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Metcalf and Flint INSECT LIFE 




Riley and Johannsen MEDICAL ENTOMOLOGY 







There are also the related series of McGraw-Hill Publications in 
the Botanical Sciences, of which Edmund W. Sinnott is Consulting 
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C>)l(/(i+( j /Vo/c.s-.s'or ot Violoyy 

Fellow of CdUnnin College, 

Yale University 






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

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

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


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

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

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

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


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




May, 1940. 


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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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






Nature of Protoplasm 3 

Protoplasmic Activities 8 


Cell Structure 20 

Human Tissues 23 

Organs and Organ Systems 32 

The Body Plan 34 


Structural Features Associated with Nutrition 41 

Functional Features Associated with Nutrition 55 

Photosynthesis 66 


Structural Features Associated with Respiration 74 

The Respiratory System of Man 76 

Breathing 81 

Functional Features Associated with Respiration 85 


Structural Features Associated with Secretion 93 

Functional Features Associated with Secretion 96 

The Liver 97 

Endocrine Glands 102 


Excretion in the Skin 119 

Excretion in the Lungs 121 

Excretion in the Liver 121 

Excretion in the Kidneys 122 


Structural Features Associated with the Vascular System 135 

Course of the Circulation in the Body 147 

Functional Features Associated with the Vascular System 152 

Transportation of Materials in the Blood 158 

Uniformity and Variation in the Blood 161 




Blood Coagulation 163 

The Spleen 167 


Structural Features Associated with Movement 170 

Functional Features Associated with Movement 178 


Structural Features Associated with the Skeletal System 189 

Exoskeleton 189 

Endoskeleton 192 

Functional Features Associated with the Skeletal System 209 


Structural Features Associated with the Nervous System 216 

Sense Organs 218 

Peripheral Nervous System 239 

Autonomic Nervous System 243 


Central Nervous System 240 

The Spinal Cord 252 

The Brain 257 

Functional Features Associated with the Nervous System 268 


Types of Reproduction 28.3 

Development of the Frog 29 1 

Development of the Chick 301 

Mammalian Development 308 

Human Reproduction 314 


Mitosis 327 

Chromosome structure 332 

Germ Cell Formation 341 

Fertilization 349 


The Particulate Nature of Inheritance 355 

Mendelian Inheritance 356 

Multiple Factors '. . 372 

Linkage 377 

Mutations 389 


Inherited Characteristics 400 

Galton and the Principles of Biometry 406 

Human Hybridization 413 

Eugenics: Negative and Positive 419 




Auto trophic Organisms 429 

Heterotrophic Organisms 435 

Enzymes 437 

The Biotic Environment 447 


Noninfectious Diseases 468 

Immunity 469 

Immunology: Uses and Techniques 473 

Epidemiology 485 

Types of Cellular Response 488 


INDEX 589 














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


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


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



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



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

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

A B C D 

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

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

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

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


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

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

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

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



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

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

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

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

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


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

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

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

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

of plants and animals. In the primitive cellular grecn p plant| Chla _ 
forms of plants and animals, many species mydomonas. Chlorophyll 

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

are found in which the entire organism ch i orop i ast . Highly magni- 

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

unassociated with other cellular units and 

completely equipped for maintaining all the life functions. These are 

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

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

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

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


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


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





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

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

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


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


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




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

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


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

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

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

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

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


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

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

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

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


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

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


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


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

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

r* ** 

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

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


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

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


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


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

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

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


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



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

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

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

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


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

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

I ' ;6/S; ''^ 

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

^W 1 1 , J 

PLATE II. Cell types. 

:ella from 

Drawings of a considerable variety of plant and animal 

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


The human body exhibits the characteristic cellular organization 
that is everywhere apparent in the living world. Examined micro- 
scopically, bits of the various body materials, such as brain, bone, 
muscle, liver, or skin, reveal the presence of the constituent cells. 
The number of these microscopic life units in even a small amount of 
tissue r3aches almost incredible figures, ylt has been estimated that in 
the body of the child at birth there are approximately 26 trillion cells, 
with a total weight 1,500 million times that of the original egg cell/ 
Both the number of constituent cells and the weight are still further 
increased before maturity is reached. All these cells are derived by 
the repeated splitting of a single cell, the fertilized egg, which marks 
the starting point of each individual life. When one considers the 
number of cellular divisions, beginning with the fertilized egg, that 
are essential to produce the 26 trillion cells of the body, the number 
does not seem so difficult of attainment; for if a cell divides forty-six 
times, and each of the resulting cells continues to divide regularly 
after each division, the number, represented by 2 45 , would be reached. 
(Fig. 13.) 

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



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

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




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

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


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



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

Cells are known, for example, in the , . 

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

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

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

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


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

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



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

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

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

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











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

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


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

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


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

1 Reticulum is also designated as linin. 

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


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

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


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



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

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

V-' 1 ''!"' "''*' T "V'l 

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

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

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

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




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

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

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


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

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

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



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

Havers! an cancrf 


tissue cor- 



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

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

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

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


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

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

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

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


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



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

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

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


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


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



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

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

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

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

Contrac+ile fibril 

A B 

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


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

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

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


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


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

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


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


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

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

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

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



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


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






..^ r . . C/oaccr 

1 Bite duct I / 
Liver I Intestine 

Heart' Stomach Spleen 


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

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


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

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

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

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


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

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

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


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



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

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











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

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


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

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

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


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

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

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

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


























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


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


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

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




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







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

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

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

1 Consult the Appendix: Metazoa. 



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


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





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

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

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



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











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

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

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


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

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


Saliva, the composite product of the SUBMAXILLARY GLAND 

salivary glands, is a liquid substance with a SUBLINGUAL GLAND 

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

charactenstic slimy or stringy quality duo to vary glands< Drawing 

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

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


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

1 Ptyalin is also known as salivary amylase. 



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

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

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

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



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


series of events which function perfectly unless interrupted by 


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

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


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



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















FIG. 30.- 

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


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


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

The mucosa forming the lining of the stomach is highly glandular. 
It is estimated that as many as 35 million microscopic gastric glands 
pour their secretions into the stomach cavity, and the total amount of 
daily secretion varies around iy 2 qt. Several types of gastric gland are 
recognized by the histologists, but essentially all of them may be 
said to be simple tubular structures which penetrate the mucosa 
perpendicularly to the surface, and so their secretion passes directly 
into the stomach cavity through these tiny openings. Three or more 
types of secretory cell may occur in these glands. Gastric juice, the 
composite product of the gastric glands, is a clear fluid, mostly water, 
but decidedly acid since it contains about 0.4 per cent hydrochloric 
acid. At least two important digestive enzymes are present in the 
gastric juice: pepsin for digestive action on the proteins, and rennin 
which coagulates the proteins of milk. The determination of the 
exact origin of the two gastric enzyniOvS and also of the hydrochloric 
acid secreted in the stomach from among the three types of secretory 
cells has proved to be a difficult problem. And it is still a mystery 
how the mucosal secretory cells are able to form hydrochloric acid 
from the materials brought to them by the alkaline blood plasma. 
(Fig. 32.) 


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



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

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




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

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

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



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








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

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

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



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

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

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

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

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

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


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

But the intestinal secretion is not the only enzyme-containing 
fluid in the small intestine, nor, in fact, is it the most important. 
That distinction belongs to the pancreatic juice which is poured into 
the pyloric end of the duodenum through the pancreatic duct. Secre- 
tions from the liver also enter the intestine near by. The structural 
and functional features of pancreas and liver are fully considered in the 
later chapter dealing with Secretion, but it will be helpful at this point 
to indicate the nature of the products of these organs so far as they are 
concerned with digestion. The secretion of the pancreas is a clear 
fluid, markedly alkaline in nature with a pH of about 9. 1 It contains 
three enzymes associated with the digestion of every type of foodstuff, 
including trypsin for the proteins, amylase for the starches, and 
lipase for the fats. The liquid received from the liver is a hetero- 
geneous mixture, termed bile, and varying in color from golden yellow 
to dark green. It is usually somewhat alkaline and contains lipoids, 
various pigments (notably bilirubin which results from the destruction 
of old red blood corpuscles), and bile salts which are used in connection 
with the digestion of fats. 

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


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

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


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

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

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

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


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


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


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

Carbohydrates. The carbohydrates are compounds of carbon, 
hydrogen, and oxygen and include three related groups of compounds, 
namely, the simple sugars (monosaccharides), commonly represented 
by glucose and fructose, with the molecular formula CcH^OeJ a more 
complex group of sugars, the disaccharides, represented by sucrose 
(cane sugar), maltose, and lactose, with the formula C^H^Ou; and 
the most complex carbohydrate group of all, the polysaccharides, 
represented by starch, cellulose, arid glycogen (animal starch) with 
the formula (C 6 HioO 5 )^. Of these three groups, only the mono- 
saccharides are absorbed from the digestive tract unchanged and so are 
ready to be assimilated at once by the body cells and oxidized as 
necessary to supply energy requirements. As will be shown below, 
digestion of any of the other carbohydrates reduces them also to the 


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

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

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

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


magnesium. A typical protein contains about 50 per cent carbon, 
25 per cent oxygen, 16 per cent nitrogen, 7 per cent hydrogen, with the 
remainder consisting of sulphur, phosphorus, and various other 
elements. Proteins are formed by the union of the somewhat simpler 
amino acids, some two dozen of which are known (page 70). The 
amino acids are characterized by a particular grouping of nitrogen 
arid hydrogen in the molecule of the acid indicated by the symbols NH 2 
(amino group). When excess proteins are eaten, as frequently occurs 
in the average diet, the liver cells are able to remove the NH 2 group 
the process of deaminization from the excess amino acids, resulting 
from the protein digestion. The final result of deaminization is the 
formation, from the excess amino acids, of a carbohydrate that can be 
used to supply the energy requirements of the body or converted into 
fat for storage if the energy requirements are supplied, just as any 
other carbohydrate. The exact composition of the proteins, which 
build the cytoplasm of the different types of cells of the body, is highly 
variable due to the fact that each cell selects the proper amino acids 
and other substances from the blood stream for the construction of 
its own particular protein or proteins. 

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


The fundamental importance of certain accessory substances in the 
diet of man and various other animals has been increasingly recognized 
since 1912 when Hopkins 1 established the basic fact that other organic 
compounds besides carbohydrates, fats, and proteins were required for 
an adequate diet. These substances, commonly designated as 
vitamins, comprise a heterogeneous group of carbon compounds. In 
the earlier periods it was supposed that they constituted a closely 
related group of essential amino acids, hence the term "vitamine," 

1 Consult the Appendix: Hopkins. 


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

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

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

Vitamin A is a fat soluble substance which is widely distributed in 
plant tissues, particularly those which contain the yellow pigment 
carotene (carrots, squash, sweet potato, etc.), and is also abundantly 
stored in various animal fats, such as egg yolk, butter, and cod-liver 
oil. Animal tissues are able to transform carotene (CsoHse) from the 
plant into vitamin A (C2oH 3 oO). It has not been artificially syn- 
thesized. This vitamin is primarily a growth-promoting substance 
and also has a basic effect upon the epithelial tissues generally. 

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



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

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




1697- 1919 





CURED 1937 






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

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


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

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

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

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

1 Consult the Appendix: Sterols. 


Vitamin K belongs to a class of compounds known as the naptho- 
quinones. It is a a1/-soluble compound which apparently is essential 
to blood clotting. In its absence, hemorrhagic conditions develop in 
the body. The isolation of vitamin K was announced only a few 
months ago when the compound was obtained from spinach and 
alfalfa and also from decomposed fish meal. In the latter source, 
the presence of vitamin K is believed to be due to the chemical activities 
of decay. Two varieties of vitamin K are recognized under the names 
Ki and K 2 . 

As was recently stated: 1 

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


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

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

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


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

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

o O o O o O o O o O 

Z3 p? 
oo oooooooo 

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

during digestion, water is added to the complex molecular associations 
found in the foodstuffs, and the result is the disassociation of the 
molecules. Hydrolysis is based on enzyme action as described in the 
following paragraphs. As an example of this essential process, let us 
again consider the relations between cane sugar (sucrose, C^H^On) 
and glucose (C 6 Hi 2 6 ). In digestion, the hydrolytic action adds 
one molecule of water to each molecule of sucrose, thus CijH^On + 
H 2 O = Ci2H 2 4Oi2; the latter does not exist as a compound but instead 
there are two separate molecules of the monsaccharide, or 2C6Hi2O6. 
One of these is glucose, but the other has a different arrangement of 
the associated atoms and is known as fructose, but both have the same 
number of atoms, and both can be absorbed from the digestive tract. 1 


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

1 Consult the Appendix: Chemical Equations. 


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

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

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

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

Pepsin 2 is secreted by the gastric glands in the mucosa of the cardiac 
portion of the stomach as an inactive substance, pepsinogcn. When 
it comes into contact with the acid gastric juice, it is changed to the 
active protein-splitting enzyme pepsin, which is effective only in the 

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

2 Consult the Appendix: Beaumont, p. 501. 


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

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

Pancreatic Enzymes. Trypsin 1 is secreted by certain cells of the 
pancreas in an inactive form, trypsinogen. It is activated after reach- 
ing the intestine by a coenzyme, enterokinase, secreted by the cells of 
the small intestine. It is known that the secretion of trypsinogen in 
the pancreas is incited by a hormone, secretin, thrown into the blood 
stream by duodenal mucosal cells. The latter arc brought into activ- 
ity by the influence of the chyme from the stomach with its high 
acidity. Trypsin is one of the most important of all the digestive 
enzymes. It disrupts the proteoses and peptones present in the chyme, 
.as well as any unchanged protein foodstuffs into the constituent 
absorbable amino acids. In association with erepsin, noted below, 
protein digestion is finally completed. The complete pancreatic juice 
is markedly alkaline, the bile weakly so; together they quickly change 
the acid chyme into an alkaline fluid, a condition which is essential for 
the pancreatic enzymes. 

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

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


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

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

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

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

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


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

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

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


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



11). Photosynthesis is based upon the unique ability of chlorophyll 1 
to utilize the energy of sunlight for synthesizing carbon compounds 
of such a nature that they can be utilized as food by living organisms. 
The substances taken into the plant cell for the photosynthetic reac- 
tions consist of carbon dioxide (CCh), which is a gas composed of the 


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

elements carbon (C) and oxygen (0) combined in the proportion of one 
carbon atom to two oxygen atoms, and water (H 2 O), which is com- 
posed of two atoms of hydrogen and one atom of oxygen. Essentially, 
chlorophyll is able to make use of the radiant energy of the sun to 
separate the carbon from the oxygen in the carbon dioxide molecules 
and to combine the carbon thus secured with the hydrogen and oxygen 
of the water to form a carbohydrate, a simple sugar, glucose, having 
1 Consult the Appendix: Chlorophyll. 



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

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

6CO 2 + 6H 2 O = C 6 H 12 O 6 + 60 2 . 

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

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

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


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

It has just been noted that various possibilities await the glucose 
after being synthesized. Let* us first consider its transformation into 
other carbohydrates. As the first step, the monosaccharide may be 
changed by the plant cells into cane sugar, that is, the disaccharide, 
sucrose (Ci2H 22 On). Synthesizing plant enzymes accomplish this by 
the removal of one molecule of water from each two molecules of glu- 
cose. Thus: 2C 6 Hi 2 6 H 2 O = Ci 2 H 22 O n . When the cane sugar 
is taken into the animal digestive tract, it will be acted upon by the 
specific digestive enzyme, sucrase; a molecule of water is added to 
each sucrose molecule, which brings back the original condition with 
two molecules of glucose (page 62). 

But the glucose of the green plant cell may be changed for storage 
into a still higher type of carbohydrate, the polysaccharides, notably 
cellulose and starch. When a polysaccharide is formed, the synthetic 
enzyme action results in the removal of one molecule of water from 
each molecule of glucose. Thus: C 6 Hi20 6 H 2 = C 6 Hi 05. But 
a polysaccharide consists of many of these anhydrous glucose residues 
joined together, and so the formula is written (CeHioOs)*. Further- 
more, the ultramicroscopic starch molecules may be combined to form 
starch grains which are of microscopic visibility. The starch grains 
of a particular species are always of characteristic size and shape. 1 

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

1 Consult the Appendix: Starch. 


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

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

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

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

1 Consult the Appendix: Cellulose. 


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

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

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

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

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

















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



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

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

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




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


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





(a) (b) 

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

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

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



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

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

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



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

C ^2^^%"Posferhr 

A Tubu/es D E 

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


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






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

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

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

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





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


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





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

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

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



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

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

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


per cent 

per cent 

Carbon dioxide, 
per cent 



20 96 




16 62 



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

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

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

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

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


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


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

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

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

muscle fibers in the diaphragm, * 

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

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

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

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



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

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

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

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


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

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

The determining factor in the behavior of the respiratory center 
appears to be the amount of carbon dioxide released into the blood 
by the body tissues, and this amount, of course, is in direct ratio to the 
cellular * activities, particularly those associated with movement. 
The carbon dioxide picked up by the blood stream is not carried as 
such, for it immediately combines with the water in the plasma to 
Form carbonic acid, thus: CO2 + H^O = CH 2 O 3 . Carbonic acid tends 
bo lower the normal alkalinity of the blood plasma, and this condition 
affects the respiratory center. The latter, in turn, stimulates the 


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

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

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


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


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


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



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

Water^L \ 
Pump V J 

Water s" 

Thermometer Pump 

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

with breathing, circulation of the blood, etc., and to produce heat 
enough to maintain the normal body temperature. The latter requires 
by far the greater energy supplies and is taken as the measurement of 
the basal metabolic rate. This rate varies with age and in accordance 
with the surface area, and has been found to be in the neighborhood 
of 40 calories 1 of heat per hour for each square meter of surface in a 
normal young adult. 

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

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



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

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

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



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

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

O s N a W <*>2 

Inlet OPoor Absorbers Absorber 

A 1 
H 2 

O 2 Ricw 

Piece *~ 

H 2 1 

m Wafer 
(to mo is 


ten air 


\C0 2 
\N 2 
\O 2 Poor 


2 \ 

Air Pump 

O 2 Poor 


- > ^ v_y ^ ji j 

HzO Absorbers 

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

This is seen from the equation C 6 Hi 2 O 6 + 6O 2 = 6CO 2 + 6H 2 O, show- 
ing that the oxidation of sugar requires six molecules of oxygen, and 
releases six molecules of carbon dioxide; accordingly the respiratory quo- 
tient is 1 .0. When a fat is burned, the amount of carbon dioxide released 
is less than the oxygen taken in. Thus each molecule of tristearin, with 
the formula CsiHnoOe, requires 81.5 molecules of oxygen for complete 
oxidation, and only 57 molecules of carbon dioxide are released, which, 
divided by the oxygen consumption, gives a respiratory quotient of 
0.70. The respiratory quotient, when protein is oxidized, is around 

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


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


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

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


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


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


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

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

U^s^ is present in the 

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

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

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








Ar/T?//? Hernberger 

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

in the human male. 


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


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



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

B C 

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

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

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



Fatty tissue* 





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

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

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

Ampul fa* 

Secretory gfandt 

FIG. 51. Vertical section through 
the mammary gland. Diagrammatic. 


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

a 6 

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


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


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

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

The term hormone 1 was first used about thirty years ago in connec- 
tion with the discovery of secretin in the digestive tract. It is 
derived from a Greek word meaning to excite, and this is essentially 
what many hormones do, as has already been noted in the action of 
secretin in stimulating the pancreas (page 64), but some of the more 
recently discovered hormones are known to inhibit a certain function 
instead of increasing it. A hormone, then, may be said to be a specific 
substance given off by an endocrine gland and carried by the blood to 
some other organ where it produces a specialized type of reaction. 
Such a definition excludes certain endocrine secretions, notably the 
secretion of glucose into ^he blood by the liver, because glucose is 
universally used by all the cells. In general, the responses to the 
hormones, that is, chemical regulation, arc slow, cumulative ones, 
which stretch over considerable periods of time, whereas the response 
to nerve control is very rapid. Exceptions are to be found, however, 
as in the adrenal secretion. 


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

1 Consult Appendix: Hormones. 


Radiating capillary 

Intralobular or 
central vein 

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

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

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

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

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


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

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

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


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

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

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

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

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



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


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

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



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


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

Secretin is formed by the duodenal mucosa cells and, when liberated 
into the blood stream, causes an active flow of pancreatic juico into the 
intestine. The flow of pancreatic juice is always exactly timed to 
follow the arrival of chyme from the stomach. The acid condition of 
the latter when it reaches the duodenal mucosa acts as an inciter for 
the hormonal activity of the mucosal cells. Accordingly the complete 
cycle of events includes the stimulation of the mucosa cells by the acid 
chyme, the liberation of the hormone, secretin, into the 4 blood stream, 
the stimulation of the pancreatic cells by the secretin received from 
the blood, and, finally, the flow of pancreatic juice into the intestine. 
Coincident with the flow of gastric juice is the flow of bile from the 
liver. Until recently, it was supposed that secretin was also respon- 
sible for inciting the bile flow. It is now believed, however, that the 
latter is due to another mucosal hormone, cholecystokinin, also 
released by the duodenal mucosa following stimulation by the acid 
chyme. Cholecystokinin received from the blood causes a con- 

1 Consult Appendix: Secretagogues. 



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


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

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

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


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

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

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

The Thyroid. The human thyroid 1 gland consists of a pair of 
ovoid bodies lying on each side of the anterior end of the trachea, 

1 Consult Appendix : Thyroid. 




.pyramid lobe 
of thyroid, 

.parathyro \ cL 

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

The thyroid follicles are nor- 
mally filled with a secreted jelly- 
like material, the colloid sub- 
stance, distinguished from all 
other compounds in the body by 
the fact that it contains a rich 
supply of iodine. Colloid sub- 
stance contains the reserve supply 
of the thyroid hormone, thyrox- 
ine, the active principle of the 
gland. The complete hormone 
consists of a protein, globulin, in 
association with the active thyrox- 
ine. The latter was completely 
analyzed almost twenty-five years 
ago and found to have the formula 
Ci 5 HnO 4 NI 4 . The distinctive 
feature, as noted, is that it contains a large amount of iodine. Thy- 
roxine is now synthesized in the laboratory, and the artificial product 
possesses all the characteristic properties of that naturally formed in 
the body, as tested by experimental animals. 

Functionally, the thyroid hormone has a powerful effect in regulat- 
ing the general metabolism of the body. A continuous supply of it is 
required at all times for normal functioning. The actual amount 
required, however, is amazingly small, due to its great potency; a 
characteristic that holds for all the hormones. It has been estimated 
that the amount of thyroxine circulating in the blood at any one 
time is about % grain (about ^750 Z 0- Variations in either direction 


..left lobe^of 
tnyroid glanct 
ro icC 

vleveet fom f^ont* 

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


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

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

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


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

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

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

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


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

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

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

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

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

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

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


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

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

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

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

The Adrenal Glands. The paired adrenal 1 glands are always found 
in close relationship with the kidneys and were, therefore, long thought 
to be linked with the kidneys functionally as well as anatomically. 
This proved not to be the case, and no functional reason seems to exist 

1 Consult Appendix: Adrenal. 


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

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

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


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

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

It has been seen above that the tissues of the medulla develop and 
remain in partnership with elements of the autonomic nervous system. 
The autonomic system controls the vital organs of the body; and, when 
necessary, its call to action is reinforced by the adrenin poured into 
the blood, thus reaching all the organs of the body in less time than it 
takes to tell it. As a result, the heart beats faster, the liver releases 
additional carbohydrate for fuel, and increased oxygen intake occurs. 
At the same time, the blood vessels supplying the skin and digestive 
organs contract so that more blood laden with essential materials can 
be sent to the muscles. In short, under the stimulus of adrenin all 
possible is done by the body tissues 1 in order to permit the maximum 
amount of carbohydrate metabolism in the muscles, with a corre- 
sponding release of energy made available for a foot race, a ball game, 
or the more serious duties with which everyone is confronted from time 
to time. 


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


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

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

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

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

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

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


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

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

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

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

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



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

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

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

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

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


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

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

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

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



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

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

/o pSpc 8 

w Spt p8pt 

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

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


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




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

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

















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


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

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


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




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








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

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


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

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


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


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


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

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

The important deaminization process in the liver should be con- 
sidered in connection with the formation of urea. It will be remem- 
bered from the previous discussion that when an excess of amino acids 
occurs in the blood, the liver splits off the NH 2 fraction and converts 
the remainder of the amino acid molecule into the carbohydrate, 
glycogen, which may be used for fuel (page 57). The NH 2 group 
split off from the amino acid is further changed to ammonia (NH 3 ) 
and united with carbon dioxide to form urea and water, as shown by 
the equation: 2NH 3 + CO 2 = CO(NH 2 ) 2 + H 2 O. 


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


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

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

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

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






















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


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

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

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

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


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

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

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

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




I. Loss of Matter. 

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

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

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

4. The tissues: constructive material (variable especially 

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

II. Loss of Heat. 

1. The skin. 

2. The lungs. 

3. The excretions by the kidney and the alimentary 



I. Gain of Matter. 

1. The lungs: oxygen (fairly constant). 

2. The alimentary canal: food (variable). 

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

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

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

according to the activity of the several tissues) . 

II. Gain of Heat. 

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

such as the muscles. 

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

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


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

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

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

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


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

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

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

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


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

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

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

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


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

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



































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


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

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

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

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




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

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

FIG. 64. 

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

FIG. 65. 

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

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

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


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


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

. . . marine jellyfish, 

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

' } J > t 9 diameter of three 

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


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



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

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

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


per cent 

Carbon dioxide, 
per cent 

per cent 










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

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

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

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


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

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


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

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



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

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

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



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

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


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

U^AI vw ancC,. 

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

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


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

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

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



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

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













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

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



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

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












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

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

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



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

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


FIG. 72- 



-Drawing illustrating the finer structure of the heart valves. 

and 77. 

Cf. Figs. 71 

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

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



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


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

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

A capillary 

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


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



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

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
















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

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


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

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


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



























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


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

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

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

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



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

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




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

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

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



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


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


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


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

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



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

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

Z N 3 | 

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

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

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



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

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

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

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


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

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

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

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



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

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

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

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


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

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

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

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


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


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

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



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











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

55 cc. of carbon dioxide is present in 100 cc. of venous blood. Of 
this amount, it is believed that not more than 2.5 cc. can be carried in 
the plasma without chemical change. An additional 20 cc. is carried 
in the plasma either as carbonic acid (H 2 CO 3 ) or sodium bicarbonate 
(NaHCO 3 ). The method used for the transportation of the remainder 
of the carbon dioxide, approximating 60 per cent of the total, presents 
difficult problems. 

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


its acidity in relation to the amount of oxygen it is carrying. Oxy- 
hemoglobin, formed in the lungs where there is an abundance of 
oxygen, is more acid than is hemoglobin with the reduced amount of 
oxygen. In the lungs the newly formed oxyhemoglobin combines 
with potassium in the red cells to form potassium hemoglobinate, 
and the amount formed of this compound is directly proportional to 
the increased acidity of the oxyhemoglobin. When the acidity of the 
hemoglobin is reduced in the tissues as the oxygen is taken up, some of 
the potassium is released, and the carbon dioxide received from the 
tissues enters the blood cells and combines with potassium to form 
potassium bicarbonate. In the lungs, when the more strongly acid 
oxyhemoglobin is again formed in the red cells, the potassium bicar- 
bonate breaks down, and the carbon dioxide is released for elimination. 
Also involved in this picture of carbon dioxide transportation is 
the so-called chloride shift 1 between the plasma and the red cells 
(page 91). 

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

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


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

1 Consult Appendix : Chloride Shift. 


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

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

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

1 Consult Appendix: Hydrogen Ion Concentration. 


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

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

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

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


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

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


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

The blood clot represents the culmination of a complicated series of 



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

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

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

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

and allowed to stand in a COn- 

x *ii i * 

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

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

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



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

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


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




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

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


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

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


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

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
































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



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

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


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




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

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

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


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



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

Dorsal vessel 
Chloragogen cells 



! Ectoderm 

' Circular muscle 

longitudinal muscle 


i i. -A- ' ' /Ventral \ Ventral A EnterO n 

Nephndmm / vessel \ nerve cord anceron 


Lateral vessel Subneural vessel 

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

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


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


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

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

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



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


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

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

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

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


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

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

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



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

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

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

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


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


. : - lP^ 

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

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

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



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



ob c 

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

The fact that each transverse striation continues across the fiber 

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

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


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

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

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


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

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



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

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

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

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


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

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


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


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

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

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

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

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


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

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

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

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



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

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

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

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



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


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

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

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



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

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

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

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


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

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

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


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

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



































A B 

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


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


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

endoskeleton (page 26). 



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




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

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

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


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



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

, - .-.,',;* 


. , : . . , '.',\ 

' ' ; ; , ;-J 

' . ...'.- 'X.U 

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

FIG. 98. Vertical soction through human skin showing microscopic structure of hau' 
and hair follicle. (Maximow-Bloom, "Histology" W. B. Saunders Company.) 


certain conditions cause a contraction of the attached muscles through 
nerve impulses. 

Examined microscopically, a hair is found to consist of two regions. 
It is covered externally by a layer of very thin cells, irregular in outline, 
which form a tile-like covering. The main body, or cortical portion, 
of the hair consists of dense, horny material, the keratin, 1 which has 
developed through the transformation of the constituent epithelial 
cells. Keratin is an important protein substance, widely distributed 
in exoskeletal structures. The amount and color of the pigment 
present in the keratin of the hair cortex varies greatly, and also air 

1 Consult Appendix: Keratin. 


spaces are not uncommon toward the center. Hair color depends 
upon the amount and quality of the pigment present and its relation 
to the transparent air spaces. In hairs with a heavier body, such as 
those of the beard, a definite central area, the medulla, is usually 
noted throughout the length of the hair. This region is characterized 
by an irregular cellular arrangement and the presence of large air 
spaces. (Fig. 98.) 

Closely related to the hair in development and structure are the 
nails of the fingers and toes, as well as the claws of lower types of 
mammals. Nails are formed from adhering keratinized epithelial 
cells of essentially the same nature as those which form hairs. The 
living tissue, which is continually forming and giving off these cells, 
lies in a fold at the root of each nail and also underneath the nail 
where it forms the nail bed. The nail is bounded on each side by the 
nail groove. Receiving additions in length by the additions of cells 
at the base and in thickness by those added underneath, the nail is 
gradually pushed forward and projects beyond the nail'bed at the tip 
of each digit. This process, unlike that of the hair follicle, is continu- 
ous throughout life, and the nail can grow indefinitely in length if left 


The bony endoskeletal system is a unique feature of the verte- 
brates. Invertebrate animals, like the insect, must attach their 
developed muscles to the inner surfaces of the nonliving exoskeleton 
and detach them periodically when the shell is shed during moulting. 
The permanent endoskelcton of the vertebrate serves largely as an 
inner supporting material, and the soft tissues can thus develop on 
the outside of the endoskeletori. It may also develop outside the soft 
tissues and serve for protection, a relationship best shown in the brain 
and skull. Though the endoskeletal tissues are relatively inactive as 
compared with the other body tissues, they are basically living tissues. 
And, by way of exception, it should be noted that a very active tissue 
is supplied in bone marrow. 

Though wide variety exists in the types of tissue associated 
together in the endoskeletal system, yet, fundamentally, all of them 
are united in the possession to a greater or less degree of a characteristic 
structural material, collagen, lying between the connective tissue 
cells, which is formed as an intercellular secretion. Collagen is pro- 
teinaceous, typically fibrillar, and comparable in its wide distribution 
in the vertebrate body (but not in its chemical composition) to the 
ubiquitous carbohydrate, cellulose, of the plant world. The endo- 


skeletal tissue system may appropriately be grouped under the term 
collagenous. Lying embedded in the collagenous ground substance 
of the connective and supporting tissues arc the cells that appear to 
be primarily responsible for the synthesis and secretion of collagen. 
These cells, constituting the so-called fibroblasts, arc typically seen in 
developing connective tissues as elongated spindle-shaped bodies, but 
both the structural and functional characteristics of the fibroblasts 
are subject to wide variation in the widely divergent types of endo- 
skeletal tissues. (Figs. 19, 99.) 

FIG. 99. Bundles of collagenous fibers in ground substance of white fibrous tissue. 


The embryologist studying the origin of the endoskelctal system 
in the embryo sees a unity throughout the various types of tissue as 
they gradually differentiate during development. The endoskeleton 
begins with a gelatinous type of embryonic connective tissue filling 
various cavities in the embryo between the germ layers. It continues 
with the formation of fibrous tissues in which distinctive bundles of 
collagenous fibers are abundantly present in the ground substance, as 
in white fibrous tissue or tendons. Later, cartilage appears in various 
regions, with a more rigid gel-condition of the intercellular material 
which may be either fibrous or homogenous in nature. Finally, bone 
tissue is formed largely by the transformation of the cartilage through 
the deposition of inorganic salts, notably calcium and phosphorus. 
Certain specialized types of connective tissue, as found in elastic and 



adipose tissue, contain a considerable amount of collagen but seem to 
be outside the main routes of endoskeletal development that culminate 
in the formation of bone. Detailed consideration may now be given 
to the structural and functional features of bone as the culmination of 


( Brain case 

Cranium -(Olfactory capsules 
( Auditory capsules 



^ i ( Upper jaw 

Visceral 1 T 
. , , < Lower jaw 
skeleton ' 

( Hyoid and larynx 
Vertebral column (including ribs when present) 





Scapula, suprascapula, cora- 
coid, procoracoid, epicora- 
coid, clavicle, episternum, 
omosternum, sternum, and 

Free limb 





Carpal s 






FIG. 100. The main division* of the bony vertebrate skeleton. (Reed and Young.} 

the endoskeletal system in man and all the vertebrates, with the excep- 
tion of the cartilaginous fish. 


The bony skeleton shows wide variation in the different classes of 
vertebrates in accordance with the size of the body and the particular 


environment for which a certain group is adapted. Thus the fish and 
other aquatic vertebrates are able to move freely in the water by a 
rhythmic back-and-forth movement of the body and tail, but this type 
of locomotor apparatus is of no use for air-living birds or for a bipedal 
and living vertebrate. Nevertheless, the basic resemblances of the 
vertebrate skeleton are much more apparent than the variations, 
which are of relatively minor importance. Thus all vertebrate 

Axial skeleton 

Vertebral column: 

Vertebrae 24 

Sacrum 1 

Coccyx 1 


Cranium 8 

Facial portion 14 

Neck and chest regions: 

Hyoid 1 

Sternum 1 

Ribs 24 

Appendicular skeleton 


Hands 28 

Wrists 26 

Arms 6 

Shoulder girdles 4 


Feet 28 

Ankles 24 

Legs 6 

Kneecaps 2 

Pelvic girdles 2 

Total 200 

skeletons are found to consist of two basic divisions: the axial skeleton, 
consisting of the skull and vertebral column; and the appcndicular 
skeleton, consisting of the fore and hind limbs and their respective 
girdles, which connect the limbs to the axial skeleton. Furthermore, 
throughout the classes of vertebrate animals it is possible to homologize 
the bones associated with a particular region or organ in one animal 
with those present in widely varying animal types. Comparative 
anatomists have long regarded the vertebrate skeleton as one of the 
most favorable organ systems for the study of homologies. l (Plate XI ; 
Fig. 100.) 

The number of separate bones in the skeleton is subject to wide 
variation in the different vertebrate classes. The variation ispartic- 

1 Consult Appendix: Comparative Anatomy. 


ularly evident in the tail region and in the bones of the hands and 
feet. Many vertebrates of high development have a long caudal 
appendage, the tail. Thus, the cat, for example, has 22 vertebral 
bones in the tail. In the birds, the forelimbs are modified as wings 
instead of hands, and the number of bones in a wing has been some- 
what reduced as compared with the typical pentadactyl appendage 
of man. The reduction of bones in the fore and hind limbs is even 
more evident in certain of the hoofed mammals (Ungulata). The 
horse, for example, retains only one functional digit on each of the 
four limbs. The number of bones in the adult human skeleton is 
usually given as 200, but 6 more bones may be added by including the 
3 tiny ossicles in each ear. The distribution of bones in the skeleton 
of man is summarized on page 195. 


Vertebral Column. One of the characteristics of the great phylum 
Chordata, to which man and the other vertebrate animals belong, is 
a longitudinal cylindrical rod, the notochord, situated near the dorsal 
surface of the body and continuing throughout the entire length. 
The notochord thus serves as a primary supporting axis. In the 
lowest vertebrates, the Cyclostomata, the notochord retains its 
original characteristics; but in the higher vertebrate classes with a 
bony skeleton, the notochord is replaced by a vsegmented vertebral 
column built up of individual vertebrae. In the more primitive types, 
each vertebra consists of a solid disc of bony tissue, the centrum, and 
these, placed end to end and held in place by ligaments of connective 
tissue, compose the segmented, rod-like vertebral columns for the 
general support of the body and the attachment of muscles. (Fig. 

The vertebral column, seen in man and most of the vertebrates, 
has developed additional bony structures which are arranged to supply 
great protection to an essential portion of the nervous system, the 
spinal cord. The most important of the new structures of the verte- 
brae consists of a bony neural arch which develops dorsally from each 
centrum. The cavity between the arch and the centrum is the verte- 
bral canal, a well-protected area in which the spinal cord lies. Pro- 
jecting from each neural arch are the dorsal and lateral articulating 
processes which are of great importance in the articulation of separate 
vertebrae with each other in forming the vertebral column and also 
in their availability for the attachment of muscles. Thus the back- 
bone, or vertebral column, is composed of independent articulated 
units, the vertebrae, each of which is essential in the support of the 



body and each also contributes, by means of the neural arch, a portion 
of the common vertebral canal for the protection of the spinal cord. 
The vertebral column, as a whole, gives firm support to the major 
axis of the body and at the same time permits considerable freedom 
of movement. (Plate XI; Fig. 101.) 

















Fia. 101. Skull and vertebral column of man, side view. Knd views of three vertebra 
are shown at the right. Skull is sectioned to show interior structure. 

Five regions are recognized in the vertebral column of the higher 
vertebrates, which, beginning at the anterior end, are known as the 
neck region (cervical), chest region (thoracic), abdominal region (lum- 
bar), pelvic region (sacral), and tail region (caudal). In the backbone 
of the human adult there are 26 vertebrae divided as follows : 7 cervical ; 
12 thoracic, with a pair of ribs attached to each; 5 lumbar; 1 sacral; 
and 1 caudal. In the early stages of development, 33 separate verte- 
brae are present. The vertebral reduction in the adult is due to a 
fusion of five sacral vertebrae to form one sacral (sacrum), and a fusion 
of four caudal vertebrae to form one caudal (coccyx). 



Skull. The vertebrate skull is an extraordinarily complex assem- 
bly of bone units designed to offer adequate protection to the most 
delicate organ of the body, the brain. The skull consists primarily 
of (1) a brain case, or cranium, enclosing the brain proper and also 
providing places of refuge for the important sense organs of the body 
eyes, nose, ears and (2) a facial portion built around the mouth 
and provided with a masticating apparatus which involves an intricate 
assemblage of bony parts and attached muscles. Studied compara- 



masto id process 
inferior max/7 /ary 


FIG. 102. Comparison of frog skull and human skull. Siclo views, 
reduced to comparable size. (Wieman.) 

Human skull 

tively, the skulls of the different vertebrate groups reveal considerable 
variety in the size relations existing between the cranial and facial 
portions. The skull of a lower vertebrate, as in the frog, shows a 
comparatively large facial portion and a vety small cranium. The 
opposite condition, with a greatly reduced facial portion and a large 
cranium to provide adequate quarters for the enlarged brain, is seen 
to best advantage in the human skull. (Fig. 102.) 

The 22 bones present in the adult human skull represent a consider- 
able fusion of the bones originally present in the embryonic skull. The 
arrangement of the eight cranial bones forming the cranium may now 
be considered, beginning with the large unpaired occipital bone which 
forms the floor of the cranium. It is characterized by a large central 


opening, the foramen magnum, through which the spinal cord enters 
the cranium and connects with the brain. The occipital bone is con- 
tinued well up on the back of the skull where it joins a pair of large 
parietal bones which form the roof of the skull. Forming the ear 
region on each side of the head is a temporal bone. It is bounded 
posteriorly by the occipital, dorsally by the parietal, and anteriorly 
by the sphenoid; the latter, together with the small ethmoid, form the 
floor of the cranium anterior to the occipital region. The frontal 
bone, continuing anteriorly from the parietal, forms the forehead and, 
laterally, at about the level of the eye, joins with the sphenoid and 
ethmoid. (Fig. 101.) 

In the facial portion of the skull the 14 constituent bones, well- 
covered with the muscles of the jaw and face, occur in pairs, with two 
exceptions. The exceptions are the single vomer, which separates the 
right and left nostrils, and the lower jaw, or mandible. The remaining 
six pairs of facial bones include, first, the pair of maxillae which form 
the upper jaw. The two members of the pair meet in the mid-line 
of the face and continue posteriorly as the hard palate, which forms 
the roof of the mouth and separates the mouth cavity from the nasal 
cavity just above. Also included are (a) the palatine bones which join 
with the maxillae posteriorly and continue the bony wall into the 
throat region; (b) the cheek bones (malar) which join with a maxilla 
bone on each side of the head, dorsally with the frontal and, posteriorly, 
with a projection of the temporal to form the zygomatic arch which 
articulates with lower jaw in the ear region; and, finally, (c) a pair each 
of nasal, lachrymal, and turbinate bones which together form the roof 
and side walls of the nose. 

Teeth. Strictly speaking, the vertebrate teeth are exoskeletal 
rather than endoskeletal structures, but their close association with the 
bony jaws makes it advisable to consider them in connection with 
the skull. Teeth are typically composed of (1) a thin outer covering, 
the enamel, which develops as a secretion from the invaginated 
epithelial cells of the mouth cavity lying over the jaws; and (2) 
dentine which develops from the underlying mcsodermal tissue. The 
latter forms by far the greater amount of material in a tooth. In the 
center of the dentine area is a pulp cavity with vascular and nerve 
elements. Teeth in the various vertebrate classes are subject to con- 
siderable variation in number, shape, and function. In man and the 
higher vertebrates, a considerable portion of each tooth is embedded 
in a deep pit, or alveolus, in the jaw bone and firmly fastened by a 
bone-like cement substance. The portion of the tooth within an 
alveolus is known as the root; and the exposed, enamel-covered portion 


is termed the crown. Four types of tooth are recognized in the human 
jaw, which, beginning anteriorly, are designated as the incisors, 
canines, premolars, and molars. (Fig. 27.) 

Commonly two sets of teeth are formed. The first set, or milk 
teeth, are replaced in the early years of childhood by the permanent 
teeth which begin to develop in the jaws before the milk teeth are lost 
and erupt shortly afterward. The number of milk teeth varies some- 
what, with a normal of 20, while in the permanent set there are 32 
teeth, divided so that each half of each jaw contains a total of eight 
teeth, including two incisors, one canine, two premolars, and three 
molars. The third molars or wisdom teeth are frequently delayed in 
their appearance or fail entirely to erupt, with consequent abnormal 
conditions. In order to state concisely the number of teeth in the 
various animals, a dental formula is made use of, in which the letters 
/, C, P, and M are used to indicate the incisors, canines, premolars, 
and molars, respectively, and the numbers of each of these teeth in 
half of the upper and lower j aws are shown by figures above and below 
a division line. Thus the dental formula of man is shown as /%, 
CM, P%, M% = 32. 

Neck and Chest. Each of the 24 ribs may be described as a 
slender bone gracefully curved to form the circular chest wall. All of 
the ribs are attached dorsally to the transverse process of the corre- 
sponding thoracic vertebra. The anterior seven pairs of ribs are 
attached vent rally by means of a short strip of cartilage to a median 
unpaired dagger-shaped bone, the sternum, which forms the "key- 
stone" of the thoracic arch surrounding the chest cavity. The 
eighth, ninth, and tenth pairs of ribs are also supplied ventrally with 
a terminal strip of cartilage, but the latter, in these ribs, is not con- 
nected directly with the sternum but with the cartilage of the rib just 
anterior. The ends of the eleventh and twelfth pairs of ribs have no 
ventral attachment but end freely in the muscle tissue of the body 
wall. In the neck region, a U-shaped hyoid bone lying just anterior 
to the larynx is without great functional significance in man, but great 
interest is associated with it from the standpoint of comparative 
anatomy because of its condition in the lower vertebrates. (Plate 
XLi, B.) 


The appendicular skeleton of man consists of the paired penta- 
dactyl fore limbs and hind limbs and the girdles by which these 
appendages are attached to the axial skeleton. (Plate XI.) 


Forelimb and Pe.ctoral Girdle.^-The arm bones consist, first, of a 
large humerus which articulates proximally with the shoulder girdle. 
The distal end of the humerus is in contact at the elbow with two bones, 
the radius and the ulna, both of which continue * to the wrist 
(carpus). The wrist consists of an aggregation of eight small carpal 
bones, followed by the hand proper with its quota of 19 bones. Five of 
the hand bones (metacarpals), which are articulated with the bones of 
the wrist, are long and slender and form the undivided main portion 
of the hand. Finally, the five digits contain a total of 14 phalanges, 
with three for each of the fingers but only two for the thumb, which 
has one less joint. The shoulder, or pectoral girdle of man has only 
two bones. There is, first, the flat, dorsal shoulder blade, or scapula, 
which is firmly anchored in the shoulder muscles but not actually 
fused with the axial skeleton. The scapula is joined at the shoulder 
by the collar bone (clavicle). The latter is slender, rod-shaped and 
extends anteriorly to the head of the dagger-shaped sternum. Articu- 
lation between arm and girdle is by a special cavity, the glenoid fossa, 
into which the head of the humerus fits to form the shoulder joint. 
(Plate XI A.) 

Hindlimb and Pelvic Girdle. The bony structure of the leg is 
very close to that of the arm, but the names given to bones are dif- 
ferent. There is, first, the large thigh bone, or femur which corre- 
sponds to the humerus of the arm and articulates at the knee with the 
tibia arid fibula. The latter are homologous with the radius and 
ulna. An additional bone, the kneecap or patella, gives added protec- 
tion to the important knee joint. In the ankle (tarsus) arc only seven 
bones, one of which, the calcaneum, is very large and forms the heel 
proper. The other tarsal bones are closely assembled, so that only a 
slight amount of movement is possible, and fastened with ligaments 
in such a manner that the inner portion of a normal foot is elevated 
to form the arch. The flat-footed condition represents a harmful 
breakdown of the normal tarsal assembly. The five metatarsal bones 
form the body of the foot and extend to the base of the toes. As in 
the hand, there is a total of 14 phalanges in the digits, the big toe 
lacking one joint and the corresponding phalangeal bone. Compared 
with the fingers, the toes show a considerable reduction in length and 
in functional adaptability. And the big toe is not opposable to the 
other digits as is the thumb with the four fingers of the hand. Each 
pelvic girdle consists of a single bone (os innominatum) which repre- 
sents a fusion of three embryonic bones. The pelvic girdle and the 
axial skeleton are firmly united by bone tissue in the sacral region of 
the vertebral column. A deep cavity, the acetabulum, is present in the 



girdle, and this receives the proximal end of the femur, thus forming 
the ball-and-socket hip joint noted below. (Plate XL4..) 

Variation in the Appendages of Vertebrates. The paired fins of 
the bony fish do not appear at first glance to show much resemblance 
to the pentadactyl appendage but nevertheless are regarded as the 
original type from which the others have arisen. The greatest 
amount of variation between the legs and arms of vertebrates .is 
found in the bones of the wrist and hand and those of ankle and 
foot. The three large b mes, which form, the main axis of an append- 
age, are relatively constant in their structure throughout the 
vertebrate series. Thus even in a highly modified appendage, as 

FIG. 103. Comparison of the bones in the forelimbs of man, dog, horse, and bird. 
Dotted lines connect the homologous bones. Appendages arc not drawn to scale. 
(Watkeya, Stern.) 

found in the forelimb of the birds, the humerus, radius, and ulna 
remain essentially typical in their structure, but the bones of the wrist 
and hand region show marked structural modifications as well as a 
reduction in numbers, changes that better adapt them to the needs of 
a wing designed for aerial locomotion. (Fig. 103.) 

In the hoofed mammals (Ungulata), both the fore- and the hind- 
limbs have departed more widely from the pentadactyl types than 
have those of most other vertebrates. Thus, for example, in the 
forelimb of the horse, the humerus is about the only bone that does 
not show wide departure from the typical condition. Only a small 
portion of the ulna remains, and it is fused with the radius. Digits 
I and V have entirely disappeared. Remnants of digits II and IV are 
present as vestigial structures, the splint bones. Digit III, essentially 
complete in its bony elements, is the only functional digit in both the 
fore- and hindlimbs, so that in locomotion the horse uses only the tip 
of the third digit, completely covered with the hoof. In the aquatic 


mammal, the whale, both the hind limbs and the girdles have almost 
completely disappeared and are not functional, whereas the furelimbs 
have become modified for locomotion in water. Even more complete 
reduction in limb structure is seen in snakes, where a complete dis- 
appearance of all functional appendagcmhas occurred in practically 
all species. The appendages of the primjles, as shown in the descrip- 
tion of the human appendages above, have remained true to the typical 
pentadactyl type, and only minor modifications and fusion of certain 
girdle bones and those in the arms and feet are in evidence. And so 
the vertebrate appendages are adapted for "all walks of life." 

Joints. The human skeleton shows various methods for the 
articulation of the 200 separate bones of which it is composed. In some 
places, as, for example, in the cranium, the bones are so rigidly articu- 
lated by toothed edges, which fit into corresponding depressions, that 
no movement between them is possible. In the vertebral column, a 
certain amount of flexibility is introduced between the separate 
vertebrae by the smoot'i articulating sin-races present on the bony 
processes of the neural arches which are nicely fitted to each other. 
In addition, pads of elastic cartilage are situated between the centra 
of the apposed vertebrae, which can be compressed in response to 
the bending movements of the trunk region. (Fig. 101.) 

We now come to the articulating surfaces of bones which are 
definitely associated with movement of the appendages. Such articu- 
lations are known as joints, and several distinct types may be indicated. 
The least differentiated type of joint and one that affords compara- 
tively little opportunity for movement is the gliding joint found in 
the bones of the wrist and ankle. The articulating bones arc fastened 
so closely by connective tissue ligaments that only a slight gliding 
movement is permitted. Much better developed are the hinge joints 
of the fingers, toes, and knees, which permit a wide latitude of back- 
and-forth movement in the same plane, like the swinging of a door. 
The very important hinge joint in the knee is protected by an addi- 
tional bony element, the kneecap (patella). A modification of the 
hinge joint which permits rotation of the hand, in addition to the 
back-and-forth movement, is found in the pivot joint at the elbow. 
The radius and ulna are so articulated with the humerus at the elbow 
that, when the palm of the hand is turned up, the distal end of the 
radius revolves around the ulna, thus following the thumb as it changes 
position. (Fig. 87.) 

The ball-and-socket type of joint permits the greatest freedom of 
movement. It is best shown in the large hip joints which are able to 
support the weight of the body and also a considerable additional 



weight when necessary; at the same time the hip joints permit the 
varying leg movements essential to locomotion. In the hip joint is 
an airtight fit between the almost spherical head of the femur and the 
deep socket, acetabulum, in the pelvic girdle. The fit is so perfect 
that, even with all the competing ligaments removed, the femur will 
be held in place by the atidHpheric pressure. Ball-and-socket joints 
of lesser degree are found in the articulation of the humerus with the 
shoulder girdle and also between the metacarpal bone of the thumb 

FIG. 104. Section through the shoulder joint of man. (Haggard, u Science of Health 
and Disease," Harper & Brothers.) 

and the wrist. This ball-and-socket thumb joint permits this impor- 
tant digit to be placed in opposition to the ends of the other digits. 
This movement is not present in the less adaptable foot, where the big 
toe has only a hinge, rather than a ball-and-socket, joint. The articu- 
lating surfaces of the hinge and ball-and-socket joints, in which bone 
movements are extensive, are covered with a layer of smooth hyaline 
cartilage. In addition, the cartilaginous surfaces which are in contact 
are in turn covered by a very thin synovial membrane supplied with 
secreting cells that continually secrete a synovial fluid for joint 
lubrication. (Fig. 104.) 

