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SCHOOL 


Dr.    J.J.   Miller 


Hmetican  Science  Series 

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AMERICAN  SCIENCE  SERIES— ADVANCED  COURSE 


THE    HUMAN    BODY 


AN  ACCOUNT  OP 


ITS  STRUCTURE  AND  ACTIVITIES  AND   THE 
CONDITIONS  OF  ITS  HEALTHY   WORKING 


H.   NEWELL  ^MARTIN,  D.Sc.,  M.A.,  M.D.,  F.R.S. 

Late  Professor  of  Biology  in  the  Johns  Hopkins  University 

and  of  Physiology  in  the  Medical  Faculty 

of  the  same 

TENTH   EDITION,  THOROUGHLY    REVISED 


ERNEST  G.  MARTIN,  PH.D. 

Professor  of  Physiology  in  Leland  Stanford  Junior  University 


QP3G 

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NEW   YORK 

HENRY  HOLT  AND  COMPANY 
1919 


COPYRIGHT.  1881,  1896. 

BY 

HENRY  HOLT  &  CO. 


COPYRIGHT,  1909.  1910,  1917. 

BY 
HENRY    HOLT   AND   COMPANY 


PREFACE   TO  THE  TENTH  EDITION 

THE  current  tendency  of  physiological  thought  is  clearly  toward 
an  increasing  emphasis  upon  the  unity  of  operation  of  the  Human 
Body.  Although  the  only  feasible  method  of  presentation  of  the 
subject  continues  to  be  by  successive  consideration  of  physiological 
systems  one  after  another,  physiologists  recognize  the  desirability 
of  keeping  constantly  before  the  mind  of  the  student  the  fact  that 
in  reality  the  .Body  is  more  than  an  aggregate  of  systems:  that 
it  is  an  integer  of  parts  so  closely  interdependent  that  the  activity 
of  any  one  calls  forth  related  activities  of  the  others.  In  the  present 
revision  the  attempt  has  been  made  to  keep  this  idea  in  the  fore- 
ground. Increased  emphasis  is  placed  upon  the  manifestations 
of  adaptation  in  the  Body;  cross  references  are  freely  used  through- 
out. As  a  further  assistance  toward  this  end  a  changed  method  of 
treating  the  subject  of  chemical  co-ordination  is  adopted.  Instead 
of  a  chapter,  or  part  of  a  chapter,  devoted  to  internal  secretions, 
the  conception  of  chemical  co-ordination  is  introduced,  concur- 
rently with  that  of  nervous  co-ordination,  in  an  early  chapter, 
and  the  various  hormones  are  described  in  connection  with  the 
bodily  processes  they  are  known  to  modify. 

Numerous  minor  changes  have  been  made  to  bring  the  pre- 
sentation abreast  of  present  physiological  knowledge.  The  fol- 
lowing more  or  less  extensive  additions  to  the  subject-matter  of 
the  former  edition  have  also  been  included.  A  section  on  the 
physical  chemistry  of  the  Body  is  added  to  the  first  chapter.  To 
this  the  paragraphs  on  filtration,  osmosis,  and  dialysis  are  trans- 
ferred from  the  chapter  on  blood  in  which  they  were  formerly 
given.  A  brief  discussion  of  crystalloids,  colloids,  solutions,  and 
the  significance  of  cell  membranes  is  also  included.  The  concep- 
tion of  the  Body  as  a  machine  in  its  energy  relationships,  obeying 
the  familiar  laws  of  mechanics,  is  emphasized  more  strongly  than 
hitherto.  In  connection  with  the  discussion  of  muscle  physiology 
a  rather  full  account  of  the  energy  transformations  in  active  muscle 
is  given.  The  section  on  the  nervous  system  has  been  modified 


vi  PREFACE  TO  THE  TENTH  EDITION 

in  two  respects.  The  description  of  the  cerebellum  and  its  func- 
tions is  introduced  before  the  account  of  the  cerebrum,  instead  of 
after  it  as  in  the  former  presentation.  The  chapter  on  the  auto- 
nomic  system  has  been  rewritten,  with  special  emphasis  on  the 
"emergency  "  function  of  the  system.  The  relation  of  the  hormone 
adrenin  to  the  autoriomic  system  is  discussed.  In  connection  with 
the  section  on  respiration  the  effect  of  muscular  exercise  on  the 
respiratory  function  is  given  fuller  consideration.  In  the  chapter 
on  foods  the  paragraphs  dealing  with  dietary  accessories  are  elab- 
orated. The  recent  work  on  the  relative  food  values  of  various 
proteins  is  described  at  some  length.  The  conception  of  basal 
metabolism  is  introduced  as  a  ground  work  for  the  discussion  of 
general  bodily  metabolism.  In  the  chapter  on  reproduction  para- 
graphs dealing  in  an  elementary  way  with  Mendelian  inheritance 
and  the  mechanism  of  sex-determination  are  included.  In  response 
to  requests  from  several  users  of  the  book  an  appendix  containing 
suggestions  for  laboratory  experiments  suitable  for  undergraduate 
classes  is  added.  The  basis  of  these  is  the  laboratory  course  given 
for  several  years  to  undergraduates  at  Harvard  and  Radcliffe 
Colleges. 

I  beg  to  acknowledge  the  receipt  of  helpful  suggestions  from 
various  colleagues.  Among  these  I  wish  to  mention  especially 
Professor  Chas.  Wright  Dodge,  who  very  kindly  furnished  me  a 
detailed  list  of  comments  based  on  his  use  of  the  former  edition 
in  his  classes  during  a  period  of  years. 

A  number  of  new  cuts  have  been  introduced.  Some  of  these 
were  drawn  especially  for  this  edition.  Others  were  kindly  fur- 
nished by  the  publishers  of  various  text-books.  I  desire  to  make 
acknowledgment  of  this  courtesy. 

ERNEST  G.  MARTIN. 
STANFORD  UNIVERSITY,  CALIF. 

Jvly,  1917. 


PREFACE  TO  THE  FIRST  EDITION 

IN  the  following  pages  I  have  endeavored  to  give  an  account  of 
the  structure  and  activities  of  the  Human  Body,  which,  while  in- 
telligible, to  the  general  reader,  shall  be  accurate,  and  sufficiently 
minute  in  details  to  meet  the  requirements  of  students  who  are  not 
making  Human  Anatomy  and  Physiology  subjects  of  special  ad- 
vanced study.  Wherever  it  seemed  to  me  really  profitable,  hy- 
gienic topics  have  also  been  discussed,  though  at  first  glance  they 
may  seem  less  fully  treated  of  than  in  many  School  or  College 
Text-books  of  Physiology.  Whoever  will  take  the  trouble,  how- 
ever, to  examine  critically  what  passes  for  Hygiene  in  the  majority 
of  such  cases  will,  I  think,  find  that,  when  correct,  much  of  it  is 
platitude  or  truism:  since  there  is  so  much  that  is  of  importance 
and  interest  to  be  said  it  seems  hardly  worth  while  to  occupy  space 
with  insisting  on  the  commonplace  or  obvious. 

It  is  hard  to  write  a  book,  not  designed  for  specialists,  without 
running  the  risk  of  being  accused  of  dogmatism,  and  some  readers 
will,  no  doubt,  be  inclined  to  think  that,  in  several  instances,  I  have 
treated  as  established  facts  matters  which  are  still  open  to  discus- 
sion. General  readers  and  students  are,  however,  only  bewildered 
by  the  production  of  an  array  of  observations  and  arguments  on 
each  side  of  every  question,  and,  in  the  majority  of  cases,  the  chief 
responsibility  under  which  the  author  of  a  text-book  lies  is  to  select 
what  seem  to  him  the  best  supported  views,  and  then  to  state  them 
simply  and  concisely :  how  wise  the  choice  of  a  side  has  been  in  each 
case  can  only  be  determined  by  the  discoveries  of  the  future. 

Others  will,  I  am  inclined  to  think,  raise  the  contrary  objection 
that  too  many  disputed  matters  have  been  discussed :  this  was  de- 
liberately done  as  the  result  of  an  experience  in  teaching  Physi- 
ology which  now  extends  over  more  than  ten  years.  It  would  have 
been  comparatively  easy  to  slip  over  things  still  uncertain  and 
subjects  as  yet  uninvestigated,  and  to  represent  our  knowledge  of 
the  workings  of  the  animal  body  as  neatly  rounded  off  at  all  its 
contours  and  complete  in  all  its  details — totus,  teres,  et  rotundus. 


viii  PREFACE  TO  THE  FIRST  EDITION 

But  by  so  doing  no  adequate  idea  of  the  present  state  of  physi- 
ological science  would  have  been  conveyed;  in  many  directions  it  is 
much  farther  traveled  and  more  completely  known  than  in  others ; 
and,  as  ever,  exactly  the  most  interesting  points  are  those  which  lie 
on  the  boundary  between  what  we  know  and  what  we  hope  to 
know.  In  Gross  Anatomy  there  are  now  but  few  points  calling  for 
a  suspension  of  judgment;  with  respect  to  Microscopic  Anatomy 
there  are  more ;  but  a  treatise  on  Physiology  which  would  pass  by, 
unmentioned,  all  things  not  known  but  sought,  would  convey  an 
utterly  unfaithful  and  untrue  idea.  Physiology  has  not  finished 
its  course.  It  is  not  cut  and  dried,  and  ready  to  be  laid  aside  for 
reference  like  a  specimen  in  an  Herbarium,  but  is  comparable 
rather  to  a  living,  growing  plant,  with  some  stout  and  useful 
branches  well  raised  into  the  light,  others  but  part  grown,  and 
many  still  represented  by  unfolded  buds.  To  the  teacher,  more- 
over, no  pupil  is  more  discouraging  than  the  one  who  thinks  there 
is  nothing  to  learn;  and  the  boy  who  has  "finished"  Latin  and 
"  done  "  Geometry  finds  sometimes  his  counterpart  in  the  lad  who 
has  "gone  through"  Physiology.  For  this  unfortunate  state  of 
mind  many  Text-books  are,  I  believe,  much  to  blame:  difficulties 
are  too  often  ignored,  or  opening  vistas  of  knowledge  resolutely 
kept  out  of  view:  the  forbidden  regions  may  be,  it  is  true,  too 
rough  for  the  young  student  to  be  guided  through,  or  as  yet  path- 
less for  the  pioneers  of  thought ;  but  the  opportunity  to  arouse  the 
receptive  mental  attitude  apt  to  be  produced  by  the  recognition 
of  the  fact  that  much  more  still  remains  to  be  learned — to  excite 
the  exercise  of  the  reasoning  faculties  upon  disputed  matters — and, 
in  some  of  the  better  minds,  to  arouse  the  longing  to  assist  in  add- 
ing to  knowledge,  is  an  inestimable  advantage,  not  to  be  lightly 
thrown  aside  through  the  desire  to  make  an  elegantly  symmetrical 
book.  While  I  trust,  therefore,  that  this  volume  contains  all  the 
more  important  facts  at  present  known  about  the  working  of  our 
Bodies,  I  as  earnestly  hope  that  it  makes  plain  that  very  much  is 
yet  to  be  discovered. 

A  work  of  the  scope  of  the  present  volume  is,  of  course,  not  the 
proper  medium  for  the  publication  of  novel  facts;  but,  while  the 
"  Human  Body,"  accordingly,  professes  to  be  merely  a  compilation, 
the  introduction  of  constant  references  to  authorities  would  have 
been  out  of  place.  I  trust,  however,  that  it  will  be  found  through- 


PREFACE  TO  THE  FIRST  EDITION  ix 

out  imbued  with  the  influence  of  my  beloved  master,  Michael 
Foster;  and  on  various  hygienic  topics  I  have  to  acknowledge  a 
special  indebtedness  to  the  excellent  series  entitled  Health  Primers. 

The  majority  of  the  anatomical  illustrations  are  from  Henle's 
Anatomie  des  Menschen,  and  a  few  from  Arendt's  Schulatlas,  the 
publishers  of  each  furnishing  electrotypes.  A  considerable  num- 
ber, mainly  histological,  'are  from  Quain's  Anatomy,  and  a  few 
figures  are  after  Bernstein,  Carpenter,  Frey,  Haeckel,  Helmholtz, 
Huxley,  McKendrick,  and  Wundt.  About  thirty,  chiefly  diagram- 
matic, were  drawn  specially  for  the  work. 

Quantities  are  throughout  expressed  first  on  the  metric  system, 
their  approximate  equivalents  in  American  weights  and  measures 
being  added  in  brackets. 

H.  NEWELL  MARTIN. 

BALTIMORE,  October,  1880. 


CONTENTS 


CHAPTER  I 

THE   GENERAL  STRUCTURE   AND   COMPOSITION   OP  THE  HUMAN  BODY 

PAGE 

Definitions.  Tissues  and  organs.  Histology.  Zoological  position  of 
man.  The  vertebrate  plan  of  structure.  The  mammalia.  Microscopic 
structure  of  the  Body.  Chemical  composition  of  the  Body.  Physico- 
chemical  constitution  of  the  Body 1 

CHAPTER  II 

THE  FUNDAMENTAL  PHYSIOLOGICAL  ACTIONS 

The  properties  of  the  living  Body.  The  Body  as  a  machine.  Cell 
growth.  Details  of  cell  structure.  Mitotic  cell  division.  Significance  of 
the  physiological  properties.  Adaptation.  Co-ordination  in  the  Body.  21 

CHAPTER  III 

TISSUES,    ORGANS,    AND   PHYSIOLOGICAL  SYSTEMS 

Development.  The  physiological  division  of  labor.  Classification  of  the 
tissues.  The  combination  of  tissues  to  form  organs.  Physiological  sys- 
tems. The  relation  of  man  to  his  environment.  Adaptive  systems.  Main- 
tenance systems.  Chemical  co-ordination.  Animals  compared  with 
plants , 29 

CHAPTER  IV 

THE   SUPPORTING  TISSUES 

Connective  tissue.  Cartilage.  Bone.  Hormones  of  the  supporting 
system.  Hygienic  remarks 43 

CHAPTER  V 

THE   SKELETON 

Exoskeleton  and  endoskeleton.  The  bony  skeleton.  Peculiarities  of  the 
human  skeleton.  Hygiene  of  the  bony  skeleton.  Articulations.  Joints. 

Hygiene  6f  the  joints 53 

xi 


xii  CONTENTS 

CHAPTER  VI 

THE   STRUCTURE   OF  THE   MOTOR   ORGANS 

PAGE 

Motion  in  animals.  The  muscles.  Histology  of  skeletal  muscle.  Struc- 
ture of  the  smooth  muscles.  Cardiac  muscular  tissue.  Ciliated  cells. 
The  physico-chemistry  of  skeletal  muscle.  The  chemistry  of  muscular 
tissue.  Rigor  mortis 78 

CHAPTER  VII 

MUSCULAR    ACTIVITY 

The  study  of  isolated  muscles.  The  necessity  of  stimulation.  A  simple 
muscular  contraction.  The  influence  of  increasing  stimulation  strength. 
The  influence  of  temperature.  Heat  rigor.  The  measure  of  muscular 
work.  Influence  of  the  form  of  the  muscle  on  its  working  power.  The 
beneficial  effect  of  exercise.  The  nature  of  fatigue.  The  response  to  rap- 
idly repeated  stimuli.  Tetanus.  Voluntary  muscular  contraction.  The 
electrical  phenomena  of  muscle.  The  source  of  muscular  energy.  The 
chemistry  of  muscular  contraction.  The  energy  relationships  of  contract- 
ing muscle.  Muscular  efficiency.  Energy  units.  The  energy  output  of 
muscle.  Significance  of  lactic  acid  in  the  contraction  process.  Summary 
of  the  contraction  process.  Oxidation  in  muscle.  Hormone  of  skeletal 
muscle.  Physiology  of  smooth  muscle.  Mechanism  of  contraction  of 
smooth  muscle.  Physiology  of  cardiac  muscle 93 


CHAPTER  VIII 

THE   USE   OF  MUSCLES  IN  THE  BODY 

Special  physiology  of  the  skeletal  muscles.  Levers  in  the  Body.  Loss 
to  the  muscles  from  the  direction  of  their  pull.  The  equilibrium  of  oppos- 
ing muscles.  Functional  muscle  groups.  Posture.  Locomotion.  Pre- 
hension. Hygiene  of  the  muscles.  Varieties  of  exercise ....: 118 


CHAPTER  IX 

ANATOMY  OF  THE  NERVOUS  SYSTEM 

General  statement.  Nerve  impulses.  Neurons.  Synapses.  The  mye- 
lin  sheath.  The  central  and  peripheral  nervous  systems.  Membranes  of 
the  central  nervous  system.  Ventricles  of  the  brain  and  central  canal  of 
the  spinal  cord.  Cerebrospinal  fluid.  The  spinal  cord.  The  brain.  The 
spinal  nerves.  Cranial  nerves.  White  and  gray  matter.  The  sympathetic 
or  autonomic  system 135 


CONTENTS  xiii 

CHAPTER  X 

GENERAL    PHYSIOLOGY    OF    THE    NERVOUS    SYSTEM.       SPINAL   AND    CEREBELLAR 

REFLEXES 

PAGE 

Conduction  within  single  neurons.  Nature  of  the  nerve  impulse.  Re- 
flexes. Reflex  arcs.  Irreversible  conduction.  Graded  synaptic  resistance. 
The  orderly  spreading  of  reflexes.  Simple  reflexes  mediated  by  the  spinal 
cord.  Significance  of  the  head  senses  in  the  control  of  reflexes.  The  sen- 
sory basis  of  locomotion.  Structure  and  connections  of  the  cerebellum. 
Functions  of  the  cerebellum.  Postural  reflexes 155 


CHAPTER  XI 

STRUCTURE,    NERVE   CONNECTIONS,    AND   FUNCTIONS   OF  THE   CEREBRUM 

The  cerebrum  in  relation  to  muscular  activity.  A  normal  animal  com- 
pared with  a  "reflex"  one.  The  cerebrum  dependent  on  the  receptor  sys- 
tem. Afferent  paths  of  the  cerebrum.  Tracing  nerve  paths.  Tracts  of 
Body  sense.  Tracts  of  the  head  senses.  General  structure  of  the  cerebrum. 
Structure  of  the  cortex.  Cortical  localization.  Cortical  reflex  paths.  Cor- 
tical reflexes  compared  with  spinal.  Memory.  Association.  Volition. 
Inhibition.  Will  power.  Habit  formation.  Language.  Consciousness. 
Emotions.  Cerebral  functions  compared  in  man  and  animals.  Nourish- 
ment of  the  brain .  .  .  169 


CHAPTER  XII 

THE   AUTONOMIC   NERVOUS  SYSTEM.      NERVOUS  FATIGUE.      HORMONES  OF  THE 

NERVOUS   SYSTEM 

The  brain  stem  (medulla  and  mid-brain).  The  autonomic  or  sympa- 
thetic system.  The  effect  of  nicotine.  Reflex  control  of  the  autonomic 
system.  Grand  divisions  of  the  autonomic  system.  This  an  emergency 
mechanism.  The  relation  of  the  autonomic  system  to  emotional  states. 
Neuro-muscular  fatigue.  Adrenin.  The  thyroid.  Emergency  action  of 
the  thyroid 192 


CHAPTER  XIII 

THE   RECEPTOR   SYSTEM.      INTERNAL  AND   CUTANEOUS   SENSATIONS 

The  receptor  system.  The  differences  between  sensations.  Psycho- 
physical  law.  Classification  of  receptors.  Internal  senses.  Muscular 
sense.  Hunger.  Thirst.  Cutaneous  senses.  Pain.  Touch.  Tempera- 
ture sense.  Peripheral  reference  of  sensations.  Perceptions.  Illusions..  204 


XIV  CONTENTS 

CHAPTER  XIV 

THE  EAR.      HEARING   AND   EQUILIBRATION.      TASTE   AND   SMELL 

PAGE 

Functions  of  the  ear.  Sounds.  Sympathetic  resonance.  The  external 
ear.  Functions  of  the  tympanic  membrane.  The  middle  ear.  Auditory 
ossicles.  Internal  ear.  Bony  labyrinth.  Membraneous  labyrinth.  Or- 
gan of  Corti.  Function  of  the  cochlea.  Auditory  perceptions.  Nerve- 
endings  in  semicircular  canals  and  vestibule.  Equilibrium  sense.  Smell. 
Taste. 223 

CHAPTER  XV 

THE   EYE   AS  AN   OPTICAL  INSTRUMENT 

The  essential  structure  of  an  eye.  Appendages  of  the  eye.  Lachrymal 
apparatus.  Muscles  of  eye.  Anatomy  of  eyeball.  Optic  nerves,  chiasma, 
and  tracts.  Retina.  Refracting  media  of  the  eye.  Ciliary  muscle.  Prop- 
erties of  light.  Refraction.  Wide  range  of  clear  vision  in  the  resting  eye. 
Accommodation.  Defects  of  the  eye.  Hygiene  of  the  eyes 243 

CHAPTER  XVI 

THE  EYE  AS  A  SENSORY  APPARATUS 

The  excitation  of  the  visual  apparatus.  Intensity  of  visual  sensations. 
Function  of  the  rods.  Visual  purple.  Duration  of  luminous  sensations. 
Localizing  power  of  retina.  Color  vision.  Function  of  the  cones.  Dis- 
tribution of  color  sense  over  the  retina.  Color  blindness.  After  images. 
Contrasts.  Theories  of  color  vision.  Visual  perceptions.  Vision  with  two 
eyes.  Perception  of  solidity 267 

CHAPTER  XVII 

THE   STRUCTURE   AND   FUNCTIONS   OF  BLOOD   AND   LYMPH 

The  external  medium.  The  internal  medium.  Blood.  Lymph.  Re- 
newal of  lymph.  Lymphatic  vessels.  Lacteals.  Composition  of  blood. 
Red  corpuscles.  Hemoglobin.  Origin  and  fate  of  red  corpuscles.  Spleen. 
Function  of  the  spleen.  Leucocytes.  Blood  plates.  Plasma.  Quantity 
of  blood.  Blood  of  other  animals.  Histology  and  chemistry  of  lymph . .  290 

CHAPTER  XVIII 

THE  HORMONE-CARRYING   AND   DISEASE-RESISTING   FUNCTIONS   OF  THE  BLOOD. 

BLOOD   CLOTTING 

Hormones.  Infection.  Resistance  to  infection.  Recovery  from  in- 
fection. Opsonins,  immune  bodies,  and  agglutinins.  Antitoxin.  Im- 
munity. Carriers.  The  use  of  antitoxin  in  disease.  Protective  inoculation. 


CONTENTS  xv 

PAGE 

Anaphylaxis.  Coagulation  of  blood.  Cause  of  coagulation.  Use  of  co- 
agulation. Source  of  blood-fibrin.  Thrombin.  Antithrombin.  Throm- 
boplastic  substance.  Methods  of  hastening  or  retarding  coagulation. 
Bleeders.  Transfusion 305 

CHAPTER  XIX 

THE  ANATOMY  OP  THE  HEART  AND  BLOOD  VESSELS 

General  statement.  Position  of  heart.  Membranes  of  heart.  Cavities 
of  heart.  Anatomy  of  heart.  Valves  of  heart.  Arterial  system.  Capil- 
laries. Veins.  Pulmonary  circulation.  Course  of  blood.  Portal  circula- 
tion. Arterial  and  venous  blood.  Structure  of  vessels 322 

CHAPTER  XX 

THE  ACTION  OF  THE  HEART.   THE  REGULATION  OF  THE  HEART-BEAT 

Beat  of  the  heart.  Cardiac  impulse.  Events  of  a  cardiac  cycle.  Use 
of  papillary  muscles.  Sounds  of  heart.  Action  of  heart-valves.  Effects 
of  valvular  insufficiency.  Function  of  auricles.  Work  done  by  heart. 
Relation  of  nerve  and  muscle  elements  within  heart.  Physiological  pe- 
culiarities of  heart.  Passage  of  beat  over  heart.  Neurogenic  and  myogenic 
theories  of  beat.  Nature  of  automatic  rhythmicity.  Extrinsic  nerves  of 
heart.  Inhibitory  and  augmentor  centers ' 339 

CHAPTER  XXI 

THE  CIRCULATION  OF  THE  BLOOD.   BLOOD  PRESSURE  AND  BLOOD  VELOCITY. 

THE  PULSE 

Circulation  seen  in  frog's  web.  Resistance  to  blood-flow.  Conversion 
of  intermittent  into  continuous  flow.  Arterial  pressure.  Weber's  schema. 
The  pulse.  Blood-pressure  in  man.  Rate  of  the  blood-flow.  Secondary 
factors  affecting  the  circulation.  Aspiration  of  the  thorax.  Proofs  of  the 
circulation  of  the  blood 355 

CHAPTER  XXII 

THE   VASOMOTOR   MECHANCISM.      SLEEP.      THE   LYMPHATIC   SYSTEM 

Distribution  of  blood  among  various  parts  of  Body.  Nerves  of  blood- 
vessels. Vasoconstrictor  nerves.  Vasoconstrictor  center.  Control  of 
vasoconstrictor  center.  Depressor  nerve.  Taking  cold.  Vasodilator 
nerves.  Vasodilator  center.  Relation  of  vasomotor  tone  to  cerebral  ac- 
tivity. Sleep.  Adrenin.  The  lymphatics.  Structure  of  lymph  vessels. 
Lymph-nodes.  Tonsils  and  adenoids.  Movements  of  lymph.  Lympha- 
gogues 373 


xvi  CONTENTS 

CHAPTER  XXIII 

RESPIRATION.     THE  MECHANISM  OF  BREATHING.      THE  REGULATION  OF 

BREATHING 

PAGE 

Definitions.  Respiratory  organs.  Air-passages  and  lungs.  Trachea 
and  bronchi.  Structure  of  lungs.  Pleura.  Respiratory  movements. 
Anatomy  of  thorax.  Changes  in  size  of  thorax.  Forced  respiration.  Res- 
piratory sounds.  Capacity  of  lungs.  Aspiration  of  thorax.  Respiratory 
center.  Excitation  of  respiratory  center.  Sensitiveness  of  respiratory 
center.  Eupnea,  hyperpnea,  dyspnea,  apnea.  Holding  the  breath.  As- 
phyxia. Artificial  respiration.  Modified  respiratory  movements 386 

CHAPTER  XXIV 

RESPIRATION.      THE   GASEOUS  INTERCHANGES 

Nature  of  the  problems.  Changes  produced  in  air  by  being  breathed. 
Ventilation.  Changes  undergone  by  blood  in  lungs.  Blood  gases.  Laws 
governing  absorption  of  gases  by  liquid.  Absorption  of  oxygen  by  blood. 
Oxygen  interchanges  in  blood.  Carbon  dioxid  in  blood.  Hormone  action 
of  carbon  dioxid.  Tissue  respiration.  Respiratory  changes  in  muscular 
exercise.  Coal  gas  poisoning 410 

CHAPTER  XXV 

FOODS!  THEIR  CLASSIFICATION 

What  constitutes  food.  Function  of  food.  Classes  of  foods.  Occur- 
rence of  nutrients  in  food.  Inorganic  essential  accessories.  Organic  essen- 
tial accessories.  Vitamines.  Occurrence  of  occasional  accessories  in  food. 
Nutrients.  Mixed  foods.  Flesh.  Eggs.  Milk.  Vegetable  foods.  Com- 
position of  foods.  Alcohol.  Tea,  coffee,  cocoa.  Food  poisoning 428 

CHAPTER  XXVI 

THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES 

General  arrangement.  Subdivisions  of  the  canal.  Mouth.  Teeth. 
Tongue.  Salivary  glands.  Pharynx.  Esophagus.  Stomach.  Small  in- 
testine. Large  intestine.  Nerves  of  intestines.  Liver.  Pancreas.  Blood- 
vessels of  canal 442 

CHAPTER  XXVIII 

THE   CHEMISTRY    OF   DIGESTION 

Object  of  digestion.  Nature  of  the  digestive  process.  Digestion  prod- 
ucts. Saliva.  Gastric  juice.  Pancreatic  juice.  Bile.  Succus  entericus. 
Summary  of  digestive  process.  Bacterial  digestion.  Prevention  of  self- 
digestion  462 


CONTENTS  xvii 

CHAPTER  XXVIII 

MOVEMENTS   OF  THE   ALIMENTARY   CANAL 

Mastication.  Hygiene  of  mouth.  Deglutition.  Movements  of  stom- 
ach. Control  of  pyloric  sphincter.  Importance  of  stomach.  Movements 
of  small  intestine.  Extrinsic  control  of  stomach  and  intestinal  movements. 
Movements  of  large  intestine.  Importance  of  roughage 469 

CHAPTER  XXIX 

THE   DIGESTIVE   SECRETIONS   AND  THEIR  CONTROL 

Organs  of  secretion.  Forms  of  glands.  Secretory  process.  Nervous 
control  of  secretory  process.  Hormone  control  of  gland  activity.  Control 
of  salivary  secretion.  Control  of  gastric  secretion.  Nature  of  chemical 
stimulus  to  gastric  secretion.  Control  of  pancreatic  secretion.  Control 
of  bile  flow.  Control  of  succus  entericus.  Digestive  history  of  a  meal. 
Maintenance  of  good  digestion 480 

CHAPTER  XXX 

THE  ABSORPTION  AND  USE  OF  FOODS 

General  statement.  Absorption  from  stomach.  Absorption  in  small 
intestine.  Nature  of  absorptive  process.  Channels  of  absorption.  Ab- 
sorption and  storage  of  carbohydrates.  Glycogen  in  muscles.  Relation 
of  kidney  to  concentration  of  sugar  in  blood.  Alimentary  glycosuria. 
Emotional  glycosuria.  Diabetes  mellitus.  Glycosuria  from  increased  per- 
meability of  kidney  cells.  Absorption  of  proteins.  Absorption  of  fats. 
Absorption  from  large  intestine.  Food  requirement  of  Body.  Protein  re- 
quirement of  Body.  Maintenance  proteins  and  growth  proteins.  Fuel 
protein.  Liberation  of  energy  in  Body.  Basal  metabolism.  Metabolism 
of  muscular  work.  Relative  food  values  of  proteins,  carbohydrates,  and 
fats.  Specific  dynamic  action  of  proteins.  Nutritive  value  of  albuminoids. 
Special  metabolism  of  fats.  Principles  of  dietetics.  Maintenance  of  con- 
stant weight.  Water  equilibrium.  Nitrogen  equilibrium.  Carbon  equi- 
librium. Influence  of  thyroid  hormone  on  metabolism.  Treatment  for 
obesity.  Source  of  Body  fat 491 

CHAPTER  XXXI 

EXCRETION  AND  THE  EXCRETORY  ORGANS 

Exogenous  and  endogenous  excreta.  Channels  of  excretion.  Liver  as  an 
excretory  organ.  General  arrangement  of  urinary  organs.  Structure  of 
kidney.  Blood-flow  through  kidney.  Urine.  Secretory  action  of  different 
parts  of  tubule.  Relation  of  blood-flow  to  secretion  of  urine.  Skin.  Hairs. 
Nails.  Glands  of  skin.  Skin  secretions.  Factors  in  sweat  secretion.  Se- 
baceous secretion.  Bathing 516 


xviii  CONTENTS 

CHAPTER  XXXII 

THE  PRODUCTION  AND  REGULATION  OP  THE  HEAT  OF  THE  BODY 

PAGE 

Cold-  and  warm-blooded  animals.  Temperature  of  Body.  Sources  of 
animal  heat.  Maintenance  of  uniform  temperature.  Local  temperature. 
Fever.  Clothing 539 

CHAPTER  XXXIII 

VOICE   AND   SPEECH 

Voice.  Larynx.  Vocal  cords.  Muscles  of  larynx.  Vowels.  Conso- 
nants    546 

CHAPTER  XXXIV 

REPRODUCTION 

Reproduction  in  general.  Germ  cells  compared  with  tissue  cells.  Sex- 
ual reproduction.  Maturation  of  the  germ-cells.  Accessory  reproductive 
organs.  Male  reproductive  organs.  Seminal  fluid.  Female  reproductive 
organs.  Mammalian  ovum.  Ovulation.  Menstruation.  Fertilization. 
Heredity.  Sex  determination.  Impregnation.  Pregnancy.  Intra- 
uterine  nutrition  of  embryo.  Parturition.  Lactation.  Puberty.  Hor- 
mones of  reproductive  system.  Stages  of  life.  Death 557 

APPENDIX 

Suggestions  for  laboratory  work 587 

Index..  .  631 


THE  HUMAN  BODY 


CHAPTER  I 

THE    GENERAL    STRUCTURE    AND    COMPOSITION    OF   THE 

HUMAN  BODY 

Definitions.  The  living  Human  Body  may  be  considered  from 
either  of  two  aspects.  Its  structure  may  be  especially  examined, 
and  the  forms,  connections  and  mode  of  growth  of  its  parts  be 
studied,  as  also  the  resemblances  or  differences  in  such  respects 
which  appear  when  it  is  compared  with  other  animal  bodies.  Or 
the  living  Body  may  be  more  especially  studied  as  an  organism 
presenting  definite  properties  and  performing  certain  actions;  and 
then  its  parts  will  be  investigated  with  a  view  to  discovering  what 
duty,  if  any,  each  fulfils.  The  former  group  of  studies  constitutes 
the  science  of  Anatomy,  and  in  so  far  as  it  deals  with  the  Human 
Body  alone,  of  Human  Anatomy;  while  the  latter,  the  science  con- 
cerned with  the  uses — or  in  technical  language  the  functions — of 
each  part  is  known  as  Physiology.  Closely  connected  with  physi- 
ology is  the  science  of  Hygiene,  which  is  concerned  with  the  con- 
ditions which  are  favorable  to  the  healthy  action  of  the  various 
parts  of  the  Body;  while  the  activities  and  structure  of  the  diseased 
body  form  the  subject-matter  of  the  science  of  Pathology. 

Tissues  and  Organs.  Histology.  Examined  merely  from  the 
outside  our  Bodies  present  a  considerable  complexity  of  structure. 
We  easily  recognize  distinct  parts  as  head,  neck,  trunk  and  limbs; 
and  in  these  again  smaller  constituent  parts,  as  eyes,  nose,  ears, 
mouth;  arm,  forearm,  hand;  thigh,  leg  and  foot.  We  can,  with 
such  an  external  examination,  go  even  farther  and  recognize  dif- 
ferent materials  as  entering  into  the  formation  of  the  larger  parts. 
Skin,  hair,  nails  and  teeth  are  obviously  different  substances; 
simple  examination  by  pressure  proves  that  internally  there  are 
harder  and  softer  solid  parts;  while  the  blood  that  flows  from  a  cut 
finger  shows  that  liquid  constituents  also  exist  in  the  Body.  The 

1 


2  THE  HUMAN  BODY 

conception  of  complexity  which  may  be  thus  arrived  at  from  ex- 
ternal observation  of  the  living,  is  greatly  extended  by  dissection 
of  the  dead  Body,  which  makes  manifest  that  it  consists  of  a  great 
number  of  diverse  parts  or  organs,  which  in  turn  are  built  up  of  a 
limited  number  of  materials;  the  same  material  often  entering  into 
the  composition  of  many  different  organs.  These  primary  build- 
ing materials  are  known  as  the  tissues,  and  that  branch  of  anatomy 
which  deals  with  the  characters  of  the  tissues  and  their  arrange- 
ment in  various  organs  is  known  as  Histology;  or,  since  it  is  mainly 
carried  on  with  the  aid  of  the  microscope,  as  Microscopic  Anatomy. 
If,  with  the  poet,  we  compare  the  Body  to  a  house,  we  may  go  on  to 
liken  the  tissues  to  the  bricks,  stone,  mortar,  wood,  iron,  glass  and 
so  on,  used  in  building;  and  then  walls  and  floors,  stairs  and  win- 
dows, formed  by  the  combination  of  these,  would  answer  to  ana- 
tomical organs. 

Zoological  Position  of  Man.  External  examination  of  the  Hu- 
man Body  shows  also  that  it  presents  certain  resemblances  to  the 
bodies  of  many  other  animals:  head  and  neck,  trunk  and  limbs, 
and  various  minor  parts  entering  into  them,  are  not  at  all  peculiar 
to  it.  Closer  study  and  the  investigation  of  internal  structure 
demonstrates  further  that  these  resemblances  are  in  many  cases  not 
superficial  only,  but  that  our  Bodies  may  be  regarded  as  built  upon 
a  plan  common  to  them  and  the  bodies  of  many  other  creatures: 
and  it  soon  becomes  further  apparent  that  this  resemblance  is 
greater  between  the  Human  Body  and  the  bodies  of  ordinary  four- 
footed  beasts,  than  between  it  and  the  bodies  of  birds,  reptiles  or 
fishes.  Hence,  from  a  zoological  point  of  view,  man's  Body  marks 
him  out  as  belonging  to  the  group  of  Mammalia  (see  Zoology), 
which  includes  all  animals  in  which  the  female  suckles  the  young ; 
and  among  mammals  the  anatomical  resemblances  are  closer  and 
the  differences  less  between  man  and  certain  apes  than  between 
man  and  the  other  mammals;  so  that  zoologists  still,  with  Lin- 
naeus, include  man  with  the  monkeys  and  apes  in  one  subdivision 
of  the  Mammalia,  known  as  the  Primates.  That  civilized  man  is 
mentally  far  superior  to  any  other  animal  is  no  valid  objection  to 
such  a  classification,  for  zoological  groups  are  defined  by  ana- 
tomical and  not  by  physiological  characters;  and  mental  traits, 
since  we  know  that  their  manifestation  depends  upon  the  struc- 
tural integrity  of  certain  organs,  are  especially  phenomena  of 


GENERAL  STRUCTURE  AND  COMPOSITION  3 

function  and  therefore  not  available  for  purposes  of  zoological 
arrangement. 

As  man  walks  erect'  with  head  upward,  while  the  great  majority 
of  Mammals  go  on  all  fours  with  the  head  forward  and  the  back 
upward,  and  various  apes  adopt  intermediate  positions,  confusion 
is  apt  to  arise  in  considering  corresponding  parts  in  man  and  other 
animals  unless  a  precise  meaning  be  given  to  such  terms  as  "  an- 
terior" and  "posterior."  Anatomists,  therefore,  give  those  words 
definite  arbitrary  significations.  The  head  end  is  always  anterior 
whatever  the  natural  position  of  the  animal,  and  the  opposite  end 
posterior;  the  belly  side  is  spoken  of  as  ventral,  and  the  opposite 
side  as  dorsal;  right  and  left  of  course  present  no  difficulty:  the 
terms  cephalic  and  caudal  as  equivalent,  respectively,  to  anterior 
and  posterior,  are  sometimes  used.  Moreover,  that  end  of  a  limb 
nearer  the  trunk  is  spoken  of  as  proximal  with  reference  to  the 
other  or  distal  end.  The  words  upper  and  lower  may  be  con- 
veniently used  for  the  relative  position  of  parts  in  the  natural 
standing  position  of  the  animal. 

The  Vertebrate  Plan  of  Structure.  Neglecting  such  merely 
apparent  differences  as  arise  from  the  differences  of  normal  posture 
above  pointed  out,  we  find  that  man's  own  zoological  class,  the 
Mammals,  differs  very  widely  in  its  broad  structural  plan  from  the 
groups  including  sea-anemones,  insects  or  oysters,  but  agrees  in 
many  points  with  the  groups  of  fishes,  amphibians,  reptiles  and 
birds.  These  four  are  therefore  placed  with  man  and  all  other 
Mammals  in  one  great  division  of  the  animal  kingdom  known  as 
the  Vertebrata.  The  main  anatomical  character  of  all  vertebrate 
animals  is  the  presence  in  the  trunk  of  the  body  of  two  cavities,  a 
dorsal  and  a  ventral,  separated  by  a  solid  partition ;  in  the  adults  of 
nearly  all  vertebrate  animals,  a  hard  axis,  the  vertebral  column 
(backbone  or  spine),  develops  in  this  partition  and  forms  a  central 
support  for  the  rest  of  the  Body  (Fig.  2,  ee) .  The  dorsal  cavity  is 
continued  through  the  neck,  when  there  is  one,  into  the  head,  and 
there  widens  out.  Within  it  are  inclosed  the  chief  organs  of  the 
nervous  system.  The  bony  axis  is  also  continued  through  the 
neck  and  extends  into  the  head  in  a  modified  form.  The  ventral 
cavity,  on  the  other  hand,  is  confined  to  the  trunk.  It  contains  the 
main  organs  connected  with  the  blood-flow  together  with  those  of 
digestion  and  respiration. 


THE  HUMAN  BODY 


Upon  the  ventral  side  of  the  head  is  the  mouth-opening  leading 
into  a  tube,  the  alimentary  canal,  f  (Fig.  2),  which  passes  back 
through  the  neck  and  trunk  and  opens 
again  on  the  outside  at  the  posterior  part 
of  the  latter.  In  its  passage  through  the 
trunk-region  this  canal  lies  in  the  ventral 
cavity. 

The  Mammalia.  In  many  vertebrate 
animals  the  ventral  cavity  is  not  sub- 
divided, but  in  the  Mammalia  it  is;  a 
membranous  transverse  partition,  the 
diaphragm  (Fig.  1,  d),  separating  it  into 
an  anterior  chest  or  thoracic  cavity,  and 
a  posterior,  or  abdominal  cavity.  The 
alimentary  canal  and  whatever  else  passes 
from  one  of  these  cavities  to  the  other 
must  therefore  perforate  the  diaphragm. 


FIG.  1. — Diagram  of  the  Body  opened  from  the 
front  to  show  the  contents  of  the  ventral  cavity. 
d,  diaphragm;  h,  heart;  lu,  lungs;  st,  stomach ;  li, 
liver-  si,  small  intestines;  c,  large  intestine. 


FIG.  2.— Diagrammatic  longi- 
tudinal section  of  the  Body,  a, 
the  neural  tube,  with  its  upper 
enlargement  in  the  skull-cavity 
at  a';  N,  the  spinal  cord;  N't 
the  brain;  ee,  vertebrae  form- 
ing the  solid  partition  between 
the  dorsal  and  ventral  cavi- 
ties; b,  the  pleural,,and  c,  the 
abdominal  division  of  the  ven- 
tral cavity,  separated  from  one 
another  by  the  diaphragm,  d;i, 
the  nasal,  and  o,  the  mouth 
chamber,  opening  behind  into 
the  pharynx,  from  which  one 
tube  leads  to  the  lungs,  I,  and 
another  to  the  stomach,  /;  h, 
the  heart;  k,  a  kidney;  s,  the 
sympathetic  nervous  chain. 
From  the  stomach,  /,  the  in- 
testinal tube  leads  through  the 
abdominal  cavity  to  the  pos- 
terior opening  of  the  alimen- 
tary canal. 


GENERAL  STRUCTURE  AND  COMPOSITION  5 

In  the  chest,  besides  part  of  the  alimentary  canal,  lie  important 
organs,  the  heart,  h,  and  lungs,  lu  (Fig.  1) ;  the  heart  being  on  the 
ventral  side  of  the  alimentary  canal.  The  abdominal  cavity  is 
mainly  occupied  by  the  alimentary  canal  and  organs  connected 
with  it  and  concerned  in  the  digestion  of  food,  as  the  stomach, 
st,  the  liver,  li,  the  pancreas,  and  the  small  and  large  intestines,  si 
and  c.  Among  the  other  more  prominent  organs  in  it  are  the  kid- 
neys and  the  spleen. 

In  the  dorsal  or  neural  cavity  lie  the  brain  and  spinal  cord,  the 
former  occupying  its  anterior  enlargement  in  the  head.  Brain 
and  spinal  cord  together  form  the  cerebrospinal  nervous  center  com- 
monly called  the  central  nervous  system;  in  addition  to  this  there 
are  found  in  the  ventral  cavity  a  number  of  small  nerve-centers 
united  to  each  other  and  to  the  cerebrospinal  center  by  connect- 
ing cords,  and  with  their  off-shoots  forming  the  sympathetic  nervous 
system. 

The  walls  of  the  three  main  cavities  are  lined  by  smooth, 
moist  serous  membranes.  That  lining  the  dorsal  cavity  is  the 
arachnoid;  that  lining  the  chest  the  pleura;  that  lining  the  abdo- 
men the  peritoneum;  the  abdominal  cavity  is  in  consequence  often 
called  the  peritoneal  cavity.  Externally  the  walls  of  these  cavities 
are  covered  by  the  skin,  which  consists  of  two  layers :  an  outer  horny 
layer  called  the  epidermis,  which  is  constantly  being  shed  on  the 
surface  and  renewed  from  below;  and  a  deeper  layer,  called  the 
dermis  and  containing  blood,  which  the  epidermis  does  not.  Be- 
tween the  skin  and  the  lining  serous  membranes  are  bones,  muscles 
(the  lean  of  meat),  and  a  great  number  of  other  structures  which 
we  shall  have  to  consider  hereafter.  All  cavities  inside  the  Body, 
as  the  alimentary  canal  and  the  air-passages,  which  open  directly 
or  indirectly  on  the  surface  are  lined  by  soft  and  moist  prolonga- 
tions of  the  skin  known  as  mucous  membranes.  In  these  two  layers 
are  found  as  in  the  skin,  but  the  superficial  bloodless  one  is  called 
epithelium  and  the  deeper  vascular  one  corium. 

Diagrammatically  we  may  represent  the  Human  Body  in  lon- 
gitudinal section  as  in  Fig.  2,  where  aaf  is  the  dorsal  or  neural 
cavity,  and  b  and  c,  respectively,  the  thoracic  and  abdominal  sub- 
divisions of  the  ventral  cavity;  d  represents  the  diaphragm  separat- 
ing them;  ee  is  the  vertebral  column  with  its  modified  prolongation 
into  the  head  beneath  the  anterior  enlargement  of  the  dorsal 


6  THE  HUMAN  BODY 

cavity;  /  is  the  alimentary  canal  opening  in  front  through  the 
nose,  i,  and  mouth,  o;  h  is  the  heart,  I  a  lung,  s  the  sympathetic 
nervous  system,  and  k  a  kidney. 

A  transverse  section  through  the  chest  is  represented  by  the 
diagram  Fig.  3,  where  x  is  the  neural  canal  containing  the  spinal 


FIG.  3. — Cross-section  of  thorax.  A,  bronchus,  entering  the  lung;  B,  the  aorta 
cut  at  its  origin  and  again  at  the  descending  part  of  its  arch;  C,  the  pericardial 
space;  D,  the  pleural  cavity;  E,  the  alimentary  canal;  PA,  the  pulmonary  artery; 
X,  the  neural  canal. 

cord.  In  the  thoracic  cavity  are  seen  the  heart,  the  lungs,  part  of 
the  alimentary  canal,  E;  bronchial  tubes,  A,  leading  to  the  lungs; 
and  blood  vessels,  B  and  P  A,  communicating  with  the  heart; 
the  heavy  line  on  each  side  covering  the  inside  of  the  chest-wall  and 
the  outside  of  the  lung  represents  the  pleura. 

Sections  through  corresponding  parts  of  any  other  Mammal 
would  agree  in  all  essential  points  with  those  represented  in  Figs. 
2  and  3. 

The  Limbs.  The  limbs  present  no  such  arrangement  of  cavities 
on  each  side-  of  a  bony  axis  as  is  seen  in  the  trunk.  They  have  an 
axis  formed  at  different  parts  of  one  or  more  bones  (as  seen  at  U 
and  R  in  Fig.  4,  which  represents  a  cross-section  of  the  forearm 
near  the  elbow-joint),  but  around  this  are  closely-packed  soft 
parts,  chiefly  muscles,  and  the  whole  is  enveloped  in  skin.  The 
only  cavities  in  the  limbs  are  branching  tubes  which  are  filled  with 


GENERAL  STRUCTURE  AND  COMPOSITION 


are  indicated  by  numbers;  n, 
n,  nerves  and  vessels. 


liquids  during  life,  either  blood  or  a  watery-looking  fluid  known  as 

lymph.    These  tubes,  the  blood  and  lymph-vessels  respectively,  are 

not,  however,  characteristic  of  the  limbs, 

for  they  are  present  in  abundance  in  the 

dorsal  and  ventral  cavities  and  in  their 

walls. 

Microscopic  Structure  of  the  Body.  R  > 
For  the  detailed  study  of   objects  too 
small  to  be  examined  with  the  unaided 
eye   the    compound    microscope  is    em- 

.  .     ,  ,  .  t  .          f  FIG.  4.  —  A  section  across 

ployed.     Important  Optical  conditions  for    the  forearm  a  short  distance 

the  successful  use  of  this  instrument  are   ^V%  tr^pporting 
adequate  illumination  and  sharpness  of  bones,  the  radius  and  ulna; 

e,  the  epidermis,  and  d,  the 
focUS.      To   Secure  these  in   the   Study  of    dermis  of  the  skin;  the  latter 

tissues    the    materials   are   cut   in   very 
thin  slices  and  observed  by  transmitted 

light.      Viewed  thus   tissues  in  their  nat- 

,  ,  ,      . 

ural  state  are  so  nearly  transparent  that 
relatively  little  of  their  detailed  structure  can  be  made  out.  The 
practice  of  histologists,  therefore,  is  first  to  subject  the  tissues  to 
the  action  of  preservatives,  and  then  to  stain  them  with  suitable 
dyes.  By  applying  the  principle  that  the  different  structures  of 

the  tissues  are  likely  to  differ  chem- 
ically as  well  as  in  other  respects, 
dyes  can  be  selected  which  have 
greater  affinity  for  some  of  the 
chemical  components  of  the  tissues 
than  for  others.  Thus  certain  of 
the  tissue  components  will  stain 
with  one  sort  of  dye;  other  com- 
FIG.  5.—  Diagram  of  a  cell  (Schafer).  ponents  are  unaffected  by  this 

P.  protoplasm;  n,  nucleus;  c,  centrosome.  ^  but    can    be   gtamed   with  an_ 

other.  This  method  of  differential  staining  enables  the  various 
features  of  tissues  to  be  made  clearly  visible. 
.  Cells.  Examination  of  the  different  tissues  with  the  aid  of  the 
microscope  reveals  that  they  are  made  up  of  minute  structures, 
the  cells.  These  vary  in  form  and  size  in  different  tissues.  They 
are  all  constructed  on  a  common  plan,  although  in  the  more 
highly  organized  tissues,  such  as  nerves  and  muscles,  this  plan  is 


8  THE  HUMAN  BODY 

so  modified  to  meet  the  special  demands  of  these  tissues  as  not  to 
be  easily  recognized.  The  typical  cell  (Fig.  5)  consists  of  a  mass 
of  living  substance,  known  as  protoplasm,  of  a  semi-liquid,  gelat- 
inous consistency,  about  0.01  millimeter  (2^  7  inch)  in  diameter. 
The  protoplasm  is  usually  not  perfectly  uniform  throughout,  but 
shows  granules  or  fine,  transparent  net  works  through  its  sub- 
stance. Imbedded  in  the  protoplasm  is  a  small  structure  of  dis- 
tinctly different  appearance  from  the  rest  of  the  cell.  This  is  the 
nucleus.  It  presents  highly  characteristic  features  which  will  be 
studied  in  later  paragraphs  (p.  23).  The  mass  of  cell  protoplasm 
outside  the  nucleus  is  called  cytoplasm.  In  general  cytoplasm 
seems  to  be,  as  stated  above,  a  relatively  simple  granular  or 
vesicular  mass.  As  figure  5  indicates,  however,  there  are  typic- 
ally certain  definite  structures  associated  with  the  cytoplasm. 
The  significance  of  these  will  be  considered  later  (p.  24).  In  the 
highly  organized  tissues,  muscle  and  nerve,  the  cytoplasm  pre- 
sents, in  addition,  complexities  of  structure  suited  to  the  special 
functions  of  these  tissues. 

Tissues.  The  individual  cells  are  grouped  into  masses  which 
are  larger  or  smaller  according  to  the  region  in  which  they  occur. 
Obviously  only  by  such  a  grouping  can  so  large  and  complex  a 
structure  as  the  body  be  built  of  microscopic  units.  Any  cell 
mass  in  which  the  cells  are  of  one  type  is  called  a  tissue.  Thus  we 
speak  of  muscle  tissue  or  gland  tissue  according  as  the  cells  which 
make  up  the  tissue  in  question  are  muscle  cells  or  gland  cells. 
Many  kinds  of  tissue  are  widely  distributed  through  the  body, 
others  occur  only  in  special  parts.  The  various  tissues  will  be 
studied  in  detail  in  later  chapters. 

Chemical  Composition  of  the  Body.  In  addition  to  the  study 
of  the  Body  as  composed  of  tissues  and  organs  which  are  optically 
recognizable,  we  may  consider  it  as  composed  of  a  number  of 
different  chemical  substances.  Ttyis  branch  of  knowledge,  which 
is  still  very  incomplete,  really  presents  two  classes  of  problems. 
On  the  one  hand,  we  may  limit  ourselves  to  the  examination  of 
the  chemical  substances  which  exist  -in  or  may  be  derived  from 
the  dead  Body,  or,  if  such  a  thing  were  possible,  from  the  living 
Body  entirely  at  rest;  such  a  study  is  essentially  one  of  structure 
and  may  be  called  Chemical  Anatomy.  But  as  long  as  the  Body 
is  alive  it  is  the  seat  of  constant  chemical  transformations  in  its 


GENERAL  STRUCTURE  AND  COMPOSITION  9 

material,  and  these  are  inseparably  connected  with  its  functions, 
the  great  majority  of  which  are  in  the  long  run  dependent  upon 
chemical  changes.  From  this  point  of  view,  then,  the  chemical 
study  of  the  Body  presents  physiological  problems,  and  might  be 
called  Chemical  Physiology.  At  present  it  is  customary  to  include 
under  the  term  Biological  Chemistry  the  study  of  the  chemical 
structure  of  living  matter  and  of  the  chemical  changes  occurring 
in  it.  At  this  point  we  may  confine  ourselves  to  the  more  im- 
portant substances  derived  from  or  known  to  exist  in  the  Body 
leaving  questions  concerning  the  chemical  changes  taking  place 
within  it  for  consideration  along  with  those  functions  which  are 
performed  in  connection  with  them. 

Elements  Composing  the  Body.  Of  the  elements  known  to 
chemists  only  seventeen  have  been  found  to  take  part  in  the 
formation  of  the  Human  Body.  These  are  carbon,  hydrogen, 
nitrogen,  oxygen,  sulphur,  phosphorus,  chlorin,  fluorin,  iodin, 
silicon,  sodium,  potassium,  lithium,  calcium,  magnesium,  iron,  and 
manganese.  Copper  and  lead  have  sometimes  been  found  in  small 
quantities,  but  are  probably  accidental  and  occasional. 

Uncombined  Elements.  Only  a  very  small  number  of  the 
above  elements  exist  in  the  Body  uncombined.  Oxygen  is  found 
in  small  quantity  dissolved  in  the  blood ;  but  even  there  most  of  it 
is  in  a  state  of  loose  chemical  combination.  It  is  also  found  in  the 
cavities  of  the  lungs  and  alimentary  canal,  being  derived  from  the 
inspired  air  or  swallowed  with  food  and  saliva;  but  while  con- 
tained in  these  spaces  it  can  hardly  be  said  to  form  a  part  of  the 
Body.  Nitrogen  also  exists  uncombined  in  the  lungs  and  alimen- 
tary canal,  and  in  small  quantity  in  solution  in  the  blood.  Free 
hydrogen  has  also  been  found  in  the  alimentary  canal,  being  there 
evolved  by  the  fermentation  of  certain  foods. 

Chemical  Compounds.  The  number  of  these  which  may  be 
obtained  from  the  Body  is  very  great;  but  with  regard  to  very 
many  of  them  we  do  not  know  that  the  form  in  which  we  extract 
them  is  really  that  in  which  the  elements  they  contain  were  united 
while  in  the  living  Body;  since  the  methods  of  chemical  analysis 
are  such  as  always  break  down  the  more  complex  forms  of  living 
matter  and  leave  us  only  its  debris  for  examination.  We  know  in 
fact,  tolerably  accurately,  what  compounds  enter  the  Body  as 
food  and  what  finally  leave  it  as  waste;  but  the  intermediate  con- 


10  THE  HUMAN  BODY 

ditions  of  the  elements  contained  in  these  compounds  during  their 
sojourn  inside  the  Body  we  know  very  little  about;  more  especially 
their  state  of  combination  during  that  part  of  their  stay  when  they 
do  not  exist  dissolved  in  the  bodily  liquids,  but  form  part  of  a  more 
or  less  compact  living  tissue. 

For  present  purposes  the  chemical  compounds  existing  in  or  de- 
rived from  the  Body  may  be  classified  as  organic  and  inorganic, 
and  the  former  be  subdivided  into  those  which  contain  nitrogen 
and  those  which  do  not. 

Inorganic  Constituents.  Of  the  simpler  substances  entering 
into  the  structure  of  the  Body  the  following  are  the  most  im- 
portant : 

Water;  in  all  the  tissues  in  greater  or  less  proportion  and  forming  about 
two-thirds  of  the  weight  of  the  whole  Body.  A  man  weighing  75  kilos  (165  Ibs.), 
if  completely  dried  would  therefore  lose  about  50  kilos  (110  Ibs.)  from  the 
evaporation  of  water.  Of  the  constituents  of  the  Body  the  enamel  of  the 
teeth  contains  least  water  (about  2  per  cent),  and  the  saliva  most  (about  99.5 
per  cent) ;  between  these  extremes  are  all  intermediate  steps — bones  contain- 
ing about  22  per  cent,  muscles  75,  blood  79. 

Common  salt — Sodium  chlorid — '(NaCl);  found  in  all  the  tissues  and 
liquids,  and  in  many  cases  playing  an  important  part  in  keeping  other  sub- 
stances in  solution  in  water. 

Potassium  chlorid  (KC1);  in  the  blood,  muscles,  nerves  and  most  liquids. 

Calcium  phosphate  [CasCPO^l;  in  the  bones  and  teeth  in  large  quantity. 
In  less  proportion  in  all  the  other  tissues. 

Besides  the  above,  ammonium  chlorid,  sodium  and  potassium  phosphates, 
magnesium  phosphate,  sodium  sulphate,  potassium  sulphate,  and  calcium 
fluoride  have  been  obtained  from  the  Body. 

Uncombined  hydrochloric  acid  (HC1)  is  found  in  the  gastric  juice. 

Nitrogenous  Organic  Compounds.  These  fall  into  several  main 
groups :  proteins  * — subdivided  into  simple  proteins,  conjugated  pro- 
teins, and  derived  proteins — nitrogenous  extractives,  and  pigments. 
The  interesting  substances  known  as  enzyms  probably  form  like- 
wise a  group  under  this  head. 

Simple  Proteins.  Under  this  head  are  grouped  those  proteins 
whose  molecules  contain  only  protein  material;  in  contradistinc- 

*  The  classification  of  proteins  here  given  is  that  recommended  by  the  joint 
committee  on  protein  nomenclature  of  the  American  Physiological  Society 
and  the  American  Society  of  Biological  Chemists,  1907. 


GENERAL  STRUCTURE  AND  COMPOSITION  11 

tion  to  the  conjugated  proteins  whose  molecules  contain  protein 
in  combination  with  a  non-protein  substance. 

Each  of  them  contains  carbon,  hydrogen,  oxygen,  and  nitrogen; 
most  of  them  contain  sulphur  also,  and  a  few  phosphorus  in  ad- 
dition. These  elements  are  united  into  very  complex  molecules, 
and  although  different  members  of  the  group  of  simple  proteins 
differ  from  one  another  in  minor  points  they  all  agree  in  their 
broad  features.  The  common  body  proteins  have  a  similar  per- 
centage composition,  falling  within  the  Jimits  given  in  the  follow- 
ing table: 

Carbon 50     to  55     per  cent. 

Hydrogen 6.5  to    7.3    "      " 

Oxygen ,  .  19     to  24       "      " 

Nitrogen • 15     to  17.6    "      " 

Sulphur 0.3  to    2.4    "     " 

In  addition  a  small  quantity  of  ash  is  usually  left  when  a  protein 
is  burned,  showing  that  some  inorganic  salts  are  held  in  combina- 
tion with  it. 

Recent  chemical  investigation  has  shown  that  the  protein 
molecule  is  a  complex,  made  up  of  a  number  of  simpler  molecules 
joined  together.  When  a  protein  is  boiled  with  a  dilute  acid  its 
molecules  are  decomposed,  and  the  resulting  solution  is  found 
when  examined  to  contain  a  mixture  of  the  substances  whose  in- 
dividual molecules  were  formerly  parts  of  the  complex  protein 
molecules.  Eighteen  such  substances  have  been  obtained  from  de- 
composed proteins;  they  all  contain  nitrogen,  and  they  all  belong 
chemically  to  the  group  of  amino  acids.  Some  proteins  contain 
all  of  them;  others  only  a  few.  The  characteristics  of  different 
proteins  are  supposed  to  depend  on  which  of  these  amino  acids 
are  present  in  the  molecules  and  also  on  their  arrangement  or 
grouping  therein. 

There  are  a  number  of  chemical  tests  that  may  be  used  in  detecting  the 
presence  of  proteins;  but  only  a  few  of  them  apply  to  the  entire  group.  Of 
these  the  so-called  biuret  reaction  is  the  most  easily  and  most  commonly 
used.  It  consists  in  making  the  protein  solution  strongly  alkaline  with  caustic 
soda  or  potash  and  adding  a  small  amount  of  very  dilute  solution  of  copper 
sulphate.  A  distinct  purple  color  is  evidence  of  the  presence  of  protein.  The 
common  proteins  of  the  body  may  also  be  recognized  by  the  following  char- 
acters: 


12  THE  HUMAN  BODY 

1.  Boiled,  either  in  the  solid  state  or  in  solution,  with  strong  nitric  acid 
they  give  a  yellow  liquid  which  becomes  orange  on  neutralization  with  am- 
monia.   This  is  the  xanthoproteic  test. 

2.  Boiled  with  a  solution  containing  subnitrate  and  pernitrate  of  mercury 
they  give  a  pink  precipitate,  or,  if  in  very  small  quantity,  a  pink  colored 
solution.    This  is  known  as  Millon's  test. 

3.  If  a  solution  containing  a  protein  be  strongly  acidulated  with  acetic 
acid  and  be  boiled  with  the  addition  of  an  equal  bulk  of  a  saturated  watery 
solution  of  sodium  sulphate,  the  protein  will  be  precipitated. 

The  simple  proteins  which  are  found  in  the  bodies  of  man  and 
the  lower  animals  fall  into  several  groups  as  follows : 

1.  Albumins.    Several  proteins  of  this  group  are  found  in  the  Body;  serum 
albumin,  one  of  the  proteins  of  the  blood,  myogen,  a  muscle  protein,  and  cell 
albumin,  found  in  the  cellular  tissues,  are  examples.    Egg  albumin  (white  of 
egg)  is  perhaps  the  most  familiar  example  of  an  albumin. 

The  albumins  are  characterized  by  being  coagulated  by  heat  (illustrated 
by  boiled  white  of  egg) ;  in  this  respect  they  are  similar  to  the  proteins  of  the 
next  group,  from  which  they  differ  by  being  soluble  in  pure  water. 

2.  Globulins.     These  proteins,  as  indicated  above,  do  not  differ  greatly 
from  albumins.    Like  them  they  are  coagulated  by  heat,  but  unlike  them,  are 
not  soluble  in  pure  water.    If  a  small  amount  of  an  inorganic  salt  is  added  to 
the  water  they  will  go  into  solution.    Two  blood  proteins,  serum  globulin  or 
paraglobulin,  and  fibrinogen  belong  to  this  group;  also  myosin,  one  of  the 
muscle  proteins,  and  cell  globulin,  found  in  cellular  tissues. 

3.  Albuminoids.     In  chemical  structure  these  simple  proteins  are  closely 
similar  to  those  already  described.    They  are  found,  however,  exclusively  in 
the  supporting  and  protective  tissues  of  the  body,  bone,  connective  tissue, 
epidermis,  and  hair,  and  evidently  have  some  important  structural  difference 
as  compared  with  the  proteins  of  the  cellular  tissues  since  the  Body  cannot 
make  use  of  them  in  building  up  its  cell  proteins  in  the  way  it  uses  other  pro- 
tein foods. 

4.  Protamins.    These  are  the  simplest  proteins  known.    They  have  thus 
far  been  found  only  in  the  spermatozoa  of  fishes.    Their  molecules  consist 
of  a  relatively  small  number  of  amino  acid  groupings  and  contain  no  sulphur. 

5.  Histons  are  intermediate  in  complexity  between  protamins  and  proteins 
of  the  albumin  class.    The  one  of  chief  importance  in  the  body  is  globin,  which 
is  combined  with  a  pigment  to  form  hemoglobin,  the  red  coloring  matter  of 
the  blood. 

Conjugated  Proteins.  In  addition  to  the  simple  proteins  de- 
scribed above  there  are  present  in  the  Body  certain  groups  of  com- 
pounds consisting  of  proteins  combined  with  non-protein  sub- 
stances. The  most  important  of  these  are : 


GENERAL  STRUCTURE  AND  COMPOSITION  13 

Nucleo  proteins,  consisting  of  protein  combined  with  nucleic  acid.    These_ 
are  of  great  interest  physiologically  since  they  form  the  chief  constituents  of 
cell  nuclei,  to  which  structures  are  assigned  the  function  of  exercising  special 
control  over  the  activities  of  living  cells. 

Glycoproteins,  consisting  of  protein  combined  with  a  carbohydrate  (see 
p.  15).  Mucin,  the  substance  which  gives  the  secretions  of  the  mouth,  nose, 
and  throat  their  peculiar  viscous  character,  is  an  example  of  this  group. 

Phosphoproteins,  consisting  of  protein  combined  with  a  phosphorus- 
containing  substance.  The  casein  of  milk,  which  forms  the  curd,  is  the  most 
familiar  member  of  this  group. 

Hemoglobins,  compounds  of  protein  with  a  pigment.  These  are  of  great 
physiological  importance  on  account  of  the  property,  common  to  all  of  them, 
of  acting  as  transporters  of  oxygen.  The  type  member  of  the  group,  the 
hemoglobin  of  Mammalian  blood,  is  of  interest  chemically  on  account  of  the 
great  size  of  its  molecules,  which  are  estimated  to  contain  not  less  than  2,300 
atoms  each  and  to  have  molecular  weight  exceeding  16,000. 

Derived  Proteins.  The  members  of  this  group  are  derived,  as 
their  name  indicates,  from  the  simple  proteins.  In  the  process  of 
protein  digestion,  by  which  the  protein  portions  of  the  food  are 
made  available  for  the  needs  of  the  Body  by  being  split  into 
simpler  substances,  the  first  steps  in  the  digestive  process  give  rise 
to  compounds  which  differ  from  the  simple  proteins  by  a  slight 
degree  only.  These  are  the  derived  proteins.  The  members  of 
the  group  which  occur  mOst  commonly  in  the  Body  are  the  proteases 
and  peptones.  These  are  present  in  the  stomach  during  protein 
digestion.  They  are  characterized  by  greater  solubility  than 
simple  proteins  possess. 

Nitrogenous  Extractives.  Under  this  head  are  grouped  various 
nitrogen-containing  substances  most  of  which  represent  materials 
that  have  done  their  work  in  the  Body  and  are  about  to  be  gotten 
rid  of.  Nitrogen  is  present  in  the  living  tissues  of  the  Body  chiefly 
as  a  part  of  their  proteins.  The  vital  activities  of  the  tissues  in- 
volve the  breaking  down  of  these  complex  proteins  into  simpler 
substances.  Part  of  their  carbon  combines  with  oxygen  and  passes 
out  through  the  lungs  as  carbon  dioxid;  their  hydrogen  is  similarly 
in  large  part  combined  with  oxygen  and  passed  out  as  water; 
while  their  nitrogen,  with  some  carbon  and  hydrogen  and  oxygen, 
is  passed  out  in  the  form  of  crystalline  extractives. 

Urea  is  the  most  important  substance  of  this  class;  fully  nine-tenths  of  all 
the  nitrogen  that  is  eliminated  from  the  Body  is  in  this  form.  It  is  a  diamide 


14  THE  HUMAN  BODY 

of  carbonic  acid,  having  the  formula  CO<MR2;  the  relationship  of  urea  to 

2  OH 

carbonic  acid  is  clear  when  the  formula  for  the  latter  is  written  thus:  CO  < ~. 

OH. 

Fully  30  grams  of  urea  are  eliminated  daily  from  the  body  of  an  adult  man. 

Creatinine  (C^NaO)  is  an  interesting  member  of  the  group  of  extractives 
because  the  amount  of  it  that  is  eliminated  from  the  body  daily  is  very  con- 
stant, regardless  of  changes  in  amount  of  food  or  exercise  taken,  and  seems 
to  depend  closely  upon  the  amount  of  muscle  tissue  present  in  the  Body; 
persons  of  great  muscular  development  have  a  larger  daily  creatinine  output 
than  those  of  smaller  build. 

Creatine  (C-iHgNsC^)  is  closely  related  chemically  to  creatinine,  but  appears 
to  play  a  very  different  part  in  the  Body.  Creatinine  is  undoubtedly  a  waste 
product  of  protein  decomposition,  being  merely  an  incidental  product  of  the 
vital  processes  which  go  on  within  the  organism.  Creatine,  on  the  other  hand, 
seems  to  be  an  essential  constituent  of  living  protoplasm,  although  just  what 
purpose  it  serves  is  not  clear.  About  one  per  cent  of  the  solid  substance  of 
muscle  is  creatine. 

The  purin  bodies,  of  which  uric  acid  (CsH^N^s)  is  the  most  familiar 
example,  are  derived  chiefly,  if  not  wholly,  from  the  decomposition  of  nucleo- 
proteins  and  are  therefore  interesting  as  being  the  end  products  of  the  vital 
activities  of  the  cell  nuclei. 

Pigments.  The  most  important  of  these  that  occur  in  the 
Body  are: 

Hemochromogen,  an  iron  containing  pigment  which  in  combination  with 
the  histon  globin  forms  hemoglobin,  the  red  coloring  matter  of  the  blood. 
When  hemochromogen  is  in  the  presence  of  oxygen  it  combines  with  it  to 
form  hematin. 

Bilirubin  and  biliverdin  are  the  bile  pigments  and  give  to  bile  its  color. 
Bilirubin  is  yellow  and  biliverdin  is  green.  The  former  usually  predominates 
in  the  bile  of  man  and  the  carnivora,  making  such  bile  yellow;  the  latter  is 
the  dominant  color  in  the  bile  of  herbivorous  animals,  which  is  green.  They 
are  closely  related  chemically  and  are  derived  from  the  decomposition  of 
hemoglobin. 

Urobilin  is  formed  in  the  intestine  as  the  result  of  the  putrefaction  there 
of  the  bile  pigments.  It  is  absorbed  thence  into  the  blood  and  excreted  by 
the  kidneys,  and  imparts  to  the  urine  its  characteristic  yellow  color. 


Enzyms  are  a  group  of  substances  which  seem  to  be  allied  in 
chemical  composition  to  the  true  proteins,  but  it  is  so  difficult  to 
be  sure  of  the  purity  of  any  specimen  that  their  composition  is  still 
in  doubt.  The  enzyms  have  the  power,  even  when  present  in  very 
small  quantity,  of  bringing  about  extensive  changes  in  other  sub- 


GENERAL  STRUCTURE  AND  COMPOSITION  15 

stances,  and  they  are  not  themselves  necessarily  used  up  or  de- 
stroyed in  the  process.  Many  enzyms  of  great  physiological  im- 
portance exist  in  the  digestive  fluids  and  play  a  part  in  fitting  food 
for  absorption  from  the  alimentary  canal.  For  example,  pepsin 
found  in  the  gastric  juice  converts,  under  suitable  conditions,  such 
complex  proteins  as  albumins  into  simpler  peptones;  ptyalin,  found 
in  the  saliva,  converts  starch  into  sugar.  We  shall  have  occasion 
later  to  study  a  number  of  enzyms  more  in  detail  in  connection 
with  their  physiological  uses.  A  characteristic  property  of  all 
enzyms  is  their  susceptibility  to  heat;  a  temperature  of  60°  C. 
suffices  to  destroy  them  completely. 

Non-Nitrogenous  Organic  Compounds.  These  may  be  con- 
veniently grouped  as  hydrocarbons  or  fatty  bodies;  carbohydrates 
or  amyloids;  and  certain  non-nitrogenous  acids. 

Fats.  The  fats  all  contain  carbon,  hydrogen,  and  oxygen,  the 
oxygen  being  present  in  small  proportion  as  compared  with  the 
hydrogen.  Three  fats  occur  in  the  Body  in  large  quantities,  viz.  : 
palmatin  (CsiHosOe),  stearin  (C5iHiioO6),  and  olein  (C^JHiQ^Oo). 
The  two  former  when  pure  are  solid  at  the  temperature  of  the 
Body,  but  in  it  are  mixed  with  olein  (which  is  liquid)  in  such  pro- 
portions as  to  be  kept  fluid.  The  total  quantity  of  fat  in  the  Body 
is  subject  to  great  variations,  but  its  average  quantity  in  a  man 
weighing  75  kilograms  (165  pounds)  is  about  2.75  kilograms  (6 
pounds). 

Each  of  these  fats  when  heated  with  a  caustic  alkali,  in  the 
presence  of  water,  breaks  up  into  a  fatty  acid  (stearic,  palmitic,  or 
oleic  as  the  case  may  be),  and  glycerin.  The  fatty  acid  unites 
with  the  alkali  present  to  form  a  soap. 

Carbohydrates.  These  may  be  defined  as  substances  composed 
of  carbon,  hydrogen,  and  oxygen,  having  the  number  of  carbon 
atoms  in  each  molecule  usually  six  or  some  multiple  thereof,  and 
having  the  hydrogen  and  oxygen  present  in  the  same  proportion 
as  in  water.  The  three  chief  groups  are  the  sugars,  starches,  and 
cellulose. 


Dextrose  or  grape  sugar  (CeH^Oe)  is  the  most  important  representative  of 
the  sugars  found  in  the  Body.  A  large  part  of  the  food  supply  is  received 
from  the  digestive  tract  into  the  blood  in  this  form.  It  occurs  constantly  in 
small  concentration  in  the  blood  and  tissues. 

Lactose,  the  sugar  of  milk,  occurs  in  considerable  quantity  in  milk. 


16  THE  HUMAN  BODY 


Glycogen  or  animal  starch  (CeHioOs)  is  the  anhydride  of  grape  sugar.  This 
is  the  form  in  which  the  excess  of  sugar  is  stored  in  the  body  to  be  drawn  upon 
at  need.  Dextrose  is  readily  converted  into  it,  and  it  in  turn  is  easily  changed 
back  into  sugar.  In  many  respects  it  resembles  common  vegetable  starch. 
It  is  present  in  the  muscles  of  the  Body  and  in  the  liver,  the  latter  organ  alone 
containing  about  as  much  as  all  the  muscles  put  together. 

Cellulose,  the  woody  fiber  of  plants,  is  not  found  in  the  Human  Body,  al- 
though a  chemically  identical  substance,  tunicin,  is  found  in  the  bodies  of 
tunicates. 

Organic  Non-Nitrogenous  Acids.  Of  these  the  most  important 
is  carbon  dioxid  (COa),  which  is  the  form  in  which  by  far  the  greater 
part  of  the  carbon  taken  into  the  Body  ultimately  leaves  it. 
United  with  calcium  it  is  found  in  the  bones  and  teeth  in  large 
proportion. 

Formic,  acetic,  and  butyric  acids  are  also  found  in  the  Body;  stearic,  palmitic, 
and  oleic  have  been  above  mentioned  as  obtainable  from  fats.  Lactic  acid, 
CsHeOs,  is  often  present  in  the  digestive  tract,  and  when  milk  turns  sour 
is  formed  from  lactose.  Virtually  the  same  substance,  (sarcolactic  acid),  is 
formed  in  muscles  when  they  work  or  die. 

Glycerin  phosphoric  acid  (CsHgPOe)  is  obtained  on  the  decomposition  of 
lecithin,  a  complex  nitrogenous  fat  found  in  nervous  tissue  and  to  some  extent 
in  all  living  cells. 

Physico-Chemical  Constitution  of  the  Body.  The  functioning 
of  the  living  body  is  the  sum  total  of  the  functioning  of  the  in- 
dividual cells.  The  activity  of  any  cell,  in  turn,  is  determined  by 
the  interactions  of  its  constituent  molecules.  In  the  living  tissues 
we  have  a  great  number  of  different  molecules  interacting  in  ways 
which  depend,  in  part,  on  the  chemical  nature  of  the  molecules, 
and  in  part,  also,  upon  the  manner  in  which  the  molecules  are 
grouped  and  interrelated  physically.  The  study  of  the  manner  in 
which  molecules  are  related  to  each  other  under  such  conditions 
as  obtain  in  the  Body  is  a  part  of  the  science  of  Physical  Chem- 
istry and  the  structure  of  the  Body  from  this  standpoint  is  its 
Physico-chemical  structure. 

Liquid  Environment.  Of  prime  importance  from  the  physico- 
chemical  standpoint  is  the  fact  that  the  active  tissues  of  the  body 
consist  largely  of  water.  Molecules  in  solution  in  a  liquid  move 
about  freely,  enter  and  leave  chemical  combinations  readily,  and 
in  general  display  the  degree  of  flexibility  essential  for  the  carry- 


GENERAL  STRUCTURE  AND  COMPOSITION  17 

ing  out  of  complex  chemical   processes   such  as   go  on   in  the— 
Body.     Although  the  water  of  the  Body  does  not  itself  take 
active  part  in  the  life  processes,  these  processes  could  not  go  on  in 
its  absence. 

Crystalloids.  Substances  which  form  crystals  when  solutions 
containing  them  are  evaporated  are  classed  as  crystalloids.  Ex- 
amples are  common  salt  and  cane  sugar.  From  the  standpoint 
of  physical  chemistry  as  applied  to  the  Body  the  important  fact 
concerning  crystalloids  is  not  their  crystal-forming  ability,  but  the 
relatively  small  size  of  their  molecules,  which  gives  them  a  cor- 
respondingly high  degree  of  freedom  of  motion  in  solution  and 
facility  in  entering  and  leaving  chemical  combinations.  All  living 
tissues  contain  crystalloids  as  part  of  their  substance.  The 
amounts  are  relatively  small,  although  the  materials  are  as  neces- 
sary to  living  protoplasm  as  are  those  that  make  up  the  greater 
part  of  its  mass. 

Colloids.  These  are  substances  which  do  not  form  crystals 
when  solutions  of  them  are  evaporated,  but  appear  as  gelatinous 
or  gummy  masses.  Examples  are  white  of  egg,  and  ordinary 
table  gelatin.  Colloids  are  composed,  in  general,  of  much  larger 
molecules  than  are  crystalloids.  They  have  correspondingly  less 
facility  of  chemical  action.  Proteins  are  colloidal  in  structure; 
hence  living  protoplasm,  which  is  chiefly  protein  in  its  constitution, 
is  colloidal. 

Cell  Membranes.  To  preserve  definite  structure  in  the  watery, 
gelatinous  protoplasm  individual  cells  are  enclosed  in  cell  mem- 
branes. These  should  not  be  confused  with  the  woody  envelopes 
in  which  many  plant  cells  are  enclosed.  Reference  here  is  to  the 
delicate  sheaths  which  surround  all  cells,  both  plant  and  animal, 
and  which  serve  to  keep  the  semi-liquid  contents  of  the  cells  from 
running  together  into  a  formless  mass.  The  chemical  nature  of 
the  membranes  is  not  certainly  known,  although  there  is  reason 
to  believe  that  they  consist  of  protoplasm  which  differs  from  that 
of  the  cells  at  large  chiefly  in  its  greater  density.  The  significance 
of  cell  membranes  in  Physiology  lies  in  the  fact  that  all  interchanges 
between  living  protoplasm  and  its  surroundings  must  take  place 
through  them.  Any  nourishment  any  cell  receives  must  pass 
through  the  cell  membrane  before  it  reaches  active  protoplasm. 
Similarly,  all  materials  which  cells  discharge  have  to  be  expelled 


18  THE  HUMAN  BODY 

through  the  membrane.  We  shall  learn  later  how  much  the  mem- 
branes affect  the  cells  which  they  enclose  through  the  influence 
they  have  on  the  passage  of  materials  into  and  out  of  the  cells. 

Intercellular  Spaces  and  Intercellular  Fluids.  In  the  grouping 
of  cells  into  tissues  (p.  8)  we  find,  even  in  those  that  are  most 
compact,  minute  spaces  among  the  cells.  There  are  points  of 
union  between  cell  and  cell,  holding  the  tissue  together,  but  these 
involve  only  relatively  small  portions  of  the  total  cell  surfaces. 
Every  cell  has  a  large  part  of  its  surface  fronting  on  intercellular 
spaces.  These  spaces  are  filled  with  watery  fluid  called  lymph, 
which  bathes  the  individual  cells,  and,  in  fact,  forms  their  sole  en- 
vironment. The  nourishment  of  the  cells  reaches  them  by  way  of 
the  lymph;  the  discharges  of  waste  materials  from  the  cells  are 
into  the  lymph.  The  interchanges  between  the  cells  and  lymph 
are  therefore  the  fundamental  interchanges  of  the  Body.  They  are 
subject,  in  part,  at  least,  to  certain  definite  laws  given  below. 

Filtration,  Osmosis,  and  Dialysis.  At  every  step  in  the  com- 
plex process  of  supplying  the  living  cells  with  nourishment  and 
removing  from  them  their  harmful  waste  products  the  membranes, 
described  above,  stand  in  the  way  of  the  substances  involved  and 
must  be  traversed  by  them.  There  are  membranes  between  the 
protoplasm  of  the  cells  and  the  lymph  which  surrounds  them. 
The  digested  food  must  pass  through  the  membranous  lining  of  the 
digestive  tract  before  it  can  enter  the  blood;  the  oxygen  of  the 
air  must  pass  through  a  membrane  in  the  lungs  on  its  way  to  the 
same  medium.  The  juices  which  are  secreted  or  excreted  have 
to  be  forced  through  membranes  in  passing  out  from  the  organs 
from  which  they  come.  The  movements  of  liquids  through  the 
membranes  of  the  Body  take  place  for  the  most  part  in  accordance 
with  certain  physical  principles  which  may  conveniently  be  stated 
at  this  point. 

FILTRATION.  If  a  membranous  bag  such  as  an  ox  bladder  be 
filled  with  a  liquid  and  pressure  be  applied  to  the  liquid  in  the 
bag  a  point  may  be  reached  where  the  liquid  is  squeezed  through 
the  membrane  and  appears  in  drops  on  its  outer  surface.  This  is 
an  example  of  filtration.  When  a  liquid  is  filtered  in  this  way  any 
solid  particles  which  may  have  been  suspended  in  it  are  left  be- 
hind, but  any  substances  which  may  be  dissolved  in  it  pass  through 
as  part  of  the  liquid.  Thus  a  salt  solution  which  contained  some 


GENERAL  STRUCTURE  AND  COMPOSITION  19 

particles  of  sand  might  be  filtered  and  the  sand  removed,  but  the 
solution  would  have  just  as  much  salt  dissolved  in  it  after  filtra- 
tion as  before. 

OSMOSIS.  If  we  should  take  such  a  membranous  bag  as  de- 
scribed above  filled  with  salt  solution  and  dip  it  into  a  vessel  of 
pure  water,  so  that  the  surfaces  within  and  without  the  bag  are 
at  the  same  level,  it  would  be  seen  after  a  while  that  the  level  of 
liquid  within  the  bag  had  risen  while  that  in  the  vessel  outside 
had  correspondingly  fallen.  That  is,  there  would  have  been  an 
actual  movement  of  water  into  the  bag  with  sufficient  force  to 
overcome  the  pressure  due  to  gravity  resulting  from  the  change  of 
water  level  on  the  two  sides  of  the  membrane.  Whenever  two  solu- 
tions of  different  concentrations  are  separated  by  a  membrane  which 
is  permeable  to  water  there  will  be  a  flow  of  water  through  the  membrane 
in  the  direction  of  the  greater  concentration.  This  phenomenon  is 
known  as  osmosis.  The  force  which  drives  the  water  is  called 
osmotic  pressure  and  is  said  to  be  exerted  by  any  solution  of  higher 
concentration  toward  any  of  lower  concentration. 

DIALYSIS.  A  membrane  which  is  permeable  to  water  but  not 
to  any  particles  which  may  be  dissolved  in  it  is  known  as  a  semi- 
permeable  membrane;  one  which  allows  dissolved  substances  as 
well  as  water  to  pass  is  a  permeable  membrane.  When  two  so- 
lutions of  different  concentration  are  separated  by  a  membrane 
of  this  latter  class  we  have  in  addition  to  the  movement  of  water 
under  the  driving  force  of  osmotic  pressure  a  movement  of  dis- 
solved particles  through  the  membrane.  This  is  a  special  case  of 
the  general  phenomenon  of  diffusion.  Diffusion  may  be  defined 
as  the  tendency  of  substances  in  solution  to  distribute  themselves 
evenly  throughout  the  solvent.  Where  this  distribution  necessi- 
tates the  passage  of  particles  through  permeable  membranes  the 
phenomenon  is  called  dialysis.  The  effect  of  both  osmosis  and 
dialysis  is  to  equalize  the  concentrations  of  the  solutions  on  the 
two  sides  of  the  membrane,  but  it  must  be  remembered  that 
they  are  entirely  distinct  phenomena.  To  illustrate:  suppose  we 
have  on  the  two  sides  of  a  permeable  membrane  solutions  re- 
spectively of  sugar  and  salt  of  the  same  concentration,  that  is, 
having  the  same  number  of  particles  in  solution ;  there  would  then 
be  no  flow  of  water  in  either  direction  since  the  osmotic  pressure 
of  both  solutions  is  the  same,  but  since  neither  the  sugar  nor  the 


20  THE  HUMAN  BODY 

salt  is  evenly  distributed  throughout  the  solvent  there  will  be 
dialysis  of  both  substances  until  an  even  distribution  is  obtained. 

Again,  both  osmosis  and  dialysis  bring  about  changes  in  the 
concentration  of  the  solutions  affected  by  them  whereas  filtration 
does  not.  In  considering  the  influence  of  the  membranes  of  the 
Body  upon  its  liquid  contents  these  facts  must  be  borne  in  mind. 

Summary.  We  may  summarize  the  physico-chemical  structure 
of  living  tissues  by  picturing  them  as  made  up  of  a  large  percentage 
of  colloids  and  a  small  percentage  of  crystalloids  dissolved  in  a 
relatively  considerable  amount  of  water,  and  subdivided  into 
units  each  of  which  is  enclosed  in  a  membrane.  Among  the  units 
(cells)  are  spaces  filled  with  a  watery  fluid  (lymph).  Interchanges 
between  cell  substance  and  lymph  take  place  through  the  cell 
membranes  in  the  main  in  accordance  with  the  laws  of  filtration, 
osmosis,  and  dialysis. 


CHAPTER  II 
THE  FUNDAMENTAL  PHYSIOLOGICAL  ACTIONS 

The  Properties  of  the  Living  Body.  Just  as  the  structure  of 
the  Body  is  the  sum  total  of  the  structures  of  its  individual  cells, 
so  the  properties,  or  functions,  of  the  Body  are  the  sum  total  of  the 
functions  of  the  constituent  tissues.  With  most  of  the  properties 
of  our  Bodies  we  are  familiar  in  a  general  way.  The  ability  to  per- 
ceive sensations  and  to  make  motions;  the  beating  of  the  heart,  the 
movements  of  breathing,  the  secretion  of  saliva  and  of  sweat,  the 
maintenance  of  bodily  warmth;  all  these  we  recognize  as  bodily 
functions.  The  power  of  performing  them  must  reside  with  the 
constituent  cells.  We  observe,  however,  that  in  our  Bodies  not 
all  these  properties  are  shared  equally  by  all  the  cells.  The  cells 
that  perceive  sensations  are  not  the  same  as  perform  active  move- 
ments; those  that  secrete  saliva  have  not  the  power  of  secreting 
sweat.  A  Body  in  which  a  large  degree  of  specialization  prevails 
we  speak  of  as  highly  organized.  In  the  simpler  animals  there  is 
less  specialization  and  we  find  individual  cells  doing  more  than  one 
kind  of  work.  In  the  simplest  animals  of  all,  the  one-celled  ani- 
mals, all  the  properties  possessed  are  necessarily  combined  in  a 
single  cell.  We  can  observe,  then,  in  the  rudimentary  form,  to  be 
sure,  in  the  one-celled  animals,  all  the  fundamental  properties  of 
living  protoplasm.  Chief  among  these  are:  (1)  assimilation,  the 
power  to  take  in  food  materials  and  to  make  them  over  into  body 
substance;  a  typical  one-celled  animal,  the  ameba,  can  be  observed 
under  the  microscope  engulfing  and  dissolving  in  its  own  proto- 
plasm minute  food  particles;  (2)  active  motion,  seen  in  all  save  a 
very  few  animal  forms  from  the  lowest  to  the  highest.  Motion  in 
animals  is  brought  about  by  forcible  contractions  of  the  moving 
parts;  the  property  of  motion  is  therefore  more  accurately  de- 
scribed as  contractility;  (3)  sensation,  excitability,  or  irritability,  the 
property  of  being  affected  by  changes  in  the  environment;  (4)  co- 
ordination, the  power  of  causing  all  the  parts  of  the  organism  to 
act  in  harmony;  (5)  reproduction,  the  property  of  separating  off  a 

21 


22  THE  HUMAN  BODY 

portion  of  the  protoplasm  which  may  become  eventually  an  inde- 
pendent organism,  like  the  parent;  (6)  growth,  the  property,  based 
on  assimilation,  of  increasing  in  size  by  the  building  up  of  the  body 
protoplasm  faster  than  it  is  broken  down. 

The  Body  as  a  Machine.  Dissimilation.  All  living  things, 
from  the  simplest  one-celled  organism  to  man  himself,  obey  the 
mechanical  law  of  the  conservation  of  energy.  By  this  we  mean 
that  whenever  energy  is  manifested,  as  in  motion,  heat  production, 
or  any  other  form  of  activity,  an  equivalent  amount  disappears 
from  some  antecedent  source.  The  source  of  bodily  energy  is 
chemical,  in  animals  being  derived,  ultimately,  from  the  food.  All 
cell  activities  involve  the  expenditure  of  energy.  All  cells,  there- 
fore, require  to  be  fed.  Energy  is  obtained  in  animal  cells  through 
the  breakdown  of  the  complex  chemical  substances  of  which  food 
is  composed  into  simpler  ones.  This  breaking^down  process  is 
described  as  dissimilation,  the  opposite  of  the  building-up  process, 
assimilation.  In  living  cells  the  two  processes,  assimilation  and 
dissimilation,  go  on  side  by  side,  and  under  ordinary  conditions 
exactly  balance,  so  there  is  neither  gain  nor  loss  of  cell  substance. 
Since  dissimilation  is  the  process  by  which  the  Body  obtains 
energy  for  its  various  activities  we  shall  have  occasion  to  study  itb 
manifestations  in  detail  as  the  different  phases  of  bodily  function 
are  considered. 

Cell  Growth.  All  cells  during  their  early  life  possess  the  power 
of  growth,  or  in  terms  of  their  chemical  activities  are  able  to  assim- 
ilate faster  than  they  dissimilate.  The  materials  that  are  to  be 
assimilated  have  to  enter  the  cell  through  its  surface  membrane, 
and  obviously,  if  there  is  no  shortage  of  food,  the  larger  the  surface 
the  more  can  enter.  The  processes  of  dissimilation,  going  on  in- 
side the  cell  are,  on  the  other  hand,  relatively  independent  of  the 
surface,  being  determined  rather  by  the  amount  or  mass  of  proto- 
plasm making  up  the  cell.  Now  a  little  consideration  will  make 
clear  to  us  that  the  smaller  an  object  is  the  larger  is  its  relative 
surface.  This  we  can  demonstrate  by  placing  two  bricks  together 
to  make  one  block.  The  dimensions  of  the  block  will  be  4"x4" 
X8",  and  the  surface  area  160  sq.  inches.  If  now  we  separate  the 
two  bricks  each  will  have  exactly  half  the  mass  of  the  former  block 
but  more  than  half  the  surface,  the  total  surface  area  of  a  single 
brick  being  112  sq.  inches.  Thinking  now  of  cells  instead  of  bricks 


THE  FUNDAMENTAL  PHYSIOLOGICAL  ACTIONS  23 

we  can  see  that  if  in  the  larger  one  the  amount  of  material  that  can 
enter  through  the  surface  is  just  sufficient  to  balance  the  dissimila- 
tion, in  the  smaller  one,  with  only  half  the  mass  to  carry  on  dis- 
similation processes,  the  relatively  larger  surface  permits  the  en- 
trance of  more  than  enough  material  for  a  balance,  and  there  is  an 
excess  which  can  be  built  into  the  substance  of  the  cell,  the  process 
which  constitutes  growth. 

Cell  Division.  When  the  relation  of  surface  to  mass  in  the 
growing  cell  reaches  the  point  where  there  is  no  excess  of  assimila- 
tion over  dissimilation,  growth  necessarily  stops.  Since  this  rela- 
tionship appears  while  both  mass  and  surface  are  very  small  single 
cells  are  always  quite  or  nearly  microscopic.  To  build  up  such 
large  structures  as  are  present  in  the  body  cell  multiplication  takes 
place.  Whenever  a  cell  which  has  reached  the  limit  of  its  growth 
divides,  the  greater  relative  reduction  of  mass  as  compared  with 
surface  gives  opportunity  for  excess  assimilation  to  occur  once 
more  and  growth  is  resumed. 

Details  of  Cell  Structure  and  Nuclear  Structure.  The  peculiar 
function  of  the  cell  nucleus  appears  to  be  the  control  of  the  processes 
of  assimilation.  Assimilation  involves  the  transformation  into 
cell  protoplasm  of  the  food  substances  which  enter  the  cell; 
furthermore,  the  differences  which  distinguish  one  sort  of  cell 
from  another  sort,  and  in  consequence,  one  sort  of  animal  or  plant 
from  another,  are  at  bottom  differences  in  the  character  of  the 
protoplasm;  the  nucleus  is,  therefore,  the  determining  factor  in  the 
establishment  of  definite  species,  and  much  study  has  been  given 
it  in  the  hope  of  obtaining  insight  into  the  conditions  upon  which 
specific  cell  differences  depend. 

Microscopically  the  most  striking  feature  of  the  nucleus  as  ob- 
served in  suitably  fixed  and  stained  preparations  is  an  irregular 
network  of  substance  which  has  greater  affinity  for  dyes  than  the 
other  constituents  of  the  cell  and  therefore  stains  much  more 
deeply.  Because  of  this  affinity  for  coloring  reagents  the  sub- 
stance has  been  named  chromatin.  It  occurs  in  the  nucleus  in  the 
form  of  excessively  minute  granules,  strung  like  beads  upon  a 
thread  of  different  material,  called  linin,  the  whole  twisted  into 
an  irregular  network  (Fig.  6). 

During  the  process  of  cell  division  the  chromatin  network  passes 
through  a  remarkable  sequence  of  events,  of  such  a  character  as  to 


24 


THE  HUMAN  BODY 


indicate  most  convincingly  that  the  chromatin  is  vitally  concerned 
in  the  determination  of  the  nature  of  the  cell. 

Outside  the  nucleus,  and  imbedded  in  the  cytoplasm  is  a  struc- 
ture, the  attraction  sphere,  which  takes  active  part  in  the  process  of 
cell  division  (Fig.  6). 

Mitotic  Cell  Division.  Since  the  chromatin  is  looked  upon  as 
determining  the  character  of  the  cell,  we  will  expect  to  find  that  in 
the  process  of  cell  division,  whereby  tissues  are  built  up  out  of  a 

Attraction -sphere  enclosing  two  centrosomes. 


Nucleus 


Plasmosome  or 

true 

nucleolus 

Chromatin- 

network 

Linin-network 


Karyosome, 
net-knot,  or 
chromatin- 
nucleolus 


Plastids  lying 
in  the  cyto- 
plasm 


Vacuole 


'assive  bodies 
(metaplasm  or 
paraplasm) 
suspended  in 
the  cyto- 
plasmic  mesh- 
work 


FIG.  6. — Minute  structure  of  a  cell  (Wilson). 


few  antecedent  cells,  care  will  be  taken  to  divide  the  chromatin 
of  the  dividing,  or  mother  cell  equally  between  the  daughter  cells, 
so  as  to  insure  that  the  lattef  shall  be  alike.  Similarly,  in  the 
earlier  stages  of  development,  when  the  different  tissues  of  the 
body  are  being  formed  from  cells  that  are  all  alike,  means  must  be 
afforded  for  dividing  the  chromatin  of  the  mother  cell  unequally 
among  the  daughter  cells,  to  make  the  desired  differentiation  pos- 
sible. The  processes  by  which  these  ends  are  secured  are  known  as 
mitosis  or  mitotic  cell  division. 

The  first  step  in  mitotic  division  is  the  splitting  of  the  attraction 
sphere  (Fig.  6)  into  two  halves,  called  the  centrosomes,  which  travel 


THE  FUNDAMENTAL  PHYSIOLOGICAL  ACTIONS  25 

to  opposite  sides  of  the  nucleus,  but  remain  connected  by  a  spindle 
of  fine,  colorless  fibers,  the  achromatic  spindle.  This  spindle,  in 
passing  from  one  centrosome  to  the  other  penetrates  the  nucleus 
and  comes  into  close  relationship  with  the  chromatic  network. 
This  latter  structure,  meanwhile,  has  arranged  itself  into  a  con- 
tinuous filament  which  presently  breaks  into  segments,  called  the 
chromosomes  (Fig.  7,  3).  An  interesting  fact  is  that  the  number  of 
chromosomes  into  which  the  chromatin  filament  divides  is  the 
same  for  all  the  cells  of  any  given  species.  In  the  cells  of  the 
guinea  pig,  for  example,  the  number  is  sixteen.  The  number  of 
chromosomes,  while  characteristic,  is  probably  not  vitally  signifi- 
cant, since  the  cells  of  the  onion  have  the  same  number  as  those 
of  the  guinea  pig. 

Each  individual  chromosome  becomes  attached  to  a  fiber  of  the 
spindle.  Often  the  chromosomes  take  the  form  of  a  V,  in  which 
case  attachment  is  at  the  apex.  The  chromosome  next  splits 
lengthwise,  each  granule  dividing  into  equal  halves  (Fig.  7,  4). 
By  a  shortening  of  the  spindle  fibers  one  subdivision  of  each 
chromosome  is  drawn  to  one  of  the  centrosomes  and  the  other  sub- 
division to  the  other  centrosome.  The  chromosomes  then  reunite 
to  form  a  continuous  filament  which,  in  turn,  shapes  itself  into  the 
characteristic  chromatin  network  of  the  resting  cell  nucleus. 
The  cell  protoplasm  divides,  meanwhile,  and  the  process  is  com- 
plete. On  the  theory  that  the  chromatin  granules  are  the  de- 
terminers of  the  cell  characteristics,  this  method  of  division  in- 
sures that  the  daughter  cells  shall  resemble  each  other  and  the 
mother  cell  very  closely.  As  supporting  this  theory  of  the  func- 
tion of  the  chromatin  is  the  interesting  observation  that  during 
the  early  stages  of  development,  while  tissue  differentiation  is  in 
progress,  those  cells  that  are  destined  to  become  the  progenitors 
of  special  tissues  lose  portions  of  their  chromatin,  by  causing  them 
to  dissolve  in  the  cell  protoplasm  and  disappear.  The  deduction 
is  that  these  specialized  tissues  are  not  going  to  need  all  the  char- 
acteristics of  undifferentiated  protoplasm  and  so  disburden  them- 
selves as  early  as  possible  of  those  determiners  for  which  they  have 
no  further  use. 

Significance  of  the  Physiological  Properties.  Adaptation.  If 
we  take  the  liberty  of  personifying  Nature  to  the  extent  of  ascrib- 
ing purposes  to  her,  we  can  say  that  the  purpose  of  Nature  with 


26 


THE  HUMAN  BODY 


reference  to  the  various  species  of  living  beings  is  to  maintain  them 
upon  earth  from  generation  to  generation.  The  life  of  the  individ- 
ual is  important  only  as  it  serves  toward  the  perpetuation  of  the 


7  : 


Y 


/  \ 


FIG.  7. — Diagram  showing  the  changes  which  occur  in  the  centrosomes  and 
nucleus  of  a  cell  in  the  process  of  mitotic  division.  (Schafer.)  The  nucleus  is  sup- 
posed to  have  four  chromosomes. 


THE  FUNDAMENTAL  PHYSIOLOGICAL  ACTIONS  27 

race.  So  long  as  the  good  of  the  individual  does  not  run  counter, 
to  that  of  the  race  the  individual  is  conserved,  but  as  soon  as  the 
good  of  the  individual  is  opposed  to  that  of  the  race  the  individual 
is  sacrificed.  Thus  in  a  state  of  nature  the  old  and  feeble  suffer 
destruction  because  their  usefulness  to  the  race  is  over,  and  they 
are  consuming  food  which  may  be  required  by  the  young  and 
vigorous.  The  elaborate  humanitarian  measures  by  which  civi- 
lized men  attempt  to  prevent  the  destruction  of  the  old,  the 
feeble,  and  the  sick  may  seem,  at  first  thought,  absolutely  opposed 
to  the  purposes  of  nature,  and  so,  perhaps,  from  a  purely  physical 
standpoint  they  are,  but  when  we  recall  that  the  really  worth 
while  part  of  Man's  life  is  intellectual  and  spiritual  rather  than 
physical,  and  consider  the  influence  of  humanitarianism  upon 
this  part,  we  realize  that  humanitarianism  represents,  after  all, 
one  phase  of  the  highest  development  of  the  purpose  of  Nature 
with  respect  to  Man. 

The  Physiological  Properties  of  organisms  are  the  means  by 
which  they  are  enabled  to  carry  out  the  purpose  of  Nature  with  re- 
gard to  themselves.  These  properties  are  peculiarly  fitted  to 
enable  organisms  to  maintain  themselves  and  their  race  upon 
earth.  Consider,  for  example,  the  usefulness  of  the  functions  of 
movement,  sensation,  and  co-ordination.  The  power  of  motion 
is  of  great  advantage  to  an  animal,  but  only  in  connection  with 
sensation  and  co-ordination.  The  chief  usefulness  of  motion  to 
the  organism  is  the  securing  of  food  and  the  avoidance  of  harm. 
Neither  of  these  ends  is  served  by  aimless  motion.  To  secure 
food  or  to  avoid  harm  the  organism  must  have  knowledge  of  its 
environment.  This  is  gained  through  the  operation  of  the  prop- 
erty of  sensation.  The  mere  possession  of  knowledge  is  of  no 
avail  unless  the  movements  can  be  directed  in  accordance  with  it. 
For  this  guidance  the  property  of  co-ordination  serves. 

Only  through  the  successful  co-operation  of  these  three  physio- 
logical properties  is  the  organism  able  to  adapt  itself  to  its  environ- 
ment and  so  to  live.  Continued  survival  requires  the  continuous 
co-operation  of  these  functions.  The  definition  sometimes  heard  of 
life  as  continuous  adaptation  emphasizes  this  truth. 

The  function  of  assimilation  and  those  phases  of  dissimilation 
not  immediately  concerned  with  the  properties  of  motion,  sensa- 
tion, and  co-ordination  are,  nevertheless,  essential  to  the  life  of 


28  *THE  HUMAN  BODY 

the  organism,  and  upon  them  in  great  degree  the  perpetuation  of 
the  race  depends.  Their  relationships  are  less  familiar  than  those 
of  the  immediately  adaptive  functions,  but  as  we  study  them,  in 
due  course,  their  fundamental  importance  will  become  clear. 

Co-ordination  in  the  Body.  A  very  little  study  of  our  most 
common  activities  shows  that  in  us  the  function  of  co-ordination 
is  developed  to  a  very  high  degree.  In  the  act  of  walking,  for 
example,  sensations  of  sight  direct  the  movements  of  the  muscles 
of  the  legs.  To  cause  the  leg  muscles  to  work  adaptively  in 
obedience  to  the  sensations  entering  the  eyes  a  special  phase  of 
co-ordination,  namely,  conduction  of  messages  from  one  point  to 
another,  enters  prominently.  This  form  of  conduction  is  accom- 
plished through  the  operation  of  the  nervous  system,  and  the  kind 
of  co-ordination  of  which  it  is  a  part  is  called  nervous  co-ordination. 
There  is  another  sort  of  co-ordination  which  is  very  important, 
and  of  which  we  shall  have  much  to  say,  but  which  is  in  many 
respects  less  familiar  in  its  workings.  In  the  growth  process,  for 
instance,  co-ordination  enters  constantly.  In  most  individuals 
the  two  legs  are  the  same  length,  so  are  the  two  arms;  the  ears  are 
about  the  same  size;  the  eyes  are  the  same  color.  These  things 
do  not  happen  by  accident,  but  because  they  are  controlled  by  a 
definite  co-ordinating  mechanism.  This  type  of  co-ordination 
differs  from  that  effected  through  the  nervous  system  chiefly  in 
that  it  is  concerned  with  processes  which  go  on  more  slowly.  The 
body  carries  on  this  type  of  co-ordination  by  means  of  chemical 
substances,  known  as  hormones,  which  are  manufactured  in  certain 
tissues  of  the  body,  specially  differentiated  for  that  purpose,  and 
conveyed  to  the  tissues  upon  which  they  exert  their  influence 
through  the  blood  stream.  This  method  of  control  is  called 
chemical  co-ordination  and  shares  with  nervous  co-ordination  the 
task  of  causing  the  different  parts  of  the  exceedingly  complex  body 
machine  to  operate  harmoniously. 

Emphasis  is  placed  upon  the  property  of  co-ordination  thus 
early  in  our  consideration  of  the  Body  because  a  true  appreciation 
of  Physiology  requires  not  only  an  understanding  of  the  working 
of  the  various  tissues,  but  even  more  a  grasp  of  the  manner  in 
which  they  co-operate  to  secure  that  continuous  adaptation  of 
the  organism  to  the  environment  upon  which  life  depends. 


CHAPTER  III 
TISSUES,  ORGANS,  AND  PHYSIOLOGICAL  SYSTEMS 

Development.  Every  Human  Body  commences  its  individual 
existence  as  a  single  nucleated  cell.  This  cell,  known  as  the  ovum, 
divides  or  segments  and  gives  rise  to  a  mass  consisting  of  a  number 


FIG.  8. — A,  an  ovum;  B  to  E,  successive  stages  in  its  segmentation  until  the 
morula,  F,  is  produced;  a,  cell-sac;  6,  cell  contents;  c,  nucleus. 

of  similar  units  and  called  the  mulberry  mass  or  the  morula.  At 
this  stage,  long  before  birth,  there  are  no  distinguishable  tissues 
entering  into  the  structure  of  the  Body,  nor  are  any  organs  recog- 
nizable. 

For  a  short  time  the  morula  increases  in  size  by  the  growth  and 
division  of  its  cells,  but  very  soon  new  processes  occur  which  ulti- 
mately give  rise  to  the  complex  adult  body  with  its  many  tissues 
and  organs.  Groups  of  cells  ceasing  to  grow  and  multiply  like 
their  parents  begin  to  grow  in  ways  peculiar  to  themselves,  and 
so  come  to  differ  both  from  the  original  cells  of  the  morula  and 
from  the  cells  of  other  groups,  and  this  unlikeness  becoming  more 
and  more  marked,  a  varied  whole  is  finally  built  up  from  one 
originally  alike  in  all  its  parts.  Peculiar  growth  of  this  kind,  form- 

29 


30  THE  HUMAN  BODY 

ing  a  complex  from  a  simple  whole,  is  called  development;  and  the 
process  itself  in  this  case  is  known  as  the  differentiation  of  the 
tissues,  since  by  it  they  are,  so  to  speak,  separated  or  specialized 
from  the  general  mass  of  mother-cells  forming  the  morula. 

As  the  differences  in  the  form  and  structure  of  the  constituent 
cells  of  the  morula  become  marked,  differences  in  property  arise, 
and  it  becomes  obvious  that  the  whole  cell-aggregate  is  not  des- 
tined to  give  rise  to  a  collection  of  independent  living  things, 
but  to  form  a  single  human  being,  in  whom  each  part,  while  main- 
taining its  own  life,  shall  have  duties  to  perform  for  the  good  of 
the  whole.  In  other  words,  a  single  compound  individual  is  to  be 
built  up  by  the  union  and  co-operation  of  a  number  of  simple  ones 
represented  by  the  various  cells,  each  of  which  thenceforth,  while 
primarily  looking  after  its  own  interests  and  having  its  own 
peculiar  faculties,  has  at  the  same  time  its  activities  subordinated 
to  the  good  of  the  entire  community. 

The  Physiological  Division  of  Labor.  As  the  differentiation 
of  tissues  proceeds  the  fundamental  physiological  properties,  orig- 
inally exhibited  in  equal  degree  by  all  the  cells,  become  distrib- 
uted among  the  various  tissues.  Thus  we  find  certain  tissues 
adapted  to  execute  movements  and  in  these  the  property  of  active 
motion  is  developed  to  an  especial  degree.  Other  tissues,  on  the 
other  hand,  show  little  or  no  active  motion  but  exhibit  a  marked 
degree  of  conductivity.  The  higher  we  look  in  the  animal  scale 
the  more  marked  becomes  this  division  of  physiological  duties 
among  the  tissues.  In  man  it  attains  its  highest  development. 

Classification  of  the  Tissues. — As  we  might  separate  the  in- 
habitants of  the  United  States  into  groups,  such  as  lawyers,  doc- 
tors, clergymen,  merchants,  farmers,  and  so  forth,  so  we  may  clas- 
sify the  tissues  by  selecting  the  most  distinctive  properties  of  each 
of  those  entering  into  the  construction  of  the  adult  Body  and 
arranging  them  into  physiological  groups;  those  of  each  group 
being  characterized  by  some  one  prominent  employment.  No 
such  classification,  however,  can  be  more  than  approximately 
accurate,  since  the  same  tissue  has  often  more  than  one  well- 
marked  physiological  property.  The  following  arrangement,  how- 
ever, is  practically  convenient. 

1.  UNDIFFERENTIATED  TISSUES.  These  are  composed  of  cells 
which  have  developed  along  no.  one  special  line,  but  retain  very 


TISSUES,  ORGANS,  AND  PHYSIOLOGICAL  SYSTEMS        31 

much  the  form  and  properties  of  the  cells  forming  the  very  young" 
Body  before  different  tissues  were  recognizable  in  it.  The  lymph- 
corpuscles  and  the  colorless  corpuscles  of  the  blood  belong  to  this 
class. 

2.  SUPPORTING  TISSUES.     Including  cartilage  (gristle),  bone,  and 
connective  tissue.     Of  the  latter  there  are  several  subsidiary  vari- 
eties, the  two  more  important  being  white  fibrous  connective  tissue, 
composed  mainly  of  colorless  inextensible  fibers,  and  yellow  fibrous 
tissue,  composed  mainly  of  yellow  elastic  fibers.     All  the  support- 
ing tissues  are  used  in  the  Body  for  mechanical  purposes;  the  bones 
and  cartilages  form  the  hard  framework  by  which  softer  tissues  are 
supported  and  protected;  and  the  connective  tissues  unite  the 
various  bones  and  cartilages,  form  investing  membranes  around 
different  organs,  and  in  the  form  of  fine  networks  penetrate  their 
substance  and  support  their  constituent  cells.     The  functions  of 
these  tissues  being  for  the  most  part  passively  to  resist  strain  or 
pressure,  none  of  them  has  any  very  marked  physiological  prop- 
erty; they  are  not,  for  example,  excitable  or  contractile,  and  their 
mass  is  chiefly  made  up  of  an  intercellular  substance  which  has 
been  formed  by  the  actively  living  cells  sparsely  scattered  through 
them,  as,  for  instance,  in  cartilage  (Fig.  11),  where  the  cells  are  seen 
imbedded  in  cavities  in  a  matrix  which  they  have  formed  around 
them;  and  this  matrix  by  its  firmness  and  elasticity  forms  the 
functionally  important  part  of  the  tissue. 

3.  NUTRITIVE  TISSUES.     These  form  a  large  group,  the  members 
of  which  fall  into  three  main  divisions,  viz. : 

Assimilative  tissues,  concerned  in  receiving  and  preparing  food 
materials,  and  including — (a)  Secretory  tissues,  composed  of  cells 
which  make  the  digestive  liquids  poured  into  the  alimentary  canal 
and  used  to  bring  about  chemical  or  other  changes  in  the  food. 
(b)  Receptive  tissues,  represented  by  cells  which  line  parts  of  the 
alimentary  canal  and  take  up  the  digested  food. 

Eliminative  or  excretory  tissues,  represented  by  cells  in  the  kid- 
neys, skin,  and  elsewhere,  whose  main  business  it  is  to  get  rid  of  the 
waste  products  of  the  various  parts  of  the  Body. 

Respiratory  tissues.  These  are  concerned  in  the  gaseous  inter- 
changes between  the  Body  and  the  surrounding  air.  They  are 
constituted  by  the  cells  lining  the  lungs  and  by  the  colored  cor- 
puscles of  the  blood. 


32  THE  HUMAN  BODY 

As  regards  the  nutritive  tissues  it  requires  especially  to  be  borne 
in  mind  that  although  such  a  classification  as  is  here  given  is  use- 
ful, as  helping  to  show  the  method  pursued  in  the  domestic  econ- 
omy of  the  Body,  it  is  only  imperfect  and  largely  artificial.  Every 
cell  of  the  Body  is  in  itself  assimilative,  respiratory,  and  excre- 
tory, and  the  tissues  in  this  class  are  only  those  concerned  in  the 
first  and  last  interchanges  of  material  between  it  and  the  external 
world.  They  provide  or  get  rid  of  substances  for  the  whole  Body, 
leaving  the  feeding  and  respiration  and  excretion  of  its  individual 
tissues  to  be  ultimately  looked  after  by  themselves,  just  as  even 
the  mandarin  described  by  Robinson  Crusoe  who  found  his  dignity 
promoted  by  having  servants  to  put  the  food  into  his  mouth,  had 
finally  to  swallow  and  digest  it  for  himself.  Many  secretory  cells, 
too,  have  no  concern  with  the  digestion  of  food,  as  for  example 
those  which  form  the  various  hormones  (p.  28). 

4.  STORAGE  TISSUES.     The  Body  does  not  live  from  hand  to 
mouth:  it  has  always  in  health  a  supply  of  food-materials  ac- 
cumulated in  it  beyond  its  immediate  needs.     This  lies  in  part  in 
the  individual  cells  themselves,  but  apart  from  this  reserve  there 
are  certain  cells,  which  store  up  considerable  quantities  of  material 
and  constitute  what  we  will  call  the  storage  tissues.     These  are 
especially  represented  by  the  liver-cells  and  fat-cells,  which  con- 
tain in  health  a  reserve  fund  for  the  rest  of  the  Body. 

5.  EXCITABLE,  OK  IRRITABLE  TISSUES.     These  include  those 
tissues  which  are  especially  susceptible  to  changes  in  their  sur- 
roundings and  are  therefore  useful  in  giving  to  the  Body  information 
of  what  is  going  on  around  it.     Any  change  in  the  environment 
which  serves  to  arouse  response  in  an  excitable  tissue  constitutes 
a  stimulus. 

6.  CONDUCTIVE  TISSUES.     While  most,  if  not  all,  of  the  cells 
of  the  Body  retain  the  property  of  conductivity  in  some  degree, 
the  nervous  tissues  exhibit  it  in  very  high  degree.     They  serve 
therefore  to  bring  into  communication  the  various  parts  of  the 
Body.     As  an  incident  in  the  conveying  of  messages  from  one 
part  of  the  Body  to  another  to  fulfil  the  requirements  of  nervous 
co-ordination  certain  nervous  structures  have  the  power  of  modify- 
ing the  messages  which  pass  through  them. 

7.  MOTOR  TISSUES.     These  have  the  contractility  of  the  orig- 
inal protoplasmic  masses  highly  developed.     The  most  important 


TISSUES,  ORGANS,  AND  PHYSIOLOGICAL  SYSTEMS        33 

are  the  ciliated  cells  and  muscular  tissue.  The  former  line  certain— 
surfaces  of  the  Body,  and  possess  on  their  free  surfaces  fine  threads 
which  are  in  constant  movement.  One  finds  such  cells,  for  ex- 
ample (Fig.  46),  lining  the  inside  of  the  windpipe,  where  their 
threads  or  cilia  serve,  by  their  motion,  to  sweep  any  fluid  formed 
there  towards  the  throat,  where  it  can  be  coughed  up  and  got  rid 
of.  Muscular  tissue  occurs  in  three  main  varieties.  One  kind  is 
found  in  the  muscles  attached  to  the  bones,  and  is  that  used  in 
the  ordinary  voluntary  movements  of  the  Body.  It  is  composed 
of  fibers  which  present  cross-stripes  when  viewed  under  the  micro- 
scope (Fig.  42),  and  is  hence  known  as  striped  or  striated  muscular 
tissue.  Because  the  muscles  which  are  made  of  this  sort  of  tissue 
are  attached  to  bones  they  are  often  called  skeletal  muscles.  A 
second  kind  of  muscular  tissue  is  found  in  the  walls  of  the  alimen- 
tary canal,  the  arteries,  and  some  other  hollow  organs,  and  consists 
of  elongated  cells  (Fig.  44)  which  present  no  cross-striation.  It  is 
known  as  smooth  or  unstriated  muscular  tissue.  The  third  sort 
occurs  only  in  the  heart.  It  consists  of  branched  cells  presenting 
some  indications  of  cross-striation  (Fig.  45)  and  is  called  cardiac 
muscular  tissue. 

The  cells  enumerated  under  the  heading  of  "  undifferentiated 
tissues"  might  also  be  included  among  the  motor  tissues,  since 
they  are  capable  of  changing  their  form. 

8.  PROTECTIVE  TISSUES.  These  consist  of  certain  cells  lining 
cavities  inside  the  body  and  called  epithelial 
cells,  and  cells  covering  the  whole  exterior  of 
the  Body  and  forming  epidermis,  hairs,  and 
nails.  The  enamel  which  covers  the  teeth 
belongs  also  to  this  group. 

The  class  of  protective  tissues  is,  however, 
even  more  artificial  than  that  of  the  nutritive 
tissues,  and  cannot  be  defined  by  positive 
characters.     Many  epithelial  cells  are  secre- 
excretory  or  receptive;  and  cilated  cells 


face  of  the  peritoneum,     have  already  been  included  among  the  motor 

a,  cell-body  ;  c,  nucleus.       ,  .  .-,-,,  .  .         ,  .  i_      T_ 

tissues.  The  protective  tissues  may  be  best 
defined  as  including  cells  which  cover  free  surfaces,  and  whose 
functions  are  mainly  mechanical  or  physical.  In  their  simplest 
form  epithelial  cells  are  flat  scales,  as,  for  example,  those  repre- 


34  THE  HUMAN  BODY 

sented  in  Fig.  9  from  the  lining  membrane  of  the  abdominal 
cavity. 

9.  THE  REPRODUCTIVE  TISSUES.  These  are  concerned  in  the 
production  of  new  individuals,  and  in  the  Human  Body  are  of  two 
kinds,  located  in  different  sexes.  The  conjunction  of  the  products 
of  each  sex  is  necessary  for  the  origination  of  offspring,  since  the 
female  product,  egg-cell  or  ovum,  which  directly  develops  into  the 
new  human  being,  remains  dormant  until  it  has  been  fertilized,  and 
fertilization  consists  essentially  in  the  fusion  of  its  nucleus  with  the 
nucleus  of  a  cell  produced  by  the  male. 

The  Combination  of  Tissues  to  Form  Organs.  The  various 
tissues  above  enumerated  form  the  building  materials  of  the  Body; 
anatomy  is  primarily  concerned  with  their  structure,  and  physi- 
ology with  their  properties.  If  this,  however,  were  the  whole 
matter,  the  problems  of  anatomy  and  physiology  would  be  much 
simpler  than  they  actually  are.  The  knowledge  about  the  living 
Body  obtained  by  studying  only  the  forms  and  functions  of  the 
individual  tissues  would  be  comparable  to  that  attained  about  a 
great  factory  by  studying  separately  the  boilers,  pistons,  levers, 
wheels,  etc.,  found  in  it,  and  leaving  out  of  account  altogether  the 
way  in  which  these  are  combined  to  form  various  machines;  for  in 
the  Body  the  various  tissues  are  for  the  most  part  associated  to 
form  organs,  each  organ  answering  to  a  complex  machine  like  a 
steam-engine  with  its  numerous  constituent  parts.  And  just  as  in 
different  machines  a  cogged  wheel  may  perform  very  different 
duties,  dependent  upon  the  way  in  which  it  is  connected  with  other 
parts,  so  in  the  Body  any  one  tissue,  although  its  essential  proper- 
ties are  everywhere  the  same,  may  by  its  activity  subserve  very 
various  uses  according  to  the  manner  in  which  it  is  combined  with 
others.  For  example :  A  nerve-fiber  uniting  the  eye  with  one  part 
of  the  brain  will,  by  means  of  its  conductivity,  when  its  end  in  the 
eye  is  excited  by  the  irritable  tissue  attached  to  it  on  which  light 
acts,  cause  changes  in  the  sensory  nerve-cells  connected  with  its 
other  end  and  so  arouse  a  sight  sensation;  but  an  exactly  similar 
nerve-fiber  running  from  the  brain  to  the  muscles  will,  also  by 
virtue  of  its  conductivity,  when  its  ending  in  the  brain  is  excited 
by  a  change  in  a  nerve-cell  connected  with  it,  stir  up  the  muscle 
to  contract  under  the  control  of  the  will.  The  different  results  de- 
pend on  the  different  parts  connected  with  the  ends  of  the  nerve- 


TISSUES,  ORGANS,  AND  PHYSIOLOGICAL  SYSTEMS        35 

fibers  in  each  case,  and  not  on  differences  in  the  properties  of  the 
nerve-fibers  themselves. 

It  becomes  necessary  then  to  study  the  arrangement  and  uses 
of  the  tissues  as  combined  to  form  various  organs,  and  this  is  fre- 
quently far  more  difficult  than  to  make  out  the  structure  and 
properties  of  the  individual  tissues.  An  ordinary  muscle,  such  as 
one  sees  in  the  lean  of  meat,  is  a  very  complex  organ,  containing 
not  only  contractile  muscular  tissue,  but  supporting  and  uniting 
connective  tissue  and  conductive  nerve-fibers,  and  in  addition  a 
complex  commissariat  arrangement,  composed  in  its  turn  of  sev- 
eral tissues,  concerned  in  the  food-supply  and  waste-removal  of  the 
whole  muscle.  The  anatomical  study  of  a  muscle  has  to  take  into 
account  the  arrangement  of  all  these  parts  within  it,  and  also  its 
connections  with  other  organs  of  the  Body.  The  physiology  of  any 
muscle  must  take  into  account  the  actions  of  all  these  parts  work- 
ing together  and  not  merely  the  functions  of  the  muscular  fibers 
themselves,  and  has  also  to  make  out  under  what  conditions  the 
muscle  is  excited  to  activity  by  changes  in  other  organs,  and  what 
changes  in  these  it  brings  about  when  it  works. 

Physiological  Systems.  Even  the  study  of  organs  added  to 
that  of  the  separate  tissues  does  not  exhaust  the  matter.  In  a  fac- 
tory we  frequently  find  machines  arranged  so  that  two  or  more 
shall  work  together  for  the  performance  of  some  one  work:  a  steam- 
engine  and  a  loom  may,  for  example,  be  connected  and  used  to- 
gether to  weave  carpets.  Similarly  in  the  Body  several  organs  are 
often  arranged  to  work  together  so  as  to  attain  some  one  end  by 
their  united  actions.  Such  combinations  are  known  as  physi- 
ological apparatuses  or  systems.  The  circulatory  system,  for  ex- 
ample, consists  of  various  organs  (each  in  turn  composed  of  several 
tissues)  known  as  heart,  arteries,  capillaries,  and  veins.  The  heart 
forms  a  force-pump  by  which  the  blood  is  kept  flowing  through 
the  whole  mechanism,  and  the  rest,  known  together  as  the  blood- 
vessels, distribute  the  blood  to  the  various  organs  and  regulate  the 
supply  according  to  their  needs.  Again,  in  the  visual  apparatus 
we  find  the  co-operation  of  (a)  a  set  of  optical  instruments  which 
bring  the  light  proceeding  from  external  objects  to  a  focus  upon 
(6)  the  retina,  which  contains  highly  irritable  parts;  these,  changed 
by  the  light,  stimulate  (c)  the  optic  nerve,  which  is  conductive  and 
transmits  a  disturbance  which  arouses  in  turn  (d)  sensory  parts  in 


36  THE  HUMAN  BODY 

the  brain.  In  the  production  of  ordinary  sight  sensations  all  these 
parts  are  concerned  and  work  together  as  a  visual  apparatus. 
So,  too,  we  find  a  respiratory  system  consisting  primarily  of  two 
hollow  organs,  the  lungs,  which  lie  in  the  chest  and  communicate 
by  the  windpipe  with  the  back  of  the  throat,  from  which  air  enters 
them.  But  to  complete  the  respiratory  apparatus  are  many  other 
organs,  bones,  muscles,  nerves,  and  nerve-centers,  which  work  to- 
gether to  renew  the  air  in  the  lungs  from  time  to  time;  and  the 
act  of  breathing  is  the  final  result  of  the  activity  of  the  whole 
apparatus. 

The  Relation  of  Man  to  His  Environment.  From  infancy  the 
human  organism  is  confronted  with  the  task  of  maintaining  itself 
alive.  To  this  end  all  the  bodily  functions  bend  themselves.  The 
maintenance  of  life  in  man,  as  in  all  animals,  presents  two  distinct 
problems:  first,  to  obtain  the  necessary  food;  and  second,  to  cope 
successfully  with  the  innumerable  perils  with  which  the  organism 
is  continually  confronted.  Failure  in  either  of. these  endeavors 
means  failure  in  maintaining  life  itself. 

The  labor  of  obtaining  food  and  the  struggle  to  escape  harm  take 
place  in  the  midst  of  a  world  filled  with  creatures  engaged  in  the 
same  labor  and  the  same  struggle.  Indeed  it  is  the  very  prevalence 
of  living  beings  that  makes  the  securing  of  food  labor,  and  the 
avoidance  of  harm  a  struggle.  All  the  living  beings  that  belong  to 
the  animal  kingdom  are  in  a  more  or  less  continuous  state  of 
activity.  Each  individual,  therefore,  finds  himself  surrounded  by 
a  continually  shifting  world  of  other  beings.  Nor  is  inanimate 
Nature  stationary;  winds  and  rains,  heat  and  cold,  come  and  go. 
To  such  a  constantly  changing  environment  the  organism  must 
adapt  itself. 

In  the  complex  of  systems  which  together  make  up  the  Body  it 
is  possible  to  distinguish  between  those  whose  immediate  function 
is  to  maintain  the  necessary  adaptation  of  the  organism  to  its 
environment  and  those  which  function  only  indirectly  to  that  end 
by  keeping  the  Body  itself  in  good  working  order  and  each  part 
well  supplied  with  the  energy  yielding  materials  without  which 
activity  is  impossible.  In  making  such  a  distinction,  however,  it 
must  be  borne  in  mind  that  all  the  bodily  functions  work  together 
for  the  good  of  the  whole  Body  so  that  no  hard  and  fast  line  can  be 
drawn  between  the  two  classes  of  systems.  It  will  be  convenient 


TISSUES,  ORGANS,  AND  PHYSIOLOGICAL  SYSTEMS        37 

to  consider  first  the  systems  which  are  particularly  concerned  in 
adapting  the  Body  to  its  environment. 

Adaptive  Systems.  The  Motor  System.  In  all  members  of  the 
animal  kingdom  with  the  exception  of  certain  parasites  adaptation 
is  secured  mainly  through  movement.  Both  for  obtaining  food 
and  for  escaping  danger  movements  either  of  the  whole  Body  or 
of  parts  of  it  are  constantly  being  resorted  to. 

In  all  higher  animals  the  motor  mechanism  is  made  up  of  skeletal 
muscles,  which  by  their  action  upon  the  movable  bones  of  the 
jointed  skeleton  bring  about  the  various  bodily  movements. 

There  are  many  types  of  movement  in  animals  which  are  not 
concerned  immediately  with  adaptation  to  the  environment.  The 
movements  of  breathing,  for  example,  the  beat  of  the  heart,  and 
the  activities  of  the  stomach.  These  have  to  do  with  maintenance.  • 
As  emphasized  in  the  last  paragraph,  however,  the  classification 
of  systems  as  adaptive  or  maintenance  is  for  convenience,  and  with 
reference  to  their  most  conspicuous  functions,  and  is  not  to  be 
taken  as  excluding  the  systems  in  one  group  from  having  important 
activities  in  the  other. 

The  Supporting  System.  In  all  but  the  very  simplest  animal 
forms  movements  are  made  effective  by  the  action  of  the  muscles 
upon  certain  of  the  supporting  tissues.  These  tissues  play,  there- 
fore, a  very  real,  although  passive  part  in  adaptation.  By  includ- 
ing the  supporting  system  among  the  adaptive  systems  of  the  Body 
we  emphasize  the  importance  of  the  supporting  structures  in  mak- 
ing muscular  action  effective,  although  here  again  we  must  bear  in 
mind  that  they  are  also  intimately  associated  with  other  systems 
whose  chief  function  is  maintenance. 

The  Receptor  System.  It  is  obvious  that  the  Body  cannot 
( 'xocute  movements  adapting  it  to  its  surroundings  unless  it  knows 
what  its  surroundings  are.  A  blind  man,  be  he  never  so  agile,  can- 
not escape  the  onward  rush  of  the  approaching  car  while  he  is 
ignorant  of  its  coming.  He  will  starve  in  the  midst  of  abundant 
food  if  he  does  not  know  where  it  is  to  be  found. 

The  Body  obtains  knowledge  of  its  environment  by  means  of  a 
set  of  structures  known  as  the  sense-organs.  In  these  the  property 
of  irritability  is  developed  to  a  high  degree,  and  so  long  as  they  all 
function  properly  not  much  that  is  important  for  the  organism 
to  know  about  need  escape  its  knowledge. 


38  THE  HUMAN  BODY 

The  Conductive  System.  Organs  for  making  movements  and 
organs  for  receiving  impressions  from  the  surroundings  are  not  of 
themselves  adequate  to  the  maintenance  of  adaptation.  It  is 
necessary  that  the  information  gained  by  the  sense-organs  be  trans- 
mitted to  the  muscles  so  that  their  movements  may  correspond  to 
the  requirements  of  the  situation.  This  function  is  performed  by 
the  nervous  system.  The  conduction  of  stimuli  from  sense-organs 
to  muscles  is  not,  however,  a  simple  matter.  Impressions  are  con- 
tinually coming  into  the  Body  by  way  of  a  number  of  different 
channels.  Movements  must  be  made  not  in  obedience  to  any  one 
of  these  impressions  by  itself  but  for  the  advantage  of  the  whole 
Body  as  indicated  by  all  of  them  taken  together.  To  this  end  a 
certain  part  of  the  nervous  system  is  adapted  for  receiving  all 
sorts  of  incoming  stimuli  and  before  passing  them  on  to  the  motor- 
organs  combining  and  modifying  them  to  produce  the  best  results. 

While  for  purposes  of  convenience,  the  conductive  system  is 
classed  as  one  of  the  adaptive  mechanisms,  we  need  to  bear  in 
mind  that  nervous  co-ordination,  for  which  this  system  is  the 
agency,  although  concerned  primarily  with  direct  adaptation,  has 
also  much  to  do  with  the  control  of  those  activities  which  are  pri- 
marily concerned  with  maintenance  and  only  indirectly  adaptive. 

Maintenance  Systems.  The  systems  which  are  not  imme- 
diately concerned  in  the  adaptation  of  the  Body  to  its  environment 
but  which  serve  rather  to  keep  it  in  proper  condition  for  activity 
may  next  be  considered. 

Activity  in  the  Body  involves  the  manifestation  of  energy,  and 
in  its  energy  relations  the  Body  is  on  exactly  the  same  plane  as 
any  machine;  it  is  without  power  to  manufacture  energy,  and 
must  receive  whatever  energy  it  obtains  from  without.  The  ulti- 
mate source  of  the  Body's  energy  is  chemical,  being  received 
in  the  complex  substances  which  serve  as  food.  This  energy 
is  made  available  for  the  use  of  the  Body  chiefly  through  the 
process  of  oxidation.  Every  living  cell  in  the  Body  must  share 
in  this  process,  for  the  energy  manifestations  of  the  Body  as  a 
whole  are  simply  the  sum-total  of  those  of  its  component  cells. 

The  systems  which  are  concerned  with  the  maintenance  of  ac- 
tivity have,  then,  the  task  of  furnishing  to  each  cell  of  the  Body 
oxidizable  substance  and  oxygen;  they  must  provide  for  making 
good  the  wear  and  tear  of  the  cells  themselves;  and  they  must 


TISSUES,  ORGANS,  AND  PHYSIOLOGICAL  SYSTEMS        39 

remove  the  waste  materials  which  are  formed  in  connection  with 
the  chemical  activities  of  the  cells  and  which  would  interfere  with 
their  proper  working  if  allowed  to  accumulate. 

The  Circulatory  System  consists  of  the  heart  and  blood-vessels. 
It  serves  to  distribute  to  all  the  parts  of  the  Body  supplies  of 
oxidizable  material,  of  repair  material,  and  of  oxygen,  and  to  re- 
move therefrom  the  accumulated  waste  products.  These  func- 
tions are  accomplished  through  the  agency  of  a  circulating  me- 
dium, the  blood. 

The  Respiratory  System  consists  of  the  lungs,  the  bronchial 
tubes,  and  the  trachea,  together  with  the  respiratory  muscles.  Its 
function  is  to  bring  the  outside  air  to  a  region  where  the  circulating 
medium  can  take  up  abundant  supplies  of  oxygen,  and  where  it 
can  get  rid  of  those  waste  products  which  are  in  gaseous  form. 

The  Digestive  System  consists  of  the  alimentary  canal  and  cer- 
tain associated  glands  (salivary,  liver,  pancreas).  It  serves  to  bring 
the  various  materials  that  are  taken  as  food  into  the  forms  best 
adapted  for  use  as  repair  materials  or  as  oxidizable  substance; 
when  it  has  so  prepared  them  it  turns  them  over  to  the  circulating 
medium  for  distribution. 

The  Excretory  System  consists  of  the  kidney  and  bladder  with 
their  connecting  tubes,  the  liver,  and  the  skin.  It  serves  to  with- 
draw from  the  circulating  medium  and  to  eliminate  from  the  body 
those  waste  products  which  are  in  liquid  form. 

Chemical  Co-ordination  is  secured,  as  previously  stated,  by 
specific  hormones  which  govern  those  bodily  activities  that  are 
either  not  readily  susceptible  to  nervous  control  or  in  which  the 
best  results  are  secured  by  supplementing  nervous  control  with 
chemical.  There  are  special  organs,  or  parts  of  organs,  which 
manufacture  hormones.  These  are  often  called  ductless  glands, 
since  they  pour  their  secretions  into  the  blood  stream  and  not  by 
ducts  to  the  surface.  They  might  be  grouped  together  as  a  sys- 
tem, although  nothing  would  be  gained  by  so  doing.  Chemical 
co-ordination  plays  a  part  in  nearly  all  forms  of  bodily  activity, 
and  the  different  hormones  will  be  studied  in  connection  with  the 
activities  over  which  they  exert  influence. 

Through  these  systems  provision  is  made  for  the  activities  of  the 
individual  cells.  These  activities  are  many  and  complex.  They 


40  THE  HUMAN  BODY 

include  oxidative  processes,  processes  involving  waste  and  repair, 
and  doubtlessly  many  others  of  which  we  know  nothing.  The 
study  of  these  cell  activities  is  comprehended  under  the  head  of 
Metabolism. 

The  chemical  activities  which  go  on  in  the  cells  of  the  Body  give 
rise  to  much  heat.  Some  cells  generate  more  heat  than  others. 
One  of  the  functions  of  the  circulating  medium  is  to  distribute  this 
heat  uniformly  over  the  Body.  There  is  constant  loss  of  heat  from 
the  surface  of  the  Body.  In  warm-blooded  animals,  which  have 
a  nearly  constant  body  temperature,  the  maintenance  of  balance 
between  heat  production  and  heat  loss  in  the  face  of  constantly 
varying  outside  temperatures  is  a  function  of  great  importance. 
It  is  studied  under  the  head  of  Heat  Production  and  Heat  Regula- 
tion. 

Not  immediately  concerned  with  the  well-being  of  the  Body  it- 
self, but  devoted  to  the  well-being  of  the  race  as  a  whole  through 
perpetuating  the  species  is  the  Reproductive  System. 

Before  we  turn  from  this  discussion  of  the  various  systems  into 
which,  for  convenience,  we  have  grouped  the  various  Bodily  struc- 
tures, we  may  well  emphasize  again  the  unity  of  operation  of  the 
Body,  so  that  we  shall  not  fall  into  the  habit  of  thinking  of  the 
different  systems  as  separate  mechanisms,  operating  independ- 
ently of  one  another.  This  unity  of  operation  is  well  illustrated 
in  one  of  our  commonest  every-day  experiences,  namely,  vigorous 
muscular  exercise.  Whenever  we  use  our  muscles  briskly  definite 
activities  of  the  various  systems  we  have  classed  as  maintenance 
systems  occur.  Thus  the  heart  is  thrown  into  rapid  beating;  the 
skin  is  flushed;  the  breathing  is  quickened;  the  sweat  glands  are 
active;  if  the  exercise  is  prolonged  and  not  too  fatiguing,  there  is 
likely  to  be  a  sharpening  of  appetite,  leading  to  a  greater  consump- 
tion of  food  and  so  to  increased  digestive  activity.  All  these  mani- 
festations accompany  muscular  exercise,  as  a  matter  of  course. 
We  shall  see  later  how  they  are  all  part  of  the  provision  whereby 
the  Body  is  able  to  use  its  muscles  effectively.  This  is  but  one  of 
many  illustrations  that  might  be  cited  to  show  the  interdependence 
of  the  various  systems.  True  insight  into  Human  Physiology  re- 
quires that  this  interdependence  be  thoroughly  realized. 

Animals  Compared  with  Plants.  We  divide  the  world  of  living 
things  into  two  kingdoms;  the  plant  kingdom  and  the  animal 


TISSUES,  ORGANS,  AND  PHYSIOLOGICAL  SYSTEMS.       41 

kingdom.  The  familiar  members  of  the  first  kingdom  seem  to  us 
to  differ  in  nearly  every  important  respect  from  the  best  known 
members  of  the  second;  an  oak  tree  and  a  horse  are  superficially 
wholly  dissimilar.  Yet  both  plants  and  animals  consist  of  living 
cells,  and  the  protoplasm  of  which  plant  cells  are  composed  is  often 
indistinguishable  from  that  found  in  animal  cells.  When  we  at- 
tempt to  analyze  the  difference  between  plants  and  animals  we  find 
that  it  cannot  be  referred  to  difference  in  the  protoplasm.  The 
simplest  plants  and  the  simplest  animals  show  the  fundamental 
properties  of  protoplasm  developed  in  about  equal  degrees.  In 
fact  it  is  by  no  means  easy  always  to  state  positively  whether  a 
given  one-celled  organism  should  be  considered  a  plant  or  an 
animal. 

As  we  go  up  the  scale  to  the  region  of  higher  organization,  how- 
ever, we  find  no  difficulty  in  deciding  whether  a  living  form  is  plant 
oj  animal.  The  fundamental  difference  between  the  higher  plants 
and  animals,  and  the  one  which  involves,  as  a  natural  sequence, 
the  superficial  differences  which  are  so  striking,  is  a  difference  in 
the  manner  of  obtaining  nourishment.  The  higher  (green)  plants 
are  able  to  use  the  energy  which  falls  upon  them  in  the  form  of 
sunlight  to  build  up  from  simple  substances  present  in  the  air,  and 
in  the  water  to  which  their  roots  penetrate,  the  complex  materials 
of  which  protoplasm  is  composed  and  which,  through  the  processes 
of  dissimilation,  provide  for  the  carrying  on  of  the  necessary  ac- 
tivities of  living  cells.  The  higher  animals,  on  the  other  hand,  are 
nourished  by  means  of  complex  materials  which  contain  within 
themselves  the  energy  required  for  the  bodily  activities.  The 
ultimate  source  of  animal  energy,  to  be  sure,  is  the  same  as  of 
plant,  for  the  complex  materials  consumed  by  animals  are  derived, 
directly  or  indirectly,  from  plants. 

The  simple  chemical  materials  needed  by  plants  are  very  widely 
distributed,  and  the  sunlight  falls,  of  course,  on  all  parts  of  the 
earth.  In  any  location,  therefore,  that  is  sufficiently  suited  to 
plant  life  to  allow  the  plant  to  get  a  start,  the  chances  of  being  able 
to  continue  to  live  are  as  good,  on  the  whole,  as  they  would  be 
anywhere.  Hence  plants  do  not  need  to  move  about.  The  giant 
sequoias  of  the  Pacific  Slope  have  lived  for  centuries  upon  the 
spots  where  they  became  established  as  seedlings. 

Animals,  on  the  other  hand,  require  materials  that  are  not  every- 


42      .  THE  HUMAN  BODY 

where  present  but  must  be  sought  in  particular  places.  The  suc- 
cessful search  for  this  sort  of  food  involves  active  motion  subject 
to  guidance  in  accordance  with  the  environment.  In  a  word, 
adaptation;  and  the  presence  of  mechanisms  for  adaptation  is  the 
most  striking  feature  of  the  higher  animals,  just  as  the  presence 
of  a  mechanism  for  utilizing  the  energy  of  sunlight  is  the  con- 
spicuous feature  of  higher  plants. 


CHAPTER  IV 
THE  SUPPORTING  TISSUES 

Connective  Tissue.  This  is  the  most  widely  distributed  of  the 
supporting  tissues.  It  envelopes  and  pervades  all  the  soft  parts  of 
the  Body.  The  various  constituents  of  individual  organs  are  held 
together  by  it,  and  the  organs  themselves,  are  supported  in  their 
places  by  the  same  tissue.  Beneath  the  skin  and  attaching  it 
rather  loosely  to  the  underlying  structures  is  a  layer  of  connective 
tissue  known  as  the  fascia.  So  completely  is  the  entire  Body  per- 
vaded by  connective  tissue  that  if  a  solvent  could  be  found  which 
would  dissolve  away  all  the  tissues  of  the  Body  except  this  one 
there  would  still  remain  in  perfect  outline  not  only  the  whole  Body 
but  also  each  organ  down  to  minutest  detail. 

This  connective  tissue  framework  is  commonly  called  areolar 
tissue.  It  is  composed,  in  the  main,  of  tough,  inelastic  strands; 
these  are  arranged,  however,  in  most  parts  of  the  Body  to  form  a 
rather  loose  network,  so  that  in  removing  the  skin  from  an  animal 
or  in  separating  one  muscle  from  another  in  making  a  dissection 
a  blunt  instrument  readily  tears  the  strands  of  areolar  tissue  apart. 

The  meshes  of  areolar  tissue  are  everywhere  filled  with  a  fluid, 
lymph.  Thus  the  various  living  tissues  of  the  Body,  all  of  which 
are  surrounded  by  areolar  tissue,  are  nourished. 

There  are  in  the  body  connective  tissue  structures  in  which  the 
individual  strands,  instead  of  forming  a  loose  network,  are  in 
parallel  bundles,  forming  the  toughest  and  strongest  of  cords  and 
bands.  These  are  the  tendons,  by  which  muscles  are  attached  to 
bones,  and  the  ligaments  which  hold  the  different  bones  of  the 
skeleton  together. 

The  functional  part  of  connective  tissue  consists  of  two  sorts  of 
fibers.  In  most  places  the  white  fibers  constitute  the  bulk  of  the 
tissue.  These  are  flexible,  inelastic  strands  composed  of  an  albu- 
minoid substance,  collagen.  The  second  sort  of  fibers  are  the 
elastic  fibers.  These  are  intermingled  with  the  white  fibers  to  some 
extent  in  nearly  all  regions  where  connective  tissue  occurs.  In 

43 


44  THE  HUMAN  BODY 

certain  structures,  where  a  high  degree  of  elasticity  is  required, 
elastic  fibers  make  up  the  entire  connective  tissue  content.  Ex- 
amples of  this  sort  are  the  walls  of  the  large  arteries  and  the  liga- 
ments connecting  the  vertebrae.  In  quadrupeds  these  fibers  form 
the  great  ligament  which  helps  to  sustain  the  head  (see  p.  69). 
Elastic  tissue  is  yellow  in  color,  and  consists  chemically  of  an  al- 
buminoid, elastin,  which  in  some  important  respects  differs  from 
the  albuminoid  of  the  white  fibers. 

Connective  tissue  fibers  are  not  living  structures.  They  owe 
their  origin  to  certain  living  cells,  the  so-called  connective  tissue 
cells,  which  lie  irregularly  interspersed  wherever  connective  tissue 
fibers  occur  (Fig.  10).  In  areolar  tissue  many  of  the  cells  have 
given  up  their  function  of  forming  fibers  and  have  devoted  them- 
selves instead  to  storing  within  their  substance  masses  of  fat. 
Adipose  tissue  consists  of  cells  of  this  sort,  and  occurs  in  regions 
where  areolar  tissue  is  most  abundant,  as  just  under  the  skin  or  in 
masses  about  certain  internal  organs. 


FIG.  10. — Connective  tissue  cells:  a,  from  areolar  tissue;  b,  from  tendon. 

Temporary  and  Permanent  Cartilages.  In  early  life  a  great 
many  parts  of  the  supporting  framework  of  the  Body,  which  after- 
wards become  bone,  consist  of  cartilage.  Such  for  example  is  the 
case  with  all  the  vertebrae,  and  with  the  bones  of  the  limbs.  In 
these  cartilages  subsequently  the  process  known  as  ossification 
takes  place,  by  which  a  great  portion  of  the  original  cartilaginous 
model  is  removed  and  replaced  by  true  osseous  tissue.  Often, 
however,  some  of  the  primitive  cartilage  is  left  throughout  the 
whole  of  life  at  the  ends  of  the  bones  in  joints  where  it  forms  the 
articular  cartilages;  and  in  various  other  places  still  larger  masses 
remain,  such  as  the  costal  cartilages,  those  in  the  external  ears 
forming  their  framework,  others  finishing  the  skeleton  of  the  nose 
which  is  only  incompletely  bony,  and  many  in  internal  parts  of 
the  Body,  as  the  cartilage  of  "  Adam's  apple/'  which  can  be  felt 
in  the  front  of  the  neck,  and  a  number  of  rings  around  the  wind- 
pipe serving  to  keep  it  open.  These  persistent  masses  are  known 


THE  SUPPORTING  TISSUES 


45 


as  the  permanent,  the  others  as  the  temporary  cartilages.  In  old  age 
many  so-called  permanent  cartilages  become  calcified — that  is, 
hardened  and  made  unyielding  by  deposits  of  lime-salts  in  them — 
without  assuming  the  histological  character  of  bone,  and  this 
calcification  of  the  permanent  cartilages  is  one  chief  cause  of  the 
want  of  pliability  and  suppleness  of  the  frame  in  advanced  life. 

Hyaline  Cartilage.  In  its  purest  form  cartilage  is  flexible  and 
elastic,  of  a  pale  bluish- white  color  when  alive  and  seen  in  large 
masses,  and  cuts  readily  with  a  knife.  In  thin  pieces  it  is  quite 
transparent.  Everywhere  except  in  the  joints  it  is  invested  by  a 
tough  adherent  membrane,  the  perichondrium. 

When  a  thin  slice  of  hyaline  cartilage  is  examined  with  a  micro- 
scope it  is  found  (Fig.  11)  to  consist  of  granular  nucleated  cells, 
often  collected  into  groups  of  two,  four,  or  more,  scattered  through 
a  homogeneous  or  faintly  granular  ground-substance  or  matrix. 

This  matrix  is  composed  of  al-  c    «      a  _b 

buminoid  substances,  and  owes 
its  origin  to  the  cells  embedded 
within  it.  At  the  time  the 
cartilage  was  in  process  of  for-  ^ 

mation  these  cells  laid  down  .--..,_.     ^  -    - 

the  matrix  substance  in  con-     Si&^isiis^r^^^^fe^  <i 
centric    layers    about    them- 
selves; thus  they  cut   them- 
selves off  from  each  other  and 
from  communication  with  the   niSd/toshwr^  ceSs 

Outside.      The  substance  of  the  homogeneous  matrix,     a,  a  cell  in  which 

.       .  ~    .  the  nucleus  has  divided;  b,  a  cell  in  which 

matrix    IS    Sufficiently    perme-  division  is  just  complete;  c,  e,  a  group  of 

_ui        r  f  i    •  four  cells  resulting  from  further  division 

able,    however,    for    a    Certain  Of  a  pair  like  6;  the  new  cells  have  formed 

interchange   of  food 'materials  ?°me    matrix    between    them,    separating 

them  from  another;  d,    d,  cavities  jn  the 

between  the  Cartilage  Cells  and  matrix  from  which  cells  have  dropped  out 

the    blood,    SO    that    the    cells  during  the  preparation  oi  the  specimen. 

are  able  to  remain  alive,  although  their  life  is  naturally  an  inac- 
tive one. 

All  temporary  cartilages  are  of  the  hyaline  type  as  are  also  the 
costal  and  articular  permanent  cartilages  and  the  cartilage  of  the 
nose  and  of  the  windpipe. 

Elastic  Cartilage  is  a  tissue  whose  cartilaginous  matrix  is  inter- 
woven with  fibers  of  elastic  connective  tissue.  The  result  of  this 


46  THE  HUMAN  BODY 

interweaving  is  to  give  the  cartilage  a  yellow  color  and  a  high  de- 
gree of  elasticity.  Cartilage  of  this  sort  is  found  in  the  external 
ear,  the  epiglottis,  and  in  certain  parts  of  the  larynx. 

Fibrocartilage  is  really  a  dense  fibrous  connective  tissue  within 
whose  spaces  a  certain  amount  of  matrix  material  has  been  de- 
posited. It  makes  up  the  intervertebral  disks,  pads  which  are 
interposed  between  the  bones  of  the  vertebral  column,  and  is 
found  also  in  certain  joints,  notably  the  knee-joints  and  the  ar- 
ticulations of  the  lower  jaw. 

Bone.  The  bones  which  make  up  the  skeleton  vary  greatly  in 
shape  and  size,  ranging  from  the  long  cylindrical  bones  of  the 
arm  and  leg  to  the  flat  skull  bones,  and  the  tiny  irregularly  shaped 
ossicles  of  the  middle  ear.  They  all,  however,  have  a  similar 
microscopic  structure  and  similar  chemical  composition. 

The  bones  may  be  classified  according  to  their  origin  as  mem- 
brane bones  or  cartilage  bones.  To  the  first  group  belong  the  flat 
bones  of  the  skull  and  the  bones  of  the  face  (see  p.  60).  They  do 
not  replace  cartilage  but  develop  upon  a  foundation  of  connective 
tissue.  The  so-called  cartilage  bones  replace  the  temporary 
cartilages  and  make  up  the  whole  of  the  bony  skeleton,  except  the 
membrane  bones  mentioned  above. 

The  Process  of  Bone  Formation  is  complicated,  and  can  be 
described  only  very  briefly  here.  At  the  beginning  of  the  develop- 
ment of  a  membrane  bone  the  strands  of  connective  tissue  upon 
which  the  bone  is  to  be  built  become  covered  with  peculiar  small 
cells  which  are  bone-producing  cells  or  osteoblasts.  These  osteo- 
blasts  deposit  upon  the  strands  whereon  they  rest  albuminoid 
material  which  constitutes  the  organic  matrix  of  bone.  There  is 
thus  produced  a  rather  open  network  of  bone  matrix.  By  the 
deposition  within  the  matrix  of  lime-salts  it  takes  on  the  character 
of  true  bone.  The  original  connective  tissue  is  thus  replaced  by  a 
network  of  bony  spicules. 

The  surfaces  of  this  bony  mass  now  become  covered  with  a  stout 
connective  tissue  membrane,  the  periosteum,  whose  inner  surface 
is  beset  with  osteoblasts.  These  deposit  upon  the  underlying  mass 
a  layer  of  compact  bone.  Thus  the  fully  formed  membrane  bone 
consists  of  outer  surfaces  of  compact  bone  inclosing  a  mass  of 
spongy  bone. 

The  replacement  of  temporary  cartilage  by  bone  proceeds  from 


THE  SUPPORTING  TISSUES  47 

certain  points  in  the  cartilage  known  as  centers  of  ossification. 
The  cartilage  itself  becomes  surrounded  by  a  periosteum  like  that 
which  incloses  membrane  bones.  At  the  center  of  ossification  the 
osteoblast  layer  of  the  periosteum  begins  to  force  its  way  into  the 
cartilage,  absorbing  much  of  the  latter  and  leaving  only  a  coarse 
network,  which  is  presently  converted  by  the  osteoblasts  into  true 
bone.  Meanwhile  the  periosteum  has  deposited  on  the  surface  of 
the  cartilage  a  layer  of  compact  bone  so  that  in  time  the  cartilage 
bone  presents  a  structure  not  unlike  that  of  membrane  bones,  a 
spongy  interior  inclosed  in  a  layer  of  compact  bone.  As  the 
cartilaginous  network  is  being  ossified,  many  osteoblasts  are  im- 
prisoned within  the  bony  substance.  The  spaces  which  they 
occupy  and  the  tiny  canals  which  radiate  therefrom  into  the  bone 
substances  are  among  the  most  characteristic  appearances  of  bone 
viewed  under  the  microscope  (Fig.  12). 

The  growth  in  thickness  of  bone  is  accomplished  by  the  addition 
of  layer  after  layer  of  compact  bone  underneath  the  periosteum. 


FIG.  12. — Cross-section  of  compact  bone  from  the  shaft  of  the  humerus.  (Sharpey, 
from  Bailey's  Text  Boole  of  Hiatology.) 

During  this  process  blood-vessels  of  the  periosteum  often  become 
embedded  within  the  bony  mass.  When  this  occurs  the  osteo- 
blasts which  accompany  the  blood-vessel  surround  it  with  con- 
centric layers  of  bone.  In  this  manner  are  formed  the  so-called 
Hwersian  Systems,  each  of  which  consists  of  the  space  through 


48 


THE  HUMAN  BODY 


which  the  blood-vessel  passes  with  its  surrounding  rings  of  bony 
material. 

The  increase  in  length  of  the  long  bones  is  brought  about  by 
plates  of  cartilage  which  persist  between  the  shaft  of  the  bone  and 
its  extremities.  There  is  a  continual  growth 
of  bone  into  these  cartilages  from  both  sides, 
but  they  grow  in  thickness  with  equal  rapidity 
until  the  adult  length  of  the  bone  is  reached 
when  their  growth  stops  and  they  are  gradually 
replaced  by  bone. 

At  the  same  time  that  the  bone  is  growing 
by  additions  to  its  outer  surfaces  a  continu- 
ous absorption  of  its  inner  portions  is  going 
on.  This  absorption  is  carried  on  by  large, 
multinuclear  cells  known  as  osteoclasts.  It 
serves  the  purpose  of  preventing  the  bone  from 
becoming  so  heavy  as  to  be  unmanageable, 
without  sacrificing  unduly  its  strength.  As 
the  result  of  this  absorption  many  adult  bones, 
especially  long  ones,  contain  little  or  no  spongy 
bone  except  at  their  ends,  the  shaft  being 
hollow  as  shown  in  Fig.  13. 

The  Repair  of  Fractured  Bone.  When,  as 
happens  with  unfortunate  frequency,  a  bone  is 
fractured,  a  sequence  of  processes  is  set  in  mo- 
tion at  the  point  of  injury  which  results  finally 
in  the  mending  of  the  break.  As  an  inevitable 
incident  of  the  injury  which  caused  the  frac- 
ture there  is  marked  laceration  of  the  perios- 
teum and  of  the  other  adjacent  tissues.  These 
lacerated  tissues  pour  out  a  mixture  of  blood 
and  lymph,  known  as  the  exudate,  which  per- 
meates the  region  of  injury.  This  exudate 
gradually  stiffens  until  it  affords  considerable 
support  to  the  injured  bone.  Osteoblasts 
from  the  inner  surface  of  the  periosteum  and  from  the  fractured 
ends  of  the  bone  penetrate  the  exudate.  These  osteoblasts,  little 
by  little,  replace  the  exudate  with  spongy  bone,  which  holds  the 
injured  parts  even  more  firmly  in  place,  and  which  in  turn  is  grad- 


FIG.  13. — The  hu- 
merus  bisected  length- 
wise, a,  marrow-cav- 
ity; 6,  hard  bone; 

ticular  cartilage. 


THE  SUPPORTING  TISSUES  49 

ually  replaced  by  hard  bone  like  that  which  was  present  before  the 
injury.  The  spongy  temporary  bone  is  absorbed  by  the  osteo- 
clasts  described  above. 

To  secure  proper  knitting  of  the  fracture  two  things  are  of 
great  importance;  the  first  of  these  is  the  reduction  of  the  fracture_, 
whereby  the  parts  are  brought  as  nearly  as  possible  into  their 
former  positions;  the  second  is  immobilization  of  the  part  by  means 
of  splints,  bandages,  or  casts  to  hold  the  broken  ends  in  place  during 
the  formation  of  the  new  bony  material. 

Chemistry  of  Bone.  Bone  is  composed  of  inorganic  and  organic 
portions  intimately  combined,  so  that  the  smallest  distinguishable 
portion  contains  both.  The  inorganic  matters  form  about  two- 
thirds  of  the  total  weight  of  a  dried  bone,  and  may  be  removed  by 
soaking  the  bone  in  dilute  hydrochloric  acid.  The  organic  portion 
left  after  this  treatment  constitutes  a  flexible  mass,  retaining  the 
form  of  the  original  bone;  it  consists  chiefly  of  an  albuminoid, 
ossein,  which  by  long  boiling,  especially  under  pressure  at  a  higher 
temperature  than  that  at  which  water  boils  when  exposed  freely 
to  the  air,  is  converted  into  gelatin,  which  dissolves  in  the  hot 
water.  Much  of  the  gelatin  of  commerce  is  prepared  in  this  man- 
ner by  boiling  the  bones  of  slaughtered  animals,  and  even  well- 
picked  bones  may  be  used  to  form  a  good  thick  soup  if  boiled  under 
pressure  in  a  Papin's  digestor;  much  nutritious  matter  being,  in 
the  common  modes  of  domestic  cooking,  thrown  away  in  the  bones. 

The  inorganic  salts  of  bone  may  be  obtained  free  from  organic 
matter  by  calcining  a  bone  in  a  clear  fire,  which  burns  away  the 
organic  matter.  The  residue  forms  a  white  very  brittle  mass,  re- 
taining perfectly  the  shape  and  structural  details  of  the  original 
bone.  It  consists  mainly  of  normal  calcium  phosphate,  or  bone- 
earth  [CaaCPO^l;  but  there  is  also  present  a  considerable  propor- 
tion of  calcium  carbonate  (CaCO3)  and  smaller  quantities  of  other 
salts. 

Hormones  of  the  Supporting  System.  We  learned  in  a  previous 
chapter  (p.  28)  that  co-ordination  of  many  bodily  processes,  and 
notably  of  the  growth  process,  is  secured  chemically  by  means  of 
substances  known  as  hormones.  The  size  of  the  Body  depends  on 
the  size  of  the  bones  which  make  up  the  skeleton.  For  that  rea- 
son any  clue  to  the  mechanism  which  governs  the  growth  of  the 
bones  is  of  great  interest.  Some  years  ago  the  discovery  was  made 


50  THE  HUMAN  BODY 

that  a  peculiar  disease,  acromegaly,  in  which  there  is  abnormal  en- 
largement of  some  of  the  bones,  notably  of  the  face  and  extrem- 
ities, is  associated  with  overgrowth  of  a  mass  of  tissue  at  the  base 
of  the  brain.  This  tissue  mass,  the  pituitary  body  (Fig.  60),  is 
formed  in  part  by  an  outgrowth  downward  from  the  brain,  and  in 
part  by  an  outgrowth  upward  from  the  roof  of  the  mouth.  The 
latter  portion,  known  separately  as  the  hypophysis,  appears  to  be 
particularly  concerned  with  bony  growth.  The  theory  of  this 
control  which  best  explains  the  known  facts  is  that  a  hormone  is 
secreted  and  poured  out  into  the  blood  which  by  its  presence 
stimulates  growth  of  the  bones.  The  more  abundant  the  hormone 
the  more  vigorous  is  the  growth.  The  enlargement  of  the  hypoph- 
ysis which  occurs  in  acromegaly  would  account  for  the  occurrence 
of  a  more  abundant  secretion  of  the  hormone.  A  similar  enlarge- 
ment appears  to  characterize  the  condition  of  gigantism,  which 
gives  rise  to  the  giants  exhibited  in  side  shows.  Although  there 
is  no  positive  proof,  it  seems  reasonable  to  suppose  that  the  oppo- 
site condition,  dwarfishness,  is  a  result  of  a  deficiency  of  the  pitui- 
tary hormone. 

We  need  to  bear  in  mind  in  discussing  hormone  action  that  when 
an  effect  is  attributed  to  a  hormone  we  have  not  offered  a  complete 
explanation  of  it,  but  only  moved  the  explanation  a  step  farther 
along.  We  can  say  that  a  man's  height  is  determined  by  the 
activity  of  his  pituitary  body,  but  we  are  still  in  the  dark  as  to  the 
factors  that  regulate  the  latter.  Particularly  are  we  ignorant  of 
the  manner  in  which  secretion  of  the  hormone  is  modified  when  the 
man  has  "gotten  his  growth." 

Another  body,  the  thyroid,  located  at  the  front  of  the  neck,  is 
believed  to  secrete  a  hormone  which  regulates  the  development  of 
connective  tissue.  This  function  of  the  thyroid  is,  however,  sub- 
ordinate to  its  major  function,  which  is  concerned  with  the  nervous 
system.  Detailed  discussion  of  the  thyroid  is  deferred,  therefore, 
to  that  connection, 

Hygienic  Remarks.  Since  in  the  new-born  infant  many  parts 
which  will  ultimately  become  bone  consist  only  of  cartilage,  the 
young  child  requires  food  which  shall  contain  a  large  proportion  of 
the  lime-salts  which  are  used  in  building  up  bone.  Nature  provides 
this  in  the  milk,  which  is  rich  in  such  salts  (see  Chap.  XXXIV), 
and  no  other  food  can  thoroughly  replace  it.  Long  after  infancy 


THE  SUPPORTING  TISSUES  51 

milk  should  form  a  large  part  of  a  child's  diet.  Many  children 
though  given  food  abundant  in  quantity  are  really  starved,  since 
their  food  does  not  contain  in  sufficient  amount  the  mineral  salts 
requisite  for  their  healthy  development. 

At  birth  even  those  bones  of  a  child  which  are  most  ossified  are 
often  not-  continuous  masses  of  osseous  tissue.  In  the  large  bone 
of  the  arm,  the  humerus,  for  example,  the  shaft  of  the  bone  is  well 
ossified  and  so  is  each  end,  but  between  the  shafts  and  each  of  the 
articular  extremities  there  still  remains  a  cartilaginous  layer,  and 
at  those  points  the  bone  increases  in  length,  new  cartilage  being 
formed  and  replaced  by  bone.  The  bone  increases  in  thickness 
by  new  osseous  tissue  formed  beneath  the  periosteum.  The 
same  thing  is  true  of  the  bones  of  the  leg.  On  account  of  the 
largely  cartilaginous  and  imperfectly  knit  state  of  its  bones,  it  is 
cruel  to  encourage  a  young  child  to  walk  beyond  its  strength,  and 
may  lead  to  " bow-legs"  or  other  permanent  distortions.  Never- 
theless here  as  elsewhere  in  the  animal  body,  moderate  exercise 
promotes  the  growth  of  the  tissues  concerned,  and  it  is  nearly  as 
bad  to  wheel  a  child  about  forever  in  a  baby-carriage  as  to  force 
it  to  overexert  ion. 

The  best  rule  is  to  let  a  healthy  child  use  its  limbs  when  it  feels 
inclined,  but  not  by  praise  or  blame  to  incite  it  to  efforts  which  are 
beyond  its  age,  and  so  sacrifice  its  healthy  growth  to  the  vanity  of 
parent  or  nurse. 

The  final  knitting  together  of  the  bony  articular  ends  with  the 
shaft  of  many  bones  takes  place  only  comparatively  late  in  life, 
and  the  age  at  which  it  occurs  varies  much  in  different  bones. 
Generally  speaking,  a  layer  of  cartilage  remains  between  the  shaft 
and  the  ends  of  the  bone,  until  the  latter  has  attained  its  full 
adult  length.  To  take  a  few  examples:  the  lower  articular  ex- 
tremity of  the  humerus  only  becomes  continuous  with  the  shaft  by 
bony  tissue  in  the  sixteenth  or  seventeenth  year  of  life.  The  upper 
articular  extremity  only  joins  the  shaft  by  bony  continuity  in  the 
twentieth  year.  The  upper  end  of  the  femur  (p.  66)  joins  the 
shaft  by  bone  from  the  seventeenth  to  the  nineteenth  year,  and 
the  lower  end  during  the  twentieth.  In  the  tibia  (p.  66)  the  upper 
extremity  and  the  shaft  unite  in  the  twenty-first  year,  and  the 
lower  end  and  the  shaft  in  the  eighteenth  or  nineteenth:  while 
in  the  fibula  (p.  66)  the  upper  end  joins  the  shaft  in  the  twenty- 


52  THE  HUMAN  BODY 

fourth  year,  and  the  lower  end  in  the  twenty-first.  The  separate 
vertebrae  of  the  sacrum  (p.  59)  are  only  united  to  form  one  bone 
in  the  twenty-fifth  year  of  life;  and  the  ilium,  ischium,  and  pubis 
unite  to  form  the  os  innominatum  about  the  same  period.  Up  to 
about  twenty-five  then  the  skeleton  is  not  firmly  "knit,"  and  is 
incapable,  without  risk  of  injury,  of  bearing  strains  which  it  might 
afterwards  meet  with  impunity.  To  let  lads  of  sixteen  or  seven- 
teen row  and  take  other  exercise  in  plenty  is  one  thing,  and  a  good 
one;  but  to  allow  them  to  undergo  the  severe  and  prolonged  strain 
of  training  for  and  rowing  a  long  race  is  quite  another,  and  not 
devoid  of  risk. 


CHAPTER  V 
THE  SKELETON 

Exoskeleton  and  Endoskeleton.  The  skeleton  of  an  animal  in- 
cludes all  its  hard  protecting  or  supporting  parts,  and  is  met  with 
in  two  main  forms.  One  is  an  exoskeleton  developed  in  connection 
with  either  the  superficial  or  deeper  layer  of  the  skin,  and  repre- 
sented by  the  shell  of  a  clam,  the  scales  of  fishes,  the  horny  plates 
of  a  turtle,  the  bony  plates  of  an  armadillo,  and  the  feathers  of 
birds.  In  man  the  exoskeleton  is  but  slightly  developed,  but  it 
is  represented  by  the  hairs,  nails,  and  teeth ;  for  although  the  latter 
lie  within  the  mouth,  the  study  of  development  shows  that  they 
are  developed  from  an  offshoot  of  the  skin  which  grows  in  and 
lines  the  mouth  long  before  birth.  Hard  parts  formed  from  struc- 
tures deeper  than  the  skin  constitute  the  endoskeleton,  which  in 
man  is  highly  developed  and  consists  of  a  great  many  bones  and 
cartilages  or  gristles,  the  bones  forming  the  mass  of  the  hard  frame- 
work of  the  Body,  while  the  cartilages  finish  it  off  at  various  parts. 
This  framework  is  what  is  commonly  meant  by  the  skeleton;  it 
primarily  supports  all  the  softer  parts  and  is  also  arranged  so  as 
to  surround  cavities  in  which  delicate  organs,  as  the  brain,  heart, 
or  spinal  cord,  may  lie  with  safety.  The  gross  skeleton  thus 
formed  is  completed  and  supplemented  by  another  made  of  the 
connective  tissues,  which  not  only,  in  the  shape  of  tough  bands  or 
ligaments,  tie  the  bones  and  cartilages  together,  but  also  in  various 
forms  pervade  the  whole  Body  as  a  sort  of  subsidiary  skeleton 
running  through  all  the  soft  organs  and  forming  networks  of  fibers 
around  their  other  constituents;  they  make,  as  it  were,  a  micro- 
scopic skeleton  for  the  individual  modified  cells  of  which  the  Body 
is  so  largely  composed,  and  also  form  partitions  between  the  muscles, 
cases  for  such  organs  as  the  liver  and  kidneys,  and  sheaths  around 
the  blood-vessels.  The  bony  and  cartilaginous  framework  with  its 
ligaments  might  be  called  the  skeleton  of  the  organs  of  the  Body, 
and  this  finer  supporting  meshwork  the  skeleton  of  the  tissues. 

The  Bony  Skeleton  (Fig.  14).  If  the  hard  framework  of  the 

53 


54 


THE  HUMAN  BODY 


2T- 


FIG.  14. — The  bony  and  cartilaginous 
skeleton. 


FIG.  15. — Side  view  of 
the  spinal  column.  C  1-7, 
cervical;  D  1-12,  dorsal; 
L  1-5,  lumbar;  S  1,  sacrum; 
Co  1-4,  coccygeal. 


THE  SKELETON  55 

Body  were  joined  together  like  the  joists  and  beams  of  a  house,  the 
whole  mass  would  be  rigid ;  its  parts  could  not  move  with  relation 
to  one  another,  and  we  should  be  unable  to  raise  a  hand  to  the 
mouth  or  put  one  foot  before  another.  To  allow  of  mobility  the 
bony  skeleton  is  made  of  many  separate  pieces  which  are  joined 
together,  the  points  of  union  being  called  articulations,  and  at 
many  places  the  bones  entering  into  an  articulation  are  movably 
hinged  together,  forming  what  are  known  as  joints.  '  The  total 
number  of  bones  in  the  Body  is  more  than  two  hundred  in  the  adult ; 
and  the  number  in  children  is  still  greater,  for  various  bones  which 
are  distinct  in  the  child  (and  remain  distinct  throughout  life  in 
many  lower  animals)  grow  together  so  as  to  form  one  bone  in  the 
full-grown  man.  The  adult  bony  skeleton  may  be  described  as  con- 
sisting of  an  axial  skeleton,  found  in  the  head,  neck,  and  trunk ;  and 
an  appendicular  skeleton,  consisting  of  the  bones  in  the  limbs  and 
in  the  arches  (u  and  s,  Fig.  14)  by  which  these  are  carried  and  at- 
tached to  the  trunk. 

Axial  Skeleton.  The  axial  skeleton  is  made  up  of  the  vertebral 
column  or  spine,  a  side  view  of  which  is  given  in  Fig.  15;  the  skull, 
Fig.  25;  the  sternum,  Fig.  28;  and  the  ribs,  Fig.  29. 

The  vertebral  column  is  the  great  supporting  center  for  the  whole 
skeleton  and  consists  of  33  bones  grouped  as  follows  from  above 
downward:  7  cervical,  12  dorsal  or  thoracic,  5  lumbar,  5  sacral,  in 
the  adult  united  into  a  single  bone,  the  sacrum,  and  4  coccygeal,  or 
rudimentary  tail  bones. 

The  vertebral  column  occupies  the  mid-dorsal  line  of  the  trunk. 
On  top  of  it  is  borne  the  skull  (22  bones)  made  up  of  two  parts ;  a 
great  box  above,  composed  of  8  bones,  which  incloses  the  brain 
and  is  called  the  cranium;  and  a  group  of  14  bones  on  the  ventral 
side  of  this  which  form  the  skeleton  of  the  face.  Attached  by  liga- 
ments to  the  underside  of  the  cranium  is  the  hyoid  bone,  to  which 
the  root  of  the  tongue  is  fixed.  There  are  12  pairs  of  ribs,  at- 
tached dorsally  to  the  12  thoracic  vertebrae,  one  pair  to  each  ver- 
tebra. The  sternum,  which  occupies  the  mid-ventral  line  of  the 
thorax  and  constitutes  the  anterior  attachment  for  the  ribs  is  made 
up  of  two  bones,  the  manubrium  and  the  body,  and  a  cartilage,  the 
ensiform  cartilage. 

Details  of  the  Vertebral  Column.  The  vertebral  column  is  in  a 
man  of  average  height  about  twenty-eight  inches  long.  Viewed 


56  THE  HUMAN  BODY 

from  the  side  (Fig.  15)  it  presents  four  curvatures;  one  with  the 
convexity  forwards  in  the  cervical  region  is  followed,  in  the  tho- 
racic, by  a  curve  with  its  concavity  towards  the  chest.  In  the 
lumbar  region  the  curve  has  again  its  convexity  turned  ventrally, 
while  in  the  sacral  and  coccygeal  regions  the  reverse  is  the  case. 
These  curvatures  give  the  whole  column  a  good  deal  of  springiness 
such  as  would  be  absent  were  it  a  straight  rod. 

All  the  vertebrae  are  built  upon  the  same  plan,  although  with 
modifications  in  various  parts  of  the  column.  Each  consists :  1,  of  a 
stout  bony  body  or  centrum  (Fig.  16,  C),  in  shape  a  cylinder  flat- 
tened at  both  ends;  2,  a  bony  arch,  the  neural  arch  (Fig.  16,  A),  at- 
tached to  the  dorsal  side  of  the  centrum  and  inclosing  the  neural 
ring  (Fig.  16,  Fv).  The  neural  rings  of  all  the  vertebrae  make  up 
together  a  long  bony  tube,  the  neural  canal,  which  contains  the 
spinal  cord.  Between  the  bodies  of  adjoining  vertebrae,  except  in 
the  sacrum  and  coccyx,  are  thick  pads  of  elastic  cartilage.  These 
permit  bending  movements  which,  while  quite  limited  at  each 
joint  may  be  very  considerable  in  the  column  as  a  whole.  They 
also  serve  to  take  up  a  great  deal  of  shock,  preventing  injury  to 
the  body  when  one  sits  down  hard  or  comes  down  on  his  heels  in 
walking  or  jumping.  During  the  hours  when  one  is  on  his  feet 
these  intervertebral  pads  are  packed  down  by  the  weight  of  the 
body,  and  especially  by  the  hammering  effect  of  the  movements 
of  walking,  running,  etc.,  so  that  a  man  may  be  from  a  half  to  three- 
quarters  of  an  inch  shorter  at  night  than  he  is  in  the  morning. 
Strong  ligaments  fasten  adjoining  vertebrae  together;  there  are 
also  muscles  passing  from  vertebrae  to  vertebrae,  which  by  their 
contractions  assist  in  bending  the  body.  These  muscles  are  ar- 
ranged in  antagonistic  groups;  that  is,  they  are  so  placed  that 
whenever  the  vertebral  column  is  bent  through  the  contraction  of 
one  group  the  muscles  of  the  antagonistic  group  are  put  on  the 
stretch.  The  neural  arch  of  each  vertebra  bears  a  dorsal  spinous 
process  (Fig.  16,  Ps),  and  a  pair  of  lateral  transverse  processes 
(Fig.  16,  Pt).  These  serve  various  purposes;  the  intervertebral 
muscles  are  attached  to  them;  they  also  bear  articular  surfaces 
(Pas  and  Pai,  Figs.  16  and  17)  which  sliding  upon  corresponding 
surfaces  of  adjoining  vertebrae  serve  to  limit  the  movements  at 
each  joint,  and  also  help  to  prevent  dislocation  of  the  vertebral 
column.  The  spinous  processes  may  be  felt  in  the  middle  of  the 


THE  SKELETON 


57 


back.  The  neural  arches  are  notched  (Fig.  17,  7s  and  Fi),  adjoin- 
ing notches  forming  rounded  openings  through  which  the  spinal 
nerves  pass  on  their  way  out  from  the  spinal  cord. 

is 


FIG.  16. 


FIG.  17. 


FIG.  16. — A  thoracic  vertebra  seen  from  behind,  i.  e.,  the  end  turned  from  the 
head. 

FIG.  17. — Two  thoracic  vertebrae  viewed  from  the  left  side,  and  in  their  natural 
relative  positions.  C,  the  body;  A,  neural  arch;  Ps,  spinous  process;  Pas,  anterior 
articular  process;  Pai,  posterior  articular  process;  Pt,  transverse  process;  Ft,  facet 
for  articulation  with  the  tubercle  of  a  rib;  Fes,  Fci,  articular  surfaces  on  the  centrum 
for  articulation  with  a  rib. 


FIG.  19. 

FIG.  18. — Diagrammatic  representation  of  a  segment  of  the  axial  skeleton 
V,  a  vertebra;  C,  Cv,  ribs  articulating  above  with  the  body  and  transverse  process 
of  the  vertebra;  S,  the  breast-bone.  The  lighter-shaded  part  between  S  and  C  is 
the  costal  cartilage. 

FIG.  19. — A  cervical  vertebra.  Frt,  vertebral  foramen;  Pai,  anterior  articular 
process;  R,  rudimentary  rib. 

The  Cervical  Vertebrae  (Fig.  19),  have  rather  small  bodies  and 
large  neural  arches;  in  some  of  them  the  spinous  process  is  bifid. 
They  move  more  freely  upon  each  other  than  do  the  vertebra 
lower  down.  A  rudimentary  rib  (R,  Fig.  19)  becomes  united 


58 


THE  HUMAN  BODY 


early  in  life  to  the  ventral  surface  of  each  transverse  process;  the 
foramina  (Fig.  19,  Frt)  thus  formed  give  passage  to  an  important 
artery  which  ultimately  passes  into  the  cranial  cavity  to  carry 
blood  to  the  brain. 

The  Atlas  and  Axis.  The  first  and  second  cervical  vertebrae 
differ  considerably  from  the  rest.  The  first,  or  atlas  (Fig.  20), 
which  carries  the  head,  has  a  very  small  body,  Aa,  and  a  large 
neural  ring.  This  ring  is  subdivided  by  a  cord,  the  transverse 
ligament,  L,  into  a  dorsal  moiety  in  which  the  spinal  cord  lies  and 


A  a    Fas 


M» 


1> 


Fas 


Frt 


Fio  20. 


Pai 


FIG  21. 
Aa,    body   of   atlas;   D,   odontoid 


FIG.  20. — The  atlas.  FIG.  21. — The  axis. 
process;  Fas,  facet  on  front  of  atlas  with  which  the  skull  articulates;  arid  in  Fig.  21 
anterior  articular  surface  of  axis;  L,  transverse  ligament;  Frt,  vertebral  foramen; 
Ap,  neural  arch;  Tp,  spinous  process. 

a  ventral  into  which  the  bony  process  D  projects.  This  is  the 
odontoid  process,  and  arises  from  the  front  of  the  axis  or  second 
cervical  vertebra  (Fig.  21).  Around  this  peg  the  atlas  rotates 
when  the  head  is  turned  from  side  to  side,  carrying  the  skull  (which 
articulates  with  the  large  hollow  surfaces  Fas)  with  it. 

The  odontoid  process  really  represents  a  large  piece  of  the  body 
of  the  atlas  which  in  early  life  separates  from  its  own  vertebra  and 
becomes  united  to  the  axis. 

The  Thoracic  Vertebrae  have  larger  bodies  and  longer  processes 
than  do  the  cervical  vertebrae.  They  are  specially  modified  for 
carrying  the  ribs.  Each  rib  is  attached  at  two  points  (Fig.  18). 
The  head  of  the  rib  fits  into  an  articulation  at  the  junction  of  two 
vertebrae,  a  part  of  the  articular  surface  being  on  the  centrum  of 
one  and  a  part  on  the  other  (Fig.  17,  Fes  and  Fci).  The  second 
attachment  is  between  a  point  on  the  neck  of  the  rib  and  an  artic- 
ular surface  at  the  end  of  the  transverse  process  of  the  posterior 
of  the  two  vertebrae  which  the  rib  touches  (Fig.  17,  Ft). 


THE  SKELETON 


59 


The  Lumbar  Vertebrae  (Fig.  22)  are  the  largest  of  all  the  mov- 
able vertebra  and  have  no  ribs  attached  to  them.  Their  spines 
are  short  and  stout  and  lie  in  a  more  horizontal  plane  than  those  of 


4 


FIG.  22. — A  lumbar  vertebra,  seen  from  the  left  side.  Ps,  spinous  process; 
Pas,  anterior  articular  process;  Pai,  posterior  articular  process;  Pi,  transverse 
process. 


FIG.  24. 
The  coccyx. 


FIG.  23. — The  last  lumbar  vertebra  and  the  sacrum  seen  from  the  ventral  side. 

the  vertebrae  in  front.     The  articular  and  transverse  processes  are 
also  short  and  stout. 

The  Sacrum,  which  is  represented  along  with  the  last  lumbar 


60  THE  HUMAN  BODY 

vertebra  in  Fig.  23,  consists  in  the  adult  of  a  single  bone;  but  cross- 
ridges  on  its  ventral  surface  indicate  the  limits  of  the  five  separate 
vertebrae  of  which  it  is  composed  in  childhood.  It  is  somewhat 
triangular  in  form,  its  base  being  directed  upwards  and  articulat- 
ing with  the  under  surface  of  the  body  of  the  fifth  lumbar  verte- 
bra. On  its  sides  are  large  surfaces  to  which  the  arch  bearing  the 
lower  limbs  is  attached  (see  Fig.  14).  Its  ventral  surface  is  con- 
cave and  smooth  and  presents  four  pairs  of  anterior  sacral  foramina, 
which  communicate  with  the  neural  canal.  Its  dorsal  surface,  con- 
vex and  roughened,  has  four  similar  pairs  of  posterior  sacral  foramina. 

The  coccyx  (Fig.  24)  calls  for  no  special  description.  The  four 
bones  which  grow  together,  or  ankylose,  to  form  it,  represent  only 
the  bodies  of  vertebrae,  and  even  those  incompletely. 

Details  of  the  Skull.  An  account  of  the  bones  which  make  up  the 
skull  can  conveniently  be  given  in  tabular  form.  Examination  of  the 
table  will  show  that  all  the  bones  are  either  single  or  paired.  Single 
bones  are  all  median,  paired  bones  occupy  corresponding  positions 
on  each  side  of  the  mid-line.  Figs.  25  and  26  will  enable  the  reader 
to  gain  a  fairly  good  notion  of  the  form  and  relations  of  individual 
bones;  for  greater  detail  works  on  anatomy  should  be  consulted. 
Cranium: 

1  Frontal,  forehead  (Fig.  25,  F). 

2  Parietal,  crown  (Fig.  25,  Pr). 

1  Occipital,  base  of  skull  (Fig.  25,  0). 

2  Temporal,  ear  region  (Fig.  25,  T). 

1  Sphenoid,  base  of  cranium  and  back  of  orbit  (Fig.  25, 

S.)  _ 
1  Ethmoid,  between  cavities  of  cranium  and  nose  (Fig. 

25,  E). 
Face: 

1  Inferior  maxilla,  lower  jaw  (Fig.  25,  Md). 

2  Maxillae,  upper  jaw,  front  of  hard  palate  (Fig.  25,  MX). 
2  Palatine,  back  of  hard  palate,  front  of  posterior  nares 

(Fig.  26,  Pi). 
2  Nasal,  bridge  of  nose  (Fig.  25,  N). 

1  Vomer,  partition  between  nostrils  (Fig.  26,  V). 

2  Inferior  turbinate,  inside  nostrils  (not  shown  in  Fig.). 
2  Malar,  cheeks  (Fig.  25,  Z). 

2  Lachrymal,  inside  wall  of  orbit  (Fig.  25,  L). 


THE  SKELETON 


61 


All  these  bones  except  the  inferior  maxilla  are  immovably 
joined  together  in  the  adult  by  irregular,  saw-tooth  like  articula- 
tions. The  inferior  maxilla  articulates  with  the  temporal  bones 
in  such  a  way  as  to  permit  not  only  rotation  about  the  points  of 
articulation  but  also  a  certain  amount  of  sliding  from  side  to  side 
and  from  back  to  front,  thus  making  the  grinding  movements 
of  chewing. 


Md. 


FIG.  25. — A  side  view  of  the  skull.  O,  occipital  bone;  T,  temporal;  Pr,  parietal; 
F,  frontal;  S,  sphenoid;  Z,  malar;  MX,  maxilla;  N,  nasal;  E,  ethmoid;  L,  lachrymal; 
Md,  inferior  maxilla. 


There  are  several  features  of  the  skull  which  call  for  special 
comment.  The  foramen  magnum  (Fig.  26)  is  a  large  opening  into 
the  cranial  cavity  through  the  occipital  bone ;  through  it  the 
spinal  cord  passes  on  its  way  to  the  brain.  On  each  side  of  the 


62 


THE  HUMAN  BODY 


foramen  magnum  is  an  occipital  condyle  (Fig.  26,  oc).     These 
are  the  points  at  which  the  skull  rests  upon  the  atlas.     The  orbits 

or  eye.  sockets  are  outlined  in  front 
by  the  frontal,  malars,  and  max- 
illae. The  space  behind  the  orbit, 
between  the  malar  and  temporal 
bones,  is  occupied  by  a  large  mus- 
cle which  closes  the  jaw.  The  shape 
of  the  face  depends  very  largely 
upon  the  malar  bones.  The  an- 
terior nares,  or  openings  of  the  nos- 
trils are  bounded  by  the  maxillae 
and  nasals.  The  posterior  nares,  by 
which  the  nose  communicates  with 
the  throat  cavity,  lie  behind  the  pal- 
FIG.  26.— The  base  of  the  skull,  ate  bones  (Fig.  23).  Enlargements 

The  lower  jaw  has  been  removed.        ,.   ,,  ,    .       ,, 

At  the  lower  part  of  the  figure  is    of  the  temporal  bones  contain  the 

the  hard  palate  forming  the  roof  of     auditory  apparatus. 


the  mouth  and  surrounded  by  the 
upper  set  of  teeth.  Above  this  are 
the  paired  openings  of  the  posterior 


The  Hyoid.    Besides  the  cranial 

,  and  a  short  way  above  the    and  facial  bones  there  is,  as  already 

pointed  out,  one  other,  the  hyoid 

(FiS'    27)>    which   really   belongS   to 
the    atlas,    on   its  sides;    V,  the    the  skull,   although   it  lies  in  the 

vonier;  Pt,  the  palatines.  ,         T,  ,        »  ,.    .       ,,       f 

neck.     It  can  be  felt  in  the  front 

of  the  throat,  just  above  "Adam's  apple."  The  hyoid  bone 
is  U-shaped,  with  its  convexity  turned  ventrally,  and  consists 
of  a  body  and  two  pairs  of  processes  called  cornua.  The  smaller 
cornua  (Fig.  27,  3)  are  attached  to  the  base  of 
the  skull  by  long  ligaments.  The  bone  serves 
as  an  attachment  for  the  base  of  the  tongue. 
The  hyoid  is  of  much  interest  from  the  stand- 
point of  comparative  anatomy  because  in  the 
very  young  Human  Body  it  is  a  part  of  a  struc- 
ture  which  corresponds  to  the  gill  mechanism  SI 
of  fish,  tadpoles,  and  similar  aquatic  animals,  consisting  of  several 
gill  arches  with  gill  clefts  between  them.  In  the  human  embryo 
the  gill  clefts  close  before  birth,  and  all  the  gill  arches  disappear 
except  those  which  persist  as  'the  hyoid.  It  is  difficult  to  explain 
the  development  and  subsequent  disappearance  of  this  structure 


FIG.  27.— The  hy- 
oid bone.     1,  body; 
cornua;  3, 


THE  SKELETON 


63 


in  the  embryo  except  upon  the  theory  which  is  part  of  the  doctrine 
of  evolution  that  each  individual  epitomizes  in  his  own  develop- 


FIG.  29. 

Fir,.  28. — The  sternum  seen  on  its  ventral  aspect.  M,  manuhrium;  C,  body; 
P,  ensiform  cartilage;  Id,  notch  for  the  collar-bone;  Ic  1-7,  notches  for  the  rib- 
cartilages. 

Fu;.  29. — The  ribs  of  the  left  side,  with  the  dorsal  and  two  lumbar  vertebrae, 
the  rib-cartilages  and  the  sternum. 

mental  history  the  evolutionary  history  of  the  race  to  which  he 
belongs. 

The  Ribs  (Fig.  29).    There  are  twelve  pairs  of  ribs,  each  being 


64  THE  HUMAN  BODY 

a  slender  curved  bone  attached  dorsally  to  the  body  and  transverse 
process  of  a  vertebra  in  the  manner  already  mentioned,  and  con- 
tinued ventrally  by  a  costal  cartilage  (Fig.  18).  In  the  case  of  the 
anterior  seven  pairs,  the  costal  cartilages  are  attached  directly  to 
the  sides  of  the  breast-bone;  the  next  three  cartilages  are  each  at- 
tached to  the  cartilage  of  the  preceding  rib,  while  the  cartilages  of 
the  eleventh  and  twelfth  ribs  are  quite  unattached  ventrally,  so 
these  are  called  the  free  or  floating  ribs.  The  convexity  of  each 
curved  rib  is  turned  outwards  so  as  to  give  roundness  to  the  sides 
of  the  chest  and  increase  its  cavity,  and  each  slopes  downwards 
from  its  vertebral  attachment,  so  that  its  sternal  end  is  consider- 
ably lower  than  its  dorsal. 

Sternum.  The  sternum  or  breast-bone  (Fig.  28  and  Fig.  14) 
is  wider  from  side  to  side  than  dorsoventrally.  It  consists  in  the 
adult  of  three  pieces,  and  seen  from  the  ventral  side  has  somewhat 
the  form  of  a  dagger.  At  the  upper  end  are  notches  for  the  articu- 
lations of  the  collar-bones  (Fig.  28,  Id),  and  along  each  side  notches 
for  the  articulations  of  the  anterior  costal  cartilages  (Fig.  28,  Ic, 

1-7). 

The  Appendicular  Skeleton.  This  consists  of  the  shoulder- 
girdle  and  the  bones  of  the  fore  limbs,  and  the  pelvic  girdle  and  the 
bones  of  the  posterior  limbs.  The  two  supporting  girdles  in  their 
natural  position  with  reference  to  the  trunk  skeleton  are  repre- 
sented in  Fig.  30. 

The  Shoulder-girdle,  or  Pectoral  Arch.  This  is  made  up,  on 
each  side,  of  the  scapula  or  shoulder-blade,  and  the  clavicle  or  collar- 
bone. 

The  scapula  (S,  Fig.  30)  is  a  flattish  triangular  bone  which  can 
readily  be  felt  on  the  back  of  the  thorax.  It  is  not  directly  articu- 
lated to  the  axial  skeleton,  but  lies  embedded  in  the  muscles  and 
other  parts  outside  the  ribs  on  each  side  of  the  vertebral  column. 
From  its  dorsal  side  arises  a  crest  to  which  the  outer  end  of  the 
collar-bone  is  fixed,  and  on  its  outer  edge  is  a  shallow  cup  into 
which  the  top  of  the  arm-bone  fits:  this  hollow  is  known  as  the 
glenoid  fossa. 

The  collar-bone  (C,  Fig.  30)  is  cylindrical  and  attached  at  its 
inner  end  to  the  sternum  as  shown  in  the  figure,  fitting  into  the 
notch  represented  at  Id  in  Fig.  28. 

The  Pelvic  Girdle  (Fig.  30).    This  consists  of  a  large  bone,  the 


THE  SKELETON 


65 


os  innominatum,  Oc,  on  each  side,  which  is  firmly  fixed  dorsally  to 
the  sacrum  and  meets  its  fellow  in  the  middle  ventral  line.  In  the 
child  each  os  innominatum  consists  of  three  bones,  viz.,  the  ilium, 
the  ischium,  and  pubis.  Where  these  three  bones  meet  and  finally 
ankylose  there  is  a  deep  socket,  the  acetabulum,  into  which  the 


FIG.  30. — The  skeleton  of  the  trunk  and  the  limb  arches  seen  from  the  front. 
C,  clavicle;  <S,  scapula;  Oc,  innominate  bone  attached  to  the  side  of  the  sacrum 
dorsally  and  meeting  its  fellow  at  the  pubic  symphysis  in  the  ventral  median  line. 


head  of  the  thigh-bone  fits  (see  Fig.  14).  Between  the  pubic  and 
ischial  bones  is  the  largest  foramen  in  the  whole  skeleton,  known  as 
the  doorlike  or  thyroid  foramen.  The  pubic  bone  lies  above  and 
the  ischial  below  it.  The  ilium  forms  the  upper  expanded  portion 
of  the  os  innominatum  to  which  the  line  drawn  from  Oc  in  Fig.  30 
points. 


66  THE  HUMAN  BODY 

Fore  and  Hind  Limbs.  Each  of  these  contains  thirty  bones, 
and  their  arrangement  is  very  similar.  This  is  clearly  seen  in  the 
figures  (31  and  32),  and  is  also  brought  out  in  the  following  table 
in  which  the  bones  of  the  extremities  are  enumerated. 

Fore  Limb  Hind  Limb 

a.  Humerus,  upper  arm.  Femur,  thigh. 

&..  Ulna,  large  bone  of  forearm.  Tibia,  shin  bone. 

c.  Radius,  smaller  bone  of  forearm.  Fibula,  small  bone  of  calf. 

d.  8  carpals,  wrist.  7  tarsals,  heel  and  upper  instep. 

e.  5  metacarpals,  hand.  5  metatarsals,  lower  instep. 

/.  14  phalanges,  fingers  and  thumb.      14  phalanges,  toes  (2  in  great  toe,  3 

(2  in  thumb,  3  in  each  finger).         in  others). 
g.  Patella,  knee-cap. 

In  general  the  bones  of  the  hind  limb  are  larger  and  stronger 
than  the  corresponding  ones  of  the  fore  limb;  the  femur  is  the 
longest  bone  in  the  body.  The  phalanges,  however,  are  smaller 
in  the  foot  than  in  the  hand.  The  tarsals  are  one  less  in  number 
than  the  carpals  because  one  of  the  tarsal  bones,  the  astragalus 
(Fig.  35,  To),  is  composed  of  two  bones  which  have  united  into  one. 
A  structure  of  the  arm  corresponding  to  the  patella  is  the  olecranon 
process  of  the  ulna  which  can  be  felt  at  the  back  of  the  elbow;  in 
early  life  this  is  a  separate  bone. 

The  differences  in  structure  between  fore  and  hind  limb  corre- 
spond to  differences  of  function;  the  fore  limb  being  a  prehensile 
organ  is  capable  of  great  freedom  of  motion;  the  hind  limb,  which 
is  a  supporting  and  locomotor  organ,  is  adapted  rather  to  main- 
tain the  weight  of  the  body  and  to  execute  the  movements  of 
walking  and  running  to  advantage.  The  special  adaptation  of  the 
arm  to  its  purpose  is  seen  particularly  in  three  things:  1,  the  com- 
paratively flexible  attachment  of  the  pectoral  girdle  to  the  axial 
skeleton  (Fig.  33),  an  attachment  composed  wholly  of  muscle  and 
ligament  except  where  the  inner  ends  of  the  clavicles  articulate 
with  the  sternum;  2,  the  rotation  of  the  radius  over  the  ulna,  an 
arrangement  which  increases  very  greatly  the  flexibility  of  the 
hand;  3,  the  articulation  of  the  thumb,  which  is  of  such  a  sort  as 
to  allow  it  to  be  opposed  to  any  of  the  fingers,  thus  enabling  the 
hand  to  manipulate  small  objects  without  difficulty.  The  leg,  on 
the  other  hand,  is  characterized  by  much  greater  firmness,  which 


THE  SKELETON 


67 


is  obtained  at  the  expense  of  flexibility.  The  pelvic  arch  (Figs.  30 
and  34)  is  not  only  heavy  and  strong,  but  is  very  firmly  fixed  to 
the  axial  skeleton,  the  sacrum  and  os  innominatum  becoming  in 
mature  life  practically  one  bone.  The  socket  into  which  the  head 


FIG.  31.  FIG.  32. 

FIG.  31. — The  bones  of  the  arm.  a,  humerus;  6,  ulna;  c,  radius;  d,  the  carpus; 
e,  the  fifth  metacarpil;  /,  the  three  phalanges  of  the  fifth  digit  (little  finger). 

FIG.  32. — Bones  oi  the  leg.  a,  femur;  6,  tibia;  c,  fibula;  d,  tarsal  bones;  e,  meta- 
tarsal  bones;  /,  phalanges;  g,  patella. 

of  the  femur  fits  is  much  deeper  than  that  which  receives  the 
head  of  the  humerus,  rendering  the  leg  much  less  liable  to  dislo- 
cation than  the  arm,  but  at  the  same  time  restricting  its  move- 
ments much  more.  The  foot  also  in  becoming  adapted  to  form  a 


68 


THE  HUMAN  BODY 


support  for  the  body  has  sacrificed  its  prehensile  structure  almost 
altogether;  the  toes  are  less  flexible  than  the  fingers  and  the  great 


FIG.  33.  —  Diagram  showing  the  relation  of  the  pectoral  arch  to  the  axial  skeleton. 

toe  cannot  be  opposed  to  the  others.     A  special  modification  of 
the  foot  for  its  particular  function  is  seen  in  the  arching  of  the 
instep.     As  Fig.  35  shows  the  bones  of  the  foot  form  a  springy  arch, 
^^^^^^  the  points  of  contact  with  the  ground 

^r  ^^^  being  at  the  extremity  of  the  heel 

f  \^^    bone  (os  calcis,  Ca  of  figure),  and  the 

€        ^j^        J      :    distal  ends  of  the  metatarsals.     The 
^^^^Jb*S  bones  of  the  leg  are  mounted  upon 

the  crown  of  the  arch  (  Ta  of  figure). 

FIG.  34.  —  Diagram  showing  the  ».,•»»  01     i 

attachment  of  the  pelvic  arch  to      Peculiarities  of  the  Human  Skele- 

ton.    These    are    largely    connected 

with  the  division  of  labor  between  the  fore  and  hind  limbs  re- 
ferred to  above,  which  is  carried  farther  in  man  than  in  any  other 
creature.  Even  the  highest  apes  frequently  use  their  fore  limbs 


Cl 


M5 


Cli 


FIG.  35. — The  bones  of  the  foot.  Ca,  calcaneum,  or  os  calcis;  Ta,  articular  surface 
for  tibia  on  the  astragalus;  N,  scaphoid  bone;  CI,  CII,  first  and  second  cuneiform 
bones;  Cb,  cuboid  bone;  Ml,  metatarsal  bone  of  great  toe. 

in  locomotion  and  their  hind  limbs  in  prehension,  and  we  find  ac- 
cordingly that  anatomically  they  present  less  differentiation  of 
hand  and  foot.  The  other  more  important  characteristics  of  the 
human  skeleton  are  correlated  for  the  most  part  with  the  mainte- 


THE  SKELETON  69 

nance  of  the  erect  posture,  which  is  more  complete  and  habitual 
in  man  than  in  the  animals  most  closely  allied  to  him  anatomically. 
These  peculiarities,  however,  only  appear  .fully  in  the  adult.  In 
the  infant  the  head  is  proportionately  larger,  which  gives  the 
center  of  gravity  of  the  Body  a  comparatively  very  high  position 
and  renders  the  maintenance  of  the  erect  posture  difficult  and  in- 
secure. The  curves  of  the  vertebral  column  are  nearly  absent, 
and  the  posterior  limbs  are  relatively  very  short.  In  all  these 
points  the  infant  approaches  more  closely  than  the  adult  to  the 
ape.  The  subsequent  great  relative  length  of  the  posterior  limbs, 
which  grow  disproportionately  fast  in  childhood  as  compared  with 
the  anterior,  makes  progression  on  them  more  rapid  by  giving  a 
longer  stride  and  at  the  same  time  makes  it  almost  impossible  to 
go  on  "all  fours "  except  by  crawling  on  the  hands  and  knees.  In 
other  Primates  this  disproportion  between  the  anterior  and  pos- 
terior limbs  does  not  occur  to  nearly  the  same  extent. 

In  man  the  skull  is  nearly  balanced  on  the  top  of  the  vertebral 
column,  the  occipital  condyles  which  articulate  with  the  atlas 
being  about  its  middle  (Fig.  25),  so  that  but  little  effort  is  needed 
to  keep  the  head  erect.  In  four-footed  beasts,  on  the  contrary,  the 
skull  is  carried  on  the  front  end  of  the  horizontal  vertebral  column 
and  needs  special  ligaments  to  sustain  it.  For  instance,  in  the  ox 
and  sheep  there  is  a  great  elastic  cord  running  from  the  cervical 
vertebrae  to  the  back  of  the  skull  and  helping  to  hold  up  the  head. 
Even  in  the  highest  apes  the  skull  does  not  balance  on  the  top  of 
the  spinal  column;  the  face  part  is  much  heavier  than  the  back, 
while  in  man  the  face  parts  are  relatively  smaller  and  the  cranium 
larger,  so  that  the  two  nearly  equipoise.  To  keep  the  head  erect 
and  look  things  straight  in  the  face,  "like  a  man/'  is  for  the  apes 
far  more  fatiguing,  and  so  they  cannot  long  maintain  that  position. 

The  human  spinal  column,  gradually  widening  from  the  neck  to 
the  sacrum,  is  well  fitted  to  sustain  the  weight  of  the  head,  upper 
limbs,  etc.,  carried  by  it;  and  its  curvatures,  which  are  peculiarly 
human,  give  it  considerable  elasticity  combined  with  strength. 
The  pelvis,  to  the  sides  of  which  the  lower  limbs  are  attached,  is 
proportionately  very  broad  in  man,  so  that  the  balance  can  be 
more  readily  maintained  during  lateral  bending  of  the  trunk.  The 
arched  instep  and  broad  sole  of  the  human  foot  are  also  very 
characteristic.  The  majority  of  four-footed  beasts,  as  horses, 


70  THE  HUMAN  BODY 

walk  on  the  tips  of  their  toes  and  fingers;  and  those  animals,  as 
bears  and  apes,  which  like  man  place  the  tarsus  also  on  the  ground, 
or  in  technical  language  are  plantigrade,  have  a  much  less  marked 
arch  there.  The  vaulted  human  tarsus,  composed  of  a  number  of 
small  bones,  each  of  which  can  glide  a  little  over  its  neighbors,  but 
none  of  which  can  move  much,  is  admirably  calculated  to  break 
any  jar  which  might  be  transmitted  to  the  spinal  column  by  the 
contact  of  the  sole  with  the  ground  at  each  step.  A  well-arched 
instep  is  therefore  rightly  considered  a  beauty;  it  makes  progres- 
sion easier,  and  by  its  springiness  gives  elasticity  to  the  step.  In 
London  flat-footed  candidates  for  appointment  as  policemen  are 
rejected,  as  they  cannot  stand  the  fatigue  of  walking  the  daily 
"beat." 

Hygiene  of  the  Bony  Skeleton.  In  early  life  the  bones  are  less 
rigid,  from  the  fact  that  the  earthy  matters  then  present  in  them 
bear  a  less  proportion  to  the  softer  organic  parts.  Hence  the  bones 
of  an  aged  person  are  more  brittle  and  easily  broken  than  those  of  a 
child.  The  bones  of  a  young  child  are  in  fact  tolerably  flexible 
and  may  be  distorted  by  any  continued  strain;  therefore  children 
should  never  be  kept  sitting  for  hours,  in  school  or  elsewhere,  on  a 
bench  which  is  so  high  that  the  feet  are  not  supported.  If  this  be 
insisted  upon  (for  no  child  will  continue  it  voluntarily)  the  thigh- 
bones will  almost  certainly  be  bent  over  the  edge  of  the  seat  by  the 
weight  of  the  legs  and  feet,  and  a  permanent  distortion  may  be 
produced.  For  the  same  reason  it  is  important  that  a  child  be 
made  to  sit  straight  while  writing,  to  avoid  the  risk  of  producing 
a  lateral  curvature  of  the  spinal  column.  The  facility  with  which 
the  bones  may  be  molded  by  prolonged  pressure  in  early  life  is 
well  seen  in  the  distortion  of  the  feet  of  the  Chinese  ladies  of  the 
old  regime,  produced  by  keeping  them  in  tight  shoes;  and  in  the 
extraordinary  forms  which  some  races  of  man  produce  in  their 
skulls,  by  tying  boards  on  the  heads  of  the  children. 

Throughout  the  whole  of  life,  moreover,  the  bones  remain  among 
the  most  easily  modified  parts  of  the  Body;  although  judging  from 
the  fact  that  dead  bones  are  the  most  permanent  parts  of  fossil 
animals  we  might  be  inclined  to  think  otherwise.  The  living  bone, 
however,  is  constantly  undergoing  changes  under  the  influence  of 
the  protoplasmic  cells  embedded  in  it,  and  in  the  living  Body  is 
constantly  being  absorbed  and  reconstructed.  The  experience  of 


THE  SKELETON  71 

physicians  shows  that  any  continued  pressure,  such  as  that  of  a 
tumor,  will  cause  the  absorption  and  disappearance  of  bone  almost 
quicker  than  that  of  any  other  tissue;  and  the  same  is  true  of  any 
other  continued  pressure.  Moreover,  during  life  the  bones  are 
eminently  plastic;  under  abnormal  pressures  they  are  found  to 
assume  abnormal  shapes  quickly,  being  absorbed  and  disappear- 
ing at  points  where  the  pressure  is  most  powerful,  and  increasing 
at  other  points;  tight  lacing  may  in  this  way  produce  a  permanent 
distortion  of  the  ribs. 

When  a  bone  is  fractured  a  surgeon  should  be  called  in  as  soon 
as  possible,  for  once  inflammation  has  set  in  and  the  parts  have  be- 
come swollen  it  is  much  more  difficult  to  place  the  broken  ends  of 
the  bone  together  in  their  proper  position  than  before  this  has 
occurred.  Once  the  bones  are  replaced  they  must  be  held  in  posi- 
tion by  splints  or  bandages,  or  the  muscles  attached  to  them  will 
soon  displace  them  again.  With  rest,  in  young  and  healthy  per- 
sons complete  union  will  commonly  occur  in  three  or  four  weeks; 
but  in  old  persons  the  process  of  healing  is  slower  and  is  apt  to  be 
imperfect. 

Articulations.  The  bones  of  the  skeleton  are  joined  together  in 
very  various  ways;  sometimes  so  as  to  admit  of  no  movement  at 
all  between  them;  in  other  cases  so  as  to  permit  only  a  limited 
range  or  variety  of  movement ;  and  elsewhere  so  as  to  allow  of  very 
free  movement  in  many  directions.  All  kinds  of  unions  between 
bones  are  called  articulations. 

Of  articulations  permitting  no  movements,  those  which  unite 
the  majority  of  the  cranial  bones  afford  a  good  example.  Except 
the  lower  jaw,  and  certain  tiny  bones  inside  the  temporal  bone  be- 
longing to  the  organ  of  hearing,  all  the  skull-bones  are  immovably, 
joined  together.  This  union  in  most  cases  occurs  by  means  of 
toothed  edges  which  fit  into  one  another  and  form  jagged  lines  of 
union  known  as  sutures.  Some  of  these  can  be  well  seen  in  Fig.  25 
between  the  frontal  and  parietal  bones  (coronal  suture)  and  be- 
tween the  parietal  and  occipital  bones  (tambdoidal  suture);  while 
another  lies  along  the  middle  line  in  the  top  of  the  crown  between 
the  two  parietal  bones,  and  is  known  as  the  sagittal  suture.  In  new- 
born children  where  the  sagittal  meets  the  coronal  and  lambdoidal 
sutures  there  are  large  spaces  not  yet  covered  in  by  the  neighboring 
bones,  which  subsequently  extend  over  them.  These  openings 


72  THE  HUMAN  BODY 

are  known  as  fontanelles.  At  them  a  pulsation  can  often  be  felt 
synchronous  with  each  beat  of  the  heart,  which,  driving  more  blood 
into  the  brain,  distends  it  and  causes  it  to  push  out  the  skin  where 
bone  is  absent.  Another  good  example  of  an  articulation  admit- 
ting of  no  movement  is  that  between  the  rough  surfaces  on  the 
sides  of  the  sacrum  and  the  innominate  bones. 

We  find  good  examples  of  the  second  class  of  articulations — 
those  admitting  of  a  slight  amount  of  movement — in  the  vertebral 
column.  Between  every  pair  of  vertebrae  from  the  second  cervical 
to  the  sacrum  is  an  elastic  pad,  the  intervertebral  disk,  which  ad- 
heres by  its  surfaces  to  the  bodies  of  the  vertebrae  between  which  it 
lies,  and  only  permits  so  much  movement  between  them  as  can 
be  brought  about  by  its  own  compression  or  stretching.  When 
the  back-bone  is  curved  to  the  right,  for  instance,  each  of  the 
intervertebral  disks  is  compressed  on  its  right  side  and  stretched 
a  little  on  its  left,  and  this  combination  of  movements,  each  in- 
dividually but  slight,  gives  considerable  flexibility  to  the  spinal 
column  as  a  whole. 

Joints.  Articulations  permitting  of  movement  by  the  gliding  of 
one  bone  over  another  are  known  as  joints,  and  all  have  the  same 
fundamental  structure,  although  the  amount  of  movement  per- 
mitted in  different  joints  is  very  different. 

Joint  Motions.  The  wide  variety  of  motions  possible  to  the 
body  group  themselves  within  a  small  number  of  classes :  flexion, 
the  bending  of  a  joint  as  at  elbow  or  knee;  extension,  the  straighten- 
ing of  a  joint,  the  opposite  of  flexion;  abduction,  the  movement  of  a 
part  away  from  the  axis  of  the  body,  as  in  moving  the  arm  out  to 
the  side  nt  right  angles,  or  the  thumb  and  fore  finger  in  spreading 
Jin1  h:md;  adduction,  the  opposite  of  abduction;  rotation,  the  rolling 
movement  seen  when  the  hand  is  turned  from  the  palm  up  to  the 
palm  down  position,  or  when  one  ankle  is  placed  on  the  opposite 
knee.  There  are  a  few  movements,  such  as  the  sliding  of  the  jaw 
from  side  to  side  in  chewing,  that  do  not  fall  in  any  of  these  classes, 
but  the  great  majority  of  joint  motions  belong  either  to  one  of 
these  groups  or  are  combinations  of  two  or  more.  Thus  flexion 
and  abduction  of  the  hip  can  occur  together,  or  extension  and 
rotation. 

Hip- joint.  We  may  take  this  as  a  good  example  of  a  true  joint 
permitting  a  great  amount  and  variety  of  movement.  On  the 


THE  SKELETON 


73 


os  iimominatum  is  the  cavity  of  the  acetabulum  (Fig.  36),  which  is 
lined  inside  by  a  thin  layer  of  articular  cartilage  which  has  an  es- 
t  remely  smooth  surface.  The  bony  cup  is  also  deepened  a  little  by 
a  cartilaginous  rim.  The  proximal  end  of  the  femur  consists  of  a 
nearly  spherical  smooth  head,  borne  on  a  somewhat  narrower  neck, 
and  fitting  into  the  acetabulum.  This  head  also  is  covered  with 
articular  cartilage;  and  it  rolls  in  the  acetabulum  like  a  ball  in  a 
socket.  To  keep  the  bones  together  and  limit  the  amount  of  move- 


FIG.  36. — Section  through  the  hip-joint,    a  and  b,  articular  cartilages;  c,  capsu- 
lar  ligament. 

men! ,  ligaments  pass  from  one  to  the  other.  These  are  composed 
of  white  fibrous  connective  tissue  (Chap.  IV)  and  are  extremely 
pliable,  but  quite  inextensible  and  very  strong  and  tough.  One  is 
the  capsular  ligament,  which  forms  a  sort  of  loose  bag  all  around  the 
joint,  and  another  is  the  round  ligament,  which  passes  from  the 
acetabulum  to  the  head  of  the  femur.  Should  the  latter  rotate 
above  a  certain  extent  in  its  socket,  the  round  ligament  and  one 
side  of  the  capsular  ligament  are  put  on  the  stretch,  and  any  fur- 
ther movement  which  might  dislocate  the  femur  (that  is,  remove 
the  head  from  its  socket)  is  checked.  Covering  the  inside  of  the 
capsular  ligament  and  the  outside  of  the  round  ligament  is  a  layer 
of  flat  cells,  which  are  continued  in  a  modified  form  over  the  ar- 


74  THE  HUMAN  BODY 

ticular  cartilages  and  form  the  synovial  membrane.  This,  which 
thus  forms  the  lining  of  the  joint,  is  always  moistened  in  health 
by  a  small  quantity  of  glairy  synovial  fluid,  something  like  the 
white  of  a  raw  egg  in  consistency,  and  playing  the  part  of  the  oil 
with  which  the  contiguous  moving  surfaces  of  a  machine  are  mois- 
tened ;  it  makes  all  run  smoothly  with  very  little  friction. 

In  the  natural  state  of  the  parts,  the  head  of  the  femur  and  the 
bottom  and  sides  of  the  acetabulum  lie  in  close  contact,  the  two 
synovial  membranes  rubbing  together.  This  contact  is  not  main- 
tained by  the  ligaments,  which  are  too  loose  and  serve  only  to 
check  excessive  movement,  but  by  the  numerous  stout  muscles 
which  pass  from  the  thigh  to  the  trunk  and  bind  the  two  firmly 
together.  Moreover,  the  atmospheric  pressure  exerted  on  the  sur- 
face of  the  Body  and  transmitted  through  the  soft  parts  to  the 
outside  of  the  air-tight  joint  helps  also  to  keep  the  parts  in  contact. 
If  all  the  muscles  and  ligaments  around  the  joint  be  cut  away,  it  is 
still  found  in  the  dead  Body  that  the  head  of  the  femur  will  be  kept 
in  its  socket  by  this  pressure,  and  so  firmly  as  to  bear  the  weight 
of  the  whole  limb  without  dislocation,  just  as  the  pressure  of  the 
air  will  enable  a  boy's  " sucker"  to  lift  a  tolerably  heavy  stone. 

Ball-and-socket  Joints.  Such  a  joint  as  that  at  the  hip  is 
called  a  ball-and-socket  joint  and  allows  of  more  free  movement 
than  any  other.  Through  movements  occurring  in  it  the  thigh  can 
be  flexed,  or  bent  so  that  the  knee  approaches  the  chest;  or  extended, 
that  is,  moved  in  the  opposite  direction.  It  can  be  abducted,  so 
that  the  knee  moves  outwards;  and  adducted,  or  moved  back  to- 
wards the  other  knee  again.  The  limb  can  also  by  movements  at 
the  hip-joint  be  made  to  describe  a  cone  of  which  the  base  is  at  the 
foot  and  the  apex  at  the  hip.  Finally,  rotation  can  occur  in  the 
joint,  so  that  with  knee  and  foot  joints  held  rigid  the  toes  can  be 
turned  in  or  out,  to  a  certain  extent,  by  a  rolling  around  of  the 
femur  in  its  socket. 

At  the  junction  of  the  humerus  with  the  scapula  is  another  ball- 
and-socket  joint  permitting  all  the  above  movements  to  even  a 
greater  extent.  This  greater  range  of  motion  at  the  shoulder-joint 
depends  mainly  on  the  shallowness  of  the  glenoid  cavity  as  com- 
pared with  the  acetabulum,  and  upon  the  absence  of  any  ligament 
answering  to  the  round  ligament  of  the  hip-joint.  Another 
ball-and-socket  joint  exists  between  the  carpus  and  the  metacarpal 


THE  SKELETON  75 

bone  of  the  thumb ;  and  others  with  the  same  variety,  but  a  much 
less  range,  of  movement  between  each  of  the  remaining  metacarpal 
bones  and  the  proximal  phalanx  of  the  finger  which  articulates 
with  it. 

Hinge- joints.  Another  form  of  synovial  joint  is  known  as  a 
hinge-joint.  In  it  the  articulating  bony  surfaces  are  of  such  shape 
as  to  permit  of  movement,  to  and  fro,  in  one  plane  only,  like  a  door 
on  its  hinges.  The  joints  between  the  phalanges  of  the  fingers  are 
good  examples  of  hinge-joints.  If  no  movement  be  allowed  where 
the  finger  joins  the  palm  of  the  hand  it  will  be  found  that  each  can 
be  bent  and  straightened  at  its  own  two  joints,  but  not  moved  in 
any  other  way.  The  knee  is  also  a  hinge-joint,  as  is  the  articula- 
tion between  the  lower  jaw  and  the  base  of  the  skull  which  allows 
us  to  open  and  close  our  mouths.  The  latter  is,  however,  not  a 
perfect  hinge-joint,  since  it  permits  of  a  small  amount  of  lateral 
movement  such  as  occurs  in  chewing,  and  also  of  a  gliding  move- 
ment by  which  the  lower  jaw  can  be  thrust  forward  so  as  to  pro- 
trude the  chin  and  bring  the  lower  row  of  teeth  outside  the  upper. 

Pivot-joints.  In  this  form  one  bone  rotates  around  another 
which  remains  stationary.  We  have  a  good  example  of  it  between 
the  first  and  second  cervical  vertebrae.  The  first  cervical  vertebra 
or  atlas  (Fig.  20)  has  a  very  small  body  and  a  very  large  arch,  and 
its  neural  canal  is  subdivided  by  a  transverse  ligament  (L,  Fig.  20) 
into  a  dorsal  and  a  ventral  portion ;  in  the  former  the  spinal  cord 
lies.  The  second  vertebra  or  axis  (Fig.  21)  has  arising  from  its 
body  the  stout  bony  peg,  D,  called  the  odontoid  process.  This 
projects  into  the  ventral  portion  of  the  space  surrounded  by  the 
atlas,  and,  kept  in  place  there  by  the  transverse  ligament,  forms 
a  pivot  around  which  the  atlas,  carrying  the  skull  with  it,  rotates 
when  we  turn  the  head  from  side  to  side.  The  joints  on  each  side 
between  the  atlas  and  the  skull  are  hinge-joints  and  permit  only 
the  movements  of  nodding  and  raising  the  head.  When  the  head 
is  leaned  over  to  one  side,  the  cervical  part  of  the  spinal  column 
is  bent. 

Another  kind  of  pivot-joint  is  seen  in  the  forearm.  If  the  limb 
be  held  straight  out,  with  the  palm  up  and  the  elbow  resting  on  the 
table,  so  that  the  shoulder-joint  be  kept  steady  while  the  hand  is  ro- 
tated until  its  back  is  turned  upwards,  it  will  be  found  that  the 
radius  has  partly  rolled  round  the  ulna.  When  the  palm  is  up- 


76 


THE  HUMAN  BODY 


wards  and  the  thumb  outwards,  the  lower  end  of  the  radius  can 
be  felt  on  the  outer  side  of  the  forearm  just  above  the  wrist,  and  if 
this  be  done  while  the  hand  is  turning  over,  it  will  be  easily  dis- 
cerned that  during  the  movement  this  end  of  the  radius,  carrying 
the  hand  with  it,  travels  around  the  lower  end  of  the  ulna  so  as  to 
got  to  its  inner  side.  The  relative  position  of  the  bones  when  the 
palm  is  upwards  is  shown  at  A  in  Fig.  37,  and  when  the  palm  is 

down  at  B.  The  former  position  is 
known  as  sypination;  the  latter  as 
pronation.  The  elbow  end  of  the 
humerus  (Fig.  37)  bears  a  large  artic- 
ular surface:  on  the  inner  two-thirds 
&  of  this,  the  ulna  fits,  and  the  ridges 
and  grooves  of  both  bones  interlocking 
form  a  hinge-joint,  allowing  only  of 
bending  or  straightening  the  forearm 
on  the  arm.  The  radius  fits  on  the 
rounded  outer  third,  and  forms  there 
a  ball-and-socket  joint  at  which  the 
movement  takes  place  when  the  hand 
is  turned  from  the  supine  to  the  prone 
position;  the  ulna  forming  a  fixed  bar 
around  which  the  lower  end  of  the 
radius  is  moved. 
T-,-8  _,  Gliding  Joints.  These  permit  as  a 

FIG.  37. — A,  arm   in    supina-  e  7 

tion;  B,  arm  in  pronation.    H,  rule  but   little   movement:    examples 

are  found  between  the"  closely  packed 

bones  of  the  tarsus  and  carpus  (Figs.  31  and  32),  which  slide  a  little 
over  one  another  when  subjected  to  pressure. 

Hygiene  of  the  Joints.  When  a  bone  is  displaced  or  dislocated 
the  ligaments  around  the  joint  are  more  or  less  torn  and  other 
soft  parts  injured.  This  soon  leads  to  inflammation  and  swelling 
which  make  not  only  the  recognition  of  the  injury  but,  after- 
diagnosis,  the  replacement  of  the  bone,  or  the  reduction  of  the  dis- 
location, difficult.  Moreover,  the  muscles  attached  to  it  constantly 
pull  on  the  displaced  bone  and  drag  it  still  farther  out  of  place;  so 
that  it  is  of  great  importance  that  a  dislocation  be  reduced  as  soon 
as  possible.  In  most  cases  this  can  only  be  attempted  with  safety 
by  one  who  knows  the  form  of  the  bones,  and  possesses  sufficient 


THE  SKELETON  77 

anatomical  knowledge  to  recognize  the  direction  of  the  displace- 
ment. No  injury  to  a  joint  should  be  neglected.  Inflammation 
once  started  there  is  often  difficult  to  check  and  runs  on,  in  a 
chronic  way,  until  the  synovial  surfaces  are  destroyed,  and  the 
two  bones  perhaps  grow  together,  rendering  the  joint  permanently 
stiff. 

Immediate  and  complete  rest  has  been  commonly  supposed  to 
be  the  only  proper  treatment  for  sprained  joints,  but  it  has  been 
shown  recently  that  massage,  properly  applied  by  one  expert  in  its 
use,  has  a  remarkably  beneficial  effect  upon  sprains.  Injuries  of 
this  sort  so  severe  that  under  the  rest  treatment  they  would  re- 
quire weeks  for  recovery  "yield  so  completely  in  a  few  days  to 
massage  treatment  that  the  injured  individual  can  participate  in 
athletic  contests.  It  should  be  borne  in  mind  that  massage  to  be 
effective  must  be  applied  by  an  expert  in  its  use. 


CHAPTER  VI 
THE  STRUCTURE  OF  THE  MOTOR  ORGANS 

Motion  in  Animals.  Motion  is  produced  in  animals  by  various 
sorts  of  motor  tissues  (p.  32),  but  in  all  the  underlying  mechanical 
principle  is  the  same,  namely,  the  forcible  contraction  of  some  ele- 
ment or  elements.  Various  means  of  making  these  contractions 
effective  exist  in  nature.  The  most  familiar  is  that  already  cited 
(p.  37)  of  causing  the  contractile  structure  to  pull  across  a  movable 
joint.  In  some  situations,  the  human  stomach,  for  example,  a 
hollow  organ  is  completely  surrounded  by  contractile  tissues,  whose 
contractions  diminish,  and  whose  relaxations  permit  increase  of 
the  capacity  of  the  organ.  Still  another  form  of  motion  is  that  of 
the  cilia  previously  mentioned  (p.  33). 

