in  an  effector,  is  a  difficult  one.  How,  for  example,  does 
the  nerve  impulse  cause  a  contraction  of  a  muscle  fiber?  Evidence  is 
accumulating  to  indicate  the  presence  of  a  mediating  hormonal  sub- 
stance, acetylcholine,  presumably  secreted  by  the  terminals  of  the 
efferent  fibers,  which  brings  the  effector  element  into  activity  (page  495) . 

INTEGRATION 

Integration  is  necessarily  the  controlling  function  of  the  entire 
body.  It  has  its  structural  basis  in  the  neurons  of  the  central  nervous 
system  and  is  an  exclusive  function  of  this  major  division.  This  is 
unlike  the  condition  noted  in  connection  with  the  receptive  and  con- 
ductive functions  which  arc  shared  by  both  the  peripheral  and  central 
systems.  Integration  as  applied  to  the  nervous  system  is  a  very 
comprehensive  term  which  includes  a  number  of  basic  associated 
functions.  Integration  may  be  regarded  as  a  process  of  internal 
unification  through  which  all  the  diverse  functional  units  of  the  body 
are  caused  to  work  together  for  the  complete  unit,  the  individual. 
Involved  in  this  master  function  are  the  reception  and  interpretation  of 
the  sensory  impulses  from  every  area  of  the  body,  the  selection  of  the 
correct  routes  for  the  efferent  impulses,  and  the  regulation  of  tho 
effector  units  associated  with  all  typos  of  movement  and  secretion  so 
that  activity  may  be  initiated,  increased,  or  inhibited  as  the  conditions 
demand.  Thus  integration  is  responsible  for  the  coordinated  control 
of  all  the  diverse  elements  of  the  organism.  Many  of  the  integrative 
adjustments  do  not  involve  consciousness,  which  means  that  they  are 
not  referred  to  the  cerebrum,  but  this  function  is  always  ready  to  be 
called  into  play  as  a  part  of  the  integrative  phenomena. 

Reflex  Arcs. — Experimental  studies  concerned  with  the  analysis  of 
the  integrative  function  in  the  vertebrate  nervous  system  give  evidence 
that  the  reflex  action  is  an  essential  element.  Apparently  reflex 
actions  are  responsible  for  most,  if  not  all,  of  the  bodily  activities. 
A  reflex  action  may  be  defined  as  an  involuntary,  or  unconscious, 
action  occurring  in  some  element  of  a  peripheral  effector  in  response 
to  a  stimulus  received  by  a  sensory  receptor  and  transmitted  to  the 
central  nervous  system  over  the  afferent  nerve  fibers.  Thus  is  set  up 
the  so-called  reflex  arc,  which,  in  the  simplest  type  of  reflex,  might 
conceivably  consist  only  of  the  afferent  fibers  of  a  sensory  neuron 
carrying  the  impulse  to  the  spinal  cord  where  synapse  is  made  with 
the  dendrites  of  a  motor  neuron.  The  latter  transmits  the  impulse, 


276 


HUMAN  BIOLOGY 


now  efferent,  to  the  connected  muscle  fibers  which  act  in  response  to 
the  original  sensory  impulse. 

It  is  doubtful  if  a  reflex  arc  that  involves  only  two  neurons,  as  just 
described,  is  more  than  a  theoretical  possibility.  Undoubtedly  the 
normal  reflex  arc  is  much  more  complicated  and  involves,  in  addition 
to  the  arc  just  described,  conduction  up  and  down  fiber  tracts  of  the 
central  nervous  system  and  synapses  at  the  proper  levels  with  other 
neurons,  thus  bringing  into  play  an  integrative  action  by  the  central 
nerve  elements.  The  sensory  axon,  on  reaching  the  cord,  may  divide 
into  an  ascending  fiber  and  a  descending  fiber  from  each  of  which  side 
branches  (collaterals)  will  be  given  off  that  synapse  with  the  motor 
neurons  at  various  levels.  It  is  thus  possible  for  an  impulse  entering 


ASSOC/AT/ON 

NEURONS 


RECEPTOR 


MOTOR  FIBER 


NEURON 


FIG.  142. — Diagram  illustrating  the  components  of  a  reflex  arc.      (Buchanan,  "Elements 
of  Biology  "  Harper  <$•  Brothers,  after  Kuhn,  redrawn.) 

the  cord  over  a  single  sensory  neuron  to  reach  a  considerable  number  of 
motor  neurons  for  transmission  to  various  peripheral  effectors.  (Fig. 
142.) 

The  integrative  action  of  the  brain  and  spinal  cord  is  best  seen  in  a 
third  type  of  reflex  arc,  probably  the  most  important  of  all,  in  which  a 
third  type  of  neuron  is  involved,  namely,  the  association  (integrative) 
neurons.  These  lie  Vholly  within  the  central  nervous  system,  as 
previously  noted,  and  mediate  between  the  sensory  and  motor  neurons 
(page  254).  Thus,  the  incoming  sensory  impulses  are  received  by  the 
dendrites  of  the  association  neurons  and  conducted  by  them  directly 
to  the  proper  motor  neurons  or  even  to  the  brain  areas  for  complete 
integration  if  complicated  reactions  are  indicated.  The  latter  may 
possibly  involve  complete  integration  of  all  body  units.  Consider, 
for  example,  the  reactions  of  a  surgeon  who  accidentally  pricks  his 


BIOLOGY  OF  THE  NERVOUS  SYSTEM  (II)  277 

finger  with  an  infected  instrument  while  operating,  in  comparison 
with  his  reactions  when  he  jabs  his  finger  against  the  point  of  his  tiepin 
while  dressing.  In  the  latter  case,  the  finger  is  quickly  drawn  away 
by  the  simple  involuntary  reflex  action,  and  the  pinprick  is  dismissed 
with  little  thought.  But  the  injury  received  while  operating  is  con- 
sidered in  an  entirely  different  light  by  the  surgeon's  nervous  system. 
The  sensory  impulses  not  only  incite  the  simple  reflex  action  but  are 
instantly  projected  to  the  highest  centers  of  the  brain  where  decisions 
are  made  and  efferent  impulses  released  which  quickly  involve  all  the 
nervous  centers  and  the  associated  effectors.  There  is  a  unification, 
an  integration,  of  all  the  body  units  in  an  endeavor  to  find  the  best 
possible  solution  for  the  serious  problem  presented.  In  this  connec- 
tion, the  association  areas  of  the  brain  are  undoubtedly  of  basic 
importance  in  causing  the  union  of  all  the  nerve  elements  to  synthesize 
the  complete  concept.  The  sensory  impulses  from  the  injured  tissues 
and  the  visual  impulses  from  the  eye  reach  the  separate  localized  areas 
in  the  cerebral  cortex,  where  they  are  interpreted.  They  are  brought 
together  through  the  association  fibers.  The  function  of  memory 
is  also  involved.  The  surgeon  remembers  what  he  has  previously 
learned  about  the  dajigers  of  an  injury  by  an  infected  instrument. 
Possibly  all  the  neurons  of  the  entire  cortex  are  instantly  brought  into 
the  picture,  and  from  them  a  mental  image,  or  consciousness,  arises 
which  is  the  basis  of  the  efferent  impulses  released  to  the  effectors. 

Reflex  actions  resulting  from  internal  and  external  stimuli  are 
continuously  occurring  throughout  life.  It  is  the  way  in  which  the 
organism  solves  its  problems  and  adapts  itself  to  the  environment. 
A  very  important  question  which  puzzles  the  physiologists  and  psychol- 
ogists is  the  determination  of  the  limits  of  reflex  activities  in  the 
human  organism.  Some  would  say  that  all  our  activities,  physical 
and  mental,  are  essentially  reflex  in  nature,  differing  in  degree  of 
complexity  but  not  in  their  basic  nature.  Others  hold  strongly  to 
the  belief  that,  in  the  higher  mental  processes  involving  such  phenom- 
ena as  intelligence,  judgment,  will,  and  memory,  the  nervous  functions 
go  beyond  the  automaticity  of  the  reflex.  At  all  events,  except  in  the 
highest  mental  activities,  reflex  actions  involving  receptors,  afferent 
conductors,  adjusters,  efferent  conductors,  and  effectors  are  highly 
important  in  determining  the  response  of  the  body  to  stimuli  of  all 
kinds. 

It  has  previously  been  shown  that  reflexes  are  of  different  degrees  of 
complexity  in  accordance  with  the  number  of  neuronic  areas  involved 
in  the  reflex  arcs.  It  is  also  apparent  that  complex  reflex  actions 
involve  the  neurons  in  various  brain  areas  or  even  those  of  the  entire 


278 


HUMAN  BIOLOGY 


brain,  for  in  it  the  adjustments  take  place  that  are  necessary  in  bring- 
ing about  the  integrated  responses  to  the  stimuli  that  referred  to  it.  In 
a  general  way,  it  can  be  stated  that  the  spinal  cord  is  dominant  over  the 
peripheral  nerve  elements,  that  the  neuronic  areas  of  the  brain  are 
dominant  to  those  of  the  spinal  cord,  and,  finally,  that  the  neurons  of 
the  cerebral  cortex  are  the  chief  controlling  and  integrating  units  of 
the  nervous  system  and,  therefore,  of  the  entire  body.  Out  of  their 
activities  develop  the  highest  function  of  the  nervous  system,  that  of 
intelligence.  The  "intelligent"  cerebral  neurons  are  able  to  determine 
what  response  is  suitable  for  a  given  condition  and  so  can  inhibit  or 
augment  the  normal  reflex  or  initiate  an  independent  action  as  seems 
best. 


FIG.  143. — Diagrams  illustrating  the  avoiding  reaction  of  Paramecium.  At  1,  the 
animal  receives  stimulation  by  corning  into  contact  with  a  solid  object  A.  The  avoiding 
reaction  is  shown  in  2  to  6;  the  direction  of  movement  indicated  by  arrows.  (Woodruff, 
after  Jennings.) 

As  shown  above,  the  automaticity  of  reflex  actions,  in  general,  is 
noteworthy.  They  are  essentially  determined  so  that  a  certain 
stimulus  induces  predictable  response.  Bright  light  invariably  causes 
the  pupils  of  the  eye  to  contract,  whereas  in  dim  light  they  enlarge; 
and  so  it  goes  in  many,  many  instances.  This  is  the  field  of  the 
tropistic  reactions1  that  has  received  so  much  attention  during  recent 
years.  Tropisms  are  defined  as  orientations,  or  directed  reactions,  in  a 
field  of  force.  Such  automatic  reflexes,  apparently  established  in  the 
basic  pattern  of  the  organism,  are  termed  unconditioned  reflexes  and 
include  the  great  majority. of  the  reflex  actions.  (Fig.  143.) 

Another  group  of  reflexes,  known  as  conditioned  reflexes,  are 
acquired  individually  through  training.  Their  establishment  involves 
consideration  by  the  higher  nerve  centers  of  tue  cerebral  cortex  just 
as  does  anything  that  is  learned.  In  addition,  the  conditioned  reflex 

1  Consult  Appendix:  Tropisms. 


BIOLOGY  OF  THE  NERVOUS  SYSTEM   (11) 


279 


is  built  upon  the  pattern  of  the  inherent  unconditioned  reflex.  The 
distinctions  just  stated  are  well  shown  with  dogs  in  the  secretion  of 
saliva  when  food  is  taken  into  the  mouth.  Under  such  conditions, 
the  secretion  of  saliva  is  a  normal  unconditioned  reflex.  The  develop- 
ment of  a  conditioned  reflex,  based  upon  this  normal  saliva  reflex,  can 
be  accomplished  by  subjecting  the  animal  to  another  stimulus,  in  this 
case  the  ringing  of  a  bell  at  the  time  food  is  given.  Thus  an  associa- 
tion between  food  and  the  sound  of  the  bell  is  gradually  built  up  in 


FIG.  144. — Apparatus  for  developing  a  conditioned  reflex  in  a  dog,  as  devised  by 
Pavlov.  A,  carmula  inserted  in  the  choek  to  collect  saliva;  B,  metal  plate  on  which 
drt>ps  of  saliva  fall.  When  this  occurs  pressure  is  exerted  through  the  tambour  (C)  on 
column  of  air  in  tube  (Z)).  This  exerts  pressure  upon  another  tarnbour  (E)  which  carries 
the  writing  lever  (F)  which  is  arranged  to  make  a  record  on  the  smoked  paper  of  the 
kymograph,  indicating  the  flow  of  saliva.  H,  glass  graduate  for  measuring  the  quantity 
of  saliva,  (Watkeys,  Berry,  after  Yerkes  and  Morgulis.) 

the  cortex  after  some  30  to  40  feedings.  When  this  association  is 
reached,  a  reflex  flow  of  saliva  will  occur  at  the  sound  of  the  bell  and  in 
the  absence  of  food.  This  is  the  conditioned  reflex.  It  is  not  a 
permanent  reaction;  its  continuance  depends  upon  receiving  food 
when  the  bell  rings.  If  the  auditory  stimulus  is  repeated  several 
times  unaccompanied  by  food,  the  conditioned  reflex  with  secretion 
of  saliva  will  gradually  disappear.  Then,  the  animal  returns  to  the 
original  unconditioned  reflex  pattern  which  is  responsible  for  the 
secretion  of  saliva  when  food  is  received.  (Fig.  144.) 


Giiiielmus  Harveus 
de 

Generatione  Animalium. 


PLATE  XV. — The  allegorical  title  page  from  De  generatione  animalium^  one  of  the  last 
publications  of  William  Harvey,  who  discovered  the  circulation  of  the  blood.  This  work 
was  published  in  1651,  six  years  before  Harvey's  death.  Jove  sits  enthroned  with  his 
eagle  and  is  releasing  various  animals,  all  of  which  come  from  the  egg — Ex  ovo  omnia. 
(Redrawn  by  L.  Krause.) 


CHAPTER  XII 
THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (I) 

The  material  presented  so  far  in  this  volume  has  been  concerned 
with  the  structural  and  functional  features  of  the  various  major  organ 
systems  that  make  up  the  human  body  and  by  which  the  life  processes 
are  continuously  maintained.  But,  in  addition,  each  individual  is 
also  supplied  with  a  reproductive  mechanism  which  is  not  concerned 
with  the  maintenance  of  the  life  functions  in  the  individual  but  with 
the  continued  propagation  of  a  particular  type  of  organism,  the  species, 
through  the  production  of  new  individuals.  Thus,  from  the  stand- 
point of  the  species,  the  reproductive  system  is  essential,  but,  from  the 
standpoint  of  the  individual,  it  is  not  essential.  However,  it  is  the 
perpetuation  of  the  species  that  carries  the  great  weight  in  nature,  and 
so  we  find  that  the  process  of  reproduction  is  dominant  in  many  organ- 
isms, far  transcending  all  other  functions  in  structural  and  functional 
development.  Furthermore,  it  should  be  noted  that  reproduction  is  a 
unique  characteristic  of  protoplasm.  It  is  difficult  to  conceive  of 
living  matter  lacking  the  power  of  reproduction,,  and,  on  the  other 
hand,  it  is  just  as  difficult  to  conceive  of  any  type  of  nonliving  material 
that  could  possibly  possess  this  amazing  life  function. 

To  the  biologist,  reproduction  is  seen  as  a  process  primarily  based 
upon  the  power  of  growth  and  cell  division.  This  was  indicated  in 
the  opening  chapter  where  it  was  shown  that  the  growth  of  cells  results 
from  the  dominance  of  the  constructive  metabolic  processes  and  that, 
when  the  cell  has  grown  to  a  certain  characteristic  size,  it  divides  to 
form  two  daughter  cells.  The  latter,  under  favorable  conditions, 
increase  in  size  until  they  equal  that  of  the  parent  cell.  It  was  also 
shown  that,  basically,  cell  division  is  reproduction.  This  fact  is 
clearly  evident  in  the  unicellular  forms  of  plants  and  animals  in  which 
the  division  (binary  fission)  of  the  one-celled  body  produces  two 
independent  daughter  individuals.  In  the  multicellular  organisms, 
however,  the  relation'  between  reproduction  and  cell  division  is 
obscured  by  the  fact  that  cell  division  normally  results  in  adding 
additional  cells  to  the  body  oT  the  individual  rather  than  in  the  forma- 
tion of  additional  individuals.  Eventually,  however,  when  the  proper 
stage  of  development  has  been  reached,  new  individuals  are  produced 

281 


282 


HUMAN  BIOLOGY 


by  cell  division  occurring  in  a  particular  region  of  the  multicellular 
parent  individual  or  by  a  particular  kind  of  cell  division  resulting  in 
the  production  of  the  highly  specialized  gametes,  eggs  and  sperm. 


FIG.  145. — Asexual  reproduction  by  cell  division  (binary  fission)  in  Amoeba.     (Wolcott, 

modified  after  Schulze.} 


FIG.  146. — Asexual  reproduction  in  unicellular  yeast  plant  by  budding.     When  buds 
remain  attached,  temporary  colonies  are  formed.      X  1,500.     (Haupt.) 

It  will  be  well  to  leave  the  consideration  of  the  important  features 
associated  with  cell  division  until  the  next  chapter  and  continue  here 
with  the  general  Consideration  of  reproduction.  (Fig.  145.) 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (/)         283 


TYPES  OF  REPRODUCTION 

In  the  first  place,  it  should  be  noted  that  reproduction  is  either 
asexual  or  sexual.  The  basic  difference  between  these  two  methods  is 
uniparental  or  biparental  inheritance,  or,  in  other  words,  whether  the 
offspring  have  one  parent  or  two  parents.  In  asexual  reproduction, 
the  offspring  are  formed  by  the  growth  and  division  of  cells  from  one 
parent.  In  sexual  reproduction,  two  types*  of  individuals  are  con- 
cerned, male  and  female,  in  reproduction,  and  each  new  individual  is 
formed  following  the  union  of  two  gametes :  the 
male  sperm  and  the  female  egg.  This  consti- 
tutes fertilization  and  produces  a  biparental 
fertilized  egg,  or  zygote,  which  divides  repeat- 
edly and  gradually  attains  parental  size. 

ASEXUAL  REPRODUCTION    i 

As  an  example  of  asexual  reproduction  in 
the  multiccllular  organism,  consideration  may 
be  given  to  the  process  of  budding,  which  is 
characterized  by  the  rapid  growth  and  division 
of  the  cells  in  certain  regions  of  the  parental 
organism  with  the  consequent  formation  of  a 
new  individual,  attached  at  first  to  the  body 
of  the  parent  but,  in  time,  separating  as  an 
independent  organism.  Budding  is  an  estab- 
lished method  of  reproduction  even  in  unicel- 
lular organisms,  as  in  the  classic  example  of  the 
yeast  cell  where  a  tiny  area  of  the  cytoplasm  of 
the  spherical  parental  cell  enlarges  to  form  a 
knob-like  protuberance  which  later  separates  as 
an  independent  daughter  cell.  In  the  multi- 
cellular  animals,  common  examples  of  budding 
are  found  in  Hydra  and  other  Coelenterates. 
The  buds  in  Hydra  are  formed  in  various  regions  in  the  wall  of  the 
tubular  body  and  gradually  develop  from  a  minute  group  of  cells  to 
multicellular  structures  almost  as  large  as  the  parent  Hydra  before 
they  finally  separate.  (Figs.  146,  147.) 

Closely  related  to  asexual  reproduction  by  budding  is  asexual 
reproduction  through  the  regenerative  process.  Thus,  in  many 
organisms,  it  is  possible  to  divide  the  body  into  several  pieces  and  have 
each  piece  gradually  grow  into  a  complete  organism.  This  process  is 
very  familiar  in  the  plant  world  where  standard  methods  of  propaga- 


FIG.  147. — -Asexual 
reproduction  by  budding 
in  the  primitive  meta- 
aean,  Hydra.  Bud  is 
seen  at  the  right  of  the 
parent  animal,  a,  ex- 
panded; b,  contracted. 
X  12.  (Wolcott.) 


284 


HUMAN  BIOLOGY 


tion  involve  the  cultivation  of  cuttings  from  the  parent  plant,  as  in 
various  common  house  plants  or,  in  the  fields,  as  in  the  potato  or  sugar 
cane.  But  many  examples  of  reproduction  through  regeneration  exist 
also  in  the  animal  kingdom.  Again  one  of  the  best  examples  is  found 
in  Hydra,  but  representatives  of  various  higher  groups,  particularly  the 
worms,  also  exhibit  great  regenerative  abilities.  Thus  in  the  marine 
flatworm,  Linens  socialiSj  it  has  been  shown  that  pieces  with  a  calcu- 
lated volume  only  1/200,000  of  the  normal  size  may  regenerate  a  com- 
plete individual.  In  this  and  many  other  instances  it  is  apparent 
that  regeneration  is  equivalent  to  asexual  reproduction.  (Fig.  148.) 


A  u    B  C 

FIG.  148. — Asexual  reproduction  in  a  marine  flatworm  (Linens  socialis)  by  fission. 
A,  mature  worm;  B,  division  into  nine  parts,  each  of  which,  as  shown  in  C,  regenerates 
to  form  a  normal  worm  which  soon  attains  full  size.  (Woodruff,  after  Coe.) 

Speaking  generally,  it  may  be  said  that  the  power  of  regeneration 
in  the  animal  kingdom  becomes  increasingly  limited  in  the  higher 
types  and  finally  reaches  a  condition,  as  in  man,  in  which  very  little 
regeneration  of  th&  highly  developed  tissue  and  organs  is  possible. 
To  the  biologist,  it  is  apparent  that  restriction  of  regeneration  is 
directly  associated  with  the  increased  tissue  differentiation  that  is 
characteristic  of  the  higher  animal  types.  That  is  to  say,  the  greater 
the  differentiation  in  an  organism  the  less  the  ability  to  regenerate 
missing  portions.  In  such  cases,  as  will  be  seen  later,  when  injury 
occurs,  unless  the  functioning  of  essential  organs  is  disturbed,  the 
continuity  of  the  tissues  is  reestablished  by  the  development  of  a 
connective,  or  scar,  tissue  which  repairs  the  wound  area  but  does  not 
have  the  functional  or  structural  characteristics  of  the  tissues  it 
replaces.  Thus  repair  but  not  regeneration  takes  place.  Differentia- 
tion becomes  increasingly  manifest  during  embryonic  development  as 
the  ultimate  adult  condition  gradually  proceeds  out  of  the  relatively 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (I)         285 

undifferentiated  embryonic  condition.  But  this  is  a  subject  that  may 
be  well  left  for  more  detailed  consideration  when  the  embryological 
processes  are  studied. 

Binary  fission,  budding,  and  regeneration,  as  discussed  in  the 
paragraphs  just  above,  are  purely  asexual  in  nature  and,  in  addition, 
are  not  dependent  upon  the  development  of  specialized  reproductive 
cells  of  any  type.  There  are  two  other  well-known  methods  of  asexual 
reproduction,  however,  namely,  spore  formation  and  parthenogenesis, 
which  involve  the  formation  of  special  reproductive  cells.  These 
are  worthy  of  some  attention  because  of  their  widespread  use  in  the 
living  world.  In  the  case  of  spore  formation,  the  organism  at  some 
period  or  periods  in  its  life  history  typically  produces  enormous  num- 


I 

FIG.  149. — A  common  bread  mold  (Rhizopus  nigricans):  horizontal  branch  with  two 
groups  of  erect  spore-bearing  branches  (hyphae) .  One  of  the  spore  cases  is  discharging 
spores.  X  15.  (Haupt.) 

bers  of  microscopic  cellular  bodies,  or  spores,  each  of  which  is  capable, 
when  given  the  proper  conditions,  of  developing  into  an  independent 
fully  formed  individual.  Typically,  spores  are  provided  with  coverings 
of  a  particular  type  which  make  these  reproductive  cells  very  resistant 
to  unfavorable  environmental  conditions,  and  so  they  are  ideal  for 
wide  dissemination  and  later  development  when  the  conditions  become 
favorable.  (Fig.  149.) 

Spore  formation  in  the  plant  kingdom  is  of  almost  universal 
occurrence.  In  all  the  higher  plant  types,  asexual  spore  formation 
is  linked  with  sexual  reproduction  in  the  complete  life  cycle,  the  latter 
consisting  of  an  asexual  spore-forming  generation  which  alternates  with 
the  sexual  gamete-producing  generation.  Thus,  the  spores  produce 
the  sexual  generation,  whereas  the  zygote,  formed  by  the  fusion  of  the 
male  and  female  gametes,  gradually  develops  into  the  spore-producing 
generation.  This  constitutes  alternation  of  generations  which,  though 
not  of  universal  occurrence,  is  extraordinarily  Apdespread  in  the  plant 
kingdom.  Furthermore,  alternating  sexual  and  asexual  generations 
are  not  uncommon  in  the  lower  animal  groups.  Spore  formation,  in 


286  HUMAN  BIOLOGY 

the  animal  kingdom,  appears  to  be  segregated  in  an  important  class  of 
unicellular  animals,  the  Sporozoa,  which  are  of  major  importance  as 
disease-producing  parasites.  Spore  formation  in  this  class  is  typically 
associated  with  complex  life  cycles  in  which  sexual  phenomena  are 
involved.  (Figs.  150,  250.) 

Finally,  asexual  reproduction  may  occur  through  the  parthenoge- 
netic  development  of  the  female  gamete,  or  egg.  Examples  of  parthe- 
nogenesis have  long  been  recognized  as  occurring  normally  in  nature, 
particularly  in  representatives  of  the  great  insect  group.  The  par- 
thenogenetic  development  of  the  male  bee,  or  drone,  (page  448)  is  the 
best  known  example  and,  in  fact,  has  long  stood  as  a  classic  instance  of 
natural  parthenogenesis.  As  a  result  of  biological  experimentation  on 


Spore- Plant  Spore        Sexual  Plant         Gametes        Spore- Plant 

.  (SporophyteX  (Gametophyty  (Sporophyte): 

FIG.  150. — Alternation  of  sexual  and  asexual  generations  in  a  fern.     (Sinnott.) 

the  eggs  of  various  animals,  several  examples  of  artificial  parthenogene- 
sis have  been  discovered,  beginning  with  the  original  work  of  J.  Loeb 
on  the  Echinoderm  egg  some  40  years  ago.  This  experimenter  found 
that  it  was  possible  to  incite  the  eggs  of  the  sea  urchin  to  4evelop  in  the 
absence  of  sperm  if  the  proper  chemical  stimulants  were  used.  In 
later  years,  the  original  results  of  Loeb  have  been  greatly  extended, 
so  it  is  now  established  that  various  types  of  egg  will  begin  to  develop 
under  the  influence  of  artificial  stimuli,  even  including  those  of  highly 
developed  vertebrates,  like  the  frog  or  rabbit.1 

SEXUAL  REPRODUCTION 

Thus  far  our  discussion  of  reproduction  has  been  confined  to  asexual 
reproduction  in  which  the  new  individual  arises  by  the  cellular  activity 
of  one  parent,  that  is,  uniparental  inheritance.  But  reproduction  is 

9 

1  The  production  of  Mature  rabbits  from  eggs  induced  to  develop  through 
artificial  parthenogenesis  was  announced  by  Dr.  Gregory  Pincus  in  the  Proceedings 
of  the  National  Academy  of  Sciences,  November,  1939. 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (I)         287 

otherwise  in  the  higher  animals,  for  asexual  reproduction  has  been 
replaced  by  sexual  reproduction,  and  each  new  individual  is  accordingly 
supplied  with  a  legacy  of  materials  from  the  two  parents,  the  condition 
of  biparental  inheritance.  The  basic  difference  between  the  male 
parent  and  the  female  parent  lies  in  the  production  of  a  characteristic 
type  of  reproductive  cell,  or  gamete,  the  sperm  or  the  egg.  The 
gametes  are  usually  produced  in  great  numbers  by  both  sexes  but  more 
particularly  in  the  male.  Underlying  gamete  formation,  just  as  in 
any  type  of  cell,  are  the  basic  processes  of  growth,  cell  division,  and 
differentiation. 

It  should  not  be  thought  that  sexual  reproduction  is  entirely 
confined  to  the  higher  plants  and  animals,  for  numerous  instances 
of  it  are  found  in  the  unicellular  organisms.  In  the  simplest  examples, 
fusion  occurs  between  two  cells,  or  gametes,  of  equal  size  and  uniform 
structural  characteristics  (isogamy),  but,  even  in  the  lowest  groups 
of  living  organisms,  examples  may  be  found  in  which  the  male  and 
female  gametes  are  as  clearly  differentiated  as 'in  the  higher  organisms. 
Accordingly,  in  fertilization,  fusion  occurs  between  a  fr£e-swimming 
sperm  and  a  passive  egg  cell  of  more  or  less  typical  cellular  pattern 
(anisogamy).  Differentiation  of  the  gametes,  for  example,  is  very 
apparent  in  the  unicellular  sporozoon,  Plasmodium  vivax,  which  is 
responsible  for  malaria,  one  of  the 
most  dangerous  of  the  human 
diseases.  In  Plasmodium,  sexual 
reproduction  takes  place  in  the 
body  of  the  mosquito.  The  male 
and  female  gametes  formed  by  this  a  b  c 

sporozoon  give  every  indication  of  •   r    Fia.isi.— Successive  stages  in  the 

^  ^.  .  .  fusion  of  two  equal-sized  cells  (isogamy) 

as  great  differentiation  as  those  of  of  the  unicellular  algae,  Chlamydomonas: 

higher  types.  Thus  it  is  clear  ^  basis  of  sexual  reproduction. 

,             „                 ,         ,                                    ,  (Watkeys,  Stern.) 

that,    from   the   lowest   animal 

groups  up  to  the  highest,  sexual  reproduction  stands  as  a  well-estab- 
lished phenomenon,  but  sexuality  is  of  increasing  importance  in  the 
higher  animal  types  and  in  the  vertebrates  constitutes  the  only  normal 
method  of  reproduction.  In  the  plant  kingdom,  on  the  other  hand, 
asexual  reproduction  by  spore  formation  and  by  regeneration  remains 
dominant  in  all  groups.  (Figs.  151,  250). 

Also  the  hermaphroditic  condition,  as  in  the  earthworm,  in  which 
the  same  individual  bears  gonads  for  the  production  of  both  male 
and  female  gametes,  disappears  with  the  advent  of  the  vertebrate. 
Numerous  examples  of  alternation  of  generations  and  also  of  herma- 
phroditism  are  well-known  in  even  the  highest  invertebrate  groups. 


288 


HUMAN  BIOLOGY 


Another  interesting  variation  of  sexual  reproduction,  particularly 
prominent  in  the  great  molluscan  group,  is  the  reversal  of  sex  which 
occurs  in  the  life  cycle  of  each  individual.  Thus  the  organism  matures 
first  as  a  male,  later  changes  to  a  female  capable  of  producing  female 
gametes.  Reversal  of  sex  does  not  occur  normally  in  vertebrate 
animals,  but  an  authentic  instance  is  recorded  in  which  a  normal  egg- 
laying  hen  changed  to  a  rooster  producing  fertile  sperm.  The  reversal 
of  sex  in  this  instance  was  due  to  the  destruction  of  the  ovarian  tissues 
following  a  tubercular  infection.  This  raises  the  question  that  has_ 
received  great  attention  in  the  last  quarter  of  a  century,  namely,  the 


>//erve  cord 


deferens 


FIG.  152. — Diagram  illustrating  male  and  female  reproductive  organs  in  the  earth- 
worm, a  hermaphroditic  organism.  They  lie  near  the  anterior  end,  segments  IX  to  XV 
as  indicated.  (Wolcott,  after  Wieman.) 

effect  of  the  gonadal  endocrine  secretions  in  altering  the  sexual  ch&rac- 
teristics  of  the  individual.  Some  consideration  has  already  been  given 
to  this  in  the  previous  chapter  on  Secretion  (page  115).  (Figs.  152, 
153.) 

Due  to  the  fact  that,  as  indicated  above,  reproduction  in  the  verte- 
brates is  exclusively  sexual  and,  furthermore,  that  the  sperm  and  eggs 
are  always  produced  in  separate  individuals,  the  variable  features  of 
vertebrate  reproduction  are  largely  associated  with  the  mechanics 
of  fertilization  and  development  of  the  zygote.  The  essential  feature  of 
fertilization  is  always  the  actual  union,  or  amphimixis,  of  the  chromatin 
materials  present  in  the  sperm  nucleus  with  the  chromatin  carried  in 
the  egg.  nucleus.  Thus  fertilization,  since  it  involves  amphimixis, 
makes  biparental  inheritance  possible;  the  nuclear  contribution  from 
each  parent  uniting  to  form  a  common  nucleus,  or  synkaryon,  in  the 
fertilized  egg,  and  each  parent  making  equal  contribution  to  the 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (I)         289 

characteristics  of  the  new  individual.  The  fertilization  of  the  verte- 
Wate  egg  may  occur  externally  or  internally.  External  fertilization, 
that  is,  outside  the  body  of  the  female,  is,  however,  the  more  common 
and  primitive  method  in  the  water-living  forms.  In  such  types,  both 
the  eggs  and  sperm  are  adapted  for  temporary  survival  and  union  in  a 
water  environment.  Since  the  gametes  of  the  two  sexes  ripen  at  the 
same  time  and  tremendous  numbers  of  them  are  usually  discharged  in 
fairly  close  proximity  to  each  other,  the  chances  are  that  the  actively 
swimming  sperm  will  come  into  contact  with  the  eggs,  thus  bringing 


FIG.  153. — Schematic  diagram  illustrating  the  sequence  of  changes  in  the  gonads  of 
an  oyster  during  sex  reversal,  oc,  oocytes  (large  cells) ;  sp>  male  cells  (small  dots) ; 
/,  gonad  without  sex  differentiation;  Fra,  goriad  of  young  animal,  bisexual;  HM,  male 
phase  but  somewhat  hermaphroditic;  (M 2,  M*)  later  stages  in  the  development  of  the 
male  gonad;  F,  first  female  phase;  later  female  stage  shown  in  F*\  F,  M2t  F2,  M*,  succes- 
sive stages  in  sex  reversal;  ctt  connective  tissue;  ep,  epithelium;  gc,  genital  gland. 
(Skull,  after  Coe.) 

about  fertilization  and  embryonic  development.  With  external 
fertilization  and  development,  the  reproductive  responsibilities  of  the 
parents  cease  with  the  formation  and  liberation  of  the  gametes.  In 
certain  instances,  notably  in  the  frog,  though  fertilization  and  develop- 
ment are  external  as  indicated,  a  temporary  pairing  (amplexus)  of  the 
male  and  female  frogs  occurs  at  the  time  the  eggs  and  sperm  are  dis- 
charged, which  insures  a  very  high  percentage  of  fertilization. 
(Fig.  154.) 

Numerous  examples  of  animals  are  found,  with  both  water  and 
land  habitats,  in  which  internal  fertilization  of  the  eggs  occurs,  but 
the  ensuing  embryonic  development  is  external,  for  the  fertilized  eggs 
leave  the  body  of  the  mother  shortly  after  fertilization  and  continue 


290  HUMAN  BIOLOGY 

their  development  externally.  In  the  warm-blooded  birds,  it  is 
necessary  to  supply  the  developing  eggs  with  the  proper  body  tem- 
perature, or  incubate  them,  in  order  for  development  to  continue  after 
the  eggs  are  laid.  Internal  fertilization  always  requires  the  pairing, 
or  copulation,  of  the  parents,  during  which  process  the  transfer  of 
sperm  from  the  male  to  the  female  takes  place.  Also,  internal  fertili- 
zation requires  the  development  of  external  male  genital  organs  for 
the  transference  of  the  sperm  to  the  female,  as  well  as  the  elaboration 
of  the  female  organs  concerned  with  the  reception  and  conduction  of 
the  sperm  to  the  eggs.  In  some  animals,  exemplified  by  the  queen 
bee,  storage  of  sperm  is  an  important  function  accomplished  by  the 
presence  of  special  sperm  receptacles. 

In  the  mammalian  female,  the  reproductive  mechanism  is  further 
complicated  by  the  retention  of  the  fertilized  egg  in  a  special  cavity, 
the  uterus  or  womb,  modeled  out  of  a  portion  of  the  egg  tubes,  or  ovi- 
ducts, through  which  the  eggs  pass  on  their  way  to  the  exterior.  The 
proper  care  and  nourishment  for  the  embryo  developing  in  the  uterus 
present  a  number  of  difficult  problems,  the  solutions  for  which  have 
been  found  through  the  formation  of  a  combination  fetal  and  maternal 
structure,  the  placenta,  which  is  described  below. 

It  is  evident  to  the  biologist  that  the  underlying  factor  primarily 
responsible  for  variation  in  the  development  of  the  vertebrate  egg  lies 
in  providing  adequate  nutrition  for  the  embryo.  The  eggs  of  various 
vertebrates  show  great  variation  in  the  amount  of  nutritive  materials 
or  yolk  that  they  contain  at  the  time  of  fertilization.  The  embryo 
that  develops  from  a  type  of  egg  in  which  very  little  reserve  food  is 
stored  must  necessarily  be  provided  with  some  method  for  quickly 
securing  nourishment  from  the  environment.  An  example  of  this  may 
be  noted  in  the  embryos  of  the  lowly  starfish  which,  lacking  reserve 
food  in  the  eggs,  are  able  to  form  a  primitive  nutritive  system  for 
securing  and  utilizing  outside  nutritive  materials  a  few  hours  after 
fertilization.  With  the  exception  of  the  mammals,  the  eggs  of  the 
vertebrates  are  heavily  yolked,  and,  accordingly,  the  embryo  is  able 
to  develop  for  a  considerable  period  without  the  necessity  of  seeking 
food  supplies  from  the  surrounding  environment. 

Food  storage  reaches  a  high  peak  in  the  hen's  egg  and  those  of  other 
birds.  This  condition  makes  it  possible  for  the  developing  embryo  to 
remain  sealed  up  in  the  original  eggshell  for  the  entire  incubation 
period  of  three  weeks,  at  the  conclusion  of  which  it  breaks  through  the 
shell,  or  hatches,  as  a  well-developed  active  individual.  On  the  other 
hand,  the  mammalian  egg  is  practically  free  from  stored  food  but  gives 
evidence  during  its  development,  as  will  be  shown  later,  of  being 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (I)         291 

closely  related  to  the  heavily  yolked  eggs  of  the  birds  and  reptiles. 
The  lack  of  stored  food  in  the  mammalian  egg  makes  it  necessary  for 
the  zygote  to  secure  nourishment  from  outside  sources  very  quickly, 
and  this  is  accomplished  by  the  rapid  development  of  an  outer  layer 
of  nutritive  cells,  the  trophoblast,  which  has  the  ability  to  secure  the 
essential  food  materials  from  the  tiny  area  in  the  maternal  uterine  walls 
in  which  the  microscopic  embryo  is  embedded. 

But  the  amount  of  stored  yolk  not  only  governs  the  nutritive 
requirements  of  the  embryo;  it  also  varies  the  pattern  of  early  develop- 
ment with  respect  to  the  numbers  and  arrangements  of  the  cells 
formed  by  the  successive  divisions  of  the  zygote.  For  the  stored  yolk 
is  inert,  nonliving  material,  and  its  presence  in  the  egg  in  any  con- 
siderable amount  retards  cell  division  and  considerably  modifies  the 
early  embryonic  stages.  Thus  we  find  that  eggs  with  a  comparatively 
small  amount  of  yolk  evenly  distributed  through  the  cytoplasm 
(homolecithal  eggs)  exhibit  a  total  cleavage  (holoblastic  cleavage) 
marked  by  the  formation  of  daughter  cells  which  are  fairly  uniform  in 
size. 

Eggs  containing  a  considerable  amount  of  yolk  show  a  tendency  for 
the  nutritive  materials  to  be  concentrated  at  one  pole  (nutritive  or 
vegetal  pole)  and  to  leave  the  cytoplasm  of  the  animal  pole  relatively 
free.  Eggs  of  this  latter  type  are  known  as  telokcithal  eggs.  When 
the  mass  of  yolk  is  not  too  large,  as  is  the  case  in  the  frog's  egg  described 
below,  the  cleavage  is  holoblastic  just  as  noted  in  the  homolecithal  eggs, 
but  a  distinct  lag  occurs  in  the  cleavage  planes  when  they  pass  through 
the  yolked  area  of  the  vegetal  pole;  this  tends  to  result  in  the  forma- 
tion of  unequal-sized  cells.  Finally,  in  eggs  with  a  very  large  amount 
of  yolk,  as  in  the  hen's  egg,  the  vegetal  pole  is  greatly  enlarged;  the 
animal  pole  comparatively  small.  The  cleavage  is  partial,  or  mero- 
blastic,  and  entirely  confined  to  the  animal  pole.  It  will  be  unneces- 
sary to  consider  cleavage  types  further  at  present,  for  in  the  following 
description  of  vertebrate  reproduction,  as  shown  in  the  frog,  bird,  and 
man,  the  various  important  differences  will  be  indicated. 

DEVELOPMENT  OF  THE  FROG 

A  close  relationship  exists  between  the  excretory  and  reproductive 
systems  in  the  vertebrate  organism.  This  fact  is  seen  to  particular 
advantage  in  the  urogenital  system  of  the  male  frog  in  which  the  paired 
testes  lie  in  close  association  with  the  kidneys.  In  fact,  the  testes  and 
kidneys  are  directly  connected  by  numerous  fine  ducts,  the  vasa 
efferentia,  which  convey  the  ripened  sperm  from  each  testis.  On 
leaving  the  testis,  these  sperm  ducts  pass  directly  into  the  kidney  tis- 


292 


HUMAN  BIOLOGY 


sues  where  they  connect  with  the  ducts  leading  to  the  urogenital  canals. 
The  latter  extend  from  the  kidneys  to  the  cloaca  and  thence  to  the 
exterior.  And  so  the  urogenital  canals,  as  the  name  indicates,  serve 
as  common  ducts  for  the  passage  of  urine  from  the  kidneys  and  sperm 
from  the  testes.  (Fig.  154.) 

Male. — The  testes  in  the  frog  are  seen  in  gross  structure  as  yel- 
lowy-white, capsule-shaped  bodies,  about  l/±  in.  in  length,  situated  on 
the  ventral  side  and  near  the  anterior  end  of  each  kidney.  Project- 
ing anteriorly  from  each  testis  is  the  so-called  fat  body  with  numerous 


'1DUCT 


OVARY 


FAT  BODY- 


BLADDER- 


FIG.  154. — Urogenital  systems  of  the  male  (right)  arid  female  (left)  frog.  In  the 
female  only  one  ovary  and  oviduct  are  shown.  In  the  male  the  testis  and  kidney  at  the 
right  have  been  opened  to  show  the  course  of  the  vasa  efferentia  through  the  kidney. 
(Redrawn  by  L.  Krause  from  ShulL  Modified.} 

tubular  branches  projecting  in  various  directions.  The  fat  bodies  are 
not  directly  concerned  with  the  reproductive  processes  but  serve  as 
storehouses  for  excess  nutriment  which  later  can  be  utilized  by  the 
body  tissues  as  needed,  chiefly,  perhaps,  in  germ  cell  formation.  Pass- 
ing now  to  the  histology  of  the  testis,  a  microscopic  examination  of 
sectioned  material  shows  that  it  consists  of  a  mass  of  very  fine,  coiled 
tubules,  intermingled  with  abundant  blood,  nervous,  and  connective 
tissue  elements.  These  are  the  essential  seminiferous  tubules  in  which 
great  numbers  of  the  sperm  develop.  They  have  their  origin  in  the 
primordial  germ  cells  present  in  the  walls  of  the  tubules. 

Female. — Turning  now  to  a  consideration  of  the  female  reproduc- 
tive organs  of  the  frog,  which  may  also  be  regarded  as  fairly  represen- 
tative of  the  vertebrates  in  general,  they  will  be  found  to  consist  of  a 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (I)         293 

pair  of  egg-producing  gonads,  or  ovaries;  a  closely  associated  pair  of 
fat  bodies  very  similar  to  those  already  noted  in  the  male;  and,  finally, 
a  pair  of  oviducts  which  carry  the  eggs  from  the  ovaries  to  the  cloaca. 
The  frog  ovary  is  essentially  a  sac-like  structure  with  a  fluid-filled 
central  area.  The  innermost  layer  of  the  covering  tissues  constitutes 
the  germinal  epithelium  in  which  the  undeveloped  germ  cells,  or 
oogonia,  have  their  origin.  As  in  the  testis,  these  unmatured  germ 
cells  conform  in  general,  to  the  cellular  pattern  of  body  cells,  and  then 
they  pass  through  a  series  of  developmental  stages  (oogonia,  primary 
oocytes,  and  secondary  oocytes)  until  the  mature  egg  stage  is  finally 
attained.  Due  to  the  gradual  storing  of  yolk  material,  as  noted 
above,  the  egg  cells  increase  in  size  until  the  mature  frog's  eggs  are 
several  times  the  size  of  the  typical  body  cells.  (Fig.  154.) 

The  eggs  reach  their  full  development  in  the  frog  at  only  one  period 
each  year.  This  is  normally  in  the  early  spring,  at  which  time  the  two 
ovaries,  distended  with  great  numbers  of  the  large  egg  cells,  fill  most  of 
the  space  in  the  abdominal  cavity.  At  the  proper  stage  of  maturity, 
the  eggs  break  directly  through  the  thin  ovarian  wall  in  large  numbers 
and  are  drawn  by  ciliary  action  into  the  opening  of  the  corresponding 
oviduct.  It  is  important  to  note  that  no  direct  connection  exists 
between  an  ovary  and  its  oviduct  as  is  the  case  in  the  testis  and  vasa 
efferentia.  Furthermore,  there  is  no  connection  between  the  oviducts 
and  the  ureters  from  the  kidneys.  The  oviducts  are  a  separate  pair 
of  tubes,  and  they  function  solely  in  the  transfer  of  the  eggs.  Each 
oviduct  is  a  comparatively  large,  convoluted  tube  connected  directly 
with  the  cloaca  and  ending  anteriorly  in  close  proximity,  but  slightly 
anterior,  to  the  ovary  of  the  corresponding  side.  The  ovarian  end  of 
each  oviduct  is  enlarged,  funnel-shaped,  and  lined  with  ciliated  cells. 
The  current  set  up  in  the  body  fluids  by  the  ciliary  action  draws  the 
eggs  into  the  oviducts  and  starts  them  on  their  journey  from  the  body. 
The  cloacal  ends  of  the  oviducts  are  enlarged  to  provide  an  egg-storage 
space  near  the  opening  into  the  cloaca  in  which  the  eggs  remain  for 
some  time  before  being  discharged  through  the  cloaca.  The  eggs, 
when  released  from  the  ovary,  are  enclosed  in  a  thin  transparent 
covering,  the  vitelline  membrane.  As  they  pass  through  the  oviduct, 
additional  covering  layers  of  a  gelatinous  material  are  secreted  by 
glandular  cells  present  in  the  lining  of  the  oviducts.  These  gela- 
tinous layers  remain  intact  until  a  well-developed,  active  embryo  is 
formed  which  is  able  to  force  its  way  through  them.  I 

Pairing  of  the  male  and  female  frogs  occurs  when  the  gametes  are 
mature.  During  amplexus  there  is  synchronous  discharge  of  the  sperm 
and  eggs  into  the  water,  and  this  greatly  enhances  the  possibility  of 


294  HUMAN  BIOLOGY 

fertilization.  In  their  movements  through  the  water,  there  is  the 
possibility  that  the  sperm  are  attracted  to  the  eggs  by  chemical  sub- 
stances released  by  the  eggs.  Such  sperm-attracting  substances  have 
been  demonstrated  in  certain  organisms. 

Development  of  the  Frog's  Egg. — The  telolecithal  frog's  egg  is 
visible  to  the  naked  eye  as  a  tiny  sphere,  about  the  size  of  a  pellet  of 
tapioca.  It  is  more  or  less  clearly  divided  into  a  dark-colored  animal 
pole  and  a  light-colored,  yolk-containing  vegetal  pole.  Following  fer- 
tilization, in  which  the  union  of  male  and  female  nuclei  to  form  the 
synkaryon  occurs,  the  fertilized  egg,  or  zygote,  begins  to  divide  mitoti- 
cally  into  daughter  cells  which  remain  in  close  contact,  enclosed  within 
the  original  vitelline  membrane.  Should  it  happen  that  some  of  the 
eggs  are  not  fertilized  owing  to  a  failure  of  the  sperm  to  reach  them, 
they  will,  of  course,  not  develop  and  soon  begin  to  disintegrate.  The 
first  cell  division,  or  cleavage,  of  the  zygote,  which  begins  shortly 
after  fertilization,  is  first  visible  externally  in  the  animal  pole.  The 
plane  of  cleavage  is  vertical;  that  is,  it  passes  through  both  the  animal 
and  vegetal  poles,  dividing  the  embryo  into  two  equal-sized  daughter 
cells.  The  second  plane  of  cleavage  is  also  vertical  and  divides  each  of 
the  first  two  cells  in  half,  thus  forming  four  equal-sized  cells.  The 
third  plane  of  cleavage  is  horizontal,  at  right  angles  to  the  first  two,  is 
entirely  in  the  animal  pole  region,  and  results  in  the  formation  of  the 
eight-cell  stage;  consisting  of  four  small  cells  from  the  animal  pole,  and 
the  same  number  of  larger  cells  from  the  vegetal  pole.  Thus,  begin- 
ning at  the  third  cleavage,  an  apparent  retardation  of  cell  division 
occurs  in  the  vegetal  pole  due  to  the  greater  concentration  of  inert 
yolk  material  in  this  region.  This  becomes  increasingly  evident  in 
the  succeeding  divisions.  (Fig.  155A  to  D.) 

Beginning  at  about  the  24-cell  stage,  formation  of  the  one-layered 
blastula  is  indicated  internally  by  the  development  of  a  central 
cavity,  or  blastocoel,  situated  largely  in  the  animal  pole.  The 
microscopic  study  of  sections  through  an  embryo  in  the  early  blastula 
stage  shows  that  the  blastocoel  is  enclosed  above  by  a  single  layer  of 
the-pigmented  cells  of  animal  pole  and,  below,  is  bounded  by  the  larger 
cells  of  the  vegetal  pole.  Thus  the  blastula  stage  of  the  embryo  may 
be  described  as  a  one-layered  organism  built  around  a  central  cavity, 
or  blastocoel.  This  first  cellular  layer  of  the  embryo  is  known  as 
ectoderm  and  constitutes  one  of  the  three  primary  germ  layers  from 
which  all  of  the  tissues  and  organs  of  the  adult  organism  gradually 
arise.  The  other  two  primary  germ  layers,  endoderm  and  mesoderm, 
develop  somewhat  later,  as  will  be  shown  in  the  following  paragraphs. 
(Fig.  155D  to  F.) 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (/)         295 

Following  the  one-layered  blastula  stage,  the  next  great  landmark 
in  embryonic  development  is  the  two-layered  gastrula  stage  which  is 
formed  by  a  turning  in,  or  invagination,  of  rapidly  dividing  ectoderm 
cells  to  form  a  new  inner  layer  of  cells,  the  endoderm,  the  second  of  the 
primary  germ  layers.  In  homolecithal  eggs,  the  process  of  gastrulation 
may  be  crudely  compared  with  pushing  in  the  wall  of  a  lightly  inflated, 
thin-walled  rubber  ball  with  the  thumb.  If  the  wall  is  thus  pushed 
in  until  it  reaches  the  opposite  pole  of  the  ball,  it  is  easy  to  see  (1)  that 
a  two-layered  condition  results;  (2)  that  the  original  cavity  of  the  ball, 
corresponding  to  the  blastocoel  of  the  blastula,  is  obliterated;  and 
(3)  that  a  new  cavity  surrounding  the  thumb  is  formed.  This  latter 
cavity  in  the  gastrula  is  lined  with  the  invaginated  endoderm  arid 
will  shortly  function  as  a  primitive  nutritive  cavity  or  enteron. 
(Fig.  1550,  H.) 

In  the  telolecithal  egg,  as  in  the  frog,  the  process  of  gastrulation 
is  considerably  modified  and  retarded  by  the  inert  mass  of  yolk  in  the 
vegetal  pole.  Basically,  of  course,  gastrulation  is  the  same  in  all 
types  of  egg  in  that  it  results  in  the  formation  of  a  two-layered  embryo. 
In  the  frog  embryo,  this  condition  is  reached  in  part  by  the  over- 
growth and  in  part  by  the  synchronous  invagination  of  the  ectoderm 
cells  from  the  animal  pole.  As  a  result  of  the  overgrowth,  the  cells 
of  the  vegetal  pole  are  gradually  and  increasingly  covered  by  the  pig- 
mented  ectoderm  cells  moving  down  from  the  animal  pole.  Synchro- 
nously, an  underlying  endoderm  layer  is  being  formed  within.  Invagi- 
nation begins  in  a  definite  region  of  the  egg,  known  as  the  gray  crescent, 
which  lies  in  a  restricted  area  of  the  animal  pole.  This  invaginating 
area  soon  becomes  circular  in  outline  and  is  gradually  reduced  in 
diameter  as  the  ectoderm  cells  cover  over  more  and  more  of  the  vegetal 
pole.  Finally,  at  the  conclusion  of  gastrulation  only  a  tiny  area,  the 
yolk  plug,  of  the  light-colored  yolk  cells  remains  visible  externally, 
surrounded  by  the  circular  opening  in  the  ectoderm.  This  opening 
through  which  the  yolk  plug  is  seen  is  known  as  the  blastopore.  It 
indicates  the  posterior  end  of  the  embryo  and  the  approximate  posi- 
tion of  the  future  anal  opening.  (Fig.  155#.) 

If  the  embryo  were  transparent,  it  would  be  possible  to  observe 
under  the  microscope  that  other  important  internal  changes,  in  addi- 
tion to  the  formation  of  the  endoderm,  were  under  way.  Thus, 
before  gastrulation  has  proceeded  very  far,  the  third  primary  germ 
layer,  or  mesoderm,  begins  to  develop  dorsally  between  the  previously 
formed  ectoderm  and  endoderm.  And  so  the  two-layered  gastrula 
gradually  changes  into  the  final  three-layered  condition.  In  a  trans- 
parent embryo,  it  would  also  be  possible  to  see  the  gradual  develop- 


296 


HUMAN  BIOLOGY 


ment  of  the  endoderm,  as  it  increases  from  a  few  cells  to  a  distinct 
layer  with  many  cells.  And,  just  as  was  noted  above  in  the  develop- 
ment of  the  homolecithal  egg,  the  continued  development  of  the  endo- 
derm results  in  the  obliteration  of  the  original  blastocoel  and  the 
formation  of  a  new  endodermal-lined  cavity,  the  enteron,  which  is  the 
forerunner  of  the  alimentary  canal.  The  latter  gradually  evolves  into 
a  tubular  structure,  but  not  until  considerably  later  does  it  open  to 


Medu/tary 
/groove 


F  E 

Notochorcf 

.Neural  tube 


Moufh 
in  votginctfion 


Archenferon 


Yolk 


FIG.  155. — Early  stages  in  the  development  of  the  frog.  A,  one  cell;  B,  two  cells; 
(7,  four  cells;  D,  blastula,  many  cells;  E,  section  of  D  showing  blastocoel,  F,  late  blastula; 
Gf;  gastrula,  early;  H,  medullary  plate;  7,  formation  of  neural  tube  and  elongation  of 
body;  «/,  tail  bud  stage;  jfiT,  median  section  through  /.  (Wolcott,  from  various  sources.} 

the  exterior  through  mouth  and  anus.  Another  important  landmark 
in  vertebrate  development  is  the  formation  of  an  anteroposterior 
rod-like  axis,  the  notochord,  which  differentiates  from  the  mesoderm 
along  the  median  dorsal  line,  between  the  ectoderm  and  endoderm. 
The  notochord  is  the  original  foundation  for  the  segemented  bony 
vertebral  column  that  later  develops.  (Fig.  155K.) 

An  external  examination  of  the  embryo  near  the  close  of  gastrula- 
tion  reveals  a  definite  flattening  of  the  future  dorsal  surface  to  form 
the  medullary  plate.  This  constitutes  the  first  visible  evidence  of 
the  establishment  of  a  central  nervous  system.  Observations  on  the 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (I)         297 

living  embryo  show  that  the  edges  of  the  medullary  plate  gradually 
become  thickened  and  elevated  above  the  surface  of  the  embryo  to 
form  the  medullary  folds.  A  little  later,  these  two  medullary  folds, 
extending  the  length  of  the  body,  meet  in  the  dorsal  mid-line  and  fuse, 
thus  forming  an  ectodermal  neural  tube.  From  the  latter,  the  brain, 
spinal  cord,  and  other  elements  of  the  highly  differentiated  nervous 
system  gradually  arise.  (Fig.  155/7,  7.) 


Oral sucker 


FIG.  156. — Late  stages  in  the  development  of  the  frog.  A,  embryo  with  oral  sucker 
for  attachment  to  water  plants  as  in  B\  C,  external  gills;  Z>,  much  later  stage  at  the 
beginning  of  metamorphosis,  with  internal  gills  and  hind  legs;  E,  embryo  with  large 
hind  legs  and  shrinking  tail;  F,  young  frog  with  four  legs  and  stump  of  tail;  Gt  adult  grass 
frog  (Rana  pipiens).  (Wolcott,  from  various  sources.) 

And  now  the  embryo  definitely  begins  to  lose  the  spherical  shape 
of  the  original  egg  stage  and  to  stretch  out  in  an  anteroposterior 
direction,  and,  shortly,  definite  body  regions  can  be  identified.  Ante- 
riorly, the  general  shape  of  the  head  is  indicated,  while  posterior  to  it 
on  each  side  of  the  .body,  the  ectoderm  is  noticably  thickened  and 
elevated  to  form  prominent  gill  arches  through  which,  later,  the  paired 
lateral  openings,  or  gill  slits,  break  through  the  body  wall  into  the  ante- 
rior pharyngeal  region  of  the  alimentary  canal.  Certain  surface 
swellings  in  the  head  region  give  evidence  of  sense  organ  formation. 


298  HUMAN  BIOLOGY 

A  depression  on  the  ventral  surface  of  the  head  region  indicates  the 
position  of  the  future  mouth  opening,  and  a  similar  depression  at  the 
posterior  end  of  the  body,  just  above  the  original  blastopore,  marks 
the  rudiment  of  the  anus.  Posterior  to  the  mouth  area,  a  crescent- 
shaped  region  indicates  the  developing  ventral  sucker  which  is  of  use 
in  the  later  tadpole  stage.  A  rounded,  knob-like  dorsal  projection  at 
the  extreme  posterior  end  of  the  body  is  known  as  the  tail  bud.  It 
gradually  extends  posteriorly  and  develops  into  the  long,  muscular 
tail.  During  all  the  changes  so  far,  the  embryo  has  not  taken  in  any 
food  from  the  environment;  nourishment  has  been  secured  by  the 
continued  utilization  of  food  materials  stored  in  the  vegetal  pole  of 
the  egg  during  ovarian  development.  Even  at  the  tail  bud  stage,  the 
ventral  and  posterior  regions  of  the  embryonic  body  still  contain  a 
considerable  quantity  of  available  yolk.  (Figs.  155J;  156A.) 

Rapid  growth  continues,  and  in  a  few  days  after  fertilization, 
depending  to  a  considerable  extent  upon  the  temperature  conditions 
in  the  environmental  waters,  the  embryos  attain  the  free-swimming 
tadpole  stage  in  which  head,  trunk,  and  tail  are  definite  structural 
entities.  The  active  tadpoles  soon  hatch;  that  is,  they  emerge  from 
the  surrounding  gelatinous  capsules  originally  secreted  in  the  oviducts. 
For  a  short  time,  external  respiratory  organs,  the  filamentous,  branched 
gills,  are  present.  These  develop  as  outgrowths,  or  projections,  from 
the  gill  arches  on  each  side  of  the  body,  just  posterior  to  the  head. 
The  external  gills  persist  only  temporarily  and  are  soon  replaced 
by  internal  gills  lying  in  the  gill  slits.  Water  currents  continually 
pass  through  the  gill  slits  en  route  from  the  pharynx  to  the  exterior. 
(Fig.  156C.) 

In  the  young  tadpoles,  rudiments  <3f  the  eyes,  nose,  and  ears  can 
be  clearly  identified  in  the  head  region.  Dorsally,  along  each  side  of 
the  body,  the  outlines  of  the  primitive  muscle  segments,  or  myotomes, 
can  be  seen  through  the  thin  outer  covering  of  ectoderm.  The 
myotomes  develop  from  the  mosoderm  layer  which  forms  in  two  sheets, 
one  lying  on  either  side,  that  is,  to  the  right  and  left  of  the  neural  tube 
and  notochord.  The  mesodermal  sheet  on  each  side  of  the  body 
grows  ventrally  and  soon  becomes  differentiated  into  a  dorsal  portion, 
the  vertebral  plate,  and  a  ventral  portion,  the  lateral  plate.  The 
vertebral  plates  soon  show  evidences  of  segmentation  and  become 
divided  into  the  segmented  myotomes  which  later  become  associated 
with  the  vertebral  column.  The  lateral  plates  do  not  become  seg- 
mented but  extend  ventrally  on  each  side  of  the  body  until  they 
finally  meet  in  the  mid-ventral  line,  thus  forming  a  complete  layer  of 
mesoderm  lying  just  under  the  ectoderm.  As  the  mesodermal  tissue 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (/)         299 


of  the  lateral  plates  is  extending  ventrally,  it  is  also  dividing  into  an 
outer  somatic  layer  and  an  inner  splanchnic  layer.  The  somatic  layer 
of  mesoderm  is  responsible,  primarily,  for  the  musculature  of  the  body 
wall,  while  the  splanchnic  layer  encloses  the  endoderm  of  the  primitive 
gut  and  gives  rise  to  the  supporting,  vascular,  and  muscular  elements 


NOTOCHORD 

NEURAL  TUBE 
AORTAn  ' 


BODY  WALL 
MYOTOME 

PRONEPHR1C  DUCT 

PRONEPHRIC  TUBULE 
GLOMUS 
LATERAL  PLATE 
ALIMENTARY  CAAJAL 

MESODERM  (SOMATIC) 
COELOM 


-MESODERM  (SPLANCHNIC) 
u ENDODERM 


•MESOAJEPrtRJC  DUCT 

•MESONEPrtRIC  TUBULE 
GLOMERULUS 


FIG.  157. — Diagrams  illustrating  the  general  body  plan  (A.)  of  a  primitive  vertebrate 
embryo  with  pronephros,  in.  which  the  tubules  open  into  the  coelom;  and  a  more  highly 
developed  vertebrate  embryo  (B),  as  in  the  frog,  with  mesoriephros,  in  which  the  meso- 
iiephric  tubules  collect  wastes  from  the  blood  stream  through  glomeruli  as  well  as  from 
the  coelorn.  The  differentiation  of  the  rnesoderm  to  form  inyotomes  and  lateral  plates 
is  clearly  shown.  (Redrawn  by  L.  Krause  from  Wilder,  "History  of  the  Human  Body," 
Henry  Holt  &  Company,  Inc.  Slightly  modified.) 

of  the  alimentary  canal.  The  space  that  develops  through  the  split- 
ting and  separation  of  the  somatic  and  splanchnic  layers  of  mesoderm 
becomes  the  coelom.  (Fig.  157.) 

Tissue  Differentiation. — Having  traced  the  main  features  of 
development  in  the  frog  from  the  fertilized  egg  to  the  well-developed, 
freo-swimming  tadpole,  it  will  be  desirable  at  this  point  to  summarize 


300  HUMAN  BIOLOGY 

the  fate  of  the  primary  germ  layers,  ectoderm,  endoderm,  and  meso- 
derm.  Thus,  embryological  studies  show  that  the  superficial  layers, 
or  epidermis,  of  the  skin  as  well  as  the  basic  tissues  of  the  exoskeletal 
structures  are  of  ectodermal  origin.  The  crowning  achievement  of 
the  ectoderm,  however,  unquestionably  lies  in  the  formation  of  the 
all-important  nervous  system  and  associated  sensory  elements  which 
permeate  every  possible  niche  and  thus  establish  control  throughout 
the  entire  organism.  As  is  evident  from  the  material  presented  in  the 
previous  chapter,  the*  nervous  system  is  not  only  the  most  highly 
differentiated  of  all  the  organ  systems  but  the  one  assigned  to  admin- 
ister all  of  the  essential  .functions  of  the  living  organism. 

The  endoderm  is  the  great  nutritive  layer  of  the  body.  The 
primitive  gut,  or  enteron,  which  begins  its  development  during  gas- 
trulation  is  entirely  endodermal,  as  has  been  noted  above,  and  the 
functional  cellular  lining  tissue  in  the  permanent  alimentary  canal  is 
directly  derived  from  these  original  endoderm  cells.  A  portion  of  the 
mouth*  cavity  (stomodaeum)  and  a  small  area  at  the  extreme  posterior 
end  of  the  alimentary  canal  (proctodaeum)  is,  however,  lined  by  ecto- 
derm. The  endoderm,  in  addition  to  forming  the  functional  lining 
of  the  alimentary  canal,  is  also  responsible  for  the  formation  of  several 
important  organs  that  are  more  or  less  closely  associated  with  the 
nutritive  system,  including  the  liver,  pancreas,  thyroid  glands,  lungs, 
and  bladder.  All  of  these  organs  develop  in  much  the  same  way  by  an 
outgrowth  of  the  endodermal  wall  of  the  enteron  at  an  early  stage  to 
form  either  single  or  paired  rudiments.  In  this  connection,  a  basic 
fact  should  be  recognized,  namely,  that  the  vertebrate  organs,  in 
general,  are  not  wholly  formed  from  a  single  tissue  but  represent  a 
structural  and  functional  mosaic  of  various  tissues.  Thus  the  hepatic 
cells  of  the  liver  -are  endodermal  in  origin,  but  the  complete  organ 
also  contains  vascular  and  connective  tissues  derived  from  the  meso- 
derm  together  with  nerve  elements  that  are  ectodermal  in  origin. 

From  the  important  mesoderm  layer  comes  three  great  organ 
systems :  the  vascular,  muscular,  and  skeletal,  all  of  which  are  widely 
distributed  throughout  the  entire  body.  Previous  mention  has  been 
made  of  the  division  of  the  mesoderm  into  the  vertebral  and  lateral 
plates  (page  298).  The  vertebral  plate  myotomes,  with  the  somatic 
layers  formed  from  the  lateral  plates,  are  responsible  for  the  muscle 
tissues  present  in  the  body  wall  and  in  the  appendages,  as  well  as  for 
the  connective  tissues  and  vascular  elements.  These  same  tissues 
surrounding  the  endodermal  lining  of  the  alimentary  canal  develop 
from  the  splanchnic  mesoderm  (Fig.  157.4).  Both  the  somatic  and 
splanchnic  mesoderm  layers  contribute  to  the  formation  of  the  peri- 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (/)         301 

toneum,  which  forms  a  continuous  lining  layer  throughout  the  coelom 
as  well  as  a  covering  tissue  for  the  various  organs!  From  the  peri- 
toneum arise  the  mesenteries  by  means  of  which  the  various  organs 
are  suspended  from  the  walls  of  the  coelom.  Finally,  the  functional 
elements  of  the  urogenital  system  are  mesodermal  in  origin.  (Fig.  157.) 
The  study  of  the  life  cycle  of  the  frog,  and  of  the  great  majority  of 
amphibia,  shows  that  the  aquatic  fish-like  tadpole  stage  is  only  tem- 
porary. Metamorphosis  occurs  after  some  weeks,  and  the  tadpole 
changes  into  the  air-breathing,  four-legged  adult  frog.  Experimental 
work  has  definitely  shown  that  the  metamorphic  processes  in  the 
amphibia  are  incited  and  regulated  to  a  great  extent  by  the  thyroid 
hormone.  The  chief  structural  changes  in  metamorphosis  are  con- 
cerned with  the  development  of  the  forelegs  and  hindlegs,  the  degenera- 
tion of  the  tail,  and  changes  in  the  alimentary  canal  and  the  respiratory 
mechanism.  The  nutritive  system  of  the  tadpole,  with  a  very  long, 
coiled  intestine,  is  equipped  for  an  herbivorous  diet.  The  meta- 
morphic changes  remodel  this  system  and  adapt  it  for  the  more  con- 
centrated carnivorous  diet  of  the  adult.  Metamorphic  changes  are 
also  of  a  radical  nature  with  respect  to  respiration.  The  gill  tissues 
degenerate,  and  so  the  animal  is  no  longer  able  to  secure  oxygen  from 
the  water.  Air  must  be  forced  into  the  lungs  which,  though  present  for 
some  time,  have  not  hitherto  functioned.  (Fig  156D  to  F.) 

DEVELOPMENT  OF  THE  CHICK 

With  the  main  features  of  amphibian  development  in  mind,  we  may 
now  pass  to  a  consideration  of  avian  embryology  which,  though  con- 
forming to  the  main  features  exhibited  in  the  frog,  presents  certain  dis- 
tinctive features  of  particular  importance  for  acquiring  a  satisfactory 
understanding  of  human  development,  our  final  goal.  The  character- 
istic developmental  processes  of  the  birds  are  basically  grounded  in  the 
provisions  for  the  internal  fertilization  and  for  nourishing  the  embryo 
and  represent  a  clim£x*in  the  storage  of  food  in  the  telolecithal  egg  as 
has  been  indicated  above. 

Reproductive  System. — We  may  begin  our  discussion  with  the 
reproductive  system  of  the  hen,  which,  it  may  be  stated,  is  an 
unpaired  structure  in  the  adult  developed  originally  on  the  left  side 
of  the  body.  The  corresponding  organs  of  the  right  side  are  present 
in  the  embryo  but  undergo  degeneration  in  the  female  before  maturity 
is  reached.  This  condition  does  not  obtain  in  the  male;  both  of  the 
testes  and  the  associated  ducts  persist  and  function  in  the  adult. 
The  ovary,  examined  with  the  naked  eye, 'is  seen  to  consist  for  the 
most  part  of  a  mass  of  projecting  yellowish  globules  of  various  sizes 


302 


HUMAN  BIOLOGY 


in  which  the  eggs  are  undergoing  development.  Lying  near  the  ovary 
is  a  large  convoluted  oviduct  which  ends  anteriorly  in  a  ciliated 
opening,  the  ostium.  Posteriorly,  the  oviduct  connects  with  the 


FIG.  158. — Reproductive  system  of  the  hen.  The  single  ovary  is  shown  above,  with 
numerous  ovarian  eggs  in  various  stages  of  development  (01,  02,  03,  o4).  The  ovi- 
duct with  opening,  or  ostium  (os),  is  showil  with  two  eggs  (Oi,  Oa),  though  normally  only 
one  egg  passes  through  the  oviduct  at  a  time.  The  oviduct  has  been  opened  at  one  place 
to  show  the  egg  (6)2)  with  blastoderm  (b)  and  the  albumin  (a)  which  is  being  secreted. 
c,  cicatrix;  cl,  cloaca;  m,  mesentery;  r,  rectum;  u1  uterus.  (Wieman,  after  Coste-Duval.) 

cloaca.  The  oviduct  consists,  first,  of  the  ostium,  just  noted.  It  is 
followed  by  a  glandular  portion  lined  with  secreting  cells  that  form 
the  so-called  white,  or  albumin,  enclosing  the  yolk  and  also  further 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (/)         303 

posteriorly,  the  outer  membranes  and  the  calcareous  shell  as  well. 
All  of  the  materials  are  formed  as  secretions  from  the  glandular  lining 
cells.  A  short,  thin-walled,  distal  region  of  the  oviduct  posterior  to 
the  secreting  areas  leads  directly  into  the  cloaca.  (Fig.  158.) 

The  egg  cell  or  yolk,,  as  it  is  commonly  termed,  when  released  from 
the  ovary  is  drawn  at  once  into  the  near-by  ostium  to  begin  its  passage 
down  the  oviduct.  The  yolk  material  is  enclosed  in  a  transparent 
vitelline  membrane.  Provided  mating  with  the  male  has  previously 
occurred,  the  sperm  deposited  in  the  cloaca  of  the  female  will  have 
found  their  way  up  the  oviduct.  Accordingly,  it  is  possible  for  the 
egg  to  be  fertilized  shortly  after  it  enters  the  oviduct.  Cleavage  in 
the  zygote  then  begins.  As  noted  earlier,  the  large  amount  of  food 
material  present  in  the  hen's  egg  makes  total  division  impossible  so 
that  only  partial,  or  meroblastic,  cleavage  takes  place.  Cell  division 
is  confined  to  the  tiny  disc-like  blastoderm  lying  on  the  upper  surface 
of  the  yolk.  As  cell  division  continues  and  additional  food  material 
is  utilized,  the  blastoderm  increases  in  size  and  gradually  spreads  over 
and  encloses  the  inert  mass  of  yolk.  Thus  at  the  end  of  about  5  or  6 
days'  incubation  it  will  be  found  that  the  actively  growing  tissues  of 
the  blastoderm  completely  cover  the  yolk  area.  (Fig.*  158.) 

Development  of  the  Hen's  Egg. — In  the  hen,  ovulation  occurs 
independently  of  pairing,  but,  of  course,  such  unfertilized  eggs  are 
infertile.  As  the  egg  passes  down  the  oviduct,  the  lining  cells  secrete 
several  layers  of  albumin.  A  portion  of  the  albumin  lying  next  to  the 
vitelline  membrane  is  drawn  out  to  form  a  pair  of  spiral-shaped  chala- 
zae,  situated  at  opposite  poles  of  the  yolk.  The  chalazae  prevent 
the  yolk-mass  from  turning  end  for  end  in  the  egg  but  at  the  same  time 
permit  it  to  revolve  with  the  shell  when  the  latter  is  turned  over  and 
over  on  its  short  axis.  Thus,  when  the  egg  comes  to  rest,  the  blasto- 
derm is  always  found  lying  above  the  yolk  and  can  be  seen  when  an 
opening  is  made  through  the  shell.  In  the  great  majority  of  hen's  eggs 
there  is  a  definite  orientation  of  the  anteroposterior  axis  of  the  embryo 
so  that,  when  the  observer  places  the  large  end  of  the  egg  to  his  left, 
the  axis  of  the  embryo  will  be  at  right  angles  to  the  long  a'xis  of  the 
egg,  with  the  head  end  pointed  away  from  the  observer.  (Fig.  159.) 

After  the  layers  of  albumin  are  secreted  around  the  yolk,  other 
glandular  cells  lying  distally  secrete  a  double  shell  membrane  which 
forms  a  flexible  resistant  covering  around  the  albumin.  At  the  large 
end  of  the  egg,  these  two  membranes  are  separated  so  that  an  air  space 
lies  between  them.  Finally,  the  hard  calcareous  eggshell  is  also  formed 
as  a  secretion,  after  which  the  egg  is  ready  to  pass  into  the  cloaca  and 
out  of  the  body  through  the  cloacal  opening.  The  passage  down  the 


304  HUMAN  BIOLOGY 

oviduct  normally  requires  24  hours,  so  that,  if  the  egg  is  fertilized  in 
the  anterior  end  of  the  oviduct,  the  blastoderm  will  have  reached  the 
24-hour  stage  of  development  at  the  time  the  egg  is  laid.  Further 
development  of  the  blastoderm  cells  then  ceases  until  the  proper 
temperature  is  supplied.  This  is  approximately  103°F.,  corresponding 
to  thatx)f  the  parental  body  tissues.  The  few  cells,  which  constitute 
the  partially  developed  embryo  in  the  blastoderm  of  the  egg  at  this 
stage  of  development,  will  remain  dormant  for  several  days  without 
injury  and  then  begin  active  developmental  processes  again  when 
incubated  in  the  normal  manner  by  a  hen  or  artificially  in  an  incubator 
if  the  proper  temperature  is  supplied.  (Fig.  159.) 


FIG.  159. — Diagram  illustrating  the  internal  structure  of  the  hen's  egg.  a,  air 
chamber;  b,  blastoderm.  Arrow  points  towards  the  head  end  of  the  embryo,  c, 
chalaza;  da,  fa,  albumin;  im,  inner  shell  membrane;  o,  the  yolk,  the4  egg  proper,  formed 
in  the  ovary;  om,  outer  shell  membrane;  s,  shell.  (Wieman.) 

The  continued  division  of  the  blastoderm  following  fertilization 
finally  results  in  the  formation  of  a  great  number  of  irregularly  shaped 
cells  with  the  smallest  ones  near  the  center  of  the  blastoderm.  Shortly, 
these  primitive  ectoderm  cells  arrange  themselves  in  a  bridge-like 
structure  which  overlies  a  small  cavity,  the  blastocoel,  near  the 
upper  surface  of  the  yolk  mass  and  just  underneath  the  original  blasto- 
derm. This  chick  blastocoel  is  comparable  to  that  found  in  the  bias- 
tula  stage  of  the  frog  embryo  as  previously  described.  The  assemblage 
of  'ectoderm  cells  above  the  blastocoel  in  the  chick  embryo  constitute 
the  blastula  proper.  Thus  the  blastula  of  the  chick  consists  of  a  flat- 
tened or  disc-like  layer  of  cells  lying  above  the  blastocoel  and  also 
above  the  large  yolk  mass,  rather  than  a  spherical  body  of  cells  enclos- 
ing the  blastocoel  as  noted  in  the  frog.  (Fig.  155J?.) 

Continuing  our  description  of  early  chick  development,  it  will  next 
be  found  that  the  ectoderm  cells,  in  the  region  of  the  blastoderm 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (/)         305 


destined  to  become  the  posterior  end  of  the  animal,  begin  to  divide 
more  rapidly  than  in  other  areas.  As  a  result,  a  sheet  of  cells  is  turned 
under  the  ectoderm  and  starts  to  invade  the  cavity  of  the  blastocoel  as 
a  second,  or  endoderm,  layer;  synchronously  forming  a  new  cavity, 
the  enteron  or  primitive  gut. 
This  is,  of  course,  the  process  of 
gastrulation.  The  new  endoderm 
layer  spreads  anteriorly  and 
laterally  under  the  outer  ecto- 
derm, and  soon  both  of  these 
layers  extend  to  the  periphery  of 
the  blastoderm.  At  about  the 
18-hour  stage  of  incubation,  a 
thicker  region,  the  primitive 
streak,  is  clearly  marked  in  the 
center  of  the  blastoderm,  and  this 
indicates  the  establishment  of  the 
embryonic  anteroposterior  axis. 
The  primitive  streak  is  primarily 
due  to  a  concentration  of  ectoderm 
cells  along  the  median  line  of  the 
blastoderm.  It  is  in  this  region 
that  the  mesoderm  layer  begins 
to  develop.  The  first  mesoderm 
cells  migrate  laterally  from  the 
posterior  part  of  the  primitive 
streak,  but,  as  more  and  more 
are  formed,  they  spread  anteriorly 


Ps 


FIG.  160. — Early  stages  in  the  develop- 
ment of  the  chick  embryo.  A,  primitive 
streak  stage  (18  hours) ;  B,  head  process 
stage  (20  hours);  C,  embryo  with  seven 
pairs  of  somites  (24  hours),  ac,  ammo- 
cardiac  vesicle;  apt  area  pellucida;  av,  area 
vasculosa;  hf,  head  fold;  hp,  head  process; 
m,  margin  of  mesoderm;  pa,  proamnion; 
ps,  primitive  streak.  (After  Wieman.) 


as  well  as  laterally  thus  in  time 
forming  a  complete  layer  of  meso- 
derm between  the  ectoderm  and 
the  endoderm  (Fig.  160A). 

Just  anteriorly  to  the  primitive  streak  region,  a  concentration  of 
ectoderm  cells  forms  an  anterior  thickening,  known  as  the  head  proc- 
ess, which  soon  involves  all  three  germ  layers.  The  region  of  the 
blastoderm  in  which  the  head  process  develops  contains  the  rudiments 
of  various  embryonic  body  structures.  The  next  notable  develop- 
ment in  this  region  is  seen  in  the  arrangement  of  the  ectoderm  cells  to 
form  the  medullary  plate  and,  from  the  latter,  the  formation  of  a 
definite  neural  tube  by  the  elevation  of  the  lateral  edges  and  their 
fusion  in  the  median  line  as  previously  described  in  the  frog.  Coinci- 
dent with  these  activities,  notochord  formation  and  the  differentiation 


306  HUMAN  BIOLOGY 

of  the  mesoderm  to  form  the  segmental  myotomes  are  started.  The 
latter,  as  in  the  frog,  soon  show  a  division  into  dorsal  and  lateral 
portions  and  also  the  formation  of  somatic  and  splanchnic  mesoderm 
with  the  coelom  between.  All  of  these  developmental  features  are 
well  under  way  during  the  first  24  hours  of  incubation  (Fig.  1605.) 

From  the  description  just  given  of  the  blastula  and  gastrula  stages, 
it  is  evident  that  the  really  distinctive  feature  of  early  chick  develop- 
ment is  the  fact  that  the  embryo  in  the  blastoderm  area  is  spread  out 
flat  on  the  surface  of  the  yolk.  As  a  result,  the  rudiments  of  the 
various  embryonic  structures  are  formed  in  right  and  left  halves  lying 
on  either  side  of  the  median  anteroposterior  axis  of  the  body.  These 
lateral,  organ-developing  areas  constitute  the  extra-embryonic  regions. 
At  the  20-  to  24-hour  stage  of  incubation  the  embryo  begins  a  process 
of  folding  which  gradually  separates  it  from  the  yolk  mass  and  brings 
the  right  and  left  half  of  each  organ  in  contact  along  the  mid-ventral 
line  where  they  unite  to  form  the  complete  structure.  The  first  of 
these  folds  (head  fold)  appears  just  anterior  to  the  head  region.  A 
little  later,  right  and  left  lateral  folds  are  indicated  which  move  in  from 
the  sides,  and,  finally,  a  tail  fold  is  formed  which  progresses  anteriorly. 
The  final  results  are  shown  in  the  formation  of  the  various  organs 
by  the  union  of  the  two  halves  and  in  the  almost  complete  separation 
of  the  body  of  the  embryo  from  the  yolk  material.  Thus,  after  about 
96  hours'  incubation,  it  is  found  that  the  well-formed  embryo  is 
attached  to  the  yolk  sac  by  only  a  short  tubular  yolk  stalk.  The 
latter  is  filled  with  blood  vessels  through  which  the  circulating  blood, 
laden  with  absorbed  food  materials  from  the  extra-embryonic  areas, 
passes  into  the  body  of  the  embryo.  (Fig.  167A.) 

Two-day  Chick  Embryo. — By  the  end  of  the  second  day  of  incuba- 
tion,, the  embryo  has  attained  a  greatly  increased  size  as  compared 
with  that  of  the  tiny  blastoderm.  The  anterior  end  of  the  embryonic 
body  is  folded  from  the  yolk  sac  and  is  lying  on  its  left  side,  whereas 
the  posterior  part  of  the  body  is  still  flat  on  the  yolk.  As  a  result, 
the  embryo  is  shaped  somewhat  like  a  reversed  question  mark,  with  the 
anterior  end  of  the  body  bent  (cervical  flexure)  toward  the  right  at  an 
angle  of  almost  90  deg.  The  rudiments  of  the  various  organ  systems 
are  now  established;  and  some  of  them,  notably  the  vascular  system, 
are  functioning.  Particularly  striking  in  the  living  two-day  embryo 
is  the  beating  heart,  which  at  this  stage  is  connected  with  blood  vessels 
running  through  the  tissues  of  the  embryonic  body  and  also  out  to  the 
yolk  regions.  The  growth  of  the  extra-embryonic  endoderm  has  put 
this  nutritive  layer  in  contact  with  the  yolk  so  that  the  latter  can  be 
digested  and  absorbed.  Also  the  extension  of  the  extra-embryonic 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (/)         307 


HB 


mesoderm  to  the  covering  of  the  yolk  sac  makes  possible  the  formation 

of  a  dense  network  of  blood  vessels  for  transporting  the  food  materials 

absorbed  from  the  yolk  to  the  growing  tissues  of  the  embryo.     The 

two-day  heart  has  two  chambers:  an  auricle  and  a  ventricle.     The 

auricle  receives  the  extra-embryonic  blood,  rich  with  absorbed  food 

material  and  laden  with  supplies  of 

oxygen  which  have  permeated  through 

the  shell  and  membranes  of  the  egg. 

From  the  auricle  the  blood  passes  into 

the  ventricle,  and  then  it  is  quickly 

forced   out    through    the    connecting 

arteries   to   all   the  body  tissues  and 

back  to  the  yolk  sac  for  new  supplies 

of  food  and  oxygen.     Gill  slits,  though 

non-functional  in  the  chick,  are  pre- 

sent at  this  stage  of  development,  and 

the  blood  leaving  the  ventricle  is  routed 

between  them.     They  soon  disappear 

as  definite  structures.     (Fig.  161.) 

The  embryo  is  mostly  enclosed  by 
this  time  in  a  fluid-filled  amniotic 
cavity.  The  amnion  starts  to  develop 
in  the  ectoderm  anterior  to  the  head 
region  and  grows  posteriorly  over  the 
embryo  as  an  ectodermal  sheet. 
Finally,  with  the  aid  of  the  lateral 
and  posterior  amniotic  folds,  a  two- 
layered  sac  is  formed  over  the  entire 
embryo.  And  so,  from  the  beginning 
of  the  second  day  of  incubation  to  the 
fourth  day,  the  embryo  is  being  sepa- 
rated from  the  yolk  sac  by  folds  that 
grow  underneath  and  at  the  same  time 
enclosed  by  the  amniotic  tissues  that 
lie  above.  The  rudiment  of  another 
important  embryonic  membrane,  the  allantois,  is  first  seen  at  about 
the  72-hour  stage  as  an  endodermal  outgrowth  from  the  primitive  gut, 
just  posterior  to  the  yolk  stalk.  The  allantois  finally  forms  a  large 
vascular-walled  sac  which  lies  close  to  the  outer  shell  of  the  egg  and 
functions,  primarily,  in  the  respiratory  interchange.  The  yolk  sac, 
the  amnion,  and  the  allantois  constitute  the  important  embryonic 
membranes  of  the  chick.  (Fig.  167.) 


FIG.  161.— Chick  embryo  with  27 
to  28  pairs  of  somites  (48  to  50  hours) . 
The  head  fold  of  the  amnion  (HFA) 
now  covers  the  anterior  two-thirds  of 
the  body.  E,  eye;  FB,  forebrain;  H, 
heart;  HB,  hindbrain;  MB,  midbrain; 
OT,  auditory  vesicle;  TFA,  tail  fold 
of  amnion.  (Wieman.) 


308 


HUMAN  BIOLOGY 


The  development  of  the  chick  embryo  continues,  enclosed  within 
the  shell,  for  a  normal  21-day  incubation  period.  It  utilizes  the  yolk 
material  for  nutrition  and  carries  on  the  essential  respiratory  exchange 
through  the  permeable  shell  materials.  When  the  proper  stage  of 
development  has  been  reached,  the  chick  breaks  through  the  shell  with 
the  aid  of  a  horny  projection  which  develops  as  a  dorsal  outgrowth, 
near  the  tip  of  the  beak.  Leaving  the  shell  behind,  the  chick  walks 
out  into  the  open  as  an  active,  independent  individual  able  to  secure 
food  for  continued  growth. 

MAMMALIAN  DEVELOPMENT 

It  has  previously  been  emphasized  that  the  essential  features  of 
vertebrate  development  present  a  background  of  uniformity  in  the 
various  types.  The  variable  features  appear  to  be  basically  associated 

F  *  v.;;  v^i 

$5# 


2  cells  4  cells  8  cells  16  cells 

FIG.  162.— Photomicrographs  of  living  eggs  of  rabbit:  2,  4,  8,  16  cells.     (Allen,  after 

Lewis  and  Gregory.     "Science  in  Progress,"  Yale  University  Press.) 

with  the  embryonic  nutrition  which,  in  turn,  is  dependent  upon  the 
amount  of  food  stored  in  the  egg.  A  comparative  study  of  mammalian 
development  shows  that  the  primitive  group,  the  Prototheria,  produce 
heavily  yolked  eggs,  structurally  very  close  to  those  of  birds.  Also, 
the  prototherian  eggs,  after  being  internally  fertilized,  pass  from  the 
body  of  the  female  and  undergo  external  development  as  do  those  of 
birds.  Thus  the  Prototheria,  which  includes  two  well-known  types, 
the  duckbill  (Ornithorhyncus)  and  the  spiny  anteater  (Echidna),  are 
oviparous.  In  all  other  mammals,  however,  both  fertilization  and 
embryonic  development  take  place  within  the  body  of  the  female; 
that  is,  they  are  viviparous.  And  it  is  found  that  the  typical  mam- 
malian egg  is  microscopic  in  size  and  with  a  minimum  amount  of  stored 
food.  The  evidence  is  clear,  however,  from  the  study  of  the  early 
developmental  stages  that  a  close  structural  relationship  exists  between 
the  mammalian  egg  arid  those  of  reptiles  and  birds.  (Fig.  162.) 

The  necessity  of  retaining  the  fertilized  egg  in  the  mother  for  intra- 
uterine  development  is  evident  from  the  fact  that  the  mammalian  eggs, 
as  noted,  contain  a  minimum  of  nutritive  materials.  Therefore,  if 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (/)         309 


development  is  to  continue  beyond  the  early  cleavage  stages,  an  addi- 
tional food  supply  must  be  established  at  once.  This  is  accomplished 
in  the  mammals  through  the  specialization  of  a  portion  of  the  oviducts 
to  form  the  womb,  or  uterus,  which  permits  the  developing  embryo  to 
tap  the  essential  nutritive  supplies  carried  in  the  maternal  blood 
stream.  The  extent  of  embryonic 
development  occurring  in  the  uterus 
varies  considerably  in  the  various 
mammalian  groups.  In  the  more 
primitive  viviparous  mammals,  as  ex- 
emplified in  such  Marsupials  as  the 
kangaroo  and  opossum,  uterine  devel- 
opment is  terminated  early,  and  the 
offspring  are  born  in  a  comparatively 
immature  condition.  Accordingly, 
after  they  are  born,  it  is  necessary  for 
them  to  find  their  place  in  a  special 
external  pouch,  the  marsupium.  This 
pouch  is  located  on  the  ventral  abdo- 
minal wall  of  the  female,  with  the 
mammary  glands  opening  into  it. 
The  embryos  are  carried  in  the  mar- 
supium for  some  time  and  there  nour- 
ished by  the  milk  from  the  mammary 
glands  until  they  reach  a  sufficient 
degree  of  maturity  to  take  care  of 
themselves.  Even  among  the  higher 
types  of  mammals,  the  maturity  of  the 
embryo  at  birth  shows  great  variation. 
The  uterine  development  of  the  off- 
spring is  highest  in  the  hoofed  mam- 
mals, or  Ungulates,  while  in  the 
Primates,  the  order  to  which  Man 
belongs,  a  comparatively  meager  uterine  development  is  found,  and 
the  child  is  born  in  an  essentially  helpless  condition  which  necessitates 
parental  care  for  a  considerable  period. 

If  mating  has  occurred  previous  to  ovulation,  great  numbers  of 
sperm  will  be  present  in  the  oviduct,  and  fertilization  will  occur  very 
quickly  after  the  egg  is  released  from  the  ovary.  The  cleavage  of  the 
yolkless  mammalian  egg  is  holoblastic  and  soon  results  in  the  formation 
of  a  tiny  spherical  body  of  cells,  the  morula,  which  externally  appears 
essentially  the  same  as  the  blastula  of  other  holoblastic  types.  Inter- 
nally, however,  the  cells  of  the  mammalian  morula  show  a  much  greater 


W  (/) 

FIG.  163. — Drawings  of  wax-plate 
models  illustrating  the  early  cleavage 
stages  of  the  mammalian  egg  (pig), 
a,  2  cells;  6,  4  cells;  c  to/  are  sections 
showing  differentiation  of  inner  cell 
mass  (dark  cells)  and  the  develop- 
ment of  the  segmentation  cavity. 
The  trophoblast  arises  from  the  cells 
shown  in  stipple.  (Stages  selected 
from  Wieman,  after  Heuser  and 
Streeter.) 


310 


HUMAN  BIOLOGY 


differentiation  than  those  of  the  blastula,  with  the  rudiments  of  the 
three  primary  germ  layers  and  the  embryonic  membranes  definitely 


FIG.  164.-— Photomicrograph  of  a  portion  of  the  uterinte  lining  (human),  indicating 
the  implantation  site  of  the  embryo  (elevated  central  area).  Cf.  Fig.  165.  (Scientific 
Monthly,  January,  1940.  Redrawn.} 


P.       **< 


-  V;  /o. 


FIG.  165. — Vertical  section  through  the  wall  of  the  uterus  showing  the  implantation 
of  a  very  early  embryo  (Bryce-Tea'cher).  a,  amniotic  cavity;  c,  trophoblast;  en,  point  of 
entrance  of  embryo  closed  by  reunion  of  edges;  g,  gland;  m,  mcsodcrm  (extra-embry- 
onic); um,  uterine  mucosa  (lining);  ys,  yolk  sac.  (Wieman.) 

established.     A  section  through  the  tiny  spherical  morula,  prepared 
for  microscopic  study,   reveals  an  outer  covering  of  the  primitive 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (I)         311 

nutritive  ectoderm,  or  trophoblast,  enclosing  a  central  cavity.  Sus- 
pended in  the  central  cavity  from  the  outer  layer  is  a  group  of  dif- 
ferentiated cells,  the  inner  cell  mass,  which  contains  the  tissue  rudiments 
of  the  embryonic  body.  Examination  of  the  inner  cell  mass  shows 
that  the  rudimentary  tissue  layers  are  spread  out  flat  over  the  yolk  sac 
and  covered  above  by  the  precociously  formed  amnion,  essentially  as 
in  the  chick  embryo.  Curiously  enough,  the  tiny  "yolkless"  yolk  sac 
under  the  embryo,  though  apparently  homologous  with  that  of  the 
chick,  is  without  nutritive  function  in  the  mammal.  (Figs.  163,  165.) 

The  trophoblast,  which,  as  just  noted,  forms  the  outer  covering, 
is  essentially  a  specialized  nutritive  tissue.  It  secretes  enzymes  which 
erode  a  tiny  area  in  the  lining  of  the  uterus.  This  enables  the  embryo 
to  embed  itself  in  the  maternal  tissues  and  to  secure  nutritive  materials 
from  them.  The  passage  of  the  zygote  down  the  oviduct  normally 
takes  two  or  more  days  so  that,  by  the  time  the  uterus  is  reached,  the 
trophoblast  is  ready  to  play  its  double  role  in  aiding  the  attachment 
of  the  embryo  and  in  securing  food  from  the  maternal  tissues.  For  a 
time,  nutritive  materials  are  secured  by  the  combined  digestive  and 
absorptive  action  of  the  trophoblast,  but  shortly,  as  the  embryo  con- 
tinues to  enlarge,  a  very  remarkable  organ  of  the  pregnant  mammalian 
female,  the  placenta,  is  formed  by  a  combination  of  fetal  and  maternal 
tissues  and  is  directly  connected  to  the  embryo  by  the  umbilical  cord. 
It  is  the  placenta  that  permits  the  embryo  to  secure  nutritive  materials 
from,  and  give  off  fetal  wastes  to,  the  maternal  blood  stream.  Early^ 
differentiation  of  the  embryonic  vascular  system,  extending  through 
the  umbilical  cord,  makes  the  rapid  transportation  of  materials  to  and 
from  the  placenta  possible.  (Figs.  164  to  167.) 

And  so  the  placenta  functions  in  the  essential  interchange  of  mate- 
rials between  the  parasitic  embryo  in  the  uterus  and  the  mother. 
Arteries  of  the  maternal  vascular  system  are  continually  bringing 
blood  to  the  placenta  with  abundant  supplies  of  food  and  oxygen. 
Maternal  blood,  carrying  embryonic  wastes  which  have  been  picked 
up  during  the  passage  through  the  placenta,  leaves  through  the  con- 
necting veins.  The  arrangement  of  the  human  placental  tissues  is 
such  that  the  maternal  blood  flows  into  large  spaces,  or  sinuses,  where 
it  directly  bathes  the  projecting  finger-like  villi.  The  latter  are 
formed  from  embryonic  tissues  and  contain  a  network  of  fetal  blood 
vessels — both  arteries  and  veins — extending  through  the  umbil- 
ical cord  and  connecting  with  the  vascular  system  of  the  embryo. 
The  main  vessels,  through  which  blood  passes  from  the  embryo  to  the 
placenta,  are  the  umbilical  arteries.  Such  blood  is  loaded  with  the- 
nitrogenous  wastes  and  carbon  dioxide  excreted  by  the  embryonic 


312 


HUMAN  BIOLOGY 


tissues.  During  its  circulation  through  the  capillary  networks  in 
the  placental  villi,  these  wastes  are  released.  They  pass  through  the 
walls  of  the  villi,  are  picked  up  by  the  maternal  blood  stream  and  then 
are  excreted  from  the  body  of  the  mother  through  the  lungs  and 
kidneys.  Synchronously,  the  supplies  of  food  and  oxygen  present  in 
the  arterial  maternal  blood  are  released  in  the  placental  tissues  and 
pass  through  the  walls  of  the  villi  and  into  the  fetal  blood.  The  latter, 


PLACENTAL  WALL 


FOETAL 
BLOOD  VESSEL 


LAKE  Of 
MATERNAL 


VEIN 


WALL  OF  UTERUS 


-ARTERY 

FIG.  166. — Vertical  section  through  the  human  placenta  attached  to  the  wall  of  the 
uterus  (below).  This  shows  the  fetal  blood  vessels  surrounded  by  "lakes"  of  maternal 
blood.  Note  that  there  is  no  direct  connection  between  the  circulatory  system  of  the* 
mother  and  child.  (Buchanan,  "Elements  of  Biology,"  Harper  &  Brothers.} 

now  freed  from  the  excess  wastes  and  laden  with  the  substances  neces- 
sary for  the  embryonic  tissues,  passes  by  way  of  the  umbilical  veins 
back  to  the  embryo  for  circulation  through  the  body.  It  should  be 
emphasized  that  the  vessels  of  the  vascular  systems  of  mother  and 
child  are  not  directly  connected  in  the  placenta  or  elsewhere.  All 
interchange  of  materials  between  fetal  and  maternal  blood  must, 
therefore,  take  place  by  diffusion  through  the  placental  tissues.  The 
duration  of  uterine  development,  or  gestation,  varies  considerably  in 
the  various  mammalian  groups.  For  example,  the  mouse  embryo 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (I)         313 


FIG.  167. — Diagrams  showing  the  relations  between  the  embryo  and  the  embryonic 
membranes  (amnion,  allantois,  and  yolk  sac)  in  (A)  the  chick  embryo  and  (B)  a  placenta! 
mammal,  as  man.  (Wilder,  "History  of  Human  Body,"  Henry  Holt  &  Company  Inc.) 


314  HUMAN  BIOLOGY 

completes  its  development  in  the  uterus  in  about  20  days,  whereas 
the  human  gestation  period  is  approximately  280  days.     (Fig.  167.) 

HUMAN  REPRODUCTION 

With  the  general  plan  of  mammalian  reproduction  in  mind,  con- 
sideration may  be  given  to  the  main  features  of  human  reproduction, 
with  particular  reference  to  the  structural  and  functional  features  of 
the  male  and  female  reproductive  systems. 

Male  Reproductive  System, — The  reproductive  system  of  the 
human  male  follows  the  general  vertebrate  pattern  in  its  basic  struc- 
tural features  but  is  more  highly  developed  and  more  closely  associated 
with  the  vascular,  endocrine,  and  nervous  systems.  The  essential 
sperm-producing  testes  are  two  in  number.  They  develop  originally 
within  the  body  cavity,  lying  close  to  the  kidneys.  Before  birth, 
however,  the  testes  migrate  posteriorly,  pass  through  the  inguinal  rings 
located  in  the  abdominal  wall  on  each  side  of  the  body,  and,  finally, 
take  their  position  outside  the  body  cavity  in  a  soft-walled  sac,  the 
scrotum,  which  is  primarily  attached  to  t'he  bony  pubic  arch.  Each 
testis  is  seen  as  an  oval-shaped  body  which  hangs  suspended  in  the 
scrotal  sac  by  the  attached  spermatic  cord.  The  latter  is  a  composite 
structure  containing  the  arteries,  veins,  and  nerve  fibers  supplying  the 
testicular  tissues  and  also  a  portion  of  the  sperm-conducting  tubule, 
or  vas  deferens.  All  the  elements  of  the  spermatic  cord  are  permeated 
and  enclosed  by  connective  tissues  which,  in  turn,  are  continuous  with 
the  tissues  of  the  body  wall  surrounding  the  inguinal  ring.  (Plate 
XVI  A.) 

The  scrotum  is  a  more  complex  organ  than  is  generally  realized. 
It  is  essentially  a  two-layered  sac  with  the  constituent  tissues  merging 
with  those  of  the  body  wall  to  which  it  is  attached.  The  outer  cover- 
ing of  the  scrotum  consists  of  a  layer  of  skin,  more  or  less  folded  and 
enclosing  a  second  layer  consisting  largely  of  smooth  muscle  tissue. 
From  the  latter,  a  median  fold  arises  which,  projecting  anteriorly, 
divides  the  scrotum  into  the  right  and  left  chambers,  each  of  which  is 
occupied  by  the  corresponding  testis.  Functionally,  it  appears  that 
the  scrotum  acts  as  a  temperature  regulator  for  the  delicate  male 
germinal  cells  undergoing  development  in  the  gonads.  Thus  when  the 
external  temperature  is  lowered,  as  in  a  cold  bath,  the  scrotal  tissues 
contract,  and  the  testes  are  drawn  anteriorly,  close  to  the  body  wall. 
The  opposite  condition  is  found  when  the  external  temperature  is  too 
high,  for  the  scrotal  tissues  then  relax,  thereby  greatly  increasing  the 
size  of  t^e  scrotum.  The  testes  fall  away  from  the  body  wall,  and 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (I)         315 


enlarged  surface  areas  of  the  scrotum  are  presented  for  cooling  by 
surface  evaporation. 

Previously,  a  description  was  given  of  the  microscopic  structure  of 
the  vertebrate  testis  as  seen  in  the  frog,  but  it  will  be  well  to  indicate 
certain  additional  features  characteristic  of  the  human  testis  though 
the  general  plan  of  structure  is  much  the  same  throughout  the  verte- 
brates. The  human  testis  consists  of  two  portions:  the  sperm-pro- 
ducing portion,  or  testis  proper;  and  the  sperm-transporting  portion,  or 
epididymis.  The  body  of  the  testis  consists  primarily  of  a  great  many 


GLANDS  OF 
INTERNAL  SECRETION 


VAS  DEFERENS 


COMPARTMENT 
OF  TESTIS 


FIG.  168. — Diagram  illustrating  tho  general  structural  plan  of  the  human  testis  and 
associated  ducts.  Diagrammatic.  (Haggard,  "Science  of  Health  and  Disease," 
Harper  &  Brothers.) 

very  fine  convoluted  seminiferous  tubules,  in  the  walls  of  which  the 
sperm  develop.     (Fig.  168.) 

Each  of  these  male  gametes  exhibits  a  typical  cellular  structure 
in  the  early  stages  of  development,  but,  after  passing  through  the 
successive  stages  associated  with  sperm  formation  (spermatogonia, 
spermatocytes,  and  spermatids),  it  becomes  a  highly  modified  sperm 
cell,  adapted  for  active  movements  in  a  liquid.  A  mature  sperm, 
though  minutely  microscopic  in  size,  is  amazingly  complex  in,  its 
structural  appointments.  Three  main  divisions  are  noted,  namely, 
(1)  an  anterior  pointed  portion,  or  head,  which  is  really  the  essential 


316  HUMAN  BIOLOGY 

part  of  a  sperm  because  it  contains  the  chromatic  material  arranged  ip. 
the  gametic  nucleus,  which,  as  we  know,  is  responsible  for  the  transfer 
of  paternal  hereditary  characters  to  the  offspring;  (2)  a  middle  piece 
carrying  the  dynamic  division  center,  or  centrosome,  to  the  fertilized 
egg;  and  (3)  a  vibratile  tail,  or  flagellum,  the  movements  of  which  are 
responsible  for  the  locomotion  of  a  sperm  in  a  suitable  liquid  medium 
and  for  the  ability  to  force  its  way  through  the  resistant  membranes  of 
the  egg.  However,  the  movements  of  the  mass  of  sperm  from  the  testis 
and  through  the  length  of  the  urogenital  canals,  as  well  as  their  later 
discharge  to  the  exterior,  are  primarily  due  to  the  action  of  the  muscle 
tissue  in  the  walls  of  the  ducts  rather  than  to  individual  locomotion. 

The  seminiferous  tubules  in  the  various  areas  are  grouped  in  some 
250  compartments,  or  lobules,  formed  by  connective  tissue  partitions 
continuous  with  the  outer  connective  tissue  sheath  (albuginea  testis). 
Intermingled  with  the  tubular  sperm  elements  throughout  the  testic- 
ular  areas  are  important  endocrine  elements  in  the  interstitial  tissue, 
which  are  responsible  for  the  development  of  the  secondary  sex 
characters  and  the  general  control  of  the  sex  phenomena,  as  previously 
described  in  the  chapter  on  Secretion  (page  115).  The  seminiferous 
tubules  converge  toward  the  posterior  testlcular  wall  and  there  connect 
with  the  tubular  network  of  the  rete  testis,  which,  in  turn,  is  in  direct 
connection  with  the  long,  greatly  coiled  tube  of  the  epididymis.  The 
latter  is  about  20  ft.  in  length  and,  distally,  leads  into  the  final  and 
larger  conducting  element,  the  vas  deferens,  which  passes  through  the 
spermatic  tubule  and  enters  the  body  cavity  through  the  inguinal 
ring.  The  vas  deferens  from  each  testis  joins  the  urethra  carrying 
liquid  wastes  from  the  bladder.  It  is  apparent  that  the  urethra  in 
the  male  serves  as  a  common  duct  for  both  urine  and  semen  in  its 
extension  from  the  vas  deferens-urethral  junction  through  the  penis 
to  the  external  opening. 

Associated  with  the  vas  deferens  are  a  number  of  other  noteworthy 
structures  which  function  in  various  ways,  as  will  be  indicated.  Thus, 
a  sperm  reservoir,  the  seminal  vesicle,  opens  into  each  vas  deferens 
shortly  before  the  latter  joins  the  urethra.  This  junction  forms  the 
ejaculatory  duct  which,  as  the  name  indicates,  forces  the  sperm  stored 
in  the  seminal  vesicles  into  the  urethra.  A  glandular  structure,  the 
prostate  gland,  surrounds  each  ejaculatory  duct  close  to  the  urethral 
opening,  with  a  duct  opening  into  the  urethra  below  the  opening  of 
the  vas  deferens.  The  prostate  glands  give  off  a  secretion  of  doubtful 
function  which  mixes  with  the  sperm.  Additional  glandular  material 
is  received  from  another  pair  of  tiny  glands  (Cowper's  glands)  situated 
close  to  each  prostate  and  also  opening  into  the  urethra.  Accordingly, 


TBE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (7)         317 

the  complete  seminal  fluid  is  found  to  be  a  milky  liquid  made  up  of 
the  various  glandular  secretions  and  normally  containing  some  70 


TESTICLE 
EPIDJDYMIS-1 


SEMINAL  VESICLE 
PROSTATE  GLAND 


URETER 

COLON 

VAS  DEFEREWS 
URETER 

BLADDER 
RECTUM 
URETHRA 
PENIS 

ANUS 
TESTICLE 
PREPUCE 
SCROTUM 


COWPER'S  GLAND       SEMINAL  VESICLE 
E JACULATORY  DUCT  -1  ^-PROSTATE  GLAND 

B 


OVIDUCT 
(FALLOPIAN  TUBE) 

OVARY 

ROUND  LIGAMENT 

UTERUS 

•URETER 

BLADDER 

RECTUM 

CLITORIS 

LABIUM  MINUS 

ANUS 

LABIUM  MAJUS 


URETHRA     VAGINA  URETHRA  VAGINA 

C  D 

PLATE  XVI. — Drawings  illustrating  the  human  reproductive  system  in  male  (A,  B) 
and  female  (C,  D).  In  A  and  C  the  various  structures  are  shown  in  perspective;  in 
B  and  D  the  structures  are  shown  as  seen  in  median  sagittal  sections. 

million  sperm  in  suspension  in  each  cubic  centimeter  of  fluid.    Since 
from  3  to  5  cc.  of  seminal  fluid  are  released  at  a  normal  emission,  the 


318  HUMAN  BIOLOGY 

number  of  contained  sperm  is  seen  to  be  very  large.  The  action  of  the 
ejaculatory  ducts  in  forcing  the  sperm  from  the  seminal  vesicles  into 
the  urethra  has  been  noted.  Movement  of  the  seminal  fluid  down  the 
muscular-walled  urethra  is  largely  due  to  a  reflex  action  of  these  con- 
tractile tissues,  under  control  of  the  autonomic  nerve  fibers.  (Plate 


The  penis,  which  serves  as  a  common  urinary  and  copulatory 
organ,  reaches  its  full  development  only  in  the  higher  mammals. 
Thus  in  the  prototherian  mammals,  in  which  a  cloaca  is  present  as 
in  the  birds,  the  penis  is  relatively  undeveloped  and  remains  concealed 
in  the  wall  of  the  cloaca  except  when  pairing  takes  place.  In  man, 
the  penis  is  external  and  is  attached  primarily  to  the  bony  elements 
of  the  pubic  bones,  ventrally  and  in  front  of  the  scrotal  sac;  the  skin 
of  the  body  wall  forming  a  continuous  extension  over  it.  Below  the 
skin  .covering,  the  body  of  the  penis  is  enclosed  by  a  firm  sheath  com- 
posed largely  of  connective  tissue  elements  and  numerous  unstriated 
muscle  tissue  fibers  which  radiate  in  all  directions.  Permeating  these 
tissues  are  many  large  blood  spaces,  or  sinuses,  which,  when  filled  with 
blood,  increase  the  turgidity  of  the  penis  and  thus  cause  its  erection. 
The  constituent  connective  and  muscle  tissues  are  arranged  to  form 
three  main  tubular  bodies  which  extend  throughout  its  length.  These 
are  designated  as  a  pair  of  corpora  cavernosa  which  lie  toward  the 
anterior  surface,  and  the  unpaired  spongy  body,  or  corpus  spongio- 
sum,  lying  below,  that  is,  toward  the  posterior  surface.  The  spongy 
body  is  penetrated  throughout  its  length  by  the  urethra  and  ends 
distally  in  a  terminal  enlargement,  the  glans  penis,  which  it  alone 
forms;  the  corpora  cavernosa  ending  just  back  of  the  glans.  The 
skin  covering  the  penis  is  not  attached  to  the  tissues  of  the  glans  but 
projects  over  it  as  a  loose  circular  fold  known  as  the  foreskin,  or  pre- 
puce, which,  normally,  can  be  pushed  back  along  the  body  of  the  penis, 
thus  exposing  the  glans  completely.  Not  infrequently  the  prepuce  is 
drawn  so  tightly  over  the  glans  that  it  cannot  be  pushed  back.  In 
such  cases,  circumcision  is  indicated,  a  comparatively  simple  operation 
to  which  Hebrew  male  babies  have  long  been  subjected. 

When  functioning  as  a  urinary  organ,  the  penis  is  soft  and  flabby 
and  in  this  position  hangs  pendant  over  the  scrotum.  As  a  copulatory 
organ,  it  is  necessary  that  erection  take  place  so  that  penetration  can 
be  made  into  the  vagina  of  the  female  where  the  sperm  are  deposited. 
The  act  of  erection  is  under  the  control  of  the  j^rasympathetic  nerve 
fibers  originating  in  the  lumbar  region  of  the  spinal  corcFanH  is  accom- 
plished  by  increasing  the  blood  flow  into  the  penis  through  nerve 
impulses  passing  ov§r  the  vasodilator  fibers  to  the  muscle  fibers  in  the 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (I)         319 


blood  vessels  and  at  the  same  time  restricting  the  outflow  of  blood. 
As  a  result,  the  large  sinuses  present  in  the  corpora  cavernosa  and  the 
corpus  spongiosum  become  filled  with  blood  under  considerable 
pressure.  Also  associated  to  some  extent  in  the  erective  phenomena 
are  the  widely  distributed  muscle  fibers. 

Female  Reproductive  System. — Comparatively  simple  in  the 
lower  types  of  animals  in  which  the  gametes  ripori  only  once  a  year  and 
are  then  released  for  fertilization 
and  development  outside  the  body 
of  the  mother,  the  reproductive 
system  of  the  vertebrate  female 
exhibits  increasing  complexity  as 
provision  is  made  for  ovulation  at 
comparatively  short  intervals  and 
for  both  fertilization  and  embryonic 
development  to  take  place  in  the 
body  of  the  mother,  as  in  the 
human  species.  The  essential  egg- 
producing  gonads  in  women  consist 
of  a  pair  of  ovaries  which  develop 
and  remain  permanently  located  in 
the  abdominal  cavity,  somewhat 
posterior  to  the  kidneys.  The 
ovaries  are  of  comparatively  small 
size,*  measuring  only  about  l/^JB* 
in  length,  %  in.  in  width,  and  with 
a  maximum^weighf^drsome  5  or  6 
g.,  or  about  %  oz.  Histologically, 
the  ovaries  are  .found  to  consist  very  largely  of  a  firm  connective 
tissue  matrix  enclosed  by  a  characteristic  covering  tissue,  the  germi- 
nal epithelium.  It  is  in  the  latter  tissue  that  the  female  gametes 
are  first  localized  as  distinct  cellular  bodies;  the  primordial  female 
germ  cells.  (Plate  XVIC,D.) 

During  the  years  of  sexual  maturity,  the  immature  primordial 
germ  cells  continuously  migrate  from  the  outer  germinal  epithelium 
centrally  into  the  peripheral  matrix  of  the  ovarian  tissue  where  each 
forms  a  Graafian  follicle.  The  latter  is  first  seen  as  a  tiny  spherical 
area  in  the  matrix  containing  a  comparatively  large  central  cell;  the 
immature  egg  or  oogonium,  surrounded  by  one  or  more  layers  of 
follicle  cells.  In  the  functional  mature  ovary,  numerous  Graafian 
follicles  of  various  sizes  and  stages  of  development  are  found  scattered 
through  the  connective  tissue  matrix.  As  the  egg  cell  passes  through 


FIG.  169. — Portion  of  a  section 
through  the  mammalian  ovary  (white 
rat)  showing  a  nearly  mature  Graafian 
follicle  with  egg  as  it  appears  under  the 
microscope,  d.p.,  discus  proligerous; 
/.c.,  follicular  cavity;  m.gr.,  membrana 
granulosa;  o,  ovurn,  or  egg;  t.e.t  t.i., 
outer  membranes.  Cf.  Figs.  60,  170. 
(Wieman.) 


320  HUMAN  BIOLOGY 

the  various  developmental  stages,  it  gradually  increases  in  size,  but 
even  more  the  size  of  the  entire  follicle  increases  until  it  becomes  a 
large  fluid-filled  cavity  bounded  by  several  layers  of  follicle  cells  and 
containing  the  large  egg  cell,  now  known  as  an  oocyte,  mounted  as  if 
on  a  pedestal  (discus  proligerus)  of  follicle  cells;  several  layers  of  these 
cells  also  enclose  the  oocyte.  (Fig.  169.) 

Though  the  germ  cells  first  migrate  into  the  ovarian  tissues  and  give 
rise  to  tiny  Graafian  follicles,  the  great  increase  in  the  size  of  these 
structures,  as  the  gametes  approach  maturity,  brings  them  not  only 
to  the  periphery  of  the  ovary  again,  but  they  actually  bulge 


FIG.  170. — Photograph  of  a  normal  pig  ovary  showing  numerous  small  transparent 
elevations,  each  of  which  is  a  Graafian  follicle  with  an  egg,  and  also  several  large  opaque 
spherical  bodies  which  are  the  corpora  lutea  and  mark  the  sites  of  previous  Graafian 
follicles.  (Cf.  Figs.  60,  169.)  Allen,  "Science  in  Progress,"  Yale  University  Press. 
After  Allen,  Kountz,  and  Francis.) 

out  from  the  surface  as  transparent,  blister-like  areas,  plainly  visible 
to  the  naked  eye.  When  the  enclosed  egg  cell  is  fully  mature  and  ready 
for  release  from  the  ovary,  it  breaks  directly  through  the  surrounding 
layers,  follicle  cells  as  well  as  the  outer  germinal  epithelium  of  the 
ovary,  at  the  most  convenient  spot.  During  this  period  of  develop- 
ment in  a  follicle,  profound  changes  are  occurring  in  the  nucleus  of  the 
gamete  that  insure  the  proper  maternal  heritage  to  the  zygote.  This 
basic  feature  of  gamete  formation  will  be  discussed  in  the  next  chapter. 
There  is  no  direct  connection  between  ovary  and  oviduct  as  there  is 
between  testis  and  sperm  duct ;  and  though  the  opening  of  the  oviduct 
lies  in  close  proximity  to  the  ovary,  there  is  always  the  chance  that  an 
egg  liberated  from  the  ovaiy  may  fall  into  the  abdominal  cavity 
instead  of  passing  into  the  oviduct. 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (I)        321 


The  two  hormones,  estrone  and  progesterone,  produced  in  the  ovary 
have  already  been  discussed  in  the  chapter  on  Secretion  (page  116). 
Particular  attention  should  be  called  to  important  ovarian  endocrine 
tissue,  the  corpus  luteum,  which  develops  in  the  cavity  of  a  follicle 
shortly  after  ovulation  occurs,  apparently  from  the  follicle  cells  that 


f 

{ 


FIG.  171. — Section  of  a  gravid  human  uterus  showing  five  weeks  embryo  and 
associated  structures.  Diagrammatic.  (Arey,  "Developmental  Anatomy"  W.  B. 
Saunders  Company.) 

remain  in  situ.  The  amount  of  the  corpus  luteum  tissue  formed  and 
the  period  of  its  retention  as  a  functional  glandular  tissue  depends 
upon  the  fate  of  the  egg.  If  the  egg  is  not  fertilized,  the  corpus  luteum 
soon  begins  to  decrease  in  size,  and,  shortly,  only  scar  tissue  remains 
in  the  ovarian  wall  to  mark  the  location  of  the  follicular  area.  On  the 
other  hand,  if  the  egg  is  fertilized  and  development  gets  under  way 
in  the  uterus,  the  corpus  luteum  continues  to  increase  in  amount  for 
some  time  after  ovulation  and  to  secrete  the  powerful  hormone  pro- 


322 


HUMAN  BIOLOGY 


gesterone  which  is  responsible  for  the  omission  of  ovulation  and  also 
for  the  cyclical  menstrual  changes  during  pregnancy.  (Figs.  60,  170.) 
The  uterine  development  of  the  mammalian  fetus  necessitates 
marked  structural  and  functional  changes  in  the  oviducts,  which,  in 
the  lower  types  function  merely  as  egg-conducting  tubes.  Thus  the 
human  oviduct  is  divided  into  three  distinct  regions:  the  oviduct 
proper,  or  fallopian  tube;  the  uterus;  and  the  vagina.  The  uterus  and 
vagina  are  single,  unpaired  structures  which  develop  from  the  union 
of  the  distal  portions  of  the  right  and  left  oviducts.  Each  oviduct 
in  the  human  female  is  a  very  small  tubular  organ  about  4  in.  in  length 
and  with  a  tiny  central  cavity,  or  lumen,  about  the  size  of  a  bristle. 
It  is  lined  with  ciliated  epithelium  which  aids  in  the  movement  of  the 


Fetal,  2 mo.     .Fetal,  5 mo,     At  birth  2yrs, 


6yrs. 


n  yrs. 


Aduli- 


FIG.  172. — Changes  in  human  body  proportions  during  prenatal  and  postnatal  periods. 

(Sherhon,  after  Stratz.) 

eggs  toward  the  uterus.  The  sperm  apparently  swim  against  the  cili- 
ary current  in  passing  up  the  oviducts  to  fertilize  tfoe  eggs.  The  walls 
of  the  uterus  are  largely  composed  of  unstriated  muscle  tissue  which  is 
covered  on  the  outside  with  the  peritoneal  epithelium  and  lined  with  a 
vascular  mucous  membrane.  The  cavity  of  the  uterus  is,  of  course, 
continuous  with  the  lumen  of  the  oviducts.  Functionally,  the  uterus 
is  characterized  by  its  great  extensibility  during  pregnancy  when  it 
may  increase  from  approximately  3  in.  to  1  ft.  or  more  in  length  in 
accordance  with  the  size  of  the  growing  fetus.  There  is  also  great 
flexibility  in  the  position  of  the  uterus.  It  is  not  firmly  attached  to 
any  bony  structure  but  rather  suspended,  as  it  were,  by  a  number  of 
flexible  ligaments  which  permit  a  shift  in  position  in  response  to  the 
various  mechanical  factors  appearing  during  pregnancy.  With  the 
completion  of  uterine  development,  rhythmic  contractions  of  the  uter- 
ine musculature,  under  the  influence  of  unknown  factors,  begin  which, 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (I)         323 


in  the  course  of  a  few  hours,  are  usually  powerful  enough  to  force  the 
embryo  from  the  uterus  and  out  through  the  vagina  to  the  exterior; 
the  process  of  childbirth.  (Plate  XVIC,  D;  Figs.  171,  172.) 

Another  characteristic  of  the  mammalian  uterus  is  a  periodical 
series  of  changes  in  the  lining  tissues  which  are  apparently  essential 
to  the  proper  preparation  of  the  uterine  wall  for  the  imbedding  and 
nourishment  of  the  fertilized  egg.  In  the  lower  mammals,  these 
cyclical  uterine  changes  occur  once  or  twice  a  year  and  are  known  as 
the  period  of  heatj  or  oestrus.  In  the  human  female,  the  menstrual 
periods  normally  occur  every  26  to  28  days  during  the  period  of  sexual 
maturity.  The  latter  typically  extends  from  about  the  twelfth  year 
to  forty-five  or  fifty  years  of  age.  As  noted  above,  the  menstrual 


IN  TUBE     IN   UTERUS  OVUM   IS  FERTILIZED 

............     JillrllUIUlllll 


FIG.  173.— Scheme  illustrating  the  ovarian  and  uterine  cycles.      (Haggard,  "Science  of 
Health  and  Disease,"  Harper  &  Brothers.) 

phenomena  do  not  occur  during  pregnancy.  Each  menstrual  period 
is  from  4  to  6  days'  duration  and  is  marked  by  a  considerable  discharge 
of  blood  from  the  uterine  cavity  which  passes  to  the  exterior  through 
the  vaginal  opening.  Histological  examination  of  the  menstrual  dis- 
charges reveals  the  presence  of  numerous  epithelial  cells  from  the  lining 
x)f  the  uterus.  As  a  matter  of  fact,  it  is  clear  that  menstruation  is 
primarily  due  to  a  degeneration  of  the  uterine  lining  with  the  conse- 
quent exposure  of,  and  leakage  from,  the  underlying  capillaries.  The 
tissue  degeneration  is  followed  by  regenerative  processes;  and  thus, 
periodically,  a  new  lining  surface  is  established  in  the  uterus  for  the 
reception  of  the  developing  embryo.  Primarily  the  menstrual  phe- 
nomena are  controlled  by  hormone  action.  The  permanent  cessation 
of  menstruation  at  forty-five  to  fifty  years  of  age  indicates  the  end  of 
ovulation  and  fertility.  This  period,  known  as  the  menopause,  or 
climacteric,  may  be  accompanied  by  certain  definite  clinical  symptoms, 
such  as  varying  temperature  reactions,  muscular  pains,  and  dizziness. 


324  HUMAN  BIOLOGY 

In  some  cases,  temporary  psychical  reactions  of  a  more  or  less  disturb- 
ing nature  also  appear  during  this  change  of  life.  (Fig.  173.) 

The  final  structural  unit  derived  from  the  oviducts  is  the  vagina, 
which  is  an  unpaired  tubular  structure  extending  from  the  uterus  to 
the  external  opening  or  vestibule.  Previous  to  sexual  union,  the 
external  opening  of  the  vagina  is  partially  closed  by  a  thin  membranous 
sheet,  the  hymen.  In  the  mammalian  female,  contrary  to  the 
condition  noted  above  in  the  male,  the  genital  organs  have  separate 
ducts  and  openings  independent  of  those  of  the  urinary  system.  The 
clitoris,  a  small  unpaired  organ  with  erectile  and  nerve  tissue,  homol- 
ogous in  its  development  with  the  penis  of  the  male,  is  situated  anterior 
to  the  opening  of  the  urethra.  (Plate  XVID). 

The  immaturity  of  most  mammalian  embryos  at  birth  makes  it 
necessary  for  the  infant  to  be  nourished  by  the  mother  for  some  time. 
And,  so,  associated  with  the  mammalian  female  reproductive  system 
are  one  or  more  pairs  of  mammary  glands  for  the  formation  and 
secretion  of  a  nutritive  fluid,  milk,  which  is  adapted  for  the  infant 
nutrition.  The  single  pair  of  mammary  glands,1  or  breasts,  of  the 
human  female  are  small  during  childhood;  but  at  the  time  of  puberty, 
thdy  begin  to  enlarge  as  a  result  of  the  growth  of  the  associated  con- 
nective tissues  together  with  the  deposition  of  fat.  The  glandular 
function,  however,  remains  inactive  until  pregnancy  occurs.  During 
gestation,  the  glandular  tissues  show  a  further  gradual  increase  in  size 
and,  under  normal  conditions,  are  ready  for  functional  activity  follow- 
ing childbirth.  (Fig.  51.) 

1  Consult  Appendix :  Mammary  Glands. 


CHAPTER  XIII 
THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II) 

In  the  chapter  just  preceding,  general  consideration  was  given  to 
the  basic  features  of  plant  and  animal  reproduction,  and,  with  this 
material  as  a  foundation,  a  rather  complete  description  was  presented 
of  the  reproductive  processes  in  three  representative  vertebrates:  the 
frog,  domestic  fowl,  and  man.  It  was  emphasized  that  reproduction 
is  not  essential  for  the  maintenance  of  the  living  state  in  the  individual 
organism  but  is  solely  concerned  with  the  perpetuation  of  a  particular 
type  of  species  through  the  production  of  new  individuals  that  are  true 
to  type.  The  perpetuation  of  a  species  means  that  the  new  individuals 


sp. 