Finally, mention should be made of a modified pivot joint, forming 
the connection between the head and the vertebral column, which per- 


mits wide latitude in head movements. The occipital bone at the base 
of the head bears two smooth articulating surfaces, the occipital con- 
dyles, near the foramen magnum. The first vertebra (atlas) articulates 
with these to give back-and-forth movements. In rotary head move- 
ments, the atlas is aided by the special odontoid process of the second 
vertebra (axis) which extends upward through the atlas and functions 
as a pivot or the rotary movements of the head and atlas. 

Development of Bone. Bones develop as a result of the deposition 
of inorganic salts in the original soft skeletal elements. Two types of 
bones are recognized. First there are membrane bones that are formed 
by the gradual transformation and hardening of soft connective tissue 
membranes as typically form the basis for the membrane bones of the 
skull. Outside of the skull bones, however, practically all of the bones 
in the human skeleton arise as modifications of cartilage and are, 
therefore, known as cartilage bones. It sounds simple enough to say 
that connective tissue membranes and cartilage are transformed into 
bone, but, as a matter of fact, the chemical and structural changes 
necessary for such a transformation are very complex and not fully 
explained as yet. Two types of bone-forming cell are involved. One 
type, the osteoblasts, is able to absorb the needed inorganic salts from 
the blood stream and *) use them in the building of bone tissue. The 
other type of bone cell, the osteoclasts, is charged with the duty of 
remodeling the original connective tissue model to conform to the new 
bone-tissue requirements. This remodeling involves the actual 
destruction of certain portions of the original cartilage, presumably by 
the use of specific enzymes. In this way, the central cavity of bone, 
which contains the highly vascularized bone marrow, is formed. 

It must be remembered, too, that the tiny bones, as first laid down 
in the embryo, must increase in size to correspond to the general body 
growth, and this growth of the bones must continue until maturity 
is reached. Increase in the length of a bone occurs principally at each 
end, where new cartilage is continually being formed and, then, 
gradually ossified. Growth also occurs at the bone surface through the 
action of an outer connective tissue covering, the periosteum, which 
supplies bone-forming cells and also the materials for the formation 
of new bone tissue. The periosteum continues, when necessary, to 
function in this manner throughout life, as in the case of bone fractures. 
The general size and shape of a bone, as well as its microscopic struc- 
ture, reveal a high degree of adaptation to the exact functional needs 
of the region in which it is found. This is not merely a passive relation, 
for bones continue to change all through life in accordance with the 
structural needs that develop. 


The examination of a typical bone, such as the long thigh bone 
(femur) of the leg, shows that it consists of a main portion, or shaft 
(diaphysis), with a terminal enlargement at each end which forms the 
articulating surfaces, or joints, with the connecting bones. The joints 
as noted, are covered with a smooth cartilage which affords the best 
possible type of all the tissues for articulating surfaces. The cartilage 
at the joints never becomes ossified. The entire bone, with the excep- 
tion of the articulating surfaces, is covered by a closely applied sheet of 
specialized connective tissue, the periosteum, which was mentioned 
above in connection with bone formation and regeneration. It can be 
shown that bunches of the connective tissue fibrils from the periosteum 
penetrate deeply into the underlying bone tissue. This penetration 
of various functional tissues by an outer layer of connective tissue is 
characteristic also of the muscles and nerves. (Fig. 89.) 

If the femur is split in halves lengthwise, it will be found that 
the bone is hollow throughout almost its entire length and ; therefore, 
the bony tissue really consists of a relatively thin outer shell surround- 
ing a central cavity. This is the marrow cavity, filled with the soft 
pinkish bone marrow and abundantly supplied with blood vessels and 
nerves. Bone marrow functions not as an endoskeletal tissue but as a 
vascular tissue primarily concerned with the formation of red blood 
cells. The marrow cavity is also lined by a layer of connective tissue, 
the endosteum. An examination of the cut surface of the bone tissue 
will show, even with the naked eye, that there is a differentiation into 
a compact bone tissue forming the walls of the shaft and a so-called 
spongy bone tissue at each end. In the spongy tissue, the bone fibers 
can be seen extending in various directions, crossing and supporting 
each other. Careful study of the arrangement of the bony fibers in 
the spongy bone tissue shows very clearly that they are arranged 
according to the best engineering principles to give the utmost strength 
with the least use of material. These- same excellent engineering 
features apply to the bone as a whole. (Kg. 104.) 

Histology of Bone. The microscopic examination of bone tissue 
reveals an extraordinarily intricate arrangement of the intercellular 
matrix forming the mass of bone tissue, with embedded bone cells and 
a puzzling array of interconnecting channels of various kinds and 
sizes. The complexity of mature bone is particularly striking when 
it is compared with the very simple arrangement of the cartilage 
elements from which it develops. In order to study bone tissue under 
the microscope, it is necessary to take small fragments and carefully 
grind them down by hand until they are thin enough to be transparent. 
After grinding, the bone fragments can be mounted on a slide for 



detailed examination. Under the low power of the microscope, con- 
siderable structural differentiation in the bone tissue is apparent. 
The general picture of bone structure, as revealed under the microscope, 
reminds one somewhat of the cyclonic areas in a weather map with 
concentric lines curving around the low-pressure areas. (Fig. 105.) 

The curved areas in the bone tissue are the lamellae and consist 
of concentric layers, or plates, of bone tissue. In the center of each of 
these areas of bony tissue, as seen in a transverse section, is the circular 
opening of the Haversian canal, measuring about 0.002 in. in diameter. 






FIG. 105. Microscopic structure of bone. (Buchanan, "Elements of Biolofjy," Harper 

& Brothers.) 

For the most part, the Haversian canals run lengthwise of the bone, 
but connecting canals, extending transversely, are also found, some 
of which continue to the surface of the bone tissue and open under- 
neath the periosteal covering. Altogether the Haversian canals form 
a ramifying tubular network throughout the bone tissue in which the 
blood vessels extending from the periosteum can enter and supply 
all regions of the bones. 

The examination of bone under a higher magnification reveals 
additional important structural elements. Many tiny cavities 
(lacunae) are revealed, each containing a living bone cell (osteoblast). 
Extending from each lacuna are many exceedingly minute channels 
which pursue irregular winding courses through the bone tissue and 
open either directly into one of the large Haversian canals or into other 
canaliculi that so open. Thus blood plasma, exuded from the tiny 
vessels in the Haversian canals, is carried by the connecting canaliculi 
to all the bone cells. It is estimated that areas of bone tissue exceed- 



ing 0.00004 in. in diameter are not found without being supplied with 
blood by a canaliculus. Directly under the periosteum, the lamellae 
and accompanying canals encircle the bones to form rings of bony 
tissue rather than longitudinal elements. 

It is apparent from the description just given that bone tissue is a 
living tissue with many cells and a network of large and small channels 
for the distribution of the essential materials. By placing a piece of 
bone in a weak acid for a time, it is possible to remove the calcareous 
bone materials and leave the organic collagenous material which always 

A B 

FIG. 106. Comparison of bone which has been soaked in acid (A) and bone which has 

been burned (B). 

forms the underlying framework. A bone treated with acid is flexible 
and can be tied in a knot without breaking. On the other hand, if 
a bone is burned, the organic materials will be destroyed and the 
inorganic materials will remain in their original shape if undisturbed. 
Burned bone is very fragile and will crumble to dust when handled. 
(Fig. 106.) 

Nowhere is the living character of the bony endoskeleton so clearly 
revealed as in the highly important bone marrow, which serves as a 
specialized tissue, largely concerned with the formation of the red cells 
of the blood. Bone marrow also functions in fat storage and is usually 
separated into the red marrow, in which the blood cells are formed, and 
yellow or fatty marrow in which fat accumulates. No clear histolog- 
ical difference exist between the two types, and a shift from one to the 
other may occur. Histologically, bone marrow consists of a con- 
nective tissue ground substance, or stroma, containing quantities of 
cells specialized for red cell formation, as well as fat-storage cells. 
Penetrating throughout the marrow tissue are open blood channels, 
or sinusoids, such as were previously noted in the liver. Since the 


sinusoids lack a definite wall as present in capillaries, newly formed, 
nonmotile red cells are able to pass directly into the circulating blood 
as the latter slowly moves through the sinusoids of the bone marrow. 
Small blood vessels enter and leave the bone by way of the periosteum 
and Haversian canals, but the blood supply of the bone marrow is 
received directly by large vessels which penetrate the bone tissue by 
definite openings, the foramina. 


The skeletal system is commonly thought of as being wholly con- 
cerned with the functions of protection, support, and muscle leverage, 
but at least two other functional features of this system are inseparably 
associated, namely, the formation of the blood cells in the bone marrow 
and the storage of calcium by the bone tissue. 
These five functions may now be considered in 
the order named. 

Protection and Support. Protection of the 
delicate tissues and organs is primarily a func- 
tion of the exoskeletal elements. This is par- > 
ticularly evident in such invertebrates as the the^t^^ 
clam and the insect. The function of support functions of protection and 

f ,i i i .. .-i support in the skeletal 

for the body tissues appears as the major sy * t p em< In the higher 
function of the vertebrate endoskeleton. animals (right) the sup- 

mi_ i 11 i i j.' i i x port becomes increasingly 

Thus, broadly speaking, relatively less protec- important. (Waiter.) 
tion is offered to the living tissues of the higher 

animals than to the lower types. In man, the greatly reduced exo- 
skeletal elements offer a minimum of protection, the living tissues 
being mostly covered by a few layers of epithelial cells. However, 
the structural development and functional importance of the central 
nervous system require that it be well protected, and this has been 
accomplished by the bony endoskeleton as was indicated above in the 
study of the skull and vertebral column (page 196). And so the endo- 
skeleton, primarily functioning for support, has also been assigned the 
protection of the central nervous system, the most important and 
delicate structure in the vertebrate body. (Fig. 107.) 

Movement. The adaptation of the bones and associated connec- 
tive tissues in the various vertebrate types, so as to serve with the 
contractile muscle tissues in providing for the numerous essential 
body movements and for efficient locomotion in water, in air, or on 
land, has reached great heights in the higher vertebrates but nowhere 
to so marked a degree as in the human skeleton. This skeletal superi- 
ority is largely centered in the human arm and hand, in which the com- 
bination of skeletal* contractile, and nerve elements has made an 


unrivaled living tool capable of coping with the myriads of duties that 
man finds necessary. It is also apparent in the skeletal changes in 
legs and bony girdles that make two4egged, bipedal, locomotion 
possible. A difference exists in the skeletal requirements for move- 
ment in an organ that remains stationary as compared with the require- 
ments in which a part of the body is moved to a new location. Thus 
the involuntary muscles present in the ducts of glands, the alimentary 
canal, and the walls of blood vessels require only flexible fibrous tissues 
for support and leverage and for binding them into compact functional 
units adapted for the stationary movements associated with these 
organs. The voluntary muscles, on the other hand, move a finger, 
a foot, an arm, a leg, or the entire body to new positions, and the 
movements may be very rapid. Necessarily associated with these 
voluntary contractile tissues are the hard bones and also the mediating 
flexible fibrous tissues, the complete skeletal assembly being con- 
tinuous from the sarcolemma of the individual muscle fibers to the 
immobile bone at the muscle origin and the movable bone at the muscle 
insertion which serves as a lever for the muscle pull. 

Bones and Levers. The physicist recognizes three types of rigid 
rods or levers that are available for leverage. These mechanical aids 
are commonly known as levers of the first, second, and third class. 
The classification depends upon the relative positions of three points 
on the lever: (1) the fixed point of support or attachment (fulcrum), 
(2) the point at which force is applied to move the lever, and (3) the 
point where work is accomplished. In levers of the first class, the 
fulcrum is situated between the other two points, as seen, for example, 
in a pair of scissors. 

Two examples may be selected to show the use of bones in the body 
as levers of the first class. Thus, when the foot is lifted and the toes 
tapped on the floor, the ankle joint is the fulcrum; the pull of the large 
gastocnemius muscle in the calf of the leg is transmitted to the heel 
bone (calcaneum) back of the fulcrum; and the point of resistance, 
where the weight is moved, is the portion of the foot anterior to the 
ankle joint fulcrum. Another example is found in the movements of 
the head in which the atlas serves as a fulcrum, lying between the 
insertion of the muscle to the occipital bone above and the origin 
below (page 188). In levers of the second class, the fulcrum is at one 
end of the lever, the force is applied at the opposite end, and the weight 
is between the other two points, a condition that is well shown in a nut- 
cracker. An instance in which bones are used as levers of the second 
class is seen when the weight of the body is raised on the toes. In 
this case, the fulcrum is at the solid surface on which the toes are 
supported; the work is accomplished at the ankle joints where the 


body weight is supported; and the force is applied dorsally at the 
calcaneum by the contraction of the gastrocnemius muscle as in 
the first example. In levers of the third class, which sacrifice power for 
speed, the fulcrum is at one end of the lever, and the weight at the 
opposite end so that the force is applied between fulcrum and weight. 
The author notes as he writes that this is the method used in mounting 
the type bars of the typewriter, which are designed to move a small 
weight rapidly. Examples in which bones are used as levers of the 
third class are well shown in the arms where the pull of the biceps 
muscle transmitted to the insertion below the elbow joint elevates the 

*\ F * r *; P- ; r /> /?/ 

Fia. 108. Illustrating the uses of bones in the three kinds of levers as described on 
page 210. /?, resistance or weight moved; F, fulcrum; P, point at which power is applied 
from muscle contraction. {Redrawn from Huxley-Barcroft.) 

forearm. The same arrangement is also seen in the extension of the 
leg by the pull of the ventral thigh muscles which is transmitted 
through the kneecap to the point of insertion below the knee. (Figs. 
87, 108.) 

Bipedal Locomotion. Generally speaking, voluntary movements 
in animals are associated with locomotion, and, in the various groups, 
the locomotor organs present wide variation in structural design in 
order to function efficiently in water, in air, or on land. It would 
seem that the two most difficult conditions to surmount in animal 
locomotion are flight through the air and bipedal locomotion on land 
by man, birds, and to some extent by a few of the higher primates. 
Bipedal locomotion involves the maintenance of the trunk and head 
of the body in an upright position, the shifting of the entire weight 
to the hind limbs, thus freeing the forelimbs for other duties. It has 
been indicated in the previous chapter that the delicate balance 
essential to the maintenance of the upright position requires that cer- 
tain muscles be kept in tone in agreement with the proprioceptive 
impulses received by the central nervous system from the outlying 
regions (page 184). More attention will be given to the nerve control 
involved in the erect posture in the following chapter. At present 
we are concerned with the complex harmony of action between the 


muscle and bone elements in human locomotion, a function that is 
learned after much effort in early childhood. 

Walking involves many muscles of the trunk and legs. When one 
starts to walk, the body is inclined or really begins to fall forward, 
which brings the center of gravity beyond the feet. Then one of the 
legs, say, the left, is flexed at the knee joint. This raises the left foot 
from the floor, and it is quickly thrust forward to support the falling 
body. Synchronously with the forward movement of the left leg, the 
gastrocnemius muscle in the calf of the right leg contracts and elevates 
the body by the pull on the heel bone, as noted in the previous para- 
graph. Then the left foot is firmly planted in its new position. At 

FIG. 109. Walking. Both FIG. 110. Running. Both feet off 

feet on the ground. the ground. 

this moment both feet are on the ground. When both feet are in the 
air at this position, the person is running rather than walking. The 
weight of the body is now shifted to the left leg, and this leaves 
the right leg free to swing forward, like a pendulum, to a new position in 
advance of the left leg. When the right leg is placed in position, the 
weight of the body is, shifted to it, and the pull of the muscles is now 
on the heel of the left leg. The elevation and flexion of the latter 
enable it to swing forward in a pendulum-like motion and to come to 
rest once more in advance of the right leg. And so the coordinated 
alternate leg motions continue. (Figs. 109, 110.) 

Blood-cell Formation. With regard to the formation of blood cells 
by the bone marrow, the concensus of opinion at the present time seems 
to be that the tissues of the bone marrow always contain large numbers 
of a special amoeboid type of cell, the hemocytoblast, which is regarded 
as the basic type, the mother cell, so to speak, of red cells and also 
of the various types of granular leucocytes. As to how the differ- 
entiative processes are carried out through the many intervening 
cellular stages, very little information is available. The end result, 
namely, the production of an abundance of red cells and granular 



leucocytes by the blood-forming, or myeloid, tissues of the bone 
marrow, is an established fact. Apparently the spleen is the only 
other tissue in the body that is equipped to play any part in the 
development of the blood cells. (Fig. Ill; page 166.) 

Mineral Reserves. It is only in recent years that the function of 
the bones as a storehouse from which the calcium content of the 
blood may be kept at normal levels has been realized, though it was 
earlier recognized that regular calcium deposition was imperative for 
normal bone development in children. Abnormalities in the calcium 
metabolism of the developing bone tissue were first found to be 
responsible for rickets. And then it was shown that a deficiency of 


I Capacity to respond 

2. State of activity 

J Degree of maturation 
of myeloid cells 

4. Injury from toxemia 



I. hjeed to combat injury 
(infection or toxemia) 

Z. Speed of withdrawal of 
qranulocytes from 
blood stream 

3. Degree of toxemia 


I. Always reflect the need 
of tissues plus capacity 
of marrouj to respond 

t May reflect condition of 
marrow if cells are free- 
ly released 

J btey reflect need of tissues 
if marrow can freely re- 

FIG. 111. Illustrating the formation of blood cells in the bone marrow (left) and 
their utilization in the body tissues (right). (Haden, "Principles of Hematology," 
Lea cfc Fehiger.) 

vitamin D was at the basis of the abnormal calcium reaction in the 
bone tissues (page 60). The basic importance of free calcium in 
the blood plasma, however, was not realized until much later, when the 
experimental studies on the parathyroid gland showed that the hor- 
mone secreted by this gland was an essential factor in the maintenance 
of the normal amount of calcium in the blood (page 109). Further- 
more, as noted previously, the absence of calcium from the blood very 
quickly produced other disastrous effects in the organism, primarily 
associated with increased muscle-nerve irritability, that, if not reme- 
died at once by supplying calcium, were invariably fatal. Linked in 
the complete picture of blood calcium is the little-understood relation- 
ship between phosphorus and calcium. In some unknown way, these 
two elements work together to maintain normal conditions in the 
blood stream and are apparently deposited in the bone or removed 
from it as the conditions demand in order to maintain the normal 
levels in the blood stream. 




















A B 

PLATE XII. Nervous system in man. A, illustrating the general plan, with the 
spinal cord and the chief nerve routes to the arms and legs; B, the chief components of 
the autonomic system; C, elements of the reflex arc; D, bipolar neuron; J5? f multipolar 
neuron. Diagrammatic. 


The nervous system as the master organ system of the body 
dominates the entire organism. It receives and interprets external 
stimuli which make known the environmental factors and also internal 
stimuli which reveal the conditions of the various organs. In order 
to do this, there are located at strategic points various types of sensory 
cells which are capable of receiving the particular stimuli, and the 
latter incite nerve impulses which travel into the central nervous 
system over the cable-like conducting nerve fibers. The central 
nervous system receives the impulses from the peripheral sensory areas, 
interprets the data, and then incites outgoing impulses which are 
transmitted over a separate set of nerve fibers to the motor tissues 
and cause coordinated action of these effector organs. Thus the basic 
function of irritability, which is common to all living matter, is largely 
assigned, in the multicellular animals, to the nervous system, and the 
extent of the assignment increases in the higher types in correspondence 
with the augmented tissue differentiation. In addition, the higher 
functions of consciousness, memory, intelligence, and volitional think- 
ing come into more and more prominence and, finally, reach their full 
flowering in the human brain. 

In order to receive the sensory impulses and to control and coordi- 
nate the activities of the muscle tissues and, as a matter of fact, all 
the tissues of the body as well, the nervous system must have direct 
connections through definite nerve fibers with all of the cooperating 
organ systems, with all of the constituent tissues, and, in many 
instances, as in the outlying sensory structures, with the individual 
cells. In a highly differentiated animal, it is easy to see that the sum 
total of all these nerve elements supplying every type of body structure 
necessitates the presence of an exceedingly intricate organ system 
far beyond anything encountered in the previous studies of the other 
organ systems. 


From the comparative standpoint, specialized nerve cells, which 
mark the beginning of nerve tissue in the multicellular animals, are 




FIG. 112. Nerve cells in the body wall 
of Hydra. The long fibrils in the stippled 
background represent contractile elements 
from the epitheliomuscular cells. (Shult, 
after Schneider, slightly modified.} 

first encountered in hydra and the related coelenterate animals. In 
these lowly metazoa, the nerve cells are not associated to form a 
definite tissue but appear as separate branched cells or as a diffuse 
nerve net, connecting directly with near-by contractile elements in the 

body wall. Each nerve cell is thus 
an independent functional unit for 
the reception and transmission of 
stimuli. A more advanced but 
still comparatively simple animal 
type, as seen in the earthworm, 
reveals important advances in the 
structure of the nerve elements. 
A definite nerve tissue is built up 
of associated cells and fibers with a 
differentiation into a central and 
a peripheral division, so that the 
principle of a central adjuster mechanism to mediate between the 
incoming sensory impulses from the receptors and the outgoing motor 
impulses to the effectors is early introduced into the animal organism, a 
development that, it may be said, persists throughout the higher types 
up to man and becomes of increasing importance. (Figs. 112, 113.) 

It will not be necessary to 
pursue the field of comparative 
neurology to greater lengths at 
present, except to indicate one 
or two basic structural differences 
between the invertebrate and 
vertebrate nervous systems. 
Thus, in the invertebrates, the 
central nervous system is seen 
to be extended throughout the 
length of the body as a ventral 
nerve cord, lying in the body 
cavity close to the body wall. In 
the vertebrate animals, the central nerve cord is dorsal in position, has 
a central cavity, and, in all except the most primitive vertebrates, is 
enclosed by the bony neural arches of the vertebral column as revealed 
earlier (page 196). 

Very early in its development, the vertebrate embryo begins to 
form a nervous system which in a short time enables it to keep in 
touch with the environment and to perform essentially adaptive 
responses. The first indications of the embryonic nervous system 


FIG. 11 3. Nerve tissue in the anterior 
end of the earthworm. Shown are: the 
ventral nerve cord with segrnental ganglia 
lying under the alimentary canal; the sub- 
pharyngeal ganglia where the nerve cord 
divides to form a nerve collar which en- 
circles the pharynx and bears the dorsal 
brain. (Buchanan, "Elements of Biology,' 1 
Harper & Brothers.) 



are seen in a thickening and later infolding of the outer ectoderm on 
the dorsal body surface. In this way, which will be described in more 
detail later when the problems of embryology are considered, a hollow 
ectodermal tube is formed, extending the length of the body. As 
the ectoderm grows over the invaginated region, the tube comes to 
lie just beneath and entirely separated from the outer ectoderm from 
which it was formed shortly before. The walls of this primitive 
nerve tube, which is the forerunner of the nervous system, increase in 
thickness, and the anterior end soon undergoes radical structural 
changes which result in a definite delimination of the forebrain, mid- 
brain, and hindbrain and the further modification and subdivision of 


D E F 

FIG. 114. Early development of the nervous system in the amphibian embryo 
(Amblystoma). A, B, (7, successive stages in the closure of the medullary folds to form 
the neural tubo; D, E, F y sections through A, B, and C showing the formation of the tube 
as observed under the microscope; FB, forebrain; MB, midbrain; HB, hindbrain; SC, 
spinal cord. (Wieman.) 

these parts to form the complete master unit of the vertebrate body, 
the brain. (Fig. 114.) 

From the nerve cells (neurons) of this newly formed neural tube, 
microscopic cell processes, the nerve fibers, rapidly develop and soon 
connect all parts and tissues of the developing embryo. These fibers 
are primarily concerned with the conduction of the sensory nerve 
impulses coming in from the peripheral regions and the motor impulses 
going out to the contractile tissues. And so the embryo is soon in 
possession of a functional nervous system, comparatively simple at 
first but increasing in complexity as the rapidly differentiating tissues 
of the embryo continue to make new demands. 

As finally constituted, the nervous system of man and other higher 
vertebrates can be separated for convenience in description into four 
major structural divisions as follows: (1) sense organs, or receptors, 
which are the outlying units specialized for the reception of external 
and internal stimuli; (2) peripheral nervous system with its network 


of nerve fibers which innervate all regions of the body and function 
in conduction, involving the transmission of nerve impulses to and 
from all parts of the body; (3) autonomic nervous system, essentially 
a part of the peripheral nervous system in that it conducts impulses to 
and from the body structures that it innervates, but, as will be seen 
later, it is also associated with the involuntary control of various 
important organ systems; (4) central nervous system which serves as 
the integrative and controlling unit of the nervous systemfrom 
the structural standpoint, its complexity is unrivaled. In our present 
consideration, it seems logical to begin with the outlying sensory 
elements, then pass to the conducting elements of the peripheral 
system, and, finally, conclude with the master units of the central 
nervous system. (Plate XII.) 


It is customary to recognize five primary senses in the animal 
organism, namely, touch (including pressure, temperature, and pain), 
taste, smell, sight, and hearing. To these should be added the sense 
of position, or equilibrium, and those internal sensory phenomena, such 
as hunger and thirst, which enable the organism to determine its own 
internal conditions. The various sense organs, associated with the 
primary senses, function only as extremely sensitive receptors. Stim- 
uli arise in them that result in the transmission of nerve impulses to 
the central nervous system, and the latter interprets the stimuli and 
determines the action to be taken. 

Sense organs throughout the body, generally speaking, are com- 
posed of essential and accessory parts. The essential element consists 
of the specialized sensory cells or terminal arborizations which arc 
capable of responding to a certain type or types of stimulus that 
impinge upon them in their particular peripheral location. Also 
generally present in the various sense organs are accessory structures 
that aid in bringing the stimuli to the sensory neurons. For example, 
the lens of the eye is an accessory structure which focuses the light 
rays on the sensitive nerve cells of the retina, the latter con- 
stituting the essential part of the eye. It should be kept in mind 
that the sense organs of the body are internal (interoceptivc) as well as 
external (exteroccptive). Thus, the sensations of hunger and thirst 
have their origin in interoceptive sense organs scattered through the 
tissues of the body. The structural features of the internal sensory 
apparatus are not apparent, but those of the various external sense 
organs are well-known. Finally, sense organs are general or specific 
in their reactions to stimuli. In the first case, as seen in the skin, a 



FIG. 115. Tactile corpuscle 
in the skin of man. Highly 
magnified. (Mitchell.) 

variety of sensory elements is subject to influence by different types 
of stimuli. In the specific type of sense organ, as seem in the eye, the 
neurons are capable of responding to one type of stimulus only that 
received from the light rays. (Figs. 98, 

Skin Sensations. Sensory elements 
located in the skin respond to a consider- 
able variety of environmental stimuli, 
notably pressure (touch), temperature 
(heat or cold), and pain. Scattered 
through the skin just under the epidermis 
are numerous tactile corpuscles, each 
covered by a connective tissue sheath 
which contains the essential network of 
delicate nerve fibers and their endings. 
The tactile organs are sensitive to pressure 
or touch, as we commonly say. They are 
more highly differentiated and more 
abundant in the skin covering the tips of 
the fingers and toes than in other regions 
of the body. By testing very small areas of the skin surface with a 
fairly stiff hair mounted on a handle, it is possible to determine the 
distribution of these tactile areas. It will be found that, in the middle 
of the back, points separated by less than 2.7 in. (68 mm.) will be felt as 
a single stimulus. Points less than half this distance apart can be felt as 
separate stimuli on the back of the hand, whereas on the tips of the 
fingers, areas around 0.1 in. in diameter are felt as separate points of 
stimulation. Each hair follicle has sensory nerve endings that are 
very sensitive to contact, and less pressure is required to stimulate 
them. Even the slightest manipulation of a single hair is noted. 
(Figs. 24, 98.) 

Temperature sensations and pain are received by separate nerve 
elements in the skin so that, altogether, four different sensory areas 
are found (pressure, heat, cold, and pain). There is some doubt as to 
whether or not pain should be regarded as a separate sensation. It 
is possible that excessive stimulation of any sensory nerve will give the 
sensation of pain. With the exception of the tactile corpuscles, noted 
above, sensory nerves in the skin do not terminate as definite sensory 
bodies, but each splits into a branching group of fibrillae (arborization) 
which lie between the epithelial cells. (Fig. 115.) 

The nerve elements in the living tissues of the body are sensitive 
to chemical agents, and, as we have seen earlier, slight changes in the 


chemical composition of the blood, due to variations in the carbon 
dioxide content, are detected and result in definite reactions to main- 
tain the proper balance. The neurons associated with the sensations 
of taste and smell are, however, specialized for the reception of external 
chemical stimuli. In the epithelium of the tongue and nasal cavities, 
these chemical receptors are grouped to form highly developed sense 
organs. Attention was given to the tasto buds of the tongue epithe- 
lium in an earlier chapter (page 44). Accordingly, attention may be 
given at once to the sense of smell (olfactory sense) localized in certain 
regions of the nasal epithelium. (Figs. 27, 116.) 

Olfactory Sense. A microscopic study of the olfactory epithelium 
shows an underlying layer of connective tissue bearing the sensory 
epithelial cells. The latter contain three cellular types: olfactory, 
basal, and interstitial. The olfactory colls are tho functional sensorv 

FIG. 116. Section of taste bud of tongue. Highly magnified. (Mitchell, after Ranvier.} 

neurons, situated between the interstitial cells. Each sensory neuron 
is an elongated cell, lying below and at right angles to the surface 
epithelium. The proximal ends of the olfactory cells, lying next to 
the connective tissue layer, connect with a very fine nerve fiber which 
is continuous with the conducting elements of the olfactory nerve 
extending to the central nervous system. The opposite end of each 
olfactory cell lies at the outer surface of the epithelium and splits into 
a terminal group of projecting olfactory cilia which are sensitive to 
chemical stimuli or odors present in the incoming air. The entire 
surface of the olfactory epithelium, bearing the exposed cilia, is kept 
continually moistened by secretions of the olfactory glands. The 
latter lie in the basal connective tissue layer and open through separate 
ducts at the surface. This secretion, of course, comes into direct 
contact with the chemical substances in the air currents and with the 



cilia. It probably acts as a solvent for the volatile substances detected 
by the sensory cells. (Fig. 117.) 

The sense of smell is unbelievably acute. It is known that a strong 
odor, such as that of vanillin, can be detected in a dilution of 1 part in 
10 million, which is a far more delicate reaction than can be performed 
by the chemist. It is also certain that the olfactory sense of other 
animals, notably dogs, is even more acute than that of man. One of 
the best examples is the ability of the bloodhound to follow a trail. 
On the other hand, the olfactory sense is unreliable in some respects 
and is easily fatigued by a particular odor so that it may become 
entirely insensitive to it while still sensitive to other odors. 

It has been stated in an earlier section that 
four primary tastes are recognized and that the 
flavors of foods and other substances are usually 
complex reactions primarily associated with the 
sense of smell (page 45). This can be proved by 
noting the absence of taste when the olfactory 
sense is not functioning. A great deal of work 
has been done in an attempt to classify the 
primary odors on the same basis as primary 
tastes. Various schemes have been proposed in 
which from three to nine primary odors were 
recognized. Recent work favors the establish- 
ment of four primary odors, thus bringing the 
olfactory sense into line with the other cutaneous 
sensations, namely, pressure, heat, cold, and pain 
in the skin; and sweet, sour, salt, and bitter in the chemical 
receptors of the tongue associated with taste. The four primary 
odors as now recognized arc termed fragrant (flowery), acid 
(vinegary), burnt (tarry), and caprylic (rancid). By using the numer- 
als from to 9 to represent the complete absence and the increasing 
concentration of the primary odors present in a certain substance, the 
perfumers have been able to standardize perfumes by designating 
them with numbers. Thus by this system the odor of oil of winter- 
green is described by the number 8442 which means that it is strongly 
fragrant (8/), with moderate amounts of the acid and burnt units 
(4a, 46), and a slight amount of caprylic (2c). Damask rose is desig- 
nated numerically as 6523, and ethyl alcohol as 5301. Experts in this 
field are able to get a conception of the odor of a new perfume by the 
number assigned. 

The Sense of Hearing and Position. The ear is a highly developed 
sense organ which contains receptors for two types of stimuli : those of 

FIG. 117. Section 
of olfactory epithe- 
lium. With types of 
cells as described on 
page 220. Diagram- 
matic. (Wieman.) 


sound and those of position, or equilibrium. The human ear consists 
of three basic structural and functional units. There is, first, the 
visible outer ear which functions as an accessory structure for collecting 
the sound waves and passing them on to the middle ear. The middle 
ear is also an accessory structure and contains the elements adapted 
for sound transference from the outer car to the inner ear. In the 
inner ear are additional accessory structures and the essential auditory 
neurons which are the receptors for the incoming sound vibrations. 
(Plate XIII A, page 229.) 

The external ear consists of a cartilaginous pinna, variable in size 
and shape, with a central passage (auditory canal) continuing to the 
middle ear like the neck of a funnel. The inner end of the auditory 
canal is closed by a vibrating drum or tympanic membrane. The 
distal portion of the auditory canal connecting with the external 
pinna is cartilaginous, but the inner portion is bony. The canal is 
uniformly lined with skin containing special areas which secrete a pro- 
tective wax. Entrance to the auditory canal is guarded by numerous 
projecting stiff hairs. The tympanic membrane, which marks the 
inner boundary of tho external ear, is so constructed that impinging 
sound waves cause corresponding vibrations. 

The middle car is an irregular-shaped cavity located in the sub- 
stance of the temporal bone. Laterally, that is, toward the outer ear, 
the cavity is terminated by the tympanic membrane. In the opposite 
direction, approach is made to the structures of the inner ear. Dor- 
sally, the middle ear cavity ends in irregular air spaces in the temporal 
bone. Opening into the cavity is an air tube (Kustachian tube) which 
connects with the throat region and serves to keep the air pressure 
equalized on the tympanic membrane. When the Eustachian tube is 
partially or entirely clogged as the result of an infection, the air pres- 
sure is gradually reduced in the cavity of the middle ear by continued 
swallowing, and the tympanic membrane is pushed inward by the 
external air pressure. This condition results in lessened vibrating 
efficiency. * 

Functionally important in the middle ear are three tiny bones, the 
auditory ossicles, which are responsible for receiving the vibrations 
from the tympanic membrane and carrying them across the cavity of 
the middle ear to that of the inner ear. Joined to the inner surface of 
the tympanic membrane is the first of these bones, the malleus (ham- 
mer) which, in turn, articulates with the incus (anvil). The third 
member of this auditory bone-bridge is the stapes (stirrup). It 
articulates with the incus laterally and then continues to the tiny 



so-called oval window (fenestra ovalis) which marks the beginning 
of the inner ear. The fenestra ovalis and another similar opening, 
the fenestra rotunda, lying just below, are tiny openings in the bone 
tissue leading from the middle ear to the inner car. Each is completely 
shut off from the middle ear by a membrane. The two ends of the 
U-shaped stapes are, attached to the fenestra ovalis membrane, which 
clearly indicates that the auditory vibrations are transmitted to the 
inner ear at this point. (Plate XIII A, B.) 

The inner ear (labyrinth) is also situated in 
the temporal bone just beyond the middle ear 
cavity. It is a structure of extreme complexity 
in the human adult. The labyrinth consists of 
an outer covering of unusually hard bone tissue 
(bony labyrinth) molded, so to speak, to 
conform to the shape of the delicate membra- 
nous labyrinth that it encloses. Between the 
membranous and the bony labyrinths is a space 
filled with a fluid, the perilyinph. The latter 
apparently serves as a medium for the transfer 
of vibrations received from the auditory ossicles 
at the oval window to the second fluid, the 
endolymph, which fills the interior of the 
membranous labyrinth. The endolymph actu- 
ally bathes the sensory cells present in the 
membranous labyrinth and, in some way, 
manages to convey the qualities of the original 
sound waves impinging upon the eardrums. 
The essential functional elements of the ear are, 
as just indicated, within the membranous 
labyrinth. (Plate X1IIC.) 

The membranous labyrinth is first seen in 
the embryo as a tiny depression in the outer 
ectoderm on each side of the head region, 
depressed area is entirely cut off from the outer surface and forms 
a tiny closed vesicle. Then the latter differentiates into an upper 
vestibular portion (utriculus) and a lower cochlear portion (sac- 
culus). From the utriculus, as development proceeds, three semicircu- 
lar canals are formed which function in connection with equilibrium; 
and, from the sacculus, the coiled cochlea, in which the auditory 
sense is localized, gradually takes its permanent structure. Both the 
semicircular canals and the cochlea are innervated by separate brari- 

118. Membra- 
nous labyrinth of human 
embryo (<M cm.). C f 
cochlea; S, sacculus; U, 
utriculus with semicir- 
cular canals; CN, coch- 
lear branch of auditory 
nerve ; VN, vestibular 
branch of auditory nerve ; 
ED, endolymphatic duct. 
(Wieman, after Streeter.) 

A little later the 


ches of the auditory nerve that comes from the central nervous system. 
The interior cavities of the labyrinth are continuous and filled with 
the common endolymph. (Fig. 118.) 

Semicircular Canals. Considering, first, the structural arrange- 
ment of the semicircular canals that develop from and remain con- 
tinuous with the utriculus, it is important to note t^at the canals lie 
in three different planes at approximately right angles to each other. 
Each canal may be described as a tubular horseshoe-shaped structure, 
both ends of which are welded to the utriculus. One end of each canal, 
near its union with the utriculus, is enlarged to form a circular ridge, or 
ampulla, in which a sensory structure (crista acustica) is located. The 

G h g f 

FIG. 119. Diagram showing sensory areas (black) in tho semicircular canals and 
cochlea of the left ear. a, 6, c, superior, lateral, and posterior semicircular canals with 
ampullae; d, macula utriculi; e, macula sacculi; /, organ of Corti, k, saccua endolymph- 
aticus. (Maximow-Bloom, "Histology," W B. Saunders Company. From Shaffer, 
after von Abner, slightly modified.) 

latter, when the ampulla is opened, is seen as an elevated ridge occupy- 
ing about one-third of the canal cavity. Covering tho top of the ridge 
and extending for a distance on each side arc ciliated sensory neurons 
with associated supporting cells. The cilia do not project directly 
into the endolymph but into a covering layer of gelatinous material 
perforated with tiny canals, through which the endolymph reaches 
the cilia. Histologists have found it impossible to make satisfactory 
microscopic preparations of the crista of the mammalian ear, and 
certain details both of structure and function are still unknown. 

It has long been known from experimental data that the sensory 
cells in the semicircular canals are influenced by changes in the body 
position and that, in some way, the stimuli received by the central 
nervous system from these stimulated sense organs are used as a basis 


for the coordinated muscle control associated with the erect posture 
of man. Presumably the more or less continuous impulses from the 
sensory cells of the semicircular canals are integrated with the pro- 
prioceptive impulses in determining the necessary course of muscle 
action. It is assumed that variations in the endolymph pressure, 
corresponding to the head movements, stimulate the canal receptors, 
but absolute proof of this and of other suggested theories of semicircu- 
lar canal functions is still lacking. The problem is further compli- 
cated by the fact that a highly developed sensory apparatus (macula 
acustica), essentially of the same nature as the crista of the canals but 
containing otoliths, is a permanent feature of the sacculus and of the 
utriculus. It is generally believed that the maculae also share in the 
equilibratory function of the ear, but again proof is lacking. (Fig. 119.) 

Cochlea. The essential auditory organ, the cochlea, as found in 
man and the higher vertebrates, is one of the most complex of all the 
organs in the body and least understood from a functional standpoint. 
The cochlea is not present at all in the lower vertebrates, where the ear 
is primarily an organ of equilibration, but it assumes increasing struc- 
tural and functional importance in the higher groups. The forerunner 
of the cochlea is seen in the lower vertebrates as a tiny teat-like pro- 
jection (lagena) of the sacculus. The mature cochlea of the human 
ear viewed externally is a coiled bony structure, fashioned like a snail 
shell with two and one-half turns around the central axis. This coiled 
cochlcar tube has the greatest diameter proximally where it connects 
with the sacculus and gradually narrows down distally to a blind 
ending in the center of the coil. All in all, it is about lj^ in. long, 
around 0.1 in. in diameter, and filled with the fluid perilymph. (Fig. 

Contained in the outer bony cochlea is an inner membranous 
cochlear canal (scala media), which contains the endolymph and the 
sensory cells grouped in the spiral organ of Corti. This cochlear canal 
is considerably smaller than the osseous cochlea that encloses it, but 
of course it follows the snail-like curvature of the latter. If, now, a 
transverse section of the cochlea is examined, it will be seen that the 
enclosed cochlear canal is not circular but roughly three-sided, or tri- 
hedral, in outline. The apex of the trihedral figure projects about two- 
thirds of the distance across the cavity of the enclosing bony cochlea. 
From the wall of the latter, however, a bony projecting shelf (spiral 
lamina) extends out the remaining distance so that a transverse section 
of the cochlear canal shows a complete partition that divides the bony 
cochlea into an upper portion (scala vestibuli) and a lower portion (scala 
tympani). The latter is considerably larger than the scala vestibuli 



because the membranous cochlear canal (scala media), noted above, 
lies entirely above the partition and occupies about one-half the space 
that would otherwise be open to the scala vestibuli. (Fig. 120.) 

We have just seen that one side of the membranous cochlear canal 
is continuous at the apex with a projecting shelf or spiral lamina of the 
bony cochlea; thus a division is formed between the scala media and 
scala tympani. The division between scala media and scala vestibuli 
is a thin undifferentiated membrane which constitutes a second side 
of the cochlear canal. The base, or third side, of the latter is in 
contact with the inner wall of the cochlea. The arrangement of 








FIG. 120. Diagrammatic section through tho cochlea to show a portion of the 
organ of Corti, as described on page 227 C/. Plato XIII D. (Buchanan, "Elements of 
Biology," Harper & Brothers.) 

chambers in a transverse section of the cochlear canal may be visual- 
ized by taking a pair of dividers, separating the points 3 or 4 in., and 
then placing them flat on the desk, with the hinge at the left and the 
separated points at the right. The hinge at the left represents the apex 
of the cochlear canal at the point where it joins the spiral lamina. 
Now place the back edge of a book in contact with the points. The 
three-sided area formed by the arms of the dividers and the edge of 
the book represents the cavity of the cochlear canal, or scala media, 
which is filled with endolymph. The upper arm of the dividers sepa- 
rates the scala media from the scala vestibuli, and the lower arm does 
the same for the scala tympani. It also bears on its upper surface, 


that is, within the scala media, the highly differentiated auditory 
elements comprising the organ of Corti. 

Organ of Corti. Many pages could be devoted to a description of 
the known structural and functional features of the organ of Corti. 
Even so, at the present time, it would not be possible to describe it 
completely, because certain of the details have never been success- 
fully worked out. For the inherent delicate nature of the tissues 
associated with the organ of Corti, together with its very inaccessible 
position, embedded as it is in the recesses of the temporal bone and 
enclosed by the bony tissues of the cochlea and also by the mem- 
branous wall of the cochlear canal, have so far made it impossible to 
fathom all of its structural complexities. Its general plan, however, 
is well known. In the first place, it must be remembered that both 
the cochlea and the enclosed cochlear canal are coiled, spiral struc- 
tures, and, therefore, the organ of Corti, lying within the cochlear 
canal, has essentially the same length and spiral pattern of the enclos- 
ing structures. In a transverse section of the cochlea, the organ of 
Corti appears as a ridge on the innor surface of the wall that separates 
the scala media from the scala tympani, as just noted in the preceding 
paragraph. The tectorial membrane, projecting from the spiral 
lamina, is in close contact with a part of the upper surface of the 
cochlear apparatus. The portion of the wall bearing the complex 
sensory mechanism of the organ of Corti is known as the basilar mem- 
brane. The latter has an unusual and complex arrangement of 
radiating connective tissues that permits vibration. Some authorities 
have held that the basilar membrane is the primary functional unit of 
the ear. (Fig. 120.) 

Five main types of sensory cell are supported by the basilar mem- 
brane. These are the inner and outer tunnel cells, the inner and 
outer hair cells, and the supporting cells. The tunnel cells are peculiar 
in shape, each with a wide base in contact with the basilar membrane 
and a narrow elongated body. The bases of the inner tunnel cells 
are separated some distance from those of the outer tunnel cells, but 
distally they bend toward each other so that the distal ends of inner 
and outer tunnel cells are in contact at the upper surface of the organ 
of Corti. The roughly triangular space thus formed between the 
cells is the tunnel of Corti. 

The arrangement, just described, may be visualized by placing the 
right hand palm down on the table. Now slightly flex the thumb and 
fingers, thus raising the palm from the table so that the hand will be 
supported by the tips of the thumb and fingers. In this position, the 
surface of the table under the hand will represent the basilar mem- 
brane; the forefinger, one of the inner tunnel cells; the thumb, one 


of the outer tunnel cells; and the space between forefinger and thumb, 
the tunnel of Corti. It is estimated that there are about 5,600 of the 
inner tunnel cells and 3,850 of the outer tunnel cells, each type arranged 
side by side in a single row and the two rows separated proximally by 
the width of the tunnel. The visualization of a spiral-shaped hand 
with this number of fingers and thumbs may possibly convey the idea. 

In close association with the inner and outer tunnel cells are the 
inner and outer hair cells. These are the specific sensory cells through 
which connection is made with the nervous system. The hair cells 
are small, fairly typical in shape, and are arranged in regular rows on 
each side of the tunnel cells. They are not in contact with the basilar 
membrane but hang down suspended by one end, as it were, from the 
upper surface of the sensory epithelium. Projecting into the endo- 
lymph from the attached upper end of each of these cells are some 40 
to 60 sensory hairs. It is estimated that there are around 20,000 of the 
hair cells. Referring once more to the analogy with the hand as 
stated in the preceding paragraph, we may let the knuckles represent 
the line of fusion of the inner and outer tunnel cells, and the back of the 
hand the upper surface of the sensory epithelium, bathed by the 
endolymph. Thus the ends of the hair cells, together with numerous 
associated supporting cells, form a cellular mosaic, marking the upper 
boundary of the organ of Corti, and the hairs on the back of the hand 
represent the position of the sensory hairs projecting from the hair 
cells into the endolymph. To complete the analogy, the hand, posi- 
tion as given, may be visualized as immersed in water. In this case, 
the portion of the container on which the hand rests will represent the 
basilar membrane, and the water will represent the endolymph that 
fills the tunnel of Corti, covers the upper surface of the hair cells with 
the sensory hairs, and completely fills the cochlear canal. (Fig. 120.) 

Ear Function. Various theories have been proposed to account for 
jbhe ability of the human ear to detect and analyze the wide variety of 
sounds that reach it. There is no question about certain features 
concerned with the transmission of sound waves through the external 
and middle ear to the perilymph and endolymph of the inner ear, 
but no adequate explanation has been made of the incredible ability of 
the organ of Corti to distinguish among the tremendous variety of 
sounds in such a way that corresponding distinctive nerve impulses 
can be sent to the brain for interpretation. Briefly, the organ of 
Corti transforms sound waves into a tremendous variety of distinctive 
nerve impulses which can be interpreted by the brain. When one 
considers that the human ear is able to distinguish sounds that vary 
in strength or loudness, in pitch, and in quality, the problem of inner 
ear function becomes highly complicated. 





















IRIS ~ """""'"-" 









PLATE XIII. Sonso organs of man. A, section showing the general structure of the 
ear; B, auditory ossicles; C, internal oar; D, section showing internal structure of the 
cochlea; E, vertical section through the eye to show internal structure. Diagrammatic. 


Detectable variations in pitch alone range from very low-pitched 
sounds with about 20 vibrations per second up to very high notes with 
some 40,000 vibrations, and all of these tones may vary in intensity 
and be associated with overtones that give a distinctive quality. It 
has been suggested that the basilar membrane acts as a vibrating 
membrane in which certain regions are responsive to particular types 
of incoming waves. Other authorities suggest that it acts like the 
vibrating disc in a telephone receiver which vibrates as a unit for 
every type of sound wave. It was long held that the peculiar tunnel 
cells (rods of Corti), described above, acted essentially as tuning 
forks, sympathetic to certain types of vibration, but there are far too 
few of them to account for the tremendous variety of sounds received 
by the ear. All in all the problems associated with our highly devel- 
oped sense of hearing are far from solved. Of the end result, there can 
be no doubt; of the methods used in the organ of Corti to obtain the 
results, there is uncertainty. 

The Sense of Sight. The eyes are special sense organs devoted 
exclusively to the function of sight, or vision. The eye is essentially 
an optical instrument of the camera type, so arranged that light can 
be focused upon a sensory tissue, the retina, which is capable of 
determining the intensity and the wave lengths of the incoming light 

Eyeball. The main structural unit is the eyeball, which, in the 
human eye, is almost spherical in shape and approximately 1 in. in 
diameter (Fig. 121). Well-developed accessory structures are neces- 
sary for the functioning of the essential sensory neurons in the retina, 
some of which lie outside the eyeball and some lie inside. The external 
accessory structures include the attached muscles which move the 
eyeball; the eyelids, with a special epithelial lining, the conjunctiva, 
which also covers the exposed anterior surface of the eyeball; arid the 
glandular lachrymal apparatus which secretes a cleansing and lubri- 
cating fluid. The internal accessory structures include the iris dia- 
phragm which regulates the amount of light admitted through the 
anterior opening, or pupil; the lens, directly back of the pupil, which is 
responsible for focusing the light rays on the retina; the important 
and highly specialized muscle tissue which is associated with the 
necessary structural changes in the iris and lens; and, finally, the 
transparent, semifluid humors which fill the interior cavities of 
the eyeball. (Plato XIII#.) 

The eyeball is enclosed externally by a strong connective tissue 
sheath, or sclera (tunica fibrosa), which forms a continuous covering, 
except for a small circular area in the back of the eye where it is 
pierced by the optic nerve running from the retina to the brain. 


The tissue of the sclera is opaque, except for a small area directly in 
the front of the eye where it forms the transparent, window-like 
cornea through which the light rays pass on their way to the lens. 
Lying within the sclera is the choroid layer. It is very vascular, 
deeply pigmented and, in the front of the eye, forms the colored, con- 
tractile iris with a circular opening, the pupil, in the center. The iris 
contains muscular elements and, in its functional aspect, may be 
compared to the adjustable iris diaphragm of the camera which can be 
regulated as desired according to the intensity of the light. When the 
light is dim, the radiating muscles in the iris contract, thus enlarging 
the pupillary opening through which light passes to the interior of the 
eye. In a bright light, the circular muscles of the iris involuntarily 
contract, and this results in a constriction of the pupil so that a 
reduced amount of light is admitted to the sensitive eye tissues. The 
third and innermost layer of the eye is the retina, characterized by the 
presence of unique functional visual elements (rods and cones) adapted 
for receiving the photic stimuli and passing the resulting impulses into 
the nervous system for transmission to the brain via the optic nerve. 
The retina lines the posterior portion of the eyeball but does not form 
a complete anterior layer as do the sclera and choroid. 