The  Muscles.  These  are  the  main  motor  organs;  their  general 
appearance  is  well  known  to  every  one  in  the  lean  of  butcher's 
meat.  The  majority  of  them  being  fixed  to  the  skeleton  can,  by 
alterations  in  their  form,  bring  about  changes  in  the  form  and  posi- 
tion of  nearly  all  parts  of  the  Body.-  With  the  skeleton  and  joints, 
they  constitute  preeminently  the  organs  of  motion  and  locomotion, 
and  are  governed  by  the  nervous  system  which  regulates  their  ac- 
tivity. In  fact  skeleton,  muscles,  and  nervous  system  are  cor- 
related parts:  the  degree  of  usefulness  of  any  one  of  them  largely 
depends  upon  the  more  or  less  complete  development  of  the  others. 
Man's  highly  endowed  senses  and  his  powers  of  reflection  and  rea- 
son would  be  of  little  use  to  him,  were  his  muscles  less  fitted  to  carry 
out  the  dictates  of  his  will  or  his  joints  less  numerous  or  mobile. 
All  the  muscles  are  under  the  control  of  the  nervous  system,  but 
all  are  not  governed  by  it  with  the  cooperation  of  will  or  con- 
sciousness; some  move  without  our  having  any  direct  knowledge 
of  the  fact.  This  is  especially  the  case  with  certain  muscles  which 
are  not  fixed  to  the  skeleton  but  surround  cavities  or  tubes  in  the 
Body,  as  the  blood-vessels  and  the  alimentary  canal,  and  by  their 
movements  control  the  passage  of  substances  through  them.  The 

78 


THE  STRUCTURE  OF  THE  MOTOR  ORGANS       79 

former  group,  or  skeletal  muscles,  are  also  from  their  microscopic 
characters  known  as  striped  muscles,  while  the  latter,  or  visceral 
muscles,  are  called  unstriped  or  smooth  muscles.  The  skeletal 
muscles  being  generally  more  or  less  subject  to  the  control  of  the 
will  (as  for  example  those  moving  the  limbs)  are  frequently  spoken 
of  as  voluntary,  and  the  visceral  muscles,  which  change  their  form 
independently  of  the  will,  as  involuntary.  The  heart  muscle  forms 
a  sort  of  intermediate  link;  it  is  not  directly  attached  to  the  skele- 
ton, but  forms  a  hollow  bag  which  drives  on  the  blood  contained 
in  it  and  that  quite  involuntarily;  but  in  its  microscopic  struc- 
ture it  resembles  somewhat  the  skeletal  voluntary  muscles.  The 
muscles  of  respiration  are  striped  skeletal  muscles  and,  as  we  all 
know,  are  to  a  certain  extent  subject  to  the  will;  any  one  can  draw 
a  deep  breath  when  he  chooses.  But  in  ordinary  quiet  breathing 
we  are  quite  unconscious  of  their  working,  and  even  when  attention 
is  turned  to  them  the  power  of  control  is  limited;  no  one  can 
voluntarily  hold  his  breath  long  enough  to  suffocate  himself.  As 
we  shall  see  hereafter,  moreover,  any  one  or  all  of  the  striped 
muscles  of  the  Body  may  be  thrown  into  activity  independently 
of  or  even  against  the  will,  as,  to  cite  no  other  instances,  is  seen  in 
the  "fidgets"  of  nervousness  and  the  irrepressible  trembling  of 
extreme  terror;  so  that  the  names  voluntary  and  involuntary  are 
not  good  ones,  but  so  far  as  we  use  them  they  indicate  no  more 
than  the  general  fact  that  the  skeletal  muscles  are  as  a  group  re- 
sponsive to  the  will  while  the  smooth  muscles  are  not. 

The  Skeletal  Muscles.  In  its  simplest  form  a  skeletal  muscle 
consists  of  a  red  soft  central  part,  the  belly,  which  tapers  at  each 
end  and  there  passes  into  one  or  more  dense  white  cords  which 
consist  almost  entirely  of  white  fibrous  connective  tissue.  These 
terminal  cords  are  called  the  tendons  of  the  muscle  and  serve  to 
attach  it  to  parts  of  the  skeleton.  In  Fig.  38  is  shown  the  biceps 
muscle  of  the  arm,  which  lies  in  front  of  the  humerus.  Its  fleshy 
belly  is  seen  to  divide  above  and  end  there  in  two  tendons,  one  of 
which,  Bl,  is  fixed  to  the  scapula,  while  the  other,  Bb,  joins  the 
tendon  of  a  neighboring  muscle  (the  coraco-brachial,  Cb),  and  is 
also  fixed  above  to  the  shoulder-blade.  Near  the  elbow-joint  the 
muscle  is  continued  into  a  single  tendon,  B',  which  is  fixed  to  the 
radius,  but  gives  an  offshoot,  B",  to  the  connective-tissue  mem- 
branes lying  around  the  elbow-joint. 


80 


THE  HUMAN  BODY 


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g>j    S^J    O 

t^.m 
II~-I^P 

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05  i^  g-^  ^ 

p— *    O-i  O  r>i  •  o3  '-^    p> 

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THE  STRUCTURE  OF  THE  MOTOR  ORGANS  81 

The  belly  of  every  muscle  possesses  the  power  of  shorten ing- 
forcibly  under  certain  conditions.  In  so  doing  it  pulls  upon  the 
tendons,  which  being  composed  of  inextensible  white  fibrous  tissue 
transmit  the  movement  to  the  hard  parts  to  which  they  are  at- 
tached, just  as  a  pull  at  one  end  of  a  rope  may  be  made  to  act  upon 
distant  objects  to  which  the  other  end  is  tied.  The  tendons  are 
merely  passive  cords  and  are  sometimes  very  long,  as  for  instance 
in  the  case  of  the  muscles  of  the  fingers,  the  bellies  of  many  of 
which  lie  away  in  the  forearm. 

If  the  tendons  at  each  end  of  a  muscle  were  fixed  to  the  same 
bone  the  muscle  would  clearly  be  able  to  produce  no  movement, 
unless  by  bending  or  breaking  the  bone;  the  probable  result  in  such 
a  case  would  be  the  tearing  of  the  muscle  by  its  own  efforts.  In 
the  Body,  however,  the  two  ends  of  a  muscle  are  always  attached 


FIG.  39. — The  biceps  muscle  and  the  arm-bones,  to  illustrate  how,  under  ordinary 
circumstances,  the  elbow-joint  is  flexed  when  the  muscle  contracts. 

to  different  parts,  usually  two  bones,  between  which  more  or  less 
movement  is  permitted,  and  so  when  the  muscle  pulls  it  alters 
the  relative  positions  of  the  parts  to  which  its  tendons  are  fixed. 
In  the  great  majority  of  cases  a  true  joint  lies  between  the  bones  on 
which  the  muscle  can  pull,  and  when  the  latter  contracts  it  produces 
movement  at  the  joint.  Many  muscles  even  pass  over  two  joints 
and  can  produce  movement  at  either,  as  the  biceps  of  the  arm 
which,  fixed  at  one  end  to  the  scapula  and  at  the  other  to  the 
radius,  can  move  the  bones  at  either  the  shoulder  or  elbow-joint. 
Where  a  muscle  passes  over  an  articulation  it  is  nearly  always  re- 
duced to  a  narrow  tendon ;  otherwise  the  bulky  bellies  lying  around 


82  THE  HUMAN  BODY 

the  joints  would  make  them  extremely  clumsy  and  limit  their 

mobility. 

Origin  and  Insertion  of  Muscles.    Almost  invariably  that  part 

of  the  skeleton  to  which  one  end  of  a  muscle  is  fixed  is  more  easily 

moved  than  the  part  on  which  it  pulls  by  its  other  tendon.  The 
ABC  less  movable  attachment  of  a  muscle  is  called  its 
origin,  the  more  movable  its  insertion.  Taking 
for  example  the  biceps  of  the  arm,  we  find  that 
when  the  belly  of  the  muscle  contracts  and  pulls 
on  its  upper  and  lower  tendons,  it  commonly 
moves  only  the  forearm,  bending  the  elbow-joint 
as  shown  in  Fig.  39.  The  shoulder  is  so  much 
more  firm  that  it  serves  as  a  fixed  point,  and  so 
that  end  is  the  origin  of  the  muscle,  and  the  fore- 

grams  '   illustrating  arm  attachment,  P,  the  insertion.     It  is  clear, 
however,  that  this  distinction  in  the  mobility 


two  terminal   ten-  of  tne  points  of  fixation  of  the  muscle  is  only  rela- 

dons.      o,    a  penm- 

form  muscle  ;c,  a  bi-  tive,  for,   by  changing  the    conditions,  the   m- 

penniform  muscle.  ,.  ,  ,,  ,.  •,     ,-, 

sertion  may  become  the  stationary  and  the 
origin  the  moved  point;  as  for  instance  in  going  up  a  rope 
"hand  over  hand."  In  that  case  the  radial  end  of  the  muscle  is 
fixed  and  the  shoulder  is  moved  through  space  by  its  contraction. 

Different  Forms  of  Muscles.  Many  muscles  of  the  Body  have 
the  simple  typical  form  of  a  belly  tapering  to  a  single  tendon  at 
each  end  as  A  ,  Fig.  40,  but  others  divide  at  one  end  and  are  called 
two-headed  or  biceps  muscles;  while  some  are  even  three-headed  or 
triceps  muscles.  On  the  other  hand,  some  muscles  have  no  tendon 
at  all  at  one  end,  the  belly  running  quite  up  to  the  point  of  attach- 
ment; and  some  have  no  tendon  at  either  end.  In  many  muscles 
a  tendon  runs  along  one  side  and  the  fibers  of  the 
belly  are  attached  obliquely  to  it:  such  muscles 
(B,  Fig.  40)  are  called  penniform  or  featherlike;  or 
a  tendon  runs  obliquely  down  the  middle  of  the 
muscle  and  has  the  fibers  of  the  belly  fixed  ob-  FIG.  41.  —  A  di- 
liquely  on  each  side  of  it  (C,  Fig.  40),  forming  a  gas 
bipenniform  muscle:  or  even  two  tendons  may  run  down  the  belly 
and  so  form  a  tripenniform  muscle.  In  a  few  cases  a  tendon  is 
found  in  the  middle  of  the  belly  as  well  as  at  each  end  of  it; 
such  muscles  are  called  digastric.  A  muscle  of  this  form  (Fig.  41) 


c 


THE  STRUCTURE  OF  THE  M'OTOR  ORGANS  83 

is  found  in  connection  with  the  lower  jaw,  It  arises  by  a 
tendon  attached  to  the  base  of  the  skull ;  from  there  its  first  belly 
runs  downwards  and  forwards  to  the  neck  by  the  side  of  the 
hyoid  bone,  where  it  ends  in  a  tendon  which  passes  through  a 
loop  serving  as  a  pulley.  This  is  succeeded  by  a  second  belly  di- 
rected upwards  towards  the  chin,  where  it  ends  in  a  tendon  in- 
serted into  the  lower  jaw.  Running  along  the  front  of  the  abdo- 
men from  the  pelvis  to  the  chest  is  a  long  muscle  on  each  side  of  the 
middle  line  called  the  rectus  abdominis:  it  is  poly  gastric,  consisting 
of  four  bellies  separated  by  short  tendons.  Many  uiuscles  more- 
over are  not  rounded  but  form  wide  flat  masses,  as  for  example  the 
muscle  Ss  seen  on  the  ventral  side  of  the  shoulder-blade  in  Fig.  38. 

Gross  Structure  of  a  Muscle.  However  the  form  of  the  skeletal 
muscles  and  the  arrangement  of  their  tendons  may  vary,  the 
essential  structure  of  all  is  the  same.  Each  consists  of  a  proper 
striated  muscular  tissue,  which  is  its  essential  part,  but  which  is 
supported  by  connective  tissue,  nourished  by  blood-vessels,  and 
has  its  activity  governed  by  nerves  so  that  a  great  variety  of 
things  go  to  form  the  complete  organ. 

A  loose  sheath  of  areolar  connective  tissue,  called  the  peri- 
mysium,  envelops  each  muscle,  and  from  this  partitions  run 
in  and  subdivide  the  belly  into  bundles  or  fasciculi  which  run 
from  tendon  to  tendon,  or  for  the  whole  length  of  the  muscle  when 
it  has  no  tendons.  The  coarseness  or  firmness  of  butcher's  meat 
depends  upon  the  size  of  these  primary  fasciculi,  which  differs  in 
different  muscles  of  the  same  animal.  These  larger  fasciculi  are 
subdivided  by  finer  connective  tissue  membranes  into  smaller 
ones,  each  of  which  consists  of  a  certain  number  of  microscopic 
muscular  fibers  bound  together  by  very  fine  connective  tissue  and 
enveloped  in  a  close  network  of  blood-vessels.  Where  a  muscle 
tapers  the  fibers  in  the  fasciculi  become  less  numerous,  and  when  a 
tendon  is  formed  disappear  altogether,  leaving  little  but  the  con- 
nective tissue. 

Histology  of  Skeletal  Muscle.  Each  muscle-fiber  is  developed 
from  a  single  cell  and  so  constitutes  a  single  histological  element. 
In  the  adult  form,  however,  a  muscle-fiber  differs  from  an  ordinary 
cell  in  that  it  contains  several  nuclei.  Muscle-fibers  vary  greatly 
in  size;  ranging  in  length  from  1  up  to  35  mm.  (^  in.  to  li  in.), 
and  in  diameter  from  0.034  to  0.055  mm.  (^^  to  ^5^  in.).  Each 


84 


THE  HUMAN  BODY 


fiber  consists  of  a  certain  amount  of  muscle  substance,  the  muscle 
plasma,  inclosed  in  a  transparent  connective  tissue  sheath,  the  sar- 
colemma.  This  latter  structure  serves  not  only  to  hold  the  semi- 
fluid muscle  plasma  in  place,  but  also  to  transit  the  pull  of  the 
contracting  fiber  to  the  point  of  attachment  of  the  muscle.  The 
most  striking  characteristic  of  a  fiber's  appearance  is  the  series 
of  alternating  light  and  dim  transverse  bands  of  nearly  equal 
width  with  which  it  is  marked,  and  from  which  its  designation  as 
striated  muscle  is  derived  (Fig.  42).  Under  the  high  power  of  the 
microscope  the  muscle  plasma  is  seen  to  be  made  up  of  a  number 


FIG.  42. — A,  Portion  of  a  Human  muscle  fiber.  B,  Separated  bundles  of  sar- 
costyles,  d,  single  sarcostyle.  x  800.  (Sharpey.) 

of  longitudinal  fibrils,  the  sarcostyles,  surrounded  by  a  homogene- 
ous medium,  the  sarcoplasm. 

Not  all  histologists  are  agreed  as  to  the  details  of  structure  of 
the  sarcostyles;  they  are  so  small  that  only  the  highest  powers  of 
the  microscope  can  be  used  in  studying  them;  they  occur  in  ordi- 
nary muscle  surrounded  always  by  sarcoplasm  and  in  company 
with  many  others.  These  circumstances  combine  to  present  to 
the  eye  of  the  observer  a  more  or  less  distorted  picture.  It  is  no 
wonder,  therefore,  that  differences  of  opinion  as  to  the  real  struc- 
ture of  the  sarcostyles  have  arisen. 


THE  STRUCTURE  OF  THE  MOTOR  ORGANS  85 

Certain  insects'  muscles  happen  to  be  so  constituted  that  the 
sarcostyles  can  be  separated  one  from  another  and  isolated  ones 
gotten  under  the  field  of  the  microscope  for  study.  When  exam- 
ined thus  singly  and  free  from  surrounding  media  which  distort 
the  view,  these  sarcostyles  are  seen  to  be  tiny  cylinders  divided  at 
regular  intervals  by  transverse  partitions,  made,  apparently,  of 
delicate  membrane.  Many  biologists  think  it  likely  that  the  sar- 
costyles of  ordinary  skeletal  muscle  have  really  this  same  struc- 
ture; that  the  position  of  the  transverse  membranes  is  indicated  by 
faint  dark  lines  in  the  middle  of  the  light  bands  and  that  the  ap 
pearance  of  light  and  dim  bands  of  nearly  equal  width  is  an  optical 


FIG.  43. — The  muscular  coat  of  the  stomach. 

illusion  due  to  the  unfavorable  conditions  of  observation.  Since 
the  fiber  as  a  whole  contains  many  sarcostyles  and  since  the  cross 
striations  are  regular  throughout  the  entire  fiber  it  follows  that  all 
the  sarcostyles  of  any  fiber  must  have  their  partitions  at  corre- 
sponding levels.  The  sarcostyles  are  probably  kept  in  place  by  an 
interfibrillar  network  of  some  sort. 

The  blood-vessels  and  nerve-fibers  supplied  to  the  skeletal 
muscles  are  numerous.  -  The  larger  blood-vessels  run  in  the  coarser 
partitions  of  the  connective  tissue  lying  between  the  fasciculi  and 
give  off  fine  branches  which  form  a  network  between  the  individual 
fibers  but  never  penetrate  the  sarcolemma. 

Connected  with  each  muscle-fiber  is  a  nerve-fiber.  The  central 
core  of  the  nerve-fiber  ends  in  an  oval  expansion  (end  plate)  which 


86  THE  HUMAN  BODY 

contains  many  nuclei  and  lies  close  under  the  sarcolcmma,  its 
deeper  side  being  in  immediate  contact  and  possibly  continuous 
with  the  striated  contents.  These  nerve-fibers  are  motor  or  con- 
cerned in  exciting  a  contraction  of  the  muscle-fiber. 
Other  nerve-fibers  are  connected  with  very  peculiar 
bodies  found  scattered  throughout  the  muscle,  but 
especially  numerous  near  the  tendons.  They  are 
usually  of  a  size  just  visible  to  the  unaided  eye  and 
from  their  form  have  been  named  muscle-spindles. 
They  are  doubtless  sensory  in  function.  Somewhat 
similar  bodies  (Golgi's  tendon-organs)  are  found  in  the 
tendons  and  are  also  richly  supplied  with  nerve- 
fibers. 

Structure  of  the  Smooth  Muscles.  Of  these  the 
muscular  coat  of  the  stomach  (Fig.  43)  is  a  good  ex- 
ample. They  have  no  definite  tendons,  but  form 
expanded  membranes  surrounding  cavities,  so  that 
they  have  no  definite  origin  or  insertion.  Like  the 
skeletal  muscles  they  consist  of  proper  contractile 
elements,  with  accessory  connective  tissue,  blood- 
vessels, and  nerves.  Their  fibers,  however,  have  a 
very  different  microscopic  structure.  They  present 
a  slightly  marked  longitudinal  but -no  cross  striation 
and  are  made  up  of  elongated  cells  (Fig.  44),  bound 
together  by  a  small  quantity  of  cementing  material. 
The  cells  vary  considerably  in  size,  but  on  the  aver- 
age are  about  yj  mm-  (STTF  in-)  in  length.  Each  is 
flattened  in  one  plane,  tapers  off  at  each  end,  and 
possesses  a  very  thin  enveloping  membrane;  in  its 
interior  lies  an  elongated  nucleus.  These  cells 
FIG.  44.  —  have  the  power  of  shortening  in  the  direction  of 

mulcleds?eUs  ^^T  ^ong  axeSj  anc*  so  °^  diminishing  the  capacity 
from      human  of  the  cavities  in  the  walls  of  which  they  lie. 

small  intestine.        ~       ,.        _, 

Cardiac  Muscular  Tissue.  This  consists  of  nu- 
•cleated  branched  cells  which  unite  to  form  a  network,  in  the  in- 
terstices of  which  blood-capillaries  and  nerve-fibers  run.  The  cells 
present  transverse  striations,  but  not  so  distinct  as  those  of  the 
skeletal  muscles,  and  are  said  to  have  no  sarcolemma  (Fig.  45). 
Ciliated  Cells.  As  the  growing  Body  develops  from  its  prim- 


THE  STRUCTURE  OF  THE  MOTOR  ORGANS  87 

itive  simplicity  we  find  that  the  cells  lining  some  of  the  tubes  and 
cavities  in  its  interior  undergo  a  very  remarkable  change,  by  which 
each  cell  differentiates  itself  into  a  nutritive  and  a  highly  motile 
portion.  Such  cells  are  found  for  example  lin- 
ing the  windpipe,  and  are  represented  in  Fig.  46. 
Each  has  a  conical  form,  the  base  of  the  cone 
being  turned  to  the  cavity  of  the  air-tube,  and 
contains  an  oval  nucleus.  On  the  broader  free 
end  are  a  number  (about  thirty  on  the  aver- 
age) of  extremely  fine  processes  called  cilia. 
During  life  these  are  in  constant  rapid  move- 
ment, lashing  to  and  fro  in  the  liquid  which 
moistens  the  interior  of  the  passage:  and  as 

FIG.     45.  —  Cardiac 

muscular  tissue,  magni-  the  cells  are  very  closely  packed,  a  bit  of  the 
ters.  a  The  ceii-boirnd-  mner  surf  ace  of  the  windpipe,  examined  with 


Ceoniyuinei  thl  a  microscope,  looks  like  a  field  of  wheat  or 
right-hand  portion  of  barley  when  the  wind  blows  over  it.  Each 
cilium  strikes  with  more  force  in  one  direction 
than  in  the  opposite,  and  as  this  direction  of  more  powerful  stroke 
is  the  same  for  all  the  cilia  on  any  one  surface,  the  resultant  effect 
is  that  the  liquid  in  which  they  move  is  driven  one  way.  In  the 
case  of  the  windpipe  for  example  it  is  driven  up  towards  the 
throat,  and  the  tenacious  liquid  or  mucus  which  is  thus  swept 
along  is  finally  coughed  or  "hawked"  up  and  got  rid  of,  instead  of 
accumulating  in  the  deeper  air-passages  away  down  in  the  chest. 

These  cells  afford  an  extremely  interesting  example  of  the  di- 
vision of  physiological  employments.     Each 
proceeds  from  a  cell   which  was  primitively 
equally  motile  and  nutritive  in  all  its  parts. 
But  in  the  fully  developed  state  the  nutritive 
duties  have  been  especially  assumed  by  the 
conical  cell-body,  while  the  contractile  prop- 
erties have  been   condensed,  so  to  speak,  in       FIG.   46.—  ciliated 
that  modified  portion  of  the  primitive  proto-    cells> 
plasmic  mass  which  forms  the  cilia.     These,  being  supplied  with 
elaborated  food  by  the  rest  of  the  cell,  are  raised  above  the  vulgar 
cares  of  life  and  have  the  opportunity  to  devote  their  whole  at- 
tention to  the  performance  of  automatic  movements;  which  are 
accordingly   far  more    rapid   and    precise   than   those  executed 


88  THE  HUMAN  BODY 

by  the  whole  cell  before  any  division  of  labor  had  occurred 
in  it. 

That  the  movements  depend  upon  the  structure  and  composi- 
tion of  the  cells  themselves,  and  not  upon  influences  reaching  them 
from  the  nervous  or  other  tissues,  is  proved  by  the  fact  that  they 
continue  for  a  long  time  in  isolated  cells,  removed  and  placed  in  a 
liquid,  as  blood-serum,  which  does  not  alter  their  physical  consti- 
tution. In  cold-blooded  animals,  as  turtles,  whose  constituent 
tissues  frequently  retain  their  individual  vitality  long  after  that 
bond  of  union  has  been  destroyed  which  constitutes  the  life  of  the 
whole  animal  as  distinct  from  the  lives  of  its  different  tissues,  the 
ciliated  cells  in  the  windpipe  have  been  found  still  at  work  three 
weeks  after  the  general  death  of  the  animal. 

The  Physico-Chemistry  of  Skeletal  Muscle.  The  activity  of  a 
muscle  is  the  sum  of  the  activities  of  its  individual  fibers.  To  un- 
derstand the  operation  of  the  muscle  engine,  therefore,  we  must 
analyze  the  activity  of  the  muscle  fiber.  Some  help  toward  this 
may  be  gained  by  studying  the  structure  of  the  fiber  from  the 
physico-chemical  standpoint.  Throughout  this  study  the  funda- 
mental fact  of  muscular  activity  should  oe  kept  before  the  mind, 
namely,  that  the  muscle  is  a  device  for  executing  a  forcible  shorten- 
ing at  the  expense  of  energy  derived  from  the  food  (p.  22). 

From  the  account  of  the  anatomical  structure  of  the  muscle- 
fiber,  given  previously  (p.  85),  we  have  learned  to  think  of  the 
fiber  as  consisting  of  a  large  number  of  tiny,  longitudinal  cylinders, 
the  sarcostyles,  inclosed  in,  and  attached  by  their  ends  to,  a  sheath, 
the  sarcolemma,  and  surrounded  by  a  fluid,  the  sarcoplasm,  which 
occupies  all  the  space  within  the  sarcolemma  not  taken  by  the 
sarcostyles.  Physico-chemical  studies  indicate  that  the  sarco- 
styles are  colloidalm  nature.  In  fact  there  is  reason  to  think  that 
the  colloids  of  which  they  are  composed  may  be  quite  dense,  con- 
taining less  water  than  do  most  of  the  colloidal  tissues  of  the  body. 
The  sarcoplasm,  on  the  other  hand,  is  thought  of  as  a  very  watery 
fluid,  with  salts  and  other  relatively  simple  substances  dissolved 
in  it,  but  containing  little  if  any  colloid.  When  we  reach  the  con- 
sideration of  the  precise  manner  in  which  the  forcible  shortening 
of  the  muscle  comes  about  we  shall  find  that  the  densely  colloidal 
consistency  of  the  sarcostyles  and  the  watery  nature  of  the  sarco- 
plasm are  of  great  significance. 


THE  STRUCTURE  OF  THE  MOTOR  ORGANS  89 

The  Chemistry  of  Muscular  Tissue.  When  we  subject  a  mass 
of  muscle  to  chemical  analysis  we,  of  course,  kill  the  fibers,  if  they 
were  not  dead  when  the  analysis  was  started.  The  difference  be- 
tween life  and  death  is  undoubtedly  at  bottom  a  chemical  differ- 
ence, so  that  we  cannot  hope  by  the  ordinary  methods  of  analysis 
as  applied  to  dead  tissues  to  learn  the  exact  chemistry  of  the  living 
muscles.  On  the  other  hand,  the  constituents  we  find  in  dead 
muscle  were  derived  from  those  of  the  living  muscle,  and  are  un- 
doubtedly nearly  related  to  them,  so  that  knowledge  of  the  chem- 
istry of  dead  muscle  cannot  fail  to  give  us  a  degree  of  insight  into 
the  nature  of  the  living  muscle. 

To  understand  clearly  the  facts  brought  out  by  an  analysis  of 
muscle  we  need  to  bear  in  mind  that  a  muscle,  as  stated  previously, 
is  an  engine,  whose  property  is  to  convert  chemical  energy  into 
mechanical.  Our  analysis  will  demonstrate  the  presence  of  some 
substances  which  are  part  of  the  machinery;  of  others  which  make 
up  the  fuel  from  which  the  energy  for  operation  is  derived;  of  still 
others  which  are  nothing  more  than  the  waste  products  from 
previous  activity.  It  is  as  though  a  locomotive  in  full  career 
suddenly  fell  into  so  deep  a  chasm  as  to  reduce  it  with  its  tender  to 
a  mass  of  indistinguishable  fragments.  Chemical  examination  of 
the  mass  would  bring  to  light  various  materials,  such  as  steel, 
brass,  and  nickel,  which  were  part  of  the  engine;  coal,  which  was 
the  fuel;  and  ashes,  representing  the  waste  products.  In  similar 
fashion  we  may  pick  out  from  a  chemical  study  of  muscle  some 
constituents  which  probably  represent  the  machinery;  others 
which  form  the  fuel;  and  still  others  which  are  waste  substances. 

The  most  abundant  single  constituent  of  muscle  is  water,  which 
forms  75  per  cent  of  its  mass.  Dissolved  in  the  water  are  small 
quantities  of  a  number  of  simple  inorganic  salts,  chiefly  phos- 
phates and  chlorides  of  potassium,  sodium,  and  magnesium. 
Since  experiment  has  shown  that  these  salts,  as  well  as  the  water 
in  which  they  are  dissolved,  are  essential  to  the  life  and  operation 
of  the  muscle  we  may  look  upon  them  as  part  of  the  machinery. 
Similarly  the  most  abundant  solid  constituents  of  muscle,  the  pro- 
teins, are  to  be  included  as  portions  of  the  mechanism. 

At  least  three  proteins  have  been  obtained  from  mammalian 
striped  muscle,  myogen,  an  albumin,  myosin,  a  globulin,  both 
coagulable  by  heat,  and  a  protein  which  is  insoluble  in  pure  water 


90  THE  HUMAN  BODY 

or  dilute  saline  solution  and  which  appears  to  form  a  framework 
within  the  fiber.  This  latter  is  called  the  muscle  stroma  and  con- 
stitutes 9  per  cent  of  the  weight  of  striated  muscle.  Muscle 
tissue  contains  three  or  four  times  as  much  inyogen  as  myosin. 
Both  of  these  proteins  possess  the  property  of  passing  over  into 
insoluble  forms  known  respectively  as  myogen  fibrin  and  myosin 
fibrin. 

Heart  muscle  contains  relatively  much  less  myogen  and  myosin 
and  much  more  stroma  than  does  ordinary  striated  muscle,  its 
stroma  constituting  56  per  cent  of  its  weight.  Smooth  muscle 
contains  an  even  larger  proportion  of  stroma,  72  per  cent.  In 
striated  muscle  the  proteins  appear  to  be  confined  largely  to  the 
colloidal  sarcostyles,  except  in  so  far  as  the  sarcolemma  is  protein 
in  constitution. 

Another  constituent  of  muscle  which  is  apparently  part  of  the 
contractile  machinery  is  the  relatively  simple  nitrogenous  com- 
pound creatine  (p.  14).  Recent  studies  of  the  part  played  by  this 
substance  in  tissue  indicate  that  living  protoplasm  always  con- 
tains it,  and  suggest  that  it  may  be  an  essential  part  of  the  chem- 
ical complex  upon  which  life  depends. 

The  known  fuel  substances  found  in  muscle  are  two,  dextrose, 
which  is  found  in  very  small  amount,  and  glycogen,  which  forms 
about  1  per  cent  of  the  weight  of  the  muscle.  The  muscle  is 
probably  able  to  use  other  substances  as  fuel,  fats,  for  example, 
and  perhaps  proteins  themselves  on  occasion,  but  chemical  analy- 
sis does  not  enable  us  to  distinguish  these  from  similar  substances 
which  belong  to  the  mechanism. 

Urea  and  other  nitrogenous  extractives,  and  sodium  carbonate 
are  found  in  muscle  also.  These  are  to  be  classed  as  waste  prod- 
ucts, formed  during  the  operation  of  the  machine.  An  interest- 
ing and  very  significant  fact  is  that  a  muscle  analyzed  imme- 
diately after  a  period  of  vigorous  activity  is  found  to  contain 
lactic  acid,  or  the  compound  of  this  acid  with  sodium,  sodium  lac- 
rate,  whereas  muscles  that  have  been  resting  do  not  contain  lac- 
tates.  This  fact,  as  we  shall  see,  has  important  bearing  on  the 
question  of  the  nature  of  muscle  contraction.  We  shall  recur  to 
it  in  connection  with  the  analysis  of  the  process  of  contraction. 

Beef  Tea.  From  the  facts  about  proteins,  stated  above,  it  is 
clear  that  when  a  muscle  is  boiled  in  water  its  myogen  and  myosin 


THE  STRUCTURE  OF  THE  MOTOR  ORGANS  91 

are  coagulated  and  left  behind  in  the  meat;  even  if  cooking  be  com- 
menced by  soaking  in  cold  water  the  myogen  still  remains,  as  it  is 
as  insoluble  in  cold  water  as  in  hot.  Beef  tea  as  ordinarily  made, 
then,  contains  little  but  the  flavoring  matters  and  salts  of  the 
meat,  traces  of  some  albumins  and  some  gelatin,  the  latter  derived 
from  the  connective  tissues  of  the  muscle.  The  flavoring  matters 
and  salts  make  it  deceptively  taste  as  if  it  were  a  strong  solution 
of  the  whole  meat,  and  the  gelatin  causes  it  to  "set"  on  cooling,  so 
the  cook  feels  quite  sure  she  has  got  out  "all  the  strength  of  the 
meat,"  whereas  the  beef  tea  so  prepared  contains  but  little  of  the 
most  nutritious  protein  portions,  which  in  an  insipid  shrunken 
form  are  left  when  the  liquid  is  strained  off.  Various  proposals 
have  been  made  with  the  object  of  avoiding  this  and  getting  a 
really  nutritive  beef  tea;  as  for  example  chopping  the  raw  meat  fine 
and  soaking  it  in  strong  brine  for  some  hours  to  dissolve  out  the 
myogen ;  or  extracting  it  with  dilute  acids  which  dissolves  the  myo- 
gen and  myosin  and  at  the  same  time  render  it  non-coagulable  by 
heat  when  subsequently  boiled.  Such  methods,  however,  make  un- 
palatable compounds  which  invalids  will  not  take.  Beef  tea  is  a 
slight  stimulant,  and  often  extremely  useful  in  preparing  the  stom- 
ach for  other  food,  but  its  direct  value  as  a  food  is  slight,  and  it 
cannot  be  relied  upon  to  keep  up  a  patient's  strength  for  any 
length  of  time.  There  can  be  no  doubt  that  thousands  of  sick 
persons  have  in  the  past  been  starved  to  death  on  it.  Liebig's  ex- 
tract of  meat  is  essentially  a  very  strong  beef  tea;  containing  much 
of  the  flavoring  substances  of  the  meat,  nearly  all  its  salts  and  the 
crystalline  nitrogenous  bodies,  such  as  creatine,  which  exist  in 
muscle,  but  hardly  any  of  its  really  nutritive  parts,  as  was  pointed 
out  by  Liebig  himself.  From  its  stimulating  effects  it  is  often 
useful  to  persons  in  feeble  health,  but  other  food  should  be  given 
with  it.  It  may  also  be  used  on  account  of  its  flavor  to  add  to  the 
''stock"  of  soup  and  for  similar  purposes;  but  the  erroneousness 
of  the  common  belief  that  it  is  a  highly  nutritious  food  cannot  be 
too  strongly  insisted  upon.  Under  the  name  of  liquid  extracts  of 
meat  other  substances  have  been  prepared  by  subjecting  meat  to 
chemical  processes  in  which  it  undergoes  changes  similar  to  those 
experienced  in  digestion:  the  myosin  is  thus  rendered  soluble  in 
water  and  uncoagulable  by  heat,  and  such  extracts  if  properly  pre- 
pared are  nutritious  and  can  often  be  absorbed  when  meat  in  the 


92  THE  HUMAN  BODY 

solid  form  cannot  be  digested:  they  may  thus  help  the  stomach 
over  a  crisis,  but  are  not,  even  the  best  of  them,  to  be  depended  on 
as  anything  but  temporary  substitutes  for  other  food;  or  in  some 
cases  as  useful  additions  to  it. 

Rigor  Mortis.  During  life  and  for  a  certain  time  after  general 
death  the  muscles  are  soft,  translucent,  extensible  and  elastic,  and 
neutral  or  feebly  alkaline  in  reaction;  after  a  period  which  in  warm- 
blooded animals  is  brief  (varying  from  a  few  minutes  to  three  or 
four  hours)  they  gradually  become  harder,  more  opaque,  less  ex- 
tensible and  less  elastic,  and  distinctly  acid  in  reaction.  The 
result  of  these  changes  is  the  well-known  cadaveric  rigidity  or 
rigor  mortis.  It  was  formerly  very  generally  believed  that  the 
cause  of  rigor  is  the  change  of  soluble  myogen  and  the  myosin  to  in- 
soluble myogen  fibrin  and  myosin  fibrin.  Quite  recently,  how- 
ever, some  physiologists  have  called  attention  to  the  strong  prob- 
ability that  death  stiffening  may  be  due  to  the  considerable 
production  of  lactic  acid  which  is  known  to  accompany  the  death 
process.  In  support  of  their  view  may  be  cited  the  well-known 
tendency  of  animals  or  men  killed  suddenly  in  the  midst  of  violent 
exertion  to  stiffen  very  quickly.  Men  killed  in  battle  often  re- 
tain the  postures  in  which  death  overtook  them.  Hard  muscular 
work  involves  a  large  production  of  lactic  acid,  a  condition  favor- 
able according  to  the  view  quoted,  to  a  prompt  onset  of  rigor. 


CHAPTER  VII 
MUSCULAR  ACTIVITY 

The  Study  of  Isolated  Muscles.  There  are  some  simple  facts  of 
muscle  activity  that  one  can  learn  by  observing  his  own  muscles; 
for  example,  when  the  arm  is  bent  at  the  elbow  the  muscle  that 
produces  the  movement,  the  biceps,  can  be  seen  under  the  skin 
to  shorten  and  thicken,  and  if  felt  will  be  found  hard  when  con- 
tracted, as  compared  with  its  soft  flabbiness  when  relaxed.  This 
knowledge  is  the  possession  of  every  school  boy. 

More  detailed  knowledge  can  be  gained  by  the  direct  examina- 
tion of  living  muscles,  dissected  away  from  their  bodily  attach- 
ments. To  isolate  the  muscles  of  the  higher  animals  thus  is  ob- 
viously impractical,  but  fortunately  the  " cold-blooded"  animals, 
notably  frogs  and  turtles,  are  peculiarly  suited  for  such  studies  as 
this.  If  a  frog  is  quickly  killed,  as  by  destroying  the  brain  with  a 
sharp  instrument,  the  large  muscle  of  the  calf  of  the  leg,  the  gastroc- 
nemius,  can  be  dissected  out,  and  if  properly  cared  for  will  remain 
alive  for  several  hours,  during  which  its  activity  can  be  studied. 
The  chief  precaution  to  be  observed  is  to  prevent  such  evaporation 
of  water  from  the  muscle  as  would  disturb  the  physico-chemical 
equilibrium  and  injure  the  tissue.  This  loss  of  water  is  prevented 
by  repeated  moistening,  but  here  again  a  precaution  must  be  ob- 
served since  the  application  of  pure  water  to  the  surface  of  the 
muscle  would  be  followed  by  a  flow  of  water  into  the  tissue  under 
the  driving  force  of  osmotic  pressure  (p.  19).  This  would  bring 
about  a  disturbance  of  equilibrium  in  the  direction  of  too  great 
dilution  as  harmful  to  the  tissue  as  the  evaporation  it  is  designed 
to  prevent.  For  moistening  the  muscle  a  liquid  of  the  same  os- 
motic pressure  as  the  tissue  fluids  must  be  employed.  A  solution 
of  common  salt  with  a  concentration  of  0.7  per  cent  satisfies  this 
condition  and  is  the  fluid  commonly  used  for  keeping  living  tissues 
moist. 

The  Necessity  of  Stimulation.  An  important  fact  about  skel- 
etal muscle,  one  indeed  which  has  much  to  do  with  the  adaptive 

93 


94  THE  HUMAN  BODY 

character  of  its  action,  is  that  it  remains  in  a  condition  of  inac- 
tivity except  when  in  receipt  of  definite  stimulation.  A  moment's 
thought  will  show  the  importance  of  this  property.  The  usefulness 
of  a  muscle  depends  quite  as  much  upon  its  ability  to  be  inactive 
when  not  wanted  as  upon  its  property  of  contracting  when  con- 
traction is  desired.  The  difficulties  frequently  experienced  by 
sufferers  from  chorea  (St.  Vitus  Dance)  illustrate  this  point  suffi- 
ciently. 

For  the  study  of  activity  in  isolated  muscles  some  form  of  arti- 
ficial stimulation  must  be  employed.  By  far  the  most  satisfac- 
tory is  an  electric  shock  such  as  may  be  generated  by  a  small  in- 
duction coil.  By  carrying  fine  wires  from  the  terminals  of  the 
coil  to  opposite  ends  of  the  muscle  the  latter  becomes  a  part  of  the 
circuit.  Thus  when  the  coil  discharges  a  spark  the  muscle  re- 
ceives it  and  is  stimulated.  Besides  being  peculiarly  effective  as 
stimuli  induction  shocks  have  the  advantage  of  being  easily  modi- 
fied in  intensity,  and,  when  not  excessive,  of  having  no  injurious 
effect  on  the  tissue. 

A  Simple  Muscular  Contraction.  When  a  single  electric  shock 
is  sent  through  a  muscle,  it  rapidly  shortens  and  then  rapidly 
lengthens  again.  The  whole  series  of  phenomena  from  the  mo- 
ment of  stimulation  until  the  muscle  regains  its  resting  form  is 
known  as  a  simple  muscular  contraction  or  a  "twitch":  it  occupies 
in  frog's  muscle  about  one-tenth  of  a  second.  So  brief  a  move- 
ment as  this  cannot  be  followed  in  its  details  by  direct  observa- 
tion, but  it  is  possible  to  record  it  and  study  its  phases  at  leisure. 
This  may  be  done  by  firmly  fixing  the  upper  tendon  of  an  isolated 
muscle,  M,  Fig.  47,  and  attaching  the  other  end  at  d  to  a  lever,  Z, 
which  can  move  about  the  fulcrum  /:  the  end  of  the  long  arm  of 
the  lever  bears  a  point,  p,  which  scratches  on  a  smooth  smoked 
surface,  S.  Suppose  the  surface  to  be  placed  so  that  the  writing 
point  of  the  lever  is  at  a;  if  the  muscle  now  contracts  it  will  raise 
the  point  of  the  lever,  and  a  line  ac  will  be  drawn  on  the  smoked 
surface,  its  vertical  height,  cm,  being  dependent,  first,  on  the  ex- 
tent of  the  shortening  of  the  muscle,  and  second,  on  the  proportion 
between  the  long  and  short  arms  of  the  lever:  the  longer  fp  is  as 
compared  with/d,  the  more  will  the  actual  shortening  of  the  muscle 
be  magnified.  With  the  lever  shown  in  the  figure  this  magnifica- 
tion would  be  about  ten  times,  so  that  one-tenth  of  cm  would  be 


MUSCULAR  ACTIVITY 


95 


the  extent  of  the  shortening  of  the  muscle.  Suppose,  next,  the 
smoked  surface  to  be  moved  to  such  position  that  the  writing 
point  of  the  lever  touches  it  at  i,  and,  the  muscle  being  left  at  rest 
the  surface  to  be  moved  evenly  from  left  to  right;  the  horizontal 
line  io  would  then  be  traced,  its  length  depending  on  the  distance 
through  which  S  moved  during  the  time  the  lever  was  marking  on 
it:  and  it  is  clear  that  if  S  move  uniformly,  and  we  know  its  rate 
of  movement,  we  can  very  readily  calculate  from  the  length  of  io 
how  long  S  was  moving  while  that  line  was  being  traced:  for  ex- 
ample, if  we  know  the  rate  of  movement  to  be  ten  centimeters  per 


FIG.  47. — Diagram  to  illustrate  the  method  of  obtaining  a  graphic  record  of  a 
muscular  contraction. 

second,  and  on  measurement  find  io  to  be  one  centimeter  long,  the 
time  during  which  the  surface  was  moving  must  have  been  VQ-  of  a 
second;  and  each  tenth  of  io  correspond  to  iVo"  of  a  second. 

If  we  set  the  recording  surface  in  motion  and  while  the  lever 
point  is  tracing  a  horizontal  line  cause  the  muscle  to  contract,  the 
point  will  be  raised  as  long  as  the  muscle  is  contracted,  and  the 
line  drawn  by  it  will  be  due  to  a  combination  of  two  simultaneous 
movements — a  horizontal,  due  to  the  motion  of  S,  a  nearly  verti- 
cal, due  to  the  shortening  of  the  muscle;  the  resulting  line  is  a 
curve  known  as  the  curve  of  a  simple  muscular  contraction.  Let  the 
surface  S  be  placed  so  that  the  writing  point  is  at  q  and  then  be  set 
in  uniform  motion  from  left  to  right  at  the  same  rate  as  before 


96  THE  HUMAN  BODY 

(ten  centimeters  per  second).  When  the  point  is  opposite  t,  stimu- 
late the  muscle  by  an  electric  shock;  the  result,  until  the  muscle  has 
fully  lengthened  again,  will  be  the  curve  tuvwxy,  from  which  many 
things  may  be  learned.  In  the  first  place  we  see  that  the  muscle 
does  not  commence  to  contract  at  the  very  instant  of  stimulation, 
but  at  an  appreciably  later  time,  and  during  the  interval  the  lever 
draws  the  horizontal  line  tu;  this  period,  occupied  by  preparatory 
changes  within  the  muscle,  is  known  as  the  latent  period.  Then  the 
muscle  begins  to  shorten  and  the  lever  to  rise,  until  the  summit  of 
the  contraction  is  reached  at  w.  The  muscle  then,  but  only  grad- 
ually passes  back  to  the  resting  state,  tracing  the  line  wxy.  The 
curve  shows  three  distinct  phases  in  the  contraction:  the  latent 
period;  the  period  of  shortening;  the  period  of  relaxation.  Know- 
ing the  rate  of  horizontal  movement,  we  can  measure  off  the  time 
occupied  by  each  phase.  The  horizontal  distance  from  t  to  u  repre- 
sents the  time  taken  by  the  latent  period;  from  u  to  z,  the  time 
occupied  in  shortening;  from  z  to  y,  the  time  taken  in  elongation; 
in  a  fresh  frog's  muscle  these  times  are  respectively  T^,  T^7,  TJo 
of  a  second.  In  the  muscles  of  warm-blooded  animals  they  are  all 
shorter,  but  the  difficulties  in  the  way  of  accurate  experiment  are 
very  great.  If  we  know  the  relative  lengths  of  the  arms  of  the 
lever  we  can  of  course  readily  calculate  from  the  height,  wz,  of  the 
curve  the  extent  of  shortening  of  the  muscle.  With  a  single  elec- 
trical stimulation  this  is  never  more  than  one-fourth  the  total 
length  of  the  muscle. 

In  Fig.  47  the  accessory  apparatus  used  in  practice  to  indicate 
on  the  moving  surface  the  exact  instant  of  stimulation  and  to 
measure  the  rate  at  which  S  moves  have  been  omitted. 

The  Influence  on  Contraction  Height  of  Increasing  Stimula- 
tion Strength.  If  an  isolated  muscle  is  stimulated  at  regular 
intervals,  as  once  in  two  seconds,  by  induction  shocks  made 
stronger  each  time,  there  will  be  at  first,  if  the  shocks  to  begin 
with  are  weak  enough,  no  visible  response.  Presently,  as  stronger 
shocks  are  used,  barely  perceptible  twitches  are  given.  These 
become  higher  and  higher  as  the  stimuli  are  increased  until  a  point 
is  reached  beyond  which  no  further  increase  in  height  appears,  no 
matter  how  much  the  intensity  of  the  shock  may  be  increased. 
These  highest  contractions  which  the  muscle  is  capable  of  giving 
as  the  result  of  any  single  stimulus,  however  strong,  are  called 


MUSCULAR  ACTIVITY  97 

maximal  contractions.  The  gradation  of  response  by  grading  the 
intensity  of  stimulation  is  obviously  the  means  employed  in  our 
own  bodies  to  produce  graded  movements.  We  realize  without 
difficulty  that  a  powerful  contraction  requires  a  great  effort  (strong 
stimulation),  while  a  gentle  contraction  is  produced  with  very 
little  effort  (weak  stimulation). 

Every  skeletal  muscle,  even  the  smallest,  is  made  up  of  a  great 
number  of  fibers.  This  must  be  borne  in  mind  when  we  attempt 
to  explain  the  production  of  graded  responses,  for  when  the  muscle 
contracts  feebly  it  may  be  that  all  the  fibers  are  contracting,  each 
one  feebly,  or  it  may  be  that  a  few  fibers  scattered  through  the 
muscle  are  contracting  powerfully  while  the  others  are  inactive, 
If  the  first  of  these  suppositions  is  correct  we  must  look  upon  all 
the  fibers  as  equally  sensitive  to  stimulation,  so  that  all  respond 
feebly  to  weak  stimuli  and  powerfully  to  strong  ones.  If  the 
second  view  is  the  true  one  we  must  picture  the  various  fibers  as 
differing  in  sensitiveness  over  a  wide  range.  Some  are  aroused 
by  very  feeble  stimuli,  others  require  stronger  ones,  and  others 
still  stronger  ones;  but  whether  aroused  by  a  weak  stimulus  or  a 
strong  one  the  response  of  the  individual  fibers  is  powerful.  Ac- 
cording to  this  view  the  muscle  fibers  could  be  compared  to  the 
cartridge  in  a  rifle  and  the  stimulus  to  the  pull  of  the  trigger.  In 
some  rifles  the  trigger  is  harder  to  pull  than  in  others,  but  the  ex- 
plosion of  the  cartridge  is  as  violent  in  the  rifle  with  the  hair 
trigger  as  in  the  stiffer  one. 

The  first  of  these  theories,  the  one  that  makes  the  gradation  of 
response  a  gradation  within  the  fibers,  is  the  older  and  the  one  that 
has  formerly  been  generally  accepted.  The  second  view,  accord- 
ing to  which  the  gradation  of  response  depends  on  the  number  of 
fibers  involved,  has  been  urged  only  recently,  and  while  many 
known  facts  are  in  accord  with  it,  it  cannot  be  said  to  be  conclu- 
sively proven. 

The  Influence  of  Temperature  on  Contraction.  If  an  isolated 
muscle  is  cooled  down  and  then  stimulated,  the  contraction  will 
occur  much  more  slowly  than  at  ordinary  temperatures.  On  the 
other  hand,  a  muscle  that  is  warmed  contracts  and  relaxes  more 
rapidly  than  one  that  is  at  room  temperature.  This  variation  in 
the  speed  of  contraction  with  change  of  temperature  is  in  accord 
with  a  general  chemical  law  which  states  that  chemical  processes 


98  THE  HUMAN  BODY 

are  more  rapid  at  higher  temperatures  than  at  lower;  a  law  which 
applies  to  muscular  contraction  because  the  mechanical  act  of 
shortening  is  based  on  a  preceding  chemical  process.  In  nature 
the  influence  of  temperature  on  muscular  contraction  is  seen  only 
in  the  lower  (cold-blooded)  animals,  whose  bodies  are  at  sub- 
stantially the  temperature  of  the  surroundings  and  which,  as  can 
easily  be  observed,  are  sluggish  in  cold  weather  and  active  in  warm. 
In  man  and  the  higher  (warm-blooded)  animals,  and  in  birds, 
which  are  also  warm-blooded,  the  body  temperature  is  high  and 
relatively  constant,  and  the  muscles  are  not  subjected,  therefore, 
to  such  temperature  variations  as  occur  in  lower  forms.  One  ad- 
vantage of  the  warm-blooded  state  is  that  it  insures  for  the  muscles 
a  favorable  temperature  for  effective  operation  in  cold  weather 
as  well  as  in  warm. 

Heat  Rigor.  If  an  isolated  muscle  is  heated  above  40-45°  C. 
(104-113°  F.)  it  is  killed  by  the  heat  and  undergoes  a  marked  con- 
traction known  as  heat  rigor.  Heat  rigor  like  the  death  stiffening 
(rigor  mortis,  p.  92)  is  accompanied  by,  and  probably  caused  by, 
a  great  production  of  lactic  acid  within  the  muscle. 

The  Measure  of  Muscular  Work.  The  work  done  by  a  muscle 
in  a  given  contraction,  when  it  lifts  a  weight  vertically  against 
gravity,  is  measured  by  the  weight  moved,  multiplied  by  the  dis- 
tance through  which  it  is  moved.  When  a  muscle  contracts  carry- 
ing no  load  it  does  very  little  work,  lifting  only  its  own  weight; 
when  loaded  with  one  gram  and  lifting  it  five  millimeters  it  does 
five  gram-millimeters  of  work,  just  as  an  engineer  would  say  an 
engine  had  done  so  many  kilogrammeters  or  foot-pounds.  If 
loaded  with  ten  grams  and  lifting  it  six  millimeters  it  would  do 
sixty  gram-millimeters  of  work.  Even  after  the  weight  becomes  so 
great  that  it  is  lifted  through  a  less  distance,  the  work  done  by  the 
muscle  goes  on  increasing,  for  the  heavier  weight  lifted  more  than 
compensates  for  the  less  distance  through  which  it  is  raised.  For 
example,  if  the  above  muscle  were  loaded  with  fifty  grams  it  would 
maybe  lift  that  weight  only  1.5  millimeters,  but  it  would  then  do 
75  gram-millimeters  of  work,  which  is  more  than  when  it  lifted 
ten  grams  six  millimeters.  A  load  is,  however,  at  last  reached 
with  which  the  muscle  does  less  work,  the  lift  becoming  very  little 
indeed,  until  at  last  the  weight  becomes  so  great  that  the  muscle 
cannot  lift  it  at  all  and  so  does  no  work  when  stimulated.  Starting 


MUSCULAR  ACTIVITY  99 

then  from  the  time  when  the  muscle  carried  no  load  and  did  no 
work,  we  pass  with  increasing  weights,  through  phases  in  which 
it  does  more  and  more  work,  until  with  one  particular  load  it  does 
the  greatest  amount  possible  to  it  with  that  stimulus:  after  that, 
with  increasing  loads  less  work  is  done,  until  finally  a  load  is 
reached  with  which  the  muscle  again  does  no  work.  What  is  true 
of  one  muscle  is  of  course  true  of  all,  and  what  is  true  of  work  done 
against  gravity  is  true  of  all  muscular  work,  so  that  there  is  one 
precise  load  with  which  a  beast  of  burden  or  a  man  can  do  the 
greatest  possible  amount  of  work  in  a  day.  With  a  lighter  or 
heavier  load  the  distance  through  which  it  can  be  moved  will  be 
more  or  less,  but  the  actual  work  done  always  less.  In  the  living 
Body,  however,  the  working  of  the  muscles  depends  so  much  on 
other  things,  as  the  due  action  of  the  circulatory  and  respiratory 
systems  and  the  nervous  energy  or  "grit"  (upon  which  the  stimu- 
lation of  the  muscles  depends)  of  the  individual  man  or  beast,  that 
the  greatest  amount  of  work  obtainable  is  not  a  simple  mechanical 
problem  as  it  is  with  the  excised  muscle. 

Influence  of  the  Form  of  the  Muscle  on  its  Working  Power. 
The  amount  of  work  that  any  muscle  can  do  depends  of  course 
largely  upon  its  physiological  state;  a  healthy  well-nourished 
muscle  can  do  more  than  a  diseased  or  starved  one;  but  allowing 
for  such  variations  the  work  which  can  be  done  by  a  muscle  varies 
with  its  form.  The  thicker  the  muscle,  that  is  the  greater  the 
number  of  fibers  present  in  a  section  made  across  the  long  axes 
of  the  fasciculi,  the  greater  the  load  that  can  be  lifted  or  the  other 
resistance  that  can  be  overcome.  On  the  other  hand,  the  extent, 
through  which  a  muscle  can  move  a  weight  increases  with  the 
length  of  its  fasciculi.  A  muscle  a  foot  in  length  can  contract 
more  than  a  muscle  six  inches  long,  and  so  would  move  a  bone 
through  a  greater  distance,  provided  the  resistance  were  not  too 
great  for  its  strength.  But  if  the  shorter  muscle  had  double  the 
thickness,  then  it  could  lift  twice  the  weight  that  the  longer 
muscle  could.  We  find  in  the  Body  muscles  .constructed  on  both 
plans;  some  to  have  a  great  range  of  movement,  others  to  over- 
come great  resistance,  besides  numerous  intermediate  forms 
which  cannot  be  called  either  long  and  slender  or  short  and  thick; 
many  short  muscles  for  example  are  not  specially  thick,  but  are 
short  merely  because  the  parts  on  which  they  act  lie  neap. to 


100  THE  HUMAN  BODY 

gether.  It  must  be  borne  in  mind,  too,  that  many  apparently 
long  muscles  are  really  short  stout  ones — those  namely  in  which 
a  tendon  runs  down  the  side  or  middle  of  the  muscle,  and  has 
the  fibers  inserted  obliquely  into  it.  The  muscle  (gastrocnemius) 
in  the  calf  of  the  leg,  for  instance  (Fig.  40,  B),  is  really  a  short  stout 
muscle,  for  its  working  length  depends  on  the  length  of  its  fasciculi 
and  these  are  short  and  oblique,  while  its  true  cross-section  is  that 
at  right  angles  to  the  fasciculi  and  is  considerable.  The  force 
with  which  a  muscle  can  shorten  is  very  great.  A  frog's  muscle 
of  1  square  centimeter  (0.39  inch)  in  section  can  just  lift  2,800 
grams  (98.5  ounces),  and  a  human  muscle  of  the  same  area  more 
than  twice  as  much. 

The  Beneficial  Effect  of  Exercise.  An  interesting  fact  about 
skeletal  muscle,  that  is  in  the  experience  of  every  athlete,  and  can 
also  be  demonstrated  upon  an  isolated  muscle,  is  that  the  response 
to  stimulation  after  a  period  of  inaction  is  less  vigorous  than  the 
response  to  precisely  the  same  amount  of  stimulation  after  the 
muscle  has  been  exercised  for  a  while.  This  fact  explains  the  ne- 
cessity under  which  base  ball  pitchers  and  other  athletes  labor  of 
" warming  up"  before  they  can  use  their  muscles  effectively.  In 
the  case  of  an  isolated  muscle  stimulated  at  regular  intervals  the 
effect  is  seen  in  a  well-marked  increase  in  the  height  of  contraction 
during  the  first  dozen  or  so  of  the  series.  This  increase  from  con- 
traction to  contraction  is  often  very  regular,  suggesting  a  flight  of 
stairs.  For  this  reason  it  is  often  spoken  of  as  the  "staircase  phe- 
nomenon." 

Fatigue  of  Muscle — Contracture.  If  an  isolated  muscle  is  sub- 
jected to  a  series  of  stimulations  at  fairly  frequent  intervals — one 
a  second  or  oftener — the  period  of  the  " staircase"  described  above, 
is  usually  followed  by  a  period  in  which  the  relaxation  of  the  muscle 
is  definitely  slowed.  So  far  as  can  be  seen  the  muscle  contracts 
as  rapidly  and  forcibly  as  ever,  but  the  relaxations  are  drawn  out. 
The  effect  of  this  in  a  regular  series  of  stimulations  is  that  the 
muscle  fails  to  relax  completely  from  one  stimulation  by  the  time 
the  next  one  is  sent  in,  so  that  it  continues  in  a  state  of  partial 
contraction  during  the  intervals  between  stimulations.  This 
condition  is  called  contradure.  It  consists  essentially  of  a  slowing 
down  of  the  relaxation  rate,  and  is  the  first  indication  of  the  con- 
cjit  on  of  impairment  that  we  call  fatigue.  Aside  from  its  signifi- 


MUSCULAR  ACTIVITY  101 

cance  as  a  phase  of  fatigue,  contracture  is  important  as  proving 
that  the  relaxation  of  muscle  is  not  a  mere  passive  falling  back 
from  the  contracted  state,  for  if  relaxation  were  just  that  the  rate 
of  the  falling  back  should  be  the  same  at  one  time  as  at  another. 
Since  the  relaxation  rate  varies,  becoming  slower  at  the  beginning 
of  fatigue,  there  must  be  a  definite  relaxation  process  and  we  are 
bound,  in  our  analysis  of  the  muscle  machine  to  take  the  relaxa- 
tion process  into  account  as  well  as  the  contraction  process  itself. 

If  the  recurrent  stimulation  of  the  isolated  muscle  is  continued 
beyond  the  phase  of  contracture  there  soon  develops  the  phase 
which  accords  more  fully  with  our  ordinary  conception  of  fatigue. 
The  muscle  contracts  more  and  more  feebly  until  finally  it  refuses 
to  respond  at  all  to  stimulation.  After  this  stage  is  reached  a 
rest  of  a  few  minutes  often  suffices  to  restore  the  muscle  to  a  con- 
dition in  which  it  will  show  a  considerable  degree  of  activity,  al- 
though usually  not  so  much  as  it  exhibited  when  fresh. 

The  Nature  of  Fatigue.  We  can  understand  why  muscles  be- 
come fatigued  if  .we  recall  the  fact  that  the  muscle  is  a  chemical 
engine.  The*  energy  for  contraction  is  furnished  by  chemical 
transformations  within  the  muscle  whereby  substances  containing 
large  amounts  of  energy  enter  combinations  of  less  energy  value, 
and  liberate  the  surplus  energy  for  the  use  of  the  muscle.  These 
resulting  compounds,  of  low  energy  value,  are  waste  products. 
Their  relation  to  the  muscle  is  that  of  the  ashes  to  the  furnace. 
Unless  they  are  gotten  rid  of  they  interfere  with  further  activity. 
This  hampering  of  a  chemical  process  by  its  own  products  is  a  well- 
known  principle  of  chemistry.  To  avoid  the  effect  the  products 
should  be  removed  as  fast  as  they  are  formed.  In  the  body  the 
agency  for  removing  waste  products  from  the  muscles  is  the  blood- 
stream flowing  through  them.  Under  ordinary  circumstances 
this  is  sufficient,  so  that  muscular  fatigue  is  not  commonly  felt. 
In  isolated  muscles,  however,  there  is  no  stream  of  blood.  The 
only  way  in  which  waste  products  can  be  gotten  rid  of  is  by  the 
slow  process  of  dialysis  from  the  fibers  into  the  surrounding  lymph. 
Fatigue  comes  on  rather  quickly,  therefore,  in  isolated  muscles 
that  are  stimulated  repeatedly. 

The  reader  should  be  cautioned  at  this  point  against  attempting 
to  apply  the  description  of  fatigue  just  given  to  all  his  own  expe- 
riences of  weariness.  The  fatigue  of  isolated  muscles  is  muscular 


102  THE  HUMAN  BODY 

fatigue,  and  its  study  is  exceedingly  instructive  in  aiding  us  to 
analyze  the  workings  of  the  muscle  machine,  but  it  is  not  the  type 
of  fatigue  that  human  beings  most  commonly  feel.  Our  muscles 
are,  to  be  sure,  so  much  like  the  muscles  of  the  frog  that  they 
would,  if  isolated  and  stimulated,  show  similar  fatigue,  but  as 
used  in  the  body  they  are  usually  saved  from  experiencing  serious 
fatigue  by  the  fact  that  the  swiftly  flowing  blood  tends  to  sweep 
out  the  " fatigue  products"  and  prevent  their  accumulation,  and 
also  because  in  the  nervous  system,  by  which  the  muscles  are 
stimulated  to  activity,  there  are  regions  which  become  fatigued 
while  the  muscles  are  yet  in  good  condition.  Only  under  excep- 
tional circumstances,  as  in  athletic  contests,  or  severe  manual 
labor,  when  the  nervous  system  is  driven  far  beyond  its  usual  ac- 
tivity and  the  blood  is  unable  to  remove  the  waste  products  as 
fast  as  they  are  formed,  do  we  experience  genuine  muscular  fatigue. 

The  Response  to  Rapidly  Repeated  Stimuli.  Tetanus.  Since 
a  simple  muscular  contraction  occupies  one-tenth  of  a  second,  it  is 
obvious  that  a  second  stimulus  following  the  first  within  that  in- 
terval will  find  the  muscle  in  a  state,  of  partial  or  complete  contrac- 
tion, depending  on  the  exact  length  of  the  interval.  In  such  a  case 
the  muscle  executes  in  response  to  the  second  stimulus  a  second 
contraction,  which  is  fused  with  the  first.  Similarly  a  third  can 
be  elicited,  which  will  be  fused  with  the  second,  and  so  on.  A 
series  of  such  fused  contractions  is  known  as  a  physiological  tetanus. 
If  the  interval  between  stimuli  does  not  exceed  yV-yV  °f  a  second 
in  fresh  frog's  muscle  the  fusion  is  complete.  That  is,  there  are 
no  signs  of  relaxation  between  stimuli,  and  the  contraction  is  per- 
fectly steady;  An  interesting  fact  about  physiological  tetanus,  or 
tetanic  contraction  as  it  is  often  called,  is  that  the  extent  of  the  con- 
traction may  be  markedly  greater  than  in  a  simple  contraction, 
even  though  the  latter  may  have  been  maximal.  Evidently  the 
so-called  maximal  simple  contraction  is  not  maximal  in  the  sense 
that  it  represents  the  mechanical  limit  of  the  muscle's  power  to 
shorten,  but  only  in  that  it  is  the  utmost  the  muscle  can  do  in 
response  to  a  single  stimulus.  In  two  respects,  then,  the  tetanic 
contraction  differs  from  the  simple  twitch:  in  being  more  pro- 
longed, and  in  being  higher. 

Voluntary  Muscular  Contraction.  In  view  of  the  superiority 
of  the  tetanus  over  the  simple  twitch  it  is  interesting  to  note  that 


MUSCULAR  ACTIVITY  103 

all  voluntary  muscular  movements,  even  the  briefest,  are  tetanic 
in  character.  They  owe  this  special  character  to  a  peculiarity 
of  the  nervous  system,  through  which  the  muscles  are  excited  to 
contract.  When  nerve-cells  discharge  their  impulses  into  muscles 
the  discharge  is  never  a  single  one  but  always  a  series  in  rapid 
succession.  The  result  in  the  muscle  is,  of  course,  a  tetanus. 

The  Electrical  Phenomena  of  Muscle.  When  a  living  muscle 
is  carefully  exposed  and  suitable  electrodes  connected  with  a 
sensitive  galvanometer  or  electrometer  are  applied  to  its  surface 
the  entire  surface  is  found  to  be  isoelectric,  i.  e.,  having  a  uniform 
electric  potential.  If,  however,  an  injury  such  as  cutting  or 
burning  is  inflicted  upon  any  part  of  the  muscle  the  injured  sur- 
face is  found  to  possess  a  different  potential  from  the  surround- 
ing uninjured  surfaces.  This  difference  of  potential  is  shown  by 
movements  of  the  indicator  of  the  galvanometer  or  electrometer. 
These  movements  are  usually  in  such  a  direction  as  to  indicate 
that  the  injured  region  has  a  lower  potential  than  uninjured  parts 
of  the  same  tissue.  This  difference  of  potential  existing  between 
injured  and  uninjured  living  tissue  is  often  referred  to  as  the 
current  of  injury,  although  no  current  actually  flows  unless  the 
two  regions  are  connected  by  an  electrical  conductor.  No  cur- 
rent of  injury  can  be  obtained  by  connecting  living  tissue  with 
dead  tissue.  Only  while  the  injured  tissue  is  in  act  of  dying  does 
it  exhibit  the  altered  potential  which  may  give  rise  to  an  injury 
current. 

The  explanation  of  the  change  of  electric  potential  accompany- 
ing an  injury  to  living  tissue  is  found  in  the  fact  that  the  death 
process  which  follows  injury  involves  extensive  chemical  changes 
in  the  tissue.  This  disturbance  in  chemical  relationship  brings 
about  corresponding  disturbance  in  the  electric  equilibrium  which 
finds  expression  in  an  altered  electric  potential  in  the  part  where 
the  chemical  activity  is  going  on. 

Just  as  the  chemical  changes  which  follow  injury  to  the  tissue 
give  rise  to  the  change  of  electrical  potential  which  we  call  the 
current  of  injury,  so  the  chemical  changes  which  accompany 
normal  activity  in  the  tissue  give  rise  to  electrical  changes  which 
are  designated  currents  of  action.  Action  currents  cannot  easily 
be  demonstrated  in  an  ordinary  contracting  muscle  because  the 
whole  muscle  goes  into  contraction  at  once  and  so  the  electric 


104  THE  HUMAN  BODY 

potential  of  its  entire  surface  rises  and  falls  uniformly.  In  the 
heart  we  have  a  muscle,  however,  which  does  not  contract  all  at 
once,  the  contraction  sweeping  over  it  from  base  to  apex.  The 
action  currents  of  the  heart,  therefore,  can  be  demonstrated  with- 
out difficulty  if  the  apparatus  used  for  detecting  them  is  able  to 
respond  quickly  enough  to  recurrent  changes  of  potential  in 
opposite  directions.  Delicate  galvanometers  have  been  devised 
which  answer  admirably  for  the  purpose.  Another  interesting 
method  of  demonstrating  the  action  currents  of  the  heart  is  by 
causing  them  to  act  as  stimuli  for  an  irritable  tissue.  If  in  a  re- 
cently killed  frog  the  sciatic  nerve  is  dissected  out  as  far  as  the  knee 
and  cut  away  from  its  connection  with  the  spinal  cord,  being  left 
in  connection  with  the  leg  below,  and  if  this  nerve  is  laid  on  the 
exposed  beating  heart  of  the  same  frog  or  some  other  recently 
killed  animal,  often  the  muscles  of  the  lower  leg  and  foot  which 
are  connected  with  the  nerve  will  contract  at  each  beat  of  the 
heart.  The  nerve  where  it  lies  on  the  heart  serves  as  a  conductor 
for  the  action  currents  as  they  are  generated  in  the  heart,  and 
the  action  currents  in  turn  stimulate  the  nerve  during  their  flow 
through  it. 

The  Source  of  Muscular  Energy.  In  the  physical  sense  a 
muscle  is  a  machine.  By  this  we  mean  that  whatever  energy  it 
gives  out  must  have  been  supplied  to  it  previously  from  the 
outside.  The  work  which  a  muscle  does  in  contracting  is  at  the 
expense  of  its  available  store  of  energy.  We  know  that  the 
energy  exhibited  by  a  steam-engine  is  derived  from  the  combus- 
tion or  oxidation  of  the  fuel  under  the  boiler.  We  know  also  that 
the  energy  exhibited  by  a  contracting  muscle  is  derived  from 
the  oxidation  of  fuel  substances  within  it.  The  physical  accom- 
paniments of  oxidation  are  not  the  same  in  the  two  cases;  the 
fuel  wnder  the  boiler  burns  with  flame  and  at  a  high  temperature; 
the  fuel  substance  within  the  muscle  burns  without  flame  and  at 
a  temperature  only  slightly  higher  than  that  of  the  body.  The 
energy  yield,  however,  for  corresponding  amounts  of  fuel  is  as 
great  in  one  case  as  in  the  other. 

The  fuel  substances  used  in  the  Body  are  fhiefly  dextrose  (grape 
sugar),  or  the  closely  related  substance  glycogen,  and  fats.  That 
the  third  group  of  energy  yielding  foods,  the  proteins,  are  not  or- 
dinarily used  as  fuel  for  the  muscles  was  proven  by  a  very  inter- 


MUSCULAR  ACTIVITY  105 

esting  experiment  which  stands  as  one  of  the  classical  experiments 
in  Physiology.  The  proof  rests  upon  the  fact  that  the  decomposi- 
tion of  proteins  gives  rise  to  compounds  which  contain  nitrogen 
and  which  are  discharged  from  the  Body,  except  for  a  negligible 
residue,  by  way  of  this  kidneys.  If  the  urine  is  collected  and  its 
nitrogen  content  determined,  the  amount  of  protein  decomposition 
that  has  occurred  in  the  Body  can  be  calculated  (see  table,  p.  11). 
Two  German  Physiologists,  Fick  and  Wislicenus,  determined 
their  average  daily  loss  of  nitrogen  over  a  period  of  several  days  of 
relative  inactivity  and  then  engaged  in  a  day  of  exceptionally 
vigorous  muscular  exercise.  The  form  of  activity  chosen  was 
mountain  climbing;  the  mountain  ascended  was  the  Faulhorn  in 
the  Alps,  1,956  meters  (6,000  feet)  high.  In  spite  of  the  very 
great  increase  in  the  amount  of  muscular  energy  manifested,  and 
the  consequent  great  increase  in  the  amount  of  fuel  consumed,  the 
total  loss  of  nitrogen  from  the  Body  was  virtually  the  same  as  on 
the  previous  days  of  inactivity.  This  experiment,  which  has  been 
repeated  and  confirmed  many  times,  shows  that  proteins  are  not 
ordinarily  used  by  the  muscles  as  fuel.  Therefore  the  other  energy 
yielding  foods  must  serve.  (See  also  Chap.  XXX). 

While  fats  are  excellent  fuel  foods,  and  are,  in  all  likelihood,  used 
by  the  muscles  when  brought  to  them  by  the  blood,  there  is  abun- 
dant evidence  that  the  muscles  can  get  along  without  fats  provided 
they  have  enough  fuel  in  the  form  of  sugar.  This  evidence  •  is 
found  in  the  experience  of  grazing  animals  which  may  never  after 
weaning  have  a  particle  of  fat  in  their  food  and  which,  nevertheless, 
are  able  to  use  their  muscles  to  the  very  best  advantage.  When 
dextrose  or  fats  are  burned  the  products  of  the  oxidation  are  car- 
bon dioxid  and  water.  The  reaction  in  the  case  of  dextrose  is 
represented  by  the  equation  C6  Hi2  O6+6  O2=6  CO2+6  H2  O. 
That  one  or  the  other  of  the  fuel  substances  mentioned  above  is  ox- 
idized during  muscular  activity  is  shown  by  an  increase  in  the  amount 
of  carbon  dioxid  produced  in  the  Body  and  breathed  out  from  the 
lungs.  By  a  comparatively  simple  device  the  amount  of  carbon 
dioxid  breathed  out  per  minute  can  be  determined,  and  there  is 
invariably  a  pronounced  increase  during  and  immediately  follow- 
ing muscular  exercise.  If  the  exercise  is  sharp  the  increase  may  be 
7  or  8  fold.  There  is,  of  course,  an  equivalent  increase  in  water 
production,  but  the  water  so  produced  merely  adds  itself  to  the 


106  THE  HUMAN  BODY     . 

abundant  water  already  present  in  the  Body  and  cannot  be  iden- 
tified as  can  the  gaseous  carbon  dioxid. 

The  Chemistry  of  Muscular  Contraction.  In  addition  to  the 
fact  just  stated,  that  there  is  oxidation  of  fuel  substances,  with 
production  of  carbon  dioxid  and  water,  about  the  only  definite 
chemical  process  we  know  to  be  associated  with  muscular  activity 
is  the  production  of  lactic  acid  (p.  16).  Lactic  acid  is  chemically 
closely  related  to  dextrose  in  that  one  molecule  of  dextrose  can 
be  split  into  two  molecules  of  lactic  acid  with  no  residue.  The  re- 
lationship is  expressed  by  the  equation  C6  Hi2  O  c=2  C3  H6  O3,  the 
latter  symbol  being  that  of  lactic  acid.  For  a  long  time  the  lactic 
acid  that  appears  in  active  muscles  was  supposed  to  be  merely  a 
stage  in  the  oxidation  of  dextrose,  and  its  invariable  appearance 
was  taken  to  mean  that  dextrose  is  the  only  fuel  that  muscles  are 
able  to  use.  This  conception  involves  the  view  that  before  muscles 
can  use  fats  as  fuel  the  fats  must  be  converted  into  dextrose. 
Chemically  the  conversion  of  fat  into  sugar  is  exceedingly  difficult, 
and  it  seemed  a  very  remarkable  thing  that  a  chemical  transforma- 
tion so  hard  to  bring  about  in  the  laboratory  should  occur  con- 
stantly in  the  Body.  As  a  result  of  this  difficulty  a  very  celebrated 
controversy  has  arisen  in  Physiology  over  the  question  of  whether 
or  not  fats  are  converted  into  sugar  in  the  Body  before  being  utilized. 

Of  recent  years  evidence  has  been  accumulating  that  the  lactic 
acid  which  appears  in  active  muscles  is  not  a  mere  incidental  stage 
in  the  transformation  of  dextrose,  but  an  essential  feature  of  the 
contraction  process.  In  fact  the  view  held  at  present  by  many 
physiologists  is  that  lactic  acid  is  associated  in  intimate  fashion 
with  the  mechanical  act  of  shortening,  but  is  not  involved  at  all  in 
the  chemical  processes  by  which  the  energy  for  the  contraction  is 
obtained.  In  other  words,  lactic  acid  is  not  a  fuel  substance,  and 
the  fact  that  it  can  be  obtained  from  sugar  by  a  simple  splitting  of 
the  molecule  does  not  prove  that  it  is  so  derived  in  the  muscle. 

The  importance  of  this  newer  conception  is  that  it  releases  us 
from  the  necessity  "of  supposing  sugar  to  be  the  only  possible  fuel 
for  muscles.  The  question  of  whether  or  not  fats  are  transformed 
into  sugar  in  the  Body,  instead  of  being  absolutely  fundamental, 
becomes  an  interesting  problem  in  Biological  Chemistry. 

The  Energy  Relationships  of  Contracting  Muscle.  Muscular 
Efficiency.  Since  the  muscle  is  an  engine  for  the  conversion  of 


MUSCULAR  ACTIVITY  107 

one  kind  of  energy  (chemical)  into  another  kind  (mechanical)  its 
energy  relationships  can  be  studied  in  the  same  manner  as  in  other 
sorts  of  engines.  We  are  familiar  with  various  classes  of  these.  A 
steam  power  plant  and  an  automobile  fall  in  the  same  group  with 
muscles  in  that  they  are  devices  for  transforming  chemical  energy 
(oxidation  of  'fuel)  into  mechanical.  A  hydro-electric  installation 
converts  the  mechanical  energy  of  the  water-fall  into  electrical 
energy.  An  ordinary  dry  cell  is  an  engine  for  the  conversion  of 
chemical  energy  into  electrical,  and  a  motor  for  the  conversion  of 
electrical  energy  into  mechanical.  All  these  engines  have  in 
common  the  feature  of  relative  inefficiency.  When  we  speak  of 
the  efficiency  of  an  engine  we  mean  the  ratio  of  the  amount  of 
useful  energy  it  gives  out  to  the  total  amount  it  uses  up.  No  en- 
gine is  100  per  cent  efficient;  in  none  can  all  the  energy  put  in  be 
recovered  in  available  form.  There  is  always  a  fraction  of  the 
total  energy  which  manifests  itself  in  the  form  of  heat.  The  hot 
flue  gases  from  the  furnace,  the  hot  bearings  on  the  locomotive, 
the  hot  water  in  the  cooling  system  of  the  automobile;  all  these 
signify  energy  which  from  the  standpoint  of  the  machine  is  wasted. 
Muscles  share  with  other  engines  this  feature  of  inefficiency.  As 
used  in  the  Body  the  muscles  are  only  about  20  per  cent  efficient. 
That  is,  in  order  to  do  a  given  amount  of  muscular  work  we  must 
burn  in  our  bodies  enough  fuel  to  give  a  total  energy  output  five 
times  as  great.  The  balance,  of  80  per  cent,  takes  the  form  of 
heat,  and  explains  why  we  find  ourselves  so  warm  after  vigorous 
exercise.  Isolated  muscles  under  favorable  circumstances  may 
show  an  efficiency  of  nearly  50  per  cent. 

Energy  Units.  In  order  to  be  able  to  discuss  energy  relation- 
ships 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  seen,  heat.  A  convenient 
energy  unit,  then,  is  the  heat  unit.  The  amount  of  heat  required 
to  raise  the  temperature  of  1  gram  (~^  oz.)  of  water  1  degree 
centigrade  (strictly  from  zero  to  1°)  is  taken  as  the  unit.  This  is 
known  as  the  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  large 
Calorie,  distinguished  from  the  small  calorie  by  the  use  of  the 
capital  initial.  Although  the  calorie  is  strictly  a  heat  unit  it  serves 


108  THE  HUMAN  BODY 

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. 

When  the  energy  is  manifested  as  mechanical  work  it  is  meas- 
ured in  terms  of  the  work  done.  Since  the  simplest  form  of  work 
is  probably  the  raising  of  weights  against  the  resistance  of  gravity 
the  units  of  work  are  based  on  weight  and  height.  Thus  the 
fool-pound  is  the  amount  of  energy  involved  in  raising  a  weight  of 
one  pound  to  a  height  of  one  foot.  A  calorie  is  approximately 
equivalent  to  3  foot-pounds.  In  the  metric  system  the  unit  of 
mechanical  energy  is  the  gram-centimeter.  In  round  numbers 
41,000  gram-centimeters  represent  the  same  amount  of  energy  as  1 
calorie. 

The  Energy  Output  of  Muscle.  Studies  of  the  mechanical 
energy  developed  by  selected  groups  of  muscles  in  the  Body  can 
be  made  directly .  An  excellent  method  is  by  means  of  a  station- 
ary bicycle.  The  wheel  can  be  made  to  revolve  against  a  measured 
resistance,  and  thus  the  work  done  can  be  readily  determined.  As 
already  stated,  muscles  in  the  Body  work  very  inefficiently.  This 
is  in  part  due  to  the  less  favorable  conditions  of  energy  liberation 
in  the  physiological  tetanus  as  compared  with  the  simple  twitch, 
but  more  because  of  the  mechanical  disadvantages  at  which 
muscles  work.  They  pull  at  the  short  arms  of  levers,  and  the 
direction  of  their  pull  is  usually  oblique  (Chap.  VIII).  The  energy 
output  of  an  isolated  muscle  is  also  easy  to  determine,  although 
to  be  certain  that  the  utmost  possible  has  been  obtained  is  not  so 
simple.  The  muscle  operates  by  forcible  shortening.  That  means 
that  the  muscle  pulls  upon  the  weight  to  which  it  is  attached.  If 
the  tension  developed  is  greater  than  the  weight  the  latter  is  lifted 
and  work  is  done.  If  the  tension  is  insufficient  to  raise  the  weight, 
there  is  no  manifestation  of  mechanical  energy.  All  the  energy 
liberated  takes  the  form  of  heat.  It  has  been  found  that  in  a 
given  muscle  under  given  conditions  the  tension  developed  is  fairly 
constant  and  directly  proportional  to  the  total  energy  manifested 
in  the  contraction;  whereas  the  actual  mechanical  work  done  de- 
pends, as  we  have  already  seen  (p.  98)  on  various  factors,  such  as 
the  relation  of  the  load  to  the  absolute  strength  of  the  muscle.  If 


MUSCULAR  ACTIVITY  109 

the  most  favorable  load  is  selected  about  half  the  energy  of  the 
contraction  may,  as  we  learned  above  (p.  107),  appear  as  mechani- 
cal work. 

In  the  study  of  the  muscle  as  an  engine  knowledge  of  the  energy 
relationships  is  of  the  greatest  moment.  We  know  that  muscular 
activity  is  based  on  chemical  transformations,  and  to  explain  the 
actions  of  the  muscle  we  must  know  what  these  chemical  trans- 
formations are.  If  we  assume  any  particular  transformation  to  be 
one  from  which  the  muscle  derives  its  energy  we  must  be  able  to 
show  that  that  transformation  yields  the  amount  of  energy  which 
the  muscle  actually  manifests.  If  it  does  not  do  so  the  assump- 
tion which  selected  it  as  the  source  of  the  muscle's  power  is  ob- 
viously erroneous. 

There  are  two  facts  of  the  chemistry  of  muscular  activity  which 
are  fully  demonstrated  and  which  have  already  been  stated.  These 
are  the  oxidation  of  sugar  or  fat  as  fuel,  and  the  production  of  lactic 
acid  as  an  essential  feature  of  the  contraction  process.  Reference 
has  already  been  made  to  the  view  formerly  held  that  the  lactic 
acid  represents  merely  a  stage  in  the  oxidation  of  sugar,  and  to  the 
replacement  of  that  view  by  the  present  one.  The  evidence  upon 
which  is  based  this  new  recognition  of  the  part  played  by  lactic 
acid  rests  in  large  part  upon  consideration  of  the  energy  relation- 
ships. 

When  a  skeletal  muscle  is  exposed  to  the  vapor  of  chloroform 
it  passes  into  a  condition  of  pronounced  rigor.  The  strong  con- 
traction of  rigor  is  the  result  of  the  production  of  lactic  acid 
within  the  muscle.  It  differs  chemically  from  ordinary  contrac- 
tions of  the  living  muscle  in  that  the  production  of  acid  goes  on 
without  concurrent  oxidation  of  fuel,  and  in  the  fact  that  the  lactic 
acid  produced  is  not  immediately  removed.  Mechanically,  as  al- 
ready pointed  out,  the  rigor  differs  from  ordinary  contraction  in  its 
persistence.  The  rigor  contraction  may  last  for  hours,  whereas 
the  ordinary  contraction  ends  with  the  cessation  of  stimulation.  If 
.  the  muscle  that  is  to  be  subjected  to  the  action  of  the  chloroform 
is  so  firmly  fixed  at  both  ends  that  no  actual  shortening  can  occur 
all  the  energy  of  the  rigor  contraction  appears  in  the  form  of  heat 
and  can  be  measured  as  such.  Because  there  is  no  concurrent 
oxidation  all  the  energy  thus  manifested  must  be  derived  directly 
from  the  chemical  transformation  which  gives  rise  to  the  lactic 


110  THE  HUMAN  BODY 

acid.  The  amount  of  energy  thus  manifested  has  been  found  to 
equal  about  1.3  calories  for  each  gram  of  muscle.  By  chemical 
analysis  the  amount  of  lactic  acid  produced  can  be  measured.  It 
does  not  exceed  0.004  gram  for  each  gram  of  muscle.  The  pro- 
duction of  0.004  gram  lactic  acid  has  given  rise,  then,  to  1.3  calories 
of  energy.  The  conversion  of  this  amount  of  sugar  to  lactic  acid, 
however,  yields  only  0.43  calories,  or  only  one-third  the  amount  of 
energy  actually  liberated.  Evidently  the  transformation  which 
actually  occurs,  the  result  of  which  is  the  production  of  0.004  gm. 
lactic  acid  and  1.3  calories  of  energy  for  each  gram  of  muscle,  is 
some  other  than  the  direct  conversion  of  sugar  into  acid. 

These  facts,  which  hold  for  the  energy  manifested  during  the 
contraction  of  rigor,  are  true  also  for  the  ordinary  contractions  of 
muscle  except  in  so  far  as  the  latter  are  attended  by  oxidation 
processes,  which  liberate  additional  energy  and  complicate  the 
determinations.  If,  however,  oxidation  be  prevented,  as  can  be 
done  by  surrounding  the  muscle  with  an  atmosphere  of  pure  nitro- 
gen, substantially  the  same  relationship  between  lactic  acid  pro- 
duction and  energy  manifestation  appears  as  in  rigor.  A  curious 
and  important  feature  of  the  energy  liberation  of  an  ordinary  con- 
traction is  that  the  oxidation  that  takes  place  in  connection  with 
the  contraction  accompanies,  not  the  contraction  phase  proper, 
but  the  relaxation  phase.  For  this  reason  it  has  been  called  the 
"  recovery  oxidation."  This  oxidation,  as  stated  previously,  is 
the  ultimate  source  of  the  energy  shown  by  the  contracting  muscle. 
How  are  we  to  connect  this  chemical  process,  occurring  after  the 
contraction  is  well  under  way-,  or  even  after  it  is  over,  with  the 
energy  shown  during  the  contraction  itself?  The  analogy  that 
has  been  suggested,  and  that  corresponds  with  the  facts  so  far  as 
we  know  them,  is  that  of  the  pile  driver.  In  this  machine  the 
energy  of  the  burning  fuel  under  the  boiler  is  used  in  raising  the 
weight  to  the  top  of  the  derrick.  When  the  trip  is  operated  the 
weight  falls  and  by  the  energy  of  its  impact  does  the  work  for 
which  the  machine  is  designed.  In  our  analogy  the  production 
of  lactic  acid  corrresponds  to  the  fall  of  the  weight.  The  stimulus 
which  excites  the  muscle  is  represented  by  the  operation  of  the 
trip  which  releases  the  weight.  The  process,  which,  in  the  muscle, 
is  analogous  to  the  raising  of  the  weight,  is  pictured  as  the  forma- 
tion of  a  substance  which  is  decomposed  into  lactic  acid  under  the 


MUSCULAR  ACTIVITY  111 

influence  of  the  stimulus  and  which  in  connection  with  this  de- 
composition yields  the  amount  of  energy  manifested  by  the  con- 
traction. The  substance  so  pictured  has  not  been  demonstrated 
chemically.  For  lack  of  a  definite  name  it  has  been  called  the 
" lactic  acid  precursor"  to  indicate  its  position  as  the  energy-yield- 
ing antecedent  substance  to  lactic  acid.  Since  this  substance 
must  contain  more  energy  than  sugar,  or  it  would  not  by  its  de- 
composition into  lactic  acid  yield  enough  energy,  it  cannot  be  one 
of  the  fuel  substances,  but  must  be  built  up  within  the  muscle  at 
the  expense  of  energy  furnished  by  the  fuel.  If  so  built  up  it  is 
not  necessary  that  the  oxidation  of  fuel  occur  in  immediate  con- 
nection with  the  contraction  process,  since  all  the  oxidation  has  to 
do  is  to  provide  energy  by  which  a  supply  of  precursor  is  kept  on 
hand.  The  muscle  is  then  ready  to  respond  whenever  stimula- 
tion occurs. 

To  complete  the  picture  we  must  account  for  the  mechanical  act 
of  relaxation.  Since  contraction  depends  on  the  production  of 
lactic  acid  in  the  muscle  relaxation  necessarily  involves  its  disap- 
pearance. That  the  removal  of  lactic  acid  is  a  definite  process 
is  shown  by  the  persistent  contraction  of  rigor,  which  is  due  to  the 
loss  in  the  dead  muscle  of  the  means  for  getting  rid  of  the  acid. 
The  immediate  discharge  of  lactic  acid  in  living  muscles  from  the 
contractile  elements  is  in  all  probability  a  simple  outward  diffusion 
from  sarcostyles  into  sarcoplasm.  Under  ordinary,  conditions 
the  relaxation  is  too  rapid  to  suggest  a  more  complicated  action. 
Unless  the  lactic  acid  is  removed  from  the  sarcoplasm,  however, 
equilibrium  will  soon  be  reached  between  it  and  the  sarcostyles 
and  further  diffusion  will  be  impossible.  There  are  at  least  three 
ways  in  which  lactic  acid  might  be  removed.  The  simplest  one, 
chemically,  is  by  reacting  with  the  alkaline  salt  of  the  muscle, 
sodium  carbonate,  to  form  neutral  sodium  lactate.  A  second 
possible  method  is  by  oxidation  with  the  formation  of  carbon 
dioxid  and  water  and  with  the  liberation  of  much  energy  that  would 
be  available  for  the  building  up  of  the  precursor.  A  third  method, 
suggested  by  the  analogy  of  the  pile  driver  above,  is  the  rebuilding 
of  the  acid  itself  into  the  precursor  from  which  it  was  originally 
derived.  The  energy  of  the  burning  fuel  might  as  welt  be  devoted 
to  the  reconversion  of  lactic  acid  into  precursor  as  to  the  building 
up  of  the  precursor  from  some  other  substance  than  lactic  acid, 


112  THE  HUMAN  BODY 

and  if  so  devoted  would  have  the  additional  advantage  of  caring 
for  the  removal  of  the  acid.  That  either  the  second  or  third  of 
these  possibilities  represents  the  method  ordinarily  operating  in 
the  muscle  seems  certain.  Either  of  them  would  satisfy  the 
known  energy  relationships  in  most  respects.  There  is,  however, 
one  fact  that  seems  to  favor  the  replacement  theory  as  against  the 
oxidation  theory.  This  is  that  if  all  the  lactic  acid  produced  in  a 
simple  contraction  were  oxidized  in  connection  with  the  relaxation 
the  total  energy  liberation  would  be  several  times  that  which  ac- 
tually occurs.  This  fact  seems  to  identify  the  oxidation  that  does 
occur  with  the  production  of  enough  energy  for  the  manufacture 
of  the  precursor,  rather  than  with  the  removal  of  the  lactic  acid, 
and  strengthens  the  view  that  the  acid  is  removed  by  being  rebuilt 
into  precursor. 

Under  conditions  of  extreme  muscular  activity  the  blood  is  not 
able  to  deliver  oxygen  to  the  tissues  fast  enough  to  enable  the 
oxidations  by  which  lactic  acid  is  removed  to  keep  up  with  the 
production  of  the  acid.  There  is,  therefore,  under  these  circum- 
stances, an  excess  of  lactic  acid  which  must  be  removed  if  normal 
activity  is  to  continue.  In  this  situation  the  method  of  removal 
suggested  first  above  comes  into  play,  namely,  the  neutralization 
of  the  acid  by  sodium  carbonate.  The  sodium  lactate  thus 
formed  escapes  from  the  muscles  into  the  blood,  is  carried  with 
the  blood  stream  to  the  kidneys,  and  there  eliminated.  The 
proof  of  this  lagging  behind  of  the  oxidations  when  the  exercise 
is  very  vigorous  is  furnished  by  the  appearance  of  considerable 
sodium  lactate  in  the  urine  of  persons  who  have  recently  undergone 
violent  exertion.  As  compared  with  the  removal  of  acid  by 
oxidation  this  is  evidently  a  wasteful  process.  The  acid  which 
escapes  in  combination  with  sodium  is  no  longer  available  for 
reconstruction  into  precursor,  and  some  other  of  the  constituents 
of  muscle  must  be  used  in  its  stead. 

Significance  of  Lactic  Acid  in  the  Contraction  Process.  The 
relationship  of  lactic  acid  to  the  liberation  of  energy  in  muscle  has 
been  discussed  in  detail.  There  remains  for  consideration  the 
manner  in  which  the  presence  of  the  acid  in  the  muscle  brings 
about  the  mechanical  act  of  contraction. 

The  most  satisfactory  explanation  of  this  mechanism  thus  far 
suggested  is  based  upon  a  property  possessed  by  colloids  of  show- 


MUSCULAR  ACTIVITY 


113 


ing  when  acidified  a  greater  affinity  for  water  than  they  show 
when  their  reaction  is  neutral.  The  cylindrical  colloidal  sarco- 
styles which  make  up  the  actual  contractile  elements  of  muscle 
are  surrounded  by  watery  sarcoplasm  (p.  88).  If  these  sarco- 
styles  are  suddenly  acidified  we  can  picture  a  rush  of  water  into 
them  from  the  surrounding  sarcoplasm,  which  would  cause  them 
to  swell. 

Studies  of  the  contraction  process  in  the  individual  sarcostyles 
of  insects'  wing  muscles  (p.  85)  show  that  during  contraction  they 
present  a  beaded  appearance.  This  beading 
could  be  brought  about  by  a  swelling  of  the 
segments  of  which  the  sarcostyles  are  composed, 
provided  the  membranes  between  the  segments 
remain  undistended  (Fig.  48).  That  such  a 
swelling  might  cause  a  forcible  shortening  can 
be  proven  with  the  aid  of  a  suitable  model.  A 
similar  swelling  of  the  sarcostyles  appears  to 
occur  during  the  contraction  of  skeletal  muscle. 
If  microscopic  cross-sections  of  relaxed  and  con- 
tracted skeletal  muscle  are  compared  the  sarco- 
styles of  the  relaxed  specimen  are  seen  to  be 

smaller  and  further  apart  than  are  those  of  the  of  sarcostyles  of  in- 
sects   wing  muscles. 

contracted  one.  This  suggests  a  transfer  of  sarco-  A,  relaxed;  B,  con- 
plasm  from  the  spaces  between  the  sarcostyles  t] 
into  the  sarcostyles  themselves.  When. we  recall  the  minute  size 
of  the  elements  and  the  correspondingly  short  distances  through 
which  any  given  particles  of  fluid  would  have  to  pass  we  can  under- 
stand without  difficulty  how  the  contraction  is  able  to  occur  in  so 
small  a  fraction  of  a  second. 

Summary  of  the  Contraction  Process.  The  mechanism  of  con- 
traction of  skeletal  muscle  will  be  more  readily  grasped  as  a  whole, 
perhaps,  if  summarized  briefly.  In  the  resting  muscle  there  has 
been  some  oxidation  of  fuel  which  has  furnished  energy  for  the 
building  up  of  a  substance  of  high  energy  value,  the  "lactic  acid 
precursor."  Stimulation  of  the  muscle  causes  decomposition  of 
some  of  this  precursor  into  lactic  acid  with  the  liberation  of  energy. 
This  energy  is  made  available  for  the  act  of  contraction  through 
the  property  the  colloidal  sarcostyles  have  of  absorbing  water 
when  acidified.  The  forcible  absorption  of  water  from  the  sur- 


B 

FIG.  48.— Diagrams 


114  THE  HUMAN  BODY 

rounding  sarcoplasm  brings  about  a  swelling  of  the  sarcostyles, 
which,  by  virtue  of  their  peculiar  segmental  structure,  results  in 
turn  in  a  forcible  contraction.  The  shortening  of  the  whole 
muscle  is  nothing  more  than  the  sum  total  of  the  contractions  of 
the  individual  sarcostyles.  Relaxation  is  brought  about  through 
the  removal  of  the  lactic  acid,  immediately  by  diffusion,  but 
ultimately  by  being  rebuilt  into  precursor  at  the  expense  of  energy 
obtained  through  further  oxidation  of  fuel. 

Oxidation  in  Muscle.  Reference  has  already  been  made  (p.  104) 
to  the  fact  that  in  the  muscle  oxidation  occurs  at  about  the  tem- 
perature of  the  body,  instead  of  at  the  high  temperature  character- 
istic of  ordinary  combustion.  This  is  true  of  all  oxidations  in 
living  cells.  It  is  accomplished  through  the  presence  of  special 
substances  known  as  oxidases,  belonging  to  the  class  of  enzyms 
(p.  14).  These  have  the  property  of  bringing  the  oxygen  and  the 
fuel  into  such  intimate  relationship  that  they  will  combine  chem- 
ically without  first  having  to  be  heated  to  a  high  temperature. 

Hormone  of  Skeletal  Muscle.  A  curious  fact  about  muscle  is 
that  the  oxidation  of  sugar  within  it  is  subject  to  the  control  of  a 
hormone.  Why  this  control  should  exist  is  not  clear,  but  that  it 
does  exist  is  proven  beyond  doubt.  The  hormone  is  secreted  by 
certain  masses  of  cells  which  are  embedded  in  one  of  the  digestive 
glands,  the  pancreas  (p.  460).  The  importance  of  the  hormone  is 
shown  by  the  dire  results  that  follow  its  absence.  A  well-known, 
and  unfortunately  rather  common  disease,  diabetes,  is  caused  by 
the  failure  of  these  cell  masses  in  the  pancreas  to  manufacture 
their  hormone  in  normal  amounts.  The  muscles  thereupon  lose 
in  greater  or  less  degree  the  power  to  utilize  sugar  as  fuel,  and 
suffer,  in  consequence,  more  or  less  serious  impairment  of  function. 
The  unused  sugar  accumulates  in  the  blood  and  is  discharged 
through  the  kidneys,  giving  rise  to  the  most  conspicuous  symptom 
of  the  disease,  sugar  in  the  urine.  This  condition  is  discussed  in 
greater  detail  in  a  later  chapter  (p.  496). 

Physiology  of  Smooth  Muscle.  Smooth  muscle  differs  strik- 
ingly from  skeletal  muscle,  not  only,  as  stated  previously,  in  struc- 
ture, but  also  in  mode  of  action.  Aside  from  the  fact  that  both 
sorts  of  muscle  produce  their  effects  by  contraction  they  have 
almost  no  features  in  common.  Weight  for  weight  smooth  muscle 
is  much  less  powerful  than  skeletal  muscle.  Its  movements  are 


MUSCULAR  ACTIVITY  115 

also  much  slower.  Smooth  muscle  constitutes  the  operating 
machinery  of  the  maintenance  systems  (except  the  respiratory 
system),  and  it  is  as  nicely  adjusted  to  the  special  requirements  of 
these  systems  as  is  skeletal  muscle  to  the  needs  of  external  adapta- 
tion. One  striking  peculiarity  of  smooth  muscle  tissue  is  illus- 
trated by  the  bladder.  This  organ  sometimes  contains  a  large 
amount  of  urine,  at  other  times  there  is  little  or  none  in  it.  When 
the  bladder  is  empty  it  is  shrunken  to  a  fraction  of  its  size  when 
full.  The  muscular  walls  are  distended  or  contracted  as  the  organ 
is  full  or  empty.  These  pronounced  changes  appear  to  be  effected, 
in  part  at  least,  by  rearrangement  of  the  cells  which  make  up  the 
muscle  coats.  When  the  organ  is  distended  there  is  a  smaller 
number  of  layers  of  cells  than  when  it  is  contracted.  Just  how 
this  rearrangement  is  brought  about  is  not  known. 

Another  feature  in  which  smooth  muscle  differs  strikingly  from 
skeletal  is  in  the  tendency  it  often  shows  to  carry  on  spontaneous 
contractions.  Skeletal  muscle,  as  previously  emphasized,  con- 
tracts only  when  subjected  to  definite  stimulation.  Smooth 
muscle,  on  the  other  hand,  often  undergoes  periods  of  rhythmic 
contraction  and  relaxation  when  no  obvious  sources  of  stimulation 
are  present. 

Still  another  peculiarity  of  smooth  muscle  is  its  ability  to  remain 
indefinitely  in  the  contracted  state  without  fatigue.  This  prop- 
erty is  seen  in  the  muscular  coats  of  the  small  arteries,  many  of 
which  are  never  relaxed.  They  may  be  more  strongly  contracted 
at  some  times  than  at  others  but  in  health  they  are  always  in  some 
degree  of  contraction. 

There  are  in  the  body  a  number  of  sphincters,  circular  bands 
of  smooth  muscle  which  guard  the  openings  of  various  organs 
such  as  the  stomach,  large  intestine,  and  bladder.  These  are 
strongly  contracted  the  greater  part  of  the  time,  relaxation  being 
for  them  only  an  occasional  occurrence.  They  maintain  their 
condition  of  strong  contraction  without  fatigue  and  apparently 
without  much  expenditure  of  energy,  offering  in  this  regard  a 
sharp  contrast  to  skeletal  muscle. 

To  excite  skeletal  muscle  sharp  stimuli,  like  the  electric  shock, 
are  most  efficient.  Smooth  muscle,  on  the  other  hand,  responds 
best  to  slower,  more  prolonged  excitants.  A  pull  or  pinch,  which 
will  ordinarily  fail  to  cause  contraction  in  such  a  muscle  as  the 


116  THE  HUMAN  BODY 

gastrocnemius,  arouses  a  strip  of  stomach  muscle  to  pronounced 
activity. 

Mechanism  of  Contraction  of  Smooth  Muscle.  The  structure 
of  smooth  muscle  is,  as  shown  formerly,  much  less  complex  than  of 
skeletal  muscle.  No  elaborate  system  of  sarcostyles,  sarcoplasm, 
and  sarcolemma  exists.  The  spindle-shaped  cells  with  their  en- 
vironment of  lymph  are  the  contractile  elements.  In  fact  when 
we  attempt  to  compare  smooth  muscle  with  skeletal  we  find  that 
the  smooth  muscle-cell  corresponds  better  to  the  sarcostyle  than  to 
the  fiber,  although  the  fiber  is  the  cell  unit.  The  lymph  which 
bathes  the  cell  of  smooth  muscle  functions  toward  it  as  does  the 
sarcoplasm  toward  the  sarcostyle.  Contraction  of  smooth  muscle 
depends,  then,  on  interaction  of  muscle-cell  with  lymph,  as  in  skele- 
tal muscle  on  interaction  of  sarcostyle  with  sarcoplasm.  This  latter 
interaction  is  of  such  a  sort  that  to  keep  the  sarcostyles  in  a  state 
of  contraction  a  continuous  expenditure  of  energy  is  necessary, 
and  fatigue  is  bound  ultimately  to  occur.  The  expenditure  of 
energy  in  smooth  muscle,  on  the  other  hand,  appears  to  take  place 
only  during  an  actual  change  in  length,  whether  the  change  is  a 
shortening  or  a  lengthening;  the  maintenance  of  a  given  length 
after  it  is  once  attained  seems  not  to  require  further  energy  libera- 
tion. This  difference  accounts  for  the  ability  of  smooth  muscle  to 
continue  in  contraction  without  fatigue. 

The  suggestion  made  above,  that  energy  expenditure  occurs 
during  change  in  length  in  smooth  muscle,  even  though  the  change 
be  a  relaxation,  is  in  harmony  with  the  interesting  fact  that  most 
smooth  muscle  tissues  are  supplied  with  two  sets  of  nerves.  Stim- 
ulation by  way  of  one  set  induces  contraction;  by  way  of  the  other, 
relaxation.  This  is  in  marked  contrast  with  the  situation  in 
skeletal  muscle,  where  the  only  function  of  stimulation  is  to  arouse 
contraction;  relaxation  following  spontaneously  upon  the  release 
from  excitation. 

Heat  rigor  in  smooth  muscle  shows  an  interesting  difference 
from  the  same  phenomenon  in  skeletal  muscle.  In  the  latter 
tissue  the  result  of  heating  above  the  death  point  is  a  pronounced 
contraction.  When  smooth  muscle,  is  thus  heated,  instead  of 
contracting  it  undergoes  marked  relaxation.  We  know  that 
when  skeletal  muscle  is  heated  there  is  production  of  lactic  acid 
within  it,  and  that  this  lactic  acid  brings  about  the  shortening 


MUSCULAR  ACTIVITY  117 

If  the  same  treatment  causes  lactic  acid  to  be  produced  in  smooth 
muscle,  we  are  obliged  to  conclude  that  the  presence  of  this  acid 
may  cause  relaxation  in  this  tissue  instead  of  contraction.  This, 
again,  is  in  harmony  with  the  general  idea  that  in  smooth  muscle 
any  change  in  length  may  require  the  liberation  of  energy. 

Physiology  of  Cardiac  Muscle.  In  some  features  of  its  activity 
heart  muscle  resembles  skeletal  muscle;  in  others  it  is  more  like 
smooth  muscle.  The  contraction  of  heart  muscle  is  rapid  and 
vigorous,  in  this  respect  corresponding  to  skeletal  muscle.  The 
liberation  of  energy  in  heart  muscle  is  associated  with  contraction 
rather  than  with  change  in  length  as  such.  Here  again  the  re- 
semblance is  with  skeletal  muscle  rather  than  with  smooth. 

Cardiac  muscle  is  like  smooth  muscle  in  that  it  has  the  power  of 
executing  spontaneous  contractions.  The  heart  receives,  also, 
the  double  innervation  referred  to  above  as  characteristic  of 
smooth  muscle. 

In  a  number  of  important  regards  the  heart  differs  from  either 
of  the  other  types  of  muscle.  For  convenience  discussion  of  these 
special  features  is  deferred  to  the  section  in  which  the  heart  is 
studied  as  an  organ  of  the  circulation  (Chap.  XX). 


CHAPTER   VIII 
THE  USE  OF  MUSCLES  IN  THE  BODY 

The  Special  Physiology  of  the  Skeletal  Muscles.  Having  now 
considered  separately  the  structure  and  properties  in  general  of  the 
skeleton,  the  joints,  and  the  muscles,  we  may  go  on  to  consider 
how  they  all  work  together  in  the  Body.  Although  the  properties 
of  muscular  tissue  are  everywhere  the  same,  the  uses  of  different 
muscles  are  very  varied,  by  reason  of  the  different  parts  with 
which  they  are  connected.  Some  are  muscles  of  respiration, 
others  of  deglutition;  many  are  known  as  flexors  because  they 
bend  joints,  others  as  extensors  because  they  straighten  them. 
The  exact  use  of  any  particular  muscle,  acting  alone  or  in  concert 
with  others,  is  known  as  its  special  physiology,  as  distinguished 
from  its  general  physiology,  or  properties  as  a  muscle  without  refer- 
ence to  its  use  as  a  muscle  in  a  particular  place.  The  functions 
of  those  muscles  forming  parts  of  the  physiological  mechanisms 
concerned  in  breathing  and  swallowing  will  be  studied  here- 
after; for  the  present  we  may  consider  the  muscles  which  co- 
operate in  maintaining  postures  of  the  Body;  in  producing 
movements  of  its  larger  parts  with  reference  to  one  another;  and 
in  producing  locomotion  or  movement  of  the  whole  Body  in 
space. 

In  nearly  all  cases  the  striated  muscles  carry  out  their  func- 
tions with  the  co-operation  of  the  skeleton,  since  nearly  all  are  fixed 
to  bones  at  each  end,  and  when  they  contract  primarily  move 
these,  and  only  secondarily  the  soft  parts  attached  to  them.  To 
this  general  rule  there  are,  however,  exceptions.  The  muscle  for 
example  which  lifts  the  upper  eyelid  and  opens  the  eye  arises 
from  bone  at  the  back  of  the  orbit,  but  is  inserted,  not  into  bone, 
but  into  the  eyelid  directly;  and  similarly  other  muscles  arising 
at  the  back  of  the  orbit  are  directly  fixed  to  the  eyeball  in  front 
and  serve  to  rotate  it  on  the  pad  of.  fat  on  which  it  lies.  Many 
facial  muscles  again  have  no  direct  attachment  whatever  to  bones, 

118 


THE  USE  OF  MUSCLES  IN  THE  BODY  119 

as  for  example  the  muscle  (orbicularis  oris)  which  surrounds  the 
mouth-opening,  and  by  its  contraction  narrows  it  and  purses  out 
the  lips;  or  the  orbicularis-  palpebrarum  which  similarly  surrounds 
the  eyes  and  when  it  contracts  closes  them. 

Levers  in  the  Body.  When  the  muscles  serve  to  move  bones 
the  latter  are  in  nearly  all  cases  to  be  regarded  as  levers  whose 
fulcra  lie  at  the  joint  where  the  movement  takes  place.  Examples 
of  all  the  three  forms  of  levers  recognized  in  mechanics  are  found 
in  the  Human  Body. 

Levers  of  the  First  Order.  Examples  of  the  first  form  of  lever 
(fulcrum  between  power  and  weight,  Fig.  49),  are  not  numerous  in 
the  Human  Body.  One  is  afforded  in  the  nodding  movements  of 


{ F \ 

P  Jj^  W 

FIG.  49. — A  lever  of  the  first  order.     F,  fulcrum;  P,  power;  W,  resistance  or 
weight. 

the  head,  the  fulcrum  being  the  articulations  between  the  skull  and 
the  atlas.  When  the  chin  is  elevated  the  power  is  applied  to  the 
skull,  behind  the  fulcrum,  by  small  muscles  passing  from  the 
vertebral  column  to  the  occiput;  the  resistance  is  the  excess  in 
the  weight  of  the  part  of  the  head  in  front  of  the  fulcrum  over 
that  behind  it,  and  is  not  great.  To  depress  the  chin  as  in  nodding 
does  not  necessarily  call  for  any  muscular  effort,  as  the  head  will 
fall  forward  of  itself  if  the  muscles  keeping  it  erect  cease  to  work, 
as  those  of  us  who  have  fallen  asleep  during  a  dull  discourse  on  a 
hot  day  have  learnt.  If  the  chin  however  be  depressed  forcibhr, 
as  in  the  athletic  feat  of  suspending  one's  self  by  the  chin,  the 
muscles  passing  from  the  chest  to  the  skull  in  front  of  the  occipital 
condyles  are  called  into  play.  Another  example  of  the  employ- 
ment of  the  first  form  of  lever  in  the  Body  is  afforded  by  the 
curtsey  with  which  formerly  one  lady  saluted  another.  In  curtseying 
the  trunk  is  bent  forward  at  the  hip-joints,  which  form  the  fulcrum; 
the  weight  is  that  of  the  trunk  acting  as  if  all  concentrated  at 
its  center  of  gravity,  which  lies  a  little  above  the  sacrum  and  be- 
hind the  hip-joints;  and  the  power  is  afforded  by  muscles  passing 
from  the  thighs  to  the  front  of  the  pelvis. 


120  THE  HUMAN  BODY 

Levers  of  the  Second  Order.  As  an  example  of  the  employ- 
ment of  such  levers  (weight  between  power  and  fulcrum,  Fig.  50) 
in  the  Body,  we  may  take  the  act  of  standing  on  the  toes.  Here 
the  foot  represents  the  lever,  the  fulcrum  is  at  the  contact  of  its 
fore  part  with  the  ground;  the  weight  is  that  of  the  Body  acting 


FIG.  50. — A  lever  of  the  second  order.  F,  fulcrum;  P,  power;  W,  weight.  The 
arrows  indicate  the  direction  in  which  the  forces  act. 

down  through  the  ankle-joints  at  Ta,  Fig.  35;  and  the  power  is  the 
great  muscle  of  the  calf  acting  by  its  tendon  inserted  into  the  heel- 
bone  (Ca,  Fig.  35).  Another  example  is  afforded  by  holding  up  the 
thigh  when  one  foot  is  kept  raised  from  the  ground,  as  in  hopping 
on  the  other.  Here  the  fulcrum  is  at  the  hip-joint,  the  power  is 
applied  at  the  knee-cap  by  a  great  muscle  (quadriceps  femoris) 
which  is  inserted  there  and  arises  from  the  pelvis;  and  the  weight  is 
that  of  the  whole  lower  limb  acting  at  its  center  of  gravity,  which 
lies  somewhere  in  the  thigh  between  the  hip  and  knee-joints, 
that  is,  between  the  fulcrum  and  the  point  of  application  of  the 
power. 

Levers  of  the  Thkd  Order.  These  are  the  levers  most  com- 
monly used  on  the  Human  Body  (power  between  weight  and 
fulcrum,  Fig.  51).  For  example,  when  the  arm  is  bent  at  the 
elbow  the  fulcrum  is  the  elbow-joint,  the  power  is  applied  at 
the  insertion  of  the  biceps  muscle  (Fig.  39)  into  the  radius  and  of 


W 


FIG.  51. — A  lever  of  the  third  order.    F,  fulcrum;  P,  power;  W,  weight. 

another  muscle  (not  represented  in  the  figure,  the  brachialis 
anticus,)  into  the  ulna,  and  the  weight  is  that  of  the  forearm  and 
hand,  with  whatever  may  be  contained  in  the  latter,  acting  at 
the  center  of  gravity  of  the  whole  somewhere  on  the  distal  side 


THE  USE  OF  MUSCLES  IN  THE  BODY 


121 


of  the  point  of  application  of  the  power.  In  the  Body  the  power- 
arm  is  usually  very  short  so  as  to  gain  speed  and  range  of  move- 
ment, the  muscles  being  powerful  enough  to  do  their  work  in  spite 
of  the  mechanical  disadvantage  at  which  they  are  placed.  The 
limbs  are  thus  made  much  more  shapely  than  would  be  the  case 
were  the  power  applied  near  or  beyond  the  weight. 

It  is  of  course  only  rarely  that  simple  movements  as  those 
described  above  take  place.  In  the  great  majority  of  those 
executed  several  or  many  muscles  co-operate. 

The  Loss  to  the  Muscles  from  the  Direction  of  their  Pull.  It 
is  worthy  of  note  that,  owing  to  the  oblique  direction  in  which 
the  muscles  are  commonly  inserted  into  the  bones,  much  of  their 
•force  is  lost  so  far  as  producing  movement  is  concerned.  Sup- 
pose the  log  of  wood  in  the  diagram  (Fig.  52)  to  be  raised  by  pull- 


Fio.  52. — Diagram  illustrating  the  disadvantage  of  an  oblique  pull. 

ing  on  the  rope  in  the  direction  a;  it  is  clear  at  first  that  the  rope 
will  act  at  a  great  disadvantage;  most  of  the  pull  transmitted 
by  it  will  be  exerted  against  the  pivot  on  which  the  log  hinges, 
and  only  a  small  fraction  be  available  for  elevating  the  latter. 
But  the  more  the  log  is  lifted,  as  for  example  into  the  position 
indicated  by  the  dotted  lines,  the  more  useful  will  be  the  direction 
of  the  pull,  and  the  more  of  it  will  be  spent  on  the  log  and  the 
less  lost  unavailingly  in  merely  increasing  the  pressure  at  the  hinge. 
If  we  now  consider  the  action  of  the  biceps  (Fig.  39)  in  flexing  the 
elbow-joint,  we  see  similarly  that  the  straighter  the  joint  is,  the 
more  of  the  pull  of  the  muscle  is  wasted.  Beginning  with  the  arm 
straight,  it  works  at  a  great  disadvantage,  but  as  the  forearm  is 


122  THE  HUMAN  BODY 

raised  the  conditions  become  more  and  more  favorable  to  the 
muscle.  Those  who  have  practiced  the  gymnastic  feat  of  raising 
one's  self  by  bending  the  elbows  when  hanging  by  the  hands 
from  a  horizontal  bar  know  practically  that  if  the  elbow-joints 
are  quite  straight  it  is  very  hard  to  start;  and  that,  on  the 
other  hand,  if  they  are  kept  a  little  flexed  at  the  beginning 
the  effort  needed  is  much  less;  the  reason  being  of  course  the 
more  advantageous  direction  of  traction  by  the  biceps  in  the 
latter  case. 

Experiment  proves  that  the  power  with  which  a  muscle  can 
contract  is  greatest  at  the  commencement  of  its  shortening,  the 
very  time  at  which,  we  have  just  seen,  it  works  at  most  mechanical 
disadvantage;  in  proportion  as  its  force  becomes  less  the  conditions 
become  more  favorable  to  it.  There  is,  however,  it  is  clear,  nearly 
always  a  considerable  loss  of  power  in  the  working  of  the  skeletal 
muscles,  strength  being  sacrificed  for  variety,  ease,  rapidity,  ex- 
tent, and  elegance  of  movement. 

The  Equilibrium  of  Opposing  Muscles.  The  muscles  are  highly 
elastic  bodies,  and  on  account  of  their  elasticity  exert,  even  while 
not  actively  contracting,  a  definite  elastic  tension  upon  their 
points  of  insertion.  We  have,  therefore,  at  every  joint,  constant 
opposing  pulls  of  the  resting  muscles.  The  efficient  manner  in 
which  the  elastic  tensions*  of  opposing  muscles  balance  each  other 
is  often  very  striking,  particularly  when  one  considers  the  marked 
differences  in  size  and  in  mechanical  advantage  apt  to  exist  in 
opposing  muscles.  Take,  for  example,  the  muscles  which  move 
the  fore  arm.  The  flexor  muscle  (biceps)  is  a  larger  and  stronger 
muscle  than  the  extensor  (triceps).  It  has,  moreover,  a  better 
leverage.  Yet  so  far  as  elastic  tension  is  concerned  the  two 
muscles  balance  perfectly.  One  appreciates  best  the  significance 
of  this  equilibrium  when  a  case  is  seen  in  which  it  has  been  lost. 
The  disease  known  as  infantile  paralysis  injures  or  destroys  the 
nervous  connections  of  the  skeletal  muscles,  and  often  affects  one 
group  while  leaving  the  opposing  group  unimpaired.  The  normal 
tension  is  lost  in  the  affected  muscles  but  persists  in  the  unaffected 
ones.  Unless  special  precautions  are  taken  to  prevent  the  occur- 
rence, the  muscles  which  still  exert  elastic  tension  will  gradually 
shorten,  pulling  the  joint  into  the  position  into  which  it  is  drawn 
by  the  normal  contractions  of  these  muscles,  and  holding  it  there 


THE  USE  OF  MUSCLES  IN  THE  BODY  123 

permanently,  and  so  forcibly  that  extreme  measures  must  often  be 
adopted  to  return  the  part  to  its  normal  resting  situation.  The 
importance  of  this  constant  tension  during  life  is  probably  in  the 
instant  readiness  it  gives  the  muscles.  There  is  never  any  slack 
to  be  taken  up;  motion  of  the  joint  is  simultaneous  with  the  con- 
traction of  the  muscle. 

Functional  Muscle  Groups.  In  attempting  to  analyze  the 
exceedingly  numerous  and  diverse  muscular  movements  of  which 
we  are  capable  we  have  to  bear  in  mind  that  in  the  state  of  nature 
for  which  our  bodies  were  primarily  adapted  men's  movements 
were  directed  to  fewer  and  simpler  ends.  Our  fundamental  ac- 
tivities fall  into  a  small  number  of  groups,  and  we  shall  see  that 
our  more  complex  movements  are  but  modifications  of  the  funda- 
mental ones. 

According  to  this  principle  we  can  classify  our  muscular  acts  as 
follows:  posture;  locomotion;  prehension  (grasping);  mastication 
(and  swallowing);  vision;  voice  production  (including  breathing). 
Of  these  groups  posture,  locomotion,  and  prehension  are  treated  in 
following  paragraphs.  A  separate  chapter  is  devoted  to  voice 
production  (Chap.  XXXIII).  The  others  are  treated  in  detail  in 
connection  with  the  discussion  of  the  particular  bodily  functions 
with  which  they  are  associated.  Some  points  of  general  interest 
concerning  them  may  not  be  out  of  place  here.  Among  the  facial 
muscles  are  found  groups  devoted  to  mastication;  to  vision;  to 
voice  production;  and  to  prehension.  The  masticatory  and  visual 
muscles  do  not  show  in  man  any  very  striking  differences  from  their 
functioning  in  other  mammals.  Prehension,  which  in  man  and 
the  higher  monkeys  is  taken  over  so  largely  by  the  front  limbs, 
manifests  itself  among  the  facial  muscles  in  the  grasping  power  of 
the  very  flexible  lips,  which  are  in  most  of  the  lower  mammals 
important  grasping  organs,  but  in  man  confined  chiefly  to  the  safe 
guidance  of  food  into  the  mouth. 

The  tongue  is  a  muscular  mass  composed  of  several  distinct  mus- 
cles which  are  interwoven  in  such  a  manner  that  by  their  interac- 
tion they  can  draw,  thrust,  or  twist  the  organ  into  the  numerous 
positions  and  shapes  it  is  capable  of  taking.  The  larger  tongue 
muscles  have  a  bony  attachment  at  one  end,  either  in  the  hyoid 
bone  (p.  62)  or  the  lower  jaw.  Their  insertions  are  within  the 
fleshy  mass  itself.  There  are  besides  these  large  muscles  a  number 


124  THE  HUMAN  BODY 

of  smaller  ones  which  are  embedded  wholly  within  the  tongue. 
These  have  no  bony  attachments  at  either  end. 

In  addition  to  its  masticatory  function  (p.  469)  the  tongue  is 
an  essential  part  of  the  speech  apparatus.  Its  importance  is  shown 
not  only  by  the  loss  of  the  power  of  articulate  speech  from  paralysis 
or  removal  of  the  tongue,  but  also  by  the  fact  that  only  those  lower 
forms  which  have  tongues  at  once  fleshy  and  flexible  (parrots, 
tongue-cut  crows)  can  learn  to  talk.  The  ape's  tongue  is  very 
similar  to  that  of  man.  The  fact  that  the  parrot  can  learn  to  talk 
while  the  ape  cannot,  or  will  not,  raises  the  interesting  question  in 
connection  with  the  lower  animals  as  to  how  much  speech  depends 
on  tongue-structure  and  how  much  on  intelligence  or  willingness 
to  imitate  sounds.  The  solution  of  this  problem  is  a  matter 
for  the  student  of  animal  behavior  rather  than  for  the  physi- 
ologist. 

Postures.  The  term  posture  is  applied  to  those  positions  of 
equilibrium  of  the  Body  which  can  be  maintained  for  some  time, 
such  as  standing,  sitting,  or  lying,  compared  with  leaping,  run- 
ning, or  falling.  In  all  postures  the  condition  of  stability  is  that 
the  vertical  line  drawn  through  the  center  of  gravity  of  the  Body 
shall  fall  within  the  basis  of  support  afforded  by  objects  with 
which  it  is  in  contact;  and  the  security  of  the  posture  is  propor- 
tionate to  the  extent  of  this  base,  for  the  wider  it  is  the  less  is  the 
risk  of  the  perpendicular  through  the  center  of  gravity  falling 
outside  of  it  on  slight  displacement. 

The  Erect  Posture.  This  is  pre-eminently  characteristic  of 
man,  his  whole  skeleton  being  modified  with  reference  to  it. 
Nevertheless  the  power  of  maintaining  it  is  only  slowly  learnt 
in  the  first  years  after  birth,  and  for  a  long  while  it  is  unsafe. 
And  though  finally  we  learn  to  stand  erect  without  conscious 
attention,  the  maintenance  of  that  posture  always  requires  the 
co-operation  of  many  muscles,  co-ordinated  by  the  nervous  system. 
The  influence  of  the  latter  is  shown  by  the  fall  which  follows  a 
severe  blow  on  the  head,  which  may  nevertheless  have  fractured 
no  bone  nor  injured  any  muscle:  the  concussion  of  the  brain,  as 
we  say,  "stuns"  the  man,  and  until  its  effects  have  passed  off 
he  cannot  stand  upright.  In  standing  with  the  arms  straight 
by  the  sides  and  the  feet  together  the  center  of  gravity  of  the 
whole  adult  Body  lies  in  the  articulation  between  the  sacrum  and 


THE  USB  OF  MUSCLES  IN  THE  BODY 


125 


V 


I  lie  last  lumbar  vertebra,  and  the  perpendicular  drawn  from  it 

will  reach  the  ground  between  the  two  feet,  within  the  basis  of 

support  afforded  by  them.     With  the  feet  close  together,  how- 

ever, the  posture  is  not  very  stable,  and  in 

standing  we  commonly  make  it  more  so  by 

slightly  separating  them  so  as  to  increase  the 

base.    The  more  one  foot  is  in  front  of  the  other 

the  more  swaying  back  and  forward  will  be 

compatible  with  safety;  and  the  greater  the 

lateral  distance  separating  them  the  greater 

will  be  the  lateral  sway  which  is  possible  with- 

out falling.     Consequently  we  see  that  a  man 

about  to  make  great  movements  with  the  upper 

part  of  his  Body,  as  in  fencing  or  boxing,  or  a 

soldier  preparing  for  the  bayonet  exercise,  al- 

ways commences  by  thrusting  one  foot  for- 

ward obliquely,  so  as  to  increase  his  basis  of 

support  in  both  directions. 

The  ease  with  which  we  can  stand  is  largely 
dependent  upon  the  way  in  which  the  head  is 
almost  balanced  on  the  top  of  the  vertebral 
column,  so  that  but  little  muscular  effort  is 
needed  to  keep  it  upright.  In  the  same  way 
the  trunk  is  almost  balanced  on  the  hip-joints, 
but  not  quite,  its  center  of  gravity  falling  rather 
behind  them;  so  that  just  as  some  muscular 
effort  is  needed  to  keep  the  head  from  falling 
forwards,  some  is  needed  to  keep  the  trunk 


1 


FlQ>  53._Diagram 


from  toppling  backwards  at  the  hips.     In  a  black     lines)     which 

.     .,  .,  ,  „    j    .    ,      pass    before    and    be- 

similar  manner  other  muscles  are  called  into  hind  the   joints   and 
play  at  other  joints:  as  between  the  vertebral 


column  and  the  pelvis,  and  at  the  knees  and  rigid  and  the  body 
ankles;  and  thus  a  certain  rigidity,  due  to 
muscular  effort,  extends  all  along  the  erect  Body:  which,  on 
account  of  the  flexibility  of  its  joints,  could  not  otherwise  be 
balanced  on  its  feet,  as  a  statue  can.  Beginning  (Fig.  53)  at 
the  ankle-joint,  we  find  it  kept  stiff  in  standing  by  the  com- 
bined and  balanced  contraction  of  the  muscles  passing  from 
the  heel  to  the  thigh,  and  from  the  dorsum  of  the  foot  to  the  shin- 


126  THE  HUMAN  BODY 

bone  (tibia).  Others  passing  before  and  behind  the  knee-joint 
keep  it  from  yielding;  and  so  at  the  hip-joints:  the  others  again, 
lying  in  the  walls  of  the  abdomen  and  along  the  vertebral  column, 
keep  the  latter  rigid  and  erect  on  the  pelvis;  and  finally  the  skull 
is  kept  in  position  by  muscles  passing  from  the  sternum  and 
vertebral  column  to  it,  in  front  of  and  behind  the  occipital  con- 
dyles. 

Posture  the  Task  of  the  Extensor  Muscles.  In  the  mainte- 
nance of  posture  the  muscles  which  bear  the  strain  are  in  general 
the  extensors,  since  posture  requires  that  the  joints  shall  be  kept 
from  bending.  So  far  as  the  flexors  co-operate  they  do  so  by  pre- 
venting overextension,  a  part  which  calls  for  relatively  little  exer- 
tion. The  degree  of  pull  manifested  by  the  extensor  muscles  in 
the  maintenance  of  posture  is  slight  but  steady.  In  the  course 
of  a  day  we  may  become  aware  of  postural  fatigue,  showing  that 
muscular  activity  has  been  present,  although  we  may  not  have 
been  conscious  of  definite  volition.  This  mild  degree  of  sustained 
contraction,  which  differs  strikingly  from  the  ordinary  rapid  and 
extensive  contractions  of  skeletal  muscle,  is  known  as  tonus.  In 
the  maintenance  of  posture  it  is  chiefly  extensor  tonus.  In  what 
respects  tonus  is  comparable  with  ordinary  contraction  and  in 
what  respects  different,  is  not  yet  known. 

Locomotion  includes  all  the  motions  of  the  whole  Body  in 
space,  dependent  on  its  own  muscular  efforts:  such  as  walking, 
running,  leaping  and  swimming. 

Walking.  In  walking  the  Body  never  entirely  quits  the  ground, 
the  heel  of  the  advanced  foot  touching  the  ground  in  each  step 
before  the  toe  of  the  rear  foot  leaves  it.  The  advanced  limb  sup- 
ports the  Body,  and  the  foot  in  the  rear  at  the  commencement  of 
each  step  propels  it. 

Suppose  a  man  standing  with  his  heels  together  to  commence 
to  walk,  stepping  out  with  the  left  foot:  the  whole  Body  is  at  first 
inclined  forwards,  the  movement  taking  place  mainly  at  the 
ankle-joints.  By  this  means  the  center  of  gravity  would  be 
thrown  in  front  of  the  base  formed  by  the  feet  and  a  fall  on  the 
face  result,  were  not  simultaneously  the  left  foot  slightly  raised 
by  bending  the  knee  and  then  swung  forwards,  the  toes  just  clear 
of  the  ground,  and,  in  good  walking  the  sole  nearly  parallel  to  it. 
When  the  step  is  completed  the  left  knee  is  straightened  and  the 


THE  USE  OF  MUSCLES  IN  THE  BODY  127 

sole  placed  on  the  ground,  the  heel  touching  it  first,  and  the  base 
of  support  being  thus  widened  from  before  back,  a  fall  is  pre- 
vented. Meanwhile  the  right  leg  is  kept  straight,  but  inclines 
forwards  above  with  the  trunk. when  the  latter  advances,  and  as 
this  occurs  the  sole  gradually  leaves  the  ground,  commencing 
with  the  heel.  When  the  step  of  the  left  leg  is  completed  the 
great  toe  of  the  right  alone  is  in  contact  with  the  support.  With 
this  a  push  is  given  which  sends  the  trunk  on  over  the  left  leg, 
which  is  now  kept  rigid,  except  at  the  ankle-joint;  and  the  right 
knee  being  bent  that  limb  swings  forwards,  its  foot  just  clearing 
the  ground  as  the  left  did  before.  The  Body  is  meanwhile  sup- 
ported on  the  left  foot  alone,  but  when  the  right  completes  its 
step  the  knee  of  that  leg  is  straightened  and  the  foot  thus  placed, 
heel  first,  on  the  ground.  Meanwhile  the  left  foot  has  been  gradu- 
ally leaving  the  ground,  and  its  toes  only  are  at  that  moment 
upon  it :  from  these  a  push  is  given,  as  before,  with  the  right  foot, 
and  the  knee  being  bent  so  as  to  raise  the  foot,  the  left  leg  swings 
forwards  at  the  hip-joint  to  make  a  fresh  step. 

During  each  step  the  whole  Body  sways  up  and  down  and  also 
from  side  to  side.  It  is  highest  at  the  moment  when  the  advanc- 
ing trunk  is  vertically  over  the  foot  supporting  it,  and  then  sinks 
until  the  moment  when  the  advancing  foot  touches  the  ground, 
when  it  is  lowest.  From  this  moment  it  rises  as  it  swings  forward 
on  this  foot,  until  it  is  vertically  over  it,  and  then  sinks  again 
until  the  other  touches  the  ground;  and  so  on.  At  the  same  time, 
as  its  weight  is  alternately  transferred  from  the  right  to  the  left 
foot  and  vice  versa,  there  is  a  slight  lateral  sway,  commonly  more 
marked  in  women  than  in  men,  and  which  when  excessive  pro- 
duces an  ugly  " waddling"  gait. 

The  length  of  each  step  is  primarily  dependent  on  the  length 
of  the  legs;  but  can  be  controlled  within  wide  limits  by  special 
muscular  effort.  In  easy  walking  little  muscular  work  is  em- 
ployed to  carry  the  rear  leg  forwards  after  it  has  given  its  push. 
When  its  foot  is  raised  from  the  ground  it  swings  on,  like  a  pen- 
dulum; but  in  fast  walking  the  muscles,  passing  in  front  of  the  hip- 
joint,  from  the  pelvis  to  the  limb,  by  their  contraction  forcibly 
carry  the  leg  forwards.  The  easiest  step,  that  in  which  there  is 
most  economy  of  labor,  is  that  in  which  the  limb  is  let  swing 
freely,  and  since  a  short  pendulum  swings  faster  than  a  longer, 


128  THE  HUMAN  BODY 

the  natural  step  of  short-legged  people  is  quicker  than  that  of 
long-legged  ones. 

In  fast  walking  the  advanced  or  supporting  leg  also  aids  in 
propulsion;  the  muscles  passing  in  front  of  the  ankle-joint  con- 
tracting so  as  to  pull  the  Body  forwards  over  that  foot  and  aid 
the  push  from  the  rear  foot.  Hence  the  fatigue  and  pain  in  front 
of  the  shin  which  is  felt  in  prolonged,  very  fast  walking.  From  the 
fact  that  each  foot  reaches  the  ground  heel  first,  but  leaves  it  toe 
last,  the  length  of  each  stride  is  increased  by  the  length  of  the  foot. 

Running.  In  this  mode  of  progression  there  is  a  moment  in 
each  step  when  both  feet  are  off  the  ground,  the  Body  being  un- 
supported in  the  air.  The  toes  alone  come  in  contact  with  the 
ground  at  each  step,  and  the  knee-joint  is  not  straight  when  the 
foot  reaches  the  ground.  When  the  rear  foot  is  to  leave  the  sup- 
port, the  knee  is  suddenly  straightened,  and  at  the  same  time  the 
ankle-joint  is  extended  so  as  to  push  the  toes  forcibly  on  the 
ground  and  give  the  whole  Body  a  powerful  push  forwards  and 
upwards.  Immediately  after  this  the  knee  is  greatly  flexed  and 
the  foot  raised  from  the  ground,  and  this  occurs  before  the  toes  of 
the  forward  foot  reach  the  latter.  The  swinging  leg  in  each  step 
is  violently  pulled  forwards  and  not  suffered  to  swing  naturally, 
as  in  walking.  By  this  the  rapidity  of  the  succession  of  steps  is 
increased,  and  at  the  same  time  the  stride  is  made  greater  by 
the  sort  of  one-legged  leap  that  occurs  through  the  jerk  given  by 
the  straightening  of  the  knee  of  the  rear  leg  just  before  it  leaves 
the  ground. 

Leaping.  In  this  mode  of  progression  the  Body  is  raised  com- 
pletely from  the  ground  for  a  considerable  period.  In  a  powerful 
leap  the  ankles,  knees,  and  hip-joints  are  all  flexed  as  a  pre- 
paratory measure,  so  that  the  Body  assumes  a  crouching  atti- 
tude. The  heels,  next,  are  raised  from  the  ground  and  the  Body 
balanced  on  the  toes.  The  center  of  gravity  of  the  Body  is  then 
thrown  forwards,  and  simultaneously  the  flexed  joints  are  straight- 
ened, and  by  the  resistance  of  the  ground,  the  Body  receives  a 
propulsion  forwards;  much  in  the  same  way  as  a  ball  rebounds 
from  a  wall.  The  arms  are  at  the  same  time  thrown  forwards. 
In  leaping  backwards,  the  Body  and  arms  are  inclined  in  that 
direction;  and  in  jumping  vertically  there  is  no  leaning  either  way 
and  the  arms  are  kept  by  the  sides. 


THE  USE  OF  MUSCLES  IN  THE  BODY  129 

Prehension.  In  man,  and  to  a  less  extent  in  monkeys,  the 
fore  limbs  are  differentiated  from  organs  of  locomotion  into  pre- 
hensile structures.  To  a  large  degree  the  effectiveness  of  the 
fore  limb  as  an  organ  of  prehension  depends  on  peculiarities  of  its 
bony  framework.  These  have  been  described  in  detail  in  a  pre- 
vious chapter  (p.  66).  The  chief  special  features,  which  it  may 
not  be  amiss  to  recall,  are  three;  the  attachment  of  the  shoulder 
girdle  to  the  trunk  by  muscles  rather  than  by  a  firm  bony  articula- 
tion; the  rotation  of  the  radius  over  the  ulna;  the  opposibility  of 
the  thumb  to  each  of  the  fingers.  These  skeletal  features,  which 
afford  a  groundwork  for  great  flexibility  of  action,  are  made  effect- 
ive by  the  arrangement  of  the  arm  muscles.  Although  detailed 
description  of  this  arrangement  is  outside  the  scope  of  this  work, 
some  general  statements  may  properly  be  presented. 

The  muscles  of  the  arm  fall  into  three  groups,  shoulder  muscles, 
muscles  of  the  upper  arm,  muscles  of  the  fore  arm  and  hand.  The 
muscles  of  the  shoulder  are  arranged  so  that  the  arm  can  be  raised 
or  lowered;  drawn  forward  or  backward.  Those  at  the  back  of 
the  shoulder  include  within  their  mass  the  shoulder  blade  (scapula] 
in  such  a  manner  that  in  ordinary  upward  movements  of  the  arm 
the  rotation  is  about  the  shoulder  joint,  but  in  extensive  upward 
movements,  as  in  raising  the  arms  above  the  head,  the  shoulder 
blade  itself  is  pulled  out  of  its  ordinary  horizontal  position  into  a 
nearly  vertical  one. 

The  muscles  of  the  upper  arm  are  simple  flexors  and  extensors 
since  the  elbow-joint  is  a  hinge-joint,  permitting  no  variety  of 
movements.  The  muscles  of  the  fore  arm  and  hand  have  a  great 
variety  of  movements  to  provide  for,  and  are  accordingly  numerous 
and  complicated.  Only  the  arrangements  for  securing  flexion 
and  extension  will  be  mentioned  here.  The  front  of  the  fore  arm 
is  made  up  of  a  number  of  muscles,  of  which  most  are  flexors  of 
the  wrist  or  of  the  fingers.  Similarly  the  back  of  the  fore  arm 
contains  the  extensors  of  wrist  and  of  fingers.  The  tendons  of 
the  latter  muscles  form  the  conspicuous  cords  at  the  back  of  the 
hand.  These  flexors  and  extensors  interact  in  an  interesting 
fashion.  Thus  the  flexors  of  the  finger  aid  in  flexion  of  the  wrist. 
If  flexion  of  the  fingers  without  accompanying  flexion  of  the  wrist 
is  desired  the  latter  must  be  prevented  by  simultaneous  contrac- 
tion of  the  wrist  extensors.  Similarly  the  extensors  of  the  fingers 


130  THE  HUMAN  BODY 

act  also  to  extend  the  wrist.  To  secure  extension  of  the  fingers 
while  the  wrist  is  flexed  the  latter  position  must  be  maintained 
by  activity  of  the  wrist  flexor  to  overcome  the  tendency  of  the 
finger  extensors  to  extend  the  wrist. 

Hygiene  of  the  Muscles.  The  healthy  working  of  the  muscles 
needs  of  course  a  healthy  state  of  the  Body  generally,  so  that 
they  shall  be  supplied  with  proper  materials  for  growth  and  re- 
pair, and  have  their  wastes  rapidly  and  efficiently  removed.  In 
other  words,  good  food  and  pure  air  are  necessary  for  a  vigorous 
muscular  system,  a  fact  which  trainers  recognize  in  insisting  upon 
a  strict  dietary,  and  in  supervising  generally  the  mode  of  life  of 
those  who  are  to  engage  in  athletic  contests.  The  muscles  should 
also  not  be  exposed  to  any  considerable  continued  pressure, 
since  this  interferes  with  the  flow  of  blood  and  lymph  through 
them. 

As  far  as  the  muscles  themselves  are  directly  concerned,  exer- 
cise is  the  necessary  condition  of  their  best  development.  The 
muscles  are  so  compactly  built  that  the  movement  of  blood  and 
lymph  through  them  is  less  free  than  in  other  tissues.  During 
the  act  of  contraction,  however,  the  circulation  both  of  blood  and 
of  lymph  is  augmented  by  the  pressure  of  the  muscle  upon  its  own 
contents.  For  their  proper  nourishment  most  of  the  muscles  are 
largely  dependent  upon  this  self-massage.  A  muscle  which  is 
permanently  unused  suffers  serious  impairment  of  nutrition,  and 
usually  degenerates  and  is  absorbed,  little  finally  being  left  but 
the  connective  tissue  of  the  organ  and  a  few  muscle-fibers  filled 
with  oil-drops.  This  is  well  seen  in  cases  of  paralysis  dependent 
on  injury  to  the  nerves.  In  such  cases  the  muscles  may  them- 
selves be  perfectly  healthy  at  first,  but  lying  unused  for  weeks 
they  become  altered,  and  finally,  when  the  nervous  injury  has  been 
healed,  the  muscles  may  be  found  incapable  of  functional  activity. 
The  physician  therefore  is  often  careful  to  avoid  this  by  exercising 
the  paralyzed  muscles  daily  by  means  of  electrical  shocks  sent 
through  the  part;  passive  exercise,  as  by  proper  massage,  is  fre- 
quently of  great  use  in  such  cases.  The  same  fact  is  illustrated 
by  the  feeble  and  wasted  condition  of  the  muscles  of  a  limb  which 
has  been  kept  for  some  time  in  splints.  After  the  latter  have  been 
removed  it  is  only  slowly,  by  judicious  and  persistent  exercise, 
that  the  long-idle  muscles  regain  their  former  size  and  power- 


THE  USE  OF  MUSCLES  IN  THE  BODY  131 

The  great  muscles  of  the  "  brawny "  arm  of  the  blacksmith  or 
wrestler  illustrate  the  reverse  fact,  the  growth  of  the  muscles  by 
exercise.  We  may  note,  incidentally,  that  in  this  growth  from 
exercise  there  is  no  increase  in  the  number  of  muscle-fibers.  The 
greater  size  is  due  to  growth  of  the  individual  fibers.  Exercise,  to 
be  effective,  must  be  judicious;  repeated  frequently  to  the  point  of 
exhaustion  it  does  harm;  the  period  of  repair  is  not  sufficient  to 
counteract  the  injurious  effects  of  fatigue,  and  the  muscles  thus 
waste  under  too  violent  exercise  as  with  too  little.  Rest  should 
alternate  with  work,  and  that  regularly,  if  benefit  is  to  be  obtained. 
Moreover,  violent  exercise  should  never  be  suddenly  undertaken 
by  one  unused  to  it,  not  only  lest  the  muscles  suffer,  but  because 
muscular  effort  greatly  increases  the  work  of  the  heart.  No  gen- 
eral rule  can  be  laid  down  as  to  the  amount  of  exercise  to  be  taken ; 
for  a  healthy  man  in  business  the  minimum  would  perhaps  be 
represented  by  a  daily  walk  of  five  miles. 

Varieties  of  Exercise.  In  walking  and  running  the  muscles 
chiefly  employed  are  those  of  the  lower  limbs  and  trunk.  This  is 
in  part  true  of  rowing,  which  when  good  is  performed  much  more 
by  the  legs  than  the  arms;  especially  when  sliding  seats  are 
used.  Hence  any  of  these  exercises  alone  is  apt  to  leave 
the  muscles  of  the  chest  and  arms  imperfectly  exercised.  Indeed, 
no  one  exercise  employs  equally  or  proportionately  all  the  muscles : 
therefore  gymnasia  in  which  various  feats  of  agility  are  practiced, 
so  as  to  call  different  parts  into  play,  have  very  great  utility.  It 
should  be  borne  in  mind,  however,  that  the  legs  especially  need 
strength;  while  the  upper  limbs,  in  which  delicacy  of  movement, 
as  a  rule,  is  more  desirable  than  power,  do  not  require  so  much 
exercise;  and  the  fact  that  gymnastic  exercises  are  commonly 
carried  on  indoors  is  a  great  drawback  to  their  value.  When  the 
weather  permits,  out-of-door  exercise  is  far  better  than  that 
carried  on  in  even  the  best  ventilated  and  lighted  gymnasium. 
For  those  who  are  so  fortunate  as  to  possess  a  garden  there  is  no 
better  exercise,  at  suitable  seasons,  than  an  hour's  daily  digging 
in  it;  since  this  calls  into  play  nearly  all  the  muscles  of  the  Body; 
while  of  games,  lawn-tennis  is  perhaps  the  best  from  a  hygienic 
view  that  has  ever  been  invented,  since  it  not  only  demands 
great  muscular  agility  in  every  part  of  the  Body,  but  trains  the 
hand  to  work  with  the  eye  in  a  way  that  walking,  running,  row- 


Ki2  THE  HUMAN  BODY 

ing  and  similar  pursuits  do  not.  For  the  same  reasons  baseball, 
cricket,  and  boxing  are  excellent. 

Exercise  in  Infancy  and  Childhood.  Young  children  have  not 
only  to  strengthen  their  muscles  by  exercise,  but  also  to  learn 
to  use  them.  Watch  an  infant  trying  to  convey  something  to  its 
mouth,  and  you  will  see  how  little  control  it  has  over  its  muscles. 
On  the  other  hand,  the  healthy  infant  is  never  at  rest  when  awake; 
it  constantly  throws  its  limbs  around,  grasps  at  all  objects  within 
its  reach,  coils  itself  about,  and  so  gradually  learns  to  exercise  its 
powers.  It  is  a  good  plan  to  leave  every  healthy  child  more  than 
a  few  months  old  several  times  daily  on  a  large  bed,  or  even  on  a  rug 
or  carpeted  floor,  with  as  little  covering  as  is  safe,  and  that  as  loose 
as  possible,  and  let  it  wriggle  about  as  it  pleases.  In  this  way  it 
will  not  only  enjoy  itself  thoroughly,  but  gain  strength  and  a 
knowledge  of  how  to  use  its  limbs.  To  keep  a  healthy  child 
swathed  all  day  in  tight  and  heavy  clothes  is  cruelty. 

When  a  little  later  the  infant  commences  to  crawl  it  is  safe  to 
permit  it  to  as  much  as  it  wishes,  but  unwise  to  tempt  it  to  do 
so  when  disinclined:  the  bones  and  muscles  are  still  feeble  and 
may  be  injured  by  too  much  work.  The  same  is  true  of  learning 
to  walk. 

From  four  or  five  to  twelve  years  of  age  almost  any  form  of 
exercise  should  be  permitted,  or  even  encouraged.  During  this 
time,  however,  the  epiphyses  of  many  bones  are  not  firmly  united 
to  their  shafts,  and  so  anything  tending  to  throw  too  great  a 
strain  on  the  joints  should  be  avoided.  After  that  up  to  com- 
mencing manhood  or  maidenhood  any  kind  of  outdoor  exercise 
for  healthy  persons  is  good,  and  girls  are  all  the  better  for  being 
allowed  to  join  in  their  brothers'  sports.  Half  of  the  debility  and 
general  ill-health  of  so  many  of  our  women  is  the  consequence  of 
deficient  exercise  during  early  life. 

Exercise  in  Youth  should  be  regulated  largely  by  sex;  not  that 
women  are  to  be  shut  up  and  made  pale,  delicate,  and  unfit  to 
share  the  duties  or  participate  fully  in  the  pleasures  of  life;  but 
the  other  calls  on  the  strength  of  the  young  woman  render  vig- 
orous muscular  work  often  unadvisable,  especially  under  con- 
ditions where  it  is  apt  to  be  followed  by  a  chill. 

A  healthy  boy  or  young  man  may  do  nearly  anything;  but 
until  twenty-two  or  twenty-three  very  prolonged  effort  is  un- 


THE  USE  OF  MUSCLES  IN  THE  BODY  133 

advisable.     The  frame  is  still  not  firmly  knit  or  as  capable  of  en- 
durance as  it  will  subsequently  become. 

( I  iris  should  be  allowed  to  ride  or  play  outdoor  games  in  mod- 
eration, and  in  any  case  should  not  be  cribbed  in  tight  stays  or 
tight  boots.  A  flannel  dress  and  proper  lawn  tennis  shoes  are  as 
necessary  for  the  healthy  and  safe  enjoyment  of  an  afternoon  at 
that  game  by  a  girl  as  they  are  for  her  brother  in  the  baseball 
field.  Rowing  is  excellent  for  girls  if  there  be  any  one  to  teach 
them  to  do  it  properly  with  the  legs  and  back,  and  not  with  the 
arms  only,  as  women  are  so  apt  to  row.  Properly  practiced  it 
strengthens  the  back  and  improves  the  carriage. 

Exercise  in  Adult  Life.  Up  to  forty  a  man  may  carry  on  safely 
the  exercises  of  youth,  but  after  that  sudden  efforts  should  be 
avoided.  A  lad  of  twenty-one  or  so  may,  if  trained,  safely  run  a 
quarter-mile  race,  but  to  a  man  of  forty-five  it  would  be  dan- 
gerous, for  with  the  rigidity  of  the  cartilages  and  blood-vessels 
which  begins  to  show  itself  about  that  time  comes  a  diminished 
power  of  meeting  a  sudden  violent  demand.  On  the  other  hand, 
the  man  of  thirty  would  more  safely  than  the  lad  of  nineteen  or 
twenty  undertake  one  of  the  long-distance  walking  matches  such 
as  used  to  be  in  vogue;  the  prolonged  effort  would  be  less 
dangerous  to  him,  though  a  six-days'  match,  with  its  attendant 
loss  of  sleep,  cannot  fail  to  be  more  or  less  dangerous  to  any  one. 
Probably  for  one  engaged  in  active  business  a  walk  of  two  or 
three  miles  to  it  in  the  morning  and  back  again  in  the  afternoon  is  • 
the  best  and  most  available  exercise.  The  habit  which  Americans 
have  everywhere  acquired,  of  never  walking  when  they  can  take 
a  street  car,  is  certainly  detrimental  to  the  general  health;  though 
the  extremes  of  heat  and  cold  to  which  we  are  subject  often  render 
it  unavoidable. 

For  women  during  middle  life  the  same  rules  apply:  there 
should  be  some  regular  but  not  violent  daily  exercise. 

In  Old  Age  the  needful  amount  of  exercise  is  less,  and  it  is 
still  more  important  to  avoid  sudden  or  violent  effort. 

Exercise  for  Invalids.  This  should  be  regulated  under  med- 
ical advice.  For  feeble  persons  gymnastic  exercises  are  especially 
valuable,  since  from  their  variety  they  permit  of  selection  accord- 
ing to  the  condition  of  the  individual;  and  their  amount  can  be 
conveniently  controlled. 


134  THE  HUMAN  BODY 

Training.  If  any  person  attempt  some  unusual  exercise  he 
soon  finds  that  he  loses  breath,  gets  perhaps  a  "stitch  in  the  side," 
and  feels  his  heart  beating  with  unwonted  violence.  If  he  perse- 
vere he  will  probably  faint — or  vomit,  as  is  frequently  seen  in  the 
case  of  imperfectly  trained  men  at  the  end  of  a  hard  boat-race. 
These  phenomena  are  avoided  by  careful  gradual  preparation 
known  as  " training."  The  immediate  cause  of  them  lies  in  dis- 
turbances of  the  circulatory  and  respiratory  organs,  on  which 
excessive  work  is  thrown. 


CHAPTER  IX 
ANATOMY  OF  THE  NERVOUS  SYSTEM 

General  Statement.  In  Chapter  III  the  special  function  of 
the  nervous  system  was  outlined,  and  was  shown  to  involve  the 
transmission  of  stimuli  from  the  sensory  regions  of  the  Body  to 
the  active  tissues  (muscles  and  glands),  and  in  the  course  of  such 
transmission  to  make  whatever  modifications  are  necessary  to  the 
production  of  the  best  .results.  The  sensory  regions  of  the  Body  are 
numerous;  there  are  likewise  many  muscles.  Successful  adaptation 
of  the  individual  to  his  surroundings  may  call  at  one  time  or  an- 
other for  the  passage  of  stimuli  from  any  sensory  region  to  any 
muscle,  or  for  the  combination  of  stimuli  from  several  sensory 
regions  to  form  stimuli  to  go  to  any  group  of  muscles.  A  somewhat 
analogous  situation  occurs  in  the  telephone  systems  which  are 
such  important  features  of  modern  life.  In  these  communication 
may  be  desired  between  any  pair  of  instruments  in  the  system. 
To  make  this  possible  all  the  telephones  in  any  one  system  are  led 
into  a  central  exchange  where  provision  is  made  for  connecting  any 
instrument  with  any  other.  Flexibility  of  communication  between 
sensory  and  motor  regions  in  the  Body  is  secured  in  somewhat 
similar  fashion.  All  nerves  from  sensory  regions  are  led  into  a 
central  "exchange"  from  which  start  all  nerves  to  the  motor 
organs. 

Nerve  Impulses.  Since  it  is  impossible  to  describe  the  nervous 
system  without  frequent  reference  to  the  messages  which  nerves 
carry  it  is  desirable  before  proceeding  farther  to  state  that  it  has 
become  the  custom  to  call  these  messages  nerve  impulses.  When 
we  speak  of  a  nerve  impulse  we  have  in  mind  the  process  by  which 
the  message  is  transmitted  along  the  nerve.  The  situation  cor- 
responds to  that  in  a  telephone  wire.  When  the  latter  is  trans- 
mitting a  message  the  words  spoken  into  the  transmitter  are 
not  carried  along,  but  an  electrical  disturbance  which  they  set 
up.  So  the  nerve  does  not  transmit  the  exact  stimulus  which  acts 
upon  it,  but  a  nerve  impulse  which  the  stimulus  arouses. 

135 


136 


THE  HUMAN  BODY 


Neurons.  The  nervous  system  as  a  whole  is  made  up  of  struc- 
lures  callod  neurons,  each  of  which  seems  to  be  a  single  nerve- 
cell. 

A  typical  neuron  consists  of  a  cell-body  containing  a  nucleus  and 
from  whose  surface  project  many  rather  short  branching  proc- 
esses called  dendrites,  and  a  single  long  process  having  few  if  any 
branches  and  known  as  the  axon  (Fig.  55 A).  Neurons  which  con- 
vey impulses  to  muscles  (motor  neurons}  have  this  structure  (Fig. 


A  B  C 

FIG.  54. — Types  of  neurons.    A,  motor;  B,  sensory;  C,  association. 

54A).  The  only  branching  of  a  motor  neuron  is  at  its  very  end, 
where  it  is  distributed  to  the  muscle  fibers  of  which  it  has  control. 
The  number  of  muscle  fibers  thus  innervated  by  one  motor  neuron 
varies  in  different  muscles,  ranging  from  a  half  dozen  to  fifty  or 
more. 

The  neurons  which  convey  impulses  from  sensory  regions  to 
the  center  (sensory  neurons}  have  a  structure  which  appears  at 
first  view,  to  be  altogether  different  from  that  of  the  typical  neuron 
just  described.  They  have  cell-bodies  with  nuclei  but  instead  of  a 
single  axon  and  numerous  much-branched  dendrites  the  cell-body 
gives  rise  to  two  long  axon-like  processes,  one  connecting  with  the 


ANATOMY  OF  THE  NERVOUS  SYSTEM  137 

receptor  and  the  other  having  a  number  of  branches.  The  bipolar 
character  of  these  neurons,  moreover,  is  concealed  in  many  through 
the  union  of  the  two  processes  for  a  short  distance  from  the  cell- 
body,  giving  an  appearance  as  though  the  latter  were  on  a  side 
branch  of  a  long  axon  (Fig.  546).  The  underlying  similarity  of 
these  to  the  type  neuron  appears  if  we  consider  that  the  dendrites 
of  the  typical  neuron  are  replaced  in  the  sensory  neuron  by  the 
axon-like  process  which  connects  with  the  receptor. 

A  third  sort  of  neurons  occurring  in  the  Body  resembles  the 
first  or  motor  type  in  the  possession  of  cell-body  and  many  branch- 
ing dendrites.  Instead  of  long,  slightly  branched  axons,  however, 
neurons  of  this  sort  have  short  and  very  much-branched  ones. 
These  neurons  occur  interposed  in  the  pathway  of  impulses  from 
sensory  to  motor  neurons  and  are  often  called  association  neurons 
(Fig.  54C):  they  are  not,  however,  the  only  sort  of  association  neu- 
rons; many  neurons  which  belong  physiologically  to  the  group  of 
association  neurons  in  that  they  form  communicating  paths  be- 
tween sensory  and  motor  neurons  are  anatomically  of  the  type  to 
which  all  motor  neurons  belong. 

If  we  adopt  the  usual  view  that  each  single  neuron  represents 
one  nerve-cell,  neurons  are  the  largest  cells  known.  Although 
axons  are  so  small  in  cross-section  as  to  be  microscopic  they  may 
have  a  length  of  three  feet  or  more,  as  in  the  nerve  trunks  which 
extend  down  the  legs  to  the  feet. 

The  nervous  system  as  a  whole  is  made  up  of  neurons  of  these 
three  types.  The  sensory  neurons,  as  stated  above,  lead  from  the 
receptors  to  the  center;  motor  neurons  lead  from  the  center  to  the 
active  tissues;  and  association  neurons  form  the  connecting  links 
wherever  such  are  necessary.  All  sensory  neurons  communicate 
with  other  neurons  at  their  central  terminations.  Since  the  central 
axons  are  branched  (see  above)  each  sensory  neuron  has  a  number 
of  such  connections.  All  motor  neurons  have  likewise  connections 
with  other  neurons  at  their  central  ends.  Association  neurons 
connect  with  other  neurons  at  both  ends,  as  they  must  if  they  are 
to  serve  as  links  in  a  chain  whose  ends  are  sensory  and  motor 
neurons. 

Synapses.  Communication  between  neuron  and  neuron  is 
always  according  to  a  certain  scheme.  The  axons  of  all  except 
motor  neurons  end  in  masses  of  fine  branches  known  as  end  arbori- 


138  THE  HUMAN  BODY 

zations.  These  are  in  contact  with  the  branching  dendrites  of  some 
other  neuron.  The  surfaces  of  contact  between  the  end  arboriza- 
tion of  one  neuron  and  the  dendrites  of  another  constitute  what  is 
called  a  synapse.  In  order  for  a  nerve  impulse  to  pass  from  one 
neuron  to  another  it  must  cross  this  synapse. 

The  Myelin  Sheath.  All  true  nerve  tissue  has  a  characteristic 
gray  color.  This  statement  applies  equally  to  cell-bodies,  den- 
drites, and  axons.  Most,  but  not  all,  of  the  long  axons  of  the 
body  are  inclosed  within  sheaths  composed  chiefly  of  a  substance, 
myelin,  which  has  a  characteristic  glossy  white  color.  The  myelin 
sheath  where  present  does  not  inclose  the  axon  throughout  its 
entire  length;  near  the  cell-body  and  again  near  its  termination 
the  axon  is  not  inclosed.  Surrounding  the  myelin  sheath,  or, 
where  it  is  absent,  the  axon  itself,  is  a  delicate  membrane,  the 
neurilemma.  The  myelin  sheath  is  made  up  of  short  seg- 
ments which  are  separated  one  from  another  by  the  nodes  of 
Ranvier. 

The  myelin  sheath  is  not  composed  of  living  cells  and  so  does 
not  contain  nuclei.  The  neurilemma,  however,  is  a  living  mem- 
brane; scattered  along  it  at  intervals  are  nuclei.  The  function 
of  the  myelin  sheath  is  not  known.  Perhaps  the  most  satisfactory 
suggestion  that  has  been  offered  is  that  it  serves  as  an  insulator 
to  keep  the  nerve  impulse  within  its  own  axon  and  prevent  its 
escape  to  adjacent  ones. 

Axons  which  are  inclosed  in  myelin  sheaths  are  spoken  of  as 
medullated  or  myelinated  nerve-fibers. 

It  is  the  presence  of  myelin  sheaths  that  gives  to  certain  parts 
of  the  nervous  system  their  characteristic  white  appearance.  All 
"  white  matter  "  is  made  up  of  medullated  axons.  "  Gray  matter," 
on  the  other  hand,  is  made  of  cell-bodies  and  dendrites,  together 
with  some  non-medullated  axons. 

The  Central  and  Peripheral  Nervous  Systems.  In  a  preceding 
paragraph  was  pointed  out  the  analogy  between  the  nervous  sys- 
tem and  a  telephone  system.  That  part  of  the  nervous  system 
corresponding  to  the  telephone  "  exchange,"  to  which  sensory 
neurons  lead  and  from  which  motor  neurons  spring  is  called  the 
central  nervous  system.  It  consists  of  the  brain  and  spinal  cord. 
(The  analogy  between  the  central  nervous  system  and  a  telephone 
exchange  should  not  be  pushed  too  far,  for  the  central  nervous 


ANATOMY  OF  THE  NERVOUS  SYSTEM  139 

system  has  numerous  functions  in  addition  to  the  simple  one  of 
making  connections  between  sensory  and  motor  neurons.  These 
special  functions  have  to  do  with  the  modification  of  the  impulses 
passing  through  it  for  the  best  advantage  of  the  organism  as  a 
whole.) 

Springing  from  the  central  nervous  system  and  corresponding 
to  the  cables  bearing  wires  to  individual  telephones  are  forty- 
three  pairs  of  nerve-trunks.  Twelve  pairs  arise  from  the  brain  and 
are  called  cranial  nerves;  the  remaining  thirty-one  pairs  arise  from 
the  spinal  cord  and  are  called  spinal  nerves.  Each  nerve-trunk 
contains  a  large  number  of  axons,  and  in  most  nerve-trunks  the 
axons  of  both  motor  and  sensory  neurons  are  present.  These 
forty-three  pairs  of  nerve-trunks  with  their  ramifications  to  all  parts 
of  the  Body  constitute  the  peripheral  nervous  system  (Fig.  55). 

There  are  in  the  Body  a  set  of  neurons  which  though  part  of 
the  peripheral  nervous  system  are  specially  adapted  for  a  certain 
function  and  are  therefore  usually  considered  independently. 
These  constitute  the  sympathetic  or  autonomic  system. 

The  Central  Nervous  System  and  its  Membranes.  Lying  in- 
side the  skull  is  the  brain  and  in  the  neural  canal  of  the  verte- 
bral column  the  spinal  cord,  the  two  being  continuous  through 
the  foramen  magnum  of  the  occipital  bone.  The  central  nervous 
system  is  bilaterally  symmetrical  throughout  except  for  slight 
differences  on  the  surfaces  of  parts  of  the  brain,  which  are  often 
found  in  the  higher  races  of  mankind.  Both  brain  and  spinal  cord 
are  very  soft  and  easily  crushed;  nervous  tissue  as  well  as  the  con- 
nective tissue  and  a  peculiar  supporting  tissue  (neuroglia)  which 
pervades  it  being  delicate;  accordingly  both  organs  are  placed  in 
nearly  completely  closed  bony  cavities  and  are  also  enveloped  by 
membranes  which  give  them  support.  These  membranes  are 
three  in  number.  Externally  is  the  dura  mater,  very  tough  and 
strong  and  composed  of  white  fibrous  and  elastic  connective  tis- 
sues. In  the  cranium  the  dura  mater  adheres  by  its  outer  surface 
to  the  inside  of  the  skull  chamber,  serving  as  the  periosteum  of  its 
bones;  this  is  not  the  case  in  the  vertebral  column,  where  the 
dura  mater  forms  a  loose  sheath  around  the  spinal  cord  and  is 
only  attached  here  and  there  to  the  surrounding  bones,  which 
have  a  separate  periosteum  of  their  own.  The  innermost  mem- 
brane lies  in  immediate  contact  with  the  proper  nervous  parts. 


140 


THE  HUMAN  BODY 


End 
,  arborization 


PIG.  55.— Diagram  illustrating  the  general  arrangement      Fia.  55a.— Diagram  of  neuron 
of  the  nervous  system.  (Stdhr.) 


ANATOMY  OF  THE  NERVOUS  SYSTEM 


141 


D    1' 


This  is  the  pia  mater,  also  made  up  of  A 
white  fibrous  tissue  interwoven  with 
elastic  fibers,  but  less  closely  than  in 
the  dura  mater,  so  as  to  form  a  less  dense 
and  tough  membrane.  The  pia  mater 
contains  many  blood-vessels  which  break 
up  in  it  into  small  branches  before  enter- 
ing the  nervous  mass  beneath.  Covering 
the  outside  of  the  pia  mater  is  a  layer  of 
flat  closely  fitting  cells;  a  similar  layer 
lines  the  inside  of  the  dura  mater,  and 
these  two  layers  make  up  the  third  mem- 
brane, called  the  arachnoid.  In  the  space 
between  the  two  layers  of  the  arachnoid 
is  contained  a  small  quantity  of  watery 
cerebrospinal  liquid.  The  surface  of  the 
brain  is  folded  and  the  pia  mater  follows 
closely  these  folds;  the  arachnoid  often 
stretches  across  them:  in  the  spaces  thus 
left  between  it  and  the  pia  mater  is  con- 
tained some  of  the  cerebrospinal  liquid. 
Ventricles  of  the  Brain  and  Central 
Canal  of  the  Spinal  Cord.  The  central 
nervous  system  begins  its  embryonic 
development  as  a  groove  in  the  layer  of 
cells  which  forms  the  upper  surface  of  the 
embryo.  This  groove  deepens,  and  finally 
cuts  itself  off  from  the  cell  layer  of  which 
it  was  at  first  a  part.  The  edges  grow  to- 
gether transforming  the  groove  into  a 
tube.  The  cell  layer  heals  over,  leaving 
the  neural  tube  beneath  it.  The  hollow  in 
the  tube  persists  throughout  life.  In  the 
adult  spinal  cord  it  is  represented  by  the 
tiny  central  canal  (Fig.  65).  As  the  front 
end  of  the  neural  tube  develops  into  the  gom  the  ventral,  and  B,  from 

^  the  dorsal  aspect  ;C  to  H,  cross- 

COmplex  brain  the  hollow  in   this  region  sections  at  different  levels. 

takes  the  form  of  a  series  of  chambers  of  extremely  irregular  shape, 
communicating  with  each  other  and  with  the  central  canal  of  the 


G 


H 


C°A, 


142  THE  HUMAN  BODY 

spinal  cord  by  narrow  channels.  The  chambers  are  four  in  number, 
and  are  known  as  the  ventricles  of  the  brain.  There  is  one  in  each 
cerebral  hemisphere  (p.  145).  These  are  called  the  lateral  ventricles. 
Numerically  they  rank  as  first  and  second.  The  lateral  ventricles 
open  into  a  narrow  chamber  in  the  base  of  the  cerebrum  in  the 
mid  line,  known  as  the  third  ventricle.  This  in  turn  communi- 
cates by  a  narrow  channel  (the  aqueduct  of  Sylvius)  with  the 
cavity  of  the  brain  stem  (p.  146)  which  is  called  the  fourth  ven- 
tricle. 

The  cavity  of  the  fourth  ventricle  communicates  with  the 
arachnoid  space  (p.  141)  by  three  small  openings  in  its  roof,  one  in 
the  mid  line  and  one  at  each  lateral  border.  By  these  openings  the 
cerebrospinal  fluid  which  occupies  the  arachnoid  space  is  con- 
tinuous with  that  which  fills  the  ventricles  and  central  canal. 

Cerebrospinal  Fluid.  This  fluid,  which  occupies  all  the  spaces 
within  and  around  the  central  nervous  system,  is  in  general  similar 
to  the  medium,  lymph,  by  which  the  other  tissues  of  the  body  are 
bathed  (p.  18).  It  represents  some  chemical  differences,  however, 
which  become  accentuated  in  certain  diseases.  In  the  lumbar 
region  there  is  room  between  the  processes  of  the  vertebrae  so  that 
a  hypodermic  needle  can  be  thrust  into  the  arachnoid  space  and 
some  of  the  cerebrospinal  fluid  withdrawn.  The  operation  is 
simple  and  by  the  application  of  cocaine  to  the  skin  made  virtually 
painless.  Chemical  examination  of  the  fluid  so  obtained  is  often 
helpful  in  diagnosing  obscure  complaints.  Under  certain  diseased 
conditions  there  is  a  great  accumulation  of  cerebrospinal  fluid. 
The  pressure  of  this  upon  the  delicate  nervous  structures  is  likely 
to  do  them  harm,  and  "lumbar  puncture"  is  often  resorted  to  to 
draw  off  the  accumulated  fluid  and  relieve  the  pressure.  Some- 
times in  young  children  the  accumulation  of  fluid  distends  the 
head  far  beyond  its  normal  size,  giving  the  condition  known  as 
"  Jiydrocephalus." 

The  Spinal  Cord  (Fig.  56)  is  nearly  cylindrical  in  form,  being 
however  a  little  wider  from  side  to  side  than  dorsiventrally,  and 
tapering  off  at  its  posterior  end.  Its  average  diameter  is  about  19 
millimeters  (f  inch)  and  its  length  0.43  meter  (17  inches).  It 
weighs  42.5  grams  (\\  ounces).  There  is  no  marked  limit  be- 
tween the  spinal  cord  and  the  brain,  the  one  passing  gradually  into 
the  other  (Fig.  62),  but  the  cord  is  arbitrarily  said  to  commence 


ANATOMY  OF  THE  NERVOUS  SYSTEM 


143 


opposite  the  outer  margin  of  the  foramen  magnum  of  the  occipital 
bone:  from  there  it  extends  to  the  articulation  between  the  first  and 
second  lumbar  vertebrae,  where  it  narrows  off  to  a  slender  non- 
nervous  filament,  the  filum  terminate  (cut  off  and  represented 
separately  at  Bf  in  Fig.  56),  which  runs  back  to  the  end  of  the 
neural  canal  behind  the  sacrum.  In  its  course  the  cord  presents 
two  expansions,  an  upper,  10,  the  cervical  enlargement,  reaching 
from  the  third  cervical  to  the  first  dorsal  vertebra,  and  a  lower  or 
lumbar  enlargement,  9,  opposite  the  last  dorsal  vertebra. 

Running  along  the  middle  line  on  both  the  ventral  and  the 
dorsal  aspects  of  the  cord  is  a  groove,  and  a  cross-section  shows 


.6' 


FIG.  57.-^-The  spinal  cord  and  nerve-roots.  A,  a  small  portion  of  the  cord  seen 
from  the  ventral  side;  B,  the  same  seen  laterally;  C,  a  cross-section  of  the  cord; 
D,  the  two  roots  of  a  spinal  nerve;  1,  ventral  fissure;  2,  dorsal  fissure;  3,  surface 
groove  along  the  line  of  attachment  of  the  ventral  nerve-roots;  4,  line  of  origin  of 
the  dorsal  roots;  5,  ventral  root  filaments  of  spinal  nerve;  6,  dorsal  root  filaments; 
(>',  ganglion  of  the  dorsal  root;  7,  7',  the  first  two  divisions  of  the  nerve-trunk  after 
its  formation  by  the  union  of  the  two  roots.  The  grooves  are  much  exaggerated. 

that  these  grooves  are  the  surface  indications  of  fissures  which 
extend  deeply  into  the  cord  (C,  Fig.  57)  and  nearly  divide  it  into 
right  and  left  halves. 

The  ventral  fissure  (1,  Fig.  57)  is  wider  and  shallower  than  the 
dorsal,  2,  which  indeed  is  hardly  a  true  fissure,  being  completely 
filled  up  by  an  ingrowth  of  pia  mater.  The  transverse  section, 


144  THE  HUMAN  BODY 

C,  shows  also  that  the  substance  of  the  cord  is  not  alike  through- 
out, but  thai  its  white  superficial  layers  envelop  a  central  gray 
substance  arranged  somewhat  in  the  form  of  a  capital  H.  Each 
half  of  the  gray  matter  is  crescent-shaped,  and  the  crescents  are 
turned  back  to  back  and  united  across  the  middle  line  by  the 
gray  commissure.  The  tips  of  each  crescent  are  called  its  horns  or 
cornua,  and  the  ventral  horn  on  each  side  is  thicker  and  larger 
than  the  dorsal.  In  the  cervical  and  lumbar  enlargements  the 
proportion  of  white  to  gray  matter  is  greater  than  elsewhere;  and 
as  the  cord  approaches  the  medulla  oblongata  its  central  gray 
mass  becomes  irregular  in  form  and  begins  to  break  up  into  smaller 


FIG.  58. — Diagram  illustrating  the  general  relationships  of  the  parts  of  the  brain. 

A,  fore-brain;  b,  midbrain;  B,  cerebellum;  C,  pons  Varolii;  D,  medulla  oblongata; 

B,  C,  and  D  together  constitute  the  hind-brain. 

portions.  If  lines  be  drawn  on  the  transverse  section  of  the  cord 
from  the  tip  of  each  horn  of  the  gray  matter  to  the  nearest  point 
of  the  surface,  the  white  substance  in  each  half  will  be  divided  into 
three  portions:  one  between  the  ventral  fissure  and  the  ventral 
cornu,  and  called  the  ventral  white  column;  one  between  the  dorsal 
fissure  and  the  dorsal  cornu,  and  called  the  dorsal  white  column; 
while  the  remaining  one  lying  in  the  hollow  of  the  crescent  and 
between  the  two  horns  is  the  lateral  column:  the  ventral  and  lateral 
columns  of  the  same  side  are  frequently  named  the  ventrolateral 
column.  A  certain  amount  of  white  substance  crosses  the  middle 


ANATOMY  OF  THE  NERVOUS  SYSTEM  145 

line  at  the  bottom  of  the  ventral  fissure;  this  forms  the  ventral  white 
commissure.  There  is  no  dorsal  white  commissure,  the  bottom  of 
the  dorsal  fissure  being  the  only  portion  of  the  cord  where  the  gray 
substance  is  uncovered  by  white.  Running  along  the  middle  of 
the  gray  commissure,  for  the  whole  length  of  the  cord,  is  the  central 
canal,  previously  described.  It  is  a  tiny  channel,  just  visible  to  the 
unaided  eye. 

The  Brain  (Fig.  58)  is  far  larger  than  the  spinal  cord  and  more 
complex  in  structure.  It  weighs  on  the  average  about  1,415  grams 
(50  ounces)  in  the  adult  male,  and  about  155  grams  (5.5  ounces) 
less  in  the  female.  In  its  simpler  forms  the  vertebrate  brain  con- 

Cb 


Po 


FIG.  59. — The  brain  from  the  left  side.  Cb,  the  cerebral  hemispheres  forming 
the  main  bulk  of  the  fore-brain;  Cbl,  the  cerebellum;  Mo,  the  medulla  oblongata; 
P,  the  pons  Varolii;  *,  the  fissure  of  Sylvius;  Ro,  the  fissure  of  Rolando;  Po,  the 
Parieto-occipital  fissure. 

sists  of  three  masses,  each  with  subsidiary  parts,  following  one 
another  in  series  from  before  back,  and  known  as  the  fore-brain, 
midbrain,  and  hind-brain  respectively.  In  man  the  fore-brain, 
A,  weighing  about  1,245  grams  (44  ounces),  is  much  larger  than 
all  the  rest  put  together  and  laps  over  them  behind.  It  consists 
mainly  of  two  large  convoluted  masses,  separated  from  one  an- 
other by  a  deep  median  fissure,  and  known  as  the  cerebral  hemi- 
spheres. The  immense  proportionate  size  of  these  is  very  char- 
acteristic of  the  human  brain.  Beneath  each  cerebral  hemisphere 
is  an  olfactory  lobe,  inconspicuous  in  man  but  in  many  animals 
larger  than  the  cerebral  hemispheres.  Buried  in  the  fore-brain 


14G 


THE  HUMAN  BODY 


on  each  side  are  two  large  gray  masses,  the  corpora  striata  and 
optic  thalami.  The  midbrain  forms  a  connecting  isthmus  between 
the  two  other  divisions  and  presents  on  its  dorsal  side  four  hemi- 
spherical eminences,  the  corpora  quadrigemina  or  colliculi.  On 
its  ventral  side  it  exhibits  two  semicylindrical  pillars  (seen  under 
the  nerve  IV  in  Fig.  62),  known  as  the  crura  cerebri.  The  hind- 
brain  consists  of  three  main  parts:  on  its  dorsal  side  is  the  cere- 
bellum, B  (Fig.  58),  consisting  of  a  right,  a  left,  and  a  median  lobe; 
on  the  ventral  side  is  the  pons  Varolii,  C  (Fig.  58),  and  behind 


Cc,      Ptj. 


,Th. 


C.b.- 


op. 


FIG.  60. — Diagram  of  the  left  half  of  a  vertical  median  section  of  the  brain, 
(Sobotta-McMurrich,  Atlas  and  Text-book  of  Human  Anatomy).  H,  H,  con- 
voluted inner  surface  of  left  cerebral  hemisphere;  Cc,  corpus  callosum;  Th,  optic 
thalamus;  e.g.,  corpora  quadrigemina;  Cb,  cerebellum;  Sp.c,  spinal  cord;  Mo. 
medulla  oblongata;  P,  pons  Varolii;  oc,  oculo-motor  nerve;  pt,  pituitary  body;  op, 
optic  nerve;  Ro,  fissure  of  Rolando;  Po,  parieto-occipital  fissure;  Fr,  frontal  lobe; 
Pa,  parietal  lobe;  O,  occipital  lobe. 

that  the  medulla  oblongata,  D  (Fig.  58),  which  is  continuous  with 
the  spinal  cord.  The  medulla  and  midbrain  together  make  up  the 
brain  stem. 

In  nature,  the  main  divisions  of  the  brain  are  not  separated  so 
much  as  has  been  represented  in  the  diagram  for  the  sake  of  clear- 
ness, but  lie  close  together,  as  represented  in  Fig.  59,  only  some 
folds  of  the  membranes  extending  between  them;  and  the  mid- 
brain  is  entirely  covered  in  on  its  dorsal  aspect.  Nearly  every- 
where the  surface  of  the  brain  is  folded,  the  folds,  known  as  gyri 


ANATOMY  OF  THE  NERVOUS  SYSTEM  147 

or  convolutions  being  deeper  and  more  numerous  in  the  brain  of 
man  than  in  that  of  the  animals  nearest  allied  to  him;  and  in  the 
human  species  more  marked  in  the  higher  than  in  the  lower  races. 
It  should  however  be  added  that  some  species  of  animals  which 
are  not  markedly  intelligent  have  much  convoluted  cerebral 
hemispheres. 

The  brain,  like  the  spinal  cord,  consists  of  gray  and  white  nervous 
matter,  but  somewhat  differently  arranged,  for  while  the  brain,  like 
the  cord,  contains  gray  matter  in  its  interior,  a  great  part  of  its 
surface  is  also  covered  with  it.  By  the  external  convolutions  of  the 
cerebellum  and  the  cerebral  hemispheres  the  surface  over  which 
this  gray  substance  is  spread  is  very  much  increased  (see  Fig.  59). 

The  Spinal  Nerves.  Thirty-one  pairs  of  spinal  nerve-trunks 
enter  the  neural  canal  of  the  vertebral  column  through  the  in- 
tervertebral  foramina  (p.  57).  Each  divides  in  the  foramen  into 
a  dorsal  and  ventral  portion  known  respectively  as  the  dorsal 
and  ventral  roots  of  the  nerve  (6  and  5,  Fig.  57),  and  these  again 
subdivide  into  finer  branches  which  are  attached  to  the  sides  of 
the  cord,  the  dorsal  root  at  the  point  where  the  dorsal  and  lateral 
white  columns  meet,  and  the  ventral  root  at  the  junction  of  the 
lateral  and  ventral  columns.  Although  the  nerve-trunks  contain 
both  sensory  and  motor  neurons  these  are  completely  separated 
in  the  roots;  the  dorsal  root  contains  only  sensory  neurons;  the 
ventral  only  motor.  At  the  lines  on  which  the  roots  are  attached 
there  are  superficial  furrows  on  the  surface  of  the  cord.  On  each 
dorsal  root  is  a  spinal  ganglion  (6',  Fig.  57),  placed  just  before 
it  joins  the  ventral  root  to  make  up  the  common  .nerve-trunk. 
This  spinal  ganglion  contains  the  cell-bodies  of  the  bipolar  sensory 
neurons.  Immediately  after  its  formation  by  the  mixture  of 
fibers  from  both  roots,  the  trunk  divides  (D,  Fig.  57),  into  a  dorsal 
primary,  a  ventral  primary,  and  a  communicating  branch.  The 
branches  of  the  first  set  go  for  the  most  part  to  the  skin  and  mus- 
cles on  the  back;  from  the  second  the  nerves  for  the  sides  and 
ventral  region  of  the  neck  and  trunk  and  for  the  limbs  arise;  the 
communicating  branches  form  part  of  the  sympathetic  system. 

The  various  spinal  nerves  are  named  from  the  portions  of  the 
vertebral  column  through  the  intervertebral  openings  of  which 
they  pass  out;  and  as  a  general  rule  each  nerve  is  named  from  the 
vertebra  in  front  of  it.  For  example,  the  nerve  passing  out  be- 


148  THE  HUMAN  BODY 

tween  the  fifth  and  sixth  thoracic  vertebrae  is  the '"fifth  thoracic" 
nerve,  and  that  between  the  last  thoracic  and  first  lumbar  verte- 
brae, the  "twelfth  thoracic."  In  the  cervical  region,  however, 
this  rule  is  not  adhered  to.  The  nerve  passing  out  between  the 
occipital  bone  and  the  atlas  is  called  the  "first  cervical"  nerve, 
that  between  the  atlas  and  axis  the  second,  and  so  on;  that  be- 
tween seventh  cervical  and  first  thoracic  vertebrae  being  the 
"eighth  cervical"  nerve.  The  thirty-one  pairs  of  spinal  nerves 
are  then  thus  distributed :  8  cervical,  12  thoracic,  5  lumbar,  5  sacral, 
and  1  coccygeal;  the  latter  passing  out  between  the  sacrum  and 
coccyx.  Since  the  spinal  cord  ends  opposite  the  upper  lumbar 
vertebrae  while  the  sacral  and  coccygeal  nerves  pass  out  from  the 
neural  canal  much  farther  back,  it  is  clear  that  the  roots  of  those 
nerves,  on  their  way  to  unite  in  the  foramina  of  exit  and  form 
nerve-trunks,  must  run  obliquely  backwards  in  the  spinal  canal  for 
a  considerable  distance.  One  finds  in  fact  the  neural  canal  in  the 
lumbar  and  sacral  regions,  behind  the  point  where  the  spinal  cord 
has  tapered  off  to  form  the  filum  terminate,  occupied  chiefly  by  a 
great  bunch  of  nerve-roots  forming  the  so-called  "horse's  tail"  or 
cauda  equina. 

Plexuses.  Very  frequently  several  neighboring  nerve-trunks 
send  off  communicating  branches  to  one  another,  each  branch 
carrying  fibers  from  one  trunk  to  the  other.  Such  networks  are 
called  plexuses  (Fig.  61),  and  through  the  interchanges  taking 
place  in  them  it  often  happens  that  the  distal  branches  of  a  nerve- 
trunk  contain  fibers  which  it  does  not  possess  as  it  leaves  the 
central  nervous  system. 

Distribution  of  the  Spinal  Nerves.  It  would  be  out  of  place 
here  to  go  into  detail  as  to  the  exact  portions  of  the  Body  sup- 
plied by  each  spinal  nerve,  but  the  following  general  statements 
may  be  made.  The  ventral  primary  branches  of  the  first  four 
cervical  nerves  form  on  each  side  the  cervical  plexus  (Fig.  61) 
from  which  branches  are  supplied  to  the  muscles  and  skin  of  the 
neck:  also  to  the  outer  ear  and  the  back  part  of  the  scalp.  The 
ventral  primary  branches  of  the  remaining  cervical  nerves  and 
the  first  dorsal  form  the  brachial  plexus,  from  which  the  upper 
limb  is  supplied.  The  roots  of  the  trunks  which  form  this  plexus 
arise  from  the  cervical  enlargement  of  the  spinal  cord. 

From  the  fourth  and  fifth  cervical  nerves  on  each  side,  small 


ANATOMY  OF  THE  NERVOUS  SYSTEM 


149 


branches  arise  and  unite  to  make  the  phrenic  nerve  (4',  Fig.  61) 

which  runs  down  through  the  chest  and  ends  in  the  diaphragm. 

The  ventral  primary  branches  of  the  thoracic  nerves,  except 

part  of  the  first  which  enters  the  brachial  plexus,  form  no  plexus, 


FIG.  61. — The  cervical  and  brachial  plexuses  of  the  left  side  of  the  Body. 

but  each  runs  along  the  posterior  border  of  a  rib  and  supplies 
branches  to  the  chest-walls,  and  the  lower  ones  to  those  of  the 
abdomen  also. 

The  ventral  primary  branches  of  the  four  anterior  lumbar 
nerves  are  united  by  branches  to  form  the  lumbar  plexus.    It  sup- 


150  THE  HUMAN  BODY 

plies  the  lower  part  of  the  trunk,  the  buttocks,  the  front  of  the 
thigh,  and  inner  side  of  the  leg. 

The  sacra/  plexus  is  formed  by  the  anterior  primary  branches 
of  the  fifth  lumbar  and  the  first  four  sacral  nerves,  which  unite 
in  one  great  cord  and  so  form  the  sciatic  nerve,  which  is  the  largest 
in  the  Body  and,  running  down  the  back  of  the  thigh,  ends  in 
branches  for  the  lower  limb.  The  roots  of  the  trunks  which  form 
the  sacral  plexus  arise  from  the  lumbar  enlargement  of  the  cord. 

Cranial  Nerves.  Twelve  pairs  of  nerves  leave  the  skull  by 
apertures  in  its  base,  and  are  known  as  the  cranial  nerves.  Most 
of  them  spring  from  the  under  side  of  the  brain,  and  so  they  are 
best  studied  in  connection  with  the  base  of  that  organ,  which  is 
represented  in  Fig.  62.  The  first  pair,  or  olfactory  nerves,  spring 
from  the  under  sides  of  the  olfactory  lobes,  /,  and  pass  out  through 
the  roof  of  the  nose.  They  are  the  nerves  of  smell.  The  second 
pair,  or  optic  nerves,  II,  spring  from  the  optic  thalami  and  corpora 
quadrigemina,  and,  under  the  name  of  optic  tracts,  run  down  to 
the  base  of  the  brain,  where  they  appear  passing  around  the  crura 
cerebri,  as  represented  in  the  figure.  In  the  middle  line  the  two 
optic  tracts  unite  to  form  the  optic  chiasma,  from  which  an  optic 
nerve  proceeds  to  each  eyeball. 

All  the  remaining  cranial  nerves  arise  from  the  hind-brain. 
The  third  pair  (motores  oculi)  arise  from  the  front  of  the  pons 
Varolii,  and  are  distributed  to  most  of  the  muscles  which  move 
the  eyeball  and  also  to  that  which  lifts  the  upper  eyelid. 

The  fourth  pair  of  nerves  (paihetici)  IV,  arise  from  behind  the 
crura  cerebri.  From  there,  each  curls  around  a  cms  cerebri  (the 
cylindrical  mass  seen  beneath  it  in  the  figure,  running  from  the 
pons  Varolii  to  enter  the  under  surface  of  the  cerebral  hemispheres) 
and  appears  on  the  base  of  the  brain.  Each  goes  to  one  muscle  of 
the  eyeball. 

The  fifth  pair  of  nerves  (trigeminales) ,  V,  resemble  the  spinal 
nerves  in  having  two  roots;  one  of  these  is  much  larger  than  the 
other  and  possesses  a  ganglion  (the  Gasserian  or  semilunar  gan- 
glion) like  the  dorsal  root  of  a  spinal  nerve.  Beyond  the  ganglion 
the  two  roots  form  a  common  trunk  which  divides  into  three 
main  branches.  Of  these,  the  ophthalmic  is  the  smallest  and  is 
mainly  distributed  to  the  muscles  and  skin  over  the  forehead  and 
upper  eyelid;  but  also  gives  branches  to  the  mucous  membrane 


ANATOMY  OF  THE  NERVOUS  SYSTEM 


151 


lining  the  nose,  and  to  the  integument  over  it.  The  second  di- 
vision (superior  maxillary  nerve)  of  the  trigeminal  gives  branches 
to  the  skin  over  the  temple,  to  the  cheek  between  the  eyebrow 
and  the  angle  of  the  mouth,  and  to  the  upper  teeth;  as  well  as  to 
the  mucous  membrane  of  the  nose,  pharynx,  soft  palate  and  roof 


nci 


FIG.  62. — The  base  of  the  brain.  The  cerebral  hemispheres  are  seen  overlapping 
all  the  rest.  /,  olfactory  lobes;  //,  optic  tract  passing  to  the  optic  chiasma  from 
which  the  optic  nerves  proceed;  ///,  the  third  nerve  or  motor  oculi;  IV,  the  fourth 
nerve  or  patheticus;  V,  the  fifth  nerve  or  trigeminalis;  VI,  the  sixth  nerve  or  ab- 
ducens;  VII,  the  seventh  or  facial  nerve;  VIII,  the  auditory  nerve;  IX,  the  ninth 
or  glossopharyngeal;  X,  the  tenth  or  pheumogastric  or  vagus;  XI,  the  spinal  ac- 
cessory; XII,  the  hypoglossal;  nci,  the  first  cervical  spinal  nerve. 

of  the  mouth.  The  third  division  (inferior  maxillary)  is  the  largest 
branch  of  the  trigeminal;  it  receives  some  fibers  from  the  larger 
root  and  all  of  the  smaller.  It  is  distributed  to  the  side  of  the 
head  and  the  external  ear,  the  lower  lip  and  lower  part  of  the  face, 
the  mucous  membrane  of  the  mouth  and  the  anterior  two-thirds 


152  THE  HUMAN  BODY 

of  the  tongue,  the  lower  teeth,  the  salivary  glands,  and  the  muscles 
which  move  the  lower  jaw  in  mastication. 

The  sixth  pair  of  cranial  nerves  VI,  or  abducentes  arises  from 
the  posterior  margin  of  the  pons  Varolii,  and  each  is  distributed 
to  one  muscle  of  the  eyeball. 

The  seventh  pair  (facial  nerves},  VII,  appear  also  at  the  posterior 
margin  of  the  pons.  They  are  distributed  to  most  of  the  muscles 
of  the  face  and  scalp. 

The  eighth  pair  (auditory  nerves),  VIII,  arise  close  to  the  facial. 
They  are  the  nerves  of  hearing  and  are  distributed  entirely  to  the 
internal  ear. 

The  ninth  pair  (glossopharyngeals) ,  IX,  arising  close  to  the 
auditories,  are  distributed  to  the  mucous  membrane  of  the  pharynx, 
the  posterior  part  of  the  tongue,  and  the  middle  ear. 

The  tenth  pair  (pneumogastric  nerves  or  vagi),  X,  arise  from  the 
sides  of  the  medulla  oblongata.  Each  gives  branches  to  the  phar- 
ynx, gullet,  and  stomach,  the  larynx,  windpipe,  and  lungs,  and 
to  the  heart.  The  vagus  runs  farther  through  the  Body  than  any 
other  cranial  nerve. 

The  eleventh  pair  (spinal  accessory  nerves),  XI,  do  not  arise 
mainly  from  the  brain  but  by  a  number  of  roots  attached  to  the 
lateral  columns  of  the  cervical  portion  of  the  spinal  cord,  be- 
tween the  ventral  and  dorsal  roots  of  the  proper  cervical  spinal 
nerves.  Each,  however,  runs  into  the  skull  cavity  alongside  of 
the  spinal  cord  and,  getting  a  few  filaments  from  the  medulla 
oblongata,  passes  out  along  with  the  glossopharyngeal  and  pneu- 
mogastric nerves.  Outside  the  skull  it  divides  into  two  branches, 
one  of  which  joins  the  pneumogastric  trunk,  while  the  other  is 
distributed  to  muscles  about  the  shoulder. 

The  twelfth  pair  of  cranial  nerves  (hypoglossa),  XII,  arise  from 
the  sides  of  the  medulla  oblongata;  they  are  distributed  mainly 
to  the  muscles  of  the  tongue  and  hyoid  bone. 

It  must  be  remembered  that  the  cranial  nerves,  like  the  spinal 
nerves,  are  really  bundles  containing  hundreds  of  axons  having 
various  destinations.  Just  as  in  the  spinal  nerve  plexuses  bundles 
of  axons  cross  over  from  one  nerve-trunk  to  another,  so  in  many 
of  the  cranial  nerves,  especially  the  fifth  and  seventh,  there  are 
branchings  from  one  nerve  to  another,  making  it  difficult  to  tell 
in  many  cases  from  what  part  of  the  brain  the  nerves  to  a  special 


ANATOMY  OF  THE  NERVOUS  SYSTEM  153 

part  have  come;  for  example,  it  was  believed  for  a  long  time  that 
the  axons  mediating  the  sense  of  taste  enter  the  brain  as  part  of 
the  trigeminal  nerve.  It  is  now  practically  certain  that  they  enter 
instead  by  way  of  the  facial  and  glossopharyngeal. 

White  and  Gray  Matter.  In  preceding  paragraphs  the  occur- 
rence of  white  and  gray  matter  in  the  central  nervous  system  has 
been  mentioned.  In  the  paragraph  on  myelin  sheaths  (p.  138) 
the  difference  between  them  was  described.  It  may  be  worth 
while,  for  emphasis,  to  state  again  this  difference  before  discussing 
more  specifically  their  distribution  in  the  nervous  system.  White 
m&tteiLjconsists  of  rnedullated  axons,  and  is  concerned  function- 
ajlvjjberefore,  with  the  conduction  of  impulses  from  point  to 
point.  Gray  matter  consists  of  cell-bodies,  dendrites,  and  parts  of 
axons,  and  in  it  and  it  alone  are  the  synapses  found  over  which 
impulses  pass  across  from  one  neuron  to  another.  Gray  matter, 
therefore,  is  concerned  with  the  distribution  of  nerve  impulses 
among  the  neurons.  In  it  also,  as  we  shall  see,  take  place  the 
modifications  which  nerve  impulses  undergo  during  their  passage 
through  the  central  nervous  system. 

Most  of  the  gray  matter  of  the  Body  is  found  in  three  special 
regions.  These  are:  (1)  the  gray  columns  of  the  spinal  cord; 
(2)  a  layer  about  2  mm.  (Ain.)  thick  over  the  entire  outer  surface 
of  the  cerebral  hemispheres,  including  the  mesial  surface  of  each, 
and  (3)  a  similar  layer  over  the  surface  of  the  cerebellum.  In 
addition  to  these  chief  gray  regions  there  are  a  number  of  small 
masses  of  gray  matter  distributed  in  various  parts  6f  the  Body. 
Some  of  these  are  embedded  in  the  brain;  others  are  outside  the 
central  nervous  system  altogether.  Those  within  the  central  nerv- 
ous system  are  known  as  nuclei,*  those  outside  it  as  ganglia. 

The  gray  nuclei  are  found  in  the  following  regions:  (1)  The 
base  of  the  cerebrum;  these  are  known  as  the  basal  nuclei  and 
include  the  optic  thalami,  the  caudate,  and  the  lenticular  nuclei; 
(2)  the  base  of  the  cerebellum;  here  are  several  pairs  of  nuclei, 
including  the  dentate  nuclei;  (3)  the  midbrain;  here  are  several 
small  nuclei,  the  superior  and  inferior  colliculi  (corpora  quadri- 
gemina),  the  external  and  internal  geniculate  bodies,  and  the  red 

*  It  must  be  understood  that  the  term  nucleus  as  applied  to  a  mass  of  gray 
nervous  matter  has  an  entirely  different  significance  than  when  applied  to  a 
part  of  a  single  cell. 


154  THE  HUMAN  BODY 

nucleus;  (4)  the  medulla;  all  the  gray  matter  of  the  medulla  is 
contained  within  its  nuclei.  They  constitute  the  so-called  deep- 
origins  of  those  cranial  nerves  which  arise  in  the  medulla. 

All  nerve-ganglia  in  the  Body,  using  the  term  ganglia  in  the 
restricted  sense  suggested  above,  fall  into  two  groups:  (1)  Those 
which  contain  the  cell-bodies  of  sensory  neurons;  in  this  group 
belong  all  dorsal  root-ganglia  of  spinal  nerves  (see  p.  143),  like- 
wise the  ganglia  which  are  found  on  some  of  the  cranial  nerves; 
(2)  the  so-called  sympathetic  ganglia  which  are  described  in  the 
next  paragraph. 

The  Sympathetic  or  Autonomic  System.  The  ganglia  which 
form  the  main  centers  of  the  sympathetic  nervous  system  lie  in 
two  rows  (s,  Fig.  2),  one  on  either  side  of  the  bodies  of  the  vertebrae. 
Each  ganglion  is  united  by  a  nerve-trunk  with  the  one  in  front  of 
it,  and  so  two  great  chains  are  formed  reaching  from  the  base  of 
the  skull  to  the  coccyx.  In  the  trunk  region  these  chains  lie  in  the 
ventral  cavity.  The  ganglia  on  these  chains  are  forty-nine  in 
number,  viz.,  twenty-four  pairs,  and  a  single  one  in  front  of  the 
coccyx  in  which  both  chains  terminate.  They  are  named  from  the 
regions  of  the  vertebral  column  near  which  they  lie;  there  being 
three  cervical,  twelve  thoracic,  four  lumbar,  and  five  sacral  pairs. 

Each  sympathetic  ganglion  is  united  by  communicating  branches 
with  the  neighboring  spinal  nerves,  and  near  the  skull  with  various 
cranial  nerves  also ;  while  from  the  ganglia  and  their  uniting  cords 
arise  numerous  trunks,  many  of  which,  in  the  thoracic  and  abdom- 
inal cavities,  form  plexuses,  from  which  in  turn  nerves  are  given 
off  to  the  viscera.  These  plexuses  frequently  possess  numerous 
ganglia  of  their  own;  two  of  the  most  important  are  the  cardiac 
plexus  which  lies  on  the  dorsal  side  of  the  heart,  and  the  solar 
plexm  which  lies  in  the  abdominal  cavity  and  supplies  nerves  to 
the  stomach,  liver,  kidneys,  and  intestines.  Many  of  the  sympa- 
thetic nerves  finally  end  in  the  walls  of  the  blood-vessels  of  various 
organs.  To  the  naked  eye  they  are  commonly  grayer  in  color 
than  the  cerebro-spinal  nerves. 


CHAPTER  X 

GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM. 
SPINAL  AND  CEREBELLAR  REFLEXES 

Conduction  within  Single  Neurons.  Since  the  nervous  system, 
whose  function  as  a  whole  is  the  conduction  of  impulses  from 
sensory  regions  to  motor  ones,  is  made  up  of  individual  neurons, 
the  study  of  its  physiology  can  best  be  begun  by  considering  the 
phenomenon  of  conduction  as  exhibited  in  single  neurons,  passing 
later  to  conduction  as  it  involves  more  neurons  than  one. 

The  passage  of  an  impulse  along  a  nerve  is  attended  by  no 
visible  alteration  of  the  nerve  itself;  it  is  impossible  to  tell  by 
looking  at  a  nerve  whether  it  is  carrying  impulses  or  not.  For 
this  reason  nerve  impulses  can  only  be  studied  indirectly.  If 
as  the  result  of  stimulating  a  motor  nerve  at  some  point  along 
its  course  the  muscle  in  which  it  terminates  is  thrown  into  con- 
traction the  obvious  conclusion  is  that  nerve  impulses  are  passing 
from  the  point  of  stimulation  to  the  muscle.  When  the  prick  of  a 
finger  gives  rise  within  the  brain  to  a  conscious  sensation  of  pain 
we  know  that  a  nerve  impulse  must  have  passed  between  the 
finger  and  the  brain,  although  we  would  be  unable  to  detect  any 
sign  of  its  passage  if  the  nerve  were  visible  throughout  its  length. 
In  addition  to  these  methods  of  studying  nerve  impulses,  in  which 
the  passage  of  the  impulse  is  made  known  through  its  effect  on 
some  other  part  of  the  Body,  we  have  a  method  which  depends 
upon  the  fact  that  activity  of  nerve,  like  activity  of  muscle  or 
any  other  living  tissue,  is  accompanied  by  changes  of  electrical 
potential  which  may  give  rise  to  action  currents.  Every  time  an 
impulse  passes  along  a  nerve  it  is  accompanied  by  this  electrical 
alteration.  Sensitive  electrometers  applied  to  nerves  will  indi- 
cate the  passage  of  impulses  under  their  points  of  contact. 

By  the  use  of  these  methods  of  studying  nerve  impulses  we  have 
learned  many  things  about  them,  although  much  more  remains 
unknown. 

155 


156  THE  HUMAN  BODY 

How  Nerve  Impulses  Are  Aroused.  We  know  that  nerve  im- 
pulses may  be  started  in  various  ways.  A  sharp  blow  on  a  living 
nerve  starts  impulses  traveling  along  it;  a  good  example  of  this  is 
the  effect  of  striking  the  "funny"  bone.  Nerves  may  be  stimu- 
lated by  heat  or  by  cold,  by  chemical  agents  or  by  an  electric 
spark.  Whatever  the  nature  of  the  stimulus  the  nerve  impulse 
which  it  arouses  is,  so  far  as  we  can  tell,  the  same  in  all  cases. 

Speed  of  Nerve  Impulses.  The  nerve  impulse  travels  from 
the  point  of  stimulation  over  the  neuron  at  a  regular  and  rather 
slow  rate  which  probably  varies  somewhat  in  different  animals 
and  in  different  nerves  of  the  same  animal.  In  frogs'  nerves  at 
ordinary  temperatures  the  rate  approximates  30  meters  (97  ft.)  per 
second.  In  human  nerves  the  rate  is  probably  two  or  three  times  as 
high. 

Spread  of  Impulses  in  Both  Directions.  Through  observations 
of  the  action  currents  of  nerves  it  has  been  shown  that  the  impulse 
spreads  from  the  point  of  stimulation  in  both  directions  along  the 
neuron,  finally  traversing  all  parts  of  it.  This  fact  could  never  have 
been  demonstrated  if  the  existence  of  the  action  currents  (p.  103) 
were  unknown  because  our  only  other  method  of  detecting  the 
presence  of  nerve  impulses  depends  upon  the  production  of  effects 
in  the  organs  to  which  the  neurons  lead;  and  in  the  body  each 
neuron  has  such  connection  only  at  one  end;  a  nerve  impulse 
imparted  to  a  motor  nerve  will  cause  contraction  in  its  connected 
muscle  but  produces  no  effect  whatever  at  its  other  end. 

Fatigue.  It  has  been  proven  beyond  question  that  the  passage 
of  impulses  over  nerve-fibers  does  not  fatigue  them  to  an  appre- 
ciable degree.  In  this  respect  the  nerve  is  comparable  to  a  tele- 
phone wire;  in  each  case  the  message  is  transmitted  without  im- 
pairing the  ability  of  the  structure  to  transmit  other  messages. 

We  learned  in  connection  with  our  study  of  muscular  fatigue 
(p.  101)  to  look  upon  fatigue  as  the  result  of  the  accumulation  of 
waste  substances.  Its  absence  from  active  nerve-fibers  indicates 
one  of  two  things.  Either  the  transmission  of  nerve  impulses  does 
not  involve  the  production  of  fatigue  substances  or  the  fiber  is 
able  to  get  rid  of  such  as  are  produced  so  quickly  that  they  cannot 
affect  its  working.  Exceedingly  delicate  tests  which  have  recently 
been  devised  indicate  that  in  nerve  trunks  there  is  a  small  produc- 
tion of  carbon  dioxid.  This  gas  is  known  to  be  a  product  of  oxida- 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM       157 

tion  and  its  occurrence  is  taken  to  mean  that  chemical  processes  do 
go  on  in  nerve-fibers.  They  must  be  of  very  small  magnitude, 
however,  for  their  products  are  so  quickly  dissipated  as  not  to 
hinder  the  functioning  of  the  nerve  at  all. 

While  indefatigability  is  thus  seen  to  be  a  property  of  axons, 
we  shall  learn  presently  that  other  nervous  structures  are  highly 
susceptible  to  it.  Nervous  fatigue  is  a  common  phenomenon,  but 
it  is  localized  in  the  region  of  the  synapses  and  not  in  the  axons. 

Nature  of  the  Nerve  Impulse.  Although  we  know  these  things 
about  nerve  impulses,  we  do  not  know  what  the  nerve  impulse  it- 
self really  is.  There  have  been  many  interesting  and  ingenious 
theories  of  its  nature  proposed.  Some  of  these  attempt  to  describe 
it  as  a  purely  physical  process,  the  transmission  of  a  physical  stress 
from  particle  to  particle  along  the  nerve;  others  would  consider  it 
as  a  chemical  process,  too  delicate  and  transitory  to  be  detected. 
All  theories  of  its  nature  agree  that  the  change  transmitted  along 
the  nerve  is  not  a  continuous  flow,  like  an  electric  current  along 
a  wire,  but  is  an  exceedingly  brief  impulse  or  series  of  impulses. 
The  name  given  to  the  nervous  discharge  implies  this  character. 
During  continuous  excitation  of  a  nerve,  as  in  prolonged  voluntary 
contraction  of  a  muscle,  the  individual  impulses  follow  each  other 
in  rapid  succession.  The  exact  rate  is  not  known,  but  is  believed 
to  be  in  the  general  neighborhood  of  50-100  a  second.  Quite  re- 
cently evidence  has  accumulated  which  indicates  that  individual 
nerve  impulses  are  on  the  whole  of  nearly  equal  intensity.  This 
is  important  in  view  of  the  familiar  fact  that  nervous  activities 
in  general  may  show  widely  different  intensities.  The  prevailing 
explanation  accounts  for  this  on  the  ground  that  nervous  activ- 
ities depend,  not  on  single  impulses  but  on  streams  of  impulses 
which  latter  may  vary  even  though  the  individual  component 
impulses  are  equal.  In  accordance  with  this  view  we  have  to 
suppose  that  a  weak  stimulus  gives  rise  to  one  sort  of  stream  of 
impulses,  a  stronger  stimulus  to  a  different  sort,  and  so  on. 

Conduction  Involving  More  Than  One  Neuron.  Reflexes.  In 
the  actual  passage  of  nerve  impulses  through  the  Body  more 
neurons  than  one  are  always  involved.  Let  us  examine  a  simple 
case  of  conduction  by  which  the  Body  adapts  itself  to  its  surround- 
ings. Accidentally  my  finger  comes  in  contact  with  a  hot  surface. 
Quite  involuntarily  I  jerk  my  hand  away.  The  chain  of  events  is 


158 


THE  HUMAN  BODY 


as  follows:  the  skin  of  the  hand  is  stimulated  by  the  heat;  the 
sensory  neurons  in  the  nerve  supplying  that  part  of  the  hand 
convey  the  nerve  impulses  thus  aroused  to  the  central  nervous 
system ;  here  the  impulses  are  conveyed  to  the  motor  neurons  lead- 
ing to  the  muscle  which  retracts  the  arm;  upon  the  arrival  of  the 
impulses  within  the  muscle  the  latter  is  stimulated  to  contract. 
The  whole  process  is  entirely  mechanical;  none  of  the  structures 
involved  has  any  knowledge  that  the  hand  is  in  danger  of  being 
severely  burnt,  or  that  retraction  of  the  arm  will  save  it.  It  is  an 
example  of  an  adaptive  mechanism.  Such  a  chain  of  events  as  the 
one  described  constitutes  a  simple  reflex  and  typifies  the  funda- 
mental basis  of  all  nervous  activity  within  the  organism.  Our 

study  of  the  operation  of  the 
nervous  system  will  consist 
throughout  of  enlargements 
and  modifications  of  this 
elementary  conception  of 
nervous  activity  as  conduc- 
tion of  nerve  impulses  from 
receptor  to  active  tissue.  In 
its  broadest  sense  any  such 
act  of  conduction  may  be 
termed  a  reflex,  and  so  we 
shall  define  the  word.  The 
neurons  involved  in  the 
transmission  of  the  impulse 
from  receptor  to  muscle 
make  up  the  reflex  arc.  The  simplest  imaginable  reflex  arc  must 
include  at  least  two  neurons,  the  sensory  neuron  which  brings 
the  impulses  from  the  receptor  to  the  center,  and  the  motor 
neuron  which  conveys  the  impulses  from  the  center  to  the 
motor  organ.  Undoubtedly  most  reflex  arcs  in  the  Body  include, 
in  addition,  one  or  more  association  neurons  interposed  between 
the  sensory  and  the  motor  neuron. 

Anatomical  Arrangement.  The  anatomical  relationships  of  the 
various  neurons  which  make  up  the  reflex  arc  are  indicated  in 
figure  63.  For  simplicity  the  spinal  cord  is  taken  as  the  part  of  the 
central  nervous  sj'stem  pictured.  The  receptor  communicates 
with  the  cell-body  of  the  sensory  neuron  by  means  of  the  axon-like 


FIG.  63. — Diagram  of  the  simple  reflex 
arc.  R,  receptor;  A,  afferent  (sensory) 
neuron;  E,  efferent  (motor)  neuron;  M, 
muscle. 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM      159 

process  previously  described  (p.  136).  The  cell-body  of  the  sensory 
neuron  always  lies  outside  the  central  nervous  system  in  a  dorsal 
root  ganglion;  its  axon  extends  thence  by  way  of  the  dorsal  root  of 
the  spinal  nerve  into  the  spinal  cord  (or  brain)  and  enters  the  dorsal 
column  of  white  matter  (p.  144). 

Within  the  central  nervous  system  is  located  the  cell-body  of 
the  motor  neuron  which  forms  the  outgoing  part  of  the  path.  This 
will  be  found  in  the  ventral  horn  of  gray  matter  of  the  spinal  cord. 
The  simple  reflex  arc  is  formed  by  one  of  the  branches  of  the 
sensory  axon  which  penetrates  the  gray  matter  and  whose  end 
arborization  makes  synaptic  connection  with  the  dendrites  of  the 
motor  neuron. 

Reflex  Arcs  Not  Rigidly  Fixed  Paths.  Although  a  given  sen- 
sory stimulus  usually  arouses  the  same  sort  of  reflex  response 
every  time  it  is  applied,  this  does  not  mean  that  the  reflex  path 
followed  in  such  a  case  is  the  only  one  into  which  that  sensory 
neuron  leads.  Very  different  reflex  responses  may  originate  in 
the  same  receptor.  A  good  illustration  of  this  is  furnished  by 
certain  reflexes  through  the  eye.  If  I  see  that  a  small  floating 
particle  threatens  my  eye  I  am  apt  to  wink;  if  a  flying  insect  ap- 
proaches I  am  more  likely  to  turn  my  head  to  one  side;  if  the 
threatening  object  is  a  swiftly  thrown  baseball  I  will  probably 
bring  the  hands  before  the  face,  or  perhaps  dodge  to  one  side. 
All  these  actions  are  performed  mechanically  and  are  therefore 
true  simple  reflexes.  The  originating  sensory  impulses  travel  in 
each  case  over  the  same  sensory  neurons,  those  of  the  optic  nerves. 
It  is  evident,  then,  that  impulses  coming  in  over  the  sensory 
neurons  of  the  optic  nerve  do  not  have  to  pass  over  to  any  partic- 
ular motor  neuron,  such  as  the  one  which  leads  to  the  muscle 
of  winking,  but  may  follow  any  one  of  various  courses,  finally 
terminating  in  muscles  far  distant  from  the  eye.  In  fact,  and  this 
is  one  of  the  most  important  things  to  remember  about  the  nervous 
system,  there  is  such  an  extraordinary  richness  of  connection 
among  the  various  neurons  within  the  central  nervous  system  that 
any  sensory  neuron  may  be  brought  into  communication  with  any 
motor  neuron. 

This  richness  of  connection  is  afforded  anatomically  through 
two  rather  simple  arrangements.  In  the  first  place  the  axons  of 
sensory  neurons  after  entering  the  central  nervous  system  continue 


160 


THE  HUMAN  BODY 


along  it  for  some  distance,  giving  off  branches,  called  collaterals,  at 
various  levels.  In  the  spinal  cord  the  dorsal  white  columns  con- 
tain these  axons.  They  extend  toward  the  brain,  but  each  gives 
off  a  branch  which  extends  a  short  distance  down  the  cord  in  the 
opposite  direction.  Each  collateral  terminates  in  an  end  arboriza- 
tion which  communicates  in  turn  with  the  dendrites  of  another 
neuron,  either  motor  or  association.  Thus  each  sensory  neuron, 
besides  its  connection  with  one  or  more  motor  neurons,  has  con- 
nection with  various  association  neurons  located  in  different  parts 
of  the  central  nervous  system.  The  association  neurons  likewise 
are  richly  branched,  each  branch  terminating  in  a  synaptic  con- 


FIG.  64. — Diagram  to  illustrate  how  a  single  sensory  neuron  may  communicate 
with  several  motor  neurons,  and  a  single  motor  neuron  with  several  sensory  neurons. 

nection  with  another  neuron,  and  this  in  turn  may  be  an  association 
neuron,  or  may  be  a  motor  neuron.  In  the  second  place  the  den- 
drites of  all  association  and  motor  neurons  doubtless  have  synaptic 
connection  with  end  arborizations  of  numerous  neurons,  sensory  or 
association  as  the  case  may  be.  Thus  a  sensory  neuron  has  a 
wide  choice  of  paths  over  which  to  send  its  impulses;  and  a  motor 
neuron  may  receive  impulses  from  a  great  variety  of  sources 
(Fig.  64). 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM      161 

Irreversible  Conduction.  In  all  this  maze  of  connections  and 
interconnections  within  the  central  nervous  system,  how  is  it 
that  the  impulses  coming  in  at  a  sensory  neuron  always  come 
out  finally  at  a  motor  neuron  instead  of  becoming  switched  some- 
times to  another  sensory  neuron?  The  orderly  progress  of  im- 
pulses is  insured  by  a  very  simple  arrangement,  namely,  that 
impulses  can  pass  freely  across  a  synapse  from  end  arborization 
to  dendrites  but  can  never  pass  in  the  reverse  direction,  from 
dendrites  to  end  arborization.  When  a  sensory  neuron  delivers 
its  impulses  to  an  association  neuron  the  impulses  doubtless  spread 
to  all  parts  of  the  latter.  They  can  leave  it,  however,  only  by  way 
of  its  end  arborizations,  and  these  communicate  only  with  the 
dendrites  of  motor  neurons  or  of  other  association  neurons.  The 
final  outcome  is  bound  to  be  a  motor  neuron  since  all  association 
neurons  lead  ultimately  to  them.  Sensory  neurons  never  receive 
impulses  from  other  neurons  because  they  have  no  dendrites 
within  the  central  nervous  system  by  which  impulses  might  be 
received.  The  portion  of  a  sensory  neuron  which  corresponds  to 
the  dendrites  of  a  motor  neuron  is  the  long  axon-like  process 
communicating  with  the  receptor. 

Graded  Synaptic  Resistance.  Another  question  which  nat- 
urally arises  when  one  considers  the  innumerable  courses  which 
an  impulse  may  take  within  the  central  nervous  system  is  what 
determines  the  course  it  actually  does  take?  Why,  for  instance, 
when  my  eye  is  threatened  do  I  wink  instead  of  opening  my  mouth, 
or  why  do  I  sometimes  wink  and  sometimes  dodge?  A  complete 
answer  to  this  question  cannot  be  made  in  the  present  state  of  our 
knowledge,  but  we  have  a  fairly  good  general  idea  of  the  way  in 
which  nerve  impulses  are  probably  guided.  A  sensory  neuron  has 
several  collaterals,  each  with  its  synaptic  connection  with  another 
neuron.  If  we  suppose  these  synapses  are  not  all  alike,  but  that 
certain  ones  transmit  the  sort  of  stream  of  impulses  generated  by 
feeble  stimuli  more  readily  than  do  the  others,  such  a  stream 
spreading  over  the  sensory  neuron  will  pass  most  easily  to  that 
connecting  neuron  whose  synapse  offers  least  resistance  to  its  pas- 
sage. Thus  we  may  imagine  a  stream  of  impulses  spreading  from 
neuron  to  neuron  following  always  the  path  of  least  resistance  until 
it  finally  terminates  in  a  muscle  which  it  arouses  to  activity.  In 
the  central  nervous  system  the  various  paths  of  least  resistance  are 


162  THE  HUMAN  BODY 

so  blocked  out  as  to  lead  to  adaptive  motions;  a  prick  on  the  finger 
causes  retraction  of  the  hurt  hand;  irritation  in  the  nose  causes  the 
convulsive  movements  of  the  respiratory  muscles  which  constitute 
a  sneeze:  in  each  case  the  motions  are  calculated  to  get  rid  of  the 
source  of  irritation. 

That  adaptive  reflexes  are  due  to  paths  of  least  resistance 
blocked  out  from  an  infinite  number  of  possible  paths  is  strik- 
ingly illustrated  by  the  effects  of  strychnine  poisoning.  This 
drug  acts  on  the  central  nervous  system  in  such  a  way  as  to  abolish 
differences  of  synaptic  resistance.  When  one  suffering  from  the 
drug  receives  a  stimulus  by  way  of  any  sensory  nerve  the  impulses, 
instead  of  following  the  usual  path,  spread  over  the  whole  central 
nervous  system;  all  the  muscles  are  stimulated  simultaneously  and 
the  well-known  strychnine  convulsion  results. 

The  Orderly  Spreading  of  Reflexes.  The  conception  of  graded 
synaptic  resistances  explains  also  in  a  very  satisfactory  way  the 
phenomenon  of  the  orderly  spreading  of  reflexes.  A  feeble  stim- 
ulus produces  reflex  movement  in  those  muscles  only  which  are 
immediately  concerned  in  the  adaptive  response;  stronger  stimuli 
involve  more  muscles,  but  only  such  as  by  their  movement  make 
the  response  more  effective.  For  example,  if  a  frog's  hind  leg  is 
touched  gently  it  will  be  drawn  away  from  the  source  of  irrita- 
tion; a  stronger  stimulus  is  likely  to  cause  contractions  of  such 
additional  muscles  as  are  required  for  jumping  away  from  the 
point  of  danger.  If  we  assume  that  the  reflex  paths  to  the  first 
set  of  muscles  have  such  low  resistances  as  to  allow  feeble  impulse 
streams  to  pass  them,  and  that  stronger  impulse  streams  can 
overcome  enough  additional  resistance  to  enter  the  paths  of 
higher  resistance  leading  to  the  jumping  muscles,  while  the  paths 
to  muscles  not  concerned  in  any  way  in  an  adaptive  response  have 
too  high  resistance  to  be  passed  at  all,  we  can  account  for  reflex 
actions  of  very  great  complexity. 

Simple  Reflexes  Mediated  by  the  Spinal  Cord.  The  simple  re- 
flexes described  in  the  preceding  paragraphs  are  all  of  a  sort  that 
can  be  carried  on  through  the  lowest  part  of  the  central  nervous 
system,  the  spinal  cord.  A  frog  whose  brain  has  been  destroyed 
and  which  is  therefore  wholly  devoid  of  feeling  and  consciousness 
can  still  perform  highly  complicated  reflex  acts;  he  will  retract  a 
foot  which  is  pinched;  he  will  wipe  off  a  bit  of  acid-soaked  paper 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM     163 

from  his  flank,  and  if  unable  to  reach  it  with  one  foot  will  bring 
the  other  into  service.  All  his  acts,  however,  are  purely  mechan- 
ical, and  are  determined  by  the  spread  of  impulses  over  reflex 
paths  of  less  or  greater  complexity. 

The  grading  of  the  synaptic  resistances  in  the  spinal  cord  of  any 
animal,  including  man,  is  established  with  the  development  of  the 
cord  itself.  The  organism  is  born  with  paths  of  least  resistance 
from  the  different  receptors  to  adaptive  muscles  laid  down.  These 
paths  are  as  much  part  of  the  hereditary  equipment  of  the  indi- 
vidual as  is  the  spinal  cord  itself.  Spinal  cord  reflexes  apparently 
do  not  require  to  be  developed  by  training.  They  seem  to  be 
performed  as  perfectly  the  first  time  as  at  any  later  time.  An 
excellent  example  of  this  class  of  reflexes  in  man  is  the  sneezing 
reflex,  and  we  all  know  that  the  new-born  infant  does  not  have 
to  learn  how  to  sneeze.  He  can  do  it  from  birth.  Moreover, 
and  this  is  important  as  regards  spinal  reflexes  as  a  class,  if  the 
stimulus  is  strong  enough  he  cannot  help  doing  it.  Although,  as 
we  shall  learn,  we  have  a  certain  degree  of  voluntary  control  over 
some  spinal  reflexes,  their  essentially  automatic  character  should 
be  emphasized. 

In  general  the  higher  we  look  in  the  animal  scale  the  less  varied 
and  extensive  are  the  spinal  reflexes.  A  large  proportion  of  all  the 
activities  of  such  animals  as  fish  and  frogs  are  in  this  class,  while  in 
man  they  are  confined  to  a  few  relatively  simple  acts,  such  as 
coughing,  sneezing,  winking,  and  simple  withdrawal  of  an  ex- 
tremity from  a  source  of  irritation. 

Significance  of  the  Head  Senses  in  the  Control  of  Reflexes. 
We  have  noted  how,  in  the  lower  animals,  highly  complicated  acts 
are  performed  automatically  through  the  operation  of  the  "  spinal 
cord"  reflexes.  When  we  study  such  activities  in  an  animal  whose 
brain  has  been  destroyed  we  note  that  on  the  sensory  side  they  are 
based  exclusively  on  the  body  senses,  touch,  temperature,  pain,  etc. 
(p.  172).  The  destruction  of  the  brain  has  cut  off  all  possibility  of 
any  action  on  the  part  of  the  head  senses,  sight,  hearing,  taste,  and 
smell  (p.  173).  One  result  of  this  dependence  on  the  body  senses  of 
spinal  cord  reflexes  is  that  they  are,  as  a  class,  immediately  pro- 
tective. The  adaptive  response  consists  of  the  withdrawal  from  or 
removal  of  a  direct  source  of  irritation.  The  chief  significance  of 
the  head  senses  is  in  their  property  of  giving  information  of  what  is 


164  THE  HUMAN  BODY 

happening  at  a  distance.  The  bodily  adjustments  based  on  them 
are  therefore  long  range  adjustments  rather  than  immediate  ones. 
Other  adaptations  than  the  simple  one  of  withdrawing  from  a 
source  of  injury  are  possible.  Notably  reflexes  concerned  with  the 
quest  for  food,  a  quest  based  in  most  animals  largely  on  the  senses 
of  smell  and  sight,  are  added  to  the  purely  protective  reflexes. 
Associated  with  these  long  range  adjustments  is  a  great  group  of 
movements  which  constitute  our  most  frequent  and,  in  general, 
most  important  muscular  acts,  movements  of  locomotion.  These 
make  up  a  separate  class  of  reflexes  and  will  be  studied  by  them- 
selves next  in  order. 