Fia.  174. — Scheme  illustrating  the  reproductive  cycle  in  a  triple blastic  animal  with 
sexual  reproduction  in  which  only  the  sperm  and  egg  bridge  the  generations  and,  there- 
fore, necessarily  carry  the  entire  body  of  heritable  materials,  e,  egg;  g.c.,  germ  cells; 
2,  zygote.  (Woodruff,  modified  from  Hegner.) 

must  conform  closely  to  the  characteristic  pattern.  Without  at 
present  going  into  the  question  as  to  just  what  constitutes  a  species, 
it  is  a  matter  of  common  knowledge  that  species  breed  true;  figs  do  not 
produce  thistles,  nor  do  mice  beget  elephants.  Thus,  a  fixity  of  species 
exists  from  generation  to  generation.  At  the  same  time,  it  is  also 
commonly  recognized  that  a  certain  amount  of  individual  variation  is 
always  present  among  the  members  of  a  particular  species. 

With  this  situation  in  mind,  it  becomes  evident  that  some  mecha- 
nism must  be  operating  in  the  specialized  germ  cells,  bridging  the 
generations,  that  makes  certain  that  the  new  individuals  develop  true 
to  the  parental  types.  Possibly  the  presence  of  an  intracellular 
mechanism  of  inheritance  is  not  so  clearly  indicated  in  certain  types  of 
asexual  reproduction  as  it  is  In  the  more  involved  processes  of  sexual 
reproduction,  for,  in  the  latter  case,  the  entire  body  of  heritable 
materials  transferred  to  the  individual  of  the  next  generation  is 
necessarily  carried  by  a  microscopic  sperm  and  egg.  (Fig.  174.) 

325 


326 


HUMAN  BIOLOGY 


On  the  other  hand  in  asexual  reproduction  in  the  unicellular 
organisms  where  reproduction  is  by  binary  fission,  or  in  the  multicellu- 
lar  organisms  where  the  processes  of  regeneration  lead  to  the  produc- 
tion of  new  individuals  through  the  gradual  growth  and  differentiation 
of  considerable  portions  of  the  parent  individual,  it  seems  reasonable 
to  expect  that  an  offspring  would  conform  closely  to  the  parental 
individual  from  which  it  sprang.  Nevertheless,  even  a  superficial 
knowledge  of  the  material  at  hand  makes  it  evident  that,  in  the  absence 
of  a  basic  intracellular  mechanism,  every  cell  division  would  offer 
almost  unlimited  opportunity  for  divergence  in  the  daughter  cells. 


FIG.  175. — Asexual  reproduction  of  the  strawberry  by  the  production  of  runners 
which  later  die  away,  thereby  isolating  a  new  plant  developed  directly  from  the  parent. 
(Haupt.) 

The  fact  of  the  matter  is  that  whenever  a  cell  undergoes  normal 
division,  or  mitosis,  be  it  plant  or  animal,  soma  or  germ,  the  inheritance 
of  the  daughter  cells  is  rigidly  and  accurately  determined.  From  the 
zygote  to  the  adult,  mitosis  is  not  a  haphazard  process.  It  is  because 
of  this  fact  that  species  do  breed  true  and  that  the  reproductive 
mechanism  in  any  particular  species  of  plant  or  animal  operates  to 
perpetuate  that  particular  species  and  no  other.  (Fig.  175.) 

Accordingly,  it  may  be  regarded  as  established  that  the  production 
of  a  new  individual  of  a  particular  species,  whether  asexually  or 
sexually  produced,  is  always  dependent  upon  the  orderly  processes  of 
mitosis  and,  though  occasionally  something  may  go  wrong  and  a  freak 
or  monster  may  appear,  it  is  a  matter  of  common  observation  that 
abnormalities  are  so  rare  as  to  cause  the  widest  interest.  The  descriu- 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II)        327 

tions  of  various  species  by  the  earliest  systematists,  if  based  on  accurate 
observations,  check  absolutely  with  the  individuals  of  the  same  species 
living  today,  though  in  the  meantime  untold  generations  have  come 
and  gone.  Or  we  may  go  back  far,  far  beyond  the  time  of  even  the 
earliest  systfcmatist  and  compare  the  structural  characteristics  of 
various  plant  arid  animal  types,  which  lived  millions  of  years  ago,  with 
their  descendants  now  living  and  still  occupying  their  particular  niches 
in  the  amazing  web  of  life. 

But  turning  to  the  other  side  of  the  picture,  lest  the  principle  of  the 
fixity  of  species  be  overemphasized  and  thereby  the  impression  given 
that  all  species  have  remained  essentially  unchanged  throughout  the 
previous  ages,  it  should  be  stated  at  once  that  plenty  of  evidence  exists 
also  to  show  that  species  have  changed  at  various  times  in  the  past  and 
that  further  changes  may  still  occur.  There  is  descent  with  change; 
a  basic  relatedness  occurs  throughout  the  living  world  that  gives 
evidence  that  in  the  beginning  there  was  a  common  life  stream  from 
which  all  the  infinite  varieties  of  organisms  now  present  in  the  world  of 
life,  as  well  as  a  great  many  types  that  have  entirely  disappeared,  have 
been  derived.  Biologists  are  convinced  that  the  intracellular  mecha- 
nism associated  with  the  mitotic  phenomena  in  cell  division  is  primarily 
responsible  for  the  production,  generation  after  generation,  of  offspring 
conforming  to  the  species  type;  for  the  individual  variation  that  is 
apparent  with  the  members  of  a  species;  and,  finally,  for  descent  with 
change,  or,  as  it  is  more  commonly  termed,  evolution.  With  the 
explanation  of  these  basically  important  features  bound  up  with 
mechanism  of  cell  division,  it  is  obvious  that  a  thorough  understanding 
of  all  phases  of  this  basic  process  is  necessary. 

MITOSIS 

Mitosis  is  primarily  a  nuclear  phenomenon  involving  an  exact 
division  of  the  chromatin ;  the  essential  substance  for  the  transfer  of  the 
heritable  materials  to  the  daughter  cells.  In  the  accomplishment  of 
this  central  aim  the  nucleus  temporarily  disappears  and  an  elaborate 
apparatus,  the  spindle,  is  temporarily  set  up  in  the  cell  cytoplasm  so 
that  the  entire  cell  is  involved  in  the  complicated  mitotic  phenomena. 
Although  mitosis  is  normally  a  continuous  process  from  the  time  it 
begins  until  the  single  cell  has  divided  to  form  two  cells,  four  stages 
during  the  process  are  generally  recognized,  namely,  prophase, 
metaphase,  anaphase,  and  telophase,  to  which  consideration  may  now 
be  given.  (Fig.  176.) 

The  Prophase. — Just  previous  to  the  beginning  of  the  prophase,  the 
nuclear  and  cytoplasmic  elements  in  a  cell  appear  in  their  normal 


328 


HUMAN  BIOLOGY 


f  8 

FIG.  176. — Diagrams  illustrating  important  stages  in  mitotic  cell  division,  a,  cell 
in  resting  stage;  b,  c,  beginning  of  cell  division  indicated  by  division  of  the  centrosome 
lying  above  the  nucleus  and  the  condensation  of  the  chromatin  to  form  definite  chromo- 
somes (prophase) ;  d,  equatorial  plate  stage  (metaphase)  with  first  cleavage  spindle  well 
formed  and  division  of  each  chromosome  into  two  parts;  e,  separation  of  the  halves  of 
each  chromosome  and  beginning  of  migration  to  the  opposite  poles  of  the  spindle 
(anaphase) ;  /,  final  stage  (telophase)  just  previous  to  the  separation  into  daughter  c(  lls, 
as  shown  in  g.  (Watkeya,  Stern,  modified.) 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II)        329 

structural  relationships;  but  as  this  stage  develops,  marked  changes  in 
the  structural  pattern  are  in  evidence  as  the  cellular  materials  are 
rearranged  for  the  approaching  climax  of  cell  division.  The  beginning 
of  the  prophase  in  animal  cells  is  first  indicated  by  the  activity  of  the 
centrosome,  a  very  minute  particle  in  the  cytoplasm,  lying  in  close 
contact  with  the  nuclear  membrane,  which  splits  into  two  daughter 
centrosomes.  The  latter,  without  delay,  begin  to  move  away  from 
each  other  toward  the  opposite  poles  of  the  cell.  Each  of  the  oentro- 
somes  is  quickly  surrounded  by  a  structurally  modified  region  of  the 
cytoplasm  from  which  fibrils,  or  rays,  soon  radiate  peripherally  to  form 
the  aster.  As  the  two  centrosomes  with  their  surrounding  astral  halo 
continue  to  move  apart,  numerous  fibers  of  another  type  appear  in  the 
cytoplasm  between  them.  These  are  the  spindle  fibers,  and  they 
extend  from  each  centrosomal  area  toward  the  center  of  the  cell  where 
they  join  with  the  nuclear  elements  and  thus  form  a  fibrillar  spindle. 
The  center  of  the  spindle  is  approximately  in  the  center  of  the  cell 
where  it  surrounds  the  nuclear  area.  During  the  peripheral  migration 
of  the  daughter  centrosomes  and  the  development  of  the  asters  and  the 
spindle,  profound  changes  have  been  occurring  in  the  nucleus.  Exter- 
nally, this  is  marked  by  the  gradual  disappearance  of  the  nuclear 
membrane,  thereby  removing  the  boundary  between  cytoplasmic 
and  nuclear  elements.  (Fig.  176a,  6,  c.) 

But  of  greater  importance  in  the  transfer  of  hereditary  characters  is 
the  decisive  rearrangement  of  the  chromatin  pattern  marked  by  the 
development  of  definite  structural  units,  the  chromosomes.  At  the 
beginning  of  mitosis,  the  chromatin  throughout  the  nuclear  area 
appears  as  an  irregular  network  with  embedded  particles  of  various 
shapes  and  sizes.  Soon  this  is  changed,  and  the  chromatin  is  con- 
solidated to  form  a  definite  number  of  long  thread-like  bodies,  the 
chromosomes,  which  with  the  proper  technique  are  beautifully  differ- 
entiated in  stained  preparations  and  can  also  be  seen  in  living  cells 
under  certain  conditions.  Expert  examination  of  a  chromosome  under 
the  highest  magnification  shows  that  it  is  essentially  a  double  structure 
with  two  chromatin  elements  in  close  contact,  or  fused,  throughout 
their  length.  As  a  mattei^of  fact,  the  double  condition  of  the  chro- 
matin is  usually  apparent  in  the  early  prophase  stage.  Furthermore, 
it  is  apparent  that  all  the  chromosomes  do  not  have  exactly  the  same 
shape  but  exhibit  a  distinct  structural  individuality;  some  are  long, 
some  short,  some  angular,  some  like  tiny  spheres.  This  individuality 
of  the  chromosomes  has  deep  significance  and  will  be  considered  later  in 
connection  with  the  development  of  the  germ  cells.  The  formation 
of  the  specific  chromosome  entities  from  the  apparently  nondescript 


330  HUMAN  BIOLOGY 

net-like  chromatin  and  their  attachment  to  the  spindle  fibers  mark  the 
end  of  the  prophase. 

The  Metaphase. — This  stage  of  mitosis  is  a  comparatively  short 
one.  It  is  characterized  by  the  definite  alignment  of  the  chromosomes 
in  the  center,  or  equator,  of  the  spindle,  equidistant  from  the  two 
asters.  This  region  of  the  spindle  is  termed  the  equatorial  plate.  The 
mechanics  involved  in  the  shifting  and  definite  arrangement  of  the 
chromosomes  in  the  equatorial  plate  are  quite  obscure,  but,  presum- 
ably, forces  applied  through  the  attached  spindle  fibers  are  responsible. 
At  all  events,  the  underlying  plan  is  evident,  for  the  chromosomes  are 
thus  placed  in  the  proper  position  for  longitudinal  cleavage,  which 
occurs  during  the  next  stage  of  nditosis,  the  anaphase.  (Fig.  176rf.) 

The  Anaphase. — The  anaphase  may,  perhaps,  be  regarded  as  the 
climax  of  mitosis  for  during  this  stage,  as  just  stated  in  the  preceding 
paragraph,  each  of  the  chromosomes  in  the  dividing  cell  splits  longi- 
tudinally to  form  two  chromosomes.  .  This  longitudinal  division 
separates  the  two  elements  of  the  double  chromosome  structure,  noted 
in  the  prophase  stage,  in  a  precise  qualitative  and  quantitative  manner 
so  that  each  of  the  two  resulting  cells  receives  the  correct  chromatin 
content.  The  two  "  daughter  "  chromosomes,  formed  by  the  division 
of  each  of  the  chromosomes  in  the  equatorial  plate,  now  move  in 
opposite  directions  toward  the  aster  from  which  the  attached  spindle 
fibers  radiate.  And  so,  when  cell  division  is  completed  and  two 
independent  cells  have  been  formed,  it  will  be  found  that  the  nucleus 
of  each  of  these  cells  does  not  contain  a  miscellaneous  array  of  chromo- 
somes but  always  one  chromosome  from  each  pair  formed  by  the 
splitting  of  the  chromosomes,  as  just  noted.  Since  it  was  apparent  in 
the  prophase  stage  that  a  chromosome  is  a  double  structure,  it  may  bo 
simpler  to  say  that,  during  the  anaphase,  the  two  elements  of  each 
chromosome  undergo  complete  separation  and  move  toward  opposite 
poles  of  the  spindle.  (Fig.  176e.) 

The  forces  involved,  both  in  the  longitudinal  splitting  and  in  the 
later  separation  of  the  half-chromosomes  during  the  anaphase,  are  by  no 
means  fully  determined  as,  indeed,  is  the  case  with  most  of  the  mitotic 
phenomena,  but  considerable  evidence  is  now  at  hand  to  show  that 
the  chromosome  elements  are  actually  pulled  apart  as  a  result  of  the 
contraction  of  the  attached  spindle  fibers.  To  visualize  this  process, 
it  is  necessary  to  assume  that  the  spindle  fibers  from  each  pole  of  the 
spindle  extend  only  to  the  equatorial  plate,  where  they  are  directly 
connected  with  the  chromosomes  in  such  a  way  that  the  pull  exerted 
by  a  contraction  of  the  spindle  fibers  will  separate  the  two  elements  of 
each  chromosome  and  draw  each  half  toward  the  aster  from  which  the 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II)        331 

particular  group  of  spindle  fibers  extends.  The  continued  movement 
of  the  daughter  chromosomes  away  from  each  other  and  toward 
opposite  poles  of  the  spindle  is  even  more  difficult  to  explain.  Appar- 
ently no  visible  evidence  exists  that  would  indicate  a  pull  on  them  as  a 
result  of  continued  spindle  fiber  contraction.  Nevertheless,  the  polar 
movement  of  the  two  chromosome  groups  continues  so  that,  as  the 
anaphase  stage  ends,  they  are  widely  separated,  with  each  group  lying 
in  close  proximity  to  the  corresponding  aster.  Evidences  of  the  route 
travelled  from  the  equatorial  plate  are  seen  in  an  orientation  of 
the  cytoplasmic  elements  parallel  to  the  direction  of  chromosome 
movement. 

The  Telophase. — This  end  phase  of  mitosis  is  characterized  by  a 
number  of  structural  changes  in  the  elements  of  the  dividing  cell. 
Externally  a  prominent  feature  in  animal  cells  is  the  formation  and 
gradual  development  of  a  cleavage  plane  that  first  appears  as  a  slight 
depression  in  the  cytoplasm  at  the  equator  of  the  cell.  This  may  be 
seen  to  excellent  advantage  in  the  holoblastic  cleavage  of  the  fertilized 
egg,  such  as  previously  described  in  the  frog.  The  continued  ingrowth 
of  the  cleavage  furrow  finally  results  in  the  division  of  the  cytoplasm  of 
the  original  cell  into  two  daughter  cells.  The  internal  nuclear  changes 
made  evident  by  a  microscopic  study  of  cells  in  the  telophase  stage 
reveal  the  final  chapter  in  the  mitotic  phenomena  previously  traced 
through  the  prophase,  metaphase,  and  anaphase  stages.  Outstanding 
is  the  reappearance  of  the  nuclear  membrane  and  also  of  that  function- 
ally obscure  body,  the  nucleolus,  in  each  of  the  daughter  nuclei. 
(Fig.  176/.) 

Along  with  these  altered  features  is  a  change  in  the  appearance  of 
the  chromosomes.  They  begin  to  lose  their  clear  visibility  and  grad- 
ually merge  into  the  irregular,  granular  network  present  in  the 
nucleus  of  the  parent  cell  at  the  beginning  of  the  prophase  stage. 
Accordingly,  so  far  as  can  be  seen  under  the  microscope  the  chromo- 
somes appear  to  be  temporary  chromatin  bodies  which  reach  their 
highest  development  in  the  metaphase  and  anaphase  and  gradually 
disappear  during  the  telephase.  They  will  not  become  prominent 
again  until  the  daughter  cells  undergo  mitosis.  However,  it  must  not 
be  thought  that  the  basic  chromatin  organization  disappears  in  a 
resting  nucleus  or  that  it  is  in  any  sense  haphazard;  for  whenever  the 
chromosomes  reappear  in  the  cells  during  succeeding  mitoses,  the 
exact  number  and  shapes  that  were  present  in  the  ancestral  cells  again 
"crystallize"  out  of  the  chromatin  net.  Finally,  in  each  daughter 
cell  a  gradual  disappearance  of  both  the  astral  fibers  and  spindle  fibers 
radiating  through  the  cytoplasm  is  evident,  and  the  tiny  centrosome, 


332  HUMAN  BIOLOGY 

shorn  of  its  astral  halo,  once  again  lies  close  to  the  nuclear  membrane, 
ready  to  divide  and  thus  inaugurate  the  prophase  of  the  next  mitosis. 
Successive  mitoses  usually  follow  each  other  in  rapid  succession  in  the 
early  stages  of  embryonic  development,  and  the  successive  generations 
of  daughter  cells  get  smaller  and  smaller,  as  can  be  seen  in  the  blastula 
and  gastrula  stages  of  the  frog  embiyo.  In  the  more  mature  cells, 
however,  long  inactive  periods  normally  follow  the  completion  of 
mitotic  activity.  In  fact,  many  highly  differentiated  types  of  cell 
completely  lose  the  power  of  mitosis  in  the  adult. 

Thus  ends  the  amazingly  exact  process  of  mitosis  on  which  the 
integrity  of  successive  generations  of  cells  and,  in  the  final  analysis, 
of  every  organism  depends.  The  fertilized  egg  cell  contains  its  exact 
quota  of  maternal  and  paternal  chromatin,  and,  during  the  successive 
cell  divisions  essential  to  the  attainment  of  the  adult  stage,  every 
daughter  cell  receives  its  rightful  share  of  the  chromatin  legacy  estab- 
lished in  the  zygote  nucleus.  And  so  it  is  apparent,  as  stated  in  the 
first  chapter,  that  "cell  division  is  an  exquisitely  beautiful  and  exact 
process  (page  13).  On  its  normal  functioning  during  all  the  stages  of 
embryonic  development  and  throughout  the  life  of  each  individual 
depends  the  structural  and  functional  integrity  of  every  tissue  and, 
organ  of  the  body,  as  well  as  the  specific  characteristics  of  the  entire 
organism." 

CHROMOSOME  STRUCTURE 

Since  the  entire  process  of  mitosis  hinges  on  an  exact  division  of 
all  the  chromosomes  so  that  the  nucleus  of  each  daughter  cell  may 
receive  its  rightful  share  of  every  chromosome,  it  is  apparent  that  the 
hereditary  materials  in  the  chromosomes  must  be  arranged  with  the 
utmost  regularity  and  precision.  Without,  for  the  moment,  bringing 
forth  the  available  experimental  data,  it  may  be  stated  that  there  is 
abundant  evidence  demonstrating  that  every  chromosome,  whether 
present  in  the  nucleus  of  a  body  cell  or  in  germ  cell,  is  composed  of  a 
great  many  independent  ultramicroscopic  hereditary  units,  the  genes, 
which  are  arranged  in  a  precise  linear  fashion  throughout  the  length 
of  the  chromosome.  It  is  well  established  that' some  2,500  genes  are 
present  in  certain  chromosomes  of  the  fruit  fly  Drosophila,  which  are 
responsible  for  the  control  of  various  hereditary  characters.  In 
addition,  it  has  been  found  possible  to  secure  data  from  various  con- 
trolled breeding  experiments  in  this  organism  that  indicate  the  posi- 
tions of  many  of  these  genes  in  a  particular  chromosome.  In  this  way, 
chromosome  maps  have  been  constructed  that  indicate  the  gene  loci. 
(Fig.  181.) 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II)       333 

With  this  linear  arrangement  of  the  gene-chromosome  complex  in 
mind,  it  is  evident  that,  when  a  chromosome  reproduces  and  splits  in 
half  longitudinally,  each  of  the  constituent  genes,  linearly  arranged 
throughout  its  length,  also  reproduces  and  splits.  Thus  each  of  the 
daughter  chromosomes  receives  its  share  of  every  gene  present  in  the 
parent  chromosome.  The  assemblage  of  a  complete  set  of  the  daugh- 
ter chromosomes  in  the  nucleus  of  each  daughter  cell  transmits  the 
complete  gene  heritage  of  the  dividing  cell.  In  a  word,  then,  the 
nucleus  of  a  daughter  cell  is  exactly  like  that  of  the  parent  cell  except 
that,  at  the  instant  it  is  formed,  it  is  only  one-half  the  parental  size. 
The  cell  cytoplasm  simply  splits  into  halves,  but  this  relatively  crude 
method  cannot  be  used  for  the  chromatin  material  in  the  nucleus 
inasmuch  as  every  one  of  the  untold  thousands  of  genes  must  undergo 
exact  qualitative  and  quantitative  division  if  the  normal  inheritance 
pattern  is  to  be  transmitted. 

The  daughter  cells  having  been  formed,  the  next  event  is  an  increase 
in  the  amount  of  both  cytoplasm  and  of  the  chromatin  until  the 
normal  cell  size  is  once  more  reached.  Growth  takes  place  during  the 
period  in  which  a  cell  is  inactive  mitotically.  >  In  such  a  condition,  as 
shown  in  the  prophase  at  the  beginning  of  mitosis  and  again  at  the 
conclusion  of  the  telophase  stage,  t,he  chromosomes  are  not  found  as 
microscopically  visible  bodies.  During  the  growth  period,  new  chro- 
matin material  is  being  formed  and  added  to  that  already  present  in 
the  nucleus.  In  some  way,  not  understood  at  present,  this  process  is 
so  exactly  controlled  that,  when  the  chromosome  structure  is  again 
rebuilt  out  of  the  irregular  network  of  the  resting  cell,  every  chromo- 
some reappears  with  its  individual  structural  characteristics  and  with 
every  gene  in  its  exact  spatial  relationship  and  containing  its  own 
specific  hereditary  substances. 

It  has  just  been  shown  that  the  number  of  chromosomes  and  their 
exact  structural  and  functional  features  pass  unchanged  through 
successive  cell  generations.  To  take  a  specific  example,  if  a  fertilized 
egg  cell  has  48  chromosomes,  as  in  Man,  every  one  of  the  trillions  of 
body  cells  in  the  adult  individual  will  have  48  chromosomes.  Fur- 
thermore, the  48  chromosomes  present  in  the  last  cells  formed  during 
embryonic  development,  whether  differentiated  as  epithelial  or  vas- 
cular or  nerve  cells,  will  have  the  exact  structural  pattern  of  the 
original  48  chromosomes  in  the  fertilized  egg.  Again,  every  individual 
belonging  to  the  human  species  will  be  found  to  have  48  chromosomes 
of  the  same  type,  no  matter  where  he  lives  or  what  his  nationality  or 
race  happen  to  be.  This  condition  is  only  what  is  to  be  expected  if, 
as  we  know,  the  composite  characteristics  of  an  organism  are  the 


334 


HUMAN  BIOLOGY 


"outcropping"  of  the  gene  complex  established  in  the  fertilized  egg. 
Or,  stated  in  another  way,  individuals  that  exhibit  the  same  character- 
istics, so  that  they  are  placed  in  a  single  species,  will  have  the  same 
chromatin  complex  as  shown  by  an  exact  uniformity  in  the  number  and 
type  of  chromosomes.  A  microscopic  examination  of  the  chromosome 
patterns  of  even  closely  related  species,  for  example  those  of  the  horse 


FIG.  177. — The  chromosome  complex  in  man. 
B,  the  arrangement  of  the  chromosomes  by  pairs. 
Family"  The  Williams  &  Wilkins  Company.) 


A,  the  normal  chromosome  pattern; 
(Painter,  "Eugenics,  Genetics,  and  the 


and  the  ass,  reveals  distinct  differences.  Possibly  all  this  is  only  a 
rather  involved  way  of  emphasizing  the  fact  that  every  type  of  organ- 
ism has  its  own  distinctive  gene  complex,  and  variations  in  these  are 
apparent  to  the  cytologist  in  the  numbers  and  shapes  of  the  chromo- 
somes. (Figs.  177,  220.) 


FIG.  178. — Illustrating  the  individuality  of  the  chromosomes.  A,  chromosomes  of  a 
plant  louse.  The  homologous  chromosomes  are  given  the  same  number.  B,  chromo- 
somes of  a  beetle;  C,  chromosomes  of  a  seed  plant.  (Wilson.) 

During  the  present  century  and  even  before,  a  great  deal  of  inten- 
sive research  work  has  been  done  by  the  cytologists  on  the  chromosome 
patterns  of  many  species  of  plants  and  animals.  The  accumulated 
data  show  that  wide  variation  occurs  in  the  different  species  both  with 
respect  to  the  number  of  chromosomes  and  their  distinctive  structural 
features.  In  a  particular  species,  however,  as  emphasized  above,  the 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II)        335 


chromosome  complex  (karyotype)  is  always  constant.     The  results 

from  the  chromosome  studies  have  yielded  important  evidence  with 

regard  to  the  degree  of  relationship  among  various  types.     Thus,  to 

take  one  important  example,  from  a  study  of  the  chromosome  patterns 

of  several  different  species  of  Drosophila,  it  is^evident  that  all  the 

specific  patterns  "bear  a  general  resemblance  to  one  another  but  show 

characteristic  minor  differences."     Since  the  various 

species  of  Drosophila  present  in  a  particular  genus 

show  specific  differences,  it  is  to  be  expected  that  the 

karyotypes  of  these  same  species  would  also   show 

minor  differences.     But  the  impression  must  not  be 

given  that  the  structural  pattern  of  the  chromosomes 

can  be  directly  associated  with  the  morphology  of  the 

individual  or,  in  other  words,  that  it  would  be  possible 

by  a  study  of  the  chromosomes  in  a  fertilized  egg  to 

arrive  at  any  conclusion  relative  to  the  characteristics 

of  an  organism  developing  from  such  a  complex.     Such 

is  not  the  case.     Nor  is  there  any  apparent  relationship 

between  the  number  of  chromosomes  and  the  relative 

complexity  of  an  organism.     (Fig.  178.) 

Inasmuch  as  the  more  complex  organism  possesses  a  greater  number 
of  characteristics  to  be  determined  than  does  a  relatively  simple 
organism,  it  might  be  expected  that  the  former  would  have  more 
determiners,  or  genes,  for  these  characters  and,  therefore/'  that  the 
chromosome  number  would  be  larger.  It  is  found,  however,  that 
some  of  the  highest  chromosome  counts  are  in  the  Protozoa,  whereas 


w 

n      m 

Fig.  179.— 
The  normal  dip- 
loid  chromosome 
complex  (karyo- 
type) of  the  fruit 
fly,  Drosophila 
melanogaster . 
(Sharp,  adapted 
from  Morgan, 
Sturtevant, 
Bridges,  and 
Stern.) 


yellow  (body  color)  prune 

aete  (bnstfe  char.)   \(eye  color) 
ie  (bristle  * 


\^,dchaete( bristle  char.)   \(eye  color)       \  facet  (eye  ch A 
\scute  (bristle  char.)         \  \       I  eel 

>T  broAd(wing)    \  \     /  I 

&j^lD]!19iD 


ite  (eye  color) 
facet  (eye  cha.r.) 

e  chin  us  (eye  char.) 


FIG.  180. — Drawing  of  a  terminal  portion  of  a  giant  X-chromosome  from  a  salivary 
gland  cell  of  Drosophila  to  show  the  position  of  certain  genes  which  determine  body  and 
eye  color  and  other  characteristics  of  wings  and  bristles.  Cf.  Figs.  181,  183.  (Painter, 
"Science  in  Progress,"  Yale  University  Press.) 

Drosophila,  a  highly  developed  invertebrate,  has  only  eight  chromo- 
somes. It  is  possible,  however,  that  the  eight  chromosomes  in  tb^ 
Drosophila  karotype  contain  many  more  genes  than  do  the  much 
more  numerous  chromosomes  of  the  protozoan  cell.  It  is  apparent 
that  chromosome  number  is  far  too  coarse  a  measure  to  use  in  estab- 
lishing a  direct  relationship  between  chromatin  structure  and  body 


336 


HUMAN  BIOLOGY 


P-,  (0.   yellowCB) 
\  SO  ±  Hairy  wing  CW) 
\  I0.t  scute  CHJ 
V,  0.3  lethal -7* 
\  '«Q6  broad  CW) 
\  1.    prune  CE) 
\  \5  White  CE) 
\\[3.    facetCE) 
\  13.*  Notch  CE) 
\  4.5  Abnormal  CB) 
,\55  echinus  CH) 
•\\a9bifid(W) 


structure.  If  the  genes  themselves  were  large  enough  to  be  directly 
studied  under  the  microscope,  just  as  we  can  now  study  the  chromo- 
somes, undoubtedly  structural  features  in  the  organism  could  be 
directly  associated  with  the  structure  pattern  in 
a  particular  gene  or  group  of  genes.  It  has  been 
previously  stated  that  it  has  been  found  possible 
in  a  few  organisms  to  determine  the  definite 
region  in  a  particular  chromosome  in  which  the 
genes  responsible  for  certain  bodily  structures 
are  situated  and  thus  to  make  a  chromosome 
map.  These  results,  as  will  be  shown  later,  were 
^rst  obtained  by  breeding  experiments  in  Droso- 
phila,  but  it  has  been  possible  to  confirm  them 
by  direct  microscopic  observations  on  the 
chromosomes.  Within  the  last  few  years,  it  has 
been  discovered  that  the  salivary  glands  of 
Drosophila  contain  cells  bearing  relatively 
enormous  giant  chromosomes  in  their  nuclei. 
Intensive  microscopical  studies  on  this  material 
have  brought  the  observations  of  the  cytologist 
very  close  to  actual  gene  visibility  and  have 
greatly  added  to  our  knowledge  of  the  minutest 
elements  of  chromosome  structure,  in  fact,  the 
way  the  genes  are  actually  arranged.  We  may 
now  briefly  indicate  the  minute  structural  pattern 
of  chromosomes  as  shown  by  the  study  of  the 
giant  chromosomes  from  the  salivary  gland  cells. 
(Figs.  179  to  182.) 

The  giant  chromosomes  are  elongated  cylin- 
drical bodies  with  more  or  less  regularly  spaced 
bands  encircling  them  throughout  their  length. 
Their  appearance  under  the  microscope  reminds 
one  of  an  earthworm  with  its  segmented  body 
indicative  of  a  characteristic  internal  arrange- 
ment of  the  various  parts.  In  the  same  way,  the 
external  segmentation  or  banding  of  the  chromo- 
somes presents  visible  evidence  to  the  micro- 
scopist  of  a  definite  arrangement  of  the  genes  or  at  least  of  the 
regions,  or  "homes,"  that  they  inhabit.  For  each  external  band  of  a 
chromosome  marks  the  outer  boundary  or  periphery  of  a  disc-like 
structure  lying  at  right  angles  to  the  main  axis  of  the  elongated 
chromosome.  Certain  evidence  goes  to  show  that  the  actual  genes 


sveinl 

\I6*  club  (W) 
Hit  deltexCW) 

20.  cut  CW) 

21.  singed  CH) 
27.5  tan  CB) 
27.7  lorenge  CE) 


33.   vermillionCE) 
36.1  miniature  CW) 
!  36.2  dusky  CW) 

-  38.±  furrowed  CE) 

43,    sable  CB) 
44.4  garnet  Cfe) 


642  small  wing 
54.5  rudim*rtBry(W) 

56.5  forked  CH) 
57.    BarCE) 
585  smalt  eye 
59.    fused  <$) 

59.6  BeadexCW) 
62,    Minute-n  (H) 

65.    cleft  CM 


TO.  bobbedCH) 
FIG.  181. — Showing 
the  complete  chromo- 
some map  as  deter- 
mined for  the  X-chro- 
mosome  of  Drosophila 
shown  in  Fig.  180. 
(Sharp,  adapted  from 
Morgan,  Sturtevant, 
Bridges,  and  Stern.) 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II)        337 

are  grouped  in  these  disc-like  bands  throughout  the  length  of  the 
chromosome.  There  seems  to  be  no  question  but  that  the  genes,  even 
in  the  giant  chromosomes,  are  somewhat  too  small  to  be  seen  under  the 
highest  magnifications.  (Fig.  183a,  6,  c,  etc.) 

Fortunately  the  giant  salivary  gland  chromosomes,  when  prepared* 
and  stained  in  the  proper  manner,  are  essentially  transparent,  which 
makes  it  possible  for  the  cytologist  to  study  their  complex  internal 
arrangement.  In  such  observations,  the  microscopist  must  make  use 
of  the  highest  magnifications  available,  and  even  then  it  is  not  possible 
to  speak  with  absolute  authority  on  the  finest  details  of  chromosome 
structure.  The  concensus  of  opinion  at  present  seems  to  be  that  the 
typical  giant  chromosome  is  essentially  cable-like  in  its  structural 
pattern  with  a  variable  number  of  linear  units,  the  chromonemata, 


FIG.  182. — The  entire  group  of  giant  salivary  gland  chromosomes  of  Drpsophila. 
They  are  attached  in  the  center  to  a  granular  material,  chromocenter,  which  lacks  genes. 
(Shull,  after  Painter,  modified.) 

spirally  wound  together  to  form  the  chromosome  body.  In  the  giant 
chromosomes,  64  chromonemata  have  been  observed,  but  the  number 
may  be  more  or  less.  Embedded  in  the  thread-like  chromonemata,  at 
rather  regularly  spaced  intervals,  are  chromatin  particles,  the  chromo- 
meres,  appearing  like  knots  tied  at  more  or  less  regular  intervals  in  an 
elastic  thread.  The  elastic  threads  are  the  chromonemata,  and  the 
simile  appears  to  be  unusually  descriptive,  for  it  is  even  possible  to 
stretch  the  fresh  salivary  gland  chromosomes  and  thus  increase  their 
length  somewhat.  Under  such  conditions,  it  is  the  chromonemata  that 
are  stretched,  and  this  stretching  increases  the  distances  between  the 
knot-like  chromomeres  present  on  the  filamentous  elastic  chromo- 
nemata. (Fig.  183.) 

Chromosomes,  chromonemata,  and  the  chromomeres,  the  latter 
being  the  smallest  elements  visible  under  the  microscope — where  are 
the  actual  genes,  and  how  can  the  external  banded  areas  be  linked  up 
with  the  internal  structure  as  just  given?  The  evidence  is  that  the 


338 


HUMAN  BIOLOGY 


genes  are  present  in  the  chromomeres.  The  latter,  as  microscopically 
visible  particles,  are  undoubtedly  too  large  to  be  the  actual  genes,  but 
they  are  believed  to  be  essentially  a  chromatin  covering  which  sur- 
rounds and  encloses  the  genes.  Finally,  what  is  the  relation  between 
the  chromomeres  and  the  banded  discs  mentioned  as  the  home  of  the 
genes  hi  the  preceding  paragraph?  This  condition  may  perhaps  be 
understood  by  saying  that  the  chromomeres  are  concentrated  in  the 
disc-shaped  areas.  For  sake  of  illustration,  let  us  visualize  the  discs  as 
many-roomed  mansions  and  the  chromomeres  as  private  rooms 
inhabited  by  the  genes.  The  chromonemata  appear  to  be  strung  along 
like  telephone  wires  from  one  "gene  dwelling"  to  the  next  throughout 
the  length  of  the  chromosome. 


9  ,  ^-^y ^^n ~^*~  — -^, ^OQ-^OO 

^  protein     protein 

FIG.  183. — Drawing  of  (1)  a  portion  of  giant  salivary  gland  chromosome  of  the  black 
fly,  Simulium  virgatum,  maximum  magnification  to  show  the  finest  details;  (2)  a  single 
chromeric  thread  further  schematized  to  show  the  linear  arrangement  of  the  constituent 
elements  of  (1),  as  described  on  page  338.  The  chromonemata  are  seen  as  longitudinal, 
parallel  lines.  (Painter,  "  Science  in  Progress,"  Yale  University  Press.) 

One  cannot  study  the  details  of  mitosis  and  the  amazingly  exact 
and  intricate  construction  of  the  chromosomes,  as  just  indicated, 
without  being  tremendously  impressed  with  the  basic  importance  of 
these  cellular  elements.  If  an  experiment  were  devised  to  show  con- 
clusively that  the  chromatin  material  is  the  basis  of  heredity,  it  would 
probably  consist  in  transferring  living  chromosomes  from  the  nucleus 
of  one  cell  to  that  of  another  cell  and  then  studying  the  effect  of  the 
transposed  chromatin  in  the  succeeding  cell  divisions.  So  far  as  can  be 
seen  at  present,  experiments  involving  the  actual  transfer  of  chromatin 
from  cell  to  cell  lie  beyond  the  range  of  the  experimental  biologist,  but, 
fortunately,  data  are  available  from  experiments  involving  the  same 
principles  that  are  continually  being  performed  in  the  greatest  labora- 
tory of  all,  the  laboratory  of  nature,  and  by  the  greatest  of  all  experi- 
menters, nature. 

In  the  past  few  years,  numerous  examples  of  chromosomal  irregu- 
larities, or  aberration^,  have  been  discovered  in  well-known  species  of 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II)        339 

plants  and  animals.  In  some  of  these,  additional  chromosomes  have 
been  added  to  the  normal  number;  in  other  cases,  the  chromatin 
material  has  been  reduced  below  that  normally  present.  Now  when 
the  normal  chromosome  complex  is  altered,  it  is  found  that  the 
individuals  exhibit  corresponding  changes  in  various  structural  charac- 
ters. Speaking  very  generally,  it  is  found  that,  when  the  cells  of  an 
organism  carry  an  increased  number  of  chromosomes,  the  individual  is 
larger  in  size  and  is  also  marked  by  other  altered  characteristics  depend- 
ing upon  the  gene  content  of  the  added  chromosomes.  Thus  fruit, 
flies  have  been  found  with  a  50  per  cent  increase  in  the  chromosome 
number  so  that  ther^were  12  chromosomes  in  each  cell  instead  of  the 
normal  number  of  eight.  Such  animals  are  larger  and  show  o>ther 
modifications  of  the  nornial  species  pattern.  Many  other  instances 
have  been  found,  more  commonly  among  plants,  and,  always,  the 
results  of  altered  chromatin  pattern  are  evident  in  the  individual. 
Some  of  the  important  results  obtained  in  this  field  will  be  considered 
in  the  next  chapter  in  dealing  with  problems  of  heredity.  (Fig. 
184.) 

In  spite  of  the  fact,  as  noted  above,  that  the  biologist  is  decidedly 
limited  in  his  ability  to  experiment  with  chromosomes  directly,  it 
has  been  found  possible  to  make  use  of  the  X  rays  in  altering  the  normal 
chromosomal  pattern  of  the  living  cells.  Some  years  ago,  it  was 
discovered  that  irradiation  of  the  living  male  germ  cells  undergoing 
development  in  the  testis  of  Drosophila  caused  decisive  changes  in  the 
chromatin  pattern  of  these  cells  so  that  later,  when  the  sperm  develop- 
ing from  these  irradiated  gametes  were  used  to  fertilize  eggs,  a  con- 
siderable proportion  of  the  resulting  offspring  were  found  to  show 
various  abnormalities,  or  mutations.  Microscopic  examination  of  the 
chromosomes  from  the  mutated  animals  revealed  visible  changes  in 
the  chromosomes.  These  epoch-making  results  from  irradiation, 
which  have  widely  extended  in  various  plant  types,  once  more  con- 
firmed the  basic  fact  that  altered  chromatin  means  an  altered  heredity 
in  the  daughter  cells  and  the  establishment  of  visible  modifications  in 
the  cells,  tissues,  organs,  and  organism  formed  from  the  mutated  cell 
or  cells.  (Fig.  207.) 

Finally,  it  is  established  that  the  characteristic  differences  between 
male  and  female  are  definitely  associated  with  what  appears  to  be, 
from  the  structural  standpoint,  very  slight  differences  in  the  chromo- 
some complex  of  the  two  sexes.  Almost  forty  years  ago,  it  was  found 
that  all  the  body  cells  of  the  females  in  a  certain  species  of  insect  had  a 
pair  of  chromosomes  that  were  visibly  different  from  those  present  in 
the  body  cells  of  the  males.  In  tracing  this  sex  individuality  back 


FIG.  184. — Photographs  of  the  jimsonweed  (Datura)  with  different  chromosome 
complexes,  as  follows:  Above,  haploid;  middle,  diploid;  below,  tetraploid.  {Sinnott  and 
Dunn,  after  BLakeslee,} 

340 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II)        341 

to  the  chromosomes  of  the  germ  cells,  it  was  found  that  all  the  mature 
eggs  contained  one  sex  chromosome  of  the  same  type,  which  we  may 
designate  as  X,  but  that  the  sperm  were  equally  divided  between  two 
types;  one  with  an  X  chromosome,  as  in  the  eggs,  and  one  with  another 
type,  known  as  the  Y  chromosome.  Union  of  an  egg  with  an  X  sperm 
gave  the  X-X  pattern  in  the  zygote,  and  this  would  develop  into  a 
female;  fertilization  with  a  Y  sperm  gave  the  X-Y  pattern  in  the  zygote 
which  resulted  in  a  male  individual.  Further  consideration  of  this 
question  will  better  be  deferred  until  later,  but  the  important  point  has 
been  established,  namely,  that  even  the  characteristic  differences 
between  the  male  and  female  have  their  origin  in  a  distinctive  gene 
pattern.  (Fig.  179.) 

GERM  CELL  FORMATION 

The  description  of  reproduction  and  chromatin  behavior  in  the 
body  cells,  based  upon  mitosis,  has  been  presented  in  sufficient  detail 
so  that  we  may  turn  to  another  and,  possibly,  even  more  important 
phase  of  reproduction,  namely,  germ  cell  formation.  To  get  the 
essential  background,  it  will  be  desirable  to  recall  for  a  moment  the 
early  stages  of  embryonic  development  as  described  in  the  previous 
chapter  (page  295).  It  was  shown  there  that  repeated  cleavages 
extending  through  the  blastula  stage  result  in  the  formation  of*  a 
considerable  number  of  cells,  all  belonging  to  the  ectoderm,  the  first  of 
the  primary  germ  layers.  Then  the  gastrula  stage  appears,  character- 
ized by  the  development  of  endoderm  cells.  A  little  later,  mesoderm 
cells  are  differentiated.  Thus,  cellular  differentiation  appears  among 
the  constituent  embryonic  cells,  all  of  which  are  direct  descendants  of 
the  original  zygote.  What  is  responsible  for  this  differentiation,  the 
appearance  of  which,  in  cells  with  the  same  chromosome  complex, 
seems  to  argue  against  the  chromosome  basis  of  heredity  as  discussed 
in  the  earlier  pages  of  this  chapter?  The  answer  seems  to  lie  in  minute 
changes  occurring  in  the  chromatin  of  the  daughter  cells;  changes  too 
small  to  be  visible  under  the  highest  magnifications.  Thus  the  most 
searching  examination  of  the  nuclei  of  ectoderm,  endoderm,  and  meso- 
derm cells  reveals  no  differences  in  the  structural  pattern  of  the  chromo- 
somes of  tho  different  types  of  cells,  but  undoubtedly  ultramicroscopic 
differences  are  present  in  the  gene  complex,  possibly  extending  down 
to  the  molecular  level.  The  apparent  uniformity  of  the  mitotic 
phenomena  really  masks  an  inherent  ability  to  segregate  specific 
differentiating  materials  to  particular  cells  during  development.  This 
segregation  is  the  basis  of  the  gradual  and  orderly  processes  of  differ- 


342 


HUMAN  BIOLOGY 


entiation  which  assume  such  amaring  proportions  in  vertebrate  ani- 
mals.    (Fig.  155.) 

The  differentiation  processes  during  development  result  in  the 
formation  of  the  six  basic  tissues  of  the  vertebrate  organism,  namely, 
epithelial,  supporting,  vascular,  muscle,  nerve,  and  reproductive 
(page  23).  Various  combinations  of  these  tissues  are  responsible  for 
the  formation  of  the  organs  and  organ  systems  that  unitedly  form 
the  functioning  organism;  and  all  of  them,  with  the  exception  of  the 
reproductive  tissues,  are  concerned  with  the  maintenance  of  the 
structural  and  functional  unity  of  the  individual  organism.  Together 


C   D 

FIG.  185. — Diagrams  illustrating  the  early  differentiation  of  somatic  and  germ 
cells  in  Ascaris  megaloeephala  during  the  cleavage  of  the  egg.  Germ  cells  at  right  in  A ,  B; 
upper  right  in  (7,  D.  Schematized.  (Shult,  after  Fogg.) 

they  comprise  the  soma  of  the  individual  that  houses  the  germ  plasm 
and,  as  a  matter  of  fact,  make  it  possible  for  the  germ  plasm  to  survive 
and  to  function  in  reproduction,  thus  maintaining  the  species.  The 
time  at  which  definite  germinal  cells  are  first  segregated  during 
embryonic  development  varies  widely  in  the  animal  kingdom.  Thus 
in  the  parasitic  worm  Ascaris,  which  supplies  such  important  material 
for  the  study  of  the  mitotic  phenomena,  the  first  cleavage  of  the  zygote 
reveals  differences  in  the  nuclei  of  the  two  daughter  cells,  and  it  is 
established  that  the  descendants  of  one  of  these  two  cells  will  form  the 
germinal  cells.  In  most  species,  visible  differentiation  between  soma 
and  germ  plasm  does  not  occur  until  very  much  later  in  development. 
(Fig.  185.) 

But  whether  it  be  early  in  development  or  later,  the  main  point  is 
that  in  the  individuals  of  species  that  reproduce  sexually,  differentia- 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II)        343 

tion  of  germinal  material  occurs,  and  the  germ  cells  thus  formed  pass 
through  a  series  of  developmental  stages,  the  process  of  gametogenesis, 
which  ends  in  the  formation  of  the  mature  gametes,  eggs  or  sperm. 
Gametogenesis  may  conveniently  be  divided  into  spermatogenesis  and 
oogenesis  in  correspondence  with  the  type  of  gamete  produced.  The 
basic  features  of  germ  cell  maturation,  involving  the  exact  preparation 
of  the  chromatin  material  for  transfer  of  hereditary  characters  to  the 
next  generation  at  the  time  of  fertilization,  are  the  same  in  the  male 
cells  as  in  the  female,  though  as  we  know,  from  the  description  given 
in  the  preceding  chapter,  the  structural  features  of  mature  sperm  and 
egg  are  markedly  different. 

The  heart  of  sexual  reproduction  is  fertilization,  and  the  essential 
feature  of  fertilization  is  the  fusion,  or  amphimixis,  of  the  chromatin 
material  in  the  nucleus  of  the  male  sperm  with  that  present  in  the 
nucleus  of  the  female  egg;  thus  forming  the  fusion  nucleus,  or  synkar- 
yon,  of  the  zygote.  Since  we  know  that  the  characteristics  of  the 
daughter  cells  formed  by  mitosis  depend  upon  their  receiving  the  nor- 
mal chromatin  complex  from  the  parental  cell,  it  is  no  less  apparent 
that,  in  reproduction,  the  integrity  of  the  new  individuals  depends 
upon  receiving  the  exact  inheritance  of  maternal  and  paternal  chro- 
matin at  fertilization.  With  this  condition  in  mind,  it  is  clear  that 
the  zygote  nucleus  cannot  possibly  receive  the  normal  chromatin 
content  of  the  species  if  both  the  sperm  nucleus  and  the  egg  nucleus 
transfer  to  it  the  full  amount  of  chromatin  material,  for  a  doubling 
of  the  chromatin  content,  evidenced  by  double  the  usual  number  of 
chromosomes,  would  be  bound  to  result.  The  upshot  of  the  matter 
is,  as  has  long  been  recognized,  that,  in  spermatogenesis  and  in  oogene- 
sis, the  chromatin  material  of  each  germ  cell  is  reduced  to  one-half 
the  normal  amount,  and  this  condition  is  visibly  shown  in  an  actual 
reduction  of  the  .chromosome  number  to  one-half  that  found  in  any  other 
cells  of  the  body  of  that  particular  species.  It  may  be  stated  very 
simply:  A  mature  sperm  or  a  mature  egg  contains  only  one-half  the 
somatic  number  of  chromosomes.  As  a  result  of  this  chromosome 
reduction,  it  is  evident  that  when  a  sperm  nucleus  unites  with  an  egg 
nucleus  in  fertilization,  the  chromosome  number  of  the  zygote  nucleus 
is  restored  to  the  characteristic  number  and  not  doubled  as  it  would 
be  otherwise.  (Fig.  186.) 

Chromatin  reduction  in  the  germ  cells,  like  chromatinic  behavior  in 
mitosis,  is  not  a  haphazard  process  but  is  so  arranged  that  each  mature 
germ  cell  will  carry  a  complete  set  of  genes  for  all  the  characters  of  that 
particular  type  of  organism,  though  the  chromosome  number  is  reduced 
one-half.  In  order  to  understand  how  this  is  accomplished,  it  will  be 


344 


HUMAN  BIOLOGY 


necessary  to  reexamine  the  normal  chromatin  complex  in  the  nucleus 
of  a  body  cell  and  see  just  what  the  condition  is  there.  Careful 
studies  on  animal  tissues  have  shown  that  the  nuclei  of  every  type 
of  somatic  cell  and  the  nuclei  in  the  immature  germ  cells,  as  well, 
always  contain  a  double,  or  diploid,  set  of  chromosomes.  The  chromo- 
somes are  in  pairs;  every  chromosome  has  a  mate  which  is  an  exact 
duplicate,  or,  as  the  cytologists  say,  a  homologous  chromosome. 
Thus,  technically,  instead  of  saying  that  Drosophila  has  eight  chromo- 
somes, it  is  preferable  to  say  that  it  has  four  pairs  of  chromosomes  or 
that  the  diploid  number  is  eight.  Also  in  Man,  since  the  diploid  num- 
ber is  48,  24  pairs  of  chromosomes  are  present.  The  individuality  of 
each  chromosome  is  so  marked  that  it  is  possible  for  the  experienced 


FIG.  186. — Illustrating  the  history  of  the  chromosomes  in  the  animal  life  cycle. 
Male,  above;  female,  below;  union  of  sperm  and  egg  to  produce  the  new  individual,  at 
the  right.  (Sharp.) 

cytologist  to  identify  the  two  members  of  each  pair  of  homologous 
chromosomes.  To  do  this  is  comparatively  simple  in  cases  like 
Drosophila  where  only  four  pairs  are  present,  but  the  chromosome 
identification  presents  considerable  difficulty  where  numerous  pairs 
are  present  as  in  man. 

A  moment's  consideration  will  show  that  the  essence  of  chromo- 
some reduction  during  germ  cell  formation  is  the  sorting  out  of  one 
complete  set  of  chromosomes,  the  haploid  number,  for  transfer  to  each 
germ  cell,  whether  egg  or  sperm.  When  the  sperm  carrying  the  pater- 
nal chromatin  later  unites  in  fertilization  with  the  nucleus  of  the  egg 
carrying  the  maternal  chromatin,  the  synkaryon  nucleus  of  the  zygotc 
is  equipped  with  two  complete  sets*  of  chromosomes,  the  diploid 
number,  and,  furthermore,  the  two  homologous  chromosomes  of  each 
pair  present  in  the  zygote  have  a  diverse  ancestry;  one  chromosome 
from  each  pair  is  of  paternal  origin  by  way  of  the  sperm,  and  the  other 
chromosome  is  of  maternal  origin  by  way  of  the  egg.  The  successive 
mitotic  cell  divisions  of  the  zygote,  during  embryonic  development, 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II)        345 

present  to  every  cell  of  the  body  exact  replicas  of  the  paternal  and 
maternal  chromosomes  received  by  the  zygote  nucleus  at  fertilization. 
Thus  biparental  inheritance,  the  unique  characteristic  of  sexual  repro- 
duction, extends  to  every  cell  of  the  individual. 

SPERMATOGENESIS 

With  the  general  relationships  between  the  chromatin  of  soma  and 
germ  plasm  in  mind,  it  is  next  in  order  to  examine  the  process  of  germ 
cell  formation  and  to  find  out  just  how  these  basic  conditions  for 
heredity  are  maintained.  The  development  of  the  functional  sperm 
may  be  considered  first.  The  structural  plan  of  the  testis  has  been 
described  in  the  previous  chapter  (page  315).  It  was  there  shown 
that  the  sperm  are  matured  in  the  seminiferous  tubules,  the  successive 
stages  of  development  culminating  in  the  production  of  free-swimming 
sperm  which  are  liberated  and  pass  from  the  tubules  to  the  exterior. 
The  problems  involved  in  the  morphological  transformation  of  the 
early,  or  primordial,  male  germ  cells,  which  appear  as  typical  cells, 
to  the  bizarre  motile  sperm  are  of  great  complexity,  but  they  are  not 
of  primary  concern  in  this  discussion  for  our  interest  lies  in  the  behavior 
of  the  chromatin.  Six  characteristic  stages  of  spermatogenesis  merit 
our  consideration;  the  first  of  these  is  the  primordial  germ  cell  which 
appears  as  a  recognizable  type  at  some  period  during  embryonic 
development  and  is  segregated  in  the  seminiferous  tubules  of  the 
testis.  The  early  history  of  the  primordial  germ  cells  is  essentially 
uneventful.  They  are  small  in  size,  but  with  a  comparatively  large 
nucleus  containing  the  diploid  number  of  chromosomes  characteristic 
of  the  species.  Repeated  mitotic  divisions  of  the  primordial  germ 
cells,  the  so-called  multiplication  period,  results  in  the  formation  of 
great  numbers  of  spermatogonia  which  are  essentially  the  same  in 
structure  as  the  primordial  cells,  and  in  all  of  which  exact  and  normal 
mitoses  ensure  the  maintenance  of  the  specific  chromatin  complex. 
(Fig.  187.) 

The  inauguration  of  the  third  stage  in  spermatogenesis  is  indi- 
cated by  an  increase  in  cell  size;  for  after  an  indefinite  number  of 
divisions  involving  normal  mitosis,  each  of  the  daughter  spermatogonia 
enlarges  to  form  a  primary  spermatocyte.  Superficially,  a  primary 
spermatocyte  appears  as  a  greatly  enlarged  spermatogonium2  but  an 
examination  of  the  nucleus  reveals  that  the  reduction  phenomena, 
essential  to  germ  cell  maturation,  are  under  way.  Furthermore,  it  is 
found  that  a  germ  cell  having  reached  the  primary  spermatocyte  st&ge 
will  divide  only  twice  more;  the  first  division  forms  two  daughter  cells 
designated  ae  secondary  spermatocytes,  and  then  each  of  these  divides 


346 


HUMAN  BIOLOGY 


Fio.  187. — Diagrams  illustrating  meiosis  during  gametogenesis.  a,  diploid  germ  cell 
in  resting  condition;  b,  condensation  of  chromatin  to  form  three  pairs  of  homologous 
chromosomes;  c,  pairing  (synapsis)  of  homologous  chromosomes  as  in  primary  spermato- 
cyte  or  odcyte;  (d),  cleavage  spindle  in  equatorial  plate  stage;  e,  f,  separation  of  the 
homologous  chromosomes  and  migration  to  opposite  poles  of  the  spindle;  g,  resulting 
daughter  cells  with  reduced  number  of  chromosomes  as  in  a  secondary  spermatocyte  or 
odcyte.  Cf  Ff«.  186.  (Watkeya,  Stern.) 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II)        347 

to  form  two  spermatids.  The  spermatids,  without  further  cleavage,  are 
gradually  transformed  into  mature  sperm.  In  a  word,  then,  the  two 
final  cell  divisions  beyond  the  primary  spermatocy te  stage  result  in  the 
formation  of  four  sperm  from  each  primary  spermatocy  te.  (Fig.  186.) 

Synapsis. — In  the  previous  description  of  mitosis,  it  was  shown 
that  the  real  crux  of  the  process  was  the  longitudinal  splitting  of  each 
chromosome  and  the  separation  of  the  resulting  halves  in  such  a  way 
that  each  daughter  cell  would  receive  one-half  of  each  chromosome, 
thus  passing  to  the  nucleus  of  each  daughter  cell  the  exact  chromatiu 
complex  of  the  parent  cell.  Accordingly,  in  typical  mitosis,  just  aftei 
each  chromosome  has  split  in  the  anaphase  stage,  there  is  a  temporary 
doubling  of  the  diploid  number  of  chromosomes  in  a  cell.  This  pumber 
is  quickly  reduced  to  the  typical  diploid  number  as  the  cytoplasm  of  the 
cell  splits  into  two  daughter  cells  and  each  nucleus  receives  its  diploid 
quota.  Now  what  appears  to  be  the  reverse  of  the  chromatin  behavior 
in  normal  mitosis  is  evident  in  the  nucleus  of  a  primary  spermatocyte 
during  the  reduction  division  (meiosis),  for,  here,  the  chromosomes, 
instead  of  splitting,  fuse  together  in  pairs — the  process  of  synapsis — 
and  the  chromosome  number  in  the  nucleus  of  the  primary  spermato- 
cyte is  thus  reduced  one-half  to  form  the  haploid  number,  though  the 
total  amount  of  chromatin  in  the  nucleus  is,  of  course,  unchanged. 

But  now,  as  the  cytoplasm  of  the  primary  spermatocyte  prepares  to 
divide  to  form  the  two  daughter  secondary  spermatocytes,  the  tempo- 
rarily paired  chromosomes  separate  and  move  in  opposite  directions, 
the  final  result  being  that  the  nucleus  of  each  of  the  secondary  sperma- 
tocytes receives  one  chromosome  from  each  pair.  Thus  the  latter  have 
the  reduced,  or  haploid,  number  of  chromosomes.  But  again  it  is  not 
the  chromosome  number  in  itself  that  is  important;  it  is  the  fact  that 
each  secondary  spermatocyte,  as  a  result  of  the  reduction  division,  has 
one  complete  set  of  chomosomes  and  genes — one  chromosome  of  every 
kind — whereas  the  somatic  cells  have  two  of  every  kind.  (Fig.  187.) 

Next  comes  the  division  of  the  secondary  spermatocytes  to  form 
the  spermatids.  This  appears  as  a  normal  mi t otic  division  in  which 
each  of  the  haploid  chromosomes  splits  longitudinally  to  form  two 
daughter  chromosomes,  the  latter  being  distributed  so  as  to  give  one 
of  the  halves  to  each  of  the  spermatids.  Their  chromatin  heritage 
is  transferred  unchanged,  since  each  spermatid  is  gradually  trans- 
formed into  a  motile  sperm.  The  essence  of  spermatogenesis  is  clear; 
each  sperm  nucleus  contains  not  only  the  reduced  or  haploid  number 
of  chromosomes  but  one  complete  set  of  chromosomes  with  not  a  gene 
missing.  When  the  sperm  fertilizes  an  egg,  which  also  contains  a 
complete  haploid  set  of  chromosomes,  the  diploid  number  of  chromo- 


348  HUMAN  BIOLOGY 

somes — two  complete  sets,  one  of  maternal  and  one  of  paternal  origin 
— will  be  restored  in  the  zygote  nucleus  to  be  maintained  in  all  the 
cells  of  the  new  individual  until  the  time  when  germ  cells  are  once  more 
formed  and  synapsis  occurs. 

OOGENESIS 

It  has  been  indicated  that  the  maturation  of  both  sperm  and  egg 
presents  a  uniformity  in  the  essential  feature,  namely,  the  formation 
of  germ  cells  bearing  a  haploid  set  of  chromosomes.  In  accomplishing 
this  primary  aim,  the  female  germ  cells  pass  through  a  series  of  matura- 
tion stages  which  conform  very  closely  to  those  just  described  in 
sperma^ogenesJs.  There  are,  however,  certain  noteworthy  variations 
in  oogenesis  which  should  be  indicated.  The  general  behavior  of  the 
female  germ  cells  as  they  migrate  into  the  body  of  the  ovary,  the  forma- 
tion of  the  Graafian  follicles,  arid  the  final  release  of  the  female  germinal 
cells  to  the  oviduct  have  been  described  in  earlier  sections  (page  319). 
In  the  mammalian  ovary,  the  multiplication  period,  marked  by  the 
formation  of  considerable  numbers  of  primordial  germ  cells  through 
successive  mitoses,  occurs  before  birth,  and  a  considerable  number 
of  the  resulting  oogonia  have  by  that  time  penetrated  into  the  ovarian 
tissue  where  each  has  become  established  in  a  Graafian  follicle.  The 
next  stage  in  maturation,  the  primary  oocyte,  is  attained  by  the  growth 
of  single  oogonia  as  was  seen  in  the  development  of  the  primary 
spermatocyte.  Synapsis,  marked  by  the  pairing  of  the  homologous 
chromosomes,  is  the  basic  feature  of  the  primary  oocyte.  (Fig.  187.) 

Typically,  the  primary  oocyte  stage  is  the  final  one  occurring  in  the 
ovary,  and  the  immature  germ  cell,  released  by  the  rupture  of  the  large 
Graafian  follicle,  passes  into  the  oviduct.  The  completion  of  the 
maturation  process  depends  upon  union  with  the  sperm  cell.  If  sperm 
are  present  in  the  oviduct,  the  entrance  of  the  sperm  head  into  the 
cytoplasm  of  the  primary  oocyte  is  the  signal  for  the  completion  of 
maturation  so  that  fertilization  may  take  place.  If  no  sperm  are 
encountered,  the  final  stages  of  maturation  do  not  occur  and  the 
degeneration  of  the  primary  oocyte  soon  follows.  (Fig.  186.) 

The  sperm  nucleus  is  temporarily  inactive  following  its  entrance, 
but  the  primary  oocyte  nucleus  at  once  begins  the  final  maturation 
phenomena.  The  homologous  chromosomes,  joined  in  synapsis,  now 
separate,  thus  forming  two  independent  haploid  sets.  One  of  these 
passes  to  each  of  the  daughter  cells  as  the  primary  oocyte  divides  into 
daughter  cells.  But  the  two  cells  formed  from  the  primary  oocyte  are 
decidedly  unequal  in  size;  they  consist  of  a  large  cell,  which  is  the 
functional  secondary  oocyte,  and  a  miniature  cell,  the  first  polar  body, 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II)        349 


which  lies  in  close  contact  with  the  oocyte  like  a  wart  on  an  apple. 
Each  receives  a  haploid  set  of  chromosomes.  Now  comes  the  final 
stage  in  maturation;  the  chromosomes  of  the  secondary  oocyte  divide 
longitudinally  as  in  normal  mitosis  to  form  two  haploid  sets,  one  set  of 
which  is  transferred  to  the  nucleus  of  the  mature  egg,  while  the  other 
goes  to  the  second  polar  body;  for  the  division  of  the  cytoplasm  of  the 
secondary  oocyte,  like  that  of  the  primary  oocyte,  is  also  very  unequal. 
Practically  all  the  cytoplasm  passes  to  the  egg,  with  only  a  tiny  portion 
forming  the  second  polar  body.  Usually  the  first  polar  body  also 
undergoes  division  at  this  time,  so  that  the  final  result  is  the  formation 


A  B  C  D  t 

FIG.  188. — Diagrammatic;  drawings  comparing  mitosis  in  the  somatic  ceils  (above) 
with  meiosis  during  the  formation  of  gametes  (below).  In  mitosis  there  is  a  temporary 
doubling  of  the  diploid  chromosome  number  (C,  D)  so  that  each  of  the  daughter  cells 
(E)  receives  the  full  diploid  number.  In  meiosis  the  homologous  chromosomes  pair  (C) 
and  then  separate  (D)  to  give  the  daughter  cells  (E)  the  reduced  or  haploid  number. 
This  haploid  condition  will  be  maintained  through  two  more  divisions  to  produce  four 
mature  sperm  in  the  male,  or  one  mature  egg  and  three  polar  bodies  in  the  female;  all 
with  the  haploid  chromosome  complex.  (Sinnott  and  Dunn,  after  Sharp.  Modified.) 

of  one  large  functional  egg  and  three  nonfunctional  polar  bodies  from 
each  primary  oocyte.  The  sperm  nucleus,  which  entered  the  cyto- 
plasm of  the  primary  oocyte,  was  transferred  to  the  secondary  oocyte 
and  then  to  the  mature  egg.  Accordingly,  with  the  completion  of 
maturation  and  the  establishment  of  the  haploid  condition  in  the  egg, 
the  gametic  nuclei  (sperm  nucleus  and  egg  nucleus)  are  ready  for  the 
formation  of  a  fusion  nucleus,  the  synkaryon,  which  determines  the 
heredity  of  the  new  individual  and  the  nature  of  the  gene  complex 
which  it  will  transmit  to  the  next  generation  by  way  of  the  gametes. 
(Fig.  188.) 

FERTILIZATION 

It  will  now  be  profitable  to  describe  the  activities  that  occur  in  the 
egg  immediately  following  the  completion  of  maturation.  Micro- 


350 


HUMAN  BIOLOGY 


scopic  observations  show  that  the  two  gametic  nuclei  present  in  the 
cytoplasm  gradually  move  toward  each  other  until  they  meet,  usually 
near  the  center  of  the  egg,  with  the  nuclear  membranes  in  contact  for  a 
time.  Synchronously  the  centrosome,  which  was  brought  into  the  egg 
cytoplasm  by  the  middle  piece  of  the  sperm,  divides,  just  as  in  normal 
mitosis,  and  the  daughter  centrosomes  separate  to  the  accompaniment 
of  ray  and  spindle  fiber  formation.  The  nuclear  membranes  of  the 
two  gametic  nuclei  disappear,  and  the  amazing  climax  of  sexual 
reproduction  and  biparental  inheritance  is  at  hand :  the  mingling  of  the 
nuclear  elements  of  the  two  sexes  in  the  synkaryon  of  the  zygote,  the 
process  of  fertilization.  The  structural  elements  of  the  spindle  for  the 
first  mitotic  division  of  the  new  individual  is  now  well  established 


FIG.  189. — Fertilization  of  an  egg  (Ascidiari)  showing  (a)  the  fusion  (amphimixis) 
of  the  egg  nucleus  ( 9rc)  with  the  sperm  nucleus  (cfw)  and  the  inauguration  of  the  first 
cleavage  spindle;  (6)  a  later  stage  showing  the  cleavage  spindle  fully  developed. 
(Seifriz,  after  Conkliji.) 

in  the  typical  prophase  stage.  This  is  quickly  followed  by  the  meta- 
phase  with  the  paternal  and  maternal  chromosomes  meeting  for  the 
first  time  in  the  equatorial  plate  and  establishing  the  diploid  nuclear 
complex.  Then  comes  the  anaphase  ^tage,  with  each  chromosome 
splitting  longitudinally  and  moving  to  the  opposite  poles  of  the  spindle; 
finally,  the  telophase,  marked  by  the  division  of  the  cytoplasm  to  form 
two  diploid  daughter  cells.  And  thus  a  new  individual  of  a  new  genera- 
tion, be  it  frog,  chick,  or  man,  is  started  on  its  way.  Growth,  cell 
division;  growth,  cell  division,  with  differentiation  appearing  between 
groups  of  cells;  primary  germ  layers,  tissues,  organs,  organ  system; 
and,  finally,  the  mature  individual  is  at  hand  capable  of  producing 
germ  cells  for  the  perpetuation  of  the  species  through  still  another 
generation.  Such  is  the  story  of  sexual  reproduction  based  on 
biparental  inheritance.  (Fig.  189.) 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (11)        351 

Fertilization  is  often  confused  with  reproduction.  From  the 
previous  discussion,  it  is  evident  that  fertilization  is  really  the  reverse 
of  reproduction,  for  reproduction  is  a  process  by  which  cells  produce 
additional  cells.  In  fertilization,  on  the  contrary,  there  is  a  fusion  of 
two  cells  to  form  a  single  composite  cell,  the  zygote.  It  may  also  be 
thought  that,  even  though  fertilization  is  not  really  reproduction,  it  is 
essential  to  this  function  in  the  multiccllular  organism.  But,  as  shown 
previously,  many  organisms  normally  reproduce  asexually.  In 
addition,  the  eggs  of  a  highly  developed  animal,  such  as  the  honeybee, 
may  develop  either  with  or  without  fertilization.  Finally,  the  experi- 
mental results,  as  previously  indicated,  have  shown  that  even  in  some 
vertebrates  it  is  possible  to  get  the  egg  to  develop  without  fertilization 
(page  286).  Under  normal  conditions,  then,  fertilization  may  be 
regarded  as  having  two  functions:  first,  as  an  activator  which  is 
responsible  for  the  inauguration  of  cell  division  in  the  zygote  and, 
second,  as  a  bearer  of  the  parental  chromatin  which  makes  biparental 
inheritance  possible.  This  latter  function,  namely,  conferring  bipar- 
ental inheritance  on  the  offspring,  can  occur  only  through 
fertilization. 

With  the  sequence  of  events  in  spermatogenesis  and  oogenesis  in 
mind,  more  detailed  attention  may  be  given  to  the  basic  feature  of 
germ  cell  maturation,  namely,  the  pairing  of  the  chromosomes  in 
synapsis.  It  is  already  apparent  that  the  union  of  the  chromosomes 
in  pairs  is  not  a  hit-or-miss  arrangement.  In  the  diploid  nucleus,  as  we 
have  seen,  are  two  chromosomes  of  every  kind,  one  of  each  pair  of  the 
homologous  chromosomes  being  received  at  fertilization  from  the  male 
parent  and  one  from  the  female  (page  346).  Synaptic  pairing  occurs 
only  between  homologous  chromosomes,  and  so  it  gives  the  opportunity 
for  the  maternal  and  paternal  genes  of  the  previous  generation,  having 
passed  through  all  the  stages  of  the  new  individual  from  zygote  to 
adult,  to  determine  the  gene  complex  of  the  germ  cells  and  through 
them  the  inheritance  of  the  next  generation.  In  a  word,  synapsis 
preserves  the  essential  continuity  between  generations  and,  at  the 
same  time,  offers  opportunity  for  variation  depending  upon  the  nature 
of  the  genes  from  the  two  lines  that  pair  at  synapsis.  (Fig.  186.) 

Taking,  as  an  example,  the  chromatin  condition  in  Drosophila  with 
the  diploid  number  of  chromosomes  established  as  eight,  it  may  be 
helpful  to  portray  the  maturation  changes  graphically.  Since  there  are 
two  of  every  kind  of  chromosome  in  the  diploid  condition,  we  may 
designate  the  four  different  chromosomes  as  A,  B,  C,  and  D.  The 
diploid  condition  will  then  be  represented  by  a  doubling  of  each  kind  of 
chromosome,  or  A, A,  JS,5,  C,C,  D,A  to  give  a  total  of  eight  chromo- 


352  HUMAN  BIOLOGY 

somes.  This  diploid  condition  is  present  in  all  the  cells  of  the  organism 
except  the  germ  cells  which  have  attained  the  stage  of  development 
when  synapsis  occurs,  as  in  the  primary  spermatocyte  or  oocyte.  This 
process,  as  shown  above,  is  characterized  by  pairing  of  the  homologous 
chromosomes  and  a  reduction  in  the  total  number  to  one-half.  This 
condition  may  be  represented  as  A  A,  BB,  CC,  DD.  The  homologous 
chromosomes  then  separate,  and  one  complete  haploid  set  of  four 
chromosomes,  A,B,C,D,  goes  to  each  secondary  spermatocyte  and, 
finally,  after  each  has  divided,  to  the  mature  sperm.  The  chromosome 
behavior  in  human  sperm  development  can  be  illustrated  in  the  same 
way  by  utilizing  24  letters  to  indicate  the  24  different  types  of  chromo- 
some present  in  the  nuclei  of  the  cells. 

Sexual  reproduction  with  biparental  inheritance  makes  possible 
wide  variation  in  the  gene  complex  of  the  chromosomes  united  in  the 
zygote  nucleus  and,  therefore,  in  the  heredity  of  the  individuals 
developing  therefrom.  No  two  individuals  of  the  human  species  or 
any  other  species  are  exactly  alike.  In  order  to  gain  some  conception 
of  the  possibilities  for  variation  during  normal  germ  cell  formation  and 
fertilization,  we  shall  continue  our  examination  of  the  basic  process  of 
synapsis  involving  the  pairing  of  homologous  chromosomes  in  the 
germ  cells  of  Drosophila  with  eight  chromosomes  in  the  diploid  condi- 
tion or,  as  they  pair  in  synapsis,  A  A,  BB,  CC,  DD.  Close  study  of  these 
synaptic  pairs  shows  that  each  of  the  homologous  chromosomes  in 
synapsis  is  split  longitudinally,  just  as  previously  noted  in  normal 
mitosis,  so  that  really  four  chromosome  elements,  or  chromatids,  are  in 
close  association,  thus  forming  the  so-called  tetrad  condition.  Viewing 
a  synaptic  pair  of  chromosomes  endwise  it  is  possible  to  see  the  ends 
of  the  four  chromatids  in  close  contact.  Thus  the  arrangement  of  the 
four  synaptic  pairs  in  Drosophila  may  be  graphically  shown  as 

AA  BB  CC  DD 
AA'  BB'  CC'  DD' 

Tetrad  formation  may  be  thought  of  as  a  precocious  longitudinal 
splitting  of  the  chromosomes  paired  in  synapsis.  In  this  way,  four 
haploid  sets  of  chromosomes  A,B,C,D  are  formed  for  later  distribution 
to  the  spermatids  and  egg  cells;  one  haploid  set  goes  to  each  spermatid 
in  the  male;  in  the  female,  one  haploid  set  goes  to  the  functional  egg 
cell  and  one  set  to  each  of  the  three  polar  bodies.  The  important  fact 
to  realize  is  that,  though  each  of  the  four  chromosomes  A, A, A, A 
coming  from  the  tetrad  condition  in  synapsis  carries  the  genes  for  the 
same  characters  with  exactly  the  same  linear  arrangement,  variation 
may  occur  in  the  way  in  which  the  characters  are  expressed.  To  take 
an  example  of  eye  color  which,  let  us  say,  is  determined  by  a  gene  or 


THE  BIOLOGY  OF  GROWTH  AND  REPRODUCTION  (II)        353 

AA 
genes  in  the  synaptic  chomosome  pair  £*  the  two  chromosomes  A  A 


of  maternal  origin  might  carry  genes  for  blue  color,  while  the  other 
two  A  A  chromosomes  of  paternal  origin  carry  genes  for  brown  eye 
color.  If  this  were  the  case,  then  two  (50  per  cent)  of  the  mature 
sperm  would  carry  genes  for  blue  eyes,  and  two  (50  per  cent)  would 
carry  genes  for  brown  eyes.  But  this  problem  in  heredity  must  be 
deferred  for  consideration  in  the  next  chapter. 


CHAPTER  XIV 
THE  BIOLOGY  OF  INHERITANCE 

In  the  two  preceding  chapters,  an  endeavor  has  been  made  to 
present  the  essential  facts  of  reproduction,  first,  as  observed  from  a 
rather  distant  reviewing  stand  from  which  only  the  larger  features  of 
the  process  could  be  observed  and,  second,  as  observed  near  at  hand, 
with  the  aid  of  the  high-power  microscope,  in  an  endeavor  to  bring  to 
light  the  basic  cellular  activities  underlying  the  process  of  cellular 
reproduction,  whether  concerned  with  the  splitting  of  a  single  cell  into 
two  daughter  cells  by  typical  mitosis  or  the  production  of  a  highly 
differentiated  multiccllular  organism  through  the  union  of  specialized 
male  and  female  germ  cells. 

It  has  been  emphasized  that  in  the  reproduction  of  cells  or  of 
multicellular  organisms,  the  new  living  units  must  be  true  to  the 
parental  type.  At  the  same  time,  the  processes  of  reproduction  must 
permit  the  introduction  of  limited  and  controlled  variation  from  the 
parental  cell  types.  It  is  evident  that,  if  all  the  cells  formed  by  the 
repeated  divisions  of  the  zygote  remained  absolutely  true  to  type,  no 
possibility  would  exist  for  the  development  of  the  many  differentiated 
types  of  cells,  tissues,  and  organs  as  seen,  for  example,  in  the  vertebrate 
organism;  for  all  of  these  cells  trace  their  origin  back  to  one  cell,  the 
zygote.  We  are  well  aware  that  a  certain  amount  of  easily  recojgnizable 
variation  exists  between  adult  individuals,  even  when  closely  related. 
Body  size,  color  of  eyes,  color  and  character  of  hair,  facial  features, 
even  the  tone  of  the  voice,  all  have  distinctive  individual  qualities, 
though  conforming  to  the  general  pattern  to  which  all  the  individuals 
of  the  group  or  species  belong. 

And  so  the  mechanism  of  heredity,  contained  in  the  chromatin 
material  of  the  nucleus,  must  be  responsible  for  conformity  to  type  and 
also  for  individual  variation.  Finally,  the  biologist  of  today  sees  no 
possibility  of  accounting  for  the  origin  of  the  enormous  variety  of  plant 
and  animal  species  now  present  in  the  world  of  life  except  through  the 
hereditary  mechanism.  Since  the  establishment  of  sexual  reproduc- 
tion, it  must  have  been  true,  just  as  it  is  now,  that  the  characters  of  an 
offspring  are  determined  by  the  gene  content  of  the  paternal  and 
maternal  chromatin  received  at  the  time  of  fertilization.  The  appear- 

354 


THE  BIOLOGY  OF  INHERITANCE  355 

ance  of  a  new  type  or  species,  past  or  present,  must,  therefore,  be  the 
result  of  some  alteration  in  the  chromatin-gene  complex  of  a  cell  or 
cells  in  the  direct  line  of  descent.  The  chromatin  pattern  having  once 
been  altered  is  transferred  to  the  successive  generations  of  daughter 
cells. 

Probably  no  area  in  the  entire  field  of  biology  has  aroused  more 
interest  and,  accordingly,  been  the  object  of  greater  speculation  in 
times  past  than  the  field  of  heredity.  As  a  matter  of  fact,  it  has  been 
only  in  the  very  recent  years  that  the  knowledge  of  the  heredity 
phenomena  has  been  sufficient  to  remove  them  from  the  realijn  of 
speculation  and  wonder  to  the  field  of  established  fact.  At  the  present 
time,  the  essential  functional  features  of  the  hereditary  mechanism 
are  known,  though  many  obscure  facts  are  yet  to  be  brought  to  light. 

THE  PARTICULATE  NATURE  OF  INHERITANCE 

Present-day  knowledge  of  heredity  rests  upon  the  discovery  near 
the  middle  of  the  last  century  that  the  many  characters  of  an  organism 
are  inherited  independently  of  each  other  and  not  as  a  composite 
group.  Thus  eye  color,  to  take  a  common  example,  is  determined 
independently  of  the  other  features  that  are  associated  in  the  complete 
organism.  Credit  for  this  discovery  of  the  particulate  nature  of 
inheritance  goes  to  Mendel,  an  Austrian  monk,  who  became  interested 
in  the  inheritance  of  certain  characters  in  peas  and  used  the  monastery 
gardens  for  his  genetics  laboratory.  Unfortunately  his  results, 
published  in  1865  in  a  scientific  periodical  with  limited  distribution, 
lay  unnoticed  by  the  scientists  of  Mendel's  time,  and  it  was  not  until 
the  beginning  of  the  present  century,  almost  forty  years  after  their 
publication,  that  Mendel's  results  were  brought  to  the  attention  of  the 
biological  world.  A  remarkable  thing  about  Mendel's  work  is  that, 
without  any  knowledge  of  the  complex  mitotic  phenomena  associated 
with  inheritance,  he  was  able  to  deduce  the  essential  facts  from  his 
breeding  experiments  and  to  establish  his  laws  of  heredity  that  stand 
essentially  unchanged  though  greatly  extended.  Biparental  inherit- 
ance, through  the  fusion  of  the  male  and  female  game  tic  nuclei,  was  not 
established  until  1879,  almost  fifteen  years  after  the  publication  of 
Mendel's  results,  and  the  behavior  of  the  chromosomes  in  the  trans- 
mission of  hereditary  characters  was  not  fully  brought  to  light  until 
1910.  In  the  past  quarter  of  a  century,  the  data  accumulated  by 
cytologists  and  geneticists  the  world  over,  culminating  in  the  recent 
discovery  of  the  giant  chromosomes  and  the  analysis  of  their  basic 
structure,  have  added  tremendously  to  our  knowledge  of  inheritance 
and  has  confirmed  and  strengthened  Mendel's  original  conclusions. 


356 


HUMAN  BIOLOGY 


Let  us  now  take  as  a  working  hypothesis  the  conclusion  reached  by 
Mendel  that  a  particular  character  of  an  organism  may  be  inherited 
independently  of  other  characters.  Mendel  found  this  to  be  the  case 
in  garden  peas,  and  a  wide  variety  of  breeding  experiments  since  then, 
with  many  species  of  plants  and  animals,  have  shown  that  the  principle 
has  universal  application.  We  cannot  do  better  at  this  point  than  to 
analyze  some  of  Mendel's  results.  In  the  first  place,  he  was  very 
fortunate  in  the  choice  of  his  experimental  material,  for  the  species  of 


Short 


Tall 


Tall  (short) 


Tall  Tall  (short)  Tall  (short)  Short 

FIG.  190.-~-Illustrating  the  inheritance  from  crosses  between  a  tall  and  dwarf 
variety  of  peas  as  described  on  page  359.  P,  parental  generation;  Fi,  first  filial  genera- 
tion: hybrid,  tall  peas:  Ft,  second  filial  generation,  produced  from  Fi  individuals  (self- 
fertilized),  show  the  expected  3:1  ratio  between  tall  and  short  plants.  Note  that  the 
two  center  plants  are  hybrids  like  the  Fi.  (Woodruff,  slightly  modified.) 

garden  peas  that  he  used  presented  several  interbreeding  varieties  with 
a  number  of  clearly  recognizable  differences,  seven  altogether,  in 
various  structural  features.  Thus,  some  of  the  plants  were  tall, 
growing  to  a  height  of  6  or  7  ft.,  while  others  were  short,  or  dwarf,  and 
never  attained  a  height  of  more  than  1J^  ft.  Again,  some  of  the 
plants  produced  yellow  seeds;  in  others,  the  seeds  were  always  green. 
Finally,  to  take  just  one  more  example  of  the  contrasting  characters, 
the  seeds  were  round  in  some  plants  and  always  wrinkled  in  others. 
Tall  or  short,  yellow  or  green,  round  or  wrinkled,  here  were  three  very 
distinct  pairs  of  alternative  characters  which  could  be  easily  observed 


THE  BIOLOGY  OF  INHERITANCE  J57 

and  the  results  from  the  breeding  experiments  recorded.  Mendel 
observed  that  these  pairs  of  characters  were  truly  alternative  in  nature ; 
that  is,  the  offspring  from  his  crosses  were  not  mixtures  but  were  either 
tall  or  short,  yellow  or  green,  round  or  wrinkled.  This  could  mean 
only  that  the  determiners  for  these  characters  did  not  mingle  from 
generation  to  generation  but  remained  as  distinct  entities  which  could 
be  segregated  indefinitely.  (Fig.  190.) 

In  breeding  experiments  with  peas  or  flies  or  wheat  or  horses  or 
any  other  type  of  organism,  it  is  essential,  for  a  correct  interpretation  of 
the  results,  that  the  breeder  know  the  condition  of  the  breeding  stock 
with  reference  to  the  particular  character  or  characters  in  question. 
Thus  it  will  be  found  in  a  general  population  that,  when  certain  indi- 
viduals are  bred  together,  all  of  the  offspring  will  be  of  one  type;  for 
example,  all  the  pea  plants  produced  by  one  cross  will  be  tall.  Con- 
trariwise, the  mating  of  other  individuals  will  produce  offspring  of  two 
types  with  reference  to  a  particular  character;  for  example,  some  will 
be  tall  and  some  will  be  short.  Individuals  that  breed  true  with 
respect  to  the  characters  in  question  are  said  to  be  pure,  or  homozygous; 
those  which  do  not  breed  true  are  termed  hybrid,  or  heterozygous. 

The  terms  homozygous  and  heterozygous  are  very  important,  for 
they  indicate  the  condition  of  the  chromatin  in  the  germ  cells,  which,  in 
turn,  is  responsible  for  the  characteristics  of  the  offspring.  Thus, 
an  individual  that  is  homozygous  can  produce  only  one  type  of  gamete. 
A  heterozygous  individual,  on  the  other  hand,  will  produce  more  than 
one  type  of  gamete.  If  it  is  heterozygous  with  respect  to  only  one 
pair  of  alternative  characters,  it  will  produce  two  types  of  gametes  and 
exactly  50  per  cent  of  each  type.  As  an  example  let  us  take  the  pair 
of  alternative  characters  Tall  and  Short.  An  individual  plant, 
heterozygous  for  this  character,  will  produce  50  per  cent  of  the  gametes 
carrying  the  determiner  or  gene  for  the  character  Tall  and  50  per  cent 
carrying  the  gene  for  Short.  The  organism  heterozygous  for  two  pairs 
of  alternative  characters,  as  in  the  case  of  Tall  and  Short,  Yellow  and 
Green,  will  produce  four  types  of  gamete  and  exactly  25  per  cent  of 
each  type  as  indicated: 

25  %  carrying  genes  for  Tall  and  Yellow 
25  %  carrying  genes  for  Tall  and  Green 
25  %  carrying  genes  for  Short  and  Yellow 
25  %  carrying  genes  for  Short  and  Green 

As  a  final  example,  we  may  consider  the  organism  heterozygous  for 
three  pairs  of  alternative  characters,  adding  the  characters  Round  and 
Wrinkled  to  the  two  pairs  used  in  the  previous  example.  The  organ- 


358  HUMAN  BIOLOGY 

ism  heterozygous  with  respect  to  three  pairs  of  alternative  characters 
will  produce  eight  types  of  gamete  in  equal  numbers,  that  is,  12)^  per 
cent  of  each  type  as  indicated: 


carrying  genes  for  Tall,  Yellow,  Round 
12  %  %  carrying  genes  for  Tall,  Yellow,  Wrinkled 
12J^  %  carrying  genes  for  Tall,  Green,  Round 
12  Ji  %  carrying  genes  for  Tall,  Green,  Wrinkled 
%  carrying  genes  for  Short,  Yellow,  Round 
%  carrying  genes  for  Short,  Yellow,  Wrinkled 
%  carrying  genes  for  Short,  Green,  Round 
%  carrying  genes  for  Short,  Green,  Wrinkled 


From  the  foregoing  examination  of  the  gametes  produced  by  the 
heterozygous  individuals,  it  is  clear  that  the  different  types  of  gamete 
are  always  produced  in  equal  numbers,  and,  second,  when  two  or  more 
pairs  of  alternative  characters  are  present,  all  possible  combinations 
of  the  genes  between  the  different  pairs  occur:  Tall  can  be  in  combina- 
tion with  Yellow  or  Green,  or  with  Round  or  Wrinkled,  but  Tall  can 
never  be  in  the  same  gamete  with  Short;  the  genes  for  alternative 
characters  (allelomorphs)  are  always  segregated  in  separate  gametes 
as  indicated  a  few  paragraphs  previously.  The  forming  of  all  possible 
combinations  between  the  genes  for  the  alternative  characters  with 
two  or  more  pairs  present  was  recognized  by  Mendel  under  the  descrip- 
tive phrase  independent  assortment  which  together  with  the  principle 
of  segregation  are  commonly  known  as  the  Mendelian  laws.  The 
number  of  possible  combinations  and,  therefore,  the  number  of  types 
of  sperm  or  egg  produced  can  be  determined  for  any  number  of  pairs 
of  alternative  characters  by  2n  where  n  equals  the  pairs  of  alternative 
characters  involved.  Thus,  as  above,  with  n  equal  to  3,  there  are  23,  or 
eight  types;  with  n  equal  to  5,  there  are  25,  or  32  types.  And,  finally, 
to  take  the  case  in  man,  if  each  of  the  24  pairs  of  homologous  chromo- 
somes carried  a  pair  of  alternative  characters,  'n  would  equal  24,  and 
the  total  types  of  gamete  produced  would  be  equal  to  224,  or  more 
than  16^  million. 

Since,  then,  as  just  shown,  homozygous  individuals  produce  only 
one  type  of  gamete,  and  heterozygous  individuals  produce  two  or  more 
types  of  gametes,  depending  upon  the  number  of  alternative  characters 
carried,  it  is  clear  why  the  former  breed  true  when  they  are  bred 
together,  while  the  hybrids  interbred  produce  a  variety  of  offspring. 
But  now  another  interesting  and  important  development  comes  to 
light,  which  Mendel  also  observed,  and  that  is  the  principle  of  domi- 
nance. This  may  be  illustrated  by  saying  that  it  is  not  always  possible 
to  determine  whether  an  organism  is  homozygous  or  heterozygous  by 


THE  BIOLOGY  OF  INHERITANCE  359 

its  appearance.  This  is  due  to  the  fact  that  one  member  of  a  pair  of 
alternative  characters  is  usually  dominant  over  the  other,  or  recessive, 
member.  Thus,  in  the  alternative  characters  Tall  and  Short,  it  is 
found  that  Tall  is  the  dominant  character,  and,  as  such,  dominates 
the  characters  of  the  body  (soma)  to  the  exclusion  of  the  recessive 
character  Short.  Therefore  two  types  of  Tall  individuals  can  be 
found;  the  Tall-Tall  type,  which  is  homozygous  producing  only  Tall 
gametes,  and  the  Tall-Short  type,  which  is  heterozygous  and  will 
produce  50  per  cent  Tall  gametes  and  50  per  cent  Short  gametes.  It 
is  obvious  that  only  one  type  of  the  recessive  short  individual  occurs, 
namely,  the  Short-Short  homozygous  type,  for  the  Short-Tall  hetero- 
zygous individuals  are  all  tall;  the  character  Tall  is  always  dominant 
over  Short.  The  breeder  who  wishes  to  establish  a  pure  line  with 
respect  to  a  certain  character  or  characters  must  find  out  the  dominant- 
recessive  relationship  and  then  make  the  appropriate  matings.  It 
must  always  be  remembered,  as  noted  previously,  that  possibly  the 
most  important  discovery  of  Mendel  was  that  the  complete  heritage 
of  an  organism  was  not  assembled  in  a  single  unit  but  consisted  of  a 
great  number  of  independent  unit  characters  (particulate  inheritance) 
each  of  which  was  separately  transmitted  to  the  offspring  in  con- 
formity with  the  general  principles  of  independent  assortment  and 
segregation.  (Fig.  190.) 

With  an  understanding  of  particulate  inheritance,  hybridization 
in  its  relationship  to  gamete  formation,  segregation,  independent 
assortment,  and  dominance,  the  basis  of  Mendelian  inheritance  is  well 
established,  and  the  results  obtained  from  the  crosses  are  easy  to 
understand.  The  starting  point  in  determining  the  characters  to  be 
expected  in  the  offspring  from  the  mating  of  any  two  individuals  is 
always:  What  genes  do  the  gametes  of  the  two  parents  carry  with 
respect  to  the  characters  in  question?  The  simplest  possible  case 
would  occur  in  the  mating  of  two  homozygous  individuals  which,  as  we 
know,  can  produce  only  one  type  of  gamete.  For  a  change,  let  us  take 
the  inheritance  of  certain  characters  in  guinea  pigs,  which  are  familiar 
to  every  one  and  which  have  been  widely  used  in  breeding  experiments 
as  well  as  in  a  wide  variety  of  other  types  of  biological  experiment. 
The  inheritance  of  certain  visible  characters  in  the  hair,  such  as  color, 
length,  and  smoothness,  is  convenient  to  use.  We  shall  consider, 
first,  the  inheritance  of  color.  The  colored,  or  Black,  condition,  which 
we  can  for  convenience  designate  by  the  letter  (7,  is  dominant  over  the 
White,  or  albino,  condition  which  we  may  designate  by  the  small  c 
since  this  condition  is  recessive.  The  small  c,  then,  represents  a  lack 
of  color,  or  pigment,  and  gives  white  hair.  Just  as  noted  above  in  the 


360  HUMAN  BIOLOGY 

case  of  the  peas,  there  will  be  two  types  of  black  animals  in  a  general 
population  that  cannot  be  distinguished  by  their  appearance :  the  pure, 
or  homozygous,  individuals  that  carry  only  dominant  genes  Colored- 
Colored,  for  convenience  represented  as  CC,  and  produce  only  one 
type  of  gamete,  namely  C;  and  the  hybrid,  or  heterozygous, 
individuals  that  $arry  both  the  dominant  gene  C  and  the  recessive 
gene  c  for  the  alternative  character,  that  is,  Colored- White  or  more 
conveniently  represented  as  Cc.  The  Cc  individuals,  of  course,  pro- 
duce gametes  of  two  kinds:  50  per  cent  C  and  50  per  cent  c.  (Fig.  190.) 
The  mating  of  a  pure  male  (CC)  with  a  pure  female  (CC)  can 
produce  only  colored  offspring  (CC)  like  the  parents  because  no  other 
types  of  gamete  are  produced  by  either  of  the  parents.  The  mating 
and  the  results  of  breeding  two  homozygous  individuals  can  be  indi- 
cated graphically  thus : 

(I)  Sperm  C  X  egg  C  =  zygote  CC 

The  same  results  are  obtained  when  a  recessive  white  male  (cc)  is 
mated  with  a  recessive  white  female  (cc)  for  they  can  produce  only 
one  type  of  gamete  (c)  and  the  results  of  all  such  matings  will  be: 

(II)  Sperm  c    X  egg  c  =  zygote  cc 

Another  possible  type  of  mating  of  homozygous  individuals  is  that 
in  which  a  homozygous  dominant  (CC)  is  mated  with  a  homozygous 
recessive  (cc).  The  result  of  such  a  cross  is  the  production  in  every 
case  of  hybrid  or  heterozygous  colored  individuals  (Cc)  thus: 

(III)  Sperm  C  X  egg  c  =  zygote  Cc 

The  reciprocal  mating  with  a  recessive  white  male  (cc)  and  a  homo- 
zygous colored  female  (CC)  will  give  the  same  results,  thus : 

(IV)  Sperm  c  X  egg  C  =  zygote  Cc 

With  these  easily  understood  results  from  homozygous  matings  in 
mind,  let  us  next  consider  the  results  of  mating  a  hybrid-colored  male 
guinea  pig  (Cc)  with  the  same  type  of  female  (Cc)  as  in  the  preceding 
paragraph.  Both  produce  two  types  of  gamete  in  equal  numbers, 
that  is,  50  per  cent  sperm  C,  50  per  cent  sperm  c  by  the  males;  and  50 
per  cent  eggs  C,  50  per  cent  eggs  c  by  the  females.  Obviously,  as  shown 
above  in  the  case  of  peas  with  two  pairs  of  alternative  characters,  the 
possible  combinations  of  sperm  and  eggs  are  as  follows: 

Sperm  C  X  egg  C  *»  zygote  CC 

/yv  Sperm  C  X  egg  c   -  zygote  Cc 

Sperm  c  X  egg  C   «•  zygote  cC 
Sperm  c  X  egg  c    »  zygote  cc 


THE  BIOLOGY  OF  INHERITANCE 


361 


It  has  been  found  convenient  in  determining  the  possible  combina- 
tions of  the  gametes  to  use  a  square  in  which  are  separate  spaces  for 
the  zygotes,  with  the  various  types  of  sperm  arranged  along  the  left 
side,  and  the  eggs  along  the  top,  as  follows: 


Sperm 


(VI) 


cc 


cC 


Cc 


An  analysis  of  the  foregoing  results  shows  that  three  different 
types  of  zygote  are  possible  from  the  mating  of  the  two  hybrids  with 


C   x'  c 


cc 


cc 


Cc 


CC  Cc  Cc  cc 

FIG.  191. — Illustrating  the  inheritance  of  hair  color  when  homozygous  black  guinea 
pigs  (CC)  are  mated  with  albino  guinea  pigs  bearing  no  pigment  (cc),  as  described  on 
page  361.  (Woodruff,  slightly  modified.) 

respect  to  one  pair  of  alternative  characters,  namely,  pure  black  (CC), 
hybrid  black  (Cc  and  cC;  the  result  is  the  same  whether  a  particular 
gene  is  carried  by  the  sperm  or  egg),  and  pure  white  (cc).  Accordingly, 
since  both  the  pure  dominant  (CC)  and  the  hybrid  dominant  (Cc) 
have  the  same  appearance — in  this  case,  they  are  black — the  offspring 
of  this  hybrid  mating  are  expected  in  the  ratio  of  3  black  animals  to  1 
white  animal  or  75  per  cent  black  to  25  per  cent  white.  (Fig.  191.) 

As  a  matter  of  fact,  this  expected  3:1  ratio  does  not  always  appear, 
because  the  union  of  the  gametes  at  fertilization  is  at  random;  that  is, 
it  is  always  a  matter  of  chance  as  to  whether  a  particular  egg  will  be 
fertilized  by  a  C  sperm  or  by  a  c  sperm,  jusi  as  it  is  always  a  matter  of 
chance  as  to  whether  heads  or  tails  will  come  up  when  a  coin  is  pitched. 


362  HUMAN  BIOLOGY 

There  is  always  a  chance  that  heads  may  appear  successively;  but  if 
the  process  is  continued  for  a  large  number  of  times,  say  1,000,  the 
likelihood  is  that  heads-tails  ratio  will  be  pretty  close  to  50  per  cent 
heads  to  50  per  cent  tails.  In  the  same  way,  if  the  numbers  of  off- 
spring obtained  from  hybrid  matings  are  large,  the  chances  are  very 
good  that  the  ratio  of  black  animals  to  white  animals  will  be  very  close 
to  the  Mendelian  ratio  of  75  per  cent  black  to  25  per  cent  white.  Thus 
Mendel  reported  from  the  offspring  of  monohybrid  individuals,  differ- 
ing with  respect  to  one  pair  of  alternative  characters,  that  the  ratio 
obtained  in  a  total  of  1,064  offspring  was  73.9  per  cent  showing  the 
dominant  character  to  26.1  per  cent  showing  the  recessive  character. 
Many  other  breeding  experiments  with  a  wide  variety  of  plants  and 
animals  have  given  comparable  results. 

Referring  again  to  the  results  of  the  hybrid  matings  as  given  above, 
it  is  clear  that  on  the  basis  of  their  appearance  the  animals  can  be 
placed  in  two  groups:  black  and  white  in  a  3 : 1  ratio.  From  the  stand- 
point of  their  germinal  constitution,  however,  three  groups,  namely, 
CC,  Cc,  and  cc,  are  present  in  a  1:2:1  ratio,  or  25  per  cent  homozygous 
dominant  (CC)  to  50  per  cent  hybrid  dominant  (Cc)  to  25  per  cent 
homozygous  recessive  (cc).  It  will  be  well  to  introduce  the  terms 
phenotype  and  genotype  at  this  point.  The  phenotype  of  an  organism 
has  reference  to  the  appearance  of  the  organism  without  regard  to  the 
germinal  conditions;  the  term  genotype  refers  to  the  actual  germinal 
constitution.  We  may  say,  then,  in  the  above  matings  that  the 
expected  phenotype  ratio  is  3  animals  showing  the  dominant  character 
black  to  1  animal  showing  the  recessive  character  white,  whereas  the 
expected  genotype  ratio  is  1  (25  per  cent)  homozygous  dominant  (CC) 
to  2  (50  per  cent)  heterozygous  dominants  (Cc)  to  1  (25  per  cent)  pure 
recessive  (cc).  From  the  standpoint  of  heredity,  it  is  obvious  that  the 
phenotypes  can  be  discarded;  it  is  only  the  actual  germinal  constitu- 
tion, or  genotype,  that  counts  in  the  formation  of  gametes  which,  in 
turn,  are  responsible  for  the  transmission  of  characters  to  the  next 
generation.  (VI.) 

The  three  genotypes  CC,  Cc,  cc  of  the  monohybrid  matings  present 
various  possibilities  for  th$  succeeding  generations.  It  is  clear  that 
the  pure  dominants  (CC)  and  the  recessives  (cc)  will  continue  to  pro- 
duce the  same  types  if  mated  with  individuals  of  the  same  genotype, 
that  is,  CC  with  CC  and  cc  with  cc.  The  hybrids  (Cc)  will  continue  to 
produce  offspring  in  the  1:2:1  ratio  if  mated  with  hybrid  animals  (Cc) 
as  shown  in  table  (VI).  Another  possibility  lies  in  the  mating  of  the 
CC  animals  with  cc  animals  which,  as  shown  in  (IV)  above,  gives,  in  all 
cases,  the  hybrid  condition  (Cc).  The  final  possibility  lies  in  the 


THE  BIOLOGY  OF  INHERITANCE 


363 


mating  of  homozygous  animals  with  heterozygous.  Thus  the  pure 
blacks  (CC)  can  be  mated  with  the  hybrid  blacks  (Cc),  or  the  white 
animals  (cc)  can  be  mated  with  the  hybrid  condition  (Cc).  In  either 
of  these  crosses  it  will  be  found  that  the  expected  ratio  of  the  offspring 
is:  50  per  cent  bearing  the  genotype  of  one  parent  and  50  per  cent 
bearing  the  genotype  of  the  other  parent.  Thus 


(VII) 


(VIII) 


CC  X  Cc        Sperm 


cc 


Cc 


Eggs 


cc  X  Cc 


Sperm 


cC 


cc 


Considering  the  homozygous-hybrid  matings  (VII),  it  is  seen  that 
there  is  only  one  phenotype — all  the  offspring  are  black — and  two 
genotypes,  CC  and  Cc,  corresponding  to  the  two  parental  types  with 
an  expected  1 : 1  ratio.  In  the  second  example  (VIII),  the  two  pheno- 
types,  black  and  white,  occur  and  also  two  genotypes,  Cc  and  cc, 
corresponding  to  the  two  parental  types  and  with  an  expected  1:1 
ratio.  Recalling  for  a  moment  the  results  of  mating  two  hybrids  as 
shown  in  square  (VI)  above,  it  will  be  seen  that  the  results  of  homo- 
zygous-hybrid matings,  as  in  (VII)  and  (VIII),  can  be  read  in  the  two 
horizontal  columns  of  (VI).  In  the  upper  horizontal  column  we  have 
the  results  shown  as  in  (VII);  the  lower  column  reads  the  same  as 
(VIII)  of  the  homozygous-hybrid  matings.  The  vertical  columns  of 
the  square  (VI)  give  the  same  results  with  the  sexes  reversed. 

The  description  of  the  various  matings  makes  it  evident  that 
Mendelian  inheritance  depends  primarily  upon  the  number  of  types  of 
gamete  produced  by  the  parents,  and  this  in  turn,  as  shown,  is  directly 
dependent  upon  the  number  of  pairs  of  genes  for  alternative  characters 
present.  When  an  organism  is  homozygous,  as  in  the  genotype  CC 
or  in  the  genotype  cc,  the  gene  for  an  alternative  character  is  not 
present,  and  all  the  gametes  must  be  of  one  kind.  The  hybrid,  or 
heterozygous,  condition,  on  the  contrary,  bears  genes  for  one  or  more 
pairs  of  alternative  characters  as  shown  above  in  the  Tall-Short  hybrids 
in  the  peas  and  the  Colored- White  (Cc)  hybrids  that  we  have  been 
considering  in  the  guinea  pigs.  The  addition  of  other  pairs  of  alterna- 
tive characters  increases  the  number  of  gametes  produced  according  to 
the  ratio  2n  where  n  represents  the  number  of  pairs  of  alternative 


364 


HUMAN  BIOLOGY 


characters  or  the  degree  of  hybridization  (page  369).  This  may  be 
indicated  by  the  terms  monohybrid  where  one  pair  of  alternative 
characters  is  present,  dihybrid  where  two  pairs  of  alternative  characters 
are  considered,  and  trihybrid  where  there  are  three  pairs  of  alternative 
characters.  Our  study  of  Mendelian  inheritance,  so  far,  has  been 
concerned  with  the  monohybrid  condition  represented  by  one  pair  of 
alternative  characters  Colored- White,  or  Cc,  and  giving  two  types  of 
gamete  C  and  c  and  four  possible  combinations.  Now  the  results 
obtained  from  matings  of  dihybrids  or  trihybrids  or  polyhybrids  with 
even  greater  hybridization  conform  exactly  to  the  results  obtained  in 
the  monohybrid  matings;  they  only  present  more  types  of  gamete  and 
correspondingly  more  possible  combinations  between  gametes,  as  will 
be  shown  in  the  following  paragraphs. 

In  the  dihybrid  condition  with  its  two  pairs  of  alternative  characters 
will  be.22,  or  four  types  of  gamete.  To  see  the  possibilities  inherent  in 
this  condition  we  may  take  another  visible  hair  character  in  the  guinea 
pig,  namely  ^  length  of  hair,  which  has  been  shown  to  be  a  Mendelian 
character  with  the  alternative  pair  of  characters  Short-Long.  The 
short-haired  condition  is  dominant  and  may  be  designated  as  $,  and 
the  long-haired  condition  which  we  shall  represent  by  s  is  recessive. 
The  dihybrid  animals,  then,  with  two  pairs  of  alternative  characters 
differing  with  respect  to  color  of  hair  (Cc)  and  length  of  hair  (Ss)  are 
indicated  in  the  genotype  constitution  CcSs.  The  four  types  of 
gamete  produced  by  these  dihybrids  are  CS  (genes  for  black  and  short), 
Cs  (genes  for  black  and  long),  cS  (genes  for  white  and  short),  and  cs 
(genes  for  white  and  long).  Mating  two  dihybrids,  then,  gives  four 
types  of  sperm  and  four  types  of  egg  and  the  possibility  of  16  combina- 
tions when  the  gametes  unite  in  random  fertilization  to  form  the  zygote. 
Again  the  square  shows  all  the  possibilities  (IX). 


(IX) 


""\lEggs 
Sperm  ^\^ 

CS 

Cs 

cS 

cs 

CS 

CCSS 

(1) 

CCSs 
(2) 

CcSS 
(3) 

CcSs 
(4) 

Cs 

CCSs 

(2) 

CCss 

(7) 

CcSs 
(4) 

Cess 

(5) 

cS 

CcSS 
(3) 

CcSs 
(4) 

ccSS 

(8) 

ccSs 
(6) 

C8 

CcSs 

(4) 

Cess 
(5) 

ccSs 

(6) 

CCSS 

(9) 

THE  BIOLOGY  OF  INHERITANCE 


365 


An  analysis  of  the  various  possibilities  in  the  dihybrid  square 
shows  that,  out  of  the  16  possible  combinations,  there  are  four  pheno- 
types  corresponding  to  the  four  types  of  gametes,  the  appearance  of 
which  may  be  described  as  follows:  black,  short-haired  animals;  black, 
long-haired  animals;  white,  short-haired  animals;  and  white,  long- 
haired animals;  or,  stated  in  another  way,  those  which  show  both 
dominant  characters,  those  which  show  one  of  the  two  dominant 
characters,  those  which  show  the  other  dominant  character,  and  thosr 


cR  cr 

FIG.  192. — Illustrating  the  inheritance  of  hair  color  and  another  alternative  character 
(rough  or  smooth),  giving  a  dihybrid  condition.  Note  that  one  of  the  parents  (/VCV) 
is  dominant  for  color  (C)  and  recessive  for  smoothness  O) ;  whereas  1  he  other  parent 
(cRcR)  is  recessive  for  color  (c)  and  has  the  dominant  character  (/if)  which  produces  a 
rough  coat.  All  of  the  Fi  dihybrids  show  the  two  dominant  characters.  (Woodruff, 
slightly  modified.) 

which  show  neither  of  the  dominant  characters.  These  occur  in 
different  proportions;  the  first  group  with  both  dominant  characters  is 
the  largest  with  9  out  of  the  16  animals,  or  56.25  per  cent.  They  are 
shown  in  the  zygotes  numbered  (1),  (2),  (3),  and  (4).  Three  out  of  the 
16,  or  18.75  per  cent,  express  the  dominant  character  Black.  They  are 
shown  in  zygotes  numbered  (5)  and  (7).  Three  out  of  the  16,  or 
18.75  per  cent,  show  the  other  dominant  character  Short.  They  are 
shown  in  zygotes  numbered  (6)  and  (8).  One  individual  out  of  16, 
or  6.25  per  cent,  may  be  expected  to  show  neither  dominant  character, 


366  HUMAN  BIOLOGY 

as  indicated  in  number  (9).     Thus  we  have  the  dihybrid  phenotype 
ratio  of  9:3:3:1.     (IX.) 

The  16  possible  dihybrid  combinations  give  9  different  genotypes 
which,  as  just  seen,  are  numbered  from  1  to  9.  Among  the  genotypes 
is  one  pure  dominant  (1)  which  has  received  the  two  dominant  genes 
CS  from  both  parents  and,  of  course,  can  form  only  CS  gametes;  also 
one  pure  recessive  (9)  which  received  the  two  recessive  genes  cs  from 
both  parents.  In  addition  to  (1)  and  (9)  there  arc  two  other  genotypes 
(7)  and  (8)  which  are  also  homozygous,  having  received  the  same  genes 
from  both  parents.  Four  of  the  genotypes  (4)  are  dihybrids  like  the 
parents,  the  genes  received,  from  either  the  sperm  or  egg,  giving  the 
genotype  OSes.  Eight  of  the  genotypes  are  inonohybrids  (2),  (3), 
(5),  (6)  with  two  each  of  four  different  combinations.  It  will  be 
helpful  to  examine  one  of  these  to  sec  just  why  they  arc  monohybrids. 
Let  us  take  the  genotype  condition  as  shown  in  (2),  namely,  CS  from 
one  parent  and  Cs  from  the  other.  If  now  the  genes  for  color  and  the 
genes  for  length  are  arranged  in  pairs,  we  have  CCSs.  It  is  thus  seen 
that  such  an  animal  is  homozygous  with  respect  to  hair  color;  that  is, 
it  can  produce  only  one  type  of  gamete  for  this  character,  namely, 
gametes  bearing  the  gene  C.  It  can  produce  two  types  of  gametes,  S 
and  s,  with  respect  to  length  of  hair.  In  brief,  then,  this  is  a  mono- 
hybrid  and  will  form  50  per  cent  gametes  with  CS  genes  and  50  per  cent 
with  Cs  genes. 

In  summarizing  the  expected  results  from  the  dihybrid  matin gs 
with  respect  to  hybridization,  it  is  thus  found  that  four  (25  per  cent) 
are  homozygous,  8  (50  per  cent)  arc  monohybrids,  and  four  (25  per 
cent)  are  dihybrids  like  the  parents.  Once  again  emphasis  should  be 
laid  on  the  fact  that  the  various  ratios  just  considered  are  not  absolute*, 
since  fertilization  is  always  at  random,  and,  therefore,  no  prediction 
can  be  made  as  to  which  one  of  the  foiir  types  of  sperm  will  fertilize  a 
particular  egg,  as  explained  above  in  the  consideration  of  the  mono- 
hybrid  ratios.  Finally,  it  should  be  noted  that  the  results  from  the 
matings  of  dihybrids  with  homozygous  individuals  and  ajso  with 
monohybrids  are  shown  in  the  various  horizontal  columns  of  the 
dihybrid  square  (IX)  as  observed  previously  in  the  moiiohybrid  square 
(VI).  Thus,  for  example,  the  zygotes  expected  from  the  matings  of  a 
dihybrid  with  a  homozygous  dominant  are  shown  in  the  top  horizontal 
column. 

The  principles  operating  in  the  moiiohybrid  and  dihybrid  matings, 
which  have  been  considered  in  some  detail,  are  unaltered  in  the 
trihybrids  with  three  pairs  of  alternative  characters  or  in  any  poly- 
hybrid  with  still  more  pairs  of  alternative  characters.  Since  this  is  the 


THE  BIOLOGY  OF  INHERITANCE 


367 


case,  it  is  not  necessary  for  the  geneticist  to  consider,  in  general, 
matings  in  which  account  has  to  be  taken  of  more  than  one,  two,  or 
three  alternative  characters.  In  nature,  however,  with  thousands  of 
heritable  characters  present  in  the  organism  it  would  be  impossible  to 
find  an  individual  homozygous  for  all  the  genes  involved  or  even 
individuals  with  only  one,  two,  or  three  pairs  of  alternative  characters; 
they  are  all  poly  hybrids. 


CSE 


cSR 


CsE 


CSr 


Csr  .  csr 

FIG.  193. — The  possible  ^2  phenotypes  from  a  trihybrid  cross  between  one  parent 
carrying  the  three  dominant  characters  for  pigmentation,  short  hair  and  rough  coat 
(upper  left-hand  corner)  and  the  other  parent  a  pure  recessive  with  no  color,  smooth 
ooat  and  long  hair  (lower  right-hand  corner),  as  described  on  page  368.  (Woodruff, 
slightly  modified.} 

Brief  consideration  of  the  trihybrid  condition  will  be  sufficient  to 
show  the  increased  possibilities  associated  with  the  addition  of  a  third 
pair  of  alternative  characters.  Continuing  with  our  study  of  the 
inheritance  of  visible  hair  characteristics  in  the  guinea  pig,  it  has  been 
found  that  a  curly  or  rough  condition  of  the  hair  coat  is  alternative  to  a 
straight  or  smooth  condition.  The  condition  Rough,  or  Ry  is  dominant 
over  Smooth,  or  r.  The  homozygous  dominant  for  colored,  short, 
rough  hair  thus  has  the  genotype  CSRCSR,  producing  only  CSR 


368 


HUMAN  BIOLOGY 


m 
m 


gametes,  and  the  recessive  has  the  genotype  csrcsr,  producing  only  csr 
gametes.  Matings  of  the  pure  dominant  and  the  pure  recessive  invari- 
ably give  offspring  in  the  next  generation  (Pi)  showing  colored,  short, 

rough  hair  like  the  dominant  parent  and 
with  genotype  CSRcsr.  This  trihybrid 
condition  with  three  pairs  of  alternative 
characters,  that  is,  with  n  equal  to  3, 
forms  2s,  or  8  types  of  gamete  and  12^ 
per  cent  of  each  type.  Making  all  the 
possible  combinations  in  an  independent 
assortment,  the  eight  types  of  gametes  are 
found  to  be:  CSR,  CSr,  CsR,  Csr,  cSR,  cSr, 
csR,  and  csr.  The  construction  of  the 
trihybrid  square  as  in  the  previous  cases, 
but  for  eight  types  of  gametes,  shows  that 
64  possible  combinations  are  represented 
in  the  zygotes  of  the  F2  generation.  The 
eight  phenotypes,  corresponding  to  the 
gametes,  may  be  expected  to  appear  in 
the  ratio  of  27:9:9:9:3:3:3:1  in  a  total 
of  64  animals,  but,  as  we  understand,  since 
fertilization  as  always  is  at  random,  the 
chance  of  getting  exactly  this  particular 
ratio  in  any  group  of  64  animals  is  very 
slight.  It  will  be  found  that  there  are  27 
genotypes  of  which  8  are  homozygous  and 
19  are  heterozygous.  Out  of  a  total  of  64 
animals  in  the  F2  generation,  the  expect- 
ancy is  that  8  (12.5  per  cent)  will  be 
homozygous;  8  (12.5  per  cent)  will  be  tri- 
hybrids  like  the  parents;  24  (37.5  per 
cent)  will  be  dihybrids;  and  24  (37.5  per 

t)    WJU    be    monohybrids.       (FigS.    193, 
'  ^  v      °  7 

194.) 

rpQ  aummarize  the  ratios  in  the  various 
hybrids  the  table  at  the  top  of  page  369  may  be  helpful. 

Blending  Inheritance. — From  the  descriptions  just  given  of  the 
various  hybrids,  it  is  evident  that  the  dominance  of  a  particular 
character  in  the  jJhenotype  is  a  striking  feature  of  Mendelian  inherit- 
ance, but  it  must  be  stated  at  once  that  dominance  .is  not  a  universal 
feature,  for  there  are  numerous  instances  in  which  the  genes  for  alterna- 
tive characters  are  of  equal  value  in  the  determination  of  the  characters 
in  the  phenotype  just  as  they  always  are  in  the  gene-complex  of  the 


648 

FIG.  194. — Diagram  illus- 
trating the  expected  distribu- 
tion of  the  64  possibilities  in 
the  Fz  generation  of  a  trihybrid. 
(Walter.) 


THE  BIOLOGY  OF  INHERITANCE 


369 


Types  of 
gamete 

Types  of 
phenotype 

Types  of 
genotype 

Possible 
combina- 
tions 

Homozygous  (CC) 

1 

1 

1 

1 

/-^  \      Monohybrid  (Cc) 

2 

2 

3 

4 

Dihybrid  (CcSs) 

4 

4 

9 

16 

Trihybrid  (CcSsRr) 

8 

8 

27 

64 

Poly  hybrids     (n    pairs     of 
alternative  characters) 

2» 

2n 

3» 

4n 

gametes.  This  condition  is  commonly  referred  to  as  blending  inherit- 
ance because  the  hybrid  offspring  show  a  mixture,  or  blend,  of  a  particu- 
lar character;  dominance  is  lacking.  One  of  the  most  common 
examples  of  this  is  seen  in  the  common  snapdragon  in  which  plants 


Parents 


White 


Pink 


Ill- 


Pink 


Pink  Whik 

FIG.    195.  —  Illustrating    blending    inheritance    in    the    snapdragon   when   homozygous 
red  and  white  individuals  are  crossed  as  described  on  Rage  369.     (Sinnott  and  Dunn.) 

are  found  with  red,  white,  or  pink  blossoms.  The  breeding  experi- 
ments show  that,  when  a  pure  red  plant  is  crossed  with  one  bearing 
white  blossoms,  all  of  the  hybrid  offspring  will  have  pink  flowers.  The 
pink  flowers  may  be  regarded  as  intermediate  in  color  between  the  red 
and  white  of  the  two  parental  plants.  It  is  apparent  from  these 


370 


HUMAN  BIOLOGY 


results  that  neither  red  nor  white  are  dominant  in  the  phenotype.  If 
now  the  hybrid  FI  plants  bearing  pink  flowers  are  crossed,  three  types 
of  colored  offspring  will  appear  in  the  F2  generation  in  a  1:2:1  ratio 
or  25  per  cent  red: 50  per  cent  pink: 25  per  cent  white.  This  is  the 
ratio  expected  from  the  mating  of  monohybrids  as  shown  in  square 
(VI)..  If  we  let  C  represent  the  gene  for  red  color  and  c  the  gene  for 
white  color,  then  the  pure  reds  have  the  genotype  CC  and  the  whites 
the  genotype  cc.  The  FI  hybrid  pink  offspring  will  contain  both 
genes,  that  is,  Cc,  and  will  produce  gametes  C  and  c.  Crossing  of  these 
monohybrids  gives  the  1:2:1  ratio  as  shown  in  table  (XI)  and  also 
in  Fig.  195. 


(XI) 


"\lEggs 
Sperm  ^^"\^ 

C 

c 

C 

CC 

(red) 

Cc 

(pink) 

c 

Cc 
(pink) 

cc 

(white) 

Thus  when  dominance  is  lacking,  the  genotype  is  revealed  by  the 
appearance  of  the  phenotype.  Many  other  examples  of  blending 
inheritance  are  known,  as  for  example,  in  the  inheritance  of  size  in 
rabbits  where  the  offspring  of  matings  between  large  and  small  races 
are  intermediate  in  size.  The  latter  when  interbred  show  a  segrega- 
tion of  size  in  the  next  generation.  Earlier,  it  was  thought  that  the 
results  obtained  from  some  of  these  examples  with  incomplete  domi- 
nance did  not  conform  to  the  Mendelian  laws,  but  further  study  has 
shown  in  practically  every  case  that  they  fall  into  line.  From  our 
standpoint,  the  important  thing  to  note  is  that  segregation  of  the  genes 
in  the  gametes  of  the  hybrids  with  dominance  lacking  occurs  just  as  in 
other  hybrids  where  complete  dominance  is  found. 

Lack^of  dominance  may  be  expressed1  in  other  ways  than  by  the 
so-ceiled -blending  inheritance,  as  just  shown  in  flower  colors.  Thus, 
in  tKe  matings  of  red  cattle  with  white,  the  FI  offspring  show  a  charac- 
teristic foan  color.  If  the  individual  hairs  of  the  hybrid  roans  are 
examined,  it  will  be  found  that  they  are  equally  divided  between  red 
hairs  and  white  hairs  so  that  both  of  the  alternative  genes  are  expressed 
in  the  color  pattern  of  the  roan  animals.  The  hybrid  roan-colored 
animals,  when  interbred,  produce  offspring  as  in  monohybrid  matings, 
namely,  25  per  cent  red  to  50  per  cent  foan  to  25  per  cent  white,  just 
as  do  the  snapdragons.  Another  well-known  example  of  the  same  type 
Of  inheritance  is  in  the  Blue  Andalusian  fowls,  which  never  breed 


THE  BIOLOGY  OF  INHERITANCE 


371 


true  but  produce  offspring  with  three  types  of  color  in  the  ratio  of 
1:2:1  as  follows :  25  per  cent  white  to  50  per  cent  blue  to  25  per  cent 


FIG.  196.— Photograph  of  a  roan,  ghort-horned  cow.  This  color  condition  occurs 
in  animals  heterozygous  for  red  and  white  hair  color,  as  described  on  page  370.  (Shull, 
after  McPhee  and  Wright.) 


FIG.  107  — Illustrating  the  inheritance  of  blue  in  Andalusian  fowls;  a  condition  shown, 
only  by  the  hybrid.      (Sinnott  and  Dunn,  modified.} 

black.     It  is  apparent  that  the  so-called  Blue  Andalusian  type  is  a 
monohybrid  with  respect  to  color,  bearing  genes  for  black  and  for 


372 


HUMAN  BIOLOGY 


white.    Accordingly,  the  blue  color  is  produced  in  all  the  offspring  from 
the  matings  of  the  homozygous  blacks  and  whites.     (Figs.  196,  197,) 

MULTIPLE  FACTORS 

The  cases  of  dominance  that  have  been  described  thus  far  have  been 
ones  in  which  a  single  pair  of  genes  was  operating,  but  it  is  well  estab- 
lished that,  in  many  instances,  several  pairs  of  genes  are  concerned 
in  the  development  of  a  particular  character,  the  condition  known  as 
multiple  genes  (multiple  factors).  This  puzzling  condition  was  first 
noted  in  the  results  obtained  from  breeding  certain  grains,  such  as  pats 
and  wheat.  When,  for  example,  a  variety  of  brown-seeded  oats  was 
crossed  with  a  white-seeded  variety,  both  being  homozygous,  all  of 
the  plants  in  the  next  generation  had  brown  seeds  that  were  distinctly 
lighter  in  color  than  the  brown-seeded  parent.  When  these  hybrid  FI 
plants,  were  interbred,  it  would  normally  be  expected,  as  we  know,  that 
a  phenotype  ratio  of  3  brown  to  1  white  would  appear  in  the  offspring. 
But  not  so  in  this  case.  The  dominant  brown  color  appeared  in  the 
ratio  of  15  dominant  brown-colored  seeds  to  1  recessive  white  seed,  or 
93.75  per  cent  to  6.25  per  cent.  An  examination  of  the  brown  seeds 
showed  considerable  variation  in  the  depth  of  color.  It  was  then  seen 
that  this  condition  was  just  what  would  be  expected  if  two  factors  or 
genes  were  associated  with  this  color  character,  thus  giving  the 
dihybrid  Condition  for  color,  with  four  different  types  of  gamete  and  16 
possible  combinations  (page  364).  Representing  the  two  factors  for 
brown  color  as  C\Ci  and  the  recessive  alternative  genes  for  white  as 
do*,  the  genotype  of  the  pure  dominant  would  be  CiCiCaC^  with  all 
gametes  CiCj;  that  of  the  pure  recessive  c\c\c^c^  with  all  gametes  c\c*. 
All  the  dihybrid  offspring  would  then  have  the  genotype  CiCiC&t  and 
would  produce  four  types  of  gamete  as  shown  in  the  dihybrid  square, 
which  conforms  to  that  shown  in  (IX)  above. 


Sperm 


(XII) 


CiC72 


(1) 


(2) 


(3) 


(4) 


(2) 


(7) 


(4) 


(5) 


(3) 


(4) 


(8) 


(6) 


(4) 


(5) 


(6) 


CiCiCjCj 

(9) 


THE  BIOLOGY  OF  INHERITANCE 


373 


When  the  results  of  the  matings  are  analyzed,  it  becomes  apparent 
that  the  brown  color  of  the  seed  increases  in  intensity  in  correspondence 
with  the  number  of  genes  for  color  present.  Thus  individuals  with 
the  genotype  CiCiCzCi  form  seeds  with  the  greatest  amount  of  brown 
pigment,  with  a  decrease  indicated  in  the  genotypes  with  fewer  of 
the  color-bearing  genes  Ci  and  C2  a§  in  the  series  CiCiCzC^  CiCiCjCj, 
CidCzCzj  and  with  no  brown  pigment  whatever  in  the  pure  recessive 
CiCiC2c2.  (Fig.  198.) 


R,R,  R 

.     Red 


V 


i 


r.n 


FIG.  198. — Illustrating  results  of  crossing  two  varieties  of  wheat,  one  with  two  fac- 
tors for  red  seeds  (RiRifaRz)  and  the  other  with  the  factors  for  white  seeds  (nr2rir2). 
This  multiple-lactor  condition  is  described  on  page  372.  (Sinnott  and  Dunn,} 

In  the  matings  of  certain  varieties  of  red-seeded  and  white-seeded 
wheat,  it  was  found  that  a  trihybrid  condition  for  color,  with  three 
pairs  of  genes,  was  present  so  that,  in  the  F2  generation,  the  ratio  of 
colored  seeds  to  pure  white  was  63  colored  out  of  64.  In  other  words, 
on  the  average  one  pure  recessive  appeared  out  of  64  possible  combina- 
tions, which  is  the  condition  in  a  trihybrid  cross.  Hair  color  in  animals 
has  been  shown,  in  some  instances,  to  be  due  to  multiple  factors,  and 
it  is  also 'evident  that  skin  color  in  man  is  another  example.  Thus,  in 
man,  thefre  are  two  or  three  pairs  of  genes  for  color,  with  the  intensity 


374  HUMAN  BIOLOGY 

of  the  color  in  the  skin  of  the  hybrid,  or  mulatto,  offspring  of  white  and 
negro  parents  being  governed  by  the  number  of  dominant  genes  pres- 
ent, as  just  seen  in  oat  seeds.  If  we  take  the  colored  conditions  as 
being  dihybrid  with  the  color  genes  CiC2,  then  the  genotype  of  the  pure 
negro  with  black  skin  can  be  given  as  CiCiCzCz  and  the  recessive 
white  as  CidC2C2,  just  as  in  the  case  of  the  dihybrid  color  factor  shown 
in  (XII)  above  and  in  Fig.  198. 