The eye lies in a deep-seated skull cavity, the socket, or orbit, 
well-protected by the surrounding bony tissues. The exposed anterior 
portion of the eyeball is covered by a special type of epithelium, the 
conjunctiva, arranged in two layers; the inner layer, thin and trans- 
parent, covers the exposed portion of the eye and is directly attached 
to the cornea and near-by regions of the sclera. Peripherally, this 
layer merges into the outer conjunct! val layer which forms the lining 
of both eyelids. At the edges of the lids, in the region of the eyelashes, 
the conjunctiva becomes continuous with the skin tissues. The eye- 
lids themselves contain muscle elements, both smooth and striated, 
together with connective tissues and numerous glands. The latter 
open on the edges of the lids in the region where the conjunctiva and 
skin merge. (Plate XIII#.) 

Lachrymal Glands. The almost continuous movements of the 
eyeball and the eyelids during the day require that the conjunct! val 
surfaces be well supplied with a moistening, lubricating, and cleansing 
medium. To furnish this medium is the function of the lachrymal 
glands which pour their secretion through the lachrymal ducts; the 
latter open through the conjunctiva near the outer edge of the upper 
eyelid. The lachrymal gland of each eye is roughly almond-shaped 
and is lodged between the eyeball and the bony socket, above and well 
to the outer edge. The lachrymal secretion flows downward over 
the exposed eye surfaces and is continuously drained off through the 



openings of the tear ducts, situated close to the inner junction of the 
upper and lower lids. The two ducts of each eye, one from the upper 
lid and one from the lower, unite to form the lachrymal sac which 
opens into the nasal cavity. An excess of the lachrymal secretion, as 
in laughing or crying, causes the tears to overflow and roll down the 

Voluntary movements of the eyeball are brought about by three 
pairs of muscles which are inserted in the sclera and anchored to the 
surrounding bony tissues. These paired muscles include (1) the 
superior and inferior recti, attached above and below, respectively, 
by means of which the eye is rolled up and down; (2) the anterior and 






FIG. 121. Dissection to show the eyeball and the arrangement of the muscles responsi- 
ble for eye movements. (Buchanan, "Elements of Biology " Harper & Brothers.) 

posterior recti, attached medially and laterally, through which the eye 
is revolved to the right or left; (3) the superior and inferior oblique, 
attached above and below in such a fashion that oblique movements of 
the eyeball are possible. A pair of strong median and lateral ligaments 
attached to the sclera near the junctions of the lids extends into the 
bony tissue of the nasal region and laterally into the bone at the outer 
edge of the orbit and limits the eye movements and keeps the eyeball 
securely lodged in its orbit. (Fig. 121.) 

With the general external structure of the eye in mind, attention 
may be directed to its important internal structural and functional 
features. Observations on a mammalian eye, which has been cut in 
half in a vertical plane, shows a division into a comparatively large 



vitreous chamber, lying posteriorly, and a small anterior chamber. 
The anterior chamber extends from the cornea posteriorly to the iris. 
Between the iris and the anterior surface of the lens, an even smaller 
chamber communicates with the anterior chamber through the opening 
of the pupil. These two chambers contain the aqueous humor, a 
transparent watery fluid secreted by certain of the surrounding tissues. 
The aqueous humor is continuously replaced, as necessary, to restore 
that lost by the drainage. Posterior to the lens is the large vitreous 
chamber, noted above, which is mostly lined by the retina. It is filled 
with the vitreous humor, a permanent, transparent, noncellular 
material of gelatinous consistency. (Plate XIII f .) 

Accomodation. The incoming light rays pass first through the 
pupillary opening and then through the lens which is directly behind. 


FIG. 122. Illustrating the mechanism of accommodation, as described on page 233. 
To the right, thickened lens, accommodation for a near object; to the left, thin lens, 
accommodation for a distant object, ch, ciliary process; i, iris; si, suspensory ligament; 
rf, ciliary muscle. (Martin, "Human Body," Henry Holt & Company, Inc.} 

The latter is constructed of highly modified epithelial cells, each drawn 
out into a transparent ribbon-like stucture, and all closely joined and 
held in position by an intercellular cement substance. The lens is a 
biconvex, crystalline body so constructed that it can change its shape 
the function of accommodation which is necessary in order to 
focus the light waves from near and distant objects on the retina. The 
lens is enclosed by a substantial membrane, or lens capsule, closely 
applied to the outer lens epithelium. The lens capsule is continuous, 
all the way around, with a tendon-like ligament attached to the 
ciliary muscles. The latter form a ridge extending toward the lens 
from the choroid layer at the base of the iris. Also these muscles are 
so arranged that, when they contract, tension is lessened on the liga- 
ment attached to the lens capsule, and the highly elastic lens is thereby 
permitted to assume a more spherical shape. The increased curvature 
makes the lens stronger, that is, capable of bending the light rays from 
near objects, as in reading, so as to bring them to a focus on the 


retina. When the ciliary muscles relax, the tension on the lens capsule 
is increased, and the lens is flattened and thus adapted for focusing 
the images of distant objects on the retina. And so the lens is able to 
accommodate for near and distant vision by altering the curvature. 
The same result is attained in a camera by adjusting the distance 
between the lens and the sensitive film surface. (Fig. 122.) 

In a considerable number of individuals, the accommodation of 
the eye is not perfect. The lenses may be consistently too strong and 
bring the objects to a focus in front of the retina. This is nearsighted- 
ness, and the individual compensates by moving the object nearer to 
the eye. Farsightedness is the opposite condition in which the lenses 

are not strong enough, and consequently 
the rays are focused back of the retina. 
The farsighted individual endeavors to 
compensate by holding the book or other 
object farther from his eyes. The faulty 

v -""'^ accommodation is not always due to an 
abnormal lens; it may be due to the fact 
that the distance between the lens and the 
retina is abnormally long or short. A third 
error in refraction, astigmatism, is due to 
FIG. i23.-~lliustrating the i rrogu i ar curvature of the leng. The oculist 

paths of parallel light rays in CT 

a normal eye (A), short- is able to measure the optical condition of 
sighted eye (B), far-sighted thc and to prescr ibe artificial lenses 

eye (C). (Martin, Human ^ ^ . 

Body" Henry Holt & Com- that will correct the deficiencies of tho 

Inc.) regular lenses. (Fig. 123.) 

Retina. The retina is ono of the most complex tissues in the entire 
body, and therefore it may be well to begin our description with its 
development in the embryo. Such a study will show that the retina 
is the first structure of the eye to develop and, also, that it is formed 
from the ectoderm of the brain region. Later, the developing retina 
is doubly enclosed by the choroid and the sclerotic layers, both of 
which are mesodermal in origin. The retina grows out laterally from 
the newly formed neural tube, in the region of the developing fore- 
brain, until it comes into contact with the epithelial cells covering the 
body surface. The epithelial cells, which are thus brought into con- 
tact with a developing retina, gradually become transformed into the 

At first, the retinal outgrowth is seen as a hollow, club-shaped 
group of cells, but the shape of the distal end is soon changed by a 
secondary invagination and develops into a double-walled, cup- 
shaped structure which becomes the retina proper. This develop- 



mental process can be visualized by holding a spherical toy balloon, 
lightly inflated, in the palm of one hand and pressing against it with 
the closed fist of the other hand. In this position, the fist represents 
the crystalline lens at the lateral surface of the head. The balloon, 
now double-walled and partially enclosing the lens, represents the 
retina, which consists essentially of an inner sensory layer (repre- 
sented by the balloon wall surrounding the fist) and an outer pig- 
mental layer (represented by the outer balloon wall in contact with 


'Pigment layer 

1 Nervous layer 

Chorioid fissure 

Optic stalk 

Lens pit 




FIG. 124. Illustrating tho development of the retina in. the human eye as an out- 
growth from the forebraiii, described on page 234. A, from 4.5 mm. embryo; B, from 
5.5 mm. embryo; C, from 7.5 rnm. embryo. (Arey, "Developmental Anatomy," W. B. 
Saunders Company.} 

the palm of the other hand), which is soon to be covered by the choroid 
layer. The optic nerve, which connects the visual cells of the retina 
with the sensory area in the brain, is gradually fashioned out of the 
proximal portion of the original outgrowth from the forebrain. (Fig. 

Our next concern is with the mature retina which gradually 
assumes an extraordinary complexity of the constituent tissues as 
noted above. The general pattern of the retinal tissues follows to a 
considerable extent the basic pattern of the brain tissue from which ;t 
developed. The microscopic study of retinal tissue is most profitable 



when a vertical section is examined, taken at right angles to the upper 
surface. Such a preparation shows that the retina consists of no less 
than 10 distinct layers. The outermost of these is the pigmented, 
nonsensory layer, which is in contact and fused with the choroid layer. 
The pigmented layer is relatively thin and consists primarily of pig- 
mented epithelial cells. All of the remaining nine layers of retina 
develop from the original inner layer of the retinal outgrowth and 
form a relatively thick sensory tissue (about 0.0125 in.) built up of 

FIG. 125. Diagram showing cellular structure in the human retina. Highly 
magnified. II to III, sensory layer; IV to VII, middle bipolar layer; and VII to X, 
inner ganglionic layer, as described 011 page 236. (Howell, "Textbook of Physiology," 
W. B. Saunders Company. After Greef, slightly modified.) 

visual cells, neurons, and cell processes, altogether so intricate in their 
varied structural types and general arrangement that it will not be 
profitable to attempt a detailed description of the separate layers in 
the present discussion. (Fig. 125.) 

The basic structural condition may perhaps be understood by 
recognizing three divisions in the functional retina, namely, (1) the 
outer photoreceptor, (2) the middle bipolar, and (3) the inner ganglionic. 
In the photoreceptor division, which lies in contact with the outer 
pigmented layer, are the specialized visual elements. These cellular 
elements are found to consist of two types: the rods and the cones, both 
of which are elongated cells with unique structural and functional 
features. They lie perpendicular to the curved surface of the eyeball, 



regularly arranged, with the distal ends in contact with the pigmented 
layer. Processes from the pigmented cells project inward for a con- 
siderable distance between the visual cells and thus partially separate 
them. The rods, as the name indicates, are slender, tubular struc- 
tures. They contain a complex substance, visual purple, which is 
known to be chemically changed by light waves. The inner or prox- 
imal end of each of these cells tapers down to a very fine proto- 
plasmic filament, or rod fiber, in which the nucleus is located. The 
rod fibers extend toward the middle of the retina where they connect 
with processes from the bipolar neurons. The cones lack the visual 
purple but are essentially similar to the rods in their - 
structure, except that the body of the cell is heavier, 
shorter, more club-shaped, and the nucleus-containing 
cone fiber is considerably thicker. There is no 
apparent reason why this should be so, and, as a 
matter of fact, certain types of cones are thin, 
elongated bodies which are similar in appearance to 
the rods. (Fig. 126.) 

The middle region of the retina contains the so- 
called bipolar cells. These are elongated nerve cells 
adapted for receiving the impulses from the terminal 
fibers of the rods and cones. The impulses are 
received from the visual cells at the fiber end by 
union, or synapse, then transferred to the neurons of 
the ganglionic division of the retina. The cell proc- 
esses of the ganglionic neurons extend to the inner 
surface of the retina and converge in great numbers 
to form the large optic nerve. Thus it is seen that 
the light rays entering from the front of the eye 
penetrate the ganglionic and bipolar divisions of the 
retina without stimulating any of the constituent cells, before finally 
reaching the distal ends of the rods and cones that are sensitive to 
them. And the resulting impulses are carried in the reverse direction 
through the neurons to reach the elements of the optic nerve over 
which they are carried to the brain for interpretation. (Fig. 125.) 

There is considerable variation in the sensory quality of the retina, 
depending upon the distribution of the rods and cones. Toward the 
front of the eye, the numbers of rods and cones are greatly reduced, and, 
in the extreme periphery of the retina, they are entirely absent. A 
definite blind spot is found in the comparatively small central area 
where the optic nerve terminates at the inner surface of the retina. 
The area of most acute vision in the retina is found in the tiny fovea 

FIG. 126. Re- 
ceptor cells from 
human retina. 
Rod cell, left; 
cone, right. 
Highly magnified. 
nu., nucleus in rod 
and cone fiber. 
(Weber- Valentine.) 


centralis, about \{\ in. in diameter, which lies directly back of the lens 
in the so-called optical axis of the eye. In the fovea, the thickness of 
the retina is greatly reduced by the complete omission of the bipolar 
and ganglionic layers so that the visual cells are almost directly exposed 
to the incoming light rays. Furthermore, only cones are present in 
this region. An object to be seen clearly must be in the line of direct 
vision, or in the optical axis, so that the image will be focused on the 
fovea, as just noted. Involuntarily, the eyes are continually being 
moved so as to bring an object in the field of acute vision. Light rays 
falling upon the more peripheral regions of the retina are seen only 
dimly. This is due primarily to the reduced numbers of visual cells, 
which correspondingly decrease the resolving power of this region. 

Light Rays. Certain important facts relative to the physical 
characteristics of the visible light rays should be noted. We may 
regard light as being produced by minute but very rapid vibrations 
which have their source in an object heated to a high temperature. 
Thus the metal filament in an electric light bulb, when heated to a 
sufficiently high temperature by the passage of an electric current, 
gives off visible light waves. In such a case, the potential energy of the 
coal or other fuel burned in the powerhouse or that of water power is 
converted into energy-bearing light waves, and the latter stimulate the 
visual elements in the eye. In nature, however, the source of the light 
energy is, as we well know, the sun. As a matter of fact, the sun is the 
source of the electric light energy, for the energy both in the fuel 
burned or the water used in order to generate the electric current came 
originally from the sun. 

The physicist, by means of the spectroscope, can analyze light and 
thus determine its nature. When, for example, sunlight is allowed to 
pass through the glass prism of a spectroscope, it emerges as a multi- 
colored spectrum with visible bands of red, orange, yellow, green, blue, 
and violet, the so-called spectral colors. These colors are commonly 
seen in the rainbow, which results from the dispersal of the rays of 
sunlight by passing through raindrops. The latter act as does the 
prism in the spectroscope of the physicist. Thus it is clear that white 
light is a mixture of different colors. Furthermore, it is known that 
the different colors are due to variations in the length of the light 
waves. In the visible spectrum, the latter range from an extreme 
length of 760 millionths of a millimeter in the case of red light to 
390 millionths at the violet end of the spectrum. Other energy- 
containing rays lie above and below the visible spectrum. Thus, 
above the red rays are longer, invisible infrared rays; below the violet 
are the ultraviolet, both of which, though ineffective in stimulating 


the visual cells of the retina of the eye, can be detected by the proper 
physical instruments. (Fig. 229.) 

From the foregoing, it is evident that the retina of the eye is 
essentially a tissue capable of responding to light rays of certain 
lengths. But how the rods and cones do this is entirely beyond our 
present knowledge. It is clear that the peculiar compound present in 
the rods, visual purple, is in some important way concerned in this 
reaction, but it is also certain that this substance is not the primary 
element of vision, for, in the fovea, the region of most acute vision, 
only cones are present, and they lack the visual purple. Also it is 
known that visual purple is entirely lacking in the retinal tissues of 
some animals, such as the pigeon and bat, which, as flying 
animals, undoubtedly have acute vision. About all that can be said 
with certainty is that the essential visual elements are located in the 
protoplasm of the rods and cones. (Fig. 126.) 

Color Vision. It has been noted above that white light represents 
a mixture of various colors of the spectrum, but wo also find that 
various combinations of two colors will give the same visual sensation 
as the mixture of all the colors. These white-producing pairs of colors 
are known as complementary colors, examples of which are seen in 
certain shades of red arid blue, yellow and blue, etc. Since two com- 
plementary colors will produce white when added together, it follows 
that, when one of the complementary colors is removed, the visual 
color sensation changes from white to that of the other member of the 
pair. This can be well shown in the phenomenon involving retinal 
fatigue. Thus if one looks intently for about one-half minute at a 
brilliant red color and then quickly transfers his vision to a white 
surface, a brilliantly colored area will be seen on the white surface, but 
the latter will be bluish-green in color, which is the complement of red. 
This is due to the fact that the retinal elements responding to the red 
color have become fatigued and do not respond, thus giving white 
minus the red, which is the green complementary. Furthermore, the 
physicist recognizes three fundamental colors (red, green, and blue) 
by the proper mixture of which white and all other colors can be 
produced. This fact has long been linked up with important theories 
of color vision which assume that the visual cells respond to the three 
primary colors and that the multiplicity of color sensations is due to 
variations in the strength of the stimuli coming from the primary colors. 


Elements of the nervous system necessarily innervate every tissue 
of the body. The previous study of the sensoiy tissues has shown 


that the invasion of the sense organs by the peripheral nerves is 
for the purpose of receiving impulses, developed in accordance with 
the external and internal environmental stimuli and transmitting the 
impulses to the central nervous system for interpretation. The inner- 
vation of the effector tissues, including the muscles and glands, is to 
make possible the coordinated control of their activities by the central 
nervous system. Thus the essential function of the peripheral nerves 
is the conduction of nerve impulses to and from the master tissues of 
the central nervous system. In order to do this, the highly differenti- 
ated animal organism possesses an extraordinarily complex system of 
nerves, containing many individual nerve fibers, from the early 
developmental stages. It has long been recognized that nerves typi- 
cally contain two types of nerve fibers : the sensory, or afferent, fibers 
over which impulses pass into the central nervous system; and the 
motor, or efferent, fibers over which impulses travel peripherally from 
the central nervous system to the outlying effectors. This condition 
clearly indicates that the individual nerve fiber is a one-way track for 
nerve impulses. (Figs. 127, 133.) 

Neuron Concept. But the nerve fibers are not to be regarded as 
the basic structural and functional units of the peripheral system, for 
histologists studying the microscopic anatomy of nerve tissue were 
able, more than fifty years ago, to establish the important fact that a 
nerve fiber is simply a greatly elongated protoplasmic process of a 
neuron. This so-called neuron concept means, in essence, that the 
nervous system, like the other systems of the body, is composed of 
cells, the neurons. The neurons, as a group, are characterized by a 
very high degree of complexity and diversity, both structurally and 
functionally, marked instances of which will be seen from the later 
description. One unifying structural feature, however, with which 
we are particularly concerned in our present discussion of the peripheral 
system is the fact that the cell bodies of the neurons are extended to 
form one or, usually, more projections for the conduction of impulses; 
these projections are the nerve fibers. (Plate XIID, E.) 

Furthermore, the neurons are not indiscriminately scattered along 
the peripheral nerve fibers as they ramify throughout the body tissues 
but are concentrated in the central nervous system or in the near-by 
nerve centers, the ganglia. This means, of course, that the cell proc- 
esses or nerve fibers of some of these centrally situated neurons must 
be several feet in length, as, for example, to take an extreme case, the 
fibers that have their origin in the neurons in the spinal cord of the cen- 
tral nervous system and extend to the tips of the fingers or toes. It 
must be remembered, however, that the nerve fibers that originally 


connected the central nervous system and the peripheral parts of the 
body in the very early embryo were not very long and that a gradual 
increase in length occurred as the embryo increased in size. Finally, 
it has already been indicated that a nerve fiber carries impulses in one 
direction only. Since the nerve fibers are cell processes, this meant* 
that there are two types of the latter: the dendrites, which carry 
impulses centripetally, that is, toward the cell body; and the axons, 
which carry impulses peripherally, that is, away from the cell body. 
Microscopic study has not revealed any structural differences between 
the axon fibers and dendrite fibers. 

Nerves and Nerve Fibers. The functional conducting elements, or 
nerves, are seen as glistening white cords extending everywhere 
throughout the body structures. Some of the nerves are tiny fila- 
ments, microscopic in size ; others, as in the nerve trunks of the leg and 
arm, may be comparable in diameter with that of the little finger. 
Since all of these nerves have their origin in the brain or spinal cord of 
the central nervous system and near-by ganglia, they tend to increase 
in size as they approach the central nervous system and to divide into 
smaller and smaller units peripherally in accordance with the fibers 
given off, or received en route to innervate various regions. Care 
must be taken to distinguish between the nerve and the nerve fiber. 
The latter, as noted, is a cell process, sometimes greatly elongated but 
always microscopic in diameter, whereas the nerve is an aggregation 
of a great many tiny individual nerve fibers, insulated from each other 
and compactly bound together by connective tissue elements. We 
have already seen an example of this type of tissue construction in the 
building of a large muscle from microscopic muscle fibrillae (page 177). 
And the telephone technicians make use of the same method in building 
the large, lead-covered telephone cables which may contain thousands 
of individual copper wires, each insulated by a special covering from 
all the others. The variation in the size of the nerves is due primarily 
to the number of associated fibers. (Plate XII A.) 

When a transverse section of a typical nerve is examined micro- 
scopically, it will be found that it is completely covered by a connective 
tissue sheath, the epineurium. Inside the sheath, considerable num- 
bers of the tiny nerve fibers are grouped ni bundles, and each of the 
latter is surrounded by the perineurium which is a continuation of 
the connective tissue elements of the epineurium. Further study 
shows that the individual fibers are separated from each other and 
more or less surrounded by a still further continuation of the connective 
tissues to form the endoneurium. In a fresh nerve, it is possible to 
separate the individual nerve fibers and note their plan of construction. 


It is found under such conditions that a delicate sheath, the neuri- 
lemma, surrounds each individual fiber and is in contact with the 
endoneurium. Lying under the ncurilemma is a comparatively thick 
layer of a fatty substance, the myelin sheath. The latter is inter- 
rupted at rather regular intervals along the nerve fiber, and the neuri- 
lemma at these points (nodes of Ranvier) is in contact with the tiny 
central process of a neuron either axon or deridrite. Each axon (or 
dendrite) is thus separated from all the other axons of a nerve by the 
myelin sheath, the neurilemma, and by the connective tissue elements 
indicated above. The axon and dendrite differ functionally, as noted 
above, but structurally are not distinguishable. As a matter of 
fact, the axons and deiidrites are not the ultimate conducting elements 
of a nerve fiber. This function is believed to be localized in cyto- 
plasmic fibrils (neurofibrils) that apparently originate in the body of 
the neurons, near the nucleus, and extend peripherally into the axons 
and dendrites. In certain types of nerve fiber, the myelin sheath is 
lacking. Such nerves are known as the nonmyelinated nerves and are 
characteristic of the autonomic system, as distinguished from the 
myelinated types of spinal nerves just described. (Fig. 131.) 

Cranial and Spinal Nerves. From the extreme anterior end of the 
brain to the posterior tip of the spinal cord, a total of 43 pairs of cranial 
and spinal nerves leave the central nervous system and extend to the 
outlying tissues; 12 pairs of these are cranial; and 31 pairs are spinal 
nerves. The majority of the cranial nerves 1 have departed quite 
widely from the typical condition of the peripheral nerves, which is to 
be seen in the spinal nerves. Thus the spinal nerves are given off from 
the spinal cord at regular intervals; that is, they are segmeiitally 
arranged. Furthermore, each spinal nerve arises in the spinal cord 
by a dorsal (posterior) root and a ventral (anterior) root. A short 
distance from the cord, the nerve fibers of the anterior and posterior 
roots unite and are bound together by the common connective tissue 
coverings, thus forming a single large nerve composed, therefore, of 
both dorsal and ventral fibers. (Plate XIII A ; Plate XI VD, page 248.) 

Early experimental work on the spinal nerves established beyond 
all doubt the fact that there is a functional differentiation between the 
posterior and anterior fibers. The posterior fibers are afferent sensory 
fibers, which bring the impulses to the central nervous system from 
the peripheral sense organs; the anterior fibers are efferent motor 
fibers, which transmit the impulses from the central nervous system 
to the peripheral effectors (muscles and glands). And so it is evident 
that each spinal jierve contains both sensory and motor fibers. This 

1 Consult Appendix : Cranial Nerves. 


mixed type of nerve is not always found in the cranial nerves, for some 
of them, as will be seen from the description below, contain either 
sensory fibers or motor fibers but not both (page 519). The later 
description of the central nervous system will give opportunity for 
consideration of the associations between the spinal nerves and spinal 

The determination of the fact that the spinal nerves contain both 
sensory and motor fibers led to the establishment of the conception of 
the reflex arc which involves a peripheral sense organ (receptor), an 
afferent sensory nerve fiber, the central nervous system (adjuster), 
an efferent motor nerve fiber, and the peripheral effector, usually 
muscle tissue. Thus, when the tip of DORSAL ROOT TO BRAIN 
the finger inadvertently touches a hot I 

surface, it is immediately jerked away SPfNAL GANGLION I 

by a reflex action which involves no 

,. r , . i SKIN 

conscious action. It is clear in a ^-=^ 

reflex action that the sensory cells in 

the skin covering the finger tip are first 

affected. The resulting impulse is 

transmitted to the central nervous ( $glP MUSCLE VENTRAL ROOT 

system over the afferent fibers of the + . F ' - 127 /T7 D ; a f f rai V sh f ng sec- 

J tion through the left side of the spinal 

root. The Central nerVOUS cord with the dorsal afferent root and 

system acts as an integrating mocha- thc moto <* effe !' en . f ' root of * 9 spinal 

~ . . . nerve as described on page 242. 

nism (adjuster), a function that in- 

volves both the interpretation of the incoming sensory impulses and 
the relaying of the outgoing impulses over the efferent fibers of the 
proper motor roots, the ones that innervate tho arm muscles. The 
contraction of the latter completes the drama. (Fig. 127.) 


The autonomic nervous system is essentially a part of the peripheral 
nervous system, but the distribution of the constituent nerves is largely 
limited to the important visceral organs present in the thoracic and 
abdominal cavities, and the impulses that pass over the autonomic 
nerves from the central nervous system are beyond our conscious 
control. It is sometimes referred to as the involuntary nervous system 
because of this condition. Since the autonomic system apparently 
exercises a complete involuntary control over the most important 
organs in the body, it is usually ranked as one of the four major divi- 
sions of the complete nervous system. Basically, however, it appears 
clear that it is not really a controlling unit but a conducting unit which 
receives efferent motor impulses from the central nervous system and 



conveys them over a network of the utmost intricacy to the proper 
effectors the involuntary muscle elements in the visceral organs. 
There appears to be no evidence that the impulses that exercise 
involuntary control over the important organs originate in the auto- 
nomic system itself. It is evident, therefore, that the autonomic 
system is essentially a part of the peripheral system. The autonomic 
system reaches its highest development in man and the mammals 
generally, where it is found to consist of two distinct conducting units, 
namely, the sympathetic division and the parasympathetic division. 
(Fig. 128.) 






FIG. 128. The general plan of the autonomic nervous system in man. Cf. Fig. 129. 

Sympathetic Division. The sympathetic division consists essen- 
tially of a pair of delicate nerve cords which lie exposed in the thoracic 
and abdominal cavities, one on each side of the vertebral column, close 
to the dorsal body wall. Examination of these sympathetic cords 
show that each consists of a rather loosely connected chain of segmental 
bead-like nerve centers, the sympathetic ganglia. Each sympathetic 
chain starts anteriorly in the neck region and extends to the posterior 
part of the abdominal cavity. Each of the constituent sympathetic 
ganglia receives one or more branches, containing efferent motor fibers, 
from the nearest spinal nerve shortly after the latter leaves the spinal 
cord. These connecting efferent fibers, which come from the spinal 
nerves and extend to the sympathetic ganglia, are known as the 



preganglionic fibers. Another group of sympathetic fibers, mostly 
nonmyelinated, have their origin in the sympathetic ganglia where they 
synapse with the preganglionic fibers and then extend to the tissues to 
be innervated. This latter group constitutes the postganglionic fibers. 
(Fig. 129.) 

The postganglionic fibers may proceed directly from the sym- 
pathetic ganglia to the peripheral tissues, as just indicated, or the 


7horacic< i 

Lumbar \ 3^ , 


Solid Lines ~ Parasympoithetic 
Dotted L ines - Sympathetic 


FIG. 129. Scheme illustrating the distribution of the autonomic nervous system of man 
and the important plexuses (Celiac Ganglion, etc., etc.) (Watkeys, Daggs.) 

postganglionic fibers from a number of ganglia may be associated to 
form an intermediate relay station, or nerve plexus, which is connected 
to the effectors by a third fiber group. There are several plexuses of 
varying size and importance in the sympathetic division, but the largest 
is the celiac ganglion, or solar plexus, from which fibers innervate the 
important abdominal organis, including the stomach, intestines, liver, 
pancreas, and certain parts of the urogenital system. 


Parasympathetic Division. The parasympathetic division of the 
autonomic nervous system consists of efferent fibers which arise in 
the extreme anterior and posterior portions of the central nervous 
system. The anterior parasympathetic fibers are given off from certain 
neuronic areas lying in the midbrain and the hindbrain, whereas the 
posterior parasympathetic fibers have their origin in the sacral portion 
of the spinal cord. The parasympathetic fibers extend directly from 
their points of origin in the central nervous system to the tissues that 
they innervate, and there they form synapses with the postganglionic 
fibers of the sympathetic division. In general, intermediate ganglia are 
not present as in the sympathetic elements. (Fig. 129.) 

The basic effect of the mammalian autonomic system is to supply 
the involuntary and cardiac muscles and also the glands with a dual 
control through the double innervation by sympathetic and para- 
sympathetic fibers. Altogether, the elements of the autonomic 
system innervate and exercise involuntary control over the smooth 
muscle tissue present in all the organs and organ systems of the 
vertebrate body, including, in addition to those already noted for tho 
sympathetic division, such diverse structures as the pupil of the eye, 
the lungs, the heart, vascular system, arid all types of gland, among 
which, of course, are those of the endocrine system. In addition it is 
known, in certain instances at least, that the autonomic system is also 
equipped with sensoiy fibers capable of transmitting afferent impulses 
from the innervated effectors, as in the case of pain stimuli, to the 
central nervous system. The pain stimuli from the viscera are not 
definitely localized as a rule and may be referred to other regions of the 

An additional fact of great functional importance associated with 
the dual control of the sympathetic and parasympathetic divisions 
is based on experimental studies which have shown that the sym- 
pathetic and parasympathetic divisions of the autonomic system are 
essentially antagonistic in their control of various organs. Thus, to 
take a well-known example, the stimulation of the sympathetic fibers 
that innervate the muscle tissue of the heart and also of the blood 
vessels results in an increased activity of the heart and a contraction of 
the muscle tissue in the walls of the blood vessels. This causes an 
increase in the blood pressure. Contrariwise, stimulation of the 
parasympathetic fibers innervating these vascular tissues results in a 
slowing down of the heart action, a dilation of the blood vessels, and a 
consequent fall in blood pressure. That the action of the sympathetic 
and parasympathetic divisions may be reversed in other regions can be 
seen, for example, when the sympathetic fibers innervating the 


bronchi of the lungs are stimulated, for, in this organ, the sympathetic; 
impulses cause a relaxation of the muscle fibers rather than a con- 
traction as noted above in the vascular system, while the stimulation 
of the parasympathetic fibers innervating these tissues causes a con- 
traction of the muscle tissue, which, again, is the reverse of its action 
in the vascular system. 

It is clear, therefore, from the experimental evidence, that the 
action of the sympathetic and parasympathetic divisions in a particular 
organ is antagonistic and, furthermore, that their action is not uniform 
in the various innervated organs. Although a great deal remains 
unknown about the functioning of the autonomic system with its 
tremendously complex interrelationships, the data are more than 
sufficient to show that it is of supreme functional importance in main- 
taining tireless, efficient, and involuntary control over an almost 
infinite number of details associated with the proper and continuous 
functioning of the important organ systems. 
















2 3 
























PLATE XIV. Central nervous system of man. A, general structure as seen from 
the under surface; B, right side of the brain; C, median sagittal section showing the cut 
surface of the right half of the brain; D, spinal cord with the roots of the paired spinal 



At long last our consideration of the various units of the human 
nervous system brings us to the central nervous system, consisting 
of the brain and spinal cord, which is the supreme structural and func- 
tional unit not only of the complete nervous system but of the entire 
body as well. It is clear that the sensory, peripheral, and autonomic 
divisions of the nervous system are of functional importance only 
because of their association with the central nervous system. Every 
sensation received by the various specialized sense organs from the 
internal and external environment is sent to the central nervous 
system for interpretation and for the determination of the proper 
action to be taken by the organism. The nervous impulses that speed 
continuously over the network of peripheral and autonomic nerves 
are directed either to (afferent) or from (efferent) the brain and spinal 

We have seen earlier that the vertebrate nervous system starts in 
the embryo by the formation of a thin-walled ectodermal tube which 
gradually becomes differentiated to form the exceedingly intricate 
adult nervous system. The numerous neuronic descendants of the 
pioneer ectodermal nerve cells become the highly differentiated neurons 
of the adult brain and spinal cord. These pioneers never leave the 
chosen land, and, protected and obeyed by the lesser members of the 
body, their decisions constitute the law of the bodily domain. Thus 
we have the primary structural and functional fact that the central 
nervous system is the cellular unit of the entire nervous system, for 
the neurons lie Actually within the brain and the spinal cord or in the 
closely associated sensory ganglia which are situated on each of the 
sensory nerve roots close to the spinal cord. Structurally, it is possible 
to divide the neurons into two main cell types, namely, the bipolar type 
and the multipolar type. Present in both types of neuron and, there- 
fore, a characteristic feature of the neuronic type of cell structure is the 
prolongation of the cell cytoplasm into two or more processes, the 
nerve fibers, which are the specialized agehts for conduction, as ha* 
previously been seen in the study of the peripheral system. From 




the functional standpoint, the neuronic cell processes, as shown above, 
are divided into the dendrites, which carry impulses into the cell body, 
and the axons, which carry impulses from the cell body to the periph- 
eral regions (page 240) . The bipolar neuron 
possesses one dendrite and one axon, while 
the multipolar neuron typically possesses 
numerous dendrites arid one axon; the latter 
may give off numerous branches. (Fig. 130.) 
The bipolar neurons are present in the 
sensory ganglia of the cranial and spinal 
nerves and also in, the retina. Thus they 
show considerable structural diversity, but 
it is clear that they are fundamentally 
bipolar, each neuron having a separate 
dendrite and axon. In the fully formed 
bipolar cell, the axon and dendrite are 
usually fused together near the cell body so 
that the cell appears to be unipolar. If, 
however, the single axon-dendrite process is 
traced some distance from the cell body, it 
will be found to split to form a T-shaped 
process with the pear-shaped cell body 
attached to the base of the T. 
The multipolar neuron is the typical motor neuron present in the 
spinal cord and in the cortex of the brain. As seen in the spinal cord, 
the multipolar neuron has a large cell body with numerous short 
dendrites and a long myelinated axon. The axon usually gives off 
some tiny branches (collaterals) near the cell body and then continues 
out of the cord, along with other associated fibers, to form an efferent 
division of a spinal nerve. In the motor areas in the cortex of the brain 
the multipolar cells, reaching their climax in the large pyramidal cells, 
are subject to wide structural variation characterized in part by pro- 
fuse branching of the axons and the dendrites. It is presumed, but 
not proved, that the primary functional differentiation between 
r dendrite and axon in direction of impulse persists throughout the 
nervous system. Since several billion neurons are present in the 
central nervous system of man, it is readily seen that the actual 
tracing of an almost infinite number of microscopic nerve fibers is an 
impossible task. (Fig. 131.) 

Neuron Histology. The protoplasm of the neuron is largely 
lacking in structural features at the level of microscopic visibility 
* Urt t give any indication of its functional ability. The nucleus is large 

FIG. 130. Diagrams of 
bipolar (above) and multipolar 
(below) neurones. Highly 
magnified. Cf. Fig. 131. 








and well formed and contains one or more prominent nucleoli but, *on 
the whole, is quite unexceptional in appearance. The cytoplasm, how- 
ever, is at times fibrillated to form numerous very fine intracellular 
fibers, the neurofibrils, which per- 
meate essentially all regions of the 
cytoplasm but particularly in the 
region around the nucleus. Toward 
the periphery of the cell the neuro- 
fibrils tend to be in parallel lines, 
thus forming bundles of neurofibrils 
which extend out into the axori and 
dendrite. Apparently there is little 
doubt that these fibrillar elements 
are directly concerned with the trans- 
mission of the nervous impulses to 
and from the cell bodies of the neu- 
rons. Another cytoplasmic feature 
is noted in the presence of the 
chromophil substance, so termed be- 
cause of its reaction to the stains. 
In most of the neurons, the chromo- 
phil substance is widely distributed 
throughout the cell cytoplasm and 
out into the dendrites, but it is not 
present in the axons. Possibly the 
chromophil substance is stored nutri- 
tive material, but no proof exists of 
either its nature or function. Possi- 
bly even more obscure in its func- 
tional characters, but prominent 
structurally, is the net-like reticular 
apparatus (Golgi body) lying close 
to the nucleus. (Fig. 131.) 

One of the most important fea- 
tures of the neurons is the terminal 
branching, or arborization in the region where the connection (synapse) 
with the nerve fibers of another cell is made, or where the cell processes 
end in the tissues of the peripheral effectors. Considering, first, the 
synapses through which a nervous impulse is passed from the axon of 
one neuron to the dendrite of another, the best evidence is that the 
processes of the synapsing cells are not actually fused with each other 
but are only in close apposition. It can be shown in many instances 

Nucleus of 





FIG. 131. Diagram of a typical 
multipolar motor neuron with a 
rnedullated axon. (Wolcott.) 




that the region of the synapse is plentifully supplied with vascular 
elements, a condition indicating that it must be a region of high 
metabolic activity. Again it should be pointed out that the enormous 
number of neurons, each with several synapses 
of microscopic dimensions, constitute a system 
of incomparable complexity in which it is possi- 
ble only to get an idea of the main outlines and 
never to trace out the impulse routes and 
synapses of the individual cells. 



The spinal cord may be said to extend from 
the foramen magnum at the base of the skull, 
where it merges into the brain, to near the pos- 
terior end of the vertebral column, where it 
shades off into a fine terminal filament (filum 
terminale), the latter continuing into the sacral 
region of the vertebral column. The spinal cord 
proper is thus some 17 or 18 in. long, about % 
in. in diameter, and weighs less than 2 oz. It is 
fairly cylindrical in shape and with two enlarged 
areas, one of which is in the shoulder region 
where the spinal nerves arise that innervate the 
arms, whereas the other is in the sacral region 
where the spinal nerves innervating the legs leave 
the cord. It has already been noted in the 
description of the peripheral system that 31 pairs 
of spinal nerves, each with a posterior sensory 
root and an anterior motor root, leave the cord 
at essentially regular intervals throughout its 
length, corresponding to the segmentation of the 
vertebral column that encloses it. (Fig. 132.) 
We may now trace the roots of a spinal nerve into the interior of 
the cord and thereby get a conception of the internal arrangements. 
This can only be accomplished by the microscopic examination of a 
transverse section of a suitable spinal cord. The study of a section of 
the cord shows it to be roughly circular in outline but with a con- 
siderable anteroposterior flattening. A rather deep indentation occurs 
on the ventral surface (anterior fissure), and a shallow indentation on 
the dorsal surface (posterior fissure). If these fissures were extended, 
they would meet in the center of the cord and thus divide it into 




FIG. 132. Human 
spinal cord with the 
roots of the paired 
spinal nerves. Com- 
pare with the spinal 
cord shown in Plate 
12 A. Diagrammatic. 



symmetrical right and left halves. The transverse section of the cord 
reveals also a clear division of the tissues into an inner portion, the 
gray matter, which is enclosed by an outer portion of white matter. 
The gray matter is roughly H-shaped, with the crossbar of the H 
pierced in the center by the tiny central canal, which is thus in the 
exact center of the cord. The central canal is the minute remnant of 
the original large central cavity of the thin-walled embryonic neural 



Motor fiber 


Ventral root 



Sensory nerve 

/fiber ending in 

sense cell 

/Motor nerve 

fiber ending 

in muscle 

FIG. 133. Illustrating the origin of the spinal nerves and the sensory and motor nerve 
paths in the spinal cord, leading to the cortex of the brain. (Woodruff.) 

tube. The white matter of the cord completely encloses the inner 
H-shaped mass of gray matter. (Fig. 133.) 

Spinal Cord Histology. Further study of the spinal cord reveals 
the important fact that the gray matter is cellular and composed of 
neurons, whereas the white matter consists entirely of myelinated 
nerve fibers (axons and dendrites) extending up and down and across 
the cord. Furthermore, it is seen that the roots of each spinal nerve 


are directly connected to the inner gray matter of the cord, the afferent 
sensory roots joining the dorsal (posterior) ends of the uprights of the 
H and the efferent motor roots joining the ventral (anterior) ends. 
The ventral horns are larger and project more deeply into the white 
matter, and in them are the large efferent multi polar motor neurons, 
the axons of which converge segmentally to form the ventral roots 
of the spinal nerves. In the smaller dorsal horns of the gray matter lie 
the multipolar association neurons with their numerous dendrites and 
axons. Through the dendrites of the association neurons, the afferent 
sensory impulses are received in the cord by means of synapses with 
the axon fibers of the sensory roots. These sensory impulses are then 
transferred to the dendrites of the motor neurons at the same level of 
the cord or, more frequently, at a different level. In the latter case, 
the sensory impulse received in the cord must be passed up or down the 
conducting fiber tracts until the synapse is made with the dendrites 
of the motor neurons innervating the desired muscles. And it may 
also be necessary that the incoming sensory impulses be conducted to 
the brain before the proper adjustment with the motor system can be 

Fiber Tracts. One of the notably difficult and important problems 
in studying nerve function has been the localization of conduction in 
the spinal cord to particular fiber bundles in the white matter. 
Though it is impossible to trace single nerve fibers in the white matter 
to individual neurons in the gray matter, it has nevertheless been found 
possible to identify bundles of fibers (fiber tracts) which connect with 
definite neuronic areas. This was accomplished experimentally by 
sectioning the spinal cord in various experimental animals and thus 
separating certain neuronic areas of the brain and spinal cord from 
the connected nerve fibers. When this is done, it has been found 
possible sometime later to trace the paths of degeneration of the 
nerve fibers up and down the cord by microscopic study of prepared 
material. This results from the fact that, when the neurons are sepa- 
rated from the fibers, it invariably follows that the fibers soon degen- 
erate. The fibers (which, after all, are only cell processes) are unable 
to nourish themselves or to maintain independent life activities when 
separated from the cell bodies. Studies on degenerating areas of 
nerve fibers, following separation from the neurons, has consistently 
revealed the presence of definite fiber tracts in the white matter of the 
injured cord. The fiber tracts in the spinal cord can be grouped into 
(1) the ascending fiber tracts, which carry impulses up the cord 
toward the brain from the peripheral regions, and (2) ttye descending 
.fiber tracts, which carry impulses down the cord, away from the brain. 



In essence, of course, the ascending tracts carry sensory impulses to 
the brain, and the descending tracts carry motor impulses from the 
brain or from posterior regions of the central nervous system to the 
effectors. (Fig. 134.) 

Afferent Fiber Tracts. The main ascending afferent fiber tracts in 
the spinal cord include (1) those which carry the proprioceptive 
impulses from the muscles of the body to the motor areas in the cerebral 
cortex of the forebrain. These fiber tracts occupy a considerable area 
in the posterior central portion of the cord. They lie above the 
" connecting bar" and between the "uprights" of the H-shaped area 
of gray matter, continuing to the periphery of the cord on each side of 

Fasciculus cuneatus 

Fasciculus gracilis 

Lateral cortico-^. 
spinal tract 


Dorsal spinocere- 
bellar tract 

Ventral cortico-.- * 

spinal tract \ 

FIG. 134. Diagram of a transverse section of the spinal cord to show the general 
arrangement of the white and gray matter (heavy black area) and the localization of 
some of the important fiber tracts. Fiber tracts in the fasciculus cuneatus and fasciculus 
gracilis carry the afferent conscious proprioceptive impulses, whereas the dorsal spino- 
cerebdlar tracts carry the afferent unconscious proprioceptive impulses. The crossed 
pyramidal fibers (efferent) are located in the lateral corticospinal tracts, and the direct 
pyramidal fibers are in the ventral cortico-spinal tracts. Cf. Fig. 133, (Crandall, 
"Human Physiology, 1 " W. B. Saunders Company. After von Geheuchtvn.) 

the dorsal fissure. These tracts carry the impulses associated with 
conscious proprioception. When they are destroyed, the individual 
loses the muscle sense that enables him to know the position of legs or 
arms without looking at them. No other route exists for the afferent 
sensory impulses arising in the muscles, joints, and tendons to reach the 
interpretive areas in the brain. 

Other ascending spinal fiber tracts include (2) those extending 
from the body muscles to the cerebellum of the hindbrain. These 
tracts lie on the right and left sides of the cord, near the periphery and 
ventral to the tracts just noted above in (1). They transmit the 
unconscious proprioceptive impulses to the cerebellum, by means of 
which equilibrium is constantly, but unconsciously, maintained. 
Finally, the ascending fiber tracts in the cord include (3) those which 


carry the impulses from the sensory areas in the skin, associated with 
the various skin sensations. In one group of these paired tracts, 
afferent impulses associated with the sensations of pain, heat, and cold 
pass upward, and, in the other group, impulses associated with the 
tactile senses appear to be segregated. These are relatively small 
tracts located ventrally in the cord; the tactile fiber tracts lying close 
to and on each side of, the ventral fissure, whereas the other pair of 
sensory tracts are situated just below the cerebellar tract indicated in 
item (2) above. The afferent impulses from the important sense 
organs located in the heg,d enter the brain directly over the corre- 
sponding spinal nerves without association with spinal cord elements. 

It should be understood that the sensory impulses coming into the 
cord through the posterior roots of the spinal nerves are not necessarily 
transmitted to the brain in all cases. For example, the afferent 
impulses associated with relatively simple reflexes may be transmitted 
up or down the cord to the proper level for synapse with a particular 
group of motor neurons. Here they are routed over the synapsing 
association neurons of the cord and then over the axons of the motor 
neurons to the effectors. Thus, as in a frog, the brain may be entirely 
destroyed, but the reflex activity for the posterior portion of the body 
will remain intact, so that irritation of the sensory elements in the 
skin will produce reflex actions in the limbs. 

Efferent Fiber Tracts, The descending, or efferent, fiber tracts of 
the cord have their origin, for the most part, in certain neurons located 
in the cerebral cortex of the brain and, accordingly, are known as the 
corticospinal pathways. All along the cord, the terminal arborizations 
of the axons of these tracts synapse with the dendrites of the motor 
neurons in the anterior horns of the gray matter. Passing posteriorly 
from the brain cortex, the corticospinal fibers reach the posterior 
portion, or medulla, of the hindbrain. In this region, about three- 
fourths of the descending fibers pass over to the opposite side of the 
cord (decussation) ; that is, the fibers coming from the right side of the 
cortex find their way to the left side of the cord and vice versa. (Fig. 

These decussating fibers of the corticospinal pathways form the 
crossed pyramidal tracts. They are situated in the cord between the 
areas of the ascending cerebellar tracts and the outer boundaries of 
the gray matter, to the right and left of the ventral fissure. The 
remaining one-fourth of the corticospinal fibers do not cross over in the 
medulla but enter the spinal cord on the side corresponding with their 
origin and accordingly constitute the uncrossed, or direct pyramidal 
tracts. As a matter of fact, however, the fibers of the direct pyramidal 
tracts also cross to the opposite side of the cord but not until they 


reach the various levels where synapse is made with the motor neurons. 
Consequently, it appears that all corticospinal fibers innervate the 
opposite side of the body from that of their cerebral origin. The 
significance of the crossing of the nerve fibers, which is a basic feature 
of the vertebrate nervous system, is not apparent. (Fig. 133.) 

Over the crossed and the direct pyramidal tracts of the spinal cord 
flow the efferent nervous impulses from the brain cortex which initiate 
voluntary movement throughout the body. Unconscious (involun- 
tary) control of the muscle tissues, associated with muscle tonus, 
equilibrium (in response to afferent impulses received from the semi- 
circular canals and proprioceptive impulses), and various other vital 
activities, notably gland secretion, are the functions of two groups of 
neurons situated in the midbrain and in the hindbrain respectively 
the latter group being situated near the vestibular division of the 
auditory nerve which innervates the semicircular canals. 

The fiber tracts of the important pathways for involuntary con- 
trol, although not so large as those previously mentioned, h^ve been 
clearly located. The cerebellar tracts are situated on each side of 
the spinal cord in close proximity to the outer anterior border of the 
crossed pyramidal tracts, whereas the fiber tracts from the midbrain 
lie at the anterior edge of the cord between the ascending sensory fiber 
tracts. The size of the fiber tracts decreases as they extend pos- 
teriorly in the cord, for, as would be expected, the maximum number 
of afferent and efferent fibers in each tract are brought together just 
posterior to the medulla. (Fig. 134.) 


The spinal cord passes through the foramen magnum at the base 
of the skull and at once merges into the medulla of the hindbrain. 
The brain as a whole may be regarded as consisting of two main 
regions. There is, first, the primitive underlying brain stem which 
appears essentially as an enlarged and differentiated projection of 
the spinal cord, ending anteriorly with the midbrain and posteriorly 
with the medulla of the hindbrain. The brain stem is essentially a 
bottle-neck through which must pass the millions of afferent and 
efferent fibers connecting the brain neurons with the body as a whole. 
On reaching the forebrain, the fibers are concentrated into a small area, 
r>r internal capsule, of the forebrain from which they spread out (corona 
radiata) into the comparatively large spaces (external capsule) of the 
cerebrum. Secondly, a much larger superstructure lies above the 
brain stem which consists of the forebrain (cerebrum) and the cere- 
bellum of the hindbrain. The forebrain has been aptly described as 
"the flowering of the brain stem." Using botanical terminology, the 



spinal cord may be thought of as a tree-like structure, from which 
the branching spinal nerves emerge for connection with every body 
structure. (Fig. 135.) 

It was previously stated that the brain and the spinal cord develop 
through the differentiation of a thin-walled ectodermal tube during 
early development. The amazing structural and functional changes 

2 3 











FIG. 135. Drawings of the human brain, 
sagittal section, showing the out 
















A, external view of the right side; J5, median 
surface of the right half of the brain. 

that occur before the adult stage is reached are due, first, to increases 
in the number and the degree of differentiation of the neurons of the 
primitive neural tube and, second, to the development of the proto- 
plasmic processes (axons and dcndrites) from the cell bodies. These 
make the proper connections with those other neurons and gradually 
become associated in the fiber tracts of the brain and spinal cord and 
also in the cranial and spinal nerves with their receptor and effector 
connections. These neuronic associations, involving ever increasing 
complexity, culminate in the human brain. 



Divisions of the Brain. In the human brain the primary divisions 
into forebrain, midbrain, and hindbrain are apparent as in the brain of 
vertebrates generally, but such a marked structural and functional 
development of the cerebral hemispheres in the forebrain has occurred 
that the other two primary brain divisions are greatly overshadowed. 

'-.. Mesencephalon 
055-J^- (raid-brain) 

(medulla oblongafea) 

Para sacralis or 
*conus medullaris 



-Cerebellum ^ 

/ Metencephalon 

Spinal cord 

FIG. 130. The main divisions of the human central nervous system shown sepa- 
rately. Left side. Diagrammatic. (Morris, "Human Anatomy" P. Blakiston's Son 
tfc Comimny, Inc.) 

Thus the complete human brain of an average sized male adult weighs 
in the neighborhood of 50 oz. Of the total weight, approximately 
nine-tenths is due to the cerebral hemispheres. The remaining 5 oz. 
are divided between the midbrain and hindbrain, with the latter much 
the larger, so that the midbrain is very small indeed. In viewing 
the brain from the upper, or dorsal, surface only the paired cerebral 
hemispheres are seen, for they are large enough completely to cover 
over the other portions. This overgrowth of the forebrain is par- 


ticularly striking when one compares the brain of man with the brain 
of a lower vertebrate, such as the frog, in which a dorsal view reveals 
the three primary divisions of the brain arranged in a linear fashion 
and merging posteriorly into the spinal cord. (Fig. 136.) 