The  Sensory  Basis  of  Locomotion.  Locomotion  takes  various 
forms  in  individual  animals  and  in  different  classes  of  animals. 
Walking,  running,  leaping,  swimming,  flying,  these  are  all  fun- 
damental locomotor  acts.  With  them  must  be  classed  also  the 
more  artificial  forms  of  locomotion  of  civilized  man,  as  bicycle 
riding  or  aviation.  All  these  have  certain  primary  features  in 
common.  They  all  require  the  accurately  co-ordinated  use  of  a 
number  of  muscles,  and  all  of  them  involve  the  maintenance  of 
equilibrium.  More,  in  fact,  than  the  simple  maintenance  of  balance 
is  involved.  In  every  sustained  locomotion  there  is  constant 
restoration  of  an  equilibrium  that  is  continually  disturbed.  Of 
great  importance  for  the  guidance  of  co-ordinated  muscular  move- 
ment is  a  sense  whose  receptors  are  embedded  within  the  muscles 
themselves  and  distributed  about  the  joints  to  which  the  contract- 
ing muscles  impart  movement.  This  is  the  muscle  and  joint  sense, 
or  briefly,  muscle  sense.  Less  well  known  than  some  of  our  other 
senses  it  is,  as  we  shall  learn  (Chap.  XIII),  of  equal  rank  with  the 
others,  and  in  connection  with  our  muscular  movements  more 
important  than  most.  Every  bodily  movement  results  in  stimula- 
tion of  the  receptors  of  muscle  sense.  Any  locomotor  act  is  accom- 
panied by  a  great  stream  of  impulses  from  these  receptors  which 
serve  not  only  to  guide  but  to  maintain  the  activity.  There  are 
definite  organs  of  equilibrium,  the  semicircular  canals  and  vestibule 
of  the  ear  (Chap.  XIV),  by  which  the  equilibrium  sense  is  mediated. 
These  two  senses  constitute  the  essential  sensory  basis  for  the 
locomotor  reflexes.  They  are  reinforced  and  modified  by  some  of 
the  other  senses,  notably  touch  and  sight.  Since  the  locomotor 
reflexes  require  the  co-operation  of  several  senses  they  are  more 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM     165 

complicated  on  the  sensory  side  than  the  most  elaborate  spinal 
cord  reflexes,  the  latter  being  based  on  stimulation  of  single  groups 
of  receptors.  On  the  motor  side,  also  they  are  more  complicated. 
The  degree  of  muscular  co-ordination  involved  is  greater  than  in 
any  spinal  cord  reflex.  For  the  translation  of  the  complex  stream 
of  sensory  impulses  into  an  equally  complicated  stream  of  motor 
impulses  a  more  elaborate  arrangement  of  interconnecting 
neurons  is  required  than  the  spinal  cord  affords.  For  this  pur- 
pose a  special  portion  of  the  brain,  the  cerebellum,  is  set  apart, 
and  our  next  concern  is  with  the  structure  and  connections 
of  this  organ,  and  its  functioning  in  the  mediation  of  reflexes  of 
locomotion. 

Structure  and  Connections  of  the  Cerebellum.  This  organ, 
as  shown  in  Figs.  58-60  is  a  distinct  portion  of  the  brain,  lying 
underneath  the  posterior  part  of  the  cerebrum,  and  behind  and 
above  the  midbrain  and  medulla.  It  consists  of  a  thin  layer  of 
gray  matter  superposed  upon  white  matter,  and  having  embedded 
within  the  white  matter  at  its  base  gray  masses,  the  nuclei  of  the 
cerebellum.  The  thin  outer  gray  layer,  known  as  the  cortex,  is  the 
region  in  which  the  incoming  streams  of  sensory  impulses  are  con- 
verted into  outgoing  streams  of  co-ordinated  motor  impulses.  The 
cerebellum  communicates  with  the  brain  stem,  as,  for  convenience, 
the  midbrain  and  medulla  together  are  often  called,  by  three  pairs 
of  stalks  or  peduncles.  These  consist  of  bundles  of  axons.  The 
upper  and  lower  stalks  (Fig.  66)  lead  directly  into  the  brain  stem. 
The  middle  peduncles  form  the  backward  extension  of  the  pons 
varolii  (Fig.  58). 

The  senses  which  are  concerned  with  locomotor  reflexes  all  have 
connection,  either  directly  or  by  means  of  association  neurons, 
with  the  brain  stem,  and  thence,  by  neurons  whose  axons  extend 
through  the  peduncles,  with  the  cerebellum  itself.  The  detailed 
anatomy  of  these  paths  will  be  presented  later  in  connection  with 
the  study  of  the  cerebrum  (p.  173). 

The  outgoing  paths  from  the  cerebellum,  the  paths  over  which 
pass  the  co-ordinated  streams  of  impulses  which  carry  on  the  acts 
of  locomotion,  consist  of  chains  of  association  neurons.  These 
begin  in  the  cerebellar  cortex  and  pass  thence  over  the  peduncles  to 
the  brain  stem.  Here  communication  is  made  with  others  which 
pass  down  the  spinal  cord  to  final  terminations  in  the  ventral  horn 


166  THE  HUMAN  BODY 

of  gray  matter  in  immediate  synaptic  connection  with  the  cell 
bodies  of  the  motor  neurons. 

We  can  trace  reflex  arcs  for  locomotor  reflexes  as  for  the  simpler 
spinal  reflexes.  In  both  cases  the  paths  begin  with  sensory  neurons 
and  terminate  with  motor  neurons.  Many  more  association  neu- 
rons are  always  involved  in  locomotor  reflexes  than  in  spinal,  and 
they  always  include  the  cerebellum  in  their  course.  As  indicated 
above,  however  (p.  164),  not  one,  but  several  senses  co-operate  in 
locomotor  reflexes,  and  many  muscles  are  concerned  in  their  per- 
formance, so  that  no  single  reflex  arc  suffices  to  carry  them  on,  but 
several  paths  into  the  cerebellum  and  a  number  out  of  it  must  be 
thought  of  as  involved. 

Functions  of  the  Cerebellum.  In  a  previous  paragraph  the 
general  function  of  the  cerebellum  was  stated,  namely,  to  translate 
the  streams  of  impulses  from  the  receptors  of  muscle  sense,  equi- 
librium, touch,  and  sight  into  co-ordinated  motor  impulses  by  which 
are  carried  on  the  important  reflexes  of  locomotion.  We  need  to 
bear  in  mind,  in  this  connection,  that  our  muscles  will  not  work 
spontaneously.  We  can  cause  them  to  contract  by  an  act  of  the 
will  (p.  184)  or  they  can  be  operated  reflexly  by  means  of  stimuli 
conducted  to  them  from  receptors.  We  know  from  our  own 
experience  that  our  common  locomotor  acts,  such  as  walking,  are 
not  volitional  in  the  sense  that  every  muscular  movement  is  volun- 
tary. We  can  see,  also,  that  in  such  an  act  as  walking  there  are 
abundant  sources  of  sensory  stimulation.  The  pressure  of  the 
feet  upon  the  ground,  the  muscular  movements  themselves,  the 
disturbances  of  equilibrium,  the  appearance  of  the  footing,  all  give 
rise  to  streams  of  sensory  impulses  which,  if  properly  co-ordinated 
can  be  made  to  operate  complicated  muscular  movements.  This 
co-ordination  is  the  function  of  the  cerebellum. 

All  the  reflexes  which  the  cerebellum  mediates  are  reflexes  of 
skeletal  muscles.  They  are  all  such  as  the  higher  parts  of  the  brain 
through  the  property  of  volition  are  competent  to  carry  on.  If 
their  performance  depended  on  the  higher  brain  regions,  however, 
these  would  have  little  time  left  for  other,  and  more  important 
activities.  We  may  view  the  cerebellum,  therefore,  as  an  organ 
which  by  taking  up  complicated  but  not  highly  intellectual  tasks 
leaves  the  higher  parts  of  the  brain  free  for  higher  forms  of  activity. 

An  important  difference  between  cerebellar  and  spinal  reflexes 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM     167 

is  that  while  the  latter  are  instinctive,  born  in  us,  the  former  are 
not.  Every  one  has  to  learn  to  stand,  walk,  run,  and  so  on;  at 
first  all  are  difficult,  but  after  a  time  become  easy  and  are  per- 
formed unconsciously.  In  standing  or  walking  very  many  muscles 
are  concerned,  and  if  the  mind  had  all  the  time  to  look  directly  after 
them  we  could  do  nothing  else  at  the  same  time;  we  have  for- 
gotten how  we  learnt  to  walk,  but  in  acquiring  a  new  mode  of 
progression  in  later  years,  as  skating,  we  find  that  at  first  it  needs 
all  our  attention,  but  when  once  learnt  we  have  only  to  start  the 
series  of  movements  and  they  are  almost  unconsciously  carried 
on  for  us.  At  first  we  had  to  learn  to  contract  certain  muscle 
groups  when  we  got  particular  sensations,  either  tactile,  from  the 
soles,  or  muscular,  from  the  general  position  of  the  limbs,  or  visual, 
or  equilibrium  sensations  from  the  semicircular  canals.  But  the 
oftener  a  given  group  of  sensations  has  been  followed  by  a  given 
muscular  contraction  the  more  close  becomes  the  association  of 
the  two;  the  path  of  connection  between  the  incoming  and  out- 
going fibers  becomes  easier  the  more  it  is  traveled,  and  at  last  the 
sensory  impulses  arouse  the  proper  movement  without  volitional 
interference  at  all,  and  while  hardly  exciting  any  consciousness;  we 
can  then  walk  or  skate  without  thinking  about  it.  The  will,  which 
had  at  first  to  excite  the  proper  motor  neurons  in  accordance  with 
the  felt  directing  sensations,  now  has  no  more  trouble  in  the  matter; 
the  sensory  impulses  stimulate  the  proper  motor  centers  in  an 
unconscious  and  unheeded  way.  Injury  or  disease  of  the  cerebel- 
lum produces  great  disturbances  of  locomotion  and  insecurity  in 
maintaining  various  postures,  as  well  as  marked  loss  of  endurance. 
The  functions  normally  performed  by  it  are  transferred  to  other 
parts  of  the  brain,  and  these,  which  are  less  fitted  for  the  task,  do  it 
less  well  and  with  more  fatigue. 

Postural  Reflexes.  In  a  previous  chapter  (p.  126)  the  depend- 
ence of  posture  on  extensor  tonus  was  described.  This  tonus  is 
maintained  reflexly,  and  to  the  extent  that  it  involves  equilibrium, 
as  in  the  erect  posture  in  man,  is  to  be  looked  upon  as  belonging  to 
the  class  of  cerebellar  reflexes.  The  most  striking  fact  about 
postural  tonus,  perhaps,  is  the  completeness  with  which  it  dis- 
appears in  the  presence  of  any  active  movement  which  would 
conflict  with  it.  This  is  a  striking  example  of  the  adaptive  char- 
acter of  the  nervous  system.  So  long  as  one  is  maintaining  any 


168  THE  HUMAN  BODY. 

posture  quietly,  the  tonus  is  present  and  suffices  to  keep  the  Body 
in  position,  but  let  any  active  movement  be  started,  either  voli- 
tionally  or  reflexly,  and  the  opposition  to  that  movement  which 
would  be  offered  by  a  persistence  of  the  tonus  is  removed  by  its 
complete  cessation.  This  fact  has  been  proven  conclusively.  For 
details  the  reader  is  referred  to  larger  works  on  the  nervous  system. 


CHAPTER  XI 

STRUCTURE,  NERVE   CONNECTIONS,  AND   FUNCTIONS   OF 
THE  CEREBRUM 

The  Cerebrum  in  Relation  to  Muscular  Activity.  In  the  pre- 
ceding chapter  two  classes  of  reflexes,  spinal  and  cerebellar,  have 
been  described.  A  fact  it  is  important  we  should  grasp,  is  that  a 
large  part  of  all  the  activities  of  all  animals  belong  to  one  or  the 
other  of  these  classes.  Indeed  as  we  go  down  the  animal  scale  and 
examine  such  animals  as  fish,  frogs,  and  turtles,  it  is  a  matter  of 
some  difficulty  to  prove  that  any  of  their  acts  involve  higher  ner- 
vous manifestations.  In  man  and  the  higher  animals,  however,  we 
recognize  many  activities  which  cannot  be  assigned  to  either 
category.  Among  these  are  all  acts  which  we  describe  as  volitional. 
For  the  performance  of  these  the  cerebrum  is  essential.  We  will 
get  an  idea  of  the  significance  of  the  cerebrum  in  relation  to  mus- 
cular activity,  by  noting  the  way  in  which  it  may  modify  such 
activity. 

A  Normal  Animal  Compared  with  a  "  Reflex  "  One.  Let  us 
imagine  that  we  have  side  by  side  before  us  two  living  animals  of 
the  same  species,  one  normal  in  every  respect,  the  other  in  the 
" reflex"  condition;  that  is,  having  had  the  cerebrum  destroyed 
but  the  remainder  of  the  nervous  system  uninjured.  Disregarding 
for  the  present  the  phenomenon  of  consciousness  and  looking  at 
both  animals  simply  as  pieces  of  machinery  three  striking  differ- 
ences between  them  are  manifest:  1.  The  "reflex"  animal  always 
responds  to  adequate  stimulation  by  a  predictable  response; 
the  intact  animal  sometimes  responds  and  sometimes  does  not. 
2.  The  "reflex"  animal  does  not  move  except  when  stimulated, 
while  the  intact  animal  often  moves  without  any  apparent  rea- 
son. 3.  The  amount  of  response  given  by  the  "reflex"  animal 
bears  some  relation  to  the  intensity  of  the  exciting  stimulus, 
whereas  in  the  normal  animal  an  apparently  feeble  stimulus  may 
arouse  a  vigorous  and  long-continued  response.  An  example  of 

169 


170  THE  HUMAN  BODY 

this  last  is  the  running  of  a  dog  to  its  master  upon  hearing  his 
whistle.  The  stimulus  may  be  a  very  faint  one,  the  motions  which 
it  arouses  are  exceedingly  vigorous  and  complicated. 

All  these  differences  depend  at  bottom  upon  a  single  funda- 
mental difference  between  the  two  animals  which  is  this:  in  the 
" reflex"  animal  the  immediate  stimulus  dominates  the  situation 
completely;  in  the  intact  animal  the  immediate  stimulus  is  only 
one  factor  of  many  which  together  determine  what  the  response 
shall  be.  The  superior  practical  efficiency  of  the  intact  animal 
as  an  adaptive  organism  depends  upon  this  power,  resident  in  the 
cerebrum,  of  modifying  immediate  stimuli  in  accordance  with  the 
demands  of  less  obvious  considerations.  To  illustrate:  a  hungry 
man  perceiving  food  would  inevitably  respond  to  the  double 
stimulus  of  hunger  and  the  sight  of  food  by  taking  the  food  and 
eating  it  if  he  acted  upon  a  purely  reflex  basis;  his  actual  response 
to  these  stimuli  .will  depend,  however,  upon  whether  they  are  in 
harmony  with  or  opposed  to  certain  more  remote  factors,  such  as 
the  question  whether  the  food  is  of  a  sort  that  will  agree  with  him, 
or  whether  he  is  likely  to  need  it  more  urgently  at  some  future 
time  than  at  present. 

Before  entering  upon  a  fuller  discussion  of  the  functions  of  the 
cerebrum,  its  structure  and  its  connections  with  lower  nerve- 
centers  must  be  described. 

The  Cerebrum  Dependent  on  the  Receptor  System.  If  the 
cerebrum  is  to  introduce  remote  considerations  as  factors  in  de- 
termining the  nature  of  reflex  responses  it  must  have  within  it  the 
knowledge  upon  which  these  remote  considerations  are  based. 
That  the  cerebrum  has  little  original  endowment  of  knowledge  is 
evident  from  study  of  infants,  who  during  the  first  months  are 
perfect  examples  of  "reflex"  organisms.  The  equipment  which 
the  cerebrum  finally  obtains  must  be  gotten  bit  by  bit  by  ex- 
perience or  the  teaching  of  others.  Since  the  receptor  system  is 
the  organism's  only  means  of  acquiring  information,  the  cere- 
brum must  be  in  communication  with  this  system  if  it  is  to  learn 
anything  whatsoever. 

Afferent  Paths  of  the  Cerebrum.  We  have  learned  in  previous 
paragraphs  that  all  sensory  neurons  lead  directly  into  the  central 
nervous  system  and  there  have  numerous  synaptic  connections 
with  association  neurons.  These  connections  are  all,  however, 


STRUCTURE  AND  FUNCTIONS  OF  THE  CEREBRUM      171 

with  the  possible  exception  of  those  of  the  sense  of  smell,  made 
in  gray  matter  of  the  spinal  cord,  the  medulla,  or  the  midbrain. 
In  order  for  impulses  coming  in  over  these  sensory  neurons  to 
reach  the  cerebrum  there  must  be  communication  by  association 
neurons  between  the  terminations  of  the  sensory  neurons  and  the 
cerebrum.  As  a  matter  of  fact  such  connections  are  richly  sup- 
plied. Some  of  the  most  conspicuous  tracts  of  white  matter  in 
the  central  nervous  system  consist  of  the  myelinated  axons  of 
association  neurons  which  form  connecting  links  between  sensory 
neurons  and  the  cerebrum.  Since  the  cerebrum  is  the  crown  of 
the  entire  nervous  system  it  is  used  as  a  landmark  in  describing 
other  nervous  structures.  Thus  nerve  paths  which  convey  im- 
pulses toward  the  cerebrum  are  called  afferent  paths;  those  carry- 
ing impulses  away  from  it  are  efferent  paths.  According  to  this 
classification  all  sensory  neurons  are  afferent  and  all  motor  ones 
efferent,  while  association  neurons  are  either  afferent  or  efferent 
according  as  they  carry  impulses  toward  the  cerebrum  or  away 
from  it. 

Tracing  Nerve  Paths.  Wallerian  Degeneration.  One  of  the 
very  satisfactory  achievements  of  biologists  has  been  the  reso- 
lution of  the  apparently  inextricable  tangle  of  gray  and  white 
matter  of  the  central  nervous  system  into  a  system  of  fairly  definite 
nerve  tracts  whose  origins,  courses,  and  terminations  are  known. 
Our  present  knowledge  is  the  result  of  various  methods  of  study. 
Perhaps  the  most  fruitful  has  rested  upon  recognition  of  three 
facts:  first,  that  white  matter  always  consists  of  myelinated  axons; 
second,  that  axons  always  are  outgrowths  of  cell-bodies  which  are 
to  be  looked  for  in  gray  matter;  and  third,  the  fact  discovered  by 
the  English  physiologist,  Waller,  in  1852,  that  axons  cut  off  from 
connection  with  their  cell-bodies  undergo  degeneration  in  a  few 
days.  Because  of  this  latter  fact  if  a  cut  be  made  anywhere  in  the 
central  nervous  system  of  an  animal,  and  the  animal  be  killed  a 
few  days  later  and  its  spinal  cord  and  brain  examined  microscop- 
ically, the  direction  and  extent  of  degeneration  reveal  the  relation 
of  the  severed  axons  to  the  rest  of  the  nervous  system.  If  the 
degeneration  is  all  toward  the  head  the  severed  tract  must  be  an 
afferent  one  with  cell-bodies  somewhere  below  the  cut.  Backward 
degeneration  would  signify  an  efferent  tract  with  its  origin  some- 
where forward  of  the  point  of  injury.  Wallerian  degeneration  is 


172  THE  HUMAN  BODY 

not  difficult  to  follow  because  it  is  fatty  and  the  drops  of  fat  in  the 
degenerated  region  can  be  plainly  revealed  by  the  application  of 
osmic  acid,  which  turns  them  black. 

Successive  Myelination.  Another  valuable  method  of  tracing 
nerve  tracts  was  discovered  by  Flechsig,  who  found  that  during  the 
embryological  development  of  the  animal  the  axons  of  individual 
tracts  all  become  myelinated  together,  while  different  tracts  re- 
ceive their  myelin  sheaths  at  different  periods  of  development. 
Thus  by  examining  a  large  series  of  embryos  in  all  stages  the  va- 
rious tracts  can  be  picked  out. 

Paths  of  the  Various  Senses.  For  convenience  in  describing 
the  paths  by  which  information  is  conveyed  from  the  various  re- 
ceptors to  the  cerebrum,  the  receptors  will  be  classified  as  body 
sense  receptors  and  head  sense  receptors.  The  group  of  body 
senses  includes  all  those  senses  such  as  touch,  pain,  muscle  sense, 
etc.,  whose  receptors  are  for  the  most  part  in  parts  of  the  Body 
other  than  the  head,  and  which  therefore  communicate  with  the 
central  nervous  system  by  way  of  spinal  nerves.  The  head  senses, 
sight,  hearing,  taste,  and  smell,  are  those  from  which  stimuli  are 
carried  over  cranial  nerves  to  the  medulla  or  midbrain,  or  in  the 
case  of  the  sense  of  smell  directly  into  the  cerebrum. 

Tracts  of  Body  Sense.  Sensory  neurons  of  body  sense  enter 
the  spinal  cord  all  along  its  length.  Afferent  paths  within  the 
cord  begin,  therefore,  at  its  extreme  end.  These  are  to  be  looked 
for,  as  previously  stated,  in  the  columns  of  white  matter  which 
make  up  the  greater  part  of  the  substance  of  the  cord.  Two  dis- 
tinct regions  of  white  matter  in  each  half  of  the  cord  have  been 
shown  to  consist  chiefly  of  afferent  neurons  leading  toward  the 
cerebrum.  These  are:  first,  the  dorsal  columns,  each  of  which  con- 
sists of  two  rather  well-marked  bundles  of  axons,  the  so-called  fas- 
ciculus gradlis  (Column  of  Goll)  next  the  dorsal  fissure,  and  the 
fasciculus  cuneatus  (Column  of  Burdach)  next  to  the  dorsal  horn 
of  gray  matter;  second,  the  ventrolateral  tracts  which  lie  next  to 
the  ventral  horns  of  gray  matter,  surrounding  them  on  the  sides 
and  below  (Fig.  65).  It  is  thought  that  the  dorsal  columns  con- 
sist chiefly  if  not  wholly  of  the  axons  of  sensory  neurons  which, 
entering  the  cord  by  the  dorsal  roots  of  spinal  nerves,  extend  for- 
ward within  the  dorsal  columns,  giving  off  collaterals  into  the  gray 
matter  at  various  levels.  Only  a  part  of  the  sensory  axons  which 


STRUCTURE  AND  FUNCTIONS  OF  THE  CEREBRUM      173 

enter  the  dorsal  columns  continue  along  them  as  far  as  the  medulla; 
the  others  after  extending  a  short  distance  plunge  into  the  gray 
matter  and  terminate  in  synaptic  connection  with  association 
neurons.  The  ventrolateral  afferent  columns  consist  chiefly  of 
association  neurons  which  communicate,  presumably,  with  those 
sensory  neurons  which  do  not  themselves  extend  all  the  way  to 
the  medulla;  these  columns  serve,  therefore,  to  afford  cerebral 
communication  to  those  sensory  neurons  which  terminate  within 
the  gray  matter  of  the  cord. 

None  of  the  afferent  axons  coming  up  the  cord  by  the  tracts 
just  described  extend  further  than  the  medulla;  they  all  termi- 
nate there  in  masses  of  gray  matter  known  as  the  gracile  and 
cuneate  nuclei;  here  they  form  synaptic  connections  with  a  new 
set  of  association  neurons  which  continue  the  path  toward  the 
cerebrum.  These  tracts,  which  from  their  ribbon-like  appearance 
have  been  named  the  fillets,  cross  the  mid-line  at  a  point  in  the 
medulla  known  as  the  sensory  decussation;  so  that  sensory  stimuli 
from  the  right  half  of  the  Body  are  carried  to  the  left  cerebral 
hemisphere,  and  those  from  the  left  half  of  the  Body  to  the  right 
hemisphere. 

In  the  lateral  margins  of  the  spinal  cord  are  tracts  known  as  the 
direct  cerebellar  tract  and  Gower's  tract  (Fig.  65),  which  consist  of 
the  axons  of  association  neurons  that  pass  up  to  the  brain  stem  and 
directly  through  it  by  way  of  the  peduncles  to  the  cerebellum.  The 
cell-bodies  of  these  axons  are  in  the  gray  matter  of  the  cord  and 
have  synaptic  connection  with  branches  of  sensory  neurons,  par- 
ticularly neurons  of  muscle  sense.  These  tracts  are  believed  to 
constitute  the  chief  channels  by  which  muscle  sense  exerts  its 
influence  on  the  cerebellum  in  the  mediation  of  locomotor  reflexes. 

Tracts  of  the  Head  Senses.  The  senses  of  sight  and  hearing 
are  the  head  senses  whose  central  connections  are  best  known. 
The  central  connections  of  the  sense  of  smell  are  imperfectly 
known;  those  of  taste  practically  not  at  all.  Axons  conveying 
visual  impulses  enter  the  midbrain  by  way  of  the  optic  nerves 
and  optic  tracts  and  terminate  for  the  most  part  in  nuclei  of  the 
midbrain,  the  external  geniculates  and  superior  colliculi;  some 
of  them  appear  to  terminate  in  basal  nuclei  of  the  cerebrum,  the 
optic  thalami.  In  all  these  nuclei  synaptic  connection  is  made 
with  new  neurons  which  carry  the  impulses  into  the  cerebrum. 


174 


THE  HUMAN  BODY 


Auditory  impulses  enter  the  medulla  by  way  of  the  auditory 
nerves.  The  axons  of  the  nerves  themselves  terminate  in  nuclei 
of  the  medulla,  the  auditory  nuclei;  new  neurons  continue  the 
path  thence  across  the  mid-line  of  the  medulla  and  forward  into 
the  midbrain  terminating  in  the  internal  geniculate  nuclei  and 
the  inferior  colliculi.  From  these  nuclei  a  third  set  of  neurons 
continue  the  path  to  the  cerebrum. 

General  Structure  of  the  Cerebrum.  This  organ  consists,  as 
previously  stated,  of  an  outer  surface  of  gray  matter,  two  milli- 

Entering  posterior 
root 

Lissauer's  tract 


anterior  root 


FIG.  65. — Diagrammatic  transverse  section  of  the  spinal  cord  showing  the  con- 
duction paths.    (Cunningham.) 

meters  thick,  overlying  a  mass  of  white  matter;  the  whole  held 
together  by  neuroglia  and  connective  tissue,  and  mounted  upon 
the  midbrain  as  upon  a  stalk.  Because  of  the  convoluted  surface 
of  the  cerebrum  the  total  amount  of  superficial  gray  matter  is 
much  greater  than  it  would  be  if  the  cerebrum  were  smooth.  This 
layer  of  gray  matter  is  the  region  wherein  occur  those  special 
activities  which  set  the  cerebrum  above  the  rest  of  the  nervous 
system.  It  is  called  the  cortex  cerebri,  or  for  convenience  simply 
the  cortex. 

Structure  of  the  Cortex.    The  cortex  cerebri  consists  for  the 
most  part  of  neurons  with  small  cell-bodies  having  much  branched 


STRUCTURE  AND  FUNCTIONS  OF  THE  CEREBRUM      175 

processes,  signifying  rich  synaptic  connections.  Many  of  these 
neurons  appear  to  be  confined  altogether  within  the  cortex;  others 
give  off  myelinated  axons  into  the  underlying  white  matter.  Inter- 
spersed with  these  small  cell-bodies  are  others  which  are  much 
larger,  which  are  pyramidal  in  shape,  and  which  always  give  off  a 
large  axon  into  the  white  matter.  From  their  shape  and  size 
these  are  known  as  large  pyramidal  cells.  In  a  certain  region  of  the 
cortex,  known  as  the  motor  area,  the  pyramidal  cells  are  relatively 
gigantic,  being  just  at  the  limit  of  naked  eye  visibility. 


FIG.  66. — Diagram  of  the  projection  fibers  of  the  cerebrum  (from  Starr).  B, 
motor  (pyramidal)  tract;  C,  body-sense  tract;  D,  visual  tract;  E,  auditory  tract; 
F,  G  and  H,  upper,  middle  and  lower  peduncles  of  cerebellum;  K,  decussation  of 
pyramids.  Numerals  refer  to  cranial  nerves. 

The  White  Matter  of  the  Cerebrum.  This  consists  of  myelin- 
ated axons  classified  according  to  their  course  and  distribution 
into  three  groups.  The  so-called  projection  fibers  (Fig.  66)  are  the 
axons  by  which  the  cortex  is  brought  into  connection  with  the 
other  parts  of  the  nervous  system.  These  include  afferent  projec- 
tion fibers,  which  are  the  continuations  within  the  cerebrum  of  the 
various  sensory  paths  described  in  previous  paragraphs  (see 
p.  172),  and  efferent  projection  fibers,  which  convey  impulses  from 
the  cortex  to  the  rest  of  the  Body. 


176  THE  HUMAN  BODY 

At  the  base  of  the  cerebrum,  where  it  rests  upon  the  midbrain, 
all  the  projection  fibers,  both  afferent  and  efferent,  are  crowded 
together  into  a  restricted  space  between  two  of  the  basal  nuclei. 
This  region  is  known  as  the  internal  capsule.  As  the  fibers  emerge 
thence  into  the  roomy  cerebrum  they  spread  apart  on  their  way 
to  the  different  parts  of  the  cortex  forming  the  corona  radiala. 

The  second  group  of  cerebral  axons  are  the  association  fibers. 
These  pass  between  one  part  of  the  cortex  and  another  within 
the  same  hemisphere,  enabling  impulses  to  travel  freely  among 
the  cortical  cells.  The  third  group  of  cerebral  axons  are  the 
commissural  fibers  which  pass  between  cortical  areas  in  opposite 
hemispheres;  these  serve  to  unify  the  anatomically  double  cere- 
brum into  a  single  physiological  organ;  the  corpus  callosum  (Cc, 
Fig.  60)  is  made  up  of  commissural  fibers. 

Lobes  of  the  Cerebrum.  The  convolutions  of  the  cerebrum  are 
sufficiently  constant  in  number  and  position  to  serve  as  land- 
marks in  locating  particular  regions.  The  individual  convolutions, 
or  gyri,  have  been  given  specific  names,  as  have  also  the  fissures, 
or  sulci,  which  separate  them.  For  our  purposes  it  is  necessary 
to  mention  by  name  only  those  fissures  which  mark  off  the  grand 
divisions,  or  lobes,  of  the  cerebrum.  The  division  of  the  cerebrum 
into  lobes  is  purely  arbitrary,  and  is  made  for  greater  ease  in 
describing  it.  In  general  the  lobes  correspond  in  position  to  the 
overlying  skull  bones  for  which  they  are  named.  The  fissures 
which  mark  the  boundaries  of  the  lobes  are  indicated  in  Fig.  59. 
They  are  the  fissure  of  Sylvius,  the  fissure  of  Rolando,  and  the 
Parieto-occipital  fissure.  The  frontal  lobe  is  that  part  of  the  cere- 
brum above  the  fissure  of  Sylvius  and  in  front  of  the  fissure  of 
Rolando;  the  parietal  lobe  is  between  the  fissure  of  Rolando  and 
the  parieto-occipital  fissure;  the  occipital  lobe  is  the  wedge-shaped 
portion  behind  the  parieto-occipital  fissure;  the  temporal  lobe  is 
below  the  fissure  of  Sylvius;  it  is  the  only  one  of  the  lobes  which 
is  sharply  set  off  as  a  distinct  region. 

Cortical  Localization.  A  problem  of  much  interest  in  connec- 
tion with  the  study  of  cerebral  functions  is  whether  there  is  di- 
vision of  labor  among  the  various  parts  of  the  cortex.  Do  certain 
groups  of  cells  perform  certain  special  functions,  or  are  all  cortical 
activities  shared  in  by  all  the  cells?  This  is  not  the  place  for  a 
history  of  the  solution  of  this  problem.  Suffice  it  to  say  that  we 


STRUCTURE  AND  FUNCTIONS  OF  THE  CEREBRUM      177 

now  have  positive  proof  of  a  high  degree  of  specialization  of  func- 
tion in  the  cortex. 

Sensory  Areas.  In  previous  paragraphs  the  paths  of  the 
various  senses  were  traced  as  far  as  their  entrance  into  the  cere- 
brum by  way  of  the  internal  capsule.  We  must  now  continue 
the  paths  to  their  cortical  terminations.  The  body  sense  fibers 
pass  to  that  part  of  the  parietal  lobe  just  behind  the  fissure  of 
Rolando;  the  region  where  they  terminate  is  the  body  sense  area. 
The  visual  tracts  end  in  the  occipital  lobes  in  the  visual  areas. 
The  auditory  tracts  terminate  in  the  temporal  lobes  in  a  region 
just  below  and  within  the  fissure  of  Sylvius;  this  region  constitutes 
the  auditory  area.  Although  the  paths  of  smell  and  taste  are 
imperfectly  known,  their  cortical  terminations  have  been  fairly 
well  established.  The  olfactory  area  is  supposed  to  be  in  the  tem- 
poral lobe,  and  possibly  at  its  very  tip.  The  area  for  taste,  gusta- 
tory area,  is  thought  to  be  also  in  the  temporal  lobe,  probably 
adjacent  to  the  area  for  smell.  Since  the  nerve-paths  of  the 
various  senses  lead  directly  to  these  areas,  and  since  destruction 
of  any  one  of  them,  by  accident  or  disease,  results  in  loss  of  the 
particular  sense  whose  area  is  involved,  we  must  conclude  that  the 
sensory  areas  are  the  receiving  stations  of  the  cerebrum.  All 
afferent  projection  fibers  entering  the  cerebrum  terminate  in  one 
or  another  of  the  sensory  areas.  Within  these  areas  they  have 
synaptic  connection  with  the  association  neurons  of  the  region. 

The  Motor  Area  and  the  Pyramidal  Tracts.  In  each  hem- 
isphere a  region  of  the  frontal  lobe  just  in  front  of  the  fissure  of 
Rolando  contains  numerous  giant  pyramidal  cells  whose  axons 
extend  into  the  white  matter  and  are  grouped  together  in  the 
internal  capsule  as  a  conspicuous  nerve  tract,  called  the  py- 
ramidal tract.  It  extends  through  the  midbrain  to  the  medulla  and 
appears  upon  the  ventral  surface  of  the  latter  as  a  well-marked 
anatomical  feature.  About  midway  of  the  medulla  the  pyramidal 
tracts  cross  the  mid-line  in  the  decussation  of  the  pyramids  (K.  fig. 
66).  This  decussation  is  not  complete;  part  of  the  fibers  of  each 
pyramidal  tract  continue  along  the  same  side  of  the  medulla  to 
the  spinal  cord  and  down  the  latter  in  the  ventral  column,  forming 
the  direct  pyramidal  tract.  That  part  of  each  pyramidal  tract 
which  crosses  over  at  the  "  decussation  "  proceeds  along  the  spinal 
cord  in  the  lateral  column  as  the  crossed  pyramidal  tract  (Fig.  65). 


178  THE  HUMAN  BODY 

It  appears  that  most  of  the  fibers  of  the  direct  pyramidal  tracts 
cross  the  mid-line  in  the  spinal  cord  before  reaching  their  termi- 
nations; so  that  the  pyramidal  tracts  are  finally  crossed  tracts. 
All  the  pyramidal  axons  have  synaptic  connection  with  the  cells 
of  motor  neurons  in  the  ventral  horns  of  gray  matter  of  the  cord. 

The  pyramidal  axons  are  branched  at  their  tips,  so  that  each 
communicates  with  several  motor  neurons.  On  page  136  we  saw 
that  each  motor  neuron  connects  with  a  number  of  muscle-fibers. 
It  follows  that  a  considerable  group  of  muscle-fibers  is  under  the 
control  of  each  pyramidal  axon.  When  we  recall  the  large  numbers 
of  fibers  which  go  to  make  up  even  our  smallest  muscles,  we  see 
that  this  arrangement,  which  cuts  down  the  number  of  nervous 
elements  required  to  operate  the  muscular  system,  does  not  at  all 
impair  the  delicacy  and  efficiency  of  our  muscular  movements. 

Since  the  pyramidal  axons  arise  from  cell-bodies  within  the 
cortex  it  is  evident  that  the  pyramidal  tracts  must  be  efferent 
paths.  The  intimate  way  in  which  the  pyramidal  fibers  connect 
with  the  cell-bodies  of  motor  neurons  indicates  that  they  form 
the  paths  by  which  the  cerebrum  exercises  control  over  bodily 
movements.  The  anatomical  evidence  for  that  view  has  been 
corroborated  and  strengthened  by  physiological  evidence.  The 
German  physiologists,  Fritsch  and  Hitzig,  showed  that  in  dogs 
electrical  excitation  of  those  areas  of  the  brain  from  which  spring 
the  pyramidal  tracts  is  followed  by  movements  of  the  muscles  of 
the  Body.  They  showed  also  that  these  are  the  only  areas  from 
which  such  movements  can  be  elicited. 

Upon  the  basis  of  all  this  evidence  we  are  justified  in  looking 
upon  the  regions  immediately  in  front  of  the  Rolandic  fissures 
as  motor  areas.  These  areas  have  been  much  studied  physiologi- 
cally in  recent  years.  The  brains  of  the  higher  apes  have  been 
preferred  in  these  studies  to  those  of  lower  animals  because  of 
their  greater  similarity  to  the  human  brain. 

There  have  been  a  few  observations  upon  the  brains  of  human 
beings  in  cases  where  the  surgical  treatment  of  certain  diseases 
has  involved  removal  of  portions  of  the  skull  overlying  the  Ro- 
landic areas. 

These  recent  studies  have  shown  that  there  is  a  considerable 
localization  within  the  motor  areas  themselves;  stimulation  of  one 
point  causes  movements  of  the  hand,  of  another  the  foot,  of  still 


STRUCTURE  AND  FUNCTIONS  OF  THE  CEREBRUM      179 

another  the  head.  They  have  shown  incidentally,  also,  that  the 
cerebral  cortex  is  not  painfully  sensitive  to  direct  stimulation. 
The  men  whose  brains  were  excited  electrically  in  the  observa- 
tions cited  above  were  conscious  throughout  the  procedure  and 
reported  no  sensations  of  pain  or  discomfort  at  any  stage. 

Cortical  Reflex  Paths.  The  various  sensory  areas  with  their 
afferent  nerve-paths  afford  means  whereby  impulses  may  enter 
the  cerebrum  from  the  different  receptors;  the  motor  areas,  one 
in  each  hemisphere,  with  their  efferent  paths,  provide  for  the 
passage  of  impulses  from  the  cerebrum  to  the  motor  organs  of  the 
Body;  the  abundant  equipment  of  association  fibers  within  the 
cerebrum  makes  possible  the  passage  of  impulses  across  from 
sensory  areas  to  motor  areas.  We  can  picture,  then,  reflex  arcs 
involving  the  cerebrum.  Such  arcs  are  necessarily  complex,  in- 
volving many  more  neurons  than  dp  the  simple  spinal  cord  reflex 
arcs  already  described.  In  a  previous  paragraph  (p.  158)  we  saw 
that  the  simplest  reflex  arc  through  the  cord  involves  at  least  two 
neurons,  one  sensory,  and  one  motor.  If  we  trace  a  reflex  arc 
involving  the  cortex  from  a  receptor  in  the  skin  of  the  right  hand, 
for  example,  to  a  retractor  muscle  of  the  right  arm,  we  find  in  it 
at  least  five  neurons  and  possibly  many  more.  The  five  which  are 
necessarily  included  are:  1,  the  sensory  neuron  which  we  suppose 
extends  all  the  way  from  the  receptor  into  the  cord  and  up  the 
dorsal  column  to  a  termination  in  the  cuneate  or  gracile  nucleus; 
2,  a  neuron  of  the  fillet  tract,  having  its  cell-body  in  the  cuneate  or 
gracile  nucleus,  and  its  axon  extending  through  the  medulla  and 
midbrain  and  the  white  matter  of  the  cerebrum,  crossing  the  mid- 
line  in  the  "  sensory  decussation"  of  the  fillet,  and  terminating  in 
synaptic  connection  with  a  neuron  of  the  body  sense  area  in  the 
left  cerebral  hemisphere;  3,  the  neuron  just  mentioned,  having  its 
cell-body  in  the  body  sense  area  and  an  axon  which  passes  by  way 
of  the  cerebral  white  matter  to  the  motor  area;  4,  a  pyramidal 
neuron  of  the  motor  area  whose  dendrites  receive  the  impulse 
from  the  body  sense  neuron  (3),  and  whose  axon  forms  part  of 
the  pyramidal  tract,  crossing  back  to  the  right  side  of  the  Body 
in  the  decussation  of  the  pyramids,  and  terminating  in  synaptic 
connection  with  the  cell-body  of  a  motor  neuron  in  the  ventral 
horn  of  gray  matter  of  the  cord;  5,  the  motor  neuron  which  forms 
the  last  link  in  the  reflex  chain,  conveying  the  impulse  from  the 


180  THE  HUMAN  BODY 

pyramidal  neuron  to  the  muscle.  It  is  doubtful  whether  any 
cortical  reflex  arcs  are  actually  composed  of  as  few  neurons  as 
five;  probably  the  simplest  ones  contain  several  additional  associa- 
tion neurons  within  the  cerebrum. 

Cortical  Reflexes  Compared  with  Spinal  Reflexes.  As  an  ex- 
ample of  a  simple  spinal  reflex  was  cited  the  involuntary  with- 
drawal of  the  hand  from  accidental  contact  with  a  hot  body. 
To  illustrate 'a  simple  cortical  reflex  suppose  that  my  finger  rests 
upon  the  terminals  of  an  apparatus  for  generating  electric  shocks; 
I  am  told  that  when  I  feel  the  shock  I  must  withdraw  my  hand. 
The  shock  may  be  so  feeble  as  to  be  barely  perceptible.  Under 
such  circumstances  the  withdrawal  must  be  voluntary  and  the  re- 
sponse, therefore,  must  involve  the  cerebrum.  The  chief  objective 
difference  between  voluntary  withdrawal  of  the  hand  in  response 
to  feeble  stimulation,  and  its  involuntary  retraction  in  response 
to  strongly  painful  stimulation  is  that  the  former  reaction  requires 
a  noticeabty  longer  time  than  does  the  latter.  The  only  simple 
reflex  whose  time  has  been  satisfactorily  measured  in  man  is 
the  winking  reflex;  this  requires  about  0.06  second  for  its  com- 
pletion. The  quickest  cortical  reflexes  take  about  0.15  second. 
This  difference  in  time  is  much  greater  than  can  be  accounted  for 
by  supposing  the  cortical  reflex  to  involve  a  greater  length  of 
nerve-fibers,  and  therefore  must  be  due  to  the  fact  that  the  cor- 
tical reflex  involves  a  greater  number  of  neurons  and  consequently 
more  synapses  to  be  crossed. 

An  additional  difference  which  we  recognize  subjectively  be- 
tween spinal  and  cortical  reflexes  is  that  while  the  former  are 
involuntary  and  unconscious,  the  latter  are  voluntary  responses 
to  stimuli  consciously  perceived.  This  difference  will  be  discussed 
more  fully  in  a  later  paragraph,  when  the  meaning  of  the  terms 
" voluntary"  and  "consciously"  shall  have  been  considered. 

Memory.  We  have  seen  that  the  primary  function  of  the 
cerebrum  is  to  .introduce  remote  considerations  as  determining 
factors  in  the  responses  of  the  organisms.  We  have  seen  also  that 
in  order  to  do  this  the  cerebrum  must  have  an  equipment  of  knowl- 
edge, which  can  be  gained  only  through  the  receptor  channels  of 
the  Body.  The  information  which  reaches  the  brain,  to  be  of 
service,  must  be  retained  there  until  needed,  and  must  be  held  in 
such  a  way  as  to  be  available  when  required. 


STRUCTURE  AND  FUNCTIONS  OF  THE  CEREBRUM      181 

The  neurons  of  the  nervous  system  generally  act,  in  the  main, 
as  conductors  pure  and  simple.  When  they  are  stimulated  nerve 
impulses  are  aroused;  these  spread  over  them  and  escape  by  those 
synapses  whose  resistance  is  not  too  high;  thus  other  neurons 
are  involved  and  so  the  impulses  advance  to  a  motor  termination. 

The  cortical  neurons  of  the  cerebrum  owe  their  dominant  po- 
sition in  the  nervous  system  chiefly  to  a  peculiar  ability  which 
they  possess  of  " holding  up"  impulses  which  come  to  them,  re- 
taining them  indefinitely,  and  giving  them  out  again  in  the  future, 
if  necessary,  over  and  over.  This  storing  of  impulses  constitutes 
memory.  The  " reflex"  animal,  because  he  is  deprived  of  this  prop- 
erty, must  always  respond  immediately  to  adequate  stimulation; 
the  intact  animal  may  respond  immediately  'or  may  retain  the 
stimulus  as  a  memory  to  modify  his  future  activities.  Since  the 
intact  animal  has  within  his  cerebrum  a  store  of  impulses  "held  in 
leash,"  he  may  at  any  time  become  active  through  the  liberation  of 
some  of  them,  without  immediate  external  stimulation. 

Association  Areas.  The  different  sensory  areas  and  the  motor 
areas  occupy  only  a  small  part  of  the  whole  cerebral  cortex.  Most 
of  the  frontal  lobes  and  large  areas  of  the  parietal  and  temporal 
lobes  are  not  involved  in  the  immediate  reception  of  impulses, 
nor  in  their  transmission  to  the  Body.  These  areas  are  as  richly 
supplied  with  interconnecting  neurons  as  any  part  of  the  cortex. 
They  are  assumed,  without  very  positive  proof,  to  be  the  seat  of  a 
function  we  know  the  cerebrum  to  possess,  that  of  association. 

The  Nature  and  Mechanism  of  Association.  At  birth  the 
brain  of  the  infant  may  be  compared  to  a  clean  page.  It  bears  no 
impressions  of  any  sort.  Such  activities  as  the  infant  shows  are 
purely  reflex.  In  course  of  time  sense  impressions  begin  to  come 
into  the  sensory  areas  of  the  cortex.  These  register  themselves 
more  or  less  definitely  as  memories,  and  presently  the  child  is  in 
possession  of  a  considerable  store  of  memories  of  various  sorts. 
He  may  know  the  sound  of  his  mother's  voice  or  may  recognize 
her  face.  As  yet,  however,  there  is  no  connection  between  these 
independent  impressions.  When  in  the  child's  mind  that  voice  is 
associated  with  that  face,  so  that  he  knows  them  as  parts  of  a 
single  whole,  he  has  performed  an  act  of  association.  From,  this 
time  throughout  his  life  his  memory  is  not  alone  of  the  simple 
sound  of  the  voice  or  the  appearance  of  the  face  but  of  the  mother 


182  THE  HUMAN  BODY 

whom    he    has    learned    to   know    by    these   associated  impres- 
sions. 

Acts  of  association  are  supposed  to  be  carried  on  within  (lie 
association  areas  of  the  cortex.  We  may  picture  the  process  in 
the  example  cited  above  somewhat  as  follows:  The  impression  of 
the  voice  is  stored  in  the  auditory  area;  that  of  the  appearance 
of  the  face  is  in  the  visual  area;  both  these  sensory  areas  have 
rich  communications  with  neurons  of  the  association  areas.  By 
some  means  impulses  from  the  sensory  cells  where  these  impres- 
sions are  stored  meet  in  a  cell  of  an  association  area.  That  cell 
builds  from  these  single  related  sense  impressions,  a  composite, 
which  is  stored  in  turn  as  a  memory.  As  additional  related  in- 
formation is  gained  the  composite,  or  concept,  is  enlarged. 

The  union  of  related  impressions  into  concepts  does  not  nec- 
essarily involve  loss  or  impairment  of  the  fundamental  impres- 
sions themselves;  the  child  in  whose  mind  is  a  definite  concept 
of  his  mother  retains  also  clear  memories  of  her  voice  and  her 
face.  The  paths  of  communication  between  the  cells  where  are 
stored  the  primary  sense  impressions  and  those  where  the  resulting 
concepts  are  formed  seem  to  remain  always  very  easy  of  passage. 
The  sound  of  the  mother's  voice  calls  up  the  entire  concept  of  the 
mother  with  great  clearness,  even  though  years  may  have  elapsed 
since  it  was  heard. 

Since  concepts  are  stored  as  memories  they  may  serve  in  their 
turn  as  bases  for  more  complex  associations;  these  again  by  be- 
coming memories  may  contribute  to  the  associative  process,  and 
so  the  complex  structure  of  the  mind  is  built  up,  resting  at  bot- 
tom always  upon  primary  sense  impressions. 

The  act  of  association  is  essentially  one  of  combining  related 
memories;  the  formed  associations  become  memories  in  their  turn. 
For  these  reasons  the  term  associative  memory  is  used  as  more 
truly  describing  the  nature  of  associative  processes  than  the  older 
expression  "the  association  of  ideas." 

The  use  of  a  memory  in  forming  one  association  does  not  inter- 
fere with  its  use  in  the  formation  of  others.  This  ability  of  the  cere- 
brum to  use  memories  over  and  over  again  is  a  very  valuable  prop- 
erty since  it  enables  us  to  make  the  utmost  of  all  our  knowledge. 

Development  of  the  Cortex.  The  increase  in  intellectual 
power  which  accompanies  the  growth  of  the  child  is  not  the  re- 


STRUCTURE  AND  FUNCTIONS  OF  THE  CEREBRUM      183 

suit  of  any  increase  in  the  number  of  nerve-cells,  for  the  child 
is  born  with  his  full  number.  It  is,  however,  based  upon  their 
continuous  development;  this  development  consisting  chiefly  of 
greater  and  greater  branching  with  correspondingly  richer  synaptic 
connections.  At  birth  scarcely  any  cortical  cells  are  sufficiently 
developed  to  be  functional.  The  sensory  areas  first  become  so. 
The  association  areas  reach  their  highest  point  of  development  at 
about  the  thirty-fifth  year.  At  this  age  the  anatomical  progress  of 
the  brain  comes  to  an  end;  all  possible  paths  of  association  have 
been  laid  down.  This  does  not  mean,  however,  that  all  possible 
associations  have  been  formed.  These  continue  to  be  formed  so 
long  as  the  brain  continues  active.  It  is  probably  true,  however, 
that  with  advancing  years  there  is  a  diminution  in  the  freedom 
of  associative  activity;  the  brain  no  longer  accomplishes  daring 
feats  of  thought,  such  as  constitute  creative  genius,  but  plods 
along  in  the  ruts  established  by  its  earlier  activities.  This  fact 
explains  why  conservative  tendencies  usually  become  more  pro- 
nounced as  age  advances.  . 

The  Functions  of  Associative  Memory.  It  is  because  the  cer- 
ebrum is  able  to  form  associative  memories  that  the  organism  can 
adjust  its  responses  with  due  regard  to  remote  as  well  as  to  im- 
mediate considerations.  Incoming  stimuli,  which  in  a  " reflex" 
animal  would  produce  a  definite  response  of  a  certain  kind,  are 
in  an  intact  animal  balanced  against  such  related  associative 
memories  as  the  animal  possesses;  if  these  indicate  that  the  natural 
reflex  response  is  the  proper  one  to  make,  the  animal  responds  as 
does  the  "reflex"  one;  if,  however,  they  indicate  a  different  line  of 
action  as  more  advantageous,  the  animal  substitutes  for  the 
natural  reflex  response  a  different  one,  suited  to  the  situation. 

Associative  memory  also  forms  the  basis  for  the  execution  of 
complex  movements  from  feeble,  immediate  stimuli,  or  in  their 
absence;  the  young  puppy  responds  to  his  master's  whistle  only 
by  a  pricking  of  the  ears;  in  the  older  dog  the  sound  of  the  whistle 
arouses  a  chain  of  associative  memories  and  under  their  impelling 
force  he  executes  the  complex  movements  which  carry  him  to  his 
master's  feet. 

In  order  that  associative  memory  may  influence  bodily  activ- 
ities it  must  have  access  to  the  efferent  nerve-paths  of  the  cere- 
brum. This  access  it  has  through  rich  connections  from  the 


184  THE  HUMAN  BODY 

association  areas  to  the  motor  areas.  It  must  have  also  the  power 
to  stimulate  the  efferent  nerves.  This  power  it  exercises  through 
the  function  of  volition. 

Volition.  Although  all  voluntary  acts  result  from  nerve  im- 
pulses which  have  come  from  the  motor  areas  of  the  cerebrum 
by  way  of  the  pyramidal  tracts,  we  cannot  suppose  that  they 
originate  in  the  cells  of  the  motor  cortex.  There  is  no  evidence 
that  these  or  any  cortical  cells  are  able  to  originate  any  activities 
whatever.  All  voluntary  acts,  as  a  matter  of  fact,  are  based  upon 
associative  memory;  the  immediate  stimulus  to  the  performance 
of  the  voluntary  act  comes,  not  from  the  motor  areas,  but  from 
that  part  of  the  association  areas  where  the  exciting  memory  is 
stored.  All  memories,  as  we  have  seen,  are  at  bottom  stored  sen- 
sory impressions.  What  happens,  then,  when  we  perform  volun- 
tary acts  is  that  we  cause  to  pass  on  to  the  motor  areas  stimuli 
which  originally  entered  the  nervous  system  by  way  of  the  re- 
ceptors, and  which  have  since  been  combined  in  various  ways, 
and  the  resulting  associations  stored  as  memories.  Voluntary  acts 
are,  therefore,  the  completion  of  reflexes. 

The  Usefulness  of  Associative  Memory  Depends  on  its  Order- 
liness. It  is  perfectly  obvious  that  associations  to  be  of  value 
must  be  formed  from  related  impressions  or  related  concepts.  We 
know  that  our  brains  normally  form  associations  in  this  orderly 
way.  How  the  brain  is  guided  in  its  selection  of  material  for  mak- 
ing associations  so  as  to  include  what  is  relevant  and  exclude  the 
rest  is  quite  beyond  our  knowledge  or  even  imagination.  That 
in  the  highly  complex  associative  processes  which  we  call  think- 
ing there  may  be  a  conscious  selection  or  rejection  of  memories 
we  know  from  our  own  experience. 

It  is  true,  of  course,  that  the  brain,  being  an  imperfect  instru- 
ment, often  makes  mistakes  and  forms  associations  that  instead 
of  being  useful  give  rise  to  harmful  activities.  The  resulting 
disaster,  through  the  additional  knowledge  it  affords,  may  enable 
the  brain  to  form  correct  associations  next  time.  Thus  we  profit 
by  our  mistakes. 

The  Interaction  of  Associative  Memories.  Inhibition.  The 
human  brain  acquires  in  the  course  of  years  such  a  wealth  of  asso- 
ciative memories,  based  upon  so  many  phases  of  experience,  that 
the  determination  of  the  conduct  to  be  employed  in  any  particular 


STRUCTURE  AND  FUNCTIONS  OF  THE  CEREBRUM      185 

situation  becomes  often  a  matter  of  much  difficulty.  One  set  of 
memories  point  toward  one  course  and  another  set  toward  quite 
the  opposite  course.  When  this  happens  it  is  necessary  to  call 
in  more  and  more  remote  considerations  until  the  balance  tips 
unmistakably  in  one  way  or  the  other.  When  even  this  procedure 
fails  to  be  decisive,  or  when  the  mind  wishes  to  avoid  the  labor  of 
deciding  by  this  method  recourse  is  often  had  to  a  selective  external 
stimulus.  Deciding  a  course  of  action  by  the  flip  of  a  coin  is  a 
case  in  point. 

Associative  memories  also  come  into  conflict  when  immediate 
considerations  point  toward  one  course  and  remote  considerations 
toward  a  different  one.  Associative  memories  are  classified  by 
placing  those  of  remote  bearing  higher  than  those  of  immediate 
bearing.  Highest  of  all,  because  most  remote,  are  abstract  con- 
ceptions of  right  and  wrong;  conceptions  of  altruism,  care  for 
mankind,  are  higher  than  conceptions  of  family  love;  these  in 
turn  rank  above  purely  personal  considerations.  Personal  con- 
siderations which  have  regard  to  the  future  are  higher  than  those 
dealing  only  with  the  immediate  present.  The  progress  of  civiliza- 
tion is  largely  measured  by  the  degree  to  which  remote  considera- 
tions outweigh  immediate  ones  in  determining  conduct. 

Because  the  cerebrum  rests  upon  an  underlying  reflex  mechan- 
ism the  tendency  of  the  organism  is  always  toward  immediate 
response  to  sensory  stimulation;  the  hungry  man  tends  to  take 
the  first  food  that  comes  to  hand;  the  cold  man  tends  to  seek  the 
nearest  available  shelter.  The  action  of  associative  memory, 
when  higher  considerations  dictate  a  different  course,  is  to  pre- 
vent or  inhibit  the  carrying  out  of  the  immediate  response.  In- 
hibition is,  then,  one  of  the  important  functions  of  associative 
memory.  The  man  who  deliberately  does  what  he  knows  to  be 
wrong,  acts  as  he  does  because  his  conceptions  of  right  are  not 
powerful  enough  to  inhibit  the  response  to  the  lower  stimulus. 
The  importance  of  inculcating  the  highest  principles  of  right 
living  by  training  and  example,  during  the  receptive  period  of 
the  brain's  development,  is  therefore  clearly  manifest. 

Will  Power.  In  some  persons  there  is  an  inborn  tendency  to 
respond  to  immediate  stimulation,  even  though  associative  mem- 
ory shows  that  such  response  is  not  for  the  best.  Such  persons  we 
describe  as  weak-willed.  Those  in  whom  the  dictates  of  associative 


186  THE  HUMAN  BODY 

memory  are  supreme  we  call  strong-willed.  The  weak-willed  per- 
son yields  to  temptations  which  are  powerless  to  move  the  one 
whose  will  power  is  great.  In  this  definition  of  will  power  we  have 
set  associative  memory  against  immediate  stimulation,  and  usually 
the  conflict  is  between  these.  Sometimes,  however,  the  struggle 
comes  between  different  immediate  stimuli.  In  the  weak-willed 
man  the  more  insistent  ones  are  likely  to  control,  rather  than  the 
more  important.  If  he  is  beseeched  by  various  friends  to  accom- 
pany them  different  ways  the  most  vociferous  is  usually  the  one  to 
carry  him  off.  The  strong-willed  man,  on  the  other  hand,  makes  his 
decision  on  other  grounds.  On  the  other  hand,  various  associative 
memories  may  be  in  conflict,  and  here  again,  whether  obedience 
will  be  to  the  most  clamorous  or  the  most  important  depends  on 
the  strength  of  the  will. 

Cerebral  Control  of  Spinal  and  Cerebellar  Reflexes.  The 
exercise  of  the  inhibitory  function  of  associative  memory  as  just 
described  involves  an  ability  on  the  part  of  the  cerebrum  to  modify 
the  reflexes  of  the  lower  parts  of  the  nervous  system.  There  is 
abundant  evidence  that  such  ability  actually  exists.  In  the  case 
of  spinal  reflexes  it  may  be  supposed  to  act  through  the  discharge  of 
impulses  from  the  motor  area  which  in  some  manner  increase 
synaptic  resistances  in  the  course  of  the  reflexes  sufficiently  to 
block  them.  A  feature  of  the  inhibition  of  spinal  reflexes  which 
points  to  this  as  the  means  of  bringing  it  about  is  that  in  the  case  of 
sharp  sensory  stimulation  the  inhibition  must  be  established  before 
the  stimulus  is  received.  If  one  unexpectedly  touches  a  hot  object 
he  automatically  and  inevitably  jerks  his  hand  away,  but  if  he 
knew  the  object  was  hot,  and  nevertheless  found  it  necessary  to 
grasp  it  he  could,  through  an  act  of  volition,  block  his  reflex  path  so 
effectively  that  the  tendency  to  draw  the  hand  away  would  be 
completely  overcome. 

Cerebellar  reflexes  are  also  subject  to  cerebral  control.  There 
is  evidence  that  part  of  the  pyramidal  tract  from  the  motor  area 
terminates  in  the  brain  stem  in  relationship  with  paths  leading  into 
and  out  of  the  cerebellum.  Apparently  thus  voluntary  control  of 
locomotion  is  exercised.  That  we  have  such  voluntary  control 
is  evident.  We  can  start,  stop,  or  modify  our  locomotor  acts  at 
will,  although  as  we  have  previously  seen,  the  performance  of  the 
reflexes  as  distinct  from  their  guidance,  is  automatic. 


STMCCTl'RE  A\D  FLXCTIOXS  OF  THE  CEREBRUM      187 


Habit   Formation.    Just 


peated  OTCT  and  over  becomes  more  firmly  fixed  in 
does  one  leceited  only  once,  there  seeming  to  be 
the  remembering  nerve-cell  which  b 


at  each  repetition  of  the  stimulus :  so  every  interaction  of 

upon  the  ecus  involved,  which  is  deepened  by  repetition  of  the 

the  strong  tendencies  of  the  brain  is  to  arrange  its 
niMii^rMR  thus  m  jjfmips  p*jj^ji rtf"  to  certain  definite 
It  is  this  tendency  which  fies  at  the  basis  of  habit 

Habits  winch  are  formed  in  this  way,  by  repeated 
of  the  same  fcn*  of  thought  to  the  «™**  artiontv,  t*^**  on  ynnrli  of 
the 

of 

of  the  habitual  action.  A  definite  act  of  inhibition  on  the  part 
01  other  assooatrve  •nnnotifift  is  muji'JUiUjjr  to  prevent  t be  response. 
AUa(B5\a^m2ny\nbit-o(lh^^^\bt?^ofibcttttc& 
Tame  in  our  dauV  fives  JbrranffP  on  account  of  them  many  thing* 
that  we  have  to  do  are  more  easuy  done  than  they  would  be  if 
the  whole  mental  process  upon  which  the  acts  depend  had  to  be 
gone  througfr  with  at  each  repetition. 

The  tendency  to  habit  formation  can  be  used  Very  effectively 
in  training  the  clnU  to  right  actions.  It  can  be  used  as  well  in 
training  to  right  thoughts,  since  thoughts  are  associative  proc- 

tend  to  follow  the  fines  laid  down  by  habit 
Of  afl  t  he  powers  of  the  human  mind  its  power  to 
has  had  as  much  to  do  with  the  progress  of  the 
it  poanraspx     Language,  from  the 

is  simply  a  special  sort  of 
of  arbitrarily  selected 


of  the  cUd  forming  the  concept  mother.    Co- 
tfae  jtnwfiitinn  of  her  roke,  appearance,  and  other 

;  the  repeated  anfitory 


188  THE  HUMAN  BODY 

stimulus  of  the  word  mother,  heard  when  she  is  present  or  when 
she  is  indicated  in  some  way.  In  course  of  time  this  particular 
succession  of  syllables  is  included  as  part  of  the  concept.  Several 
years  later  the  group  of  written  symbols  making  up  the  word 
mother  is  included  in  the  same  concept.  Thus  language,  spoken 
and  written,  becomes  indissolubly  included  in  our  whole  mental 
equipment. 

It  is  a  curious  fact  that  in  man  the  use  of  language  seems  to  be 
not  optional,  but  a  necessary  factor  in  his  mental  development. 
Two  lines  of  evidence  favor  this  view.  The  first  is  the  common 
experience  of  all  of  us  that  we  are  incapable  of  thought  except 
in  terms  of  words;  coupled  with  the  observation  that  no  race  of 
men  exists  or  is  known  to  have  existed  without  some  form  of  lan- 
guage. The  second,  and  more  striking,  fact  is  that  certain  regions 
in  the  association  areas  of  the  cerebrum  are  specially  devoted  to 
language  associations.  Four  such  regions  are  known,  having 
been  revealed  by  the  physiological  effects  of  their  impairment 
through  accident  or  disease.  Two  of  these  areas  have  to  do  with 
spoken,  and  two  with  written,  language.  One  of  the  two  areas 
for  each  form  of  language  is  sensory  and  the  other  motor.  An 
interesting  thing  about  these  language  association  areas  is  that 
they  seem  to  be  confined  to  one  of  the  cerebral  hemispheres;  in 
right-handed  people  the  left  hemisphere  contains  them,  and  in 
left-handed  people  they  occur  in  the  right  hemisphere.  It  has 
been  observed,  moreover,  that  the  development  of  right  or  left- 
handedness  in  infants  is  coincident  with  their  learning  to  use 
language.  Just  what  the  relationship  between  the  two  properties 
may  be  is  not  clear. 

Impairment  of  a  sensory  language  area  results  in  word-deafness 
or  word-blindness;  the  sounds  are  heard,  or  the  words  are  seen, 
but  they  are  without  meaning  because  the  power  to  associate 
language  with  concepts  is  affected.  When  motor  language  areas 
are  injured  the  power  of  expression  is  lost.  The  commonest  of  all 
these  abnormalities  is  the  loss  of  power  to  use  spoken  language,  a 
condition  known  as  motor  aphasia.  The  sufferer  from  this  con- 
dition knows  what  he  wants  to  say  but  is  unable  to  recall  the 
words  by  which  to  express  his  ideas.  Embarrassment  often  gives 
rise  to  a  momentary  inhibition  of  the  motor  aphasia  region,  re- 
sulting in  the  same  inability  to  recall  the  needful  words. 


STRUCTURE  AND  FUNCTIONS  OF  THE  CEREBRUM      189 

Since  all  our  mental  processes  are  dependent  on  language,  im- 
pairment of  the  language  areas  would  be  expected  to  lower  the 
whole  mental  power.  This  appears  to  be  the  case  in  most  sufferers 
from  this  condition. 

There  is  no  evidence  that  any  species  of  animals  except  the 
human  species  possesses  the  power  to  use  language.  This  differ- 
ence sets  man  sharply  apart  from  the  animals  most  ^nearly  ap- 
proaching him  in  intelligence. 

Consciousness.  This  is  a  phenomenon  that  we  all  recognize 
as  existing  in  ourselves  and  as  accompanying  most  if  not  all  of 
our  cerebral  activities.  That  it  is  present  when  the  cortical  cells 
are  actively  functioning  and  absent  when  they  are  inactive  is 
indicated  by  the  fact  that  any  treatment,  such  as  anaesthesia, 
which  depresses  nerve-cells,  tends  to  abolish  consciousness.  It 
is  a  phenomenon  whose  nature  is  wholly  unknown,  and  for  whose 
existence,  even,  there  is  no  objective  evidence.  We  cannot  prove 
that  any  lower  animal  has  the  same  sort  of  consciousness  that  we 
have.  We  can  only  suppose  from  the  general  similarity  of  their 
cerebral  processes  to  ours  that  this  particular  phenomenon  is 
also  in  them  as  in  us.  As  we  go  down  the  animal  scale  where 
mental  processes  become  simpler  and  simpler  does  it  follow  that 
consciousness  becomes  dimmer  and  dimmer?  We  ordinarily  as- 
sume this  to  be  true,  but  without  any  positive  evidence  upon 
which  to  base  the  assumption. 

Emotions.  Another  set  of  phenomena  accompanying  cerebral 
activity,  but  known  chiefly  by  subjective  experience,  are  the 
emotions.  We  knoW  that  certain  sensory  stimuli  give  us  pleasure, 
others  arouse  disgust.  Love  and  hate,  sorrow  and  joy,  are  mental 
states  which  are  associated  with  certain  sense  impressions  im- 
mediate or  remembered.  Emotion,  like  consciousness,  does  not 
lend  itself  to  objective  study,  and  therefore  does  not  come  within 
the  realm  of  physiology  beyond  simple  recognition  of  the  exist- 
ence of  the  phenomenon.  It  is  true  that  emotional  states  are 
usually  accompanied  by  reactions  of  other  parts  of  the  body,  the 
blush  which  accompanies  embarrassment  being  an  example,  but 
this  allows  us  only  to  judge  whether  an  emotion  is  present,  and 
tells  us  nothing  about  its  actual  nature. 

Cerebral  Functions  Compared  in  Man  and  Animals.  In  the 
higher  animals  as  well  as  in  man  associative  memory  is  the  rep- 


190  THE  HUMAN  BODY 

resentative  cerebral  activity.  So  far  as  we  can  judge  it  represents 
in  animals  the  climax  of  intellectual  achievement.  No  animal 
has  ever  been  seen  to  perform  any  act,  not  purely  reflex,  as  are  all 
"  instinctive "  actions,  which  associative  memory  cannot  account 
for.  The  activities  of  man  are  for  that  matter  based  upon  as- 
sociative memory  almost  as  fully  as  are  those  of  animals.  The 
important  •  intellectual  difference  between  man  and  animals  is 
the  possession  by  man  of  the  faculty  of  reason,  which  is  denied 
to  animals.  The  power  to  reason  is  itself,  however,  based  upon 
associative  memory.  It  may  be  roughly  explained  as  the  as- 
sociation of  concepts  whose  relationship  is  not  obvious.  An 
animal,  according  to  this  idea,  cannot  reason  because  he  cannot 
form  associations  except  of  concepts  that  manifestly  belong  to- 
gether. The  man  reasons  by  perceiving  relationships  in  appar- 
ently unrelated  facts  or  ideas. 

We  must  admit,  however,  that  the  most  complicated  acts  of 
associative  memory  that  have  been  observed  in  animals  simulate  so 
closely  mental  processes  which  in  man  are  ordinarily  thought  of  as 
reason,  that  no  hard  and  fast  limit  of  the  latter  can  be  set.  We 
would  scarcely  venture  to  establish  any  point  as  marking  the  ut- 
most intellectual  achievement  in  animals,  although  we  would  not 
hesitate  to  say  that  in  comparison  with  the  possibilities  of  the 
human  brain  the  extremest  mental  process  of  the  most  intelligent 
animal  dwindles  to  insignificance. 

The  powers  of  using  language  and  of  reasoning  are  the  only 
cerebral  functions  possessed  by  man  and  not  by  animals  of  which 
we  have  positive  objective  proof.  They  are  therefore  the  only  ones 
of  which  physiology  can  take  account  at  the  present  time.  Physiol- 
ogy does  not  thereby  deny,  however,  the  existence  of  many  ac- 
tivities in  the  human  brain  which  are  without  counterpart  in  the 
brains  of  lower  animals.  While  associative  memory  accounts 
completely  for  all  non-instinctive  actions  of  the  lower  animals,  the 
history  of  the  human  race  and  the  experience  of  individuals  con- 
tain much  that  baffles  explanation  in  terms  of  associative  memory 
or  of  reason.  The  factors  which  lead  the  race  always  onward  and 
upward  to  greater  and  greater  heights  of  spiritual  achievement  are 
beyond  the  power  of  present-day  physiology  to  analyze  or  even 
discuss. 

Nourishment  of  the  Brain.    The  cells  of  the  cerebral  cortex 


STRUCTURE  AND  FUNCTIONS  OF  THE  CEREBRUM      191 

are  very  dependent  upon  their  blood  supply.  A  slight  diminution 
in  the  rate  of  blood  flow  through  the  brain  may  depress  the  cor- 
tical cells  to  such  an  extent  that  consciousness  is  lost.  The  prob- 
lem of  retaining  consciousness  is,  then,  the  problem  of  keeping 
the  cerebral  circulation  up  to  the  proper  level.  How  this  is  ac- 
complished during  our  waking  hours,  and  how  its  falling  off  af- 
fords opportunity'  for  needed  intervals  of  sleep  will  be  discussed 
in  connection  with  the  circulation  of  the  blood  (Chap.  XXII). 


CHAPTER  XII 

THE  AUTONOMIC  NERVOUS  SYSTEM.    NERVOUS  FATIGUE. 
HORMONES  OF  THE  NERVOUS  SYSTEM 

The  Brain  Stem  (Medulla  and  Midbrain).  If  our  attention 
had  been  called  to  the  matter  when  the  courses  of  the  various 
afferent  and  efferent  pathways  of  the  cerebrum  and  cerebellum 
were  being  described,  we  should  have  noted  that  the  brain  stem 
forms  a  great  highway  through  which  pass  virtually  all  impulses  on 
their  way  to  or  from  the  higher  brain  structures.  Moreover,  most 
of  the  nerve  tracts  leading  through  the  brain  stem  do  not  pass 
directly  through,  but  suffer  interruption  in  one  or  the  other  of  the 
many  nuclei  which  occur  therein.  Wherever  a  nerve  tract  is 
interrupted  by  a  nucleus  the  axons  leading  into  the  nucleus  ter- 
minate in  synaptic  connection  with  new  neurons  by  which  the 
tract  is  continued.  There  is  always  the  possibility,  where  such 
connections  are  being  formed,  of  a  certain  amount  of  diversion 
from  the  main  channel  into  side  channels.  The  medulla  and  mid- 
brain,  then,  are  strategically  located  for  concentrating  into  small 
areas  influences  from  all  the  receptors  of  the  Body.  This  region  has 
also  its  own  efferent  pathways.  It  affords,  therefore,  an  additional 
field  for  the  establishment  of  reflex  arcs,  but,  as  we  shall  see,  of  a 
somewhat  less  specialized  sort  than  are  afforded  by  the  cerebrum 
and  cerebellum. 

There  are  a  number  of  so-called  " vital  processes"  going  on  in 
the  Body.  These  are  activities  whose  continuance  is  essential 
to  the  maintenance  of  life,  and  which  must,  therefore,  go  on  quite 
independently  of  the  will;  they  are  of  a  sort,  however,  to  require 
modification  in  accordance  with  the  demands  of  the  Body.  Ex- 
amples of  such  activities  are  the  beating  of  the  heart,  breathing, 
the  secretion  of  sweat. 

Many  of  these  so-called  "vital"  activities  are  really  as  purely 
reflex  as  any  of  the  ordinary  reflex  acts  of  the  Body,  and  those 
that  are  truly  automatic  are  subject  to  constant  reflex  influence. 

192 


THE  AUTONOMIC  NERVOUS  SYSTEM  193 

Their  immediate  control  is  vested  in  certain  centers  located  in 
the  medulla.  This  location  for  the  centers  insures  that  they  shall 
never  be  wholly  free  from  sensory  stimulation,  for  no  matter  how 
quiet 'the  surroundings  of  the  Body  may  be  the  processes  going 
on  within  it  give  rise  to  sensory  stimuli,  and,  as  we  have  seen, 
whatever  impulses  are  aroused  are  sure  to  pass  through  the  brain 
stem.  Detailed  consideration  of  the  various  centers  is  not  neces- 
sary here  as  each  will  be  treated  in  connection  with  the  vital 
process  with  which  it  is  related. 

The   Autonomic   or   Sympathetic   System.     This   system   is 
treated  as  a  distinct  portion  of  the  nervous  system  because  to  a 
rather  special  physiological  function  it  adds  peculiar  anatomical 
relationships.     In  spite  of  its  anatomical  and  physiological  pe- 
culiarities, however,  it  forms  an  integral  part  of  the  whole  nervous 
system,  and  interacts  with  other  parts  as  completely  as  though 
nothing  distinguished  it  from  them.    Its  old  name  has  no  present 
significance,  having  been  given  to  it  in  the  erroneous  belief  that 
its  function  is  to  bring  remote  organs  into  sympathy  with  each 
other.     The  name  autonomic,  by  which  it  is  at  present  known, 
signifies  a  mechanism  not  under  voluntary  control,  and  in  thus 
emphasizing  an  important  feature  of  the  system,  constitutes  a  more  | 
satisfactory  designation.    The  special  physiological  function  of  the  | 
autonomic  system  may  be  stated  in  a  sentence :  it  forms  the  efferent  j 
connection  between  the  central  nervous  system  and  all  the  smooth^ 
muscles  and  glands  of  the  Body,  and  the  heart. 

It  will  be  recalled  that  the  skeletal  muscles  have  motor  con- 
nection with  the  central  nervous  system  by  means  of  motor  neu- 
rons, structures  whose  cell-bodies  lie  in  the  ventral  horns  of  gray 
matter  and  whose  axons  extend  directly  to  the  muscles.  The 
autonomic  system  differs  from  the  motor  system  to  skeletal  mus- 
cles in  that  each  pathway  from  the  central  nervous  system  to  a 
smooth  muscle  or  to  a  gland  is  made  up  of  a  succession  of  two 
neurons.  The  first  neuron  has  its  cell-body  in  the  ventral  horn  of 
gray  matter;  its  axon  passes  out  by  way  of  the  ventral  root  of 
the  spinal  nerve  and  the  communicating  branch  (see  p.  147)  to 
one  of  the  sympathetic  ganglia  where  it  forms  synaptic  connection 
with  the  second  neuron  of  the  chain.  This  neuron  sends  its  axon 
back  over  the  communicating  branch  to  the  spinal  nerve  along 
which  it  passes  to  its  destination  in  a  smooth  muscle  or  a  gland. 


194  THE  HUMAN  BODY 

Because  of  their  positions  with  regard  to  sympathetic  ganglia  the 
first  and  second  neurons  are  known  respectively  as  pre-ganglionic 
and  post-ganglionic  neurons.  The  latter  present  the  anatomical 
peculiarity  of  being  for  the  most  part  devoid  of  myelin  sheaths; 
nerve-trunks  made  up  of  post-ganglionic  fibers  can  therefore  be 
distinguished  from  other  nerve-trunks  by  their  gray  color. 

The  structures  innervated  by  the  autonomic  system  perform 
their  functions  by  acting  to  a  considerable  extent  in  groups  to- 
gether; not  individually  as  do  skeletal  muscles.  To  enable  them 
to  be  stimulated  in  groups  single  autonomic  pathways  commonly 
involve  numerous  end  structures.  This  is  accomplished  by  rich 
branching  of  the  pre-ganglionic  fibers,  enabling  each  to  have 
synaptic  connection  with  a  number  of  post-ganglionic  neurons,  and 
so  to  influence  simultaneously  numerous  end  organs. 

The  Effect  of  Nicotine.  Much  of  our  knowledge  of  the  auto- 
nomic system  has  resulted  from  the  discovery  that  application  of 
the  drug  nicotine  to  sympathetic  ganglia  prevents  the  passage  of 
impulses  over  whatever  synapses  may  be  contained  therein.  By 
the  use  of  this  drug,  therefore,  the  point  of  contact  of  pre-ganglionic 
with  post-ganglionic  fibers  in  the  pathway  to  any  particular  organ 
can  be  determined.  To  illustrate  how  its  use  brings  out  these 
points  of  contact  we  may  take  the  autonomic  innervation  of  the 
eye.  The  size  of  the  pupil  is  regulated  by  opposing  autonomic 
fibers;  one  set  tending  to  constrict  it,  the  other  to  dilate  it.  By  the 
use  of  nicotine  it  has  been  shown  that  the  contact  of  pre-ganglionic 
with  post-ganglionic  fibers  in  the  constrictor  pathway  is  in  the 
ciliary  ganglion,  which  is  in  the  orbit,  while  for  the  dilator  pathway 
the  connection  between  pre-ganglionic  and  post-ganglionic  fibers 
is  in  one  of  the  sympathetic  ganglia  of  the  neck. 

Reflex  Control  of  the  Autonomic  System.  The  autonomic 
system,  as  we  have  seen,  forms  only  the  last  step  in  the  conduct- 
ing pathway  by  which  influences  are  brought  to  bear  on  the  struc- 
tures it  innervates.  Like  the  motor  system  for  the  skeletal  muscles 
it  conveys  only  those  impulses  which  are  imparted  to  it  from 
without.  It  is,  in  other  words,  the  efferent  portion  of  a  reflex 
mechanism. 

The  so-called  "vital"  processes  of  the  Body  are,  with  the  ex- 
ception of  respiration,  largely  carried  on  through  the  agency  of 
smooth  muscles  and  glands.  The  autonomic  system  is,  therefore, 


THE  AUTONOMIC  NERVOUS  SYSTEM 


195 


the  system  through  which  these  processes  have  their  nervous  con- 
trol. In  the  paragraph  dealing  with  the  brain  stem  the  existence 
therein  of  reflex  " centers"  for  the  various  "vital"  processes  was 
mentioned.  On  the  afferent  side  these  centers  are  subject  to  all 
sensory  stimulations  which  affect  the  Body.  On  the  efferent  side 
they  act  through  the  autonomic  system. 

This  reflex  mechanism  is  not  subject,  to  voluntary  control  except 
for  the  single  case  of  the  muscle  of  accommodation  of  the  eye,  the 
ciliary  muscle.  This  muscle  is  innervated  through  the  autonomic 
system,  but  can  be  voluntarily  controlled  as  completely  as  any 
muscle  in  the  Body.  When  we  say  that  the  autonomic  system  is 
not  under  voluntary  control  we  are  simply  stating  in  other  words 
that  the  motor  area  of  the  cerebrum  is  not  able  to  establish  con- 
nection through  the  pyramidal  tracts  with  the  neurons  of  this 
system.  Since  this  sytem  is  outside  the  control  of  the  motor  area 
all  reflexes  which  affect  it  must  be  immediate  ones.  Only  present 
stimuli  can  arouse  it  to  activity.  When  we  bear  in  mind  that  the 
proper  functioning  of  the  Body  requires  its  vital  activities  to  be 
adjusted  to  its  immediate  circumstances  and  not  to  its  circum- 
stances of  a  week  or  a  year  ago,  the  necessity  that  autonomic  re- 
flexes be  immediate  is  manifest. 

Grand  Divisions  of  the  Autonomic  System.  Not  all  parts  of 
the  central  nervous  system  give  rise  to  autonomic  pre-ganglionic 
neurons.  A  group  originates  in  the  brain  stem.  Its  fibers  are  dis- 
tributed through  cranial  nerves.  These  are  called  cranial  auto- 
nomies. The  vagus  nerve  (p.  152)  consists  largely  of  cranial  auto- 
nomic fibers  distributed  to  various  organs  of  the  trunk.  A  second 
group  arises  in  the  thoracic  and  lumbar  regions  of  the  spinal  cord. 
These  are  distributed  through  the  sympathetic  system  of  the  old 
classification.  They  are  known  as  thoracico-lumbar  autonomies. 
The  third  group  of  fibers  arises  in  the  sacral  portion  of  the  cord. 
These  are  distributed  to  the  pelvic  region  and  constitute  the  sacral 
autonomies. 

In  a  previous  paragraph  (p.  116)  attention  was  called  to  the 
peculiar  feature  of  smooth  muscle,  shared  by  heart  muscle,  of  re- 
quiring double  innervation.  These  tissues  must  have  stimulation 
to  augment  their  activity  and  other  stimulation  to  inhibit  it.  Both 
sorts  of  innervation  are  furnished  through  the  autonomic  system. 
An  important  feature  of  the  system  as  a  whole  is  that  the  opposing 


196  THE  HUMAN  BODY 

innervations  to  any  organ  come  to  it  by  way  of  different  grand 
divisions.  The  cranial  and  sacral  groups  stand  in  opposition  to  1  lie 
thoracico-lumbar  group.  It  does  not  follow  that  one  grand  divi- 
sion is  always  inhibitory  and  the  other  augmentory.  As  a  matter  of 
fact  the  thoracico-lumbar  system  augments  some  activities  and 
inhibits  others.  Whichever  are  augmented  by  the  thoracico- 
lumbar  are  inhibited  by  the  cranial  or  sacral,  and  vice  versa. 

Significance  of  Thoracico-Lumbar  and  Cranial-Sacral  Func- 
tions. During  the  ordinary  life  of  the  individual  the  balanced 
action  of  the  opposing  subdivisions  of  the  autonomic  system  serves 
to  carry  on  the  normal  quiet  functioning  of  the  maintenance 
mechanisms  of  the  Body,  which,  as  stated  previously  (p.  115),  are 
Operated  by  smooth  muscle.  As  we  study  in  detail  these  structures 
in  later  chapters  we  shall  see  how  admirably  through  this  balancing 
of  opposing  innervations  they  are  kept  in  just  the  desired  degree  of 
.  activity. 

In  addition  to  this,  which  we  may  call  its  routine  function,  the 
thoracico-lumbar  system  shows  a  special,  and  very  significant, 
property  which  can  be  made  clear  through  a  tabulation  of  a  few  of 
the  many  bodily  changes  governed  by  the  autonomic  system,  with 
the  respective  parts  played  by  the  thoracico-lumbar  and  cranial- 
sacral  systems  set  down. 

Thoracico-lumbar  Cranial-sacral 

Pupil  of  eye dilated contracted 

Salivary  secretion inhibited excited     . 

Hair erected depressed 

Blood-vessels  of  skin  .  .  constricted  (pallor)  . .  dilated  (flushing) 

Heart accelerated slowed 

Digestive  organs inhibited excited  (to  normal  activity) 

If  we  consider  an  individual  whose  pupils  are  dilated,  whose 
mouth  is  dry,  whose  hair  tends  to  stand  on  end,  whose  face  is  pale, 
whose  heart  is  racing,  and  whose  stomach  is  a  leaden  weight  within 
him  we  have  no  difficulty  in  recognizing  the  picture.  Only  terror, 
rage,  or  sharp  pain  could  bring  about  precisely  this  condition.  We. 
have  then  in  this  property  of  reacting  characteristically  to  condi- 
tions involving  strong  emotions  a  feature  of  the  thoracico-lumbar 
system. 

This  an  Emergency  Mechanism.  At  first  thought  there  may 
appear  little  utility  in  the  characteristic  reactions  of  excitement 


THE  AUTONOMIC  NERVOUS  SYSTEM  197 

described  in  the  last  paragraph.  If  we  note,  however,  that  the 
conditions  that  arouse  these  reactions  are  such  as  call  for  a  rallying 
of  the  Body  for  flight  or  struggle  we  begin  to  see  wherein  the  im- 
portance of  this  mechanism  lies.  In  general  its  effect  on  the  Body 
is  a  diversion  of  resources  from  the  maintenance  organs  to  those  of 
external  adaptation,  the  latter  making  up  the  mechanism  on  which 
the  Body  must  depend  for  salvation  in  time  of  stress.  The  dilation 
of  the  pupil  may  be  supposed  to  enhance  the  sensitiveness  of 
vision.  The  dryness  of  the  mouth  signifies  that  energy  ordinarily 
employed  in  producing  saliva  is  now  set  free  for  use  elsewhere.  The 
erection  of  the  hair,  of  no  importance  in  man,  is  in  many  of 
the  lower  animals  an  important  part  of  the  scheme  of  defense. 
The  pallor  of  the  face  is  the  result  of  the  diversion  of  blood  from  the 
skin,  whence  it  can  be  spared,  to  the  muscles  and  brain  where  it  is 
greatly  needed.  The  acceleration  of  the  heart  results  in  a  quick- 
ened circulation  of  blood  through  the  regions  of  heightened  activity. 
The  inhibition  of  the  digestive  organs  is  another  example,  like  the 
cessation  of  salivary  secretion,  of  the  suspension  of  functions  not 
immediately  essential,  in  order  that  all  the  Bodily  energy  shall  be 
available  for  the  emergency.  Numerous  other  reactions  of  the 
thoracico-lumbar  mechanism  also  contribute  to  the  general  plan  of 
defense.  These  we  shall  examine  in  due  course.  Here  we  need  only 
note  that  all  of  them  tend  toward  increasing  the  efficiency  of  the 
skeletal  muscles  and  the  central  nervous  system,  which  together 
make  up  the  emergency  mechanism. 

The  Relation  of  the  Autonomic  System  to  Emotional  States. 
In  a  previous  paragraph  (p.  189)  the  fact  was  noted  that  emotion  in 
general  is  accompanied  by  activity  of  the  autonomic  system.  We 
have  just  examined  the  basis  for  this  relationship  in  those  emotions 
that  are  associated  with  the  immediate  need  of  self-preservation. 
An  interesting  fact,  and  one  of  great  practical  importance,  is  that 
the  emotion  of  worry  or  anxiety,  which  is  responsible  for  much  of 
the  discomfort  of  life,  has  significance  as  a  means  of  preparing 
before  hand  for  a  time  of  trouble.  We  may  describe  it  as  an  an- 
ticipatory emotion.  We  bring  about  in  our  Bodies  through  worry 
the  characteristic  reactions  of  the  thoracico-lumbar  autonomic 
system.  Unfortunately,  these  reactions,  useful  indeed  when  the 
actual  stress  is  at  hand,  are  inimical  to  the  carrying  on  of  the 
ordinary  bodily  processes,  so  that  their  occurrence  in  advance  of 


198  THE  HUMAN  BODY 

the  emergency  does  no  particular  good,  and  when,  as  usually  hap- 
pens, the  worry  proves  to  have  been  needless,  real  harm. 

Emotion  of  satisfaction  and  contentment  appear  to  manifest 
themselves  chiefly  through  the  cranial  autonomies.  The  sacral 
autonomies  control  the  activities  of  the  generative  organs.  Their 
emotional  associations  are  for  the  most  part  those  concerned  with 
reproduction. 

Neuro  Muscular  Fatigue.  In  a  previous  chapter  (p.  101)  mus- 
cular fatigue  was  discussed,  and  the  fact  pointed  out  that  under 
ordinary  circumstances  the  muscles  are  protected  from  fatigue  by 
precurring  nervous  fatigue.  Twojgeneral  locations  are  recognized 
in  the  nervous  system  for  the  occurrence  of  fatigue.  The  first  of 
these  is  in  the  synapses.  The  delicate  junctions  between  neuron 
and  neuron  are  Relieved  to  be  highly  susceptible  to  fatigue.  In 
terms  of  the  prevailing  theory  we  would  say  that  the  accumulation 
of  waste  products  at  the  synapses  increases  their  resistance  to  the 
passage  of  nervous  impulses,  and  that  the  resulting  hindrance  to 
nervous  action  constitutes  fatigue.  The  second  place  of  fatigue 
is  at  the  junctions  between  mator_nerves  and  the  fibers  of  skeletal 
muscle,  These  junctions  consist  of  minute  flat  plates  pressed 
against  the  muscle-fibers  and  in  which  the  nerve-fibers  terminate. 
They  are  known  as  motor  end  plates.  There  is  ample  proof  that  as 
the  result  of  continued  excitation  of  a  muscle  the  neuro  muscular 
junctions  show  a  falling  off  in  the  ease  with  which  impulses  pass 
through  them  to  muscle-fibers.  Synaptic  fatigue  and  end  plate 
fatigue  occur  in  such  minute  structures  that  we  would  be  apt  to 
expect  recovery  to  be  rather  rapid.  As  a  matter  of  fact  quick 
recovery  seems  often  to  occur.  It  is  a  common  experience  to  obtain 
marked  relief  from  fatigue  by  the  briefest  sort  of  a  nap.  Neverthe- 
less, we  are  bound  to  recognize  that  although  the  feelings  of  fatigue 
may  be  quickly  dissipated  actual  restoration  of  the  fatigued  struc- 
tures requires  time.  An  ordinary  night's  rest  is  none  too  long  for 
recovery  from  nervous  fatigue.  In  fact  there  is  definite  evidence 
that  under  many  conditions  a  single  night's  sleep  does  not  suffice 
for  complete  restoration.  The  almost  universal  and  very  valuable 
habit  of  abstaining  from  ordinary  duties  one  day  in  seven  has  its 
physiological  significance  in  the  necessity  of  allowing  at  intervals  a 
longer  period  of  restoration  than  the  usual  nightly  ones,  in  order 
that  any  fatigue  which  failed  to  be  overcome  in  ordinary  course 


THE  AUTONOMIC  NERVOUS  SYSTEM  199 

might  be  gotten  rid  of  therein.  That  this  longer  rest  period  be 
spent  in  sleep  is  by  no  means  always  desirable.  When  we  recall 
that  the  synapses  which  experience  fatigue  primarily  are  the  ones 
that  are  being  used  we  realize  that  the  essential  for  rest  is  often 
diversion  rather  than  sleep.  During  the  rest  periods  one's  mental 
activities  should  be  along  as  different  lines  as  possible  from  those  of 
his  ordinary  workaday  life.  Thus  his  fatigued  synapses  can  be 
resting  while  others  are  busy.  This  same  fact  emphasizes  the 
importance  of  diversity  of  interests.  Where  one's  thoughts  cling  in 
certain  ruts  mental  fatigue  is  apt  to  be  more  pronounced  than 
where  various  lines  can  be  followed.  In  those  whose  occupation 
requires  prolonged  concentration  it  is  particularly  advantageous  to 
have  widely  different  interests  to  turn  to  during  the  intervals  of 
relaxation. 

Hormones  of  the  Nervous  System.  Adrenin.  This  hormone 
is  interesting  chemically  because  it  was  the  first  hormone  to  be 
obtained  pure,  and  is  even  yet  by  far  the  best  known  of  the  nu- 
merous hormones  produced  in  the  Body.  Various  names  have 
been  applied  to  it  (suprarenin,  epinephrin,  adrenalin).  The  name 
given  in  the  paragraph  heading  is  coming  into  general  use  at  pres- 
ent. 

The  Suprarenal  Capsules  or  Adrenals  are  a  pair  of  small  organs, 
weighing  together  about  12  grams  Q  oz.)  placed  one  on  the  top 
of  each  kidney.  They  have,  however,  no  intimate  connection 
with  the  kidneys,  and  in  many  animals  are  placed  at  some  dis- 
tance from  them.  Each  consists  of  a  denser  less  colored  external 
cortex,  and  a  central  deep  yellow-brown  softer  medulla.  The  cor- 
tex is  subdivided  into  chambers  by  connective  tissue,  and  the 
chambers  are  filled  by  closely  packed,  polygonal  nucleated  cells. 
Similar  cells  are  found  in  the  medulla,  which  is,  moreover,  closely 
connected  with  the  sympathetic  system  and  is  richly  supplied 
with  nerves. 

It  was  noticed  some  seventy-five  years  ago  by  a  physician  named 
Addison  that  certain  obscure  diseased  conditions  characterized  by 
great  debility  and  by  the  appearance  of  bronzed  patches  on  the 
skin,  and  leading  to  death,  were  found  on  post-mortem  examina- 
tion to  be  accompanied  by  disease  of  the  adrenals.  The  disease 
has  since  been  named  Addison's  disease.  When  the  suprarenal 
capsules  are  completely  removed  from  animals  a  similar  fatal 


200  THE  HUMAN  BODY 

diseased  condition  results,  death  taking  place  in  warm-blooded 
animals  within  two  or  three  days,  and  being  preceded  by  muscu- 
lar weakness,  dilation  of  the  arteries,  mental  feebleness  and  general 
prostration. 

These  symptoms  show  that  the  hormone  is  essential  to  life,  al- 
though they  do  not  afford  any  very  positive  evidence  as  to  the 
manner  of  its  working.  Careful  studies  have  shown  that  adrenin 
is  present  in  the  blood  under  ordinary  circumstances  in  almost 
inconceivably  minute  amounts.  A  striking  feature  of  this,  and  of 
hormones  in  general,  is  their  remarkable  potency  as  chemical  stim- 
ulants. Our  detailed  knowledge  of  the  functioning  of  adrenin 
has  been  gained  chiefly  by  observing  the  results  of  its  introduction 
into  the  blood  in  larger  than  normal  amounts.  The  Body  responds 
to  these  enlarged  doses  by  a  considerable  number  of  very  definite 
reactions  which,  when  first  observed,  seemed  to  be  quite  unrelated, 
but  are  now  recognized  as  combining  to  bring  about  a  particular 
bodily  condition,  and  one  which,  as  we  shall  see,  is  sometimes  of 
great  importance  to  the  organism.  Not  all  these  effects  of  adrenin 
can  be  described  in  this  place,  some  will  have  to  be  deferred  to 
later  chapters;  but  enough  can  be  presented  to  make  clear  the 
significance  of  its  action. 

One  of  the  properties  of  adrenin  is  to  stimulate  chemically  the 
terminations  of  the  thbracico-lumbar  autonomic  system.  It  is 
thus  able  to  bring  about  the  same  bodily  reactions  as  are  called 
forth  through  thoracico-lumbar  autonomic  activity.  Dilation  of 
the  pupil,  acceleration  of  the  heart,  constriction  of  the  blood-ves- 
sels, with  consequent  heightened  blood  pressure,  all  are  brought 
about  by  the  injection  of  adrenin  into  the  Body.  These  manifesta- 
tions, as  we  saw  above,  are  part  of  what  we  have  described  as  the 
emergency  reaction  of  the  Body,  and  the  ability  of  adrenin  to  bring 
them  about  reveals  its  function  as  the  emergency  hormone.  The 
emergency  reaction  is  so  vital  in  time  of  stress  that  the  Body  does 
not  depend  wholly  on  the  nervous  system  to  evoke  it.  The  action 
of  the  thoracico-lumbar  autonomies  is  reinforced  by  the  chemical 
stimulation  of  adrenin.  This  adrenin  action  depends,  as  we  have 
seen,  on  the  presence  in  the  blood  of  larger  than  normal  amounts  of 
the  hormone.  The  adrenal  bodies  are  under  the  control  of  nerves 
which  form  part  of  the  thoracico-lumbar  system.  Whenever,  in  a 
time  of  excitement,  there  is  an  outrush  of  impulses  over  this  sy&- 


THE  AUTONOMIC  NERVOUS  SYSTEM  201 

tern,  the  adrenals  are  stimulated  to  great  activity,  and  pour  out 
their  product  into  the  blood  stream.  Thus  at  the  time  when  in- 
creased adrenin  is  advantageous  to  the  organism  it  is  provided. 
The  persistence  of  the  bodily  effects  of  strong  emotion  after  the 
emotion  itself  has  subsided  may  be  explained  by  the  continued 
presence  of  adrenin  in  the  blood. 

The  reinforcement  of  the  thoracico-lumbar  autonomic  mechan- 
ism is  only  one  phase  of  the  emergency  function  of  adrenin.  An- 
other, and  very  interesting,  feature  of  its  action  is  in  connection 
with  the  fatigue  of  the  neuro-muscular  junctions  described  in  an 
earlier  paragraph  (p.  198).  We  saw  there  that  the  effect  of  fatigue 
on  these  junctions  is  to  make  the  passage  of  impulses  over  them 
difficult.  Recently  the  important  discovery  has  been  made  that 
adrenin  has  the  property  of  counteracting  this  fatigue,  and  thus 
making  the  muscles  more  accessible  to  nervous  impulses.  The 
value  of  this  property  in  time  of  emergency  is  obvious.  It  explains 
a  familiar  fact  that  was  unexplained  before,  namely,  the  "strength 
of  desperation."  Why  a  man  in  a  tight  place  should  suddenly  ex- 
perience an  access  of  strength  we  now  know  is  because  in  connection 
with  the  powerful  emotions  engendered  by  his  situation  there  is  an 
outpouring  of  impulses  over  his  thoracico-lumbar  autonomic  sys- 
tem. His  adrenal  bodies  are  stimulated  thereby  'to  abundant 
production  of  adrenin;  the  adrenin  is  carried  by  his  blood  to  all  his- 
muscles,  and  there  makes  the  access  of  nerve  impulses  to  the 
muscles  more  ready.  The  gain  is  not  in  -actual  muscular  strength, 
but  in  ability  to  use  to  the  full  the  strength  already  present. 

The  Thyroid.  This  organ  lies  in  the  neck  on  the  sides  of  the 
windpipe  and  consists  usually  of  a  right  and  a  left  lobe  united  by  a 
narrow  isthmus  across  the  front  of  the  air-tube.  It  is  about  thirty 
grams  (one  ounce)  in  weight;  in  the  disease  known  as  goiter  it  is 
greatly  enlarged  and  its  structure  altered.  The  thyroid  is  dark  red 
in  color  and  very  vascular,  richly  supplied  with  nerves,  and  is 
subdivided  by  connective  tissue  into  cavities  or  alveoli,  the  largest 
of  which  are  just  visible  to  the  unaided  eye.  Each  alveolus  is  lined 
by  a  single  layer  of  cuboidal  cells,  and  filled  by  a  glairy  fluid  known 
as  the  thyroid  colloid. 

From  the  gland  can  be  obtained,  in  addition  to  the  usual  or- 
ganic compounds,  a  peculiar  substance  containing  a  large  percen- 
tage of  iodine,  and  known  as  iodothyrin.  This  compound  was 


202  THE  HUMAN  BODY 

thought,  when  first  discovered,  to  be  the  hormone  of  the  gland, 
but  fuller  study  showed  that  iodothyrin  as  such  is  not  the  hormone 
although  it  probably  has  to  do  in  some  way  with  it.  Although 
the  chemistry  of  the  hormone  is  not  perfectly  known  its  physiology 
can  be  studied  indirectly  by  observing  the  effect  of  changes  in  the 
amount  present  in  the  Body.  These  changes  may  be  brought 
about  experimentally  or  may  occur  as  the  result  of  disease. 

Studies  thus  made  show  that  the  hormone  of  the  thyroid  gland 
has  a  great  deal  to  do  with  the  proper  carrying  on  of  those  chemical 
activities  of  living  cells  which  constitute  their  "  vital"  processes  and 
which  are  grouped  together  under  the  term  metabolism.  The 
nervous  system  is  peculiarly  dependent  upon  this  hormone  for  its 
proper  development  and  for  the  proper  carrying  on  of  its  metabolic 
activities.  This  fact  appears  strikingly  in  cases  in  which  the 
hormone  is  deficient  in  amount.  In  adults  a  condition  known  as 
myxedema  is  the  result  of  such  deficiency;  its  chief  manifestation  is 
distressing  mental  deterioration.  Sometimes  children  are  born  in 
whom  the  thyroid  gland  fails  to  develop  properly;  they  grow  into 
dwarfish,  misshapen  idiots.  To  such  a  condition  the  name  cretinism 
is  applied.  The  sufferers  are  called  cretins.  Thanks  to  the  dis- 
covery that  by  simple  feeding  of  thyroid  material  the  hormone  can 
be  supplied  in  ample  quantity,  sufferers  from  myxedema  and 
•cretinism  are  now  restored  to  perfectly  normal  condition;  although 
it  is  said  that  for  the  treatment  to  be  wholly  successful  for  cretins 
it  must  be  begun  quite  early  in  life. 

There  is  a  disease  known  as  exophthalmic  goiter  (Grave's  dis- 
ease), named  from  the  protrusion  of  the  eyes  which  is  a  prominent 
symptom.  This  disease  is  due  to  an  increase  in  the  amount  of  the 
thyroid  hormone.  The  effects  on  the  Body  are  just  the  opposite  of 
those  seen  in  myxedema.  There  is  heightened  nervous  activity, 
often  proceeding  so  far  beyond  the  normal  as  to  constitute  mental 
instability.  One  of  the  triumphs  of  modern  surgery  is  the  establish- 
ment of  a  method  whereby  enough  of  the  thyroid  can  be  removed 
to  reduce  the  hormone  to  normal  amount,  and  so  cure  the  com- 
plaint. In  this  connection  only  the  effects  of  the  hormone  on  the 
nervous  system  are  discussed.  In  a  later  chapter  (p.  513)  its  in- 
fluence on  general  metabolism  is  considered. 

Emergency  Action  of  the  Thyroid.  An  interesting  fact  of  re- 
cent discovery  is  that  during  the  outpouring  of  autonomic  in- 


THE  AUTONOMIC  NERVOUS  SYSTEM  203 

fluences  in  time  of  stress  the  thyroid  shows  augmented  secretory 
activity.  The  organ  is  innervated  by  the  thoracico-lumbar  system, 
and  so  may  be  excited  directly.  In  addition  to  this  means  of 
arousing  it,  the  thyroid  may  be  stimulated  to  activity  chemically 
by  means  of  adrenin.  Whenever  the  blood  is  charged  with  this 
latter  hormone  the  thyroid  is  thrown  into  activity.  So  far  as  we 
are  able  to  judge  from  present  knowledge  the  importance  of  this 
emergency  action  of  the  thyroid  is  in  the  general  speeding  up  of  the 
chemical  activities  of  the  Body;  and  possibly  also  in  heightening 
nervous  irritability,  although  that  the  latter  effect  can  be  brought 
about  so  promptly  as  would  be  necessary  in  an  emergency 
mechanism  has  not  been  demonstrated. 


CHAPTER  XIII 

THE  RECEPTOR  SYSTEM.     INTERNAL  AND  CUTANEOUS 
SENSATIONS 

The  Receptor  System  constitutes  the  Body's  means  of  gain- 
ing information  of  its  surroundings  and  of  such  internal  condi- 
tions as  it  needs  to  know  about.  Since  the  surroundings  may  play 
upon  the  Body  in  many  different  ways  and  through  the  operation 
of  many  forms  of  energy,  receptors  are  provided  which  respond 
to  all  sorts  of  stimuli.  Inasmuch  as  proper  adaptation  requires 
that  different  ^orts  of  stimuli  affect  the  Body  differently  partic- 
ular receptors  are  specialized  to  respond  most  readily  to  partic- 
ular kinds  of  stimulation. 

An  interesting  thing  about  the  responses  of  the  different  re- 
ceptors is  that  while  their  adaptation  to  special  forms  of  stimula- 
tion does  not  exclude  the  possibility  of  their  being  aroused  by 
other  sorts  of  stimuli. than  the  normal  ones,  when  so  aroused  the 
effect  in  consciousness  is  as  though  the  normal  stimulus  had  been 
applied.  Pressure  on  the  eyes  gives  rise  to  sensations  of  light; 
electrical  stimulation  of  the  tongue  may  cause  sensations  of  taste. 
This  fact  has  led  physiologists  to  take  the  view  that  the  quality 
of  any  sensation  depends  on  the  region  of  the  cerebrum  to  which 
it  comes,  and  that  it  is  quite  independent  of  the  structure  of  the 
receptor  or  the  manner  of  its  stimulation.  If  this  is  true  it  ac- 
'cords  well  with  another  conception  which  most  physiologists 
find  very  attractive,  that  the  nerve  impulse,  whatever  it  may  be, 
is  the  same  sort  of  process  wherever  it  occurs.  It  is,  of  course, 
evident  that  this  idea,  the  so-called  "doctrine  of  specific  nerve 
energies,"  cannot  be  true  if  the  quality  of  sensation  depends  in 
any  manner  upon  the  nature  of  the  receptor  or  the  way  in  which 
it  is  stimulated.  It  must  be  confessed  that  many  known  facts 
about  the  senses,  that  of  sight  particularly,  cannot  at  present  be 
explained  upon  any  basis  which  excludes  differences  in  the  re- 
ceptor as  determining  factors  of  the  quality  of  sensation. 

The  Differences  between  Sensations.  We  distinguish  among 

204 


THE  RECEPTOR  SYSTEM  205 

our  sensations  kinds  which  are  absolutely  distinct  for  our  con- 
sciousness, and  not  comparable  mentally.  We  can  never  get  con- 
fused between  a  sight,  a  sound,  and  a  touch,  rior  between  pain 
and  hunger;  nor  can  we  compare  them  with  one  another:  each  is 
sui  generis.  The  fundamental  difference  which  thus  separates  one 
sensation  from  another  is  its  modality.  Sensations  of  the  same 
modality  may  differ;  but  they  shade  imperceptibly  into  one  an- 
other, and  are  comparable  between  themselves  in  two  ways. 
First,  as  regards  quality:  while  a  high  and  a  low  pitched  note  are 
both  auditory  sensations,  they  are  nevertheless  different  and  yet 
intelligibly  comparable;  and  so  are  blue,  purple,  and  red  objects. 
In  the  second  place,  sensations  of  the  same  modality  are  distin- 
guishable and  comparable  as  to  amount  or  intensity:  we  readily 
recognize  and  compare  a  loud  and  a  weak  sound  of  the  same  pitch; 
a  bright  and  feeble  light  of  the  same  color;  an  acute  and  a  slight 
pain  of  the  same  general  character.  Our  sensations  thus  differ  in 
the  three  aspects  of  modality,  quality  within  the  same  modality, 
and  intensity.  Certain  sensations  also  differ  in  what  is  known  as 
the  "local  signs,"  a  difference  by  which  we  tell  a  touch  on  one  part 
of  the  skin  from  a  similar  touch  on  another;  or  an  object  exciting 
one  part  of  the  eye  from  an  object  like  it,  but  in  a  different  location 
in  space  and  exciting  another  part  of  the  visual  surface. 

As  regards  modality,  we  commonly  distinguish  five  senses,  those 
of  sight,  sound,  touch,  taste,  and  smell;  to  these  at  least  six  others 
must  be  added  to  make  the  list  approximately  complete.  These  addi- 
tional senses  are  temperature,  pain,  hunger,  thirst,  muscle  sense, 
and  equilibrium  sense.  The  last  five  of  this  list  were  formerly  set 
apart  as  common  sensations,  but  there  seems  to  be  no  good  reason 
for  viewing  them  in  any  different  light  from  the  others. 

The  Psychophysical  Law.  Although  our  sensations  are,  in 
modality  or  kind,  independent  of  the  force  exciting  them,  they  are 
not  so  in  degree  or  intensity,  at  least  within  certain  limits.  We 
cannot  measure  the  amount  of  a  sensation  and  express  it  in  foot- 
pounds or  calories,  but  we  can  get  a  sort  of  unit  by  determining 
how  small  a  difference  in  sensation  can  be  perceived.  This  smallest 
perceptible  difference  varies  in  the  different  senses  and  for  different 
amounts  of  stimulation  in  the  same  sense.  Its  variation  in  any 
single  sense  follows,  however,  a  certain  law.  The  increase  of  stimu- 
lus necessary  to  produce  the  smallest  perceptible  change  in  a  sensation 


206  THE  HUMAN  BODY 

is  proportional  to  the  strength  of  the  stimulus  already  acting;  for  ex- 
ample, the  heavier  a  pressure  already  acting  on  the  skin  the  more 
must  it  be  increased  or  diminished  in  order  that  the  increase  or 
diminution  may  be  felt.  Examples  of  this,  which  is  known  as 
"Weber's"  or  "Fechner's  psychophysical  law"  will  be  hereafter 
pointed  out,  and  are  readily  observable  in  daily  life;  we  have,  for 
example,  a  luminous  sensation  of  certain  intensity  when  a  lighted 
candle  is  brought  into  a  dark  room;  this  sensation  is  not  doubled 
when  a  second  candle  is  brought  in;  and  is  hardly  affected  at  all  by 
a  third.  The  law  is  only  true,  however  (and  then  but  approxi- 
mately), for  sensations  of  medium  intensity;  it  is  applicable,  for 
example,  to  light  sensations  of  all  degrees  between  those  aroused 
by  the  light  of  a  candle  and  ordinary  clear  daylight:  but  it  is  not 
true  for  luminosities  so  feeble  as  only  to  be  seen  at  all  with  diffi- 
culty, or  so  bright  as  to  be  dazzling. 

Besides  their  variations  in  intensity,  dependent  on  variations 
in  the  strength  of  the  stimulus,  our  sensations  also  vary  with  the 
irritability  of  the  sensory  apparatus  itself;  which  is  not  constant 
from  time  to  time  or  from  person  to  person.  In  the  above  state- 
ments the  condition  of  the  sense-organ  and  its  nervous  connections 
is  presumed  to  remain  the  same  throughout. 

Classification  of  Receptors.  It  is  possible  to  group  the  sense- 
organs  in  several  different  ways  according  to  the  properties  upon 
which  the  classification  is  based.  If  we  group  them  according  to 
the  forms  of  energy  to  which  they  respond  they  fall  into  four 
classes:  I,  the  senses  aroused  by  mechanical  stimulation,  touch, 
pain,  hunger,  muscle  sense,  equilibrium,  and  hearing;  2,  those 
aroused  by  chemical  stimulation,  taste,  smell,  and  probably  the 
sensation  of  thirst;  3,  the  temperature  sense,  aroused  by  thermal 
stimuli;  4,  the  sense  of  sight,  aroused  by  stimuli  of  light. 

Another  classification,  and  a  more  convenient  one  to  follow  in 
describing  the  receptors,  is  based  upon  their  position  in  the  Body. 
This  classification  gives  us  two  main  groups:  1,  the  internal  senses, 
whose  receptors  lie  within  the  Body;  here  belong  muscle  sense, 
equilibrium,  pain,  hunger,  and  thirst;  2,  the  external  senses,  whose 
receptors  are  on  the  surface  of  the  Body  and  which  therefore  obtain 
information  of  the  outside  world.  These  senses  fall  again  into  two 
subgroups;  the  first  includes  the  contact  senses  which  are  stimulated 
only  by  things  in  immediate  contact  with  the  Body;  the  second  in- 


THE  RECEPTOR  SYSTEM  207 

eludes  the  projecting  senses  which  tell  us  of  the  surroundings  not 
immediately  touching  us. 

The  group  of  contact  senses  includes  the  cutaneous  senses,  touch, 
temperature,  and  pain,  the  latter  being  both  external  and  internal, 
and  the  sense  of  taste.  The  group  of  projecting  senses  includes 
hearing,  smell,  and  sight. 

It  is  not  desirable  to  follow  this  classification  exactly  in  the 
discussion  of  the  various  senses,  but  it  represents  in  the  main  the 
order  of  their  consideration. 

Not  Included  in  this  Classification  are  a  group  of  feelings  which 
in  consciousness  have  features  in  common  with  the  senses,  although 
from  the  standpoint  of  physiology  they  seem  not  to  fall  in  the  same 
category.  Examples  are  fatigue,  nausea,  and  the  general  state  of 
ill-feeling  called  malaise.  While  these  are  well-marked  sensations 
there  is  reason  to  doubt  whether  they  are  mediated  by  definite 
receptors  as  are  the  senses.  They  are  more  probably  induced  by 
general  bodily  states  in  some  manner  not  now  understood. 

The  Internal  Senses.  Of  these  only  muscle  sense,  hunger,  and 
thirst  will  be  considered  here.  The  sense  of  pain  is  treated  more 
satisfactorily  in  connection  with  the  cutaneous  senses.  The 
equilibrium  sense  requires  an  account  of  the  structure  of  the  ear 
and  will  be  given  in  connection  with  the  sense  of  hearing.  The 
functions  of  these  senses  are  to  inform  the  Body  of  its  own  con- 
dition. They  are  recognized  in  consciousness  as  bodily  states, 
being  in  this  respect  very  different  from  the  external  senses,  which 
we  interpret  altogether  in  terms  of  the  sources  from  which  the 
stimuli  arise.  The  difference  in  consciousness  between  internal  and 
external  senses  may  be  illustrated  by  supposing  that  a  knife  is 
hold  in  the  hand.  The  sensations  we  have  are  referred  in  our 
consciousness  to  the  knife.  It  is  hard,  cold,  etc.  Let  the  knife  now 
cut  through  the  skin.  The  stimulus  arises  from  the  knife  as  much 
as  before,  but  it  is  to  the  hand  and  not  to  the  knife  that  we  refer 
the  feeling  of  pain. 

The  Muscular  Sense.  From  the  muscles  arise  sensations  of 
great  importance,  although  they  do  not  often  become  so  obtrusive 
in  consciousness  as  to  arouse  separate  attention.  They  are  due  to 
the  excitation  of  sensory  nerves  ending  within  the  muscles  them- 
selves, or  in  the  tendons  or  joints  with  which  the  muscles  are  con- 
nected. 


208  THE  HUMAN  BODY 

We  have  at  any  moment  a  fairly  accurate  knowledge  of  the 
position  of  various  parts  of  our  Bodies,  even  when  we  do  not  see 
them;  and  we  can  also  judge  fairly  accurately  the  extent  of  a 
movement  made  with  the  eyes  shut.  The  afferent  nerve  impulses 
concerned  in  the  development  of  such  judgments  may  be  various; 
different  parts  of  the  skin  are  pressed  or  creased;  different  joints 
are  subjected  to  pressure;  different  tendons  are  put  on  the  stretch 
and  different  muscles  are  in  different  states  of  contraction,  and  it 
is  by  no  means  easy  to  determine  the  part  played  in  each  case  by 
the  sensory  nerves  of  the  different  organs.  Moreover,  when  we 
push  against  an  object,  or  lift  it,  we  are  able  to  form  a  judgment 
as  to  the  amount  of  effort  exerted;  but  here  again  pressure  on 
skin  and  joints  and  tension  of  tendons  come  in.  Although  under 
normal  circumstances  the  skin  sensations  are  undoubtedly  of  im- 
portance, they  are  not  necessary:  persons  with  cutaneous  paralysis 
can,  apart  from  sight,  judge  truly  the  position  of  a  limb  and  the 
extent  of  movement  made  by  it;  and  in  many  movements  change 
in  joint  pressure  must  be  very  little  if  any.  We  have  then  to  look 
to  muscles  and  tendons  themselves  for  an  important  part  of  the 
sensations,  and  in  both  muscles  and  tendons  there  are  organs  in 
connection  with  nerve-fibers  which  are  certainly  sensory  in  nature : 
moreover,  muscle  sensory  nerves  appear  to  be  excited  by  mere 
passive  change  of  form  in  the  muscle;  with  the  eyes  closed  each  of 
us  can  tell  how  much  another  person  has  lifted  one  of  our  arms. 

The  sensations  by  which  we  judge  the  extent  of  a  muscular 
movement  enable  us  to  determine  very  minute  differences  of  con- 
traction; the  ocular  determination  of  the  distance  of  an  object  not 
too  far  off  to  have  its  absolute  distance  determined  with  con- 
siderable accuracy,  depends  almost  entirely  upon  judgments  based 
upon  very  small  changes  in  the  degree  of  contraction  of  the  internal 
and  external  straight  (recti)  muscles,  converging  or  diverging  the 
eyeballs.  A  singer,  too,  must  be  able  to  judge  with  great  minute- 
ness the  degree  of  contraction  of  the  small  muscles  of  the  larynx 
necessary  to  produce  a  certain  tension  of  the  vocal  cords.  It  may 
be  well  to  point  out  that  we  do  not  refer  a  muscular  sensation  to 
any  given  muscle  or  muscles;  it  is  merely  associated  with  a  certain 
movement  or  position,  arid  a  person  who  knows  nothing  about  his 
ocular  muscles  can  judge  distance  through  sensations  derived  from 
them,  quite  as  well  as  any  anatomist.  This  fact  is  of  course  cor- 


THE  RECEPTOR  SYSTEM  209 

related  with  the  fact  that  in  voluntary  movement  we  do  not  make 
a  conscious  effort  to  contract  any  particular  muscles:  the  higher 
nerve-centers  are  merely  concerned  with  the  initiation  of  a  given 
movement  of  a  given  extent,  and  all  the  details  are  carried  out  by 
lower  co-ordinating  centers.  In  ordinary  daily  life  in  fact  we  have 
no  interest  whatever  in  a  muscular  contraction  per  se;  all  we  are 
concerned  with  is  the  result,  and  consciousness  has  never  had  need 
to  trouble  itself,  if  it  could,  with  associating  a  particular  feeling  or 
a  particular  movement  with  any  individual  muscle. 

Muscular  feelings  are,  as  already  pointed  out,  frequently  and 
closely  combined  not  only  with  visual  but  also  with  tactile,  in  pro- 
viding sensations  on  which  to  base  judgments:  in  the  dark,  when 
an  object  is  of  such  size  and  form  that  it  cannot  be  felt  all  over  by 
any  one  region  of  the  skin,  we  deduce  its  shape  and  extent  by  com- 
bining the  tactile  feelings  it  gives  rise  to,  with  the  muscular  feelings 
accompanying  the  movements  of  the  hands  over  it.  Even  when 
the  eyes  are  used  the  sensations  attained  through  them  mainly 
serve  as  short-cuts  which  we  have  learned  by  experience  to  inter- 
pret, as  telling  us  what  tactile  .and  muscular  feelings  the  object 
seen  would  give  us  if  felt;  and,  in  regard  to  distant  points,  although 
we  have  learnt  to  apply  arbitrarily  selected  standards  of  measure- 
ment, it  is  probable  that  distance,  in  relation  to  perception,  is 
primarily  a  judgment  as  to  how  much  muscular  effort  would  be 
needed  to  come  into  contact  with  the  thing  looked  at. 

When  we  wish  to  estimate  the  weight  of  an  object  we  always, 
when  possible,  lift  it,  and  so  combine  muscular  with  tactile,  sensa- 
tions. By  this  means  we  can  form  much  better  judgments.  While 
with  touch  alone  just  perceptibly  different  pressures  have  the 
ratio  1 :3,  with  the  muscular  sense  added  differences  of  TV  can  be 
perceived. 

Hunger.  In  discussing  this  sense  we  must  first  draw  a  distinc- 
tion between  true  hunger  and  appetite.  The  latter  is  a  feeling  that 
food  would  be  acceptable,  with  usually  a  degree  of  pleasurable 
anticipation  included.  It  is  often  heightened  by  the  odor  and 
taste  of  food.  There  is  reason  to  doubt  whether  appetite  should  be 
called  a  sense  in  the  strict  meaning  of  that  term.  It  might,  per- 
haps, be  better  classed  with  fatigue,  nausea,  and  the  other  feelings 
mentioned  in  a  former  paragraph  as  not  representing  the  results  of 
definite  receptor  stimulation.  True  hunger,  on  the  other  hand,  is  a 


210  THE  HUMAN  BODY 

definite  sense  aroused  in  a  specific  manner.  In  consciousness  it 
takes  the  form  of  sensations  arising  from  the  stomach,  which,  when 
pronounced,  are  of  a  character  sufficiently  disagreeable  to  justify 
their  description  as  "  pangs  of  hunger."  During  a  period  of  hunger 
the  feeling  is  not  continuous,  but  comes  and  goes,  usually  at  fairly 
regular  intervals.  By  means  of  interesting  experiments,  in  which 
records  were  obtained  of  the  movements  of  the  stomach,  the  fact 
was  demonstrated  that  spasms  of  hunger  are  the  result  of  vigorous 
contractions  of  the  muscular  walls  of  the  organ.  Apparently  these 
contractions  stimulate,  mechanically,  receptors  embedded  in  the 
stomach  walls.  Most  of  the  facts  about  hunger  are  readily  ex- 
plicable in  accordance  with  this  idea  of  its  nature  when  we  recall 
that  the  stomach,  whose  contractions  evoke  the  sensations,  is 
governed  by  the  autonomic  system,  which,  in  turn,  is  subject  to 
emotional  as  well  as  to  reflex  influences.  The  well-known  capri- 
ciousness  of  hunger  can  thus  be  accounted  for.  If  the  need  for 
food  were  the  necessary  incitement  to  hunger  we  should  expect 
the  greatest  hunger  to  be  after  the  longest  fast,  but  the  experience 
of  a  great  many  people  is  that  their  least  hunger  before  any  meal  is 
before  breakfast,  which  is  the  meal  at  the  end  of  the  longest  inter- 
val. Moreover,  those  who  have  endured  long  fasts  testify  that 
hunger  disappears  completely  after  a  period  of  two  or  three  days, 
particularly  if  not  much  exercise  is  taken. 

The  function  of  hunger  is  to  insure  the  taking  of  food.  This 
is  an  act  essential  to  life,  but  in  the  lower  animals,  and  in  children,  is 
not  recognized  as  such  through  the  operation  of  associative  mem- 
ory, and,  therefore,  is  not  to  be  depended  on  to  be  performed 
volitionally.  It  is  essentially  a  reflex  act  and  hunger  is  the  sensory 
basis  for  the  reflex.  As  we  shall  learn  in  a  later  chapter,  an  im- 
portant feature  of  proper  eating  is  the  maintenance  of  regular 
habits  in  regard  to  it.  Hunger  serves  as  a  powerful  aid  to  regular- 
ity, for  it  tends  to  come  on  at  about  the  time  we  are  in  the  habit  of 
eating.  Many  people  suffer  rather  severely  if  obliged  to  wait 
through  the  period  of  a  usual  meal,  although  the  interval  measured 
in  hours  may  be  no  longer  than  others  to  which  they  are  accus- 
tomed and  which  cause  no  discomfort. 

Thirst.  This  sense,  in  its  ordinary  form,  arises  from  dryness  of 
the  throat.  Apparently  there  are  receptors  in  that  region  whioh  are 
stimulated  by  deficiency  of  moisture.  The  throat  is  moistened  by 


THE  RECEPTOR  SYSTEM  211 

the  saliva  which  is  swallowed  at  frequent  intervals,  and  the  sense  of 
thirst  thus  kept  in  abeyance.  There  is  a  constant  loss  of  water 
from  the  Body  by  means  of  the  various  channels  of  excretion, 
lungs,  sweat  glands,  etc.  When  the  resultant  diminution  in  the 
water  content  of  the  tissues  reaches  a  certain  point  the  swallowing 
of  saliva  no  longer  prevents  stimulation  of  the  thirst  receptors,  and 
liquid  from  outside  the  Body  must  be  taken  if  the  thirst  is  to  be  re- 
lieved. The  liquid  need  not  necessarily  be  swallowed.  Injections 
directly  into  the  veins  are  effective  in  abolishing  thirst  sensations. 

If,  as  the  result  of  prolonged  deprivation,  the  water  content  of 
the  Body  is  seriously  diminished,  ordinary  thirst  gives  way  to  much 
more  pronounced  and  finally  very  painful  sensations.  From  these 
there  is  no  relief  with  the  passage  of  time  as  there  is  in  case  of 
hunger.  The  distress  becomes  more  and  more  marked  leading 
ultimately,  it  is  said,  to  mental  breakdown.  Thirst  is  believed  to 
be  the  only  sense  of  which  the  Body  may  not  be  deprived  through 
accident  or  disease. 

The  Cutaneous  Senses.  These  occur  over  the  entire  Body,  not 
uniformly  distributed  but  scattered  in  fine  dots  over  the  surface. 
This  punctiform  arrangement  can  be  demonstrated  by  exploring 
the  skin  with  fine  needles.  Such  a  procedure  shows  that  the  dif- 
ferent cutaneous  senses  occur  in  distinct  spots  which  do  not  over- 
lap, but  which  in  most  parts  of  the  Body  are  so  intermingled  as  to 
leave  no  area  of  any  size  devoid  of  any  one  of  the  senses.  Sensory 
spots  are  much  more  numerous  and  more  closely  packed  together 
in  such  regions  as  the  hands  and  face  which  are  liable  to  come  in 
contact  with  foreign  bodies,  than  they  are  in  the  better  protected 
surfaces  of  the  trunk  and  limbs.  Four  sorts  of  cutaneous  sense 
spots  are  recognized:  those  of  pain,  touch,  warmth,  and  cold. 
Pain  spots  are  more  numerous  than  any  of  the  others;  touch  spots 
rank  next  in  number,  it  being  estimated  that  on  the  trunk  and 
limbs  there  are  a  half  million  of  them;  cold  spots  are  only  half  as 
numerous  as  touch  spots;  warmth  spots  are  fewest  of  all,  their 
number  being  estimated  at  thirty  thousand  for  the  entire  Body. 

Pain.  When  the  skin  is  powerfully  stimulated  by  heat,  cold  or 
pressure,  or  is  inflamed,  we  get  a  sensation  which  we  call  pain. 
This  is  something  quite  different  from  the  unpleasantness  caused 
by  a  dazzling  light  or  a  musical  discord  or  a  disagreeable  odor  or 
taste.  We  recognize  these  as  being  still  sight  or  sound  or  smell . 


212  THE  HUMAN  BODY 

or  taste  sensations.  Pain,  however,  is  always  recognized  as  a 
distinct  sensation  having  its  own  modality.  Its  function  seems 
to  be  wholly  one  of  warning;  only  when  something  is  amiss  do 
we  feel  it.  Since  danger  results  from  strong  stimulation  but  not 
from  feeble  stimulation  pain  receptors  are  less  irritable  than  other 
sorts:  it  is  estimated  that  the  sense  of  touch  is  one  thousand  times 
as  delicate  as  the  sense  of  pain.  Harm  may  result  from  excessive 
stimulation  of  any  sort.  Pain  receptors,  therefore,  are  irritable 
to  all  forms  of  energy  except  that  of  light. 

Because  pain  results  from  any  sort  of  stimulation,  but  only 
when  excessive,  it  was  formerly  thought  to  be  not  a  distinct  sense 
but  the  result  of  overstimulation  of  the  other  senses.  On  this 
theory  it  would  be  hard  to  account  for  the  fact  that  skin  pain  is 
so  very  different  in  modality  from  a  touch  or  temperature  feeling, 
and  to  understand  why  it  gives  rise  in  consciousness  to  concep- 
tions concerning  a  condition  of  the  Body  and  not  of  some  external 
object:  it  is  not  extrinsically  referred  by  the  mind  to  a  quality  of 
anything  but  the  painful  part  itself,  as  a  dazzling  light  sensation 
or  a  fetid  odor  is.  There  is  also  experimental  and  pathological 
evidence  that  the  paths  taken  in  the  spinal  cord  by  nerve  impulses 
causing  pain  are  different  from  those  leading  to  a  consciousness 
of  touch.  If  certain  parts  of  the  cord  are  cut  in  the  thoracic  region 
of  a  rabbit,  gentle  touches  on  the  hind  limb  appear  to  be  felt;  the 
animal  erects  its  ears  or  moves  its  head :  but  powerful  stimulation 
of  the  sciatic  nerve  causes  no  signs  of  pain,  while  if  the  dorsal  white 
columns  be  cut  the  animal  still  can  feel  stimuli  applied  to  the  hind 
limb  and  sufficient  to  cause  pain  under  normal  conditions,  but  it 
appears  insensible  to  gentle  pressure  on  the  skin.  In  human  beings 
very  similar  phenomena  have  been  observed  in  cases  of  spinal 
cord  disease:  and  in  a  certain  stage  of  chloroform  or  ether  narcosis 
the  patient  feels  the  surgeon's  hand  or  his  knife  where  it  touches 
the  skin,  but  he  experiences  no  pain  when  deeper  parts  are  cut. 
/Such  considerations  seem  to  lead  to  the  conclusion  that  the 
nerve-fibers  and  receptors  concerned  with  painful  sensations  are 
quite  distinct  from  those  of  the  other  senses.  If  that  be  so  we 
must  assume  that  there  are  "pain"  fibers  very  widely  distributed 
over  the  skin  and  through  most  other  parts  of  the  Body.  In 
accident  or  disease  these  are  stimulated  powerfully  enough  to 
arouse  perception  and  imperiously  call  attention  to  danger. 


THE  RECEPTOR  SYSTEM 


213 


The  pain  nerves  of  the  skin  do  not  seem  to  be  provided  with 
special  end  organs  but  to  end  nakedly  among  the  cells  of  the 
epidermis.  Such  a  mode  of  termination  accords  with  the  low 
irritability  of  the  pain  mechanism  and  with  its  absence  of  adapta- 
tion to  particular  forms  of  energy,  since  nerve-tissue  proper  ex- 
hibits these  same  qualities. 

The  interior  of  the  Body,  in  certain  regions  at  least,  seems  to 
be  provided  with  special  pain  receptors.  These  are  the  Pacinian 
corpuscles  (see  Fig.  67).  They  are  specially  numerous  in  the  mesen- 
tery, the  connective  tissue  membrane 
which  supports  the  abdominal  viscera. 

Pains  can  be  localized,  though  only 
imperfectly,  and  the  less  perfectly  the 
more  severe  they  are.  The  exact  place 
of  a  needle  prick  after  removal  of  the 
needle  (so  that  there  is  no  guiding 
concomitant  touch  sensation)  cannot 
be  recognized  as  well  as  a  pin  touch 
on  the  same  region  of  the  skin,  but 
still  fairly  well;  while  the  acute  pain 
caused  by  a  small  abscess  (bone  felon) 
under  the  periosteum  of  a  finger  bone 
is  often  felt  all  over  the  forearm;  and 
a  single  diseased  tooth  may  cause  pain 

felt  over  the  whole  of  that  side  of  the     FlG.  67._A  Pacinian  corpus- 
face.  cle»  magnified. 

Many  internal  pains  instead  of  being  felt  as  coming  from  the 
organ  where  they  originate  are  referred  to  areas  of  the  skin.  So 
constant  is  this  misreference  that  the  physician  is  able  to  judge 
of  the  seat  of  many  disturbances  from  the  particular  skin  areas 
that  exhibit  tenderness.  The  explanation  of  this  misreference 
of  internal  pain  to  the  skin  is  not  easy  to  make.  It  has  been  sug- 
gested that  the  nerve-paths  over  which  internal  pain  reach  the 
body  sense-area  of  the  cortex  lie  close  to  those  of  pains  from  cer- 
tain skin  areas;  and  that  since  painful  skin  stimulation  is  much 
more  common  than  internal  pains,  the  brain  interprets  all  im- 
pulses reaching  it  over  a  restricted  nerve-path  as  coming  from 
the  particular  skin  area  whose  nerve-path  forms  part  of  the  whole 
nerve-path  in  question. 


214  THE  HUMAN  BODY 

Touch,  or  the  Pressure  Sense.  Through  touch  proper  we 
recognize  pressure  or  traction  exerted  on  the  skin,  and  the  force 
of  the  pressure,  the  softness  or  hardness,  roughness  or  smoothness, 
of  the  body  producing  it;  and  the  form  of  this,  when  not  too 
large  to  be  felt  all  aver.  When  to  learn  the  form  of  an  object 
we  move  the  hand  over  it,  muscular  sensations  are  combined 
with  proper  tactile,  and  such  a  combination  of  the  two  sensations 
iarfrfciuent ;  moreover,  we  rarely  touch  anything  without  at  the 
same  time  getting  temperature  sensations;  therefore  pure  tactile 
feelings  are  rare. 

From  an  evolutign,, point  of  view,  touch  is  probably  the  first 
distinctly  differentiated  sensation,  and  this  primary-  position 
it  still  largely  holds  in  our  mental  life;  we  mainly  think  of  the 
tilings  about  us  as  objects  which  would  give  us  certain  tactile 
/Sensations  if  we  were  in  contact  with  them.  Though  the  eye 
/  tells  us  much  quicker,  and  at  a  greater  range,  what  are  the  shapes 
of  objects  and  whether  they  are  smooth,  rough,  and  so  on,  our 
real  conceptions  of  round  and  square  and  rough  bodies  are  de- 
rived through  touch,  and  we  largely  translate  unconsciously  the 
teachings  of  the  eye  into  mental  terms  of  the  tactile  sense. 

The  delicacy  of  the  pressure  sense  varies  on  different  parts 
of  the  skin;  it  is  greatest  on  the  forehead,  temples,  and  back  of 
the  forearm,  where  a  weight  of  2  milligr.  (0.03  grain)  pressing  on 
an  area  of  9  sq.  millim.  (0.0139  sq.  inch)  can  be  felt.  On  the  front 
of  the  forearm  3  milligr.  (0.036  grain)  can  be  similarly  felt,  and 
on  the  front  of  the  forefinger  5  to  15  milligr.  (0.07-0.23  grain). 

In  order  that  the  sense  of  touch  may  be  excited  neighboring 
sjdn  areas  must  be  differently  pressed;  when  we  lay  the  hand 
<on  a  table  this  is  secured  by  the  inequalities  of  the  skin,  which 
prevent  end  organs,  lying  near  together,  from  being  equally  com- 
pressed. When,  however,  the  hand  is  immersed  in  a  liquid,  as 
mercury,  which  fits  into  all  its  inequalities  and  presses  with 
practically  the  same^weight  on_  .ail,  neighboring  iminameiLareas, 
the  sense  of  pressure  is  only  felt  at  a  line  along  the  surface,  where 
the  immersed  and  non-immersed  parts  of  the  skin  meet. 

It  was  in  connection  with  the  tactile  sense  that  the  facts  on 
which  the  so-called  psychophysical  law  (p.  205)  is  based,  were  first 
observed.  The  smallest  perceptible  difference  of  pressure  recog- 
nizable when  touch  alone  is  used,  is  about  ^,  i.  e.,  we  can  just  tell 


THE  RECEPTOR  SYSTEM  215 

a  weight  of  20  grams  (310  grains)  from  one  of  30  (465  grains)  or 
of  40  grams  (620  grains)  from  one  of  60  (930  grains) ;  the  change 
which  can  just  be  recognized  being  thus  the  same  fraction  of  that 
already  acting  as  a  stimulus.  The  rate  only  holds  good,  how- 
ever, for  a  certain  mean  range  of  pressure;  it  is  not  true  for  very 
small  or  very  great  pressures.  The  experimental  difficulties  in 
determining  the  question  are  considerable;  muscular  sensations 
must  be  rigidly  excluded;  the  time  elapsing  between  laying  the 
different  weights  on  the  skin  must  always  be  equal;  the  same 
region  and  area  of  the  skin  must  be  used;  the  weights  must  have 
the  same  temperature;  and  fatigue-^f  the  organs  must  be  elimi- 
nated. Considerable  individual  variatidns^afe i  also  observed,  the 
least  perceptible  difference  not  being  the  same  in  all  persons. 

The  Localizing  Power  of  the  Skin.  When  the  eyes  are  closed 
and  a  point  of  the  skin  is  touched  we  can  with  some  accuracy 
indicate  the  region  stimulated;  although  tactile  feelings  are  in 
general  characters  alike,  they  differ  in  something  (local  sign) 
besides  intensity  by  which  we  can  distinguish  them;  some  "sensa- 
tion quality  must  be  present  enabling  us  to  tell  from  one  another 
two  precisely  similar  contacts  of  an  external  object  when  ap^" 
plied,  say,  to  the  tips  of  the  fore  and  ring  fingers  respectively. 
The  accuracy  of  the  localizing  power  is  not  nearly  so  great  as  in 
the  eye  and  varies  widely  in  different  skin  regions;  it  may  be 
measured  by  observing  the  least  distance  which  must  separate 
two  objects  (as  the  blunted  points  of  a  pair  of  compasses)  in  order 
that  they  may  be  felt  as  two.  The  following  table  illustrates 
some  of  the  differences  observed: 

Tongue-tip .  .  . 1.1  mm.  (0.04  inch) 

Palm  side  of  last  phalanx  of  finger 2.2  mm.  (0.08  inch) 

Red  part  of  lips 4.4  mm.  (0.16  inch) 

Tip  of  nose 6.6  mm.  (0.24  inch) 

Back  of  second  phalanx  of  finger 11.0  mm.  (0.44  inch) 

Heel 22.0  mm.  (0.88  inch) 

Back  of  hand 30.8  mm.  (1.23  inches) 

Forearm 39.6  mm.  (1.58  inches) 

Sternum 44.0  mm.  (1.76  inches) 

Back  of  neck 52.8  mm.  (2.11  inches) 

Middle  of  back 66.0  mm.  (2.64  inches) 

The  localizing  power  is  a  little  more  acute  across  the  long  axis, 
of  a  limb,  and  is  better  when  the  pressure  is  only  strong  enougn 


216  THE  HUMAN  BODY 

just  to  cause  a  distinct  tactile  sensation,  than  when  it   is  more. 
^powerful;  it  is  also  very  readily  and  rapidly  improvable  by  practice. 
It  might  be  thought  that  this  localizing  power  depended  di- 
rectly on  nerve  distribution;  that  each  touch  nerve  had  connec- 
tion with  a  special  brain-center  at  one  end  (the  excitation  of 
which  caused  a  sensation  with  a  characteristic  local  sign),  and 
at  the  other  end  was  distributed  over  a  certain  skin  area,  and 
^at  the  larger  this  area  the  farther  apart 
might  two  points  be  and  still  give  rise  to 
only  one  sensation.    If  this  were  so,  how- 
ever, the  peripheral  tactile  areas  (each  be- 
ing determined  by  the  anatomical  distribu- 
tion of  a  nerve-fiber)  must  have  definite 
unchangeable    limits,    which    experiment 
shows  that  they  do  not  possess.    Suppose 
each  of  the  small  areas  in  Fig.  68  to  repre- 
sent a  peripheral  area  of  nerve  distribu- 
tion.   If  any  two  points  in  c  were  touched 
we  would  according  to  the  theory  get  but 
a  single  sensation;  but  if,  while  the  compass 
points  remained  the  same  distance  apart,  or  were  even  approxi- 
mated, one  were  placed  in  c  and  the  other  on  a  contiguous  area, 
two  fibers  would  be  stimulated  and  we  ought  to  get  two  sensa- 
tions; but  such  is  not  the  case;  on  the  same  skin  region  the  points 
\must  be  always  the  same  distance  apart,  no  matter  how  they 
xbe  shifted,  in  order  to  give  rise  to  two  just  distinguishable  sen- 
sations. 

^  It  is  probable  that  the  nerve  areas  are  much  smaller  than  the 
tactile;  and  that  several  unstimulated  must  intervene  between 
the  excited,  in  order  to  produce  sensations  which  shall  be  di§ 
tinct.  If  we  suppose  twelve  unexcited  nerve  areas  must  inter- 
vene, then,  in  Fig.  68,  a  and  b  will  be  just  on  the  limits  of  a  single 
tactile  area;  and  no  matter  how  the  points  are  moved,  so  long  as 
eleven,  or  fewer,  unexcited  areas  come  between,  we  would  get  a 
single  tactile  sensation;  in  this  way  we  can  explain  the  fact  that 
tactile  areas  have  no  fixed  boundaries  in  the  skin,  although  the 
nerve  distribution  in  any  part  must  be  constant.  We  also  see 
why  the  back  of  a  knife  laid  on  the  surface  causes  a  continuous 
linear  sensation,  although  it  touches  many  distinct  nerve  areas; 


THE  RECEPTOR  SYSTEM  217 

if  we  could  discriminate  the  excitations  of  each  of  these  from  that 
of  its  immediate  neighbors  we  would  get  the  sensation  of  a  series 
of  points  touching  us,  one  for  each  nerve  region  excited;  but  in 
the  absence  of  intervening  unexcited  nerve  areas  the  sensations 
are  fused  together. 

The  Temperature  Sense.  By  this  we  mean  our  faculty  of 
perceiving  cold  and  warmth;  and,  with  the  help  of  these  sensa- 
tions, of  perceiving  temperature  differences  in  external  objects. 
Its  organ  is  the  whole  skin,  the  mucous  membrane  of  mouth  and 
fauces,  pharynx  and  upper  part  of  gullet,  and  the  entry  of  the 
nares.  Direct  heating  or  cooling  of  a  sensory  nerve  may  stimulate 
it  and  cause  pain,  but  not  a  true  temperature  sensation;  and  the 
amount  of  heat  or  cold  requisite  is  much  greater  than  that  neces- 
sary when  a  temperature-perceiving  surface  is  acted  upon;  hence 
we  must  assume  the  presence  of  temperature  receptors.  As 
previously  stated  these  are  of  two  kinds,  those  that  are  stimulated 
by  cold,  and  those  that  are  stimulated  by  warmth. 

In  a  comfortable  room  we  feel  at  no  part  of  the  Body  either 
heat  or  cold,  although  different  parts  of  its  surface  are  at  differ- 
ent temperatures;  the  fingers  and  nose  being  cooler  than  the 
trunk  which  is  covered  by  clothes,  and  this,  in  turn,  cooler  than 
the  interior  of  the  mouth.  The  temperature  which  a  given  region 
of  the  temperature  organ  has  (as  measured  by  a  thermometer) 
when  it  feels  neither  hot  nor  cold  is  its  temperature-sensation  zero 
for  that  time,  and  is  not  associated  with  any  one  objective  tem- 
perature; for  not  only,  as  we  have  just  seen,  does  it  vary  in  dif- 
ferent parts  of  the  organ,  but  also  on  the  same  part  from  time  to 
time.  Whenever  a  skin  region  passes  with  a  certain  rapidity  to 
a  temperature  above  its  sensation  zero  we  feel  warmth;  and  vice 
versa:  the  sensation  is  more  marked  the  greater  the  difference, 
and  the  more  suddenly  it  is  produced;  touching  a  metallic  body, 
which  conducts  heat  rapidly  to  or  from  the  skin,  causes  a  more 
marked  hot  or  cold  sensation  than  touching  a  worse  conductor, 
as  a  piece  of  wood,  of  the  same  temperature. 

The  change  of  temperature  in  the  organ  may  be  brought  about 
by  changes  in  the  circulatory  apparatus  (more  blood  flowing 
through  the  skin  warms  it  and  less  leads  to  its  cooling),  or  by 
temperature  changes  in  gases,  liquids,  or  solids  in  contact  with  it. 
Sometimes  we  fail  to  distinguish  clearly  whether  the  cause  is 


218  THE  HUMAN  BODY 

external  or  internal;  a  person  coming  in  from  a  windy  walk  often 
feels  a  room  uncomfortably  warm  which  is  not  really  so;  the 
exercise  has  accelerated  his  circulation  and  tended  to  warm  his 
skin,  but  the  moving  outer  air  has  rapidly  conducted  off  the  extra 
heat;  on  entering  the  house  the  stationary  air  there  does  this  less 
quickly,  the  skin  becomes  hotter,  and  the  cause  is  supposed  to  be 
oppressive  heat  of  the  room.  Hence,  frequently,  opening  of  win- 
dows and  sitting  in  a  draught,  with  its  concomitant  risks;  whereas 
keeping  quiet  for  five  or  ten  minutes,  until  the  circulation  had 
returned  to  its  normal  rate,  would  attain  the  same  end  without 
danger. 

The  acuteness  of  the  temperature  sense  is  greatest  at  tempera- 
tures within  a  few  degrees  of  30°  C.  (86°  F.);  at  these  differences  of 
less  than  0.1°  C.  can  be  discriminated.  As  a  means  of  measuring 
absolute  temperature,  however,  the  skin  is  very  unreliable,  on 
account  of  the  changeability  of  its  sensation  zero.  We  can 
localize  temperature  sensations  much  as  tactile,  but  not  so  ac- 
curately. 

The  receptors  for  cold  are  near  the  surface  of  the  skin;  those 
for  warmth  are  embedded  deeply  within  it.  While  the  latter  re- 
spond only  to  temperatures  above  their  own,  the  cold  receptors 
are  stimulated  not  only  by  temperatures  below  their  own  but 
also  by  temperatures  above  45°  C.  (140°  F.).  It  is  for  this  reason 
that  a  sensation  of  cold  is  felt  when  one  first  steps  into  a  hot  bath; 
the  receptors  for  cold  being  nearer  the  surface  than  those  for 
warmth  are  stimulated  an  instant  before  them.  It  is  said  that  the 
sensation  of  "hot"  as  distinguished  from  "warm''  results  from 
simultaneous  stimulation  of  warmth  and  cold  spots  by  tempera- 
tures above  45°  C. 

The  Peripheral  Reference  of  our  Sensations.  Repeated  men- 
tion has  been  made  of  the  fact  that  we  refer  our  external  sensa- 
tions to  the  outside  world;  this  is  only  one  case  of  a  more  general 
law,  in  accordance  with  which  we  do  not  ascribe  our  sensations, 
as  regards  their  locality,  to  the  brain,  where  the  sensation  is 
actually  aroused,  but  to  a  peripheral  part.  With  respect  to  the 
brain,  other  parts  of  the  Body  are  external  objects  as  much  as 
the  rest  of  the  material  universe,  yet  we  locate  the  majority  of 
our  internal  sensations  at  the  places  where  the  sensory  nerves 
concerned  are  irritated,  and  not  in  the  brain.  Even  if  a  nerve- 


THE  RECEPTOR  SYSTEM  219 

trunk  be  stimulated  in  the  middle  of  its  course,  we  refer  the  re- 
sulting sensation  to  its  outer  endings.  A  blow  on  the  inside  of 
the  elbow-joint,  injuring  the  ulnar  nerve,  produces  not  only  a 
local  pain,  but  a  sense  of  tingling  ascribed  to  the  fingers  to  which 
the  ends  of  the  fibers  go.  Persons  with  amputated  limbs  have 
feelings  in  their  fingers  and  toes  long  after  they  have  been  lost, 
if  the  nerve-trunks  in  the  stump  be  irritated.  This  persistent- 
reference  is  commonly  ascribed  to  the  results  of  experience.  The 
events  of  life  have  taught  us  that  in  the  great  majority  of  in- 
stances the  sensory  impulses  which  excite  a  given  tactile  sensa- 
tion, for  example,  have  acted  upon  the  tip  of  a  finger.  The  sen- 
sation goes  when 'the  finger  is  removed,  and  returns  when  it  is 
replaced;  and  the  eye  confirms  the  contact  of  the  external  object 
with  the  finger-tip  when  we  get  the  tactile  sensation  in  question. 
We  thus  come  firmly  to  associate  a  particular  region  of  the  skin 
with  a  given  sensation,  and  whenever  afterwards  the  nerve-fibers 
coming  from  the  finger  are  stimulated,  no  matter  where  in  their 
course,  we  ascribe  the  origin  of  the  sensation  to  something  acting 
on  the  finger-tip. 

Perceptions.  In  every  sensation  we  have  to  distinguish  care- 
fully between  the  pure  sensation  and'  certain  judgments  founded 
upon  it;  we  have  to  distinguish  between  what  we  really  feel  and 
what  we  think  we  feel;  and  very  often  firmly  believe  we  do  feel 
when  we  do  not. 

The  most  important  of  these  judgments  is  that  which  leads  us 
to  ascribe  certain  sensations,  those  aroused  through  organs  of 
special  sense,  to  external  objects — that  outer  reference  of  our 
sensations  which  leads  us  to  form  ideas  concerning  the  existence, 
form,  position,  and  properties  of  external  things.  Such  represen- 
tations as  these,  founded  on  our  senses,  are  called  perceptions. 
Since  these  always  imply  some  mental  activity  in  addition  to  a 
mere  feeling,  their  full  discussion  belongs  to  the  domain  of  Psy- 
chology. Physiology,  however,  is  concerned  with  them  so  far  as 
it  can  determine  the  conditions  of  stimulation  under  which  a 
given  mental  representation  concerning  a  sensation  is  made. 
It  is  quite  certain  that  we  can  feel  nothing  but  states  of  ourselves, 
but,  as  already  pointed  out,  we  have  no  hesitation  in  saying  we 
feel  a  hard  or  a  cold,  a  rough  or  smooth  body.  When  we  look  at 
a  distant  object  we  usually  make  no  demur  to  saying  that  we 


220  THE  HUMAN  BODY 

perceive  it.  What  we  really  feel  is,  however,  the  change  produced 
by  it  in  our  eyes.  There  are  no  parts  of  our  Bodies  reaching  to 
a  tree  or  a  house  a  mile  off — and  yet  we  seem  to  feel  all  the  while 
that  we  are  looking  at  the  tree  or  the  house  and  feeling  them,  and 
not  merely  experiencing  modifications  of  our  own  eyes  or  brains. 
When  reading  we  feel  that  what  we  really  see  is  the  book;  and  yet 
the  existence  of  the  book  is  a  judgment  founded  on  a  state  of  our 
Body,  which  alone  is  what  we  truly  feel. 

We  have  the  same  experience  in  other  cases,  for  example  with 
regard  to  touch. 

Hairs  are  quite  insensible,  but  are  embedded  in  the  sensitive 
skin,  which  is  excited  when  they  are  moved.  But  if  the  tip  of  a 
hair  be  touched  by  some  external  object  we  believe  we  feel  the 
contact  at  its  insensible  end,  and  not  in  the  sensitive  skin  at  its 
root.  So,  the  hard  parts  of  the  teeth  are  insensible;  yet  when  we 
rub  them  together  we  refer  the  seat  of  the  sensation  aroused  to  the 
points  where  they  touch  one  another,  and  not  to  the  sensitive  parts 
around  the  sockets  where  the  sensory  nerve  impulse  is  really 
started. 

Still  more,  we  may  refer  tactile  sensations,  not  merely  to  the 
distal  ends  of  insensible  bodies  implanted  in  the  skin,  but  to  the 
far  ends  of  things  which  are  not  parts  of  our  Bodies  at  all;  for 
instance,  the  distant  end  of  a  rod  held  between  the  finger  and  a 
table  while  the  finger  is  moved  a  little  from  side  to  side.  We  then 
believe  we  feel  touch  or  pressure  in  two  places;  one  where  the  rod 
touches  our  finger,  and  the  other  where  it  comes  in  contact  with 
the  table.  A  blind  man  gropes  his  way  along  by  feeling  at  the  end 
of  his  stick. 

This  irresistible  mental  tendency  to  refer  certain  of  our  states 
of  feeling  to  causes  outside  of  our  Bodies,  whether  in  contact  with 
them  or  separated  from  them  by  a  certain  space,  is  known  as  the 
phenomenon  of  the  extrinsic  reference  of  our  sensations.  It  seems 
largely  to  depend  on  the  fact  that  the  sensations  extrinsically 
referred  can  be  modified  by  movements  of  our  Bodies.  Hunger, 
thirst,  and  toothache  all  remain  the  same  whether  we  turn  to  the 
right  or  left,  or  move  away  from  the  place  we  are  standing  in. 
But  a  sound  is  altered.  We  may  find  that  in  a  certain  position 
of  the  head  it  is  heard  more  by  the  right  ear  than  the  left ;  but  on 
turning  round  the  reverse  is  the  case;  and  halfway  round  the 


THE  RECEPTOR  SYSTEM  221 

loudiiess  in  each  ear  is  the  same.  Hence  we  are  led,  by  mental 
laws  outside  of  the  physiological  domain,  to  suspect  that  its  cause 
is  not  in  our  Body,  but  outside  of  it;  and  depends  not  on  a  condi- 
tion of  the  Body  but  on  something  else. 

Sensory  Illusions.  "I  must  believe  my  own  eyes"  and  "we 
can't  always  believe  our  senses"  are  two  expressions  frequently 
heard,  and  each  expressing  a  truth.  No  doubt  a  sensation  in 
itself  is  an  absolute  incontrovertible  fact:  if  I  feel  redness  or  hot- 
ness  I  do  feel  it,  and  that  is  an  end  of  the  matter:  but  if  I  go  be- 
yond the  fact  of  my  having  a  certain  sensation  and  conclude  from 
it  as  to  properties  of  something  else— if  I  form  a  judgment  from 
my  sensation — I  may  be  totally  wrong;  and  in  so  far  be  unable  to 
believe  my  eyes  or  skin.  Such  judgments  are  almost  inextricably 
woven  up  with  many  of  our  sensations,  and  so  closely  that  we 
cannot  readily  separate  the  two;  not  even  when  we  know  that 
the  judgment  is  erroneous. 

For  example,  the  moon  when  rising  or  setting  appears  bigger 
than  when  high  in  the  heavens — we  seem  to  feel  directly  that  it 
arouses  more  sensation,  and  yet  we  know  certainly  that  it  does 
not.  With  a  body  of  a  given  brightness  the  amount  of  change 
produced  in  the  end  organs  of  the  eye  will  depend  on  the  size  of 
the  image  formed  in  the  eye,  provided  the  same  part  of  its  sensory 
surface  is  acted  upon.  Now  the  size  of  this  image  depends  on  the 
distance  of  the  object;  it  is  smaller  the  farther  off  it  is  and  greater 
the  nearer,  and  measurements  show  that  the  area  of  the  sensitive 
surface  affected  by  the  image  of  the  rising  moon  is  no  larger  than 
that  affected  by  it  when  overhead.  Why  then  do  we,  even  after 
we  know  this,  see  it  bigger?  The  reason  is  that  when  the  moon  is 
near  the  horizon  we  imagine,  unconsciously  and  irresistibly,  that 
it  is  farther  off;  even  astronomers  who  know  perfectly  well  t hat- 
it  is  not,  cannot  help  forming  this  unconscious  and  erroneous 
judgment — and  to  them  the  moon  appears  in  consequence  larger 
when  near  the  horizon,  just  as  it  does  to  less  well-informed  mor- 
tals. In  fact  we  have  a  conception  of  the  sky  over  which  the  moon 
seems  to  travel,  not  as  a  half  sphere  but  as  somewhat  flattened, 
and  hence  when  the  moon  is  at  the  horizon  we  unconsciously 
judge  that  it  is  farther  off  than  when  overhead.  But  any  body 
which  excites  the  same  extent  of  the  sensitive  surface  of  the  eye 
at  a  great  distance  that  another  does  at  less,  must  be  larger  than 


222  THE  HUMAN  BODY 

the  latter;  and  so  we  conclude  that  the  moon  at  the  horizon  is 
larger  than  the  moon  in  the  zenith,  and  are  ready  to  declare  that 
we  see  it  so. 

Erroneous  perceptions  of  this  sort  are  known  as  sensory  illu- 
sions; and  we  ought  to  be  constantly  on  guard  against  them. 


CHAPTER  XIV 

THE  EAR.    HEARING  AND  EQUILIBRATION.    TASTE  AND 

SMELL 

Functions  of  the  Ear.  The  ear  is  not  solely  an  organ  of  hearing. 
It  includes,  in  addition,  the  highly  important  structures  by  which 
is  mediated  the  sense  of  equilibrium.  Hearing  is,  however,  its 
familiar  function,  and  we  will  consider  it  first  in  order.  To  be  able 
to  discuss  intelligently  the  apparatus  for  hearing  we  must  have  in 
mind  the  fundamental  facts  about  the  agency  by  which  the  sense 
is  aroused,  namely,  sound. 

The  Loudness,  Pitch,  and  Timbre  of  Sounds.  Sounds,  as  sensa- 
tions, fall  into  two  groups — notes  and  noises.  Physically,  sounds 
consist  of  vibrations,  and  these,  under  most  circumstances,  when 
they  first  reach  our  auditory  organs,  are  alternating  rarefactions 
and  condensations  of  the  air,  or  aerial  waves.  When  the  waves  fol- 
low one  another  uniformly,  or  periodically,  the  resulting  sensation 
(if  any)  is  a  note;  when  the  vibrations  are  irregular  it  is  a  noise. 
In  notes  we  recognize  (1)  loudness  or  intensity;  (2)  pitch;  (3)  qual- 
ity or  timbre,  or,  as  it  has  been  called,  tone  color;  a  note  of  a  given 
loudness  and  pitch  produced  by  a  flute  and  by  a  violin  has  a  dif- 
ferent character  or  individuality  in  each  case;  this  quality  is  its 
timbre.  Before  understanding  the  working  of  the  auditory  mech- 
anism we  must  get  some  idea  of  the  physical  qualities  in  ob- 
jective sound  of  which  the  subjective  differences  of  auditory 
sensations  are  signs. 

The  loudness  of  a  sound  depends  on  the  force  of  the  aerial  waves; 
the  greater  the  intensity  of  the  alternating  condensations,  and  rare- 
factions of  these,  the  louder  the  sound.  The  pitch  of  a  note  depends 
on  the  length  of  the  waves,  that  is,  the  distance  from  one  point  of 
greatest  condensation  to  the  next,  or  (what  amounts  to  the  same 
thing)  on  the  number  of  waves  reaching  the  ear  in  given  time,  say 
a  second.  The  shorter  the  waves  the  more  rapidly  they  follow 
one  another,  and  the  higher  the  pitch  of  the  note.  When  audible 

223 


224  THE  HUMAN  BODY 

vibrations  bear  the  ratio  1 :2  to  one  another,  we  hear  the  musical 
interval  called  an  octave.  The  middle  C  of  the  musical  scale  is 
due  to  256  vibrations  per  second.  Its  octave  has  512  vibrations. 

Sound  vibrations  may  be  too  rapid  or  too  slow  in  succession  to 
produce  sonorous  sensations.  The  highest-pitched  audible  note 
answers  to  about  38,000  vibrations  in  a  second,  but  it  differs  in 
individuals;  many  persons  cannot  hear  the  cry  of  a  bat  nor  the 
chirp  of  a  cricket,  which  lie  near  this  upper  audible  limit.  On  the 
other  hand,  sounds  of  vibrational  rate  about  40  per  second  are  not 
well  heard,  and  a  little  below  this  become  inaudible.  The  highest 
note  used  in  orchestras  is  the  dv  of  the  fifth  accented  octave,  pro- 
duced by  the  piccolo  flute,  due  to  4,752  vibrations  in  a  second; 
and  the  lowest-pitched  is  the  E\,  of  the  contra  octave,  produced 
by  the  double  bass.  Modern  grand  pianos  and  organs  go  down  to 
C,  in  the  contra  octave  (33  vibrations  per  second)  or  even  A", 
(271),  but  the  musical  quality  of  such  notes  is  imperfect;  they  pro- 
duce rather  a  "hum"  than  a  true  tone  sensation,  and  are  only  used 
along  with  notes  of  higher  octaves  to  which  they  give  a  character 
of  greater  depth. 

Timbre.  Since  the  loudness  of  a  tone  depends  on  the  vibra- 
tional amplitude  of  its  physical  antecedent,  and  its  pitch  on  the 
vibrational  rate,  we  have  still  to  seek  the  cause  of  timbre;  the 
quality  by  which  we  recognize  the  human  voice,  the  violin,  the 
piano,  and  the  flute,  even  when  all  sound  the  same  note  and  of 
the  same  loudness.  Helmholtz  showed  that  the  quality  of  any 
tone  is  determined  by  the  particular  overtones  or  harmonic  partials 
that  are  combined  in  it  with  the  fundamental  tone.  Most  vibrating 
bodies  are  able  to  vibrate  both  as  a  whole  and  in  sections.  Since 
the  sections  are  smaller  than  the  whole  body  their  vibrations  are 
more  rapid  than  those  of  the  body  as  a  whole.  The  vibrating 
sections  may  be  halves,  thirds,  fourths,  or  any  other  fraction  of 
the  whole  body.  Also  one  and  the  same  body  may  be  vibrating 
at  once  in  halves,  quarters,  and  several  other  smaller  divisions. 
These  vibrations  in  parts  are  the  sources  of  overtones,  the  pitch 
of  the  tone  being  determined  by  its  vibration  as  a  whole,  the  so- 
called  fundamental  vibration. 

The  air  waves  set  in  motion  by  a  body  vibrating  in  such  com- 
plex fashion  must  necessarily  be  themselves  very  complex.  Since 
they  are  periodic,  however,  they  produce  audible  notes,  if  rapid 


THE  EAR,  HEARING,  TASTE  AND  SMELL  225 

and  intense  enough.  The  actual  form  of  air  wave  which  proceeds 
from  a  body  vibrating  thus  depends  upon  the  particular  com- 
ponents which  make  it,  and  it  has  been  shown  that  any  complex 
periodic  vibration  can  be  analyzed  mathematically  into  its  con- 
stituents, and  these  unerringly  determined.  The  timbre  of  a  tone 
depends,  then,  according  to  our  former  definition,  upon  the  form  of 
air  wave  which  enters  the  ear.  A  tone  composed  of  a  fundamental 
and  three  overtones  will  come  to  the  ear  as  a  wave  having  quite  a 
different  form  from  one  having  in  addition  to  the  fundamental 
five  partials. 

Whereas  we  ordinarily  hear  compound  tones  merely  as  tones  of 
certain  quality,  the  trained  ear  is  able  to  hear  and  pick  out  the 
overtones  by  which  the  quality  is  determined.  It  is  evident, 
therefore,  that  the  ear  is  able  to  analyze  compound  tones  into  their 
individual  constituents. 

Sympathetic  Resonance.  Imagine  slight  taps  to  be  given  to  a 
pendulum;  if  these  be  repeated  at  such  intervals  of  time  as  al- 
ways to  help  the  swing  and  never  to  retard  it,  the  pendulum  will 
soon  be  set  in  powerful  movement.  If  the  taps  are  irregular,  or 
when  regular  come  at  such  intervals  as  sometimes  to  promote  and 
sometimes  retard  the  movement,  no  great  swing  will  be  produced; 
but  if  they  always  push  the  pendulum  in  the  way  it  is  going  at  that 
instant,  they  need  not  come  every  swing  in  order  to  set  up  a  power- 
ful vibration;  once  in  two,  three,  or  four  swings  will  do.  A  stretched 
string,  such  as  that  of  a  piano,  is  so  far  like  a  pendulum  that 
it  tends  to  vibrate  at  one  rate  and  no  other;  if  aerial  waves  hit 
it  at  exactly  the  right  times  they  soon  set  it  in  sufficiently  power- 
ful vibrations  to  cause  it  to  emit  an  audible  note.  By  using  such 
strings  we  can  analyze  compound  tones  and  thus  prove  objectively 
that  they  are  made  up  of  partials.  If  the  dampers  of  a  piano  be 
raised  and  a  note  be  sung  loudly  to  it,  it  will  be  found  that  several 
strings  are  set  in  vibration,  such  vibrations  being  called  sympa- 
thetic. The  human  voice  emits  compound  tones  which  can  be 
mathematically  analyzed  into  simple  vibrations,  and  if  the  piano 
strings  set  in  movement  by  it  be  examined,  they  will  be  found  to 
be  exactly  those  which  answer  to  these  vibrations  and  to  no 
others.  We  thus  get  experimental  grounds  for  believing  that  com- 
pound tones  are  really  made  up  of  a  number  of  simple  vibrations, 
and  get  an  additional  justification  for  the  supposition  that  in 


226  THE  HUMAN  BODY 

the  ear  each  note  is  analyzed  into  its  components;  and  that  the 
difference  of  sensation  which  we  call  timbre  is  due  to  the  effect  of 
the  secondary  partial  tones  thus  perceived.  If  so,  the  ear  must 
have  in  it  an  apparatus  adapted  for  sympathetic  resonance. 

The  External  Ear.  The  auditory  organ  in  man  consists  of 
three  portions,  known  respectively  as  the  external  ear,  the  middle 
ear  or  tympanic  cavity,  and  the  internal  ear  or  labyrinth;  the  latter 
contains  the  end  organs  of  the  auditory  nerve.  The  external  ear 
consists  of  the  expansion  seen  on  the  exterior  of  the  head,  called 
the  concha,  M,  Fig.  69,  and  a  passage  leading  in  from  it,  the  ex- 


FIG.  69. — Semidiagrammatic  section  through  the  right  ear  (Czermak).  M,  con- 
cha; G,  external  auditory  meatus;  T,  tympanic  membrane;  P,  middle  ear;  o,  oval 
foramen;  r,  round  foramen;  R,  pharyngeal  opening  of  Eustachian  tube;  V,  vesti- 
bule; B,  a  semicircular  canal;  S,  the  cochlea;  Vt,  scala  vestibuli;  Pt,  scala  tympani; 
A,  auditory  nerve. 

ternal  auditory  meatus,  G.  This  passage  is  closed  at  its  inner  end 
by  the  tympanic  or  drum  membrane,  T.  It  is  lined  by  skin,  through 
which  numerous  small  glands,  secreting  the  wax  of  the  ear,  open. 
The  Functions  of  the  Tympanic  Membrane.  If  a  stretched 
membrane,  such  as  a  drumhead,  be  struck,  it  will  be  thrown  into 
periodic  vibration  and  emit  for  a  time  a  note  of  a  determined  pitch. 
The  smaller  the  membrane  and  the  tighter  it  is  stretched  the  higher 
the  pitch  of  its  note;  every  stretched  membrane  thus  has  a  rate  of 
its  own  at  which  it  tends  to  vibrate,  just  as  a  piano  or  violin  string 


THE  EAR,  HEARING,  TASTE  AND  SMELL  227 

has.  When  a  note  is  sounded  in  the  air  near  such  a  membrane,  the 
alternating  waves  of  aerial  condensation  and  rarefaction  will  move 
it;  and  if  the  waves  succeed  at  the  vibratioiial  rate  of  the  membrane 
the  latter  will  be  set  in  powerful  sympathetic  vibration;  if  they  do 
not  push  the  membrane  at  the  proper  times,  their  effects  will 
neutralize  one  another:  hence  such  membranes  respond  well  to 
only  one  note.  The  tympanic  membrane,  however,  responds 
equally  well  to  a  large  number  of  notes;  at  the  least  for  those  due 
to  aerial  vibrations  of  rates  from  60  to  4,000  per  second,  running 
over  eight  octaves  and  constituting  those  commonly  used  in 
music.  This  faculty  depends  on  two  things:  (1)  the  membrane  is 
comparatively  loosely  and  not  uniformly  stretched;  (2)  it  is  loaded 
by  the  tympanic  bones. 

The  drum-membrane  is  a  shallow  funnel  with  its  sides  convex 
towards  the  external  auditory  meatus;  something  like  an  umbrella 
turned  inside  out;  in  such  a  membrane  the  tension  is  not  uniform 
but  increases  towards  the  center,  and  it  has  accordingly  no  proper 
note  of  its  own.  Further,  whatever  tendency  such  a  membrane 
may  have  to  vibrate  rather  at  one  rate  than  another,  is  almost  com- 
pletely removed  by  " damping"  it,  i.  e.,  placing  in  contact  with  it 
something  comparatively  heavy  and  which  has  to  be  moved  when 
the  membrane  vibrates.  This  is  effected  by  the  tympanic  bones, 
fixed  to  the  drum-membrane  by  the  handle  of  the  malleus.  An- 
other advantage  is  gained  by  the  damping;  once  a  stretched  mem- 
brane is  set  vibrating  it  continues  so  doing  for  some  time;  but  if 
loaded  its  movements  cease  almost  as  soon  as  the  moving  impulses. 
The  dampers  of  a  piano  are  for  this  purpose;  and  violin-players 
have  to  "damp"  with  the  fingers  the  strings  they  have  used  when 
they  wish  the  note  to  cease.  The  tympanic  bones  act  as  dampers. 

The  Middle  Ear  (P,  Fig.  69)  is  an  irregular  cavity  in  the  tem- 
poral bone,  closed  externally  by  the  drum  membrane.  From  its 
inner  side  the  Eustachian  tube  (R)  proceeds  to  the  pharynx,  and 
the  mucous  membrane  of  that  cavity  is  continued  up  the  tube  to 
line  the  middle  ear;  the  proper  tympanic  membrane  composed 
of  connective  tissue  is  therefore  covered  by  mucous  membrane  on 
its  inner,  as  it  is  by  very  thin  skin  on  its  outer,  side.  In  the  bony 
inner  wall  of  the  middle  ear  are  two  small  apertures,  the  oval  and 
round  foramens,  o  and  r,  which  lead  into  the  labyrinth.  During 
life  the  round  aperture  is  closed  by  the  lining  mucous  membrane, 


228  THE  HUMAN  BODY 

and  the  oval  in  another  way,  to  be  described  presently.  The 
tympanic  membrane,  T,  stretched  across  the  outer  side  of  the 
middle  ear,  forms  a  shallow  funnel  with  its  concavity  outwards. 
It  is  pressed  by  the  external  air  on  its  exterior,  and  by  air  enter- 
ing the  tympanic  cavity  through  the  Eustachian  tube  on  its  inner 
side.  If  the  middle  ear  were  closed  the  pressures  on  the  inner  and 
outer  sides  of  the  drum  membrane  would  not  be  always  equal 
when  barometric  pressure  varied,  and  the  membrane  would  be 
bulged  in  or  out  according  as  the  external  or  internal  pressure  on 
it  were  the  greater.  This  unequal  pressure  would  interfere  se- 
riously with  the  freedom  of  vibration  of  the  membrane  and  so 
impair  hearing.  On  the  other  hand,  were  the  Eustachian  tube 
always  open  the  sounds  of  our  own  voices  would  be  loud  and  dis- 
concerting, so  it  is  usually  closed;  but  every  time  we  swallow  it  is 
opened,  and  thus  the  air-pressure  in  the  cavity  is  kept  equal  to 
that  in  the  external  auditory  meatus.  By  holding  the  nose,  keep- 
ing the  mouth  shut,  and  forcibly  expiring,  air  may  be  forced  un- 
der pressure  into  the  middle  ear,  and  will  be  held  in  part  im- 
prisoned there  until  the  next  act  of  swallowing.  On  making  a 
balloon  ascent  or  going  rapidly  down  a  deep  mine,  the  sudden 
and  great  change  of  aerial  pressure  outside  frequently  causes 
painful  tension  of  the  drum  membrane,  which  may  be  greatly 
alleviated  by  frequent  swallowing  movements. 

The  great  importance  of  the  Eustachian  tubes  in  hearing  is 
illustrated  by  the  deafness  that  results  from  their  continued  closure. 
This  condition  is  frequently  brought  about  in  children  by  the 
growth  of  adenoids  (see  p.  383)  in  the  back  of  the  throat,  which 
press  upon  and  close  the  Eustachian  tubes. 

Essential  as  these  tubes  are  for  good  hearing  they  constitute  a 
frequent  source  of  ear  trouble.  The  congestion  of  the  mucous 
membranes  of  the  throat  and  nose  in  a  "cold  in  the  head"  is  apt 
to  involve  the  Eustachian  tubes  and  the  lining  of  the  middle  ear. 
Sometimes  an  exudate  from  the  congested  membranes  fills  the 
middle  ear  completely,  and  by  its  pressure  causes  acute  pain,  as 
well  as  deafness.  Unless  relief  is  obtained  the  tympanic  membrane 
may  be  ruptured.  Earache  resulting  from  a  cold  should  therefore 
not  be  neglected.  Less  commonly  but  more  seriously  actual  in- 
fection (p.  306)  of  the  middle  ear  may  occur,  the  infection  invading 
the  region  by  way  of  the  Eustachian  tubes.  The  mastoid  process 


THE  EAR,  HEARING,  TASTE  AND  SMELL 


229 


(the  prominent  bony  mass  just  behind  the  ear)  has  many  hollows 
in  it  which  communicate  with  the  cavity  of  the  middle  ear  and 
may  become  infected  from  it.  Infection  of  these  hollows  gives 
rise  to  the  extremely  grave  condition,  mastoiditis. 

The  Auditory  Ossicles.  Three  small  bones  lie  in  the  middle 
ear  forming  a  chain  from  the  drum  membrane  to  the  oval  fora- 
men. The  external  bone  (Fig.  70)  is  the  malleus  or  hammer;  the 
middle  one,  the  incus  or  anvil;  and  the  internal,  the  stapes  or 
stirrup.  The  malleus,  M,  has  an  upper  enlargement  or  head, 
which  carries  on  its  inner  side  an  articular  surface  for  the  incus; 
below  the  head  is  a  constriction,  the  neck,  and  below  this  two 
processes  complete  the  bone;  one,  the  long  or  slender  process,  is 
embedded  in  a  ligament  which  reaches  from  it  to  the  front  wall  of 
the  tympanic  cavity;  the  other  process,  the  handle,  reaches  down 
between  the  mucous  membrane  lining  the  inside  of  the  drum 
membrane  and  the  membrane  proper,  and  is  firmly  attached  to 
the  latter  near  its  center  and  keeps  the  membrane  dragged  in 
there  so  as  to  give  it  its  peculiar 
concave  form,  as  seen  from  the 
outside.  The  incus  has  a  body 
and  two  processes,  and  is  much 
like  a  molar  tooth  with  two  roots. 
On  its  body  is  an  articular  hollow 
to  receive  the  head  of  the  malleus; 
its  short  process  ( Jb)  is  attached  by 
ligament  to  the  back  wall  of  the 
tympanum;  the  long  process  (Jl) 
is  directed  inwards  to  the  stapes; 
on  the  tip  of  this  process  is  a  little 
knob,  which  represents  a  bone  (os 
orbiculare)  distinct  in  early  life. 
The  stapes  (»S)  is  extremely  like  a 
stirrup,  and  its  base  (the  footpiece 
of  the  stirrup)  fits  into  the  oval  foramen,  to  the  margin  of  which 
its  edge  is  united  by  a  fibrous  membrane,  allowing  of  a  little  play 
in  and  out. 

From  the  posterior  side  of  the  neck  of  the  malleus  a  ligament 
passes  to  the  back  wall  of  the  middle  ear:  this,  with  the  ligament 
embedding  the  slender  process  and  fixed  to  the  front  wall  of  the 


Mm 

FIG.  70. — The  auditory  ossicles  of 
the  right  ear,  seen  from  the  front.  M , 
malleus;  J,  incus;  S,  stapes;  Mcp, 
head  of  the  malleus;  Me,  neck  of 
ditto;  Ml,  long  process;  Mm,  handlo; 
Jc,  body;  Jb,  short,  and  Jl,  long 
process  of  incus;  Jpl,  os  orbiculare; 
Scp,  head  of  stapes. 


230  THE  HUMAN  BODY 

cavity,  forms  an  anteroposterior  axial  ligament,  on  which  the 
malleus  can  rotate  slightly,  so  that  the  handle  can  be  pushed  in 
and  the  head  out  and  vice  versa.  If  a  pin  be  driven  through  Fig.  70 
just  below  the  neck  of  the  malleus  and  perpendicular  to  the  paper 
it  will  very  fairly  represent  this  axis  of  rotation.  Connected  with 
the  malleus  is  a  tiny  muscle,  called  the  tensor  tympani;  it  is  inserted 
on  the  handle  of  the  bone  below  the  axis  of  rotation,  and  when  it 
contracts  pulls  the  handle  in  and  tightens  the  drum  membrane. 
Another  muscle  (the  stapedius)  is  inserted  into  the  outer  end  of  the 
stapes,  and  when  it  contracts  fixes  the  bone  so  as  to  limit  its  range 
of  movement  in  and  out  of  the  fenestra  ovalis. 

Functions  of  the  Auditory  Ossicles.  When  the  air  in  the  ex- 
ternal auditory  meatus  is  condensed  it  pushes  in  the  tympanic 
membrane  which  carries  with  it  the  handle  of  the  malleus.  This 
bone  then  slightly  rotates  on  the  axial  ligament  and,  locking 
into  the  incus  where  the  two  bones  articulate,  causes  the  long 
process  (Jl,  Fig.  70)  of  the  latter  to  move  inwards.  The  incus 
thus  pushes  in  the  stapes;  the  reverse  occurs  when  air  in  the  au- 
ditory passage  is  rarefied.  Aerial  vibrations  thus  set  the  chain  of 
bones  swinging,  and  push  in  and  pull  out  the  base  of  the  stapes, 
which  sets  up  waves  in  the  fluid  of  the  labyrinth.  This  fluid  being 
chiefly  water,  and  practically  incompressible,  the  end  of  the  stapes 
could  not  work  in  and  out  at  the  oval  foramen,  were  the  labyrinth 
elsewhere  completely  surrounded  by  bone:  but  the  membrane 
covering  the  round  foramen  bulges  out  when  the  base  of  the  stapes 
is  pushed  in,  and  vice  versa;  and  so  allows  of  waves  being  set  up  in 
the  labyrinthic  fluid.  These  correspond  in  period  and  form  to 
those  in  the  auditory  meatus;  their  amplitude  is  determined  by  the 
extent  of  the  vibrations  of  the  drum  membrane. 

The  form  of  the  tympanic  membrane  causes  it  to  transmit  to  its 
center,  where  the  malleus  is  attached,  vibrations  of  its  lateral 
parts  in  diminished  amplitude  but  increased  power;  so  that  the 
tympanic  bones  are  pushed  only  a  little  way  but  with  considerable 
force.  Its  area,  too,  is  about  twenty  times  as  great  as  that  of  the 
oval  foramen,  so  that  force  collected  on  the  large  area  is,  by  push- 
ing the  tympanic  bones,  all  concentrated  on  the  smaller.  The 
ossicles  also  form  a  bent  lever  (Fig.  70)  of  which  the  fulcrum  is  at 
the  axial  ligament  and  the  effective  outer  arm  of  this  lever,  is  about 
half  as  Jong  again  as  the  inner,  and  so  the  movements  transmitted 


THE  EAR,  HEARING,  TASTE  AND  SMELL 


231 


by  the  drum  membrane  to  the  handle  of  the  malleus  are  com- 
municated with  diminished  range,  but  increased  power,  to  the  base 
of  the  stapes. 

Ordinarily  sound-waves  reach  the  labyrinth  through  the  tym- 
panum, but  they  may  also  be  transmitted  through  the  bones  of  the 
head;  if  the  handle  of  a  vibrating  tuning-fork  be  placed  on  the  top 
of  the  head,  the  sounds  heard  by  the  person  experimented  upon 
seem  to  have  their  origin  inside  his  own  cranium.  Similarly  when  a 
vibrating  body  is  held  between  the  teeth,  sound  reaches  the  end 
organs  of  the  auditory  nerve  through  the  skull-bones;  and  persons 
who  are  deaf  from  disease  or  injury  of  the  tympanum  can  thus  be 
made  to  hear,  as  with  the  audiphone.  Of  course  if  deafness  be  due 

A 


Co 


FIG.  71. — Casts  of  the  bony  labyrinth.  A,  left  labyrinth  seen  from  the  outer 
side;  B,  right  labyrinth  from  the  inner  side;  C,  left  labyrinth  from  above;  Fc,  round 
foramen;  Fv,  oval  foramen;  h,  horizontal  semicircular  canal;  ha,  its  ampulla; 
Vaa,  ampulla  of  anterior  vertical  semicircular  canal;  Vpa,  ampulla  of  posterior 
vertical  semicircular  canal;  Vc,  conjoined  portion  of  the  two  vertical  canals. 

to  disease  of  the  proper  nervous  auditory  apparatus  no  device  can 
make  the  person  hear. 

The  Internal  Ear.  The  labyrinth  consists  primarily  of  cham- 
bers and  tubes  hollowed  out  in  the  temporal  bone  and  inclosed  by 
it  on  all  sides,  except  for  the  oval  and  round  foramina  on  its  ex- 
terior, and  certain  apertures  on  its  inner  side  by  which  blood- 
vessels and  branches  of  the  auditory  nerve  enter;  during  life  all 
these  are  closed  water-tight  in  one  way  or  another.  Lying  in  the 
bony  labyrinth  thus  constituted,  are  membranous  parts,  of  the 
same  general  form  but  smaller,  so  that  between  the  two  a  space  is 
left;  this  is  filled  with  a  watery  fluid,  called  the  perilymph;  and  the 
membranous  internal  ear  is  filled  by  a  similar  liquid,  the  endolymph. 

The  Bony  Labyrinth.  The  bony  labyrinth  is  described  in  three 
portions,  the  vestibule,  the  semicircular  canals,  and  the  cochlea; 


232  THE  HUMAN  BODY 

casts  of  its  interior  are  represented  from  different  aspects  in  Fig.  71. 
The  vestibule  is  the  central  part  and  has  on  its  exterior  the  oval 
foramen  (Fv)  into  which  the  base  of  the  stirrup-bone  fits.  Behind 
the  vestibule  are  three  bony  semicircular  canals,  communicating 
with  the  back  of  the  vestibule  at  each  end,  and  dilated  near  one 
end  to  form  an  ampulla  (Vpa,  Vaa,  and  ha).  The  horizontal 
canal  lies  in  the  plane  which  its  name  implies,  and  has  its  am- 
pulla at  the  front  end.  The  two  other  canals  lie  vertically,  the 
anterior  at  right  angles,  and  the  posterior  parallel,  to  the  median 
anteroposterior  vertical  plane  of  the  head.  Their  ampullary  ends 
are  turned  forwards  and  open  close  together  into  the  vestibule; 
their  posterior  ends  unite  (Vc)  and  have  a  common  vestibular 
opening. 

The  bony  cochlea  is  a  tube  coiled  on  itself  somewhat  like  a  snail's 
shell,  and  lying  in  front  of  the  vestibule. 

The  Membranous  Labyrinth.  The  membranous  vestibule, 
lying  in  the  bony  one,  consists  of  two  sacs  communicating  by  a 
narrow  aperture.  The  posterior  is  called  the  utriculus,  and  into  it 
the  membranous  semicircular  canals  open.  The  anterior,  called  the 
sacculus,  communicates  by  a 
tube  with  the  membranous 
cochlea.  The  membranous  sem- 
icircular canals  much  resemble 
the  bony,  and  each  has  an  am- 
pulla; in  most  of  their  extent 
they  are  only  united  by  a  few  ir- 
regular connective-tissue  bands 
with  the  periosteum  lining  the 
bony  canals;  but  in  the  ampulla 

One  side  of  the  membranous  tube         FIG.  72. — A  section  through  the  cochlea 
,        ,  . ,      ,  in  the  line  of  its  axis. 

js  closely  adherent  to  its  bony 

protector;  at  this  point  nerves  enter  the  former.  The  relations 
of  the  membranous  to  the  bony  cochlea  are  more  complicated. 
A  section  through  this  part  of  the  auditory  apparatus  (Fig.  72) 
shows  that  its  osseous  portion  consists  of  a  tube  wound  two  and 
a  half  times  (from  left  to  right  in  the  right  ear  and  vice  versa)  around 
a  central  bony  axis,  the  modiolus.  From  the  axis  a  shelf,  the 
lamina  spiralis,  projects  and  partially  subdivides  the  tube, 
tending  farthest  across  in  its  lower  coils.  Attached  to  the  ou1 


THE  EAR,  HEARING,  TASTE  AND  SMELL  233 

edge  of  this  bony  plate  is  the  membranous  cochlea  (scala  media), 
a  tube  triangular  in  cross-section  and  attached  by  its  base  to  the 
outer  side  of  the  bony  cochlear  spiral.  The  spiral  lamina  and  the 
membranous  cochlea  thus  subdivide  the  cavity  of  the  bony  tube 
(Fig.  73)  into  an  upper  portion,  the  scala  vestibuli,  SV,  and  a  lower, 
the  scala  tympani,  ST.  Between  these  lie  the  lamina  spiralis  (Iso) 
and  the  membranous  cochlea  (CC),  the  latter  being  bounded 
above  by  the  membrane  of  Reissner  (R)  and  below  by  the  basilar 
membrane  (6).  The  free  edge  of  the  lamina  spiralis  is  thickened 
and  covered  with  connective  tissue  which  is  hollowed  out  so  as  to 
form  a  spiral  groove  (the  sulcus  spiralis,  ss)  along  the  whole  length 
of  the  membranous  cochlea.  The  latter  does  not  extend  to  the 


FIG.  73. — Section  of  one  coil  of  the  cochlea,  magnified.  SV,  scala  vestibuli; 
R,  membrane  of  Reissner;  CC,  membranous  cochlea  (scala  media);  Us,  limbus 
lamince  spiralis;  t,  tectorial  membrane;  ST,  scala  tympani;  Iso,  spiral  lamina; 
Co,  rods  of  Corti;  b,  basilar  membrane. 

tip  of  the  bony  cochlea;  above  its  apex  the  scala  vestibuli  and 
scala  tympani  join;  both  are  filled  with  perilymph,  and  the  former 
communicates  below  with  the  perilymph  cavity  of  the  vestibule, 
while  the  scala  tympani  abuts  below  on  the  round  foramen,  which, 
as  has  already  been  pointed  out,  is  closed  by  a  membrane.  The 
membranous  cochlea  contains  certain  solid  structures  seated  on 
the  basilar  membrane  and  forming  the  organ  of  Corti;  the  rest  of 
its  cavity  is  filled  with  endolymph,  which  has  free  passage  to  that 
in  the  sacculus. 

The  Organ  of  Corti.  This  contains  the  end  organs  of  the  coch- 
lear nerves.  Lining  the  sulcus  spiralis  are  cuboidal  cells;  on  the 
inner  margin  of  the  basilar  membrane  the  cells  become  columnar, 
and  then  are  succeeded  by  a  row  which  bear  on  their  upper  ends  a 


234  THE  HUMAN  BODY 

set  of  short  stiff  hairs,  and  constitute  the  inner  hair-cells,  which  are 
fixed  below  by  a  narrow  apex  to  the  basilar  membrane;  nerve-fibers 
enter  them.  To  the  inner  hair-cells  succeed  the  rods  of  Corti  (Co, 
Fig.  73),  which  are  represented  much  magnified  in  Fig.  74.  These 
rods  are  stiff  and  arranged  side  by  side  in  two  rows,  leaned  against 
one  another  by  their  upper  ends  so  as  to  cover  in  a  tunnel;  they  are 
known  respectively  as  the  inner  and  outer  rods,  the  former  being 
nearer  the  lamina  spiralis.  Each  has  a  somewhat  dilated  base, 
firmly  fixed  to  the  basilar  membrane;  an  expanded  head  where  it 
meets  its  fellow  (the  inner  rod  presenting  there  a  concavity  into 
which  the  rounded  head  of  the  outer  fits);  and  a  slender  shaft 
uniting  the  two,  slightly  curved  like  an  italic  /.  The  inner  rods  arc 
more  slender  and  more  numerous  than  the  outer,  the  numbers  be- 


** 


FIG.  74.— The  rods  of  Corti.  A,  a  pair  of  rods  separated  from  the  rest;  B,  a  bit 
of  the  basilar  membrane  with  several  rods  on  it,  showing  how  they  cover  in  the 
tunnel  of  Corti;  i,  inner,  and  e,  outer  rods;  b,  basilar  membrane;  r,  reticular  mem- 
brane. 

ing  about  6,000  and  4,500  respectively.  Attached  to  the  external 
sides  of  the  head  of  the  outer  rods  is  the  reticular  membrane  (r, 
Fig.  74),  which  is  stiff  and  perforated  by  holes.  External  to  the 
outer  rods  come  four  rows  of  outer  hair-cells,  connected  like  the 
inner  row  with  nerve-fibers;  their  bristles  project  into  the  holes  of 
the  reticular  membrane.  Beyond  the  outer  hair-cells  is  ordinary 
columnar  epithelium,  which  passes  gradually  into  cuboidal  cells 
lining  most  of  the  membranous  cochlea.  The  upper  lip  of  the 
sulcus  spiralis  is  uncovered  by  epithelium,  and  is  known  as  the 
linibus  lamince  spiralis;  from  it  projects  the  tectorial  membrane 
(t,  Fig.  73)  which  extends  over  the  rods  of  Corti  and  the  hair-cells. 
Function  of  the  Cochlea.  We  have  already  seen  reason  to  be- 
lieve that  in  the  ear  there  is  an  apparatus  adapted  for  sympathetic 
resonance,  by  which  we  recognize  different  musical  tone  colors;  the 


THE  EAR,  HEARING,  TASTE  AND  SMELL  235 

minute  structure  of  the  membranous  cochlea  is  such  as  to  lead  us  to 
look  for  it  there.  Of  the  various  structures  making  up  the  mem- 
branous cochlea  the  basilar  membrane  seems  to  satisfy  best  the 
requirements  of  an  apparatus  for  registering  sounds  by  sympa- 
thetic resonance.  It  increases  in  breadth  twelve  times  from  the 
base  of  the  cochlea  to  its  tip  (the  less  width  of  the  lamina  spiralis 
at  the  apex  more  than  compensating  for  the  less  size  of  the  bony 
tube  there).  Careful  histological  examination  has  shown  that  in- 
stead of  being  a  true  membrane  it  is  really  made  up  of  a  large 
number  of  transverse  strands  tightly  stretched,  and  varying  in 
length  as  the  space  between  the  lamina  spiralis  and  the  wall  of  the 
bony  cochlea  varies. 

Probably  each  strand  vibrates  to  simple  tones  of  its  own  period, 
and  excites  the  hair-cells  which  lie  on  it,  and  through  them  the 
nerve-fibers.  Perhaps  the  rods  of  Corti,  being  stiff,  and  carrying 
the  reticular  membrane,  rub  that  against  the  upper  ends  of  the 
hair-cells  which  project  into  its  apertures  and  so  help  in  a  sub- 
sidiary way,  each  pair  of  rods  being  especially  moved  when  the 
band  of  basilar  membrane  carrying  it  is  set  in  vibration.  The 
tectorial  membrane  is  probably  a  " damper";  it  is  soft  and  in- 
elastic, and  suppresses  the  vibrations  as  soon  as  the  moving  force 
ceases. 

According  to  various  estimates  that  have  been  made,  from  six 
thousand  to  eleven  thousand  different  tones  can  be  distinguished 
in  the  whole  range  of  the  ear.  The  basilar  membrane  is  more  than 
adequate  to  distinguish  this  number  as  it  consists  of  twenty-four 
thousand  strands.  Fourteen  thousand  nerve-fibers  communicate 
with  the  hair-cells  of  the  organ  of  Corti. 

We  must  suppose  that  compound  tones  entering  the  ear  set  the 
fluids  of  the  cochlea  into  vibrations  whose  form  depends  upon  the 
make-up  of  the  tone  producing  them.  These  vibrations  are 
analyzed  by  the  basilar  membrane,  the  particular  strands  having 
the  vibration  rates  of  the  fundamental  and  the  partials  which  are 
present  being  set  into  sympathetic  vibration  and  stimulating  the 
nerve-fibers  with  which  they  communicate. 

Auditory  Perceptions.  Sounds,  as  a  general  rule,  do  not  seem 
to.  us  to  originate  within  the  auditory  apparatus;  we  refer  them  to 
an  external  source,  and  to  a  certain  extent  can  judge  the  distance 
and  direction  of  this.  As  already  mentioned,  the  extrinsic  reference 


236 


THE  HUMAN  BODY 


of  sounds  which  reach  the  labyrinth  through  the  general  skull- 
bones  instead  of  through  the  tympanic  chain  is  imperfect  or 
absent.  The  recognition  of  the  distance  of  a  sounding  body  is  pos- 
sible only  when  the  sound  is  well  known,  and  then  not  very  accu- 
rately; from  its  faintness  or  loudness  we  may  make  in  some  cases  a 
pretty  good  guess.  Judgments  as  to  the  direction  of  a  sound  are 
also  liable  to  be  grossly  wrong,  as  most  persons  have  experienced. 
However,  when  a  sound  is  heard  louder  by  the  left  than  the  right 
ear  we  can  recognize  that  its  source  is  on  the  left;  when  equally 
with  both  ears,  that  it  is  straight  in  front  or  behind;  and  so  on. 

The  concha  has  perhaps  something 
to  do  with  enabling  us  to  detect 
whether  a  sound  originates  before 
or  behind  the  ear,  since  it  col- 
lects, and  turns  with  more  in- 
tensity into  the  external  auditory 
meatus,  sound-waves  coming  from 
the  front.  By  turning  the  head 
and  noting  the  accompanying 
changes  of  sensation  in  each  ear 
we  can  localize  sounds  better  than 
if  the  head  be  kept  motionless. 
The  large  movable  concha  of  many 
animals,  as  a  rabbit  or  a  horse, 
which  can  be  turned  in  several 
directions,  is  probably  an  important  aid  to  them  in  detecting  the 
position  of  the  source  of  a  sound.  That  the  recognition  of  the 
direction  of  sounds  is  not  a  true  sensation,  but  a  judgment, 
founded  on  experience,  is  illustrated  by  the  fact  that  we  can 
estimate  much  more  accurately  the  direction  of  the  human 
voice,  which  we  hear  and  heed  most,  than  that  of  any  other 
sound. 

Nerve-Endings  in  the  Semicircular  Canals  and  the  Vestibule. 
Myelinated  fibers  (/,  Fig.  75)  from  the  vestibular  branch  of  the 
auditory  nerve  are  distributed  along  a  line  across  the  ampulla  of 
each  semicircular  canal.  They  lose  their  myelin  sheath  close 
to  the  basement  membrane,  a,  which  the  axons  pierce.  The 
axons  branch  among  the  epithelium  cells,  which  at  this  place  are 
several  rows  thick,  but  have  not  yet  been  traced  into  direct  con- 


FIG.  75. — Diagram  of  epithelium 
in  nervous  region  of  ampulla  of  a 
semicircular  canal. 


THE  EAR,  HEARING,  TASTE  AND  SMELL  237 

tinuity  with  any  of  them.  The  cells  of  the  epithelium  are  of  two 
varieties.  The  columnar  cells  or  hair-cells,  c,  do  not  reach  the 
basement  membrane,  are  nucleated  or  slightly  granular :  from  the 
free  end  of  each  projects  a  rigid  hair  process,  d.  The  remaining 
cells,  rod-cells,  b,  are  in  several  rows:  each  has  a  slender  inner 
process  extending  to  the  basement  membrane  and  an  outer 
which  reaches  to  the  bases  of  the  columnar  cells  and  appears 
there  to  end  in  a  rigid  membrane,  e,  which  is  perforated  for  the 
passage  of  the  hairs.  They  probably  are-  mere  supporting 
structures. 

In  some  parts  of  the  utricle  and  saccule  are  regions  of  epithelium 
very  similar  to  that  above  described,  and  also  supplied  with  nerve- 
fibers.  In  connection  with  them  are  found  minute  calcareous 
particles, — otoliths  or  ear-stones. 

The  Equilibrium  Sense.  An  important  group  of  afferent  im- 
pulses concerned  with  the  maintenance  of  bodily  equilibrium  is 
derived  through  the  semicircular  canals  and  vestibule  of  the  ear, 
which  are  supplied  by  the  vestibular  portion  of  the  auditory 
nerve. 

Experiment  shows  that  cutting  a  semicircular  canal  is  followed 
by  violent  movements  of  the  head  in  the  plane  of  the  canal  di- 
vided; the  animal  staggers,  also,  if  made  to  walk;  and,  if  a  pigeon 
and  thrown  into  the  air,  cannot  fly.  All  its  muscles  can  contract 
as  before,  but  they  are  no  longer  so  co-ordinated  as  to  enable  the 
animal  to  maintain  or  regain  a  position  of  equilibrium.  It  is  like 
a  creature  suffering  from  giddiness;  and  similar  phenomena  fol- 
low, in  man,  electrical  stimulation  of  the  regions  of  the  skull  in 
which  the  semicircular  canals  lie. 

If,  moreover,  a  person  lie  perfectly  quiet  with  closed  eyes  OR 
a  table  which  can  be  rotated,  he  is  able  to  tell  when  the  table  is 
turned  and  in  which  direction,  and  often  with  considerable  ac- 
curacy through  what  angle.  If  the  rotation  be  continued  for  a 
time  the  feeling  of  it  is  lost,  and  then  when  the  movement  ceases 
there  is  a  sense  of  rotation  in  the  opposite  direction.  In  such 
case  neither  tactile,  muscular,  nor  visual  sensations  can  help,  and 
in  the  semicircular  canals  we  seem  to  have  a  mechanism  through 
which  rotation  of  the  head  could  give  origin  to  afferent  impulses, 
whether  the  head  be  passively  moved  with  the  rest  of  the  Body 
or  independently  by  its  own  muscles.  Movements  of  endolymph 


238  THE  HUMAN  BODY 

in  relation  to  the  walls  of  the  canals  may  act  as  stimuli  by  caus- 
ing a  swaying  of  the  projecting  hairs  of  the  ampullae  (Fig.  75). 
Place  a  few  small  bits  of  cork  in  a  tumbler  of  water,  and  rotate 
the  tumbler;  at  first  the  water  does  not  move  with  it;  then  it  be- 
gins to  go  in  the  same  direction,  but  more  slowly;  and,  finally, 
moves  at  the  same  angular  velocity  as  the  tumbler.  Then  stop 
the  tumbler,  and  the  water  will  go  on  rotating  for  some  time. 
Now  if  the  head  be  turned  or  rotated  in  a  horizontal  plane  simi- 
lar phenomena  will  occur  in  the  endolymph  of  the  horizontal 
canal;  if  it  be  bent  sidewise  in  the  vertical  plane,  in  the  ante- 
rior vertical  canal;  and  if  nodded,  in  the  posterior  vertical;  the 
hairs  moving  with  the  canal  would  meet  the  more  stationary 
water  and  be  pushed  and  so,  possibly,  excite  the  nerves  at  the 
deep  ends  of  the  cells  which  bear  them,  and  generate  afferent 
impulses  which  will  cause  the  general  nerve-centers  of  bodily 
equilibration  to  be  differently  acted  upon  in  each  case.  Under 
ordinary  circumstances  the  results  of  these  impulses  do  not  be- 
come prominent  in  consciousness  as  definite  sensations;  but  they 
are  probably  always  present.  If  one  spins  round  for  a  time,  the 
endolymph  takes  up  the  movement  of  the  canals,  as  the  water  in 
the  tumbler  does  that  of  the  glass;  on  stopping,  the  liquid  still 
goes  on  moving  and  stimulates  the  hairs  which  are  now  stationary; 
and  we  feel  giddy,  from  the  ears  telling  us  we  are  rotating  and  the 
eyes  that  we  are  not;  hence  difficulty  in  standing  erect  or  walking 
straight.  A  common  trick  illustrates  this  very  well:  make  a  per- 
son place  his  forehead  on  the  handle  of  an  umbrella,  the  other 
end  of  which  is  on  the  floor,  and  then  walk  three  or  four  times 
round  it,  rise,  and  try  to  go  out  of  a  door;  he  will  nearly  always 
fail,  being  unable  to  combine  his  muscles  properly  on  account  of 
the  conflicting  afferent  impulses.  This  and  the  feeling  of  rotation 
in  the  contrary  direction  when  a  previous  rotation  ceases  become 
readily  intelligible  if  we  suppose  feelings  to  be  excited  by  relative 
movements  of  the  endolymph  and  the  canals  inclosing  it. 

The  sense  of  equilibrium  as  mediated  by  the  semicircular  canals 
is  a  dynamic  sense,  one  dealing  with  equilibrium  of  motion.  That 
we  have  also  a  static  sense  of  equilibrium,  which  tells  us  our  posi- 
tion when  at  rest  is  well  known.  The  swimmer  immersed  in 
water  knows  perfectly  whether  he  is  on  his  face  or  on  his  back; 
whether  his  head  is  up  or  down.  This  static  equilibrium  sense  is 


THE  EAR,  HEARING,  TASTE  AND  SMELL 


239 


thought  to  be  mediated  by  structures  of  the  vestibule,  the  utricle 

and  saccule.     These  are  hollow  structures  having  stiff  hairs  pro- 

jecting into  their  cavities  and  tiny  stones  caught  among  the  hairs. 

The  weight  of  the  stones  will  affect  the  hairs  among  which  it  rests 

in  one  way  when  the  head  is  erect,  in  quite  another  way  when 

the    head    is    horizontal.     Thus    the 

nerves  may  be  stimulated  differently 

for  different  positions  of  the  head,  ful- 

filling the  conditions  that  the  sense  re- 

quires.   In  many  invertebrate  animals 

structures  similar  to  the  utricle  and 

saccule  represent  their  only  organs  re- 

sembling our  ears  in  any  way.    Experi- 

ments upon  these  animals  have  shown 

that  in  them  these  .structures  are  not 

hearing  organs  but  organs  of  equilib- 

rium. 

Smell.  The  region  of  the  nostril 
•nearest  its  outer  end  possesses  the 
sense  of  touch:  the  olfactory  organ 
proper  consists  of  the  upper  portions 
of  the  two  nasal  cavities,  over  which 
the  endings  of  the  olfactory  nerves 
are  spread  and  where  the  mucous 
membrane  has  a  brownish-yellow  color. 
This  region  (regio  olfactoria)  covers 
the  upper  and  lower  turbinate  bones, 

Which  are  expansions  of  the  ethmoid  On 

„      ..   .  ,  ,Mi          i 

the  outer  wall  of  the  nostril  chamber, 
the  opposite  part  of  the  partition  between  the  nares,  and  the  part 
of  the  roof  of  the  nose  separating  it  from  the  cranial  cavity.  The 
epithelium  covering  the  mucous  membrane  contains  three  varieties 
of  cells  (2,  Fig.  76).  The  cells  of  one  set  are  much  like  ordinary 
columnar  epithelium,  but  with  long  branched  processes  attached  to 
their  deeper  ends;  mixed  with  these  are  peculiar  cells,  each  of  which 
has  a  large  nucleus  surrounded  by  a  little  protoplasm;  a  slender 
external  process  reaching  to  the  surface;  and  a  very  slender  deep 
one.  The  latter  cells  have  been  supposed  to  be  the  proper  olfac- 
tory end  organs,  and  to  be  connected  with  the  fibers  of  the  ol- 


FIG.  76. — Cells  from  the  ol- 
factory epithelium.  1,  from  the 
frog;  2,  from  man;  a,  columnar 
cell,  with  its  branched  deep 
process;  6,  so-called  olfactory 
cell;  c,  its  narrow  outer  process; 


tory   nerve,   seen  dividing  into 
fine  peripheral  branches  at  a. 


240  THE  HUMAN  BODY 

factory  nerve,  which  enter  the  deeper  strata  of  the  epithelium 
and  there  divide.  In  Amphibia  the  corresponding  cells  have  fine 
filaments  on  their  free  ends.  The  cells  of  the  third  kind  are  irreg- 
ular in  form  and  lie  in  several  rows  in  the  deeper  parts  of  the 
epithelium.  It  may  be  that  the  cylindrical  cells  if  not  (as  is 
possible)  directly  concerned  in  olfaction,  have  important  functions 
in  regard  to  the  nourishment  of  the  olfactory  cells  which  they 
surround;  they  may  supply  them  with  needful  material. 

Odorous  substances,  the  stimuli  of  the  olfactory  apparatus,  are 
always  gaseous  and  frequently  act  powerfully  when  present  in  very 
small  amount.  We  cannot,  however,  classify  them  by  the  sensa- 
tions they  arouse,  or  arrange  them  in  series;  and  smells  are  but 
minor  sensory  factors  in  our  mental  life,  although  very  powerful 
associations  of  memory  are  often  aroused  by  odors.  We  com- 
monly refer  them  to  external  objects,  since  we  find  that  the  sen- 
sation is  intensified  by  "sniffing"  air  into  the  nose,  and  ceases 
when  the  nostrils  are  closed.  Their  peripheral  localization  is, 
however,  imperfect,  for  we  confound  many  smells  with  tastes  (see 
below);  nor  can  we  well  judge  of  the  direction  of  an  odorous 
body  through  the  olfactory  sensations  which  it  arouses. 

Although  the  sense  of  smell  in  man  is  aroused  by  inconceivably 
small  amounts  of  odoriferous  substance,  one  part  of  mercaptan 
to  thirty  billion  of  air  being  detectible,  it  is  much 'less  keen  than 
the  sense  of  smell  in  many  animals,  canines  in  particular.  In 
such  animals  the  sense  of  smell  as  a  source  of  information  seems 
to  be  of  the  first  importance,  approaching  our  eyes  in  rank. 

A  striking  thing  about  the  sense  of  smell  is  the  ease  with  which 
it  is  fatigued.  One  may  notice  a  bad  odor  upon  entering  a  room, 
but  in  a  few  minutes  ceases  to  perceive  it  because  his  olfactory 
apparatus  has  become  fatigued.  For  this  reason  the  sense  of 
smell  is  wholly  untrustworthy  as  a  guide  by  which  to  regulate 
the  ventilation  of  a  room. 

Taste.  The  organ  of  taste  is  the  mucous  membrane  on  the 
dorsum  of  the  tongue  *  and,  in  some  persons,  of  the  soft  palate 
and  fauces.  The  nerves  concerned  are  the  glossopharyngeals, 
distributed  over  the  hind  part  of  the  tongue,  and  the  lingual 
branches  of  the  inferior  maxillary  division  of  the  trigeminals  on 
its  anterior  two-thirds.  It  has  been  shown  that  the  nerves  of 
*  A  description  of  the  tongue  will  be  found  on  page  446. 


THE  EAR,  HEARING,  TASTE  AND  SMELL  241 

taste  which  reach  the  tongue  by  way  of  the  trigeminal  nerve  spring 
from  the  medulla  as  part  of  the  sensory  branch  of  the  facial. 

On  the  tongue  most  of  the  sensory  nerves  run  to  papillae;  the 
circumvallate  have  the  richest  supply,  and  on  these  are  peculiar 
end  organs  (Fig.  77)  known  as  taste-buds;  they  are  oval  and  em- 
bedded in  the  epidermis  covering  the  side  of  the  papilla.  Each 
consists,  externally,  of  a  number  of  flat,  fusiform,  nucleated  cells 
and,  internally,  of  six  or  eight  so-called  taste-cells.  The  latter  are 
much  like  the  olfactory  cells  of  the  nose,  and  are  probably  con- 
nected with  nerve-fibers  at  their  deeper  ends.  The  capsule  formed 
by  the  enveloping  cells  has  a  small  opening  on  the  surface;  each 
taste-cell  terminates  in  a  very  fine  thread  which  there  protrudes. 
Taste-buds  are  also  found  on  some  of  the  fungiform  papillae,  and 


FIG.  77. — Taste-buds. 

it  is  possible  that  simpler  structures,  not  yet  recognized,  and  con- 
sisting of  single  taste-cells  are  widely  spread  over  the  tongue, 
since  the  sense  of  taste  exists  where  no  taste-buds  can  be  found. 
The  filiform  papillae  are  probably  tactile. 

In  order  for  substances  to  be  tasted  they  must  be  in  solution: 
wipe  the  tongue  dry  and  put  a  crystal  of  sugar  on  it;  no  taste  will 
be  felt  until  exuding  moisture  has  dissolved  some  of  the  crystal. 
Excluding  the  feelings  aroused  by  acid  substances,  tastes  proper 
may  be  divided  into  sweet,  bitter,  acid,  and  saline.  Although  con- 
tributing much  to  the  pleasures  of  life,  they  are  intellectually  of 
small  value;  the  perceptions  we  attain  through  them  as  to  quali- 
ties of  external  objects  being  of  little  use,  except  as  aiding  in  the 
selection  of  food,  and  for  that  purpose  they  are  not  safe  guides  at 
all  times. 

Many  so-called  tastes  (flavors)  are  really  smells;  odoriferous 


242  THE  HUMAN  BODY 

particles  of  substances  which  are  being  eaten  reach  the  olfactory 
region  through  the  posterior  nares  and  arouse  sensations  which, 
since  they  accompany  the  presence  of  objects  in  the  mouth,  we 
take  for  tastes.  Such  is  the  case,  e.  g.,  with  most  spices;  when  the 
nasal  chambers  are  blocked  or  inflamed  by  a  cold  in  the  head,  or 
closed  by  compressing  the  nose,  the  so-called  taste  of  spices  is  not 
perceived  when  they  are  eaten;  all  that  is  felt,  when  cinnamon, 
e.  g.,  is  chewed  under  such  circumstances  is  a  certain  pungency 
due  to  its  stimulating  nerves  of  touch  in  the  tongue.  This  fact 
is  sometimes  taken  advantage  of  in  the  practice  of  domestic 
medicine  when  a  nauseous  dose,  as  rhubarb,  is  to  be  given  to  a 
child. 

As  the  tongue,  in  addition  to  taste  functions,  possesses  tactile 
and  temperature  sensibility,  its  nerve  apparatus  must  be  complex; 
and  there  is  even  reason  to  believe  that  different  nerve-fibers 
with  presumably  different  end  organs  are  concerned  in  the  differ- 
ent true  tastes.  Most  persons  taste  bitter  things  better  with  the 
back  part  of  the  tongue  and  sweet  things  with  the  tip,  and  in 
some  persons  the  separation  of  function  is  quite  complete.  Chem- 
ical compounds  are  known  which  in  such  persons  cause  a  pure 
sweet  sensation  if  placed  on  the  tongue  tip  and  a  pure  bitter  sen- 
sation if  placed  in  the  region  of  the  circumvallate  papillae;  these 
facts  seem  to  show  that  the  fibers  concerned  in  bitter  and  sweet 
sensation  are  distinct.  Again,  if  leaves  of  a  certain  plant  (Gym- 
nema  sylvestre)  be  chewed,  the  capacity  to  taste  sweet  or  bitter 
things  is  lost  for  some  time,  but  salts  and  acids  are  tasted  as  well 
as  usual;  and  most  persons  taste  salines  better  at  the  sides  of  the 
tongue  than  elsewhere;  so  that  the  salt  and  acid  sensations  seem 
to  have  a  different  apparatus,  not  only  from  the  sweet  and  bitter, 
but  from  one  another. 


CHAPTER  XV 
THE  EYE  AS  AN  OPTICAL  INSTRUMENT 

The  Essential  Structure  of  an  Eye.  Every  visual  organ  con- 
sists primarily  of  a  nervous  expansion,  provided  with  end  organs 
by  means  of  which  light  is  enabled  to  excite  nervous  impulses, 
and  exposed  to  the  access  of  objective  light;  such  an  expansion  is 
called  a  retina.  By  itself,  however,  a  retina  would  give  no  visual 
sensations  referable  to  distinctly  limited  external  objects;  it 
would  enable  its  possessor  to  tell  light  from  darkness,  more  light 
from  less  light,  and  (at  least  in  its  highly  developed  forms)  light 
of  one  color  from  light  of  another  color;  but  that  would  be  all. 
Were  our  eyes  merely  retinas  we  could  only  tell  a  printed  page 
from  a  blank  one  by  the  fact  that,  being  partly  covered  with 
black  letters  (which  reflect  less  light),  it  would  excite  our  visual 
organ  less  powerfully  than  the  spotless  white  page  would.  In 
order  that  distinct  objects  and  not  merely  degrees  of  luminosity 
may  be  seen,  some  arrangement  is  needed  which  shall  bring  all 
light  entering  the  eye  from  one  point  of  a  luminous  surface  to  a 
focus  again  on  one  point  of  the  sensitive  surface.  If  A  and  B 
(Fig.  78)  be  two  red  spots  on  a  black  surface,  K,  and  rr  be  a  ret- 
ina, then  rays  of  light  diverging  from  A  would  fall  equally  on  all 
parts  of  the  retina  and  excite  it  all  a  little;  so  with  rays  starting 
from  B.  The  sensation  aroused,  supposing  the  retina  in  connec- 
tion with  the  rest  of  the  nervous  visual  apparatus,  would  be  one 
of  a  certain  amount  of  red  light  reaching  the  eye;  the  red  spots, 
as  definite  objects,  would  be  indistinguishable.  If,  however,  a 
convex  glass  lens  L  (Fig.  79)  be  put  in  front  of  the  retina,  it  will 
cause  to  converge  again  to  a  single  point  all  the  rays  from  A  fall- 
ing upon  it;  so,  too,  with  the  rays  from  B:  and  if  the  focal  distance 
of  the  lens  be  properly  adjusted  these  points  of  convergence  will 
both  lie  on  the  retina,  that  for  rays  from  A  at  a,  and  that  for  rays 
from  B  at  b.  The  sensitive  surface  would  then  only  be  excited  at 
two  limited  and  separated  points  by  the  red  light  emanating  from 
the  spots;  consequently  only  some  of  its  end  organs  and  nerve- 

243 


244 


THE  HUMAN  BODY 


fibers  would  be  stimulated  and  the  result  would  be  the  recognition 
of  two  separate  red  objects.    In  our  eyes  there  are  certain  refract- 


FIG.  78. — Diagram  illustrating  the  indistinctness  of  vision  with  a  retina  alone. 
K,  a  surface  on  which  are  two  spots,  A  and  B;  r  r,  the  retina.  The  diverging  lines 
represent  rays  of  light  spread  uniformly  over  the  retina  from  each  spot. 

ing  media  which  lie  in  front  of  the  retina  and  take  the  place  of  the 
lens  L  in  Fig.  79.     That  portion  of  physiology  which  treats  of  the 


FIG.  79. — Illustrating  the  use  of  a  lens  in  giving  definite  retinal  images.  A,  B,  K, 
r  r,  as  in  Fig.  78.  L,  a  biconvex  lens  so  placed  that  it  brings  to  a  focus  on  the 
points  a  and  6  of  the  retina,  rays  of  light  diverging  from  A  and  B  respectively. 

physical  action  of  these  media  or,  in  other  words,  of  the  eye  as  an 
optical  instrument,  is  known  as  the  dioptrics  of  the  eye. 

The  Appendages  of  the  Eye.  The  eyeball  itself  consists  of  the 
retina  and  refracting  media,  together  with  supporting  and  nutri- 
tive structures  and  other  accessory  apparatuses,  as,  for  example, 
some  controlling  the  light-converging  power  of  the  media,  and 
others  regulating  the  size  of  the  aperture  (pupil)  by  which  light 
enters.  Outside  the  ball  lie  muscles  which  bring  about  its  move- 
ments, and  other  parts  serving  to  protect  it. 

Each  orbit  is  a  pyramidal  cavity  occupied  by  connective  tissue, 
muscles,  blood-vessels,  and  nerves,  and  in  great  part  by  fat,  which 
forms  a  soft  cushion  on  which  the  back  of  the  eyeball  lies  and 
rolls  during  its  movements.  The  contents  of  the  orbit  being  for 
the  most  part  incompressible,  the  eye  cannot  be  drawn  into  its 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT  245 

socket.  It  simply  rotates  there,  as  the  head  of  the  femur  does  in 
the  acetabulum.  When  the  orbital  blood-vessels  are  gorged, 
however,  the  eyeball  may  protude  (as  in  strangulation) ;  and  when 
these  vessels  empty  it  recedes  somewhat,  as  is  commonly  seen 
after  death.  The  front  of  the  eye  is  exposed  for  the  purpose  of 
allowing  light  to  reach  it,  but  can  be  covered  up  by  the  eyelids, 
which  are  folds  of  integument,  movable  by  muscles  and  strength- 
ened by  plates  of  fibrocartilage.  At  the  edge  of  each  eyelid  the 
skin  which  covers  its  outside  is  turned  in,  and  becomes  continu- 
ous with  a  mucous  membrane,  the  conjunctiva,  which  lines  the 
inside  of  each  lid,  and  also  covers  all  the  front  of  the  eyeball  as  a 
closely  adherent  layer. 

The  upper  eyelid  is  larger  and  more  mobile  than  the  lower,  and 
when  the  eye  is  closed  covers  all  its  transparent  part.  It  has  a 
special  muscle  to  raise  it,  the  levator  palpebrce  superioris.  The 
eyes  are  closed  by  a  flat  circular  muscle,  the  orbicularis  palpebra- 
rum  which,  lying  on  and  around  the  lids,  immediately  beneath 
the  skin,  surrounds  the  aperture  between  them.  At  their  outer 
and  inner  angles  (canthi)  the  eyelids  are  united,  and  the  apparent 
size  of  the  eye  depends  upon  the  interval  between  the  canthi,  the 
eyeball  itself  being  nearly  of  the  same  size  in  -all  persons.  Near 
the  inner  canthus  the  line  of  the  edge  of  each  eyelid  changes  its 
direction  and  becomes  more  horizontal.  At  this  point  is  found  a 
small  eminence,  the  lachrymal  papilla,  on  each  lid.  For  most  of 
their  extent  the  inner  surfaces  of  the  eyelids  are  in  contact  with 
the  outside  of  the  eyeball,  but  near  their  inner  ends  a  red  vertical 
fold  of  conjunctiva,  the  semilunar  fold  (plica  semilunaris)  inter- 
venes. This  is  a  representative  of  the  third  eyelid,  or  nictitating 
membrane,  found  largely  developed  in  many  animals,  as  birds,  in 
which  it  can  be  drawn  all  over  the  exposed  part  of  the  eyeball. 
At  the  inner  or  nasal  corner  is  a  reddish  elevation,  the  caruncula 
lachrymalis,  caused  by  a  collection  of  sebaceous  glands  *  embedded 
in  the  semilunar  fold.  Opening  along  the  edge  of  each  eyelid  are 
from  twenty  to  thirty  minute  compound  sebaceous  glands,  named 
the  Meibomian  follicles.  Their  secretion  is  sometimes  abnormally 
abundant,  and  then  appears  as  a  yellowish  matter  along  the  edges 
of  the  eyelids,  which  often  dries  in  the  night  and  causes  the  lids 
to  be  glued  together  in  the  morning.  The  eyelashes  are  short 
*  For  a  description  of  the  glands  see  p.  535, 


246  THE  HUMAN  BODY 

curved  hairs,  arranged  in  one  or  two  rows  along  each  lid  where 
the  skin  joins  the  conjunctiva. 

The  Lachrymal  Apparatus  consists  of  the  tear-gland  in  each 
orbit,  the  ducts  which  carry  its  secretion  to  the  upper  eyelid, 
and  the  canals  by  which  the  tears,  unless  when  excessive,  are 
carried  off  from  the  front  of  the  eye  without  running  down  over 
the  face.  The  lachrymal  or  tear-gland,  about  the  size  of  an  almond, 
lies  in  the  upper  and  outer  part  of  the  orbit,  near  the  front  end. 
It  is  a  compound  racemose  gland,  (see  Chap.  XXIX)  from  which 
twelve  or  fourteen  ducts  run  and  open  in  a  row  at  the  outer  corner 
of  the  upper  eyelid.  The  secretion  there  poured  out,  is  spread 
evenly  over  the  exposed  part  of  the  eye  by  the  movements  of 
winking,  and  keeps  it  moist;  finally  the  tear  is  drained  off  by  two 
lachrymal  canals,  one  of  which  opens  by  a  small  pore  (punctum 
lachrymalis)  on  each  lachrymal  papilla.  The  aperture  of  the  lower 
canal  can  be  readily  seen  by  examining  the  corresponding  papilla 
by  the  aid  of  a  looking-glass.  The  canals  run  inwards  and  open 
into  the  lachrymal  sac,  which  lies  just  outside  the  nose,  in  a  hollow 
where  the  lachrymal  and  superior  maxillary  bones  (L  and  MX, 
Fig.  25)  meet.  From  the  sac  the  nasal  duct  proceeds  to  open  into 
the  nose-chamber,  below  the  inferior  turbinate  bone  and  within 
the  nostril. 

Tears  are  constantly  being  secreted,  but  ordinarily  in  such 
quantity  as  to  be  drained  off  into  the  nose,  from  which  they  flow 
into  the  pharynx  and  are  swallowed.  When  the  lachrymal  ducts 
are  stopped  up,  however,  their  continual  presence  makes  itself 
unpleasantly  felt,  and  may  need  the  aid  of  a  surgeon  to  clear  the 
passage.  In  weeping  the  secretion  is  increased,  and  then  not  only 
more  of  it  enters  the  nose,  but  some  flows  down  the  cheeks.  The 
frequent  swallowing  movements  of  a  crying  child,  sometimes 
spoken  of  as  "gulping  down  his  passion,"  are  due  to  the  need 
of  swallowing  the  extra  tears  which  reach  the  pharynx. 

The  Muscles  of  the  Eye  (Fig.  80).  The  eyeball  is  spheroidal 
in  form  and  attached  behind  to  the  optic  nerve,  n,  somewhat  as 
a  cherry  might  be  to  a  thick  stalk.  On  its  exterior  are  inserted 
the  tendons  of  six  muscles,  four  straight  and  two  oblique.  The 
straight  muscles  lie,  one  (superior  rectus),  s,  above,  one  (inferior 
rectus),  not  appearing  in  the  figure,  below,  one  (external  rectus), 
a,  outside,  and  one  (internal  rectus},  i,  inside  the  eyeball.  Each 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT  247 

arises  behind  from  the  bony  margin  of  the  foramen  through  which 
the  optic  nerve  enters  the  orbit.  In  the  figure,  which  represents 
the  orbits  opened  from  above,  the  superior  rectus  of  the  right 
side  has  been  removed.  The  superior  oblique  or  pulley  (trochlear) 
muscle,  t,  arises  behind  near  the  straight  muscles  and  forms  an- 
teriorly a  tendon,  u,  which  passes  through  a  fibrocartilaginous 
ring,  or  pulley,  placed  at  the  notch  in  the  frontal  bone  where  it 


FIG.  80. — The  eyeballs  and  their  muscles  as  seen  when  the  roof  of  the  orbit 
has  been  removed  and  the  fat  in  the  cavity  has  been  partly  cleared  away.  On  the 
right  side  the  superior  rectus  muscle  has  been  cut  away,  a,  external  rectus;  s,  su- 
perior rectus;  i,  internal  rectus;  t,  superior  oblique. 

bounds  superiorly  the  front  end  of  the  orbit.  The  tendon  then 
turns  back  and  is  inserted  into  the  eyeball  between  the  upper  and 
outer  recti  muscles.  The  inferior  oblique  muscle  does  not  arise, 
like  the  rest,  at  the  back  of  the  orbit,  but  near  its  front  at  the 
inner  side,  close  to  the  lachrymal  sac.  It  passes  thence  outwards 
and  backwards  beneath  the  eyeball  to  be  inserted  into  its  outer 
and  posterior  part. 

The  inner,  upper,  and  lower  straight  muscles,  the  inferior 
oblique,  and  the  elevator  of  the  upper  lid  are  supplied  by  branches 
of  the  third  cranial  nerve.  The  sixth  cranial  nerve  goes  to  the 
outer  rectus;  and  the  fourth  to  the  superior  oblique. 

The  eye  may  be  moved  from  side  to  side ;  up  or  down ;  obliquely, 


248  THE  HUMAN  BODY 

that  is,  neither  truly  vertically  nor  horizontally,  but  partly  both; 
or,  finally,  it  may  be  rotated  on  its  anteroposterior  axis.  The 
oblique  movements  are  always  accompanied  by  a  slight  amount 
of  rotation.  When  the  glance  is  turned  to  the  left,  the  left  external 
rectus  and  the  right  internal  contract,  and  rice  versa;  when  up, 
both  superior  recti;  when  down,  both  the  inferior.  The  superior 
oblique  muscle  acting  alone  will  roll  the  front  of  the  eye  down- 
wards and  outwards  with  a  certain  amount  of  rotation;  the  infe- 
rior oblique  does  the  reverse.  In  oblique  movements  two  of  the 
recti  are  concerned,  an  upper  or  lower  with  an  inner  or  outer;  at 
the  same  time  one  of  the  oblique  also  always  contracts.  Move- 
ments of  rotation  rarely,  if  ever,  occur  alone. 