The  discovery  of  multiple  genes,  that  is,  the  knowledge  that  more 
than  one  pair  of  genes  were  concerned  with  the  production  of  a  partic- 
ular character,  brought  a  great  many  cases  of  inheritance  into  the 
Mendelian  fold  that  did  not,  at  first,  appear  to  conform  to  the  estab- 
lished principles.  At  the  same  time,  increased  emphasis  came  to  be 
laid  upon  the  fact  that  the  characters  exhibited  in  the  organism,  in 
most  cases  at  least,  were  not  the  result  of  one  pair  of  genes  acting 
during  development  but  of  many  genes.  In  the  determination  of  eye 
color  in  Drosophila,  for  example,  it  is  known  that  more  than  20  pairs 
of  genes  are  operating,  and  essentially  the  same  situation  exists  with 
regard  to  the  determination  of  wing  characters.  Furthermore,  the 
various  genes  for  eye  and  wing  characters  are  not  even  located  in  the 
same  pair  of  homologous  chromosomes.  In  the  determination  of  sex, 
however,  it  is  clear  that  the  gene  mechanism  is  localized  in  one  pair  of 
chromosomes,  though  various  genes  may  be  at  work.  But  aside  from 
the  determination  of  sex,  no  evidence  exists  that  all  the  genes  for  a 
particular  character  or  associated  with  the  development  of  a  certain 
organ  are  located  in  a  particular  chromosome.  Thus  to  refer  again  to 
the  eye  of  Drosophila,  the  numerous  genes  concerned  with  color  and 
various  other  characteristics  have  been  found  to  be  located  in  all  four 
of  the  synaptic  pairs.  Furthermore,  the  genes  in  a  particular  chromo- 
some, though  maintaining  a  rigid  linear  relationship  as  shown  by  the 
synaptic  phenomena,  appear  to  be  promiscuously  arranged  with  respect 
to  the  characters  that  they  determine,  so  that,  for  example,  a  gene  for 
eye  color  may  be  situated  close  to  a  gene  responsible  for  the  develop- 
ment of  some  character  in  the  body  or  wings.  Since,  however,  com- 
paratively few  of  the  total  number  of  genes  have  been  located,  it  may 
be  that  the  apparent  irregularities  do  not  give  a  true  picture. 

But  possibly  the  most  important  result  coming  from  an  under- 
standing of  multiple  factors  was  the  light  thrown  upon  the  problem 
of  selection.  Practically  all  types  of  domesticated  varieties  of  plant 
and  animal,  which  man  has  found  valuable  for  his  multitudinous  needs, 
have  been  subjected  to  selection  since  the  earliest  times;  certain  indi- 
viduals being  chosen  in  each  generation  for  the  production  of  the  next 
generation.  Such  selection  has  been  made  in  order  to  strengthen  and 


THE  BIOLOGY  OF  INHERITANCE 


375 


16  17  18 

FIG.  199. — Illustrating  variations  in  hair  color  in  Dutch  rabbits.  By  systematic 
selection  the  average  pigmentation  "of  a  race  of  Dutch  rabbits  may  be  gradually  but 
permanently  changed  either  in  a  plus  or  in  a  minus  direction."  The  extreme  of  pig- 
mentation is  shown  in  1,  and  the  opposite  condition  in  18.  (Castle,  "Genetics  and 
Eugenics''  Harvard  University  Press.) 


376 


HUMAN  BIOLOGY 


Pure  Line 


2 


8 


establish  a  certain  desirable  attribute  which  was  particularly  well 
displayed  in  the  organisms  selected  for  breeding;  more  milk  in  cows, 
more  speed  in  race  horses,  more  power  in  draft  horses,  more  returns  per 
acre  in  the  grains  due  to  higher  productivity,  to  greater  ability  to  resist 
disease,  or  to  earlier  ripening  qualities;  the  examples  are  almost 

unlimited.     (Fig.  199.) 

Now  the  fact  is,  of  course,  as 
almost  everyone  knows,  that 
selection  as  practiced  by  practical 
breeders,  who  knew  nothing  of 
the  Mendelian  principles  or  genes, 
has  been  successful  and  is  re- 
sponsible for  most  of  the  im- 
provement that  has  been  made 
in  our  present-day  varieties  of 
domesticated  plants  and  animals 
over  the  undomesticated  types. 
But  it  is  now  clear  that  the 
improvement  of  types  by  selec- 
tion is  based  upon  the  gradual 
accumulation,  generation  by  gen- 
eration, of  the  genes  responsible 
for  the  production  of  the  desired 
character  for  which  selection  is 
being  made.  In  other  words, 
where  selection  is  effective,  sev- 
eral pairs  of  genes,  multiple 
genes,  are  associated  with  the 
development  of  the  character  in 
question  so  that  the  selection,  for 
example,  of  certain  animals  for 
breeding,  which  show  the  opti- 
mum expression  of  a  desired 
character,  means  the  accumula- 
tion of  more  genes  for  this 
character  and  a  correspondingly 
greater  expression  of  it  in  the  next 
generation.  Selection,  then,  to  be  effective  must  occur  in  a  mixed 
population,  or,  in  Mendelian  terms,  the  individuals  must  be  hetero- 
zygous. In  a  pure  line,  where  all  the  individuals  are  homozygous, 
selection  can  have  no  effect,  for  all  the  genes  governing  the  character  in 
question  are  then  present  in  the  genotypes  of  the  homozygous  indi- 


Fio.  200. — Diagrams  illustrating  varia- 
tion in  weight  of  pure  lines  of  beans  (1-5). 
The  test  tubes  containing  beans  of  the 
same  weight  are  arranged  vertically. 
The  general  population  formed  by  com- 
bining the  pure  lines  is  shown  below. 
(Walter  t  after  Jokannsen.) 


THE  BIOLOGY  OF  INHERITANCE  377 

viduals.  Thus,  to  take  a  very  simple  example,  selection  would  have 
no  effect  in  increasing  the  color  in  the  brown-seeded  oats  with  the 
genotype  CiCiC2C2,  for  all  the  genes  for  color  are  present,  or  in  decreas- 
ing the  color  in  plants  with  the  genotype  CiCic2cf.  (Figs.  198  to  200.) 

LINKAGE 

If  the  preceding  descriptions  of  the  Mendelian  phenomena  have 
been  followed  carefully,  it  is  probably  already  apparent  to  the  reader 
that  the  segregation  of  the  genes  in  gamete  formation  and  their  recom- 
bination in  the  zygote  nucleus  at  the  time  of  fertilization  exactly 
parallel  the  behavior  of  the  chromosomes  as  described  in  the  closing 
pages  of  the  previous  chapter.  Of  course,  this  must  be  true  if  it  is 
kept  in  mind  that  the  chromosomes  are  really  strings  of  genes  arranged 
in  linear  fashion,  like  beads  on  a  necklace.  Thus  the  same  terminology 
might  be  used  with  a  pair  of  homologous  chromosomes  as  with  genes  in 
the  case  of  a  pair  of  alternative  characters,  such  as  Colored  (C)  and 
White  (c).  In  the  parent  hybrid  individual  with  the  diploid  condition 
Cc,  both  chromosomes  C  and  c  would  be  present  in  all  the  cells,  but 
when  the  gametes  are  formed  following  synapsis  and  reduction,  each 
gamete  with  the  haploid  condition  will  receive  only  one  chromosome, 
that  is,  either  C  or  c,  just  as  each  gamete  contains  either  the  gene  C 
or  the  gene  c.  Fertilization,  following  mating  with  an  organism 
carrying  the  same  chromosomes,  will  restore  the  diploid  condition  and 
give  the  opportunity  for  a  sperm  with  the  C  chromosome  to  combine 
either  with  an  egg  with  the  C  chromosome  or  with  an  egg  bearing  the  c 
chromosome,  thus  giving  zygotes  with  the  three  genotypes  ,as  given 
above  (VI)  in  the  nxonohybrid  matings.  This  can  be  carried  further 
with  additional  homologous  pairs  of  chromosomes  for  thejShort  and 
Long  (Ss)  as  in  the  dihybrid,  and  the  Rough  and  Smooth  (Rr)  as  in 
the  trihybrid.  The  point  is  clear;  a  particular  gene  is  always  in  a 
particular  chromosome  and  cannot  act  independently  of  that 
association. 

This  condition  leads  at  once  to  the  question  of  the  association,  or 
linkage,  of  the  genes  present  in  a  particular  chromosome.  It  was  early 
recognized  in  breeding  experiments  that  certain  characters  always 
appeared  together  in  an  individual;  both  were  present  or  both  were 
absent,  but  never  one  without  the  other.  The  explanation  was  long 
lacking,  but  it  is  now  known  that  the  linking  of  certain  characters  in 
an  individual  is  due  to  the  fact  that  the  genes  for  these  characters 
happen  to  lie  in  the  same  chromosome  and  must  accompany  this 
chromosome  wherever  it  goes.  Since  the  number  of  characters  and  the 


378  HUMAN  BIOLOGY 

corresponding  genes  in  the  highly  developed  organism  runs  into  the 
thousands  and  there  are  only  a  relatively  few  chromosomes  in  which 
all  these  genes  lie,  it  becomes  apparent  that  many  genes  are  situated,  or 
linked,  in  a  particular  chromosome  and  must  always  go  along  with  it. 
It  was  stated  in  the  previous  chapter  that  some  2,500  genes  were 
arranged  in  linear  fashion  in  one  chromosome  of  Drosophila  (page  336). 
All  these  genes  must,  therefore,  go  into  one  gamete  with  that  chromo- 
some and  give  rise  to  a  corresponding  group  of  associated  characters 
in  the  offspring.  The  genes  in  a  particular  chromosome  constitute  a 
linkage  group.  The  total  number  of  linjcage  groups  in  an  organism,  as 
determined  by  a  study  of  linked  characters  in  breeding  experiments, 
must,  then,  correspond  to  the  haploid  number  of  chromosomes  as  seen 
under  the  microscope.  This  has  been  found  to  be  the  case  in  two 
important  instances  where  data  are  available.  In  Drosophila,  as  we 
knowr  four  different  chromosomes  are  to  be  observed  when  the  haploid 
nuclei  are  observed  under  the  microscope.  The  breeding  experiments 
with  Drosophila  likewise  show  four  linkage  groups.  It  has  been  found 
in  Indian  corn  that  the  10  haploid  chromosomes  found  by  the  cytologist 
check  with  b^eding  experiments  of  the  geneticist  showing  10  linkage 
groups.  (Fig.  179.) 

Linkage  necessarily  restricts  the  independent  assortment  of  genes 
in  gamete  formation.  Independent  assortment  can  occur  only  in 
cases  where  the  pairs  of  alternative  characters  that  are  being  considered 
lie  in  different  chromosomes  or  linkage  groups.  To  take  a  specific 
example  in  the  dihybrid  CScs,  which  we  studied  in  (VI)  above,  if  the 
two  pairs  of  alternative  genes  are  present  in  the  same  pair  of  homo- 
logous chromosomes,  only  two  types  of  gamete,  CS  and  cs,  can 
be  formed.  This  will  be  understood  if  it  is  remembered  that  the 
homologous  chromosomes  separate  after  having  been  temporarily 
fused  in  synapsis,  and  then  each  divides  to  form  a  total  of  four  chromo- 
somes which  are  distributed  to  the  spermatids  of  the  male  or  to  the  egg 
and  the  three  polar  bodies  of  the  female.  In  this  case,  then,  the  chro- 
mosome bearing  the  genes  CS  unites  with  its  synaptic  mate  bearing  the 
genes  cs.  Following  this  temporaiy  union,  they  separate  and  are 
segregated  in  the  secondary  spermatocytes  or,  in  the  female,  in  the 
secondary  oocyte  and  the  first  polar  body.  In  this  case,  one  of  the 
secondary'  spermatocytes  would  carry  the  CS  chromosome,  and 
the  other  would  receive  the  cs  chromosome.  In  the  final  stage,  each 
chromosome  divides  longitudinally  so  that  two  CS  chromosomes  and 
two  cs  chromosomes  are  distributed  to  the  four  spermatids  or,  in  the 
female,  to  the  egg  and  three  polar  bodies.  Thus,  as  stated  above,  when 
two  genes  are  present  in  the  same  chromosome,  there  is  no  possibility 


THE  BIOLOGY  OF  INHERITANCE  379 

of  independent  assortment,  and  only  two  types  of  gamete  are  formed 
in  a  1 :1  ratio,  as  in  the  case  of  the  monohybrid. 

Though  it  had  long  been  observed  that  certain  characters  were 
associated  in  the  organism,  the  first  evidence  obtained  by  the  labora- 
tory geneticist  that  supplied  an  explanation  of  the  phenomenon  was  in 
connection  with  the  so-called  sex-linked  characters.  A  sex-linked 
character  is  one  in  which  the  gene  is  carried  on  the  sex  chromosome 
(X  chromosome)  which  determines  the  sex  of  the  zygote  at  the  time  of 
fertilization,  as  stated  previously  (page  339).  Before  discussing  the 
behavior  of  sex-linked  characters,  it  may  be  well  to  reexamine  the 
inheritance  of  sex  in  the  light  of  our  added  knowledge  of  inheritance 
phenomena.  It  will  be  recalled  that  the  female  produces  only  one  type 
of  gamete  with  respect  to  sex;  all  eggs  contain  an  X  chromosome. 
Thus  the  female  is  homozygous  in  this  respect.  The  male,  on  the  other 
hand,  is  heterozygous  for  sex  and  produces  two  types  of  sperm :  the  X 
sperm  and  the  Y  sperm  (which  is  the  synaptic  mate)  in  equal  numbers 
as  in  the  normal  monohybrid  condition.  As  shown  above  in  matings 
between  homozygous  and  monohybrid  individuals,  the  expected  Men- 
delian  ratio  is  1:1,  or,  in  this  case,  50  per  cent  males  and  50  per  cent 
females.  Since,  however,  fertilization  is  always  at  random,  the  sex  of 
any  particular  mating  depends  on  whether  the  X  sperm  or  the  Y  sperm 
reached  the  egg  first.  The  possibilities  of  sex  inheritance,  as  just 
described,  may  be  shown  graphically  as  follows: 

Sperm  ^^ 


(XIII)  X 


XX  9 


XYd" 


Now  in  the  case  of  sex-linked  characters  it  is  established  that  the 
X  chromosome  carries  other  genes  beside  those  which  determine  sex. 
Since  these  genes  actually  lie  in  the  X  chromosome  with  those  which 
determine  sex,  they  must  be  carried  along  just  as  in  ar^y  case  of  link- 
age, discussed  above.  One  of  the  best  examples  of  this  is  found  in 
the  inheritance  of  red  and  white  eye  color  in  Drosophila.  In  this  case, 
the  gene  for  red  in  the  X  chromosome  is  dominant  over  the  gene  for 
white,  and  so,  whenever  an  X  chromosome  bearing  the  gene  for  red  is 
present  in  the  genotype,  the  animal  has  red  eyes.  Accordingly,  if  a 
red-eyed  male  is  mated  with  a  white-eyed  female,  all  the  sons  will  have 
white  eyes  and  all  the  daughters  will  have  red  eyes,  as  we  shall  now 
see.  Since  the  gene  for  eye  color  and  the  gene  determining  sex  are  on 


380 


HUMAN  ECOLOGY 


the  same  chromosome,  independent  assortment  of  the  two  genes  can- 
not occur.  The  red-eyed  male  acts  as  a  monohybrid  and  produces 
equal  numbers  of  two  types  of  gamete;  one  type  bearing  X  with  gene 
R  for  red  which  we  may  indicate  as  X*,  and  the  other,  gametes  bear- 


FIG.  201. — Illustrating  the  possibilities  of  sex-linked  inheritance  from  crosses 
between  a  white-eyed  female  Drosophila  and  a  red-eyed  male.  All  of  the  Fi  females 
are  red-eyed  (left);  ail  of  the  males  are  white-eyed  (right).  The  results  of  matings 
between  these  two  genotypes  are  shown  in  the  Ft  generation  and  described  on  page  381. 
(Slightly  modified  from  Morgan,  "Scientific  Basis  of  Evolution,'1  W.  W.  Norton  &  Com- 
pany, Inc.) 

ing  Y.  The  white-eyed  female  is  homozygous  with  respect  to  sex  and 
also  eye,  color  and,  therefore,  produces  only  one  type  of  gamete,  indi- 
cated as  Xr,  all  of  which  contain  the  gene  for  the  recessive  white  eye 
color.  This  is  another  case  of  mating  a  monohybrid  with  a  homozy- 


THE  BIOLOGY  OF  INHERITANCE 


38J 


gous  individual  (VII),  (VIII),  (XIII),  and  the  expected  results  in  the 
Fi  generation,  namely,  50  per  cent  white-eyed  males  and  50  per  cent 
red-eyed  females  are  illustrated  in  the  following  square.  (Fig.  201.) 


(XIV) 


X'Y 


Let  us  carry  sex-linked  inheritance  a  step  further  by  next  mating 
these  males  and  females  of  the  Fi  generation  (XIV).  It  will  be  seen 
that  the  male,  as  always,  is  heterozygous  with  respect  to  sex,  produc- 
ing two  types  of  gametes  X*  and  Y.  But  neither  of  these  carry  the 
red  gene,  so  the  male  is  homozygous  with  respect  to  eye  color.  The 
female,  as  always,  is  homozygous  with  respect  to  sex,  producing  only 
one  type  of  gamete,  X,  but  is  heterozygous  for  eye  color  wit.h  two 
types  of  gamete,  XR  and  Xr.  The  union  of  these  male  and  female 
gametes  involves  a  typical  monohybrid  crossing  (VI)  with  four  possi- 
ble zygotes,  as  shown  in  the  square. 


(XV) 


X' 


XR 


XB  Y 


X' 


X'Xr 


X'Y 


It  is  seen  that  the  offspring  show  the  following  possibilities;  25  per 
cent  with  the  genotype  Xr  XR  will  be  red-eyed  females;  25  per  cent 
with  the  genotype  X*  Xr  will  be  white-eyed  females;  25  per  cent 
with  the  genotype  XR  Y  will  be  red-eyed  males;  and  25  per  cent  with 
the  genotype  Xr  Y  will  be  white-eyed  males.  All  of  these  possibilities 
are  illustrated  in  Figure  201.  Mating  between  a  white-eyed  male, 
X*  Y ,  and  a  red-eyed  female  homozygous  for  the  red-eyed  condition, 
X*XR,  gives  the  same  results  for  color  inheritance  as  in  any  mating  of 
two  individuals  homozygous  for  a  particular  character.  Thus  all  of  the 
hybrid  FI  offspring  of  both  sexes  from  this  cross  will  show  the  domi- 
nant character,  red  eye,  and  these,  when  interbred,  will  normally  give, 
in  the  next  generation,  3  red-eyed  individuals  to  1  white;  made  up  of 
one  homozygous  red-eyed  female,  XRXR,  one  hybrid  red-eyed 
female,  XR  X' ;  one  red-eyed  male,  XR  Y,  and  one  white-eyed  male, 
X'  Y.  (Fig.  201.) 

Color  Blindness. — Long  before  the  mechanism  of  sex-linked 
characters  was  understood,  it  had  been  noted  that  a  defect  in  human 


382 


HUMAN  BIOLOGY 


vision,  known  as  color  blindness  and  characterized  by  an  inability  to 
distinguish  between  red  and  green  color,  was  inherited  in  a  peculiar 
fashion.  In  the  first  place,  this  defect  is  much  more  common  in  men 
than  it  is  in  women,  and,  secondly,  it  is  transmitted  between  the  two 
sexes  in  an  unusual  way  which  can  be  explained  on  the  assumption 
that  the  defective  gene  is  carried  in  the  X  chromosome.  There  is 
an  added  feature,  however,  not  found  in  the  inheritance  of  eye  color 
in  Drosophila  which,  as  we  have  just  seen,  behaves  as  a  simple  Men- 
delian  dominant;  that  is,  the  red  eye  appears  in  either  sex  when  the 


Color  blind 


Normal 


FIG.  202. — Diagram  illustrating  the  inheritance  of  color  blindness  from  a  color-blind 
father  and  normal  mother  as  described  on  page  383.  X-chromosomes  carrying  this 
defect  are  shown  in  solid  black.  (Sinnott  and  Dunn,  after  Dunn,  Courtesy  of  the  Uni- 
versity Society.) 

gene  is  present  in  the  X  chromosome.  The  gene  for  color  blindness, 
which  we  may  designate  as  c,  for  some  unknown  reason  acts  as  a 
recessive  in  the  female  and  must,  therefore,  be  present  in  both  of  the 
X  chromosomes  in  order  to  produce  color  blindness,  as  Xc  Xc.  If 
it' is  present  in  only  one  of  the  X  chromosomes,  as  Xc  X,  the  woman 
will  not  have  defective  color  vision,  but  she  will  be  a  carrier  for'  color 
blindness.  These  carrier  females  will  form  two  types  of  gamete  with 
respect  to  this  character,  namely,  gametes  bearing  the  normal  X  and 
gametes  bearing  the  defective  Xc,  with  the  result  that  the  gene  for 
color  blindness  will  be  transmitted  to  the  next  generation.  In  the 


THE  BIOLOGY  OF  INHERITANCE 


383 


male,  the  gene  for  color  blindness  acts  as  a  dominant  so  that  the  man 
is  always  color -blind  when  the  X  chromosome  carries  the  defective 
gene  as  Xc  Y . 

Various  possibilities  for  the  offspring  result  from  matings  between 
normal  males,  color-blind  males,  normal  females,  carrier  females,  and 
color-blind  females.  One  common  example  may  be  taken  to  show 
the  inheritance  from  a  color-blind  father  with  the  genotype  Xc  Y- 
Two  types  of  gamete  will  be  produced,  namely,  Xc  and  Y.  If  union 
occurs  with  a  normal  female  with  the  genotype  XX  and  therefore 
producing  only  X  gametes,  the  same  possibilities  exist  as  in  a  mono- 
hybrid-homozygous  combination  as  already  indicated  in  (VII)  and 
(VIII).  As  shown  below  (XVI),  all  the  children  of  both  sexes  will 
have  normal  vision,  but  the  daughters  will  be  carriers. 


(XVI) 


X 


X  Y 


Continuing  this  analysis  one  step  further  to  the  grandchildren,  we 
consider  the  possibilities  for  color  blindness  inherent:  in  the  offspring 
of  normal  fathers  and  carrier  mothers  (XVI).  It  will  be  seen  that 
the  male  gametes  are  free  from  the  defective  genes  and  form  normal  X 
gametes  and  Y  gametes.  The  mother,  as  a  carrier  with  the  genotype 
Xc  X,  produces  Xc  gametes  and  X  gametes,  and  the  results  in  the 
offspring  can  be  shown  in  the  monohybrid  square  (XVII). 


(XVII) 


X  X« 


X 


XX 


XY 


It  is  evident  that  the  defective  gene  of  the  male  grandparent  (XVI) 
under  these  conditions  may  be  expected  to  produce  color  blindness 
in  one-half  of  the  grandsons  and  carriers  in  one-half  of  the  grand- 
daughters. All  the  other  grandchildren,  both  male  and  female,  will 
be  free  from  the  defective  gene.  Another  defective  gene  associated 
with  the  sex  chromosome  is  responsible  for  hemophilia  or  bleeding. 
It  behaves  in  the  same  way  in  inheritance.  (Fig.  202.) 

Lethal  Genes. — The  typical  monohybrid  ratio  of  3  dominant  to 
1  recessive,  which  has  been  so  thoroughly  established  in  various  organ-- 
isms, is  supplanted  by  a  2:1  ratio  in  the  expression  of  certain  char- 
acters. The  explanation  of  this  ratio  was  not  clear  for  some  time,  and 


384 


HUMAN  BIOLOGY 


then  it  was  discovered  from  the  proper  experimental  crosses  that  one  of 
the  expected  F2  genotypes  was  entirely  missing  in  the  offspring  from 
certain  monohybrid  crosses,  and  this  was  due,  it  became  evident,  to  the 
presence  of  a  so-called  lethal  gene  which  resulted  in  the  death  of  all 
individuals  when  present  in  a  homozygous  condition.  This  lethal 
condition  was  first  shown  in  the  gene  for  yellow  hair  color  in  mice.  It 
is  known  that  yellow  color  is  dominant,  and  accordingly  it  would  be 
expected  that  the  two  genotypes  YY  and  Yy  would  be  found  among 
the  yellow  animals.  But  yellow  animals  crossed  with  black  invariably 
gave  a  1:1  ratio  instead  of  all  yellow  as  expected  with  matings  of 
homozygous  dominants  (YY)  and  homozygous  recessives  (yy).  Also 
when  the  yellow  animals  were  crossed  with  yellow,  the  color  ratio  in 
the  offspring  was  always  2  yellow  to  1  recessive  (black  or  brown  or 


O 

Dies 

YY  Yy  Yy  yy 

FIG.  203. — Illustrating  inheritance  of  a  lethal  factor  in  mice.  The  homozygous 
yellow  embryos  (YY)  die,  as  described  on  page  384.  (Sinnott  and  Dunn,  slightly 
modified.} 

gray).  In  a  study  of  over  1,200  mice,  the  2:1  ratio  was  always  closely 
maintained.  The  breeding  results  are  just  what  would  be  expected  if 
all  the  yellow  animdls  were  heterozygous  (Yy).  It  is  clear  that  such  is 
the  situation.  The  homozygous  yellow  (YY)  is  lethal,  and  the  zygotes 
that  receive  this  combination  at  fertilization  die  before  birth.  (Fig. 
203.) 

Another  very  interesting  example  of  a  lethal  gene  that  has  been  very 
thoroughly  studied  is  associated  with  the  so-called  creeper  chicken, 
which  is  characterized  by  a  marked  reduction  in  the  length  of  the  legs 
(also  the  wings)  so  that  the  animals  appear  to  be  squatting  on  the 
ground,  and  their  locomotion  is  more  of  a  creep  than  a  walk.  It  has 
been  shown  by  the  proper  crosses  that  the  gene  for  the  creeper  charac- 
ter always  acts  as  a  dominant  when  present.  Normal  animals  never 
have  creeper  offspring,  but  normals  crossed  with  creepers  give  a  1:1 
ratio,  and  creepers  crossed  with  creepers  give  a  ratio  of  2  creepers  to  1 
normal,  as  in  the  yellow  mice.  The  creeper  condition  is  therefore 


THE  BIOLOGY  OF  INHERITANCE 


385 


heterozygous  (Dd).  The  homozygous  creeper  (DD)  dies.  An 
examination  of  the  eggs  being  incubated  shows  that  the  homozygous 
creeper  embryos  die  at  about  the  fourth  day  of  incubation.  If  account 
is  taken  of  the  dead  creeper  embryos,  then  the  expected  3 : 1  ratio  from 
breeding  monohybrids  is  obtained.  The  square  gives  the  results  when 
creepers,  all  bearing  the  genotype  Dd,  are  crossed: 


(XVIII) 


NX\Eggs 
Sperm^^x^^ 

D 

d 

D 

DD 

(dies) 

Dd 
(creeper) 

d 

Dd 
(creeper) 

dd 
(normal) 

Crossing  Over. — The  description  of  Mendelian  inheritance  given  so 
far  shows  an  amazingly  rigid  system  for  the  transmission  of  characters 
from  generation  to  generation.  Thus  it  has  been  shown  that  the  genes 
are  arranged  in  definite  linear  fashion  in  the  chromosomes  and  are 
sorted  out  or  segregated  in  mechanical  fashion  during  gamete  forma- 
tion in  correspondence  with  the  pairs  of  alternative  characters. 
Furthermore,  it  has  been  shown  that  there  are  relatively  few  chromo- 
somes as  compared  with  a  large  number  of  genes,  and,  accordingly, 
great  numbers  of  genes  are  linked  together  in  each  chromosome.  Thus 
linkage  limits  the  possibility  of  independent  assortment  and,  thereby, 
the  possible  types  of  gamete  produced.  The  rigidity  of  the  system  is 
somewhat  lessened  by  the  fact  that  fertilization  is  at  random;  but  with 
a  small  number  of  chromosomes,  as  in  Drosophila  with  a  haploid 
number  of  4,  only  16  combinations  are  possible  with  other  gametes  as 
shown  in  the  dihybrid  square  (IX).  And  so  a  Drosophila  sperm  from  a 
pure  dominant,  with  four  haploid  chromosomes,  A,  B,  C,  D,  has  the 
possibility  of  uniting  with  an  egg  from  an  individual  with  the  same 
chromosome  grouping  or  with  an  egg  from  one  of  the  15  other  chromo- 
some combinations  such  as  A,  B,  C,  d;  A,/?,c,d;  etc.;  and,  finally,  to 
a,  6,  c,  d,  in  which  all  four  chromosomes  carry  the  recessive  gene. 
In  man,  with  24  different  types  of  chromosomes,  the  possibilities  of 
chromosome  combinations  are,  of  course,  greatly  increased,  but  even 
so  there  appears  to  be  no  way  to  get  new  heritable  characters  intro- 
duced into  the  system.  The  purpose  of  this  section  is  to  introduce 
certain  important  features  associated  with  inheritance  that  markedly 
decrease  the  rigidity  of  inheritance,  which  has  just  been  summarized, 
for  we  now  know  a  number  of  important  phenomena  associated  with 


386 


HUMAN  BIOLOGY 


the  hereditary  mechanism  that  introduce  new  and,  to  some  extent, 
unpredictable  heritable  features  into  the  germ  plasm  that,  thereafter, 
become 'part  and  parcel  of  the  hereditary  melange  for  transmission  to 
succeeding  generations.  (Fig.  204.) 

The  first  of  these  flexible  features  is  a  process  associated  with 
synapsis  and  aptly  designated  as  crossing  over.  The  title  is  descriptive 
and  means  that,  under  certain  conditions,  genes  located  in  one  of  the 
synaptic  mates  may  cross  to  the  other  member  of  the  pair  during 
synapsis.  That  such  occurred  in  certain  insects  during  gamete  forma- 
tion was  observed  by  the  cytologist  Janssens  nearly  thirty  years  ago, 


Gamete 


Gamete 


Homologous 

Chromosomes 

Paired 


Fia.  204. — Diagram  illustrating  the  16  possible  types  of  gametes  resulting  from 
the  union,  in  the  previous  generation,  of  two  gametes,  each  bearing  four  chromosomes, 
as  described  on.  page  385.  (Woodruff.) 

but  it  was  the  more  recent  results  obtained  by  breeding  experiments 
in  fruit  flies  that  confirmed  the  cytological  findings.  Crossing  over, 
since  it  occurs  with  predictable  frequency  in  the  case  of  certain  matings, 
is  to  be  regarded  as  a  normal  feature  of  synapsis.  It  produces  new 
possibilities,  recombinations,  for  the  genevS  concerned.  (Fig.  205.) 

The  classic  example  of  crossing  over,  which  will  serve  admirably  for 
our  purpose,  occurs  in  Drosophila.  Female  flies  with  two  dominant 
linked  genes  for  Gray  Body  ((?)  and  Long  Wings  (TF),  when  mated  with 
the  so-called  vestigial  male  flies  that  carry  linked  recessive  genes  for  the 
alternative  characters  Black  Body  (g)  and  Short  Wings  (w),  produce 
only  hybrid  gray,  long-winged  offspring.  This  is  just  what  would  be 


THE  BIOLOGY  OF  INHERITANCE 


387 


expected  since  the  pure  gray,  long-winged  flies  with  the  genes  G  and  W 
in  the  same  chromosome  can  produce  only  one  type  of  gamete,  namely, 
GW;  and  the  recessive  black,  short-winged  animals,  under  the  same 
conditions,  can  also  produce  only  one  type  of  gamete,  namely,  gw. 
Fertilization  between  these  gametes  produces  100  per  cent  hybrid  FI 
offspring  with  the  genotype  GWgw.  These  individuals,  of  course, 


cm     u 


123 

FIG.  205. — Diagram  illustrating  crossing  over  between  homologous  chromosomes 
during  syiiapsis,  giving  new  arrangements  of  the  genes  (recombinations).  If  110 
crossing  over  occurs  the  chromosomes  separate  after  synapsis  carrying  the  same  genes 
(1).  Crossing  over  at  two  points  (double  crossing  over)  is  shown  in  (2)  and  the  results 
of  the  recombinations  in  (3).  (Sinnott  and  Dunn,  slightly  modified.} 

behave  as  monohybrids  and  produce  two  types  of  gamete  in  equal 
numbers:  GW  gametes  and  gw  gametes.  Now,  if  the  monohybrid 
females  (GWgw)  are  mated  with  the  pure  recessive  males  (gwgw),  the 
expected  Mendelian  ratio  is  50  per  cent  hybrid  dominants  (GWgw) 
and  50  per  cent  pure  recessives  (gwgw)  as  in  the  typical  monohybrid- 
homozygous  mating,  as  shown  in  the  square. 


(XIX) 


Sperm 


gw 


GW 


GWgw 


gw 


gwgw 


Actually,  however,  repeated  experiments  showed  that  the  expected 
1 : 1  ratio  did  not  appear.  In  all  cases,  approximately  82  per  cent  of  the 
animals  were  divided  equally  between  the  hybrid  dominants  and  the 


388 


HUMAN  BIOLOGY 


pure  recessives,  whereas  in  18  per  cent  of  the  offspring  two  unexpected 
varieties  appeared  in  equal  numbers,  namely,  Gray  animals  with  short 
wings,  that  is,  the  genotype  Gwgw,  and  black  flies  bearing  long  wings, 
or  the  genotype  gWgw.  The  results  were  decisive,  and,  therefore,  it 
was  apparent  that  either  something  was  wrong  with  the  accepted  ideas 
of  Mendelian  inheritance  or  else  some  condition  existed  that  had  not 
been  taken  into  account.  The  latter  proved  to  be  correct.  The 
unknown  factor  was  the  phenomenon  of  crossing  over.  It  is  amply 
confirmed  that  synapsis  results  not  only  in  the  pairing  of  the  genes  for 
alternative  characters  but  also,  in  many  instances,  in  the  actual 
transfer,  or  cross  over,  of  portions  of  the  pairing  chromosomes  from 
one  to  the  other  so  that  when  the  chromosomes  separate  after  synapsis, 
each  actually  contains  one  or  more  pieces  of  the  other  member  of  the 
pair.  Necessarily  the  genes  in  the  detached  pieces  of  the  chromosomes 
are  also  transferred  to  the  other  sy  nap  tic  mate.  Thus,  in  the  example 
above,  one  of  the  homologous  chromosomes  with  the  linked  genes  GW 
became  Gw  after  synapsis,  and,  in  the  other,  the  linkage  gw  was  changed 
to  gW.  Consequently,  as  shown  in  the  square,  there  were  four 
types  of  gametes  for  union  with  the  gib  gametes  of  the  recessive 
male. 


(XX) 


^^^Eggs 
Sperm  ^. 

GW 

Gw 

gW 

gw 

gw 

GWgw 

Gwgw 

gWgw 

gwgw 

41% 

9% 

9% 

41% 

Sex-linked  inheritance  gave  the  clue  to  linkage;  linkage  gave  the 
clue  to  crossing  over;  crossing  over  gave  the  clue  to  the  arrangement 
of  genes  in  the  chromosomes  and  enabled  the  investigators  in  this  field 
to  accumulate  data  for  the  preparation  of  chromosome  maps  through 
the  discovery  that  the  relative  frequency,  or  percentage,  of  crossing 
over  was  in  direct  relation  to  the  distance  between  the  genes  concerned. 
Genes  lying  close  together  have  very  little  chance  to  shift  their  position, 
or  cross  over,  to  thfe  other  chromosome  of  the  pair;  genes  lying  rela- 
tively far  apart  have  a  much  greater  chance.  As  an  example  of  the 
methods  that  have  been  employed,  let  us  consider  the  position  of  the 
genes  for  another  character  in  the  same  chromosomes,  in  relation  to 
the  genes  Gg  and  Ww.  Designating  one  end  of  the  chromosome  asf  zero, 
we  can  arbitrarily  locate  genes  0  and  W  at  two  points  along  the 
chromosome,  but  they  must  he  separated  by  18  units  from  each  other 


THE  BIOLOGY  OF  INHERITANCE  389 

in  correspondence  with  the  percentage  of  crossovers  as  determined  by 
the  breeding  experiments.  The  appropriate  breeding  experiments 
with  the  genes  for  the  new  character,  which  we  shall  indicate  as  Nn, 
show  a  percentage  crossover  in  relation  to  Gg  of  30,  whereas  the  per- 
centage of  crossovers  of  Nn  in  relation  to  Ww  is  only  12.  Since  the 
percentage  of  crossovers  is  higher  when  the  distance  between  the  genes 
involved  is  greater,  the  results  from  the  breeding  experiments  show  that 
the  gene  N  must  be  located  12  linear  units  below  W  and  30  points 
below  G.  This  is  the  only  point  that  will  conform  to  the  percentages 
of  crossovers,  that  is,  18  per  cent  between  G  and  TF,  12  per  cent 
between  W  and  N,  and  30  per  cent  between  G  and  N.  Cumulative 
data  from  a  large  number  of  characters  in  Drosophila  have  established 
the  positions  of  the  corresponding  genes  in  certain  chromosomes  and 
the  development  of  the  so-called  chromosome  map.  Chromosome 
maps  have  also  been  established  for  certain  characters  in  other  organ- 
isms, notably  Indian  corn.  Particularly  gratifying  to  the  biologist  is 
the  fact  that  the  positions  of  the  genes  shown  in  the  chromosome  maps, 
which  were  determined  on  the  basis  of  crossing  over,  have  been  shown 
in  many  instances  to  rest  on  a  firm  basis  by  the  data  derived  from  the 
microscopic  study  of  the  chromosome  complex  which  will  now  be 
indicated.  (Figs.  181,  211.) 

MUTATIONS 

Leaving  the  phenomenon  of  crossing  over,  which  has  been  found 
to  be  a  normal  and  predictable  feature  of  Mendelian  inheritance,  con- 
sideration must  next  be  given  to  a  variety  of  unpredictable  and  abnor- 
mal inheritance  patterns  that  occur  as  the  result  of  several  types  of 
irregularity  or  aberration  in  the  chromosomal  complex.  These  are 
commonly  grouped  under  the  term  mutation;  the  term  coming  from  the 
Latin  verb  mutare,  meaning  "to  change."  Chromosomal  aberrations, 
or  mutations,  in  the  germ"  cells  are  effectual  in  producing  somatic 
mutations  in  the  resulting  offspring.  The  latter,  in  turn,  in  producing 
gametes  with  the  mutated  chromosomes,  continue  to  pass  the  muta- 
tions along.  Once  in  the  germ  plasm,  the  mutation  is  there  to  stay 
inless  a  later  mutation  in  this  same  region  again  alters  the  chromatin 
pattern.  It  is  possible  for  mutations  to  occur  also  in  the  chromatin 
rf  one  or  more  of  the  body  cells  of  an  organism.  Succeeding  genera- 
tions of  cells,  directly  descended  from  the  mutated  cell,  will  have  the 
same  change  impressed  upon  them,  but,  in  this  case,  the  mutation 
3annot  get  into  the  stream  of  germ  plasm  for  transmission  to  offspring. 
However,  somatic  mutations  of  this  kind  are  not  uncommon  in  plants 


390  HUMAN  BIOLOGY 

and  are  known  as  bud  mutations.  Since  plant  tissues  are  usually 
capable  of  regeneration,  it  is  possible  to  remove  the  mutated  tissue 
and  to  grow  it  independently  or  as  a  graft  on  another  plant  of  the 
same  species  where  it  will  produce  the  mutated  type  of  cells  and 
tissues.  Bud  mutations  of  this  type  are  responsible  for  many  standard 
varieties  of  apples  and  other  fruits. 

Mutations  have  their  origin  in  the  various  types  of  chromosomal 
aberrations.  Some  of  these  involve  changes  in  considerable  areas  of 
the  chromosomes  and  so  come  within  the  range  of  microscopic  vision  ; 
marked  structural  abnormality  being  visible  in  one  or  more  of  the 
chromosomes.  Suppose,  for  example,  that  the  offspring  produced 
by  the  mating  of  individuals  with  known  genotypes  exhibit  some 
unusual  character.  Microscopic  examination  of  the  chromosome 
complex  from  both  germinal  and  somatic  cells  may  reveal  changes  in 


FIG.  206. — Diagrams  illustrating  nondisjunctioii  in  which  both  members  of  a  pair 
of  homologous  chromosomes  go  to  one  cell  (to  the  left),  thus  giving  one  cell  with  four 
chromosomes  and  one  cell  with  two  chromosomes.  (Shull.) 

the  normal  pattern.  *  Thus  it  was  found  some  years  ago  that  certain 
females  of  Drosophila  had  unexpected  eye  color  and  various  other 
abnormalities.  The  basis  of  this  altered  inheritance  was  revealed  by 
the  microscopic  studies  that  showed  that  such  females  had  a  Y  chromo- 
some in  addition  to  the  XX,  thus  giving  an  XXY  instead  of  either  XX 
of  normal  females  or  XY  of  normal  males.  This  abnormal  condition 
was  traced  to  the  fact  that  the  parental  females  were  producing  mature 
eggs  carrying  XX  instead  of  X.  A  failure  of  the  X  chromosomes  to 
separate  (nondisj  unction)  at  the  last  maturation  division  caused  the 
production  of  XX  gametes.  Instead  of  a  single  chromosome  being 
added  to  the  chromatin  complex  of  the  gametes,  as  in  the  case  of  the 
XX  condition  just  described,  many  instances  are  known  in  which 
an  altered  heredity  pattern  is  the  result  of  a  doubling  or  tripling  or 
even  a  larger  multiple  of  the  normal  chromosome  number,  the  condition 
of  heteroploidy  (page  340).  In  this  way,  distinct  varieties  of  a 
particular  species  are  established  and  continue  as  a  standard 
type,  as  seen,  for  example,  in  the  well-known  varieties  of  wheat  with 
7,  14,  and  21  chromosomes  or  in  certain  distinct  species  of  Chrysan- 
themum with  18,  36,  54,  72,  and  90  chromosomes  in  the  diploid 


THE  BIOLOGY  OF  INHERITANCE 


391 


condition.     The  case  of  Drosophila  with  12  chromosomes  instead  of 
8  has  been  mentioned  previously.     (Figs.  184,  206.) 

Other  chromosomal  aberrations  that 
result  in  altered  heredity  may  involve 
the  breaking,  or  fragmentation,  of  a 
chromosome  and  the  later  association  of 
one  or  more  of  the  detached  pieces 
with  a  chromosome  of  another  linkage 
group.  Thus  chromatin  material  may  A  B 

i         1   i    ,     i    <•  i  '        f  •  FIG.     207. — Drawings    of    the 

be  deleted  from  one  member  of  a  pair     diploid  chromosomes  Of  the  Dro- 

of    homologous     chromosomes    and    at-      sophila  female   (XX)   to  illustrate 
11,  i  i     i          •          x         the     translocation     of     chromatin 

tached  to  a  chromosome  belonging  to     material.    In  A  it  is  noted  that 

another    pair.      As    a    result,    a    Certain      there  is  a  normal  sized  chromosome 


(IV)  and  an  abnormally  large  one 
(IV  -f  X) ;  the  increased  size  of  the 
latter  is  due  to  a  trarislocatiori  of  a 
piece  from  one  of  the  X  chromo- 


percentage of  the  gametes  will  be  defi- 
cient for  certain  genes,  but  other  gametes 

will  have  genes  added.     Deletion,  trans-    80mes.    This    tranfllocation    was 
location,  inversion   are   all  established     duo  to  the  irradiation  of  germ 

.,.*...  /.       i  ,•  A.    *.•  plasm  of  one  of  the  parents.     In  B 

possibilities    of    chromatin   mutations,     it  is  noted  that  th£  translocation 
many    of    which   involve   chromosome     nas  affected  both  of  the  IV  chromo- 

, .         . .  /.          /*•    •      j.        •          j.        i         somes,    and    therefore    this    indi- 

alterations    of    sufficient    size    to     be    vidual  was  homozygous  for  the 

checked  with  the  microscope.      In  Very      altered    chromatin    condition.    Cf. 
,  ,  11  /.     ,1          Fig.    179.     (Painter,    "Science    in 

recent    years,    our    knowledge    of   the    Progre88t»  Yale  University  Press.) 
relations  between  chromosomal  abnor- 
malities and  altered  somatic  structures  has  been  markedly  increased 
by  two  things;  first,  the  discovery  that  the  X-ray  irradiation  of  germ 

cells  undergoing  development  would  change 
the  normal  chromatin  setup  in  the  gametes, 
and  second,  the  discovery  of  giant  chromo- 
somes in  the  nuclei  of  the  salivary  gland  cells 
that  were  large  enough  to  make  visible  altered 
chromatin  patterns  in  areas  much  too  small  to 
be  studied  in  normal-sized  chromosomes  (page 
336).  In  addition  to  their  extraordinary  size, 
the  giant  chromosomes  are  undergoing  syn- 
apsis,  though  present  in  somatic  cells,  arid  this 


FIG.  208. — Drawing  of 
the  terminal  portion  from 
one  of  the  giant  chromo- 
somes (II),  of  a  salivary 
gland  cell,  Drosophila, 

showing  the  translocation  synaptic  oondition  is  of  the  highest  value  in 
the  detection  of  chromatin  irregularities  in 
either  of  the  pairing  chromosomes  (page  347). 
For,  in  syriapsis,  absolute  exactness  of  gene 
position  throughout  the  length  of  the  pairing  chromosomes  is  neces- 
sary. The  genes  for  all  of  the  alternative  characters  linked  together 
in  a  particular  pair  of  chromosomes  must  be  at  exactly  the  same  levels 


of  a  piece  of  the  fourth 
chromosome  (IV) .  (Painter, 
1 '  Science  in  Progress, ' '  Yale 
University  Press.) 


392 


HUMAN  BIOLOGY 


in  the  synaptic  mates.  With  size  of  the  genes  well  below  microscopic 
visibility,  this  means  an  exactness  of  construction  with  relation  to  gene 
position  in  chromosomes  far  beyond  anything  that  can  be  achieved  in 
machine  construction.  (Figs.  207,  208.) 

Since  every  gene  must  be  in  its  exact  position  when  synapsis  occurs, 
this  process  is  now  recognized  as  one  of  the  most  important  tools  for 

|K 


f 


aocdefghlmno 
Fio.  209. — Diagram  illustrating  synpasis  between  chromosomes  in  which  the  genes 
are  not  identical.     In  the  lower  chromosome  it  will  be  noted  that  deletion  has  occurred 
involving  genes  j,  k,  and  I.     This  portion  of  the  upper  chromosome  is  drawn  to  one  side 
so  that  the  paired  genes  match  exactly  (page  392) .     (Shull.) 

determining  an  altered  gene  complex  in  either  member  of  the  synaptic 
mates.  If  some  of  the  genes  in  one  of  the  pairing  chromosomes  are 
out  of  place  or  if  they  are  missing,  then  the  corresponding  or  alternative 
genes  in  the  normal  chromosome  cannot  join  in  synapsis.  Accordingly, 
this  region  of  the  normal  chromosome  pulls  away  from  the  abnormal 

roughoid 


FIG.  210. — Drawings  of  the  terminal  portion  of  one  of  the  giant  chromosomes  (II), 
salivary  gland  cell,  Drosophila,  showing  actual  deletion  as  diagrammed  in  Fig.  209. 
The  normal  condition  of  synapsis  is  shown  in  the  upper  figure.  The  affected  region, 
which  carries  the  genes  for  roughoid,  an  eye  character,  is  indicated  at  Del.  A  and  Del.  B. 
Modification  in  synapsis  as  the  result  of  the  deletions  is  shown  at  a  maximum  magnifica- 
tion in  the  lower  figures.  In  Deletion  A  one  of  the  synapsing  chromosomes  is  normal, 
whereas  one  band  is  missing  from  the  other.  In  Deletion  B,  one  chromosome  is  normal 
and  three  bands  are  missing  from  the  other  one.  (Painter,  "Science  in  Progress,11 
Yale  University  Press.) 

» 

synaptic  mate  and  forms  a  loop-like  structure  off  to  one  side,  and  this 
permits  those  genes  which  are  present  and  normally  locatld  in  both 
chromosomes  to  meet  in  synapsis.  The  synaptic  behavior  of  the 
normal  chromosome  gives  visible  evidence  to  the  cytologist  of  regions 
with  an  altered  gene  arrangement  in  the  synaptic  mate.  Intensive 
study  of  the  latter,  particularly  in  the  giant  chromosomes,  has  brought 


THE  BIOLOGY  OF  INHERITANCE 


393 


altered  heredity  patterns  almost  down  to  the  actual  genes  concerned 
(page  335).     (Figs.  209  to  211.) 

We  have  just  been  dealing  with  mutations  that  involve  areas 
in  the  chromosomes  of  sufficient  size  to  be  observed  under  the  micro- 
scope. Mutations  occur,  however,  in  which  the  chromatin  pattern 
of  the  mutants  shows  no  visible  alteration  even  under  the  highest 
magnifications.  Such  mutations  must  have  their  sole  basis  in  the 
chromatin  complex,  but  they  may  result  from  a  change  in  only  a  single 
gene  and,  therefore,  cannot  possibly  be  brought  within  the  limits  of 
microscopic  visibility.  Such  mutations  are  said  to  be  due  to  point 
changes,  the  point  being  the  exact  region  in  the  chromosome  at  which 


px  sp 

FIQ.  211. — Diagram  illustrating  the  determination  of  gene  loci  by  different  methods. 
Above  is  shown  an  outline  drawing  of  Chromosome  II.  Below,  the  chromosome  map 
of  this  chromosome  is  given,  with  the  positions  of  several  gene  loci  indicated.  The 
positions  of  the  genes  in  the  chromosome  map  have  been  determined  by  data  from 
matings  involving  crossovers  (page  387);  the  positions  of  certain  of  these  genes  have 
also  been  determined  by  cytological  studies  following  translocation  (Figs.  207,  208). 
It  will  be  noted  from  the  positions  of  the  vertical  lines  (1  to  9)  that  genetical  and  cyto- 
logical data  agree  in  the  order  of  gene  loci  but  indicate  certain  differences  in  the  dis- 
tances separating  them.  Sf,  spindle  fiber  attachment;  Bl,  bristle-  dp,  dumpy — a  body 
character;  b,  black;  sp,  speck — color  on  wing,  etc.  (After  Dobzhan$ky.  Redrawn.) 

the  mutating  gene  is  located.  It  is  impossible  to  speak  with  absolute 
certainty,  but  it  appears  most  likely  that,  in  a  point  change,  an  actual 
chemical  change  takes  place  in  the  mutating  gene.  In  the  final 
analysis,  a  gene  can  be  nothing  more  or  less  than  the  tiniest  bit  of  a 
specific  chemical  compound,  possibly  a  single  molecule.  In  addition 
to  point  changes  as  the  result  of  chemical  change,  the  possibility  also 
exists  of  a  mutation  due  to  a  change  in  the  position  of  a  gene  or  genes, ' 
the  so-called  position  effect.  In  other  words,  the  mutation  may  be  due 
to  a  change  in  the  position  of  a  gene  rather  than  a  change  in  its  chemical 
nature.  Considerable  evidence  has  been  very  recently  accumulated 
that  indicates  the  importance  of  gene  position.  The  possibilities 
involved  may  be  visualized  by  realizing  the  changes  that  would  follow 


394 


HUMAN  BIOLOGY 


the  removal  of  a  factory  manufacturing  chemicals  from  one  town  to 
another,  even  though  the  two  places  might  not  be  far  apart.  The 
importance  of  gene  position  in  synapsis  has  just  been  emphasized. 

The  discussion  of  inheritance,  so  far  presented,  has  been  concerned 
with  formation  and  transmission  from  generation  to  generation  of  the 
gene  complex  responsible  for  the  characters  expressed  in  the  individual. 
At  this  point,  some  attention  may  profitably  be  given  to  the  environ- 
ment with  which  each  living  organism  is  in  continuous  adjustment  and 
on  which  it  depends  for  a  constant  supply  of  the  materials  essential 


FIG.  212. — Photograph  of  a  corn  field  showing  plants  growing  under  favorable 
environmental  conditions  (left)  and  plants  growing  under  unfavorable  environmental 
conditions  as  the  result  of  crowding  (right).  (Woodruff,  after  Blakeslee.} 

to  the  life  processes.  The  question  is:  Do  the  characters  shown  in 
the  mature  organism  depend  exclusively  upon  the  gene  complex?  The 
answer  is  clearly  in  the  negative.  Each  individual  represents  the 
results  obtained  from  a  partnership  in  which  the  gene  complex  has 
been  working  in  close  association  with  the  environment.  If  the 
environment  is  favorable,  the  gene  complex  will  come  to  full  fruition, 
but,  if  the  environment  is  unfavorable,  the  normal  group  of  characters 
expected  from  a  particular,  gene  complex  will  be  restricted  or  modified 
in  various  ways.  (Fig.  212.) 

But  now  we  come  to  another  question  with  reference  to  environ- 
mental effects:  Is  the  chromatin  complex  with  its  thousands  of  con- 


THE  BIOLOGY  OF  INHERITANCE  395 

stituent  genes  changed  in  the  organism  by  the  environmental  factors 
so  that  gametes  will  be  formed  with  an  altered  gene  complex.  The 
answer  is  "no"  but  with  certain  reservations.  Thus,  the  geneticist 
has  discovered  that  if  the  immediate  environment  of  Drosophila 
contains  X  rays  of  certain  strength,  the  chromatin  in  the  germ  cells 
will  be  altered  (page  339).  In  this  connection  it  was  suggested  some 
years  ago,  following  the  discovery  of  the  cosmic  rays  with  their  great 
power  of  penetration,  that  possibly  these  rays  had  been  directly  affect- 
ing the  chromatin  of  organisms  from  the  earliest  times. 

However,  the  condition  just  considered,  in  which  the  biologist  has 
used  X  rays  to  penetrafte  directly  to  the  germ  cells  and  alter  the  normal 
chromatin  pattern,  is  very  different  from  environmental  effects  which 
affect  only  the  somatic  cells.  In  such  cases,  if  the  heredity  of  the 
succeeding  generations  is  to  be  changed  by  environmental  action,  it 
would  be  necessary  for  the  chromatin  changes  in  the  affected  somatic 
cells  to  be  transferred  to  the  gene  complex  in  the  gametes  and  corre- 
spondingly to  modify  the  gene  pattern  in  them.  Suppose,  for  example, 
the  somatic  cells  present  in  the  thyroid  gland  of  a  particular  individual 
arc  subjected  to  an  unfavorable  environment  as  the  result  of  a  greatly 
decreased  iodine  content  in  the  blood  stream,  the  latter,  in  turn,  being 
due  to  faulty  nutrition.  In  time,  an  abnormal  condition  of  the  thyroid 
develops.  There  is  no  reason  to  suppose  that  this  environmental  effect 
directly  affects  the  germ  plasm  of  the  individual  in  any  way  whatso- 
ever, and,  since  the  gene  complex  of  the  individual  remains  unchanged, 
there  appears  to  be  no  possibility  of  the  hereditary  transfer  of  the 
thyroid  goiter  to  later  generations.  The  unsuitable  environment 
results  merely  in  an  individual  modification  that  is  doomed  to  extinc- 
tion with  the  passing  of  the  individual  concerned.  So  far  as  the 
biologist  can  see,  the  same  result,  namely,  unchanged  heredity,  is  to  be 
expected  from  all  sorts  of  individual  modifications  that  appear  in  the 
soma  following  exposure  to  abnormal  environmental  influences. 
(Fig.  207.) 

As  late  as  the  beginning  of  the  second  decade  of  the  present  century, 
it  was  not  possible  to  discuss  evolution1  with  any  authority  because  a 
knowledge  of  the  underlying  mechanism  responsible  for  the  production 
of  new  types  was  not  available.  It  was  clear  that  species  had  changed, 
that  new  ones  had  developed,  but  the  functioning  of  the  mechanism 
responsible  for  the  tremendous  variety  of  living  forms  coming  from  a 
common  life  stream  was  not  known.  To  the  paleontologists  busily 
engaged  in  discovering,  collecting,  and  mounting  the  fossil  remains  of 
plants  and  animals,  the  Lamarkian  doctrine  of  the  inheritance  of 
1  Consult  Appendix:  Organic  Evolution. 


396  HUMAN  BIOLOGY 

acquired  characters  still  persisted  with  almost  undiminished  power. 
To  the  biologist  of  today,  however,  it  is  clear  that  successive  genera- 
tions of  giraffes  could  continue  to  stretch  their  necks,  in  order  to  secure 
the  more  plentiful  food  supplies  at  the  tops  of  tall  trees,  for  billions  of 
years  without  in  any  way  affecting  the  genes  responsible  for  the  length 
of  the  neck. 

The  crux  of  the  matter  was  stated  by  Professor  E.  G.  Conklin  a  few 
years  ago  when  he  wrote  that  "The  germ  cells  are  the  only  living  bonds 
not  only  between  generations  but  also  between  species,  and  they 
contain  the  physical  basis  not  only  of  heredity  but  also  of  evolution/1 
Any  evolution  that  has  occurred  in  the  past,  therefore,  has  occurred  as 
the  result  of  the  same  mechanism  that  will  bring  it  about  today, 
namely,  changes  in  the  germ  plasm.  The  geneticist  and  the  cytologist 
have  made  clear  the  main  features  responsible  for  variations  in  the 
germ  .plasm1  and,  in  addition,  have  teen  able  to  link  these  microscopic 
changes  involving  the, genes  in  the  chromosomes  directly  with  visible 
alterations  in  the  resulting  offspring.  From  the  material  presented  in 
the  two  preceding  chapters,  it  is  apparent  that  the  condition  of  our 
knowledge  in  this  field  at  the  end  of  the  third  decade  of  this  century  is 
very  different  from  that  a  few  years  earlier,  for  we  do  have  a  knowledge 
of  the  underlying  mechanism  responsible  for  the  production  of  new 
types.  It  is  the  same  mechanism  that  is  responsible  for  the  production 
of  each  new  individual. 

1  Consult  Appendix:  Germ  Plasm. 


CHAPTER  XV 
HUMAN  HEREDITY 

The  knowledge  gained  from  the  consideration  of  reproduction  and 
heredity  in  the  preceding  chapters  can  now  be  used  as  a  basis  for  an 
inquiry  into  the  established  facts  relative  to  inheritance  in  the  human 
organism.  Human  heredity  has  long  been  a  subject  of  the  keen- 
est interest,  with  the  result  that  a  great  amount  of  data  has  been 
accumulated.  For  the  most  part,  however,  the  earlier  data  rest  upon 
upon  observations  of  a  more  or  less  random  nature  which  were  not 
subject  to  rigid  scientific  scrutiny.  With  the  establishment  of  the 
Mendelian  laws  and  the  realization  of  their  universal  application  in  the 
living  world,  geneticists  everywhere  have  given  increasing  attention  to 
the  collection  and  correlation  of  data  that  would  throw  light  upon  the 
behavior  of  the  genes  in  human  germ  plasm  in  determining  the  charac- 
ters of  the  offspring.  As  a  result,  a  considerable  number  of  characters 
have  been  found  in  man  that  are  known  to  be  transmitted  to  offspring 
in  accordance  with  the  established  Mendelian  laws.  Two  of  these 
characters,  namely,  the  inheritance  of  sex  and  the  inheritance  of  a 
sex-linked  character,  color  blindness,  have  been  discussed  in  the 
previous  chapter.  (Fig.  202.) 

It  should  be  emphasized  that  the  difficulties  inherent  in  securing 
accurate  knowledge  of  human  heredity  are  very  great.  In  the  first 
place,  the  geneticist  has  no  control  over  the  matings;  he  can  only  sit 
on  the  side  lines  and  observe  the  results.  Then,  too,  observations  are 
possible  only  on  the  characteristics  of  a  comparatively  small  number  of 
offspring  extending  over  a  few  generations.  Information  regarding 
the  previous  generations  of  a  family  is  rarely  a  matter  of  written 
record  unless  it  be  concerning  the  occurrence  of  some  particularly 
striking  characteristic,  usually  abnormal  in  nature.^  On  the  whole, 
then,  the  data  regarding  the  ancestors  of  a  particular  couple  are  apt  to 
be  sketchy  and  hearsay  rather  than  detailed  and  accurate.  Neverthe- 
less, considerable  reliable  information  relative  to  human  heredity  is 
now  at  the  disposal  of  the  geneticist,  and  the  inheritance  of  a  variety 
of  characters  in  man  well  established.  Primarily,  it  should  be  recog- 
nized that  the  analysis  of  the  relatively  unsatisfactory  data  dealing 
with  human  heredity  and  the  recognition  of  the  general  applicability 

397 


398  HUMAN  BIOLOGY 

of  the  Mendelian  laws  would  not  have  been  possible  except  for  the 
results  obtained  by  the  geneticist  from  controlled  breeding  experiments 
in  a  wide  variety  of  organisms.  In  our  present  examination  of  Men- 
delian inheritance  in  man,  it  will  be  possible  to  consider  the  inheritance 
of  only  relatively  few  characters  but  sufficient,  perhaps,  to  show  the 
broad  application  of  the  principles  that  have  been  thoroughly  estab- 
lished in  various  other  organisms.  The  inheritance  of  pigmentation, 
eye  defects,  skeletal  characteristics,  and  blood  groups  are  suited  for 
our  present  discussion  and  will  be  considered  in  the  order  named, 
following  which  consideration  can  be  given  to  certain  general  problems 
associated  with  human  hybridization.  (Fig.  213.) 

INHERITED  CHARACTERISTICS 

Pigmentation. — It  has  become  evident  that  the  visible  pigmenta- 
tion of  all  degrees  and  colors  present  in  the  eye,  skin,  and  hair  of  the 
white,  yellow,  or  black  races  results  from  the  presence  of  varying 
amounts  of  two  basic  pigments,  melanin  and  carotene,  which  are 
formed  by,  and  remain  in,  the  cytoplasm  of  various  types  of  cells. 
Presumably  the  presence  of  the  gene  or  genes  responsible  for  pigment 
formation  results  primarily  in  the  formation  of  a  specific  enzyme,  and 
the  latter  works  in  association  with  a  pigment-forming  substance 
(chromogen)  in  the  cell  cytoplasm.  Occasional  instances  in  which  blue 
eye  color  is  associated  in  an  individual  with  heavily  pigmented,  or 
brunette,  skin  and  hair  give  good  evidence  that  separate  genes  are 
responsible  for  the  production  of  pigment  in  these  three  structures,  but 
the  method  of  pigment  formation  is  believed  to  be  the  same  in  each 
instance.  In  the  case  of  melanin,  which  is  a  widely  distributed  brown 
pigment,  the  chromogen  has  been  identified  as  the  ammo  acid  tyrosine, 
hormally  present  in  cytoplasm.  The  reaction  between  tyrosine  and  a 
specific  enzyme,  tyrosinase,  forms  melanin.  (Plate  XVII,  page  415.) 

The  other  human  pigment,  carotene,  is  yellow  in  color  and  is  found 
in  both  plant  and  animal  cells.  It  is  particularly  prominent  in  the 
carrot  from  which  it  derives  its  name.  Presumably,  a  specific  enzyme 
(not  as  yet  isolated)  which  functions  in  association  with  a  cytoplasmic 
chromogen  is  necessaiy  for  its  formation.  Carotene  has  recently 
become  of  increasing  interest  because  of  its  close  relationship  to  vitamin 
A  (page  58).  The  latter,  in  turn,  is  associated  with  night  blindness 
and  other  pathological  conditions  of  the  eyes  and  is  required  for  the 
formation  of  the  visual  purple  in  the  retina  of  the  eye.  Carotene  is 
regarded  as  the  mother  substance  of  vitamin  A.  The  actual  synthesis 
of  the  latter  occurs  in  the  liver  cells. 


HUMAN  HEREDITY  399 

In  rare  cases,  a  hereditary  defect,  albiriisrri,  results  in  the  absence 
of  both  pigments.  Albinism  behaves  as  a  simple  Mendelian  recessive. 
Accordingly,  all  the  children  of  albino  parents,  since  the  latter  must  be 
homozygous  to  show  the  defect,  are  albino.  With  both  parents  nor- 
mally pigmented,  but  carrying  the  recessive  gene  for  albinism,  25  per 
cent  of  the  offspring  would  be  expected  to  show  the  defect  as  in  a 
typical  monohybrid.  The  examination  of  data  from  a  considerable 
number  of  families  has  shown  this  percentage  to  be  about  29.  It  is 
not  clear  why  it  should  be  considerably  in  excess  of  the  expected  ratio. 

Human  skin  color  may  vary  from  the  deepest  black  to  the  purest 
white,  with  a  wide  intermediate  range  of  browns,  tans,  and  yellows 
between  these  two  extremes.  The  pigmented  cells  of  the  skin,  for  the 
most  part,  are  found  in  the  epithelial  cells,  but  they  are  present  to  some 
extent  also  in  the  outer  layers  of  the  underlying  dermal  cells.  The 
offspring  from  unions  of  homozygous  colored  individuals  and  homo- 
zygous whites  show  an  intermediate,  or  mulatto,  condition  with  respect 
to  skin  color.  Children  of  the  hybrid  mulattoes  show  varying  degrees 
of  color  ranging  from  deep  black,  as  in  one  of  the  grandparents,  to  clear 
white  as  in  the  other  grandparent. 

Consideration  of  data  involving  a  large  number  of  Fa  children  from 
mulatto  marriages  shows  that  about  6  per  cent  are  deep  black.  Essen- 
tially the  same  percentage  is  white-skinned.  This  clearly  indicates  a 
multiple  gene  condition  in  which  two  pairs  of  genes  determine  the 
pigmentation  of  the  skin.  Thus  a  dihybrid  condition  for  color  is 
believed  to  be  present  as  in  the  brown-seeded  grains,  described  in  the 
previous  chapter  (page  372).  Other  authorities  have  accumulated 
evidence  indicating  that  a  trihybrid  condition  with  three  pairs  of  genes 
is  associated  with  human  skin  color.  It  is  also  clear  that  the  produc- 
tion of  pigment  in  the  skin  of  individuals  is  directly  associated  with 
sunlight.  The  temporary  coloring,  or  tanning,  of  the  skin,  following 
repeated  exposures  to  sunlight,  is  a  matter  of  common  observation,  as 
is  also  the  formation  of  freckles.  It  is  also  noteworthy  that  skin 
pigmentation,  even  in  the  children  of  the  colored  races,  is  much  reduced 
at  birth.  (Fig.  194.) 

Hair  Qualities. — Possibly  wider  variations  are  found  in  hair  color 
than  in  either  the  eye  or  the  skin;  for  in  the  white  race,  all  gradations 
from  deep  black  to  a  very  light  yellow  or  flaxen  and  branching  off  to  a 
decided  red  are  of  normal  occurrence.  It  appears  that  red  hair  color 
is  due  to  a  special  derivative  of  the  carotene  pigment,  whereas  all  the 
other  shades  have  their  origin  in  varying  proportions  of  the  melanin 
and  carotene  pigments.  The  absence  of  both  pigments  gives  the 
abnormal  albino  condition  noted  above.  Failure  to  produce  hair 


400  HUMAN  BIOLOGY 

INHERITED  CHARACTERS  IN  MAN 

1.  Blending 

General  body  size,  stature,  weight,  skin-color,  hair-form  (in  cross- 
section,  correlated  with  straightness,  curliness,  etc.),  shape  of  head 
and  proportions  of  its  parts  (features). 


Skin 
and 
hair 


2.  Mendelian 


Dominant 


Recessive 


Dark. 


Spotted  with  white. 

Tylosis  and  ichthyosis  (thick- 
ened or  scaly  skin). 

Epidermolysis  (excessive  for-    Normal  skin, 
mation  of  blisters). 

Hair   beaded    (diameter   not    Normal  hair, 
uniform). 


Blonde  or  albino  (probably 

multiple  allelomorphs). 
Uniformly  colored. 
Normal  skin. 


Eyes 


Front  of  iris  pigmented  (eye: 

black,  brown,  etc.). 
Hereditary  cataract. 
Night  blindness  (when  not  sex 

limited). 
Normal. 


Only  back  of  iris  pigmented 

(eye  blue). 
Normal. 
Normal. 

Pigmentary  degeneration  of 
retina. 


Skeleton 


Brachydactyly    (short    digits     Normal, 
and  limbs). 

Polydactyly  (extra  digits).          Normal. 

Syndactyly  (fused,  webbed,  or    Normal, 
reduced  number  of  digits). 

Symphalangy  (fused  joints  of    Normal, 
digits,  stiff  digits). 

Exostoses  (abnormal  out- 
growths of  long  bones). 

Hereditary  fragility  of  bones.     Normal. 

a.  213. — Showing  the  behavior  of  various  characters  which  are  known  to  be 


Kidneys 


HUMAN  HEREDITY  401 

Dominant  Recessive 

Diabetes  insipidus  (excessive    Normal 

production  of  urine). 

Normal,  Alkaptonuna    (urii^e   black 

on  oxidation). 


^            (Huntington's  chorea.  Normal. 

<  Normal.  Hereditary    feeble-minded- 
system  I 

J          (  ness. 


3.  Mendelian  and  Sex-Linked 

(Appearing  in  males  when  simplex,  but  in  females  only  when  duplex.) 

Normal.  Gower's  muscular  atrophy. 

Normal.  Haemophilia  (bleeding). 

Normal.  Color  blindness  (inability  to 

distinguish      red      from 

green). 
Normal.  Night  blindness  (inability  to 

see  in  faint  light). 


4.  Probably  Mendelian  but  Dominance  Uncertain  or  Imperfect 

Defective  hair  and  teeth  or  teeth  alone,  extra  teeth,  a  double  set 
of  permanent  teeth,  hare-lip,  cryptorchism  and  hypospadias  (imper- 
fectly developed  male  organs),  tendency  to  produce  twins  (in  some 
families  determined  by  the  father,  in  others  by  the  mother),  left- 
handedness,  otosclerosis  (hardness  of  hearing  owing  to  thickened 
tympanum). 


5.  Subject  to  Heredity,  but  to  what  Extent  or  how  Inherited  Uncertain 

General  mental  ability,  memory,  temperament,  musical  ability, 
literary  ability,  artistic  ability,  mathematical  ability,  mechanical 
ability,  congenital  deafness,  liability  to  abdominal  hernia,  cretinism 
(due  to  defective  or  diseased  thyroids),  defective  heart,  some  forms 
of  epilepsy  and  insanity,  longevity. 

heritable  in  man.     (Cattle,  "Genetics  and  Eugenics''  Harvard  University -Press.) 


402  HUMAN  BIOLOGY 

pigment  in  the  later  years  of  life  results  in  grayness.  Not  uncom- 
monly, premature  grayness  appears  as  a  definite  hereditary  character, 
The  factors  for  human  hair  color  have  not  been  determined  as  definitely 
as  they  have  in  the  case  of  the  eyes  and  skin,  but  a  very  great  deal  of 
work  has  been  done  upon  the  inheritance  of  hair  color  in  various  other 
mammals.  In  the  rabbit,  to  take  a  well-known  example,  it  is  found 
that  no  le&  than  four  pairs  of  genes  are  concerned  with  the' develop- 
ment of  hair  color.  Speaking  in  general  terms,  the  results  show  that 
darker  shades  of  hair  color  tend  to  dominate  over  the  lighter  shades  in 
inheritance. 

In  addition  to  wide  variation  in  color,  the  character  of  human  hair 
differs  markedly  with  respect  to  form.  Straight,  curly,  kinky,  coarse, 
fine,  short,  and  long  are  terms  commonly  used  to  describe  various  hair 
types.  Each  of  these  has  its  basis  in  the  distinctive  shape  and  charac- 
ter of  the  hair  follicles,  and  the  latter,  in  turn,  is  the  product  of  the  gene 
complex  (page  191).  Again,  some  types  of  hair  grow  for  a  short  time 
and  are  shed  while  very  short;  other  varieties  are  retained  for  long 
periods  and  grow  to  great  lengths.  A  certain  type  of  baldness  in  the 
human  male,  known  as  hereditary,  or  pattern  baldness,  presents  an 
interesting  type  of  inheritance  in  which  the  genes  producing  this 
condition  are  influenced  by  the  sex  chromosomes,  though  not  directly 
linked  with  them,  and  so  we  have  sex-influenced  characters  as  well  as 
sex-linked  characters  (page  380).  The  underlying  fact  in  the  hered- 
itary transmission  of  a  sex-influenced  character  is  that  identical 
genotypes  produce  different  phenotypes  in  the  two  sexes.  Thus  in 
the  case  of  hereditary  baldness,  the  heterozygous  condition  (£6) 
results  in  baldness  in  the  male  where  it  acts  as  a  dominant  but  not  in 
the  female  where  it  acts  as  a  recessive. 

Eye  Color. — The  inheritance  of  eye  color  in  man  has  been  the 
subject  of  much  interest,  partiqularly  since  the  establishment  of  the 
Mendelian  laws.  An  examination  of  the  iris  shows  that  pigmented 
epithelial  cells  containing  melanin  particles  may  be  present  both  in 
front  and  in  back  of  the  eyes.  This  double  pigmentation,  or  duplex 
condition,  produces  brown  eyes.  In  a  so-called  simplex  eye,  the 
melanin  is  found  only  in  cells  located  at  the  back  of  the  iris.  The 
Deflection  of  the  light  rays  in  the  simplex  eye  from  the  anterior  unpig- 
mejcited  tissues  of  the  iris  gives  blue  eye  color.  In  the  albino  condition, 
no  Digment  is  present  in  any  region  of  the  iris,  and  the  pinkish  eye 
color  results  from  the  reflection  of  the  blood  in  the  iris  vessels.  Con- 
sequently, the  albino  iris  offers  very  little  protection  to  the  sensitive 
retina  cells  from  the  incoming  light  rays,  and  the  affected  individual 
finds  it  necessary  to  keep  the  eyelids  partially  closed. 


HUMAN  HEREDITY  403 

In  general,  it  is  found  that  the  brown-eyed  condition  is  dominant 
oVer  the  reduction  of  pigment  that  gives  blue  eyes.  On  this  basis,  two 
types  of  brown  eyes  occur;  one  being  homozygous,  and  the  other 
hybrid  carrying  a  recessive  gene.  The  various  possibilities  of  inherit- 
ance from  the  mating  of  the  two  types  correspond  to  those  shown  in 
the  monohybrid  square.  The  results  obtained  from  a  study  of  eye 
color  in  a  large  number  of  Danish  families  have  shown  close  conformity 
to  expected  Mendelian  ratios  (page  361).  However,  it  is  recognized 
^Jso  that  various  other  factors  are  often  bound  up  with  inheritance  of 
eye  color  and,  when  present,  greatly  complicate,  the  analysis.  Thus, 
pigmented  cells  carrying  the  yellow  carotene  sometimes  occur  in  the 
iris.  Also,  partial  pigmentation  in  the  front  wall  of  the  iris  results  in 
scattered  specks  or  streaks  or  even  a  complete  ring  of  color.  The 
control  of  this  additional  pigmentation  is  undoubtedly  lodged  in  other 
genes — thus  giving  a  multiple  gene  condition  for  eye  color. 

Eye  Defects. — The  inheritance  of  various  eye  defects,  both  struc- 
tural and  functional  in  nature,  have  been  studied  by  various  investiga- 
tors. In  the  previous  chapter,  one  of  the  most  interesting  of  these, 
color  blindness,  was  used  as  an  example  of  sex-linked  inheritance 
(page  382).  Altogether,  the  inheritance  ratios  have  been  studied  for 
some  20  different  eye  abnormalities,  including  such  structural  defects 
as  displacement  of  the  lens,  opacity  of  the  lens  (cataract),  and  partial 
or  total  absence  of  the  iris.  Among  the  functional  defects  (all  of 
which,  of  course,  have  a  structural  basis  of  some  kind)  are  color  blind- 
ness, shortsightedness,  night  blindness,  degeneration  of  the  optic  nerve 
(Leber's  disease),  degeneration  of  the  retina,  and  paralysis  of  the  eye 
muscles.  The  last  defect  has  appeared  as  a  recessive  character  in  the 
offspring  of  cousin  marriages.  Undoubtedly  the  most  data  have  been 
collected  bearing  on  the  inheritance  of  the  shortsighted  (myopic) 
condition,  night  blindness,  and  color  blindness. 

In  connection  with  myopia,  which  is  one  of  the  least  serious  of  the 
various  eye  defects,  the  study  in  Berlin,  a  few  years  ago,  of  over  900 
family  histories  showed  that  the  character  behaved  in  all  cases  as  a 
Mendelian  recessive  but  that  more  than  one  pair  of  genes  were  involved 
in  producing  the  defect.  Night  blindness,  which  is  due  to  a  defect 
in  the  visual  cells  of  the  retina,  has  been  traced  through  10  generations 
of  the  Nougaret  family  in  France  by  the  study  of  more  than  2,000  case 
histories.  The  results  clearly  show  that  the  defect  was  inherited  in 
this  family  as  a  simple  Mendelian  dominant.  Much  less  complete 
records  of  other  families  in  the  United  States  give  evidence  that  the 
defect  may  be  sex-linked.  It  is  apparent  from  the  examples  cited  that 
no  general  statement  can  be  made  to  cover  the  inheritance  of  human 


404 


HUMAN  BIOLOGY 


eye  defects,  for  they  may  be  recessive  or  dominant  or  sex-linked,  and 
also  considerable  evidence  indicates  that  inheritance  of  the  same  type 
of  defect  is  not  uniform  in  different  families. 


A  B 

FIG.  214. — Portion  of  the  skeleton  of  the  cat's  forelimb.  A,  normal  condition ;  #, 
polydactylous  condition  as  a  result  of  mutation  producing  twinning  of  certain  digits. 
(Coe,  "Evolution  of  Earth  and  MOM"  Yale  University  Press.) 

Skeletal  Characteristics. — The  inheritance  of  defects  in  skeletal 
structures  has  been  mostly  studied  in  the  bones  of  the  hands  and  feet. 
The  departures  from  the  normal  gene  complex  may  result  in  the  occur- 
rence of  extra  digits  (polydactyly)  or  the  opposite  extreme  in  which  a 
complete  absence  of  hands  and  feet  occurs,  as  has  been  recorded  in  the 
members  of  one  Brazilian  family.  In  addition  to  polydactyly,  fairly 
frequent  examples  are  found  in  which  the  number  of  digits  has  been 
reduced  through  a  fusion  of  the  bones  (syndactyly).  In  other  cases,  a 
webbed  condition  in  the  hands  or  feet  is  inherited.  This  latter  defect 
is  due  to  the  persistence  of  a  web  of  skin  tissue  between  the  digits, 


HUMAN  HEREDITY 


405 


4  B 

FIG.  215. — Illustrating  human  polydactyly.  A,  heritable  mutation  producing 
twinning  of  thumb;  J?,  twinned  little  finger  produced  apparently  as  a  defect  in  develop- 
ment and,  therefore,  not  heritable.  (Coe,  "Evolution  of  Earth  and  Man,"  Yale  Univer- 
sity Press.  After  MUles.) 


FIG.  216. — Drawings  showing  external  structure  (left)  and  also  the  skeleton  of  the 
foot  of  the  "mule-footed"  pig.  This  condition  is  produced  by  a  mutation  which  pro- 
duces a  fusion  of  the  terminal  phalanges  and  the  hoof  covering  this  region.  (Coe, 
"Evolution  of  Earth  and  Man"  Yale  University  Press.) 


406  HUMAN  BIOLOGY 

usually  the  second  and  third,  and  is  not  a  skeletal  defect.  Finally, 
an  abnormally  short  type  of  digit  (brachydactyly)  occurs,  in  which  one 
joint  is  missing  from  each  digit,  that  appears  as  a  hereditary  character. 
The  data  from  a  large  number  of  individuals  show  that  skeletal  defects, 
with  certain  exceptions,  behave  as  simple  Mendelian  dominants.  The 
presence  of  a  skeletal  mutation  in  an  individual  is,  therefore,  very  good 
evidence  that  the  defect  will  be  transmitted  to  the  next  generation. 
(Figs.  213  to  216.) 

GALTON  AND  THE  PRINCIPLES  OF  BIOMETRY 

The  studies  of  Gal  ton  on  human  inheritance  are  probably  the  most 
thorough  of  any  of  the  pre-Mendelian  studies,  and  his  "Laws  of 
Ancestral  Heredity/'  published  in  1897,  just  a  few  years  before  the 
rediscovery  of  Mendel's  results,  were  considered  of  the  highest  impor- 
tance by  biologists  in  the  early  years  of  the  present  century.  But 
Gal  toil's  laws  have  not  continued  to  be  of  major  importance  because  he 
failed  to  recognize  the  participate  nature  of  inheritance  based  upon  the 
gene  mechanism  (page  355).  Nevertheless,  Galton  has  a  great  deal  to 
his  credit.  He  was  the  first  to  distinguish  between  alternative  and 
blending  inheritance.  An  example  of  the  former  was  found  in  the 
inheritance  of  hair,  color  in  Basset  hounds  and  of  the  latter  in  human 
stature.  From  his  extended  and  thorough  studies  on  the  inheritance 
of  stature,  or  tallness,  in  man,  Galton  established  the  principles  of 
biometry,  the  measurement  of  living  things. 

Biometrical  data,  correlated  in  accordance  with  mathematical 
formulae  developed  by  Pearson,  have  proved  to  be  of  the  greatest 
importance  in  statistical  studies  of  variation  in  heredity.  Since,  as 
shown  earlier,  the  inheritance  of  various  characters  in  which  multiple 
genes  are  concerned  results  in  a  graded  series  of  variations  in  the  off- 
spring, the  answers  to  the  problems  involved  require  the  determination 
of  the  characteristics  of  a  large  population  rather  than  of  an  individual 
(page  376).  Biontetrical  methods  must  be  used.  The  inheritance  of 
stature  is  a  very;  good  example.  If  a  large  number  of  individuals  are 
measured  for  height,  it  will  be  found  that  a  small  percentage  are  very 
short,  a  corresponding  number  are  very  tall,  but  the  heights  of  most  of 
those  measured  fall  between  these  two  extremes.  Thus,  for  example, 
in  a  certain  population,  the  extremes  for  height  might  be  found  to  be 
58  and  76  in.,  with  the  greatest  number  measuring  around  67  in.  From 
the  data  thus  secured,  a  curve  can  be  constructed  that  will  show  the 
results  graphically.  The  same  methods  were  applied  in  determining 
the  inheritance  of  color  in  the  brown-seeded  oats,  described  in  the 
previous  chapter,  through  the  examination  of  large  numbers  of  seeds 


HUMAN  HEREDITY 


407 


and  establishing  the  percentages  with  respect  to  the  amount  of  color 
present.     In  this  case,  the  two  extremes  were  very  dark-colored  seeds 


Number  of 
Individ- 
ual* 

180 
ICO 
140 
J20 
100 
80 
CO 
40 
20 
0 

TT 

/ 

\ 

; 

t 

\ 

~T] 

i 

• 

\ 

i 

\ 

\ 
\ 

j 

j 

\ 

/ 

\ 

/ 

J. 

\ 
\ 

-r 

\ 

-K.  — 

166     169 

162 

165     168      171      174     177     180     L83     186     189     192     195     198 
Height  in  Centimeters 

FIG.  217. — Illustrating  variation  in  heights  of  1,000  Harvard  students,  ages  18  to  26, 
The  curve  (dotted  line)  is  computed  from  the  number  of  individuals  at  a  particulai 
height.  (Castle,  "Genetics  and  Eugenics,"  Harvard  University  Press.} 

and  seeds  with  no  color.  Each  of  these  was  found  to  comprise  about 
6  per  cent  of  the  total  population.  The  greatest  percentage  of  seeds 
was  found  to  be  intermediate  in  color,  from  which  they  graded  toward 


408 


HUMAN  BIOLOGY 


the  dark  and  toward  the  light-colored.  The  percentages  obtained 
gave  the  clue  to  the  number  of  genes  involved  (page  372).  (Figs.  217, 
218.) 

There  is  every  evidence  that  height,  general  body  build,  shape  of 
head,  and  various  other  body  characters  are  determined  by  multiple 
genes.  But  in  some  instances,  there  is  also  evidence  of  alternative 


tiofa 
160 

140 

iao 

n* 

T" 

•^ 

V 

/ 

^-1 

\ 

) 

1 
% 

i 
• 

"T" 

\ 

*! 

40 
40 
20 
0 

i 
i 

/ 

\ 

•^- 

i 

\ 

1 
rf- 

-\i 
\i 

J_ 

W_ 

4ft   48    51    64    67    60    63    66    69    72    76    78    81    84    87    90    93    96    99  102  10ft 

Weight  in  Kilogram* 

Fio.  218. — Illustrating  variation  in  weights  of  1,000  Harvard  students,  ages  18  to  25. 
Cf.  Fig.  217.     (Castle,  "Genetics  and  Eugenics,"  Harvard  University  Press.) 

inheritance.  In  the  inheritance  of  stature,  for  example,  some  data 
show  that  the  progeny  from  matings  of  tall  and  short  individuals  tend 
to  be  below  the  average  in  height,  which  indicates  that  the  genes  for 
shortness  are  dominant,  to  some  extent  at  least.  But  the  problem  is 
even  more  complicated  because  the  size  of  the  vertebrate  body  is 
markedly  affected  by  hormonal  action.  This  is  well  shown  in  the 
gigantism  resulting  from  hyperactivity  of  the  pituitary  gland  or  in 


HUMAN  HEREDITY  409 

dwarfing  that  results  from  other  hormonal  factors  (page  113).  Hor- 
monal action,  in  turn,  is  directly  influenced  by  environmental  condi- 
tions. The  failure  of  the  environment  to  provide  iodine  in  the  food 
will  produce  a  cretinous  condition,  no  matter  what  the  gene  complex 
of  the  individual  happens  to  be  (page  106). 

Blood  Groups. — Nearly  forty  years  ago,  it  was  discovered  that  a 
very  serious  reaction,  which  resulted  in  the  sticking  together,  or 
agglutination,  of  red  blood  corpuscles,  took  place  when  blood  from 
certain  individuals  was  mixed.  Later  it  was  shown  that  the  agglutina- 
tion of  the  red  cells  was  not  a  haphazard  phenomenon,  that  there  were 
four  types  of  blood  found  in  man,  now  known  as  Groups  A,  B,  AB, 
and  0,  and  that  the  agglutination  reaction  always  occurred  when 
certain  groups  were  mixed.  Extensive  investigation  has  shown  that 
agglutination  depends  upon  the  presence  of  two  blood  substances: 
the  antigen,  carried  in  the  red  blood  cells;  and  the  antibody,  carried 
in  the  serum  (page  166).  Agglutination  requires  the  presence  of  the 
antigen  and  its  specific  antibody.  Both  antigen  and  antibody  cannot, 
of  course,  be  present  in  the  blood  of  the  same  individual  for,  if  they 
were,  agglutination  would  occur.  The  condition  of  the  various  types 
of  blood  may  be  summarized  as  follows: 

Group  A  carries  antigen  A  and  antibodies  for  Groups  B  and  AB. 

Group  B  carries  antigen  B  and  antibodies  for  Groups  A  and  AB. 

Group  AB  carries  antigen  for  the  three  other  groups  but  lacks  anti- 
bodies. It  will  be  agglutinated  by  any  of  the  other  groups.  Group 
AB  individuals  are  undesirable  as  blood  donors. 

Group  0  lacks  antigen  but  carries  antibodies  for  the  three  other 
groups.  It  will  not  be  agglutinated  by  any  of  the  other  three  groups. 
Group  0  individuals  are  important  blood  donors.  (Fig.  219.) 

Genetical  studies  have  brought  the  data  from  the  inheritance  of 
the  different  blood  groups  into  line  with  the  Mendelian  principles  by 
the  establishment  through  experimental  breeding  of  a  method  of 
inheritance  that  involves  an  extension  of  the  Mendelian  laws  beyond 
those  considered  in  the  previous  chapter.  This  condition  may  be 
explained  in  a  few  words  by  the  statement  that  the  paired  genes 
(allelomorphs)  for  a  certain  character  may  be  present  in  different 
varieties  or  forms  in  the  various  individuals  of  a  population.  Instances 
of  this  genie  variation  have  been  demonstrated,  for  example,  in  the 
genes  for  certain  eye  colors  in  Drosophila  and  also  in  hair  color  in  the 
rabbit.  The  data  there  obtained  fit  with  that  of  human  blood  group 
inheritance  which  can  now  be  stated. 

In  the  transmission  to  offspring,  one  mky  say  that  the  genes  for 
Group  A  act  as  a  dominant,  Group  0  as  a  recessive,  and  Groups  B  and 


410 


HUMAN  BIOLOGY 


AB  are  intermediate.  If  the  dominant  gene  associated  with  the  blood 
groups  is  designated  as  A  and  the  recessive  designated  as  a,  then,  just 
as  in  a  typical  monohybrid,  Group  A  individuals  may  occur  either  as 
pure  dominants  with  the  genotype  A  A  or  as  hybrids  with  the  genotype 


Serum  of 
Group  A 


Serum  of 
Group  B 


B 


AB 


FIG.  219. — Illustrating  the  technique  for  determining  the  blood  group  to  which  the 
blood  obtained  from  a  particular  donor  belongs.  "Two  drops  of  serum,  one  of  Group 
A  and  one  of  Group  B,  are  put  on  a  glass  slide  and  a  bit  of  the  unknown  blood  placed 
in  each.  If  the  red  blood  cells  agglutinate  in  one  (A  or  B)  or  both  (AB)  or  neither  (0), 
the  group  of  the  unknown  blood  is  determined  in  accordance  with  the  scheme  illus- 
trated." (Skull,  modified  from  Snyder,  "Blood  Grouping"  The  Williams  &  Wilkins 
Company,} 

A  a.  All  Group  0  individuals  act  as  pure  recessives  with  the  genotype 
aa.  Now  in  the  individuals  belonging  to  Group  B  and  AB,  the  paired 
genes  determining  the  blood  group  are  present  in  a  different  form 
which,  for  convenience,  may  be  indicated  as  a'.  The  homozygous 


HUMAN  HEREDITY 


411 


Group  B  individual  has  the  genotype  a'a',  and  the  hybrid  Group  B  has 
the  genotype  a'a.  All  Group  AB  individuals  are  hybrid  and  have  the 
genotype  Aaf.  If  now  the  groups  and  genotypes  are  arranged  in 
tabular  form,  it  will  be  possible  to  note  the  genotypes  and  the  corre- 
sponding types  of  gametes  produced. 


(XXI) 


Blood  group 

Genotypes 

Gametes 

A 

AA 

A 

Aa 

A 

a 

B 

a'a' 

a' 

aa' 

a 

a' 

AB 

Aa' 

A 

a' 

0 

aa 

a 

In  addition  to  their  basic  importance  in  making  the  highly  valuable 
blood  transfusions  possible,  the  methods  used  in  determining  blood 
groups  are  sometimes  important  in  medicolegal  work  for  establishing 
parentage.  In  approximately  one-third  of  the  cases  involving  the 
question  of  parentage  it  is  possible  to  speak  with  authority. 

Using  the  various  gametes  in  the  square  (XXI)  will  show  the  vari- 
ous possibilities  with  regard  to  inheritance.  Let  us  analyze  two  or 
three  of  the  possibilities.  Thus  it  can  be  shown  that,  if  one  parent 
belongs  to  Group  A  and  the  other  to  Group  #,  it  is  possible  for  a  child 
to  belong  to  any  of  the  four  groups.  Obviously  both  of  the  parents  in 
such  a  case  must  be  hybrids,  for  the  mating  of  a  homozygous  A  A  and  a 
homozygous  a'a'  could  give  only  children  with  the  genotype  A  a'  and 
belonging,  therefore,  to  Group  AB  as  shown  in  the  square: 


Eggs 


(XXII) 


Sperm 


Aa1 


With  heterozygous  A  and  B  parents,  each  producing  two  types  of 
gametes  as  shown  above,  the  children  may  belong  to  any  one  of  the  four 
groups  as  follows: 


HUMAN  BIOLOGY 


Sperm 


(XXIII) 


Aa 


aa 


Aa' 


aa' 


Again  it  is  evident  that,  if  the  mother  belongs  to  Group  A  and  the 
child  belongs  to  Group  J3,  then  the  father  cannot  belong  to  Group  0. 
Thus,  if  the  father  belongs  to  Group  0  and  the  mother  is  homozygous, 
as  AA,  then  the  child  will  have  to  be  Aa,  or  a  member  of  the  A  group 
as  indicated: 


Eggs 


(XXIV) 


Sperm 


A 


Aa 


If  the  mother  is  heterozygous,  or  A  a,  the  children  will  have  to  be  either 
Group  A  (Ad)  or  Group  0  (ad) : 


(XXV) 


Sperm 


Aa 


aa 


Thus  it  is  shown  that  the  association  of  a  Group  A  parent  and  a 
Group  B  child  excludes  the  other  parent  from  Group  0.  Likewise,  it 
can  be  established  that  the  other  parent  cannot  be  a  member  of  Group 
A. 

Numerous  other  patterns  for  blood-group  inheritance  are  estab- 
lished that  we  cannot  take  the  space  to  analyze,  but  a  few  words  should 
be  said  with  reference  to  the  possibility  of  establishing  a  relationship 
between  blood  type  of  child  and  father.  The  blood-group  methods 
cannot,  of  course,  definitely  establish  that  a  particular  individual  is 
a  parent  of  the  child  in  question.  All  that  can  be  said  is  that  certain 
combinations  of  the  blood  groups  of  one  parent  and  child  make  it  either 
possible  or  impossible  for  a  member  of  a  particular  blood  group  to 
have  been  the  other  parent.  To  cite  a  few  more  instances,  if  both  the 
parents  have  Group  0  as  their  blood  group,  it  is  impossible  for  the 
children  to  belong  to  any  other  group.  With  one  Group  0  parent  and* 
one  Group  A  parent,  the  child  cannot  belong  to  either  the  B  or  the  AB 
groups;  If  the  mother  and  child  are  both  type  0,  then  the  father 
cannot  belong  to  group  AB.  Conversely,  type  A  B  in  both  mother  and 


HUMAN  HEREDITY  413 


child  excludes  type  0  as  the  male  parent.  Eveii  uiuoci  A  c»  dictions  of 
relationship  have  been  brought  about  by  the  discovery  of  two  addi- 
tional antigens  and  the  establishment  of  the  M  and  N  blood  groups. 

That  the  genes  for  a  particular  character,  which  necessarily  lie  in 
the  same  chromosome  position,  may  be  'found  in  more  than  one  form, 
as  indicated  by  a  and  a',  is  not  surprising  from  the  chemical  standpoint 
when  it  is  realized  that  a  very  slight  change  in  the  position  of  an  atom 
or  atoms  in  a  molecule  may  result  in  the  production  of  a  different 
substance  with  distinctive  characteristics.  Various  results  have 
shown  that  a  gene  for  a  particular  character  may  exhibit  four  or  even 
more  varieties.  Technically,  this  condition  of  gene  variety  is  known 
as  multiple  allelomorphs  (multiple  alleles).  Careful  distinction  must  be 
made  between  the  multiple  genes  condition,  which  involves  separate 
pairs  of  genes  as  in  seed  color,  and  the  multiple  allelomorphs,  which  are 
concerned  with  different  varieties  of  a  single  pair  of  genes  as  just  shown 
in  the  blood  groups.  Furthermore,  it  must  be  understood  that  an 
individual  can  never  form  gametes  that  carry  more  than  one  variety  of 
a  particular  allelomorph.  Thus  the  gametes  determining  the  blood 
groups  carry  either  A  or  a  or  a'  but  never  any  combination  of  these. 
Accordingly,  not  more  than  two  allelomorphic  varieties  are  ever 
present  in  the  resulting  zygote  and  the  mature  individual.  The  latter, 
for  example,  may  be  A  A,  A  a,  or  A  a',  as  shown  in  the  table  above, 
but  never  Aaa'.  (XXI.) 

Human  Hybridization.  —  The  various  races  of  Man  inhabiting  the 
world  today  belong  to  one  genus,  Homo,  and  one  species,  sapiens. 
The  most  widely  diverse  human  types  interbreed  and  produce  fertile 
offspring.  The  ability  to  interbreed  and  to  produce  fertile  offspring 
has  long  been  recognized  as  one  of  the  most  decisive  characteristics  in 
the  determination  of  a  distinct  species.  Modern  knowledge  of  the 
specificity  of  the  chromatin  complex,  as  shown  in  the  previous  discus- 
sions, has  added  additional  prestige  to  interbreeding  as  a  species 
limitation  (page  334).  In  general,  it  is  found  that  two  individuals 
belonging  to  widely  separated  species  cannot  produce  offspring; 
individuals  from  species  that  are  more  closely  related  may  produce 
offspring,  but  the  latter  are  usually  infertile  as  sfyown  by  the  mule. 
The  production  of  fertile  offspring  represents  an  even  closer  relation- 
ship, and  the  individuals  participating  must  possess  homologous 
chromosomes  bearing  the  same  gene  complex.  Accordingly  they  may 
be  assigned  to  membership  in  a  common  group,  the  species.  Member- 
ship in  a  species  does  not  mean  absolute  uniformity  by  any  means; 
different  varieties,  or  races,  are  included  within  the  species  limits,  and 
even  smaller  subdivisions  with  recognizable  differences  continue  down 


414 


HUMAN  BIOLOGY 


to  individual  differences.  In  the  final  analysis,  as  has  often  been  said, 
no  two  individuals  in  any  species  are  exactly  alike.  It  may  be  of 
interest  in  this  connection  to  call  attention  to  the  results  obtained  when 
two  closely  related  species  of  Ungulates,  the  horse  and  the  ass,  are 
mated.  The  two  species  are  fertile  when  mated  and  produce  a  charac- 
teristic hybrid  offspring,  the  mule,  when  the  male  ass  (jack)  is  mated 
with  the  female  horse  (mare).1  But  the  hybrid  mule  is  practically 
always  sterile,  and  the  reason  lies  in  the  fact  that  the  chromatin 
complex  is  different  in  the  two  species.  Gametogeriesis  in  the  mule 
cannot  occur  because  homologous  mates  for  the  chromosomes  received 
from  the  diverse  parents  are  lacking;  this  prevents  synapsis  and  the 
formation  of  functional  gametes — the  mule  is  sterile.  However,  even 


FIG.  220. — Illustrating  the  basis  of  sterility  in  species  hybrids,  as  described  on  page 
414.  Chromosomes  of  the  mule  in  the  developing  germ  cells.  A,  early  stage  showing 
the  maternal  chromosomes  of  the  horse  (large)  and  the  paternal  chromosomes  of  the 
jack  (small) ;  J5,  later  stage  of  germ  cell  formation  showing  degeneration  of  chromatin 
due  to  the  inability  of  the  chromosomes  to  pair  in  synapsis  with  homologous  mates. 
(Jennings,  "Genetics,"  W.  W.  Norton  &  Company,  Inc.  After  Wodsedalek.) 

though  sterile,  the  mule  has  been  found  to  be  a  very  desirable  hybrid 
type  because  of  its  strength  and  high  resistance  to  unfavorable 
conditions,  (Fig.  220.) 

The  human  species,  Homo  sapiens,  as  seen  in  the  world  today, 
includes  three  great  primary  subdivisions  or  races :  the  whites  (Cauca- 
sian), the  yellow-browns  (Mongolian),  and  the  blacks  (Negroid). 
Chief  subdivisions  of  the  Caucasians  include  the  Nordic,  Alpine,  and 
Mediterranean  peoples;  subdivisions  of  the  Mongolians  include  the 
Mongolic  (Chinese  and  the  Japanese),  the  Malay,  (Hawaiians  and 
other  South  Sea  Islanders,  American  Indians,  and  Eskimos) ;  subdivi- 
sions of  the  Negroid  race  include  the  Negroes  proper  and  a  diverse 
group,  the  Pygmies.  These  various  peoples  have  long  since  ceased  to 
exist  as  completely  segregated  groups.  Almost  every  conceivable 
racial  mixture  has  occurred  at  one  time  or  another  during  the  thousands 


1  Essentially  the  same  condition  obtains  with  the  reciprocal  cross  between  the 
male  horse  (stallion)  and  the  female  (jenny). 


HUMAN  HEREDITY 


415 


1,  Nordic 


2. 


3 .  Medi 


4. 


5.  Negroid  6.  Half-breed  (page  417.) 

PLATE  XVII. — Representatives  of  various  human  races.      (Baur,  Fischer,  and  Lenz, 
"Human  Heredity"  George  Allen  &  Unwin,  London.) 


416 


HUMAN  BIOLOGY 


of  years  that  Homo  sapiens  has  roamed  the  earth,  and,  so  far  as  the 
scientist  is  aware,  there  are  no  barriers  to  the  production  of  fertile 
offspring  from  the  union  of  individuals  from  even  the  most  diverse 
races.  Well-authenticated  examples  of  human  hybridization,  involv- 
ing considerable  numbers  of  individuals  and  extending  over  many 
years,  have  been  studied  by  the  experts  in  this  field.  A  few  of  the 
more  important  of  these  may  be  noted.  (Plate  XVII;  Fig.  221.) 


Ewing  Galloway 
FIG.  221. — Photograph  of  the  African  pygmies. 

The  Pitcairn'  Islanders  represent  a  mixture  of  British  and  Polyne- 
sian stock,  which  was  instigated  in  1790  following  the  mutiny  on  a 
British  ship,  the  Bounty.  Some  of  the  mutineers,  in  order  to  escape 
punishment,  made  their  way  to  the  then  unknown  Pitcairn  Island  and 
took  with  them  twelve  native  Polynesian  women  and  six  men.  The 
descendants  of  the  motley  group,  now  numbering  a  thousand  or  so, 
occupy  Pitcairn  Island  and  the  neighboring  Norfolk  Island  as  well. 
The  racial  mixture  appears  to  have  established  a  healthy,  vigorous 


HUMAN  HEREDITY 


417 


stock.  Many  other  examples  of  racial  mixtures  involving  the  Polyne- 
sian peoples  are  known,  particularly  in  Hawaii.  Thus,  offspring  from 
Hawaiian-Chinese  unions  give  notable  evidence  of  the  establishment 
of  a  very  favorable  hybrid  type. 

Hybridization  between  Dutch  colonists  and  Hottentots  in  South- 
west Africa  has  resulted  in  the  establishment  of  a  distinct  group  which 
has  held  together  and  developed  certain  distinctive  and,  on  the  whole, 
favorable  features  (page  415,  6).  In  many  of  the  alternative  parental 
characters  exhibited  by  the  very  diverse  parental  types,  it  is  clear  that 


FIG.  222. — Drawings  illustrating  hybrids  in  the  Fi  and  Fz  generations  produced  by 
mating  the  French  bulldog  with  the  Dachshund.  (Stockard,  "Physical  Basis  of  Per- 
sonality," W.  W.  Norton  &  Company,  Inc.) 

the  hybrids  are  intermediate.  Hybridization  of  the  native  Filipino,  on 
the  contrary,  produces  offspring  that  are  often  notably  inferior  to  the 
parental  types  in  various  respects.  This  hybrid  degeneration  is  not 
so  apparent  in  the  physical  characteristics  as  it  is  in  the  mental  trails. 
Considerable  data  are  available  relative  to  the  hereditary  pattern  in 
the  offspring  from  crosses  between  the  White  and  Negro  races  in  North 
America.  The  hybrid  individual,  or  mulatto,  shows  a  blend  of  certain 
characters  as  in  the  skin  pigmentation  and  various  facial  features, 
whereas  in  their  general  body  build,  the  hybrids  tend  more  toward  the 
Negro  ancestry.  There  appears  to  be  no  scientific  evidence  that 
sterility  appears  in  the  offspring  from  crosses  involving  less  of  the  negro 
ancestry  as  sometimes  stated. 


418 


HUMAN  BIOLOGY 


Controlled  breeding  experiments  with  dogs  have  yielded  interesting 
and  important  results  in  recent  years  which  should  be  considered  in 

connection  with  the  problems  of 
human  hybridization.  The  many 
varieties  of  dogs  distributed  in 
every  land  are  assigned  to  one 
species,  Canis  familiaris.  All  va- 
rieties interbreed  freely  and  prod- 
uce hybrid  offspring  in  which 
many  of  the  parental  characters 
are  clearly  inherited  in  accordance 
with  Mendelian  laws.  But  the 
hybrid  offspring  from  certain 
crosses  do  not  appear  to  have  assembled  a  very  satisfactory  set 
of  characters  in  their  composite  inheritance.  The  original  parental 
types  are  much  better.  There  is  a  certain  structural  disharmony  in  the 


FIG.  223. — Illustrating  hybrid  off- 
spring from  the  mating  of  the  St.  Bernard 
and  Dachshund.  (Jennings,  "Genetics," 
W.  W.  Norton  &  Company,  Inc.  After 
Lang.) 


FIG.  224. — Drawings  illustrating  F\  hybrids  produced  by  the  mating  of  the  giant 
St.  Bernard  with  the  Great  Dane.  The  hybrids  are  vigorous  for  the  first  three  months, 
but  later  develop  varying  degrees  of  overgrowth,  and  all  become  paralyzed  in  the  hind 
legs.  (Stockard,  "Physical  Basis  of  Personality,"  W.  W.  Norton  &  Company,  Inc.) 

offspring  of  diverse  races  that  gives,  to  say  the  least,  an  unprepossessing 
appearance.  Thus,  when  the  short-legged  Dachshund  is  crossed  with 
the  longer  legged  French  bulldog,  the  Ft  offspring  are  moro  or  less 


HUMAN  HEREDITY  419 

intermediate.  The  latter,  when  interbred,  produce  several  bizarre 
types  with  ears,  legs,  and  bodies  that  are  far  from  harmonious.  The 
segregation  of  these  characters  in  the  F2  animals  is  in  accord  with 
Mendelian  laws.  Even  more  unsuitable  are  the  offspring  produced 
from  matings  of  the  very  large  St.  Bernard  and  the  small  short-legged 
Dachshuud.  The  hybrids  inherit  the  dominant  stump  legs  of  the 
Dachshund  in  association  with  the  long,  heavy  St.  Bernard  body. 
The  latter  hangs  so  low  that  it  may  even  drag  on  the  ground.  An  even 
more  serious  hybrid  defect  appears  when  the  St.  Bernard  and  the 
Great  Dane  are  mated.  A  defective  gene  becomes  apparent  in  this 
mating,  which  results  in  the  partial  paralysis  of  the  offspring  when 
about  three  months  old.  (Figs.  222  to  224.) 

Ancl  so  the  results  of  the  breeding  experiments  with  a  highly 
developed  mammal,  like  the  dog,  indicates  with  considerable  clarity 
that  the  mixing  of  established  diverse  races  within  the  species  is  not 
always  helpful  and  may,  in  fact,  be  decidedly  harmful.  Possibly  the 
same  condition  applies  in  the  case  of  the  human  organism.  Various 
authorities  are  convinced  that  the  matings  of  diverse  types  give  oppor- 
tunity for  the  production  of  hybrid  progeny  that  tend  to  be  badly 
assembled,  as  it  were.  The  different  parts  of  the  hybrid  body  may  not 
harmonize;  there  is  lack  of  a  unifying  life  architecture.  This  condition 
may  not  appear  so  serious  in  the  first  generation  when  many  of  the 
diverging  parental  characters  show  a  blending  inheritance  with  the 
production  of  an  intermediate  type.  But  the  succeeding  generations, 
produced  by  matings  between  hybrids,  or  between  hybrids  and  either 
of  the  diverse  parental  types,  are  apt  to  result  in  the  production  of 
poorly  adapted  offspring  as  the  result  of  the  segregation  of  the  diverse 
genes  and  the  random  union  of  the  gametes. 

INBREEDING 

Quite  the  opposite  of  the  condition  just  considered,  with  hybridiza- 
tion of  diverse  races,  is  of  common  occurrence  in  various  plant  and 
animal  types,  including  man.  This  is  inbreeding  by  the  union  of 
related  individuals  such  as  occurs  in  cousin  marriages.  It  might  be 
thought  that,  if  mating  between  individuals  belonging  to  diverse  races 
has  its  dangers,  inbreeding  would  be  helpful.  It  is  clear,  however,  that 
this  is  not  always  the  case.  Inbreeding  may  be  helpful  or  harmful 
depending  upon  the  genotype  of  the  individuals  concerned.  If 
recessive  genes  for  harmful  characters  lurk  in  the  genotypes  of  a 
particular  family,  then  the  union  with  a  related  individual  carrying 
this  defective  genotype  is  liable  to  end  disastrously,  since  it  will  give 
the  opportunity  for  the  paired  recessive  genes  coming  from  both 


420  HUMAN  BIOLOGY 

Controlled  breeding  experiments  with  dogs  have  yielded  interesting 
ad  important  results  in  recent  years  which  should  be  considered  in 
duced  well-endowed  offspring  lor  generations,  with  no  hint  ol  any 
undesirable  characters,  offers  very  little  danger  from  inbreeding.  This 
assumption,  however,  is  not  based  on  solid  ground,  for  it  has  been 
shown  in  Drosophila  that  a  recessive  character  may  remain  concealed 
in  the  genotype  for  many  generations  only  to  appear  in  full  force  in  the 
phenotype  of  a  certain  stock  when  the  homozygous  condition  is 
attained  by  the  union  of  two  gametes  both  carrying  the  recessive  gene. 
The  gist  of  the  matter  may  be  given  in  the  statement  that  inbreeding 
d^es  not  produce  harmful  characters;  it  only  gives  an  opportunity  for 
them  to  be  shown  as  somatic  characters  in  the  offspring  if  they  are 
present  in  the  parental  germ  plasm. 

Inbreeding  involving  varying  degrees  of  relationship  is  exhibited 
in  the  living  world.  The  closest  inbreeding  occurs  in  self-fertilizing 
types  of  plant  and  animal  that  produce  both  the  male  and  the  female 
gametes  in  the  same  individual.  This  is  commonly  found  in  the  plant 
kingdom  and  is  not  unusual  among  animals,  as  in  the  hermaphroditic 
earthworm  and  the  parasitic  flat  worms.  Self-fertilization  may  or  may 
not  be  practiced  in  the  hermaphroditic  types.  Earthworms  go  to 
great  lengths  to  mate  with  another  individual  and  thus  to  secure 
foreign  sperm  for  the  fertilization  of  their  own  eggs.  The  parasitic 
flatworms,  on  the  other  hand,  depend  upon  self-fertilization. 

A  similar  condition  is  found  in  the  plant  kingdom.  Darwin  made 
an  extensive  study  of  the  effects  of  self-fertilization  and  cross-fertiliza- 
tion in  a  wide  variety  of  plants  and  found  great  divergence  in  the 
different  types.  Some  plants  will  not  tolerate  self-fertilization;  that  is, 
they  are  self-sterile;  others  utilize  self-fertilization  exclusively  and  do 
not  appear  to  receive  any  benefit  when  they  are  artificially  cross- 
fertilized.  Wide  variation  in  fertilization  requirements  exists  among 
relatively  close  plant  groups,  as  in  the  domesticated  grains,  or  cereals. 
For  example,  in  wheat  and  oats  self-fertilization  occurs.  In  fact,  it  is 
quite  difficult  to  carry  on  artificial  cross-fertilization  in  these  species 
because  the  flowers  are  structurally  adapted  for  self-fertilization.  In 
corn,  quite  the  contrary  condition  is  found;  for  iu  this  plant,  cross- 
fertilization  is  essential  for  the  production  of  normal  progeny.  The 
self-fertilization  of  corn  may  be  forced  artificially,  but  the  plants  so 
produced  are  inferior.  Mendel,  in  his  original  experiments,  used  peas 
that  normally  were  self-fertilized  by  pollen  coming  from  the  same 
flower.  Self-fertilization  occurring  in  the  FI  hybrids  was  a  very  impor- 
tant factor  in  enabling  Mendel  to  interpret  the  results  of  his  experi- 
ments correctly. 


HUMAN  HEREDITY  421 

In  organisms  in  which  male  and  female  individuals  occur,  as  in  all 
the  higher  animals,  it  is  obvious  that  self-fertilization  is  impossible. 
The  closest  possible  inbreeding  occurs  in  matings  between  brother  and 
sister  or  between  parent  and  offspring.  A  considerable  amount  of 
experimental  breeding  has  been  directed  in  an  endeavor  to  discover  the 
results  from  long  continued,  close  inbreeding.  Drosophila  has  been 
inbred  between  brother  and  sister  for  59  generations  without  producing 
degeneration,  provided  care  is  taken  to  select  vigorous  individuals  from 
each  generation  for  propagation.  Likewise,  inbreeding  involving 
brother  and  sister  matings  have  been  studied  in  mammals,  particularly 
in  rats  and  guinea  pigs,  for  some  25  generations  and  has  given  essen- 
tially the  same  result  as  in  Drosophila.  The  crucial  point  of  these 
inbreeding  experiments  lies  in  the  selection  of  vigorous  normal  indi- 
viduals in  each  generation  to  carry  along  the  line.  If  parental  selec- 
tions are  not  made,  defective  types  will  increase  in  the  population. 
This  result  is  not  due  to  the  inherent  harmf ulness  of  the  inbreeding  for, 
as  we  have  seen,  self-fertilization  involving  the  closest  possible  inbreed- 
ing is  the  accepted  method  in  various  types  of  plants  and  animals. 
Defective  types  which  appear  in  the  progeny  of  inbred  animals  result 
from  the  outcropping  of  recessive  genes  present  in  the  closely  related 
chromatin  of  the  two  parents.  In  the  controlled  breeding  experiments, 
the  defective  individuals,  homozygous  for  the  undesirable  character, 
are  discarded  and  not  allowed  to  propagate.  Thus,  under  the  experi- 
mental conditions,  inbreeding  aided  by  the  selection  of  the  most 
desirable  individuals  for  reproduction  tends  to  rid  the  germ  plasm  of 
undesirable  recessive  genes  and  to  produce  a  homozygous  condition 
carrying  only  desirable  characters. 

Suppose  now  that  a  vigorous  individual  from  an  inbred  race, 
selected  over  a  considerable  number  of  generations,  and  therefore 
highly  homozygous,  is  crossed  with  an  individual  from  another  homo- 
zygous race,  which  is  not  closely  related.  Considerable  evidence 
exists  that  offspring  from  matings  of  such  individuals  will  be  more 
vigorous  and,  in  general,  more  desirable  than  those  obtained  from 
continued  inbreeding  in  either  of  the  parental  lines.  This  is  the  phe- 
nomenon of  heterosis  or,  more  commonly,  hybrid  vigor  which  is  most 
strikingly  shown  in  the  experimental  breeding  of  the  corn  plant. 
Strains  of  this  plant,  which  have  been  developed  by  artificial  self- 
fertilization  through  several  generations  until  homozygosity  is  well 
established,  will  continue  to  produce  normal  progeny  indefinitely,  but 
the  ears  formed  will  be  small,  and  the  individual  plants  lacking  in  size 
and  vigor.  When  two  of  these  homozygous  lines  are  crossed,  the 
hybrid  FI  plants  will  be  larger  and  more  vigorous.  And  in  the  next 


422  HUMAN  BIOLOGY 

generation,  grown  from  the  Fi  seeds,  a  strikingly  superior  type  of  corn 
will  always  be  produced.  Of  course,  it  is  apparent  that  the  unre- 
lated homozygous  parents  must  be  desirable  types  and  also  that  they 
should  not  belong  to  widely  diverse  races  within  the  species.  Thus,  as 
noted  above,  the  progeny  of  the  St.  Bernard-Great  Dane  cross  are  not 
going  to  be  satisfactory  even  though  each  of  the  parents  is  homozygous 
(page  418). 

In  the  human  race,  it  is  evident  that  the  peoples  of  the  various 
nations  carry  a  high  degree  of  hybridization  in  their  gene  complex  as 
the  result  of  racial  mixtures  following  migrations  to  other  lands  at 
various  times  in  the  past.  This  is  particularly  true  in  a  melting  pot 
of  the  races  such  as  is  found  in  the  United  States.  But  even  a  well- 
established  people,  like  the  English,  contains  additions  to  the  genotype 
from  the  Mediterranean  peoples  brought  in  by  the  Roman  invaders. 
At  various  times,  the  Germanic,  Norman,  French,  and  other  nationali- 
ties have  added  to  the  racial  mixture  that  is  more  or  less  stabilized  in 
the  British  type  of  today.  Everywhere  among  the  Caucasian  peoples, 
the  story  is  much  the  same,  whether  they  live  in  Germany  or  France 
or  Italy  or  Spain.  Evidence  that  these  established  nationalities  have 
developed  from  marked  racial  mixtures  in  the  past  may  be  used  as  an 
argument  for  the  belief  that  the  final  result  of  the  racial  mixture  in  this 
country  will  not  necessarily  be  unsatisfactory.  It  is,  in  fact,  impossible 
to  draw  any  definite  conclusions  as  to  the  future,  for  no  one  knows 
what  the  blending  of  the  diverse  genotypes  will  bring  forth  in  the 
generations  that  lie  ahead. 

In  addition  to  lack  of  knowledge  of  the  individual  human  genotypes, 
the  question  of  mutations  is  to  be  considered,  because  the  appearance 
of  some  unfavorable  character  in  the  progeny  may  be  due  to  a  sudden 
chromosomal  aberration,  in  one  or  the  other  of  the  parental  gametes, 
that  has  never  been  in  the  germ  plasm  of  either  parent  previously. 
The  classic  example  of  a  mammalian  mutation  occurring  naturally 
was  noted  in  1791  when  a  mutant  type  of  sheep  was  born  to  normal 
parents.  The  animals  belonged  to  a  farmer,  Seth  Wright,  of  the 
Massachusetts  colony.  This  mutation  produced  a  short-legged,  or 
ancon,  type  of  sheep  which  was  highly  regarded  for  a  time  because  it 
lacked  the  fence-jumping  ability  of  its  long-legged  relatives.  But  the 
ancon  sheep  were  lacking  in  other  ways  and,  all  things  considered,  did 
not  measure  up  to  the  standards  of  the  normal  animals.  Accordingly, 
the  mutant  type  was  propagated  for  some  years  until  the  advantages 
and  disadvantages  became  apparent  and  better  methods  of  fence 
building  were  devised  and  then  was  discarded  in  favor  of  the  generally 
more  desirable  long-legged  varieties.  This  particular  mutation  in  the 


HUMAN  HEREDITY  423 

germ  plasm,  producing  short  logs,  appeared  without  warning  and  was 
easily  propagated  because  it  was  dominant  over  the  normal  gene 
complex  responsible  for  length  of  leg.  Most  mutations,  which  have 
been  studied,  are  found  to  be  recessive  in  nature,  and,  therefore,  they 
are  unable  to  alter  a  particular  character  unless  it  is  present  in  the 
zygote  in  a  homozygous  condition.  (Fig.  225.) 

The  upshot  of  the  matter  is  apparent;  it  is  impossible  to  determine 
when  the  recessive  gene  for  an.  unfavorable  character  became  estab- 
lished in  a  particular  human  gene  complex.  It  may  have  been  during 
gamete  formation  in  the  previous  generation,  or  the  recessive  gene  may 


FIG.  225. — Photograph  showing  the  sho^-legged  ancon  sheep  (left)  in  compari- 
son with  the  normal  condition  (right).  (From  photograph  by  Dr.  W.  Landauer,  Univer- 
sity of  Connecticut.} 

have  been  there  for  untold  generations  before  getting  the  opportunity 
to  be  present  in  a  zygote  in  a  homozygous  condition.  But  it  is  always 
true  that  the  chances  of  an  undesirable  recessive  gene  finding  a  homo- 
zygous mate  are  much  greater  when  the  parental  chromatin  is  related 
than  when  the  gametes  have  a  diverse  ancestry.  Hence  it  seems  wise 
to  accept  the  established  belief,  which  in  various  regions  has  crystal- 
lized into  law,  that  marriage  between  cousins  is  not  desirable.  That 
is  not  to  say  that  the  progeny  of  cousin  marriages  are  always  below 
grade.  Quite  the  contrary  is  the  case,  as  can  be  seen  in  examples  from 
various  distinguished  families.  It  may  be  that  statistics  would  show 
no  higher  percentage  of  defectives  in  the  children  from  cousin  mar- 
riages than  from  the  union  of  unrelated  persons.  Nevertheless,  our 
present-day  knowledge  of  the  heredity  mechanism  makes  clear  the 
nherent  dangers. 


424  HUMAN  BIOLOGY 

Fortunately  the  history  of  a  family  over  a  number  of  generations 
usually  reveals  with  considerable  accuracy  the  desirability  or  undesir- 
ability  of  the  gene  complex  even  though  all  the  family  pedigrees  have 
not  been  studied  with  scientific  exactness.  The  extreme  examples  of 
an  undesirable  gene  complex  with  respect  to  mental  characteristics, 
which  after  all  is  the  final  consideration,  are  afforded  by  the  studies  that 
have  been  made  of  members  of  the  " Jukes"  and  "Kallikak"  families 
through  several  generations.  Possibly  these  examples  have  been  held 
up  to  view  so  frequently  in  the  past  twenty-five  years  that  they  have 
lost  their  effectiveness.  And  perhaps  a  great  deal  of  the  trouble  in 
these  families  was  the  result  of  very  unfavorable  environmental  condi- 
tions producing  individual  modifications.  But  it  would  seem  that  an 
unprejudiced  observer  on  the  side  lines  would  have  to  conclude  that 
something  was  wrong  with  the  family  genotypes  when  more  than  40 
per  cent  of  the  individuals  in  successive  generations  are  mentally 
defective. 

That  the  genotype  is  responsible  for  mental  inadequacy  and  other 
departures  from  the  normal  human  pattern  is  possibly  even  more 
strikingly  shown  by  the  careful  studies  that  have  been  made  on  the 
behavior  of ,  twins  with  respect  to  criminal  tendencies.  It  should  be 
emphasized  that  two  types  of  human  twins  are  recognized,  namely, 
fraternal,  or  dizygotic,  twins;  and  identical,  or  monozygotic,  twins. 
Dizygotic  twins,  as  the  name  indicates,  develop  from  two  zygotes;  that 
is,  two  eggs  were  fertilized  at  the  same  time.  Accordingly,  except  for 
the  fact  that  they  are  of  the  same  age,  dizygotic  twins  are  no  more 
alike  than  other  members  of  the  family.  Monozygotic  twins  develop 
from  the  same  zygote  and,  therefore,  have  identical  genotypes.  Such 
a  condition  is  believed  to  arise  by  the  separation  of  the  two  daughter 
cells,  following  the  first  cleavage  of  the  fertilized  egg,  and  the  independ- 
ent development  of  the  two  cells  thereafter  so  that  each  cell  forms  a 
twin.  Identical  twins  are  always  of  the  same  sex  and  so  nearly  alike  in 
appearance  that  it  is  usually  impossible  for  strangers  to  tell  them 
apart. 

An  examination  of  the  prisons  in  Bavaria  some  years  ago  revealed 
that  one  or  both  members  of  30  pairs  of  twins  were  imprisoned  or  had 
prison  records.  Very  complete  information  was  secured  with  respect 
to  these  individuals.  Of  the  30  pairs  of  twins  with  criminal  records, 
it  was  found  that  17  pairs  were  fraternal  and  13  pairs  were  identical. 
Examination  of  the  prison  records  of  the  17  pairs  of  fraternal. twins 
showed  that  criminal  tendencies  in  one  member  of  the  pair  gave  no 
evidence  that  the  other  twin  would  likewise  be  a  burden  to  society,  for 
it  was  found  that  in  only  two  cases  had  both  members  of  the  pair  been 


HUMAN  HEREDITY  425 

imprisoned.  Quite  the  reverse  condition  was  found  «*  the  criminality 
of  the  13  pairs  of  identical  twins.  The  investigation  showed  that,  in 
10  cases,  both  members  of  the  pair  had  prison  records,  and  in  only 
3  cases  was  imprisonment  confined  to  one  twin.  The  numbers  of 
cases  used  as  a  basis  of  these  investigations  were  necessarily  small,  but 
the  trend  is  so  decisive  that  it  appears  safe  to  conclude  that  the 
behavior  of  an  individual  as  well  as  his  structural  pattern  is  largely  the 
outgrowth  of  the  gene  complex  received  at  the  time  of  fertilization. 

EUGENICS:  NEGATIVE  AND  POSITIVE 

Society  has  recognized  more  and  more  that  some  individuals  are 
inherently  burdened  by  an  undesirable  gene  complex,  though  that  term 
may  not  have  been  used  to  express  the  situation,  and  accordingly  has 
taken  measures  to  protect  future  generations  against  further  trans- 
mission of  the  undesirable  germ  plasm.  The  traditional  method  of 
accomplishing  this  desirable  aim  has  been  through  the  segregation  of 
the  afflicted  individuals  in  government  institutions  of  one  kind  and 
another.  The  trouble  with  segregation  has  been,  so  far,  that  there  are 
far  too  many  afflicted — with,  for  example,  an  estimated  2  to  5  per  cent 
of  the  population  feeble-minded — to  make  segregation  effective. 
Furthermore,  great  pressure  is  continually  brought  to  bear  to  bring 
about  the  release  or  parole  of  individuals  who  are  lightly  afflicted  but, 
nevertheless,  potentially  dangerous  individuals  from  the  standpoint  of 
heredity.  Laws  with  regard  to  the  requirements  for  marriage  differ 
widely,  with  the  result  that  a  license  denied  in  one  locality  can  usually 
be  secured  in  another.  Accordingly,  many  cases  are  found  in  which 
afflicted  individuals,  who  should  be  permanently  segregated,  find  it 
possible  to  marry  and  produce  offspring.  And  the  alarming  fact  for 
the  future  is  that  the  rate  of  reproduction  of  mentally  deficient  couples 
is  probably  twice  that  of  couples  with  high  mentality. 