From the functional standpoint, the neurologist finds that the two 
main divisions of the human brain are the forebrain and the hindbrain, 
with the midbrain functioning in the main as a fiber tract and appar- 
ently without major independent integrating functions of its own. 
The midbrain is seen in gross structure as a short section of the under- 
lying brain stem situated between the fore- and hindbrain. On the 
under surface of the midbrain region are two rounded, pillar-like 
prominences, the crura cerebri, a term that literally means the "legs 
of the cerebral hemispheres, " and so named because the latter seem 
to rest upon them. As a matter of fact, the crura cerebri are fiber 
tracts with connections extending to the forebrain. The dorsal 
surface of the midbrain shows four rounded projections, the corpora 
quadrigemina, which are also important neurological landmarks 
because of their fiber tracts. 

Hindbrain. The hindbrain consists of three main portions: the 
medulla oblongata, the pons (pons Varolii), and the cerebellum. Of 
these, the medulla is the most posterior part of the brain stem and, 
as stated above, joins the anterior end of the spinal cord. Viewed from 
the ventral surface, the medulla appears as an enlarged portion of the 
spinal cord. Viewed dorsally, it is found to possess a triangular cavity, 
the fourth ventricle, which is continuous with the central canal of the 
spinal cord. The ventricle is covered by a thin membrane. The 
lateral walls and floor of the medulla contain important fiber tracts arid, 
in addition, contain neuronic areas which exercise involuntary control 
over such vital functions as respiration, heart action, blood pressure, 
and various complex reflex actions of a more difficult nature than those 
handled by the ^eurons in the spinal cord. Partially covering the 
anterior portion of the medulla, as seen from the ventral side, and 
extending along the brain stem to the midbrain is the pons, which 
consists almost entirely of two main groups of nerve fibers, designated 
as the longitudinal tracts and the transverse tracts. The latter appear 
from the ventral surface as a ring-like structure surrounding the brain 
stem and connecting dorsally through the middle peduncle with the 
cerebellum lying above the brain stem. (Figs. 135#; 136.) 

Thus the transverse fiber tracts of the pons make possible a close 
functional association of the three divisions of the hindbrain (medulla, 
pons, and cerebellum). Of these, the cerebellum is the largest and 
possibly the most important unit. It is a trilobed structure with right, 



left, and median portions, held in position and connected with the 
other elements of the nervous system by three fiber tracts, one of 
which, the middle peduncle, consists of the transverse fibers from the 
pons. The superior peduncle extends directly anteriorly into the 
cerebrum, while the inferior peduncle runs posteriorly, connecting 
with certain fiber tracts of the spinal cord. (Fig. 140.) 

The cerebellum is a very important brain center for the control of 
personally acquired reflexes and for general supervision over muscular 
movements, particularly those associated with equilibrium. Many 
of the afferent fibers from the sensory structures of the body form 
synapses in the cerebellum, and it is apparent that it has a wide range 













Fia. 137. The human brain viewed from the under surface. Roots of the cranial 

nerves shown to the left. 

of influence on both sensory and motor impulses passing to and from 
the forebrain. No conclusive evidence appears to exist, however, 
that the cerebellum acts otherwise than in a subsidiary manner to the 
higher nerve centers of the forebrain. 

Before passing to a consideration of the forebrain, it will be well to 
indicate the important relationships between the hindbrain and the 
12 pairs of cranial nerves, for all but t the first two pairs of cranial nerves 
emerge from the hindbrain. All of the cranial nerves have their real 
origin in definite neuronic areas 1 which are located, with the exception 
of the first pair (olfactory), in the midbrain and hindbrain. The 
neuronic areas, located deeply in the brain tissue, may be some distance 
from the point where the nerves emerge from the brain. 2 (Fig. 137.) 

1 Neurologists commonly refer to the neuronic areas as nuclei, but the use of 
the term nuclei in this connection is somewhat confusing to the biology student. 
The term neuronic area is therefore preferable, indicating an area in which a great 
many of the nucleated cell bodies of the neurons are concentrated. 

2 Consult Appendix: Cranial Nerves. 


Forebrain. The forebrain, or cerebrum, greatly overshadows all 
other parts of the brain in its functional aspects and is clearly the 
superorgan of the human nervous system. The cerebrum consists 
primarily of a pair of cerebral hemispheres, with which three lesser 
structures are associated, namely, the olfactory lobes, the thalami, and 
the corpora striata. The olfactory lobes, as the name indicates, are 
concerned with the sense of smell. In some of the lower vertebrates 
they are very highly developed and constitute the largest part of the 
forebrain. In man, they are of comparatively small size, lying under- 
neath and near the anterior border of each cerebral hemisphere. Just 
below this position is the sensitive area of the nasal epithelium, sepa- 
rated from the brain cavity by the bony perforated cribiform plate. 
Through the latter great numbers of minute sensory nerve fibers from 
the olfactory lobes project into the nasal cavity and innervate the 
nasal epithelium (page 220). 

The thalami (optic thalami) consist of a pair (one for each cerebral 
hemisphere) of neuronic areas deeply embedded in the tissue of each 
cerebral hemisphere, close to the median line and near the midbrain 
region of the brain stem. As will be seen later, the thalami are impor- 
tant association centers for synapses with afferent fibers connecting 
with the forebrain particularly with respect to the so-called body- 
sense (pain, warmth). Another pair of neuronic areas present in the 
forebrain are the corpora striata which are situated in close proximity, 
lateral and anterior to the thalami. The corpora striata are concerned 
as association areas for synapses with efferent fibers from the forebrain 
controlling muscle action. 

Cerebral Cortex, An examination of the outer tissue layer, or 
cortex, of the cerebral hemispheres shows an uneven or convoluted 
surface due to elevations (gyri) and depressions (sulci) which are quite 
uniformly distributed over the cerebral areas. Four main regions of 
the cortex can be noted, namely, the frontal, parietal, occipital, and 
temporal, corresponding in position to the areas of the skull as pre- 
viously given (page 197). In each cerebral hemisphere the boundaries 
of the regions just noted are more or less distinctly indicated by con- 
spicuous depressions (fissures) which are deeper than the sulci. 
Particularly prominent is the fissure of Rolando which marks the 
boundary between the frontal and parietal lobes and constitutes an im- 
portant landmark for the study of functional localization in the cortex. 
It should be said in this connection that the results obtained from 
numerous experimental studies on cerebral function show beyond a 
doubt that the neurons concerned with the control of a particular 
function are localized in definite cortical areas of the cerebral hemi- 


spheres. It is possible to indicate ; only a few of the many important 
areas that have been identified. In each hemisphere, the area of the 
frontal lobe lying just anterior to the fissure of Rolando is known to be 
definitely associated with the control of voluntary muscles in various 
regions of the body. Thus the muscles of the leg, body, and arms are 
controlled by neurons in the cortex lying along the anterior margin of 
the fissure of Rolando, whereas neurons along the posterior margin of 
this fissure in the parietal lobe serve as interpreters of the afferent 
sensory impulses which give the tactile sense. In the parietal lobes, 
posterior to the fissure of Rolando, other neuronic areas are associated 
with facial control and speech. The auditory center of the cortex is 
localized near by in the anterior portion of each temporal lobe, and 
the incoming visual impulses from the retina are received by cortical 

..Central Fissure (Rolando) 
Parietal Lobe 


Lo be Y.r "\ ^S^Jf*, TK^ronM 


-Temporal Lobe 


FIG. 138. Human brain. View of right side showing localization of various important 
areas in the cortex. (Watkeys, Dogga.) 

neurons located in the posterior portion of the occipital lobes. (Fig. 

Cerebral Fiber Tracts. It is apparent that the segregation of certain 
cerebral functions in definite and corresponding areas of the cerebral 
hemispheres means that the entire brain is not ordinarily involved in 
the interpretation of the incoming impulses or in the transmission of 
the efferent impulses. Thus, to take one example, afferent impulses 
from the sensory areas associated with the voluntary muscles of the 
arms and legs on the right side of the body are primarily received by 
neurons in the posterior portion of the frontal lobe of the left cerebral 
hemisphere. But some of the incoming impulses from the peripheral 
regions may be so important as to require additional consideration, 
and this will necessarily involve other neuronic areas of the cerebrum. 
Accordingly, it is found that the various localized neuronic areas in 
the cerebral hemispheres are interconnected by special association 
fiber tracts composed of bundles of nerve fibers through which the 



impulses from one cerebral region are transferred to other regions as 
required. In the forebrain, separate groups of the association fiber 
tracts are established for communication between the right and left 
cerebral hemispheres and also for communication between the various 
neuronic areas in each of the hemispheres. This condition is easily 
understandable when one thinks of the telephone central of a large 
city containing a number of separate functional units, the exchanges, 
each of which is connected to all the other exchanges in the central 
office so that messages may be transmitted back and forth throughout 
the entire switchboard. (Fig. 139.) 

In addition to the association fiber tracts, Another very important 
and even more widely distributed system of fiber tracts, the projection 



temporal area 

FIG. 139. Schematic section of the human brain showing the association fiber 
tracts. A, between adjacent areas; B, connecting frontal arid occipital areas; C, D, 
connecting frontal and temporal areas; E, connecting occipital and temporal areas. 
The corpus callosum also contains association fibers connecting the cortex of the right 
and left cerebral hemispheres. C N, caudate nucleus; OT, thalamus. (Hunter, Walter, 
and Hunter, "Biology," American Book Company. Modified from Starr.) 

fibers, connects important regions of the forebrain, midbrain, hind- 
brain, and spinal cord into a unified whole. Some of the more impor- 
tant tracts of the projection fibers may now be briefly indicated. 
(1) In the first place, there is present in each cerebral hemisphere a 
cerebro-cortico-pontal tract which originates from the neurons in the 
cortex of each frontal lobe and extends posteriorly to the pons of the 
hindbrain where the constituent axon fibers synapse so that the efferent 
impulses from the cerebrum reach the cerebellum through the fiber 
tract of the middle peduncle (page 260). This tract carries efferent 
impulses, which, since they originate in the frontal neurons, are pre- 
sumed to be of a very high order. (2) The pyramidal tracts originate 
in the so-called pyramidal neurons of the cortex of the frontal lobes 
near the fissure of Rolando, pass ventrally through the corpora striata, 



and then form the crossed and uncrossed fiber tracts of the spinal 
cord (page 256). The crossing of the pyramidal tracts occurs in the 
medulla of the hind brain. The pyramidal tracts transmit efferent 
impulses for the control of voluntary motion. (Fig. 140.) 

(3) The cutaneous sensory projection tracts, carrying afferent 
impulses, originate in the neurons situated in the anterior portion 
of each parietal lobe and extend to the optic thalami where the main 
sensory fiber tracts of the spinal cord terminate anteriorly. It should 


FIG. 140. Schematic section of the human brain showing the projection fiber tracts 
connecting the cerebrum and other parts of the brain and spinal cord. A, tracts con- 
necting cortex of frontal lobe to the pons varolii arid thence to the cerebellum via the 
transverse fibers (middle peduncle) at G', B, pyramidal motor tracts; C, sensory tracts; 
Z), visual tracts; E, auditory tracts; F, projection fibers (anterior peduncle) connecting 
cerebellum and anterior portions of the brain; G, transverse fibers of pons varolii; H, 
projection fibers (posterior peduncle) connecting cerebellum and the spinal cord; J, 
projection fibers between auditory nucleus and the brain; K t crossing over (decussation) 
of pyramidal motor tracts in the brain; Vt t fourth ventricle. Roman numerals refer to 
cranial nerves. (Hunter, Walter, and Hunter, "Biology," American Book Company. 
Modified from Starr.) 

also be remembered that a crossing of these sensory fibers occurs in the 
hindbrain, just anterior to the decussation of the pyramidal tracts. 
(4) The visual tracts originate from neurons lying in the posterior part 
of the occipital lobes and extend posteriorly to special neuronic areas 
in the midbrain where synapse is made with the afferent fibers of the 
optic nerves, carrying impulses from the retina. It is probable that 
each visual cell in the retina has a direct connection through this 
projection path to one or more cerebral neurons where the interpreta- 
tion of the visual images takes place. (5) The auditory tracts originate 


in the neurons of the temporal lobes of the cerebrum from which each 
extends to neuronic areas located in the midbrain. Here synapse 
occurs with the fibers of the auditory nerve transmitting auditory 
impulses from the hindbrain. The routes of the afferent impulses 
from the cochlea and semicircular canals are difficult to trace through 
the intricate fiber tracts of the hindbrain and midbrain, but the course 
of the auditory tracts from the temporal lobes t6 the midbrain is well 
established. (6) Finally, the projection tracts include the superior, 
middle, and inferior peduncles of the cerebellum, concerned with the 
transmission of impulses to the forebrain, to the midbrain, and to the 
pons and medulla of the hindbrain, as has been indicated previously 
(page 261). (Fig. 140.) 

Histology of the Cortex. In the previous study of the spinal cord, 
it was noted that the gray matter, composed essentially of the motor 
and association neurons, formed an H-shaped body in the interior of 
the cord which was enclosed by a layer of white matter consisting of the 
fiber tracts. This same arrangement of the white and gray matter 
persists in the medulla, pons, and midbrain, but it is reversed in the 
cerebellum of the hindbrain and in the cerebral hemispheres of the 
forebrain, in both of which the gray matter, containing the neurons, 
forms the outer cortex, while the fibrous white matter is enclosed in 
the interior. It is this cortical gray matter in the cerebral hemispheres 
and the cerebellum that constitutes the dominant, controlling, Integra- 
tive, and interpretive tissue of the body, the seat of consciousness and 
intelligence. Histological studies of cortical tissue reveal essentially 
the same general arrangement of tissues throughout. Forming the 
thin outer covering is the molecular layer, about 0.01 in. thick, consist- 
ing largely of interlacing nerve fibers but also containing numerous 
comparatively small neurons. Below the molecular layer are several 
nuclear layers composed of various specialized types of neurons. As 
might be expected, the vascularization of the gray matter is much 
greater than that of the white matter. 

The cortex of the cerebral hemispheres, which is somewhat more 
highly differentiated than that of the cerebellum, may be subdivided 
into four or even more rather well-defined layers in correspondence 
with the various types of neuron that are present. Characteristic 
structural features are evident in the neurons from different regions of 
the cortex. Thus in the cortex of the cerebral motor area of the frontal 
lobes are found the multipolar pyramidal neurons, the axon fibers of 
which form the crossed and uncrossed pyramidal tracts. The 
pyramidal neurons are the largest neurons in the entire nervous system 
and almost reach naked-eye visibility. On the other hand, in the 


visual area of the cortex the pyramidal type of neuron is almost entirely 
absent. It is estimated that between 13 and 14 billion neurons are 
present in the cerebral cortex of the human brain, which, with the 
associated dendrites and axons, make a complexity of organization 
which defies complete analysis. A further complication is found in 
the specialized supporting tissue (neuroglia) which is widely distributed 
throughout the neurons and fiber tracts of the central nervous system. 
The neuroglia (literally, nerve glue) cells are of various types but all are 
characterized by the presence of cellular processes of varying number 
and complexity which are intermingled with the true nerve fibers in 
many cases. 

Brain Ventricles. The central cavity of the embryonic neural 
tube is comparatively large at first but during development is gradually 
reduced in size as the walls are thickened by the increased number of 
neurons. Remnants of the original cavity persist as (1) the tiny 
central canal in the spinal cord and (2) the much larger ventricles of the 
brain. The posterior ventricle is in the medulla of the hindbrain where 
the vertebral canal enlarges to form the fourth ventricle. Proceeding 
anteriorly from the fourth ventricle, the central cavity narrows again 
to form the aqueduct of Sylvius, which extends to the midbrain region; 
here it forms the third ventricle. Anterior to the third ventricle, the 
brain cavity divides into a pair of lateral ventricles which extend into 
and throughout the length of the cerebral hemispheres. (Fig. 1355.) 

It is thus evident that the central canal of the spinal cord together 
with the ventricles of the brain form a continuous central cavity 
throughout the length of the central nervous system. The vertebral 
canal of the spinal cord is very small and apparently without definite 
function, but the brain ventricles serve an important function in the 
collection and distribution of the cerebrospinal fluid which fills the 
central cavities and surrounds the entire central nervous system, thus 
affording protection from mechanical shocks and also as a medium 
from which nutrient materials are obtained for the neurons. 

Meninges. In considering the cerebrospinal fluid, attention must 
be given to the three membranes (meninges) that enclose the central 
nervous system. The outermost membrane (dura mater) completely 
lines the bony tissue of the skull and, to a lesser degree, the vertebral 
column. The middle meninges is the arachnoid, and within it is the 
innermost one, the pia mater, lying in close contact with the nerve 
tissues of the brain and cord; in fact, the pia mater follows the con- 
voluted surface of the cerebral cortex, while the arachnoid stretches 
across from "peak to peak" of the sulci. Between the arachnoid and 
pia mater is the subarachnoid space filled with cerebrospinal fluid. 


The latter thus forms a liquid layer completely surrounding the central 
nervous system. 

The ccrebrospinal fluid is a product of the blood and is received 
from the vascular system through thin-walled, high-vascularized areas 
(choroid plexuses) present in the walls of the brain ventricles, par- 
ticularly in the lateral ventricles of the cerebral hemispheres. From 
the lateral ventricle of each hemisphere, the cerebrospinal fluid flows 
posteriorly, passing through the third, and finally reaching the fourth 
ventricle. Here it slowly flows through three tiny openings in the thin 
ventricular covering and then into the extended spaces of the sub- 
arachnoid cavity. The route of the cerebrospinal fluid in returning 
to the blood stream has not been fully determined. The pia mater 
serves not only as an intimate membranous covering of the central 
nervous system but also as a highly vascularized tissue in which the 
blood vessels supplying the nerve elements of the cord and brain have 
their origin. 


From the functional standpoint, the human nervous system is 
primarily receptive, conductive, and integrative. We have seen that 
the function of reception is localized in the many and varied types of 
peripheral sense organ, capable of receiving the continuous and multi- 
tudinous internal and external stimuli. Conduction of nerve impulses 
is usually regarded as being primarily the function of the peripheral 
nerve fibers, and rightly so. It must be remembered, however, that 
the nerve fibers are not independent elements of the nervous system 
but merely the processes of the neurons located either in the central 
nervous system or in near-by ganglia. Furthermore, conduction is 
also an essential function of the fiber tracts extending through the 
brain and spinal cord. The function of integration, which involves 
the coordination of all the body structures so as to unify the entire 
organism, is of paramount importance and is exclusively a function of 
the central nervous system. Ramifications of the integration func- 
tion involve the higher functions of intelligence, consciousness, mem- 
ory, volition, and sensation. Consideration may now be given to 
these primary functions of the nervous system reception, conduction, 
and integration in the order named. 


Concerned with the reception of stimuli are two groups of receptors, 
namely, those specialized for external stimuli, which comprise the 
exteroceptive system, and those influenced by the stimuli arising 


internally, which comprise the inter6ceptive and proprioceptive sys- 
tems. The receptors of the exteroceptive system are located near the 
body surface where they are in a position to be stimulated by 
the various environmental stimuli that impinge upon them, whereas the 
receptors of the other two sensory systems are located at strategic 
points throughout the body where they receive the internal stimuli 
and thus furnish information relative to the condition of the various 
tissues and organs and of the needs of the body as a whole. 

It is at once apparent that the so-called primary senses of the 
body, namely, touch, taste, smell, temperature, sight, and hearing, 
are all components of the exteroceptive system with the receptors 
located peripherally where they are influenced by the particular types 
of external stimulus for which they are adapted. Furthermore these 
receptors may be divided into those equipped to receive stimuli from 
a distance, as in hearing and seeing in which the incoming sound and 
light waves reach the sensory tissues from varying distances, and into 
the receptors which are stimulated only by actual contact with certain 
substances, as in touch, taste, and smell. It might be thought that 
the olfactory receptors belonged to the distance receptors; but as a 
matter of fact, the olfactory epithelium is stimulated only when tiny 
particles of a volatile material are brought into actual contact with the 
olfactory cells. It appears probable that the sensation of pain arises 
from specialized sensory cells which are more or less widely distributed 
among the various types of receptors and are affected whenever a par- 
ticular stimulus reaches excessive strength. Adequate consideration 
has been given to the exteroceptive sense organs in the first section of 
the previous chapter (page 218), but the interoceptive system remains 
for brief discussion at this time. 

The receptors of the interoceptive system are associated with less 
generally recognized sensations, notably hunger, thirst, equilibrium, 
and one that may be referred to as the muscle sense. The receptors 
associated with equilibrium and muscle sense are conveniently grouped 
as the proprioceptors. There seems to be no reason to regard the 
interoceptive receptors as essentially different in nature from those 
responsible for the reception of external stimuli. However, it is clear 
that the structural elements of the interoceptive receptors are very 
simple compared with those of the exteroceptive receptors in which, 
in most cases, the stimuli are first received by highly specialized sen- 
sory cells, as in the retina, and then released to the afferent sensory 
fibers for transmission to the central nervous system. An examination 
of the visceral, muscular, and supporting elements of the body does 
not reveal the presence of definite sense organs with sensory cells, and 


so it is evident that the internal stimuli are directly received by the 
dendrites of the sensory nerve fibers at their highly developed terminal 
arborizations. The latter are abundantly distributed and in intimate 
contact with the various body tissues. In the voluntary muscles and 
skeletal elements of the body the sensory fibers are elements of the 
spinal nerves, but throughout the viscera of the body, in which involun- 
tary muscle tissue forms the effector units, the sensory fibers are 
associated with the autonomic system. The three basic interoceptive 
sensations of hunger, thirst, and proprioception may now be considered. 

Hunger. The sensation of hunger appears to be due primarily to 
involuntary muscular contractions in the wall of the stomach in the 
absence of the normal intake of food. As the result of the hunger 
contractions the sensory nerve fibers in the stomach tissues are stimu- 
lated, and a discharge of sensory impulses reaching the central nervous 
system causes a distinctly unpleasant, even painful, sensation which is 
interpreted as a demand by the nutritive tissues for food. However, 
the sensation of hunger appears to arise primarily from an empty 
stomach, which incites a distinct type of muscular contraction, rather 
than from an actual demand from the tissues for nutritive materials. 
Thus, when one fasts for several days, it is found that the hunger sensa- 
tions disappear after a time. If they were primarily associated with 
the body tissues, it would be expected that the sensations would 
increase with the continued failure to supply additional nutritive 
materials. It seems evident also that the unpleasant hunger sensa- 
tion is not directly associated with the distinctly pleasant mental 
phenomenon which we term appetite. The latter involves the mem- 
ory of pleasant tastes, odors, and companionship around the festive 
board and seems to be linked up with a general feeling of well-being. 
There in, however, an indirect connection between hunger and appetite 
in that the sight or smell of food, when one is hungry, will incite various 
activities associated with eating, such as a flow from the salivary 
glands, whereas, if one is surfeited with food, the presence of additional 
food brings no response. 

Thirst. It has been shown, in an earlier chapter, how necessary 
water is to animal metabolism. Any failure of the water supply so 
that the fluid reserves in the tissues begin to decrease results in a very 
definite and early warning, the sensation of thirst. This sensation 
apparently is not localized in the stomach but in the pharynx. Accord- 
ingly, the intake of dry or salty food or hot dry air very quickly results 
in a dry feeling in the throat, which is immediately associated with 
the sensation of thirst. At first, the thirst sensation is only a gentle 
warning which may be satisfied with a slight amount of water. When, 


however, the supply of water is lacking for a longer time, the demand 
is increasingly insistent and soon becomes one of the most powerful 
and painful of all the sensations, with all the sensory elements of the 
body gradually stimulated by the demands of the tissues for water. 

Proprioception. Proprioception is primarily concerned with coor- 
dinated control of voluntary muscles and with the muscular sense that 
makes one aware of the position of a particular muscle or group of 
muscles, as in the leg or arm, without looking at them. When the 
proprioceptive apparatus is destroyed in any region, sensation is lost. 
The sensory impulses arising in the proprioceptors originate in specific 
dendritic arborizations of the sensory fibers, abundantly distributed 
through the voluntary muscle tissues and, also, in the attached ten- 
dons. Also bound up in the complicated proprioceptive association is 
the function of equilibrium which has its primary interpretive area in 
the cerebellum of the hindbrain. The cerebellum also receives by way 
of the auditory nerve the afferent impulses originating in the sensory 
cells of the semicircular canals, which, as we have seen, are the organs 
of equilibration. The exact role of the cerebellum in maintaining 
equilibrium is difficult to determine with exactness, but that it is of 
prime importance is established by the fact that the complete or partial 
destruction of the cerebellum decidedly mars not only the normal pic- 
ture of equilibrium but also the essential integrated operation of 
practically all types of voluntary muscle movement. Again, the 
maintenance of normal muscle tonus is also bound up in the proprio- 
ceptive phenomena. On the whole, the cerebellum apparently should 
be regarded as the general interpretive center of the afferent impulses 
from the proprioceptors and the organs of equilibrium. Out of the 
complete picture thus obtained by the cerebellum, suitable efferent 
impulses are released which govern tonus, equilibrium, and coordinated 
voluntary movements, particularly those associated with locomotion 
and other complex movements that have been gradually learned. 


From the external and internal receptors of the body, scattered far 
and wide, widely variable in design and almost innumerable, sensory 
impulses are continuously being received and conducted over the 
afferent nerve fibers to the central nervous system, and, from the lat- 
ter, efferent impulses are released for transmission over the motor and 
autonomic nerves to the effector units of muscle and glandular tissue. 
And, as we have already seen, the central nervous system contains 
important fiber tracts over which countless nerve impulses continu- 
ously pass from one end to the other. It is apparent that in the 


absence of conduction the receptive and integrative functions would 
be without effect. The underlying phenomena responsible for the 
conduction of nerve impulses are, to a considerable extent, bound up 
with secrets of the living state which, as yet, are undisclosed. Never- 
theless numerous important facts pertaining to conduction, which were 
established by years of research, are now recorded, and a few of the 
most important of these may now be considered. 

In the first place, the conduction of impulses over the nerve fiber is 
unqestionably a vital process in which oxygen is used, carbon dioxide 
is released, and a slight rise in temperature occurs, as in other cellular 
activities. Accordingly, conduction must be basically a process in 
which potential chemical energy present in the protoplasmic com- 
pounds of the neurons concerned is released by oxidation. The chem- 
ical changes associated with the movement of a nerve impulse along 
the fiber has been compared to the burning of the explosive in a fuse 
as it proceeds in regular fashion from the lighted end. However, it is 
certain that, when the explosive compound has undergone the chem- 
ical changes associated with combustion, the original substance can- 
not automatically be restored; whereas the chemical changes in the 
cytoplasm of a nerve fiber, when the impulse is transmitted, are only 
temporary, so that another impulse may be transmitted almost 

It has long been established that the conduction of nerve impulses 
is accompanied by electrical phenomena. This can be shown by plac- 
ing the electrodes of a galvanometer of the proper sensitivity in con- 
tact with an active living nerve fiber and, then, artificially stimulating 
the latter at some point beyond the electrodes so that nerve impulses 
will pass along the fiber. When the advancing impulse reaches the 
first electrode, an electric current, known as the action current, will be 
registered moving toward this spot from the second electrode. As 
the impulse passes to the portion of the nerve lying between the two 
electrodes, no current is detected; but when the impulse reaches the 
point of the nerve where the second electrode is attached, a current is 
detected flowing from the region of the first electrode toward the 
second; that is, the direction of the current is the reverse of that first 
indicated. Since the action current flows toward the region of the 
fiber over which the impulse is passing, it is evident that the latter 
temporarily reduces the electric potential of successive points of the 
fiber (that is, they become negative) as it moves along. If no impulse 
is passing over the fiber, the galvanometer shows an absence of current, 
which, of course, means that all regions of the fiber have the same elec- 
tric potential. The action current is distinct from the nerve impulse 


but is induced by the conduction of the latter along the fiber. (Fig. 
141A, B.) 

In the earlier chapter dealing with the Muscular System, a descrip- 
tion was given of the muscle-nerve preparation in which a voluntary 
muscle and the attached nerve were removed from an experimental 
animal and used for the study of muscle contraction (page 179). 
Such a preparation is also of great value, as was there indicated, for 
the study of conduction in nerve fibers. It is possible, in the first 
place, to ascertain the effects on nerve fibers of various types of stimu- 
lus, such as electrical, thermal, chemical, and mechanical, all of which 
will stimulate nerve tissue and produce nerve impulses. In general, 

FIG. 141A. Illustrating electrical phenomena associated with the passage of a nerve 
impulse, as described on page 272. a, electrodes for applying stimulus to nerve; 6, c, 
electrodes connecting with galvanometer G. (Howell, "Physiology " W. B. Saunders 

FIG. 141B. Schematic diagram illustrating the changes in the electric potential 
during the passage of a nerve impulse. (Buchanan, "Elements oj Biology" Harper & 
Brothers. Slightly modified after Gerard.} 

it has been found that the electric current offers the best type of artifi- 
cial stimulus, since it can be controlled accurately with respect to the 
area of stimulation, the strength of stimulus, and the time during 
which it acts. It should be noted that the continuous flow of an elec- 
tric current through a nerve does not stimulate it unless the current is 
of considerable strength, in which case it is probably injurious to the 
nerve tissue. On the other hand, nerves are readily stimulated by 
interruptions in the flow of an electric current. Thus nerve stimulation 
occurs at the instant when the switch is closed and the electric current 
begins to flow (make-shock) and again when the flow of the current is 
stopped by opening the switch (break-shock). The physiologist using 
the proper electrical stimuli can study the nature and characteristics 


of the artificially incited nerve impulses, both in excised nerves and 
also in those in their normal location in the body. In the latter case, 
electrodes placed on the skin may be used to stimulate particular nerves 
and muscles. (Figs. 91, 92.) 

Speed of Nerve Impulse. Experimental studies on nerve conduc- 
tion show that the speed of the impulse is subject to wide variation. 
In the first place, the rate is much slower in a cold-blooded animal, 
like the frog, than it is in a warm-blooded organism. Thus the maxi- 
mum speed of the nerve impulse in the frog is in the neighborhood of 
140 ft. per second, while in man the impulse may attain a speed more 
than twice as rapid, or almost 300 ft. per second. Other experiments 
have shown that the conduction of the nerve impulse under identical 
conditions is more rapid in myelinated than in nonmyelinated nerves 
and that the rate also varies in accordance with the size of the nerve; 
in the larger nerves, with the correspondingly greater number of nerve 
fibers, the impulse is conducted more rapidly than in the smaller ones. 
The figure, given just above, is for conduction in a large medulla ted 
nerve. The measurements taken on small nonmyelinated nerves show 
an impulse rate of less than 5 ft. per second in some cases. Again the 
speed of the nerve impulse may be progressively reduced by decreasing 
the temperature of the nerve fiber until, at a point a little above freez- 
ing, conduction is entirely stopped. With all conditions equal, no 
detectable difference exists between the transmission phenomena of 
the sensory fibers carrying afferent impulses -toward the central 
nervous system and those occurring in the motor fibers in which 
efferent impulses are conducted toward the peripheral effectors. 

In the earlier discussion of muscle function, it was noted that the 
repeated contraction of a muscle, without adequate rest periods 
between, involved muscle fatigue which considerably altered and, 
finally entirely prevented, contraction (page 184). The evidence for 
fatigue in the nerve fiber following repeated conduction of impulses 
over considerable periods, even up to several hours, is not clear. In 
fact, the results obtained by various investigators have shown that a 
nerve fiber is competent to receive stimulation and to conduct the 
resulting impulses over long periods. Slight fatigue immediately 
following conduction, as shown in recent experimental results, possibly 
decreases conduction somewhat. Apparently protecting the nerve 
against too rapid onset of conduction is a refractory period following a 
stimulus, during which a complete loss of excitability occurs and, 
accordingly, another stimulus cannot be received for conduction. The 
refractory period lasts from 0.002 to 0.003 second, after which the 
nerve fiber can again be stimulated. It is evident that the refractory 


period, brief as it is, is sufficient to permit the recuperation of the 
conducting elements in the nerve fiber. The question as to the nature 
of the activity at the terminal arborization of a nerve fiber, which 
incites activity in an effector, is a difficult one. How, for example, does 
the nerve impulse cause a contraction of a muscle fiber? Evidence is 
accumulating to indicate the presence of a mediating hormonal sub- 
stance, acetylcholine, presumably secreted by the terminals of the 
efferent fibers, which brings the effector element into activity (page 495) . 


Integration is necessarily the controlling function of the entire 
body. It has its structural basis in the neurons of the central nervous 
system and is an exclusive function of this major division. This is 
unlike the condition noted in connection with the receptive and con- 
ductive functions which arc shared by both the peripheral and central 
systems. Integration as applied to the nervous system is a very 
comprehensive term which includes a number of basic associated 
functions. Integration may be regarded as a process of internal 
unification through which all the diverse functional units of the body 
are caused to work together for the complete unit, the individual. 
Involved in this master function are the reception and interpretation of 
the sensory impulses from every area of the body, the selection of the 
correct routes for the efferent impulses, and the regulation of tho 
effector units associated with all typos of movement and secretion so 
that activity may be initiated, increased, or inhibited as the conditions 
demand. Thus integration is responsible for the coordinated control 
of all the diverse elements of the organism. Many of the integrative 
adjustments do not involve consciousness, which means that they are 
not referred to the cerebrum, but this function is always ready to be 
called into play as a part of the integrative phenomena. 

Reflex Arcs. Experimental studies concerned with the analysis of 
the integrative function in the vertebrate nervous system give evidence 
that the reflex action is an essential element. Apparently reflex 
actions are responsible for most, if not all, of the bodily activities. 
A reflex action may be defined as an involuntary, or unconscious, 
action occurring in some element of a peripheral effector in response 
to a stimulus received by a sensory receptor and transmitted to the 
central nervous system over the afferent nerve fibers. Thus is set up 
the so-called reflex arc, which, in the simplest type of reflex, might 
conceivably consist only of the afferent fibers of a sensory neuron 
carrying the impulse to the spinal cord where synapse is made with 
the dendrites of a motor neuron. The latter transmits the impulse, 



now efferent, to the connected muscle fibers which act in response to 
the original sensory impulse. 

It is doubtful if a reflex arc that involves only two neurons, as just 
described, is more than a theoretical possibility. Undoubtedly the 
normal reflex arc is much more complicated and involves, in addition 
to the arc just described, conduction up and down fiber tracts of the 
central nervous system and synapses at the proper levels with other 
neurons, thus bringing into play an integrative action by the central 
nerve elements. The sensory axon, on reaching the cord, may divide 
into an ascending fiber and a descending fiber from each of which side 
branches (collaterals) will be given off that synapse with the motor 
neurons at various levels. It is thus possible for an impulse entering 






FIG. 142. Diagram illustrating the components of a reflex arc. (Buchanan, "Elements 
of Biology " Harper <$ Brothers, after Kuhn, redrawn.) 

the cord over a single sensory neuron to reach a considerable number of 
motor neurons for transmission to various peripheral effectors. (Fig. 

The integrative action of the brain and spinal cord is best seen in a 
third type of reflex arc, probably the most important of all, in which a 
third type of neuron is involved, namely, the association (integrative) 
neurons. These lie Vholly within the central nervous system, as 
previously noted, and mediate between the sensory and motor neurons 
(page 254). Thus, the incoming sensory impulses are received by the 
dendrites of the association neurons and conducted by them directly 
to the proper motor neurons or even to the brain areas for complete 
integration if complicated reactions are indicated. The latter may 
possibly involve complete integration of all body units. Consider, 
for example, the reactions of a surgeon who accidentally pricks his 


finger with an infected instrument while operating, in comparison 
with his reactions when he jabs his finger against the point of his tiepin 
while dressing. In the latter case, the finger is quickly drawn away 
by the simple involuntary reflex action, and the pinprick is dismissed 
with little thought. But the injury received while operating is con- 
sidered in an entirely different light by the surgeon's nervous system. 
The sensory impulses not only incite the simple reflex action but are 
instantly projected to the highest centers of the brain where decisions 
are made and efferent impulses released which quickly involve all the 
nervous centers and the associated effectors. There is a unification, 
an integration, of all the body units in an endeavor to find the best 
possible solution for the serious problem presented. In this connec- 
tion, the association areas of the brain are undoubtedly of basic 
importance in causing the union of all the nerve elements to synthesize 
the complete concept. The sensory impulses from the injured tissues 
and the visual impulses from the eye reach the separate localized areas 
in the cerebral cortex, where they are interpreted. They are brought 
together through the association fibers. The function of memory 
is also involved. The surgeon remembers what he has previously 
learned about the dajigers of an injury by an infected instrument. 
Possibly all the neurons of the entire cortex are instantly brought into 
the picture, and from them a mental image, or consciousness, arises 
which is the basis of the efferent impulses released to the effectors. 

Reflex actions resulting from internal and external stimuli are 
continuously occurring throughout life. It is the way in which the 
organism solves its problems and adapts itself to the environment. 
A very important question which puzzles the physiologists and psychol- 
ogists is the determination of the limits of reflex activities in the 
human organism. Some would say that all our activities, physical 
and mental, are essentially reflex in nature, differing in degree of 
complexity but not in their basic nature. Others hold strongly to 
the belief that, in the higher mental processes involving such phenom- 
ena as intelligence, judgment, will, and memory, the nervous functions 
go beyond the automaticity of the reflex. At all events, except in the 
highest mental activities, reflex actions involving receptors, afferent 
conductors, adjusters, efferent conductors, and effectors are highly 
important in determining the response of the body to stimuli of all 

It has previously been shown that reflexes are of different degrees of 
complexity in accordance with the number of neuronic areas involved 
in the reflex arcs. It is also apparent that complex reflex actions 
involve the neurons in various brain areas or even those of the entire 



brain, for in it the adjustments take place that are necessary in bring- 
ing about the integrated responses to the stimuli that referred to it. In 
a general way, it can be stated that the spinal cord is dominant over the 
peripheral nerve elements, that the neuronic areas of the brain are 
dominant to those of the spinal cord, and, finally, that the neurons of 
the cerebral cortex are the chief controlling and integrating units of 
the nervous system and, therefore, of the entire body. Out of their 
activities develop the highest function of the nervous system, that of 
intelligence. The "intelligent" cerebral neurons are able to determine 
what response is suitable for a given condition and so can inhibit or 
augment the normal reflex or initiate an independent action as seems 

FIG. 143. Diagrams illustrating the avoiding reaction of Paramecium. At 1, the 
animal receives stimulation by corning into contact with a solid object A. The avoiding 
reaction is shown in 2 to 6; the direction of movement indicated by arrows. (Woodruff, 
after Jennings.) 

As shown above, the automaticity of reflex actions, in general, is 
noteworthy. They are essentially determined so that a certain 
stimulus induces predictable response. Bright light invariably causes 
the pupils of the eye to contract, whereas in dim light they enlarge; 
and so it goes in many, many instances. This is the field of the 
tropistic reactions 1 that has received so much attention during recent 
years. Tropisms are defined as orientations, or directed reactions, in a 
field of force. Such automatic reflexes, apparently established in the 
basic pattern of the organism, are termed unconditioned reflexes and 
include the great majority. of the reflex actions. (Fig. 143.) 

Another group of reflexes, known as conditioned reflexes, are 
acquired individually through training. Their establishment involves 
consideration by the higher nerve centers of tue cerebral cortex just 
as does anything that is learned. In addition, the conditioned reflex 

1 Consult Appendix: Tropisms. 



is built upon the pattern of the inherent unconditioned reflex. The 
distinctions just stated are well shown with dogs in the secretion of 
saliva when food is taken into the mouth. Under such conditions, 
the secretion of saliva is a normal unconditioned reflex. The develop- 
ment of a conditioned reflex, based upon this normal saliva reflex, can 
be accomplished by subjecting the animal to another stimulus, in this 
case the ringing of a bell at the time food is given. Thus an associa- 
tion between food and the sound of the bell is gradually built up in 

FIG. 144. Apparatus for developing a conditioned reflex in a dog, as devised by 
Pavlov. A, carmula inserted in the choek to collect saliva; B, metal plate on which 
drt>ps of saliva fall. When this occurs pressure is exerted through the tambour (C) on 
column of air in tube (Z)). This exerts pressure upon another tarnbour (E) which carries 
the writing lever (F) which is arranged to make a record on the smoked paper of the 
kymograph, indicating the flow of saliva. H, glass graduate for measuring the quantity 
of saliva, (Watkeys, Berry, after Yerkes and Morgulis.) 

the cortex after some 30 to 40 feedings. When this association is 
reached, a reflex flow of saliva will occur at the sound of the bell and in 
the absence of food. This is the conditioned reflex. It is not a 
permanent reaction; its continuance depends upon receiving food 
when the bell rings. If the auditory stimulus is repeated several 
times unaccompanied by food, the conditioned reflex with secretion 
of saliva will gradually disappear. Then, the animal returns to the 
original unconditioned reflex pattern which is responsible for the 
secretion of saliva when food is received. (Fig. 144.) 

Giiiielmus Harveus 

Generatione Animalium. 

PLATE XV. The allegorical title page from De generatione animalium^ one of the last 
publications of William Harvey, who discovered the circulation of the blood. This work 
was published in 1651, six years before Harvey's death. Jove sits enthroned with his 
eagle and is releasing various animals, all of which come from the egg Ex ovo omnia. 
(Redrawn by L. Krause.) 


The material presented so far in this volume has been concerned 
with the structural and functional features of the various major organ 
systems that make up the human body and by which the life processes 
are continuously maintained. But, in addition, each individual is 
also supplied with a reproductive mechanism which is not concerned 
with the maintenance of the life functions in the individual but with 
the continued propagation of a particular type of organism, the species, 
through the production of new individuals. Thus, from the stand- 
point of the species, the reproductive system is essential, but, from the 
standpoint of the individual, it is not essential. However, it is the 
perpetuation of the species that carries the great weight in nature, and 
so we find that the process of reproduction is dominant in many organ- 
isms, far transcending all other functions in structural and functional 
development. Furthermore, it should be noted that reproduction is a 
unique characteristic of protoplasm. It is difficult to conceive of 
living matter lacking the power of reproduction,, and, on the other 
hand, it is just as difficult to conceive of any type of nonliving material 
that could possibly possess this amazing life function. 

To the biologist, reproduction is seen as a process primarily based 
upon the power of growth and cell division. This was indicated in 
the opening chapter where it was shown that the growth of cells results 
from the dominance of the constructive metabolic processes and that, 
when the cell has grown to a certain characteristic size, it divides to 
form two daughter cells. The latter, under favorable conditions, 
increase in size until they equal that of the parent cell. It was also 
shown that, basically, cell division is reproduction. This fact is 
clearly evident in the unicellular forms of plants and animals in which 
the division (binary fission) of the one-celled body produces two 
independent daughter individuals. In the multicellular organisms, 
however, the relation' between reproduction and cell division is 
obscured by the fact that cell division normally results in adding 
additional cells to the body oT the individual rather than in the forma- 
tion of additional individuals. Eventually, however, when the proper 
stage of development has been reached, new individuals are produced 




by cell division occurring in a particular region of the multicellular 
parent individual or by a particular kind of cell division resulting in 
the production of the highly specialized gametes, eggs and sperm. 

FIG. 145. Asexual reproduction by cell division (binary fission) in Amoeba. (Wolcott, 

modified after Schulze.} 

FIG. 146. Asexual reproduction in unicellular yeast plant by budding. When buds 
remain attached, temporary colonies are formed. X 1,500. (Haupt.) 

It will be well to leave the consideration of the important features 
associated with cell division until the next chapter and continue here 
with the general Consideration of reproduction. (Fig. 145.) 



In the first place, it should be noted that reproduction is either 
asexual or sexual. The basic difference between these two methods is 
uniparental or biparental inheritance, or, in other words, whether the 
offspring have one parent or two parents. In asexual reproduction, 
the offspring are formed by the growth and division of cells from one 
parent. In sexual reproduction, two types* of individuals are con- 
cerned, male and female, in reproduction, and each new individual is 
formed following the union of two gametes : the 
male sperm and the female egg. This consti- 
tutes fertilization and produces a biparental 
fertilized egg, or zygote, which divides repeat- 
edly and gradually attains parental size. 


As an example of asexual reproduction in 
the multiccllular organism, consideration may 
be given to the process of budding, which is 
characterized by the rapid growth and division 
of the cells in certain regions of the parental 
organism with the consequent formation of a 
new individual, attached at first to the body 
of the parent but, in time, separating as an 
independent organism. Budding is an estab- 
lished method of reproduction even in unicel- 
lular organisms, as in the classic example of the 
yeast cell where a tiny area of the cytoplasm of 
the spherical parental cell enlarges to form a 
knob-like protuberance which later separates as 
an independent daughter cell. In the multi- 
cellular animals, common examples of budding 
are found in Hydra and other Coelenterates. 
The buds in Hydra are formed in various regions in the wall of the 
tubular body and gradually develop from a minute group of cells to 
multicellular structures almost as large as the parent Hydra before 
they finally separate. (Figs. 146, 147.) 

Closely related to asexual reproduction by budding is asexual 
reproduction through the regenerative process. Thus, in many 
organisms, it is possible to divide the body into several pieces and have 
each piece gradually grow into a complete organism. This process is 
very familiar in the plant world where standard methods of propaga- 

FIG. 147. -Asexual 
reproduction by budding 
in the primitive meta- 
aean, Hydra. Bud is 
seen at the right of the 
parent animal, a, ex- 
panded; b, contracted. 
X 12. (Wolcott.) 



tion involve the cultivation of cuttings from the parent plant, as in 
various common house plants or, in the fields, as in the potato or sugar 
cane. But many examples of reproduction through regeneration exist 
also in the animal kingdom. Again one of the best examples is found 
in Hydra, but representatives of various higher groups, particularly the 
worms, also exhibit great regenerative abilities. Thus in the marine 
flatworm, Linens socialiSj it has been shown that pieces with a calcu- 
lated volume only 1/200,000 of the normal size may regenerate a com- 
plete individual. In this and many other instances it is apparent 
that regeneration is equivalent to asexual reproduction. (Fig. 148.) 

A u B C 

FIG. 148. Asexual reproduction in a marine flatworm (Linens socialis) by fission. 
A, mature worm; B, division into nine parts, each of which, as shown in C, regenerates 
to form a normal worm which soon attains full size. (Woodruff, after Coe.) 

Speaking generally, it may be said that the power of regeneration 
in the animal kingdom becomes increasingly limited in the higher 
types and finally reaches a condition, as in man, in which very little 
regeneration of th& highly developed tissue and organs is possible. 
To the biologist, it is apparent that restriction of regeneration is 
directly associated with the increased tissue differentiation that is 
characteristic of the higher animal types. That is to say, the greater 
the differentiation in an organism the less the ability to regenerate 
missing portions. In such cases, as will be seen later, when injury 
occurs, unless the functioning of essential organs is disturbed, the 
continuity of the tissues is reestablished by the development of a 
connective, or scar, tissue which repairs the wound area but does not 
have the functional or structural characteristics of the tissues it 
replaces. Thus repair but not regeneration takes place. Differentia- 
tion becomes increasingly manifest during embryonic development as 
the ultimate adult condition gradually proceeds out of the relatively 


undifferentiated embryonic condition. But this is a subject that may 
be well left for more detailed consideration when the embryological 
processes are studied. 

Binary fission, budding, and regeneration, as discussed in the 
paragraphs just above, are purely asexual in nature and, in addition, 
are not dependent upon the development of specialized reproductive 
cells of any type. There are two other well-known methods of asexual 
reproduction, however, namely, spore formation and parthenogenesis, 
which involve the formation of special reproductive cells. These 
are worthy of some attention because of their widespread use in the 
living world. In the case of spore formation, the organism at some 
period or periods in its life history typically produces enormous num- 


FIG. 149. A common bread mold (Rhizopus nigricans): horizontal branch with two 
groups of erect spore-bearing branches (hyphae) . One of the spore cases is discharging 
spores. X 15. (Haupt.) 

bers of microscopic cellular bodies, or spores, each of which is capable, 
when given the proper conditions, of developing into an independent 
fully formed individual. Typically, spores are provided with coverings 
of a particular type which make these reproductive cells very resistant 
to unfavorable environmental conditions, and so they are ideal for 
wide dissemination and later development when the conditions become 
favorable. (Fig. 149.) 

Spore formation in the plant kingdom is of almost universal 
occurrence. In all the higher plant types, asexual spore formation 
is linked with sexual reproduction in the complete life cycle, the latter 
consisting of an asexual spore-forming generation which alternates with 
the sexual gamete-producing generation. Thus, the spores produce 
the sexual generation, whereas the zygote, formed by the fusion of the 
male and female gametes, gradually develops into the spore-producing 
generation. This constitutes alternation of generations which, though 
not of universal occurrence, is extraordinarily Apdespread in the plant 
kingdom. Furthermore, alternating sexual and asexual generations 
are not uncommon in the lower animal groups. Spore formation, in 


the animal kingdom, appears to be segregated in an important class of 
unicellular animals, the Sporozoa, which are of major importance as 
disease-producing parasites. Spore formation in this class is typically 
associated with complex life cycles in which sexual phenomena are 
involved. (Figs. 150, 250.) 

Finally, asexual reproduction may occur through the parthenoge- 
netic development of the female gamete, or egg. Examples of parthe- 
nogenesis have long been recognized as occurring normally in nature, 
particularly in representatives of the great insect group. The par- 
thenogenetic development of the male bee, or drone, (page 448) is the 
best known example and, in fact, has long stood as a classic instance of 
natural parthenogenesis. As a result of biological experimentation on 

Spore- Plant Spore Sexual Plant Gametes Spore- Plant 

. (SporophyteX (Gametophyty (Sporophyte): 

FIG. 150. Alternation of sexual and asexual generations in a fern. (Sinnott.) 

the eggs of various animals, several examples of artificial parthenogene- 
sis have been discovered, beginning with the original work of J. Loeb 
on the Echinoderm egg some 40 years ago. This experimenter found 
that it was possible to incite the eggs of the sea urchin to 4evelop in the 
absence of sperm if the proper chemical stimulants were used. In 
later years, the original results of Loeb have been greatly extended, 
so it is now established that various types of egg will begin to develop 
under the influence of artificial stimuli, even including those of highly 
developed vertebrates, like the frog or rabbit. 1 


Thus far our discussion of reproduction has been confined to asexual 
reproduction in which the new individual arises by the cellular activity 
of one parent, that is, uniparental inheritance. But reproduction is 


1 The production of Mature rabbits from eggs induced to develop through 
artificial parthenogenesis was announced by Dr. Gregory Pincus in the Proceedings 
of the National Academy of Sciences, November, 1939. 


otherwise in the higher animals, for asexual reproduction has been 
replaced by sexual reproduction, and each new individual is accordingly 
supplied with a legacy of materials from the two parents, the condition 
of biparental inheritance. The basic difference between the male 
parent and the female parent lies in the production of a characteristic 
type of reproductive cell, or gamete, the sperm or the egg. The 
gametes are usually produced in great numbers by both sexes but more 
particularly in the male. Underlying gamete formation, just as in 
any type of cell, are the basic processes of growth, cell division, and 

It should not be thought that sexual reproduction is entirely 
confined to the higher plants and animals, for numerous instances 
of it are found in the unicellular organisms. In the simplest examples, 
fusion occurs between two cells, or gametes, of equal size and uniform 
structural characteristics (isogamy), but, even in the lowest groups 
of living organisms, examples may be found in which the male and 
female gametes are as clearly differentiated as 'in the higher organisms. 
Accordingly, in fertilization, fusion occurs between a fre-swimming 
sperm and a passive egg cell of more or less typical cellular pattern 
(anisogamy). Differentiation of the gametes, for example, is very 
apparent in the unicellular sporozoon, Plasmodium vivax, which is 
responsible for malaria, one of the 
most dangerous of the human 
diseases. In Plasmodium, sexual 
reproduction takes place in the 
body of the mosquito. The male 
and female gametes formed by this a b c 

sporozoon give every indication of r Fia.isi. Successive stages in the 

^ ^. . . fusion of two equal-sized cells (isogamy) 

as great differentiation as those of of the unicellular algae, Chlamydomonas: 

higher types. Thus it is clear ^ basis of sexual reproduction. 