The  natural  combined  movements  of  the  eyes  by  which  both 
are  directed  simultaneously  towards  the  same  point  depends  on 
the  accurate  adjustment  of  all  its  nervo-muscular  apparatus. 
When  the  coordination  is  deficient  the  person  is  said  to  squint. 
A  left  external  squint  would  be  caused  by  paralysis  of  the  inner 
rectus  of  that  eye,  for  then,  after  the  eyeball  had  been  turned 
out  by  the  external  rectus,  it  would  not  be  brought  back  again 
to  its  median  position.  A  left  internal  squint  would  be  caused, 
similarly,  by  paralysis  of  the  left  external  rectus;  and  probably 
by  disease  of  the  sixth  cranial  nerve  or  its  brain-centers.  Drop- 
ping of  the  upper  eyelid  (ptosis)  indicates  paralysis  of  its  special 
elevator  muscle  and  is  often  a  serious  symptom,  pointing  to 
disease  of  the  brain-parts  from  which  it  is  innervated. 

The  Globe  of  the  Eye  is  on  the  whole  spherical,  but.  consists  of 
segments  of  two  spheres  (see  Fig.  81),  a  portion  of  a  sphere  of 
smaller  radius  forming  its  anterior  transparent  part  and  being 
set  on  to  the  front  of  its  posterior  segment,  which  is  part  of  a 
larger  sphere.  From  before  back  it  measures  about  22.5  milli- 
meters (J  inch),  and  from  side  to  side  about  25  millimeters  (1 
inch).  Except  when  looking  at  near  objects,  the  anteroposterior 
axes  of  the  eyeballs  are  nearly  parallel,  though  the  optic  nerves 
diverge  considerably  (Fig.  80);  each  nerve  joins  its  eyeball,  not 
at  the  center,  but  about  2.5  mm.  (^  inch)  on  the  nasal  side  of  the 
posterior  end  of  its  anteroposterior  axis.  In  general  terms  the 
eyeball  may  be  described  as  consisting  of  three  coats  and  three 
refracting  media. 

The  outer  coat,  1  and  3,  Fig.  81,  consists  of  the  sclerotic  and  the 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT  249 

cornea,  the  latter  being  transparent  and  situated  in  front;  the 
former  is  opaque  and  white  and  covers  the  back  and  sides  of  the 
globe  and  part  of  the  front,  where  it  is  seen  between  the  eyelids 


FIG.  81. — The  left  eyeball  in  horizontal  section  from  before  back.  1,  sclerotic; 
2,  junction  of  sclerotic  and  cornea;  3,  cornea;  4,  5,  conjunctiva;  7,  ciliary  muscle; 
10,  choroid;  11,  13,  ciliary  processes;  14,  iris;  15,  retina;  16,  optic  nerve;  17,  artery 
entering  retina  in  optic  nerve;  18,  fovea  centralis;  19,  20,  region  where  sensory  part 
of  retina  ends;  22,  suspensory  ligament;  24,  the  anterior  part  of  the  hyaloid  mem- 
brane; 26,  the  lens;  29,  vitreous  humor;  30,  aqueous  humor. 

as  the  white  of  the  eye.     Both  are  tough  and  strong,  being  com- 
posed of  dense  connective  tissue. 

The  second  coat  consists  of  the  choroid,  10,  the  ciliary  proc- 
esses, 11,  13,  and  the  iris,  14.  The  choroid  is  made  up  of  blood- 
vrssels  supported  by  loose  connective  tissue  containing  numerous 
corpuscles,  which  in  its  inner  layers  are  richly  filled  with  dark- 
brown  or  black  pigment  granules.  Towards  the  front  of  the  eye- 
ball, where  it  begins  to  diminish  in  diameter,  the  choroid  is  thrown 
into  plaits,  the  ciliary  processes,  11,  13.  Beyond  these  it  con- 
tinues as  the  iris,  which  forms  the  colored  part  of  the  eye  seen 
through  the  cornea;  and  in  the  center  of  the  iris  is  a  circular  aper- 
ture, the  pupil:  so  its  second  coat  does  not,  like  the  outer  one, 
completely  envelop  the  eyeball.  In  the  iris  is  a  ring  of  plain  mus- 
cular tissue  encircling  the  aperture  of  the  pupil:  when  its  fibers 
contract  they  narrow  the  pupil.  Radiating  from  this  ring  to  the 


250  THE  HUMAN  BODY 

edges  of  the  iris  are  muscle-fibers  which  by  their  contraction  en- 
large the  pupil.  Both  sets  of  muscles  are  under  the  control  of 
autonomic  nerves.  Those  to  the  constrictor-fibers  reach  the  eye 
by  way  of  the  third  cranial  nerve  and  belong  to  the  cranial  au- 
tonomic system;  those  innervating  the  dilator-fibers  enter  by  way 
of  the  ophthalmic  branch  of  the  fifth  nerve.  These  latter  fibers  be- 
long to  the  thoracico-lumbar  autonomic  system  and  have  a  rather 
tortuous  connection  with  the  central  nervous  system.  The  path- 
way starts  in  the  upper  thoracic  region  of  the  spinal  cord  where  the 
cell-body  of  the  preganglionic  neuron  lies.  The  axon  of  this  neuron 
passes  out  from  the  cord  to  the  sympathetic  chain  and  in  this  chain 
up  the  neck  to  the  superior  cervical  ganglion  at  the  base  of  the 
skull.  Here  the  preganglionic  neuron  terminates  in  connection  with 
a  post-ganglionic.  The  axon  of  the  latter  passes  to  the  fifth  nerve 
and  along  this  to  its  termination  in  the  pupillo-dilator  muscle. 

The  iris  contains  pigment  which  is  yellow,  or  of  lighter  or  darker 
brown,  according  to  the  color  of  the  eye,  and  more  or  less  abun- 
dant according  as  the  eye  is  black,  brown,  or  gray.  In  blue  eyes 
the  pigment  is  confined  to  the  deeper  layers,  and  modified  in  tint 
by  light  absorption  in  the  anterior  colorless  strata  through  which 
the  light  passes. 

The  third  coat  of  the  eye,  the  retina,  15,  is  its  essential  portion, 
being  the  part  in  which  the  light  produces  those  changes  that  give 
rise  to  impulses  in  the  optic  nerve.  It  is  a  still  less  complete  en- 
velope than  the  choroid,  extending  forwards  only  as  far  as  the 
commencement  of  the  ciliary  processes,  at  least  in  its  typical 
form.  It  is  extremely  soft  and  delicate;  and,  when  fresh,  trans- 
parent. Usually  when  an  eye  is  opened  the  retina  is  colorless; 
but  when  the  eye  has  been  cut  open  in  faint  yellow  light  and  the 
exposed  retina  quickly  examined  in  white  light  it  is  seen  to  be 
purple.  The  coloring  substance  (visual  purple]  very  rapidly 
bleaches  when  a  dead  eye  is  exposed  to  daylight.  On  the  front  or 
inner  surface  of  the  human  retina  two  special  areas  can  be  dis- 
tinguished in  a  fresh  eye.  One  is  the  point  of  entry  of  the  optic 
nerve,  16,  the  fibers  of  which,  penetrating  the  sclerotic  and  cho- 
roid, spread  out  in  the  retina.  At  this  place  the  retina  is  whiter 
than  elsewhere  and  presents  an  elevation,  the  optic  disk.  The 
other  peculiar  region  is  the  fovea  centralis,  18,  which  lies  nearly 
at  the  posterior  end  of  the  axis  of  the  eyeball  and  therefore  out- 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT  251 

side  the  optic  disk;  in  it  the  retina  is  thinner  than  elsewhere 
and  so  a  pit  is  formed.  This  appears  black,  the  thinned  retina 
there  allowing  the  choroid  to  be  seen  through  it  more  clearly  than 
elsewhere.  In  Fig.  82  is  represented  the  right  retina  as  seen  from 
the  front,  the  elliptical  darker  patch  about  the  center  indicating 
the  fovea  and  the  white  circle  on  one  side,  the  optic  disk.  The 
vessels  of  the  retina  arise  from  an  artery  (17,  Fig.  81)  which  runs 
in  with  the  optic  nerve  and  from  which  branches  diverge  as  shown 
in  Fig.  82. 

The  Optic  Nerves,  Chiasma,  and  Tracts.  The  optic  nerves 
converge  to  meet  in  the  optic  chiasma  (ra,  Fig.  80),  from  which 
the  optic  tracts  pass  to  the  region  of  the  midbrain.  They  termi- 
nate mainly  in  the  anterior  corpora  quadrigemina,  (superior  col- 
liculi)  (Chap.  IX)  and  in  the  corpora  geniculata.  The  behavior 
of  the  nerve-fibers  in  the  chiasma  is  interesting  in  that  part  of 
them  cross  to  the  opposite  side  and  part  continue  into  the  tract 
of  the  same  side.  The  fibers  which  cross  over  in  each  optic  nerve 
are  those  coming  from  the  inner  half  of  the  retina,  the  right  half 
of  the  left  retina  and  the  left  half  of  the  right  retina.  The  effect 
of  this  arrangement  is  to  include  in  the  right  optic  tract,  behind 
the  chiasma,  the  nerve-fibers  from  the  right  halves  of  both  retinas, 
and  in  the  left  optic  tract  those  from  the  left  halves  of  both 
retinas.  Cutting  the  right  optic  nerve,  therefore,  causes  total 
blindness  of  the  right  eye,  but  cutting  the  right  optic  tract  blind- 
ness of  the  right  half  of  each  retina  (hemianopia) . 

The  half  crossing  of  the  optic  nerve-fibers  in  man  is  correllated 
with  the  fact  that  his  eyes  are  so  placed  that  most  of  the  field  of 
vision  is  common  to  both.  In  mammals  whose  eyes  are  so  lat- 
erally placed  that  at  any  given  moment  the  objects  seen  by  the 
two  eyes  are  quite  different,  the  crossing  at  the  commissure  is 
complete;  this  condition  obtains  also  in  birds  with  the  exception 
of  owls,  whose  eyes  like  those  of  man  have  their  visual  axes 
parallel;  in  owls  the  crossing  is  only  partial.  It  should  be  noted 
that  the  fovea  centralis,  which  is  the  center  of  distinct  vision,  has 
nerve  connections  from  both  eyes  with  both  optic  tracts.  For 
this  reason  unilateral  injury  to  the  visual  mechanism  back  of 
the  chiasma  interferes  practically  not  at  all  with  ordinary  vision, 
and  sufferers  from  hemianopia  may  be  unaware  of  their  infirmity 
until  careful  examination  by  a  physician  reveals  it. 


252 


THE  HUMAN  BODY 


The  Microscopic  Structure  of  the  Retina.  This,  the  sensitive 
portion  of  the  eye,  has  the  form  of  a  thin  membrane  lining  the 
entire  back  part  of  the  cavity  of  the  eyeball  as  far  forward  as 
the  ciliary  processes.  Although  only  0.15  millimeter  (0.006  inch) 

thick  it  presents  a  very  complex 
structure,  ten  distinct  layers  ap- 
pearing upon  microscopic  exam- 
ination. The  membrane  as  a 
whole  includes  supporting  tissues 
as  well  as  sensitive  and  nervous 
tissues  proper;  we  are  concerned 
only  with  the  latter  and  shall 
confine  our  discussion  to  them. 
The  retina  develops  in  such  a 
way  that  the  actual  sensitive 
structures  instead  of  being  on 
its  front  surface  where  light 
would  strike  them  immediately 

FIG.  82.  —  The  right  retina  as  it  would  11Tinn    roanVn'nn-  +ko  rofino 
be  seen  if  the  front  part  of  the  eyeball  UP°n  reading  tne  ]  Ctina,  are 
with  the  lens  and  vitreous  humor  were  its  posterior  Surface,  next  to  the 
removed.  .  .  . 

choroid  coat,  and  interposing  be- 

tween themselves  and  the  source  of  light  the  nerve  structures 
which  connect  them  with  the  optic  nerve,  and  the  supporting 
tissues  and  blood-vessels  of  the  retina.  Fortunately  all  these 
structures  are  so  transparent  or  so  placed  as  not  to  interfere  ser- 
iously with  vision.  In  the  fovea,  where  all  clear  sight  is  located, 
blood-vessels  are  absent  and  the  other  structures  are  much  reduced. 
The  sensitive  elements  of  the  eye  are  called,  from  their  shape, 
rods  and  cones.  The  rods  consist  of  basal  enlarged  portions  from 
which  slender  rod-like  processes  project  toward  the  choroid  coat. 
These  processes  contain  a  peculiar  reddish  substance  (visual  pur- 
ple), which  has  the  property  of  bleaching  out  when  exposed  to 
light  (R,  Fig.  83).  The  cones  have  somewhat  thicker  basal  por- 
tions than  the  rods  and  much  shorter  processes  containing  no 
visual  purple  (C,  Fig.  83).  Rods  and  cones  make  up  layer  number 
two  of  the  ten  retinal  layers.  The  first  layer,  which  is  between 
the  rods  and  cones  and  the  choroid  coat,  is  a  layer  of  pigment 
cells  which  send  processes  in  among  the  rods,  and  seem  to  have 
something  to  do  with  forming  the  visual  purple. 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT 


253 


The  rods  and  cones  appear  to  constitute  the  peripheral  or 
dendritic  portions  of  bipolar  sensory  neurons.  They  communi- 
cate with  cell-bodies  from  which  in  turn  pass  typical,  though 
very  short,  axons.  The  third  retinal  layer  is  composed  of  these 
cell-bodies  with  their  axons.  The  axons  of  the  rod  and  cone 
neurons  come  into  synaptic  connection  with  dendrites  of  a  second 


FIG.  83. — Diagram  of  the  structure  of  the  human  retina  (Greeff) :  /,  pigment 
layer;//,  rod  and  cone  layer; \R.  rods;C,  cones ;III-IX.  intraretinal  nerve-elements; 
X,  axons  which  pass  to  optic  nerve. 

set  of  retinal  neurons,  the  synapses  making  up  the  fourth  retinal 
layer.  The  fifth,  sixth,  and  seventh  retinal  layers  contain  the 
cell-bodies  and  short  axons  of  these  second  retinal  neurons;  in 
the  eighth  layer  these  come  into  synaptic  connection  with  the 
dendrites  of  the  third  set  of  retinal  neurons.  The  large  cell- 
bodies  of  these  neurons  make  up  the  ninth  retinal  layer,  and 
their  axons,  converging  from  all  parts  of  the  retina  upon  the  optic 


254  THE  HUMAN  BODY 

disk,  constitute  the  tenth  and  front  layer  of  the  retina.  These 
axons  continue  uninterrupted  to  terminations  in  the  midbrain 
ganglia.  The  relations  of  the  three  sets  of  retinal  neurons  are 
shown  in  the  diagram  (Fig.  83). 

Rods  and  cones  are  not  uniformly  distributed  over  the  retina. 
The  fovea,  where  distinct  vision  is  centered,  contains  only  cones. 
The  peripheral  portions  of  the  retina  contain  a  larger  and  larger 
proportion  of  rods  as  the  margin  is  approached,  until  the  outer- 
most regions  contain  only  rods.  This  difference  of  distribution 
indicates  a  differentiation  of  function  between  the  two  sorts  of 
sensitive  structures.  The  'probability  of  such  differentiation  is 
strengthened  by  the  observation  that  each  cone  communicates 
through  the  intervening  retinal  neuron  with  a  single  and  sepa- 
rate neuron  of  the  optic  nerve,  whereas  the  connection  of  the  rods 
is  such  that  several  of  them  may  send  impulses  into  a  single  optic 
neuron. 

The  blood-vessels  of  the  retina  lie  almost  entirely  in  the  ninth 
and  tenth  retinal  layers. 

The  Refracting  Media  of  the  Eye  are,  in  succession  from  before 
back,  the  aqueous  humor,  the  crystalline  lens,  and  the  vitreous  humor. 

The  aqueous  humor  fills  the  space  between  the  front  of  the  lens, 
and  the  back  of  the  cornea  (30,  Fig.  81).  Chemically,  it  consists 
of  water  holding  in  solution  a  small  amount  of  solid  matters, 
mainly  common  salt. 

The  crystalline  lens  (26,  Fig.  81)  is  colorless,  transparent,  and  bi- 
convex, with  its  anterior  surface  less  curved  than  the  posterior.  It 
is  surrounded  by  a  capsule,  and  the  inner  edge  of  the  iris  lies  in 
contact  with  it  in  front.  In  consistence  it  is  soft,  but  its  central 
layers  are  rather  more  dense  than  the  outer. 

The  capsule  is  continuous  at  the  margin  of  the  lens  with  the 
suspensory  ligament  which  in  turn  is  attached  all  around  to  the 
ciliary  processes.  The  suspensory  ligament  is  stretched  and  its 
pull  upon  the  capsule  keeps  the  lens  more  flattened  than  it  would 
be  if  free. 

The  vitreous  humor  (29,  Fig.  81)  is  a  soft  jelly  enveloped  in  a  thin 
capsule,  the  hyaloid  membrane.  It  consists  mainly  of  water  and 
contains  some  salts,  a  little  albumin,  and  some  mucin.  It  is  di- 
vided up,  by  delicate  membranes,  into  compartments  in  which  its 
more  liquid  portions  are  imprisoned. 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT  255 

The  Ciliary  Muscle.  (7,  Fig.  81.)  Between  the  sclerotic  and 
rhoroid  coats,  just  where  the  former  merges  into  the  cornea,  are 
small  masses  of  smooth  muscle-fibers  which  make  up  the  ciliary 
muscle.  These  fibers  are  attached  in  front  to  the  sclerotic  coat  and 
pass  back  a  short  distance  to  an  insertion  in  the  choroid  coat  just 
in  front  of  the  ciliary  processes.  The  contraction  of  the  ciliary 
muscle  pulls  the  margin  of  the  choroid  coat  forward  and  inward. 
The  effect  of  this  is  to  bring  the  ciliary  processes  nearer  together 
and  loosen  the  suspensory  ligament,  which  is  attached  to  them. 
The  tension  upon  the  capsule  of  the  crystalline  lens  is  thus  di- 
minished. 

The  ciliary  muscle  is  interesting  as  being  the  only  voluntary 
muscle  in  the  Body  which  is  innervated  through  the  autonomic 
system. 

The  Properties  of  Light.  Before  proceeding  to  the  study  of 
the  eye  as  an  optical  instrument,  it  is  necessary  to  recall  briefly 
certain  properties  of  light. 

Light  is  considered  as  a  form  of  movement  of  the  particles  of 
an  hypothetical  medium,  or  ether,  the  vibrations  being  in  planes 
at  right  angles  to  the  line  of  propagation  of  the  light.  Starting 
from  a  luminous  point  light  travels  in  all  directions  along  the 
radii  of  a  sphere  of  which  the  point  is  the  center;  the  light  propa- 
gated along  one  such  radius  is  called  a  ray,  and  in  each  ray  the 
ethereal  particles  vibrate  from  side  to  side  in  a  plane  perpendic- 
ular to  the  direction  of  the  ray. 

Any  ray,  all  of  whose  particles  are  vibrating  at  the  same  rate, 
is  a  ray  of  monochromatic  light.  It  has  a  pure  spectral  color.  The 
wave  length  of  a  beam  of  monochromatic  light  is  measured  by  the 
distance  between  any  ethereal  particle  of  the  beam  and  the  next 
one  which  is  in  precisely  the  same  phase  of  vibration.  Since  the 
rate  at  which  light  travels  is  nearly  fixed,  the  wave  length  must 
vary  inversely  as  the  vibration  rate.  Light  of  high  vibration  rate 
has  short  wave  length  and  vice  versa.  The  color  of  monochro- 
matic light  depends  upon  its  wave  length.  Where  lights  of  va- 
rious wave  lengths  are  mixed  together  in  a  beam  a  compound  light 
results.  To  the  eye  such  a  beam  gives  a  definite  color  sensation 
but  not  one  of  the  pure  spectral  colors. 

Refraction.  When  light  passes  obliquely  from  one  transpar- 
ent medium  into  another  of  different  density  it  is  bent  from  its 


256 


THE  HUMAN  BODY 


course,  or  refracted.     The  amount  of  refraction  depends  upon 
the  optical  nature  of  the  two  media  and  also  upon  the  angle  at 

which  the  ray  strikes  the  surface 
of  separation.  This  angle,  meas- 
ured between  the  incident  ray  and 
a  line  drawn  at  right  angles  to 
the  surface  between  the  media,  is 
known  as  the  angle  of  incidence. 
The  angle  which  the  refracted  ray 
makes  with  this  same  perpendicu- 
lar is  the  angle  of  refraction.  If 

the   ray    is   PaSsinS   from   a  .leSS  re~ 

fractive  to  a  more  refractive  me- 


media;  c  D,  the  perpendicular  to  the   dium  it  is  bent  toward  the  normal; 

surface  at  the  point  of  incidence;  x, 
a  x,  incident  ray;  x  d,  refracted  ray, 

ilthfiST.ondmedJum,l!fdenser/hf?;n   refractive  medium  it  is  bent  away 


FIG.  84. — Diagram  illustrating  the 
refraction  of  light.  A  B,  surface  of 
separation  between  two  transparent 
media ;  C  D,  the  perpendicular  to  the 

L?e;  *'   if  passing  from   a  more  to  a  less 

the  first;  x  g,  refracted  ray,  if  the 

second  medium  is  less  refractive  than    from    the    normal    (Fig.    84).       The 

amount  of  bending  is  determined 

by  the  law  of  refraction  which  is:  the  ratio  of  the  sine  of  the  angle 
of  incidence  to  that  of  the  angle  of  refraction  is  always  constant  for 
the  same  two  media  and  for  light  of  the  same  wave  length. 

This  ratio  of  sines  is  the  index  of  refraction.     It  is  usually  ex- 


FIG.  85. — Diagram  illustrating  the  dispersion  of  mixed  light  by  a  prism. 

pressed  for  various  refractive  media  with  air  as  the  second  and 
less  refractive  one. 

Dispersion  of  Mixed  Light.     The  shorter  the  vibration  periods 
of  light-rays  the  more  they  are  deviated  by  refraction.     Hence 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT  257 

mixed  light  when  sent  through  a  prism  is  spread  out,  and  decom- 
posed into  its  simple  constituents.  For  let  ax  (Fig.  85)  be  a  ray 
of  mixed  light  composed  of  a  set  of  short  and  a  set  of  long  ethereal 
waves.  When  it  falls  on  the  surface  AB  of  the  prism,  that  por- 
tion which  enters  will  be  refracted  towards  the  normal  ED,  but 
the  short  waves  more  than  the  longer.  Hence  the  former  will 
take  the  direction  xy,  and  the  latter  the  direction  xz.  On  emerg- 
ing from  the  prism  both  rays  will  again  be  refracted,  but  now 
from  the  normals  Fy  and  Gz,  since  the  light  is  passing  from  a 
more  to  a  less  refracting  medium.  Again  Jthe  ray  xy,  made  up 
of  shorter  waves,  will  be  most  deviated,  as  in  the  direction  yv, 
and  the  long  waves  less,  in  the  direction  zr.  If  a  screen  were  put 
at  SS',  we  would  receive  on  it  at  separate  points,  v  and  r,  the  two 
simple  lights  which  were  mixed  together  in  the  compound  inci- 
dent ray  ax.  Such  a  separation  of  light-rays  is  called  dispersion. 

Ordinary  white  light,  such  as  that  of  the  sun,  is  composed  of 
ethereal  vibrations  of  every  rate,  mixed  together.  When  such 
light  is  sent  through  a  prism  it  gives  a  continuous  band  of  light- 
rays,  known  as  the  solar  spectrum,  reaching  from  the  least  refracted 
to  the  most  refracted  and  shortest  waves.  The  exceptions  to  this 
statement  due  to  Frauenhofer's  lines  (see  Physics)  are  unessential 
for  our  present  purpose.  Not  all  the  rays  of  the  solar  spectrum 
are  visible  to  the  human  eye.  The  least  refracted  ones,  called 
the  ultra  red,  and  the  most  refracted  ones,  the  ultra  violet,  do  not 
stimulate  the  retina;  they  are  determined  by  their  physical  and 
chemical  effects.  The  visible  spectrum  includes  in  order  of  in- 
creasing refrangibility  the  seven  spectral  colors  red,  orange, 
yellow,  green,  blue,  indigo,  and  violet.  These  merge  insensibly 
into  one  another,  showing  the  sun's  light  to  be  a  mixture  of  all 
possible  wave  lengths,  and  not  of  certain  selected  ones. 

Refraction  of  Light  by  Lenses.  In  the  eye  the  refracting 
media  have  the  form  of  lenses  thicker  in  the  center  than  towards 
the  periphery;  and  we  may  here  confine  ourselves,  therefore,  to 
such  convex  lenses.  If  simple  light  from  a  point  A,  Fig.  79, 
fall  on  such  a  lens  its  rays,  emerging  on  the  other  side,  will  take 
new  directions  after  refraction  and  meet  anew  at  a  point,  a,  after 
which  they  again  diverge.  If  a  screen,  rr,  be  held  at  a  it  will 
therefore  receive  an  image  of  the  luminous  point  A.  For  every 
convex  lens  there  is  such  a  point  behind  it  at  which  the  rays 


258  THE  HUMAN  BODY 

from  a  given  point  in  front  of  it  meet:  the  point  of  meeting  is 
called  the  conjugate  focus  of  the  point  from  which  the  rays  start. 
If  instead  of  a  luminous  point  a  luminous  object  be  placed  in 
front  of  the  lens  an  image  of  the  object  will  be  formed  at  a  certain 

distance  behind  it,  for  all  rays  proceed- 
ing from  one  point  of  the  object  will 
meet  in  the  conjugate  focus  of  that 
point  behind.  The  image  is  inverted, 
as  can  be  readily  seen  from  Fig.  86. 

FIG.  86. — Diagram  illustrat-  ATI  r{,v<a  frnrn  tV»P  nnint  A  nf  tViA  nhippf 
ing  the  formation  of  an  image  Ail  raVS  3DJec 

by  a  convex  lens.  meet  at  the  point  a  of  the  image ;  those 

from  B  at  6,  and  those  from  intermediate  points  at  intermediate 
positions.  If  the  single  lens  were  replaced  by  several  combined 
so  as  to  form  an  optical  system  the  general  result  would  be  the 
same,  provided  the  system  were  thicker  in  the  center  than  at  the 
periphery. 

A  moment's  consideration  of  the  diagram  (Fig.  86)  shows  us 
that  the  nearer  any  luminous  point  is  to  the  lens  the  further  be- 
hind the  lens  its  conjugate  focus  will  be.  The  rays  from  near 
points  are  more  divergent  when  they  strike  the  lens  than  are  those 
from  far  points,  they  are  therefore  not  so  much  bent  toward  each 
other  upon  emerging,  and  their  point  of  meeting  is  further  back. 
There  must  be  some  point  near  the  lens  from  which  rays  are  so 
divergent  that  after  emerging  they  do  not  meet  at  all,  but  con- 
tinue to  diverge  or  form  a  parallel  beam.  A  plane  so  located  with 
reference  to  a  lens  that  rays  from  any  point  in  it  striking  the  lens 
emerge  in  a  parallel  beam  is  the  principal  focal  plane  of  the  lens. 
The  thicker  a  lens  the  nearer  to  it  is  its  principal  focal  plane. 

The  Ordinary  Photographic  Camera  is  an  instrument  which 
serves  to  illustrate  the  formation  of  images  by  converging  sys- 
tems of  lenses.  It  consists  of  a  box  blackened  inside  and  having 
on  its  front  face  a  tube  containing  the  lenses;  the  posterior  wall 
is  made  of  ground  glass.  If  the  front  of  the  instrument  be  di- 
rected on  exterior  objects,  inverted  and  diminished  images  of 
them  will  be  formed  on  the  ground  glass;  those  images  only  are 
well  denned,  at  any  one  time,  which .  are  at  such  a  distance  in 
front  of  the  instrument  that  the  conjugate  foci  of  points  on  them 
fall  exactly  on  the  glass  behind  the  lens:  objects  nearer  or  farther 
off  give  confused  and  indistinct  images;  but  by  altering  the  dis- 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT  259 

tance  between  the  lenses  and  the  ground  glass,  in  common  lan- 
guage " focussing  the  instrument,"  either  can  be  made  distinct. 
For  near  objects  the  lenses  must  be  farther  from  the  surface  on 
which  the  image  is  to  be  received,  and  for  distant  nearer. 

The  Refracting  Media  of  the  Eye  Form  a  Convergent  Optical 
System,  made  up  of  cornea,  aqueous  humor,  lens,  and  vitreous 
humor.  These  four  media  are  reduced  to  three  practically,  by 
the  fact  that  the  indices  of  refraction  of  the  cornea  and  aqueous 
humor  are  the  same,  so  that  they  act  together  as  one  converging 
lens.  The  surfaces  at  which  refraction  occurs  are:  (1)  that  be- 
tween the  air  and  the  cornea;  (2)  that  between  the  aqueous  humor 
and  the  front  of  the  lens;  (3)  that  between  the  vitreous  humor 
and  the  back  of  the  lens.  The  refractive  indices  of  those  media 
are:  the  air,  1;  the  aqueous  humor,  1.3379;  the  lens  (average), 
1.4545;  the  vitreous  humor,  1.3379.  From  the  laws  of  the  re- 
fraction of  light  it  therefore  follows  that  (Fig.  87)  the  rays  Cd 
will  at  the  corneal  surface  be  refracted  towards  the  normals  N,  N, 
and  take  the  course  de.  At  the  front  of  the  lens  they  will  again 
be  refracted  towards  the  normals  to  that  surface  and  take  the 
course  ef;  at  the  back  of  the  lens,  passing  from  a  more  refracting 
to  a  less  refracting  medium,  they  will  be  bent  from  the  normals 
TV"  and  take  the  course  fg.  If  the  retina  be  there,  these  parallel 
rays  will  therefore  be  brought  to  a  focus  on  it.  In  the  resting 
condition  of  the  natural  eye  this  is  what  happens  to  parallel  rays 
entering  it:  and,  since  distant  objects  send  into  the  eye  rays  which 
are  practically  parallel,  such  objects  are  seen  distinctly  without 
any  effort,  because  all  rays  emanating  from  a  point  of  the  object 
meet  again  in  one  point  on  the  retina. 

Wide  Range  of  Clear  Vision  in  the  Resting  Eye.  While  in  the 
normal  resting  eye  only  parallel  rays  focus  exactly  on  the  retina, 
the  fact  is  that  it  sees  clearly  all  objects  that  are  as  far  as  18-20  feet 
away.  The  rays  of  light  from  points  on  such  objects  are  divergent 
when  they  strike  the  cornea,  and  their  focus  is  therefore  behind  the 
retina.  How  then  can  the  resting  eye  see  such  objects  clearly? 
The  explanation  is  found  in  the  structure  of  the  retina.  The  light 
perceiving  elements,  the  rods  and  cones  (Fig.  83),  although  ex- 
tremely minute,  are  not  mathematical  points,  but  objects  with 
measurable  diameter.  When  light  falls  on  one  of  them  the  effect 
is  the  same  whether  only  a  part  of  the  element  is  illuminated  or  the 


260 


THE  HUMAN  BODY 


whole,  provided  the  light  does  not  lap  over  into  adjacent  elements. 
Any  beam  of  light  entering  the  eye  forms  a  cone  with  its  apex  at 
the  focus.  Near  the  focus  a  cross-section  of  the  cone  consists  of 
a  small  circular  area,  which  is  larger  the  further  away  from  the 
focus  it  is  taken.  Such  an  area  is  known  as  a  dispersion  circle. 
The  convergent  beams  from  the  points  of  an  object  18  feet  away 
strike  the  retina  before  reaching  their  exact  focus,  but  the  disper- 
sion circles  formed  by  them  are  too  small  to  stimulate  more  than 
one  element;  the  effect  is  therefore  the  same  as  though  an  accurate 
focus  had  been  reached,  and  objects  at  this  distance  are  seen  clearly. 
Accommodation.  Points  on  objects  nearer  than  18  feet  send 
into  the  eye  beams  so  diverging,  and  therefore  focussing  so  far 


FIG.  87. — Diagram  illustrating  the  surfaces  at  which  light  is  refracted  in  the 
eye. 

behind  the  retina,  that  the  dispersion  circles  formed  at  the  retina 
are  too  large  to  stimulate  only  single  elements.  Near  objects, 
therefore,  would  not  be  seen  distinctly,  did  not  some  change  occur 
in  the  eye;  since  we  can  see  them  quite  plainly  if  we  choose  (unless 
they  be  very  near  indeed),  there  must  exist  some  means  by  which 
the  eye  is  focussed  or  accommodated  for  looking  at  objects  at  dif- 
ferent near  distances.  That  some  change  does  occur  one  can,  also, 
readily  prove  by  observing  that  we  cannot  see  distinctly,  at  the 
same  moment,  both  near  and  distant  objects.  For  example,  stand- 
ing behind  a  lace  curtain,  at  a  window,  we  can  as  we  choose  look  at 
the  threads  of  the  lace  or  at  the  houses  across  the  street;  but  when 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT  261 

we  look  at  the  one  we  see  the  other  only  indistinctly ;  and  if,  after- 
looking  at  the  more  distant  object,  we  look  at  the  nearer  we  expe- 
rience a  distinct  sense  of  effort.  It  is  clear,  then,  that  something 
in  the  eye  is  different  in  the  two  cases.  The  resting  eye,  suited  for 
seeing  distinctly  distant  objects,  might  conceivably  be  accommo- 
dated for  near  vision  in  several  ways.  The  refracting  indices  of  its 
media  might  be  increased;  that  of  course  does  not  happen;  the 
physical  properties  of  the  media  are  the  same  in  both  cases:  or  the 
distance  of  the  retina  from  the  refracting  surfaces  might  be  in- 
creased, for  example,  by  compression  of  the  eyeball  by  the  muscles 
around  it;  however,  experiment  shows  that  changes  of  accommoda- 


cs 


cb 


FIG.  88. — Diagram  to  illustrate  the  mechanism  of  accommodation;  on  the 
right  half  of  the  figure  for  a  near,  on  the  left  for  a  distant,  object;  rf,  ciliary  muscle; 
ch,  ciliary  process  of  choroid;  si,  suspensory  ligament;  i,  iris. 

tion  can,  by  stimulating  the  third  cranial  nerve,  be  brought  about 
in  the  fresh  excised  eyes  of  animals  from  which  the  muscles  lying 
outside  the  eyeball  have  been  removed,  in  which  no  such  compres- 
sion is  possible;  we  are  thus  reduced  to  the  third  explanation,  that 
the  refracting  surfaces,  or  some  of  them,  become  more  curved,  and 
so  bring  diverging  rays  sooner  to  a  focus.  Observation  shows 
that  this  is  what  actually  happens:  the  corneal  surface  remains 
unchanged  when  a  near  object  is  looked  at  after  a  distant  one, 
but  the  lens  becomes  considerably  more  convex. 

Accommodation  is  brought  about  by  the  ciliary  muscle  (Fig.  88) . 
In  the  resting  eye  it  is  relaxed  and  the  suspensory  ligament  of  the 
lens  is  taut,  and,  pulling  on  its  edge,  drags  it  out  laterally  a  little 
and  flattens  its  surfaces,  especially  the  anterior,  since  the  ligament 


262  THE  HUMAN  BODY 

is  attached  a  little  in  front  of  the  edge.  To  see  a  nearer  object  the 
ciliary  muscle  is  contracted,  and  according  to  the  degree  of  its 
contraction  slackens  the  suspensory  ligament,  and  then  the  elastic 
lens,  relieved  from  the  lateral  drag,  bulges  out  a  little  in  the  center. 
When  the  eye  is  focussed  for  seeing  a  near  object  the  circular 
muscle  of  the  iris  contracts,  narrowing  the  pupil,  but  this  has 
nothing  directly  to  do  with  the  accommodation. 

Short  Sight  and  Long  Sight.  In  the 
normal  eye  parallel  rays  meet  on  the 
retina  when  the  ciliary  muscle  is  com- 
pletely relaxed  (A,  Fig.  89).  Such  eyes 
are  emmetropic.  In  other  eyes  the  eye- 
ball is  too  long  from  before  back;  in  the 
resting  state  parallel  rays  meet  in  front 
of  the  retina  (B).  Persons  with  such 
eyes,  therefore,  cannot  see  distant  ob- 
jects distinctly  without  the  aid  of  diverg- 
FIG.  89.-Diagram  mustrat-  inS  (concave)  spectacles;  they  are  short- 
ing the  path  of  parallel  rays  siqhted  or  myopic.  Or  the  eyeball  may 

after   entering  an   emmetropic 

(A),  a  myopic  (#),  and  a  hy-  be  too  short  from  before  back;  then,  in 

the  resting  state,  parallel  rays  are 

brought  to  a  focus  behind  the  retina  (C).  To  see  even  infinitely 
distant  objects,  such  persons  must  therefore  use  their  accommodat- 
ing apparatus  to  increase  the  converging  power' of  the  lens;  and 
when  objects  are  near  they  cannot,  with  the  greatest  effort,  bring 
the  divergent  rays  proceeding  from  them  to  a  focus  soon  enough. 
To  get  distinct  retinal  images  of  near  objects  they  therefore  need 
converging  (convex)  spectacles.  Such  eyes  are  called  hypermetropic 
or  in  common  language  long-sighted. 

Optical  Defects  of  the  Eye.  The  eye,  though  it  answers  ad- 
mirably as  a  physiological  instrument,  is  by  no  means  perfect 
optically;  not  nearly  so  good,  for  example,  as  a  good  microscope 
objective.  The  main  defects  in  it  are  due  to: 

1.  Chromatic  Aberration.  As  already  pointed  out,  the  rays  at 
the  violet  end  of  the  solar  spectrum  are  more  refrangible  than  those 
at  the  red  end.  Hence  they  are  brought  to  a  focus  sooner.  The 
light  emanating  from  a  point  on  a  white  object  does  not,  therefore, 
all  meet  in  one  point  on  the  retina;  but  the  violet  rays  come  to  a 
focus  first,  then  the  indigo,  and  so  on  to  the  red,  farthest  back  of 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT  263 

all.  If  the  eye  is  accommodated  so  as  to  bring  to  a  focus  on  the 
retina  parallel  red  rays,  then  violet  rays  from  the  same  source  will 
meet  half  a  millimeter  in  front  of  it,  and  crossing  and  diverging 
there  make  a  little  violet  circle  of  diffusion  around  the  red  point  on 
the  retina.  In  optical  instruments  this  defect  is  remedied  by  com- 
bining together  lenses  made  of  different  kinds  of  glass;  such  com- 
pound lenses  are  called  achromatic. 

The  general  result  of  chromatic  aberration,  as  may  be  seen  in  a 
bad  opera-glass,  is  to  cause  colored  borders  to  appear  around  the 
edges  of  the  images  of  objects.  In  the  eye  we  usually  do  not  notice 
such  borders  unless  we  especially  look  for  them;  but  if,  while  a 
white  surface  is  looked  at,  the  edge  of  an  opaque  body  be  brought 
in  front  of  the  eye  so  as  to  cover  half  the  pupil,  colorations  will  be 
seen  at  its  margin.  If  accommodation  be  inexact  they  appear  also 
when  the  boundary  between  a  white  and  a  black  surface  is  ob- 
served. The  phenomena  due  to  chromatic  aberration  are  much 
more  easily  seen  if  light  containing  only  red  and  violet  rays  be 
used  instead  of  white  light  containing  all  the  rays  of  intermediate 
refrangibility.  Ordinary  blue  glass  only  lets  through  these  two 
kinds  of  rays.  If  a  bit  of  it  be  placed  over  a  very  small  hole  in  an 
opaque  shutter  and  sunlight  be  admitted  through  the  hole,  it  will 
be  found  that  with  one  accommodation  (that  for  the  red  rays)  a 
red  point  is  seen  with  a  violet  border,  and  with  another  (that  at 
which  violet  rays  are  brought  to  a  focus  on  the  retina)  a  violet 
point  is  seen  with  a  red  aureole. 

2.  Spherical  Aberration.  It  is  not  quite  correct  to  state  that 
ordinary  lenses  bring  to  a  focus  in  one  point  behind  them  rays 
proceeding  from  a  point  in  front,  even  when  these  are  all  of  the 
same  refrangibility.  Convex  lenses  whose  surfaces  are  segments 
of  spheres,  as  are  those  of  the  eye,  bring  to  a  focus  sooner  the  rays 
which  pass  through  their  marginal  than  those  passing  through  their 
central  parts.  If  rays  proceeding  from  a  point  and  traversing  the 
lateral  part  of  a  lens  be  brought  to  a  focus  at  any  point,  then  those 
passing  through  the  center  of  the  lens  will  not  meet  until  a  little 
beyond  that  point.  If  the  retina  receive  the  image  formed  by  the 
peripheral  rays  the  others  will  form  around  this  a  small  luminous 
circle  of  light — such  as  would  be  formed  by  sections  of  the  cones 
of  converging  rays  in  Fig.  78,  taken  a  little  in  front  of  r  r.  This 
defect  exists  in  all  glass  lenses,  as  it  is  found  impossible  in  practice 


264 


THE  HUMAN  BODY 


to  grind  them  of  the  non-spherical  curvatures  necessary  to  avoid 
it.  In  our  eyes  its  effect  is  to  a  large  extent  corrected  in  the  follow- 
ing ways:  (a)  The  opaque  iris  cuts  off  many  of  the  external  and 
more  strongly  refracted  rays,  preventing  them  from  reaching  the 
retina,  (b)  The  outer  layers  of  the  lens  are  less  refracting  than 
the  central;  hence  the  rays  passing  through  its  peripheral  parts 
are  less  refracted  than  those  passing  nearer  its  axis. 

3.  Irregularities  in  Curvature.     The  refracting  surfaces  of  our 
eyes  are  not  even  truly  spherical;  this  is  especially  the  case  with 

the  cornea,  which  is  very  rarely 
curved  to  the  same  extent  in  its 
vertical  and  horizontal  diameters. 
Suppose  the  vertical  meridian  to 
be  the  most  curved;  then  the  rays 
proceeding  from  points  along  a  ver- 
tical line  will  be  brought  to  a 
focus  sooner  than  those  from  points 
on  a  horizontal  line.  If  the  eye  is 
accommodated  to  see  distinctly 
the  vertical  line,  it  will  see  indis- 
tinctly the  horizontal  and  vice 
versa.  Few  people  therefore  see 
equally  clearly  at  once  two  lines  crossing  one  another  at  right 
angles.  The  phenomenon  is  most  obvious,  however,  when  a  series 
of  concentric  circles  (Fig.  90)  is  looked  at:  then  when  the  lines 
appear  sharp  along  some  sectors,  they  are  dim  along  the  rest. 
This  defect  is  known  as  astigmatism;  it  is  corrected  by  the  use  of 
lenses  which  are  curved  only  in  one  plane.  The  lens  is  so  adjusted 
that  its  curvature  combines  with  the  less  curvature  of  the  eye  to 
equal  the  greater. 

4.  Opaque  Bodies  in  the  Refracting  Media.    In  diseased  eyes  the 
lens  may  be  opaque  (cataract)  and  need  removal;  or  opacities  from 
ulcers  or  wounds  may  exist  on  the  cornea.    But  even  in  the  best 
eye  there  are  apt  to  be  small  opaque  bodies  in  the  vitreous  humor 
causing  muscce  volitantes;  that  is,  the  appearance  of  minute  bodies 
floating  in  space  outside  the  eye,  but  changing  their  position  when 
the  position  of  the  eye  changes,  by  which  fact  their  origin  in  in- 
ternal causes  may  be  recognized.    Many  persons  never  see  them 
until  their  attention  is  called  to  their  sight  by  some  weakness  of  it, 


FIG.  90. 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT  265 

and  then  they  think  they  are  new  phenomena.  Visual  phenomena 
due  to  causes  in  the  eye  itself  are  called  entoptic;  the  most  interest- 
ing are  those  due  to  the  retinal  blood-vessels  (Chap.  XVI).  Tears, 
or  bits  of  the  secretion  of  the  Meibomian  glands,  on  the  front  of  the 
eyeball  often  cause  distant  luminous  objects  to  look  like  ill-defined 
luminous  bands  or  patches  of  various  shape.  The  cause  of  such 
appearances  is  readily  recognized,  since  they  disappear  or  are 
changed  after  winking. 

Hygienic  Remarks.  Since  muscular  effort  is  needed  by  the 
normal  eye  to  see  near  objects,  it  is  clear  why  the  prolonged  con- 
templation of  such  is  more  fatiguing  than  looking  at  more  distant 
things.  If  the  eye  be  hypermetropic  still  more  is  this  apt  to  be  the 
case,  for  then  the  ciliary  muscle  has  no  rest  when  the  eye  is  used, 
and  to  read  a  book  at  a  distance  such  that  enough  light  is  reflected 
from  it  into  the  eye  in  order  to  enable  the  letters  to  be  seen  at  all, 
requires  an  extraordinary  effort  of  accommodation.  Such  persons 
complain  that  they  can  read  well  enough  for  a  time,  but  soon  fail 
to  be  able  to  see  distinctly.  This  kind  of  weak  sight  should  always 
lead  to  examination  of  the  eyes  by  an  oculist,  to  see  if  glasses  are 
needed ;  otherwise  severe  neuralgic  pains  about  the  eyes  are  apt  to 
come  on,  and  the  overstrained  organ  may  be  permanently  injured. 
Old  persons  are  apt  to  have  such  eyes;  but  young  children  fre- 
quently also  possess  them,  and  if  so  should  at  once  be  provided 
with  spectacles.  Astigmatism  is  another  fruitful  source  of  eye 
strain.  Although  sharp  focussing  is  impossible  the  eye  constantly 
strives  for  it.  This  involves  great  activity  of  the  muscles  of  accom- 
modation, which  suffer  from  the  effort.  The  occurrence  of  head- 
ache at  frequent  intervals,  particularly  in  connection  with  the  use 
of  the  eyes,  as  in  reading  or  sewing,  is  more  often  than  not  an  indi- 
cation of  visual  defects  which  proper  glasses  would  overcome. 
Sufferers  from  such  headaches  should  therefore  have  their  eyes 
examined  and  if  glasses  are  necessary  should  wear  them. 

Short-sighted  eyes  appear  to  be  much  more  common  now  than 
formerly,  especially  in  those  given  to  literary  pursuits.  Myopia 
is  rare  among  those  who  cannot  read  or  who  live  mainly  out  of 
doors.  It  is  not  so  apt  to  lead  to  permanent  injury  of  the  eye  as  is 
the  opposite  condition,  but  the  effort  to  see  distinctly  objects  a 
little  distant  is  apt  to  produce  headaches  and  other  symptoms  of 
nervous  exhaustion.  If  the  myopia  become  gradually  worse  the 


266  THE  HUMAN  BODY 

eyes  should  be  rested  for  several  months.  Short-sighted  persons 
are  apt  to  have,  or  acquire,  peculiarities  of  appearance :  their  eyes 
are  often  prominent,  indicative  of  the  abnormal  length  of  the  eye- 
ball. They  also  get  a  habit  of  "screwing"  up  the  eyelids,  probably 
an  indication  of  an  effort  to  compress  the  eyeball  from  before  back 
so  that  distant  objects  may  be  better  seen.  They  often  stoop,  too, 
from  the  necessity  of  getting  their  eyes  near  objects  they  want  to 
see.  The  acquirement  of  such  habits  may  be  usually  prevented 
by  the  use  of  proper  glasses.  On  the  other  hand,  "it  is  said  that 
myopia  even  induces  peculiarities  of  character,  and  that  myopes 
are  usually  unsuspicious  and  easily  pleased;  being  unable  to  ob- 
serve many  little  matters  in  the  demeanor  or  expression  of  those 
with  whom  they  converse,  which,  being  noticed  by  those  of  quicker 
sight,  might  induce  feelings  of  distrust  or  annoyance." 

In  old  age  the  lens  loses  some  of  its  elasticity  and  becomes  more 
rigid.  This  leads  to  the  long-sightedness  of  old  people,  known  as 
presbyopia.  The  stiffer  lens  does  not  become  as  convex  as  it  did 
in  early  life,  when  the  ciliary  muscle  contracts  and  the  suspensory 
ligament  is  relaxed.  In  order  to  adapt  the  eye  to  see  near  objects 
distinctly,  therefore,  convex  glasses  are  required. 

In  all  forms  of  defective  vision  too  strong  glasses  will  injure 
the  eyes  irreparably,  increasing  the  defects  they  are  intended  to 
relieve.  Skilled  advice  should  therefore  be  invariably  obtained 
in  their  selection,  except  perhaps  in  the  long-sightedness  of  old 
age,  when  the  sufferer  may  tolerably  safely  select  for  himself  any 
glasses  that  allow  him  to  read  easily  a  book  about  30  centimeters 
(12  inches)  from  the  eye.  As  age  advances  stronger  lenses  must 
usually  be  obtained. 


CHAPTER  XVI 
THE  EYE  AS  A  SENSORY  APPARATUS 

The  Excitation  of  the  Visual  Apparatus.  The  excitable  visual 
apparatus  for  each  eye  consists  of  the  retina,  the  optic  nerve,  and 
the  brain-centers  connected  with  the  latter;  however  stimulated, 
if  intact,  it  causes  visual  sensations.  In  the  great  majority  of 
cases  its  excitant  is  objective  light,  and  so  we  refer  all  stimula- 
tions of  it  to  that  cause,  unless  we  have  special  reason  to  know 
the  contrary.  As  already  pointed  out  pressure  on  the  eyeball 
causes  a  luminous  sensation  (phosphene),  which  suggests  itself 
to  us  as  dependent  on  a  luminous  body  situated  in  space  where 
such  an  object  must  be  in  order  to  excite  the  same  part  of  the 
retina.  Since  all  rays  of  light  penetrating  the  eye,  except  in  the 
line  of  its  long  axis,  cross  that  axis,  if  we  press  the  outer  side  of 
the  eyeball  we  get  a  visual  sensation  referred  to  a  luminous  body 
on  the  nasal  side;  if  we  press  below  we  see  the  luminous  patch 
above,  and  so  on. 

Of  course  different  rays  entering  the  eye  take  different  paths 
through  it,  but  on  general  optical  principles,  which  cannot  here  be 
detailed,  we  may  trace  all  oblique  rays  through  the  organ  by 
assuming  that  they  meet  and  leave  the  optic  axis  at  what  are 
known  as  the  nodal  points  of  the  system;  these  (kkf,  Fig.  91)  lie 
near  together  in  the  lens.  If  we  want  to  find  where  rays  of  light 
from  A  will  meet  the  retina  (the  eye  being  properly  accommodated 
for  seeing  an  object  at  that  distance)  we  draw  a  line  from  A  to  k 
(the  first  nodal  point)  and  then  another,  parallel  to  the  first,  from 
kf  (the  second  nodal  point)  to  the  retina.  The  nodal  points  of 
the  eye  lie  so  near  together  that  for  practical  purposes  we  may 
treat  them  as  one  (k,  Fig.  92),  placed  near  the  back  of  the  lens. 
By  manifold  experience  we  have  learnt  that  a  luminous  body 
(A,  Fig.  92)  which  we  see,  always  lies  on  the  prolongation  of  the 
line  joining  the  excited  part  of  the  retina,  a,  and  the  nodal  point  k. 
Hence  any  excitation  of  that  part  of  the  retina  makes  us  think 
of  a  luminous  body  somewhere  on  the  line  a  A,  and,  similarly,  any 

267 


268 


THE  HUMAN  BODY 


excitation  of  6,  of  a  body  on  the  line  6  B  or  its  prolongation.    It  is 
only  other  conflicting  experiences,  as  that  with  the  eyes  closed 


FIG.  91. — Diagram  illustrating  the  points  at  which  incident  rays  meet  the 
retina,  xx,  optic  axis;  k,  first  nodal  point;  k' ,  second  nodal  point;  b,  point  where 
the  image  of  B  would  be  formed,  were  the  eye  properly  accommodated  for  it; 
a,  the  retinal  point  where  the  image  of  A  would  be  formed. 

external  bodies  do  not  excite  visual  sensations,  and  the  constant 
connection  of  the  pressure  felt  on  the  eyelid  with  the  visual  sen- 


FIG.  92. — Diagrammatic  section  through  the  eyeball,  xx,  optic  axis;  k,  nodal 
point. 

sation,  that  enable  us  when  we  press  the  eyeball  to  conclude  that, 
in  spite  of  what  we  seem  to  see,  the  luminous  sensation  is  not  due 
to  objective  light  from  outside  the  eye. 

The  Excitation  of  the  Visual  Apparatus  by  Light.  Light  only 
excites  the  retina  when  it  reaches  its  nerve  end  organs,  the  rods 
and  cones.  The  proofs  of  this  are  several. 

1 .  Light  does  not  arouse  visual  sensations  when  it  falls  directly  on 
the  fibers  of  the  optic  nerve.  Where  this  nerve  enters  there  is  a 
retinal  part  possessing  only  nerve-fibers,  and  this  part  is  blind. 
Close  the  left  eye  and  look  steadily  with  the  right  at  the  cross  in 


THE  EYE  AS  A  SENSORY  APPARATUS 


269 


Fig.  93,  holding  the  book  vertically  in  front  of  the  face,  and  mov- 
ing it  to  and  fro.    It  will  be  found  that  at  about  25  centimeters 


FIG.  93. 

(10  inches)  off  the  white  circle  disappears;  but  when  the  page  is 
nearer  or  farther,  it  is  seen.  During  the  experiment  the  gaze  must 
be  kept  fixed  on  the  cross.  There  is  thus  in  the  field  of  vision  a 
blind  spot,  and  it  is  easy  to  show  by  measurement  that  it  lies  where 
the  optic  nerve  enters. 

When  the  right  eye  is  fixed  on  the  cross,  it  is  so  directed  that 
rays  from  this  fall  on  the  fovea  (y,  Fig.  94).  The  rays  from  the 
circle  then  cross  the  visual  axis  at  the  nodal  point,  x+ 
n,  and  meet  the  retina  at  o.  If  the  distance  of 
the  nodal  point  of  the  eye  from  the  paper  be  /, 
and  from  the  retina  (which  is  15  mm.)  be  F,  then 
the  distance,  on  the  paper,  of  the  cross  from  the 
circle  will  be  to  the  distance  of  y  from  o  as  /  is  to  F. 
Measurements  made  in  this  way  show  that  the 
circle  disappears  when  its  image  is  thrown  on  the 
entry  of  the  optic  nerve,  which  lies  to  the  nasal 
side  of  the  fovea. 

2.  The  above  experiment  having  shown  that 
light  does  not  act  directly  on  the  optic  nerve- 
fibers  any  more  than  it  does  on  any  other  nerve- 
fibers,  we  have  next  to  see  in  what  part  of  the 
retina  those  changes  do  first  occur  which  form 
the  link  between  light  and  nervous  impulses. 
They  occur  in  the  outer  part  of  the  retina,  in  the  rods  and  cones. 
This  is  proved  by  what  is  called  Purkinje's  experiment.  Take 
a  candle  into  a  dark  room  and  look  at  a  surface  not  covered 
with  any  special  pattern,  say  a  whitewashed  wall  or  a  plain 


FIG.  94. 


270  THE  HUMAN  BODY 

window-shade.  .  Hold  the  candle  to  the  side  of  one  eye  and  close 
to  it,  but  so  far  back  that  no  light  enters  the  pupil  from 
it;  that  is,  so  far  back  that  the  flame  just  cannot  be  seen,  but 
so  that  a  strong  light  is  thrown  on  the  white  of  the  eye  as  far  back 
as  possible.  Then  move  the  candle  a  little  to  and  fro.  The  sur- 
face looked  at  will  appear  luminous  with  reddish-yellow  light, 
and  on  it  will  be  seen  dark  branching  lines  which  are  the  shadows 
of  the  retinal  vessels.  Now  in  order  that  these  shadows  may  be 
seen  the  parts  on  which  the  light  acts  must  be  behind  the  vessels, 
that  is,  in  the  layers  of  the  retina  next  the  choroid  since  the  blood- 
vessels lie  in  its  front  strata. 

If  the  light  be  kept  steady  the  vascular  shadows  soon  disappear; 
in  order  to  continue  to  see  them  the  candle  must  be  kept  moving. 
The  explanation  of  this  fact  may  readily  be  made  clear  by  fixing 
the  eyes  for  ten  or  fifteen  seconds  on  the  clot  of  an  "  i "  somewhere 
about  the  middle  of  this  page :  at  first  the  distinction  between  the 
slightly  luminous  black  letters  and  the  highly  luminous  white 
page  is  very  obvious;  in  other  words,  the  different  sensations 
arising  from  the  strongly  and  the  feebly  excited  areas  of  the 
retina.  But  if  the  glance  be  not  allowed  to  wander,  very  soon 
the  letters  become  indistinct  and  at  last  disappear  altogether; 
the  whole  page  looks  uniformly  grayish.  The  reason  of  this  is 
that  the  powerful  stimulation  of  the  retina  by  the  light  reflected 
from  the  white  part  of  the  page  soon  fatigues  the  part  of  the 
visual  apparatus  it  acts  upon;  and  as  this  fatigue  progresses  the 
stimulus  produces  less  and  less  effect.  The  parts  of  the  retina, 
on  the  other  hand,  which  receive  light  only  from  the  black  letters 
are  but  little  stimulated  and  retain  much  of  their  original  ex- 
citability, so  that,  at  last,  the  feebler  excitation  acting  upon 
these  more  irritable  parts  produces  as  much  sensation  as  the 
stronger  stimulus  acting  upon  the  fatigued  parts;  and  the  letters 
become  indistinguishable.  To  see  them  continuously  we  must 
keep  shifting  the  eyes  so  that  the  parts  of  the  visual  apparatus  are 
alternately  fatigued  and  rested,  and  the  general  irritability  of 
the  whole  is  kept  about  the  same.  So,  in  Purkinje's"  experiments 
if  the  position  of  the  shadows  remain  the  same,  the  shaded  part 
of  the  retina  soon  becomes  more  irritable  than  the  more  excited 
unshaded  parts,  and  its  relative  increase  of  irritability  makes  up 
for  the  less  light  falling  on  it,  so  that  the  shadows  cease  to  be  per- 


THE  EYE  AS  A  SENSORY  APPARATUS  271 

ceived.  It  is  for  this  reason  that  we  do  not  see  the  retinal  vessels 
under  ordinary  circumstances.  When  light,  as  usual,  enters  the 
eye  from  the  front  through  the  pupil  the  shadows  always  fall  on 
the  same  parts  of  the  retina,  and  these  parts  are  thus  kept  suffi- 
ciently more  excitable  than  the  rest  to  make  up  for  the  less  light 
reaching  them  through  the  vessels. 

Further  evidence  that  the  rod  and  cone  layer  is  the  true  .recep- 
tor of  the  eye  is  furnished  by  the  fact  that  the  seat  of  most  acute 
vision  is  the  fovea  centralis,  where  only  this  layer  and  the  cone- 
fibers  diverging  from  it  are  present.  When  we  want  to  see  any- 
thing distinctly  we  always  turn  our  eyes  so  that  its  image  shall 
fall  on  the  fovese. 

The  Intensity  of  Visual  Sensations.  Light  considered  as  a 
form  of  energy  may  vary  in  quantity;  physiologically,  also,  we 
distinguish  quantitative  differences  in  light  as  degrees  of  bright- 
ness, but  the  connection  between  the  intensity  of  the  sensation 
excited  and  the  quantity  of  energy  represented  by  the  stimulat- 
ing light  is  not  a  direct  one.  In  the  first  place,  some  rays  excite 
our  visual  apparatus  more  powerfully  than  others :  a  given  amount 
of  energy  in  the  form  of  yellow  light,  for  example,  causes  more 
powerful  visual  sensations  than  the  same  quantity  of  energy  in 
the  form  of  violet  light. 

Furthermore,  the  sense  of  vision,  like  all  the  other  senses,  obeys 
the  psychophysical  law  (Chap.  XIII).  That  is,  differences  of 
sensation  are  proportional  not  to  absolute  but  to  relative  changes 
in  the  amount  of  stimulating  energy.  If  a  room  is  lighted  by  one 
candle  and  another  is  brought  in  we  perceive  an  increase  of  il- 
lumination, but  if  it  is  lighted  by  an  arc  light  the  bringing  in  of  a 
single  lighted  candle  makes  no  perceptible  difference  in  the  il- 
lumination. Another  illustration  of  the  application  of  the  psycho- 
physical  law  to  the  visual  sense  is  found  in  the  fact  that  the  stars 
which  are  ordinarily  invisible  in  the  daytime  can  be  seen  from 
the  bottom  of  deep  wells  or  from  deep  and  narrow  canons.  The 
explanation  is  that  in  open  day  the  general  illumination  of  the 
sky  is  so  intense  that  the  additional  light  of  the  stars  is  unper- 
ceived.  To  one  in  the  bottom  of  a  well,  however,  the  general 
illumination  is  cut  down  so  much  as  to  bring  the  additional  light 
from  the  stars  within  the  limits  of  perception.  The  smallest  dif- 
ference in  luminous  intensity  which  we  can  perceive  is  about 


272  THE  HUMAN  BODY 

jJo  of  the  whole,  for  all  the  range  of  lights  we  use  in  carrying 
on  our  ordinary  occupations.  For  strong  lights  the  smallest  per- 
ceptible fraction  is  considerably  greater;  finally  we  reach  a  limit 
where  no  increase  in  brightness  is  felt.  For  weak  illumination  the 
sensation  is  more  nearly  proportioned  to  the  total  differences  of 
the  objective  light.  Thus  in  a  dark  room  an  object  reflecting  all 
the  little  light  that  reaches  it  appears  almost  twice  as  bright  as 
one  reflecting  only  half;  in  a  stronger  light  it  would  not  so  appear. 
Bright  objects  in  general  obscurity  thus  appear  unnaturally 
bright  when  compared  with  things  about  them,  and  indeed  often 
look  self-luminous.  A  cat's  eyes,  for  example,  are  said  to  "  shine 
in  the  dark";  and  painters  to  produce  moonlight  effects  always 
make  the  bright  parts  of  a  picture  relatively  brighter,  when  com- 
pared with  things  about  them,  than  would  be  the  case  if  a  sunny 
scene  were  to  be  represented;  by  a  relatively  excessive  use  of 
white  pigment  they  produce  the  relatively  great  brightness  of 
those  things  which  are  seen  at  all  in  the  general  obscurity  of  a 
moonlight  landscape. 

Function  of  the  Rods.  Inasmuch  as  the  rods  are  absent  from 
the  fovese,  they  cannot  be  concerned  with  ordinary  conscious  vi- 
sion since  clear  vision,  as  we  know  from  experience,  is  confined  to 
these  areas.  It  is  easy  to  demonstrate  by  a  simple  experiment 
that  the  parts  of  the  retina  containing  rods  are  more  susceptible 
to  feeble  lights  than  is  the  fovea,  which  is  devoid  of  them.  The 
constellation  of  the  Pleiades  consists  of  seven  stars ;  one  of  these  is 
so  faint,  however,  as  to  be  invisible  to  most  eyes  when  the  con- 
stellation is  looked  at  directly.  If  the  gaze  be  turned  to  a  point 
in  the  sky  a  degree  or  two  to  one  side  of  the  constellation,  so  as 
to  throw  its  image  off  the  fovea  unto  a  rod-containing  area,  the 
seventh  star  becomes  immediately  visible. 

This  evidence  indicates  that  the  function  of  the  rods  is  some 
how  related  to  the  reception  of  light  stimuli  of  feeble  intensity. 
The  portions  of  the  retina  outside  the  fovea  seem  to  function  for 
the  most  part  more  reflexly  than  consciously;  stimuli  striking 
these  portions  of  the  retina  bring  about  reflex  movements  of  the 
eyes  and  head  so  that  the  source  of  stimulation  throws  its  light 
upon  the  foveae,  and  we  derive  conscious  perceptions  as  to  its 
nature.  For  such  reflex  activity  a  high  degree  of  irritability  is 
desirable. 


THE  EYE  AS  A  SENSORY  APPARATUS  273 

It  is  said  that  some  animals,  such  as  snakes,  have  no  rods  in 
their  retinas,  while  the  retinas  of  others  of  nocturnal  habits,  such 
as  owls,  consist  exclusively  of  rods. 

Visual  Purple.  If  a  perfectly  fresh  retina  be  excised  rapidly, 
its  outer  layers  will  be  found  of  a  rich  purple  color.  In  daylight 
this  rapidly  bleaches,  but  in  the  dark  persists  even  when  putre- 
faction has  set  in.  In  pure  yellow  li£ht  it  also  remains  unbleached 
a  long  time,  but  in  other  lights  disappears  at  different  rates.  If  a 
rabbit's  eye  be  fixed  immovably  and  exposed  so  that  an  image 
of  a  window  is  focussed  on  the  same  part  of  its  retina  for  some 
time,  and  then  the  eye  be  rapidly  excised  in  the  dark  and  placed 
in  solution  of  potash  alum,  a  colorless  image  of  the  window  is 
found  on  the  retina,  surrounded  by  the  visual  purple  of  the  rest 
which  is,  through  the  alum,  fixed  or  rendered  incapable  of  change 
by  light.  Photographs,  or  optograms,  are  thus  obtained  which 
differ  from  the  photographer's  only  in  the  nature  of  the  chemical 
substances  and  processes  involved.  Both  depend  on  the  produc- 
tion of  a  chemical  reaction  by  light.  If  the  eye  be  not  rapidly  ex- 
cised and  put  in  the  alum  after  its  exposure,  the  optogram  will 
disappear;  the  vision  purple  being  rapidly  regenerated  at  the 
bleached  part.  This  reproduction  of  it  is  due  mainly  to  the  cells 
of  the  pigmentary  layer  of  the  retina,  which  in  living  eyes  ex- 
posed to  light  thrust  long  processes  between  the  rods  and  cones. 
Portion  of  frogs'  retinas  raised  from  this,  bleach  more  rapidly 
than  those  left  in  contact  with  it,  but  become  soon  purple  again 
if  let  fall  back  upon  the  pigment-cells. 

The  visual  purple,  as  stated  previously,  occurs  only  in  the  outer 
segments  of  the  rods.  Whatever  function  it  has  is  probably  con- 
nected, therefore,  with  their  special  property  of  reacting  to  feeble 
lights.  The  nature  of  its  function  is,  further  than  this,  unknown. 

The  Duration  of  Luminous  Sensations.  This  is  greater  than 
that  of  the  stimulus,  a  fact  taken  advantage  of  in  making  fire- 
works: an  ascending  rocket  produces  the  sensation  of  a  trail  of 
light  extending  far  behind  the  position  of  the  bright  part  of  the 
rocket  itself  at  the  moment,  because  the  sensation  aroused  by  it 
in  a  lower  part  of  its  course  still  persists.  So,  shooting  stars  ap- 
pear to  have  luminous  tails  behind  them.  By  rotating  rapidly 
before  the  eye  a  disk  with  alternate  white  and  black  sectors  we 
get  for  each  point  of  the  retina  on  which  a  part  of  its  image  falls, 


274  THE  HUMAN  BODY 

alternating  stimulation  (due  to  the  passage  of  white  sector)  and 
rest  (when  a  black  sector  is  passing).  If  the  rotation  be  rapid 
enough  the  sensation  aroused  is  that  of  a  uniform  gray,  such  as 
would  be  produced  if  the  white  and  black  were  mixed  and  spread 
evenly  over  the  disk.  In  each  revolution  the  eye  gets  as  much 
light  as  if  that  were  the  case,  and  is  unable  to  distinguish  that  this 
light  is  made  up  of  separate  portions  reaching  it  at  intervals: 
the  stimulation  due  to  each  lasts  until  the  next  begins  and  so 
all  are  fused  together.  If,  while  looking  at  the  flame,  one  turns 
out  suddenly  the  gas  in  a  room  containing  no  other  light,  the  image 
of  the  flame  persists  a  short  time  after  the  flame  itself  is  extin- 
guished. 

The  Localizing  Power  of  the  Retina.  As  already  pointed  out 
a  necessary  condition  of  seeing  definite  objects,  as  distinguished 
from  the  power  of  recognizing  differences  of  light  and  darkness,  is 
that  all  light  entering  the  eye  from  one  point  of  an  object  shall  be 
focussed  on  one  point  of  the  retina.  This,  however,  would  not  be 
of  any  use  had  we  not  the  faculty  of  distinguishing  the  stimula- 
tion of  one  part  of  the  retina  from  that  of  another  part.  This 

i« 


FIG.  95. 

power  the  visual  apparatus  possesses  in  a  very  high  degree;  while 
with  the  skin  we  cannot  distinguish  from  one,  two  points  touching 
it  less  than  1  mm.  (^  inch)  apart,  with  our  eyes  we  can  distin- 
guish two  points  whose  retinal  images  are  not  more  than  .004  mm. 
(.00016  inch)  apart.  The  distance  between  the  retinal  images  of 
two  points  is  determined  by  the  "  visual  angle "  under  which 
they  are  seen;  this  angle  is  that  included  between  lines  drawn 
from  them  to  the  nodal  point  of  the  eye.  If  a  and  6  (Fig.  95)  are 
luminous  points,  the  image  of  a  will  be  formed  at  a'  on  the  pro- 
longation of  the  line  a  n  joining  a  with  the  node,  n.  Similarly  the 
image  of  b  will  be  formed  at  6'.  If  a  and  b  still  remaining  the  same 
distance  apart,  be  moved  nearer  the  eye  to  c  and  d,  then  the 
visual  angle  under  which  they  are  seen  will  be  greater  and  their 
retinal  images  will  be  farther  apart,  at  c'  and  d'.  If  a  and  6  are 


THE  EYE  AS  A  SENSORY  APPARATUS  275 

the  highest  and  lowest  parts  of  an  object,  the  distance  between 
their  retinal  images  will  then  depend,  clearly,  not  only  on  the 
size  of  the  object,  but  on  its  distance  from  the  eye;  to  know  the 
discriminating  power  of  the  retina 'we  must  therefore  measure 
the  visual  angle  in  each  case.  In  the  fovea  centralis  two  objects 
seen  under  a  visual  angle  of  50  to  70  seconds  can  be  distinguished 
from  one  another;  this  gives  for  the  distance  between  the  retinal 
images  that  above  mentioned,  and  corresponds  pretty  accurately 
to  the  diameter  of  a  cone  in  that  part  of  the  retina.  We  may 
conclude,  therefore,  that  when  two  images  fall  on  the  same  cone 
or  on  two  contiguous  cones  they  are  not  discriminated;  but  that 
if  one  or  more  unstimulated  cones  intervene  between  the  stimu- 
lated, the  points  may  be  perceived  as  distinct.  The  diameter  of 
a  rod  or  cone,  in  fact,  marks  the  anatomical  limit  up  to  which 
we  can  by  practice  raise  our  acuteness  of  visual  discrimination; 
and  in  the  fovea  which  we  constantly  use  all  our  lives  in  looking 
at  things  which  we  want  to  see  distinctly,  we  have  educated  the 
visual  apparatus  up  to  about  its  highest  power.  Elsewhere  on 
the  retina  our  discriminating  power  is  much  less  and  diminishes 
as  the  distance  from  the  fovea  increases. 

While  we  can  tell  the  stimulation  of  an  upper  part  of  the  retina 
from  a  lower,  or  a  right  region  from  a  left,  it  must  be  borne  in 
mind  that  we  have  no  direct  knowledge  of  which  is  upper  or  lower 
or  right  or  left  in  the  ocular  image.  All  our  visual  sensations  tell 
us  is  that  they  are  aroused  at  different  points,  and  nothing  at  all 
about  the  actual  positions  of  these  on  the  retina.  There,  is  no 
other  eye  behind  the  retina  looking  at  it  to  see  the  inversion  of  the 
image  formed  on  it.  Suppose  I  am  looking  at  a  pane  in  a  second- 
story  window  of  a  distant  house:  its  image  will  then  fall  on  the 
fovea  centralis;  the  line  joining  this  with  the  pane  is  called  the 
visual  axis.  The  image  of  the  roof  will  be  formed  on  a  part  of  the 
retina  below  the  fovea,  and  that  of  the  front  door  above  it.  I 
distinguish  that  the  images  of  all  these  fall  on  different  parts  of 
the  retina  in  certain  relative  positions,  and  have  learnt,  by  the 
experience  of  all  my  life,  that  when  the  image  of  anything  arouses 
the  sensation  due  to  excitation  of  part  of  the  retina  below  the 
fovea  the  object  is  above  my  visual  axis,  and  vice  versa;  similarly 
with  right  and  left.  Consequently  I  interpret  the  stimulation  of 
lower  retinal  regions  as  meaning  high  objects,  and  of  right  retinaJ 


276  THE  HUMAN  BODY 

regions  as  meaning  left  objects,  and  never  get  confused  by  the 
inverted  retinal  image  about  which  directly  I  know  nothing.  A 
new-born  child,  even  supposing  it  could  use  its  muscles  perfectly, 
could  not,  except  by  mere  chance,  reach  towards  an  object  which 
it  saw;  it  would  grasp  at  random,  not  yet  having  learnt  that  to 
reach  an  object  exciting  a  part  of  the  retina  above  the  fovea 
needed  movement  of  the  hand  towards  a  position  in  space  below 
the  visual  axis;  but  very  soon  it  learns  that  things  near  its  brow, 
that  is  up,  excite  certain  visual  sensations,  and  objects  below  its 
eyes  others,  and  similarly  with  regard  to  right  and  left;  in  time 
it  learns  to  interpret  retinal  stimuli  so  as  to  localize  accurately 
the  direction,  with  reference  to  its  eyes,  of  outer  objects,  and 
never  thenceforth  is  puzzled  by  retinal  inversion. 

Color  Vision.  Sunlight  reflected  from  snow  gives  us  a  sensa- 
tion which  we  call  white.  The  same  light  sent  through  a  prism 
and  reflected  from  a  white  surface  excites  in  us  no  white  sensation 
but  a  number  of  color  sensations,  gradating  insensibly  from  red  to 
violet,  through  orange,  yellow,  green,  blue-green,  blue,  and  indigo. 
The  prism  separates  from  one  another  light-rays  of  different 
periods  of  oscillation  and  each  ray  excites  in  us  a  colored  visual 
sensation,  while  all  mixed  together,  as  in  sunlight,  they  arouse 
the  entirely  different  sensation  of  white.  If  the  light  fall  on  a 
piece  of  black  velvet  we  get  still  another  sensation,  that  of  black; 
in  this  case  the  light-rays  are  so  absorbed  that  but  few  are  reflected 
to  the  eye  and  the  visual  apparatus  is  left  at  rest.  Physically 
black  represents  nothing :  it  is  a  mere  zero — the  absence  of  ethereal 
vibrations;  but,  in  consciousness,  it  is  as  definite  a  sensation  as 
white,  red,  or  any  other  color.  We  do  not  feel  blackness  or  dark- 
ness except  over  the  region  of  the  possible  visual  field  of  our  eyes. 
In  a  perfectly  dark  room  we  only  feel  the  darkness  in  front  of  our 
eyes,  and  in  the  light  there  is  no  such  sensation  associated  with 
the  back  of  our  heads  or  the  palms  of  our  hands,  though  through 
these  we  get  no  visual  sensations.  It  is  obvious,  therefore,  that 
the  sensation  of  blackness  is  not  due  to  the  mere  absence  of  lu- 
minous stimuli,  but  to  the  unexcited  state  of  the  retinas,  which  are 
alone  capable  of  being  excited  by  such  stimuli  when  present. 
This  fact  is  a  very  remarkable  one,  and  is  not  paralleled  in  any 
other  sense.  Physically,  complete  stillness  is  to  the  ear  what 
darkness  is  to  the  eye;  but  silence  impresses  itself  on  us  as  the  ab- 


THE  EYE  AS  A  SENSORY  APPARATUS  277 

sence  of  sensation,  while  darkness  causes  a  definite  feeling  of 
"blackness." 

Our  color  sensations  insensibly  fade  into  one  another;  starting 
with  black  we  can  insensibly  pass  through  lighter  and  lighter 
shades  of  gray  to  white:  or  beginning  with  green  through  darker 
and  darker  shades  of  it  to  black  or  through  lighter  and  lighter 
to  white:  or  beginning  with  red  we  can  by  imperceptible  steps 
pass  to  orange,  from  that  to  yellow  and  so  on  to  the  end  of  the 
solar  spectrum :  and  from  the  violet,  through  purple  and  carmine, 
we  may  get  back  again  to  red.  Black  and  white  appear  to  bo 
fundamental  color  sensations  mixed  up  with  all  the  rest:  we 
never  imagine  a  color  but  as  light  or  dark,  that  is,  as  more  or  less 
near  white  or  black;  and  it  is  found  that  as  the  light  thrown  on 
any  given  colored  surface  weakens,  the  shade  becomes  deeper 
until  it  passes  into  black;  and  if  the  illumination  be  increased,  the 
color  becomes  "lighter"  until  it  passes  into  white.  Of  all  the 
colors  of  the  spectrum  yellow  most  easily  passes  into  white  with 
strong  illumination.  Black  and  white,  with  the  grays  which  are 
mixtures  of  the  two,  thus  seem  to  stand  apart  from  all  the  rest  as 
the  fundamental  visual  sensations,  and  the  others  alone  are  in 
common  parlance  named  "colors."  It  has  even  been  suggested 
that  the  power  of  differentiating  them  in  sensation  has  only  lately 
been  acquired  by  man,  and  a  certain  amount  of  evidence  has  been 
adduced  from  passages  in  the  Iliad  to  prove  that  the  Greeks  in 
Homer's  time  confused  together  colors  that  are  very  different  to 
most  modern  eyes;  at  any  rate  there  seems  to  be  no  doubt  that 
the  color  sense  can  be  greatly  improved  by  practice ;  women  whose 
mode  of  dress  causes  them  to  pay  more  attention  to  the  matter, 
have,  as  a  general  rule,  a  more  acute  color  sense  than  men. 

Leaving  aside  black,  white,  gray,  and  the  various  browns 
(which  are  only  dark  tints  of  other  colors),  we  may  enumerate 
our  color  sensations  as  red,  orange,  yellow,  green,  blue,  violet, 
and  purple;  between  each  there  are,  however,  numerous  transi- 
tion shades,  as  yellow-green,  blue-green,  etc.,  so  that  the  number 
which  shall  have  definite  names  given  to  them  is  to  a  large  extent 
arbitrary.  Of  the  above,  all  but  purple  are  found  in  the  spec- 
trum given  when  sunlight  is  separated  by  a  prism  into  its  rays 
of  different  refrangibility;  rays  of  a  certain  wave-length  cause  in 
us  the  feeling  red;  others  yellow,  and  so  on;  for  convenience  we 


278  THE  HUMAN  BODY 

may  speak  of  these  as  red,  yellow,  blue,  etc.,  rays;  ail  together, 
in  about  equal  proportions,  they  arouse  the  sensation  of  white. 

Peculiarities  of  Color  Vision.  A  remarkable  fact  is  that  most 
color  feelings  can  be  aroused  in  several  ways.  White,  for  ex- 
ample, not  only  by  the  above  general  mixture,  but  red  and  blue- 
green  rays,  or  orange  and  blue,  or  yellow  and  violet,  taken  in  pairs 
in  certain  proportions,  and  acting  simultaneously  or  in  very  rapid 
succession  on  the  same  part  of  the  retina,  cause  the  sensation  of 
white:  such  colors  are  called  complementary  to  one  another.  The 
mixture  may  be  made  in  several  ways;  as,  for  example,  by  caus- 
ing the  red  and  blue-green  parts  of  the  spectrum  to  overlap,  or  by 
painting  red  and  blue-green  sectors  on  a  disk  and  rotating  it 
rapidly;  they  cannot  be  made,  however,  by  mixing  pigments, 
since  what  happens  in  such  cases  is  a  very  complex  phenomenon. 
Painters,  for  example,  are  accustomed  to  produce  green  by  mix- 
ing blue  and  yellow  paints,  and  some  may  be  inclined  to  ridicule 
the  statement  that  yellow  and  blue  when  mixed  give  white. 
When,  however,  we  mix  the  pigments  we  do  not  combine  the 
sensations  of  the  same  name,  which  is  the  matter  in  question.  Blue 
paint  is  blue  because  it  absorbs  all  the  rays  of  the  sunlight  except 
the  blue  and  some  of  the  green ;  yellow  is  yellow  because  it  absorbs 
all  but  the  yellow  and  some  of  the  green,  and  when  blue  and  yel- 
low are  mixed  the  blue  absorbs  all  the  distinctive  part  of  the 
yellow  and  the  yellow  does  the  same  for  the  blue ;  and  so  only  the 
green  is  left  over  to  reflect  light  to  the  eye,  and  the  mixture  has 
that  color.  Grass-green  has  no  complementary  color  in  the  solar 
spectrum;  but  with  purple,  which  is  made  by  mixing  red  and  blue, 
it  gives  white.  Several  other  colors  taken  three  together,  give 
also  the  sensation  of  white.  If  then  we  call  the  light-rays  which 
arouse  in  us  the  sensation  red,  a,  those  giving  us  the  sensation 
orange  6,  yellow  c,  and  so  on,  we  find  that  we  get  the  sensation 
white  with  a,  b,  c,  d,  e,  f,  and  g  all  together;  or  with  b  and  e,  or 
with  c  and  /,  or  with  a,  d,  and  e;  our  sensation  white  has  no  deter- 
minate relation  to  ethereal  oscillations  of  a  given  period,  and  the 
same  is  true  for  several  other  colors;  yellow  feeling,  for  example, 
may  be  excited  by  ethereal  vibrations  of  one  given  wave-length 
(spectral  yellow) ,  •  or  by  a  light  containing  only  such  waves  as 
taken  separately  cause  the  sensations  red  and  grass-green;  in 
otner  words,  a  physical  light  in  which  there  are  no  waves  of  the 


THE  EYE  AS  A   SENSORY  APPARATUS  279 

"yellow"  length  may  cause  in  us  the  sensation  yellow,  which  is 
only  one  more  instance  of  the  general  fact  that  our  sensations, 
as  such,  give  us  no  direct  information  as  to  the  nature  of  external 
forces;  they  are  but  signs  which  we  have  to  interpret. 

Function  of  the  Cones.  These  structures,  since  they  are  the 
only  sensitive  elements  of  the  fovea  centralis,  must  be  the  recep- 
tors for  all  ordinary  conscious  vision.  Their  special  function  is 
doubtless  the  perception  of  color.  This  perception  is  a  part  of 
all  our  conscious  visual  sensations.  We  never  think  of  a  luminous 
object  as  being  merely  light;  but  always  as  having  some  color. 

Distribution  of  Color  Sense  over  the  Retina.  By  means  of  an 
apparatus  called  the  perimeter  it  is  possible  to  determine  the 
boundaries  of  visual  sensation  in  the  retina.  In  using  this  ap- 
paratus the  subject  with  head  supported  in  one  position  looks 
fixedly  at  a  point  straight  in  front;  the  observer  then  brings 
small  squares  of  paper  from  the  side  toward  the  front  and  the  sub- 
ject reports  the  instant  the  square  of  paper  comes  into  his  field 
of  vision.  The  angle  is  marked  on  a  specially  prepared  chart  and 
the  observation  repeated  along  different  radii.  By  this  means 
the  field  of  vision  is  mapped  out.  The  visual  field  for  any  par- 
ticular color  can  be  determined  similarly,  the  subject  in  this  case 
being  required  to  report  as  soon  as  he  is  certain  what  the  color 
of  the  square  of  paper  is.  Such  studies  have  brought  out  the  in- 
teresting fact  that  ability  to  perceive  the  different  colors  is  un- 
equally distributed  over  the  retina.  The  margins  of  the  visual 
field  are  sensitive  only  to  white  and  black,  and  to  their  mixtures 
of  gray;  the  fields  for  blue  and  yellow  cover  the  whole  area  except 
the  margins;  the  fields  for  red  and  green  sensation  are  the  smallest 
of  all,  occupying  only  the  central  part  of  the  field  and  covering 
about  half  its  entire  surface.  According  to  most  determinations 
the  boundaries  of  the  yellow  and  blue  fields  do  not  coincide  ex- 
actly, nor  those  of  green  and  red;  but  it  is  quite  probable  that 
they  do  coincide  exactly  in  reality,  and  that  experimental  errors 
account  for  their  apparent  divergences. 

It  is  clear  from  these  observations  that  the  cones  in  the  central 
part  of  the  visual  field  are  sensitive  to  all  colors;  that  those  further 
out  are  sensitive  to  all  except  red  and  green;  and  that  the  marginal 
ones  are  insensitive  to  color  as  such,  and  distinguish  only  degrees 
of  light  and  darkness. 


280  THE  HUMAN  BODY 

Color  Blindness.  This  is  a  deficiency  in  color  vision  whereby 
certain  colors  fail  to  produce  the  characteristic  color  sensations 
that  they  do  in  normal  eyes.  The  commonest  sort  of  color  blind- 
ness is  so-called  red-green  blindness.  In  it  neither  red  nor  green 
has  the  same  value  as  in  normal  eyes.  Both  colors  seem  to  give 
the  sensation  of  "  neutral "  tints,  grays  and  browns.  Two  varieties 
of  red-green  blindness  are  recognized;  the  difference  between 
them  is,  however,  apparently  one  of  perception  of  luminosity 
rather  than  of  color.  To  the  red-blind  person  a  red  object  looks 
dim  as  well  as  of  neutral  tint;  to  the  green-blind  person  a  red  ob- 
ject appears  to  be  bright,  although  in  color  of  neutral  tint  likewise. 

Red-green  blindness  is  the  common  form.  It  is  usually  con- 
genital and  occurs  more  frequently  in  males  than  in  females. 
One  male  in  twenty-five,  on  the  average,  is  color  blind,  and  less 
than  one  female  in  a  hundred.  It  has  been  suggested  that  this 
difference  is  at  bottom  one  of  training;  women  have  from  time 
immemorial  used  brighter  colors  and  more  colors  in  their  clothing 
than  have  men,  and  have  therefore  become  more  accustomed  to 
making  nice  color  discriminations. 

A  form  of  violet  blindness  has  been  described  as  occurring  in 
rare  pathological  conditions.  It  can  be  brought  on  temporarily, 
it  is  said,  by  taking  the  drug  santonin.  This  form  of  color  blind- 
ness has  not  been  thoroughly  studied.  Monochromatic  blindness, 
in  which  the  only  sensation  is  of  degrees  of  grayness,  shading  at 
one  end  into  white,  at  the  other  into  black,  is  also  described.  This 
is  accompanied  in  most  cases  by  blindness  of  the  fovea,  and  is 
probably  therefore  the  result  of  complete  loss  of  cone  function. 

A  full  explanation  of  red-green  blindness  cannot  be  had,  of 
course,  until  the  mechanism  of  color  vision  is  understood.  From 
what  was  said  about  the  distribution  of  color  perception  in  the 
retina  it  is  clear,  however,  that  in  all  eyes  there  is  an  area  of  red- 
green  blindness  between  the  area  of  complete  color  perception 
and  the  area  of  white-black  vision.  If  we  suppose  the  cones  in 
the  central  area  to  be  undifferentiated  from  those  of  this  im- 
mediately surrounding  zone  we  have  a  condition  of  red-green 
blindness  involving  the  whole  eye  and  corresponding  to  that  of  the 
color-blind  person. 

The  detection  of  color  blindness  is  often  a  matter  of  considerable 
importance,  especially  in  sailors  and  railroad  operators  since  the 


THE  EYE  AS  A  SENSORY  APPARATUS  281 

two  colors  most  commonly  confounded,  red  and  green,  are  those 
used  in  maritime  and  railroad  signals.  Persons  attach  such  dif- 
ferent names  to  colors  that  a  decision  as  to  color  blindness  can- 
not be  safely  arrived  at  by  simply  showing  a  color  and  asking  its 
name.  The  best  plan  is  to  take  a  heap  of  worsted  of  all  tints, 
select  one,  say  a  red,  and  tell  the  man  to  put  alongside  it  all  those 
of  the  same  color,  whether  of  a  lighter  or  a  darker  shade;  if  red 
blind  he  will  select  not  only  the  reds  but  the  greens,  especially 
the  paler  tints,  as  well  as  the  grays  and  browns.  This  test,  which 
is  almost  universally  used,  was  devised  by  the  Swedish  physiologist, 
Holmgren. 

After-images  and  Contrasts.  These  are  well-marked  visual 
phenomena,  and  have  to  be  taken  into  account  in  attempting  to 
explain  the  mechanism  of  color  vision.  After-images  are  visual 
sensations  which  remain  after  the  withdrawal  of  the  stimulus. 
They  are  best  seen  after  looking  at  bright  objects,  or  fixedly  for 
several  seconds  at  the  same  object.  After-images  are  of  two 
sorts,  positive  and  negative.  Positive  after-images  are  always 
the  same  color  as  the  object  looked  at;  if  one  looks  for  an  instant 
at  an  incandescent  filament  and  then  shuts  his  eyes  he  perceives 
a  positive  after  image  of  the  filament.  This  is  due,  probably,  to 
the  persistence  of  the  chemical  process  in  the  retina  after  the  light 
which  causes  it  is  withdrawn.  Negative  after-images,  instead  of 
being  the  color  of  the  object  looked  at,  are  always  of  its  compli- 
mentary color;  if  a  red  paper  is  looked  at  fixedly  for  several 
seconds  and  the  eyes  then  turned  to  a  white  wall,  a  bluish  green 
after-image  is  seen,  instead  of  a  red  one.  Negative  after-images 
can  also  be  seen  by  closing  the  eyes  after  looking  fixedly  at  a 
bright  object  for  some  seconds. 

Contrasts  are  effects  produced  by  bringing  side  by  side  different 
colors ;  blue  appears  bluer  when  near  yellow  than  when  near  other 
shades  or  the  same  shade  of  blue.  Red  and  green  heighten  each 
other  in  a  similar  way.  If  a  large  black  square  and  a  large  white 
square  are  placed  side  by  side  the  black  square  looks  blacker  on 
the  edge  next  the  white  than  elsewhere,  and  the  white  looks 
whiter  next  the  black  than  elsewhere. 

Theories  of  Color  Vision.  A  theory  of  color  vision  to  be  ac- 
ceptable must  explain  first  the  fundamental  facts  of  color  per- 
ception, our  ability  to  distinguish  innumerable  shades  of  color, 


282  THE  HUMAN  BODY 

and  the  fact  that  pairs  or  groups  of  fused  colors  give  rise  to  sen- 
sations entirely  unrelated  to  any  of  the  constituent  colors.  The 
theory  must  account  for  the  distribution  of  color  perception  over 
the  retina  and  for  the  facts  of  color  blindness ;  it  must  also  explain 
after-images  and  contrasts.  The  fact  that  black,  the  absence  of 
stimulation,  has  all  the  subjective  qualities  of  a  true  sensation  is 
also  to  be  explained  in  some  way.  No  theory  yet  proposed  is 
satisfactory  in  accounting  for  all  the  known  facts.  Each  one 
lays  special  emphasis  on  some  group  of  visual  phenomena  and 
disregards  such  facts  as  cannot  be  harmonized  with  it.  Three 
interesting  theories  will  be  briefly  summarized  for  the  sake  of 
showing  how  such  a  problem  is  attacked.  Each  of  them  assumes 
that  the  excitation  of  the  visual  nerve  endings  depends  upon  the 
action  of  light  upon  certain  photochemical  substances  in  the  cones. 

The  Young-Helmholtz  Theory.  This  theory,  proposed  by 
Young  in  1807  and  elaborated  by  Helmholtz  many  years  later, 
may  be  described  rather  as  an  attempt  to  apply  the  doctrine  of 
specific  nerve  energies  to  color  vision  than  as  an  attempt  to  ex- 
plain the  facts  of  color  vision  as  we  know  them. 

It  is  an  interesting  illustration  of  the  extent  to  which  this  doc- 
trine has  come  to  physiologists  to  seem  fundamental  in  forming 
conceptions  of  the  nervous  system  that  the  theory  of  Young, 
manifestly  impossible  as  it  is,  because  of  the  numerous,  facts  with 
which  it  cannot  be  harmonized,  has  received  much  more  atten- 
tion and  consideration  than  other  theories,  agreeing  with  many  of 
the  facts  as  we  know  them,  but  not  in  accord  with  the  doctrine 
of  specific  nerve  energies. 

The  theory  assumes  all  our  color  sensations  to  be  based  on  three 
primary  ones,  red,  green,  and  violet,  each  of  which  is  aroused  by 
the  decomposition  of  its  special  photochemical  substance,  and 
each  having  distinct  nervous  connection  with  the  visual  area  of 
the  cerebrum.  Since  anatomical  study  shows  that  each  cone  has 
a  single  nerve-fiber  leading  from  it  we  must  either  suppose  that 
there  are  three  sorts  of  cones,  one  red-perceiving,  one  green- 
perceiving,  and  one  violet-perceiving,  and  that  these  are  scattered 
in  groups  of  three  over  the  retina;  or  we  must  conclude  that  the 
nerve-fiber  is  not  the  unit  of  nervous  conduction  but  that  it  is 
made  up  of  smaller  units  in  the  same  way  that  the  nerve-trunk  is 
made  up  of  fibers.  The  originators  of  the  theory  held  the  first 


THE  EYE  AS  A  SENSORY  APPARATUS  283 

of  these  views;  they  assumed  that  any  method  of  stimulating  a 
red-perceiving  cone  would  give  rise  to  red  sensations;  if  red-  and 
green-perceiving  cones  were  stimulated  simultaneously  the  effect 
in  consciousness  would  be  very  different  from  that  of  stimulating 
either  one  alone,  the  red  cone  and  the  green  cone  together,  giving 
yellow;  and  if  all  three  sorts  were  stimulated  at  once  in  equal 
amounts  the  effect  would  be  a  sensation  of  white.  All  our  color 
perceptions  are  supposed  to  be  based  on  proper  combinations  of 
stimuli  acting  on  the  groups  of  three  cones.  To  explain  some  facts, 
such  as  that  pure  red  light  as  it  becomes  brighter  and  brighter 
approaches  and  finally  becomes  white,  the  theory  supposes  that 
no  light  stimulates  only  one  cone ;  all  three  of  the  group  are  stim- 
ulated by  light  of  any  color,  and  the  effect  in  consciousness  de- 
pends on  which  is  more  strongly  stimulated. 

It  is  easy  to  demonstrate  that  the  color  of  a  spot  of  light  whose 
retinal  image  is  of  such  a  size  as  to  fall  within  the  boundaries  of  a 
single  cone  can  be  accurately  distinguished.  According  to  the 
theory  white  light  should  seem  to  be  one  or  the  other  fundamental 
color  under  such  circumstances,  instead  of  looking  white  as  it 
actually  does. 

When  the  theory  was  proposed  it  was  thought  that  red-blindness 
and  green-blindness  were  entirely  distinct  forms  of  color  blind- 
ness. The  theory  fits  that  idea  very  well,  since  it  supposes  dis- 
tinct red-perceiving  and  green-perceiving  cones.  Now  that  we 
know  that  both  red  and  green  blindness  are  really  forms  of  red- 
green  blindness  in  which  neither  red  nor  green  gives  normal  color 
sensations  the  theory  does  not  agree  at  all  with  the  facts  in  this 
regard.  The  theory  also  fails  to  explain  the  distribution  of  color 
vision  over  the  retina  or  the  fact  that  black  is  a  true  sensation. 
It  explains  very  well  on  the  basis  of  fatigue  the  negative  after- 
images that  one  sees  when  the  eyes  are  turned  to  a  white  surface 
after  looking  at  a  colored  body ;  for  if  one  particular  set  of  cones  is 
fatigued  by  looking  steadily  at  any  color,  white  light  coming  upon 
the  retina  stimulates  the  unfatigued  ones  more  powerfully  than 
the  fatigued  ones,  and  instead  of  the  sensation  of  white  which 
follows  equal  stimulation  of  all  cones,  the  complimentary  color 
to  that  one  which  fatigued  the  cones  in  the  first  place  is  seen. 
The  theory  does  not  explain  well  the  negative  after-images  seen 
with  closed  eyes,  nor  does  it  explain  the  phenomena  of  contrast. 


284  THE  HUMAN  BODY 

The  Bering  Theory.  This  theory  frankly  makes  no  attempt  to 
accord  with  the  doctrine  of  specific  nerve  energies,  but  seeks 
rather  to  explain  on  a  rational  basis  those  visual  phenomena 
which  the  Young-Helmholtz  theory  explains  poorly  or  not  at  all. 
It  is  based  upon  the  observation  that  whereas  we  recognize  certain 
colors  as  being  combinations  of  two  others,  as  bluish-green,  or 
reddish-yellow,  there  are  no  colors  which  we  recognize  as  com- 
binations of  complementary  colors;  greenish-red  or  yellowish-blue 
do  not  occur.  The  existence  of  these  mutually  exclusive  colors 
suggested  to  the  author  of  this  theory  that  there  might  be  two 
opposing  processes  going  on  in  the  retina,  one  a  process  of  chemical 
breaking  down  or  dissimilation;  the  other  a  process  of  building 
up,  or  assimilation. 

He  therefore  postulated  three  photochemical  substances,  a 
white-black  substance,  a  yellow-blue  substance,  and  a  red-green 
substance.  He  supposed  that  white  light  falling  on  the  retina 
breaks  down  the  white-black  substance  and  gives  rise  to  the 
sensation  of  white ;  whenever  no  white  light  is  falling  on  the  retina 
this  substance  is  building  itself  up;  this  gives  rise  to  the  sensation 
of  black.  Similarly  the  sensation  of  red  is  the  result  of  breaking 
down  the  red-green  substance,  and  green  of  its  assimilation.  The 
white  sensation  resulting  from  stimulation  of  complementary  colors 
is  explained  as  due  to  neutralization  of  opposing  effects.  When  red 
and  green  light  come  together  into  the  retina  the  red-green  sub- 
stance is  neither  broken  down  nor  built  up.  Both  red  and  green 
light  have  a  dissimilatory  effect  on  the  white-black  substance  as 
do  rays  of  all  colors,  according  to  the  theory.  The  only  effect, 
therefore,  of  the  complementary  colors,  is  to  produce  a  sensation 
of  white.  Contrast  is  explained  as  due  to  the  maintenance  of  a 
sort  of  chemical  balance  in  the  retina  whereby  a  breaking  down  of 
one  of  the  elements  in  part  of  it  is  accompanied  by  building  up  of 
the  same  element  in  neighboring  areas.  So,  if  the  yellow-blue 
substance  is  being  broken  down  in  part  of  the  retina  by  yellow 
light,  and  built  up  in  adjoining  part  by  blue  light,  at  the  border 
between  them  each  process  is  heightened  by  the  near  presence  of 
the  other. 

The  theory  explains  very  well,  also,  the  facts  of  negative  after- 
images, of  color  blindness,  and  of  the  distribution  of  color  vision 
in  the  retina.  The  chief  criticism  that  has  been  offered  against  it, 


THE  EYE  AS  A  SENSORY  APPARATUS  285 

apart  from  its  failure  to  accord  with  the  doctrine  of  specific  nerve 
energies,  is  that  its  assumption  of  similar  nervous  activities  result- 
ing from  opposing  chemical  processes  is  unwarranted  by  any 
knowledge  that  we  have  of  the  relation  between  chemical  processes 
and  nervous  activities  in  other  parts  of  the  body. 

The  Franklin  Theory  is  based  on  the  idea  that  the  peculiar  dis- 
tribution of  color  vision  over  the  retina  is  significant  as  suggesting 
that  the  more  complex  color  perceptions  are  evolved  from  simpler 
ones.  According  to  this  theory  the  primary  photochemical  sub- 
stance is  a  gray-perceiving  substance;  white  and  black  represent- 
ing the  ends  of  the  gray  color  series.  This  substance  is  in  all  the 
rods,  and  in  the  cones  of  the  retinal  margin  where  only  gray  per- 
ception occurs.  In  the  cones  of  the  yellow-blue  field,  the  funda- 
mental gray-perceiving  photochemical  substance  is  supposed  to 
be  dissociated  into  two  different  photochemical  substances,  one 
yellow-perceiving,  the  other  blue-perceiving.  Since  these  are 
products  of  the  gray-perceiving  substance  when  both  are  stimu- 
lated together  the  effect  is  the  same  as  when  the  gray-perceiving 
substance  itself  is  stimulated,  namely,  a  shade  of  gray. 

In  the  central  cones  of  the  retina  a  still  further  decomposition 
is  assumed  to  have  occurred,  of  the  yellow-perceiving  substance 
into  red  and  green-perceiving  substances.  The  central  cones,  then, 
contain  three  photochemical  substances,  a  red-perceiving  one,  a 
green-perceiving  one,  and  a  blue-perceiving  one.  Since  all  are 
ultimately  derived  from  the  gray-perceiving  substance  their  com- 
bined stimulation  produces  gray  sensations;  simultaneous  stimu- 
lation of  the  red  and  green  substances  gives  the  same  result  as 
stimulation  of  their  parent  substance,  that  for  perceiving  yellow. 

This  theory  puts  the  distribution  of  color  vision  in  the  retina  and 
the  phenomenon  of  color  blindness,  which  it  explains  as  due  to 
failure  of  dissociation  of  the  yellow-perceiving  substance,  upon  a 
more  rational  basis  than  do  either  of  the  other  theories  described. 
In  most  other  respects  it  offers  little  advantage  over  them. 

While  we  must  admit  that  at  present  a  full  understanding  of 
color  vision  is  beyond  us  we  may  properly  look  forward  to  its  ulti- 
mate mastery,  as  physiology  is  able  to  penetrate  more  deeply  the 
processes  which  underly  it. 

Visual  Perceptions.  The  sensations  which  light  excites  in  us  we 
interpret  as  indications  of  the  existence,  form,  and  position  of  ex- 


286  THE  HUMAN  BODY 

ternal  objects.  The  conceptions  which  we  arrive  at  in  this  way  are 
known  as  visual  perceptions.  The  full  treatment  of  perceptions  be- 
longs to  the  domain  of  Psychology,  but  Physiology  is  concerned 
with  the  conditions  under  which  they  are  produced. 

The  Visual  Perception  of  Distance.  With  one  eye  our  perception 
of  distance  is  very  imperfect,  as  illustrated  by  the  common  trick  of 
holding  a  ring  suspended  by  a  string  in  front  of  a  person's  face,  and 
telling  him  to  shut  one  eye  and  pass  a  rod  from  one  side  through 
the  ring.  If  a  penholder  be  held  erect  before  one  eye,  while  the 
other  is  closed,  and  an  attempt  be  made  to  touch  it  with  a  finger 
moved  across  towards  it,  an  error  will  nearly  always  be  made.  (If 
the  finger  be  moved  straight  on  towards  the  pen  it  will  be  touched 
because  with  one  eye  we  can  estimate  direction  accurately  and 
have  only  to  go  on  moving  the  finger  in  the  proper  direction  till  it 
meets  the  object.)  In  such  cases  we  get  the  only  clue  from  the 
amount  of  effort  needed  to  "  accommodate "  the  eye  to  see  the 
object  distinctly.  When  we  use  both  eyes  our  perception  of  dis- 
tance is  much  better;  when  we  look  at  an  object  with  two  eyes  the 
visual  axes  are  converged  on  it,  and  the  nearer  the  object  the 
greater  the  convergence.  We  have  a  pretty  accurate  knowledge 
of  the  degree  of  muscular  effort  required  to  converge  the  eyes  on 
all  tolerably  near  points.  When  objects  are  farther  off,  their 
apparent  size,  and  the  modifications  of  their  retinal  images  brought 
about  by  aerial  perspective,  come  in  to  help..  The  relative  distance 
of  objects  is  easiest  determined  by  moving  the  eyes;  all  stationary 
objects  then  appear  displaced  in  the  opposite  direction  (as  for  ex- 
ample when  we  look  out  of  the  window  of  a  railway  car)  and  those 
nearest  most  rapidly;  from  the  different  apparent  rates  of  move- 
ment we  can  tell  which  are  farther  and  nearer.  We  so  inseparably 
and  unconsciously  bind  up  perceptions  of  distance  with  the  sensa- 
tions aroused  by  objects  looked  at,  that  we  seem  to  see  distance; 
it  seems  at  first  thought  as  definite  a  sensation  as  color.  That  it  is 
not  is  shown  by  cases  of  persons  born  blind,  who  have  had  sight 
restored  later  in  life  by  surgical  operations.  Such  persons  have  at 
first  no  visual  perceptions  of  distance:  all  objects  seem  spread  out 
on  a  flat  surface  in  contact  with  the  eyes,  and  they  only  learn 
gradually  to  interpret  their  sensations  so  as  to  form  judgments 
about  distances,  as  the  rest  of  us  did  unconsciously  in  childhood 
before  we  thought  about  such  things. 


THE  EYE  AS  A  SENSORY  APPARATUS  287 

The  Visual  Perception  of  Size.  The  dimensions  of  the  retinal 
image  determine  primarily  the  sensations  on  which  conclusions 
as  to  size  are  based;  and  the  larger  the  visual  angle  the  larger  the 
retinal  image ;  since  the  visual  angle  depends  on  the  distance  of  an 
object  the  correct  perception  of  size  depends  largely  upon  a  correct 
perception  of  distance;  having  formed  a  judgment,  conscious  or 
unconscious,  as  to  that,  we  conclude  as  to  size  from  the  extent  of 
the  retinal  region  affected.  Most  people  have  been  surprised  now 
and  then  to  find  that  what  appeared  a  large  bird  in  the  clouds  was 
only  a  small  insect  close  to  the  eye;  the  large  apparent  size  being 
due  to  the  previous  incorrect  judgment  as  to  the  distance  of  the 
object.  The  presence  of  an  object  of  tolerably  well-known  height, 
as  a  man,  also  assists  in  forming  conceptions  (by  comparison)  as  to 
size;  artists  for  this  purpose  frequently  introduce  human  figures  to 
assist  in  giving  an  idea  of  the  size  of  other  objects  represented. 

The  Visual  Perception  of  a  Third  Dimension  of  Space.  This 
is  very  imperfect  with  one  eye;  still  we  can  thus  arrive  at  conclu- 
sions from  the  distribution  of  light  and  shade  on  an  object,  and 
so  that  amount  of  knowledge  as  to  the  relative  distance  of  dif- 
ferent points  which  is  attainable  monocularly;  the  different  visual 
angles  under  which  objects  are  seen  also  assist  us  in  concluding  that 
objects  are  farther  and  nearer,  and  so  are  not  spread  out  on  a  plane 
before  the  eye,  but  occupy  depth  also.  Painters  depend  mainly 
on  devices  of  these  kinds  for  representing  solid  bodies,  and  objects 
spread  over  the  visual  field  in  the  third  dimension  of  space. 

Single  Vision  with  Two  Eyes.  When  we  look  at  a  flat  object 
with  both  eyes  we  get  a  similar  retinal  image  in  each.  Under  ordi- 
nary circumstances  we  see,  however,  not  two  objects  but  one.  In 
the  habitual  use  of  the  eyes  we  move  them  so  that  the  images  of 
the  object  looked  at  fall  on  the  two  fovese.  A  point  to  the  left 
of  this  object  forms  its  image  on  the  inner  (right)  side  of  the  left 
eye  and  the  outer  (right)  side  of  the  right.  An  object  vertically 
above  that  looked  at  would  form  an  image  straight  below  the 
fovea  of  each  eye;  an  object  to  the  left  and  above,  its  image  to 
the  inner  side  and  below  in  the  left  eye  and  to  the  outer  side  and 
below  in  the  right  eye;  and  so  on.  We  have  learned  that  similar 
simultaneous  excitations  of  these  corresponding  points  mean  single 
objects,  and  so  interpret  our  sensations.  When  the  eyes  do  not 
work  together,  as  in  the  muscular  incoordination  of  one  stage  of 


288 


THE  HUMAN  BODY 


intoxication,  then  they  are  not  turned  so  that  images  of  the  same 
objects  fall  on  corresponding  retinal  points,  and  the  person  sees 
double.  When  a  squint  comes  on,  as  from  paralysis  of  the  external 
rectus  of  one  eye,  the  sufferer  at  first  sees  double  for  the  same 
reason,  but  after  a  time  he  makes  new  associations  of  correspond- 
ing retinal  points. 

When  a  given  object  is  looked  at,  lines  drawn  from  it  through 
the  nodal  points  reach  the  fovea  centralis  in  each  eye.  Lines  so 
drawn  at  the  same  time  from  a  more  distant  object  diverge  less  and 
meet  each  retina  on  the  inner  side  of  its  fovea;  but  as  above  pointed 
out  the  corresponding  points  for  each  retinal  region  on  the  inside 
of  the  left  eye,  are  on  the  outside  of  the  right,  and  vice  versa. 
Hence  the  more  distant  object  is  seen  double.  So,  also,  is  a  nearer 
object,  because  the  more  diverging  lines  drawn  from  it  through  the 
nodal  points  lie  outside  of  the  fovea  in  each  eye.  Most  people  go 
through  life  unobservant  of  this  fact ;'  we  only  pay  attention  to 
what  we  are  looking  at,  and  nearly  always  this  makes  its  images 


FIG.  96. 

on  the  two  foveas.  That  the  fact  is  as  above  stated  ma,y,  however, 
be  readily  observed.  Hold  one  finger  a  short  way  from  the  face 
and  the  other  a  little  farther  off;  looking  at  one,  observe  the  other 
without  moving  the  eyes;  it  will  be  seen  double.  For  every  given 
position  of  the  eyes  there  is  a  surface  in  space,  all  objects  on  which 
produce  images  on  corresponding  points  of  the  two  retinas:  this 
surface  is  called  the  horopter  for  that  position  of  the  eyes:  all  ob- 
jects in  it  are  seen  single;  all  others  in  the  visual  field,  double. 

The  Perception  of  Solidity.  When  a  solid  object  is  looked  at  the 
two  retinal  images  are  different.  If  a  truncated  pyramid  be  held 
in  front  of  one  eye  its  image  will  be  that  represented  at  P,  Fig.  96. 
If,  however,  it  be  held  midway  between  the  eyes,  and  looked  at 
with  both,  then  the  left-eye  image  will  be  that  in  the  middle  of  the 
figure,  and  the  right-eye  image  that  to  the  right.  The  small  sur- 


THE  EYE  AS  A  SENSORY  APPARATUS  289 

face,  b  d  c  a,  in  one  answers  to  the  large  surface,  b'  d'  cf  a',  in  the 
other.  This  may  be  readily  observed  by  holding  a  small  cube  in 
front  of  the  nose  and  alternately  looking  at  it  with  each  eye.  In 
such  cases,  then,  the  retinal  images  do  not  correspond,  and  yet  we 
combine  them  in  consciousness  so  as  to  see  one  solid  object.  This 
is  known  as  stereoscopic  vision,  and  the  illusion  of  the  common 
stereoscope  depends  on  it.  Two  photographs  are  taken  of  the  same 
object  from  two  different  points  of  view,  one  as  it  appears  when 
seen  from  the  left,  and  the  other  when  seen  from  the  right.  These 
are  then  mounted  for  the  stereoscope  so  that  each  is  looked  at  by 
its  proper  eye,  and  the  object  appears  in  distinct  relief,  as  if,  in- 
stead of  flat  pictures,  solid  objects,  occupying  three  dimensions  of 
space,  were  looked  at. 


CHAPTER  XVII 

THE     STRUCTURE     AND    FUNCTIONS     OF     BLOOD     AND 

LYMPH 

Introductory.  We  turn  at  this  point  from  study  of  the  mech- 
anism by  which  the  Body  adapts  itself  to  its  surroundings  to  a 
consideration  of  the  structures  and  processes  engaged  in  body 
maintenance.  These  have  the  task  of  providing  the  living  tissues 
of  the  Body  with  supplies  of  energy-yielding  material  and  of 
keeping  them  in  good  working  order.  Their  dependence  upon 
the  environment  is  not  so  obvious  perhaps  as  is  that  of  the  adaptive 
mechanism  proper,  although  as  a  matter  of  fact,  changes  in  the 
environment  do  influence  the  maintenance  mechanisms,  and  often 
very  promptly  and  strikingly.  For  example,  variations  in  the  sur- 
rounding temperature  bring  about  adaptive  responses  in  the 
mechanism  for  keeping  the  Body  at  the  proper  degree  of  warmth. 
We  shall  have  constant  occasion,  therefore,  to  recall  the  facts 
brought  out  in  preceding  chapters. 

The  External  Medium.  During  the  whole  of  life  interchanges 
of  material  go  on  between  every  living  being  and  the  external 
world;  by  these  exchanges  material  particles  that  one  time  con- 
stitute parts  of  inanimate  objects  come  at  another  to  form  part 
of  a  living  being;  and  later  on  these  same  atoms,  after  having 
been  a  part  of  a  living  thing,  are  passed  out  from  it  in  the  form 
of  lifeless  compounds.  As  the  foods  and  wastes  of  various  or- 
ganisms differ  more  or  less,  so  are  more  or  less  different  environ- 
ments suited  for  their  existence;  and  there  is  accordingly  a  re- 
lationship between  the  plants  and  animals  living  in  any  one  place 
and  the  conditions  of  air,  earth,  and  water  prevailing  there.  Even 
such  simple  unicellular  animals  as  the  amoebae  live  only  in  water 
or  mud  containing  in  solution  certain  gases,  and  in  suspension 
solid  food-particles;  and  they  soon  die  if  the  water  be  changed 
either  by  essentially  altering  its  gases  or  by  taking  out  of  it  the 
solid  food.  So  in  yeast  we  find  a  unicellular  plant  which  thrives 

290 


STRUCTURE  AND  FUNCTIONS  OF  BLOOD  AND  LYMPH    291 

and  multiplies  only  in  liquids  of  certain  composition,  and  which 
in  the  absence  of  organic  compounds  of  carbon  in  solution  will  not 
grow  at  all.  Each  of  these  simple  living  things,  which  corresponds 
to  one  only  of  the  innumerable  cells  composing  the  full-grown 
Human  Body,  thus  requires  for  the  manifestation  of  its  vital 
properties  the  presence  of  a  surrounding  medium  suited  to  itself: 
the  yeast  would  die,  or  at  the  best  lie  dormant,  in  a  liquid  con- 
taining only  the  solid  organic  particles  on  which  the  amoeba  lives; 
and  the  amceba  would  die  in  such  solutions  as  those  in  which  yeast 
thrives  best. 

The  Internal  Medium.  A  similar  close  relationship  between 
the  living  being  and  its  environment,  and  an  interchange  between 
the  two  like  that  which  we  find  in  the  amceba  and  the  yeast-cell, 
we  find  also  in  even  the  most  complex  living  beings.  When, 
however,  an  animal  comes  to  be  composed  of  many  cells,  some 
of  which  are  placed  far  away  from  the  surface  of  its  body  and 
from  immediate  contact  with  the  environment,  there  arises  a  new 
need — a  necessity  for  an  internal  medium  or  plasma  which  shall 
play  the  same  part  toward  the  individual  cells  as  the  surrounding 
air,  water  and  food  to  the  whole  animal.  This  internal  medium 
kept  in  movement  and  receiving  at  some  regions  of  the  bodily 
surfaces  materials  from  the  exterior,  while  losing  substances  to 
the  exterior  at  the  same  or  other  surfaces,  forms  a  sort  of  middle- 
man between  the  individual  tissues  and  the  surrounding  world, 
and  stands  in  the  same  relationship  to  each  of  the  cells  of  the  Body 
as  the  water  in  which  an  amceba  lives  does  to  that  animal,  or 
beer-wort  does  to  a  yeast-cell.  We  find  accordingly  the  Human 
Body  pervaded  by  a  liquid  plasma,  containing  gases  and  food- 
material  in  solution,  the  presence  of  which  is  necessary  for  the 
maintenance  of  the  life  of  the  tissues.  Any  great  change  in  this 
medium  will  affect  injuriously  few  or  many  of  the  groups  of  cells 
in  the  Body,  or  may  even  cause  their  death;  just  as  altering  the 
media  in  which  they  live  will  kill  an  amceba  or  a  yeast-cell. 

In  a  body  so  large  and  complex  as  that  of  man,  moreover,  the 
internal  medium  must  do  more  than  merely  bring  food  to  the 
individual  cells  and  carry  waste  materials  away  from  them.  All 
the  cells  have  to  be  kept  at  just  the  right  degree  of  warmth,  but 
some  produce  more  heat  than  others;  so  part  of  its  work  is  to 
maintain  an  even  distribution  of  heat  over  the  whole  Body  or 


292  THE  HUMAN  BODY 

when  excess  is  generated  to  provide  for  its  escape.  Many  bodily 
processes,  particularly  the  slower  ones,  such  as  growth,  are  not 
of  a  nature  to  be  conveniently  controlled  by  the  nervous  system. 
Their  control  is  vested  in  the  hormones  with  which  we  are  already 
familiar.  These  are  conveyed  by  the  internal  medium  to  all  parts 
of  the  Body,  being  thus  sure  of  reaching  the  structures  upon  which 
their  influence  is  to  be  exerted.  Finally,  the  environment  which  is 
favorable  for  the  life  and  growth  of  the  body-cells  is  also  favorable 
for  the  life  and  growth  of  foreign  and  harmful  organisms.  That 
the  Body  is  subject  to  invasion  by  such  organisms  is  only  too 
well  known,  and  but  for  the  system  of  defense  which  the  internal 
medium  affords  these  invasions  could  not  fail  to  be  even  more 
disastrous  than  they  are. 

We  can  summarize  the  functions  of  the  internal  medium  as 
follows: 

1.  To  convey  to  all  the  living  cells  their  needed  supplies  of 
food  material  and  oxygen. 

2.  To  convey  away  from  the  body-cells  the  waste  materials  gen- 
erated by  their  activities. 

3.  To  distribute  heat  uniformly  over  the  Body  and  provide  for 
getting  rid  of  the  excess. 

4.  To  convey  from  the  regions  where  they  are  produced  to  those 
where  they  are  used  the  special  substances,   hormones,   which 
regulate  many  bodily  processes. 

5.  To  defend  the  Body  against  the  inroads  of  disease-producing 
micro-organisms.  - 

The  Blood.  In  the  Human  Body  the  internal  medium  is  pri- 
marily furnished  by  the  blood  which,  as  every  one  knows,  is  a 
red  liquid  very  widely  distributed  over  the  frame,  since  it  flows 
from  any  part  when  the  skin  is  cut  through.  There  are  in  fact 
very  few  portions  of  the  Body  into  which  the  blood  is  not  carried. 
One  of  the  exceptions  is  the  epidermis  or  outer  layer  of  the  skin : 
if  a  cut  be  made  through  it  only,  leaving  the  deeper  skin-layers 
intact,  no  blood  will  flow  from  the  wound.  Hairs  and  nails  also 
contain  no  blood.  In  the  interior  of  the  Body  the  epithelial  layers 
lining  free  surfaces,  such  as  the  inside  of  the  alimentary  canal, 
contain  no  blood,  nor  do  the  hard  parts  of  the  teeth,  the  cartilages, 
and  the  refracting  media  of  the  eye  (see  Chap.  XV),  but  these 
interior  parts  are  moistened  with  liquid  of  some  kind,  and  unlike 


STRUCTURE  AND  FUNCTIONS  OF  BLOOD  AND  LYMPH    293 

flu*  epidermis  are  protected  from  rapid  evaporation.  All  these 
bloodless  parts  together  form  a  group  of  non-vascular  tissues; 
they  alone  excepted,  a  wound  of  any  part  of  the  Body  will  cause 
bleeding. 

In  many  of  the  lower  animals  there  is  no  need  that  the  liquid 
representing  their  blood  should  be  renewed  very  rapidly  in  dif- 
ferent parts.  Their  cells  live  slowly,  and  so  require  but  little  food 
and  produce  but  little  waste.  In  a  sea-anemone,  for  example, 
there  is  no  special  arrangement  to  keep  the  blood  moving;  it  is 
just  pushed  about  from  part  to  part  by  the  general  movements 
of  the  body  of  the  animal.  But  in  higher  animals,  especially 
warm-blooded  ones,  such  an  arrangement,  or  rather  absence  of 
arrangement,  as  this  would  not  suffice.  In  them  the  constituent 
cells  live  very  fast,  making  much  waste  and  using  much  food,  and 
altering  the  medium  in  their  neighborhood  very  rapidly.  Besides, 
we  have  seen  that  in  complex  animals  certain  cells  are  set  apart 
to  get  food  for  the  whole  organism  and  certain  others  to  remove 
its  wastes,  and  there  must  be  a  sure  and  rapid  interchange  of 
material  between  the  feeding  and  excreting  tissues  and  all  the 
others.  This  can  only  be  brought  about  by  a  rapid  movement 
of  the  blood  in  a  definite  course,  and  that  is  accomplished  by 
shutting  it  up  in  a  closed  set  of  tubes,  and  placing  somewhere  a 
pump,  which  constantly  takes  in  blood  from  one  end  of  the  sys- 
tem of  tubes  and  forces  it  out  again  into  the  other.  Sent  by  this 
pump,  the  heart,  through  all  parts  of  the  Body  and  back  to  the 
heart  again,  the  blood  gets  food  from  the  receptive  cells,  takes 
it  to  the  working  cells,  carries  off  the  waste  of  these  latter  to  the 
excreting  cells;  and  so  the  round  goes  on. 

The  Lymph.  The  blood,  however,  lies  everywhere  in  closed 
tubes  formed  by  the  vascular  system,  and  does  not  come  into 
direct  contact  with  any  cells  of  the  Body  except  those  which  float 
in  it  and  those  which  line  the  interior  of  the  blood-vessels.  At 
one  part  of  its  course,  however,  the  vessels  through  which  it  passes 
have  extremely  thin  coats,  and  through  the  walls  of  these  capil- 
laries liquid  transudes  from  the  blood  and  bathes  the  various 
tissues.  The  transuded  liquid  is  the  lymph,  and  it  is  this  which 
forms  the  immediate  nutrient  plasma  of  the  tissues  except  the 
few  which  the  blood  moistens  directly. 

The  Renewal  of  the  Lymph.    Osmotic  phenomena  (p.  18)  play 


294  THE  HUMAN  BODY 

a  great  part  in  the  nutritive  processes  of  the  Body.  The  lymph 
present  in  any  organ  gives  up  things  to  the  cells  there  and  gets 
things  from  them ;  and  thus,  although  it  may  have  originally  been 
tolerably  like  the  liquid  part  of  the  blood,  it  soon  acquires  a  differ- 
ent chemical  composition.  Diffusion  or  dialysis  then  commences 
between  the  lymph  outside  and  the  blood  inside  the  capillaries,  and 
the  latter  gives  up  to  the  lymph  new  materials  in  place  of  those 
which  it  has  lost  and  takes  from  it  the  waste  products  it  has  re- 
ceived from  the  tissues.  When  this  blood,  altered  by  exchanges 
with  the  lymph,  gets  again  to  the  neighborhood  of  the  receptive 
cells,  having  lost  some  food-materials  it  is  poorer  in  these  than 
the  richly  supplied  lymph  around  those  cells,  and  takes  up  a 
supply  by  dialysis  from  it.  When  it  reaches  the  excretory  organs 
it  has  previously  picked  up  a  quantity  of  waste  matters  and  loses 
these  by  dialysis  to  the  lymph  there  present,  which  is  specially 
poor  in  such  matters,  since  the  excretory  cells  constantly  deprive 
it  of  them.  In  consequence  of  the  different  wants  and  wastes  of 
various  cells,  and  of  the  same  cells  at  different  times,  the  lymph 
must  vary  considerably  in  composition  in  various  organs  of  the 
Body,  and  the  blood  flowing  through  them  will  gain  or  lose  dif- 
ferent things  in  different  places.  But  renewing  during  its  circuit 
in  one  what  it  loses  in  another,  its  average  composition  is  kept 
pretty  constant,  and,  through  interchange  with  it,  the  average 
composition  of  the  lymph  also. 

The  Lymphatic  Vessels.  The  blood,  on  the  whole,  loses  more 
liquid  to  the  lymph  through  the  capillary  walls  than  it  receives 
back  the  same  way.  This  depends  mainly  on  the  fact  that  the 
pressure  on  the  blood  inside  the  vessels  is  greater  than  that  on 
the  lymph  outside,  and  so  a  certain  amount  of  nitration  of  liquid 
from  within  out  occurs  through  the  vascular  wall  in  addition  to 
the  dialysis  proper.  The  excess  is  collected  from  the  various 
organs  of  the  Body  into  a  set  of  lymphatic  vessels  which  carry  it 
directly  back  into  some  of  the  larger  blood-vessels  near  where 
these  empty  into  the  heart;  in  this  way  the  liquid  which  is  forced 
out  of  the  blood  stream  in  the  capillaries  gets  back  into  it  again. 

The  Lacteals.  In  the  walls  of  the  alimentary  canal  certain 
food-materials  after  passing  through  the  receptive  cells  into  the 
lymph  are  not  transferred  locally,  like  the  rest,  by  dialysis  into 
the  blood,  but  are  carried  off  bodily  in  the  lymph-vessels  and 


STRUCTURE  AND  FUNCTIONS  OF  BLOOD  AND  LYMPH    295 

poured  into  the  veins  of  a  distant  part  of  the  Body.  The  lymphatic 
vessels  concerned  in  this  work,  being  frequently  filled  with  a  white 
liquid  during  digestion,  are  called  the  milky  or  lacteal  vessels. 

Summary.  To  sum  up:  the  blood  and  lymph  form  the  internal 
medium  in  which  the  tissues  of  the  Body  live;  the  lymph  is  pri- 
marily derived  from  the  blood  and  forms  the  immediate  plasma 
for  the  great  majority  of  the  living  cells  of  the  Body;  and  the  ex- 
cess of  it  is  finally  returned  to  the  blood.  The  lymph  moves  but 
slowly,  but  is  constantly  renovated  by  the  blood,  which  is  kept  in 
rapid  movement,  and  which,  besides  containing  a  store  of  new 
food-matters  for  the  lymph,  carries  off  the  wastes  which  the 
various  cells  have  poured  into  the  latter,  and  thus  is  also  a  sort 
of  sewage  stream  into  which  the  wastes  of  the  whole  Body  are 
primarily  collected. 

Composition  of  the  Blood.  The  average  specific  gravity  of 
human  blood  is  1,055.  It  has  an  alkaline  reaction  to  litmus. 
About  one-third  its  mass  consists  of  moist  corpuscles  and  the  re- 
mainder of  the  liquid  part  or  plasma.  Exposed  in  a  vacuum, 
100  volumes  of  blood  yield  about  60  of  gas  consisting  of  a  mixture 
of  oxygen,  carbon  dioxid,  and  nitrogen. 

Microscopic  Characters  of  Blood.  If  a  finger  be  pricked,  and 
the  drop  of  blood  flowing  out  be  spread  on  a  glass  slide,  covered, 
protected  from  evaporation,  and  examined  with  a  microscope 
magnifying  about  400  diameters,  it  will  be  seen  to  consist  of  in- 
numerable solid  bodies  floating  in  a  liquid.  The  solid  bodies  are 
the  blood-corpuscles,  and  the  liquid  is  the  blood-plasma. 

The  corpuscles  are  not  all  alike.  While  currents  still  exist  in 
the  freshly-spread  drop  of  blood,  the  great  majority  of  them  are 
readily  carried  to  and  fro;  but  a  certain  number  more  commonly 
stick  to  the  glass  and  remain  in  one  place.  The  former  are  the 
red,  the  latter  the  pale  or  colorless  blood-corpuscles.  With  proper 
precautions  a  third  sort  of  corpuscles,  the  blood-plates,  may  also 
be  seen. 

Red  Corpuscles.  Form  and  Size.  The  red  corpuscles  as  they 
float  about  frequently  seem  to  vary  in  form,  but  by  a  little  at- 
tention it  can  be  made  out  that  this  appearance  is  due  to  their 
turning  round  as  they  float,  and  so  presenting  different  aspects  to 
view;  just  as  a  silver  dollar  presents  a  different  outline  according  as 
it  is  looked  at  from  the  front  or  edgewise  or  in  three-quarter  profile. 


296 


THE  HUMAN  BODY 


Sometimes  the  corpuscle  (Fig.  97,  B)  appears  circular;  then  it  is 
seen  in  full  face;  sometimes  linear  (C),  and  slightly  narrowed  in 
the  middle;  sometimes  oval,  as  the  dollar  when  halfway  between 
a  full  and  a  side  view.  These  appearances  show  that  each  red 
corpuscle  is  a  circular  disk,  slightly  hollowed  in  the  middle  (or 
biconcave)  and  about  four  times  as  wide  as  it  is  thick.  The  av- 
erage transverse  diameter  is  0.008  millimeter  (swff  inch).  Shortly 
after  blood  is  drawn  the  corpuscles  tend  to  arrange  themselves  in 
rows,  or  rouleaux,  adhering  to  one  another  by  their  broader  sur- 
faces. 

Color.  Seen  singly  each  red  corpuscle  is  of  a  pale  yellow  color; 
it  is  only  when  collected  in  masses  that  they  appear  red.  The 


FIG.  97. — Blood-corpuscles.  A,  magnified  about  400  diameters.  The  red  corpus- 
cles have  arranged  themselves  in  rouleaux;  a,  a,  colorless  corpuscles;  B,  red  cor- 
puscles more  magnified  and  seen  in  focus;  E,  a  red  corpuscle  slightly  out  of  focus. 
Near  the  right-hand  top  corner  is  a  red  corpuscle  seen  in  three-quarter  face,  and  at 
C  one  is  seen  edgewise.  F,  G,  H,  I,  white  corpuscles  highly  magnified. 

blood  owes  its  red  color  to  the  great  numbers  of  these  bodies  in  it; 
if  it  is  spread  out  in  a  very  thin  layer  it,  too,  is  yellow. 

Structure.  There  is  no  satisfactory  evidence  that  these  cor- 
puscles have  any  enveloping  sac  or  cell-wall.  All  the  methods 
used  to  bring  one  into  view  under  the  microscope  are  such  as 
would  coagulate  the  outer  layers  of  the  substance  composing  the 
corpuscle  and  so  make  an  artificial  envelope.  So  far  as  optical 


STRUCTURE  AND  FUNCTIONS  OF  BLOOD  AND  LYMPH    297 

analysis  goes,  then,  each  corpuscle  is  homogeneous  throughout. 
By  other  means  we  can,  however,  show  that  at  least  two  materials 
enter  into  the  structure  of  each  red  corpuscle.  If  the  blood  be 
diluted  with  several  times  its  own  bulk  of  water  and  examined 
with  the  microscope,  it  will  be  found  that  the  formerly  red  cor- 
puscles are  now  colorless  and  the  plasma  colored.  The  dilution 
has  caused  the  coloring  matter  to  pass  out  of  the  corpuscles  and 
dissolve  in  the  liquid.  This  coloring  constituent  of  the  corpuscle  is 
hemoglobin,  and  the  colorless  residue  which  it  leaves  behind  and 
which  swells  up  into  a  sphere  in  the  diluted  plasma  is  the  stroma. 
In  the  living  corpuscle  the  two  are  intimately  mingled  through- 
out it,  and  so  long  as  this  is  the  case  the  blood  is  opaque;  but 
when  the  coloring  matter  dissolves  in  the  plasma,  then  the  blood 
becomes  transparent,  or,  as  it  is  called,  laky.  The  difference  may 
be  very  well  seen  by  comparing  a  thin  layer  of  fresh  blood  diluted 
with  ten  times  its  volume  of  ten  per  cent  salt  solution  with  a 
similar  layer  of  blood  diluted  with  ten  volumes  of  water.  The 
watery  mixture  is  a  dark  transparent  red;  the  other,  in  which  the 
coloring  matter  still  lies  in  the  corpuscles,  is  a  brighter  opaque  red. 

Red  corpuscles  do  not  possess  nuclei;  they  are  not,  therefore, 
living  cells  in  the  ordinary  sense.  Whether  they  contain  any 
living  protoplasm  cannot  be  told  certainly.  So  far  as  we  can 
judge  their  activities  they  are  purely  mechanical  and  do  not  re- 
quire the  participation  of  living  substance. 

Consistency.  Each  red  corpuscle  is  a  soft  jelly-like  mass  which 
can  be  readily  crushed  out  of  shape.  Unless  the  pressure  be  such 
as  to  rupture  it,  the  corpuscle  immediately  reassumes  its  proper 
form  when  the  external  force  is  removed.  The  corpuscles  are, 
then,  highly  elastic;  they  frequently  can  be  seen  much  dragged 
out  of  shape  inside  the  vessels  when  the  circulation  of  the  blood  is 
watched  in  a  living  animal  (Chap.  XX),  but  immediately  spring- 
ing back  to  their  normal  form  when  they  get  a  chance. 

Composition.  In  the  fresh  moist  state  there  are  in  100  parts 
of  red  corpuscles  57  to  64  of  water  and  36  to  43  of  solids.  Of  the 
solids  nearly  ninety  per  cent  is  hemoglobin,  about  one  per  cent 
inorganic  salts,  chiefly  phosphates  and  chlorides  of  potassium,  the 
residue  the  proteins  and  other  materials  of  the  stroma. 

Number.  There  is  considerable  variation  in  the  number  of  red 
corpuscles  in  any  given  volume  of  the  blood.  The  average  for  the 


298  THE  HUMAN  BODY 

adult  male  is  stated  at  5,000,000  per  cubic  millimeter  (-,  R/Uo  cubic 
inch);  for  the  adult  female  the  figure  is  half  a  million  less.  The 
method  of  determining  this  number  is  to  draw  from  the  ear  or 
finger  tip  an  accurately  measured  volume  of  blood;  this  is  diluted 
to  a  known  volume  and  the  number  of  corpuscles  in  a  known 
amount  of  this  diluted  blood  is  counted  under  the  microscope. 
From  this  the  total  number  in  any  volume  of  undiluted  blood 
can  easily  be  calculated. 

It  must  be  remembered  that  the  liquid  part  of  the  blood  is 
subject  to  changes  of  volume,  either  in  the  way  of  increase  as 
liquid  is  received  into  it  from  the  digestive  tract,  or  decrease  as 
liquid  passes  from  it  into  the  lymph;  therefore  a  variation  in  the 
number  of  red  corpuscles  per  cubic  millimeter  does  not  necessarily 
mean  a  corresponding  variation  in  the  total  number  in  the  Body. 

Hemoglobin.  This  substance,  which  is  a  compound  of  a  pig- 
ment with  a  protein  (see  Chap.  I),  is  the  functionally  important 
part  of  the  red  corpuscle,  the  stroma  serving  merely  as  a  frame- 
work upon  which  it  is  carried.  Its  importance  lies  in  the  fact 
that  it  combines  readily  with  oxygen,  forming  a  loose  combina- 
tion which  can  easily  be  broken  up,  thus  it  serves  to  transport 
oxygen  from  the  lungs  to  the  tissues  of  the  Body  (see  Respira- 
tion). This  property  seems  to  be  associated  with  the  presence 
of  iron  in  the  pigment  part  of  the  hemoglobin  molecule. 

In  the  adult  male  about  fourteen  parts  in  the  hundred  by 
weight  of  the  blood  are  hemoglobin.  It  has  been  estimated  that 
a  man  weighing  68  kilograms  (150  Ibs.)  has  in  his  blood  750  grams 
(1.64  Ibs.)  of  hemoglobin,  which  is  distributed  among  some 
25,000,000,000,000  red  corpuscles,  giving  a  total  superficial  area 
of  about  3,200  sq.  meters  (3,800  sq.  yds.)  of  hemoglobin.  On  ac- 
count of  the  very  rapid  circulation  of  the  blood  (see  Circulation) 
practically  the  whole  of  this  great  area  of  hemoglobin  is  poured 
through  the  capillaries  of  the  lungs  every  thirty  seconds,  so  it  is 
apparent  that  we  have  here  a  remarkably  efficient  arrangement 
for  supplying  the  Body  with  oxygen. 

There  is  a  pathological  condition  known  as  anemia  in  which 
there  is  a  considerable  reduction  in  the  number  of  red  corpuscles. 
This  is  usually  accompanied  by  a  diminution  in  the  amount  of 
hemoglobin  contained  in  each  corpuscle,  so  that  as  a  result  there 
is  a  serious  shortage  in  the  hemoglobin  content  of  the  Body.  Per- 


STRUCTURE  AND  FUNCTIONS  OF  BLOOD  AND  LYMPH    299 

sons  suffering  from  this  condition  usually  have  little  or  no  color, 
and  because  the  oxygen-carrying  mechanism  of  the  Body  is  be- 
low normal  there  is  a  loss  of  bodily  strength  and  endurance.  The 
condition  is  more  common  between  the  ages  of  twelve  and  twenty 
years  than  at  other  periods,  and  in  girls  than  in  boys.  It  is  not 
always  easily  overcome  and  should  have  the  care  of  a  physician. 
An  outdoor  life  and  plenty  of  nourishing  food,  in  which  iron  con- 
taining substances  are  included,  are  beneficial  in  such  cases. 

Origin  and  Fate  of  the  Red  Corpuscles.  Mammalian  red 
corpuscles  are  cells  which  have  lost  their  nuclei.  In  the  red  mar- 
row of  certain  bones  is  the  so-called  hematopoietic  (corpuscle- 
forming)  tissue  where  red  corpuscles  are  constantly  being  formed. 
The  cells  of  this  corpuscle-forming  tissue  are  continually  multi- 
plying by  mitotic  division  (see  Chap.  II),  and  the  daughter  cells 
thus  formed  store  up  within  themselves  hemoglobin,  lose  their 
nuclei,  either  by  disintegration  or  extrusion,  and  are  cast  off  into 
the  blood  stream.  It  is  not  known  how  rapidly  they  are  formed, 
nor  how  long  any  individual  corpuscle  remains  actively  at  work 
in  the  blood  stream;  but  it  is  known  that  sooner  or  later  the  red 
corpuscles  become  worn  out,  and  disintegrate;  the  hemoglobin  is 
decomposed  in  the  liver  in  such  fashion  as  to  save  the  iron,  and 
the  residue  is  converted  into  the  bile  pigments  and  excreted  (see 
Chap.  XXXI). 

After  hemorrhage  or  as  the  result  of  certain  pathological  con- 
ditions the  rate  of  production  of  red  corpuscles  may  be  much 
increased.  When. this  occurs  some  corpuscles  are  liberated  into 
the  blood  stream  in  an  immature  condition,  and  so  the  blood  will 
be  found  at  such  times  to  Contain  nucleated  as  well  as  non-nucleated 
red  corpuscles. 

In  the  human  embryo  the  labor  of  making  red  corpuscles  is 
shared  by  many  of  the  organs  of  the  Body,  notably  the  liver  and 
spleen. 

The  Spleen.  This  large  and  conspicuous  abdominal  organ 
(L,  Fig.  134)  has  presented  to  physiologists  a  problem  of  classi- 
fication, in  that  its  function  has  been  and  still  is  so  obscure  as  to 
cause  uncertainty  under  what  general  heading  it  should  be  dis- 
cussed. The  most  satisfactory  present  view  assigns  it  a  function 
in  connection  with  the  blood,  and  it  will,  therefore,  be  described 
here.  The  spleen  consists  of  an  outer  coating  or  sheath  of  con- 


300  THE  HUMAN  BODY 

nective  tissue,  part  fibrous  and  part  elastic,  interspersed  with 
smooth  muscle-fibers.  Projections  of  the  sheath  extend  into  the 
cavity  of  the  organ,  subdividing  it  into  numerous  spaces;  these 
are  filled  with  masses  of  cells  which  make  up  the  spleen  pulp. 
Numerous  blood-corpuscles,  both  red  and  white,  are  found  mingled 
with  the  cells  of  the  spleen  pulp.  The  spleen  has  a  very  rich  blood 
supply  which  differs  from  that  of  all  other  organs  of  the  Body  in 
that  the  small  arteries  instead  of  communicating  with  capillaries, 
which  lead  in  turn  into  veins,  open  directly  into  the  spleen  pulp. 
This  tissue  is  bathed,  therefore,  with  blood  instead  of  with  lymph 
as  are  all  other  tissues.  The  spleen  pulp  is  drained  by  tiny  veins 
which  collect  the  blood  into  larger  ones  and  so  return  it  to  the 
portal  vein  (p.  335)  whence  it  passes  through  the  liver  and  so  on 
into  the  general  circulation. 

Function  of  the  Spleen.  The  peculiarly  intimate  way  in  which 
the  spleen  cells  are  brought  into  relationship  with  the  blood  suggests 
that  the  organ  is  specially  concerned  somehow  in  maintaining  the 
normal  constitution  of  the  blood.  Moreover,  this  concern  would 
appear  to  be  with  the  formed  elements  rather  than  with  the  plasma, 
for  the  delicate  membranes  which  form  the  capillary  walls,  and 
which,  in  all  organs  except  this,  stand  between  the  blood  and  the 
tissue  cells,  oppose  no  difficulty  to  the  passage  of  dissolved  sub- 
stances, but  only  to  the  passage  of  corpuscles.  The  spleen  is  the 
only  region,  then,  aside  from  the  red  marrow,  in  which  they  were 
formed,  that  the  red  corpuscles  have  direct  contact  with  tissue 
cells,  other  than  those  that  form  the  lining  membrane  of  the  blood- 
vessels. The  most  satisfactory  theory  of  spleen  function  that  we 
have  is  that  it  picks  out  from  the  blood  stream  and  disintegrates 
those  red  corpuscles  that  are  "worn  out"  and  no  longer  able  to 
carry  on  efficiently  their  function  as  oxygen  carriers.  In  support 
of  this  theory  is  the  observation  that  the  spleen  always  shows 
within  its  meshes  numerous  cells  that  have  engulfed  red  corpuscles 
and  are  apparently  in  the  process  of  disintegrating  them.  More- 
over, in  some  cases  of  pernicious  anemia,  a  blood  disease  in  which 
there  is  excessive  destruction  of  red  corpuscles,  virtual  cures  have 
been  wrought  by  operative  removal  of  the  spleen.  The  hemoglobin 
that  is  set  free  by  the  disintegration  of  the  corpuscles  is  carried  to 
the  liver  and  there  decomposed  as  described  above. 

The  spleen  shows  rhythmic  contractions  and  relaxations  which 


STRUCTURE  AND  FUNCTIONS  OF  BLOOD  AND  LYMPH    301 

have  been  thought  to  aid  the  circulation  of  blood  through  it.  It 
also  becomes  congested  during  the  period  of  digestion.  Whether 
this  is  important  or  merely  incidental  is  not  known. 

The  Colorless  Blood-Corpuscles  or  Leucocytes  (Fig.  97,  F, 
H,  G).  The  colorless  or  white  corpuscles  of  the  blood  are  far  less 
numerous  than  the  red ;  in  health  there  is  on  the  average  about  one 
white  to  three  hundred  red,  but  the  proportion  may  vary  con- 
siderably. Each  is  finely  granular  and  consists  of  a  soft  mass  of 
protoplasm  enveloped  in  no  definite  cell-wall,  but  containing  a 
nucleus.  The  granules  in  the  protoplasm  commonly  hide  the 
nucleus  in  a  fresh  corpuscle,  but  dilute  acetic  acid  dissolves  most 
of  them  and  brings  the  nucleus  into  view.  These  colorless  cor- 
puscles belong  to  the  group  of  undifferentiated  tissues,  and  differ 
in  no  important  recognizable  character  from  the  cells  which  make 
up  the  whole  very  young  Human  Body,  nor  indeed  from  such  a 
unicellular  animal  as  an  Amoeba.  They  have  the  power  of  slowly 
changing  their  form  spontaneously.  At  one  moment  a  leucocyte 
will  be  seen  as  a  spheroidal  mass;  a  few  seconds  later  (Fig.  98) 
processes  will  be  seen  radiating  from  this,  and  soon  after  these 
processes  may  be  retracted  and  others  thrust  out;  and  so  the  cor- 
puscle goes  on  changing  its  shape.  These  slow  amoeboid  movements 
are  greatly  promoted  by  keeping  the  specimen  of  blood  at  the  tem- 
perature of  the  Body.  By  thrusting  out  a 
process  on  one  side,  then  drawing  the  rest  of 
its  body  up  to  it,  and  then  sending  out  a 
process  again  on  the  same  side,  the  corpus- 
cle can  slowly  change  its  place  and  creep 
across  the  field  of  the  microscope.  Inside 
the  blood-vessels  these  corpuscles  often  ex- 

.     .,  ,    ,,  FIG.  98.— Awhiteblood- 

ecute  similar  movements;  and  they  some-  corpuscle  sketched  at  sue- 
times  bore  right  through  the  capillary  walls  ^M**.'  £ 
and,  getting  out  into  the  lymph-spaces,  changes  of  form  due  to  its 

2,  rnu-      amoeboid  movements. 

creep  about  among  the  other  tissues.     Ihis 

migration  is  especially  frequent  in  inflamed  parts,  and  the  pus 
or  ''matter"  which  collects  in  abscesses  is  largely  made  up  of 
white  blood-corpuscles  which  have  in  this  way  got  out  of  the 
blood-vessels.  The  average  diameter  of  the  white  corpuscles  is 
one-third  greater  than  that  of  the  red. 

The  colorless  corpuscles,  or  some  of  them,  are  capable  of  tak- 


302  THE  HUMAN  BODY 

ing  into  themselves  foreign  particles  present  in  the  blood;  this 
they  do  in  a  manner  similar  to  that  in  which  an  amoeba  feeds: 
the  process  is  known  as  phagocytosis  and  the  cells  exhibiting  it  as 
phagocytes.  Among  the  substances  observed  to  be  taken  up  by 
white  corpuscles  are  the  minute  organisms  known  as  Bacteria, 
certain  species  of  which  have  been  proved  to  be  the  causes  of  some 
diseases.  The  white  corpuscles  doubtless  in  this  way  play  an 
important  part  in  the  cure  of  such  diseases,  or  in  their  preven- 
tion in  persons  exposed  to  infection.  The  accumulation  of  white 
corpuscles  in  inflamed  or  injured  parts  is  probably  primarily  as- 
sociated with  the  removal  of  dead  and  broken-down  tissues, 
though  it  may  be  carried  to  excess  as  in  the  case  of  purulent  ac- 
cumulations. 

The  Blood-Plates.  These  are  a  third. kind  of  corpuscle  which 
remained  undiscovered  for  a  long  time  after  the  others  were  known 
because  they  break  up  and  disappear- very  soon  after  the  blood 
is  shed  unless  special  precautions  are  taken  to  preserve  them. 
They  are  smaller  than  the  red  corpuscles;  in  structure  and  com- 
position they  appear  to  resemble  somewhat  the  colorless  corpus- 
cles, although  they  do  not  possess  a  well-marked  nucleus.  They 
are  said  to  exhibit  amoeboid  movements  under  certain  conditions. 
The  only  function  that  is  known  for  them  is  in  connection  with 
the  process  of  blood-clotting  (see  page  317).  They  are  fairly 
numerous,  the  blood  containing  perhaps  one-tenth  as  many  plate- 
lets as  red  corpuscles.  The  promptness  with  which  they  disin- 
tegrate when  exposed  to  a  foreign  environment  is  their  most 
marked  characteristic. 

The  Blood-Plasma.  This  is  a  very  complex  liquid,  containing 
as  it  does  all  the  varied  substances  which  are  associated  in  the 
carrying  out  of  the  blood's  many  functions.  The  plasma  is  90 
per  cent  water;  of  the  remaining  10  per  cent  the  chief  portion  con- 
sists of  organic  compounds,  mostly  protein,  serum  albumin,  para- 
globulin,  and  fibrinogen;  the  remainder  is  sugar,  about  0.15 
per  cent,  and  fat;  the  latter  constituent  varies  greatly,  being 
considerable  after  a  meal  rich  in  fats,  and  slight  at  other 
times.  About  0.8  of  1  per  cent  of  the  plasma  consists  of  sodium, 
potassium,  and  calcium  salts,  the  sodium  salts  constituting  by 
far  the  greatest  part  of  the  inorganic  content.  The  small  residue 
is  made  up  mostly  of  the  waste  materials  which  have  been  received 


STRUCTURE  AND  FUNCTIONS  OF  BLOOD  AND  LYMPH    303 

into  the  blood  from  the  tissues,  and  which  are  to  be  gotten  rid  of 
in  the  excretory  organs.  The  most  important  of  these  are  urea, 
creatinine,  uric  acid,  and  similar  bodies.  The  plasma  contains 
also  the  various  hormones,  mentioned  in  a  previous  paragraph, 
and  a  group  of  substances,  known  as  biological  reagents,  which  are 
part  of  the  disease-resisting  mechanism.  These  will  be  considered 
in  detail  in  later  paragraphs. 

The  plasma  carries  in  solution  a  certain  amount  of  oxygen, 
nitrogen,  and  carbon  dioxid,  but  no  more  than  a  similar  amount 
of  pure  water  would  dissolve  at  the  same  temperature.  Most  of 
the  oxygen  carried  by  the  blood  is  in  combination  with  the  hemo- 
globin of  the  red  corpuscles;  most  of  the  carbon  dioxid  is  in  com- 
bination with  sodium,  forming  sodium  carbonate  or  bicarbonate. 

Summary.  Practically  the  composition  of  the  blood  may  be 
thus  stated:  It  consists  of  (1)  plasma,  consisting  of  watery  solu- 
tions of  serum  albumin,  paraglobulin,  fibrinogen,  grape-sugar, 
sodium  and  other  salts,  and  extractives  of  which  the  most  constant 
are  urea,  creatinine,  and  uric  acid;  (2)  red  corpuscles,  contain- 
ing rather  more  than  half  their  weight  of  water,  the  remainder 
being  mainly  hemoglobin,  other  proteins,  and  potash  salts;  (3) 
white  corpuscles,  consisting  of  water,  various  proteins,  glycogen, 
and  potash  salts;  (4)  the  platelets;  (5)  gases,  partly  dissolved  in 
the  plasma  or  combined  with  its  sodium  salts,  and  partly  com- 
bined (oxygen)  with  the  hemoglobin  of  the  red  corpuscles. 

Quantity  of  Blood.  The  total  amount  of  blood  in  the  Body  is 
difficult  of  accurate  determination.  It  is  about  ^V  of  the  whole 
weight  of  the  Body,  so  the  quantity  in  a  man  weighing  75  kilos 
(165  Ibs.)  is  about  3.8  kilos  (8.5  Ibs.).  Of  this  at  any  given  moment 
about  one-fourth  would  be  found  in  the  heart,  lungs,  and  larger 
blood-vessels;  and  equal  quantities  in  the  vessels  of  the  liver,  and 
in  those  of  the  muscles  which  move  the  skeleton;  while  the  remain- 
ing fourth  is  distributed  among  the  remaining  parts  of  the  Body. 

Blood  of  Other  Animals.  In  all  animals  with  blood  the  white 
corpuscles  are  pretty  much  alike,  but  the  red  corpuscles,  which 
with  rare  exceptions  are  found  only  in  Vertebrates,  vary  con- 
siderably. In  all  the  classes  of  the  mammalia  they  are  circular 
biconcave  disks,  with  the  exception  of  the  camel  tribe,  in  which 
they  are  oval.  They  vary  in  diameter  from  0.02  mm.  dinnr  fach) 
(musk  deer)  to  0,011  mm.  GrzW  inch)  (elephant).  In  the  dog  they 


304  THE  HUMAN  BODY 

are  nearly  the  same  size  as  those  of  man.  In  no  mammals  do  the 
fully-developed  red  corpuscles  possess  a  nucleus.  In  all  other 
vertebrate  classes  the  red  corpuscles  possess  a  central  nucleus,  and 
are  oval  slightly  biconvex  disks,  except  in  a  few  fishes  in  which  they 
are  circular.  They  are  largest  of  all  in  the  amphibia.  Those  of  the 
frog  are  0.022  mm.  (T  AD  inch)  long  and  0.015  mm.  ( i  ^  inch)  broad. 

The  blood  of  certain  crustaceans  contains  instead  of  hemo- 
globin a  substance  of  similar  physiological  action,  hemocyanin, 
which  is  blue  instead  of  red,  and  contains  copper  in  place  of  iron. 

Histology  and  Chemistry  of  Lymph.  Pure  lymph  is  a  color- 
less watery-looking  liquid;  examined  with  a  microscope  it  is  seen 
to  contain  numerous  white  corpuscles  closely  resembling  those  of 
the  blood,  and  no  doubt  many  are  leucocytes  which  have  mi- 
grated. For  the  most  part,  however,  these  lymph-corpuscles  or 
lymphocytes  have  another  more  important  origin.  In  many 
parts  of  the  Body  there  are  collections  of  a  peculiar  lymphoid  or 
adenoid  tissue  (p.  383),  sometimes  in  nodular  masses  (lymphatic 
glands).  This  tissue  consists  essentially  of  a  fine  network,  the 
meshes  of  which  are  occupied  with  lymphocytes  which  frequently 
shows  signs  of  division.  The  meshes  of  the  network  communi- 
cate with  lymphatic  vessels  and  the  lymph  flowing  through  picks 
up  and  carries  off  the  new-formed  lymphocytes.  The  function  of 
the  lymphocytes  is  not  clear.  They  are  believed  not  to  share 
in  the  phagocytic  function  of  the  leucocytes. 

The  lymph  flowing  from  the  intestines  during  digestion  is,  as 
already  mentioned,  not  colorless,  but  white  and  milky.  It  will  be 
considered  with  the  process  of  digestion.  During  fasting  the 
lymph  from  the  intestines  is  colorless,  like  that  from  other  parts 
of  the  Body. 

Lymph  is  feebly  alkaline,  and  has  a  specific  gravity  of  about 
1,045.  The  chief  chemical  difference  between  lymph  and  blood- 
plasma  is  that  the  former  contains  somewhat  more  waste  ma- 
terials and  less  food  stuffs  than  the  latter.  This  is  because  the 
consumption  of  food  by  the  cells  and  their  production  of  waste 
keep  slightly  ahead  of  the  interchange  of  these  substances  between 
blood  and  lymph  by  the  processes  of  filtration  and  dialysis.  Lymph 
contains  carbon  dioxid  in  solution  but  no  uncombined  oxygen,  the 
latter  substance  being  taken  up  by  the  living  cells  as  fast  as  it  enters 
the  lymph  from  the  blood. 


CHAPTER  XVIII 

THE    HORMONE-CARRYING    AND    DISEASE-RESISTING 
FUNCTIONS  OF  THE  BLOOD.    BLOOD-CLOTTING 

Hormones.  The  chemical  control  of  bodily  processes  by  means 
of  hormones  has  assumed  great  importance  of  recent  years  and 
is  at  present  the  subject  of  active  investigation.  For  a  long  time 
it  has  been  recognized  that  many  processes  are  subject  to  hormone 
influence,  but  the  number  of  such  processes  is  being  constantly 
added  to  as  our  knowledge  advances.  Although  a  few  of  the 
hormones  have  been  isolated  and  their  chemistry  studied,  by  far 
the  greater  number  are  known  only  by  their  physiological  effects. 
Most  of  the  hormones  are  special  substances,  formed  in  organs 
whose  sole  function,  so  far  as  we  can  judge,  is  their  production. 
A  few  of  them  exercise  their  hormone  function  only  incidentally  to 
their  chief  bodily  destiny. 

As  stated  previously  the  organs  whose  exclusive  function  is  to 
secrete  hormones  are  known  as  ductless  glands.  In  spite  of  a  great 
amount  of  investigation  in  recent  years  our  knowledge  of  the 
chemical  co-ordination  of  the  Body  is  still  very  incomplete  and 
there  are  some  ductless  glands  concerning  whose  function  we  have 
virtually  no  information.  Among  these  may  be  mentioned  the 
parathyroids,  small  bodies,  usually  four  in  number,  which  are  found 
on  or  near  the  thyroids,  sometimes  embedded  within  them.  That 
these  produce  an  essential  hormone  is  proven  by  the  fact  that  their 
complete  removal  is  followed  by  acute  toxic  symptoms,  with  mus- 
cular convulsions,  ending  in  death.  Of  the  normal  functioning  of 
the  hormone  which  they  produce  nothing  significant  is  known. 

There  are  some  hormones,  which,  instead  of  being  elaborated  in 
specific  ductless  glands,  are  made  by  cells  embedded  in  organs 
whose  primary  functions  have  no  relation  with  those  of  the  hor- 
mones made  within  their  mass.  The  special  hormone-producing 
cells  in  these  cases,  although  anatomically  parts  of  the  organs  within 
which  they  lie,  are  physiologically  as  distinct  as  though  they 

305 


306  THE  HUMAN  BODY 

were  grouped  into  specific  masses,  recognizable  as  independent 
organs. 

Since  the  hormones  whose  functions  are  at  all  understood  are 
discussed  in  connection  with  the  bodily  processes  with  which  they 
are  associated  no  further  account  of  them  need  be  given  here. 

Infection.  Bacteriology  has  taught  us  that  we  are  continually 
surrounded  by  myriads  of  micro-organisms  of  various  kinds. 
They  are  on  the  skin  and  mucous  membranes;  they  are  breathed 
in  with  the  air  and  swallowed  in  the  food  and  water;  colonies  of 
them  flourish  in  the  intestinal  tracts.  Not  all  of  them  are  disease 
producing  (pathogenic),  but  these  are  always  present  along  with 
the  harmless  varieties. 

Not  only  are  these  organisms  always  present,  but  small  num- 
bers of  them  frequently  find  their  way  into  the  lymph  spaces  of  the 
Body,  whence  they  get  into  the  blood.  The  entry  of  pathogenic 
organisms  into  the  Body  does  not  constitute  infection.  It  is  only 
when  they  gain  a  foothold  and  begin  to  multiply  that  the  infection 
is  established  and  the  disease  under  way. 

It  is  recognized  that  the  ill  effects  of  an  infection  are  not  due  to 
the  presence  of  the  organisms  merely,  but  to  poisonous  substances, 
or  toxins,  which  they  produce  as  incidents  in  their  vital  processes. 
Some  sorts  give  off  this  poison  to  the  blood,  themselves  remaining 
out  of  the  blood-stream;  the  diphtheria  organism  is  of  this  sort. 
Others  retain  the  toxin  within  themselves,  and  it  is  only  when  they 
die  and  decompose  that  the  poison  is  liberated. 

Resistance  to  Infection.  In  order  for  organisms  to  attack  the 
Body  they  have  first  to  get  within  it.  So  long  as  the  skin  and  the 
lining  membranes  of  the  respiratory  and  digestive  tracts  are  intact 
the  entry  of  organisms  is  difficult,  if  not  impossible.  A  prime  fea- 
ture in  the  resistance  to  infection,  therefore,  is  the  preservation  of 
the  membranes  intact.  The  great  danger  from  an  ordinary  cold; 
which  in  itself  is  usually  a  mild  infection,  is  in  the  damage  to  the 
mucous  membranes  which  accompanies  it,  and  which  may  afford 
channels  of  entry7  to  organisms  which  otherwise  would  not  be  able 
to  gain  admission.  In  ^uninjured  membranes,  then,  we  have  the 
" first  line  of  defense"  against  infection. 

Even  though  organisms  do  succeed  in  penetrating  the  mem- 
branes infection  does  not  always  or  even  usually  follow.  If  it  did 
infection  would  be  our  fate  much  more  frequently  than  it  is.  The 


DISEASE-RESISTING  FUNCTIONS  OF  THE  BLOOD       307 

tissues  of  the  Body  form,  however,  excellent  culture  media;  or- 
ganisms that  do  establish  themselves  flourish  mightily,  at  least  for  a 
time.  It  follows,  therefore,  that  ordinarily  organisms  are  forcibly 
prevented  from  establishing  themselves.  This  prevention  of  infec- 
tion is  in  part  a  function  of  the  tissue  generally  and  in  part  a  func- 
tion of  the  blood.  It  must  be  sharply  differentiated  from  an 
additional  disease-resisting  function  possessed  also  by  the  blood, 
which  is  the  overcoming  of  infection  after  it  is  once  established.  In 
the  absence  of  this  latter  function  every  infection  would  result 
fatally. 

The  Infection-Resisting  Mechanism.  Although,  as  stated 
above,  the  bodily  tissues  form  excellent  culture  media  they  do  not 
all  yield  readily  to  the  attacks  of  the  invading  micro-organisms. 
Some  tissues  are  more  susceptible  than  others,  and  some  kinds  of 
organisms  attack  certain  tissues  more  readily  than  they  do  others. 
The  curious  fact  has  recently  been  demonstrated  that  the  organism 
of  "  blood-poisoning "  or  septicemia,  which  often  attacks  nearly  all 
the  tissues  of  the  Body,  shows  a  decided  preference  for  the  par- 
ticular tissue  in  which  it  formerly  grew.  Thus  if  from  an  animal 
killed  by  the  infection  some  of  the  organisms  found  in  the  kidney 
be  injected  into  the  veins  of  a  second  animal,  the  kidneys  of  the 
latter  will  be  first  attacked.  If  the  organisms  came  from  the  liver 
they  will  strike  first  at  the  liver. 

The  tissues  of  some  people  are  in  general  more  resistant  than 
those  of  others.  It  is  believed  that  this  resistance  is  to  a  certain 
degree  inherited.  At  any  rate  the  experience  of  peoples  exposed  for 
the  first  time  to  particular  infections  suggests  this.  Whenever  in 
the  history  of  the  world  races  have  been  brought  into  contact  with 
new  diseases  they  have  suffered  severely  therefrom,  although  in 
many  cases  the  diseases  which  wrought  the  havoc  were  lightly  es- 
teemed by  races  that  had  been  accustomed  to  them  for  generations. 

In  addition  to  this  general  mode  of  resisting  infection,  which 
we  may  call  tissue  resistance,  there  are  two  sorts  of  structures  in  the 
blood  specially  devoted  to  the  destruction  of  invading  micro- 
organisms. They  work  independently  but  in  co-operation.  The 
first  of  these  are  the  phagocytes  previously  mentioned  (p.  302) 
which  engulf^and  thus  dispose  of  the  invading  foreign  bodies.  The 
second  sort  is  not  made  up  of  formed  elements  like  the  phagocytes, 
but  is  in  solution  in  the  plasma.  It  attacks  and  destroys  bacteria 


308  THE  HUMAN  BODY 

IdWJt^/vv-^^AJt^  CM     ^su± 

by  chemical  action.  To  this  substance  is  given  the  name 
It  has  been  shown  to  be  made  up  of  two  other  substances.  The 
first  of  these,  the  complements,  are  present  in  the  blood  in  variable, 
but  considerable,  amounts  and  are  actively  destructive  agents. 
Their  destructive  power  is  limited,  however,  by  the  circumstance 
that  they  are  unable  to  attack  foreign  organisms  directly,  but  must 
first  be  in  combination  with  the  second  bodies,  known  as  inter- 
mediary or  immune  bodies,  through  which  they  gain  the  necessary 
chemical  grasp  on  the  cells  which  are  attacked.  An  important 
feature  of  the  immune  bodies  is  that  each  kind  can  combine  with 
only  one  sort  of  foreign  cell.  Unless  immune  bodies  of  the  right 
kind  are  present,  the  complements  are  helpless.  The  analogy  of 
the  yale  lock  which  can  be  opened  only  by  its  own  key  suggests 
itself.  Clearly  the  scope  of  this  protective  mechanism  is  limited  to 
the  varieties  of  immune  bodies  that  happen  to  be  present. 

Why  Does  Infection  Ever  Occur?  The  establishment  of  an 
infection  in  the  face  of  this  elaborate  protective  mechanism  can  be 
explained  in  one  of  two  ways.  Either  the  mechanism  falls  off  in 
efficiency,  which  is  the  condition  present  when  we  say  "the  re- 
sistance of  the  Body  is  lowered,"  or  the  invading  organisms  are  so 
virulent  that  the  Body  is  unable  to  overcome  them.  Lowered 
body  resistance  may  result  from  a  number  of  conditions;  under- 
nutrition,  prolonged  exposure  to  extremes  of  heat  or  cold,  alco- 
holism, severe  local  inflammations,  all  of  these  may  diminish  the 
number  of  phagocytes  or  the  quantity  of  alexins,  or  may  lessen 
their  activity.  Bacteria  may  vary  from  time  to  time  in  virulence. 
It  appears  that  the  virulence  of  most  sorts  is  much  increased  by  a 
period  of  growth  in  a  living  Body.  It  is  because  of  this  increase 
of  virulence  that  "exposure"  to  an  infected  individual  is  so  often 
followed  by  infection.  The  fact  of  increased,  virulence  explains 
also  the  occurrence  of  "epidemics." 

Recovery  from  Infection  involves  two  processes:  1.  Destroying 
and  getting  rid  of  the  enormous  numbers  of  bacteria  which  de- 
velop during  the  progress  of  the  disease;  2.  Getting  rid  of  or  neu- 
tralizing the  poison,  or  toxin,  which  the  bacteria  produce  and  which 
is  the  real  cause  of  trouble.  The  course  of  every  infection  is  a 
struggle  between  the  Body  on  one  hand  and  the  micro-organisms 
on  the  other.  The  outcome  is  recovery  or  death  according  as  one 
side  or  the  other  proves  victorious. 


DISEASE-RESISTING  FUNCTIONS  OF  THE  BLOOD       309 

For  destroying  and  getting  rid  of  the  bacteria  the  Body  makes 
use  of  the  same  structures,  the  complements  and  phagocytes, 
that  it  uses  in  resisting  infection  in  the  first  place;  but  the  effi- 
ciency of  these  is  enormously  increased  through  the  development 
of  special  aids  to  their  activity. 

Opsonins,  Immune  Bodies,  and  Agglutinins.  The  presence  and 
growth  of  foreign  organisms  stimulate  the  cells  of  the  Body  to 
produce  and  set  free  in  the  blood  large  numbers  of  bodies  of  prob- 
ably at  least  three  sorts.  The  first  of  these,  called  opsonins,  act 
upon  the  invading  bacteria  in  such  a  way  as  to  increase  very 
greatly  the  "appetite"  of  the  phagocytes  for  them.  It  is  possible 
to  obtain  living  phagocytes  in  salt  solution,  free  from  the  other 
elements  of  blood.  If  to  a  slide  containing  some  of  these  a  num- 
ber of  bacteria  be  added  and  the  whole  kept  at  body  temperature, 
the  average  number  of  bacteria  ingested  by  each  phagocyte  can 
be  determined  by  actual  observation.  It  is  found  that  if  the 
bacteria,  before  being  placed  on  the  slide,  are  treated  with  a 
liquid  containing  the  proper  opsonin,  the  average  ingestion  per 
phagocyte  is  multiplied  many  fold.  It  is  necessary,  for  this  effect 
to  be  produced,  that  the  opsonin  be  applied  to  the  bacteria;  treat- 
ment of  the  phagocytes  with  opsonin,  with  subsequent  washing, 
does  not  increase  at  all  their  tendency  to  ingest  bacteria. 

Under  the  stimulus  afforded  by  the  presence  of  foreign  or- 
ganisms are  produced,  also,  great  quantities  of  the  special  immune 
bodies  needed  to  give  the  complements  access  to  those  particular 
organisms.  Thus  a  defensive  agency  which  if  present  at  all  before 
the  infection  was  only  slightly  effective  becomes  the  chief  reliance 
of  the  Body  in  its  struggle  to  rid  itself  of  the  invaders. 

In  addition  to  opsonins  and  immune  bodies,  the  cells  under 
bacterial  stimulation  produce  what  is  thought  to  be  a  third  sub- 
stance, agglutinin,  which  causes  the  bacteria  to  clump  together, 
becoming  thus  more  subject  to  the  action  of  the  phagocytes  or 
complements.  The  development  of  these  various  bodies  is  the 
process  of  immui^zation. 

Antitoxin.  Beside  the  destruction  of  the  invading  bacteria 
it  is  necessary,  before  the  Body  is  cured  of  an  infection,  that 
the  toxins  produced  by  the  rapidly  multiplying  organisms  be  neu- 
tralized. This  neutralization  of  poison  is  a  simpler  process  than 
the  destruction  of  formed  elements  as  carried  on  by  the  phago- 


310  THE  HUMAN  BODY 

cytes  and  complements.  It  is  brought  about  in  the  Body,  how- 
ever, in  much  the  same  way.  The  foreign  toxin  stimulates  the 
cells  of  the  Body  to  produce  and  pour  into  the  blood  an  antitoxin 
which  neutralizes  the  toxin.  Antitoxins,  like  opsonins  and  im- 
mune bodies,  are  specific  for  the  toxin  which  stimulated  their 
development. 

Immunity.  An  individual  who  has  gone  through  an  infection, 
and  by  the  co-operation  of  the  forces  described  above  has  over- 
come and  destroyed  the  invaders  with  their  harmful  toxins,  re- 
tains for  a  long  time  afterward  in  his  blood  the  special  opsonins, 
immune  bodies,  and  antitoxins  which  were  developed  therein 
during  the  course  of  the  infection.  He  is,  during  this  time,  im- 
mune toward  that  particular  disease.  The  existence  of  this  im- 
munity has  been  known  for  centuries;  its  explanation  is  the  result 
of  the  work  of  the  last  twenty  years. 

The  duration  of  immunity  varies  greatly  in  different  infections. 
There  is  every  degree  from  the  extremely  brief  immunity  toward 
common  colds,  an  immunity  that  apparently  terminates  with  the 
period  of  convalescence;  to  an  immunity  that  is  life  long  as  in  the 
case  of  yellow  fever. 

Carriers.  A  fact  of  interest,  as  well  as  of  great  moment  in  the 
problem  of  caring  for  the  public  health,  is  that  an  occasional  in- 
fected individual,  instead  of  destroying  the  invading  organisms, 
becomes  so  adapted  to  them  that  he  continues  in  perfect  health 
with  his  Body  swarming  with  pathogenic  organisms.  Such  a  per- 
son is  known  as  a  carrier.  He  is  a  constant  source  of  danger,  since 
the  organisms  he  carries,  and  by  which  he  is  unaffected,  are  liable 
to  be  transferred  to  some  susceptible  individual  and  cause  severe 
illness  or  even  a  widespread  epidemic. 

The  Use  of  Antitoxin  in  Disease.  In  some  diseases,  of  which 
diphtheria  is  the  best  known  example,  the  bacteria  do  not  spread 
through  the  Body  but  take  up  their  abode  on  a  convenient  surface 
where  they  develop  and  whence  they  discharge  their  toxin  into 
the  blood.  Successful  combating  of  such  diseases  requires  only 
that  the  toxin  be  neutralized.  In  course  of  time  the  bacteria  will 
reach  the  end  of  their  development  and  die. 

The  antitoxin  for  any  particular  kind  of  toxin  will  neutralize 
it  whether  produced  in  the  body  which  is  infected  or  in  some 
other  body  from  which  it  is  transferred  to  the  infected  one.  This 


DISEASE-RESISTING  FUNCTIONS  OF  THE  BLOOD        311 

fact  has  made  possible  the  development  of  the  well-known  anti- 
toxin treatment.  Animals,  usually  horses,  receive  doses  of  toxin 
obtained  by  growing  the  bacteria  on  culture  media  in  proper 
vessels.  These  doses  are  small  at  first,  but  are  gradually  increased 
as  the  animal  acquires  immunity.  In  course  of  time  the  blood  of 
an  animal  so  treated  contains  large  quantities  of  antitoxin.  Con- 
siderable amounts  of  blood  can  be  withdrawn  from  animals  the 
size  of  horses  without  their  suffering  the  slightest  inconvenience. 
It  is  thus  possible  to  obtain  abundant  supplies  of  antitoxin. 

The  methods  of  purifying  antitoxin-containing  solutions  are 
so  perfect  at  the  present  time  that  no  one  should  feel  the  least 
hesitation  at  the  prospect  of  its  use.  The  percentage  of  deaths 
from  diphtheria  has  fallen  from  more  than  fifty  to  about  two 
since  its  introduction. 

Protective  Inoculation.  It  has  been  found  practicable  in  some 
diseases,  notably  smallpox,  to  develop  immunity  by  infecting  the 
Body  with  an  organism  which  is  not  virulent  enough  to  endanger 
life  but  which  produces  immune  substances  that  protect  the  Body 
against  the  more  virulent  infection.  On  account  of  the  specific 
character  of  immunity  this  method  can  only  be  used  where  vir- 
tually the  same  organism  occurs  in  virulent  and  non-virulent 
forms. 

The  most  hopeful  path  of  progress  at  present  toward  the  mas- 
tery of  disease  is  along  the  lines  here  indicated.  We  may  look 
forward  confidently  to  a  time  when  most  if  not  all  the  acute  in- 
fections will  be  brought  under  medical  control  through  applica- 
tion of  the  principles  of  immunity. 

Anaphylaxis.  Although  in  our  discussion  of  immunity  thus  far 
emphasis  has  been  laid  on  it  as  a  means  of  destroying  disease  germs 
and  their  toxins,  the  fact  is  that  the  immunity  reaction,  considered 
as  a  reaction,  may  manifest  itself  toward  foreign  protein  substances 
in  general,  whether  they  have  any  relation  to  disease  or  not. 
Thus  it  is  possible  by  injection  of  egg-white  into  the  blood  to  cause 
the  Body  to  develop  immunity  toward  that  substance. 

In  the  development  of  immunity  toward  toxins  of  disease  the 
Body  is  under  the  influence  of  the  toxin  more  or  less  continuously 
for  a  time,  and  this  continuous  influence  seems  essential  to  the 
normal  progress  of  the  immunity  reaction.  If,  instead  of  such 
continuous  influence  the  Body  receives  a  single  dose  of  foreign 


312  THE  HUMAN  BODY 

protein  which  is  not  repeated,  there  may  appear  a  marked  increase 
of  sensitiveness  toward  the  immunizing  substance,  so  that  although 
it  may  not  have  had  any  noteworthy  effect  on  the  Body  formerly, 
after  this  sensitization  has  occurred  injection  of  the  protein  may 
cause  violent  or  even  fatal  disturbances.  This  reversal  of  the 
ordinary  course  of  immunization  is  called  anaphylaxis.  The  tis- 
sues which  are  most  markedly  affected  are  the  involuntary  muscles, 
and  death,  when  it  occurs,  is  the  result  of  cardiac  or  bronchial 
spasms,  or  other  smooth  muscle  involvements. 

Anaphylaxis  is  of  practical  importance  because  the  administra- 
tion of  antitoxin  involves  the  introduction  of  foreign  proteins  into 
the  system,  and  if  sensitization  should  take  place,  a  second  dose 
would  have  serious,  or  even  fatal  consequences.  The  serum  (see 
next  paragraph)  of  horses  forms  the  basis  of  diphtheria  antitoxin. 
Sometimes  persons  who  are  much  about  horses  develop  the  condi- 
tion, apparently  from  inhaling  the  effluvium  from  the  animals. 
Such  persons  cannot  endure  injections  of  antitoxin.  They  often 
suffer  disagreeable  bronchial  disturbances  from  the  mere  pres- 
ence of  horses.  Hay  fever  is  a  similar  sensitization  toward  the 
proteins  contained  in  the  pollen  grains  of  plants. 

The  Coagulation  of  Blood.  When  blood  is  first  drawn  from  the 
living  Body  it  is  perfectly  liquid,  flowing  in  any  direction  as  readily 
as  water.  This  condition  is,  however,  only  temporary;  in  a  few 
minutes  the  blood  becomes  viscid  and  sticky,  and  the  viscidity 
becomes  more  and  more  marked  until,  after  the  lapse  of  five  or 
six  minutes,  the  whole  mass  sets  into  a  jelly  which  adheres  to  the 
vessel  containing  it,  so  that  this  may  be  inverted  without  any 
blood  whatever  being  spilled.  This  stage  is  known  as  that  of 
gelatinization  and  is  also  not  permanent.  In  a  few  minutes  the 
top  of  the  jelly-like  mass  will  be  seen  to  be  hollowed  or  "cupped" 
and  in  the  concavity  will  be  seen  a  small  quantity  of  nearly  color- 
less liquid,  the  blood-serum.  The  jelly  next  shrinks  so  as  to  pull 
itself  loose  from  the  sides  and  bottom  of  the  vessel  containing 
it,  and  as  it  shrinks  squeezes  out  more  and  more  serum.  Ulti- 
mately we  get  a  solid  dot,  colored  red  and  smaller  in  size  than 
the  vessel  in  which  the  blood  coagulated  though  retaining  its 
form,  floating  in  a  quantity  of  pale  yellow  serum.  If,  however, 
the  blood  be  not  allowed  to  coagulate  in  perfect  rest,  a  certain 
number  of  red  corpuscles  will  be  rubbed  out  of  the  clot  into  the 


DISEASE-RESISTING  FUNCTIONS  OF  THE  BLOOD       313 

serum  and  the  latter  will  be  more  or  less  reddish.  The  longer  the 
clot  is  kept  the  more  serum  will  be  obtained:  if  the  first  quantity 
exuded  be  decanted  off  and  the  clot  put  aside  and  protected  from 
evaporation,  it  will  in  a  short  time  be  found  to  have  shrunk  to  a 
smaller  size  and  to  have  pressed  out  more  serum;  and  this  goes  on 
until  putrefactive  changes  commence. 

Cause  of  Coagulation.  If  a  drop  of  fresh-drawn  blood  be  spread 
out  very  thin  and  watched  for  a  few  minutes  with  a  microscope 
magnifying  600  or  700  diameters,  it  will  be  seen  that  the  coagu- 
lation is  due  to  the  separation  of  very  fine  solid  threads  which 
run  in  every  direction  through  the  plasma  and  form  a  close  net- 
work entangling  all  the  corpuscles.  These  threads  are  composed 
of  the  protein  substance  fibrin.  When  they  first  form,  the  whole 
drop  is  much  like  a  sponge  soaked  full  of  water  (represented  by 
the  serum)  and  having  solid  bodies  (the  corpuscles)  in  its  cavi- 
ties. After  the  fibrin  threads  have  been  formed  they  tend  to 
shorten;  hence  when  blood  clots  in  mass  in  a  vessel,  the  fibrinous 
network  tends  to  shrink  in  every  direction  just  as  a  network 
formed  of  stretched  india-rubber  bands  would,  and  this  shrinkage 
is  greater  the  longer  the  clotted  blood  is  kept.  At  first  the  threads 
stick  too  firmly  to  the  bottom  and  sides  of  the  vessel  to  be  pulled 
away,  and  thus  the  first  sign  of  the  contraction  of  the  fibrin  is 
seen  in  the  cupping  of  the  surface  of  the  gelatinized  blood  where 
the  threads  have  no  solid  attachment,  and  there  the  contracting 
mass  presses  out  from  its  meshes  the  first  drops  of  serum.  Finally 
the  contraction  of  the  fibrin  overcomes  its  adhesion  to  the  vessel 
and  the  clot  pulls  itself  loose  on  all  sides,  pressing  out  more  and 
more  serum,  in  which  it  ultimately  floats.  The  great  majority 
of  the  red  corpuscles  are  held  back  in  the  meshes  of  the  fibrin, 
but  a  good  many  leucocytes,  by  their  amoeboid  movements, 
work  their  way  out  and  get  into  the  serum. 

Whipped  Blood.  The  essential  point  in  coagulation  being  the 
formation  of  fibrin  in  the  plasma,  and  blood  only  forming  a  cer- 
tain amount  of  fibrin,  if  this  be  removed  as  fast  as  it  forms  the 
remaining  blood  will  not  clot.  The  fibrin  may  be  separated  by 
what  is  known  as  " whipping"  the  blood.  For  this  purpose  fresh- 
drawn  blood  is  stirred  up  vigorously  with  a  bunch  of  twigs,  and 
to  these  the  sticky  fibrin  threads  as  they  form,  adhere.  If  the 
twigs  be  withdrawn  after  a  few  minutes  a  quantity  of  stringy 


314  THE  HUMAN  BODY 

material  will  be  found  attached  to  them.  This  is  at  first  colored 
red  by  adhering  blood-corpuscles:  but  by  washing  in  water  they 
may  be  removed,  and  the  pure  fibrin  thus  obtained  is  perfectly 
white  and  in  the  form  of  highly  elastic  threads.  It  is  insoluble  in 
water  and  in  dilute  acids,  but  swells  up  to  a  transparent  jelly  in 
the  latter.  The  "whipped"  or  "  defibrinated  blood"  from  which 
the  fibrin  has  been  in  this  way  removed,  looks  just  like  ordinary 
blood,  but  has  lost  the  power  of  coagulating  spontaneously. 

The  Buffy  Coat.  That  the  red  corpuscles  are  not  an  essential 
part  of  the  clot,  but  are  merely  mechanically  caught  up  in  it, 
seems  clear  from  the  microscopic  observation  of  the  process  of 
coagulation;  and  from  the  fact  that  perfectly  formed  fibrin  can 
be  obtained  free  from  corpuscles  by  whipping  the  blood  and 
washing  the  threads  which  adhere  to  the  twigs.  Under  certain 
conditions,  moreover,  one  gets  a  naturally  formed  clot  containing 
no  red  corpuscles  in  one  part  of  it.  The  corpuscles  of  human  blood 
are  a  little  heavier,  bulk  for  bulk,  than  the  plasma  in  which  they 
float;  hence,  when  the  blood  is  drawn  and  left  at  rest  they  sink 
slowly  in  it;  and  if  for  any  reason  clotting  take  place  more  slowly 
or  the  corpuscles  sink  more  rapidly  than  usual,  a  colorless  top 
stratum  of  plasma,  with  no  red  corpuscles  in  it,  is  left  before 
gelatinization  occurs  and  stops  the  further  sinking  of  the  cor- 
puscles. The  uppermost  part  of  the  clot  formed  under  such  cir- 
cumstances is,  colorless  or  pale  yellow,  and  is  known  as  the  buffy 
coatj,  it  is  especially  apt  to  be  formed  in  the  blood  drawn  from 
febrile  patients,  and  was  therefore  a  point  to  which  physicians 
paid  much  attention  in  the  olden  times  when  blood-letting  was 
thought  to  be  almost  a  panacea.  In  horse's  blood  the  difference 
between  the  specific  gravity  of  the  corpuscles  and  that  of  the 
plasma  is  greater  than  in  human  blood,  and  horse's  blood  also 
coagulates  more  slowly,  so  that  its  clot  has  nearly  always  a  buffy 
coat.  The  colorless  buffy  coat  seen  sometimes  on  the  top  of  the 
clot  must,  however,  not  be  confounded  with  another  phenomenon. 
When  a  blood-clot  is  left  floating  exposed  to  the  air  its  top  be- 
comes bright  scarlet,  while  the  part  immersed  in  the  serum  has 
a  dark  purple-red  color.  The  brightness  of  the  top  layer  is  due 
to  the  action  of  the  oxygen  of  the  air,  which  forms  a  scarlet  com- 
pound with  the  coloring  matter  of  the  red  corpuscles.  If  the 
clot  be  turned  upside  down  and  left  for  a  short  time,  the  pre- 


DISEASE-RESISTING  FUNCTIONS  OF  THE  BLOOD        315 

viously  dark  red  bottom  layer,  now  exposed  to  the  air,  becomes 
bright. 

Uses  of  Coagulation.  The  clotting  of  the  blood  is  so  important 
a  process  that  its  cause  has  been  frequently  investigated;  but  it  is 
not  yet  completely  understood.  The  living  circulating  blood  in 
the  healthy  blood-vessels  does  not  clot;  it  contains  no  solid  fibrin, 
but  this  forms  in  it,  sooner  or  later,  when  the  blood  gets  by  any 
means  out  of  the  vessels  or  when  the  lining  of  these  is  injured. 
In  this  way  the  mouths  of  the  small  vessels  opened  in  a  cut  are 
clogged  up,  and  the  bleeding,  which  would  otherwise  go  on  in- 
definitely, is  stopped.  So,  too,  when  a  surgeon  ties  up  an  artery 
before  dividing  it,  the  tight  ligature  crushes  or  tears  its  delicate 
inner  surface,  and  the  blood  clots  where  that  is  injured,  and  from 
there  a  coagulum  is  formed  reaching  up  to  the  next  highest  branch 
of  the  vessel.  This  becomes  more  and  more  solid,  and  by  the  time 
the  ligature  is  removed  has  formed  a  firm  plug  in  the  cut  end  of 
the  artery,  which  greatly  diminishes  the  risk  of  bleeding. 

The  Source  of  Blood-Fibrin.  Since  fresh  blood-plasma  contains 
no  fibrin  but  does  contain  considerable  quantities  of  other  pro- 
teins, we  look  first  to  these  as  a  possible  source  of  the  fibrin  formed 
during  coagulation.  If  horse's  blood  be  drawn  directly  from  the 
living  animal  into  a  cold  vessel  and  kept  just  above  freezing 
temperature  it  does  not  clot  and  after  a  time  the  corpuscles  settle 
to  the  bottom  leaving  a  supernatant  portion  of  clear  plasma.  This 
plasma  retains  the  power  of  clotting,  as  is  shown  when  it  is  warmed ; 
but  if  before  it  clots  it  be  saturated  with  sodium  chlorid  and  filtered, 
the  liquid  that  remains  will  no  longer  clot.  The  precipitate  formed 
by  the  saturation  with  sodium  chlorid  must  contain,  therefore, 
some  essential  in  the  process  of  clotting.  This  precipitate  if 
examined  will  be  found  to  be  a  mixture  containing  all  the  fibrinogen 
of  the  plasma  and  part  of  the  globulin.  These  two  substances  may 
be  separated  by  proper  treatment,  and  after  this  has  been  done  it 
is  found  that  a  solution  of  the  fibrinogen  can  be  made  to  clot, 
while  one  containing  only  paraglobulin  cannot.  During  the 
clotting  of  the  fibrinogen  solution  the  fibrinogen  disappears,  giv- 
ing place  to  fibrin. 

We  are  thus  led  to  the  conclusion  that  the  natural  clotting  of 
fresh  blood  is  due  to  the  formation  of  fibrin  from  fibrinogen  which 
existed  in  solution  in  the  plasma  of  the  circulating  blood  and  has 


316  THE  HUMAN  BODY 

been  altered  in  the  clotted,  giving  origin  to  fibrin.  But  as  normal 
blood  circulating  in  healthy  uninjured  blood-vessels  does  not  clot 
nor  do  pure  solutions  of  fibrinogen,  we  have  still  to  seek  the  ex- 
citing cause  of  the  change. 

If  to  a  solution  of  fibrinogen  there  be  added  a  few  drops  of 
blood  or  of  blood-serum,  or  of  the  washings  of  a  blood-clot,  fibrin 
will  be  formed;  therefore  drawn  blood  and  serum  and  natural 
clot  each  contain  something  which  can  effect  the  conversion  of 
fibrinogen  into  fibrin.  This  substance  is  thrombin,  frequently 
called  also  the  fibrin  ferment. 

Thrombin.  When  blood-serum  is  treated  with  several  times 
its  volume  of  strong  alcohol  its  various  proteins  and  most  of  its 
salts  are  precipitated :  if  the  precipitate  be  left  standing  in  alcohol 
for  some  days  the  proteins  become  almost  entirely  insoluble  in 
water,  but  a  few  drops  of  the  watery  extract  cause  clotting  in  a 
saline  solution  of  fibrinogen,  and  clearly  contain  some  of  the 
thrombin.  This  substance  was  for  a  long  time  believed  to  be 
an  enzym,  hence  its  name  of  " fibrin  ferment."  Recent  careful 
study  shows,  however,  that  it  does  not  correspond  to  enzyms  in 
either  of  their  two  cardinal  characteristics,  namely,  the  ability 
of  a  small  amount  of  the  substance  to  produce  a  very  large  amount 
of  chemical  activity,  and  the  destruction  of  the  substance  by 
heating  above  60°  C.  It  has  been  definitely  proven  that  the 
amount  of  fibrinogen  that  is  converted  to  fibrin  bears  a  direct 
relationship  to  the  amount  of  thrombin  present,  and  that  throm- 
bin solutions  free  from  protein  impurities  can  be  boiled  without 
destroying  the  thrombin. 

Source  of  Thrombin.  If  fresh  blood  is  drawn  directly  from  the 
veins  of  an  animal  into  strong  alcohol,  and  the  resulting  pre- 
cipitate treated  as  described  above  for  preparing  thrombin  from 
serum,  it  yields  no  thrombin;  this  substance,  therefore,  which  is 
present  in  blood-serum,  is  absent  from  the  blood  within  the  Body 
and  must  be  formed  after  the  blood  is  shed  and  before  the  forma- 
tion of  the  clot.  When  the  process  of  clotting  is  watched  under 
the  microscope  the  fibrin  threads  will  usually  be  seen  to  form 
about  certain  centers.  These  centers  consist  of  disintegrating 
blood-plates,  and  the  observation  that  fibrin  formation  proceeds 
from  them  in  this  fashion  led  to  the  idea  that  the  blood-plates  are 
in  some  way  associated  with  the  process. 


DISEASE-RESISTING  FUNCTIONS  OF  THE  BLOOD       317 

The  natural  conclusion  drawn  from  this  observation  was  that 
the  blood-plates  contain  thrombin  which  is  inactive  so  long  as 
they  are  intact,  and  is  liberated  by  their  disintegration.  This 
simple  conclusion  was  upset  by  the  further  observation  that  fresh 
blood  drawn  into  a  solution  of  sodium  oxalate  will  not  clot.  So- 
dium oxalate  does  not  hinder  the  process  of  blood-plate  disin- 
tegration. In  fact  its  sole  effect  upon  blood,  so  far  as  can  be  de- 
termined, is  to  precipitate  out  its  calcium,  as  calcium  oxalate. 
That  the  prevention  of  clotting  is  due  to  this  precipitation  of  cal- 
cium is  shown  by  the  fact  that  addition  of  excess  of  a  soluble 
calcium  salt  to  " oxalate"  blood  causes  it  to  clot  with  great 
promptness.  The  formation  of  active  thrombin  is  dependent, 
then,  upon  the  presence  of  calcium  in  the  blood,  and  the  substance 
contained  in  the  blood-plates  is  not  true  thrombin,  but  a  prepara- 
tory substance  which  we  may  call  prothrombin. 

Antithrombin.  A  feature  of  the  coagulation  process  that  pre- 
sents some  difficulty  is  that  although  the  circulating  blood  contains 
all  the  essential  factors  of  the  process,  clotting  does  not  occur  in  it 
so  long  as  it  circulates  normally,  but  only  when  it  escapes  from  the 
vessels  or  when  the  lining  of  these  is  injured.  To  say  that  the 
prothrombin  is  stored  in  the  platelets  and  so  kept  from  combining 
with  calcium  to  form  thrombin  seems  an  insufficient  protection 
against  the  possible  accident  of  a  decomposition  of  platelets  in  the 
blood-stream. 

Definite  evidence  has  been  obtained  of  the  existence  of  a  sub- 
stance which  will  prevent  clotting.  This  substance  is  present  in 
the  salivary  glands  of  leeches,  and  serves  to  keep  the  blood  which 
they  ingest  liquid  in  their  stomachs.  To  it  has  been  given  the 
name  antithrombin.  Snake  venom  contains  similar  material. 
What  is  believed  to  be  the  same  substance  is  produced  within  the 
bodies  of  some  animals  (dogs)  by  injecting  unpurified  peptone 
solutions  into  their  veins.  There  is  reason  to  believe  that  normal 
blood  contains  antithrombin  in  sufficient  amounts  to  prevent 
clotting  within  the  blood-vessels.  If  this  substance  is  present  in 
the  blood  it  must  be  neutralized  when  the  blood  is  shed  to  allow 
coagulation  to  proceed. 

Thromboplastic  substance.  An  observation  that  throws  light 
on  the  manner  in  which  antithrombin  is  neutralized  when  blood 
is  shed  is  that  if  the  blood  is  drawn  from  a  vessel  directly  into  a 


318  THE  HUMAN  BODY 

glass  tube  with  care  to  avoid  contamination  from  the  wound  clot- 
ting takes  place  very  slowly,  or  in  birds  and  reptiles  may  not  occur 
at  all.  The  deduction  is  that  the  tissues  over  which  the  blood  flows 
in  ordinary  hemorrhage  contain  something  that  neutralizes  the 
antithrombin.  For  this  the  name  thromboplastic  substance  has  been 
suggested.  The  leucocytes,  and  probably  also  the  platelets,  of 
mammalian  blood  contain  enough  of  this  substance  so  that  their 
disintegration  will  neutralize  the  antithrombin  and  allow  clot- 
ting to  occur  even  though  the  blood  may  not  have  come  at  all 
into  contact  with  the  tissues.  In  ordinary  bleeding,  however, 
the  escaping  blood  must  flow  directly  over  the  raw  tissue  sur- 
faces so  that  the  antithrombin  is  promptly  neutralized  and 
clotting  can  proceed  at  once.  Apparently  all  tissues  contain 
thromboplastic  substance  except  those  that  form  the  lining 
membranes  of  the  blood-vessels.  This  rather  cumbersome  mech- 
anism appears  to  be  necessary  to  insure  prompt  clotting  when 
the  blood-vessels  are  ruptured  and  at  the  same  time  immunity 
from  the  disaster  of  clot-formation  within  the  circulation. 

The  formation  of  blood-clots  (thrombi)  within  the  vessels  is 
likely  to  be  followed  by  serious  effects,  due  to  the  plugging  of 
important  vessels  by  the  clotted  blood,  but  the  occurrence  of 
thrombi  in  the  intact  healthy  circulation  is  unknown;  their  forma- 
tion presupposes  some  injury  to  the  walls  of  the  blood-vessels,  as 
by  crushing  them  or  tying  ligatures  about  them. 

Summary  of  the  Process  of  Coagulation.  We  may  picture  the 
entire  process  of  blood-clotting  somewhat  as  follows: 

1.  As  the  result  of  rupture  of  the  blood-vessels  and  contact  of 
the  blood  with  raw  tissue  surfaces  the  antithrombin  is  neutralized 
by  thromboplastic  substance  and  the  blood-plates  disintegrate, 
yielding  prothrombin. 

2.  The  prothrombin  thus  set  free  reacts  with  the  calcium  of  the 
blood  and  forms  thrombin. 

3.  By  a  reaction  between  thrombin  and  fibrinogen  insoluble 
fibrin  is  precipitated  in  the  form  of  a  sticky  network. 

4.  The  fibrin  network  entangles  corpuscles  within  it,  forming  a 
typical  clot. 

Methods  of  Hastening  or  Retarding  Coagulation.  Since  the 
process  of  clotting  is  in  several  steps  there  are  a  corresponding 
number  of  points  at  which  its  normal  course  may  be  broken  into, 


DISEASE-RESISTING  FUNCTIONS  OF  THE  BLOOD       319 

either  with  the  effect  of  hastening  the  result  or  of  retarding  it 
or  even  preventing  it  altogether.  Anything  which  quickens  the 
disintegration  of  the  blood-plates,  as  the  application  of  a  hand- 
kerchief to  a  wound,  which  acts  by  increasing  the  foreign  surface 
in  contact  with  the  blood,  makes  the  blood  clot  more  quickly. 
The  application  of  heat  has  this  same  effect;  probably  it  acts  both 
by  increasing  the  rate  of  destruction  of  blood-plates  and  by  has- 
tening the  chemical  reactions  involved  in  the  process  as  a  whole. 
Cold,  as  would  be  expected,  has  the  converse  effect.  An  increase 
in  the  calcium  content  of  the  blood  shortens  the  coagulation  time. 
Coagulation  may  be  retarded,  as  we  have  seen,  by  cold  or  by  de- 
priving the  blood  of  its  calcium  content.  Blood  drawn  into  a 
strong  solution  of  sodium  or  magnesium  sulphate  and  well  mixed 
will  not  clot,  these  salts  appearing  to  interfere  in  some  way  with 
the  formation  of  the  thrombin;  such  "  sal  ted"  blood  will  clot  if 
thrombin  is  added  or  if  diluted  sufficiently  with  water. 

An  interesting  fact,  recently  established,  is  that  an  increase  in 
the  amount  of  adrenin  (p.  199)  in  the  blood  hastens  its  coagulation. 
This  result  cannot  be  secured  by  adding  the  hormone  to  the  blood 
as  it  is  drawn,  but  only  by  introducing  it  into  the  circulation;  show- 
ing that  the  quickening  of  the  clotting  process  is  not  a  direct  result 
of  the  chemical  action  of  adrenin  on  the  blood,  but  is  brought  about 
indirectly  through  the  influence  of  the  adrenin  on  some  of  the 
tissues  through  which  the  blood  circulates.  This  property  of 
adrenin  is  looked  upon  as  a  phase  of  its  general  function  as  an 
emergency  hormone,  for  in  time  of  stress  and  possible  bodily  injury 
prompt  coagulation  of  the  blood  would  tend  to  stop  a  hemorrhage 
quickly  and  so  conserve  the  precious  liquid. 

"  Bleeders."  There  is  a  pathological  condition,  fortunately  not 
very  common,  known  as  hemophilia,  in  which  the  blood  will  not 
clot.  Persons  suffering  from  this  disease  are  called  bleeders.  Such 
persons  are  in  danger  of  bleeding  to  death  from  slight  wounds;  a 
nosebleed,  or  the  bleeding  which  follows  the  extraction  of  a  tooth, 
becomes  in  such  persons  an  affair  of  the  utmost  gravity.  Various 
explanations  have  been  offered  to  account  for  this  disease;  at 
present  it  is  believed  to  be  due  to  a  deficiency  of  prothrombin. 

This  condition  is  usually  hereditary.  An  interesting  fact  in 
connection  with  it  is  that  whereas  the  disease  itself  appears  only 
in  males,  its  transmission  seems  to  be  confined  wholly  to  females; 


320  THE  HUMAN  BODY 

a  father  who  was  a  "bleeder"  would  have  no  children  suffering 
from  the  condition  nor  would  his  sons,  but  if  his  daughters  had 
sons  they  would  probably  be  bleeders. 

Blood  Transfusion.  The  restoration  of  blood  lost  in  severe 
hemorrhage,  or  the  replacement  of  diseased  blood  by  healthy 
blood  through  transfusion  from  the  veins  of  one  individual  to 
those  of  another  has  long  been  a  dream  of  physicians  and  physi- 
ologists. The  early  attempts  to  treat  disease  by  this  method  were 
more  often  fatal  than  not  because  the  blood  to  be  introduced  into 
the  circulation  had  to  be  defibrinated.  This  process,  as  we  have 
seen,  preserves  the  blood  in  a  liquid  condition,  but  it  leaves  in  it 
large  quantities  of  the  exciting  agent  to  coagulation,  thrombin. 
When  such  blood  was  introduced  into  the  circulation  it  usually 
induced  prompt  clotting  of  the  blood  already  there,  with  im- 
mediately fatal  results.  The  fuller  knowledge  of  the  mechanism 
of  blood-clotting  gained  of  late  years  has  made  it  clear  that  blood 
transfusion  need  not  be  followed  by  clotting  if  the  transfer  of 
blood  be  made  without  exposing  it  at  any  time  to  a  foreign  sur- 
face, such  as  favors  the  disintegration  of  the  blood-plates.  In 
accordance  with  this  idea  an  ingenious  method  has  recently  been 
developed  whereby  an  artery  of  one  individual  can  be  brought 
into  communication  with  a  vein  of  another  and  the  blood  allowed 
to  flow  naturally  across  the  living  channel  thus  formed.  Many 
lives  have  been  saved  by  this  method  during  the  few  years  since 
its  first  application,  and  it  promises  to  fulfil  in  some  degree,  at 
least,  the  early  hopes  of  the  medical  world.  It  should  be  noted 
that  successful  blood  transfusion  requires  that  the  blood  to  be 
introduced  be  taken  from  an  individual  of  the  same  species  as  the 
one  who  is  to  receive  it;  hence  human  beings  who  require  blood 
must  receive  it  from  other  human  beings,  and  not  from  animals. 
One  of  the  most  curious  facts  brought  out  in  connection  with  the 
study  of  the  disease-resisting  mechanism  of  the  Body  is  that  to 
this  mechanism  the  red  corpuscles  of  animals  of  a  different  species 
are  as  much  foreign  bodies  to  be  attacked  and  destroyed  as  are 
the  most  malignant  bacteria.  The  introduction  of  foreign  blood, 
even  if  not  attended  by  coagulation,  is  therefore  more  apt  than 
not  to  be  fatal,  through  the  destruction  of  each  kind  of  corpuscles 
by  the  liquid  portion  of  the  other  sort  of  blood.  Moreover,  the 
operation  is  much  more  likely  to  prove  successful  if  the  donor  is  a 


DISEASE-RESISTING  FUNCTIONS  OF  THE  BLOOD       321 

near  relative  of  the  recipient;  since  different  human  strains  may 
behave  toward  each  other  as  do  different  species.  Fortunately  the 
operation  of  transfusing  blood  is  neither  excessively  painful  nor 
accompanied  by  untoward  after  effects  to  the  donor,  and  persons 
can  always  be  found  who  are  willing  to  undergo  the  discomfort 
involved  for  the  sake  of  restoring  a  fellow-being  to  health. 


CHAPTER  XIX 
THE  ANATOMY  OF  THE  HEART  AND  BLOOD-VESSELS 

General  Statement.  During  life  the  blood  is  kept  flowing  with 
great  rapidity  through  all  parts  of  the  Body  (except  the  few  non- 
vascular  tissues  already  mentioned)  in  definite  paths  prescribed 
for  it  by  the  heart  and  blood-vessels.  These  paths,  which  under 
normal  circumstances  it  never  leaves,  constitute  a  continuous  set  of 
closed  tubes  (Fig.  99)  beginning  at  and  ending  again  in  the  heart, 
and  simple  only  close  to  that  organ.  Elsewhere  it  is  greatly 
branched,  the  most  numerous  and  finest  branches  being  the 
capillaries.  The  heart  is  essentially  a  bag  with  muscular  walls, 
internally  divided  into  four  chambers  (see  figure).  Those  at  one 
end  receive  blood  from  vessels  opening  into  them  and  known  as  the 
veins.  From  there  the  blood  passes  on  to  the  remaining  chambers 
which  have  very  powerful  walls  and,  forcibly  contracting,  drive  the 
blood  out  into  vessels  which  communicate  with  them  and  are 
known  as  the  arteries.  The  big  arteries  divide  into  smaller;  these 
into  smaller  again  (Fig.  100)  until  the  branches  become  too  small 
to  be  traced  by  the  unaided  eye,  and  these  smallest  branches 
end  in  the  capillaries,  through  which  the  blood  flows  and  enters 
the  commencements  of  the  veins;  and  these  convey  it  again  to 
the  heart.  At  certain  points  in  the  course  of  the  blood-paths 
valves  are  placed,  which  prevent  a  back-flow.  This  alternating 
reception  of  blood  at  one  end  by  the  heart  and  its  ejection  from 
the  other  go  on  during  life  steadily  about  seventy  times  in  a  minute, 
and  so  keep  the  liquid  constantly  in  motion. 

The  vascular  system  is  completely  closed  except  at  two  points 
in  the  neck  where  lymph-vessels  open  into  the  veins;  there  some 
lymph  is  poured  in  and  mixed  directly  with  the  blood.  Accord- 
ingly everything  which  leaves  the  blood  must  do  so  by  passing 
through  the  walls  of  the  blood-vessels,  and  everything  which  enters 
it  must  do  the  same,  except  matters  conveyed  in  by  the  lymph 
at  the  points  above  mentioned.  This  interchange  through  the 

322 


THE  ANATOMY  OF  THE  HEART  AND  BLOOD-VESSELS    323 


walls  of  the  vessels  takes  place  only  in  the  capillaries,  which  form 
a  sort  of  irrigation  system  all  through  the  Body.  The  heart, 
arteries,  and  veins  are  all  merely  arrangements  for  keeping  the 
capillaries  full  and  renewing  the  blood  within  them.  It  is  in  the 
capillaries  alone  that  the  blood 
does  its  physiological  work. 

The  Position  of  the  Heart. 
The  heart  (h,  Fig.  1)  lies  in  the 
chest  immediately  above  the 
diaphragm  and  opposite  the 
lower  two-thirds  of  the  breast- 
bone. It  is  conical  in  form  with 
its  base  or  broader  end  turned 
upwards  and  projecting  a  little 
on  the  right  of  the  sternum, 
while  its  narrow  end  or  apex, 
turned  downwards,  projects  to 
the  left  of  that  bone,  where 
it  may  be  felt  beating  between 
the  cartilages  of  the  fifth  and 
sixth  ribs.  The  position  of  the 
organ  in  the  Body  is  therefore 
oblique  with  reference  to  its 
long  axis.  It  does  not,  how- 
ever, lie  on  the  left  side  as  is  so 
commonly  supposed  but  very 
nearly  in  the  middle  line,  with 
the  upper  part  inclined  to  the 
right,  and  the  lower  (which 
may  be  more  easily  felt  beating 
—hence  the  common  belief)  to 
the  left. 

The  Membranes  of  the 
Heart.  The  heart  does  not  lie 
bare  in  the  chest  but  is  sur- 
rounded by  a  loose  bag  com- 
posed of  connective  tissue  and  FlG  99._The  hcart  and  blood-vessels 

called    the    pericardium.        This  diagrammatically    represented       L,  lung; 

.      ,   M,  intestine;  P,  liver;  dotted  lines  repre- 
like    the    heart,    IS    COniCal  sent  lymphatic  vessels. 


324 


THE  HUMAN  BODY 


but  turned  the  other  way,  its  broad  part  being  lowest  and 
attached  to  the  upper  surface  of  the  diaphragm.  Internally  it  is 
lined  by  a  smooth  serous  membrane  like  that  lining  the  abdominal 
cavity,  and  a  similar  layer  (the  visceral  layer  of  the  pericardium) 


mdu 


del 


mva 


mvp 


imv 


FIG.  100. — The  arteries  of  the  hand,  showing  the  communications  or  anasto- 
moses of  different  arteries  and  the  fine  terminal  twigs  given  off  from  the  larger 
trunks;  these  twigs  end  in  the  capillaries  which  would  only  become  visible  if  mag- 
nified. R,  the  radial  artery  on  which  the  pulse  is  usually  felt  at  the  wrist;  U,  the 
ulnar  artery. 

covers  the  outside  of  the  heart  itself,  adhering  closely  to  it.  Each 
of  the  serous  layers  is  covered  by  a  stratum  of  flat  cells,  and  in  the 
space  between  them  is  found  a  small  quantity  of  liquid  which 
moistens  the  contiguous  surfaces,  and  diminishes  the  friction  which 
would  otherwise  occur  during  the  movements  of  the  heart. 

Internally  the  heart  is  also  lined  by  a  fibrous  membrane,  covered 
with  a  single  layer  of  flattened  cells,  and  called  the  endocardium. 


THE  ANATOMY  OF  THE  HEART  AND  BLOOD-VESSELS    325 


Between  the  endocardium  and  the  visceral  layer  of  the  pericar- 
dium the  bulk  of  the  wall  of  the  heart  lies  and  is  made  up  mainly 
of  the  special  cardiac  muscular  tissue  previously  described  (p.  86) ; 
but  connective  tissues,  blood-vessels,  nerve-cells,  and  nerve-.fibers 
are  also  abundant  in  it. 

Note.  Sometimes  the  pericardium  becomes  inflamed,  this  af- 
fection being  known  as  pericarditis.  It  is  extremely  apt  to  occur 
in  acute  rheumatism,  and  great  care  should  be  taken  never,  even 
for  a  moment,  except  under  medical  advice,  to  expose  a  patient 
to  cold  during  that  disease,  since  any  chill  is  then  especially  apt 
to  set  up  pericarditis.  In  the  earlier  stages  of  pericardiac  inflam- 
mation the  rubbing  surfaces  on  the  outside  of  the  heart  and  the 
inside  of  the  pericardium  become  roughened,  and  their  friction 
produces  a  sound  which  can  be  recognized  through  the  stethoscope. 
In  later  stages  great  quantities  of  liquid  may  accumulate  in  the 
pericardium  so  as  seriously  to  impede  the  heart's  beat. 

The  Cavities  of  the  Heart.  On  opening  the  heart  (see  diagram, 
Fig.  101)  it  is  found  to  be  subdivided  by  a  longitudinal  partition 
or  septum  into  completely  separated  right  and  left  halves,  the 
partition  running  from  about 
the  middle  of  the  base  to  a  point 
a  little  on  the  right  of  the  apex. 
Each  of  the  chambers  on  the 
sides  of  the  septum  is  again  in- 
completely divided  transversely, 
into  a  thinner  basal  portion  into 
which  veins  open,  known  as 
the  auricle,  and  a  thicker  ap- 
ical portion  from  which  arteries 
arise,  called  the  ventricle.  The 
heart  thus  consists  of  a  right 
auricle  and  ventricle  and  a  left 
auricle  and  ventricle,  each  auricle  communicating  by  an  auric- 
uloventricular  orifice  with  the  ventricle  on  its  own  side,  and  there 
is  no  direct  communication  whatever  through  the  septum  between 
the  opposite  sides  of  the  heart.  To  get  from  one  side  to  the  other 
the  blood  must  leave  the  heart  and  pass  through  a  set  of  capillaries, 
as  may  readily  be  seen  by  tracing  the  course  of  the  vessels  in 
Fig.  99. 


Pd 


FIG.  101. — Diagram  representing  a  sec- 
tion through  the  heart  from  base  to  apex. 


326 


THE  HUMAN  BODY 


The  Heart  as  seen  from  its  Exterior.  When  the  heart  is  viewed 
from  the  side  turned  towards  the  sternum  (Fig.  102)  the  two 
auricles,  Aid  and  As,  are  seen  to  be  separated  by  a  deep  groove 
from  the  ventricles,  Vd  and  Vs.  A  more  shallow  furrow  runs 
between  the  ventricles  and  indicates  the  position  of  the  internal 


vd 


FIG.  102. — The  heart  and  the  great  blood-vessel  attached  to  it,  seen  from  the 
side  towards  the  sternum.  The  left  cavities  and  the  vessels  connected  with  them 
are  colored  red;  the  right  black.  Atd,  right  auricle;  Adx  and  As,  the  right  and 
left  auricular  appendages;  Vd,  right  ventricle;  Vs,  left  ventricle;  Aa,  aorta;  Ab,  in- 
nominate artery;  Cs,  left  common  carotid  artery;  Ssi,  left  subclavian  artery ;  P,  main 
trunk  of  the  pulmonary  artery,  and  Pd  and  Ps,  its  branches  to  the  right  and  left 
lungs;  cs,  superior  vena  cava;  Ade  and  Asi,  the  right  and  left  innominate  veins; 
pd  and  ps,  the  right  and  left  pulmonary  veins;  crd  and  crs,  the  right  and  left  coro- 
nary arteries. 

longitudinal  septum.  On  the  dorsal  aspect  of  the  heart  (Fig.  103) 
similar  furrows  may  be  noted,  and  on  one  or  other  of  the  two  fig- 
ures the  great  vessels  opening  into  the  cavities  of  the  heart  may  be 
seen.  The  pulmonary  artery,  P,  arises  from  the  right  ventricle, 
and  very  soon  divides  into  the  right  and  left  pulmonary  arteries, 
Pd  and  Ps,  which  break  up  into  smaller  branches  and  enter  the 


THE  ANATOMY  OF  THE  HEART  AND  BLOOD-VESSELS    327 

corresponding  lungs.  Opening  into  the  right  auricle  are  two 
great  veins  (see  also  Fig.  101),  cs  and  ci,  known  respectively  as 
the  upper  and  lower  vence  cavce,  or  " hollow"  veins;  so  called  by  the 
older  anatomists  because  they  are  frequently  found  empty  after 


FKJ.  KK3. — The  heart  viewed  from  its  dorsal  aspect.  Aid,  right  auricle;  ci,  in- 
ferior vena  cay  a;  Vc,  coronary  vein.  The  remaining  letters  of  reference  have  the 
same  signification  as  in  Fig.  102. 

death.  Into  the  back  of  the  right  auricle  opens  also  another  vein, 
Vc,  called  the  amtn-nry  vein  or  .sinus,  which  brings  back  blood 
that  has  circulated  in  the  walls  of  the  heart  itself.  Springing  from 
the  left  ventricle,  and  appearing  from  beneath  the  pulmonary 
artery  when  the  heart  is  looked  at  from  the  ventral  side,  is  a  great 


328  THE  HUMAN  BODY 

artery,  the  aorta,  Aa.  It  forms  an  arch  over  the  base  of  the  heart 
and  then  runs  down  behind  it  at  the  back  of  the  chest.  From  the 
convexity  of  the  arch  of  the  aorta  several  great  branches  are 
given  off,  Ssi,  Cs,  Ab;  but  before  that,  close  to  the  heart,  the  aorta 
gives  off  two  coronary  arteries,  branches  of  which  are  seen  at  crd 
and  crs  lying  in  the  groove  over  the  partition  between  the  ventri- 
cles, and  which  carry  to  the  substance  of  the  organ  that  blood 
which  comes  back  through  the  coronary  sinus.  Into  the  left  au- 
ricle open  two  right  and  two  left  pulmonary  veins,  ps  and  pd, 
which  are  formed  by  the  union  of  smaller  veins  proceeding  from 
the  lungs. 

In  the  diagram  Fig.  101  from  which  the  branches  of  the  great 
vessels  near  the  heart  have  been  omitted  for  the  sake  of  clearness, 
the  connection  of  the  various  vessels  with  the  chambers  of  the 
heart  can  be  better  seen.  Opening  into  the  right  auricle  are  the 
superior  and  inferior  venae  cavae  (cs  and  ci)  and  proceeding  from 
the  right  ventricle  the  pulmonary  artery,  P.  Opening  into  the 
left  auricle  are  the  right  and  left  pulmonary  veins  (pd  and  ps)  and 
springing  from  the  left  ventricle  the  aorta,  A. 

The  Interior  of  the  Heart.  The  communication  of  each  auricle 
with  its  ventricle  is  also  represented  in  the  diagram  Fig.  101,  and 
the  valves  which  are  present  at  those  points  and  at  the  origin  of 
the  pulmonary  artery  and  that  of  the  aorta.  Internally  the  auricles 
are  for  the  most  part  smooth,  but  from  each  a  hollow  pouch,  the 
auricular  appendage,  projects  over  the  base  of  the  corresponding 
ventricle  as  seen  at  Adx  and  As  in  Figs.  102  and  103.  These 
pouches  have  somewhat  the  shape  of  a  dog's  ear  and  have  given 
their  name  to  the  whole  auricle.  Their  interior  is  roughened  by 
muscular  elevations,  covered  by  endocardium,  known  as  the  fleshy 
columns  (columnce  carnce).  On  the  inside  of  the  ventricles  (Fig. 
104)  similar  fleshy  columns  are  very  prominent. 

The  Auriculoventricular  Valves.  These  are  known  as  right 
and  'left,  or  as  the  tricuspid  and  mitral  valves  respectively.  The 
mitral  valve  (Fig.  104)  consists  of  two  flaps  of  the  endocardium 
fixed  by  their  bases  to  the  margins  of  the  auriculoventricular 
aperture  and  with  their  edges  hanging  down  into  the  ventricle 
when  the  heart  is  empty.  These  unattached  edges  are  not,  how- 
ever, free,  but  have  fixed  to  them  a  number  of  stout  connective- 
tissue  cords,  the  cordce  tendinece,  which  are  fixed  below  to  muscular 


THE  ANATOMY  OF  THE  HEART  AND  BLOOD-VESSELS    329 

elevations,  the  papillary  muscles,  Mpm  and  Mpl,  on  the  interior 
of  the  ventricle.  The  cords  are  long  enough  to  let  the  valve  flaps 
rise  into  a  horizontal  position  and  so  close  the  opening  between 
auricle  and  ventricle  which  lies  between  them,  and  passes  up  be- 


Sd 


Mpm 


Fio.  104.— The  left  ventricle  and  the  commencement  of  the  aorta  laid  open. 
Mpm,  Mpl,  the  papillary  muscles.  From  their  upper  ends  are  seen  the  cordoe 
tendinece  proceeding  to  the  edges  of  the  flaps  of  the  mitral  valve.  The  opening 
into  the  auricle  lies  between  these  flaps.  At  the  beginning  of  the  aorta  are  seen  its 
three  pouch-like  semilunar  valves. 

hind  the  opened  aorta,  Sp,  represented  in  the  figure.    The  tricus- 
pid  valve  is  like  the  mitral,  but  with  three  flaps  instead  of  two. 

Semilunar  Valves.  These  are  six  in  number :  three  at  the  mouth 
of  the  aorta,  Fig.  104,  and  three,  quite  like  them,  at  the  mouth 
of  the  pulmonary  artery.  Each  is  a  strong  crescentic  pouch  fixed 


330  THE  HUMAN  BODY 

by  its  more  curved  border,  and  with  its  free  edge  turned  away 
from  the  heart.  When  the  valves  are  in  action  these  free  edges 
meet  across  the  vessel  and  prevent  blood  from  flowing  back  into 
the  ventricle.  In  the  middle  of  the  free  border  of  each  valve  is  a 
little  cartilaginous  nodule,  the  corpus  Arantii,  and  on  each  side  of 
this  the  edge  of  the  valve  is  very  thin  and  when  it  meets  its  neigh- 
bor turns  up  against  it  and  so  secures  the  closure. 

The  Arterial  System.  All  the  arteries  of  the  Body  arise  either 
directly  or  indirectly  from  the  aorta  or  pulmonary  artery,  and  the 
great  majority  of  them  from  the  former  vessel.  The  pulmonary 
artery  carries  blood  only  to  the  lungs,  to  undergo  exchanges  with 
the  air  in  them  after  it  has  circulated  through  the  Body  generally. 

After  making  its  arch  the  aorta  continues  back  through  the 
chest,  giving  off  many  branches  on  its  way.  Piercing  the  dia- 
phragm it  enters  the  abdomen  and  after  supplying  the  parts  in 
and  around  that  cavity  with  branches,  it  ends  opposite  the  last 
lumbar  vertebra  by  dividing  into  the  right  and  left  common  iliac 
arteries,  which  carry  blood  to  the  lower  limbs.  We  have  then  to 
consider  the  branches  of  the  arch  of  the  aorta,  and  those  of  the 
descending  aorta,  which  latter  is  for  convenience  described  by 
anatomists  as  consisting  of  the  thoracic  aorta,  extending  from  the 
end  of  the  arch  to  the  diaphragm,  and  the  abdominal  aorta,  extend- 
ing from  the  diaphragm  to  the  final  subdivision  of  the  vessel. 

Branches  of  the  Arch  of  the  Aorta.  From  this  arise  first  the 
coronary  arteries  (crd  and  crs,  Figs.  102  and  103)  which  spring 
close  to  the  heart,  just  above  two  of  the  pouches  of  the  semilunar 
valve,  and  carry  blood  into  the  substance  of  that  organ.  The 
remaining  branches  of  the  arch  are  three  in  number,  and  all  arise 
from  its  convexity.  The  first  is  the  innominate  artery  (Ab,  Fig. 
102),  which  is  very  short,  immediately  breaking  up  into  the  right 
subclavian  artery,  and  the  right  common  carotid.  Then  comes  the 
left  common  carotid,  Cs,  and  finally  the  left  subclavian,  Ssi. 

Each  subclavian  artery  runs  out  to  the  arm  on  its  own  side  and 
after  giving  off  a  vertebral  artery  (which  runs  up  the  neck  to  the 
head  in  the  vertebral  canal  of  the  transverse  processes  of  the 
cervical  vertebrae),  crosses  the  armpit  and  takes  there  the  name  of 
the  axillary  artery.  This  continues  down  the  arm  as  the  brachial 
artery,  which,  giving  off  branches  on  its  way,  runs  to  the  front  of 
the  arm,  and  just  below  the  elbow-joint  divides  into  the  radial 


THE  ANATOMY  OF  THE  HEART  AND  BLOOD-VESSELS    331 

and  ulnar  arteries,  the  lower  ends  of  which  are  seen  at  R  and  U  in 
Fig.  100.  These  supply  the  forearm  and  end  in  the  hand  by  unit- 
ing to  form  an  arch,  from  which  branches  are  given  off  to  the 
fingers. 

The  common  carotid  arteries  pass  out  of  the  chest  into  the  neck, 
along  which  they  ascend  on  the  sides  of  the  windpipe.  Opposite 
the  angle  of  the  lower  jaw  each  divides  into  an  internal  and  exter- 
nal carotid  artery,  right  or  left  as  the  case  may  be.  The  external 
ends  mainly  in  branches  for  the  face,  scalp,  and  salivary  glands, 
one  great  subdivision  of  it  with  a  tortuous  course,  the  temporal 
artery,  being  often  seen  in  thin  persons  beating  on  the  side  of  the 
brow.  The  internal  carotid  artery  enters  the  skull  through  an 
aperture  in  its  base  and  supplies  the  brain,  which  it  will  be  re- 
membered gets  blood  also  through  the  vertebral  arteries. 

Branches  of  the  Thoracic  Aorta.  These  are  numerous  but 
small.  Some,  the  intercostal  arteries,  run  out  between  the  ribs  and 
supply  the  chest-walls;  others,  the  bronchial  arteries,  carry  blood  to 
the  lungs  for  their  nourishment,  that  carried  to  them  by  the  pul- 
monary arteries  being  brought  there  for  another  purpose;  and  a  few 
other  small  branches  are  given  to  other  neighboring  parts. 

Branches  of  the  Abdominal  Aorta.  These  are  both  large  and 
numerous,  supplying  not  only  the  wall  of  the  posterior  part  of  the 
trunk,  but  the  important  organs  in  the  abdominal  cavity.  The 
larger  are :  the  celiac  axis  which  supplies  stomach,  spleen,  liver,  and 
pancreas;  the  'Superior  mesenteric  artery,  which  supplies  a  great  part 
of  the  intestine;  the^renal  arteries,  one  for  each  kidney;  and  finally 
the  inferior  mesenteric  artery,  which  supplies  the  rest  of  the  in- 
testine. Besides  these  the  abdominal  aorta  gives  off  very  many 
smaller  branches. 

Arteries  of  the  Lower  Limbs.  Each  common  iliac  divides  into 
an  internal  and  external  iliac  artery.  The  former  ends  mainly  in 
branches  to  parts  lying  in  the  pelvis,  but  the  latter  passes  into  the 
thighs  and  there  takes  the  name  of  the  femoral  artery.  At  first  this 
lies  on  the  ventral  aspect  of  the  limb,  but  lower  down  passes  to  the 
ba?k  of  the  femur,  and  above  the  knee-joinfc  (where  it  is  called  the 
popliteal  artery]  divides  into  the  anterior  and  posterior  tibial  ar- 
teries, which  supply  the  leg  and  foot. 

The  Capillaries.  As  the  arteries  are  followed  from  the  heart, 
their  branches  become  smaller  and  smaller,  and  finally  cannot  be 


332 


THE  HUMAN  BODY 


traced  without  the  aid  of  a  microscope.  The  smallest  arteries  are 
called  artenoles.  These  pass  into  the  capillaries,  the  walls  of 
which  are  simpler  than  those  of  the  arterioles,  and  which  form  very 
close  networks  in  nearly  all  parts  of  the  Body;  their  immense  num- 
ber compensating  for  their  small  size.  The  average  diameter  of  a 
capillary  vessel  is  .016  mm.  (i^j  inch)  so  that  only  two  or  three 


FIG.  105.— A  small  portion  of  the  capillary  network  as  seen  in  the  frog's  web 
when  magnified  about  25  diameters,  a,  a  small  artery  feeding  the  capillaries; 
v,  v,  small  veins  carrying  blood  back  from  the  latter. 

blood-corpuscles  can  pass  through  it  abreast,  and  in  many  parts 
they  are  so  close  that  a  pin's  point  could  not  be  inserted  between 
two  of  them  (Fig.  105).  It  is  while  flowing  in  these  delicate  tubes 
that  the  blood  does  its  nutritive  work,  the  arteries  being  merely 
supply-tubes  for  the  capillaries. 