Increasingly,  the  tendency  in  the  United  States  during  the  last 
twenty-five  years  has  been  to  pass  laws  requiring  or  permitting  the 
sterilization  of  certain  classes  of  defectives  of  both  sexes  in  such  a  way 
that  the  production  of  offspring  is  impossible,  though  the  normal 
sexual  relations  of  the  married  state  are  in  no  way  disturbed.  In  both 
sexes,  the  sterilization  operation  consists  of  cutting  the  ducts  from  the 
gonads  so  that  the  germ  cells  cannot  pass  through  them.  In  the  male, 
the  operation  is  a  very  simple  one,  since  the  testes  lie  outside  the 
abdominal  cavity  and  the  connecting  ducts  are  easily  exposed.  In  the 
female,  an  abdominal  operation  is  involved  which  may  fairly  be 
compared  in  severity  with  an  operation  for  appendicitis.  In  1909, 
only  four  states  had  sterilization  laws;  in  1934,  the  number  had 


426  HUMAN  BIOLOGY 

increased  to  27.  These  laws  have  been  opposed  chiefly  on  three 
grounds.  It  has  been  argued  that  sterilization  would  tend  to  increase 
sexual  immorality,  that  it  conflicted  with  the  Constitution  in  that  it 
constituted  " cruel  and  unusual  punishment/'  and  that  it  represented 
a  dangerous  infringement  of  personal  liberty,  The  legality  of  the 
sterilization  laws  of  one  state,  Virginia,  was  carried  to  the  Supreme 
Court  in  1927  and  there  upheld,  a  decision  that  carried  the  famous 
remark  of  Justice  Holmes  that  "  three  generations  of  imbeciles  are 
enough." 

The  advisability  of  sterilization  is  a  question  that  cannot  be  settled 
in  a  few  years,  but  a  careful  study  of  the  results  in  California,  based 
on  nearly  10,000  legal  sterilizations,  indicates  a  much  more  favorable 
result  than  might  have  been  expected.  The  application  of  sterilization 
laws  throughout  the  past  ages  would  undoubtedly  have  prevented  the 
appearance  of  many  unfortunates,  but,  at  the  same  time,  it  is  also 
possible  that  some  of  the  geniuses  of  the  first  rank,  who  have  greatly 
enriched  civilization,  would  never  have  been  known.  The  incompre- 
hensibly complex  chromatin  of  the  human  race  can  never  be  analyzed 
to  the  extent  that  all  of  the  possibilities  inherent  in  the  offspring  of 
two  individuals  can  be  determined  previous  to  their  appearance.  The 
offspring  of  a  particular  marriage  will  always  be  a  gamble.  But  on  the 
other  hand,  it  should  be  remembered  that  the  principles  of  selective 
breeding,  which  man  has  rigorously  applied  in  order  to  obtain  desirable 
domesticated  plant  and  animal  types,  have  been  tremendously  effective 
in  establishing  new  varieties  far  superior  to  the  original  stocks.  The 
biologist,  knowing  that  the  same  hereditary  mechanism  is  at  work  in 
the  human  organism,  is  certain  that  the  same  methods,  if  it  were 
possible  to  apply  them,  would  be  effective  with  the  heritable  qualities 
in  man. 

The  discussion  so  far  has  dealt  with  the  prevention  of  the  trans- 
mission of  defective  chromatin  to  another  generation.  To  some 
authorities  this  is  " negative  eugenics,"  which  may  be  helpful  to  some 
extent  but  should  be  augmented  by  a  policy  of  " positive  eugenics"  in 
which  selected  human  stocks  would  be  encouraged  to  transmit  the 
desirable  genotype  to  an  increasing  number  of  progeny.  In  other 
words,  every  possible  measure  should  be  taken  to  increase  the  birth  rate 
among  the  better  endowed  families  rather  than  to  let  it  continue  to  sink 
to  lower  levels,  as  appears  to  be  the  case  at  present.  Admittedly,  if  this 
were  possible,  much  could  be  accomplished  in  improving  the  human 
race,  but  just  how  it  could  be  effected  seems  to  test  the  limits  of  human 
intelligence.  Some  countries,  notably  France,  have  in  recent  ^years 
been  experimenting  with  a  general  family  allowance  plan  for  additional 


HUMAN  HEREDITY  427 

children,  but  it  will  be  a  long  time  "before  any  conclusion  can  be  safely 
drawn  as  to  the  desirability  of  such*  a  plan.  It  is  one  thing  to  adopt  a 
plan  that  will  give  an  allowance  to  all  families  with  a  large  number  of 
children,  but  quite  another  to  select  families  that  are  thought  to  have  a 
more  desirable  gene  complex  and  to  reward  them  for  increased  numbers 
of  progeny,  while  at  the  same  time  restricting  other  families  and  requir- 
ing them  to  share  in  the  increased  expense  of  maintaining  a  subsidy  for 
the  selected  families. 


CHAPTER  XVI 
THE  WEB  OF  LIFE 

In  the  previous  chapters,  attention  has  been  primarily  centered  on 
the  structural  and  functional  features  of  the  human  organism.  It  is 
now  time  to  broaden  this  viewpoint  somewhat  and  give  attention  to 
man's  relationships  to  other  members  of  the  living  world,  infinite  in 
number,  which  surround  him  on  every  side  and  with  which  he  is  indis- 
solubly  linked  in  a  complex  living  fabric,  aptly  termed  the  web  of  life. 
Man  is  a  part  of,  not  apart  from,  the  living  world.  His  basic  require- 
ments for  food  and  clothing  are  supplied  by  materials  produced  by 
other  living  organisms.  And  all  plants  and  animals  are  necessarily 
dependent  upon  the  constructive  photosynthetic  activities  of  green 
plants  for  the  formation  of  the  essential  foodstuffs  carrying  abundant 
supplies  of  potential  energy  which  may  be  utilized  in  maintaining  the 
varied  life  activities  and  for  the  construction  and  maintenance  of 
protoplasm  itself. 

In  the  final  analysis,  whether  or  not  an  organism  is  successful,  as 
evidenced  by  its  ability  to  survive  and  to  propagate  its  kind,  depends 
directly  upon  its  ability  to  secure  adequate  food  supplies  in  the  particu- 
lar environment  to  which  it  is  permanently  adapted.  The  abundance 
of  life  and  the  relative  scarcity  of  suitable  energy-supplying,  proto- 
plasm-building foodstuffs  make  it  necessary  for  organisms  to  compete 
for  their  nutritive  requirements.  The  innumerable  living  organisms 
surviving  today  are  adapted  for  every  possible  environment  in  which 
energy-containing  substances  are  to  be  found.  As  the  English 
biologist  Dendy  has  well  said: 

At  the  present  day  we  see  the  surface  of  the  earth  teeming  with  hosts  of 
living  things,  incalculable  in  number  and  of  endless  diversity  in  form  and 
sftucture.  Every  situation  where  life  is  possible  is  occupied  by  plants  or 
animals  of  some  kind  or  other,  all  specially  adapted  in  bodily  organization  to 
the  conditions  under  which  they  have  to  maintain  their  existence.  From  the 
bleak  and  inhospitable  summits  of  high  mountain  ranges  to  ocean  depths 
which  can  be  measured  in  miles,  from  the  perpetually  frozen  circumpolar 
regions  to  the  torrid  zone  on  either  side  of  the  equator,  living  things  abound. 
Seas,  rivers,  lakes,  dry  land,  and  air  have  all  alike  been  taken  possession  of 
by  representatives  of  the  animal  and  vegetable  kingdoms.1 

1  Dendy,  "Outlines  of  Evolutionary  Biology,"  D.  Appleton-Century  Com- 
pany, Inc. 

428 


THE  WEB  OF  LIFE 


429 


Thus  it  is  clear  that  the  world  of  life,  as  seen  today,  presents  a 
bewildering  array  of  species  that  are  able  to  supply  their  specific 
nutritive  requirements  under  very  different  environmental  conditions. 
Basically,  however,  all  living  organisms  may  be  regarded  as  either 
autotrophic  or  heterotrophic  in  their  nutrition.  Autotrophic  organ- 
isms are  those  which  possess  the  ability  to  construct,  or  synthesize,  the 
essential  nutritive  substances  from  the  abundant  inorganic  elements 
and  compqunds  in  their  environment  and  are,  therefore,  independent 
in  their  nutrition.  Heterotrophic  organisms  require  complex  organic 


FIG.  226. — Diagram  illustrating  formation  of  carbon  compounds  by  photosynthesis 
in  the  green  plants  and  their  destruction  by  bacteria  and  other  colorless  plants.  (Re- 
drawn from  Lutman;  slightly  modified.) 

compounds  as  the  basis  for  their  food  supply  and  consequently  are 
dependent  in  their  nutrition  upon  the  synthetic  activities  of  the  auto- 
trophic forms.  It  is  apparent,  therefore,  that  the  autotrophic  organ- 
isms manufacture  food  materials  for  themselves  and  also  for  the 
heterotrophic  forms.  (Fig.  226.) 

AUTOTROPHIC  ORGANISMS 

Autotrophic  organisms  consist  almost  entirely  of  the  green,  chloro- 
phyll-bearing plants,  equipped  for  photosynthesis.  There  is,  however, 
another  group  of  autotrophic  plants  which  though  inconspicuous  are, 
nevertheless,  of  considerable  importance,  namely,  the  autotrophic 
bacteria.  These  unicellular  colorless  plants  we  able  to  disrupt  various 
highly  stable  inorganic  substances  through  the  action  of  powerful 


430 


HUMAN  BIOLOGY 


intracellular  enzymes  and  to  utilize  the  energy  thus  released  for  the 
synthesis  of  the  complex  carbon  compounds  which  are  essential  for  the 
repair  and  growth  of  their  protoplasm.  Possibly  the  autotrophic 
bacteria  are  to  be  regarded  as  the  most  primitive  of  all  forms  of  life. 
Presumably,  they  were  the  first  type  to  appear  on  this  earth.  The 
development  of  chlorophyll  and  the  associated  processes  of  photo- 
synthetic  food  formation  apparently  represent  later  stages  of  proto- 
plasmic phenomena.  The  nutritive  activities  of  two  important  groups 
of  autotrophic  bacteria  may  now  be  described. 

Sulphur  Bacteria. — Sulphur  is  one  of  the  essential  elements  of 
living  tissues,  and  it  is  through  the  activities  of  a  large  and  diverse 


B  c 

FIG.  227. — Sulphur  bacteria.  A,  (Spirillum  granulatum) ,  with  dividing  cell; 
B,  C,  D,  giant  sulphur  bacterium  (HUlhousia  mirabilis);  B,  normal  cell  with  sulphur 
bodies  filling  the  entire  cell;  (7,  an  individual  in  which  the  sulphur  globules  have  been 
used  in  respiration  after  being  kept  in  tap  water  for  a  week;  £>,  sulphur  crystals  obtained 
when  animals  are  dried.  (Lutman,  B,  C,  D  after  West  and  Griffiths.) 

group  of  sulphur  bacteria  that  suitable  compounds  of  sulphur  are 
supplied  for  animal  and  plant  nutrition.  Thus,  in  the  formation  of 
plant  proteins,  the  green  plants  utilize  the  supplies  of  sulphur  obtained 
from  certain  soluble  sulphates  dissolved  in  the  soil  waters.  The  auto- 
trophic sulphur  bacteria  are  able  by  enzyme  action  to  oxidize  the 
hydrogen  sulphide  gas  (H2S),  released  into  the  air  during  the  decay  of 
organic  compounds,  to  form  water  and  sulphur.  In  so  doing,  energy  is 
obtained  for  the  vital  activities  of  the  organism.  Then  the  sulphur 
may  be  combined  with  water  and  oxygen  to  form  sulphuric  acid.  The 
latter  is  released  into  the  soil  where  it  combines  with  various  mineral 


THE  WEB  OF  LIFE 


431 


Nucleus  - 


elements  to  form  the  soluble  sulphates,  noted  abdve,  which  are 
absorbed  by  the  root  tissues  of  the  green  plants  and  utilized  in  protein 
formation.  Sulphur  bacteria  are  unable  to  survive  without  an 
adequate  supply  of  sulphur  compounds  for  their  energy  requirements. 
(Fig.  227.) 

Nitrifying  Bacteria. — The  soil- 
living  nitrifying  bacteria  constitute 
another  important  group  of  auto- 
trophic  bacteria  that  make  their  liv- 
ing by  salvaging  the  nitrogen  in  the 
residues  resulting  from  the  decay  of 
plant  and  animal  tissues.  There 
are  various  species  which  can  be 
separated  into  two  groups:  the  ni- 
trite bacteria  and  the  nitrate  bacte-  Chloroplas-r  -~j 
ria.  During  the  decay  of  proteins, 
ammonia  gas  (NH3)  is  formed. 
From  the  latter,  the  nitrite  bacteria 
are  able  to  form  nitrous  acid  (HN02) 
by  oxidative  processes.  The  ni- 
trate bacteria  find  the  nitrous  acid 
suitable  for  their  metabolic  activi- 
ties and  add  additional  oxygen  to 
form  nitric  acid  (HNO3).  The  lat- 
ter is  released  into  the  soil  where 
it  combines  with  mineral  elements 
to  form  soluble  nitrates  which  are 
in  time  absorbed  by  the  green  plants 
and  utilized  in  protein  synthesis. 
(Fig.  226.) 

The  Photosynthetic  Organisms. 
It  is  the  presence  of  chlorophyll  in 
the  cells  of  green  plants  that  is 
responsible  for  photosynthesis. 
Unquestionably,  chlorophyll  is  the 
most  important  pigment  known  to 
man,  for  it  is  essential  to  the 
formation  of  the  foodstuffs  required  by  every  living  organism 
with  the  exception  of  the  autotrophic  bacteria  as  just  noted. 
Chlorophyll  is  also  responsible  for  the  liberation  of  free  oxygen 
into  the  atmosphere  during  the  photosynthietic  processes.  Respira- 
tion, involving  the  utilization  of  oxygen,  is  essential  for  every 


FIG.  228. — Drawing  of  an  active 
photosynthetic  cell  (palisade  cell)  from  a 
leaf.  The  chloroplasts  lie  embedded  in 
a  thin,  transparent  layer  of  cytoplasm 
(not  shown)  which  also  surrounds  the 
nucleus.  The  center  of  the  typical  plant 
cell  is  largely  occupied  by  the  fluid-filled 
cell  vacuole.  (Sinnott.) 


432  HUMAN  BIOLOGY 

living  cell,  for  ho  other  method  is  available  to  release  the  potential 
chemical  energy  stored  in  the  complex  organic  molecules.  Oxygen  is 
an  active  element  and  combines  readily  with  other  elements,  so  that 
the  free  oxygen  in  the  atmosphere  would  quickly  disappear  were  it  not 
for  its  continuous  release  during  photosynthesis.  (Figs.  226,  228.) 

The  analyses  of  chlorophyll1  show  it  to  be  a  very  complex  substance 
in  which  two  chlorophyll  compounds  are  associated.  These  are  known 
as  chlorophyll  a  (CbsH^Os^Mg)  and  chlorophyll  6  (C55H7o06N4Mg). 
Chemical  analyses,  however,  shed  no  light  on  the  basic  problem, 
namely,  why  this  particular  assemblage  of  common  elements  is  the 
only  one  of  all  the  innumerable  compounds  known  to  the  chemist  able 
to  bring  about  the  photosynthetic  reaction.  Of  particular  interest,  as 
previously  noted,  is  the  fact  that  the  chemical  composition  of  hemo- 
globin, the  essential  oxygen-carrying  pigment  present  in  the  red  blood 
cells  of  man  and  the  vertebrates  generally,  is  closely  related  to  that  of 
chlorophyll.  Two  other  yellowish  pigments,  carotene  and  xantho- 
phyll,  of  doubtful  function,  are  associated  with  chlorophyll. 

In  the  earlier  discussion  of  retinal  function,  consideration  was  given 
to  the  physical  characteristics  of  the  energy-bearing  light  waves  with 
particular  reference  to  their  wave  lengths  and  associated  colors  in  the 
visible  spectrum  (page  238).  In  the  utilization  of  the  radiant  energy 
by  the  green  plant  cells,  the  function  of  absorption  of  the  light  rays  of 
the  proper  wave  length  is  of  primary  importance.  As  is  well  known, 
an  object  appears  of  a  certain  color  because  it  reflects  that  particular 
color  of  the  spectrum  and  absorbs  the  other  colors.  An  object  that 
appears  black  absorbs  all  the  colors  of  the  spectrum  and  reflects  none. 
The  reverse  condition  obtains  with  white  objects,  which  absorb  none 
and  reflect  all  the  spectral  colors  equally,  thus  producing  the  sensation  of 
white.  It  is  obvious,  therefore,  that  chlorophyll  appears  green  because 
it  reflects  the  light  rays  from  the  green  portion  of  the  spectrum  and 
absorbs  the  rays  from  the  not-green  portions,  the  latter  containing  the 
energy-bearing  rays  essential  to  the  photosynthetic  reactions.  (Fig. 
229.) 

The  absorption  of  these  rays  can  be  demonstrated  by  examining 
the  spectrum  obtained  when  the  rays  of  sunlight  are  passed  through  a 
chlorophyll  solution.  Under  such  conditions,  it  will  be  found  that  the 
resulting  spectrum  is  incomplete,  for  the  red  and  orange  rays  have 
been  absorbed  by  the  chlorophyll  from  one  eftd  of  the  spectrum  and 
violet  rays  from  the  other.  Accordingly,  it  is  evident  that  the  red- 
orange  and  the  blue-violet  rays  absorbed  by  the  chlorophyll  are  the 
ones  that  function  in  photosynthesis.  The  radiant  energy  actually 
1  Consult  Appendix:  Chlorophyll. 


THE  WEB  OF  LIFE  433 

used  by  leaves,  under  the  most  favorable  condition  of  photosynthesis, 
probably  never  exceeds  3  per  cent  of  the  amount  available  and  usually 
is  considerably  less  than  that.  Thus  the  sun  continuously  supplies  an 
incredible  amount  of  radiant  energy,  only  a  very  small  portion  of  which 
is  utilized  by  the  green  plants  for  photosynthesis  and  stored  as  poten- 
tial chemical  energy  in  the  compounds  associated  with  the  plant  tissues. 
Protoplasm  has  a  great  capacity  to  do  work  so  long  as  it  is  supplied 
with  the  energy-containing  foodstuffs.  Life  is  characterized  by  a 


Red 

Orange 

Yellow 

Green 

Blue 

Violet 


FIG.  229. — Diagram  showing  tho  colors  produced  in  the  spectrum  when  a  ray  of 
light  is  passed  through  the  prism  of  a  spectroscope  (above) ;  (below)  diagram  showing 
the  bands  absorbed  from  the  spectrum  when  the  ray  of  light  is  first  passed  through 
chlorophyll  solution.  Described  011  page  432.  (Sinnott). 

continuous  supply  of  energy.  Living  organisms  have  no  method 
for  creating  energy  but  only  for  the  transformation  of  radiant  energy 
received  from  the  sun.  Furthermore,  as  just  stated,  the  ability  to 
utilize  radiant  energy  is  limited  to  the  green  plants.  They  perform 
this  essential  function  through  the  synthesis  of  simple  inorganic  sub- 
stances to  form  complex  organic  compounds  which  are  suitable  for  food, 
and  thus  available  to  keep  the  wheels  of  life  turning. 

The  physicist  defines  energy1  as  the  capacity  to  do  work  and  sees 
that  it  may  be  manifested  as  energy  of  position,  shown  in  gravitation, 
motion,  etc. ;  as  chemical  energy,  which  is  evidenced  in  molecular  and 

'-Consult  Appendix:  Energy. 


434  HUMAN  BIOLOGY 

heat  phenomena;  and  as  radiant  energy,  illustrated  in  the  phcnomen 
associated  with  light  and  electricity.  Energy  is  accumulated  a 
potential  energy  and  later  released  as  active,  or  kinetic,  energy.  Th 
biologist  is  particularly  interested  in  chemical  energy  and  in  the  radian 
energy  present  in  the  sun's  rays,  for  the  chloroplasts  in  the  green  plan 
cells  have  discovered  the  secret  of  transforming  radiant  energy  int 
potential  chemical  energy  and  storing  the  latter  in  complex  organi 
nutritive  compounds,  the  carbohydrates,  fats,  and  proteins ;  synthese 
that  result  from  the  essential,  but  poorly  understood,  process  o 
photosynthesis. 

The  conventional  equation  for  photosynthesis,  namely, 

6H20  +  6CO«  =  C6H1206  +  602 

does  not  show  the  energy  relations  that  are  basic  for  the  maintenanc< 
of  the  life  functions.  Each  molecule  of  glucose  that  is  formed  fy 
photosynthesis  requires  677.2  calories1  of  radiant  energy.  Accord 
ingly,  the  equation  for  the  photosynthetic  reaction  will  read : 

6H2O  +  6CO2  +  677.2  calories  =  C6H12O6  +  6O2 

The  oxidation  of  the  carbohydrate  molecule  in  the  living  tissues  during 
respiration  results  in  the  liberation  of  this  amount  of  heat  energy  fo 
the  maintenance  of  the  life  functions,  as  shown  in  the  following 
equation: 

C6Hi2O6  +  6O2  =  6CO2  +  6H2O  +  677.2  calories 

Thus,  in  the  living  tissues,  the  potential  chemical  energy  of  glucose  is 
transformed  into  kinetic  energy  and  used  to  maintain  the  life  activities 

But  the  living  organism  requires  more  from  the  foods  that  are  taker 
in  than  the  mere  release  of  energy — for  materials  must  also  be  suppliec 
for  the  repair  and  growth  of  the  tissues.  Our  previous  consideratior 
of  human  nutrition  has  made  it  evident  that  the  tissue  requirements 
are  supplied  in  full  only  when  an  adequate  assortment  of  proteins  i* 
secured  from  the  utilization  of  various  plant  and  animal  tissues  (page 
56).  In  a  word,  it  is  recognized  that  universal  food  requirement* 
operate  throughout  the  world  of  life  and  that  the  food  supply  of  al 
organisms  rests  finally  upon  the  photosynthetic  activities  of  the  greer 
plants.  Here,  then,  is  a  basic  interdependence  binding  together  al 
living  organisms. 

Furthermore,  the  materials  accumulated  in  the  tissues  of  ever} 
living  organism,  together  with  the  wastes  continually  formed  during 
life,  must  be  returned  to  the  great  storehouses  of  nature  for  latei 

1  Small  calories:  see  footnote,  p.  86. 


THE  WEB  OF  LIFE  435 

reassembling  in  another  cycle  of  life.  This  requires  the  services  of  the 
colorless  plants,  or  Fungi,  which  secure  their  own  nutrition  by  dis- 
integrating— the  processes  of  decay — the  complex  organic  compounds 
built  up  in  other  organisms,  thus  making  the  constituent  materials 
again  available.  This  function  o?  the  colorless  plants  is  responsible 
for  the  cycle  of  elements*  in  nature  and  is  just  as  important  as  the 
opposite  process  involving  the  constructive  activities  of  the  green 
plants. 

HETEROTROPHIC  ORGANISMS 

Turning  our  attention  to  the  heterotrophic  organisms  which  are 
dependent  upon  the  photosynthetic  organisms  for  supplying  their 
nutritive  requirements,  it  may  be  noted  at  once  that  they  include  the 
organisms  belonging  to  two  widely  separated  groups,  namely,  animals 
and  colorless  plants  (except  for  the  relatively  few  types  of  autotrophic 
bacteria  noted  above).  It  will  not  be  necessary  to  give  further  atten- 
tion to  animal  nutrition,  inasmuch  as  this  subject  was  fully  considered 
in  the  earlier  chapter  on  Nutrition,  but  brief  mention  of  colorless  plant 
nutrition  will  be  helpful.  (Fig.  230.) 

Representatives  of  the  Fungi  are  extraordinarily  abundant  in 
nature.  At  the  same  time,  they  exhibit  wide  diversity  in  their 
structural  patterns  and  in  their  nutritive  requirements.  Throughout, 
however,  there  is  a  common  lack  of  the  basic  food-synthesizing  chloro- 
phyll of  the  green  plants,  and  hence  the  colorless  plants  find  it  necessary 
to  satisfy  their  nutritive  requirements  by  utilizing  complex  foodstuffs 
as  do  animals,  but,  unlike  the  latter,  the  Fungi  are  unable  to  ingest 
solid  particles  of  food.  Accordingly,  it  is  necessary  for  the  fungal  cells 
to  secrete  specific  extracellular  enzymes  which  digest  the  solid  nutritive 
substances  in  their  environment,  thus  liquefying  the  foods  so  that  they 
can  be  absorbed  through  the  unbroken  cell  membranes.  Commonly, 
the  Fungi  are  termed  decay  organisms  because  the  enzyme  actions 
associated  with  their  nutrition  result  in  the  disintegration  or  decay  of 
the  organic  materials  stored  in  the  dead  animal  and  plant  tissues.  The 
compounds  thus  utilized  for  the  life  activities  of  the  Fungi  are  later 
returned  to  the  soil  and  air  in  a  greatly  simplified  form  which  permits 
them  to  be  utilized  in  the  synthetic  processes  of  the  green  plants—the 
cycle  of  elements  in  nature.  In  many  instances,  the  Fungi  are  para- 
sitic, which  means,  in  a  word,  that  to  supply  their  nutritive  require- 
ments they  invade  and  destroy  the  tissues  of  living  plants  and  animals 
and  cause  disease,  as  will  be  discussed  at  length  below.  In  either  of  the 
conditions  noted  above,  the  essentials  of  nutrition  remain  unchanged  in 
that  the  enzymes  secreted  by  the  fungous  cells  are  able  to  digest  the 


436 


HUMAN  BIOLOGY 


complex  materials  whether  the  latter  are  present  in  dead  or  living 
organisms.     This  is  termed  saprophytic  nutrition. 

The  colorless  plants  adapted  for  saprophytic  nutrition  include  such 
apparently  diverse  types  as  bacteria,  yeasts,  molds,  mildews,  mush- 
rooms, smuts,  rusts,  and  various  others,  totaling,  altogether,  many 
thousands  of  species.  From  among  these,  the  common  bread  mold 
may  be  selected  for  further  consideration.  It  is  so  widely  distributed 
that  usually  it  is  necessary  to  expose  a  piece  of  bread  to  the  air  for  a 


8° 

OO/D 


0 


B 


CO 


<9 


G  H 

FIG.  230. — Various  types  of  bacteria.  A,B,C,D  are  virulent  pathogenic  (disease- 
producing)  bacteria  as  follows:  A,  Staphylococcus;  B,  Myobacterium  leprae;  C,  Pneumo- 
coccus;  D,  Streptococcus.  E,G,H  are  common  nonpathogeriic,  or  saprophytic,  forms  as 
follows:  E,  Spirillum;  G,  the  colon  bacillus  (Bacillus  coli);  H,  the  hay  bacillus  (Bacillus 
subtilis).  F,  three  types  of  spores.  Highly  magnified.  (Sinnott.) 

few  minutes  only  in  order  to  infect  it  with  the  minute,  floating  spores 
of  the  bread  mold  which  are  almost  invariably  present  in  the  dust  and 
air.  If  sufficient  moisture  is  present,  the  spores  in  the  bread  soon  swell, 
disrupting  the  cell  wall,  and  then  each  releases  a  bit  of  active  proto- 
plasm which  immediately  begins  to  permeate  the  bread  substance  to 
secure  the  essential  nutritive  materials.  In  order  to  obtain  the  latter, 
the  mold  protoplasm  secretes  digestive  enzymes  which  pass  into  the 
bread  and  digest  the  solid  foodstuffs,  thus  rendering  them  soluble. 
The  liquid  foods  are  absorbed  by  the  mold  cytoplasm  and  utilized 


THE  WEB  OF  LIFE 


437 


for  the  energy  requirements  and  for  the  formation  of  additional 
protoplasm.     (Fig.  149.) 

The  example  just  given  of  the  use  of  extracellular  enzymes  by  the 
bread  mold  to  secure  soluble  food  materials  from  suitable  solid  sub- 
stances in  the  environment  has  wide  application  in 
the  world  of  life,  and,  as  a  matter  of  fact,  it  is 
exactly  what  occurs  in  the  holozoic  nutrition  of  man 
and  other  animals  in  the  digestion  of  foods  in  the 
alimentary  tract.  Solid  materials  taken  into  the 
alimentary  tract  cannot  be  regarded  as  being  within 
the  body  until  digestion  has  taken  place  and  the 
resulting  nutrient  liquids  have  been  absorbed  by  the 
nutritive  epithelium  that  lines  the  intestine.  To 
all  intents  and  purposes,  therefore,  animal  digestion 
is  extracellular,  and  the  digestive  enzymes  are 
secreted  for  external  use  just  as  are  those  of  the 
bread  mold  or  other  colorless  plants. 

ENZYMES 


^  H  HK>CH2OH 


It  should  be  recognized  that  all  types  of  nutri- 
tion   exhibited   in    the    living   world   are   directly 
dependent  upon  enzyme1  action.    Accordingly,  an 
organism  is  limited  in  its  selection  of  foodstuffs  by 
the  nutritive  enzymes  that  it  is  able  to  synthesize 
and  to  employ.     The  adaptation  of  an  organism, 
therefore,    to    a    particular    environment    may    be    said    to    rest 
primarily  upon  the  ability  of  the  nutritive  enzymes  to  digest  the 
available  materials.     The  most  powerful  enzymes  are  undoubtedly 


Fro.  231. —Mo- 
lecular structure  of 
cellulose  as  deter- 
mined by  x-ray 
studies.  (Seifriz.) 


FIG.  232. — Diagram  to  illustrate  possible  arrangement  of  cellulose  chains  into  larger 
units  of  cellulose,  as  in  the  plant  cell  wall,  a,  cellulose  chain  as  in  Fig.  231;  b,c,dt  posi- 
tion of  forces  holding  larger  cellulose  units  together.  (Seifriz.) 

associated  with  the  life  chemistry  of  the  autotrophic  bacteria,  for,  as 
noted  above,  these  enzymes  are  able  to  break  down  certain  very  stable 
1  Consult  Appendix:  Enzymes. 


438  HUMAN  BIOLOGY 

inorganic  compounds  and  make  them  available  to  the  organisms. 
Possibly  at  the  opposite  end  of  the  scale  are  the  digestive  enzymes  of 
the  flesh-eating  mammals  which  are  limited  in  their  chemical  activities 
to  reactions  with  organic  compounds  possessing  relatively  large  and 
unstable  molecules.  In  the  chapter  on  Nutrition,  considerable  atten- 
tion was  directed  toward  enzyme  actions  in  digestion  (page  63).  The 
earlier  discussion  may  now  be  broadened  somewhat  in  an  endeavor 
to  give  brief  answers  to  four  questions  relative  to  the  enzymes,  namely, 
What  are  they?  How  do  they  work?  What  do  they  accomplish? 
Where  do  they  work?  (Figs.  231,  232.  Pages  70,  510.) 

In  the  first  place,  enzymes  may  be  described  as  nonliving  com- 
pounds which  are  formed  by  the  synthetic  activities  of  cell  protoplasm. 
Every  cell  must  be  equipped  with  its  battery  of  enzymes  in  order  to 
maintain  the  essential  chemical  processes  associated  with  the  main- 
tenance of  the  life  processes.  The  chemist  recognizes  them  as 
catalysts,  a  group  that  includes  many  inorganic  compounds  and  even 
certain  elements.  The  enzymes  associated  with  chemical  reactions  in 
living  organisms  are  much  more  elaborate  in  their  chemical  structure. 
A  catalyst  may  be  defined  as  any  substance  that  hastens  the  attain- 
ment of  equilibrium  in  a  chemical  reaction.  In  so  doing,  the  catalyst 
itself  is  not  changed. 

A  well-known  example  of  catalytic  action  is  found  in  the  greatly 
accelerated  reaction  between  hydrogen  and  oxygen  in  the  presence  of 
finely  divided  platinum  particles  which  act  as  a  catalyst.  Another 
catalytic  action,  and  one  that  can  easily  be  demonstrated,  is  to  be  seen 
in  the  oxidation  of  cane  sugar.  An  attempt  to  ignite  a  lump  of  cane 
sugar  with  a  match  will  be  unsuccessful  without  the  aid  of  a  catalyst 
which  will  bring  about  a  chemical  reaction  between  oxygen  and  the 
sugar  molecules  at  a  comparatively  low  temperature.  An  efficient 
catalyst1  for  this  reaction  is  found  in  powdered  ashes.  The  test  may 
be  made  by,  first,  attempting  to  ignite  the  pure  sugar  by  the  match 
flame;  the  sugar  will  melt  but  not  burn.  If,  now,  the  end  of  the  sugar 
lump  is  rubbed  in  some  powdered  ashes,  it  can  be  ignited  and  will  burn 
vigorously.  The  oxidative  reaction  in  the  presence  of  the  ash-catalyst 
will  continue  until  all  of  the  sugar  is  burned.  This  shows  that  the 
catalyst  is  not  destroyed  in  the  reaction  but  continues  to  function  in 
the  presence  of  sugar  and  oxygen. 

The  enzymes  of  living  organisms  differ  from  inorganic  catalysts,  as 
just  described,  in  being  much  more  complex  in  their  chemical  structure. 
They  are  colloidal,  probably  proteinaceous  substances.  Accordingly, 

1  The  author  is  indebted  to  Dr.  0.  W.  Richards  for  calling  his  attention  to 
this  striking  example  of  catalytic  action. 


THE  WEB  OF  LIFE  439 

the  process  of  adsorption,  in  which  a  precipitation  of  the  combining 
substances  on  the  finely  dispersed  particles  of  the  colloidal  enzyme 
occurs,  appears  to  be  primarily  responsible  for  the  acceleration  of  the 
chemical  reactions.  To  the  chemist,  probably  the  most  amazing 
characteristic  of  life  is  the  ability  to  maintain  vigorous  chemical 
reactions  at  comparatively  low  temperatures.  The  same  reactions 
in  the  laboratory,  without  the  catalytic  enzyme  phenomena,  will  occur 
only  under  a  very  much  higher  temperature.  Another  important 
characteristic  of  enzyme  activity  is  its  rigid  specificity.  In  general, 
each  enzyme  is  concerned  with  a  single  reaction  which  takes  place  in  a 
particular  substance  or  substrate.  Thus,  in  digestion,  the  enzyme 
sucrase  is  required  for  the  splitting  of  the  cane  sugar,  or  sucrose, 
molecule.  In  any  other  substrate  than  a  sucrose  solution,  this  enzyme 
is  an  inert  substance. 

Though  a  great  many  enzymes  are  known,  around  100  being  avail- 
able for  the  various  chemical  processes  associated  with  the  human 
organism,  and  though  they  are,  as  we  have  seen,  markedly  specific  in 
their  selection  of  a  substrate,  nevertheless,  they  accomplish  their 
results  almost  entirely  by  two  processes,  namely,  hydrolysis  and  oxida- 
tion. By  far  the  greatest  number  of  enzymes  are  hydrolyzers,  which, 
as  indicated  by  the  descriptive  term,  perform  their  chemical  magic  by 
the  use  of  water  molecules,  which  may  be  added  or  removed  from  a 
particular  compound.  Thus,  in  digestion,  as  we  know,  water  is  added 
to  the  complex  organic  solids  (page  62),  whereas  in  the  synthetic 
reactions  the  opposite  condition  obtains  and  water  is  released  (page 
66).  Or  in  some  enzymes,  as  in  those  responsible  for  intracellular 
respiration,  the  reactions  are  brought  about  by  oxidative  processes 
which  result  in  an  increase  or  decrease  in  the  oxygen  present  in  the 
substrate. 

Enzymes  are  often  divided  into  two  groups  on  the  basis  of  their 
ability  to  synthesize  more  complex  compounds  from  less  complex 
materials  or  the  opposite  condition  in  which  disintegration  of  the 
complex  substances  is  incited.  Comparatively  little  is  known  about 
the  synthesizing  enzymes,  though  every  cell  must  carry  its  complement 
of  these  essential  catalytic  agents  in  order  to  build  the  protoplasmic 
materials  require^,  for  repair  and  growth.  Unquestionably,  the  amino 
acids  selected  from  the  environment  are  synthesized  by  intracellular 
enzymes  to  form  the  exact  type  of  protein  required  for  each  cell. 
Furthermore,  the  basic  process  for  all  life,  photosynthesis  in  the  green 
plant  cells,  is  undoubtedly  dependent  upon  synthesizing  enzymes. 
The  presence  of  one  of  these  enzymes  (ehlorophyllase)  has  been 
definitely  established.  Blood  coagulation,  with  the  formation  of  the 


440  HUMAN  BIOLOGY 

insoluble  protein  fibrin,  is  also  seen  as  the  result  of  synthetic  activity 
incited  by  a  synthesizing  enzyme,  kinase. 

Finally,  enzymes  may  be  divided  into  two  groups  on  the  basis  of 
intracellular  or  intercellular  activity.  The  term  enzyme,  coming  from 
the  Greek,  literally  means  "in  yeast"  and  refers  to  the  fact  that  a 
substance  is  present  in  yeast  that  is  responsible  for  the  chemical 
activities  resulting  in  alcoholic  fermentation.  This  intracellular 
enzyme,  zymase,  can  be  obtained  from  the  yeast  cells  when  the  cell 
walls  are  destroyed  by  grinding.  Zymase  is  only  one  of  many  intra- 
cellular enzymes  that  are  necessarily  present  in  the  cytoplasm  of  the 
yeast  cells  in  order  for  them  to  synthesize  or  to  disintegrate  the  various 
substances  essential  to  the  life  activities  of  these  cells.  Essentially  the 
same  thing  is  true  for  every  type  of  living  cell  no  matter  where  found. 
In  addition  to  the  essential  intracellular  enzymes,  heterotrophic 
organisms,  as  just,noted,  must  be  able  to  form  and  to  secrete  into  their 
environment  various  extracellular  enzymes  for  the  digestive  functions 
so  that  the  available  nutritive  materials  can  be  absorbed  and  assimi- 
lated by  the  cells. 

As  already  indicated,  the  disintegrative  enzymes  include  the  diges- 
tive enzymes.  These  have  been  the  subject  of  a  great  deal  of  investiga- 
tion. Primarily,  this  is  due  to  the  fact  that  many  of  the  digestive 
enzymes  are  formed  and  secreted  in  considerable  quantities  in  the 
vertebrate  animals  so  that  they  have  been  relatively  easy  for  the 
investigator  to  secure.  However,  the  first  digestive  enzyme  dis- 
covered, more  than  one  hundred  years  ago  (1833),  was  found  in  plant 
tissues.  It  was  noted  that  germinating  seeds  contained  a  substance 
able  to  change  the  stored  starch  grains  into  sugar.  This  action  was  due 
to  the  enzyme  amylase  which  is  also  present  in  the  human  digestive 
tract,  where  it  performs  the  same  function.  Some  thirty  years  later, 
Pasteur  discovered  that  enzyme  action  was  responsible  for  the  forma- 
tion of  alcohol  from  the  sugar  molecule  and  that  the  enzyme  was 
formed  in  the  cytoplasm  of  yeast  cells.  It  was  not  until  1897  that  this 
enzyme,  zymase,  was  extracted  from  the  yeast  cells  by  grinding.  It 
was  further  shown  that  zymase,  though  normally  intracellular,  was 
able  *to  incite  the  same  reaction  outside  the  cell.  Other  important 
enzymes  associated  with  carbohydrate  disruption  include  sucrase, 
lactase,  and  maltase.  These,  together  with  lipase,  the  fat-splitting 
enzyme,  and  the  battery  of  important  proteolytic  enzymes,  pepsin, 
rennin,  trypsin,  and  erepsin,  constitute  the  complement  of  digestive 
enzymes  associated  with  digestion  in  the  human  organism  as  discussed 
in  the  earlier  chapter  (pages  63  to  66).  Similar  enzymes  are  widely 


THE  WEB  OF  LIFE  441 

distributed  in  every  type  of  organism  and  apparently  are  the  basis  of 
nutrition  throughout  the  living  world. 

Brief  mention  should  be  made  of  two  other  activities  of  disintegra- 
tive  enzymes  associated  with  animal  respiration  and  with  the  so-called 
deaminization  process.  The  respiratory  enzymes  are  intracellular  and 
are  often  termed  oxidases  because  they  disrupt  the  glucose  molecule 
by  the  addition  of  oxygen,  as  indicated  in  the  equation  CeH^Oe  + 
602  =  6C(>2  +  6H20.  Deaminization  enzymes  are  present  in  the 
cytoplasm  of  the  liver  cells.  Acting  on  the  amino  acid  molecule,  when 
a  surplus  is  absorbed  by  these  cells,  they  are  able  to  split  off  the  amino 
acid  radical,  NH2,  and  leave  the  remainder  of  the  molecule,  with  the 
carbon,  hydrogen,  and  oxygen  elements  to  be  utilized  as  a  carbo- 
hydrate. In  this  way  excess  proteins  are  disposed  of.  Also,  attention 
was  directed  previously  to  the  pigment-producing  enzyme,  tyrosinase 
(page  400). 

Food  Chains. — It  is  hoped  that  the  preceding  discussion  in  this 
chapter  makes  it  entirely  clear  that,  from  the  nutritive  standpoint, 
organisms  are  largely  restricted  in  supplying  their  essential  require- 
ments by  the  digestive  enzymes  with  which  they  are  equipped.  This 
nutritive  adaptation  is  responsible  for  the  cycle  of  elements  in  nature 
that  maintains  a  continuous  supply  of  the  essential  elements  and,  at 
the  same  time,  binds  all  organisms  together  in  a  nutritive  web  of  life. 
The  latter  is  woven  of  innumerable  strands,  the  food  chains,  by  which 
the  nutritive  requirements  of  the  associated  organisms  are  supplied 
and  to  which  each  particular  group  of  organisms  makes  a  contribution 
to  the  nutrition  of  the  other  groups  possessing  different  nutritive 
requirements.  The  food  chains  of  all  animals  start  from  the  organic 
foodstuffs  synthesized  by  the  green  plants,  which,  as  we  know,  are 
capable  of  supplying  the  energy  requirements  and  the  building  mate- 
rials as  well. 

A  temporary,  but  very  interesting,  web  of  life  with  many  food  chains 
is  readily  observed  in  a  laboratory  hay  infusion.  The  latter  is  easily 
started  by  introducing  a  few  wisps  of  hay  or  grass  into  a  battery  jar  or 
other  suitable  receptacle  containing  tap  or  pond  water.  The  dried 
plant  tissue  contains  substances  formed  by  photosynthetic  activity 
suitable  for  the  nutrition  of  various  microscopic  organisms,  particu- 
larly bacteria,  which  may  be  present  in  the  water  or,  in  an  inactive 
state,  on  the  hay.  The  bacteria  and  other  types  of  fungi,  finding  the 
hay  infusion  environment  suitable,  quickly  become  active  and  start 
the  secretion  of  enzymes  which,  in  turn,  begin  the  digestion  of  the 
nutritive  materials  of  the  hay.  The  soluble  compounds  thus  formed 


442  HUMAN  BIOLOGY 

diffuse  through  the  water,  from  which  they  are  absorbed  and  assimi- 
lated by  the  bacterial  cells.  Such  conditions,  provided  the  temperature 
remains  suitable,  are  highly  favorable  for  bacteria.  As  a  con- 
sequence, the  bacterial  cells  reproduce  with  great  rapidity  so  that,  in 
the  course  of  a  few  days,  untold  billions  are  present  in  the  liquid,  and 
these  congregate  at  the  surface  of  the  liquid  infusion  to  form  a  scum. 
It  is  known  that  under  optimum  conditions  a  bacterial  cell  may  divide 
approximately  every  half  hour. 

Marked  increase  in  the  numbers  of  bacterial  cells  will  continue  until 
the  stored  food  materials  in  the  hay,  suitable  for  digestion  by  the 
bacterial  enzymes,  are  exhausted  or  until  some  larger  organism  appears 
in  the  infusion  which  finds  the  bacterial  cells  suitable  for  food  and 
begins  to  prey  upon  them.  Thus,  in  a  hay  infusion,  it  will  be  found 
that  various  types  of  protozoa,  present  on  the  materials  at  the  time 
the  infusion  was  made,  soon  become  abundant,  for  they  find  a  very 
satisfactory  food  supply  in  the  bacterial  cells.  Accordingly,  the 
protozoa  feed  on  the  bacteria  and  begin  to  increase  in  numbers  with 
amazing  rapidity.  The  first  protozoan  types  to  appear  in  large 
numbers  will  usually  be  very  tiny  flagellated  forms,  not  much  larger 
than  the  largest  bacterial  cells  on  which  they  feed.  Soon,  however, 
much  larger  ciliated  protozoa  appear  in  increasing  numbers,  all  direct 
descendants  of  a  few  cells  present  when  the  infusion  was  started.  And 
these  ciliates  get  their  food  by  devouring  the  smaller  protozoa  and 
also,  to  some  extent,  the  bacteria.  (Fig.  10.) 

Life  in  this  microcosm  becomes  increasingly  abundant  for  a  time 
until  the  supply  of  food  stored  in  the  hay  begins  to  be  exhausted. 
When  this  occurs,  as  is  inevitable  unless  more  hay  is  added,  then  the 
organisms  in  the  food  chain  rapidly  decrease  in  number.  In  the  course 
of  a  few  weeks,  it  will  be  found  that  all  the  active  forms  of  life  have 
disappeared,  and  the  water  in  the  infusion,  with  no  scum  at  the  top, 
is  clear,  though  inactive  spores  and  cysts  await  the  restoration  of 
adequate  nutritive  supplies.  The  energy  stored  by  photosynthesis  in 
the  complex  compounds  has  been  dissipated,  and  only  comparatively 
simple  substances  and  elements  remain  which  are  resistant  to  the 
enzyme  action. 

But  the  old  infusion  contains  the  inorganic  materials  that  the 
green  plants  utilize  in  photosynthesis.  Accordingly,  if  some  suitable 
green  water  plants  are  introduced,  and  the  aquarium  is  placed  in  the 
sunlight,  the  process  of  utilizing  the  radiant  energy  of  sunlight  to  form 
the  nutritive  carbon  compounds  proceeds  at  a  rapid  rate  in  the  presence 
of  the  chlorophyll-containing  chloroplasts;  radiant  energy  is  trans- 
formed to  potential  chemical  energy.  In  time,  a  body  of  plant  tissues. 


THE  WEB  OF  LIFE 


443 


suitable  for  animal  or  colorless-plant  nutrition,  will  be  synthesized. 
By  the  introduction  of  animal  life  at  this  stage,  there  is  the  possibility 
of  establishing  a  balanced  aquarium  in  which  the  cycle  of  elements  will 
be  more  or  less  permanently  maintained  provided  the  aquarium  is  kept 
in  sunlight  so  that  the  energy  dissipated  by  the  maintenance  of  the 
life  activities  of  the  various  organisms  will  be  continually  restored. 

In  a  balanced  aquarium,  the  various  food  chains  are  woven  into  a 
complete  pattern — a  web  of  life.  Green  plant  tissues,  built  up  by  the 
photosynthetic  actions,  may  be  consumed  directly  by  certain  animal 
types,  such  as  the  protozoa,  snails,  fish,  and  others.  Also,  the  smaller 
herbivorous  animals  are  preyed  upon  by  larger  carnivorous  species, 
and  these,  in  turn,  by  the  largest  types  present  which  represent  the 
climax  of  a  particular  food  chain. 
The  oxygen  released  into  the  water  by 
the  photosynthetic  activities  is  utilized 
by  the  animals  for  respiration;  the 
release  of  waste  carbon  dioxide  by  the 
animal  cells  is  essential  to  photo- 
synthesis in  the  plants.  The  nitro- 
genous wastes  excreted  by  the  animals 
are  immediately  attacked  by  the  ™ 
enzymes  of  the  colorless  plants,  as  are 
also  the  plant  or  animal  tissues  when 
an  organism  dies.  In  time,  the  dead 
tissues  are  reduced  to  the  inorganic 
materials  suitable  for  the  photosyn- 
thetic activities  of  the  green  plants.  (Fig.  233). 

The  cycle  of  elements  and  food  chains,  as  just  described  in  a  bal- 
anced aquarium,  are  basically  no  different  from  those  present  in 
typical  bodies  of  fresh  and  salt  water  widely  distributed  over  the 
earth.  Representatives  of  the  colorless  plants,  green  plants,  and 
animals  are  everywhere  present;  and  the  nutritive  requirements  of  all 
the  organisms  in  a  particular  biotic  association,  if  maintained,  are  so 
interlinked  as  to  complete  a  balanced  web  of  life.  These  nutritive 
linkages  haYe  been  very  carefully  worked  out  in  many  instances  by 
the  ecologists,  who  have  as  their  goal  the  unraveling  of  the  complex  and 
almost  innumerable  patterns  found  in  the  world  of  life.  And,  of 
course,  it  is  apparent  that  the  organisms  in  the  world  in  which  we  live 
are  balanced  in  essentially  the  same  way  as  just  noted  in  the  individual 
associations;  that  so  long  as  the  conditions  remain  adapted  for , the 
synthesis  of  foodstuffs  the  heterotrophic  animal  and  colorless  plant 
life  will  be  maintained  by  the  autotrophic  green  plants.  (Fig.  234.) 


Fia.  233.— Scheme  illustrating 
the  cycle  of  elements  in  a  balanced 
aquarium.  (Hunter,  Walter,  and 
Hunter,  "Biology,"  American  Book 
Company.) 


444 


HUMAN  BIOLOGY 


And  so  in  a  very  real  sense  one  can  speak  of  the  aquatic  pastures 
in  the  fresh  and  salt  waters  just  as  we  speak  of  the  pastures  present  on 
the  soil  on  which  the  animals  graze.  The  limnological  biologist,  con- 
cerned with  the  study  of  aquatic  life  in  waters,  notes  that  the  abun- 
dance of  life  is  relatively  greater  near  the  surface.  Here  are  found 
many  forms  of  microscopic  floating  organisms,  both  animal  and  plant, 
which  collectively  constitute  the  plankton,  the  term  meaning  "that 
which  is  drifted  about. "  The  plankton  contains  many  holophytic 
unicellular  plants  which  constitute  the  basic  source  of  food  for  the 
heterotrophic  forms;  here  as  elsewhere  photosynthesis  is  the  basis  of 


GEESE  AND  OTHER  \    \  FOX 
BIRDS 


ROTIFERS 
SMALL  ARTHROPODS 


FIG.  234. — Scheme  to  illustrate  the  various  food  chains  in  the  Arctic  Bear  Island, 
an  isolated  community.  Arrows  indicate  the  derivation  of  the  food  supply  by  the 
numerous  types  of  life.  (Buchanan, ' '  Elements  of  Biology, ' '  Harper  &  Brothers.  Adapted 
from  Summerhayes  and  Elton.) 

nutrition.  Though  the  surface  waters  may  contain  a  tremendous 
fauna  and  flora  of  organisms  in  the  plankton  microcosm,  the  appear- 
ance to  the  unaided  eye  is  not  impressive.  But  a  fine-meshed  tow  net 
drawn  through  the  waters  will  collect  an  abundance  of  protistan  types, 
which,  under  the  microscope,  will  reveal  the  amazing  prodigality  of 
aquatic  life.  Even  so,  many  of  the  plankton  organisms  are  so  small 
that  they  will  pass  through  the  finest  nets.  They  can,  however,  be 
collected  by  other  methods,  such  as  centrifuging. 

The  plankton  microcosm  has  its  own  food  chains  which  bind  these 
microscopic  forms  in  a  composite  web  of  life,  and  the  plankton  as  a 
whole  is,  in  turn,  preyed  upon  by  larger  swimming  forms  which 
collectively  constitute  the  nekton.  The  latter  consists  of  active 


THE  WEB  OF  LIFE  445 

types  ranging  in  size  irom  large  species  of  Protozoa,  barely  visible  to 
the  naked  eye,  through  a  wide  variety  of  Crustacea,  to  fish  of  con- 
siderable size  which  represent  the  climax  types.  Finally,  in  large 
bodies  of  water  with  considerable  depth,  a  third  association  of  animals 
is  recognized  in  the  bottom  feeders,  or  benthos  forms,  which  are,  so  to 
speak,  dependent  upon  "the  crumbs  dropped  from  the  rich  man's 
table. "  At  the  depths  at  which  the  benthos  types  exist  the  light  rays 
penetrate  but  feebly  if  at  all,  and,  consequently,  the  environmental 
conditions  are  not  suitable  for  the  green  plants.  Therefore,  any 
animals  present  must  receive  their  food  supplies  from  the  abundant 
life  streams  in  the  upper  regions.  Fish,  which  are  adapted  for  bottom 
feeders,  typically  exhibit  marked  structural  and  functional  adaptations. 
Some  of  the  most  bizarre  types  of  fish  occur  in  this  group.  Also  in 
the  ocean  depths,  as  in  the  surface  areas,  the  fungi  are  well  represented, 
and  organic  materials,  whatever  their  source,  are  soon  reduced  to  the 
inorganic  elements  and  compounds.  The  latter,  eventually  reaching 
the  surface  waters,  pass  once  more  into  the  stream  of  life  through 
photosynthetic  organisms.  Again  the  cycle  of  elements  is  evident. 

At  the  beginning  of  the  chapter,  it  was  stated  that  the  abundance 
of  life  upon  the  earth  makes  it  necessary  for  all  organisms  to  enter  into 
competition  to  secure  the  foodstuffs  necessary  for  their  existence.  The 
available  food  supply  is  the  decisive  factor  that  limits  the  abundance  of 
life.  This  fact  is  particularly  apparent  in  the  unicellular  forms  of 
life,  such  as  the  bacteria  and  the  protozoa.  The  prodigious  abundance 
of  organisms  in  the  microcosm  of  a  hay  infusion  culture  quickly 
disappear  as  the  food  supply  dwindles.  Woodruff,  the  famed  pro- 
tozoologist,  calculated  some  years  ago  that  the  descendants  of  a  single 
paramecium,  which  he  cultured  in  the  laboratory  for  many  years, 
would  have  formed  a  mass  of  protoplasm  in  a  period  of  5  years  equal 
to  101'000  times  the  volume  of  the  earth,  if  it  had  been  possible  to 
provide  all  the  daughter  cells  with  "food  and  shelter."  Even  more 
prolific  are  the  bacteria  which,  under  suitable  conditions  of  food  and 
temperature,  will  divide  every  20  to  30  minutes.  It  has  been  cal- 
culated that,  under  these  conditions,  the  descendants  of  a  single 
bacterial  cell  have  the  potentiality  of  producing  more  than  280  trillion 
individuals  in  24  hours.  At  this  rate,  the  descendants  would  form  a 
mass  of  bacterial  protoplasm  as  large  as  the  earth  every  few  days. 
Even  large  types  of  animals,  such  as  elephants,  which  reproduce  at  a 
comparatively  slow  rate  will  in  time  overrun  the  earth  if  optimum 
conditions  are  provided. 

And  so,  with  diverse  types  of  living  orgaiiisms  invading  every  nook 
and  cranny  of  this  earth  where  adequate  food  supplies  may  be  secured 


446  HUMAN  BIOLOGY 

and  where  the  environmental  conditions  permit  the  maintenance  of  the 
living  processes,  it  becomes  apparent  that  the  term  life  pressure, 
which  has  been  used  by  eeologists  to  indicate  the  force  directing  the 
organisms  into  a  possible  environment,  is  a  very  apt  one.  It  will  be 
well  at  this  point  to  consider  the  question  of  environmental  relation- 
ships, for  a  close  relationship  always  exists  between  a  particular 
environment  and  the  organisms  subjected  to  it.  Basically,  of  course, 
certain  fundamental  requirements  must  be  supplied  by  every  environ- 
ment, since  they  are  essential  to  life  as  we  know  it.  These  require- 
ments may  be  listed  as  follows:  (1)  suitable  temperature;  (2)  necessary 
elements  for  the  growth  and  repair  of  protoplasm;  (3)  suitable  con- 
ditions for  the  formation  of  the  carbon  compounds,  the  basic  one  being 
carbon  dioxide;  (4)  the  presence  of  a  liquid  or  water  environment,  the 
latter  being  the  principal  constituent  of  all  living  things. 

Speaking  generally,  the  earth  supplies  these  basic  requirements  in 
abundance.  It  is  always  of  interest  to  the  biologist  to  speculate  on 
the  possibility  of  the  distribution  of  these  primary  life  requirements 
through  the  unknown  spaces  of  the  universe.  The  opinion  of  those 
best  fitted  to  know  appears  to  be  quite  unanimous  that  the  conditions 
necessary  for  the  maintenance  of  life  must  be  very  closely  restricted 
and  possibly  are  present  only  on  this  tiny  pin  point  of  matter,  com- 
pared with  the  universe  as  a  whole,  which  we  designate  as  the  earth. 
The  following  quotation  from  the  noted  British  scientist,  Sir  James 
Jeans,  summarizes  the  situation  as  he  sees  it.  He  says: 

The  physical  conditions  under  which  life  is  possible  form  only  a  tiny 
fraction  of  the  range  of  physical  conditions  which  prevail  in  the  universe  as  a 
whole.  The  very  concept  of  life  implies  duration  in  time ;  there  can  be  no  life 
where  the  atoms  change  their  make-up  millions  of  times  a  second  and  no  pairs 
of  atoms  can  ever  become  joined  together.  It  also  implies  a  certain  mobility 
in  space,  and  these  two  implications  restrict  life  to  the  small  range  of  physical 
conditions  in  which  the  liquid  state  is  possible.  Our  survey  of  the  universe 
has  shown  how  small  this  range  is  in  comparison  with  the  range  of  the  whole 
universe.  Primeval  matter  must  go  on  transforming  itself  into  radiation  for 
millions  of  millions  of  years  to  produce  an  infinitesimal  amount  of  the  inert 
ash  on  which  life  can  exist.  Even  then  this  residue  of  ash  must  not  be  too  hot 
or  too  cold  or  life  will  be  impossible.  It  is  difficult  to  imagine  life  of  any  high 
order  except  on  planets  warmed  by  a  sun,  and  even  after  a  star  has  lived  its 
life  of  millions  of  years,  the  chance,  so  far  as  we  can  calculate  it,  is  still  about  a 
hundred  thousand  to  one  against  its  being  a  sun  surrounded  by  planets.  In 
every  respect — space,  time,  physical  conditions — life  is  limited  to  an  almost 
inconceivably  small  corner  of  the  universe. 

The  earth  not  only  supplies  the  basic  requirements  of  the  living 
state  in  abundance,  but,  as  was  pointed  out  some  years  ago  by  L.  J. 


THE  WEB  OF  LIFE  447 

Henderson  in  his  noteworthy  book  "  The  Fitness  of  the  Environment/'1 
there  is  a  maximum  fitness  in  the  earth-environment  fitness.  He  has 
summarized  his  views  on  the  matter  as  follows: 

The  fitness  of  the  environment  results  from  characteristics  which  con- 
stitute a  series  of  maxima — unique  or  nearly  unique  properties  of  water,  car- 
bonic acid,  the  compounds  of  carbon,  hydrogen,  and  oxygen  and  the  ocean-*- 
so  numerous,  so  varied,  so  nearly  complete  among  all  things  which  are  con- 
cerned in  the  problem  that  together  they  form  certainly  the  greatest  possible 
fitness.  No  other  environment  consisting  of  primary  constituents  made  up 
of  other  known  elements,  or  lacking  water  and  carbonic  acid,  could  possess  a 
like  number  of  fit  characteristics,  or  in  any  mariner  such  great  fitness  to  pro- 
mote complexity,  durability,  and  active  metabolism  in  the  organic  mechanism 
which  we  call  life. 

THE  BIOTIC  ENVIRONMENT 

The  previous  discussion  has  emphasized  the  abundance  of  life  an<£ 
the  relative  scarcity  of  food,  and  we  have  seen  that  the  latter  is  a 
decisive  limiting  factor  which  definitely  restricts  living  organisms. 
It  is  evident  that,  in  this  world  of  today,  a  life  pressure  forces  organ- 
isms into  every  possible  environment.  It  must  also  be  emphasized 
that  the  environment  is  not  wholly  lifeless;  there  is  a  living  or  biotic 
environment  which  is  possibly  of  equal  importance.  Life  presses  on 
life!  In  fact,  to  a  considerable  extent,  the  environment  of  any  organ- 
ism consists  of  other  living  organisms.  The  same  quest  for  adequate 
food  and  shelter  that  forces  them  into  every  possible  position  in  the 
inorganic  world  from  mountain  tops  to  ocean  depths  also  forces  them 
into  all  sorts  of  diversified  interrelationships  with  other  living  organ- 
isms— the  biotic  environment.  The  associations  thus  formed  among 
living  organisms  may  be  only  transitory  and  casual,  or  they  may  be 
obligatory  and  accompanied  by  marked  structural  and  functional 
adaptations  which  make  independent  survival  impossible.  For  our 
present  consideration,  the  various  biotic  associations  in  the  living 
world  may  be  assembled  in  four  main  types,  namely,  the  communal, 
the  commensal,  the  symbiotic,  and  the  parasitic,  which  will  be  con- 
sidered in  the  order  named. 

Communal  Associations. — These  may  be  regarded  as  beneficial 
groupings  of  individuals  of  the  same  species  to  form  various  types 
of  casual  and  obligatory  association.  Casual  associations  are  seen  in 
flocks,  herds,  droves,  and  even  in  human  communities,  all  of  which  are 
more  or  less  variable.  The  individuals  thus  associated  are  free  to  come 

1The  Macmillan  Company. 


448  HUMAN  BIOLOGY 

and  go,  while  the  group  as  a  whole,  which  has  been  found  helpful, 
persists.  Communal  associations  also  include  colonies  of  various  types, 
composed  of  individuals  that  are  structurally  modified  so  that  member- 
ship in  the  colony  is  obligatory;  an  individual  so  adapted  cannot  long 
survive  away  from  the  colony.  Many  striking  examples  of  this 
condition  are  to  be  found  in  the  societies  or  colonies  developed  by 
various  of  the  so-called  social  insects,  such  as  the  wasps,  bees,  ants,  and 
termites.  (Figs.  235,  236.) 

Take  one  of  the  most  common  examples  of  insect  colonies,  as 
shown  by  the  honeybee.  A  colony  of  bees  in  a  hive  consists  of  three 
types  of  individuals  which  exhibit  distinct  structural  and  functional 
adaptations.  There  is  one  fertile  female,  the  queen,  on  which  the 
life  of  the  colony  depends  because  she  is  the  only  one  capable  of  pro- 


Worker  Queen  Drone 

Fio.  235. — The  honeybee.      X  2.     (Wieman,  after  Phillips.     U.  S.  Dept.  Agr.,  Farmers' 

Bulletin  447.) 

ducing  new  individuals.  The  death  of  the  queen,  therefore,  means  the 
death  of  the  colony  as  the  older  individuals  continue  to  die  off.  There 
are  several  hundred  mature  males  or  drones  in  an  active  colony,  and 
one  of  these  mates  with  the  young  virgin  queen  during  the  nuptial 
flight  which  takes  place  following  swarming.  All  of  the  drones  are 
expelled  from  the  hive  at  the  close  of  the  active  season  in  the  fall  so 
that  only  the  queen  and  workers  remain  as  members  of  the  colony 
during  the  inactive  season.  The  workers  carry  on  all  the  activities  of 
the  hive  with  the  exception  of  reproduction.  They  are  infertile 
females  and  never  attain  sexual  maturity.  It  has  long  been  held 
that  the  queen  may  lay  either  fertilized  or  unfertilized  eggs.  The 
latter  undergo  parthenogenetic  development,  giving  rise  to  the  drones, 
while  the  fertilized  eggs  are  potentially  queens  or  workers.  The 
development  of  queen  or  worker  appears  to  be  determined  by  the  food 
supplied  to  the  female  larvae  during  early  stages  by  the  nurse-workers, 


THE  WEB  OF  LIFE 


449 


but  there  is  also  the  possibility  that  hormonal  secretion  may  be 
involved. 

Young  queens  are  desired  only  when  the  colony  becomes  so  large 
that  the  hive  is  crowded.  Under  this  condition,  workers  construct  a 
special  queen  cell,  in  which  the  queen  lays  a  fertilized  egg.  The 
attainment  of  sexual  maturity  by  the  young  virgin  queen  is  the  signal 
for  swarming.  The  main  feature  of  this  phenomenon  is  the  departure 
of  the  old  queen  from  the  parental  hive,  accompanied  by  some  thou- 
sands of  workers  who  remain  her  loyal  subjects.  The  swarm  seeks  a 


a 


FIG.  236.  FIG.  237. 

FIG.  236.— Various  types  of  cells  in  the  comb  of  the  honeybee.  Three  large  queen 
cells,  numerous  honey  cells  (capped),  and  worker  brood  cells  (uncapped)  are  shown. 
(Wieman,  after  Phillips.  U.  5.  Dept.  Agr.,  Farmers'  Bulletin  447.) 

FIG.  237. — Development  of  the  honeybee,  a,  egg;  b,  young  larva;  c,  old  larva 
just  before  pupation;  d,  pupa.  X  3.  (Wieman,  after  Phillips.  U.  £.  Dept.  Agr., 
Farmers'  Bulletin  447.) 

new  hive;  and  when  a  suitable  one  is  found,  normal  routines  are  once 
more  established.  And  now  the  virgin  queen  leaves  the  old  hive  on 
her  nuptial  flight,  accompanied  by  the  drones.  Mating  occurs  with 
one  of  the  drones  during  the  flight,  and  then  the  young  queen  returns 
to  the  parental  hive  and  begins  her  reign  over  the  less  adventurous 
workers  who  remained  behind  when  swarming  took  place.  (Fig.  237.) 
The  duties  of  the  workers  are  almost  legion  in  number,  since  they 
are  responsible  for  the  maintenance  of  the  hive,  supplying  food,  nursing 
the  larvae,  and  protecting  the  interests  of  the  colony  in  every  way. 
The  life  of  the  colony  depends  upon  the  collection  of  various  substances 
from  flowering  plants.  Among  these  is  nectar,  a  scented  liquid  rich  in 
sugar  which,  slightly  modified  by  the  evaporation  of  some  of  the  water, 


450 


HUMAN  BIOLOGY 


is  stored  as  honey  in  the  cells  of  the  wax  honeycomb.  Abundant 
collections  are  also  made  of  pollen.  This  activity  is  essential  to  bees 
and  plants  alike.  Since  pollen,  or  bee  bread,  is  rich  in  nitrogenous 
substances  which  are  not  present  in  honey,  it  is  an  essential  bee  food. 
For  the  flowers,  pollen  is  an  essential  element  in  sexual  reproduction 
as  it  contains  the  male  nuclei.  The  distribution  of  pollen  from  flower 
to  flower,  by  bees  and  other  insects,  is  required  for  cross-fertilization. 
An  important  and  interesting  commensal  association  exhibited 
between  insects  and  flowering  plants  is  considered  below  (page  452). 
Finally,  the  workers  collect  resinous  secretions  of  various  plants  which, 


FIG.  238. — Example  of  commensalism  existing  between  hermit  crab  and  sea  ane- 
mones. Frequently,  the  anemones  completely  rover  the  mollusc  shell  which  the  crab 
has  appropriated.  (Wieman.) 

as  bee  glue,  or  propolis,  are  used  to  cover  the  interior  on  the  hive  and 
to  fill  the  cracks,  much  as  paint  and  putty. 

Commensal  Associations. — In  the  present  discussion,  this  large 
group  of  associations  will  be  restricted  to  external  partnerships  between 
individuals  of  diverse  species,  which  are  of  mutual  benefit.  The  sym- 
biotic associations  considered  below  have  the  same  general  basis  but 
are  essentially  more  intimate,  since  the  association  is  internal  rather 
than  external.  Commensalism  may  be  obligatory  but  frequently 
appears  to  be  purely  casual  and  accidental  in  nature.  A  classic  exam- 
ple of  this  condition  is  seem  in  the  well-known  partnership  frequently 
found  between  hermit  crabs  and  sea  anemones.  The  latter  attach 
themselves  to  the  mollusk  shell  which  the  hermit  crab  has  appropriated 
as  a  trailer  home  and  are  carried  from  place  to  place  as  the  crab  searches 
for  his  daily  food  supply.  Crumbs  from  the  latter  are  noteworthy 


THE  WEB  OF  LIFE 


451 


additions  to  the  diet  of  the  anemones.  The  advantages  of  this  arrange- 
ment appear  to  be  all  with  the  anemones,  and  possibly  this  is  the  condi- 
tion, but  the  general  belief  is  that  the  batteries  of  stinging  cells  borne 
by  the  anemones  serve  as  highly  desirable  arsenals  for  the  defense  of 
the  hermit  crab.  The  crabs  and  anemones  survive  irr  the  absence  of 


FIG.  239. — The  corn-root  aphid  (Anuraphia).  A,  winged  form;  B,  wingless  form; 
both  enlarged.  C,  diagram  illustrating  the  care  of  the  adults,  underground,  by  the 
ants  during  winter.  In  the  spring  the  aphids  are  placed  on  the  young  corn  plants. 
(Wolcott,  after  Davis.  U.  S.  Dept.  Agr.,  Farmers'  Bulletin  891.) 

their  casual  commensalism,  but  probably  they  are  more  successful  in 
life's  battles  when  they  are  associated.     (Fig.  238.) 

Another  frequently  cited  example  of  commensalism  is  found  in  the 
relations  existing  between  certain  species  of  ants  and  the  plant  lice, 
or  aphids.  The  latter  secure  their  nourishment  by  piercing  young 
plant  tissues  with  their  specialized  mouth  parts  and  sucking  up  the 
cellular  juices.  The  aphids  convert  much  of  the  ingested  food  into  a 


452  HUMAN  BIOLOGY 

nutritious  "honey dew,"  which  the  ants  find  highly  desirable  for  their 
nutrition.  Accordingly  the  ants  endeavor  to  maintain  herds  of  the 
aphid  "ant-cows,"  thus  applying  the  same  principle  that  man  does  with 
his  herds  of  milk  cows.  In  some  instances,  as  in  the  corn-root  aphis, 
the  ants  maintain  the  aphids  in  their  own  colonies  during  the  winter. 
In  the  fall,  the  aphids  lay  eggs  in  the  galleries  of  the  ant  colonies, 
which  hatch  in  the  spring.  The  young  aphids  are  carefully  nurtured 
by  the  ants  and,  at  the  proper  time,  are  placed  on  the  corn  roots. 
Here  the  aphids  begin  to  feed  and  to  produce  the  honeydew  for  their 
owners.  The  ants  lick  the  honeydew  from  the  leaves,  where  it  was 
secreted  by  the  aphids,  or  they  may  "milk"  the  aphids  by  stroking 
them  with  their  antennae  and  thus  secure  the  droplets  of  liquid  food 
just  as  they  are  released.  (Fig.  239.) 

Some  of  the  most  interesting  commensal  relationships  have  been 
established  between  insects  and  flowering  plants.  These  provide  the 
insects  with  an  abundant  food  supply  of  pollen,  which  is  very  rich  in 
protein,  and,  at  the  same  time,  as  noted  above  with  the  honeybee, 
insure  cross-fertilization  for  the  plants  by.means  of  the  pollen  grains 
containing  the  male  nuclei,  which  the  insects  carry  from  flower  to 
flower.  Insects  are  often  attracted  to  the  flowers  by  the  scented 
nectarg  that  also  serve  as  food.  Nectar,  slightly  modified,  is  stored  as 
honey  and  is  a  highly  nutritious  energy  food.  The  commensal  rela- 
tionships between  insects  and  flowers  may  be  purely  casual,  as  seen 
in  the  honeybee  which  collects  nectar  and  scatters  pollen  from  a  wide 
variety  of  flowers.  Or  the  insect-flower  relationship  may  be  obliga- 
tory, as  shown  for  example,  in  the  yucca  plant  and  the  Pronuba  moth, 
where  it  is  associated  with  an  almost  unbelievable  degree  of  specializa- 
tion. (Fig.  240.) 

There  are  several  species  of  liliaceous  plants  belonging  to  the  genus 
Yucca,  each  of  which  is  dependent  for  fertilization  upon  the  females 
of  a  particular  species  of  Pronuba.  The  pronuban  females,  unlike  any 
other  moth,  are  provided  with  prehensile  mouth  parts  adapted  for 
grasping  and  also  with  a  peculiar  egg-laying  apparatus,  or  ovipositor, 
which  may  be  used  to  penetrate  the  delicate  plant  tissues  of  the  yucca 
flower.  Even  more  remarkable  than  the  structural  adaptations  of  this 
amazing  insect  are  the  instincts  directed  toward  insuring  the  fertiliza- 
tion of  the  plant  in  order  to  produce  seeds,  a  certain  portion  of  which 
are  used  as  food  by  the  larval  insects.  The  yucca  blossoms  are  open 
at  night.  The  night-flying  female  moth  enters  a  flower  soon  after 
dark  and  begins  to  collect  the  pollen  grains  from  the  stamens.  The 
sticky  pollen  grains  are  carefully  formed  into  tiny  pellets.  When 
sufficient  pollen  has  been  collected,  she  forms  a  hole  with  the  ovipositor 


THE  WEB  OF  LIFE 


453 


in  the  pistil  of  the  flower  in  which  the  seeds  are  produced  and  lays  one 
or  more  eggs  in  close  proximity  to  the  embryonic  seeds.  The  produc- 
tion of  seeds  with  the  stored  food  depends,  of  course,  upon  pollination. 
Accordingly,  the  female  moth,  having  laid  the  eggs,  next  insures  seed 
development  by  inserting  the  previously  collected  pollen  balls  in  the 


FIG.  240. — Photograph  of  the  Yucca  in  flower.     The  plant  may  reach  about  18  ft.  in 
height.     Southern  California.     (Plaupt.) 

tip,  or  style,  of  the  pistil.  Here  the  pollen  grains  germinate,  and 
the  male  nuclei  later  unite  in  fertilization  with  the  female  nuclei  of  the 
embryonic  seeds.  The  seeds,  with  a  large  amount  of  food  available 
for  the  insect  larvae,  soon  develop.  It  is  important  to  note  that  the 
female  moth  lays  only  a  few  eggs  in  the  pistil  of  any  one  flower  so  that 


454 


HUMAN  BIOLOGY 


the  larva  do  not  require  all  of  the  seeds  for  their  nutrition,  the  remainder 
being  available  for  the  propagation  of  the  plant. 

Symbiotic  Associations. — The  term  symbiosiSj  which  literally  means 
"living  together,"  will  be  restricted  in  the  present  discussion  to 
mutually  beneficial  internal  partnerships  between  diverse  species. 
Thus,  as  a  rule,  the  symbiotic  associations  are  much  more  intimate  in 
nature  than  in  the  commensalism  noted  above.  Some  of  the  best 


FIG.  241. —  Drawing  illustrating  vertical  section  through  the  tissues  of  a  lichen 
(Physcia).  The  green  alga  cells  (stippled)  are  seen  to  be  surrounded  by  interlacing 
fungus  filaments;  the  latter  form  the  main,  body  of  the  lichen.  X  500.  (Haupt.) 

examples  of  the  symbiotic  conditions  are  found  in  the  plant  world. 
Among  these,  the  classic  example  ot  a  symbiotic  plant,  the  lichen,  may 
be  selected  for  consideration.  Microscopic  examination  of  the  lichen 
plant  body  shows  that  it  consists  of  a  filamentous  fungous  plant  living 
in  close  association  with  a  unicellular  green  alga.  The  intertwining 
fungal  filaments  form  the  mass  of  the  body  of  the  lichen  but  with 
numerous  green  alga  cells  interspersed.  And  so,  the  lichen  is  a  "  double 
plant, "  since  it  consists  of  two  distinct  plant  types  living  together  and 


THE  WEB  OF  LIFE 


455 


forming  a  composite  organism  in  which  each  retains  its  identity.  It  is 
generally  held  that  this  association  between  fungus  and  lichen  is  of 
mutual  benefit.  Possibly,  however,  the  partnership  is  not  on  an  equal 
basis,  for  the  continued  life  of  the  fungus  is  entirely  dependent  upon 
the  photosynthetic  activities  of  the  alga,  whereas  the  latter  can  survive 
independently.  At  all  events,  the  lichens  are  a  very  successful  type  of 
plant  organism  for  they  can  survive  in  exposed  surfaces,  as  on  rocks, 
where  no  other  plant  life  is  possible.  Accordingly,  they  are  the 
pioneers  in  the  colonization  of  a  new  region  and  pave  the  way  for  the 
later  immigration  of  the  more  highly  developed  types  of  plants  as  soil 
conditions  become  suitable.  (Fig.  241.) 

Another  important  symbiotic  plant  association  is  found  in  the 
Mycorrhiza,  or  root-fungi,  which  have  developed 
a  symbiotic  relationship  with  various  important 
trees,  such  as  the  oaks  and  beeches.  The  growth 
of  the  root  fungi  forms  a  felt-like  covering  over 
the  root  tissues  from  which  they  receive  nutritive 
materials.  At  the  same  time,  they  make  various 
essential  salts  from  the  soil  available  to  the  host 
tree.  Another  important  symbiotic  relationship 
between  green  and  colorless  plants  occurs  in  the 
roots  of  the  so-called  leguminous  plants,  such  as 
the  pea,  bean,  alfalfa,  and  clover,  which  harbor 
certain  bacteria,  the  symbiotic  nitrifiers.  These 
nitrogen-fixing  bacteria  live  in  great  numbers 
in  special  root  tubercles  where  they  are  protected 
and  nourished  by  nutritive  materials  from  the 
host  tissues.  The  importance  of  the  nitrogen-fixing  bacteria  to  the 
green  plants  and  to  man  lies  in  the  fact  that  they  are  able  to  synthesize 
soluble  nitrogenous  compounds  from  the  inert  nitrogen  of  the  air. 
These  nitrogenous  compounds  are  utilized  in  the  synthesis  of  plant 
proteins  which  are  available  for  animal  food  or  for  the  enrichment  of 
the  soil  if  the  leguminous  plants  are  allowed  to  remain  and  decay  later 
takes  place.  (Fig.  242.) 

But  symbiotic  associations  are  by  no  means  restricted  to  the  plant 
world;  they  exist  between  plants  and  animals  and  also  between  various 
species  of  animals.  One  of  the  most  common  examples  of  a  symbiotic 
plant-animal  relationship  is  to  be  noted  in  the  ubiquitous  green  hydra. 
The  green  color  of  hydra  is  due  to  the  presence  of  a  holophytic  sym- 
biont,  the  unicellular  alga  Chlorella,  which  occurs  in  great  numbers, 
Now,  the  interesting  fact  is  that  these  tiny  plant  cells,  though  actually 
living  within  the  body  walls  of  the  hydra,  are  not  injurious.  On  the 


FIG.  242. — Root  tip 
of  beech  covered  with 
the  micorrhiza  fila- 
ments. (Woodruff, 
after  Pfeffer.) 


456  HUMAN  BIOLOGY 

contrary,  it  is  clear  that  the  association  between  plant  and  animal  is 
of  mutual  benefit,  for  the  metabolic  wastes  of  the  hydra  cells  are  needed 
by  the  plant  for  photosynthesis.  On  the  other  hand,  the  excess  oxygen 
liberated  by  the  Chlorella  cells  during  photosynthesis  is  utilized  by  the 
hydra  cells  in  respiration.  The  photosynthetic  activities  of  the  alga 
cells  necessarily  cease  when  the  animals  are  placed  in  the  dark.  Such 
hydra  soon  lose  their  green  color  and,  though  they  are  able  to  survive 
if  food  is  available,  do  not  show  so  great  vitality  as  those  kept  in  light 
in  which  the  symbiotic  condition  is  maintained.  (Fig.  147.) 

Parasitic  Associations. — A  parasite  may  be  denned  as  an  organ- 
ism that  lives  at  the  expense  of  another  organism,  the  host.  Thus  the 
host-parasite  relationship  is  not  one  of  mutual  advantage,  as  exempli- 
fied in  the  various  types  of  biological  associations  previously  noted, 
but  the  balance  turns  toward  the  parasite.  No  sharp  line  of  division 
can  be  drawn  between  symbiosis  and  parasitism  but  rather  a  gradual 
shading  from  a  mutually  beneficial  condition  to  one  that  is  slightly 
parasitic,  with  the  series  ending  finally  in  a  parasitic  association  in 
which  the  parasite  contributes .  nothing  to  the  host  and  takes  all. 
Parasitic  organisms  may  be  divided  into  two  groups:  external  para- 
sites (ectoparasites)  and  internal  parasites  (endoparasites).  In 
general,  an  ectoparasite  exhibits  relatively  slight  adaptation  for  the 
parasitic  relationships,  and  the  association  with  a  particular  host 
species  is  more  or  less  casual  and  transitory.  Thus  such  external 
parasites  of  man  as  the  mosquito,  flea,  or  louse  find  it  possible,  when 
the  occasion  demands,  to  supply  their  nutritive  requirements  from 
various  other  host  species.  Or  the  destructive  insect  pests  of  trees 
may  prefer  a  certain  species;  failing  that,  they  will  find  the  plant  tis- 
sues of  other  species  hardly  less  suitable. 

Contrariwise,  the  typical  endoparasite  shows  marked  structural 
and  functional  adaptations  which  make  it  necessary  for  it  to  inhabit 
the  internal  tissues  of  a  particular  host  species.  Thus  the  endopara- 
site is  usually  an  obligate  parasite,  since  survival  is  not  possible  except 
in  one  host  species.  Furthermore,  many  of  the  obligatory  endopara- 
sites have  complicated  life  histories  which  may  involve  obligatory 
habitation  in  a  certain  host  species  at  one  time  and  in  one  or  more 
separate  species  at  another  period  in  the  life  cycle.  Typically,  the 
type  of  reproduction  in  the  parasite  varies  from  sexual  to  asexual  or 
vice  versa  when  the  parasite  passes  to  the  new  host.  In  the  malaria 
parasite,  for  example,  the  reproduction  is  entirely  asexual  in  man;  but 
in  the  body  of  the  mosquito,  sexual  reproduction  i&  encountered.  In 
general,  the  structural  changes  associated  with  parasitism  are  markedly 
degenerative  ID  nature.  This  condition  is  particularly  evident  in  the 


THE  WEB  OF  LIFE 


457 


nutritive  and  sensory  organs  of  the  parasite,  which  are  essentially 
without  function  since  those  of  the  host  supply  both.  On  the  other 
hand,  there  is  usually  a  tremendous  elaboration  of  the  reproductive 


To  cerebrospinal  fluid 
causing  sleeping  sickness  and  death 


Trypanosomes^ 

in  human  blood 

causing  Trypanosome  fever 


Infection  of  man  by  _ 
bite  of  tsetse  fly 


Man,  Antelope,  etc* 


Tsetse  Fly 


Transmission  by 
bite  of  tsetse  fly 


Forms  in  salivary  glands 

ready  for  transmission  to  man 

(20th- 30th  day) 


Crithidial  forms  hi 

salivary  glands 
(2  or  3  days  later) 


Forms  in  mid  gut  of  fly 
(48  hrs.  after  infective  meal) 


Newly  arrived 
trypanosomes  in 

salivary  gland 
(12th  to  20th  days) 


Long,  slender  forms  in  proventriculus 

(about  10th  to  15th  days) 

FIG.  243. — Diagram  illustrating  the  life  history  of  Trypanosoma  gambiense,  respon- 
sible for  African  sleeping  sickness.  (Chandler,  "Animal  Parasites  in  Human  Disease" 
John  Wiley  &  Sons,  Inc.) 

mechanism  so  that  the  highly  specialized  endoparasite  is  little  more 
than  a  mechanism  for  the  production  of  germ  cells. 

Wide  variation  is  also  found  in  the  so-called  host-parasitic  rela- 
tionship, primarily  with  reference  to  the  host  tolerance.     It  is  erident 


458  HUMAN  BIOLOGY 

to  the  parasitologist  that  a  correlation  exists  between  the  degree  of 
tolerance  and  the  length  of  time  that  the  association  between  a  par- 
ticular host  species  and  the  parasite  species  has  persisted.  In  host- 
parasite  relationships  of  long  standing,  tolerance  of  the  parasite  by 
the  host  seems  to  have  developed.  Under  the  conditions,  the  para- 
sites secure  a  good  living  from  the  host,  but  their  numbers  do  not 
become  so  great  that  they  destroy  vital  organs  and  thus  kill  the  host 
and  make  it  necessary  to  secure  a  new  home.  A  well-known  example 
of  this  condition  is  seen  in  the  relations  existing  between  certain  blood- 
dwelling  protozoa,  the  trypanosomes,  which  are  the  causative  agents 
of  African  sleeping  sickness,  and  the  domesticated  animals  in  the 
affected  regions.  The  latter  harbor  considerable  numbers  of  these 
parasites  in  their  blood,  but  the  host-parasite  relationship  is  such  that 
the  parasitic  infection  is  in  some  way  kept  down  to  a  point  where  fatal 
injury  is  not  sustained  by  the  host.  Individuals  of  the  same  host 
species  brought  into  this  region  from  areas  outside  the  sleeping-sickness 
zone  will  quickly  become  parasitized  with  the  blood-dwelling  trypano- 
somes. But,  under  these  circumstances,  the  parasites  increase  with 
great  rapidity  in  the  foreign  hosts  and  soon  kill  them.  In  general, 
then,  it  seems  probable  that  the  death  of  a  host  by  parasitic  invasion 
indicates  a  relatively  new  host-parasite  relationship.  (Fig.  243.) 

It  may  be  opportune  at  this  point  to  indicate  certain  distinctions 
between  a  parasitic  organism  and  a  predator.  A  predator  is  an  animal 
that  preys  upon  and  speedily  kills  individuals  of  the  same  or  other 
types,  which  are  suitable  for  food,  as  in  the  case  of  the  flesh-eating, 
or  carnivorous,  animals.  When  the  predator  again  becomes  hungry, 
it  repeats  the  process.  The  true  predator  is  therefore  different  in  its 
behavior  from  that  of  the  true  parasite  which,  as  just  noted,  is  best 
served  when  the  host  is  long-lived. 

Parasitism  is  very  widespread  in  the  living  world.  Exact  data  are 
not  easily  obtained,  but  a  conservative  estimate  would  probably  show 
that  at  least  50  per  cent  of  the  plant  and  animal  species  supply  all  or 
part  of  their  nutritive  requirements  by  parasitizing  other  species. 
In  the  plant  kingdom,  as  would  naturally  be  expected,  most  of  the 
parasites  are  found  among  the  colorless  plants,  which,  as  has  been 
shown  earlier,  are  dependent  upon  the  organic  foodstuffs  for  their 
nutrition  just  as  are  animals,  and  they  secure  the  latter  from  dead 
tissues  of  animals  and  plants,  or,  in  the  many  parasitic  species,  from 
the  tissues  of  living  organisms.  In  either  instance,  the  essentials  of 
the  nutritive  activities  are  unchanged;  that  is,  Fungi,  whether  sapro- 
phytic  or  parasitic,  supply  their  nutritive  requirements  by  subjecting 
complex  substances  to  extracellular  digestion,  as  described  above  for 


THE  WEB  OF  LIFE 


459 


bread  mold.  From  the  standpoint  of  animal  parasitism,  the  unicellu- 
lar fungi,  in  particular,  the  bacteria,  are  of  the  greatest  importance. 
The  major  diseases  that  affect  mankind  are  for  the  most  part  due  to 
bacterial  invasions  of  various  tissues  and  organs  of  the  body.  Among 
these  are  such  important  diseases  as  typhoid,  tuberculosis,  diphtheria, 
anthrax,  various  virulent  streptococci  infections,  and  a  host  of  others. 
Hardly  less  important  to  man  are  numerous  other  fungal  parasites  that 
produce  disease  and  destruction  among  domesticated  animals  or  in 
important  domesticated  plant  types  such,  for  example,  as  the  wheat 
rust,  chestnut  blight,  white  pine  blister,  and  the  comparatively  recent 
Dutch  elm  disease.  (Fig.  244.) 

SUMMER 


Th*  Clmlct  Cup  Ste««  of  R«nt 
Develop*  en  Uit  B«(b«ny  LMYW 


7)»  BUd,  Ste««  el  fttist  on  $*•»,  Stubfcl. 
*nd  Wild  Gr»u«  in  Wlnttr 

FIG.  244. — Diagrams  illustrating  the  life  history  of  the  wheat  rust,  an  important  para- 
site.    (E.  T.  Smith,  "Exploring  Biology"     After  U.  S.  Dept.  Agr.) 

Though  much  less  common,  parasitism  is  also  in  evidence  among 
the  chlorophyll-bearing  plants.  This  is  very  interesting  because, 
when  it  occurs,  the  amount  of  chlorophyll  is  correspondingly  reduced. 
Three  well-known  examples,  as  seen  in  the  mistletoe,  dodder,  and 
rafflesia,  will  be  sufficient  to  emphasize  the  relationship  between  para- 
sitism and  chlorophyll  and  the  changes  associated  with  parasitism. 

The  mistletoe,  which  has  found  such  favor  at  the  holiday  season, 
is  partially  parasitic.  It  fastens  itself  upon  the  host  tree  and  develops 
highly  specialized  peg-like  roots,  the  haustoria,  which  push  through 
the  outer  bark  and  into  the  underlying  vascular  tissues.  Through 
the  haustoria,  the  mistletoe  secures  essential  supplies  of  water  and  dis- 
solved salts  from  the  host  and  builds  these  up  into  food  materials  by 


460 


HUMAN  BIOLOGY 


FIG,  245. — Mistletoe,  a  parasite 
on  various  deciduous  trees. 


its  own  photosynthetic  apparatus.     It  is,  then,  parasitic  in  that  the 

raw  materials  necessary  for  nutrition  are 
taken  from  the  host.  A  rather  common 
weed,  the  dodder,  exhibits  a  greater 
degree  of  parasitism  in  the  adult  stage 
when  it  becomes  entirely  dependent  upon 
the  host  plant  for  the  essential  foodstuffs. 
In  the  early  stages  of  development,  the 
dodder  is  an  independent  plant  growing 
in  the  soil,  with  chlorophyll  and  active 
photosynthesis.  Increasingly,  however, 
as  the  plant  matures,  the  chlorophyll 
disappears,  and  the  dodder,  twining 
around  a  host  plant,  develops  haustoria 
which  invade  the  host  tissues.  In  time, 
the  plants  completely  lose  connection 
with  the  soil  and  become  entirely  parasitic  upon  the  host  plant. 
(Figs.  245,  246.) 

Parasitism  among  the  higher 
plants  probably  reaches  a  climax  in 
the  tropical  plant,  Raffiesia  arnoldii, 
which  is  closely  restricted  in  its 
distribution  to  Sumatra.  The  de- 
generative changes,  typically  as- 
sociated with  the  endoparasitic 
condition,  are  nowhere  more  strik- 
ingly illustrated  than  in  this 
parasitic  spermatophyte.  It  has 
entirely  lost  the  chlorophyll-bearing 
tissues,  and,  in  addition,  the  char- 
acteristic plant  body,  with  root, 
stems,  and  leaves,  has  been  trans- 
formed into  a  mass  of  colorless 
filaments.  These  lie  under  the 
bark  of  the  host  tree,  ramify 
through  the  host  tissues,  and  con- 


B  A 

FIG.  246. — The  dodder. 


C,  young 


tinUOUSly  rob  them  of  a   portion   of      »eedli^s;    \  mature   parasite   twining 

^  ^  around  the  host  plant;  B,  microscopic 

their      nutritive      materials.      Thus      section  of  the  host  tissues  with  dodder 

rafflesia  is  entirely  dependent  upon    tissue  attached  by  haustoria.    (After 

J         *  \  Strasberger.     Redrawn  by  L.  Krause.) 

the    host.     But    the    reproductive 

organs  have  become  greatly  enlarged,  and  this  parasite  develops  the 

largest  known  flowers,  measuring  as  much  as  3  ft.  in  diameter  and 


THE  WEB  OF  LIFE 


461 


weighing  some  25  Ib.  There  are  no  external  indications  of  the  pre- 
sence of  rafflesia  in  the  host  plant  until  the  parasite  blooms  and  the 
enormous  blossom  breaks  through  the  bark.  Fertilization  and  seed 
formation  can  thus  be  effected.  It  is  clear  that  rafflesia  exhibits  a 
complete  adaptation  to  the  parasitic  life.  (Fig.  247.) 

In  the  Animal  kingdom,  as  a  whole,  parasitism  is  widely  distributed 
and  apparently  more  common  than  in  plants.     Parasitic  species  are 


Ewing  Galloway 
FIG.  247. — The  enormous  flower  of  Rafflesia,  the  parasitic  seed  plant. 

known  to  occur  in  every  animal  phylum  with  the  exception  of  two: 
the  Porifera  (sponges)  and  the  Echinodermata  (starfish,  sea  urchins, 
etc.).  However,  from  the  standpoint  of  parasitism,  the  four  most 
important  phyla  are  the  Protozoa,  Platyhelminthes,  Nemathelminthes, 
and  Arthropoda,  all  containing  numerous  important  parasitic  species 
which  infect  man  and  important  domesticated  animals  and  produce 
many  virulent  and  infective  diseases.  The  description  of  parasitism 
in  aberrant  plant  types,  as  given  in  the  preceding  paragraphs,  strongly 


462 


HUMAN  BIOLOGY 


HEAD  OR  SCO  LEX 


emphasizes  the  degenerative  changes  that  accompany  increasing 
adaptation  to  parasitism,  with  the  fully  adapted  parasite  existing 
solely  for  its  reproductive  activities  and  bleeding  the  host  white  to 
secure  nutritive  materials  for  conversion  into  the  reproductive  elements 
of  the  parasite. 

The  varities  of  animal  parasites  are  legion  in  number,  with  every 
conceivable  modification  of  body  plan.  It  will  be  possible  to  indicate 
only  three  or  four  common  examples  which  will,  perhaps,  give  some 
inkling  of  the  condition  associated  with  animal  parasitism.  In  the 
first  place,  the  body  tissues  of  parasites,  whether  in  a  plant  or  in  an 
animal,  are  admirably  suited' for  absorbing  the  nutritive  materials  of 

the  host.  This  condition  is  well 
exemplified  in  tapeworms,  which  are 
commonly  found  as  intestinal  para- 
sites in  man  and  other  vertebrates. 
The  body  of  the  tapeworm  is  long, 
flattened,  cylindrical — some  species 
reaching  a  length  of  several  feet. 
It  consists  of  a  large  number  of 
segmental  structures,  the  proglot- 
tids;  which  contain  the  reproductive 
mechanism.  The  mature  proglot- 
tids  are  continually  detached  from 
the  posterior  end  of  the  animal 
when  they  are  mature,  and  the  repro- 
ductive elements  are  ready  to  func- 
tion. Anteriorly,  the  tapeworm 
ends  in  the  scolex,  a  unique  structure  about  the  size  of  a  pinhead, 
which  is  embedded  in  the  wall  of  the  alimentary  canal  of  the  host. 
Having  once  gained  attachment  to  the  alimentary  canal  of  the  host, 
the  parasite  floats  idly  in  the  nutritive  stream  supplied  by  the  host  and 
absorbs  such  nutritive  materials  as  are  necessary  to  maintain  the 
operations  of  the  reproductive  mechanism  at  full  speed!  (Fig.  248.) 
Even  more  striking  are  the  degenerative  changes  found  in  the 
amazing  parasite,  Sacculina,  which  infects  the  crab,  Carcinus.  This 
situation  is  all  the  more  impressive  because  Sacculina,  which  is  itself  a 
crustacean  closely  related  to  the  barnacles,  hatches  from  the  egg  as  an 
active  free-swimming  individual,  giving  no  indications  that  it  is  soon 
to  become  a  degenerate  internal  parasite.  But,  nevertheless,  after  a 
short  period  of  independent  existence,  Sacculina  attaches  itself  to 
some  membranous  structure  on  the  body  of  the  crab  and  then  the 
degenerative  changes  begin.  These  continue  until  the  body  and  legs 
disappear,  and  only  the  head,  denuded  of  all  sense  organs  and  other 


FIG.  248. — Diagram  illustrating 
structure  of  the  tapeworm,  A;  JB, 
head  enlarged.  (Buchanan,  "Elements 
of  Biology  "  Harper  <fe  Brothers.} 


THE  WEB  OF  LIFE 


463 


external  structures,  remains  as  a  tiny  sac-like  body.  .  In  this  condi- 
tion, entrance  is  made  through  the  membranous  tissues  of  the  body 
wall,  and  Sacculina  becomes  an  internal  parasite.  From  the  point  of 
entrance,  wherever  that  may  be,  the  parasite  gradually  works  its  way 
among  the  tissues  until  it  reaches  the  abdominal  region  of  the  crab. 
Here  it  begins  to  grow  and  produces  innumerable  fine  filamentous 
branches  which  extend  to  all  regions  of  the  body  by  way  of  the  blood 
channels  and  continually  absorb  nourishment  from  the  blood  for  the 
formation  of  reproductive  cells.  (Fig.  249.) 


FIG.  249. — Diagrammatic  drawing  of  a  crab  parasitized  by  another  crustacean, 
Sacculina.  The  latter  produces  filamentous  structures  which  ramify  throughout  the 
tissues  of  the  body  of  the  crab.  Note  the  large  brood  sac.  Cf.  page  462.  Appendages 
shown  on  one  side.  (Lane,  "Animal  Biology,"  P.  Blakiston's,  Son  &  Company,  Inc.) 

In  preparing  for  reproduction,  Sacculina  in  time  develops  an 
external,  tumor-like  brood  sac  on  the  ventral  surface  of  the  abdomen. 
First,  the  tissues  of  the  body  wall  of  the  crab  are  dissolved  in  a  small 
area,  through  which  the  developing  brood  sac  protrudes,  gradually 
forming  a  large  brownish-colored  oval-shaped  body  in  which  the  male 
and  female  gametes  are  formed.  After  fertilization  and  early  develop- 
ment, the  embryos  are  released  in  the  free-swimming  stage  with  which 
the  description  began.  Earlier  zoologists,  unaware  of  the  life  history 
of  Sacculina  and  having  no  conception  of  the  origin  of  the  brood  sac, 
regarded  the  parasitized  crabs  as  a  distinct  species. 

However,  even  though  Sacculina  undergoes  this  incredible  trans- 
formation, its  life  cycle  is  not  complicated  by  obligatory  association 
with  a  second  host  as  is  the  condition  in  many  parasites.  Possibly 
the  two  best  known  examples  of  parasites  with  more  than  one  host  are 


464 


HUMAN  BIOLOGY 


the  malaria  parasite  and  the  liver  fluke,  each  of  which  may  now  be 
briefly  considered. 

The  malaria  parasite,  Plasmodium  vivax,1  belongs  to  the  Protozoa 
and  is  of  microscopic  size.  Great  numbers  of  the  parasitic  cells  may 
be  introduced  into  the  blood  stream  of  man  by  the  attacks  of  infected 
mosquitoes.  In  man,  the  parasite  finds  the  blood  stream  a  suitable 
habitat  and  lives  as  an  intracellular  parasite  of  the  red  cells  where  it 
grows  and  reproduces  asexually,  finally  destroying  the  host  cell  as  the 


FIG.  250. — Diagram  illustrating  the  life  cycle  of  the  malaria  parasite;  in  man 
(below)  and  in  the  female  mosquito  (above).  (Hunter,  Walter,  and  Hunter,  "Biology" 
American  Book  Company.") 

newly  formed  parasites  are  liberated  into  the  blood  stream.  New  red 
cells  are  entered,  and  the  cycle  of  asexual  reproduction  and  cell  destruc- 
tion is  repeated,  possibly  many  times.  If  the  patient  suffering  from 
malaria  is  again  bitten  by  a  mosquito,  some  of  the  infected  red  cells 
may  be  obtained  by  the  mosquito.  If  so,  these  cells  will  undergo 
sexual  reproduction  in  the  walls  of  the  alimentary  tract  of  the  mos- 
quito. In  time,  the  zygotes  thus  formed  divide  repeatedly,  and  each 
quickly  produces  great  quantities  of  active  cells  that  finally  assemble 
near  the  salivary  glands.  They  may  then  be  introduced  into  the 
human  blood  stream  again  at  the  first  opportunity  as  originally.  Thus 
1  Consult  Appendix:  Plasmodium. 


THE  WEB  OF  LIFE  465 

the  malaria  parasite  has  two  obligatory  hosts:  a  vertebrate  in  which 
asexual  reproduction  occurs  and  an  invertebrate  where  sexual  repro- 
duction followed  by  asexual  reproduction  is  found.  (Fig.  250.) 

The  liver  fluke,  Fasciola  hepatica,  another  member  of  the  flat- 
worm  phylum  (Platyhelminthes)  to  which  the  tapeworms  belong,  is 
an  important  parasite  producing  a  serious  infection  in  sheep.  The 
adult  stage  of  the  liver  fluke  occurs  in  great  numbers  in  the  liver  of  the 
infected  animals  and  is  seen  as  a  small,  flattened,  disc-shaped  struc- 
ture without  noticeable  external  organs.  The  adults  are  herma- 
phroditic and  produce  gametes  in  great  numbers.  The  fertilized  eggs 
pass  down  the  bile  duct,  intp  the  alimentary  canal,  and  the  partially 
developed  embryos  are  egested  with  the  feces.  They  cannot  sur- 
vive more  than  a  few  hours  unless  they  find  their  way  into  water. 
If  their  quest  is  successful,  they  quickly  develop  into  tiny,  ciliated 
bodies,  the  miracidia,  whose  survival  is  dependent  upon  finding  their 
next  host,  which  is  a  particular  species  of  fresh-water  snail.  If  the 
snail  is  found,  the  miracidia  bore  their  way  into  the  soft  tissues  and 
then  by  utilizing  nutritive  materials  from  the  host  begin  to  grow 
rapidly.  Parthenogenetic  reproduction  occurs  repeatedly  with  the  for- 
mation of  several  types.  Finally,  great  numbers  of  parasites,  all 
asexually  produced  from  the  miracidia,  leave  the  snail  as  active 
cercariae  and  endeavor  to  attach  themselves  to  stalks  of  grass.  Here 
they  encyst  and  await  introduction  into  the  sheep  when  the  infected 
grass  is  eaten.  (Fig.  251.) 

Finally,  it  should  be  recognized  that  parasites  are,  in  turn,  para- 
sitized themselves,  and  so  they  serve  as  the  host  for  other  species  of 
parasites.  This  condition,  known  as  hyperparasitism,  is  possibly  most 
clearly  in  evidence  among  the  insects,  where  it  is  often  used  to  advan- 
tage by  the  entomologist  of  today  in  the  endeavor  to  control  insect 
pests  that  parasitize  important  plants.  The  introduction  of  another 
organism  that  will  parasitize  the  injurious  insect  may  serve  to  control 
the  spread  of  the  latter.  An  example  of  hyperparasitism,  the  tussock 
moth,  may  be  cited.  In  the  larval,  or  caterpillar,  stage  this  insect 
feeds  upon  the  leaves  of  trees  and  causes  great  destruction.  The 
entomologists  know  of  more  than  20  insect  species  that  are  adapted 
for  securing  their  nutrition  by  parasitizing  the  tussock  moth.  These 
are  known  as  primary  parasites,  and  some  of  them  have  been  found  of 
value  in  controlling  the  destructive  activities  of  the  tussock  moth, 
when  introduced  under  proper  conditions.  But  the  primary  para- 
sites have  secondary  parasites  and  so  on  ad  infinitum,  as  the  old  rhyme 
goes.  For  thp  smallest  insect  species  are  often  parasitized  by  unicellu- 
lar protozoa  and  bacteria,  parasite  on  parasite  forming  one  of  the 
innumerable  food  chains  in  nature,  all  based  ultimately  on  the  utiliza- 


466 


HUMAN  BIOLOGY 


tion  of  the  green  plant  tissues.  It  might  be  thought  that  the  chain 
of  parasitism  ends  with  the  extremely  minute  bacterial  cells,  but,  in 
late  years,  it  has  been  shown  that  the  bacteria  are  attacked  by  the 


'..-Intestine 


,— Germ-cells 


FIG.  251. — Diagrams  illustrating  the  life  cycle  of  the  liver  fluke  (Faaciola  hepatica} 
parasitic  in  the  sheep,  as  described  on  page  465.  a,  egg;  6,  miracidium  which  enters 
snail  and  produces  stages  c-/asexually;  c,  sporocyst;  d  and  e,  rediae;/,  cercaria  developed 
in  rediae;  g,  inactive  stage  encysted  on  grass;  h,  adult  stage  in  sheep's  liver.  (Hegner, 
after  Kerr.) 

very  much  smaller  ultramicroscopic  units  of  the  bacteriophage  which, 
like  the  viruses,  are  below  the  cellular  level  in  organization  and  appar- 
ently at  the  vague  border  line  between  living  and  nonliving. 


CHAPTER  XVII 
BIOLOGY  OF  DISEASE 

The  consideration  of  parasitism  in  the  previous  chapter  leads 
naturally  to  the  problems  .associated  with  disease,  for  an  infectious 
disease  is  always  the  result  of  an  invasion  by  some  parasite.  Only  in 
comparatively  recent  times  has  this  condition  been  fully  recognized 
even  by  the  scientist;  as  a  matter  of  fact,  it  is  still  unrecognized  by 
the  great  majority  of  people  living  outside  the  sphere  of  scientific 
knowledge.  Among  the  latter,  the  age-old  demonic  theory  of  disease 
still  holds  sway.  This  theory  is  based  on  the  belief  that  disease  is 
due  to  the  indwelling  of  sundry  evil  spirits  and  that  recovery  is,  there- 
fore, to  be  expected  when  the  demons  are  forced  to  leave  the  body  of 
the  unfortunate  victim.  The  recent  hex  trials  in  a  neighboring  state 
emphasize  the  fact  that  there  is  no  necessity  for  looking  to  some 
remote,  uncivilized  region  in  order  to  find  adherents  to  this  ancient 
belief  of  the  origins  of  disease. 

Numerous  other  theories  of  disease  have  been  proposed  during  the 
ages  that  have  passed  since  attention  first  began  to  be  focused  on  this 
problem.  In  the  present  discussion,  it  will  be  possible  to  mention 
only  two  or  three  of  these.  Thus  there  is  the  theory  of  the  humors 
taught  by  that  illustrious  father  of  medicine,  Hippocrates.1  He 
reached  the  conclusion  that  disease  was  due  to  an  improper  mixture  of 
the  four  hypothetical  body  fluids,  or  humors,  namely,  blood,  phlegm, 
yellow  bile,  black  bile.  Another  theory  of  disease  which  was  widely 
held  for  a  time,  and  possibly  still  has  its  adherents,  is  the  terrestrial 
disturbance  theory  which  was  notably  espoused  in  this  country  by 
that  illustrious  student  of  the  Eftglish  language  Noah  Webster.  This 
theory  held  that  disease  was  the  result  of  violent  terrestrial  disturb- 
ances, ranging  from  windstorms  to  earthquakes.  As  a  matter  of  fact, 
it  is  entirely  evident  that  epidemics  of  disease  do  tend  to  follow  dis- 
turbances of  one  kind  or  another,  but  it  is  also  clear  that  the  epidemics 
which  appear  under  these  conditions  are  the  indirect  and  not  the 
direct  result  of  the  preceding  disturbances.  For  these  catastrophes 
make  possible  the  wide  distribution  of  parasitic  disease-producing 
organisms,  and  the  latter,  under  the  disturbed  conditions  temporarily 
present,  find  it  possible  to  incite  widespread  epidemics. 
1  Consult  Appendix:  Biology  and  Medicine;  Hippocrates. 

467 


468  HUMAN  BIOLOGY 

Then  there  was  the  famous  Hahnemann  theory  of  disease  which 
taught  that  disease  results  from  the  "  derangement  of  a  spiritual  vital 
principle,"  whatever  that  phrase  may  mean.  Hahnemann  used  as 
treatment  for  diseases  a  great  many  natural  substances,  highly  unusual 
and  obscure  in  nature,  which  were  prepared  for  use  by  repeated  dilu- 
tion and  shaking.  As  a  matter  of  fact,  the  methods  of  Hahnemann 
undoubtedly  represented  a  great  advance  over  the  treatments  for 
disease  prevailing  at  that  time,  for  many  of  the  prescriptions  in  general 
use  by  the  physicians  were  virulent  concoctions  essentially  dangerous, 
which  had  been  handed  down  over  long  periods  of  time.  Apparently 
each  succeeding  generation  of  physicians  had  felt  free  to  add  other 
doubtful  ingredients  to  the  prescriptions  until  it  almost  became  a  fact, 
as  stated  by  Oliver  Wendell  Holmes,  that  "if  all  the  drugs  that  had 
ever  been  used  for  the  cure  of  human  ills,  were  gathered  together  and 
thrown  into  the  sea,  it  would  be  ever  so  much  better  for  humanity  and 
ever  so  much  worse  for  the  fishes. " 

It  was  not  until  after  the  middle  of  the  last  century  that  the  many 
and  highly  varied  theories  of  disease  were  finally  directed  into  a  defi- 
nite channel.  This  was  primarily  due  to  the  work  of  Louis  Pasteur, 
who,  to  quote  a  recent  author1 : 

...  by  the  brilliance  of  his  genius,  by  the  clearness  and  breadth  of  his 
vision  .  .  .  formulated  the  bacterial  or  germ  theory  of  infectious  diseases 
which  must  forever  dominate  medicine.  No  longer  could  evil  spirits  be  held 
responsible  for  disease  nor  could  an  improper  mixture  of  the  four  humors  be 
regarded  as  the  cause  of  ill  health.  Disease  was  but  another  example  of  the 
struggle  for  existence;  it  was  life  preying  on  life;  the  invasion  of  the  macro- 
organism  by  the  microorganism.  The  cause  of  typhoid,  cholera,  diphtheria, 
tuberculosis,  meningitis,  and  many  other  diseases  have  today  passed  from  the 
realm  of  theory  into  the  field  of  established  fact,  and  each  year  finds  the  list 
of  vague  indefinite  diseases  growing  shorter  and  the  list  of  germ  diseases 
longer. 

NONINFECTIOU*S  DISEASES 

Thus  far,  our  discussion  has  been  concerned  with  infectious  diseases 
which  result  from  the  invasion  of  a  living  parasite.  It  must  be  recog- 
nized, however,  that  many  important  diseases  are  not  communicable 
— that  a  disease  may  be  produced  in  the  body  by  various  factors  that 
do  not  involve  attacks  by  living  agents.  But  the  diseases  so  produced 
are  necessarily  localized  in  one  individual  only;  there  is  no  possibility 
of  direct  transfer  of  an  infective  agent  to  another  individual.  It  will 

1  Quoted  by  Greaves,  "  Elementary  Bacteriology,"  p.  349,  W.  B.  Oaunders 
Company,  Philadelphia,  1928. 


BIOLOGY  OF  DISEASE  469 

be  helpful  to  indicate  the  nature  of  a  very  few  noncommunicable 
diseases  at  the  present  time.  Outstanding  examples  may  be  noted 
in  the. various  dietary  deficiency  diseases  which,  as  we  know,  have 
their  origin  in  diets  that  do  not  contain  all  the  essential  nutritive 
materials.  In  particular,  in  the  last  few  years,  a  great  deal  of  atten- 
tion has  been  given  to  securing  di^ts  adequately  balanced  with  respect 
to  the  vitamins  (page  57).  The  establishment  of  the  direct  relation- 
ship between  vitamin  deficiency  and  serious  pathological  conditions 
has  worked  a  revolution  in  dietary  questions  the  world  over. 

But  it  is  also  very  apparent  that  vitamins  are  not  the  only  materials 
that  may  be  lacking  from  an  apparently  adequate  diet.  For  example, 
it  was  noted  in  the  earlier  chapter  on  Secretion  that  lack  of  iodine  in 
the  diet  restricts  the  thyroid  gland  in  its  production  of  the  thyroid 
hormone  thyroxine;  and  this,  in  turn,  is  responsible  for  the  cretinous 
condition  (page  106).  Numerous  other  noninfectious  diseases  may 
originate  through  functional  abnormalities,  such,  for  example,  as 
athlete's  heart  or  kidney  failure.  And  there  is  a  whole  host  of  phys- 
ical and  chemical  agents  in  the  environment  which  injure  the  cells  in 
various  ways  and  thus  induce  disease.  Further  consideration  would 
take  us  too  far  afield,  but  it  should  be  clearly  recognized  that  whether 
a  disease  is  parasitic  in  nature  or  is  produced  by  some  other  abnormal 
condition  as  just  noted,  it  is  always  definitely  associated  with  and 
localized  in  some  group  or  groups  of  cells.  All  the  functions  of  the 
body,  whether  in  health  or  in  disease,  are  the  result  of  cellular  action; 
and  if  these  cellular  activities  are  abnormal,  then  we  have  disease. 
Thus  it  is  evident  from  the  biological  standpoint  that  the  picture  pre- 
sented by  any  disease  results  from  the  adaptation  of  the  affected  cells 
to  the  pathological  conditions  and  their  attempts  to  repair  the  damage 
and  to  regenerate  new  tissues  in  the  injured  areas  so  that  normal  activi- 
ties may  be  resumed. 

IMMUNITY 

The  host  is  not  defenseless  in  the  warfare  inaugurated  when  para- 
sitic organisms  attempt  to  invade  the  tissues  of  the  body.  The 
foundation  of  the  host  defense  is  believed  to  center  around  the  pres- 
ence of  specific  chemical  substances,  the  antibodies,  some  of  which 
appear  to  be  always  available,  whereas  others  are  not  formed  until 
the  invader  actually  enters  the  host  tissues.  An  antibody  wages 
chemical  warfare  on  the  parasite,  and  accordingly  the  invader  may  be 
limited  in  its  activities  or  entirely  destroyed.  It  has  long  been  recog- 
nized that  immunity  to  a  particular  disease-producing  parasite  may 
be  inherent  or,  in  other  words,  a  species  characteristic  (natural  immun- 


470  HUMAN  BIOLOGY 

ity)  or  it  may  be  acquired  by  experiencing  the  disease  (acquired 
immunity).  Presumably  the  most  important  factor  in  either  natural 
or  acquired  immunity  is  an  antibody  reaction.  On  this  basis,  natural 
immunity  is  present  when  an  organism  inherently  possesses  an  anti- 
body against  the  disease  in  question;  acquired  immunity,  when  the 
organism  is  forced  to  synthesize  ^  specific  antibody,  following  the 
attack  of  the  parasite,  in  order  to  survive.  The  antibody  once  formed 
in  the  host  may  remain  and  thus  render  the  environment  permanently 
unsuitable  for  the  activities  of  that  particular  parasite.  In  other 
words,  the  individual  possesses  an  acquired  immunity. 

Perhaps  the  situation  may  be  clarified  by  one  or  two  specific  exam- 
ples. Thus,  typhoid  fever  is  a  dangerous  disease  to  which  the  human 
species  is  very  susceptible,  but  the  domesticated  animals,  with  which 
man  is  closely  associated,  have  a  natural  immunity.  Typhoid  is  due 
to  a  bacterial  invasion  which  centers  primarily  in  the  mucosa  lining' 
the  alimentary  tract.  The  typhoid  bacteria  gain  entrance  by  the 
ingestion  of  infected  foods.  In  the  alimentary  canal  of  the  dog,  how- 
ever, there  is  a  natural  immunity  and  the  typhoid  organism  is  unable 
to  secure  a  foothold;  the  ingestion  of  food  materials  bearing  the 
typhoid  bacteria  has  no  ill  effects.  But  the  reverse  is  true  for  dis- 
temper, a  virulent  disease  in  dogs  which  invades  the  body  tissues  by 
way  of  the  respiratory  tract.  Man,  fortunately,  has  a  species,  or 
natural,  immunity  to  this  disease. 

Even  though  a  species  may  be  susceptible  to  the  attacks  of  a 
disease-producing  organism,  some  groups  or  races  included  within  the 
species  may  have  a  natural  immunity.  Even  individuals  within  these 
subdivisions  show  marked  variation  in  either  direction;  that  is,  they 
may  be  more  resistant  or  less  resistant  than  other  individuals  in  the 
group.  Thus  none  of  the  disease  epidemics  so  far  encountered  by 
the  human  species  has  been  able  to  infect  every  individual.  If  it  had 
been  otherwise,  man  would  long  since  have  been  swept  from  the  earth 
as  a  result  of  epidemics  that  have  appeared  in  past  times.  The 
Negro  race  is  much  more  resistant  to  yellow  fever  than  are  members  of 
the  White  race.  Eskimos  as  a  group  are  particularly  susceptible  to 
tuberculosis,  and  the  same  condition  obtains  with  reference  to  influ- 
enza and  certain  other  diseases,  such  as  measles,  among  the  South  Sea 
Islanders. 

As  is  well  known,  susceptible  individuals  commonly  acquire  an 
immunity  against  further  attacks  by  experiencing  the  disease.  This 
naturally  acquired  immunity  is  frequently  of  a  permanent  nature. 
Unfortunately,  however,  a  number  of  infective  diseases  do  not  give 
the  victim  a  permanent  immunity.  Such  is  the  situation  following 


BIOLOGY  OF  DISEASE  471 

attacks  of  the  common  respiratory  diseases,  including  various  types 
of  colds  and  influenza  that  afflict  the  human  organism.  Even  the 
much  more  serious  invasion  of  virulent  pneumonia  parasites  does  not 
grant  immunity  against  later  infections  to  those  who  were  fortunate 
enough  to  survive  'the  first  attack.  For  the  great  majority  of  diseases, 
however,  the  survivor  unquestionably  does  acquire  immunity,  be 
it  temporary  or  permanent.  The  underlying  basis  for  immunity 
acquired  by  the  individual  is  believed  to  center  in  the  antibodies 
developed  in  the  host  as  a  result  of  the  parasitic  invasion  and  which 
remain  temporarily  or  permanently  to  ward  off  later  attacks. 

But  the  fact  to  be  emphasized  in  our  present  consideration  is  that 
laboratory  methods  have  been  developed  by  the  researches  of  special- 
ists in  this  field,  the  immunologists,  that  can  be  used  to  confer  an 
artificial  acquired  immunity  without -experiencing  the  disease.  These 
immunological  methods  are  of  such  fundamental  importance  for  the 
control  of  infectious  disease  that  it  will  be  worth  while  to  give  them 
full  consideration.  In  the  first  place,  it  should  be  emphasized  that 
whether  immunity  against  a  particular  parasite  is  naturally  acquired 
by  having  the  disease  or  by  treatment  with  established  artificial 
methods,  it  is  the  presence  of  a  specific  antibody  that  is  the  basis  of 
the  immunity.  The  production  of  an  antibody  may  be  incited  either 
in  the  tissues  of  the  individual  desiring  immunity  (active  immunity) 
or  in  the  tissues  of  certain  other  animals  that  have  been  found  suitable, 
such  as  the  horse  or  goat,  and  then  transferred  to  the  human  organism 
for  conferring  immunity  (passive  immunity). 

Considering,  first,  the  active  type  of  artificial  acquired  immunity, 
which,  as  just  noted,  involves  the  formation  of  antibodies  in  the  body 
tissues  just  as  when  a  particular  disease  is  experienced,  it  is  found  that 
the  antibody  response  is  invoked  by  inoculation  or  vaccination  with 
the  actual  living  parasitic  organisms.  However,  the  virulence  of  the 
latter  has  been  reduced  by  special  methods  so  that  the  individuals 
vaccinated  do  not  experience  so  severe  a  case  of  the  disease  as  would 
occur  if  they  were  inoculated  with  fully  active  organisms.  The  reac- 
tion of  the  body  tissues  to  the  attentuated  organisms  is,  however, 
sufficient  to  confer  active  immunity  through  antibody  formation. 
The  best  known  example  of  an  acquired  active  immunity  is  that  con- 
ferred by  vaccination  against  smallpox,  which  will  be  considered  later. 

An  acquired  passive  immunity  is  the  result  of  a  treatment  in  which 
the  antibody  against  a  particular  disease  has  been  developed  in  some 
suitable  experimental  animal  and 'then  transferred  to  the  human  organ- 
ism by  way  of  the  blood  serum.  This  is  particularly  well  shown  in 
the  antibody  used  in  the  fight  against  diphtheria.  Diphtheria  anti- 


472  HUMAN  BIOLOGY 

toxin  is  synthesized  in  the  horse  and  then  transferred  to  the  human 
organism  for  use  in  developing  immunity  in  children  or  even  for  the 
treatment  of  diphtheria  if  it  has  unfortunately  been  contracted.  Pas- 
sive immunity  of  this  type  is  typically  less  permanent  than  is  active 
immunity. 

ANTIBODIES 

Frequent  mention  has  been  made  in  the  paragraphs  above  of  the 
ability  of  antibodies  to  combat  invading  parasites  and  also  to  confer 
active  or  passive  immunity.  Thus  the  valuable  functions  of  the  anti- 
bodies are  recognized  even  though  very  little  is  actually  known  as  to 
their  chemical  nature  or  as  to  how  and  where  they  are  synthesized  in 
the  animal  tissues.  It  is  generally  recognized  that  there  are  four 
main  types  of  antibody  reactions,  any  one  of  which  may  be  called 
forth  following  the  entrance  of  parasites  or  other  foreign  substances 
into  the  host  tissues.  Presumably,  there  are  four  types  of  antibodies 
corresponding  to  the  different  types  of  reaction.  These  are  designated 
as  the  antitoxins,  the  opsonins,  the  agglutinins  or  precipitins,  and  the 
lysins,  and  may  be  considered  in  the  order  named. 

Antitoxins. — The  term  antitoxin  given  to  this  group  of  antibodies 
indicates  that  they  react  against  and  thus  neutralise  the  poisonous 
substances  (toxins)  formed  by  a  parasitic  organism  in  the  tissues  of 
the  host.  The  damage  wrought  in  certain  diseases,  notably  diphtheria 
and  tetanus,  is  due  to  the  action  of  a  toxin  given  off  by  the  parasitic 
agent.  It  is  also  a  function  of  the  antitoxins  to  neutralize  injurious 
foreign  proteins  which  gain  entrance  to  the  body  tissues  in  various 
ways,  as  happens  for  example,  when  venom  is  injected  by  the  snake 
bite. 

Opsonins. — The  term  given  to  this  type  of  antibody  is  derived  from 
a  Greek  verb  meaning  " to  prepare  food."  The  term  refers  to  the  action 
of  the  opsonins  in  modifying  the  invading  bacterial  cells  so  that  they 
become  " palatable"  to  the  phagocytic  leucocytes  of  the  blood.  The 
latter  readily  ingest  and  destroy  bacterial  cells  following  their  contact 
with  an  opsonin.  The  phagocytic  reaction  may  be  observed  under  a 
microscope,  as  described  later. 

Agglutinins. — The  name  refers  to  the  ability  possessed  by  this 
group  of  antibodies  to  cause  the  permanent  clumping  together,  or 
agglutination,  of  bacterial  cells  and  thus  to  bring  about  their  destruc- 
tion. Presumably  the  agglutinin  reactions  cause  chemical  changes  at 
the  surfaces  of  bacterial  cells.  As  'a  result,  the  cells  stick  together 
when  they  com6  into  contact,  and  so  great  numbers  become  insepa- 
rably associated,  forming  large  irregular  masses.  Closely  related  to 


BIOLOGY  OF  DISEASE  473 

the  agglutinating  reaction  of  living  cells  is  the  important  precipitin 
reaction,  presumably  incited  by  the  same  type  of  antibody,  which  is 
visibly  indicated  by  the  formation  of  an  insoluble  precipitate  when  a 
specific  protein  in  solution  is  encountered  by  the  antibody.  A  precipi- 
tate is  built  up  by  the  adherence  of  ultramicroscopic  protein  molecules 
to  form  visible  particles. 

Lysins  (Cytolysins). — This  group  of  antibodies  is  responsible  for 
the  most  powerful  and  complex  reactions  of  all.  The  lytic  reaction 
causes  the  destruction  of  an  invading  cell  through  an  actual  disruption 
of  the  cell  wall  which  results,  in  turn,  in  the  dispersal  of  the  proto- 
plasmic content.  Commonly,  lysins  act  as  destroyers  of  bacterial 
cells  and  are  known,  therefore,  as  the  bacteriolysins,  but  of  great  impor- 
tance in  diagnosis  are  the  hemolysins  which  cause  the  destruction 
(hemolysis)  of  red  blood  cells.  The  hemolysins  are  the  basis  of  the 
so-called  complement-fixation  tests  which  are  of  primary  importance 
in  the  Wassermann  test,  as  indicated  below.  Lysins  are  character- 
ized by  the  fact  that  they  have  two  combining  affinities:  first,  with 
the  invading  type  of  foreign  cell  and,  second,  with  aij  obscure  sub- 
stance, the  complement,  normally  present  in  the  blood  plasma.  The 
combination  of  lysin,  foreign  cell  (antigen),  and  complement  results  in 
lysis,  that  is,  the  destruction  of  the  antigen. 

Though  it  has  generally  been  held  by  most  authorities  that  four 
different  types  of  antibody  are  active  in  the  endeavor  to  destroy  invad- 
ing cells  or  foreign  substances,  as  just  described,  the  so-called  Unitarian 
viewpoint  has  been  increasingly  emphasized  in  recent  years.  It  holds 
that  only  one  basic  type  of  antibody  is  formed  in  the  animal  body  but 
that  this  powerful  substance  has  the  possibility  of  functioning  in  the 
four  different  ways  just  indicated,  that  is,  as  an  antitoxin,  an  agglu- 
tinin,  an  opsonin,  or  a  lysin.  For  the  present  discussion,  the  question 
is  not  of  primary  importance  since  the  results  are  the  same  in  either 
case. 

IMMUNOLOGY:  USES  AND  TECHNIQUES 

The  fact,  as  shown  in  the  earlier  discussion,  that  immunity  to 
various  dangerous  diseases  may  be  obtained  through  the  use  of  arti- 
ficial methods  involving  antibody  formation,  so  that  the  individual 
does  not  need  to  experience  the  disease,  has  led  to  a  very  rapid  develop- 
ment of  a  new  science,  Immunology.  It  is  also  increasingly  evident 
that  the  methods  employed  in  this  field  have  important  applications 
outside  the  medical  field.  The  present  situation  may  be  briefly 
summarized  as  follows: 

1.  It  is  possible  to  determine  by  immunological  methods  whether 
or  not  a  person  possesses  a  natural  immunity  to  certain  diseases.  This 


474  HUMAN  BIOLOGY 

is  best  exemplified  by  the  well-known  Schick  test  for  diphtheria, 
described  below. 

2.  It  is  possible,  by  the  use  of  various  immunological  methods, 
to  acquire  immunity  to  some  of  the  worst  diseases  known  to  man. 
Thus,  if  the  Schick  test  shows  that  a  child  is  susceptible  to  diphtheria, 
artificial  acquired  immunity  may  be  obtained  through  the  proper 
treatments. 