, , , , (Watkeys, Stern.) 

that, from the lowest animal 

groups up to the highest, sexual reproduction stands as a well-estab- 
lished phenomenon, but sexuality is of increasing importance in the 
higher animal types and in the vertebrates constitutes the only normal 
method of reproduction. In the plant kingdom, on the other hand, 
asexual reproduction by spore formation and by regeneration remains 
dominant in all groups. (Figs. 151, 250). 

Also the hermaphroditic condition, as in the earthworm, in which 
the same individual bears gonads for the production of both male 
and female gametes, disappears with the advent of the vertebrate. 
Numerous examples of alternation of generations and also of herma- 
phroditism are well-known in even the highest invertebrate groups. 



Another interesting variation of sexual reproduction, particularly 
prominent in the great molluscan group, is the reversal of sex which 
occurs in the life cycle of each individual. Thus the organism matures 
first as a male, later changes to a female capable of producing female 
gametes. Reversal of sex does not occur normally in vertebrate 
animals, but an authentic instance is recorded in which a normal egg- 
laying hen changed to a rooster producing fertile sperm. The reversal 
of sex in this instance was due to the destruction of the ovarian tissues 
following a tubercular infection. This raises the question that has_ 
received great attention in the last quarter of a century, namely, the 

>//erve cord 


FIG. 152. Diagram illustrating male and female reproductive organs in the earth- 
worm, a hermaphroditic organism. They lie near the anterior end, segments IX to XV 
as indicated. (Wolcott, after Wieman.) 

effect of the gonadal endocrine secretions in altering the sexual ch&rac- 
teristics of the individual. Some consideration has already been given 
to this in the previous chapter on Secretion (page 115). (Figs. 152, 

Due to the fact that, as indicated above, reproduction in the verte- 
brates is exclusively sexual and, furthermore, that the sperm and eggs 
are always produced in separate individuals, the variable features of 
vertebrate reproduction are largely associated with the mechanics 
of fertilization and development of the zygote. The essential feature of 
fertilization is always the actual union, or amphimixis, of the chromatin 
materials present in the sperm nucleus with the chromatin carried in 
the egg. nucleus. Thus fertilization, since it involves amphimixis, 
makes biparental inheritance possible; the nuclear contribution from 
each parent uniting to form a common nucleus, or synkaryon, in the 
fertilized egg, and each parent making equal contribution to the 


characteristics of the new individual. The fertilization of the verte- 
Wate egg may occur externally or internally. External fertilization, 
that is, outside the body of the female, is, however, the more common 
and primitive method in the water-living forms. In such types, both 
the eggs and sperm are adapted for temporary survival and union in a 
water environment. Since the gametes of the two sexes ripen at the 
same time and tremendous numbers of them are usually discharged in 
fairly close proximity to each other, the chances are that the actively 
swimming sperm will come into contact with the eggs, thus bringing 

FIG. 153. Schematic diagram illustrating the sequence of changes in the gonads of 
an oyster during sex reversal, oc, oocytes (large cells) ; sp> male cells (small dots) ; 
/, gonad without sex differentiation; Fra, goriad of young animal, bisexual; HM, male 
phase but somewhat hermaphroditic; (M 2 , M*) later stages in the development of the 
male gonad; F, first female phase; later female stage shown in F*\ F, M 2 t F 2 , M*, succes- 
sive stages in sex reversal; ct t connective tissue; ep, epithelium; gc, genital gland. 
(Skull, after Coe.) 

about fertilization and embryonic development. With external 
fertilization and development, the reproductive responsibilities of the 
parents cease with the formation and liberation of the gametes. In 
certain instances, notably in the frog, though fertilization and develop- 
ment are external as indicated, a temporary pairing (amplexus) of the 
male and female frogs occurs at the time the eggs and sperm are dis- 
charged, which insures a very high percentage of fertilization. 
(Fig. 154.) 

Numerous examples of animals are found, with both water and 
land habitats, in which internal fertilization of the eggs occurs, but 
the ensuing embryonic development is external, for the fertilized eggs 
leave the body of the mother shortly after fertilization and continue 


their development externally. In the warm-blooded birds, it is 
necessary to supply the developing eggs with the proper body tem- 
perature, or incubate them, in order for development to continue after 
the eggs are laid. Internal fertilization always requires the pairing, 
or copulation, of the parents, during which process the transfer of 
sperm from the male to the female takes place. Also, internal fertili- 
zation requires the development of external male genital organs for 
the transference of the sperm to the female, as well as the elaboration 
of the female organs concerned with the reception and conduction of 
the sperm to the eggs. In some animals, exemplified by the queen 
bee, storage of sperm is an important function accomplished by the 
presence of special sperm receptacles. 

In the mammalian female, the reproductive mechanism is further 
complicated by the retention of the fertilized egg in a special cavity, 
the uterus or womb, modeled out of a portion of the egg tubes, or ovi- 
ducts, through which the eggs pass on their way to the exterior. The 
proper care and nourishment for the embryo developing in the uterus 
present a number of difficult problems, the solutions for which have 
been found through the formation of a combination fetal and maternal 
structure, the placenta, which is described below. 

It is evident to the biologist that the underlying factor primarily 
responsible for variation in the development of the vertebrate egg lies 
in providing adequate nutrition for the embryo. The eggs of various 
vertebrates show great variation in the amount of nutritive materials 
or yolk that they contain at the time of fertilization. The embryo 
that develops from a type of egg in which very little reserve food is 
stored must necessarily be provided with some method for quickly 
securing nourishment from the environment. An example of this may 
be noted in the embryos of the lowly starfish which, lacking reserve 
food in the eggs, are able to form a primitive nutritive system for 
securing and utilizing outside nutritive materials a few hours after 
fertilization. With the exception of the mammals, the eggs of the 
vertebrates are heavily yolked, and, accordingly, the embryo is able 
to develop for a considerable period without the necessity of seeking 
food supplies from the surrounding environment. 

Food storage reaches a high peak in the hen's egg and those of other 
birds. This condition makes it possible for the developing embryo to 
remain sealed up in the original eggshell for the entire incubation 
period of three weeks, at the conclusion of which it breaks through the 
shell, or hatches, as a well-developed active individual. On the other 
hand, the mammalian egg is practically free from stored food but gives 
evidence during its development, as will be shown later, of being 


closely related to the heavily yolked eggs of the birds and reptiles. 
The lack of stored food in the mammalian egg makes it necessary for 
the zygote to secure nourishment from outside sources very quickly, 
and this is accomplished by the rapid development of an outer layer 
of nutritive cells, the trophoblast, which has the ability to secure the 
essential food materials from the tiny area in the maternal uterine walls 
in which the microscopic embryo is embedded. 

But the amount of stored yolk not only governs the nutritive 
requirements of the embryo; it also varies the pattern of early develop- 
ment with respect to the numbers and arrangements of the cells 
formed by the successive divisions of the zygote. For the stored yolk 
is inert, nonliving material, and its presence in the egg in any con- 
siderable amount retards cell division and considerably modifies the 
early embryonic stages. Thus we find that eggs with a comparatively 
small amount of yolk evenly distributed through the cytoplasm 
(homolecithal eggs) exhibit a total cleavage (holoblastic cleavage) 
marked by the formation of daughter cells which are fairly uniform in 

Eggs containing a considerable amount of yolk show a tendency for 
the nutritive materials to be concentrated at one pole (nutritive or 
vegetal pole) and to leave the cytoplasm of the animal pole relatively 
free. Eggs of this latter type are known as telokcithal eggs. When 
the mass of yolk is not too large, as is the case in the frog's egg described 
below, the cleavage is holoblastic just as noted in the homolecithal eggs, 
but a distinct lag occurs in the cleavage planes when they pass through 
the yolked area of the vegetal pole; this tends to result in the forma- 
tion of unequal-sized cells. Finally, in eggs with a very large amount 
of yolk, as in the hen's egg, the vegetal pole is greatly enlarged; the 
animal pole comparatively small. The cleavage is partial, or mero- 
blastic, and entirely confined to the animal pole. It will be unneces- 
sary to consider cleavage types further at present, for in the following 
description of vertebrate reproduction, as shown in the frog, bird, and 
man, the various important differences will be indicated. 


A close relationship exists between the excretory and reproductive 
systems in the vertebrate organism. This fact is seen to particular 
advantage in the urogenital system of the male frog in which the paired 
testes lie in close association with the kidneys. In fact, the testes and 
kidneys are directly connected by numerous fine ducts, the vasa 
efferentia, which convey the ripened sperm from each testis. On 
leaving the testis, these sperm ducts pass directly into the kidney tis- 



sues where they connect with the ducts leading to the urogenital canals. 
The latter extend from the kidneys to the cloaca and thence to the 
exterior. And so the urogenital canals, as the name indicates, serve 
as common ducts for the passage of urine from the kidneys and sperm 
from the testes. (Fig. 154.) 

Male. The testes in the frog are seen in gross structure as yel- 
lowy-white, capsule-shaped bodies, about l / in. in length, situated on 
the ventral side and near the anterior end of each kidney. Project- 
ing anteriorly from each testis is the so-called fat body with numerous 





FIG. 154. Urogenital systems of the male (right) arid female (left) frog. In the 
female only one ovary and oviduct are shown. In the male the testis and kidney at the 
right have been opened to show the course of the vasa efferentia through the kidney. 
(Redrawn by L. Krause from ShulL Modified.} 

tubular branches projecting in various directions. The fat bodies are 
not directly concerned with the reproductive processes but serve as 
storehouses for excess nutriment which later can be utilized by the 
body tissues as needed, chiefly, perhaps, in germ cell formation. Pass- 
ing now to the histology of the testis, a microscopic examination of 
sectioned material shows that it consists of a mass of very fine, coiled 
tubules, intermingled with abundant blood, nervous, and connective 
tissue elements. These are the essential seminiferous tubules in which 
great numbers of the sperm develop. They have their origin in the 
primordial germ cells present in the walls of the tubules. 

Female. Turning now to a consideration of the female reproduc- 
tive organs of the frog, which may also be regarded as fairly represen- 
tative of the vertebrates in general, they will be found to consist of a 


pair of egg-producing gonads, or ovaries; a closely associated pair of 
fat bodies very similar to those already noted in the male; and, finally, 
a pair of oviducts which carry the eggs from the ovaries to the cloaca. 
The frog ovary is essentially a sac-like structure with a fluid-filled 
central area. The innermost layer of the covering tissues constitutes 
the germinal epithelium in which the undeveloped germ cells, or 
oogonia, have their origin. As in the testis, these unmatured germ 
cells conform in general, to the cellular pattern of body cells, and then 
they pass through a series of developmental stages (oogonia, primary 
oocytes, and secondary oocytes) until the mature egg stage is finally 
attained. Due to the gradual storing of yolk material, as noted 
above, the egg cells increase in size until the mature frog's eggs are 
several times the size of the typical body cells. (Fig. 154.) 

The eggs reach their full development in the frog at only one period 
each year. This is normally in the early spring, at which time the two 
ovaries, distended with great numbers of the large egg cells, fill most of 
the space in the abdominal cavity. At the proper stage of maturity, 
the eggs break directly through the thin ovarian wall in large numbers 
and are drawn by ciliary action into the opening of the corresponding 
oviduct. It is important to note that no direct connection exists 
between an ovary and its oviduct as is the case in the testis and vasa 
efferentia. Furthermore, there is no connection between the oviducts 
and the ureters from the kidneys. The oviducts are a separate pair 
of tubes, and they function solely in the transfer of the eggs. Each 
oviduct is a comparatively large, convoluted tube connected directly 
with the cloaca and ending anteriorly in close proximity, but slightly 
anterior, to the ovary of the corresponding side. The ovarian end of 
each oviduct is enlarged, funnel-shaped, and lined with ciliated cells. 
The current set up in the body fluids by the ciliary action draws the 
eggs into the oviducts and starts them on their journey from the body. 
The cloacal ends of the oviducts are enlarged to provide an egg-storage 
space near the opening into the cloaca in which the eggs remain for 
some time before being discharged through the cloaca. The eggs, 
when released from the ovary, are enclosed in a thin transparent 
covering, the vitelline membrane. As they pass through the oviduct, 
additional covering layers of a gelatinous material are secreted by 
glandular cells present in the lining of the oviducts. These gela- 
tinous layers remain intact until a well-developed, active embryo is 
formed which is able to force its way through them. I 

Pairing of the male and female frogs occurs when the gametes are 
mature. During amplexus there is synchronous discharge of the sperm 
and eggs into the water, and this greatly enhances the possibility of 


fertilization. In their movements through the water, there is the 
possibility that the sperm are attracted to the eggs by chemical sub- 
stances released by the eggs. Such sperm-attracting substances have 
been demonstrated in certain organisms. 

Development of the Frog's Egg. The telolecithal frog's egg is 
visible to the naked eye as a tiny sphere, about the size of a pellet of 
tapioca. It is more or less clearly divided into a dark-colored animal 
pole and a light-colored, yolk-containing vegetal pole. Following fer- 
tilization, in which the union of male and female nuclei to form the 
synkaryon occurs, the fertilized egg, or zygote, begins to divide mitoti- 
cally into daughter cells which remain in close contact, enclosed within 
the original vitelline membrane. Should it happen that some of the 
eggs are not fertilized owing to a failure of the sperm to reach them, 
they will, of course, not develop and soon begin to disintegrate. The 
first cell division, or cleavage, of the zygote, which begins shortly 
after fertilization, is first visible externally in the animal pole. The 
plane of cleavage is vertical; that is, it passes through both the animal 
and vegetal poles, dividing the embryo into two equal-sized daughter 
cells. The second plane of cleavage is also vertical and divides each of 
the first two cells in half, thus forming four equal-sized cells. The 
third plane of cleavage is horizontal, at right angles to the first two, is 
entirely in the animal pole region, and results in the formation of the 
eight-cell stage; consisting of four small cells from the animal pole, and 
the same number of larger cells from the vegetal pole. Thus, begin- 
ning at the third cleavage, an apparent retardation of cell division 
occurs in the vegetal pole due to the greater concentration of inert 
yolk material in this region. This becomes increasingly evident in 
the succeeding divisions. (Fig. 155A to D.) 

Beginning at about the 24-cell stage, formation of the one-layered 
blastula is indicated internally by the development of a central 
cavity, or blastocoel, situated largely in the animal pole. The 
microscopic study of sections through an embryo in the early blastula 
stage shows that the blastocoel is enclosed above by a single layer of 
the-pigmented cells of animal pole and, below, is bounded by the larger 
cells of the vegetal pole. Thus the blastula stage of the embryo may 
be described as a one-layered organism built around a central cavity, 
or blastocoel. This first cellular layer of the embryo is known as 
ectoderm and constitutes one of the three primary germ layers from 
which all of the tissues and organs of the adult organism gradually 
arise. The other two primary germ layers, endoderm and mesoderm, 
develop somewhat later, as will be shown in the following paragraphs. 
(Fig. 155D to F.) 


Following the one-layered blastula stage, the next great landmark 
in embryonic development is the two-layered gastrula stage which is 
formed by a turning in, or invagination, of rapidly dividing ectoderm 
cells to form a new inner layer of cells, the endoderm, the second of the 
primary germ layers. In homolecithal eggs, the process of gastrulation 
may be crudely compared with pushing in the wall of a lightly inflated, 
thin-walled rubber ball with the thumb. If the wall is thus pushed 
in until it reaches the opposite pole of the ball, it is easy to see (1) that 
a two-layered condition results; (2) that the original cavity of the ball, 
corresponding to the blastocoel of the blastula, is obliterated; and 
(3) that a new cavity surrounding the thumb is formed. This latter 
cavity in the gastrula is lined with the invaginated endoderm arid 
will shortly function as a primitive nutritive cavity or enteron. 
(Fig. 1550, H.) 

In the telolecithal egg, as in the frog, the process of gastrulation 
is considerably modified and retarded by the inert mass of yolk in the 
vegetal pole. Basically, of course, gastrulation is the same in all 
types of egg in that it results in the formation of a two-layered embryo. 
In the frog embryo, this condition is reached in part by the over- 
growth and in part by the synchronous invagination of the ectoderm 
cells from the animal pole. As a result of the overgrowth, the cells 
of the vegetal pole are gradually and increasingly covered by the pig- 
mented ectoderm cells moving down from the animal pole. Synchro- 
nously, an underlying endoderm layer is being formed within. Invagi- 
nation begins in a definite region of the egg, known as the gray crescent, 
which lies in a restricted area of the animal pole. This invaginating 
area soon becomes circular in outline and is gradually reduced in 
diameter as the ectoderm cells cover over more and more of the vegetal 
pole. Finally, at the conclusion of gastrulation only a tiny area, the 
yolk plug, of the light-colored yolk cells remains visible externally, 
surrounded by the circular opening in the ectoderm. This opening 
through which the yolk plug is seen is known as the blastopore. It 
indicates the posterior end of the embryo and the approximate posi- 
tion of the future anal opening. (Fig. 155#.) 

If the embryo were transparent, it would be possible to observe 
under the microscope that other important internal changes, in addi- 
tion to the formation of the endoderm, were under way. Thus, 
before gastrulation has proceeded very far, the third primary germ 
layer, or mesoderm, begins to develop dorsally between the previously 
formed ectoderm and endoderm. And so the two-layered gastrula 
gradually changes into the final three-layered condition. In a trans- 
parent embryo, it would also be possible to see the gradual develop- 



ment of the endoderm, as it increases from a few cells to a distinct 
layer with many cells. And, just as was noted above in the develop- 
ment of the homolecithal egg, the continued development of the endo- 
derm results in the obliteration of the original blastocoel and the 
formation of a new endodermal-lined cavity, the enteron, which is the 
forerunner of the alimentary canal. The latter gradually evolves into 
a tubular structure, but not until considerably later does it open to 


F E 


.Neural tube 

in votginctfion 



FIG. 155. Early stages in the development of the frog. A, one cell; B, two cells; 
(7, four cells; D, blastula, many cells; E, section of D showing blastocoel, F, late blastula; 
G f ; gastrula, early; H, medullary plate; 7, formation of neural tube and elongation of 
body; /, tail bud stage; jfiT, median section through /. (Wolcott, from various sources.} 

the exterior through mouth and anus. Another important landmark 
in vertebrate development is the formation of an anteroposterior 
rod-like axis, the notochord, which differentiates from the mesoderm 
along the median dorsal line, between the ectoderm and endoderm. 
The notochord is the original foundation for the segemented bony 
vertebral column that later develops. (Fig. 155K.) 

An external examination of the embryo near the close of gastrula- 
tion reveals a definite flattening of the future dorsal surface to form 
the medullary plate. This constitutes the first visible evidence of 
the establishment of a central nervous system. Observations on the 


living embryo show that the edges of the medullary plate gradually 
become thickened and elevated above the surface of the embryo to 
form the medullary folds. A little later, these two medullary folds, 
extending the length of the body, meet in the dorsal mid-line and fuse, 
thus forming an ectodermal neural tube. From the latter, the brain, 
spinal cord, and other elements of the highly differentiated nervous 
system gradually arise. (Fig. 155/7, 7.) 

Oral sucker 

FIG. 156. Late stages in the development of the frog. A, embryo with oral sucker 
for attachment to water plants as in B\ C, external gills; Z>, much later stage at the 
beginning of metamorphosis, with internal gills and hind legs; E, embryo with large 
hind legs and shrinking tail; F, young frog with four legs and stump of tail; G t adult grass 
frog (Rana pipiens). (Wolcott, from various sources.) 

And now the embryo definitely begins to lose the spherical shape 
of the original egg stage and to stretch out in an anteroposterior 
direction, and, shortly, definite body regions can be identified. Ante- 
riorly, the general shape of the head is indicated, while posterior to it 
on each side of the .body, the ectoderm is noticably thickened and 
elevated to form prominent gill arches through which, later, the paired 
lateral openings, or gill slits, break through the body wall into the ante- 
rior pharyngeal region of the alimentary canal. Certain surface 
swellings in the head region give evidence of sense organ formation. 


A depression on the ventral surface of the head region indicates the 
position of the future mouth opening, and a similar depression at the 
posterior end of the body, just above the original blastopore, marks 
the rudiment of the anus. Posterior to the mouth area, a crescent- 
shaped region indicates the developing ventral sucker which is of use 
in the later tadpole stage. A rounded, knob-like dorsal projection at 
the extreme posterior end of the body is known as the tail bud. It 
gradually extends posteriorly and develops into the long, muscular 
tail. During all the changes so far, the embryo has not taken in any 
food from the environment; nourishment has been secured by the 
continued utilization of food materials stored in the vegetal pole of 
the egg during ovarian development. Even at the tail bud stage, the 
ventral and posterior regions of the embryonic body still contain a 
considerable quantity of available yolk. (Figs. 155J; 156A.) 

Rapid growth continues, and in a few days after fertilization, 
depending to a considerable extent upon the temperature conditions 
in the environmental waters, the embryos attain the free-swimming 
tadpole stage in which head, trunk, and tail are definite structural 
entities. The active tadpoles soon hatch; that is, they emerge from 
the surrounding gelatinous capsules originally secreted in the oviducts. 
For a short time, external respiratory organs, the filamentous, branched 
gills, are present. These develop as outgrowths, or projections, from 
the gill arches on each side of the body, just posterior to the head. 
The external gills persist only temporarily and are soon replaced 
by internal gills lying in the gill slits. Water currents continually 
pass through the gill slits en route from the pharynx to the exterior. 
(Fig. 156C.) 

In the young tadpoles, rudiments <3f the eyes, nose, and ears can 
be clearly identified in the head region. Dorsally, along each side of 
the body, the outlines of the primitive muscle segments, or myotomes, 
can be seen through the thin outer covering of ectoderm. The 
myotomes develop from the mosoderm layer which forms in two sheets, 
one lying on either side, that is, to the right and left of the neural tube 
and notochord. The mesodermal sheet on each side of the body 
grows ventrally and soon becomes differentiated into a dorsal portion, 
the vertebral plate, and a ventral portion, the lateral plate. The 
vertebral plates soon show evidences of segmentation and become 
divided into the segmented myotomes which later become associated 
with the vertebral column. The lateral plates do not become seg- 
mented but extend ventrally on each side of the body until they 
finally meet in the mid-ventral line, thus forming a complete layer of 
mesoderm lying just under the ectoderm. As the mesodermal tissue 


of the lateral plates is extending ventrally, it is also dividing into an 
outer somatic layer and an inner splanchnic layer. The somatic layer 
of mesoderm is responsible, primarily, for the musculature of the body 
wall, while the splanchnic layer encloses the endoderm of the primitive 
gut and gives rise to the supporting, vascular, and muscular elements 










FIG. 157. Diagrams illustrating the general body plan (A.) of a primitive vertebrate 
embryo with pronephros, in. which the tubules open into the coelom; and a more highly 
developed vertebrate embryo (B), as in the frog, with mesoriephros, in which the meso- 
iiephric tubules collect wastes from the blood stream through glomeruli as well as from 
the coelorn. The differentiation of the rnesoderm to form inyotomes and lateral plates 
is clearly shown. (Redrawn by L. Krause from Wilder, "History of the Human Body," 
Henry Holt & Company, Inc. Slightly modified.) 

of the alimentary canal. The space that develops through the split- 
ting and separation of the somatic and splanchnic layers of mesoderm 
becomes the coelom. (Fig. 157.) 

Tissue Differentiation. Having traced the main features of 
development in the frog from the fertilized egg to the well-developed, 
freo-swimming tadpole, it will be desirable at this point to summarize 


the fate of the primary germ layers, ectoderm, endoderm, and meso- 
derm. Thus, embryological studies show that the superficial layers, 
or epidermis, of the skin as well as the basic tissues of the exoskeletal 
structures are of ectodermal origin. The crowning achievement of 
the ectoderm, however, unquestionably lies in the formation of the 
all-important nervous system and associated sensory elements which 
permeate every possible niche and thus establish control throughout 
the entire organism. As is evident from the material presented in the 
previous chapter, the* nervous system is not only the most highly 
differentiated of all the organ systems but the one assigned to admin- 
ister all of the essential .functions of the living organism. 

The endoderm is the great nutritive layer of the body. The 
primitive gut, or enteron, which begins its development during gas- 
trulation is entirely endodermal, as has been noted above, and the 
functional cellular lining tissue in the permanent alimentary canal is 
directly derived from these original endoderm cells. A portion of the 
mouth* cavity (stomodaeum) and a small area at the extreme posterior 
end of the alimentary canal (proctodaeum) is, however, lined by ecto- 
derm. The endoderm, in addition to forming the functional lining 
of the alimentary canal, is also responsible for the formation of several 
important organs that are more or less closely associated with the 
nutritive system, including the liver, pancreas, thyroid glands, lungs, 
and bladder. All of these organs develop in much the same way by an 
outgrowth of the endodermal wall of the enteron at an early stage to 
form either single or paired rudiments. In this connection, a basic 
fact should be recognized, namely, that the vertebrate organs, in 
general, are not wholly formed from a single tissue but represent a 
structural and functional mosaic of various tissues. Thus the hepatic 
cells of the liver -are endodermal in origin, but the complete organ 
also contains vascular and connective tissues derived from the meso- 
derm together with nerve elements that are ectodermal in origin. 

From the important mesoderm layer comes three great organ 
systems : the vascular, muscular, and skeletal, all of which are widely 
distributed throughout the entire body. Previous mention has been 
made of the division of the mesoderm into the vertebral and lateral 
plates (page 298). The vertebral plate myotomes, with the somatic 
layers formed from the lateral plates, are responsible for the muscle 
tissues present in the body wall and in the appendages, as well as for 
the connective tissues and vascular elements. These same tissues 
surrounding the endodermal lining of the alimentary canal develop 
from the splanchnic mesoderm (Fig. 157.4). Both the somatic and 
splanchnic mesoderm layers contribute to the formation of the peri- 


toneum, which forms a continuous lining layer throughout the coelom 
as well as a covering tissue for the various organs! From the peri- 
toneum arise the mesenteries by means of which the various organs 
are suspended from the walls of the coelom. Finally, the functional 
elements of the urogenital system are mesodermal in origin. (Fig. 157.) 
The study of the life cycle of the frog, and of the great majority of 
amphibia, shows that the aquatic fish-like tadpole stage is only tem- 
porary. Metamorphosis occurs after some weeks, and the tadpole 
changes into the air-breathing, four-legged adult frog. Experimental 
work has definitely shown that the metamorphic processes in the 
amphibia are incited and regulated to a great extent by the thyroid 
hormone. The chief structural changes in metamorphosis are con- 
cerned with the development of the forelegs and hindlegs, the degenera- 
tion of the tail, and changes in the alimentary canal and the respiratory 
mechanism. The nutritive system of the tadpole, with a very long, 
coiled intestine, is equipped for an herbivorous diet. The meta- 
morphic changes remodel this system and adapt it for the more con- 
centrated carnivorous diet of the adult. Metamorphic changes are 
also of a radical nature with respect to respiration. The gill tissues 
degenerate, and so the animal is no longer able to secure oxygen from 
the water. Air must be forced into the lungs which, though present for 
some time, have not hitherto functioned. (Fig 156D to F.) 


With the main features of amphibian development in mind, we may 
now pass to a consideration of avian embryology which, though con- 
forming to the main features exhibited in the frog, presents certain dis- 
tinctive features of particular importance for acquiring a satisfactory 
understanding of human development, our final goal. The character- 
istic developmental processes of the birds are basically grounded in the 
provisions for the internal fertilization and for nourishing the embryo 
and represent a climx*in the storage of food in the telolecithal egg as 
has been indicated above. 

Reproductive System. We may begin our discussion with the 
reproductive system of the hen, which, it may be stated, is an 
unpaired structure in the adult developed originally on the left side 
of the body. The corresponding organs of the right side are present 
in the embryo but undergo degeneration in the female before maturity 
is reached. This condition does not obtain in the male; both of the 
testes and the associated ducts persist and function in the adult. 
The ovary, examined with the naked eye, 'is seen to consist for the 
most part of a mass of projecting yellowish globules of various sizes 



in which the eggs are undergoing development. Lying near the ovary 
is a large convoluted oviduct which ends anteriorly in a ciliated 
opening, the ostium. Posteriorly, the oviduct connects with the 

FIG. 158. Reproductive system of the hen. The single ovary is shown above, with 
numerous ovarian eggs in various stages of development (01, 02, 03, o 4 ). The ovi- 
duct with opening, or ostium (os), is showil with two eggs (Oi, Oa), though normally only 
one egg passes through the oviduct at a time. The oviduct has been opened at one place 
to show the egg (6)2) with blastoderm (b) and the albumin (a) which is being secreted. 
c, cicatrix; cl, cloaca; m, mesentery; r, rectum; u 1 uterus. (Wieman, after Coste-Duval.) 

cloaca. The oviduct consists, first, of the ostium, just noted. It is 
followed by a glandular portion lined with secreting cells that form 
the so-called white, or albumin, enclosing the yolk and also further 


posteriorly, the outer membranes and the calcareous shell as well. 
All of the materials are formed as secretions from the glandular lining 
cells. A short, thin-walled, distal region of the oviduct posterior to 
the secreting areas leads directly into the cloaca. (Fig. 158.) 

The egg cell or yolk,, as it is commonly termed, when released from 
the ovary is drawn at once into the near-by ostium to begin its passage 
down the oviduct. The yolk material is enclosed in a transparent 
vitelline membrane. Provided mating with the male has previously 
occurred, the sperm deposited in the cloaca of the female will have 
found their way up the oviduct. Accordingly, it is possible for the 
egg to be fertilized shortly after it enters the oviduct. Cleavage in 
the zygote then begins. As noted earlier, the large amount of food 
material present in the hen's egg makes total division impossible so 
that only partial, or meroblastic, cleavage takes place. Cell division 
is confined to the tiny disc-like blastoderm lying on the upper surface 
of the yolk. As cell division continues and additional food material 
is utilized, the blastoderm increases in size and gradually spreads over 
and encloses the inert mass of yolk. Thus at the end of about 5 or 6 
days' incubation it will be found that the actively growing tissues of 
the blastoderm completely cover the yolk area. (Fig.* 158.) 

Development of the Hen's Egg. In the hen, ovulation occurs 
independently of pairing, but, of course, such unfertilized eggs are 
infertile. As the egg passes down the oviduct, the lining cells secrete 
several layers of albumin. A portion of the albumin lying next to the 
vitelline membrane is drawn out to form a pair of spiral-shaped chala- 
zae, situated at opposite poles of the yolk. The chalazae prevent 
the yolk-mass from turning end for end in the egg but at the same time 
permit it to revolve with the shell when the latter is turned over and 
over on its short axis. Thus, when the egg comes to rest, the blasto- 
derm is always found lying above the yolk and can be seen when an 
opening is made through the shell. In the great majority of hen's eggs 
there is a definite orientation of the anteroposterior axis of the embryo 
so that, when the observer places the large end of the egg to his left, 
the axis of the embryo will be at right angles to the long a'xis of the 
egg, with the head end pointed away from the observer. (Fig. 159.) 

After the layers of albumin are secreted around the yolk, other 
glandular cells lying distally secrete a double shell membrane which 
forms a flexible resistant covering around the albumin. At the large 
end of the egg, these two membranes are separated so that an air space 
lies between them. Finally, the hard calcareous eggshell is also formed 
as a secretion, after which the egg is ready to pass into the cloaca and 
out of the body through the cloacal opening. The passage down the 


oviduct normally requires 24 hours, so that, if the egg is fertilized in 
the anterior end of the oviduct, the blastoderm will have reached the 
24-hour stage of development at the time the egg is laid. Further 
development of the blastoderm cells then ceases until the proper 
temperature is supplied. This is approximately 103F., corresponding 
to thatx)f the parental body tissues. The few cells, which constitute 
the partially developed embryo in the blastoderm of the egg at this 
stage of development, will remain dormant for several days without 
injury and then begin active developmental processes again when 
incubated in the normal manner by a hen or artificially in an incubator 
if the proper temperature is supplied. (Fig. 159.) 

FIG. 159. Diagram illustrating the internal structure of the hen's egg. a, air 
chamber; b, blastoderm. Arrow points towards the head end of the embryo, c, 
chalaza; da, fa, albumin; im, inner shell membrane; o, the yolk, the 4 egg proper, formed 
in the ovary; om, outer shell membrane; s, shell. (Wieman.) 

The continued division of the blastoderm following fertilization 
finally results in the formation of a great number of irregularly shaped 
cells with the smallest ones near the center of the blastoderm. Shortly, 
these primitive ectoderm cells arrange themselves in a bridge-like 
structure which overlies a small cavity, the blastocoel, near the 
upper surface of the yolk mass and just underneath the original blasto- 
derm. This chick blastocoel is comparable to that found in the bias- 
tula stage of the frog embryo as previously described. The assemblage 
of 'ectoderm cells above the blastocoel in the chick embryo constitute 
the blastula proper. Thus the blastula of the chick consists of a flat- 
tened or disc-like layer of cells lying above the blastocoel and also 
above the large yolk mass, rather than a spherical body of cells enclos- 
ing the blastocoel as noted in the frog. (Fig. 155J?.) 

Continuing our description of early chick development, it will next 
be found that the ectoderm cells, in the region of the blastoderm 


destined to become the posterior end of the animal, begin to divide 
more rapidly than in other areas. As a result, a sheet of cells is turned 
under the ectoderm and starts to invade the cavity of the blastocoel as 
a second, or endoderm, layer; synchronously forming a new cavity, 
the enteron or primitive gut. 
This is, of course, the process of 
gastrulation. The new endoderm 
layer spreads anteriorly and 
laterally under the outer ecto- 
derm, and soon both of these 
layers extend to the periphery of 
the blastoderm. At about the 
18-hour stage of incubation, a 
thicker region, the primitive 
streak, is clearly marked in the 
center of the blastoderm, and this 
indicates the establishment of the 
embryonic anteroposterior axis. 
The primitive streak is primarily 
due to a concentration of ectoderm 
cells along the median line of the 
blastoderm. It is in this region 
that the mesoderm layer begins 
to develop. The first mesoderm 
cells migrate laterally from the 
posterior part of the primitive 
streak, but, as more and more 
are formed, they spread anteriorly 


FIG. 160. Early stages in the develop- 
ment of the chick embryo. A, primitive 
streak stage (18 hours) ; B, head process 
stage (20 hours); C, embryo with seven 
pairs of somites (24 hours), ac, ammo- 
cardiac vesicle; ap t area pellucida; av, area 
vasculosa; hf, head fold; hp, head process; 
m, margin of mesoderm; pa, proamnion; 
ps, primitive streak. (After Wieman.) 

as well as laterally thus in time 
forming a complete layer of meso- 
derm between the ectoderm and 
the endoderm (Fig. 160A). 

Just anteriorly to the primitive streak region, a concentration of 
ectoderm cells forms an anterior thickening, known as the head proc- 
ess, which soon involves all three germ layers. The region of the 
blastoderm in which the head process develops contains the rudiments 
of various embryonic body structures. The next notable develop- 
ment in this region is seen in the arrangement of the ectoderm cells to 
form the medullary plate and, from the latter, the formation of a 
definite neural tube by the elevation of the lateral edges and their 
fusion in the median line as previously described in the frog. Coinci- 
dent with these activities, notochord formation and the differentiation 


of the mesoderm to form the segmental myotomes are started. The 
latter, as in the frog, soon show a division into dorsal and lateral 
portions and also the formation of somatic and splanchnic mesoderm 
with the coelom between. All of these developmental features are 
well under way during the first 24 hours of incubation (Fig. 1605.) 

From the description just given of the blastula and gastrula stages, 
it is evident that the really distinctive feature of early chick develop- 
ment is the fact that the embryo in the blastoderm area is spread out 
flat on the surface of the yolk. As a result, the rudiments of the 
various embryonic structures are formed in right and left halves lying 
on either side of the median anteroposterior axis of the body. These 
lateral, organ-developing areas constitute the extra-embryonic regions. 
At the 20- to 24-hour stage of incubation the embryo begins a process 
of folding which gradually separates it from the yolk mass and brings 
the right and left half of each organ in contact along the mid-ventral 
line where they unite to form the complete structure. The first of 
these folds (head fold) appears just anterior to the head region. A 
little later, right and left lateral folds are indicated which move in from 
the sides, and, finally, a tail fold is formed which progresses anteriorly. 
The final results are shown in the formation of the various organs 
by the union of the two halves and in the almost complete separation 
of the body of the embryo from the yolk material. Thus, after about 
96 hours' incubation, it is found that the well-formed embryo is 
attached to the yolk sac by only a short tubular yolk stalk. The 
latter is filled with blood vessels through which the circulating blood, 
laden with absorbed food materials from the extra-embryonic areas, 
passes into the body of the embryo. (Fig. 167A.) 

Two-day Chick Embryo. By the end of the second day of incuba- 
tion,, the embryo has attained a greatly increased size as compared 
with that of the tiny blastoderm. The anterior end of the embryonic 
body is folded from the yolk sac and is lying on its left side, whereas 
the posterior part of the body is still flat on the yolk. As a result, 
the embryo is shaped somewhat like a reversed question mark, with the 
anterior end of the body bent (cervical flexure) toward the right at an 
angle of almost 90 deg. The rudiments of the various organ systems 
are now established; and some of them, notably the vascular system, 
are functioning. Particularly striking in the living two-day embryo 
is the beating heart, which at this stage is connected with blood vessels 
running through the tissues of the embryonic body and also out to the 
yolk regions. The growth of the extra-embryonic endoderm has put 
this nutritive layer in contact with the yolk so that the latter can be 
digested and absorbed. Also the extension of the extra-embryonic 



mesoderm to the covering of the yolk sac makes possible the formation 

of a dense network of blood vessels for transporting the food materials 

absorbed from the yolk to the growing tissues of the embryo. The 

two-day heart has two chambers: an auricle and a ventricle. The 

auricle receives the extra-embryonic blood, rich with absorbed food 

material and laden with supplies of 

oxygen which have permeated through 

the shell and membranes of the egg. 

From the auricle the blood passes into 

the ventricle, and then it is quickly 

forced out through the connecting 

arteries to all the body tissues and 

back to the yolk sac for new supplies 

of food and oxygen. Gill slits, though 

non-functional in the chick, are pre- 

sent at this stage of development, and 

the blood leaving the ventricle is routed 

between them. They soon disappear 

as definite structures. (Fig. 161.) 

The embryo is mostly enclosed by 
this time in a fluid-filled amniotic 
cavity. The amnion starts to develop 
in the ectoderm anterior to the head 
region and grows posteriorly over the 
embryo as an ectodermal sheet. 
Finally, with the aid of the lateral 
and posterior amniotic folds, a two- 
layered sac is formed over the entire 
embryo. And so, from the beginning 
of the second day of incubation to the 
fourth day, the embryo is being sepa- 
rated from the yolk sac by folds that 
grow underneath and at the same time 
enclosed by the amniotic tissues that 
lie above. The rudiment of another 
important embryonic membrane, the allantois, is first seen at about 
the 72-hour stage as an endodermal outgrowth from the primitive gut, 
just posterior to the yolk stalk. The allantois finally forms a large 
vascular-walled sac which lies close to the outer shell of the egg and 
functions, primarily, in the respiratory interchange. The yolk sac, 
the amnion, and the allantois constitute the important embryonic 
membranes of the chick. (Fig. 167.) 

FIG. 161. Chick embryo with 27 
to 28 pairs of somites (48 to 50 hours) . 
The head fold of the amnion (HFA) 
now covers the anterior two-thirds of 
the body. E, eye; FB, forebrain; H, 
heart; HB, hindbrain; MB, midbrain; 
OT, auditory vesicle; TFA, tail fold 
of amnion. (Wieman.) 



The development of the chick embryo continues, enclosed within 
the shell, for a normal 21-day incubation period. It utilizes the yolk 
material for nutrition and carries on the essential respiratory exchange 
through the permeable shell materials. When the proper stage of 
development has been reached, the chick breaks through the shell with 
the aid of a horny projection which develops as a dorsal outgrowth, 
near the tip of the beak. Leaving the shell behind, the chick walks 
out into the open as an active, independent individual able to secure 
food for continued growth. 


It has previously been emphasized that the essential features of 
vertebrate development present a background of uniformity in the 
various types. The variable features appear to be basically associated 

F * v.;; v^i 


2 cells 4 cells 8 cells 16 cells 

FIG. 162. Photomicrographs of living eggs of rabbit: 2, 4, 8, 16 cells. (Allen, after 

Lewis and Gregory. "Science in Progress," Yale University Press.) 

with the embryonic nutrition which, in turn, is dependent upon the 
amount of food stored in the egg. A comparative study of mammalian 
development shows that the primitive group, the Prototheria, produce 
heavily yolked eggs, structurally very close to those of birds. Also, 
the prototherian eggs, after being internally fertilized, pass from the 
body of the female and undergo external development as do those of 
birds. Thus the Prototheria, which includes two well-known types, 
the duckbill (Ornithorhyncus) and the spiny anteater (Echidna), are 
oviparous. In all other mammals, however, both fertilization and 
embryonic development take place within the body of the female; 
that is, they are viviparous. And it is found that the typical mam- 
malian egg is microscopic in size and with a minimum amount of stored 
food. The evidence is clear, however, from the study of the early 
developmental stages that a close structural relationship exists between 
the mammalian egg arid those of reptiles and birds. (Fig. 162.) 

The necessity of retaining the fertilized egg in the mother for intra- 
uterine development is evident from the fact that the mammalian eggs, 
as noted, contain a minimum of nutritive materials. Therefore, if 


development is to continue beyond the early cleavage stages, an addi- 
tional food supply must be established at once. This is accomplished 
in the mammals through the specialization of a portion of the oviducts 
to form the womb, or uterus, which permits the developing embryo to 
tap the essential nutritive supplies carried in the maternal blood 
stream. The extent of embryonic 
development occurring in the uterus 
varies considerably in the various 
mammalian groups. In the more 
primitive viviparous mammals, as ex- 
emplified in such Marsupials as the 
kangaroo and opossum, uterine devel- 
opment is terminated early, and the 
offspring are born in a comparatively 
immature condition. Accordingly, 
after they are born, it is necessary for 
them to find their place in a special 
external pouch, the marsupium. This 
pouch is located on the ventral abdo- 
minal wall of the female, with the 
mammary glands opening into it. 
The embryos are carried in the mar- 
supium for some time and there nour- 
ished by the milk from the mammary 
glands until they reach a sufficient 
degree of maturity to take care of 
themselves. Even among the higher 
types of mammals, the maturity of the 
embryo at birth shows great variation. 
The uterine development of the off- 
spring is highest in the hoofed mam- 
mals, or Ungulates, while in the 
Primates, the order to which Man 
belongs, a comparatively meager uterine development is found, and 
the child is born in an essentially helpless condition which necessitates 
parental care for a considerable period. 

If mating has occurred previous to ovulation, great numbers of 
sperm will be present in the oviduct, and fertilization will occur very 
quickly after the egg is released from the ovary. The cleavage of the 
yolkless mammalian egg is holoblastic and soon results in the formation 
of a tiny spherical body of cells, the morula, which externally appears 
essentially the same as the blastula of other holoblastic types. Inter- 
nally, however, the cells of the mammalian morula show a much greater 

W (/) 

FIG. 163. Drawings of wax-plate 
models illustrating the early cleavage 
stages of the mammalian egg (pig), 
a, 2 cells; 6, 4 cells; c to/ are sections 
showing differentiation of inner cell 
mass (dark cells) and the develop- 
ment of the segmentation cavity. 
The trophoblast arises from the cells 
shown in stipple. (Stages selected 
from Wieman, after Heuser and 



differentiation than those of the blastula, with the rudiments of the 
three primary germ layers and the embryonic membranes definitely 

FIG. 164.- Photomicrograph of a portion of the uterinte lining (human), indicating 
the implantation site of the embryo (elevated central area). Cf. Fig. 165. (Scientific 
Monthly, January, 1940. Redrawn.} 

P. **< 

- V; /o. 

FIG. 165. Vertical section through the wall of the uterus showing the implantation 
of a very early embryo (Bryce-Tea'cher). a, amniotic cavity; c, trophoblast; en, point of 
entrance of embryo closed by reunion of edges; g, gland; m, mcsodcrm (extra-embry- 
onic); um, uterine mucosa (lining); ys, yolk sac. (Wieman.) 

established. A section through the tiny spherical morula, prepared 
for microscopic study, reveals an outer covering of the primitive 


nutritive ectoderm, or trophoblast, enclosing a central cavity. Sus- 
pended in the central cavity from the outer layer is a group of dif- 
ferentiated cells, the inner cell mass, which contains the tissue rudiments 
of the embryonic body. Examination of the inner cell mass shows 
that the rudimentary tissue layers are spread out flat over the yolk sac 
and covered above by the precociously formed amnion, essentially as 
in the chick embryo. Curiously enough, the tiny "yolkless" yolk sac 
under the embryo, though apparently homologous with that of the 
chick, is without nutritive function in the mammal. (Figs. 163, 165.) 

The trophoblast, which, as just noted, forms the outer covering, 
is essentially a specialized nutritive tissue. It secretes enzymes which 
erode a tiny area in the lining of the uterus. This enables the embryo 
to embed itself in the maternal tissues and to secure nutritive materials 
from them. The passage of the zygote down the oviduct normally 
takes two or more days so that, by the time the uterus is reached, the 
trophoblast is ready to play its double role in aiding the attachment 
of the embryo and in securing food from the maternal tissues. For a 
time, nutritive materials are secured by the combined digestive and 
absorptive action of the trophoblast, but shortly, as the embryo con- 
tinues to enlarge, a very remarkable organ of the pregnant mammalian 
female, the placenta, is formed by a combination of fetal and maternal 
tissues and is directly connected to the embryo by the umbilical cord. 
It is the placenta that permits the embryo to secure nutritive materials 
from, and give off fetal wastes to, the maternal blood stream. Early^ 
differentiation of the embryonic vascular system, extending through 
the umbilical cord, makes the rapid transportation of materials to and 
from the placenta possible. (Figs. 164 to 167.) 

And so the placenta functions in the essential interchange of mate- 
rials between the parasitic embryo in the uterus and the mother. 
Arteries of the maternal vascular system are continually bringing 
blood to the placenta with abundant supplies of food and oxygen. 
Maternal blood, carrying embryonic wastes which have been picked 
up during the passage through the placenta, leaves through the con- 
necting veins. The arrangement of the human placental tissues is 
such that the maternal blood flows into large spaces, or sinuses, where 
it directly bathes the projecting finger-like villi. The latter are 
formed from embryonic tissues and contain a network of fetal blood 
vessels both arteries and veins extending through the umbil- 
ical cord and connecting with the vascular system of the embryo. 
The main vessels, through which blood passes from the embryo to the 
placenta, are the umbilical arteries. Such blood is loaded with the- 
nitrogenous wastes and carbon dioxide excreted by the embryonic 



tissues. During its circulation through the capillary networks in 
the placental villi, these wastes are released. They pass through the 
walls of the villi, are picked up by the maternal blood stream and then 
are excreted from the body of the mother through the lungs and 
kidneys. Synchronously, the supplies of food and oxygen present in 
the arterial maternal blood are released in the placental tissues and 
pass through the walls of the villi and into the fetal blood. The latter, 







FIG. 166. Vertical section through the human placenta attached to the wall of the 
uterus (below). This shows the fetal blood vessels surrounded by "lakes" of maternal 
blood. Note that there is no direct connection between the circulatory system of the* 
mother and child. (Buchanan, "Elements of Biology," Harper & Brothers.} 

now freed from the excess wastes and laden with the substances neces- 
sary for the embryonic tissues, passes by way of the umbilical veins 
back to the embryo for circulation through the body. It should be 
emphasized that the vessels of the vascular systems of mother and 
child are not directly connected in the placenta or elsewhere. All 
interchange of materials between fetal and maternal blood must, 
therefore, take place by diffusion through the placental tissues. The 
duration of uterine development, or gestation, varies considerably in 
the various mammalian groups. For example, the mouse embryo 


FIG. 167. Diagrams showing the relations between the embryo and the embryonic 
membranes (amnion, allantois, and yolk sac) in (A) the chick embryo and (B) a placenta! 
mammal, as man. (Wilder, "History of Human Body," Henry Holt & Company Inc.) 


completes its development in the uterus in about 20 days, whereas 
the human gestation period is approximately 280 days. (Fig. 167.) 


With the general plan of mammalian reproduction in mind, con- 
sideration may be given to the main features of human reproduction, 
with particular reference to the structural and functional features of 
the male and female reproductive systems. 

Male Reproductive System, The reproductive system of the 
human male follows the general vertebrate pattern in its basic struc- 
tural features but is more highly developed and more closely associated 
with the vascular, endocrine, and nervous systems. The essential 
sperm-producing testes are two in number. They develop originally 
within the body cavity, lying close to the kidneys. Before birth, 
however, the testes migrate posteriorly, pass through the inguinal rings 
located in the abdominal wall on each side of the body, and, finally, 
take their position outside the body cavity in a soft-walled sac, the 
scrotum, which is primarily attached to t'he bony pubic arch. Each 
testis is seen as an oval-shaped body which hangs suspended in the 
scrotal sac by the attached spermatic cord. The latter is a composite 
structure containing the arteries, veins, and nerve fibers supplying the 
testicular tissues and also a portion of the sperm-conducting tubule, 
or vas deferens. All the elements of the spermatic cord are permeated 
and enclosed by connective tissues which, in turn, are continuous with 
the tissues of the body wall surrounding the inguinal ring. (Plate 
XVI A.) 

The scrotum is a more complex organ than is generally realized. 
It is essentially a two-layered sac with the constituent tissues merging 
with those of the body wall to which it is attached. The outer cover- 
ing of the scrotum consists of a layer of skin, more or less folded and 
enclosing a second layer consisting largely of smooth muscle tissue. 
From the latter, a median fold arises which, projecting anteriorly, 
divides the scrotum into the right and left chambers, each of which is 
occupied by the corresponding testis. Functionally, it appears that 
the scrotum acts as a temperature regulator for the delicate male 
germinal cells undergoing development in the gonads. Thus when the 
external temperature is lowered, as in a cold bath, the scrotal tissues 
contract, and the testes are drawn anteriorly, close to the body wall. 
The opposite condition is found when the external temperature is too 
high, for the scrotal tissues then relax, thereby greatly increasing the 
size of t^e scrotum. The testes fall away from the body wall, and 


enlarged surface areas of the scrotum are presented for cooling by 
surface evaporation. 

Previously, a description was given of the microscopic structure of 
the vertebrate testis as seen in the frog, but it will be well to indicate 
certain additional features characteristic of the human testis though 
the general plan of structure is much the same throughout the verte- 
brates. The human testis consists of two portions: the sperm-pro- 
ducing portion, or testis proper; and the sperm-transporting portion, or 
epididymis. The body of the testis consists primarily of a great many 




FIG. 168. Diagram illustrating tho general structural plan of the human testis and 
associated ducts. Diagrammatic. (Haggard, "Science of Health and Disease," 
Harper & Brothers.) 

very fine convoluted seminiferous tubules, in the walls of which the 
sperm develop. (Fig. 168.) 