The  Veins.  The  first  veins  arise  from  the  capillary  networks 
and  like  the  last  arteries  are  very  small.  They  soon  increase  in  size 
by  union,  and  so  form  larger  and  larger  trunks.  These  in 


THE  ANATOMY  OF  THE  HEART  AND  BLOOD-VESSELS    333 

many  places  lie  near  or  alongside  the  main  artery  of  the 
part,  but  there  are  many  more  large  veins  just  beneath  the  skin 
than  there  are  large  arteries.  This  is  especially  the  case  in  the 
limbs,  the  main  veins  of  which  are  superficial,  and  can  in  many 
persons  be  seen  as  faint  blue  marks  through  the  skin.  Fig.  106 
represents  the  arm  at  the  front  of  the  elbow-joint  after  the  skin 
and  subcutaneous  areolar  tissue  and  fat  have  been  removed.  The 
brachial  artery,  B,  colored  red,  is  seen  lying  tolerably  deep,  and 
accompanied  by  two  small  veins  (vence  comites)  which  communi- 
cate by  cross-branches.  The  great  median  nerve,  1,  a  branch  of  the 
brachial  plexus  which  supplies  several  muscles  of  the  forearm  and 
hand,  the  skin  over  a  great  part  of  the  palm  and  the  three  inner 
fingers,  is  seen  alongside  the  artery.  The  larger  veins  of  the  part 
are  seen  to  form  a  more  superficial  network,  joined  here  and  there, 
as  for  instance  at  *,  by  branches  from  deeper  parts.  Several 
small  nerve-branches  which  supply  the  skin  (2,  3,  4)  are  seen 
among  these  veins.  It  is  from  the  vessel,  cep,  called  the  cephalic 
vein,  just  above  the  point  where  it  crosses  the  median  nerve,  that 
surgeons  usually  bleed  a  patient. 

A  great  part  of  the  blood  of  the  lower  limb  is  brought  back  by  the 
long  saphenous  vein,  which  can  be  seen  in  thin  persons  running 
from  the  inner  side  of  the  ankle  to  the  top  of  the  thigh.  All  the 
blood  which  leaves  the  heart  by  the  aorta,  except  that  flowing 
through  the  coronary  arteries,  is  finally  collected  into  the  superior 
and  inferior  vence  cavce  (cs  and  a',  Figs.  102  and  103),  and  poured 
into  the  right  auricle.  The  jugular  veins  which  run  down  the  neck, 
carrying  back  the  blood  which  went  out  along  the  carotid  arteries, 
unite  below  with  the  arm-vein  (subclavian)  to  form  on  each  side  an 
innominate  vein  (Asi  and  Ade,  Fig.  102)  and  the  innominates  unite 
to  form  the  superior  cava.  The  coronary-artery  blood  after  flow- 
ing through  the  capillaries  of  the  heart  itself  also  returns  to  this 
auricle  by  the  coronary  veins  and  sinus. 

The  Pulmonary  Circulation  (L,  Fig.  99).  Through  this  the 
blood  gets  back  to  the  left  side  of  the  heart  and  so  into  the  aorta 
again.  The  pulmonary  artery,  dividing  into  branches  for  each 
lung,  ends  in  the  capillaries  of  those  organs.  From  these  the 
blood  is  collected  by  the  pulmonary  veins,  which  carry  it  back  to 
the  left  auricle,  whence  it  passes  to  the  left  ventricle  to  recom- 
mence its  flow  through  the  Body  generally. 


334 


THE  HUMAN  BODY 


The  Course  of  the  Blood.  From  what  has  been  said  it  is  clear 
that  the  movement  of  the  blood  is  a  circulation.  Starting  from  any 
one  chamber  of  the  heart  it  will  in  time  return  to  it;  but  to  do  this 


bas 


oep 


FIG.  106. — The  superficial  veins  in  front  of  the  elbow-joint.  B',  tendon  of  biceps 
muscle;  Bi,  brachialis  interims  muscle;  Pt,  prpnator  teres  muscle;  1,  median  nerve; 
2,  3,  4,  nerve-branches  to  the  skin;  B,  brachial  artery,  with  its  small  accompany- 
ing veins;  cep,  cephalic  vein;  bas,  basilic  vein;  m',  median  vein;  *,  junction  of  a 
deep-lying  vein  with  the  cephalic. 

it  must  pass  through  at  least  two  sets  of  capillaries;  one  of  these 
is  connected  with  the  aorta  and  the  other  with  the  pulmonary 
artery,  and  in  its  circuit  the  blood  returns  to  the  heart  twice. 
Leaving  the  left  side  it  returns  to  the  right,  and  leaving  the  right  it 
returns  to  the  left;  and  there  is  no  road  for  it  from  one  side  of  the 


THE  ANATOMY  OF  THE  HEART  AND  BLOOD-VESSELS    335 

heart  to  the  other  except  through  a  capillary  network.  Moreover, 
it  always  leaves  from  a  ventricle  through  an  artery,  and  returns  to 
an  auricle  through  a  vein. 

There  is  then  really  only  one  circulation;  but  it  is  not  uncommon 
to  speak  of  two,  the  flow  from  the  left  side  of  the  heart  to  the  right, 
through  the  Body  generally,  being  called  the  systemic  circulation, 
and  from  the  right  to  the  left,  through  the  lungs,  the  pulmonary 
circulation.  But  since  after  completing  either  of  these  alone  the 
blood  is  not  back  at  the  point  from  which  it  started,  but  is  sepa- 
rated from  it  by  the  septum  of  the  heart,  neither  is  a  "circulation" 
in  the  proper  sense  of  the  word. 

The  Portal  Circulation.  A  certain  portion  of  the  blood  which 
leaves  the  left  ventricle  of  the  heart  through  the  aorta  has  to  pass 
through  three  sets  of  capillaries  before  it  can  again  return  there. 
This  is  the  portion  which  goes  through  the  stomach,  spleen,  pan- 
creas, and  intestines  (M,  Fig.  99).  After  traversing  the  capillaries 
of  those  organs  it  is  collected  into  the  portal  vein  which  enters  the 
liver,  and  breaking  up  in  it  into  finer  and  finer  branches  like  an 
artery,  ends  in  the  capillaries  of  that  organ,  forming  the  second  set 
which  this  blood  passes  through  on  its  course  (P,  Fig.  99).  From 
these  it  is  collected  by  the  hepatic  veins,  which  pour  it  into  the 
inferior  vena  cava,  which  carries  it  to  the  right  auricle,  so  that 
it  has  still  to  pass  through  the  pulmonary  capillaries  to  get  back 
to  the  left  side  of  the  heart.  The  flow  from  the  stomach  and  in- 
testines through  the  liver  to  the  vena  cava  is  often  spoken  of  as 
the  portal  circulation. 

Diagram  of  the  Circulation.  Since  the  two  halves  of  the  heart 
are  actually  completely  separated  from  one  another  by  an  im- 
pervious partition,  although  placed  in  proximity  in  the  Body, 
we  may  conveniently  represent  the  course  of  the  blood  as  in  the 
accompanying  diagram  (Fig.  107),  in  which  the  right  and  left 
halves  of  the  heart  are  represented  at  different  points  in  the 
vascular  system.  Such  an  arrangement  makes  it  clear  that  the 
heart  is  really  two  pumps  working  side  by  side,  each  engaged  in 
forcing  the  blood  to  the  other.  Starting  from  the  left  auricle, 
la,  and  following  the  flow,  we  trace  it  through  the  left  ventricle 
and  along  the  branches  of  the  aorta  into  the  systemic  capil- 
laries, sc;  from  thence  it  passes  back  through  the  systemic 
veins,  vc.  Reaching  the  right  auricle,  ra,  it  is  sent  into  the  right 


336 


THE  HUMAN  BODY 


ventricle,  rv,  and  thence  through  the  pulmonary  artery,  pa,  to 
the  lung  capillaries,  pc,  from  which  the  pulmonary  veins,  pv, 
carry  it  to  the  left  auricle,  which  drives  it  into  the  left  ventricle,  Iv, 
and  this  again  into  the  aorta. 

Arterial  and  Venous  Blood.  The  blood  when  flowing  in  the  pul- 
monary capillaries  gives  up  carbon  dioxid  to  the  air  and  receives 
oxygen  from  it;  and  since  its  coloring  mat- 
ter (hemoglobin)  forms  a  scarlet  compound 
with  oxygen,  it  flows  to  the  left  auricle 
through  the  pulmonary  veins  of  a  bright 
red  color.  This  color  it  maintains  until  it 
reaches  the  systemic  capillaries,  but  in 
these  it  loses  much  oxygen  to  the  sur- 
rounding tissues  and  gains  much  carbon 
dioxid  from  them.  But  the  blood  coloring- 
matter  which  has  lost  its  oxygen  has  a 
dark  purple  color,  and  since  this  unoxidized 
or  " reduced"  hemoglobin  is  now  in  excess, 
the  blood  returns  to  the  heart  by  the  venae 
cavae  of  a  dark  purple-red  color.  This 
hue  it  keeps  until  it  reaches  the  lungs, 
when  the  reduced  hemoglobin  becomes 
+v,FliS'  3°7'~Diiagram,  of  again  oxidized.  The  bright  red  blood,  rich 

the  blood  vascular  system, 

showing  that  it   forms  a  in  oxygen  and  poor  in  carbon  dioxid,  is 

single    closed    circuit   with    ,  •   i   i  i        i »          i    ,  i         11 

two  pumps  in  it,  consisting  known  as  "arterial  blood"  and  the  dark 

of  the  right  and  left  halves  ,,ori  a<s  "-rr™™^  Klr^r!"-  anrl  it  miicf  Ka 
of  the  heart,  which  are  rep-  recl  as  VenOUS  t 

resented   separate  in  the  borne  in  mind  that  the  terms  have  this 

diagram,    ra  and  rv,  right 

auricle  and  ventricle ;  la  and  peculiar  technical  meaning,  and  that  the 

cle;  ao,  aorta ?  «c?  systemic  pulmonary  veins    contain   arterial   blood, 

capillaries ;^vc,  ven»  cavse;  an(j  ^e  pulmonary  arteries,  venous  blood; 

pulmonary  capillaries;  pv',  the  change  from  arterial  to  venous  taking 

pulmonary  veins.  ,          .      ,,  .  .„  ,   - 

place  in  the  systemic  capillaries,  and  from 

venous  to  arterial  in  the  pulmonary  capillaries.  The  chambers 
of  the  heart  and  the  great  vessels  containing  arterial  blood  are 
shaded  red  in  Figs.  102  and  103. 

The  Structure  of  the  Arteries.  A  large  artery  can  by  careful 
dissection  be  separated  into  three  coats :  an  internal,  a  middle,  and 
an  outer.  The  internal  coat  tears  readily  across  the  long  axis  of  the 
artery  and  consists  of  an  inner  lining  of  flattened  nucleated  cells, 


THE  ANATOMY  OF  THE  HEART  AND  BLOOD-VESSELS    337 

known  as  the  intima,  enveloped  by  a  variable  number  of  layers 
composed  of  membranes  or  networks  of  elastic  tissue.  The  middle 
coat  is  made  up  of  alternating  layers  of  elastic  fibers  and  plain 
muscular  tissue;  the  former  running  for  the  most  part  longitu- 
dinally and  the  latter  across  the  long  axis  of  the  vessel.  The  outer 
coat  is  the  toughest  and  strongest  because  it  is  mainly  made  up  of 
white  fibrous  connective  tissue;  it  contains  a  considerable  amount 
of  elastic  tissue  also,  and  gradually  shades  off  into  a  loose  areolar 
tissue  which  forms  the  sheath  of  the  artery,  or  the  tunica  adventitia, 
and  packs  it  between  surrounding  parts.  The  smaller  arteries 
have  all  the  elastic  elements  less  developed.  The  internal  coat  is 
consequently  thinner,  and  the  middle  coat  is  made  up  mainly  of 
smooth  muscular  fibers.  As  a  result  the  large  arteries  are  highly 
elastic,  the  aorta  being  physically  much  like  a  piece  of  india- 
rubber  tubing,  while  the  smaller  arteries  are  highly  contractile,  in 
the  physiological  sense  of  the  word. 

Structure  of  the  Capillaries.  In  the  smaller  arteries  the  outer 
and  middle  coats  gradually  disappear,  and  the  elastic  layers  of 
the  inner  coat  also  go.  Finally  in  the  capillaries  the  intima  alone 
is  left,  with  a  more  or  less  developed  layer  of  connective-tissue 
corpuscles  around  it,  representing  the  remnant  of  the  tunica 
adventitia.  These  vessels  are  thus  extremely  well  adapted  to 
allow  of  filtration  or  diffusion  taking  place  through  their  thin 
walls. 

Structure  of  the  Veins.  In  these  the  same  three  primary  coats 
as  in  the  arteries  are  found;  the  inner  and  middle  coats  are  less  de- 
veloped, while  the  outer  one  remains  thick,  and  is  made  up  almost 
entirely  of  white  fibrous  tissue.  Hence  the  venous  walls  are  much 
thinner  than  those  of  the  corresponding  arteries,  and  the  veins 
collapse  when  empty  while  the  stouter  arteries  remain  open. 
But  the  toughness  of  their  outer  coats  gives  the  veins  great 
strength. 

Except  the  pulmonary  artery  and  the  aorta,  which  possess  the 
semilunar  valves  at  their  cardiac  orifices,  the  arteries  possess  no 
valves.  Many  veins,  on  the  contrary,  have  such,  formed  by  semi- 
lunar  pouches  of  the  inner  coat,  attached  by  one  margin  and  hav- 
ing the  edge  turned  towards  the  heart  free.  These  valves,  some- 
times single,  oftener  in  pairs,  and  rarely  three  at  one  level,  per- 
mit blood  to  flow  only  towards  the  heart,  for  a  current  in  that 


338  THE  HUMAN  BODY 

direction  (as  in  the  upper  diagram,  Fig.  108)  presses  the  valve 
close   against   the  side  of   the   vessel   and  meets   with   no   ob- 
struction from  it.     Should  any  back-flow  be  attempted,  how- 
A  _,       ever,  the  current  closes  up  the  valve  and 

bars  its  own  passage  as  indicated  in 
the  lower  figure.  These  valves  are  most 
numerous  in  superficial  veins  and  those 
of  muscular  parts.  They  are  absent  in 
the  venae  cava?  and  the  portal  and  pul- 
ac°  monary  veins.  Usually  the  vein  is  a  little 


and  H,  the  heart  end  of  the  parts    where    the    valves    are    numerous 
gets    a    knotted    look.      On    compressing 

the  forearm  so  as  to  stop  the  flow  in  its  subcutaneous  veins  and 
cause  their  dilatation,  the  points  at  which  valves  are  placed  can 
be  recognized  by  their  swollen  appearance.  They  are  most  fre- 
quently situated  where  two  veins  communicate. 


CHAPTER  XX 

THE  ACTION  OF  THE  HEART.     THE  REGULATION  OF  THE 

HEART-BEAT 

The  Beat  of  the  Heart.  It  is  possible  with  some  little  skill  and 
care  to  open  the  chest  of  a  living  narcotized  animal,  such  as  a 
rabbit,  and  see  its  heart  at  work,  alternately  contracting  and  re- 
laxing. As  observed  under  ordinary  conditions  these  phases  fol- 
low one  another  so  rapidly  as  seemingly  to  defy  analysis.  When 
Harvey,  the  discoverer  of  the  circulation,  first  looked  upon  the 
beating  heart  of  a  mammal  he  was  so  impressed  by  the  complex- 
ity and  rapidity  of  its  action  as  to  believe  for  the  moment  that 
the  human  mind  could  never  fathom  it. 

By  proper  treatment  the  beat  of  the  heart  can  be  much  slowed. 
When  this  has  been  done  it  is  observed  that  each  beat  commences 
at  the  mouths  of  the  great  veins;  from  there  runs  over  the  rest' 
of  the  auricles,  and  then  over  the  ventricles;  the  auricles  dilating 
the  moment  the  ventricles  commence  to  contract.  Having  fin- 
ished their  contraction  the  ventricles  also  dilate,  and  so  for  some 
time  neither  they  nor  the  auricles  are  contracting,  but  the  whole 
heart  is  at  rest.  The  contraction  of  any  part  of  the  heart  is 
known  as  its  systole  and  the  relaxation  as  its  diastole. 

The  average  heart-rate  in  man  is  72  beats  per  minute,  giving 
for  each  beat  0.8  second.  The  two  sides  of  the  heart  work  syn- 
chronously, the  auricles  together  and  the  ventricles  together.  In 
describing  the  "  cardiac  cycle,"  therefore,  the  auricles  are  treated 
as  one  organ  and  the  ventricles  as  one.  The  auricular  systole 
occupies  about  0.1  second,  its  diastole  lasts  0.7  second.  The 
ventricular  systole  begins  at  the  end  of  the  auricular  contraction; 
it  occupies  about  0.3  second;  the  diastole  of  the  ventricle  lasts 
about  0.5  second.  During  fully  half  of  each  cardiac  cycle,  then, 
there  is  no  muscular  activity  going  on  in  any  part  of  the  heart. 
During  diastole  the  heart  if  taken  between  the  finger  and  thumb 
feels  soft  and  flabby,  but  during  systole  it  (especially  its  ventric- 
ular portion)  becomes  hard  and  rigid. 

339 


340  THE  HUMAN  BODY 

Change  of  Form  of  the  Heart.  During  its  systole  the  heart 
becomes  shorter  and  rounder,  mainly  from  a  change  in  the  shape 
of  the  ventricles,  which  from  having  an  elliptical  cross-section 
take  on  a  circular  one.  At  the  same  time  the  length  of  the  ven- 
tricles is  lessened,  the  apex  of  the  heart  approaching  the  base  and 
becoming  blunter  and  rounder. 

The  Cardiac  Impulse.  The  human  heart  lies  with  its  apex 
touching  the  chest-wall  between  the  fifth  and  sixth  ribs  on  the 
left  side  of  the  breast-bone.  At  every  beat  a  sort  of  tap,  known 
as  the  "cardiac  impulse"  or  "apex  beat,"  may  be  felt  by  the 
finger  at  that  point.  There  is,  however,  no  actual  "tapping, 
since  the  heart's  apex  never  leaves  the  chest-wall.  During  the 
diastole  the  soft  ventricles  yield  to  the  chest-wall  where  they 
touch  it,  but  during  the  systole  they  become  hard  and  tense  and 
push  it  out  a  little  between  the  ribs,  and  so  cause  the  apex  beat. 
Since  the  heart  becomes  shorter  during  the  ventricular  systole,  it 
might  be  supposed  that  at  that  time  the  apex  would  move  up  a 
little  in  the  chest.  This,  however,  is  not  the  case,  the  ascent  of 
the  apex  towards  the  base  of  the  ventricles  being  compensated 
for  by  a  movement  of  the  whole  heart  in  the  opposite  direction. 
If  water  be  pumped  into  an  elastic  tube,  already  moderately  full, 
the  tube  will  be  distended  not  only  transversely  but  longitudi- 
nally. This  is  what  happens  in  the  aorta:  when  the  left  ventricle 
contracts  and  pumps  blood  forcibly  into  it,  the  elastic  artery  is 
elongated  as  well  as  widened,  and  the  lengthening  of  that  limb  of 
its  arch  attached  to  the  heart  pushes  the  latter  down  towards  the 
diaphragm,  and  compensates  for  the  upward  movement  of  the 
apex  due  to  the  shortening  of  the  ventricles.  Hence  if  the  ex- 
posed living  heart  be  watched  it  appears  as  if  during  the  systole 
the  base  of  the  heart  moved  towards  the  tip,  rather  than  the  re- 
verse. 

Events  occurring  within  the  Heart  during  a  Cardiac  Cycle. 
Let  us  commence  at  the  end  of  the  ventricular  systole.  At  this 
moment  the  semilunar  valves  at  the  orifices  of  the  aorta  and  the 
pulmonary  artery  are  closed,  so  that  no  blood  can  flow  bajck  from 
those  vessels.  The  whole  heart,  however,  is  soft  and  distensible 
and  yields  readily  to  blood  flowing  into  it  from  the  pulmonary 
veins  and  the  venee  cavse;  this  passes  on  through  the  open  mitral 
and  tricuspid  valves  and  fills  up  the  dilating  ventricles,  as  well  as 


THE  ACTION  OF  THE  HEART  341 

the  auricles.  As  the  ventricles  fill,  back  currents  are  set  up  along 
their  walls  and  these  carry  up  the  flaps  of  the  valves  so  that  by 
the  end  of  the  pause  they  are  nearly  closed.  At  this  moment  the 
auricles  contract,  and  since  this  contraction  commences  at  and 
narrows  the  mouths  of  the  veins  opening  into  them,  and  at  the 
same  time  the  blood  in  those  vessels  opposes  some  resistance  to 
a  back-flow  into  them,  while  the  still  flabby  and  dilating  ventricles 
oppose  much  less  resistance,  the  general  result  is  that  the  con- 
tracting auricles  send  blood  into  the  ventricles,  and  not  back  into 
the  veins.  At  the  same  time  the  increased  direct  current  into  the 
ventricles  produces  a  greater  back  current  on  the  sides,  which, 
when  the  auricles  cease  their  contraction  and  the  filled  ventricles 
become  tense  and  press  on  the  blood  inside  them,  completely  closes 
the  auriculo ventricular  valves.  That  this  increased  filling  of  the 
ventricles,  due  to  auricular  contractions  will  close  the  valves  may 
be  seen  easily  in  a  sheep's  heart.  If  the  auricles  be  carefully  cut 
away  from  this  so  as  to  expose  the  mitral  and  tricuspid  valves, 
and  water  be  then  poured  from  a  little  height  into  the  ventricles, 
it  will  be  seen  that  as  these  cavities  are  filled  the  valve-flaps  are 
floated  up  and  close  the  orifices. 

The  auricular  contraction  now  ceases  and  the  ventricular  com- 
mences. The  blood  in  each  ventricle  is  imprisoned  between  the 
auriculoventricular  valves  behind  and  the  semilunar  valves  in 
front.  The  former  cannot  yield  on  account  of  the  cordse  tendineae 
fixed  to  their  edges :  the  semilunar  valves,  on  the  other  hand,  can 
open  outwards  from  the  ventricle  and  let  the  blood  pass  on,  but 
they  are  kept  tightly  shut  by  the  pressure  of  the  blood  on  their 
other  sides,  just  as  the  lock-gates  of  a  canal  are  by  the  pressure  of 
the  water  on  them.  In  order  to  open  the  canal-gates  water  is  let 
in  or  out  of  the  lock  until  it  stands  at  the  same  level  on  each  side 
of  them;  but  of  course  they  might  be  forced  open  without  this 
by  applying  sufficient  power  to  overcome  the  higher  water  pres- 
sure on  one  side.  It  is  in  this  latter  way  that  the  semilunar  valves 
are  opened.  The  contracting  ventricle  tightens  its  grip  on  the 
blood  inside  it  and  becomes  rigid  to  the  touch.  As  it  squeezes 
harder  and  harder,  at  last  the  pressure  on  the  blood  within  it  be- 
comes greater  than  the  pressure  exerted  on  the  other  side  of  the 
valves  by  the  blood  in  the  arteries,  the  flaps  are  forced  open  and 
the  blood  begins  to  pass  out:  the  ventricle  continues  its  contrac- 


342  THE  HUMAN  BODY 

tion  until  it  has  obliterated  its  cavity  and  completely  emptied 
itself;  this  total  emptying  appears,  at  least,  to  occur  in  the  nor- 
mally beating  heart,  but  in  some  pathological  conditions  and 
under  the  influence  of  certain  drugs  the  emptying  of  the  ventri- 
cles is  incomplete.  After  the  systole  the  ventricle  commences  to 
relax  and  blood  immediately  to  flow  back  towards  it  from  the 
highly  stretched  arteries.  This  return  current,  however,  catches 
the  pockets  of  the  semilunar  valves,  drives  them  back  and  closes 
the  valve  so  as  to  form  an  impassable  barrier;  and  so  the  blood 
which  has  been  forced  out  of  either  ventricle  cannot  flow  directly 
back  into  it. 

Use  of  the  Papillary  Muscles.  In  order  that  the  contracting 
ventricles  may  not  force  blood  back  into  the  auricles  it  is  essential 
that  the  flaps  of  the  mitral  and  tricuspid  valves  be  maintained 
in  position  across  the  openings  which  they  close,  and  be  not 
pushed  back  into  the  auricles.  At  the  commencement  of  the 
ventricular  systole  this  is  provided  for  by  the  cordse  tendineae, 
which  are  of  such  a  length  as  to  keep  the  edges  of  the  flaps  in  ap- 
position, a  position  which  is  further  secured  by  the  fact  that  each 
set  of  cordse  tendinese  (Fig.  104)  radiating  from  a  point  in 
the  ventricle,  is  not  attached  around  the  edges  of  one  flap  but 
on  the  contiguous  edges  of  two  flaps,  and  so  tends  to  pull  them 
together.  But  as  the  contracting  ventricles  shorten,  the  cordas 
tendinese,  if  directly  fixed  to  their  interior,  would  be  slackened 
and  the  valve-flaps  pushed  up  into  the  auricle.  The  little 
papillary  muscles  prevent  this.  Shortening  as  the  ventricular 
systole  proceeds,  they  keep  the  cordse  taut  and  the  valves 
closed. 

Sounds  of  the  Heart.  If  the  ear  be  placed  on  the  chest  over 
the  region  of  the  heart  during  life,  two  distinguishable  sounds 
will  be  heard  during  each  cardiac  cycle.  They  are  known  re- 
spectively as  the  first  and  second  sounds  of  the  heart.  The  first  is 
of  lower  pitch  and  lasts  longer  than  the  second  and  sharper  sound : 
vocally  their  character  may  be  tolerably  imitated  by  the  words 
lubb,  dup.  The  cause  of  the  second  sound  is  the  closure,  or,  as  one 
might  say,  the  "clicking  up,"  of  the  semilunar  valves,  since  it 
occurs  at  the  moment  of  their  closure  and  ceases  if  they  be  hooked 
back  in  a  living  animal.  The  origin  of  the  first  sound  is  still  un- 
certain: it  takes  place  during  the  ventricular  systole  and  is  prob- 


THE  ACTION  OF  THE  HEART  343 

ably  due  to  vibrations  of  the  tense  ventricular  wall  at  that  time. 
It  is  not  due,  at  least  not  entirely,  to  the  auriculoventricular 
valves,  since  it  may  still  be  heard  in  a  beating  heart  empty  of 
blood,  and  in  which  there  could  be  no  closure  or  tension  of  those 
valves.  In  various  forms  of  heart  disease  these  sounds  are  mod- 
ified or  cloaked  by  additional  "murmurs"  which  arise  when  the 
cardiac  orifices  are  roughened  or  narrowed  or  dilated,  or  the 
valves  inefficient.  By  paying  attention  to  the  character  of  the 
new  sound  then  heard,  the  exact  period  in  the  cardiac  cycle  at 
which  it  occurs,  and  the  region  of  the  chest-wall  at  which  it  is 
heard  most  distinctly,  the  physician  can  often  get  important  in- 
formation as  to  its  cause. 

Action  of  the  Heart  Valves.  The  valves  of  the  heart  are  en- 
tirely without  rigidity.  They  consist  of  tough,  but  perfectly 
flaccid  membranes,  so  that  they  respond  perfectly  to  the  forces 
which  act  upon  them.  This  structure  makes  it  inevitable  that  the 
valves  will  open  whenever  the  pressure  behind  them  is  greater 
than  that  in  front,  and  will  close  whenever  the  pressure  in  front 
is  greater  than  that  behind.  During  the  whole  diastole  of  the  heart 
the  pressure  behind  the  auriculoventricular  valves  is  greater  than 
that  in  front  of  them;  for  in  front  is  only  the  gradually  filling 
cavity  of  the  ventricle,  while  behind  is  the  onward  flow  of  blood 
from  the  great  veins.  During  this  time,  therefore,  these  valves 
stand  open.  The  systole  of  the  auricle,  by  increasing  the  pressure 
behind,  keeps  them  open  until  its  end.  During  this  same  time  the 
aortic  valves  are  shut,  because  in  front  of  them  are  arteries  whose 
walls  are  stretched  with  their  load  of  blood  and  which,  therefore, 
exert  high  pressure  upon  the  valves,  while  behind  are  only  the 
ventricular  cavities,  filling  with  blood.  At  the  instant  the  ven- 
tricles begin  to  contract  the  situation  with  regard  to  the  auriculo- 
ventricular valves  changes.  The  relaxing  auricles  make  room  for 
the  blood  coming  in  from  the  great  veins  and  so  release  the  pressure 
behind  these  valves;  the  contracting  ventricle  exerts  pressure  in 
front  of  them;  they  therefore  close  instantly.  Since  the  semilunar 
valves  remain  closed  until  the  rising  pressure  in  the  ventricle  be- 
comes greater  than  that  in  the  aorta  there  is  an  instant  at  the  be- 
ginning of  ventricular  systole  when  all  the  valves  are  shut.  Again, 
at  the  beginning  of  ventricular  diastole  there  is  an  instant  when  the 
ventricular  pressure  has  fallen  below  that  in  the  aorta  but  is  still 


344 


THE  HUMAN  BODY 


above  the  pressure  in  the  auricles;  during  this  time,  again,  the 
valves  of  the  heart  are  all  shut. 

Effects  of  Valvular  Insufficiency.  The  commonest  heart 
troubles  are  due  to  failure  of  one  or  other  of  the  valves  to  close 
perfectly.  The  mechanical  effect  of  such  inefficiency  is,  of  course,  a 
back  rush  of  blood  through  the  leaky  valve.  The  effects  on  the 
Body  at  large  will  depend  on  which  valve  is  inefficient.  Leakage 
of  the  semilunars  means  a  return  into  the  ventricle  of  part  of  the 
blood  just  pumped  out.  The  circulation  is  to  that  extent  less 
effectively  maintained.  The  heart  usually  compensates  for  this 
defect  by  muscular  growth  (hypertrophy)  by  which  it  becomes 
enough  more  powerful  than  normally  to  make  up  for  the  lessened 
efficiency.  Leakage  of  an  auriculoventricular  valve  is  much  more 
serious  because  it  permits  a  jet  of  blood  to  be  driven  backward 
into  the  veins  at  each  heart-beat  under  the  driving  force  of  the 
powerful  ventricular  contraction.  The  small  veins  and  capillaries 
are  not  adapted  to  receive  such  a  hammering  and  are  injured 
thereby.  If  the  leaky  valve  is  the  mitral  the  lung  capillaries  are 
the  ones  affected.  If  the  tricuspid  is  inefficient  the  backward  surge 
makes  itself  felt  in  distant  organs.  Kidney  impairment  is  a  com- 
mon sequel  to  this  type  of  valvular  disease.  Inflammatory  rheuma- 
tism frequently  brings  on  valve  trouble.  In  fact  the  danger  of  this 
outcome  is  so  great  that  every  pains  should  be  taken  to  avoid  it,  by 
giving  the  patient  the  best  of  care  and  treatment. 

Diagram  of  the  Events  of  a  Cardiac  Cycle.  In  the  following 
table  the  phenomena  of  the  heart's  beat  are  represented  with  ref- 
erence to  the  changes  of  form  which  are  seen  on  an  exposed  working 
heart.  Events  in  the  same  vertical  column  occur  simultaneously; 
on  the  same  horizontal  line,  from  left  to  right,  successively. 


Auricular 
Systole 

Commence- 
ment of 
Ventricular 
Systole 

Ventricular 
Systole 

Cessation 
of  Ven- 
tricular 
Systole 

Pause 

Auricles     .        

Contracting 
and 
emptying. 
Dilating  and 
filling. 

Dilating  and 
filling. 

Contracting. 

Apex  beat. 
Closed. 
Closed. 
First  sound. 

Dilating  and 
filling. 

Contracting 
and 
emptying. 

Closed.' 
Open. 

Dilating  and 
filling. 

Dilating. 

Dilating  and 
filling. 

Dilating  and 
filling. 

Ventricles                  

Impulse 

Auriculoventricular  valves  . 
Semilunar  valves      

Open. 
Closed. 

Closed. 
Closed. 
Second 
sound. 

Open. 

Closed. 

Sounds                        

THE  ACTION  OF  THE  HEART  345 

Function  of  the  Auricles.  The  ventricles  have  to  do  the  work 
of  pumping  the  blood  through  the  blood-vessels.  Accordingly  their 
walls  are  far  thicker  and  more  muscular  than  those  of  the  auricles; 
and  the  left  ventricle,  which  has  to  force  the  blood  over  the  Body 
generally,  is  stouter  than  the  right,  which  has  only  to  send  blood 
around  the  comparatively  short  pulmonary  circuit.  The  circu- 
lation of  the  blood  is  in  fact  maintained  by  the  ventricles,  and  we 
have  to  inquire  what  is  the  use  of  the  auricles.  Not  unfrequently 
the  heart's  action  is  described  as  if  the  auricles  first  filled  with 
blood  and  then  contracted  and  filled  the  ventricles;  and  then  the 
latter  contracted  and  drove  the  blood  into  the  arteries.  From  the 
account  given  above,  however,  it  will  be  seen  that  the  events  are 
not  accurately  so  represented,  but  that  during  all  the  pause  blood 
flows  on  through  the  auricles  into  the  ventricles,  which  latter  are 
already  nearly  full  when  the  auricles  contract;  this  contraction 
merely  completing  their  filling.  The  real  use  of  the  auricles 
is  to  afford  a  reservoir  into  which  the  veins  may  empty  while 
the  comparatively  long-lasting  ventricular  contraction  is  taking 
place. 

If  the  heart  consisted  of  the  ventricles  only,  with  valves  at  the 
points  of  entry  and  exit  of  the  blood,  the  circulation  could  be 
maintained.  During  disatole  the  ventricle  would  fill  from  the 
veins,  and  during  systole  empty  into  the  arteries.  But  in  order 
to  accomplish  this,  during  the  systole  the  valves  at  the  point  of 
entry  must  be  closed,  or  the  ventricle  would  empty  itself  into  the 
veins  as  well  as  into  the  arteries;  and  this  closure  would  necessitate 
a  great  loss  of  time  which  might  be  utilized  for  feeding  the  pump. 
This  is  avoided  by  the  auricles,  which  are  really  reservoirs  at  the 
end  of  the  venous  system,  collecting  blood  when  the  ventricular 
pump  is  at  work.  When  the  ventricles  relax,  the  blood  entering 
the  auricles  flows  on  into  them;  but  previously,  during  the  part  of 
the  cardiac  cycle  occupied  by  the  ventricular  systole,  the  auricles 
have  accumulated  blood,  and  when  they  at  last  contract  they  send 
on  into  the  ventricles  this  accumulation.  Even  were  the  flow  from 
the  veins  stopped  during  the  auricular  contraction  this  would  be  of 
comparatively  little  consequence,  since  that  event  occupies  so 
brief  a  time.  But,  although  no  doubt  somewhat  lessened,  the 
emptying  of  the  veins  into  the  heart  does  not  seem  to  be,  in  health, 
stopped  while  the  auricle  is  contracting.  The  heart  in  fact  con- 


346  THE  HUMAN  BODY 

sists  of  a  couple  of  "feed-pumps" — the  auricles — and  a  couple  of 
"force-pumps" — the  ventricles;  and  so  wonderfully  perfect  is  the 
mechanism  that  the  supply  to  the  feed-pumps  is  never  stopped. 
The  auricles  are  never  empty,  being  supplied  all  the  time  of  their 
contraction,  which  is  never  so  great  as  to  obliterate  their  cavities; 
while  the  ventricles  contain  little  or  no  blood  at  the  end  of  their 
systole. 

The  Work  Done  by  the  Heart.  According  to  the  physical 
definition  work  is  measured  by  the  weight  lifted  times  the  height 
to  which  it  is  raised.  In  estimating  the  work  of  the  heart  we  sub- 
stitute for  the  height  the  resistance  against  which  the  heart  works. 
This  resistance  is  equivalent  in  the  case  of  the  left  ventricle  to  that 
of  a  column  of  blood  about  2  meters  high,  and  for  the  right  ven- 
tricle about  0.8  meter.  The  mass  of  blood  ejected  from  each  ven- 
tricle during  systole  probably  averages  about  100  gms.  The 
work  done  by  the  left  ventricle  per  beat  equals,  then,  about 
100x2  =  200  grammeters,  and  that  by  the  right  ventricle  equals 
about  100x0.8  =  80  grammeters.  Since  the  heart  in  addition  to 
moving  the  weight  of  blood  imparts  to  it  a  considerable  velocity,  it 
is  necessary  to  add  to  the  amounts  of  work  calculated  above  an 
additional  amount  to  represent  that  required  to  impart  to  the 
blood  its  velocity.  This  latter  amount  approximates  3  gram- 
meters.  The  total  work  output  of  the  heart  per  beat  is,  therefore, 
roughly  283  grammeters,  equivalent  in  the  English  scale  to  about 
2  foot-pounds.  When  the  heart  is  beating  at  the  rate  of  70  per 
minute  it  does  140  foot-pounds  per  minute,  making  it  a  240th 
horse-power  engine.  If  it  maintained  this  rate  throughout  the 
entire  twenty-four  hours  of  the  day  it  would  do  in  that  time 
200,000  foot-pounds  of  work,  an  amount  equivalent  to  that  done 
by  the  leg  muscles  of  a  man  weighing  150  pounds  in  climbing  a 
mountain  1,300  feet  high. 

That  the  heart  is  able  to  do  this  amount  of  work  daily  without 
fatigue,  and  keep  it  up  day  in  and  day  out  for  seventy  or  more 
years,  is  due  to  its  ability  to  recover  quickly  from  the  effects  of  its 
activity,  coupled  with  the  fact  that  in  a  whole  day  its  resting  time 
considerably  outweighs  the  time  during  which  it  is  active.  The 
heart-beat  is  ordinarily  much  slower  during  sleep  than  during 
bodily  activity;  as  the  result  the  heart  enjoys  an  "eight  hour  day" 
if  only  its  actual  contraction  time  be  counted. 


THE  ACTION  OF  THE  HEART  347 

Relations  of  Nerve  and  Muscle  Elements  within  the  Heart. 

The  heart-muscle  consists,  as  previously  stated,  of  muscle-cells  of 
small  size,  intimately  communicating  with  one  another  through 
their  branches,  and  showing  signs  of  cross-striation.  At  the  junc- 
tion of  the  great  veins  with  the  heart,  a  region,  as  we  shall  see,  of 
great  importance  in  the  heart's  activity,  these  muscular  elements 
form  thin  sheets;  in  the  auricles  the  heart-muscle  is  somewhat 
heavier  and  thicker;  but  it  attains  its  greatest  development  in  the 
ventricles,  where  the  muscular  walls  are  exceedingly  heavy,  and 
very  stout.  In  mammals  the  only  pulsating  heart  structures  are 
the  auricles  and  ventricles;  in  lower  vertebrates,  such  as  the  frog, 
the  great  veins  near  the  heart  are  differentiated  into  a  pulsating 
structure,  the  sinus  venosus,  and  the  outlet  from  the  ventricle,  the 
bulbus  arteriosus,  also  pulsates.  Although  in  mammals  these 
structures  no  longer  pulsate,  the  region  of  the  great  veins  which 
corresponds  to  the  sinus  venosus  still  seems  to  preserve  to  some 
degree  the  physiological  properties  it  has  in  lower  animals,  and 
observations  made  upon  frogs'  hearts  are  interpreted  for  mammals' 
hearts  upon  that  basis. 

Embedded  within  the  tissue  of  the  heart  are  numerous  nerve- 
cells.  These  are  most  numerous  in  the  region  of  the  sinus  venosus 
and  auricles;  the  base  of  the  ventricles  contains  some  of  them,  but 
the  apex  of  the  ventricles  is  said  to  be  wholly  free  from  them. 
Nerve-fibers,  communicating  with  these  cells,  penetrate  all  parts 
of  the  cardiac  musculature.  It  has  not  been  possible  by  histologic 
means  to  show  that  these  fibers  are  dendrites  and  axons  such  as 
occur  in  the  general  nervous  system,  and  many  histologists  and 
physiologists  believe  that  they  form  a  continuous  network  or 
plexus  involving  all  parts  of  the  heart  and  so  constituted  that  a 
stimulus  applied  at  any  point  spreads  over  the  whole  organ.  Ac- 
cording to  this  view  the  nervous  mechanism  of  the  heart  is  not  a 
"synaptic  system"  and  so  does  not  show  the  irreversibility  of  con- 
duction which  is  a  cardinal  feature  of  the  general  nervous  system. 
Some  support  for  this  idea  is  had  in  the  fact  that  certain  other 
viscera,  notably  the  stomach  and  intestines,  have  within  their 
walls  nerve  plexuses  showing  similar  physiological  properties. 

Physiological  Peculiarities  of  the  Heart.  The  most  striking  of 
these  is  its  automatic  rhythmicity.  The  heart  may  be  removed  com- 
pletely from  the  Body  without  its  regular  beating  being  at  all  in- 


348  THE  HUMAN  BODY 

terfered  with.  In  cold-blooded  animals  such  as  frogs  or  turtles 
this  activity  outside  the  Body  may  continue  for  hours.  While  we 
refer  to  this  activity  as  automatic  we  do  not  mean  by  the  word 
anything  more  than  the  fact  just  stated,  that  the  heart  continues 
to  beat  independently  of  the  rest  of  the  Body.  The  rhythmic  na- 
ture of  the  heart's  activity  is  as  characteristic  as  its  automaticity. 
The  regular  succession  of  contractions  and  relaxations  is  its  normal 
response  to  continuous  or  rapidly  recurring  stimulation.  In  this 
respect  it  differs  strikingly  from  skeletal  muscle,  which  remains 
strongly  contracted  throughout  the  period  of  such  stimulation  un- 
less fatigue  sets  in  to  release  it. 

Another  peculiarity  of  heart-muscle,  and  one  that  probably  ex- 
plains in  part  its  rhythmic  property,  is  that  its  contractions  are 
always  maximal.  By  this  is  meant  that  whenever  heart-muscle 
contracts  it  always  does  so  to  the  full  extent  of  its  ability  at  the 
time.  In  this  respect  we  may  compare  its  energy  liberation  with 
the  discharge  of  a  gun.  When  the  trigger  is  pulled  all  the  powder 
in  the  cartridge  is  exploded;  similarly  whenever  the  heart  contracts 
it  uses  up  all  the  energy  available  at  the  time.  Because  of  this 
it  is  necessary  that  the  contraction  be  followed  by  a  relaxation 
during  which  an  accumulation  of  energy  may  prepare  for  the  next 
contraction. 

The  evidence  that  all  the  available  energy  of  the  heart-muscle  is 
used  up  at  each  systole  is  furnished  by  the  existence  of  the  refrac- 
tory period.  During  this  period,  which  coincides  with  the  systole, 
external  stimulation  of  the  heart-muscle  is  altogether  ineffective, 
although  during  diastole  the  heart  responds  to  adequate  stimu- 
lation by  contraction.  It  is  observed,  also,  that  the  irritability  of 
the  heart  increases  steadily  from  the  end  of  the  refractory  period 
to  the  beginning  of  the  next  systole.  We  may  assume,  then,  that 
during  diastole  there  is  a  gradual  replacement  of  the  energy  supply 
used  up  during  the  preceding  systole,  and  that  the  more  energy  has 
accumulated  the  more  irritable  is  the  tissue. 

The  Passage  of  the  Beat  over  the  Heart.  In  the  first  paragraph 
of  the  chapter  it  was  stated  that  the  beat  of  the  heart  takes  a 
certain  course,  beginning  at  the  mouths  of  the  great  veins,  spread- 
ing thence  over  the  auricles,  and  passing  from  them  to  the  ven- 
tricles. In  all  vertebrates  there  is  a  distinct  pause  between  the 
contraction  of  the  auricles  and  of  the  ventricles.  In  animals,  such 


THE  ACTION  OF  THE  HEART  349 

as  the  frog  and  turtle  that  have  a  pulsating  sinus,  there  is  likewise 
a  pause  between  the  contraction  of  the  sinus  and  of  the  auricles. 

If  in  a  beating  heart  a  cut  be  made  between  the  sinus  and  the 
auricles  so  that  they  are  completely  separated,  the  sinus  con- 
tinues to  beat  exactly  as  before;  the  other  chambers  of  the  heart 
may  not  beat  for  a  moment,  but  after  a  short  interval  usually 
resume  activity.  The  rate  of  beat  of  these  chambers  under  such 
circumstances  is  slower  than  that  of  the  sinus.  Similarly  the  ven- 
tricles may  be  separated  from  the  auricles  without  affecting  the 
auricular  beat,  but  with  the  result  that  the  ventricles  either  fail  to 
beat  at  all,  or  beat  at  a  much  slower  rate  than  the  auricles.  Such 
experiments  as  these  show  that  the  rhythmic  power  increases  the 
nearer  we  go  toward  the  venous  end  of  the  heart,  and  also  that  in 
the  normal  heart  the  most  rhythmic  portion  imposes  its  rate  on 
the  rest  of  the  organ.  In  order  for  the  heart-rate  to  be  determined 
as  a  whole  by  the  beat  of  the  venous  end  it  is  evident  that  there 
must  be  a  conduction  of  the  impulse  to  activity  from  one  chamber 
to  the  next  throughout  the  heart.  This  conduction  moves  over 
the  heart  in  the  form  of  a  wave. 

There  are  in  the  frog's  heart  two  places  and  in  that  of  the  mam- 
mal one  place  where  there  is  a  delay  in  the  passage  of  the  con- 
traction wave.  These  are,  as  already  noted,  at  the  junction  of  the 
sinus  with  the  auricles  and  of  the  auricles  with  the  ventricles. 
Anatomical  study  shows  that  at  these  junctions  most  of  the 
cardiac  tissue  proper  is  replaced  by  connective  tissue,  so  that 
physiological  communication  between  one  chamber  and  another 
is  restricted  to  small  bundles  of  conducting  heart  tissue.  The  de- 
lay at  the  junctions  is  usually  explained  as  resulting  from  the 
small  size  of  these  conducting  paths,  which  offer  on  that  account 
considerable  resistance  to  the  passage  of  the  contraction  wave. 

Neurogenic  and  Myogenic  Theories  of  the  Heart  Beat.  There 
are  two  questions  of  fundamental  importance  to  an  understanding 
of  the  mechanism  of  the  heart's  action.  These  are:  (1)  Does  the 
rhythmic  property  of  the  heart  reside  in  its  muscular  elements 
or  in  its  nervous  elements?  and  (2)  Is  the  contraction  wave  con- 
ducted over  the  heart  by  muscle  or  by  nerve-tissue?  By  the 
early  students  of  the  heart  both  these  properties  were  attributed 
to  its  nervous  elements  as  being  more  like  nerve  activities  in  gen- 
eral than  like  those  of  muscle;  and  also  because  the  venous  end  of 


350  THE  HUMAN  BODY 

the  heart,  where  the  beat  originates,  contains  more  nervous  matter 
than  do  the  other  chambers.  More  recently  the  view  that  both 
rhythmicity  and  conductivity  are  cardinal  functions  of  the  heart's 
musculature  began  to  receive  considerable  attention,  chiefly 
through  such  observations  as  that  the  apex  of  the  ventricle,  which 
is  devoid  of  nerve-cells,  may  be  made  to  show  true  rhythmicity, 
and  that  a  series  of  zigzag  cuts,  sufficient  to  sever  all  direct  nerve 
paths  although  leaving  ample  muscular  connections,  can  be  made 
in  the  ventricle  without  preventing  the  passage  of  the  contraction 
wave  over  it.  With  recognition  of  the  probability  that  the  nervous 
elements  of  the  heart  form,  not  a  synaptic  system  with  irreversible 
conduction,  but  an  intercommunicating  plexus  which  may  con- 
duct in  all  directions,  most  of  the  evidence  in  favor  of  the  myogenic 
theory  seems  less  conclusive  than  it  did  at  first,  so  that  the  prob- 
lems of  which  is  the  rhythmic  and  conducting  tissue,  or  whether 
both  properties  are  possessed  by  both  tissues,  are  still  far  from 
settled. 

The  Nature  of  Automatic  Rhythmicity.  It  should  be  clearly 
understood  that  the  question  whether  rhythmicity  is  a  property  of 
cardiac  muscle  or  of  cardiac  nerve-tissue  is  quite  distinct  from  the 
question  of  the  underlying  nature  of  rhythmicity  itself.  Much 
study  has  been  given  to  this  latter  problem  and  here  again  two 
opposing  views  are  held.  One  of  these  is  that  the  heart  is  sub- 
ject to  the  influence  of  a  constant  stimulus,  its  property  of  "maxi- 
mal" contractions  with  their  accompanying  refractory  periods 
sufficing  to  bring  about  rhythmic  responses  to  such  constant 
stimulation.  The  other  view  is  that  the  heart  is  a  truly  automatic 
organ,  the  metabolic  processes  going  on  within  the  heart  tissue 
being  of  such  a  nature  as  to  produce  rhythmic  activity  quite 
independently  of  "stimulation"  as  we  ordinarily  understand  it. 

Those  who  believe  the  heart  to  be  under  the  influence  of  a 
constant  stimulus  look  to  the  blood  as  its  source,  and  especially 
to  the  inorganic  blood-salts,  it  having  been  shown  that  the  heart- 
beat can  be  maintained  for  an  astonishing  length  of  time  when 
the  heart  is  fed  solutions  containing  only  inorganic  salts  of  sodium, 
potassium,  and  calcium  in  proper  proportion.  Those  who  look 
upon  the  heart  as  a  truly  automatic  organ  take  the  position  that 
their  view  is  more  in  accordance  with  general  physiological  prin- 
ciples than  the  other,  and  that  no  evidence  yet  brought  forth 


THE  ACTION  OF  THE  HEART  351 

disproves  their  claim.  They  put  the  burden  of  proof  upon  the 
supporters  of  the  "constant  stimulus "  theory.  It  must  be  ad- 
mitted that  at  present  no  conclusive  evidence  for  either  view  is 
available,  nor  are  the  supporters  of  either  able  to  picture  a  satis- 
factory mechanism  of  rhythmicity  in  terms  of  their  particular 
theory. 

The  Extrinsic  Nerves  of  the  Heart.  The  heart,  as  stated  pre- 
viously, is  under  the  control  of  the  autonomic  system.  It  receives 
nerve-fibers  both  from  the  cranial  and  thoracico-lumbar  systems. 
The  cranial  autonomic  fibers  reach  it  by  way  of  the  tenth  cranial 
nerves,  the  vagi,  the  thoracico-lumbar  by  way  of  sympathetic 
ganglia.  The  vagus  nerves  give  off  their  cardiac  branches  in  the 
neck;  the  cardiac  nerves  from  the  thoracico-lumbar  system  arise 
from  the  inferior  cervical  ganglion,  a  sympathetic  ganglion  lying 
in  the  lower  neck  region.  Both  anatomically  and  physiologically 
the  two  sets  of  nerve-fibers  are  distinct.  Anatomically  the  vagus 
fibers  are  pre-ganglionic;  they  arise  from  cell-bodies  in  the  nucleus 
of  the  tenth  nerve  in  the  medulla  and  are  myelinated.  They 
terminate  about  nerve-cells  lying  on  or  within  the  heart  itself. 
The  fibers  from  the  sympathetic  system  are  post-ganglionic ;  they 
arise  from  cell-bodies  in  sympathetic  ganglia,  the  inferior  cervical 
for  the  most  part,  and  are  non-myelinated.  They  terminate  in 
the  tissues  of  the  heart  directly.  Since  nicotine  cuts  the  connection 
between  pre-  and  post-ganglionic  fibers,  application  of  that  drug 
to  the  nerve-cells  of  the  heart  abolishes  the  influence  of  the  vagi, 
but  does  not  affect  the  thoracico-lumbar  control  at  all. 

Physiologically  the  vagus  fibers  are  inhibitory;  their  stimula- 
tion slows  and  weakens  the  heart-beat.  When  very  strongly 
stimulated  they  may  bring  the  heart  to  a  complete  standstill, 
although  in  mammals  the  standstill  is  maintained  for  a  few  sec- 
onds only,  the  heart  soon  " breaking  through"  the  inhibition. 
The  thoracico-lumbar  fibers  have  precisely  the  opposite  function, 
being  augmentor;  their  stimulation  accelerates  and  strengthens  the 
beat  of  the  heart. 

In  addition  to  the  efferent  autonomic  innervation  just  de- 
scribed the  heart  is  provided  with  a  set  of  afferent  nerve-fibers. 
These  reach  the  central  nervous  system  either  by  way  of  the 
vagus  nerves,  or  in  some  species  of  animals,  rabbits  for  example, 
as  separate  nerve-trunks  known  as  the  depressor  nerves.  The 


352  THE  HUMAN  BODY 

function  of  these  afferent  fibers  will  be  discussed  in  Chap.  XXII 
in  connection  with  the  nervous  control  of  the  blood-vessels. 

The  Inhibitory  and  Augmentor  Centers.  The  control  of  the 
heart-beat  is  reflex  in  its  nature,  and  like  most  other  " vital" 
processes  which  are  subject  to  reflex  control  is  vested  in  certain 
''centers"  of  the  medulla.  Two  heart-regulating  centers  are 
recognized,  the  cardio-mhibitory  center  and  the  cardio-augmentor 
center.  The  inhibitory  center  is  in  the  nuclei  of  the  tenth  nerve. 
It  is  bilateral,  each  side  containing  half  of  it.  The  exact  position 
of  the  augmentor  center  has  not  been  determined.  It  is  probably 
not  a  compact  mass  of  cells  as  is  the  inhibitory  center,  but  is  scat- 
tered diffusely  through  the  medulla. 

Both  these  centers  are  in  the  path  of  all  incoming  impulses, 
and  there  is  evidence  that  both  of  them  are  kept  in  constant 
" tonic"  activity  through  the  incessant  play  of  stimuli  upon  them. 
Of  recent  years  the  view  has  been  gaining  ground  that  the  tonic 
activity  of  the  "vital"  centers  is  maintained,  in  part  at  least,  by 
chemical  influences  exerted  through  the  blood.  This  influence 
has  long  been  known  to  exist  in  the  case  of  the  respiratory  center. 
That  it  is  a  factor  in  the  regulation  of  the  heart  is  only  now  com- 
ing to  be  believed. 

The  heart  is  thus  constantly  receiving  both  inhibitory  and 
augmentor  impulses,  the  former  tending  to  diminish  its  activity, 
the  latter  to  increase  it.  The  actual  heart-beat  is  the  expression, 
therefore,  of  the  balance  between  two  opposing  tendencies,  and 
its  increase  or  decrease  indicates  that  one  or  the  other  has  gained 
the  advantage. 

In  attempting  to  analyze  the  causes  of  changes  in  the  heart- 
rate  it  must  be  remembered  that  an  increase  in  rate  may  mean 
either  an  increase  in  the  activity  of  the  augmentor  center,  or  a 
depression  of  the  inhibitory  center.  Conversely,  a  decrease  in 
rate  may  mean  either  a  depression  of  the  augmentor  center  or  an 
increase  in  the  activity  of  the  inhibitory  center.  An  observation 
that  helps  us  in  deciding  which  of  the  centers  may  have  been  re- 
sponsible for  any  observed  change  is  that  the  inhibitory  mechanism 
acts  much  more  promptly  than  does  the  augmentor.  Any  change 
that  follows  quickly  an  exciting  cause  is,  therefore,  to  be  attrib- 
uted to  the  inhibitory  mechanism.  Since  the  heart,  as  a  matter 
of  fact,  responds  almost  instantly  to  most  influences  we  are  in 


THE  ACTION  OF  THE  HEART  353 

the  habit  of  looking  upon  the  augmentor  mechanism  as  affording  a 
fairly  steady  background  of  augmentor  excitation  upon  which 
the  inhibitory  mechanism  may  play  in  delicate  adjustment  to  the 
needs  of  the  circulation.  .There  is  a  perceptible  quickening  of 
the  beat  with  any  muscular  movement,  at  least  with  any  as  ex- 
tensive as  that  required  to  press  a  telegraph  key.  The  quickening 
shows  itself  in  the  next  beat  after  the  beginning  of  the  movement. 
The  suggestion  has  been  made  that  during  the  discharge  of  the 
exciting  nervous  impulses  from  the  brain  to  the  muscles,  there  is 
irradiation  in  the  brain  stem  unto  the  inhibitory  center;  that  this 
irradiation  depresses  the  center,  and  so  allows  a  quickening  of  the 
beat. 

There  are  certain  conditions  in  which  the  augmentor  center 
seems  to  show  heightened  activity.  After  muscular  exercise  there 
is  a  more  or  less  persistent  acceleration  of  the  heart  that  appears 
to  be  due  to  stimulation  of  the  augmentor  center  by  the  waste 
products  of  muscular  activity  which  persist  for  a  time  in  the  cir- 
culating blood.  The  acceleration  of  the  heart  in  time  of  emotional 
stress  or  of  great  pain  is  to  be  explained,  as  stated  previously, 
through  the  connection  of  the  augmentor  mechanism  with  the 
thoracico-lumbar  autonomic,  the  emergency,  system. 

The  familiar  changes  of  heart-rate  with  changes  of  posture, 
slowed  when  lying,  quickened  with  sitting  or  standing,  are  ap- 
parently the  results  of  the  redistribution  of  the  blood  over  the 
Body  under  the  influence  of  gravity.  The  quickening  of  the  beat 
when  one  stands  erect  is  undoubtedly  an  adaptation  designed  to 
overcome  the  tendency  of  the  blood  to  accumulate  in  the  lower 
parts  of  the  Body  when  in  this  position;  but  how  the  adaptation 
is  brought  about  is  not  known.  Successive  swallowing,  as  in  sip- 
ping water,  increases  the  heart-rate  by  depressing  the  inhibitory 
center.  A  blow  over  the  stomach  (the  solar  plexus)  gives  rise  to 
afferent  impulses  which  stimulate  the  inhibitory  center;  the  heart- 
rate  is  therefore  diminished. 

These  are  all  illustrations  of  the  general  rule  that  the  heart- 
beat may  be  modified  by  sensory  stimulations.  It  is  a  matter  of 
ordinary  observation  that  many  experiences,  particularly  those 
involving  sensory  impressions  of  high  intensity,  are  accompanied 
by  marked  changes  in  heart-rate. 

In  connection  with  this  analysis  of  the  control  of  the  heart- 


354  THE  HUMAN  BODY 

beat  the  importance  of  obtaining  the  proper  viewpoint  for  con- 
sidering physiological  processes  may  well  be  emphasized.  If  one 
who  has  not  studied  the  subject  particularly  be  asked  why  run- 
ning makes  the  heart  beat  faster  he  will  probably  answer  that 
exercising  muscles  require  more  blood  than  resting  ones,  and  that 
the  heart  beats  faster  to  furnish  this  extra  amount.  A  moment's 
thought  shows  that  this  statement,  though  quite  true,  does  not 
really  answer  the  question.  It  implies  that  the  heart  has  knowl- 
edge of  the  needs  of  the  tissues,  which,  of  course,  it  cannot  have. 
The  increased  heart-rate  which  accompanies  exercise  is  undoubt- 
edly an  adaptive  response,  as  are  most  reflex  responses,  but  its 
explanation  resides,  not  in  the  adaptation,  but  in  the  reflex  mech- 
anism which  brings  it  about.  We  should  be  continually  on 
guard  against  the  tendency  to  explain  physiological  processes  by 
their  results  rather  than  by  the  means  by  which  the  results  are 
accomplished. 


CHAPTER  XXI 

THE   CIRCULATION  OF  THE  BLOOD.      BLOOD  PRESSURE 
AND  BLOOD-VELOCITY.    THE  PULSE 

The  Flow  of  the  Blood  Outside  of  the  Heart.  The  blood  leaves 
the  heart  intermittently  and  not  in  a  regular  stream,  a  quantity 
being  forced  out  at  each  systole  of  the  ventricles :  before  it  reaches 
the  capillaries,  however,  this  rhythmic  movement  is  transformed 
into  a  steady  flow,  as  may  readily  be  seen  by  examining  under  the 
microscope  thin  transparent  parts  of  various  animals,  as  the  web 
of  a  frog's  foot,  a  mouse's  ear,  or  the  tail  of  a  small  fish.  In  conse- 
quence of  the  steadiness  with  which  the  capillaries  supply  the 
veins  the  flow  in  these  is  also  unaffected,  directly,  by  each  beat 
of  the  heart;  if  a  vein  be  cut  the  blood  wells  out  uniformly,  while 
from  a  cut  artery  the  blood  spurts  out  not  only  with  much  greater 
force,  but  in  jets  which  are  much  more  powerful  at  regular  inter- 
vals corresponding  with  the  systoles  of  the  ventricles. 

The  Circulation  of  the  Blood  as  seen  in  the  Frog's  Web.  There 
is  no  more  fascinating  or  instructive  phenomenon  than  the  circu- 
lation of  the  blood  as  seen  with  the  microscope  in  the  thin  mem- 
brane between  the  toes  of  a  frog's  hind  limb.  Upon  focusing 
beneath  the  epidermis  a  network  of  minute  arteries,  veins,  and 
capillaries,  with  the  blood  flowing  through  them,  comes  into  view 
(Fig.  105).  The  arteries,  a,  are  readily  recognized  by  the  fact 
that  the  flow  in  them  is  fastest  and  from  larger  to  smaller  branches. 
The  latter  are  seen  ending  in  capillaries,  which  form  networks, 
the  channels  of  which  are  all  nearly  equal  in  size.  While  in  the 
veins  arising  from  the  capillaries  the  flow  is  from  smaller  to  larger 
trunks,  and  slower  than  in  the  arteries,  but  faster  than  in  the 
capillaries. 

The  reason  of  the  slower  flow  of  the  capillaries  is  that  their 
united  area  is  considerably  greater  than  that  of  the  arteries 
supplying  them,  so  that  the  same  quantity  of  blood  flowing 
through  them  in  a  given  time  has  a  wider  channel  to  flow  in  and 
moves  slowly.  The  area  of  the  veins  is  smaller  than  that  of  the 

355 


356  THE  HUMAN  BODY 

capillaries  but  greater  than  that  of  the  arteries,  and  hence  the 
rate  of  movement  in  them  is  also  intermediate.  Almost  always 
when  an  artery  divides,  the  area  of  its  branches  is  greater  than 
that  of  the  main  trunk,  and  so  the  arterial  current  becomes 
slower  and  slower  from  the  heart  onwards.  In  the  veins,  on  the 
other  hand,  the  area  of  a  trunk  formed  by  the  union  of  two  or 
more  branches  is  less  than  that  of  the  branches  together,  and  the 
flow  becomes  quicker  and  quicker  towards  the  heart.  But  even 
at  the  heart  the  united  cross-sections  of  the  veins  entering  the 
auricles  are  greater  than  those  of  the  arteries  leaving  the  ventricles, 
so  that,  since  as  much  blood  returns  to  the  heart  in  a  given  time 
as  leaves  it,  the  rate  of  the  current  in  the  pulmonary  veins  and 
the  vena3  cavse  is  less  than  in  the  pulmonary  artery  and  aorta. 
We  may  represent  the  vascular  system  as  a  double  cone,  widen- 
ing from  the  ventricles  to  the  capillaries  and  narrowing  from  the 
latter  to  the  auricles.  Just  as  water  forced  in  at  a  narrow  end  of 
this  would  flow  quickest  there  and  slowest  at  the  widest  part,  so 
the  blood  flows  quickest  in  the  aorta  and  slowest  in  the  capillaries, 
which  taken  together  form  a  much  wider  channel. 

The  Axial  Current  and  the  Inert  Layer.  If  a  small  artery  in 
the  frog's  web  be  closely  examined  it  will  be  seen  that  the  rate  of 
flow  is  not  the  same  in  all  parts  of  it.  In  the  center  is  a  very 
rapid  current  carrying  along  all  the  red  corpuscles  and  known  as 
the  axial  stream,  while  near  the  wall  of  the  vessel  the  flow  is  much 
slower,  as  indicated  by  the  rate  at  which  the  pale  blood-corpuscles 
are  carried  along  in  it.  This  is  a  purely  physical  phenomenon. 
If  any  liquid  be  forcibly  driven  through  a  fine  tube  which  it  wets, 
water  for  instance  through  a  glass  tube,  the  outermost  layers  of 
the  liquid  will  remain  nearly  motionless  in  contact  with  the  tube; 
the  next  layers  of  molecules  will  move  a  little,  the  next  faster 
still;  and  so  on  until  a  rapid  current  is  found  in  the  center.  If 
solid  bodies,  as  powdered  sealing-wax,  be  suspended  in  the  water, 
these  will  all  be  carried  on  in  the  central  faster  current  or  axial 
stream,  just  as  the  red  corpuscles  are  in  the  artery.  The  white 
corpuscles,  partly  because  of  their  less  specific  gravity,  and  partly 
because  of  their  sometimes  irregular  form,  due  to  amceboid  move- 
ments, get  frequently  pushed  out  of  the  axial  current,  so  that 
many  of  them  are  found  in  the  inert  layer. 

The  Resistance  to  the  Blood-Flow.    As  liquid  flows  through  a 


THE  CIRCULATION  OF  THE  BLOOD  357 

tube  there  is  a  certain  amount  of  friction  between  the  moving 
liquid  and  the  walls  of  the  tube.  There  is  also  friction  between 
the  different  concentric  layers  of  the  liquid,  since  each  of  them  is 
moving  at  a  different  rate  from  that  in  contact  with  it  on  each 
side.  This  form  of  friction  is  known  in  hydrodynamics  as  "  in- 
ternal friction/'  and  it  is  of  great  importance  in  the  circulation 
of  the  blood.  The  friction  increases  very  fast  as  the  caliber  of  the 
tube  through  which  the  liquid  flows  diminishes:  so  that  with  the 
same  rate  of  flow  it  is  disproportionately  much  greater  in  a  small 
tube  than  in  a  larger  one.  Hence  a  given  quantity  of  liquid  forced 
in  a  minute  through  one  large  tube  would  experience  much  less 
resistance  from  friction  than  if  sent  in  the  same  time  through 
four  or  five  smaller  tubes,  the  united  transverse  sections  of  which 
were  together  equal  to  that  of  the  single  larger  one.  In  the  blood- 
vessels the  increased  total  area,  and  consequently  slower  flow,  in 
the  smaller  channels  partly  counteracts  this  increase  of  friction, 
but  only  to  a  comparatively  slight  extent;  so  that  the  friction, 
and  consequently  the  resistance  to  the  blood-flow,  is  far  greater 
in  the  capillaries  and  arterioles  than  in  the  small  arteries,  and  in 
the  small  arteries  than  in  the  large  ones.  Practically  we  may  re- 
gard the  arteries  as  tubes  ending  in  a  sponge:  the  united  areas  of 
all  the  channels  in  the  latter  might  be  considerably  larger  than 
that  of  the  supplying  tubes,  but  the  friction  to  be  overcome  in 
the  flow  through  them  would  be  much  greater. 

The  Conversion  of  the  Intermittent  into  a  Continuous  Flow. 
Since  the  heart  sends  blood  into  the  aorta  intermittently,  we 
have  still  to  inquire  how  it  is  that  the  flow  in  the  capillaries  is 
continuous.  In  the  larger  arteries  it  is  not,  since  we  can  feel 
them  dilating  as  the  "pulse,"  on  applying  the  finger  over  the 
radial  artery  at  the  wrist,  or  over  the  temporal  artery  on  the  side 
of  the  brow. 

The  first  explanation  which  suggests  itself  is  that  since  the 
capacity  of  the  blood-vessels  increases  from  the  heart  to  the 
capillaries,  an  acceleration  of  the  flow  during  the  ventricular 
contraction  which  might  be  very  manifest  in  the  vessels  near  the 
heart  would  become  less  and  less  obvious  in  the  more  distant 
vessels.  But  if  this  were  so,  then  when  the  blood  was  collected 
again  from  the  wide  capillary  sponge  into  the  great  veins  near 
the  heart,  which  together  are  but  little  bigger  than  the  aorta,  we 


358 


THE  HUMAN  BODY 


FIG.  109. 


ought  to  find  a  pulse,  but  we  do  not :  the  venous  pulse  which  some- 
times occurs  having  quite  a  different  cause,  being  due  to  a  back- 
flow  from  the  auricles,  or  a  checking  of  the  on-flow  into  them, 
during  the  cardiac  systole.  The  rhythm  of  the  flow  caused  by 
the  heart  is  therefore  not  merely  cloaked  in  the  small  arteries  and 
capillaries,  but  abolished  in  them. 

We  can,  however,  readily  contrive  conditions  outside  the  Body 
under  which  an  intermittent  supply  is  transformed  into  a  con- 
tinuous flow.  Suppose  we  have  two 
vessels,  A  and  B  (Fig.  109)  contain- 
ing water  and  connected  below  in 
two  ways:  through  the  tube  a  on 
which  there  is  a  pump  provided  with 
valves  so  that  it  can  only  drive  liquid 
from  A  to  B;  and  through  b,  which 
may  be  left  wide  open  or  narrowed  by 
the  clamp  c,  at  will.  If  the  apparatus 
be  left  at  rest  the  water  will  lie  at 
the  same  level,  d,  in  each  vessel. 
If  now  we  work  the  pump,  at  each  stroke  a  certain  amount  of 
water  will  be  conveyed  from  A  to  B,  and  as  result  of  the  lower- 
ing of  the  level  of  liquid  in  A  and  its  rise  in  B,  there  will  be 
immediately  a  return  flow  from  B  to  A  through  the  tube  b.  A,  in 
these  circumstances,  would  represent  the  venous  system,  from 
which  the  heart  constantly  takes  blood  to  pump  it  into  B,  repre- 
senting the  arterial  system;  and  b  would  represent  the  capillary 
vessels  through  which  the  return  flow  takes  place;  but,  so  far,  we 
should  have  as  intermittent  a  flow  through  the  capillaries,  6,  as 
through  the  heart-pump,  a.  Now  imagine  b  to  be  narrowed  at 
one  point  so  as  to  oppose  resistance  to  the  back-flow,  while  the 
pump  goes  on  working  steadily.  The  result  will  be  an  accumula- 
tion of  water  in  B,  and  a  fall  of  its  level  in  A .  But  the  more  the 
difference  of  level  in  the  two  vessels  increases,  the  greater  is  the 
force  tending  to  drive  water  back  through  6  to  A,  and  more  will 
flow  back,  under  the  greater  difference  of  pressure,  in  a  given  time, 
until  at  last,  when  the  water  in  B  has  reached  a  certain  level,  df, 
and  that  in  A  has  correspondingly  fallen  to  d",  the  current  through 
b  will  carry  back  in  one  minute  just  so  much  water  as  the  pump 
sends  the  other  way,  and  this  back-flow  will  be  nearly  constant; 


THE  CIRCULATION  OF  THE  BLOOD  359 

it  will  not  depend  directly  upon  the  strokes  of  the  pump,  but 
upon  the  head  of  water  accumulated  in  B;  which  head  of  water 
will,  it  is  true,  be  slightly  increased  at  each  stroke  of  the  pump, 
but  the  increase  will  be  very  small  compared  with  the  whole  driv- 
ing force,  and  its  influence  will  be  inappreciable.  We  thus  gain 
the  idea  that  an  incomplete  impediment  to  the  flow  from  the 
arteries  to  the  veins  (from  B  to  A  in  the  diagram),  such  as  is 
afforded  by  friction  in  the  capillaries,  may  bring  about  conditions 
which  will  lead  to  a  steady  flow  along  the  latter  vessels. 

But  in  the  arterial  system  there  can  be  no  accumulation  of 
blood  at  a  higher  level  than  that  in  the  veins,  such  as  is  supposed  in 
the  above  apparatus;  and  we  must  next  consider  if  the  "head  of 
water"  can  be  replaced  by  some  other  form  of  driving  force.  It 
is  in  fact  replaced  by  the  elasticity  of  the  large  arteries.  Suppose 
an  elastic  bag  instead  of  the  vessel  B  connected  with  the  pump 
"a."  If  there  be  no  resistance  to  the  back-flow  the  current 
through  6  will  be  discontinuous.  But  if  resistance  be  interposed, 
then  the  elastic  bag  will  become  distended,  since  the  pump  sends 
in  a  given  time  more  liquid  into  it  than  it  passes  back  through  b. 
But  the  more  it  becomes  distended  the  more  will  the  bag  squeeze 
the  liquid  inside  and  the  faster  will  it  send  that  back  to  A,  until 
at  last  its  squeeze  is  so  powerful  that  each  minute  or  two  or  five 
minutes  it  sends  back  into  A  as  much  as  it  receives.  Thenceforth 
the  back-flow  through  b  will  be  practically  constant,  being  im- 
mediately dependent  upon  the  elastic  reaction  of  the  bag,  and  only 
indirectly  upon  the  action  of  the  pump  which  keeps  it  distended. 
Such  a  state  of  things  represents  very  closely  the  phenomena  oc- 
curring in  the  blood-vessels.  The  highly  elastic  large  arteries  are 
kept  stretched  with  blood  by  the  heart ;  and  the  reaction  of  their 
elastic  walls,  steadily  squeezing  on  the  blood  in  them,  forces  it  con- 
tinuously through  the  small  arteries  and  capillaries.  The  steady 
flow  in  the  latter  depends  thus  on  two  factors :  first,  the  elasticity 
of  the  large  arteries;  and  secondly,  the  resistance  to  their  empty- 
ing, dependent  upon  internal  friction  in  the  small  arteries  and  the 
capillaries,  which  calls  into  play  the  elasticity  of  the  large  vessels. 
Were  the  capillary  resistance  or  the  arterial  elasticity  absent  the 
blood-flow  in  the  capillaries  would  be  rhythmic. 

Weber's  Schema.  It  is  clear  from  the  statements  made  in  the 
last  paragraph  that  it  is  the  pressure  exerted  by  the  elastic  arteries 


360  THE  HUMAN  BODY 

upon  the  blood  inside  them  which  keeps  up  the  flow  through  the 
capillaries,  the  heart  serving  to  keep  the  big  arteries  tightly  filled 
and  so  to  call  the  elastic  reaction  of  their  walls  into  play.  The 
whole  circulation  depends  primarily,  of  course,  upon  the  beat  of 
the  heart,  but  this  only  indirectly  governs  the  capillary  flow,  and 
since  the  latter  is  the  aim  of  the  whole  vascular  apparatus,  it 
is  of  great  importance  to  know  as  much  as  possible  about  arterial 
pressure;  not  only  how  great  it  is  on  the  average,  but  how  it  is 
altered  in  different  vessels  in  various  circumstances  so  as  to  make 
the  flow  through  the  capillaries  of  a  given  part  greater  or  less 
according  to  circumstances;  for,  as  blushing  and  pallor  of  the  face 
(which  frequently  occur  without  any  change  in  the  skin  elsewhere) 
prove,  the  quantity  of  blood  flowing  through  a  given  part  is  not 
always  the  same,  nor  is  it  always  increased  or  diminished  in  all 
parts  of  the  Body  at  the  same  time.  Most  of  what  we  know  about 
arterial  pressure  has  been  ascertained  by  experiments  made  upon 
the  lower  animals,  from  which  deductions  are  then  made  concern- 
ing what  happens  in  man,  since  Anatomy  shows  that  the  circula- 
tory organs  are  arranged  upon  the  same  plan  in  all  the  mammalia. 
A  great  deal  can,  however,  be  learnt  by  studying  the  flow  of  liq- 
uids through  ordinary  elastic  tubes.  Suppose  we  have  a  set  of 
such  (Fig.  110)  supplied  at  one  point  with  a  pump,  c,  possessing 
valves  of  entry  and  exit  which  open  only  in  the  direction  indi- 
cated by  the  arrows,  and  that  the  whole  system  is  slightly  over- 
filled with  liquid  so  that  its  elastic  walls  are  slightly  stretched. 
These  will  in  consequence  press  upon  the  liquid  inside  them  and 
the  amount  of  this  pressure  will  be  indicated  by  the  gauges;  so 
long  as  the  pump  is  at  rest  it  will  be  the  same  everywhere  (and 
therefore  equal  in  the  gauges  on  B  and  A),  since  liquid  in  a  set  of 
horizontal  tubes  communicating  freely,  as  these  do  at  D,  always 
distributes  itself  so  that  the  pressure  upon  it  is  everywhere  the 
same.  Let  the  pump  c  now  contract  once,  and  then  dilate:  dur- 
ing the  contraction  it  will  empty  itself  into  B  and  during  the  dila- 
tation fill  itself  from  A.  Consequently  the  pressure  in  B,  indi- 
cated by  the  gauge  x,  will  rise  and  that  in  A  will  fall.  But  very 
rapidly  the  liquid  will  redistribute  itself  from  B  to  A  through  D, 
until  it  again  exists  everywhere  under  the  same  pressure.  Every 
time  the  pump  works  there  will  occur  a  similar  series  of  phenom- 
ena, and  there  will  be  a  disturbance  of  equilibrium  causing  a 


THE  CIRCULATION  OF  THE  BLOOD  361 

wave  to  flow  round  the  tubing;  but  there  will  be  no  steady  main- 
tenance of  a  pressure  on  the  side  B  greater  than  that  in  A .  Now 
let  the  upper  tube  D  be  closed  so  that  the  liquid  to  get  from  B  to 
A  must  flow  through  the  narrow  lower  tubes  D',  which  oppose 
considerable  resistance  to  its  passage  on  account  of  their  frequent 
branchings  and  the  great  friction  in  them ;  then  if  the  pump  works 
frequently  enough  there  will  be  produced  and  maintained  in  B  a 
pressure  considerably  higher  than  that  in  A.  If,  for  example, 
the  pump  works  60  times  a  minute  and  at  each  stroke  takes  180 


FIG.  110. — Diagram  of  Weber's  Schema. 

cubic  centimeters  of  liquid  (6  ounces)  from  A  and  drives  it  into 
B,  the  quantity  sent  in  at  the  first  stroke  will  not  (on  account  of 
the  resistance  to  its  flow  offered  by  the  small  branched  tubes), 
have  all  got  back  into  A  before  the  next  stroke  takes  place,  send- 
ing 180  more  cubic  centimeters  (6  ounces)  into  B.  Consequently 
at  each  stroke  B  will  become  more  and  more  distended  and  A  more 
and  more  emptied,  and  the  gauge  x  will  indicate  a  much  higher 
pressure  than  that  on  A.  As  B  is  more  stretched,  however,  it 
squeezes  harder  upon  its  contents,  until  at  last  a  time  comes  when 
this  squeeze  is  powerful  enough  to  force  through  the  small  tubes 
just  180  cubic  centimeters  (6  ounces)  in  a  second.  Then  further 
accumulation  in  B  ceases.  The  pump  sends  into  it  10,800  cubic 
centimeters  (360  ounces)  in  a  minute  at  one  end  and  it  squeezes 
out  exactly  that  amount  in  the  same  time  from  its  other  end;  and 
so  long  as  the  pump  works  steadily  the  pressure  in  B  will  not  rise, 
nor  that  in  A  fall,  any  more.  But  under  such  circumstances  the 
flow  through  the  small  tubes  will  be  nearly  constant  since  it  de- 
pends upon  the  difference  in  pressure  prevailing  between  B  and 


362  THE  HUMAN  BODY 

A,  and  only  indirectly  upon  the  pump  which  serves  simply  to 
keep  the  pressure  high  in  B  and  low  in  A.  At  each  stroke  of  the 
pump  it  is  true  there  will  be  a  slight  increase  of  pressure  in  B  due 
to  the  fresh  180  cubic  centimeters  (6  ounces)  forced  into  it,  but 
this  increase  will  be  but  a  small  fraction  of  the  total  pressure  and 
so  have  but  an  insignificant  influence  upon  the  rate  of  flow  through 
the  small  connecting  tubes. 

Arterial  Pressure.  The  condition  of  things  just  described  repre- 
sents very  closely  the  phenomena  presented  in  the  blood-vascular 
system,  in  which  the  ventricles  of  the  heart,  with  their  auriculo- 
ventricular  and  semilunar  valves,  represent  the  pump,  the  small- 
est arteries  and  the  capillaries  the  resistance  at  D',  the  large 
arteries  the  elastic  tube  B,  and  the  veins  the  tube  A.  The  ventri- 
cles constantly  receiving  blood  through  the  auricles  from  the 
veins,  send  it  into  the  arteries,  which  find  a  difficulty  in  emptying 
themselves  through  the  capillaries,  and  so  blood  accumulates  in 
them  until  the  elastic  reaction  of  the  stretched  arteries  is  able  to 
squeeze  in  a  minute  through  the  capillaries  just  so  much  blood  as 
the  left  ventricle  pumps  into  the  aorta,  and  the  right  into  the 
pulmonary  artery,  in  the  same  time.  Accordingly  in  a  living 
animal  a  pressure-gauge  connected  with  an  artery  shows  a  much 
higher  pressure  than  one  connected  with  a  vein,  and  this  persist- 
ing difference  of  pressure,  only  increased  by  a  small  fraction  of 
the  whole  at  each  heart-beat,  brings  about  a  steady  flow  from  the 
arteries  to  the  veins.  The  heart  keeps  the  arteries  stretched  and 
the  stretched  arteries  maintain  the  flow  through  the  capillaries, 
and  the  constancy  of  the  current  in  them  depends  on  two  factors : 
(1)  the  resistance  experienced  by  the  blood  in  its  flow  from  the 
ventricles  to  the  veins,. and  (2)  the  elasticity  of  the  larger  arteries 
which  allows  the  blood  to  accumulate  in  them  under  a  high  pres- 
sure, in  consequence  of  this  resistance. 

Since  the  blood  flows  from  the  aorta  to  its  branches  and  from 
these  to  the  capillaries  and  thence  to  the  veins,  and  liquids  in  a 
set  of  continuous  tubes  flow  from  points  of  greater  to  those  of 
less  pressure,  it  is  clear  that  the  blood-pressure  must  constantly 
diminish  from  the  aorta  to  the  right  auricle;  and  similarly  from 
the  pulmonary  artery  to  the  left  auricle.  At  any  point,  in  fact, 
the  pressure  is  proportionate  to  the  resistance  in  front,  and  since 
the  farther  the  blood  has  gone  the  less  of  this,  due  to  impediments 


THE  CIRCULATION  OF  THE  BLOOD  363 

at  branchings  and  to  internal  friction,  it  has  to  overcome  in  finish- 
ing its  round,  the  pressure  on  the  blood  diminishes  as  we  follow 
it  from  the  aorta  to  the  venae  cavae.  In  the  larger  arteries  the  fall 
of  pressure  is  gradual  and  small,  since  the  amount  of  resistance 
met  with  in  the  flow  through  them  is  but  little.  In  the  small 
arteries  and  capillaries  the  resistance  overcome  and  left  behind 
is  (on  account  of  the  great  internal  friction  due  to  their  small 
caliber)  very  great,  and  consequently  the  fall  of  pressure. between 
the  medium-sized  arteries  and  the  veins  is  rapid  and  considerable. 
Modifications  of  Arterial  Pressure  by  Changes  in  the  Heart-beat. 
A  little  consideration  will  make  it  clear  that  the  pressure  prevail- 
ing at  any  time  in  a  given  artery  depends  on  two  things — the  rate 
at  which  the  vessel  is  filled,  i.  e.,  upon  the  amount  of  work  done 
by  the  heart;  and  the  ease  or  difficulty  with  which  it  is  emptied, 
that  is,  upon  the  resistance  in  front.  A  third  factor  has  to  be 
taken  into  account  in  some  cases;  namely,  that  when  the  muscular 
coats  of  the  small  arteries  contract  the  local  capacity  of  the  vas- 
cular system  is  diminished,  and  has  to  be  compensated  for  by 
greater  distention  elsewhere,  and  vice  versa.  This  would  of  itself 
of  course  bring  about  changes  in  the  pressure  exerted  on  the 
contained  liquid,  but  for  the  present  it  may  be  left  out  of  con- 
sideration. If  we  suppose  a  system  such  as  represented  in  Fig.  110, 
to  be  in  equilibrium,  with  the  pump  injecting  into  B  a  certain 
volume  of  liquid  per  minute,  and  the  elastic  tension  of  the  tube  B 
just  sufficient  to  force  that  volume  through  the  resistance  D'  in 
the  same  time,  it  is  clear  that  the  pressure  indicated  on  the 
gauge  x  will  be  very  nearly  constant.  If,  now,  the  volume  of 
liquid  forced  into  B  in  a  minute  be  increased,  either  by  the  pump 
working  faster  or  by  its  pumping  more  at  each  stroke,  there  will 
evidently  be  an  accumulation  in  B,  since  its  tension  is  adjusted 
to  force  out  the  less  volume  per  minute,  but  this  accumulation, 
by  stretching  the  tube  still  more,  increases  its  elastic  tension,  so 
that  this  is  presently  great  enough  to  force  out  the  added  volume 
as  fast  as  it  comes  in.  The  pressure-gauge  will  now  stand  at  a 
higher  point,  showing  that  the  contents  of  the  tube  are  under 
greater  pressure  than  before.  Similarly,  a  diminution  in  the  in- 
flux of  liquid  into  B  will  be  followed  by  a  fall  of  pressure  within 
it  as  the  walls  of  the  tube  adjust  themselves  to  the  smaller  volume 
to  be  forced  out  per  minute.  Precisely  the  same  reasoning  may 


364  THE  HUMAN  BODY 

be  applied  to  the  vascular  system  for  determining  the  effects  upon 
arterial  pressure  of  changes  in  the  heart-beat. 

.Modifications  of  Arterial  Pressure  by  Changes  in  the  Peripheral 
Resistance.  If  while  the  pump  c  in  Fig.  110  is  steadily  sending 
a  given  volume  of  liquid  per  minute  into  B  the  resistance  at  D' 
increase,  it  is  clear  arterial  pressure  must  rise.  For  B  is  only 
stretched  enough  to  squeeze  out  in  a  minute  the  given  quantity 
of  liquid  against  the  original  resistance,  and  cannot  at  first  send 
out  that  quantity  against  the  greater.  Liquid  will  consequently 
accumulate  in  it  until  at  last  it  becomes  stretched  enough  to  send 
out  as  much  in  a  minute  as  before  in  spite  of  the  greater  resistance 
to  be  overcome.  A  new  mean  pressure  at  a  higher  level  will  then 
be  established.  If,  on  the  contrary,  the  resistance  diminishes 
while  the  pump's  work  remains  the  same,  then  B  will  at  first 
squeeze  out  in  a  minute  more  than  it  receives,  until  finally  its 
elastic  pressure  is  reduced  to  the  point  at  which  its  receipts  and 
losses  balance,  and  a  new  and  lower  mean  pressure  will  be  estab- 
lished in  B. 

Similarly  in  the  vascular  system,  increase  of  the  peripheral 
resistance  by  narrowing  of  the  small  arteries  will  increase  arterial 
pressure  in  all  parts  nearer  the  heart,  while  dilatation  of  the  small 
arteries  will  have  the  contrary  effect. 

Summary.  We  find  then  that  arterial  pressure  at  any  moment 
is  dependent  upon:  (1)  the  quantity  of  blood  forced  into  the  ar- 
teries in  a  given  time;  (2)  the  caliber  of  the  smaller  vessels.  Both 
of  these  and  consequently  the  capillary  circulation  which  depends 
upon  arterial  pressure,  are  under  the  control  of  the  nervous  sys- 
tem (see  Chaps.  XX  and  XXII). 

The  Pulse.  When  the  left  ventricle  contracts  it  forces  a  cer- 
tain amount  of  blood  into  the  aorta,  which  is  already  distended 
and  on  account  of  the  resistance  in  front  cannot  empty  itself  as 
fast  as  the  contracting  ventricle  fills  it.  As  a  consequence  its 
elastic  walls  yield  still  more — it  enlarges  both  transversely  and 
longitudinally  and  if  exposed  in  a  living  animal  can  be  seen  and 
felt  to  pulsate,  swelling  out  at  each  systole  of  the  heart,  and 
shrinking  and  getting  rid  of  the  excess  during  the  pause.  A 
similar  phenomenon  can  be  observed  in  all  the  other  large  arteries, 
for  just  as  the  contracting  ventricle  fills  the  aorta  faster  than  the 
latter  empties  (the  whole  period  of  diastole  and  systole  being 


THE  CIRCULATION  OF  THE  BLOOD  365 

required  by  the  aorta  to  pass  on  the. blood  sent  in  during  systole), 
so  the  increased  tension  in  the  aorta  immediately  after  the  cardiac 
contraction  drives  on  some  of  its  contents  into  its  branches,  and 
fills  these  faster  than  they  are  emptying,  and  so  causes  a  dilatation 
of  them  also,  which  only  gradually  disappears  as  the  aortic  tension 
falls  before  the  next  systole.  Hence  after  each  beat  of  the  heart 
there  is  a  sensible  dilatation  of  all  the  larger  arteries,  known  as 
the  pulse,  which  becomes  less  and  less  marked  at  points  on  the 
smaller  branches  farther  from  the  heart,  but  which  in  health  can 
readily  be  recognized  on  any  artery  large  enough  to  be  felt  by 
the  finger  through  the  skin.  The  radial  artery  near  the  wrist, 
for  example,  will  always  be  felt  tense  by  the  finger,  since  it  is 
kept  overfilled  by  the  heart  in  the  way  already  described.  But 
after  each  heart-beat  it  becomes  more  rigid  and  dilates  a  little, 
the  increased  distension  and  rigidity  gradually  disappearing  as 
the  artery  passes  on  the  excess  of  blood  before  the  next  heart- 
beat. The  pulse  is  then  a  wave  of  increased  pressure  started  by 
the  ventricular  systole,  radiating  from  the  semilunar  valves  over 
the  arterial  system,  and  gradually  disappearing  in  the  smaller 
branches.  In  the  aorta  the  pulse  is  most  marked,  for  the  resist- 
ance there  to  the  transmission  onwards  of  the  blood  sent  in  by 
the  heart  is  greatest,  and  the  elastic  tube  in  which  it  consequently 
accumulates  is  shortest,  and  so  the  increase  of  pressure  and  the 
dilatation  caused  are  considerable.  The  aorta,  however,  gradually 
squeezes  out  the  excess  blood  into  its  branches,  and  so  this  be- 
comes distributed  over  a  wider  area,  and  these  branches  having 
less  resistance  in  front  find  less  and  less  difficulty  in  passing  it  on; 
consequently  the  pulse-wave  becomes  less  and  less  conspicuous 
and  finally  altogether  disappears  before  the  capillaries  are  reached, 
the  excess  of  liquid  in  the  whole  arterial  system  after  a  ventricular 
systole  being  too  small  to  raise  the  mean  pressure  sensibly  once  it 
has  been  widely  distributed  over  the  elastic  vessels,  which  is  the 
case  by  the  time  the  wave  has  reached  the  small  branches  which 
supply  the  capillaries. 

The  pulse-wave  travels  over  the  arterial  system  at  the  rate  of 
about  9  jneters  (29.5  feet)  in  a  second,  commencing  at  the  wrist 
0.159  second,  and  in  the  posterior  tibial  artery  at  the  ankle  0.193 
second,  after  the  ventricular  systole.  The  blood  itself  does  not 
of  course  travel  as  fast  as  the  pulse-wave,  for  that  quantity  sent 


366  THE  HUMAN  BODY 

into  the  aorta  at  each  heart-beat  does  not  immediately  rush  on 
over  the  whole  arterial  system,  but  by  raising  the  local  pressure 
causes  the  vessel  to  squeeze  out  faster  than  before  some  of  the 
blood  it  already  contains,  and  this  entering  its  branches  raises 
the  pressure  in  them  and  causes  them  more  quickly  to  fill  their 
branches  and  raise  the  pressure  in  them;  the  pulse-wave  or  wave 
of  increased  pressure  is  transmitted  in  this  way  much  faster  than 
any  given  portion  of  the  blood.  How  the  wave  of  increased 
pressure  and  the  liquid  travel  at  different  rates  may  be  made 
clearer  perhaps  by  picturing  what  would  happen  if  liquid  were 
pumped  into  one  end  of  an  already  full  elastic  tube,  closed  at 
the  other  end.  At  the  closed  end  of  the  tube  a  dilatation  and  in- 
creased tension  would  be  felt  immediately  after  each  stroke  of 
the  pump,  although  the  liquid  pumped  in  at  the  other  end  would 
have  remained  about  its  point  of  entry;  it  would  cause  the  pulsa- 
tion not  by  flowing  along  the  tube  itself,  but  by  giving  a  push  to 
the  liquid  already  in  it.  If  instead  of  absolutely  closing  the  distal 
end  of  the  tube  one  brought  about  a  state  of  things  more  nearly 
resembling  that  found  in  the  arteries  by  allowing  it  to  empty 
itself  against  a  resistance,  say  through  a  narrow  opening,  the  phe- 
nomena observed  would  not  be  essentially  altered;  the  increase 
of  pressure  would  travel  along  the  distended  tube  far  faster  than 
the  liquid  itself. 

The  pulse  being  dependent  on  the  heart's  systole,  "feeling  the 
pulse"  of  course  primarily  gives  a  convenient  means  of  counting 
the  rate  of  beat  of  that  organ.  To  the  skilled  touch,  however,  it 
may  tell  a  great  deal  morej  as  for  example  whether  it  is  a  readily 
compressible  or  "soft  pulse"  showing  a  low  arterial  pressure,  or 
tense  and  rigid  ("  a  hard  pulse  ")  indicative  of  high  arterial  pres- 
sure, and  so  on.  In  adults  the  normal  pulse-rate  may  vary  from 
sixty-five  to  seventy-five,  the  most  common  number  being  seventy- 
two.  In  the  sanie  individual  it  is  faster  when  standing  than  when 
sitting,  and  when  sitting  than  when  lying  down.  Any  exercise 
increases  its  rate  temporarily,  and  so  does  excitement;  a  sick 
person's  pulse  should  not  therefore  be  felt  when  he  is  nervous  or 
excited  (as  the  physician  knows  when  he  tries  first  to  get  his 
patient  calm  and  confident),  as  it  is  then  difficult  to  draw  correct 
conclusions  from  it.  In  children  the  pulse  is  quicker  than  in 
adults,  and  in  old  age  slower  than  in  middle  life. 


THE  CIRCULATION  OF  TUB  BLOOD 


367 


The  Measurement  of  Blood-Pressure.  Direct  determinations 
of  arterial  and  venous  pressures  are  made  in  living,  anesthetized 
animals  by  inserting  into  a  large  artery  or  vein  a  glass  tube  con- 
nected with  a  pressure-gauge.  The  usual  form  of  gauge  for  such 
work  is  the  mercury  manometer  represented  in  Fig.  111.  This 


a .          c 


FIG.  111. — Mercury  manometer  for  recording  blood-pressure,  d  g,  glass  U-tube 
partly  filled  with  mercury.  In  one  limb  is  borne  a  float,  e,  bearing  a  recording  de- 
vice /;  the  other  limb  is  filled  with  a  suitable  liquid  and  connected  water-tight  with 
the  heart  end  of  a  divided  artery  b,  by  means  of  glass  connection  a.  Changes  in 
the  mercury  level  indicate  changes  of  arterial  pressure. 

instrument,  on  account  of  the  great  inertia  of  mercury,  follows 
only  slightly  the  rapid  fluctuations  of  pressure  due  to  the  beats 
of  the  heart.  It  therefore  gives  mean  or  average  pressures.  Re- 
sults obtained  with  mercury  manometers  are  expressed  in  terms 
of  the  height  of  the  mercury  column  sustained  by  the  blood- 
pressure.  To  reduce  them  to  columns  of  blood  they  must  be 
multiplied  by  13.6,  the  number  of  times  mercury  is  heavier  than 
blood.  The  mean  aortic  pressure  in  average-sized  dogs  is  ordi- 


368  THE  HUMAN  BODY 

narily  not  far  from  170  millimeters  of  mercury.  The  pressure  in 
the  veins  diminishes  from  3  or  4  millimeters  of  mercury  in  the 
large  veins  of  the  front  leg  to  zero  at  the  entrance  to  the  auricle 
(see  p.  362). 

Blood-Pressure  in  Man.  In  man  it  is  necessary  to  determine 
blood-pressures  by  methods  that  do  not  involve  operative  pro- 
cedure. Various  devices  are  in  use  for  this  purpose.  Most  of 
them  depend  on  the  fact  that  bodily  tissues,  being  for  the  most 
part  liquid,  are  virtually  incompressible  and  so  transmit  through- 
out their  extent  pressures  applied  to  them.  For  determining 
arterial  pressures  the  upper  arm  is  inclosed  in  a  cuff  of  hollow 
rubber  tubing  so  arranged  that  its  inflation  presses  from  all  sides 
on  the  arm.  The  cuff  is  inflated  until  its  pressure  on  the  arm  is 
just  sufficient  to  squeeze  shut  the  brachial  artery.  By  means  of 
a  manometer  attached  to  the  cuff  the  amount  of  pressure  applied 
can  be  determined.  The  differences  between  the  various  forms  of 
instruments  depend  chiefly  on  their  methods  for  determining 
exactly  when  the  artery  is  occluded.  These  instruments  do  not 
give  mean  blood-pressures,  as  does  the  mercury  manometer,  but 
maximum  (systolic)  and  minimum  (diastolic)  pressures.  It  is 
found  that  in  man  the  systolic  pressure  averages  from  110  to 
120  mms.  of  mercury,  and  the  diastolic  about  65  mms.  of  mercury. 

Determinations  of  capillary  and  venous  pressures  in  man  can 
be  made  more  easily  than  determinations  of  arterial  pressure 
because  there  are  superficial  capillaries  and  veins  whose  occlusion 
can  be  observed  directly;  in  capillaries  by  whitening  of  the  skin, 
in  veins  by  the  disappearance  of  the  vein-ridge  along  it.  The 
basis  of  the  method  is  the  same  as  for  arterial  pressure,  namely, 
determination  of  the  pressure  necessary  to  occlude  the  vessel. 
Capillary  pressures  measured  by  this  method  average  about 
30  mms.  of  mercury;  venous  pressures  10  mms.  or  less. 

The  Rate  of  the  Blood-Flow.  As  the  vascular  system  be- 
comes more  capacious  from  the  aorta  to  the.  capillaries  the  rate 
of  flow  in  it  becomes  proportionately  slower,  and  as  the  total 
area  of  the  channels  diminishes  again  from  the  capillaries  to  the 
venae  cavse,  so  does  the  rate  of  flow  quicken,  just  as  a  river  current 
slackens  when  it  spreads  out,  and  flows  faster  when  it  is  confined 
to  a  narrower  channel;  a  fact  taken  advantage  of  in  the  construc- 
tion of  Eads'  jetties  at  the  mouth  of  the  Mississippi,  the  object 


THE  CIRCULATION  OF  THE  BLOOD  369 

of  which  is  to  make  the  water  flow  in  a  narrower  channel  and  so 
with  a  more  rapid  current  in  that  part  of  the  river.  Actual  meas- 
urements as  to  the  rate  of  flow  in  the  arteries  cannot  be  made  on 
man,  but  from  experiments  on  lower  animals  it  is  calculated  that 
in  the  human  carotid  the  blood  flows  about  400  millimeters 
(16  inches)  in  a  second.  In  the  capillaries  the  current  travels  only 
from  0.5  to  0.75  mm.  (-^  to  ^  inch)  in  a  second.  The  total  time 
taken  by  a  portion  of  blood  in  making  a  complete  circulation  has 
been  measured  by  injecting  some  easily  detected  substance  into 
an  artery  on  one  side  of  the  body  and  noting  the  time  which 
elapses  before  it  can  be  found  in  a  corresponding ,  vein  on  the 
opposite  side.  In  dogs  this  time  is  15  seconds,  and  it  is  calcu- 
lated for  man  at  about  23  seconds.  Of  this  total  about 
a  second  is  spent  in  the  systemic  and  another  second  in  the 
pulmonary  capillaries,  as  each  portion  of  blood  on  its  course 
from  the  last  artery  to  the  first  vein  passes  through  a  length  of 
capillary  which  on  the  average  is  0.5  mm.  (^  inch).  The  rate  of 
flow  in  the  great  veins  is  about  100  mm.  (4  inches)  in  a  second, 
but  is  subject  to  considerable  variations  dependent  on  the  respira- 
tory and  other  movements  of  the  Body;  in  the  small  veins  it  is 
much  slower. 

Secondary  Factors  Affecting  the  Circulation.  While  the  heart's 
beat  is  the  great  driving  force  of  the  circulation,  certain  other 
things  help  more  or  less — viz.,  gravity,  compression  of  the  veins, 
and  aspiration  of  the  thorax.  All  of  them  are,  however,  quite 
subsidiary;  experiment  on  the  dead  Body  shows  that  the  injection 
of  defibrinated  blood  into  the  aorta  under  a  less  force  than  that 
exerted  by  the  left  ventricle  during  life  is  more  than  sufficient 
to  drive  it  round  and  back  by  the  vena  cavse. 

The  Influence  of  Gravity.  Under  ordinary  circumstances  this 
may  be  neglected,  since  in  parts  of  the  Body  below  the  level  of 
the  heart  it  will  assist  the  flow  in  the  arteries  and  impede  it  equally 
in  the  veins,  while  the  reverse  is  the  case  in  the  upper  parts  of  the 
Body.  In  certain  cases,  however,  it  is  well  to  bear  these  points  in 
mind.  A  part  "  congested  "  or  gorged  with  blood  should  if  possi- 
ble be  raised  so  as  to  make  the  back-flow  in  its  veins  easier;  and 
sometimes  when  the  heart  is  acting  feebly  it  may  be  able  to  drive 
blood  along  arteries  in  which  gravity  helps,  but  not  otherwise. 
Accordingly  in  a  tendency  to  fainting  it  is  best  to  lie  down,  and 


370  THE  HUMAN  BODY 

make  it  easier  for  the  heart  to  send  blood  up  to  the  brain,  defi- 
ciency in  its  blood-supply  being  the  cause  of  the  loss  of  conscious- 
ness in  a  fainting-fit.  '  In  fact,  so  long  as  the  breathing  continues, 
the  aspiration  of  the  thorax  will  keep  up  the  venous  flow  (see 
below),  while,  in  the  circumstances  supposed,  a  slight  diminution 
in  the  resistance  opposed  to  the  arterial  flow  may  be  of  impor- 
tance. The  head  of  a  person  who  has  fainted  should  accordingly 
never  be  raised  until  he  has  undoubtedly  recovered,  a  fact  rarely 
borne  in  mind  by  spectators,  who  commonly  rush  at  once  to  lift 
any  one  whom  they  see  fall  in  the  street  or  elsewhere. 

The  Influence  of  Transient  Compression  of  the  Veins.  The 
valves  of  the  veins  being  so  disposed  as  to  permit  only  a  flow 
towards  the  heart,  when  external  pressure  empties  a  vein  it  assists 
the  circulation.  Continuous  pressure,  as  by  a  tight  garter,  is  of 
course  bad,  since  it  checks  all  subsequent  flow  through  the  vessel; 
but  intermittent  pressure,  such  as  is  exerted  on  many  veins  by 
muscles  in  the  ordinary  movements  of  the  Body,  acts  as  a  pump 
to  force  on  the  blood  in  them. 

The  value  of  this  pumping  of  the  blood  out  of  the  veins  by 
muscular  movements  is  well  illustrated  by  comparing  two  classes 
of  workers  whose  occupations  require  that  they  be  upon  their 
feet  continuously  for  hours.  The  condition  of  varicose  veins, 
which  is  a  stasis  of  blood  in  the  superficial  veins  of  the  lower 
extremities,  is  very  prevalent  among  motormen,  and  others  who 
must  stand  still  for  long  periods,  but  is  virtually  unknown  among 
postmen,  who  are  walking  during  the  time  spent  on  their  feet. 

The  valves  of  the  veins  have  another  use  in  diminishing  the 
pressure  on  the  lower  part  of  those  vessels  in  many  regions.  If, 
for  instance,  there  were  no  valves  in  the  long  saphenous  vein  of 
the  leg  the  considerable  weight  of  the  column  of  blood  in  it, 
which  in  the  erect  position  would  be  about  a  meter  (39  inches) 
high,  would  press  on  the  lower  part  of  the  vessel.  But  each  set  of 
valves  in  it  carries  the  weight  of  the  column  of  blood  between  it 
and  the  next  set  of  valves  above,  and  relieves  parts  below,  and 
so  the  weight  of  the  column  of  blood  is  distributed  and  does  not 
all  bear  on  any  one  point. 

Aspiration  of  the  Thorax  (see  also  p.  399).  Whenever  a  breath 
is  drawn  the  pressure  of  the  air  on  the  vessels  inside  the  chest  is 
diminished,  while  that  on  the  other  vessels  of  the  Body  is  unaf- 


THE  CIRCULATION  OF  THE  BLOOD  371 

fected.  In  consequence  blood  tends  to  flow  into  the  chest.  It 
cannot,  however,  flow  back  from  the  arteries  on  account  of  the 
semilunar  valves  of  the  aorta,  but  it  can  readily  be  pressed,  or  in 
common  language  " sucked,"  into  the  great  veins  close  to  the 
heart  and  into  the  right  auricle  of  the  latter.  The  details  of  this 
action  must  be  omitted  until  the  respiratory  mechanism  has  been 
considered.  All  parts  of  the  pulmonary  circuit  being  within  the 
thorax,  the  respiratory  movements  do  not  directly  influence  it, 
except  in  so  far  as  the  distention  or  collapse  of  the  lungs  alters 
the  caliber  of  their  vessels. 

The  considerable  influence  of  the  respiratory  movements  upon 
the  venous  circulation  can  be  readily  observed.  In  thin  persons 
the  jugular  vein  in  the  neck  can  often  be  seen  to  empty  rapidly 
and  collapse  during  inspiration,  and  fill  up  in  a  very  noticeable 
way  during  expiration,  exhibiting  a  sort,  of  venous  pulse.  Every 
one,  too,  knows  that  by  making  a  violent  and  prolonged  expira- 
tion, as  exhibited  for  example  by  a  child  with  whooping-cough, 
the  flow  in  all  the  veins  of  the  head  and  neck  may  be  checked, 
causing  them  to  swell  up  and  hinder  the  capillary  circulation  until 
the  person  becomes  "black  in  the  face/'  from  the  engorgement  of 
the  small  vessels  with  dark-colored  venous  blood. 

In  diseases  of  the  tricuspid  valve  another  form  of  venous  pulse 
is  often  seen  in  the  superficial  veins  of  the  neck,  since  at  each 
contraction  of  the  right  ventricle  some  blood  is  driven  back 
through  the  right  auricle  into  the  veins. 

Proofs  of  the  Circulation  of  the  Blood.  The  ancient  physiolo- 
gists believed  that  the  movement  of  the  blood  was  an  ebb  and 
flow,  to  and  from  each  side  of  the  heart,  and  out  and  in  by  both 
arteries  and  veins.  They  had  no  idea  of  a  circulation,  but  thought 
pure  blood  was  formed  in  the  lungs  and  impure  in  the  liver,  and 
that  these  partially  mixed  in  the  heart  through  minute  pores  sup- 
posed to  exist  in  the  septum.  Servetus,  who  was  burnt  alive  by 
Calvin  in  1553,  first  stated  that  there  was  a  continuous  passage 
through  the  lungs  from  the  pulmonary  artery  to  the  pulmonary 
veins,  but  the  great  Englishman  Harvey  first,  in  lectures  delivered 
in  the  College  of  Physicians  of  London  about  1616,  demonstrated 
that  the  movement  of  the  blood  was  a  continuous  circulation  as 
we  now  know  it,  and  so  laid  the  foundation  of  modern  Physi- 
ology. In  his  time,  however,  the  capillary  vessel3  tad  not  been 


372  THE  HUMAN  BODY 

discovered,  so  that  although  he  was  quite  certain  that  the  blood 
got  somehow  from  the  final  branches  of  the  aorta  to  the  radicles 
of  the  -venous  system,  he  did  not  exactly  know  how. 

The  proofs  of  the  course  of  the  circulation  are  at  present  quite 
conclusive,  and  may  be  summed  up  as  follows:  (1)  Blood  injected 
into  an  artery  in  the  dead  Body  will  return  by  a  vein;  but  injected 
into  a  vein  will  not  pass  back  by  an  artery.  (2)  The  anatomical 
arrangement  of  the  valves  of  the  heart  and  of  the  veins  shows 
that  the  blood  can  only  flow  from  the  heart,  through  the  arteries 
and  back  to  the  heart  by  the  veins.  (3)  A  cut  artery  spurts  from 
the  end  next  the  heart,  a  cut  vein  bleeds  most  from  the  end 
farthest  from  the  heart.  (4)  A  portion  of  a  vein  when  emptied 
fills  only  from  the  end  farthest  from  the  heart.  This  observation 
can  be  made  on  the  veins  on  the  back  of  the  hand  of  any  thin 
person,  especially  if  the  vessels  be  first  gorged  by  holding  the 
hand  in  a  dependent  position  for  a  few  seconds.  Select  then  a 
vein  which  runs  for  an  inch  or  so  without  branching,  place  a  finger 
on  its  distal  end,  and  then  empty  it  up  to  its  next  branch  (where 
valves  usually  exist)  by  compressing  it  from  below  up.  The  ves- 
sel will  then  be  found  to  remain  empty  as  long  as  the  finger  is  kept 
on  its  lower  end,  but  to  fill  immediately  when  it  is  removed; 
which  proves  that  the  valves  prevent  any  filling  of  the  vein  from 
its  -heart-end  backwards.  (5)  If  a  bandage  be  placed  around  the 
arm,  so  as  to  close  the  superficial  veins,  but  not  tight  enough  to 
occlude  the  deeper-seated  arteries,  the  veins  on  the  distal  side 
of  the  bandage  will  become  gorged  and  those  on  its  proximal  side 
empty,  showing  again  that  the  veins  only  receive  blood  from  their 
ends  turned  towards  the  capillaries.  (6)  In  the  lower  animals 
direct  observation  with  the  microscope  shows  the  steady  flow 
of  blood  from  the  arteries  through  the  capillaries  to  the  veins,  but 
never  in  the  opposite  direction. 


CHAPTER  XXII 

THE  VASOMOTOR  MECHANISM.     SLEEP.     THE  LYMPHATIC 

SYSTEM 

The  Distribution  of  Blood  Among  Various  Parts  of  the  Body. 

In  the  nervous  control  of  the  heart-beat  we  have,  as  already 
noted,  a  mechanism  whereby  the  blood-flow  through  the  Body  as 
a  whole  can  be  modified  in  accordance  with  the  needs  of  the 
organism.  In  the  vasomotor  mechanism  we  have  an  arrangement, 
equally  important,  whereby  individual  organs  or  regions  can  be 
furnished  with  more  or  less  blood  as  their  activities  require  with- 
out the  necessity  of  involving  the  whole  circulation. 

The  Nerves  of  the  Blood- Vessels.  The  arteries,  as  already 
pointed  out,  possess  a  muscular  coat  composed  of  fibers  arranged 
around  them,  so  that  their  contraction  can  narrow  the  vessels. 
This  coat  is  most  prominent  in  the  smaller  vessels,  the  arterioles. 
These  vascular  muscles  are  under  the  control  of  certain  special 
nerves  called  vasomotor,  and  these  latter  can  thus  govern  the 
amount  of  blood  reaching  any  organ  at  a  given  time.  The  vaso- 
motor nerves  belong  to  the  autonomic  system.  Their  physi- 
ology is  therefore  the  application  to  special  structures  of  the 
general  principles  laid  down  in  connection  with  that  system 
(Chap.  XII). 

In  the  heart  we  had  to  consider  a  rhythmically  contracting 
organ  the  force  of  whose  contractions  could  be  increased  or  dimin- 
ished by  the  influence  of  extrinsic  nerves;  in  the  arteries,  speak- 
ing broadly,  we  have  to  deal  with  muscle  in  a  condition  of  tonic 
or  constant  contraction,  which  contraction  can  be  increased  by 
impulses  coming  through  excitor  or  vasoconstrictor  nerves,  and 
diminished  through  the  activity  of  inhibitory  or  vasodilator 
nerves.  The  general  tonic  contraction  of  the  arterial  muscle  is, 
however,  much  more  dependent  on  the  vasoconstrictor  nerve- 
fibers  than  is  the  beat  of  the  heart  on  the  cardio-excitor  nerves. 
The  inhibitory  (dilator)  set  of  vasomotor  nerves  have  a  much  less 
extensive  distribution  over  the  arterial  system  than  the  constrictor. 

373 


374  THE  HUMAN  BODY 

The  Vasoconstrictor  Nerves.  If  the  ear  of  a  white  rabbit  be 
held  up  against  the  light  while  the  animal  is  kept  quiet  and  not 
alarmed,  the  red  central  artery  can  be  seen  coursing  along  the 
translucent  organ,  giving  off  branches  which  by  subdivision  be- 
come too  small  to  be  separately  visible,  and  the  whole  ear  has  a 
pink  color  and  is  warm  from  the  abundant  blood  flowing  through 
it.  Attentive  observation  will  show  also  that  the  caliber  of  the 
main  artery  is  not  constant;  at  somewhat  irregular  periods  of  a 
minute  or  more  it  dilates  and  contracts  a  little. 

If  the  sympathetic  trunk  have  been  previously  divided  on  the 
other  side  of  the  neck  of  the  animal,  the  ear  on  that  side  will  pre- 
sent a  very  different  appearance.  Its  arteries  will  be  much  dilated 
and  the  whole  ear  fuller  of  blood,  redder,  and  distinctly  warmer; 
the  slow  alternating  variations  in  arterial  diameter  also  have 
disappeared.  We  get  thus  evidence  that  the  normal  mean  caliber 
of  the  artery  is  maintained  by  influences  reaching  its  muscular 
coat  through  the  cervical  sympathetic.  Stimulation  of  the  upper 
end  of  the  cut  nerve  confirms  this  opinion.  It  is  then  seen  that 
the  arteries  of  the  corresponding  ear  gradually  contract  until 
even  the  main  vessel  can  hardly  be  seen,  and  in  consequence  the 
whole  ear  becomes  pale  and  cold.  After  the  stimulation  is  stopped 
the  arteries  again  slowly  dilate  until  they  have  regained  their 
full  paralytic  size. 

Quite  similar  phenomena  can  be  observed  in  transparent  parts 
of  other  living  animals,  as  in  the  web  of  a  frog's  foot,  the  arteries 
of  which  dilate  after  section  of  the  sciatic  nerve  and  constrict 
when  the  peripheral  end  of  the  nerve  is  stimulated.  In  the  case 
of  9ther  parts  changes  in  temperature  may  be  used  to  detect 
alterations  in  the  flow  of  blood.  In  a  dog  or  cat,  for  example,  a 
sensitive  thermometer  placed  between  the  toes  indicates  a  rise 
of  temperature,  owing  to  increased  flow  of  warm  blood  through 
the  skin,  after  section  of  the  chief  nerve  of  the  limb,  and  a  fall  of 
temperature  (usually)  .during  stimulation  of  the  peripheral  end 
of  the  divided  nerve. 

When  the  vasoconstrictor  nerves  cut  are  those  controlling  a 
large  number  of  arteries,  the  dilatation  of  the  latter  so  much 
diminishes  peripheral  resistance  to  the  blood-flow  as  to  lead  to  a 
marked  fall  of  general  arterial  pressure;  and,  due  care  being  taken 
to  avoid  or  to  allow  for  concomitant  variations  in  the  rate  or 


THE  VASOMOTOR  MECHANISM  375 

force  of  the  heart's  beat,  this  gives  us  another  useful  method  of 
studying  the  distribution  of  the  nerves  concerned.  For  example, 
the  splanchnic  nerves  are  branches  which  spring  from  the  thoracic 
portion  of  the  sympathetic  chain  and  pass  through  the  diaphragm 
to  end  in  the  solar  plexus  from  which  nerves  pass  to  the  arteries 
of  most  of  the  abdominal  viscera.  The  region  whose  blood-vessels 
are  innervated  by  these  nerves  is  often  spoken  of  as  the  splanchnic 
region.  When  the  splanchnic  nerves  are  cut  on  both  sides  arterial 
pressure  falls  enormously,  from  say  120  millimeters  of  mercury  in 
the  carotid  of  a  dog  to  15  or  20  millimeters,  most  of  the  blood  of 
the  Body  lying  almost  stagnant  in  the  dilated  blood-vessels  of 
the  abdomen.  On  the  other  hand,  stimulation  of  the  splanchnic 
nerves  so  diminishes  the  paths  open  for  the  circulation  of  the 
blood  as  to  increase  general  blood-pressure  enormously. 

The  skin  and  the  abdominal  organs  seem  to  be  the  predominant 
localities  of  distribution  of  the  vasoconstrictor  nerves:  other 
parts  have  them,  but  not  in  quantity  sufficient  to  bring  about 
any  great  general  change  in  the  blood-flow. 

The  Vasoconstrictor  Center.  This,  one  of  the  most  important 
of  the  " vital"  centers  of  the  medulla,  has  not  been  identified 
anatomically  with  any  particular  group  of  nerve-cells,  but  its 
location  is  quite  sharply  denned  physiologically.  There  is  a  small 
region  of  the  medulla,  known  as  the  "vital  knot,"  whose  destruc- 
tion is  promptly  fatal  to  the  life  of  the  organism.  This  region 
includes,  in  addition  to  at  least  one  other  "center,"  the  vaso- 
constrictor center.  From  this  center  there  is  a  constant  outflow 
of  impulses  to  all  those  arterioles  of  the  Body  whose  muscles 
contain  vasoconstrictor  nerve-endings.  This  constant  stream  of 
constrictor  impulses  is  the  chief  factor  in  the  maintenance  of  so- 
called  vasomotor  tone,  a  condition  of  continuous  moderate  con- 
striction of  the  arterioles  by  which  general  arterial  pressure  is  kept 
at  the  proper  level. 

It  is  probable  that  the  vasoconstrictor  center  consists  physi- 
ologically of  a  number  of  associated  centers  which  may  act  as  a 
unit  or  separately.  These  "partial"  centers  are  in  connection 
with  restricted  vasomotor  areas,  and  thus  are  enabled  to  bring 
about  local  vasomot  3r  effects. 

The  Control  of  the  Vasoconstrictor  Center.  This  center,  like 
the  other  "vital  '  centers  of  the  medulla,  is  kept  in  activity  in 


376  THE  HUMAN  BODY 

part  reflexly,  and  in  part,  probably,  through  chemical  stimulation 
brought  by  way  of  the  blood.  The  whole  stream  of  afferent  im- 
pulses passing  through  the  medulla  plays  upon  it.  Like  the  centers 
for  controlling  the  heart-beat  its  activity  may  be  increased  through 
the  influx  of  stimuli  into  it,  or  it  may  suffer  depression  for  the 
same  cause.  We  divide  afferent  impulses  affecting  the  vasocon- 
strictor center,  therefore,  into  two  groups:  those  increasing  its 
activity,  pressor  impulses,  and  those  diminishing  it,  depressor 
impulses.  Certain  sorts  of  stimuli  are  generally  pressor  in  effect; 
pain,  for  example,  usually  brings  about  a  reflex  rise  of  blood- 
pressure  through  stimulating  the  vasoconstrictor  center;  cold  on 
the  skin  acts  similarly.  It  is  possible  that  other  stimuli  may  be 
pressor  or  depressor  according  to  circumstances. 

The  Depressor  Nerve.  The  best  known  nerve-tract  which 
carries  depressor  impulses  uniformly  has  already  been  mentioned. 
It  is  the  afferent  tract  from  the  heart  known,  in  animals  where  it 
is  present  as  a  separate  trunk,  as  the  depressor  nerve.  Stimulation 
of  this  nerve  brings  about,  always,  a  reflex  fall  of  blood-pressure, 
which  is  due  mainly  to  vasodilation  resulting  from  depression  of 
the  vasoconstrictor  center.  This  nerve  rises,  not  in  heart  tissue 
proper,  but  in  the  walls  of  the  aorta  near  where  that  vessel  springs 
from  the  heart.  An  undue  increase  in  blood-pressure,  such  as 
might  affect  the  heart  injuriously,  subjects  the  aortic  wall  to  un- 
usual tension.  This  seems  to  stimulate  the  depressor  nerve  me- 
chanically. Thus  the  heart  is  protected  against  injury  arising 
from  working  against  too  great  resistance. 

Taking  Cold.  This  common  condition  is  not  unfrequently  the 
indirect  result  of  undue  reflex  excitement  of  the  vasomotor  center. 
Chilling  of  the  skin  beyond  a  certain  point  stimulates,  through  the 
afferent  nerves,  the  portion  of  the  vasomotor  center  governing  the 
skin  arteries,  and  the  latter  become  contracted,  as  shown  by  the 
pallor  of  the  surface.  This  has  a  twofold  influence — in  the  first 
place,  more  blood  is  thrown  into  internal  parts,  and  in  the  second, 
contraction  of  the  arteries  over  so  much  of  the  Body  considerably 
raises  the  general  blood-pressure.  Consequently  the  vessels  of 
internal  parts  become  overgorged  or  "congested,"  a  condition 
which  is  especially  favorable  to  invasion  by  the  organisms  which 
cause  colds.  The  best  preventive  is  to  wear,  when  exposed  to 
great  changes  of  temperature,  a  woolen  or  at  least  a  cotton  gar- 


THE  VASOMOTOR  MECHANISM  377 

ment  over  the  trunk  of  the  Body;  linen  is  so  good  a  conductor  of 
heat  that  it  permits  any  change  in  the  external  temperature  to 
act  almost  at  once  upon  the  surface  of  the  Body.  After  an  un- 
avoidable exposure  to  cold  or  wet  the  thing  to  be  done  is  of  course 
to  restore  the  cutaneous  circulation;  for  this  purpose  movement 
should  be  persisted  in,  and  a  thick  dry  outer  covering  put  on,  until 
warm  and  dry  underclothing  can  be  obtained. 

For  healthy  persons  a  temporary  exposure  to  cold,  as  a  plunge 
in  a  bath,  is  good,  since  in  them  the  sudden  contraction  of  the 
cutaneous  arteries  soon  passes  off  and  is  succeeded  by  a  dilatation 
causing  a  warm  healthy  glow  on  the  surface.  If  the  bather  remain 
too  long  in  cold  water,  however,  this  reaction  passes  off  and  is  suc- 
ceeded by  a  more  persistent  chilliness  of  the  surface,  which  may 
even  last  all  day.  The  bath  should  therefore  be  left  before  this 
occurs,  but  no  absolute  time  can  be  stated,  as  the  reaction  is  more 
marked  and  lasts  longer  in  strong  persons,  and  in  those  used  to 
cold  bathing,  than  in  others. 

Vasodilator  Nerves.  We  have  already  noticed,  in  connection 
with  the  control  of  the  vasoconstrictor  center,  one  method  by 
which  dilation  of  arterioles  may  be  secured,  namely,  by  inhibition 
of  the  tonic  activity  of  vasoconstrictor  fibers.  Frequently,  how- 
ever, in  the  Body  this  is  managed  in  another  way;  by  efferent 
vasodilator  nerves  which  inhibit,  not  the  vasoconstrictor  center, 
but  the  muscles  of  the  blood-vessels  directly.  The  nerves  of  the 
skeletal  muscles  for  example  contain  two  sets  of  efferent  fibers :  one 
motor  proper  and  the  other  vasodilator.  When  the  muscle  contracts 
in  a  reflex  action  or  under  the  influence  of  the  will  both  sets  of 
fibers  are  excited;  so  that  when  the  organ  is  set  at  work  its  arteries 
are  simultaneously  dilated  and  more  blood  flows  through  it. 
But  if  the  animal  have  previously  administered  to  it  such  a  dose 
of  curare  as  just  to  throw  out  of  function  the  true  motor-fibers, 
stimulation  of  the  nerve  produces  dilation  of  the  arteries  with- 
out a  corresponding  muscular  contraction.  Quite  a  similar  thing 
occurs  in  the  salivary  glands.  Their  cells,  which  form  the  saliva, 
are  aroused  to  activity  by  special  nerve-fibers;  but  the  gland-nerve 
also  contains  a  quite  distinct  set  of  vasodilator  fibers  which  nor- 
mally cause  a  simultaneous  dilation  of  the  gland-artery,  though 
either  can  be  artificially  stimulated  by  itself  and  produce  its 
effect  alone. 


378  THE  HUMAN  BODY 

Since  the  effect  of  stimulating  vasodilator  nerves  is  the  same 
as  inhibiting  the  constrictor  mechanism  we  might  ask  why  there 
should  be  two  distinct  means  thus  provided  for  securing  the  same 
result.  As  a  matter  of  fact  the  two  mechanisms  do  not  seem  to 
overlap  to  any  great  extent;  they  rather  supplement  each  other. 
The  vasoconstrictor  mechanism  is  confined,  in  the  main,  to  the 
blood-vessels  of  the  skin  and  viscera;  the  dilator  mechanism  is 
distributed  chiefly  to  the  muscles,  the  glands,  and  the  genital 
organs. 

Through  such  arrangements  the  distribution  of  the  blood  in  the 
Body  at  any  moment  is  governed :  so  that  working  parts  shall  have 
abundance  and  other  parts  less,  while  at  the  same  time  the  general 
arterial  pressure  remains  the  same  on  the  average;  since  the  ex- 
pansion of  a  few  small  local  branches  but  little  influences  the  total 
peripheral  resistance  in  the  vascular  system.  Moreover,  com- 
monly when  one  set  of  organs  is  at  work  with  its  vessels  dilated, 
others  are  at  rest  with  their  arteries  comparatively  contracted, 
and  so  a  general  average  blood-pressure  is  maintained.  Few  per- 
sons, for  example,  feel  inclined  to  do  brain-work  after  a  heavy  meal; 
for  then  a  great  part  of  the  blood  of  the  whole  Body  is  led  off  into 
the  dilated  vessels  of  the  digestive  organs,  and  the  brain  gets  a 
smaller  supply. 

The  Vasodilator  Center.  There  is  reason  to  believe  that  the 
vasodilator  nerves  are  under  the  control  of  a  center  in  the  medulla, 
which  is  in  turn  subject  to  the  influence  of  afferent  impulses  of 
various  sorts.  The  exact  location  of  this  center  has  not  been  de- 
termined. So  far  as  can  be  judged  from  observation  of  vaso- 
dilator phenomena  the  vasodilator  center  is  probably  not  in  con- 
stant tonic  activity,  as  is  the  constrictor  center,  but  is  aroused  to 
activity  only  when  afferent  stimuli  come  to  it  from  certain  par- 
ticular regions. 

The  Relation  of  Vasomotor  Tone  to  Cerebral  Activity.  The 
circulation  through  the  brain  differs  in  some  important  respects 
from  that  of  the  rest  of  the  Body.  The  differences  arise  from  the 
fact  that  the  brain,  a  fluid  and  therefore  incompressible  mass,  is 
inclosed  in  an  unyielding  receptacle,  the  skull,  which  it  fills  com- 
pletely. The  result  is  that  the  cerebral  blood-vessels  occupy  their 
allotted  space,  which  cannot  be  either  increased  or  diminished  ap- 
preciably. The  total  volume  of  blood  in  the  brain  at  any  time  is 


THE  VASOMOTOR  MECHANISM  379 

therefore  practically  constant,  and  the  circulation  through  the 
brain  can  only  be  altered  by  changing  the  rate  at  which  the  blood 
flows  through  it.  In  such  an  arrangement  as  this,  where  local 
vasodilation  cannot  occur,  the  only  way  in  which  the  rate  of 
blood-flow  can  be  altered  is  by  changes  in  the  pressure  at  which 
the  blood  is  forced  into  the  region.  The  arteries  feeding  the  brain 
spring  directly  from  the  aorta;  it  follows,  therefore,  that  variations 
in  aortic  pressure,  in  other  words,  in  general  blood-pressure,  are 
reflected  exactly  in  the  rate  of  cerebral  blood-flow. 

General  blood-pressure,  as  we  have  seen,  is  maintained  by  vaso- 
motor  tone,  the  state  of  moderate  constriction  of  arterioles  gener- 
ally. Variations  in  the  tone  of  restricted  areas,  such  as  occur  in 
connection  with  the  functioning  of  individual  organs,  do  not  or- 
dinarily affect  general  blood-pressure  enough  to  alter  the  circula- 
tion through  the  brain  to  any  extent. 

There  is  good  evidence  that  the  degree  of  activity  of  the  cells  of 
the  cerebral  cortex  is  directly  and  immediately  dependent  upon 
the  rate  of  blood-flow  through  the  organ.  A  rapid  circulation 
means  alertness  and  efficiency  of  mental  processes;  as  the  flow 
becomes  slower  and  slower  the  cells  work  less  and  less  actively; 
when  a  certain  point  of  sluggishness  is  reached  consciousness  dis- 
appears, the  ce/ls,  if  not  altogether  quiescent,  working  too  freely 
to  arouse  that  state. 

The  phenomenon  of  fainting,  which  has  already  been  men- 
tioned, is  the  result  usually  of  a  sudden  inhibition  of  the  vaso- 
constrictor center  whereby  over  a  large  area,  the  whole  splanchnic 
region,  for  instance,  there  is  general  vasodilation  and  a  resulting 
fall  in  blood-pressure.  The  rate  of  cerebral  blood-flow  falls  to  a 
point  below  that  required  for  the  maintenance  of  consciousness 
and  the  individual  falls  in  a  faint. 

Sleep.  This  periodic  loss  of  consciousness,  so  important  for  the 
proper  restoration  of  the  fatigued  organs  and  tissues  of  the  Body, 
has  been  the  subject  of  considerable  attention  and  investigation. 
Its  explanation  is  not  simple,  involving  as  it  does  a  number  of 
questions,  as,  for  instance,  why  fatigue,  which  ordinarily  induces 
sleep,  may,  if  extreme,  prevent  it;  and  what  it  is  that  causes  one 
to  awake  after  the  proper  number  of  hours  of  sleep. 

Objectively  sleep  is  marked  by  its  well-known  signs,  which  are 
not  very  instructive  as  to  its  cause,  and  also  by  certain  vaso- 


380  THE  HUMAN  BODY 

motor  changes  which  have  been  looked  upon  as  very  instructive; 
and  as  affording  us,  indeed,  our  only  satisfactory  method  of  study- 
ing sleep  experimentally.  Observations  upon  sleeping  individuals 
have  shown  that  normal  sleep  is  frequently  accompanied  by  a 
considerable  fall  in  general  blood-pressure,  resulting  from  exten- 
sive vasodilation.  This  is  itself  sufficient  to  account  for  the  dimin- 
ished cerebral  activity  with  its  accompanying  loss  of  conscious- 
ness which  constitutes  sleep,  and  many  physiologists  are  inclined 
to  believe,  therefore,  that  the  vasomotor  changes  may  form  the 
underlying  basis  for  the  phenomenon.  A  theory  which  expresses 
this  view  looks  upon  the  vasoconstrictor  center  as  the  controlling 
mechanism  of  sleep.  When  this  center  is  in  good  condition  the 
constant  stream  of  afferent  impulses  playing  upon  it  maintains  it 
in  strong  activity,  and  vasomotor  tone  is  kept  high.  With  the 
passage  of  hours  of  such  ceaseless  activity  the  center  becomes 
fatigued  and  tends  to  respond  less  strongly  to  the  afferent  impulses 
coming  to  it.  The  result  will  be  a  falling  off  of  vasomotor  tone, 
unless  by  an  effort  of  the  will  or  an  increase  in  the  stream  of  af- 
ferent impulses,  such  as  follows  muscular  exercise,  for  example, 
the  center  is  whipped  up  to  renewed  activity.  "Keeping  awake" 
when  one  is  sleepy  is,  according  to  this  view,  a  matter  of  stimu- 
lating the  tired  vasoconstrictor  center  to  continued  effort.  The 
effect  may  be  produced  by  an  artificial  stimulant,  such  as  coffee, 
or  by  an  act  of  the  will.  The  usual  preparations  for  sleep  are  such 
as  favor  diminished  activity  of  the  vasoconstrictor  center  by  les- 
sening the  afferent  impulses  coming  to  it.  Lying  in  a  comfort- 
able position  removes  most  of  the  impulses  of  muscle  sense;  by 
closing  the  eyes  visual  stimuli  are  gotten  rid  of.  Thus  unless  the 
center  is  so  irritable  that  the  small  stream  of  inevitable  afferent 
impulses  keeps  it  up  to  the  mark  the  essential  condition  for  sleep, 
loss  of  vasomotor  tone,  is  fulfilled.  The  act  of  waking,  according 
to  this  theory,  results  either  from  an  undue  stimulation  of  the 
vasoconstrictor  center,  as  when  one  is  waked  by  being  violently 
shaken,  or  from  a  gradual  restoration  of  the  irritability  of  the 
center  during  its  period  of  rest,  to  a  point  where  the  minimal 
stream  of  afferent  impulses,  inseparable  from  the  living  Body,  is 
sufficient  to  stimulate  it  to  the  maintenance  of  waking  vasomotor 
tone. 
It  must  be  admitted  that  not  all  experiments  upon  sleep  have 


THE  VASOMOTOR  MECHANISM  381 

shown  marked  loss  of  vasomotor  tone,  but  even  if  we  consider 
vasomotor  fatigue  the  primary  factor  we  must  grant,  of  course, 
that  there  are  numerous  additional  factors  modifying  sleep.  The 
condition  of  the  cerebral  cells  and  the  nature  of  their  activity 
doubtless  have  much  to  do  with  the  phenomenon.  These,  how- 
ever, are  factors  which  physiology  at  present  is  unable  to  analyze 
completely,  so  that  the  vasomotor  theory  affords  our  most  satisfac- 
tory explanation  of  sleep  from  the  physiological  standpoint. 

Adrenin.  The  effect  of  this  hormone  upon  the  vascular  system, 
as  stated  previously  (Chap.  XII),  is  to  stimulate  the  vaso- 
constrictor fibers  at  their  terminations  in  the  muscles  of  the  ar- 
terioles.  The  constant  presence  of  this  hormone  in  the  blood  is 
probably  an  important  factor  in  maintaining  that  degree  of  vaso- 
motor tone  upon  which  the  well-being  of  the  Body  depends.  The 
great  outpouring  of  adrenin  into  the  blood  under  emotional  stress 
so  much  increases  the  constriction  of  the  blood  vessels  in  the  skin 
and  the  splanchnic  area  as  to  produce  a  pronounced  rise  in  blood- 
pressure,  with  a  correspondingly  augmented  cerebral  circulation. 
The  same  influence  acts  to  divert  the  blood  largely  from  these 
regions  to  the  skeletal  muscles.  The  vessels  of  these  latter  being 
unprovided  with  vasoconstrictor  fibers  are  not  involved  in  the 
adrenin  effect.  Since  the  brain  and  the  skeletal  muscles  are  the 
regions  specially  in  need  of  adequate  nourishment  in  crises  the 
adaptive  character  of  this  reaction  is  obvious.  The  substance 
adrenin  as  used  experimentally  shows  several  striking  characteris- 
tics. In  the  first  place  a  very  small  concentration  of  it  (one  part 
in  ten  thousand),  introduced  into  a  capillary  region,  brings  about 
so  strong  a  constriction  in  the  immediate  neighborhood  as  to  stop 
the  flow  of  blood  completely  through  that  region.  It  is  possible 
thus  to  prevent  troublesome  bleeding  in  small  operations.  The 
effect  of  adrenin  used  in  this  way  is,  however,  very  transient;  re- 
peated injections  are  necessary  to  maintain  the  constricted  state. 

The  Lymphatics.  The  living  cells  of  the  Body,  as  previously 
pointed  out  (Chap.  XVII),  are  bathed  in  lymph,  a  liquid  derived 
from  the  blood  and  serving  as  the  intermediary  by  which  inter- 
changes of  food  materials,  gases  and  waste  substances  between 
it  and  the  cells  are  carried  on.  At  the  same  place  it  was  shown 
that  there  is  a  continuous  movement  of  liquid  from  the  blood  into 
the  lymph  spaces,  necessitating  a  system  whereby  the  accumu- 


382  THE  HUMAN  BODY 

lation  can  be  drained  away  from  the  tissues  and  carried  back  to 
the  blood.  This  drainage  is  afforded  by  the  lymphatic  system.  At 
its  beginning  this  system  is  without  definite  structure,  consisting 
simply  of  intercellular  spaces.  These  communicate  with  one  an- 
other, and  at  intervals  with  minute  vessels  having  definite  walls. 
These  latter  are  the  beginnings  of  definite  lymph-channels. 

The  Structure  of  Lymph- Vessels.  The  smallest  lymph-vessels 
proper  are  the  lymph-capillaries;  tubes  rather  wider  than  the 
blood-capillaries,  but  like  them  having  a  wall  consisting  of  a  single 
layer  of  flattened  epithelium  cells.  The  cells  have,  however,  a 
wavy  margin  and  are  not  as  a  rule  much  longer  in  one  diameter 
than  another,  in  both  of  which  respects  they  differ  from  the  cells  of 
the  corresponding  blood-vessels.  In  some  regions,  as  in  many 
glands,  the  lymph-capillaries  are  much  dilated  and  form  irregular 
lymph  lacunw,  everywhere  bounded  by  their  peculiar  wavy  cells, 
lying  in  the  interstices  of  organs;  and  sometimes  they  form  tubes 
around  small  blood-vessels,  as  in  the  brain  (perivascular  lymph- 
channel).  In  some  places  they  commence  by  blind  ends  as  in  the 
lacteal  vessels  of  the  villi  of  the  small  intestine  (Fig.  131)  which 
are  lymph-capillaries;  but  usually  they  branch  and  join  to  form 
networks.  Lymph  from  the  intercellular  spaces  enters  them 
(probably  by  passing  through  their  boundary  cells)  and  is  passed 
on  to  larger  vessels  which  much  resemble  veins  of  corresponding 
size,  having  the  same  three  coats,  and  being  abundantly  provided 
with  valves. 

The  Thoracic  Duct.  The  lymph-vessels  proceeding  from  the 
capillaries  in  various  organs  become  larger  and  fewer  by  joining 
together,  and  all  end  finally  in  two  main  trunks  which  open  into 
the  venous  system  on  the  sides  of  the  neck,  at  the  point  of  junction 
of  the  jugular  and  subclavian  veins.  The  trunk  on  the  right  side 
is  much  smaller  than  the  other  and  is  known  as  the  "right  lymphatic 
duct.11  It  collects  lymph  from  the  right  side  of  the  thorax,  from 
the  right  side  of  the  head  and  neck,  and  the  right  arm.  The  lymph 
from  all  the  rest  of  the  Body  is  collected  into  the  thoracic  duct.  It 
commences  at  the  upper  part  of  the  abdominal  cavity  in  a  dilated 
reservoir  (the  receptaculum  chyli),  into  which  the  lacteals  from  the 
intestines,  and  the  lymphatics  of  the  rest  of  the  lower  part  of  the 
Body,  open.  From  thence  the  thoracic  duct,  receiving  tributaries 
on  its  course,  runs  up  the  thorax  alongside  of  the  aorta  and,  pass- 


THE  VASOMOTOR  MECHANISM  383 

ing  on  into  the  neck,  ends  on  the  left  side  at  the  point  already  indi- 
cated; receiving  on  its  way  the  main  stems  from  the  left  arm  and 
the  left  side  of  the  head  and  neck.  The  thoracic  duct,  thus,  brings 
back  to  the  blood  much  more  lymph  than  the  right  lymphatic  duct. 

Lymph-Nodes.  At  intervals  along  the  course  of  various  lym- 
phatic vessels  are  structures  consisting  of  cells  so  arranged  as  to 
leave  interspaces  among  them,  through  which  interspaces  the 
lymph  is  forced  to  flow.  These  structures  are  the  lymph-nodes 
or  lymph-glands  and  the  peculiar  tissue  of  which  they  are  com- 
posed is  lymphoid  or  adenoid  tissue.  Lymph-nodes  occur  in  the 
neck,  the  groin,  the  axilla  (arm  pit)  and  in  various  other  regions 
of  the  Body.  Certain  structures  in  the  wall  of  the  small  intestine 
near  its  lower  end,  the  so-called  Peyer's  Patches,  are  composed  of 
lymphoid  tissue  as  are  also  the  structures  in  the  throat  making 
up  the  tonsillar  ring. 

Functions  of  Lymph-Nodes.  Two  quite  different  functions 
are  attributed  to  the  lymph-nodes.  The  first  of  these  is  that 
previously  mentioned  (p.  304)  of  serving  as  the  seat  of  lymph- 
ocyte production. 

The  lymph-nodes  have  also  the  additional  function  of  filtering 
the  lymph  that  passes  through  them.  This  filtering  action  is 
probably  of  great  importance  in  confining  micro-organisms  to 
the  region  which  they  first  enter,  since  if  they  get  into  the  lymph 
stream  they  are  arrested  at  the  first  lymph-node.  It  is  thought 
that  the  lymph-nodes  are  able  also  to  arrest,  for  a  time  at  least, 
the  spread  of  cancer-cells  over  the  Body.  The  lymph-nodes  located 
on  the  channels  draining  the  lungs  become  filled  with  dust  that 
has  worked  its  way  through  the  pulmonary  walls  into  the  lymph, 
and  that  is  prevented  thus  from  spreading  throughout  the  Body. 

Tonsils  and  Adenoids.  The  irregular  ring  of  lymphoid  tissue 
surrounding  the  throat  was  referred  to  above.  This  at  the  front 
shows  two  enlargements,  one  on  each  side,  known  as  the  tonsils. 
At  the  back  of  the  throat  this  same  ring  often  in  children  becomes 
enlarged  by  overgrowth  until  it  obstructs  the  nasal  passage  and 
interferes  with  the  breathing.  It  may  also  obstruct  the  Eustachian 
tubes  and  cause  partial  deafness.  This  overgrowth  is  known  as  ade- 
noids. The  removal  of  adenoids  is  a  simple  matter  surgically,  and 
is  advisable  wherever  there  is  evident  obstruction  of  the  breathing. 

The  tonsils,  which  function  in  the  manner  of  lymphoid  tissue 


384  THE  HUMAN  BODY 

generally,  to  filter  out  organisms  from  the  lymph  stream,  are 
peculiarly  liable  to  invasion  by  the  organisms  of  common  colds 
and  also  by  those  which  form  pus  (streptococcus).  When  any  of 
these  become  established  in  the  tonsils  and  set  up  inflammation 
therein  the  very  painful  condition  called  tonsilitis  results.  In 
many  cases  the  tonsils  become  permanently  infected.  In  such 
there  is  a  steady  production  of  toxins  which  are  discharged  into 
the  lymph  stream  and  thence  pervade  the  Body.  Malnutrition 
in  children  and  adults  is  often  to  be  accounted  for  solely  on  the 
basis  of  chronic  poisoning  from  infected  tonsils.  There  is  also 
reason  to  believe  that  acute  rheumatism  is  caused  similarly.  In 
such  cases  the  possible  good  that  may  come  to  the  Body  from  the 
normal  functioning  of  the  tonsils  is  so  far  outweighed  by  the  harm 
they  do  as  seats  of  infection  that  they  should  obviously  be  removed. 
The  Movement  of  the  Lymph.  This  is  no  doubt  somewhat 
irregular  in  the  commencing  vessels,  but,  on  the  whole,  sets  on 
to  the  larger  trunks  and  through  them  to  the  veins.  In  many 
animals  (as  the  frog)  at  points  where  the  lymphatics  communicate 
with  the  veins,  there  are  found  regularly  contractile  "lymph- 
hearts"  which  beat  with  a  rhythm  independent  of  that  of  the 
blood-heart,  and  pump  the  lymph  into  a  vein.  In  the  Human 
Body,  however,  there  are  no  such  hearts,  and  the  flow  of  the 
lymph  is  dependent  on  less  definite  arrangements.  It  seems  to 
be  maintained  mainly  by  three  things:  (1)  The  pressure  on  the 
blood-plasma  in  the  capillaries  is  greater  than  that  in  the  great 
veins  of  the  neck;  hence  any  plasma  filtered  through  the  capillary- 
walls  will  be  under  a  pressure  which  will  tend  to  make  it  flow  to 
the  venous  termination  of  the  thoracic  or  the  right  lymphatic 
duct.  (2)  On  account  of  the  numerous  valves  in  the  lymphatic 
vessels  (which  all  only  allow  the  lymph  to  flow  past  them  to 
larger  trunks)  any  movement  compressing  a  lymph-vessel  will 
cause  an  onward  flow  of  its  contents.  The  influence  thus  exerted 
is  very  important.  If  a  tube  be  put  in  a  large  lymphatic,  say  at 
the  top  of  the  leg  of  an  animal,  it  will  be  seen  that  the  lymph  only 
flows  out  very  slowly  while  the  animal  is  quiet;  but  as  soon  as  it 
moves  the  leg  the  flow  is  greatly  accelerated.  (3)  During  each 
inspiration  the  pressure  on  the  thoracic  duct  is  less  than  that  in 
the  lymphatics  in  parts  of  the  Body  outside  the  thorax  (see 
Chap.  XXIII).  Accordingly,  at  that  time,  lymph  is  pressed,  or, 


THE  VASOMOTOR  MECHANISM  385 

in  common  phrase,  is  "sucked,"  into  the  thoracic  duct.  During 
the  succeeding  expiration  the  pressure  on  the  thoracic  duct  be- 
comes greater  again,  and  some  of  its  contents  are  pressed  out;  but 
on  account  of  the  valves  of  the  vessels  which  unite  to  form  the 
duct,  they  can  only  go  towards  the  veins  of  the  neck. 

During  digestion,  moreover,  contractions  of  the  villi  and  of 
the  intestinal  walls  press  on  the  lymph  or  chyle  within  them  and 
force  it  on;  and  in  certain  parts  of  the  Body  gravity,  of  course, 
aids  the  flow,  though  it  will  impede  it  in  others. 

The  Action  of  Lymphagogues.  Any  substance  that  causes  a 
pronounced  increase  in  the  rate  of  lymph  formation  is  known 
as  a  lymphagogue.  The  source  of  lymph,  we  have  already  seen 
(p.  294),  is  in  the  main  by  filtration  through  the  capillary  walls. 
Evidently  lymphagogues  act  by  increasing  this  filtration.  There 
are  two  ways  in  which  this  might  be  brought  about,  and  lympha- 
gogues are  assigned  to  one  of  two  classes  according  to  which  of  the 
ways  they  use.  The  first  is  by  making  the  capillary  walls  more 
permeable,  and  so  increasing  the  outpouring  of  lymph.  Sub- 
stances which  have  this  effect  are  shell  fish,  strawberries,  some  meat 
extracts,  egg-white  and  related  organic  compounds.  Not  all 
people  are  affected  by  these  lymphagogues.  Nor  are  those  that 
are  susceptible  to  one  necessarily  susceptible  to  all.  Where  the 
capillaries  whose  permeability  is  increased  are  superficial  the  out- 
pouring lymph  forms  blotches  on  the  skin.  The  condition  is  known 
as  urticaria  or  hives.  Mechanical  injury  to  the  capillaries  may 
cause  a  similar  outpouring,  as  seen  in  the  swelling  from  a  bruise. 

The  second  method  of  increasing  the  flow  of  lymph  is  by  pro- 
ducing an  engorgement  of  the  capillaries,  a  condition  known  as 
hydremic  plethora.  This  can  be  brought  about  by  raising  the 
osmotic  pressure  of  the  blood,  as  by  injecting  into  it  a  strong 
sugar  solution.  The  effect  is  to  cause  a  rush  of  lymph  into  the 
blood  through  the  capillaries.  The  lymph  thus  withdrawn  is 
made  good  by  an  outpouring  of  tissue  fluids  into  the  lymph  spaces. 
It  has  been  shown  that  in  this  situation  the  plethora  is  relieved 
chiefly  by  an  increased  filtration  through  the  capillaries  of  the  liver. 
The  conclusion  is  drawn  that  these  are  the  most  permeable  in  the 
Body.  The  lymph  thus  formed  passes  to  the  thoracic  duct  and  back 
to  the  blood,  so  that  evidently  no  permanent  advantage  is  gained. 
The  excess  of  fluid  is  finally  discharged  through  the  kidneys. 


CHAPTER  XXIII 

RESPIRATION.    THE  MECHANISM  OF  BREATHING.    THE 
REGULATION  OF  BREATHING. 

Definitions.  The  blood  as  it  flows  from  the  right  ventricle  of 
the  heart,  through  the  lungs,  to  the  left  auricle,  loses  carbon 
dioxid  and  gains  oxygen.  In  the  systemic  circulation  exactly 
the  reverse  changes  take  place,  oxygen  leaving  the  blood  to  supply 
the  living  tissues;  and  carbon  dioxid,  generated  in  them,  passing 
back  into  the  blood  capillaries.  The  oxygen  loss  and  carbon 
dioxid  gain  are  associated  with  a  change  in  the  color  of  the  blood 
from  bright  scarlet  to  purple-red,  or  from  arterial  to  venous;  and 
the  opposite  changes  in  the  lungs  restore  to  the  dark  blood  its 
bright  tint.  The  whole  set  of  processes  through  which  blood  be- 
comes venous  in  the  systemic  circulation  and  arterial  in  the 
pulmonary — in  other  words,  the  processes  concerned  in  the  gaseous 
reception,  distribution,  and  elimination  of  the  Body — constitute 
the  function  of  respiration;  so  much  of  this  as  is  concerned  in  the 
interchanges  between  the  blood  and  air  being  known  as  external 
respiration;  while  the  interchanges  occurring  between  the  tissues 
and  the  systemic  capillaries  through  the  lymph,  constitute  internal 
respiration,  and  the  processes  in  general  by  which  oxygen  is  fixed 
and  carbon  dioxid  formed  by  the  living  tissues,  are  known  as 
tissue  respiration.  When  the  term  respiration  is  used  alone, 
without  any  limiting  adjective,  the  external  respiration  only,  is 
commonly  meant. 

Respiratory  Organs.  The  blood  being  kept  poor  in  oxygen 
and  rich  in  carbon  dioxid  by  the  action  of  the  living  tissues,  a 
certain  amount  of  gaseous  interchange  will  nearly  always  take 
place  when  it  comes  into  close  proximity  to  the  surrounding 
medium;  whether  this  be  the  atmosphere  itself  or  water  contain- 
ing air  in  solution.  When  an  animal  is  small  there  are  often  no 
special  organs  for  its  external  respiration,  its  general  surface  being 
sufficient  (especially  in  aquatic  animals  with  a  moist  skin)  to 
permit  of  all  the  gaseous  exchange  that  is  necessary.  In  the 

386 


RESPIRATION:  THE  MECHANISM  OF  BREATHING       387 

simplest  creatures,  indeed,  there  is  even  no  blood,  the  cell  or  cells 
composing  them  taking  up  for  themselves  from  their  environ- 
ment the  oxygen  which  they  need,  and  passing  out  into  it  their 
carbon  dioxid  waste;  in  other  words,  there  is  no  differentiation 
of  the  external  and  internal  respirations.  When,  however,  an 
animal  is  larger  many  of  its  cells  are  so  far  from  a  free  surface 
that  they  cannot  transact  this  give-and-take  with  the  surround- 
ing medium  directly,  and  the  blood,  or  some  liquid  representing 
it  in  this  respect,  serves  as  a  middleman  between  the  living  tissues 
and  the  external  oxygen;  and  then  one  usually  finds  special 
respiratory  organs  developed,  to  which  the  blood  is  brought  to 
make  good  its  oxygen  loss  and  get  rid  of  its  excess  of  carbon 
dioxid.  In  aquatic  animals  such  organs  take  commonly  the  form 
of  gills;  these  are  protrusions  of  the  body  over  which  a  constant 
current  of  water,  containing  oxygen  in  solution,  is  kept  up;  and 
in  which  blood  capillaries  form  a  close  network  immediately  be- 
neath the  surface.  In  air-breathing  animals  a  different  arrange- 
ment is  usually  found.  In  some,  as  frogs,  it  is  true,  the  skin  is 
always  moist  and  serves  as  an  important  respiratory  organ,  large 
quantities  of  venous  blood  being  sent  to  it  for  aeration.  But  for 
the  occurrence  of  the  necessary  gaseous  diffusion,  the  skin  must 
be  kept  very  moist,  and  this,  in  a  terrestrial  animal,  necessitates 
a  great  amount  of  secretion  by  the  cutaneous  glands  to  com- 
pensate for  evaporation;  accordingly  in  most  land  animals  the 
air  is  carried  into  the  body  through  tubes  with  narrow  external 
orifices  and  so  the  drying  up  of  the  breathing  surfaces  is  greatly 
diminished;  just  as  water  in  a  bottle  with  a  narrow  neck  will 
evaporate  much  more  slowly  than  the  same  amount  exposed 
in  an  open  dish.  In  insects  (as  bees,  butterflies,  and  beetles) 
the  air  is  carried  by  tubes  which  split  up  into  extremely  fine 
branches  and  ramify  all  through  the  body,  even  down  to  the 
individual  tissue  elements,  which  thus  carry  on  their  gase- 
ous exchanges  without  the  intervention  of  blood.  But  in  the 
great  majority  of  air-breathing  animals  the  arrangement  is  dif- 
ferent; the  air-tubes  leading  from  the  exterior  of  the  body 
do  not  subdivide  into  branches  which  ramify  all  through  it, 
but  open  into  one  or  more  large  sacs  to  which  the  venous 
blood  is  brought,  and  in  whose  walls  it  flows  through  a  close 
capillary  network.  Such  respiratory  sacs  are  called  lungs,  and  it 


388 


THE  HUMAN  BODY 


is  a  highly  developed  form  of  them  which  is  employed  in  the 

Human  Body. 

The  Air-Passages  and  Lungs.  In  our  own  Bodies  the  es- 
sential gaseous  interchanges  between 
the  Body  and  the  atmosphere  take 
place  in  the  lungs,  two  large  sacs  (lu, 
Fig.  1)  lying  in  the  thoracic  cavity,  one 
on  each  side  of  the  heart.  To  these 
sacs  the  air  is  conveyed  through  a  series 
of  passages.  Entering  the  pharynx 
through  the  nostrils  or  mouth,  it  passes 
out  of  this  by  the  opening  leading  into 
the  larynx,  or  voice-box  (a,  Fig.  112), 
lying  in  the  upper  part  of  the  neck  (the 
communication  of  the  two  is  seen  in 
Fig.  121)  ;  from  the  larynx  passes  back 
the  trachea  or  windpipe,  b,  which,  after 
entering  the  chest  cavity,  divides  into 
FiG."il2.-The  lungs  and  air-  the  right  and  left  bronchi,  d,  e.  Each 

OnSSthrieTteVth?fihurlr°tnhe  bronchus  divides   up  into  smaller   and 
pulmonary  tissue  has  been  dis-  smaller  branches,  called  bronchial  tubes. 

sected  away  to  show  the  rami-       .  ,  .         ,        ,  .  .  . 

fications  of  the  bronchial  tubes.  within  the  lung  on  its   own  side;  and 


the  smallest  bronchial  tubes  end  in 
seen  entering  the  roo<  of  its  lung.  sacculated  dilatations,  the  infundibula  of 
the  lungs,  the  sacculations  (Fig.  114)  being  the  alveoli.  On  the 
walls  of  the  alveoli  the  pulmonary  capillaries  ramify,  and  it  is 
in  them  that  the  interchanges  of  the 
external  respiration  take  place. 

Structure  of  the  Trachea  and  Bronchi. 
The  windpipe  may  readily  be  felt  in 
the  middle  line  of  the  neck,  a  little  be- 
low Adam's  apple,  as  a  rigid  cylindrical 
mass.  It  consists  fundamentally  of  a 
fibrous  tube  in  which  cartilages  are 

FIG.  113.  —  A  small  bronchial 

embedded,   SO   as   tO   keep   it     from   Col-  tube,  a,  dividing  into  its  terminal 
i         .  j     •      v        i    •    x  ii       -i  branches,  c;  these  have  pouched 

lapsing;  and   is  lined  internally   by    a  Or  sacculated  walls  and  end  in 
mucous  membrane  covered  by  several  the  sacculated  infundibula,  b. 
layers  of  epithelium  cells,  of  which  the  superficial  is   ciliated. 
The  elastic  cartilages  embedded  in  its  walls  are  imperfect  rings, 


RESPIRATION.     THE  MECHANISM  OF  BREATHING      389 


each  somewhat  the  shape  of  a  horseshoe  and  the  deficient  part 
of  each  ring  being  turned  backwards,  it  comes  to  pass  that  the 
deeper  or  dorsal  side  of  the  windpipe  has  no  hard  parts  in  it. 
Here  the  wall  consists  of  smooth  muscle.  Against  this  the  gul- 
let lies,  the  absence  of  cartilage  facilitating  swallowing.  The 
bronchi  are  similar  in  structure. 

The  Structure  of  the  Lungs.  These  consist  of  the  bronchial 
tubes  and  their  terminal  dilations;  numerous  blood-vessels,  nerves, 
and  lymphatics;  and  an  abundance  of  connective  tissue,  rich  in 
elastic  fibers,  binding  all  together.  The  bronchial  tubes  ramify 
in  a  tree-like  manner  (Fig.  112).  The  larger  ones  resemble  the 
trachea,  except  that  the  cartilage  rings  do  not  have  their  open 
parts  all  turned  one  way,  and  the  smooth  muscle  encircles  the 
tube  completely.  As  the  tubes  become  smaller  their  constituents 
thin  away;  the  cartilages  become  less  frequent  and  finally  dis- 
appear; the  epithelium  is  reduced  to  a  single  layer  of  cells  which, 
though  still  ciliated,  are  much  shorter  than  the  columnar  super- 
ficial cell-layer  of  the  larger  tubes.  The  terminal  alveoli  (a,  a,  Fig. 
114)  have  walls  composed  mainly  of 
elastic  tissue  and  lined  by  a  single 
layer  of  flat,  non-ciliated  epithelium, 
immediately  beneath  which  is  a  very 
close  network  of  capillary  blood-ves- 
sels. The  air  entering  by  the  bronchial 
tube  is  thus  only  separated  from  the 
blood  by  the  thin  capillary  wall  and 
the  thin  epithelium,  both  of  which 
are  moist,  and  well  fitted  for  gaseous 
diffusion. 

The  Pleura.  Each  lung  is  covered, 
except  at  one  point,  by  an  elastic  se- 
rous  membrane  which  adheres  tightly 
to  it  and  is  called  the  pleura;  that 

point  at  which  the  pleura  is  Wanting  ity;   c,    terminal    branches   of   a 
n    j   .  i  .     ,.  ,  ,      !  j  .        ~    bronchial  tube. 

is  called  the  root  of  the  lung  and  is  on 

its  median  side;  it  is  there  that  its  bronchus,  blood-vessels  and 
nerves  enter  it.  At  the  root  of  the  lung  the  pleura  turns  back 
and  lines  the  inside  of  the  chest  cavity,  as  represented  by  the 
heavy  black  line  in  the  diagram  Fig.  3.  The  part  of  the  pleura  at- 


390  THE  HUMAN  BODY 

tached  to  each  lung  is  its  visceral,  and  that  attached  to  the 
chest-wall  its  parietal  layer.  Each  pleura  thus  forms  a  closed 
sac  surrounding  a  pleural  cavity,  in  which,  during  health,  there 
are  found  a  few  drops  of  lymph,  keeping  its  surfaces  moist.  This 
lessens  friction  between  the  two  layers  during  the  movements 
of  the  chest-walls  and  the  lungs;  for  although,  to  insure  dis- 
tinctness, the  visceral  and  parietal  layers  of  the  pleura  are  rep- 
resented in  the  diagram  as  not  in  contact,  that  is  not  the  nat- 
ural condition  of  things;  the  lungs  are  in  life  distended  so  that 
the  visceral  pleura  rubs  against  the  parietal,  and  the  pleural 
cavity  is  practically  obliterated.  This  is  due  to  the  pressure  of 
the  atmosphere  exerted  through  the  air-passages  on  the  interior 
of  the  lungs.  The  lungs  are  extremely  elastic  and  distensible,  and 
when  the  chest  cavity  is  perforated  each  shrivels  up  just  as  an 
india-rubber  bladder  does  when  its  neck  is  opened;  the  reason 
being  that  then  the  air  presses  on  the  outside  of  each  with  as 
much  force  as  it  does  on  the  inside.  These  two  pressures  neutral- 
izing one  another,  there  is  nothing  to  overcome  the  tendency  of 
the  lungs  to  collapse.  So  long  as  the  chest-walls  are  whole,  how- 
ever, the  lungs  remain  distended.  The  pleural  sac  is  air-tight 
and  contains  no  air,  and  the  pressure  of  the  air  around  the  Body 
is  borne  by  the  rigid  walls  of  the  chest  and  prevented  from  reach- 
ing the  lungs;  consequently  no  atmospheric  pressure  is  exerted 
on  their  outside.  On  their  interior,  however,  the  atmosphere 
presses  with  its  full  weight,  equal  to  about  90  centigrams  on  a 
square  centimeter  (14.5  Ibs.  on  the  square  inch),  and  this  is  far 
more  than  sufficient  to  distend  the  lungs  so  as  to  make  them 
completely  fill  all  the  parts  of  the  thoracic  cavity  not  occupied  by 
other  organs.  Suppose  A  (Fig.  115)  to  be  a  bottle  closed  air- 
tight by  a  cork  through  which  two  tubes  pass,  one  of  which,  b, 
leads  into  an  elastic  bag,  d,  and  the  other,  c,  provided  with  a  stop- 
cock, opens  freely  below  into  the  bottle.  When  the  stop-cock,  c, 
is  open  the  air  will  enter  the  bottle  and  press  there  on  the  outside 
of  the  bag,  as  well  as  on  its  inside  through  b.  The  bag  will  there- 
fore collapse,  as  the  lungs  do  when  the  chest  cavity  is  opened. 
But  if  some  air  be  sucked  out  through  c  the  pressure  of  that  re- 
maining in  the  bottle  will  diminish,  and  of  that  inside  the  bag 
will  be  unchanged,  and  the  bag  will  thus  be  blown  up,  because 
the  atmospheric  pressure  on  its  interior  will  not  be  balanced  by 


RESPIRATION:  THE  MECHANISM  OF  BREATHING       391 

that  on  its  exterior.  At  last,  when  all  the  air  is  sucked  out  of  the 
bottle  and  the  stop-cock  on  c  closed,  the  bag,  if  sufficiently  dis- 
tensible, will  be  expanded  so  as  to  completely  fill  the  bottle  and 
press  against  its  inside,  and  the  state  of  things  . 
will  then  answer  to  that  naturally  found  in  the 
chest.  If  the  bottle  were  now  increased  in  size 
without  letting  air  into  it,  the  bag  would  ex- 
pand still  more,  so  as  to  fill  it,  and  in  so  doing 
would  receive  air  from  outside  through  b;  and 
if  the  bottle  then  returned  to  its  original  size,  FIG  115  —  Dia- 
its  walls  would  press  on  the  bag  and  cause  it  to  gram  illustrating  the 

,     .    ,  ,  <••-•-!  i    7        T-I        p  r  e  s  s  u  r  e  relation- 

shrink  and  expel  some  of  its  air  through  6.    Ex-  ships  of  the  lungs  in 

actly  the  same  must  of  course  happen,  under  t] 

similar  circumstances,  in  the   chest,  the  windpipe  answering   to 

the  tube  b  through  which  air  enters  or  leaves  this  elastic  sac. 

The  Respiratory  Movements.  The  air  taken  into  the  lungs 
soon  becomes  laden  in  them  with  carbon  dioxid,  and  at  the  same 
time  loses  much  of  its  oxygen;  these  interchanges  take  place 
mainly  in  the  deep  recesses  of  the  alveoli,  far  from  the  exterior 
and  only  communicating  with  it  through  a  long  tract  of  narrow 
tubes.  The  alveolar  air,  thus  become  unfit  any  longer  to  convert 
venous  blood  into  arterial,  could  only  very  slowly  be  renewed  by 
gaseous  diffusion  with  the  atmosphere  through  the  long  air- 
passages — not  nearly  fast  enough  for  the  requirements  of  the 
Body,  as  one  learns  by  the  sensation  of  suffocation  which  follows 
holding  the  breath  for  a  short  time  with  mouth  and  larynx  open. 
Consequently  cooperating  with  the  lungs  is  a  respiratory  mechan- 
ism, by  which  the  air  within  them  is  periodically  mixed  with  fresh 
air  taken  from  the  outside,  and  also  the  air  in  the  alveoli  is  stirred 
up  so  as  to  bring  fresh  layers  of  it  in  contact  with  the  walls  of  the 
air-cells.  This  mixing  is  brought  about  by  the  breathing  move- 
ments, consisting  of  regularly  alternating  inspirations,  during 
which  the  chest  cavity  is  enlarged  and  fresh  air  enters  the  lungs, 
and  expirations,  in  which  the  cavity  is  diminished  and  air  expelled 
from  the  lungs.  When  the  chest  is  enlarged  the  air  the  lungs 
contain  immediately  distends  them  so  as  to  fill  the  larger  space; 
in  so  doing  it  becomes  rarefied  and  less  dense  than  the  external 
air;  and  since  gases  flow  from  points  of  greater  to  those  of  less 
pressure,  some  outside  air  at  once  flows  in  by  the  air-passages 


392  THE  HUMAN  BODY 

and  enters  the  lungs.  In  expiration  the  reverse  takes  place.  The 
chest  cavity,  diminishing,  presses  on  the  lungs  and  makes  the 
air  inside  them  denser  than  the  external  air,  and  so  some  passes 

out  until  an  equilibrium  of  pres- 
sure is  restored.  The  chest,  in  fact, 
acts  very  much  like  a  bellows. 
When  the  bellows  are  opened  air 
FIG  lie— Diagram  to  illustrate  enters  in  consequence  of  the  rare- 

^11^  of  that  in  the  interior> 
which  is  expanding  to  fill  the  larger 

space;  and  when  the  bellows  are  closed  again  it  is  expelled.  To 
make  the  bellows  quite  like  the  lungs  we  must,  however,  as  in 
Fig.  116,  have  only  one  opening  in  them,  that  of  the  nozzle,  for 
both  the  entry  and  exit  of  the  air;  and  this  opening  should  lead, 
not  directly  into  the  bellows-cavity,  but  into  an  elastic  bag  ly- 
ing in  it,  and  tied  to  the  inner  end  of  the  nozzle-pipe.  This  sac 
would  represent  the  lungs  and  the  space  between  its  outside  and 
the  inside  of  the  bellows,  the  pleural  cavities. 

We  have  next  to  see  how  the  expansion  and  contraction  of  the 
chest  cavity  are  brought  about. 

The  Structure  of  the  Thorax.  The  thoracic  cavity  has  a  conical 
form  determined  by  the  shape  of  its  skeleton  (Fig.  117),  its  nar- 
rower end  being  turned  upwards.  Dorsally,  ventrally,  and  on  the 
sides,  it  is  supported  by  the  rigid  framework  afforded  by  the 
thoracic  vertebrae,  the  breast-bone,  and  the  ribs.  Between  and 
over  these  lie  muscles,  and  the  whole  is  covered  in,  air-tight,  by 
the  skin  externally,  and  the  parietal  layers  of  the  pleurae  inside. 
Above,  its  aperture  is  closed  by  muscles  and  by  various  organs 
passing  between  the  thorax  and  the  neck;  and  below  it  is  bounded 
by  the  diaphragm,  which  forms  a  movable  bottom  to  the,  other- 
wise, tolerably  rigid  box.  In  inspiration  this  box  is  increased  in 
all  its  diameters — dorsiventrally,  laterally,  and  from  above  down. 

The  Vertical  Enlargement  of  the  Thorax.  This  is  brought 
about  by  the  contraction  of  the  diaphragm  which  (Figs.  1  and  118) 
is  a  thin  muscular  sheet,  with  a  fibrous  membrane,  serving  as  a 
tendon,  in  its  center.  In  rest,  the  diaphragm  is  dome-shaped, 
with  its  concavity  towards  the  abdomen,  being  supported  in  that 
position  by  the  pressure  of  the  underlying  abdominal  organs. 
From  the  tendon  on  the  crown  of  the  dome  striped  muscular  fibers 


RESPIRATION:  THE  MECHANISM  OF  BREATHING       393 

radiate,  downwards  and  outwards,  to  all  sides;  and  are  fixed  by 
their  inferior  ends  to  the  lower  ribs,  the  breast-bone,  and  the  ver- 
tebral column.  In  expiration  the  lower  lateral  portions  of  the 
diaphragm  lie  close  against  the  chest-walls,  no  lung  intervening 
between  them.  In  inspiration  the  muscular  fibers,  shortening, 
flatten  the  dome  and  enlarge  the  thoracic  cavity,  room  for  the 


FIG.  117. — The  skeleton  of  the  thorax,  a,  g,  vertebral  column;  6,  first  rib;  e. 
clavicle;  e,  seventh  rib;  i,  glenoid  fossa. 

viscera  thus  displaced  being  secured  by  stretching  the  abdominal 
walls;  at  the  same  time  its  lateral  portions  are  pulled  away  from 
the  chest-walls,  leaving  a  space  into  which  the  lower  ends  of  the 
lungs  expand.  The  contraction  of  the  diaphragm  thus  increases 
greatly  the  size  of  the  thorax  chamber  by  adding  to  its  lowest  and 
widest  part. 

The  Dorsiventral  Enlargement  of  the  Thorax.  The  ribs  on  the 
whole  slope  downwards  from  the  vertebral  column  to  the  breast- 
bone, the  slope  being  most  marked  in  the  lower  ones.  During 
inspiration  the  breast-bone  and  the  sternal  ends  of  the  ribs  at- 
tached to  it  are  raised,  and  so  the  distance  between  the  sternum 
and  the  vertebral  column  is  increased.  That  this  must  be  so  will 
readily  be  seen  on  considering  the  diagram  Fig.  119,  where  ab 
represents  the  vertebral  column,  c  and  d  two  ribs,  and  st  the  ster- 


394 


THE  HUMAN  BODY 


num.  The  continuous  lines  represent  the  natural  position  of  the 
ribs  at  rest  in  expiration,  and  the  dotted  lines  the  position  in 
inspiration.  It  is  clear  that  when  their  lower  ends  are  raised,  so 


Ql 
FIG.  118. — The   diaphragm   seen   from   below. 

as  to  make  the  bars  lie  in  a  more  horizontal  plane,  the  sternum  is 
pushed  away  from  the  spine,  and  so  the  chest  cavity  is  increased 
dorsiventrally.  The  inspiratory  elevation  of  the  ribs  is  mainly 
due  to  the  action  of  the  scalene  and  external  intercostal  muscles. 
The  scalene  muscles,  three  on  each  side,  arise 
from  the  cervical  vertebrae,  and  are  inserted 
into  the  upper  ribs.  The  external  intercos- 
tals  (Fig.  120,  A)  lie  between  the  ribs  and 
extend  from  the  vertebral  column  to  the 
costal  cartilages;  their  fibers  slope  downwards 
and  forwards.  During  an  inspiration  the 
scalenes  contract  and  fix  the  upper  ribs 
firmly;  then  the  external  intercostals  shorten 
and  each  raises  the  rib  below  it.  The  muscle, 


lustrat'mg    the    dorsi-  in  fact,  tends  to  pull  together  the  pair  of  ribs 

ventral    increase  in  the  .  .  .   ,      .      ,. 

diameter  of  the  thorax  between  which  it  lies,  but  as  the  upper  one 

when  the  ribs  are  raised.  Qjf    thege    ig 


tight   by  the    gcalenes   and 

other  muscles  above,  the  result  is  that  the  lower  rib  is  pulled  up, 
and  not  the  upper  down.  In  this  way  the  lower  ribs  are  raised 
much  more  than  the  upper,  for-  the  whole  external  intercostal 
muscles  on  each  side  may  be  regarded  as  one  great  muscle  with 
many  bellies,  each  belly  separated  from  the  next  by  a  tendon, 


RESPIRATION:  THE  MECHANISM  OF  BREATHING        395 

represented  by  the  rib.  When  the  whole  muscular  sheet  is  fixed 
above  and  contracts,  it  is  clear  that  its  lower  end  will  be  raised 
more  than  any  intermediate  point,  since  there  is  a  greater  length 
of  contracting  muscle  above  it.  The  elevation  of  the  ribs  tends 


FIG.  120. — Portions  of  four  ribs  of  a  dog  with  the  muscles  between  them,  a,  a, 
ventral  ends  of  the  ribs,  joining  at  c  the  rib  cartilages,  b,  which  are  fixed  to  carti- 
laginous portions,  d,  of  the  sternum.  A,  external  intercostal  muscle,  ceasing  be- 
tween the  rib  cartilages,  where  the  internal  intercostal,  B,  is  seen.  Between  the 
middle  two  ribs  the  external  intercostal  muscle  has  been  dissected  away,  so  as  to 
display  the  internal  which  was  covered  by  it. 

to  diminish  the  vertical  diameter  of  the  chest;  this  is  more  than 
compensated  for  by  the  simultaneous  descent  of  the  diaphragm. 
The  Lateral  Enlargement  of  the  Chest  is  brought  about  by  a 
rotation  of  the  middle  ribs  which,  as  they  are  raised,  roll  round  a 
little  at  their  vertebral  articulations  and  twist  their  cartilages. 
Each  rib  is  curved  and,  if  the  bones  be  examined  in  their  natural 
position  in  a  skeleton,  it  will  be  seen  that  the  most  curved  part 
lies  below  the  level  of  a  straight  line  drawn  from  the  vertebral  to 
the  sternal  attachment  of  the  bone.  By  the  rotation  of  the  rib, 
during  inspiration,  this  curved  part  is  raised  and  turned  out,  and 
the  chest  widened.  The  mechanism  can  be  understood  by  clasp- 
ing the  hands  opposite  the  lower  end  of  the  sternum  and  a  few 
inches  in  front  of  it,  with  the  elbows  bent  and  pointing  down- 
wards. Each  arm  will  then  answer,  in  an  exaggerated  way,  to  a 


396  THE  HUMAN  BODY 

curved  rib,  and  the  clasped  hands  to  the  breast-bone.  If  the 
hands  be  simply  raised  a  few  inches  by  movement  at  the  shoulder- 
joints  only,  they  will  be  separated  farther  from  the  front  of  the 
Body,  and  rib  elevation  and  the  consequent  dorsiventral  en- 
largement of  the  cavity  surrounded  will  be  represented.  But  if, 
simultaneously,  the  arms  be  rotated  at  the  shoulder-joints  so  as 
to  raise  the  elbows  and  turn  them  out  a  little,  it  will  be  seen  that 
the  space  surrounded  by  the  two  arms  is  considerably  increased 
from  side  to  side,  as  the  chest  cavity  is  in  inspiration  by  the  simi- 
lar elevation  of  the  most  curved  part  or  " angle"  of  the  middle 
ribs. 

Expiration.  To  produce  an  inspiration  requires  considerable 
muscular  effort.  The  ribs  and  sternum  have  to  be  raised;  the 
elastic  rib  cartilages  bent  and  somewhat  twisted;  the  abdominal 
viscera  pushed  down;  and  the  abdominal  wall  pushed  out  to 
make  room  for  them.  In  expiration,  on  the  contrary,  no  muscu- 
lar effort  is  needed.  As  soon  as  the  muscles,  which  have  raised 
the  ribs  and  sternum  relax,  these  tend  to  return  to  their  natural 
unconstrained  position,  and  the  rib  cartilages,  also,  to  untwist 
themselves  and  bring  the  ribs  back  to  their  position  of  rest;  the 
elastic  abdominal  wall  presses  the  contained  viscera  against  the 
under  side  of  the  diaphragm,  and  pushes  that  up  again  as  soon 
as  its  muscular  fibers  cease  contracting.  By  these  means  the 
chest  cavity  is  restored  to  its  original  capacity  and  the  air  sent 
out  of  the  lungs,  by  the  elasticity  of  the  parts  which  were  stretched 
or  twisted  in  inspiration,  and  not  by  any  special  expiratory 
muscles. 

Forced  Respiration.  When  a  very  deep  breath  is  drawn  or 
expelled,  or  when  there  is  some  impediment  to  the  entry  or  exit 
of  the  air,  a  great  many  muscles  take  part  in  producing  the  respir- 
atory movements;  and  expiration  then  becomes,  in  part,  an  ac- 
tively muscular  act.  The  main  expiratory  muscles  are  the  internal 
intercostals  which  lie  beneath  the  external  between  each  pair  of 
ribs  (Fig.  120,  B),  and  have  an  opposite  direction,  their  fibers 
running  upwards  and  forwards.  In  forced  expiration  the  lower 
ribs  are  fixed  or  pulled  down  by  muscles  running  in  the  abdominal 
wall  from  the  pelvis  to  them  and  to  the  breast-bone.  The  internal 
intercostals,  contracting,  pull  down  the  upper  ribs  and  the  ster- 
num, and  so  diminish  the  thoracic  cavity  dorsiventrally.  At 


RESPIRATION:  THE  MECHANISM  OF  BREATHING       397 

the  same  time,  the  contracted  abdominal  muscles  press  the  walls 
of  that  cavity  against  the  viscera  within  it,  and  pushing  these  up 
forcibly  against  the  diaphragm  make  it  very  convex  towards  the 
chest,  and  so  diminish  the  latter  in  its  vertical  diameter.  In  very 
violent  expiration  many  other  muscles  may  co-operate,  tending 
to  fix  points  on  which  those  muscles  which  can  directly  dimmish 
the  thoracic  cavity,  pull.  In  violent  inspiration,  also,  many  extra 
muscles  are  called  into  play.  The  neck  is  held  rigid  to  give  the 
scalenes  a  firm  attachment;  the  shoulder-joint  is  held  fixed  and 
muscles  going  from  it  to  the  chest-wall,  and  commonly  serving 
to  move  the  arm,  are  then  used  to  elevate  the  ribs;  the  head  is 
held  firm  on  the  vertebral  column  by  the  muscles  going  between 
the  two,  and  then  other  muscles,  which  pass  from  the  collar-bone 
and  sternum  to  the  skull,  are  used  to  pull  up  the  former.  The 
muscles  which  are  thus  called  into  play  in  labored  but  not  in 
quiet  breathing  are  called  extraordinary  muscles  of  respiration. 

The  Respiratory  Sounds.  The  entry  and  exit  of  air  are  accom- 
panied by  respiratory  sounds  or  murmurs,  which  can  be  heard  on 
applying  the  ear  to  the  chest-wall.  The  character  of  these  sounds 
is  different  and  characteristic  over  the  trachea,  the  larger  bron- 
chial tubes,  and  portions  of  lung  from  which  large  bronchial  tubes 
are  absent.  They  are  variously  modified  in  pulmonary  affections, 
and  hence  the  value  of  auscultation  of  the  lungs  in  assisting  the 
physician  to  form  a  diagnosis. 

The  Capacity  of  the  Lungs.  Since  the  chest  cavity  never  even 
approximately  collapses,  the  lungs  are  never  completely  emptied 
of  air:  the  space  they  have  to  occupy  is  larger  in  inspiration  than 
during  expiration,  but  is  always  considerable,  so  that  after  a 
forced  expiration  they  still  contain  a  large  amount  of  air  which 
can  only  be  expelled  from  them  by  opening  the  pleural  cavities; 
then  they  collapse  almost  completely,  retaining  within  them  only  a 
small  quantity  of  air  imprisoned  within  the  alveoli  by  the  collapse 
of  the  small  bronchi. 

The  capacity  of  the  chest,  and  therefore  of  the  lungs,  varies 
much  in  different  individuals,  but  in  a  man  of  medium  height 
there  remain  in  the  lungs  after  the  most  violent  possible  expira- 
tion, about  1,000  cub.  cent,  of  air,  called  the  residual  air.  After 
an  ordinary  expiration  there  will  be  in  addition  to  this  about 
1,600  cub.  cent,  of  supplemental  air;  the  residual  and  supplemental 


398  THE  HUMAN  BODY 

together  forming  the  stationary  air,  which  remains  in  the  chest 
during  quiet  breathing.  In  an  ordinary  inspiration  500  cub.  cent. 
(30  cub.  inches)  of  tidal  air  are  taken  in,  and  about  the  same 
amount  is  expelled  in  natural  expiration.  By  a  forced  inspira- 
tion about  1,600  cub.  cent.  (98  cub.  inches)  of  complemental  air 
can  be  added  to  the  tidal  air.  After  a  forced  inspiration,  therefore, 
the  chest  will  contain  1,000+1,600+500+1,600=4,700  cub.  cent. 
(300  cub.  inches)  of  air.  The  amount  which  can  be  taken  in  by 
the  most  violent  possible  inspiration  after  the  strongest  possible 
expiration,  that  is,  the  supplemental,  tidal,  and  complemental 
air  together,  is  known  as  the  vital  capacity.  For  a  healthy  man 
1.7  meters  (5  feet  8  inches)  high  it  is  about  3,700  cub.  cent.  (225 
cub.  inches)  and  increases  60  cub.  cent,  for  each  additional  centi- 
meter of  stature ;  or  about  9  cub.  inches  for  each  inch  of  height. 
These  figures  are,  of  course,  average  figures.  Individual  variations 
from  them  are  numerous. 

The  Quantity  of  Air  Breathed  Daily.  Knowing  the  quantity 
of  air  taken  in  at  each  breath  and  expelled  again  (after  more  or 
less  thorough  admixture  with  the  stationary  air)  we  have  only  to 
know,  in  addition,  the  rate  at  which  the  breathing  movements 
occur,  to  be  able  to  calculate  how  much  air  passes  through  the 
lungs  in  twenty-four  hours.  The  average  number  of  respira- 
tions in  a  minute  is  found  by  counting  on  persons  sitting  quietly, 
and  not  knowing  that  their  breathing  rate  is  under  observation, 
to  be  fifteen  in  a  minute.  In  each  respiration  half  a  liter  (30  cub. 
inches)  of  air  is  concerned;  therefore  0.5X15X60X24=10,800 
liters  (375  cub.  feet)  is  the  quantity  of  air  breathed  under  ordi- 
nary circumstances  by  each  person  in  a  day. 

Hygienic  Remarks.  Since  the  diaphragm  when  it  contracts 
pushes  down  the  abdominal  viscera  beneath  it,  these  have  to  make 
room  for  themselves  by  pushing  out  the  soft  front  of  the  abdomen 
which,  accordingly,  protudes  when  the  diaphragm  descends. 
Hence  breathing  by  the  diaphragm,  being  indicated  on  the  exte- 
rior by  movements  of  the  abdomen,  is  often  called  "abdominal 
respiration,"  as  distinguished  from  breathing  by  the  ribs,  called 
"  costal "  or  "  chest  breathing."  In  both  sexes  the  diaphragmatic 
breathing  is  the  most  important,  but,  as  a  rule,  men  and  children 
use  the  ribs  less  than  adult  women.  Since  both  abdomen  and 
chest  alternately  expand  and  contract  in  healthy  breathing,  any- 
thing which  impedes  their  free  movement  is  to  be  avoided;  and 
the  tight  lacing  which  used  to  be  thought  elegant  a  few  years 


RESPIRATION:  THE  MECHANISM  OF  BHLATHING        399 

back,  and  is  still  indulged  in  by  some  who  think  a  distorted  form 
beautiful,  seriously  impedes  one  of  the  most  important  functions- 
of  the  Body,  leading,  if  nothing  worse,  to  shortness  of  breath  and 
an  incapacity  for  muscular  exertion.  In  extreme  cases  of  tight 
lacing  some  organs  are  often  directly  injured,  weals  of  fibrous 
tissue  being,  for  example,  not  unfrequently  found  developed  on 
the  liver,  from  the  pressure  of  the  lower  ribs  forced  against  it  by 
a  tight  corset. 

The  Aspiration  of  the  Thorax.  As  already  pointed  out,  the 
external  air  cannot  press  directly  upon  the  contents  of  the  thoracic 
cavity,  on  account  of  the  rigid  framework  which  supports  its 
walls;  it  still,  however,  presses  on  them  indirectly  through  the 
lungs.  Pushing  on  the  interior  of  these  with  a  pressure  equal  to 
that  exerted  on  the  same  area  by  a  column  of  mecury  760  mm. 
(30  inches)  high,  it  distends  them  and  forces  them  against  the  in- 
side of  the  chest-walls,  the  heart,  the  great  thoracic  blood-vessels, 
the  thoracic  duct,  and  the  other  contents  of  the  chest  cavity. 
The  pressure  against  these  organs  is  not  equal  to  that  of  the  ex- 
ternal air,  since  some  of  the  total  air-pressure  on  the  inside  of  the 
lungs  is  used  up  in  overcoming  their  elasticity,  and  it  is  only  the 
residue  which  pushes  them  against  the  things  outside  them.  In 
expiration  this  residue  is  equal  to  that  exerted  by  a  column  of 
mercury  754  mm.  (29.8  inches)  high.  On  most  parts  of  the  Body 
the  atmospheric  pressure  acts,  however,  with  full  force.  Pressing 
on  a  limb  it  pushes  the  skin  against  the  soft  parts  beneath,  and 
these  compress  the  blood-  and  lymph-vessels  among  them ;  and  the 
yielding  abdominal  walls  do  not,  like  the  rigid  thoracic  walls, 
carry  the  atmospheric  pressure  themselves,  but  transmit  it  to  the 
contents  of  the  cavity.  It  thus  comes  to  pass  that  the  blood  and 
lymph  in  most  parts  of  the  Body  are  under  a  higher  atmospheric 
pressure  than  they  are  exposed  to  in  the  chest,  and  consequently 
these  liquids  tend  to  flow  into  the  thorax,  until  the  extra  disten- 
tion  of  the  vessels  in  which  they  there  accumulate  compensates 
for  the  less  external  pressure  to  which  those  vessels  are  exposed. 
An  equilibrium  would  thus  very  soon  be  brought  about  were  it 
not  for  the  respiratory  movements,  in  consequence  of  which  the 
intrathoracic  pressure  is  alternately  increased  and  diminished, 
and  the  thorax  comes  to  act  as  a  sort  of  suction-pump  on  the 
contents  of  the  vessels  of  the  Body  outside  it;  thus  the  respira- 


400  THE  HUMAN  BODY 

tory  movements  influence  the  circulation  of  the  blood  and  the 
flow  of  the  lymph. 

Influence  of  the  Respiratory  Movements  upon  the  Circulation. 
Suppose  the  chest  in  a  condition  of  normal  expiration  and  the 
external  pressure  on  the  blood  in  the  blood-vessels  within  it  and 
in  the  heart,  to  have  come,  in  the  manner  pointed  out  in  the  last 
paragraph,  into  equilibrium  with  the  atmospheric  pressure  exerted 
on  the  blood-vessels  of  the  neck  and  abdomen.  If  an  inspiration 
now  occurs,  the  chest  cavity  being  enlarged  the  pressure  on  all  of 
its  contents  will  be  diminished.  In  consequence,  air  enters  the 
lungs  from  the  windpipe,  and  blood  enters  the  venae  caya?  and  the 
right  auricle  of  the  heart  from  the  outlying  veins.  When  the 
next  expiration  occurs,  and  the  pressure  in  the  thorax  again  rises, 
air  and  blood  both  tend  to  be  expelled  from  the  cavity.  What- 
ever extra  blood  has,  to  use  the  common  phrase,  been  "  sucked  " 
into  the  intrathoracic  venae  cavaB  in  inspiration  and  has  not  been 
sent  already  on  into  the  right  ventricle  before  expiration  occurs, 
is,  however,  on  account  of  the  venous  valves,  prevented  from 
flowing  back  whence  it  came,  and  is  imprisoned  in  the  cavse  under 
an  increased  pressure  during  expiration;  and  this  tends  to  make 
it  flow  faster  into  the  auricle  during  the  diastole  of  the  latter.  How 
much  the  alternating  respiratory  movements  assist  the  venous 
flow  is  shown  by  the  dilatation  of  the  veins  of  the  head  and  neck 
which  occurs  when  a  person  is  holding  his  breath;  and  the  black- 
ness of  the  face,  from  distention  of  the  veins  and  stagnation  of 
the  capillary  flow,  which  occurs  during  a  prolonged  fit  of  cough- 
ing, which  is  a  series  of  expiratory  efforts  without  any  inspira- 
tions. 

The  vencricles  and  arteries  are  not  directly  affected  to  any 
appreciable  extent  by  the  respiratory  movements;  their  walls 
are  too  thick  and  the  arterial  pressure  too  great  to  respond  to 
these  small  variations  of  intrathoracic  pressure.  The  increase 
in  venous  flow  which  occurs  during  inspiration  does,  however, 
by  supplying  the  heart  with  more  blood  at  that  time,  bring  about 
a  small  increase  in  arterial  pressure  during  each  inspiration.  The 
increased  blood-supply  is  handled  by  the  heart  through  an  aug- 
mentation of  its  beat.  This  has  been  shown  to  be  brought  about 
by  an  irradiation  to  the  cardiac  centers  of  the  influences  that 
govern  the  act  of  inspiration.  To  a  marked  extent  the  vigor  of 
breathing  and  the  heart-rate  run  parallel. 


RESPIRATION:  THE  MECHANISM  OF  BREATHING       401 

Influence  of  Breathing  Movements  on  the  Lymph-Flow.  D  uring 
inspiration,  when  intrathoracic  pressure  is  lowered,  lymph  is 
pressed  into  the  thoracic  duct  from  the  abdominal  lymphatics. 
In  expiration,  when  thoracic  pressure  rises  again,  the  extra  lymph 
cannot  flow  back  on  account  of  the  valves  in  the  lymphatic  ves- 
sels, and  it  is  consequently  driven  on  to  the  cervical  ending  of  the 
thoracic  duct.  The  breathing  movements  thus  pump  the  lymph 
on. 

The  Respiratory  Center.  The  respiratory  movements  are  to  a 
certain  extent  under  the  control  of  the  will;  we  can  breathe  faster 
or  slower,  shallower  or  more  deeply,  as  we  wish,  and  can  also  "  hold 
the  breath  "  for  some  time — but  the  voluntary  control  thus  exerted 
is  limited  in  extent;  no  one  can  commit  suicide  by  holding  his 
breath.  In  ordinary  quiet  breathing  the  movements  are  quite  in- 
voluntary; they  go  on  perfectly  without  the  least  attention  on  our 
part,  and,  not  only  in  sleep,  but  during  the  unconsciousness  of 
fainting  or  of  an  apoplectic  fit.  The  natural  breathing  movements 
are  therefore  either  reflex  or  automatic. 

The  muscles  concerned  in  producing  the  changes  in  the  chest 
which  lead  to  the  entry  or  exit  of  air  are  of  the  ordinary  striped 
kind;  and  these,  as  we  have  seen,  only  contract  in  the  Body  under 
the  influence  of  the  nerves  going  to  them;  the  nerves  of  the  dia- 
phragm are  the  two  phrenic  nerves,  one  for  each  side  of  it ;  the  ex- 
ternal intercostal  muscles  are  supplied  by  certain  branches  of  the 
thoracic  spinal  nerves,  called  the  intercostal  nerves.  If  the  phrenic 
nerves  be  cut  the  diaphragm  ceases  its  contractions,  and  a  similar 
paralysis  of  the  external  intercostals  follows  section  of  the  inter- 
costal nerves. 

Since  the  inspiratory  muscles  only  act  when  stimulated  by 
nervous  impulses  reaching  them,  we  have  next  to  seek  where  these 
impulses  originate;  and  experiment  shows  that  it  is  in  the  medulla 
oblongata.  All  the  brain  of  a  cat  or  a  rabbit  in  front  of  the  medulla 
can  be  removed,  and  it  will  still  go  on  breathing;  and  children  are 
sometimes  born  with  the  medulla  oblongata  only,  the  rest  of  the 
brain  being  undeveloped,  and  yet  they  breathe  for  a  time.  If,  on 
the  other  hand,  the  spinal  cord  be  divided  immediately  below  the 
medulla  of  an  animal,  all  breathing  movements  of  the  chest  cease 
at  once.  We  conclude,  therefore,  that  the  nervous  impulses  calling 
forth  contractions  of  the  respiratory  muscles  arise  in  the  medulla 


402  THE  HUMAN  BODY 

oblongata,  and  travel  down  the  spinal  cord  and  thence  out  along 
the  phrenic  and  intercostal  nerves.  This  is  confirmed  by  the  fact 
that  if  the  spinal  cord  be  cut  across  below  the  origin  of  the  fourth 
pair  of  cervical  spinal  nerves  (from  which  the  phrenics  mainly 
arise)  but  above  the  first  thoracic  spinal  nerves,  the  respiratory 
movements  of  the  diaphragm  continue,  but  those  of  the  intercostal 
muscles  cease;  this  phenomenon  has  sometimes  been  observed  on 
men  so  stabbed  in  the  back  as  to  divide  the  spinal  cord  in  the 
region  indicated.  Finally,  that  the  nervous  impulses  exciting  the 
inspiratory  muscles  originate  in  the  medulla,  is  proved  by  the  fact 
that  if  a  small  portion  of  that  organ,  the  so-called  vital  point,  be 
destroyed,  all  the  respiratory  movements  cease  at  once  and  for- 
ever, although  all  the  rest  of  the  brain  and  spinal  cord  may  be  left 
uninjured.  This  part  of  the  medulla  is  known  as  the  respiratory 
center. 

Is  the  Respiratory  Center  Reflex?  Since  this  center  goes  on 
working  independently  of  the  will,  we  have  next  to  inquire,  Is  it  a 
reflex  center  or  not?  Are  the  efferent  discharges  it  sends  along  the 
respiratory  nerves  due  to  afferent  impulses  reaching  it  by  centrip- 
etal nerve-fibers?  Or  does  it  originate  efferent  nervous  impulses 
independently  of  excitation  through  afferent  nerves? 

We  know,  in  the  first  place,  that  the  respiratory  center  is  largely 
under  reflex  control;  a  dash  of  cold  water  on  the  skin,  the  irritation 
of  the  nasal  mucous  membrane  by  snuff,  or  of  the  larynx  by  a 
foreign  body,  will  each  cause  a  modification  in  the  respiratory 
movements — a  long  indrawn  breath,  a  sneeze,  or  a  cough.  But, 
although  thus  very  subject  to  influences  reaching  it  by  afferent 
nerves,  the  respiratory  center  seems  essentially  independent  of 
such.  In  many  animals,  as  rabbits  (and  in  some  men),  marked 
breathing  movements  take  place  in  the  nostrils,  which  dilate  during 
inspiration;  and  when  the  spinal  cord  of  a  rabbit  is  cut  close  to  the 
medulla,  thus  cutting  off  all  afferent  nervous  impulses  to  the  re- 
spiratory center  except  such  as  may  reach  it  through  cranial 
nerves,  the  respiratory  movements  of  the  nostrils  still  continue 
until  death.  The  movements  of  the  ribs  and  diaphragm  of  course 
cease,  and  so  the  animal  dies  very  soon  unless  artificial  respiration 
be  maintained.  Moreover,  if  after  cutting  the  spinal  cord  as  above 
described,  the  chief  sensory  cranial  nerves  be  divided,  so  as  to  cut 
off  the  respiratory  center  from  almost  all  possible  afferent  nervous 


RESPIRATION.     THE  MECHANISM  OF  BREATHING      403 

impulses,  regular  breathing  movements  of  the  nostrils  continue. 
We  conclude,  therefore,  that  the  activity  of  the  respiratory  center, 
however  much  it  may  be  capable  of  modification  through  sensory 
nerves,  is  essentially  independent  of  them. 

What  it  is  that  Excites  the  Respiratory  Center.  It  has  long 
been  recognized  that  the  activity  of  the  respiratory  center  is  re- 
lated to  the  condition  of  the  blood  flowing  through  it;  arterial 
blood  excites  it  feebly  or  not  at  all;  venous  blood  excites  it  power- 
fully, and  more  and  more  strongly  as  its  venosity  increases.  The 
difference  between  arterial  and  venous  blood  is  wholly  a  differ- 
ence in  the  relative  amounts  of  oxygen  and  of  carbon  dioxid  present 
therein.  The  question  is:  Does  venous  blood  owe  its  ability  to 
stimulate  the  respiratory  center  to  its  low  oxygen  content  or  to 
its  high  content  of  carbon  dioxid?  Experiment  has  shown  that 
both  factors  enter  somewhat,  but  that  the  center  is  more  affected 
by  small  changes  in  the  amount  of  carbon  dioxid  than  by  small 
changes  in  the  amount  of  oxygen. 

We  might  look  upon  carbon  dioxid  as  the  main  regulator  of  the 
respiratory  center,  and  for  convenience  of  description  shall  do  so. 
As  a  matter  of  exactness,  however,  not  carbon  dioxid  as  such  but 
an  acid  condition  of  the  blood,  dependent  chiefly  on  carbon  dioxid, 
determines  the  activity  of  the  center.  Lack  of  oxygen  may  pro- 
duce indirectly  the  same  acid  condition  that  is  brought  about  by 
excess  of  carbon  dioxid.  So  we  see  that  we  are  not  entitled  to  as- 
sign the  control  of  the  center  exclusively  to  carbon  dioxid,  although 
that  substance  determines  under  ordinary  circumstances,  its 
stimulation. 

Why  are  the  Respiratory  Discharges  Rhythmic?  If  carbon 
dioxid  is  the  stimulus  for  the  respiratory  center,  why  does  that 
center  act  rhythmically?  Does  the  carbon  dioxid  content  of  the 
circulating  blood  increase  and  decrease  fifteen  times  or  more  a 
minute?  The  answer  to  this  question  is  afforded  by  a  simple  ex- 
periment. If  in  an  animal  breathing  naturally  under  anesthesia 
both  vagus  nerves  are  cut  there  is  an  immediate  change  in  the 
character  of  the  respirations.  From  being  rapid  and  shallow  they 
become  very  deep  and  take  on  a  much  slower  rate.  Under  this 
condition  we  may  properly  assume  that  the  respirations  do  follow 
the  carbon  dioxid  content  of  the  blood;  the  center  begins  to  dis- 
charge when  the  blood  contains  enough  carbon  dioxid  to  stimulate 


404  THE  HUMAN  BODY 

it,  and  continues  its  discharge  until  the  aeration  of  the  blood,  re- 
sulting from  the  inspiration,  lowers  the  carbon  dioxid  below  the 
point  of  stimulation.  There  follows  a  period  of  expiration  and 
rest  which  continues  until  sufficient  carbon  dioxid  has  again  ac- 
cumulated to  start  the  action  anew. 

Since  with  the  vagus  nerves  cut  the  respirations  follow  the  car- 
bon dioxid  concentration  of  the  blood,  but  with  the  nerves  intact 
do  not,  being  much  more  shallow  and  rapid,  we  must  determine 
the  influence  of  the  vagus  nerves  upon  the  center  in  order  to  un- 
derstand ordinary  breathing.  It  has  been  shown  that  the  influence 
of  the  vagus  nerves  is  a  simple  reflex  one.  These  nerves  contain 
sensory  fibers  arising  in  the  lung  tissue  and  so  situated  as  to  be 
stimulated  mechanically  every  time  the  lung  is  inflated.  The  im- 
pulses conveyed  over  these  fibers  to  the  central  nervous  system 
are  inhibitory  to  the  respiratory  center.  Bearing  this  action  of  the 
vagus  fibers  in  mind  we  may  account  for  normal  breathing  thus; 
the  blood  contains  enough  carbon  dioxid  all  the  time,  under  ordi- 
nary circumstances,  to  stimulate  the  respiratory  center;  when- 
ever the  center  discharges  under  this  stimulus  it  brings  about 
the  movements  of  inspiration  which  result  in  expansion  of  the 
lungs;  whenever  the  lungs  expand  the  sensory  fibers  contained  in 
their  walls  are  stimulated  and  so  inhibitory  influences  are  sent  to 
the  respiratory  center.  Inspiration  proceeds,  then,  until  the  in- 
hibitory impulses  from  the  lungs  overcome  the  stimulus  of  carbon 
dioxid,  when  it  comes  to  an  end  and  the  thorax  falls  back  to  the 
position  of  rest.  This  falling  back,  which  constitutes  normal  ex- 
piration, collapses  the  lungs  somewhat;  the  inhibitory  impulses 
diminish  or  disappear;  and  the  stimulating  action  of  the  carbon 
dioxid  again  becomes  effective.  Thus  in  normal  breathing  in- 
spiration and  expiration  follow  one  another  without  any  pause 
between,  and  the  respirations  are  shallow  because  the  inhibition 
cuts  them  off  almost  as  soon  as  started. 

The  entire  purpose  of  breathing  is  to  ventilate  the  lungs.  It  is 
relatively  a  minor  matter  whether 'a  system  of  rapid  shallow  breaths 
or  of  slow  deep  ones  is  used  so  long  as  the  result  is  secured.  The 
necessary  amount  of  air  would  be  taken  into  and  discharged  from 
the  lungs  every  minute  by  either  arrangement.  There  may  pos- 
sibly be  some  advantage  to  the  Body  in  the  rapid  shallow  type  in 
avoiding  such  wide  fluctuations  in  the  concentrations  of  the  respira- 


RESPIRATION.    THE  MECHANISM  OF  BREATHING      405 

tory  gases  in  the  blood  and  the  alveoli  as  would  occur  with  slower 
and  deeper  breathing. 

Forced  Expiration.  Although  in  ordinary  quiet  breathing,  as 
we  have  seen,  expiration  is  a  passive  collapse  of  the  chest,  active 
expiratory  effort  is  of  frequent  occurrence.  In  talking,  singing, 
whistling,  as  well  as  in  coughing,  sneezing,  and  straining,  the  ex- 
piratory muscles  are  functioning.  The  ease  with  which  these  are 
brought  into  play  suggests  that  the  part  of  the  center  which 
controls  them,  although  not  normally  in  action,  is  hung  on  a 
"hair  trigger"  so  to  speak,  requiring  very  slight  additional  in- 
fluence to  arouse  it  to  action. 

Sensitiveness  of  the  Respiratory  Center.  The  respiratory 
center  is  responsive  to  very  slight  changes  in  the  amount  of  carbon 
dioxid  in  the  blood.  A  small  increase  quickens  the  breathing 
notably.  The  effect  of  the  quickened  breathing  is  to  ventilate  the 
lungs  more  thoroughly,  and  thus  to  bring  about  a  more  rapid 
movement  of  carbon  dioxid  from  the  blood  to  the  alveolar  air  (see 
next  chapter).  In  this  manner  the  increased  amount  of  carbon 
dioxid  is  gotten  rid  of.  The  respiratory  center  may  be  thought  of 
as  a  delicate  governor  which  serves  to  keep  the  carbon  dioxid 
content  of  the  blood  at  a  uniform  level.  The  chief  source  of  carbon 
dioxid  is  in  the  active  muscles.  Muscular  exercise  is,  therefore, 
the  most  common  cause  of  quickened  breathing.  If  so  careful  a 
regulation  of  the  amount  of  carbon  dioxid  in  the  blood  seems  at 
first  not  very  important  we  can  better  appreciate  its  significance 
by  recalling  that  carbon  dioxid  is  the  end  product  of  oxidation, 
so  that  any  change  in  the  amount  of  carbon  dioxid  in  the  blood 
means  a  corresponding  change  in  the  consumption  of  oxygen  by 
the  Body;  and,  furthermore,  that  the  changes  in  lung  ventilation 
which  serve  to  keep  the  carbon  dioxid  level  steady  have  the  effect 
at  the  same  time  of  increasing  or  diminishing  the  supply  of  avail- 
able oxygen.  Thus  an  increase  in  carbon  dioxid  brings  about  a 
more  pronounced  ventilation  of  the  lungs,  and  thus,  in  turn  pro- 
vides more  oxygen  than  is  brought  to  the  lungs  during  ordinary 
breathing.  There  will  be  no  need  for  a  change  in  the  amount  of 
oxygen  unless  there  is  a  change  in  the  bodily  oxidations,  so  when- 
ever more  oxygen  is  needed  the  need  is  signalized  by  an  increase  in 
the  carbon  dioxid,  and  this  acts  to  bring  into  play  the  mechanism 
by  which  the  required  oxygen  is  supplied. 


406  THE  HUMAN  BODY 

Eupnea,  Hyperpnea,  Dyspnea,  Apnea.  Ordinary  quiet  breath^ 
ing  is  known  as  eupnea.  Rapid  breathing,  such  as  follows 
moderate  exercise,  is  designated  as  hyperpnea.  When  the  breath- 
ing is  forced,  and  especially  when  forced  expiration  enters,  we 
have  the  condition  called  dyspnea.  This  results  from  abnormal 
excitement  of  the  respirator}*-  center  either  reflexly,  as  from  stimula- 
tion of  pain  nerves,  or  by  a  greater  increase  in  the  carbon  dioxid 
content  of  the  blood  than  that  which  causes  simple  hyperpnea. 
The  dyspnea  of  the  early  stages  of  suffocation  arises  from  this 
latter  cause.  Apnea,  or  absence  of  breathing,  may  result  from 
one  of  two  conditions  or  from  both  acting  together.  The  first 
of  these  is  a  deficiency  of  carbon  dioxid  in  the  blood,  so  that  the 
respiratory  center  is  not  stimulated.  The  second  is  inhibition  of 
the  center  through  vigorous  and  repeated  inflation  of  the  lungs. 
Since  inflation  of  the  lungs  with  ordinary  air  brings  about  both 
conditions  the  apnea  which  results  from  this  treatment  is  partly 
chemical  and  partly  inhibitory.  That  inhibition  enters  in  the 
production  of  apnea  in  this  way  is  shown  by  the  greater  difficulty 
of  producing  the  condition  in  animals  with  both  vagi  cut. 

Holding  the  Breath.  When  one  holds  his  breath  he  is  sending 
impulses  to  the  respiratory  center  which  inhibit  its  discharge. 
Meanwhile  the  bodily  oxidations  go  right  on,  so  the  longer  this 
inhibition  continues  the  greater  becomes  the  amount  of  carbon 
dioxid  in  the  blood,  and  the  more  powerfully  does  the  normal 
excitation  of  the  center  act.  In  a  very  short  time  the  carbon 
dioxid  stimulation  becomes  more  p6tent  than  the  volitional  in- 
hibition, and  when  that  time  comes  a  breath  must  be  taken  in 
spite  of  the  effort  to  hold  it.  Evidently  any  procedure  that  will 
diminish  the  amount  of  carbon  dioxid  in  the  blood  to  begin  with 
will  prolong  the  time  the  breath  can  be  held.  This  can  be  done  by 
forced  breathing  for  several  minutes.  The  over-ventilation  of  the 
lungs  thus  carried  on  sweeps  out  so  much  carbon- dioxid  from  the 
blood  that  a  much  longer  time  elapses  than  ordinarily  before  the 
accumulation  overcomes  the  volitional  inhibition.  Since,  as  we 
shall  learn  (p.  421),  over-ventilation  of  the  lungs  does  not  ma- 
terially increase  the  supply  of  available  oxygen,  this  procedure 
may  bring  about  severe  oxygen  deficiency,  which  shows  itself  by 
blueness  of  the  skin  and  mucous  membranes,  a  blueness  caused  by 
the  venous  condition  of  the  blood  in  the  arteries. 


RESPIRATION.     THE  MECHANISM  OF  BREATHING      407 

Asphyxia.  Asphyxia  is  death  from  suffocation,  or  want  of 
oxygen  by  the  tissues.  It  may  be  brought  about  in  various  ways; 
as  by  strangulation,  which  prevents  the  entry  of  air  into  the  lungs; 
or  by  exposure  in  an  atmosphere  containing  no  oxygen;,  or  by 
putting  an  animal  in  a  vacuum;  or  by  making  it  breathe  air  con- 
taining a  gas  which  has  a  stronger  affinity  for  hemoglobin  than 
oxygen  has,  and  which,  therefore,  turns  the  oxygen  out  of  the  red 
corpuscles  and  takes  its  place.  The  gases  which  do  the  latter  are 
very  interesting  since  they  serve  to  prove  conclusively  that  the 
Body  can  live  only  by  the  oxygen  carried  around  by  the  hemo- 
globin of  the  red  corpuscles;  the  amount  dissolved  in  the  blood- 
plasma  being  insufficient  for  its  needs.  Of  such  gases  carbon 
monoxid  is  the  most  important  and  best  studied;  in  the  frequent 
mode  of  committing  suicide  by  stopping  up  all  the  ventilation 
holes  of  a  room  and  turning  on  the  gas,  it  is  poisoning  by  carbon 
monoxid  which  causes  death. 

The  Phenomena  of  Asphyxia.  As  soon  as  the  carbon  dioxid  in 
the  blood  rises  above  the  normal  amount  the  breathing  becomes 
hurried  and  deeper,  and  the  extraordinary  muscles  of  respiration 
are  called  into  activity.  The  dyspnea  becomes  more»and  more 
marked,  and  this  is  especially  the  case  with  the  expirations  which, 
almost  or  quite  passively  performed  in  natural  breathing,  become 
violently  muscular.  At  last  nearly  all  the  muscles  in  the  Body  are 
set  at  work;  the  rhythmic  character  of  the  respiratory  acts  is  lost, 
and  general  convulsions  occur,  but,  on  the  whole,  the  contractions 
of  the  expiratory  muscles  are  more  violent  than  those  of  jtheii> 
spiratory. 

The  violent  excitation  of  the  nerve-centers  soon  exhausts  them, 
and  all  the  more  readily  since  their  oxygen  supply  (which  they  like 
all  other  tissues  need  in  order  to  continue  their  activity)  is  cut  off. 
The  convulsions  therefore  gradually  cease,  and  the  animal  be- 
comes calm  again,  save  for  an  occasional  act  of  breathing:  these 
final  movements  are  inspirations  and,  becoming  less  and  less  fre- 
quent, at  last  cease,  and  the  animal  appears  dead.  Its  heart,  how- 
ever, though  gorged  with  extremely  dark  venous  blood  still  makes 
some  slow  feeble  pulsations.  So  long  as  it  beats  artificial  respira- 
tion can  restore  the  animal,  but  once  the  heart  has  finally  stopped 
restoration  is  impossible.  There  are  thus  three  distinguishable 
stages  in  death  from  asphyxia.  (1)  The  stage  of  dyspnea.  (2) 


408  THE  HUMAN  BODY 

The  stage  of  convulsions.  (3)  The  stage  of  exhaustion;  the  con- 
vulsions having  ceased  but  there  being  from  time  to  time  an  in- 
spiration. The  end  of  the  third  stage  occurs  in  a  mammal  about 
five  minuter  after  the  oxygen  supply  has  been  totally  cut  off.  If 
the  asphyxia  be  due  to  deficiency,  and  not  absolute  want  of  oxy- 
gen, of  course  all  the  stages  take  longer. 

Artificial  Respiration.  Asphyxia  from  drowning  and  other 
causes  occurs  with  lamentable  frequency  these  days,  and  there  is 
no  doubt  that  many  lives  are  sacrificed  through  ignorance  on  the 
part  of  bystanders  of  the  proper  restorative  procedures.  There 
are  several  methods  of  applying  artificial  respiration  to  human 
beings.  The  method  of  Schaefer  is  as  effective  as  any.  The  follow- 
ing description  is  quoted  from  his  paper  on  the  subject:  "The 
method  consists  in  laying  the  subject  in  the  prone  posture,  prefer- 
ably on  the  ground,  with  a  thick  folded  garment  underneath  the 
chest  and  epigastrium.  The  operator  puts  himself  athwart  or  at 
the  side  of  the  subject,  facing  his  head  and  places  his  hands  on 
each  side  over  the  lower  part  of  the  back  (lowest  ribs).  He  then 
slowly  throws  the  weight  of  his  Body  forward  to  bear  upon  his  own 
arms,  and  thus  presses  upon  the  thorax  of  the  subject  and  forces 
air  out  of  the  lungs.  This  being  effected,  he  gradually  relaxes  the 
pressure  by  bringing  his  own  Body  up  again  to  a  more  erect  posi- 
tion, but  without  moving  the  hands."  These  movements  should 
be  repeated  about  fifteen  times  a  minute  until  normal  breathing  is 
resumed,  and  should  not  be  given  up  for  at  least  a  half  hour  if  re- 
covery does  not  occur  sooner.  If  there  is  water  in  the  lungs  it 
should  be  allowed  to  drain  out  before  the  artificial  respiration  is 
begun.  Otherwise  it  may  be  churned  into  a  foam  by  the  move- 
ments, and  defeat  the  desired  ventilation  of  the  lungs. 

Modified  Respiratory  Movements.  Sighing  is  a  deep,  long- 
drawn  inspiration  followed  by  a  shorter  but  correspondingly  large 
expiration.  Yawning  is  similar,  but  the  air  is  mainly  taken  in  by 
the  mouth  instead  of  the  nose,  and  the  lower  jaw  is  drawn  down  in 
a  characteristic  manner.  Hiccough  depends  upon  a  sudden  con- 
traction of  the  diaphragm,  while  the  aperture  of  the  larynx  closes; 
the  entering  air,  drawn  through  the  narrowing  opening,  causes  the 
peculiar  sound.  Coughing  consists  of  a  full  inspiration  followed  by 
a  violent  and  rapid  expiration,  during  the  first  part  of  which  the 
laryngeal  opening  is  kept  closed;  being  afterwards  suddenly  opened, 


RESPIRATION.     TIJK  MECHANISM  OF  BREATHING      409 

the  air  issues  forth  with  a  rush,  tending  to  carry  out  with  it  any- 
thing lodged  in  the  windpipe  or  larynx.  Sneezing  is  much  like 
coughing,  except  that,  while  in  a  cough  the  isthmus  of  the  fauces 
is  held  open  and  the  air  mainly  passes  out  through  the  mouth, 
in  sneezing  the  fauces  are  closed  and  the  blast  is  driven  through 
the  nostrils.  It  is  commonly  excited  by  irritation  of  the  nasal 
mucous  membrane,  but  in  many  persons  a  sudden  bright  light 
falling  into  the  eye  will  produce  a  sneeze.  Laughing  consists  of 
a  series  of  short  expirations  following  a  single  inspiration;  the 
larynx  is  open  all  the  time,  and  the  vocal  cords  (Chap.  XXXIII) 
are  set  in  vibration.  Crying  is,  physiologically,  much  like  laughing 
and,  as  we  all  know,  one  often  passes  into  the  other.  The  accom- 
panying contractions  of  the  face  muscles  giving  expression  to  the 
countenance  are,  however,  different  in  the  two. 

All  these  modified  respiratory  acts  are  essentially  reflex  and 
they  serve  to  show  to  what  a  great  extent  the  discharges  of  the 
respiratory  center  can  be  modified  by  afferent  nerve  impulses;  but, 
with  the  exception  of  hiccough,  they  are  to  a  certain  extent,  like 
natural  breathing,  under  the  control  of  the  will.  Most  of  them, 
too,  can  be  imitated  more  or  less  perfectly  by  voluntary  muscular 
movements;  though  a  good  stage  sneeze  or  cough  is  rare. 


CHAPTER  XXIV 
RESPIRATION.     THE  GASEOUS  INTERCHANGES 

Nature  of  the  Problems.  The  study  of  the  respiratory  process 
from  a  chemical  standpoint  has  for  its  object  to  discover  what  are, 
in  kind  and  extent,  the  interchanges  between  the  air  in  the  lungs 
and  the  blood  in  the  pulmonary  capillaries;  and  the  nature  and 
amount  of  the  corresponding  gaseous  changes  between  the  living 
tissues,  and  the  blood  in  the  systemic  capillaries.  Neglecting  some 
oxygen  used  up  otherwise  than  in  forming  carbon  dioxid,  and  some 
carbon  dioxid  eliminated  by  other  organs  than  the  lungs,  these 
processes  in  the  long  run  balance,  the  blood  losing  as  much  carbon 
dioxid  gas  in  the  lungs  as  it  gains  elsewhere,  and  gaining  as 
much  oxygen  in  the  lungs  as  it  loses  in  the  systemic  capillaries. 
To  comprehend  the  matter  it  is  necessary  to  know  the  physical 
and  chemical  conditions  of  these  gases  in  the  lungs,  in  the  blood, 
and  in  the  tissues  generally;  for  only  so  can  we  understand  how 
it  is  that  in  different  localities  of  the  Body  such  exactly  contrary 
processes  occur.  So  far  as  the  problems  connected  with  the  exter- 
nal respiration  are  concerned  our  knowledge  is  tolerably  complete; 
but  as  regards  the  internal  respiration,  taking  place  all  through 
the  Body,  much  has  yet  to  be  learnt;  we  know  that  a  muscle  at 
work  gives  more  carbon  dioxid  to  the  blood  than  one  at  rest  and 
takes  more  oxygen  from  it,  but  how  much  of  the  one  it  gives  and 
of  the  other  it  takes  is  only  known  approximately ;  as  are  the  con- 
ditions under  which  this  greater  interchange  during  the  activity 
of  the  muscular  tissue  is  effected:  and  concerning  nearly  all  the 
other  tissues  we  know  even  less  than  about  muscle.  In  fact,  as 
regards  the  Body  as  a  whole,  it  is  comparatively  easy  to  find  how 
great  its  gaseous  interchanges  with  the  air  are  during  work  and 
rest,  waking  and  sleeping,  while  fasting  or  digesting,  and  so  on, 
but  when  it  comes  to  be  decided  what  organs  are  concerned  in 
each  case  in  producing  the  greater  or  less  exchange,  and  how 
much  of  the  whole  is  due  to  each  of  them,  the  question  is  one  far 
more  difficult  to  settle  and  still  very  far  from  completely  answered. 

410 


RESPIRATION.     THE  GASEOUS  INTERCHANGES         411 

The  Changes  Produced  in  Air  by  Being  Once  Breathed.  These 
are  fourfold — changes  in  its  temperature,  in  its  moisture,  in  its 
chemical  composition,  and  its  volume. 

The  air  taken  into  the  lungs  is  nearly  always  cooler  than  that 
expired,  which  has  a  temperature  of  about  36°  C.  (97°  R).  The 
temperature  of  a  room  is  usually  less  than  21°  C.  (70°  R).  The 
warmer  the  inspired  air  the  less,  of  course,  the  heat  which  is  lost 
to  the  Body  in  the  breathing  process;  its  average  amount  is  calcu- 
lated as  about  equal  to  ,50  Calories  in  twenty-four  hours;  a  Calory 
being  as  much  heat  as  will  raise  the  temperature  of  one  kilogram 
(2.2  Ibs.)  of  water  one  degree  centigrade  (1.8°  R). 

The  inspired  air  always  contains  more  or  less  water  vapor,  but 
is  rarely  saturated;  that  is,  rarely  contains  so  much  but  it  can 
take  up  more  without  showing  it  as  mist;  the  warmer  air  is,  the 
more  water  vapor  is  required  to  saturate  it.  The  expired  air  is 
nearly  saturated  for  the  temperature  at  which  it  leaves  the  Body, 
as  is  readily  shown  by  the  water  deposited  when  it  is  slightly 
cooled,  as  when  a  mirror  is  breathed  upon;  or  by  the  clouds  seen 
issuing  from  the  nostrils  on  a  frosty  day,  these  being  due  to  the 
fact  that  the  air,  as  soon  as  it  is  cooled,  cannot  hold  all  the  water 
vapor  which  it  took  up  when  warmed  in  the  Body.  Air,  therefore, 
when  breathed  once,  gains  water  vapor  and  carries  it  off  from  the 
lungs;  the  actual  amount  being  subject  to  variation  with  the 
temperature  and  saturation  of  the  inspired  air:  the  cooler  and  drier 
that  is,  the  more  water  will  it  gain  when  breathed.  On  an  aver- 
age the  amount  thus  carried  off  in  twenty-four  hours  is  about  255 
grams  (9  ounces).  To  evaporate  this  water  in  the  lungs  an  amount 
of  heat  is  required,  which  disappears  for  this  purpose  in  the  Body, 
to  reappear  again  outside  it  when  the  water  vapor  condenses. 
The  amount  of  heat  taken  off  in  this  way  during  the  day  is  about 
148  Calories.  The  total  daily  loss  of  heat  from  the  Body  through 
the  lungs  averages  therefore  198  Calories,  50  in  warming  the  in- 
spired air  and  148  in  the  evaporation  of  water. 

The  most  important  changes  brought  about  in  the  breathed  air 
are  those  in  its  chemical  composition.  Pure  air  when  completely 
dried  consists  in  each  100  parts  of: 

By  Volume        By  Weight 

Oxygen 21  23 

Nitrogen 79  77 


412  THE  HUMAN  BODY 

Ordinary  atmospheric  air  contains  in  addition  4  volumes  of 
carbon  dioxid  in  10,000,  or  0.04  in  100,  a  quantity  which,  for  prac- 
tical purposes,  may  be  neglected.  When  breathed  once,  such 
air  gains  rather  more  than  4  volumes  in  100  of  carbon  dioxid, 
and  loses  a  little  less  than  5  of  oxygen.  More  accurately,  100  vol- 
umes of  expired  air  after  drying  contain: 

Oxygen 16. 

Nitrogen 79. 

Carbon  dioxid 4.4 

Since  10,800  liters  (375  cubic  feet)  of  air  are  breathed  in  twenty- 
four  hours  and  lose  5  per  cent  of  oxygen,  the  total  quantity  of 
this  gas  taken  up  in  the  lungs  daily  is  10,800  X  5  -f-  IPO  =  540 
liters.  One  liter  of  oxygen  measured  at  0°  C.  (32°  F.)  and  under  a 
pressure  equal  to  one  atmosphere,  weighs  1.43  grams,  so  the  total 
weight  of  oxygen  taken  up  by  the  lungs  daily  is  540  X  1.43  =  772 
grams  (27  ounces). 

The  amount  of  carbon  dioxid  excreted  from  the  lungs  being 
4.4  per  cent  of  the  volume  of  the  air  breathed  daily,  is  10,800  x 
4.4  -f-  100  =  475  liters  measured  at  the  normal  temperature  and 
pressure.  This  volume  weighs  930  grams,  or  32.5  ounces.  If  all 
the  oxygen  taken  in  were  breathed  out  again  as  carbon  dioxid 
the  volume  of  the  latter  should  equal  that  of  the  oxygen  breathed 
in.  The  discrepancy  results  from  the  fact  that  not  all  the  oxygen 
combines  with  carbon;  some  of  it  unites  with  hydrogen  to  form 
water.  The  water  thus  formed  simply  adds  itself  to  the  general 
water  content  of  the  Body,  and  has  no  bearing  on  the  amount  dis- 
charged in  the  expired  air;  this  latter  depending,  as  already  stated, 
on  the  rate  of  evaporation  from  the  lung  surface. 

If  the  expired  air  be  measured  as  it  leaves  the  Body  its  bulk 
will  be  found  greater  than  that  of  the  inspired  air,  since  it  not 
only  has  water  vapor  added  to  it,  but  is  expanded  in  consequence 
of  its  higher  temperature.  If,  however,  it  be  dried  and  reduced 
to  the  same  temperature  as  the  inspired  air  its  volume  will  be 
found  diminished,  since  it  has  lost  5  volumes  per  cent  of  oxygen 
and  gained  only  4.4  of  carbon  dioxid.  In  round  numbers,  100 
volumes  of  dry  inspired  air  at  zero,  give  99  volumes  of  dry  expired 
air  measured  at  the  same  temperature  and  pressure. 