3.  To  a  limited  extent,  it  is  possible  to  cure  diseases  through 
immunological  methods.     The  outstanding  example  again  is  diph- 
theria, but  progress  has  been  made  with  pneumonia,  scarlet  fever,  dog 
distemper,  and  other  diseases. 

4.  Immunological   methods    are   increasingly   important   in    the 
medicolegal  field,  for  they  are  able  to  solve  problems  dealing  with  the 
identification  of  various  proteins.     This  is  particularly  important  in 
the  identification  of  blood  stains. 

5.  Diagnosis  of  disease  has  been  greatly  advanced  by  the  use  of 
the  proper  immunological  methods.     This  field  has  become  of  par- 
ticular importance  in  the  diagnosis  of  veneral  disease,  and  the  result  is 
shown  by  the  laws  passed  in  an  increasing  number  of  states  requiring 
that  freedom  from  such  disease  be  established  by  these  diagnostic 
tests  previous  to  the  granting  of  a  marriage  licence.     For  example, 
the  present  law  in  Connecticut  is  as  follows : 

No  application  shall  be  accepted  by  the  registrar  until  he  has  in  his  pos- 
session a  statement  or  statements  signed  by  a  licensed  physician  that  each 
applicant  has  submitted  to  a  Wassermann  or  Kahn  or  other  similar  standard 
laboratory  blood  test  and  that,  in  the  opinion  of  the  physician,  the  person  is 
not  infected  with  syphilis  or  in  a  stage  of  that  disease  that  may  become  com- 
municable and  such  statements  shall  be  accompanied  by  a  record  of  the 
standard  laboratory  blood  tests  made,  which  record  shall  contain  the  exact 
name  of  the  applicant. 

The  immunological  methods  are  dependent  upon  reactions  that 
occur  in  the  liquid  blood  serum.  It  will  be  remembered  from  the  dis- 
cussion in  an  earlier  chapter  (Chap.  VII)  that  blood  consists  of  various 
types  of  blood  cells  and  a  liquid  plasma  in  which  the  cells  "live  and 
move  and  have  their  being. "  It  was  also  shown  that  the  phenomenon 
of  clotting  is  a  function  of  the  plasma.  Following  clotting,  a  non- 
coaguable  liquid,  the  blood  serum,  is  squeezed  out  of  the  clot  by  the 
gradual  contraction  of  the  fibrin  elements.  Blood  serum  is  of  primary 
importance  in  immunology,  for  it  contains  the  antibodies  that  may 
have  been  developed  in  an  individual  as  the  result  of  parasitic  infec- 


BIOLOGY  OF  DISEASE 


475 


tions  or  in  response  to  foreign  substances  that  have  gained  entrance 
into  the  tissues.  How  and  where  antibodies  are  formed  in  an  organ- 
ism is  not  known,  but  it  is  evident  that  they  are  finally  present  in  the 
blood  serum. 

Securing  blood  serum  from  experimental  animals  involves  opening 
a  suitable  blood  vessel  and  the  insertion  of  a  small  glass  cannula 
through  which  blood  will  flow  into  a  container.  It  is  necessary  to 
use  a  technique  that  will  prevent  clotting  until  the 
blood  cells  have  been  removed  by  centrifuging  (page 
163).  This  involves  placing  the  blood  in  a  centrifuge 
where  it  is  revolved  at  high  speed  for  a  few  minutes. 
The  centrifugal  force  thus  developed  will  throw  the 
blood  cells  to  the  bottom  of  the  centrifuge  tubes.  The 
cell-free  plasma,  a  straw-colored  liquid,  is  now  with- 
drawn and  allowed  to  clot.  The  fibrin  elements  in  the 
clot  gradually  contract  to  form  a  firm,  jelly-like  mass, 
from  which  the  permanently  liquid  blood  serum  gradu- 
ally separates.  (Fig.  252.) 

Now  the  important  thing  from  the  standpoint  of 
immunology  is  that  the  antibodies,  formed  in  the 
organism  from  which  the  blood  was  obtained,  are  in  the 
blood  serum  rather  than  in  the  cells  or  fibrin  and  may 
therefore  be  transferred  when  the  serum  is  injected  into 
another  organism.  To  refer  once  more  to  the  technique 
developed  for  the  control  of  diphtheria:  The  specifip 
antibody  against  diphtheria  is  developed  in  the  horse 
and  then  transferred  by  means  of  the  horse  serum  to  the 
human  blood  stream  where  the  antitoxin  will  be  effec- 
tive against  the  disease.  Essentially  this  same  tech- 
nique is  used  in  various  other  immunological  activities. 

Another  example  of  the  serum-antibody  relationship 
is  seen  in  the  use  of  convalescent  serum,  which  is  the 
blood  serum  obtained  from  an  individual  that  has  had 
a  particular  infectious  disease  and  recovered  from  it. 
The  antibody  responsible  for  the  control  of  the  disease  remains 
in  the  blood  serum.  Accordingly,  the  convalescent  serum  is  of 
value  in  treating  individuals  that  have  contracted  the  same  disease. 
Possibly  the  most  extensive  use  of  convalescent  serum  is  in  connection 
with  infantile  paralysis,  or  poliomyelitis,  in  which  serum  obtained 
from  a  child  who  has  recovered  from  the  disease  is  supplied  to  the  one 
who  is  ill.  The  antibody  against  the  infantile  paralysis  parasite  thus 
fights  against  the  disease  when  transferred  to  another  individual. 


FIG.  252.— 
Diagram  show- 
ing test  tube 
with  clotted 
blood.  The  fi- 
brin (A)  has 
shrunken,  leav- 
ing clear  serum 
(#).  (Frobish- 
er,  "Bacteri- 
ology," W.  B. 
Saunders  Com- 
pany.) 


476  HUMAN  BIOLOGY 

At  this  point,  it  will  be  well  to  emphasize  certain  f^cts  relative 
to  the  operation  of  the  immunological  techniques  as  a  basis  for  the 
description  of  a  few  representative  examples  of  their  use  in  the  control 
of  human  disease.  In  the  first  place,  as  emphasized  in  the  preced- 
ing paragraphs,  an  antibody  developed  in  an  organism  is  present 
in  the  blood  serum  and  is  effective  when  transferred  to  another 
individual.  Again,  antibody  formation  occurs  in  response  to  the  inva- 
sion of  a  particular  organism  or  foreign  substance  and  is  a  specific 
reaction  to  each  invader.  Finally,  it  follows  from  the  statement 
just  made  that  the  tissues  of  an  organism  must  be  able  to  detect 
an  invasion  and  to  react  in  a  specific  way  to  each  invader.  It  is, 
in  essence,  the  ability  to  detect  a  particular  protein  out  of  all  the 
possible  proteins,  almost  infinite  in  number  (page  70).  This  ability 
of  living  organisms  to  detect  and  react  to  a  foreign  substance  is  almost 
unbelievable  in  its  specificity. 

Hypersensitivity. — Furthermore,  a  hypersensitivity  develops  under 
certain  conditions  that  greatly  exceeds  the  normal  immunological 
reactions  in  its  delicacy  of  response.  This  condition  of  hypersensi- 
tivity is  technically  known  as  allergy  (anaphylaxis) .  An  individual 
may  be  naturally  allergic  toward  certain  foods  such  as  strawberries, 
clams,  eggs,  cereals,  and  even  fatty  substances.  These  individual 
nutritive  idiosyncracies,  when  present,  greatly  complicate  the  feed- 
ing of  children  because  the  ingestion  of  minute  amounts  of  foods 
to  which  they  are  allergic  will  cause  a  violent  reaction.  Again,  indi- 
viduals may  be  normal  in  their  nutrition  but  be  highly  allergic  to 
certain  wind-blown  protein  particles,  such  as  the  pollen  from  plants 
or  dust  of  various  kinds,  particularly  from  animal  hair.  This  respira- 
tory hypersensitivity  is  responsible  for  hay  fever  and  certain  types 
of  asthma. 

Also,  hypersensitivity  may  be  developed  in  any  individual  by 
sensitization  to  a  foreign  protein  through  injection.  Thus,  as  Wells 
says, 

If  the  foreign  protein  is  injected  into  the  body  of  an  animal  which  has  been 
sensitized  by  previous  injection  of  a  minute  amount  of  the  same  protein,  the 
animal  may  exhibit  a  profound  reaction,  often  fatal.  Unbelievably  small 
amounts  of  protein  may  accomplish  this  sensitization  .  .  .  and  hence  it 
serves  as  a  remarkably  delicate  test  for  the  presence  or  character  of  proteins 
in  a  solution. 

This  artificial  hypersensitivity  developed  by  sensitization  through 
previous  injection  of  a  particular  protein  is  particularly  important  in 
immunology. 


BIOLOGY  OF  DISEASE 


477 


To  takfc  a  specific  example  of  this  acquired  hypersensitivity, 
reference  may  be  made  to  serum  sickness  which  results  from  a 
previous  sensitization  to  a  particular  serum,  such  as  horse  serum. 
The  horse  has  been  found  to  be  a  particularly  favorable  animal  for  the 
development  of  various  antibodies 
used  in  the  treatment  of  human 
disease,  in  particular,  the  treatment 
of  diphtheria  and  tetanus,  and  is 
widely  used.  Accordingly,  if  a  child 
is  given  diphtheria  antitoxin  in  horse 
serum,  it  may  become  sensitized  to 
horse  serum.  At  a  later  time,  the 
child  may  be  so  seriously  injured  that 
antitoxin  protection  against  the 
tetanus  organism,  which  produces 
lockjaw,  is  deemed  advisable.  Teta- 
nus antitoxin  is  also  developed  in 
horse  serum.  Accordingly,  if  the 
child  has  become  sensitized,  or  al- 
lergic, to  horse  serum  by  the  previous 
diphtheria  antitoxin  treatment,  injec- 
tion of  horse  serum  with  the  tetanus 
antitoxin  will  cause  a  violent  reac- 
tion, serum  sickness,  which  may  have 
serious  results. 

It  is  thus  apparent  that  the  tis- 
sues detect  and  react  almost  imme- 
diately to  a  foreign  substance  to 
which  they  are  sensitized.  Fortu- 
nately, a  simple  test  will  tell  whether 
or  not  the  individual  is  allergic  to  a 
serum  or  other  substance.  This  is 
done  by  injecting  a  slight  amount 
in  solution  under  the  skin  of  the  fore- 
arm. The  resulting  reaction  as  in- 
dicated by  the  extent  of  inflamma- 
tory area  that  develops  gives  the 
answer.  It  is  sometimes  found  necessary  to  test  a  great  many 
substances  in  this  way  in  order  to  determine  those  responsible 
for  asthmatic  conditions.  The  phenomena  involved  in  hypersensi- 
tivity are  by  no  means  entirely  clear,  but  primarily  they  center  around 
the  need  of  an  organism  for  protection  from  foreign  substances, 


FIG.  253. — Illustrating  the  reactions 
in  the  skin  of  the  forearm  after  injec- 
tion of  various  proteins  in  a  test  for 
allergic  substances.  The  letters  indi- 
cate the  injection  of  solutions  of  the 
following  substances:  A,  milk;  B,  pro- 
tein of  pork;  C,  protein  of  straw- 
berries; Z>,  hen's  egg;  E,  codfish;  F, 
pollen  of  the  rose;  G,  cat  dandruff;  H, 
pollen  of  the  goldenrod.  The  test 
shows  that  the  patient  is  hypersensi- 
tive, or  allergic,  to  the  egg  protein. 
The  other  reactions  are  not  regarded 
as  significant.  (Frobiaher,  "Bacteri- 
ology" W.  B.  Saunders  Company. 
Redrawn  by  L.  Krause,  modified.) 


478  HUMAN  BIOLOGY 

particularly  if  the  latter  are  placed  directly  into  the  tissues  without 
having  been  altered  through  enzyme  action  in  the  alimentary  canal. 
Proteins,  in  particular,  are  generally  not  welcome  unless  they  have 
entered  by  way  of  the  alimentary  canal  and  there  broken  down  into 
their  constituent  amino  acids  by  the  digestive  enzymes.  In  individ- 
uals with  marked  nutritive  idiosyncracies,  the  allergic  antipathy  is  so 
marked  that  even  entrance  into  the  alimentary  canal  is  "  efficient  to 
incite  the  allergic  reactions.  (Fig.  253.) 

With  the  general  conditions  governing  immunological  reactions  in 
mind,  it  is  next  in  order  to  describe  a  few  of  the  important  materials 
and  methods  that  have  been  found  of  value  in  this  field  and  commonly 
used  by  the  immunologists  in  their  attempts  to  eradicate  germ  diseases. 

Killed  Cultures  of  Bacteria. — Since,  as  stated  above,  the  tissues 
detect  and  react  to  foreign  substances,  the  possibility  was  early 
recognized  that  the  injection  of  the  killed  cells  of  a  certain  disease- 
producing  organism  might  incite  an  antibody  formation  that  would 
give  an  immunity  against  living  cells  of  the  species  injected.  An  out- 
standing example  of  this  i$  found  in  the  development  of  typhoid 
vaccine  which  gives  a  temporary  immunity  against  this  dangerous 
disease.  The  striking  results  that  have  been  achieved  are  best  told 
by  comparing  the  number  of  deaths  from  typhoid  in  the  Spanish- 
American  with 'those  in  the  World  War.  In  the  former,  with  no 
typhoid  vaccination,  there  was  one  death  from  typhoid  for  every  71 
men,  while  in  the  World  War,  with  the  soldiers  vaccinated  against 
typhoid,  there  was  only  one  death  for  every  25,641  soldiers. 

Typhoid  fever  is  produced  by  certain  bacterial  organisms  which 
cause  the  formation  of  ulcers  in  the  lining  of  the  alimentary  tract. 
From  these  localized  regions  of  infection,  the  bacteria  find  their  way 
into  the  blood  stream  and  thus  are  widely  distributed  through  the 
body.  In  the  preparation  of  typhoid  vaccine,  the  typhoid  organisms 
are  grown  in  pure  laboratory  cultures  by  standardized  techniques  and 
killed  at  the  proper  time  by  heating.  Sterile  salt  solution  is  added  to 
make  a  suspension  of  the  bacterial  cells.  The  number  of  organisms 
per  unit  volume  of  the  suspension  is  determined  so  that  the  correct 
amount  may  be  sealed  in  vials  for  individual  dosage.  To  secure  the 
optimum  immunization,  three  injections  of  the  typhoid  vaccine  are 
given;  the  first  dose  contains  approximately  one-half  billion  bacteria, 
and  the  other  two,  given  at  later  periods,  a  billion  each. 

A  number  of  other  vaccines  prepared  in  essentially  the  same  manner 
are  in  rather  common  use,  particularly  for  protection  against  colds 
and  influenza,  but  the  results  so  far  obtained  do  not  give  conclusive 
evidence  of  their  value  as  does  the  typhoid  vaccine.  Possibly  this  is 


BIOLOGY  OF  DISEASE  479 

to  be  expected,  inasmuch  as  even  severe  attacks  of  such  diseases  as 
colds,  influenza,  and  pneumonia  do  not  confer  immunity  upon  the 
individual.  Under  such  circumstances,  it  is  evident  that  the  use  of 
vaccines  to  acquire  immunity  is  bound  to  be  of  doubtful  value. 

Living  Organisms  with  Reduced  Virulence. — It  has  long  been 
known  that  immunity  to  smallpox  may  be  acquired  by  vaccination 
with  material  containing  living  organisms  having  reduced  virulence. 
In  1796,  Edward  Jenner,  an  English  physician,  vaccinated  James 
Phipps,  an  eight-year-old  child,  against  smallpox  by  rubbing  into  a 
scratch  on  his  arm  a  tiny  bit  of  infective  material  from  a  patient  having 
cowpox.  The  boy  developed  cowpox,  a  disease  closely  related  to 
smallpox  but  far  less  virulent.  It  was  later  shown  that  he  had 
acquired  immunity  to  smallpox  by  having  had  cowpox. 

This  was  the  start  of  modern  vaccination  against  smallpox  which 
has  proved  to  be  so  successful  that,  in  a  comparatively  short  time,  it 
has  practically  eliminated  one  of  the  most  dangerous  diseases  ever 
known  and  one  that,  for  untold  centuries,  took  an  annual  toll  of 
millions  of  lives.  The  preparation  of  smallpox  vaccine1  today  is 
far  different,  in  the  rigid  controls,  from  those  inaugurated  following 
Jenner's  results  at  the  close  of  the  eighteenth  century,  but  the  basic 
principle  underlying  the  treatment  remains  the  same,  namely,  sub- 
jecting the  individuals  to  infection  with  relatively  harmless  organisms, 
thereby  inciting  antibody  formation  which  will  protect  against  an 
invasion  of  highly  virulent  organisms.  The  immunity  is  usually  not 
permanent,  and,  accordingly,  it  is  necessary  to  revaccinate  every  few 
years  to  see  if  the  immunity  persists.  This  will  be  shown  by  whether 
or  not  a  later  vaccination  "takes."  Vaccination  every  5  to  10  years  is 
essential  to  insure  immunity. 

Another  example  of  the  use  of  living  cultures  of  a  disease-producing 
organism  is  found  in  the  treatment  used  to  prevent  hydrophobia  follow- 
ing the  bite  of  a  rabid  dog.  This  very  dangerous  disease  is  produced 
by  a  virus  that  develops  slowly  in  man.  Consequently,  there  is  time 
for  the  treatment,  designed  to  incite  antibody  formation,  before  the 
rabies  organism  invades  the  central  nervous  system  where  it  produces 
its  deadly  effect.  The  mortality  from  rabies  is  stated  to  be  almost 
100  per  cent  when  the  disease  is  allowed  to  run  its  course,  but  cure  is 
almost  certain  when  treatment  is  begun  in  time.  The  present  treat- 
ment is  essentially  the  s.ame  as  devised  by  the  great  Pasteur  near  the 
middle  of  the  last  century.  The  modern  immunologist  knows  a  great 
deal  more  than  Pasteur  about  the  principles  underlying  the  immuno- 
logical  reactions,  but  nevertheless  Pasteur  was  sufficiently  well 
>  Consult  Appendix:  Smallpox  Vaccine. 


480  HUMAN  BIOLOGY 

informed  to  devise  a  treatment  that  prevents  the  onset  of  rabies. 
The  Pasteur  treatment  makes  use  of  a  vaccine  containing  the  living 
virus  of  hydrophia  in  which  the  virulence  of  the  virus  organism  has 
been  greatly  reduced  by  drying.1 

ANTIBODIES 

In  the  preceding  paragraphs,  it  has  been  shown  how  the  causative 
organisms  of  dangerous  diseases,  either  killed  or  alive  (but  greatly 
reduced  in  virulence),  may  be  used  to  incite  the  formation  of  a  specific 
antibody  in  the  individual  and  thus  render  the  environment  unsuitable 
for  the  development  of  the  organism  in  question.  But  in  the  treat- 
ment of  certain  diseases  and  also  for  diagnostic  tests,  the  immunologist 
has  found  that  it  is  necessary  to  have  the  antibodies  formed  in  experi- 
mental animals  and  then  transferred  to  the  human  body.  This  field 
of  immunology,  unlike  those  established  by  Jenner  and  fasteur,  has 
been  of  comparatively  recent  development  and  represents  the  cul- 
mination of  immunological  research  with  contributions  from  scientists 
in  many  fields. 

Antitoxins. — This  group  of  antibodies,  as  stated  previously, 
functions  in  the  neutralization  of  the  toxins  produced  by  an  invading 
organism.  In  certain  diseases,  notably  diphtheria  and  tetanus,  the 
deadly  effects  are  due  primarily  to  the  toxins  released  in  the  host  by 
the  parasite,  rather  than  to  an  actual  invasion  and  destruction  of  a 
particular  tissue.  To  aid  the  patient  in  overcoming  certain  toxins,  it  is 
possible  to  produce  the  specific  antitoxin  needed  in  the  serum  of  some 
suitable  experimental  animal  and  then  transfer  it  to  the  human  blood 
stream.  The  best  example  of  this  is  found  in  the  development  of 
antitoxin  for  diphtheria.2  A  child  may  be  seriously  ill  with  this 
disease  and  quickly  brought  back  to  normal  by  the  proper  treatment 
with  a  serum  containing  the  antitoxin  against  the  diphtheria  toxin. 
Unfortunately,  few  diseases  can  be  cured  by  this  method  because  the 
foreign  antitoxin  is  ineffective. 

Diphtheria  antitoxin  thus  produced  in  the  horse  has  changed 
diphtheria  from  one  of  the  world's  worst  diseases  to  its  present  con- 
dition which  is  indicated  by  the  statement  that  "no  child  need  have 
diphtheria/'  This  is  due  to  the  fact  that  it  is  also  possible  to  deter- 
mine by  the  combined  use  of  toxin  and  antitoxin  if  children  are 
susceptible  and,  if  so,  render  them  immune.  Susceptibility  to  diph- 
theria is  determined  by  the  now  well-known  Schick  test  in  which  a 
standardized  dose  of  diphtheria  toxin  in  solution  is  introduced  under 

1  Consult  Appendix:  Rabies  Vaccine. 

s  Consult  Appendix:  Diphtheria  Antitoxin. 


BIOLOGY  OF  DISEASE  481 

the  skin  of  the  child's  forearm.  The  extent  of  the  resulting  reaction, 
visible  to  the  naked  eye  as  an  inflamed  area,  is  observed.  If,  as  is 
usually  the  case,  the  child  is  thus  found  to  be  susceptible,  the  toxin- 
antitoxin  treatment  is  given.  This  will  confer  an  acquired  immunity 
against  diphtheria,  which  may  last  a  lifetime.  The  basis  for  the 
immunity  thus  conferred  is,  of  course,  antibody  formation  in  response 
to  the  diphtheria  toxin  injected,  but  it  is  not  desirable  to  inject  this 
powerful  toxin  into  the  human  tissues  without  partial  neutralization 
by  the  a/ntitoxin.  It  is  probable  that  in  the  years  ahead  the  use  of 
antitoxin  for  the  control  of  various  other  germ  diseases  will  be  greatly 
extended,  but  it  must  be  admitted  that  the  results  so  far  achieved 
limit  the  applications  rather  closely  to  two  very  important  diseases, 
namely,  diphtheria  and  tetanus.  Tetanus  antitoxin,  furthermore,  is 
effective  as  a  preventative  but  not  as  a  cure  for  the  disease  when  the 
latter  has  become  established. 

Another  increasingly  important  use  of  an  antitoxin  is  found  in 
the  treatment  developed  for  snake  bite.  In  preparing  the  antitoxin,  the 
venom  is  collected  from  the  snakes  under  laboratory  conditions.  The 
venom  is  then  reduced  in  strength  and  small  amounts  are  introduced 
into  a  suitable  animal,  such  as  a  goat,  which  will  incite  the  production 
of  an  antitoxin  for  neutralizing  the  venom.  Goat  serum  with  the 
antibody  is  now  widely  distributed  commercially  and  has  proved  to  be 
highly  efficacious  when  quickly  supplied  to  the  victim  of  a  snake  bite. 
Supplying  an  adequate  amount  of  the  venom  antitoxin  without  delay 
results  in  the  neutralization  of  the  venom  before  it  has  opportunity  to 
destroy  the  body  tissues. 

Agglutinins  and  Precipitins. — In  the  earlier  discussion,  it  was 
stated  that  the  agglutinins  and  precipitins  probably  constitute  a 
single  type  of  antibody  which  is  characterized  by  the  ability  to  induce 
surface  changes  that  cause  bacterial  cells  or  protein  molecules  to 
adhere  and  thus  build  up  large  clumps  or  masses.  The  antibody  action 
is  described  as  agglutination  when  cells  are  affected  and  as  precipita- 
tion when  the  phenomenon  is  associated  with  molecular  changes. 
Both  reactions  are  of  value  in  various  immunological  reactions,  a  few 
of  which  will  be  briefly  indicated. 

The  well-known  Widal  test  for  typhoid  fever,  which  is  not  always 
easy  to  diagnose,  is  based  on  the  action  of  an  agglutinin  formed  in  the 
tissues  of  the  host  following  the  entrance  of  the  typhoid  bacilli. 
Evidence  that  the  antibody  is  present  may  be  obtained  by  noting  the 
reactions  between  the  patient's  serum  a$d  typhoid  bacilli.  The 
serum  will  contain  an  agglutinin  against  typhoid  bacilli  if  the  patient 
is  suffering  from  this  disease.  Consequently  the  addition  of  serum 


482 


HUMAN  BIOLOGY 


with  the  agglutinin  to  a  suspension  of  typhoid  bacilli  will  give  a  posi- 
tive reaction  and  cause  their  agglutination,  thus  forming  large  clumps 
of  cells.  In  the  absence  of  the  agglutinin  antibody,  the  cells  will  not 
adhere.  The  process  can  be  observed  under  the  microscope  when 
active  typhoid  bacilli  are  added  to  a  drop  of  serum  containing  the 
typhoid  agglutinin.  (Fig.  254.) 

Pneumonia  is  rightly  regarded  as  one  of  the  worst  of  the  infectious 
diseases.  It  results  from  an  infection  of  the  lung  tissues  by  various 
types  of  pneumococci.  Noteworthy  progress  has  been  made  by  the 
immunologist  in  its  control.  In  the  first  place,  it  has  been  established 
that  of  the  four  known  types  of  pneumonia,  which  result  from  infec- 
tions by  different  species  of  pneumococci,  three  distinct  serological 
types,  known  as  Types  I,  II,  and  III,  may  be  diagnosed  by  the  agglu- 


FIG.  254. — Illustrating  the  agglutination  reaction  with  bacteria  as  in  Widal  test  for 
typhoid,  a,  normal;  6,  agglutination  .of  the  bacterial  cells  following  introduction  of  the 
antibody.  Highly  magnified.  (Grefives,  "Bacteriology,"  W.  B.  Saunders  Company.} 

tination  tests.  By  the  use  of  agglutinins,  the  immunologist  has  made 
noteworthy  progress  in  the  control  of  pneumonia.  (Fig.  255.)  If  the 
serum  reactions  show  that  the  patient  is  infected  by  pneumococci  .of 
either  Type  I  or  Type  II,  beneficial  results  may  be  expected  from  treat- 
ment with  horse  serum  containing  the  corresponding  antibody.  Serum 
treatment  for  pneumonia  appears  to  be  of  doubtful  value  when  used 
against  Type  III  infections.  It  has  not  been  found  possible  to  develop 
a  serum  treatment  for  Type  IV.  Pneumonia  of  this  type  is  probably 
due  to  a  mixed  bacterial  infection  and,  fortunately,  has  a  compara- 
tively low  mortality  rate. 

The  Agglutinin  Tests  for  Blood  Transfusion. — It  is  often  necessarjf 
to  supply  additional  blood  to  a  patient  who  has  lost  a  great  deal 
following  some  serious  accident;  or  transfusion  may  be  indicated  as  a 
result  of  various  diseases,  particularly  anemia,  which  is  characterized 
by  a  marked  deficiency  of  the  red  cells  and,  correspondingly,  a  deficient 
oxygen  supply  to  the  tissues.  In  the  earlier  chapter  on  Human 
Heredity,  it  was  shown  that  four  types  of  blood  are  commonly  found 


BIOLOGY  OF  DISEASE 


483 


in  man  and  that,  in  transfusion,  the  blood  supplied  to  the  patient  by 
the  donor  must  be  of  the  proper  type  (page  409).  Otherwise,  agglu- 
tination of  the  red  cells  will  occur,  and  the  patient  will  be  injured 
rather  than  helped  by  the  transfusion.  The  clumping  of  the  red 
corpuscles  is  due,  as  pointed  out  previously,  to  an  antibody  reaction. 
Accordingly  it  is  possible  to  determine  the  type  of  blood  present  in 
patient  and  donor  before  transfusion.  In  fact,  hospitals  find  it 
necessary  to  have  a  group  of  blood  donors  available,  comprising 
individuals  with  the  various  blood  types,  so  that,  when  a  transfusion 


.,255. — Microscopic  preparation  of  peritoneal  fluid  from  a  mouse  killed  by  a 
pn^imococcus  infection.-  The  numerous  pneumococci  are  seen  as  black  bodies.  The 
laps  gray  bodies  are  cells.  Page  567.  X  GOO.  (Frobisher,  "Bacteriology,"  W  B. 
Scevjnfdei\p  Company.) 

is  indicated,  a  donor  with  the  correct  blood  type  can  be  summoned  as 
soon  as  the  patient's  blood  has  been  typed. 

Precipitin  Reactions. — Increasingly  important  in  various  fields  of 
immunology  are  certain  precipitin  reactions  that,  as  stated  above, 
result  in  the  formation  of  a  visible  precipitate  when  the  test  is  positive. 
Such  tests  are  used  in  medicolegal  work  when-  it  is  necessary  to  identify 
certain  proteins,  blood  stains,  etc.  The  test  involves  the  formation 
in  some  experimental  animal  of  the  specific  antibody  by  repeated 
injections  of  the  substance  that  it  suspected  and  the  consequent  local- 
ization of  the  antibody  in  the  serum.  When  the  latter  is  matched 
with  a  solution  of  the  material  to  be  identified,  the  formation  of  a 
precipitate  makes  the  identification  positive. 

But  the  precipitin  tests  are  also  of  great  value  for  the  diagnosis  of 
certain  diseases.  In  particular,  the  Kahn  test  for  syphilis,  which  is 
based  on  a  precipitin  reaction,  has  become  increasingly  important. 


484 


HUMAN  BIOLOGY 


In  this  test,  serum  from  the  patient  is  tested  under  the  proper  condi- 
tions with  a  prepared  syphilitic  antigen.  The  presence  of  the  syphilitic 
antibody  in  the  patient's  blood — indicating,  of  course,  that  the  disease 
is  present — will  cause  the  formation  of  a  visible  precipitate  when  the 
antigen  and  serum  solutions  are  mixed.  The  amount  of  the  precipi- 
tate formed  is,  in  general,  indicative  of  the  activity  of  the  disease;  the 
absence  of  a  precipitate  under  the  conditions  of  the  test  means  that  no 
syphilitic  antibody  in  the  serum  is  present,  or,  in  other  words,  that 
the  patient  is  free  from  the  disease.  (Fig.  256.) 


ABC 

FIG.  256. — Photograph  illustrating  the  Kahn  reaction,  a  precipitin  test.  A,  show- 
ing heavy  precipitation  at  bottom  of  tube  which  indicates  a  positive  test;  B,  lighter 
precipitation  indicating  a  positive  test,  but  less  virulent;  C,  no  precipitation,  negative 
test.  (Kahn,  "The  Kahn  Test"  The  Williams  &  Wilkins  Company.) 

Lysins. — It  is  recognized  that  the  lysins  are  the  most  powerful 
group  of  antibodies  and  also  the  most  complex  in  their  reactions. 
It  will  be  remembered  that  the  lytic  antibody  must  combine  with  two 
substances  before  a  particular  reaction,  directed  toward  the  destruction 
of  the  invader,  will  take  place.  One  of  these  combining  substances  is 
a  normal  constituent  of  the  blood,  known  as  complement,  or  alexin;  and 
the  other,  designated  as  the  antigen,  may  be  either  invading  cells  or 
foreign  protein.  Lysin  is  formed  only  as  a  result  of  the  invasion  by  an 
antigen.  The  reaction  against  foreign  cells  when  sufficient  lysin  has 
been  formed  may  be  indicated  as  follows : 

Complement  +  antigen  +  lysin  =  cytolysis 


BIOLOGY  OF  DISEASE  485 

Cytolysis  involves  an  actual  destruction  of  the  invading  cells;  the 
cell  membranes  are  ruptured  and  the  enclosed  cytoplasm  flows  out 
and  is  destroyed.  When  bacterial  cells  are  destroyed,  the  action  is 
known  as  bacteriolysis.  One  of  the  most  interesting  and  important  of 
the  lytic  reactions,  because  of  its  use  in  diagnostic  tests,  has  been 
developed  in  the  laboratory  by  the  immunologist  and  is  directed 
towards  the  destruction  of  red  blood  cells  (hemolysis).  The  important 
thing  about  this  reaction  is  that,  when  the  walls  of  the  red  cells  are 
destroyed,  the  hemoglobin  is  released  and  it  colors  the  liquid  in  which 
the  reaction  occurs,  thereby  giving  visible  evidence  of  hemolysis.  The 
hemolytic  reaction  is  basically  the  same  as  bacteriolysis,  but  it  requires 
the  presence  of  a  specific  lysin  (hemolysin)  against  red  blood  cells. 
The  reaction  may  be  indicated  as  follows: 

Complement  +  antigen  +  hemolysin  =  hemolysis 

Some  of  the  most  valuable  diagnostic  tests  at  the  disposal  of  the 
physician  are  based  upon  the  lytic  reactions.  These  include  the 
Wassermann  blood  test,  which  was  the  first  one  devised  and  is  still 
regarded  as  the  standard  test  for  syphilis,  though  this  duty  is  now 
shared  by  the  Kahn  precipitin  test  noted  above.  The  lytic  tests  are 
generally  known  as  the  complement-fixation1  tests  because  all  of  them 
involve  the  permanent  combination  or  fixation  of  the  complement  with 
lysin  and  antigen. 

EPIDEMIOLOGY 

The  science  of  epidemiology  is  concerned  with  the  nature  and  the 
control  of  infectious  or  epidemic  diseases.  The  specialist  in  this  field, 
the  epidemiologist,  must  have  broad  training  in  both  biology  and 
medicine  so  as  to  be  able  to  ascertain  the  characteristics  of  the  causa- 
tive organisms  and  thus  be  capable  of  applying  the  available  scientific 
data  for  their  control.  In  attacking  and  striving  for  the  eradication 
of  a  parasitic  disease,  answers  must  be  found  to  the  following  problems : 

What  is  the  infective  organism  whose  invasion  causes  the  disease? 
The  parasite  must  be  completely  identified,  its  life  cycle  determined, 
and  the  other  hosts,  if  there  are  any,  discovered.  The  complete 
morphology  and  physiology  of  the  parasite  during  all  stages  of  its  life 
cycle  should  be  ascertained.  It  is  apparent  that  it  would  never  have 
been  possible  to  bring  about  any  measure  of  control  of  the  malaria 
parasite  until  it  was  established  that  during  one  stage  of  the  life  cycle 
this  organism  parasitized  a  certain  species  of  mosquito,  which,  in 
turn,  transmitted  the  disease  to  man  (page  464). 

1  Consult  Appendix:  Complement  Fixation. 


486  HUMAN  BIOLOGY 

What  is  the  portal  of  entry  that  the  parasite  uses  in  gaining  entrance 
into  the  human  organism?  A  common  entrance  to  the  internal  tissues 
is  through  the  skin  by  insect  bites,  as  noted  with  malaria,  or  through 
breaks  in  the  skin  when  wounds  occur.  In  certain  instances,  but  rather 
rarely,  parasites  are  able  to  pierce  the  unbroken  skin.  This  is  well 
illustrated  by  the  hookworm  which  manages  to  pierce  the  soles  of  the 
feet.  Again,  parasites  are  adapted  to  gain  entrance  through  the 
alimentary  tract.  Amoebic  dysentery,  typhoid  fever,  and  tuber- 
culosis are  notable  instances  of  invasion  through  this  postal.  Finally, 
the  respiratory  tract  is  used  by  a  variety  of  parasitic  organisms  as  a 
suitable  point  for  beginning  their  invasion.  Colds,  influenza,  and 
pneumonia  organisms,  in  particular,  make  use  of  this  portal. 

What  are  the  host-parasite  relations?  The  answer  to  this  question 
involves  a  complete  study  of  the  nature  of  the  injury  to  the  host 
tissues;  what  tissues  and  organs  are  affected;  how  the  parasite  pro- 
duces the  injury;  and  how  the  host  reacts  to  overcome  the  parasitic 
invasion. 

What  methods  may  be  used  for  determining  susceptibility,  for  con- 
ferring immunity;  for  diagnosis;  and,  finally,  for  the  treatment  of  those 
who  have  been  unfortunate  to  contract  the  infection?  It  is  at  once 
apparent  that  the  results  obtained  by  immunologists,  as  indicated  in 
the  preceding  paragraphs,  arc  of  major  importance  for  the  solution  of 
problems  in  this  field.  But  even  so,  there  are  only  a  very, limited 
number  of  diseases  in  which  these  methods  have  been  found  to  be 
completely  applicable.  In  fact  at  the  present  time,  diphtheria  is 
possibly  the  only  disease  in  which  an  answer  to  all  the  problems  has 
been  obtained  by  the  methods  of  immunology. 

Chemotherapy. — But  increasingly  important  in  the  treatment  of 
disease  are  the  discovery  and  use  of  substances — the  field  of  chemo- 
therapy— that  have  been  found  to  be  specific  poisons  to  a  parasitic 
organisms  but,  at  the  same  time,  essentially  harmless  to  the  host. 
Such  substances  may  be  compounds  found  in  nature,  or  they  may  be 
entirely  new  compounds  developed  by  the  biochemist  in  his  laboratory 
researches.  Thus  quinine,  a  natural  compound  found  in  the  bark 
of  the  cinchona  tree,  is  a  specific  poison  to  the  malaria  parasite.  On 
the  other  hand,  the  important  arsenic  compound  salvarsan,  which  is  a 
specific  poison  for  the  syphilitic  organism,  was  developed  in  the  chem- 
ical laboratory  by  the  extensive  researches  of  the  great  research 
scientist  Ehrlich.  Researches  in  chemotherapy  are  constantly  yield- 
ing results  of  the  highest  value  for  the  treatment  of  disease.  One  of 
the  latest  additions  is  sulphanilamide  and  related  compounds  which 


BIOLOGY  OF  DISEASE  487 

are  now  widely  used  in  the  treatment  of  various  infections  because 
of  their  lethal  action  toward  the  bacterial  cells  concerned. 

As  a  result  of  all  these  advances  in  immunology  and  chemotherapy, 
the  great  epidemics  of  infectious  diseases,  which  from  the  earliest 
times  have  swept  over  the  peoples  of  the  world  and  wrought  untold 
destruction  to  human  life,  appear  to  be  past  history.  The  last  epi- 
demic of  world-wide  scope  was  the  influenza  epidemic  of  1917-1919, 
and  it  is  still  possible  that  other  influenza  epidemics  may  occur,  for 
medical  scientists  have  not  yet  learned  the  methods  for  the  control 
of  this  infection.  But  such  major  plagues  of  the  past  as  the  bubonic 
plague,  yellow  fever,  diphtheria,  smallpox,  and  malaria  appear  to  be 
under  control  except  for  localized  outbreaks.  This  result  has  been 
achieved  through  the  combined  researches  of  scientists  in  almost  every 
field.  Medical  science  is  continually  absorbing  and  putting  into 
practical  application  the  results  obtained  from  research  in  scientific 
laboratories  all  over  the  world. 

TYPES  OF  CELLULAR  RESPONSE 

Whatever  the  type  of  disease  that  affects  an  individual,  it  appears 
that  relatively  few  types  of  cellular  reaction  are  exhibited  by  the  cells 
and  «tissues  concerned.  These  standardized  reactions  are  designated 
by  the  terms  inflammation,  fever,  repair,  hypertrophy,  atrophy. 

Inflammation  is  the  primary  and  almost  universal  reaction  of  the 
tissue  cells  to  any  unfavorable  condition.  It  is  essentially  a  localized 
response  at  or  near  the  site  of  the  injury  particularly  by  the  elements 
of  the  vascular  system,  so  that  the  region  becomes  congested  with 
blood  fluids  and  the  accompanying  cells.  An  increase  takes  place  in 
the  metabolic  activity  of  the  cells;  more  heat  is  liberated;  and  conse- 
quently the  affected  region  feels  hot  or  inflamed.  In  essence,  it 
appears  that  inflammation  is  an  attempt  to  localize  the  disease- 
producing  conditions  through  the  phagocytic  action  of  the  leucocytes 
and  by  the  secretion  of  specific  chemical  substances,  the  antibodies, 
which  are  synthesized  by  the  cells  concerned.  (Fig.  257.) 

Fever  is  a  systemic  response  following  a  more  serious  injury  to  the 
tissues  and  one  that  has  not  been  successfully  controlled  by  the 
localized  inflammatory  reactions.  In  a  sense,  fever  may  be  regarded 
as  a  general  inflammation,  involving  the  entire  organism  and  resulting 
in  increased  metabolic  activity  and  the  consequent  elevation  of 
the  body  temperature,  the  latter  corresponding,  in  general,  with  the 
severity  of  the  infection  and  resulting  injury.  High  body  tempera- 
tures are,  therefore,  regarded  with  apprehension  not  because  of  the 
fever  primarily  but  because  of  the  underlying  condition  that  they 


488 


HUMAN  BIOLOGY 


indicate.  Fever  involves  complicated  relations  between  the  vascular 
and  nervous  systems,  as  is  clearly  indicated  by  the  fact  that  vigorous 
exercise  results  in  greatly  increased  heat  production — much  more  so 
than  does  a  fever — and  yet  the  body  temperature  remains  normal,  for 
the  excess  heat  generated  in  the  tissues  by  muscular  activity  is  quickly 
dissipated  at  the  body  surface.  The  elevation  of  the  body  tempera- 
ture during  fever  is  due  to  the  fact  that  the  comparatively  slight 
amount  of  excess  h£at  resulting  from  increased  cell  metabolism  is 


A  B 

FIG.  257. — Photomicrographs  of  microscopic  preparations  of  portion  of  human 
diaphragm  showing  contrast  in  the  vascularization  in11  the  normal  diaphragm  (A)  and 
in  an  inflamed  diaphragm  (B).  (MacCallum,  "Pathology"  W.  B.  Saunders  Company. 
Slightly  modified.) 

largely  conserved.  The  capillaries  in  the  skin  are  contracted,  thereby 
reducing  the  flow  of  blood  through  them  and  preventing  dissipation 
of  the  excess  heat  at  the  body  surface.  This  condition  is  responsible 
for  the  common  association  between  chills  and  fever;  the  skin  feels 
cold,  due  to  the  decreased  blood  supply,  though  the  body  temperature 
is  actually  above  normal. 

Commonly  regarded  as  basically  harmful,  it  has  become  increas- 
ingly evident  that,  speaking  generally,  fever  is  a  highly  important  and 
beneficial  response  to  an  invasion  or  injury  of  the  body  tissues — an 
attempt  to  overcome  an  abnormal  condition  in  the  body  by  destruction 
of  the  invader  or  by  the  neutralization  of  poisonous  materials  through 


BIOLOGY  OF  DISEASE  489 

the  production  of  antibodies.  Many  of  the  phenomena  associated 
with  the  fever  reaction  remain  obscure,  but  it  is  evident  that  they  are 
primarily  directed  toward  the  restoration  of  normal  conditions. 

Repair  is  an  essential  process  following  the  destruction  of  tissues, 
whatever  be  the  cause  of  the  injury.  Even  a  slight  pinprick,  with 
the  resulting  local  inflammation,  is  accompanied  by  a  certain  amount 
of  tissue  injury  through  cell  destruction,  so  that  later  when  the  "fire  is 
under  control/7  the  injuries  must  be  repaired  and  the  continuity  of  the 
tissues  reestablished.  Obviously  the  need  for  repair  is  correspondingly 
greater  when  more  extensive  damages  are  incurred  following  wide- 
spread destruction  of  body  tissues.  But  repair  does  not  necessarily 
mean  that  regeneration  of  the  original  type  of  tissue  takes  place;  the 
essential  thing  is  that  the  continuity  of  tissues  be  reestablished  in  the 
injure'd  area.  As  a  matter  of  fact,  comparatively  little  regeneration 
occurs  following  injury  to  the  highly  differentiated  human  tissues. 
The  loss  of  lung  tissue,  kidney  tissue,  muscle  tissue,  or  even  a  tooth, 
to  take  a  few  examples,  is  permanent.  The  highly  differentiated 
cells  are  unable  to  build,  to  regenerate,  additional  tissue  of  the  type 
destroyed.  But,  if  the  injured  individual  survives,  the  process  of 
repair,  by  which  the  continuity  of  the  tissues  is  again  brought  about 
in  the  affected  region,  is  a  necessity.  Repair  is  accomplished  through 
the  utilization  of  the  ubiquitous  connective  tissues,  aided,  to  some 
extent,  at  least,  by  the  blood  fibrin.  The  regions  of  injury  are  invaded 
by  connective  tissue  cells  which  gradually  form  a  wound  or  scar  tissue 
to  serve  as  a  permanent  connection  between  the  free  surfaces.  Thus 
continuity  is  established,  but  the  scar  tissue  cannot  supply  the  func- 
tional activities  of  the  original  tissue  that  it  replaces. 

Usually  the  cycle  involving  inflammation,  fever,  and  repair  is 
completed  without  great  delay;  but  with  unfavorable  conditions  and 
extensive  injury,  the  restoration  of  normal  activities  may  be  indefi- 
nitely delayed.  This  may  result  in  an  overgrowth  or  hypertrophy  of 
the  affected  areas,  or  quite  .the  opposite  reaction  may  occur,  in  which 
marked  tissue  degeneration  or  atrophy  is  increasingly  evident. 

Hypertrophy. — The  term  hypertrophy  is  used  to  indicate  an  over- 
growth of  tissues  to  such  an  extent  that  a  particular  region  or  organ 
becomes  abnormally  large.  The  excess  formation  of  tissue  under 
these  conditions  is  primarily  the  result  of  cell  growth  and  consequent 
mitosis.  Certain  types  of  hypertrophy,  however,  may  arise  from 
the  accumulation  of  tissue  fluids,  as  in  edema,  or  from  the  accumula- 
tion of  fatty  materials  stored  in  the  cells  of  adipose  tissue,  and  in  such 
instances  are  probably  accompanied  with  little  or  no  actual  increase 
in  cell  numbers.  Hypertrophy  is  sometimes  evident  as  a  very  impor- 


490  HUMAN  BIOLOGY 

tant  inherent  regulatory  process  by  which  the  organism  is  able  to 
maintain  the  normal  activities  of  an  essential  function,  even  though 
an  important  organ  may  be  missing.  Thus,  following  the  surgical 
removal  of  a  kidney,  compensatory  regulation  is  responsible  for  the 
hypertrophy  of  the  remaining  kidney  so  that  it  is  able  to  carry  on  the 
excretion  of  nitrogenous  wastes.  The  same  essential  process  occurs 
in  other  organs,  as  when  the  loss  of  one  lung  results  from  tuberculosis. 
Compensatory  hypertrophy  is  particularly  evident  during  embryonic 
development  as  can  be  shown  by  the  experimental  embryologist. 
(Fig.  258.)  ,..  ,  _ 


FIG.  258.— Drawing  of  a  human  heart  showing  groat  hypertrophy  of  the  walls  of  the 
left  ventricle,  which  has  been  laid  open.  Hypertrophy  in  this  instance  was  due  to 
increased  activity  as  a  result  of  chronic  kidney  disease  (nephritis).  (MacCallum, 
Pathology,  W.  B.  Saundera  Company.  Redrawn  by  L.  Krausc.) 

But,  specifically,  from  the  standpoint  of  disease,  various  instances 
are  found  in  which  hypertrophy  is  due  to  widely  differing  conditions. 
Thus,  hypertrophy  may  be  the  direct  result  of  a  parasitic  invasion. 
A  startling  example  of  this  is  seen  in  the  widespread  tropical  disease 
elephantiasis,  which  is  caused  by  an  invasion  of  a  species  of  the 
microscopic  roundworm  Filaria.  The  latter,  entering  the  tissues  of 
the  legs  and  feet,  gradually  accumulate  in  great  numbers  in  the  lymph 
channels  of  the  legs  and  thus  prevent  the  normal  return  of  the  lymph 
to  the  other  body  regions.  The  accumulation  of  the  lyrnph  gradually 
brings  about  the  formation  of  relatively  enormous  masses  of  connective 


BIOLOGY  OF  DISEASE 


491 


tissue  in  the  leg  and  scrotal  regions.  Again,  hypertrophy  may  have 
its  origin  in  a  nutritive;  deficiency.  Thus,  a  deficiency  in  the  iodine 
content  is  responsible  for  an  abnormal  growth  of  the  thyroid.  Again, 
an  abnormal  functioning  of  an  endocrine  gland  will  result  in  the  hyper- 
trophy of  certain  tissues,  as  was  previously  considered  in  acromegaly; 
in  this  instance  a  too  abundant  secretion  of  the  growth  hormone  from 
the  pituitary  gland  is  the  inciting  force  (page  113).  (Fig.  259.) 

Atrophy. — This  pathological  condition  is  marked  by  tissue  destruc- 
tion resulting  from  various  causes,  to  certain  of  which  attention  may 
now  be  directed.  A  continued  failure  to  supply  the  cells  of  a  particular 
tissue  with  the  proper  nutritive  materials  will  necessarily  result  in 
degenerative,  or  atrophic,  processes.  This  nutritive  deficiency  may 


FIG.  259. — Illustrating  extreme  examples  of  hypertrophy  of  the  legs  antl  scrota! 
regions  (elephantiasis),  the  result  of  filarial  infection.  (Chandler,  " Animal  Parasites 
and  Human  Disease"  John  Wiley  &  Sons,  Inc.,  after  Manson.) 

be  caused  by  a  failure  to  secure  the  proper  foodstuffs  or  by  the  inability 
of  the  nutritive  system  to  digest  them  so  that  assimilation  is  possible. 
The  same  result  may  occur  from  a  failure  of  the  vascular  system  to 
supply  a  particular  area  with  an  adequate  flow  of  blood  containing  the 
essential  materials;  cell  destruction  must  follow.  Another  striking 
example  of  tissue  atrophy  is  found  in  the  degeneration  of  muscle 
tissue  when  the  flow  of  stimuli  through  the  associated  nerve  fibers  is 
interrupted,  owing  to  section  of  the  nerve  or  destruction  of  the  neurons. 
Normal  muscle  tissue  is  directly  dependent  upon  its  connection  with 
the  central  nervous  system  so  that  muscle  tonus  may  be  maintained  by 
the  incoming  nerve  stimuli.  An  all  too  common  example  of  muscle 
atrophy  is  seen  in  the  degeneration  of  the  leg  muscles  following  severe 
infantile  paralysis  that  has  destroyed  the  neurons  -in  the  central 
nervous  system.  Still  another  source  of  tissue  atrophy  is  found  in  the 


492  HUMAN  BIOLOGY 

cumulative  action  of  toxins  secreted  by  parasitic  organisms  and  dis- 
tributed throughout  the  body.  In  addition,  there  is  a  great  variety 
of  poisonous  inorganic  and  organic  substances  that  may  gain  entrance 
to  the  body  and  poison  the  tissues.  The  concentration  may  be  high 
enough  to  produce  serious  results  at  once;  or  with  lower  concentra- 
tions, a  chronic  toxemia  may  develop,  which  results  in  the  gradual 
atrophy  of  the  affected  tissues. 

Senescence. — The  consideration  of  atrophy  leads  naturally  to  an 
inquiry  as  to  the  real  nature  of  the  degenerative  changes  associated 
with  old  age.  Is  senescence  a  normal  process,  or  is  it  essentially  a 
pathological  condition?  Possibly  the  gradual  wasting  away  of  the 
body  tissues,  which  is  evident  in  old  age,  is  primarily  due  to  the 
exhaustion  of  essential  substances  or  to  the  accumula- 
tion of  toxic  substances  formed  in  the  metabolic 
processes  and  not  excreted.  The  biologists  have 
accumulated  considerable  evidence  that  such  may  be 
the  true  state  of  affairs.  It  has  been  shown  by 
conclusive  experiments  with  the  cultures  of  para- 
mecium  extending  over  many  years,  that,  given  suit- 
a^je  environmental  conditions,  these  cells  are  able  to 
maintain  a  continued  high  rate  of  growth  indefinitely. 
These  results  mean  that,  under  the  proper  conditions, 
together."  the  protoplasm  does  not  become  senescent,  and  the 
Camper!)'  so-called  life  cycle,  which  was  supposed  invariably  to 

end  in  the  death  of  the  organism  after  a  certain  number 
of  generations,  can  be  prolonged  without  limit.     (Fig.  260.) 

Of  even  more  interest  in  this  connection  is  the  experimental  evi- 
dence that  senescence  can  be  prevented  even  in  the  tissues  of  highly 
differentiated,  multicellular  animals  by  the  use  of  tissue  culture 
methods.  It  has  been  found  possible  over  a  period  of  several  years 
to  secure  active  and  continuous  growth  of  connective  tissue  cells 
obtained  from  the  heart  of  a  chick  embryo.  The  original  fragment 
of  tissue  was  placed  in  a  culture  medium  composed  of  blood  plasma 
and  embryonic  extract  of  chick  tissues,  which  proved  to  be  an  extremely 
favorable  medium.  Now  the  crucial  advantage  of  this  method  of 
cultivating  tissues  lies  in  the  fact  that  every  few  days,  or  as  often  as  is 
necessary,  some  of  the  actively  growing  cells  from  the  bit  of  explanted 
tissue  can  be  removed  from  the  gradually  aging  culture,  in  which  the 
environmental  conditions  are  becoming  unsuitable,  and  transferred 
to  a  new  culture  with  fresh  culture  medium.  By  this  process  of  sub- 
culturing  or  transplanting  from  time  to  time,  it  is  possible  continually 
to  subject  the  cells  to  a  highly  favorable  environment,  and  thus, 


BIOLOGY  OF  DISEASE 


493 


PLATE  XVIII.— Drawings  of  a  living  tissue  culture  as  seen  under  the  microscope. 
In  the  upper  figure,  the  dark  colored  area  is  a  tiny  piece  of  living  frog  tissue  embedded 
in  a  drop  or  so  of  blood  plasma  which  has  clotted.  Fibrin  filaments  are  visible.  Two 
large  cells  (right)  are  seen  which  have  moved  from  the  tissue  into  the  clot.  Later 
changes  in  the  shapes  of  these  active  migrating  cells  are  shown  in  two  lower  figures. 


494  HUMAN  BIOLOGY 

apparently,  they  can  be  protected  from  the  onset  of  senescence. 
(Plate  XVIII.) 

Attention  should  also  be  called  to  the  fact  that  the  germ  cells  of  all 
organisms  possess  a  method  by  which  they  remain  young  and  embry- 
onic in  character,  although  the  somatic  cells  of  organisms  that  pro- 
duced them  gradually  become  senescent.  The  complicated  processes 
of  germ  cell  development  in  the  two  sexes  result  in  the  formation  of  the 
highly  specialized  male  and  female  germ  cells.  And  the  extraordinary 
thing  is  that  by  the  union  of  these  two  cells,  a  composite  cell  is  pro- 
duced in  which  the  protoplasm  is  apparently  as  young  as  the  first 
protoplasm  on  this  earth  and  a  cell  that  has  the  potentiality  of  grad- 
ually producing  a  complete  new  organism  through  growth  and  differen- 
tiation. Biologists  have  no  experimental  data  to  show  how  the  germ 
cells  are  able  to  carry  on  in  this  matter,  while,  in  the  same  environ- 
ment that  nourished  them,  a  gradual  aging  and  senescence  of  the 
somatic  cells  occurs;  and  although  we  are  not  on  proved  ground  in 
attempting  to  explain  this  inherent  ability  of  the  germ  cells  to  bridge 
the  generations,  it  appears  reasonable  to  suppose  that  the  primary 
factor  lies  in  being  able  to  break  away  from  the  gradually  aging  body 
in  which  they  were  produce*!  and  to  secure  a  new  and  more  favorable 
environment. 

It  appears  from  the  foregoing  data  that  the  suitability  of  the 
environment  may  be  of  fundamental  importance  in  determining 
whether  or  not  senescent  changes  are  to  appear  in  animal  cells.  On 
this  basis,  then,  senescence  i^  to  be  regarded  not  as  an  inherent  char- 
acteristic of  protoplasm  but  essentially  as  a  pathological  condition 
incited  by  unfavorable  environmental  conditions.  Possibly  the 
unsuitability  of  the  environment  in  a  highly  specialized  animal  is 
due  to  some  slight,  but  accumulative,  deficiency  of  substances  essential 
to  the  cell  metabolism.  Or,  again,  the  possibility  exists  that  senes- 
cence is  due  to  imperfect  excretion,  thus  resulting  in  a  gradual  accumu- 
lation of  the  excretory  products  and  a  consequent  increasing  restriction 
of  the  normal  life  processes  as  the  wastes  accumulate.  At  all  events, 
we  are  not  in  a  position  at  present  to  do  more  than  indicate  certain 
possibilities.  Death  is  still  certain. 


APPENDIX 

Abiogenesis.  The  term  used  to  designate  the  discarded  belief  that  living 
matter  arises  spontaneously  from  nonliving  matter.  See  Biogenesis. 
Organic  Evolution. 

Acetylcholine.  ''These  observations  have  seemed  to  us  to  lead  inevitably 
to  the  conclusion  that,  in  spite  of  the  considerations  which  made  the  idea 
initially  difficult  to  entertain,  the  excitatory  process  is  actually  transmitted 
across  a  synapse  in  an  autonomic  ganglion,  by  the  liberation  of  acetylcholine 
as  the  impulse  reaches  the  endings  of  the  preganglionic  nerve  fibres.  With 
regard  to  the  mechanism  by  which  acetylcholine  is  thus  liberated  from  the 
inactivating  and  protective  complex,  in  which  we  must  suppose  it  to  be  held 
in  the  neighbourhood  of  the  preganglionic  nerve  ending,  Brown  and  Feldberg 
(1936b)  have  made  the  very  suggestive  observation  that,  if  the  potassium 
content  of  the  perfusion  fluid  is  suddenly  augmented,  acetylcholine  promptly 
appears  in  the  venous  effluent  from  the  perfused  ganglion,  in  a  manner  strongly 
reminiscent  of  its  appearance  when  the  preganglionic  nerve  is  "stimulated. 
There  is  evidence  connecting  the  propagated  impulse  along  a  nerve  fibre  with  a 
wave  of  mobilization  of  potassium  ions;  and  it  is  tempting  to  picture  this 
process  arriving  at  the  ending  of  the  preganglionic  fibre,  there  immediately 
liberating  a  small  charge  of  acetylcholine,  which  causes  the  discharge,  from  the 
nerve  cell  sensitive  to  its  action,  of  a  new  propagated  impulse,  perhaps  a  new 
wave  of  potassium  mobilization,  passing  along  the  postganglionic  fibre." 


"The  case  of  the  voluntary  muscle  presented  additional  difficulties.  A 
sympathetic  ganglion  is  a  small  structure,  and  the  synaptic  endings  of  pre- 
ganglionic fibres  are  closely  packed  in  it.  If  acetylcholine  were  liberated  by 
the  arrival  of  preganglionic  impulses  at  these  endings,  we  might  expect  to  find 
it  in  reasonable  concentration,  in  the  fluid  slowly  percolating  through  the 
very  small  vascular  bed  of  the  ganglion;  and  my  colleagues  had,  in  fact,  so 
found  it.  In  the  voluntary  muscle,  on  the  other  hand,  the  motor  nerve  endings 
are  thinly  scattered,  one  to  each  fibre,  through  a  relatively  enormous  mass, 
and  only  a  very  small  part  of  the  perfusion  needed  to  keep  the  muscle  alive 
makes  any  contact  with  them.  If  acetylcholine  were  liberated  at  these 
endings  by  the  arrival  there  of  motor  nerve  impulses,  we  should,  accordingly, 
expect  to  find  it,  if  at  all,  only  in  very  low  concentration  in  the  fluid  flowing 
rapidly  from  the  vein.  The  concentration  which  we  found  was  small,  indeed, 
but  not  too  small  to  be  detected  and  measured  by  the  delicate  physiological 
tests  available.  The  substance  so  detected  showed  the  physiological  activities 
of  acetylcholine,  not  only  on  one  test  object,  but  in  the  characteristic  pro- 
portions on  several,  including  some  .reacting  to  its  " nicotine"  and  others  to 

495 


496  HUMAN  BIOLOGY 

its  "  muscarine  "  effects.  It  was  rapidly  destroyed  by  cholinesterase  or  by  alkali ; 
there  could  be  no  reasonable  doubt,  indeed,  as  to  its  identity.  Though  the 
quantity  obtained  was  small,  it  was  of  the  order  to  be  expected.  Calculating, 
as  in  the  case  of  the  ganglion,  the  quantity  liberated  in  a  muscle  by  a  single, 
maximal  motor  volley,  and  then,  from  the  number  of  muscle  fibres,  the  quan- 
tity liberated  by  one  impulse  arriving  at  a  single  motor  nerve  ending,  w^obtain 
a  number  of  the  same  order  as  that  obtained  for  a  single  impulse  impinging 
on  a  single  ganglion  cell,  namely  10~15  gramme,  or  about  3  million  molecules/' 
(Quoted  from  an  article  by  Sir  Henry  Dale,  entitled  "Transmission  of  Nervous 
Effects  by  Acetylcholine,"  which  is  printed  in  The  Hafvey  Lectures,  Series  32, 
pp.  237-239,  Williams  &  Wilkins  Company,  Baltimore,  1937.) 

Adrenal  Glands — Historical.  "  During  the  fifteenth  century,  as  one  phase 
of  the  revival  of  learning,  a  novel  fashion  sprang  up  in  the  field  of  medicine. 
It  became  the  vogue  to  consult  nature  rather  than  the  old  Greek  authorities. 
The  human  body  was  subjected  to  a  new  scrutiny  from  head  to  heel.  From 
this  scrutiny  emerged  the  recognition  of  numerous  structures,  the  names  of 
which  still  serve  as  monuments  to  their  finders.  Among  the  scholastic 
radicals,  one  of  the  most  eminent  was  Eustachio.  His  name  is  perpetuated 
as  the  first  to  describe  the  passage  leading  from  the  throat  to  the  inner  ear — 
the  Eustachian  tube.  It  was  he  who  also — in  the  year  1563 — reported  the 
discovery  of  the  adrenal  (suprarenal)  glands.  With  the  recognition  of  their 
existence  and  such  a  description  as  naked  eye  observation  permitted,  however, 
progress  ceased.  Many  a  slow  step  in  the  development  of  biologic  method- 
ology had  to  be  taken  before  the  first  inkling  of  their  functions  emerged. 
The  experimental  method,  to  which  medicine  chiefly  owes  its  modern  progress, 
was  introduced  by  John  Hunter  only  in  the  late  seventeenth  century.  Prior 
to  that  time,  clinical  experience  was  seldom  productive  of  anything  more 
substantial  than  uncritical  lore.  Physiology  mostly  consisted,  as  it  had  for 
centuries,  of  picturesque  speculations.  Some  of  these  are  well  illustrated  in 
the  story  of  the  adrenal  itself. 

"In  1716,  as  Sharpey-Schafer  tells  the  tale,  the  Academy  of  Sciences  of 
Bordeaux  proposed  as  a  subject  for  competitive  essays:  'What  is  the  Use  of 
the  Suprarenal  Glands?'  The  manuscripts  were  submitted  for  judgment  to 
the  young  president  of  the  society,  the  budding  satirist,  Montesquieu.  In  a 
spirit  foreshadowing  his  future  renown,  Montesquieu  reported  'Some  have 
imagined  that  these  glands  are  placed  in  the  situation  where  they  occur  in 
order  to  hold  up  the  stomach  which  would  otherwise  press  too  hard  on  the 
veins  of  the  kidneys.  Others  have  imagined  them  to  strengthen  and  con- 
solidate the  venus  complex  which  is  in  contact  with  them — conclusions  which 
have  appeared  to  escape  the  ancients  who  were  content  with  simply  expressing 
ignorance  of  the  functions  of  these  glands.  Bartholin  was  the  first  to  relieve 
them  of  the  stigma  of  performing  so  menial  an  office.  He  is  of  the  opinion 
that  a  humour  which  he  terms  'black  bile'  is  preserved  within  their  cavity 
and  believes  that  there  exists  a  communication  between  the  capsules  and  the 
kidneys,  this  humour  serving  to  dilute  the  urine. 


APPENDIX  497 

"'Some  anatomists  teach  that  the  only  use  of  the  glands  is  to  collect  the 
humidities  which  leak  out  of  the  great  vessels  surrounding  them;  others  have 
held  that  a  bilious  juice  is  formed  within  them  and  being  carried  to  the  heart 
mingles  with  acidity  which  is  there  present  and  excites  fermentation,  this 
being  the  cause  of  the  heart's  movements.  Others  consider  that  the  humour 
within  the  glands  is  nothing  more  than  a  lacteal  juice  which  is  distributed  by 
the  mesenteric  glands. 

"'We  have  one  author  who  affirms  the  existence  of  two  kinds  of  bile,  one, 
grosser,  secreted  by  the  liver;  the  other  more  subtile,  secreted  by  the  kidneys 
with  the  aid  of  a  ferment.  This  ferment  flows  from  the  glands  through  ducts, 
the  existence  of  which  is  completely  unknown  to  us — arid  as  to  which  we  are 
threatened  with  perpetual  ignorance/  adds  Montesquieu.  Finally  con- 
fiding his  opinion  that  none  of  the  memoirs  submitted  could  be  looked  upon 
as  satisfying  the  legitimate  curiosity  of  the  Academy,  he  concludes:  Terhaps 
chance  may  some  day  effect  whaVall  these  labors  have  been  unable  to  per- 
form/ Nearly  a  century  and  a  half  elapsed  before  that  chance  was  realized." 
(Hoskins,  "The  Tides  Of  Life,"  pp.  25-27,  W.  W.  Norton  &  Company,  Inc., 
New  York,  1933.) 

Aerobes  and  Anaerobes.  "With  respect  to  these* sources  of  oxygen  it 
may  be  said  that  there  are  two  main  classes  of  bacteria.  First,  there  are  the 
aerobic  bacteria  which,  like  most  other  creatures,  utilize  the  oxygen  of  the  air. 
Some  of  these  cannot  grow  at  all  well  unless  perfectly  free  access  to  air  is  had 
at  all  times.  These  are  called  strict  aerobes.  Second,  there  are  the  anaerobic 
bacteria  which  can  live  and  grow  in  the  absence  of  free  oxygen.  Some  species 
of  anaerobes  are  extremely  sensitive  to  the  presence  of  air  and  will  not  multiply 
at  all  if  the  least  trace  of  oxygen  be  present.  Some  are  so  sensitive  that  a  few 
minutes  exposure  to  the  air  kills  them.  These  are  called  strictly  anaerobic 
bacteria  or  strict  anaerobes.  .  .  . 

"Although  strictly  anaerobic  bacteria  thrive  best  only  in  the  absence  of 
free  oxygen,  it  must  not  be  supposed  that  they  differ  from  other  living  crea- 
tures in  not  requiring  oxygen  for  growth.  It  is  only  that  they  are  sensitive 
to  free  oxygen.  In  anaerobic  metabolism,  whether  by  strict  or  facultative 
anaerobes,  oxygen  is  believed  to  be  obtained  through  the  hydrolysis  of  carbo- 
hydrates, and  also  of  nitrogenous  compounds  such  as  proteins.  Substances 
like  sodium  nitrate  are  also  easily  reduced  by  many  species.  Such  reactions 
may  proceed  according  to  the  following  equation: 

NaN03  =  NaN02  +  0. 

"The  oxygen  thus  obtained  may  be  utilized  directly  in  oxidating  some 
other  substance  inside  the  cell.  This  yields  energy.  The  process  of  taking 
oxygen  from  one  molecule  requires  energy,  but  more  is  gained  when  the  oxygen 
is  used  to  oxidize  some  other  compound.  Furthermore,  by  hydrolyzing 
carbohydrates  and  proteins,  oxygen  may  be  taken  from  one  readily  reduced 
part  of  a  hydrolyzed  molecule  and  used  to  oxidize  another  radical  of  the  same 
molecule.  This  results  in  a  liberation  of  energy  to  the  bacteria  through  the 


498  HUMAN  BIOLOGY 

formation  of  a  less  highly  oxidized  compound;  a  molecular  rearrangement 
having  occurred  to  permit  this,  the  bacteria  gaining  energy  thereby/'  (Frob- 
isher,  "  Fundamentals  of  Bacteriology/'  pp.  47, 48,  Courtesy  of  W.  B,  Saunders 
Company,  Philadelphia,  1937.) 

Alternation  of  Generations.  The  alternation  of  sexual  and  asexual 
generations  in  the  life  cycle  of  an  organism.  In  the  plant  kingdom,  the 
phenomenon  is  widespread  and  is  exhibited  by  all  the  higher  plants.  Many 
examples  occur  also  in  the  animal  kingdom,  notably  among  the  Coelenterates. 
The  classic  example  is  found  in  Obelia  in  which  the  asexual  generation  is  a 
sessile,  branched,  colonial  form.  Buds  are  formed  asexually  which  develop 
into  tiny  jellyfish  (medusae).  These  are  detached  from  the  parent  organism 
when  mature  and  become  free-swimming  sexual  individuals.  Sperm  and 
eggs  formed  by  the  medusae  are  released  into  the  water,  fertilization  occurs, 
and  the  zygote  develops  into  the  asexual  colonial  stage.  (See  Coelenterata.) 

Amino  Acids.  Nitrogenous  compounds  that  are  associated  to  form 
proteins.  They  are  characterized  by  the  presence  of  the  NH2  group  (amino 
group).  See  Proteins. 

Amoeba.  "There  is  probably  no  better  introduction  to  the  study  of  the 
biology  of  an  animal  than  that  afforded  by  Amoeba  proteiis,  a  common  organ- 
ism of  ponds,  ditches,  and  decaying  vegetable  infusions.  Amoebae,  fre- 
quently referred  to  as  the  simplest  animals,  are  representatives  of  the  great 
group  of  single-celled  animals,  or  Protozoa.  Members  of  this  group  are 
found  in  almost  every  niche  in  nature  and,  like  the  Protophyta,  as  the  uni- 
cellular plants  are  sometimes  called,  are  important  because,  although  small 
in  size,  the  number  of  individuals  is  inconceivably  large.  Collectively,  they 
produce  profound  changes  in  their  environment. 

"In  order  to  study  an  Amoeba  it  is  necessary  to  magnify  it  several  hundred 
times.  This  done,  it  appears  as  a  more  or  less  irregular  mass  of  granular 
jelly-like  material,  rather  slowly  changing  its  shape  and  thereby  moving 
along.  As  a  matter  of  fact  it  is  essentially  a  naked  bit  of  protoplasm,  without 
obviously  specialized  parts.  However,  careful  study  reveals  that  the  organ- 
ism really  consists  of  a  single  protoplasmic  unit  differentiated  into  cytoplasm 
and  nucleus — it  is  a  cell:  an  animal. 

"But  there  are  no  specialized  locomotor  organs — merely  now  and  again 
the  clear  outer  layer  of  protoplasm  or  ECTOPLASM,  flows  out,  followed  by  the 
internal  granular  ENDOPLASM,  so  that  a  projection,  or  PSEUDOPODIUM,  is 
formed.  There  is  no  permanent  mouth;  food  being  engulfed  by  the  proto- 
plasm flowing  about  it  as  opportunity  offers.  There  is  no  permanent  digestive 
or  excretory  apparatus. 

"Amoeba,  under  favorable  conditions,  grows  rapidly  and,  when  it  has 
attained  the  size  limit  characteristic  of  the  species,  cell  division,  termed 
BINARY  FISSION,  takes  place,  with  the  result  that  from  the  single  large  cell 
there  are  formed  two  smaller  individuals  which  soon  become  complete  in  all 
respects.  These  in  turn,  grow  and  repeat  the  process  so  that  .  .  .  within 
a  few  days  the  original  Amoeba  has  divided  its  individuality,  so  to  speak, 


APPENDIX  499 

among  a  multitude  of  descendants. "  (Woodruff,  "Animal  Biology,"  pp. 
35-36,  The  Macmillan  Company,  New  York,  1938.)  See  Protozoa. 

Anaerobes.    See  Aerobes  and  Anaerobes. 

Anions.    See  Dissociation. 

Annelida.    See  Earthworm. 

Appendix,  Human.  "Near  the  junction  of  the  large  and  small  intestine 
in  man  there  is  a  narrow,  blind  process,  about  three  and  a  half  inches  in 
length,  known  as  the  vermiform  appendix.  The  appendix  in  man  is  a  vestigial 
structure  and  represents  the  functionless,  shriveled,  terminal  remains  of  the 
caecum,  the  blind  beginning  of  the  large  intestine.  In  an  herbivorous  animal, 
the  caecum  is  a  large,  nutritive  organ  of  great  importance.  In  carnivorous 
animals,  the  caecum  is  reduced.  The  reduction  of  the  terminal  portion  of 
the  caecum  to  form  an  appendix  occurs  only  in  man,  the  anthropoid  apes,  and 
some  rodents.  The  frequent  pathological  condition  of  the  appendix  in  man 
has  given  rise  to  the  aphorism  'that  vestigial  structures  are  particularly  prone 
to  disease/"  (Ferris,  "The  Evolution  of  Earth  and  Man,"  Chap.  VI,  p.  221, 
Yale  University  Press,  New  Haven,  Conn.,  1929.) 

Aristotle.  "Aristotle  (384-322  B.C.),  the  most  famous  pupil  of  Plato  and 
dissenter  from  the  Platonic  School,  represents  the  high-water  mark  of  the 
Greek  students  of  nature  and  is  justly  called  the  Father  of  Natural  History. 
Aristotle's  contributions  to  biology  are  manifold.  He  took  a  broad  survey  of 
the  existing  facts  and  welded  them  into  a  science  by  relying,  to  a  considerable 
extent,  on  the  direct  study  of  organisms  and  by  insisting  that  the  only  true 
path  of  advance  lay  in  accurate  observation  and  description.  But  mere 
observation  without  interpretation  is  not  science.  Aristotle's  generalizations 
— his  elaboration  of  broad  philosophical  conceptions  of  organisms — give  to  his 
biological  works  their  perennial  significance.  Among  the  facts  and  supposed 
facts  there  are  interspersed  questions,  answers,  theories  which  involve  a 
recognition  and  remarkable  grasp  of  fundamental  biological  problems; 
though  of  course  there  are  many  crudities  because  adequate  apparatus  and 
biological  technique  were  of  the  distant  future.  A  study  of  Aristotle's  works 
shows  ancient  pedigrees  for  some  of  the  most  'modern'  questions  of  biology, 
though  it  is  undoubtedly  true,  as  Sachs  insists,  that  one  must  continually 
inhibit  the  tendency  to  read  the  present  viewpoint  into  the  past,  and  not 
assign  to  earlier  writers  merits  which,  if  they  were  alive,  they  themselves 
would  not  claim. 

"We  have  not  mentioned  a  single  discovery  made  by  Aristotle — and  with 
purpose.  Aristotle's  position  as  the  founder  of  biology  rests  chiefly  on  his 
viewpoint  and  his  methods.  Plato  relied  on  intuition  as  the  basis  of  knowl- 
edge. Aristotle  emphasized  observation  and  induction,  insisting  that  errors 
arise  not  from  the  false  testimony  of  our  sense  organs  but  from  false  interpreta- 
tions of  the  data  they  afford.  'We  must  not  accept  a  general  principle  from 
logic  only,  but  musfr  prove  its  application  to  each  fact;  for  it  is  in  facts  that 
we  must  seek  general  principles,  and  these  must  always  accord  with  facts 
from  which  induction  is  the  pathway  to  general  laws.'  But  it  is  not  to  be 


500  HUMAN  BIOLOGY 

imagined  that  Aristotle  always  followed  his  own  advice;  few  great  men  do — 
'no  pilot  can  explore  unsurveyed  channels  without  a  confidence  which  some- 
times leads  to  disaster/  It  must  be  admitted  that  Aristotle  frequently 
lapsed  into  unbridled  speculation  which  tended  to  obscure  the  methods  that 
time  has  shown  produce  the  most  enduring  results,  though,  as  Huxley  has 
well  said,  'It  is  a  favorite  popular  delusion  that  the  scientific  enquirer  is  under 
a  sort  of  moral  obligation  to  abstain  from  going  beyond  that  generalization  of 
observed  facts  which  is  absurdly  called  'Baconian'  induction.  But  any  one 
who  is  practically  acquainted  with  scientific  work  is  aware  that  those  who 
refuse  to  go  beyond  fact,  rarely  get  as  far  as  fact ;  and  any  one  who  has  studied 
the  history  of  science  knows  that  almost  every  step  therein  has  been  made  by 
the  'Anticipation  of  Nature/  that  is,  by  the  invention  of  hypotheses,  which, 
though  verifiable,  often  had  very  little  foundation  to  start  with;  and  not 
infrequently,  in  spite  of  a  long  career  of  usefulness,  turned  out  to  be  wholly 
erroneous  in  the  long  run. 

"With  the  Greeks,  then,  biology  emerged  from  the  shadows  of  the  past 
and  took  concrete  form — a  fact  which  apparently  the  discerning  mind  of 
Aristotle  appreciated  since,  though  frequently  referring  to  the  ancients,  he 
wrote;  'I  found  no  basis  prepared;  no  models  to  copy.  .  .  .  Mine  is  the  first 
step,  and  therefore  a  small  one,  though  worked  out  with  much  thought  and 
hard  labor.  It  must  be  looked  at  as  a  first  step  and  judged  with  indulgence/  " 
{Woodruff,  "The  Development  of  the  Sciences/'  Chap.  VI.,  pp.  216-218, 
Yale  University  Press,  New  Haven,  Conn.,  1923.) 

Arthropoda.  "  The  great  phylum  Arthropoda  is  the  largest  phylum  of  the 
animal  kingdom  in  the  number  of  known  species  and  possibly  also  in  the  total 
number  of  individuals.  Thus  the  waters  of  the  earth,  both  fresh  and  salt, 
swarm  with  myriads  of  Arthropods  consisting  for  the  most  part  of  microscopic 
Crustacea,  close  relatives  of  the  Crayfish,  while  the  soil  and  air  are  dominated, 
at  least  in  numbers,  by  an  almost  infinite  variety  of  Insects  which  also  belong 
to  the  Arthropods.  In  the  study  of  the  Crayfish  we  have  seen  that  to  the 
segmented  body,  first  noted  in  the  Annelida,  the  Arthropods  have  added 
paired,  jointed  appendages  which  are  highly  modified  in  many  instances  for 
the  performance  of  definite  functions.  Moreover  the  principles  of  segmental 
specialization  and  cephalization,  which  were  introduced  in  certain  of  the 
Annelida,  are  further  exemplified  and  firmly  established  in  the  Arthropods. 
It  is  noteworthy,  however,  that  in  the  more  primitive  members  of  this  phylum, 
as  in  most  of  the  Annelida,  segmental  specialization  is  lacking  except  in  the 
head  segments.  Also  characteristic  of  the  arthropodan  body  is  a  secreted 
exoskeleton  with  definite  body  regions,  and  with  joints  provided  for  flexi- 
bility, but  requiring  molting  for  growth  of  the  body.  Finally  a  great  reduc- 
tion of  the  coelom,  as  compared  with  the  Annelids,  is  characteristic. 

"Commonly,  five  classes  of  Arthropods  are  recognized:  (1)  the  Crustacea, 
represented  by  the  Crayfish,  most  of  which  are  watef-living  forms  which 
breathe  by  gills;  (2)  the  Arachnoidea,  represented  by  the  Spider,  most  of  whicli 
are  land-living  forms  breathing  by  peculiar  book-lungs;  (3)  the  Onychophora, 
a  very  small  class  represented  by  Peripatus — a  living  link  between  Annelid 


APPENDIX  501 

and  Arthropod;  (4)  Myriapoda,  represented  by  the  Centipedes,  with  a  mini- 
mum of  segmental  and  appendage  specialization;  (5)  the  Insecta,  represented 
by  Grasshopper  and  Honey  Bee  and,  on  the  whole,  exhibiting  the  acme  of 
invertebrate  development."  (Baitsell,  "  Manual  of  Biology,"  pp.  208-209, 
The  Macmillan  Company,  New  York,  1936.) 

Atoms.    See  Matter,  Dissociation. 

Beaumont,  William.  "William  Beaumont  (1785-1853),  a  physician  sta- 
tioned at  a  military  post  in  the  primeval  forest  of  northern  Michigan,  grasped 
the  unique  opportunity  of  studying  the  processes  of  digestion  in  the  victim  of 
a  gunshot  wound  that  had  caused  a  permanent  gastric  fistula.  The  story  of 
Alexis  St.  Martin's  accident  and  of  Beaumont's  wisdom  in  the  management 
of  the  case  are  clearly  set  forth  in  Beaumont's  own  words,  which  follow. 
The  accident  occurred  on  June  6,  1822.  The  patient  had  recovered  within  a 
few  months,  and  Beaumont  attempted  during  the  next  two  years,  by  repeated 
dressings,  to  close  the  wound.  The  first  observations  of  a  physiological 
nature  were  begun  in  May,  1825.  Full  details  of  his  studies  were  published 
in  a  separate  volume  in  1833.  After  giving  the  case  history,  Beaumont 
described  briefly  the  work  of  his  predecessors  in  the  same  field  and  then  put 
down  his  epoch-making  observations  upon  movements  of  the  stomach  during 
digestion,  the  normal  appearance  of  the  gastric  mucous  membrane,  the  fact 
that  gastric  juice  is  secreted  only  as  a  result  of  the  taking  of  food,  mechanical 
irritation  being  ineffective.  By  a  series  of  ingenious  arguments  he  concluded 
that,  in  addition  to  free  hydrochloric  acid,  which  Prout  had  previously 
observed,  there  was  also  present  in  the  gastric  juice  another  active  chemical 
substance,  to  which  Schwann  in  1835  gave  the  name  of  pepsin.  Beaumont's 
observations  illustrate  the  enormous  contribution  that  may  come  from  pains- 
taking clinical  observation,  and  those  who  read  his  little  book  must  inevitably 
feel  the  inspiration  of  his  great  example. 

"'Whilst  stationed  at  Michillimackinac,  Michigan  Territory,  in  1822,  in 
the  military  service  of  the  United  States,  the  following  case  of  surgery  came 
under  my  care  and  treatment. 

"'Alexis  St.  Martin,  who  is  the  subject  of  these  experiments,  was  a  Cana- 
dian, of  French  descent,  at  the  above  mentioned  time  about  eighteen  years  of 
age,  of  good  constitution,  robust  and  healthy.  He  had  been  engaged  in  the 
service  of  the  American  Fur  Company,  as  a  voyageur,  and  was  accidentally 
wounded  by  the  discharge  of  a  musket,  on  the  6th  of  June,  1822.  .  .  .  The 
whole  mass  of  materials  forced  from  the  musket,  together  with  fragments  of 
clothing  and  pieces  of  fractured  ribs,  were  driven  into  the  muscles  and  cavity 
of  the  chest. 

"'I  saw  him  in  twenty-five  or  thirty  minutes  after  the  accident  occurred, 
and,  on  examination,  found  a  portion  of  the  lung,  as  large  as  a  Turkey's  egg, 
protruding  through  the  external  wound,  lacerated  and  burnt;  and  imme- 
diately below  this,  another  protrusion,  which,  on  further  examination,  proved 
to  be  a  portion  of  the  stomach,  lacerated  through  all  its  coats,  and  pouring 
out  the  food  he  had  taken  for  his  breakfast,  through  an  orifice  large  enough 
to  admit  the  fore  finger. 


502  HUMAN  BIOLOGY 

EXPEKIMENT  I 

"  'August  1,  1825.  At  12  o'clock  M.,  I  introduced  through  the  perforation, 
into  the  stomach,  the  following  articles  of  diet,  suspended  by  a  silk  string, 
and  fastened  at  proper  distances,  so  as  to  pass  in  without  pain — viz.: — a  piece 
of  high  seasoned  a  la  mode  beef;  a  piece  of  raw,  salted,  fat  pork;  a  piece  of  raw, 
salted,  lean  beef;  a  piece  of  boiled,  salted  beef;  a  piece  of  stale  bread;  and  a 
bunch  of  raw,  sliced  cabbage;  each  piece  weighing  about  two  drachms;  the  lad 
continuing  his  usual  employment  about  the  house. 

"'At  1  o'clock  P.M.,  withdrew  and  examined  them — found  the  cabbage 
and  bread  about  half  digested:  the  pieces  of  meat  unchanged.  Returned 
them  into  the  stomach. 

" '  At  2  o'clock  P.M.,  withdrew  them  again — found  the  cabbage,  bread,  pork, 
and  boiled  beef,  all  cleanly  digested,  and  gone  from  the  string;  the  other  pieces 
of  meat  but  very  little  affected.  Returned  them  into  the  stomach  again. 

"'At  3  o'clock  P.M.,  examined  again — found  the  a  la  mode  beef  partly 
digested:  the  raw  beef  was  slightly  macerated  on  the  surface,  but  its  general 
texture  was  firm  and  entire.  The  smell  and  taste  of  the  fluids  of  the  stomach 
were  slightly  rancid ;  and  the  boy  complained  of  some  pain  and  uneasiness  at 
the  breast.  Returned  them  again. 

"'The  lad  complaining  of  considerable  distress  and  uneasiness  at  the 
stomach,  general  debility  and  lassitude,  with  some  pain  in  his  head,  I  with- 
drew the  string,  and  found  the  remaining  portions  of  aliment  nearly  in  the 
same  condition  as  when  last  examined ;  the  fluid  more  rancid  and  sharp.  The 
boy  still  complaining,  I  did  not  return  them  any  more. 

"  'August  2.  The  distress  at  the  stomach  and  pain  in  the  head  continuing, 
accompanied  with  costiveness,  a  depressed  pulse,  dry  skin,  coated  tongue,  and 
numerous  white  spots,  or  pustules,  resembling  coagulated  lymph,  spread  over 
the  inner  surface  of  the  stomach,  I  thought  it  advisable  to  give  medicine;  and, 
accordingly,  dropped  into  the  stomach,  through  the  aperture,  half  a  dozen 
calomel  pills,  four  or  five  grains  each;  which,  in  about  three  hours,  had  a 
thorough  cathartic  effect,  and  removed  all  the  foregoing  symptoms,  and  the 
diseased  appearance  of  the  inner  coat  of  the  stomach.  The  effect  of  the 
medicine  was  the  same  as  when  administered  in  the  usual  way,  by  the  mouth 
and  oesophagus,  except  the  nausea  commonly  occasioned  by  swallowing 
pills.'"  ...  (Fulton,  "Selected  Readings  in  the  History  of  Physiology," 
pp.  164-169,  Courtesy  of  Charles  C.  Thomas,  Springfield,  111.,  1930.) 

Binomial  Nomenclature.    See  Taxonomy. 

Biogenesis.  The  term  given  by  Huxley  to  designate  the  now  generally 
accepted  belief  that  life  comes  only  from  preexisting  life,  as  opposed  to  the 
view,  firmly  established  until  the  middle  of  the  nineteenth  century,  that  proto- 
plasm was  continually  being  formed  spontaneously  from  nonliving  matter. 
The  final  establishment  of  biogenesis  and  the  downfall  of  abiogenesis  was  due 
very  largely  to  the  researches  of  Pasteur.  "In  the  two-thirds  of  a  century 
that  have  since  elapsed,  it  has  been  shown  in  various  ways  that  if  due  pre- 


APPENDIX 


503 


cautions  be  taken  to  exclude  living  organisms  and  their  eggs,  spores,  or  seeds, 
no  fermentation,  putrescence,  or  other  production  of  minute  life  ever  takes 
place.  It  is  all  a  question  of  the  adequacy  of  the  precautions.  This  adequacy 
is  a  question  of  technique."  (Singer,  " Story  of  Living  Things,"  p.  441, 
Harper  &  Brothers,  New  York,  1931.) 


Biological 
Sciences. 


Biology,  the 
science  of  life 


of  animals 
Zoology 


of  plants 
Botany 


Physiology  (chemical  and  physical  processes) 
Anatomy  (gross  structure) 
Histology  (microscopic  structure) 

Morphology   Embryology  (development  of  struc- 
(structure)        ture,    studied    partly   by   physio- 
logical method) 

Cytology  (morphology  and  physiology  of  cells) 

Pathology  (morbid  morphology  and  physiology) 

Psychology  (mental  phenomena,  studied  largely 
by  physiological  method) 

Ecology  (adaptation  and  other  relationships  of 
organism  to  its  environment,  studied  chiefly  by 
physiological  method) 

Taxonomy  (classification,  based  chiefly  on  com- 
parative anatomy  but  partly  on  physiology) 

Physiology 

Morphology 

Cytology      (Each  of  these  has  its  physiological 

Pathology     f     aspects  as  in  corresponding  zoologi- 

Ecology  cal  studies 

Taxonomy 


of   those  microorganisms 

and  other  forms,  difficult 

to  classify  as  true  plants 

or  true  animals, 
Bacteriology,    dealing    with 

plant-like  microscopic 

forms, 
Mycology,  dealing  with 

fungi, 
Protozoology,   dealing   with 

animal-like  microscopic 

forms 


Each  of  these  branches  of 
biology  subdivides  into  physi- 
ology, morphology,  cytology, 
pathology,  ecology  and  tax- 
onomy, and  each  of  these  sub- 
divisions has  its  physiological 
aspects. 


(Mitchell,   A   Textbook   of  General  Physiology,  p.  xiv,   McGraw-Hill  Book 
Company,  Inc.,  New  York,  1938.) 

Biological  Elements.  The  problem  of  determining  which  chemical  ele- 
ments are  essential  to  the  activities  of  living  matter  is  difficult.  Certain 
elements,  such  as  silicon  and  aluminum,  are  so  widespread  in  nature  that  it 
would  be  hard  for  organisms  to  prevent  their  entering  their  cells  in  small 


504  HUMAN  BIOLOGY 

quantities.  Such  substances  are  usually  reported  in  chemical  analyses  of 
tissues,  including  those  of  man,  but  there  is  no  evidence  that  they  perform 
any  biological  functions  in  animal  protoplasm.  Certain  other  elements, 
though  undoubtedly  of  great  functional  importance,  exist  in  such  small 
quantities  in  organisms  that  an  investigation  of  their  role  is  extremely  difficult. 
The  essential  part  played  by  such  an  element  can  be  determined  only  by 
restricting  the  supply  of  this  element  in  the  environment  of  a  plant  or  animal 
and  then  observing  whether  or  not  the  restriction  has  a  deleterious  effect  on 
the  organism.  Sometimes  natural  environments  are  deficient  in  particular 
elements.  Thus  a  restricted  iodine  supply  in  certain  regions  produces 
endemic  goiter  in  man;  a  restricted  cobalt  supply,  bush-sickness  in  sheep;  a 
restricted  boron  supply,  various  diseases  of  fruit  trees  and  edible  roots. 
These  diseases  can  all  be  easily  cured  by  adding  the  missing  element.  Experi- 
mental investigations  of  such  deficiencies  are  very  difficult,  as  it  is  hard  to 
prepare  pure  solutions  for  the  growth  of  plants  and  far  harder  to  prepare  pure 
solid  organic  food  stuffs,  free  from  the  element  in  question,  for  the  higher 
animals.  At  present,  information  of  this  sort  has  been  obtained  chiefly  for 
green  plants;  for  fungi;  and,  with  considerably  less  accuracy,  for  the  rat. 
Apart  from  such  experiments,  the  probability  of  the  essential  role  of  an  element 
is  enhanced  by  the  existence  of  special  organic  compounds  of  the  element  in 
the  tissues  of  organisms  and  of  high  concentration  of  the  element  in  the  organs. 
As  a  provisional  and  rough  classification,  the  generally  essential  biological 
elements  may  be  classified  as: 

1.  Universal  primary  constituents,  occurring  in  all  organisms  and  consti- 
tuting more  than  1  per  cent  of  the  living  matter  of  the  earth:  oxygen,  hydro- 
gen, carbon,  nitrogen,  phosphorus,  sulphur.     (In  man,  more  calcium  and  less 
sulphur  are  present  than  in  the  organic  world  as  a  whole.) 

2.  Universal  secondary  constituents,  occurring  in  all  organisms  and  con- 
stituting between  0.05  and  1.0  per  cent  of  the  living  matter  of  the  earth: 
sodium,  magnesium,  chlorine,  potassium,  calcium,  iron. 

3.  Universal    viicroconstituents,    occurring    in    all    organisms    adequately 
studied,  and  apparently  essential  to  living  matter,  but  present  in  very  minute 
amounts,  less  than  0.05  per  cent:  manganese,  copper,  zinc,  iodine,  probably 
cobalt,  and  possibly  arsenic  and  fluorine. 

4.  Elements  of  probable  biological  significance  in  plants  or  some  species  of 
animals,  but,  as  far  as  is  known,  having  no  function  in  man,  though  further 
research  may  indicate  the  universal  importance  of  some  of  them:  boron, 
silicon,    vanadium,    gallium,   selenium,   bromine,   strontium,   molybdenum, 
barium,  and  possibly  aluminum  and  scandium. 

5.  In  addition  to  these  30  elements,  the  atoms  of  which  appear  to  be  the 
ultimate  building  stones  from  which  all  organisms  are  constructed,  evidence 
for  the  existence  of  at  least  28,  and  possibly  42,  other  elements  in  the  bodies 
of  animals  or  plants  has  from  time  to  time  been  obtained,  thus  making  a  total 
of  at  least  58  and  posvsibly  72  of  the  92  elements  believed  to  exist.     Most  of 
these,  such  as  the  rare  earth  elements  deposited  with  calcium  in  bones  or  the 
argon  dissolved  from  the  air  by  blood  and  other  body  fluids,  certainly  have  no 
biological  function;  some  others,  however,  may  turn  out  to  be  true  micro- 


APPENDIX 


505 


constituents.  In  man,  the  most  constantly  present  of  such  accidental  elements 
are  lithium,  rubidium,  nickel,  silver,  tin,  lead,  and  mercury.  Minute  quan- 
tities* of  radium  occur  in  organisms,  but  the  commoner,  though  less  soluble, 
radioactive  element,  thorium,  seems  to  be  absent  from  biological  material. 

A  few  elements  exist  in  nature  in  quantities  deleterious  to  'organisms. 
Fluorine  in  water  supplies  has  been  found  to  cause  mottled  teeth,  a  serious 
dental  condition  common  in  the  Southwestern  states.  Selenium,  which  is 
essential  to  some  species  of  the  small  pea-like  plant  Astragalus,  is  highly 
poisonous  'to  most  animals.  Domestic  animals  eating  the  plant  become 
afflicted  with  "loco  disease. "  Selenium  may  enter  wheat  plants  from  sele- 
nif erous  soils,  derived  in  part  from  rocks  containing  fossil  Astragalus ;  and  such 
wheat  is  unfit  for  human  consumption.  At  least  one  case  has  been  recorded 
of  stock  suffering  from  an  overdose  of  molybdenum  naturally  accumulating  in 
pasture  grasses  from  a  molybdenum-rich  soil. 

In  general,  the  biological  elements  are  the  light  and  common  elements  of 
the  universe.  In  both  the  universe  as  a  whole  and  the  chemical  composition 
of  organisms,  the  common  elements  are  the  elements  of  low  atomic  weight. 
The  only  important  exceptions  to  the  rule  of  decreasing  abundance  of  elements 
with  increasing  atomic  weights  are  the  elements  lithium,  beryllium,  and  to  a 
less  extent,  perhaps,  boron,  which  are  lighter  but  much  rarer  in  the  universe 
than  carbon  and  nitrogen.  These  three  elements  can  be  disintegrated  into 
helium  and  hydrogen  in  the  interiors  of  stars  and  so  have  tended  to  disappear 
from  the  universe.  It  is  therefore  very  interesting  to  find  that  lithium  and 
beryllium  are  the  only  light  elements  for  which  no  indication  of  any  biological 
function  has  ever  been  found.  Boron,  however,  which  is  important  to  plants, 
is  abnormally  available  at  the  earth's  surface,  owing  to  the  great  solubility 
of  the  borates,  and  so  has  entered  into  the  living  world. 

The  following  table  indicates  the  order  of  abundance  of  the  commoner 
elements  in  the  universe  as  a  whole,  in  the  earth's  crust,  in  sea  water,  in  the 
atmosphere,  and  in  the  human  body. 


Universe  as  a 
whole 

Earth's  crust 

Sea  water 

Atmosphere 

Human  body 

Hydrogen 

Oxygen,  49.5  % 

Oxygen,  84.2  % 

Nitrogen,  78% 

Oxygen,  65  % 

Helium 

Silicon,  25.7  % 

Hydrogen,  12.0% 

Oxygen,  21  % 

Carbon,  18.3% 

Oxygen 

Aluminum,  7.5  % 

Chlorine,  2.1% 

Argon,  1.0% 

Hydrogen,  10% 

Carbon 

Iron,  4.7  % 

Sodium,  1.2% 

Hydrogen,  ca.0.1  % 

Nitrogen,  2.65% 

Nitrogen 

Calcium,  3.4% 

Magnesium,  0.14  % 

Carbon,  ca.0.01  % 

Calcium,  1.4% 

Neon 

Sodium,  2.0% 

Sulphur,  0.097  % 

Other  constituents, 

Phosphorus  0.8  % 

Iron 

Potassium,  2.4% 

Calcium,  0.046% 

less  than  0.01  % 

Potassium,  0.3  % 

Silicon 

Magnesium,  2.0  % 

Potassium,0.041  % 

Sodium,  0.3% 

Magnesium 

Hydrogen,  1.0  % 

Carbon,  0.010% 

Chlorine,  0.3  % 

Argon 

Titanium,  0.5% 

Other  constituents, 

Sulphur,  0.2  % 

Nickel 

Carbon,  0.4  % 

less  than  0.01  % 

Magnesium,  0.04  % 

Aluminum 

Chlorine,  0.2  % 

Iron,  0.04  % 

Calcium 

Sulphur,  0.15% 

Other  constituents, 

Sodium 

Manganese,  0.1  % 

less  than  0.04  % 

Other  constituents 

Phosphorus,  0.1  % 

\ 

probably      less 

Other  constituents, 

than  0.01  % 

less  than  0.1% 

506  HUMAN  BIOLOGY 

Owing  to  the  fact  that  it  is  difficult  to  estimate  the  relative  amounts  of 
hydrogen  and  helium  in  the  universe,  though  it  is  known  that  these  are  the 
two  commonest  elements,  no  percentages  are  given  for  the  first  column.  The 
14  elements  given  in  that  column,  however,  are  probably  the  only  ones  that 
constitute  at  least  0.01  per  cent  of  the  universe.  Except  for  the  presence 
of  large  amounts  of  the  inert  gases,  helium  and  neon,  the  composition  of  man 
is  not  unlike  that  of  the  average  inorganic  matter  of  the  universe;  more  like 
the  average,  in  fact,  than  are  the  earth's  crust,  the  sea,  or  the  air.  In  both 
the  universe  and  in  living  organisms,  hydrogen,  oxygen,  carbon,  and  nitrogen 
are  the  chief  active  elements  existing  in  large  amounts  and  in  more  or  less 
comparable  proportions. 

The  importance  of  the  distribution  of  elements  is  best  seen  if  we  compare 
the  composition  of  a  unit  volume,  say,  1,000  cc.  of  sea  water  with  an  equal 
volume  of  living  organisms,  say,  fish.  The  marine  animal  has  had  no  diffi- 
culty in  obtaining  its  hydrogen,  oxygen,  chlorine,  sulphur,  magnesium,  or 
sodium;  its  calcium  content  will  be  from  five  to  ten  times  that  of  the  sea  water; 
its  carbon  content  and  content  of  microconstituents,  about  one  thousand 
times;  its  combined  nitrogen,  phosphorus,  and  iron  contents,  about  ten 
thousand  times.  These  last  three  elements,  then,  ultimately  set  a  limit  to  the 
amount  of  life  in  the  sea  and  so  to  the  number  of  fish  available  for  human 
consumption.  This  aspect  of  the  biological  elements  is  of  great  importance 
in  the  study  of  biogeochemistry,  the  science  that  considers  the  transformations 
of  chemical  substances  through  organisms  in  nature.  (Hutchinson,  Osborn 
Zoological  Laboratory,  Yale  University,  New  Haven,  Conn.,  January,  1940.) 

Biology  and  Medicine.  "  Before  leaving  the  Greeks  we  must  mention 
Hippocrates  (460-370  B.C.),  the  Father  of  Medicine.  Writing  a  generation 
before  Aristotle,  at  the  height  of  the  Age  of  Pericles,  Hippocrates  crystallized 
the  knowledge  of  medicine  into  a  science,  dissociated  it  from  philosophy,  and 
gave  to  physicians  'the  highest  moral  inspiration  they  have/  To  him  medi- 
cine owes  the  art  of  clinical  inspection  and  observation,  and  he  is,  above  all, 
the  exemplar  of  that  flexible,  critical,  well-poised  attitude  of  mind,  ever  on  the 
lookout  for  sources  of  error,  which  is  the  very  essence  of  the  scientific  spirit. 
.  .  .  The  revival  of  the  Hippocratic  methods  in  the  seventeenth  century  and 
t.heir  triumphant  vindication  by  the  concerted  scientific  movement  of  the 
nineteenth,  is  the  whole  history  of  internal  medicine/ 

"Medicine,  the  most  important  aspect  of  applied  biology,  is  the  foster 
parent  of  zoology  and  botany,  since  a  large  proportion  of  biological  advances 
have  been  the  work  of  physicians.  Until  relatively  recently  the  schools  of 
medicine  afforded  the  only  training,  and  the  practice  of  medicine  the  chief 
livelihood  for  men  especially  interested  in  general  biological  problems.  The 
history  of  medicine  and  of  biology  as  a  so-called  pure  science  are  so  inex- 
tricably interwoven  that  the  consideration  of  one  involves  that  of  the  other. 
Indeed,  the  physicians  form  the  only  bond  of  continuity  in  biological  history 
between  Greece  and  Rome.  The  chief  interest  of  the  Romans  lay  in  tech- 
nology, and  therefore  it  is  natural  that  the  practical  advantages  to  be  gained 


APPENDIX  507 

should  ensure  the  advance  of  medicine.  As  it  happens,  however,  two  Greek 
physicians  were  destined  to  have  the  most  influence:  Dioscorides  (c.  64  A.D.), 
an  army  surgeon  under  Nero,  and  Galen  (131-201  A.D.),  physician  to  the 
Emperor  Marcus  Aurelius  and  his  son,  Commodus. 

"  Just  as  Theophrastus  established  botany  as  a  pure  science,  so  Dioscorides 
was  the  originator  of  the  pharmacopoeia,  writing,  as  he  did,  not  only  a  work 
which  was  the  first  one  on  medical  botany,  but  one  which,  gaining  authority 
with  age,  was  the  sole  standard  'botany'  for  fifteen  centuries.  Theophrastus 
was  long**  overshadowed.  Most  of  the  botanical  writings  up  to  the  seven- 
teenth century  were  annotations  on  the  text  of  Dioscorides. 

"  Galen  was  the  most  famous  physician  of  the  Roman  Empire  and  his 
voluminous  works  represent  both  the  depository  for  the  anatomical  and 
physiological  knowledge  of  his  predecessors,  rectified  and  worked  over  into  a 
system,  and  a  large  amount  of  original  investigation.  Galen  was  a  practical 
anatomist  who  described  from  dissections  and  insisted  on  the  importance  of 
vivisection  and  experiment,  and  therefore  he  may  be  considered  the  first 
experimental  physiologist  and  the  founder  of  experimental  medicine.  Galen 
gave  to  medicine  its  standard  anatomy  and  physiology  for  fifteen  centuries. 

"  Any  consideration  of  the  biological  science  of  Rome  would  be  incomplete 
without  a  reference  to  the  vast  compilation  of  a  fact  and  fiction,  indiscrimi- 
nately mingled,  made  by  Pliny  the  Elder  (23-79).  It  was  beside  the  path  of 
biological  advance,  but  long  the  recognized  'Natural  History/  passing  through 
some  eighty  editions  after  the  invention  of  printing.  Its  prestige  was  largely 
due  to  the  fact  that  it  was  written  in  Latin,  whereas  the  great  works  on 
biological  subjects  were  in  Greek. 

"For  all  practical  purposes  we  may  consider  that  biology  at  the  decline 
of  the  Roman  Empire  was  represented  in  the  works  of  Aristotle,  Theophrastus, 
Dioscorides,  Galen,  and  Pliny.  Even  these  exerted  little  influence  during 
the  Middle  Ages,  being  saved  from  total  loss  for  future  generations  chiefly 
by  Arabian  scientists,  and  in  the  monasteries  of  Italy  and  Britain.  We  cannot 
pause  to  consider  the  various  causes  which  resulted  in  the  almost  complete 
break  in  the  continuity  of  learning  in  general  and  science  in  particular  during 
the  dormant  period  in  western  Europe.  Suffice  it  to  say  that  contributing 
factors  were  wars  and  rumors  of  wars,  the  destruction  of  the  libraries  of 
Alexandria,  the  antagonism  of  Christian  and  pagan  ideals,  and  the  empha- 
sis by  the  Church,  which  held  the  gates  of  learning,  of  the  written  word 
in  place  of  observation  of  nature  as  it  is.  To  a  very  large  extent  'truth 
and  science  came  to  mean  simply  that  which  was  written,  and  inquiry 
became  mere  interpretation/  though  recent  historical  studies  are  reveal- 
ing medieval  scientific  manuscripts  which  may  necessitate  a  reappraisal  of 
the  period. 

"In  so  far  as  science  reached  the  people  in  general,  it  was  almost  solely 
from  small  compilations  of  corrupt  texts  of  ancient  authors  interspersed  with 
anecdotes  and  fables.  Quite  characteristic  of  the  times  is  the  oft-quoted 
Physiologus,  found  in  many  forms  and  languages,  that  evolved  into  a  collection 


508  HUMAN  BIOLOGY 

of  natural  history  stories  in  which  the  centaur  and  phoenix  take  their  place 
with  the  frog  and  crow  in  affording  allegorical  illustrations  of  texts  and  in 
pointing  out  more  or  less  evident  morals.  The  line  of  demarcation  between 
the  Physiologus  and  the  Bestiaries  is  ill  defined,  while  the  remnants  of  the  latter 
are  incorporated  in  the  early  works  of  the  Renaissance  encyclopaedists. 

"The  scientific  Renaissance  may  be  said  to  owe  its  origin  to  the  revival  of 
classical  learning  and  to  the  translation  and  study  of  the  writings  of  Aristotle 
and  others  which  had  been  under  eclipse  for  a  thousand  years.  These  were  so 
superior  to  the  existing  science  that,  in  accord  with  the  spirit  of  the  time, 
Aristotle  and  Galen  became  the  bible  of  biology.  The  first  works  were  merely 
commentaries  on  the  classical  authors,  but  as  time  went  on  more  and  more 
new  observations  were  interspersed  with  the  old  until  elaborate  and  volumi- 
nous treatises  describing  all  known  forms  of  plants  and  animals  were  produced. 
In  short,  the  climax  of  the  scientific  Renaissance  involved  a  turning  away  from 
the  authority  of  Aristotle  and  an  adoption  of  the  Aristotelian  method  of 
observation  and  induction."  .  .  .  (Woodruff,  "The  Development  of  the 
Sciences,"  Chap.  VI,  pp.  218-221,  Yale  University  Press,  New  Haven,  Conn., 
1923.) 

Blood  Pressure.  Stephen  Hales  (1677-1761)  took  the  next  important 
step  after  Harvey  and  Malpighi  in  elucidating  the  physiology  of  the  circula- 
tion. The  determination  of  blood  pressure  made  it  possible  to  calculate  the 
work  done  by  the  heart,  and  to  estimate  for  the  first  time  the  magnitude  of  the 
peripheral  resistance.  The  following  selection  is  taken  from  his  "Haemasta- 
ticks,"  published  in  1733.  (Spelling  modernized.) 

"1.  In  December  I  laid  a  common  field  gate  on  the  ground,  with  some 
straw  upon  it,  on  which  a  white  mare  was  cast  on  her  right  side,  and  in  that 
posture  bound  fast  to  the  gate;  she  was  fourteen  hands  and  three  inches  high; 
lean,  tho'  not  to  a  great  degree,  and  about  ten  or  twelve  years  old.  This  and 
the  above  mentioned  horse  and  mare  were  to  have  been  killed,  as  being  unfit 
for  service. 

"2.  Then  laying  open  the  left  jugular  vein,  I  fixed  to  that  part  of  it  which 
comes  from  the  head,  a  glass  tube,  which  was  four  feet,  and  two  inches  long. 

"3.  The  blood  rose  in  it,  in  three  or  four  seconds  of  time,  about  a  foot, 
and  then  was  stationary  for  two  or  three  seconds;  then  in  three  or  four  seconds 
more,  it  rose  sometimes  gradually,  and  sometimes  with  an  unequally  acceler- 
ated motion  nine  inches  more,  on  small  strainings  of  the  mare:  Then  upon 
greater  strainings  it  rose  about  a  yard,  and  would  subside  five  or  six  inches: 
Then  upon  a  larger  strain  or  struggle  of  the  mare,  it  rose  so  high,  as  to  flow 
a  little  out  at  the  top  of  the  tube ;  so  that  had  the  tube  been  a  few  inches  higher, 
it  would  have  risen  probably  to  that  height. 

"4.  When  the  mare  ceased  to  strain  and  struggle,  the  blood  subsided 
about  eighteen  or  twenty  inches ;  so  the  return  of  the  blood  into  the  vein  was 
not  hindered  by  the  valves;  which  I  have  also  observed  in  other  parts  where 
there  are  valves,  tho'  sometimes  they  absolutely  hinder  the  return  of  any 
fluid. 


APPENDIX  509 

"5.  The  diameter  of  the  brass  pipe  and  tube  which  were  fixed  to  the  vein, 
were  nearly  one  seventh  of  an  inch:  The  diameter  of  the  jugular  vein  about 
half  an  inch. 

"6.  Then  laying  bare  the  left  carotid  artery,  I  fixed  to  it  towards  the 
heart  the  brass  pipe,  and  to  that  the  wind-pipe  of  a  goose ;  to  the  other  end  of 
which  a  glass  tube  was  fixed,  which  was  twelve  feet  nine  inches  long.  The 
design  of  using  the  wind-pipe  was  by  its  pliancy  to  prevent  the  inconven- 
iencies  that  might  happen  when  the  mare  struggled;  if  the  tube  had  been 
immediately  fixed  to  the  artery,  without  the  intervention  of  this  pliant  pipe. 

"7.  There  had  been  lost  before  the  tube  was  fixed  to  the  artery,  about 
seventy  cubic  inches  of  blood.  The  blood  rose  in  the  tube  in  the  same  manner 
as  in  the  case  of  the  two  former  horses,  till  it  reached  to  nine  feet  six  inches 
height.  I  then  took  away  the  tube  from  the  artery,  and  let  out  by  measure 
sixty  cubic  inches  of  blood,  and  theft  immediately  replaced  the  tube  to  see  how 
high  the  blood  would  rise  in  it  after  each  evacuation ;  this  was  repeated  several 
times,  till  the  mare  expired,  as  follows,  viz. 

"8.  We  may  observe,  that  these  three  horses  all  expired,  when  the  per- 
pendicular height  of  the  blood  in  the  tube  was  about  two  feet. 

"9,  These  833  cubic  inches  of  blood  weigh  28.89  pounds,  and  are  equal  to 
fourteen  wine  quarts,  the  large  veins  in  the  body  of  the  mare  were  full  of 
blood,  there  was  some  also  in  the  descending  aorta,  and  in  both  ventricles  and 
auricles."  (Fulton,  "Selected  Readings  in  the  History  of  Physiology," 
pp.  58-60,  Charles  C.  Thomas,  Springfield,  111.,  1930.) 

Brownian  Movement.  "In  1827,  a  British  botanist,  Robert  Brown, 
observed  that  microscopically  small  particles  of  pollen  dust,  when  suspended 
in  water,  are  in  a  state  of  constant  agitation.  They  move  incessantly  in  a 
random,  zigzag  manner.  The  smaller  the  particles  are  the  greater  is  the 
activity.  Now  it  can  be  shown  that  the  Brownian  movement,  quantitatively 
as  well  as  qualitatively,  is  just  what  we  should  expect  on  the  basis  of  the  kinetic 
molecular  theory  of  matter.  The  suspended  particle  is  being  constantly 
bombarded  from  all  sides  by  the  moving  molecules  of  the  liquid.  If  the 
particle  is  so  large  as  to  be  visible  to  the  unaided  eye,  no  motion  will  be  per- 
ceptible, since  the  number  of  molecular  blows  to  which  the  particle  is  sub- 
jected at  any  given  moment  is  so  large  that  they  practically  balance  one 
another.  However,  if  the  suspended  particle  is  very  minute,  such  an  equali- 
zation of  impacts  is  not  likely  to  occur,  the  less  so  the  smaller  the  particle. 
In  response  to  this  unequal  bombardment,  greater  now  on  one  side,  now  on 
another,  the  particle  darts  about  in  a  zigzag  course,  thus  revealing  to  us  the 
movement  of  the  molecules  of  the  liquid  surrounding  it."  (Watkeys,  "An 
Orientation  in  Science,"  Chap.  Ill,  p.  137,  McGraw-Hill  Book  Company, 
Inc.,  New  York,  1938.) 

Buffon.     See  Organic  Evolution. 

Calorie.  "  In  order  to  be  able  to  discuss  energy  relationships  intelligently 
we  need  to  have  some  means  of  designating  definite  amounts.  The  form  of 
energy  into  which  all  other  forms  tend  to  convert  themselves  is,  as  we  have 


510  HUMAN  BIOLOG^ 

seen,  heat.  A  convenient  energy  unit,  then,  is  the  heat  unit.  The  amount  of 
heat  required  to  raise  the  temperature  of  1  gram  (%B  oz.)  of  water  1  degree 
centigrade  (strictly  from  14°  to  15°)  is  taken  as  the  unit.  This  is  known  as 
the  gram  calorie.  For  convenience  when  large  amounts  of  heat  are  involved  a 
second  unit  just  one  thousand  times  as  great  is  also  used.  This  is  called  the 
kilocalorie  or  simply  the  Calorie,  usually  distinguished  from  the  gram  calorie 
by  the  use  of  the  capital  initial.  Although  the  calorie  is  strictly  a  heat  unit 
it  serves  as  an  expression  for  any  form  of  energy.  If  we  speak  of  any  engine 
as  able  to  furnish  a  certain  number  of  calories  we  mean  that  if  all  the  energy 
were  to  appear  as  heat  that  many  calories  would  be  liberated.  As  a  matter 
of  fact  much  of  the  energy  may  actually  take  other  forms,  as  it  does  in  the 
case  of  the  contracting  muscle."  (Martin,  "The  Human  Body,"  p.  103, 
Henry  Holt  &  Company,  New  York,  1935.) 

Carbohydrates.    See  Cellulose,  Glucose",  Lactose,  Starch,  Sucrose. 

Cellulose.  "  Cellulose  chemists  recognize  not  one  substance  that  is  cellu- 
lose but  a  group  of  substances,  the  celluloses.  Modern  research  has  produced 
an  alpha  cellulose  that  is  as  near  a  chemical  entity  as  any  cellulose  heretofore 
attained;  but  although  this  may  be  regarded  as  a  definite  cellulose,  there  are 
others.  The  naturally  occurring  celluloses  are  of  three  groups:  the  true,  the 
compound,  and  the  hemi-  or  reserve  celluloses.  Among  the  first,  that  of  the 
cotton  fiber  is  the  purest,  being  90  per  cent  true  cellulose.  Compound 
celluloses  are  true  celluloses  impregnated  with  other  substances.  The  hemi- 
ce^uloses  are  incompletely  developed  forms  of  cellulose  and  other  carbohydrate 
materials  such  as  araban  and  xylan.  In  spite  of  this  apparent  variety,  it 
does  not  appear  that  the  celluloses  of  the  various  seed-bearing  plants  are 
actually  different  chemical  substances;  that  is,  although  physical  differences 
(for  example,  fiber  length)  exist,  and  chemical  differences  in  the  constitution 
of  the  cellulose  of  the  original  wood  may  exist,  the  residues,  termed  cellulose, 
obtained  from  different  woods  are  probably  identical  in  chemical  structure. 

"Protoplasm,  as  it  builds  the  plant-cell  wall,  simultaneously  or  subse- 
quently secretes  substances  that  occur  either  as  distinct  layers  alternating  with 
the  cellulose  or,  more  usually,  as  an  impregnation  of  it.  Such  substances  are 
lignin,  suberin,  pectin,  and  cutin.  Old  wood  is  lignified  cellulose,  and  cork 
is  suberized  cellulose.  Pectin  may  form  distinct  layers  in  the  cell  wall  alternat- 
ing with  cellulose,  or  it  may  be  separately  deposited.  In  general,  pectin 
compounds  impregnate  the  wall,  forming  so-called  pectocelluloses.  Cutin  is 
often  a  surface  deposit  and  occurs  as  the  waxy  coating  on  glossy  leaves  and 
fruits.  To  be  superficially  deposited,  it  must  pass  through  the  cellulose  wall 
and  in  so  doing  adds  to  the  chemical  complex  that  we  call  natural  cellulose. 

"The  hemi-  or  reserve  celluloses  constitute  an  interesting  group  which 
differs  structurally  from  the  fibrous  celluloses.  They  are  more  readily 
hydrolyzed  than  the  true  celluloses  and  break  down  into  sugars  (galactose 
and  pentose)  of  which  they  are  regarded  as  the  anhydrides  and  from  which  they 
receive  their  names  (galactosans  and  pentosans). 

"Associated  with  cellulose,  in  a  manner  similar  to  that  just  described  for 
pectin,  are  numerous  other  compounds  generally  regarded,  like  the  hemi- 


APPENDIX  511 

celluloses,  as  derivatives  of  cellulose.  Among  them  are  the  gums,  mucilages, 
and  gelatinous  substances,  usually  produced  during  heartwood  formation. 
Their  origin  and  chemical  constitution  are  not  well  understood. 

"Cellulose  is  almost  wholly  a  plant  product,  yet,  like  most  features  used 
to  distinguish  plants  from  animals,  it  is  not  an  infallible  criterion  of  what 
is  a  plant  and  what  an  animal.  Tunicates  and  insects  are  reported  to  have 
tunicin  in  their  tests  or  pellicles.  This  substance  is  said  to  be  identical  with 
cellulose. 

"Although  cellulose  is  used  primarily  by  the  plant  as  a  material  for  wall 
building,  it  may  serve,  probably  in  some  modified  form,  as  a  reserve  food. 
Cellulose  is  also  food  for  certain  animals  which,  though  lacking  the  capacity 
to  digest  it  themselves,  are  nevertheless  able  to  use  it  because  of  their  intestinal 
flora.  There  is  no  digestive  enzyme  in  the  fermentation  fluids  of  higher 
animals  that  will  act  upon  cellulose,  nor  indeed  is  any  intestinal  ferment  known 
that  will  attack  the  hemicelluloses,  the  pentosans,  or  the  galactans,  yet  these 
last  two  carbohydrates  certainly,  and  probably  some  of  the  higher  celluloses, 
not  only  are  utilized  by  animals  but  form  an  important  part  of  the  dietary  of 
herbivora.  This  is  possible  because  the  digestion  of  the  cellulose  is  carried 
out  by  microorganisms.  It  is  said  that  the  intestinal  juices  of  the  horse  dis- 
solve 70  per  cent  of  favorable  nonlignified  cellulose  but  that  the  ferments  are 
produced  by  bacteria  or  Protozoa.  The  cow  is  another  example  of  a  higher 
animal  that  digests  cellulose.  In  all  such  cases,  the  fermentation  is  done  by 
microorganisms.  The  digestion  products  apparently  are  not  monosaccharides, 
as  one  would  expect,  but  carbon  dioxide,  methane,  and  fatty  acids,  the  last 
only  being  suitable  for  nutrition. 

"The  classical  example  of  the  wood-feeding  habit  in  animals  is  that  of 
termites.  Intestinal  Protozoa  make  it  possible  for  these  insects  to  live  on 
wood.  When  defaunated  (robbed  of  their  protozoan  companions)  by  heat  or 
oxygen,  they  cannot  digest  wood  and  die  from  starvation  when  fed  it,  but  they 
can  then  live  on  rotted  wood,  that  is  to  say,  wood  predigested  by  fungi.  If 
intestinal  Protozoa  of  the  same  kind  as  were  removed  are  returned  to  the 
termites,  they  can  again  transform  wood.  This  experiment,  done  by  Cleve- 
land, led  to  the  further  conclusion  that  wood-ingesting  Protozoa  form  glycogen 
by  splitting  the  cellulose  into  cellobiose  and  decomposing  this,  in  turn,  to 
glucose,  from  which  they  build  up  glycogen."  (Seifriz,  "Protoplasm,"  pp. 
459-460,  McGraw-Hill  Book  Company,  Inc.,  New  York,  1936.) 

Chemical  Equations.  "By  the  use  of  symbols  and  formulae,  it  is  possible 
to  express  concisely  chemical  changes  in  the  form  of  chemical  equations. 
For  example,  the  combination  of  hydrogen  with  oxygen  to  form  water  is 
expressed  as  follows: 

2H2  +  02  =  2H20 

"In  terms  of  our  theoretical  conception  of  matter,  the  equation  states  that 
two  molecules  of  hydrogen,  each  consisting  of  two  atoms,  react  with  one 
molecule  of  oxygen,  consisting  of  two  atoms,  to  form  two  molecules  of  water, 
each  of  which  is  composed  of  two  hydrogen  atoms  and  one  oxygen  atom.  In 


512  HUMAN  BIOLOGY 

agreement  with  the  law  of  the  conservation  of  mass,  tne  number  of  atoms  of 
each  element  on  the  right-hand  side  of  the  equation  is  the  same  as  that  on  the 
left;  in  other  words,  the  equation  is  balanced. 

"Five  types  of  chemical  reactions  can  be  distinguished. 

1.  The  combination  of  two  or  more  substances  to  form  a  more  complex 
substance. 

C          +      02      =  C02 

carbon  oxygen         carbon  dioxide 

2Mg        +      02      =  2MgO 

magnesium       oxygen       magnesium  oxide 

2.  The  decomposition  of  a  more  complex  substance  into  two  or  more 
simpler  substances. 

2HgO          =       2Hg      +      02 
mercuric  oxide        mercury        oxygen 

2H20          =       2H2       +      O2 
water  hydrogen       oxygen 

3.  The  replacement  of  an  element  in  a  compound  by  another  element. 

Mg         +        2HC1        =        IT2        +      MgQ2 
magnesium       hydrochloric       hydrogen       magnesium 
acid  chloride 

4.  The  double  decomposition  of  two  compounds  resulting  in  the  formation 
of  two  new  compounds. 

MgO       +        2HC1        =   H20     +    MgCl2 

magnesium       hydrochloric       water       magnesium 

oxide  acid  chloride 

5.  Molecular  rearrangement.     This  kind  of  chemical  change  consists  in 
the  transformation  of  one  compound  into  another  compound  having  the  same 
molecular  composition  but  a  different  arrangement  of  the  atoms  within  the 
molecule.     The  illustration  of  this  phenomenon  would  involve  the  introduc- 
tion of  more  complicated  formulae  than  those  which  we  have  been  considering." 
(Watkeys  and  Associates,  "An  Orientation  in  Science,"  pp.  150-151,  McGraw- 
Hill  Book  Company,  Inc.,  New  York,  1938.) 

Chloride  Shift.  The  transportation  of  carbon  dioxide  in  the  blood  stream 
is  a  difficult  problem.  Apparently  one  of  the  important  factors  in  this  process 
is  a  shift  in  either  direction  of  the  chloride  ions  between  the  red  cells  and  the 
blood  plasma,  the  direction  of  the  shift  depending  upon  the  amount  of  carbon 
dioxide  present.  Thus  in  the  lungs,  when  the  carbon  dioxide  is  released,  the 
chloride  ions  leave  the  red  cells  and  combine  with  the  sodium,  which  has  been 
in  combination  with  potassium  in  the  plasma,  to  form  sodium  chloride.  In  the 
tissues,  when  carbon  dioxide  is  received  into  the  blood,  it  combines  at  once 


APPENDIX  513 

with  water  in  the  plasma  to  form  carbonic  acid.  The  latter  has  the  power  to 
displace  the  chloride  from  the  sodium  chloride.  The  chloride  ions  now  enter 
the  red  cells  and  combine  with  potassium,  which  is  released  as  the  oxyhemoglo- 
bin  changes  to  the  less  acid  hemoglobin,  and  the  potassium  chloride  (KC1)  thus 
formed  remains  until  the  lungs  are  reached,  when  the  chloride  is  again  released 
to  the  plasma  in  correspondence  with  the  increased  acidity  of  the  oxyhemo- 
globin  and  its  ability  to  combine  with  the  potassium  in  the  red  cells. 

Chlorophyll.  "Chemists  from  the  time  of  Berzelius  (1839)  have  struggled 
with  the  chemistry  of  chlorophyll.  Willstatter  made  the  first  great  advance 
in  the  determination  of  the  structure  of  chlorophyll.  During  the  past  ten 
years,  Conant,  Hans  Fischer,  Stoll,  and  Inrnan,  to  mention  only  a  few  of  the 
workers,  have  advanced  our  knowledge  of  the  structure  of  chlorophyll,  and  the 
actual  synthesis  of  the  chlorophyll  molecule  appears  imminent. 

"  Chlorophyll  probably  exists  in  the  colloidal  state  in  plants,  or  at  least 
adsorbed  upon  colloids.  It  can  be  extracted  with  certain  organic  solvents. 
The  earlier  workers  thought  that  various  plants  were  characterized  by  different 
varieties  of  chlorophyll.  Willstatter  showed,  however,  that  there  is  only  one 
variety.  This  exists,  at  least  as  isolated  in  the  laboratory,  in  two  forms  which 
have  been  designated  chlorophyll-a  and  chlorophyll-^. 

"  Chlorophyll,  possessing  the  same  properties,  may  be  prepared  from  either 
fresh  or  dried  leaves.  One  kilo  of  fresh  leaves  gives  a  yield  of  0.9  to  2.1  grams ; 
dried  leaves  yield  5  to  10  grams.  The  most  suitable  solvent  for  extraction  is 
acetone  (80  per  cent)  for  dried  leaves,  and  pure  acetone  for  fresh  leaves,  suffi- 
cient acetone  being  added  so  that,  allowing  for  the  moisture  in  the  fresh  leaves, 
the  resulting  solution  is  80  per  cent  acetone.  Chlorophyll  can  be  now  isolated 
as  readily  as  can  any  alkaloid  or  any  sugar,  and  within  a  few  hours  a  kilo  of 
dried  leaves  should  yield  about  6.5  grams  of  practically  pure  chlorophyll. 