Each of these male gametes exhibits a typical cellular structure 
in the early stages of development, but, after passing through the 
successive stages associated with sperm formation (spermatogonia, 
spermatocytes, and spermatids), it becomes a highly modified sperm 
cell, adapted for active movements in a liquid. A mature sperm, 
though minutely microscopic in size, is amazingly complex in, its 
structural appointments. Three main divisions are noted, namely, 
(1) an anterior pointed portion, or head, which is really the essential 


part of a sperm because it contains the chromatic material arranged ip. 
the gametic nucleus, which, as we know, is responsible for the transfer 
of paternal hereditary characters to the offspring; (2) a middle piece 
carrying the dynamic division center, or centrosome, to the fertilized 
egg; and (3) a vibratile tail, or flagellum, the movements of which are 
responsible for the locomotion of a sperm in a suitable liquid medium 
and for the ability to force its way through the resistant membranes of 
the egg. However, the movements of the mass of sperm from the testis 
and through the length of the urogenital canals, as well as their later 
discharge to the exterior, are primarily due to the action of the muscle 
tissue in the walls of the ducts rather than to individual locomotion. 

The seminiferous tubules in the various areas are grouped in some 
250 compartments, or lobules, formed by connective tissue partitions 
continuous with the outer connective tissue sheath (albuginea testis). 
Intermingled with the tubular sperm elements throughout the testic- 
ular areas are important endocrine elements in the interstitial tissue, 
which are responsible for the development of the secondary sex 
characters and the general control of the sex phenomena, as previously 
described in the chapter on Secretion (page 115). The seminiferous 
tubules converge toward the posterior testlcular wall and there connect 
with the tubular network of the rete testis, which, in turn, is in direct 
connection with the long, greatly coiled tube of the epididymis. The 
latter is about 20 ft. in length and, distally, leads into the final and 
larger conducting element, the vas deferens, which passes through the 
spermatic tubule and enters the body cavity through the inguinal 
ring. The vas deferens from each testis joins the urethra carrying 
liquid wastes from the bladder. It is apparent that the urethra in 
the male serves as a common duct for both urine and semen in its 
extension from the vas deferens-urethral junction through the penis 
to the external opening. 

Associated with the vas deferens are a number of other noteworthy 
structures which function in various ways, as will be indicated. Thus, 
a sperm reservoir, the seminal vesicle, opens into each vas deferens 
shortly before the latter joins the urethra. This junction forms the 
ejaculatory duct which, as the name indicates, forces the sperm stored 
in the seminal vesicles into the urethra. A glandular structure, the 
prostate gland, surrounds each ejaculatory duct close to the urethral 
opening, with a duct opening into the urethra below the opening of 
the vas deferens. The prostate glands give off a secretion of doubtful 
function which mixes with the sperm. Additional glandular material 
is received from another pair of tiny glands (Cowper's glands) situated 
close to each prostate and also opening into the urethra. Accordingly, 


the complete seminal fluid is found to be a milky liquid made up of 
the various glandular secretions and normally containing some 70 






















C D 

PLATE XVI. Drawings illustrating the human reproductive system in male (A, B) 
and female (C, D). In A and C the various structures are shown in perspective; in 
B and D the structures are shown as seen in median sagittal sections. 

million sperm in suspension in each cubic centimeter of fluid. Since 
from 3 to 5 cc. of seminal fluid are released at a normal emission, the 


number of contained sperm is seen to be very large. The action of the 
ejaculatory ducts in forcing the sperm from the seminal vesicles into 
the urethra has been noted. Movement of the seminal fluid down the 
muscular-walled urethra is largely due to a reflex action of these con- 
tractile tissues, under control of the autonomic nerve fibers. (Plate 

The penis, which serves as a common urinary and copulatory 
organ, reaches its full development only in the higher mammals. 
Thus in the prototherian mammals, in which a cloaca is present as 
in the birds, the penis is relatively undeveloped and remains concealed 
in the wall of the cloaca except when pairing takes place. In man, 
the penis is external and is attached primarily to the bony elements 
of the pubic bones, ventrally and in front of the scrotal sac; the skin 
of the body wall forming a continuous extension over it. Below the 
skin .covering, the body of the penis is enclosed by a firm sheath com- 
posed largely of connective tissue elements and numerous unstriated 
muscle tissue fibers which radiate in all directions. Permeating these 
tissues are many large blood spaces, or sinuses, which, when filled with 
blood, increase the turgidity of the penis and thus cause its erection. 
The constituent connective and muscle tissues are arranged to form 
three main tubular bodies which extend throughout its length. These 
are designated as a pair of corpora cavernosa which lie toward the 
anterior surface, and the unpaired spongy body, or corpus spongio- 
sum, lying below, that is, toward the posterior surface. The spongy 
body is penetrated throughout its length by the urethra and ends 
distally in a terminal enlargement, the glans penis, which it alone 
forms; the corpora cavernosa ending just back of the glans. The 
skin covering the penis is not attached to the tissues of the glans but 
projects over it as a loose circular fold known as the foreskin, or pre- 
puce, which, normally, can be pushed back along the body of the penis, 
thus exposing the glans completely. Not infrequently the prepuce is 
drawn so tightly over the glans that it cannot be pushed back. In 
such cases, circumcision is indicated, a comparatively simple operation 
to which Hebrew male babies have long been subjected. 

When functioning as a urinary organ, the penis is soft and flabby 
and in this position hangs pendant over the scrotum. As a copulatory 
organ, it is necessary that erection take place so that penetration can 
be made into the vagina of the female where the sperm are deposited. 
The act of erection is under the control of the j^rasympathetic nerve 
fibers originating in the lumbar region of the spinal corcFanH is accom- 
plished by increasing the blood flow into the penis through nerve 
impulses passing ovr the vasodilator fibers to the muscle fibers in the 


blood vessels and at the same time restricting the outflow of blood. 
As a result, the large sinuses present in the corpora cavernosa and the 
corpus spongiosum become filled with blood under considerable 
pressure. Also associated to some extent in the erective phenomena 
are the widely distributed muscle fibers. 

Female Reproductive System. Comparatively simple in the 
lower types of animals in which the gametes ripori only once a year and 
are then released for fertilization 
and development outside the body 
of the mother, the reproductive 
system of the vertebrate female 
exhibits increasing complexity as 
provision is made for ovulation at 
comparatively short intervals and 
for both fertilization and embryonic 
development to take place in the 
body of the mother, as in the 
human species. The essential egg- 
producing gonads in women consist 
of a pair of ovaries which develop 
and remain permanently located in 
the abdominal cavity, somewhat 
posterior to the kidneys. The 
ovaries are of comparatively small 
size,* measuring only about l/^JB* 
in length, % in. in width, and with 
a maximum^weighf^drsome 5 or 6 
g., or about % oz. Histologically, 
the ovaries are .found to consist very largely of a firm connective 
tissue matrix enclosed by a characteristic covering tissue, the germi- 
nal epithelium. It is in the latter tissue that the female gametes 
are first localized as distinct cellular bodies; the primordial female 
germ cells. (Plate XVIC,D.) 

During the years of sexual maturity, the immature primordial 
germ cells continuously migrate from the outer germinal epithelium 
centrally into the peripheral matrix of the ovarian tissue where each 
forms a Graafian follicle. The latter is first seen as a tiny spherical 
area in the matrix containing a comparatively large central cell; the 
immature egg or oogonium, surrounded by one or more layers of 
follicle cells. In the functional mature ovary, numerous Graafian 
follicles of various sizes and stages of development are found scattered 
through the connective tissue matrix. As the egg cell passes through 

FIG. 169. Portion of a section 
through the mammalian ovary (white 
rat) showing a nearly mature Graafian 
follicle with egg as it appears under the 
microscope, d.p., discus proligerous; 
/.c., follicular cavity;, membrana 
granulosa; o, ovurn, or egg; t.e. t t.i., 
outer membranes. Cf. Figs. 60, 170. 


the various developmental stages, it gradually increases in size, but 
even more the size of the entire follicle increases until it becomes a 
large fluid-filled cavity bounded by several layers of follicle cells and 
containing the large egg cell, now known as an oocyte, mounted as if 
on a pedestal (discus proligerus) of follicle cells; several layers of these 
cells also enclose the oocyte. (Fig. 169.) 

Though the germ cells first migrate into the ovarian tissues and give 
rise to tiny Graafian follicles, the great increase in the size of these 
structures, as the gametes approach maturity, brings them not only 
to the periphery of the ovary again, but they actually bulge 

FIG. 170. Photograph of a normal pig ovary showing numerous small transparent 
elevations, each of which is a Graafian follicle with an egg, and also several large opaque 
spherical bodies which are the corpora lutea and mark the sites of previous Graafian 
follicles. (Cf. Figs. 60, 169.) Allen, "Science in Progress," Yale University Press. 
After Allen, Kountz, and Francis.) 

out from the surface as transparent, blister-like areas, plainly visible 
to the naked eye. When the enclosed egg cell is fully mature and ready 
for release from the ovary, it breaks directly through the surrounding 
layers, follicle cells as well as the outer germinal epithelium of the 
ovary, at the most convenient spot. During this period of develop- 
ment in a follicle, profound changes are occurring in the nucleus of the 
gamete that insure the proper maternal heritage to the zygote. This 
basic feature of gamete formation will be discussed in the next chapter. 
There is no direct connection between ovary and oviduct as there is 
between testis and sperm duct ; and though the opening of the oviduct 
lies in close proximity to the ovary, there is always the chance that an 
egg liberated from the ovaiy may fall into the abdominal cavity 
instead of passing into the oviduct. 


The two hormones, estrone and progesterone, produced in the ovary 
have already been discussed in the chapter on Secretion (page 116). 
Particular attention should be called to important ovarian endocrine 
tissue, the corpus luteum, which develops in the cavity of a follicle 
shortly after ovulation occurs, apparently from the follicle cells that 



FIG. 171. Section of a gravid human uterus showing five weeks embryo and 
associated structures. Diagrammatic. (Arey, "Developmental Anatomy" W. B. 
Saunders Company.) 

remain in situ. The amount of the corpus luteum tissue formed and 
the period of its retention as a functional glandular tissue depends 
upon the fate of the egg. If the egg is not fertilized, the corpus luteum 
soon begins to decrease in size, and, shortly, only scar tissue remains 
in the ovarian wall to mark the location of the follicular area. On the 
other hand, if the egg is fertilized and development gets under way 
in the uterus, the corpus luteum continues to increase in amount for 
some time after ovulation and to secrete the powerful hormone pro- 



gesterone which is responsible for the omission of ovulation and also 
for the cyclical menstrual changes during pregnancy. (Figs. 60, 170.) 
The uterine development of the mammalian fetus necessitates 
marked structural and functional changes in the oviducts, which, in 
the lower types function merely as egg-conducting tubes. Thus the 
human oviduct is divided into three distinct regions: the oviduct 
proper, or fallopian tube; the uterus; and the vagina. The uterus and 
vagina are single, unpaired structures which develop from the union 
of the distal portions of the right and left oviducts. Each oviduct 
in the human female is a very small tubular organ about 4 in. in length 
and with a tiny central cavity, or lumen, about the size of a bristle. 
It is lined with ciliated epithelium which aids in the movement of the 

Fetal, 2 mo. .Fetal, 5 mo, At birth 2yrs, 


n yrs. 


FIG. 172. Changes in human body proportions during prenatal and postnatal periods. 

(Sherhon, after Stratz.) 

eggs toward the uterus. The sperm apparently swim against the cili- 
ary current in passing up the oviducts to fertilize tfoe eggs. The walls 
of the uterus are largely composed of unstriated muscle tissue which is 
covered on the outside with the peritoneal epithelium and lined with a 
vascular mucous membrane. The cavity of the uterus is, of course, 
continuous with the lumen of the oviducts. Functionally, the uterus 
is characterized by its great extensibility during pregnancy when it 
may increase from approximately 3 in. to 1 ft. or more in length in 
accordance with the size of the growing fetus. There is also great 
flexibility in the position of the uterus. It is not firmly attached to 
any bony structure but rather suspended, as it were, by a number of 
flexible ligaments which permit a shift in position in response to the 
various mechanical factors appearing during pregnancy. With the 
completion of uterine development, rhythmic contractions of the uter- 
ine musculature, under the influence of unknown factors, begin which, 


in the course of a few hours, are usually powerful enough to force the 
embryo from the uterus and out through the vagina to the exterior; 
the process of childbirth. (Plate XVIC, D; Figs. 171, 172.) 

Another characteristic of the mammalian uterus is a periodical 
series of changes in the lining tissues which are apparently essential 
to the proper preparation of the uterine wall for the imbedding and 
nourishment of the fertilized egg. In the lower mammals, these 
cyclical uterine changes occur once or twice a year and are known as 
the period of heatj or oestrus. In the human female, the menstrual 
periods normally occur every 26 to 28 days during the period of sexual 
maturity. The latter typically extends from about the twelfth year 
to forty-five or fifty years of age. As noted above, the menstrual 


............ JillrllUIUlllll 

FIG. 173. Scheme illustrating the ovarian and uterine cycles. (Haggard, "Science of 
Health and Disease," Harper & Brothers.) 

phenomena do not occur during pregnancy. Each menstrual period 
is from 4 to 6 days' duration and is marked by a considerable discharge 
of blood from the uterine cavity which passes to the exterior through 
the vaginal opening. Histological examination of the menstrual dis- 
charges reveals the presence of numerous epithelial cells from the lining 
x)f the uterus. As a matter of fact, it is clear that menstruation is 
primarily due to a degeneration of the uterine lining with the conse- 
quent exposure of, and leakage from, the underlying capillaries. The 
tissue degeneration is followed by regenerative processes; and thus, 
periodically, a new lining surface is established in the uterus for the 
reception of the developing embryo. Primarily the menstrual phe- 
nomena are controlled by hormone action. The permanent cessation 
of menstruation at forty-five to fifty years of age indicates the end of 
ovulation and fertility. This period, known as the menopause, or 
climacteric, may be accompanied by certain definite clinical symptoms, 
such as varying temperature reactions, muscular pains, and dizziness. 


In some cases, temporary psychical reactions of a more or less disturb- 
ing nature also appear during this change of life. (Fig. 173.) 

The final structural unit derived from the oviducts is the vagina, 
which is an unpaired tubular structure extending from the uterus to 
the external opening or vestibule. Previous to sexual union, the 
external opening of the vagina is partially closed by a thin membranous 
sheet, the hymen. In the mammalian female, contrary to the 
condition noted above in the male, the genital organs have separate 
ducts and openings independent of those of the urinary system. The 
clitoris, a small unpaired organ with erectile and nerve tissue, homol- 
ogous in its development with the penis of the male, is situated anterior 
to the opening of the urethra. (Plate XVID). 

The immaturity of most mammalian embryos at birth makes it 
necessary for the infant to be nourished by the mother for some time. 
And, so, associated with the mammalian female reproductive system 
are one or more pairs of mammary glands for the formation and 
secretion of a nutritive fluid, milk, which is adapted for the infant 
nutrition. The single pair of mammary glands, 1 or breasts, of the 
human female are small during childhood; but at the time of puberty, 
thdy begin to enlarge as a result of the growth of the associated con- 
nective tissues together with the deposition of fat. The glandular 
function, however, remains inactive until pregnancy occurs. During 
gestation, the glandular tissues show a further gradual increase in size 
and, under normal conditions, are ready for functional activity follow- 
ing childbirth. (Fig. 51.) 

1 Consult Appendix : Mammary Glands. 


In the chapter just preceding, general consideration was given to 
the basic features of plant and animal reproduction, and, with this 
material as a foundation, a rather complete description was presented 
of the reproductive processes in three representative vertebrates: the 
frog, domestic fowl, and man. It was emphasized that reproduction 
is not essential for the maintenance of the living state in the individual 
organism but is solely concerned with the perpetuation of a particular 
type of species through the production of new individuals that are true 
to type. The perpetuation of a species means that the new individuals 


Fia. 174. Scheme illustrating the reproductive cycle in a triple blastic animal with 
sexual reproduction in which only the sperm and egg bridge the generations and, there- 
fore, necessarily carry the entire body of heritable materials, e, egg; g.c., germ cells; 
2, zygote. (Woodruff, modified from Hegner.) 

must conform closely to the characteristic pattern. Without at 
present going into the question as to just what constitutes a species, 
it is a matter of common knowledge that species breed true; figs do not 
produce thistles, nor do mice beget elephants. Thus, a fixity of species 
exists from generation to generation. At the same time, it is also 
commonly recognized that a certain amount of individual variation is 
always present among the members of a particular species. 

With this situation in mind, it becomes evident that some mecha- 
nism must be operating in the specialized germ cells, bridging the 
generations, that makes certain that the new individuals develop true 
to the parental types. Possibly the presence of an intracellular 
mechanism of inheritance is not so clearly indicated in certain types of 
asexual reproduction as it is In the more involved processes of sexual 
reproduction, for, in the latter case, the entire body of heritable 
materials transferred to the individual of the next generation is 
necessarily carried by a microscopic sperm and egg. (Fig. 174.) 




On the other hand in asexual reproduction in the unicellular 
organisms where reproduction is by binary fission, or in the multicellu- 
lar organisms where the processes of regeneration lead to the produc- 
tion of new individuals through the gradual growth and differentiation 
of considerable portions of the parent individual, it seems reasonable 
to expect that an offspring would conform closely to the parental 
individual from which it sprang. Nevertheless, even a superficial 
knowledge of the material at hand makes it evident that, in the absence 
of a basic intracellular mechanism, every cell division would offer 
almost unlimited opportunity for divergence in the daughter cells. 

FIG. 175. Asexual reproduction of the strawberry by the production of runners 
which later die away, thereby isolating a new plant developed directly from the parent. 

The fact of the matter is that whenever a cell undergoes normal 
division, or mitosis, be it plant or animal, soma or germ, the inheritance 
of the daughter cells is rigidly and accurately determined. From the 
zygote to the adult, mitosis is not a haphazard process. It is because 
of this fact that species do breed true and that the reproductive 
mechanism in any particular species of plant or animal operates to 
perpetuate that particular species and no other. (Fig. 175.) 

Accordingly, it may be regarded as established that the production 
of a new individual of a particular species, whether asexually or 
sexually produced, is always dependent upon the orderly processes of 
mitosis and, though occasionally something may go wrong and a freak 
or monster may appear, it is a matter of common observation that 
abnormalities are so rare as to cause the widest interest. The descriu- 


tions of various species by the earliest systematists, if based on accurate 
observations, check absolutely with the individuals of the same species 
living today, though in the meantime untold generations have come 
and gone. Or we may go back far, far beyond the time of even the 
earliest systfcmatist and compare the structural characteristics of 
various plant arid animal types, which lived millions of years ago, with 
their descendants now living and still occupying their particular niches 
in the amazing web of life. 

But turning to the other side of the picture, lest the principle of the 
fixity of species be overemphasized and thereby the impression given 
that all species have remained essentially unchanged throughout the 
previous ages, it should be stated at once that plenty of evidence exists 
also to show that species have changed at various times in the past and 
that further changes may still occur. There is descent with change; 
a basic relatedness occurs throughout the living world that gives 
evidence that in the beginning there was a common life stream from 
which all the infinite varieties of organisms now present in the world of 
life, as well as a great many types that have entirely disappeared, have 
been derived. Biologists are convinced that the intracellular mecha- 
nism associated with the mitotic phenomena in cell division is primarily 
responsible for the production, generation after generation, of offspring 
conforming to the species type; for the individual variation that is 
apparent with the members of a species; and, finally, for descent with 
change, or, as it is more commonly termed, evolution. With the 
explanation of these basically important features bound up with 
mechanism of cell division, it is obvious that a thorough understanding 
of all phases of this basic process is necessary. 


Mitosis is primarily a nuclear phenomenon involving an exact 
division of the chromatin ; the essential substance for the transfer of the 
heritable materials to the daughter cells. In the accomplishment of 
this central aim the nucleus temporarily disappears and an elaborate 
apparatus, the spindle, is temporarily set up in the cell cytoplasm so 
that the entire cell is involved in the complicated mitotic phenomena. 
Although mitosis is normally a continuous process from the time it 
begins until the single cell has divided to form two cells, four stages 
during the process are generally recognized, namely, prophase, 
metaphase, anaphase, and telophase, to which consideration may now 
be given. (Fig. 176.) 

The Prophase. Just previous to the beginning of the prophase, the 
nuclear and cytoplasmic elements in a cell appear in their normal 



f 8 

FIG. 176. Diagrams illustrating important stages in mitotic cell division, a, cell 
in resting stage; b, c, beginning of cell division indicated by division of the centrosome 
lying above the nucleus and the condensation of the chromatin to form definite chromo- 
somes (prophase) ; d, equatorial plate stage (metaphase) with first cleavage spindle well 
formed and division of each chromosome into two parts; e, separation of the halves of 
each chromosome and beginning of migration to the opposite poles of the spindle 
(anaphase) ; /, final stage (telophase) just previous to the separation into daughter c( lls, 
as shown in g. (Watkeya, Stern, modified.) 


structural relationships; but as this stage develops, marked changes in 
the structural pattern are in evidence as the cellular materials are 
rearranged for the approaching climax of cell division. The beginning 
of the prophase in animal cells is first indicated by the activity of the 
centrosome, a very minute particle in the cytoplasm, lying in close 
contact with the nuclear membrane, which splits into two daughter 
centrosomes. The latter, without delay, begin to move away from 
each other toward the opposite poles of the cell. Each of the oentro- 
somes is quickly surrounded by a structurally modified region of the 
cytoplasm from which fibrils, or rays, soon radiate peripherally to form 
the aster. As the two centrosomes with their surrounding astral halo 
continue to move apart, numerous fibers of another type appear in the 
cytoplasm between them. These are the spindle fibers, and they 
extend from each centrosomal area toward the center of the cell where 
they join with the nuclear elements and thus form a fibrillar spindle. 
The center of the spindle is approximately in the center of the cell 
where it surrounds the nuclear area. During the peripheral migration 
of the daughter centrosomes and the development of the asters and the 
spindle, profound changes have been occurring in the nucleus. Exter- 
nally, this is marked by the gradual disappearance of the nuclear 
membrane, thereby removing the boundary between cytoplasmic 
and nuclear elements. (Fig. 176a, 6, c.) 

But of greater importance in the transfer of hereditary characters is 
the decisive rearrangement of the chromatin pattern marked by the 
development of definite structural units, the chromosomes. At the 
beginning of mitosis, the chromatin throughout the nuclear area 
appears as an irregular network with embedded particles of various 
shapes and sizes. Soon this is changed, and the chromatin is con- 
solidated to form a definite number of long thread-like bodies, the 
chromosomes, which with the proper technique are beautifully differ- 
entiated in stained preparations and can also be seen in living cells 
under certain conditions. Expert examination of a chromosome under 
the highest magnification shows that it is essentially a double structure 
with two chromatin elements in close contact, or fused, throughout 
their length. As a mattei^of fact, the double condition of the chro- 
matin is usually apparent in the early prophase stage. Furthermore, 
it is apparent that all the chromosomes do not have exactly the same 
shape but exhibit a distinct structural individuality; some are long, 
some short, some angular, some like tiny spheres. This individuality 
of the chromosomes has deep significance and will be considered later in 
connection with the development of the germ cells. The formation 
of the specific chromosome entities from the apparently nondescript 


net-like chromatin and their attachment to the spindle fibers mark the 
end of the prophase. 

The Metaphase. This stage of mitosis is a comparatively short 
one. It is characterized by the definite alignment of the chromosomes 
in the center, or equator, of the spindle, equidistant from the two 
asters. This region of the spindle is termed the equatorial plate. The 
mechanics involved in the shifting and definite arrangement of the 
chromosomes in the equatorial plate are quite obscure, but, presum- 
ably, forces applied through the attached spindle fibers are responsible. 
At all events, the underlying plan is evident, for the chromosomes are 
thus placed in the proper position for longitudinal cleavage, which 
occurs during the next stage of nditosis, the anaphase. (Fig. 176rf.) 

The Anaphase. The anaphase may, perhaps, be regarded as the 
climax of mitosis for during this stage, as just stated in the preceding 
paragraph, each of the chromosomes in the dividing cell splits longi- 
tudinally to form two chromosomes. . This longitudinal division 
separates the two elements of the double chromosome structure, noted 
in the prophase stage, in a precise qualitative and quantitative manner 
so that each of the two resulting cells receives the correct chromatin 
content. The two " daughter " chromosomes, formed by the division 
of each of the chromosomes in the equatorial plate, now move in 
opposite directions toward the aster from which the attached spindle 
fibers radiate. And so, when cell division is completed and two 
independent cells have been formed, it will be found that the nucleus 
of each of these cells does not contain a miscellaneous array of chromo- 
somes but always one chromosome from each pair formed by the 
splitting of the chromosomes, as just noted. Since it was apparent in 
the prophase stage that a chromosome is a double structure, it may bo 
simpler to say that, during the anaphase, the two elements of each 
chromosome undergo complete separation and move toward opposite 
poles of the spindle. (Fig. 176e.) 

The forces involved, both in the longitudinal splitting and in the 
later separation of the half-chromosomes during the anaphase, are by no 
means fully determined as, indeed, is the case with most of the mitotic 
phenomena, but considerable evidence is now at hand to show that 
the chromosome elements are actually pulled apart as a result of the 
contraction of the attached spindle fibers. To visualize this process, 
it is necessary to assume that the spindle fibers from each pole of the 
spindle extend only to the equatorial plate, where they are directly 
connected with the chromosomes in such a way that the pull exerted 
by a contraction of the spindle fibers will separate the two elements of 
each chromosome and draw each half toward the aster from which the 


particular group of spindle fibers extends. The continued movement 
of the daughter chromosomes away from each other and toward 
opposite poles of the spindle is even more difficult to explain. Appar- 
ently no visible evidence exists that would indicate a pull on them as a 
result of continued spindle fiber contraction. Nevertheless, the polar 
movement of the two chromosome groups continues so that, as the 
anaphase stage ends, they are widely separated, with each group lying 
in close proximity to the corresponding aster. Evidences of the route 
travelled from the equatorial plate are seen in an orientation of 
the cytoplasmic elements parallel to the direction of chromosome 

The Telophase. This end phase of mitosis is characterized by a 
number of structural changes in the elements of the dividing cell. 
Externally a prominent feature in animal cells is the formation and 
gradual development of a cleavage plane that first appears as a slight 
depression in the cytoplasm at the equator of the cell. This may be 
seen to excellent advantage in the holoblastic cleavage of the fertilized 
egg, such as previously described in the frog. The continued ingrowth 
of the cleavage furrow finally results in the division of the cytoplasm of 
the original cell into two daughter cells. The internal nuclear changes 
made evident by a microscopic study of cells in the telophase stage 
reveal the final chapter in the mitotic phenomena previously traced 
through the prophase, metaphase, and anaphase stages. Outstanding 
is the reappearance of the nuclear membrane and also of that function- 
ally obscure body, the nucleolus, in each of the daughter nuclei. 
(Fig. 176/.) 

Along with these altered features is a change in the appearance of 
the chromosomes. They begin to lose their clear visibility and grad- 
ually merge into the irregular, granular network present in the 
nucleus of the parent cell at the beginning of the prophase stage. 
Accordingly, so far as can be seen under the microscope the chromo- 
somes appear to be temporary chromatin bodies which reach their 
highest development in the metaphase and anaphase and gradually 
disappear during the telephase. They will not become prominent 
again until the daughter cells undergo mitosis. However, it must not 
be thought that the basic chromatin organization disappears in a 
resting nucleus or that it is in any sense haphazard; for whenever the 
chromosomes reappear in the cells during succeeding mitoses, the 
exact number and shapes that were present in the ancestral cells again 
"crystallize" out of the chromatin net. Finally, in each daughter 
cell a gradual disappearance of both the astral fibers and spindle fibers 
radiating through the cytoplasm is evident, and the tiny centrosome, 


shorn of its astral halo, once again lies close to the nuclear membrane, 
ready to divide and thus inaugurate the prophase of the next mitosis. 
Successive mitoses usually follow each other in rapid succession in the 
early stages of embryonic development, and the successive generations 
of daughter cells get smaller and smaller, as can be seen in the blastula 
and gastrula stages of the frog embiyo. In the more mature cells, 
however, long inactive periods normally follow the completion of 
mitotic activity. In fact, many highly differentiated types of cell 
completely lose the power of mitosis in the adult. 

Thus ends the amazingly exact process of mitosis on which the 
integrity of successive generations of cells and, in the final analysis, 
of every organism depends. The fertilized egg cell contains its exact 
quota of maternal and paternal chromatin, and, during the successive 
cell divisions essential to the attainment of the adult stage, every 
daughter cell receives its rightful share of the chromatin legacy estab- 
lished in the zygote nucleus. And so it is apparent, as stated in the 
first chapter, that "cell division is an exquisitely beautiful and exact 
process (page 13). On its normal functioning during all the stages of 
embryonic development and throughout the life of each individual 
depends the structural and functional integrity of every tissue and, 
organ of the body, as well as the specific characteristics of the entire 


Since the entire process of mitosis hinges on an exact division of 
all the chromosomes so that the nucleus of each daughter cell may 
receive its rightful share of every chromosome, it is apparent that the 
hereditary materials in the chromosomes must be arranged with the 
utmost regularity and precision. Without, for the moment, bringing 
forth the available experimental data, it may be stated that there is 
abundant evidence demonstrating that every chromosome, whether 
present in the nucleus of a body cell or in germ cell, is composed of a 
great many independent ultramicroscopic hereditary units, the genes, 
which are arranged in a precise linear fashion throughout the length 
of the chromosome. It is well established that' some 2,500 genes are 
present in certain chromosomes of the fruit fly Drosophila, which are 
responsible for the control of various hereditary characters. In 
addition, it has been found possible to secure data from various con- 
trolled breeding experiments in this organism that indicate the posi- 
tions of many of these genes in a particular chromosome. In this way, 
chromosome maps have been constructed that indicate the gene loci. 
(Fig. 181.) 


With this linear arrangement of the gene-chromosome complex in 
mind, it is evident that, when a chromosome reproduces and splits in 
half longitudinally, each of the constituent genes, linearly arranged 
throughout its length, also reproduces and splits. Thus each of the 
daughter chromosomes receives its share of every gene present in the 
parent chromosome. The assemblage of a complete set of the daugh- 
ter chromosomes in the nucleus of each daughter cell transmits the 
complete gene heritage of the dividing cell. In a word, then, the 
nucleus of a daughter cell is exactly like that of the parent cell except 
that, at the instant it is formed, it is only one-half the parental size. 
The cell cytoplasm simply splits into halves, but this relatively crude 
method cannot be used for the chromatin material in the nucleus 
inasmuch as every one of the untold thousands of genes must undergo 
exact qualitative and quantitative division if the normal inheritance 
pattern is to be transmitted. 

The daughter cells having been formed, the next event is an increase 
in the amount of both cytoplasm and of the chromatin until the 
normal cell size is once more reached. Growth takes place during the 
period in which a cell is inactive mitotically. > In such a condition, as 
shown in the prophase at the beginning of mitosis and again at the 
conclusion of the telophase stage, t,he chromosomes are not found as 
microscopically visible bodies. During the growth period, new chro- 
matin material is being formed and added to that already present in 
the nucleus. In some way, not understood at present, this process is 
so exactly controlled that, when the chromosome structure is again 
rebuilt out of the irregular network of the resting cell, every chromo- 
some reappears with its individual structural characteristics and with 
every gene in its exact spatial relationship and containing its own 
specific hereditary substances. 

It has just been shown that the number of chromosomes and their 
exact structural and functional features pass unchanged through 
successive cell generations. To take a specific example, if a fertilized 
egg cell has 48 chromosomes, as in Man, every one of the trillions of 
body cells in the adult individual will have 48 chromosomes. Fur- 
thermore, the 48 chromosomes present in the last cells formed during 
embryonic development, whether differentiated as epithelial or vas- 
cular or nerve cells, will have the exact structural pattern of the 
original 48 chromosomes in the fertilized egg. Again, every individual 
belonging to the human species will be found to have 48 chromosomes 
of the same type, no matter where he lives or what his nationality or 
race happen to be. This condition is only what is to be expected if, 
as we know, the composite characteristics of an organism are the 



"outcropping" of the gene complex established in the fertilized egg. 
Or, stated in another way, individuals that exhibit the same character- 
istics, so that they are placed in a single species, will have the same 
chromatin complex as shown by an exact uniformity in the number and 
type of chromosomes. A microscopic examination of the chromosome 
patterns of even closely related species, for example those of the horse 

FIG. 177. The chromosome complex in man. 
B, the arrangement of the chromosomes by pairs. 
Family" The Williams & Wilkins Company.) 

A, the normal chromosome pattern; 
(Painter, "Eugenics, Genetics, and the 

and the ass, reveals distinct differences. Possibly all this is only a 
rather involved way of emphasizing the fact that every type of organ- 
ism has its own distinctive gene complex, and variations in these are 
apparent to the cytologist in the numbers and shapes of the chromo- 
somes. (Figs. 177, 220.) 

FIG. 178. Illustrating the individuality of the chromosomes. A, chromosomes of a 
plant louse. The homologous chromosomes are given the same number. B, chromo- 
somes of a beetle; C, chromosomes of a seed plant. (Wilson.) 

During the present century and even before, a great deal of inten- 
sive research work has been done by the cytologists on the chromosome 
patterns of many species of plants and animals. The accumulated 
data show that wide variation occurs in the different species both with 
respect to the number of chromosomes and their distinctive structural 
features. In a particular species, however, as emphasized above, the 


chromosome complex (karyotype) is always constant. The results 

from the chromosome studies have yielded important evidence with 

regard to the degree of relationship among various types. Thus, to 

take one important example, from a study of the chromosome patterns 

of several different species of Drosophila, it is^evident that all the 

specific patterns "bear a general resemblance to one another but show 

characteristic minor differences." Since the various 

species of Drosophila present in a particular genus 

show specific differences, it is to be expected that the 

karyotypes of these same species would also show 

minor differences. But the impression must not be 

given that the structural pattern of the chromosomes 

can be directly associated with the morphology of the 

individual or, in other words, that it would be possible 

by a study of the chromosomes in a fertilized egg to 

arrive at any conclusion relative to the characteristics 

of an organism developing from such a complex. Such 

is not the case. Nor is there any apparent relationship 

between the number of chromosomes and the relative 

complexity of an organism. (Fig. 178.) 

Inasmuch as the more complex organism possesses a greater number 
of characteristics to be determined than does a relatively simple 
organism, it might be expected that the former would have more 
determiners, or genes, for these characters and, therefore/' that the 
chromosome number would be larger. It is found, however, that 
some of the highest chromosome counts are in the Protozoa, whereas 


n m 

Fig. 179. 
The normal dip- 
loid chromosome 
complex (karyo- 
type) of the fruit 
fly, Drosophila 
melanogaster . 
(Sharp, adapted 
from Morgan, 
Bridges, and 

yellow (body color) prune 

aete (bnstfe char.) \(eye color) 
ie (bristle * 

\^,dchaete( bristle char.) \(eye color) \ facet (eye ch A 
\scute (bristle char.) \ \ I eel 

>T broAd(wing) \ \ / I 


ite (eye color) 
facet (eye cha.r.) 

e chin us (eye char.) 

FIG. 180. Drawing of a terminal portion of a giant X-chromosome from a salivary 
gland cell of Drosophila to show the position of certain genes which determine body and 
eye color and other characteristics of wings and bristles. Cf. Figs. 181, 183. (Painter, 
"Science in Progress," Yale University Press.) 

Drosophila, a highly developed invertebrate, has only eight chromo- 
somes. It is possible, however, that the eight chromosomes in tb^ 
Drosophila karotype contain many more genes than do the much 
more numerous chromosomes of the protozoan cell. It is apparent 
that chromosome number is far too coarse a measure to use in estab- 
lishing a direct relationship between chromatin structure and body 



P-, (0. yellowCB) 
\ SO Hairy wing CW) 
\ I0.t scute CHJ 
V, 0.3 lethal -7* 
\ 'Q6 broad CW) 
\ 1. prune CE) 
\ \5 White CE) 
\\[3. facetCE) 
\ 13.* Notch CE) 
\ 4.5 Abnormal CB) 
,\55 echinus CH) 

structure. If the genes themselves were large enough to be directly 
studied under the microscope, just as we can now study the chromo- 
somes, undoubtedly structural features in the organism could be 
directly associated with the structure pattern in 
a particular gene or group of genes. It has been 
previously stated that it has been found possible 
in a few organisms to determine the definite 
region in a particular chromosome in which the 
genes responsible for certain bodily structures 
are situated and thus to make a chromosome 
map. These results, as will be shown later, were 
^ rst obtained by breeding experiments in Droso- 
phila, but it has been possible to confirm them 
by direct microscopic observations on the 
chromosomes. Within the last few years, it has 
been discovered that the salivary glands of 
Drosophila contain cells bearing relatively 
enormous giant chromosomes in their nuclei. 
Intensive microscopical studies on this material 
have brought the observations of the cytologist 
very close to actual gene visibility and have 
greatly added to our knowledge of the minutest 
elements of chromosome structure, in fact, the 
way the genes are actually arranged. We may 
now briefly indicate the minute structural pattern 
of chromosomes as shown by the study of the 
giant chromosomes from the salivary gland cells. 
(Figs. 179 to 182.) 

The giant chromosomes are elongated cylin- 
drical bodies with more or less regularly spaced 
bands encircling them throughout their length. 
Their appearance under the microscope reminds 
one of an earthworm with its segmented body 
indicative of a characteristic internal arrange- 
ment of the various parts. In the same way, the 
external segmentation or banding of the chromo- 
somes presents visible evidence to the micro- 
scopist of a definite arrangement of the genes or at least of the 
regions, or "homes," that they inhabit. For each external band of a 
chromosome marks the outer boundary or periphery of a disc-like 
structure lying at right angles to the main axis of the elongated 
chromosome. Certain evidence goes to show that the actual genes 


\I6* club (W) 
Hit deltexCW) 

20. cut CW) 

21. singed CH) 
27.5 tan CB) 
27.7 lorenge CE) 

33. vermillionCE) 
36.1 miniature CW) 
! 36.2 dusky CW) 

- 38. furrowed CE) 

43, sable CB) 
44.4 garnet Cfe) 

642 small wing 
54.5 rudim*rtBry(W) 

56.5 forked CH) 
57. BarCE) 
585 smalt eye 
59. fused <$) 

59.6 BeadexCW) 
62, Minute-n (H) 

65. cleft CM 

TO. bobbedCH) 
FIG. 181. Showing 
the complete chromo- 
some map as deter- 
mined for the X-chro- 
mosome of Drosophila 
shown in Fig. 180. 
(Sharp, adapted from 
Morgan, Sturtevant, 
Bridges, and Stern.) 


are grouped in these disc-like bands throughout the length of the 
chromosome. There seems to be no question but that the genes, even 
in the giant chromosomes, are somewhat too small to be seen under the 
highest magnifications. (Fig. 183a, 6, c, etc.) 

Fortunately the giant salivary gland chromosomes, when prepared* 
and stained in the proper manner, are essentially transparent, which 
makes it possible for the cytologist to study their complex internal 
arrangement. In such observations, the microscopist must make use 
of the highest magnifications available, and even then it is not possible 
to speak with absolute authority on the finest details of chromosome 
structure. The concensus of opinion at present seems to be that the 
typical giant chromosome is essentially cable-like in its structural 
pattern with a variable number of linear units, the chromonemata, 

FIG. 182. The entire group of giant salivary gland chromosomes of Drpsophila. 
They are attached in the center to a granular material, chromocenter, which lacks genes. 
(Shull, after Painter, modified.) 

spirally wound together to form the chromosome body. In the giant 
chromosomes, 64 chromonemata have been observed, but the number 
may be more or less. Embedded in the thread-like chromonemata, at 
rather regularly spaced intervals, are chromatin particles, the chromo- 
meres, appearing like knots tied at more or less regular intervals in an 
elastic thread. The elastic threads are the chromonemata, and the 
simile appears to be unusually descriptive, for it is even possible to 
stretch the fresh salivary gland chromosomes and thus increase their 
length somewhat. Under such conditions, it is the chromonemata that 
are stretched, and this stretching increases the distances between the 
knot-like chromomeres present on the filamentous elastic chromo- 
nemata. (Fig. 183.) 

Chromosomes, chromonemata, and the chromomeres, the latter 
being the smallest elements visible under the microscope where are 
the actual genes, and how can the external banded areas be linked up 
with the internal structure as just given? The evidence is that the 



genes are present in the chromomeres. The latter, as microscopically 
visible particles, are undoubtedly too large to be the actual genes, but 
they are believed to be essentially a chromatin covering which sur- 
rounds and encloses the genes. Finally, what is the relation between 
the chromomeres and the banded discs mentioned as the home of the 
genes hi the preceding paragraph? This condition may perhaps be 
understood by saying that the chromomeres are concentrated in the 
disc-shaped areas. For sake of illustration, let us visualize the discs as 
many-roomed mansions and the chromomeres as private rooms 
inhabited by the genes. The chromonemata appear to be strung along 
like telephone wires from one "gene dwelling" to the next throughout 
the length of the chromosome. 

9 , ^-^y ^^n ~^*~ -^, ^OQ-^OO 

^ protein protein 

FIG. 183. Drawing of (1) a portion of giant salivary gland chromosome of the black 
fly, Simulium virgatum, maximum magnification to show the finest details; (2) a single 
chromeric thread further schematized to show the linear arrangement of the constituent 
elements of (1), as described on page 338. The chromonemata are seen as longitudinal, 
parallel lines. (Painter, " Science in Progress," Yale University Press.) 

One cannot study the details of mitosis and the amazingly exact 
and intricate construction of the chromosomes, as just indicated, 
without being tremendously impressed with the basic importance of 
these cellular elements. If an experiment were devised to show con- 
clusively that the chromatin material is the basis of heredity, it would 
probably consist in transferring living chromosomes from the nucleus 
of one cell to that of another cell and then studying the effect of the 
transposed chromatin in the succeeding cell divisions. So far as can be 
seen at present, experiments involving the actual transfer of chromatin 
from cell to cell lie beyond the range of the experimental biologist, but, 
fortunately, data are available from experiments involving the same 
principles that are continually being performed in the greatest labora- 
tory of all, the laboratory of nature, and by the greatest of all experi- 
menters, nature. 

In the past few years, numerous examples of chromosomal irregu- 
larities, or aberration^, have been discovered in well-known species of 


plants and animals. In some of these, additional chromosomes have 
been added to the normal number; in other cases, the chromatin 
material has been reduced below that normally present. Now when 
the normal chromosome complex is altered, it is found that the 
individuals exhibit corresponding changes in various structural charac- 
ters. Speaking very generally, it is found that, when the cells of an 
organism carry an increased number of chromosomes, the individual is 
larger in size and is also marked by other altered characteristics depend- 
ing upon the gene content of the added chromosomes. Thus fruit, 
flies have been found with a 50 per cent increase in the chromosome 
number so that ther^were 12 chromosomes in each cell instead of the 
normal number of eight. Such animals are larger and show o>ther 
modifications of the nornial species pattern. Many other instances 
have been found, more commonly among plants, and, always, the 
results of altered chromatin pattern are evident in the individual. 
Some of the important results obtained in this field will be considered 
in the next chapter in dealing with problems of heredity. (Fig. 

In spite of the fact, as noted above, that the biologist is decidedly 
limited in his ability to experiment with chromosomes directly, it 
has been found possible to make use of the X rays in altering the normal 
chromosomal pattern of the living cells. Some years ago, it was 
discovered that irradiation of the living male germ cells undergoing 
development in the testis of Drosophila caused decisive changes in the 
chromatin pattern of these cells so that later, when the sperm develop- 
ing from these irradiated gametes were used to fertilize eggs, a con- 
siderable proportion of the resulting offspring were found to show 
various abnormalities, or mutations. Microscopic examination of the 
chromosomes from the mutated animals revealed visible changes in 
the chromosomes. These epoch-making results from irradiation, 
which have widely extended in various plant types, once more con- 
firmed the basic fact that altered chromatin means an altered heredity 
in the daughter cells and the establishment of visible modifications in 
the cells, tissues, organs, and organism formed from the mutated cell 
or cells. (Fig. 207.) 

Finally, it is established that the characteristic differences between 
male and female are definitely associated with what appears to be, 
from the structural standpoint, very slight differences in the chromo- 
some complex of the two sexes. Almost forty years ago, it was found 
that all the body cells of the females in a certain species of insect had a 
pair of chromosomes that were visibly different from those present in 
the body cells of the males. In tracing this sex individuality back 

FIG. 184. Photographs of the jimsonweed (Datura) with different chromosome 
complexes, as follows: Above, haploid; middle, diploid; below, tetraploid. {Sinnott and 
Dunn, after BLakeslee,} 



to the chromosomes of the germ cells, it was found that all the mature 
eggs contained one sex chromosome of the same type, which we may 
designate as X, but that the sperm were equally divided between two 
types; one with an X chromosome, as in the eggs, and one with another 
type, known as the Y chromosome. Union of an egg with an X sperm 
gave the X-X pattern in the zygote, and this would develop into a 
female; fertilization with a Y sperm gave the X-Y pattern in the zygote 
which resulted in a male individual. Further consideration of this 
question will better be deferred until later, but the important point has 
been established, namely, that even the characteristic differences 
between the male and female have their origin in a distinctive gene 
pattern. (Fig. 179.) 


The description of reproduction and chromatin behavior in the 
body cells, based upon mitosis, has been presented in sufficient detail 
so that we may turn to another and, possibly, even more important 
phase of reproduction, namely, germ cell formation. To get the 
essential background, it will be desirable to recall for a moment the 
early stages of embryonic development as described in the previous 
chapter (page 295). It was shown there that repeated cleavages 
extending through the blastula stage result in the formation of* a 
considerable number of cells, all belonging to the ectoderm, the first of 
the primary germ layers. Then the gastrula stage appears, character- 
ized by the development of endoderm cells. A little later, mesoderm 
cells are differentiated. Thus, cellular differentiation appears among 
the constituent embryonic cells, all of which are direct descendants of 
the original zygote. What is responsible for this differentiation, the 
appearance of which, in cells with the same chromosome complex, 
seems to argue against the chromosome basis of heredity as discussed 
in the earlier pages of this chapter? The answer seems to lie in minute 
changes occurring in the chromatin of the daughter cells; changes too 
small to be visible under the highest magnifications. Thus the most 
searching examination of the nuclei of ectoderm, endoderm, and meso- 
derm cells reveals no differences in the structural pattern of the chromo- 
somes of tho different types of cells, but undoubtedly ultramicroscopic 
differences are present in the gene complex, possibly extending down 
to the molecular level. The apparent uniformity of the mitotic 
phenomena really masks an inherent ability to segregate specific 
differentiating materials to particular cells during development. This 
segregation is the basis of the gradual and orderly processes of differ- 



entiation which assume such amaring proportions in vertebrate ani- 
mals. (Fig. 155.) 

The differentiation processes during development result in the 
formation of the six basic tissues of the vertebrate organism, namely, 
epithelial, supporting, vascular, muscle, nerve, and reproductive 
(page 23). Various combinations of these tissues are responsible for 
the formation of the organs and organ systems that unitedly form 
the functioning organism; and all of them, with the exception of the 
reproductive tissues, are concerned with the maintenance of the 
structural and functional unity of the individual organism. Together 

C D 

FIG. 185. Diagrams illustrating the early differentiation of somatic and germ 
cells in Ascaris megaloeephala during the cleavage of the egg. Germ cells at right in A , B; 
upper right in (7, D. Schematized. (Shult, after Fogg.) 

they comprise the soma of the individual that houses the germ plasm 
and, as a matter of fact, make it possible for the germ plasm to survive 
and to function in reproduction, thus maintaining the species. The 
time at which definite germinal cells are first segregated during 
embryonic development varies widely in the animal kingdom. Thus 
in the parasitic worm Ascaris, which supplies such important material 
for the study of the mitotic phenomena, the first cleavage of the zygote 
reveals differences in the nuclei of the two daughter cells, and it is 
established that the descendants of one of these two cells will form the 
germinal cells. In most species, visible differentiation between soma 
and germ plasm does not occur until very much later in development. 
(Fig. 185.) 

But whether it be early in development or later, the main point is 
that in the individuals of species that reproduce sexually, differentia- 


tion of germinal material occurs, and the germ cells thus formed pass 
through a series of developmental stages, the process of gametogenesis, 
which ends in the formation of the mature gametes, eggs or sperm. 
Gametogenesis may conveniently be divided into spermatogenesis and 
oogenesis in correspondence with the type of gamete produced. The 
basic features of germ cell maturation, involving the exact preparation 
of the chromatin material for transfer of hereditary characters to the 
next generation at the time of fertilization, are the same in the male 
cells as in the female, though as we know, from the description given 
in the preceding chapter, the structural features of mature sperm and 
egg are markedly different. 

The heart of sexual reproduction is fertilization, and the essential 
feature of fertilization is the fusion, or amphimixis, of the chromatin 
material in the nucleus of the male sperm with that present in the 
nucleus of the female egg; thus forming the fusion nucleus, or synkar- 
yon, of the zygote. Since we know that the characteristics of the 
daughter cells formed by mitosis depend upon their receiving the nor- 
mal chromatin complex from the parental cell, it is no less apparent 
that, in reproduction, the integrity of the new individuals depends 
upon receiving the exact inheritance of maternal and paternal chro- 
matin at fertilization. With this condition in mind, it is clear that 
the zygote nucleus cannot possibly receive the normal chromatin 
content of the species if both the sperm nucleus and the egg nucleus 
transfer to it the full amount of chromatin material, for a doubling 
of the chromatin content, evidenced by double the usual number of 
chromosomes, would be bound to result. The upshot of the matter 
is, as has long been recognized, that, in spermatogenesis and in oogene- 
sis, the chromatin material of each germ cell is reduced to one-half 
the normal amount, and this condition is visibly shown in an actual 
reduction of the .chromosome number to one-half that found in any other 
cells of the body of that particular species. It may be stated very 
simply: A mature sperm or a mature egg contains only one-half the 
somatic number of chromosomes. As a result of this chromosome 
reduction, it is evident that when a sperm nucleus unites with an egg 
nucleus in fertilization, the chromosome number of the zygote nucleus 
is restored to the characteristic number and not doubled as it would 
be otherwise. (Fig. 186.) 

Chromatin reduction in the germ cells, like chromatinic behavior in 
mitosis, is not a haphazard process but is so arranged that each mature 
germ cell will carry a complete set of genes for all the characters of that 
particular type of organism, though the chromosome number is reduced 
one-half. In order to understand how this is accomplished, it will be 



necessary to reexamine the normal chromatin complex in the nucleus 
of a body cell and see just what the condition is there. Careful 
studies on animal tissues have shown that the nuclei of every type 
of somatic cell and the nuclei in the immature germ cells, as well, 
always contain a double, or diploid, set of chromosomes. The chromo- 
somes are in pairs; every chromosome has a mate which is an exact 
duplicate, or, as the cytologists say, a homologous chromosome. 
Thus, technically, instead of saying that Drosophila has eight chromo- 
somes, it is preferable to say that it has four pairs of chromosomes or 
that the diploid number is eight. Also in Man, since the diploid num- 
ber is 48, 24 pairs of chromosomes are present. The individuality of 
each chromosome is so marked that it is possible for the experienced 

FIG. 186. Illustrating the history of the chromosomes in the animal life cycle. 
Male, above; female, below; union of sperm and egg to produce the new individual, at 
the right. (Sharp.) 

cytologist to identify the two members of each pair of homologous 
chromosomes. To do this is comparatively simple in cases like 
Drosophila where only four pairs are present, but the chromosome 
identification presents considerable difficulty where numerous pairs 
are present as in man. 