Ventilation.    Since  at  every  breath  some  oxygen  is  taken  from 


RESPIRATION.    THE  GASEOUS  INTERCHANGES         413 

the  air  and  some  carbon  dioxid  given  to  it,  were  the  atmosphere 
around  a  living  man  not  renewed  he  would,  at  last,  be  unable  to 
get  from  the  air  the  oxygen  he  required;  he  would  die  of  oxygen 
starvation  or  be  suffocated,  as  such  a  mode  of  death  is  called,  as 
surely,  though  not  quite  so  fast,  as  if  he  were  put  under  the  re- 
ceiver of  an  air-pump  and  all  the  air  around  him  removed.  Hence 
the  necessity  of  ventilation  to  supply  fresh  air  in  place  of  that 
breathed,  and  clearly  the  amount  of  fresh  air  requisite  must  be 
determined  by  the  number  of  persons  collected  in  a  room;  the 
supply  which  would  be  ample  for  one  person  would  be  insufficient 
for  two.  Moreover,  fires,  gas,  and  oil  lamps,  all  use  up  the  oxygen 
of  the  air  and  give  carbon  dioxid  to  it,  and  hence  calculation 
must  be  made  for  them  in  arranging  for  the  ventilation  of  a  build- 
ing in  which  they  are  to  be  employed. 

In  order  that  air  be  unwholesome  to  breathe,  it  is  by  no  means 
necessary  that  it  have  lost  so  much  of  its  oxygen  as  to  make  it 
difficult  for  the  Body  to  get  what  it  wants  of  that  gas.  The  evil 
results  of  insufficient  air-supply  are  rarely,  if  ever,  due  to  that 
cause  even  in  the  worst-ventilated  room  for,  as  we  shall  see  here- 
after, the  blood  is  able  to  take  what  oxygen  it  wants  from  air 
containing  comparatively  little  of  that  gas.  The  headache  and 
drowsiness  which  come  on  from  sitting  in  a  badly  ventilated  room 
appear  to  be  due  chiefly  to  the  high  percentage  of  water  vapor 
present  under  such  circumstances,  and  the  want  of  energy  and 
general  ill-health  which  result  from  permanently  living  in  such 
surroundings  are  probably  the  result  of  a  slow  poisoning  of  the 
Body  by  absorption  of  gaseous  substances  given  off  to  the  air,  not 
from  the  lungs,  but  from  the  skin  in  evaporating  sweat  and  from 
the  alimentary  tract.  The  idea,  formerly  held  very  generally, 
that  volatile  poisons  are  given  off  by  the  lungs  in  quantities  too 
small  for  chemical  detection,  has  been  largely  abandoned  partly 
because  of  the  failure  of  the  most  careful  experiments  to  demon- 
strate any  such  substances,  but  more  because  there  are  enough 
injurious  materials  given  off  from  other  channels  of  the  Body  to 
explain  all  the  ill  effects  of  insufficient  ventilation. 

That  the  air  of  rooms  occupied  by  persons  becomes  injurious 
long  before  the  amount  of  carbon  dioxid  in  it  is  sufficient  to  do 
any  harm  has  been  abundantly  demonstrated.  Breathing  air 
containing  one  or  two  per  cent  of  that  gas  produced  by  ordinary 


414  THE  HUMAN  BODY 

chemical  methods  does  no  particular  injury,  but  air  containing 
one  per  cent  of  it  produced  by  respiration  is  decidedly  injurious, 
because  of  the  other  things  present  in  it  at  the  same  time.  Carbon 
dioxid  itself,  at  least  in  any  such  percentage  as  is  commonly  found 
in  a  room,  is  not  poisonous,  as  used  to  be  believed,  but,  since  it  is 
tolerably  easily  estimated  in  air,  while  the  actually  injurious 
substances  also  present  are  not,  the  purity  or  foulness  of  the 
air  in  a  room  is  usually  determined  by  finding  the  percentage 
of  carbon  dioxid  in  it:  it  must  be  borne  in  mind  that  to  mean 
much  this  carbon  dioxid  must  have  been  produced  by.  breathing; 
the  amount  of  it  found  is  in  itself  no  guide  to  the  quantity  of  really 
important  injurious  substances  present.  Of  course  when  a  great 
deal  of  carbon  dioxid  is  present  the  air  is  irrespirable :  as  for  ex- 
ample sometimes  at  the  bottom  of  wells  or  brewing-vats. 

In  one  minute  .5  x  15  =  7.5  liters  (0.254  cubic  feet)  of  air  are 
breathed  and  this  is  vitiated  with  carbon  dioxid  to  the  extent  of 
rather  more  than  four  per  cent;  mixed  with  three  times  its  volume 
of  external  air,  it  would  give  thirty  liters  (a  little  over  one  cubic 
foot)  vitiated  to  the  extent  of  one  per  cent,  and  such  air  is  not 
respirable  for  any  length  of  time  with  safety.  The  result  of  breath- 
ing it  for  an  evening  is  headache  and  general  malaise;  of  breath- 
ing it  weeks  or  months  a  lowered  tone  of  the  whole  Body — less 
power  of  work,  physical  or  mental,  and  less  power  of  resisting 
disease;  the  ill  effects  may  not  show  themselves  at  once,  and  may 
accordingly  be  overlooked,  or  considered  scientific  fancies,  by 
the  careless;  but  they  are  nevertheless  there  ready  to  manifest 
themselves.  In  order  to  have  air  to  breathe  in  an  even  moder- 
ately pure  state  every  man  should  get  for  his  own  allowance  at 
least  23,000  liters  of  space  to  begin  with  (about  800  cubic  feet) 
and  the  arrangements  for  ventilation  should,  at  the  very  least, 
renew  this  at  the  rate  of  30  liters  (one  cubic  foot)  per  minute.  In 
the  more  recently  constructed  hospitals,  as  a  result  of  experience, 
twice  the  above  minimum  cubic  space  is  allowed  for  each  bed  in  a 
ward,  and  the  replacement  of  the  old  air  at  a  far  more  rapid  rate, 
100,000  liters  per  hour  per  person,  is  also  provided  for. 

Ventilation  does  not  necessarily  imply  draughts  of  cold  air,  as 
is  often  supposed.  In  warming  by  indirect  radiation  (the  ordi- 
nary hot-air  furnace)  it  may  readily  be  secured  by  arranging,  in 
addition  to  the  registers  from  which  the  warmed  air  reaches  the 


RESPIRATION.     THE  GASEOUS  INTERCHANGES         415 

room,  proper  openings  at  the  opposite  side,  by  which  the  old  air 
may  pass  off  to  make  room  for  the  fresh.  An  open  fire  in  a  room 
will  always  keep  up  a  current  of  air  through  it,  and  is  the  healthiest, 
though  not  the  most  economical,  method  of  warming  an  apart- 
ment. 

In  severe  weather,  when  there  is  a  tendency  to  keep  rooms  rather 
tightly  closed,  a  good  plan  is  to  open  widely  all  doors  and  windows 
for  a  few  minutes  each  day,  allowing  fresh  air  to  penetrate  to 
every  corner,  sweeping  out  the  old  air  before  it.  This  daily  re- 
newing, helped  out  by  such  ventilation  as  is  afforded  by  ill-fitting 
doors  and  windows,  usually  keeps  the  air  of  rooms  in  respirable 
condition  when  not  occupied  by  too  many  persons.  The  modern 
habit  of  sleeping  summer  and  winter  in  rooms  with  open  windows 
is  to  be  highly  commended,  and  should  be  even  more  generally 
adopted.  In  fact  the  more  outdoor  air  one  can  have,  and  at 
the  same  time  keep  warm,  the  better  for  the  bodily  well-being. 
The  beneficial  effects  of  fresh  air  and  sunshine,  especially  in  pul- 
monary tuberculosis,  cannot  be  too  strongly  emphasized. 

Reference  was  made  above  to  the  fact  that  discomfort  in  illy 
ventilated  rooms  is  more  a  matter  of  the  amount  of  water  vapor 
present  than  of  excess  carbon  dioxid  or  other  poisons,  or  of  de- 
ficient oxygen.  Recent  careful  studies  have  emphasized  this 
fact  so  clearly  as  to  bring  about  marked  changes  in  the  practice 
of  ventilation  experts,  particularly  in  their  treatment  of  the  prob- 
lem of  ventilating  auditoriums,  and  other  places  where  large  num- 
bers of  people  gather  temporarily.  To  secure  highest  bodily  com- 
fort there  should  be  a  certain  degree  of  humidity  in  association 
with  a  certain  temperature.  If  the  temperature  changes  the 
amount  of  water  vapor  in  the  air  should  change  to  correspond. 
Too  low  humidity  is  to  be  avoided  as  well  as  too  high.  An  or- 
dinary fault  in  ventilation  is  that  the  air  is  allowed  to  become  too 
dry.  This  is  particularly  true  during  the  winter  months  when 
artificial  heat  is  used.  To  maintain  the  desired  humidity  in  dwell- 
ing houses  of  ordinary  size  during  cold  weather  from  lJ/£  to  2 
gallons  of  water  should  be  evaporated  in  the  house  daily.  Where 
large  numbers  of  house  plants  are  kept  the  evaporation  from  their 
leaves  will  contribute  materially  toward  this  amount. 

While  comfort  depends  on  proper  relationship  of  temperature 
and  moisture,  we  must  not  lose  sight  of  the  fact  that  ultimate 


416  THE  HUMAN  BODY 

well-being  requires  that  the  air  that  we  breathe  be  reasonably 
pure  also.  Provisions  for  renewing  the  air  of  occupied  rooms 
must  not  be  neglected,  therefore,  in  working  out  ventilation  plans. 

Changes  undergone  by  the  Blood  in  the  Lungs.  These  are  the 
exact  reverse  of  those  undergone  by  the  breathed  air — what  the 
air  gains  the  blood  loses,  and  vice  versa.  Consequently,  the  blood 
loses  heat,  and  water,  and  carbon  dioxid  in  the  pulmonary  capil- 
laries; and  gains  oxygen.  These  gains  and  losses  are  accompanied 
by  a  change  of  color  from  the  dark  purple  which  the  blood  ex- 
hibits in  the  pulmonary  artery,  to  the  bright  scarlet  it  possesses  in 
the  pulmonary  veins. 

The  dependence  of  this  color  change  upon  the  access  of  fresh 
air  to  the  lungs  while  the  blood  is  flowing  through  them,  can  be 
readily  demonstrated.  If  a  rabbit  be  rendered  unconscious  by 
chloroform,  and  its  chest  be  opened,  after  a  pair  of  bellows  has 
been  connected  with  its  windpipe,  it  is  seen  that,  so  long  as  the 
bellows  are  worked  to  keep  up  artificial  respiration,  the  blood  in 
the  right  side  of  the  heart  (as  seen  through  the  thin  auricle)  and 
that  in  the  pulmonary  artery,  is  dark  colored,  while  that  in  the 
pulmonary  veins  and  the  left  auricle  is  bright  red.  Let,  however, 
the  artificial  respiration  be  stopped  for  a  few  seconds  and,  conse- 
quently, the  renewal  of  the  air  in  the  lungs  (since  an  animal  can- 
not breathe  for  itself  when  its  chest  is  opened),  and  very  soon  the 
blood  returns  to  the  left  auricle  as  dark  as  it  left  the  right.  In  a 
very  short  time  symptoms  of  suffocation  show  themselves  and  the 
animal  dies,  unless  the  bellows  be  again  set  at  work. 

In  a  former  paragraph  (p.  412)  we  saw  that  about  5  volumes  in 
100  of  oxygen  are  absorbed  from  the  alveolar  air  into  the  blood, 
and  4.4  in  100  of  carbon  dioxid  given  off  to  the  alveolar  air  from  the 
blood.  If  we  put  the  amount  of  air  inhaled  and  exhaled  with  each 
breath  (tidal  air)  at  500  c.c.  and  the  respiratory  rate  at  15  per 
minute,  we  have  7,500  c.c.  of  air  involved  each  minute,  5  per  cent 
of  this,  or  375  c.c.  would  give  the  oxygen  consumption  and  4.4 
per  cent,  or  330  c.c.  the  carbon  dioxid  output  in  the  same  time. 
As  a  matter  of  fact  direct  determinations  of  the  oxygen  absorp- 
tion and  the  carbon  dioxid  output  of  persons  at  rest  ordinarily 
give  somewhat  smaller  figures  than  these,  280-325  c.c.  per  minute 
for  oxygen  and  250-280  c.c.  for  carbon  dioxid.  This  discrepancy 
can  be  explained  by  recalling  that  the  figures  for  tidal  air,  for  the 


RESPIRATION.     THE  GASEOUS  INTERCHANGES         417 

breathing  rate  and  for  the  percentages  of  oxygen  and  carbon  dioxid, 
are  round  numbers  and  somewhat  higher  than  the  actual  averages. 
The  Blood  Gases.  If  fresh  blood  be  rapidly  exposed  to  as  com- 
plete a  vacuum  as  can  be  obtained,  it  gives  off  certain  gases,  known 
as  the  gases  of  the  blood.  These  are  the  same  in  kind,  but  differ  in 
proportion,  in  venous  and  arterial  blood;  there  being  more  carbon 
dioxid  and  less  oxygen  obtainable  from  the  venous  blood  going  to 
the  lungs  by  the  pulmonary  artery,  than  from  the  arterial  blood 
coming  back  to  the  heart  by  the  pulmonary  veins.  The  gases  given 
off  by  venous  and  arterial  blood,  measured  under  the  normal  pres- 
sure and  at  the  normal  temperature,  amount  to  from  58  to  60 
volumes  for  every  100  volumes  of  blood,  and  in  the  two  cases  are 
about  as  follows: 

Venous  Blood  Arterial  Blood 

Oxygen 12  20 

Carbon  dioxi:! 45  38 

Nitrogen 1.7  1.7 

It  is  important  to  bear  in  mind  that  while  arterial  blood  contains 
some  carbon  dioxid  that  can  be  removed  by  the  air-pump,  venous 
blood  also  contains  some  oxygen  removable  in  the  same  way;  so 
that  the  difference  between  the  two  is  only  one  of  degree.  When 
an  animal  is  killed  by  suffocation,  however,  the  last  trace  of  oxygen 
which  can  be  yielded  up  in  a  vacuum  disappears  from  the  blood 
before  the  heart  ceases  to  beat.  All  the  blood  of  such  an  animal 
is  what  might  be  called  suffocation  blood,  and  has  a  far  darker 
color  than  ordinary  venous  blood. 

The  Cause  of  the  Bright  Color  of  Arterial  Blood.  The  color  of 
the  blood  depends  on  its  red  corpuscles,  since  pure  blood-plasma 
or  blood-serum  is  colorless,  or  at  most  a  very  faint  straw  yellow. 
Hence  the  color  change  which  the  blood  experiences  in  circulating 
through  the  lungs  must  be  due  to  some  change  in  its  red  corpuscles. 
We  have  already  seen  (Chap.  XVII)  that  the  functional  sub- 
stance of  the  red  corpuscles  is  hemoglobin,  which  has  the  prop- 
erty of  combining  with  oxygen.  Hemoglobin  itself  is  of  a  dark 
purplish  color,  when  combined  with  oxygen  the  resulting  com- 
pound is  a  bright  scarlet.  Hemoglobin  combined  with  oxygen  is 
known  as  oxy  hemoglobin,  and  it  is  on  its  predominance  that  the 
color  of  arterial  blood  depends.  Hemoglobin  uncombined  with 


418  THE  HUMAN  BODY 

oxygen,  sometimes  named  reduced  hemoglobin,  predominates  in 
venous  blood,  and  is  the  only  kind  found  in  the  blood  of  a  suffo- 
cated mammal. 

The  Laws  Governing  the  Absorption  of  Gases  by  a  Liquid.  In 
order  to  understand  the  condition  of  the  gases  in  the  blood  liquid 
it  is  necessary  to  recall  the  general  laws  in  accordance  with  which 
liquids  absorb  gases.  They  are  as  follows : 

1.  A  given  volume  of  a  liquid  at  a  definite  temperature  if  it 
absorbs  any  of  a  gas  to  which  it  is  exposed,  and  yet  does  not  com- 
bine chemically  with  it,  takes  up  an  amount  of  the  gas  which  de- 
pends upon  two  things:  (1)  the  solubility  of  the  gas  in  the  liquid; 
and  (2)  the  pressure  of  the  gas  upon  the  surface  of  the  liquid.    As 
the  pressure  of  the  gas  is  increased  the  amount  of  it  which  goes  in 
solution  in  the  liquid  is  increased  in  exactly  the  same  proportion. 
If  a  complete  vacuum  be  formed  above  a  liquid  all  the  gas  con- 
tained within  it  is  given  off.    This  law,  that  the  quantity  of  a  gas 
dissolved  by  a  liquid  varies  directly  as  the  pressure  of  that  gas  on 
the  surface  of  the  liquid  is  known  as  Henry's  law. 

2.  The  amount  of  a  gas  dissolved  by  a  liquid  depends,  not  on 
the  total  pressure  exerted  by  all  the  gases  pressing  on  its  surface, 
but  on  the  fraction  of  the  total  pressure  which  is  exerted  by  the 
particular  gas  in  question.    For  example,  the  average  atmospheric 
pressure  is  equal  to  that  of  a  column  of  mercury  760  mm.  (30 
inches)  high.    But  100  volumes  of  air  contain  approximately  80 
volumes  of  nitrogen  and  20  of  oxygen;  therefore  £  of  the  total 
pressure  is  due  to  oxygen  and  |  to  nitrogen:  and  the  amount  of 
oxygen  absorbed  by  water  is  just  the  same  as  if  all  the  nitrogen 
were  removed  from  the  air  and  its-  total  pressure  therefore  reduced 
to  J  of  760  mm.  (30  inches)  of  mercury;  that  is,  to  152  mm.  (6 
inches)  of  mercury  pressure.    It  is  only  the  fraction  of  the  total 
pressure  exerted  by  the  oxygen  itself  which  affects  the  quantity  of 
oxygen  absorbed  by  water  at  any  given  temperature.    So,  too,  of 
all  the  atmospheric  pressure  f  is  due  to  nitrogen,  and  all  the 
oxygen  might  be  removed  from  the  air  without  affecting  the  quan- 
tity of  nitrogen  which  would  be  absorbed  from  it  by  a  given  volume 
of  water.     The  atmospheric  pressure  would  then  be  f  of  760  mm. 
of  mercury,  or  608  mm.  (24  inches),  but  it  would  all  be  due  to 
nitrogen  gas — and  be  exactly  equal  to  the  fraction  of  the  total 
pressure  due  to  that  gas  before  the  oxygen  was  removed  from  the 


RESPIRATION.     THE  GASEOUS  INTERCHANGES         419 

air.  When  several  gases  are  mixed  together  the  fraction  of  the 
total  pressure  exerted  by  each  one  is  known  as  the  partial  pressure 
of  that  gas;  and  it  is  this  partial  pressure  which  determines  the 
amount  of  each  individual  gas  dissolved  by  a  liquid.  If  a  liquid 
exposed  to  the  air  for  some  time  had  taken  up  all  the  oxygen  and 
nitrogen  it  could  at  the  partial  pressures  of  those  gases  in  the  air, 
and  were  then  put  in  an  atmosphere  in  which  the  oxygen  had  all 
been  replaced  by  nitrogen,  it  would  now  give  off  all  its  oxygen, 
since,  although  the  total  gaseous  pressure  on  it  was  the  same,  no 
part  of  it  was  any  longer  due  to  oxygen ;  and  at  the  same  time  it 
would  take  up  one-fifth  more  nitrogen,  since  the  whole  gaseous 
pressure  on  its  surface  was  now  due  to  that  gas,  while  before  only 
four-fifths  of  the  total  was  exerted  by  it.  If.  on  the  contrary,  the 
liquid  were  exposed  to  pure  hydrogen  under  a  pressure  of  one 
atmosphere  it  would  give  off  all  its  previously  dissolved  oxygen 
and  nitrogen,  since  none  of  the  pressure  on  its  surface  would  now 
be  due  to  those  gases;  and  would  take  up  as  much  hydrogen  as 
corresponded  to  a  pressure  of  that  gas  equal  to  760  mm.  of  mercury 
(30  inches). 

3.  The  amount  of  gas  taken  up  by  a  liquid  varies,  other  things 
being  equal,  inversely  as  the  temperature. 

4.  A  liquid  may  be  such  as  to  combine  chemically  with  a  gas. 
Then  the  amount  of  the  gas  absorbed  is  independent  of  the  partial 
pressure  of  the  gas  on  the  surface  of  the  liquid.    The  quantity  ab- 
sorbed will  depend  upon  how  much  the  liquid  can  combine  with. 
Or,  a  liquid  may  be  composed  partly  of  things  which  simply  dis- 
solve a  gas  and  partly  of  things  which  combine  with  it  chemically. 
Then  the  amount  of  the  gas  taken  up  under  a  given  partial  pres- 
sure will  depend  on  two  things;  a  certain  portion,  that  merely  dis- 
solved, will  vary  with  the  pressure  of  the  gas  in  question;  but 
another  portion,  that  chemically  combined,  will  remain  the  same 
under  different  pressures. 

.  5.  Bodies  are  known  which  combine  chemically  with  certain 
gases  when  the  partial  pressure  of  these  is  considerable,  forming 
compounds  which  break  up,  or  dissociate,  liberating  the  gas,  when 
its  partial  pressure  falls  below  a  certain  limit.  Oxygen  forms  such 
a  compound  with  hemoglobin. 

6.  A  membrane,  moistened  by  a  liquid  in  which  a  gas  is  soluble, 
does  not  essentially  alter  the  laws  of  absorption,  by  a  liquid  on  one 


420  THE  HUMAN  BODY 

side  of  it  of  a  gas  present  on  its  other  side,  whether  the  absorption 
be  due  to  mere  solution  or  to  chemical  combinations  or  to  both. 

The  Absorption  of  Oxygen  by  the  Blood.  Applying  the  phys- 
ical and  chemical  facts  stated  in  the  preceding  paragraph  to  the 
blood,  we  find  that  the  blood  contains  (1)  plasma,  which  simply 
dissolves  oxygen,  and  (2)  hemoglobin,  which  combines  with  it  un- 
der some  partial  pressures  of  that  gas,  but  gives  it  up  under  lower. 

Blood-plasma  or,  what  comes  to  the  same  thing,  fresh  serum, 
exposed  to  the  air,  takes  up  no  more  oxygen  than  so  much  water: 
about  0.56  volumes  of  the  gas  for  every  100  of  the  liquid,  at  a 
temperature  of  20°  C.  At  the  temperature  of  the  Body  the  volume 
absorbed  would  be  still  less.  This  quantity  obeys  Henry's  law. 

If  fresh  defibrinated  blood  be  employed,  the  quantity  of  oxygen 
taken  up  is  much  greater;  this  extra  quantity  must  be  taken  up 
by  the  red  corpuscles  and  it  does  not  obey  Henry's  law.  If  the 
partial  pressure  of  oxygen  on  the  surface  of  the  defibrinated  blood 
be  doubled,  only  as  much  more  oxygen  will  be  taken  up  as  corre- 
sponds to  that  dissolved  in  the  serum;  and  if  the  partial  pressure 
of  oxygen  on  its  surface  be  reduced  to  one-half,  only  a  very  small 
amount  of  oxygen  (orie-half  of  that  dissolved  by  the  serum)  will 
be  given  off.  All  the  much  larger  quantity  taken  up  by  the  red 
corpuscles  will  be  unaffected  and  must  therefore  be  chemically 
combined  with  something  in  them.  Since  90  per  cent  of  their 
dry  weight  is  hemoglobin,  and  this  body  when  prepared  pure  is 
found  capable  of  combining  with  oxygen,  there  is  no  doubt  that  it 
is  the  hemoglobin  in  the  circulating  blood  which  carries  around 
most  of  its  oxygen.  The  red  corpuscles  are  so  many  little  packages 
in  which  oxygen  is  stowed  away. 

The  compound  formed  between  oxygen  and  hemoglobin  is,  how- 
ever, a  very  feeble  one;  the  two  easily  separate,  and  always  do 
so  completely  when  the  oxygen  pressure  in  the  liquid  or  gas  to 
which  the  oxyhemoglobin  is  exposed  falls  below  25  mm.  of  mer- 
cury. There  is  some  slight  dissociation  at  pressures  of  70  mm. 
of  mercury.  Hence,  in  an  air-pump,  the  blood  only  gives  off  a 
little  of  its  oxygen,  until  the  pressure  falls  to  about  J  of  an  at- 
mosphere, that  is  to  -f-  =  125  mm.  (5  inches)  of  mercury,  of 
which  total  pressure  one-fifth  (25  mm.  or  1  inch)  is  due  to  the 
oxygen  present.  As  soon  as  this  limit  is  passed  the  hemoglobin 
gives  up  its  remaining  oxygen  with  a  rush. 


RESPIRATION.     THE  GASEOUS  INTERCHANGES         421 

Consequences  of  the  Peculiar  Way  in  Which  the  Oxygen  of  the 
Blood  is  Held.    The  first,  and  most  important,  is  that  the  blood 
can  take  up  far  more  oxygen  in  the  lungs  than  would  otherwise  be 
possible.    Blood-serum  exposed  to  the  air  would  take  up  only  one- 
half  volume  of  oxygen  per  hundred  of  liquid  at  ordinary  tempera- 
tures, and  still  less  at  the  temperature  of  the  Body,  were  it  not  for 
its  hemoglobin.    In  the  lungs  even  less  would  be  taken  up,  since 
the  air  in  the  air-cells  of  those  organs  is  poorer  in  oxygen  than  the 
external  air;  and  consequently  the  partial  pressure  of  that  gas  in 
it  is  lower.    The  tidal  air  taken  in  at  each  breath  serves  merely  to 
renew  directly  the  air  in  the  big  bronchi;  the  deeper  we  examine 
the  pulmonary  air  the  less  oxygen  and  more  carbon  dioxid  will 
be  found;  in  the  layers  farthest  from  the  exterior  and  only  re- 
newed by  diffusion  with  the  air  of  the  large  bronchi,  it  is  estimated 
that  the  oxygen  only  exists  in  such  quantity  that  its  partial  pres- 
sure is  equal  to  about  100  mm.  of  mercury  (j  atmos.)  instead  of 
152  (J  atmos.)  as  in  ordinary  air.    In  the  second  place,  on  account 
of  the  way  in  which  hemoglobin  combines  with  oxygen,  the  quan- 
tity of  that  gas  taken  up  by  the  blood  is  independent  of  such  varia- 
tions of  its  partial  pressure  in  the  atmosphere  as  we  are  subjected 
to  in  daily  life.    At  the  top  of  a  high  mountain,  for  example,  the 
atmospheric  pressure  is  greatly  diminished,  but  still  mountaineers 
can  breathe  freely  and  get  all  the  oxygen  they  want;  the  distress 
felt  for  a  time  by  persons  unused  to  living  in  high  altitudes  is  due 
in  part  to  circulatory  disturbances  resulting  from  the  low  atmos- 
pheric pressure  and  in  part  to  another  condition  to  be  described 
presently,  but  not  at  all  to  deficiency  of  oxygen.     So  long  as  the 
partial  pressure  of  that  gas  in  the  lung  air-cells  is  well  above  25 
mm.  of  mercury,  the  amount  of  it  taken  up  by  the  blood  depends 
on  how  much  hemoglobin  there  is  in  that  liquid  and  not  on  how 
much  oxygen  there  is  in  the  air.    So,  too,  breathing  pure  oxygen 
under  a  pressure  of  one  atmosphere,  or  air  compressed  to  one-half 
or  a  fourth  its  normal   bulk,  does  not  increase  the  quantity  of 
oxygen  absorbed  by  the  blood,  apart  from  the  small  extra  quan- 
tity dissolved  by  the  plasma. 

The  General  Oxygen  Interchanges  in  the  Blood.  Suppose  we 
have  a  quantity  of  arterial  blood  in  the  aorta.  This,  fresh  from  the 
lungs,  will  have  its  hemoglobin  practically  saturated  with  oxygen 
and  in  the  state  of  oxyhemoglobin.  In  the  blood-plasma  some 


422  THE  HUMAN  BODY 

more  oxygen  will  be  dissolved,  viz.,  so  much  as  answers  to  a  pres- 
sure of  that  gas  equal  to  100  mm.  of  mercury,  which  is  the  partial 
pressure  of  oxygen  in  the  pulmonary  air-cells.  This  tension  of  the 
gas  in  the  plasma  will  be  more  than  sufficient  to  keep  the  hemo- 
globin from  giving  off  its  oxygen.  Suppose  the  blood  now  enters 
the  capillaries  of  a  muscle.  In  the  liquid  moistening  this  organ 
the  oxygen  tension  is  practically  nil,  since  the  tissue  elements  are 
steadily  taking  the  gas  up  from  the  lymph  around  them.  Conse- 
quently, through  the  capillary  walls,  the  plasma  will  give  off 
oxygen  until  the  tension  of  that  gas  in  it  falls  below  25  mm.  of 
mercury.  Immediately  some  of  the  oxyhemoglobin  is  decom- 
posed, and  the  oxygen  liberated  is  dissolved  in  the  plasma,  and 
from  there  next  passed  on  to  the  lymph  outside;  and  so  the  tension 
in  the  plasma  is  once  more  lowered  and  more  oxyhemoglobin 
decomposed.  This  goes  on  .so  long  as  the  blood  is  in  the  capillaries 
of  the  muscle,  but  on  account  of  the  shortness  of  this  interval, 
about  one  second,  not  all  the  oxyhemoglobin  has  time  to  decom- 
pose before  the  blood  has  passed  on  into  the  veins.  Here  further 
decomposition  is  quickly  brought  to  an  end  by  the  rising  tension  of 
the  oxygen  dissolved  in  the  plasma,  the  last  oxygen  given  off  from 
the  corpuscles  not  being  taken  up  by  the  lymph  because  of  the 
passage  of  the  blood  on  out  of  the  capillaries.  The  blood  will  now 
go  on  as  ordinary  venous  blood  into  the  veins  of  the  muscle  and 
so  back  to  the  lungs.  It  will  consist  of  (1)  plasma  with  oxygen 
dissolved  in  it  at  a  tension  of  about  25  mm.  (1  inch)  of  mercury,. 

(2)  A  number  of  red  corpuscles  containing  reduced  hemoglobirt. 

(3)  A  number  of  red  corpuscles  containing  oxyhemoglobin.     Or 
perhaps  all  of  the  red  corpuscles  will  contain  some  reduced  and 
some   oxidized   hemoglobin.     This   venous  blood,   returning  to 
the  heart,  is  sent  on  to  the  pulmonary  capillaries.     Here,  the 
partial  pressure  of  oxygen  in  the  air-cells  being  100  mm.  and 
that  in  the  blood-plasma  much  less,  oxygen  will  be  taken  up 
by  the  latter,  and  the  tension  of  that  gas  in  the  plasma  tend  to 
be  raised  above  the  limit  at  which  hemoglobin  combines  with  it. 
Hence,  as  far  as  the  plasma  gets  oxygen  thoL3  red  corpuscles 
which  contain  any  reduced  hemoglobin  rob  it,  and  so  its  oxygen 
tension  is  kept  down  below  that  in  the  air-cells  until  all  the  hemo- 
globin is  saturated.    Then  the  oxygen  tension  of  the  plasma  rises 
to  that  of  the  gas  in  the  air-cells;  no  more  oxygen  is  absorbed, 


RESPIRATION.    THE  GASEOUS  INTERCHANGES         423 

and  the  blood  returns  to  the  left  auricle  of  the  heart  in  the  same 
condition,  so  far  as  oxygen  is  concerned,  as  when  we  commenced 
to  follow  it. 

The  Carbon  Dioxid  of  the  Blood.  The  same  general  laws  apply 
to  this  as  to  the  blood  oxygen.  The  gas  is  partly  merely  dissolved 
and  partly  in  a  loose  chemical  combination  with  some  one  or  more 
of  the  constituents  of  blood.  Carbon  dioxid  is  about  twenty  times 
as  soluble  in  blood-plasma  as  is  oxygen  under  equivalent  conditions 
of  temperature  and  pressure.  We  can  therefore  account  for  more 
of  it  than  of  oxygen  in  the  state  of  simple  solution.  Not  more 
than  6  per  cent  of  the  total  amount  present  in  venous  blood  can  be 
accounted  for,  however,  in  this  way.  The  remainder  must  be  in 
some  easily  dissociable  chemical  combination.  Two  such  combina- 
tions are  known  to  exist  in  blood.  The  first  is  a  combination  of 
carbon  dioxid  with  sodium,  forming  sodium  carbonate;  the  second 
of  carbon  dioxid  with  the  blood  proteins,  including  hemoglobin, 
forming  a  compound  somewhat  analogous  with  oxyhemoglobin. 
This  latter  compound  is  more  readily  dissociable  than  sodium 
carbonate,  and  since,  as  we  have  seen,  there  is  always,  even  in 
arterial  blood,  a  considerable  percentage  of  carbon  dioxid,  we 
may  suppose  that  under  ordinary  circumstances  the  sodium  car- 
bonate circulates  as  such,  and  the  protein  compound  serves  as 
the  carrier  of  carbon  dioxid  from  tissues  to  lungs. 

We  may  summarize  the  carbon  dioxid  interchanges  as  follows: 

1.  The  tissues  constantly  produce  and  give  off  to  the  lymph 
carbon  dioxid.    It  is  present  in  lymph,  therefore,  at  all  times  in 
considerable   quantity,  probably  amounting  to  a  carbon   dioxid 
tension  of  70  mm.  of  mercury. 

2.  The  blood  entering  the  capillaries  contains  carbon  dioxid 
under  much  less  tension  than  this  (about  35  mm.),  there  is  there- 
fore a  movement  of  carbon  dioxid  from  lymph  to  blood.     This 
movement,  by  raising  the  tension  of  carbon  dioxid  in  the  blood 
brings  about  conditions  under  which  chemical  combination  may 
take  place,  chiefly  with  the  blood  proteins. 

3.  The  venous  blood  as  it  enters  the  lungs  contains  carbon 
dioxid  under  a  higher  tension  than  that  of  alveolar  air,  70  mm. 
for  venous  blood,  35  mm.  for  the  alveoli;  there  is  therefore  a 
movement  of  carbon  dioxid  from  the  blood  to  the  alveoli.     This 
movement,  by  lowering  the  carbon  dioxid  tension  of  the  blood, 


424  THE  HUMAN  BODY 

favors  the  dissociation  of  the  chemical  compounds  formed  during 
the  passage  of  the  blood  through  the  tissue  capillaries;  thus  the 
carbon  dioxid  taken  up  in  the  systemic  capillaries  is  gotten  rid  of 
in  the  lung  capillaries. 

The  Hormone  Action  of  Carbon  Dioxid.  We  have  already 
learned  (Chap.  XXIII)  that  carbon  dioxid  has  an  important 
action  in  connection  with  maintaining  the  activity  of  the  respira- 
tory center.  Recent  work  has  shown  that  it  has  other  functions 
as  well.  The  carbon  dioxid  tension  of  alveolar  air  is  ordinarily 
about  35  mm.  of  mercury.  The  carbon  dioxid  tension  of  the  blood 
does  not,  of  course,  fall  below  that  of  the  alveoli,  so  that  arterial 
blood  under  normal  conditions  contains  a  considerable  amount  of 
carbon  dioxid.  Under  exceptional  circumstances,  as  at  high 
altitudes,  where  the  atmospheric  pressure  as  a  whole  is  less  than 
at  the  earth's  surface,  the  tension  of  carbon  dioxid  in  the  alveoli 
may  be  considerably  less  than  35  mm.,  and  that  of  the  blood 
correspondingly  diminished.  There  is  a  condition  known  as 
mountain  sickness,  characterized  by  nausea  and  other  distressing 
symptoms,  which  is  due  to  this  diminution  of  the  carbon  dioxid 
content  of  the  blood.  Any  one,  by  taking  a  number  of  deep 
breaths  in  rapid  succession,  can  lower  the  carbon  dioxid  tension 
of  his  alveolar  air,  and  consequently  of  his  blood,  to  a  point  where 
very  disagreeable  sensations  are  felt.  Just  how  the  carbon  dioxid 
of  the  blood  prevents  these  symptoms  is  not  clear.  That  it  has 
the  power  to  do  so  is,  however,  well  demonstrated. 

The  normal  breathing  mechanism  is  an  adaptation  by  which  the 
blood  is  continuously  provided  with  all  the  oxygen  it  is  able  to 
carry,  and  by  which  also  its  carbon  dioxid  content,  while  never 
allowed  to  become  excessive,  is  kept  high  enough  for  the  proper 
performance  of  its  hormone  function.  Deep  breathing  is  there- 
fore of  no  particular  value  from  the  standpoint  of  respiration.  As 
an  exercise  for  the  chest  muscles;  as  a  means  of  insuring  ventila- 
tion of  the  remotest  alveoli;  and  most  of  all  as  an  aid  to  the  flow 
of  venous  blood  and  lymph,  through  the  aspiration  of  the  thorax, 
(p.  370)  the  practice  has  great  value.  We  should  remember,  how- 
ever, that  shallow  breathing  is  the  normal  mode,  and  that  only 
while  we  are  thinking  about  it  can  we  breathe  deeply.  As  soon  as 
our  attention  is  diverted  to  other  matters  we  recur  at  once  to  the 
automatic  shallow  type. 


RESPIRATION.     THE  GASEOUS  INTERCHANGES         425 

Tissue  Respiration.  Our  knowledge  of  the  use  of  oxygen  and 
the  production  of  carbon  dioxid  by  the  tissues  is  not  very  com- 
plete. The  following  general  facts  maybe  stated  here:.  (1)  The 
tissues  take  up  oxygen  from  the  lymph  as  fast  as  it  is  brought  by 
the  blood  and  use  it  in  oxidative  processes  at  the  same  rate; 
careful  experiments  fail  to  show  that  there  is  any  storage  of  oxy- 
gen in  the  tissues  for  future  use.  (2)  Tissue  oxidations  differ  from 
ordinary  oxidative  processes,  such  as  occur  when  fuel  is  burned  in 
a  furnace,  for  example,  in  that  they  are  carried  on  through  the 
agency  of  enzyms  known  as  oxidases.  The  chemical  process  of  oxi- 
dation carried  on  thus  is  not  direct  as  in  ordinary  burning;  it  oc- 
curs at  a  lower  temperature,  and  requires  a  longer  time;  but  it  must 
be  remembered  that  the  amount  of  heat  produced  by  the  oxidation 
of  a  given  weight  of  fuel  is  always  the  same  whether  the  process 
be  rapid  or  slow.  Tissue  oxidations,  therefore,  are  not  necessarily 
inefficient  because  they  go  on  slowly.  (3)  The  amount  of  work 
that  a  man's  organs  do,  is  not  dependent  on  the  amount  of  oxygen 
supplied  to  them,  but  the  amount  of  oxygen  used  by  him  depends 
on  how  much  he  uses  his  organs.  It  is  necessary  to  emphasize 
this  fact  because  of  the  notion,  which  seems  to  be  rather  wide- 
spread, that  bodily  processes  are  augmented  by  increasing  the 
supply  of  oxygen  to  them.  The  man  who  goes  from  his  ill-ven- 
tilated office  to  the  open  country,  and  feels  the  impulse  to  vigorous 
exercise  as  he  breathes  the  pure  country  air,  is  apt  to  attribute 
his  sensations  of  virility  to  an  imagined  augmentation  of  all  his 
bodily  processes  through  the  increased  amount  of  oxygen  breathed 
in.  The  fact  is  that  whatever  augmentation  of  activity  he  may 
experience  is  the  result  of  the  agreeable  sensory  stimulations  com- 
ing to  him,  which  arouse  his  tissues  to  activity,  either  reflexly  or 
voluntarily.  Increased  oxygen  consumption  is,  therefore,  never 
the  cause,  but  always  the  result  of  augmented  tissue  activity. 

Respiratory  Changes  in  Muscular  Exercise.  With  every  in- 
crease in  degree  of  muscular  activity  there  is  corresponding  in- 
crease in  oxygen  consumption  and  carbon  dioxid  production  up 
to  a  limit  which  is  set  by  the  ability  of  the  blood  to  carry  oxygen. 
Since,  as  already  noted  (p.  421),  the  blood  as  it  leaves  the  lungs  is 
virtually  saturated  with  the  gas  under  resting  conditions,  an  in- 
crease in  the  amount  transported  by  it  can  come  about  only  by 
a  more  rapid  flow  of  the  blood  or  by  a  more  complete  use  by  the 


426  THE  HUMAN  BODY 

tissues  of  that  brought  to  them.  Both  these  methods  enter  as  a 
matter  of  fact.  The  familiar  increase  in  the  rate  of  the  heart,  in 
combination  with  a  slight  increase  in  the  amount  of  blood  dis- 
charged with  each  beat,  suffices  to  augment  the  blood-flow  about 
2|-2f  times.  We  saw  above  (p.  417)  that  ordinarily  venous  blood 
contains  about  60  per  cent  as  much  oxygen  as  arterial.  In  exercise 
the  amount  of  oxygen  in  venous  blood  is  very  much  reduced;  in 
extreme  cases  none  at  all  may  remain.  By  this  more  complete 
utilization,  in  connection  with  the  more  rapid  flow,  the  total 
oxygen  carrying  power  may  be  raised  about  7-7-J  times.  The 
other  factors,  carbon  dioxid  transport,  and  lung  ventilation,  have 
much  wider  limits,  so  that  the  bound  is  established,  as  stated 
above,  by  the  oxygen  carrying  power. 

In  connection  with  this  an  interesting  point  arises.  Repeated 
reference  has  been  made  to  the  fact  that  ordinarily  during  the 
passage  of  the  blood  through  the  tissue  capillaries  it  gives  up  only 
40  per  cent  of  its  oxygen.  The  suggestion  was  made  (p.  422)  that 
this  relatively  small  disbursement  is  due  to  the  short  stay  of  the 
blood  in  the  capillaries.  During  muscular  activity  there  is  a 
much  more  rapid  blood-flow,  with  a  corresponding  shortening  of 
the  time  required  for  the  blood  to  pass  through  the  capillaries, 
yet  in  spite  of  this  we  find  the  blood  giving  up  virtually  all  its 
oxygen,  instead  of  only  40  per  cent  of  it.  A  recent  discovery  may 
help  us  to  explain  this  apparent  paradox.  It  has  been  shown  that 
the  dissociation  of  oxyhemoglobin  is  much  more  rapid  in  an  en- 
vironment rich  in  carbon  dioxid  than  in  one  containing  only  small 
amounts  of  this  substance.  One  result  of  muscular  exercise  is  a 
great  outpouring  of  carbon  dioxid  from  the  active  tissues.  It  may 
be  supposed  that  in  the  presence  of  this  outpouring  the  dissocia- 
tion of  oxyhemoglobin  is  so  greatly  accelerated  that  the  increased 
rate  of  blood-flow  is  more  than  counter-balanced. 

If,  as  may  readily  happen,  the  activity  becomes  so  great  that 
the  oxygen  supply  cannot  keep  pace  with  the  needs  of  the  muscles 
we  have  the  result  already  discussed  in  Chap.  VII  (p.  112),  namely, 
an  outpouring  of  sodium  lactate  into  the  blood.  The  effect  of  this 
is  to  make  more  pronounced  that  acid  condition  which,  as  stated 
previously  (p.  403),  constitutes  the  real  stimulus  to  the  respiratory 
center.  .  The  dyspnea  is,  therefore,  markedly  increased  with  the 
appearance  of  this  substance  in  the  blood.  We  would  probably 


RESPIRATION.    THE  GASEOUS  INTERCHANGES         427 

be  safe  in  assuming  the  point  of  onset  of  marked  respiratory  dis- 
tress as  indicating  the  passage  of  the  laboring  muscles  beyond  the 
limit  at  which  their  immediate  need  for  oxygen  can  be  fully  sup- 
plied. 

Coal  Gas  Poisoning.  In  the  paragraph  on  asphyxia  (Chap. 
XXIII)  the  possibility  of  suffocation  by  carbon  monoxid  was 
mentioned.  This  substance,  which  is  an  important  constituent  of 
illuminating  gas,  has  a  greater  affinity  for  hemoglobin  than  has 
oxygen,  and  forms  with  it  a  more  stable  compound,  carbon  monoxid 
hemoglobin.  The  result  of  breathing  illuminating  gas  is,  then,  the 
conversion  of  hemoglobin  of  the  blood  into  carbon  monoxid  hemo- 
globin, and  the  consequent  abolishment  of  the  oxygen-carrying 
function  of  the  red  corpuscles.  If  the  breathing  of  carbon  monoxid 
has  gone  on  long  enough  for  practically  all  the  hemoglobin  of  the 
blood  to  be  combined  with  it,  death  from  lack  of  oxygen  is  inevit- 
able unless  by  the  prompt  performance  of  blood  transfusion  a 
fresh  supply  of  properly  functioning  red  corpuscles  be  introduced 
into  the  circulation.  Exposure  to  the  gas  for  a  shorter  time,  not 
enough  to  prove  fatal,  but  to  the  point  of  unconsciousness,  is  often 
followed  by  a  long  period,  weeks  or  months,  of  serious  functional 
impairment  of  the  tissues  of  the  Body,  due  to  the  injury  suffered 
by  them  during  the  period  of  oxygen  deficiency. 


CHAPTER  XXV 
FOODS:  THEIR  CLASSIFICATION 

What  Constitutes  Food.  Material  is  taken  into  the  Body  in 
three  physical  states:  solid,  liquid,  gaseous.  We  have  considered 
the  gaseous  intake  under  the  head  of  respiration,  and  turn  now 
to  the  use  by  the  Body  of  solid  and  liquid  substances.  From  the 
standpoint  of  physiology  we  may  include  under  the  head  of  food 
everything,  either  solid  or  liquid,  which  is  taken  into  the  Body  and 
used  there  for  its  normal  functioning.  This  classification  includes 
with  the  foods  liquid  substances,  such  as  milk  and  water,  which 
we  ordinarily  classify  separately  as  drinks.  It  is  clear,  however, 
that  from  the  standpoint  of  the  Body  a  classification  on  this 
basis,  the  physical  nature  of  the  substance  taken,  is  not  very 
helpful,  and  we  shall  therefore  disregard  the  distinction  commonly 
made  between  liquid  and  solid  foods. 

The  Function  of  Food.  If  we  have  gotten  the  viewpoint  which 
the  earlier  chapters  of  this  book  have  attempted  to  instil,  and 
are  able  to  look  upon  the  Body  as  a  piece  of  machinery,  we  appre- 
ciate that  materials  must  be  furnished  it  for  at  least  two  pur- 
poses: (1)  to  supply  what  it  needs  for  the  liberation  of  energy;  and 
(2)  to  provide  for  its  maintenance  and  repair.  The  first  of  these 
requirements  is  a  simple  fuel  demand;  anything  that  the  Body  is 
able  to  burn  can  be  used  if  its  burning  or  mere  presence  does  not 
injure  the  delicate  machinery.  The  second  requirement  is  not  so 
simple;  the  repair  of  the  complex  body  mechanism  calls  for  par- 
ticular repair  materials;  in  the  carrying  on  of  the  Body's  func- 
tions there  is  a  continuous  loss  from  it  of  substances,  such  as  water, 
which  must  be  continuously  replaced;  moreover,  we  often  see  fit 
to  introduce  substances  which  we  think  will  aid  the  Body  in  carry- 
ing out  its  functions,  as  coffee,  tea,  spices,  and  condiments. 

In  the  ^case  of  the  child  an  additional  factor  enters,  namely, 
growth,  or  the  manufacture  of  new  tissue.  As  we  shall  learn,  this 
is  not  precisely  equivalent  to  the  repair  of  tissue  already  present, 

428 


FOODS:  THEIR  CLASSIFICATION  429 

so  that  we  shall  have  also  to  consider  foods  in  their  relationship 
to  growth. 

Classes  of  Foods.  We  are  aware  that  the  materials  which  com- 
pose our  meals  include  indigestible  substances  as  well  as  true  foods. 
These  indigestible  materials  serve,  as  we  shall  see,  an  important 
function  through  the  bulk  they  impart  to  the  food;  it  would  be 
extremely  difficult  to  maintain  the  Body  in  health  upon  a  diet 
from  which  they  were  excluded;  we  may  borrow  for  them  an  ex- 
pressive term  used  by  feeders  of  cattle  for  bulky  stuffs  of  little 
nutritive  value,  and  designate  them  as  roughage. 

The  true  foods  fall  into  two  classes,  energy  yielders  and  non- 
energy  yielders.  The  latter  class  includes  all  the  inorganic  con- 
stituents of  the  diet,  such  as  water  and  the  various  salts;  and  a 
number  of  organic  substances  which  serve  definite  purposes  not 
involving  the  liberation  of  energy  by  them.  These  non-energy 
yielders  are  commonly  classed  as  accessories  of  the  diet,  to  signify 
their  subordinate  relation  to  the  energy  yielding  food.  We  have 
to  recognize,  however,  that  some  of  the  so-called  accessories  are 
necessary  to  health.  These  we  may  call  the  essential  accessories. 
They  include  water,  the  various  salts,  and  a  group  of  organic  sub- 
stances to  be  described  in  detail  in  a  later  paragraph,  known  as 
the  vitamines.  The  other  accessories  of  the  diet,  chiefly  organic, 
may  be  designated  as  occasional  accessories.  Among  these  are 
included  the  special  substances  which  give  flavor  to  the  food,  and 
by  making  it  palatable  aid  in  its  digestion.  All  drugs,  including 
the  essential  principles  of  tea,  coffee,  and  cocoa,  fall  also  into  this 
class,  as  do  the  substances  classed  as  condiments,  pepper,  mustard, 
etc. 

The  organic  constituents  of  our  food  not  included  among  the 
group  of  accessory  articles  of  diet  belong  chemically  to  one  or 
other  of  three  great  subdivisions.  They  are  either  carbohydrates, 
fats,  or  proteins.  The  entire  supply  of  energy  for  the  Body,  and 
its  repair  and  maintenance  to  great  extent  are  derived  from  these 
three  classes  of  food  stuffs.  Because  of  their  prime  importance 
they  are  usually  set  apart  from  the  other  foods  as  nutrients  proper. 

Occurrence  of  Nutrients  in  Food.  The  articles  which  in  com- 
mon language  we  call  foods  are,  in  most  cases,  mixtures  of  several 
nutrients  with  inorganic  and  organic  accessory  substances  and 
with  roughage.  Bread,  for  example,  contains  water,  salts,  gluten 


430  THE  HUMAN  BODY 

(a  protein),  some  fats,  much  starch,  and  a  little  sugar;  all  true 
food  stuffs:  but  mixed  with  these  is  a  quantity  of  cellulose  (the 
chief  chemical  constituent  of  the  walls  which  surround  vegetable 
cells),  and  this  is  not  a  true  food  since  it  is  incapable  of  digestion. 
Chemical  examination  of  all  the  common  articles  of  diet  shows 
that  the  actual  number  of  important  food  stuffs  is  but  small: 
they  are  repeated  in  various  proportions  in  the  different  things  we 
eat,  mixed  with  small  quantities  of  different  flavoring  substances, 
and  so  give  us  a  pleasing  variety  in  our  meals;  but  the  essential 
substances  are  much  the  same  in  the  fare  of  the  workman  and 
in  the  "delicacies  of  the  season."  These  primary  food  stuffs, 
which  are  found  repeated  in  so  many  different  foods,  belong  to  one 
or  the  other  of  the  classes  of  nutrients  mentioned  above;  and  the 
food  value  of  any  article  of  diet  depends  on  them  far  more  than  on 
the  traces  of  flavoring  matters  which  cause  certain  things  to  be 
especially  sought  after  and  so  raise  their  market  value.  We 
cannot,  however,  conclude  that  the  possession  of  flavor  by  foods 
is  wholly  unnecessary.  We  shall  see  it  plays  a  very  real  and  very 
important  role  in  our  use  of  foods  in  general. 

The  Inorganic  Essential  Accessories.  Two  inorganic  sub- 
stances, water  and  sodium  chlorid  (common  salt),  are  taken 
separately  and  consciously  as  constituents  of  the  diet.  We  re- 
quire such  large  amounts  of  these  substances  that  they  have  to  be 
taken  thus  purposely  to  insure  that  enough  be  gotten.  The  other 
inorganic  materials,  the  chlorids,  phosphates,  and  sulphates  of 
potassium,  magnesium,  and  calcium,  occur  in  most  ordinary  arti- 
cles of  diet,  so  that  we  do  not  swallow  them  in  a  separate  form. 
Phosphates,  for  example,  exist  in  nearly  all  animal  and  vegetable 
foods;  moreover  certain  foods,  as  casein,  contain  phosphorus  in 
combinations  which  in  the  Body  yield  it  up  to  be  oxidized  to  form 
phosphoric  acid.  The  same  is  true  of  sulphates,  which  are  par- 
tially swallowed  as  such  in  various  articles  of  diet,  and  are  partly 
formed  in  the.  Body  by  the  oxidation  of  the  sulphur  of  various  pro- 
teins. Calcium  salts  are  abundant  in  bread  and  milk,  and  are  also 
found  in  many  drinking-waters.  That  these  salts  are  essential  to 
life  is  proven  by  the  results  of  feeding  animals  on  diets  which 
have  been  carefully  made  salt-free,  but  are  otherwise  fully  ade- 
quate. Such  diets  invariably  cause  a  steady  decline,  and,  if  not 
discontinued,  death.  The  remarkable  fact  is  that  under  these 


FOODS:  THEIR  CLASSIFICATION  431 

circumstances  death  comes  much  sooner  than  in  complete  starva- 
tion, if  there  is  no  lack  of  water.  When  starvation  threatens  the 
Body  conserves  all  its  substance  carefully.  This  power  is  not 
shown  when  the  only  elements  lacking  are  the  salts.  The  Body 
then  continues  to  eliminate  them  at  the  usual  rate  along  with 
the  waste  products  from  the  other  food  stuffs,  and  a  fatal  defi- 
ciency comes  on  quickly. 

In  general  the  craving  for  salt  is  associated  with  a  vegetable 
diet.  This  is  shown  very  strikingly  in  the  case  of  grazing  animals 
that  in  the  wild  state  are  known  to  travel  long  distances  in  quest 
of  "salt  licks."  Carnivorous  animals,  on  the  other  hand,  not  only 
have  no  craving  for  salt,  but  will  reject  food  containing  an  excess 
of  it.  This  is  said  to  be  true  also  of  Eskimos,  whose  diet  is  exclu- 
sively of  flesh. 

The  relationship  of  the  salt  craving  to  a  vegetable  diet  is  ex- 
plained on  the  basis  of  the  high  potash  content  of  vegetables.  The 
potash  salts  react  in  the  Body  with  the  sodium  chlorid,  forming 
compounds  which  are  rapidly  eliminated  by  the  kidneys.  This 
constant  drain  on  the  sodium  chlorid  of  the  Body  gives  rise  to  a 
craving  which  insures  its  adequate  replenishment.  We  must 
admit,  nevertheless,  that  civilized  man  habitually  consumes  much 
more  salt  than  is  absolutely  necessary.  The  excess  should  be 
classed  as  a  condiment,  among  the  occasional  accessories. 

The  Organic  Essential  Accessories.  Vitamines.  For  a  long 
time  it  has  been  known  that  rigid  confinement  to  certain  restricted 
diets  leads  to  serious  bodily  disturbances,  even  though  the  amounts 
of  food  are  ample  and  all  the  nutrients  sufficiently  represented. 
Outbreaks  of  scurvy  among  ships'  companies  on  long  voyages 
were  early  recognized  as  due  to  inadequacies  of  diet;  specifically 
to  lack  of  fresh  meats  and  vegetables.  The  precise  reason  for  the 
disturbed  metabolism  of  scurvy  was  not  made  clear  until  another 
dietary  disease  came  under  investigation  in  which  the  situation 
could  be  analyzed  more  exactly.  This  is  the  disease  beri-beri,  a 
disease  in  which  the  nerve  trunks  become  inflamed,  with  conse- 
quent impairment  of  conductivity.  Paralyses  and  various  dis- 
turbances in  the  normal  nutrition  of  the  tissues  follow.  There  is 
definite  proof  that  this  disease  is  the  result  of  limiting  the  diet  too 
strictly  to  polished  rice.  The  inclusion  of  rice  hulls,  or  of  almost 
any  other  food  substance  prevents  its  occurrence  or  cures  it  if 


432  THE  HUMAN  BODY 

present.  From  rice  hulls  has  been  prepared  a  relatively  simple 
extract  which  has  the  curative  properties  of  the  entire  hulls.  The 
explanation  seems  to  be  that  certain  organic  substances  are  essen- 
tial to  normal  metabolism.  They  are  present  in  most  foods,  but 
are  wanting  from  some.  When  the  diet  is  restricted  to  these 
latter  nutritional  disturbances  arise.  A  third  dietary  disease, 
pellagra,  appears  to  be  due  to  a  diet  composed  too  largely  of  the 
products  of  maize,  grits,  hominy,  and  cornmeal.  For  the  sub- 
stance or  substances  thus  essential  the  name  vitamines  has  been 
proposed.  We  have  no  definite  knowledge  as  to  their  mode  of 
action,  although  the  suggestion  has  been  made  that  they  may 
either  act  directly  as  hormones,  or  may  be  essential  constituents  of 
some  or  all  of  the  hormones  manufactured  in  the  Body. 

Recently  the  interesting  discovery  has  been  made  that  growth 
of  young  animals  is  much  favored  by  the  presence  in  the  diet  of 
certain  unpurified  fats,  notably  the  fat  of  milk  (butter  fat),  of  egg 
yolk,  or  of  the  liver  (cod  liver  oil).  Fats  of  exactly  the  same  chem- 
ical composition  as  these  but  from  other  sources,  or  these  same 
fats  after  careful  purification,  do  not  show  this  growth-favoring 
property.  The  conclusion  is  that  a  vitamine-like  substance  is 
present  with  these  particular  fats,  and  that  the  effect  observed  is 
due  to  it. 

Occurrence  of  Occasional  Accessories  in  Food.  Variety  in  the 
diet  depends  practically  altogether  upon  the  occasional  accessories, 
for  the  primary  food  stuffs  are  few  in  number  and  for  the  most 
part  without  very  pronounced  tastes  or  flavors,  with  the  single 
exception  of  sugar,  whose  sweet  taste  makes  it,  to  the  eyes  of 
most  children  at  least,  the  most  desirable  of  all  foods.  To  civilized 
man  variety  of  diet  is  a  virtual  necessity;  the  accessories,  there- 
fore, are  to  him  of  great  importance.  Both  meats  and  vegetables 
owe  their  characteristic  flavors,  in  the  main,  to  organic  substances 
present  in  them.  We  do  not,  however,  depend  wholly  on  these 
substances  for  securing  the  needed  variety  in  our  food.  Condi- 
ments, pepper  and  mustard  for  example,  and  spices  are  used  very 
largely  in  all  civilized  countries.  Chocolate,  coffee,  and  tea  are 
taken  by  most  people  more  for  their  agreeable  flavor  than  for  their 
stimulating  properties. 

The  Nutrients.  The  actual  nourishment  of  the  Body  depends, 
as  stated  above,  primarily  upon  the  taking  of  sufficient  quantities 


FOODS:  THEIR  CLASSIFICATION  433 

of  the  nutrients  proper.  Of  the  three  groups  of  nutrients  two, 
carbohydrates  and  fats,  are  exclusively  energy  yielders.  Their 
function  is  to  be  oxidized  in  the  Body  and  thus  to  furnish  the 
energy  by  which  the  machine  does  its  work.  The  third  nutrient 
group,  the  proteins,  furnishes  all  the  material  by  which  waste  of 
living  tissues  is  made  good,  and  provides  likewise  a  very  con- 
siderable proportion  of  the  fuel  supply  of  the  Body.  Because  of 
the  twofold  function  of  proteins  it  is  possible  for  a  person  or 
animal  to  live  for  a  long  time  upon  an  exclusively  protein  diet. 
Since  repair  of  tissue  waste  can  be  made  only  by  proteins,  an  ani- 
mal or  a  man  would  starve  to  death  upon  a  protein-free  diet,  no 
matter  how  much  of  the  other  food  stuffs  he  might  have.  For 
that  matter  not  all  proteins  are  tissue-formers;  reference  to  the 
classification  of  proteins  in  Chap.  I  shows  that  only  the  first 
two  classes,  the  albumins  and  globulins,  are  sufficiently  complex 
to  yield  all  the  constituents  needed  for  the  formation  or  repair  of 
living  tissues.  Albuminoids  form  a  constant  part  of  all  flesh  food, 
but  they  can  be  used  by  the  Body,  in  the  long  run,  only  as  it  'uses 
carbohydrates  and  fats,  for  fuel. 

Carbohydrates.  These  are  mainly  of  vegetable  origin.  The 
most  important  are  starch,  found  in  nearly  all  vegetable  foods,  and 
having  the  chemical  formula  (C6Hi0O5)n;  the  dextrins,  or  gums; 
and  two  classes  of  sugars;  double  sugars,  having  the  formula 
CnHaOiii  and  represented  by  cane-sugar,  sucrose,  and  milk-sugar, 
lactose;  and  single  sugars,  having  the  formula  C6Hi2O6,  and  repre- 
sented by  grape-sugar,  dextrose.  Glycogen,  animal  starch,  is  a 
constituent  of  muscle  tissue  and  is  eaten  as  a  part  of  flesh.  It  and 
milk-sugar  are  the  only  carbohydrates  commonly  eaten  which  are 
of  animal  origin.  Cellulose,  a  very  abundant  vegetable  carbo- 
hydrate, is  to  the  human  alimentary  tract  practically  indigestible. 

Fats.  The  most  important  are  stearin,  palmatin,  and  olein, 
which  exist  in  various  proportions  in  animal  fats  and  vegetable 
oils;  the  more  fluid  containing  more  olein.  Butter  contains  also  a 
little  of  a  fat  named  butyrin.  Fats  are  compounds  of  glycerin  and 
fatty  acids,  and  any  such  substance  which  is  fusible  at  the  temper- 
ature of  the  Body  will  serve  as  a  food.  The  stearin  of  beef  and 
mutton  fats  is  not  by  itself  fusible  at  the  body  temperature,  but 
is  mixed  in  those  foods  with  so  much  olein  as  to  be  melted  in  the 
alimentary  canal.  Beeswax,  on  the  other  hand,  is  a  fatty  body 


434  THE  HUMAN  BODY 

which  will  not  melt  in  the  intestines  and  so  passes  on  unabsorbed; 
although  from  its  composition  it  would  be  useful  as  a  food  could  it 
be  digested.  A  distinction  is  sometimes  made  between  fats  proper 
(the  adipose  tissue  of  animals  consisting  of  fatty  compounds  in- 
closed in  albuminous  cell-walls)  and  oils,  or  fatty  bodies  which  are 
not  so  organized. 

Proteins  occur  as  the  chief  constituent  of  animal  foods,  lean 
meat  for  example  being  90  per  cent  protein  after  its  large  water 
content  is  removed.  Eggs  and  milk  contain  considerable  amounts 
of  protein  also.  Proteins  occur  to  a  greater  or  lesser  degree  in 
most  vegetable  foods.  The  gluten  of  wheat  is  protein;  beans  and 
peas  contain  a  larger  percentage  of  protein  than  any  other  food 
except  cheese. 

The  albuminoid  of  connective  tissue,  which  is  present  in  all 
meat,  is  by  cooking  converted  into  gelatin,  a  digestible  protein. 

Mixed  Foods.  These,  as  already  pointed  out,  include  nearly  all 
common  articles  of  diet;  they  contain  more  than  one  nutrient. 
Among  them  we  find  great  differences;  some  being  rich  in  pro- 
teins, others  in  starch,  others  in  fats,  and  so  on.  The  formation 
of  a  scientific  dietary  depends  on  a  knowledge  of  these  charac- 
teristics. The  foods  eaten  by  man  are,  however,  so  varied  that  we 
cannot  do  more  than  consider  the  most  important. 

Flesh.  This,  whether  derived  from  bird,  beast,  or  fish,  consists 
essentially  of  the  same  things — muscular  fibers,  connective  tissue 
and  tendons,  fats,  blood-vessels,  and  nerves.  It  contains  several 
proteins,  especially  myosin  and  myogen;  gelatin-yielding  matters 
in  the  white  fibrous  tissue;  stearin,  palmatin,  and  olein  as  repre- 
sentatives of  the  fats;  and  a  small  amount  of  carbohydrates  in  the 
form  of  glycogen  and  grape-sugar,  or  some  chemically  allied  sub- 
stances. Flesh  also  contains  much  water  and  a  considerable 
number  of  salines,  the  most  important  and  abundant  being  po- 
tassium phosphate.  The  nitrogenous  extractives  (Chap.  I)  give 
much  of  its  taste  to  flesh;  and  small  quantities  of  various  of  these 
substances  exist  in  different  kinds  of  meat.  There  is  also  more 
or  less  yellow  elastic  tissue  in  flesh;  it  is  indigestible  and  useless 
as  food. 

When  meat  is  cooked  its  white  fibrous  tissue  is  turned  into 
gelatin,  and  the  whole  mass  becomes  thus  softer  and  more  easily 
disintegrated  by  the  teeth.  When  boiled  some  of  the  protein 


FOODS:  THEIR  CLASSIFICATION  435 

matters  of  the  meat  pass  out  into  the  broth,  and  there  in  part 
coagulate  and  form  the  scum;  this  loss  may  be  prevented  in  great 
part  by  putting  the  raw  meat  at  once  into  boiling  water  which 
coagulates  the  surface  albumen  before  it  dissolves  out,  and  this 
keeps  in  the  rest,  while  the  subsequent  cooking  is  continued 
slowly.  In  any  case  the  myosin,  being  insoluble  in  water,  remains 
behind  in  the  boiled  meat.  In  baking  or  roasting,  all  the  solid 
parts  of  the  flesh  are  preserved  and  certain  agreeably  flavored 
bodies  are  produced,  as.  to  the  nature  of  which  little  is 
known. 

Eggs.  These  contain  a  large  amount  of  egg  albumen  and,  in  the 
yolk,  another  protein,  known  as  vitellin.  Also  fats,  and  a  sub- 
stance known  as  lecithin,  which  is  important  as  containing  a  con- 
siderable quantity  of  phosphorus.  Lecithin,  or  rather  a  sub- 
stance yielding  it,  is  an  important  constituent  of  the  nervous 
tissues. 

Milk  contains  at  least  two  proteins,  lactalbumin  and  casein; 
several  fats  in  the  butter;  a  carbohydrate;  milk-sugar;  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  one  per  cent  of  a  special  fat,  butyrin.  In  the  milk 
it  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  surrounded  by  a  pellicle 
of  albuminous  matter;  by  churning,  these  pellicles  are  broken 
up  and  the  fat-droplets  then  run  together  to  form  the  butter. 
Casein  is  insoluble  in  water;  in  milk  it  is  dissolved  by  the  alkaline 
salts  present.  When  milk  is  kept,  its  sugar  ferments  and  gives 
rise  to  lactic  acid,  which  neutralizes  the  alkali  and  precipitates 
the  casein  as  curds.  In  cheese-making  the  casein  is  acted  upon 
by  a  ferment  present  in  the  extract  of  stomach  used,  and  con- 
verted into  tyrein  which  is  precipitated:  this  clotting  does  not 
take  place  unless  a  calcium  salt  be  present.  Tyrein,  which  forms 
the  main  bulk  of  a  true  cheese,  is  different  from  the  curd  pre- 
cipitated from  milk  by  acids;  cheese  made  from  the  latter  does 
not  " ripen." 

Vegetable  Foods.  Of  these  wheat  affords  the  best;  not  that  it 
contains  more  of  any  particular  nutrient  but  because  of  a  peculiar 
property  of  its  protein.  The  protein  of  wheat  is  mainly  gluten, 


436  THE  HUMAN  BODY 

which  when  moistened  with  water  forms  a  tenacious  mass,  and 
this  it  is  to  which  wheaten  bread  owes  its  superiority.  When  the 
dough  is  made,  yeast  is  added  to  it,  and  produces  a  fermentation 
by  which,  among  other  things,  carbon  dioxid  gas  is  produced. 
This  gas,  imprisoned  in  the  tenacious  dough,  and  expanded  during 
baking,  forms  cavities  in  it  and  causes  it  to  "rise"  and  make 
"light  bread,"  which  is  not  only  more  pleasant  to  eat  but  more 
digestible  than  heavy  bread.  Other  cereals  may  contain  a  larger 
percentage  of  starch,  but  none  have. so  much  gluten  as  wheat; 
when  bread  is  made  from  them  the  carbon  dioxid  gas  escapes  so 
readily  from  the  less  tenacious  dough  that  it  does  not  expand  the 
mass  properly.  Corn  and  rice  are  valuable  chiefly  for  their  high 
carbohydrate  content;  beans  and  peas,  on  the  other  hand,  have 
a  high  per  cent  of  protein.  Potatoes  contain  less  actual  nutri- 
ment for  their  weight  than  do  any  of  the  other  important  foods. 
Their  cheapness  and  digestibility  have  combined  to  give  them  a 
place  in  the  average  dietary  out  of  all  proportion  to  their  real 
value.  Other  fresh  vegetables,  as  carrots,  turnips,  and  cabbages, 
are  valuable  mainly  for  the  salts  they  contain;  their  weight  is 
mainly  due  to  water,  and  they  contain  but  little  starch,  proteins, 
or  fats.  Fruits,  like  most  fresh  vegetables,  are  mainly  valuable 
for  their  saline  constituents,  the  other  food  stuffs  in  them  being 
only,  present  in  small  proportion.  The  cellulose  which  they  con- 
tain makes  up  the  major  portion  of  the  roughage  of  the  diet,  and 
is  valuable  on  that  account. 

The  Cooking  of  Vegetables.  This  is  of  more  importance  even 
than  the  cooking  of  flesh,  since  in  most  the  main  alimentary 
principle  is  starch,  and  raw  starch  is  difficult  of  digestion.  In 
plants  starch  is  stored  up  within  the  walls  of  the  plant-cells,  which 
are  of  cellulose  and  therefore  indigestible.  When  vegetables  are 
cooked  the  contents  of  the  cells  swell,  the  cellulose  walls  are  rup- 
tured and  the  starch  is  set  free  to  be  acted  upon  by  the  digestive 
mechanism  of  the  Body. 

Composition  of  Foods.  The  following  table  gives  the  per- 
centage composition  of  some  of  the  common  foods. 


FOODS:  THEIR  CLASSIFICATION 


437 


In  100  Parts 

Water 

Protein 

Fat 

Digestible 
Carbohydrate 

Inorganic 
Material 

76.7 

20.8 

1.5 

0.3 

1.3 

Esss 

73.7 

126 

12  1 

1.1 

Cheese 

36-60 

25-33 

7-30 

3-7 

3-4 

Cow's  Milk 

87  7 

3  4 

3  2 

4  8 

0  7 

Human  Milk    

89.7 

2  0 

3  1 

5.0 

0.2 

Wheat  Flour  
Wheat  Bread  

13.3 
35.6 

10.2 
7.1 

0.9 
0  2 

74.8 
55.5 

0.5 
1.1 

Rye  Flour  

13  7 

11  5 

2  1 

69  7 

1.4 

Rye  Bread 

42  3 

6  1 

0  4 

49  2 

1  5 

Rice  

13.1 

7.0 

0.9 

77.4 

1.0 

Corn 

13  1 

9  9 

4  6 

68  4 

1.5 

Macaroni  

10.1 

9.0 

0.3 

79.0 

0.5 

Peas  and  Beans  
Potatoes  . 

12-15 

75  5 

23-26 
2  0 

l|-2 
0  2 

49-54 
20  6 

2-3 
1.0 

Carrots 

87  1 

1  0 

0  2 

9  3 

0.9 

Cabbages  

90.0 

2-3 

0.5 

4-6 

1.3 

Mushrooms  
Fruit 

73-91 
84  0 

4-8 
0  5 

0.5 

3-12 
10  0 

1.2 
0.5 

In  a  bulletin  of  the  U.  S.  Department  of  Agriculture  more  de- 
tailed analyses  can  be  found.  (Bull.  28.) 

Alcohol.  Perhaps  no  single  question  in  physiology  has  aroused 
more  discussion  than  that  of  the  physiological  position  of  alcohol. 
Its  use  from  time  immemorial  as  a  beverage,  and  the  long  history 
of  misery  and  crime  which  has  followed  its  use  to  excess,  make  the 
problem  of  its  true  place  one  of  very  great  practical  importance. 

We  must  recognize  at  the  outset  that  alcohol  has  very  diverse 
immediate  effects  according  as  it  is  taken  in  large  or  small  amounts. 
The  miserable  spectacle  presented  by  an  intoxicated  man  em- 
phasizes only  too  clearly  the  harm  of  excessive  indulgence;  on 
the  other  hand,  the  taking  of  a  small  quantity  often  leads  to  an 
appearance  of  heightened  mental  and  physical  ability.  Both 
Mind  and  Body  seem  more  alert  than  commonly.  No  one  ques- 
tions the  injurious  effects  of  large  amounts  of  alcohol;  the  diversity 
of  opinion  is  with  reference  to  its  use  in  small  doses. 

It  has  been  demonstrated  that  alcohol  in  moderate  amounts  is 
oxidized  in  the  Body  with  the  liberation  of  energy,  and  is  there- 
fore a  fuel  in  the  true  sense  of  the  word.  That  it  may  serve  as 
fuel  is  not  in  itself,  however,  justification  for  its  use,  even  in  small 


438  THE  HUMAN  BODY 

quantities.  It  must  be  shown  that  its  direct  physiological  effects 
are  not  harmful  to  the  Body,  before  it  can  be  accepted  as  a  food. 

The  action  of  alcohol  in  small  doses  appears  to  be  chiefly  upon 
the  nervous  system,  and  particularly  upon  the  higher  portions  of 
the  central  nervous  system.  Its  effect  upon  nerve-centers  seems 
to  be  a  depressing  one;  the  generally  accepted  view  that  alcohol 
is  a  stimulant  being  based  upon  bodily  effects  which  follow  nerve- 
center  depression  rather  than  stimulation.  For  example,  cutane- 
ous vasodilation,  with  flushing  of  the  skin,  such  as  is  commonly 
seen  after  taking  alcohol,  is  the  result  of  depression  of  the  vaso- 
constrictor center.  The  rapid  heart-beat,  which  is  another  usual 
phenomenon,  results  from  depression  of  the  cardio-inhibitory 
center.  Even  the  sparkle  of  wit  and  repartee,  which  is  reputed 
to  be  very  marked  after  partaking  of  wine,  is  the  result  of  removal 
of  the  brakes  of  judgment  and  caution  through  depression  of  those 
regions  of  the  brain  where  these  functions  reside.  It  is  intimated, 
in  fact,  that  after-dinner  wit  is  ordinarily  appreciated  at  more 
than  its  due  desert,  because  of  the  depression  of  judgment  in  the 
brains  of  the  hearers. 

The  depressing  effect  of  alcohol  upon  the  brain  appears  to  be 
progressively  from  higher  to  lower  centers.  The  first  traits  to  be 
dulled  are  those  acquired  through  precept  and  moral  training; 
therefore  the  individual  is  apt  to  reveal  his  "true  self,"  stripped 
of  the  veneer  of  education.  With  increasing  indulgence  in  alcohol 
lower  and  lower  tendencies  come  to  the  fore,  set  free  by  the  de- 
pression of  the  higher,  and  ordinarily  controlling  ones.  Thus  it 
comes  to  pass  that  man  may  sink  to  the  level  of  the  beast. 

The  question  of  the  moderate  use  of  alcohol  resolves  itself,  then, 
from  a  physiological  standpoint,  into  one  of  the  desirability  of 
setting  free  the  lower  mental  traits  and  activities  through  depres- 
sion of  the  higher  inhibitory  ones.  It  is  sometimes  argued  that  in 
America  where  the  dominant  mental  obsession  of  a  considerable 
proportion  of  the  population  is  in  affairs  of  business  the  setting 
free  of  the  brain  from  business  cares  during  leisure  hours  is  a 
virtual  necessity,  and  that  the  use  of  alcohol  is  the  most  direct 
method  of  bringing  this  about.  Even  though  we  grant  the  first 
part  of  the  argument  it  does  not  necessarily  follow  that  the  second 
part  is  to  be  accepted  also.  For  the  real  objection  to  the  use  of 
alcohol,  even  in  small  quantities,  is  that  the  desire  for  alcohol, 


FOODS:  THEIR  CLASSIFICATION  439 

unlike  the  desire  for  food,  increases  as  more  is  taken  instead  of 
decreasing  when  satiety  is  reached.  It  thus  requires  a  stronger 
effort  of  will  to  leave  off  as  more  is  taken,  and  since  the  alcohol 
at  the  same  time  depresses  the  power  of  the  will  the  danger  of 
Overindulgence  is  continually  present.  Only  where  the  will  is 
sufficiently  strong  to  set  a  limit  and  adhere  rigidly  to  it  is  the  con- 
tinuous moderate  use  of  alcohol  in  any  degree  safe. 

Returning  to  the  question  of  the  desirability  of  the  practice  of 
removing  the  brakes  from  the  brain  periodically,  it  may  be  said 
that  the  opinion  seems  to  be  becoming  more  and  more  prevalent 
among  neurologists  that  the  use  of  alcohol  for  such  a  purpose, 
particularly  in  early  and  middle  life,  is  more  of  an  injury  than  a 
benefit.  The  normal  interactions  among  the  different  parts  of  the 
mental  apparatus  should  be  permitted,  according  to  these  ob- 
servers, to  proceed  without  artificial  interference,  at  least  during 
the  period  of  the  most  active  associative  processes.  There  seems 
to  be  no  vital  objection  to  the  moderate  use  of  alcohol  on  the  part 
of  persons  who  have  passed  the  age  of  fifty  or  thereabouts.  The 
danger  of  acquiring  the  alcohol  habit  is  practically  nil  at  that  age, 
and  the  predominant  mental  traits  are  by  that  time  so  completely 
in  control  that  occasional  release  from  them  may  operate  as  a 
distinct  advantage.  This  is  particularly  true  in  the  case  of  those 
dderly  persons  who  find  themselves  disposed  to  a  somewhat 
gloomy  outlook  upon  life.  The  temperate  use  of  alcohol  may 
make  life  more  enjoyable  for  themselves  and  also  for  those  about 
them. 

Tea,  Coffee,  and  Cocoa.  These  beverages  all  owe  their  special 
physiological  properties  to  certain  alkaloids  present  in  them. 
The  active  principle  of  tea  and  coffee  is  the  same,  caffein;  that  of 
cocoa,  and  its  derivative,  chocolate,  is  a  closely  related  substance 
theobromin.  Caffein  and  theobromin  appear  to  be  direct  nerve- 
stimulants.  They  cause  a  rise  of  blood-pressure  through  stimu- 
lation of  the  vasoconstrictor  center.  Their  use,  like  that  of  al- 
cohol, constitutes  an  artificial  interference  with  normal  processes, 
and  is  subject,  therefore,  to  the  general  objections  which  arise 
against  such  interference.  Their  effects  are  of  varying  intensity; 
cocoa  is  an  exceedingly  mild  stimulant;  tea,  properly  made,  is 
somewhat  stronger;  and  coffee,  properly  made,  is  stronger  yet. 
Their  use  is  borne  much  better  by  some  persons  than  by  others. 


440  THE  HUMAN  BODY 

They  are  not  dangerous  in  the  sense  that  alcohol  is,  through 
an  increasing  craving  which  readily  leads  to  overindulgence  and 
resulting  disaster,  although  they,  like  alcohol,  are  often  taken 
to  excess.  Temperance  in  the  use  of  these  beverages  is  as  much 
the  part  of  wisdom  as  in  the  use  of  alcohol.  Again,  like  alcohol, 
they  are  best  left  alone  during  early  life. 

The  improper  preparation  of  tea  and  coffee,  by  boiling  them  in 
water,  carries  into  solution,  in  addition  to  the  stimulating  prin- 
ciple, a  substance,  tannin,  whose  effect  upon  the  system  is  apt  to 
be  distinctly  harmful.  These  beverages  should  therefore  always 
be  prepared  by  methods  which  do  not  involve  prolonged  or  even 
brief  boiling  while  the  tea  leaves  or  coffee  grounds  are  actually 
present  in  the  liquid. 

Food  Poisoning.  There  are  several  conditions  under  which 
foods,  instead  of  being  of  benefit  to  the  Body,  may  become  ac- 
tually harmful.  They  need  to  be  guarded  against,  both  by  in- 
dividuals and  by  the  public.  The  latter  because,  with  the  present 
organization  of  society,  virtually  all  foods  pass  through  many 
hands  before  they  finally  reach  the  consumer,  and  there  are  cor- 
respondingly many  possibilities  of  contamination.  The  deliberate 
introduction  into  foods  of  injurious  adulterants  is  probably  much 
less  common  than  some  people  have  supposed.  Unintentional 
contamination  may  occur,  although  it  is  much  less  likely  under 
modern  scientific  conditions  than  formerly  when  rule-of-thumb 
methods  obtained.  A  historical  example  of  accidental  contamina- 
tion was  the  ergot  poisoning  that  used  occasionally  to  ravage 
certain  parts  of  Europe.  Ergot  is  a  poisonous  constituent  of  a 
parasitic  growth  sometimes  found  on  rye.  When  the  affected 
grain  was  made  into  rye  flour  and  eaten  regularly  over  a  long  pe- 
riod, whole  populations  underwent  typical  ergot  poisoning.  The 
symptoms  were  in  many  respects  similar  to  those  of  leprosy;  dry 
gangrene,  with  loss  of  fingers  and  toes,  and  ultimate  death. 

More  dangerous,  because  more  difficult  to  guard  against,  are 
chemical  changes  that  may  occur  in  food  between  the  time  of  its 
preparation  and  its  consumption.  These  are  usually  the  result  of 
bacterial  growth  within  the  food  mass,  this  growth  giving  rise  to 
toxins  as  waste  products  in  much  the  same  manner  as  does  the 
growth  of  pathogenic  organisms  within  the  Body.  There  are  at 
least  two  conditions  of  poisoning  from  such  toxins.  The  first  is 


FOODS:  THEIR  CLASSIFICATION  \  \  I 

ptomain  poisoning;  the  toxins  formed  are  known  as  ptomains.  The 
chief  symptom  is  the  very  pronounced  gastro-intestinal  upset. 
This  is  beneficial  in  that  it  acts  to  rid  the  Body  promptly  of  the 
contaminated  material,  and  so  to  reduce  the  amount  of  poison 
absorbed.  A  second  form  of  poisoning  from  bacterial  decomposi- 
tion is  botulism,  so  named  from  its  occasional  occurrence  in  sausage. 
The  effect  of  this  poison  on  the  digestive  tract  is  just  opposite  to 
that  of  ptomain.  It  paralyzes  instead  of  exciting;  so  the  poisoned 
mass  is  not  expelled,  but  remains  in  the  intestinal  tract  and  allows 
absorption  to  continue.  For  this  reason  botulism  has  ordinarily 
much  more  serious  effects  than  has  ptomain. 

An  additional  type  of  food  poisoning  is  that  seen  in  individual 
susceptibilities,  or  idiosyncracies.  Some  people  are  poisoned  by 
veal,  others  by  shell  fish,  occasionally  a  case  of  susceptibility  to  egg 
albumen  is  seen.  Various  other  foods  may  act  similarly.  The 
poisonous  elements  in  these  cases  appear  to  be  identical  with  the 
lymphagogues  described  in  a  previous  chapter  (p.  385).  The 
lymphagogue  action  was  there  stated  to  show  itself  only  in  sus- 
ceptible individuals.  In  general  acute  poisoning  is  probably -only 
a  more  marked  manifestation  of  the  sensitiveness  which  takes 
the  form  of  increased  permeability  of  the  capillaries  in  those  in 
whom  the  action  is  purely  that  of  a  lymphagogue. 


CHAPTER  XXVI 
THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES 

General  Arrangement.  The  alimentary  canal  is  essentially  a 
tube  running  through  the  Body  (Fig.  2)  and  lined  by  a  vascular 
membrane,  most  of  which  is  specially  adapted  for  absorption;  it 
communicates  with  the  exterior  at  three  points  (the  nose,  the 
mouth,  and  the  anal  aperture),  at  which  the  lining  mucous  mem- 
brane is  continuous  with  the  general  outer  integument.  Support- 
ing the  absorbent  membrane  are  layers  which  strengthen  the  tube, 
and  are  in  part  muscular  and,  by  their  contractions,  serve  to  pass 
materials  along  it  from  one  end  to  the  other.  In  the  walls  of  the 
canal  are  numerous  blood  and  lymphatic  vessels  which  carry  off 
the  matters  absorbed  from  its  cavity;  and  there  also  exist  in  con- 
nection with  it  numerous  glands,  whose  function  it  is  to  pour  into 
it  various  secretions  by  which  the  chemical  act  of  digestion  is 
carried  on.  Some  of  these  glands  are  minute  and  embedded  in  the 
walls  of  the  alimentary  tube  itself,  but  others  (such  as  the  salivary 
glands)  are  larger  and  lie  away  from  the  main  channel,  into  which 
their  products  are  carried  by  ducts  of  various  lengths. 

The  alimentary  tube  is  not  uniform  but  presents  several  dilata- 
tions on  its  course ;  nor  is  it  straight,  since,  being  much  longer  than 
the  Body,  a  large  part  of  it  is  packed  away  by  being  coiled  up  in 
the  abdominal  cavity. 

Subdivisions  of  the  Alimentary  Canal.  The  mouth-opening 
leads  into  a  chamber  containing  the  teeth  and  tongue,  the  mouth- 
chamber  or  buccal  cavity.  This  is  succeeded  by  the  pharynx  or 
throat-cavity,  which  narrows  at  the  top  of  the  neck  into  the  gullet  or 
esophagus;  this  runs  down  through  the  thorax  and,  passing 
through  the  diaphragm,  dilates  in  the  upper  part  of  the  abdominal 
cavity  into  the  stomach.  Beyond  the  stomach  the  channel  again 
narrows  to  form  a  long  and  greatly  coiled  tube,  the  small  intestine, 
which  terminates  by  opening  into  the  large  intestine,  much  shorter 
although  wider  than  the  small,  and  terminating  by  an  opening  on 
the  exterior. 

442 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES     443 


The  Mouth- Cavity  (Fig.  121)  is  bounded  in  front  and  on  the 
sides  by  the  lips  and  cheeks,  below  by  the  tongue,  k,  and  above 

by  the  palate;  which  latter  consists 
of  an  anterior  part,  Z,  supported 
by  bone  and  called  the  hard  palate, 
and  a  posterior,  /,  containing  no 
bone,  and  called  the  soft  palate. 
The  two  can  readily  be  distinguished 
by  applying  the  tip  of  the  tongue 
to  the  roof  of  the  mouth  and  draw- 
ing it  backwards.  The  hard  palate 
forms  the  partition  between  the 
mouth  and  nose.  The  soft  palate 
arches  down  over  the  back  of  the 
mouth,  hanging  like  a  curtain  be- 
tween it  and  the  pharynx,  as  can 
be  seen  by  holding  the  mouth  open 
in  front  of  a  looking-glass.  From 
the  middle  of  its  free  border  a 
conical  process,  the  uvula,  hangs 
down. 
The  Teeth.  Immediately  within 

FIG.  121.— The  mouth,  nose  and  the  cheeks  and  lips  are  two  semi- 
pharynx,  with  the  commencement  .  r 

of  the  gullet  and  larynx,  as  exposed  circles,  formed   by   the   borders  of 

by  a  section,  a  little  to  the  left  of  ,  ,    ,                           , 

the  median   plane  of  the  head,     a,  the      upper  and   lower  jaw-bones, 

vertebral  column;  b,  gullet;  c,  wind-  wV>ipVi     QT-A  Prv^Arprl     KIT-  tV»A     niivn* 

pipe;  d,  larynx;  e,  epiglottis,  /,  soft  Wn]  'OVGI           DV           \    gums, 

palate;  g,  opening  of  Eustachian  except  at  intervals  along  their  edges 

tube;  k,  tongue;  I,  hard  palate;  m,  .  . 

the  sphenoid  bone  on  the  base  of  where  they  contain  sockets  in  which 
^iVca^y^^Tth^turWnaS  the  teeth  are   implanted.     During 

bones  of  the  outer  side  of  the  left   Hfc  two  getg  of  teeth  are  developed: 

nostril-chamber. 

the  first  or  milk   set  appears   soon 

after  birth  and  is  shed  during  childhood,  when  the  second  or 
permanent  set  appears. 

The  teeth  differ  in  minor  points  from  one  another,  but  in  each 
three  parts  are  distinguishable;  one,  seen  in  the  mouth  and  called 
the  crown  of  the  tooth;  a  second,  embedded  in  the  jaw-bone  and 
called  the  root  or  fang;  and  between  the  two,  embraced  by  the  edge 
of  the  gum,  is  a  narrowed  portion,  the  neck  or  cervix.  From  dif- 
ferences in  their  forms  and  uses  the  teeth  are  divided  into  incisorst 


444  THE  HUMAN  BODY 

canines,  bicuspids,  and  molars,  arranged  in  a  definite  order  in  each 
jaw.  Beginning  at  the  middle  line  we  meet  in  each  half  of  each 
jaw  with,  successively,  two  incisors,  one  canine,  and  two  molars 
in  the  milk  set;  making  twenty  altogether  in  the  two  jaws.  The 
teeth  of  the  permanent  set  are  thirty-two  in  number,  eight  in  each 
half  of  each  jaw,  viz. — beginning  at  the  middle  line — two  incisors, 
one  canine,  two  bicuspids,  and  three  molars.  The  bicuspids,  or 
premolars,  of  the  permanent  set  replace  the  milk-molars,  while 
the  permanent  molars  are  new  teeth  added  on  as  the  jaw  grows, 
and  not  substituting  any  of  the  milk-teeth.  The  hindmost  per- 
manent molars  are  often  called  the  wisdom-teeth. 

Characters  of  Individual  Teeth.  The  incisors  (Fig.  122)  are 
adapted  for  cutting  the  food.  Their  crowns  are  chisel-shaped  and 
have  sharp  horizontal  cutting  edges,  which  become  worn  away  by 


FIG.  122  FIG.  123  FIG.  124  FIG.  125 

FIG.  122. — An  incisor  tooth. 
FIG.  123. — A  canine  or  eye-tooth. 

FIG.  124. — A  bicuspid  tooth  seen  from  its  outer  side;  the  inner  cusp  is,  accord- 
ingly, not  visible. 

Fig.  125. — A  molar  tooth. 

use  so  that  they  are  beveled  off  behind  in  the  upper  row,  and  in  the 
opposite  direction  in  the  lower.  Each  has  a  long  root.  The 
canines  (Fig.  123)  are  somewhat  larger  than  the  incisors.  Their 
crowns  are  thick  and  somewhat  conical,  having  a  central  point  or 
cusp  on  the  cutting  edge.  In  dogs,  cats,  and  other  carnivora  the 
canines  are  very  large  and  adapted  for  seizing  and  holding  prey. 
The  bicuspids  or  premolars  (Fig.  124)  are  rather  shorter  than  the 
canines  and  their  crowns  are  somewhat  cuboidal.  Each  has  two 
cusps,  an  outer  towards  the  cheek,  and  an  inner  on  the  side  turned 
towards  the  interior  of  the  mouth.  The  root  is  compressed  later- 
ally, and  has  usually  a  groove  partially  subdividing  it  into  two. 
At  its  tip  the  separation  is  often  complete.  The  molar  teeth  or 
grinders  (Fig.  125)  have  large  crowns  with  broad  surfaces,  on  which 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES      445 


are  four  or  five  projecting  tubercles,  which  roughen  them  and 
make  them  better  adapted  to  crush  the  food.  Each  has  usually 
several  roots.  The  milk-teeth  differ  only  in  subsidiary  points  from 
those  of  the  same  names  in  the  permanent  set. 

The  Structure  of  a  Tooth.  If  a  tooth  be  broken  open,  a  cavity 
extending  through  both  crown  and  root  will  be  found  in  it.  This 
is  filled  during  life  with  a  soft  vascular  pulp,  and  hence  is  known 
as  the  "pulp-cavity"  (c,  Fig.  126).  The  hard  parts  of  the  tooth 
disposed  around  the  pulp-cavity  consist  of  three  different  tissues. 
Of  these  one  immediately  surrounds  the  cavity  and  makes  up  most 
of  the  bulk  of  the  tooth;  it  is  dentine  (2, 
Fig.  126);  covering  the  dentine  on  the 
crown  is  the  enamel  (1,  Fig.  126)  and  on 
the  root,  the  cement  (3,  Fig.  126). 

The  pulp-cavity  opens  below  by  a  narrow 
aperture  at  the  tip  of  the  root,  or  at  the  tip 
of  each  if  the  tooth  have  more  than  one. 
The  pulp  consists  mainly  of  connective 
tissue,  but  its  surface  next  the  dentine  is 
covered  by  a  layer  of  columnar  cells. 
Through  the  opening  on  the  root  blood- 
vessels and  nerves  enter  the  pulp. 

The  dentine  (ivory)  yields  on  analysis  the 
same  materials  as  bone  but  is  somewhat 
harder,  earthy  matters  constituting  72  per 
cent  of  it  as  against  66  per  cent  in  bone.  FIG.  126,-Section  through 
Under  the  microscope  it  is  recognized  by  a  premoiar  tooth  of  the 

_  _        ^    .  ...  ^     cat  still  embedded   in  its 

the  fine  dentinal  tubules  which,  radiating  socket,  i,  enamel;  2,  den- 
from  the  pulp-cavity,  perforate  it  through-  ^m:.  |;  thTbone  tf  the 
out,  finally  ending  in  minute  branches  which  J^?*  Jaw;  c>  the  pulp~ 
open  into  irregular  cavities,  the  interglobular 
spaces,  which  lie  just  beneath  the  enamel  or  cement.  At  their 
widest  ends,  close  to  the  pulp-cavity,  the  dentinal  tubules  are  only 
about  0.005  millimeter  (*  5Vir  °f  an  inch)  in  diameter.  The  cement 
is  much  like  bone  in  structure  and  composition.  It  is  thickest  at 
the  tip  of  the  root  and  thins  away  towards  the  cervix.  Enamel  is 
the  hardest  tissue  in  the  Body,  yielding  on  analysis  only  from  2 
per  cent  to  3  per  cent  of  organic  matter,  the  rest  being  mainly 
calcium  phosphate  and  carbonate.  Its  histological  elements  are 


446  THE  HUMAN  BODY 

minute  hexagonal  prisms,  closely  packed,  and  set  on  vertically  to 
the  surface  of  the  subjacent  dentine.  It  is  thickest  over  the  free 
end  of  the  crown,  until  worn  away  by  use.  Covering  the  enamel  in 
unworn  teeth  is  a  thin  structureless  horny  layer,  the  enamel  cuticle. 

The  Tongue  (Fig.  127)  is  a  muscular  organ  covered  by  mucous 
membrane,  extremely  mobile,  and  endowed  not  only  with  a  deli- 
cate tactile  sensibility  but  with  the  terminal  organs  of  the  special 
sense  of  taste;  it  is  attached  by  its  root  to  the  hyoid  bone.  On  itn 
upper  surface  are  numerous  small  eminences  or  papillae,  such  an' 
are  found  more  highly  developed  on  the  tongue  of  a  cat,  where  they 
may  be  readily  felt.  On  the  human  tongue  there  are  three  forma 
of  papillae,  the  circumvallate,  the  fungiform,  and  the  filiform.  The 
circumvallate  papillae,  1  and  2  (Fig.  127),  the  largest  and  least 
numerous,  are  from  seven  to  twelve  in  number  and  lie  near  the 
root  of  the  tongue  arranged  in  the  form  of  a  V  with  its  open  angle 
turned  forwards.  Each  is  an  elevation  of  the  mucous  membrane, 
covered  by  epithelium,  and  surrounded  by  a  trench.  On  the  sides 
of  these  papillae,  embedded  in  the  epithelium,  are  many  small  oval, 
bodies  richly  supplied  with  nerves  and  supposed  to  be  concerned 
in  the  sense  of  taste,  and  hence  called  the  taste-buds  (Chap.  XIV). 
The  fungiform  papillae,  3,  are  rounded  elevations  attached  by 
somewhat  narrowed  stalks,  and  found  all  over  the  middle  and  fore 
part  of  the  upper  surface  of  the  tongue.  They  are  easily  -recog- 
nized on  the  living  tongue  by  their  bright  red  color.  The  filiform 
papillae,  4,  most  numerous  and  smallest,  are  scattered  all  over  the 
dorsum  of  the  tongue  except  near  its  base.  Each  is  a  conical 
eminence  covered  by  a  thick  horny  layer  of  epithelium.  It  is  these 
papillae  which  are  so  highly  developed  on  the  tongues  of  Carnivora, 
and  serve  them  to  scrape  bones  clean  of  even  such  tough  structures 
as  ligaments. 

In  health  the  surface  of  the  tongue  is  moist,  covered  by  little 
"fur,"  and  in  childhood  of  a  red  color.  In  adult  life  the  natural 
color  of  the  tongue  is  less  red,  except  around  the  edges  and  tip;  a 
bright-red  glistening  tongue  being  then,  usually  a  symptom  of 
disease.  When  the  digestive  organs  are  deranged  the  tongue  is 
commonly  covered  with  a  thick  yellowish  coat,  composed  of  a  little 
mucus,  some  cells  of  epithelium  shed  from  the  surface,  and 
numerous  microscopic  organisms  known  as  bacteria;  and  there 
is  frequently  a  "bad  taste  in  the  mouth."  The  whole  alimen^ 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES      447 

tary  mucous  membrane  is  in  close  physiological  relationship;  and 
anything  disordering  the  stomach  is  likely  to  produce  a  "  furred 
tongue." 

The  Salivary  Glands.     The  saliva,  which  is  poured  into  the 
mouth  and  which,  mixed  with  the  secretion  of  minute  glands  em- 


FIG.  127. — The  upper  surface  of  the  tongue  with  part  of  the  pillars  of  the  fauces 
and  the  tonsils.  1,  2,  circumvallate  papillae;  3,  fungiform  papillae;  4,  filiform  pa- 
pillae; 6,  mucous  glands;  7,  tonsils;  8,  tip  of  epiglottis. 

bedded  in  its  lining  membrane,  moistens  it,  is  secreted  by  three 
pairs  of  glands,  the  parotid,  the  submaxillary,  and  the  sublingual. 
The  parotid  glands  lie  in  front  of  the  ear  behind  the  ramus  of  the 
lower  jaw;  each  sends  its  secretion  into  the  mouth  by  a  tube  known 


448  THE  HUMAN  BODY 

as  Stenson's  duct,  which  crosses  the  cheek  and  opens  opposite  the 
second  upper  molar  tooth.  In  the  disease  known  as  mumps  *  the 
parotid  glands  are  inflamed  and  enlarged.  The  submaxillary 
glands  lie  between  the  halves  of  the  lower  jaw-bone,  near  its  an- 
gles, and  their  ducts  open  beneath  the  tongue  near  the  middle  line. 
The  sublingual  glands  lie  beneath  the  floor  of  the  mouth,  covered 
by  its  mucous  membrane,  between  the  back  part  of  the  tongue 
and  the  lower  jaw-bone.  Each  has  many  ducts  (8  to  20),  some  of 
which  join  the  submaxillary  duct,  while  the  rest  open  separately 
in  the  floor  of  the  mouth. 

The  Fauces  is  the  name  given  to  the  aperture  which  can  be  seen 
at  the  back  of  the  mouth  below  the  soft  palate  (Fig.  121),  and 
leading  into  the  pharynx.  It  is  bounded  above  by  the  soft  palate 
and  uvula,  below  by  the  root  of  the  tongue,  and  on  the  sides  by 
muscular  elevations  covered  by  mucous  membrane,  which  reach 
from  the  soft  palate  to  the  tongue.  These  elevations  are  the  pillars 
of  the  fauces.  Each  bifurcates  below,  and  in  the  hollow  between 
its  divisions  lies  a  tonsil  (7,  Fig.  127),  a  soft  rounded  body  about 
the  size  of  an  almond,  composed  of  lymphoid  tissue  (Chap.  XXII). 

The  Pharynx  or  Throat-Cavity  (Fig.  121).  This  portion  of  the 
alimentary  canal  may  be  described  as  a  conical  bag  with  its  broad 
end  turned  upwards  towards  the  base  of  the  skull,  and  its  narrow 
end  downwards  and  passing  into  the  gullet.  Its  front  is  imperfect, 
presenting  openings  which  lead  into  the  nose,  the  mouth,  and 
(through  the  larynx  and  windpipe)  the  lungs.  Except  during 
swallowing  or  speech  the  soft  palate  hangs  down  between  the  mouth 
and  pharynx;  during  deglutition  it  is  raised  into  a  horizontal  posi- 
tion and  separates  an  upper  or  respiratory  portion  of  the  pharynx 
fronj  the  rest.  Through  this  upper  part,  therefore,  air  alone  passes, 
entering  it  from  the  posterior  ends  of  the  two  nostril-chambers; 
while  through  the  lower  portion  both  food  and  air  pass,  one  on 
its  way  to  the  gullet,  6,  Fig.  121,  the  other  through  the  larynx,  d, 
to  the  windpipe,  c;  when  a  morsel  of  food  "goes  the  wrong  way" 
it  takes  the  latter  course.  Opening  into  the  upper  portion  of  the 
pharynx  on  each  side  is  an  Eustachian  tube,  g  (p.  227) ;  so  that  the 
apertures  leading  out  of  it  are  seven  in  number;  the  two  pos- 
terior nares,  the  two  Eustachian  tubes,  the  fauces,  the  opening 
of  the  larynx,  and  that  of  the  gullet.  At  the  root  of  the  tongue, 
*  Parotitis,  in  technical  language. 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES      449 


over  the  opening  of  the  larynx,  is  a  plate  of  cartilage,  the  epiglottis, 
e,  which  can  be  seen  if  the  mouth  is  widely  opened  and  the  back 
of  the  tongue  pressed  down  by  some  such  thing  as  the  handle  of  a 
spoon.  During  swallowing  the  epiglottis  is  pressed  down  like  a  lid 
over  the  air-tube  and  helps  to  keep  food  or  saliva  from  entering  it. 
In  structure  the  pharynx  consists  essentially  of  a  bag  of  connective 
tissue  lined  by  mucous  membrane,  and  having  muscles  in  its  walls 
which  drive  the  food  on. 

The  Esophagus  or  Gullet  is  a  tube  commencing  at  the  lower 
termination  of  the  pharynx  and  which,  passing  on  through  the 
neck  and  chest,  ends  below  the  diaphragm  by  joining  the  stomach. 
In  the  neck  it  lies  close  behind  the  windpipe.  It  consists  of  three 
coats — a  mucous  membrane  within;  next,  a  submucous  coat  of 
areolar  connective  tissue:  and,  outside,  a  muscular  coat  made  up 
of  two  layers,  an  inner  with  transversely  and  an  outer  with  longi- 
tudinally arranged  fibers.  In  and  beneath  its  mucous  membrane 
are  numerous  small  mucous  glands  whose  ducts  open  into  the 
tube. 

The  Stomach  (Fig.  128)  is  a  somewhat  conical  bag  placed  trans- 
versely in  the  upper  part  of  the  abdominal  cavity.  Its  larger  end 
is  turned  to  the  left  and  lies  close  beneath  the  diaphragm;  opening 
into  its  upper  border,  through  the 
cardiac  orifice  at  a,  is  the  gullet  d. 
The  narrower  right  end  is  con- 
tinuous at  c  with  the  small  intes- 
tine; the  aperture  between  the 
two  is  the  pyloric  orifice.  The 
pyloric  end  of  the  stomach  lies 
lower  in  the  abdomen  than  the 
cardiac,  and  is  separated  from 
the  diaphragm  by  the  liver  (see 
Fig.  1).  The  concave  border  be- 
tween the  two  orifices  is  known 
as  the  small  curvature,  and  the 
convex,  as  the  great  curvature  of 
the  stomach.  From  the  latter  hangs  down  a  fold  of  peritoneum 
known  as  the  great  amentum.  It  is  spread  over  the  rest  of  the 
abdominal  contents  like  an  apron.  After  middle  life  much  fat 
frequently  accumulates  in  the  omentum,  so  that  it  is  largely  re- 


FIG.  128. — The  stomach,  d,  lower 
end  of  the  gullet;  a,  position  of  the 
cardiac  aperture;  b,  the  fundus;  c,  the 
pylorus;  e,  the  commencement  of 
the  small  intestine;  along  a,  b,  c,  the 
great  curvature;  between  the  pylorus 
and  d,  the  lesser  curvature. 


450  THE  HUMAN  BODY 

sponsible  for  the  "fair  round  belly  with  good  capon  lin'd."  The 
protrusion  b  to  the  left  side  of  the  cardiac  orifice,  Fig.  128,  is  the 
fundus.  The  size  of  the  stomach  varies  greatly  with  the  amount 
of  food  in  it;  when  empty  it  is  little  more  than  a  tube;  just  after  a 
moderate  meal  it  is  about  ten  inches  long,  by  five  wide  at  its 
broadest  part. 

Since  the  cardiac  end  of  the  stomach  lies  immediately  beneath 
the  diaphragm,  which  has  the  heart  on  its  upper  side,  its  over- 
distension,  due  to  indigestion  or  flatulence,  may  impede  the  action 
of  the  thoracic  organs,  and  cause  feelings  of  oppression  in  the 
chest,  or  palpitation  of  the  heart. 

Structure  of  the  Stomach.  This  organ  has  four  coats,  known 
successively  from  without  in  as  the  serous,  the  muscular,  the  sub- 
mucous,  and  the  mucous.  The  serous  coat  is  formed  by  a  reflection 
of  the  peritoneum,  a  double  fold  of  which  slings  the  stomach;  after 
separating  to  envelop  it  the  two  layers  again  unite  and,  hanging 
down  beyond  it,  form  the  great  omentum.  The  muscular  coat 
(Fig.  43)  consists  of  unstriped  muscular  tissue  arranged  in  three 
layers:  an  outer,  longitudinal,  most  developed  about  the  curva- 
tures; a  circular,  evenly  spread  over  the  whole  organ,  except 
around  the  pyloric  orifice  where  it  forms  a  thick  ring;  and  an  inner, 
oblique  and  very  incomplete,  radiating  from  the  cardiac  orifice. 
The  submucous  coat  is  made  up  of  lax  areolar  tissue  and  binds 
loosely  the  mucous  coat  to  the  muscular.  The  mucous  coat  is  a 
moist  pink  membrane  which  is  inelastic,  and  large  enough  to  line 
the  stomach  evenly  when  it  is  fully  distended.  Accordingly,  when 
the  organ  is  empty  and  shrunken,  this  coat  is  thrown  into  folds, 
which  disappear  when  the  organ  is  distended.  During  digestion 
the  arteries  supplying  the  stomach  become  dilated  and,  its  capil- 
laries being  gorged,  its  mucous  membrane  is  then  much  redder 
than  during  hunger. 

The  blood-vessels  of  the  stomach  run  to  it  between  the  folds  of 
peritoneum  which  sling  it.  After  giving  off  a  few  branches  to  the 
outer  layers,  most  of  the  arteries  break  up  into  small  branches  in 
the  submucous  coat,  from  which  twigs  proceed  to  supply  the  close 
capillary  network  of  the  mucous  membrane. 

The  nerves  of  the  stomach  belong  to  the  autonomic  system,  and 
like  most  other  structures  supplied  by  this  system,  the  stomach 
has  double  innervation  (p.  195).  The  cranial  autonomic  fibers  are 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES      451 


derived  from  the  vagi.  In  the  lower  part  of  the  thorax  these  nerves 
consist  mainly  of  non-medullated  fibers,  and  lie  on  the  sides  of 
the  gullet,  across  which  they  interchange  fibers  by  means  of 
several  branches.  On  entering  the  abdomen  the  left  vagus  passes 
to  the  ventral  side  of  the  stomach,  in  which  it  ends :  the  right  sup- 
plies the  dorsal  side  of  the  stomach,  but  a  considerable  portion  of 
it  passes  on  to  enter  the  solar  plexus,  which  lies  behind  the  stomach 
and  contains  several  large  ganglia.  The  thoracico-lumbar  auto- 
nomic  fibers  pass  to  the  stomach  as  branches  from  the  great  splanch- 
nic nerves,  which  serve  as  the  chief  paths  of  distribution  for  these 
fibers  in  the  abdomen. 

Histology  of  the  Gastric  Mucous  Membrane.  Examination  of 
the  inner  surface  of  the  stomach  with  a  hand  lens  shows  it  to  be 
covered,  except  in  the  fundic  region,  with  minute 
shallow  pits.  Into  these  open  the  mouths  of 
minute  tubes,  the  gastric  glands,  which  are  closely 
packed  side  by  side  in  the  mucous  membrane ; 
something  like  the  cells  of  a  honeycomb,  except 
that  each  is  open  at  one  end.  Between  them  lie 
a  small  amount  of  connective  tissue,  a  close 
network  of  lymph-channels,  and  capillary  blood- 
vessels. The  whole  surface  of  the  mucous  mem- 
brane is  lined  by  a  single  layer  of  columnar 
mucus-making  epithelium  cells  (m,  Fig.  129). 
These  dip  down  and  line  the  necks  of  the 
tubular  glands.  The  deeper  portions  of  the 
glands  are  lined  by  a  layer  of  shorter  and  some- 
what cuboidal  cells,  the  central  or  chief  cells.  In 
specimens  taken  from  a  healthy  animal  killed 
during  digestion  these  cells  are  large  and  do 
not  stain  deeply  with  carmine.  Similar  speci- 
mens taken  from  an  animal  an  hour  or  two  the  gland;  m,  mu- 

,  ,        ,  ,,  -,     *  ,1        cous  cells  lining  the 

after  a  good  meal  has  been  swallowed  show  the  mouth  of  the  gland 
chief  cells  shrunken  and  staining  more  deeply.  f£*er  surface gof  the 
They  thus  store  up  during  rest  a  material  which  ™uc<gus  ce™s™p  ^vai 
they  get  rid  of  when  the  gastric  juice  is  being  cells, 
secreted. 

In  the  pyloric  end  of  the  stomach  only  the  chief  cells  line  the 
glands,  but  elsewhere  there  is  found  outside  of  them,  in  most 


-P 


FIG.    129.  — Dia- 
of 
D, 


452  THE  HUMAN  BODY 

of  the  glands,  an  incomplete  layer  of  larger  oval  cells  (p,  Fig.  129). 
The  glands  frequently  branch  at  their  deeper  ends. 

The  Pylorus.  If  the  stomach  be  opened  it  is  seen  that  the 
mucous  membrane  projects  in  a  fold  around  the  pyloric  orifice  and 
narrows  it.  This  is  due  to  a  thick  ring  of  the  circular  muscular 
layer  there  developed,  and  forming  around  the  orifice  a  sphincter 
muscle,  which,  by  its  contraction,  keeps  the  passage  to  the  small 
intestine  closed  except  when  portions  of  food  are  to  be  passed  on 
from  the  stomach  to  succeeding  divisions  of  the  alimentary  canal. 

The  Small  Intestine  (Fig.  136),  commencing  at  the  pylorus, 
ends,  after  many  windings,  in  the  large  intestine.  It  is  about  six 
meters  (twenty  feet)  long,  and  about  five  centimeters  (two  inches) 
wide  at  its  gastric  end,  narrowing  to  about  two-thirds  of  that 
width  at  its  lower  portion.  Externally  there  are  no  lines  of  sub- 
division on  the  small  intestine,  but  anatomists  arbitrarily  describe 
it  as  consisting  of  three  parts;  the  first  twelve  inches  being  the 
duodenum,  D,  the  succeeding  two-fifths  of  the  remainder  the 
jejunum,  J,  and  the  rest  the  ileum,  I. 

Like  the  stomach,  the  small  intestine  possesses  four  coats;  a 
serous,  a  muscular,  a  submucous,  and  a  mucous.  The  serous  coat 
is  formed  by  a  duplicature  of  the  peritoneum,  but  presents  noth- 
ing answering  to  the  great  omentum;  this  double  fold  slinging  the 
intestine  is  named  the  mesentery.  The  muscular  coat  is  composed 
of  plain  muscular  tissue  arranged  in  two  strata,  an  outer  longitu- 
dinal, and  an  inner  transverse  or  circular.  The  submucous  coat  is 
like  that  of  the  stomach;  consisting  of  loose  areolar  tissue,  binding 
together  the  mucous  and  muscular  coats,  and  forming  a  bed  in 
which  the  blood  and  lymphatic  vessels  (which  reach  the  intestine 
in  the  fold  of  the  mesentery)  break  up  into  minute  branches  be- 
fore entering  the  mucous  membrane. 

The  Mucous  Coat  of  the  Small  Intestine.  This  is  pink,  soft  and 
extremely  vascular.  It  does  not  present  temporary  or  effaceable 
folds  like  those  of  the  stomach,  but  is,  throughout  a  great  portion 
of  its  length,  raised  up  into  permanent  transverse  folds  in  the  form 
of  crescentic  ridges,  each  of  which  runs  transversely  for  a  greater 
or  less  way  round  the  tube  (Fig.  130).  These  folds  are  the  valvulce 
conniventes.  They  are  first  found  about  two  inches  from  the 
pylorus,  and  are  most  thickly  set  and  largest  in  the  upper  half  of 
the  jejunum,  in  the  lower  half  of  which  they  become  gradually 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES      453 

less  conspicuous;  and  they  finally  disappear  altogether  about  the 
middle  of  the  ileum.  The  folds  serve  greatly  to  increase  the  sur- 
face of  the  mucous  membrane  both  for  absorption  and  secretion, 
and  they  also  delay  the  food  somewhat  in  its  passage,  since  it  must 
collect  in  the  hollows  between  them,  and  so  be  longer  exposed  to 
the  action  of  the  digestive  liquids.  Examined  closely  with  the  eye 
or,  better,  with  aid  of  a  lens,  the  mucous  membrane  of  the  small 
intestine  is  seen  to  be  not  smooth  but  shaggy,  being  covered  every- 
where (both  over  the  valvulae  conniventes  and  between  them) 
with  closely  packed  minute  processes,  standing  up  somewhat  like 
the  "pile"  on  velvet,  and  known  as  the  villi.  Each  villus  is  from 
0.5  to  0.7  millimeter  (^  to  ^  inch)  in  length;  some  are  conical 


FIG.  130. — A  portion  of  the  small  intestine  opened  to  show  the  valvulce  conniventes. 

and  rounded,  but  the  majority  are  compressed  at  the  base  in  one 
diameter  (Fig.  131).  In  structure  a  villus  is  somewhat  complex. 
Covering  it  is  a  single  layer  of  columnar  epithelial  cells,  the  ex- 
posed ends  of  the  majority  having  a  peculiar  bright  striated  border 
and  being  probably  of  great  importance  in  absorption.  Mixed 
with  these  cells  are  others  in  which  most  of  the  cell  has  become  filled 
with  a  clear  mass  which  does  not  stain  readily  with  reagents;  the 
deep  narrow  end  of  the  cell  stains  easily  and  contains  the  nucleus. 
From  time  to  time  the  clear  substance  (mucigen)  is  converted  into 
mucus  and  discharged  into  the  intestine,  leaving  behind  only  the 
nucleus  and  the  protoplasm  around  it.  These  reconstruct  the  cell 
and  form  more  mucigen.  These  mucus-forming  cells  are  named 
goblet-cells,  from  their  shape*.  Beneath  the  epithelium  the  villus 
may  be  regarded  as  made  up  of  a  framework  of  connective  tissue, 
supporting  the  more  essential  constituents.  Near  the  surface  is 
an  incomplete  layer  of  plain  muscular  tissue,  continuous  below 
with  a  muscular  stratum  forming  the  deepest  layer  of  the  mucous 
membrane  and  named  the  muscularis  mucosce.  In  the  center  is  an 


454 


THE  HUMAN  BODY 


off-shoot  of  the  lymphatic  system;  sometimes  in  the  form  of  a 
single  vessel  with  a  closed  dilated  end,  and  sometimes  as  a  net- 
work formed  by  two  main  vessels  with  cross-branches.  During 
digestion  these  lymphatics  are  filled  with  a  milky-white  liquid  ab- 
sorbed from  the  intestines,  and  they  are  accordingly  called  the 
lacteals.  They  communicate  with  larger  branches  in  the  sub- 
mucous  coat,  which  end  in  trunks  that  pass  out  through  the  mes- 
entery to  join  the  main  lymphatic  system.  Finally,  in  each  villus, 


FIG.  131. — Villi  of  the  small  intestine;  magnified  about  80  diameters.  In  the 
right-hand  figure  the  lacteals,  a,  &,  c,  are  filled  with  white  injection;  d,  blood- 
vessels. In  the  left-hand  figure  the  lacteals  alone  are  represented,  filled  with  a 
dark  injection.  The  epithelium  covering  the  villi,  and  their  muscular  fibers,  are 
omitted. 

outside  the  lacteals  and  beneath  the  muscular  layer  of  the  villus, 
is  a  close  network  of  blood-vessels. 

Opening  on  the  surface  of  the  small  intestine,  between  the  bases 
of  the  villi,  are  small  glands,  the  crypts  of  Lieberkuhn.  Each  is  a 
simple  unbranched  tube  lined  by  a  layer  of  columnar  cells  some  of 
which  have  a  striated  free  border,  though  less  marked  than  that 
on  the  corresponding  cells  of  the  villi,  and  others  are  goblet-cells. 
The  crypts  of  Lieberkuhn  are  closely  packed,  side  by  side,  like  the 
glands  of  the  stomach.  In  the  duodenum  are  found  other  minute 
glands,  the  glands  of  Brunner.  They  lie  in  the  submucous  coat 
and  send  their  ducts  through  the  mucous  membrane  to  open  on 
its  inner  side. 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES      455 

The  Large  Intestine  (Fig.  136),  forming  the  final  portion  of  the 
alimentary  canal,  is  about  1.5  meters  (5  feet)  long,  and  varies  in 
diameter  from  about  6  to  4  centimeters  (2J^  to  1^  inches).  Anato- 
mists describe  it  as  consisting  of  the  ccecum  with  the  vermiform 
appendix,  the  colon,  and  the  rectum.  The  small  intestine  does  not 
open  into  the  commencement  of  the  large  but  into  its  side,  some 
distance  from  its  closed  upper  end,  and  the  caecum,  CC,  is  that 
part  of  the  large  intestine  which  extends  beyond  the  communica- 
tion. From  it  projects  the  vermiform  appendix,  a  narrow  tube  not 
thicker  than  a  lead  pencil,  and  about  10  centimeters  (4  inches) 
long.  The  colon  commences  on  the  right  side  of  the  abdominal 
cavity  where  the  small  intestine  communicates  with  the  large, 
runs  up  for  some  way  on  that  side  (ascending  colon,  AC),  then 
crosses  the  middle  line  (transverse  colon,  TC)  below  the  stomach, 
and  turns  down  (descending  colon,  DC)  on  the  left  side  and  there 
makes  an  S-shaped  bend  known  as  the  sigmoid  flexure,  SF;  from 
this  the  rectum,  R,  the  terminal  straight  portion  of  the  intestine, 
proceeds  to  the  anal  opening,  by  which  the  alimentary  canal  com- 
municates with  the  exterior.  In  structure  the  large  intestine 
presents  the  same  coats  as  the  small.  The  external  stratum  of  the 
muscular  coat  is  not,  however,  developed  uniformly  around  it, 
except  on  the  rectum,  but  occurs  in  three  bands  separated  by  in- 
tervals in  which  it  is  wanting.  These  bands  being  shorter  than 
the  rest  of  the  tube  cause  it  to  be  puckered,  or  saccullated,  between 
them.  The  mucous  coat  possesses  no  villi  or  valvulae  conniventes, 
but  is  usually  thrown  into  effaceable  folds,  like  those  of  the  stomach 
but  smaller.  It  contains  numerous  closely  set  glands  much  like 
the  crypts  of  Lieberkiihn  of  the  small  intestine. 

The  Ileocolic  Valve.  Where  the  small  intestine  joins  the  large 
there  is  a  valve,  formed  by  two  flaps  of  the  mucous  membrane 
sloping  down  into  the  colon,  and  so  disposed  as  to  allow  matters 
to  pass  readily  from  the  ileum  into  the  large  intestine  but  not  the 
other  way. 

The  Nerves  of  the  Intestines.  The  intestines,  like  the  stomach, 
have  the  double  autonomic  innervation;  the  paths  of  approach 
are  in  general  the  same  as  for  the  stomach,  by  way  of  the  vagus 
for  the  cranial  autonomies,  and  the  splanchnics  for  the  thoracico- 
lumbar.  Both  these  sets  of  nerves  ramify  in  the  solar  plexus; 
from  here  nerve  strands  pass  to  the  intestine,  as  well  as  to  the 


456 


THE  HUMAN  BODY 


stomach,  along  the  mesentery.  This  innervation  extends  through- 
out the  small  intestine  and  the  ascending  and  transverse  colons. 
The  descending  colon  has  its  thoracico-lumbar  innervation  by  way 
of  the  hypogastric  nerve  which  extends  to  the  hypogastric  plexus 
in  the  lower  portion  of  the  abdominal  cavity  and  ramifies  thence 
over  the  descending  colon  and  rectum.  The  opposing  autonomic 
innervation  for  this  region  is  derived  from  the  sacral  part  of  the 
system.  The  nervus  erigens  is  the  path  from  the  sacral  part  of  the 


FIG.  132. — The  under  surface  of  the  liver,  d,  right,  and  s,  left  lobe;  Vh,  hepatic 
vein;  Vp,  portal  vein;  Vc,  vena  cava  inferior;  Dch,  common  bile-duct;  DC,  cystic 
duct;  Dh,  hepatic  duct;  Vf,  gall-bladder. 

spinal  cord  to  the  hypogastric  plexus.  Thence  the  distribution 
is  the  same  as  for  the  thoracico-lumber  autonomies. 

The  intestines  are  provided,  in  addition,  with  an  intrinsic  inner- 
vation consisting  of  two  nervous  networks  or  plexuses  lying,  one 
between  the  mucosa  and  the  muscular  coat,  the  plexus  of  Meissner, 
and  the  other  between  the  circular  and  longitudinal  muscle  layers, 
the  plexus  of  Auerbach. 

The  Liver.  Besides  the  secretions  formed  by  the  glands  em- 
bedded in  its  walls,  the  small  intestine  receives  those  of  two  large 
glands,  the  liver  and  the  pancreas,  which  lie  in  the  abdominal 
cavity.  The  ducts  of  both  open  by  a  common  aperture  into  the 
duodenum  about  10  centimeters  (4  inches)  from  the  pylorus. 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES      457 

The  liver  is  the  largest  gland  in  the  Body,  weighing  from  1,400  to 
1,700  grams  (50  to  64  ounces).  It  is  situated  in  the  upper  part  of 
the  abdominal  cavity  (k,  le',  Fig.  1),  rather  more  on  the  right  than 
on  the  left  side  and  immediately  below  the  diaphragm,  into  the 
concavity  of  which  its  upper  surface  fits,  and  reaches  across  the 
middle  line  above  the  pyloric  end  of  the  stomach.  It  is  of  dark 
reddish-brown  color,  and  of  a  soft  friable  texture.  A  deep  fissure 
incompletely  divides  the  organ  into  right  and  left  lobes,  of  which 


FIG.  133. — A  lobule  of  the  liver  (pig),  magnified,  showing  the  hepatic  cells 
radiately  arranged  around  the  central  intralobular  vein,  and  the  connective  tissue 
surrounding  the  lobule.  (Scymonowicz.) 

the  right  is  much  the  larger;  on  its  under  surface  (Fig.  132)  shal- 
lower grooves  mark  off  several  minor  lobes.  Its  upper  surface  is 
smooth  and  convex.  The  vessels  carrying  blood  to  the  liver  are 
the  portal  vein,  Vp,  and  the  hepatic  artery;  both  enter  it  at  a  fissure 
(the  portal  fissure)  on  its  under  side,  and  there  also  a  duct  passes 
out  from  each  half  of  the  organ.  The  ducts  unite  to  form  the 
hepatic  duct,  Dh,  which  meets  at  an  acute  angle,  the  cystic  duct,  DC, 
proceeding  from  the  gall-bladder,  Vf,  a  pear-shaped  sac  in  which 
the  bile,  or  gall,  formed  by  the  liver,  accumulates  when  food  is  not 


458  THE  HUMAN  BODY 

being  digested  in  the  intestine.  The  common  bile-duct,  Dch,  formed 
by  the  union  of  the  hepatic  and  cystic  ducts,  opens  into  the  duode- 
num. The  blood  which  enters  the  liver  by  the  portal  vein  and 
hepatic  artery  passes  out  by  the  hepatic  veins,  Vh,  which  leave  the 
posterior  border  of  the  organ  close  to  the  vertebral  column,  and 
there  open  into  the  inferior  vena  cava  just  before  it  passes  up 
through  the  diaphragm. 

The  Structure  of  the  Liver.  On  closely  examining  the  surface 
of  the  liver,  it  will  be  seen  to  be  marked  out  into  small  angular 
areas  from  one  to  two  millimeters  (-^  to  ^  inch)  in  diameter. 

; ^  These  are  the  outer  sides  of  the  superficial 

layer  of  a  vast  number  of  minute  polygonal 
masses,  or  lobules,  of  which  the  liver  is 
km^  UP5  similar  areas  are  seen  on  the 
surface  of  any  section  made  through  the 
organ.  Each  lobule  (Fig.  133)  consists  of 
a  number  of  hepatic  cells  supported  by  a 
close  network  of  capillaries;  and  is  sepa- 
•nFl4G\13fu~Diia^amu*°  rated  from  neighboring  lobules  by  con- 

illustrate  the  relationship 

of  blood-capillaries,  bile-    nective   tissue,   larger    blood-vessels,    and 
branches  of  the  hepatic  duct.    The  hepatic 

CelU  are  the  Pr°P6r  tisSU6  elementS  °f  the 

which  a  blood   capillary    liver,  all  the  rest  being  subsidiary  arrange- 

extends  to  L;  D,  a  minute  „          ,     .  .'•,•  ±* 

bile-duct  with  which  a  bile-  ments  for  their  nutrition  and  protection, 
capillary  communicates.  Each  ig  poiygo^  nucleated  and  very  gran- 
ular, and  has  a  diameter  of  about  0.025  millimeter  (T^TT  °f  an 
inch).  In  each  lobule  they  are  arranged  in  rows  or  strings,  which 
form  a  network,  in  the  meshes  of  which  the  blood-capillaries  and 
bile-capillaries  run.  The  blood  carried  in  by  the  portal  vein 
(which  has  already  circulated  through  the  capillaries  of  the 
stomach,  spleen,  intestines  and  pancreas)  is  conveyed  to  a 
fine  vascular  interlobular  plexus  around  the  liver-lobules,  from 
which  it  flows  on  through  the  capillaries  of  the  lobules  them- 
selves. These  (Fig.  134)  unite  in  the  center  of  the  lobule  to  form 
a  small  intralobular  vein,  which  carries  the  blood  out  and  pours 
it  into  one  of  the  branches  of  origin  of  the  hepatic  vein,  called 
the  sublobular  vein.  Each  of  the  latter  has  many  lobules  emptying 
blood  into  it,  and  if  dissected  out  with  them  would  look  something 
like  a  branch  of  a  tree  with  apples  attached  to  it  by  short  stalks, 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES      459 

represented  by  the  intralobular  veins.  The  blood  is  finally  carried, 
as  already  pointed  out,  by  the  hepatic  veins  into  the  inferior  vena 
cava.  The  hepatic  artery,  a  direct  off-shoot  of  the  celiac  axis 


FIG.  135. — The  stomach,  pancreas,  liver,  and  duodenum,  with  part  of  the  rest 
of  the  small  intestine  and  the  mesentery;  the  stomach  and  liver  have  been  turned 
up  so  as  to  expose  the  pancreas.  V,  stomach;  D,  D',  D",  duodenum;  L,  spleen; 
P,  pancreas;  R,  right  kidney;  T,  jejunum;  Vf,  gall-bladder;  h,  hepatic  duct;  c, 
cystic  duct;  ch,  common  bile-duct;  1,  aorta,  2,  an  artery  (left  coronary)  of  the 
stomach;  3,  hepatic  artery;  4,  splenic  artery;  5,  superior  mesenteric  artery;  6,  su- 
perior mesenteric  vein;  7,  splenic  vein;  Vp,  portal  vein. 

(p.  331)  supplies  some  blood  to  the  lobular  plexuses,  but  by  no 
means  so  much  as  the  portal  vein;  it  all  finally  leaves  the  liver  by 
the  hepatic  veins. 

The  bile-ducts  can  be  readily  traced  to  the  periphery  of  the 


460 


THE  HUMAN  BODY 


lobules,  and  there  communicate  with  a  network  of  extremely 
minute  commencing  bile-capillaries,  ramifying  in  the  lobule  be- 
tween the  hepatic  cells  composing  it.  The  relation  of  the  bile- 
capillaries  to  the  blood-capillaries  within  the  lobule  is  such  that 
there  is  always  a  liver-cell  interposed  between  them. 
This  arrangement  is  illustrated  diagrammatically  in  Fig.  134. 

From  the  arrangement  of 
blood-capillaries  and  bile- 
capillaries  with  their  con- 
nections we  can  picture  the 
movement  of  blood  and  bile 
through  the  lobules;  the 
blood,  both  from  the  portal 
vein  and  the  hepatic  artery, 
is  delivered  to  the  lobule 
at  its  periphery  and  flows 
thence  from  all  sides  toward 
the  center,  where  it  enters 
the  interlobular  vein  and  is 
conveyed  away.  The  bile, 
on  the  other  hand,  is  se- 
creted by  the  liver-cells 
and  from  them  passed  into 
the  bile-capillaries;  it  flows 
along  these  toward  the  per- 
iphery where  it  enters  small 
bile-ducts,  and  so  is  carried 
toward  the  great  outlet  of 

FIG.  136. — Diagram  of  abdominal  part  of  al-  +u     o-lanrl    +hp  hpnafip  Hunt 
imentary  canal.     C,  the  cardiac,  and  P,  the  ttie  Slana>  tJ 
pyloric  end  of  the  stomach,  A;  D,  the  duode-        The  Pancreas  Or  Sweet- 
num;  J,  I,  the  convolutions  of  the  small  intes-  .  . 

tine;  CC,  the  caecum  with  the  vermiform  ap-  bread.  This  IS  an  elon- 
pendix;  AC,  ascending,  TC,  transverse,  and  ,  i  «^4?4.  „  ^e  « 

DC,  descending   colon;   8F,   sigmoid    flexure;  gated  soft  Organ  of  a  pmk- 

R,  the  rectum.  ish-yellow  color,  lying  along 

the  great  curvature  of  the  stomach.  Its  right  end  is  the  larger, 
and  is  embraced  by  the  duodenum  (Fig.  135),  which  there  makes 
a  curve  to  the  left.  A  duct  traverses  the  gland  and  joins  the  com- 
mon bile-duct  close  to  its  intestinal  opening.  The  pancreas  pro- 
duces a  watery-looking  secretion  which  is  of  great  importance 
in  digestion;  the  gland  also  secretes  a  hormone  which  exerts  an 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES      461 

important  influence  on  the  general  nutritional  processes  of  the 
Body  (Chap.  XXX). 

The  Blood-Vessels  of  Alimentary  Canal,  Liver,  Spleen,  and 
Pancreas.  The  portal  vein  (Vp,  Fig.  135)  has  already  been  referred 
to  as  differing  from  all  other  veins  in  that  it  not  only  receives  blood 
from  a  system  of  capillaries  but  ends  in  a  second  set  of  capillaries, 
which  lie  in  the  liver.  The  quantity  of  blood  brought  to  supply 
the  hepatic  capillaries  by  the  hepatic  artery  is  in  fact  much  less  than 
that  brought  by  the  portal  vein.  The  stomach,  the  intestines,  the 
pancreas,  and  the  spleen  are  supplied  with  arterial  blood  from 
three  great  branches  of  the  aorta.  The  most  anterior  of  these,  the 
celiac  axis,  springs  from  the  aorta  close  beneath  the  diaphragm  and 
divides  into  the  hepatic  artery,  splenic  artery,  and  arteries  for  the 
stomach;  some  of  these  divisions  may  be  seen  in  Fig.  135.  The 
pancreas  is  supplied  partly  from  the  hepatic,  partly  from  the 
splenic  artery.  The  two  other  branches  (superior  and  inferior 
mesenteric  artery)  are  given  off  from  the  aorta  lower  down  in  the 
abdominal  cavity;  the  former  (5,  Fig.  135)  supplies  the  small  in- 
testine and  half  of  the  large,  the  latter  the  remainder  of  the  large. 
The  blood  passing  through  all  these  arteries  becomes  venous  in  the 
capillaries  of  the  organs  they  supply,  and  is  gathered  into  corre- 
sponding veins  (Fig.  135)  which  unite  near  the  liver  to  form  the 
portal  vein.  The  further  course  of  the  blood  carried  to  the  liver 
(partly  arterial  from  the  hepatic  artery,  partly  venous  from  the 
portal  system)  has  been  described  already  (p.  335). 


CHAPTER  XXVII 
THE  CHEMISTRY  OF  DIGESTION 

The  Object  of  Digestion  is  twofold ;  to  prepare  the  various  foods 
for  absorption  by  the  lining  of  the  digestive  tract,  which  means 
that  they  must  be  made  soluble  if  not  already  so ;  and  to  convert 
them  into  forms  in  which  the  Body  can  make  use  of  them  after 
they  have  been  absorbed.  Digestion  is  confined  to  the  nutrients; 
the  inorganic  salts  of  the  food  are  soluble,  and  are  used  by  the 
Body  in  essentially  the  same  form  as  eaten,  they  therefore  need  no 
digestion.  The  accessories  either  perform  their  function  in  con- 
nection with  the  process  of  digestion  itself,  or  are  absorbed  and 
used  by  the  Body  in  the  form  in  which  they  are  taken. 

Nature  of  the  Digestive  Process.  Although  the  foods  requiring 
digestion  are  of  very  different  sorts  chemically,  the  method  of 
digestion  is  at  bottom  the  same  for  all  of  them.  It  consists  of  the 
process  known  in  chemistry  as  hydrolysis.  Hydrolysis  is  a  chemi- 
cal reaction  in  which  one  molecule  of  the  substance  involved  com- 
bines with  one  molecule  of  water  and  the  resulting  compound  splits 
into  two  or  more  simpler  molecules.  By  repeated  hydrolyses  very 
complex  substances  may  be  split  into  comparatively  simple 
ones. 

Hydrolysis  is  a  common  reaction  of  organic  chemistry.  It  is 
probable  that  it  is  the  most  frequently  occurring  reaction  of  the 
living  Body.  Not  only  the  digestive  processes,  but  many  of  the  ac- 
tivities of  living  cells  are  of  this  nature.  The  digestive  hydrolyses 
are  all  carried  on  through  the  agency  of  enzyms.  There  is  a  special 
and  specific  enzym  for  each  particular  reaction;  the  enzym  that 
splits  starch  is  without  effect  on  protein  or  fat.  These  enzym 
reactions  are  all  simple  chemical  reactions;  they  are  carried  on  in 
the  alimentary  tract  as  in  a  chemical  laboratory,  and  will  go  on 
just  as  well  in  test-tubes  kept  at  Body  temperature  as  in  the  Body 
itself.  They  are  not  therefore  "  vital "  processes  in  the  sense  that 
they  cannot  occur  except  in  the  presence  of  living  cells. 

Digestion  Products.  Before  beginning  a  detailed  description 

462 


THE  CHEMISTRY  OF  DIGESTION  463 

of  the  digestive  process  as  it  affects  the  different  food-stuffs  It 
will  perhaps  be  helpful  to  call  attention  to  the  comparatively  few 
and  simple  substances  which  are  finally  produced  as  the  result  of 
the  numerous  reactions  that  go  on  in  the  alimentary  tract.  All 
carbohydrates  (except  the  single  sugars),  the  starches,  gums,  and 
'double  sugars,  are  hydrolyzed  into  single  sugars  during  their  di- 
gestion, so  that  absorption  of  carbohydrates  is  altogether  in  the 
form  of  single  sugars.  All  fats  are  split  into  fatty  acid  and  glycerin, 
in  which  state  they  are  ready  to  be  taken  up  by  the  intestinal 
walls.  The  proteins,  as  we  saw  in  Chap.  I,  are  complexes  built  up 
of  a  large  number  of  amino  acids.  The  digestive  process  splits 
them  into  simpler  molecules  each  of  which  is  composed  either  of  a 
single  amino  acid,  or  of  two  or  three  of  them  together.  We  may 
say,  in  general,  that  proteins  are  split  into  their  constituent 
amino  acids. 

Tabulating  the  digestion  products  we  have : 
from  carbohydrates,  single  sugars; 
from  fats,  fatty  acid  and  glycerin; 
from  proteins,  amino  acids. 

The  Saliva.  The  first  digestive  fluid  that  the  food  meets  with 
is  the  saliva,  which,  as  found  in  the  mouth,  is  a  mixture  of  pure 
saliva,  formed  in  parotid,  submaxillary,  and  sublingual  glands, 
with  the  mucus  secreted  by  small  glands  of  the  buccal  mucous 
membrane.  This  mixed  saliva  is  a  colorless,  cloudy,  feebly  alkaline 
liquid,  "  ropy  "  from  the  mucin  present  in  it,  and  usually  contain- 
ing air-bubbles.  Pure  saliva,  as  obtained  by  putting  a  fine  tube  in 
the  duct  of  one  of  the  salivary  glands,  is  more  fluid  and  contains 
no  imprisoned  air. 

The  uses  of  the  saliva  are  in  part  physical  and  mechanical.  It 
keeps  the  mouth  moist  and  allows  us  to  speak  with  comfort;  it 
also  dissolves  such  bodies  as  salt,  and  sugar,  when  they  are  taken 
into  the  mouth  in  solid  form,  and  enables  us  to  taste  them ;  undis- 
solved  substances  are  not  tasted,  a  fact  which  any  one  can  verify 
for  himself  by  wiping  his  tongue  dry  and  placing  a  fragment  of 
sugar  upon  it.  No  sweetness  will  be  felt  until  a  little  moisture  has 
exuded  and  dissolved  part  of  the  sugar. 

In  addition  to  such  actions  the  saliva  exerts  a  chemical  one  on 
an  important  food-stuff.  It  contains  an  enzym,  ptyalin,  which 
has  the  power  of  turning  starch  into  a  double  sugar,  maltose.  This 


464  THE  HUMAN  BODY 

change,  like  all  digestive  reactions,  is  a  hydrolysis.  It  does  not 
occur  in  a  single  stage;  that  is,  the  starch  molecule  is  not  split 
directly  into  maltose,  but  first  into  a  dextrin  which  is  hydrolyzed 
into  a  simpler  dextrin,  and  this  in  turn  into  maltose.  In  effecting 
the  change  the  ptyalin  is  not  altered ;  a  very  small  amount  of  it  can 
convert  a  vast  amount  of  starch,  and  does  not  seem  to  have  its 
activity  impaired  in  the  process. 

In  order  that  the  ptyalin  may  act  upon  starch  certain  conditions 
are  essential.  Water  must  be  present,  and  the  liquid  must  be 
neutral  or  feebly  alkaline;  acids  retard,  or  if  stronger,  entirely 
stop  the  process.  The  change  takes  place  most  quickly  at  about 
the  temperature  of  the  Human  Body,  and  is  greatly  checked  by 
cold.  Boiling  the  saliva  destroys  its  ptyalin  and  renders  it  quite 
incapable  of  converting  starch.  Cooked  starch  is  changed  more 
rapidly  and  completely  than  raw. 

It  will  be  noted  that  salivary  digestion  is  only  a  stage  in  the 
preparation  of  starch  for  the  use  of  the  Body,  since  starch,  in 
common  with  the  other  carbohydrates  taken  as  food,  is  finally 
converted  to  single  sugar  before  it  is  absorbed. 

The  Gastric  Juice.  The  food  having  entered  the  stomach  is 
subjected  to  the  action  of  the  gastric  juice,  which  is  a  thin,  color- 
less or  pale  yellow  liquid,  of  a  strongly  acid  reaction.  It  contains 
as  specific  elements  free  hydrochloric  acid  (about  0.2  per  cent),  and 
an  enzym  called  pepsin  which,  in  acid  liquids,  has  the  power  of 
converting  the  ordinary  proteins  which  we  eat,  by  hydrolysis,  into 
closely  allied  bodies,  proteoses  and  peptones. 

In  neutral  or  alkaline  media  the  pepsin  is  inactive;  and  cold 
checks  its  activity.  Boiling  destroys  it.  In  addition  to  pepsin, 
gastric  juice  contains  another  enzym  (rennin)  which  coagulates 
the  casein  of  milk,  as  illustrated  by  the  use  of  "rennet,"  prepared 
from  the  mucous  membrane  of  the  calf's  digestive  stomach,  in 
cheese-making.  The  acid  of  the  natural  gastric  juice  would,  it  is 
true,  precipitate  the  casein,  but  such  precipitate  is  quite  different 
from  the  true  tyrein,  and  neutralized  gastric  juice  still  possesses 
this  power;  moreover,  boiled  gastric  juice  loses  the  milk-clotting 
property,  and  a  very  little  normal  juice  can  coagulate  a  great 
quantity  of  milk.  The  curdled  condition  of  the  milk  regurgitated 
by  infants  is,  therefore,  not  any  sign  of  a  disordered  state  of  the 
stomach,  as  nurses  commonly  suppose.  It  is  proper  for  milk  to 


THE  CHEMISTRY  OF  DIGESTION  465 

undergo  this  change,  before  the  pepsin  and  acid  of  the  gastric 
juice  digest  it. 

Since  muscle-fibers  are  enclosed  within  connective  tissue  (al- 
buminoid) envelopes,  it  is  necessary  that  the  albuminoid  cover- 
ings be  digested  off  before  the  protein  contents  are  exposed  to  the 
action  of  the  digestive  enzyms.  There  is  reason  to  think  that 
pepsin,  which  converts  proteins,  including  albuminoids,  into  pro- 
teoses  and  peptones,  soluble  substances,  but  does  not  carry  the 
digestion  to  completion,  has  as  an  important  part  of  its  function 
this  removal  from  animal  proteins  of  their  albuminoid  coverings. 

The  Pancreatic  Juice.  In  the  intestine  the  food  is  subjected 
to  the  action  of  the  pancreatic  juice.  This  is  clear,  watery,  alka- 
line, and  much  like  saliva  in  appearance.  The  Germans  call  the 
pancreas  the  "abdominal  salivary  gland."  In  digestive  prop- 
erties, however,  the  pancreatic  secretion  is  far  more  important 
than  the  saliva,  or  even  the  gastric  juice.  It  contains  three  di- 
gestive enzyms;  amylopsin,  a  starch-splitting  enzym  whose  action 
is  identical  with  that  of  salivary  ptyalin,  and  which  is  thought  to 
be,  perhaps,  itself  identical  with  ptyalin;  lipase,  a  fat-splitting 
enzym,  converting  fats  to  fatty  acid  and  glycerin;  trypsin,  a 
protein-splitting  (proteolytic)  enzym  whose  action  is  much  more 
powerful  than  that  of  pepsin,  as  it  is  able  to  carry  the  process  of 
protein  hydrolysis  clear  to  the  amino  acid  stage.  It  acts  upon 
such  proteins  as  escape  the  influence  of  pepsin  in  the  stomach. 

The  Bile.  This  fluid,  which  is  poured  into  the  intestine  from 
the  liver  does  not  contain  any  digestive  enzym,  but  it  does  have 
an  important  role  in  connection  with  fat  digestion;  it  has  been 
shown  that  pancreatic  lipase  splits  fats  several  times  as  rapidly 
when  bile  is  present  as  when  it  is  absent. 

The  Succus  Entericus  (Intestinal  juice).  This  fluid,  which  is 
secreted  by  the  minute  glands  of  the  intestinal  wall,  is  the  last  of 
the  digestive  fluids  to  come  in  contact  with  the  food,  and  by  its 
enzyms  whatever  foods  are  not  completely  digested  must  be 
finally  prepared  for  absorption.  By  the  enzyms  thus  far  described 
none  of  the  carbohydrate  digestion  is  carried  to  completion,  and 
only  part  of  the  proteins  are  made  ready  for  use,  for  proteose 
and  peptone  are  not  end  products,  but  only  intermediate  products 
of  digestion.  Fats  are  the  only  foods  which  do  not  require  the 
aid  of  the  succus  entericus  for  their  complete  digestion. 


466 


THE  HUMAN  BODY 


The  digestive  enzyms  of  the  succus  entericus  are  four;  one  pro- 
teolytic,  erepsin,  which  acts  particularly  on  proteoses  and  pep- 
tones, thus  completing  the  work  of  the  gastric  pepsin;  and  three 
so-called  inverting  enzyms,  which  change  double  sugars  to  single 
sugars.  These  enzyms  are  specific  in  their  action,  each  affecting 
only  its  particular  sugar.  Maltose  inverts  maltose,  thus  com- 
pleting the  starch  digestion  begun  by  ptyalin  and  amylopsin; 
sucrase  splits  cane-sugar  or  sucrose,  and  lactose  converts  milk-sugar, 
lactose,  to  single  sugar.  The  result  of  the  action  of  these  three  en- 
zyms is  to  bring  all  the  carbohydrates  of  the  food,  except  cellu- 
lose, into  the  condition  of  single  sugars,  in  which  form  they  are 
ready  for  the  use  of  the  Body. 

Summary  of  the  Digestive  Process.  The  chemical  reactions  by 
which  the  various  food  stuffs  are  made  ready  for  absorption  and 
use  by  the  Body  can  be  conveniently  summarized  in  tabular  form : 


Region 

Secretion 

Enzyms 

Substances 
Affected 

Products 
Formed 

Mouth 
Stomach 

Small 
Intestine 

Saliva 
Gastric  Juice 

Pancreatic 
Juice 

Ptyalin 
Pepsin 

Amylopsin 

Starch 
Albuminoid 
Protein 
Starch 

Maltose  l 
Proteoses  l 
Peptones  l 
Maltose  l 

Lipase 
Trypsin 

Fats 
Proteins 

Fatty  acid  2 
Glycerin  2 
Amino  Acids  2 

Succus 
Entericus 

Erepsin 

Proteoses 
Peptones 

«         « 

Maltase 
Sucrase 
Lactase 

Maltose 
Cane-Sugar 
Milk-Sugar 

Single  Sugar  2 
«          « 

U                     It 

1  Intermediate  products. 

2  Final  products. 

Bacterial  Digestion.  The  human  intestines  normally  contain 
enormous  numbers  of  bacteria.  In  the  small  intestine  the  action 
of  these  is  for  the  most  part  fermentation  of  carbohydrates  with 
the  production  of  carbon  dioxid,  alcohol,  and  acetic  and  lactic 
acids.  There  is  no  doubt  that  even  in  perfect  health  a  considerable 
fermentation  goes  on  in  the  intestine.  So  far  as  appears  it  is 


THE  CHEMISTRY  OF  DIGESTION  467 

neither  particularly  harmful  nor  beneficial.  The  fermentation 
products  are  probably  absorbed  and  used  by  the  Body,  but  they 
would  be  used  equally  well  if  absorbed  as  sugar  without  fermenta- 
tion. In  the  case  of  one  particular  carbohydrate,  however,  cellu- 
lose, bacterial  fermentation  affords  the  only  means  by  which  it 
can  be  made  available  in  man  for  the  use  of  the  Body.  It  seems 
to  be  well  established  that  tender  cellulose,  such  as  is  eaten  in 
lettuce,  for  example,  may  be  digested  by  bacteria  to  a  considerable 
extent;  where  it  is  less  tender,  as  in  most  fruits  and  vegetables,  it 
remains,  as  stated  earlier,  practically  undigested. 

Intestinal  fermentation  is  not  essential  to  health  as  is  shown 
by  the  possibility  of  living  normally  in  arctic  regions,  where,  it  is 
said,  intestinal  bacteria  are  sometimes  wholly  wanting.  When 
the  fermentation  becomes  excessive  intestinal  disturbances  may 
readily  result.  The  production  of  fermentation  acids  in  too  great 
concentration  leads  to  irritation  of  the  intestinal  wall  and  causes 
diarrhea. 

In  the  large  intestine  the  bacterial  action  is  chiefly  putrefaction 
of  proteins,  rather  than  fermentation  of  carbohydrates.  The 
difference  is  not  due  to  the  presence  of  different  species  of  bac- 
teria, but  to  the  different  nature  of  the  available  food.  Where 
carbohydrate  is  present  in  excess,  as  in  the  small  intestine,  fer- 
mentation is  the  normal  action.  By  the  time  the  intestinal  con- 
tents reach  the  large  intestines  the  digestible  carbohydrates  are, 
as  we  shall  learn  (p.  500),  all  absorbed  out  into  the  blood.  There 
remains,  however,  a  portion  of  the  protein,  including  all  indigestible 
meat  fragments.  In  this  environment,  largely  protein,  the  normal 
bacterial  action  is  of  the  nature  of  putrefaction.  The  character- 
istic features  of  the  contents  of  the  large  intestine  are  the  results 
of  this  putrefaction.  In  connection  with  it  various  toxic  substances 
are  formed  which  may  be^-absorbed  from  the  intestine  into  the 
blood.  The  symptoms  of  heaviness  and  general  ill-feeling  that 
frequently  accompany  sluggishness  of  the  large  intestine  are  to 
be  referred  to  the  presence  of  these  toxins  in  the  blood.  The  con- 
dition is  known  as  autointoxication.  The  obvious  method  of 
avoiding  this  condition  is  by  using  care  that  material  shall  not 
stagnate  in  the  colon. 

The  Prevention  of  Self -Digestion.  A  question  of  much  in- 
terest to  physiologists  has  been  why  the  stomach  and  intestinal 


468  THE  HUMAN  BODY 

walls  and  the  gastric  and  pancreatic  glands  are  not  themselves 
digested  by  the  powerful  proteolytic  enzyms  which  they  produce, 
in  the  case  of  the  glands,  or  which  are  poured  out  unto  them,  in  the 
case  of  the  walls  of  the  digestive  organs.  It  has  been  shown  that 
the  prevention  of  self-digestion  of  stomach  and  intestine  depends 
upon  the  continuance  of  life,  for  animals  killed  in  the  midst  of  di- 
gesting a  meal  often  do  digest  great  parts  of  their  stomach  and  in- 
testinal walls.  Just  how  self-digestion  of  these  structures  is 
normally  prevented  is  not  clear,  except  in  so  far  as  the  mechanism 
to  be  described  presently  (Chap.  XXIX),  which  limits  the  out- 
pouring of  the  secretions  to  periods  when  food  is  present,  may 
be  efficacious.  The  self-digestion  of  the  pancreatic  and  gastric 
glands  is,  however,  prevented  by  an  interesting  arrangement 
which  has  been  recently  analyzed.  It  appears  that  neither  pepsin 
nor  trypsin  is  formed  in  the  gland  as  an  active  enzym  but  in  an 
inactive  pro-enzym  or  zymogen  form,  pepsinogen  or  trypsinogen, 
which  becomes  active  only  when  converted  into  pepsin  or  trypsin 
by  some  activating  agent.  It  has  been  shown  that  the  conversion 
of  trypsinogen  to  trypsin  occurs  only  when  the  pancreatic  juice  is 
poured  into  the  small  intestine,  and  that  it  is  brought  about 
through  a  constituent  of  the  succus  entericus,  enter okinase.  This 
substance  is  believed  to  be  an  enzym  having  the  sole  function  of 
activating  trypsinogen  to  trypsin.  The  conversion  of  pepsinogen 
to  pepsin  is  a  similar  activation,  carried  on  by  the  hydrochloric 
acid  of  gastric  juice. 


CHAPTER  XXVIII 
MOVEMENTS  OF  THE  ALIMENTARY  CANAL 

Mastication  serves  to  break  the  food  into  fine  particles  and  by 
mixing  it  intimately  with  saliva  to  reduce  it  to  a  semi-liquid  state. 
It  consists  primarily  of  cutting  and  grinding  the  food  between 
the  upper  and  lower  teeth,  a  process  which  is  performed  by  move- 
ments of  the  lower  jaw.  The  articulation  of  the  lower  jaw  with 
the  skull  and  its  equipment  of  muscles  permit  both  up  and  down 
cutting  movements  and  sidewise  grinding  movements.  The  ac- 
tual chewing  process  involves,  in  addition,  motions  of  the  lips, 
cheeks,  and  tongue  in  holding  the  food  in  position  for  the  teeth 
to  act  upon  it.  The  whole  process  is  carried  on  by  skeletal  muscles 
and  is,  therefore,  under  the  control  of  the  will. 

It  ought  not  to  be  necessary  to  emphasize  the  importance  of 
thorough  mastication  of  the  food.  Salivary  digestion  depends 
wholly,  of  course,  upon  the  bringing  of  saliva  into  contact  with 
the  starch  particles,  and  it  can  easily  be  shown  experimentally 
that  gastric  digestion  is  'several  times  more  rapid  when  the  ma- 
terial exposed  to  the  action  of  gastric  juice  is  finely  divided  than 
when  it  is  in  large  masses. 

The  interesting  fact  has  recently  been  brought  out  that  the 
more  the  process  of  masticating  each  mouthful  is  prolonged  the 
less  food  is  required  to  satisfy  the  appetite.  Since  many  people 
doubtless  eat  too  much  there  is  here  a  suggestion  as  to  a  way  of 
reducing  the  amount  taken  without  serious  sacrifice  of  appetite. 

Hygiene  of  the  Mouth.  The  mouth  cavity  is  almost  never  free 
from  micro-organisms.  The  alkaline  reaction  of  saliva  is  favor- 
able to  their  growth,  and  they  scarcely  ever  lack  for  food.  The 
irregularly  shaped  teeth,  packed  closely  along  the  jaw,  have  be- 
tween them  spaces  where  material  that  is  being  chewed  readily 
lodges,  and  where  it  stays  unless  special  care  is  taken  to  remove 
it.  Such  lodged  food-masses  shortly  harbor  flourishing  colonies 
of  bacteria.  These  in  connection  with  their  growth  and  multi- 
plication produce  substances  which  attack  the  protective  enamel 

469 


470  THE  HUMAN  BODY 

of  the  teeth  and  so  gain  foothold  within  the  tooth  substance  itself, 
and  we  have  under  way  the  too-familiar  process  of  tooth  decay. 
Good  teeth  are  so  important  for  efficient  mastication,  as  well  as 
for  the  appearance  of  the  face,  that  no  pains  should  be  spared  to 
preserve  them.  Evidently  the  way  to  do  this  is  to  prevent  the 
accumulation  of  bacteria  in  the  spaces  between  them.  Thorough 
cleaning,  desirably  after  each  meal,  with  the  occasional  use  of  an 
antiseptic  mouth-wash  is  fairly  but  not  completely  satisfactory. 
Half  yearly  inspection  and  cleaning  by  a  dentist  are  usually  neces- 
sary to  supplement  one's  own  efforts,  because  of  the  practical 
impossibility  of  keeping  every  one  of  the  small  mouth  spaces  clear. 
Such  inspection  also  insures  the  discovery  of  decay  while  the  cavi- 
ties are  still  small,  and  makes  possible  the  preservation  of  the 
teeth  in  approximately  normal  condition  for  many  years. 

Recently  evidence  has  been  advanced  showing  that  the  saliva 
varies  slightly  in  alkalinity  in  different  people,  and  that  the  sus- 
ceptibility of  the  teeth  to  decay  depends  largely  on  the  degree  of 
alkalinity.  Three  general  groupings  are  suggested.  Those  who 
fall  within  the  limits  of  the  first  group  are  likely  to  have  perfect 
teeth  even  though  no  care  is  taken  of  them.  The  second  group 
can  have  good  teeth  by  the  exercise  of  reasonable  care.  The  third 
group  have  difficulty  in  preserving  the  teeth  in  good  condition 
in  spite  of  unremitting  attention  to  them.  This  observation  ex- 
plains the  frequent,  occurrence  of  perfect  teeth  in  savages  and 
others  who  never  pay  them  the  slightest  attention,  and  the  prev- 
alence of  decay  among  the  most  highly  civilized.  It  is  probable, 
although  not  proven,  that  the  nature  of  the  diet  has  much  to  do 
with  the  degree  of  alkalinity  of  the  saliva. 

Of  late  years  a  great  deal  of  attention  has  been  paid  to  indirect 
harm  that  may  follow  neglect  of  the  teeth.  Allowing  colonies  of 
bacteria  to  flourish  among  them  undisturbed  means,  of  course, 
that  any  toxins  these  may  produce  will  be  absorbed  into  the  sys- 
tem. The  result  of  continuous  absorption  of  such  toxins  is  often 
manifested  in  lowering  of  the  general  health.  Specifically,  acute 
rheumatism  is  said  frequently  to  follow. 

Deglutition.  A  mouthful  of  solid  food  is  broken  up  by  the 
teeth,  and  rolled  about  the  mouth  by  the  tongue,  until  it  is  thor- 
oughly mixed  with  saliva  and  made  into  a  soft  pasty  mass.  This 
mass  is  sent  on  from  the  mouth  to  the  stomach  by  the  process  of 


MOVEMENTS  OF  THE  ALIMENTARY  CANAL  471 

deglutition,  which  is  described  as  occurring  in  three  stages.  The 
first  stage  includes  the  passage  from  the  mouth  into  the  pharynx. 
The  food  being  collected  into  a  heap  on  the  tongue,  the  tip  of  that 
organ  is  placed  against  the  front  of  the  hard  palate,  and  then  the 
rest  of  the  tongue  is  raised  from  before  back,  so  as  to  press  the 
food-mass  between  it  and  the  palate,  and  drive  it  back  through 
the  fauces.  This  portion  of  the  act  of  swallowing  is  voluntary,  or 
at  least  is  under  the  control  of  the  will,  although  it  commonly 
takes  place  unconsciously.  The  second  stage  of  deglutition  is 
that  in  which  the  food  passes  through  the  pharynx;  it  is  the  most 
rapid  part  of  its  progress,  since  the  pharynx  has  to  be  emptied 
quickly  so  as  to  clear  the  opening  of  the  air-passages  for  breathing 
purposes.  The  food-mass,  passing  back  over  the  root  of  the  tongue, 
pushes  down  the  epiglottis;  at  the  same  time  the  larynx  (or  voice- 
box  at  the  top  of  the  windpipe)  is  raised,  so  as  to  meet  it,  and  thus 
the  passage  to  the  lungs  is  closed;  muscles  around  the  aperture 
probably  also  contract  and  narrow  the  opening.  The  raising  of 
the  larynx  can  be  readily  felt  by  placing  the  finger  on  the  large 
cartilage  forming  " Adam's  apple"  in  the  neck,  and  then  swallow- 
ing something.  The  soft  palate  is  at  the  same  time  raised  and 
stretched  horizontally  across  the  pharynx,  thus  cutting  oft"  com- 
munication with  its  upper,  or  respiratory  portion,  leading  to  the 
nostrils  and  Eustachian  tubes.  Finally,  the  isthmus  of  the  fauces 
is  closed  as  soon  as  the  food  has  passed  through,  by  the  contrac- 
tion of  the  muscles  on  its  sides  and  the  elevation  of  the  root  of  the 
tongue.  All  passages  out  of  the  pharynx  except  the  gullet  are 
thus  blocked,  and  by  a  sharp  contraction  of  the  mylohyoid  mus- 
cles, in  the  floor  of  the  mouth,  such  great  pressure  is  put  upon 
the  food-mass  as  to  shoot  it  clear  through  the  pharynx  into  the 
opening  of  the  esophagus.  Liquids  or  very  soft  foods,  under  the 
impetus  given  by  the  contraction  of  these  muscles,  are  propelled 
the  whole  length  of  the  gullet  to  the  sphincter  which  guards  the 
entrance  to  the  stomach;  more  solid  masses  are  thrown  only  into 
the  entrance  of  the  gullet  whence  the  third  stage  of  swallowing 
conveys  them  to  the  stomach.  The  muscular  movements  con- 
cerned in  this  part  of  deglutition  are  all  reflexly  excited;  food 
coming  in  contact  with  the  mucous  membrane  of  the  pharynx 
stimulates  afferent  nerve-fibers  in  it;  these  excite  efferent  nerve- 
fibers  proceeding  to  the  muscles  concerned  and  cause  them 


472  THE  HUMAN  BODY 

to  contract  in  proper  sequence.  The  pharyngeal  muscles,  although 
of  the  striped  variety,  are  but  little  under  the  control  of  the  will; 
it  is  extremely  difficult  to  go  through  the  movements  of  swallow- 
ing without  something  (if  only  a  little  saliva)  to  swallow  and  thus 
excite  the  movements  reflexly.  Many  persons,  after  having  got 
the  mouth  completely  empty  cannot  perform  the  movements  of 
the  second  stage  of  deglutition  at  all.  On  account  of  the  reflex 
nature  of  the  contractions  of  the  pharynx,  any  food  which  has 
once  entered  it  must  be  swallowed :  the  isthmus  of  the  fauces  is  a 
sort  of  Rubicon;  food  that  has  passed  it  must  continue  its  course 
to  the  stomach,  although  the  swallower  learnt  immediately  that 
he  was  taking  poison.  The  third  stage  of  deglutition  is  that  by 
which  solid  food  is  passed  along  the  gullet,  and  is  comparatively 
slow.  The  movements  of  the  eosphagus  are  of  the  kind  known 
as  peristaltic.  Its  circular  muscular  fibers  contract  behind  the 
morsel  and  narrow  the  passage  there;  and  the  constriction  then 
travels  along  to  the  stomach,  pushing  the  food  in  front  of  it. 
Simultaneously  the  longitudinal  fibers,  at  the  point  where  the 
food-mass  is  at  any  moment  and  immediately  in  front  of  that, 
relax,  tending  to  widen  the  passage.  This  peristaltic  wave  re- 
quires about  six  seconds  in  man  for  its  passage  along  the  esophagus. 
It  is  part  of  the  reflex  act  of  swallowing  and  takes  place  whenever 
the  act  occurs,  whether  there  be  any  food-mass  to  be  conveyed 
to  the  stomach  or  not.  The  ring  of  smooth  muscle  of  the  circular 
coat  at  the  entrance  of  the  stomach  acts  as  a  sphincter  (cardiac 
sphincter).  This  is  ordinarily  tightly  contracted  when  there  is 
food  in  the  stomach,  holding  the  esophagus  shut,  and  only  opens 
at  the  approach  of  the  peristaltic  wave  to  allow  the  food-mass 
to  pass  through  into  the  stomach.  Liquids,  which  pass  very 
quickly  down  the  esophagus  (in  0.1  sec.),  usually  do  not  get  into 
the  stomach  at  once,  but  are  held  by  the  sphincter  until  the  arrival 
of  the  peristaltic  wave  opens  a  passage  for  them.  The  relaxation 
of  the  cardiac  sphincter  under  these  circumstances  does  not  open 
a  free  communication  between  the  stomach  and  the  throat,  for 
there  is  always  a  descending  peristaltic  wave  holding  the  esophagus 
closed.  This  is  important  because,  as  we  shall  see,  the  stomach 
contents  are  under  pressure,  and  would  be  forced  up  into  the  esoph- 
agus were  the  sphincter  to  relax  with  no  peristaltic  wave  present. 
As  a  matter  of  fact  this  sometimes  happens,  particularly  in  per- 


MOVEMENTS  OF  THE  ALIMENTARY  CANAL  473 

sons  suffering  from  indigestion,  or  certain  nervous  disorders,  or 
in  users  of  tobacco.  The  upward  rush  of  the  acid  stomach  con- 
tents into  the  esophagus  gives  rise  to  a  burning  sensation  which  is 
generally  known  as  "heart  burn,"  although  the  heart  has  really 
nothing  whatever  to  do  with  it. 

Movements  of  the  Stomach.  When  the  stomach  is  empty  of 
food  its  normal  condition  is  as  a  flabby  pouch.  Its  walls  are  neither 
much  relaxed  nor  strongly  distended.  There  are  probably  always 
a  small  amount  of  liquid  and  some  bubbles  of  swallowed  air  in  the 
stomach,  even  at  the  time  when  we  speak  of  it  as  empty.  Shortly 
before  the  usual  time  for  taking  a  meal  the  circular  muscle  coat 
of  the  stomach  goes  into  a  state  of  tonus,  probably  as  a  result  of 
a  flow  of  impulses  over  the  vagus  nerve,  which  is  the  motor  nerve 
of  the  organ.  The  effect  of  this  tonus  is  to  contract  the  stomach 
until  it  is  little  more  than  a  tube.  Usually  about  this  same  time 
the  active  contractions  which  give  rise  to  hunger  sensations  (p.  209) 
begin.  As  food  enters  the  contracted  stomach  it  makes  room  for 
itself  by  stretching  the  walls,  and  the  more  food  is  taken,  the  more 
the  stomach  is  distended.  One  result  of  this  manner  of  filling  the 
stomach  is  that  the  food  is  deposited  in  it  in  layers,  the  first  food 
taken  being  next  to  the  walls,  subsequent  amounts  being  toward 
the  center,  and  further  from  the  walls  the  more  has  entered  before 
them. 

The  gastric  glands  are  located  in  the  middle  and  to  some  extent 
in  the  pyloric  regions  of  the  stomach.  Such  food  as  is  in  the  fundus 
is  not  exposed  directly,  therefore,  to  the  action  of  gastric  juice, 
and  so  is  not  very  rapidly  acidified.  The  action  of  salivary  ptyalin, 
which  is  brought  to  an  end  when  the  food  becomes  acid,  may  thus 
continue  in  the  fundic  region  for  a  considerable  time  after  the 
food  is  swallowed,  especially  in  those  portions  of  food  which  are 
swallowed  late  in  the  meal. 

The  movements  of  the  stomach  have  been  watched  by  means 
of  the  X-rays.  Food  which  has  been  mixed  with  bismuth  subni- 
trate  is  opaque  to  these  rays  and  its  movements  in  response  to  the 
movements  of  the  stomach  walls  can  be  readily  followed.  By 
this  means  it  has  been  learned  that  the  walls  of  the  stomach  show 
peristaltic  waves;  these  begin  at  about  the  middle,  in  a  strong 
contraction  of  a  ring  of  circular  muscles  at  that  point,  and  sweep 
to  the  pylorus.  The  fundic  end  is  not  involved  at  all  in  them. 


474  THE  HUMAN  BODY 

In  man  they  recur  regularly,  so  long  as  food  is  in  the  stomach,  at 
intervals  of  about  twenty  seconds.  For  a  considerable  period 
after  food  enters  the  stomach  the  pyloric  sphincter,  which  guards 
the  exit  into  the  small  intestine,  remains  perfectly  tight.  During 
this  time  the  peristaltic  waves  crowd  the  food  caught  by  them  up 
to  the  pylorus  but  cannot  force  any  through.  As  the  constriction 
approaches  the  pylorus  the  food-mass  in  front  of  it  escapes  back 
through  the  opening  at  its  center,  the  waves  not  being  deep  enough 
to  close  this  entirely,  and  so  the  food  in  the  central  and  pyloric 
portions  of  the  stomach  is  thoroughly  churned. 

During  this  churning  the  food,  already  semi-liquid  from  the 
mixture  with  saliva  and  with  such  liquid  as  was  taken  with  the 
meal,  is  mixed  with  the  gastric  juice  and  made  still  more  liquid, 
being  called  at  this  stage  chyme.  The  effect  of  the  gastric  juice  is 
to  give  the  food  an  acid  reaction,  stopping  the  action  of  ptyalin 
and  permitting  that  of  the  pepsin  which  it  also  pours  out  upon 
the  food. 

The  Control  of  the  Pyloric  Sphincter.  The  way  in  which  the 
sphincter  of  the  pylorus  is  regulated  so  that  after  the  food  has  been 
thoroughly  mixed  with  gastric  juice  it  opens  and  allows  a  small 
amount  to  pass,  and  then  promptly  closes  to  give  opportunity  for 
this  to  be  influenced  by  the  intestinal  secretions  before  more  is 
admitted,  is  one  of  the  most  interesting  adaptations  that  we  know 
of  in  the  Body.  The  mechanism  of  this  action  is  a  special  case 
of  a  peculiar  reflex  which  apparently  obtains  throughout  the  ali- 
mentary canal,  and  is  probably  dependent  on  special  properties 
of  the  nerve  plexus  which  is  embedded  therein.  This  so-called 
myenteric  reflex,  is  of  such  a  sort  that  a  stimulus  applied  to  any 
point  along  the  alimentary  canal  causes  a  contraction  of  the 
muscles  immediately  in  front  of  (anterior  to)  the  stimulated  point, 
and  a  relaxation  of  those  immediately  behind  (posterior  to)  it. 
The  reflex  was  worked  out  first  for  the  small  intestine,  and  has 
since  been  shown  to  apply  to  the  other  parts  of  the  canal.  It  is  a 
so-called  " local  reflex,"  as  the  central  nervous  system  has  noth- 
ing whatever  to  do  with  it. 

The  adequate  stimulus  for  arousing  the  reflex  in  the  pyloric 
sphincter  is  the  presence  of  free  hydrochloric  acid.  When  there- 
fore the  originally  alkaline  food  in  the  pyloric  part  of  the  stomach 
has  been  completely  neutralized  by  the  acid  of  the  gastric  juice, 


MOVEMENTS  OF  THE  ALIMENTARY  CANAL  475 

and  excess  acid  begins  to  accumulate,  the  pyloric  sphincter  is 
stimulated,  but  from  the  stomach  side,  and  according  to  the  work- 
ing of  the  myenteric  reflex  a  stimulus  from  that  side  produces 
relaxation  of  a  region  just  posterior  to  it.  As  soon  as  the  sphincter 
relaxes  under  this  stimulation  that  part  of  the  food  lying  in  the 
pylorus  is  forced  through  into  the  intestine,  but  it  carries  with  it 
the  free  acid  with  which  the  food  is  mixed  and  stimulates  the 
sphincter  from  the  intestinal  side,  namely,  from  behind,  and  there- 
fore tends  to  cause  it  to  close.  A  feature  of  the  myenteric  reflex  is 
that  where,  as  just  described,  a  point  is  simultaneously  stimulated 
from  in  front  and  from  behind,  the  stimulus  causing  contraction, 
that  from  behind,  is  dominant.  Therefore  as  soon  as  food  enters 
the  intestine  the  sphincter  of  the  pylorus  contracts  and  prevents 
more  from  passing.  Before  it  will  relax  again  the  acid  on  its  in- 
testinal side  must  be  neutralized;  but  this  is  rapidly  done  by  the 
strongly  alkaline  bile  and  pancreatic  juice,  and  so  as  fast  as  the 
food  in  the  intestine  is  mixed  with  these  juices  more  is  admitted 
from  the  stomach. 

The  fundus  of  the  stomach,  which  stores  the  bulk  of  the  food 
while  that  in  the  pylorus  is  being  thus  treated  and  passed  on  to 
the  intestine,  is  on  the  stretch  all  the  time,  so  that  as  fast  as  food 
is  passed  out  through  the  pyloric  sphincter  more  is  pushed  to  the 
pylorus  from  the  fundus  until  at  last  the  stomach  is  wholly  emp- 
tied. The  time  required  for  emptying  the  stomach  completely 
varies  with  different  foods  and  under  different  bodily  conditions. 
An  average  meal  is  probably  all  out  of  the  stomach  about  four 
to  six  hours  after  eating. 

An  interesting  incidental  feature  of  this  mechanism  is  that  it 
operates  automatically  to  pass  quickly  on  into  the  small  intestine 
carbohydrate  food  stuffs,  which  undergo  no  digestive  action  in 
the  stomach,  while  proteins,  upon  which  the  pepsin  of  gastric  juice 
acts,  remain  long  enough  to  ensure  their  thorough  mixture  with 
the  juice.  This  differentiation  depends  on  the  fact  that  the  acid 
does  not  enter  any  chemical  combination  with  carbohydrates,  and 
therefore  begins  to  appear  in  excess  as  soon  as  the  alkali  present 
has  been  neutralized.  Proteins,  on  the  other  hand,  do  combine 
chemically  with  the  acid,  and  there  can  be  no  excess,  therefore, 
until  this  combination  has  occurred.  Meanwhile  thorough  mix- 
ture with  pepsin  is  taking  place.  This  difference  does  not  show, 


476  THE  HUMAN  BODY 

of  course,  in  the  case  of  a  mixed  meal,  but  a  meal  of  pure  carbo- 
hydrates will  begin  to  leave  the  stomach  much  sooner  after  Di- 
gestion than  a  meal  of  pure  protein  (10-15  minutes  as  compared 
with  J/2  hour),  and  will  be  discharged  completely  in  half  the  time 
(2-23/2  hours  as  against  4-5);  and  a  meal  in  which  the  carbohy- 
drates are  eaten  before  the  proteins  may  show  a  definite  interval 
between  the  discharge  of  the  last  carbohydrates  and  the  first 
proteins.  The  admixture  of  fats  with  the  other  food  stuffs  delays 
considerably  the  rate  of  discharge. 

The  pyloric  sphincter  does  not  hold  against  pure  water  nor 
against  substances  of  the  consistency  of  raw  egg-white  or  raw 
oysters.  These,  unless  mixed  with  other  materials,  pass  promptly, 
therefore,  from  the  stomach  into  the  small  intestine. 

Importance  of  the  Stomach.  Aside  from  its  function  of  begin- 
ning the  digestion  of  proteins,  a  function  which,  as  we  have  seen 
(p.  465)  is  subordinate  to  the  more  efficient  digestive  action  of  the 
small  intestine,  the  chief  significance  of  the  stomach  is  that  it  en- 
ables us  to  take  our  daily  supply  of  food  in  three  meals,  more  or  less, 
according  to  our  habit.  The  small  intestine  is  a  narrow  tube.  The 
ducts  of  pancreas  and  liver  open  into  its  upper  end.  If  our  food 
when  swallowed  passed  directly  into  the  intestine  each  mouthful 
would  crowd  the  preceding  ones  along  at  such  a  rate  that  no  ade- 
quate admixture  with  the  essential  juices  of  the  pancreas  and 
liver  could  occur,  and  very  little  digestion  would  take  place.  To 
avoid  this  difficulty  the  food  would  have  to  be  eaten  little  by 
little,  and  to  get  enough  for  the  needs  of  the  Body  would  require 
hours  of  steady  nibbling.  By  affording  storage  to  a  considerable 
amount  of  food,  which  is  automatically  passed  along  to  the  intes- 
tine at  just  the  rate  at  which  that  region  can  handle  it,  the  stomach 
permits  us  to  follow  eating  habits  much  less  time  consuming,  and 
more  convenient. 

Movements  of  the  Small  Intestine.  The  food  entering  the 
small  intestine  is  subjected  to  two  sorts  of  movements  whose 
combined  effect  is  to  churn  it  very  thoroughly  and  to  move  it 
slowly  along  the  gut  so  as  to  make  room  for  more  to  come  in  from 
the  stomach.  The  churning  is  effected  mainly  by  movements  of 
the  intestine  known  as  rhythmic  segmentation.  In  these  move- 
ments rings  of  the  circular  muscle  coat  about  an  inch  apart  con- 
strict simultaneously,  splitting  the  contained  food  into  a  series  of 


MOVEMENTS  OF  THE  ALIMENTARY  CANAL  477 

segments;  an  instant  later  these  constrictions  disappear,  and  new 
ones,  midway  between  the  first,  are  formed,  by  which  the  food  is 
again  segmented,  but  in  a  shifted  position.  These  rhythmic  move- 
ments may  recur  as  often  as  thirty  times  a  minute.  Their  effect  is 
to  bring  every  particle  of  the  contained  food  into  intimate  con- 
tact with  the  intestinal  walls,  insuring  thorough  mixing  with  the 
intestinal  secretions,  and  also  favoring  absorption. 

The  onward  movement  of  the  food  is  secured  by  peristaltic  waves 
which  start  at  the  pylorus  and  run  rather  slowly  along  the  intes- 
tine. They  are  normally  gentle  movements,  which  do  not  carry 
the  food  bodily  before  them,  but  move  it  forward  little  by  little. 
During  digestion  the  two  sorts  of  movements  alternate  more  or 
less  irregularly.  After  the  segmentation  has  churned  a  food-mass 
thoroughly  in  one  section  it  dies  away  and  a  peristaltic  wave  de- 
velops, which  carries  the  food  ahead  of  it  into  a  fresh  section;  then 
the  peristalsis,  in  turn,  subsides,  and  segmentation  is  resumed. 

The  mechanism  of  these  intestinal  movements  is  not  entirely 
clear,  the  peristaltic  waves,  and  possibly  also  the  segmentations, 
are  special  manifestations  of  the  myenteric  reflex  described  above, 
but  the  conditions  that  govern  their  appearance  and  disappear- 
ance, first  in  one  part  of  the  intestine  and  then  in  another,  are 
not  known. 

Observations  with  the  X-rays  have  shown  that  the  rate  of  prog- 
ress of  the  food  through  the  human  small  intestine  is  about  4J^ 
feet  in  the  hour,  so  that  the  first  food  from  any  meal  may  appear  at 
the  ileocolic  valve  about  4J^  hours  after  it  begins  to  leave  the 
stomach. 

Extrinsic  Control  of  Stomach  and  Intestinal  Movements.  It 
has  been  shown  that  normal  movements  of  both  stomach  and  in- 
testine may  go  on  in  animals  in  which  the  nerves  leading  to  these 
organs  from  the  central  nervous  system  are  cut.  To  a  certain  ex- 
tent, therefore,  they,  like  the  heart,  contain  within  themselves  the 
essential  requirements  for  normal  activity.  Like  the  heart,  how- 
ever, they  are  subject  to  reflex  control  through  the  central  nervous 
system. 

The  vagus  nerves  carry  cranial  autonomic  fibers  which  when 
stimulated  arouse  the  stomach  and  intestine  to  activity.  The  op- 
posing thoracico-lumbar  autonomies,  which,  as  we  have  already 
seen  (p.  455),  come  by  way  of  the  splanchnics,  are  inhibitory.  A 


478  THE  HUMAN  BODY 

part  of  the  emergency  reaction  of  the  Body,  therefore,  consists  in 
suspension  of  activity  in  these  organs.  This  has  been  noted  pre- 
viously (p.  196). 

Movements  of  the  Large  Intestine.  During  the  passage  of  the 
food  through  the  small  intestine  the  greater  part  of  its  nutritive 
content  is  absorbed,  but  practically  none  of  the  water,  so  that  it 
is  delivered  through  the  ileocolic  valve  to  the  large  intestine  in  a 
very  watery  condition.  The  parts  of  the  large  intestine  next  to 
the  small  intestine,  the  ascending  and  transverse  colon,  show  an 
interesting  movement  in  the  form  of  an  antiperistalsis.  This  is  a 
peristaltic  wave  which  begins  in  the  transverse  colon  and  sweeps 
toward  the  ileocolic  valve.  It  would  tend  to  force  the  material 
within  the  colon  back  into  the  small  intestine  did  not  the  ileocolic 
valve  prevent.  The  result  of  this  movement  is  a  churning  and 
mixing  of  the  contents  whereby  the  absorption  of  the  last  useful 
materials,  including  the  water,  is  promoted.  As  the  large  in- 
testine is  filled  more  and  more  from  the  small,  some  of  its  contents 
are  crowded,  in  spite  of  the  antiperistalsis,  into  the  descending 
colon,  where  regular  peristaltic  waves  carry  them  on  to  the  sigmoid 
flexure  and  the  rectum,  whence  they  are  discharged  from  the  Body. 
There  is  evidence  that  the  stimulus  for  these  intestinal  waves  is 
mechanical,  depending  on  stretching  of  the  walls  by  the  intes- 
tinal contents. 

Importance  of  Roughage.  As  the  result  of  the  absorption  of 
water  from  the  contents  of  the  large  intestine  the  material  re- 
maining, which  consists  of  undigested  substances,  bacteria,  the 
products  of  bacterial  action,  and  some  waste  products  excreted  in 
the  bile  (p.  518),  tends  to  become  dry  and  closely  packed.  If  the 
diet  is  poor  in  roughage  (p.  429)  so  little  room  is  occupied  by  this 
material  that  the  necessary  mechanical  stimulation  fails  to  be 
forthcoming  for  the  movements  by  which  it  should  be  carried  along 
to  the  region  of  discharge.  There  is,  therefore,  stagnation  in  the 
large  intestine,  and  this,  by  permitting  time  for  a  more  complete 
absorption  of  water,  makes  the  condition  of  affairs  still  worse,  and 
the  evacuation  of  the  colon  still  more  difficult.  The  inclusion  of 
considerable  roughage  in  the  diet  (bran,  the  pulp  of  vegetables 
and  fruits,  particularly  apples,  popcorn)  by  increasing  the  bulk 
of  the  intestinal  contents  favors  the  onward  movement  of  the 
material,  and  tends  against  stagnation.  We  need  to  remember 


MOVEMENTS  OF  THE  ALIMENTARY  CANAL  479 

in  this  connection  that  the  colon  is  a  smooth  muscle  structure, 
under  the  control  of  the  autonomic  system,  and  subject,  therefore, 
to  the  disturbing  influences  characteristic  of  such  structures.  The 
inclusion  of  ample  roughage  in  the  diet  does  not  always  suffice  to 
secure  adequate  evacuations,  particularly  where  neglect  or  im- 
proper treatment  has  affected  the  colon  so  that  it  no  longer  re- 
sponds normally  to  mechanical  stimulation  from  its  contents.  The 
means  commonly  used  to  induce  evacuations,  the  taking  of  purga- 
tive drugs,  is  objectionable,  although  sometimes  necessary,  be- 
cause the  drugs  act  by  irritating  the  intestinal  lining.  Such  irrita- 
tion, if  repeated  regularly,  brings  on  a  chronic  inflammation, 
which  seriously  impairs  the  ability  of  the  colon  to  react  normally. 
The  habit  of  taking  purgative  drugs  should  be  strenuously  avoided. 
If  persisted  in  it  is  sure  to  lead  to  much  discomfort  or  even  severe 
suffering.  It  is  probable  that  much  of  the  trouble  from  intestinal 
sluggishness  could  be  avoided  by  proper  supervision  and  care  in 
childhood,  when  regular  habits  are  easy  to  establish'  and  enforce. 
Regularity,  even  more  than  ample  roughage,  is  a  prime  requisite 
to  the  proper  functioning  of  the  colon. 


CHAPTER  XXIX 
THE  DIGESTIVE  SECRETIONS  AND  THEIR  CONTROL 

Organs  of  Secretion.  The  simplest  form  in  which  a  secreting 
organ  occurs  (A,  Fig.  137)  is  that  of  a  flat  membrane  provided  with 
a  layer  of  cells,  a,  on  one  side  (that  on  which  the  secretion  is  poured 
out)  and  with  a  nelbwork  of  capillary  blood-vessels,  c,  on  the  other. 
The  dividing  membrane,  b,  is  known  as  the  basement  membrane 
and  is  usually  made  up  of  flat,  closely  fitting  connective-tissue 
corpuscles;  supporting  it  on  its  deep  side  is  a  layer  of  connec- 
tive tissue,  d,  in  which  the  blood-vessels  and  lymphatics  are  sup- 
ported. Such  simple  forms  of  secreting  surfaces  are  found  on  the 
serous  membranes,  but  are  not  common;  in  most  cases  an  extended 
area  is  required  to  form  the  necessary  amount  of  secretion,  and  if 
this  were  attained  simply  by  spreading  out  plane  surfaces,  these 
from  their  number  and  extent  would  be  hard  to  pack  conveniently 
in  the  Body.  Accordingly  in  most  cases,  the  greater  area  is  at- 
tained by  folding  the  secreting  surface  in  various  ways  so  that  a 
large  area  can  be  packed  in  a  small  bulk,  just  as  a  Chinese  lantern 
when  shut  up  occupies  much  less  space  than  when  extended,  al- 
though its  actual  surface  remains  of  the -same  extent.  In  a  few 
cases  the  folding  takes  the  form  of  protrusions  into  the  cavity  of 
the  secreting  organ  as  indicated  at  C,  Fig.  137,  and  found  on  some 
synovial  membranes;  but  much  more  commonly  the  surface  ex- 
tension is  attained  in  another  way,  the  basement  membrane,  cov- 
ered by  its  epithelium,  being  pitted  in  or  involuted  as  at  B.  Such 
a  secreting  organ  is  known  as  a  gland. 

Forms  of  Glands.  In  some  cases  the  surface  involutions  are 
uniform  in  diameter,  or  nearly  so,  throughout  (B,  Fig.  137).  Such 
glands  are  known  as  tubular;  examples  are  found  in  the  lining  coat 
of  the  stomach  (Fig.  129);  also  in  the  skin  (Fig.  142),  where  they 
form  the  sweat-glands.  In  other  cases  the  involution  swells  out  at 
its  deeper  end  and  becomes  more  or  less  sacculated;  (E)  such 
glands  are  racemose  or  acinous.  The  small  glands  which  form  the 
oily  matter  poured  out  on  the  hairs  are  of  this  type.  In  both  kinds 

480 


THE  DIGESTIVE  SECRETIONS  AND  THEIR  CONTROL    481 


FIG.  137. — Forms  of  glands.  A,  &  simple  secreting  surface;  a,  its  epithelium; 
b,  basement  membrane;  c,  capillaries;  B,  a  simple  tubular  gland;  C,  a  secreting 
surface  increased  by  protrusions;  E,  a  simple  racemose  gland;  D  and  G,  com- 
pound tubular  glands;  F,  a  compound  racemose  gland.  In  all  but  A,  B,  and  C 
the  capillaries  are  omitted  for  the  sake  of  clearness.  H,  half  of  a  highly  developed 
racemose  gland ;  c,  its  main  duct. 


482  THE  HUMAN  BODY 

the  lining  cells  near  the  deeper  end  are  commonly  different  in 
character  from  the  rest;  and  around  that  part  of  the  gland  the 
blood-vessels  form  a  closer  network.  These  deeper  cells  form  the 
true  secreting  elements  of  the  gland,  and  the  passage,  lined  with 
different  cells,  leading  from  them  to  the  surface,  and  serving  merely 
to  carry  off  the  secretion,  is  known  as  the  gland-duct.  When  the 
duct  is  undivided  the  gland  is  simple;  but  when,  as  is  more  usual, 
it  is  branched  and  each  branch  has  a  true  secreting  part  at  its 
end,  we  get  a  compound  gland,  tubular  (G)  or  racemose  (F,  H) 
as  the  case  may  be.  In  such  cases  the  main  duct,  into  which 
the  rest  open,  is  often  of  considerable  length,  so  that  the  se- 
cretion is  poured  out  at  some  distance  from  the  main  mass  of  the 
gland. 

A  fully  formed  gland,  H,  thus  comes  to  be  a  complex  structure, 
consisting  primarily  of  a  duct,  c,  ductules,  dd,  and  secreting  re- 
cesses, ee.  The  ducts  and  ductules  are  lined  with  epithelium  which 
is  merely  protective  and  differs  in  character  from  the  secreting 
epithelium  which  lines  the  deepest  parts.  Surrounding  each  sub- 
division and  binding  it  to  its  neighbors  is  the  gland  stroma  formed 
of  connective  tissue,  a  layer  of  which  also  commonly  envelops  the 
whole  gland,  as  its  capsule.  Usually  on  looking  at  the  surface  of  a 
large  gland  it  is  seen  to  be  separated  by  partitions  of  its  stroma, 
coarser  than  the  rest,  into  lobes,  each  of  which  answers  to  a  main 
division  of  the  primary  duct;  and  the  lobes  are  often  similarly  di- 
vided into  smaller  parts  or  lobules.  In  the  connective  tissue  be- 
tween the  lobes  and  lobules  blood-vessels  penetrate,  to  end  in  fine 
capillary  vessels  around  the  terminal  recesses.  They  never  pene- 
trate the  basement  membrane.  Lymphatics  and  nerves  take  a 
similar  course;  there  is  reason  to  believe  that  the  nerve-fibers 
penetrate  the  basement  membrane  and  become  directly  united 
with  the  secreting  cells  of  some  glands. 

The  Secretory  Process.  The  function  of  glands  is  to  elaborate 
and  pour  out  a  liquid,  the  secretion.  It  is  obvious  that  the  ulti- 
mate source  of  the  secretion  is  the  blood  circulating  through  the 
gland.  The  digestive  secretions,  as  we  have  already  seen,  contain, 
in  addition  to  water,  and  inorganic  salts,  special  chemical  sub- 
stances, the  enzyms,  which  are  different  in  different  glands.  It 
is  easy  to  believe  that  the  water  and  salts  of  the  gland,  since  they 
are  precisely  the  same  as  occur  in  blood,  may  be  withdrawn  from 


THE  DIGESTIVE  SECRETIONS  AND  THEIR  CONTROL    483 

the  blood  through  simple  physical  processes,  filtration  and  dialysis 
(Chap.  I).  The  special  constituents  of  each  secretion,  being 
different  from  anything  contained  in  the  blood,  must,  on  the 
other  hand,  be  produced  by  chemical  processes  within  the  gland 
itself.  It  is  easy  to  show  microscopically  that  the  cells  of  most 
glands  during  rest  become  filled  with  small  granules,  and  that 
when  the  gland  is  active  these  granules  for  the  most  part  disap- 
pear. We  can  picture  the  entire  secretory  process  as  occurring 
in  two  stages :  the  first,  a  chemical  stage,  during  which  the  peculiar 
constituents  of  the  secretion  are  elaborated  and  deposited  within 
the  cells  of  the  gland;  and  a  second  physical  stage  consisting  of  a 
rapid  flow  of  water  with  its  dissolved  salts  from  the  blood  through 
the  gland  into  its  duct,  carrying  with  it  the  special  materials  pre- 
viously prepared  by  the  gland. 

Nervous  Control  of  the  Secretory  Process.  Considerable  ev- 
idence has  accumulated  indicating  that  gland  tissue,  like  skeletal 
muscle  tissue,  carries  on  its  function  only  when  stimulated  to  do 
so,  and  that  the  stimulus  is  in  many  glands  nervous.  It  has  been 
shown  for  the  salivary  glands  of  dogs,  for  example,  that  proper 
stimulation  of  certain  nerve-fibers  leading  to  them  causes  them 
to  produce  and  store  within  themselves  granules,  while  stimula- 
tion of  quite  different  nerve-fibers  causes  them  to  pour  out  their 
secretion.  The  chemical  part  of  secretion  is  thus  controlled  by  one 
set  of  nerves,  often  called  trophic  nerves,  and  the  physical  part  by 
another.  It  is  interesting  to  note  that  the  nerves  which  cause  the 
gland  to  pour  out  its  secretion  usually  cause  also  vasodilation 
within  it;  an  increased  flow  of  blood  through  the  gland  therefore 
usually  accompanies  the  physical  part  of  secretion.  That  this  in- 
creased blood-flow  is  not  the  sole  cause  of  the  outpouring  of  the 
secretion,  as  might  easily  be  supposed,  is  proved  by  the  possibility 
under  proper  conditions  of  stimulating  a  gland  to  pour  out  its  fluid 
without  any  accompanying  vasodilation.  We  must  recognize, 
then,  that  the  physical  act  of  secretion  is  the  result  of  the  action  of 
definite  secretory  nerves,  as  distinct  from  vasodilator  nerves.  Just 
how  these  function  to  bring  about  the  more  rapid  passage  of  water 
and  salts  through  the  gland-cell  is  not  clear.  As  we  should  expect, 
continued  stimulation  of  the  secretory  fibers  leading  to  a  gland, 
without  accompanying  stimulation  of  the  trophic  fibers,  results 
soon  in  the  production  of  a  secretion  which  is  very  watery,  virtu- 


484  THE  HUMAN  BODY 

ally  free  from  the  special  chemical  substances  that  usually  are 
present  in  the  secretion. 

The  efferent  nerves  to  glands  belong,  without  exception,  to  the 
autonomic  system.  Glands  are,  therefore,  under  reflex  control, 
and  not  subject  to  the  will. 

Hormone  Control  of  Gland  Activity.  Some  of  the  digestive 
glands,  notably  the  pancreas,  appear  to  be  wholly,  or  at  least 
chiefly,  independent  of  nervous  influences.  Their  control  is  vested 
in  hormones.  The  details  of  this  method  of  control  will  be  de- 
scribed in  connection  with  the  glands  themselves.  It  may  be 
noted  here,  however,  that  in  general  those  glands  whose  secre- 
tions are  needed  early  in  the  digestive  process  are  under  reflex 
control,  and  those  whose  secretions  may  not  be  required  for  some 
time  after  are  under  hormone  control. 

Control  of  the  Salivary  Secretion.  The  salivary  glands  are  sub- 
ject to  reflex  stimulation.  We  must  inquire,  therefore,  what  sen- 
sory stimuli  may  excite  the  reflex.  At  least  three  sorts  of  stimuli 
are  effective  to  this  end;  mechanical,  the  presence  of  dry  sub- 
stances in  the  mouth,  or  merely  the  rubbing  of  the  tongue  against 
the  palate  and  jaws;  chemical,  the  presence  of  sapid  substances 
upon  the  tongue;  and  psychic,  the  thought  of  savory  food,  as 
when  the  mouth  "waters."  It  is  an  interesting  fact  that  the 
character  of  the  saliva  varies  somewhat  with  the  nature  of  the 
exciting  stimulus;  mechanical  stimulation  causes  the  production 
of  an  abundant  but  very  watery  secretion;  whereas  the  chemical 
stimulus  of  food  in  the  mouth  calls  forth  a  secretion  rich  in  ptyalin. 
By  this  mechanism  the  character  of  the  secretion  is  adapted  to 
the  need  which  excites  it.  The  mucous  lining  of  the  mouth  and 
throat  requires  constant  moistening.  For  this  a  watery  saliva  is 
adequate,  and  such  a  saliva  is  poured  out  whenever  the  dryness 
of  the  mouth  becomes  pronounced  enough  to  act  as  a  stimulus. 
When  food  is  taken,  on  the  other  hand,  the  proper  function- 
ing of  saliva  requires  that  it  be  rich  in  ptyalin.  Chemical 
stimulation,  therefore,  excites  a  secretion  containing  this  sub- 
stance. 

The  watering  of  the  mouth  at  the  thought  of  food  is  an  example 
of  an  emotional  reflex  through  the  autonomic  system  such  as  was 
discussed  earlier  (Chap.  XII).  Inhibition  of  the  salivary  glands, 
leading  to  dryness  of  the  mouth,  as  the  result  of  excitation  of  the 


THE  DIGESTIVE  SECRETIONS  AND  THEIR  CONTROL    485 

thoracico-lumbar  autonomic  system  in  time  of  stress,  has  also 
been  described. 

The  Control  of  the  Gastric  Secretion.  Our  present  knowledge 
of  the  mechanism  for  controlling  the  secretion  of  gastric  juice  is 
the  result  of  some  of  the  most  interesting  investigations  of  modern 
Physiology.  Many  workers  have  had  a  share  in  the  solution  of 
the  problem  but  the  name  of  one  of  them,  the  Russian  physiologist 
Pawlow  (Pavloff),  is  more  closely  associated  with  it  than  that  of 
any  other  one  man.  Pawlow's  chief  contribution  was  the  demon- 
stration that  the  secretion  of  gastric  juice  is  in  its  early  stages  ex- 
cited reflexly,  and  by  only  one  particular  sort  of  stimulus,  namely, 
the  psychical  state  accompanying  the  eating  of  food  which  is 
enjoyed.  Pawlow  gained  this  information  through  feeding  ex- 
periments on  dogs  which  had  been  prepared  in  a  special  way  for 
the  study.  The  preparation  consisted  of  making  a  fistulous  open- 
ing into  the  stomach,  through  which  the  secretion  of  gastric  juice 
could  be  followed,  and  of  cutting  the  esophagus  in  the  neck  and 
bringing  the  cut  ends  to  the  surface  in  such  fashion  that  all  the 
food  swallowed  reappeared  at  the  upper  esophageal  opening,  and 
none  reached  the  stomach  unless  it  was  placed  within  the  lower 
section  of  the  esophagus  through  its  opening.  Dogs  thus  operated 
upon  recovered  promptly  and  completely  and  could  be  studied 
very  satisfactorily.  It  was  found  that  one  of  these  dogs  would 
eat  with  the  greatest  enjoyment,  although  none  of  the  food  reached 
the  stomach,  and  that  within  a  few  minutes  of  the  beginning  of 
eating  a  secretion  of  gastric  juice  began  to  be  poured  into  the 
stomach.  That  this  secretion  was  excited  reflexly  was  proved 
by  cutting  the  vagus  nerves,  after  which  it  never  appeared.  That 
it  depends  upon  a  certain  psychical  state,  and  not  upon  the  mere 
eating  of  food  was  shown  in  various  ways.  Dogs  which  were 
not  hungry  would  chew  and  swallow  food,  but  without  signs  of 
much  interest  in  it;  no  secretion  was  evoked.  Meat  which  had 
been  boiled  till  it  was  tasteless  was  eaten  without  the  production 
of  a  secretion.  These  results  make  it  clear  that  the  stimulus  is  a 
psychical  one,  and  that  it  depends  upon  active  enjoyment  of  food. 
Equally  important  is  the  observation,  made  upon  these  same  dogs, 
that  unfavorable  emotional  states  prevent  the  secretion  of  the 
juice.  If  the  dog  was  angered  while  eating  no  juice  appeared; 
even  the  presence  of  an  attendant  for  whom  he  had  an  aversion 


486  THE  HUMAN  BODY 

sufficed  to  prevent  the  secretion.    All  these  facts,  established  first 
upon  dogs,  have  been  proved  true  likewise  for  human  beings. 

Pawlow's  studies  showed,  moreover,  that  the  psychical  secre- 
tion is  not  the  only  secretion  of  gastric  juice  which  occurs  during 
the  digestion  of  a  meal.  This  was  proved  by  the  simple  observa- 
tion that  the  amount  of  juice  secreted  during  the  eating  of  a 
"fictitious  meal"  is  much  less  than  that  produced  if  the  food 
eaten  enters  the  stomach.  We  must  look,  then,  for  some  other 
stimulating  agency  additional  to  the  psychical  one.  In  such  a 
search  the  attention  turns  naturally  to  the  foods  swallowed.  Do 
they  serve  as  chemical  stimuli  for  the  production  of  the  additional 
secretion?  It  has  been  shown  that  some  foods,  milk  and  water 
very  slightly,  the  juices  of  meat  more,  do  excite  the  secreting 
mechanism  somewhat,  but  the  really  effective  excitant  appears 
to  be  something  produced  during  the  process  of  gastric  digestion 
itself.  Thus  if  the  taking  of  food  is  attended  with  pleasure,  so  that 
a  psychical  secretion  is  produced,  the  digestive  process  is  started 
and  itself  furnishes  the  stimulating  agent  for  the  additional  se- 
cretion needed  to  complete  the  digestion.  On  the  other  hand, 
food  eaten  under  conditions  not  favorable  to  the  production  of  a 
psychical  secretion  may  fail  of  digestion  completely,  through  the 
absence  of  all  factors  which  may  lead  to  an  outpouring  of  the  juice. 

Nature  of  the  Chemical  Stimulus  to  Gastric  Secretion.  It  has 
been  shown  that  the  substances  mentioned  in  the  last  paragraph 
as  chemical  excitants  of  gastric  secretion  do  not  stimulate  the 
glands  directly  but  indirectly  through  a  hormone,  gastric  secretin. 
This  hormone  is  apparently  derived  from  some  substance  in  the 
mucous  membrane  of  the  pyloric  region,  which  reacts  with  the 
exciting  substances  derived  from  the  food  in  such-  fashion  as  to 
produce  the  hormone,  which  is  then  taken  up  by  the  blood  and 
carried  to  the  gastric  glands. 

Control  of  the  Pancreatic  Secretion.  Proper  regulation  of  the 
outpouring  of  pancreatic  juice  requires  that  it  begin  about  the 
time  food  begins  to  pass  from  the  stomach  into  the  small  intes- 
tine. Since  this  may  occur  at  a  variable  time  after  the  eating  of 
the  meal,  it  would  seem  to  call  for  a  regulating  mechanism  quite 
independent  of  the  act  of  eating.  It  has  been  shown  that  this 
requirement  is  fulfilled  through  the  action  of  a  hormone  which 
is  produced  in  active  form  during  the  time  that  food  is  passing 


THE  DIGESTIVE  SECRETIONS  AND  THEIR  CONTROL    487 

from  the  stomach  into  the  small  intestine,  and  only  then.  The 
mucous  membrane  of  the  small  intestine  at  its  upper  end  contains 
a  substance  which  has  been  named  prosccretin.  This  substance 
reacts  with  hydrochloric  acid  to  form  pancreatic  secretin,  the 
hormone  for  exciting  the  pancreas  to  secrete.  But  this  region  of 
the  small  intestine  comes  in  contact  with  hydrochloric  acid  only 
at  the  moment  when  a  mass  of  food  is  entering  it  from  the  stomach; 
we  have  previously  seen  that  this  passage  of  food  occurs  only 
when  the  food  is  mixed  with  excess  of  hydrochloric  acid.  Thus 
the  production  of  the  hormone  is  confined  to  the  time  when  its 
stimulating  function  is  required. 

The  Control  of  the  Bile  Flow.  It  has  been  shown  recently 
that  the  bile,  which,  although  secreted  continuously,  is  poured 
out  only  when  food  enters  the  small  intestine,  is  controlled  by  the 
same  hormone,  secretin,  which  excites  the  flow  of  pancreatic  juice. 
Under  the  stimulation  of  this  hormone  the  gall  bladder  contracts, 
forcing  its  contents  through  the  bile-duct  into  the  intestine. 

The  Control  of  the  Succus  Entericus  is  at  present  wholly  un- 
known. Whether  it  is  constantly  present  in  the  intestine  or 
whether  its  secretion  is  controlled  by  a  hormone  remains  to  be 
determined.  It  is  worth  noting,  however,  that  there  are  probably 
not  many  periods,  except  during  prolonged  fasting,  when  intestinal 
digestion  is  not  going  on,  so  that  a  continuous  secretion  of  intes- 
tinal juice  would  be  less  wasteful  than  of  the  other  digestive  juices. 

Digestive  History  of  a  Meal.  We  can  summarize  the  whole 
process  of  digestion  as  well,  perhaps,  by  following  the  course  of 
an  ordinary  meal  through  the  digestive  tract  as  in  any  other  way. 
We  shall  disregard  the  accessories,  and  consider  only  the  nutrients 
proper,  since,  as  we  have  seen,  the  digestive  process  concerns  it- 
self with  these  alone.  The  meal,  then,  is  a  mixture  of  carbo- 
hydrates, proteins,  albuminoids,  and  fats. 

In  the  mouth  the  food  is  reduced  to  a  semi-liquid  alkaline  mass, 
containing  no  large  particles,  by  the  combined  action  of  chewing 
and  mixing  with  the  saliva.  The  salivary  glands  are  reflexly 
excited  to  secrete  their  juice  by  the  presence  of  the  food  in  the 
mouth.  The  enzym  of  saliva,  ptyalin,  begins  its  digestive  action 
on  the  starch,  converting  it  to  maltose.  By  the  act  of  deglutition 
the  food,  when  sufficiently  mixed  with  saliva,  is  passed  on  to  the 
stomach.  If  the  chewing  and  swallowing  of  the  food  is  attended 


488  THE  HUMAN  BODY 

with  agreeable  emotions,  there  is  aroused  a  reflex  secretion  of 
gastric  juice;  the  so-called  "psychical"  secretion. 

The  food  enters  the  stomach  in  very  much  the  same  condition 
chemically  as  when  taken  into  the  mouth;  a  small  amount  of 
maltose  added  to  it  through  the  action  of  salivary  ptyalin,  and  a 
correspondingly  diminished  amount  of  starch,  being  the  only 
differences.  That  part  of  the  food  which  is  crowded  down  into 
the  pyloric  region  begins  at  once  to  be  churned  by  the  peristaltic 
waves  which  sweep  over  that  region;  by  the  churning  it  is  mixed 
with  gastric  juice.  The  food  which  remains  in  the  fundic  end  of 
the  stomach  does  not  come  into  contact  with  the  gastric  juice; 
its  reaction,  therefore,  continues  alkaline,  and  the  splitting  of 
starch  by  pytalin  goes  on  uninterruptedly.  In  the  portion  of 
food  (chyme)  which  becomes  impregnated  with  gastric  juice  there 
is  an  acid  reaction  and  the  changes  which  the  gastric  enzyms, 
pepsin,  and  rennin  are  capable  of  producing  take  place.  Rennin 
clots  any  milk  that  may  be  present;  pepsin  attacks  albuminoids 
and  proteins,  converting  them  into  proteoses  and  peptones.  Any 
fats  present  are  liquefied,  not  by  enzyms  but  by  the  stomach 
warmth.  Some  of  the  substances  produced  during  this  peptic 
digestion  react  with  other  substances  in  the  mucosa  of  the  pyloric 
region,  forming  a  hormone,  gastric  secretin.  This  hormone  is 
taken  up  by  the  blood,  passes  in  the  blood-stream  to  the  gastric 
glands,  and  stimulates  them  to  further  outpouring  of  juice;  thus 
enough  for  the  whole  meal  is  secured.  Finally  as  the  hydrochloric 
acid  of  the  gastric  juice  accumulates  in  excess  the  pyloric  sphincter 
is  stimulated  to  relax;  the  mass  of  chyme  next  to  it  is  pushed 
through;  and  more  material  from  the  fundic  end  comes  down  to 
fill  its  place.  Too  much  chyme  is  prevented  from  passing  the 
sphincter  at  once  by  the  powerful  stimulus  to  contraction  which  is 
exerted  on  the  sphincter  by  the  acid  chyme  in  contact  with  the 
upper  intestine.  The  acid  of  this  same  chyme  reacts  with  the 
prosecretin  of  the  intestinal  mucosa  to  form  secretin,  a  hormone 
which  is  carried  by  the  blood  to  the  pancreas  and  excites  it  to 
activity. 

The  chyme  which  enters  the  intestine  contains  some,  at  least,  of 
all  the  food  stuffs  originally  making  up  the  meal,  and  in  addition 
maltose,  proteose,  and  peptone.  The  strongly  alkaline  bile  and 
pancreatic  juice  quickly  neutralize  its  acid  and  the  various  en- 


77//<;  DKiKKTIYK  SECRETIONS  AND  TIIKUf  CUM'ltOL     481) 

zyms  of  the  intestinal  tract  act  upon  it.  The  amylopsin  of  the; 
pancreatic  juice  converts  to  maltose  all  starch  not  affected  by 
ptyalin;  the  lipase  of  the  same  secretion  splits  the  fats  to  fatty 
acid  and  glycerin;  the  trypsin  of  pancreatic  juice,  in  co-operation 
with  erepsin  of  the  succus  entericus  reduces  all  proteins,  includ- 
ing proteoses  and  peptones,  to  amino  acids;  the  inverting  enzyms, 
maltase,  sucrase,  and  lactase,  change  all  the  double  sugars,  and 
therefore  all  the  carbohydrates  of  the  meal,  to  single  sugars.  The 
intestinal  contents  are  churned  and  kept  in  onward  progress  by 
movements  of  segmentation  and  peristalsis  performed  by  the 
muscular  walls  of  the  gut. 

The  Maintenance  of  Good  Digestion.  In  the  preceding  par- 
agraph the  various  activities  essential  to  the  proper  performance 
of  the  digestive  function  have  been  outlined.  If  they  are  reviewed 
carefully  it  will  'be  seen  that  most  of  them,  after  the  food  reaches 
the  stomach,  are  affected,  directly  or  indirectly,  by  the  conditions 
upon  which  depend  the  proper  production  of  the  psychical  secre- 
tion of  gastric  juice.  If,  through  anxiety  or  anger  at  meal-time, 
this  secretion  is  inhibited,  the  whole  sequence  of  the  digestive 
process  is  upset.  Without  a  psychic  secretion  little  or  no  chemical 
secretion  of  gastric  juice  will  appear;  there  is  therefore  not  the 
necessary  hydrochloric  acid  to  stimulate  the  pyloric  sphincter  to 
relax,  nor  to  react  with  prosecretin  to  form  pancreatic  secretin, 
should  any  food  by  any  means  get  through  into  the  intestine. 
Moreover,  the  same  conditions  which  inhibit  the  psychical  secre- 
tion inhibit  also,  as  stated  previously,  the  motions  of  the  stomach 
and  intestines.  That  indigestion  usually  follows  the  eating  of 
meals  under  unfavorable  emotional  conditions  is  well  known  to 
all;  the  reason  for  it  we  have  just  seen.  Of  as  great  importance, 
though  not  so  generally  recognized,  is  that  the  psychical  secretion, 
and  hence  good  digestion,  depends  upon  an  active  emotional  state 
of  enjoyment  of  the  meal.  Preoccupation,  allowing  the  mind  to 
dwell  upon  business  or  household  cares,  may  interfere  witli 
the  digestive  processes  only  less  seriously  than  worry  or  angry 
discussion. 

The  value  of  soups  in  aiding  digestion  is  twofold.  By  exciting 
the  appetite  they  help  to  arouse  the  psychical  secretion;  their  con- 
tent of  meat  juice  is  itself  in  some  measure  an  excitant  of  the 
hormone  to  chemical  gastric  secretion,  thus  they  are  usually 


490  THE  HUMAN  BODY 

effective  in  starting  the  chain  of  events  which  make  up  the 
digestion  of  a  meal.  The  practice  of  using  them  at  the  be- 
ginning rather  than  elsewhere  in  the  meal,  although  long  ante- 
dating our  knowledge  of  their  real  value  is  thus  seen  to  be  physi- 
ologically sound. 


CHAPTER  XXX 
THE  ABSORPTION  AND  USE  OF  FOODS 

General  Statement.  The  digestive  process,  as  we  have  con- 
sidered it  in  preceding  chapters,  is  purely  one  of  preparation.  Its 
completion  finds  the  food  still  within  the  alimentary  tract,  but 
ready  for  the  use  of  the  Body.  It  is  conveyed 'to  the  tissues,  as  we 
have  seen  (Chap.  XVII),  by  the  blood.  The  passage  of  digested 
food  from  the  alimentary  tract,  through  its  walls,  into  the  blood 
or  lymph,  is  known  as  absorption.  The  use  of  the  food  by  the 
tissues,  since  it  involves  chemical  activities  on  the  part  of  the 
tissues  themselves,  is  spoken  of  as  metabolism.  The  discussion  of 
these  two  processes  is  the  purpose  of  the  present  chapter. 

Absorption  from  the  Stomach.  Although  the  food  remains  in 
the  stomach  for  several  hours  after  each  meal,  in  fact  is  often  not 
wholly  discharged  before  the  taking  of  another  one,  it  appears  that 
absorption  from  the  stomach  into  the  blood  normally  occurs  to  a 
very  limited  degree,  if  at  all.  The  fact  that  the  digestive  process 
is  for  no  foods  completed  in  the  stomach  affords  sufficient  reason 
why  absorption  should  not  take  place  there.  We  might  suppose 
that  the  single  great  group  of  food-stuffs  not  requiring  digestion, 
the  single  sugars,  could  advantageously  be  absorbed  from  the 
stomach,  but  experiment  shows  that  even  they  are  absorbed  very 
slightly  unless  in  rather  high  concentration,  5  per  cent,  in  which 
case  the  walls  of  the  stomach  do  take  them  up  rather  rapidly.  The 
presence  of  alcohol  in  the  stomach  is  said  to  increase  markedly  its 
absorptive  power,  but  this  is  at  best  a  doubtful  benefit,  since  the 
single  sugars  form  ordinarily  a  minor  part  of  the  meal,  and  the 
other  food-stuffs  are  not  ready  for  the  use  of  the  Body,  and  are 
wasted,  therefore,  if  they  are  absorbed. 

Absorption  in  the  Small  Intestine.  The  small  intestine,  being 
the  chief  and  final  digestive  laboratory  of  the  Body,  is  naturally 
the  place  from  which  absorption  most  largely  goes  on.  It  is,  in 
fact,  specially  adapted  structurally,  as  is  no  other  region  of  the 

491 


492  THE  HUMAN  BODY 

alimentary  tract,  for  the  absorptive  processes.  The  innumerable 
projecting  villi,  each  containing  a  capillary  network  and  a  lymph- 
channel,  afford  a  total  absorbing  surface  many  times  greater  than 
would  the  same  area  if  lined  with  ordinary  mucous  membrane; 
they  also,  by  projecting  into  the  intestinal  cavity,  are  brought 
more  readily  into  intimate  contact  with  the  intestinal  contents. 

Nature  of  the  Absorptive  Process.  There  is  very  good  reason 
to  believe  that  the  process  of  absorption  is  not  a  simple  physical 
one,  involving  only  filtration,  osmosis,  and  dialysis,  but  that  it  is 
carried  on  actively  by  the  living  cells  which  form  the  innermost 
intestinal  lining,  the  columnar  epithelium  (Chap.  XXVI).  The 
support  for  this  idea  is  chiefly  experimental:  the  observation  that 
blood-serum  placed  in  the  intestine  is  absorbed  completely  through 
its  walls  into  the  blood  so  long  as  the  mucous  lining  is  alive  and 
functioning,  but  fails  to  be  absorbed  if  the  cells  are  injured,  as  by 
sodium  fluorid,  or  some  similar  poison.  Since  the  blood-serum 
placed  in  the  intestine  has  presumably  precisely  the  same  osmotic 
pressure  and  percentage  composition  as  the  animal's  own  it  is 
difficult  to  see  how  purely  physical  factors  could  bring  about  the 
absorption. 

Channels  of  Absorption.  We  noted  above  that  each  villus 
contains  a  capillary  network  and  a  lymph-channel.  The  absorbed 
food  stuffs  might  pass,  therefore,  either  to  the  blood-stream  directly 
or  by  the  lymph-channels  be  conveyed  to  the  receptaculum  chyli 
(p.  382),  and  thence  by  way  of  the  thoracic  duct  enter  the  blood- 
stream at  the  great  vein  of  the  shoulder.  The  essential  difference 
between  these  two  pathways  is  that  the  intestinal  blood-stream 
drains  into  the  portal  vein,  and  must  pass,  therefore,  through  the 
capillaries  of  the  liver  before  reaching  the  general  circulation,  while 
the  lymph-stream  reaches  the  general  circulation  without  first 
traversing  the  liver.  The  significance  of  these  two  pathways  will 
appear  presently. 

The  entire  phenomenon  of  absorption  from  the  small  intestine 
presents  so  many  phases  that  it  will  be  convenient  to  consider  it  in 
sections,  one  class  of  nutrients  at  a  time. 

The  Absorption  and  Temporary  Storage  of  Carbohydrates. 
Carbohydrate  digestion  reduces  all  foods  of  the  class  to  single 
sugars.  It  is  in  this  form,  then,  that  they  undergo  absorption. 
However  the  process  may  be  carried  on  it  results  in  a  flow  of  single 


THE  ABSORPTION  AND  USE  OF  FOODS  493 

sugars  from  the  intestinal  cavity  into  the  blood-capillaries  of  the 
villi.  These  capillaries  all  drain,  as  previously  stated  (Chap.  XIX), 
into  the  portal  vein,  which  in  turn  passes  to  the  liver  and  breaks  up 
therein  into  the  liver-capillaries  (Chap.  XXVI) ;  so  that  all  blood 
from  the  intestine,  with  whatever  it  may  have  taken  up  there,  is 
forced  to  traverse  the  liver,  and  to  come  into  intimate  contact  with 
the  liver-cells,  before  it  reaches  any  of  the  other  living  tissues  of 
the  Body. 

The  amount  of  sugar  present  in  the  blood  of  the  portal  vein  is,  of 
course,  variable,  there  being  a  higher  concentration  at  times  when 
sugar  is  being  actively  absorbed  from  the  intestine  than  at  other 
times.  Curiously,  the  blood  flowing  away  from  the  liver,  in  the 
hepatic  vein,  is  always  found,  normally,  to  contain  a  certain  small 
percentage,  about  0.15  per  cent,  of  sugar,  whether  the  sugar  con- 
tent of  the  portal  vein  is  high  or  low. 

It  is  evident  that  the  liver  must  be  able  to  store  within  itself  the 
excess  sugar  that  comes  to  it  during  active  absorption  from  the 
intestine,  and  to  give  this  out  again  between  times.  The  sugar 
is  retained  in  the  liver,  not  as  such,  but  in  the  form  of  glycogen  or 
animal  starch.  The  conversion  of  sugar  into  glycogen  is  a  simple 
dehydration  (C6Hi2O6— H2O  =  C6Hi0O&),  and  is  doubtless  easily  ef- 
fected by  the  liver-cells.  The  purpose  of  the  change  from  sugar 
to  starch  seems  to  be  to  make  the  retention  by  the  liver  easier; 
sugar  is  too  soluble  to  be  held  readily,  whereas  the  liver  can  hold 
the  glycogen  without  trouble.  The  liver  is  said  to  be  able  to  hold 
10  per  cent  of  its  weight  of  glycogen. 

The  use  of  the  sugar  is,  as  we  have  already  seen,  for  fuel  for  the 
Body.  Oxidations  are  constantly  going  on  in  the  living  tissues, 
therefore  there  is  a  steady  withdrawal  of  sugar  from  the  blood, 
and  the  liver  must  be  continually  making  good  the  depletion  by 
reconverting  some  of  its  glycogen  into  sugar.  That  the  sugar 
content  of  the  blood  is  kept  up  at  the  expense  of  liver-glycogen  is 
proven  by  observations  on  fasting  animals.  A  comparatively 
short  period  of  starvation  results  in  the  complete  disappearance 
of  glycogen  from  the  liver.  That  in  fact  is  the  first  fuel  supply  to 
be  drawn  upon  in  the  absence  of  food. 

Just  how  the  chemical  process  of  converting  glycogen  to  sugar 
is  performed  is  not  certain;  although  an  enzym  capable  of  effect- 
ing the  transformation  is  said  to  be  present  in  the  liver.  If  the 


494  THE  HUMAN  BODY 

process  is  carried  on  by  an  enzym  it  is  under  closer  control  than 
the  enzym  reactions  we  have  studied  in  connection  with  digestion, 
for  it  does  not  go  on  rapidly  till  all  the  glycogen  is  used  up,  but 
only  so  fast  as  is  necessary  to  make  good  the  loss  of  sugar  from 
the  blood. 

Storage  of  Glycogen  in  the  Muscles.  These  organs,  as  we 
learned  when  studying  them  (Chap.  VII),  perform  their  work 
through  the  oxidation  of  sugar,  and  since  they  are  likely  to  be 
called  upon  for  prolonged  activity  need  to  have  immediately  avail- 
able a  supply  of  their  special  fuel.  Such  a  supply  they  have,  in 
the  form  of  glycogen,  whi.ch  makes  up  about  1  per  cent  of  the 
weight  of  muscle  tissue.  This  glycogen  is,  of  course,  derived  from 
the  sugar  of  the  blood,  so  that  the  muscle-cells  must  have  the  same 
power  that  liver-cells  have  of  changing  sugar  to  glycogen  and 
glycogen  back  to  sugar. 

The  Relation  of  the  Kidney  to  the  Concentration  of  Sugar  in  the 
Blood.  As  we  have  seen,  the  sugar  content  of  the  blood  remains 
practically  constant  all  the  time  at  a  relatively  low  concentration, 
about  0.15  per  cent.  It  is  an  interesting  fact  that  the  kidney,  the 
great  excretory  organ  of  the  Body,  is  so  constructed  that  if  for  any 
reason  the  sugar  content  of  the  blood  rises  much  above  normal, 
to  0.2  per  cent  or  more,  the  excess  of  sugar  is  withdrawn  from  the 
blood  by  the  kidney  and  appears  in  the  urine.  The  kidney  stands 
to  the  sugar  of  the  blood  in  the  relation  of  a  spillway;  it  allows  the 
concentration  to  rise  just  so  high,  but  no  higher.  This  property 
of  the  kidney  makes  such  a  storage  mechanism  for  sugar  as  we 
have  described  virtually  necessary  to  the  Body,  since  without  it 
the  tissues  could  not  be  provided  with  fuel  at  once  continuously 
and  economically. 

Just  why  the  kidney  should  have  this  function  is  not  very  clear, 
but  a  suggestion  is  found  in  the  observation  that  the  continued 
presence  of  excess  sugar  in  the  tissue  fluids,  as  in  diabetes  (p.  496)  is 
inimical  to  the  highest  welfare  of  the  tissues. 

The  Assimilation  Limit.  Alimentary  Glycosuria.  The  ability 
of  the  liver  to  convert  into  glycogen  the  sugar  delivered  to  it  by 
the  portal  vein  is  not  without  limit.  If  the  absorption  from  the 
intestine  is  so  rapid  as  to  raise  the  sugar  content  of  the  portal 
blood  to  an  abnormally  high  point,  the  liver  is  not  able  to  handle 
all  the  sugar;  and  the  excess  escapes  into  the  hepatic  vein  and  so 


THE  ABSORPTION  AND  USE  OF  FOODS  495 

into  the  general  circulation.  Should  this  excess  be  sufficient  to 
raise  the  sugar  percentage  of  the  blood  above  0.2  per  cent  there  is 
excretion  of  sugar  from  the  kidney,  a  condition  known  as  gly- 
cosuria.  It  is  found  that  the  rate  of  absorption  of  sugar  depends 
chiefly  on  how  much  of  it  is  present  at  one  time  in  absorbable 
form  in  the  intestine.  Thus  if  large  amounts  of  single  sugar  are 
eaten  the  essential  condition  for  excessive  absorption  is  likely  to 
be  fulfilled.  Honey,  a  sweet  containing  considerable  single  sugar, 
is  thus  apt  to  cause  glycosuria  if  too  freely  eaten.  The  greatest 
amount  that  can  be  eaten  without  causing  glycosuria  marks  the 
assimilation  limit.  The  other  carbohydrates,  since  they  require 
digestion  before  they  are  absorbed,  are  less  apt  to  give  rise  to  too 
rapid  absorption.  It  is  found,  however,  that  there  is  a  great  dif- 
ference in  the  amounts  that  can  be  taken  without  exceeding  the 
assimilation  limit.  The  inversion  of  milk-sugar  gives  rise  to  a 
special  single  sugar,  galactose,  which  is  converted  into  glycogen 
very  slowly.  The  assimilation  limit  for  milk-sugar  is  correspond- 
ingly low.  Starch  is  digested  so  slowly  that  the  assimilation  limit 
for  it  is  quite  difficult  to  exceed.  Glycosuria  resulting,  not  from 
disease,  but  merely  from  overconsumption  of  carbohydrates,  is 
called  alimentary  glycosuria. 

Other  Types  of  Glycosuria.  An  analysis  of  the  carbohydrate- 
storage  mechanism  just  described  reveals  three  points  where  an 
upset  of  the  normal  sequence  might  give  rise  to  glycosuria;  and 
three  corresponding  varieties  are  known.  The  three  conditions 
which  may  cause  glycosuria  are:  (1)  A  disturbance  of  the  mech- 
anism which  controls  the  rate  of  conversion  of  liver-glycogen 
into  sugar,  so  that  more  is  poured  into  the  blood  than  the  tissues 
are  able  to  use;  (2)  a  diminution  in  the  consumption  of  sugar  by 
the  tissues,  so  that  more  accumulates  than  the  liver  can  store; 
(3)  an  alteration  of  the  kidney  such  that  it  excretes  all  the  sugar 
that  comes  to  it,  and  thus  drains  sugar  from  the  blood  continuously. 
Much  insight  into  the  working  of  the  carbohydrate-storing  mech- 
anism, as  well  as  the  use  of  carbohydrates  by  the  Body,  has 
been  gained  by  study  of  these  three  forms  of  glycosuria. 