"  Chlorophyll  is  a  bluish-black  substance  with  a  strong  metallic  luster, 
powdering  to  a  greenish-  or  bluish-black  powder.  It  has  no  definite  melting 
point,  ranging  from  93°  to  106°C.  for  various  samples,  and  is  soluble  in  absolute 
alcohol  to  a  blue-green  solution.  It  shows  neither  acidic  nor  basic  properties. 
Acids  change  its  color  to  olive  brown  and  split  off  magnesium  which  is  asso- 
ciated with  the  molecule."  (Gortner,  "Outlines  of  Biochemistry,"  pp.  732- 
733,  John  Wiley  &  Sons,  Inc.,  New  York,  1938.  Reprinted  by  permission.) 

Cholecystokinin.  "During  digestion,  bile  is  needed  in  the  intestine.  The 
sphincter  of  Oddi  relaxes,  and  peristaltic  waves  pass  over  the  duodenum  milk- 
ing the  bile  in  the  common  duct  into  the  duodenum.  The  major  factor  in 
emptying  the  contents  of  the  gallbladder  into  the  duodenum  is  a  hormone, 
cholecystokinin,  that  is  liberated  from  the  duodenal  mucosa  when  acid  enters 
the  intestine.  The  discharge  of  acid  chyme  from  the  stomach  sets  this  hor- 
mone free,  and  it  is  absorbed  into  the  blood  stream;  some  of  it  reaches  the 
gallbladder,  where  it  causes  a  contraction  of  the  smooth  muscle  in  the  gall- 
bladder wall.  There  is  not  very  much  muscle  in  the  gallbladder,  and  the  con- 
tractions are  not  vigorous  but  slow  and  continuous  so  that  the  bile  is  very 
gradually  expelled  during  the  process  of  digestion.  Cholecystokinin  is  also 
very  effectively  liberated  from  the  intestinal  mucosa  by  the  presence  of  fat." 


514  HUMAN  BIOLOGY 

(Crandall,  "An  Introduction  to  Human  Physiology,"  p.  159,  Courtesy  of 
W.  B.  Saunders  Company,  Philadelphia,  1934.) 

Cholesterol.     See  Sterols. 

Chondriosomes.  "These  bodies,  or  their  products,  are  among  the  most 
characteristic  of  the  formed  components  of  the  cytosome  and  are  known  to 
occur  in  nearly  all  kinds  of  cells,  among  both  plants  and  animals,  and  every- 
where showing  the  same  general  characters.  They  have  attracted  much 
attention  in  recent  years  because  of  the  questions  raised  by  Altmann,  Benda, 
Meves,  and  their  followers  concerning  their  possible  significance  in  histogenesis 
and  heredity;  but  opinion  concerning  them  is  still  in  a  very  unsettled  state. 
Morphologically  they  appear  in  the  form  of  small  granules  (mitochondria), 
rods  or  filaments  (chondrioconts)  and  other  bodies.  .  .  .  More  recent  studies 
have  shown  that  they  consist  of  a  specific  material  showing  definite  cytological 
and  microchemical  characters  but  morphologically  highly  plastic,  so  that  it 
may  appear  under  many  forms,  which  are  probably  to  be  regarded  as  only 
different  phases  of  the  same  material.  The  most  common  of  these  are 
separate  mitochondria  and  chondrioconts,  both  of  which  may  often  be  observed 
in  the  same  cell;  and  all  gradations  between  them  may  be  observed  in  sec- 
tions. .  .  . 

"The  physico-chemical  nature  of  chondriosomes  has  been  the  object  of 
numerous  researches.  .  .  .  They  are  soluble  in  various  degrees  in  dilute  acetic 
acid,  ether,  acetone,  alcohol  and  other  fat-solvents;  hence  the  fact  that  they 
are  often  imperfectly  fixed  or  even  destroyed  by  many  of  the  ordinary  fixing 
agents  containing  acetic  acid,  and  were  often  overlooked  until  a  more  appro- 
priate technique  had  been  devised."  (Wilson,  "The  Cell  in  Development  and 
Heredity,"  pp.  45-47,  The  Macmillan  Company,  New  York,  1925.) 

Classification.     See  Taxonomy. 

Coelenterata.  "The  phylum  Coelenterata,  to  which  Hydra  and  Obelia 
belong,  includes  a  wide  variety  of  relatively  simple  Metozoa,  almost  all  of  which 
are  marine  in  habitat.  Three  classes  are  generally  recognized,  namely:  (1) 
the  Hydrozoa,  represented  by  the  independent  polyp  type,  like  Hydra,  and  the 
colonial  type  consisting  of  many  attached,  dependent  polyps,  as  in  Obelia;  (2) 
the  Scyphozoa,  represented  by  many  species  of  large  jellyfishes;  and  (3)  the 
Actinozoa,  represented  by  an  independent  polyp  type,  like  the  Sea-anemone 
and  the  important  '  island-building/  colonial  Corals,  in  all  of  which  a  con- 
siderably greater  cellular  specialization  is  exhibited  than  in  Hydra.  Through- 
out these  three  classes  of  Coelenterates,  basic  structural  likeness  is  evident  in 
the  body  plan  of  the  individuals,  which  is  always  diploblastic  and  radially 
symmetrical.  The  enteric  cavity  is  a  sac-like  structure  with  one  opening  for 
ingestion  and  egestion,  and  is  encircled  by  tentacles.  All  species  possess 
stinging  cells  (nematocysts).  Finally,  an  alternation  of  generations,  well- 
shown  in  Obelia,  but  subject  to  great  variation,  is  often  found."  (Baitsell, 
"Manual^of  Biology,"  pp.  175,  The  Macmillan  Company,  New  York,  1936.) 
See  Hydra. 

Coelom.  The  body  cavity  of  the  triploblastic  animals.  It  is  formed 
originally  in  the  embryo  as  a  result  of  the  splitting  of  the  mesoderra  into  an 


APPENDIX  515 

outer  layer,  which  is  associated  with  the  body  wall,  and  an  inner  layer,  which 
forms  the  wall  of  the  alimentary  canal.  The  cavity  developed  between  the 
two  mesoderm  layers  is  the  coelom.  In  man,  the  coelom  is  divided  into  three 
portions:  namely,  the  pericardial  cavity,  or  sac,  in  which  the  heart  lies;  the 
thoracic  cavity  containing  the  lungs  and  heart;  and  the  abdominal,  or  peri- 
toneal, cavity  with  various  important  abdominal  organs.  The  thoracic  and 
abdominal  cavities  are  separated  by  the  diaphragm.  Strictly  speaking,  none 
of  the  visceral  organs  lie  in  the  coelom  proper  but  rather  in  the  cavity  formed 
between  the  two  layers  of  the  serous  membranes  that  cover  them.  See  Serous 
Membranes. 

Colloids.  "  Matter  is  said  to  be  in  the  colloidal  state  when  it  is  perma- 
nently dispersed  and  so  finely  divided  that  the  individual  particles,  though 
larger  than  molecules,  cannot  be  seen.  The  water  of  the  Mississippi  River 
is  forever  muddy  because  the  clay  particles  contained  in  it  are  so  small  that 
they  do  not  settle  until  they  meet  the  salts  of  the  sea,  when  they  quickly 
fall  and  form  the  Mississippi  delta.  Both  the  suspension  of  the  finely  divided 
clay  particles  in  the  river  water  and  their  precipitation  by  the  salts  of  the  sea 
are  colloidal  phenomena.  A  threatening  cloud  is  made  up  of  droplets  of  water 
finely  dispersed  and  in  relatively  permanent  suspension  in  the  air;  the  water 
is  in  the  colloidal  state.  When  the  droplets,  through  coalescence,  become  too 
large,  they  fall  as  rain.  The  tails  of  comets  consist  of  particles  so  small  that 
when  our  earth  sweeps  through  them  we  see  nothing  of  them,  yet  illuminated 
against  the  black  background  of  the  night  sky  they  become  brilliant.  The 
cosmic  particles  of  the  comet's  tail  are  in  the  colloidal  state,  and  their  lumi- 
nosity is  due  to  the  scattering  of  light,  a  colloidal  phenomenon.  The  blue  color 
of  tobacco  smoke  or  pale  forest-fire  smoke,  of  mist,  blue  eyes,  feathers,  and 
skimmed  milk  is  due  to  the  presence  of  tiny  particles  in  permanent  suspension, 
in  other  words,  to  matter  in  the  colloidal  state.  Metals  may  be  so  finely 
dispersed  in  water  as  to  remain  in  permanent  suspension.  Gold  so  dispersed 
forms  a  classical  colloidal  suspension.  Where  dispersed  particles  settle,  as 
does  sand  in  water,  or  rise,  as  does  cream  in  milk,  the  system  is  a  coarse  sus- 
pension. Only  the  smaller  particles  which  remain  behind  in  permanent 
suspension  are  colloidal.  Minuteness  in  size  of  particles  and  relative  perma- 
nency in  suspension  characterize  the  colloidal  state. 

"The  medium  in  which  the  particles  of  a  colloidal  system  are  scattered  is 
termed  the  dispersion  medium,  or  continuous  phase;  and  the  scattered  particles 
are  the  dispersed,  or  discontinuous  phase;  thus,  the  air  of  clouds  is  the  dispersion 
medium,  and  the  droplets  of  water  are  the  dispersed  phase. 

"Matter  finely  divided  and  in  permanent  suspension  is  said  to  be  col- 
loidally  dispersed  rather  than  in  solution,  because  the  particles  are  above 
the  molecule  in  size,  though  one  may  speak  of  colloidal  solutions;  furthermore, 
a  molecular  dispersion  may  be  colloidal  if  the  molecules  are  exceedingly  large, 
as  in  the  case  of  proteins. 

"As  particle  size  is  characteristic  of  the  colloidal  state,  the  latter  may  be 
(somewhat  arbitrarily)  defined  in  terms  of  the  forhier.  The  maximum  size 
of  colloidal  particles  is  conveniently  placed  at  the  limit  of  microscopic 


516  HUMAN  BIOLOGY 

visibility.  The  minimum  size  is  above  that  of  the  average  molecule.  Thij 
means  that  the  largest  colloidal  particles  are  below  0.1  u  or  0.0001  mm.  in 
diameter  and  therefore  invisible  and  above  1  m^u  or  0.000001  mm."  (Seifriz, 
" Protoplasm,"  pp.  88-89,  McGraw-Hill  Book  Company,  Inc.,  New  York, 
1936.)  See  Measurements;  Energy. 

Comparative  Anatomy.  "The  first  step  towards  scientific  classification 
was  made  ...  by  Aristotle  in  emphasizing  anatomical  characters  as  tax- 
onomic  criteria,  so  that  to  all  intents  and  purposes  classification  implies  com- 
parison of  structural  details.  Indeed,  Aristotle  recognized  the  unity  of 
structural  plan  throughout  the  chief  animal  groups,  and  in  reference  to  man 
he  says,  ' whatever  parts  a  man  has  before,  a  quadruped  has  beneath;  those 
that  are  behind  in  man  form  the  quadruped's  back.'  Not  only  did  he  appreci- 
ate homology,  but  also  correlation  of  parts  and  division  of  labor  in  the  economy 
of  the  animal  body.  And  Theophrastus  approached  plant  morphology  in 
the  same  philosophical  spirit.  .  .  .  But  it  probably  would  be  reading  too  much 
into  the  past  to  assign  the  origin  of  comparative  anatomy  of  animals  in  the 
modern  sense  of  the  term  to  Greek,  Roman,  or  early  Renaissance  science, 
since  description  rather  than  comparison  was  the  keynote.  The  same  may 
be  said  of  the  anatomical  work  of  Vesalius,  Harvey,  and  Malpighi,  though  the 
latter  compared  the  microscopic  structure  of  various  organs,  and  in  his 
Anatomy  of  Plants,  which  shares  with  Grew's  Anatomy  the  honor  of  founding 
vegetable  histology,  emphasized  the  importance  of  the  comparative  method. 
Owing  to  the  less  marked  structural  differentiation  of  plants  in  comparison 
with  animals,  plant  anatomy  does  not  lend  itself  so  readily  to  descriptive 
analysis  and  therefore  an  epoch  in  the  study  of  comparative  anatomy  is  less 
defined  in  botany  than  in  the  sister  science.  Accordingly  both  reason  and 
expediency  warrant  confining  our  attention  to  the  comparative  anatomy  of 
animals. 

"Probably  the  first  consistent  attempt  to  make  a  comparative  study  of  the 
form  and  arrangements  of  the  parts  of  animals  is  represented  in  a  volume 
published  in  1645  by  Severinus  (1580-1656)  of  Naples,  in  which  he  concluded 
that  many  vertebrates  are  constructed  on  the  same  plan  as  man,  though  Belon, 
nearly  a  century  earlier,  figured  and  compared  the  skeletons  of  bird  and  man 
side  by  side  in  the  same  posture,  and  as  nearly  as  possible  bone  for  bone. 
Tyson  (1650-1708)  of  Cambridge  at  the  end  of  the  seventeenth  century 
definitely  instituted  the  monographic  treatment  of  comparative  morphological 
problems  in  his  study  of  the  anatomy  of  man  and  monkeys. 

"  Comparative  anatomy,  however,  as  a  really  important  aspect  of  biological 
work,  in  fact  as  a  science  in  itself,  was  the  result  of  the  life  work  of  Cuvier 
(1769-1832)  of  Paris  during  the  first  quarter  of  the  last  century.  It  is  true 
that  his  immediate  predecessors,  such  as  John  Hunter  (1728-1793),  the  founder 
of  the  Hunterian  Collection,  the  nucleus  of  the  Anatomical  Museum  of  the 
Royal  College  of  Surgeons  in  London,  Camper  (1722-1789)  of  Groningen  and 
Vicq  d'Azyr  (1748-1794)  of  Paris,  added  synthesis  to  analysis  and  reached  a 
broader  viewpoint  in  anatomical  study,  but  Cuvier's  claim  to  fame  rests  on 
the  remarkable  breadth  of  his  investigations — his  grasp  of  the  comparative 


APPENDIX  517 

anatomy  of  the  whole  series  of  animal  forms.  And  not  content  merely  with 
the  living,  he  made  himself  the  first  real  master  of  the  anatomy  of  fossil 
vertebrates  and  as  such  is  the  founder  of  vertebrate  paleontology,  while  his 
contemporary,  Lamarck,  holds  the  same  relation  to  invertebrate  paleontology. 

"  Cuvier's  position  in  the  history  of  anatomy  is  largely  due  to  his  emphasiz- 
ing, as  Aristotle  had  done  before  him,  the  functional  unity  of  organisms — that 
the  interdependence  of  organs  results  from  the  interdependence  of  function 
and  that  structure  and  function  are  two  aspects  of  the  living  machine  which 
go  hand  in  hand.  Cuvier's  famous  principle  of  correlation — 'Give  me  a 
tooth/  said  he,  'and  I  will  construct  the  whole  animal' — is  really  an  outcome 
of  this  viewpoint.  Every  change  of  function  involves  a  change  in  structure 
and  therefore,  given  extensive  knowledge  of  function  and  of  the  interdepend- 
ence of  function  and  structure,  it  is  possible  to  infer  from  the  form  of  one  organ 
that  of  most  of  the  other  organs  of  an  animal.  'In  a  word,  the  form  of  the 
tooth  implies  the  form  of  the  condyle ;  that  of  the  shoulder  blade  that  of  the 
claws,  just  as  the  equation  of  a  curve  implies  all  its  properties. ' 

"Although  Cuvier  undoubtedly  allowed  himself  to  exaggerate  his  guiding 
principle  until  it  exceeded  the  bounds  of  facts,  he  was  above  all  in  his  science 
and  philosophy  a  hard-headed  conservative  and  autocrat.  He  opposed  with 
equal  vigor  the  influence  of  the  Natur philosophic  of  Schelling  and  his  school 
with  its  transcendental  anatomy,  Platonic  archetypes,  and  the  like,  as  well  as 
the  evolutionary  speculations  of  Lamarck  and  his  school.  From  the  vantage 
points  of  today  we  know  that  in  one  case  he  was  right  and  in  the  other  wrong — 
though,  in  so  far  as  the  facts  then  available,  his  opposition  was  justified  in 
both  cases. 

"Cuvier's  immediate  successors  in  France  were  Milne-Edwards  (1800- 
1885)  and  Lacaze-Duthiers  (1821-1901);  in  Germany,  Mcckel  (1781-1833), 
Rathke  (1793-1860),  Miiller,  and  Gegenbaur  (1826-1903);  in  England,  Owen 
and  Huxley,  and  in  America,  Agassiz  (1807-1873);  Cope  (1840-1897),  and 
Marsh  (1831-1899).  Among  these,  Owen  (1804-1892)  perhaps  demands 
special  mention.  At  once  a  peculiar  combination  of  Cuverian  obstinacy  in 
regard  to  facts  arid  of  transcendental  imagination,  Owen  spent  a  long  life 
dissecting  with  untiring  patience  and  skill  a,  remarkable  series  of  animal  types, 
as  well  as  in  reconstructing  extinct  forms  from  fossil  remains.  Aside  from  the 
facts  accumulated,  probably  his  greatest  contribution  was  making  concrete 
the  distinction  between  homologous  and  analogous  structures,  which  has  been 
of  the  first  importance  in  working  out  the  pedigrees  of  plants  as  well  as  animals 
— though  Owen  himself  took  an  enigmatical  position  in  regard  to  organic 
evolution."  (Woodruff,  "The  Development  of  the  Sciences,"  Chap.  VI, 
pp.  233-236,  Yale  University  Press,  New  Haven,  1923.) 

Complement  Fixation.  The  complement-fixation  tests,  such  as  the  Wasser- 
tnann  test,  have  long  since  become  highly  standardized  and  a  matter  of  routine 
in  laboratories  and  hospitals  throughout  the  world.  Such  tests  require 
solutions  of  complement  and  antigen  with  measured  content  and  with  the 
antigen  made  specifically  for  the  disease  in  question.  It  is  necessary  to  keep 
in  mind  certain  characteristics  that  serve  to  differentiate  between  comple- 


518  HUMAN  BIOLOGY 

ment  and  lysin.  Thus:  (1)  Complement  is  not  specific  for  a  particular  lysin 
but  reacts  whenever  any  lysin  is  present  together  with  antigen;  (2)  it  is  easily 
destroyed  by  heating;  (3)  it  may  be  standardized  by  the  proper  laboratory 
methods,  and  the  necessary  amount  supplied  to  react  with  a  determined 
amount  of  antigen. 

Lysin,  on  the  other  hand,  (1)  is  specific  against  a  particular  invader.  Con- 
sequently, a  lytic  antibody  is  never  present  in  the  blood  unless  the  tissues  have 
been  invaded  by  foreign  material;  (2)  it  is  not  easily  destroyed  by  heating. 
Therefore,  by  heating  blood  serum,  the  normal  complement  will  be  destroyed, 
while  the  lysin  remains. 

With  the  antigen  and  complement  standardized  and  of  known  strength, 
the  only  unknown  factor  in  the  test  is  the  presence  of  lysin  in  the  serum 
obtained  from  the  patient,  and  that  is  what  the  test  aims  to  determine.  The 
components  of  a  complement-fixation  test  may  be  outlined  as  follows: 

A.  Standardized  complement  in  solution. 

B.  Standardized  solution  of  antigen  for  the  specific  disease,  in  sufficient 
amount  to  combine  with  the  complement,  provided  lysin  is  present. 

C.  Blood  serum  from  patient,  which  has  been  heated  to  destroy  the  com- 
plement normally  present,  but  which  will  contain  the  lysin  if  the  patient  has 
the  disease. 

When  these  substances  A,  B,  and  C  are  combined  under  the  proper  condi- 
tions in  a  test  tube,  there  are  two  possibilities: 

1.  No  reaction  will  occur  if  the  serum  C  is  free  from  lysin. 

2.  Reaction  will  occur  if  lysin  is  present,  and  the  complement  will  be  bound 
or  fixed  (complement-fixation) ;  that  is,  complement  A  and  lysin  C  will  com- 
bine to  destroy  antigen  B  just  as  in  the  body. 

It  is  impossible  to  tell  from  the  appearance  of  the  liquid  in  the  test  tube 
as  to  whether  or  not  a  reaction  has  occurred,  but  the  answer  will  be  given  by 
the  introduction  of  the  hemolytic  system  as  a  visible  indicator.  It  will  be 
necessary  to  add  two  additional  substances  to  those  previously  combined 
(A,  B,  C),  namely; 

D.  Blood  serum  from  a  rabbit  containing  a  hemolysin  against  red  blood 
cells  of  the  shef>^      This  hemolysin  has  been  previously  developed  in  the  rabbit 
in  response  to  repeated  injections  of  sheep  corpuscles.     It  is  strong  enough  to 
cause  the  hemolysis  of  these  corpuscles  when  complement  is  present.     The 
rabbit  serum  has  been  heated  to  destroy  all  complement  normally  present. 

E.  Finally,  the  red  blood  cells  of  the  sheep  are  added  as  the  hemolytic 
antigen. 

With  this  very  ingenious  hemolytic  indicator  it  will  be  possible  to  secure 
visible  evidence  in  a  short  time  as  to  whether  or  not  the  patient  has  the  dis- 
ease. For,  if  the  disease  is  present,  a  reaction  will  previously  have  occurred 
between  A,  B,  and  C,  as  stated  in  (2)  above,  and  the  complement  will  have 
been  used.  In  this  case,  the  addition  of  the  hemolytic  rabbit  serum  D  and 
the  sheep  blood  cell  antigen  E  will  cause  no  further  reaction,  since  there  will 
be  no  complement.  Accordingly,  the  red  cells  will  remain  intact  at  the. 
bottom  of  the  tube  with  a  clear  liquid  above. 


APPENDIX  519 

If,  however,  the  patient  is  free  from  the  disease,  a  further  reaction  will 
occur  when  the  two  substances  D,  E,  associated  with  the  hemolytic  system  are 
added,  for  unbound  complement  A  will  be  present  to  react  with  the  hemolytic 
antibody  D  and  antigen  E.  It  will  be  remembered  that  complement  is  not 
specific  for  a  particular  antibody,  and  consequently  it  will  react  whenever  any 
lysin  and  antigen  are  present.  The  reaction  in  this  instance  will  cause  the 
disruption  of  the  sheep  corpuscles  (antigen)  and  the  release  of  the  hemoglobin 
into  the  solution  which  will  gradually  be  uniformly  colored  as  the  hemoglobin 
diffuses  throughout,  as  indicated  in  the  following  equation: 

Complement  A  +  rabbit  hemolysin  D  +  sheep  red  cells  antigen  E  = 

«  hemolysis 

A  highly  colored  red  solution,  with  no  intact  corpuscles  at  the  bottom  of  the 
tube,  indicates  complete  freedom  from  the  disease,  that  is,  a  negative  test. 
On  the  contrary,  a  transparent  solution  with  no  corpuscles  destroyed  indicates 
a  severe  active  infection  (+4).  Less  severe  infections  are  indicated  by  a  corre- 
sponding increase  in  hemolysis  and  are  commonly  designated  +3,  +2,  and 

+  1. 

Cranial  Nerves,  Human.  I.  Olfactory.  The  first  pair  of  cranial  nerves 
has  its  origin  in  the  forebrain.  Only  sensory  fibers  are  present  in  this  pair 
which  innervate  the  olfactory  cells  in  restricted  areas  of  the  nasal  epithelium. 

II.  Optic.     These  nerves  contain  only  sensory  fibers  that  innervate  the 
retina  and  thus  receive  the  impulses  from  the  visual  cells.     They  have  their 
origin  in  the  midbrain.     Emerging  from  the  brain  stem,  the  optic  nerves  form 
the  optic  chiasma  on  the  ventral  surface  of  the  midbrain  and  then  continue 
to  each  eye. 

III.  Oculomotor.     As  the  name  indicates,  this  pair  of  nerves  innervates 
certain  eye  muscles  concerned  with  movements  of  the  eyeball.     They  are 
concerned  also  with  lens  accommodation  and  pupillary  changes.     In  addition, 
sensory  fibers  are  present  that  carry  afferent  impulses  from  the  eye  muscles. 
The  oculomotor  nerves  have  their  origin  in  the  anterior  portion  of  the 
hindbrain. 

IV.  Trochlear.     These  nerves  consist  largely  of  motor  fibers  which,  in 
conjunction  with  fibers  from  III  and  VI,  innervate  certain  eye  muscles.     Sen- 
sory fibers  possibly  present  carrying  impulses  from  the  eye  muscles.     They 
emerge  from  the  hindbrain  but  extend  anteriorly  for  a  distance  before  reaching 
the  eyes. 

V.  TrigeminaL    This  pair  of  nerves  consists  of  both  motor  and  sensory 
fibers  which  have  their  origin  in  the  hindbrain.     Each  trigeminal  nerve  has 
sensory  and  motor  roots,  with  a  large  ganglion  (Gasserian)  on  the  sensory  root. 
The  two  roots  of  each  nerve  unite  distally  to  the  ganglion,  and  then  the  nerve 
divides  into  three  main  branches  with  motor  and  sensory  fibers  (opthlamic, 
inferior  maxillary,  superior  maxillary)  that  terminate  in  the  muscles  and  sense 
cells  of  the  eyes,  tongue,  jaws,  and  skin  of  the  face, 

VI.  Abducens.     A  small  pair  of  nerves,  primarily  motor  in  function,  which 
Enervates  one  pair  of  the  eyeball  muscles  but,  possibly,  also  carries  afferent 


520  HUMAN  BIOLOGY 

sensory  fibers  from  these  same  muscles.     Origin  in  hindbrain,  near  the  posterior 
margin  of  the  pons. 

VII.  Facial.     An  important  pair  of  cranial  nerves  carrying  motor  fibers 
which  innervate  various  muscles  of  face  and  scalp;  also  sensory  fibers  are 
present  carrying  afferent  impulses  from  the  taste  buds  of  the  tongue/    Origin 
in  hindbrain,  just  posterior  to  the  abducens  (VI). 

VIII.  Auditory.     The  auditory  nerves  contain  only  sensory  fibers  that 
carry  auditory  and  equilibratory  impulses  to  the  central  nervous  system. 
Each  auditory  nerve  divides  into  two  main  branches:  the  vestibular,  which 
innervates  the  semicircular  canals,  and  the  auditory,  which  innervates  the 
cochlea.     Origin  in  hindbrain,  just  posterior  to  the  facial  (VII). 

IX.  Glotsopharyngeal.     Both  motor  and  sensory  fibers  are  present  in  this 
pair  of  nerves.     The  former  control  muscles  in  the  pharynx  concerned  with  the 
process  of  swallowing  and  also  motor  elements  in  the  salivary  glands.     The 
sensory  fibers  innervate  the  taste  buds  in  the  posterior  third  of  the  tongue, 
together  with  certain  membranes  lining  the  pharynx.     Origin  in  hindbrain, 
in  close  association  with  VII  and  VIII. 

X.  Vagus.     This  is  a  large  and  important  pair  of  nerves  containing  both 
motor  and  sensory  fibers.     The  motor  fibers  of  the  vagus  are  important  in 
the  control  of  heart  action  and  also  of  the  muscle  tissue  in  certain  regions  of 
the  alimentary  canal,  notably  the  larynx  (speech),  esophagus,  stomach,  and 
small  intestine.     Efferent  vagus  fibers  are  also  concerned  with  the  secretions 
of  the  gastric  glands.     Sensory  fibers  of  the  vagus  innervate  the  arch  of  the 
aorta  and  aid  in  maintaining  proper  blood  pressure. 

XI.  Spinal  Accessory.     This  pair  of  cranial  nerves  innervates  certain  mus- 
cles of  the  shoulder  region.     Sensory  fibers  also  carry  afferent  impulses  from 
these  muscles.     Origin,  partly  in  medulla  and  partly  from  anterior  portion  of 
spinal  cord. 

XII.  Hypoglossal.     This  final  pair  of  spinal  nerves  carries  motor  and  sen- 
sory fibers  which  innervate  certain  tongue  muscles. 

Cuvier.     See  Comparative  Anatomy. 

Darwin,  Charles.     See  Organic  Evolution. 

Darwin,  Erasmus.     See  Organic  Evolution. 

Diffusion.  "Diffusion  is  the  process  by  which  molecules  in  the  gaseous  or 
liquid  state  tend  to  attain  a  uniform  distribution  throughout  the  region  acessi- 
ble  to  them.  Diffusion  of  gases  is  familiar.  Diffusion  of  solutes  in  the  solvent 
is  not  so  often  noticed  because  the  process  is  comparatively  slow  and  is  com- 
monly hastened  by  stirring;  but,  given  time  enough,  any  solute  can  become 
uniformly  distributed  in  its  solvent  without  any  mechanical  aid  to  diffusion. 
This  results  in  equal  concentration  in  all  parts  except  in  so  far  as  prevented 
by  surface  tension,  adsorption,  or  other  interfering  factors.  Diffusion  is  a 
necessary  accompaniment  to  the  equal  partition  of  energy  between  molecules 
which  is  completely  attained  in  any  system  only  when  each  species  of  mole- 
cule or  ion  is  uniformly  distributed. 

"In  living  matter  and  in  many  other  colloidal  systems,  however,  the  inter- 
fering structures  in  the  form  of  micellae,  gels,  and  membranes  make  the  non- 


APPENDIX  521 

uniformity  of  distribution  of  molecules  and  ions  more  significant  than  is  the 
tendency  to  equal  distribution.  Inequalities  of  distribution  in  living  things 
have  important  consequences.  Among  them  is  the  development  of  osmotic 
pressure  which  partly  determines  the  movement  of  water  in  and  out  of  cells. 
Furthermore,  localized  concentrations  of  fat  or  carbohydrate  and  of  enzymes 
and  other  reactants  in  the  cell  are  of  importance  in  regulating  vital  chemical 
reactions.  The  definitely  restricted  concentration  of  ions  resulting  from  col- 
loidal structure  causes  accumulation  of  electrical  charges  so  that  potential 
differences  arise  and  produce  the  electrical  phenomena  of  life.  Other  conse- 
quences of  the  interferences  with  free  diffusion  might  be  mentioned,  but  these 
are  sufficient  to  draw  attention  to  the  physiological  significance  of  colloidal 
structure. 

"  Of  the  factors  that  prevent  free  diffusion  in  living  things,  membranes  have 
attracted  especial  attention.  The  belief  that  matter  can  be  alive  only  when 
enclosed  in  protective  cell  membranes  has  become  more  and  more  firmly 
established  by  all  the  developments  of  physiology  since  the  statement  of  the 
cell  theory  (1833-1839).  Fundamental  qualities  of  living  matter  are  depend- 
ent upon  some  protection  from  the  environment.  Metabolism  is  constantly 
proceeding,  so  that  the  chemical  composition  of  living  matter  is  ever  changing. 
Yet  these  changes  must  be  largely  reversible ;  for  the  chemical  composition  of 
every  bit  of  protoplasm  varies  only  within  the  narrow  limits  that  permit  the 
maintenance  of  that  integrity  of  composition  consonant  with  the  highly 
developed  individuality  of  every  different  kind  of  living  structure.  Main- 
tenance of  integrity  demands  that  protoplasm  shall  constantly  " select"  its 
specialized  requirements  and  "reject"  other  materials,  while  providing  simul- 
taneously for  ejecting  its  equally  specialized  products  and  wastes.  Free 
diffusion  between  protoplasm  and  its  environment  spells  instant  death.  The 
limiting  membrane  of  protoplasm  appears  to  be  the  very  guardian  of  life,  not 
a  mere  dead  partition.  Its  intricate  architectiire  has  so  far  defied  all  attempts 
at  artificial  imitations,  even  though  membranes  possessing  some  of  the  proper- 
ties of  living  ones  can  be  prepared.  Hoping  eventually  to  understand  the 
nature  of  living  matter,  one  finds  no  problem  more  alluring  than  those  relat- 
ing to  the  nature  and  behavior  of  cell  membranes.  No  wonder,  then,  that  a 
considerable  portion  of  physiological  research  in  recent  years  has  been  devoted 
to  such  studies. 

"One  reason  for  belief  in  the  existence  of  protoplasmic  membranes  is  fur- 
nished by  the  phenomena  of  bio-osniotic  pressure.  Living  cells  can  exhibit 
internal  pressures  such  as  are  developed  by  an  artificial  sac  composed  of  a 
semipermeable  membrane  containing  a  solution  and  immersed  in  water. 
Although  the  pressure  in  cells  is  explained  by  some  physiologists  as  due  to 
imbibition  and  this  doubtless  is  one  of  the  forces  involved,  yet  much  evidence 
points  to  osmotic  pressure  as  an  important  factor  and  sometimes  the  pre- 
dominating one  in  the  production  of  intracellular  pressures.  It  thus  gives 
presumptive  evidence  for  the  existence  of  protoplasmic  membranes.  Osmotic 
pressure  is  one  of  the  forces  that  determine  the  movements  of  water  into  and 
out  of  living  structures.  This,  as  previously  explained,  has  important  effects 


522  HUMAN  BIOLOGY 

on  every  type  of  vital  activity  so  that  an  attempt  to  understand  its  funda- 
mental nature  is  worth  while. 

"To  begin  with,  one  should  recall  the  kinetic  conception  of  matter  that 
postulates,  in  accordance  with  the  laws  of  thermodynamics,  that  all  molecules 
are  constantly  in  motion  which  could  cease  only  at  a  temperature  of  absolute 
zero.  As  a  corollary  of  this  idea,  we  conceive  of  the  molecules  of  matter  in 
the  fluid  state  as  exhibiting  not  only  vibratory  motion  but  translatory  motion 
as  well.  This  results  in  the  phenomenon  of  diffusion.  Suppose,  however, 
that  the  solution  is  in  contact  with  a  semipermeable  membrane,  permeable  to 
the  molecules  of  the  solvent  but  not  to  those  of  the  solute.  The  conditions 
that  then  prevail  are  typified  by  the  following  specific  instance.  Suppose  a 
dialyzer  is  provided  with  a  copper  ferrocyanide  membrane  that  is  permeable 
to  water  but  impermeable  to  sugar.  On  one  side  of  the  membrane  is  a  10  per 
cent  sugar  solution,  and  on  the  other  is  distilled  water.  Computing  on  the 
basis  of  the  molecular  weights  of  water  (18)  and  sugar  (342),  we  find  that  there 
are  approximately  169  molecules  of  water  in  the  solution  to  1  of  sugar.  Molec- 
ular motion  results  in  the  continual  bombardment  of  the  membrane  on  both 
sides;  but  out  of  every  170  hits  on  the  inside,  169  are  made  by  water  molecules 
and  1  by  a  sygar  molecule,  whereas,  on  the  outside,  all  hits  would  be  made  by 
water  molecules  alone.  Since  the  membrane  is  permeable  to  water  but  not  to 
sugar,  the  chances  are  in  favor  of  the  passage  of  water  from  the  outside  to  the 
inside  of  the  membrane.  The  actually  observed  result  is  in  accord  with  this 
conception,  because  the  level  of  the  solution  inside  the  dialyzer  rises.  The 
solution  then  exerts  a  pressure  measured  by  the  difference  in  level  between  it 
and  the  water  outside  the  membrane.  This  is  osmotic  pressure. "  (Mitchell, 
"  A  Textbook  of  General  Physiology,"  pp.  433-437,  McGraw-Hill  Book  Com- 
pany, Inc.,  New  York,  1938.) 

Diphtheria  Antitoxin.  Consideration  of  the  preparation  of  diphtheria 
antitoxin  in  the  horse  will  serve  as  an  example  of  methods  used  in  the  prepara- 
tion of  various  other  antitoxins.  As  the  first  step  in  the  process,  it  is  neces- 
sary to  secure  the  toxin  produced  by  the  diphtheria  bacilli.  This  is  accom- 
plished by  the  cultivation  of  pure  laboratory  cultures  of  the  bacilli  in  the 
proper  nutrient  solution.  The  growth  of  the  bacteria  is  accompanied  by  the 
liberation  of  the  diphtheria  toxin  in  the  nutrient  solution  just  as  occurs  in  the 
tissues  of  the  body.  The  increasing  strength  of  the  toxin  can  be  determined 
from  time  to  time  by  injecting  a  measured  amount  into  a  guinea  pig  of  stand- 
ard weight  and  noting  the  length  of  time  it  takes  to  kill  the  animal.  This  unit 
of  toxicity,  known  as  the  minimum  lethal  dose  (M.L.D.),  is  the  least  amount 
that  will  kill  a  standard  guinea  pig  in  a  certain  time.  When  the  toxicity  of 
the  solution  has  reached  the  desired  standard,  the  diphtheria  organisms  are 
killed  and  then  filtered  off  from  the  nutrient  solution  in  which  they  have 
grown  and  that  now  contains  the  diphtheria  toxin. 

The  next  step  involves  the  transfer,  over  a  period  of  some  days,  of  meas- 
ured amounts  of  the  toxin  to  the  horse.  Only  a  comparatively  small  dose  of 
this  extremely  powerful  poison  can  be  tolerated  at  first,  but,  as  the  tissues 
react  by  the  synthesis  of  antitoxin,  the  amount  of  diphtheria  toxin  injected 


APPENDIX  523 

into  the  horse  can  be  gradually  increased  without  danger.  When  the  anti- 
toxin in  the  horse's  blood  has  reached  the  optimum  strength,  the  horse  is  bled. 
Several  quarts  of  the  blood  containing  the  diphtheria  antitoxin  can  be  secured 
without  injury  to  the  animal;  then  the  blood  corpuscles  are  removed;  and, 
finally,  the  serum  obtained  after  the  plasma  has  clotted.  The  blood  corpus- 
cles thus  removed  are  usually  restored  at  once  to  the  vascular  system  of  the 
horse.  The  production  of  antitoxin,  as  just  described,  does  not  injure  the 
experimental  animals,  so  that  the  horses  may  be  used  for  years  in  the  produc- 
tion of  antitoxin,  as  described  in  the  following  interesting  quotation  from  the 
New  York  Times. 

"'Old  Doc  Dobbin/  a  large  black  work-horse,  whose  life  was  considered 
a  notable  contribution  to  public  health,  is  dead.  He  died  suddenly  today 
in  his  stable  at  a  biological  laboratory  near  New  Brunswick  where  he  was 
employed  to  supply  antitoxin  material  for  the  treatment  of  children  against 
diphthera. 

"'Old  Doc/  a  native  of  the  Western  Plains,  was  21  years  old.  During 
his  lifetime  his  blood  had  supplied  antitoxin  for  the  treatment  of  more  than 
41,000  children.  Two  years  ago  he  was  the  guest  of  honor  at  a  birthday 
party  attended  by  school  children  of  the  city.  At  that  time  'Old  Doc'  was 
escorted  to  a  table  decorated  with  apples  and  a  huge  cake  decked  with  candles. 
Greeted  with  'happy  birthdays'  and  congratulations,  Dobbin  munched  a  big 
red  apple  while  a  eulogy  for  him  was  delivered. 

"A  12-year-old-bay,  known  as  'Mickey/  has  been  chosen  as  the  successor 
of  'Old  Doc'  at  the  farm  where  150  horses  are  kept  for  making  antitoxin. 
Mickey,  too,  hails  from  the  Western  Plains  and  was  chosen  because  of  the 
strength  of  the  serum  made  from  his  blood  and  his  strong  constitution.  The 
successor  to  'Old  Doc'  is  gentle  despite  his  weight  of  nearly  1,400  pounds,  and 
has  been  on  the  laboratory  farm  for  five  years." 

Dissociation.  "It  has  been  found  that  acids,  bases,  and  salts,  when  dis- 
solved in  water,  have  the  power  to  conduct  an  electrical  current,  whereas  cer- 
tain other  substances,  for  example,  sugar,  fail  to  do  so.  It  has  also  been  found 
that  the  substances  that  conduct  the  electrical  current  when  in  solution  exert, 
for  the  same  molecular  concentration,  a  higher  osmotic  pressure  than  sub- 
stances that  do  not  conduct  the  electric  current. 

"It  was  suggested  by  Arrhenius  that  substances  that  conduct  the  electric 
current  do  so  by  virtue  of  the  fact  that  in  solution  there  is  a  splitting  of  their 
molecules  into  two  or  more  portions,  atoms,  which,  becoming  associated  with 
a  number  of  molecules  of  water,  are  called  ions.  These  then  behave  like 
molecules,  so  far  as  diluting  the  solvent  is  concerned.  These  ions  are  of  two 
sorts:  those  bearing  positive  charges,  the  cations;  and  those  bearing  negative 
charges,  the  anions.  The  names  of  the  ions  receive  their  respective  prefixes 
from  the  fact  that,  if  electrodes  are  placed  in  a  solution  of  an  electrolyte  and  a 
current  is  passed  through  the  circuit,  it  is  found  that  the  ions  bearing  the 
positive  charges,  such  as  H+,  Na+,  Ca++,  collect  and  even  give  up  their  charges 
at  the  cathode,  and  those  bearing  the  negative  charges,  such  as  Cl~,  S04~~, 
COa — ,  at  the  anode.  From  what  has  just  been  said,  it  will  appear  that  in  a 


524  HUMAN  BIOLOGY 

solution  of  an  electrolyte  three  varieties  of  particles  'are  present :  positive  ions, 
or  cations;  negative  ions,  or  anions;  and  undissociated  molecules.  The 
degree  to  which  the  dissociation  may  take  place  is,  of  course,  a  variable  and 
depends  upon  a  variety  of  factors,  such  as  the  nature  of  the  substance  and  its 
concentration  in  the  solution. 

"  Dr.  Lewis  Jones  has  given  a  very  vivid  picture  of  the  processes  that  go  on 
in  an  electrolytic  solution  when  an  electric  current  is  passing.  He  likens  the 
molecules  in  solution  to  dancing  couples  on  the  floor  of  a  ballroom.  Here 
and  there  couples  are  separated,  and  the  isolated  individuals  are  moving  about 
by  themselves.  Suppose  a  mirror  at  one  end  of  the  room  and  a  buffet  at  the 
other;  the  ladies  will  gradually  accumulate  around  the  mirror  and  the  gentle- 
men around  the  buffet.  Moreover,  the  dancing  couples  will  gradually  be 
dissociated  to  follow  this  movement.  Although  Dr.  Jones's  example  gives  a 
good  picture,  it  should  not  be  taken  too  literally,  for  not  all  dissociations  are 
of  the  same  type.  It  is  of  importance  for  the  student  of  physiology  to 
remember  that 

1.  All  acids  give  a  free  H+  ion. 

2.  All  bases  give  a  free  OH~  ion. 

3.  All  sodium  salts  give  a  free  Na+  ion. 

4.  All  potassium  salts  give  a  free  K+  ion. 

5.  All  calcium  salts  give  a  free  Ca++  ion. 

6.  All  ammonium  salts  give  a  free  NH4+  ion. 

7.  All  nitrates  give  a  free  NO3~  ion. 

8.  All  chlorides  give  a  free  Cl~~  ion. 

9.  All  sulphates  give  a  free  SO4      ion. 

"The  degree  to  which  any  electrolyte  dissociates  in  solution  depends  very 
largely  upon  the  degree  to  which  the  solution  is  diluted.  For  example,  at 
infinite  dilution  such  a  salt  as  potassium  chloride  would  be  completely  dissoci- 
ated so  that  there  would  be  twice  as  many  particles  in  solution  as  there  were 
molecules  originally  introduced.  The  solutions  that  the  physiologist  uses, 
however,  are  not  infinitely  dilute — and  do  not  contain  completely  dissociated 
salts.  The  dissolved  salt  molecules  are,  in  general,  dissociated  to  about  86 
per  cent. 

"The  ionic  condition  is  of  great  importance  in  living  matter  because,  in  the 
living  substance,  so  many  different  substances  are  brought  into  close  relations. 
There  is  great  opportunity  for  a  vast  number  of  new  ionic  combinations  to  be 
formed.  The  formation  of  these  new  combinations  is  a  part  of  the  normal 
metabolic  activity  of  every  living  organism. 

"In  the  solutions  that  will  be  dealt  with  here,  water  is  the  principal  solvent. 
It  is  the  one  universal  solvent.  It  is  not  only  the  chief  constituent  of  living 
organisms,  but  it  is,  as  well,  the  solvent  and  carrier  of  the  chief  food  and 
excretory  products.  In  it,  in  the  animal  body,  are  dissolved  gases;  inorganic 
salts;  a  great  variety  of  organic  compounds,  including  carbohydrates  and  pro- 
teins; products  of  digestion,  such  as  amino  acids  and  simple  sugars;  and 
various  metabolic  wastes.  Henderson  well  states:  "Indeed,  as  clearer  ideas 


APPENDIX  525 

of  the  physical-chemical  organization  of  protoplasm  have  developed,  it  has 
become  evident  that  the  organism  itself  is  essentially  an  aqueous  solution  in 
which  are  spread  out  colloidal  substances  of  vast  complexity." 

"Water  is  not  only  a  solvent,  but  it  is  itself  an  electrolyte.  A  few  of  the 
H20  molecules  dissociate  into  IP  and  OH~  ions.  The  number  of  the  mole- 
cules so  dissociated  is  relatively  very  small  and  is  measured  by  the  concen- 
tration of  H+  or  of  OH"  ions  in  the  water.  At  temperatures  of  22  or  23°C., 
this  dissociation  is  sufficient  to  give  1  g.  of  weight  of  free  H+  ions  in  10,000,000 
liters  of  water,  that  is,  a  solution  having  a  concentration  of  Af/10,000,OOOH+. 
Inasmuch  as  there  is  an  equal  concentration  of  OH~  ions  in  the  solution,  the 
liquid  will  be  neutral  in  reaction.  And  since  this  dissociation  of  molecules 
occurs,  water  may  be  spoken  of  as  an  electrolyte. 

"Water  seems  to  aid  also  in  the  dissociation  of  the  molecules  of  many  sub- 
stances  that  may  be  dissolved  in  it.  Such  substances  are  spoken  of  as  elec- 
trolytes because  they  will  conduct  an  electric  current. 

"Other  properties  possessed  by  water  that  render  it  valuable  as  a  com- 
ponent of  living  matter  are  its 

1.  High  surface  tension,  exceeded  only  by  that  of  mercury. 

2.  Low  internal  friction,  resulting  in  low  viscosity. 

3.  Great  heat  capacity. 

4.  High  heat  conductivity. 

5.  Latent  heat. 

6.  Greatest  density  at  4.0°C. 

(Rogers,  "Textbook  of  Comparative  Physiology,"  pp.  14-17,  McGraw-Hill 
Book  Company,  Inc.,  New  York,  1938.)  Sec  Matter;  Hydrogen  Ion. 

Earthworm.  "Earthworms,  of  which  there  are  a  great  many  species 
widely  distributed  in  the  soil  of  practically  every  region  of  the  globe,  belong  to 
a  phylum  of  segmented  animals  known  as  the  Annelida.  Due  to  the  fact  that 
the  Earthworm  possesses  a  number  of  structural  features  which  are  of  con- 
siderable importance  in  interpreting  those  of  still  higher  types  of  animal  life, 
it  is  an  especially  valuable  form  for  study.  These  structural  features  may  be 
enumerated  as  follows: 

"The  Earthworm  is  a  triploblastic  animal;  the  three  primary  germ  layers, 
ectoderm,  mesoderm,  and  endoderm,  being  present  as  in  higher  animals,  and 
in  contrast  to  diploblastic  animals  like  Hydra. 

"The  Earthworm  possesses  a  body  cavity,  or  coelom,  lying  between  the 
body  wall  and  the  tubular  alimentary  canal.  Thus,  the  body  plan  may  be 
described  as  a  tube  within  a  tube.  This  type  of  structure  is  present  in  higher 
forms,  but  it  is  not  found  in  the  Coelenterates,  in  which  the  body  may  be  said 
to  consist  of  a  single  tube. 

"The  Earthworm  shows  a  definite  segmentation,  or  metamerism,  of  the 
body;  that  is  to  say,  the  body  is  composed  of  a  large  number  of  distinct  seg- 
ments which  .are  arranged  in  a  linear  series.  Varying  degrees  of  segmentation 
are  present  in  most  of  the  higher  forms  of  animals. 


526  HUMAN  BIOLOGY 

"The  Earthworm  shows  a  two-sided,  or  bilateral,  symmetry.  As  a  rule, 
the  organs  in  such  a  case  are  paired :  one  situated  on  the  right  side  of  the  body 
and  one  on  the  left  side.  Accordingly  there  is  only  one  plane  which  will 
divide  the  animal  into  symmetrical  halves.  Bilateral  symmetry  is  even  more 
pronounced  in  the  higher  animal  types. 

"The  Earthworm  possesses  a  number  of  highly  developed  organ  systems 
for  performing  various  vital  functions,  such  as  nutrition,  transportation, 
excretion,  etc.  These  arise  by  a  grouping  of  certain  tissues,  and  are  charac- 
teristic of  all  the  higher  organisms."  (Baitsell,  "  Manual  of  Animal  Biology," 
p.  81.  The  Macmillan  Company,  New  York,  1932.) 

,  Electrocardiogram.  "A  graphic  record  of  the  electrical  variations  pro- 
duced by  the  beating  heart  is  called  an  electrocardiogram.  These  variations 
are  the  result  of  the  development  of  electrical  negativity  of  excited  muscle  as 
compared  with  unexcited  tissues.  The  electrical  variations  of  the  heart  are 
thus  entirely  comparable  to  the  negative  variation  or  action  current  of  other 
muscles.  When  the  skeletal  muscles  are  at  rest,  save  for  quiet  breathing 
movements,  the  action  currents  of  the  heart  can  be  satisfactorily  recorded  and 
accurately  measured  by  means  of  a  string  galvanometer.  Such  an  instrument, 
when  especially  adapted  for  observations  on  the  heart,  is  called  an  electro- 
cardiograph. The  movements  of  the  string  of  the  galvanometer  are  photo- 
graphed upon  a  moving  sensitive  film  to  give  the  electrocardiogram.  The 
changes  of  electrical  potential  in  the  heart  can  be  communicated  to  the 
galvanometer  through  electrodes  applied  to  the  surface  of  the  body.  This 
is  the  case  because  animal  tissues  and  fluids  are  able  to  conduct  electrical 
currents.  Large  nonpolarizable  electrodes  are  applied  to  the  two  hands'  or  to 
one  hand  and  one  foot  in  order  to  connect  the  human  body  to  the  apparatus. 
In  studies  upon  experimental  animals,  small  nonpolarizable  electrodes  can  be 
applied  to  definite  locations  of  the  exposed  or  excised  heart.  The  electro- 
cardiograms that  have  been  obtained  by  this  latter  method  have  been  espe- 
cially useful  in  studies  designed  to  show  the  point  at  which  the  heart  beat 
originates  and  to  show  the  course  that  the  wave  of  contraction  takes  as  it 
progresses  over  the  heart  muscles.  (Mitchell,  "Textbook  of  General  Phy- 
siology," pp.  589ff.,  McGraw-Hill  Book  Company,  Inc.,  New  York,  1938.) 

Electrolytes.     See  Dissociation. 

Electrons.     See  Matter. 

Energy.  "The  phases  of  reality  with  which  the  student  of  science  has  to 
deal  are  matter  and  energy.  The  term  commonly  used  to  distinguish  force  is 
energy,  and  by  energy  is  meant  the  ability  or  the  power  to  do  work.  It  will 
be  noted  that  the  word  energy  involves  the  concept  of  motion,  either  existent 
or  potential.  Some  of  the  earlier  writers  believed  not  only  in  the  existence  of 
forms  of  matter  that  had  no  common  factor  but  in  different  manifestations  of 
force,  which  were  not  and  could  not  be  related  to  each  other.  The  necessary 
conclusion  now  is  not  only  that  all  matter  is  composed  of  the  same  sorts  of 
ultimate  units  but  also  that  all  forms  of  energy  have  a  common  origin,  the 
energy  of  the  electron;  that  is,  matter  and  energy  are  but  different  manifesta- 
tions of  the  same  thing  (electricity). 


APPENDIX  527 

"Energy  is  found  to  exist  in  two  chief  types: 


1.  Kinetic  energy,  the  energy  of  motion. 

2.  Potential  energy,  the  energy  of  position. 


"Energy  of  motion  appears  in  a  variety  of  forms,  as  electrical,  magnetic, 
atomic,  molecular,  radiant,  chemical,  gravitational,  mechanical,  and  thermal. 
It  is  possible  that  all  these  different  manifestations  may  be  explained  upon 
the  assumption  of  the  energy  of  the  electrons.  It  is  certainly  not  difficult  to 
transform  one  form  of  energy  into  another.  Some  authors  would  add  to  the 
list  given  biotic  energy,  as  a  distinct  form  of  energy  found  only  in  living  matter. 
This,  too,  may  be  derived  from  the  energy  of  the  electrons  that  make  up  the 
living  matter.  At  any  rate,  there  is  not  at  the  present  time  sufficient  reason 
for  distinguishing  this  form  of  energy  from  the  chemical  energies  of  the  sub- 
stances that  are  to  be  found  undergoing  change  in  the  living  substance. 

"Energy  of  position  may  be  that  of  a  weight  in  an  elevated  position,  which 
may,  if  properly  harnessed,  accomplish  work  when  allowed  to  move;  or  it  may 
be  that  of  electricity  accumulated  in  a  storage  battery  or  the  energy  of  chem- 
ical substances  that  may  be  released  in  the  formation  of  some  new  chemical 
substances;  or  that  of  food  substances  that  may  be  transformed,  with  the 
production  of  heat  or  light  or  electricity  or  the  accomplishment  of  work;  or 
that  of  storage  substance  in  living  cells,  which,  when  drawn  into  the  vortex 
of  the  metabolic  activities  of  the  cell,  may  furnish  the  energy  for  the  perform- 
ance of  a  great  variety  of  cellular  activities. 

"Energy  of  one  form  may,  under  suitable  conditions,  be  converted  into 
energy  of  another  form.  Thus,  kinetic  energy  of  moving  air  or  of  falling  water 
may  be  converted  into  electrical  energy.  By  the  use  of  the  proper  type  of 
transformer,  the  electrical  energy  may  be  converted  into  mechanical,  thermal, 
or  radiant  energy.  The  energy  of  the  sunlight  may,  in  a  similar  way,  be 
converted  into  mechanical  energy,  or  it  may  be  stored  up  by  the  green  plant 
and  form  a  supply  of  chemical  potential  energy.  The  energy  of  chemical  com- 
pounds, bound  in  the  molecules  through  the  attractions  of  the  different  atoms 
for  one  another,  may  be  transformed  into  heat  energy  or  mechanical  energy. 
In  all  these  transformations  of  energy,  there  appears  to  be  a  tendency  for 
energy  to  be  degraded  into  heat.  All  forms  of  energy  may  ultimately  appear 
as  heat,  but  it  does  not  seem  possible  at  present  to  convert  heat  energy  into 
all  the  other  forms  of  energy. 

"Any  physical  system  through  which  energy  is  transformed  from  one  sort 
to  another  may  be  spoken  of  as  a  transformer.  Such  transformers  are  of  great 
variety — some  very  simple,  and  some  very  complex.  Many  machines  have 
been  constructed  as  the  result  of  human  ingenuity.  These  serve  in  a  mechan- 
ical way  as  energy  transformers.  Many  energy  transformers  are  the  result 
of  long-continued  evolutionary  processes,  dealing  in  particular  with  what  is 
commonly  known  as  living  matter.  It  has  been  customary  to  think  of  living 
cells  as  the  most  wonderful  of  these  energy  transformers  and  to  attribute  to 
them  very  special  powers  and  properties  because  of  the  things  they  have  been 
observed  to  do.  Living  cells  are  themselves  highly  complex  colloidal  systems. 


528  HUMAN  BIOLOGY 

As  such,  they  exhibit  properties  that  are  common  to  other  colloidal  systems. 
Among  these  properties  is  the  ability  to  synthesize  more  complex  substances 
out  of  their  less  complex  components.  The  more  there  is  discovered  con- 
cerning the  nature  of  the  living  substance  the  more  likely  it  seems  that  the 
inorganic  colloids  are  the  substances  that  must  be  looked  to  for  the  first 
appearance  of  those  remarkable  energy  transformations  commonly  attributed 
to  living  cells.  The  very  intimate  way  in  which  the  components  of  the 
colloidal  systems  are  related  to  one  another  and  the  great  expanse  of  internal 
surface  exposed  render  the  colloidal  condition  ideal  for  transformations  of 
both  material  and  energy.  Certain  it  is  that  some  of  the  inorganic  colloids, 
such  as  ferric  hydrate,  have  the  power  to  synthesize  carbon  dioxide  and  water 
into  more  complex  molecules.  It  has  been  shown  that  the  synthetic  power 
of  the  chloroplast  of  the  green  plant  is  due,  not  to  the  chlorophyll  held  in  the 
chloroplast,  but  to  an  iron  compound  of  a  simpler  sort.  The  earliest  forms  of 
matter  that  had  the  power  to  synthesize  even  very  complex  substances  must 
have  been  very  simple  as  compared  with  the  simplest  forms  of  living  matter 
as  known  at  the  present  time.  The  fact  is  also  becoming  apparent  that,  in 
living  cells,  chemical  operations  are  initiated  and  controlled  not  by  the  active 
protoplasm  as  a  whole  but  by  certain  special  substances  produced  by  the 
activity  of  the  protoplasm,  termed  enzymes.  These  are  the  energy  trans- 
formers of  living  matter."  (Rogers,  "  Textbook  of  Comparative  Physiology," 
pp.  10-12.  McGraw-Hill  Book  Company,  Inc.,  New  York,  1938.) 

Enzymes.  The  inauguration  of  the  modern  science  of  enzymology  began  to 
take  shape  during  the  first  half -of  the  nineteenth  century.  Scientific  methods 
of  study  were  applied  to  a  variety  of  phenomena  that  common  people  had 
accepted  for  centuries:  the  use  of  yeast  to  leaven  bread  or  ferment  wine;  the 
digestive  action  of  the  stomach  juice;  the  ability  of  organs  supplied  only  from 
the  blood  to  produce  a  multitude  of  different  chemical  substances,  as  in 
saliva,  milk,  or  urine.  Simultaneously,  in  the  field  of  inorganic  chemistry, 
came  the  recognition  that  certain  chemical  reactions  were  peculiar  in  that  their 
progress  was  profoundly  affected  by  the  presence  of  some  ingredient  that 
apparently  remained  itself  unchanged.  A  little  sulphuric  acid  facilitated  the 
breakdown  of  starch  into  glucose;  hydrogen  peroxide  decomposed  with  great 
rapidity  in  the  presence  of  an  apparently  inert  substance  like  platinum. 

In  1837,  Berzelius  recognized  that  the  many  diverse  phenomena  outlined 
above  had  in  common  a  single  feature  which  he  described  as  an  unknown 
chemical  "force":  the  ability  of  a  substance  present  in  relatively  small 
amounts  to  cause  an  enormous  increase  in  the  rate  of  a  chemical  reaction. 
Berzelius  proposed  the  term  catalysis  to  describe  such  reactions  which  take 
place,  therefore,  under  the  influence  of  a  catalyst.  The  term  catalyst  was 
thus  from  its  inception  applicable  to  similar  phenomena  in  the  organic  and 
inorganic  worlds  alike.  Indeed,  Berzelius  predicted  the  fundamental  part 
that  must  be  played  by  catalytic  processes  in  living  plants  and  animals,  a 
prediction  that  has  since  received  overwhelming  verification. 

At  this  time,  the  organic  catalyst  that  received  the  greatest  attention 
was  what  was  then  known  as  the  yeast  ferment,  responsible  for  alcoholic 


APPENDIX  529 

fermentation.  Berzelius  still  believed  that  yeast  was  a  nonliving  substance; 
but  in  the  same  year,  Cagnaird-Latour,  Theodor  Schwann,  and  Kiitzing  dis- 
covered, independently  of  each  other,  that  yeast  was  a  microscopic  form  of 
life.  For  some  time  after  this,  the  word  ferment  was  reserved  for  such  cases 
in  which  the  presence  of  uninjured  cells  was  supposed  to  be  essential  for  the 
action  of  the  organic  catalyst.  Active  extracts,  such  as  malt,  or  secretions, 
as  of  the  stomach,  were  frequently  referred  to  as  unorganized  or  unformed 
ferments.  In  order  to  clarify  current  concepts,  Kuhne,  in  1878,  proposed  that 
the  word  enzyme  should  be  used  for  the  unorganized  ferments.  All  confusion 
was  finally  resolved  in  1897,  when  Biichner  was  able  to  show  that  extracted 
juices  of  yeast  contained  the  yeast  ferment;  the  term  ferment  and  enzyme  thus 
came  to  mean  the  same  thing  and  are  used  interchangeably  at  the  present  day, 
although,  in  English-speaking  countries,  the  latter  is  more  generally  accepted. 

In  accordance  with  Berzelius,  it  is  customary  to  introduce  the  student  to 
the  concept  of  an  enzyme  by  pointing  out  that  it  is  an  organic  catalyst.  Some 
authors  prefer  to  elaborate  this  statement  by  saying  that  an  enzyme  is  an 
organic  catalyst  present  in  or  produced  by  living  organisms.  Furthermore, 
the  statement  that  enzymes  are  produced  by  living  cells  must,  at  best,  be  only 
a  partial  truth;  the  growing  acceptance  of  the  idea  that  enzymes  are  as  essential 
in  the  building  up  as  in  the  breakdown  of  protoplasm  would  suggest  the 
existence  of  some  more  intimate  relationship.  Indeed,  some  authors,  as,  for 
example,  Wright,  have  speculated  on  the  possibility  of  a  close  connection 
between  the  genes,  or  genie  complexes,  which  determine  the  hereditary 
potentialities  of  the  organism,  and  the  intracellular  constructive  enzymes 
which  effect  the  practical  realization  of  these  potentialities. 

The  essential  feature,  common  to  all  catalyzed  reactions,  is,  as  already 
explained,  the  effect  of  the  catalyst  on  the  rate  of  the  reaction.  In  all  the 
more  familiar  cases,  this  effect  is  one  of  pronounced  acceleration,  but  it  is 
important  to  bear  in  mind  that  the  effect  may  equally  well  be  one  of  retarda- 
tion; the  term  negative  catalysis  being  applied  in  such  cases.  According  to 
the  generally  accepted  theory,  no  catalyst  can  initiate  a  reaction,  nor  can  it 
change  in  any  way  the  state  of  equilibrium  that  marks  the  completion  of  the 
reaction.  On  this  basis,  therefore,  the  sole  action  of  a  catalyst  is  upon  the 
rate  at  which  chemical  equilibrium  is  attained.  In  practice,  however,  in 
the  absence  of  a  positive  catajyst,  a  reaction  rate  may  be  infinitely  slow  and  the 
effect  of  the  catalyst  thus  frequently  appears  to  be  an  initiation  as  well  as  an 
acceleration  of  the  reaction.  It  is  this  feature  of  enzyme  actions  which 
makes  them  such  important  factors  in  the  metabolism  of  living  organisfns; 
reactions  that  can  be  performed  in  the  laboratory  only  with  great  difficulty, 
if  at  all,  and  then  frequently  only  with  the  aid  of  high  temperatures  and 
powerful  reagents  destructive  to  life,  can  be  performed  with  the  greatest  ease 
and  efficiency  by  living  cells. 

The  concept  of  a  catalyst  as  an  agent  that  changes  the  rate  of  a  chemical 
reaction  is  accompanied  by  the  postulate  that  catalyzed  reactions  must  obey 
the  lavvs  that  govern  chemical  processes.  The  fact  that  these  laws  are  not 


530  HUMAN  BIOLOGY 

always  followed  by  enzymic  reactions  has,  in  the  past,  greatly  impeded  the 
progress  of  knowledge  in  this  field.  One  difficulty  in  the  way  of  critical 
investigation  of  the  kinetics  of  enzyme  action  has  been  the  relative  impurity 
of  the  enzyme  preparations.  A  major  achievement  of  the  last  decade  has  been 
the  development  of  suitable  technical  methods  for  the  concentration  and 
isolation  of  enzymes. 

A  crystalline  preparation  of  the  enzyme  urease  was  isolated  by  Sumner  in 
1926.  Urease  is  responsible  for  the  breakdown  of  urea  with  the  liberation  of 
ammonia  and  carbon  dioxide.  It  is  abundant  in  certain  seeds,  like  soybean 
and  jack  bean  from  which  it  is  conveniently  isolated.  A  bacterium,  Micro- 
COCCUA  ureae,  utilizes  this  enzyme  to  split  the  urea  that  is  excreted  as  nitrog- 
enous waste  in  the  urine  and  is  thus  responsible  for  the  ammoniacal  smell  of 
stale  urine. 

Crystalline  urease  proved,  upon  examination,  to  beji  protein  of  the  globulin 
group.  Formerly  there  had  been  much  dispute  as  to  whether  or  not  enzymes 
were  of  a  protein  nature,  and,  since  much  supposed  evidence  had  been  accumu- 
lated to  the  contrary,  Sumner's  discovery  did  not  immediately  receive  a  ready 
acceptance.  It  was  not  long,  however,  before  parallel  results  were  obtained 
through  the  isolation  of  other  enzymes  in  pure,  or  relatively  pure,  crystalline 
form;  the  most  important  contributions,  perhaps,  being  those  of  Northrop 
and  his  coworkers,  beginning  in  1930  with  the  isolation  of  crystalline  pepsin. 
Although  great  progress  has  been  made,  it  must  be  admitted  that  the  study  of 
purified  enzymes  is  still  in  its  infancy;  the  fact  that  the  best  known  prepara- 
tions have  all  proved,  up  to  the  present  time,  to  be  proteins  or  simple  protein 
derivatives  must  not  be  taken  to  exclude  the  possibility  that  other  types  of 
organic  compound  may  also  function  as  enzymes.  Furthermore,  the  dispute 
remains  as  to  whether  the  protein  is  itself  the  enzyme  or  merely  an  essential 
carrier  for  an  active  (prosthetic)  group.  In  Northrop's  laboratory,  it  has 
been  shown  that  any  reaction  that  destroys  or  radically  changes  the  protein 
causes,  in  similar  proportion,  a  corresponding  loss  of  enzyme  activity.  Such 
experiments  strongly  support  the  view  that  the  protein  is  itself  the  enzyme. 
In  order  to  function  as  a  catalyst,  it  is  evident,  however,  that  the  protein 
must  possess  some  quite  special  configuration  of  the  molecule,  the  key,  as  it 
were,  to  the  catalyzed  reaction.  Active  groups  attached  to  a  protein  molecule 
have  been  demonstrated  in  the  case  of  certain  respiratory  enzymes,  and  the 
distinction  between  a  prosthetic  group  that  is  loosely  attached  and  one  that 
forms  a  more  integral  part  of  the  protein  molecule  may,  after  all,  be  an  aca- 
demic rather  than  a  fundamental  concept. 

It  is  important  for  the  student  of  living  matter  to  have  a  clear  idea  of  the 
nature  of  reversible  chemical  reactions.  It  is  misleading  to  think  of  a  chem- 
ical reaction  as  a  process  that  continues  to  completion  in  one  direction;  how- 
ever near  to  completion  the  end  point.may  be,  it  is  nevertheless  an  equilibrium. 
Reactions  can  only  proceed  to  completion  when  the  products  are  continually 
being  removed  from  the  scene  of  the  action.  Large  numbers  of  reactions, 
especially  in  the  field  of  organic  chemistry,  do  not  proceed  to  an  end  point  that 


APPENDIX  531 

even  remotely  resembles  completion.  Such  reactions  are  known  as  balanced, 
or  reversible,  reactions;  a  state  of  equilibrium  is  reached,  for  example,  when  the 
rate  of  combination  of  two  substances  A  and  B  to  form  the  compound  AB 
is  exactly  balanced  by  the  tendency  of  AB  to  decompose  with  the  formation  of 
A  and  B.  It  will  be  evident  from  general  considerations,  without  the  applica- 
tion of  mathematical  formulae,  that  the  relative  amounts  of  A,  B,  and  AB  that 
may  be  present  in  the  mixture  at  equilibrium  must  depend  in  part  on  the 
relative  rates  of  the  two  opposing  reactions,  in  part  on  the  initial  concentration 
of  the  ingredients  at  the  start  of  the  reaction.  An  irreversible  reaction  can, 
from  this  point  of  view,  be  regarded  as  one  in  which  the  rate  of  the  opposing 
reaction  is  negligibly  small.  It  will  be  clear,  in  the  case  of  all  reversible  reac- 
tions, that  equilibrium  may  be  reached  from  either  direction;  by  breakdown, 
if  the  product  AB  is  in  excess  at  the  start;  by  combination,  if  the  breakdown 
products  A  and  B  are  in  excess. 

Whenever  a  catalyzed  reaction  is  of  the  reversible  type,  the  action  of  the 
catalyst  is  to  accelerate  (or,  in  rare  cases,  retard)  the  rate  at  which  equilibrium 
is  attained.  A  catalyst  must  therefore  facilitate  both  the  combination  and 
the  breakdown  phases  of  the  reaction  in  equal  measure,  since  it  has  no  effect 
whatever  on  the  final  state  of  equilibrium.  If  enzymes  are  true  catalysts, 
it  is  evident  that  this  important  concept  should  be  applicable  to  large  Cumbers 
of  reactions  taking  place  within  the  living  organism.  Furthermore,  an 
enzyme  should  be  able  to  effect  either  breakdown  or  synthesis  according  to 
the  conditions  under  which  it  is  permitted  to  operate.  In  a  few  well-estab- 
lished cases,  the  reversible  nature  of  enzymic  catalysis  has  been  clearly 
demonstrated.  Enzymes  known  as  phosphatases  facilitate  the  reactions  that 
take  place  between  phosphoric  acid  and  certain  organic  substances  such  as 
sugar  or  glycerin.  An  extract  containing  intestinal  phosphatase  can,  in  the 
presence  of  sodium  phosphate  and  glycerin,  effect  a  partial  synthesis  of 
sodium  glycerophosphate.  Conversely,  in  the  presence  of  sodium  glycero- 
phosphate,  it  will  effect  a  partial  breakdown  to  glycerin  and  sodium  phosphate 
The  equilibrium  will  be  the  same  in  either  case,  in  accordance  with  the  laws 
governing  such  reactions.  If  one  of  the  end  products  is  continuously  being 
removed,  the  reaction  could  be  carried  to  completion  in  either  direction. 
This  is  presumably  what  actually  happens  at  the  surface  of  the  intestinal 
mucosa;  the  breakdown  products  are  absorbed*  by  the  intestinal  epithelium 
and  transferred  to  the  blood  or  underlying  tissues;  and  this  process  continues 
until  digestion  of  those  organic  phosphates  which  form  the  substrate  for  this 
reaction  is  completed. 

The  reversibility  of  enzyme  action  has  been  demonstrated  in  relatively 
few  cases;  its  theoretical  importance  is,  however,  outstanding.  Various 
examples  will  occur  to  the  student  as  being  readily  susceptible  to  interpreta- 
tion along  these  lines.  Thus,  during  the  day,  the  green  leaf  of  a  plant  stores 
up  starch  because  glucose  is  being  formed  by  photosynthesis  more  rapidly 
than  it  can  be  distributed  and  utilized.  At  night,  the  starch  disappears  from 
the  leaf  because  the  rate  of  removal  is  now  in  excess.  It  is  possible,  and  in 
fact  probable,  that  this  reversible  behavior  is  under  the  control  of  a  single 


532  HUMAN  BIOLOGY 

enzyme  complex.  Similar  processes  may  govern  the  storage  and  subsequent 
utilization  of  other  temporary  food  reserves,  for  example,  the  temporary 
increase  of  glycogen  (animal  starch)  in  the  liver  after  a  meal  when  the  blood 
is  laden  with  sugar.  The  reaction  in  this  case  is  the  familiar  union  of  a  large 
number  of  glucose  molecules  to  form  a  complex  polysaccharide  with  the 
elimination  of  water,  according  to  the  following  expression  : 

+  nH20 


In  the  case  of  starch  the  number  n  may  be  between  26  and  30;  it  is  prob- 
ably somewhat  less  for  glycogen  but  is  not  definitely  known.  Synthetic 
proteins,  the  plasteins,  have  been  prepared  by  Wastenys  and  Borsook  by  the 
reversible  action  of  pepsin  or  trypsin.  From  the  point  of  view  of  the  biologist, 
the  most  significant  aspect  of  enzymic  synthesis  is,  perhaps,  the  synthesis  of 
the  complex  system  of  protoplasm  itself.  Almost  nothing  is  yet  known  in 
this  field. 

Protein-splitting  enzymes  can  readily  be  extracted  from  all  living  cells. 
The  conditions  under  which  they  perform  this  operation  have  been  extensively 
studied;  they  differ  markedly  from  the  protein  splitt&ng  enzymes  of  the 
digestive  juices  as  noted  below;  for  example,  they  are  activated  by  substances 
bearing  the  reducing  group  —  SH  (hydrosulphide)  in  the  molecaile  and  by 
hydrogen  cyanide.  One  of  the  best  known  of  these  intracellular  proteinases 
is  papain,  which  may  be  extracted  from  the  fruit  of  the  papaya  and  which  has 
recently  been  isolated  in  the  form  of  crystals  with,  as  might  be  expected,  the 
properties  of  a  protein.  Dried  preparations  are  sold  for  medicinal  purposes, 
and  the  fruit  has  often  been  advertised  as  an  aid  to  digestion  although  it  is 
riot  known  to  what  extent  it  can  profitably  be  used  to  supplement  defective 
secretion  of  normal  digestive  juices. 

Extracellular  Enzymes.  Because  the  digestive  enzymes  are  so  powerful  and 
so  relatively  easy  to  study,  the  science  of  enzymology  has  proceeded  more 
rapidly  in  this  field  than  in  any  other,  not  excepting  the  immense  field  of  the 
respiratory  enzymes.  It  must  not  be  supposed  that  extracellular  digestive 
secretions  should  figure  as  largely  as  they  do  in  a  balanced  survey  of  organic 
catalysis.  The  ability  to  form  and  secrete  extracellular  enzymes  appears 
rather  to  be  a  special  property  which  has  been  evolved  in  various  ways  by 
different  types  of  living  organisms.  Extracellular  enzymes  are  not  produced 
at  all  as  a  rule  by  green  plants.  It  is  possible  that  a  few  plants  that  live  in 
nitrogen-deficient  environments  and  that  supplement  their  normal  holophytic 
methods  of  nutrition  by  the  assimilation  of  the  decomposition  products  of 
captured  organisms  may  also  produce  extracellular  digestive  secretions. 
This  is  denied  in  the  case  of  the  pitcher  plant;  the  activities  of  other  carnivor- 
ous plants,  the  sundew,  flytrap,  and  bladderwort,  certainly  call  for  reinvestiga 
tion  in  the  light  of  modern  methods  of  microenzymology. 

The  colorless  plants,  such  as  fungi,  molds,  yeasts,  and  bacteria,  typically 
liberate  powerful  extracellular  enzymes  which  effect  the  breakdown  of  organic 
substances  in  their  environment.  The  products  of  hydrolysis  can  then  be 
taken  up  by  the  living  cells.  Many  of  these  organisms,  especially  among  the 


APPENDIX  533 

yeasts  and  bacteria,  are  either  facultative  or  obligatory  anaerobes;  that  is, 
they  can  or  must  obtain  their  metabolic  energy  by  some  other  method  than 
the  utilization  of  atmospheric  oxygen.  Many  of  them  thus  contain  or  secrete 
enzymes  which  can  perform  remarkable  metabolic  feats.  A  familiar  and 
economically  important  example  is  the  anaerobic  utilization  of  sugar  by  yeast. 
A  complex  system  of  enzymes,  activators,  and  co-enzymes,  which,  in  the 
light  of  modern  knowledge,  must  now  be  called  the  zymase  complex,  effects 
the  breakdown  of  sugar  into  alcohol  with  the  liberation  of  carbon  dioxide. 
The  reaction  undoubtedly  proceeds  in  several  stages,  and  only  its  final  prod- 
ucts are  expressed  by  the  following  formulation: 

C6Hi2O6  ->  2C2H5OH  +  2CO2 

A  surprisingly  similar  series  of  operations  are  apparently  performed  by 
the  enzyme  complexes  of  muscle  tissue.  Glucose  is  broken  down,  through  a^ 
long  chain  of  intermediary  reactions,  into  the  end  product,  lactic  acid.  The 
initial  and  final  phases  of  the  reaction  can  be  expressed  as  follows: 

CcHuOe  ^  2C3H603 

The  similarities  between  the  action  of  yeast  zymase  to  that  of  muscle 
enzymes  does  not  appear  clearly  when  the  end  products  of  the  two  reactions 
are  indicated  crudely,  as  in  the  foregoing  equations.  The  mode  of  formation 
of  alcohol  in  the  one  case  and  of  lactic  acid  in  the  other  is  nevertheless  achieved 
through  an  almost  parallel  series  of  hydrolyses  and  transformations.  In  both 
cases,  an  essential  intermediary  step  is  the  formation  of  hexosephosphoric 
acid;  in  both  cases,  the  presence  of  magnesium  appears  to  play  an  essential 
but  little-understood  role;  in  both  cases  a  co-enzyme  has  been  isolated,  and, 
although  not  identical,  they  have  proved  to  be  similar  kinds  of  substance. 
Finally,  in  both  cases,  a  stage  of  the  reaction  can  be  blocked  by  the  addition 
of  monoiodo-acetic  acid. 

Special  attention  has  been  given  to  yeast  fermentation  because  of  its 
economic  importance  as  well  as  because  of  the  resemblances  that  it  shows  to 
processes  that  accompany  muscular  contraction.  Many  other  microorganisms 
are  able  to  produce  substances  of  use  and  interest  to  man.  Vinegar  is  pro- 
duced from  alcohol  by  the  oxidative  action  of  the  acetic  acid-forming  bac- 
terium. Another  bacterium  is  used  commercially  in  the  fermentation  of 
starch  to  form  butyl  alcohol.  A  mold,  Aspergillus  niger,  assists  in  the  produc- 
tion of  citric  and  oxalic  acids  from  sugar. 

The  Protozoa  do  not  appear  to  secrete  extracellular  enzymes.  Holozoic 
species  produce  digestive  enzymes  in  their  food  vacuoles,  whereas  the  saprozoic 
forms  must  live  in  an  environment  rich  in  the  diffusible  products  of  digestion. 
As  we  pass  to  the  multicellular  animals,  there  seems  to  have  been  a  gradual 
evolution  of  extracellular  digestion.  Sponges  produce  no  extracellular 
enzymes;  the  cells  lining  the  gut  are  provided  with  flagellae  and  are  able  to 
pick  up  and  ingest  food  particles  in  the  same  way  as  individual  protozoans. 
Among  the  Coelenterates,  we  find  the  first  appearance  of  an  extracellular 


534  HUMAN  BIOLOGY 

digestive  secretion.  In  hydra  and  among  the  corals,  the  only  enzyme  con- 
tained in  this  secretion  is  a  protease;  all  other  phases  of  digestion  are  carried 
on  within  the  cells  lining  the  alimentary  tract  that  phagocytose  the  food 
particles  and  initial  products  of  protein  hydrolysis.  The  number  of  extra- 
cellular enzymes  and  the  degree  to  which  digestion  proceeds  in  the  lumen  of  the 
alimentary  canal  vary  greatly  among  different  groups  of  invertebrates.  In 
the  vertebrates,  there  is  also  evidence  that  some  of  the  final  phases  of  intestinal 
digestion  may  even  be  carried  on  within  the  cells  of  the  intestinal  wall. 

Some  animals  produce  remarkable  enzymes  that  enable  them  to  digest 
substances  that  are  entirely  useless  as  food  for  man.  Thus  snails  possess  an 
enzyme  that  attacks  cellulose,  hydrolyzing  it  to  simple  sugars.  An  extract 
of  the  digestive  gland  of  such  an  animal  cannot  be  filtered  through  ordinary 
filter  paper  because  the  paper  itself  will  be  digested.  Wood-boring  insects 
also  can  digest  cellulose,  and,  in  the  case  of  the  termites,  this  has  been  shown 
to  depend  on  the  presence  of  a  symbiotic  protozoan  fauna  and  bacterial  flora 
that  inhabit  the  gut.  The  wax  moth  can  digest  beeswax,  a  substance  that 
is  totally  resistant  to  the  powerful  lipase  of  the  mammalian  pancreas.  The 
clothes  moth  has  a  peculiar  type  of  proteinase  which,  in  a  strongly  alkaline 
medium,  is  able  to  effect  the  breakdown  of  hair  and  horn.  This  enzyme  is 
called  keratinase  because  of  its  ability  to  attack  these  materials.  Some  snails 
and  a  few  other  invertebrates  can  digest  the  resistant  material  chitin,  a  nitrog- 
enous derivative  of  carbohydrates  which  is  an  important  constituent  of  the 
external  skeleton  of  arthropods. 

Proteases.  The  protein  splitting  enzymes  of  the  digestive  juices  deserve 
special  consideration.  The  enzyme  pepsin  was  isolated  in  crystalline  form 
by  Northrop  and  has  been  extensively  examined  by  this  investigator  and  his 
colleagues.  It  is  secreted  from  peptic  cells  in  the  gastric  mucosa  in  an  inactive 
form,  pepsinogen,  which  has  been  isolated  in  crystalline  form  also.  Pep- 
sinogen  is  transformed  into  active  pepsin  by  the  action  of  hydrochloric  acid 
secreted  by  the  oxyntic  cells  of  the  gastric  mucosa.  The  correlation  of  the 
type  of  cell,  peptic  or  oxyntic,  with  the  type  of  secretion,  pepsinogen  or  hydro- 
chloric acid,  respectively,  was  achieved  by  the  application  of  the  modern 
methods  of  histochemical  technique  developed  by  Linderstr0m-Lang  and  his 
colleagues  during  the  last  decade.  Pepsin  can  only  attack  proteins  in  an  acid 
medium.  Northrop  believes  that  this  is  due  to  the  fact  that  pepsin  can 
react  only  with  the  positive  protein  ion  which  appears,  of  course,  in  a  medium 
that  is  on  the  acid  side  of  the  isoelectric  point.  Trypsin,  on  the  other  hand, 
attacks  proteins  in  an  alkaline  medium  and  would  appear  to  react  with  the 
negative  protein  ion.  The  situation  can  be  expressed  with  the  aid  of  a  simple 
diagram: 
Thus 

Partially  Neutral  protein  Partially 

ionized  NaOH             at             HC1  ionized 

Sodium         < isoelectric >  Protein 

Proteinate  point  Hydrochloride 


APPENDIX  535 

Or 

+  ~     +          + 

PandNa      <-  P      ~>      P  and  Cl 

(attacked  (attacked         (attacked 

by  by  by 

trypsin)  papairi)  pepsin) 

Although  crystalline  pepsin  is  now  believed  to  be  a  pure  substance,  slight 
elements  of  doubt  still  remain.  Northrop  himself  was  able  to  effect  a  partial 
separation  of  another  proteolytic  enzyme  which  was  specially  active  in  the 
digestion  of  gelatin;  this  gelatinase  was  present  in  minute  amounts;  it  was 
separated  but  never  completely  purified;  and  mystery  still  surrounds  the  true 
meaning  of  its  presence  and  discovery. 

Another  and  completely  distinct  gastric  proteinase  can  be  isolated  from 
the  stomachs  of  calves.  This  is  the  enzyme  rennin.  It  was  formerly  believed 
that  rennin  was  the  only  enzyme  that  could  cause  the  clotting  of  milk;  and  as  a 
result  of  this,  much  confusion  arose  in  the  early  literature  on  the  subject. 
Investigators  would  test  the  power  of  different  extracts  on  the  clotting  of  milk 
and  would  then  state  that  the  enzyme  rennin  was  present,  sometimes  in 
the  most  unexpected  places.  For  example,  rennin  has  been  listed  among  the 
digestive  enzymes  of  many  invertebrates,  including  spiders  and  earthworms. 
The  situation  was  greatly  clarified  when  it  was  recognised  that  all  proteolytic 
enzymes  can,  under  certain  circumstances,  cause  the  clotting  of  milk,  some, 
it  is  true,  with  greater  facility  than  others.  This  discovery  raised  doubt  as  to 
whether  or  not  a  separate  milk-clotting  enzyme  existed  in  the  gastric  juice  of 
young  animals.  Through  the  investigations  of  Tauber  and  others,  it  is  now 
clear  that  a  separate  milk-clotting  enzyme  does  exist,  distinct  from  pepsin, 
in  the  fourth  stomach  of  the  calf.  On  the  other  hand,  it  has  not  proved 
possible  to  separate  rennin  from  pepsin  as  a  distinct  enzyme  in  the  gastric 
juice  of  various  other  mammals,  young  or  old.  Rennin  is  not  present  in 
children  or  in  adult  human  beings;  nor  is  it  found  in  dogs  or  in  pigs.  In  all 
these  animals,  the  digestion  of  milk  is  apparently  a  function  of  the  enzyme 
pepsin. 

The  chemistry  of  milk  clotting  is  of  some  interest  because  of  its  practical 
application.  Junkets  are  prepared  by  the  addition  of  powdered  preparations 
of  calves'  stomachs  to  warm  milk.  In  a  few  hours,  the  junket  "sets"  to  a 
semisolid  mass;  an  insoluble  substance,  the  curd,  holds  in  its  interstices  the 
fluid  whey.  Clotting  can  take  place  only  in  the  presence  of  calcium  salts, 
which  are,  of  course,  normal  constituents  of  milk.  A  simplified  explanation 
of  the  course  of  events  is  as  follows :  The  proteolytic  enzyme,  normally  pepsin 
or  rennin,  first  hydrolyzes  the  milk  protein  casein  to  another  soluble  derivative 
known  as  paracasein;  this  is  apparently  achieved  by  the  splitting  off  of  a  pro- 
tecting substance  also  of  a  protein  nature.  The  soluble  paracasein  then 
reacts  with  calcium  to  form  the  insoluble  curd  calcium  paracaseinate.  Rennin 
has  little  ability  to  effect  other  proteolytic  hydrolyses  and  is  unable  to  carry 


536  HUMAN  BIOLOGY 

the  process  beyond  the  simple  stage  of  clotting.  Pepsin  and  other  proteolytic 
enzymes  can  continue  the  breakdown  to  Jower  stages  provided  they  are 
allowed  to  work  at  the  pH  suitable  to  their  mode  of  action.  Clotting  of  milk 
by  pepsin  is  not  observed  unless  the  reaction  is  allowed  to  proceed  in  a  nearly 
neutral  medium;  the  reason  for  this  is  that  calcium  paracaseinate  is  redissolved 
by  acid;  and  although  the  casein  is  split,  no  curd  makes  its  appearance. 

When  the  protein  digest  leaves  the  stomach,  it  is  acted  on  by  at  least  three 
proteases  that  are  secreted  by  the  pancreas;  these  three  have  all  been  isolated 
by  Northrop  and  his  colleagues  in  crystalline  form.  Tlie  pancreatic  protease 
complex  was  formerly  called  trypsin;  it  was  known  to  work  in  an  alkaline 
medium,  resulting  from  the  secretion  of  sodium  carbonate  in  the  pancreatic 
juice,  and  it  was  believed  to  be  more  powerful  than  pepsin  and  to  be  able  to 
carry  the  breakdown  of  proteins  all  the  way  to  simple  peptides  and  amino 
acids.  Two  components  of  the  trypsin  complex,  trypsin  proper  and  chymo- 
trypsin,  act  upon  native  proteins  and  hydrolyze  them,  in  general,  to  peptones 
and  polypeptides.  In  this  respect,  they  resemble  pepsin,  although,  as  indi- 
cated above,  they  apparently  attack  the  negative  rather  than  the  positive 
protein  ion.  Trypsin  and  chymotrypsin  differ  markedly  in  their  crystalline 
form  and  in  other  physical  and  chemical  properties;  the  fact  that  their  action 
on  the  protein  digest  seems,  at  first  sight,  similar  is  due  to  our  lack  of  knowl- 
edge of  the  structure  of  the  complex  protein  molecules  and  of  the  types  of 
linkage  that  are  attacked.  It  can  be  shown  that  the  two  enzymes  attack 
different  parts  of  the  protein  molecule  by  submitting  a  substrate  first  to  the 
action  of  one  of  these  enzymes  and  then  to  the  other.  If  the  reaction  is  allowed 
to  come  to  equilibrium  and  if  the  two  enzymes  catalyzed  the  same  breakdown, 
it  is  evident  that  no  change  would  be  brought  about  by  the  second  enzyme. 
In  practice,  it  is  found  that  the  second  enzyme  will  continue  the  hydrolysis 
to  a  new  equilibrium,  thus  showing  that  entirely  different  linkages  are  attacked. 

In  a  similar  way,  it  can  be  demonstrated  that  pepsin  attacks  the  protein 
molecule  in  yet  another  way,  dissimilar  to  the  action  of  either  trypsin  or 
chymotrypsin.  It  is  thus  clear  that  all  the  proteolytic  enzymes  may  be 
necessary  for  complete  digestion  since  their  actions  are  supplementary. 

The  third  proteolytic  enzyme  of  the  pancreatic  juice  is  a  carboxypolypep- 
tidase;  this  is  an  enzyme  that  splits  a  few  dipeptides  and  a  large  number  of 
polypeptides,  all  of  them  compounds  that  possess  an  unsubstituted —  COOH 
group  at  one  end  of  the  molecule  (page  537).  Substitution  of  the  — NH2 
group  of  the  pep  tide  does  not  prevent  the  action  of  the  carboxypolypeptidase. 
The  pancreatic  carboxypolypeptidase  has  been  isolated  in  crystalline  form  by 
Anson;  it  liberates  free  amino  acids  from  that  end  of  the  peptide  molecule 
which  it  is  able  to  attack. 

By  the  time  the  protein  digest  leaves  the  duodenum,  it  has  been  reduced 
by  the  pancreatic  proteases  to  dipeptides,  polypeptides,  and  a  percentage  of 
free  amino  acids.  The  final  stages  of  protein  hydrolysis  are  carried  out  by 
the  intestinal  juices,  most  active  at  the  surface  of  the  intestinal  mucosa.  The 
active  proteolytic  enzyme  of  the  intestine  was  formerly  believed  to  be  a  single 
substance,  erepsin.  As  in  the  case  of  trypsin,  it  has  now  been  shown  to  be  a 


APPENDIX  537 

complex  of  several  distinct  enzymes.  None  of  these  have  been  isolated  in 
crystalline  form,  and  the  methods  of  separation  have  been  of  a  different  nature 
from  those  employed  in  the  isolation  of  gastric  and  pancreatic  proteases. 

At  least  three  different  proteolytic  enzymes  can  be  recognised  as  distinct 
entities  in  the  erepsin  complex.  The  first  of  these  attacks  remaining  polypep- 
tides  at  the  end  of  the  molecule  opposite  to  that  which  was  attacked  by 
carboxypolypeptidase ;  that  is,  it  releases  an  amino  acid  that  bears  a  free 
— NH2  group.  Enzymes  of  this  type  are  known  as  aminopolypeptidases. 
The  final  breakdown  of  the  proteins  is  completed  by  intestinal  dipeptidases 
that  attack  the  simple  dipeptides,  like  glycylglycine,  as  described  in  the  follow- 
ing paragraph.  A  third  component  of  the  erepsin  complex  is  a  little  known 
enzyme  called  prolinase,  whose  peptide  linkage  is  atypical  on  account  of  the 
presence  of  the  heterocyclic  amino  acid  proline. 

Dipeptidases  are  enzymes  that  facilitate  the  hydrolysis  of  dipeptides  into 
their  component  amino  acids,  for  example,  in  the  splitting  of  such  substances 
as  glycylglycine  by  the  intestinal  mucosa,  according  to  the  following  scheme: 

2  glycine  molecules  ^  glycylglycine  +  water 

The  student  is  well  aware  that  the  amino  acids  are  the  bricks  out  of  which 
protein  molecules  are  built.  Although  differing  widely  from  one  another 
in  chemical  composition,  they  have  in  common  a  particular  configuration  of 
atomic  groupings  at  one  end  of  their  molecule;  this  is  represented  diagram- 
matically  in  the  following  way: 

H 

Radicle  of)  | 

variable   [(R)— C— COOH 
character)  | 

NH2 

Of  these  groupings,  the  — COOH  (carboxyl  group)  is  acidic  in  character, 
while  the  NEU  (amino  group)  has  basic  properties.  Amino  acids  can  therefore 
behave  either  as  acids  or  as  bases,  according  to  the  conditions  under  which  they 
are  reacting  (page  568).  Most  important  of  all,  they  combine  with  each 
other,  head  to  tail  as  it  were,  and  it  is  in  this  way  that  dipeptides,  polypeptides, 
and  finally,  in  all  probability,  proteins  are  built  up.  This  head-to-tail  linkage, 
which  is  effected  with  the  elimination  of  a  molecule  of  water,  is  known  as  the 
peptide  linkage.  It  might  be  supposed  that  it  could  be  attacked  by  the  same 
enzyme  irrespective  of  whether  it  was  present  in  the  simple  union  of  two  amino 
acid  molecules  to  form  a  dipeptide  or  in  the  union  of  long  chains  forming 
polypeptides.  This,  however,  is  not  the  case.  It  seems  that  the  configura- 
tions of  parts  of  the  molecule  adjacent  to  the  linkage  are  of  fundamental 
importance.  The  enzyme,  dipeptidase,  is  merely  the  last  in  a  long  series  of 
enzymes,  as.  noted  above,  that  are  involved  in  the  Breakdown  of  the  proteins 
of  the  food  to  their  constituent  amino  acids.  The  initial  step  is  taken  by 
pepsin  in  the  stomach  juice.  Pepsin  can  carry  the  hydrolysis  of  proteins  only 


538  HUMAN  BIOLOGY 

as  far  as  intermediary  products  proteases  and  peptones,  still  of  large  molecular 
size. 

Other  Digestive  Enzymes.  It  is  not  possible  to  supplement  this  survey  of 
the  digestive  proteases  with  an  equally  extensive  account  of  those  other  diges- 
tive enzymes  which  are  responsible  for  the  hydrolysis  of  carbohydrates,  fats,  - 
compounds  of  nucleic  acid,  organic  phosphates,  and  the  like.  A  few  points  of 
interest  may  be  mentioned.  The  chief  starch-splitting  enzyme  is  the  pan- 
creatic amylase;  the  enzyme  ptyalin,  found  in  the  saliva  of  a  few  animals, 
including  man,  is  believed  to  be  identical.  Amylase  has  recently  been 
obtained  in  crystalline  form  and  is,  as  might  be  expected  from  what  has  gone 
before,  a  protein. 

The  final  breakdown  of  the  disaccharide  maltose,  released  by  amylytic 
hydrolysis  of  starch  and  glycogen,  is  effected  through  the  aid  of  an  intestinal 
enzyme,  maltase.  The  splitting  of  maltose  releases  two  molecules  of  glucose. 
In  a  similar  manner,  cane  sugar  is  split  by  intestinal  sucrase,  yielding  one 
molecule  of  glucose  and  one  of  fructose.  Milk  sugar,  lactose,  is  split  by  yet 
another  enzyme,  lactase,  yielding  glucose  and  galactose.  There  is  evidence 
that,  in  the  case  of  these  and  other  intestinal  enzymes,  the  catalysis  takes  place 
either  on  the  surface  of  the  intestinal  mucosa  or  possibly  within  the  cells 
themselves.  The  intestinal  juice  has  very  little  digestive  action.  A  weak 
maltase  is  also  present  in  human  saliva. 

The  action  of  pancreatic  lipase  is  essential  for  the  digestion  of  fats.  Bile 
salts  have  an  important  effect  on  the  action  of  lipase;  they  perform  a  double 
function.  On  the  one  hand,  they  lower  the  surface  tension  and  thus  facilitate 
the  emulsion  of  the  fat;  this  reduces  the  fat  droplets  to  submicroscopic  size  and 
enormously  increases  the  area  of  surface  exposed  for  lipolytic  action.  In 
another  capacity,  the  bile  salts  combine  readily  with  the  fatty  acids  that  are 
liberated  by  the  hydrolysis  of  the  fat;  the  combination  products,  known  as 
choleic  acids,  are  soluble  in  water  and  readily  diffusible,  and  it  is  probably 
in  this  form  that  the  fatty  acids  are  absorbed  by  the  intestinal  mucosa. 

A  weak  lipase  is  also  secreted  by  the  stomach.  It  appears  to  be  most 
active  in  weakly  acid  media  and  is  destroyed  by  the  higher  acidity  of  the 
gastric  juice  during  periods  of  digestion.  Its  function  may  be  to  attack  fats 
left  clinging  to  the  mucosa  between  periods  of  proteolytic  digestion.  Pan- 
creatic lipase  is  carried  down  into  the  intestine,  but  the  intestinal  mucosa  does 
not  appear  to  secrete  a  lipolytic  enzyme. 

The  intestinal  enzyme  phosphatase  supplements  the  action  of  two  other 
intestinal  enzymes.  A  polynucleotidase  attacks  nucleic  acid,  releasing  sub- 
stances that  are,  in  turn,  hydrolyzed  by  phosphatase,  with  the  release  of 
phosphoric  acid  and  further  breakdown  products  known  as  nudeosides.  The 
latter  may,  in  part,  be  further  attacked  by  another  enzyme,  nucleosidase. 

Intracellular  Enzymes.  A  complete  account  of  the  intracellular  enzymes 
would  of  necessity  involve  a  full  understanding  of  all  the  metabolic  processes 
that  occur  within  the  living  body.  Such  a  feat  could  not  be  attempted  in  the 
scope  of  the  present  article,  even  if  it  could  be  claimed  that  knowledge  was 
complete,  which  is,  of  course,  very  far  from  the  case.  After  the  soluble  prod- 


APPENDIX  539 

nets  of  digestion  have  been  absorbed  through  the  walls  of  the  intestine,  they 
are  distributed  through  the  blood  and  lymph  to  the  tissues  and  there  utilized 
in  various  ways. 

As  is  well  known,  only  comparatively  few  of  the  amino  acids  derived  from 
the  food  are  required  by  the  human  body.  Some  10  amino  acids  cannot  be 
synthesized  by  the  body  cells  and  must  be  derived  ready  made  from  the  food; 
the  requirements  for  these  are  relatively  small  except  during  periods  of  rapid 
growth  or  tissue  repair.  Similarly,  the  requirements  of  amino  acids  as  sources 
of  nitrogen,  sulphur,  etc.,  are  relatively  small.  The  bulk  of  the  amino  acids 
undergo  an  enzymatic  breakdown  (deaminization)  by  which  the  nitrogen 
group  — NH2  is  removed  from  the  molecule  and  converted  at  first  into 
ammonia;  later  the  ammonia  is  synthesized  into  urea  and  excreted  through 
the  kidneys  (page  100).  The  enzymes  that  effect  deaminization  are  known 
as  amidases;  a  wide  variety  of  such  enzymes  can  be  extracted  from  liver  and 
other  tissues,  and  they  are  classified  according  to  the  substrate  that  they 
attack.  Some  attack  different  sorts  of  amino  acids;  others  separate  the 
— NH2  group  from  related  types  of  organic  bases.  The  enzyme  urease, 
referred  to  previously,  is  an  example  of  a  particular  kind  of  amidase  that  splits 
the  — NH2  group  from  urea. 

The  intracellular  phosphatases  and  esterases  are  also  very  interesting. 
Phosphatases  undoubtedly  play  an  important  part  in  the  calcification  and 
decalcification  of  bone.  Blood  phosphatases  rise  to  abnormally  high  levels  in 
cases  of  rickets,  when  a  deficiency  of  vitamin  D  is,  in  some  little-understood 
way,  the  cause  of  imperfect  ossification.  The  acetylcholine  esterase  is  con- 
cerned in  the  destruction  of  the  chemical  transmitter  acetylcholine  that  is 
released  at  nerve  endings  in  skeletal  and  cardiac  muscle  and  at  the  synapse 
(page  495).  It  is  evident  that  accumulation  of  so  potent  a  drug  as  acetyl- 
choline would  be  harmful;  moreover,  its  rapid  removal  is  obviously  essential 
to  the  recovery  phase  of  transmission. 

The  vast  subject  of  the  respiratory  enzymes  is  in  reality  a  field  to  itself. 
Nearly  all  of  the  enzymes  discussed  previously  have  catalyzed  reactions  that 
are  hydrolyses,  that  is,  are  effected  by  the  addition  (or  removal)  of  water. 
The  respiratory  enzymes  are  oxidation-reduction  systems  which  effect  the 
transfer  of  oxygen  (or,  anaerobically,  its  equivalent  in  removable  electrons 
from  some  other  source)  to  the  tissues  where,  in  the  last  analysis  and  under 
the  influence  of  yet  other  oxidizing  enzymes,  it  is  finally  used  to  complete  the 
breakdown  of  carbohydrates  and  fats,  with  equivalent  release  of  energy.  The 
enzymes  involved  in  these  respiratory  processes  must  be  subjected  to  arbitrary 
classification.  In  the  first  place,  there  are  the  oxidases  whose  function  is  to 
activate  molecular  oxygen.  They  function  only  in  the  presence  of  a  substance 
that  can  receive  the  activated  oxygen.  Oxidases  appear  to  owe  their  ability 
to  activate  oxygen  to  the  presence  of  an  iron-porphyrin,  or  prosthetic  hematin, 
group. 

Although  the  hemoglobin  of  the  blood  is  not  usually  treated  as  an  enzyme, 
it  shares  many  features  in  common  with  the  oxidases.  Hemoglobin  is  a 
protein  to  which  is  attached  a  prosthetic  iron-containing  hematin  group.  By 


540  HUMAN  BIOLOGY 

virtue  of  this  group,  the  hemoglobin  (like  the  oxidases,  perhaps),  is  able  to 
enter  into  labile  union  with  molecular  oxygen.  In  the  presence  of  oxygen 
acceptors  in  the  tissues,  hemoglobin  gives  up  its  oxygen  and  reappears  in  the 
reduced  form.  Like  the  oxidases,  hemoglobin  is  inactivated  by  cyanide  which 
reacts  with  the  prosthetic  group.  The  differences  between  hemoglobin  and 
an  oxidase  reside  chiefly  in  the  fact  that  the  intermediate  stage,  corresponding 
to  the  hypothetical  "  enzyme-substrate "  combination,  is  prolonged  and  is,  in 
fact,  carried  from  the  lungs  to  the  tissues  by  the  blood  stream.  In  the  case  of 
oxidases,  the  intermediary  compound,  if  formed,  is  ephemeral.  It  is  by  no 
means  certain  that  hemoglobins  cannot  in  some  cases  behave  as  oxidases; 
thus,  red  muscles  contain  a  hemoglobin  that  may  act  as  a  temporary  oxygen 
reserve  but  may  equally  well  play  an  essential  role  in  the  transfer  of  oxygen  to 
the  tissue. 

Interesting  examples  of  oxidizing  enzymes  are  those  which  are  responsible 
for  the  formation  of  brown  or  black  melanin  pigments  through  the  oxidation 
of  tyrosine.  Such  enzymes  are  referred  to  as  tyrosinases;  it  is  tyrosinase  that 
causes  the  blackening  of  potatoes  and  other  vegetables  when  they  are  cut  and 
left  exposed  to  the  air.  An  animal  tyrosinase  ("  dopa-oxidase  ")  is  responsible 
for  the  development  of  melanin  in  the  skin  (page  400).  Onslow  was  able  to 
show  that  albino  rabbits,  which  are  genetically  recessives,  lack  the  enzyme, 
while  dominant  albinos  possess  enzyme  plus  an  enzyme  inhibitor. 

Tissues  also  contain  enzymes  that  effect  oxidations  not  by  the  addition 
of  oxygen  but  by  the  removal  of  hydrogen  from  the  substrate.  Such  enzymes 
are  known  as  dehydrogenases.  They  can  act  only  in  the  presence  of  a  sub- 
stance capable  of  accepting  hydrogen.  Oxygen  can  act  as  a  hydrogen  aceep-, 
tor,  but,  in  the  absence  of  oxygen,  other  reversible  hydrogen  acceptors  occur 
in  tissues.  In  the  laboratory,  methylene  blue  has  frequently  been  used  as  a 
labile  hydrogen  acceptor;  it  is  readily  reduced  to  a  colorless  compound.  A 
number  of  important  dehydrogenases  have  been  studied  in  various  tissue 
preparations;  one  of  the  first  discovered  was  the  Schardinger  enzyme  of  milk. 
The  so-called  "yellow  enzyme"  extracted  from  many  types  of  tissue  appears 
either  to  be  an  oxidase  or  to  behave  like  methylene  blue  as  a  hydrogen  acceptor 
in  relation  to  other  tissue  oxidases.  It  does  not  contain  iron  but  is  a  protein 
which  has  a  labile  prosthetic  group,  the  yellow  vitamin  B  pigment,  riboflavin, 
in  combination  with  phosphoric  acid  (page  59). 

Reduced    +  Hydrogen       Oxidized     +  Reduced 
substrate         acceptor  ~*  substrate         acceptor 

Or 

XH2  +  Y  ->  X  +  YH2 

The  foregoing  scheme  represents  the  reaction  catalyzed  by  a  dehydrogenase 
in  the  presence  of  a  hydrogen  acceptor.  If  the  hydrogen  acceptor  is  molecular 
oxygen ,  it  will  be  seen  that  the  reaction  takes  place  as  follows : 

XH2  +  02  ->  X  +  H202 


APPENDIX  541 

There  is  no  doubt  that  such  reactions  must  occur  under  aerobic  conditions 
in  living  tissues,  and,  since  hydrogen  peroxide  is  a  toxic  substance,  the  impor- 
tance of  enzymes  capable  of  effecting  its  removal  is  evident.  The  enzyme 
catalase  is  present  in  almost  all  tissues  and  liberates  molecular  oxygen  and 
harmless  water  by  decomposition  of  the  hydrogen  peroxide. 

Considerations  outlined  above  lead  to  the  conclusion  that  oxidizing  and 
reducing  enzymes  provide  complementary  mechanisms  that,  together,  regulate 
the  respiratory  processes  of  the  cell.  Respiratory  pigments,  known  as 
cytochromes,  are  found  in  the  cells  of  all  except  anaerobic  organisms.  Keilin 
has  developed  the  theory  that  the  cytochromes  form  an  essential  link  between 
the  oxidases  and  the  dehydrogenases  since,  like  hemoglobin,  they  can  take  up 
and  release  oxygen  according  to  the  needs  of  their  immediate  environment. 
Compounds  bearing  the  — SH  group  also  appear  to  have  an  important  inter- 
mediary part  in  the  respiratory  mechanism  of  the  cell.  The  best  known  of 
these  substances  is  glutathione,  a  tripeptide  which  has  been  synthesized. 

The  ultimate  result  of  tissue  oxidation  is  always  the  release  of  energy. 
As  a  rule,  this  energy  is  either  utilized  in  cell  metabolism  or  released  in  part  as 
heat.  The  enzyme  luciferase,  which  is  found  in  many  luminescent  organisms, 
effects  the  removal  of  hydrogen  from  a  substrate  of  unknown  chemical  com- 
position that  is  called  luciferin;  the  oxidation  takes  place  according  to  the 
following  scheme  with  oxygen  behaving  as  a  hydrogen  acceptor;  it  is  accom- 
panied by  the  production  of  light : 

Luciferin  +  0  ^  oxyluciferin  +  H20 

The  most  familiar  example  of  the  production  of  light  by  an  organism  is 
of  course  the  case  of  the  firefly.  Many  other  luminous  plants  and  animals  are 
known. 

Another  enzyme  connected  with  respiration  is  found  in  the  red  blood  cells 
from  which  it  may  be  extracted  in  a  partially  purified  form.  This  enzyme, 
carbonic  anhydrase,  facilitates  the  release  of  carbon  dioxide  from  carbonic 
acid  according  to  the  following  equation: 

H2C03  ^±  C02  +  H20 

Carbon  dioxide  is  thus  readily  released;  as  fast  as  it  can  be  removed  in 
the  air  exhaled  from  the  lungs.  (Pickford,  Osborn  Zoological  Laboratory, 
Yale  University,  New  Haven,  Conn.,  January,  1940.) 

Evolution.     See  Organic  Evolution. 

Galen.     See  Biology  and  Medicine. 

Germ  Plasm.  "Although  clearly  suggested  by  a  number  of  workers, 
the  conception  of  the  continuity  of  the  germ  cells — or  germ  plasm — was  first 
forced  upon  the  attention  of  biologists  and  given  greater  precision  by  Weis- 
mann  (1834-1914)  in  a  series  of  essays  culminating  in  1892  in  his  volume 
entitled  The  Germ  Plasm.  He  identified  the  chromatin  material  which 
constitutes  the  chromosomes  of  the  cell  nucleus  as  the  specific  bearer  of 
hereditary  characters,  and  emphasized  a  sharp  distinction  between  the 
cellular  derivatives  of  the  fertilized  egg — on  the  one  hand,  the  somatic  cells 


542  HUMAN  BIOLOGY 

which  by  division  and  differentiations  build  up  the  body  of  a  higher  plant  or 
animal;  and  on  the  other,  the  germ  cells  which  are  destined  to  play  but  little 
part  in  the  life  of  the  individual  which  bears  them,  but  instead  are  to  be 
liberated  and  give  rise  to  the  next  generation.  The  importance  of  this  dis- 
tinction can  hardly  be  overemphasized,  for  at  once  it  makes  clear  that,  for  all 
practical  purposes,  the  bodily  characteristics  of  an  individual  are  negligible 
from  the  standpoint  of  heredity,  since  the  offspring  are  descendants  not  from 
the  body  cells,  but  from  the  germ  cells  and  these  in  turn  from  the  germ  cells  of 
the  preceding  generation.  As  Weismann  insisted,  this  view  makes  it  difficult 
to  conceive  how  modifications  of  the  soma  can  so  specifically  affect  the  germ 
cells  which  it  bears  that  the  latter  can  reproduce  the  modifications — in  other 
words  that  so-called  'acquired  characters'  cannot  be  inherited.  And  there  is 
no  satisfactory  evidence  that  such  characters  are  inherited.  The  practical 
bearings  of  this  conclusion  are  obviously  of  the  highest  importance,  lying 
as  they  do  at  the  very  root  of  many  questions  in  regard  to  the  factors  of 
evolution,  not  to  mention  such  practical  ones  as  education  and  eugenics. 

"While  this  viewpoint  has  been  gradually  gaining  content  and  prevision, 
the  science  of  heredity  has  been  advancing  not  only  by  exact  studies  of  the 
structure  and  physiology  of  the  germ  cells,  but  also  by  statistical  investiga- 
tions of  the  results  of  heredity — the  various  characters  of  animals  and  plants 
in  parent  and  offspring."  (Woodruff,  "The  Development  of  the  Sciences/' 
Chap.  VI,  pp.  248-249.  Yale  University  Press,  New  Haven,  1923.) 

Glucose  (Dextrose).  One  of  the  two  basic  nutritive  carbohydrates  formed 
in  photosynthesis;  the  other  being  fructose.  Both  are  monosaccharides  with 
the  chemical  formula  C6Hi206,  but  they  differ  in  their  molecular  structure, 
that  is,  the  arrangement  of  the  atoms  in  the  molecule.  Glucose  is  also  known 
as  dextrose  because,  in  solution,  it  turns  the  plane  of  polarized  light  to  the 
right,  while  fructose  under  the  same  conditions  turns  the  plane  to  the  left 
and  is  therefore  often  designated  as  levulose.  Glucose  is  widely  distributed  in 
the  tissues  of  many  plants.  The  digestion  of  the  higher  carbohydrates 
results  in  the  formation  of  glucose  (and  fructose)  which  is  the  only  carbohy- 
drate available  for  energy. 

Golgi  Bodies,  Golgi  Apparatus.  "By  these  various  names  are  designated 
a  group  of  cell-components,  as  yet  imperfectly  known,  which  show  some 
points  of  resemblance  to  the  chondriosomes  though  morphologically  quite 
distinct  from  them.  Like  the  chondriosomes  the  Golgi-elements  are  in  con- 
siderable degree  polymorphic,  though  always  consisting,  apparently,  of  the 
same  specific  material.  .  .  . 

"The  Golgi  apparatus  is  of  very  wide  distribution  among  the  cells  of 
higher  animals  and  is  known  in  the  Protozoa,  everywhere  showing  the  same 
general  characters;  and  there  is  reason  to  believe  that  the  same  may  be  true  of 
plant  cells  though  considerable  doubt  concerning  this  still  exists.  It  appears 
in  two  principal  forms,  the  localized  and  the  diffuse,  which  may  be  converted 
into  one  another  in  changing  phases  of  cell-activity  and  are  therefore  to  be 
regarded  as  merely  different;  phases  of  the  same  structural  element.  In  its 
localized  form,  as  first  described  by  Golgi  ('98)  in  nerve-cells  of  the  spinal 


APPENDIX  543 

ganglia  of  vertebrates,  it  commonly  gives  the  appearance  of  a  localized  net-like 
structure,  composed  of  more  or  less  contorted  and  varicose  fibrils,  which 
appear  intensely  black  after  silver  impregnation  or  prolonged  treatment  by 
osmic  acid.  .  .  . 

"Concerning  the  functional  significance  of  the  Golgi-elements  even  less  is 
known  than  in  case  of  the  chondriosomes."  (Wilson,  "The  Cell  in  Development 
and  Heredity,"  pp.  48,  49,  52,  The  Macmillan  Company,  New  York,  1925). 

Harvey.  The  epoch-making  work  of  Vesalius  on  anatomy  was  matched 
on  the  functional  side  in  1628  "with  the  publication  of  Harvey's  tract,  Exer- 
citatio  Anatomica  de  Motu  Cordis  et  Sanguinis  in  Animalibus.  No  rational 
conception  of  the  economy  of  the  animal  organism  was  possible  under  the 
influence  of  the  Galenic  system,  and  it  remained  for  Harvey  (1578-1657)  to 
demonstrate  by  a  series  of  experiments,  logically  planned  and  ingeniously 
executed,  that  the  blood  flows  in  a  circle  from  heart  back  to  heart  again,  and 
thus  to  supply  the  groundwork  for  a  proper  understanding  of  the  dynamics  of 
the  organism  as  a  whole.  A  new  picture  of  the  function  of  the  blood  was 
presented  which  quickly  led  to  the  discovery  of  the  lymphatic  system,  and 
gave  content  to  the  study  of  the  nutrition  of  the  body.' 

"Harvey's  use  of  distinctively  quantitative  factors  is  so  important  in  its 
establishment  of  the  experimental  method  in  biology  that  his  own  statement  is 
of  great  historical  interest: 

"'I  frequently  and  seriously  bethought  me,  and  long  revolved  in  my  mind, 
what  might  be  the  quantity  of  blood  which  was  transmitted,  in  how  short  a 
time  its  passage  might  be  effected,  and  the  like;  and  not  finding  it  possible 
that  this  could  be  supplied  by  the  juices  of  the  ingested  aliment  without  the 
veins  on  the  one  hand  becoming  drained,  and  the  arteries  on  the  other  hand 
getting  ruptured  through  the  excessive  charge  of  blood,  unless  the  blood  should 
somehow  find  its  way  from  the  arteries  into  the  veins,  and  so  return  to  the  right 
side  of  the  heart;  I  began  to  think  whether  there  might  not  be  a  motion,  as  it 
were,  in  a  circle.  Now  this  I  afterwards  found  to  be  true;  and  I  finally  saw 
that  the  blood,  forced  by  the  action  of  the  left  ventricle  into  the  arteries,  was 
distributed  to  the  body  at  large,  and  its  several  parts,  in  the  same  manner  as 
it  is  sent  through  the  lungs  impelled  by  the  right  ventricle  into  the  pulmonary 
artery,  and  that  it  then  passed  through  the  veins  and  along  the  vena  cava, 
and  so  round  to  the  left  ventricle  in  the  manner  already  indicated.  Which 
motion  we  may  be  allowed  to  call  circular/"  (Woodruff,  "The  Development 
of  the  Sciences,"  Chap.  VI.,  pp.  224-225,  Yale  University  Press,  New  Haven, 
Conn.,  1923.) 

Hippocrates.     See  Biology  and  Medicine. 

Histology.  "  Histology  is  the  science  that  treats  of  the  minute  structure  of 
the  tissues  and  organs  of  the  plant  and  animal  body.  The  study  of  the  living 
cells  lacks  the  factor  of  permanency  of  record,  except  in  those  instances  where 
this  has  been  accomplished  by  photographic  methods.  This  difficulty  and 
that  of  distinguishing  the  different  parts  of  the  cell  in  the  living  condition 
have  been  overcome  to  some  extent  by  the  study  of  cells  and  tissues  which 
have  been  killed,  that  is,  "fixed,"  and  then  stained  in  various  ways.  A  study 


544  HUMAN  BIOLOGY 

of  both  living  and  fixed  cells  is  necessary  for  knowing  the  structure  and  func- 
tion of  particular  cells  and  tissues. 

"The  great  mass  of  work  done  today  in  both  normal  and  pathologic  his- 
tology depends  on  the  fixation  of  the  tissues  and  their  subsequent  staining  in 
an  elective  manner.  All  of  the  fixatives  in  use  precipitate  the  proteins;  many 
of  them  leave  the  lipins  unaffected;  but  most  of  them  remove  the  carbohy- 
drates and  many  of  the  salts.  Accordingly,  to  study  all  of  these  constituents 
of  a  cell,  various  fixation  methods  must  be  used. 

"The  next  step  in  the  preparation  of  fixed  tissues  for  study  consists  in 
slicing  them  into  very  thin  layers.  This  is  usually  accomplished  by  freezing 
a  bit  of  tissue,  &fter  which  it  can  be  sectioned  in  a  special  instrument,  or  by 
infiltrating  it  with  a  solution  of  gelatin,  paraffin,  or  celloidin  which  is  later 
solidified  so  that  the  tissue  and  the  embedding  matrix  may  be  sectioned 
together.  The  use  of  both  paraffin  and  celloidin  requires  that  the  tissue  shall 
be  dehydrated  in  alcohol,  which  'removes  most  of  the  lipins.  The  use  of 
paraffin  permits  the  tissues  to  be  sectioned  relatively  rapidly  and  in  very  thin 
slices.  Celloidin,  on  the  other  hand,  disturbs  the  arrangement  of  the  cells  less 
and  causes  less  shrinkage  than  does  the  paraffin  method. 

"These  thin  slices  may  be  stained  to  demonstrate  the* various  parts  of  the 
cell  and  the  intercellular  substance.  The  most  usual  staining  method — 
hematoxylin  and  eosin — stains  the  nucleus  blue  and  the  cytoplasm  pink. 
Special  staining  methods  are  necessary  to  demonstrate  certain  cellular  con- 
stituents that  are  present  in  the  dead  cell  body  but  are  not  made  visible  by 
hematoxylin  and  eosin.  A  host  of  such  staining  methods  has  been  devised; 
a  few  are  indispensable,  but  most  of  them  are  of  questionable  value."  (Maxi- 
mow  and  Bloom,  "A  Textbook  of  Histology/7  pp.  2-3,  W.  B.  Saunders  Com- 
pany, Philadelphia,  1934.) 

History  of  Biology.    See  Biology  and  Medicine. 

Hooke,  Robert  (1635-1703).  "Intellectually  Robert  Hooke  was  unques- 
tionably the  most  distinguished  of  the  classical  microscopists.  He  was, 
however,  primarily  a  physical -experimenter,  and  most  of  his  best  work  lies 
outside  our  field.  Sickly  from  childhood,  his  health  prevented  him  from 
receiving  a  normal  education.  He  was,  however,  a  precocious  and  rapid 
worker.  At  Oxford  he  attracted  the  attention  of  Robert  Boyle.  When  the 
Royal  Society  was  founded,  he  entered  its  service  as  a  salaried  'curator  of 
instruments/  This  country  has  produced  no  more  brilliant,  ingenious,  and 
inventive  experimenter,  and  in  certain  important  matters  he  anticipated  New- 
ton. He  was  a  virulent  and  acrimonious  controversialist,  jealous  and  censorious 
beyond  all  tolerable  limits,  with  a  spirit  warped  by  congenital  infirmities  of 
body  and  temper. 

"Hooke's  Micrographia,  published  in  London  in  1665,  opens  with  a 
description  and  figure  of  his  microscope.  This  account  is  a  valuable  landmark 
in  the  history  of  the  subject.  The  book  is  made  up  of  a  number  of  observa- 
tions. Their  chief  biological  importance  is  in  the  accuracy  and  beauty  of  his 
figures,  which  formed  a  standard  for  generations.  Biology  is  the  loser  from 
the  application  of  his  great  intellect  to  other  departments. 


APPENDIX  545 

"Hooke  has  a  figure  of  the  microscopic  structure  of  cork,  showing  the  walls 
bounding  the  cells.  He  refers  to  these  as  cells.  That  word  in  our  modern 
biological  nomenclature  comes  from  him."  (Singer,  "The  Story  of  Living 
Things,  p.  168,  Harper  &  Brothers,  New  York,  1931.) 

Hopkins,  Frederick  Gowland.  Feeding  experiments  illustrating  the  impor- 
tance of  accessory  factors  in  normal  dietaries.  The  experiments  described  in 
this  paper  confirm  the  work  of  others  in  showing  that  animals  cannot  grow 
when  fed  upon  so-called  " synthetic"  dietaries  consisting  of  mixtures  of  pure 
proteins,  fats,  carbohydrates,  and  salts.  But  they  show  further  that  a  >sub- 
stance  or  substances  present  in  normal  foodstuffs  (for  example,  milk)  can, 
when  added  to  the  dietary  in  astonishingly  small  amount,  secure  the  utilization 
for  growth  of  the  protein  and  energy  contained  in  such  artificial  mixtures.  .  .  . 

"  Convinced  of  the  importance  of  accurate  diet  factors  by  my  own  earlier 
observations,  I  ventured,  in  an  address  delivered  in  November,  1906,  to 
make  the  following  remarks: 

'"But,  further,  no  animal  can  live  upon  a  mixture  of  pure  protein,  fat, 
and  carbohydrate,  and  even  when  the  necessary  inorganic  material  is  carefully 
supplied  the  animal  still  cannot  flourish.  The  animal  body  is  adjusted  to 
live  either  upon  plant  tissues  or  the  tissues  of  other  animals,  and  these  con- 
tain countless  substances  other  than  the  proteins,  carbohydrates,  and  fats. 
Physiological  evolution,  I  believe,  has  made  some  of  these  well-nigh  as  essen- 
tial as  are  the  basal  constituents  of  diet;  lecithin,  for  instance,  has  been 
repeatedly  shown  to  have  a  marked  influence  upon  nutrition,  and  this  just 
happens  to  be  something  already  familiar,  and  a  substance  that  happens  to 
have  been  tried.  The  field  is  almost  unexplored;  only  is  it  certain  that  there 
are  many  minor  factors  in  all  diets,  of  which  the  body  takes  account.  In 
diseases  such  as  rickets,  and  particularly  in  scurvy,  we  have  had  for  long  years 
knowledge  of  a  dietetic  factor;  but  though  we  know  how  to  benefit  these  con- 
ditions empirically,  the  scale  errors  in  the  diet  are  to  this  day  quite  obscure. 
They  are,  however,  certainly  of  the  kind  which  comprises  these  minimal 
qualitative  factors  that  I  am  considering.  Scurvy  and  rickets  are  conditions 
so  severe  that  they  force  themselves  upon  our  attention;  but  many  other 
nutritive  errors  affect  the  health  of  individuals  to  a  degree  most  important  to 
themselves,  and  some  of  them  depend  upon  unsuspected  dietetic  factors/"  .  .  . 

"Evidence  has  now  accumulated  from  various  sides  to  justify  these  views. 
That  a  deficiency  in  quite  other  factors  can  induce  disease  is  a  fact  which  is 
now  upon  a  firm  experimental  basis.  That  a  deficiency,  quite  as  little  related 
to  energy  supply,  may  result  in  the  failure  of  so  fundamental  a  phenomenon  as 
growth  in  young  animals  seems  equally  certain."  (Fulton, "  Selected  Readings 
in  the  History  of  Physiology,"  pp.  299-301,  Courtesy  of  Charles  C.  Thomas, 
Springfield,  111.,  1930.) 

Hormones — Historical.  "  Organ  magic  has  figured  in  the  folk  supersti- 
tions of  many  peoples.  A  primitive  form  of  the  belief  is  that  man  can  increase 
the  store  of  his  own  virtues  by  consuming  various  organs  of  his  fellow  man 
or  of  animals  taken  in  the  chase.  The  warrior  eats  the  heart  of  his  enemy  to 
add  to  his  own  courage.  As  early  as  the  beginning  of  the  Christian  era  the 


546  HUMAN  BIOLOGY 

practice  in  a  less  nai've  form  had  come  under  the  sanction  of  orthodox  medicine. 
Diseases  of  one  sort  or  another  were  believed  to  be  due  to  the  lack  of  mysterious 
substances  supplied  to  the  body  as  a  whole  by  different  individual  organs; 
it  followed  that  the  resulting  diseases  were  to  be  cured  by  supplying  artificially 
these  lacking  substances.  As  a  system  of  treatment  this  came  ultimately  to 
be  known  as  opotherapy.  It  was  employed  systematically  by  Celsus  and 
Dioscorides.  Wolf's  liver  was  prescribed  for  diseases  of  that  organ,  hare's 
brain  for  nervousness,  and  fox's  lung  for  respiratory  disorders.  Throughout 
the  ages,  sex  gland  material  has  been  given  as  an  antidote  for  loss  of  virility. 
As  Paracelsus  phrased  the  doctrine,  'heart  cures  heart,  spleen  spleen,  lungs 
lungs/ 

"  During  the  Middle  Ages  in  Europe  a  large  number  of  revolting  organic 
substances  came  into  standard  use  in  the  treatment  of  diseases.  The  ingre- 
dients of  the  witches'  brew  listed  in  Macbeth  may  serve  as  a  fair  sample  of 
these,  though  even  more  disgusting  materials  were  actually  used.  Altgether, 
they  made  up  the  'filth  pharmacopeias.' 

"It  is  historically  interesting  that  modern  medical  interest  in  the  internal 
secretions  grew  out  of  the  ancient  practice  of  opotherapy.  Brown-S6quard 
was  a  French  physiologist  who  was  at  one  time  a  professor  at  Harvard  Univer- 
sity. Later  he  established  himself  in  Paris  where  he  carried  out  a  series  of 
brilliant  researches.  Toward  the  end  of  his  life  he  was  overtaken  by  general 
debility  while  many  interesting  things  remained  yet  to  be  done.  He  was  led 
to  treat  himself  by  injections  of  extract  of  sex  glands.  The  experiments  were 
reported  before  the  Socie*te*  de  Biologic  of  Paris  on  May  31,  1889 — a  date  that 
is  sometimes  cited  as  'the  birthday  of  endocrinology.'  So  eminent  was  the 
scientist  and  so  spectacular  the  beneficial  results  he  claimed  to  have  experi- 
enced that  world- wide  interest  was  immediately  aroused.  The  very  meagre 
stream  of  contributions  that  up  to  this  time  had  been  devoted  to  the  science 
of  endocrinology  soon  was  swollen  to  a  flood. 