A moment's consideration will show that the essence of chromo- 
some reduction during germ cell formation is the sorting out of one 
complete set of chromosomes, the haploid number, for transfer to each 
germ cell, whether egg or sperm. When the sperm carrying the pater- 
nal chromatin later unites in fertilization with the nucleus of the egg 
carrying the maternal chromatin, the synkaryon nucleus of the zygotc 
is equipped with two complete sets* of chromosomes, the diploid 
number, and, furthermore, the two homologous chromosomes of each 
pair present in the zygote have a diverse ancestry; one chromosome 
from each pair is of paternal origin by way of the sperm, and the other 
chromosome is of maternal origin by way of the egg. The successive 
mitotic cell divisions of the zygote, during embryonic development, 


present to every cell of the body exact replicas of the paternal and 
maternal chromosomes received by the zygote nucleus at fertilization. 
Thus biparental inheritance, the unique characteristic of sexual repro- 
duction, extends to every cell of the individual. 


With the general relationships between the chromatin of soma and 
germ plasm in mind, it is next in order to examine the process of germ 
cell formation and to find out just how these basic conditions for 
heredity are maintained. The development of the functional sperm 
may be considered first. The structural plan of the testis has been 
described in the previous chapter (page 315). It was there shown 
that the sperm are matured in the seminiferous tubules, the successive 
stages of development culminating in the production of free-swimming 
sperm which are liberated and pass from the tubules to the exterior. 
The problems involved in the morphological transformation of the 
early, or primordial, male germ cells, which appear as typical cells, 
to the bizarre motile sperm are of great complexity, but they are not 
of primary concern in this discussion for our interest lies in the behavior 
of the chromatin. Six characteristic stages of spermatogenesis merit 
our consideration; the first of these is the primordial germ cell which 
appears as a recognizable type at some period during embryonic 
development and is segregated in the seminiferous tubules of the 
testis. The early history of the primordial germ cells is essentially 
uneventful. They are small in size, but with a comparatively large 
nucleus containing the diploid number of chromosomes characteristic 
of the species. Repeated mitotic divisions of the primordial germ 
cells, the so-called multiplication period, results in the formation of 
great numbers of spermatogonia which are essentially the same in 
structure as the primordial cells, and in all of which exact and normal 
mitoses ensure the maintenance of the specific chromatin complex. 
(Fig. 187.) 

The inauguration of the third stage in spermatogenesis is indi- 
cated by an increase in cell size; for after an indefinite number of 
divisions involving normal mitosis, each of the daughter spermatogonia 
enlarges to form a primary spermatocyte. Superficially, a primary 
spermatocyte appears as a greatly enlarged spermatogonium 2 but an 
examination of the nucleus reveals that the reduction phenomena, 
essential to germ cell maturation, are under way. Furthermore, it is 
found that a germ cell having reached the primary spermatocyte st&ge 
will divide only twice more; the first division forms two daughter cells 
designated ae secondary spermatocytes, and then each of these divides 



Fio. 187. Diagrams illustrating meiosis during gametogenesis. a, diploid germ cell 
in resting condition; b, condensation of chromatin to form three pairs of homologous 
chromosomes; c, pairing (synapsis) of homologous chromosomes as in primary spermato- 
cyte or odcyte; (d), cleavage spindle in equatorial plate stage; e, f, separation of the 
homologous chromosomes and migration to opposite poles of the spindle; g, resulting 
daughter cells with reduced number of chromosomes as in a secondary spermatocyte or 
odcyte. C f Ff. 186. (Watkeya, Stern.) 


to form two spermatids. The spermatids, without further cleavage, are 
gradually transformed into mature sperm. In a word, then, the two 
final cell divisions beyond the primary spermatocy te stage result in the 
formation of four sperm from each primary spermatocy te. (Fig. 186.) 

Synapsis. In the previous description of mitosis, it was shown 
that the real crux of the process was the longitudinal splitting of each 
chromosome and the separation of the resulting halves in such a way 
that each daughter cell would receive one-half of each chromosome, 
thus passing to the nucleus of each daughter cell the exact chromatiu 
complex of the parent cell. Accordingly, in typical mitosis, just aftei 
each chromosome has split in the anaphase stage, there is a temporary 
doubling of the diploid number of chromosomes in a cell. This pumber 
is quickly reduced to the typical diploid number as the cytoplasm of the 
cell splits into two daughter cells and each nucleus receives its diploid 
quota. Now what appears to be the reverse of the chromatin behavior 
in normal mitosis is evident in the nucleus of a primary spermatocyte 
during the reduction division (meiosis), for, here, the chromosomes, 
instead of splitting, fuse together in pairs the process of synapsis 
and the chromosome number in the nucleus of the primary spermato- 
cyte is thus reduced one-half to form the haploid number, though the 
total amount of chromatin in the nucleus is, of course, unchanged. 

But now, as the cytoplasm of the primary spermatocyte prepares to 
divide to form the two daughter secondary spermatocytes, the tempo- 
rarily paired chromosomes separate and move in opposite directions, 
the final result being that the nucleus of each of the secondary sperma- 
tocytes receives one chromosome from each pair. Thus the latter have 
the reduced, or haploid, number of chromosomes. But again it is not 
the chromosome number in itself that is important; it is the fact that 
each secondary spermatocyte, as a result of the reduction division, has 
one complete set of chomosomes and genes one chromosome of every 
kind whereas the somatic cells have two of every kind. (Fig. 187.) 

Next comes the division of the secondary spermatocytes to form 
the spermatids. This appears as a normal mi t otic division in which 
each of the haploid chromosomes splits longitudinally to form two 
daughter chromosomes, the latter being distributed so as to give one 
of the halves to each of the spermatids. Their chromatin heritage 
is transferred unchanged, since each spermatid is gradually trans- 
formed into a motile sperm. The essence of spermatogenesis is clear; 
each sperm nucleus contains not only the reduced or haploid number 
of chromosomes but one complete set of chromosomes with not a gene 
missing. When the sperm fertilizes an egg, which also contains a 
complete haploid set of chromosomes, the diploid number of chromo- 


somes two complete sets, one of maternal and one of paternal origin 
will be restored in the zygote nucleus to be maintained in all the 
cells of the new individual until the time when germ cells are once more 
formed and synapsis occurs. 


It has been indicated that the maturation of both sperm and egg 
presents a uniformity in the essential feature, namely, the formation 
of germ cells bearing a haploid set of chromosomes. In accomplishing 
this primary aim, the female germ cells pass through a series of matura- 
tion stages which conform very closely to those just described in 
sperma^ogenesJs. There are, however, certain noteworthy variations 
in oogenesis which should be indicated. The general behavior of the 
female germ cells as they migrate into the body of the ovary, the forma- 
tion of the Graafian follicles, arid the final release of the female germinal 
cells to the oviduct have been described in earlier sections (page 319). 
In the mammalian ovary, the multiplication period, marked by the 
formation of considerable numbers of primordial germ cells through 
successive mitoses, occurs before birth, and a considerable number 
of the resulting oogonia have by that time penetrated into the ovarian 
tissue where each has become established in a Graafian follicle. The 
next stage in maturation, the primary oocyte, is attained by the growth 
of single oogonia as was seen in the development of the primary 
spermatocyte. Synapsis, marked by the pairing of the homologous 
chromosomes, is the basic feature of the primary oocyte. (Fig. 187.) 

Typically, the primary oocyte stage is the final one occurring in the 
ovary, and the immature germ cell, released by the rupture of the large 
Graafian follicle, passes into the oviduct. The completion of the 
maturation process depends upon union with the sperm cell. If sperm 
are present in the oviduct, the entrance of the sperm head into the 
cytoplasm of the primary oocyte is the signal for the completion of 
maturation so that fertilization may take place. If no sperm are 
encountered, the final stages of maturation do not occur and the 
degeneration of the primary oocyte soon follows. (Fig. 186.) 

The sperm nucleus is temporarily inactive following its entrance, 
but the primary oocyte nucleus at once begins the final maturation 
phenomena. The homologous chromosomes, joined in synapsis, now 
separate, thus forming two independent haploid sets. One of these 
passes to each of the daughter cells as the primary oocyte divides into 
daughter cells. But the two cells formed from the primary oocyte are 
decidedly unequal in size; they consist of a large cell, which is the 
functional secondary oocyte, and a miniature cell, the first polar body, 


which lies in close contact with the oocyte like a wart on an apple. 
Each receives a haploid set of chromosomes. Now comes the final 
stage in maturation; the chromosomes of the secondary oocyte divide 
longitudinally as in normal mitosis to form two haploid sets, one set of 
which is transferred to the nucleus of the mature egg, while the other 
goes to the second polar body; for the division of the cytoplasm of the 
secondary oocyte, like that of the primary oocyte, is also very unequal. 
Practically all the cytoplasm passes to the egg, with only a tiny portion 
forming the second polar body. Usually the first polar body also 
undergoes division at this time, so that the final result is the formation 

A B C D t 

FIG. 188. Diagrammatic; drawings comparing mitosis in the somatic ceils (above) 
with meiosis during the formation of gametes (below). In mitosis there is a temporary 
doubling of the diploid chromosome number (C, D) so that each of the daughter cells 
(E) receives the full diploid number. In meiosis the homologous chromosomes pair (C) 
and then separate (D) to give the daughter cells (E) the reduced or haploid number. 
This haploid condition will be maintained through two more divisions to produce four 
mature sperm in the male, or one mature egg and three polar bodies in the female; all 
with the haploid chromosome complex. (Sinnott and Dunn, after Sharp. Modified.) 

of one large functional egg and three nonfunctional polar bodies from 
each primary oocyte. The sperm nucleus, which entered the cyto- 
plasm of the primary oocyte, was transferred to the secondary oocyte 
and then to the mature egg. Accordingly, with the completion of 
maturation and the establishment of the haploid condition in the egg, 
the gametic nuclei (sperm nucleus and egg nucleus) are ready for the 
formation of a fusion nucleus, the synkaryon, which determines the 
heredity of the new individual and the nature of the gene complex 
which it will transmit to the next generation by way of the gametes. 
(Fig. 188.) 


It will now be profitable to describe the activities that occur in the 
egg immediately following the completion of maturation. Micro- 



scopic observations show that the two gametic nuclei present in the 
cytoplasm gradually move toward each other until they meet, usually 
near the center of the egg, with the nuclear membranes in contact for a 
time. Synchronously the centrosome, which was brought into the egg 
cytoplasm by the middle piece of the sperm, divides, just as in normal 
mitosis, and the daughter centrosomes separate to the accompaniment 
of ray and spindle fiber formation. The nuclear membranes of the 
two gametic nuclei disappear, and the amazing climax of sexual 
reproduction and biparental inheritance is at hand : the mingling of the 
nuclear elements of the two sexes in the synkaryon of the zygote, the 
process of fertilization. The structural elements of the spindle for the 
first mitotic division of the new individual is now well established 

FIG. 189. Fertilization of an egg (Ascidiari) showing (a) the fusion (amphimixis) 
of the egg nucleus ( 9rc) with the sperm nucleus (cfw) and the inauguration of the first 
cleavage spindle; (6) a later stage showing the cleavage spindle fully developed. 
(Seifriz, after Conkliji.) 

in the typical prophase stage. This is quickly followed by the meta- 
phase with the paternal and maternal chromosomes meeting for the 
first time in the equatorial plate and establishing the diploid nuclear 
complex. Then comes the anaphase ^tage, with each chromosome 
splitting longitudinally and moving to the opposite poles of the spindle; 
finally, the telophase, marked by the division of the cytoplasm to form 
two diploid daughter cells. And thus a new individual of a new genera- 
tion, be it frog, chick, or man, is started on its way. Growth, cell 
division; growth, cell division, with differentiation appearing between 
groups of cells; primary germ layers, tissues, organs, organ system; 
and, finally, the mature individual is at hand capable of producing 
germ cells for the perpetuation of the species through still another 
generation. Such is the story of sexual reproduction based on 
biparental inheritance. (Fig. 189.) 


Fertilization is often confused with reproduction. From the 
previous discussion, it is evident that fertilization is really the reverse 
of reproduction, for reproduction is a process by which cells produce 
additional cells. In fertilization, on the contrary, there is a fusion of 
two cells to form a single composite cell, the zygote. It may also be 
thought that, even though fertilization is not really reproduction, it is 
essential to this function in the multiccllular organism. But, as shown 
previously, many organisms normally reproduce asexually. In 
addition, the eggs of a highly developed animal, such as the honeybee, 
may develop either with or without fertilization. Finally, the experi- 
mental results, as previously indicated, have shown that even in some 
vertebrates it is possible to get the egg to develop without fertilization 
(page 286). Under normal conditions, then, fertilization may be 
regarded as having two functions: first, as an activator which is 
responsible for the inauguration of cell division in the zygote and, 
second, as a bearer of the parental chromatin which makes biparental 
inheritance possible. This latter function, namely, conferring bipar- 
ental inheritance on the offspring, can occur only through 

With the sequence of events in spermatogenesis and oogenesis in 
mind, more detailed attention may be given to the basic feature of 
germ cell maturation, namely, the pairing of the chromosomes in 
synapsis. It is already apparent that the union of the chromosomes 
in pairs is not a hit-or-miss arrangement. In the diploid nucleus, as we 
have seen, are two chromosomes of every kind, one of each pair of the 
homologous chromosomes being received at fertilization from the male 
parent and one from the female (page 346). Synaptic pairing occurs 
only between homologous chromosomes, and so it gives the opportunity 
for the maternal and paternal genes of the previous generation, having 
passed through all the stages of the new individual from zygote to 
adult, to determine the gene complex of the germ cells and through 
them the inheritance of the next generation. In a word, synapsis 
preserves the essential continuity between generations and, at the 
same time, offers opportunity for variation depending upon the nature 
of the genes from the two lines that pair at synapsis. (Fig. 186.) 

Taking, as an example, the chromatin condition in Drosophila with 
the diploid number of chromosomes established as eight, it may be 
helpful to portray the maturation changes graphically. Since there are 
two of every kind of chromosome in the diploid condition, we may 
designate the four different chromosomes as A, B, C, and D. The 
diploid condition will then be represented by a doubling of each kind of 
chromosome, or A, A, JS,5, C,C, D,A to give a total of eight chromo- 


somes. This diploid condition is present in all the cells of the organism 
except the germ cells which have attained the stage of development 
when synapsis occurs, as in the primary spermatocyte or oocyte. This 
process, as shown above, is characterized by pairing of the homologous 
chromosomes and a reduction in the total number to one-half. This 
condition may be represented as A A, BB, CC, DD. The homologous 
chromosomes then separate, and one complete haploid set of four 
chromosomes, A,B,C,D, goes to each secondary spermatocyte and, 
finally, after each has divided, to the mature sperm. The chromosome 
behavior in human sperm development can be illustrated in the same 
way by utilizing 24 letters to indicate the 24 different types of chromo- 
some present in the nuclei of the cells. 

Sexual reproduction with biparental inheritance makes possible 
wide variation in the gene complex of the chromosomes united in the 
zygote nucleus and, therefore, in the heredity of the individuals 
developing therefrom. No two individuals of the human species or 
any other species are exactly alike. In order to gain some conception 
of the possibilities for variation during normal germ cell formation and 
fertilization, we shall continue our examination of the basic process of 
synapsis involving the pairing of homologous chromosomes in the 
germ cells of Drosophila with eight chromosomes in the diploid condi- 
tion or, as they pair in synapsis, A A, BB, CC, DD. Close study of these 
synaptic pairs shows that each of the homologous chromosomes in 
synapsis is split longitudinally, just as previously noted in normal 
mitosis, so that really four chromosome elements, or chromatids, are in 
close association, thus forming the so-called tetrad condition. Viewing 
a synaptic pair of chromosomes endwise it is possible to see the ends 
of the four chromatids in close contact. Thus the arrangement of the 
four synaptic pairs in Drosophila may be graphically shown as 

AA' BB' CC' DD' 

Tetrad formation may be thought of as a precocious longitudinal 
splitting of the chromosomes paired in synapsis. In this way, four 
haploid sets of chromosomes A,B,C,D are formed for later distribution 
to the spermatids and egg cells; one haploid set goes to each spermatid 
in the male; in the female, one haploid set goes to the functional egg 
cell and one set to each of the three polar bodies. The important fact 
to realize is that, though each of the four chromosomes A, A, A, A 
coming from the tetrad condition in synapsis carries the genes for the 
same characters with exactly the same linear arrangement, variation 
may occur in the way in which the characters are expressed. To take 
an example of eye color which, let us say, is determined by a gene or 


genes in the synaptic chomosome pair * the two chromosomes A A 

of maternal origin might carry genes for blue color, while the other 
two A A chromosomes of paternal origin carry genes for brown eye 
color. If this were the case, then two (50 per cent) of the mature 
sperm would carry genes for blue eyes, and two (50 per cent) would 
carry genes for brown eyes. But this problem in heredity must be 
deferred for consideration in the next chapter. 


In the two preceding chapters, an endeavor has been made to 
present the essential facts of reproduction, first, as observed from a 
rather distant reviewing stand from which only the larger features of 
the process could be observed and, second, as observed near at hand, 
with the aid of the high-power microscope, in an endeavor to bring to 
light the basic cellular activities underlying the process of cellular 
reproduction, whether concerned with the splitting of a single cell into 
two daughter cells by typical mitosis or the production of a highly 
differentiated multiccllular organism through the union of specialized 
male and female germ cells. 

It has been emphasized that in the reproduction of cells or of 
multicellular organisms, the new living units must be true to the 
parental type. At the same time, the processes of reproduction must 
permit the introduction of limited and controlled variation from the 
parental cell types. It is evident that, if all the cells formed by the 
repeated divisions of the zygote remained absolutely true to type, no 
possibility would exist for the development of the many differentiated 
types of cells, tissues, and organs as seen, for example, in the vertebrate 
organism; for all of these cells trace their origin back to one cell, the 
zygote. We are well aware that a certain amount of easily recojgnizable 
variation exists between adult individuals, even when closely related. 
Body size, color of eyes, color and character of hair, facial features, 
even the tone of the voice, all have distinctive individual qualities, 
though conforming to the general pattern to which all the individuals 
of the group or species belong. 

And so the mechanism of heredity, contained in the chromatin 
material of the nucleus, must be responsible for conformity to type and 
also for individual variation. Finally, the biologist of today sees no 
possibility of accounting for the origin of the enormous variety of plant 
and animal species now present in the world of life except through the 
hereditary mechanism. Since the establishment of sexual reproduc- 
tion, it must have been true, just as it is now, that the characters of an 
offspring are determined by the gene content of the paternal and 
maternal chromatin received at the time of fertilization. The appear- 



ance of a new type or species, past or present, must, therefore, be the 
result of some alteration in the chromatin-gene complex of a cell or 
cells in the direct line of descent. The chromatin pattern having once 
been altered is transferred to the successive generations of daughter 

Probably no area in the entire field of biology has aroused more 
interest and, accordingly, been the object of greater speculation in 
times past than the field of heredity. As a matter of fact, it has been 
only in the very recent years that the knowledge of the heredity 
phenomena has been sufficient to remove them from the realijn of 
speculation and wonder to the field of established fact. At the present 
time, the essential functional features of the hereditary mechanism 
are known, though many obscure facts are yet to be brought to light. 


Present-day knowledge of heredity rests upon the discovery near 
the middle of the last century that the many characters of an organism 
are inherited independently of each other and not as a composite 
group. Thus eye color, to take a common example, is determined 
independently of the other features that are associated in the complete 
organism. Credit for this discovery of the particulate nature of 
inheritance goes to Mendel, an Austrian monk, who became interested 
in the inheritance of certain characters in peas and used the monastery 
gardens for his genetics laboratory. Unfortunately his results, 
published in 1865 in a scientific periodical with limited distribution, 
lay unnoticed by the scientists of Mendel's time, and it was not until 
the beginning of the present century, almost forty years after their 
publication, that Mendel's results were brought to the attention of the 
biological world. A remarkable thing about Mendel's work is that, 
without any knowledge of the complex mitotic phenomena associated 
with inheritance, he was able to deduce the essential facts from his 
breeding experiments and to establish his laws of heredity that stand 
essentially unchanged though greatly extended. Biparental inherit- 
ance, through the fusion of the male and female game tic nuclei, was not 
established until 1879, almost fifteen years after the publication of 
Mendel's results, and the behavior of the chromosomes in the trans- 
mission of hereditary characters was not fully brought to light until 
1910. In the past quarter of a century, the data accumulated by 
cytologists and geneticists the world over, culminating in the recent 
discovery of the giant chromosomes and the analysis of their basic 
structure, have added tremendously to our knowledge of inheritance 
and has confirmed and strengthened Mendel's original conclusions. 



Let us now take as a working hypothesis the conclusion reached by 
Mendel that a particular character of an organism may be inherited 
independently of other characters. Mendel found this to be the case 
in garden peas, and a wide variety of breeding experiments since then, 
with many species of plants and animals, have shown that the principle 
has universal application. We cannot do better at this point than to 
analyze some of Mendel's results. In the first place, he was very 
fortunate in the choice of his experimental material, for the species of 



Tall (short) 

Tall Tall (short) Tall (short) Short 

FIG. 190.-~-Illustrating the inheritance from crosses between a tall and dwarf 
variety of peas as described on page 359. P, parental generation; Fi, first filial genera- 
tion: hybrid, tall peas: Ft, second filial generation, produced from Fi individuals (self- 
fertilized), show the expected 3:1 ratio between tall and short plants. Note that the 
two center plants are hybrids like the Fi. (Woodruff, slightly modified.) 

garden peas that he used presented several interbreeding varieties with 
a number of clearly recognizable differences, seven altogether, in 
various structural features. Thus, some of the plants were tall, 
growing to a height of 6 or 7 ft., while others were short, or dwarf, and 
never attained a height of more than 1J^ ft. Again, some of the 
plants produced yellow seeds; in others, the seeds were always green. 
Finally, to take just one more example of the contrasting characters, 
the seeds were round in some plants and always wrinkled in others. 
Tall or short, yellow or green, round or wrinkled, here were three very 
distinct pairs of alternative characters which could be easily observed 


and the results from the breeding experiments recorded. Mendel 
observed that these pairs of characters were truly alternative in nature ; 
that is, the offspring from his crosses were not mixtures but were either 
tall or short, yellow or green, round or wrinkled. This could mean 
only that the determiners for these characters did not mingle from 
generation to generation but remained as distinct entities which could 
be segregated indefinitely. (Fig. 190.) 

In breeding experiments with peas or flies or wheat or horses or 
any other type of organism, it is essential, for a correct interpretation of 
the results, that the breeder know the condition of the breeding stock 
with reference to the particular character or characters in question. 
Thus it will be found in a general population that, when certain indi- 
viduals are bred together, all of the offspring will be of one type; for 
example, all the pea plants produced by one cross will be tall. Con- 
trariwise, the mating of other individuals will produce offspring of two 
types with reference to a particular character; for example, some will 
be tall and some will be short. Individuals that breed true with 
respect to the characters in question are said to be pure, or homozygous; 
those which do not breed true are termed hybrid, or heterozygous. 

The terms homozygous and heterozygous are very important, for 
they indicate the condition of the chromatin in the germ cells, which, in 
turn, is responsible for the characteristics of the offspring. Thus, 
an individual that is homozygous can produce only one type of gamete. 
A heterozygous individual, on the other hand, will produce more than 
one type of gamete. If it is heterozygous with respect to only one 
pair of alternative characters, it will produce two types of gametes and 
exactly 50 per cent of each type. As an example let us take the pair 
of alternative characters Tall and Short. An individual plant, 
heterozygous for this character, will produce 50 per cent of the gametes 
carrying the determiner or gene for the character Tall and 50 per cent 
carrying the gene for Short. The organism heterozygous for two pairs 
of alternative characters, as in the case of Tall and Short, Yellow and 
Green, will produce four types of gamete and exactly 25 per cent of 
each type as indicated: 

25 % carrying genes for Tall and Yellow 
25 % carrying genes for Tall and Green 
25 % carrying genes for Short and Yellow 
25 % carrying genes for Short and Green 

As a final example, we may consider the organism heterozygous for 
three pairs of alternative characters, adding the characters Round and 
Wrinkled to the two pairs used in the previous example. The organ- 


ism heterozygous with respect to three pairs of alternative characters 
will produce eight types of gamete in equal numbers, that is, 12)^ per 
cent of each type as indicated: 

carrying genes for Tall, Yellow, Round 
12 % % carrying genes for Tall, Yellow, Wrinkled 
12J^ % carrying genes for Tall, Green, Round 
12 Ji % carrying genes for Tall, Green, Wrinkled 
% carrying genes for Short, Yellow, Round 
% carrying genes for Short, Yellow, Wrinkled 
% carrying genes for Short, Green, Round 
% carrying genes for Short, Green, Wrinkled 

From the foregoing examination of the gametes produced by the 
heterozygous individuals, it is clear that the different types of gamete 
are always produced in equal numbers, and, second, when two or more 
pairs of alternative characters are present, all possible combinations 
of the genes between the different pairs occur: Tall can be in combina- 
tion with Yellow or Green, or with Round or Wrinkled, but Tall can 
never be in the same gamete with Short; the genes for alternative 
characters (allelomorphs) are always segregated in separate gametes 
as indicated a few paragraphs previously. The forming of all possible 
combinations between the genes for the alternative characters with 
two or more pairs present was recognized by Mendel under the descrip- 
tive phrase independent assortment which together with the principle 
of segregation are commonly known as the Mendelian laws. The 
number of possible combinations and, therefore, the number of types 
of sperm or egg produced can be determined for any number of pairs 
of alternative characters by 2 n where n equals the pairs of alternative 
characters involved. Thus, as above, with n equal to 3, there are 2 3 , or 
eight types; with n equal to 5, there are 2 5 , or 32 types. And, finally, 
to take the case in man, if each of the 24 pairs of homologous chromo- 
somes carried a pair of alternative characters, 'n would equal 24, and 
the total types of gamete produced would be equal to 2 24 , or more 
than 16^ million. 

Since, then, as just shown, homozygous individuals produce only 
one type of gamete, and heterozygous individuals produce two or more 
types of gametes, depending upon the number of alternative characters 
carried, it is clear why the former breed true when they are bred 
together, while the hybrids interbred produce a variety of offspring. 
But now another interesting and important development comes to 
light, which Mendel also observed, and that is the principle of domi- 
nance. This may be illustrated by saying that it is not always possible 
to determine whether an organism is homozygous or heterozygous by 


its appearance. This is due to the fact that one member of a pair of 
alternative characters is usually dominant over the other, or recessive, 
member. Thus, in the alternative characters Tall and Short, it is 
found that Tall is the dominant character, and, as such, dominates 
the characters of the body (soma) to the exclusion of the recessive 
character Short. Therefore two types of Tall individuals can be 
found; the Tall-Tall type, which is homozygous producing only Tall 
gametes, and the Tall-Short type, which is heterozygous and will 
produce 50 per cent Tall gametes and 50 per cent Short gametes. It 
is obvious that only one type of the recessive short individual occurs, 
namely, the Short-Short homozygous type, for the Short-Tall hetero- 
zygous individuals are all tall; the character Tall is always dominant 
over Short. The breeder who wishes to establish a pure line with 
respect to a certain character or characters must find out the dominant- 
recessive relationship and then make the appropriate matings. It 
must always be remembered, as noted previously, that possibly the 
most important discovery of Mendel was that the complete heritage 
of an organism was not assembled in a single unit but consisted of a 
great number of independent unit characters (particulate inheritance) 
each of which was separately transmitted to the offspring in con- 
formity with the general principles of independent assortment and 
segregation. (Fig. 190.) 

With an understanding of particulate inheritance, hybridization 
in its relationship to gamete formation, segregation, independent 
assortment, and dominance, the basis of Mendelian inheritance is well 
established, and the results obtained from the crosses are easy to 
understand. The starting point in determining the characters to be 
expected in the offspring from the mating of any two individuals is 
always: What genes do the gametes of the two parents carry with 
respect to the characters in question? The simplest possible case 
would occur in the mating of two homozygous individuals which, as we 
know, can produce only one type of gamete. For a change, let us take 
the inheritance of certain characters in guinea pigs, which are familiar 
to every one and which have been widely used in breeding experiments 
as well as in a wide variety of other types of biological experiment. 
The inheritance of certain visible characters in the hair, such as color, 
length, and smoothness, is convenient to use. We shall consider, 
first, the inheritance of color. The colored, or Black, condition, which 
we can for convenience designate by the letter (7, is dominant over the 
White, or albino, condition which we may designate by the small c 
since this condition is recessive. The small c, then, represents a lack 
of color, or pigment, and gives white hair. Just as noted above in the 


case of the peas, there will be two types of black animals in a general 
population that cannot be distinguished by their appearance : the pure, 
or homozygous, individuals that carry only dominant genes Colored- 
Colored, for convenience represented as CC, and produce only one 
type of gamete, namely C; and the hybrid, or heterozygous, 
individuals that $arry both the dominant gene C and the recessive 
gene c for the alternative character, that is, Colored- White or more 
conveniently represented as Cc. The Cc individuals, of course, pro- 
duce gametes of two kinds: 50 per cent C and 50 per cent c. (Fig. 190.) 
The mating of a pure male (CC) with a pure female (CC) can 
produce only colored offspring (CC) like the parents because no other 
types of gamete are produced by either of the parents. The mating 
and the results of breeding two homozygous individuals can be indi- 
cated graphically thus : 

(I) Sperm C X egg C = zygote CC 

The same results are obtained when a recessive white male (cc) is 
mated with a recessive white female (cc) for they can produce only 
one type of gamete (c) and the results of all such matings will be: 

(II) Sperm c X egg c = zygote cc 

Another possible type of mating of homozygous individuals is that 
in which a homozygous dominant (CC) is mated with a homozygous 
recessive (cc). The result of such a cross is the production in every 
case of hybrid or heterozygous colored individuals (Cc) thus: 

(III) Sperm C X egg c = zygote Cc 

The reciprocal mating with a recessive white male (cc) and a homo- 
zygous colored female (CC) will give the same results, thus : 

(IV) Sperm c X egg C = zygote Cc 

With these easily understood results from homozygous matings in 
mind, let us next consider the results of mating a hybrid-colored male 
guinea pig (Cc) with the same type of female (Cc) as in the preceding 
paragraph. Both produce two types of gamete in equal numbers, 
that is, 50 per cent sperm C, 50 per cent sperm c by the males; and 50 
per cent eggs C, 50 per cent eggs c by the females. Obviously, as shown 
above in the case of peas with two pairs of alternative characters, the 
possible combinations of sperm and eggs are as follows: 

Sperm C X egg C * zygote CC 

/yv Sperm C X egg c - zygote Cc 

Sperm c X egg C zygote cC 
Sperm c X egg c zygote cc 



It has been found convenient in determining the possible combina- 
tions of the gametes to use a square in which are separate spaces for 
the zygotes, with the various types of sperm arranged along the left 
side, and the eggs along the top, as follows: 






An analysis of the foregoing results shows that three different 
types of zygote are possible from the mating of the two hybrids with 

C x' c 




CC Cc Cc cc 

FIG. 191. Illustrating the inheritance of hair color when homozygous black guinea 
pigs (CC) are mated with albino guinea pigs bearing no pigment (cc), as described on 
page 361. (Woodruff, slightly modified.) 

respect to one pair of alternative characters, namely, pure black (CC), 
hybrid black (Cc and cC; the result is the same whether a particular 
gene is carried by the sperm or egg), and pure white (cc). Accordingly, 
since both the pure dominant (CC) and the hybrid dominant (Cc) 
have the same appearance in this case, they are black the offspring 
of this hybrid mating are expected in the ratio of 3 black animals to 1 
white animal or 75 per cent black to 25 per cent white. (Fig. 191.) 

As a matter of fact, this expected 3:1 ratio does not always appear, 
because the union of the gametes at fertilization is at random; that is, 
it is always a matter of chance as to whether a particular egg will be 
fertilized by a C sperm or by a c sperm, jusi as it is always a matter of 
chance as to whether heads or tails will come up when a coin is pitched. 


There is always a chance that heads may appear successively; but if 
the process is continued for a large number of times, say 1,000, the 
likelihood is that heads-tails ratio will be pretty close to 50 per cent 
heads to 50 per cent tails. In the same way, if the numbers of off- 
spring obtained from hybrid matings are large, the chances are very 
good that the ratio of black animals to white animals will be very close 
to the Mendelian ratio of 75 per cent black to 25 per cent white. Thus 
Mendel reported from the offspring of monohybrid individuals, differ- 
ing with respect to one pair of alternative characters, that the ratio 
obtained in a total of 1,064 offspring was 73.9 per cent showing the 
dominant character to 26.1 per cent showing the recessive character. 
Many other breeding experiments with a wide variety of plants and 
animals have given comparable results. 

Referring again to the results of the hybrid matings as given above, 
it is clear that on the basis of their appearance the animals can be 
placed in two groups: black and white in a 3 : 1 ratio. From the stand- 
point of their germinal constitution, however, three groups, namely, 
CC, Cc, and cc, are present in a 1:2:1 ratio, or 25 per cent homozygous 
dominant (CC) to 50 per cent hybrid dominant (Cc) to 25 per cent 
homozygous recessive (cc). It will be well to introduce the terms 
phenotype and genotype at this point. The phenotype of an organism 
has reference to the appearance of the organism without regard to the 
germinal conditions; the term genotype refers to the actual germinal 
constitution. We may say, then, in the above matings that the 
expected phenotype ratio is 3 animals showing the dominant character 
black to 1 animal showing the recessive character white, whereas the 
expected genotype ratio is 1 (25 per cent) homozygous dominant (CC) 
to 2 (50 per cent) heterozygous dominants (Cc) to 1 (25 per cent) pure 
recessive (cc). From the standpoint of heredity, it is obvious that the 
phenotypes can be discarded; it is only the actual germinal constitu- 
tion, or genotype, that counts in the formation of gametes which, in 
turn, are responsible for the transmission of characters to the next 
generation. (VI.) 

The three genotypes CC, Cc, cc of the monohybrid matings present 
various possibilities for th$ succeeding generations. It is clear that 
the pure dominants (CC) and the recessives (cc) will continue to pro- 
duce the same types if mated with individuals of the same genotype, 
that is, CC with CC and cc with cc. The hybrids (Cc) will continue to 
produce offspring in the 1:2:1 ratio if mated with hybrid animals (Cc) 
as shown in table (VI). Another possibility lies in the mating of the 
CC animals with cc animals which, as shown in (IV) above, gives, in all 
cases, the hybrid condition (Cc). The final possibility lies in the 



mating of homozygous animals with heterozygous. Thus the pure 
blacks (CC) can be mated with the hybrid blacks (Cc), or the white 
animals (cc) can be mated with the hybrid condition (Cc). In either 
of these crosses it will be found that the expected ratio of the offspring 
is: 50 per cent bearing the genotype of one parent and 50 per cent 
bearing the genotype of the other parent. Thus 



CC X Cc Sperm 




cc X Cc 




Considering the homozygous-hybrid matings (VII), it is seen that 
there is only one phenotype all the offspring are black and two 
genotypes, CC and Cc, corresponding to the two parental types with 
an expected 1 : 1 ratio. In the second example (VIII), the two pheno- 
types, black and white, occur and also two genotypes, Cc and cc, 
corresponding to the two parental types and with an expected 1:1 
ratio. Recalling for a moment the results of mating two hybrids as 
shown in square (VI) above, it will be seen that the results of homo- 
zygous-hybrid matings, as in (VII) and (VIII), can be read in the two 
horizontal columns of (VI). In the upper horizontal column we have 
the results shown as in (VII); the lower column reads the same as 
(VIII) of the homozygous-hybrid matings. The vertical columns of 
the square (VI) give the same results with the sexes reversed. 

The description of the various matings makes it evident that 
Mendelian inheritance depends primarily upon the number of types of 
gamete produced by the parents, and this in turn, as shown, is directly 
dependent upon the number of pairs of genes for alternative characters 
present. When an organism is homozygous, as in the genotype CC 
or in the genotype cc, the gene for an alternative character is not 
present, and all the gametes must be of one kind. The hybrid, or 
heterozygous, condition, on the contrary, bears genes for one or more 
pairs of alternative characters as shown above in the Tall-Short hybrids 
in the peas and the Colored- White (Cc) hybrids that we have been 
considering in the guinea pigs. The addition of other pairs of alterna- 
tive characters increases the number of gametes produced according to 
the ratio 2 n where n represents the number of pairs of alternative 



characters or the degree of hybridization (page 369). This may be 
indicated by the terms monohybrid where one pair of alternative 
characters is present, dihybrid where two pairs of alternative characters 
are considered, and trihybrid where there are three pairs of alternative 
characters. Our study of Mendelian inheritance, so far, has been 
concerned with the monohybrid condition represented by one pair of 
alternative characters Colored- White, or Cc, and giving two types of 
gamete C and c and four possible combinations. Now the results 
obtained from matings of dihybrids or trihybrids or polyhybrids with 
even greater hybridization conform exactly to the results obtained in 
the monohybrid matings; they only present more types of gamete and 
correspondingly more possible combinations between gametes, as will 
be shown in the following paragraphs. 

In the dihybrid condition with its two pairs of alternative characters 
will be.2 2 , or four types of gamete. To see the possibilities inherent in 
this condition we may take another visible hair character in the guinea 
pig, namely ^ length of hair, which has been shown to be a Mendelian 
character with the alternative pair of characters Short-Long. The 
short-haired condition is dominant and may be designated as $, and 
the long-haired condition which we shall represent by s is recessive. 
The dihybrid animals, then, with two pairs of alternative characters 
differing with respect to color of hair (Cc) and length of hair (Ss) are 
indicated in the genotype constitution CcSs. The four types of 
gamete produced by these dihybrids are CS (genes for black and short), 
Cs (genes for black and long), cS (genes for white and short), and cs 
(genes for white and long). Mating two dihybrids, then, gives four 
types of sperm and four types of egg and the possibility of 16 combina- 
tions when the gametes unite in random fertilization to form the zygote. 
Again the square shows all the possibilities (IX). 


Sperm ^\^ 



































An analysis of the various possibilities in the dihybrid square 
shows that, out of the 16 possible combinations, there are four pheno- 
types corresponding to the four types of gametes, the appearance of 
which may be described as follows: black, short-haired animals; black, 
long-haired animals; white, short-haired animals; and white, long- 
haired animals; or, stated in another way, those which show both 
dominant characters, those which show one of the two dominant 
characters, those which show the other dominant character, and thosr 

cR cr 

FIG. 192. Illustrating the inheritance of hair color and another alternative character 
(rough or smooth), giving a dihybrid condition. Note that one of the parents (/VCV) 
is dominant for color (C) and recessive for smoothness O) ; whereas 1 he other parent 
(cRcR) is recessive for color (c) and has the dominant character (/if) which produces a 
rough coat. All of the Fi dihybrids show the two dominant characters. (Woodruff, 
slightly modified.) 

which show neither of the dominant characters. These occur in 
different proportions; the first group with both dominant characters is 
the largest with 9 out of the 16 animals, or 56.25 per cent. They are 
shown in the zygotes numbered (1), (2), (3), and (4). Three out of the 
16, or 18.75 per cent, express the dominant character Black. They are 
shown in zygotes numbered (5) and (7). Three out of the 16, or 
18.75 per cent, show the other dominant character Short. They are 
shown in zygotes numbered (6) and (8). One individual out of 16, 
or 6.25 per cent, may be expected to show neither dominant character, 


as indicated in number (9). Thus we have the dihybrid phenotype 
ratio of 9:3:3:1. (IX.) 

The 16 possible dihybrid combinations give 9 different genotypes 
which, as just seen, are numbered from 1 to 9. Among the genotypes 
is one pure dominant (1) which has received the two dominant genes 
CS from both parents and, of course, can form only CS gametes; also 
one pure recessive (9) which received the two recessive genes cs from 
both parents. In addition to (1) and (9) there arc two other genotypes 
(7) and (8) which are also homozygous, having received the same genes 
from both parents. Four of the genotypes (4) are dihybrids like the 
parents, the genes received, from either the sperm or egg, giving the 
genotype OSes. Eight of the genotypes are inonohybrids (2), (3), 
(5), (6) with two each of four different combinations. It will be 
helpful to examine one of these to sec just why they arc monohybrids. 
Let us take the genotype condition as shown in (2), namely, CS from 
one parent and Cs from the other. If now the genes for color and the 
genes for length are arranged in pairs, we have CCSs. It is thus seen 
that such an animal is homozygous with respect to hair color; that is, 
it can produce only one type of gamete for this character, namely, 
gametes bearing the gene C. It can produce two types of gametes, S 
and s, with respect to length of hair. In brief, then, this is a mono- 
hybrid and will form 50 per cent gametes with CS genes and 50 per cent 
with Cs genes. 

In summarizing the expected results from the dihybrid matin gs 
with respect to hybridization, it is thus found that four (25 per cent) 
are homozygous, 8 (50 per cent) arc monohybrids, and four (25 per 
cent) are dihybrids like the parents. Once again emphasis should be 
laid on the fact that the various ratios just considered are not absolute*, 
since fertilization is always at random, and, therefore, no prediction 
can be made as to which one of the foiir types of sperm will fertilize a 
particular egg, as explained above in the consideration of the mono- 
hybrid ratios. Finally, it should be noted that the results from the 
matings of dihybrids with homozygous individuals and ajso with 
monohybrids are shown in the various horizontal columns of the 
dihybrid square (IX) as observed previously in the moiiohybrid square 
(VI). Thus, for example, the zygotes expected from the matings of a 
dihybrid with a homozygous dominant are shown in the top horizontal 

The principles operating in the moiiohybrid and dihybrid matings, 
which have been considered in some detail, are unaltered in the 
trihybrids with three pairs of alternative characters or in any poly- 
hybrid with still more pairs of alternative characters. Since this is the 



case, it is not necessary for the geneticist to consider, in general, 
matings in which account has to be taken of more than one, two, or 
three alternative characters. In nature, however, with thousands of 
heritable characters present in the organism it would be impossible to 
find an individual homozygous for all the genes involved or even 
individuals with only one, two, or three pairs of alternative characters; 
they are all poly hybrids. 





Csr . csr 

FIG. 193. The possible ^2 phenotypes from a trihybrid cross between one parent 
carrying the three dominant characters for pigmentation, short hair and rough coat 
(upper left-hand corner) and the other parent a pure recessive with no color, smooth 
ooat and long hair (lower right-hand corner), as described on page 368. (Woodruff, 
slightly modified.} 

Brief consideration of the trihybrid condition will be sufficient to 
show the increased possibilities associated with the addition of a third 
pair of alternative characters. Continuing with our study of the 
inheritance of visible hair characteristics in the guinea pig, it has been 
found that a curly or rough condition of the hair coat is alternative to a 
straight or smooth condition. The condition Rough, or R y is dominant 
over Smooth, or r. The homozygous dominant for colored, short, 
rough hair thus has the genotype CSRCSR, producing only CSR 




gametes, and the recessive has the genotype csrcsr, producing only csr 
gametes. Matings of the pure dominant and the pure recessive invari- 
ably give offspring in the next generation (Pi) showing colored, short, 

rough hair like the dominant parent and 
with genotype CSRcsr. This trihybrid 
condition with three pairs of alternative 
characters, that is, with n equal to 3, 
forms 2 s , or 8 types of gamete and 12^ 
per cent of each type. Making all the 
possible combinations in an independent 
assortment, the eight types of gametes are 
found to be: CSR, CSr, CsR, Csr, cSR, cSr, 
csR, and csr. The construction of the 
trihybrid square as in the previous cases, 
but for eight types of gametes, shows that 
64 possible combinations are represented 
in the zygotes of the F 2 generation. The 
eight phenotypes, corresponding to the 
gametes, may be expected to appear in 
the ratio of 27:9:9:9:3:3:3:1 in a total 
of 64 animals, but, as we understand, since 
fertilization as always is at random, the 
chance of getting exactly this particular 
ratio in any group of 64 animals is very 
slight. It will be found that there are 27 
genotypes of which 8 are homozygous and 
19 are heterozygous. Out of a total of 64 
animals in the F 2 generation, the expect- 
ancy is that 8 (12.5 per cent) will be 
homozygous; 8 (12.5 per cent) will be tri- 
hybrids like the parents; 24 (37.5 per 
cent) will be dihybrids; and 24 (37.5 per 

t ) W J U be mono hybrids. (FigS. 193, 
' ^ v 7 


rp Q aummar i ze the ratios in the various 
hybrids the table at the top of page 369 may be helpful. 

Blending Inheritance. From the descriptions just given of the 
various hybrids, it is evident that the dominance of a particular 
character in the jJhenotype is a striking feature of Mendelian inherit- 
ance, but it must be stated at once that dominance .is not a universal 
feature, for there are numerous instances in which the genes for alterna- 
tive characters are of equal value in the determination of the characters 
in the phenotype just as they always are in the gene-complex of the 


FIG. 194. Diagram illus- 
trating the expected distribu- 
tion of the 64 possibilities in 
the Fz generation of a trihybrid. 



Types of 

Types of 

Types of 


Homozygous (CC) 





/-^ \ Monohybrid (Cc) 





Dihybrid (CcSs) 





Trihybrid (CcSsRr) 





Poly hybrids (n pairs of 
alternative characters) 


2 n 


4 n 

gametes. This condition is commonly referred to as blending inherit- 
ance because the hybrid offspring show a mixture, or blend, of a particu- 
lar character; dominance is lacking. One of the most common 
examples of this is seen in the common snapdragon in which plants 






Pink Whik 

FIG. 195. Illustrating blending inheritance in the snapdragon when homozygous 
red and white individuals are crossed as described on Rage 369. (Sinnott and Dunn.) 

are found with red, white, or pink blossoms. The breeding experi- 
ments show that, when a pure red plant is crossed with one bearing 
white blossoms, all of the hybrid offspring will have pink flowers. The 
pink flowers may be regarded as intermediate in color between the red 
and white of the two parental plants. It is apparent from these 



results that neither red nor white are dominant in the phenotype. If 
now the hybrid FI plants bearing pink flowers are crossed, three types 
of colored offspring will appear in the F 2 generation in a 1:2:1 ratio 
or 25 per cent red: 50 per cent pink: 25 per cent white. This is the 
ratio expected from the mating of monohybrids as shown in square 
(VI).. If we let C represent the gene for red color and c the gene for 
white color, then the pure reds have the genotype CC and the whites 
the genotype cc. The FI hybrid pink offspring will contain both 
genes, that is, Cc, and will produce gametes C and c. Crossing of these 
monohybrids gives the 1:2:1 ratio as shown in table (XI) and also 
in Fig. 195. 


Sperm ^^"\^ 












Thus when dominance is lacking, the genotype is revealed by the 
appearance of the phenotype. Many other examples of blending 
inheritance are known, as for example, in the inheritance of size in 
rabbits where the offspring of matings between large and small races 
are intermediate in size. The latter when interbred show a segrega- 
tion of size in the next generation. Earlier, it was thought that the 
results obtained from some of these examples with incomplete domi- 
nance did not conform to the Mendelian laws, but further study has 
shown in practically every case that they fall into line. From our 
standpoint, the important thing to note is that segregation of the genes 
in the gametes of the hybrids with dominance lacking occurs just as in 
other hybrids where complete dominance is found. 

Lack^of dominance may be expressed 1 in other ways than by the 
so-ceiled -blending inheritance, as just shown in flower colors. Thus, 
in tKe matings of red cattle with white, the FI offspring show a charac- 
teristic foan color. If the individual hairs of the hybrid roans are 
examined, it will be found that they are equally divided between red 
hairs and white hairs so that both of the alternative genes are expressed 
in the color pattern of the roan animals. The hybrid roan-colored 
animals, when interbred, produce offspring as in monohybrid matings, 
namely, 25 per cent red to 50 per cent foan to 25 per cent white, just 
as do the snapdragons. Another well-known example of the same type 
Of inheritance is in the Blue Andalusian fowls, which never breed 



true but produce offspring with three types of color in the ratio of 
1:2:1 as follows : 25 per cent white to 50 per cent blue to 25 per cent 

FIG. 196. Photograph of a roan, ghort-horned cow. This color condition occurs 
in animals heterozygous for red and white hair color, as described on page 370. (Shull, 
after McPhee and Wright.) 

FIG. 107 Illustrating the inheritance of blue in Andalusian fowls; a condition shown, 
only by the hybrid. (Sinnott and Dunn, modified.} 

black. It is apparent that the so-called Blue Andalusian type is a 
monohybrid with respect to color, bearing genes for black and for 



white. Accordingly, the blue color is produced in all the offspring from 
the matings of the homozygous blacks and whites. (Figs. 196, 197,) 


The cases of dominance that have been described thus far have been 
ones in which a single pair of genes was operating, but it is well estab- 
lished that, in many instances, several pairs of genes are concerned 
in the development of a particular character, the condition known as 
multiple genes (multiple factors). This puzzling condition was first 
noted in the results obtained from breeding certain grains, such as pats 
and wheat. When, for example, a variety of brown-seeded oats was 
crossed with a white-seeded variety, both being homozygous, all of 
the plants in the next generation had brown seeds that were distinctly 
lighter in color than the brown-seeded parent. When these hybrid FI 
plants, were interbred, it would normally be expected, as we know, that 
a phenotype ratio of 3 brown to 1 white would appear in the offspring. 
But not so in this case. The dominant brown color appeared in the 
ratio of 15 dominant brown-colored seeds to 1 recessive white seed, or 
93.75 per cent to 6.25 per cent. An examination of the brown seeds 
showed considerable variation in the depth of color. It was then seen 
that this condition was just what would be expected if two factors or 
genes were associated with this color character, thus giving the 
dihybrid Condition for color, with four different types of gamete and 16 
possible combinations (page 364). Representing the two factors for 
brown color as C\Ci and the recessive alternative genes for white as 
do*, the genotype of the pure dominant would be CiCiCaC^ with all 
gametes CiCj; that of the pure recessive c\c\c^c^ with all gametes c\c*. 
All the dihybrid offspring would then have the genotype CiCiC&t and 
would produce four types of gamete as shown in the dihybrid square, 
which conforms to that shown in (IX) above. 



CiC7 2 




















When the results of the matings are analyzed, it becomes apparent 
that the brown color of the seed increases in intensity in correspondence 
with the number of genes for color present. Thus individuals with 
the genotype CiCiCzCi form seeds with the greatest amount of brown 
pigment, with a decrease indicated in the genotypes with fewer of 
the color-bearing genes Ci and C 2 a in the series CiCiCzC^ CiCiCjCj, 
CidCzCzj and with no brown pigment whatever in the pure recessive 
CiCiC 2 c 2 . (Fig. 198.) 

R,R, R 

. Red 




FIG. 198. Illustrating results of crossing two varieties of wheat, one with two fac- 
tors for red seeds (RiRifaRz) and the other with the factors for white seeds (nr 2 rir 2 ). 
This multiple-lactor condition is described on page 372. (Sinnott and Dunn,} 

In the matings of certain varieties of red-seeded and white-seeded 
wheat, it was found that a trihybrid condition for color, with three 
pairs of genes, was present so that, in the F 2 generation, the ratio of 
colored seeds to pure white was 63 colored out of 64. In other words, 
on the average one pure recessive appeared out of 64 possible combina- 
tions, which is the condition in a trihybrid cross. Hair color in animals 
has been shown, in some instances, to be due to multiple factors, and 
it is also 'evident that skin color in man is another example. Thus, in 
man, thefre are two or three pairs of genes for color, with the intensity 


of the color in the skin of the hybrid, or mulatto, offspring of white and 
negro parents being governed by the number of dominant genes pres- 
ent, as just seen in oat seeds. If we take the colored conditions as 
being dihybrid with the color genes CiC 2 , then the genotype of the pure 
negro with black skin can be given as CiCiCzCz and the recessive 
white as CidC2C 2 , just as in the case of the dihybrid color factor shown 
in (XII) above and in Fig. 198. 