Glycosuria  from  Disturbance  of  the  Liver  Function.  Emotional 
Glycosuria.  It  has  been  shown  that  injury  to  a  definite  point  in 
the  medulla  destroys  the  co-ordination  between  the  output  of 
sugar  from  the  liver  and  the  use  of  sugar  by  the  tissues,  with 


496  THE  HUMAN  BODY 

resulting  glycosuria.  This  suggests,  of  course,  that  the  liver 
carries  on  its  function  of  storing  and  delivering  sugar  under  the 
control  of  a  reflex  " center."  Such  a  method  of  control  seems 
reasonable  inasmuch  as  increased  activity  of  the  tissues  involves 
increased  consumption  of  sugar,  with  a  greater  call  upon  the  liver 
for  supplies,  and,  as  we  know,  the  tissues  most  involved,  the 
muscles,  send  into  the  medulla  streams  of  afferent  impulses  when- 
ever they  are  active,  which  would  serve  to  excite  the  center.  In 
corroboration  of  this  idea  it  may  be  stated  that  certain  diseases 
of  the  central  nervous  system  in  man  result  in  an  upset  of  the  liver 
function  of  precisely  this  sort. 

Recent  observations  have  brought  out  the  interesting  fact  that 
this  "nervous  control"  of  the  conversion  of  glycogen  to  sugar  by 
the  liver,  is  not  direct,  but  operates  through  the  intervention  of 
the  hormone  adrenin.  Some  time  ago  the  discovery  was  made 
that  during  great  emotional  excitement  sugar  is  apt  to  appear  in 
the  urine.  A  test  was  made  recently  on  the  members  of  the  foot- 
ball squad  of  a  great  university  immediately  following  the  crucial 
game  of  the  year.  Of  the  men  examined,  players  and  substitutes, 
nearly  all  showed  pronounced  glycosuria.  This  fact,  in  conjunc- 
tion with  the  known  outpouring  of  adrenin  in  times  of  stress,  sug- 
gested a  causal  relationship,  and  the  demonstration  was  shortly 
afforded  that  the  increased  production  of  sugar  from  liver  glycogen 
is  the  result  of  stimulation  by  the  hormone.  This  we  recognize  at 
once  as  a  part,  and  an  important  part,  of  the  general  emergency 
reaction  of  the  Body.  At  a  time  when  the  utmost  muscular  exer- 
tion is  likely  to  be  demanded  it  is  imperative  that  there  be  no  fail- 
ure from  a  shortage  of  fuel.  The  flooding  of  the  blood  with 
sugar  as  the  result  of  the  outpouring  of  adrenin  assures  that  the 
fuel  supply  for  the  laboring  muscles  shall  be  ample.  That  there  is 
an  overproduction,  so  that  much  passes  out  by  the  kidneys  and  is 
wasted,  merely  emphasizes  the  general  principle  that  in  time  of 
emergency  the  Body  scorns  economy,  directing  its  resources 
lavishly  toward  successful  meeting  of  the  immediate  situation. 

Glycosuria  from  Inability  of  the  Tissues  to  Use  Sugar.  Di- 
abetes Mellitus.  This  condition,  the  usual  pathological  cause  of 
glycosuria,  and  unfortunately  not  of  rare  occurrence,  has  been 
much  studied,  chiefly  because  it  involves  the  relation  of  the  tissues 
to  their  chief  fuel  supply,  sugar,  and  a  complete  understanding  of 


THE  ABSORPTION  AND  USE  OF  FOODS  497 

the  disease  should  throw  much  light  on  the  mechanism  of  the 
consumption  of  fuel  by  them.  The  presence  of  sugar  in  the  urine 
is  only  one  of  the  symptoms  of  diabetes  mellitus.  A  symptom  of 
equal  importance  is  the  muscular  weakness,  and  particularly  the 
lack  of  endurance,  which  results  from  the  failure  of  the  tissues  to 
make  use  of  their  fuel  supply  to  advantage. 

A  very  interesting  feature  of  this  condition  is  that  it  can  be  in- 
duced experimentally  in  a  quite  unexpected  way,  namely,  by  in- 
juring or  removing  the  pancreas.  Complete  destruction  of  this 
organ  is  followed  by  an  apparent  total  loss  of  the  power  of  the 
tissues  to  use  sugar;  there  is  excessive  muscular  weakness,  and 
death  occurs  in  a  few  days  after  the  operation.  The  effects  of 
partial  destruction  are  less  severe;  in  fact  no  symptoms  appear 
unless  fully  three-fourths  of  the  gland  are  destroyed.  The  func- 
tion of  the  gland  in  connection  with  the  prevention  of  diabetes  is 
wholly  independent  of  its  function  as  a  digestive  gland.  The 
duct  of  the  pancreas  may  be  tied  without  the  production  of  dia- 
betes, or  the  gland  may  be  transplanted  from  its  usual  location 
to  some  other,  quite  abnormal  one,  where,  if  it  lives  and  estab- 
lishes connections  with  the  circulation,  it  suffices  to  prevent 
diabetes  perfectly. 

The  interpretation  of  this  function  of  the  pancreas  is  that  it  is 
a  hormone  action.  The  gland  produces  the  hormone,  and  this, 
when  carried  by  the  blood  to  the  tissues,  in  some  way  enables 
them  to  use  sugar;  perhaps  by  activating  some  tissue  enzym  or 
enzyms  upon  which  the  oxidation  of  sugar  depends.  It  is  not 
thought  that  the  ordinary  secreting  cells  of  the  pancreas  produce 
the  hormone,  but  that  certain  peculiar  groups  of  cells  embedded 
in  the  gland,  the  Islands  of  Langerhans,  have  this  function.  Al- 
though not  all  physiologists  agree  in  assigning  the  production  of 
the  hormone  to  the  Islands  of  Langerhans,  the  general  trend  of 
opinion  seems  to  be  that  that  is  their  function. 

Diabetes  mellitus  in  man  not  only  shows  symptoms  agreeing 
precisely  with  those  seen  in  animals  with  injuries  to  the  pancreas, 
but  many  cases  show  on  autopsy  very  well  marked  lesions  of  the 
Islands  of  Langerhans.  We  may  thus  conclude  with  fair  cer- 
tainty that  the  disease  is  one  affecting  these  Islands,  and  that  its 
symptoms  are  the  result  of  more  or  less  complete  failure  of  the 
hormone  formed  by  them. 


498  THE  HUMAN  BODY 

Glycosuria  from  Increased  Permeability  of  the  Kidney-Cells  to 
Sugar.  The  injection  of  a  certain  drug,  phlorhizin,  into  the  cir- 
culation is  followed  by  a  glycosuria  which  is  due  chiefly,  although 
probably  not  wholly,  to  alterations  in  the  kidney.  These  are  of 
such  a  sort  that  the  kidney-cells,  instead  of  removing  only  sugar 
in  excess  of  0.2  per  cent,  take  all  that  comes  to  them.  The  result, 
of  course,  is  a  great  waste  of  this  valuable  fuel,  requiring  greatly 
increased  consumption  of  carbohydrates  to  make  it  good.  This 
form  of  glycosuria  has  been  produced  experimentally  in  animals, 
for  purposes  of  study,  but  occurs  rarely,  if  at  all,  as  a  disease  of 
man. 

The  Absorption  of  Proteins.  We  have  learned  that  the  digest- 
ive process  splits  proteins  into  their  constituent  amino  acids 
(p.  466).  The  advantage  of  this  is  obvious,  when  we  recall  the 
fact  that  an  important  function  of  proteins  is  to  repair  tissue  waste, 
and  the  further  fact  that  to  do  this  the  food  protein  must  be  con- 
verted into  the  characteristic  tissue  protein  of  which  it  becomes  a 
part.  We  saw  in  Chap.  I  (p.  11),  that  the  difference  between 
one  protein  and  another  is  in  the  number,  proportions,  or  arrange- 
ment of  the  amino  acids  which  make  up  their  molecules.  While 
the  food  proteins,  as  such,  would  not  serve  for  tissue  repair,  the 
amino  acids  of  which  they  are  composed  are  precisely  what  the 
Body  needs  for  rebuilding  its  own  substance.  Furthermore,  the 
different  tissues  must  differ  somewhat  in  the  constitution  of  their 
characteristic  proteins,  and  for  the  repair  of  all  the  different 
tissues  a  mixture  of  amino  acids  is  evidently  much  more 
useful  than  a  small  number  of  undigested  food  proteins  could 
possibly  be. 

There  is  abundant  evidence  that  the  digested  amino  acids  are 
absorbed  directly  into  the  blood-stream,  and  not  into  the  lacteals. 
This  has  been  proven  by  inserting  a  tube  into  the  thoracic  duct  of 
an  animal  and  draining  off  all  the  lymph  produced  during  the 
absorption  of  a  protein-rich  meal.  No  increase  in  the  percentage 
of  nitrogen  (the  characteristic  element  of  proteins)  could  be  de- 
tected in  the  lymph;  conclusive  proof  that  the  amino  acids  do  not 
follow  that  pathway.  Moreover,  chemical  methods  recently  de- 
vised have  proven  the  presence  of  amino  acids  in  the  blood-stream, 
and  that  during  the  absorption  of  a  meal  of  meat,  they  are  in- 
creased in  amount.  The  use  the  Body  makes  of  these  amino  acids 


THE  ABSORPTION  AND  USE  OF  FOODS  499 

will  be  considered  in  detail  in  a  later  paragraph,  as  will  also  the 
relation  of  the  liver  to  them. 

The  Absorption  of  Fats.  The  result  of  fat  digestion  is  to  split 
the  fats  to  fatty  acid  and  glycerin.  It  is  believed  that  they  are 
taken  up  by  the  cells  of  the  intestinal  lining  partly  in  this  form; 
but  not  wholly  so,  since  free  fatty  acid  in  the  presence  of  free 
alkali,  such  as  is  furnished  by  the  bile  and  pancreatic  juice,  reacts 
with  the  alkali  to  form  soap.  That  there  is  in  the  small  intestine 
a  certain  amount  of  soap  formation  cannot  be  doubted.  The  ad- 
vantage of  soap  formation  is  one  of  increased  solubility;  fatty 
acids  are  insoluble  in  water,  soap  quite  soluble.  There  is  reason 
to  believe,  however,  that  only  part  of  the  fatty  acid  is  combined 
into  soap,  and  that  the  remainder  is  absorbed,  as  stated  above, 
as  fatty  acid.  This  direct  fatty  acid  absorption  seems  to  be  ef- 
fected largely  through  the  agency  of  the  bile.  It  is  known  that 
fatty  acids  are  soluble  in  bile,  and  can  thus  be  brought  in  solution 
into  contact  with  the  absorbing  cells;  and  a  very  common  observa- 
tion of  physicians  is  that  stoppage  of  the  flow  of  bile  into  the  in- 
testine, as  by  occlusion  of  the  bile-duct,  is  followed  by  an  almost 
complete  failure  of  fat  absorption.  The  glycerin  part  of  the  de- 
composed fat  is  quite  soluble  in  water  and  is  doubtless  absorbed 
readily. 

After  the  absorbing  cells  of  the  intestinal  wall  have  taken  up 
the  fatty  acid  and  glycerin,  these  are  recombined  within  the  cells 
into  fat.  The  presence  of  fat  droplets  in  the  absorbing  cells  can  be 
demonstrated  microscopically.  We  know  that  the  fat  droplets  are 
not  absorbed  as  such,  but  are  formed  after  their  constituents  have 
been  separately  taken  up,  because  these  fat  droplets  are  always 
observed  in  the  part  of  the  cells  away  from  the  intestinal  cavity, 
and  never  in  the  part  next  to  it;  also  because  we  know  that  the 
digestive  splitting  to  acid  and  glycerin  takes  place,  a  meaning- 
less process  if  not  necessary  to  absorption. 

The  fat  finds  its  way  into  the  circulation  by  way  of  the  lymph- 
channels  of  the  villi,  the  lacteals,  and  the  thoracic  duct,  entering 
the  blood-stream  at  the  point  of  emptying  of  the  thoracic  duct  in 
the  large  vein  of  the  shoulder.  The  fats  alone,  of  all  the  food 
stuffs,  take  this  course,  and  we  may  suppose  the  difference  to 
mean  that  the  liver  has  no  special  function  to  carry  out  in  con- 
nection with  the  fats  as  it  has  for  carbohydrates  and  proteins. 


500  THE  HUMAN  BODY 

Therefore  the  fats  are  shunted  into  another  course  which  carries 
them  into  the  blood  stream  without  having  first  to  traverse  the  liver. 

Absorption  from  the  Large  Intestine.  The  mass  that  passes 
through  the  ileocolic  valve  into  the  large  intestine  contains  com- 
paratively little  absorbable  food  material.  The  carbohydrates  and 
fats  are  very  completely  removed  during  the  passage  of  the  small 
intestine,  and  fully  ninety  per  cent  of  the  proteins  as  well.  There 
remains  for  the  large  intestine,  then,  only  the  absorption  of  the 
protein  residue  and  the  absorption  of  water.  It  is  probable  that 
this  latter  function,  that  of  absorbing  water,  is  in  reality  the  chief 
one  possessed  by  the  large  intestine.  There  is  virtually  no  ab- 
sorption of  water  in  the  small  intestine;  the  intestinal  contents 
pass  the  ileocolic  valve  as  liquid  as  when  leaving  the  stomach. 
This  maintenance  of  a  liquid  consistency  is,  of  course,  essential  to 
the  absorptive  processes,  and  it  is  only  after  all  absorbable  food 
has  been  removed  that  the  water,  which  is  also  needed  by  the 
Body,  is  taken  up. 

The  Food  Requirement  of  the  Body.  If  we  know  how  much 
energy  the  Body  liberates  in  a  day,  and  how  much  tissue  break- 
down it  suffers,  we  ought  to  be  able  to  estimate  how  much  energy- 
yielding  food,  and  how  much  tissue-repair  food  is  required  daily; 
assuming,  of  course,  that  we  know  the  amount  of  energy  yielded 
by  definite  weights  of  food  stuffs.  By  the  use  of  devices  called 
calorimeters  the  total  energy  liberation  of  the  Body  per  day  has 
been  determined  under  various  conditions,  and  the  energy  content 
of  the  various  foods  has  also  been  found.  We  learned  in  a  previous 
chapter  (p.  107)  that  a  unit  of  heat  energy  commonly  used  in 
physiology  is  the  Calorie;  the  amount  of  heat  required  to  raise 
1,000  grams  of  water  through  1°  centigrade.  In  terms  of  this 
unit  the  energy  output  of  man  in  24  hours  averages  from  about 
2,400  Calories  for  men  of  sedentary  occupation  to  5,000  Calories 
for  those  doing  heavy  manual  labor.  The  energy  yield  of  the 
various  foods  is  as  follows: 

Carbohydrates 4. 1  C.  per  gram. 

Proteins 4. 1  C.    "      " 

Fats 9.3C.    "      " 

It  is  therefore  a  matter  of  simple  calculation  to  determine  how 
much  of  any  one  food  stuff  is  needed  to  supply  the  required  en- 


THE  ABSORPTION  AND  USE  OF  FOODS  501 

ergy,  or  to  arrange  suitable  mixtures  of  the  three.  By  reference 
to  the  table  of  food  compositions  (Chap.  XXV),  the  amounts  of 
actual  food  materials  needed  can  be  found. 

The  Protein  Requirement  of  the  Body.  Before  proceeding  with 
a  further  discussion  of  the  energy  relationships  of  the  Body  it  will 
be  well  to  consider  the  tissue-maintenance  requirement,  which  as 
we  have  seen,  is  wholly  a  protein  need.  In  order  to  analyze  this 
requirement  intelligently  we  need  to  know,  first  of  all,  what  use 
the  Body  makes  of  protein,  and  second,  how  much  is  required. 
We  have  already  seen  that  protein  can  be  oxidized  in  the 
Body  with  the  liberation  of  energy,  and  constitutes,  therefore,  a 
good  fuel.  In  this  respect,  however,  it  is  in  no  degree  superior 
to  the  other  nutrients,  fats  and  carbohydrates.  Our  special  in- 
terest in  it  is  for  the  function  which  it  alone  can  exercise,  that  of 
making  good  tissue  wear  and  tear. 

Since  protein  is  the  only  food  stuff  that  contains  nitrogen  we 
can  tell  how  much  of  it  is  used  up  in  the  Body  by  measuring  the 
amount  of  nitrogen  eliminated  (p.  512).  All  except  a  very  small 
portion  (roughly  2  per  cent)  is  discharged  in  the  urine.  Chemical 
tests  of  the  urine  will  furnish  us,  then,  with  the  data  we  seek. 
Evidently  if  a  man  abstains  wholly  from  food  for  a  while  all  the 
nitrogen  in  his  urine  must  come  from  tissue  break-down.  We  have 
a  means,  thus,  of  finding  out  how  rapidly  this  break-down  occurs. 
A  moment's  thought  will  show  us,  however,  that  the  tissue  break- 
down in  complete  starvation  is  not  necessarily  the  same  as  in 
ordinary  life.  When  no  food  is  eaten  the  energy  requirements  of 
the  Body  must  be  met  at  the  expense  of  its  own  tissues;  particularly 
in  prolonged  starvation,  when  the  stored  fuels,  fat  and  glycogen 
have  been  used  up;  so  that  in  addition  to  the  usual  loss  of  sub- 
stance by  wear  and  tear  there  is  a  further  consumption  of  ma- 
terial as  fuel.  To  get  at  the  amount  of  tissue  break-down  under 
ordinary  conditions  by  this  method  the  starvation  must  not  be 
complete.  The  subject  must  be  given  abundant  supplies  of  fat, 
carbohydrates,  and  essential  accessories,  but  no  proteins.  When 
this  is  done  the  nitrogen  eliminated  from  the  Body  can  be  as- 
sumed to  represent  the  normal  tissue  break-down.  Experiments 
conducted  along  this  line  have  shown  that  in  an  adult  man  of 
ordinary  size  (70  kilos,  165  Ibs.)  the  protein  lost  from  the  Body 
daily  by  tissue  wear  and  tear  amounts  to  about  20-25  grams 


502  THE  HUMAN  BODY 

(|-j?  oz.).  Theoretically  it  should  be  possible  to  sustain  life 
indefinitely  on  a  diet  containing  this  amount  of  protein,  provided 
adequate  fats  and  carbohydrates  for  the  fuel  requirements  of  the 
Body  are  also  furnished.  This  theoretical  minimum  of  protein 
does  not  agree  at  all  well  with  the  amounts  of  protein  actually 
consumed.  In  fact  dietary  studies  show  that  most  people  take 
four  to  five  times  this  amount.  The  great  discrepancy  between 
the  amount  of  protein  theoretically  required  and  that  actually 
ingested  has  occasioned  a  great  deal  of  discussion  among  dietitians 
as  to  whether  the  human  race  is  habitually  consuming  proteins 
to  excess.  Since  the  proteins  are  the  most  expensive  food  stuffs 
the  question  is  one  of  great  economic  importance. 

Numerous  experiments  have  been  performed  to  see  what  is 
the  effect  of  cutting  the  protein  intake  approximately  to  the 
theoretical  minimum.  In  all  the  early  experiments  along  this 
line  when  the  protein  content  of  the  diet  was  reduced  to  about 
35-40  grams  daily,  a  figure  well  above  the  theoretical  minimum, 
the  tissue  wear  and  tear  was  no  longer  made  good  completely. 
This  was  evidenced  by  the  daily  elimination  from  the  Body  of 
more  nitrogen  than  was  taken  in.  The  only  possible  source  of 
the  excess  was  from  break-down  of  the  Body's  own  tissues.  There 
was  also  a  steady  loss  of  body  weight.  When  an  explanation  of 
this  was  sought  it  was  found  that  the  chief  difficulty  lay  in  the 
manner  of  administering  the  protein.  This  substance  when  taken 
with  the  diet  in  the  usual  rather  large  amounts  functions  in  part 
to  replace  worn-out  tissues  and  in  part,  as  we  have  seen,  as  fuel 
for  supplying  energy,  the  latter  use  being  made  of  all  surplus  after 
the  tissue  repair  has  been  provided  for.  The  wear  and  tear  of 
tissues  goes  on  throughout  the  twenty-four  hours  of  the  day.  The 
ordinary  method  of  taking  the  protein,  on  the  other  hand,  is  in 
three  meals  during  the  daytime  portion  of  the  day.  When  the 
consumption  is  cut  down  to  a  low  level  evidently  if  there  is  any 
use  of  protein  as  fuel  a  shortage  for  tissue  repair  is  likely  to  occur. 
When  the  protein  is  ingested  in  connection  with  the  usual  meals 
there  is  absorbed  into  the  Body  after  each  an  amount  which  is  in 
excess  of  the  immediate  needs  for  tissue  repair,  and  accordingly 
some  is  used  as  fuel.  The  result  is  that  before  the  time  for  the 
next  meal  arrives  all  the  absorbed  protein  is  used  up  and  there  is 
none  to  carry  on  the  work  of  tissue  repair.  To  avoid  this  the 


THE  ABSORPTION  AND  USE  OF  FOODS  503 

experiment  was  tried  of  dividing  the  protein  into  six  equal  parts 
and  administering  one  part  every  four  hours.  When  this  was 
done,  so  that  there  was  a  practically  continuous  though  small 
absorption  of  protein  into  the  Body,  it  was  found  possible  to  make 
good  completely  the  tissue  break-down  with  an  amount  of  protein 
very  little  in  excess  of  the  theoretical  requirement. 

The  Replacement  Value  of  Different  Proteins.  Obviously  to 
make  good  the  wear  and  tear  of  the  tissues  with  the  least  possible 
amount,  the  proteins  ingested  must  correspond  as  closely  as  pos- 
sible in  composition  with  those  of  the  Body  itself.  We  have 
already  noted  (p.  11)  that  the  differences  between  different  pro- 
teins are  in  the  number  or  relative  amounts  of  amino  acids  present. 
These  differences  are  in  some  cases  very  pronounced.  In  general 
meat  proteins  resemble  those  of  man  more  closely  than  do  pro- 
teins of  vegetable  origin.  Any  protein  that  contains  only  a  small 
proportion  of  some  amino -acid  that  is  present  in  human  protein 
in  large  proportion  must  evidently  be  fed  in  sufficient  amount  to 
satisfy  the  requirement  for  that  particular  amino  acid.  The  other 
constituents,  meanwhile,  are  in  excess  and  afford  a  surplus  to  be 
used  as  fuel.  It  appears  that  the  Body  possesses  a  limited  ability 
to  convert  some  kinds  of  amino  .acids  into  others,  but  this  is  ap- 
plicable to  so  few  of  the  many  which  make  up  the  protein  molecule 
as  to  have  little  practical  bearing. 

Maintenance  Proteins  and  Growth  Proteins.  Evidently,  from 
what  has  been  said  above,  any  protein  that  is  completely  lacking 
in  some  essential  amino  acid  or  acids  cannot  serve  to  replace  worn- 
out  tissues.  Such  a  protein  is  ordinary  table  gelatin.  This  is  a 
protein  derived  from  bone  and  connective  tissue  (p.  49).  It  is 
deficient  in  three  of  the  amino  acids  which  are  essential  to  living 
protein  (tryptophan,  tyrosin,  and  cystein).  No  matter  how  much 
gelatin  may  be  included  in  the  diet,  if  there  is  not  provided  also 
some  protein  which  contains  these  essential  acids  there  will  be  a 
wasting  of  the  tissues. 

Related  to  this  fact  is  the  even  more  remarkable  discovery  that 
there  are  certain  proteins  which  are  fully  adequate  for  the  main- 
tenance of  the  Body,  but  will  not  suffice  for  the  formation  of  new 
tissues.  Young  animals  fed  upon  diets  whose  protein  components 
are  of  this  character  will  maintain  a  constant  weight,  but  will 
not  grow.  A  good  example  of  such  a  protein  is  gliadin,  one  of  the 


504  ,  THE  HUMAN  BODY 

proteins  of  wheat.  This  protein  lacks  the  amino  acid  lysin.  The 
conclusion  drawn  from  this  observation  is  that  the  lysin  which  is 
one  of  the  constituents  of  living  protein  is  not  involved  in  the 
processes  of  tissue  break-down.  After  the  tissue  is  once  formed, 
therefore,  it  does  not  require  continual  supplies  of  this  substance. 
No  new  tissue  can  be  made,  however,  unless  lysin  is  provided.  A 
curious  incidental  discovery  in  connection  with  the  experiments 
by  which  this  was  established  was  that  the  rats  which  were  used 
as  subjects  could  be  maintained  in  health  with  the  weight  and 
bodily  dimensions  of  young  animals  for  months  after  they  would 
have  become  full  grown  on  an  ordinary  diet.  If  then  they  were 
changed  from  maintenance  proteins  to  proteins  that  were  ade- 
quate for  growth  they  promptly  began  growing  and  presently 
attained  full  size.  The  significance  of  this  is  that  the  ability  to 
grow  is  not  restricted  to  the  early  periods  of  life  and  does  not  come 
to  an  end  with  the  attainment  of  a  certain  age. 

Fuel  Protein.  We  have  learned  that  proteins  are  absorbed  from 
the  digestive  tract  into  the  blood  as  amino  acids.  Of  these  a 
portion  are  destined  to  provide  for  tissue  repair  and  growth.  The 
excess  is  used  as  fuel.  The  nitrogenous  portion  is  virtually  devoid 
of  value  as  a  source  of  energy.  To  fit  the  remainder  to  serve  as 
fuel  the  nitrogen-containing  radicals  are  dissociated  from  the 
molecules,  leaving  non-nitrogenous  residues  of  high  energy  value. 
This  process  of  setting  aside  the  nitrogenous  radicals  is  known  as 
deaminization.  It  was  formerly  believed  to  occur  during  the  pas- 
sage of  the  amino  acids  through  the  intestinal  walls  in  the  process 
of  absorption,  but  recent  investigations  have  shown  that  the 
amino  acids  are  absorbed  as  such  and  that  deaminization  is  prob- 
ably carried  on  by  the  tissues  generally.  The  further  history  of 
the  nitrogen-containing  radicals  will  be  considered  in  a  later  chap- 
ter (p.  517).  The  non-nitrogen  residues  join  themselves  with  the 
other  energy-yielding  food  stuffs  and  will  be  discussed  together 
with  them  (p.  507). 

Should  the  Diet  Include  Much  or  Little  Protein?  We  have  seen 
that  it  is  possible  to  maintain  the  tissues  adequately  upon  a  diet 
containing  only  a  fraction  of  the  amount  of  protein  ordinarily 
taken.  Are  we  to  conclude  from  this  that  the  human  race  eats 
too  much  protein?  To  this  question  no  final  answer  can  be  given 
at  present.  Eminent  dietitians  have  argued  on  both  sides  of  it. 


THE  ABSORPTION  AND  USE  OF  FOODS  505 

One  consideration  that  has  been  suggested  as  probably  significant 
is  that  the  low  protein  diet,  although  adequate  for  immediate 
maintenance,  does  not  afford  the  Body  sufficient  reserve  vitality 
to  place  it  in  the  best  situation  for  resisting  infections  or  other 
debilitating  influences.  Emphasis  has  also  been  placed  on  the 
fact  that  the  poorer  inhabitants  of  Bengal,  who  live  of  necessity 
on  a  low-protein  diet,  are  deficient  both  in  strength  and  endurance. 
Conservative  students  of  the  subject  are  inclined  to  the  opinion 
that  our  present  dietary  habits,  based  as  they  are  upon  centuries 
of  experience,  are  probably  in  the  long  run  better  suited  to  our 
needs  than  radically  altered  dietaries,  which  may  be  theoretically 
sound,  but  lack  the  confirmation  of  long  experience. 

The  allowance  of  protein  in  standard  diets  varies  from  80-90 
grams  daily,  which  is  the  average  amount  consumed  by  American 
College  students,  to  the  115-120  grams  considered  by  some  Euro- 
pean dietitians  suitable  for  the  European  laborer.  In  contrast 
with  these  figures  are  the  allowances  of  40-60  grams  proposed 
by  the  advocates  of  a  low-protein  diet.  While  we  may  properly 
adopt  a  conservative  attitude  with  reference  to  the  low-protein 
controversy,  we  are  not  thereby  justified  in  going  to  the  opposite 
extreme.  Excessive  consumption  of  meat,  particularly  by  people 
who  lead  sedentary  lives,  undoubtedly  is  attended  by  various 
evils,  although  most  of  these  are  referable  to  other  causes  than  over- 
consumption  of  proteins. 

The  Liberation  of  Energy  in  the  Body.  We  have  seen  that  all 
the  energy  liberated  by  the  Body  can  be  expressed  in  terms  of 
heat-units,  but  it  is  not  to  be  concluded,  therefore,  that  heat  energy 
is  the  only  form  manifested  by  the  Body.  As  a  matter  of  fact  the 
Body  undoubtedly  converts  the  potential  energy  of  the  food  into 
at  least  three  forms  of  kinetic  energy;  chemical,  the  carrying  on  of 
the  digestive  and  other  chemical  processes  of  the  Body;  mechanical, 
the  working  of  the  skeletal  muscles,  as  well  as  of  the  heart,  the 
muscles  of  respiration,  and  the  muscles  of  the  viscera;  and  thermal, 
the  direct  production  of  heat  by  oxidation  processes.  This  latter 
form  of  energy,  although  far  exceeding  in  amount  both  the  others 
together,  may  be  looked  upon  as  in  large  degree  a  by-product  of 
the  mechanical  work  of  the  Body,  and  arising  through  the  ineffi- 
ciency of  the  body  machinery.  We  know  that  most  of  the  heat  of 
the  Body  is  produced  in  the  muscles,  and  that  though  these  are 


506  THE  HUMAN  BODY 

producing  some  heat  even  when  at  rest,  they  produce  enormously 
more  when  they  are  active.  A  characteristic  of  all  machines  is 
that  they  work  more  or  less  wastefully;  not  all  the  energy  imparted 
to  them  appears  again  as  useful  work;  the  part  that  is  lost,  more- 
over, appears  always  as  hec^t.  In  the  Body  there  is  this  same  in- 
ability to  convert  food  energy  into  mechanical  energy  without 
there  being  at  the  same  time  a  large  heat  production. 

Studies  of  the  metabolism  of  the  Body  must  necessarily  take 
into  account  these  two  main  forms  in  which  the  energy  of  the  food 
is  manifested.  For  physical  reasons  which  need  not  be  considered 
here  all  muscular  and  chemical  activities  occurring  wholly  within 
the  Body  manifest  themselves  ultimately  to  the  exterior  in  the 
form  of  heat.  The  total  energy  turnover  of  the  Body  can  be  de- 
termined, therefore,  if  the  external  mechanical  work  and  the  entire 
heat  output  can  be  measured.  Theoretically  these  should  exactly 
balance  the  energy  content  of  the  ingested  food.  Metabolism 
studies  are  devoted  in  part  to  demonstrating  that  this  balance 
actually  exists,  and  in  part  to  determinations  of  the  individual 
factors  concerned. 

Basal  Metabolism.  A  necessary  starting  point  for  any  study  of 
energy  manifestation  in  the  Body  is  the  determination  of  the 
amount  liberated  when  the  Body  is  as  inactive  as  possible.  The 
metabolism  which  gives  rise  to  this  energy  represents  that  which 
is  essential  to  the  life  processes.  It  is  known  as  the  basal  metab- 
olism. Its  energy  all  appears  in  the  form  of  heat.  As  measured  in 
an  adult  man  of  average  size,  who  eats  nothing  during  the  day 
of  observation,  it  amounts  to  about  1,700  Calories  (p.  500).  The 
necessary  activities  of  eating  and  digesting  enough  food  to  main- 
tain the  Body  involve  an  expenditure  of  about  10  per  cent  addi- 
tional energy,  bringing  the  total  practical  basal  metabolism  up  to 
about  1,870  Calories  per  day.  Any  energy  liberation  in  excess  of 
this  amount  must  represent  either  actual  muscular  work  or  the 
by-product  of  heat  which  always  attends  it  on  account  of  the  in- 
efficiency of  the  muscles. 

We  shall  see  in  the  chapter  on  Heat  Regulation  (Chap.  XXXII) 
that  the  Body  makes  very  good  use  *of  this  by-product  of  heat  in 
keeping  itself  at  a  proper  temperature  the  year  round,  and  so  the 
extra  amounts  of  food  we  have  to  eat  on  account  of  the  inefficiency 
of  our  bodily  machines  are  not  wholly  wasted  after  all. 


THE  ABSORPTION  AND  USE  OF  FOODS  507 

The  Metabolism  of  Muscular  Work.  The  total  energy  turn- 
over per  day  of  any  individual  is  made  up,  as  we  have  just  seen, 
of  his  basal  metabolism,  together  with  the  metabolism  of  his 
active  muscles.  The  first  factor  is  practically  constant;  the  second 
is  extremely  variable.  Some  average  figures  may,  however,  be 
presented.  If  we  reckon  the  muscular  efficiency  at  20  per  cent 
every  Calorie  of  energy  manifested  in  the  form  of  muscular  work 
means  a  consumption  of  5  Calories  altogether,  and  a  liberation 
of  4  Calories  as  the  by-product  of  heat.  A  man  who  leads  a  de- 
cidedly sedentary  life,  making  no  more  movements  than  necessary, 
is  calculated  to  do  an  amount  of  work  in  a  day  equivalent  to  about 
40  Calories  (120,000)  foot-pounds.  This  work  consists  in  large 
part  of  the  labor  involved  in  the  maintenance  of  the  sitting  and 
standing  positions.  The  performance  of  40  Calories  of  muscular 
work  requires,  on  account  of  the  bodily  inefficiency,  previously 
noted,  an  energy  liberation  of  200  Calories.  This,  added  to  the 
practical  basal  metabolism  of  1,900  Calories,  brings  the  total  to 
2,100  Calories.  When  allowance  is  made  for  a  moderate  amount 
of  exercise;  no  more  than  must  be  taken  if  good  health  is  to  be 
maintained;  the  daily  metabolism  amounts  to  2,500  Calories.  This 
figure  is  believed  to  represent  the  average  for  adults  of  all  classes 
other  than  manual  laborers.  An  interesting  fact  is  that  calcula- 
tions of  the  average  daily  metabolism  per  individual  of  the  in- 
habitants of  cities,  based  on  estimates  of  the  amounts  of  food 
brought  in  each  day  to  the  markets,  indicate  this  same  figure, 
2,500  Calories,  as  the  average  metabolism  for  the  city  dweller. 
The  energy  liberation  of  the  manual  laborer  varies  greatly,  of 
course,  with  the  nature  of  the  toil.  The  range  is  usually  set  at 
3,500  to  5,000  Calories  per  day.  The  latter  figure  probably  repre- 
sents a  high  limit  which  is  rarely  exceeded  by  any  worker  day 
after  day  for  long  periods,  although  trained  men  may  show  a 
much  greater  metabolism  for  a  day  or  two.  An  output  at  the 
rate  of  10,000  Calories  is  believed  not  to  be  impossible  for  a  brief 
spurt. 

The  Relative  Food  Values  of  Proteins,  Carbohydrates  and  Fats. 
Disregarding  the  use  of  protein  as  a  tissue-repairer,  and  consider- 
ing all  three  varieties  of  food  simply  as  furnishers  of  energy,  we 
may  inquire  whether  any  one  of  them  is  superior  to  the  others, 
or  whether  any  particular  proportion  of  the  three  food  stuffs  is 


508  THE  HUMAN  BODY 

specially  desirable.  From  the  purely  mechanical  standpoint  there 
is  evidently  no  choice  among  them;  the  Body  requires  2,500  or 
more  Calories  of  energy  each  day;  each  food  stuff  yields  definite 
amounts  of  energy;  therefore  all  we  have  to  do  to  supply  the 
Body's  requirement  is  to  eat  enough  grams  of  one  or  the  other  food 
stuff,  or  of  a  mixture  of  them.  The  answer  to  the  question  goes 
back,  then,  to  other  considerations  than  that  of  the  energy  content 
of  the  foods.  The  first  of  these  is  the  matter  of  relative  digesti- 
bility and  absorbability;  it  is  of  little  avail  to  eat  a  food  if  it  fails 
to  be  properly  digested  and  absorbed.  Experiments  have  shown 
that  carbohydrates,  exclusive,  of  course,  of  cellulose,  are  the  most 
completely  absorbed  of  all  foods,  97  per  cent  of  the  amount  eaten 
finding  its  way  into  the  Body;  fats  come  next  in  order,  94.4  per 
cent  being  absorbed ;  proteins  are  taken  up  least  completely  of  all, 
the  Body  getting  only  92.6  per  cent  of  the  protein  eaten.  There 
are  also  differences  of  digestibility  and  absorbability  of  different 
foods  within  the  same  class;  the  protein  of  lean  meat,  for  example, 
being  more  readily  digested  and  absorbed  than  that  of  beans  and 
peas.  Cheese,  which  contains  the  highest  per  cent  of  protein  of 
any  common  food,  has  a  reputation,  perhaps  undeserved,  for  in- 
digestibility.  Graham  bread  is,  by  many,  supposed  to  be  more 
nutritious  than  white.  It  is  true  that  graham  flour  contains  a 
higher  percentage  of  protein  than  does  white  flour,  but  the  extra 
protein  of  the  graham  flour  is  in  the  bran,  whence  the  human 
digestive  process  fails  to  extract  it;  so  as  a  matter  of  fact  white 
bread  yields  more  actual  nourishment  to  the  Body  than  doe? 
graham.  The  special  importance  of  graham  flour  or  of  whole 
wheat  is  in  the  roughage  it  contains.  Some  fats  are  much  more 
digestible  than  others;  olive  oil  and  pork  fat,  for  example,  are 
more  completely  utilized  by  the  Body  than  is  mutton  fat.  Fat  of 
any  sort,  taken  in  the  meal  with  other  foods,  seems  for  some  reason 
to  delay  the  whole  digestive  process,  and  the  delay  is  greater  the 
more  fat  is  present.  For  this  reason  it  is  desirable  to  limit  some- 
what the  amount  of  fat  used. 

Another  question  which  may  affect  the  choice  of  foods  is  the 
degree  to  which  they  tax  the  excretory  organs  of  the  Body.  We 
have  seen  that  fuel  proteins  yield  a  nitrogenous  residue  which  must 
be  gotten  rid  of  by  the  excretory  organs.  There  seems  to  be  a 
rather  general  belief  that  this  task  constitutes  a  somewhat  serious 


THE  ABSORPTION  AND  USE  OF  FOODS  509 

strain  upon  these  organs,  and  if  it  does  tend  to  throw  upon  them 
excessive  labor  it  is  clear  that  the  consumption  of  proteins  ought 
on  this  account  to  be  kept  as  low  as  possible.  The  idea  that  the 
excretory  organs  are  endangered  by  ordinary  amounts  of  protein 
in  the  diet  is  not  sustained  by  any  very  convincing  evidence.  In 
fact  there  is  at  least  one  race  of  men,  the  Eskimos,  in  which  huge 
consumption  of  flesh  proteins  is  the  rule  and  in  which  no  tend- 
ency to  gout  and  the  other  diseases  ordinarily  attributed  to  over- 
use of  meat  is  discoverable. 

In  the  matter  of  cost,  which  must  also  be  taken  into  considera- 
tion, carbohydrates  have  a  marked  advantage  over  the  other 
food  stuffs.  For  example,  bread,  which  is  chiefly  carbohydrate, 
yields,  dollar  for  dollar,  about  ten  times  as  many  Calories  as  lean 
beef,  a  protein.  The  cheapest  proteins  are  the  vegetable  ones;  a 
given  weight  of  protein  costing  about  five  times  as  much  when 
bought  as  beef  as  when  purchased  in  the  form  of  beans. 

Still  another  factor  to  be  taken  into  account  is  the  appetizing 
quality  of  the  different  foods.  The  dependence  of  the  whole  di- 
gestive process  upon  a  proper  initial  psychic  secretion  of  gastric 
juice  emphasizes  the  importance  of  the  use  of  appetizing  foods. 
Boiled  meat  contains  as  much  nourishment  as  the  same  weight 
of  roasted  meat,  but  the  former  is  less  desirable  as  a  food  because 
the  process  of  boiling  extracts  from  it  the  substances  which  impart 
to  meat  its  flavor.  Eggs  are  exceedingly  nutritious,  but  to  some 
people  they  are  practically  valueless  as  food,  because  they  inspire 
aversion  rather  than  appetite. 

The  Specific  Dynamic  Action  of  Proteins.  A  feature  of  protein 
metabolism  that  is  both  interesting  and  of  great  dietary  impor- 
tance is  a  stimulating  power  it  exercises  toward  the  whole  meta- 
bolic process.  Whenever  in  the  Body  active  consumption  of 
proteins  is  going  on  there  occurs,  in  addition  to  the  metabolism 
of  the  proteins  themselves  a  further  metabolism  of  some  of  the 
reserve  fuel  supply  of  the  Body,  with,  of  course,  a  corresponding 
increase  in  the  total  heat  production.  This  stimulating  property 
of  protein  has  been  called  its  specific  dynamic  action.  Practically 
it  is  important  in  regulating  the  heat  production  at  different  sea- 
sons of  the  year.  In  winter,  when  we  naturally  eat  protein  freely,  a 
large  amount  of  heat  is  necessary  to  maintain  the  Bodily  warmth. 
In  summer,  when  we  wish  to  produce  no  more  heat  within  our 


510  THE  HUMAN  BODY 

Bodies  than  absolutely  necessary,  the  amount  of  protein  is  cut 
down.  The  very  large  protein  intake  of  Eskimos  probably  serves 
to  insure  for  them  a  heat  production  adequate  to  the  extreme 
climate  in  which  they  live. 

The  Nutritive  Value  of  Albuminoids.  These  proteins,  as  stated 
above  (p.  503),  lack  some  of  the  essential  constituents  of  cell  pro- 
teins, and  cannot,  therefore,  serve  as  tissue-restorers.  We  can 
imagine,  however,  that  they  ought  to  satisfy  the  Body's  demand 
for  protein  fuel,  and  so  be  substituted  for  the  major  part  of  the 
protein  of  the  diet.  Various  attempts  have  been  made  to  substitute 
gelatin  for  proteins  in  this  way,  and  it  seems  to  be  highly  efficacious 
in  satisfying  the  Body's  protein-fuel  demand.  But  curiously 
gelatin  can  be  used  thus  for  only  a  few  meals;  presently  there 
is  a  revolt  of  the  appetite  against  it  and  no  more  can  be  eaten. 
Experiments  have  shown  that  dogs  will  starve  rather  than  take 
continuously  a  diet  whose  chief  constituent  is  gelatin. 

The  Special  Metabolism  of  Fats.  Fats  are  very  useful  fuel 
foods.  Their  energy  content  is  twice  that  of  the  other  nutrients. 
As  we  saw  in  an  early  chapter  (p.  106)  there  is  no  present  reason 
to  suppose  that  they  have  to  be  changed  to  sugar  before  they  can 
be  used  as  sources  of  muscular  energy.  There  is,  however,  a 
feature  of  their  metabolism  which  negatives  their  consumption 
in  large  excess.  In  the  process  of  oxidation  of  fats  there  is  a  stage 
in  which  certain  organic  acids  are  formed.  These,  if  produced  in 
amounts  so  large  that  the  alkalies  of  the  Body  cannot  neutralize 
them  successfully,  bring  about  a  condition  known  as  acidosis, 
which  is  harmful  and,  when  pronounced,  fatal.  The  acid  forma- 
tion is  kept  in  check  if  there  is  an  accompanying  metabolism  of 
carbohydrates.  Acidosis  is  not  so  likely  to  occur  on  a  mixed  diet, 
therefore,  as  on  one  in  which  fat  is  the  chief  item. 

A  practical  difficulty  that  arises  in  prescribing  a  diet  in  diabetes 
(p.  496)  is  due  to  this  feature  of  fat  metabolism.  The  diabetic, 
as  we  have  seen,  cannot  utilize  carbohydrates.  To  feed  him  upon 
a  carbohydrate  diet  is,  therefore,  not  only  wasteful  but  positively 
harmful,  since  it  involves  the  constant  presence  in  his  body  fluids 
of  injurious  quantities  of  sugar.  The  same  difficulty  inheres,  al- 
though in  less  degree,  in  a  diet  of  protein,  since  the  fuel  residue 
of  this  substance  is,  as  we  have  noted  (p.  504)  essentially  carbo- 
hydrate. The  most  feasible  source  of  energy  to  the  diabetic  is, 


THE  ABSORPTION  AND  USE  OF  FOODS  511 

therefore,  fat,  and  his  diet  usually  consists  largely  of  this  sub- 
stance. He  is  thus  confronted  with  the  ever-present  possibility 
of  developing  acidosis.  As  a  matter  of  fact  sooner  or  later 
practically  every  pronounced  diabetic  has  this  experience.  Fatal 
acidosis  is  the  recognized  cause  of  death  in  the  disease. 

Principles  of  Dietetics.  From  the  various  considerations 
presented  above  we  may  summarize  the  general  rule  that  the 
choice  of  food  should  be  such  as  to  yield  sufficient  protein  for  the 
Body's  protein  requirement,  without  containing  an  amount  so 
excessive  as  to  throw  an  undue  burden  on  the  excretory  organs; 
that  the  amount  of  fat  should  be  somewhat  limited;  and  that 
enough  carbohydrate  should  be  added  to  bring  the  sum  total 
up  to  the  Body's  energy  requirement;  finally,  that  the  most  ap- 
petizing foods  obtainable  within  a  reasonable  limit  of  cost  should 
be  selected.  Fortunately  for  the  well-being  of  the  race,  mankind 
has  always  selected  just  such  a  diet  under  no  other  guidance  than 
his  appetite  and  his  means,  and  these,  to  a  healthy  person,  make 
trustworthy  guides,  so  long  as  they  are  accompanied  by  temper- 
ance as  a  third. 

The  importance  of  dietetics  as  a  science  is  chiefly  in  connection 
with  the  feeding  of  the  sick,  or  providing  for  the  maintenance  of 
large  numbers  of  individuals,  as  in  armies  or  public  institutions, 
where  a  slight  error  in  selecting  food,  in  greater  amounts,  or  at 
greater  cost  than  needed,  amounts  in  the  aggregate  to  a  very  large 
waste. 

The  Maintenance  of  Constant  Weight.  It  is  the  experience  of 
most  adults  that  during  periods  of  unbroken  health  the  body 
weight  remains  practically  unchanged  day  in  and  day  out.  It  is 
clear  that  this  condition  depends  on  the  maintenance  of  an  exact 
balance  between  the  intake  and  outgo  of  the  Body,  since  if  more 
is  taken  in  than  is  given  out  there  must  be  a  gain  in  weight,  and 
vice  versa.  It  is  customary  to  consider  the  question  of  weight 
maintenance  under  three  heads :  water  equilibrium,  nitrogen  equilib- 
rium, and  carbon  equilibrium. 

Water  Equilibrium.  For  a  Body  to  be  in  water. equilibrium 
the  amount  of  water  lost  per  day  must  be  exactly  replaced  by  the 
amount  drunk.  In  large  measure  the  sudden  and  transient 
changes  of  weight  which  occur  are  due  to  upsets  of  water  equi- 
librium. Any  violent  exercise  in  hot  weather  reduces  the  weight 


512  THE  HUMAN  BODY 

by  inducing  a  profuse  perspiration  with  resulting  loss  of  water. 
The  intense  thirst  which  follows  the  exercise  leads  to  abundant 
ingestion  of  water  and  a  speedy  restoration  of  the  lost  weight. 

Nitrogen  Equilibrium.  Those  metabolic  activities  of  living 
tissues  which  result  in  tissue  break-down  are  particularly  associ- 
ated with  the  use  of  protein  foods,  since,  as  we  have  seen,  their 
repair  can  be  accomplished  only  by  proteins.  The  characteristic 
constituent  of  protein  is  nitrogen;  and  the  simplest  way  to  esti- 
mate the  amount  of  protein  contained  in  any  food  mass,  or  repre- 
sented by  any  particular  amount  of  excretion,  is  to  determine  the 
nitrogen  and  multiply  the  weight  of  it  present  by  6.25,  the  fraction 
of  protein  which  is  nitrogen.  We  shall  learn  in  the  chapter  on 
Excretion  (Chap.  XXXI),  that  in  the  healthy  Body  an  accumula- 
tion of  nitrogen-containing  excretory  products  never  occurs;  as 
fast  as  wastes  are  formed  they  are  gotten  rid  of.  It  follows, 
then,  that  if  there  is  less  nitrogen  being  given  off  than  taken  in, 
the  living  tissues  of  the  Body  must  be  increasing  in  amount,  and 
if  more  is  given  off  than  is  obtained  in  the  food  the  living  tissues 
must  be  wasting  away.  In  the  healthy  adult  Body,  neither  of 
these  conditions  is  at  all  usual;  the  intake  and  outgo  of  nitrogen 
balance  each  other  and  the  Body  is  in  nitrogen  equilibrium. 

It  has  been  chiefly  through  experimental  studies  of  nitrogen 
equilibrium  that  our  ideas  of  the  twofold  function  of  protein,  as 
tissue-restorer  and  as  fuel,  have  been  gained.  If  an  animal  be 
fed  large  enough  quantities  of  protein  he  requires  no  other  food, 
and  if  healthy  maintains  nitrogen  equilibrium  upon  this  high  level, 
the  large  nitrogen  intake  being  exactly  balanced  by  an  equally 
large  outgo.  Now  by  substituting  other  foods,  as  carbohydrates 
or  fats,  for  part  of  the  protein,  the  nitrogen  intake  and  outgo  are 
each  less  in  quantity,  but  they  still  balance;  the  animal  is  in  nitro- 
gen equilibrium  upon  a  lower  level.  If  the  substitution  of  other 
foods  for  protein  is  increased  a  point  is  presently  reached  when 
the  nitrogen  outgo  exceeds  its  intake;  the  animal  is  not  getting 
enough  protein  for  his  needs,  and  so  his  own  tissues  are  breaking 
down  (p.  501).  During  the  growth  period,  on  the  other  hand,  or 
after  a  wasting  illness,  when  new  tissue  is  being  formed,  the  nitro- 
gen balance  is  the  other  way;  the  amount  of  balance  lost  from  the 
Body  daily  is  less  than  that  consumed.  The  hearty  appetites  of 
children  and  convalescents  are  associated  with  this  necessity  of 


THE  ABSORPTION  AND  USE  OF  FOODS  513 

taking  sufficient  nourishment  to  insure  a  supply  of  protein  for 
tissue  building. 

Carbon  Equilibrium.  For  an  animal  to  be  in  carbon  equilibrium 
only  needs  that  all  the  fuel  taken  in  be  burned,  and  that  no  reserve 
store  be  called  upon.  Aside  from  the  temporary  storage  of  carbo- 
hydrate food  as  glycogen  all  the  fuel  taken  into  the  Body  must 
look  forward  to  one  of  two  fates,  either  to  be  oxidized  promptly  or 
to  be  stored  in  the  form  of  fat  for  future  use.  Just  as  nitrogen 
equilibrium  may  be  established  on  a  high  or  a  low  level  so  carbon 
equilibrium  can  be  maintained  in  the  face  of  variations  in  the  in- 
take of  fuel.  It  is  easily  seen,  however,  that  the  limits  of  carbon 
equilibrium  must  be  narrower  than  of  nitrogen  equilibrium.  The 
actual  protein  requirement  of  the  Body  is  so  much  less  than  the 
usual  protein  intake  that  considerable  variations  in  the  protein 
consumed  can  be  made  without  affecting  the  nitrogen  equilibrium; 
but  the  energy  requirement  of  the  Body  is  quite  definite,  varying 
with  the  work  done  rather  than  with  the  food  eaten.  Thus  it 
follows  that  the  fuel  intake  and  the  energy  requirement  are  harder 
to  keep  balanced  than  are  the  nitrogen  intake  and  outgo.  It  may 
easily  be  a  matter  of  astonishment  how  successfully  the  Body,  un- 
der the  guidance  of  the  appetite,  manages  to  make  its  fuel  con- 
sumption balance  its  fuel  need. 

There  is  a  difference  of  opinion  among  Physiologists  as  to 
whether  every  accidental  excess  consumption  of  fuel  results  in 
the  normal  individual  in  the  deposition  of  the  surplus  in  the  form 
of  fat,  or  whether  the  Body  has  the  power  to  carry  on  oxidations 
in  excess  of  the  normal  basal  metabolism  and  of  the  amount  of 
muscular  exercise. 

Such  positive  information  as  we  have  on  this  point  (see  next 
paragraph)  is  based  on  observations  on  abnormal  individuals  and 
cannot  be  taken  as  necessarily  applying  to  persons  in  normal 
health. 

The  Influence  of  the  Thyroid  Hormone  upon  Metabolism. 
Whether  excess  fuel  shall  be  stored  as  fat  or  be  burned,  has  been 
shown  to  depend,  to  a  large  extent,  at  least,  on  the  amount 
of  the  thyroid  hormone  that  is  produced.  When  the  hor- 
mone is  abundant  the  bodily  oxidations  are  so  vigorous  that 
no  surplus  of  fuel  remains  to  be  converted  into  fat.  An 
inactive  thyroid  gland,  on  the  other  hand,  signifies  a  likelihood 


514  THE  HUMAN  BODY 

to  fat  formation  whenever  the  consumption  of  food  happens  to 
exceed  the  immediate  energy  requirement.  In  the  disease  known 
as  exophthalmic  goiter  (Grave's  Disease)  the  thyroid  gland  is  ab- 
normally active.  The  chief  symptoms  of  the  disease  are  those 
that  are  associated  with  a  greatly  augmented  metabolism.  Suf- 
ferers from  the  condition  eat  hugely  and  yet  are  emaciated. 
Measurements  of  the  daily  energy  turn-over  show  a  heat  produc- 
tion that  may  be  virtually  double  that  of  normal  persons. 

Recently  the  interesting  fact  has  been  brought  out  that  the 
thyroid  gland  is  subject  to  nervous  stimulation  by  way  of  the 
thoracico-lumbar  autonomic  system.  Artificial  Grave's  Disease 
has  been  produced  in  animals  by  causing  persistent  excitation  of 
those  branches  of  the  system  that  innervate  the  thyroid.  Simi- 
larly, the  hormone  adrenin,  which  stimulates  tissues  innervated 
by  the  thoracico-lumbar  autonomies,  has  been  shown  to  excite 
the  thyroid  to  activity.  The  suggestion  has  been  made  that  this 
reaction  is  a  part  of  the  general  emergency  function  of  the  Body. 
Evidently  a  heightened  metabolism,  by  increasing  the  outpouring 
of  energy,  might  be  beneficial  in  time  of  stress.  At  present,  how- 
ever, this  emergency  action  of  the  thyroid  must  be  looked  upon 
as  suggested  rather  than  proved. 

The  Treatment  for  Obesity  is  obviously  to  make  the  energy  re- 
quirement equal,  or  even  exceed,  the  fuel  intake.  Vigorous  mus- 
cular exercise  accompanied  by  strict  dietary  limitation  may  pro- 
duce the  desired  result,  but  the  good  effects  continue  only  so  long 
as  the  flesh-reducing  measures  are  persisted  in.  Exercise  and 
dieting  are  both  conducive  to  good  appetite,  therefore  as  soon  as 
the  treatment  is  relaxed  a  return  to  the  former  condition  is  vir- 
tually inevitable.  Persistent  semi-starvation,  unaccompanied  by 
active  exercise,  is  an  efficient  weight  reducer.  It  should  be  re- 
sorted to  with  intelligence,  however,  for  undesirable  impairment 
of  strength  may  follow  its  injudicious  employment.  A  good  rule 
for  those  who  wish  to  avoid  gaining  flesh  is  never  to  satisfy  the 
appetite  wholly.  On  account  of  its  specific  dynamic  action  (p.  509) 
protein  is  usually  made  the  chief  constituent  of  the  diet  in  the 
treatment  of  obesity. 

The  administration  of  thyroid  extract  is  a  means  of  reducing 
flesh  by  stimulating  the  oxidation  processes  of  the  Body.  Since 
the  thyroid  hormone  has  effects  upon  the  nervous  system  (p.  202) 


THE  ABSORPTION  AND  USE  OF  FOODS  515 

as  well  as  upon  general  metabolism  this  treatment  should  never 
be  undertaken  except  under  competent  medical  advice. 

Source  of  the  Body  Fat.  For  a  long  time  there  was  much  dis- 
cussion as  to  which  of  the  three  sorts  of  food  stuffs,  proteins,  car- 
bohydrates, or  fats,  is  the  source  of  the  fat  which  is  stored  in  the 
Body.  The  natural  conclusion  that  body  fat  is  derived  from  food 
fat  is  shown  to  be  not  universally  true,  at  any  rate,  by  the  ability 
of  cattle  to  produce  milk,  with  its  abundant  fat  content,  upon  a 
diet  of  hay  and  grain  in  which  no  trace  of  fat  occurs.  The  ques- 
tion whether  in  these  animals  the  protein  or  the  carbohydrate  of 
the  food  gives  rise  to  the  fat  was  formerly  much  studied ;  but  with 
the  rise  of  the  modern  view  of  normal  protein  metabolism,  accord- 
ing to  which  all  but  a  small  percentage  of  the  protein  taken  in  the 
food  is  deaminized  and  used  as  carbohydrate,  the  question  has 
lost  much  of  its  force.  There  can  be  little  doubt  that  body  fat 
represents  stored  fuel/  and  since  the  whole  fuel  supply  of  the 
bovine  Body  is  often  represented  by  carbohydrates,  these  must 
be  the  source  of  the  fat  which  the  Body  elaborates. 

It  seems  to  be  the  general  opinion  that  even  in  animals  whose 
diet  includes  some  fat  the  normal  source  of  the  body  fat  is  for  the 
most  part  carbohydrate.  It  is  supposed,  without  very  definite 
evidence  to  prove  it,  that  the  fat  absorbed  after  a  meal  is  retained 
in  the  blood  till  taken  up  by  the  tissues  and  burned,  and  that  the 
somewhat  leisurely  process  of  fat  deposition  is  carried  on  in  con- 
nection with  the  carbohydrate,  which  is  transferred  from  its 
temporary  storehouse  in  the  liver  to  a  more  permanent  one  in 
the  adipose  tissues.  There  is  no  reason  to  doubt  that  when  large 
amounts  of  fat  are  included  in  the  diet  there  may  be  direct  storage 
of  some  of  the  fat  absorbed.  In  fact  it  has  been  shown  that  under 
these  circumstances  foreign  fats,  such  as  linseed-oil,  for  example, 
can  be  deposited  in  the  adipose  tissues  of  animals. 


CHAPTER  XXXI 
EXCRETION  AND  THE  EXCRETORY  ORGANS 

Exogenous  and  Endogenous  Excreta.  It  is  usual  to  include 
under  the  general  head  of  excreta  all  waste  materials  of  any  kind 
that  are  given  out  from  the  Body.  We  shall  see,  however,  that 
under  this  general  definition  come  two  very  distinct  classes  of  ma- 
terials. Many  substances  are  taken  into  the  Body  with  the  food 
which  have  of  themselves  no  food  value,  and  escape  absorption 
during  the  passage  of  the  food  through  the  alimentary  tract;  thes^ 
appear,  of  course,  among  the  excreta.  Other  substances  have  an 
accessory  food  value,  in  arousing  appetite,  or  in  stimulating  some 
of  the  bodily  processes;  these  may  be  absorbed  from  the  alimentary 
tract  into  the  blood,  but  they  do  not  enter  in  any  intimate  fashion 
into  the  metabolic  activities  of  the  living  tissues,  and  after  a  longer 
or  shorter  sojourn  in  the  blood  they  appear  among  the  excreta. 
The  third  substances  to  be  grouped  with  those  just  described  are 
the  nitrogen  containing  compounds  which  are  split  off  from  the 
fuel-proteins  in  the  process  of  deaminization.  These,  from  the 
moment  of  their  separation,  are  waste  products,  to  be  conveyed 
as  rapidly  as  possible  to  the  excretory  organs  and  gotten  rid  of. 
All  these  excretory  materials  are  grouped  together  as  exogenous 
excreta,  the  term  suggesting  that  they  are  derived  from  sources 
outside  the  actual  life  processes  of  the  tissues. 

The  second  group  of  excreta,  the  endogenous  excreta,  includes 
those  substances  that  are  produced  by  the  living  cells  of  the  Body 
in  the  course  of  their  metabolic  activities.  Most  of  our  knowledge 
of  cell  metabolism  has  been  gained  through  studies  of  the  en- 
dogenous excreta. 

The  Channels  of  Excretion.  Four  channels  are  recognized 
through  which  the  body  discharges  waste  materials;  these  are:  the 
lungs,  the  skin,  the  urinary  system,  the  rectum.  The  lungs  are  the 
channel  for  the  discharge  of  gaseous  wastes,  carbon  dioxid,  and 
water  vapor;  the  skin  eliminates  a  part  of  the  water  and  traces  of 
the  nitrogenous  excreta;  the  urinary  system  disposes  of  the  major 
part  of  the  endogenous  excreta  other  than  gaseous,  and  also  of 

516 


EXCRETION  AND  THE  EXCRETORY  ORGANS  517 

those  exogenous  excreta  that  are  absorbed  from  the  alimentary 
tract  into  the  blood.  From  the  rectum  are  discharged  all  exog- 
enous excreta  that  fail  of  absorption,  and  likewise  a  number  of 
endogenous  excretory  substances  received  into  the  intestine  from 
the  liver,  by  way  of  the  bile  duct.  The  chapter  on  Respiration 
contains  the  discussion  of  the  excretory  function  of  the  lungs. 
It  is  not  necessary,  therefore,  to  consider  it  here. 

The  Liver  as  an  Excretory  Organ.  To  the  functions  previously 
described  of  aiding  the  digestive  and  absorptive  processes,  and  of 
serving  as  a  temporary  storehouse  for  carbohydrates,  the  liver 
adds  a  very  important  excretory  function.  This  is  in  part  direct, 
the  separation  from  the  blood  of  waste  materials  contained  in  it, 
and  in  part  the  working  over  of  harmful  excretory  substances  into 
harmless  ones  which  it  does  not  excrete  but  returns  to  the  blood 
to  be  discharged  through  the  urinary  system  and  skin.  This  latter 
function  will  be  considered  before  the  direct  excretions  of  the  liver 
are  discussed.  It  will  be  recalled  that  by  the  process  of  deaminiza- 
tion  the  "  fuel-protein "  is  split  into  a  nitrogenous  waste  portion, 
and  a  non-nitrogenous  oxidizable  portion.  The  nitrogenous  part 
takes  the  form  largely  of  ammonia  compounds,  chief  of  which  is 
ammonium  carbonate  (NH4)2CO3.  These  ammonia  compounds 
are  discharged  into  the  blood.  In  connection  with  the  putrefactive 
processes  that  go  on  in  the  large  intestine  there  is  a  considerable 
production  of  ammonia  which  is  also  absorbed  into  the  blood.  It 
is  well  known  that  ammonia  compounds  are  very  poisonous  to 
animals  into  whose  circulating  blood  they  are  introduced,  and  it 
has  been  proven  that  an  animal  would  be  seriously  affected  if  all 
the  ammonia  produced  in  the  Body  were  allowed  to  remain  in 
the  circulation  in  that  form.  It  is  through  the  action  of  the  liver 
that  the  Body  is  protected  from  the  harmful  effects  of  ammonia. 
During  the  passage  of  the  blood  through  the  liver  its  ammonia  is 
converted  by  dehydration  into  urea,  a  compound  harmless  to  the 
Body  if  not  present  in  the  blood  in  too  great  concentration.  The 
conversion  of  ammonium  carbonate  by  dehydration  to  urea  is  made 
clear  if  we  compare  the  chemical  formulae  of  the  two  substances: 


C0< 


NH40-H20  NH2 

NH4O-H2O~          NH2 


(ammonium  (urea) 

carbonate) 


518  THE  HUMAN  BODY 

The  urea  formed  thus  from  the  ammonia  compounds  of  the  blood 
belongs  to  the  group  of  exogenous  excreta,  since  it  does  not  repre- 
sent a  product  of  true  cell  metabolism  in  the  Body.  From  the 
liver  it  is  delivered  to  the  blood  of  the  general  circulation  where  it 
remains  till  excreted  by  the  kidneys. 

The  direct  excretory  function  of  the  liver  consists  in  the  with- 
drawal from  the  blood  and  the  delivery  to  the  intestine  through 
the  bile  of  certain  endogenous  excretory  substances.  The  most 
marked  of  these  are  the  bile-pigments,  which,  as  stated  in  Chap. 
XVII,  are  derived  from  the  worn-out  red  corpuscles  of  the  blood, 
and  consist  essentially  of  the  pigment  portion  of  hemoglobin  minus 
its  iron.  Two  bile-pigments  occur,  of  very  similar  chemical  con- 
stitution; bilirubin,  golden-brown  in  color,  is  the  predominating 
pigment  of  carnivorous  bile,  and  of  human  bile  on  a  mixed  diet; 
biliverdin,  a  green  pigment,  predominates  in  the  bile  of  herbiverous 
animals.  Recent  investigations  have  indicated  that  the  bile- 
pigments,  although  primarily  waste  products,  serve  some  useful 
purpose  during  their  stay  in  the  alimentary  tract.  The  nature  of 
their  use  is  not  yet  clear. 

Beside  the  bile-pigments  the  liver  excretes  small  amounts  of 
various  substances  which  are  interesting  chiefly  on  account  of 
their  insolubility  in  the  ordinary  fluids  of  the  Body,  and  the  fact 
that  they  are  soluble  in  bile.  These  are  found  in  the  Body  for  the 
most  part  in  nervous  tissues,  and  they  may  be  excretory  products 
of  nerve-cell  metabolism.  The  most  abundant  of  them  is  the  non- 
nitrogenous  substance  cholesterin. 

The  chief  constituents  of  bile  not  heretofore  mentioned  are  the 
bile  salts,  sodium  salts  of  peculiar  acids  found  only-  in  bile,  glyco- 
cholic  add  and  taurocholic  add.  These  do  not  appear  to  be  excreta 
pure  and  simple,  inasmuch  as  they  are  reabsorbed  in  part  by  the 
intestinal  walls,  and  returned  by  the  portal  vein  to  the  liver  whence 
they  again  appear  as  constituents  of  the  bile.  They  are  thought 
to  give  to  bile  its  special  ability  to  promote  fat  absorption  by  dis- 
solving the  fatty  acids,  and  it  is  also  by  virtue  of  their  presence 
that  the  bile  is  able  to  dissolve  cholesterin. 

General  Arrangement  of  the  Urinary  Organs.  These  consist 
of  (1)  the  kidneys,  the  glands  which  secrete  the  urine;  (2)  the 
ureters  or  ducts  of  the  kidneys,  which  carry  their  secretion  to 
(3)  the  urinary  bladder,  a  reservoir  in  which  it  accumulates  and 


EXCRETION  AND  THE  EXCRETORY  ORGANS 


519 


from  which  it  is  expelled  from  time  to  time  through  (4)  an  exit 
tube,  the  urethra.     The  general  arrangement  of  these  parts,  as 


Ua 


FIG.  138. — The  renal  organs,  viewed  from  behind.  R,  right  kidney;  A,  aorta; 
Ar,  right  renal  artery;  Vc,  inferior  vena  cava;  Vr,  right  renal  vein;  U,  right  ureter; 
Vu,  bladder;  Ua,  commencement  of  urethra. 

seen  from  behind,  is  represented  in  Fig.  138.     The  two  kidneys, 
R,  lie  in  the  dorsal  part  of  the  lumbar  region  of  the  abdominal 


520  THE  HUMAN  BODY 

cavity,  one  on  each  side  of  the  middle  line.  Each  is  a  solid  mass, 
with  a  convex  outer  and  a  concave  inner  border,  and  its  upper  end 
a  little  larger  than  the  lower.  From  the  abdominal  aorta,  A,  a 
renal  artery,  Ar,  enters  the  inner  border  of  each  kidney,  to  break 
up  within  it  into  finer  branches,  ultimately  ending  in  capillaries. 
The  blood  is  collected  from  these  into  the  renal  veins,  Vr,  one  of 
which  leaves  each  kidney  and  opens  into  the  inferior  vena  cava, 
Vc.  From  the  concave  border  of  each  kidney  proceeds  also  the 
ureter,  U,  a  slender  tube  from  28  to  34  cm.  (11  to  13.5  inches) 
long,  opening  below  into  the  bladder,  Vu,  on  its  dorsal  aspect,  and 
near  its  lower  end.  From  the  bladder  proceeds  the  urethra,  at 
Ua.  The  channel  of  each  ureter  passes  very  obliquely  through 
the  wall  of  the  bladder  to  open  into  it;  accordingly  if  the  pressure 
inside  the  latter  organ  rises  above  that  of  the  liquid  in  the  ureter, 
the  walls  of  the  oblique  passage  are  pressed  together  and  it  is 
closed.  Usually  the  bladder,  which  has  a  thick  coat  of  unstriped 
muscular  tissue  lined  by  a  mucous  membrane,  is  relaxed,  and 
the  urine  flows  readily  into  it  from  the  ureters.  While  urine  is 
collecting,  the  beginning  of  the  urethra  is  kept  closed,  in  part  at 
least,  by  bands  of  elastic  tissue  around  it:  some  of  the  muscles 
which  surround  the  commencement  of  the  urethra  assist,  being 
kept  in  reflex  contraction;  it  is  found  that  in  a  dog  the  urinary 
bladder  can  retain  liquid  under  considerably  higher  pressure  when 
the  spinal  cord  is  intact  than  after  destruction  of  its  lumbar  por- 
tion. The  contraction  of  these  urethra  constricting  muscles  can 
be  reinforced  voluntarily.  When  some  amount  of  urine  has  ac- 
cumulated in  the  bladder,  it  contracts  and  presses  on  its  content; 
the  ureters  being  closed  in  the  way  above  indicated,  the  elastic 
fibers  closing  the  urethral  exit  are  overcome,  and  the  urethral 
muscles  simultaneously  relaxing,  the  liquid  is  forced  out. 

Naked  Eye  Structure  of  the  Kidneys.  These  organs  have  ex- 
ternally a  red-brown  color,  which  can  be  seen  through  the  trans- 
parent capsule  of  peritoneum  which  envelops  them.  When  a 
section  is  carried  through  a  kidney  from  its  outer  to  its  inner 
border  (Fig.  139)  it  is  seen  that  a  deep  fissure,  the  hilus,  leads  into 
the  latter.  In  the  hilus  the  ureter  widens  out  to  form  the  pelvis, 
D,  which  breaks  up  again  into  a  number  of  smaller  divisions,  the 
cups  or  calices.  The  cut  surface  of  the  kidney  proper  is  seen  to 
consist  of  two  distinct  parts:  an  outer  or  cortical  portion,  and  an 


EXCRETION  AND  THE  EXCRETORY  ORGANS 


521 


inner  or  medullary.  The  medullary  portion  is  less  red  and  more 
glistening  to  the  eye,  is  finely  striated  in  a  radial  direction,  and 
does  not  consist  of  one  continuous  mass  but  of  a  number  of  con- 
ical portions,  the  pyramids  of  Malpighi,  2',  each  of  which  is  sep- 
arated from  its  neighbors  by  an  inward  prolongation,  4,  of  the 

,6 
2' 


FIG.  139. — Section  through  the  right  kidney  from  its  outer  to  its  inner  border, 
1,  cortex;  2,  medulla;  2',  pyramid  of  Malpighi;  2",  pyramid  of  Ferrein;  5,  small 
branches  of  the  renal  artery  entering  between  the  pyramids;  A,  a  branch  of  the 
renal  artery;  D,  the  pelvis  of  the  kidney;  U,  ureter;  C,  a  calyx. 

cortical  substance:  this,  however,  does  not  reach  to  the  inner  end 
of  the  pyramid,  which  projects,  as  the  papilla,  into  a  calyx  of  the 
ureter.  At  its  outer  end  each  pyramid  separates  into  smaller 
portions,  the  pyramids  of  Ferrein,  2",  separated  by  thin  layers  of 
cortex  and  gradually  spreading  everywhere  into  the  latter.  The 
cortical  substance  is  redder  and  more  granular  looking  and  less 


522  THE  HUMAN  BODY 

shiny  than  the  medullary,  and  forms  everywhere  the  outer  layer 
of  the  organ  next  its  capsule,  besides  dipping  in  between  the 
pyramids  in  the  way  described. 

The  renal  artery  divides  in  the  hilus  into  branches  (5)  which 
run  into  the  kidney  between  the  pyramids,  giving  off  a  few  twigs 
to  the  latter  and  ending  finally  in  a  much  richer  vascular  network 
in  the  cortex.  The  branches  of  the  renal  vein  have  a  similar 
course. 

The  Minute  Structure  of  the  Kidney.  The  kidneys  are  com- 
pound tubular  glands,  composed  essentially  of  branched  micro- 
scopic uriniferous  tubules,  lined  by  epithelium.  Each  tubule 
commences  at  a  small  opening  on  a  papilla  and  from  thence  has 
a  very  complex  course  to  its  other  extremity :  usually  about  twenty 
open,  side  by  side,  on  one  papilla,  where  they  have  a  diameter 
of  about  0.125  mm.  (200  inch).  Running  from  this  place  into 
the  pyramid  each  tubule  divides  repeatedly;  the  ultimate  branches, 
which  are  the  secreting  tubules,  pursue  a  tortuous  course  (Fig.  140) 
to  terminations  in  the  cortex  of  the  kidney  in  peculiar  spherical 
dilatations,  the  Malpighian  capsules,  each  containing  a  tuft  of 
capillaries,  the  glomerulus  (Fig.  141).  Throughout  its  course  the 
tubule  is  lined  by  a  single  layer  of  epithelium  cells  differing  in 
character  in  its  different  sections:  they  are  flat  and  clear  in  the 
capsules,  and  very  granular  in  the  convoluted  parts,  where  their 
appearance  suggests  that  they  are  not  mere  lining  cells  but  cells 
with  active  work  to  do;  in  the  collecting  and  discharging  tubules 
they  are  somewhat  cuboidal  in  form  and  have  no  active  secretory 
function.  All  the  tubes  are  bound  together  by  a  sparse  amount 
of  connective  tissue  and  by  blood-vessels  to  form  the  gland.  The 
lymph-spaces  are  large  and  numerous,  especially  about  the  con- 
voluted portions  of  the  tubules. 

The  Blood-Flow  Through  the  Kidney.  The  amount  of  blood 
brought  to  the  kidney  is  large  relatively  to  the  size  of  the  organ 
and  enters  under  a  very  high  pressure  almost  direct  from  the  aorta, 
and  leaves  under  a  very  low,  into  the  inferior  cava  (Fig.  138). 
The  final  twigs  of  the  renal  artery  in  the  cortex,  giving  off  a  few 
branches  which  end  in  a  capillary  network  around  the  convoluted 
tubules  and  in  the  pyramids,  are  continued  as  the  afferent  ves- 
sels of  Malpighian  capsules,  the  walls  of  which  are  doubled  in  be- 
fore them  (Fig.  141);  there  each  breaks  up  into  a  little  knot  of 


EXCRETION  AND  THE  EXCRETORY  ORGANS 


523 


Lobule. 


Lobule. 


Arched   col- 
ecting  tubule. 


Descending 
limb. 


Collecting 
tubule. 


Papillary  duct. 


Tunica  fibrosa 


Stellate  vein. 


Interlobular 

artery. 

Interlobular 

vein. 


Arcif  orm  artery. 
Arciformvein 


Interlobar  artery. 
Interlobar  vein. 


Fia.  140. — ^Diagram  of  kidney  tubule  and  renal  blood-vessels  (Lewis  and  Stohr). 


524 


THE  HUMAN  BODY 


capillary  vessels  called  the  glomerulus,  from  which  ultimately 
an  efferent  vessel  proceeds.  Where  the  wall  of  the  capsule,  w, 
Fig.  141,  is  doubled  in  before  the  blood-vessels,  its  lining  cells 
continue  as  a  covering,  c,  to  the  latter,  closely  adhering  to  the 
vascular  walls.  A  space,  A,  is  left  between  the  epithelial  cells  of 
the  outside  of  the  capsule  and  those  involuted  on  the  vessels,  as 
there  would  be  in  the  interior  of  a  rubber  ball  one  side  of  which 
was  pushed  in  so  as  to  nearly  meet  the  other;  this  cleft,  into  which 
any  liquid  transuded  from  the  vessels  must  enter,  opens  by  a 
narrow  nec%,  d,  into  the  commencement  of  the  first  contorted 

part  of  an  uriniferous  tubule.  The  ef- 
ferent vein,  carrying  blood  away  from 
the  glomerulus,  breaks  up  into  a  close 
capillary  network  around  the  neighbor- 
ing tubules  of  the  cortex  (Fig.  140). 
From  these  capillaries  the  blood  is  col- 
lected into  the  renal  vein.  Most  of  the 
blood  flowing  through  the  kidney  thus 
goes  through  two  sets  of  capillaries;  one 
found  in  the  capsules,  and  the  second 
formed  by  the  breaking  up  of  their  ef- 
ferent veins.  The  capillary  network  in 
FIG.  141.—  Diagram  showing  the  pyramids  is  much  less  close  than 

a  kidney  glomerulus  and  the  ^  at  in  thp  rortpY  whirh  ffivps  reason  to 
commencement  of  an  urinifer-  tnat  m  tne  jGX>  Wni  n  1Ves 


ous  tubule,  a,  afferent  blood-  suspect  that  most  of  the  secretory  work 

vessel  pushing  ia  the  wall,  w,       .,,,.,  .     ,          .      .  , 

of  a  Malpighian  capsule  and  of  the  kidneys  is  done  in  the  capsules  and 

fr^hi^h^e^TLi^  convoluted    tubules.      The    pyramidal 

c,  involuted  epithelium  cover-  blood  flows  only  through  one  set  of 

mg  the  vascular  tuft;  for  the  .*•  ••   • 

sake  of  distinctness  it  is  rep-  capillaries,  there  being  no  glomeruli  in 

resented  as  a  general  wrapping  ,,      i  .,  j    ,, 

for  the  whole  tuft,  but  in  na-  the  kidney  medulla. 
^SS^S^JfSfSSi      The  Renal  Excretion.     The  amount 

erulus;  A,  space  in  capsule  into  of  this  carried  off  from  the  Body  in 

which   liquid  transuded  from  rt  .     ,  .  .  .  •111 

the  vessels  of  the  glomerulus  24    hours    is    subject    to    considerable 

passes;  d,  neck  of  capsule  pass-  Vorjflt;on      hpincr    psnppiallv    diminished 

ing  into  commencement  of  first  variation,     I   .ing    especially 

convoluted  portion,  /  /,  of  an  by  anything  which  promotes  perspira- 

unmferous  tubule;  o,  granular  .  . 

epithelial   cells;  6,  basement  tion,  and  increased  by  conditions,  as 

cold  to  the  surface,  which  diminish  the 

skin  excretion.     Its  average  daily  quantity  varies  from  1,200  to 
1,750  cub.  cent.  (40  to  60  fluid  ounces).    The  urine  is  a  clear  amber- 


EXCRETION  AND  THE  EXCRETORY  ORGANS  525 

colored  liquid,  of  a  slightly  acid  reaction;  its  specific  gravity  is 
about  1,022,  being  higher  when  the  total  quantity  excreted  is 
small  than  when  it  is  greater,  since  the  amount  of  solids  dissolved 
in  it  remains  nearly  the  same  in  health;  the  changes  in  its  bulk 
being  dependent  mainly  on  changes  in  the  amount  of  water 
separated  from  the  blood  by  the  kidneys. 

Normal  Urine  consists  of  about  96  per  cent  water  and  4  per  cent 
dissolved  solids.  Chemically  it  is  a  very  complex  liquid,  the  4 
per  cent  of  dissolved  materials  including  a  large  variety  of  dif- 
ferent substances.  This  is  to  be  expected  when,  we  recall  that  the 
kidney  is  the  excretory  channel,  not  only  for  the  chief  part  of  the 
endogenous  excreta,  but  also  for  virtually  all  the  exogenous  waste 
materials  that  are  absorbed  into  the  blood-stream.  Among  these 
latter  are  found  the  substances  that  lend  flavor  to  our  food;  like- 
wise most  drugs  that  are  taken  find  their  way  ultimately  into  the 
urine.  One  group  of  exogenous  urinary  substances,  the  ethereal 
sulphates,  are  interesting  since  they  are  derived  from  compounds 
formed  in  the  large  intestine  in  the  course  of  the  putrefactive 
processes  which  normally  go  on  there;  these  compounds  are  ab- 
sorbed into  the  blood-stream  and  are  excreted  by  the  kidney. 
The  extent  of  their  occurrence  in  the  urine  measures  the  amount 
of  putrefaction  in  the  large  intestine.  These  substances  are  toxic 
if  present  in  quantity  and  it  may  be  that  the  ill  feeling  which 
often  accompanies  constipation  is  the  result  of  their  presence  in 
considerable  concentration  in  the  blood. 

Urea  is  the  constituent  of  urine  most  abundant  next  to  the 
water.  About  two  per  cent  of  urine,  half  of  all  the  dissolved  ma- 
terials, is  urea.  The  greater  part  of  this  is  of  exogenous  origin, 
being  formed  in  the  liver  from  the  ammonia  residues  of  fuel- 
protein.  The  amount  of  exogenous  urea  varies  from  time  to 
time  according  as  the  amount  of  protein  undergoing  absorption 
varies.  It  is  thought  that  a  certain  amount  of  endogenous 
urea  is  produced  during  the  course  of  cell  metabolism.  How 
much  of  the  total  urea  of  the  excretion  is  of  this  origin  cannot 
be  told. 

Creatinin.  In  some  respects  the  most  interesting  of  the  en- 
dogenous excreta  found  in  the  urine  is  the  compound  creatinin. 
This  substance,  as  stated  in  Chap.  I,  is  excreted  during  health  at 
a  rate  which  is  practically  constant  for  a  given  individual,  and 


526  THE  HUMAN  BODY 

which  appears  to  be  determined  chiefly  by  the  amount  of  muscle  • 
tissue  present  in  the  Body.  The  conclusion  with  regard  to  creat- 
inin  which  has  been  drawn  from  these  facts  is  that  it  is  a  product 
of  the  life  of  muscles  as  distinct  from  their  special  function.  In 
other  words,  the  muscle  in  doing  its  work  uses  up  sugar  and  pro- 
duces carbon  dioxid  and  water,  but  in  living  it  uses  up  protein 
and  produces,  among  other  things,  creatinin.  Since  the  amount 
of  creatinin  is  constant,  regardless  of  the  extent  to  which  the 
muscles  are  used,  unless  they  are  used  to  excess,  it  is  believed  that 
muscle-cells,  and  perhaps  other  cells  as  well,  live  at  a  rate  which 
varies  scarcely  at  all  from  day  to  day,  and  is  independent  of  their 
functional  activity.  The  interesting  observation  that  the  amount 
of  creatinin  excreted  is  roughly  proportional  to  the  bulk  of  the 
muscle  tissues  may  be  taken  to  indicate  that  all  muscle-cells  live 
at  about  the  same  rate,  the  temperamental  differences  noted  in 
different  individuals  not  involving  differences  in  the  metabolic 
activities  of  their  muscle-tissues. 

The  Purin  Bodies,  of  which  uric  acid  is  the  best  known,  are 
other  endogenous  excreta  found  in  urine.  They  show  chemical 
characteristics  which  indicate  that  they  represent  probably  the 
end  products  of  the  metabolism  of  cell  nuclei.  Caffein,  the  active 
principle  of  coffee  and  tea,  and  theobromin,  the  active  principle 
of  cocoa,  are  very  closely  related  chemically  to  the  purin  bodies 
excreted  from  the  kidney. 

Since  all  the  endogenous  excreta  are  produced  in  the  living 
tissues  they  occur  in  the  flesh  of  animals  eaten  for  food.  In  fact 
the  flavor  of  meat  is  largely  the  result  of  their  presence.  When 
eaten  with  meat  they  are,  of  course,  absorbed  into  the  blood  from 
the  intestine  and  become  part  of  the  exogenous  excreta.  For 
this  reason  it  is  often  necessary,  when  studying  metabolism  ex- 
perimentally, to  exclude  meat  from  the  diet,  so  that  the  endoge- 
nous excreta  may  be  obtained  pure. 

The  Urinary  Salts  are  chiefly  sodium  chlorid,  and  the  sulphates 
and  acid  phosphates  of  sodium,  potassium,  calcium,  and  magne- 
sium. Whatever  salt  is  taken  with  the  food,  unless  stored  perma- 
nently in  the  Body,  as  in  bone  formation,  finally  is  excreted  by  the 
kidneys.  The  acid  phosphates  of  sodium  and  potassium  are  in 
part  responsible  for  the  acid  reaction  of  urine. 

In  various  diseases  abnormal  substances  are  found  in  the  urine: 


EXCRETION  AND  THE  EXCRETORY  ORGANS  527 

the  more  important  are  albumens  in  albuminuria  or  nephritis; 
grape  sugar  or  glucose  in  diabetes;  bile  salts;  bile  pigments. 

The  Secretory  Actions  of  Different  Parts  of  a  Uriniferous 
Tubule.  The  microscopic  structure  of  the  kidneys  is  such  as  to 
suggest  that  in  those  organs  we  have  to  do  with  two  essentially 
distinct  secretory  apparatuses:  one  represented  by  the  glomeruli, 
with  their  capillaries  separated  only  by  a  single  layer  of  flat  epi- 
thelial cells  from  the  cavity  of  the  capsule  and  especially  adapted 
for  filtration  and  dialysis;  the  other  represented  by  the  contorted 
portions  of  the  tubules,  with  their  large  granular  cells,  which 
clearly  have  some  more  active  part  to  play  than  that  of  a  mere 
passive  transudation  membrane.  And  we  find  in  the  urine  sub- 
stances which  like  the  water  and  mineral  salts  may  easily  be  ac- 
counted for  by  mere  physical  processes,  and  others,  urea  especially, 
which  are  present  in  such  proportion  as  must  be  due  to  some  active 
physiological  work  of  the  kidney.  More  direct  evidence  does,  in 
fact,  justify  us  in  saying  that  in  general  the  glomeruli  are  transuda- 
tion organs,  the  contorted  portions  of  the  tubuli  secretory  organs, 
while  the  collecting  and  discharging  tubules  are  merely  passive 
channels  for  the  gathering  and  transmission  of  liquid.  In  calling 
the  capsules  transudation  organs  we  do  not  intend  to  assert  that 
the  passage  of  water  and  salts  through  them  is  necessarily  a  phys- 
ical process  pure  and  simple.  Although  many  physiologists  have 
supposed  it  to  be  nothing  more,  there  is  abundant  evidence  that 
the  cells  of  the  capsule  exercise  a  controlling  function  over  the 
passage  of  the  salts  through  them  if  not  of  the  water. 

Several  lines  of  evidence  indicate  that  the  organic  constituents 
of  urine  are  excreted  through  the  secretory  portions  of  the  tubules. 
One  of  the  best  of  these  has  come  from  work  on  frogs.  Urea,  the 
most  important  and  most  abundant  of  the  characteristic  ingre- 
dients of  urine,  has  a  very  marked  influence  on  kidney  activity,  the 
injection  of  some  of  it  into  blood  causing  a  greatly  increased  se- 
cretion of  urine,  in  which  the  injected  urea  is  quickly  passed  out. 
In  amphibia  the  blood  carried  to  the  kidney,  like  that  supply- 
ing the  mammalian  liver,  has  two  sources,  one  venous  and  one 
arterial;  the  arterial  supply  comes  from  the  renal  arteries,  -the 
venous  from  the  veins  of  the  leg  by  the  reniportal  vein.  Both 
bloods  leave  the  organ  by  the  renal  veins,  but  their  distribution 
in  it  is  in  great  part  distinct;  the  arteries  supply  the  glomeruli, 


528  THE  HUMAN  BODY 

the  reniportal  vein  the  tubules  of  the  cortex,  though  mixed  there 
with  blood  from  the  efferent  vessels  of  the  glomeruli.  On  tying 
the  renal  arteries  of  one  of  these  animals  urinary  secretion  ceases, 
there  being  then  no  blood-pressure  in  the  glomeruli  to  cause  the 
transudation  of  liquid;  but  if  some  urea  be  now  injected  into  the 
blood  the  epithelial  cells  of  the  tubules  are  stimulated  to  secrete, 
and  urine  rich  in  urea  is  formed ;  but  in  these  circumstances  it  can- 
not come  from  the  Malpighian  bodies.  It  would  seem  then  that 
urea  is  a  special  stimulant  to  some  cells  of  the  tubules,  and  that  an 
excess  of  it  in  the  blood  can  stir  them  up  to  its  elimination  along 
with  some  water,  quite  independently  of  any  formation  of  trans- 
udation urine. 

The  Relation  of  Renal  Blood-Flow  to  the  Secretion  of  Urine. 
The  kidneys  have  probably  a  richer  blood  supply  than  any  other 
organs  of  the  Body.  It  has  been  estimated  that  under  proper 
circumstances  their  own  weight  of  blood  may  flow  through  them 
each  minute.  This  rich  blood  supply  is,  of  course,  an  adaptation 
to  secure  the  withdrawal  of  waste  substances  from  the  blood  at  a 
rapid  rate.  From  the  structure  of  the  glomeruli  and  the  fact  that 
most  of  the  water  of  the  urine  is  derived  from  them  it  is  a  priori 
probable  that  anything  tending  to  increase  the  pressure  of  blood 
in  them  will  increase  the  bulk  of  urine  secreted,  and  anything 
diminishing  that  pressure  will  decrease  the  urine.  The  structure 
of  the  glomeruli  themselves  is  such  that  the  pressure  of  blood  with 
them  tends  to  be  higher  than  in  the  capillaries  in  general.  Refer- 
ence to  Figure  141  shows  that  the  vessel  which  drains  the  glom- 
erulus,  the  efferent  vessel  e,  is  smaller  than  the  afferent  vessel  a. 
This  means  that  there  is  a  resistance  to  the  outflow  from  the  capsule 
greater  than  that  at  the  point  of  entrance.  According  to  the  rela- 
tion between  pressure  and  resistance  (p.  364)  there  must  be  a  cor- 
respondingly greater  pressure  in  the  glomeruli  than  in  the  other 
capillaries  whose  outlet  is  not  similarly  restricted.  This  high 
glomerular  pressure  favors  filtration.  Experiment  shows,  more- 
over, that  the  vigor  of  urine  formation  depends  on  the  pressure  'of 
blood  within  the  capsule.  The  kidney  is  supplied  with  both  vaso- 
constrictor and  vasodilator  nerves  which  reach  it  mainly  through 
the  solar  plexus.  When  the  spinal  cord  is  cut  in  the  neck  region 
of  a  dog  the  kidney  vessels  as  well  as  those  of  the  rest  of  its  Body 
dilate  and  blood-pressure  everywhere  is  very  low.  Under  these 


EXCRETION  AND  THE  EXCRETORY  ORGANS  529 

circumstances  the  secretion  of  urine  is  suppressed.  If  the  lower 
end  of  the  cut  cord  be  stimulated  the  vessels  all  over  the  Body  of 
the  animal  contract,  and  blood-pressure  everywhere  becomes  very 
high.  But  the  kidney  vessels  being  constricted  with  the  rest  allow 
very  little  blood  to  enter  the  glomeruli  in  spite  of  the  high  aortic 
pressure,  and  little  or  no  urine  is  secreted.  If,  however,  the  vaso- 
constrictor nerves  of  the  kidney  be  cut  before  the  stimulation  of 
the  cord,  we  get  a  dilation  of  the  kidney  vessels  with  a  constric- 
tion of  vessels  elsewhere,  and  abundant  blood  flows  through  the 
glomeruli  under  high  pressure:  the  whole  kidney  swells  and  abun- 
dant urine  is  formed.  When  the  skin  vessels  contract  on  exposure 
to  cold,  more  blood  flows  through  internal  organs,  the  kidneys 
included,  and  the  blood-pressure  in  these  is  if  anything  increased, 
the  expansion  of  internal  arteries  not  at  the  most  more  than 
counterbalancing  the  constriction  of  the  cutaneous.  Hence  the 
greater  secretion  of  urine  in  cold  weather. 

Diuretics.  Various  substances,  caffein,  digitalis,  urea,  salts, 
and  even  water,  stimulate  the  kidney  to  increased  activity.  Sub- 
stances which  have  this  effect  are  known  as  diuretics.  It  appears 
that  these  act  for  the  most  part  by  stimulating  the  secreting  cells 
of  the  tubules  to  greater  activity. 

The  Skin,  which  covers  the  whole  exterior  of  the  Body,  con- 
sists everywhere  of  two  distinct  layers;  an  outer,  the  cuticle  or 
epidermis,  and  a  deeper,  the  dermis,  cutis  vera,  or  corium.  A  blister 
is  due  to  the  accumulation  of  liquid  between  these  two  layers.  The 
hairs  and  nails  are  excessively  developed  parts  of  the  epidermis. 

The  Epidermis,  Fig.  142,  consists  of  cells,  arranged  in  many 
layers,  and  united  by  a  small  amount  of  cementing  substance. 
The  deepest  layer,  d,  is  composed  of  elongated  or  columnar  cells, 
set  on  with  their  long  axes  perpendicular  to  the  corium  beneath. 
To  it  succeed  several  layers  of  roundish  cells,  b,  the  deepest  of 
which,  prickle-cells,  are  covered  by  minute  processes  (not  indicated 
in  the  figure)  which  do  not  interlock  but  join  end  to  end  so  as  to 
leave  narrow  spaces  between  the  cells;  in  more  external  layers  the 
cells  become  more  and  more  flattened  in  a  plane  parallel  to  the 
surface.  The  outermost  epidermic  stratum  is  composed  of  many 
layers  of  extremely  flattened  cells  from  which  the  nuclei  (conspic- 
uous in  the  deeper  layers)  have  disappeared.  These  superficial 
cells  are  dead  and  are  constantly  being  shed  from  the  surface  of 


530 


THE  HUMAN  BODY 


the  Body,  while  their  place  is  taken  by  new  cells,  formed  in  the 
deeper  layers,  and  pushed  up  to  the  surface  and  flattened  in  their 
progress.  The  change  in  the  form  of  the  cells  as  they  travel  out- 
wards is  accompanied  by  chemical  changes,  and  they  finally  con- 


FIG.  142. — A  section  through  the  epidermis,  somewhat  diagrammatic,  highly 
magnified.  Below  is  seen  a  papilla  of  the  dermis,  with  its  artery,  /,  and  veins,  g  g; 
a,  the  horny  layer  of  the  epidermis;  b,  the  rete  mucosum  or  Malpighian  layer;  d,  the 
layer  of  columnar  epidermic  cells  in  immediate  contact  with  the  dermis;  h,  the 
duct  of  a  sweat-gland. 

stitute  a  semitransparent  dry  horny  stratum,  a,  distinct  from  the 
deeper,  more  opaque  and  softer  Malpighian  or  mucous  layer,  b  and 
d,  of  the  epidermis. 

The  rolls  of  material  which  are  peeled  off  the  skin  in  the  "  sham- 
pooing" of  the  Turkish  bath,  or  by  rubbing  with  a  rough  towel 


•       EXCRETION  AND  THE  EXCRETORY  ORGANS  531 

after  an  ordinary  warm  bath,  are  the  dead  outer  scales  of  the 
horny  stratum  of  the  epidermis. 

In  dark  races  the  color  of  the  skin  depends  mainly  on  minute 
pigment-granules  lying  in  the  cells  of  the  deeper  part  of  the  Mal- 
pighian  layer. 

No  blood  or  lymphatic  vessels  enter  the  epidermis,  which  is  en- 
tirely nourished  by  matters  derived  from  the  subjacent  corium. 
Fine  nerve-fibers  run  into  it  and  end  there  among  the  cells. 

The  Corium,  Dennis,  or  True  Skin,  Fig.  143,  consists  funda- 
mentally of  a  close  feltwork  of  elastic  and  white  fibrous  tissue, 
which,  becoming  wider  meshed  below,  passes  gradually  into  the 
subcutaneous  areolar  tissue  (Chap.  IV)  which  attaches  the  skin 
loosely  to  parts  beneath.  In  tanning  it  is  the  dermis  which  is 
turned  into  leather,  its  white  fibrous  tissue  forming  an  insoluble 
and  tough  compound  with  the  tannin  of  the  oak-bark  employed. 

Wherever  there  are  hairs,  bundles  of  smooth  muscular  tissue  are 
found  in  the  corium;  it  contains  also  a  close  capillary  network 
and  numerous  lymphatics  and  nerves.  In  shaving,  so  long  as  the 
razor  keeps  in  the  epidermis  there  is  no  bleeding;  but  a  deeper  cut 
shows  at  once  the  vascularity  of  the  true  skin. 

The  outer  surface  of  the  corium  is  almost  everywhere  raised  into 
minute  elevations,  called  the  papillce,  on  which  the  epidermis  is 
molded,  so  that  its  deep  side  presents  pits  corresponding  to  the 
projections  of  the  dermis.  In  Fig.  142  is  shown  a  papilla  of  the 
corium  containing  a  knot  of  blood-vessels,  supplied  by  the  small 
artery,  /,  and  having  the  blood  carried  off  from  them  by  the  two 
little  veins,  g  g.  Other  papillae  contain  no  capillary  loops  but 
special  organs  connected  with  nerve-fibers,  and  supposed  to  be 
concerned  in  the  cutaneous  senses  (Chap.  XIII).  On  the  pal- 
mar surface  of  the  hand  the  dermic  papillae  are  especially  well  de- 
veloped (as  they  are  in  most  parts  where  the  sense  of  touch  is 
r.cute)  and  are  frequently  compound,  or  branched  at  the  tip.  On 
the  front  of  the  hand,  they  are  arranged  in  rows;  the  epidermis  fills 
up  the  hollows  between  the  papillae  of  the  same  row,  but  dips  down 
between  adjacent  rows,  and  thus  are  produced  the  finer  ridges  seen 
on  the  palms.  In  many  places  the  corium  is  also  furrowed,  as  op- 
posite the  finger-joints  and  on  the  palm.  Elsewhere  such  furrows 
are  less  marked,  but  they  exist  over  the  whole  skin.  The  epidermis 
closely  follows  all  the  hollows,  and  thus  they  are  made  visible 


532 


THE  HUMAN  BODY 


from  the  surface.  The  wrinkles  of  old  persons  are  due  to  the  ab- 
sorption of  subcutaneous  fat  and  of  other  soft  parts  beneath  the 
skin,  which,  not  shrinking  itself  at  the  same  rate,  is  thrown  into 
folds. 

Hairs.  Each  hair  is  a  long  filament  of  epidermis  developed  on 
the  top  of  a  special  dermic  papilla,  seated  at  the  bottom  of  a  de- 
pression reaching  down  from  the  skin  into  the  tissue  beneath,  and 


a 


FIG.  143. — A  section  through  the  skin  and  subcutaneous  areolar  tissue,  h, 
horny  stratum,  and  m,  deeper  more  opaque  layer  of  the  epidermis;  d,  dermis  passing 
below  into  sc,  loose  areolar  tissue,  with  fat,  /,  in  its  meshes;  above,  dermic  papilla? 
are  seen,  projecting  into  the  epidermis  which  is  molded  on  them,  o,  opening  of  a 
sweat-gland;  gl,  the  gland  itself. 

called  the  hair-follicle.  The  portion  of  a  hair  buried  in  the  skin  is 
called  its  root;  this  is  succeeded  by  a  stem  which,  in  an  uncut  hair, 
tapers  off  to  a  point .  The  stem  is  covered  by  a  single  layer  of  over- 
lapping scales  forming  the  hair-cuticle;  the  projecting  edges  of 
these  scales  are  directed  towards  the  top  of  the  hair.  Beneath  the 
hair-cuticle  comes  the  cortex,  made  up  of  greatly  elongated  cells 
united  to  form  fibers;  and  in  the  center  of  the  shaft  there  is  found, 
in  many  hairs;  a  medulla,  made  up  of  more  or  less  rounded  cells. 


EXCRETION  AND  THE  EXCRETORY  ORGANS  533 

The  color  of  hair  is  mainly  dependent  upon  pigment-granules 
lying  between  the  fibers  of  the  cortex.  All  hairs  contain  some  air 
cavities,  especially  in  the  medulla.  They  are  very  abundant  in 
white  hairs  and  cause  the  whiteness  by  reflecting  all  the  incident 
light,  just  as  a  liquid  beaten  into  fine  foam  looks  white  because 
of  the  light  reflected  from  the  walls  of  all  the  little  air  cavities  in  it. 
In  dark  hairs  the  air  cavities  are  few. 

The  hair-follicle  (Fig.  144)  is  a  narrow  pit  of  the  dermis,  pro- 
jecting down  into  the  subcutaneous  areolar  tissue,  and  lined  by  an 
involution  of  the  epidermis.  At  the  bottom  of  the  follicle  is  a 
papilla,  and  the  epidermis,  turning  up  over  this,  becomes  con^ 
tinuous  with  the  hair.  On  the  papilla  epidermic  cells  multiply 
rapidly  so  long  as  the  hair  is  growing,  and  the  whole  hair  is  there 
made  up  of  roundish  cells.  As  these  are  pushed  up  by  fresh  ones 
formed  beneath  them,  the  outermost  layer  become  flattened  and 
form  the  hair-cuticle;  several  succeeding  layers  elongate  and  form 
the  cortex;  while,  in  hairs  with  a  medulla,  the  middle  cells  retain 
pretty  much  their  original  form  and  size.  Pulled  apart  by  the 
elongating  cortical  cells,  these  central  ones  then  form  the  medulla 
with  its  air-cavities.  The  innermost  layer  of  the  epidermis  lining 
the  follicle,  has  its  cells  projecting,  C 
with  overlapping  edges  turned 

downwards.      Accordingly    these  ~"  

interlock  with  the  upward  directed 
edges  of  the  cells  of  the  hair- 
cuticle;  consequently  when  a  hair 
is  pulled  out  the  epidermic  lining 
of  the  follicle  is  usually  brought 
with  it.  So  long  as  the  dermic 
papilla  is  left  intact  a  new  hair  O 

FIG.  144. — Parts  of  two  hairs  em- 
Will  be  formed,  but  not  Otherwise,  bedded  in  their  follicles,    a,  the  skin, 

Slender  bundles  of  smooth  muscle 
(c,  Fig.  144)  run  from  the  dermis 

to  the  side  of  the  hair-follicles,  o,  sebaceous  gland. 
The  latter  are  in  most  regions  obliquely  implanted  in  the  skin  so 
that  the  hairs  lie  down  on  the  surface  of  the  Body,  and  the  muscles 
are  so  fixed  that  when  they  shorten,  they  erect  the  hair  and  cause 
it  to  bristle,  as  may  be  seen  in  an  angry  cat,  or  sometimes  in  a 
greatly  terrified  man.  Opening  into  each  hair-follicle  are  usually  a 


534  THE  HUMAN  BODY 

couple  of  sebaceous  or  oil-glands.  Hairs  are  found  all  over  the  skin 
except  on  the  palms  of  the  hands  and  the  soles  of  the  feet;  the 
back  of  the  last  phalanx  of  the  fingers  and  toes,  the  upper  eyelids, 
and  one  or  two  other  regions. 

Nails.  Each  nail  is  a  part  of  the  epidermi's,  with  its  horny 
stratum  greatly  developed.  The  back  part  of  the  nail  fits  behind 
into  a  furrow  of  the  dermis  and  is  called  its  root.  The  visible  part 
consists  of  a  body,  fixed  to  the  dermis  beneath  (which  forms  the 
bed  of  the  nail),  and  of  a  free  edge.  Near  the  root  is  a  little  area 
whiter  than  the  rest  of  the  nail  and  called  the  lunula.  The  white- 
ness is  due  in  part  to  the  nail  being  really  more  opaque  there  and 
partly  to  the  fact  that  its  bed,  which  seen  through  the  nail  causes 
its  pink  color,  is  in  this  region  less  vascular. 

The  portion  of  the  corium  on  which  the  nail  is  formed  is  called 
its  matrix.  Posteriorly  this  forms  a  furrow  lodging  the  root,  and  it 
is  by  new  cells  added  on  there  that  the  nail  grows  in  length.  The 
part  of  the  matrix  lying  beneath  the  body  of  the  nail,  and  called 
its  bed,  is  highly  vascular  and  raised  up  into  papillae  which,  except 
in  the  region  of  the  lunula,  are  arranged  in  longitudinal  rows, 
slightly  diverging  as  they  run  towards  the  tip  of  the  finger  or  toe. 
It  is  by  new  cells  formed  on  its  bed  and  added  to  its  under  surface 
that  the  nail  grows  in  thickness,  as  it  is  pushed  forward  by  the  new 
growth  in  length  at  its  root.  The  free  end  of  a  nail  is  therefore  its 
thickest  part.  If  a  nail  is  "cast"  in  consequence  of  an  injury,  or 
torn  off,  a  new  one  is  produced,  provided  the  matrix  is  left. 

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, 
Fig.  145,  lie  in  the  subcutaneous  tissue,  where  they  form  little 
globular  masses  composed  of  a  coiled  tube.  From  the  coil  a  duct 
(sometimes  doubb)  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  along 
the  duct  with  the  dermis;  within  this  is  a  basement  membrane; 
and  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  hands 
and  on  the  brow,  than  elsewhere:  there  are  altogether  about 


EXCRETION  AND  THE  EXCRETORY  ORGANS 


535 


two  and  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  open- 
ing near  the  mouth  of  a  hair-follicle  and  branching  at  its  other  end  : 
the  final  branches  lead  into  globular  secreting  saccules,  which,  like 
the  ducts,  are  lined  with  epithelium.  In  the  saccules  the  substance 
of  the  cells  becomes  charged  with  oil-drops,  the  protoplasm  disap- 
pearing; and  finally  the  whole  cell  falls  to  pieces,  its  detritus  con- 
stituting the  secretion.  New  cells  are,  meanwhile,  formed  to  take 
the  place  of  those  destroyed.  Usually  two  glands 
are  connected  with  each  hair-follicle,  but  there 
may  be  three  or  only  one.  A  pair  of  sebaceous 
glands  are  represented  on  the  sides  of  each  of 
the  hair-follicles  in  Fig.  142. 

The  Skin  Secretions.  The  skin  besides  form- 
ing a  protective  covering  and  serving  as  a  sense 
organ  (Chap.  XIII)  also  plays  an  important 
part  in  regulating  the  temperature  of  the  Body, 
and  a  less  important  function  as  an  excretory 
organ,  in  carrying  off  water  and  traces  of  other 
waste  products. 

The  sweat  poured  out  by  the  sudoriparous 
glands  is  a  transparent  colorless  liquid,  with  a 
peculiar  odor,  varying  in  different  races  and,  in 

J?  .     ,;  '         J,     .       ,.«.  .  .  '  FIG.  145.—  A  sweat- 

the  same  individual,  in  different  regions  of  the  gland,  a,  horny  layer 


Body.    Its  quantity  in  twenty-four  hours  is  sub- 

ject  to  great  variations,  but  usually  lies  between  ™k-     The  coils  of 

700  and  2,000  grams  (10,850  and  31,000  grains),  embedded  in  t 


rr,,  ,_    •      •    a  j  -IT  cutaneous    fat,     are 

The  amount  is  influenced  mainly  by  the  sur-  seen  below  the  der- 
rounding  temperature,  being  greater  when  this  mis* 
is  high;  but  it  is  also  increased  by  other  things  tending  to 
raise  the  temperature  of  the  Body,  as  muscular  exercise.  The 
sweat  may  or  may  not  evaporate  as  fast  as  it  is  secreted;  in 
the  former  case  it  is  known  as  insensible,  in  the  latter  as  sen- 
sible perspiration.  By  far  the  most  passes  off  in  the  insensi- 
ble form,  drops  of  sweat  only  accumulating  when  the  secretion 
is  very  profuse,  or  the  surrounding  atmosphere  so  humid  that 
it  does  not  readily  take  up  more  moisture.  The  perspiration 


536  THE  HUMAN  BODY 

is  acid,  and  in  1,000  parts  contains  990  of  water  to  10  of  solids. 
Among  the  latter  are  found  urea  (1.5  in  1,000),  fatty  acids,  sodium 
chlorid,  and  other  salts.  In  diseased  conditions  of  the  kidneys  the 
urea  may  be  greatly  increased,  the  skin  supplementing  to  a  certain 
extent  deficiencies  of  those  organs. 

The  Nervous  and  Circulatory  Factors  in  the  Sweat  Secretion. 
It  used  to  be  believed  that  an  increased  flow  of  blood  through  the 
skin  would  suffice  of  itself  to  cause  increased  perspiration;  but 
against  this  view  are  the  facts  that,  in  terror  for  example,  there 
may  be  profuse  sweating  with  a  cold  pallid  skin ;  and  that  in  many 
febrile  states  the  skin  may  be  hot  and  its  vessels  full  of  blood,  and 
yet  there  may  be  no  sweating. 

Direct  experiment  shows  that  the  secretory  activity  of  the 
sweat-glands  is  under  immediate  control  of  nerve-fibers,  and  is 
only  indirectly  dependent  on  the  blood-supply  in  their  neighbor- 
hood. Stimuating  the  sciatic  nerve  of  the  freshly  amputated 
leg  of  a  cat  will  cause  the  balls  of  its  feet  to  sweat,  although  there 
is  no  blood  flowing  through  the  limb.  On  the  other  hand,  if  the 
sciatic  nerve  be  cut  so  as  to  paralyze  it,  in  a  living  animal,  the 
skin  arteries  dilate  and  the  food  gets  more  blood  and  becomes 
warmer;  but  it  does  not  sweat.  The  sweat-fibers  doubtless  com- 
municate with  sweat-centers  in  the  medulla,  which  may  either  be 
directly  excited  by  blood  of  a  higher  temperature  than  usual  flow- 
ing through  them  or,  reflexly,  by  warmth  acting  on  the  exterior 
of  the  Body  and  stimulating  the  sensory  nerves  there.  Both  of 
these  agencies  commonly  also  excite  the  vasodilator  nerves  of  the 
sweating  part,  and  so  the  increased  blood-supply  goes  along  with 
the  secretion;  but  the  two  phenomena  are  fundamentally  inde- 
pendent. Since  the  sweat-glands  are  innervated  through  the 
autonomic  system  they  share  in  the  emotional  reactions  which 
are  characteristic  of  this  system.  The  effect  of  embarrassment  to 
cause  profuse  sweating  is  too  well  known  to  require  comment. 

The  Sebaceous  Secretion.  This  is  oily,  semifluid,  and  of  a 
special  odor.  It  contains  about  50  per  cent  of  fats  (olein  and 
palmatin).  It  lubricates  the  hairs  and  usually  renders  them 
glossy.  No  doubt,  too,  it  gets  spread  more  or  less  over  the  skin 
and  makes  the  cuticle  less  permeable  by  water.  Water  poured 
on  a  healthy  skin  does  not  wet  it  readily  but  runs  off  it,  as  "off 
a  duck's  back"  though  to  a  less  marked  degree. 


EXCRETION  AND  THE  EXCRETORY  ORGANS  537 

Hygiene  of  the  Skin.  The  sebaceous  secretion,  and  the  solid 
residue  left  by  evaporating  sweat,  constantly  form  a  solid  film 
over  the  skin,  which  must  tend  to  choke  the  mouths  of  the  sweat- 
glands  (the  so-called  " pores"  of  the  skin)  and  impede  their  ac- 
tivity. Hence  the  value  to  health  of  keeping  the  skin  clean:  a 
daily  bath  should  be  taken  by  every  one. 

Bathing.  The  general  subject  of  bathing  may  be  considered 
here.  One  object  of  it  is  that  above  mentioned — to  cleanse  the 
skin;  but  it  is  also  useful  to  strengthen  and  invigorate  the  whole 
frame.  For  strong  healthy  persons  a  cold  bath  is  the  best,  except 
in  extremely  severe  weather,  when  the  temperature  of  the  water 
should  be  raised  to  15°  C.  (about  60°  F.),  at  which  it  still  feels 
quite  cold  to  the  surface.  The  first  effect  of  a  cold  bath  is  to  con- 
tract all  the  skin-vessels  and  make  the  surface  pallid.  This  is 
soon  followed  by  a  reaction,  in  which  the  skin  becomes  red  and 
congested,  and  a  glow  of  warmth  is  felt  in  it.  The  proper  time  to 
come  out  is  while  this  reaction  lasts,  and  after  emersion  it  should 
be  promoted  by  a  good  rub.  If  the  stay  in  the  cold  water  be  too 
prolonged  the  state  of  reaction  passes  off,  the  skin  becomes  cold 
and  pale  and  the  person  feels  chilly,  uncomfortable,  and  depressed 
all  day.  Then  bathing  is  injurious  instead  of  beneficial;  it  lowers 
instead  of  stimulating  the  activities  of  the  Body.  How  long  a 
stay  in  the  cold  water  may  be  made  with  benefit  depends  greatly 
on  the  individual:  a  vigorous  man  can  bear  and  set  up  a  healthy 
reaction  after  much  longer  immersion  than  a  feeble  one;  moreover, 
being  used  to  cold  bathing  renders  a  longer  stay  safe,  and,  of 
course,  the  temperature  of  the  water  has  a  great  influence:  water 
called  "cold"  may  vary  within  very  wide  limits  of  temperature, 
as  indicated  by  the  thermometer;  and  the  colder  it  is  the  shorter 
is  the  time  which  it  is  wise  to  remain  in  it.  Persons  who  in  the 
comparatively  warm  water  of  Narragansett  during  the  summer 
months  stay  with  benefit  and  pleasure  in  the  sea,  have  to  content 
themselves  with  a  single  plunge  on  parts  of  the  coast  where  the 
water  is  colder.  The  nature  of  the  water  has  some  influence;  the 
salts  contained  in  sea-water  stimulate  the  skin-nerves  and  pro- 
mote the  afterglow.  Many  persons  who  cannot  stand  a  simple 
cold  fresh-water  bath  take  one  with  benefit  when  some  salines  are 
previously  dissolved  in  the  water.  The  best  for  this  purpose  are 
probably  those  sold  in  the  shops  under  the  name  of  "sea-salts." 


538  THE  HUMAN  BODY 

It  is  perfectly  safe  to  bathe  when  warm,  provided  the  skin  is 
not  perspiring  profusely,  the  notion  commonly  prevalent  to  the 
contrary  notwithstanding.  On  the  other  hand,  no  one  should 
enter  a  cold  bath  when  feeling  chilly,  or  in  a  depressed  vital  con- 
dition. It  is  not  wise  to  take  a  bath  immediately  after  a  meal, 
since  the  afterglow  tends  to  draw  away  too  much  blood  from  the 
digestive  organs,  which  are  then  actively  at  work.  The  best  time 
for  a  long  bath  is  about  three  hours  after  breakfast;  but  for  an 
ordinary  daily  dip,  lasting  but  a  short  time,  there  is  no  better 
period  than  on  rising  and  while  still  warm  from  bed. 

The  shower-bath  abstracts  less  heat  from  the  skin  than  an  or- 
dinary cold  bath  and,  at  the  same  time,  gives  it  a  greater  stimulus : 
hence  it  has  certain  advantages. 

Persons  in  feeble  health  may  diminish  the  shock  to  the  system 
by  raising  the  temperature  of  the  water  they  bathe  in  up  to  any 
point  at  which  it  still  feels  cool  to  the  skin.  The  very  hot  bath 
is  occasionally  useful  as  the  most  efficient  means  for  cleansing  the 
skin.  There  is  no  doubt,  however,  that  its  effect  tends  to  be  ener- 
vating, and  it  should  not  be  indulged  in  too  frequently. 


CHAPTER  XXXII 

THE  PRODUCTION  AND  REGULATION  OF  THE  HEAT  OF 

THE  BODY 

Cold-  and  Warm-Blooded  Animals.  All  animals,  so  long  as 
they  are  alive,  are  the  seat  of  chemical  changes  by  which  heat  is 
liberated;  hence  all  tend  to  be  somewhat  warmer  than  their  or- 
dinary surroundings,  though  the  difference  may  not  be  noticeable 
unless  the  heat  production  is  considerable.  A  frog  or  a  fish  is  a 
little  hotter  than  the  air  or  water  in  which  it  lives,  but  not  much; 
the  little  heat  that  it  produces  is  lost,  by  radiation  or  conduction, 
almost  at  once.  Hence  such  animals  have  no  proper  temperature 
of  their  own;  on  a  warm  day  they  are  warm,  on  a  cold  day  cold, 
and  are  accordingly  known  as  changeable-temper -atured  (poikilo- 
thermous)  or,  in  ordinary  language,  "cold-blooded"  animals. 
Man  and  other  mammals,  as  well  as  birds,  on  the  contrary,  are 
the  seat  of  very  active  chemical  changes  by  which  much  heat  is 
produced,  and  so  maintain  a  tolerably  uniform  temperature  of 
their  own,  much  as  a  fire  does  whether  it  be  burning  in  a  warm  or 
a  cold  room ;  the  heat  production  during  any  given  time  balancing 
the  loss,  a  normal  body  temperature  is  maintained,  and  usually 
one  considerably  higher  than  that  of  the  medium  in  which  they 
live;  such  animals  are  commonly  named  "warm-blooded."  This 
name, '  however,  does  not  properly  express  the  facts;  a  lizard 
basking  in  the  sun  on  a  warm  summer's  day  may  be  quite  as  hot 
as  a  man  usually  is;  but  on  the  cold  day  the  lizard  becomes  cold, 
while  the  average  temperature  of  the  healthy  Human  Body  is, 
within  a  degree,  the  same  in  winter  or  summer;  within  the  arctic 
circle  or  on  the  equator.  Hence  it  is  better  to  call  such  animals 
"  homothermous  "  or  of  uniform  temperature. 

Moderate  warmth  accelerates  protoplasmic  activity;  compare 
a  frog  dormant  in  the  winter  with  the  same  animal  active  in  the 
warm  months :  what  is  true  of  the  whole  frog  is  true  of  each  of  its 
living  cells.  Its  muscles  contract  more  rapidly  when  warmed, 
and  the  white  corpuscles  of  its  blood  when  heated  up  to  the  tern- 

639 


540  THE  HUMAN  BODY 

perature  of  the  Human  Body  are  seen  (with  the  microscope)  to 
exhibit  much  more  active  amoeboid  movements  than  they  do  at 
the  temperature  of  frog's  blood.  In  summer  a  frog  or  other  cold- 
blooded animal  uses  much  more  oxygen  and  evolves  much  more 
carbon  dioxid  than  in  winter,  as  shown  not  only  by  direct  meas- 
urements of  its  gaseous  exchanges,  but  by  the  fact  that  in  winter 
a  frog  can  live  a  long  time  after  its  lungs  have  been  removed 
(being  able  to  breathe  sufficiently  through  its  moist  skin),  while 
in  warm  weather  it  dies  of  asphyxia  very  soon  after  the  same  loss. 
The  warmer  weather  puts  its  tissues  in  a  more  active  state;  and 
so  the  amount  of  work  the  animal  does,  and  therefore  the  amount 
of  oxygen  it  needs,  depend  to  a  great  extent  upon  the  temperature 
of  the  medium  in  which  it  is  living.  With  the  warm-blooded 
animal  the  reverse  is  the  case.  Within  very  wide  limits  of  expo- 
sure to  heat  or  cold  it  maintains  its  temperature  at  that  at  which 
its  tissues  live  best;  accordingly  in  cold  weather  it  uses  more 
oxygen  and  sets  free  more  carbon  dioxid  because  it  needs  a  more 
active  internal  combustion  to  compensate  for  its  greater  loss  of 
heat  to  the  exterior.  And  it  does  not  become  warmer  in  warm 
weather,  partly  because  its  oxidations  are  less  than  in  cold  (other 
things  being  equal),  and  partly  because  of  physiological  arrange- 
ments by  which  it  loses  heat  faster  from  its  body.  In  fact  the 
living  tissues  of  a  man  may  be  compared  to  hothouse  plants, 
living  in  an  artificially  maintained  temperature;  but  they  differ 
from  the  plants  in  the  fact  that  they  themselves  are  the  seats  of 
the  combustions  by  which  the  temperature  is  kept  up.  Since, 
within  wide  limits,  the  Human  Body  retains  the  same  tempera- 
ture no  matter  whether  it  be  in  cold  or  warm  surroundings,  it  is 
clear  that  it  must  possess  an  accurate  arrangement  for  heat  reg- 
ulation; either  by  controlling  the  production  of  heat  in  it,  or  the 
loss  of  heat  from  it,  or  both. 

The  Temperature  of  the  Body.  The  parts  of  the  Body  are  all 
either  in  contact  with  one  another  directly  or,  if  not,  at  least  in- 
directly through  the  blood,  which,  flowing  from  part  to  part, 
carries  heat  from  warmer  to  colder  regions.  Thus,  although  at 
one  time  one  group  of  muscles  may  especially  work,  liberating 
heat,  and  at  other  times  another,  or  the  muscles  may  be  at  rest 
and  the  glands  the  seat  of  active  oxidation,  the  temperature  of 
the  whole  Body  is  kept  pretty  much  the  same.  The  skin,  however, 


THE  HEAT  OF  THE  BODY  541 

which  is  in  direct  contact  with  external  bodies,  usually  colder  than 
itself,  is  cooler  than  the  internal  organs;  its  temperature  in  health 
is  from  36°  to  37°  C.  (96.8-98.5°  F.),  being  warmer  in  more  pro- 
tected parts,  as  the  hollow  of  the  armpit.  In  internal  organs,  as 
the  liver  and  brain,  the  temperature  is  somewhat  higher.  In  the 
lungs  there  is  loss  of  the  heat  carried  out  by  the  expired  air  and 
that  used  up  in  evaporating  the  water  carried  out  in  the  breath, 
so  the  blood  returned  to  the  heart  by  the  pulmonary  veins  is 
slightly  colder  than  that  carried  from  the  right  side  of  the  heart 
to  the  lungs. 

The  Sources  of  Animal  Heat.  Apart  from  heat  received  from 
its  surroundings  and  in  hot  food  and  drink,  the  source  of  heat  in 
the  Body  is  the  oxidation  of  fuel.  The  1,750  Calories  which  repre- 
sent the  basal  metabolism  (p.  506)  all  appear  as  heat;  whenever 
muscular  work  is  done  there  is  an  additional  by-product  of  heat. 
Moreover,  except  in  those  who  store  surplus  fuel  as  fat,  any  excess 
of  food  consumed  is  burned,  with  the  production  of  still  more  heat. 

The  Maintenance  of  a  Uniform  Temperature.  Obviously  if 
the  Body  is  to  preserve  the  same  temperature  during  any  period 
of  time  the  production  of  heat  within  it  must  exactly  balance  the 
loss  of  heat  from  it  during  that  time.  In  ourselves  this  balance 
is  actually  maintained  within  narrow  limits  of  fluctuation  through- 
out healthy  life.  Only  in  fevers,  or  as  the  result  of  prolonged  ex- 
posure to  cold,  is  the  balance  upset.  In  fact  its  preservation  is 
necessary  for  the  continuance  of  the  life  of  a  warm-blooded  ani- 
mal; should  the  temperature  rise  above  certain  limits  chemical 
changes,  incompatible  with  life,  occur  in  the  tissues;  for  example, 
at  about  49°  C.  (120°  F.)  the  muscles  begin  to  become  rigid.  On 
the  other  hand,  death  ensues  if  the  Body  be  cooled  down  to  about 
19°  C.  (66°  F.). 

Since  we  live  in  an  environment  of  constantly  varying  tem- 
perature a  rather  delicate  adjustment  between  heat  production 
and  heat  loss  is  required. 

This  adjustment  is  attained  through  the  interaction  of  two 
sorts  of  regulatory  devices,  one  for  controlling  the  loss  of  heat  from 
the  Body,  the  other  its  production  in  the  Body.  Both  of  these 
are  partly  voluntary  and  partly  involuntary.  As  regards  heat- 
loss,  by  far  the  most  important  regulating  organ  is  the  skin :  under 
ordinary  circumstances  nearly  90  per  cent  of  the  total  heat  given 


542  THE  HUMAN  BODY 

off  from  the  Body  in  24  hours  goes  by  the  skin  (73  by  radiation 
and  conduction,  14.5  by  evaporation).  This  loss  may  be  con- 
trolled: 

1.  By  clothing;  we  naturally  wear  more  in  cold  and  less  in  warm 
weather;  the  effect  of  clothes  being,  of  course,  not  to  warm  the 
Body  but  to  diminish  the  rate  at  which  the  heat  produced  in  it  is 
lost. 

2.  Warmth  through  reflex  vasomotor  actions  leads  to  dila- 
tion of  the  skin  vessels  and  cold  to  contraction.     In  a  warm 
room  the  vessels  on  the  surface  dilate  as  shown  by  its  redness, 
while  in  a  cold  atmosphere  they  contract  and  the  skin  becomes 
pale.    But  the  more  blood  that  flows  through  the  skin  the  greater 
will  be  the  heat  lost  from  the  surface — and  vice  versa. 

3.  Heat  induces  sweating  and  cold  checks  it;  the  heat  appears 
to  act,  for  the  most  part,  reflexly  through  afferent  cutaneous  nerve- 
fibers  exciting  the  sweat-centers  from  which  the  secretory  nerves 
for  the  sudoriparous  glands  arise;  it  may  also  act  to  some  extent 
directly  on  those  centers,  as  they  are  thrown  into  activity,  at  least 
in  health,  as  soon  as  the  temperature  of  the  blood  flowing  through 
the  spinal  cord  is  raised.    In  fever  of  course  we  may  have  a  high 
temperature  with  a  dry  non-sweating  skin.    The  more  sweat  is 
poured  out,  the  more  heat  is  used  up  in  evaporating  it  and  the 
more  the  Body  is  cooled. 

Of  less  importance  in  man,  but  of  great  importance  in  fur- 
bearing  animals,  is  the  loss  of  heat  through  the  lungs.  In  warm 
weather  there  is  quickened  respiration,  brought  about  reflexly 
through  the  play  of  cutaneous  sensory  impulses  of  warmth  upon 
the  respiratory  center.  This  quickened  respiration  carries  off 
heat  more  rapidly  both  by  increasing  the  amount  of  air  warmed 
to  body  temperature  in  a  given  time,  and  by  increasing  the  evap- 
oration of  water  from  the  lungs. 

Our  sensations  induce  us  to  add  to  or  diminish  the  heat  in  the 
Body  according  to  circumstances;  as  by  cold  or  warm  baths,  and 
iced  or  hot  drinks. 

As  regards  temperature  regulation  by  modifying,  the  rate  of 
heat  production  in  the  Body,  the  following  points  may  be  noted; 
on  the  whole,  such  regulation  is  far  less  important  than  that 
brought  about  by  changes  in  the  rate  of  loss,  since  the  necessary 
vital  work  of  the  Body  always  necessitates  the  continuance  of 


THE  HEAT  OF  THE  BODY  543 

oxidative  processes  which  liberate  a  tolerably  large  quantity  of 
heat.  The  Body  cannot  therefore  be  cooled  by  diminishing  such 
oxidations;  nor,  on  the  other  hand,  can  it  be  safely  warmed  by 
largely  increasing  them.  Still,  within  certain  limits,  the  heat 
production  may  be  controlled  in  several  ways: 

1.  In  cold  weather  there  is  an  increased  appetite  for  protein 
foods.      The  increased  consumption   of  proteins  leads,  through 
their  specific  dynamic  action  (p.  509)  to  greater  oxidative  activity, 
and  so  to  increased  heat  production. 

2.  Cold  inclines  us  to  voluntary  exercise;  warmth  to  muscular 
idleness;  and  the  more  the  muscles  are  worked  the  more  heat  is 
produced  in  the  Body. 

3.  Cold  tends  to  produce  reflex  muscular  movements,  and  so  in- 
creased heat  production ;  as  chattering  of  the  teeth  and  shivering. 

4.  Certain  drugs,  as  salicylic  acid,  and  perhaps  quinine,  diminish 
the  heat  production  of  the  Body.    Their  mode  of  action  is  still 
obscure. 

On  the  whole,  however,  the  direct  heat-regulating  mechanisms 
of  the  Human  Body  itself  are  not  very  efficient,  especially  as 
protections  against  excessive  cooling.  Man  needs  to  supplement 
them  in  cold  climates  by  the  use  of  clothing,  fuel,  and  exercise. 

Local  Temperatures.  Although,  by  the  means  above  described, 
a  wonderfully  uniform  bodily  temperature  is  maintained,  and  by 
the  circulating  blood  all  parts  are  kept  at  nearly  the  same  warmth, 
variations  in  both  respects  do  occur.  The  arrangements  for  equal- 
ization are  not  by  any  means  fully  efficient.  External  parts,  as  the 
skin,  the  lungs  (which  are  really  external  in  the  sense  of  being  in 
contact  with  the  air),  the  mouth,  and  the  nose  chambers,  are  al- 
ways cooler  than  internal,  and  even  all  parts  of  the  skin  have  not 
the  same  temperature,  such  hollows  as  the  armpit  being  warmer 
than  more  exposed  regions.  On  the  other  hand,  a  secreting  gland 
or  a  working  muscle  becomes  warmer,  for  the  time,  than  the  rest 
of  the  Body,  because  more  heat  is  liberated  in  it  than  is  carried  off 
by  the  blood  flowing  through.  In  such  organs  the  venous  blood 
leaving  is  warmer  than  the  arterial  coming  to  them;  while  the 
reverse  is  the  case  with  parts,  like  the  skin,  in  which  the  blood  is 
cooled.  An  organ  colder  than  the  blood  is  of  course  warmed  by 
an  increase  in  its  circulation,  as  seen  in  the  local  rise  of  tempera- 
ture in  the  skin  of  the  face  in  blushing. 


544  THE  HUMAN  BODY 

Fever.  The  condition  of  fever  or  pyrexia,  as  an  abnormally 
high  temperature  is  named,  could  conceivably  be  brought  about 
by  increased  heat  production,  decreased  heat  loss,  or  both;  or  by 
a  greater  increase  of  production  than  of  loss.  Direct  experiments 
on  animals  prove  that  there  is  always  increased  production  of 
heat,  in  febrile  diseases.  This  is  shown  by  the  fact  that  the  animal 
uses  more  oxygen  and  gives  off  more  carbon  dioxid  in  a  given  time 
than  when  in  health.  It  also  usually  gives  off  more  heat,  but  not 
enough  to  compensate  for  the  increase  of  oxidative  processes  going 
on  in  its  body,  and  so  its  temperature  rises.  The  regulating  mech- 
anism which  in  health  keeps  heat  production  and  heat  dissipa- 
tion proportionate  is  out  of  gear.  The  increased  heat  production 
during  fever  is  usually  attributed  to  stimulation  of  the  oxidative 
processes  of  the  Body  by  toxins  in  the  blood,  but  the  mechanism 
of  their  action  is  not  known.  It  has  been  suggested  that  fever  is 
a  protective  reaction  in  that  it  raises  the  body  temperature  above 
that  which  is  most  favorable  to  the  growth  of  the  invading  or- 
ganisms, while  at  the  same  time  favoring  the  development  of  the 
resisting  mechanism  of  the  Body  itself. 

Clothing.  While  the  majority  of  other  warm-blooded  animals 
have  coats  of  their  own,  formed  of  hairs  or  feathers,  over  most  of 
man's  Body  his  hairy  coating  is  merely  rudimentary  and  has  lost 
nearly  all  physiological  importance  as  a  protection  from  cold;  ex- 
cept in  tropical  regions  he  has  to  protect  himself  by  artificial  gar- 
ments, which  his  esthetic  sense  has  led  him  to  utilize  also  for  pur- 
poses of  adornment.  Here,  however,  we  must  confine  ourselves 
to  clothes  from  a  physiological  point  of  view.  In  civilized  societies 
every  one  is  required  to  cover  most  of  his  Body  with  something, 
and  the  question  is  what  is  the  best  covering;  the  answer  will  vary, 
of  course,  with  the  climatic  conditions  of  the  country  dwelt  in. 
In  warm  countries,  clothing,  in  general  terms,  should  allow  free 
radiation  or  conduction  of  heat  from  the  surface;  in  cold  it  should 
do  the  reverse;  and  in  temperate  climates,  with  varying  temper- 
atures, it  should  vary  with  the  season.  If  the  surface  of  the 
Body  be  exposed  so  that  currents  of  air  can  freely  traverse  it 
much  more  heat  will  be  carried  off  (under  those  usual  conditions 
in  which  the  air  is  cooler  than  the  skin)  than  if  a  stationary  layer 
of  air  be  maintained  in  contact  with  the  surface.  As  every  one 
knows,  a  " draught"  cools  much  faster  than  air  of  the  same  tern- 


THE  HEAT  OF  THE  BODY  545 

perature  not  in  motion.  All  clothing,  therefore,  tends  to  keep  up 
the  temperature  of  the  Body  by  checking  the  renewal  of  the  layer 
of  air  in  contact  with  it.  Apart  from  this,  however,  clothes  fall 
into  two  great  groups:  those  which  are  good,  and  those  which  are 
bad,  conductors  of  heat.  The  former  allow  changes  in  the  external 
temperature  to  cool  or  heat  rapidly  the  air  stratum  in  actual  con- 
tact with  the  Body,  while  the  latter  only  permit  these  changes  to 
act  more  slowly.  Of  the  materials  used  for  clothes,  linen  is  a  good 
conductor;  calico  not  quite  so  good;  and  silk,  wool,  and  fur  are 
bad  conductors. 

Whenever  the  surface  of  the  Body  is  suddenly  chilled  the  skin- 
vessels  are  contracted  and  those  of  internal  parts  reflexly  dilated; 
hence  internal  organs  tend  to  become  congested;  this  within  limits 
is  a  protective  physiological  process,  but  if  excessive  it  is  danger- 
ous since  the  congested  membranes  of  the  nose,  throat,  and  lungs 
are  especially  liable  to  fall  victims  to  the  agencies  which  pro- 
duce colds,  influenza  or  even  pneumonia.  When  hot,  therefore, 
the  most  unadvisable  thing  to  do  is  to  sit  in  a  draught,  throw  off 
the  clothing,  or  in  other  ways  to  strive  to  get  suddenly  cooled. 
Moreover,  while  in  the  American  summer  it  is  tolerably  safe  to 
wear  good-conducting  garments,  and  few  people  take  cold  then, 
this  is  by  no  means  safe  in  the  spring  or  autumn,  when  the  tem- 
perature of  the  air  is  apt  to  vary  considerably  within  the  course  of 
a  day.  A  person  going  out,  clad  only  for  a  warm  morning,  may 
have  to  return  in  a  very  much  colder  evening;  and  if  his  clothes 
be  not  such  as  to  prevent  a  sudden  surface  chill,  will  get  off  lightly 
if  he  only  "take"  one  of  the  colds  so  prevalent  at  those  seasons. 
In  the  great  majority  of  cases,  no  doubt,  he  suffers  nothing  worse, 
but  persons,  especially  of  the  female  sex,  often  thus  acquire  far 
more  serious  diseases.  When  sudden  changes  of  temperature  are 
at  all  probable,  even  if  the  prevailing  weather  be  warm,  the  trunk 
of  the  Body  should  be  always  protected  by  some  tolerably  close- 
fitting  garment  of  non-conducting  material.  Those  whose  skins 
are  irritated  by  anything  but  linen  should  wear  immediately  out- 
side the  under-garments  a  jacket  of  silken  or  woolen  material. 


CHAPTER  XXXIII 
VOICE  AND  SPEECH 

Voice  consists  of  sounds  produced  by  the  vibrations  of  two 
elastic  bands,  the  true  vocal  cords,  placed  in  the  larynx,  an  upper 
modified  portion  of  the  passage  which  leads  from  the  pharynx  to 
the  lungs.  When  the  vocal  cords  are  put  in  a  certain  position,  air 
driven  past  them  sets  them  in  vibration,  and  they  emit  a  musical 
note;  the  lungs  and  respiratory  muscles  are,  therefore,  accessory 
parts  of  the  vocal  apparatus:  the  strength  of  the  blast  produced 
by  them  determines  the  loudness  of  the  voice.  The  larynx  itself 
is  the  essential  voice-organ:  its  size  primarily  determines  the  pitch 
of  the  voice,  which  is  lower  the  longer  the  vocal  cords;  and,  hence, 
shrill  in  children,  and  usually  higher  pitched  in  women  than  in 
men;  the  male  larynx  grows  rapidly  at  commencing  manhood, 
causing  the  change  commonly  known  as  the  "breaking  of  the 
voice."  Every  voice,  while  its  general  pitch  is  dependent  on  the 
length  of  the  vocal  cords,  has,  however,  a  certain  range,  within 
limits  which  determine  whether  it  shall  be  soprano,  mezzo-soprano, 
alto,  tenor,  baritone,  or  bass.  This  variety  is  produced  by  muscles 
within  the  larynx  which  alter  the  tension  of  the-vocal  cords.  Those 
characters  of  voice  which  we  express  by  such  phrases  as  harsh, 
sweet,  or  sympathetic,  depend  on  the  structure  of  the  vocal  cords 
of  the  individual ;  cords  which  in  vibrating  emit  only  harmonic 
partial  tones  (Chap.  XIV)  are  pleasant;  while  those  in  which  in- 
harmonic partials  are  conspicuous  are  disagreeable. 

The  vocal  cords  alone  would  produce  but  feeble  sounds;  those 
that  they  emit  are  strengthened  by  sympathetic  resonance  of  the 
air  in  the  pharynx  and  mouth,  the  action  of  which  may  be  com- 
pared to  that  of  the  sounding-board  of  a  violin.  By  movements 
of  throat,  soft  palate,  tongue,  cheeks,  and  lips  the  sounds  emitted 
from  the  larynx  are  altered  or  supplemented  in  various  ways,  and 
converted  into  articulate  language  or  speech. 

The  Larynx  lies  in  front  of  the  neck,  beneath  the  hyoid  bone 
and  above  the  windpipe;  in  many  persons  it  is  prominent,  caus- 

540 


VOICE  AND  SPEECH 


547 


Ci 


ing  the  projection  known  as  "Adam's  apple."  It  consists  of  a 
framework  of  cartilages,  partly  joined  by  true  synovial  joints 
and  partly  bound  together  by  membranes;  muscles  are  added 

Cs  which  move  the  cartilages  with 
reference  to  one  another;  and 
the  whole  is  lined  by  a  mucous 
membrane. 

The  cartilages  of  the  larynx 
(Fig.  146)  are  nine  in  number; 
three  single  and  median,  and 
Pv  three  pairs.  The  largest  (t)  is 
called  the  thyroid,  and  consists 
of  two  halves  which  meet  at  an 
angle  in  front,  but  separate  be- 
hind so  as  to  inclose  a  V-shaped 
space,  in  which  most  of  the  re- 
maining cartilages  lie.  The 
epiglottis  (not  represented  in  the 

FIG.  146. — The  more  important   carti-   n  \     •       /»        i  ,  i  /• 

lages  of  the  larynx  from  behind,     t,  thy-  Hgure)     IS     fixed     to  the     top    Of 
roid;  Cs,  its  superior,  and  Ci,  its  inferior,   fUp  tVivrniH    partilao-P   onH  mr^r 
horn  of  the   right  side;  **,  cricoid  carti-  l         m^n         Cartilage  and  OV6I 
lage;t,  arytenoid  cartilage  ;Pv,  the  corner  hangs  the  entry  from  the  phar- 
to  which  the  posterior  end  of  a  vocal  cord  .        ,  .  . 

is  attached;  Pm,  corner  on  which  the  ynx  to  the  larynx;  it-  may  be 

muscles  which    approximate  or   separate  covered  bv  niUCOUS  mem- 

the  vocal  cords  are  inserted;  co,  cartilage  &    ""i  ^UV^J         UJ   mucuua 

of  Santorini.  brane,  projecting  at  the  base  of 

the  tongue,  if  the  latter  be  pushed  down  while  the  mouth  is  held 
open  in  front  of  a  mirror;  and  is,  similarly  covered,  represented,  as 
seen  from  behind,  at  a  in  Fig.  147.  The  cricoid,  the  last  of  the  un- 
paired cartilages,  has  the  shape  of  a  signet-ring;  its  broad  part 
(**,  Fig.  146)  is  on  the  posterior  side  and  lies  at  the  lower  part  of 
the  opening  between  the  halves  of  the  thyroid;  in  front  and  on 
the  sides  it  is  narrow,  and  a  space,  occupied  by  the  cricothyroid 
membrane,  intervenes  between  its  upper  border  and  the  lower 
edge  of  the  thyroid  cartilage.  The  angles  of  the  latter  are 
produced  above  and  below  into  projecting  horns  (Cs  and  Ci, 
Fig.  148),  and  the  lower  horn  on  each  side  forms  a  joint  with  the 
cricoid.  The  thyroid  can  be  rotated  on  an  axis,  passing  through 
the  joints  on  each  side,  and  rolled  down  so  that  its  lower  front 
edge  shall  come  nearer  the  cricoid  cartilage,  the  membrane  there 
intervening  being  folded.  The  arytenoids  (t,  Fig.  146)  are  the 


548 


THE  HUMAN  BODY 


largest  of  the  paired  cartilages;  they  are  seated  on  the  upper 
edge  of  the  posterior  wide  portion  of  the  cricoid,  and  form 
true  joints  with  it.  Each  is 
pyramidal  with  a  triangular 
base,  and  has  on  its  tip  a  small 
nodule  (co,  Fig.  146),  the  carti- 
lage of  Santorini.  From  the  tip 
of  each  arytenoid  cartilage  the 
aryteno-epiglottic  fold  of  mucous 
membrane  (10,  Fig.  147)  extends 
to  the  epiglottis ;  the  cartilage  of 
Santorini  causes  a  projection 
(8,  Fig.  147)  in  this,  and  a  little 
farther  on  (9)  is  a  similar  emi- 
nence on  each  side,  caused  by 
the  remaining  pair  of  cartilages, 
known  as  the  cuneiform,  or  car- 
tilages of  Wrisberg. 

The  Vocal  Cords  are  bands  of 
elastic  tissue  which  reach  from 
the  inner  angle  (Pv,  Fig.  146)  of 
the  base  of  each  arytenoid  carti- 
lage to  the  angle  on  the  inside 
of  the  thyroid  where  the  sides 

f  ,1        TT-         "i        J.T          xi  FIG.   147* — The    larynx  viewed    from 

Of  the   V  Unite;  they  thus  meet  its  pharyngeal  opening.     The   back  wall 

in  front  but  are  separated   at  **£  ?nfS£^d  ^de^^bSdy  of 

their    Other     ends.      The      COrds  hyoid ;  2,  its  small,  and 3,  its  great,  horns; 

,    .  4,  upper  and  lower  horns  of  thyroid  car- 

are  not,    however,    bare   Strings,   tilage;  5,  mucous  membrane  of  front  of 

like  those  of  a  harp,  but  covered  1S£; c  ^uppe^'end^of  ^iiSf?! 

Over    With     the     lining    muCOUS  windpipe,   lying  in  front   of  the   gul'let'; 

8,  eminence  caused  by  cartilage  of  ban- 
membrane  Of  the  larynx,   a  Silt,   torini;  9,  eminence  caused  by  cartilage 
11     i     ,i  7    ,, .      /       TV        i  AI\     of  Wrisberg;  both  lie  in,  10,  the  aryteno- 

Called    the    glottis    (C,    Jblg.    147),   epigiottic  fold  of  mucous  membrane,  sur- 

being  left  between  them.    It  is  1S^^^^x°^x^I^di^8 p^^^ 

the    projecting    Cushions   formed  tip  of  epiglottis ;c,  the  glottis,  the   lines 
.  .     leading  from  the  latter  point  to  the  free 

by  tnem  On  each  Side  OI  this  vibratory  edges  of  the  vocal  cords,  b', 
«?lit  whiph  nrp  Qpt  in  vihratirm  the  ventricles  of  the  larynx;  their  upper 
oiiu  wiiii/ii  cut;  ecu  in  viui cttujii  orjjyog  marking  them  off  from  the  emi- 

during  phonation.       Above  each  nences  b,  are  the  false  vocal  cords. 

vocal  cord  is  a  depression,  the  ventricle  of  the  larynx  (&',  Fig.  147); 
this  is  bounded  above  by  a  somewhat  prominent  edge,  the  false 


VOICE  AND  SPEECH  549 

vocal  cord.  Over  most  of  the  interior  of  the  larynx  its  mucous 
membrane  is  thick  and  covered  by  ciliated  epithelium,  and  has 
many  mucous  glands  embedded  in  it.  Over  the  vocal  cords, 
however,  it  is  represented  only  by  a  thin  layer  of  flat  non- 
ciliated  cells,  and  contains  no  glands.  In  quiet  breathing,  and 
after  death,  the  free  inner  edges  of  the  vocal  cords  are  thick  and 
rounded,  and  seem  very  unsuitable  for  being  readily  set  in  vibra- 
tion. They  are  also  tolerably  widely  separated  behind,  the  aryte- 
noid  cartilages,  to  which  their  posterior  ends  are  attached,  being 
separated.  Air  under  these  conditions  passes  through  without  pro- 
ducing voice.  If  they  are  watched  with  the  laryngoscope  during 
phonation,  it  is  seen  that  the  cords  approximate  behind  so  as  to 
narrow  the  glottis;  at  the  same  time  they  become  more  tense,  and 
their  inner  edges  project  more  sharply  and  form  a  better-defined 
margin  to  the  glottis,  and  their  vibrations  can  be  seen.  These 
changes  are  brought  about  by  the  delicately  coordinated  activity 
of  a  number  of  small  muscles,  which  move  the  cartilages  to  which 
the  cords  are  fixed. 

The  Muscles  of  the  Larynx.  In  describing  the  direction  and 
action  of  these  it  is  convenient  to  use  the  words  front  or  anterior 
and  back  or  posterior  with  reference  to  the  larynx  itself  (that  is, 
as  equivalent  to  ventral  and  dorsal)  and  not  with  reference  to  the 
head,  as  usual.  The  base  of  each  arytenoid  cartilage  is  triangular 
and  fits  on  a  surface  of  the  cricoid,  on  which  it  can  slip  to  and  fro 
to  some  extent,  the  ligaments  of  the  joint  being  lax.  One  corner 
of  the  triangular  base  is  directed  inwards  and  forwards  (i.  e.,  to- 
wards the  thyroid)  and  is  called  the  vocal  process  (Pv,  Fig.  146),  as 
to  it  the  vocal  cords  are  fixed.  The  outer  posterior  angle  (Pm, 
Fig.  146)  has  several  muscles  inserted  on  it  and  is  called  the  mus- 
cular process.  If  it  be  pulled  back  and  towards  the  middle  line 
the  arytenoid  cartilage  will  rotate  on  its  vertical  axis,  and  roll 
its  vocal  processes  forwards  and  outwards,  and  so  widen  the 
glottis;  the  reverse  will  happen  if  the  muscular  process  be  drawn 
forwards.  The  muscle  producing  the  former  movement  is  the 
posterior  crico-arytenoid  (Cap,  Fig.  148);  it  arises  from  the  back 
of  the  cricoid  cartilage,  and  narrows  to  its  insertion  into  the  mus- 
cular process  of  the  arytenoid  on  the  same  side.  The  opponent 
of  this  muscle  is  the  lateral  crico-arytenoid,  which  arises  from  the 
side  of  the  cricoid  cartilage,  on  its  inner  surface,  and  passes  up- 


550 


THE  HUMAN  BODY 


wards  and  backwards  to  the  muscular  process.  The  posterior 
crico-arytenoids,  working  alone,  pull  inwards  and  downwards  the 
muscular  processes,  turn  upwards  and  outwards  the  vocal  proc- 
esses, and  separate  the  posterior  ends  of  the  vocal  cords.  The 
lateral  cricothyroid,  working  alone,  pulls  downwards  and  for- 
wards the  muscular  process,  and  rotates  inwards  and  upwards 
the  vocal  process,  and  narrows  the  glottis;  it  is  the  chief  agent  in 
producing  the  approximation  of  the. cords  necessary  for  the  pro- 
duction of  voice.  When  both  pairs  of  muscles  act  together,  how- 
ever, each  neutralizes  the  tendency  of  the  other  to  rotate  the 


aep 


FIG.  148. — The  larynx  seen  from  behind  and  dissected  so  as  to  display  some  of 
its  muscles.  The  mucous  membrane  of  the  front  of  the  pharynx  (5,  Fig.  146)  has 
been  dissected  away,  so  as  to  display  the  laryngeal  muscles  beneath  it.  Part  of 
the  left  half  of  the  thyroid  cartilage  has  been  cut  away,  co,  cartilage  of  San- 
torini ;  cu,  cartilage  of  Wrisberg. 


arytenoid  cartilage ;  the  downward  part  of  the  pull  of  each  is,  thus, 
alone  left,  and  this  causes  the  arytenoid  to  slip  downwards  and 
outwards,  off  the  eminence  on  the  cricoid  with  which  it  articu- 
lates, as  far  as  the  loose  capsular  ligament  of  the  joint  will  allow. 
The  arytenoid  cartilages  are  thus  moved  apart  and  the  glottis 
greatly  widened  and  brought  into  its  state  in  deep  quiet  breathing. 


VOICE  AND  SPEECH  551 

Other  muscles  approximate  the  arytendid  cartilages  after  the  car- 
tilages have  been  separated.  The  most  important  is  the  transverse 
arytenoid  (A,  Fig.  148),  which  runs  across  from  one  arytenoid  car- 
tilage to  the  other.  Another  is  the  oblique  arytenoid  (Taep),  which 
runs  across  the  middle  line  from  the  base  of  one  arytenoid  to  the 
tip  of  the  other;  thence  certain  fibers  continue  in  the  aryteno- 
epiglottic  fold  (10,  Fig.  147)  to  the  base  of  the  epiglottis;  this, 
with  its  fellow,  embraces  the  whole  entry  to  the  larynx;  when 
they  contract  they  bend  inwards  the  tips  of  the  arytenoid  carti- 
lages, approximate  the  edges  of  the  aryteno-epiglottic  fold,  and 
draw  down  the  epiglottis,  and  so  close  the  passage  from  the 
pharynx  to  the  larynx.  When  the  epiglottis  has  been  removed, 
food  and  drink  rarely  enter  the  larynx  in  swallowing,  the  folds  of 
mucous  membrane  being  so  brought  together  as  to  effectually  close 
the  aperture  between  them. 

Increased  tension  of  the  vocal  cords  is  produced  by  the  crico- 
thyroid  muscles,  one  of  which  lies  on  each  side  of  the  larynx,  over 
the  cricothyroid  membrane.  Their  action  may  be  understood 
by  help  of  the  diagram,  Fig.  149,  in  which  t  represents  the  thyroid 

cartilage,  c  the  cricoid,  a  an  arytenoid, 
and  vc  a  vocal  cord.    The  muscle  passes 
obliquely  backwards  and  upwards  from 
about  d  near  the  front  end  of  c,  to  t, 
about  I,  near  the  pivot  (which  represents 
the  joint  between  the  cricoid  cartilage 
£*>^  ^^//*/    and  the  inferior  horn  of  the  thyroid). 
^---^'  /       When  the  muscle  contracts  it  pulls  to- 
"^  \       gether  the  anterior  ends  of  t  and  c;  either 
by  depressing  the  thyroid  (as  represented 
by  the  dotted  lines)  or  by  raising  the  front 

end  of  the  cricoid ;  and  thus  stretches  the  vocal  cord,  if  the  aryte- 
noid cartilages  be  held  from  slipping  forwards.  The  antagonist  of 
the  cricothyroid  is  the  thyro-arytenoid  muscle;  it  lies,  on  each  side, 
embedded  in  the  fold  of  elastic  tissue  forming  the  vocal  cord,  and 
passes  from  the  inside  of  the  angle  of  the  thyroid  cartilage  in  front, 
to  the  anterior  angle  and  front  surface  of  the  arytenoid  behind. 
If  the  latter  be  held  firm,  the  muscle  raises  the  thyroid  cartilage 
from  the  position  into  which  the  cricothyroid  pulls  it  down,  and  so 
slackens  the  vocal  cords.  If  the  thyroid  be  held  fixed  by  the 


vc 


552  THE  HUMAN  BODY 

cricothyroid  muscle,  the  thyro-arytenoid  will  help  to  approxi- 
mate the  vocal  cords,  rotating  inwards  the  vocal  processes  of  the 
arytenoids. 

The  lengthening  of  the  vocal  cords  when  the  thyroid  cartilage 
is  depressed  tends  to  lower  their  pitch;  the  increased  tension,  how- 
ever, more  than  compensates  for  this  and  raises  it.  There  seems, 
however,  still  another  method  by  which  high  notes  are  produced. 
Beginning  at  the  bottom  of  his  register,  a  singer  can  go  on  up  the 
scale  some  distance  without  a  break;  but,  then,  to  reach  his  higher 
notes,  must  pause,  rearrange  his  larynx,  and  begin  again.  What 
happens  is  that,  at  first,  the  vocal  processes  are  turned  in,  so  as  to 
approximate  but  not  to  meet;  the  whole  length  of  each  edge  of  the 
glottis  then  vibrates,  and  its  tension  is  increased,  and  the  pitch 
of  the  note  raised,  by  increasing  contraction  of  the  cricothyroid. 
At  last  this  attains  its  limit  and  a  new  method  has  to  be  adopted. 
The  vocal  processes  are  more  rolled  in,  until  they  touch.  This 
produces  a  node  (see  Physics)  at  that  point  and  shortens  the 
length  of  vocal  cord  which  vibrates.  The  shorter  string  emits  a 
higher  note;  so  the  cricothyroid  is  relaxed,  and  then  again  gradu- 
ally tightened  as  the  notes  sung  are  raised  in  pitch  from  the  new 
starting-point.  To  pass  easily  and  imperceptibly  from  one  such 
arrangement  of  the  larynx  to  another  is  a  great  art  in  singing. 
There  is  some  reason  to  believe  that  a  second  node  may,  for  still 
higher  notes,  be  produced  at  a  more  anterior  point  on  the  vocal 
cords. 

The  method  of  production  of  falsetto  notes  is  uncertain;  dur- 
ing their  emission  the  free  border  of  the  vocal  cords  alone  vi- 
brates. 

The  range  of  the  human  voice  is  about  three  octaves,  from 
e  (80  vib.  per  1")  on  the  unaccented  octave,  in  male  voices, 
to  c  on  the  thrice-accented  octave  (1,024  vib.  per  1"),  in  fe- 
male. Great  singers  of  course  go  beyond  this  range;  basses 
have  been  known  to  take  a  on  the  great  octave  (55  vib.  per  1") ; 
and  Nilsson  in  "II  Flauto  Magico"  used  to  take  /  on  the 
fourth  accented  octave  (1,408  vib.  per  1").  Mozart  heard  at 
Parma,  in  1770,  an  Italian  songstress  whose  voice  had  the  ex- 
traordinary range  from  g  in  the  first  accented  octave  (198  vib. 
per  1")  to  c  on  the  fifth  accented  octave  (2,112  vib.  per  1"). 
An  ordinary  good  bass  voice  has  a  compass  from  /  (88  vib. 


VOICE  AND  SPEECH  553 

per  V)  to  d"  (297  vib.  per  V) ;  and  a  soprano  from  b'  (248  vib. 
per  1")  to  g"  (792). 

Vowels  arc,  primarily,  compound  musical  tones  produced  in 
the  larynx.  Accompanying  the  primary  partial  of  each,  which 
determines  its  pitch  When  said  or  sung,  are  a  number  of  upper 
partials,  the  first  five  or  six  beingj  recognizable  in  good  full  voices. 
Certain  of  these  upper  partials  are  reinforced  in  the  mouth  to 
produce  one  vowel,  and  others  for  other  vowels;  so  that  the  va- 
rious vowel  sounds  are  really  musical  notes  differing  from  one  an- 
other in  timbre.  The  mouth  and  throat  cavities  form  an  air- 
chamber  above  the  larnyx,  and  this  has  a  note  of  its  own  which 
varies  with  its  size  and  form,  as  may  be  observed  by  opening  the 
mouth  widely,  with  the  lips  retracted  and  the  cheeks  tense;  then 
gradually  closing  it  and  protruding  the  lips,  meanwhile  tapping 
the  cheek.  As  the  mouth  changes  its  form  the  note  produced 
changes,  tending  in  general  to  pass  from  a  higher  to  a  lower  pitch 
and  suggesting  to  the  ear  at  the  same  time  a  change  from  the 
sound  of  a  (father)  through  o  (more)  to  oo  (moor).  When  the 
mouth  and  throat  chambers  are  so  arranged  that  the  air  in  them 
has  a  vibratory  rate  in  unison  with  any  partial  in  the  laryngeal 
tone,  it  will  be  set  in  sympathetic  vibration,  that  partial  will  be 
strengthened,  and  the  vowel  characterized  by  it  uttered.  As  the 
mouth  alters  its  form,  although  the  same  note  be  still  sung,  the 
vowel  changes.  In  the  above  series  (a,  o,  oo)  the  tongue  is  de- 
pressed and  the  cavity  forms  one  chamber ;  for  a  this  has  a  wide 
mouth  opening;  for  o  it  is  narrowed;  for  oo  still  more  narrowed, 
and  the  lips  protruded  so  as  to  increase  the  length  of  the  resonance 
chamber.  The  partial  tones  reinforced  in  each  case  are,  accord- 
ing to  Helmholtz — 


tl/  1 

|     • 

/L'b 

inv 

r 

SJz 

1 

^Y^ 

00 

In  other  cases  the  mouth  and  throat  cavity  is  partially  subdi- 
vided, by  elevating  the  tongue,  into  a  wide  posterior  and  a  nar- 
row anterior  part,  each  of  which  has  its  own  note;  and  the  vowels 
thus  produced  owe  their  character  to  two  reinforced  partials. 


554  THE  HUMAN  BODY 

This  is  the  case  with  the  series  a  (man),  e  (there),  and  i  (machine), 
the  tones  reinforced  by  resonance  in  the  mouth  being— 


The  usual  i  of  English,  as  in  spire,  is  not  a  true  simple  vowel 
but  a  diphthong,  consisting  of  &  (pad)  followed  by  e  (feet),  as 
may  be  observed  by  trying  to  sing  a  sustained  note  to  the  sound  i; 
it  will  then  be  seen  that  it  begins  as  a"  and  ends  as  ee.  A  simple 
vowel  can  be  maintained  pure  as  long  as  the  breath  holds  out. 

In  uttering  true  vowel  sounds  the  soft  palate  is  raised  so  as  to 
cut  off  the  air  in  the  nose,  which,  thus,  does  not  take  part  in  the 
sympathetic  resonance.  For  some  other  sounds  (the  semi-vowels 
or  resonants)  the  initial  step  is,  as  in  the  case  of  the  true  vowels, 
the  production  of  a  laryngeal  tone;  but  the  soft  palate  is  not 
raised,  and  the  mouth  exit  is  more  or  less  closed  by  the  lips  or  the 
tongue;  hence  the  blast  partly  issues  through  the  nose,  and  the 
air  there  takes  part  in  the  vibrations  and  gives  them  a  special 
character;  this  is  the  case  with  m,  n,  and  ng. 

Consonants  are  sounds  produced  not  mainly  by  the  vocal  cords, 
but  by  modifications  of  the  expiratory  blast  on  its  way  through 
the  mouth.  The  current  may  be  interrupted  and  the  sound 
changed  by  the  lips  (labials) ;  or,  at  or  near  the  teeth,  by  the  tip 
of  the  tongue  (dentals) ;  or,  in  the  throat,  by  the  root  of  the  tongue 
and  the  soft  palate  (gutturals) .  Consonants  are  also  characterized 
by  the  kind  of  movement  which  gives  rise  to  them.  In  explosives 
an  interruption  to  the  passage  of  the  air-current  is  suddenly  in- 
terposed or  removed  (P,  T,  B,  D,  K,  G).  Other  consonants  are 
continuous  (as  F,  S,  R),  and  may  be  subdivided  into:  (1)  Aspirates, 
characterized  by  the  sound  produced  by  a  rush  of  air  through  a 
narrow  passage,  as  when  the  lips  are  approximated  (F),  or  the 
teeth  (S) ,  or  the  tongue  is  brought  near  the  palate  (Sh) ,  or  its  tip 
against  the  two  rows  of  teeth,  they  not  being  quite  in  contact 
(Th).  For  L  the  tongue  is  put  against  the  hard  palate  and  the 


VOICE  AND  SPEECH 


555 


air  escapes  on  its  sides.  For  Ch  (as  in  the  proper  Scotch  pronun- 
ciation of  loch)  the  passage  between  the  back  of  the  tongue  and 
the  soft  palate  is  narrowed.  To  many  of  the  above  pure  conso- 
nants answer  others,  in  whose  production  true  vocalization  (i.  e., 
a  laryngeal  tone)  takes  a  part.  F  with  some  voice  becomes  V; 
S  becomes  Z,  Th  soft  (teeth)  becomes  Th  hard;  and  Ch  becomes 
Gh.  (2)  Resonants;  these  have  been  referred  to  above.  (3)  Vibra- 
tories  (the  different  forms  of  R),  which  are  due  to  vibrations  of 
parts  bounding  a  constriction  put  in  the  course  of  the  air-current. 
Ordinary  R  is  due  to  vibrations  of  the  tip  of  the  tongue  held  near 
the  hard  palate;  and  guttural  R  to  vibrations  of  the  uvula  and 
parts  of  the  pharynx. 

The  consonants  may  physiologically  be  classified  as  in  the  fol- 
lowing table  (Foster) : 

Explosives.    Labials,  without  voice P. 

"        with  voice B. 

Dentals,  without  voice T. 

"       with  voice  ........ .D. 

Gutturals,  without  voice K, 

"        with  voice G  (hard). 

Aspirates.      Labials,  without  voice F. 

"      'with  voice V. 

Dentals,  without  voice S,  L,  Sh,  Th  (hard). 

with  voice Z,   Zh    (azure),   Th    (softt, 

Gutturals,  without  voice Ch  (loch). 

"        with  voice. Ch. 

Resonants.    Labial • M. 

Dental N. 

Gutteral NG. 

Vibratories.   Labial — not  used  in  European  languages. 

Dental R  (common). 

Guttural R  (guttural). 

H  is  a  laryngeal  sound:  the  vocal  cords  are  separated  for  its 
production,  yet  not  so  far  as  in  quiet  breathing.  The  air-current 
then  produces  a  friction  sound  but  not  a  true  note,  as  it  passes 
the  glottis;  and  this  is  again  modified  when  the  current  strikes 
the  wall  of  the  pharynx.  Simple  sudden  closure  of  the  glottis, 
attended  with  no  sound,  is  also  a  speech  element,  though  we  do 
not  indicate  it  with  a  special  letter,  since  it  is  always  understood 
when  a  word  begins  with  a  vowel,  and  only  rarely  is  used  at  other 
times.  The  Greeks  had  a  special  sign  for  it,  ',  the  soft  breathing; 


556  THE  HUMAN  BODY 

and  another,  ',  the  hard  breathing,  answering  somewhat  to  our  h 
and  indicating  that  the  larynx  was  to  be  held  open,  so  as  to  give 
a  friction  sound,  but  not  voice. 

In  whispering  there  is  no  true  voice;  the  latter  implies  true 
tones,  and  these  are  only  produced  by  periodic  vibrations ;  whisper- 
ing is  a  noise.  To  produce  it  the  glottis  is  considerably  narrowed 
but  the  cords  are  not  so  stretched  as  to  produce  a  sharply  defined 
edge  on  them,  and  the  air  driven  past  is  then  thrown  into  irregular 
vibrations.  Such  vibrations  as  coincide  in  period  with  the  air 
in  the  mouth  and  throat  are  always  present  in  sufficient  number 
to  characterize  the  vowels;  and  the  consonants  are  produced  in 
the  ordinary  way,  though  the  distinction  between  such  letters  as 
P  and  B,  F  and  V,  remains  imperfect. 


CHAPTER  XXXIV 
REPRODUCTION 

Reproduction  in  General.  In  all  cases  reproduction  consists, 
essentially,  in  the  separation  of  a  portion  of  living  matter  from  a 
parent;  the  separated  part  bearing  with  it,  or  inheriting,  certain 
tendencies  to  repeat,  with  more  or  less  variation,  the  life  history 
of  its  progenitor.  In  the  more  simple  cases  a  parent  merely  di- 
vides into  two  or  more  pieces,  each  resembling  itself  except  in 
size;  these  then  grow  and  repeat  the  process;  as,  for  instance,  in 
the  case  of  Amoeba  and  our  own  white  blood  corpuscles  (p.  19). 
Such  a  process  may  be  summed  up  in  two  words  as  discontinu- 
ous growth;  the  mass,  instead  of  increasing  in  size  without  seg- 
mentation, divides  as  it  grows,  and  so  forms  independent  living 
beings.  In  some  tolerably  complex  multicellular  animals  we  find 
essentially  the  same  thing;  at  times  certain  cells  of  the  fresh- 
water Polyp  multiply  by  simple  division  in  the  manner  above 
described,  but  there  is  a  certain  concert  between  them :  they  build 
up  a  tube  projecting  from  the  side  of  the  parent,  a  mouth-opening 
forms  at  the  distal  end  of  this,  tentacles  sprout  out  around  it, 
and  only  when  thus  completely  built  up  and  equipped  is  the  young 
Hydra  set  loose  on  its  own  career.  How  closely  such  a  mode  of 
multiplication  is  allied  to  mere  growth  is  shown  by  other  polyps 
in  which  the  young,  thus  formed,  remain  permanently  attached" 
to  the  parent  stem,  so  that  a  compound  animal  results.  This 
mode  of  reproduction  (known  as  gemmation  or  budding)  may  be 
compared  to  the  method  in  which  many  of  the  ancient  Greek 
colonies  were  founded;  carefully  organized  and  prepared  at  home, 
they  were  sent  out  with  a  due  proportion  of  artificers  of  various 
kinds;  so  that  the  new  commonwealth  had  from  its  first  separa- 
tion a  considerable  division  of  employments  in  it,  and  was,  on  a 
small  scale,  a  repetition  of  the  parent  community.  -In  the  great 
majority  of  animals,  however  (even  those  which  at  times  multi- 
ply by  budding),  a  different  mode  of  reproduction  occurs,  one 
more  like  that  by  which  our  western  lands  were  settled  and  grad- 

557 


558  THE  HUMAN  BODY 

ually  built  up  into  Territories  and  States.  The  new  individual 
in  the  political  world  began  with  little  differentiation ;  it  consisted 
of  units,  separated  from  older  and  highly  organized  societies,  and 
these  units  at  first  did  pretty  much  everything,  each  man  for  him- 
self, with  more  or  less  efficiency.  As  growth  took  place  develop- 
ment also  occurred;  persons  assumed  different  duties  and  per- 
formed different  work  until,  finally,  a  fully  organized  State  was 
formed.  Similarly,  the  body  of  one  of  the  higher  animals  is,  at 
an  early  stage  of  life,  merely  a  collection  of  undifferentiated  cells, 
each  capable  of  multiplication  by  division,  and  more  or  less  re- 
taining all  its  original  protoplasmic  properties;  and  with  no  spe- 
cific individual  endowment  or  function.  The  mass  (Chap.  Ill) 
then  slowly  differentiates  into  the  various  tissues,  each  with  a 
predominant  character  and  duty;  at  the  same  time  the  majority 
of  the  cells  lose  their  primitive  powers  of  reproduction,  though 
exactly  how  completely  is  a  problem  not  yet  sufficiently  studied. 
In  adult  Vertebrates  it  seems  certain  that  the  white  blood  cor- 
puscles multiply  by  division:  and  in  some  cases  (in  the  newts  or 
tritons,  for  example)  a  limb  is  reproduced  after  amputation. 
But  exactly  what  cells  take  part  in  such  restorative  processes  is 
uncertain;  we  do  not  know  whether  or  not  the  old  bone  corpuscles 
left  form  new  bones,  old  muscle-fibers  new  muscles,  and  so  on. 
In  Mammals  no  such  restoration  occurs;  an  amputated  leg  may 
heal  at  the  stump  but  does  not  form  again.  In  the  healing  proc- 
esses the  connective  tissues  play  the  main  part,  as  we  might 
expect;  their  cellular  elements  being  but  little  modified  from 
their  primitive  state  can  still  multiply  and  develop.  New  blood- 
capillaries,  however,  sprout  out  from  the  sides  of  old,  and  new 
epidermis  seems  only  to  be  formed  by  the  multiplication  of 
epidermic  cells;  hence  the  practice,  frequently  adopted  by  sur- 
geons, of  transplanting  little  bits  of  skin  to  points  on  the  surface 
of  an  extensive  burn  or  ulcer.  In  blood-capillaries  and  epidermis 
the  departure  from  the  primary  undifferentiated  cell  is  but  slight  ; 
and,  as  regards  the  cuticle,  one  of  the  permanent  physiological 
characters  of  the  cells  of  the  rete  mucosum  is  their  multiplication 
throughout  the  whole  of  life;  that  is  a  main  physiological  char- 
acteristic of  the  tissue :  the  same  is  very  probably  true  of  the  pro- 
toplasmic cells  forming  the  walls  of  the  capillaries.  When  a  highly 
differentiated  tissue  is  replaced  in  the  body  of  mammals  after 


REPRODUCTION  559 

breaking  down  or  removal,  it  is  usually  by  the  activity  of  special 
cells  set  apart  for  that  purpose,  or  by  repair  or  outgrowth  of  the 
cells  affected  and  not  by  their  division.  The  red  blood-corpuscles 
are  constantly  being  broken  down  and  replaced,  but  the  new  ones 
are  not  formed  by  the  division  of  already  fully  formed  corpuscles 
but  by  certain  special  hematoblastic  cells  retained  throughout 
life  in  the  red  marrow  of  bone.  The  nervous  tissues  are  highly 
differentiated  and  a  nerve  is  often  regenerated  after  division,  but 
this  is  by  outgrowth  of  the  ends  of  axons  still  attached  to  their 
cells  and  by  secondary  formation  of  a  myelin  sheath  around  these, 
and  not  by  division  or  multiplication  of  already  existing  fibers. 
A  striped  muscle  when  cut  across  is  healed  by  the  formation  of  a 
band  of  connective  tissue;  after  a  very  long  time  it  is  said  that 
true  muscular  fibers  may  be  found  in  the  cicatrix,  but  their  origin 
is  not  known ;  it  is  probably  not  from  previously  developed  muscle- 
fibers.  On  the  other  hand,  the  less  differentiated  unstriated 
muscle  has  been  observed  to  be  repaired  in  some  cases  after 
injury  by  true  karyokinetic  division  of  previously  formed  muscle- 
cells.  Although  many  gland-cells  in  the  performance  of  their  phys- 
iological work  are  partially  broken  down  and  lost  in  their  secre- 
tion, and  then  repaired  by  the  residue  of  the  cell,  multiplication  by 
division  of  fully  differentiated  gland-cells  does  not  appear  to  occur, 
if  we  except  such  organs  as  the  testes,  the  secretion  of  which  con- 
sists essentially  of  cells.  An  excised  portion  of  a  salivary  or  pa- 
rotid gland  is  never  regenerated:  the  wound  is  repaired  by  con- 
nective tissues. 

We  find,  then,  as  we  ascend  in  the  animal  scale  a  diminishing 
reproductive  power  in  the  tissues  generally:  with  the  increasing 
division  of  physiological  labor,  with  the  changes  that  fit  pre- 
eminently for  one  work,  there  is  a  loss  of  other  faculties,  and  this 
one  among  them.  The  more  specialized  a  tissue  the  less  the  re- 
productive power  of  its  elements,  and  the  most  differentiated 
tissues  are  either  not  reproduced  at  all  after  injury,  or  only 
by  the  specialization  of  amoeboid  cells,  and  not  by  a  progeni- 
tive activity  of  survivors  of  the  same  kind  as  those  destroyed. 
In  none  of  the  higher  animals,  therefore,  do  we  find  multi- 
plication by  simple  division,  or  by  budding:  no  one  cell,  and 
no  group  of  cells  used  for  the  physiological  maintenance  of 
the  individual,  can  build  up  a  new  complete  living  being;  but 


560  THE  HUMAN  BODY 

the  continuance  of  the  race  is  specially  provided  for  by  setting 
apart  certain  cells  which  shall  have  this  one  property — cells  whose 
duty  is  to  the  species  and  not  to  any  one  representative  of  it — an 
essentially  altruistic  element  in  the  otherwise  egoistic  whole. 

Germ-Cells  Compared  with  Tissue-Cells.  Those  cells  which 
are  set  apart  for  the  maintenance  of  the  race  are  called  germ-cells 
to  distinguish  them  from  the  cells  which  make  up  the  Body  gen- 
erally and  which  are  designated  as  somatic  cells.  Each  individual 
is  derived  from  a  single  germ-cell,  as  noted  in  an  earlier  chapter 
(p.  29),  by  a  process  of  cell  multiplication  and  cell  differentiation. 
The  controlling  factor  in  these  processes  was  shown  to  be  the 
chromatin  network  of  the  nucleus,  made  up  of  a  definite  number 
of  chromosomes.  An  important  feature  of  the  difference  between 
germ-cells  and  somatic  cells  is  that  the  former  contain  a  much 
larger  amount  of  chromatin  material  than  do  the  latter.  At  the 
beginning  of  cell  multiplication  in  the  ovum  (p.  25)  the  daughter 
cells  are  alike  in  chromatin  content,  but  a  stage  is  soon 
reached,  very  early  in  some  forms,  in  which  many  of  the  daughter 
cells  discharge  a  part  of  their  chromatin.  The  part  eliminated 
passes  out  of  the  nucleus  into  the  mass  of  the  cell  and  is  dissolved. 
Those  cells  in  which  this  occurs  are  destined  to  develop  into  the 
somatic  or  general  tissues.  Those  that  retain  their  full  comple- 
ment of  chromatin  become  the  germ-cells  of  the  adult  organism. 

Sexual  Reproduction.  In  some  cases,  especially  among  insects, 
the  specialized  reproductive  cells  can  develop,  each  for  itself, 
under  suitable  conditions,  and  give  rise  to  new  individuals;  such 
a  mode  of  reproduction  is  called  parthenogenesis:  but  in  the  major- 
ity of  cases,  and  always  in  the  higher  animals,  this  is  not  so; 
the  fusion  of  two  cells,  or  of  products  of  two  cells,  is  a  necessary 
preliminary  to  development.  Commonly  the  coalescing  cells 
differ  considerably  in  size  and  form,  and  one  takes  a  more  direct 
share  in  the  developmental  processes;  this  is  the  egg-cell  or  ovum; 
the  other  is  the  sperm-cell  or  spermatozoon.  The  fusion  of  the 
two  is  known  as  fertilization.  Animals  producing  both  ova  and 
spermatozoa  are  hermaphrodite;  those  bearing  ova  only,  female; 
and  those  spermatozoa  only,  male:  hermaphroditism  is  not  found 
in  Vertebrates,  except  in  rare  and  doubtful  cases  of  monstrosity. 

Maturation  of  the  Germ-Cells.  In  the  germinal  tissues  of  the 
sexually  mature  individual  cell  multiplication  goes  on  actively 


REPRODUCTION  561 

by  the  process  of  mitosis  previously  described  (p.  24).  At  a  cer- 
tain stage,  however,  every  germ-cell  passes  through  a  modified 
mitosis  to  fit  it  for  taking  part  in  the  reproductive  cycle.  An  es- 
sential feature  of  reproduction  in  higher  forms,  as  noted  in  the 
last  paragraph,  is  fertilization,  or  the  fusion  of  two  germ-cells, 
male  and  female.  This  fusion,  by  adding  the  chromosomes  of  the 
male  cell  to  those  of  the  female,  would  double  the  number  in  the 
fertilized  egg  were  not  some  arrangement  provided  to  avoid  it. 
This  arrangement  is  found  in  the  modified  mitosis  mentioned 
above,  to  which  is  given  the  name  maturation  or  ripening  of  the 
germ-cell.  We  will  recall  from  the  earlier  description  of  mitosis 
that  the  chromatin  forms  itself  into  a  definite  number  of 
chromosomes,  and  that  each  chromosome  splits  lengthwise  in 
such  fashion  that  half  of  it  goes  to  each  daughter  cell.  Thus  the 
daughter  cells  are  exactly  alike  so  far  as  chromosome  content  goes. 
In  the  process  of  maturation  the  chromosomes  do  not  split  length- 
wise. Instead  half  of  them  pass  to  one  daughter  cell  and  half  to 
the  other.  Thus  we  have  a  reduction  of  the  number  of  chromo- 
somes to  half  the  original.  Moreover  the  daughter  cells  of  this 
division  are  not  alike  in  chromosome  content,  a  fact  that  is  sup- 
posed to  be  highly  significant  in  heredity,  since  the  chromosomes 
are  looked  upon  as  determiners  of  hereditary  traits. 

In  the  formation  of  sperm  the  'daughter  cells  of  the  maturation 
or  reduction  division  (so  called  because  it  reduces  the  number  of 
chromosomes),  are  of  equal  value.  Each  undergoes  an  additional 
mitosis  of  the  ordinary  type,  so  that  from  each  primary  sperm- 
forming  cell  four  functional  spermatozoa  are  derived.  The  ovum 
is  the  active  agent  in  the  reproductive  process.  Its  maturation 
proceeds  in  such  a  fashion  that  the  cell  mass  as  a  whole  is  undis- 
turbed by  the  changes  taking  place  in  the  chromatin.  The  reduc- 
tion division  occurs  with  the  chromosomes  of  the  ovum,  precisely 
as  in  the  primary  sperm-cell ;  but  instead  of  this  chromosome  divi- 
sion being  followed  by  ordinary  cell  division,  the  division  is  un- 
equal. Most  of  the  cell  substance  continues  as  before,  retaining 
half  the  chromosomes.  A  very  small  amount  is  set  apart  with 
the  other  half  of  the  chromosomes,  and  serves  no  useful  purpose. 
This  is  known  as  the  first  polar  body.  At  this  stage  the  ovum  and 
the  polar  body  are  comparable  to  the  daughter  cells  of  the  reduc- 
tion division  of  the  primary  sperm-forming  cell.  There  is  an  addi- 


562  THE  HUMAN  BODY 

tional  mitosis  in  ovum  and  polar  body  just  as  in  the  sperm  formers. 
The  polar  body  divides  into  two  of  equal  size.  The  division  of 
the  ovum  is  again  unequal,  and  an  additional  polar  body  is  formed. 
At  the  end  of  maturation  of  the  egg,  therefore,  we  have  four  cells 
corresponding  to  the  four  sperm,  but  one  of  them  has  retained 
virtually  all  the  cell  substance,  and  is  the  functional  ovum.  Figure 
150  is  a  diagram  illustrating  the  stages  of  maturation.  The  ovum 
and  the  sperm  each  contain  half  the  original  number  of  chromo- 
somes. When  they  fuse  in  the  process  of  fertilization  the  full 
number  is  restored. 

Accessory  Reproductive  Organs.  The  organ  in  which  ova  are 
produced  is  known  as  the  ovary,  that  forming  spermatozoa,  as 
the  testis  or  testicle;  but  in  different  groups  of  animals  many  addi- 


-— — -——Ovarian  .egg. 


«**»"** 

polar  body. 
Second  polar  body  (abortive  ovum). 

FIG.  150. — Diagram  showing  the  genesis  of  the  egg  (after  Boveri).  A  similar 
diagram  in  which  all  the  daughter  cells  were  of  equal  size  would  serve  to  illustrate 
the  genesis  of  spermatozoa. 

tional  accessory  parts  may  be  developed.  Thus,  in  all  but  the 
very  lowest  Mammalia,  the  offspring  is  nourished  for  a  consid- 
erable portion  of  its  early  life  within  the  body  of  its  mother,  a 
special  cavity,  the  uterus  or  womb,  being  provided  for  this  purpose : 
the  womb  communicates  with  the  exterior  by  a  passage,  the  va- 
gina; and  two  tubes,  the  oviducts  or  Fallopian  lubes,  convey  the 
eggs  to  it  from  the  ovaries.  In  addition,  mammary  glands  provide 
milk  for  the  nourishment  of  the  young  in  the  first  months  after  birth. 
In  the  male  mammal  we  find  as  accessory  reproductive  organs,  vasa 
deferentia  which  convey  from  the  testes  the  seminal  fluid  contain- 
ing spermatozoa;  vesicular  seminales  (not  present  in  all  Mammalia), 
glands  whose  secretion  is  mixed  with  that  of  the  testes  or  is  ex- 
pelled after  it  in  the  sexual  act;  a  prostate  gland,  whose  secretion 
is  added  to  the  semen;  and  an  erectile  organ,  the  penis,  by  which 
the  fertilizing  liquid  is  conveyed  into  the  vagina  of  the  female. 


REPRODUCTION  563 

The  Male  Reproductive  Organs.  The  testes  in  man  are  paired 
tubular  glands,  which  lie  in  a  pouch  of  skin  called  the  scrotum. 
This  pouch  is  subdivided  internally  by  a  partition  into  right  and 
left  chambers,  in  each  of  which  a  testicle  lies.  The  chambers  are 
lined  inside  by  a  serous  membrane,  the  tunica  vaginalis,  and  this 
doubles  back  (like  the  pleura  round  the  lung)  and  covers  the  ex- 
terior of  the  gland.  Between  the  external  and  reflected  layers  of 
the  tunica  vaginalis  is  a  space  containing  a  small  quantity  of  lymph. 

The  testicles  develop  in  the  abdominal  cavity,  and  only  later 
(though  commonly  before  birth)  descend  into  the  scrotum, 
passing  through  apertures  in  the  muscles,  etc.,  of  the  abdominal 
wall,  and  then  sliding  down  over  the  front  of  the  pubes,  beneath 
the  skin.  The  cavity  of  the  tunica  vaginalis  at  first  is  a  mere 
offshoot  of  the  peritoneal  cavity,  and  its 
serous  membrane  is  originally  a  part  of  the 
peritoneum.  In  the  early  years  of  life  the 
passage  along  which  the  testis  passes  usually 
becomes  nearly  closed  up,  and  the  com- 
munication between  the  peritoneal  cavity 
and  that  of  the  tunica  vaginalis  is  also  ob- 
literated. Traces  of  this  passage  can,  how- 
ever, readily  be  observed  in  male  infants; 
if  the  skin  inside  the  thigh  be  tickled  a 
muscle  lying  beneath  the  skin  of  the  scrotum 
is  made  to  contract  reflexly,  and  the  testis 
is  jerked  up  some  way  towards  the  abdo-  FIQ  151_Diagran. 
men  and  quite  out  of  the  scrotum.  Some-  a  vertical  section  through 

j-  ,v  •  ji  the   testis.      a,   a,    tubuli 

times  the  passage  remains  permanently  open  seminiferi;  6,  vasa  recta; 
and  a  coil  of  intestine  may  descend  along  f^^^l^os^e, 
it  and  enter  the  scrotum,  constituting  an  epididymis.  h,  vas  def- 
inguinal  hernia  or  rupture.  A  hydrocele  is 
an  excessive  accumulation  of  liquid  in  the  serous  cavity  of  the 
tunica  vaginalis. 

Beneath  its  covering  of  serous  membrane  each  testis  has  a 
proper  fibrous  tunic  of  its  own.  This  forms  a  thick  mass  on 
the  posterior  side  of  the  gland,  from  which  partitions  or  septa 
(i,  Fig.  151)  radiate,  subdividing  the  gland  into  many  chambers. 
In  each  chamber  lie  several  greatly  coiled  seminiferous  tubules,  a, 
a,  averaging  in  length  0.68  meter  (27  inches)  and  in  diameter  only 


564  THE  HUMAN  BODY 

0.14  mm.  (i Fff  inch).  Their  total  number  in  each  gland  is  about 
800.  Near  the  posterior  side  of  the  testis  the  tubules  unite  to 
form  about  20  vasa  recta  (6),  and  these  pass  out  of  the  gland  at  its 
upper  end,  as  the  vasa  efferentia  (d),  which  become  coiled  up  into 
conical  masses,  the  coni  vasculosi;  these,  when  unrolled,  are  tubes 
from  15  to  20  cm.  (6-8  inches)  in  length;  they  taper  somewhat 
from  their  commencements  at  the  vasa  efferentia,  where  they  are 
0.5  mm.  (^V  inch)  in  diameter,  to  the  other  end  where  they  ter- 
minate in  the  epididymis  (e,  e,  Fig.  151).  The  latter  is  a  narrow 
mass,  slightly  longer  than  the  testicle,  which  lies  along  the  posterior 
side  of  that  organ,  near  the  lower  end  of  which  it  passes  (g)  into  the 
vas  defer  ens,  h.  If  the  epididymis  be  carefully  unravelled  it  is 
found  to  consist  of  a  tube  about  6  meters  (20  feet)  in  length,  and 
varying  in  diameter  from  0.35  to  0.25  mm.  (?V  to  9V  inch). 

The  vas  deferens  (h,  Fig.  151)  commences  at  the  lower  part  of 
the  epididymis  as  a  coiled  tube,  but  it  soon  ceases  to  be  convo- 
luted and  passes  up  beneath  the  skin  covering  the  inner  part  of 
the  groin,  till  it  gets  above  the  pelvis  and  then,  passing  through 
the  abdominal  walls,  turns  inwards,  backwards,  and  downwards, 
to  the  under  side  of  the  urinary  bladder,  where  it  joins  the  duct 
of  the  seminal  vesicle;  it  is  about  0.6  meter  (2  feet)  in  length  and 
2.5  mm.  (^  inch)  in  diameter.  Its  lining  epithelium  is  ciliated. 

The  vesiculce  seminales,  two  in  number,  are  membranous  recepta- 
cles which  lie,  one  on  each  side,  beneath  the  bladder,  between  it  and 
the  rectum.  They  are  commonly  about  5  cm.  (2  inches)  long  and  a 
little  more  than  a  centimeter  wide  (or  about  0.5  inch)  at  their 
broadest  part.  The  narrowed  end  of  each  enters  the  vas  deferens  on 
its  own  side,  the  tube  formed  by  the  union  being  the  ejaculatory 
duct,  which,  after  a  course  of  about  an  inch,  enters  the  urethra  near 
the  neck  of  the  bladder.  In  some  animals  the  vesiculce  seminales 
form  a  liquid  which  is  added  to  the  secretion  of  the  testis.  In  man 
they  appear  to  be  merely  reservoirs  in  which  the  semen  collects. 

The  prostate  gland  is  a  dense  body,  about  the  size  of  a  large 
chestnut,  which  surrounds  the  commencement  of  the  urethra; 
the  ejaculatory  ducts  pass  through  it.  It  is  largely  made  up  of 
fibrous  and  unstriped  muscular  tissues,  but  contains  also  a  num- 
ber of  small  secreting  saccules  whose  ducts  open  into  the  urethra. 
The  prostatic  secretion  though  small  in  amount  would  appear  to 
be  of  importance:  at  least  the  gland  remains  undeveloped  in  per- 


REPRODUCTION  565 

sons  who  have  been  castrated  in  childhood;  and  atrophies  after 
removal  of  the  testicles  later  in  life. 

The  male  urethra  leads  from  the  bladder  to  the  end  of  the  penis, 
where  it  terminates  in  an  opening,  the  meatus  urinarius.  It  is  de- 
scribed by  anatomists  as  made  up  of  three  portions,  the  prostatic, 
the  membranous,  and  the  spongy.  The  first  is  surrounded  by 
the  prostate  gland  and  receives  the  ejaculatory  ducts.  On  its 
posterior  wall,  close  to  the  bladder,  is  an  elevation  containing 
erectile  tissues  (see  below)  and  supposed  to  be  dilated  during 
sexual  congress,  so  as  to  cut  off  the  passage  to  the  urinary  recep- 
tacle. On  this  crest  is  an  opening  leading  into  a  small  recess,  the 
utricle,  which  is  of  interest,  since  the  study  of  its  embryology 
shows  it  to  be  an  undeveloped  male  uterus.  The  succeeding  mem- 
branous portion  of  the  urethra  is  about  1.8  cm.  (f  inch)  long;  the 
spongy  portion  lies  in  the  penis. 

The  penis  is  composed  mainly  of  erectile  tissue,  i.  e.,  tissues  so 
arranged  as  to  inclose  cavities  which  can  be  distended  by  blood. 
Covered  outside  by  the  skin,  internally  it  is  made  up  of  three 
elongated  cylindrical  masses,  two  of  which,  the  corpora  cavernosa, 
lie  on  its  anterior  side;  the  third,  the  corpus  spongiosum,  surrounds 
the  urethra  and  lies  on  the  posterior  side  of  the  organ  for  most  of 
its  length;  it,  however,  alone  forms  the  .terminal  dilatation,  or 
glans,  of  the  penis.  Each  corpus  cavernosum  is  closely  united  to 
its  fellow  in  the  middle  line  and  extends  from  the  pubic  bones, 
to  which  it  is  attached  behind,  to  the  glans  penis  in  front.  It  is 
enveloped  in  a  dense  connective-tissue  capsule  from  which  nu- 
merous bars,  containing  white  fibrous,  elastic,  and  unstriped 
muscular  tissues,  radiate  and  intersect  in  all  directions,  dividing 
its  interior  into  many  irregular  chambers  called  venous  sinuses. 
Into  these  blood  is  conveyed  partly  through  open  capillaries, 
partly  directly  by  the  open  ends  of  small  arteries;  this  blood  is 
carried  off  by  veins  proceeding  from  the  sinuses. 

The  arteries  of  the  penis  are  supplied  with  vasodilator  nerves, 
the  nervi  erigentes,  derived  from  the  sacral  plexus.  Under  cer- 
tain conditions  these  are  stimulated  and,  the  arteries  expanding, 
blood  is  poured  into  the  venous  sinuses  faster  than  the  veins  drain 
it  off;  the  latter  are  probably  also  at  the  same  time  compressed 
where  they  leave  the  penis  by  the  contraction  of  certain  muscles 
passing  over  them.  Simultaneously  the  involuntary  muscular 


566  THE  HUMAN  BODY 

tissue  of  the  bars  ramifying  through  the  erectile  masses  relaxes. 
As  a  result  the  whole  organ  becomes  distended  and  finally  rigid 
and  erect.  The  co-ordinating  center  of  erection  lies  in  the  lumbar 
region  of  the  spinal  cord,  and  may  be  excited  reflexly  by  mechan- 
ical stimulation  of  the  penis,  or  under  the  influence  of  nervous 
impulses  originating  in  the  brain  and  associated  with  sexual  emo- 
tions. The  corpus  spongiosum  resembles  the  corpora  cavernosa 
in  essential  structure  and  function. 

The  skin  of  the  penis  is  thin  and  forms  a  simple  layer  for  some 
distance;  towards  the  end  of  the  organ  it  separates  and  forms  a 
fold,  the  foreskin  or  prepuce,  which  doubles  back,  and,  becoming 
soft,  moist,  red,  and  very  vascular,  covers  the  glans  to  the  meatus 
urinarius,  where  it  becomes  continuous  with  the  mucous  mem- 
brane of  the  urethra;  in  it,  near  the  projecting  posterior  rim  of 
the  glans,  are  embedded  many  sebaceous  glands.  It  possesses 
nerve  end  organs  (genital  corpuscles)  which  must  resemble  end 
bulbs  in  structure. 

The  Seminal  Fluid.  The  essential  elements  of  the  testicular 
secretion  are  much  modified  cells,  the  spermatozoa,  which  are 
passed  out  with  some  albuminous  liquid.  The  spermatozoa 
(Fig.  152)  are  motile  bodies  about  0.04  mm.  (^  inch)  in  length. 
They  have  a  flattened  clear  body  or  head  and  a 
long  vibratile  tail  or  cilium;  the  portion  of  the 
tail  nearest  to  the  head  is  thicker  than  the  rest, 
and  is  known  as  the  neck.  The  mode  of  develop- 
ment of  a  spermatozoon  shows  that  the  head  is  a 
cell-nucleus  and  the  neck  and  tail  a  modified  cell- 


FIG.  152.-Sper- 

matozoa,  seen  from       Qn   cross-section   a   seminiferous   tubule   pre- 

the   front   and   in- 

side view,  a,  head;  sents  externally  a  well-marked  basement  mem- 
brane, upon  which  are  borne  several  layers  of 
cells;  the  lumen  or  bore  of  the  tubule  is  in  great  part  occupied 
by  the  tails  of  spermatozoa  projecting  from  some  of  the  lining 
cells.  The  outer  cells,  those  next  the  basement  membrane,  are 
arranged  in  a  single  layer,  and  are  usually  found  in  one  or  other 
stage  of  active  mitosis  (p.  24).  The  result  of  the  division  is  an 
outer  cell,  which  remains  next  the  basement  membrane  to  repeat 
the  process,  and  an  inner,  which  is  the  mother-cell  of  spermatozoa. 
The  latter  by  the  process  of  maturation  described  in  a  former 


REPRODUCTION  567 

paragraph  (p.  561)  gives  rise  to  four  cells  each  of  which  develops 
into  a  functional  spermatozoon. 

The  Reproductive  Organs  of  the  Female.  Each  ovary  (o, 
Fig.  153)  is  a  dense  oval  mass  about  3.25  cm.  (1.5  inches)  in 
length,  2  cm.  (0.75  inch)  in  width,  and  1.27  cm.  (0.5  inch)  in 
thickness;  it  weighs  from  4  to  7  grams  (60-100  grains).  The 
organs  lie  in  the  pelvic  cavity  enveloped  in  a  fold  of  peritoneum 
(the  broad  ligament),  and  receive  blood-vessels  and  nerves  along 
one  border.  From  time  to  time  ova  reach  the  surface,  burst 
through  the  enveloping  peritoneum,  and  are  received  by  the  wide 
fringed  aperture,  fi,  of  the  oviduct  or  Fallopian  tube,  od.  This 
tube  narrows  towards  its  inner  end,  where  it  communicates  with 
the  uterus,  and  is  lined  by  a  mucous  membrane,  covered  by 
ciliated  epithelium;  plain  muscular  tissue  is  also  developed  in  its 
wall.  The  uterus  (u,  c,  Fig.  153)  is  a  hollow  organ,  with  relatively 
thick  muscular  walls  (left  unshaded  in  the  figure) ;  it  contains  the 
fetus  during  pregnancy  and  expels  it  at  birth;  it  lies  in  the  pelvis 
between  the  urinary  bladder  and  the  rectum  (Fig.  154);  the  Fal- 
opian  tubes  open  into  its  anterior  corners.  It  is  free  above,  but 
its  lower  end  is  attached  to  and  projects  into  the  vagina.  In  the 
fully  developed  virgin  state  the  organ  is  somewhat  pear-shaped, 
but  flattened  from  before  back;  about  7.5  cm.  (3  inches)  in  length, 
5  cm.  (2  inches)  in  breadth  at  its  upper  widest  part,  and  2.5  cm. 
(1  inch)  in  thickness;  it  weighs  from  25  to  42  grams  (J  to  1J  oz.). 
The  upper  wider  portion  of  the  womb  is  known  as  its  body;  the 
cavity  of  this  is  produced  at  each  side  to  meet  the  openings  of  the 
Fallopian  tubes,  and  narrows  below  to  the  neck,  or  cervix  uteri, 
opposite  c  (Fig.  153),  the  communication  between  neck  and  body 
cavities  being  known  as  the  os  internum.  Below  this  the  neck 
dilates  somewhat:  it  forms  no  part  of  the  cavity  in  which  the  em- 
bryo is  retained  and  nourished.  The  lowest  part  of  the  cervix 
reaches  into  the  vagina  and  communicates  with  it  by  a  transverse 
aperture,  the  os  uteri.  During  gestation  or  pregnancy  the  fetus 
develops  in  the  body  of  the  womb,  which  becomes  greatly  enlarged 
and  rises  high  into  the  abdomen:  the  virgin  womb  lies  mainly 
below  the  level  of  the  bones  of  the  pelvis. 

The  chief  bulk  of  the  non-gravid  uterus  consists  of  a  coat  of 
plain  muscular  tissue,  arranged  in  a  thin  outer  longitudinal  layer, 
and  an  inner,  thicker,  consisting  of  oblique  and  circular  fibers. 


568  THE  HUMAN  BODY 

Between  the  layers  is  an  extensive  vascular  network,  with  many 
dilated  veins  or  venous  sinuses.  The  muscular  coat  is  lined  in- 
ternally by  a  ciliated  mucous  membrane,  and  is  covered  externally 
by  the  peritoneum,  bands  of  which  project  from  each  side  of  it 
as  the  broad  ligaments  (II,  Fig.  153).  The  outer  layer  of  the  mucous 
membrane  presents  a  very  well  developed  muscularis  mucosce, 
much  thicker  than  the  corresponding  layer  in  the  gastric  or  intes- 
tinal mucous  membranes  and  much  less  sharply  marked  off  from 
the  true  muscular  coat  outside  it.  The  main  thickness  of  the 
mucous  membrane  consists  of  closely  set,  simple  or  slightly 
branched,  tubular  glands;  between  these  is  a  close  blood-vascular 


FIG.  153. — The  uterus,  in  section,  with  the  right  Fallopian  tube  and  ovary,  as 
seen  from  behind,  about  I  the  natural  size,  u,  upper  part  of  uterus;  c,  cervix; 
v,  upper  part  of  vagina;  od,  Fallopian  tube;  fi,  its  fimbriated  extremity;  o,  ovary; 
po,  parovarium. 

and  lymphatic  network.  The  glands  open  on  the  interior  of  the 
womb;  they  and  the  mucous  membrane  between  their  mouths  are 
lined  by  a  single  layer  of  columnar  ciliated  cells,  with  some  gob- 
let cells  between  them.  In  the  cervix  the  glands  are  shorter,  and 
many  of  the  epithelial  cells  not  ciliated.  The  viscid  mucus  se- 
creted by  the  uterine  glands  is  alkaline  or  neutral. 

The  vagina  is  a  distensible  passage,  extending  .from  the  uterus 
to  the  exterior;  dorsally  it  rests  on  the  rectum,  and  ventrally  is 
in  contact  with  the  bladder  and  urethra.  It  is  lined  by  mucous 
membrane,  the  epithelium  of  which  is  much  like  the  epidermis 
but  thinner;  outside  the  mucous  membrane  the  vagina  is  made 
up  of  areolar,  erectile,  and  unstriped  muscular  tissues.  Around 


REPRODUCTION 


569 


its  lower  end  is  a  ring  of  striated  muscular  tissue,  the  sphincter 
vagince. 

The  vulva  is  a  general  term  for  all  the  portions  of  the  female  gen- 
erative organs  visible  from  the  exterior.  Over  the  front  of  the  pel- 
vis the  skin  is  elevated  by  adipose  tissue  beneath  it,  and  forms  the 
mons  Veneris.  From  this  two  folds  of  skin  (I,  Fig.  154),  the  labia 


FIG.  154. — The  viscera  of  the  female  pelvis  as  exposed  by  a  dorsiventral  me- 
dian section,  s,  symphysis  pubis;  v,  v',  urinary  bladder;  n,  urethra;  u,  uterus; 
va,  vagina;  r,  r',  rectum;  a,  anal  opening;  I,  right  labium  major;  n,  right  nympha; 
h,  hymen;  cl,  divided  cilitoris. 

majora,  extend  downwards  and  backwards  on  each  side  of  a  median 
cleft,  beyond  which  they  again  unite.  On  separating  the  labia 
majora  a  shallow  genito-urinary  sinus,  into  which  the  urethra  and 
vagina  open,  is  exposed.  At  the  upper  portion  of  this  sinus  lies  the 
clitoris,  a  small  and  very  sensitive  erectile  organ,  resembling  a 
miniature  penis  in  structure,  except  that  it  has  no  corpus  spon- 
giosum  and  is  not  traversed  by  the  uretha.  From  the  clitoris  de- 
scend two  folds  of  mucous  membrane,  the  nymphce  or  labia  interna, 


570 


THE  HUMAN  BODY 


between  which  is  the  vestibule,  a  recess  containing  above,  the  open- 
ing of  the  short  female  urethra,  and,  below,  the  aperture  of  the 
vagina,  which  is  in  the  virgin  more  or  less  closed  by  a  thin  dupli- 
cature  of  mucous  membrane,  the  hymen. 

Microscopic  Structure  of  the  Ovary.  The  main  mass  of  the 
ovary  consists  of  a  dense  connective-tissue  stroma,  containing  un- 
striped  muscle,  blood-vessels,  and  nerves:  it  is  covered  externally 
by  a  peculiar  germinal  epithelium,  and  contains  embedded  in  it 
many  minute  cavities,  the  Graafian  follicles,  in  which  ova  lie.  If  a 
thin  section  of  an  ovary  be  examined  with  the  microscope  many 


FIG.  155. — A  section  of  a  Mammalian  ovary,  considerably  magnified.  1,  outer 
capsule  of  ovary;  2,  3,  3',  stroma;  4,  blood-vessels;  5,  rudimentary  Graafian  fol- 
licles; 6,  7,  8,  follicles  beginning  to  enlarge  and  mature,  and  receding  from  the  sur- 
face; 9,  a  nearly  ripe  follicle  which  is  extending  towards  the  surface  preparatory  to 
discharging  the  ovum;  a,  membrana  granulosa;  b,  discus  proligerus;  c,  ovum,  with 
d,  germinal  vesicle,  and  e,  germinal  spot.  The  general  cavity  of  the  follicle  (in 
which  9  is  printed)  is  filled  with  lymph-like  transudation  liquid  during  life. 

hundreds  of  small  Graafian  follicles,  each  about  0.25  mm.  (ife 
inch)  in  diameter,  will  be  found  embedded  in  it  near  the  surface. 
These  are  lined  by  cells,  and  each  contains  a  single  ovum.  In  a 
woman  of  child-bearing  age  there  will  be  found  also,  deeper  in, 
larger  follicles  (7,  8,  9,  Fig.  155),  their  cavities  being  distended, 
during  life,  by  liquid;  in  these  the  essential  structure  may  be  more 
readily  made  out.  Each  has  an  external  fibrous  coat  constituted 
by  a  dense  and  vascular  layer  of  the  ovarian  stroma;  within  this 
come  several  layers  of  lining  cells  (9,  a,  Fig.  155)  constituting  the 
membrana  granulosa.  At  one  point,  b,  the  cells  of  this  layer  are 


REPRODUCTION  571 

heaped  up,  forming  the  discus  proligerus,  which  projects  into  the 
liquid  filling  the  cavity  of  the  follicle.  Buried  among  the  cells  of 
the  discus  proligerus  the  ovum,  c,  lies. 

The  Mammalian  Ovum.  As  the  Graafian  follicles  enlarge  the 
ova  grow  but  not  proportionately,  so  that  they  occupy  relatively 
less  of  the  cavities  of  the  larger  follicles :  the  cells  of  the  discus  pro- 
ligerus probably  elaborate  food  for  the  egg-cell  from  material  de- 
rived from  the  blood-vessels  which  form  a  close  network  around 
most  of  each  enlarging  Graafian  follicle  and  transude  crude  nutri- 
tive matter  into  the  liquid  filling  most  of  the  follicle.  The  fully 
formed  ovum  (Fig.  156)  is  about  0.2  mm.  (y|o  inch)  in  diameter: 
it  has  a  well-marked  outer  coat  or  sac,  a,  the  zona  radiata,  zona 
pellucida  or  vitelline  membrane,  surrounding  a  very  granular  cell- 
body  or  vitellus,  b,  in  which  is  a  conspicuous  nucleus,  c,  with  its 
characteristic  network  of  chromatin.  The 
main  bulk  of  the  vitellus  or  yolk  consists  of 
highly  refracting  spheroidal  particles  of 
nutritive  matter  (deutoplasm)  embedded 
in  and  concealing  a  true  protoplasmic 
reticulum.  In  the  eggs  of  birds  and  reptiles 
the  deutoplasm  is  in  very  large  amount 
and  forms  nearly  all  the  yolk,  the  proto- 
plasm being  for  the  most  part  aggregated  FlG-  156.  —  A  human 

ovum;  somewhat  diagram- 

around  the  nucleus  at  a  small  area  on  one  matic.  a,  zona  radiata;  6, 
side  of  the  yolk.  It  is  in  this  area  that  new  viteUu3  or  yolk;  c'  m 
cell-formation  occurs  and  the  embryo  is  built  up,  the  rest  of  the 
yolk  being  gradually  absorbed  by  it;  such  eggs  are  known  as 
mesoblastic  or  partly  dividing  eggs.  In  all  the  higher  mammalia 
the  deutoplasm  is  relatively  sparse  and  tolerably  evenly  mingled 
with  the  protoplasm,  and  the  whole  fertilized  ovum  divides  to 
form  the  first  cells  of  the  embryo:  such  eggs  are  named  holoblastic. 
Ovulation.  From  puberty,  during  the  whole  child-bearing 
period  of  life,  certain  comparatively  very  large  Graafian  follicles 
may  nearly  always  be  found  either  close  to  the  surface  of  the  ovary 
or  projecting  on  its  exterior.  These,  by  accumulation  of  liquid 
within  them,  have  become  distended  to  a  diameter  of  about  4  mm. 
(i  inch);  finally,  the  thinned  projecting  portion  of  the  wall  of  the 
follicle,  which  differs  from  the  rest  in  containing  few  blood-vessels, 
gives  way  and  the  ovum  is  discharged,  surrounded  by  some  cells  of 


572  THE  HUMAN  BODY 

the  discus  proligerus.  The  emptied  follicle  becomes  filled  up  with 
a  reddish-yellow  mass  of  cells,  and  constitutes  the  corpus  luteum, 
which  recedes  again  to  the  interior  of  the  ovary  and  disappears  in 
three  or  four  weeks,  unless  pregnancy  occur;  in  that  case  the  corpus 
luteum  increases  for  a  time,  and  persists  during  the  greater  part  of 
the  gestation  period. 

The  discharged  ovum  enters  the  Fallopian  tube  and  passes  down 
it  to  the  uterus.  Just  how  the  passage  from  the  ovary  to  the  tube 
occurs  is  not  clear,  although  it  is  suggested  that  the  cilia  which 
line  the  tube  set  up  by  their  motion  a  current  sufficient  to  convey 
the  ovum  across  the  intervening  space  and  into  its  mouth.  Having 
entered  the  Fallopian  tube  the  egg  slowly  passes  on  to  the  uterus, 
moved  by  the  cilia  lining  the  oviduct;  its  descent  probably  takes 
about  four  or  five  days;  if  not  fertilized,  it  dies  and  is  passed  out. 

Menstruation.  Ovulation  occurs  during  the  sexual  life  of  a 
healthy  woman  at  intervals  of  about  four  weeks,  and  is  attended 
with  important  changes  in  other  portions  of  the  generative  ap- 
paratus. The  ovaries  and  Fallopian  tubes  become  congested. 
The  mucous  membrane  of  the  uterus  at  or  just  before  the  periods 
of  ovulation  becomes  swollen  and  soft,  and  minute  hemorrhages 
occur  in  its  substance.  The  superficial  layers  are  broken  down, 
and  discharged  along  with  more  or  less  blood,  constituting  the 
menses,  or  monthly  sickness,  which  commonly  lasts  from  three  to 
five  days.  During  this  time  the  vaginal  secretion  is  also  increased, 
and,  mixed  with  the  blood  discharged,  more  or  less  alters  its  color 
and  usually  destroys  its  coagulating  power.  Except  during  preg- 
nancy and  while  suckling,  menstruation  occurs  at  the  above  in- 
tervals, from  puberty  up  to  about  the  forty-fifth  year;  the  periods 
then  become  irregular,  and  finally  the  discharges  cease;  this  is  an 
indication  that  ovulation  has  come  to  an  end,  and  that  the  sexual 
life  of  the  woman  is  completed.  This  time,  the  climacteric  or  "  turn 
of  life,"  is  a  critical  one;  various  local  disorders  are  apt  to  super- 
vene, and  even  mental  derangement. 

Hygiene  of  Menstruation.  During  menstruation  there  is  apt 
to  be  more  or  less  general  discomfort  and  nervous  irritability;  the 
woman  is  not  quite  herself,  and  those  responsible  for  her  happiness 
ought  to  watch  and  tend  her  with  special  solicitude,  forbearance, 
and  tenderness,  and  protect  her  from  anxiety  and  agitation.  Any 
strong  emotion,  especially  of  a  disagreeable  character,  is  apt  to 


REPRODUCTION  573 

check  the  flow,  and  this  is  always  liable  to  be  followed  by  serious 
consequences.  A  sudden  chill  often  has  the  same  effect;  hence  a 
menstruating  woman  ought  always  to  be  warmly  clad,  and  take 
more  than  usual  care  to  avoid  draughts  or  getting  wet.  At  these 
periods,  also,  the  uterus  is  enlarged  and  heavy,  and  being  (as  may 
be  seen  in  Fig.  152)  but  slightly  supported,  and  that  near  its  lower 
end,  it  is  especially  apt  to  be  displaced  or  distorted;  it  may  tilt 
forwards  or  sideways  (versions  of  the  uterus),  or  be  bent  where  the 
neck  and  body  of  the  organ  meet  (flexion) .  Hence  violent  exercise 
at  this  time  should  be  avoided,  though  there  is  no  reason  why  a 
properly  clad  woman  should  not  take  her  usual  daily  walk. 

The  absence  of  the  menstrual  flow  (amenorrhea)  is  normal  dur- 
ing pregnancy  and  while  suckling;  and  in  some  rare  cases  it  never 
occurs  throughout  life,  even  in  healthy  women  capable  of  child- 
bearing.  Usually,  however,  the  non-appearance  of  the  menses  at 
the  proper  periods  is  a  serious  symptom,  and  one  which  calls  for 
prompt  measures.  In  all  such  cases  it  cannot  be  too  strongly  im- 
pressed upon  women  that  the  most  dangerous  thing  to  do  is  to  take 
drugs  tending  to  induce  the  discharge,  except  under  skilled  ad- 
vice; to  excite  the  flow,  in  many  cases,  as  for  example  occlusion  of 
the  os  uteri,  or  in  general  debility  (when  its  absence  is  a  conserva- 
tive effort  of  the  system),  may  have  the  most  disastrous  results. 

Fertilization.  As  the  ovum  descends  the  Fallopian  tube  the 
changes  of  menstruation  are  taking  place  in  the  uterus.  Fertiliza- 
tion usually  takes  place  in  a  Fallopian  tube.  The  spermatozoa  are 
carried  along  partly,  perhaps,  by  the  contractions  of  the  muscular 
walls  of  the  female  cavities,  but  mainly  by  their  own  activity. 
Occasionally  the  ovum  is  fertilized  before  reaching  the  Fallopian 
tube  and  fails  to  enter  it,  giving  rise  to  an  extra-uterine  pregnancy. 

The  actual  process  of  the  fertilization  of  the  ovum  has  only  been 
observed  in  the  lower  animals,  but  there  is  no  doubt  that  the  phe- 
nomena are  the  same  in  all  essentials  in  all  cases.  Some  of  the 
spermatozoa  penetrate  the  zona  radiata  and  one  of  them  enters  the 
ovum.  After  the  entrance  of  a  single  spermatozoan  a  membrane 
forms  inside  the  zona  radiata  (Fig.  156)  which  prevents  others 
from  entering.  The  head  of  the  spermatozoan,  which  is  chiefly 
chromatin,  becomes  separated  from  the  tail;  the  latter  disappears, 
probably  by  absorption  into  the  substance  of  the  ovum.  The  head, 
now  known  as  the  male  pronucleus,  approaches  the  chromatin  mass 


574  THE  HUMAN  BODY 

of  the  ovum,  at  this  stage  called  the  female  pronucleus,  and  the  two 
fuse  into  a  single  fertilization  or  segmentation  nucleus.  This  process 
restores  to  the  ovum  the  typical  number  of  chromosomes. 

In  addition  to  the  chromatin  material  the  spermatozoan  also 
brings  with  it  the  stimulus  to  cell  division,  so  that  immediately 
after  the  formation  of  the  fertilization  nucleus  segmentation  begins. 
In  the  first  and  subsequent  divisions,  which,  as  stated  earlier,  are 
by  the  process  of  mitosis,  the  chromatin  is  so  distributed  that  each 
daughter  cell  receives  equal  amounts  from  ovum  and  sperm. 

Heredity.  The  relative  influence  of  the  two  parents  upon  the 
characteristics  of  the  offspring  has  been  studied  and  speculated 
upon  for  ages.  With  the  discovery  of  the  chromosomes  it  has 
become  evident  that  to  them  we  must  look  largely,  if  not  wholly, 
for  the  agency  of  hereditary  transmission.  So  far  as  paternal 
characters  impress  themselves  they  must  do  so  through  the 
chromosomes  since  the  sperm  contributes  virtually  nothing  else. 
To  the  Austrian  monk  Mendel  and  the  Dutch  botanist  DeVries 
we  owe  the  conception  of  the  machinery  of  heredity  which  has 
clarified  our  ideas  on  the  subject  more  than  all  the  previous  work 
has  done. 

According  to  this  conception  the  chromosomes  are  to  be  looked 
upon  as  made  up  of  groups  of  determiners  of  hereditary  traits.  If 
in  the  union  of  maternal  and  paternal  chromosomes  all  the  factors 
are  harmonious  the  offspring  will  be  a  perfect  blend  of  the  parents. 
This  condition  is  not  realized,  however,  unless  the  parents  are 
alike  in  practically  all  respects.  Thus  if  one  is  light  haired  and 
the  other  dark,  or  if  one  has  blue  eyes  and  the  other  brown  the 
chromosomes  which  bear  these  traits  are  in  conflict.  Mendel 
found  that  under  these  circumstances  usually  one  of  the  conflicting 
traits  appears  in  the  offspring  and  the  other  is  suppressed.  The 
one  which  appears  is  called  dominant,  the  other  recessive.  More 
rarely  there  is  a  blending  of  the  characters,  as  seen  in  the  inter- 
mediate skin  coloration  in  mulattos.  Even  though  the  recessive 
traits  are  not  apparent  in  the  presence  of  dominant  conflicting 
characters  the  chromosomes  which  determine  them  persist  un- 
changed, and  will  be  found  in  the  germ-cells.  An  individual  whose 
germ  plasm  contains  such  conflicting  chromosomes  is  known  as  a 
hybrid.  Experiment  has  shown  that  during  the  development  of 
the  germ  plasm  of  hybrids  there  is  a  separation  of  conflicting  char- 


REPRODUCTION  575 

acters  so  that  any  given  germ-cell  may  contain  either  the  deter- 
miners for  the  dominant  character  or  those  for  the  recessive,  but 
not  both.  This  principle  of  "the  purity  of  the  germ-cell"  is  the 
corner  stone  of  Mendelian  inheritance.  When  such  hybrids  mate 
it  is  evident  that  there  are  four  possible  combinations  of  germ-cells 
that  may  occur.  If  we  designate  the  dominant  by  D  and  the 
recessive  by  R,  the  maternal  germ-cell  by  m  and  the  paternal  by 
p,  we  may  represent  the  four  possible  combinations  thus  Dm  + 
Dp;  Dm  +  Rp;  Rm  +  Dp;  Em  +  Rp.  Of  these  four  the  first 
and  last  are  pure;  the  second  and  third  are  hybrid.  Since  the 
dominant  character  is  present  in  the  hybrids  they  will  have  the 
same  appearance  as  number  1,  which  is  pure  dominant.  Number  4, 
however,  which  is  pure  recessive,  will  have  the  appearance  char- 
acteristic of  the  recessive  trait.  A  simple  illustration  is  furnished 
by  eye  color.  Brown  eyes  are  dominant  and  blue  eyes  recessive. 
According  to  the  principles  just  stated  brown-eyed  persons  may 
be  pure  dominant  or  hybrid,  but  all  blue-eyed  persons  are  pure 
recessive.  If  both  parents  are  blue-eyed  all  the  offspring  must 
therefore  be  blue-eyed  also.  If  both  parents  are  brown-eyed  the 
eye  color  of  the  offspring  will  depend  on  whether  the  parents  are 
pure  dominant  or  hybrid.  If  one  or  both  are  pure  dominant  all  off- 
spring will  have  brown  eyes.  If  both  are  hybrid  one  in  four  of  the 
offspring  may  have  blue  eyes. 

The  actual  situation  is  complicated  by  the  numerous  factors  that 
may  be  in  conflict,  but  extension  of  the  principle  stated  above  is 
believed  to  cover  all  forms  of  hereditary  transmission  that  are 
susceptible  of  modification  by  breeding.  Whether  the  fundamental 
features  of  inheritance;  those  that  make  the  offspring  of  dogs  dogs 
and  of  roses  roses,  are  also  Mendelian;  is  at  present  a  subject  of 
discussion. 

Sex  Determination.  An  interesting  application  of  the  prin- 
ciples of  Mendel  is  in  the  determination  of  sex.  It  appears  that 
in  general  in  the  germ-cells  of  males  there  is  one  less  chromosome 
than  in  the  cells  of  females  of  the  same  species.  Human  females, 
for  example,  have  48  chromosomes  and  human  males  47.  In  the 
reduction  division  that  occurs  in  connection  with  maturation 
(p.  561)  one  of  the  daughter  cells  that  is  formed  from  the  division 
of  the  primary  sperm  cell  has  only  23  chromosomes,  while  the 
other  has  24.  The  subsequent  division  of  the  daughter  cells  to 


576  THE  HUMAN  BODY 

form  sperm  preserves  the  same  relation  of  numbers.  Half  of  the 
spermatozoa,  therefore,  will  have  23  chromosomes  and  the  other 
half  24.  Since  the  female  germ-cells  contain  48  chromosomes  each 
ovum  will  have  24.  In  the  fertilization  of  the  ovum,  if  the  pene- 
trating sperm  contains  24  chromosomes  the  offspring  will  be 
female;  if  only  23  the  offspring  will  be  male.  Obviously  this  has 
little  practical  bearing  on  the  problem  of  artificial  sex  determina- 
tion, except  in  so  far  as  it  shows  the  futility  of  attempting  to 
bring  it  about.  It  serves,  however,  to  explain  a  number  of  facts 
of  inheritance.  For  example,  in  certain  species  of  insects  all  the 
fertilized  eggs  give  rise  to  females;  the  males  being  derived  from 
eggs  that  develop  without  fertilization.  This  is  explained  by  the 
fact  that  only  those  spermatozoa  that  have  the  full  number  of 
chromosomes  develop  to  functional  maturity. 

Impregnation.  The  fertilized  ovum,  which,  as  we  have  seen 
(p.  573),  receives  the  sperm  in  the  Fallopian  tube,  continues  its 
descent  to  the  uterine  cavity,  but,  instead  of  lying  dormant  like 
the  unfertilized,  segments  (p.  29),  and  forms  a  morula.  This  be- 
comes embedded  in  the  soft,  vascular  uterine  mucous  membrane 
from  which  it  imbibes  nourishment,  and  which,  instead  of  being 
cast  off  in  subsequent  menstrual  discharges,  is  retained  and  grows 
during  the  whole  of  pregnancy,  having  important  duties  to  dis- 
charge in  connection  with  the  nutrition  of  the  embryo. 

Sexual  congress  is  most  apt  to  be  followed  by  pregnancy  if  it 
occur  immediately  after  a  menstrual  period;  at  those  times  a  ripe 
ovum  is  usually  in  the  Fallopian  tube,  near  the  upper  end  of  which 
it  is  probably  fertilized  in  the  majority  of  cases.  There  is  some 
difference  of  opinion  as  to  whether  the  rupture  of  the  Graafian 
follicle  occurs  most  frequently  immediately  before  the  appearance 
of  the  menstrual  flow,  or  towards  its  close;  but  the  preponderance 
of  evidence  favors  the  latter  view.  The  menstrual  process  probably 
is  a  special  preparation  of  the  womb  for  the  reception  of  an  embryo 
and  its  nourishment.  There  is,  however,  evidence  that  ova  are 
occasionally  discharged  at  other  than  the  regular  monthly  periods 
of  ovulation  and  may  be  fertilized  and  cause  a  pregnancy. 

Pregnancy.  When  the  mulberry  mass  reaches  the  uterine  cav- 
ity the  mucous  membrane  lining  the  latter  grows  rapidly  and 
forms  a  new,  thick,  very  vascular  lining  to  the  womb,  known  as  the 
decidua.  At  one  point  on  this  the  morula  becomes  attached,  the 


REPRODUCTION  577 

decidua  growing  up  around  it.  As  pregnancy  advances  and  the 
embryo  grows,  it  bulges  out  into  the  uterine  cavity  and  pushes 
before  it  that  part  of  the  decidua  which  has  grown  over  it  (the 
decidua  reflexa) ;  at  about  the  end  of  the  third  month  this  coalesces 
with  the  decidua  lining  the  opposite  sides  of  the  uterine  cavity  so 
that  the  two  can  no  longer  be  separated.  That  part  of  the  decidua 
(decidua  serotina)  against  which  the  morula  is  first  attached  sub- 
sequently undergoes  a  great  development  in  connection  with  the 
formation  of  the  placenta  (see  below).  Meanwhile  the  whole 
uterus  enlarges;  its  muscular  coat  especially  thickens.  At  first  the 
organ  still  lies  within  the  pelvis,  where  there  is  but  little  room  for 
it;  it  accordingly  presses  on  the  bladder  and  rectum  (see  Fig.  152) 
and  the  nerves  in  the  neighborhood,  frequently  causing  consider- 
able discomfort  or  pain;  and,  reflexly,  often  exciting  nausea  or 
vomiting  (the  morning  sickness  of  pregnancy).  Later  on,  the  preg- 
nant womb  escapes  higher  into  the  abdominal  cavity,  and  although 
then  larger,  the  soft  abdominal  walls  more  readily  make  room  for 
it,  and  less  discomfort  is  usually  felt,  though  there  may  be  short- 
ness of  breath  and  palpitation  of  the  heart  from  interference  with 
the  diaphragmatic  movements.  All  tight  garments  should  at  this 
time  be  especially  avoided;  the  woman's  breathing  is  already  suffi- 
ciently impeded,  and  the  pressure  may  also  injure  the  developing 
child.  Meanwhile,  changes  occur  elsewhere  in  the  Body.  The 
breasts  enlarge  and  hard  masses  of  developing  glandular  tissue 
can  be  felt  in  them;  and  there  may  be  mental  symptoms:  depres- 
sion, anxiety,  and  an  emotional  nervous  state. 

During  the  whole  period  of  gestation  the  woman  is  not  merely 
supplying  from  her  blood  nutriment  for  the  fetus,  but  also,  through 
her  lungs  and  kidneys,  getting  rid  of  its  wastes;  the  result  is  a 
strain  on  her  whole  system  which,  it  is  true,  she  is  constructed  to 
bear  and  will  carry  well  if  in  good  health,  but  which  is  severely 
felt  if  she  be  feeble  or  suffering  from  disease.  The  healthy  married 
woman  who  endeavors  to  evade  motherhood  because  she  thinks 
she  will  thus  preserve  her  personal  appearance,  or  because  she  dis- 
likes the  trouble  of  a  family,  deserves  but  little  sympathy;  she  is 
trying  to  escape  a  duty  voluntarily  undertaken,  and  owed  to  her 
husband,  her  country,  and  her  race;  but  she  whose  strength  is  un- 
dermined and  whose  life  is  made  one  long  discomfort  for  the  sexual 
gratification  of  her  husband  deserves  every  consideration,  and  the 


578  THE  HUMAN  BODY 

family  physician  ought  perhaps  to  warn  the  husband  more  fre- 
quently than  he  does  of  the  risk  to  a  delicate  wife's  health,  or  even 
life,  of  frequent  pregnancies :  and  the  husband  should  control  him- 
self accordingly. 