"Whether  Brown-S6quard's  results  were  more  than  a  triumph  of  stfgges- 
tive  therapy  is  doubtful.  But  growing  out  of  his  error,  if  error  it  was,  has  come 
a  development  in  the  field  of  medicine  more  significant  than  any  other  since 
the  discovery  of  the  bacterial  origin  of  disease.  The  evidence  is  now  conclu- 
sive that  what  we  are — physically,  mentally,  sexually  and  emotionally — 
depends  in  no  small  measure  upon  the  functions  of  our  endocrine  glands. 
They  cooperate  in  an  important  way  in  the  regulation  *of  our  activities  in 
health,  and  modify  the  course  when  they  do  not  primarily  determine  our 
diseases.  A  fundamental  new  principle  has  been  added  to  physiology. 

"In  a  fascinating  account  of  the  history  of  the  endocrine  doctrine,  Garrison 
points  out  that  the  first  clearly  to  state  the  function  of  the  internal  secretions 
was  the  fashionable  physician  at  the  Court  of  Louis  the  Fifteenth,  Theophile 
de  Bordeu.  Of  him,  Garrison  writes:  'It  was  his  ambition  to  confirm  and 
ujJhold  the  humoral  pathology  of  Hippocrates.  .  .  .  Bordeu's  slender  reputa- 
tion today  is  centered  in  a  single  idea — the  doctrine  that  not  only  each  gland, 
but  each  organ  of  the  body,  is  the  workshop  of  a  specific  substance  or  secretion 
which  passes  into  the  blood  and  that  upon  these  secretions  the  physiological 


APPENDIX  547 

integration  of  the  body  as  a  whole  depends/"     (Hoskins,  "The  Tides  of 
Life,"  pp.  15-17,  W.  W.  Norton  &  Company,  Inc.,  New  York,  1933.) 

Hydra.  "The  body  of  Hydra  somewhat  resembles  a  long  narrow  sac,  the 
base  constituting  the  FOOT,  and  the  opening  at  the  opposite  end  forming  the 
MOUTH.  Surrounding  the  mouth  is  a  circle  of  out-pocketings  of  the  body  wall 
termed  TENTACLES.  The  main  axis  of  the  body  extends  from  foot  to  mouth, 
and  every  plane  passing  through  this  axis  divides  the  body  into  symmetrical 
halves.  In  other  words,  the  parts  of  the  body  are  symmetrically  disposed 
about,  or  radiate  from  the  main  axis,  and  so  Hydra  affords  an  example  of 

RADIAL  SYMMETRY. 

"The  body  wall  of  Hydra  is  composed  of  two  distinct  cell  layers,  ectoderm 
and  endoderm,  separated  by  a  thin  non-cellular  supporting  layer  of  jelly-like 
material  (MESOGLOEA)  secreted  by  the  cells  of  both  ectoderm  and  endoderm. 
Hydra  thus  illustrates  a  simple  type  of  metazoan  structure  in  which  but  two 
primary  tissues  exist;  such  specializations  as  are  necessary  for  the  performance 
of  the  essential  life  functions  being  confined  to  the  various  cells  that  compose 
these  layers.  The  majority  of  the  cells  of  the  endoderm  which  line  the 
ENTERIC  CAVITY  are  concerned  with  the  digestion  of  solid  food  taken  in  through 
the  mouth,  while  those  of  the  ectoderm  are  variously  modified  for  protection, 
and  the  other  relations  of  the  individual  to  its  surroundings,  as  well  as  for 
reproduction. 

"In  short,  in  the  organization  of  Hydra  the  primary  tissues  (ectoderm  and 
endoderm)  have  not  become  differentiated  into  secondary  specialized  tissues 
(muscular  tissue,  nerve  tissue,  etc.)  for  one  function  or  another — the  simple 
life  processes  of  the  animal  are  adequately  provided  for  by  the  specialization 
of  isolated  cells  or  small  groups  within  ectoderm  and  endoderm."  (Woodruff, 
"Foundations  of  Biology,"  pp.  104-105,  The  Macmillan  Company,  New  York, 
1936.) 

Hydrogen  Ion.  * '  The  reaction  of  a  solution,  that  is,  the  degree  of  its  acidity 
or  alkalinity,  depends  only  upon  the  relative  concentrations  of  the  electro- 
positive hydrogen  ion  (H+)  and  the  electronegative  hydroxyl  ion  (OH")  in 
the  solution.  An  excess  of  hydrogen  ions  causes  an  acid  reaction;  an  excess  of 
hydroxyl  ions  causes  an  alkaline  reaction.  If  the  concentration  of  the  hydro- 
gen ions  equals  the  concentration  of  the  hydroxyl  ions,  the  solution  is  said  to 
be  neutral.  Water  molecules  on,  dissociation  furnish  an  equal  number  of 
hydrogen  and  hydroxyl  ions,  and  water  is  neutral  in  reaction.  The  number  of 
water  molecules  that  dissociate  electrolytically  (ionize)  is  very  small.  At 
22°C.,  only  0.0000001  per  cent  of  the  water  molecules^  are  ionized,  and  this  is 
the  reason  why  pure  water  does  not  conduct  a  measurable  electric  current. 
The  extent  of  ionization  on  part  of  an  acid  or  alkali  added  to  water  determines 
the  degree  of  acidity  or  alkalinity  of  the  resulting  solution.  Any  excess  of 
acid  or  base  added  to  water  represses  the  ionization  on  part  of  the  water,  and 
the  product  of  the  concentrations  of  the  two  ions  remains  a  constant.  The 
acidity  or  alkalinity,  that  is,  the  reaction  of  a  solution,  therefore  can  be 
expressed  in  terms  of  either  of  these  two  ions.  Knowing  the  concentration 
of  hydrogen  ions,  the  concentration  of  hydroxyl  ions  is  likewise  given.  The 


548  HUMAN  BIOLOGY 

concentration  of  hydrogen  ions  can  be  determined  with  far  greater  accuracy 
and  ease  than  the  concentration  of  hydroxyl  ions,  and  the  reaction  of  a 
solution  therefore  is  usually  stated  in  terms  of  hydrogen  ion  concentration. 

"In  biological  fluids,  the  concentrations  of  hydrogen  ions  and  hydroxyl 
ions  are  of  the  order  of  water;  they  are  extremely  small.  On  that  account, 
the  reaction  of  a  biological  fluid  is  best  stated  by  the  method  designated  as  the 
pH  of  the  solution.  The  pH  expresses  the  reciprocal  value  of  the  hydrogen 
ion  concentration  in  grams  per  liter  in  logarithmic  notation.  That  is  to  say 
the  pH  equals  1/cH  expressed  as  its  logarithm.  It  is  evident  that  in  this 
manner  one  obtains,  instead  of  a  minute  decimal  fraction,  a  large  whole 
number.  It  must  be  remembered  that  the  lower  the  pH  the  higher  the  con- 
centration of  hydrogen  ion  in  the  solution,  and  vice  versa. 

"The  blood  and  tissue  fluids  are  almost  neutral  in  reaction,  and  the  varia- 
tions in  acidity  (or  alkalinity)  consistent  with  life  are  extremely  small.  Of  all 
the  body  fluids,  the  pH  of  the  arterial  blood  plasma  normally  varies  from 
7.42,  while  the  body  is  at  rest,  to  7.35,  while  the  body  is  at  work.  This 
corresponds  to  a  variation  of  0.00000007  g.  of  hydrogen  ion  concentration 
in  1  liter  of  blood  plasma.  The  variations  of  the  pH  in  the  venous  blood 
plasma  lie  between  7.39  and  7.28.  The  pH  of  the  blood  plasma  of  a  comatose 
diabetic  patient,  dying  of  acidosis,  may  be  lowered  to  pH  7.00.  If  the  alka- 
linity of  the  blood  is  increased  and  the  pH  of  the  blood  plasma  is  raised  to 
7.60,  tetanic  spasms  occur.  The  range  of  the  pH  of  the  blood  plasma,  com- 
patible with  life,  may  be  said  to  lie  between  pH  7.00  and  pil  7.70.  In  terms 
of  hydrogen  concentration,  this  amounts  to  a  variation  of  about  five  ten- 
millionths  of  a  gram  of  hydrogen  ions  in  the  total  volume  of  the  blood  of  an 
adult  human  being  of  average  height. 

"The  average  pH  of  the  lymph  exceeds  that  of  the  blood  plasma  by  about 
0.05  units.  Lymph,  therefore,  is  slightly  more  alkaline.  The  variations  in 
pH  of  the  different  fluids  secreted  and  excreted  by  the  body  cells  are  greater. 
Pancreatic  juice  has  an  average  pH  of  8.3.  It  is  much  more  alkaline  than 
the  blood  or  tissue  fluid.  The  pH  of  the  saliva  is  about  6.8;  that  is  say,  saliva 
is  slightly  acid.  The  pH  of  the  urine  varies  during  the  day  from  pH  5.00  to 
pH  7.00  with  an  average  pH  for  the  24-hour  urine  of  about  pH  6.00.  The 
gastric  juice  is  extremely  acid;  its  pH  varies  between  0.90  to  1.60.  The  main- 
tenance of  the  physiological  neutrality  of  blood  and  tissue  fluid,  therefore,  is  a 
constant  battle  with  disturbing  factors  that  tend  to  shift  the  reaction  from  its 
physiological  level.  One  of  the  main  mechanisms  whereby  the  blood  and 
tissue  fluids  maintain  the  physiological  reaction  is  the  buffer  action  of  certain 
salts.  (Eulenburg- Wiener,  "Fearfully  and  Wonderfully  Made/'  pp.  216-218. 
The  Macmillan  Company,  New  York,  1938.)  See  Dissociation. 

Infusoria.    See  Protozoa;  Paramecium. 

Insecta.    See  Arthropoda. 

Interstitial  Cells.  1.  "The  testis,  besides  producing  spermia,  causes  the 
development  and  maintenance  of  the  so-called  'secondary  sexual  characters' 
and  of  the  sex  impulse.  In  the  developing  organ^ms  it  is  supposed  to  regulate 
the  growth  of  the  skeleton  and  of  other  parts.  After  excision  of  both  testes 


APPENDIX  549 

in  the  prepubertal  age,  the  normal  cessation  of  the  growth  period  of  the  long 
bones  of  the  extremities  is  delayed,  and  the  secondary  sexual  characters  do  not 
develop.  If  this  is  done  after  puberty,  the  libido  gradually  disappears,  the 
secondary  sexual  characters  and  the  auxiliary  sex  glands  undergo  partial 
involution,  and  disorders  of  metabolism  eventually  appear,  as  obesity,  etc. 
The  implantation  of  a  testis  into  such  an  individual  may  restore  normal  con- 
ditions to  a  certain  extent.  In  experimental  animals  the  injection  of  tes- 
ticular  hormone  prevents  many  of  these  changes  from  occurring. 

"  Experiments  on  animals  have  shown  that  implantation  of  a  testis  may 
cause  the  appearance  of  secondary  male  characters  even  in  a  spayed  female. 
This  is  due  to  a  hormone  secreted  by  the  testis.  Some  authors  ascribe  the 
production  of  this  hormone  to  the  interstitial  cells;  others,  to  the  seminiferous 
epithelium  (spermatogenic  and  Sertoli  cells).  A  third  possibility  is,  of  course, 
the  participation  of  both  elements. 

"  Most  of  the  data  favor  the  first  hypothesis.  It  is  known  that  individuals 
with  cryptorchid  testes  display,  in  most  cases,  a  normal  sexual  behavior  and 
normal  secondary  characters;  they  usually  retain  their  virility,  although 
sterility  is  the  rule.  The  seminiferous  tubules  in  the  testes  of  such  males  are 
always  atrophic,  as  a  result  of  the  higher  temperature  in  the  abdomen.  In 
experimental  animals  with  cryptorchid  testes  of  long  duration,  the  seminiferous 
tubules  seem  to  disappear  completely,  leaving  large  masses  of  interstitial 
cells.  Such  individuals,  as  a  rule,  keep  their  libido,  the  potcntia  coeundi,  and 
the  secondary  sexual  characters.  Similar  results  were  obtained  after  ligation 
of  the  vas  deferens  or  the  ductuli  efferentes  and  after  large  doses  of  x-rays. 
Grafts  of  testicular  tissue  into  castrated  animals  are  supposed  to  act  through 
their  interstitial  cells,  which  proliferate,  while  the  seminiferous  tubules 
become  atrophic.  These  and  many  other  facts  indicate  that  the  male  sexual 
hormone  is  very  probably  secreted  by  the  interstitial  cells  rather  than  the 
seminiferous  epithelium/' 

2.  "Much  has  been  written  on  the  endocrine  nature  of  the  'interstitial 
cells'  of  the  ovary.  Recent  investigations  have  shown  that  they  do  not  play 
any  particular  role  as  endocrine  elements  in  influencing  the  secondary  sexual 
characters  or  in  regulating  the  sexual  cycle.  It  is  impossible  to  separate 
them  experimentally  from  the  other  constituents  of  the  ovary  and  to  test  their 
physiologic  importance.  It  is  possible  that  they  have  something  to  do  with 
the  nutrition  of  the  follicles  and  perhaps  also  of  the  corpora  lutea."  (Max- 
imow  and  Bloom,  "A  Textbook  of  Histology,"  pp.  507,  539,  W.  B.  Saunders 
Company,  Philadelphia,  1934.) 

Intracellular.  The  term  refers  to  the  materials  contained  within  the 
boundary  of  each  cell  or  to  vital  phenomena  occurring  within  the  cell  as,  for 
example,  intracellular  movements  (cydosis).  Some  authorities  hold  that  all 
tissue  materials,  whether  intercellular  or  intracellular,  are  living,  but  the 
more  common  conception  is  that  protoplasm  is  intracellular,  that  is,  occurs 
within  the  cell  boundary.  On  this  conception,  such  intercellular  materials 
as  the  collagenou&  intercellular  tissues  of  the  connective  tissues  and  blood 
plasma  are  nonliving  substances  secreted  by  the  intracellular  protoplasm, 


550  HUMAN  BIOLOGY 

In  a  resting  cell,  cytoplasm  and  nucleus  are  seen  as  the  primary  intracellular 
units  of  protoplasm,  and  each  contains  numerous  formed  bodies.  Included 
in  the  cytoplasm  are  chondriosomes,  Golgi  bodies,  plastids,  centrosome,  metaplasm 
(ergastic  substance),  and  cell  vacuoles.  In  the  nucleus  is  the  nucleolus, 
chromatin,  and  the  nuclear  protoplasm. 

Ions.    See  Dissociation;  Hydrogen  Ion. 

Keratin*  "Keratin  is  a  nitrogenous  organic  substance  which  may  be 
formed  by  epithelial  cells.  It  is  the  basis  of  horny  structures.  Its  most 
characteristic  development  is  seen  in  the  epidermis  of  vertebrates.  Pro- 
duced within  the  cell,  the  keratin  is  deposited  in  the  peripheral  region  of 
the  cell  and  at  the  expense  of  the  cytoplasm.  As  the  process  reaches  its 
limit,  the  nucleus  and  remnant  of  cytoplasm  die  and  dry  up.  What  was 
a  living  cell  is  then  merely  a  minute  horny  scale — in  contrast  to  the  fact 
that  cells  which  produce  a  cuticula  remain  alive.  As  the  keratin  is  deposited, 
adjacent  cells  somehow  become  strongly  adherent  so  that  the  entire  kera- 
tinized  or  ' horny'  layer  (stratum  corneum)  acquires  a  high  degree  of  mechan- 
ical resistance.  The  process  may  involve  only  the  outermost  tier  of  cells 
of  the  epidermis,  as  in  some  amphibians,  or,  as  in  reptiles,  several  or  many 
of  the  upper  layers  of  cells  become  horny.  On  the  human  body  the  stratum 
corneum  varies  from  a  thin  and  flexible  layer,  as  on  the  back  of  the  hand, 
to  a  thick  hard  and  tough  layer,  as  in  the  callosities  of  the  palm  and  sole. 

"The  stratum  corneum  is  one  of  the  most  important  epithelial  products 
of  a  vertebrate.  Fishes  have  merely  a  cuticular  outer  layer  on  the  epidermis. 
Apparently  ampbjftians  introduced  the  stratum  corneum.  The  character- 
istic superficial  sc^Ses  of  reptiles  and  feathers,  hair,  claws,  hoofs,  nails,  and 
the  hollow  horns  of  ruminant  ungulates  are  all  differentiations  of  the  stratum 
corneum — they  are  epithelial  products. 

"In  amphibians  and  reptiles  the  horny  layer  is  shed  periodically  and 
either  entire  or  in  large  fragments.  In  birds  and  mammals  minute  particles 
of  the  layer  are  constantly  sloughing  off.  The  material  thus  lost  is  replaced 
by  growth  in  the  deeper  part  of  the  epidermis.  In  animals  which  shed  peri- 
odically, a  new  horny  layer  is  well  established  beneath  the  old  before  the 
old  is  shed.  Thew  animal  therefore  passes  through  no  such  critical  period  as 
the  ' soft-Shelled*  stage  of  a  crab.  li  is  this  ease  of  repair  and  replacement 
of  the  outermost  layer  of  the  body  which  makes  the  stratum  corneum  incom- 
parably superior  to  a  cuticular  layer  for  the  uses  of  large  heavy  land  animals.7' 
(Neal  and  Rand,  "Comparative  Anatomy,"  p.  135,  Copyright  P.  Blakiston's 
Son  &  Company,  Philadelphia,  1936.) 

Lactation.    See  Mammary  Glands. 

Lacteals.    See  Lymph. 

Lactose  (Milk  Sugar),  "Lactose  occurs  in  milk  and  is  made  commer- 
cially from  the  whey  of  milk  used  in  the  manufacture  of  cheese  or  casein. 
In  the  body  lactose  is  digested  into  equal  parts  of  glucose  and  galactose,  the 
nutritive  functions  of  which  have  been  noted  above.  Lactose  has  special 
interest  for  the  student  of  nutrition  for  at  least  two  reasons,  It  is  not  found 


APPENDIX  551 

in  the  blood  or  body  tissues  generally,  but  is  evidently  formed  only  in  the 
mammary  gland  for  secretion  in  the  milk,  which  suggests  its  especial  impor- 
tance in  the  nourishment  of  the  young.  It  also  appears  to  be  unique  among 
the  sugars  in  its  property  of  favoring  the  development  of  the  most  desirable 
species  of  bacteria  in  the  intestine."  (Sherman,  "Food  Products/'  pp.  9-10, 
The  Macmillan  Company,  New  York,  1926.) 

Lamarck.    See  Organic  Evolution. 

Linnaeus.     See  Taxonomy. 

Malaria,     gee  Plasmodium. 

Malpighi.  "The  versatility  as  well  as  the  genius  of  Malpighi  (1628-1694) 
is  illustrated  by  his,  studies  on  the  anatomy  of  plants,  the  function  of  leaves, 
the  development  of  the  plant  embryo,  the  embryology  of  the  chick,  the 
anatomy  of  the  silkworm,  the  structure  of  glands.  Master  of  morphology 
but  with  prime  interest  in  physiology,  his  lasting  contribution  lies  in  his 
dependence  on  the  microscope  for  the  elucidation  of  problems  where  structure 
and  function,  so  to  speak,  merge.  This  is  well  illustrated  by  his  ocular 
demonstration  of  the  capillary  circulation  in  the  lungs,  at  once  his  first  and 
greatest  discovery  and  the  first  of  prime  importance  ever  made  with  a  micro- 
scope— since  it  completed  Harvey's  work  on  the  circulation  of  the  blood. 
Malpighi  wrote:  'I  see  with  my  own  eyes  a  truly  great  thing.  ...  It  is 
clear  to  the  senses  that  the  blood  flowed  away  along  tortuous  vessels  and  was 
not  poured  into  spaces,  but  was  always  contained  within  tubules,  and  that  its 
dispersion  is  due  to  the  multiple  winding  of  the  vessels/"  (Woodruff,  The 
Development  of  the  Sciences,"  Chap.  VI,  p.  229,  Yale  University  Press, 
New  Haven,  1923.) 

Maltose.  "Maltose  occurs  in  malted  or  germinated  grains,  in  malt 
extracts,  etc.,  but  the  amount  of  maltose  eaten  as  such  is  not  likely  to  be 
large.  It  is  formed  in  quantity  by  the  digestion  of  starch  by  the  saliva  or 
the  pancreatic  juice.  Maltose,  however,  whether  eaten  or  formed  in  the 
course  of  digestion,  is  not  absorbed  as  such  to  any  important  extent,  but  is 
split  by  a  digestive  ferment  of  the  intestinal  juice,  each  molecule  of  maltose 
yielding  two  molecules  of  glucose."  (Sherman,  "Food  Products,"  p.  10, 
The  Macmillan  Company,  New  York,  1926.)  * 

Mammary  Glands.  "The  mammary  glands  for  several  years  after  birth 
remain  small,  and  alike  in  both  sexes.  Towards  puberty  under  the  stimulus 
of  the  newly-present  sex  hormones  they  begin  to  enlarge  in  the  female,  and 
when  fully  developed  form  in  that  sex  two  rounded  eminences,  the  breasts, 
placed  on  the  thorax.  A  little  below  the  center  of  each  projects  a  small 
eminence,  the  nipple,  and  the  skin  around  this  forms  a  colored  circle,  the 
areola.  In  virgins  the  areolae  are  pink;  they  darken  in  tint  and  enlarge  during 
the  first  pregnancy  and  never  quite  regain  their  original  hue.  The  mammary 
glands  are  constructed  on  the  compound  racemose  type.  Each  consists  of 
from  fifteen  to  twenty  distinct  lobes,  made  up  of  smaller  divisions;  from  each 
main  lobe  a  separate  galactophorous  duct,  made  T^y  the  union  of  smaller 
branches  from  the  lobules,  runs  towards  the  nipple,  all  converging  beneath 


552  HUMAN  BIOLOGY 

the  areola.  There  each  dilates  and  forms  a  small  elongated  reservoir  in  which 
the  milk  may  temporarily  collect.  Beyond  this  the  ducts  narrow  again, 
and  each  continues  to  a  separate  opening  on  the  nipple.  Imbedding  and 
enveloping  the  lobes  of  the  gland  is  a  quantity  of  firm  adipose  tissue  which 
gives  the  whole  breast  its  rounded  form. 

"  During  maidenhood  the  glandular  tissue  remains  imperfectly  developed 
and  dormant.  Early  in  pregnancy  it  begins  to  increase  in  bulk;  this  secondary 
development  being  due  to  stimulation  by  lutein,  secreted  by  the*  persistent 
corpus  luteum  of  pregnancy;  and  the  gland-lobes  can  be  felt  as  hard  masses 
through  the  superajacent  skin  and  fat.  Even  at  parturition,  however,  their 
f unction?  1  activity  is  not  fully  established.  The  mammary  glands  are 
modified  sebaceous  glands.  The  oil-globules  of  the  milk  are  formed  by  a 
sort  of  fatty  degeneration  of  the  gland-cells,  which  finally  fall  to  pieces;  the 
cream  is  thus  set  free  in  the  watery  and  albuminous  secretion  formed  simul- 
taneously, while  newly  developed  gland-cells  take  the  place  of  those  partially 
destroyed.  In  the  milk  first  secreted  after  accouchement  (the  colostrum) 
many  cells  float  in  the  liquid,  which  has  a  yellowish  color;  this  first  milk  acts 
as  a  purgative  on  the  infant,  and  probably  thus  serves  a  useful  purpose,  as  a 
certain  amount  of  substances  (biliary  and  other),  excreted  by  its  organs 
during  development,  are  found  in  the  intestines  at  birth."  (Martin,  "The 
Human  Body,"  pp.  631-632,  Henry  Holt  &  Company,  New  York,  1935.) 

Matter,  Structure  of.  "For  more  than  2,500  years,  men  interested  in 
metaphysics  have  speculated  concerning  the  ultimate  constitution  of  matter. 
Ancient  Hindu  and  Greek  philosophers  held  that  the  physical  universe  (or, 
as  sometimes  taught,  the  whole  universe,  both  physical  and  mental)  is  com- 
posed of  very  small  indivisible  particles,  or  atoms,  that  are  in  constant  motion. 
The  first  clear  statements  of  this  idea  came  in  the  writings  of  Leucippus  and 
Democritus,  who  taught  that  all  phenomena  are  to  be  explained  by  the 
incessant  movements  of  atoms,  which  differ  only  in  shape,  order,  and  position. 

"The  beginnings  of  a  scientific  understanding  of  the  structure  of  matter 
date  back  only  to  the  work  of  Lomonosov,  a  Russian  physical  chemist  (1743), 
and  to  that  of  John  Dalton,  an  Englishman  (1803-1807)  who  has  given  to 
us  the  basis  of  fche  modern  atomic  theory.  The  Russian  work  was  to  all 
practical  purposes  buried  and  unknown  and  was  resurrected  only  in  1904. 
Dalton's  work  has  played  a  most  important  part  in  correlating  and  interpret- 
ing the  known  facts  of  chemistry.  Dalton  states  that  all  material  substances 
are  composed  of  minute  particles  or  atoms  of  a  comparatively  small  number 
of  kinds.  All  the  atoms  of  the  same  kind  have  the  same  size,  weight,  and 
other  properties. 

"The  theory,  as  at  first  stated,  has  been  developed,  added  to,  and  modified 
by  the  work  of  many  investigators.  During  recent  years,  a  very  great  amount 
of  scientific  evidence  has  been  published  of  such  a  character  as  to  establish 
beyond  any  reasonable  doubt  the  facts  of  the  atomic  and  molecular  structure 
of  matter.  It  is  impossible  here  to  trace  the  steps  in  the  development  of  this 
bit  of  scientific  truth;  we  can  merely  make  a  very  simple  and  brief  statement 
of  the  facts  as  they  are  known  at  the  present  time. 


APPENDIX  553 

"Probably  the  most  important  addition  to  our  information  as  to  the  struc« 
ture  of  matter  is  the  idea  that  the  atom  is  not  a  simple  indivisible  thing. 
It  has  been  shown  to  be  a  very  complex  system,  whose  components,  subatoms 
or  electrons,  are  in  very  rapid  orbital  motion.  The  electrons  are  of  two  kinds, 
positive  and  negative.  These  are  alike  in  the  strength  of  the  electrical  charge 
that  they  bear  but  wholly  different  in  mass.  The  negative  electron  is  asso- 
ciated with  a  mass  that  is  1/1,845  that  of  the  hydrogen,  the  lightest  known, 
atom.  The  diameters  of  some  of  these  atoms  have  been  calculated  and  found 
to  be:  for  helium,  2  X  10~8  cm.;  for  the  hydrogen  atom,  slightly  less;  and  for 
oxygen  and  nitrogen  atoms,  slightly  more.  Sir  Ernest  Rutherford  suggested 
the  theory  that  the  atom  is  constructed  somewhat  upon  the  plan  of  a  solar 
system,  having  at  its  center  a  nucleus  bearing  positive  electrical  charges  and 
negative  electrons  whirling  in  orbits  about  it.  The  rate  of  movement  of  these 
negative  electrons  appears  to  approach  closely  that  of  light.  The  electronic 
constituents  are  as  small  in  comparison  with  the  dimensions  of  the  atomic  sys- 
tems as  are  the  sun  and  planets  in  comparison  with  the  dimensions  of  the  solar 
system.  Of  course,  in  such  a  system,  the  electronic  or  other  particles  can 
occupy  but  a  very  small  portion  of  the  space  enclosed  within  the  system. 

"As  concerns  the  structure  of  matter,  physicists  have  experienced  rapidly 
changing  thought  in  recent  years.  They  now  claim  the  existence  of  four, 
at  least,  instead  of  two  kinds  of  elementary  particles.  These  are 

"1.  Electrons.  Units  of  electricity  negatively  charged  and  considered  to 
form  the  '  outer  shell'  of  atoms  or  to  revolve  about  atomic  hearts,  or  nuclei, 
like  satellites  about  a  sun.  These  have  many  of  the  properties  of  light  and 
partake  of  the  nature  of  a  wave  motion. 

"2.  Protons.  Positive  particles  or  corpuscles,  the  nuclei,  or  hearts,  of 
hydrogen  atoms.  The  mass  of  the  proton  is  approximately  1,850  times  that 
of  the  electron. 

"3.  Neutrons.  Neutral  particles  of  matter,  consisting  of  a  close  combina- 
tion of  electron  and  proton,  whose  electrical  charges  neutralize  each  other. 

"4.  Positive  Electrons,  Positrons.  Positively  charged  particles  or  cor- 
puscles or  rays  discovered  in  cosmic  rays.  They  have  the  mass  of  electrons 
but  the  opposite  electrical  charge."  (Rogers,  "  Textbook  of  Comparative 
Physiology/'  pp.  6-7,  McGraw-Hill  Book  Company,  Inc.,  New  York,  1938.) 

Measurements.  "In  order  better  to  grasp  the  dimensions  that  charac- 
terize the  colloidal  state,  it  will  be  well  to  stop  a  moment  and  recall  the  ultra- 
microscopic  scale.  A  micron  is  one-millionth  of  a  meter,  or  one-thousandth  of 
a  millimeter,  and  has  the  symbol  ju.  This  unit  does  for  microscopic  objects; 
thus,  a  human  blood  corpuscle  or  an  average  globule  of  butterfat  in  milk  is 
8  /*  across,  and  a  bacterium  is  between  1  /x  and  5  /-i  long.  Ultramicroscopic 
particles  require  a  smaller  scale,  such  as  was  developed  for  measuring  the 
wave  length  of  light.  The  physicist  uses  the  symbol  m/*,  the  so-called  milli- 
micron, for  the  thousandth  part  of  a  millionth  part  of  a  meter.  He  also  uses 
the  symbol  /*/*  to  indicate  the  millionth  part  of  a  ^millionth  part  of  a  meter, 
the  so-called  micromicron. 


554  HUMAN  BIOLOGY 

"  It  is  impossible  to  grasp  the  true  size  of  such  minute  dimensions,  but  some 
idea  of  them  can  be  gained  if  we  approach  them  from  objects  of  appreciable 
size.  The  following  table  may  help  to  do  this: 

1  meter  (m.)  =  1,000  millimeters Sound  waves  are  16  to  17  mm.  in 

length 

1  millimeter  (mm.)  =  1,000  microns Cells  range  from  0.15  mm.  to  1  /* 

1  micron  GU)  —  1,000  millimicrons Colloidal  particles  range  between 

0.1  fj.  and  1  m/A 
1  millimicron  (m^i)  —  1,000  micromicrons  GUM)  -   Molecules    range    from    2.5    m/* 

(protein)  to  46  up  (water)  and 

less 

"To  this  scale,  each  member  of  which  is  a  thousand  times  the  one  below  it, 
may  be  added  the  Angstrom  unit,  A.U.,  used  chiefly  in  indicating  the  wave 
length  of  light.  It  is  0.1  m/z  and  therefore  100  w>  The  light  waves  of  the 
visible  spectrum  are  7,500  A.U.  long  at  the  red  end  and  3,900  A.U.  long  at 
the  violet  end."  (Seifriz,  "  Protoplasm,"  pp.  99-100,  McGraw-Hill  Book 
Company,  Inc.,  New  York,  1936.) 

Mendel.  "The  first  studies  of  this  type  which  attracted  the  attention 
of  biologists  were  made  by  Galton  (1822-1911),  who  in  the  eighties  and 
nineties  of  the  last  century  amassed  a  large  amount  of  data  in  regard,  for 
example,  to  the  stature  of  children  with  reference  to  that  of  their  parents,  and 
formulated  his  well-known  ( laws'  of  inheritance.  But  the  epoch-making 
work  which  eventually  created  the  science  of  genetics  was  that  of  an  Austrian 
monk,  Gregor  Mendel  (1822-1884),  who  combined  in  a  masterly  manner  the 
experimental  breeding  of  pedigree  strains  of  plants  and  the  statistical  treat- 
ment of  the  data  thus  secured  in  regard  to  the  inheritance  of  sharply  con- 
trasting characters,  such  as  the  flower  color  in  sweet  peas.  Mendel's  work 
was  published  in  1865  in  an  obscure  natural  history  periodical  and  he  himself 
abandoned  his  teaching  and  research  to  become  the  abbot  of  his  monastery. 
Thus  terminated  prematurely  the  scientific  work  of  one  of  the  epoch  makers 
of  biology,  and  the  now  famous  '  Mendelian  laws'  of  inheritance  were  unknown 
to  science  until  1900  when  other  biologists,  coming  to  similar  results,  unearthed 
his  forty-year-old  paper.  We  can  pause  only  to  say  that  the  fundamental 
principle  of  the  segregation  of  the  genes  of  the  '  alternative'  characters  within 
the  germ  cells,  which  Mendel's  work  indicated,  has  been  extended  to  other 
plants  and  to  animals,  and  from  being,  as  at  first  thought,  a  principle  of  rather 
limited  application,  now  seems  to  be  the  key  to  all  inheritance.  And  the 
present  results  are  extremely  convincing  because  cytological  studies  on  the 
architecture  of  the  chromosome-complex  of  the  germ  cells  keep  pace  and 
afford  a  picture  of  the  physical  basis — of  the  mechanism  by  which  the  segre- 
gation and  distribution  of  genes  by  the  Mendelian  formula  takes  place." 
(Woodruff,  "The  Development  of  the  Sciences,"  Chap.  VI.,  pp.  249-250, 
Yale  University  Press,  New  Haven,  Conn.,  1923.) 

Metazoa.  "The  group  Metazoa  includes  all  animals  above  Protozoa  and 
therefore  has  the  rank  of  a  subkingdom.  On  this  basis  Protozoa  is  both  a 


APPENDIX  555 

subkingdom  and  a  phylum.  The  general  features  that  distinguish  Metazoa 
from  Protozoa  are  as  follows: 

"1.  The  body  of  the  Metazoan  is  composed  of  many  cells  that  may  be 
divided  into  two  general  classes:  somatic  cells  and  germ  cells. 

"2.  The  somatic  cells  are  differentiated  into  tissues  and  organs,  in  which 
there  is  specialization  of  structure  and  function. 

"3.  The  germ  cells  are  the  reproductive  cells,  which  in  many  forms  are 
segregated  from  the  somatic  cells  early  in  ontogeny. 

"4.  Though  asexual  reproduction  by  fission  or  budding  occurs,  there  is 
always  sexual  reproduction  from  a  fertilized  egg  or,  less  commonly,  an  unfer- 
tilized egg. 

"5.  The  developing  egg  undergoes  cleavage;  the  cells  or  blastomeres  thus 
formed  adhere  to  one  another  to  produce  a  multicellular  complex. 

"6.  At  least  two  germ  layers  develop:  an  ectoderm,  forming  the  external 
covering,  and  an  endoderm,  lining  the  alimentary  canal  and  its  outgrowths. 
Between  these,  in  the  majority  of  metazoans,  a  third  germ  layer,  the  meso- 
derm,  is  formed  from  which  muscles,  vascular  and  other  tissues  and  organs 
develop. 

"The  distinction  between  Metazoa  and  Protozoa  is  not  sharp  since  colonial 
protozoans,  such  as  Volvox,  consist  of  groups  of  different  kinds  of  cells, 
organically  connected  with  one  another.  Colonial  Protozoa  to  a  certain 
extent  bridge  the  gap  between  solitary  Protozoa  and  Metazoa,  but  in  the 
latter  there  is  a  greater  degree  of  interdependence  among  the  cells  of  the 
individual  organism  than  there  is  between  the  individual  members  of  a  pro- 
tozoan colony."  (Wieman,  "General  Zoology,"  pp.  384-385,  McGraw-Hill 
Book  Company,  Inc.,  New  York,  1938.)  See  Protozoa. 

Microscope,  Development  of.  "The  microbiologist  is  able  to  see  these 
minute  forms  only  by  the  help  of  the  compound  microscope,  but  some  of  them 
are  too  small  even  for  its  magnifications,  and  reliance  must  be  placed  on  the 
ultramicroscope  and  filters  to  determine  their  presence,  size,  and  form. 

Since  all  of  these  organisms  have  to  be  studied  by  the  help  of  the 
microscope,  the  advance  of  microbiology  has  been  practically  a  history  of 
the  microscope.  Whether  the  Dutch  lens  grinder,  Jannsen,  discovered  the 
principle  of  the  compound  microscope  or  it  was  an  Italian  invention,  the 
improvements  that  were  made  about  the  beginning  of  the  seventeenth  century 
were  by  a  number  of  workers.  Simple  lenses  and  their  properties  had  been 
known  to  the  Romans,  but  the  compound  microscope  with  its  magnification 
of  50  to  3,000  diameters  opened  a  new  world  which  amateur  lens  makers 
explored,  as  amateurs  today  spend  their  time  with  radio  or  electrical  -con- 
trivances. A  training  in  lens  grinding  preceded  any  examination  of  this  new 
and  fascinating  region,  but  some  amateurs  became  so  skillful  in  grinding 
lenses  accurately  that  they  produced  microscopes  with  a  magnification  of  300 
or  400  diameters.  The  high-power  lens  on  the  ordinary  student  instrument  of 
today  gives  a  magnification  of  400  to  500  diameters,  so  that  these  homemade 
instruments  were  real  compound  microscopes.  They  were  ground,  however, 
to  no  fixed  formula  as  to  the  shape  and  glass.  A  modern  lens  is  not  an  instru- 


556  HUMAN  BIOLOGY 

ment  that  is  made  and  improved  by  progressive  trials  but  is  ground  to  an 
exact  curvature  from  glass  with  known  exact  refractive  index  and  dispersion. 
With  these  primitive  instruments,  however,  one  enthusiastic  amateur  was 
able  to  see  bacteria.  Leeuwenhoek,  a  Delft  Dutchman  (1632-1723),  drew 
them  with  an  accuracy  and  skill  that  would  be  a  credit  to  a  student  equipped 
with  a  much  better  outfit.  On  Sept.  14,  1683,  he  communicated  his  discovery 
in  a  letter  to  the  Royal  Society  of  London. 

"It  is  interesting  to  note  that,  although  Leeuwenhoek  discovered  bacteria 
in  1683,  Linnaeus,  the  great  Swedish  botanist,  when  writing  a  systematic 
account  of  the  plant  kingdom,  named  and  classified  a  few  of  the  larger  fleshy 
fungi  and  then  grouped  all  the  other  small,  unknown,  and  undescribed  plants 
into  the  great  order  of  '  Chaos/ 

"The  bacteria  continued  to  be  studied  by  such  lenses  as  were  available 
during  the  eighteenth  and  nineteenth  centuries,  and  the  botanists  who 
specialized  in  bacteria  laid  the  foundations  for  our  modern  classification. 
About  fifty  years  ago  (1879),  the  microscope  was  further  improved  by  the 
homogeneous  oil  immersion  objectives,  and  magnifications  up  to  1,000  diam- 
eters could  be  obtained.  The  very  smallest  living  organisms  could  be  studied 
with  greater  ease  when  they  were  placed  under  such  relatively  high  powers. 
This  principle  was  not  a  new  one,  for  water  had  been  used  as  a  substitute  for 
the  air  intervening  between  objective  and  cover  glass  by  Amici  in  1840  and 
Hartnack  in  1855,  glycerin  by  Gundlach  in  1867,  and  various  oils  by  Amici 
in  1869.  The  new  principle  was  to  use  an  oil  that  had  approximately  the  same 
refractive  index  as  glass,  cedar  oil  most  nearly  answering  this  requirement. 

"In  1886,  a  still  further  improvement  in  the  lenses  was  made  by  the  inven- 
tion of  apochromatic  objectives  with  compensating  oculars.  These  apo- 
chromatic  oil-immersion  objectives,  made  up  of  at  least  eight  different  lenses, 
were  so  perfect  that  a  magnification  of  about  3,000  diameters  was  possible. 
This  improvement  was  due  to  Ernst  Abbe,  a  physics  professor  in  the  University 
of  Jena,  so  of  whatever  laurels  bacteriology  and  cytology  have  gathered  in 
the  past  50  years  a  large  part  must  be  laid  at  the  feet  of  the  man  who  has  given 
microbiology  the  tools  with  which  to  work.  Today,  the  bacteria  and  even 
many  details  inside  them,  in  spite  of  their  almost  incredible  smallness,  can 
be  seen.  This  advance  was  made  possible  by  the  invention  of  the  new  types 
of  glass  which  had  different  refractive  indexes  and  dispersions  from  the  old 
crown  and  flint  glasses  which  were  all  that  lens  makers  had  formerly  at  their 
disposal.  Otto  Schott,  a  practical  glassmaker  and  chemist,  found  that, 
by  adding  the  proper  chemicals,  baryta,  borate,  phosphate,  and  zinc,  glasses 
could  be  produced  that  had  very  different  properties  from  the  flint  and  crown 
glasses.  By  combining  some  of  these  glasses,  Abbe  was  able  to  build  up  a 
lens  that  produced  an  image  that  was  not  only  color  free,  achromatic,  but  that 
had  extraordinary  definition  and  freedom  from  distortion. 

"These  high-power  objectives  are  smaller  than  the  head  of  an  ordinary  brass 
pin  so  that  they  could  not  have  been  used,  if,  at  the  same  time,  some  system 
of  concentrating  the  light  had  not  been  invented.  We  owe  to  Abbe  the 
illuminating  apparatus,  or  condenser,  which  furnishes  better  illumination 


APPENDIX  557 

and  a  better  microscopic  image."  This  was  invented  in  1872.  (Lutman, 
"  Microbiology ,"  pp,  8-10,  McGraw-Hill  Book  Company,  Inc.,  New  York, 
1929.) 

Milk.  "Milk  contains  at  least  two  proteins,  lactalbumin  and  casein; 
several  fats  in  the  butter;  a  carbohydrate,  milk  sugar  or  lactose;  much  water; 
and  salts,  especially  potassium  and  calcium  phosphates.  Butter  consists 
mainly  of  the  same  fats  as  those  in  beef  and  mutton  but  has  in  it  about  1 
per  cent  of  a  special  fat,  butyrin.  In  the  milk,  the  fat  is  disseminated  in  the 
form  of  minute  globules  which,  for  the  most  part,  float  up  to  the  top  when  the 
milk  is  let  stand  and  then  form  the  cream.  In  this,  each  fat  droplet  is  sur- 
rounded by  a  pellicle  of  albuminous  matter;  by  churning,  these  pellicles  are 
broken  up  and  the  fat  droplets  then  run  together  to  form  the  butter.  Milk 
is  also  rich  in  vitamins;  the  presence  of  vitamins  in  milk  and  eggs  is  obviously 
essential  for  the  proper  growth  and  development  of  mammals  and  birds, 
respectively,  during  the  early  periods  of  life. 

"  Casein  is  insoluble  in  water,  but  when  acted  on  by  rennin,  an  enzyme  of 
gastric  juice,  is  converted  into  paracasein,  which  forms  an  insoluble  compound 
with  the  lime  salts  of  the  milk,  and  so  is  precipitated  as  the  curd.  Casein 
itself  is  rendered  insoluble  by  acid;  when  milk  is  kept,  its  sugar  ferments, 
giving  rise  to  lactic  acid,  the  familiar  process  of  souring;  after  this  reaches  a 
certain  point  the  casein  is  precipitated  as  'clabber.'  There  is  sufficient 
difference  between  the  two  forms  of  curd  so  that  cheeses  made  from  them  are 
quite  unlike;  cottage  cheese,  made  from  sour  milk,  cannot  be  'ripened'  as 
can  cheese  made  from  milk  acted  upon  by  'rennet'  in  the  ordinary  com- 
mercial process  of  cheese  making. 

"  Human  milk  is  undoubtedly  the  best  food  for  an  infant  in  the  early  months 
of  life;  and  to  suckle  her  child  is  useful  to  the  mother  if  she  be  a  healthy 
woman.  Many  women  refuse  to  suckle  their  children  from  a  belief  that  so 
doing  will  injure  their  personal  appearance,  but  skilled  medical  opinion  is  to 
the  contrary  effect;  the  natural  course  of  events  is  the  best  for  this  purpose, 
unless  lactation  be  too  prolonged.  Of  course  in  many  cases  there  are  justifia- 
ble grounds  for  a  mother's  not  undertaking  this  part  of  her  duties;  a  physician 
is  the  proper  person  to  decide."  (Martin,  "The  Human  Body,"  pp.  472-473, 
632,  Henry  Holt  &  Company,  New  York,  1935.) 

Mucous  Membrane.  "The  skin  can  be  readily  enough  removed  from  all 
parts  of  the  exterior,  but  at  the  margins  of  the  apertures  of  the  body  it  seems 
to  stop,  and  to  be  replaced  by  a  layer  which  is  much  redder,  more  sensitive, 
bleeds  more  readily,  and  which  keeps  itself  continually  moist  by  giving  out  a 
more  or  less  tenacious  fluid,  called  mucus.  Hence,  at  these  apertures,  the 
skin  is  said  to  stop,  and  to  be  replaced  by  mucous  membrane,  which  lines  all 
those  interior  cavities,  such  as  the  alimentary  canal,  into  which  the  apertures 
open.  But,  in  truth,  the  skin  does  not  really  come  to  an  end  at  these  points, 
but  is  directly  continued  into  the  mucous  membrane,  which  last  is  simply 
an  integument  of  greater  delicacy,  but  consisting  fundamentally  of  the  same 
two  layers — a  deep,  fibrous  layer,  containing  blood-vessels,  and  a  superficial 
bloodless  one,  now  called  the  epithelium.  Thus  every  part  of  the  body  might 


558  HUMAN  BIOLOGY 

be  said  to  be  contained  between  the  walls  of  a  double  bag,  formed  by  the 
epidermis,  which  invests  the  outside  of  the  body,  and  the  epithelium,  its 
continuation,  which  lines  the  alimentary  canal."  (Huxley,  " Lessons  in 
Elementary  Physiology,"  p.  12,  The  Macmillan  Company,  New  York,  1918.) 

Nucleolus.  "  Nearly  all  metabolic  nuclei  contain  one  or  more  true  nucleoli^ 
or  plasmosomes.  In  a  young  nucleus  just  formed  by  division  there  are  often 
several  small  nucleoli  which  may  unite  to  form  two  or  one  as  the  nucleus 
becomes  fully  developed.  In  the  living  nucleus  the  nucleolus  appears  as  a 
dull,  viscous  droplet,  usually  round  but  frequently  irregular  in  shape.  Centri- 
fuging  and  the  position  it  naturally  assumes  in  certain  eggs  show  it  to  be 
heavier  than  the  rest  of  the  nuclear  matter.  It  may  be  homogeneous  through- 
out, or  it  may  contain  vacuole-like  masses  and  occasionally  small  granules. 
Chemically,  it  is  composed  mainly  of  proteins  and  lipides.  It  commonly 
SHOWS  an  affinity  for  acid  dyes,  but  in  some  procedures  it  takes  the  basic  ones. 
Of  greater  interest  is  the  fact  that  its  chromaticity  undergoes  marked  altera- 
tions during  the  nuclear  division  cycle.  .  .  .  Such  alterations  are  thought  to 
indicate  interactions  of  some  sort  with  the  reticulum,  with  which  the  nucleolus 
is  in  contact  at  one  or  more  points.  .  .  . 

"The  functions  of  the  nucleolus  are  obscure,  but  there  is  obviously  some 
relation  between  its  behavior  and  the  cycle  of  alterations  undergone  by  the 
other  nuclear  constituents.  ...  It  should,  however,  be  pointed  out  here 
that  the  nucleolar  matter  arising  in  the  young  nucleus  develops  principally 
in  close  association  with  definitely  localized  regions  of  particular  chromosomes, 
a  fact  which  has  only  recently  come  to  light.  The  nucleolus  or  nucleoli  tend 
to  remain  attached  to  such  chromosomes  and  consequently  to  the  metabolic 
reticulum  which  the  latter  form.  As  the  reticulum  again  develops  condensed 
chromosomes,  the  nucleolar  matter  diminishes  in  amount  and  commonly 
disappears  completely.  This  long  ago  suggested  that  it  is  with  the  chromo- 
somal changes  especially  that  the  nucleolus  is  concerned."  (Sharp,  " Intro- 
duction to  Cytology,"  pp.  57-58,  McGraw-Hill  Book  Company,  Inc.,  New 
York,  1934.) 

Organic  Evolution.  "Since  we  have  every  reason  to  believe  that  all  life 
now  arises  from  pre-existing  life  and  has  done  so  since  matter  first  assumed 
the  living  state,  it  apparently  follows  that  the  stream  of  life  is  continuous 
from  the  remote  geological  past  to  the  present  and  that  all  organisms  of 
today  have  an  ancient  pedigree.  This  leads  up  to  a  question  which  has 
interested  and  perplexed  thinking  men  of  all  times:  how  things  came  to  be  as 
they  are  today.  It  was  the  Greek  natural  philosophers  who  projected  the 
idea  of  history  into  science  and  attempted  to  substitute  a  naturalistic  explana- 
tion of  the  earth  and  its  inhabitants  for  the  established  theogenies,  and  thus 
started  the  uniformitarian  trend  of  thought  which  culminated  in  the  establish- 
ment of  organic  evolution  during  the  past  century. 

"Again  it  is  Aristotle  who  is  singled  out  among  the  Greeks  for  his  com- 
bination of  sound  philosophy  and  induction  which  reaches  no  higher  expres- 
sion than  in  his  statements  regarding  the  relationships  of  organisms.  He 
says,  in  substance:  Although  the  line  of  demarcation  is  broadly  defined,  yet 


APPENDIX  559 

nature  passes  by  ascending  steps  from  one  to  the  other.  The  first  step  is 
that  of  plants;  which,  compared  to  animals,  seem  inanimate.  The  second 
step  nature  takes  is  from  plants  to  plant-animals,  the  zoophytes.  The  third 
step  is  the  development  of  animals,  which  arise  from  an  increased  activity 
of  the  vital  principle,  resulting  in  sensibility;  and  with  sensibility,  desire; 
and  with  desire,  locomotion.  Man  is  the  head  of  animal  creation.  To  him 
belongs  the  God-like  nature.  He  is  pre-eminent  by  thought  and  volition. 
But  although  all  are  dwarf-like  and  incomplete  in  comparison  with  man,  he 
is  only  the  highest  point  of  one  continuous  ascent. 

"  Broadly  speaking,  Aristotle  apparently  held  substantially  the  modern 
idea  of  the  evolution  of  life  from  a  primordial  mass  of  living  matter  to  the 
highest  forms,  and  believed  that  evolution  is  still  going  on — the  highest  has 
not  yet  been  attained.  In  looking  for  the  effective  cause  of  evolution  Aristotle 
rejected  Empedocles'  hypothesis  of  the  chance  play  of  forces,  which  embodied 
in  crude  form  the  idea  of  the  survival  of  the  fittest,  and  substituted  secondary 
natural  laws  to  account  for  the  fact  that '  Nature  produces  those  things  which, 
being  continually  moved  by  a  certain  principle  contained  in  themselves, 
arrive  at  a  certain  end/  Aristotle's  rejection  of  the  hypothesis  of  the  survival 
of  the  fittest  to  account  for  adaptations  of  organisms  was  a  sound  induction 
from  his  necessarily  limited  knowledge  of  nature — but  had  he  accepted  it  he 
would  have  been  the  l  literal  prophet  of  Darwinism/ 

"  Although  the  thread  of  continuity  of  evolutionary  thought  is  not  broken 
from  Aristotle  to  the  present,  no  historical  interest  will  be  served  in  following 
.  .  .  the  Renaissance  naturalists  and  speculative  evolutionists,  who,  with  a 
minimum  of  fact  and  a  plethora  of  imagination  were  the  worst  enemies  of 
the  evolution  idea.  In  truth,  the  great  natural  philosophers  from  Bacon 
and  Leibnitz  to  Kant  and  Hegel  laid  the  broad  foundation  for  our  modern 
attack  on  evolution,  but  from  the  strictly  biological  viewpoint,  two  French- 
men, Buffon  and  Lamarck,  and  two  Englishmen,  Erasmus  Darwin  and  his 
grandson,  Charles  Darwin,  stand  pre-eminent — and  the  greatest  is  Charles 
Darwin. 

"Buffon  (1707-1788)  was  a  peculiarly  happy  combination  of  popular 
writer  and  scientist — entertaining  by  each  new  volume  of  his  great  Histoire 
Naturelle  the  social  set  of  Paris,  and  instructing  them  at  the  same  time.  And 
it  was  largely  between  the  lines  of  his  Natural  History  that  Buffon's  evolu- 
tionary ideas  found  expression:  but  expressed  they  were,  though  sometimes 
difficult  to  decipher — beyond  the  ken,  Buffon  hoped,  of  the  censor  and  dilet- 
tante, for  apparently  he  was  not  of  martyr  stuff.  It  is  not  strange,  therefore, 
that  there  are  some  differences  of  opinion  amongst  biologists  today  as  to  just 
how  much  weight  is  to  be  placed  on  some  of  Buffon's  statements,  but  certainly 
it  is  not  exaggerating  to  ascribe  to  him  not  only  the  recognition  of  the  factors 
of  geographical  isolation,  struggle  for  existence,  artificial  and  natural  selection 
in  the  origin  of  species,  but  also,  which  is  equally  important,  the  propounding 
of  a  theory  of  the  origin  of  variations.  He  thought  that  the  direct  action  of 
the  environment  brings  about  modifications  of  the  structure  of  animals  and 
plants  and  these  are  transmitted  to  the  offspring. 


560  HUMAN  BIOLOGY 

"  When  Buffon's  influence  was  at  its  zenith,  Erasmus  Darwin  (1731-1802), 
a  successful  medical  practitioner,  expressed  consistent  views  on  the  evolution 
of  organisms  in  several  volumes  of  prose  and  poetry.  Although  a  contem- 
porary* critic  in  the  Edinburgh  Review  remarked  that  Darwin's  '  reveries  in 
science  have  probably  no  other  chance  of  being  saved  from  oblivion,  but  by 
having  been  married  to  immortal  verse/  today  biologists  recognize  him  as  the 
anticipator  of  Lamarck's  doctrine  that  variations  spring  from  within  the 
organism  through  its  reaction  to  environmental  conditions.  'All  animals 
undergo  perpetual  transformations  which  are  in  part  produced  by  their 
exertions  in  consequence  of  their  desires  and  aversions,  of  their  pleasures  and 
their  pains,  or  of  irritations,  or  of  associations;  and  many  of  these  acquired 
forms  or  propensities  are  transmitted  to  their  posterity/  'Thus  it  would 
appear  that  all  nature  exists  in  a  state  of  perpetual  improvement  by  laws 
impressed  on  the  atoms  of  matter  by  the  great  Cause  of  Causes ;  and  that  the 
world  may  still  be  in  its  infancy,  and  continue  to  improve  forever  and  ever/ 

"While  Cuvier  was  extending  and  synthesizing  the  knowledge  of  anatomy 
of  living  and  extinct  forms,  and  founding  the  so-called  school  of  facts,  his 
fellow  countryman,  Lamarck  (1744-1829),  on  the  basis  of  work  first  on  plants 
and  then  on  animals,  carried  on  in  a  fearless  manner  the  evolutionary  inspira- 
tion of  Buffon  and  Erasmus  Darwin  (though  the  latter's  works  may  not  havo 
been  known  to  him),  and  established  the  coterie  of  evolutionists  in  Paris  each 
of  whose  essays  Cuvier  hailed  as  a  'new  folly/  Lamarck  developed  with 
great  care  the  first  complete  and  logical  theory  of  organic  evolution,  and  is 
the  one  outstanding  figure  in  biological  uniformitarian  thought  between 
Aristotle  and  Charles  Darwin.  'For  nature/  he  writes,  'time  is  nothing. 
It  is  never  a  difficulty,  she  always  has  it  at  her  disposal;  and  it  is  for  her  the 
means  by  which  she  has  accomplished  the  greatest  as  well  as  the  least  of  her 
results.  For  all  the  evolution  of  the  earth  and  of  living  beings,  nature  needs 
but  three  elements — space,  time,  and  matter/ 

"In  regard  to  the  factors  of  evolution,  Lamarck  emphasized  the  indirect 
action  of  the  environment  in  case  of  animals,  and  the  direct  action  in  the 
case  of  plants.  The  former  are  induced  to  react  and  thus  adapt  themselves, 
while  the  latter,  without  a  nervous  system,  are  molded  directly  by  their 
surroundings.  And,  so  Lamarck  believed,  such  bodily  modifications — 
acquired  characters — are  transmitted  to  the  next  generation  and  bring  about 
the  evolution  of  organisms.  .  .  . 

"And  then  appeared  the  greatest  work  of  Charles  Darwin  (1809-1882) 
the  result  of  twenty  years'  labor.  The  Origin  of  Species  (1859)  presented  a 
huge  amount  of  data  which  most  reasonably  could  be  explained  by  assuming 
the  origin  of  existing  species  by  descent  with  modifications  from  others,  and 
also  offered  as  the  explanation  of  their  origin  the  theory  of  c  natural  selection, 
or  the  preservation  of  favored  races  in  the  struggle  for  life/  In  Darwin's 
words:  'As  many  more  individuals  of  each  species  are  born  than  can  possibly 
survive,  and  as,  consequently,  there  is  frequently  recurring  struggle  for 
existence,  it  follows  that  any  being,  if  it  vary  however  slightly  in  any  manner 


APPENDIX  561 

profitable  to  itself,  under  the  complex  and  sometimes  varying  conditions  of 
life,  will  have  a  better  chance  of  surviving,  and  thus  be  naturally  selected. 
From  the  strong  principle  of  inheritance  any  selected  variety  will  tend  to 
propagate  its  new  and  modified  form/ 

"  Facts  and  theories  had  been  brought  forward  before  in  support  of 
evolution — indeed,  the  theory  of  natural  selection  had  been  suggested  before 
Darwin's  time  and  again  independently  by  Wallace  (1822-1913)  just  as 
Darwin  was  completing  his  long  studies  preparatory  to  publication.  But 
the  stupendous  task  of  thinking  evolution  through  for  the  endless  realm  of 
living  nature  remained  to  be  done,  and  Darwin  did  it  convincingly  by  his 
brilliant,  scholarly,  open-minded,  and  cautious  marshaling  and  interpreting 
of  data. 

"It  was  the  combination  of  the  facts  and  the  theory  to  account  for  the 
'facts  which  won  the  thinking  world  to  organic  evolution  and  'made  the  old 
idea  current  intellectual  coin/  Darwin  supplied  the  Ariadne  thread  which 
led  from  the  maze  of  transcendental  affinity  to  genetic  continuity.  Now  we 
know  that  evolution  is  a  bird;s-eye  view  of  the  results  of  heredity  since  the 
origin  of  life  and  that  the  facts  of  inheritance  hold  the  key  to  the  factors  of 
evolution. 

"Darwin  spent  the  twenty  years  subsequent  to  the  publication  of  the 
Origin  of  Species,  as  he  had  spent  the  preceding  twenty  years,  in  study  and 
research,  the  results  of  which  appeared  in  nine  additional  volumes.  Three 
of  these  perhaps  may  be  singled  out  as  primarily  an  elaboration  of  the  Origin: 
The  Variation  of  Animals  and  Plants  under  Domestication  (1868),  The  Descent 
of  Man  (1871),  and  The  Expression  of  the  Emotions  (1872).  Singly  and 
collectively  these  volumes  are  a  monument  to  genius  and  labor.  Erasmus 
Darwin  was  wont  to  say  that  the  world  is  not  governed  by  brilliancy  but  by 
energy.  His  grandson  revolutionized  biological  thought  through  their 
combination.  • 

"Among  Darwin's  early  converts  from  the  ranks  of  professional  biologists 
must  be  mentioned  Huxley  (1825-1895)  and  Hooker  (1817-1911)  in  England, 
Haeckel  (1834-1919)  and  Weismann  in  Germany,  and  Gray  (1810-1888)  in 
America — men  with  the  courage  of  their  convictions  when  courage  was 
necessary,  whose  support  did  so  much  for  the  promulgation  of  evolutionary 
ideas.  .  .  . 

"Today  rno  representative  biologist  questions  the  fact  of  evolution — 
'evolution  knows  only  one  heresy,  the  denial  of  continuity' — though  in 
regard  to  the  factors  there  is  much  difference  of  opinion.  It  may  well  be  that 
we  shall  have  reason  to  depart  widely  from  Darwin's  interpretation  of  the 
effective  principles  at  work  in  the  origin  of  species,  but  withal  this  will  have 
little  influence  on  his  position  in  the  history  of  biology.  The  great  value 
which  he  placed  upon  facts  was  exceeded  only  by  his  demonstration  that  this 
value  is  due  to  their  power  of  guiding  the  mind  to  a  further  discovery  of 
principles/  Darwin  brought  biology  into  line  with  the  other  inductive 
sciences,  recast  practically  all  of  its  problems,  and  instituted  new  ones/' 


562  HUMAN  BIOLOGY 

(Woodruff,  "The  Development  of  the  Sciences/'  Chap.  VI,  pp.  251-259, 
Yale  University  Press,  New  Haven,  1923.) 

Osmosis.     See  Diffusion. 

Oxidation.  "In  recent  years  the  important  discovery  has  been  made 
that  oxidation  processes  yield  energy  because  they  are  chemical  reactions  in 
which  electrons  shift  from  a  position  of  high  potential  energy  to  one  of  lower, 
and  in  the  shift  set  energy  free.  Modern  physics  has  revealed  that  electrons 
possess  much  inherent  energy;  they  tend  strongly  to  exert  force  on  one 
another.  Each  individual  electron  of  an  atom  is  subject  to  stresses  and  strains 
arising  from  all  the  other  electrons,  and  the  atom  as  a  whole  is  thus  the  seat 
of  a  whole  series  of  delicately  balanced  electron  stresses.  When  atoms  or 
molecules  come  into  contact  with  other  atoms  or  molecules  the  stresses  are 
necessarily  rearranged  as  the  two  sets  of  electrons  exert  their  inherent  forces 
on  each  other.  It  may  happen  that  in  the  course  of  this  rearrangement 
stresses  will  develop  leading  directly  to  electron  shifts  of  the  sort  indicated, 
in  other  words  to  oxidation  processes.  Whether  such  electron  shifts  will 
occur  depends  primarily  on  what  kinds  of  atoms  or  molecules  come  into  con- 
tact. Thus  oxygen  has  an  electronic  organization  which  fits  it  particularly 
to  enter  into  reactions  in  which  its  atoms  take  up  electrons.  This  special 
fitness,  together  with  the  very  great  abundance  in  which  oxygen  occurs  in 
nature,  explains  why  oxygen  is  one  of  the  constituents  in  a  great  majority  of 
oxidation  reactions.  The  name  oxidation  derives  from  an  early  period  of 
chemistry,  long  before  the  existence  of  electrons  was  known  or  even  suspected, 
and  before  it  was  realized  that  the  oxidations  with  which  everyone  is  familiar 
are  not  the  only  reactions  of  the  type  but  only  the  most  common  examples. 

"Oxygen  and  other  substances  which,  like  it,  can  take  up  electrons  under 
proper  conditions,  are  known  in  modern  chemistry  as  oxidants.  Substances 
which  tend  under  suitable  rearrangements  of  electron  stresses  to  transfer 
electrons  «to  oxidants  with  liberation  of  energy  are  known  as  reductants.  All 
substances  commonly  called  fuels  belong  in  this  latter  class."  (Martin, 
"The  Human  Body,"  pp.  23-25,  Henry  Holt  &  Company,  New  York,  1935.) 
See  Matter. 

Paramecium.  "The  ciliated  Protozoa,  constituting  the  class  Infusoria, 
probably  represent  the  most  complex  development  of  the  unicellular  plan  of 
animal  structure.  Infusoria  have  afforded  ready  material  for  the  study  of 
various  physiological  problems,  not  only  because  some  of  the  species  are 
relatively  large,  but  also  because,  in  general,  they  lend  themselves  readily  to 
experiment.  Most  of  the  Infusoria  are  free-living  in  fresh  and  salt  water, 
though  not  a  few  are  parasitic.  There  is  a  highly  complex  fauna  in  the 
digestive  tract  of  sheep  and  cattle,  and  man  is  not  immune. 

"  The  organization  of  the  group  may  be  illustrated  by  Paramecium  which  is 
a  giant  among  the  Protozoa,  being  just  visible  to  the  naked  eye  as  a  whitish 
speck  if  the  water  in  which  it  is  swimming  is  properly  illuminated.  When 
magnified  several  hundred  times  it  appears  as  a  more  or  less  slipper-shaped 
organism  which  one  would  not  consider,  at  first  glance,  a  single  cell  because  it 
shows  highly  specialized  parts. 


APPENDIX  563 

"The  nuclear  material  in  Paramecium,  instead  of  forming  a  single  body 
as  it  does  in  most  cells,  is  distributed  in  two  parts:  a  large  MACRONUCLEUS, 
and  one  or  more  small  MICRONUCLEI.  Strictly  speaking,  the  macronucleus 
and  micronuclei  together  constitute  the  nucleus  of  the  cell,  and  represent  a 
sort  of  physiological  division  of  labor  of  the  chromatic  complex  which  is 
characteristic  of  the  Infusoria. 

"But  it  is  in  the  cytoplasm  that  specialization  is  most  conspicuous.  Not 
only  are  there  general  differentiations  into  ectoplasm  and  endoplasm,  but 
these  regions  also  have  local  specializations  such  as  thousands  of  hair-like, 
vibratile  CILIA  for  locomotion  and  securing  food,  TRICHOCYSTS  for  defense, 
PERISTOME,  MOUTH,  and  GULLET  for  the  intake  of  solid  food,  GASTRIC  VACUOLES 
for  digestion,  and  CONTRACTILE  VACUOLES  for  excretion.  And  withal,  recent 
investigations  indicate  that  various  parts  of  the  cell  are  coordinated  by  a 

NEUROMOTOR  SYSTEM. 

"Paramecium,  under  normal  conditions,  grows  rapidly  and,  when  it  has 
attained  the  size  limit  characteristic  of  the  species,  cell  division  takes  place — a 
process  of  reproduction  that  can  continue  indefinitely  if  the  environment  is 
favorable.  But  periodically  Paramecium  undergoes  an  internal  nuclear 
reorganization  process  (ENDOMIXIS).  Also  now  and  then  individuals  tem- 
porarily fuse  in  pairs  and  interchange  nuclear  material  (CONJUGATION) — an 
expression  of  fundamental  sex  phenomena,  involving  fertilization. 

"Indeed  the  Infusoria  seem,  so  to  speak,  to  have  made  the  most  of  their 
unicellular  plan  of  structure,  for  Paramecium  is  fairly  representative:  it  is 
not  the  most  simple  nor  yet  the  most  complex.  Specialization  of  one  part 
and  another  of  the  cell  has  produced  in  the  Infusoria  a  group  of  animals  that, 
judged  by  distribution  and  numbers,  is  highly  successful  in  the  microscopic 
world  of  life."  (Woodruff,  "Foundations  of  Biology,"  pp.  473,  475-476, 
The  Macmillan  Company,  New  York,  1936.) 

Pericardium.  The  mesodermal  covering  tissue  that  forms  the  thin, 
transparent  pericardial  sac  in  which  the  heart  lies.  See  Serous  Membranes. 

Peritoneum.  The  mesodermal  lining  tissues  of  the  abdominal  or  peri- 
toneal cavity  that  are  reflected  as  a  covering  over  the  various  enclosed  organs. 
See  Serous  Membranes. 

Peritonitis.  An  infection  of  the  peritoneum  that  lines  the  abdominal 
cavity.  For  example,  a  ruptured  appendix  will  release  infective  agents  into 
the  body  cavity  which  attack  the  peritoneum  and  cause  peritonitis. 

pH.     See  Hydrogen  Ion. 

Physiology,  Development  of.  "Animal  and  plant  physiology  were  dis- 
cussed by  Aristotle,  but  as  might  be  expected,  since  physiology  is  more  depend- 
ent than  anatomy  upon  progress  in  other  branches  of  science,  with  less  happy 
results.  Similarly,  Galen  was  hampered  in  his  attempt  to  make  physiology  a 
distinct  department  of  learning  based  on  a  thorough  study  of  anatomy,  and 
the  corner-stone  of  medicine,  Like  Aristotle  he  attempted  to  develop  a 
picture  of  the  modus  operandi  of  the  organism,  and  with  such  success  that 
fate  foisted  it  upon  uncritical  generations  through  fifteen  centuries.  And  the 
unfortunate  fact  was  not  that  Galen's  physiology  and  anatomy  were  largely 


564  HUMAN  BIOLOGY 

incorrect,  but  that  to  question  his  authority  was  little  less  than  sacrilege  until 
the  labors  of  Vesalius  and  Harvey  brought  a  realization  that  Galen  had  not 
quite  finished  the  work. 

"Neither  Vesalius  nor  Harvey  made  an  attempt  to  explain  the  workings 
of  the  body  by  appeal  to  so-called  physical  and  chemical  laws;  and  for  good 
reason.  Chemistry  had  not  yet  thrown  off  the  shackles  of  alchemy  and 
taken  its  legitimate  place  among  the  elect  sciences,  while  during  Harvey's 
lifetime,  under  the  influence  of  Galileo,  the  new  physics  arose.  But  by  the 
end  of  the  seventeenth  century  both  physics  and  chemistry,  aided  by  the 
philosophical  systems  of  Bacon  and  Descartes,  had  forced  their  way  into 
physiology  and  split  it  into  two  schools:  the  iatro-mechanical  founded  by 
Borelli  (1608-1679),  who  by  incisive  physical  methods  attacked  a  long  series 
of  problems,  frequently  with  brilliant  results;  and  the  iatro-chemical  school, 
which  developed  from  the  influence  of  Franciscus  Sylvius  (1614-1672)  as  a 
teacher  rather  than  as  an  investigator. 

"This  awakening  brought  a  host  of  workers  into  the  field  and  the  harvest 
of  the  century  was  garnered  and  enriched  by  Haller  (1708-1777),  the  'abyss  of 
learning'  of  the  time,  in  a  comprehensive  treatise  fwhich  at  once  indicated 
the  erudition  and  critical  judgment  of  its  author  and  established  physiology 
as  a  distinct  and  important  branch  of  biological  science,  rather  than  as  a 
mere  adjunct  of  medicine.  Great  as  was  this  contribution  of  Haller  in 
crystallizing  physiology  and  setting  the  dividing  line  between  the  old  and  the 
modern,  unfortunately  the  weight  of  the  author's  authority  was  ranged  in 
favor  of  two  theories  which  were  in  crude  form,  attracting  the  attention  of 
biologists — the  idea  of  special  vital  force  and  the  preformation  theory  of 
development. 

"  Perhaps  the  most  significant  lines  of  advance  in  Haller's  century  were  in 
setting  the  physiology  of  nutrition  and  respiration — both  of  which  waited 
upon  the  work  of  the  chemists — well  upon  their  way  towards  modern  form.  .  .  . 

"Most  of  the  foundation  on  which  the  physiology  of  animals  rests  today 
has  been  built  up  by  works  on  vertebrates,  though  since  the  middle  of  the 
nineteenth  century,  when  the  versatile  M  tiller  showed  the  value  of  studying 
the  physiology  of  higher  and  lower  animals  alike,  the  science  of  comparative 
physiology  may  be  said  to  have  been  established.  Perhaps  it  is  not  an 
exaggeration  to  say  that  the  tendency  to  focus  evidence,  in  so  far  as  possible, 
from  all  forms  of  life  on  general  problems  of  function  represents  the  present 
trend  of  physiological  inquiry. 

"The  less  obvious  structural  and  functional  differentiation  of  plants 
retarded  progress  in  plant  physiology  as  it  did  in  plant  anatomy.  Probably 
of  most  historical,  and  certainly  of  most  general  interest  is  the  development 
of  our  knowledge  of  the  tiutrition  of  green  plants. 

"Aristotle's  theory  that  the  food  of  plants  is  prepared  for  them  in  the 
ground  was  still  prevalent  at  the  end  of  the  sixteenth  century  when  Cesalpino, 
the  most  philosophic  botanist  of  his  day,  thought  that  food  enters  and  passes 
through  vessels  and  fibers  of  plants  much  as  oil  in  a  lamp  wick,  and  Jung 
conceded  that  plants  are  not  mere  passive  absorbers  of  ready-made  food, 


APPENDIX  565 

but  possess  the  power  of  selecting  from  the,  soil  the  ingredients  needed.  But 
it  was  van  Helmont,  on  the  border  line  between  alchemist  and  chemist,  who 
precociously  brought  to  bear  the  chemical  point  of  view  on  animal  nutrition. 
He  planted  a  small  tree  in  a  large  vessel  and  weighed  it.  Then  after  five 
years,  during  which  time  it  had  only  been  supplied  with  water,  he  found  that 
it  had  increased  some  thirty-fold  in  weight  and  '  not  suspecting  that  the  plant 
drew  a  great  part  of  its  materials  from  the  air  was  forced  to  exaggerate  the 
virtues  of  rain-water/  Malpighi,  however,  from  his  studies  on  plant  histology, 
gave  the  first  hint  of  the  fact  of  supreme  importance  that  the  crude  sap,  which 
enters  by  the  roots,  is  carried  to  the  leaves  where,  by  the  action  of  sunlight, 
evaporation,  and  some  sort  of  a  fermentation,  it  is  'digested'  and  then  dis- 
tributed as  food  to  the  plant  as  a  whole.  But  it  is  Hales  (1677-1761)  to  whom 
the  botanist  looks  as  the  Harvey  of  plant  physiology,  for  in  his  Vegetable 
Staticks,  published  in  1727,  he  laid  the  foundations  of  the  physiology  of  plants 
by  making  '  plants  speak  for  themselves  through  his  incisive  experiments/ 
For  the  first  time  it  became  clear  that  green  plants  derive  an  important 
element  of  their  food  from  the  atmosphere,  and  also  that  the  leaves  play  an 
active  role  in  the  movements  of  fluids  up  the  stem  and  in  eliminating  superflu- 
ous wlter  through  evaporation. 

"  Still  the  picture  was  incomplete,  and  so  it  remained  until  the  biologist 
had  recourse  to  further  data  from  the  chemist.  In  1779  Priestley,  the  dis- 
coverer of  oxygen,  showed  that  this  gas  under  certain  conditions  is  liberated 
by  plants.  This  fact  was  seized  upon  by  Ingen-Housz  (1730-1799),  who 
demonstrated  that  carbon  dioxide  from  the  air  is  broken  down  in  the  leaf 
during  exposure  to  sunlight;  the  plant  retaining  the  carbon  and  returning 
oxygen — the  process  of  carbon  getting  being  quite  distinct  from  that  of 
respiration  in  which  carbon  dioxide  is  eliminated.  It  remained  then  for  de 
Saussure  to  show,  by  quantitative  studies  of  the  plant's  income,  that,  in 
addition  to  the  fixation  of  carbon,  the  elements  of  water  are  also  employed, 
while  from  the  soil  various  salts,  including  the  element  nitrogen,  are  obtained. 
But  it  was  nearly  the  middle  of  the  last  century  before  the  influence  of  Liebig 
(1803-1873),  and  the  crucial  experiments  of  Boussingault  (1802-1887)  estab- 
lished the  part  played  by  the  chlorophyll  of  the  green  leaf  in  making  certain 
chemical  elements  available  to  animals.  The  realization  of  the  cosmical 
function  of  green  plants — the  link  they  supply  in  the  circulation  of  the  ele- 
ments in  nature — is  a  landmark  in  biological  progress,  and  we  may  leave  the 
subject  here  since,  except  for  details  in  regard  to  some  of  the  more  evident 
chemical  products  of  photosynthesis  and  the  influence  of  external  factors,  the 
matter  still  stands  essentially  where  it  was  in  de  Saussure' s  day."  (Wood- 
ruff, "The  Development  of  the  Sciences,"  Chap.  VI,  pp.  236-242,  Yale 
University  Press,  New  Haven,  Conn.,  1923.) 

Pituitary — Historical.  "  The  existence  of  the  pituitary  was  known  as  early 
as  the  time  of  Galen  (200  A.D.).  The  name  pituitary,  assigned  to  it  by  Vesa- 
lius,  perpetuates  an  erroneous  theory  of  its  function  that  was  long  held.  This 
designation  ascribes  to  the  gland  the  lowly  office  of  secreting  a  fluid  to  lubri- 
cate the  throat  (Latin:  pituita,  mucus).  The  secretion  was  supposed  to  be 


566  HUMAN  BIOLOGY 

poured  by  minute  channels  into  the  nose  cavity.  This  misapprehension  was 
fostered  by  the  porous  nature  of  the  bone  that  intervenes  between  the  pituitary 
and  the  nasal  cavity  (cribriform  plate  of  the  ethmoid  bone).*.  Actually,  there 
exist  no  such  passages  as  the  ancients  surmised.  The  idea  of  the  mucus- 
secreting  function  was  overthrown  by  Conrad  Schneider  in  a  treatise  on  the 
membranes  of  the  nose  as  early  as  1660.  No  other  function  for  the  gland  being 
known,  it  came  ultimately  to  be  regarded  as  merely  a  vestigial  relic  that  had 
no  particular  importance  to  its  possessor. 

"  One  of  the  important  functions  of  the  pituitary  is  the  promotion  of  growth. 
Accordingly,  we  find  the  earliest  beginnings  of  our  knowledge  going  back  into 
primitive  legendary  lore.  Giants  have  been  known  from  time  immemorial. 
Many  peoples  have  held  the  belief  that  humanity  has  descended  from  races  of 
enormous  height.  Curiously,  the  legend  carried  over  into  relatively  modern 
times  through  misinterpretation  of  fossil  remains.  Actually,  the  giants  of 
the  neomythology  were  such  huge  creatures  as  the  dinosaurs.  A  picturesque 
version  of  the  belief  is  reported  in  the  book  of  Deuteronomy.  'For  only  Og 
king  of  Bashan  remained  of  the  remnant  of  giants;  behold  his  bedstead  was 
a  bedstead  of  iron;  nine  cubits  was  the  length  thereof,  and  four  cubits  the 
breadth  of  it,  after  the  cubit  of  a  man/  This  description  implies  that  the 
king  was  about  11  ft.  in  height.  Goliath  of  Gath  is  another  gigantic  figure 
of  popular  lore.  Pliny  mentioned  by  name  an  Arabian  giant  9J^  ft.  tall  and 
reported,  from  hearsay,  two"  others  who  had  reached  10  ft.  This  gradual 
attenuation  brings  us  to  a  scientifically  verified  case,  that  of  Kayanus,  a 
Finn,  who  was  authentically  nine  and  two-tenths  feet  tall. 

"Actually,  our  knowledge  of  the  involvement  of  the  pituitary  in  bodily 
overgrowth  was  first  appreciated  in  connection  with  a  modified  type  of  gigan- 
tism  known  as  acromegaly.  This  condition  had  been  recognized  as  a  growth 
anomaly  by  Verga  in  1864,  and  he  made  the  further  important  observation 
that  the  pituitary  gland  in  the  patient  he  studied  was  abnormal.  The 
eminent  pathologist,  Professor  Klebs,  thirty  years  later,  wrote  an  excellent 
monographic  study  of  a  case  of  acromegaly.  Klebs  emphasized  the  fact 
that  the  pituitary  gland  was  excessively  large,  but  he  was  unable  to  decide 
whether  this  abnormality  was  the  cause  of  or  merely  a  part  of  the  patient's 
general  overgrowth.  To  the  French  neurologist  Pierre  Marie  is  usually  given 
the  credit  for  finally  determining  the  relationship  of  the  pituitary  to  the 
disease.  His  studies  were  reported  in  1886."  (Hoskins,  "  The  Tides  of  Life/' 
pp.  118-120,  W.  W.  Norton  &  Company,  Inc.,  New  York,  1933.) 

Plasmodium.  "  There  are  several  recognized  types  of  malaria  parasites, 
all  of  which  belong  to  the  genus  Plasmodium.  The  life  cycle  of  this  important 
organism  involves  a  SEXUAL  PHASE  which  must  occur  in  the  mosquito  and 
ASEXUAL  PHASES  which  occur  in  man  and  mosquito.  The  essential  features 
of  the  life  history  may  be  outlined  as  follows:  Man  is  infected  by  the  bite  of  a 
female  Anopheles  mosquito  which  harbors  the  parasite  in  its  salivary  glands. 
The  parasites  when  received  into  the  blood  stream  are  motile  SPOROZOITES 
which  are  able  to  pass  through  the  cell  wall  of  the  red  blood  corpuscles  and 
into  the  cytoplasm  where  they  remain  as  intracellular  parasites  and  absorb 


APPENDIX  567 

the  life  substance  of  the  red  cells  as  food.  They  increase  in  size  until  each 
sporozoite  occupies  almost ^the  entire  corpuscle.  They  then  divide  to  form  a 
great  number  of  active  MEROZOITES  which  burst  the  wall  of  the  blood  cor- 
puscles and  escape  free  into  the  blood  stream  where  each  attacks  a  new 
corpuscle. 

"This  asexual  cycle  in  man  occurs  very  rapidly  and  may  be  repeated 
indefinitely  to  produce  a  serious  or  even  fatal  case  of  malaria.  Finally,  sexual 
elements,  the  male  and  female  GAMETOCYTES,  appear  in  the  blood  corpuscles 
of  the  infected  person  and  these  must  be  transmitted  to  the  mosquito  for 
maturation.  Accordingly  if  the  victim  is  bitten  once  more  by  a  mosquito 
the  latter,  in  drawing  the  blood,  will  receive  some  of  the  gametocytes,  which 
pass  into  the  stomach  and  soon  form  free-swimming  sperm  and  non-motile 
eggs.  Fertilization  occurs,  and  the  newly  formed  zygotes,  tiny  active 
individuals,  penetrate  into  the  epithelial  cells  lining  the  stomach  where  each 
becomes  an  OOCYST.  A  great  increase  in  size  occurs  followed  by  asexual 
reproduction,  or  SPOROGONY,  which  results  in  the  production  within  four  or 
five  days  of  numerous  sporozoites.  Breaking  out  of  the  oocyst  wall  into 
the  body  cavity  of  the  mosquito,  they  soon  find  their  way  to  the  salivary 
glands  from  where  they  pass  into  the  human  victim.  Inasmuch  as  spore 
formation  in  the  body  of  the  mosquito  is  obligatory  in  the  life  cycle  of  the 
parasite,  it  is  apparent  that  an  infected  person  does  not  transmit  malaria  to 
others  except  indirectly  by  way  of  the  mosquito. 

"A  rather  close  relative  of  the  malaria  parasite  is  the  sporozoon,  Babesia, 
which  has  a  wide  distribution  and  formerly  caused  great  losses  among  domestic 
animals.  In  the  United  States  it  is  the  cause  of  a  very  serious  disease  in 
cattle,  Texas  fever.  To  the  biologist  the  determination  of  the  life  cycle  of 
this  organism  represents  a  landmark  in  achievement  because  it  was  the  first 
protozoan  disease  known  to  be  transmitted  by  an  arthropod,  in  this  case  a 
tick.  The  results  led  to  the  development  of  successful  methods  of  control  and 
laid  the  foundation  for  many  advances  against  disease-carrying  organisms." 
(Baitsell,  "  Manual  of  Biology/'  pp.  80-82,  The  Macmillan  Company,  New 
York,  1936.) 

Pleura.  The  mesodermal  lining  tissue  of  the  thoracic  cavity  that  is 
reflected  as  a  covering  over  the  lungs.  See  Serous  Membranes. 

Pneumonia,  Determination  of  the  Type.  It  has  been  established  that 
there  are  three  serological  types  of  pneumonia  which  may  be  determined  by 
agglutination  tests.  Horse  serum,  in  which  an  agglutination  has  been 
developed  by  previous  injections  of  a  particular  type  of  pneumococcus,  is 
used  in  the  typing.  To  carry  out  the  test,  it  is  necessary  to  secure  an  abundant 
supply  of  the  pneumococci  from  the  patient.  This  is  quickly  accomplished 
by  securing  sputum,  emulsifying  with  salt  solution,  and  injecting  into  the 
abdominal  cavity  of  white  mice.  These  animals  are  very  susceptible  to  all 
types  of  pneumococci.  In  the  abdominal  cavity  of  the  mouse,  the  bacteria 
grow  very  rapidly,  and  24  hours  later  the  peritoneal  fluid  will  contain  great 
quantities  of  pneumococci,  which  may  be  concentrated  by  centrifuging  and 
then  tested  with  each  of  the  three  types  of  horse  serum,  noted  above,  for  the 


568  HUMAN  BIOLOGY 

agglutinating  reaction.  If  the  agglutinating  reaction  does  not  occur  with  any 
of  the  three  types  of  serum,  then  the  infection  is  d\je  to  Type  IV  pneumococci 
(Fig.  255). 

Proteins.  "These  are  the  most  important  and  most  characteristic  com- 
ponents of  living  matter.  No  living  matter  is  known  that  does  not  contain 
them.  They  differ  from  the  carbohydrates,  fats,  and  lipoids  in  the  facts  that 
the  molecules  of  most  of  them  are  much  larger  and  more  complex  in  their 
arrangement  and  that  nitrogen  forms  a  constituent  part  of  the  molecule. 
The  average  percentage  composition  of  proteins  is  as  follows : 

Per  Cent 

Carbon 50  to  55 

Hydrogen 6.5  to  7.3 

Nitrogen 15  to  17.6 

Oxygen 19  to  24 

Sulphur 0.3  to  2.4 

"Other  elements,  for  example,  phosphorus,  may  be  included  in  small 
amounts. 

"The  size  of  the  protein  molecule  is  always  very  large,  since  it  is  built  up  by 
the  linkage  of  a  number  of  relatively  simple  substances  known  as  amino  acids. 
Some  22  different  amino  acids  are  now  known,  and,  as  these  are  combined  in 
varying  numbers,  proportions,  and  relations  to  each  other,  it  will  be  apparent 
that  the  possible  number  of  combinations  that  may  be  formed  is  almost  beyond 
calculation.  The  various  amino  acids  are  linked  in  the  protein  by  the  joining 
of  the  COOH  group  of  one  to  an  NH2  group  of  another,  with  the  elimina- 
tion of  water,  thus  CO[OH  H]HN.  Some  COOH  and  NH2  groups  are  left 
uncombined. 

"Proteins,  because  they  contain  both  basic  (NH2)  and  acid  (COOH) 
groups,  are  considered  amphoteric  electrolytes,  capable  of  forming  ionizable 
salts  with  acids  as  well  as  with  alkalies,  according  to  the  hydrogen-ion  con- 
centration of  the  solution  in  which  they  are  placed.  They  exist,  therefore,  in 
three  conditions: 

"L  They  form  salts  with  acids,  as  gelatin  chloride.  This  occurs  when 
the  hydrogen-ion  concentration  exceeds  a  certain  critical  value,  in  this  case 
pH  4.7. 

"2.  They  form  salts  with  bases,  as  sodium  gelatinate.  This  action  takes 
place  when  the  hydrogen-ion  concentration  is  below  the  critical  value. 

"3.  At  the  critical  value  of  the  hydrogen-ion  concentration,  the  protein  is 
not  able  to  combine  with  either  an  acid,  a  base,  or  a  neutral  salt.  This 
critical  hydrogen-ion  concentration  is  called  the  isoelectric  point  of  the  protein. 

"Among  the  chemical  properties  of  the  proteins  of  importance  to  the 
physiologist  should  be  mentioned  their  general  color  reactions.  These  color 
reactions  are  due  to  a  reaction  between  some  one  or  more  of  the  constituent 
groups  of  the  complex  protein  molecule  and  the  chemical  reagent  used  in  the 
particular  test.  Inasmuch  as  not  all  proteins  contain  all  of  the  same  groups, 
it  is  found  that  the  tests  vary  in  intensity  of  reaction.  In  case  of  any  doubt  it 


APPENDIX  569 

is  always  wise  to  submit  the  sample  under  examination  to  more  than  one  of 
the  recognized  tests  before  making  a  decision  as  to  the  nature  of  the  substance." 
(Rogers,  "  Textbook  of  Comparative  Physiology,"  pp.  33-34,  36-37,  McGraw- 
Hill  Book  Company,  Inc.,  New  York,  1938.) 

Protoplasm.  The  material  substance  in  which  life  phenomena  are  exhib- 
ited. It  is  characterized  structurally  by  its  division  into  microscopic  cells, 
which  are  the  primary  structural  and  functional  units  of  living  organisms. 
The  term  was  first  used  in  1839  by  Purkinje,  a  Bohemian  biologist,  to  designate 
the  vital  substance  that  he  found  in  various  animal  and  plant  materials. 
It  was  not,  however,  until  1861,  from  the  researches  of  Max  Schultze,  that  the 
concept  of  protoplasm  as  the  universal  life  stuff  of  both  plants  and  animals 
became  firmly  established.  The  cell  is  an  organized  mass  of  protoplasm  typi- 
cally differentiated  into  cytoplasm  and  nucleus  and  enclosed  by  a  secreted 
nonliving  cell  wall;  the  thicker  cell  walls  of  plant  cells  consisting  of  cellulose, 
whereas  those  of  animals  are  proteinaceous  in  nature.  There  are  noteworthy 
differences  between  a  so-called  resting  cell  and  one  that  is  undergoing  mitosis 
in  that,  in  the  latter,  the  nucleus  disappears  and  the  nuclear  contents  are 
united  with  the  cytoplasm  in  the  structural  forms  associated  with  mitosis. 
See  Proteins;  Biological  Elements. 

Protoplast.  The  protoplasmic  mass  in  a  cell  exclusive  of  the  nonliving  cell 
wall.  The  term  is  most  frequently  employed  by  botanists  in  describing  the 
contents  of  a  plant  cell  enclosed  within  the  heavy  cellulose  wall. 

Protozoa.  "The  first  great  phylum  is  the  Protozoa  which  comprises  the 
most  primitive  forms  of  animal  life,  each  individual  being,  as  we  know,  a 
single  unit  of  living  matter.  But  it  does  not  follow  that  the  Protozoa  are 
devoid  of  complex  organization.  Indeed  some  exhibit  a  complexity  of 
structure  within  the  confines  of  a  cell  that  probably  is  not  exceeded  in  the 
cells  of  higher  animals.  The  Protozoa  are  the  simplest,  but  by  no  means 
simple,  animals,  and  their  study  forms  the  science  of  PROTOZOOLOGY. 

"All  of  the  Protozoa,  since  they  are  single  cells,  demand  for  active  life  a 
more  or  less  fluid  medium,  and  are  typically  aquatic  animals.  However, 
different  species  exhibit  all  gradations  of  adaptation  to  variations  in  moisture 
from  those  that  thrive  in  oceans  and  lakes,  or  pools  and  puddles,  to  those 
which  find  sufficient  the  dew  on  soil  or  grass  blade,  or  the  fluids  within  the 
tissues  and  cells  of  higher  animals  and  plants. 

"The  phylum  Protozoa  is  divided,  largely  on  the  basis  of  the  locomotor 
organs,  into  four  CLASSES:  The  Mastigophora,  Sarcodina,  Sporozoa,  and 
Infusoria.  In  general,  we  may  regard  the  MASTIGOPHORA  as  cells  with  flagella 
as  locomotive  organs,  such  as  Euglena;  the  SARCODINA  as  forms,  like  Amoeba, 
that  move  about  by  means  of  pseudopodia;  and  the  INFUSORIA  as  organisms, 
like  Paramecium,  that  swim  by  cilia.  The  SPOROZOA,  all  of  which  are  parasitic, 
such  as  the  organisms  causing  malaria,  possess  no  characteristic  type  of  organ 
for  locomotion  though  all  are  motile  at  some  stage  in  their  life  history." 
(Woodruff,  "Foundations  of  Biology,"  pp.  467-468,  The  Macmillan  Company, 
New  York,  1936.)  See  Metazoa. 


570  HUMAN  BIOLOGY 

Rabies  Vaccine.  The  preparation  of  rabies  vaccine  begins  with  the 
securing  of  infected  nerve  tissue  from  the  central  nervous  system  of  a  rabid 
dog,  containing  the  virus  in  its  most  active  state.  The  inoculation  of  an 
individual  with  this  material  would  be  as  dangerous  as  the  bite  of  a  mad  dog. 
In  order  to  reduce  the  virulence,  the  infected  nerve  tissue  is  emulsified  and 
then  injected  into  a  rabbit  where  it  develops  slowly.  Repeated  transfers 
of  the  virus  are  made  from  one  rabbit  to 'another;  altogether,  a  series  of  pas- 
sages involving  from  30  to  50  animals  may  be  required.  In  time,  the  rabies 
virus  becomes  standardized  as  a  "  fixed  virus"  which  will  kill  a  rabbit  6  days 
after  the  infection.  A  final  transfer  of  the  fixed  virus  is  then  made.  The 
rabbit  receiving  the  fixed  virus  is  not  allowed  to  die  from  the  infection  but  is 
anesthetized  just  before  that  time,  and  the  infected  spinal  cord  with  the 
virulent  fixed  virus  is  removed  from  the  body  and  sectioned,  usually  into  18 
pieces.  These  spinal  cord  sections  are  now  dried  from  1  to  18  days  to  reduce 
the  virulence  of  the  virus  so  that  it  may  be  used  for  human  vaccination.  In 
sections  dried  for  1  day,  the  virus  remains  almost  full  strength  but  is  progres- 
sively weakened  by  continued  drying  and  is  almost  destroyed  in  sections 
dried  for  18  days.  Solutions  are  now  made  from  each  of  the  sections  dried 
for  the  various  periods,  and  these  constitute  the  rabies  vaccine.  The  treat- 
ment is  begun  with  the  vaccine  made  from  the  section  dried  for  18  days;  and 
on  successive  days,  increasingly  stronger  vaccines  are  given.  Finally,  at  the 
conclusion  of  an  18-day  period,  the  patient  will  receive  an  injection  contain- 
ing practically  full-strength  virus.  But  this  is  safe  because,  during  the 
intervening  days,  the  antibody  formation  has  been  incited,  and  the  rabies 
virus  in  the  vaccine  as  well  as  that  injected  by  the  bite  of  the  mad  dog  will  be 
destroyed  before  damage  is  done  to  the  tissues  of  the  central  nervous  system. 

Secretagogues.  "It  seems  that  some  foods  contain  substances  designated 
as  secretagogues,  that  are  able  to  cause  a  secretion  of  gastric  juice  when  taken 
into  the  stomach.  Thus,  meat  extracts,  meat  juices,  soups,  etc.,  are  par- 
ticularly effective  in  this  respect;  milk  and  water  cause  less  secretion.  In 
other  foods  these  ready-formed  secretagogues  are  lacking.  Certain  common 
articles  of  food,  such  as  bread  and  white  of  eggs,  have  no  effect  of  this  kind  at 
all.  If  introduced  into  the  stomach  of  a  dog  through  a  fistula  so  as  not  to 
arouse  a  psychical  secretion — for  instance,  while  the  dog's  attention  is  diverted 
or  while  he  is  sleeping — they  cause  no  flow  of  gastric  juice  and  are  not  digested. 
If  such  articles  of  food  are  eaten,  however,,  they  cause  a  psychical  secretion, 
and  when  this  has  acted  upon  the  foods  some  products  of  their  digestion,  in 
turn,  become  capable  of  arousing  a  further  flow  of  gastric  juice.  The  steps 
in  the  mechanism  of  secretion  are,  therefore,  three:  (1)  The  psychical  secretion 
or  appetite  secretion;  (2)  the  secretion  from  secretagogues  contained  in  the 
food;  (3)  the  secretion  from  secretagogues  contained  in  the  products  of 
digestion.  The  manner  in  which  the  secretagogues  act  cannot  be  stated 
positively."  (Howell,  "A  Textbook  of  Physiology/'  p.  824,  Courtesy  of 
W.  B.  Saunders  Company,  Philadelphia,  1937.) 

Serous  Membranes.  "The  Tissue  of  the  Serous  Membranes.  The 
serous  membranes  (the  peritoneum,  the  pleura,  and  the  pericardium}  are 


APPENDIX  571 

thin  layers  of  loose  connective  tissue  covered  on  their  free  surfaces  by  a  layer 
of  mesothelium.  When  the  membranes  are  folded,  as  the  omentum  or  the 
mesentery,  both  of  the  free  surfaces  are  covered  with  mesothelium.  The 
serous  cavities  always  contain  a  small  amount  of  serous  liquid,  the  serous 
exudate.  The  cells  floating  in  it  originate  from  the  serous  membrane. 

"All  the  elements  of  the  loose  connective  tissue  are  found  in  the  serous 
membranes,  where  they  are  arranged  in  a  thin  layer.  The  mesentery  con- 
tains a  loose  network  of  collagenous  and  elastic  fibers,  scattered  fibroblasts, 
fixed  macrophages,  mast  cells,  and  a  varying  number  of  fat  cells  along  the 
blood  vessels. 

"  Physiologically  the  most  important,  and  histologically  the  most  interest- 
ing part  of  the  serous  membranes  in  mammals  is  the  omentum."  (Maximow 
and  Bloom,  "A  Textbook  of  Histology,"  p.  64,  Courtesy  of  W.  B.  Saunders 
Company^  Philadelphia,  1934.) 

Sexual  Characteristics  (Secondary).  "In  the  folk  thought  of  all  times, 
the  testes  have  figured  as  the  source  of  virility.  Not  only  because  of  the 
resulting  sterility  but  even  more,  perhaps,  because  of  the  effect  upon  the 
personality  of  the  subject,  emasculation  has  always  been  regarded  as  a  major 
calamity.  From  time  immemorial,  removal  of  the  testes,  castration,  has  been 
practiced  on  boys  and  the  common  farm  animals.  The  operation  has  been  a 
religious  rite  among  various  sects,  for  "example,  until  recently,  the  Skoptzs 
of  Russia.  The  eunuch  thus  produced  has  had  a  special  utility  in  various 
organizations  of  society,  particularly  as  guardians  of  harems.  As  late  as 
1870,  the  operation  was  practiced  to  conserve  the  high-pitched  singing  voice 
of  boys  of  a  famous  choir.  It  was  from  observations  of  ^individuals  who  had 
undergone  such  mutilation  that  the  popular  impression  was  derived.  .  .  . 

"  Wheelon  has  defined  sex  as  'dependent  upon  the  sum  total  of  the  somatic 
characteristics  and  differences  associated  with  the  reproductive  tissue.' 
He  continues:  'In  addition  to  the  evolution  of  male  and  female  genital  organs 
arose  other  phenomena  by  which  the  sexes  are  characterized.  Such  charac- 
ters were  designated  by  John  Hunter  as  secondary  sexual  characteristics. 
This  term  embraces  all  those  specific  differences  between  the  male  and  the 
female  .  .  .  which  are  not  directly  concerned  with  the  processes  of  reproduc- 
tion. Such  characters  are  usually  more  elaborate  in  the  male  than  in  the 
female.  Familiar  examples  of  these  characters  are  found  .  in  insects  and 
vertebrates  but  are  rare  or  absent  in  the  lower  invertebrates.  The  horns  of 
the  stag,  the  mane  of  the  lion,  the  great  variation  of  color  among  birds,  the 
phosphorescent  organs  of  the  fire-fly,  and  the  distribution  of  hair  in  man,  are 
typical  examples  of  secondary  sexual  characteristics.' 

"In  addition  to  the  anatomical  and  physiological  differences  between  males 
and  females,  distinctive  sexual  instincts  have  arisen  also.  In  the  higher 
forms,  these  include  the  impulses  that  bring  the  male  and  female  together 
at  the  breeding  season.  They  control  the  behavior  of  the  individuals  in  their 
reciprocal  relationships,  such  as  courting  and  mating.  Finally,  they  initiate 
the  various  activities  involved  in  the  building  of  the  nest  and  the  rearing 
of  the  young.  In  many  animals,  the  sexual  instincts  are  operative  only  during 


572  HUMAN  BIOLOGY 

the  breeding  season.  The  utility  of  this  adaptation  is  obviously  to  insure 
that  offspring  will  not  be  produced  at  unfavorable  times  of  the  year.  Various 
of  the  secondary  sex  characteristics,  too,  are  exaggerated  during  the  breeding 
season  only  to  wane  with  its  passing.  In  many  of  the  higher  forms,  the 
periodical  changes  in  the  secondary  sex  characteristics  are  directly  under  the 
control  of  the  primary  sex  glands."  (Hoskins,  "The  Tides  of  Life,"  pp. 
170-173,  W.  W.  Norton  &  Company,  Inc.,  New  York,  1933.) 

Skin  Glands.  "The  glands  of  the  skin  are  of  two  kinds,  the  sudoriparous 
or  sweat-glands,  and  the  sebaceous  or  oil-glands.  The  former  belong  to  the 
tubular,  the  latter  to  the  racemose  type.  The  sweat-glands  lie  in  the  subcu- 
taneous tissue,  where  they  form  little  globular  masses  composed  of  a  coiled 
tube.  %  From  the  coil  a  duct  (sometimes  double)  leads  to  the  surface,  being 
usually  spirally  twisted  as  it  passes  through  the  epidermis.  The  secreting 
part  of  the  gland  consists  of  a  connective-tissue  tube,  continuous  with  the 
dermis  along  the  duct;  within  this  is  a  basement  membrane;  ana  the  final 
secretory  lining  consists  of  several  layers  of  gland-cells.  A  close  capillary 
network  intertwines  with  the  coils  of  the  gland.  Sweat-glands  are  found  on 
all  regions  of  the  skin,  but  more  closely  set  in  some  places,  as  the  palms  of 
the  hand  and  on  the  brow,  than  elsewhere:  there  are  altogether  about  two 
aftd  a  half  millions  of  them  opening  on  the  surface  of  the  body. 

"The  sebaceous  glands  nearly  always  open  into  hair-follicles,  and  are  found 
wherever  there  are  hairs.  Each  consists  of  a  duct  opening  near  the  mouth 
of  a  hair-follicle  and  branching  at  its  other  end:  the  final  branches  lead  into 
globular  secreting  sacules,  which,  like  the  ducts,  are  lined  with  epithelium. 
In  the  saccules  the. substance  of  the  cells  becomes  charged  with  oil-drops, 
replacing  all  the  protoplasm  except  the  basal  part  where  the  nucleus  is  located; 
finally  the  whole  outer  part  falls  to  pieces,  its  detritus  constituting  the  secre- 
tion. This  outer  portion  is  then  reformed,  to  provide  material  for  further 
secretion,  usually  two  glands  are  connected  with  each  hair-follicle,  but 
there  may  be  three  or  only  one."  (Martin,  "  The  Human  Body,"  pp.  592-593, 
Henry  Holt  &  Company,  New  York,  1935.) 

Smallpox  Vaccine.  In  the  preparation  of  smallpox  vaccine  by  the  great 
commercial  biological  laboratories,  every  precaution  is  taken  to  insure  sterile 
cultures  of  smallpox  virus  that  have  been  so  treated  as  to  reduce  their  virulence 
to  a  safe  level.  Several  steps  are  involved  which  may  be  briefly  summarized. 
In  the  first  place,  it  is  necessary  to  secure  the  virus  either  from  cowpox  mate- 
rial or  from  vaccination  scabs.  The  latter  are  obtained  from  healthy  children 
about  19  days  after  vaccination,  at  which  time  the  scabs  are  well  dried  and 
about  ready  to  slough  off,  but  they  still  contain  active  virus  of  smallpox. 
The  virus-containing  material  is  then  emulsified  with  a  salt  solution  to  form  a 
vaccination  paste.  If  this  material  were  used  for  human  vaccination,  the 
virus  would  probably  develop  rapidly  and  give  a  severe  case  of  smallpox  to 
the  recipient.  Accordingly,  methods  have  been  devised  to  reduce  its  virulence. 
As  the  first  step,  the  virus  paste  is  inoculated  into  the  sterile  shaved  skin 
on  the  abdomen  of  a  calf.  After  the  virus  has  developed  here  for  a  specified 
time,  the  human-bovine  virus  is  collected  from  the  inoculated  areas,  but  it  is 


APPENDIX  573 

not  yet  suitable  for  human  inoculation.  The  virus  material  is  next  diluted 
with  sterile  salt  solution  and  used  to  vaccinate  rabbits;  the  virus  solution 
being  rubbed  into  the  skin  from  which  the  hair  has  just  been  shaved.  In  a 
few  days  the  human-bovine-rabbit  virus  is  collected  and  mixed  in  the 
proper  proportions  with  a  sterile  water  solution  of  glycerin,  together  with  a 
minute  amount  of  carbolic  acid.  At  this  stage,  the  material  is  known  as 
seed  vaccine,  but  it  is  still  not  used  for  human  inoculation.  For  the  final 
animal  passage,  young  female  calves,  which  have  been  under  observation  for 
some  time  in  order  to  be  sure  of  their  freedom  from  disease,  are  taken.  The 
hair  is  closely  clipped  from  the  flanks  and  abdominal  regions,  and  these  areas 
thoroughly  washed  until  the  skin  is  sterile.  Now  the  seed  vaccine  is  used  to 
inoculate  about  100  tiny  areas  in  the  prepared  skin.  In  a  few  days,  pustules 
containing  human-bovine-rabbit-bovine  smallpox  virus  will  develop  in  each 
inoculated  area.  The  calf  is  then  killed  and  the  smallpox  vaccine  collected 
under  the  most  rigid  aseptic  conditions.  After  mixing  with  glycerin,  the 
vaccine  is  allowed  to  stand  for  some  time  until  exact  tests  have  been  made  to 
determine  its  purity  and  strength. 

These  necessarily  elaborate  methods  result  in  the  production  of  smallpox 
vaccine  that  contains  living  smallpox  virus  of  reduced  virulence  and  suitable 
for  human  inoculation.  Implanted  in  a  minute  area  of  the  human  skin, 
the  smallpox  vaccine  produces  a  highly  inflamed,  but  localized,  reaction. 
The  inflammatory  reaction  results  in  antibody  formation  that  destroys  the 
implanted  virus  and  also  renders  the  individual  immune  to  smallpox  for  a 
considerable  period. 

Spontaneous  Generation.     See  Biogenesis. 

Staining.     See  Histology. 

Starch.  "The  higher  carbohydrates — the  polysaccharides  or  nonsugars — 
are  mostly  insoluble  in  water,  but  they  take  up  water  readily  and  form  pastes 
and  jellies;  they  are  therefore  colloidal. 

"The  starches  are  sometimes  referred  to  as  amyloses  and,  together  with  the 
celluloses,  as  hexosans,  because  on  hydrolysis  they  yield  hexose  sugars: 

(CeHioOs),  +  (H20)x  =  xC6H1206 

Starch  is  one  of  the  most  widely  distributed  of  substances  in  the  vegetable 
kingdom ;  it  is  the  chief  storage  food  of  plants  and  may  constitute  70  per  cent 
of  the  dry  weight  of  seed.  The  structure  of  the  starch  grain,  as  it  occurs  in 
the  plant,  is  very  characteristic  and  is  used  as  a  means  of  identification. 
Its  chief  distinguishing  feature  is  its  layered,  or  lamellar,  structure.  Starch 
is  of  biological  importance  because  of  its  nutritive  qualities,  its  extraordinarily 
high  imbibition  pressure,  and  its  paste-forming  qualities.  Whether  or  not 
the  imbibition  pressure  of  starch  is  in  part  responsible,  as  has  been  maintained, 
for  the  carrying  of  water  to  the  tops  of  trees  cannot  be  said,  but  it  certainly 
plays  a  part  in  bringing  water  into  the  cell.  The  gelatinous  properties  of 
starch  may,  to  a  great  extent,  be  responsible  for  the  highly  viscous  properties 
of  protoplasm.  Starch  paste  has  some  of  the  properties  of  a  true  elastic 
jelly  and  some  of  those  of  a  plastic  mass,  but  much  of  the  viscous,  glutinous, 


574  HUMAN  BIOLOGY 

and  elastic  properties  that  one  might  be  inclined  to  attribute  to  starch,  for 
example,  in  such  substances  as  bread  dough,  are  in  great  measure  due  to 
associated  matter.  Gluten  comprises  10  per  cent  of  wheat.  When  flour 
is  freed  of  starch,  gluten  remains  behind  as  a  tenacious  sticky  mass.  It  is 
less  abundant  in  foods  than  is  starch  but  an  equally  valuable  foodstuff. 

Another  amylose  is  dextrin;  it  is  an  intermediate  product  between  starch 
and  glucose.  Some  of  the  so-called  "  soluble"  starches  are  probably  dextrins. 
They  are  not  abundant  in  plants.  Dextrin  is  used  as  a  substitute  for  gum. 

Glycogen,  or  animal  starch,  occurs  rarely  in  plants — in  only  a  few  of  the 
fungi.  It  has  risen  to  great  prominence  of' late  as  the  fuel  for  muscular 
action,  though  it  has  long  been  recognized  as.  a  substance  of  great  physiological 
importance,  especially  in  the  liver  where  formerly  it  was  thought  to  exist 
simply  as  stored  excess  carbohydrate  but  now  is  viewed  dynamically,  that  is 
to  say,  as  fuel  for  energy."  (Seifriz,  "Protoplasm,"  pp.  456-457,  McGraw- 
Hill  Book  Company,  Inc.,  New  York,  1936.) 

Sterols.  The  word  sterol  means  solid  alcohol.  The  sterols  are  widely 
distributed  in  nature.  Because  their  solubilities  are  similar  to  those  of  fats, 
they  were  formerly  classified  as  lipoids.  But  they  are  really  alcohols  and  not 
chemically  related  to  fats.  Cholesterol,  formerly  called  cholesterin,  is  the 
best  known  member  of  the  group  of  sterols.  Its  formula,  C27H45OH,  shows 
one  —OH  group  indicative  of  an  alcohol.  It  is  related  in  its  chemical  struc- 
ture to  many  substances  of  biological  interest,  including  bile  acids,  certain 
hormones  and  vitamins.  It  resists  chemical  agents,  save  concentrated  mineral 
acid  or  powerful  oxidizing  reagents.  It  is  a  very  stable  substance. 

Cholesterol  appears  to  occur  in  every  animal  cell,  also  in  blood,  lymph, 
and  bile.  It  occurs  abundantly  in  gallstones  which  are  usually  the  result  of 
crystallization  of  cholesterol  from  the  bile.  (Mitchell,  "Textbook  of  General 
Physiology,"  p.  259,  McGraw-Hill  Book  Company,  Inc.,  New  York, 
1938.) 

Suprarenals.     See  Adrenal  Glands. 

Sucrose  (Cane  Sugar).  "Sucrose  occurs  commonly  in  the  vegetable 
kingdom,  being  found  in  considerable  quantity  in  many  familiar  fruits  and 
vegetables.  Usually  these  sweet  fruits  and  plant  juices  contain  glucose  and 
fructose  along  with  the  sucrose,  and  also  other  substances  which  make  it 
difficult  to  separate  the  sucrose  in  crystalline  form.  The  juices  of  the  sugar 
cane,  the  sugar  beet,  and  to  a  less  extent  certain  maple  and  palm  trees,  con- 
tain enough  sucrose  and  little  enough  of  other  substances  to  make  it  practica- 
ble to  manufacture  sugar  from  them  commercially.  On  hydrolysis  a  molecule 
of  sucrose  yields  one  molecule  each  of  glucose  and  fructose.  The  process  is 
often  called  ' inversion'  and  the  product  'invert  sugar/  When  eaten,  sucrose 
is  digested  into  glucose  and  fructose.  .  .  .  (Sherman  "Food  Products,"  p.  9, 
The  Macmillan  Company,  New  York,  1926.) 

Syphilis — Historical.  "Few  diseases  mean  more  to  the  human  race  as  a 
whole  than  syphilis,  owing  in  part  to  its  almost  universal  distribution  and  in 
part  to  its  insidious  and  deceiving  course,  thereby  leading  to  untold  misery 
and  disaster.  Rosenau  says  'civilization  and  syphilization  have  been  close 


APPENDIX  575 

companions';  the  one  has  followed  in  the  wake  of  the  other  like  the  guerillas 
behind  an  army.  Unlike  most  diseases,  syphilis  is  one  of  whose  origin  among 
civilized  nations  we  have  strong  evidence.  There  are  many  reasons  for 
believing  that  syphilis  was  acquired  by  the  members  of  Columbus'  crew  when 
they  discovered  the  island  of  Haiti  and  that  it  was  carried  back  to  Spain  by 
them  on  their  return.  These  adventurers  promptly  joined  the  army  of  Charles 
VIII  of  France  in  its  invasion  of  Italy  in  1494.  Soon  after  the  army  had 
triumphantly  set  up  a  court  in  Naples,  it  became  weakened  through  the 
ravages  of  a  terrible  venereal  disease  of  unusual  intensity,  hitherto  apparently 
unknown  in  Europe.  The  following  year,  the  army  retreated  almost  in  a 
rout  and  was  broken  up,  the  miscellaneous  troops  scattering  all  over  Europe 
to  their  respective  home  countries  and  carrying  the  new  disease  with  them. 
In  the  next  four  years,  the  disease  had  spread  to  practically  every  country 
in  Europe  and  was  soon  carried  by  the  Portuguese  to  Africa  and  the  Orient. 
The  venereal  nature  of  the  disease  was  fully  recognized,  and  its  foreign  origin 
was  well  known,  each  nation  trying  to  shift  the  responsibility  to  another  by 
name,  many  peoples  calling  it  the  'French  disease/  others  the  'Spanish 
disease/  etc.,  whereas  the  Spanish  alone  seemed  aware  of  its  real  origin  in 
America  and  called  it  espanola  which  then  meant  Haiti.  The  absence  of 
any  reference  to  a  disease  resembling  syphilis  in  the  historical  records  before 
the  discovery  of  America;  the  absence  of  any  bones  showing  evidence  of 
syphilitic  attack  in  the  abundant  pre-Columbian  remains  in  Europe,  and 
abundance  of  such  bones  in  American  remains,  many  of  which  must  certainly 
be  pre-Columbian;  the  positive  evidence  of  Spanish  physicians  and  historians 
at  the  time  of  the  return  of  Columbus ;  and  the  severity  of  the  great  epidemic 
in  the  latter  part  of  the  fifteenth  century — it  being  almost  invariable  for  an 
infectious  disease,  when  first  introduced  among  a  new  people,  to  rage  with 
unwonted  severity;  all  these  facts  point  strongly  to  the  American  origin  of 
syphilis. 

"  Interesting  as  is  the  early  history  of  the  disease,  the  recent  history  is 
infinitely  more  so.  By  the  beginning  of  the  twentieth  century,  medical  men 
had  come  to  the  end  of  their  rope  in  knowledge  and  treatment  of  the  disease 
and  found  themselves  at  a  standstill.  But,  in  1902,  the  disease  was  success- 
fully trarpmitted  to  animals  where  it  could  be  conveniently  studied;  in  1905, 
Schaudinn  discovered  the  causative  organism,  Treponema  pallidum,  which  is 
believed  to  cause  the  disease.  In  1906,  Wassermann  demonstrated  the  possi- 
bility of  detecting  latent  syphilis  by  the  reaction  that  bears  his  name;  in  1910, 
Ehrlich  made  the  epoch-making  discovery  of  his  famous  drug,  'No.  606/ 
or  Salvarsan,  a  deadly  poison  for  spirochetes  of  all  kinds  and  a  cure  for  syphilis 
in  nearly  all  stages;  in  1913,  the  direct  relation  of  syphilis  to  insanity,  paraly- 
sis, and  other  diseased  conditions  of  the  central  nervous  system  was  demon- 
strated by  the  discovery  of  the  organisms  in  the  cerebrospinal  fluid;  and  in 
the  same  year,  a  method  of  destroying  the  parasites  in  the  central  nervous 
system  was  discovered.  There  is  no  other  instance  in  the  history  of  medical 
science  where  such  wonderful  strides  have  been  made  in  such  a  short  time 
in  the  knowledge  and  control  of  a  disease.  At  the  beginning  of  the  twentieth 


576  HUMAN  BIOLOGY 

century,  syphilis  was  one  of  the  most  horrible,  hopeless,  and  tragic  diseases 
known  to  ravage  the  human  body;  it  is  now  a  disease  that  can  be  readily 
recognized  even  in  latent  stages;  it  can  be  cured  in  its  early  stages;  and  the 
terrible  tragedies  resulting  from  apparent  but  imperfect  cure  can  be  avoided. 
Its  eradication,  however,  will  not  soon,  if  ever,  be  accomplished,  since  in  this 
are  involved  some  of  the  most  intricate  moral  and  social  questions  with  which 
we  have  to  deal."  (Chandler,  "  Animal  Parasites  and  Human  Disease,"  pp. 
48-49,  John  Wiley  &  Sons,  Inc.,  New  York,  1926.  Reprinted  by  permission.) 
See  Complement  Fixation. 

Taxonomy.  "Classification  has  as  its  object  the  bringing  together  of 
things  which  are  alike  and  the  separating  of  those  which  are  unlike.  It  is 
'discrimination,  description,  and  illustration — the  necessary  census  task  which 
forms  the  groundwork  on  which  great  theories  may  be  built  up' — a  problem 
of  no  mean  proportions  when  a  conservative  estimate  today  shows  upwards  of 
a  million  species  of  animals  and  plants,  leaving  out  of  account  the  myriads 
of  forms  represented  only  by  fossil  remains.  Naturally  the  earliest  classifica- 
tions were  utilitarian,  or  more  or  less  physiological:  edible  and  harmful,  useful 
and  useless,  fish  of  the  sea  and  beasts  of  the  earth.  But  as  knowledge 
increased,  emphasis  was  shifted  to  the  anatomical  criterion  of  specific  differ- 
ences and  thenceforth  classification  became  at  once  an  important  aspect  of 
natural  history — a  central  thread  both  practical  and  theoretical.  Practical,  in 
that  it  involved  the  arranging  of  living  forms  so  that  a  working  catalog  was 
formed  which  required  nice  anatomical  discrimination,  and  therefore  the 
amassing  of  a  large  body  of  facts  concerning  animals  and  plants.  Theoretical 
because  in  the  process  botanists  and  zoologists  were  impressed,  almost  uncon- 
sciously at  first,  with  the  'affinity'  of  various  types  of  animals  and  of  plants 
and  so  were  led  to  problems  of  their  origin. 

"From  Aristotle,  who  emphasized  the  grouping  of  organisms  on  the  basis 
of  structural  similarities,  we  must  pass  over  some  seventeen  centuries,  in  which 
the  only  work  of  interest  was  done  by  herbalists  and  encyclopaedists,  to  the 
time  of  Ray  (1628-1705)  of  Cambridge.  As  a  matter  of  fact,  the  Theophras- 
tan  classification  of  plants  as  trees,  shrubs,  and  herbs  persisted  until  the  end  of 
the  seventeenth  century.  Previous  to  Ray  the  term  'species'  was  used  some- 
what indefinitely;  and  his  chief  contribution  was  to  make  the  w,ord  more 
concrete  by  applying  it  solely  to  groups  of  similar  individuals  which  exhibit 
constant  characters  from  generation  to  generation.  Covering,  as  Ray's 
labors  did,  the  classification  of  both  animals  and  plants,  it  is  probably  not  an 
exaggeration  to  regard  him  as  the  seventeenth  century  precursor  of  the  great 
Swedish  taxonomist,  Linnaeus,  for  whom  he  paved  the  way. 

"Like  many  another  genius,  Linnaeus  (1707-1778)  was  a  product  of  his 
time  and,  perhaps,  one  of  the  very  best  examples  of  the  fact  that  '  the  most 
original  people  are  frequently  those  who  are  able  to  borrow  the  most  freely' — 
to  see  a  great  deal  in  what  to  others  appears  commonplace.  Linnaeus  was 
first  and  foremost  a  botanist.  Garnering  much  of  the  best  which  the  past  had 
to  offer  in  taxonomy,  and  bringing  to  bear  on  it  his  supreme  talent  for  '  classi- 
fying, coordinating,  and  subordinating/  Linnaeus  gave  botanical  students 


APPENDIX.  577 

at  once  a  practical  method  of  classification  of  flowering  plants,  based  chiefly 
on  the  number  and  arrangement  of  the  stamens.  At  the  same  time  he 
insisted  on  brief  descriptions  and  the  scheme  of  giving  each  kind  of  organism 
a  name  composed  of  two  words,  in  which  the  second  word  indicates  the  species 
and  the  first,  the  genus,  a  group  of  closely  similar  species.  In  short,  to  name 
an  organism  is  to  classify.  Linnaeus'  success  with  botanical  taxonomy  led 
him  to  extend  the  principles  to  animals  and  even  to  the  so-called  mineral 
kingdom,  the  latter  showing  at  a  glance  his  lack  of  appreciation  of  any  genetic 
relationship  between  species. 

"Indeed,  the  terms  genus  and  species  to  Linnaeus  expressed  a  transcen- 
dental affinity  since  he  believed  that  species,  genera,  and  even  higher  groups 
represented  distinct,  consecutive  thoughts  of  the  Creator.  Accordingly,  the 
ultimate  goal  of  taxonomy  was  to  determine  the  so-called  scala  naturae. 
This  viewpoint  is  somewhat  whimsically  expressed  by  an  old  naturalist  who, 
finding  a  beetle  which  did  not  seem  to  agree  exactly  with  any  species  in  his 
collection,  solved  the  difficulty  by  crushing  the  unorthodox  individual  under 
his  foot.  Thus,  Linnaeus  crystallized  two  dogmas — constancy  and  con- 
tinuity of  species — which  permeated  biology  and  reached,  in  slightly  different 
form,  their  high-water  mark,  indeed  a  reductio  ad  absurdum,  in  Agassiz's 
Essay  on  Classification  a  century  later — as  fate  would  have  it,  just  a  year 
before  Darwin's  Origin  of  Species  appeared. 

" Though  today  Linnaeus'  conception  of  fixity  has-been  replaced  by 
modifiability  of  species,  the  affinity  which  he  recognized  and  expressed  in 
transcendental  terms  has  given  place  to  similarity  based  on  descent,  and  his 
artificial  classifications  have  been  superseded  by  natural  classifications,  which 
express,  or  attempt  to  express,  this  genetic  connection  between  species — 
nevertheless  his  greatest  works,  the  Sy sterna  Naturae  and  Species  Plantarum, 
created  an  epoch  in  biological  history,  and  are  by  common  consent  the  base 
line  of  priority  in  zoological  and  botanical  nomenclature."  (Woodruff,  "The 
Development  of  the  Sciences,"  Chap.  VI.  pp.  230-232,  Yale  University  Press, 
New  Haven,  Conn.,  1923.) 

Thyroid — Historical.  "The  swelling  of  the  thyroid  gland,  commonly 
known  as  goiter,  has  been  familiar  to  physicians  as  well  as  laymen  from  time 
immemorial.  Juvenal  reflected  this  familiarity  in  the  line  Quis  turnidum 
gutter  miratur  in  Alpibus — 'Who  wonders  at  goiter  in  the  Alps'/  Beyond 
the  existence  of  the  glands,  however,  and  their  liability  to  goitrous  swelling, 
the  knowledge  of  the  ancients  did  not  go.  During  the  centuries  of  antiquity, 
numerous  theories  engaged  the  imagination  of  thinking  men,  but  no  one 
descended  to  the  unfashionable  procedure  of  grubbing  for  facts.  Some 
scholars  regarded  the  thyroid  as  a  protective  device  to  keep  the  throat  warm, 
'to  cherish  the  vocal  cords/  Others  ascribed  the  gland  to  the  aesthetic 
impulse  of  the  Creator  who  established  it  for  the  sole  purpose  of  rounding  out 
the  neck  in  a  beautiful  contour.  The  theory  that  gained  most  favorably 
currency  in  the  nineteenth  century  was  that  the  thyroid,  like  the  adrenals 
and  the  thymus,  has  no  significance  except  during  the  stage  of  life  that  precedes 
birth. 


578  HUMAN  BIOLOGY 

"Professor  Schiff  of  Geneva  was  the  first  to  put  this  theory  to  the  serious 
test  of  experiment.  He  removed  the  glands  from  a  series  of  animals,  follow- 
ing which  death  soon  ensued.  This  fact  he  first  communicated  verbally 
to  the  Academy  at  Copenhagen,  then  later  published — in  1858.  But  Schiffs 
observations  made  no  impression  on  contemporary  physiology.  As  Meltzer 
says,  physiologists  at  that  particular  time  had  neither  any  great  interest  in 
purfc  biologic  researches  nor  special  confidence  in  their  results.  The  fashion 
then  was  to  try  to  explain  all  the  phenomena  of  life  in  terms  of  inanimate 
machinery. 

"For  awakening  practical  interest  in  the  functions  of  the  thyroid,  the 
world  is  indebted  to  clinicians  rather  than  physiologists.  In  1873,  Sir  William 
Gull,  the  surgeon,  reported  the  cases  of  five  middle-aged  women  whose 
puffy  faces,  bulky  forms,  and  physical  lethargy  indicated  the  presence  of  a 
common  disease.  Five  years  later,  Ord,  another  British  physician,  who  had 
had  similar  patients  under  observation  for  ten  years  or  more,  performed  a 
post-mortem  examination  on  one  of  the  victims.  He  noted  that  the  thyroid 
gland  was  atrophic  and  that  the  general  puffiness  of  the  external  layer  of 
the  body  was  due  to  the  accumulation  of  mucilaginous  material  in  the  tissues 
under  the  skin.  It  was  this  characteristic  that  caused  him  to  designate  the 
new  disease  as  myxedema  (mucoid  swelling). 

"The  next  step  in  discovery  came  also  from  surgery.  With  the  general 
introduction  of  antisepsis  by  Lister  in  the  beginning  of  the  'seventies,  surgeons 
were  emboldened  to  carry  operative  technic  into  regions  of  the  body  up  till 
then  recognized  by  common  consent  as  inaccessible.  They  now  began  to 
treat  goiters  by  radical  operation.  Rederdin  of  Geneva  reported  on  a  few 
of  these  cases  in  1883.  The  work  received  little  notice,  but,  during  the  same 
year,  Kocher  of  Bern  gave  a  more  extensive  report  that  included  a  discussion 
of  the  after-effects  of  complete  removal  of  the  goitrous  glands.  He  empha- 
sized especially  the  marked  interference  with  nutrition.  In  November  of 
that  same  year,  Semon  called  the  attention  of  the  Clinical  Society  of  London 
to  the  similarity  between  the  symptoms  of  myxedema  and  those  following 
surgical  removal  of  the  thyroids.  He  suggested  that  the  glands  might  be  of 
fundamental  importance  to  life. 

"The  topic  by  this  time  had  become  one  of  keen  interest  among  members 
of  the  medical  profession,  and  Professor  Schiff  was  led  to  repeat  and  extend 
his  earlier  experiments.  He  found  that,  in  dogs,  complete  removal  of  the 
thyroid  was  commonly  followed  by  death  and  that  the  symptoms  in  various 
respects  resembled  those  following  complete  removal  of  goitrous  glands  in  man. 