The  Intra-Uterine  Nutrition  of  the  Embryo.  At  first  the  em- 
bryo is  nourished  by  absorption  of  materials  from  the  soft  vas- 
cular lining  of  the  womb;  as  it  increases  in  size  this  is  not  suffi- 
cient, and  a  new  organ,  the  placenta,  is  formed  for  the  purpose. 
A  fetal  outgrowth,  the  allantois,  plants  itself  firmly  against  the 
decidua  serotina,  and  villi  developed  on  it  burrow  from  its  surface 
into  the  uterine  mucous  membrane.  In  the  deeper  layer  of  this 
latter  are  large  sinuses  through  which  the  maternal  blood  flows, 
and  into  which  the  allantoic  villi  project.  Blood  is  brought  from 
the  fetus  to  the  allantois  by  arteries  and  carried  back  by  veins 
after  traversing  the  capillaries  of  the  villi,  and  while  flowing 
through  these  receives,  by  dialysis,  oxygen  and  food  materials 
from  the  maternal  blood,  and  gives  up  to  it  carbon  dioxid,  urea, 
and  other  wastes.  There  is  thus  no  direct  intermixture  of  the  two 
bloods;  the  embryo  is  from  the  first  an  essentially  separate  and 
independent  organism.  The  allantois  and  decidua  serotina  be- 
coming inseparably  united  together  form  the  placenta,  which  in 
the  human  species  is,  when  fully  developed,  a  round  thick  mass 
about  the  size  of  a  large  saucer,  connected  to  the  embryo  by  a 
narrow  stalk,  the  umbilical  cord,  in  which  blood-vessels  run  to  and 
from  the  placenta. 

Parturition.  At  the  end  of  from  275  to  280  days  from  fertiliza- 
tion of  the  ovum  (conception)  pregnancy  terminates,  and  the  child 
is  expelled  by  powerful  contractions  of  the  uterus,  assisted  by 
those  of  the  muscles  in  the  abdominal  walls.  When  the  child  is 
born,  it  has  attached  to  its  navel  the  umbilical  cord,  which  is 
then  usually  ligatured  and  cut  across:  some  good  authorities, 
however,  maintain  that  this  should  not  be  done  until  after  the 
contractions  which  expel  the  placenta,  as. otherwise  a  quantity 
of  the  infant's  blood  remains  in  that  organ;  the  loss  of  which 
might  be  serious  to  a  feeble  infant.  Shortly  after  the  birth  of  the 
child  renewed  uterine  contractions  detach  and  expel  the  placenta, 
both  its  fetal  or  allantoic  and  maternal  or  decidual  part,  as  the 
afterbirth.  Where  it  is  torn  loose  from  the  uterine  wall  large  blood 
sinuses  are  left  open;  hence  a  certain  amount  of  bleeding  occurs, 


REPRODUCTION  579 

but  in  normal  labor  this  is  speedily  checked  by  firm  contraction 
of  the  uterus.  Should  this  fail  to  take  place  profuse  hemorrhage 
occurs  (flooding)  and  the  mother  may  bleed  to  death  in  a  few 
minutes  unless  prompt  measures  are  adopted. 

For  a  few  days  after  delivery  there  is  some  discharge  (the 
lochia)  from  the  uterine  cavity:  the  whole  decidua  being  broken 
down  and  carried  off,  to  be  subsequently  replaced  by  new  mucous 
membrane.  The  muscular  fibers  developed  in  the  uterine  wall  in 
such  large  quantities  during  pregnancy  undergo  rapid  fatty  de- 
generation and  are  absorbed,  and  in  a  few  weeks  the  organ  re- 
turns almost  to  its  original  size.  The  parturient  woman  is  es- 
pecially apt  to  take  infectious  diseases;  and  these,  should  they 
attack  her,  are  fatal  in  a  very  large  percentage  of  cases.  Very 
special  care  should  therefore  be  taken  to  keep  all  contagion  from 
her. 

There  is  a  current  impression  that  a  pregnancy,  once  com- 
menced, can  be  brought  to  a  premature  end,  especially  in  its  early 
stages,  without  any  serious  risk  to  the  woman.  That  belief  is 
erroneous.  Premature  delivery,  early  or  late  in  pregnancy,  is 
always  more  dangerous  than  natural  labor  *at  the  proper  term; 
the  physician  has  sometimes  to  induce  it,  as  when  a  malformed 
pelvis  makes  normal  parturition  impossible,  or  the  general  de- 
rangement of  health  accompanying  the  pregnancy  is  such  as  to 
threaten  the  mother's  life;  but  the  occasional  necessity  of  decid- 
ing whether  it  is  his  duty  to  procure  an  abortion  is  one  of  the  most 
serious  responsibilities  he  meets  with  in  the  course  of  his  profes- 
sional work. 

The  production  of  abortion,  even  in  the  first  stages  of  preg- 
nancy, by  the  taking  of  drugs,  the  so-called  abortifacients,  a  prac- 
tice which  seems  to  have  gained  considerable  headway  through 
the  widespread  advertisement  of  their  wares  by  unscrupulous 
vendors  of  " patent  medicines,"  is  so  dangerous  to  the  health, 
and  even  the  life,  of  the  woman  who  practices  it  that  no  consid- 
eration sanctions  it. 

Lactation.  The  mammary  glands  for  several  years  after  birth 
remain  small,  and  alike  in  both  sexes.  Towards  puberty  they  be- 
gin 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 


580  THE  HUMAN  BODY 

nipple,  and  the  skin  around  this  forms  a  colored  circle,  the  areola. 
In  virgins  the  areolie  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  sep- 
arate galactophorous  duct,  made  by  the  union  of  smaller  branches 
from  the  lobules,  runs  towards  the  nipple,  all  converging  beneath 
the  areola.  There  each  dilates  and  forms  a  small  elongated  reser- 
voir in  which  the  milk  may  temporarily  collect.  Beyond  this  the 
ducts  narrow  again,  and  each  continues  to  a  separate  opening  on 
the  nipple.  Embedding  and  enveloping  the  lobes  of  the  gland  is  a 
quantity  of  firm  adipose  tissue  v/hich  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,  and  the  gland-lobes  can  be  felt  as  hard  masses  through 
the  super jacent  skin  and  fat.  Even  at  parturition,  however,  their 
functional  activity  is  not  fully  established.  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  simultaneously,  while 
newly  developed  gland-cells  take  the  place  of  those  destroyed. 
In  the  milk  first  secreted  after  accouchement  (the  colostrum)  the 
cell  destruction  is  incomplete,  and  many  cells  still  float  in  the 
liquid,  which  has  a  yellowish  color;  this  first  milk  acts  as  a  pur- 
gative 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. 

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  ap- 
pearance, 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 
justifiable  grounds  for  a  mother's  not  undertaking  this  part  of  her 
duties;  a  physician  is  the  proper  person  to  decide. 


REPRODUCTION  581 

In  a  healthy  woman,  not  suckling  her  child,  ovulation  and 
menstruation  recommence  about  six  weeks  after  childbirth;  a 
nursing  mother  usually  does  not  menstruate  for  ten  or  twelve 
months;  the  infant  should  then  be  weaned. 

When  an  infant  cannot  be  suckled  by  its  mother  or  a  wet-nurse 
an  important  matter  is  to  decide  what  is  the  best  food  to  substi- 
tute. Good  cow's  milk  contains  rather  more  fats  than  that  of  a 
woman,  and  much  more  casein;  the  following  table  gives  averages 
in  1,000  parts  of  milk: 

Woman  Cow 

Casein 28.0  54.0 

Butter 33.5  43.0 

Milk-sugar 44.5  42.5 

Inorganic  matters 4.75  7.75 

The  inorganic  matters  of  human  milk  yield,  on  analysis,  in 
100  parts — calcium  carbonate,  6.9;  calcium  phosphate,  70.6; 
sodium  chlorid,  9.8;  sodium  sulphate,  7.4;  other  salts,  5.3.  The 
lime  salts  are  of  especial  importance  to  the  child,  which  has  still 
to  build  up  nearly  all  its  bony  skeleton. 

When  undiluted  cow's  milk  is  given  to  infants  they  rarely  bear 
it  well;  the  too  abundant  casein  is  vomited  in  loose  coagula.  The 
milk  should  therefore  be  diluted  with  half  or,  for  very  young 
children,  even  two-thirds  its  bulk  of  water.  This,  however,  brings 
down  the  percentage  of  sugar  and  fat  below  the  proper  amount. 
The  sugar  is  commonly  replaced  by  adding  cane-sugar;  but  sugar 
of  milk  is  readily  obtainable  and  is  better  for  the  purpose.  If 
used  at  all  it  should,  however,  be  employed  from  the  first;  it 
sweetens  much  less  than  cane-sugar,  and  infants  used  to  the  latter 
often  refuse  milk  in  which  milk-sugar  is  substituted.  In  order  to 
bring  the  percentage  of  fat  up  to  normal  it  is  usual  to  dilute,  not 
" whole  milk"  but  "top  milk."  The  latter,  after  the  milk  has 
stood  for  a  few  hours,  contains  enough  of  the  rising  cream  to 
supply  the  needed  fat.  As  the  infant  grows  older  less  diluted 
cow's  milk  may  gradually  be  given;  after  the  seventh  or  eighth 
month  no  water  need  be  added. 

It  should  not  be  necessary  to  emphasize  the  vital  importance  of 
giving  to  infants  only  the  purest  milk  obtainable.  It  is.  unfor- 
tunately true  that  the  milk  produced  in  the  average  dairy  is  not 


582  THE  HUMAN  BODY 

only  dirty  but  swarming  with  micro-organisms.  In  cities  it  has 
become  the  practice  for  medical  societies  to  inspect  various  dairies 
and  set  their  seal  of  approval  upon  those  that  fulfil  the  sanitary 
conditions  essential  to  the  production  of  pure,  clean  milk.  The 
slightly  higher  cost  of  such  " certified"  milk  should  not  be  allowed 
to  bar  it  from  homes  where  children  are  to  be  fed  except  where 
extreme  poverty  makes  its  procurement  impossible.  In  small 
towns  and  in  the  country  personal  inspection  of  the  source  of 
the  milk  supply  on  the  part  of  parent  or  physician  should  give 
assurance  of  its  cleanliness.  Where  it  is  impossible  to  procure 
milk  free  from  suspicion,  pasteurization  (heating  to  120°  F.  for 
20  minutes)  should  be  resorted  to.  This  destroys  most  of  the 
dangerous  organisms,  but  of  course  is  not  a  complete  substitute 
for  cleanliness  and  care  in  the  production  of  the  milk  in  the  be- 
ginning. 

In  the  first  weeks  after  birth  it  is  no  use  to  give  an  infant  starchy 
foods,  as  arrowroot.  The  greater  part  of  the  starch  passes  through 
the  bowels  unchanged;  apparently  because  the  pancreas  has  not 
yet  fully  developed,  and  has  not  commenced  to  make  its  starch- 
converting  enzym.  Later  on,  starchy  substances  may  be  added 
to  the  diet  with  advantage,  but  it  should  be  borne  in  mind  that 
they  cannot  form  the  chief  part  of  the  child's  food;  it  needs  pro- 
teins for  the  formation  of  its  tissues,  and  amyloid  foods  contain 
none  of  these.  Many  infants  are,  ignorantly,  half  starved  by 
being  fed  almost  entirely  on  such  things  as  corn-flour  or  arrowroot. 

Puberty.  The  condition  of  the  reproductive  organs  of  each 
sex  described  in  preceding  pages  is  that  found  in  adults;  although 
mapped  out,  and,  to  a  certain  extent,  developed  before  birth  and 
during  childhood,  these  parts  grow  but  slowly  and  remain  func- 
tionally incapable  during  the  early  years  of  life;  then  they  com- 
paratively rapidly  increase  in  size  and  become  physiologically 
active;  the  boy  or  girl  becomes  man  or  woman. 

This  period  of  attaining  sexual  maturity,  known  as  puberty, 
takes  place  from  the  eleventh  to  the  sixteenth  year,  and  is  accom- 
panied by  changes  in  many  parts  of  the  Body.  Hair  grows  more 
abundantly  on  the  pubes  and  genital  organs,  and  in  the  armpits, 
in  the  male  also  on  various  parts  of  the  face.  The  lad's  shoulders 
broaden ;  his  larynx  enlarges,  and  lengthening  of  the  vocal  cords 
causes  a  fall  in  the  pitch  of  his  voice;  all  the  reproductive  organs 


REPRODUCTION  583 

increase  in  size;  fully  formed  seminal  fluid  is  secreted,  and  erec- 
tions of  the  penis  occur.  As  these  changes  are  completed  spon- 
taneous nocturnal  seminal  emissions  take  place  from  time  to  time 
during  sleep,  being  usually  associated  with  voluptuous  dreams. 
Many  a  young  man  is  alarmed  by  these;  he  has  been  kept  in  ig- 
norance of  the  whole  matter,  is  too  bashful  to  speak  of  it,  and 
getting  some  quack  advertisement  thrust  into  his  hand  in  the 
street  is  alarmed  to  learn  that  his  strength  is  being  drained  off, 
and  that  he  is  on  the  highroad  to  idiocy  and  impotence  unless 
he  place  himself  in  the  hands  of  the  advertiser.  Lads  at  this 
period  of  life  should  have  been  taught  that  such  emissions,  when 
not  too  frequent  and  not  excited  by  any  voluntary  act  of  their 
own,  are  natural  and  healthy.  They  may,  however,  occur  too 
often;  if  there  is  any  reason  to  suspect  this,  the  family  physician 
should  be  consulted,  as  the  healthy  activity  of  the  sexual  organs 
varies  so  much  in  individuals  as  to  make  it  impossible  to  lay  down 
numerical  rules  on  the  subject.  The  best  preventives  in  any  case 
are,  however,  not  drugs,  but  an  avoidance  of  too  warm  and  soft 
a  bed,  plenty  of  muscular  exercise,  and  keeping  out  of  the  way  of 
anything  likely  to  excite  the  sexual  instincts. 

In  the  woman  the  pelvis  enlarges  considerably  at  puberty,  and, 
commonly,  more  subcutaneous  adipose  tissue  develops  over  the 
Body  generally,  but  especially  on  the  breasts  and  hips;  conse- 
quently the  contours  become  more  rounded.  The  external  genera- 
tive organs  increase  in  size,  and  the  clitoris  and  nympha3  become 
erectile.  The  uterus  grows  considerably,  the  ovaries  enlarge,  some 
Graafian  follicles  ripen,  and  menstruation  commences. 

Hormones  of  the  Reproductive  System.  The  interrelations  of 
various  processes  in  the  functioning  of  the  reproductive  mechan- 
ism are  many  of  them  very  striking  and  they  have  long  been  the 
subject  of  investigation.  The  development  of  the  so-called  second- 
ary sexual  characters  at  puberty,  where  in  a  few  weeks  the  vocal 
cords  change  and  hair  develops  over  various  parts  of  the  body,  is 
a  good  example  of  the  sort  of  interrelations  that  occur  in  this  sys- 
tem. The  fact,  known  for  centuries,  that  castration  in  early  life 
prevents  the  appearance  of  the  secondary  sexual  characters,  shows 
that  they  are  directly  dependent  on  the  reproductive  organs.  Be- 
fore the  idea  of  hormone  action  had  crystallized  to  its  present  form 
some  such  mechanism  had  been  postulated  for  the  reproductive 


584  THE  HUMAN  BODY 

system.  For  it  is  difficult  to  explain  such  effects  as  those  of  castra- 
tion on  any  other  basis  than  that  the  generative  organs  elaborate 
some  control-exercising  substance  of  which  the  body  is  deprived 
by  castration.  Perhaps  the  best  known  examples  of  hormones 
concerned  with  reproduction  are  those  that  have  to  do  with  lacta- 
tion. It  has  been  proven  that  the  development  of  the  mammary 
glands  during  pregnancy  is  caused  by  a  hormone  produced  in  the 
body  of  the  embryo.  This  hormone  is  attended  apparently  by 
another  one,  which,  while  permitting  the  development  of  the 
glands  inhibits  their  active  functioning.  At  the  birth  of  the  child 
this  second  hormone  is  withdrawn,  and  the  glands  are  thus  left 
free  to  pour  forth  their  secretion. 

The  Stages  of  Life.  Starting  from  the  ovum  each  human  being, 
apart  from  accident  or  disease,  runs  through  a  life-cycle  which 
terminates  on  the  average  after  a  course  of  from  75  to  80  years. 
The  earliest  years  are  marked  not  only  by  rapid  growth  but  by 
differentiating  growth  or  development;  then  comes  a  more  station- 
ary period,  and  finally  one  of  degeneration.  The  life  of  various 
tissues  and  of  many  organs  is  not,  however,  coextensive  with  that 
of  the  individual.  At  birth  numerous  bones  are  represented  mainly 
by  cartilage.  The  pancreas  has  not  attained  its  full  development; 
and  some  of  the  sense-organs  seem  to  be  in  the  same  case;  at  least 
new-born  infants  appear  to  hear  very  imperfectly.  The  reproduc- 
tive organs  only  attain  full  development  at  puberty,  and  de- 
generate and  lose  all  or  much  of  their  functional  importance  as 
years  accumulate.  Certain  organs  have  even  a  still  shorter  range 
of  physiological  life;  the  thymus,  for  example,  attains  its  fullest 
development  at  the  end  of  the  second  year  and  then  gradually 
dwindles  away,  so  that  in  the  adult  scarcely  a  trace  of  it  is  to  be 
found.  The  milk-teeth  are  shed  in  childhood,  and  their  so-called 
permanent  successors  rarely  last  to  ripe  old  age. 

During  early  life  the  Body  increases  in  mass,  at  first  very  rapidly, 
and  then  more  slowly,  till  the  full  size  is  attained,  except  that  girls 
make  a  sudden  advance  in  this  respect  at  puberty.  Henceforth 
the  woman's  weight  (excluding  exceptional  cases  of  accumulation 
of  non-working  adipose  tissue)  remains  about  the  same  until  the 
climacteric.  After  that  there  is  often  an  increase  of  weight  for 
several  years  due  mainly  to  increased  formation  of  fat;  a  man's 
weight  usually  slowly  increases  until  forty. 


REPRODUCTION  585 

As  old  age  comes  on  a  general  decline  sets  in,  the  rib  cartilages 
become  calcified,  and  lime  salts  are  laid  down  in  the  arterial  walls, 
which  thus  lose  their  elasticity;  the  refracting  media  of  the  eye 
become  more  or  less  opaque;  the  physiological  irritability  of  the 
sense-organs  in  general  diminishes;  and  fatty  degeneration,  di- 
minishing their  working  power,  occurs  in  many  tissues.  In  the 
brain  we  find  signs  of  less  plasticity;  the  youth  in  whom  few  lines 
of  least  resistance  have  been  firmly  established  is  ready  to  accept 
novelties  and  form  new  associations;  but  the  longer  he  lives,  the 
more  difficult  does  this  become  to  him.  A  man  past  middle  life 
may  do  good,  or  even  his  best  work,  but  almost  invariably  in 
some  line  of  thought  which  he  has  already  accepted;  it  is  ex- 
tremely rare  for  an  old  man  to  take  up  a  new  study  or  change  his 
views,  philosophical,  scientific,  or  other.  Hence,  as  we  live,  we  all 
tend  to  lag  behind  the  rising  generation. 

Death.  After  the  prime  of  life  the  tissues  dwindle  (or  at  least 
the  most  important  ones)  as  they  increased  in  childhood. 

Before  any  great  diminution  takes  place,  however,  a  breakdown 
occurs  somewhere,  the  enfeebled  community  of  organs  and  tissues 
forming  the  man  is  unable  to  meet  the  contingencies  of  life,  and 
death  supervenes.  "It  is  as  natural  to  die  as  to  be  born,"  Bacon 
wrote  long  since;  but  though  we  all  know  it,  few  realize  the  fact 
until  the  summons  comes.  To  the  popular  imagination  the  pros- 
pect of  dying  is  often  associated  with  thoughts  of  extreme  suf- 
fering; personifying  life,  men  picture  a  forcible  and  agonizing 
rending  of  it,  as  an  entity,  from  the  bodily  frame  with  which  it  is 
associated.  As  a  matter  of  fact,  death  is  probably  rarely  asso- 
ciated with  any  immediate  suffering.  The  sensibilities  are  grad- 
ually dulled  as  the  end  approaches;  the  nervous  tissues,  with  the 
rest,  lose  their  functional  capacity,  and,  before  the  heart  ceases  to 
beat,  the  individual  has  commonly  lost  consciousness. 

The  actual  moment  of  death  is  hard  to  define:  that  of  the  Body 
generally,  of  the  mass  as  a,  whole,  may  be  taken  to  be  the  moment 
when  the  heart  makes  its  last  beat;  arterial  pressure  then  falls 
irretrievably,  the  capillary  circulation  ceases,  and  the  tissues,  no 
longer  nourished  from  the  blood,  gradually  die,  not  all  at  one  in- 
stant, but  one  after  another,  according  as  their  individual  respira- 
tory or  other  needs  are  great  or  little. 

While  death  is  the  natural  end  of  life,  it  is  not  its  aim — we  should 


586 


THE  HUMAN  BODY 


not  live  to  die,  but  live  prepared  to  die.  Life  has  its  duties  and 
its  legitimate  pleasures,  and  we  better  play  our  part  by  attending 
to  the  fulfilment  of  the  one  and  the  enjoyment  of  the  other,  than 
by  concentrating  a  morbid  and  paralyzing  attention  on  the  in- 
evitable, with  the  too  frequent  result  of  producing  indifference 
to  the  work  which  lies  at  hand  for  each.  Our  organs  and  faculties 
are  not  talents  which  we  may  justifiably  leave  unemployed;  each 
is  bound  to  do  his  best  with  them,  and  so  to  live  that  he  may  most 
utilize  them.  An  active,  vigorous,  dutiful,  unselfish  life  is  a  good 
preparation  for  death;  when  that  time,  at  which  we  must  pass 
from  the  realm  controlled  by  physiological  laws,  approaches,  when 
the  hands  tremble  and  the  eyes  grow  dim,  when  "the  grasshopper 
shall  be  a  burden  and  desire  shall  fail,"  then,  surely,  the  conscious- 
ness of  having  quitted  us  like  men  in  the  employment  of  our 
faculties  while  they  were  ours  to  use,  will  be  no  mean  consolation. 


APPENDIX 

SUGGESTIONS  FOR  LABORATORY  WORK 

To  a  greater  and  greater  extent  colleges  and  normal  schools  are 
supplementing  text-book  instruction  in  Physiology  with  practical 
work  in  the  laboratory.  For  such  work  laboratory  instructions 
must  be  provided.  There  are  upon  the  market  numerous  excellent 
laboratory  manuals  of  Physiology,  but  most  of  them  have  been 
prepared  for  use  in  Medical  Schools,  and  with  the  needs  of  Medical 
Students  primarily  in  mind.  The  aim  of  this  appendix  is  to  furnish 
a  basis  for  the  preparation  of  a  manual  suited  to  the  needs  of  the 
students  who  are  pursuing  the  subject  non-prof essionally.  The 
complete  equipment  of  a  Physiological  laboratory  is  rather  ex- 
tensive, and  in  many  cases  not  available  in  entirety.  The  time 
which  can  be  devoted  to  laboratory  work  in  Physiology  varies 
greatly  in  different  institutions.  For  these  two  reasons  it  has 
seemed  best  not  to  offer  in  this  connection  a  manual  of  se- 
lected experiments,  but  rather  to  suggest  simple  exercises  that 
can  be  adapted  by  the  teacher  to  his  particular  require- 
ments. 

The  special  character  of  much  of  the  work  of  the  physiological 
iboratory  demands  a  certain  amount  of  special  preparation  on  the 

irt  of  the  teacher.    In  describing  the  following  experiments  this 
reparation  is  assumed.    The  majority  of  teachers  of  Biology  have 
it.    For  those  who  have  not,  and  wish  to  introduce  in  their  classes 
iboratory  work  in  Physiology,  resort  may  be  had  to  one  of  the 
imer  courses  in  Physiology  now  offered  by  the  larger  Universi- 
ties in  various  parts  of  the  country. 

Fortunately  for  the  successful  development  of  laboratory  Physi- 
ology in  America  the  special  apparatus  required  can  be  obtained  of 
American  manufacture  and  at  a  moderate  price.  Information  as 
to  where  such  apparatus  may  be  sought  can  be  had  upon  applica- 
tion to  the  department  of  Physiology  in  any  of  the  larger  Uni- 
versities. 

587 


588  APPENDIX 

INTRODUCTION 

General  Histology  of  Body.  Microscopic  study  of  typical  cells 
and  tissues.  Desirable  for  students  who  have  no  previous  histo 
logical  training. 

General  Chemistry  of  Body.  Inorganic  Constituents.  Water. 
The  percentage  of  water  in  flesh  may  be  determined  roughly  by 
placing  a  weighed  piece  of  meat  in  a  dessicator  in  a  warm  place  and 
weighing  it  at  intervals  until  thoroughly  dry. 

Inorganic  Salts.  A  weighed  piece  of  meat  may  be  incinerated 
under  a  hood  and  the  ash  weighed.  The  solubility  of  the  ash  in 
water  may  be  ascertained.  Tests  for  chlorids  with  silver  nitrate, 
and  for  calcium  with  sodium  oxalate  solution  may  be  applied. 

Organic  Constituents.  Carbohydrates.  Dextrose  is  present  i:i 
honey  and  corn  syrup.  The  test  for  its  presence  is  by  means  cf 
Fehling's  Solution  (see  any  laboratory  manual  of  Organic  Chem- 
istry). The  presence  of  a  reddish  precipitate  in  the  test  tube  in 
which  mixed  Fehling's  Solution  (alkaline  copper  tartrate)  and 
dextrose  are  combined  and  heated  to  boiling  is  the  test. 

Glycogen  is  a  constituent  of  oysters.  Grind  a  raw  oyster  with 
sand  in  a  mortar.  Extract  with  water.  Add  to  a  few  drops  of  the 
juice  in  a  test  tube  a  few  drops  of  iodine  solution  (tincture  may  be 
used).  A  mahogany-brown  color  is  the  test  for  glycogen. 

Lactose.  If  sweet  milk  is  clotted  with  rennet  and  filtered  the 
whey  contains  milk  sugar.  Test  with  Fehling's  solution  as  for 
dextrose. 

Fats.    Solubility  of  Fats.    Shake  a  small  piece  of  lard  with  so 
ether  in  a  test  tube.    Pour  some  of  the  solution  on  filter  pa 
and   allow  it  to  evaporate.     If  the  ether  dissolved  the  fat,  a 
grease  spot  will  be  left  on  the  filter  paper  after  the  evaporatio 
of  the  ether. 

Moisten  a  second  filter  paper  with  ether,  allow  it  to  evaporate. 
Compare  the  two  filter  papers.  The  test  may  be  repeated  with 
olive  oil  instead  of  lard. 

Test  for  Fats.  Melt  a  small  piece  of  lard  in  a  test  tube  and  to  it 
add  a  drop  of  dilute  osmic  acid.  This  acid  turns  fat  black. 

Composition  of  Fats.  Fats  are  compounds  of  fatty  acids  with 
glycerin.  To  separate  the  constituents  heat  on  a  water  bath  a 
small  piece  of  lard  with  an  alcoholic  solution  of  caustic  potash. 


" 

3. 


SUGGESTIONS  FOR  LABORATORY  WORK  589 

After  forty  minutes  pour  the  mixture  into  a  dish  containing  100  c.  c. 
water.  Drive  off  the  alcohol  by  heating  over  water.  While  still 
hot  acidify  with  sulphuric  acid.  On  cooling  the  fatty  acid  forms 
a  solid  crust  on  the  surface  of  the  liquid.  Remove  it.  The  glycerin 
is  dissolved  in  the  liquid. 

The  fatty  acid  is  insoluble  in  water  as  may  be  shown  by  test. 

Saponification.  Soap  is  a  compound  of  fatty  acid  with  alkali. 
Shake  some  of  the  fatty  acid  produced  above  with  warm  dilute 
caustic  soda.  Filter.  The  filtrate  shows  the  characteristic  features 
of  a  soap  solution.  Soaps  form  an  insoluble  compound  with  calcium 
chlorid.  This  can  be  shown  by  adding  a  few  drops  of  calcium 
chlorid  to  some  of  the  soap  solution  in  a  test  tube.  Hard  water 
contains  calcium  salts.  This  explains  the  difficulty  of  using  soap 
with  hard  water. 

Proteins.  Tests  for  Proteins.  Raw  egg  white  is  a  satisfactory 
protein  for  these  tests. 

Heat  Test.  To  a  small  amount  of  a  solution  of  protein  add  a 
drop  of  a  two-tenths  per  cent  solution  of  acetic  acid.  Heat  the 
upper  portion  of  the  solution.  The  protein  is  rendered  insoluble 
and  is  now  a  coagulated  protein. 

Biuret  Test.  To  a  solution  of  protein  add  a  drop  or  two  of  a 
one  per  cent  solution  of  copper  sulphate  (CuS04)  and  then  strong 
sodic  hydrate  (NaOH).  A  characteristic  violet  color  is  the  test. 

Xanthoproteic  Reaction.  To  a  solution  of  protein  add  a  few  drops 
of  strong  nitric  acid  (HNO3),  and  boil;  after  cooling,  add  ammonic 
hydrate  (NH4OH).  A  yellow  color  which  deepens  to  orange  when 
the  ammonia  is  added  is  the  test. 

Physico-Chemical  Principles.  Osmosis.  Tie  an  osmotic  mem- 
brane (gold  beater's  skin,  bladder)  across  the  mouth  of  a  funnel 
or  thistle  tube.  Fill  with  a  sugar  solution  of  the  consistency  of 
syrup.  Attach  a  glass  tube  to  the  nozzle  of  the  funnel.  Fasten  in 
an  upright  position  and  surround  the  funnel  with  distilled  water 
in  a  beaker. 

Drops  of  blood  on  a  microscope  slide  may  be  used  to  demonstrate 
losis  indirectly.  Three  such  drops  should  be  examined;  one 
diluted  with  distilled  water;  a  second  with  0.9  per  cent  sodium 
chlorid ;  the  third  with  10  per  cent  sodium  chlorid.  Avoid  excessive 
evaporation.  The  destruction  of  the  corpuscles  in  the  dilute  solu- 
tion by  over-distension  (plasmolysis) ;  their  preservation  in  the  0.9 


590  APPENDIX 

per  cent  saline;  and  their  shrinkage  (crenatiori)  in  the  concentrated 
solution  illustrate  osmosis. 

Dialysis. — Sausage  casings  make  good  dialyzing  tubes,  or  special 
tubes  may  be  purchased  from  chemical  supply  houses.  To  illus- 
trate the  separation  of  crystalloids  from  colloids  by  dialysis  make 
a  mixture  of  raw  egg  white  with  moderately  concentrated  sodium 
chlorid  solution.  Place  in  the  dialyzing  tube;  suspend  the  tube  in 
a  beaker  of  distilled  water.  Stir  the  water  frequently.  After 
a  sufficient  interval  the  presence  of  sodium  chlorid  in  the  water 
can  be  demonstrated  with  silver  nitrate,  but  the  biuret  test  for 
protein  (a  very  delicate  test)  continues  negative. 

THE  SUPPORTING  TISSUES 

Gross  and  microscopic  studies  of  bones  and  cartilage,  and  micro- 
scopic studies  of  various  forms  of  connective  tissue  may  be  made 
as  the  time  and  available  material  permit,  and  as  the  previous 
training  of  the  students  requires. 

That  bone  consists  of  inorganic  salts  deposited  in  an  organic 
matrix  may  be  shown  by  dissolving  out  the  inorganic  salts  with 
dilute  hydrochloric  acid.  That  calcium  is  an  important  constituent 
of  the  inorganic  portion  may  be  shown  by  testing  the  hydrochloric 
acid  solution  with  sodium  oxalate. 

Gelatin  is  a  product  obtained  from  bones  by  cooking  them  in 
water  heated  above  the  normal  boiling  point  by  inclosing  in 
sealed  vessel.    The  ordinary  protein  tests  applied  to  gelatin  shoi 
that  it  is  a  member  of  the  group  of  proteins. 

THE  SKELETON 

The  general  arrangement  of  the  bones  of  the  skeleton,  and  d< 
tails  of  selected  regions,  as  the  skull,  may  be  studied. 

Joints.    In  connection  with  work  on  the  skeleton  the  varioi 
types  of  joints  should  be  studied  in  detail. 

Typical  joints  are  those  at  hip,  knee,  and  ankle. 

Hip  Joint.  Observe  on  the  outer  surface  of  the  innominate  bon( 
a  deep  depression,  the  acetabulum,  into  which  fits  the  smooi 
nearly  spherical  head  of  the  femur,  making  a  ball  and  socket  joit 
Study  the  possible  movements  of  the  joint.  Note  (1)  flexion,  simple 
bending  of  the  joint  as  in  walking;  (2)  extension,  the  opposite  of 


SUGGESTIONS  FOR  LABORATORY  WORK  591 

flexion;  (3)  abduction,  drawing  the  leg  outward  from  the  body; 
(4)  adduction,  the  opposite  of  abduction;  (5)  rotation,  twisting  at 
the  joint  as  in  placing  ankle  on  knee. 

In  addition  to  these  elementary  joint  movements,  the  hip  joint 
permits  various  combination  movements,  as  flexion  with  abduc- 
tion. Note  these. 

Knee  Joint.  Observe  the  surfaces  on  femur  and  tibia  which  come 
together  at  the  knee.  Analyze  the  possible  movements  of  this 
joint. 

Ankle  Joint.    Observe  the  possible  movements  of  this  joint. 

MUSCLES 

For  dissection  studies  the  cat  is  the  most  available  mammal.* 
Detailed  descriptions  are  given  in  "The  Anatomy  of  the  Cat"  by 
Reighard  and  Jennings.  For  purposes  of  illustration  directions 
for  a  single  region  are  included  here. 

Dissection  of  Hind  Leg  of  Cat.  Directions  for  dissection. 
Muscles  are  inclosed  in  and  bound  together  by  sheets  of  connective 
tissue,  which  make  up  the  fascia.  To  dissect  out  a  muscle  cut  or 
tear  through  the  fascia  which  joins  it  to  its  neighboring  muscles 
until  the  desired  muscle  can  be  lifted  clear  except  for  the  attach- 
ments at  its  ends.  Do  not  cut  attachments  until  you  have  finished 
the  study  of  the  muscle. 

The  attachment  of  the  muscle  nearer  the  trunk  is  called  the 
origin;  the  attachment  further  from  the  trunk  is  the  insertion. 

In  dissecting  the  hind  leg  of  the  cat,  consider  it  as  made  up  of 
four  regions:  pelvis,  thigh,  shank,  and  foot.  The  muscles  are  to  be 
dissected  in  order  as  directed  below.  As  each  is  cleared  from  its 
surrounding  muscles  the  region  in  which  it  has  its  origin  is  to  be 
determined,  also  the  region  of  its  insertion. 

Muscles  are  grouped  functionally,  according  to  the  joint-motions 
they  produce,  as  flexors,  extensors,  abductors,  adductors,  and  ro- 
tators, f  Determine  for  each  muscle  the  region  it  moves  and  the 
motion  it  produces. 

On  the  outside  of  the  thigh  is  the  biceps  femoris.    Abductor  of 

*  If  there  is  a  pork-packing  establishment  in  the  vicinity  embryo  pigs  can 
usually  be  obtained  in  ample  numbers.  These  make  exceedingly  satisfactory 
dissection  material,  especially  for  elementary  classes. 

t  Elevators,  depressors,  and  sphincters  occur  in  the  Body  but  not  in  the  leg. 


592  APPENDIX 

leg  and  flexor  of  shank.    Directly  beneath  the  biceps  femoris  ob- 
serve the  large  nerve,  the  sciatic. 

Along  the  front  edge  of  the  thigh  and  on  its  inner  surface  is  the 
sartorius  or  tailor's  muscle.  Extensor  of  shank,  adductor  of  leg, 
and  rotator  of  leg. 

The  entire  mass  of  muscle  in  front  of  the  femur  after  the  sartorius 
has  been  removed  is  the  quadriceps  femoris.  Extensor  of  shank. 

On  the  inner  surface  of  the  thigh  toward  the  back  is  the  gracilis. 
Adductor  of  leg. 

On  the  inner  surface  of  the  thigh  after  the  removal  of  the  gracilis 
appear  the  following  muscles  from  front  to  back.  Small  adductor 
longus.  Adductor  of  leg.  The  larger  triangular  adductor  femoris. 
Extensor  of  thigh.  Large  flat  semi-membranosus.  Extensor  of 
thigh. 

After  dissection  of  above  muscles  there  remains  on  the  thigh 
only  the  semi-tendinosus.  Flexor  of  shank. 

The  great  mass  of  muscle  forming  the  calf  consists  of  three 
muscles,  the  plantaris,  gastrocnemiust  and  soleus.  Do  not  try  to 
separate  these.  The  tendo  achilles  is  their  common  tendon.  Ex- 
tensor of  foot. 

After  removing  above  muscles  there  will  be  found  against  the 
shank  bones  at  the  back  the  flexor  longus  digitorum.  Flexor  of  toes. 

Along  the  front  of  the  shank,  a  superficial  muscle  with  its  tendon 
toward  the  inner  side,  is  the  tibialis  anterior.  Flexor  of  foot. 

Partly  underneath  the  above,  with  its  tendon  toward  the  outer 
side,  is  the  extensor  digitorum  longus.  Extensor  of  toes. 

The  Contraction  of  Muscles.    The  tissues  of  cold-blooded  ani- 
mals are  well  suited  for  studies  of  function  since  they  survive 
sometimes  for  hours,  the  general  death  of  the  animal. 

From  the  hind  leg  of  a  recently  killed  frog  make  a  femur-gastn 
nemius  preparation.    This  preparation  is  used  for  the  study 
muscular  contraction. 

To  study  muscular  contraction  adequately  the  motions  of  th< 
muscle  must  be  magnified  and  must  be  recorded.  For  obtaining 
magnified  record  of  its  movements  the  muscle  is  fastened  in 
clamp  and  its  tendon  attached  to  the  short  arm  of  a  lever,  whos 
long  arm  presses  lightly  at  its  tip  against  a  smoked  paper  on  whicl 
every  movement  of  the  muscle  is  recorded  as  a  line  (p.  94).  T( 
avoid  superposing  separate  tracings  the  smoked  paper  is  mounl 


SUGGESTIONS  FOR  LABORATORY  WORK  593 

on  a  drum  which  can  be  moved  by  hand,  or  driven  at  various  speeds 
by  a  clockwork.  This  apparatus  is  called  a  kymograph.  Tracings 
made  with  it  can  be  preserved  by  passing  the  paper  bearing  them 
through  a  solution  of  shellac  in  alcohol. 

Skeletal  muscles  contract  only  when  stimulated.  A  suitable 
artificial  stimulus  is  the  shock  from  an  induction  coil.  Induced 
currents  are  generated  in  the  secondary  coil  of  an  inductorium 
when,  and  only  when,  a  current  is  made  or  broken  in  the  primary 
coil.  An  ordinary  dry  cell  is  a  good  source  of  current.  The  strength 
of  the  induced  current  varies  with  the  position  of  the  secondary  coil 
relative  to  the  primary,  being  greatest  when  the  secondary  is  di- 
rectly over  the  primary,  and  least  when  the  secondary  is  with- 
drawn as  far  as  possible  from  the  primary  and  turned  at  right  angles 
to  it. 

Mount  the  frog's  gastrocnemius  muscle  in  readiness  for  obtaining 
tracings  of  its  contractions.  By  means  of  fine  copper  wires  es- 
tablish a  circuit  from  the  secondary  coil  of  the  inductorium  through 
the  tissue.  Be  careful  to  avoid  short  circuits. 

With  the  secondary  coil  in  the  position  of  least  effectiveness 
make  and  break  the  primary  circuit.  If  the  muscle  does  not  re- 
spond shift  the  secondary,  little  by  little,  toward  the  position  of 
greatest  effectiveness,  making  and  breaking  the  primary  circuit 
with  each  shift.  As  soon  as  the  muscle  responds,  recording  its 
contraction  on  the  smoked  paper,  move  the  drum  forward  by  hand 
so  that  new  contractions  will  not  be  superposed  on  the  first  one. 

Continue  increasing  the  strength  of  the  shock,  obtaining  records 
of  each  contraction. 

Thus  the  relation  of  contraction  height  to  stimulation  strength 
is  shown. 

A  current  is  induced  in  the  secondary  at  make  and  at  break  of 
the  primary  current.  Determine  which  is  a  more  powerful  stimu- 
lus, a  "make"  shock  or  a  "break"  shock. 

The  inductorium  as  used  in  the  physiological  laboratory  is  pro- 
vided with  an  automatic  circuit  breaker  which  can  be  included  in 
the  primary  circuit.  Make  the  proper  connections  for  doing  this. 
Now  when  the  circuit  is  closed  shocks  are  sent  into  the  muscle  in 
rapid  succession. 

Set  the  clockwork  of  the  kymograph  to  drive  the  drum  at  a  slow 
speed.  Obtain  a  tracing  showing  the  response  of  the  muscle  to 


594  APPENDIX 

rapidly  repeated  stimuli.    This  sort  of  contraction  is  known  as  a 
physiological  tetanus  or  a  tetanic  contraction  (p.  102). 

The  Staircase  Effect,  Contracture,  Fatigue.  Prepare  and 
mount  a  gastrocnemius  muscle  as  in  the  previous  exercise. 
Set  the  secondary  of  the  inductorium  at  a  position  that 
gives  a  sharp  but  not  excessive  break  stimulus.  With  the  drum 
moving  at  its  slowest  speed  make  and  break  the  primary  circuit 
twice  per  second,  allowing  the  contractions  to  record  themselves 
on  the  slowly  moving  drum.  Continue  the  series  of  stimuli  until 
the  muscle  ceases  to  respond.  Make  the  record  permanent. 

The  increase  in  the  height  of  the  contractions  during  the  first 
part  of  the  record,  due  to  "warming  up"  is  called  the  "staircase 
effect." 

A  rise  in  the  base  line  during  the  later  part  of  the  curve  is  the 
phenomenon  of  contracture  (p.  100). 

The  final  failure  of  the  muscle  to  contract  is  the  result  of  fatigue. 
This  may  be  explained  either  as  due  to  the  exhaustion  of  the  fuel 
supply  or  to  the  accumulation  of  harmful  "fatigue  products." 

To  distinguish  between  these  alternatives  replace  the  fatigued 
muscle  with  a  fresh  one.  Arrange  the  primary  circuit  for  rapidly 
repeated  shocks  of  moderate  intensity.  With  the  muscle  recording 
on  a  slowly  moving  drum  close  the  key,  throwing  the  muscle  into 
tetanus.  Continue  the  stimulation  until  the  muscle  is  well  fatigued. 
Let  the  muscle  rest  for  five  minutes.  Repeat  the  stimulation. 
Recovery  under  this  condition  proves  that  the  previous  fatigue 
could  not  have  been  due  to  exhaustion  of  the  fuel  supply. 

The  production  of  acid  in  an  active  muscle  may  be  demonstrated 
with  sensitive  litmus  paper.  A  resting  isolated  muscle  is  neutral  or 
slightly  alkaline  to  litmus.  The  same  muscle,  exercised  to  fatigue, 
is  acid  to  litmus. 

The  Influence  of  Temperature  on  Contraction.  For  thi 
study  arrangement  must  be  made  for  immersing  the  muscle 
in  a  liquid  and  at  the  same  time  recording  its  contractions. 
A  method  of  doing  this  is  to  mount  an  L-shaped  glass  rod  in  a  clamp. 
Fasten  the  femur  end  of  the  frog's  gastrocnemius  to  the  horizontal 
end  of  the  L.  A  small  pulley  must  be  mounted  above  the  rod, 
over  which  a  thread  can  be  led  from  the  tendon  of  the  muscle 
the  recording  lever.  Connect  the  ends  of  the  muscle  by  fine  cop 
wire  to  the  terminals  of  the  secondary  coil.  Set  this  so  that  th 


pj- 

I 


SUGGESTIONS  FOR  LABORATORY  WORK  595 

muscle  gives  a  vigorous  contraction  upon  stimulation.  Bring 
around  the  muscle  a  beaker  of  ice-cold  Ringer's  solution  (NaCl 
0.7%;  CaCl2  0.026%;  KC1  0.03%),  which  has  the  same  osmotic 
pressure  as  frog's  blood. 

With  the  drum  moving  at  the  fastest  clockwork  speed  obtain  a 
record  of  a  single  contraction. 

Withdraw  the  cold  solution  and  allow  the  muscle  tissue  to  return 
to  room  temperature.  With  the  drum  moving  at  the  same  rate 
as  before  obtain  a  record  of  a  single  contraction.  Now  surround 
the  muscle  with  Ringer's  solution  warmed  to  30°  C.,  and  obtain  a 
record  of  a  single  contraction,  again  with  the  drum  at  the  same 
speed  as  before. 

Let  the  drum  make  a  complete  revolution  at  the  speed  used  in 
making  those  records.  Determine  as  accurately  as  possible  the 
time  consumed.  After  the  smoked  paper  has  been  removed  meas- 
ure its  length,  and  calculate  the  speed  of  the  drum  in  centimeters 
per  second. 

Varnish  the  tracings  and  mount  side  by  side  the  records  of  con- 
traction at  the  three  temperatures  used. 

Compute  the  time  required  at  each  temperature  for  a  complete 
contraction. 

Set  the  drum  moving  at  its  slowest  speed.  Surround  the  muscle 
with  Ringer's  solution,  and  without  stimulating  the  muscle  allow 
it  to  record  on  the  drum  while  the  solution  is  gradually  warmed  to 
65°  C.  The  contraction  brought  about  by  warming  above  40°  is 
called  heat  rigor. 

THE  NERVOUS  SYSTEM 

Dissections  of  PERIPHERAL  NERVES  may  be  made  according  to  the 
description  in  Reighard  and  Jennings.  By  way  of  suggestion  direc- 
tions for  dissecting  a  typical  spinal  nerve  and  a  typical  cranial 
nerve  are  given. 

A  representative  spinal  nerve  is  the  great  sciatic  with  its 
branches.  (To  be  dissected  in  the  cat.) 

Cut  through  the  skin  on  the  outer  side  of  the  hind  leg  from  the 
heel  to  the  middle  of  the  back.  Remove  the  biceps  femoris  muscle. 
The  nerve  trunk  thus  exposed  is  the  sciatic.  Follow  the  nerve 
trunk  upward,  cutting  away  overlying  tissues  where  necessary. 


596  APPENDIX 

The  nerve  can  be  traced  to  where  it  passes  through  a  hole  in  the 
pelvic  bone.  Thrust  a  seeker  through  this  hole  and  then  cut  away 
the  tissues  on  the  front  surface  of  the  pelvis  until  the  seeker  is 
exposed.  The  nerve  will  thus  be  brought  into  view,  and  can  be 
traced  for  a  short  distance  to  the  point  where  it  emerges  from  the 
spinal  canal. 

Returning  to  the  nerve  on  the  outer  surface  of  the  thigh  follow 
it  downward,  noting  that  it  gives  off  occasional  branches  to  contigu- 
ous muscles.  A  short  distance  behind  the  knee  the  nerve  divides 
into  two  branches.  One  of  these,  the  peroneus,  passes  across  the 
gastrocnemius  muscle  on  its  outer  surface,  then  plunges  beneath 
it  and  passes  down  the  shank  close  to  the  fibula.  It  can  be  followed 
by  cutting  away  the  overlying  tissue.  The  second  branch  of  the 
sciatic,  the  tibialis,  passes  directly  into  the  muscular  mass  of  the 
calf,  then  turns  downward  toward  the  foot.  Follow  it  as  far  as 
possible,  cutting  away  overlying  tissues  carefully. 

A  Typical  Cranial  Nerve:  The  Vagus.  Make  an  incision  along 
the  mid-line  of  the  under  surface  of  the  body  its  entire  length. 
Separate  the  muscles  of  the  neck  until  the  trachea  (wind-pipe)  is 
exposed  in  the  mid-line.  Follow  around  the  left  side  of  the  trachea, 
separating  the  muscles,  but  not  cutting  them,  until  a  sheath  of 
connective  tissue  inclosing  blood-vessels  and  nerves  is  found.  This 
sheath  is  usually  in  close  contact  with  the  side  of  the  trachea. 
With  a  blunt  instrument  open  the  sheath  and  separate  the  nerves 
from  the  adjacent  artery  for  the  space  of  an  inch  or  more.  Careful 
observation  discloses  two  nerves  in  close  contact  with  each  other. 
The  larger  is  the  vagus.  Follow  it  toward  the  head.  At  the  level  of 
the  larynx  (vocal  apparatus)  it  gives  off  a  branch,  the  superior 
laryngeal.  Continue  the  dissection  forward  to  the  point  where  the 
nerve  enters  the  skull.  An  enlargement,  the  ganglion  nodosum,  is 
seen  just  here. 

Return  to  the  point  where  the  vagus  was  first  separated,  and 
carry  the  dissection  backward.  Separate  overlying  tissues,  but  do 
not  cut  through  them  except  when  absolutely  necessary  for  prog- 
ress. At  the  junction  of  neck  with  thorax  there  are  some  large 
veins  which  may  bleed.  If  any  such  are  cut  accidentally  the  blood 
should  be  carefully  wiped  away  with  cotton  to  keep  the  field  of 
dissection  clear.  Follow  the  nerve  into  the  thorax  to  the  level  of  the 
root  of  the  lung,  A  short  distance  above  this  point  a  branch  of 


SUGGESTIONS  FOR  LABORATORY  WORK  597 

the  vagus,  the  inferior  laryngeal,  passes  behind  a  large  artery,  the 
aorta,  and  turns  back  toward  the  head. 

Branches  of  the  vagus  can  be  traced  into  the  root  of  the  lung. 
At  about  the  level  of  the  root  of  the  lung  the  nerve  divides  into  two 
branches,  each  of  which  can  be  traced  to  the  surface  of  the  esoph- 
agus, where  each  unites  with  a  corresponding  branch  from  the 
right  vagus.  The  nerve  trunks  thus  formed  continue  backward 
along  the  esophagus  to  the  stomach  where  they  break  up  into  fine 
branches  which  supply  the  stomach  and  upper  portion  of  the  small 
intestine. 

For  the  CENTRAL  NERVOUS  SYSTEM  the  sheep's  brain  is  a  satisfac- 
tory object  of  dissection.  The  brain  should  be  carefully  removed 
from  the  skull  and  hardened  in  formalin  before  use. 

Sheep's  Brain.  With  the  aid  of  the  figures  on  pages  144,  145, 
146,  and  151  make  out  the  grand  divisions  of  the  brain:  cerebrum, 
or  fore  brain;  midbrain,  overlain  in  front  by  the  pom,  and  behind 
by  the  cerebellum;  medulla  oblongata,  forming  the  connecting  link 
between  brain  and  spinal  cord. 

Observe  on  the  surface  of  the  cerebrum  the  irregular  convolutions 
which  serve  to  increase  its  surface  relative  to  its  bulk. 

On  the  base  of  the  brain  make  out  with  the  aid  of  the  figure  on 
page  151  the  optic  tracts  and  optic  chiasma;  also  as  many  other  roots 
of  cranial  nerves  as  possible.  Note  that  all  cranial  nerves  behind 
the  optic  nerve  spring  from  the  brain  stem  (midbrain  and  medulla). 
In  front  of  the  optic  tracts  and  springing  from  the  cerebrum,  the 
olfactory  lobes  may  be  seen. 

Cut  the  brain  through  the  vertical  median  plane.  Note  the 
corpus  callosum. 

Conduction  in  the  Nerve  Trunk.  The  sciatic  nerve  in  the  frog 
can  be  exposed  by  removing  the  skin  from  the  leg  and  separating 
carefully  the  two  large  muscles  on  the  dorsal  surface  of  the  thigh. 
Dissect  the  nerve  out  carefully  from  the  upper  end  of  the  leg  toward 
the  knee.  Use  great  care  to  avoid  injuring  the  nerve  by  stretching 
or  squeezing.  Cut  the  nerve  away  at  its  upper  end.  Leave  it  in 
connection  below  with  the  muscles  of  the  shank.  The  ability  of 
the  nerve  to  conduct  impulses  may  be  demonstrated  by  stimulating 
it  as  far  as  possible  from  the  muscles  and  observing  their  response. 
The  susceptibility  of  the  nerve  to  different  forms  of  energy  may 
be  shown  by  stimulating  it  with  forceps  (mechanical),  a  hot  rod 


698  APPENDIX 

(thermal),  or  shocks  from  a  pair  of  electrodes  leading  from  the 
terminals  of  the  induction  coil  (electrical). 

The  impairment  of  conductivity  by  cold  may  be  shown  by  bring- 
ing against  the  nerve,  between  the  point  of  stimulation  and  the 
muscle,  a  small  test  tube  filled  with  cold  brine. 

Motor  Points  on  the  Body.  To  one  terminal  of  the  secondary 
coil  of  an  inductorium  attach  a  flat  electrode  which  has  been 
wrapped  with  gauze  well  moistened  with  saline  solution.  Bare  the 
forearm  and  lay  it,  palm  up,  in  contact  with  the  electrode.  To  the 
other  terminal  of  the  inductorium  attach  a  rod  electrode.  With  the 
use  of  tetanizing  stimuli  of  moderate  strength  explore  the  surface 
of  the  forearm  by  means  of  the  rod  electrode.  At  certain  points 
individual  muscles  will  be  thrown  into  contraction.  These  points 
are  "  mo  tor  points." 

SPINAL  REFLEXES 

Suspend  a  frog,  whose  brain  has  been  recently  destroyed,  by 
means  of  a  hook  through  the  jaw.  Pinch  the  toes  of  the  right 
foot.  The  foot  is  retracted.  Repeat  the  experiment  on  the  other 
toes.  In  each  case  note  the  relation  of  the  muscles  that  respond 
to  the  region  stimulated. 

Tie  two  fine  copper  wires  J4  inch  (6  mm.)  apart,  about  the  right 
hind  toes.  Carry  these  to  the  terminals  of  the  secondary  coil. 
Send  in  tetanizing  shocks  of  increasing  strength.  As  more  and 
more  widespread  movements  are  elicited  note  the  order  in  which 
various  parts  of  the  body  become  involved. 

Moisten  a  bit  of  porous  paper  with  acid.  Place  the  acid  on 
the  frog's  back  near  the  legs.  Note  the  adaptive  response. 

Wash  the  skin  thoroughly  with  water.  Repeat  the  experiment, 
this  time  placing  the  acid  paper  on  the  belly. 

Destroying  the  frog's  brain  has  destroyed  his  intelligence.  These 
responses,  though  adaptive,  are  purely  automatic. 

By-  means  of  a  fine-pointed  pipette  introduce  a  few  drops  of 
strychnine  solution  under  the  skin  of  the  frog's  back.  After  allow- 
ing a  few  minutes  for  the  drug  to  take  effect  pinch  one  of  the 
toes. 

The  widespread  convulsive  responses  signify  the  breaking  down 
of  synaptic  resistances  to  a  uniform  level  (p.  162). 


SUGGESTIONS  FOR  LABORATORY  WORK  599 


SUMMATION  AND  INHIBITION  OF  REFLEXES 

Summation.  Suspend  a  frog,  whose  brain  has  recently  been 
destroyed,  by  a  hook  through  the  lower  jaw.  Tie  fine  copper  wires 
34  inch  (6  mm.)  apart,  about  the  right  toes.  Connect  the  wires 
with  the  secondary  coil  of  an  inductorium,  taking  care  to  avoid 
short-circuits.  The  primary  circuit  should  be  arranged  to  give 
single  shocks. 

Set  the  secondary  coil  so  that  a  small  twitch  follows  each 
stimulus.  Make  and  break  the  primary  circuit  repeatedly  and 
rapidly. 

Note  that  a  series  of  stimuli  produce  an  effect  that  a  single 
stimulus  could  not.  This  is  summation. 

Inhibition.  Bring  dilute  acid  in  a  beaker  in  contact  with  the 
left  toes  of  the  frog  used  in  the  preceding  experiment. 

Determine  in  seconds  the  time  required  for  the  foot  to  be  with- 
drawn. 

Immediately  wash  thoroughly  with  water  the  acidified  foot. 

Now  bring  the  beaker  of  acid  again  in  contact  with  the  left  toes, 
at  the  same  time  stimulating  the  right  toes  with  an  interrupted 
current  of  moderate  strength. 

Determine  the  time  for  withdrawal  of  the  foot  from  the  acid. 
A  delay  is  due  to  inhibition.  To  prove  that  the  acid  has  not  injured 
the  foot  repeat  the  immersion  without  simultaneous  stimulation. 
Prompt  withdrawal  should  occur. 

NEURO-MUSCULAR  FATIGUE 

Dissect  out  a  gastrocnemius-sciatic  preparation  (p.  597).  Ar- 
range to  secure  a  record  of  the  contraction  of  the  muscle.  Moisten 
the  nerve  frequently  with  salt  solution  to  prevent  drying  and 
consequent  loss  of  irritability.  With  the  drum  moving  at  a  slow 
speed  stimulate  the  nerve  of  the  preparation  with  fairly  strong 
stimuli  until  the  muscle  no  longer  responds.  Now  quickly  bring 
the  electrodes  in  contact  with  the  muscle  itself.  A  contraction 
shows  that  the  muscle  is  not  fatigued. 

Since  nerve  trunks  are  indefatigable  the  fatigue  must  have  oc- 
curred in  the  neuro-muscular  junctions  (p.  198). 


600 


APPENDIX 


TIME  RELATIONS  IN  NERVOUS  PROCESSES 

Determine  accurately  by  repeated  trials  the  time  in  seconds  re- 
quired for  a  single  revolution  of  the  kymograph  drum  at  its  highest 
speed.  Be  careful  to  use  this  determined  speed  in  the  observations 
below.  Each  experiment  requires  a  subject  and  an  operator. 

Simple  Reaction  Time.  Arrange  the  inductorium  for  single 
shocks,  and  select  a  strength  of  stimulus  distinctly  felt  on  the 
tongue  at  the  break  of  the  primary  circuit.  The  apparatus  is  so 
arranged  that  the  operator  can  make  and  break  the  circuit  at 
one  place  and  the  subject  at  another  (see  diagram,  Fig.  157).  A 
signal  records  on  the  drum  the  instant  of  making  and  breaking 


FIG.  157. — Diagram  of  reaction  time  apparatus.  K'  and  K",  keys  for  making 
or  breaking  primary  circuit;  C,  dry  cell;  /,  inductorium;  T,  wires  to  tongue  elec- 
trodes; S,  signal  magnet,  writing  on  drum. 

the  circuit.  Let  the  subject  press  the  electrodes  on  his  tongue, 
place  his  hand  on  his  key,  and  close  his  eyes.  The  operator  should 
now  start  the  drum  at  known  speed  and  close  the  circuit  at  his 
contact.  While  the  drum  is  in  motion  the  operator  should  break 
the  circuit  at  his  contact.  This  break  shock  stimulates  the  subject. 
The  instant  the  stimulus  is  felt  the  subject  should  close  his  key. 
The  points  of  stimulus  and  of  response  are  shown  in  the  record 
traced  by  the  signal  on  the  drum.  Repeat  the  experiment  several 
times  with  each  member  of  the  pair  acting  as  subject.  After  the 
tracing  has  been  varnished  measure  with  care  the  length  of  each 
reaction  record.  By  comparing  these  lengths  with  the  drum  cir- 
cumference compute  the  reaction  times  in  hundredths  of  a  second. 
Average  the  results  from  each  individual. 


SUGGESTIONS  FOR  LABORATORY  WORK  601 

Thought  Time  Compared  with  Speech  Time.  With  the  apparatus 
arranged  as  before  let  the  subject,  when  stimulated,  think  the 
first  ten  letters  of  the  alphabet  before  pressing  his  key.  Repeat 
the  experiment;  this  time  having  the  subject  say  the  ten  letters 
aloud.  Make  a  number  of  trials  and  determine  the  average  results. 


THE  SPECIAL  SENSES 

In  the  following  experiments  one  member  of  a  pair  is  to  act  as 
subject,  the  other  as  experimenter.  Members  of  the  pair  should 
alternate  in  these  functions. 

TOUCH 

Localizing  delicacy.  Let  the  subject  sit  with  his  hand  on  the 
table,  and  with  eyes  closed.  Apply  carefully  to  the  back  of  the 
hand  the  points  of  small  dividers  separated  about  %  mm.  The 
subject  reports  whether  he  feels  one  point  or  two  points,  or  is  in 
doubt.  Record  the  result.  Change  the  distance  between  the 
points  gradually,  in  successive  tests  applied  to  the  same  region, 
until  the  subject  reports  a  change  in  sensation.  The  minimal  dis- 
tance at  which  the  two  points  can  be  felt  as  two  points  is  the 
threshold. 

Record  the  results  of  testing  for  the  threshold  on  the  finger-tips, 
palm,  flexor  and  extensor  surfaces  of  the  forearm,  cheek,  and  lips. 

TEMPERATURE 

Cold  and  Warmth  Spots.  Outline  an  area  on  the  back  of  the 
wrist  about  2  cm.  square.  Let  a  blunt  pointed  metal  rod  stand  in 
cold  water  until  it  has  become  cooled.  Dry  it  and  examine  point 
by  point  the  selected  area.  Mark  the  spots  at  which  the  cool  rod 
causes  sensations  of  cold. 

Let  the  rod  stand  in  hot  water  until  it  can  be  felt  as  hot  when 
dried  and  touched  to  the  skin  lightly,  but  not  so  hot  as  to  cause 
burning  or  pain.  Explore  point  by  point  with  very  light  contact 
an  equal  area  contiguous  to  that  examined  for  cold  spots.  Mark 
with  ink  the  spots  at  which  the  rod  causes  sensations  of 
warmth. 


602  APPENDIX 

EQUILIBRIUM  SENSE 

Compensating  Movements.  Let  the  subject  with  head  erect  ro- 
'tate  his  body  for  15  seconds  about  the  vertical  axis.  When  rota- 
tion is  stopped,  note  the  movements  of  the  eyeballs  and  the  arms 
and  legs.  Describe  the  after-sensation. 

Influence  of  Vision  on  the  Maintenance  of  Equilibrium.  Try  to 
stand  on  one  foot  for  a  minute  with  the  eyes  closed.  Repeat  the 
trial  with  the  eyes  open.  Record  the  experiences. 

HEARING 

Threshold.  In  a  quiet  room  determine  the  greatest  distance  at 
which  the  subject,  who  sits  with  eyes  closed  and  a  hand  pressed 
tightly  over  the  right  ear  can  hear  the  ticking  of  a  watch  held 
opposite  the  left  ear.  Repeat  the  experiment  with  the  right  ear, 
holding  the  left  ear  tightly  closed. 

Bone  Transmission.  Hold  a  ticking  watch  between  the  teeth. 
Close  both  ears  with  finger-tips.  Note  the  effect  on  loudness  of 
closing  both  ears. 

Unstop  one  ear.    Compare  the  loudness  in  the  two  ears. 

Spate  Perception.  Let  a  student  click  together  two  coins  in 
various  positions  with  reference  to  the  ears  of  another  student,  who 
acts  as  subject  and  keeps  his  eyes  closed.  The  subject  should  point 
in  the  direction  to  which  he  refers  the  sound.  Compare  the  ac- 
curacy of  judgment  at  the  sides  with  that  in  the  median  plane. 

With  a  finger-tip  stop  the  ear  on  one  side,  and  observe  whether 
the  power  of  localizing  sound  is  diminished. 

TASTE 

Localization.  Apply  to  different  parts  of  the  tongue  samples  of 
the  following  solutions:  a  solution  of  quinine  sulphate  (bitter),  a 
5  per  cent  solution  of  cane  sugar  (sweet),  a  10  per  cent  solution  of 
NaCl  (saline),  and  a  1  per  cent  solution  of  acetic  acid  (sour).  Note 
the  region  on  the  tongue  on  which  each  substance  is  tasted  most 
acutely. 

Let  the  student  wipe  the  surface  of  his  tongue  as  dry  as  possible, 
and  then  let  another  student  apply  crystals  of  salt  and  of  sugar  to 
the  dry  surface. 

Undissolved  substances  are  not  tasted. 


SUGGESTIONS  FOR  LABORATORY  WORK  603 

SMELL 

Fatigue.  With  one  nostril  stopped,  smell  tincture  of  iodine 
through  the  other.  Hold  the  bottle  near  the  nose,  inhale  evenly 
and  somewhat  rapidly,  and  exhale  through  the  mouth. 

Note  the  time  required  to  produce  exhaustion. 

Allow  a  minute  for  recuperation  and  repeat  the  above  test. 
Repeat  until  a  minute  does  not  suffice  for  recuperation. 

Note  the  successive  exhaustion  times. 

This  experiment  explains  failure  to  perceive  closeness  in  a  room 
through  fatigue  of  the  sense  of  smell. 

VISION 

A  good  introduction  to  the  study  of  vision  is  the  dissection  of 
the  eye. 

Sheep's  eyes  hardened  in  formalin  are  satisfactory.  Directions 
for  dissection  are  given  below. 

Conjunctiva.  This  is  the  smooth  membrane  which  is  loosely 
attached  to  the  eye  in  front.  It  lines  the  lids,  and  is  reflected 
from  the  lid  upon  the  surface  of  the  ball. 

Muscles.  Remove  the  fat  which  is  adherent  to  the  ball,  so  that 
the  external  smooth  coat  will  be  exposed.  The  cut  ends  of  several 
muscles  will  be  seen. 

The  Cornea  and  Sclera.  On  the  free  surface  of  the  ball  the 
elliptical  area  includes  the  cornea,  transparent  during  life  but 
rendered  opaque  during  preservation;  the  rest  of  the  surface  of 
the  ball  is  constituted  by  the  naturally  white  and  opaque  sclera, 
commonly  called  the  sclerotic  coat. 

Optic  Nerve.  With  the  finger  and  forceps  tear  apart  the  muscu- 
lar masses  surrounding  the  optic  nerve,  and  remove  with  the 
scissors.  Notice  the  fibrous  constitution  of  the  nerve  and  the  firm- 
ness of  the  sheath ;  also,  that  the  nerve  does  not  enter  the  center  of 
the  eye. 

Aqueous  Humor.  Press  the  eye  so  as  to  make  the  cornea  tense. 
Cut  through  the  cornea  with  the  point  of  the  scalpel;  a  clear  fluid 
will  ooze  out. 

Iris  and  Pupil.  Raise  the  cut  edge  of  the  cornea  with  the  forceps 
and  remove  it  with  the  scissors;  a  dark  lamina,  the  iris,  with  a 
central  orifice,  the  pupil,  will  be  seen. 


604  APPENDIX 

Anterior  Chamber.  This  is  the  space  between  the  iris  and  cornea 
and  is  filled  with  the  aqueous  humor.  Through  the  pupil  will  be 
seen  the  crystalline  lens.  The  space  between  the  iris  and  the  lens 
is  called  the  posterior  chamber  and  also  contains  aqueous  humor. 

The  Crystalline  Lens  and  the  Coats  of  the  Eye.  Make  a  median 
section  through  the  remaining  part  of  the  eye.  The  lens  in  cross 
section,  the  cut  edges  of  three  coats,  and  a  transparent  jelly-like 
mass,  the  vitreous  humor,  will  be  seen. 

The  coats  from  within  outward  are : 

1.  The  retina,  a  very  thin,  white  membrane,  covering  the  inside 
of  the  eye,  except  the  anterior  part.    The  retina  is  a  continuation 
of  the  optic  nerve.    At  its  posterior  part  where  the  nerve  enters 
may  be  seen  a  small  area,  the  blind  spot,  from  which  several  minute 
blood-vessels  radiate. 

2.  The  choroid  coat,  which  is  the  middle  tunic  of  the  eye,  is  pig- 
men  ted,,  and  firmer  than  the  retina.    This  coat  appears  black  or 
blue  in  the  specimen,  and  is  continued  into  the  ciliary  body  and 
the  iris,  the  former  supporting  and  controlling  the  shape  of  the 
lens  in  accommodation. 

3.  The  outer  coat  is  the  solera,  the  anterior  part  of  which  is 
transparent  and  called  the  cornea.    It  is  thick  and  fibrous,  giving 
strength  and  form  to  the  eye.    It  is  white  in  appearance,  is  pierced 
by  the  optic  nerve  at  the  back,  and  gives  attachment  to  the  muscles 
on  its  outer  surface. 

Lens.  Separate  the  halves  of  the  lens  from  the  vitreous  humor 
in  which  they  are  embedded.  Note  that  the  lens  is  composed  of 
concentric  layers,  like  an  onion.  It  is  surrounded  by  a  capsule. 
Note  that  the  anterior  surface  is  flatter  than  the  posterior. 

REFRACTION  IN  THE  EYE 

The  eye  is  an  instrument  for  producing  upon  a  sensitive  surface, 
the  retina,  images  of  objects  in  space.  The  production  of  an  image 
requires  a  device  for  focussing.  In  the  eye  the  cornea  and  lens 
together  make  up  the  focussing  apparatus.  The  eye  is  so  con- 
structed that  rays  of  light  coming  from  points  more  than  18  feet 
away  are  focussed  naturally  upon  the  retina. 

The  fundamental  fact  of  vision,  the  formation  of  images  by 
lenses  can  be  demonstrated  with  the  aid  of  a  double  convex  lens, 


SUGGESTIONS  FOR  LABORATORY  WORK  605 

a  candle  flame,  and  a  screen  in  a  darkened  room.  .The  visual  de- 
fects of  myopia  and  hypermetropia  (p.  262)  may  be  illustrated  by 
shifting  the  screen  in  such  a  manner  as  to  throw  the  image  on  it 
out  of  focus.  When  the  screen  is  too  far  away  from  the  lens  for 
the  image  to  be  clear  the  situation  is  as  in  myopia.  A  double  con- 
cave lens  placed  in  the  path  of  the  rays  illustrates  the  correction 
for  this  defect.  Throwing  the  image  out  of  focus  by  bringing  the 
screen  too  near  the  lens  gives  the  situation  seen  in  hypermetropia. 
This  may  be  corrected  with  a  double  convex  lens. 


SOME  PHENOMENA  OF  VISION 

Visual  Reference.  The  eye  learns  by  experience  to  refer  visual 
stimuli  outward  through  a  point  called  the  "nodal  point"  (p.  267). 
This  point  is  within  the  crystalline  lens,  three-quarters  of  the  dis- 
tance from  retina  to  cornea. 

Schemer's  Experiment.  The  fact  that  images  on  the  right  side 
of  the  retina  are  interpreted  as  coming  from  objects  to  the  left  of 
the  visual  axis,  and  vice  versa,  was  demonstrated  by  Scheiner  in 
1619. 

Pierce  a  card  with  two  pin  holes  about  one-tenth  inch  apart. 
Look  through  the  pin  holes  at  a  distant  object.  Place  in  the  line 
of  vision  about  a  foot  from  the  eye  a  pin  mounted  in  a  block.  Do 
not  accommodate  for  the  pin.  Two  images  of  the  pin  are  seen. 
Slide  a  card  over  the  right  hand  pin  hole.  The  left  hand  image 
disappears. 

Place  the  pin  four  feet  from  the  eye.  Look  through  the  pin 
holes  at  a  second  pin  in  line  with  the  first  one,  but  only  four  to  six 
inches  distant  from  the  eye.  Two  images  of  the  far  pin  are  seen. 
Slide  a  card  over  the  right  hand  pin  hole.  Now  the  right  hand  image 
disappears.  For  the  explanation  see  diagram,  Fig.  158. 

The  Blind  Spot.  Make  a  small  black  spot  near  the  left  margin 
of  a  sheet  of  note  paper.  Place  the  paper  on  the  desk.  Let  one 
student  of  a  pair  look  fixedly  at  the  spot  with  the  right  eye,  holding 
the  head  stationary,  about  twelve  inches,  over  the  spot.  The  other 
member  of  the  pair  should  move  a  black-headed  hatpin  from  a 
point  in  the  right  margin  of  the  paper  directly  opposite  the  black' 
spot  toward  the  spot  itself.  The  subject  should  report  the  instant 
the  head  of  the  pin  disappears,  and  the  place  should  be  marked 


606 


APPENDIX 


with  a  pencil.  Continue  moving  the  pin  head  toward  the  black  spot 
and  fix  in  the  same  manner  the  place  of  reappearance. of  the  head 
of  the  pin.  These  two  spots  indicate  the  lateral  extremities  of  the 
blind  spot.  Its  outline  is  to  be  determined  by  moving  the  pin  head 
toward  it  from  various  directions  and  fixing  the  point  of  disap- 
pearance in  each  of  them. 

The  Field  of  Color  Vision.  Let  one  student  of  a  pair  look  fixedly 
with  the  right  eye  at  a  spot  in  an  upright  screen,  supporting  the 
chin  firmly.  The  other  student  moves  a  small  square  of  colored 
paper  toward  the  "spot"  from  the  margin  of  the  screen.  The 
squares  of  paper  should  be  fixed  to  straws  or  stiff  wires  so  they 


FIG.  158. — Diagrams  illustrating  Schemer's  experiment.  In  A  the  eye  is  ac- 
commodated for  distant  vision  and  rays  from  the  pin  P  strike  the  retina  before 
they  meet.  The  image  through  the  right  hand  pin  hole  falls  upon  the  right  hand 
side  of  the  retina,  just  opposite  to  the  usual  manner.  In  B  the  rays  cross  before  strik- 
ing the  retina.  The  relation  of  images  to  pin  holes  is  therefore  the  same  as  in 
ordinary  vision. 

can  be  handled  readily.  Red,  green,  blue,  yellow,  and  white  should 
be  the  colors  provided.  The  subject  should  not  know  which  color 
is  being  used.  As  soon  as  he  recognizes  the  color  he  should  report 
it,  and  the  color  should  be  marked  on  the  screen.  If  the  color  is 
incorrectly  named,  continue  moving  the  square  inward  until  it  is 
correctly  perceived.  Repeat  the  test  along  different  meridians  and 
with  different  colors  until  the  field  of  each  color  has  been  roughly 
outlined. 

BLOOD 

Histological  Structure.  Blood  is  composed  of  a  liquid,  the  plasma, 
in  which  are  several  kinds  of  minute  structures,  the  corpuscles,  red 
and  colorless,  and  the  platelets. 

A  sample  of  blood  for  observation  is  prepared  as  follows:  Provide 


SUGGESTIONS  FOR  LABORATORY  WORK  607 

two  clean  microscopic  slides.  Congest  the  blood  in  the  middle 
finger  of  the  left  hand  by  wrapping  a  handkerchief  tightly  about 
it,  beginning  at  the  base.  Prick  the  congested  region  sharply  with 
a  clean  needle  and  squeeze  out  a  drop  of  blood.  Bring  the  surface 
of  one  of  the  slides  near  one  end  in  contact  with  the  drop  of  blood. 
The  blood  will  adhere  to  the  slide.  Quickly  place  an  edge  of  the 
second  slide  against  the  surface  of  the  first  one  and  move  it  along 
till  it  comes  in  contact  with  the  blood-drop.  The  latter  should 
spread  out  along  the  edge  of  the  second  slide.  Now  draw  this 
slide  along  the  first  one.  The  blood  will  follow,  and  thus  be  spread 
out  in  a  thin  layer.  Let  the  slide  dry  for  a  few  minutes  before  be- 
ginning observations.  No  cover  glass  is  needed,  but  the  slide 
should  be  kept  free  from  dust. 

Place  the  prepared  slide  on  the  stage  of  the  microscope  and  focus 
on  it  with  the  low  power.  Numerous  pale  yellow  specks  will  be 
seen.  These  are  red  corpuscles.  Some  idea  of  their  great  numbers 
in  the  blood  can  be  gained  by  comparing  the  area  of  the  field  of  the 
microscope  with  that  over  which  the  original  blood-drop  was 
spread,  and  that  drop,  in  turn,  with  the  whole  volume  of  blood 
in  the  body. 

Change  from  the  low-power  objective  to  the  high  power.  Note 
that  by  this  change  the  field  is  much  reduced.  After  obtaining  a 
sharp  image,  study  individual  red  corpuscles  carefully.  Compare 
the  margin  of  a  corpuscle  with  its  center.  The  different  appearance 
of  the  two  regions  signifies  that  the  corpuscle  is  a  disk  thicker  at 
the  edges  than  in  the  center. 

Look  for  groups  of  corpuscles  arranged  in  rows,  edge  to  edge. 
These  are  rouleaux.  Corpuscles  in  shed  blood  tend  to  cling  together 
thus. 

By  exploring  the  slide  carefully,  colorless  corpuscles  can  be  found 
and  studied.  They  are  transparent,  colorless  bodies  about  twice 
the  diameter  of  red  corpuscles.  They  are  much  less  numerous  than 
red  corpuscles.  The  ratio  is  about  1  to  300. 

The  various  kinds  of  colorless  corpuscles  can  be  distinguished 
by  their  appearance  after  treatment  with  suitable  stains.  On  pre- 
pared slides  different  types  of  colorless  corpuscles  can  be  studied. 

Chemical  Structure.     Blood  plasma  *  is  an  exceedingly  complex 

*The  liquid  part  of  clotted  blood  is  called  serum.  That  of  unclotted 
blood  is  called  plasma. 


608  APPENDIX 

liquid.  It  carries  in  solution  all  substances  absorbed  from  the  diges- 
tive tract;  all  waste  products  of  cell  activity;  all  hormones;  and 
all  of  the  great  group  of  unidentified  materials  that  are  concerned 
with  the  control  of  infection.  The  most  prominent  constituents 
of  blood  belong  in  the  chemical  group  of  proteins. 

Tests  for  Blood  Proteins.  The  Xanthoproteic  Reaction.  Pour  in  a 
test  tube  concentrated  blood  serum  to  the  depth  of  J4  inch.  Add 
3  or  4  drops  of  concentrated  nitric  acid  (HNO3).  (Handle  with 
care.)  Boil  and  cool.  A  yellow  precipitate  is  formed  and  the  solu- 
tion becomes  yellow.  Add  strong  sodium  hydrate  (NaOH). 
(Handle  with  care.)  When  sufficient  has  been  added  the  color 
becomes  much  deeper. 

The  Biuret  Reaction.  Pour  a  little  blood  serum  into  a  test  tube. 
Add  a  few  drops  of  very  dilute  copper  sulphate  (CuS04).  Make 
alkaline  with  sodium  hydrate  (NaOH).  A  rose  color  is  produced. 
These  two  tests  are  characteristic  for  proteins. 

Iron  in  Hemoglobin.  The  essential  substance  of  red  corpuscles 
is  an  iron-containing  pigment  compound,  hemoglobin.  The  prop- 
erty of  hemoglobin  as  an  oxygen-carrier  depends  on  its  iron 
content. 

A  Chemical  Test  for  Iron.  Place  a  few  iron  filings  in  a  test  tube. 
Pour  into  the  test  tube,  under  the  hood,  a  few  drops  of  aqua  regia 
(nitric  and  hydrochloric  acids).  The  fumes  of  aqua  regia  are  very 
irritating  and  the  fluid  is  very  corrosive.  Handle  with  great  care, 
always  under  the  hood,  and  avoid  inhaling  the  fumes.  After  al- 
lowing a  few  minutes  for  some  iron  to  be  dissolved,  dilute  with  an 
inch  of  water  and  add  a  few  drops  of  potassium  ferrocyanide  solu- 
tion. A  characteristic  deep  blue  precipitate  of  iron  ferrocyanide 
(prussian  blue)  is  the  test  for  iron. 

To  demonstrate  the  presence  of  iron  in  blood,  place  a  few  lumps 
of  dried  blood  in  a  porcelain  crucible  over  a  bunsen  flame  under  a 
hood.  Continue  heating  the  mass,  stirring  occasionally  with  a 
glass  rod,  until  only  a  reddish  ash  is  left.  Allow  the  crucible  to 
cool.  Add  a  few  drops  of  aqua  regia.  Warm  gently.  After  the 
solution  has  cooled  again  dilute  with  water,  pour  into  a  test  tube 
and  add  potassium  ferrocyanide  solution.  The  appearance  of 
abundant  prussian  blue  shows  the  presence  of  iron  in  the  ash  of 
blood. 


SUGGESTIONS  FOR  LABORATORY  WORK  609 

COAGULATION  OF  BLOOD 

Draw  a  drop  of  blood,  as  described  above,  but  do  not  remove 
it  from  the  finger.  Rest  the  hand  in  a  comfortable  position.  Test 
the  consistency  of  the  blood-drop  by  drawing  a  hair  through  it 
Repeat  the  test  at  intervals  of  one  minute  till  no  further  change 
occurs.  Observe  the  changes  that  take  place  in  the  drop  of  blood 
during  the  process  of  coagulation. 

Coagulation  Time.  When  blood  clots  it  sets  first  into  a  soft  jelly 
which  is  firm  enough,  however,  to  support  a  small  weight.  Ad- 
vantage is  taken  of  this  fact  in  the  Dak  Coagulometer.  A  fine 
glass  tube,  about  J/2  inch  long,  is  to  be  filled,  by  suction,  with 
freshly  drawn  blood.  A  small  shot  placed  in  the  tube  will  run 
along  it  when  the  tube  is  inclined  so  long  as  the  blood  is  uncoagu- 
lated,  but  will  remain  stationary  as  soon  as  clotting  occurs. 

Congest  the  finger  as  above,  and  after  two  sharp  prickings  with 
a  needle,  near  together,  squeeze  out  a  good  sized  drop  of  blood. 
Suck  the  fine  tube  nearly  full.  Insert  the  small  shot  as  quickly  as 
possible.  The  ends  of  the  tube  need  not  be  plugged,  since  surface 
tension  will  retain  the  shot  in  the  blood.  Note  the  minute  and 
second  at  which  the  blood  was  drawn.  Thirty  seconds  thereafter 
hold  the  tube  upright  with  the  shot  at  the  top.  If  the  shot  runs 
down  repeat  at  thirty  second  intervals  until  it  fails  to  move.  The 
elapsed  time  is  coagulation  time.  To  check  the  result  the  experi- 
ment may  be  repeated. 

The  Importance  of  Calcium  in  the  Coagulation  Process.  Prepare  to 
repeat  the  above  described  experiment  on  coagulation  time.  After 
the  blood  is  drawn  and  before  sucking  it  into  the  tube  sprinkle  into  it 
three  or  four  grains  of  powdered  sodium  oxalate.  This  substance 
removes  the  calcium  from  the  blood  by  precipitation.  Continue 
the  experiment  for  twice  the  coagulation  time  previously  deter- 
mined. If  the  blood  has  not  then  clotted  discontinue  the  experi- 
ment. Interpret  the  result. 

Coagulation  Time  in  a  Lower  Animal.  The  experiment  on  coag- 
ulation time  may  be  varied  by  using,  instead  of  human  blood,  blood 
drawn  directly  from  the  vessels  of  a  turtle,  with  brain  destroyed 
and  plastron  removed,  into  the  tube  of  the  coagulometer.  Deter- 
mine carefully  the  coagulation  time  for  the  turtle.  Compare  with 
the  coagulation  time  of  human  blood. 


510  APPENDIX 

THE  CIRCULATORY  SYSTEM 

A  good  introduction  to  the  study  of  the  circulation  is  the  dis- 
section of  the  chief  arteries  and  veins  of  the  cat. 

Preparation.  Inject  the  arterial  system  with  a  starch  mass 
colored  red.  To  do  this  expose  the  heart  and  tie  the  nozzle  of  the 
injecting  syringe  directly  into  the  tip  of  the  left  ventricle.  The 
injection  drives  the  blood  into  the  veins  so  that  they  retain  their 
natural  blue  color.  Trace  the  vessels  by  tearing  cautiously  with 
the  handle  of  a  scalpel  or  some  blunt  instrument.  Do  not  cut  un- 
less directions  are  given.  Lay  the  thorax  and  abdomen  wide  open 
by  a  median  incision. 

Heart.  The  process  of  injection  mutilates  the  heart.  A  separate 
exercise  on  the  sheep's  heart  is  described  below. 

ARTERIES  OF  THE  THORAX 

The  Aorta.  This  is  a  single  great  artery  arising  from  the  left 
chamber  of  the  heart.  It  curves  sharply  to  the  left,  thus  making 
the  arch  of  the  aorta. 

Coronary  Artery.  Two  in  number  arising  within  the  heart;  they 
are  small  and  the  first  branches  seen.  They  supply  the  heart. 

Innominate.  This  arises  from  the  convexity  of  the  arch  very 
near  its  origin;  it  gives  rise  to  the  right  and  left  carotid  arteries. 

Subclavians.  The  right  subclavian  is  a  continuation  of  the  in- 
nominate;  the  left  subclavian,  the  next  large  branch  of  the  aorta,  is 
given  off  close  to  the  innominate. 

Intercostal  Arteries.    These  are  seen,  one  below  each  pair  of  ribs. 

ARTERIES  OF  THE  ABDOMEN 

Abdominal  Aorta.  This  is  a  continuation  of  the  thoracic  aorta. 
Turn  the  stomach  and  intestines  to  the  right,  press  upon  the  median 
line  against  the  spinal  column,  and  the  injected  aorta  will  be  felt. 
Tear  away  the  peritoneum  and  follow  the  vessel  from  the  dia- 
phragm and  note  branches. 

Celiac  Axis.  As  the  aorta  enters  the  abdomen  there  is  given 
off  a  large  branch,  the  celiac  axis.  This  divides  into  three 
branches,  the  first  being  the  gastric,  which  goes  to  the  stomach. 
The  second  goes  to  the  liver  and  is  called  the  hepatic  artery.  Turn 


SUGGESTIONS  FOR  LABORATORY  WORK  611 

the  liver  upward,  and  near  the  lesser  curvature  of  the  stomach 
this  vessel  will  be  seen.  The  third  and  largest  is  the  splenic. 

Superior  Mesenteric.  Turn  the  stomach  and  intestines  to  the 
right.  The  artery  rises  from  the  aorta  just  below  the  celiac  axis. 
It  has  an  extensive  distribution  to  the  coils  of  the  small  intestine. 

Renal  Arteries.  Rise  from  the  sides  of  the  aorta  and  enter  the 
hilum  of  the  kidneys. 

Inferior  Mesenteric.  It  arises  from  the  abdominal  aorta  about 
opposite  the  iliac  crest,  and  has  two  large  branches  which  supply 
the  large  intestine. 

ARTERIES  OP  THE  LOWER  EXTREMITY 

Just  before  leaving  the  abdomen  the  aorta  sends  off  four 
branches,  two  external  iliacs  and  two  .internal  iliacs,  and  then  it 
becomes  the  caudal. 

External  Iliac.  This  passes  downward  a  short  distance  and  be- 
comes the  femoral  artery,  it  runs  down  the  leg  as  the  femoral,  and 
behind  the  knee  it  becomes  the  popliteal,  which  divides  into  the 
anterior  and  posterior  tibial.  The  anterior  tibial  becomes  the 
dorsalis  pedis  on  the  upper  surface  of  the  foot. 

Internal  Iliac.  This  arises  from  the  aorta  just  below  the  origin 
of  the  preceding  and  passes  obliquely  downward  into  the  pelvis, 
and  supplies  the  organs  of  the  pelvis  with  the  following  branches: 
vesical  to  bladder,  internal  pubic  to  internal  genital  organs,  external 
pubic  to  external  genital  organs,  sciatic,  with  gluteal  branch,  to 
muscles  of  back  of  pelvis,  hip,  and  thigh. 

Caudal  Artery.    A  continuation  of  the  aorta  to  the  tail. 

ARTERIES  OF  THE  HEAD  AND  UPPER  EXTREMITIES 

Carotid  Arteries.  These  arise  from  the  innominate  artery,  a 
short  distance  above  the  arch  of  the  aorta,  and  pass  upward  on 
either  side  of  the  trachea,  supplying  the  neck  and  head.  Follow 
one  of  these  arteries  forward,  noting  its  branches. 

Subclavian  Arteries.  The  right  subclavian  is  a  continuation  of 
the  innominate.  The  left  arises  from  the  arch  of  the  aorta.  They 
supply  the  upper  extremities.  Follow  the  artery  down  one  arm, 
note  the  vertebral,  a  branch  running  up  the  foramina  in  the  trans- 
verse processes  of  the  cervical  vertebrae  to  the  brain.  The  sub- 


612  APPENDIX 

clavian  changes  its  name  to  axillary  in  axilla,  brachial  in  upper  arm, 
and  divides  at  elbow  into  radial  and  ulnar. 

These  arteries  give  off  branches  to  muscles  and  surrounding 
tissues  in  their  course. 

VEINS  OF  THE  EXTREMITIES 

These  veins  follow  the  general  course  of  the  arteries  and  usually 
have  similar  names  in  their  corresponding  positions. 

Common  Iliac  Veins.  These  form  the  inferior  vena  cava  at  a 
point  opposite  the  junction  of  the  sixth  and  seventh  lumbar 
vertebrae,  where  the  internal  and  external  iliac  veins  which  bring 
the  blood  from  the  leg  and  pelvis  unite  to  form  the  common  iliac 
vein. 

Inferior  Vena  Cava.  This  is  formed  by  the  union  of  the  common 
iliacs.  Turn  stomach  and  intestines  to  the  left.  It  will  be  seen 
accompanying  the  aorta,  and  running  to  the  right  auricle  of  the 
heart. 

Renal  Veins.  These  extend  laterally  -from  the  kidneys  and 
empty  into  the  inferior  vena  cava. 

Portal  Vein.  Formed  near  the  outlet  from  the  stomach  by  union 
of  veins  from  the  stomach  and  intestines,  and  goes  to  the  liver. 

Veins  of  the  Thorax.  The  superior  vena  cava  is  a  prominent  vessel 
extending  from  a  point  opposite  the  first  rib  to  the  upper  part  of 
the  right  auricle.  It  conducts  the  blood  from  the  head  and  upper 
extremities  back  to  the  heart.  It  is  formed  by  the  union  of  the 
two  innominate  veins. 

Innominate.  Formed  by  union  of  the  subclavian  and  jugular 
veins. 

Subclavian.    This  returns  the  blood  from  the  arm. 

External  Jugular.    Returns  the  blood  from  head  and  brain. 

ANATOMY  OF  THE  SHEEP'S  HEART 

Removal  of  the  Pericardium.  At  about  the  middle  of  the  length 
of  the  heart,  slit  the  pericardium  and  with  the  scissors  girdle  it 
completely.  Remove  the  lower  portion  and  note  the  smoothness 
of  its  internal  surface.  It  and  the  apposed  external  surface  of  the 
heart  are  covered  by  a  serous  membrane  which  secretes  a  fluid 
during  life.  Turn  the  upper  portion  of  the  pericardium  inside  out, 


SUGGESTIONS  FOR  LABORATORY  WORK  613 

like  the  finger  of  a  glove.  At  varying  distances  from  the  base  it 
is  attached  to  the  heart  and  vessels.  Trim  the  pericardium  along 
or  near  the  line  of  attachment. 

General  Topography  of  the  Heart.  The  apex  is  conical,  smooth, 
firm,  and  fleshy,  and  is  formed  by  the  muscular  ventricles.  Notice 
that  the  left  ventricle  is  larger  and  has  thicker  walls  than  the  right 
ventricle.  The  base  is  irregular  and  wider.  It  presents  not  only 
the  thin-walled  auricles  but  also  vessels  and  fat. 

The  Vessels.  The  aorta  and  pulmonary  artery  maintain  a  cylin- 
drical form  and  their  cut  ends  are  naturally  circular.  The  great 
veins  have  thinner  walls  in  proportion  to  their  size  and  collapse 
more  or  less  completely.  The  inferior  vena  cava  forms  nearly  a 
right  angle  with  the  long  axis  of  the  heart.  The  superior  vena  cava 
is  at  the  base  on  the  ventral  surface.  The  pulmonary  artery  is  the 
prominent  vessel  on  the  ventral  aspect  between  the  two  auricles, 
extending  from  the  base  of  the  right  ventricle.  The  aorta,  with 
its  principal  branch,  will  be  seen  more  distinctly  at  a  later  stage. 

Dissection  of  the  Heart.  The  order  of  dissection  follows  the 
course  of  the  blood  through  the  organ.  Bear  in  mind  that,  al- 
though anatomically  united  and  acting  together  as  muscles,  as 
to  their  cavities,  the  right  and  left  sides  of  the  heart  are  entirely 
separated  by  complete  partitions  between  the  two  auricles  and 
between  the  two  ventricles.  When  the  cavities  are  open  wash 
their  interiors  with  running  water.  The  flowing  water  will  show 
how  the  valves  close  the  openings  between  the  cavities  and  also 
the  action  of  both  sets  of  semilunar  valves. 

Opening  the  Right  Auricle.  Hold  the  heart  with  its  ventral  side 
toward  you.  Push  the  point  of  the  scissors  into  the  upper  and  outer 
angle  of  the  appendix  and  cut  toward  the  median  line  of  the  heart. 
Lift  the  edge  of  the  flap  and  note  the  wide  mouths  of  the  great 
veins.  The  right,  the  superior;  the  left,  the  inferior  vena  cava. 
In  the  back  wall  of  the  auricle  at  the  base  of  the  inferior  vena  cava 
will  be  found  an  oval  scar,  the  fossa  ovalis.  Near  the  orifice  of  the 
inferior  vena  cava  is  a  ridge,  the  eustachian  valve. 

Opening  the  Right  Ventricle.  This  must  be  done  very  carefully. 
At  a  point  on  the  ventricle  at  a  short  distance  from  the  pulmonary 
artery,  insert  the  point  of  a  scalpel  and  cut  parallel  with  the  fur- 
row, extending  to  the  apex  of  the  right  ventricle.  Keep  the  cut 
edges  of  ventricular  walls  apart  while  studying  the  cavity. 


614  APPENDIX 

Musculi  Papillares.  These  are  muscular  columns  attached  at 
one  end  to  the  walls  of  the  ventricle,  and  at  the  other  to  the  chordce 
tendinece.  These  are  delicate  tendinous  cords  which  pass  from 
the  musculi  papillares  to  the  edge  of  the  valve  segments.  The 
muscles  and  cords  prevent  the  segments  of  the  valve  being  forced 
into  the  auricle  by  the  weight  of  the  blood  behind  them. 

The  Tricuspid  Valve.  Pass  the  finger  from  the  auricle  into  the 
ventricle  and  distend  the  auricula-ventricular  orifice.  Note  that 
it  is  surrounded  by  three  fibrous  sheets  which  hang  down  into  the 
ventricle  and  are  connected  at  the  sides  to  the  musculi  papillares 
by  the  chorda  tendinew.  This  is  the  tricuspid  valve. 

The  Pulmonary  Artery.  With  the  scissors,  extend  the  incision 
upward  through  the  pulmonary  artery  and  note  that  the  mouth  of 
the  artery  is  surrounded  by  three  membranous  cups,  the  semilunar 
valve.  Each  constitutes  a  sort  of  pocket,  and  all  three  together 
when  distended  close  the  opening  of  the  artery  completely. 

Opening  the  Left  Auricle.  Make  an  incision  in  the  appendix  so 
as  to  see  the  interior  of  the  auricle.  Note  the  two  portions,  the 
appendix  and  the  atrium.  It  resembles  the  right  auricle. 

The  Pulmonary  Veins.  Hold  the  heart  so  as  to  see  the  depth  of 
the  atrium  and  note  that  it  presents  a  ridge  at  right  angles  to  it. 
Hold  the  organ  to  the  light  and  note  the  openings  of  the  pulmonary 
veins  near  the  ridge. 

Opening  the  Left  Ventricle.  With  the  scalpel  transect  the  left 
ventricle,  carrying  the  incision  to  the  apex.  The  left  ventricle 
forms  the  apex 'of  the  heart.  The  auriculo-vbntricular  opening  of 
this  side  is  surrounded  by  the  mitral  or  bicuspid  valves.  Note  the 
chordce  tendinece  and  musculi  papillares. 

The  Aorta.  Pass  a  probe  into  the  aorta  and  follow  the  course 
of  the  probe  with  the  scissors,  thus  opening  the  aorta.  Note  the 
semilunar  valves  guarding  its  orifice.  Note  the  openings  of  other 
blood  vessels  from  the  aorta,  the  coronary  arteries,  just  above  the 
valve.  Observe  the  smooth  lining  of  the  aorta,  as  well  as  of  the 
auricles  and  ventricles. 

THE  CIRCULATION  OF  BLOOD  IN  THE  FROG'S  FOOT 

A  live  frog  is  wrapped  carefully  in  moist  cloth  except  one  leg. 
The  projecting  foot  is  secured  on  the  microscope  stage  in  such 


SUGGESTIONS  FOR  LABORATORY  WORK  615 

fashion  that  a  portion  of  the  web  is  in  the  field.  Study  the  field 
with  the  low  power. 

Note  the  rate  and  character  of  the  blood  flow  in  different  ves- 
sels. The  larger  vessels  are  either  arteries  or  veins.  In  arteries  the 
blood  flow  is  intermittent,  and  the  rate  of  pulsation  agrees  with 
that  of  the  heart-beat.  In  veins  the  flow  is  more  or  less  steady. 
The  smallest  vessels  are  capillaries.  These  communicate,  in 
general,  between  arteries  and  veins.  In  the  frog's  web,  however, 
the  circulation  is  anastomosing.  In  such  a  circulation  some  cap- 
illaries can  be  seen  connecting  one  vein  with  another.  The  direc- 
tion of  flow  in  anastomosing  capillaries  is  sometimes  forward,  and 
sometimes  backward. 

Study  a  region  showing  arteries,  veins,  and  typical,  non-anas- 
tomosing capillaries. 

THE  CIRCULATION  OF  BLOOD  IN  MAN 

Pulse  Tracings.  To  obtain  a  graphic  record  of  the  pulse  the  pres- 
sure changes  in  the  beating  artery  are  transmitted  to  a  delicate 
recording  device  known  as  a  tambour.  For  the  transmitter  a 
thistle  tube,  with  sheet  rubber  tied  tightly,  without  stretching, 
over  the  mouth,  may  be  used.  Secure  a  bone  or  wood  button  to 
the  center  of  the  rubber.  Connect  the  transmitter  with  the  re- 
corder by  a  rubber  tube.  Insert  in  the  course  of  this  tube  a  glass 
T  with  a  short  rubber  tube  on  the  side  neck. 

Gently  press  the  button  on  the  transmitter  on  the  front  surface 
of  the  wrist  directly  over  the  point  where  the  pulse  can  be  felt  in 
the  radial  artery.  The  carotid  artery  in  the  neck  may  be  used  if 
preferred.  When  the  system  is  made  air  tight  by  closing  the  side 
tube  the  tambour  lever  should  pulsate  in  synchronism  with  the 
artery.  Obtain  a  record  of  these  pulsations  on  a  drum  moving  at 
moderate  speed.  The  notch  in  the  descending  limb, of  the  pulse 
curve  marks  the  closing  of  the  semilunar  valves,  and  therefore 
the  end  of  systole. 

Determinations  of  Pulse  Rate.  In  these  determinations  two  stu- 
dents should  alternate  as  subject  and  observer.  Trustworthy 
readings  cannot  be  made  upon  oneself.  The  pulse  should  be 
taken  by  pressing  the  tips  of  three  fingers  against  the  radial  artery 
at  the  wrist.  Do  not  use  the  thumb. 


616  APPENDIX 

With  the  subject  sitting  quietly  in  a  comfortable  position 
count  the  pulse  during  the  first  twenty  seconds  of  three  con- 
secutive minutes.  Compute  the  rate  per  minute  for  each 
minute.  If  there  is  much  variation  wait  three  minutes  and 
try  again  for  three  minutes.  When  the  pulse  is  reasonably 
steady  for  three  consecutive  minutes  the  average  rate  may  be 
taken  as  the  pulse  rate  for  that  subject  in  that  particular 
condition. 

The  Effect  of  Posture.  Using  the  method  described  above  deter- 
mine the  pulse  rate  with  the  subject  lying  down,  sitting,  and  stand- 
ing. 

The  Effect  of  Exercise.  Determine  the  pulse  rate  with  the  subject 
in  the  sitting  position.  When  the  rate  is  uniform  let  the  subject 
raise  and  lower  his  legs  six  times  without  rising  from  his  seat. 
Count  the  pulse  for  twenty  seconds  as  quickly  as  possible  after 
the  movement  ceases. 

This  shows  the  effect  of  slight  exercise. 

After  the  pulse  rate  has  returned  to  normal  and  become  steady 
let  the  subject  run  up  and  down  stairs  for  two  or  three  minutes. 
Count  the  pulse  for  twenty  seconds  as  soon  as  possible  after  he 
returns  to  his  seat  and  at  two-minute  intervals  for  ten  minutes 
thereafter. 

This  shows  the  effect  on  pulse  rate  of  vigorous  exercise. 

The  Direction  of  Blood  Flow  in  Arteries  and  Veins.  Expose  the 
arm  to  the  shoulder.  Find  in  the  upper  arm  a  place  where  the  pulse 
can  be  felt.  Learn  by  practice  to  occlude  the  artery  by  pressing 
it  against  the  arm  bone.  Now  with  one  hand  on  the  artery  in  the 
upper  arm  and  the  other  on  the  artery  at  the  wrist  determine  the 
direction  of  arterial  flow  by  showing  which  pulse  disappears  whei 
the  other  artery  is  occluded. 

Occlude  a  prominent  vein  on  the  hand  or  forearm  by  pressing 
upon  it  with  a  finger. 

Observe  on  which  side  of  the  occluded  point  the  vein  becomes 
congested. 

Attempt  to  empty  the  vein  above  and  below  the  occluded  point 
by  pressing  a  finger  along  it. 

Note  on  which  side  of  the  point  this  can  be  done. 

Thus  is  determined  the  direction  of  venous  flow. 

The  Control  of  Hemorrhage.   Bleeding  can  be  checked  by  com- 


SUGGESTIONS  FOR  LABORATORY  WORK  617 

pressing  the  ruptured  blood  vessel  on  the  side  of  the  injury  from 
which  blood  comes. 

Arterial  bleeding  can  be  distinguished  from  venous  by  the 
brighter  color  of  the  blood  and  by  the  fact  that  the  blood  escapes 
in  jets.  When  bleeding  is  from  veins  the  flow  is  steady  and  the 
blood  is  dark. 

Veins  which  are  likely  to  be  injured  are  on  the  surface.  A  band- 
age tied  tightly  around  the  arm  or  leg  in  proper  position  relative 
to  the  injury  usually  suffices  to  check  venous  bleeding.  Select  a 
point  on  the  arm  as  a  possible  seat  of  injury.  Adjust  a  bandage 
in  such  a  position  as  to  cut  off  venous  flow  to  the  chosen  point. 
Show  on  a  diagram  the  position  of  injury  point  and  of  bandage. 
Indicate  direction  of  venous  flow  by  arrows. 

Arteries  are  deep  seated  through  most  of  their  course,  and  blood 
pressure  within  them  is  high.  To  check  arterial  bleeding  strong 
pressure  on  properly  selected  points  is  necessary.  Expose  the 
entire  arm.  Find  a  point  near  the  upper  and  inner  margin  of  the 
biceps  muscle  where  the  pulse  can  be  felt.  Strong  pressure  on 
this  point  if  properly  applied  will  check  arterial  bleeding  below. 
For  a  more  permanent  check  make  a  hard  knot  the  size  of  an 
egg  in  the  middle  of  a  handkerchief.  Tie  firmly  around  the  arm 
just  above  the  elbow  with  the  knot  in  front.  Bend  the  forearm 
so  that  it  presses  hard  against  the  knot.  If  the  procedure  is  suc- 
cessful the  pulse  at  the  wrist  disappears.  Another  method  is  to 
make  a  hard  knot  of  cloth  the  size  of  a  fist.  A  round  stone  or  other 
hard  substance  may  be  used.  Push  it  hard  into  the  arm  pit. 
Bring  the  elbow  straight  down  and  hold  it  firmly  against  the  side. 
The  success  of  the  procedure  should  be  tested  by  observing  whether 
the  pulse  at  the  wrist  disappears.  (For  the  control  of  bleeding  in 
other  regions  than  the  arm  see  Dulles:  Accidents  and  Emergencies, 
Philadelphia,  1899.) 

The  Effect  of  Muscular  Movement  on  Venous  Flow.  Tie  a  cloth 
tightly  about  the  arm  at  the  elbow.  Note  the  rate  at  which  the 
superficial  veins  become  congested.  Loosen  the  cloth  until  the 
veins  return  to  normal.  Replace  the  cloth  about  the  arm  and 
close  and  open  the  hand  several  times. 

Compare  the  rate  of  venous  congestion  with  that  seen  in  the 
quiet  arm. 


618  APPENDIX 

HEART  BEAT  AND  BLOOD  PRESSURE  IN  THE  TURTLE 

A  turtle  whose  brain  has  been  destroyed  and  plastron  (lower 
shell)  removed,  is  fastened  to  a  board  back  down,  and  with  neck 
extended.  The  heart  can  be  seen  beating  through  its  enclosing 
membrane,  the  pericardium.  With  a  fine  scissors  cut  away  the 
pericardium,  taking  care  not  to  injure  the  heart.  The  turtle  has 
two  auricles  and  a  single  ventricle.  Identify  these.  Determine 
by  careful  observation  the  sequence  of  beat  of  the  different  cham- 
bers. 

Compare  the  periods  of  systole  and  diastole. 

In  the  turtle  the  great  veins  pulsate  as  well  as  do  auricles  and 
ventricle.  To  observe  these  veins  the  ventricle  must  be  lifted 
out  of  the  way.  Find  the  connective  tissue  frenum  which  at- 
taches the  tip  of  the  ventricle  to  the  pericardium.  Cut  this  as 
far  from  the  heart  as  possible.  Seize  the  frenum  with  a  pair  of 
forceps  and  by  means  of  it  lift  the  ventricle  till  the  underlying 
veins  can  be  seen.  The  beat  is  seen  to  originate  in  these  veins 
whence  it  sweeps  over  the  heart  in  the  form  of  a  wave. 

Vagus  Inhibition.  Find  the  carotid  artery  where  it  passes  up 
the  neck.  Associated  with  it  is  a  nerve,  the  vagus.  Expose  care- 
fully a  short  portion  of  this  nerve  and  stimulate  it  with  repeated 
induction  shocks.  In  a  normal  preparation  one  or  the  other  vagus 
nerve  contains  inhibitory  fibers  whose  stimulation  will  stop  tl 
heart. 

Note  whether  the  standstill  occurs  in  systole  or  in  diastole. 

Graphic  Record.    Catch  a  pin-hook  either  through  the  frenui 
or  through  the  very  tip  of  the  ventricle.     Connect  this  hook 
thread  or  fine  wire  to  the  short  arm  of  the  heart  lever,  whicl 
should  be  directly  above  the  heart.    Obtain  on  a  slowly  movii 
drum  a  record  of  the  heart  beat.    While  the  record  is  being  trac 
stimulate  the  vagus  nerve.    Indicate  on  the  record  the  period 
stimulation. 

Determine   carefully  the  interval  between  the  beginning 
stimulation  and  the  cessation  of  beat.    Note  also  how  long  it  tak* 
the  heart  to  recover  after  cessation  of  vagus  stimulation. 

The  Effect  of  Nicotine.  By  means  of  a  camel's  hair  brush  apply 
a  few  drops  of  nicotine  solution  to  the  surface  of  the  heart.  In 
two  minutes  repeat  the  vagus  stimulation.  The  inhibition  is 


SUGGESTIONS  FOR  LABORATORY  WORK  619 

longer  effective.    The  junctions  of  preganglionic  and  postganglionic 
neurons  of  the  vagus  path  are  in  the  heart  tissue  itself  (p.  194). 

Arterial  Pressure.  Disconnect  the  heart  from  the  recording 
lever.  If  the  heart  is  beating  feebly  or  very  slowly  bathe  with 
warm  water.  If  no  response  take  a  freshly  prepared  turtle.  Trace 
one  of  the  large  arteries  to  a  point  at  least  one  inch  from  the  heart. 
At  this  point  separate  the  artery  carefully  from  surrounding  tissues 
for  another  inch.  Squeeze  the  artery  shut  at  the  cardiac  end  of 
the  prepared  portion  by  means  of  a  spring  clip.  Pass  a  stout 
thread  around  the  prepared  artery.  Fill  with  salt  solution  a 
small  L-shaped  glass  tube,  15  inches  high.  Hold  the  solution  in 
the  tube  by  closing  the  long  end.  Make  a  transverse  cut  half- 
way through  the  prepared  artery,  slip  the  short  end  of  the  glass 
tube  through  this  out  into  the  artery  and  tie  it  in  place  by  means 
of  the  thread  previously  prepared.  The  tube  may  now  be  opened 
and  the  clip  removed  from  the  artery.  The  height  at  which  the 
column  of  salt  solution  is  maintained  measures  arterial  pressure. 

THE  RESPIRATORY  SYSTEM 

Dissection  of  Air  Passages  and  Lungs  of  Sheep.  Note  the  large 
trachea  (windpipe)  with  open  rings  of  cartilage  in  its  wall.  Ob- 
serve carefully  the  surface  of  the  lung.  Note  the  cellular  appear- 
ance and  the  delicate  texture.  Note  the  subdivision  of  each  lung 
into  lobes. 

Inflate  the  specimen. 

Dissection.  In  one  lung,  follow  the  trachea  down  the  bronchus 
to  its  smallest  branches. 

Starting  at  the  heart  follow  the  pulmonary  artery  and  vein  to 
their  finest  branches.  Use  a  probe  and  scissors. 

On  the  other  lung  tear  away  the  tissues  so  as  to  show  the  inter- 
lacing of  these  vessels.  Use  the  forceps  to  pick  with. 

Miser oscopic  Study.  The  final  subdivisions  of  the  bronchi,  and 
their  terminations  in  infundibula  and  alveoli,  are  microscopic. 
Observe  in  prepared  and  stained  sections  the  relatively  thick 
walled  bronchioles,  and  the  extremely  delicate  walls  of  the  air 
cells  (infundibula  and  alveoli).  The  capillaries  which  run  in  the 
alveolar  walls  cannot  be  seen  in  ordinary  microscopic  sections. 

Breathing  Movements  in  Man.    The  following  observations  may 


620 


APPENDIX 


be  made  by  a  group  of  students  on  a  single  subject.    Let  the  latt 
strip  the  upper  part  of  the  body  to  the  undershirt.     With  the 
subject  sitting  erect  on  a  stool  observe  the  chest  and  abdom< 
closely  during  quiet  breathing. 

Note  the  general  direction  of  movement  of  the  body  walls  dui 
ing  inspiration,  and  during  expiration. 

Observe  the  movements  of  chest  and  abdomen  during  fon 
inspiration  and  expiration. 

Abdominal  movements  are  caused  by  contraction  and  rel 
tion  of  the  diaphragm.    Explain  the  relationship. 

Costal  and  Diaphragmatic  Breathing.  Whereas  in  normal  11 
voluntary  breathing  the  diaphragm  and  the  chest  muscles  are  ii 
action  simultaneously,  it  is  possible  in  voluntary  breathing  move 
ments  to  use  one  or  the  other  at  will.  Compare  abdominal  move 
ments  in  normal  breathing  and  in  breathing  with  the  chest  hele 
stationary  (diaphragmatic  breathing).  Compare  chest 
ments  in  normal  breathing  and  in  breathing  with  abdomen  helc 
stationary  (costal  breathing). 

Volumes  of  Respired  Air.    These  are: 

1.  Volume  passing  in  and  out  in  quiet  breathing — tidal  air. 

2.  Volume  that  can  be  breathed  in  after  a  normal  inspi] 

tion — complemental  air. 

3.  Volume  that  can  be  breathed  out  after  a  normal  expirati< 

— supplemental  air. 

4.  Sum  of  above:  volume  that  can  be  breathed  out  aft 

forced  inspiration — vital  capacity. 

Determine  the  supplemental  air  by  blowing  into  a  spiromel 
after  normal  expiration.    Determine  tidal  air  by  blowing  into  tl 
spirometer  after  normal  inspiration  and  subtracting  previously 
determined  supplemental  air.     Make  several  trials.     Determine 
vital    capacity    by    blowing    into    spirometer   after   forced    ii 
spiration. 

Determine  complemental  air  by  subtracting  combined  supple 
mental  and  tidal  air  from  above. 

Graphic  Record  of  Breathing.     For  recording  breathing  an  aj 
paratus  known  as  a  pneumograph  is  fastened  around  the  chest 
that  its  volume  will  be  changed  by  breathing  movements.    It 
connected  with  a  recording  tambour  which  indicates  on  a  kyme 
graph  the  movements  of  breathing. 


SUGGESTIONS  FOR  LABORATORY  WORK  621 

•With  the  pneumograph  in  place  and  the  back  to  the  apparatus 
get  a  record  of  normal  breathing. 

Note  the  rate  per  minute. 

Sip  the  contents  of  a  glass  of  water.  Observe  the  effect  of  swal- 
lowing on  breathing. 

Obtain  records  of  reading  aloud,  of  coughing.  Compare  these 
with  normal,  quiet  breathing. 

The  Control  of  Breathing.  The  students  of  a  pair  should  act 
alternately  as  subject  and  observer  in  the  following  tests.  Obtain, 
by  counting  the  breathing  movements,  the  normal  rate  per  minute. 
Let  the  subject  run  down  and  up  stairs.  Determine  the  rate  per 
minute  every  three  minutes  till  the  normal  rate  is  restored. 

Determine  how  many  seconds  the  subject  can  hold  his  breath. 
Note  the  difference  according  to  whether  he  begins  to  hold  it  at 
the  end  of  inspiration  or  the  end  of  expiration. 

Let  the  subject  breathe  rapidly  and  deeply  for  two  or  three 
minutes  and  then  hold  the  breath. 

Note  the  time  of  holding  it  as  compared  with  the  former  trials. 

Repeat  the  experiment,  this  time  breathing  into  and  from  a 
paper  bag.  Compare  the  time  the  breath  can  be  held  with  former 
trials. 

The  automatic  stimulus  for  breathing  depends  on  the  amount 
of  carbon  dioxide  in  the  blood,  and  indirectly  on  the  amount  in 
the  alveoli  of  the  lungs.  The  experiments  on  breath-holding  can 
be  explained  on  this  basis. 

Artificial  Respiration.  The  Schafer  Method.  In  this  experiment 
the  subject  should  cease  breathing  so  far  as  possible.  The  treat- 
ment may  be  considered  successful  when  no  active  breathing 
movements  are  necessary. 

Lay  the  subject  prone,  with  a  thick  roll  of  clothing  under  the 
chest  and  the  epigastrium.  Take  a  position  over  or  beside  the 
subject's  legs,  and  facing  his  head.  Place  the  hands  on  either 
side  over  his  lowest  ribs.  Slowly  throw  the  weight  of  the  body 
onto  the  hands,  and  thus  compress  the  subject's  thorax  and  force 
air  from  the  lungs.  Without  removing  the  hands,  release  the 
pressure.  The  chest  by  its  own  elasticity  will  perform  the  func- 
tion of  inspiration.  Repeat  this  procedure  at  the  normal  rate  of 
respiration  until  the  issue  is  determined.  In  case  of  drowning 
it  may  be  a  half  hour  before  respiration  is  restored.  Schafer'a 


622  APPENDIX 

method  is  specially  applicable  to  cases  of  drowning  because  the 
face  is  downwards  and  water  in  the  air  passages  readily  runs  out. 

Carbon  Dioxide  (COz)  in  Expired  Air.  Dip  a  tube  below  the 
surface  of  a  bottle  containing  lime  water  (CaO2H2).  Exhale 
through  the  tube.  The  C02  in  the  expired  air  combines  with  th< 
lime  water  in  the  bottle  and  forms  an  insoluble  carbonate  of  lime 
(CaC03)  which  makes  the  solution  cloudy,  later  this  will  settle 
as  a  precipitate  (powdered  chalk). 

When  present  in  excess  the  CO2  makes  the  solution  acid 
the  CaCO3  redissolves. 

THE  DIGESTIVE  SYSTEM 

Dissection  of  the  Digestive  System  in  the  Cat 

Exposure  of  the  Viscera.     Make  an  incision  the  length  of  th< 
abdomen  in  the  mid  line.    Separate  the  edges  of  the  opening 
as  to  get  a  good  view  of  the  abdominal  contents. 

Peritoneum.    This  is  a  membrane  lining  the  abdomen.    It  give 
the  abdominal  wall  a  smooth  glistening  appearance  and  may 
easily  separated  from  the  muscles  forming  the  walh    The  mesen- 
teries and  the  ligaments  of  the  liver,  the  bladder  and  uterus 
formed  by  duplicatures  of  the  peritoneum. 

The  Great  Omentum.  This  is  a  double  fold  of  peritoneum  form- 
ing a  sac  which  is  called  the  lesser  peritoneal  cavity.  It  is  attache 
to  the  posterior  abdominal  wall  and  the  greater  curvature  of  tl 
stomach.  Demonstrate  the  sac-like  character  of  the  omentum  b] 
tearing  it  open.  Each  wall  of  the  sac  is  also  composed  of  tw< 
layers.  Notice  the  distribution  of  fat  through  the  omentum. 

Spleen.    This  is  a  deep  red,  usually  single-lobed  organ,  situai 
on  the  left  of  the  stomach  in  the  great  omentum. 

Drawing.    Make  a  drawing  without  disturbing  anything,  show- 
ing position  of  liver,  stomach,  spleen,  and  great  omentum  covei 
ing  the  coils  of  small  intestine. 

Stomach.    Turn  the  left  lobe  of  the  liver  toward  the  head  anc 
the  abdominal  oesophagus  will  be  seen  emerging  from  the  diaphi 
and  entering  the  cardiac  end  of  the  stomach.    The  stomach,  as 
whole,  is  pear-shaped  and  curved  upon  itself.    The  great  curvati 
is  at  the  lower  border  of  the  stomach,  and  has  the  great  omentui 
attached  to  it,  while  the  lesser  curvature  is  the  upper  border.    Th< 


SUGGESTIONS  FOR  LABORATORY  WORK  623 

larger  or  cardiac  end  is  next  to  the  diaphragm.  The  pyloric  or 
smaller  end  is  curved  sharply  upon  itself.  It  is  firm  to  the  touch 
and  appears  as  an  annular  constriction. 

Small  Intestine.  Very  carefully  turn  the  ornentum  over  toward 
the  thorax.  The  greatly  coiled  cylindrical  small  intestine  will  be 
exposed.  It  is  divided  into  three  regions,  the  duodenum,  the 
jejunum,  and  the  ileum. 

Duodenum.  This  is  the  first  portion  of  the  small  intestine  along 
which  the  pancreas  extends.  It  is  held  rather  firmly  in  position. 
Into  the  duodenum  empty  the  common  bile  duct  and  the  pan- 
creatic duct. 

Jejunum.  This  is  an  ill-defined  portion  of  the  small  intestine 
immediately  following  the  duodenum.  It  is  so  called  because  in 
man  it  is  often  found  empty  after  death. 

The  Ileum.  This  is  the  last  part  of  the  small  intestine.  It  ter- 
minates in  the  large  intestine,  entering  it  obliquely.  At  its  ter- 
mination is  the  ileo-coecal  valve  which  allows  the  alimentary  con- 
tents to  pass  from  the  small  to  the  large  intestine,  but  not  easily 
in  the  opposite  direction. 

Large  Intestine.  Turn  the  coil  of  small  intestine  toward  the 
left  leg.  The  large  intestine  extends  from  the  caecum  to  the  anus. 
It  is  divided  into  five  parts, — ccecum,  ascending,  transverse  and 
descending  colon,  and  rectum. 

Ccecum.  This  is  a  somewhat  conical  blind  sac  at  the  beginning 
of  the  large  intestine.  It  lies  on  the  right  side  and  in  about  the 
middle  of  the  abdominal  cavity. 

Ascending  Colon.  This  is  the  part  of  the  large  intestine  which 
extends  upward  from  the  caecum. 

Transverse  Colon.  This  is  a  continuation  of  the  preceding.  It 
extends  transversely  across  the  abdomen  in  front  of  the  duodenum 
and  below  the  stomach. 

Descending  Colon  and  Rectum.  After  extending  nearly  across 
the  abdomen  from  right  to  left,  the  large  intestine  passes  obliquely 
downward.  The  last  and  straighter  part  is  called  the  rectum. 

Pancreas.  The  pancreas  will  appear  as  a  pinkish,  finely  lobulated 
and  elongated  body  after  the  great  omentum  has  been  turned 
toward  the  thorax.  It  extends  from  the  spleen  under  the  stomach 
to  the  pylorus,  in  the  great  omentum,  and  then  downward  for  a 
short  distance  along  the  duodenum  in  the  mesentery. 


624  APPENDIX 

Mesentery.  This  is  a  duplicature  of  peritoneum  supporting  the 
different  portions  of  the  small  intestine.  It  is  a  double-walled 
membrane  and  carries  blood-vessels  and  lymphatics. 

Mesenteric  Glands.  The  so-called  mesenteric  glands  belong  to 
the  lymphatic  system.  They  are  between  the  layers  of  the  mesen- 
tery and  are  especially  large  near  the  caecum. 

Internal  Structure  of  the  Stomach.  Open  the  stomach  and  wash 
out  its  contents.  It  is  composed  of  a  muscular  coat  covered  by 
the  peritoneum,  and  an  internal,  mucous  coat,  which  is  thrown  into 
folds  or  rugce. 

Interior  of  the  Small  Intestine.  Open  and  wash  in  water.  It  is 
composed  of  three  coats  like  the  stomach.  It  has  a  velvety  feel 
due  to  villi,  which  are  microscopic  finger-like  processes  found  only 
in  the  small  intestine  and  most  abundantly  in  the  upper  part. 

Interior  of  the  Ccecum.  Open  the  caecum  and  observe  the  ileo- 
caecal  valve. 

Interior  of  the  Large  Intestine.  Wash  the  contents  out.  The 
structure  of  the  large  intestine  is  like  the  small,  excepting  that  it 
has  no  villi. 

The  Liver.  The  liver  is  a  deep  red  and  multi-lobular  organ  oc- 
cupying nearly  all  the  upper  part  of  the  abdomen  but  especially  the 
right  side.  It  is  supported  in  various  parts  by  folds  of  peritoneum. 
It  is  composed  of  right  and  left  lobes,  each  of  which  is  subdivided 
into  smaller  lobes  by  fissures.  The  cystic  lobe  is  one  of  the  divi- 
sions of  the  right  lobe  near  the  front,  and  contains  the  gall-bladder. 

If  good  sections  are  available  the  gross  study  outlined  above 
may  be  followed  by  microscopic  studies  of  the  structure  of  the 
stomach  wall,  the  small  intestine,  salivary  gland,  pancreas,  and 
liver. 

STUDY  OF  FOODS 

Carbohydrates.  The  food  carbohydrates  are  starch,  glycogen, 
dextrin,  double  sugars,  single  sugars. 

The  following  experiments  illustrate  tests  for  the  different 
carbohydrates,  and  the  application  of  these  tests  to  different  foods 
to  determine  which  of  the  carbohydrates  is  present. 

Test  for  Starch.  To  a  solution  of  starch  add  a  drop  or  two  of  a 
solution  of  iodine.  A  deep  blue  color  shows  the  presence  of  starch. 

Various  foods  such  as  potato,  bread,  egg  white,  may  be  tested 


SUGGESTIONS  FOR  LABORATORY  WORK  625 

for  starch  by  adding  hot  water  to  a  small  amount  of  each  (solids) , 
and  then,  after  cooling,  applying  the  iodine  test. 

Dextrin.  This  is  an  intermediate  product  in  the  conversion  of 
starch  into  sugar,  and  during  the  change  different  forms  of  dextrin 
are  produced. 

Test  for  Dextrin.  To  a  solution  of  commercial  dextrin  add  a  few 
drops  of  a  solution  of  iodine.  A  reddish  color  is  the  dextrin  test. 

Tests  for  glycogen,  single  sugars,  and  lactose  (milk-sugar)  are 
described  above  (p.  588) .  Cane-sugar  (sucrose)  does  not  give  the 
Fehling  test.  It  can  be  split  into  single  sugars  (dextrose  and 
levulose)  by  boiling  with  a  mineral  acid,  and  the  sugars  thus  pro- 
duced will  respond  to  the  Fehling  test.  Tests  for  proteins  and 
fats  are  described  above  (p.  589). 


STUDY  OF  DIGESTION 

Salivary  Digestion.  Add  to  an  inch  of  dilute  starch  solution  in 
a  test  tube  a  quarter  of  an  inch  of  saliva.  At  four  minute  intervals 
test  a  few  drops  of  this  mixture  with  dilute  iodine  solution.  Note 
the  time  it  takes  for  the  appearance  of  dextrin,  and  then  for  the 
disappearance  of  the  dextrin. 

The  nature  of  the  final  product  of  the  salivary  digestion  of  starch 
may  be  shown  by  keeping  a  test  tube  of  starch  solution  and  saliva, 
as  prepared  above,  at  a  temperature  of  40°  C.  for  30  minutes  or 
more.  Fehling's  test  will  show  the  presence  of  sugar. 

Gastric  Digestion  of  Protein.  Gastric  digestion  proceeds  slowly. 
Allow  ample  time  for  the  following  observation;  a  good  plan  is  to 
begin  the  experiment  one  day  and  complete  it  on  the  next. 

Fill  each  of  four  test  tubes  about  one-third  full  of  water.  Add 
to  the  first  a  few  drops  of  commercial  pepsin  solution;  to  the 
second  ten  drops  of  a  0.5%  solution  of  hydrochloric  acid;  to  the 
third  and  fourth  both  pepsin  and  acid.  Place  in  each  tube  a  cube 
of  boiled  egg  white.  The  cubes  should  be  approximately  the  same 
size.  Mark  the  tubes  and  place  numbers  one,  two,  and  three  in 
a  thermostat  at  body  temperature,  number  four  in  a  cool  place. 
Shake  each  tube  at  intervals.  Several  hours  later  examine  all  the 
tubes  for  evidences  of  digestion.  It  will  be  found  that  acid  pepsin 
is  essential,  and  warmth  desirable  for  gastric  digestion. 

Movements  of  the  Stomach.    Expose  the  stomach  of  a  recently 


626  APPENDIX 

killed  frog.  Tie  a  ligature  about  the  pyloric  end.  Make  an  open- 
ing into  the  stomach  at  the  cardiac  end  and  by  means  of  a  pipette 
fill  the  stomach  with  0.7%  salt  solution.  Tie  a  second  ligature  so 
as  to  close  the  opening  into  the  stomach,  and  cut  the  stomach 
from  the  body. 

By  means  of  a  thread  attached  to  the  pylorus  hang  the  stomach 
so  that  the  cardiac  end  just  dips  into  a  solution  of  0.7%  sodium 
chloride  and  0.1%  sodium  carbonate.  Look  for  the  peristaltic 
waves  passing  over  the  stomach.  Note  the  time  required  for  a 
single  wave  to  pass,  and  the  number  of  waves  per  minute.  This 
observation  is  more  striking  in  early  fall  or  late  spring  than  in 
mid  winter  when  the  frogs  are  in  the  midst  of  hibernation. 

Absorption  from  Stomach  and  from  Small  Intestine.  Lay  bare 
the  stomach  and  intestines  of  a  large  turtle  whose  brain  has  been 
destroyed  and  plastron  removed  without  loss  of  blood. 

Using  great  care  to  avoid  tearing  the  mesentery  (supporting 
membrane  of  stomach  and  intestines)  find  the  points  of  union  of 
stomach  and  intestine  and  of  intestine  and  rectum.  Tie  stout 
threads  tightly  about  these  points. 

Find  the  junction  of  esophagus  with  stomach.  Place  a  thread 
about  the  junction  ready  for  tying.  Make  an  opening  into  the 
esophagus  above  the  thread.  Introduce  into  the  stomach  through 
this  opening  by  means  of  a  graduated  pipette,  water  to  moderate 
distension.  Note  the  exact  amount  of  water  introduced.  With- 
draw the  pipette  and  tie  off  the  junction  with  care  that  no  water 
escapes. 

Place  a  thread  ready  for  tying  about  the  intestine  one-half  inch 
below  the  one  previously  tied  about  the  point  of  union  of  stomach 
with  intestine.  Introduce  a  known  amount  of  water  into  the  in- 
testine through  a  hole  made  just  above  the  thread  last  placed. 
Tie  this  thread.  Allow  absorption  to  proceed  for  one  hour.  At 
the  expiration  of  the  period  of  absorption  cut  between  the  threads 
tied  about  the  upper  end  of  the  intestine  and  dissect  out  the 
stomach  and  intestine.  Empty  each  into  a  separate  vessel.  Com- 
pare the  amount  of  water  introduced  with  the  amount  recovered. 
If  the  mesentery  is  intact  and  the  circulation  good  the  intestine 
will  be  virtually  empty  at  the  end  of  an  hour.  The  stomach  will 
contain  practically  the  entire  amount  introduced.  Similar  tests 
may  be  made  with  solutions  of -dextrose. 


SUGGESTIONS  FOR  LABORATORY  WORK  627 

METABOLISM 

The  Influence  of  Muscular  Exercise  on  Carbon  Dioxide  Produc- 
tion. Make  the  first  observation  described  below  after  at  least 
one  hour  of  relative  inactivity. 

Fill  a  large  wide-mouthed  bottle  with  water  and  invert  over  a 
reservoir  of  water.  Insert  a  good  sized  rubber  or  bent  glass  tube 
into  the  neck  of  the  bottle.  Place  the  other  end  in  the  mouth. 
Hold  the  nose  during  each  expiration  and  blow  all  the  expired  air 
into  the  bottle.  Note  carefully  the  number  of  expirations  and  the 
time  required  for  filling  the  bottle  with  expired  air.  Cork  the 
bottle  tightly  and  turn  right  side  up. 

Fill  a  narrow  test  tube  with  strong  sodium  hydroxide  solution. 
Cork.  Tie  a  thread  about  the  test  tube  and  by  means  of  this 
thread  lower  the  tube  carefully  into  the  bottle  of  expired  air. 
Restopper  this  bottle  tightly  and  then  by  jarring  it  break  the  test 
tube,  liberating  the  alkali.  Shake  thoroughly.  The  alkali  takes 
up  the  carbon  dioxide.  Invert  the  bottle  again  over  the  reservoir 
of  water.  With  the  neck  under  water  remove  the  stopper.  Water 
rushes  in  to  replace  the  absorbed  carbon  dioxide.  Lower  the 
bottle  till  the  water  is  at  the  same  level  inside  and  outside.  With 
the  bottle  in  this  position  replace  the  stopper.  The  volume  of 
water  enclosed  equals  that  of  the  carbon  dioxide  in  the  entire 
bottle  of  expired  air.  Determine  this  volume  with  the  aid  of  a 
graduated  vessel.  Calculate  the  volume  of  carbon  dioxide 
exhaled  with  each  breath  and  also  the  volume  exhaled  per 
minute. 

Rinse  the  bottle  out  thoroughly  to  remove  all  traces  of  sodium 
hydroxide.  Take  several  minutes  of  very  brisk  exercise.  As 
quickly  as  possible  after  the  cessation  of  the  exercise  repeat  the 
experiment  above.  Care  must  be  taken  that  all  the  air  expired 
during  the  period  of  collection  enters  the  bottle.  The  augmented 
breathing  makes  this  a  matter  of  some  difficulty.  There  are  numer- 
ous sources  of  error  in  this  experiment,  but  if  carefully  performed 
it  demonstrates  a  marked  increase  in  the  carbon  dioxide  produc- 
tion per  minute  with  exercise,  and  a  suggestion  as  to  the  amount 
produced. 

A  Study  of  Urine.  Urine  is  the  chief  excretion  of  the  body.  It 
contains  the  greater  portion  of  the  end-products  of  protein  metab- 


628  APPENDIX 

olism  and  is  also  the  medium  in  which  the  accessories  of  the  diet 
are  discharged  from  the  body. 

The  chief  end-products  of  protein  metabolism  are  urea,  creatinin, 
uric  acid,  and  ammonia.  Among  the  chief  excreted  accessories  are 
(a)  inorganic:  water,  and  sodium,  potassium,  calcium,  and  mag- 
nesium chlorides,  sulphates,  and  phosphates;  (b)  organic:  non- 
nutrient  constituents  of  food  which  serve  to  give  it  flavor;  drugs. 
Examples  of  this  class  of  excreta  are  the  purin  bodies,  which  rep- 
resent the  excreted  alkaloids  of  tea,  coffee,  and  cocoa;  and  the 
substance  which  gives  urine  its  peculiar  odor  after  the  eating  of 
asparagus. 

Test  for  Urea.  To  15  c.c.  of  urine  add  J^  its  volume  of  baryta 
mixture  to  remove  inorganic  constituents  which  would  interfere 
with  the  test.  Filter.  To  a  portion  of  the  filtrate  add  a  solution 
of  mercuric  nitrate,  a  precipitate  forms  which  is  a  compound  of 
urea  and  mercury. 

Test  for  Creatinin.  To  8  c.c.  of  urine  add  2  c.c.  of  a  solution  of 
sodium  nitro-prussiate,  then  1  c.c.  NaOH;  a  red  color  appears. 
Boil  the  solution.  The  color  fades;  while  boiling  add  about  1  c.c. 
of  acetic  acid;  the  color  changes  to  blue. 

Test  for  Ammonia.  To  4  c.c.  of  fresh  urine  in  a  test  tube  add  a 
little  dry  sodium  carbonate.  Heat.  Hold  in  the  neck  of  the  test 
tube,  without  touching  the  sides,  a  strip  of  moistened,  neutral 
litmus  paper.  The  blue  coloration  shows  the  presence  of  ammonia 
gas. 

Test  for  Chlorides.  To  4  c.c.  of  urine  add  an  excess  of  nitric 
acid  (HNO3),  then  a  drop  or  two  of  silver  nitrate  solution  (AgNO3). 

Test  for  Sulphates.  Add  to  4  c.c.  of  urine  a  few  drops  of  barium 
chloride  solution  and  then  an  excess  of  HC1.  The  latter  redis- 
solves  the  phosphates,  leaving  the  sulphate  of  barium  alone  in 
the  precipitate. 

Test  for  Phosphates.  Make  10  c.c.  of  urine  alkaline  with  am- 
monium hydroxide  (NH4OH).  Heat.  The  precipitate  that  forms 
is  a  mixture  of  calcium  and  magnesium  phosphates.  Filter.  To 
the  filtrate  add  a  small  amount  of  magnesia  mixture  (MgS04, 
NH4C1,  and  NH4OH  in  water).  Heat.  The  precipitate  is  due 
to  the  presence  of  sodium,  potassium,  and  ammonium  phosphates. 

Test  for  Purin  Bodies.  Add  to  10  c.c.  of  urine  an  excess  of 
magnesia  mixture.  Filter  off  the  precipitate  of  phosphates.  Add 


SUGGESTIONS  FOR  LABORATORY  WORK  629 

to  the  filtrate  ammoniacal  silver  nitrate  solution.  The  precipitate 
consists  of  the  silver  salts  of  the  purin  bodies,  and  will  be  more 
abundant  after  tea,  coffee,  or  cocoa  have  been  taken. 


A  STUDY  OF  MILK 

This  is  a  secretion  of  the  mammary  gland  and  contains  Protein, 
Fat,  Carbohydrate  (Sugar),  and  salts  in  solution. 

The  opaque  white  appearance  of  milk  is  due  to  the  presence  in 
it  of  a  protein,  caseinogen.  This  may  be  precipitated  out  with 
acid  or  by  the  enzyme  rennin.  In  the  latter  case  the  caseinogen 
combines  with  some  of  the  calcium  of  the  milk  forming  casein. 

Fill  each  of  two  test  tubes  1/3  full  of  milk  and  add  an  equal 
volume  of  water. 

To  one  of  the  tubes  add  five  drops  of  hydrochloric  acid  (HC1) ; 
to  the  other  ten  drops  rennin  solution.  Allow  both  tubes  to  stand 
in  a  water  bath  at  body  temperature. 

After  twenty  minutes  filter  the  contents  of  both  tubes.  Make 
the  following  tests  on  each. 

Demonstrate  the  presence  of  protein  in  the  precipitate  (curd) 
with  the  xanthoproteic  test. 

Test  for  fat  with  osmic  acid. 

Apply  the  biuret  test  to  a  few  drops  of  the  filtrate  (whey). 

Test  2  c.c.  of  filtrate  for  sugar  with  Fehling's  test. 

Add  to  5  c.c.  of  the  filtrate  three  drops  ammonia  (NH4OH), 
and  5  drops  sodium  oxalate  solution.  A  white  precipitate  proves 
the  presence  of  calcium  in  the  filtrate.  A  careful  comparison  of 
acid  whey  with  rennin  whey  will  show  that  the  former  contains 
more  calcium. 


INDEX 


Abdominal  cavity,  4;  contents  of,  5. 

Abdominal  respiration,  398. 

Abducens  nerve,  152. 

Abduction,  72,  74. 

Aberration,  chromatic,  262;  spher- 
ical, 263. 

Abortion,  danger  of,  579. 

Absorption,  491;  of  carbohydrates, 
492;  channels  of,  492;  nature  of, 
492;  of  proteins,  498;  of  fats,  499; 
from  small  intestine,  491;  from 
stomach,  491;  from  large  intestine, 
500. 

Accessories  of  diet,  429;  inorganic, 
430;  organic,  431. 

Accessory  reproductive  organs,  562. 

Accommodation,  260. 

Acetabulum,  65,  73,  74. 

Acetic  acid,  16. 

Achromatic  lenses,  263. 

Achromatic  spindle,  25. 

Acid,  acetic,  16;  animo,  11,  463; 
butyric,  16;  fatty,  463;  formic,  16; 
glychocolic,  518;  hydrochloric,  10, 
464;  lactic,  16,  90,  92,  435;  tauro- 
cholic,  518;  uric,  14,  526. 

Acidosis,  510. 

Acromegaly,  50. 

Action  currents,  103. 

Activity,  maintenance  of,  38. 

Adam's  apple,  547. 

Adaptation,  25,  42. 

Adaptive  systems,  37. 

Addison's  disease,  199. 

Adduction,  72,  74. 

Adenoids,  228,  383. 

Adenoid  tissue,  304. 

Adipose  tissue,  44. 

Adrenals,  199. 

Adrenin,  199,  319;  effect  of,  on  vas- 
cular system,  381. 

Afferent  nerve  paths,  170. 

After  birth  (placenta),  578. 

After  images,  281. 

Agglutinins,  309. 

Air,  composition  of,  411;  changes  in 
when  breathed,  411;  quantity 
breathed  daily,  398. 

Albumin,  12;  serum,  302. 

Albuminoid,  12;  nutritive  value  of, 
510. 

631 


Albuminuria,  471. 
Alcohol,  437. 

Alexin,  308. 

Alimentary  canal,  4,  39;  general  ar- 
rangement of,  442;  blood-vessels 
of,  461 ;  subdivisions  of,  442. 

Alimentary  glycosuria,  494. 

Allant9is,  578. 

Alveoli  of  lungs,  388,  389. 

Ameba,  21. 

Ameboid  movements,  301. 

Amenorrhea,  573. 

Animo  acid,  11,  463. 

Ammonia  compounds,  517. 

Ampulla,  232. 

Amylopsin,  465. 

Anal  opening,  455. 

Anaphylaxis,  311. 

Anatomy,  definition  of,  1. 

Anatomy,  of  alimentary  canal,  442; 
of  brain,  145;  of  ear,  226;  of  eye, 
248;  of  joints,  72;  of  lymphatic 
system,  382;  of  nervous  system, 
138;  of  respiratory  organs,  388;  of 
skeleton,  53;  of  skin,  529;  of  urinary 
organs,  518;  of  vascular  system, 
322. 

Anemia,  298. 

Animal  heat,  sources  of,  541. 

Animal,  normal  compared  with  "re- 
flex," 169. 

Animals  compared  with  plants,  40. 

Ankle  bones,  66. 

Antiperistalsis,  478. 

Antithrombin,  317. 

Antitoxin,  309;  uses  of,  in  disease,  310. 

Anus,  455. 

Anvil  bone,  229. 

Aorta,  328,  331;  abdominal,  330; 
thoracic,  330;  branches  of,  331. 

Apex  beat  of  heart,  340. 

Aphasia,  188. 

Apnea,  406. 

Apparatus,  lachrymal,  246. 

Appendages  of  eye,  244. 

Appendicular  skeleton,  64. 

Appendix,  vermiform,  455. 

Appetite,  209. 

Aqueous  humor,  254. 

Arachnoid,  5,  141;  space,  142. 

Arborization,  terminal,  137. 


632 


INDEX 


Arc,  reflex,  158;  variability  in,  159. 

Areas  of  cerebrum,  association,  181; 
motor,  177;  sensory,  177. 

Areola,  580. 

Areolar  tissue,  41. 

Arm,  skeleton  df,  66. 

Arterial  blood,  336;  color  of,  417. 

Arterial  pressure,  362;  influence  of 
capillary  resistance  on,  364;  in- 
fluence of  heart-rate  on,  363; 
measurement  of,  367. 

Arterial  system,  330. 

Arterioles,  332. 

Artery,  axillary,  330;  brachial,  330; 
bronchial,  331;  carotid,  330,  331; 
celiac,  331,  461;  coronary,  328,  330; 
femoral,  331;  hepatic,  457,  461; 
iliac,  330,  331;  innominate,  330; 
intercostal,  331;  mesenteric,  331, 
461;  popliteal,  331;  pulmonary, 
326,  333;  radial,  330;  renal,  331; 
splenic,  461;  subclavian,  330;  tem- 
poral, 331;  tibial,  331;  ulnar,  330; 
vertebral,  330. 

Artery,  332;  structure  of,  336. 

Articular  cartilage,  73. 

Articulations,  71;  of  skull,  61. 

Artificial  respiration,  408. 

Aryteno-epiglottic  fold,  548. 

Arytenoid  cartilages,  547. 

Arytenoid  muscles,  551. 

Asphyxia,  407. 

Aspirates,  554. 

Aspiration  of  thorax,  370,  399. 

Assimilation,  21. 

Assimilation  limit,  494. 

Association  areas  of  cerebrum,  181; 
fibers  of  cerebrum,  176. 

Association,  nature  of,  181. 

Association  neurons,  137. 

Associative  memory,  182;  functions 
of,  183;  interactions  of,  184. 

Astigmatism,  264. 

Astragalus,  66. 

Atlas,  58,  75. 

Attraction  sphere,  24. 

Auditory  apparatus,  62. 

Auditory  area  of  cerebrXim,  177. 

Auditory  nerve,  152. 

Auditory  ossicles,  229;  functions  of, 
230. 

Auditory  perceptions,  235. 

Auerbach's  plexus,  456. 

Augmenter  center,  352;  nerves,  351. 

Auricle,  325;  function  of,  345. 

Auriculo-yentricular  valves,  328. 

Auscultation  of  lungs,  397. 

Automatic  rhythmicity  of  heart,  347; 
nature  of,  350. 


Autonomic  nervous  system,  139,  154, 
193;  divisions  of,  195;  reflex  control 
of,  194;  cranial,  195;  sacral,  195; 
thoracico-lumbar,  195;  in  relation 
to  emotions,  197. 

Axial  current,  356. 

Axial  ligament,  230. 

Axillary  artery,  330. 

Axis,  58,  75. 

Axis,  visual,  275. 

Axon,  136;  collaterals  of,  160. 

Bacterial  digestion,  466. 

Ball-and-socket  joints,  74. 

Basal  metabolism,  506. 

Basement  membran*,  480,  566. 

Basilar  membrane,  233. 

Bathing,  537. 

Beat,  "apex,"  340. 

Beat  of  heart,  339. 

Beef  tea,  90. 

Beri-beri,  431. 

Biceps,  79,  82. 

Bicuspid  teeth,  444. 

Bile,  457,  465;  capillaries,  460;  con- 
trol of,  487;  duct,  458;  pigments, 
14,  518;  acids,  518;  salts,  518. 

Bilirubin,  14,  518. 

Biliverdin,  14,  518. 

Binocular  vision,  287. 

Biological  chemistry,  definition  of,  9. 

Biuret  reaction,  1 1 . 

Blackness,  sensation  of,  276. 

Bladder,  urinary,  39,  518. 

Bleeders,  319. 

Blind  spot,  269. 

Blood,  39,  292,  of  animals  other  than 
man,  303;  arterial,  336;  carbon 
dioxid  of,  423;  changes  in,  in  lungs, 
416;  chemical  composition  of,  295; 
coagulation  of,  312;  course  of,  334; 
distribution  of,  in  body,  373; 
fibrin,  source  of,  315;  functions  of, 
292;  gases,  417;  microscopic  char- 
acters of,  295;  oxygen  interchanges 
in,  420;  plates,  302;  plasma,  295, 
302;  quantity  of,  303;  reactipn  of, 
295;  serum,  312;  specific  gravity  of, 
295;  structure  of,  295;  transfusion, 
320;  venous,  336;  whipped,  313. 

Blood-clot,  312. 

Blood-corpuscles,  295;  colorless,  301; 
red,  295. 

Blood-fibrin,  313. 

Blood-flow,  rate  of,  368. 

Blood-flow,  see  Circulation. 

Blood-plasma,  295,  302. 

Blood-plates,  302. 

Blood-pressure,  362;  in  man,  determi- 
nation of,  368;  measurement  of,  367. 


INDEX 


633 


Blood-vessels,  35,  39,  322. 

Blood-vessels  of  alimentary  tract, 
461. 

Blood-vessels,  nerves  of,  373. 

Body,  composition  of,  8;  compounds 
in,  9;  elements  in,  9;  microscopic 
structure  of,  7;  physico-chemical 
constitution  of,  16;  physiological 
properties  of,  21;  liquid  environ- 
ment of,  16;  water  in,  10;  levers  in, 
119;  food  requirements  of,  500; 
liberation  of  energy  in,  505;  pro- 
tein requirement  of,  500;  tempera- 
ture of,  540. 

Body  fat,  source  of,  515. 

Body  senses,  163,  172;  tracts  of,  172. 

Body  sense  area,  177. 

Body  temperature,  540. 

Bone,  31,  46,  49;  chemistry  of,  49; 
formation  of,  46,  51;  structure  of, 
47;  repair  of,  48,  71. 

Bones,  of  cranium,  60;  of  face,  60;  of 
limbs,  66;  of  pectoral  arch,  64;  of 
pelvic  girdle,  64;  of  skull,  60;  of 
vertebral  column,  55. 

Bony  labyrinth,  231. 

Botulism,  441. 

Bow  legs,  cause  of,  51. 

Brachial  artery,  330. 

Brachial  plexus,  148. 

Brain,  5,  36,  138,  145,  174;  mem- 
branes of,  138;  nourishment  of, 
190;  ventricles  of,  142;  convolu- 
tions of,  176;  white  and  gray  mat- 
ter in,  174,  175. 

Brain  stem,  192. 

Bread,  436. 

Breast,  579. 

Breast-bone,  64. 

Breath,  holding,  406. 

Breathing,  391;  forced,  396;  hygiene 
of,  398. 

Broad  ligament,  567,  568. 

Bronchial  artery,  331. 

Bronchial  tubes,  39,  388;  structure  of, 
388. 

Brunner's  glands,  454. 

Buccal  cavity,  442. 

Buffy  coat,  314. 

Bulbus  arteriosus,  347. 

Burdach,  column  of,  172. 

Butter,  433,  435. 

Butyric  acid,  16. 

Caffein,  439. 

Calcium  phosphate,  10,  49. 

Calcium  salts,  relation  of  to  blood- 
clotting,  317. 

Calorie,  107,  500. 

Calorimeter,  500. 


Camera,  photographic,  258. 

Canals,  lachrymal,  246;  semicircular, 
232. 

Canal,  neural,  56;  central  of  spinal 
cord,  141,  144. 

Canine  teeth,  444. 

Canthi  of  eyelids,  245. 

Capacity  of  lungs,  397. 

Capillaries,  bile,  460;  blood,  293,  322, 
331;  structure  of,  337. 

Capillary  blood-flow,  357. 

Capsule,  internal,  176. 

Carbohydrates,  15,  429,  433;  ab- 
sorption of,  492;  food  value  of,  507; 
storage  of,  492. 

Carbohydrate  foods,  433. 

Carbon  dioxid,  16;  of  blood,  423;  in- 
fluence of  on  respiratory  center, 
403;  hormone  action  of,  424. 

Carbon  equilibrium,  513. 

Carbon  monoxid  hemoglobin,  427; 
poisoning  by,  427. 

Carbonate  of  sodium,  90. 

Cardiac  cycle,  diagram  of,  344; 
events  of,  340;  time  relations  of, 
339. 

Cardiac  impulse,  340. 

Cardiac  murmurs,  343. 

Cardiac  muscle,   86;  physiology  of, 

Cardiac  orifice  of  stomach,  449. 

Cardiac  plexus,  154. 

Cardio-augmentor  center,  352;  nerves, 
351. 

Cardio-inhibitory  center,  352;  nerves, 
351. 

Care  of  teeth,  470. 

Carotid  artery,  330,  331. 

Carpals,  66. 

Carriers  of  infection,  310. 

Cartilage,  31,  44,  49;  articular,  73; 
arytenoid,  547;  costal,  64;  cricoid, 
547;  cuneiform,  548;  elastic,  45; 
ensiform,  55;  fibro-,  46;  hyaline, 
45;  structure  of,  45;  temporary 
and  permanent,  44;  thyroid,  547; 
of  Wrisberg,  548. 

Caruncula  lachrymalis,  245. 

Casein,  13,  435. 

Castration,  583. 

Cataract,  264. 

Cauda  equina,  148. 

Caudate  nucleus,  153. 

Celiac  axis,  331,  461. 

Cells,  7;  structure  of,  23;  ciliated,  86 

Cell-body,  of  neuron,  136. 

Cell  division,  23. 

Cell  growth,  22. 

Cell  membranes,  17. 


634 


INDEX 


Cell-nucleus,  23. 

Cellulose,  16,  430,  433,  467. 

Cement  of  tooth,  445. 

Center  of  gravity  of  Body,  125. 

Centers,  cardio-augmentor,  352; 
cardio-inhibitory,  352;  respiratory, 
401;  sweat,  536;  vasoconstrictor, 
375;  vasodilator,  378. 

Central  nervous  system,  138;  mem- 
branes of,  139. 

Centrosome,  24. 

Cephalic  vein,  333. 

Cerebellar  reflexes,  cerebral  control 
of,  186. 

Cerebellum,  165;  functions  of,  166. 

Cerebral  activity,  relation  of  to  vaso- 
motor  tone,  378. 

Cerebral  circulation,  relation  of  to 
consciousness,  191. 

Cerebral  control  of  spinal  and  cere- 
bellar  reflexes,  186. 

Cerebral  functions  compared  in  man 
and  animals,  189. 

Cerebrospinal  liquid,  141,  142. 

Cerebrum,  145;  afferent  paths  of, 
170;  cortex  of,  174;  development 
of,  182;  projection  fibers  of,  175; 
commissural  fibers  of,  176;  lobes  of, 
176;  relation  of,  to  muscular  ac- 
tivity, 169;  relation  of,  to  receptor 
system,  170;  motor  areas  of,  177; 
reflex  paths  of,  179;  white  matter 
of,  175. 

Cervical  plexus,  148. 

Cervical  vertebrae,  57. 

Channels,  of  absorption,  492;  of  ex- 
cretion, 516. 

Characters,  hereditary,  574. 

Characteristics  of  human  skeleton, 
68. 

Cheeks,  443;  bones  of,  60,  62. 

Cheese,  435. 

Chemical  changes  in  respired  air,  411. 

Chemical  composition  of  body,  8. 

Chemical  co-ordination,  28,  39. 

Chemistry,  biological,  definition  of, 
9. 

Chemistry,  of  bile,  518;  of  blood,  302; 
of  bone,  49;  of  fats,  15;  of  gastric 
juice,  464;  of  lymph,  304;  of  muscle, 
89;  of  pancreatic  juice,  465;  of 
saliva,  418;  of  teeth,  445;  of  urine, 
525. 

Chemistry  of  muscular  contraction, 
106. 

Chest,  4. 

Childbirth,  578. 

Chloroform  rigor,  109. 

Cholesterin,  518. 


Chorea,  94. 

Choroid,  249. 

Chromatic  aberration,  262. 

Chromatin,  23,  560. 

Chromosomes,  25,  560. 

Chyme,  488. 

Cilia,  33,  87. 

Ciliary  muscle,  195,  255;  action  of, 
in  accommodation,  261. 

Ciliary  processes,  249. 

Ciliated  cells,  86. 

Circle  of  dispersion,  260. 

Circulation,  334;  appearance  under 
microscope,  355;  diagram  of,  335; 
influence  of  gravity  on,  369;  of 
vein  compression  on,  370;  of  res- 
piratory movements  on,  370,  400; 
portal,  335;  proofs  of,  371;  pul- 
monary, 333;  resistance  to,  356; 
rate  of,  368;  renal,  522;  systemic, 
335;  outside  heart,  355. 

Circulation  scheme,  359. 

Circulatory  system,  39. 

Circumvallate  papillae,  446. 

Classification  of  tissues,  30. 

Clavicle,  64. 

Climacteric,  572. 

Clitoris,  569. 

Clot,  of  blood,  312. 

Clothing,  542,  544. 

Coagulation  of  blood,  312;  cause  of, 
313;  summary  of,  318;  use  of,  315; 
methods  of  hastening  and  retard- 
ing, 318;  within  blood-vessels,  318; 
influence  of  adrenin  on,  319. 

Coal-gas  poisoning,  427. 

Coccyx,  60. 

Cochlea,  bony,  231;  membraneous, 
233;  functions  of,  234. 

Cocoa,  439. 

Coecum,  455. 

Coffee,  439. 

Cold-blooded  animals,  539. 

Cold  receptors,  218. 

Colds,  common,  376,  545. 

Collagen,  41. 

Collar-bone,  64. 

Collaterals,  of  axon,  160. 

Colliculi,  146,  153. 

Colloids,  17. 

Colon,  455. 

Color  blindness,  280;  tests  for,  281. 

Color,  sensations,  277;  sense,  dis- 
tribution of,  in  retina,  279;  vision, 
276;  peculiarities  of,  278;  theories 
of,  281. 

Colors,  complementary,  278. 
,  Colostrum,  580. 

Columnae  carnae,  328. 


INDEX 


635 


Columns,  of  spinal  cord,  144,  172;  of 
Burdach,  172;  of  Goll,  172. 

Commissures,  of  cerebrum,  176;  of 
spinal  cord,  144. 

Common  bile  duct,  458. 

Common  sensations,  205. 

Complemental  air,  398. 

Complemental  colors,  278. 

Complements,  of  blood,  308. 

Conception,  576,  578. 

Concepts,  182. 

Concha,  226. 

Condiments,  429. 

Conduction,  nervous,  irreversibility 
of,  161. 

Conductive  system,  38. 

Conductive  tissues,  32. 

Condyle,  occipital,  62,  69. 

Cones,  252;  functions  of,  279;  excita- 
tion of,  268. 

Coni  vasculosi,  564. 

Conjugate  focus,  258. 

Conjunctiva,  245. 

Connective  tissue,  31,  41,  49. 

Consciousness,  189;  dependence  of  on 
blood  supply,  190. 

Consonants,  554. 

Constant  weight,  maintenance  of,  511. 

Contact  senses,  206. 

Contractile  tissues,  see  Muscles. 

Contractility,  21. 

Contraction  of  muscle,  94;  effect  of 
temperature  on,  97;  effect  of  in- 
creasing stimuli  on,  96;  graphic 
record  of,  95. 

Contraction,  maximal,  97;  tetanic, 
102;  voluntary,  102;  summary  of, 
113. 

Contracture,  100. 

Contrasts,  281. 

Convolutions  of  brain,  146,  176. 

Cooking  of  meats,  434;  of  vegetables, 
436. 

Co-ordination,  27,  28;  chemical,  28, 
39;  nervous,  28. 

Cordae  tendinae,  328. 

Cord,  spinal,  5,  138,  142. 

Cords,  vocal,  546. 

Corium,  5,  531. 

Cornea,  249. 

Corona  radiata,  176. 

Coronary  artery,  328,  330;  vein,  327. 

Corpora  quadrigemina,  146,  153,  251. 

Corpora  striata,  146. 

Corpus,  arantii,  330;  callosum,  176; 
cavernosum,  565;  luteum,  572; 
spongiosum,  565. 

Corpuscles  of  blood,  295;  colorless, 
301;  red,  295. 


Corpuscles,  Pacinian,  213. 

Corresponding  points  of  retina,  287. 

Cortex,  of  cerebrum,  174;  develop- 
ment of,  182;  of  cerebellum,  165. 

Corti,  organ  of,  233;  rods  of,  234. 

Cortical  localization,  176. 

Cortical  reflex  paths,  179. 

Cortical  reflexes,  180. 

Costal  breathing,  398. 

Costal  cartilages,  64. 

Coughing,  408. 

Course  of  blood,  322. 

Cranial  autonomies,  195. 

Cranial  nerves,  139,  150. 

Cranium,  55. 

Cream,  435. 

Creatine,  14,  90. 

Creatinine,  14,  525. 

Cretinism,  202. 

Crico-arytenoid  muscles,  549. 

Cricoid  cartilage,  547. 

Cricothyroid  membrane,  547;  muscle, 
551. 

Crossed  pyramidal  tracts,  177. 

Crura  cerebri,  146. 

Crying,  409. 

Crypts  of  Lieberkiihn,  454. 

Crystalline  lens,  254. 

Crystalloids,  17. 

Cuneate  nucleus,  173. 

Cuneiform  cartilage,  548. 

Currents  of  action,  103;  of  injury, 
103. 

Curve  of  muscular  contraction,  95. 

Cutaneous  senses,  211. 

Cuticle,  529. 

Cystic  duct,  457. 

Cytoplasm,  8. 

Deaminization  of  protein,  504. 

Death,  585;  rigor,  92. 

Decidua,  576. 

Decussation,  of  pyramids,  177;  sen- 
sory, 173. 

Defects  of  eye,  optical,  262 

Degeneration  of  nerves,  171. 

Deglutition,  470. 

Dendrites,  136. 

Dentals,  554. 

Dentate  nucleus,  153. 

Dentine,  445. 

Depressor  nerve,  351,  376;  impulses, 
376. 

Depth,  perception  of,  287. 

Dermis,  5,  531. 

Desperation,  strength  of,  201. 

Determination  of  sex,  575. 

Development,  29. 

De  Vries,  574. 

Dextrin,  433,  464. 


636 


INDEX 


Dextrose,  15,  90,  433. 

Diabetes,  114,  496,  527. 

Dialysis,  19. 

Diaphragm,  4,  5,  392. 

Diastole  of  heart,  339. 

Dietary  accessories,  429. 

Dietetics,  511. 

Differentiation  of  tissues,  25,  30. 

Diffusion,  19. 

Digastric  muscles,  82. 

Digestion,  462;  auto,  467;  bacterial, 
466;  of  cellulose,  467;  good,  main- 
tenance of,  489;  in  intestine,  488; 
in  mouth,  487;  object  of,  462; 
products,  463;  in  stomach,  488; 
summary  of,  466. 

Digestive  system,  39. 

Djoptrics  of  eye,  244. 

Direct  cerebellar  tract,  173. 

Discus  proligerus,  571. 

Disks,  intervertebral,  56,  72. 

Dislocations,  73. 

Dispersion  circles,  260. 

Dispersion  of  light,  256. 

Dissimulation,  23. 

Distance,  perception  of,  286. 

Distribution  of  blood  over  body,  373. 

Diuretics,  529. 

Diversion,  importance  of,  199. 

Division  of  labor,  physiological,  30, 
87.  ^ 

Divisions  of  autonomic  system,  195. 

Doctrine  of  specific  nerve  energies, 
204. 

Dominant  characters,  574. 

Dorsal  (neural)  cavity,  5. 

Dorsal  (thoracic)  vertebrae,  58. 

Drum  of  ear,  226. 

Duct,  bile,  458;  cystic,  457;  hepatic, 
457;  of  pancreas,  460;  of  salivary 
glands,  448;  of  Stenson,  448; 
thoracic,  382. 

Ductless  glands,  39,  305. 

Duodenum,  452. 

Dura  mater,  139. 

Duration  of  luminous  sensations,  273. 

Dwarfishness,  50. 

Dynamic  action  of  protein,  509,  543. 

Dyspnea,  406. 

Ear,  226;  drum,  226;  external,  226; 
functions  of,  223;  internal,  231; 
middle,  227. 

Ear-ache,  228. 

Efferent  nerve  paths,  171. 

Efficiency,  of  muscle,  106. 

Eggs,  435. 

Elastic  cartilage,  45. 

Elastic  connective  tissue,  44. 

Elastin,  44. 


Electrical  phenomena  of  muscle,  103. 

Elements  found  in  body,  9. 

Embryo,  nutrition  of,  578. 

Emergency  mechanism  of  body,  196; 
reaction,  478. 

Emmetropia,  262. 

Emotion,  189;  in  relation  to  auto- 
nomic system,  197. 

Emotional  glycosuria,  495. 

Enamel,  33,  445. 

End  arborization,  137. 

End  plate,  85,  198. 

Endocardium,  324. 

Endogenous  excreta,  516. 

Endolymph,  231,  233. 

Endoskeleton,  53. 

Energy,  manifestation  in  body,  22, 
38;  in  contracting  muscle,  106,  108; 
units,  107. 

Energy-yielding  foods,  429. 

Ensiform  cartilage,  55. 

Enterokinase,  468. 

Entoptic  phenomena,  265. 

Environment,  relation  of  man  to,  36. 

Enzyms,  14;  digestive,  462. 

Epidermis,  5,  33,  529. 

Epididymis,  564. 

Epiglottis,  449,  547. 

Epithelium,  5,  33. 

Equilibrium,  maintenance  of,  125. 

Equilibrium,  of  carbon,  513;  of  ni- 
trogen, 512;  of  water,  511. 

Equilibrium,  of  opposing  muscles, 
122. 

Equilibrium  organs,  164,  236. 

Equilibrium  sense,  205,  206,  237. 

Erect  posture,  124. 

Erectile  tissue,  565. 

Erepsin,  466. 

Ergot,  440. 

Esophagus,  442,  449. 

Etherial  sulphates,  525. 

Ethmoid  bone,  60. 

Eupnea,  406. 

Eustachian  tube,  227,  448. 

Excitability,  a  physiological  prop- 
erty, 21. 

Excitation  of  visual  apparatus,  267. 

Excreta,  endogenous  and  exogenous, 
516. 

Excretion,  channels  of,  516;  from 
lungs,  516;  renal,  524. 

Excretory  function  of  liver,  517. 

Excretory  system,  39;  tissue,  31. 

Exercise,  beneficial  effects  of,  100; 
proper  kinds  for  various  ages,  132, 
133;  respiratory  changes  in,  425; 
varieties  of,  131. 

Exogenous  excreta,  516. 


INDEX 


637 


Exophthalmic  goiter,  202,  514. 

Exoskeleton,  53. 

Expiration,  396. 

Expired  air,  composition  of,  412. 

Extension,  of  joint,  72,  74. 

Extensor  muscles,  118;  relation  of 
to  posture,  126. 

External  auditory  meatus,  226. 

External  ear,  226. 

External  medium,  290. 

External  rectus  muscle,  246. 

External  respiration,  386. 

External  senses,  206. 

Extract  of  meat,  food  value  of,  91. 

Extractives,  13. 

Extrinsic  reference  of  sensations,  220. 

Eye,  248;  appendages  of,  244;  de- 
fects of,  262;  hygiene  of,  265;  mo- 
tions of,  247 ;  muscles  of,  246; 
nodal  points  of,  267;  physiology  of, 
267;  refracting  media  of,  244,  254, 
259;  structure  of,  243,  248;  wide 
range  of  clear  vision  in,  259. 

Eyelashes,  245. 

Eyelids,  245;  muscles  of,  245. 

Eyestrain,  265. 

Face,  bones  of,  60. 

Facial  nerve,  152,  241. 

Fallopian  tube,  567. 

False  vocal  cords,  549. 

Falsetto,  552. 

Far-sightedness,  262. 

Fasciculus  cuneatus,  172;  gracilis, 
172. 

Fat,  absorption  of,  499;  food  value 
of,  507;  food,  429,  433;  of  body, 
source  of,  515;  chemistry  of,  15; 
special  metabolism  of,  510. 

Fatigue,  of  muscle,  100;  of  nerves, 
156;  nature  of,  101;  neuro-muscu- 
lar,  198;  sense  of,  207. 

Fattv  tissue,  44. 

Fauces,  448. 

Fechner's  law,  205,  214. 

Feeding  of  infants,  581. 

Female  reproductive  organs,  567. 

Femoral  artery,  331. 

Femur,  86;  dislocation  of,  73. 

Fermentation,  466. 

Ferments,  see  Enzyms. 

Ferrein,  pyramids  of,  521. 

Fertilization,  573. 

Fetus,  nutrition  of,  578. 

Fever,  544. 

Fiber,  of  muscle,  83. 

Fibrin,  313;  source  of,  315. 

Fibrin  ferment,  316. 

Fibrinogen,  302. 

Fibrocartilage,  46. 


Fibula,  66. 

Filiform  papillae,  446. 

Fillet,  173. 

Filtration,  18. 

Filum  terminale,  143,  148. 

First  order  levers  in  body,  119. 

Fissures,  of  cerebrum,  176;  of  spinal 
cord,  143. 

Flavor,  241;  importance  of,  in  food, 
432. 

Flesh  food,  434. 

Flexion,  72,  74. 

Flexors,  118. 

Flexure,  sigmoid,  455. 

Flooding,  579. 

Fluid,  cerebrospinal,  141,  142;  syno- 
vial,  74;  seminal,  566. 

Focal  plane  of  lens,  258. 

Focus,  of  lens,  207;  conjugate,  258. 

Follicle,  Graafian,  570;  Meibomian, 
245. 

Follicle  of  hair,  532. 

Fontanelles,  72. 

Food,  carbohydrate,  433;  composi- 
tion of,  436;  classes  of,  429;  defini- 
tion of,  428;  energy  yielding,  429; 
fat,  389;  flesh,  434;  functions  of, 
428;  inorganic,  430;  maintenance, 
429;  protein,  434;  requirement  of 
body,  500;  the  source  of  energy, 
38;  values,  507;  vegetable,  435. 

Food  poisoning,  440. 

Foot-pound,  108. 

Foot,  skeleton  of,  68. 

Foramen  magnum,  61, 139;  oval,  227, 
229;  round,  227. 

Fore  brain,  145. 

Fore  limb,  66. 

Fore  skin,  566. 

Formation  of  bone,  46. 

Formic  acid,  16. 

Forms  of  muscles,  82. 

Fossa,  glenoid,  64,  74. 

Fovea  centralis,  250,  251,  254,  271. 

Fractured  bone,  repair  of,  48. 

Franklin  theory  of  color  vision,  285. 

Frontal  bone,  60. 

Frontal  lobe,  176. 

Fuel  of  body,  428;  of  muscles,  104. 

Fundamental  vibrations,  224. 

Fundus  of  stomach,  450. 

Fungiform  papillae,  446. 

Fur  on  tongue,  446. 

Gall  (bile),  457,  465. 

Gall  bladder,  457. 

Ganglia,  spinal,  147;  sympathetic, 
154. 

Ganglion,  definition  of,  153;  Gas- 
serian,  150;  semilunar,  150. 


638 


INDEX 


Gas,  absorption  of,  by  liquid,  418; 
partial  pressure  of,  419. 

Gases  of  blood,  417. 

Gasserian  ganglion,  150. 

Gastric,  digestion,  488;  glands,  451; 
juice,  464;  secretin,  486;  secretion, 
control  of,  485. 

Gastric  mucous  membrane,  histology 
of,  451. 

Gastrocnemius  muscle,  93. 

Gelatin,  49. 

Gemmation,  557. 

Geniculate  bodies,  153,  251. 

Germ  cells,  compared  with  tissue 
cells,  560;  maturation  of,  560. 

Gestation,  567. 

Gigantism,  50. 

Girdle,  pelvic,  64,  69. 

Gland-duct,  482. 

Glands,  480;  of  Brunner,  454;  duct- 
less, 39;  gastric,  451;  mammary, 
579;  prostate,  564;  pancreatic,  39; 
salivary,  39,  447;  sebaceous,  245, 
535;  of  skin,  534;  sweat,  480,  534; 
tear,  246;  thyroid,  201. 

Glenoid  fossa,  64,  74. 

Gliadin,  503. 

Gliding  joints,  76. 

Globe  of  eye,  248. 

Globin,  12. 

Globulin,  12. 

Glomerulus,  522,  524. 

Glossopharyngeal  nerve,  152,  240. 

Glottis,  548. 

Glucose,  see  Dextrose. 

Gluten,  435. 

Glycerin,  433,  463. 

Glycocholic  acid,  518. 

Glycogen,  16,  90,  433;  storage  of  in 
liver,  493;  in  muscles,  494. 

Glycoprotein,  13. 

Glycosuria,  alimentary,  494;  emo- 
tional, 495;  pancreatic,  496;  phlor- 
hizin,  498. 

Goblet  cells,  453. 

Goiter,  201;  exophthalmic,  202. 

Golgi,  tendon  organs  of,  85. 

Goll,  column  of,  172. 

Gower's  tract,  173. 

Graafian  follicle,  570. 

Gracile  nucleus,  173. 

Graded  synaptic  resistance,  161. 

Graham  flour,  508. 

Gram-centimeter,  108. 

Grape  sugar  (dextrose),  15,  433. 

Graphic  record,  94. 

Grave's  Disease,  202,  514. 

Gravity,  influence  of,  on  circulation, 
369. 


Gray  matter,  153;  definition  of,  153; 
distribution  of,  153;  of  spinal  cord, 
144. 

Growth  of  cells,  22. 

Growth  proteins,  503. 

Gullet,  442,  449. 

Gums,  443. 

Gustatory  area  of  cerebrum,  177. 

Gutterals,  554. 

Gyri,  146,  176. 

Habit  formation,  187. 

Hairs,  33,  532. 

Hair  cells  of  cochlea,  234;  of  semi- 
circular canals,  237. 

Hammer  bone,  229. 

Hand,  see  Fore  limb. 

Harmonic  partials,  224. 

Haversian  system,  47. 

Hay  fever,  312. 

Head  senses,  163,  172;  relation  of  to 
control  of  reflexes,  163;  tractsof,  173. 

Hearing,  163,  172,  205,  206;  nerve 
paths  of,  174;  range  of,  224. 

Heart,  5,  35,  39,  293,  322;  anatomy 
of,  328;  augmentor  center  of,  352; 
augmentor  nerves  of,  351;  autom- 
aticity  of,  347;  beat  of,  339; 
cavities  of,  325;  change  in  form  of, 
340;  contractions  maximal,  348; 
extrinsic  nerves  of,  351;  hyper- 
trophy of,  344;  influence  of  salts 
on,  350;  inhibitory  center  of,  352; 
inhibitory  nerves  of,  351 ;  interior  of, 
328;  membranes  of,  323;  passage  of 
beat  over,  348;  physiological  pecul- 
iarities of,  347;  position  of,  323; 
rate,  339;  refractory  period  of,  348; 
relation  of  nerve  and  muscle  ele- 
ments within,  347;  rhythmic  action 
of,  347;  septum  of,  325;  sounds  of, 
342;  valves  of,  328;  work  of,  346. 

Heart-beat,  theories  of,  349. 

Heart-valves,  action  of,  343. 

"Heart  burn,"  472. 

Heat,  animal,  sources  of,  541;  k 
regulation  of,  542;  productic 
control  of,  542. 

Heat  rigor,  98;  in  smooth  muscle,  IK 

Hematin,  14. 

Hematopoietic  tissue,  298. 

Hemianopia,  251. 

Hemispheres,  cerebral,  145. 

Hemoblastic  cells,  559. 

Hemochromogen,  14. 

Hemocyanin,  304. 

Hemoglobin,  13,  298,  336,  417, 
absorption    of    oxygen    by, 
amount  of  in  body,  298;   carbon 
monoxid,  427;  reduced,  418. 


INDEX 


639 


Hemophilia,  319. 

Henry's  law,  418. 

Hepatic  artery,  457,  461;  cells,  458; 
duct,  457;  vein,  335,  458. 

Heredity,  574. 

Bering's  theory  of  color  vision,  284. 

Hermaphrodite,  560. 

Hernia,  inguinal,  563. 

Hiccough,  408. 

Hilus  of  kidney,  520. 

Hind-brain,  146. 

Hind  limb,  structure  of,  66. 

Hinge  joints,  75. 

Hip  joint,  72. 

Histological  methods,  7. 

Histology,  definition,  2;  of  adipose 
tissue,  44;  of  adenoid  tissue,  304; 
of  areolar  tissue,  43;  of  blood,  295; 
of  bone,  47;  of  cardiac  muscle,  86; 
of  cartilage,  45;  of  connective 
tissue,  43;  of  ear,  232,  237;  of 
elastic  tissue,  44;  of  hairs,  532;  of 
heart,  86;  of  kidney,  522;  of  liver, 
458;  of  lungs,  389;  of  lymph,  304; 
of  lymph  glands,  304;  of  nails,  534; 
of  nervous  tissue,  136;  of  retina, 
252;  of  skeletal  muscle,  83;  of 
small  intestine,  452;  of  smooth 
muscle,  85;  of  stomach,  451;  of 
tongue,  446. 

Histon,  12. 

Hives,  385. 

Holding  the  breath,  406: 

Holmgren  test  for  color  blindness, 
281. 

Holoblastic  ova,  571. 

Homothermous  animals,  539. 

Hormones,  definition,  39,  292,  305; 
action  of  on  glands,  484;  affecting 
metabolism,  496,  513;  emergency, 
200,  319;  production  of,  305;  of 
reproductive  system,  583;  of  skel- 
etal muscle,  114;  of  supporting 
system,  49. 

Hormone  action  of  carbon  dioxid, 
424. 

Horopter,  288. 

Humerus,  66,  79. 

Humor,  aqueous  and  vitreous,  254. 

Hunger,  205,  206,  209. 

Hyaline  cartilage,  45. 

Hyaloid  membrane,  254. 

Hybrid,  574. 

Hydremic  plethora,  385. 

Hydrocarbons,  15. 

Hydrocele,  563. 

Hydrocephalus,  142. 

Hydrochloric  acid,  10,  464. 

Hydrogen,  9. 


Hydrolysis,  462. 

Hygiene,  definition,  1;  of  bones,  51; 
of  clothing,  544;  of  digestion,  489; 
of  exercise,  131;  of  eyes,  265;  of 
joints,  76;  of  menstruation,  572; 
of  muscles,  130;  of  mouth,  469;  of 
respiration,  398;  of  skeleton,  70; 
of  skin,  537. 

Hymen,  570. 

Hyoid  bone,  55,  62. 

Hypermetropia,  262. 

Hyperpnea,  406. 

Hypertrophy  of  heart,  344. 

Hypogastric  nerve,  456;  plexus,  456. 

Hypoglossal  nerve,  152. 

Hypophysis,  50. 

Idiosyncracy,  441. 

Ileocolic  valve,  455. 

Ileum,  452. 

Iliac  artery,  330,  331. 

Ilium,  52,  65. 

Illusions,  sensory,  221. 

Images,  after,  281. 

Immune  bodies,  308,  309. 

Immunity,  310. 

Immunization,  309. 

Impregnation,  576. 

Impulse,  cardiac,  340;  nervous,  135; 
passage  of  along  neuron,  155;-  na- 
ture of,  157;  how  aroused,  156; 
speed  of,  156;  spread  of  in  both 
directions,  156. 

Incisor  teeth,  444. 

Incus,  229. 

Index  of  refraction,  256. 

Inert  layer,  356. 

Infant  feeding,  581. 

Infection,  306;  carriers  of,  310;  re- 
covery from,  308;  resistance  to, 
306. 

Infection-resisting  mechanism,  307. 

Inferior  maxilla,  60. 

Inferior  maxillary  nerve,  151. 

Inferior  mesenteric  artery,  331. 

Inferior  oblique  muscle,  247. 

Inferior  rectus  muscle,  246. 

Inferior  turbinate  bone,  60. 

Inflammatory  rheumatism,  344. 

Infundibulum,  388. 

Inhibition,  184. 

Inhibitory  center,  352;  nerves,  351. 

Injury  currents,  103. 

Innervation  of  iris,  250. 

Innominate  artery,  330;  vein,  333. 

Inoculation,  protective,  311. 

Inorganic  food,  430. 

Insertion  of  muscles,  82. 

Inspiration,  391. 

Insufficiency,  valvular,  effects  of,  344. 


640 


INDEX 


Instinctive  reactions,  190. 

Intensity  of  sensations.  205;  visual, 
271. 

Intercellular  spaces,  1$. 

Intercostal  arteries,  331;  muscles, 
394. 

Interior  of  heart,  328. 

Intermediary  bodies,  307. 

Internal  capsule,  176. 

Internal  ear,  231. 

Internal  medium,  291. 

Internal  rectus  muscle,  246. 

Internal  senses,  206;  effect  of,  in 
consciousness,  207. 

Intervertebral  pads,  56,  72. 

Intestinal  digestion,  489;  juice,  465. 

Intestine,  5,  442;  large,  455;  absorp- 
tion from,  500;  movements  of,  478; 
small,  452;  absorption  from,  491; 
digestion  in,  488;  movements  of, 
476;  nervous  control  of,  477; 
mucous  coat  of,  452. 

Intestines,  nerves  of,  455. 

Intermittent  flow  converted  to  con- 
tinuous, 357. 

Intima,  337. 

lodothyrin,  201. 

Iris,  249,  250;  innovation  of,  250; 
muscles  of,  250;  pigment  of,  214. 

Irradiation,  nervous,  353. 

Irritable  tissues,  32. 

Irritability,  21. 

Ischium,  52,  65. 

Islands  of  Langerhans,  497. 

Jaw,  60. 

Jejunum,  452. 

Joint  motions,  72. 

Joints,  72;  ball-and-socket,  74;  glid- 
ing, 76;  hip,  72;  hinge,  75;  hy- 
giene of,  76;  knee,  75;  pivot,  75. 

Judgments,  221. 

Jugular  vein,  333. 

Kidney,  5,  39,  518;  blood-flow 
through,  522;  blood  supply  of,  520; 
relation  of  to  sugar  in  blood,  494; 
structure  of,  520,  522. 

Kidney  secretion,  mechanism  of,  527; 
relation  of  to  blood-flow,  528. 

Kilocalorie,  107. 

Knee-cap,  66. 

Knee  joint,  75. 

Labia  majora,  569;  interna,  569. 

Labials,  554. 

Labyrinth,  226;  bony,  231;  mem- 
braneous, 232. 

Lachrymal  apparatus,  246;  bone,  60; 
canals,  246;  papilla,  245;  sac,  246. 

Lactase,  466. 

Lactation,  579. 


Lacteals,  294,  454. 

Lactic  acid,  90,  92,  435;  significance 
of  in  contraction,  112;  precursor, 
111. 

Lactose,  15,  466. 

Lamina  spiralis,  232. 

Langerhans,  Islands  of,  497. 

Language,  187. 

Large  intestine,  455;  absorption  from, 
500;  movements  of,  478. 

Larynx,  546;  cartilages  of,  547; 
muscles  of,  549. 

Latent  period,  96. 

Laughing,  409. 

Leaping,  128. 

Lecethin,  16. 

Leg  bones,  66. 

Lens,  crystalline,  254. 

Lens,  refraction  by,  257. 

Lenticular  nucleus,  153. 

Leucocytes,  301;  movements  of,  301. 

Levator  palpebrae  superioris,  245. 

Levers  in  body,  119. 

Lieberkiihn,  crypts  of,  454. 

Life,  stages  of,  584. 

Ligament,  53,  73;  axial,  230;  broad, 
567;  capsular,  73;  round,  73;  sus- 
pensory, of  eye,  254,  261. 

Light,  255;  dispersion  of,  256;  mono- 
chromatic, 255;  refraction  of,  255; 
wave-length  of,  255. 

Limbs,  6;  skeleton  of,  66. 

Lime,  in  diet,  50,  581. 

Linin,  23. 

Lipase,  465. 

Lips,  443. 

Liquid  environment  of  body  cells,  16. 

Liver,  5,  39,  456;  excretory  function 
of,  517;  glycogenetic  function  of, 
493;  histology  of,  458. 

Lobes,  of  cerebrum,  176;  olfactory, 
145. 

Lobules  of  liver,  458. 

Local  sign  in  sensation,  205,  215. 

Localization  of  function  in  cerebrum, 
176. 

Localizing  power  of  retina,  274;  of 
skin,  215. 

Local  temperatures,  543. 

Lochia,  579. 

Locomotion,  118,  123,  126;  .sensory 
basis  of,  164. 

Locomotor  reflexes,  166. 

Long-sight,  262. 

Lumbar  plexus,  149;  vertebrae,  59. 

Lungs,  5,  36,  39,  388;  capacity  of, 
397;  changes  of  blood  in,  416;  ex- 
cretory functions  of,  517;  structure 
of,  389. 


INDEX 


641 


Lymph,  18,  142,  293;  chemistry  of, 
304;  histology  of,  304;  movements 
of,  384;  nodes,  383;  relation  of  to 
blood,  294;  renewal  of,  293;  vessels, 
322,  382. 

Lymphagogue,  385,  441. 

Lymphatics,  294,  381. 

Lymph-flow,  influence  of  respiratory 
movements  on,  401. 

Lymph-nodes,  383;  functions  of,  383; 
-vessels,  322,  382 

Lymphocytes,  304. 

Lymphoid  tissue,  304. 

Lysin,  504. 

Maintenance  food,  429;  proteins, 
503;  systems,  38,  40. 

Malaise,  207. 

Malar  bone,  60. 

Male  reproductive  organs,  563. 

Malleus,  229. 

Malphigian  capsule,  522 

Malphigian  layer  of  epidermis,  530; 
pyramids  of  kidney,  521. 

Maltase,  466. 

Maltose,  463,  487.    . 

Mammal,  characteristics  of,  4. 

Mammary  gland,  579. 

Man,  zoological  position,  2;  relation 
to  environment,  36. 

Manometer,  367. 

Mastication,  123,  469. 

Mastoiditis,  229. 

Maturation  of  germ  cells,  560. 

Maxilla,  60. 

Maximal  contraction,  97;  of  heart, 
348. 

Meal,  digestive  history  of,  487. 

Measurement  of  blood-pressure,  367. 

Meatus,  external  auditory,  226. 

Meatus  urinarius,  565,  566. 

Media,  refracting,  of  eye,  254;  re- 
fractive indices  of,  259. 

Median  nerve,  333. 

Medium,  external,  290;  internal,  291. 

Medulla  oblongata,  146,  192;  centers 
of,  192,  352,  401;  nuclei  in,  154. 

Medullated  (myelinated)  nerve  fibers, 
138. 

Meibomian  follicles,  245. 

Meissner's  plexus,  456. 

Membrane,  aryteno-epiglottic,  548; 
basilar,  233;  cell,  17;  cricothyroid, 
547;  hyaloid,  254;  mucous,  5; 
nictitating,  245;  permeable,  19;  of 
Reissner,  233;  semipermeable,  19; 
serous,  5;'  synovial,  74;  tectorial, 
234;  tympanic,  226,  228;  vitelline, 
571. 

Membraneous  labyrinth,  232. 


Membrane?  of  central  nervous  sys- 
tem, 139;  of  heart,  323 

Mendel,  574. 

Memory,  180;  associative,  182. 

Menstruation,  572;  hygiene  of,  572. 

Mesenteric  artery,  331.  461. 

Mesentery,  452. 

Mesoblastic  ova,  571 . 

Metabolism,  40;  basal,  506;  of  fats, 
510;  of  muscular  work,  507;  rela- 
tion of  thyroid  to,  202,  513. 

M  eta  car  pals,  66. 

Metatarsals,  66. 

Microscopic  anatomy,  see  Histology . 

Mid  brain.  192. 

Middle  ear,  227. 

Milk,  composition  of,  435,  580;  for  in- 
fants, 581 ;  in  diet,  51 ;  pasteurization 
of,  582;  pure,  importance  of,  581. 

Millon's  test  for  proteins,  12. 

Mitosis,  24. 

Mitral  valve,  328. 

Modality  of  sensations,  205 

Modified  respiratory  movements,  408. 

Modiolus,  232. 

Molar  teeth,  444. 

Mons  Veneris,  569 

Monochromatic  light,  255. 

Morula,  29. 

Motion,  27;  in  animals,  78. 

Motions  of  joints,  72. 

Motor  area  of  cortex,  177;  neurons, 
136;  system,  37;  tissues,  32. 

Motores  oculi,  150. 

Mountain  sickness,  424. 

Mouth,  443;  digestion  in,  487;  hy- 
giene of,  469. 

Movements,  intestinal,  476,  478; 
respiratory,  391;  influence  of  on  cir- 
culation, 400;  on  lymph  flow,  401. 

Mucin,  13. 

Mucous  layer  of  epidermis.  529;  of 
intestine,  452;  of  stomach,  451. 

Mucous  membrane,  5. 

Mulberry  mass,  29. 

Mumps,  448. 

Murmurs,  cardiac,  343. 

Musca3  volitantes,  264. 

Muscle,  anatomy  of,  82;  chemistry 
of,  89;  cardiac,  86,  347;  electrical 
phenomena  of,  103;  end  plates  of, 
85;  energy  output  of,  108;  fatigue 
of,  100;  forms  of,  82;  fuel  of,  104, 
105;  glycogen  in,  494;  heat  rigor  of, 
98;  histology  of,  83;  hormones  of, 
114;  oxidation  in,  114;  plasma,  84; 
relation  of  form  to  working  power, 
99;  relaxation  of,  111;  skeletal,  79; 
smooth,  79;  spindles,  85;  stroma,  89. 


642 


INDEX 


Muscles,  classification  of,  79;  biceps, 
79;  ciliary,  195,  255,  261 ;  digastric, 
82;  extensor,  126;  of  eyeball,  246; 
flexor,  118;  hygiene  of,  130;  of  iris, 
249;  of  larynx,  549;  opposing, 
equilibrium  of,  122;  origin  and  in- 
sertion, 82;  papillary,  329,  342; 
paralysis  of,  130;  relation  of  to 
bones,  79;  respiratory,  394;  special 
physiology  of,  118. 

Muscle-fiber,  83. 

Muscle  groups,  functional,  123. 

Muscle  sense,  164,  172,  205,  206,  207. 

Muscle,  smooth,  79;  heat  rigor  in, 
116;  mechanism  of  contraction  of, 
116;  physiology  of,  114. 

Muscle  spindle,  85. 

Muscle  stroma,  89;  tissue,  33. 

Muscular  contraction,  94;  chemistry 
of,  106;  energy  relations  in,  106; 
extent  of,  96. 

Muscular  efficiency,  106. 

Muscular  energy,  source  of,  104; 
exercise,  respiratory  changes  in, 
425. 

Muscular  tissue,  33. 

Muscular  work,  98;  metabolism  of, 
507. 

Muscularis  mucosse,  453. 

Myelin  sheath,  138. 

Myelination,  successive,  172. 

Myenteric  reflex,  474 

Myogen,  89. 

Myogenic  theory  of  heart-beat,  349. 

Myopia,  262. 

Myosin,  89. 

Myxedema,  202. 

Nails,  33,  534. 

Nares,  62. 

Nasal  bone,  60;  duct,  246. 

Nausea,  207. 

Nearsightedness,  262. 

Nerve,  135;  abducens,  150;  auditory, 
152;  depressor,  351,  376;  facial, 
152;  glossopharyngeal,  152;  hypo- 
glossal,  152;  inferior  maxillary,  151; 
median,  333;  oculomotor,  150;  ol- 
factory, 150;  ophthalmic,  150;  op- 
tic, 35,  150;  patheticus,  150; 
phrenic,  149;  pneumogastric,  152; 
sciatic,  150;  spinal  accessory,  152; 
splanchnic,  375,  451;  superior 
maxillary,  151;  trigeminal,  150; 
vagus,  152. 

Nerve-cells,  136;  sensory,  cell-bodies 
of,  136. 

Nerve  end  plate,  85,  198. 

Nerve  elements  within  heart,  347. 

Nerve  energies,  specific,  204. 


Nerve-fibers,  indefatigability  of,  156. 

Nerve  impulse,  definition,  135;  how 
aroused,  156;  nature  of,  157;  speed 
of,  156;  spread  of  in  both  directions, 
156;  methods  of  studying,  155. 

Nerve  paths,  afferent,  170;  efferent, 
170;  method  of  tracing,  171. 

Nerves,  autonomic,  139,  193;  cardiac, 
351;  cranial,  139,  150;  of  intestines, 
455;  of  respiration,  402;  secretory, 
483;  spinal,  139,  147;  sympathetic, 
139,  193;  trophic,  483;  vasocon- 
strictor, 374;  vasodilator,  377; 
vasomotor,  373. 

Nervus  erigens,  456,  565. 

Nervous  co-ordination,  28,  38. 

Nervous  fatigue,  156,  198. 

Nervous  irradiation,  353. 

Nervous  system,  135;  autonomic, 
139,  154,  193;  central  and  periph- 
eral, 138;  membranes  of,  139;  sym- 
pathetic, 139,  154,  193. 

Neural  canal,  56;  tube,  141. 

Neurilemma,  138. 

Neurogenic  theory  of  heart-beat,  349. 

Neuroglia,  139. 

Neuromuscular  fatigue,  198. 

Neurons,  136;  association,  137;  bi- 
polar, 137;  conduction  in,  155; 
motor,  136;  post  ganglionic,  194; 
preganglionic,  194;  sensory,  136. 

Njcotine,  194,  351. 

Nictitating  membrane,  245. 

Nipple,  580. 

Nitrogen,  9. 

Nitrogen  equilibrium,  512. 

Nitrogenous  extractives,  13. 

Nodal  points  of  eye,  267. 

Nodes  of  Ran vier,  138. 

Noise,  223. 

Nose,  bones  of,  60. 

Notes,  musical,  223. 

Nourishment,  of  brain,  190. 

Nuclear  spindle,  25. 

Nuclei,  nervous,  153;  in  medulla,  154; 
auditory,  174;  caudate,  153;  cu- 
neate,  173;  dentate,  153;  gracile, 
173;  lenticular,  153;  red,  153;  of 
cerebellum,  165. 

Nucleoprotein,  13. 

Nucleus  of  cell,  8;  structure  of,  23. 

Nutrients,  429,  432;  functions  of,  433; 
occurrence  of  in  foods,  429. 

Nutrition,  see  Metabolism. 

Nutrition  of  embryo,  578. 

Nutritive  tissues,  31. 

Nutritive  value,  of  albuminoids,  510; 
of  carbohydrates,  fats,  and  pro- 
teins, 507. 


INDEX 


643 


Nymphse,  569. 

Obesity,  514. 

Oblique  muscles  of  eye,  247. 

Occipital  bone,  60;  condyle,  62,  69; 

lobe  of  cerebrum,  176. 
Oculomotor  nefve,  150. 
Odontoid  process,  58,  75. 
Odors,  nature  of,  240. 
Old-sightedness,  266. 
Olecranon,  66. 
Olein,  15. 

Olfactory  areas  of  cerebrum,  157. 
Olfactory    lobes,    145;   nerves,    150; 

organ,  239. 
Omentum,  449. 
Ophthalmic  nerve,  150. 
Opsonins,  309. 
Optical  defects  of  eye,  262. 
Optical  system,  258;  of  eye,  259. 
Optic  chiasma,  150,  251;  disk,  250; 
nerve,  35,  150,  251;  thalami,  146, 
153;  tracts,  150,  251. 
Optogram,  273. 
Orbicularis  oris.   119;  palpebrarum, 

119,  245. 
Orbit,  62. 

Organ  of  Corti,  233. 
Organs,  1,  34;  of  circulation,  322;  of 
digestion,  442;  of  excretion,  516;  of 
movement,  78;  of  nervous  system, 
135;  of  reproduction,  562;  of  res- 
piration, 386;  of  secretion,  480;  of 
sensation,  204,  226,  243. 
Origin  of  muscles,  82. 
Os  innpminatum,  52,  65,  67. 
Osmosis,  19. 

Osmotic  pressure,  19,  93. 
Os  orbiculare,  229. 
Ossein,  49. 
Ossicles,  auditory,  229:  functions  of, 

230. 

Osteoblast,  46. 
Osteoclast,  48. 
Otoliths,  237. 
Oval  foramen,  227. 
Ovary,  567;  structure  of,  570. 
Overtones,  224. 
Oviduct,  567. 
Ovulation,  571. 
Ovum,  29,  571;  fertilization  of,  573; 

maturation  of,  560. 
Oxidase,  425. 

Oxidation,  38;  in  muscle,  104,  114; 
as    source    of    animal    heat,    541; 
"recovery,"  110. 
Oxygen,  9;  absorption  of  by  blood, 

420;  interchanges  in  blood,  421. 
Oxyhemoglobin,  417. 
Pacinian  corpuscles,  213. 


Pain,  172,  205,  206,  211;  internal,  213; 
localization  of,  213. 

Palate,  443. 

Palatine  bones,  60. 

Palmatin,  15. 

Pancreas,  5,  39,  114,  460. 

Pancreatic  juice,  465;  secretin,  487; 
secretion,  control  of,  486. 

Papillae,  lachrymal,  245;  of  skin,  531; 
of  tongue,  241,  446. 

Papillary  muscles,  329;  use  of,  342. 

Paraglobulin,  302. 

Parathyroids,  305. 

Parietal  bones,  60;  lobe,  176. 

Parieto-occipital  fissure,  176. 

Parotid  gland,  447. 

Parthenogenesis,  560. 

Partial  tones,  224. 

Parturition,  578. 

Pasteurization  of  milk,  582. 

Patella,  66. 

Patheticus  nerve,  150. 

Pathogenic  organisms,  306. 

Pathology',  definition,  1. 

Paths,  nervous,  of  various  senses,  172. 

Pawlow  (Pavloff),  485. 

Peas,  508. 

Pectoral  arch,  64. 

Peduncles  of  cerebellum,  165. 

Pelvic  girdle,  64,  67. 

Pelvis  of  kidney,  520. 

Penis,  565. 

Pepsin,  15,  464. 

Pepsinogen,  468. 

Peptone,  13,  464. 

Perceptions,     219;    auditory,     235; 
visual,  285,  286,  287,  288. 

Pericarditis,  325. 

Pericardium,  323. 

Perichondrium,  45. 

Perilymph,  231,  233. 

Perimeter,  279. 

Perimysium,  83. 

Periosteum,  46. 

Peripheral  nervous  system,  139;  ref- 
erence of  sensations,  218. 

Peristalsis,  472. 

Peritoneum,  5,  450. 

Permanent  cartilage,  44. 

Permeable  membrane,  19. 

Perspiration,  535. 

Peyer's  patches,  383. 

Phagocytes,  302,  307;  action  of  in 
resisting  infection,  307. 

Phagacytosis,  302. 

Phalanges,  66. 

Pharynx,  442,  448. 

Phlorhizin,  498. 

Phosphoproteins,  13. 


644 


INDEX 


Photochemical  substances,  282. 

Phrenic  nerve,  149. 

Physico-chemistry  of  Body,  16;  of 
skeletal  muscle,  88. 

Physiological  division  of  labor,  30,  87. 

Physiological  systems,  35. 

Physiology,  1;  of  brain,  166,  179;  of 
digestion,  462;  of  ear,  226,  237;  of 
eye,  267;  of  heart,  347;  of  kidney, 
527;  of  metabolism,  500;  of  muscle, 
93;  of  nerve,  155;  of  respiration, 
386;  of  sensation,  204;  of  skin, 
535;  of  smell,  239;  of  spinal  cord, 
157;  of  taste,  240;  of  touch,  214. 

Pia  mater,  141. 

Pigment,  14;  of  iris,  250. 

Pitch,  audible  limits  of,  224;  definition 
of,  223;  range  of,  in  human  voice, 
552. 

Pituitary  body,  50. 

Pivot  joints,  75. 

Placenta,  578. 

Plain  muscular  tissue,  see  Smooth 
Muscle. 

Plantigrade  animals,  70. 

Plants  compared  with  animals,  40. 

Plasma,  295,  302. 

Platelets,  blood,  302. 

Pleura,  5,  6,  389. 

Plexus,  148;  of  Auerbach,  456; 
brachial,  148;  cardiac,  154;  cer- 
vical, 148;  lumbar,  149;  of  Meiss- 
ner,  456;  sacral,  150;  solar,  154,  411. 

Pneumogastric  nerves,  152. 

Poikilothermous  animals,  539. 

Poisoning,  by  coal  gas,  427;  by  food, 
440. 

Polar  bodies,  561. 

Pons  varolii,  146. 

Popliteal  artery,  331. 

Portal  circulation,  335;  vein,  335,  457, 
461. 

Post  ganglionic  neurons,  194. 

Postural  reflexes,  167. 

Posture,  123,  124;  the  task  of  extensor 
muscles,  126. 

Potassium  chlorid,  10. 

Precursor,  lactic  acid,  111. 

Preganglionic  neurons,  194. 

Pregnancy,  567,  576;  extra-uterine, 
573. 

Prehension,  123,  129. 

Prepuce,  566. 

Presbyopia,  266. 

Pressor  impulses,  376. 

Pressure,  of  blood,  362,  367;  intra- 
thoracic,  399;  osmotic,  19;  partial, 
of  gases,  419. 

Pressure  sense,  214. 


Primates,  2. 

Principles  of  dietetics,  511. 

Process,  olecranon,  66;  odontoid,  58, 
75;  ciliary,  249. 

Production  of  heat  in  Body,  40,  541. 

Projecting  senses,  207: 

Projection  fibers  of  cerebrum,  175. 

Pronation,  76. 

Proofs  of  circulation,  371. 

'Properties  of  Body,  21. 

Prosecretin,  487 

Prostate,  564. 

Protamin,  12. 

Protective  inoculation,  311. 

Protective  tissues,  33. 

Protein,  10,  429,  434;  absorption  of, 
498;  amount  of  in  diet,  504;  con- 
jugated, 12;  deaminization  of,  504; 
derived,  13;  digestion  of,  464;  foods, 
434;  food  value  of,  507;  fuel,  504; 
growth,  503;  maintenance,  503;  of 
muscle,  89;  requirements  of  Body 
for,  500;  specific  dynamic  action 
of,  509,  543;  subdivisions  of,  10; 
tests  for,  11;  use  of  in  Body,  501. 

Proteose,  13,  464. 

Prothrombin,  317. 

Protoplasm,  8. 

Psychic  secretion,  485. 

Psycho-physical  law,  205,  214,  271. 

Ptomain,  441. 

Ptosis,  248. 

Ptyalin,  15,  463,  487. 

Puberty,  582. 

Pubis,  52,  65. 

Pulmonary  artery,  326,  333,  334; 
circulation,  333,  335;  veins,  323, 
333,  336. 

Pulse,  364;  use  of  in  diagnosis,  36G; 
-wave,  rate  of  movement,  365. 

Pupil,  244,  249. 

Purin  bodies,  14,  526. 

Purkinje's  experiment,  269. 

Pus,  301. 

Putrefaction,  467. 

Pyloric  sphincter,  474;  control  of,  474. 

Pylorus,  449,  452. 

Pyramidal  nerve  cells,  175;  tracts, 
177. 

Pyramids,  decussation  of,  177. 

Pyramids  of  Ferrein,  521;  of  Mal- 
phigi,  521. 

Pyrexia,  544. 

Quadriceps  femoris,  120. 

Qualities  of  sensation,  205. 

Quantity,  of  air  breathed  daily,  398; 
of  blood,  303;  of  food  needed  daily, 
500,  511. 

Racemose  glands,  480. 


INDEX 


645 


Radial  artery,  330. 

Radio-ulnar  articulation,  76. 

Radius,  66. 

Range  of  human  voice,  552. 

Ranvier,  nodes  of,  138. 

Rate  of  blood  flow,  368;  of  nerve  im- 
pulse, 156;  of  pulse-wave,  365. 

Reaction  of  blood,  295. 

Reactions,  emotional,  197. 

Reason,  faculty  of,  190. 

Receptaculum  chyli,  382. 

Receptive  tissues,  see  Irritable  tissues. 

Receptor  system,  204. 

Receptors,  classification  of,  206. 

Recessive  characters,  574. 

Record,  graphic,  94. 

"Recovery"  oxidation  in  muscle, 
110. 

Rectum,  455. 

Rectus  abdominis,  83. 

Rectus  muscles  of  eye,  246. 

Red  blood  corpuscles,  295;  color,  296; 
composition,  297;  consistency,  297; 
form  and  size,  295;  number,  297; 
origin  and  fate,  299;  structure,  296. 

Red  nucleus,  153. 

Reduced  hemoglobin,  418. 

Reduction  division,  561. 

Reflex,  definition  of,  157;  myenteric, 
474. 

Reflex  animal,  compared  with  nor- 
mal, 169. 

Reflex  control  of  autonomic  system, 
194. 

Reflex  arcs,  158;  of  cortex,  179;  va- 
riability in,  159. 

Reflex  time,  180. 

Reflexes,  control  of  by  head  senses, 
163;  cortical  compared  with  spinal, 
180;  locomotor,  166;  mediated  by 
spinal  cord,  162;  postural,  167; 
spreading  of,  162. 

Refracting  media  of  eye,  254. 

Refraction,  255;  in  the  eye,  259; 
index  of,  256;  law  of,  256;  of  lenses, 
257;  of  light,  255. 

Refractory  period  of  heart,  348. 

Regeneration,  558. 

Regio  olfactoria,  239. 

Regulation  of  temperature,  40,  539. 

Reissner,  membrane  of,  233. 

Relaxation  of  muscle,  111. 

Renal  artery,  331,  520;  excretion, 
524;  organs,  518;  vein,  520. 

Rennin,  464. 

Repair  of  fractured  bone,  48. 

Reproduction,  21,  557;  sexual,  560. 

Reproductive  organs,  accessory,  562; 
female,  567;  male,  563. 


Reproductive  system,  40;  hormones 
of,  583. 

Reproductive  tissues,  34. 

Residual  air,  397. 

Resistance,  capillary,  356. 

Resistance,  synaptic,  161;  to  in- 
fection, 306. 

Resonance,  225. 

Resonants,  554,  555. 

Respiration,  386;  abdominal,  398; 
artificial,  408;  chemistry  of,  410; 
costal,  398;  external,  386;  forced, 
396;  hygiene  of,  398;  influence  of 
on  circulation,  400;  on  lymph  flow, 
401;  internal,  386;  nerves  of,  402; 
rhythmic  character  of,  403;  tissue. 
386,  425. 

Respiratory  center,  401;  action  of 
vagi  on,  404;  excitation  of,  403; 
reflex  influence  on,  402;  sensitive- 
ness of,  405;  changes  in  muscular 
exercise,  425;  movements,  391; 
movements,  modified,  408;  organs, 
386;  sounds,  397;  tissue,  31. 

Response  of  muscle  to  rapidly  re- 
peated stimuli,  102. 

Reticular  membrane,  234. 

Retina,  35,  243,  250;  blood  vessels  of, 
251 ;  distribution  of  color  sense  over, 
279;  localizing  power  of,  274;  mi- 
croscopic structure  of,  252;  nervous 
elements  of,  253. 

Rheumatism,  344. 

Rhythmic  segmentation,  476. 

Rhythmicity  of  heart,  347;  nature  of, 
350. 

Rib  cartilage,  64. 

Ribs,  55,  63. 

Rigor,  chloroform,  109;  mortis.  92. 

Rods  of  Corti,  234. 

Rods  of  retina,  252;  excitation  of,  268; 
function  of,  272. 

Rolando,  fissure  of,  176. 

Roots  of  spinal  nerves,  147. 

Rotation,  movements  of,  72,  74;  of 
radius  over  ulna,  66,  75,  129. 

Roughage,  429;  importance  of,  478. 

Round  foramen,  227. 

Running,  128. 

Rupture,  563. 

Sac,  lachrymal,  246. 

Sacculus,  232. 

Sacral  autonomies,  195;  plexus,  150; 
vertebrae,  60. 

Sacrum,  52,  55,  59,  67. 

Saint  Vitus'  dance,  94. 

Saliva,  463. 

Salivary  glands,  39,  447. 

Salivary  secretion,  control  of,  484. 


640 


INDEX 


Salt,  common,  10;  importance  of  in 
diet,  431. 

Salts,  of  Body,  10;  of  urine,  526. 

Santorini,  cartilages  of,  548. 

Saphenous  vein,  333. 

Sarcolemma,  84,  88. 

Sarcoplasm,  84,  88. 

Sarcostyle,  84,  85,  88;  behavior  of  in 
contraction,  113. 

Scalae  of  cochlea,  233. 

Scalene  muscles,  394. 

Scapula,  64,  129. 

Sciatic  nerve,  150. 

Sclerotic,  248. 

Scrotum,  563. 

Scurvy,  431. 

Sebaceous  glands,  245,  535;  secretion, 
536. 

Secondary  sexual  characters,  583. 

Secretin,  gastric,  486;  pancreatic, 
487. 

Secretion,  482;  cutaneous,  535;  gas- 
tric, 464;  control  of,  485;  intestinal, 
465;  control  of,  487;  organs  of,  480; 
pancreatic,  465;  control  of,  486; 
psychic,  485;  renal,  527;  salivary, 
463;  control  of,  484;  sebaceous,  536; 
sweat,  535. 

Secretory  nerves,  483. 

Secretory  process,  482;  hormone  con- 
trol of,  484;  nervous  control  of,  483. 

Secretory  tissues,  31. 

Sections  of  Body,  5,  6. 

Segmentation,  of  ovum,  29,  576; 
rhythmic,  of  intestine,  476. 

Self-digestion,  prevention  of,  467. 

Semicircular  canals,  164,  231;  bony, 
232;  epithelium  of,  237;  function 
of,  237;  membraneous,  232;  nerve 
endings  in,  236. 

Semilunar  ganglion,  150;  valves  of 
heart,  329. 

Seminal  fluid,  566;  vesicle,  564. 

Seminiferous  tubule,  563. 

Semipermeable  membrane,  19. 

Sensations,  27;  of  color,  277;  com- 
mon, 205;  differences  between,  204; 
extrinsic  reference  of,  220;  inten- 
sity of,  205;  local  sign  of,  205,  215; 
modality  of,  205;  peripheral  ref- 
erence of,  218;  quality  of,  205;  vis- 
ual, duration  of,  273;  intensity  of, 
271. 

Sense,  of  equilibrium,  237;  of  hearing, 
226,  234;  of  hunger,  209;  muscular, 
207;  of  pain,  211;  of  sight,  267;  of 
smell,  239;  of  taste,  240;  of  thirst, 
210;  of  touch,  214;  of  temperature, 
206,  217. 


Senses,  205;  classification  of,  206; 
contact,  206;  cutaneous,  211;  ex- 
ternal, 206;  internal,  206,  207; 
nerve  paths  of,  172;  projecting,  207. 

Sensory  areas  of  cortex,  177. 

Sensory  decussation,  173;  illusions, 
221;  neurons,  136. 

Septicemia,  307. 

Septum  of  heart,  325. 

Serous  membranes,  5,  450. 

Serum,  312;  albumin,  302. 

Sex  determination,  575. 

Sexual  characters,  secondary,  583; 
reproduction,  560. 

Sheath,  myelin,  138. 

Shivering,  543. 

Short  sight,  262. 

Shoulder-blade,  64;  -girdle,  64;  at- 
tachment to  axial  skeleton,  66. 

Sighing,  408. 

Sight,  163.  164,  172,  205,  206;  nerve 
paths  of,  173;  hygiene  of,  265. 

Sigmoid  flexure,  455. 

Simple  contraction,  94. 

Sinus  venosus,  347. 

Size,  perception  of,  287. 

Skeletal  muscles,  79;  chemistry  of,  89; 
histology  of,  83;  hormone  of,  114; 
physico-chemistry  of,  88;  special 
physiology  of,  118. 

Skeleton,  53;  appendicular,  60;  axial, 
55;  of  face,  60;  hygiene  of,  70;  pecu- 
liarities of,  68;  of  skull,  60;  of 
thorax,  63,  392. 

Skin,  5,  39,  529;  glands,  534;  hygiene 
of,  537;  localizing  power  of,  215; 
papillae  of,  531;  secretions,  535. 

Skull,  55;  details  of,  60;  articulations 
of,  61. 

Sleep,  379. 

Small  intestine,  452;  absorption  from, 
491;  digestion  in,  488;  movements 
of,  476;  control  of,  477. 

Smell,  163,  172,  205,  206,  239;  fatigue 
of,  240;  keenness  of,  240. 

Smooth  muscle,  85;  heat  rigor  in,  116; 
mechanism  of  contraction  of,  116; 
physiology  of,  114. 

Sneezing,  408. 

Soap,  production  of  from  fat,  15;  in 
small  intestine,  499. 

Sodium  carbonate,  90,  423. 

Sodium  chlorid,  10,  431. 

Sodium  lactate,  formation  of  in 
muscle,  112,  426. 

Solar  plexus,  154,  375,  451;  effect  of 
blow  on,  353. 

Solidity,  perception  of,  288. 

Somatic  cells,  560. 


INDEX 


647 


Sound,  223;  intensity  of,  223;  pitch 
of,  223;  quality  of ,~223. 

Sounds,  of  heart,  342;  respiratory,397. 

Soup,  value  of,  498. 

Source,  of  animal  heat,  541;  of  body 
fat,  515;  of  glycogen,  493;  of  mus- 
cular energy,  104;  of  urea,  517. 

Spaces,  arachnoid,  142;  intercellular, 
18. 

Special  senses,  205. 

Specific  dynamic  action  of  proteins, 
ou y  f  o4o . 

Specific  gravity  of  blood,  295. 

Specific  nerve  energies,  204. 

Spectacles,  265. 

Speech,  546. 

Speed  of  nerve  impulse,  156. 

Sperm,  561. 

Spermatozoa,  560,  566. 

Sphenoid  bone,  60. 

Sphere,  attraction,  24. 

Spherical  aberration,  263. 

Sphincter,  115;  pyloric,  474. 

Spinal  accessory  nerve,  152. 

Spinal  column,  see  Vertebral  column. 

Spinal  cord,  5,  138,  142;  central  canal 
of,  141,  144;  columns  of,  144;  com- 
missures of,  144;  fissures  of,  143; 
functions  of,  160,  172;  gray  matter 
of,  144;  membranes  of,  139;  white 
matter  of,  144. 

Spinal  cord  reflexes,  162;  cerebral 
control  of,  186;  compared  with 
cortical,  180. 

Spinal  ganglia,  147;  nerve  roots,  147; 
nerves,  139,  147;  distribution  of, 
148. 

Spindle,  nuclear,  25. 

Spindles,  muscle,  85. 

Spine,  curvature  of,  70. 

Splanchnic  nerves,  375,  451;  region, 
375. 

Spleen,  5,  299;  function  of,  300. 

Splenic  artery,  461. 

Sprains,  77. 

Spread  of  nerve  impulse  in  both  direc- 
tions, 156. 

Squinting,  248. 

Staining,  differential,  7. 

Stages  of  life,  584. 

Staircase  phenomenon,  100. 

Stapedius  muscle,  230. 

Stapes,  229. 

Starch,  animal,  16;  digestion  of,  463, 
465,  487;  as  food,  433. 

Stearin,  15. 

Stenson's  duct,  448. 

Stereoscopic  vision,  289. 

Sternum,  55,  64. 


Stimulation,  necessity  of,  93. 

Stimuli,  94,  204;  increasing,  influence 
on  contraction,  96;  rapidly  re- 
peated, effect  of,  102. 

Stirrup  bone,  229. 

Stomach,  5,  442,  449;  absorption 
from,  491;  digestion  in,  488;  his- 
tology of,  451;  importance  of,  476; 
movements  of,  473;  nervous  con- 
trol of,  477;  salivary  digestion  in, 
473. 

Storage,  of  carbohydrates,  493;  of 
glycogen  in  muscles,  494. 

Storage  tissues,  32. 

Strabismus,  248. 

Strength  of  desperation,  201. 

Structure,  vertebrate,  3. 

Strychnine  poisoning,  162. 

Subclavian  artery,  330;  vein,  333. 

Subcutaneous  areolar  tissue,  531. 

Sublingual  gland,  448. 

Submaxillary  gland,  448. 

Successive  myelination,  172. 

Succus  entericus,  465;  control  of,  487. 

Sucrase,  466. 

Sucrose,  433. 

Sudoriparous  glands,  534. 

Sugar,  433;  as  food,  493;  as  fuel  for 
muscles,  105;  elimination  of 
through  kidney,  494. 

Sulcus  spiralis,  233. 

Superior  maxillary  nerve,  151;  mesen- 
teric  artery,  461;  oblique  muscle, 
247;  rectus  muscle,  246. 

Supination,  76. 

Supplemental  air,  397. 

Supporting  tissue,  31;  system,  37. 

Suprarenal  capsules,  199. 

Suspensory  ligament  of  eye,  254,  261. 

Sutures,  71. 

Swallowing,  470. 

Sweat,  535;  center,  536;  glands,  480, 
534;  nervous  and  circulatory  factors 
in,  536. 

Sweating,  relation  of  to  heat  loss,  542. 

Sweet  bread,  460. 

Sylvius,  fissure  of,  176. 

Sympathetic  ganglion,  154. 

Sympathetic  resonance,  225,  227. 

Sympathetic  system,  see  Autonomic 
system. 

Synapse,  137;  relation  of  to  gray 
matter,  153. 

Synaptic  fatigue,  198;  resistance,  161. 

Synovia!  fluid,  74;  membrane,  74. 

System,  alimentary,  39,  442;  auto- 
nomic,  139,  154,  193;  circulatory, 
39,  322;  conductive,  38,  135;  diges- 
tive, 39,  442;  excretory,  39,  516; 


648 


INDEX 


motor,  37,  78;  nervous,  38,  135; 
receptor,  37,  204;  respiratory,  39, 
386;  supporting,  37,  43. 

Systemic  circulation,  335. 

Systems,  Haversian,  47. 

Systems,  physiological,  35;  adaptive, 
37;  maintenance,  38. 

Systole  of  heart,  339. 

Taking  cold,  376,  545. 

Tannin,  440. 

Tarsus,  66,  70. 

Taste,  163,  172,  205,  206,  240;  -buds, 
241,  446. 

Taurocholic  acid,  518. 

Tea,  439. 

Tear-glands,  246. 

Tears,  246. 

Tectorial  membrane,  234. 

Teeth,  443;  structure  of,  445;  care  of, 
470. 

Temperature,  of  Body,  540;  effect  of 
on  muscular  contraction,  97;  sensa- 
tion zero,  217;  sense,  206,  217; 
bodily,  regulation  of,  541 ;  local,  543. 

Temporal  artery,  331;  bone,  60;  lobe, 
176. 

Temporary  cartilage,  44. 

Tendon-organs  of  Golgi,  85. 

Tendons,  79. 

Tension  of  blood-gases,  422. 

Tensor  tympani  muscle,  230. 

Test  for  color  blindness,  281. 

Testis,  563. 

Tests  for  proteins,  12. 

Tetanus,  102. 

Thalami,  optic,  153. 

Theobromin,  439. 

Theories,  of  color  vision,  281;  of 
heart-beat,  349;  of  sleep,  380. 

Thirst,  205,  206,  210. 

Thoracic  cavity,  4;  duct,  382;  verte- 
brae, 58. 

Thoracico-lumbar  autonomies,  195. 

Thorax,  aspiration  of,  370,  399;  con- 
tents of,  5;  movements  of,  392; 
skeleton  of,  392. 

Throat,  448. 

Thrombin,  316;  source  of,  316. 

Thromboplastic  substance,  317. 

Thumb,  articulation  of,  66. 

Thyro-arytenoid  muscle,  551. 

Thyroid  cartilage,  547. 

Thyroid  gland,  50,  201. 

Thyroid  hormone,  emergency  func- 
tion of,  202;  influence  of  on  me- 
tabolism, 513. 

Tibia,  66. 

Tibial  artery,  331. 

Tidal  air,  398. 


Timbre,  223,  224. 

Tissue  cells  compared  with  germ 
cells,  560. 

Tissue  differentiation,  25;  resistance, 
307;  respiration,  425. 

Tissues,  1,  8;  adenoid,  304,  383;  cadi- 
pose,  44;  areolar,  43;  assimilative, 
31;  bony,  46;  cartilaginous,  45; 
classification  of,  30;  conductive,  32; 
connective,  31,  41;  contractile,  32, 
78;  elastic,  44;  erectile,  565;  ex- 
cretory, 31,  518;  irritable,  32; 
lymphoid,  304,  383;  motor,  32,  78; 
nervous,  135;  nutritive,  31;  pro- 
tective, 33;  reproductive,  34;  res- 
piratory, 31,  389;  secretory,  31, 
480;  storage,  32;  supporting,  31, 
41;  undifferentiated,  30. 

Tone,  vasomotor,  375;  relation  of  to 
cerebral  activity,  378. 

Tones;  number  distinguishable,  235. 

Tongue,  123,  240,  443,  446. 

Tonsil,  383,  448. 

Touch,  164,  172,  205,  206,  214. 

Toxins,  308. 

Trachea,  39,  388. 

Tracing  nerve  paths,  171. 

Tracts,  of  body  sense,  172;  of  Gower, 
173;  of  head  senses,  173;  pyram- 
idal, 177. 

Training,  124. 

Transfusion  of  blood,  320. 

Triceps,  82. 

Tricuspid  valve,  328,  329. 

Trigeminal  nerve,  150,  240. 

Trochlear  muscle,  247. 

Trophic  nerves,  483. 

Trypsin,  465. 

Trypsinogen,  468. 

Tube,  Eustachian,  227,  228;  Fallo- 
pian, 567;  neural,  141. 

Tubular  glands,  480. 

Tubules,  uriniferous,  522;  seminifer- 
ous, 563. 

Tunica,  adventitia,  337;  vaginalis,  563. 

Turbinate  bones,  60. 

Twitch  of  muscle,  94. 

Tympanic  membrane,  226,  228;  func- 
tions of,  226. 

Tyrein,  39. 

Ulna,  66. 

Ulnar  artery,  331. 

Umbilical  cord,  578. 

Undifferentiated  tissues,  30. 

Uniform  temperature,  maintenance 
of,  541. 

Units  of  energy,  107. 

Unstriped  muscle,  see  Smooth  muscle. 

Urea,  13,  90,  517,  525. 


INDEX 


649 


Ureter,  518. 

Urethra,  519;  male,  565. 

Uric  acid,  14,  526. 

Urinary  organs,  518;  salts,  526. 

Urine,  '525. 

Uriniferous  tubules,  522;  secretory 
action  of,  527. 

Urobilin,  14. 

Urticaria,  385. 

Uterus,  567. 

Utriculus,  232. 

Uvula,  443. 

Vaccination,  311. 

Vagina,  568. 

Vagus  nerve,  152,  477;  relation  of  to 
heart,  351. 

Valve,  ileocolic,  455. 

Valves  of  heart,  328;  action  of,  343. 

Valves  of  veins,  337. 

Valvulse  conniventes,  452. 

Valvular  insufficiency,  effects  of,  344. 

Varicose  veins,  370. 

Vasa  efferentia,  564. 

Vasa  recta,  564. 

Vas  deferens,  564. 

Vaso  constrictor  center,  375;  control 
of,  375;  nerves,  374. 

Vasodilator  center,  378;  nerves,  377. 

Vasomotor  tone,  375;  relation  of  to 
cerebral  activity,  378. 

Vegetable  foods,  435;  cooking  of,  436. 

Veins,  322,  332;  cephalic,  333;  coro- 
nary, 327;  hepatic,  335,  458;  in- 
nominate, 333;  jugular,  333;  por- 
tal, 335,  457;  pulmonary,  328,  333; 
saphenqus,  333;  structure  of,  337; 
subclavian,  333;  valves  of,  337; 
varicose,  370. 

Vena  cava,  327,  833. 

Venous  blood,  336. 

Venous  sinus,  565. 

Ventilation,  412. 

Ventricle  of  heart,  325;  functions  of, 
341,  346. 

Ventricles  of  brain,  142. 

Vermiform  appendix,  455. 

Vertebrae,  56;  cervical,  57;  lumbar, 
59;  sacral,  59;  structure  of,  56; 
thoracic,  58. 

Vertebral  artery,  330;  column,  3,  55. 

Vertebrate,  3.  " 

Vesicle,  seminal,  564. 

Vessels,  blood,  322;  lymphatic,  294. 

Vestibule  of  ear,  164,  231,  232;  func- 
tion of,  237;  nerve  endings  in,  236. 

Vibrations,  analysis  of,  224;  the  basis 
of  sound,  223 ;  sympathetic,  225, 227. 

Vibratories,  555. 

Villi  of  intestines,  453. 


Vision,  123;  binocular,  287;  color,  276; 
stereoscopic,  289 ;  wide  range  of,  259. 

Visual  angle,  274;  apparatus,  excita- 
tion of,  267;  area  of  cerebrum,  177; 
axis,  275;  contrasts,  281;  defects, 
262;  perceptions,  285;  purple,  250, 
252,  273;  sensations,  271. 

Vital  capacity,  398;  centers,  193; 
point,  402;  processes,  192. 

Vitamines,  429,  431. 

Vitelline  membrane,  571. 

Vitreous  humor,  254. 

Vocal  cords,  546,  548;  false,  548;  re- 
lation of,  to  pitch,  552. 

Voice,  546;  production,  123;  range  of, 
552. 

Volition,  166,  169,  184. 

Voluntary  acts,  reflex  at  bottom,  184. 

Voluntary  muscular  contraction,  102. 

Vomer,  60. 

Vowels,  553. 

Vulva,  569. 

Walking,  126. 

Wallerian  degeneration,  171. 

Warm-blooded  animals,  539. 

"Warming  up,"  100. 

Warmth  receptors,  218. 

Water,  equilibrium,  511;  proportion 
of  in  Body,  10. 

Wave-length  of  light,  255. 

Waves,  peristaltic,  472. 

Wax  of  ear,  226. 

Weber's  law,  205,  214;  circulation 
scheme,  359. 

Weeping,  246. 

Weight,  maintenance  of,  511. 

Whipped  blood,  313. 

Whispering,  556. 

White,  sensations,  276;  blood-cor- 
puscles, 301;  of  eye,  249;  fibrous 
connective  tissue,  31,  43;  matter, 
138,  153;  matter  of  cerebrum,  175; 
of  spinal  cord,  144. 

Will  power,  185. 

Wind  pipe,  36,  388. 

Work,  of  heart,  346;  muscular,  meas- 
ure of,  98. 

Worry,  significance  of,  197. 

Wrisberg,  cartilage  of,  548. 

Wrist  bones,  66. 

Xanthoproteic  test,  12. 

Yawning,  408. 

Yellow  elastic  tissue,  31,  44. 

Yolk,  571. 

Young-Helmholtz  theory  of  color 
vision,  282. 

Zona  pellucida,  571;  radiata,  571. 

Zoological  position  of  man,  2. 

Zymogen,  468. 


DATE  DUE  SLIP 

UNIVERSITY  OF  CALIFORNIA  MEDICAL  SCHOOL  LIBRARY 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


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•87  1  5  1951 


2m-5,'31