"A  German,  Bruns,  then  entered  into  the  discussion.  He  had  noted,  in  the 
literature,  a  report  of  a  case  of  a  boy  from  whom  a  goitrous  thyroid  had  been 
perilously  removed  ten  years  before  the  advent  of  the  antiseptic  period. 
He  sought  out  the  subject  and  obtained  confirmation  of  the  growing  conviction 
that  the  thyroid  plays  an  important  role  in  body  metabolism.  The  lad  had 
managed  to  survive  the  operation  and  was  then  nearly  forty  years  old;  but 
in  size  and  appearance,  he  resembled  a  mentally  and  physically  backward 
boy.  In  short,  he  presented  the  typical  picture  of  myxedema. 


APPENDIX  579 

"At  the  end  of  1884,  then,  the  new  knowledge  could  have  been  summarized 
to  this  effect:  Natural  absence  of  the  thyroid  glands  in  adults  causes  the 
disease  myxedema;  in  children,  it  results  in  arrested  growth;  complete  removal 
of  the  norrnal  thyroid  in  animals  results  in  death;  in  children  or  in  adults, 
complete  removal  of  the  glands  is  soon  followed  by  surgical  myxedema  identi- 
cal with  that  occurring  spontaneously  from  thyroid  deficiency.  Clearly 
enough  it  was  evident  that  the  thyroid  gland  is  of  fundamental  importance  to 
the  health — perhaps  even  the  life — of  man  or  animal. 

"Schiff,  the  physiologist,  immediately  brought  forward  still  stronger 
evidence  in  favor  of  this  thesis.  He  found  that  if  one  of  the  lobes  of  the 
thyroid  gland  was  transplanted  into  the  body  cavity  of  an  animal,  it  could 
survive  for  a  long  time  the  removal  of  the  other  lobe — final  proof  that  the 
disturbance  following  the  thyroid  operation  was  due  to  a  lack  of  the  gland 
tissue  and  not  to  general  operative  injury  as  such. 

"In  the  years  that  followed,  numerous  experimental  investigations  were 
made  that  confirmed  the  main  facts  and  added  many  new  details.  It  is 
interesting  that  these  physiologic  investigations  were  mostly  made,  however, 
by  surgeons  rather  than  by  professional  physiologists  whose  special  business 
it  is  to  study  such  problems.  Curiously,  Munk,  the  only  physiologist  to 
enter  the  lists,  is  remembered  now  for  his  share  in  the  work  only  because  of 
the  erroneous  claim  that  he  persistently  supported,  namely,  that  the  results 
following  removal  of  the  thyroid  were  due  merely  to  incompetent  surgery  that 
resulted  in  injury  of  the  important  nerves  coursing  near  the  glands. 

"The  disastrous  effects  of  thyroid  deficiency  having  become  known,  the 
next  logical  step  in  research  was  clearly  enough  seen.  ,  This  was  to  attempt  the 
treatment  of  naturally  occurring  thyroid  deficiency  "by  replacing  the  missing 
tissue.  In  1889,  the  first  case  of  successful  thyroid  grafting  was  reported. 
After  implantation  of  living  thyroid  tissue,  all  symptoms  disappeared  for  a 
considerable  period,  but  ultimately  the  grafts  were  absorbed  and  the  symptoms 
reappeared.  It  seemed  then  that  the  thyroid  graft  amounted  in  effect  merely 
to  the  injection  of  thyroid  material,  and  the  next,  step  was  obviously  to 
employ  simple  injections.  These  proved  to  be  successful  and  might  have 
continued  to  this  day  to  be  the  treatment  of  choice  for  the  symptoms  of 
thyroid  deficiency  had  it  not  soon  been  learned  that  the  administration  of 
gland  substance  by  mouth  is  equally  effective  and  much  less  troublesome. 
This  fact  was  first  reported  by  Fox  in  1892. 

"In  one  short  decade,  then,  more  was  learned  about  the  thyroid  gland  by 
the  methods  of  biologic  research  than  in  all  the  centuries  that  had  gone 
before  when  men  were  content  simply  with  observations  upon  such  patients 
as  passed  before  their  eyes."  (Hoskins,  "The  Tides  of  Life/'  pp.  64-67, 
W.  W.  Norton  &  Company,  Inc.,  New  York,  1933.) 

Tropism.  "When  a  sessile  animal  or  a  fixed  plant  bends  or  grows  in  a 
definite  direction  in  response  to  a  definite  stimulus,  turning,  for  example, 
toward  the  sun,  the  movement  is  called  a  tropism.  This  term  has  been 
extended  to  include  the  definite  oriented  movements  of  motile  organisms. 
If  a  plant  or  animal  turns  or  moves  toward  the  source  of  the  stimulus,  it  is 


580  HUMAN  BIOLOGY 

said  to  show  positive  tropism;  if  it  turns  or  moves  away  from  the  source  of  the 
stimulus,  it  is  said  to  show  negative  tropism.  Positive  heliotropism  or 
phototropism  is  the  tendency  to  turn  or  move  toward  light.  Positive  gal- 
vanotropism  is  the^tendency  to  turn  or  move  toward  the  positive  pole  (anode) 
when  in  the  stream  of  an  electric  current.  Positive  geotropism  is  the  tend- 
ency of  the  roots  of  a  plant  or  of  parts  of  animals  to  grow  or  to  bend  downward 
under  the  influence  of  gravity.  Similar  upward  growth  or  bending  is  negative 
geotropism.  Positive  chemotropism  is  the  tendency  of  an  organism  to  turn 
or  move  toward  the  source  of  a  given  chemical  substance  which  is  diffusing 
from  its  source  into  the  surrounding  medium.  Stereotropism  is  the  tendency 
of  an  organism  to  orient  itself  in  a  certain  definite  way  with  respect  to  solid 
bodies.  Numerous  other  tropisms  have  been  described,  for  example,  rheo- 
tropism  or  orientation  with  respect  to  stream  flow  and  thermotropism  or 
orientation  with  respect  to  a  source  of  radiating  heat  in  the  environ- 
ment; but  as  many  of  these  reactions  can  be  shown  to  be  due  to  other  tropisms 
or  to  behavior  which  is  not  typical  of  a  tropism,  they  will  not  be  discussed 
here.  The  terms,  phototaxis,  chemotaxis,  etc.,  are  sometimes  used  with  the 
same  meanings  as  those  of  the  corresponding  tropisms. 

"Tropisms  afford  an  explanation  for  many  aspects  of  animal  behavior, 
which  the  physiology  of  reflexes  alone  cannot  explain.  In  a  certain  sense,  a 
tropism  is  itself  a  reflex  when  it  occurs  in  an  animal  with  a  central  nervous 
system,  but  differs  from  a  reflex  in  that  it  involves  the  coordinated  action  of 
so  many  reflex  arcs  that  it  may  be  regarded  as  a  reaction  of  the  organism  as  a 
whole.  A  reflex,  on  the  other  hand,  need  not  necessarily  involve  so  many 
different  reflex  arcs  as  a  tropism,  so  that  only  a  segment  or  small  portion  of  an 
animal  responds."  (Mitchell,  "Textbook  of  General  Physiology,"  pp.  128- 
129,  McGraw-Hill  Book  Company,  Inc.,  New  York,  1938.) 

Vaccines.    See  Rabies;  Small  pox. 

Vertebrates  in  General.  The  animal  kingdom  is  commonly  said  to  be 
divided  into  the  INVERTEBRATES  and  the  VERTEBRATES.  The  basic  distinction 
between  these  two  groups  may  be  said  to  be  the  presence  in  the  latter  of  a 
dorsal  supporting  axis,  the  vertebral  column,  which  is  of  paramount  impor- 
tance in  its  relations  to  the  general  supporting  structures  of  the  body  and  in 
the  protection  rendered  to  the  delicate  spinal  cord  of  the  nervous  system. 
The  Vertebrates,  on  the  other  hand,  belong  to  one  phylum,  the  CHORD  ATA. 
This  phylum  also  includes,  in  addition  to  the  important  vertebrate  division, 
a  small  number  of  types  that,  for  the  most  part,  are  aberrant  structurally  but 
possess,  nevertheless,  certain  basic  features  which  seem  to  link  all  of  them 
together.  The  distinctive  chordate  features  may  now  be  noted: 

1.  A  dorsal  supporting  axis,  the  NOTOCHORD,  is  present  either  throughout 
life  or  during  early  development.    The  notochord  is  a  rod-like  structure  which 
lies  dorsal  to  the  alimentary  canal  and  typically  extends  the  entire  length 
of  the  animal,  but  it  is  subject  to  considerable  variation  in  the  different 
ehordate  groups. 

2.  'A  tubular  CENTRAL  NERVOUS  SYSTEM,  which  lies  dorsal  to  the  notochord 
Bind  alimentary  canal,  is  present  either  during  embryonic  development  or 


APPENDIX  581 

throughout  life.  In  the  Invertebrates  that  possess  a  central  nervous  system, 
the  nerve  cord  always  lies  ventral  to  the  alimentary  canal  and  is  a  solid  cord, 
instead  of  a  tube. 

3.  At  some  period  in  their  life  history,  the  Chor dates  typically  possess 
paired  lateral  openings,  GILL  SLITS,  which  connect  the  cavity  of  the  pharynx 
directly  with  the  exterior.  These  openings,  when  functional,  permit  the 
water  taken  in  through  the  mouth  to  pass  over  vascularized  tissues,  which 
are  developed  in  or  near  the  walls  of  the  gill  slits,  and  then  to  the  exterior. 
In  the  aquatic  Chordates,  this  is  essential  in  respiration. 

Four  major  divisions,  or  subphyla,  of  Chordata  are  recognized,  the  first 
three  of  which  are  interesting  from  the  comparative  standpoint,  but  otherwise 
unimportant.  They  are  as  follows: 

A.  ENTEROPNEUSTA.     A  small  group  of  marine  animals,  usually  worm-like 
in  size  and  appearance,  but  which  may  show  great  variation.     The  best  known 
representative  of  the  Enteropneusta  is  Dolichoglossus  which  is  fairly  common. 
It  lives  along  the  shore  embedded  in  the  sand  or  mud  and  secures  its  food  in 
much   the  same  way  as  the  earthworm,  that  is,  by  digesting  the  organic 
material  from  the  debris  that  passes  through  the  tubular  alimentary  canal. 
The  systematists  are  far  from  agreement  as  to  the  taxonomic  position  of  this 
group. 

B.  TUNICATA.     This  subphylum  contains  several  rather  abundant  marine 
organisms  commonly  known  as  the  sea-squirts,  due  to  their  habit  of  ejecting 
a  stream  of  water  when  disturbed.     The  mature  individual  of  this  group  shows 
a  degenerate  condition  as  compared  with  the  larva  but  possesses  many  gill 
slits  and  associated  organs  that  serve  both  for  respiration  and  for  the  capture 
of  food.     The  adult  is  enclosed  by  a  peculiar  tunic  largely  composed  of  cellu- 
lose.    This  is  possibly  the  only  example  in  the  animal  kingdom  of  this  material 
which  is  so  very  abundant  in  plant  tissues. 

C.  LEPTOCABDIA.     Only  one  genus  consisting  of  a  few  marine  species  are 
classified  in  this  subphylum,  but  included  among  these  is  the  important  species 
Branchiostoma  lanceolatus,  more  commonly  known  as  Amphioxus,  or  the 
lancelet.     The  importance  of  Amphioxus  from  the  zoological  aspect  lies  in  its 
possession  throughout  life  of  structural  features  that  seem  to  link  it  with  the 
true  Vertebrates  as  well  as  with  the  Chordates.     It  is  a  small  fish-like  animal 
a  few  inches  in  length  and  is  able  to  dart  about  quite  rapidly  when  disturbed. 
The  adult,  however,  usually  lies  vertically  in  a  sand  burrow  with  only  the 
anterior  end  of  the   body  projecting.     Amphioxus  lacks  a  definite  head, 
jaws,  and  limbs. 

D.  VERTEBKATA.     The  Vertebra ta,  by  far  the  most  important  subphylum 
of  the  Chordates,  including  all  the  familiar  animal  types  such,  for  exam  pie, 
as  the  fishes,  frogs,  snakes,  birds,  rabbits,  and  Man.     Certain  important 
diagnostic  features  of  this  subphylum — in  addition  to  the  three  fundamental 
chordate  characteristics — may  now  be  briefly  noted : 

1.  As  indicated  by  the  term  VERTEBRATE  and  noted  above,  all  these  forms 
possess  a  backbone,  or  vertebral  column.  This  important  supporting  struc- 
ture is  composed,  except  in  the  most  primitive  vertebrates,  of  a  considerable 


582  HUMAN  BIOLOGY 

number  of  bony  segments,  or  VERTEBRAE,  which  develop  in  close  relation  to 
the  unsegmented  notochord  of  the  embryo  and  usually  supplant  it  in  the 
adult.  The  backbone  ends  in  a  postanal  projection,  the  TAIL. 

2.  Vertebrate  animals  possess  an  internal  supporting  skeleton,  the  ENDO- 
SKELETON    composed  essentially  of  living  matter,  and  forming  the  bones, 
tendons,  cartilage,  and  the  very  abundant  connective  tissues.     Vertebrates 
also  have  either  a  partial  or  complete  EXOSKELETON  composed  essentially  of 
nonliving  matter. 

3.  The   vertebrate   appendages   are   restricted   in   number.     There   are 
never  more  than  two  pairs  present,  and  in  many  cases  there  are  less.     Thus 
in  certain  reptiles,  such  as  the  snake,  limbs  are  lacking.     It  appears  that  the 
five-fingered  PENTADACTYL  limb  is  to  be  considered  as  the  basic  vertebrate 
type.     The  entire  series   of  vertebrate  limbs  is   regarded  as  homologous, 
including  those  of  the  horse  which  have  retained  only  one  functional  digit  on 
each  limb. 

4.  The  vertebrate  heart  is  ventral,  and  the  blood  has  a  new  type  of  red 
cell.     The  color  of  these  cells  is  due  to  the  important  respiratory  compound, 
hemoglobin,  which,  in  the  invertebrates,  is  carried  as  a  dissolved  substance 
in  the  blood  plasma,  rather  than  in  specific  cells. 

5.  Reproduction  is  always  sexual.     There  is  also  an  absence  of  herma- 
phroditism  in  the  Vertebrates.     The  abandonment  of  both  asexual  reproduc- 
tion and  hermaphroditism  appears  to  have  been  a  comparatively  recent  step 
in  the  history  of  animal  development. 

We  may  now  note  the  animals  included  in  the  main  divisions  of  the  sub- 
phylum  Vertebrata. 

CYCLOSTOMATA.  This  is  a  small  class  but  nevertheless  contains  a  number 
of  fish-like  species  which  are  noteworthy  because  of  certain  primitive  charac- 
ters that  differ  from  those  of  other  vertebrates.  Thus  (a)  the  notochord 
persists  throughout  life;  (6)  a  cartilaginous 'endoskeleton  develops,  is  func- 
tional, and  never  replaced  by  bone;  (c)  the  circular  mouth  opening,  with  no 
jaws  present,  shows  a  superficial  resemblance  to  that  of  Amphioxus;  (d) 
dorsal  and  caudal  fins  are  present,  but  paired  fins  are  lacking. 

ELASMOBRANCHII.  This  class  includes  a  number  of  species,  some  of  which 
occur  in  great  abundance  in  most  marine  waters.  The  sharks,  dogfish,  and 
rays  (skates)  belong  to  the  Elasmobranchs.  They  show  considerable  advance 
over  the  Cyclostomes.  Thus  (a)  the  notochord  is  segmented,  only  partially 
persistent,  and  cartilaginous  vertebrae  have  arisen;  (6)  a  well-developed 
lower  jaw  is  present  and  possesses  modified  scales  that  serve  as  teeth;  (c) 
two  pairs  of  lateral  fins  are  found. 

The  common  Dogfish,  Squalus  acanthias,  has  been  found  very  satisfactory 
for  laboratory  study  in  comparative  anatomy  as  an  important  example  of  the 
lower  Vertebrates.  Among  the  Rays,  the  Torpedo  is  particularly  noteworthy 
because  of  the  amazing  modification  of  certain  muscles,  lying  in  the  head 
region,  which  permit  them  to  accumulate  charges  of  electrical  energy  sufficient 
to  paralyze  large  animals. 


APPENDIX  583 

PISCES.  This  is  by  far  the  largest  and  most  important  group  of  fish 
including  some  15,000  species  of  the  so-called  bony  fishes,  among  which  are 
the  perch,  cod,  trout,  mackerel,  and  salmon.  Again  considerable  advances 
in  organization  over  both  the  Cyclostomes  and  Elasmobranchs  are  to  be 
noted.  In  fact,  the  Pisces  are  often  referred  to  as  the  true  fishes.  Of  out- 
standing importance  is  the  fact  that,  for  the  first  time,  bone  is  developed 
in  the  endoskeleton.  In  most  species  of  this  class,  the  skeleton  is  almost 
entirely  ossified,  although  in  a  few  the  original  cartilaginous  skeleton  is 
replaced  in  part  only.  As  a  rule,  the  notochord  is  entirely  replaced  by  the 
segmented,  bony  vertebral  column.  The  external  openings  of  the  gill  slits 
are  covered,  on  each  side  of  the  body,  by  a  fold  of  tissue,  the  OPEECULUM. 

AMPHIBIA.  This  vertebrate  class  includes  tailed  forms  (salamanders, 
newts)  and  also  tailless  types  (frogs,  toads).  The  tailed  Amphibia  are  for  the 
most  part  aquatic,  and  the  gills  are  functional  throughout  life  in  some  cases. 
The  frogs  and  toads  are  fish-like  in  the  tadpole  stage;  then  they  metamorphose 
into  air-breathing,  adult  individuals  which  are  different  from  the  tadpole, 
particularly  in  the  absence  of  the  tail  and  the  presence  of  two  pairs  of  penta- 
dactyl  limbs.  With  very  few  exceptions,  the  amphibian  skin  is  smooth  and 
shows  no  exoskeletal  structures,  such  as  the  scales  of  fishes  or  of  reptiles. 
Even  more  noteworthy  are  the  pentadactyl  limbs,  which  mark  a  wide  advance 
over  the  fish  fin,  and  the  development  of  lungs. 

REPTILIA.  In  this  class,  we  find  a  group  of  vertebrate  animals  that  are 
air-breathing  at  all  stages  in  their  life  history.  The  embryonic  gills  never 
function.  The  skin  is  marked  by  a  considerable  development  of  exoskeletal 
structures,  such  as  are  shown  in  the  bony  plates  of  the  turtle  or  the  scaly 
snake  skin.  Undoubtedly,  reptilian  development  reached  its  peak  in  pre- 
historic periods  when  the  living  representatives  included  the  enormous  land- 
living  Dinosaurs  and  related  types. 

Three  important  orders  of  the  Reptilia  are  recognized,  namely: 

1.  The  Testudinata,  consisting  of  the  turtles  and  tortoises; 

2.  The  Crocodilia,  which  includes  the  alligators  and  crocodiles; 

3.  The  Squamata,  represented  by  snakes,  chameleons,  and  lizards. 

AVES.  Since  the  Aves  are  the  only  animals  that  possess  feathers,  this  one 
character  serves  to  differentiate  the  birds  from  all  other  groups.  The  main 
portion  of  a  feather  develops  in  the  dermis  of  the  skin,  and  is  covered  externally 
by  an  epidermal  layer.  Birds  possess  two  pairs  of  limbs,  but  the  fore  limbs 
are  highly  modified  for  flying.  Even  the  most  primitive  fossil  birds  show 
this  important  development,  and  it  persists  throughout  all  species. 

Another  interesting  and  important  features  found  in  this  class  is  the 
maintenance  of  a  uniform  body  temperature  (homothermal),  a  condition  that 
elsewhere  is  found  only  in  the  mammalian  group.  In  all  other  animals,  the 
body  temperature  varies  with  the  environment  (poikilothermal).  Attention 
should  also  be  called  to  the  fact  that  birds  and  mammals  possess  a  four- 


584  HUMAN  BIOLOG? 

chambered  heart.  Although  teeth  can  be  demonstrated  in  certain  fossil 
birds,  they  are  lacking  in  present-day  species.  The  birds  represent  an 
extremely  homogeneous  group,  so  much  so  that  it  is  very  difficult  to  find  a 
sufficient  number  of  differentiating  structural  characters  to  construct  a  satis- 
factory scheme  of  classification  for  the  nearly  20,000  species  that  are  known. 

MAMMALIA.  This  most  important  vertebrate  class  is  characterized  by 
the  development  of  hair  in  the  skin.  Abundant  in  many  species,  where  it 
forms  a  heavy  external  covering,  in  certain  other  types  hair  may  be  con- 
siderably restricted  or  in  an  extreme  case,  such  as  in  certain  whales,  be  entirely 
lacking.  Another  mammalian  characteristic  is  noted  in  the  mammary  glands 
which  form  a  secretion  for  nourishing  the  young  after  birth.  A  constant 
body  temperature  is  maintained  by  all  Mammals. 

Probably  the  greatest  amount  of  external  variation  is  to  be  found  in  the 
structure  of  the  appendages.  These  may  vary -from  two  pairs  of  pentadactyl 
limbs,  as  in  man,  to  the  condition  found  in  the  whale,  where  the  fore  limbs  are 
paddle-shaped  structures,  although  maintaining  the  fundamental  pentadactyl 
arrangement,  and  the  hind  limbs  are  entirely  lacking.  Or  again  a  reduction 
of  digits  may  occur,  as  in  the  horse,  where  only  the  third  digit  of  each  limb  is 
functional.  In  fact,  a  very  complete  series  of  mammalian  appendages 
can  be  arranged  to  show  the  ADAPTIVE  RADIATION  from  a  basic  or  generalized 
type  to  the  highly  specialized  types  in  conformity  to  the  chosen  environ- 
mental conditions. 

Again,  the  exoskeletal  structures  show  great  variation  and  are  used  in 
classifying  this  group.  Thus  we  have  Mammals  with  claws,  or  UNGUICULATA 
(dog);  Mammals  with  hoofs,  or  UNGULATA  (horse);  Mammals  with  nails 
(monkey,  ape,  man).  The  teeth  are  also  important  in  this  connection. 

In  all  Mammals,  except  a  few  of  the  most  primitive  species,  the  fertilized 
egg  is  retained  in  the  body  of  the  mother  for  early  development.  It  is  interest- 
ing to  note  that  in  the  primitive  mammalian  types,  large-yolked  eggs  are  laid 
that  are  very  similar  to  those  of  reptiles  and  birds. 

The  Mammalia  are  commonly  divided  into  two  subclasses,  the  Prototheria 
and  the  Eutheria.  The  former  are  the  egg-laying  mammals.  There  are 
only  a  few  species  known,  notably,  Ornithorhyncus,  the  Duckbill,  and  Echidna, 
the  Spiny  Anteater.  There  are  two  basic  groups  of  Eutheria:  the  more 
primitive  Marsupials  and  the  Placentals.  The  first  named  are  the  so-called 
pouched  mammals.  The  young  are  born  in  a  very  immature  condition 
and  are  carried  for  a  time  by  the  mother  in  a  special  pouch,  the  marsupium, 
present  on  the  ventral  surface  of  the  female  body.  This  is  also  a  very  small 
mammalian  group  with  the  kangaroo  and  the  opossum  as  typical  examples. 
The  Placentals  are  born  in  a  comparatively  high  developed  condition.  This 
is  due  to  the  PLACENTA  in  the  female  which  enables  the  embryo  to  be  retained 
for  a  longer  period,  of  uterine  growth.  Great  importance  attaches  to  the 
placental  group,  and  further  attention  may  now  be  given  to  its  classification. 
Considerable  variation  in  the  arrangement  of  the  mammalian  orders  is  found 
among  the  systematists.  Present  purposes  may  be  served  by  indicating 
nine  orders  as  follows: 


APPENDIX  585 

Order  1.  Insectivora.  This  order  includes  certain  well  known  species,  such 
as  the  mole,  hedgehog,  and  shrew. 

Order  2.  Edentata.  Examples  of  his  class  are  noted  in  such  distinctive 
species  as  the  armadillo,  anteater,  and  sloth. 

Order  3.  Chiroptera.  The  bats  which  constitute  this  order  are  charac- 
terized by  a  modification  of  the  fore  limbs  which  adapt  them  for  flight. 

Order  4.  Rodentia.  This  is  the  largest  mammalian  order  in  the  number 
of  species  included.  Also  the  number  of  individuals  and  the  extent  of  the 
geographical  distribution  of  certain  species  are  extraordinary,  for  example, 
the  mouse  and  the  rat.  Rodents  are  characterized  as  the  gnawing  mammals. 
They  possess  one  or  two  pairs  of  long  incisor  teeth  particularly  adapted  for 
this  purpose.  Claws  are  present  on  the  digits.  The  destructiveness,  high 
fertility,  and  disease-carrying  ability  of  certain  species  make  them  pests  of 
the  first  rank.  All  things  considered,  the  rat  is  probably  the  most  destructive 
and  dangerous  animal  with  which  we  have  to  contend.  Additional  examples 
of  rodents  are  found  in  the  squirrel,  rabbit,  guinea  pig,  beaver,  gopher,  and 
porcupine. 

Order  5.  Carnivora.  The  Carnivores  are  typically  characterized  as  the 
flesh-eating  mammals  and  possess  teeth  adapted  for  tearing  animal  tissues. 
On  the  whole,  they  are  fairly  large,  clawed  animals  with  a  heavy  coat  of  fur 
which  is  frequently  of  considerable  commercial  value.  The  Carnivora  are 
clearly  divided  into  the  terrestrial  forms,  a  few  important  examples  of  which 
may  be  noted  in  the  dog,  wolf,  cat/ lion,  tiger,  bear,  raccoon,  mink,  and  into 
the  aquatic  types,  such  as  the  seal,  sea-lion,  and  walrus. 

Order  6.  Cetacea.  This  is  a  rather  small  order  of  exclusively  marine  ani- 
mals and  includes  the  whale,  porpoise,  and  dolphin.  The  largest  living 
animal,  the  Sulphur-bottom  Whale,  may  reach  a  length  of  nearly  100  ft.  and  a 
weight  close  to  300,000  Ib. 

Order  7.  Ungulata.  For  ages  Man  has  found  some  of  his  most  important 
animal  allies  among  the  Ungulates,  including  such  almost  indispensable 
species  as  horses,  cattle,  sheep,  hogs,  and  camels.  These  and  other  species 
Have  long  since  been  domesticated  to  provide  constant  supplies  of  animal 
food,  materials  for  clothing,  and  transportation.  We  can  characterize  the 
Ungulata  as  the  hoofed  mammals  and  divide  them  into  the  even-toed  and  odd- 
toed  types.  Thus  the  cow,  pig,  and  camel  may  be  given  as  examples  of  the 
even-toed  Ungulates,  while  the  horse,  rhinoceros,  and  elephant  are  examples 
of  the  odd-toed.  Among  the  outstanding  structural  features  of  economic 
importance  are  the  character  of  the  flesh  of  certain  species  which  makes  it 
desirable  for  human  consumption;  the  character  of  the  skin,  particularly  in 
cattle,  which  makes  it  suitable  k>  tan  for  leather;  and,  finally,  the  mammary 
glands  of  a  few  species  which  provide  a  supply  of  milk  for  human  nutrition. 

Order  8.  Sirenia.  A  small,  relatively  unimportant  order  of  aquatic 
Mammals,  including  the  Manatee  and  Dugong,  which  appears  to  be  closely 
related  to  the  Ungulates. 

Order  9.  Primates.  This  final  order  of  Mammals,  which  includes  Man,  is 
primarily  characterized  by  the  great  development  of  the  brain;  a  feature 


586  HUMAN  BIOLOGY 

generally  regarded  as  being  of  sufficient  importance  to  make  it  necessary  to 
place  this  group  as  the  highest  order  of  the  Mammals,  even  though  in  various 
other  features,  such  as  the  development  of  the  muscular  tissue,  character  of 
teeth,  and  condition  of  young  at  birth,  the  Primates  are  less  advanced  than 
certain  other  orders,  particularly  the  Ungulates.  Other  characteristics  are 
noted  in  the  digits,  which  bear  nails  rather  than  claws  or  hoofs,  and  also  in 
that  the  first  digits  (toe  or  thumb)  are  opposable  (one  or  both)  to  the  other 
digits.  The  primate  appendages  are  primarily  adapted  for  grasping,  a  func- 
tion that  corresponds  to  the  arboreal  habitat  of  the  great  majority  of  species. 

The  Primates  may  be  divided  into  two  suborders  on  the  basis  of  a  compara- 
tively minor  structural  feature,  namely,  the  separation  or  contact  of  the 
front  teeth  in  the  anterior  median  line.  Thus  in  the  suborder  Lemuroidea 
the  teeth  are  separated,  while  in  the  suborder  Anthropoidea  the  teeth  are  in 
contact. 

The  Anthropoidea  includes  the  tailed  monkeys,  with  numerous  species  in 
South  America  and  various  regions  in  the  Old  World,  and  the  short-tailed 
anthropoid  Apes,  represented  by>  the  gibbon,  orang-utan,  chimpanzee,  and  the 
rare  gorilla.  Man  is  classified  as  a  separate  family  (Hominidae)  of  the 
Anthropoidea.  Only  one  species,  Homo  sapiens,  is  recognized  at  present. 
The  anthropoid  Apes  are  regarded  as  the  closest  structurally  to  Man.  This 
is  based  on  such  features  as  the  absence  of  a  tail,  the  frequent  occurrence  of 
bipedal  locomotion,  the  very  high  degree  of  intelligence,  and  the  almost 
human  facial  structure  and  expression  due  to  the  enlarged  cranial  bones  and 
reduced  facial  bones. 

In  Man,  the  bipedal  locomotion  is  universal,  the  big  toes  are  not  opposable 
to  the  other  digits,  the  formation  of  hair  is  not  so  abundant,  and,  above  all, 
the  tremendous  development  in  the  size  and  quality  of  the  forebrain  has  given 
a  mental  superiority  that  far  transcends  that  of  any  other  living  organism. 
The  superior  mental  equipment  of  Man  has  enabled  him  to  dominate  other 
types  of  living  organisms  and  to  surmount  highly  diverse  climatic  conditions 
so  that  his  distribution  is  world-wide.  (Baitsell,  "  Manual  of  Biology," 
selected  from  pp.  253-267,  The  Macmillan  Company,  New  York,  1936.) 

Viscosity.  "  Viscosity  may  be  roughly  defined  as  the  resistance  of  matter 
in  the  liquid  or  semiliquid  state  to  change  in  shape.  It  is  usually  measured 
by  the  time  required  for  the  passage  of  a  standard  volume  of  the  liquid 
through  a  narrow-bore  tube  under  standard  conditions  of  temperature  and 
pressure.  Viscosity  is  really  the  internal  friction  of  a  liquid,  its  resistance  to 
flowing  or  to  shearing  stresses.  It  may  thus  involve  not  only  the  mutual 
attraction,  cohesion,  of  molecules  but  also  their  tendency  to  maintain  a 
certain  arrangement  or  "  pattern,"  that  is,%their  tendency  to  orient  them- 
selves with  respect  to  one  another. 

"Although  the  usual  type  of  viscosimeter  is  a  narrow  tube,  other  forms  are 
in  use.  One  of  them  measures  the  torsion  of  a  wire  that  suspends  a  cylinder 
immersed  in  the  liquid  to  be  measured  while  the  latter  is  kept  rotating  at 
constant  speed.  The  viscosity  of  the  liquid  causes  friction  as  it  rotates  around 
the  cylinder  so  as  to  drag  it  along.  The  resulting  torsion  in  the  suspending 


APPENDIX  587 

wire,  observed  in  angular  degrees,  can  be  converted  by  the  use  of  an  equation 
into  viscosity  units."  (Mitchell,  "Textbook  of  General  Physiology,"  pp. 
392-393,  McGraw-Hill  Book  Company,  Inc.,  New  York,  1938.) 

Volvox.  Colony  formation  is  widespread  among  the  flagellated  organisms. 
This  phenomenon  reaches  its  climax  in  the  beautiful  fresh-water  form  Volvox, 
the  spherical  body  of  which  consists  of  several  thousand  attached  cells,  very 
similar  to  those  in  various  unicellular  flagellates  in  their  structural  features. 
By  the  botanist,  Volvox  is  classified  as  the  highest  of  the  colonial  Green  Algae, 
whereas  the  zoologists  generally  place  it  among  the  Mastigophora  and  regard 
it  as  the  most  plant-like  of  the  colonial  Phytomastigina.  Our  interest  in 
Volvox  lies  in  the  fact  that  it  represents  a  primitive  type  of  multicellular 
organism  in  which  the  constituent  cells  have  become  somewhat  dependent 
upon  each  other  and  which  also  exhibits  a  certain  amount  of  intercellular 
differentiation  in  that  specialized  reproductive  cells,  which  are  unlike  the 
normal  body  cells,  are  developed  in  the  mature  colonies.  Thus  Volvox  may 
be  said  to  represent  the  beginnings  of  the  true  multicellular  organism  with 
slight  indications  of  the  cellular  dependence  and  specialization  so  prominently 
shown  in  all  the  higher  plant  or  animal  forms. 

In  the  true  multicellular  plants  and  animals,  Metaphyta  and  Metazoa, 
all  of  which  in  an  early  stage  are  essentially  colonies  of  undifferentiated  cells, 
more  and  more  intercellular  specialization  takes  place  as  the  organisms 
gradually  attain  maturity.  This  process,  of  course,  results  in  a  division  of 
labor  between  the  cells,  so  that,  in  the  mature  organism,  the  various  cellular 
groups  do  not  perform  all  the  functions  essential  to  the  life  of  the  organism 
but  only  the  particular  functions  for  which  they  are  structurally  adapted. 

Volvox  is  large  enough  to  be  seen  with  the  naked  eye.  It  appears  as  a 
small,  green,  hollow  sphere,  the  wall  of  which  is  composed  of  some  10  or  12 
thousand  microscopic,  flagellated,  chlorophyll-bearing  cells,  arranged  in  a 
single  layer  and  surrounded  by  a  transparent,  gelatinous,  intercellular  mate- 
rial, the  MATRIX.  The  latter  is  formed  as  a  cellular  secretion  and  serves 
apparently  to  hold  the  cells  of  the  colony  together.  (Baitsell,  "Manual  of 
Biology,"  pp.  63-64,  The  Macmillan  Company,  New  York,  1936.) 

Wassermann  Test.     See  Complement-Fixation. 

Water.     See  Dissociation. 


INDEX 

Boldface  numbers  indicate  pages  on  which  illustrations  appear 


Abdominal  region,  197 
Abdominal  viscera,  146 
Abducens  nerve,  261,  519 
Abductor  muscle,  31 
Aberration,  338 

chromosomal,  389,  391,  422 
Ability,  inheritance  of,  399 
Abiogenesis,  495 
Absorption,  and  digestion,  61-66 
Acapnia,  80 
Accommodation,  233 
Acetabulum,  201,  204 
Acetylcholine,  275,  495 
Achondroplast,  113 
Acid,  amino,  70 

ascorbic,  60 

hydrochloric,  49 

nicotinic,  59 

Acromegaly,  113,  491,  566 
Actinozoa,  514 
Action  current,  272 
Active  immunity,  471 
Adaptation,  14,  277 
Adaptive  radiation,  584 
Addison's  disease,  110 
Adductor  muscles,  30,  174 
Adipose  tissue,  27,  28,  194 
Adjuster  neurons,  31,  243,  276 
Adrenal  glands,  92,  109-111 

cortex,  110 

historical,  496 
Adrenaline  (see  Adrenin) 
Adrenin,  111 

autonomic  system,  111 

formula,  irt 
Adrenosterone,  111 
Aerobes,  497 

bacteria,  73  ' 
Afferent  fibers,  240 


Agglutination,  409,  472,  481 

reaction,  482 
Agglutinin,  472,  481-484 
Agglutinin  reactions,  472 
Agglutinin  tests,  blood  transfusion,  482 
Air,  complemental,  79 

expired,  80 

gases,  79,  80 

inspired,  80 

residual,  79 

tidal,  79 

Albino  (ism),  398,  401 
Albuginea  testis,  316 
Albumin,  302,  304 

serum,  138 
Alcohol,  533 
Alexin,  484 
Alga,  455 

cells,  454 

green,  587 

kelp,  133 
Alimentary  canal,  299 

regions  of,  42 

tract,  172 
Alkaptonuria,  399 
AUantois,  307,  313 
Alleles  (see  Allelomorphs) 
Allelomorphs,  358,  413 
Allergy,  476,  477 

tests  for,  477 
Alpine  race,  415 

Alternation  of  generations,  285,  498 
Alternative  inheritance,  358,  361,  406 
Alveola,  78 

pancreas,  103 

sac,  78 

tooth,  199 
Amblystoma,  217 
Amidases,  539 
Amine  group,  57 
Amino  acid,  70,  441,  498,  537,  539,  568 


589 


590 


HUMAN  BIOLOGY 


Ammopolypeptidases,  537 
Ammonia,  130,  431 
Amniocardiac  vesicle,  305 
Amnion,  307,  311,  313,  321 
Amniotic  cavity,  307,  310 
Amoeba,  8,  18,  21,  498 

asexual  reproduction,  282 

binary  fission,  282 
Amphibia,  583 

development,  291-301 

heart,  140 

Amphimixis,  288,  343,  350 
Amphioxus,  581 
Amplexus,  289,  293 
Ampulla,  224 
Amylase,  64,  440,  538 

salivary  (see  Ptyalin) 
Amyloses,  573 
Anabolism,  9 
Anaerobes,  497 
Anaerobic  bacteria,  73 
Anaphase,  328,  330,  350 
Anaphylaxis,  476 
Anatomy,  comparative,  516 
Ancon  sheep,  422,  423 
Andalusian  fowls,  371 
Androsterone,  111 
Angina  pectoris,  152 
Angstrom  unit,  554 
Anhydrase,  carbonic,  541 
Anisotropic,  178 
Anopheles  mosquito,  566 
Anteater,  308 
Anterior  chamber,  233 
Anterior  fissure,  252 
Anthrax,  18 
Anthropoidea,  586 

Antibody,  166,  409,  469,  471-474,  480- 
Antigen,  409,  473,  484 
Antitoxin,  472,  475,  480 
Anur aphis,  451 
Anus,  54,  317 

frog,  298 

Aorta,  118,  142,  143,  145,  146,  299 
Aphid,  451 

ant-cow,  452 

Apochromatic  objectives,  556 
Appendages,  vertebrate,   comparative, 

202 
Appendicular  skeleton,  200-209 


Appendicular  vertebrate,  195 
Appendix,  48,  53,  54 

human,  499 

ruptured,  563 
Appetite,  270 
Aquarium,  balanced,  443 
Aqueduct  of  Sylvius,  267 
Arachnoid,  267 
Arborization,  218,  251 
Archenteron,  296 

Arctic  Bear  Island,  food  chains,  444 
Arcuate  artery,  118,  124 
Arcuate  vein,  118,  124 
Area  pellucida,  305 
Area  vasculosa,  305 
Arginase,  122 
Arginine,  122 

Aristotle,  499,  558,  559,  564 
Arm,  173,  201,  202 
Arteriolae  rectae  spuriae,  124 
Artery,  146,  148 

arcuate,  118,  124 

carotid,  143,  149 

coeliac,  146 

hepatic,  98 

iliac,  149 

innominate,  149 

renal,  118 

structure  of,  144 

subclavian,  143,  149 
Arthropoda,  461,  500 
Ascaris,  342 

mitosis  in,  13 

Ascidian,  egg  fertilization,  350 
Ascorbic  acid,  60 
Asexual  generations,  498 
Asexual  phase,  566 
Asexual  reproduction,  283-286 

amoeba,  282 

flat  worm,  284 

yeast,  282 

Aspergillus  niger,  533 
Ass,  414 
Association,  commensal,  450—454 

commynal,  447-450 

fiber  tracts,  263 

fibers,  277 

neurons,  254,  276 

parasitic,  456-466 

symbiotic,  454-456 


INDEX 


591 


Aster,  329 
Astigmatism,  234 
Atlas,  205 

Atom,  523,  552,  553,  562 
Atrophy,  489,  491,  492 

Gower's,  399 
Auditory  canal,  222 
Auditory  cells,  18 
Auditory  meatus,  229 
'Auditory  nerve,  223,  229,  261,  520 
Auditory  ossicles,  222 
Auditory  tracts,  265 
Auerbach  plexus,  51 
Auricle,  141,  143 
Aoiriculo-ventricular  bundle,  154 
Auriculo-ventricular  node,  154 
Autonomic   nervous  system,   243-247, 
244,  245,  270 

adrenin,  111 

Autotrophic  organisms,  429-435 
Aves,  583 

Avoiding  reaction,  Paramecium,  278 
Axial  skeleton,  196-200 
Axial  vertebrate,  195 
Axis,  205 
Axon,  32,  242,  250 

medullated,  251 

nerve,  251 

sensory,  276 


Babesia,  567 
Bacillus  coli,  436 
Bacillus  subtilis,  436 
Bacillus,  typhoid,  481 
Backbone,  196 
Bacon,  559 
Bacteria,  12 

aerobic,  73 

anaerobic,  73 

killed  cultures,  478 

nitrate,  431 

nitrifying,  431 

nitrite,  431 

nitrogen-fixing,  455 

sulphur,  430,  431 

types  of,  436 
Bacteriolysins,'  473 
Bacteriolysis,  485 


Bacteriophage,  466 
Balanced  aquarium,  443 
Baldness,  hereditary,  402 

pattern  of,  402 

Ball  and  socket  joint,  203,  204 
Basal  cell,  220 
Basal  disc,  42 

Basal  metabolic  rate,  85-89,  86,  88 
Basilar  artery,  261 
Basilar  membrane,  226 
Beans,  pure  line  breeding,  376 
Beaumont,  William,  501 
Bee,  honey,  448-450 
Beetle,  chromosomes,  334 
Benedict,  F.  G.,  87 
Benzoic  acid,  131 
Beriberi,  59 
Berzelius,  513 
Bestiaries,  505 
Biceps  muscle,  168,  173,  174 
Bicuspid  valve,  143,  144,  151 
Bicycle-ergometer,  185 
Bilateral  symmetry,  34,  526 
Bile,  53,  99 
Bile  duct,  465 
Bile  salts,  65,  99,  538 
Bilirubin,  100,  122 
Binary  fission,  281,  282,  498 
Biogenesis,  502 
Biological  elements,  7,  505 
Biological  sciences,  503 
Biology,  definition,  3 

divisions  of,  503 

Biometry,  principles  of,  406-409 
Bio-osmotic  pressure,  521 
Biotic  environment,  447-466 
Biparental  inheritance,  350,  352 
Bipedal  locomotion,  211,  212 
Bipolar  cell,  eye,  236,  237 
Bird,  crop,  47 

egg,  301 
,  feathers,  190 

gizzard,  47 

heart,  140 

wing,  202 

Black  fly,  giant  chromosome,  338 
Bladder,  118,  123,  146,  317 

capacity,  131 
Blastocoel,  294 

chick,  304 


592 


HUMAN  BIOLOGY 


Blastoderm,  302,  303,  304 
Blastopore,  295 
Blastula,  294 
chick,  304 
Bleeding,  383 

Blending  inheritance,  368-372,  406 
genotype,  370 
man,  398 
phenotype,  370 
snapdragon,  369 
Blind  spot,  237 
Blindness,  color,  399 

night,  398,  399 
Blood,  29,  136-140 
cell,  29,  137-139 
formation,  212,  213 
frog,  18 
number,  138 
hemoglobin,  138 
centrifuged,  136 
circulation,    with    all    main    vessels 

labelled,  148 
clotting,  61 

chemical  reactions,  164 
fibrin,  476 
serum,  476 

coagulation,  29,  163-166 
counts,  139 
donor,  483 
fibrin,  489 
gases  in,  136 
groups,  409-413 
determination,  410 
gametes,  411 
genotypes,  411 

inheritance  of,  409 

patterns,  412 
loss  and  gain,  127 
plasma,  135-137 

solids,  136 
platelets,  139 
pressure,  155-158,  508 
determination,  157 
normal,  157 
proteins,  137 
serum,  474 

transfusion,  tests  for,  482 
transportation  in,  158 
types  of,  482,  483 
uniformity,  161 


Blood,  variation,  162 

vessel,  140-147,  172 
fetal,  311 

water-plasma  relations,  129 
Body,  functions  and  hormones,    111- 
117 

human,  divisions  of,  35 

number  of  cells,  19 

plan  of  vertebrate,  34 

stalk,  321 

wall,  earthworm,  43 
Bolus,  46 
Bones,  27,  28 

and  acid,  208 

burned,  208 

carpal,  201 

cartilage,  28,  205 

cells,  18 

development  of,  205 

fragility,  398 

histology  of,  206-208 

levers,  210,  211 

marrow,  28,  192,  205,  213,  218 

membrane,  28,  204,  205 

metacarpal,  201 

metatarsal,  201 

mineral  reserves,  213 

tissue,  193 

spongy,  206 

Bony  labyrinth,  223,  229 
Bounty,  mutiny,  416 
Boussingault,  565 
Bowman's  capsule,  124 
Brachial  plexus,  214 
Brachydactyly,  398,  406 
Brain,  34,  257-268,  258,  263,  264,  265 

case,  198 

cells,  14 

chick,  307 

divisions,  269 

gray  matter,  266 

male,  average  weight,  259 

stem,  257 

under  surface,  261 

ventricles,  267 

white  matter,  266 
Bread  mold  (Rhizopus  nigricans),  286, 

436 

Break-shock,  273 
Breasts,  324 


INDEX 


593 


Breathing,  81-84 

control,  83 

rate,  80 

Breeders,  practical,  376 
Breeding,  pure  line,  376 

selective,  426 
Bronchi,  77 
Bronchioles,  77,  78 
Brown,  Robert,  609 
Brownian  movement,  509 
Bruns,  578 

Bryce-Teacher,  embryo,  310 
Budding,  283 

Hydra,  42,  283 

mutations,  390 
Buffering,  161 
Buffon,  559 

"Histoire  Naturelle,"  559 
Bulldog,  hybrids,  417,  418 
Bundle,  auriculo- ventricular,  154 
Bursa,  176 
Butter,  4,  557 


Caecum,  48,  50,  53,  499 
Caffein,  126 
Calcaneum,  201 
Calciferol,  60 
Calcium,  193 

metabolism,  109,  213 

paracaseinate,  535 
Calories,  86,  434,  509 

required,  89,  185 
Calorimeter,  86,  87,  186 
Canal,  auditory,  222 
Canaliculi,  bone,  207 

liver,  99 
Cane  sugar,  574 

(See  also  Sucrose) 
Canines,  200 
Cam's  familiar  is,  418 
Capillary,  146,  146-147 
Capsule,  brain,  257 
Carbohydrates,  6,  55,  186,  192,  573 

digestion  of,  65 
Carbon  dioxide,  186 

transportation,  160 
Carbon  monoxide,  and  respiration,  91 
Carbon  compounds,  formation,  429 


Carbonic  acid,  respiration,  83 
Carbonic  anhydrase,  541 
Carboxypolypeptidase,  536 
Cardnus,  462 
Cardiac  cycle,  152,  163 
Carnivora,  585 
Carnivorous  plants,  532 
Carotene,  58,  400,  401,  403 
Carotid  artery,  3^3,  147 
Carpal  bones,  201 
Carpus,  201 
Carrier  females,  382 
Cartilage,  18,  27,  28,  193 

bone,  28,  205 

ribs,  200 

thyroid,  106 
Casein,  557 
Castania,  18 
Castration,  571 
Cat,  forelimb,  normal,  404 

polydactylous,  404 
Catalysis,  528 
Catalyst,  63,  438,  528 
Cataract,  403 

hereditary,  398 
Caucasian  peoples,  lips,  39 
Caucasian  race,  414 

subdivisions  of,  414 
Caudal  appendage,  196 
Caudate  nucleus,  264 
Cell-bridge,  18 
Cells,  7 

auditory,  18 

blood,  29 

brain,  14 

daughter,  333 

division,  13,  14,  281,  282 
mitotic,  328 

goblet,  26,  26,  93 

Kupffer,  101,  121 

leaf,  18,  431 

liver,  18, 101 

nerve,  32 

(See  also  Neuron) 

nuclear  elements  of,  22 

photosynthetic,  431 

resting,  328 

retinal,  18 

Sertoli,  549 

structure,  2,  18,  20 


594 


HUMAN  BIOLOGY 


Cells,  theory,  521 

types,  18 

Cellular  differentiation,  20,  341 
Cellular  response,  types  of,  487-494 
Cellulose,  56,  69,  192,  510 

chains,  437 

molecular  structure,  437 
Central  canal,  253 
Central  fissure  (Rolanojp),  258,  263 
Central  nervous  system,  development, 

216-217 

Central  structure,  249-268 
Centrifuging,  475 
Centrosome,  22,  328,  329,  350 
Centrum,  196 
Cercariae,  465 
Cerebellar  artery,  261 
Cerebellum,  268,  269,  260,  261,  271 
Cerebral  artery,  261 
Cerebral  cortex,  262 
Cerebral  fiber  tracts,  263 
Cerebro-cortico-pontal  tract,  264 
Cerebrospinal  fluid,  267,  268 
Cerebrum,  257,  269 
Cervical  canal,  321 
Cervical  flexure,  306 
Cesalpino,  564 
Cetacea,  585 
Chalaza,  303,  304 
Chambers,  eye,  229 
Characteristics,  sexual,  571 
Chemical  energy,  433 
Chemical  equations,  511 
Chemotherapy,  486 
Chemotropism,  positive,  580 
Chest  and  neck,  200 
Chest  region  (thoracic),  197 
Chest  wall,  200 
Chick,  blastocoel,  304 

blastula,  304 

development,  301-308 

embryo,  306,  307,  313 
,    mesenchyme,  18 
Child,  metabolic  rate,  89 
Childbirth,  323 
Chiroptera,  585 
Chitin,  190 

Chlamydomonaa,  7,  287 
Chlorella,  455 
Chloride  shift,  160,  512 


Chlorogogen  cells,  171 

Chlorophyll,  12,  22, 67,  91,  431, 459,  514 

analyses,  432 
Chlorophyllase,  68,  439 
Chloroplast,  7,  18,  22 

leaf,  431 

Cholecystokinin,  99,  102,  513 
Choleic  acids,  538 
Cholesterin,  574 
Cholesterol,  574 
Cholinesterase,  496 
Chondrioconts,*  514 
Chondrin,  27 
Chondriosomes,  22,  514 
Chordae  tendinae,  144 
Chordata,  196,  580 
Chorea,  Huntington's,  399 
Chorion  laeve,  321 
Chorion  frondosum,  321 
Chorionic  cavity,  321 
Choroid,  229 

eye,  231 

plexus,  258,  268 
Chromatin,  13,  22,  23 

complex,  426 

degeneration  of,  414 

Drosophila,  351 

pattern,  395 

reduction,  343 
Chromogen,  400 
Chromoineres,  337 
Chromonemata,  338 
Chromophil,  251 

Chromosomal  aberration,  389,  391,  422 
Chromosomal  abnormalities,  391 
Chromosomal  irregularities,  338 
Chromosome  maps,  388,  389,  393 

Drosophila,  336 

Indian  corn,  389 
Chromosomes,  23,  328,  329,  331 

crossing  over,  387 

and  genes,  377 

giant,  336,  337 

history  of,  344 

horse,  414 

individuality,  329,  334 

jack,  414 

man,  333,  334 

mutated,  389 

pairing,  351,  387 


INDEX 


595 


Chromosomes,  reduction,  344 

structure,  332-341 

translocation,  391 

X,  giant,  335 

Chrysanthemum,  chromosomes  in,  390 
Chyme,  102 
Chymotrypsin,  536 
Cicatrix,  302 
Cilia,  9,  221,  563,  569 
Ciliary  body,  229 
Ciliary  muscles,  233 
Ciliary  processes,  229 
Ciliated  epithelium,  25 
Circulation,   blood,  with  main  vessels 
labelled,  148 

kidney,  125 

pulmonary,  148 

routes,  147-152 

systemic,  149 
Circumcision,  318 
Clam,  shell,  189 
Clavicle,  201 
Claws,  190 
Cleavage,  holoblastic,  291 

mammalian  egg,  309 

meroblastic,  291 

partial,  291 
•  plane,  294,  331 

spindle,  350 

zygote,  294,  303 
Climacteric,  323 
Clitoris,  317,  324 
Cloaca,  34,  302 
Coagulation,  blood,  29,  163-166 

milk,  64 
Coccyx,  197 
Cochlea,  223,  225 
Cochlear  canal,  226 

(See  also  Seal  a  media) 
Coe,  W.  R.,  284,  289 
Caelenterates,  170,  498,  525 
Coelenterata,  514 
Coeliac  artery,  146 
Coeliac  plexus,  244 
Coelom,  34,  42,  43,  171,  299,  514,  525 
Coelomate  structure,  34 
Coenzyme,  64 
Cold,  256 
Collagen,' 192,  194 
Collagenoiis  fibers,  193 


Collagenous  tissues,  549 
Collaterals,  nerves,  250,  251 
Colloid,  515 

thyroid,  105 
Colloidal  emulsion,  4 
Colon,  48,  54,  146,  317 
Colony  formation,  19,  448 
Color  blindness,  381-383,  399 

gene,  382 
Color  vision,  239 
Column,  vertebral,  35     * 
Columnar  epithelium,  24 
Commensal  associations,  450-454 
Commensalism,  460,  451 
Communal  associations,  447-450 
Communicating  artery,  261 
Complement,  484 

fixation,  517 
Complernental  aii^  79 
Complementary  colors,  239 
Complex,  vitamin  B,  59 
Conduction,  nerve  impulse,  271-275 
Condyles,  occipital,  205 
Cones,  and  rods,  236,  237 
Conjunctiva,  229,  230 
Conklin,  E.  G.,  396 
Connective  tissue,  26,  27,  175,  189,  489 

subcutaneous,  37 
Contractile,  elements,  170 
Contractile  filament,  169,  170 
Contractile  tissues,  169 
Contractile  vacuole,  8,  9,  563 
Contraction,  170 
Conus  medullaris,  269 
Corals,  189,  534 
Corium,  37,  172,  191 
Corn,  pith  cells,  18 

root-hair,  18 

Corn  field,  environment,  394 
Corn-root  aphid,  461 
Cornea,  229 
Corona  radiata,  257 
Coronary  orifice,  143,  160 
Corpora  cavernosa,  318 
Corpora  lutea,  320 
Corpora  quadrigemina,  268 
Corpora  striata,  262 
Corpus  callosum,  268,  264 
Corpus  luteum,  117,  320,  321 
Corpus  spongiotsum,  318.  319 


596 


HUMAN  BIOLOGY 


Corpuscle,  blood  (see  Cells) 

tactile,  37,  219 
Cortex,  123,  253 

adrenal,  110 

cerebral,  262 

histology  of,  260-267 

kidney,  118 
Corti,  organ  of,  224,  226,  227,  229 

rods  of,  226 
funnel  of,  226,  228 
Cortico-spirial  pathways,  256 
Cortico-spinal  tract,  255 
Cortin,  110 
Cough,  83 

Cowper's  gland,  316,  317 
Crab,  189,  462 
Cranial  nerves,  261 

human,  519 

roots  of,  261 
Cranium,  35,  198 
Crayfish,  shell,  190 
Cream,  4,  557 
Creatine,  122,  187 
Creeper  chicken,  384 
Crenation,  138 
Cretinism,  107,  399,  469 
Cribiform  plate,  262 
Crista  acustica,  224 
Crop,  bird,  47 
Cross-fertilization,  420,  450 
Crossing  over,  387 

Drosophila,  386 
process,  386-389 
Crura  cerebri,  260 
Crustacea,  500 
Cryptorchid  testes,  549 
Cryptorchism,  399 
Cuboidal  epithelium,  24 
Curd,  557 
Cuvier,  516,  560 
Cycle,  cardiac,  152,  153 

of  elements,  12,  443 
Cyclosis,  9,  15,  549 
Cyclostomata,  196,  582 
Cystic  duct,  99 
Cytochromes,  541 
Cytolysins,  473 
Cytolysis,  484 
Cytoplasm,  20,  22,  569 


D 


Dachshund,  hybrid,  417,  418,  419 
Dale,  Sir  Henry,  496 
Darwin,  Charles,  420,  559 

"Origin  of  Species,"  560,  561 
Darwin,  Erasmus,  559,  560 
Datura,  chromosome  complexes,  340 
Daughter  cells,  333 
Deafness,  399 

Deaminization,  57,  122,  441 
Decidua  basalis,  321 
Decidua  capsularis,  321 
Decidua  parietalis,  321 
Decussation,  256,  265 
Deficiency,  dietary,  469 
Dehydrogenases,  540 
Deletion,  391,  392 
Dendrite,  32,  250,  251,  254 
Dental  formula,  200 
Dentine,  199 
Depressors,  174 
Dermis,  skin,  190 
De  Saussure,  565 
Descent  with  change,  327 
Determination,  gene  loci,  393 

sex,  374 
Development,  chick  embryos,  305-308 

frog,  291-301,  296,  297 

hen's  egg,  303-308 

viviparous,  308 
Dextrin,  574 
Dextrose,  542 
Diabetes,  104,  130 

insipidus,  399 
Diaphragm,  35,  48,  142 

movements,  82 

vascularization  in,  488 
Diaphysis,  206 
Diastasis,  152 
Diastole,  152 
Diatom,  18  ' 
Dietary  deficiency,  469 
Differentiation,  cellular,  19,  20,  341 

germ  cells,  342 

somatic  cells,  342 
Diffusion,  62,  520 
Digestion,  and  absorption,  61-66 

hormones,  102 
Digestive  enzymes,  63-66 


INDEX 


597 


Digestive  tract,  man,  48 
Digit,  192 
Digitalis,  126 
Dihybrid,  364-366 

analysis,  366 

condition,  365,  401 
Dioscorides,  504 
Dipeptidases,  537 
Diphtheria,  antitoxin,  471-472,  522 

control  of,  475 

treatment  of,  477 
Diploblastic  animals,  525 
Diploid  chromosomes,  391 
Diploid  complex,  340 
Disaccharides,  55 

digestion,  65 

Discus  proligerous,  319,  320 
Disease,  467-494 

Addison's,  110 

cause  of,  468 

demonic  theory,  467 

Hahnemann  theory,  468 

noninfectious,  468-469 

nutritive  deficiency,  58 
Dissociation,  523 
Distomum  hepatica  (see  Fasciola) 

Diuretics,  126 

Dodder,  460 

Dog,  conditioned  reflex,  279 

forelimb,  202 
Dolichoglossus,  581 
Dominance,  lack  of,  370 

principal  of,  358 
Dorsal  fissure,  255 
Dorsal  root,  253 
Dorsal  vessel,  171 
Drone,  448 

parthenogenesis,  286 
Drosophila,  abnormalities,  390 

chromatin,  351 

chromosome,  map,  336 
translocation,  391 

crossing  over,  386 

deletion,  392 

diploid  chromosomes,  391 

eye  color,  374,  379 

genes,  332,  336,  378 

giant  chromosomes,  337,  391 

inbreeding,  421 


Drosophila,  irradiated  gametes,  339 

melanogaster,  335 

nondisjunction,  390 

recessive  character,  420 

salivary  glands,  336 

sex-linked  inheritance,  380 
Duckbill,  308 

Duodenum,  48,  49,  51,  102,  146 
Duplex  eyes,  402 
Dura  mater,  267 
Dutch  rabbits,  hair  color,  375 
Dwarfism,  113 
Dwarfs,  achondroplastic,  113,  114 


E 


Ear,  221-230,  229 

drum,  229 

external,  222 

function,  228 

middle,  71 

ossicles,  196 
Earthworm,  525 

body  plan,  43 

hermaphroditic  organsm,  288 

intestine,  43 

mating,  420 

muscles,  171 

nephridia,  122, 171 

nutrition,  41-42 

reproductive  organs,  288 

segment,  43 

transverse  section,  171 
Echidna,  308 
Echinodermata,  461 
Ectoderm,  23,  42,  294,  300,  525,  547 

cells,  295 

Hydra,  170 
Ectoparasites,  456 
Ectoplasm,  8,  498 
Edema,  489 
Edentata,  585 
Effector,  muscle,  276 
Efferent  fibers,  240 
Egestion  and  excretion,  119 
Eggs,  325 

birds,  301 

cat,  18,  21 

frog,  293 

hen,  food  storage,  290 


598 


HUMAN  BIOLOGY 


Eggs,  homolecithal,  291 

incubation,  290 

mammalian,  cleavage,  309 

nucleus,  350 

ovarian,  117,  302 

and  pituitary,  114 

production,  114 

rabbit,  308 

telolecithal,  291,  301 
Ehrlich,  486,  575 
Eijkman,  59 

Ejaculatory  duct,  316,  317 
Elasmobranchii,  582 
Elastic  tissue,  27,  193 
Electric  potential,  273 
Electric  stimulus,  273 
Electrocardiogram,  526 
Electrolytes,  amphoteric,  568 
Electrons,  553,  562 
Elements,  abundance  of,  507 

biological,  505 

cycle  of,  12,  443 
Elephant,  basal  metabolism,  87 

heart,  142 

Elephantiasis,  490,  491 
Elodea,  68 
Embryo,  chick,  307 
24-hour,  306 

human,  321 

muscle  rudiments,  175 

nutrition,  290 

primitive  vertebrate,  299 

starfish,  290 

Empedocles'  hypothesis,  559 
Emulsion,  5 

colloidal,  4 

End  plate,  motor,  251 
Endocardium,  143 

Endocrine  glands,  92,  95,  96,  102-117 
Endocrines  (see  Hormones) 
Endoderm,  23,  42,  299,  300,  305,  525, 

547 

Endolymph,  223 
Endolymphatic  duct,  223 
Endomixis,  563 
Endomysium,  176 
Endoneurium,  241 
Endoparasites,  456 
Endoplasm,  8,  498 


Endoskeleton,  26,  35,  192-194,  582 
Endoskeletai  system,  in  embryo,  193 
Endoskeletal  tissue,  189 
Endosteum,  206 
Endothelium,  29,  145 
Energy,  186,  433,  434,  526 

basis  of,  16 

chemical,  433 

kinetic,  527 

of  position,  433 

potential,  527 

radiant,  432,  434 

relations,  10 

source  of,  11 

transformer,  527 
Enteric  cavity,  42,  547 
Enterokinase,  65 
Enteron,  171,  295,  300,  305 
Enteropneusta,  581 
Environment,  bio  tic,  447-466 

effect  of,  394 
Enzymes,  62-66,  437-447,  528-541 

digestive,  63-66 

extracellular,  63 

intestinal,  65 

intracellular,  63,  430 

pancreatic,  64 

proteolytic,  71 

salivary  and  gastric,  63 

synthesizing,  66 
Epicardium,  143 
Epidemics,  467,  470 
Epidemiology,  485-487 

science  of,  485 
Epidermis,  37 
Epidermolysis,  398 
Epididymis,  316,  316,  317 
Epiglottis,  46,  48,  77,  142 
Epilepsy,  399 
Epinephrine,  111 

(See  also  Adrenin) 
Epineurium,  241 
Epiphysis,  204 
Epithelial  tissue,  24 
Epitheliomuscular  cells,  170 
Epithelium,  24-26,  557 

ciliated,  18,  25 

columnar,  24 

cuboidal,  24 


INDEX 


599 


Epithelium,  glandular,  25 

nervous,  26 

olfactory,  221,  269 

respiratory,  78 

squamous,  24 

stratified,  18,  26 
Equations,  chemical,  511 
Equatorial  plate,  328,  330,  346 
Equilibrium,  257,  271 
Erepsin,  65,  440,  536 
Ergograph,  183 

records,  184 
Ergometer,  186 
Ergosterol,  60 
Erythrocyte,  101,  137,  138 

(See  also  Red  cells) 
Esophagus,  35,  46,  47,  48 
Esterases,  539 
Estrone,  116,  321 
Ethmoid  bone,  197,  199 
Eugenics,  425-426 
Eunuchs,  115 

Eustachian  tube,  46,  222,  229 
Kustachio,  496 
Eutheria,  584 
Evolution,  327,  395,  396,  561 

organic,  558 
Excretion,  kidneys,  122-131 

liver,  121-122 

lungs,  121 

skin,  119-121,  120 
Excretory  system,  118 
Exocrine  glands,  94,  95 
Exoskeleton,  26,  189-192,  582 
Exostoses,  398 
Extensor,  174 
External  ear,  222 
External  gills,  297 
Exteroceptive  system,  268,  269 
Exudate,  serous,  571 
Eye,  33,  229 

chambers,  229 

of  chick  embryo,  307 

color,  402 

Drosophila,  374,  379 

defects,  403 

muscles,  232 
Eyeball,  230,  232 
Eyelids,  230 


F 

Facial  nerve,  261,  520 
Feces,  465 

Fallopian  tube,  317,  322 
Farsightedness,  234 
Fascia,  175,  176 
Fasciculi,  muscle,  176,  177 
Fasciculi  proprii,  265 
Fasciculus  cuneatus,  265 
Fasciculus  gracilis,  266 
Fasciola  hepatica,  465,  466 
(See  also  Liver  fluke) 
Fatigue,  182 

nerve,  274 

in  finger  muscles,  184 
Fats,  6,  56,  70 

digestion  of,  70 
Fauna,  444 

Feeble-minded,  segregation,  425 
Feet,  bones  of,  196 
Females,  carrier,  382    * 
Femoral  nerve,  214 
Femur,  201,  206 
Fenestra  ovalis,  223 
Fenestra  rotunda,  223 
Ferment,  529 
Ferrf,  134 

alternation  of  generations,  286 
Fertilization,  343,  349-351 

Ascidian  egg,  350 

cross,  420 

essential  feature,  288 

external,  289 

functions,  351 

internal,  289 

random,  361 

and  reproduction,  351 

self,  420 

Fetal  blood  vessels,  311 
Fever,  487 
Fiber  tracts,  254-257 

afferent,  255 

ascending,  254 

association,  263 

cerebral,  263 

descending,  254 

efferent,  256 

projection,  265 


600 


HUMAN  BIOLOGY 


Fibrin,  blood  clot,  475 

protein,  440 
Fibrinogen,  100,  137 
Fibula,  201 
Filaria,  490,  491 
Filum  terminate,  252 
Finger,  fatigue,  184 
Fish,  cartilaginous,  194 

gill  structure,  74 

heart,  140 

movements,  195 

scales,  190 
Fission,  binary,  282,  498 

flatworm,  284 
Fissure  of  Rolando,  262 
Fixity  of  species,  325,  327 
Flagellates,  70 
Flagellum,  7 
Flatworm,  420 

asexual  reproduction,  284 
Flexion,  rate  of,  184 
Flexor,  174 
Flora,  444 
Fluorine,  506 
Follicle,  Graafian,  116,  117 

thyroid,  1O5 
Follicular  cavity,  319 
Food,  11 

chains,  441-447 

Arctic  Bear  Island,  444 

storage,  290,  301 
hen's  egg,  290 
Foodstuffs,  55 
Foramen  magnum,  197,  199,  205,  252, 

257 

Foramina,  bones,  209 
Forearm,  opposed  muscles,  173 
Forebrain,  217,  257,  262 
Forelimb,  201 

bird,  202 

dog,  202 

horse,  202 

man,  202 
Foreskin,  318 
Fossa,  glen  old,  201 
Fourth  ventricle,  258,  267 
Fovea  centralis,  237 
Fox,  579 
Fragility,  bones,  398 


Frog,  blood  cells,  18, 

development,  291-301 
early  stages,  296 
late  stages,  297 

eggs,  293  • 

development,  294-299 

female,  292-294 

life  cycle,  301 

male,  292 

urogenital  systems,  292 
Frohlich's  syndrome,  113 
Frontal  bone,  197 

lobe,  268,  261 

nerve,  214 

sinus,  197 
Fructose,  62,  574 
Fruit  fly  (see  Drosophila) 
Fuels,  562 
Fundus,  48 
Fungal  parasites,  459 
Fungus,  12,  435,  455 

filaments,  454 

G 

Galactophorous  duct,  551 

Galactose,  538 

Galen,  504,  563,  564 

Galileo,  564 

Gall  bladder,  48,  99 

Gal  ton,  laws,  406 

Galvanometer,  272,  273 

string,  526 

Galvanotropism,  positive,  580 
Gametes,  465 

blood  group,  411 

possible  types,  386 

types  produced,  357,  358 
Gametocytes,  567 
Gametogenesis,  343,  346 
Gametophyte,  286 
Ganglia  cell,  253 

coeliac,  245 

Gasserian,  519 

mesenteric,  245 

sensory,  241,  276 
Gases,  respiratory,  79,  159 
Gasserian  ganglion,  519 
Gastric  juice,  49 
Gastric  vaouoles,  563 


INDEX 


601 


Gastrin,  102 

Gastrocnemius  muscle,  179,  181 

Gastrulation,  295,  305 

Gel,  3 

Generations,  alternation  of,  498 

asexual,  498 

sexual,  498 
Genes,  and  chromosomes,  377 

complex,  423,  424 
variation,  352 

Drosophila,  332,  335,  378 

lethal,  383-389 

linear  arrangement,  333 

linkage,  377 

loci,  determination  of,  393 

multiple,  372,  401,  403,  413 

mutating,  393 

recombinations,  386,  387 

synapsis,  392 
Genie  variation,  409 
Genital  gland,  oyster,  289 
Genital  organs,  290 
Genotypes,  362 

blending  inheritance,  370 

blood  group,  411 

family,  424 
Germ  cells,  345 

formation,  341-353 

irradiation,  391 

Germ  layers,  primary,  23,  34,  300 
Germ  plasm,  541 
Germinal  epithelium,  frog,  293 

ovary,  320 
Gestation,  324 

human,  314 

mammalian,  312 
Giant  chromosomes,  336,  355 

black  fly,  338 

Drosophila,  391 
Gigantism,  113 
Gill  arches,  298 
Gill  slits,  34,  581 
Gill  structure,  fish,  74 
Girdle,  pectoral,  201 

pelvic,  201 
Gizzard,  bird,  47 
Glands,  adrenal,  109-111 
historical,  496 

cell,  42  ' 

comparative.  94 


Glands,  compound,  95 

endocrine,  95,  96,  102-117 

exocrine,  94,  95 

intestinal,  52 

Lieberkuhn,  52 

mammary,  96,  551 

mixed,  95,  97-101 

oil,  572 

parathyroid,  108-109 

parotid,  45 

pituitary,  111-115,  112 

salivary,  human,  45 

sebaceous,  26,  37,  191,  572 

secretion,  257 

skin,  572 

sublingual,  45 

submaxillary,  45 

sudoriparous,  572 

sweat,  26,  38,  120,  572 
Glans  penis,  318 
Glenoid  fossa,  201 
Gliding  joint,  203 
Globulin,  105 

serum,  137 
Glomerular  fluid,  128 
Glomerulus,  124,  125,  299 

filtrate,  129 
Glomus,  299 

Glossopharyngeal  nerve,  261,  520 
Glottis,  46 
Glucose,  61,  62,  67,  68,  187,  434,  538, 

542,  574 
Glutathione,  541 
Gluten,  574 

Glycogen,  66,  99,  187,  532,  574 
Glycosuria,  104, 
Goblet  cell,  25,  26,  52,  93 
Goiter,  colloid  type,  106 

exopthalmic,  107 
Golgi  body,  22,  542 
Golgi  apparatus,  22,  251,  542 
Gonads,  115-117 
hormones,  115 
Gower's  atrophy,  399 
Graafian  follicle,  116,  117,  319,  320,  348 

with  egg,  319 
Gray,  Asa,  561 
Gray  crescent,  295 
Gray  matter,  brain,  266 
spinal  cord,  253,  255 


602 


HUMAN  BIOLOGY 


Great  Dane,  hybrid,  418,  419,  422 
Green  algae,  587 
Growth,  12 

and  reproduction,  281-324,  325-353 
Guinea  pigs,  inbreeding,  421 

inheritance,  359 
hair  color,  361 
Gull,  Sir  William,  578 
Gullet,  47,  563 

(See  also  Esophagus)  fr 

Gyri,  262 


H 


Haeckel,  561 

Hahnemann,  theory  of  disease,  468 

Hair,  37,  38,  190-192 

air  spaces,  192 

bulb,  191 

cells,  226,  227 

color,  361,  365,  373,  375,  384,  402 
mice,  384 

follicle,  172,  190 
structure  of,  191 

muscle,  191 

papilla,  191 

qualities,  401 

root,  191 

shaft,  191 

Hales,  Stephen,  508,  565 
Half-breed,  415 
Haller,  564 
Hand,  36 

Haploid  complex,  340 
Hard  palate,  199 
Hare-lip,  399 

Harvey,  William,  280,  543,  564 
Haustoria,  459 
Haversian  canal,  207  • 
Hawaiian-Chinese  hybrid  type,  417 
Hay  infusion,  441 
Head,  35 

fold,  306,  306 

movements,  205 

muscles,  174 

process,  305 
chick,  305 

Hearing  and  position,  sense  of,  221 
Heart,  34,  140-144,  142 

amphibian,  140 


Heart,  bird,  140 

chick  embryo,  306 

comparative,  141 

contraction,  141 

elephant,  142 

fish,  140 

function,  152 

histology,  143 

humming  bird,  142,  153 

hypertrophy,  490 

innominate  artery,  143 

reptile,  140 

size,  142 

sounds,  153 

valve,  vascularization,  161 
Heat,  186,  256 

production,  86 
Hegel,  559 

Heights,  variation  in,  407 
Heliotropism,  positive,  580 
Hematin,  91 

prosthetic,  539 
Heine,  100 

Hemocytoblast,  212     • 
Hemoglobin,  57,  90,  91,  138,  539,  582 
Hemolysins,  473 
Hemolysis,  138,  473,  485 
Hemophilia,  383,  401 
Hen,  egg,  development,  303-308 
internal  structure,  304 

food  storage  in  egg,  290 

ovary,  301 

reproductive  system,  301-303,  302 
Henle,  loop  of,  124 
Henderson,  L.  J.,  447 
Heparin,  165 
Hepatic  artery,  98 
Hepatic  duct,  99 
Hepatic  lobule,  98 
Hepatic  vessels,  146 
Heredity,  human,  397-427 
Hermaphrodism,  earthworm,  287 
Hermaphroditic,  465 
Hermit  crabs,  460 
Hernia,  399 
Heteroploidy,  390 
Heterosis,  421 

Heterotrophic  organisms,  429,  435-437 
Hexosans,  573 
Hiccough,  83 


INDEX 


603 


Hitthousia  mirabilis,  430 
Hilus,  123 
Hind  limb,  188,  201 
Hindbrain,  217,  268,  269,  260 

Hinge  joint,  203 
Hippocrates,  467,  503 
Hippuric  acid,  122,  126,  130 
Hirudin,  165 
Histology,  543 
Holoblastic  cleavage,  291 
Homo  sapiens,  413,  416,  586 

primary  subdivisions,  414 
Homolecithal  eggs,  291 
Homologies,  195 
Homothermal  temperature,  583 
Homozygous,  357 
Honey,  450 
Honeybee,  448 

larvae,  448 
Honey  dew,  452 
Hoof,  fusion  of,  405 
Hooke,  Robert,  544 
Hooker,  561 
Hopkins,  F.  G.,  57,  545 
Hormone,  97 

and  body  functions,  111-117 

arid  digestion,  102 

emergency,  111 

gonads,  116 

historical,  545 

intestinal  mucosa,  102 

and  metabolism,  103-111 

parathyroid,  108 
Horse,  414 

chromosomes,  414 

forelimb,  202 

limb,  evolution,  202 
Hoskins,  R.  G.,  108 
Human  body,  changes  in  proportions, 
323 

divisions  of,  35 
Human  heredity,  397-427 
Human  tissues,  23 
Human  trachea,  77 
Humerus,  201,  204 

eye;  23,0 
Humming  bird,  heart,  142,  153 

wings,,  170 

Humors,  theory  of,  467 
Hunger,  270 


Hunter,  John,  496 
Huntington's  chorea,  399 
Huxley,  T.  H.,  561 
Hybridization,  369 

Dutch  and  Hottentots,  417 

Filipino,  417 

human,  413-419,  422 
Hybrids,  basis  of  sterility,  414 

bulldog,  417,  418 

Dachshund,  417,  419 

mule,  414 

St.  Bernard  and  Dachshund,  418,  422 

St.  Bernard  and  Great  Dane,  418,  422 

vigor,  421 
Hydra,  31,  170,  455,  525,  534,  647 

bud,  42 

budding,  283 

foot,  547 

mouth,  547 

nutrition,  41,  42 
Hydrogen  ion,  53,  547 
Hydrogen  peroxide,  541 
Hydrogen  sulphide  gas,  430 
Hydrolysis,  62,  439,  532 
Hydrophobia,  479 
Hydroxyl  ion,  547 
Hydrozoa,  514 
Hymen,  324 

Hyoid  bone,  142,  197,  200 
Hyperparasitism,  465 
Hypersensitivity,  476 
Hypertension,  157 
Hyperthyroidism,  107 
Hypertrophy,  489-491 

human  heart,  490 

legs  arid  scrotal  regions,  491 
Hyphae,  286 

Hypoglossal  nerve,  261,  520 
Hypophysis,  111 
Hypospadias,  399 
Hypothyroidism,  106 


/-band,  muscle,  178 
Ileocaecal  valve,  49,  53 
Hewn,  48,  49,  53,  146 
Iliac  arteries,  149 
Immunity,  469-472 
acquired,  470 


604 


HUMAN  BIOLOGY 


Immunity,  active,  471 

natural,  469 

passive,  471 

Immunological  methods,  474 
Immunology,     uses     and     techniques, 

473-485 

Implantation  site,  uterus,  310 
Inbreeding,  419-422 

close,  420 

Drosophila,  421 

guinea  pigs,  421 

rats,  421 
Incisors,  200 
Incubation,  290,  306 
Incus,  222,  229 

Independent  assortment,  358,  378 
Indian  corn,  chromosome  maps,  389 
Individual  variation,  354 
Induction  coil,  180 

for  muscle  stimulation,  181 
Infantile  paralysis,  475,  491 
Infection,  filarial,  491 
Infectious  diseases,  485 
Inflammation,  487 
Inflation,  lung,  82 
Infra  orbital  nerve,  214 
Infundibulum,  77,  78,  112 
Infusoria,  562,  563,  569 
Ingen-Housz,  565 
Inguinal  rings,  314 
Inheritance,  354-396,  554 

alternative,  406 

biparental,  287,  350,  352 

blending,  368-372,  406 

blood  groups,  409 

guinea  pigs,  359 

hair  color,  365 

man,  398 

particulate,  359 

nature  of,  355-396 

sex-linked,  Drosophila,  380 

skeletal  defects,  404 

stature,  408 

Inherited  characteristics,  400-427 
Inherited  characters,  man,  398-399 
Inner  cell  mass,  309,  311 
Innominate  artery,  143,  149 
Insanity,  399 
Insect,  189 

leg  muscles,  190 


Insect,  trachea,  76 
Insecta,  501 
Insectivora,  585 
Insulin,  103,  187 

isolation,  104 
Integration,  275-279 
Integumental  muscles,  174 

skin,  174 
Intelligence,  277 
Intercellular  specialization,  587 
Intercostal  muscle,  142 
Intercostal  nerve,  214 
Interlobular  vessels,  124 
Internal  carotid  artery,  261 
Internal  secretions  (see  Hormone 
Interoceptive  system,  269 
Interstitial  cells,  220,  548 
Interstitial  tissue,  116,  316,  316 

testis,  115 
Intestine,  earthworm,  43 

large,  53-55 

man,  43-55 

Intestinal  enzymes,  65 
Intestinal  glands,  52 
Intestinal  peristalsis,  172 
Intestinal  secretion,  52 
Inversion,  391,  574 
Involuntary  muscle,  171-173 
Iodine,  105,  106 

deficiency,  469,  491 
Iris,  229,  230 

absence  of,  403 

diaphragm,  231 

pigmented,  398 
Iron-porphyrin,  539 
Irradiation,  60 

germ  cells,  391 
Irritability,  14,  215 

nofrmal,  182 

Island  of  Langerhans,  103,  104 
Isoelectric  point,  568 
Isogamy,  287 
Isotonic  salt  solution,  179 
Isotropic,  178 
Isthmus,  thyroid,  106 


Jack,  paternal  chromosomes,  414 
Janssens,  386 


INDEX 


605 


Jaws,  44 

Jeans,  Sir  James,  446 

Jejunum,  48,  49 

Jelly  fish,  134,  135 

Jenner,  Edward,  479 

Jimsonweed,  chromosomes,  340 

Joints,  203-205 

ball  and  socket,  203 

gliding,  203 

hinge,  203 

pivot,  205 
Judgment,  277 
Jugular  vein,  169 
Juice,  gastric,  49 

intestinal,  52 

pancreatic,  53 
"Jukes"  family,  424 
Junket,  535 

K 

K-band  muscle,  178 
Kahn  reaction,  484 
Kahn  test,  474,  483 
"Kallikak"  family,  424 
Kangaroo,  309 
Kant,  559 
Karyolymph,  23 
Karyotype,  335 
Katabolism,  9 
Kelps,  133 

Keratin,  190,  191,  550 
Keratinase,  534 
Ketones,  104 
Kidneys,  118 

circulation,  125 

cortex,  118 

and  excretion,  122-131 

frog,  292 

function,  126-131 

histology,  125 

medulla,  118 

and  nephridia,  123 

papilla,  118 

pelvis,  118 

structure,  123-126 

synthesis,  130 

tissue,  124 
Kinase,  44ft 
Kinetic  energy,  527 


Klebs,  566 

Krause's  membrane,  178 
Kupffer  cell,  101,  121 
Kymograph,  179,  180,  279 

curves,  183 

drum,  181 

record,  182 


Labium  majus,  317 
Labium  minus,  317 
Labor,  division  of,  19 
Labyrinth,  bony,  223 

membranous,  223 
Lachrymal  bones,  199 
Lachrymal  glands,  230,  231 
Lact albumin,  557 
Lactase,  65,  440,  538 
Lactation,  115 
Lacteal,  52,  159 
Lactic  acid,  187,  633 
Lactose,  550,  557 
Lacuna,  18,  207 
Lagena,  225 
Lamarck,  559,  560 
Lamella,  207 
Lamina,  spiral,  225,  226 
Langerhans,  Island  of,  103,  104 
Large  intestine,  53-55 
Larynx,  48,  77,  84 
Latent  perkxj,  182 
Lateral  fissure  (Sylvius),  258 
Lateral  folds,  306 
Lateral  plate,  298,  299 
Leaf,  cells,  18 

photosynthesis,  67 
Leber's  disease,  403 
Left-handedness,  399 
Legs,  hind,  frog,  297 

muscles,  190 
Leguminous  plants,  455 
Leibnitz,  559 
Lemuroidea,  586 
Lens,  229,  230,  233 

capsule,  233 
Leptocardia,  581 
Lethal  gene,  383-389 
Leucocytes,  138,  139,  213 

phagocytic,  472 


606 


HUMAN  BIOLOGY 


Levators,  174 

Levers,  types,  210,  211 

Levulose,  542 

Lichen,  section,  464 

Lieberkiihn,  glands  of,  52 

Liebig,  565 

Life,  definitions  of,  8,  9 

essentials  of,  446 

web  of,  428-466 
Life  cycle,  frog,  301 

history  of  chromosomes,  344 
Life   history,    Trypanosoma  gambiense, 

457 

Life  pressure,  446 
Light  rays,  238 
Linens  socialis,  284 
Linin  (see  Reticulum) 
Linkage,  377-385 

genes,  377 

group,  378 

restriction  of,  378 
Linnaeus,  576 
Lip,  43 

hare-,  399 

nrncous  membrane,  39 
Lipase,  65,  440,  538 
Lipoids,  56 

Little  finger,  twinned,  406 
Liver,  34,  48,  92,  97-101,  169 

blood  supply,  98 

canaliculi,  99 

cells,  18 

and  excretion,  121—122 

functions,  9&-101 

and  pituitary,  112 

protein  metabolism,  122 

rabbit,  101 
Liver  fluke,  465 

life  cycle,  466 
Lobster,  189 
Lobule,  hepatic,  98 
Lockjaw,  477 
Locomotion,  169 
Loop  of  Henle,  124 
Lucif erase,  541 
Luciferin,  541 
Lungs,  34,  142 

bird,  76 

capacity,  man,  79 

excretion  in,  121 


Lungs,  frog,  76 

inflation,  82 

lizard,  76 

mammal,  76 

man,  77 

Necturus,  76 

vertebrate,  75,  76 
Lymph,  147 

Lymph  node,  60,  61,  166 
Lymphatics,  60,  169 

in  arms,  166 

in  chest,  166 
Lymphatic  vessel,  147 
Lysin,  473,  484,  518 
Lysis,  473 
Lytic  reactions,  485 
Lytic  tests,  485 

M 

Macronucleus,  9,  563 
Macroorganisrn,  468 
Macula  acustica,  224 
Make-shock,  273 
Malaria,  287 

control  of,  485 
Malaria  parasite,  456,  464 

life  cycle,  464 
Malleus,  222,  229 
Malpighi,  551,  565 
Malpighian  bodies,  124,  125 

functions,  128 
Malpighian  region,  37 
Maltase,  65,  440.  538 
Maltose,  63,  538,  551 
Mammal,  testes,  116 
Mammalia,  584 

Mammalian  development,  308-314 
Mammary  glands,  96,  324,  551 
Man,  alimentary  canal,  regions  of,  42 

chromosomes,  333 

mouth,  43-45 

respiratory  system,  76-81 

small  intestine,  49-53,  60 

stomach ,  47-49 

trunk,  3o 

Mandible,  197,  199 
Mandibular  nerve,  214 
Manometer,  166 
Marie,  Pierre,  566 


INDEX 


607 


Marriages,  cousin,  419,  423 
Marrow,  blood  cells,  140 

bone,  28,  205,  208,  213 

cavity,  206 
Marsupials,  309,  584 
Marsupium,  309,  584 
Mason,  K.  E.,  61 
Massa  intermedia,  258 
Mastigophora,  7,  569,  587 
Matings,  homozygous,  360 
hybrid,  363 

reciprocal,  360 

types,  360 
Matrix,  26,  587 

collagenous,  26 
Matter,  structure  of,  552 
Maxilla,  197,  199 
Measurements,  553 
Median  nerve,  214 
Medicolegal  work,  474,  480 
Mediterranean  race,  415 
Medulla,  83,  110,  123,  260 

hair,  192 

kidney,  118 

oblongata,  268,  261 

and  respiration,  83 
Medullary  groove,  296 
Medullary  plate,  296,  305 
Medullary  sheath,  251 
Meiosis,  346,  347 

and  mitosis,  349 
Meissner  plexus,  51 
Melanin,  400,  401 
Membrana  granulosa,  319 
Membranes,  bones,  28,  205 

mucous,  38 

outer,  319 

plasma,  21 

protoplasm,  521 

semipermeable,  62,  521 

serous,  570 

Membranous  labyrinth,  223 
Memory,  277 
Mendel,  362,  554 

results  of,  355,  356 
Mendelian  laws,  358^. 
Meninges,  267-268 
Menopause,  323 
Menstruation,  323 
Merozoites,  567 


Mesencephalon  (midbrain),  259 
Mesenchyme,  chick,  18 
Mesenteric  plexus,  244 
Mesenteric  vessels,  146 
Mesentery,  50,  51,  302,  571 
Mesoderm,  23,  298,  300,  525 

development,  295 

somatic,  299 

splanchnic,  299 
Mesogloea,  42,  547 
Mesonephric  duct,  299 
Mesonephric  tubule,  299 
Mesonephros,  34,  123,  299 
Metabolism,  9,  521 

calcium,  109,  213 

and  hormones,  103-111 
Metabolic  rate,  basal,  85-89,  88 

normal,  89 

Metacarpal  bones,  201 
Metamerism,  525 
Metamorphosis,  297,  301 

frog,  301 

Metanephros,  123 
Metaphase,  328,  330 
Metaplasm,  18,  22 
Metatarsal  bone,  201 
Metazoa,  554 
Metencephalon,  259 
Mice,  hair  color,  384 
Mycorrhiza,  455 
Microenzymology,  532 
"Micrographia,"  544 
Micronucleus,  9,  564 
Microorganism,  468 
Microscope,  development  of,  555 
Micturition,  131 
Midbrain,  217,  257,  259,  260 
Middle  ear,  222 
Midgets,  113,  114 
Milk,  324,  557 

coagulation,  64 

sugar,  550,  557 
Mineral  reserves,  213 

bones,  213 
Miracidia,  465 
Mistletoe,  459,  460 
Mitochondria,  514 
Mitosis,  13,  326,  327-332,  328,  347,  489 

compared  with  meiosis,  349 
Mixed  glands,  95 


608 


HUMAN  BIOLOGY 


Modifications,  effect  of,  395 

individual,  424 
Molars,  200 
Mold,  bread,  436 
Molecular  layer,  266 
Molecular  motion,  522 
Mollusks,  189 
Mongolian  race,  414,  416 

subdivisions  of,  414 
Monohybrid,  361,  364 
Monosaccharides,  55 
Montesquieu,  496 
Morula,  309 
Mosquito,  464 

Anopheles,  566 
Moss,  134 
Moth,  tussock,  465 
Motion,  bodily,  176 
Motor  fiber,  253 
Motor  nerve  impulses,  216 
Motor  neuron,  31 

multipolar,  260,  261 
Motor  tracts,  pyramidal,  265 
Moulting,  192 
Mouth,  44,  563 

frog,  298 

invagination,  296 

man,  43-45 
Movement,  15 

muscular,  209 
Mucigen,  93 
Mucin,  93 
Mucosa,  39,  49,  51,  52,  54 

stomach,  49 

(See  also  Nutrition) 
Mucosae  muscularis,  51 
Mucous  membrane,  38,  557 
Mucus,  93,  557 
Mulatto,  401,  417 
Mule,  414 

sterility,  414 
Multiple  alleles,  413 
Multiple  allelomorphs,  413 
Multiple  factors,  372-377 
Multiple  genes,  372,  374,  401,  413 
Munk,  579 
Muscles,  abductor,  31 

adductor,  30,  174 

cardiac,  18,  21 

circular,  51 


Muscles,  complex,  176 

contraction,  180-187,  274 
basis  of,  186 
chemistry  of,  186-187 

earthworm,  171 

effector,  276 

efficiency,  185 

eye,  232 

fatigue,  182 

human  body,  168 

integumental,  174 

involuntary  movement,  169 

longitudinal,  51 

and  nerve,  preparation,  179 

opposed,  173 

rudiments,  embryo,  175 

segmental,  174 

striated,  171 

tendons,  bones,  connections,  176 

tissue,  21,  29,  169 

tonus,  257 

twitch,  184 

types,  174-176 

unstriated,  171 

voluntary  movement,  169 
Muscular  system,  169-187 

functional  features,  178-187 

principal  muscles  labelled,  168 

structural  features,  170-178 
Muscularis,  uterine,  321 
Muscular  s,  mucosae,  61 
Mutating  gene,  393 
Mutation,  389-394 

bud,  390 

mammalian,  422 
Myelencephalon    (medulla  oblongata), 

259 

Myelin  sheath,  242 
Myelinated  nerves,  242 
Myeloid  tissue,  213 
Myobacterium  leprae,  436 
Myocardium,  143 
Myofibrils,  176,  177 

changes  in,  187 
Myoneme  fibers,  170 
Myopia,  403 

Myotome,  298,  299,  300,  306 
Myriapoda,  501 
Myxedema,  106,  678 


INDEX 


609 


N 

Nail,  190,  192 
Napthoquinones,  61 
Nasal  bones,  199 
Nasal  septum,  46 
Natural  selection,  561 
Nearsightedness,  234 
Neck,  35,  200 

region  (cervical),  197 
Nectar,  449 
Negroid  race,  414,  415 

subdivisions  of,  414 
Nematocyst,  42 
Nemathelminthes,  461 
Nephridium,  171 

earthworm,  122 

and  kidneys,  123 
Nerve  cell,  32 
Nerve  cord,  35 
Nerve  impulse,  speed,  2t4 
Nerve  tissue,  31 
Nerves,  abducens,  519 

auditory,  520 

cranial,  242,  261 
human,  519 

earthworm,  288 

facial,  520 

fatigue,  274 

glossopharyngeal,  520 

histology,  241 

hypoglossal,  520 

motor  root,  252 

and  nerve  fibers,  241 

oculomotor,  519 

olfactory,  519 

optic,  519 

plexus,  78 

reception,  268-271 

spinal,  242 
accessory,  520 

trigeminal,  519 

trochlear,  519 

vagus,  520 

Nervous  epithelium,  26 
Nervous  system,  214 

central;  248,  249-268,  580 

divisions,  of,  217 

functional  features,  268-279 

protection,  196 


Nervous  system,  structural  features, 

216-218jf. 
Neural  arch,  196 
Neural  tube,  34,  296,  299 
Neurilemma,  241,  251 
Neurofibril,  242,  251 
Neuroglia,  267 
Neuromotor  system,  563 
Neuromuscular  apparatus,  164 
Neuronic  areas,  261,  262,  277 
Neurons,  31,  32,  240,  276 

adjuster,  31 

association,  254,  276 

concept,  240 

histology,  250-252 

"intelligent,"  278 

motor,  31 

pyramidal,  266 

rabbit,  18 

sensory,  31 
Neutral  solution,  547 
Neutrons,  553 
Nicotinic  acid,  59 
Night  blindness,  398,  399,  400,  403 
Nitrate  bacteria,  431 
Nitric  acid,  431 
Nitrifying  bacteria,  431 
Nitrite  bacteria,  431 
Nitrogen-fixing  bacteria,  455 
Nitrous  acid,  431 
Nodes,  auriculo-ventricular,  154 

of  Ranvier,  242 

sinoauriculo,  154 
Nondisjunction,  390 
Nonmyelinated  nerves,  242 
Nordic  race,  415 
Norfolk  Island,  416 
Normal  metabolic  rate,  89 
Notochord,  34, 196,  296, 299, 580 
Nougaret  family,  403 
Nuclear  membrane,  22,  329,  331 
Nuclei,  261 

Nucleolus,  22,  23,  331,  558 
Nucleus,  21,  23,  569 

caudate,  264 

egg,  350 

leaf  cell,  431 

nerve,  251 

sperm,  360 
Nurse-workers,  448 


610 


HUMAN  BIOLOGY 


Nutrition,  429 

earthworm,  41—42 

embryo,  290 

functional  features,  55-66 

green  plants,  564 

saprophytic,  436 

structural  features  of,  41—55 
Nutritive  deficiency  diseases,  58 
Nutritive  epithelium  (see  Mucosa) 
Nutritive  system, 

man,  40 


Oats,  heredity,  372 

Obelia,  498 

Objectives,  apochromatic,  556 

Oblique  muscles,  232 

Occipital  bone,  197,  199 

condyles,  205 

lobe,  268 

Oculomotor  nerve,  261,  519 
Odors,  primary,  221 
Oestrus,  323 
Oil  glands,  572 
Olfactory  cell,  220 
Olfactory  lobes,  262 
Olfactory  nerve,  261,  519 
Olfactory  sense,  220 
Omentum,  50,  571 
Onychophora,  500 
Oocyst,  567 
Oocyte,  289,  320,  348 

frog,  293 

primary,  348,  349 

secondary,  348,  349 
Oogenesis,  348-349 
Oogonia,  319,  348 

frog,  293 
Operculum,  583 
Opossum,  309 
Opotherapy,  546 
Opposed  muscles,  173 
Opsonins,  166,  472 
Optic  chiasma,  258 
Optic  nerve,  229,  236,  261,  511 
Oral  sucker,  297 
Orbit,  eye,  231 
Organic  evolution,  558 
Organisms,  unicellular,  7,  569 


Organization,  cellular,  7 
Organs,  20,  32,  35 

development,  300 

of  Corti,  224,  226,  227,  229 

system,  32 

vertebrate,  39 

tissues  in,  33 
Ornithine,  122 
Ornithorhyncus,  308 
Os  innominatuni,  201 
Osmotic  pressure,  521,  522 
Osteoblast,  205,  207 
Osteoclasts,  205 
Ostium,  302 
Otoschlerosis,  399 
Ovarian  cycle,  323 
Ovarian  eggs,  302 
Ovaries,  34,  42,  116,  117,  317,  319 

earthworm,  288 

frog,  292,  293 

germinal  epithelium,  320 

hen,  301,  302 

mammalian,  section,  319 

pig,  320 
Oviduct,  302,  317 

earthworm,  288 

frog,  292,  293 

mammalian,  322 
Ovulation,  303 
Ovum,  319 

(See  also  Eggs) 
Owen,  517 
Oxidaiits,  562 
Oxidases,  441 
Oxidation,  439,  562 
Oxygen,  562 

deficit,  186 

and  photosynthesis,  68 
Oxy hemoglobin,  91,  160 
Oyster,  genital  gland,  289 

gonad,  289 

sex  reversal,  289 

shell,  189 


Pain,  256 

sensation  of,  269 
Pairing,  chromosomes,  351,  387 

synaptic,  351 


INDEX 


611 


Palate,  43,  44 

hard,  199 

Palatine  bones,  199 
Palisade  cell,  431 
Pancreas,  34,  48,  92,  103,  146,  536 

alveola,  103 
Pancreatic  enzymes,  64 
Pancreatic  juice,  53 
Papilla,  44,  123 

hair,  190 

kidney,  118 
Papillary  muscle,  144 
Paracasein,  535,  557 
Paralysis,  dog  hybrids,  418 

eye  muscles,  403 
Paramecium,  9,  15,  16,  562 

avoiding  reaction,  278 

rate  of  growth,  492 
Parasite,  456 

fungal,  459 

malaria,  456 

portal  of  entry,  486 

primary,  465 

secondary,  465 
Parasitism,  456-466 
Parasympathetic  division,  246 
Parathyroid,  92,  105 

glands,  108-109 

tetany,  109 

Parental  selections,  421 
Parietal  bone,  197,  199 
Parietal  lobe,  268 
Parotid  gland,  46 
Pars  cervicalis,  259 
Pars  lumbalis,  269 
Pars  sacralis,  259 
Pars  thoracalis,  259 
Parthenogenesis,  285 

artificial,  286 
rabbits,  286 

drone,  286 

Particulate  inheritance,  355^. 
Passive  immunity,  471 
Pasteur,  Louis,  468,  479 
Patella,  201,  203 
Pearson,  406 
Peas,  contrasting  characters,  356 

tall  and  dwarf,  356 
Pectoral  girdle,  201 


Peduncle,  266 

inferior,  261 

middle,  261,  266 

posterior,  265 

superior,  261 
Pellagra,  60 
Pelvic  girdle,  201 
Pelvic  region  (sacral),  197 
Pelvis,  123 

kidney,  118 
Penis,  317 

birds,  318 

mammals,  318 
Pentadactyl  limb,  582 
Pepsin,  63,  440,  501,  534 
Pepsinogen,  63,  534 
Peptide  linkage,  537 
Peptone,  64 
Pericardium,  563,  570 
Perilymph,  223 
Perimysium,  175,  176 
Perineurium,  241 

Periosteum,  28,  175,  204,  205,  206 
Peripatus,  500 
Peripheral  effector,  243 
Peripheral  nervous  system,  239-243 
Peristalsis,  47 

intestinal,  172 

reversed,  47 

ureter,  131 

Peristaltic  actions,  172 
Peristome,  563 
Peritoneum,  60,  171,  570 
Peritonitis,  563 
Peroneal  nerve,  214 
pH  (see  Hydrogen  ion) 
Phalanges,  201 

fusion  of,  406 
Pharyngeal  slits,  296 
Pharynx,  45,  46,  48 
Phenotype,  362 

blending  inheritance,  370 
Phosphagen,  187 
Phosphatase,  531,  538,  539 
Phospholipines,  56 
Phosphoric  acid,  187 
Phosphorus,  193 
Photosynthesis,  11,  66-69,  429,  431,  439 

equation,  434 

leaf,  67,  &1 


612 


HUMAN  BIOLOGY 


Photosynthesis,  and  oxygen,  68 

Phototropism,  580 

Phrenic  nerve,  214,  244 

Physcia,  454 

Physiologus,  505 

Physiology,  development  of,  563 

Pia  mater,  267,  268 

Pig,  mule-footed,  405 

ovary,  320 

Pigment,  respiratory,  90 
Pigmentation,  400-403 

skin,  401 

Pincus,  Gregory,  286 
Pinna,  222 
Pinnularia,  18 
Pisces,  583 
Pistil,  453 

style,  453 

Pitcairn  Islanders,  416 
Pitcher  plant,  532 
Pith  cells,  corn,  18 
Pituitary,  92 

body,  258,  261 

control,  112 

deficiency,  113 

and  eggs,  114 

functions,  113—115 

gland,  111-115,  112 

historical,  565 

and  liver,  112 

set,  114 

Pivot  joint,  205 
Placenta,  311,  3,21,  584 

human,  vertical  section,  312 

mammal,  313 
Placental,  583 
Plankton,  444 
Plants,  carnivorous,  532 

colorless,  12,  532 

pitcher,  532 

unicellular,  7 

vascular  tissues,  133,  134 
Plant  louse,  chromosomes,  334 
Plasma,  29 

membrane,  22 
^lasmodium,  566 

vivax,  287,  464 
Plasmosomes,  558 
Plastids,  22 
Platelets,  blood,  139 


Platyhelminthes,  461,  465 
Pleura,  60,  78,  142,  567,  570 
Pleurisy,  78 
Plexus,  Auerbach,  61 

choroid,  268 

celiac,  244 

Meissner,  61 

mesenteric,  244 
Pliny  the  Elder,  504 
Pneumococcus,  436 

infection,  mouse,  483 
Pneumonia,  482,  567 
Poikilothermal  temperature,  583 
Point  changes,  393 
Polar  body,  348 
Polarized  light,  178 
Poliomyelitis,  475 
Pollen,  450 

lily,  18 

Pollination,  453 
Polydactyly,  398,  404 

human,  406 
Polyhybrids,  367 
Polynesian  stock,  416 
Polynucleotidase,  538 
Polysaccharide,  55,  69,  573 
Poiis,  259,  260 
Pons  varolii,  268,  261,  265 
Porifera,  461 
Portal  vein,  146,  159 
Position  effect,  393      * 
Positive  electrons,  553 
Positrons,  553 
Posterior  fissure,  252 
Postgaiiglionic  fiber,  245 
Posture,  erect,  36 
Potential,  electric,  273 
Potential  energy,  527 
Potential  differences,  521 
Precipitin,  481-484 
Precipitin  reaction,  472,  483 
Precipitin  test,  484 
Predator,  458 
Preganglionic  fiber,  245 
Pregnancy,  322 
Premolars,  200 
Prepuce,  317,  318 
Priestley,  565 

Primary  germ  layers,  23,  34 
Primary  odors,  221 


INDEX 


613 


Primary  parasites,  465 
Primary  spermatocyte,  34£> 
Primates,  309,  585 
Primitive  streak,  306 
Primordial  germ  cell,  345 

female,  319 
Proctodaeum,  300 
Progesterone,  116,  321 
Proglottids,  462 
Projection  fibers,  264 
Projection  tracts,  265 

sensory,  265 
Prolactin,  115 
Prolan,  A  and  B,  114 
Prolinase,  537 
Pronephros,  123,  299 
Pronephric  duct,  299 
Pronephric  tubule,  299 
Pronuba,  452 
Prophase,  327-330,  328 
Propolis,  450 
Proportions,  changes  in  human  body, 

322 
Proprioception,  271 

conscious,  255 

unconscious,  255 
Proprioceptive  impulses,  184 
Proprioceptive  system,  269 
Prosencephalon  (forebrain),  269 
Prostate  gland,  316,  317 
Prosthetic  group,  530 
Prosthetic  hematin,  539 
Proteases,  534 
Protection,  209 
Proteins,  6,  56,  57,  568 

blood,  137 

conjugated,  57 

derived,  57 

digestion  of,  71 

fibrin,  440 

metabolism,  liver,  122 

synthesis,  70 

Proteolytic  enzymes,  71,  534 
Proteose,  64 
Prothrombin,  164 
ProiococcuSj  18 
Protons,  553 
Protophy ta,  498 
Protoplasm,  3,  6,  569 

constituents,  6 


Protoplasm,  fibrillar,  6 

living,  6 
Protoplast,  569 
Prototheria,  308,  584 
Protozoa,  7,  8,  9,  10,  18,  461,  464,  498, 
533,  542,  554,  562,  569 

chromosomes,  335 

in  hay  infusion,  442 
Protozoology,  569 
Pseudopodium,  8,  498,  569 
Psychical,  secretions,  570 
Ptyalin,  45,  63,  538 
Pulmonary  vessels,  142,  143 
Pulse,  144 
Pupil,  229,  230 
Purkinjef  569 
Pygmies,  African,  416 
Pylorus,  48 
Pyramid,  123 

thyroid,  106 
Pyramidal  neurons,  266 
Pyramidal  tracts,  256,  264 

direct,  256 

motor,  266 

Q 

Q-band,  muscle,  178 
Queen  bee,  448,  449 
Quinine,  486 

R 

Rabbits,  artificial  parthenogenesis,  286 

eggs,  photomicrographs,  308 

liver,  101 

neuron,  18 
Rabies,  479 

vaccine,  570 
Races,  human,  416,  416 
Racial  mixtures,  422 
Radial  nerve,  214 
Radial  symmetry,  547 
Radiant  energy,  434 
Radiation,  adaptive,  584 
Radiolaria,  18 
Radius,  201 
Rafflesia  arnoldii,  460 

flower  of,  461 
Rana  pipiens,  297 


614 


HUMAN  BIOLOGY 


Rats,  inbreeding,  421 

section  of  ovary,  319 

testis,  116 
Rays,  329 
Reactions,  balanced,  531 

reversible,  531 
Receptaculum  chyli,  159 
Reception,  nerve,  268-271 
Receptor,  243,  276 
Recessive  character,  420 
Recombinations,  genes,  386,  387 
Recti,  muscles,  232 
Rectum,  54,  302,  317 
Red  cells,  destruction,  100,  167 
Reductants,  562 
Reflexes,  580 

action,  275,  277 

arc,  243,  275-279,  276 

conditioned,  278,  279 
dog,  279 

unconditioned,  278,  279 
Refractory  period,  274 
Regeneration  and  differentiation,  284 
Renaissance,  scientific,  505 
Renal  artery,  118 
Renal  tubule,  125 

detail,  124 

functions,  128 
Renal  vein,  118 
Rennin,  64,  440,  535,  557 
Repair,  489 
Reproduction,  12 

asexual,  283-286 
strawberry,  326 

basic  difference,  283 

and  fertilization,  351 

and  growth,  281jjf. 

human,  314-324 

regenerative,  283 

types  of,  283-291 

Reproductive   cycle,   triploblastic   ani- 
mal, 325 

Reproductive  system,  female,  317,  319- 
324 

hen,  301-303,  302 

male,  314-319,  317 
Reptiles,  heart,  140 

scaly  armor,  190 
Reptilia,  583 
Reserves,  mineral,  213 


Residual  air,  79 
Respiration,  10 

and  carbon  monoxide,  91 

carbonic  acid,  83 

diaphragm,  81 

functional  features,  85-91 

and  medulla,  83 

movements  in,  81 

and  ribs,  81 

structural  features,  74-76 
Respiratory  epithelium,  78 
Respiratory  gases,  79,  159 
Respiratory  quotient,  88,  186 
Respiratory  system,  man,  76-81,  72 
Resting  cell,  328 
Rete  testis,  316 
Reticulum,  23,  48,  146 
Retina,  229 

bipolar  division,  236 

degeneration,  403 

development,  234,  235 

ganglionic  division,  236 

photoreceptor,  division  of,  236 

sensory  division,  236 

structures,  235-238 
Retinal  cells,  18 
Rhetropism,  580 
Rhizopus  nigricans,  285 
Riboflavin,  59 
Ribs,  35,  200 

cartilage,  200 

and  respiration,  81 
Rickets,  60,  213 
Roans,  371 

hybrid,  370 
Rodentia,  585 
Rods,  and  cones,  retina,  231,  236,  237 

of  Corti,  226 

fiber,  237 

Root  hair,  corn,  18 
Rosenau,  574 
Rouleaux,  138 
Round  ligament,  317 
Running,  212 
Ruszicka,  115 

S 

Sacculina,  462,  463 
Sacculus,  223 
Sacral  plexus,  214 


INDEX 


615 


Sacrum,  197 

St.  Bernard,  hybrid,  418,  419,  422 

St.  Martin,  Alexis,  501 

Saliva,  45 

Salivary  and  gastric  enzymes,  63 

Salivary  glands,  Drosophila,  336 

human,  45 
Salts,  bile,  65 
Salvarsan,  486,  575 
Saprophytic  nutrition,  436 
Sarcodina,  569 
Sarcolemma,  176 
Sarcomeres,  178 
Sarcoplasm,  177 
Sartorius  muscle,  175 
Scala  media,  225,  229 
Scala  naturae,  577 
Scala  tympani,  225,  229 
Scala  vestibuli,  225,  229 
Scapula,  201,  204 
Scar  tissue,  489 
Schaudinn,  575 
Schick  test,  480 
Schiff,  578,  579 
Schneider,  Conrad,  566 
Schultze,  Max,  569 
Schwann,  501 
Sciatic  nerve,  214 
Sclera,  229,  230 
Scolex,  462 
Scrotum,  314,  317 
Scurvy,  60 
Scyphozoa,  135,  514 
Sea  anemones,  450 
Sea  squirts,  581 

Sebaceous  glands,  26,  37,  191,  572 
Secondary  characteristics,  571 
Secondary  parasites,  465 
Sccretagogues,  102,  570 
Secretin,  64,  102 
Secretion,  and  excretion,  96 

functional,  96-97 

intestinal,  52 

psychical,  570 

structural,  93-96 
Seed  plant,  chromosomes,  334 
Seed  vaccine,  573 
Segment,  earthworm,  43 
Segrnental  muscles,  174 
Segmentation,  525 


Segregation,  358 

cavity,  309 

feebleminded,  425 

vertebrate,  34 
Selection,  domesticated  animals,  374 

domesticated  plants,  374 

effective,  376 

problem  of,  374 

systematic,  376 
Selenium,  506 
Self-fertilization,  420 
Sella  tursica,  197 
Semicircular  canals,  223,  224 
Semilunar  valves,  149,  160 
Seminal  fluid,  317,  318 
Seminal  receptacles,  earthworm,  288 
Seminal  vesicles,  316,  317 

earthworm,  288 

Seminiferous  tubules,  116,  315,  316 
Semipermeable  membrane,  62 
Senescence,  492-494 
Sense,  sight,  230 
Sense  organs,  218 

exteroceptive,  218 

interoceptive,  218 
Sensory  axon,  276 
Sensory  fiber,  253 
Sensory  ganglia,  241,  276 
Sensory  impulses,  256 
Sensory  nerve  impulses,  216 
Sensory  neurons,  31 

bipolar,  250 
Sensory  tracts,  266 
Septum,  nasal,  46 
Serosa,  50,  61 
Serous  exudate,  571 
Sertoli  cells,  549 
Serum,  409 

albumin,  137 

blood,  474 

blood  clot,  476 

convalescent,  475 

globulin,  137 
Seta,  171 
Severinus,  516 
Sex,  determination  of,  379 
Sex  characters,  secondary,  116,  571 
Sex-linked  characters,  379-383,  380,  382 
Sex  reversal,  oyster,  289 
Sexual  characteristics,  571 


616 


HUMAN  BIOLOOY 


Sexual  generations,  498 
Sexual  phase,  566 
Sharpey-Schafer,  496 
Sheep,  Ancon,  422,  423 

normal,  423 
Shell,  304 

membrane,  303,  304 
Shortsightedness,  403 
Shoulder  joint,  204 
Simplex  eye,  402 
Simulium  virgatum,  338 
Singing,  $4 

Sinoauricular  node,  154 
Sinusoids,  98,  101,  110 

bone,  208 
Sirenia,  585 

Skeletal  characteristics,  404 
Skeletal  defects,  inheritance,  404 
Skeletal  muscles,  173 
Skeletal  system,  188 

functional,  209-213 

structural,  189-209 
Skeleton,  adult,  196 

appendicular,  200-205 

bony,  194-196 

divisions  of,  194 
Skin,  36-39,  37 

color,  373,  401 

corium,  172 

dermis,  190 

excretion  in,  119-121,  120 

glands,  572 

pigmentation,  401 

reactions,  allergy  test,  477 

sensory  areas,  219 
Skoptzs,  571 
Skull,  35 

frog  and  human  compared,  198 

man,  188, 197 

Small  intestine,  man,  49-53,  50 
Smallpox,  immunity,  479 

vaccine,  479,  572 
Smooth  muscle,  30,  171-173 
Snails,  534 

Snake  bite,  antitoxin,  481 
Snapdragon,  inheritance,  369 
Sneeze,  83 
Sol,  colloidal,  3 
Soma,  342,  359 
Somatic  layer,  299 


Somatic  mesoderm,  300,  306 
Somites,  chick,  305 
Sounds,  letter,  85 
Species,  413,  576 

appearance  of,  355 

conforming  to,  354 

origin,  354 

Spectroscope,  238,  433 
Spectrum,  238,  432 

colors  in,  433 
Speech,  84 
Spencer,  Herbert,  9 
Sperm,  315,  325,  345,  347 

of  cat,  18,  21 

head,  315 

middle  piece,  316 

nucleus,  350 

receptacles,  290 

structure,  315,  316 

tail,  316 

Spermatic  cord,  314,  316 
Spermatids,  315,  347 
Spermatocyte,  315 

primary,  345 

secondary,  345 
Spermatogenesis,  345-348 
Spermatogonia,  315,  345 
Sphenoid  sinus,  197,  199 
Sphincter  muscle,  urethra,  131 
Sphincter  valves,  pylorus,  184 

urethra,  184 

Sphygmomanometer,  157 
Spinal  accessory  nerve,  261,  520 
Spinal  cord,  196,  252,  263,  257,  268,  259 

central  canal,  253 

conduction,  254 

gray  matter,  253,  255 

histology,  253 

section,  255 

white  matter,  253,  255 
Spinal  nerves,  262,  253 
Spindle,  327,  328,  329,  346 

cleavage,  350 

fibers,  330,  331 

Spinocerebellar  tract,  dorsal,  255 
Spiracle,  trachea,  75 
Spiral  lamina,  225,  226 
Spirilla,  18 
Spirillum,  436 
Spirillum  granulatum,  430 


INDEX 


617 


Splanchnic  layer,  299 
Splanchnic  mesoderm,  300,  306 
Spleen,  34,  48,  166 
Splenic  vessels,  146 
Sponges,  533 
Spongy  bone,  204 
Spongy  tissue,  206 
Spore,  285 

case,  285 

formation,  285 
Sporogony,  567 
Sporophyte,  286 
Sporozoa,  286,  569 
Sporozoites,  566 
Sqitalus  acanthias,  582 
Squamous  epithelium,  24 
Stamens,  452 
Stapes,  222,  229 
Staphylococcus,  436 
Starch,  69,  573 
Starfish,  embryo,  290 
Stature,  inheritance  of,  408 
Stearic  acid,  56 
Sterile,  self,  420 
Sterility,  60 

basis  of,  in  hybrids,  414 
Sterilization,  425 

advisability,  426 

defectives,  425 

laws,  425 
legality,  426 

operation,  425 
Sternum,  200 
Sterols,  56,  574 
Stethoscope,  153,  157 
Stimulus,  electric,  273 

of  muscle,  182 
Stockard,  C.  R.,  114 
Stomach,  47,  48 

man,  47-49 

mucosa,  49 

peristalsis,  48 

regions,  48 
Stomodaeum,  300 
Stratified  epithelium,  25 
Stratum  corneum,  550 
Strawberry,  reproduction,  326 
Streptococcus,  436 
Striated  muscle,  30,  171,  177 

histology,  176-178 


Stroma,  138 
Style,  pistil,  453 
Subarachnoid,  267 
Subclavian  artery,  143,  149 
Subcutaneous  tissue,  37 
Subfingual  gland,  45 
Submaxillary  gland,  45 
Submucosa,  51 
Sucker,  298 

Sucrase,  65,  440,  538,  549 
Sucrose,  574 

molecules,  62 
Sudan  III,  159 
Sudoriparous  glands,  572 
Sulci,  248,  262 
Sulphanilamide,  486 
Sulphur  bacteria,  430,  431 
Sulphuric  acid,  430 
Sun-stroke,  129 
Support,  209 

Supporting  tissue,  26,  189 
Suprarenal  glands  (see  Adrenal  glands) 
Swallowing,  46,  47 
Sweat,  119 

control,  121 

gland,  26,  37,  38,  120,  572 
Symbiosis,  454 

Symbiotic  associations,  454-456 
Symmetry,  bilateral,  34,  526 
Sympathetic  divisions,  244 
Sympathetic  trunk,  244 
Symphalangy,  398 
Synapse,  251,  253 
Synapsis,  346,  347,  351-353,  414 

deletion,  392 

genes,  392 

position,  391 
Synaptic  pairing,  351 
Syndactyly,  398,  404 
Syndrome,  Frohlich's,  113 
Synkaryon,  288,  294,  349 
Synovial  fluid,  204 
Synovial  membrane,  204 
Synthesis,  kidney,  130 
Synthesizing  enzymes,  66 
Syphilis,  474,  483 

historical,  574 
Systematic  selection,  375 
Systemic  circulation,  149 
Systole,  152 


618 


HUMAN  BIOLOGY 


Tactile  corpuscle,  37,  219 
Tadpole,  nutritive  system,  301 
Tadpole  stage,  298 
Tail,  196,  582 
bud,  298 
frog,  297 

region  (caudal),  197 
Tapeworm,  structure,  462 
Tarsus,  201 
Taste  bud,  44,  219 
Taxonomy,  576 
Tectorial  membrane,  226 
Teeth,  44,  190,  199 
enamel,  199 
pulp  cavity,  199 
Telolecithal  eggs,  291,  301 
Telophase,  328,  331 
Temperature  control,  129 
Temperature  regulation,  120 
Temporal  bone,  197,  199 
Temporal  lobe,  268,  261 
Tendon,  27,  176,  193,  204 
of  Achilles,  190 
attachments,  176 
Tentacle,  42,  547 
Termites,  70,  534 
Testicle,  317 
Testis,  42,  92,  314,  548 
cryptorchid,  549 
earthworm,  288 
frog,  292 

interstitial  tissue,  115 
mammal,  116 
rat,  116 
rete,  316 
structure,  316  • 
Testosterone,  115 
Tetanus,  182,  472 
development,  182 
treatment,  477 
Tetany,  and  parathyroid,  109 
Tetrad  formation,  352 
Tetrakaidecahedra,  2,  21,  22 
Tetraploid  complex,  340 
Texas  fever,  567 
Thalami,  optic,  262 
Thalamus  (third  ventricle),  268 
Thalassicolla,  18 


Theophrastus,  504 
Thermodynamics,  522 
Thermotropism,  580 
Thiamin,  69 
Thiamin  chloride,  60 
Thirst,  270-271 
Thoracic  duct,  159 
Thorax,  35 
Throat,  45-47 

openings,  46 
Thumb,  twinning,  406 
Thyroid,  92,  104-108,  106,  491 

cartilage,  106 

gland,  395 

historical,  577 

pyramid,  106 
Thyroxine,  105,  106,  469 
Tibia,  175,  201 
Tibial  nerve,  214 
Tidal  air,  79 
Tissues,  20 

adipose  fatty,  27,  28 

basic,  vertebrate,  342 

collagenous,  193,  549 

connective,  26,  27 

culture,  492 
living,  493 

differentiation,  299-301 

elastic,  27 

epithelial,  24 

human,  23 

kidney,  124 

muscle,  21,  29,  30 

nerve,  31 

supporting,  26 

types  of,  24 

vascular,  28 

white  fibrous,  27 
Tocopherol,  60 
Tongue,  44 
Tonsil,  44 
Tonus,  184 

muscle,  271 
Tooth,  alveolus,  199 

crown,  200 

root,  199 

Toxemia,  chronic,  492 
Toxins,  472 
Trachea,  45,  46,  48,  76,  106t  142 

human,  77 


INDEX 


619 


Trachea,  insect,  75 
Transformer,  energy,  527 
Translocation,  gene,  391 
Triceps  muscle,  173,  174 
Trichocysts,  563 
Tricuspid  valve,  143 
Trigeminal  nerve,  261,  519 
Trihybrid,  364,  366,  367 

condition,  401 

distribution,  368 

Triploblastic  animals,  34,  170,  525 
Tristearin,  b8 
Trochlear  nerve,  261,  519 
Trophoblast,  291,  309,  310,  311 
Tropism,  15,  278,  579,  580 
Trunk,  35 

man,  35 
Trypanosoma   gambiense,    life    history, 

457 

Trypanosomes,  458 
Trypsin,  64,  104,  440,  534,  536 
Tubules,  seminiferous,  116 
Tunica  fibrosa,  230 
Tunicata,  581 
Tunnel,  of  Corti,  226,  228 
Tunnel  cells,  226,  227 
Turbinate,  197 
Turbinate  bones,  199 
Turtle,  shell,  190 
Tussock  moth,  465 
Twins,  behavior,  424 

criminal  records,  424,  425 
,     dizygotic,  424 

fraternal,  424 

identical,  424 

monozygotic,  424 
Tylosis,  398 

Tympanic  membrane,  222 
Types  of  muscles,  174-176 
Typhlosole,  43,  171 
Typhoid  bacilli,  481 
Typhoid  fever,  470,  478 
Typhoid  vaccine,  478 
Tyrosinase,  400,  441,  540 
Tyrosine,  400,  540 


U 


Ulna,  201      ' 
Ulnar  nerve,  214 


Ultramicroscopic,  scale,  553 
Umbilical  cord,  313 
Umbilical  vein,  311 
Unguiculata,  584 
Ungulata,  202,  309,  414,  584,  585 
Unit  characters,  359 
Unstriated  muscle,  171 
Urea,  100,  122,  126 

formation,  122 

synthesis,  126 
Urease,  530,  539 
Ureter,  34,  118,  123 
Urethra,  118,  123,  316 
Uric  acid,  122 
Urine,  126 

analysis,  126 

pH  of,  130 

variations  in,  129 
Uriniferous  tubule,  detail,  124 
Urogenital  canals,  frog,  291 
Urogenital  systems,  frog,  292 
Uterine  cavity,  321 
Uterine  cycle,  323 
Uterine  development,  309,  322 
Uterine  lining,  photomicrograph,  310 
Uterus,  172,  290,  302,  309,  317,  322 

gravid  human,  321 

section  through  wall,  310 
Utriculus,  223 
Uvula,  43,  44,  46 


Vaccination,  479 

Vaccines,  478,  479,  570,  572,  573 

Vacuole,  of  cell,  22,  23 

contractile,  8,  9 
Vagina,  322,  324 
Vagus  nerve,  214,  244,  261,  520 
Valve,  145 

bicuspid,  143,  151 

ileocaecal,  49,  53 

semilunar,  149,  150 

tricuspid,  143 
Van  Helmont,  565 
Variation,  gene  complex,  352 

in  heights,  407 

individual,  354 

in  weights,  408 


HUMAN  BIOLOGY 


Vas  deferens,  314,  315,  316,  317 

earthworm,  288 
Vasa  efferentia,  291,  292 
Vascular  system,  132 

functional  features,  152-167 

structural  features,  135-152 
Vascular  tissue,  28 

.plants,  133,  134 
Vascularization,  diaphragm,  488 

heart  valve,  151 
Vasodilator,  120 

fibers,  318 
Vegetal  pole,  294, 
Vein,  145 

arcuate,  118,  124 

jugular,  159 

portal,  146,  159 

renal,  118 

structure,  145-146 
Vena  cava,  118,  142,  143,  146 
Venereal  disease,  474 
Venom,  472 
Ventral  root,  253 
Ventricle,  143 

brain,  267 

fourth,  260 
Venulae  rectae,  124 
Verga,  566 
Vertebra,  35,  582 

caudal,  197 

lumbar,  197 

sacral,  197 

seventh  cervical,  197 

twelfth  thoracic,  197 
Vertebral  canal,  196 
Vertebral  column,  35,  196,  580 

man,  197 

Vertebral  plate,  298 
Vertebrata,  581 
Vertebrate,  580 

body  plan,  34 

embryo,  primitive,  299 

lung  structure,  76 

organ  systems,  39 

segmentation,  34 
Vesalius,  543,  564,  565 
Vestigial  male,  386 
Vestigial  structure,  499 
Villi,  51,  52,  311 

placental,  312 


Virulence,  reduced,  459 
Viruses,  4*66 
Viscosity,  586 
Visual  purple,  237 
Visual  tracts,  265 
Vitamins,  57-61,  469 

A,  58,  400 

B-complex,  59 

C,  60 

D,  60,  69 

E,  60 
K,  60 

Vitelline  membrane,  293 

Vitreous  chamber,  233 

Vitreous  humor,  233 

Viviparous  development,  308 

Vocal  cords,  48,  84 

Voice,  84 

Voluntary  muscles,  division  of,  174 

striated,  172-178 
Volvox,  16,  555,  587 
Vomer,  199 
Vorticella,  169,  170 

W 

Walking,  212 
Wallace,  561 

Wasserman  test,  474,  485,  575 
Web  of  life,  428-466 
Webster,  Noah,  467 
Weights,  variation  in,  408 
Weismann,  561 
Wheat,  heredity,  373 
Wheat  rust,  life  history,  459 
White  fibrous  tissue,  27 
White  matter,  brain,  266 

spinal  cord,  253,  255 
Widal  test,  481,  482 
Will,  277 

Williams,  R.  R.,  60 
Wind  pipe  (see  Trachea) 
Wing,  bird,  202 
Wohler,  126 
Womb,  290 
Wood,  69 
Worker,  448 
Wright,  Seth,  422 


__    ,  .     A   __.  nucleus,  18 

X-chromosome  giant,  385  pl       295  2 

(See  also  Chromosomes)  O*Q  g^ 


INDEX  621 

Yolk,  globule,  18 


Y  ^1 

X-X  pattern,  341  Yucca  453 

X-Y  pattern,  341  Yucca,  453 


Zygomatic  arch,  199 

Yeast,  18  Zygote,  294,  326,  464 

asexual  reproduction,  282  cleavage,  294,  342 

budding,  282  Zymase,  440 

Yolk,  296,  303,  313  complex,  533