The discovery of multiple genes, that is, the knowledge that more 
than one pair of genes were concerned with the production of a partic- 
ular character, brought a great many cases of inheritance into the 
Mendelian fold that did not, at first, appear to conform to the estab- 
lished principles. At the same time, increased emphasis came to be 
laid upon the fact that the characters exhibited in the organism, in 
most cases at least, were not the result of one pair of genes acting 
during development but of many genes. In the determination of eye 
color in Drosophila, for example, it is known that more than 20 pairs 
of genes are operating, and essentially the same situation exists with 
regard to the determination of wing characters. Furthermore, the 
various genes for eye and wing characters are not even located in the 
same pair of homologous chromosomes. In the determination of sex, 
however, it is clear that the gene mechanism is localized in one pair of 
chromosomes, though various genes may be at work. But aside from 
the determination of sex, no evidence exists that all the genes for a 
particular character or associated with the development of a certain 
organ are located in a particular chromosome. Thus to refer again to 
the eye of Drosophila, the numerous genes concerned with color and 
various other characteristics have been found to be located in all four 
of the synaptic pairs. Furthermore, the genes in a particular chromo- 
some, though maintaining a rigid linear relationship as shown by the 
synaptic phenomena, appear to be promiscuously arranged with respect 
to the characters that they determine, so that, for example, a gene for 
eye color may be situated close to a gene responsible for the develop- 
ment of some character in the body or wings. Since, however, com- 
paratively few of the total number of genes have been located, it may 
be that the apparent irregularities do not give a true picture. 

But possibly the most important result coming from an under- 
standing of multiple factors was the light thrown upon the problem 
of selection. Practically all types of domesticated varieties of plant 
and animal, which man has found valuable for his multitudinous needs, 
have been subjected to selection since the earliest times; certain indi- 
viduals being chosen in each generation for the production of the next 
generation. Such selection has been made in order to strengthen and 



16 17 18 

FIG. 199. Illustrating variations in hair color in Dutch rabbits. By systematic 
selection the average pigmentation "of a race of Dutch rabbits may be gradually but 
permanently changed either in a plus or in a minus direction." The extreme of pig- 
mentation is shown in 1, and the opposite condition in 18. (Castle, "Genetics and 
Eugenics'' Harvard University Press.) 



Pure Line 



establish a certain desirable attribute which was particularly well 
displayed in the organisms selected for breeding; more milk in cows, 
more speed in race horses, more power in draft horses, more returns per 
acre in the grains due to higher productivity, to greater ability to resist 
disease, or to earlier ripening qualities; the examples are almost 

unlimited. (Fig. 199.) 

Now the fact is, of course, as 
almost everyone knows, that 
selection as practiced by practical 
breeders, who knew nothing of 
the Mendelian principles or genes, 
has been successful and is re- 
sponsible for most of the im- 
provement that has been made 
in our present-day varieties of 
domesticated plants and animals 
over the undomesticated types. 
But it is now clear that the 
improvement of types by selec- 
tion is based upon the gradual 
accumulation, generation by gen- 
eration, of the genes responsible 
for the production of the desired 
character for which selection is 
being made. In other words, 
where selection is effective, sev- 
eral pairs of genes, multiple 
genes, are associated with the 
development of the character in 
question so that the selection, for 
example, of certain animals for 
breeding, which show the opti- 
mum expression of a desired 
character, means the accumula- 
tion of more genes for this 
character and a correspondingly 
greater expression of it in the next 
generation. Selection, then, to be effective must occur in a mixed 
population, or, in Mendelian terms, the individuals must be hetero- 
zygous. In a pure line, where all the individuals are homozygous, 
selection can have no effect, for all the genes governing the character in 
question are then present in the genotypes of the homozygous indi- 

Fio. 200. Diagrams illustrating varia- 
tion in weight of pure lines of beans (1-5). 
The test tubes containing beans of the 
same weight are arranged vertically. 
The general population formed by com- 
bining the pure lines is shown below. 
(Walter t after Jokannsen.) 


viduals. Thus, to take a very simple example, selection would have 
no effect in increasing the color in the brown-seeded oats with the 
genotype CiCiC 2 C 2 , for all the genes for color are present, or in decreas- 
ing the color in plants with the genotype CiCic 2 c f . (Figs. 198 to 200.) 


If the preceding descriptions of the Mendelian phenomena have 
been followed carefully, it is probably already apparent to the reader 
that the segregation of the genes in gamete formation and their recom- 
bination in the zygote nucleus at the time of fertilization exactly 
parallel the behavior of the chromosomes as described in the closing 
pages of the previous chapter. Of course, this must be true if it is 
kept in mind that the chromosomes are really strings of genes arranged 
in linear fashion, like beads on a necklace. Thus the same terminology 
might be used with a pair of homologous chromosomes as with genes in 
the case of a pair of alternative characters, such as Colored (C) and 
White (c). In the parent hybrid individual with the diploid condition 
Cc, both chromosomes C and c would be present in all the cells, but 
when the gametes are formed following synapsis and reduction, each 
gamete with the haploid condition will receive only one chromosome, 
that is, either C or c, just as each gamete contains either the gene C 
or the gene c. Fertilization, following mating with an organism 
carrying the same chromosomes, will restore the diploid condition and 
give the opportunity for a sperm with the C chromosome to combine 
either with an egg with the C chromosome or with an egg bearing the c 
chromosome, thus giving zygotes with the three genotypes ,as given 
above (VI) in the nxonohybrid matings. This can be carried further 
with additional homologous pairs of chromosomes for thejShort and 
Long (Ss) as in the dihybrid, and the Rough and Smooth (Rr) as in 
the trihybrid. The point is clear; a particular gene is always in a 
particular chromosome and cannot act independently of that 

This condition leads at once to the question of the association, or 
linkage, of the genes present in a particular chromosome. It was early 
recognized in breeding experiments that certain characters always 
appeared together in an individual; both were present or both were 
absent, but never one without the other. The explanation was long 
lacking, but it is now known that the linking of certain characters in 
an individual is due to the fact that the genes for these characters 
happen to lie in the same chromosome and must accompany this 
chromosome wherever it goes. Since the number of characters and the 


corresponding genes in the highly developed organism runs into the 
thousands and there are only a relatively few chromosomes in which 
all these genes lie, it becomes apparent that many genes are situated, or 
linked, in a particular chromosome and must always go along with it. 
It was stated in the previous chapter that some 2,500 genes were 
arranged in linear fashion in one chromosome of Drosophila (page 336). 
All these genes must, therefore, go into one gamete with that chromo- 
some and give rise to a corresponding group of associated characters 
in the offspring. The genes in a particular chromosome constitute a 
linkage group. The total number of linjcage groups in an organism, as 
determined by a study of linked characters in breeding experiments, 
must, then, correspond to the haploid number of chromosomes as seen 
under the microscope. This has been found to be the case in two 
important instances where data are available. In Drosophila, as we 
know r four different chromosomes are to be observed when the haploid 
nuclei are observed under the microscope. The breeding experiments 
with Drosophila likewise show four linkage groups. It has been found 
in Indian corn that the 10 haploid chromosomes found by the cytologist 
check with b^eding experiments of the geneticist showing 10 linkage 
groups. (Fig. 179.) 

Linkage necessarily restricts the independent assortment of genes 
in gamete formation. Independent assortment can occur only in 
cases where the pairs of alternative characters that are being considered 
lie in different chromosomes or linkage groups. To take a specific 
example in the dihybrid CScs, which we studied in (VI) above, if the 
two pairs of alternative genes are present in the same pair of homo- 
logous chromosomes, only two types of gamete, CS and cs, can 
be formed. This will be understood if it is remembered that the 
homologous chromosomes separate after having been temporarily 
fused in synapsis, and then each divides to form a total of four chromo- 
somes which are distributed to the spermatids of the male or to the egg 
and the three polar bodies of the female. In this case, then, the chro- 
mosome bearing the genes CS unites with its synaptic mate bearing the 
genes cs. Following this temporaiy union, they separate and are 
segregated in the secondary spermatocytes or, in the female, in the 
secondary oocyte and the first polar body. In this case, one of the 
secondary' spermatocytes would carry the CS chromosome, and 
the other would receive the cs chromosome. In the final stage, each 
chromosome divides longitudinally so that two CS chromosomes and 
two cs chromosomes are distributed to the four spermatids or, in the 
female, to the egg and three polar bodies. Thus, as stated above, when 
two genes are present in the same chromosome, there is no possibility 


of independent assortment, and only two types of gamete are formed 
in a 1 :1 ratio, as in the case of the monohybrid. 

Though it had long been observed that certain characters were 
associated in the organism, the first evidence obtained by the labora- 
tory geneticist that supplied an explanation of the phenomenon was in 
connection with the so-called sex-linked characters. A sex-linked 
character is one in which the gene is carried on the sex chromosome 
(X chromosome) which determines the sex of the zygote at the time of 
fertilization, as stated previously (page 339). Before discussing the 
behavior of sex-linked characters, it may be well to reexamine the 
inheritance of sex in the light of our added knowledge of inheritance 
phenomena. It will be recalled that the female produces only one type 
of gamete with respect to sex; all eggs contain an X chromosome. 
Thus the female is homozygous in this respect. The male, on the other 
hand, is heterozygous for sex and produces two types of sperm : the X 
sperm and the Y sperm (which is the synaptic mate) in equal numbers 
as in the normal monohybrid condition. As shown above in matings 
between homozygous and monohybrid individuals, the expected Men- 
delian ratio is 1:1, or, in this case, 50 per cent males and 50 per cent 
females. Since, however, fertilization is always at random, the sex of 
any particular mating depends on whether the X sperm or the Y sperm 
reached the egg first. The possibilities of sex inheritance, as just 
described, may be shown graphically as follows: 

Sperm ^^ 


XX 9 


Now in the case of sex-linked characters it is established that the 
X chromosome carries other genes beside those which determine sex. 
Since these genes actually lie in the X chromosome with those which 
determine sex, they must be carried along just as in ar^y case of link- 
age, discussed above. One of the best examples of this is found in 
the inheritance of red and white eye color in Drosophila. In this case, 
the gene for red in the X chromosome is dominant over the gene for 
white, and so, whenever an X chromosome bearing the gene for red is 
present in the genotype, the animal has red eyes. Accordingly, if a 
red-eyed male is mated with a white-eyed female, all the sons will have 
white eyes and all the daughters will have red eyes, as we shall now 
see. Since the gene for eye color and the gene determining sex are on 



the same chromosome, independent assortment of the two genes can- 
not occur. The red-eyed male acts as a monohybrid and produces 
equal numbers of two types of gamete; one type bearing X with gene 
R for red which we may indicate as X*, and the other, gametes bear- 

FIG. 201. Illustrating the possibilities of sex-linked inheritance from crosses 
between a white-eyed female Drosophila and a red-eyed male. All of the Fi females 
are red-eyed (left); ail of the males are white-eyed (right). The results of matings 
between these two genotypes are shown in the Ft generation and described on page 381. 
(Slightly modified from Morgan, "Scientific Basis of Evolution,' 1 W. W. Norton & Com- 
pany, Inc.) 

ing Y. The white-eyed female is homozygous with respect to sex and 
also eye, color and, therefore, produces only one type of gamete, indi- 
cated as X r , all of which contain the gene for the recessive white eye 
color. This is another case of mating a monohybrid with a homozy- 



gous individual (VII), (VIII), (XIII), and the expected results in the 
Fi generation, namely, 50 per cent white-eyed males and 50 per cent 
red-eyed females are illustrated in the following square. (Fig. 201.) 



Let us carry sex-linked inheritance a step further by next mating 
these males and females of the Fi generation (XIV). It will be seen 
that the male, as always, is heterozygous with respect to sex, produc- 
ing two types of gametes X* and Y. But neither of these carry the 
red gene, so the male is homozygous with respect to eye color. The 
female, as always, is homozygous with respect to sex, producing only 
one type of gamete, X, but is heterozygous for eye color wit.h two 
types of gamete, XR and X r . The union of these male and female 
gametes involves a typical monohybrid crossing (VI) with four possi- 
ble zygotes, as shown in the square. 








It is seen that the offspring show the following possibilities; 25 per 
cent with the genotype X r X R will be red-eyed females; 25 per cent 
with the genotype X* X r will be white-eyed females; 25 per cent 
with the genotype X R Y will be red-eyed males; and 25 per cent with 
the genotype X r Y will be white-eyed males. All of these possibilities 
are illustrated in Figure 201. Mating between a white-eyed male, 
X* Y , and a red-eyed female homozygous for the red-eyed condition, 
X*X R , gives the same results for color inheritance as in any mating of 
two individuals homozygous for a particular character. Thus all of the 
hybrid FI offspring of both sexes from this cross will show the domi- 
nant character, red eye, and these, when interbred, will normally give, 
in the next generation, 3 red-eyed individuals to 1 white; made up of 
one homozygous red-eyed female, X R X R , one hybrid red-eyed 
female, X R X' ; one red-eyed male, X R Y, and one white-eyed male, 
X' Y. (Fig. 201.) 

Color Blindness. Long before the mechanism of sex-linked 
characters was understood, it had been noted that a defect in human 



vision, known as color blindness and characterized by an inability to 
distinguish between red and green color, was inherited in a peculiar 
fashion. In the first place, this defect is much more common in men 
than it is in women, and, secondly, it is transmitted between the two 
sexes in an unusual way which can be explained on the assumption 
that the defective gene is carried in the X chromosome. There is 
an added feature, however, not found in the inheritance of eye color 
in Drosophila which, as we have just seen, behaves as a simple Men- 
delian dominant; that is, the red eye appears in either sex when the 

Color blind 


FIG. 202. Diagram illustrating the inheritance of color blindness from a color-blind 
father and normal mother as described on page 383. X-chromosomes carrying this 
defect are shown in solid black. (Sinnott and Dunn, after Dunn, Courtesy of the Uni- 
versity Society.) 

gene is present in the X chromosome. The gene for color blindness, 
which we may designate as c, for some unknown reason acts as a 
recessive in the female and must, therefore, be present in both of the 
X chromosomes in order to produce color blindness, as X c X c . If 
it' is present in only one of the X chromosomes, as X c X, the woman 
will not have defective color vision, but she will be a carrier for' color 
blindness. These carrier females will form two types of gamete with 
respect to this character, namely, gametes bearing the normal X and 
gametes bearing the defective Xc, with the result that the gene for 
color blindness will be transmitted to the next generation. In the 



male, the gene for color blindness acts as a dominant so that the man 
is always color -blind when the X chromosome carries the defective 
gene as X c Y . 

Various possibilities for the offspring result from matings between 
normal males, color-blind males, normal females, carrier females, and 
color-blind females. One common example may be taken to show 
the inheritance from a color-blind father with the genotype X c Y- 
Two types of gamete will be produced, namely, Xc and Y. If union 
occurs with a normal female with the genotype XX and therefore 
producing only X gametes, the same possibilities exist as in a mono- 
hybrid-homozygous combination as already indicated in (VII) and 
(VIII). As shown below (XVI), all the children of both sexes will 
have normal vision, but the daughters will be carriers. 



X Y 

Continuing this analysis one step further to the grandchildren, we 
consider the possibilities for color blindness inherent: in the offspring 
of normal fathers and carrier mothers (XVI). It will be seen that 
the male gametes are free from the defective genes and form normal X 
gametes and Y gametes. The mother, as a carrier with the genotype 
X c X, produces Xc gametes and X gametes, and the results in the 
offspring can be shown in the monohybrid square (XVII). 


X X 




It is evident that the defective gene of the male grandparent (XVI) 
under these conditions may be expected to produce color blindness 
in one-half of the grandsons and carriers in one-half of the grand- 
daughters. All the other grandchildren, both male and female, will 
be free from the defective gene. Another defective gene associated 
with the sex chromosome is responsible for hemophilia or bleeding. 
It behaves in the same way in inheritance. (Fig. 202.) 

Lethal Genes. The typical monohybrid ratio of 3 dominant to 
1 recessive, which has been so thoroughly established in various organ-- 
isms, is supplanted by a 2:1 ratio in the expression of certain char- 
acters. The explanation of this ratio was not clear for some time, and 



then it was discovered from the proper experimental crosses that one of 
the expected F 2 genotypes was entirely missing in the offspring from 
certain monohybrid crosses, and this was due, it became evident, to the 
presence of a so-called lethal gene which resulted in the death of all 
individuals when present in a homozygous condition. This lethal 
condition was first shown in the gene for yellow hair color in mice. It 
is known that yellow color is dominant, and accordingly it would be 
expected that the two genotypes YY and Yy would be found among 
the yellow animals. But yellow animals crossed with black invariably 
gave a 1:1 ratio instead of all yellow as expected with matings of 
homozygous dominants (YY) and homozygous recessives (yy). Also 
when the yellow animals were crossed with yellow, the color ratio in 
the offspring was always 2 yellow to 1 recessive (black or brown or 



YY Yy Yy yy 

FIG. 203. Illustrating inheritance of a lethal factor in mice. The homozygous 
yellow embryos (YY) die, as described on page 384. (Sinnott and Dunn, slightly 

gray). In a study of over 1,200 mice, the 2:1 ratio was always closely 
maintained. The breeding results are just what would be expected if 
all the yellow animdls were heterozygous (Yy). It is clear that such is 
the situation. The homozygous yellow (YY) is lethal, and the zygotes 
that receive this combination at fertilization die before birth. (Fig. 

Another very interesting example of a lethal gene that has been very 
thoroughly studied is associated with the so-called creeper chicken, 
which is characterized by a marked reduction in the length of the legs 
(also the wings) so that the animals appear to be squatting on the 
ground, and their locomotion is more of a creep than a walk. It has 
been shown by the proper crosses that the gene for the creeper charac- 
ter always acts as a dominant when present. Normal animals never 
have creeper offspring, but normals crossed with creepers give a 1:1 
ratio, and creepers crossed with creepers give a ratio of 2 creepers to 1 
normal, as in the yellow mice. The creeper condition is therefore 



heterozygous (Dd). The homozygous creeper (DD) dies. An 
examination of the eggs being incubated shows that the homozygous 
creeper embryos die at about the fourth day of incubation. If account 
is taken of the dead creeper embryos, then the expected 3 : 1 ratio from 
breeding monohybrids is obtained. The square gives the results when 
creepers, all bearing the genotype Dd, are crossed: 


NX \Eggs 










Crossing Over. The description of Mendelian inheritance given so 
far shows an amazingly rigid system for the transmission of characters 
from generation to generation. Thus it has been shown that the genes 
are arranged in definite linear fashion in the chromosomes and are 
sorted out or segregated in mechanical fashion during gamete forma- 
tion in correspondence with the pairs of alternative characters. 
Furthermore, it has been shown that there are relatively few chromo- 
somes as compared with a large number of genes, and, accordingly, 
great numbers of genes are linked together in each chromosome. Thus 
linkage limits the possibility of independent assortment and, thereby, 
the possible types of gamete produced. The rigidity of the system is 
somewhat lessened by the fact that fertilization is at random; but with 
a small number of chromosomes, as in Drosophila with a haploid 
number of 4, only 16 combinations are possible with other gametes as 
shown in the dihybrid square (IX). And so a Drosophila sperm from a 
pure dominant, with four haploid chromosomes, A, B, C, D, has the 
possibility of uniting with an egg from an individual with the same 
chromosome grouping or with an egg from one of the 15 other chromo- 
some combinations such as A, B, C, d; A,/?,c,d; etc.; and, finally, to 
a, 6, c, d, in which all four chromosomes carry the recessive gene. 
In man, with 24 different types of chromosomes, the possibilities of 
chromosome combinations are, of course, greatly increased, but even 
so there appears to be no way to get new heritable characters intro- 
duced into the system. The purpose of this section is to introduce 
certain important features associated with inheritance that markedly 
decrease the rigidity of inheritance, which has just been summarized, 
for we now know a number of important phenomena associated with 



the hereditary mechanism that introduce new and, to some extent, 
unpredictable heritable features into the germ plasm that, thereafter, 
become 'part and parcel of the hereditary melange for transmission to 
succeeding generations. (Fig. 204.) 

The first of these flexible features is a process associated with 
synapsis and aptly designated as crossing over. The title is descriptive 
and means that, under certain conditions, genes located in one of the 
synaptic mates may cross to the other member of the pair during 
synapsis. That such occurred in certain insects during gamete forma- 
tion was observed by the cytologist Janssens nearly thirty years ago, 






Fia. 204. Diagram illustrating the 16 possible types of gametes resulting from 
the union, in the previous generation, of two gametes, each bearing four chromosomes, 
as described on. page 385. (Woodruff.) 

but it was the more recent results obtained by breeding experiments 
in fruit flies that confirmed the cytological findings. Crossing over, 
since it occurs with predictable frequency in the case of certain matings, 
is to be regarded as a normal feature of synapsis. It produces new 
possibilities, recombinations, for the genevS concerned. (Fig. 205.) 

The classic example of crossing over, which will serve admirably for 
our purpose, occurs in Drosophila. Female flies with two dominant 
linked genes for Gray Body ((?) and Long Wings (TF), when mated with 
the so-called vestigial male flies that carry linked recessive genes for the 
alternative characters Black Body (g) and Short Wings (w), produce 
only hybrid gray, long-winged offspring. This is just what would be 



expected since the pure gray, long-winged flies with the genes G and W 
in the same chromosome can produce only one type of gamete, namely, 
GW; and the recessive black, short-winged animals, under the same 
conditions, can also produce only one type of gamete, namely, gw. 
Fertilization between these gametes produces 100 per cent hybrid FI 
offspring with the genotype GWgw. These individuals, of course, 

cm u 


FIG. 205. Diagram illustrating crossing over between homologous chromosomes 
during syiiapsis, giving new arrangements of the genes (recombinations). If 110 
crossing over occurs the chromosomes separate after synapsis carrying the same genes 
(1). Crossing over at two points (double crossing over) is shown in (2) and the results 
of the recombinations in (3). (Sinnott and Dunn, slightly modified.} 

behave as monohybrids and produce two types of gamete in equal 
numbers: GW gametes and gw gametes. Now, if the monohybrid 
females (GWgw) are mated with the pure recessive males (gwgw), the 
expected Mendelian ratio is 50 per cent hybrid dominants (GWgw) 
and 50 per cent pure recessives (gwgw) as in the typical monohybrid- 
homozygous mating, as shown in the square. 








Actually, however, repeated experiments showed that the expected 
1 : 1 ratio did not appear. In all cases, approximately 82 per cent of the 
animals were divided equally between the hybrid dominants and the 



pure recessives, whereas in 18 per cent of the offspring two unexpected 
varieties appeared in equal numbers, namely, Gray animals with short 
wings, that is, the genotype Gwgw, and black flies bearing long wings, 
or the genotype gWgw. The results were decisive, and, therefore, it 
was apparent that either something was wrong with the accepted ideas 
of Mendelian inheritance or else some condition existed that had not 
been taken into account. The latter proved to be correct. The 
unknown factor was the phenomenon of crossing over. It is amply 
confirmed that synapsis results not only in the pairing of the genes for 
alternative characters but also, in many instances, in the actual 
transfer, or cross over, of portions of the pairing chromosomes from 
one to the other so that when the chromosomes separate after synapsis, 
each actually contains one or more pieces of the other member of the 
pair. Necessarily the genes in the detached pieces of the chromosomes 
are also transferred to the other sy nap tic mate. Thus, in the example 
above, one of the homologous chromosomes with the linked genes GW 
became Gw after synapsis, and, in the other, the linkage gw was changed 
to gW. Consequently, as shown in the square, there were four 
types of gametes for union with the gib gametes of the recessive 


Sperm ^. 














Sex-linked inheritance gave the clue to linkage; linkage gave the 
clue to crossing over; crossing over gave the clue to the arrangement 
of genes in the chromosomes and enabled the investigators in this field 
to accumulate data for the preparation of chromosome maps through 
the discovery that the relative frequency, or percentage, of crossing 
over was in direct relation to the distance between the genes concerned. 
Genes lying close together have very little chance to shift their position, 
or cross over, to thfe other chromosome of the pair; genes lying rela- 
tively far apart have a much greater chance. As an example of the 
methods that have been employed, let us consider the position of the 
genes for another character in the same chromosomes, in relation to 
the genes Gg and Ww. Designating one end of the chromosome as f zero, 
we can arbitrarily locate genes and W at two points along the 
chromosome, but they must he separated by 18 units from each other 


in correspondence with the percentage of crossovers as determined by 
the breeding experiments. The appropriate breeding experiments 
with the genes for the new character, which we shall indicate as Nn, 
show a percentage crossover in relation to Gg of 30, whereas the per- 
centage of crossovers of Nn in relation to Ww is only 12. Since the 
percentage of crossovers is higher when the distance between the genes 
involved is greater, the results from the breeding experiments show that 
the gene N must be located 12 linear units below W and 30 points 
below G. This is the only point that will conform to the percentages 
of crossovers, that is, 18 per cent between G and TF, 12 per cent 
between W and N, and 30 per cent between G and N. Cumulative 
data from a large number of characters in Drosophila have established 
the positions of the corresponding genes in certain chromosomes and 
the development of the so-called chromosome map. Chromosome 
maps have also been established for certain characters in other organ- 
isms, notably Indian corn. Particularly gratifying to the biologist is 
the fact that the positions of the genes shown in the chromosome maps, 
which were determined on the basis of crossing over, have been shown 
in many instances to rest on a firm basis by the data derived from the 
microscopic study of the chromosome complex which will now be 
indicated. (Figs. 181, 211.) 


Leaving the phenomenon of crossing over, which has been found 
to be a normal and predictable feature of Mendelian inheritance, con- 
sideration must next be given to a variety of unpredictable and abnor- 
mal inheritance patterns that occur as the result of several types of 
irregularity or aberration in the chromosomal complex. These are 
commonly grouped under the term mutation; the term coming from the 
Latin verb mutare, meaning "to change." Chromosomal aberrations, 
or mutations, in the germ" cells are effectual in producing somatic 
mutations in the resulting offspring. The latter, in turn, in producing 
gametes with the mutated chromosomes, continue to pass the muta- 
tions along. Once in the germ plasm, the mutation is there to stay 
inless a later mutation in this same region again alters the chromatin 
pattern. It is possible for mutations to occur also in the chromatin 
rf one or more of the body cells of an organism. Succeeding genera- 
tions of cells, directly descended from the mutated cell, will have the 
same change impressed upon them, but, in this case, the mutation 
3annot get into the stream of germ plasm for transmission to offspring. 
However, somatic mutations of this kind are not uncommon in plants 


and are known as bud mutations. Since plant tissues are usually 
capable of regeneration, it is possible to remove the mutated tissue 
and to grow it independently or as a graft on another plant of the 
same species where it will produce the mutated type of cells and 
tissues. Bud mutations of this type are responsible for many standard 
varieties of apples and other fruits. 

Mutations have their origin in the various types of chromosomal 
aberrations. Some of these involve changes in considerable areas of 
the chromosomes and so come within the range of microscopic vision ; 
marked structural abnormality being visible in one or more of the 
chromosomes. Suppose, for example, that the offspring produced 
by the mating of individuals with known genotypes exhibit some 
unusual character. Microscopic examination of the chromosome 
complex from both germinal and somatic cells may reveal changes in 

FIG. 206. Diagrams illustrating nondisjunctioii in which both members of a pair 
of homologous chromosomes go to one cell (to the left), thus giving one cell with four 
chromosomes and one cell with two chromosomes. (Shull.) 

the normal pattern. * Thus it was found some years ago that certain 
females of Drosophila had unexpected eye color and various other 
abnormalities. The basis of this altered inheritance was revealed by 
the microscopic studies that showed that such females had a Y chromo- 
some in addition to the XX, thus giving an XXY instead of either XX 
of normal females or XY of normal males. This abnormal condition 
was traced to the fact that the parental females were producing mature 
eggs carrying XX instead of X. A failure of the X chromosomes to 
separate (nondisj unction) at the last maturation division caused the 
production of XX gametes. Instead of a single chromosome being 
added to the chromatin complex of the gametes, as in the case of the 
XX condition just described, many instances are known in which 
an altered heredity pattern is the result of a doubling or tripling or 
even a larger multiple of the normal chromosome number, the condition 
of heteroploidy (page 340). In this way, distinct varieties of a 
particular species are established and continue as a standard 
type, as seen, for example, in the well-known varieties of wheat with 
7, 14, and 21 chromosomes or in certain distinct species of Chrysan- 
themum with 18, 36, 54, 72, and 90 chromosomes in the diploid 



condition. The case of Drosophila with 12 chromosomes instead of 
8 has been mentioned previously. (Figs. 184, 206.) 

Other chromosomal aberrations that 
result in altered heredity may involve 
the breaking, or fragmentation, of a 
chromosome and the later association of 
one or more of the detached pieces 
with a chromosome of another linkage 
group. Thus chromatin material may A B 

i 1 i , i < i ' f FIG. 207. Drawings of the 

be deleted from one member of a pair diploid chromosomes O f the Dro- 

of homologous chromosomes and at- sophila female (XX) to illustrate 
11, i i i x the translocation of chromatin 

tached to a chromosome belonging to material . In A it is noted that 

another pair. As a result, a Certain there is a normal sized chromosome 

(IV) and an abnormally large one 
(IV -f X) ; the increased size of the 
latter is due to a trarislocatiori of a 
piece from one of the X chromo- 

percentage of the gametes will be defi- 
cient for certain genes, but other gametes 

will have genes added. Deletion, trans- 80mes . This tranfllocation was 
location, inversion are all established duo to the irradiation of germ 

.,.*... /. i , A. *. plasm of one of the parents. In B 

possibilities of chromatin mutations, it is noted that th translocation 
many of which involve chromosome nas affected both of the IV chromo- 

, . . . /. /* j. j. i somes, and therefore this indi- 

alterations of sufficient size to be vidual was h omozygous for the 

checked with the microscope. In Very altered chromatin condition. Cf. 
, , 11 /. ,1 Fig. 179. (Painter, "Science in 

recent years, our knowledge of the Progre88t Ya le University Press.) 
relations between chromosomal abnor- 
malities and altered somatic structures has been markedly increased 
by two things; first, the discovery that the X-ray irradiation of germ 

cells undergoing development would change 
the normal chromatin setup in the gametes, 
and second, the discovery of giant chromo- 
somes in the nuclei of the salivary gland cells 
that were large enough to make visible altered 
chromatin patterns in areas much too small to 
be studied in normal-sized chromosomes (page 
336). In addition to their extraordinary size, 
the giant chromosomes are undergoing syn- 
apsis, though present in somatic cells, arid this 

FIG. 208. Drawing of 
the terminal portion from 
one of the giant chromo- 
somes (II), of a salivary 
gland cell, Drosophila, 

showing the translocation synaptic oondition is of the highest value in 
the detection of chromatin irregularities in 
either of the pairing chromosomes (page 347). 
For, in syriapsis, absolute exactness of gene 
position throughout the length of the pairing chromosomes is neces- 
sary. The genes for all of the alternative characters linked together 
in a particular pair of chromosomes must be at exactly the same levels 

of a piece of the fourth 
chromosome (IV) . (Painter, 
1 ' Science in Progress, ' ' Yale 
University Press.) 



in the synaptic mates. With size of the genes well below microscopic 
visibility, this means an exactness of construction with relation to gene 
position in chromosomes far beyond anything that can be achieved in 
machine construction. (Figs. 207, 208.) 

Since every gene must be in its exact position when synapsis occurs, 
this process is now recognized as one of the most important tools for 



Fio. 209. Diagram illustrating synpasis between chromosomes in which the genes 
are not identical. In the lower chromosome it will be noted that deletion has occurred 
involving genes j, k, and I. This portion of the upper chromosome is drawn to one side 
so that the paired genes match exactly (page 392) . (Shull.) 

determining an altered gene complex in either member of the synaptic 
mates. If some of the genes in one of the pairing chromosomes are 
out of place or if they are missing, then the corresponding or alternative 
genes in the normal chromosome cannot join in synapsis. Accordingly, 
this region of the normal chromosome pulls away from the abnormal 


FIG. 210. Drawings of the terminal portion of one of the giant chromosomes (II), 
salivary gland cell, Drosophila, showing actual deletion as diagrammed in Fig. 209. 
The normal condition of synapsis is shown in the upper figure. The affected region, 
which carries the genes for roughoid, an eye character, is indicated at Del. A and Del. B. 
Modification in synapsis as the result of the deletions is shown at a maximum magnifica- 
tion in the lower figures. In Deletion A one of the synapsing chromosomes is normal, 
whereas one band is missing from the other. In Deletion B, one chromosome is normal 
and three bands are missing from the other one. (Painter, "Science in Progress, 11 
Yale University Press.) 

synaptic mate and forms a loop-like structure off to one side, and this 
permits those genes which are present and normally locatld in both 
chromosomes to meet in synapsis. The synaptic behavior of the 
normal chromosome gives visible evidence to the cytologist of regions 
with an altered gene arrangement in the synaptic mate. Intensive 
study of the latter, particularly in the giant chromosomes, has brought 



altered heredity patterns almost down to the actual genes concerned 
(page 335). (Figs. 209 to 211.) 

We have just been dealing with mutations that involve areas 
in the chromosomes of sufficient size to be observed under the micro- 
scope. Mutations occur, however, in which the chromatin pattern 
of the mutants shows no visible alteration even under the highest 
magnifications. Such mutations must have their sole basis in the 
chromatin complex, but they may result from a change in only a single 
gene and, therefore, cannot possibly be brought within the limits of 
microscopic visibility. Such mutations are said to be due to point 
changes, the point being the exact region in the chromosome at which 

px sp 

FIQ. 211. Diagram illustrating the determination of gene loci by different methods. 
Above is shown an outline drawing of Chromosome II. Below, the chromosome map 
of this chromosome is given, with the positions of several gene loci indicated. The 
positions of the genes in the chromosome map have been determined by data from 
matings involving crossovers (page 387); the positions of certain of these genes have 
also been determined by cytological studies following translocation (Figs. 207, 208). 
It will be noted from the positions of the vertical lines (1 to 9) that genetical and cyto- 
logical data agree in the order of gene loci but indicate certain differences in the dis- 
tances separating them. Sf, spindle fiber attachment; Bl, bristle- dp, dumpy a body 
character; b, black; sp, speck color on wing, etc. (After Dobzhan$ky. Redrawn.) 

the mutating gene is located. It is impossible to speak with absolute 
certainty, but it appears most likely that, in a point change, an actual 
chemical change takes place in the mutating gene. In the final 
analysis, a gene can be nothing more or less than the tiniest bit of a 
specific chemical compound, possibly a single molecule. In addition 
to point changes as the result of chemical change, the possibility also 
exists of a mutation due to a change in the position of a gene or genes, ' 
the so-called position effect. In other words, the mutation may be due 
to a change in the position of a gene rather than a change in its chemical 
nature. Considerable evidence has been very recently accumulated 
that indicates the importance of gene position. The possibilities 
involved may be visualized by realizing the changes that would follow 



the removal of a factory manufacturing chemicals from one town to 
another, even though the two places might not be far apart. The 
importance of gene position in synapsis has just been emphasized. 

The discussion of inheritance, so far presented, has been concerned 
with formation and transmission from generation to generation of the 
gene complex responsible for the characters expressed in the individual. 
At this point, some attention may profitably be given to the environ- 
ment with which each living organism is in continuous adjustment and 
on which it depends for a constant supply of the materials essential 

FIG. 212. Photograph of a corn field showing plants growing under favorable 
environmental conditions (left) and plants growing under unfavorable environmental 
conditions as the result of crowding (right). (Woodruff, after Blakeslee.} 

to the life processes. The question is: Do the characters shown in 
the mature organism depend exclusively upon the gene complex? The 
answer is clearly in the negative. Each individual represents the 
results obtained from a partnership in which the gene complex has 
been working in close association with the environment. If the 
environment is favorable, the gene complex will come to full fruition, 
but, if the environment is unfavorable, the normal group of characters 
expected from a particular, gene complex will be restricted or modified 
in various ways. (Fig. 212.) 

But now we come to another question with reference to environ- 
mental effects: Is the chromatin complex with its thousands of con- 


stituent genes changed in the organism by the environmental factors 
so that gametes will be formed with an altered gene complex. The 
answer is "no" but with certain reservations. Thus, the geneticist 
has discovered that if the immediate environment of Drosophila 
contains X rays of certain strength, the chromatin in the germ cells 
will be altered (page 339). In this connection it was suggested some 
years ago, following the discovery of the cosmic rays with their great 
power of penetration, that possibly these rays had been directly affect- 
ing the chromatin of organisms from the earliest times. 

However, the condition just considered, in which the biologist has 
used X rays to penetrafte directly to the germ cells and alter the normal 
chromatin pattern, is very different from environmental effects which 
affect only the somatic cells. In such cases, if the heredity of the 
succeeding generations is to be changed by environmental action, it 
would be necessary for the chromatin changes in the affected somatic 
cells to be transferred to the gene complex in the gametes and corre- 
spondingly to modify the gene pattern in them. Suppose, for example, 
the somatic cells present in the thyroid gland of a particular individual 
arc subjected to an unfavorable environment as the result of a greatly 
decreased iodine content in the blood stream, the latter, in turn, being 
due to faulty nutrition. In time, an abnormal condition of the thyroid 
develops. There is no reason to suppose that this environmental effect 
directly affects the germ plasm of the individual in any way whatso- 
ever, and, since the gene complex of the individual remains unchanged, 
there appears to be no possibility of the hereditary transfer of the 
thyroid goiter to later generations. The unsuitable environment 
results merely in an individual modification that is doomed to extinc- 
tion with the passing of the individual concerned. So far as the 
biologist can see, the same result, namely, unchanged heredity, is to be 
expected from all sorts of individual modifications that appear in the 
soma following exposure to abnormal environmental influences. 
(Fig. 207.) 

As late as the beginning of the second decade of the present century, 
it was not possible to discuss evolution 1 with any authority because a 
knowledge of the underlying mechanism responsible for the production 
of new types was not available. It was clear that species had changed, 
that new ones had developed, but the functioning of the mechanism 
responsible for the tremendous variety of living forms coming from a 
common life stream was not known. To the paleontologists busily 
engaged in discovering, collecting, and mounting the fossil remains of 
plants and animals, the Lamarkian doctrine of the inheritance of 
1 Consult Appendix: Organic Evolution. 


acquired characters still persisted with almost undiminished power. 
To the biologist of today, however, it is clear that successive genera- 
tions of giraffes could continue to stretch their necks, in order to secure 
the more plentiful food supplies at the tops of tall trees, for billions of 
years without in any way affecting the genes responsible for the length 
of the neck. 

The crux of the matter was stated by Professor E. G. Conklin a few 
years ago when he wrote that "The germ cells are the only living bonds 
not only between generations but also between species, and they 
contain the physical basis not only of heredity but also of evolution/ 1 
Any evolution that has occurred in the past, therefore, has occurred as 
the result of the same mechanism that will bring it about today, 
namely, changes in the germ plasm. The geneticist and the cytologist 
have made clear the main features responsible for variations in the 
germ .plasm 1 and, in addition, have teen able to link these microscopic 
changes involving the, genes in the chromosomes directly with visible 
alterations in the resulting offspring. From the material presented in 
the two preceding chapters, it is apparent that the condition of our 
knowledge in this field at the end of the third decade of this century is 
very different from that a few years earlier, for we do have a knowledge 
of the underlying mechanism responsible for the production of new 
types. It is the same mechanism that is responsible for the production 
of each new individual. 

1 Consult Appendix: Germ Plasm. 


The knowledge gained from the consideration of reproduction and 
heredity in the preceding chapters can now be used as a basis for an 
inquiry into the established facts relative to inheritance in the human 
organism. Human heredity has long been a subject of the keen- 
est interest, with the result that a great amount of data has been 
accumulated. For the most part, however, the earlier data rest upon 
upon observations of a more or less random nature which were not 
subject to rigid scientific scrutiny. With the establishment of the 
Mendelian laws and the realization of their universal application in the 
living world, geneticists everywhere have given increasing attention to 
the collection and correlation of data that would throw light upon the 
behavior of the genes in human germ plasm in determining the charac- 
ters of the offspring. As a result, a considerable number of characters 
have been found in man that are known to be transmitted to offspring 
in accordance with the established Mendelian laws. Two of these 
characters, namely, the inheritance of sex and the inheritance of a 
sex-linked character, color blindness, have been discussed in the 
previous chapter. (Fig. 202.) 

It should be emphasized that the difficulties inherent in securing 
accurate knowledge of human heredity are very great. In the first 
place, the geneticist has no control over the matings; he can only sit 
on the side lines and observe the results. Then, too, observations are 
possible only on the characteristics of a comparatively small number of 
offspring extending over a few generations. Information regarding 
the previous generations of a family is rarely a matter of written 
record unless it be concerning the occurrence of some particularly 
striking characteristic, usually abnormal in nature.^ On the whole, 
then, the data regarding the ancestors of a particular couple are apt to 
be sketchy and hearsay rather than detailed and accurate. Neverthe- 
less, considerable reliable information relative to human heredity is 
now at the disposal of the geneticist, and the inheritance of a variety 
of characters in man well established. Primarily, it should be recog- 
nized that the analysis of the relatively unsatisfactory data dealing 
with human heredity and the recognition of the general applicability 



of the Mendelian laws would not have been possible except for the 
results obtained by the geneticist from controlled breeding experiments 
in a wide variety of organisms. In our present examination of Men- 
delian inheritance in man, it will be possible to consider the inheritance 
of only relatively few characters but sufficient, perhaps, to show the 
broad application of the principles that have been thoroughly estab- 
lished in various other organisms. The inheritance of pigmentation, 
eye defects, skeletal characteristics, and blood groups are suited for 
our present discussion and will be considered in the order named, 
following which consideration can be given to certain general problems 
associated with human hybridization. (Fig. 213.) 


Pigmentation. It has become evident that the visible pigmenta- 
tion of all degrees and colors present in the eye, skin, and hair of the 
white, yellow, or black races results from the presence of varying 
amounts of two basic pigments, melanin and carotene, which are 
formed by, and remain in, the cytoplasm of various types of cells. 
Presumably the presence of the gene or genes responsible for pigment 
formation results primarily in the formation of a specific enzyme, and 
the latter works in association with a pigment-forming substance 
(chromogen) in the cell cytoplasm. Occasional instances in which blue 
eye color is associated in an individual with heavily pigmented, or 
brunette, skin and hair give good evidence that separate genes are 
responsible for the production of pigment in these three structures, but 
the method of pigment formation is believed to be the same in each 
instance. In the case of melanin, which is a widely distributed brown 
pigment, the chromogen has been identified as the ammo acid tyrosine, 
hormally present in cytoplasm. The reaction between tyrosine and a 
specific enzyme, tyrosinase, forms melanin. (Plate XVII, page 415.) 

The other human pigment, carotene, is yellow in color and is found 
in both plant and animal cells. It is particularly prominent in the 
carrot from which it derives its name. Presumably, a specific enzyme 
(not as yet isolated) which functions in association with a cytoplasmic 
chromogen is necessaiy for its formation. Carotene has recently 
become of increasing interest because of its close relationship to vitamin 
A (page 58). The latter, in turn, is associated with night blindness 
and other pathological conditions of the eyes and is required for the 
formation of the visual purple in the retina of the eye. Carotene is 
regarded as the mother substance of vitamin A. The actual synthesis 
of the latter occurs in the liver cells. 


In rare cases, a hereditary defect, albiriisrri, results in the absence 
of both pigments. Albinism behaves as a simple Mendelian recessive. 
Accordingly, all the children of albino parents, since the latter must be 
homozygous to show the defect, are albino. With both parents nor- 
mally pigmented, but carrying the recessive gene for albinism, 25 per 
cent of the offspring would be expected to show the defect as in a 
typical monohybrid. The examination of data from a considerable 
number of families has shown this percentage to be about 29. It is 
not clear why it should be considerably in excess of the expected ratio. 

Human skin color may vary from the deepest black to the purest 
white, with a wide intermediate range of browns, tans, and yellows 
between these two extremes. The pigmented cells of the skin, for the 
most part, are found in the epithelial cells, but they are present to some 
extent also in the outer layers of the underlying dermal cells. The 
offspring from unions of homozygous colored individuals and homo- 
zygous whites show an intermediate, or mulatto, condition with respect 
to skin color. Children of the hybrid mulattoes show varying degrees 
of color ranging from deep black, as in one of the grandparents, to clear 
white as in the other grandparent. 

Consideration of data involving a large number of F a children from 
mulatto marriages shows that about 6 per cent are deep black. Essen- 
tially the same percentage is white-skinned. This clearly indicates a 
multiple gene condition in which two pairs of genes determine the 
pigmentation of the skin. Thus a dihybrid condition for color is 
believed to be present as in the brown-seeded grains, described in the 
previous chapter (page 372). Other authorities have accumulated 
evidence indicating that a trihybrid condition with three pairs of genes 
is associated with human skin color. It is also clear that the produc- 
tion of pigment in the skin of individuals is directly associated with 
sunlight. The temporary coloring, or tanning, of the skin, following 
repeated exposures to sunlight, is a matter of common observation, as 
is also the formation of freckles. It is also noteworthy that skin 
pigmentation, even in the children of the colored races, is much reduced 
at birth. (Fig. 194.) 

Hair Qualities. Possibly wider variations are found in hair color 
than in either the eye or the skin; for in the white race, all gradations 
from deep black to a very light yellow or flaxen and branching off to a 
decided red are of normal occurrence. It appears that red hair color 
is due to a special derivative of the carotene pigment, whereas all the 
other shades have their origin in varying proportions of the melanin 
and carotene pigments. The absence of both pigments gives the 
abnormal albino condition noted above. Failure to produce hair 



1. Blending 

General body size, stature, weight, skin-color, hair-form (in cross- 
section, correlated with straightness, curliness, etc.), shape of head 
and proportions of its parts (features). 


2. Mendelian 




Spotted with white. 

Tylosis and ichthyosis (thick- 
ened or scaly skin). 

Epidermolysis (excessive for- Normal skin, 
mation of blisters). 

Hair beaded (diameter not Normal hair, 

Blonde or albino (probably 

multiple allelomorphs). 
Uniformly colored. 
Normal skin. 


Front of iris pigmented (eye: 

black, brown, etc.). 
Hereditary cataract. 
Night blindness (when not sex 


Only back of iris pigmented 

(eye blue). 

Pigmentary degeneration of 


Brachydactyly (short digits Normal, 
and limbs). 

Polydactyly (extra digits). Normal. 

Syndactyly (fused, webbed, or Normal, 
reduced number of digits). 

Symphalangy (fused joints of Normal, 
digits, stiff digits). 

Exostoses (abnormal out- 
growths of long bones). 

Hereditary fragility of bones. Normal. 

a. 213. Showing the behavior of various characters which are known to be 



Dominant Recessive 

Diabetes insipidus (excessive Normal 

production of urine). 

Normal, Alkaptonuna (urii^e black 

on oxidation). 

^ (Huntington's chorea. Normal. 

< Normal. Hereditary feeble-minded- 
system I 

J ( ness. 

3. Mendelian and Sex-Linked 

(Appearing in males when simplex, but in females only when duplex.) 

Normal. Gower's muscular atrophy. 

Normal. Haemophilia (bleeding). 

Normal. Color blindness (inability to 

distinguish red from 

Normal. Night blindness (inability to 

see in faint light). 

4. Probably Mendelian but Dominance Uncertain or Imperfect 

Defective hair and teeth or teeth alone, extra teeth, a double set 
of permanent teeth, hare-lip, cryptorchism and hypospadias (imper- 
fectly developed male organs), tendency to produce twins (in some 
families determined by the father, in others by the mother), left- 
handedness, otosclerosis (hardness of hearing owing to thickened 

5. Subject to Heredity, but to what Extent or how Inherited Uncertain 

General mental ability, memory, temperament, musical ability, 
literary ability, artistic ability, mathematical ability, mechanical 
ability, congenital deafness, liability to abdominal hernia, cretinism 
(due to defective or diseased thyroids), defective heart, some forms 
of epilepsy and insanity, longevity. 

heritable in man. (Cattle, "Genetics and Eugenics'' Harvard University -Press.) 


pigment in the later years of life results in grayness. Not uncom- 
monly, premature grayness appears as a definite hereditary character, 
The factors for human hair color have not been determined as definitely 
as they have in the case of the eyes and skin, but a very great deal of 
work has been done upon the inheritance of hair color in various other 
mammals. In the rabbit, to take a well-known example, it is found 
that no le& than four pairs of genes are concerned with the' develop- 
ment of hair color. Speaking in general terms, the results show that 
darker shades of hair color tend to dominate over the lighter shades in 

In addition to wide variation in color, the character of human hair 
differs markedly with respect to form. Straight, curly, kinky, coarse, 
fine, short, and long are terms commonly used to describe various hair 
types. Each of these has its basis in the distinctive shape and charac- 
ter of the hair follicles, and the latter, in turn, is the product of the gene 
complex (page 191). Again, some types of hair grow for a short time 
and are shed while very short; other varieties are retained for long 
periods and grow to great lengths. A certain type of baldness in the 
human male, known as hereditary, or pattern baldness, presents an 
interesting type of inheritance in which the genes producing this 
condition are influenced by the sex chromosomes, though not directly 
linked with them, and so we have sex-influenced characters as well as 
sex-linked characters (page 380). The underlying fact in the hered- 
itary transmission of a sex-influenced character is that identical 
genotypes produce different phenotypes in the two sexes. Thus in 
the case of hereditary baldness, the heterozygous condition (6) 
results in baldness in the male where it acts as a dominant but not in 
the female where it acts as a recessive. 

Eye Color. The inheritance of eye color in man has been the 
subject of much interest, partiqularly since the establishment of the 
Mendelian laws. An examination of the iris shows that pigmented 
epithelial cells containing melanin particles may be present both in 
front and in back of the eyes. This double pigmentation, or duplex 
condition, produces brown eyes. In a so-called simplex eye, the 
melanin is found only in cells located at the back of the iris. The 
Deflection of the light rays in the simplex eye from the anterior unpig- 
mejcited tissues of the iris gives blue eye color. In the albino condition, 
no Digment is present in any region of the iris, and the pinkish eye 
color results from the reflection of the blood in the iris vessels. Con- 
sequently, the albino iris offers very little protection to the sensitive 
retina cells from the incoming light rays, and the affected individual 
finds it necessary to keep the eyelids partially closed. 


In general, it is found that the brown-eyed condition is dominant 
oVer the reduction of pigment that gives blue eyes. On this basis, two 
types of brown eyes occur; one being homozygous, and the other 
hybrid carrying a recessive gene. The various possibilities of inherit- 
ance from the mating of the two types correspond to those shown in 
the monohybrid square. The results obtained from a study of eye 
color in a large number of Danish families have shown close conformity 
to expected Mendelian ratios (page 361). However, it is recognized 
^Jso that various other factors are often bound up with inheritance of 
eye color and, when present, greatly complicate, the analysis. Thus, 
pigmented cells carrying the yellow carotene sometimes occur in the 
iris. Also, partial pigmentation in the front wall of the iris results in 
scattered specks or streaks or even a complete ring of color. The 
control of this additional pigmentation is undoubtedly lodged in other 
genes thus giving a multiple gene condition for eye color. 

Eye Defects. The inheritance of various eye defects, both struc- 
tural and functional in nature, have been studied by various investiga- 
tors. In the previous chapter, one of the most interesting of these, 
color blindness, was used as an example of sex-linked inheritance 
(page 382). Altogether, the inheritance ratios have been studied for 
some 20 different eye abnormalities, including such structural defects 
as displacement of the lens, opacity of the lens (cataract), and partial 
or total absence of the iris. Among the functional defects (all of 
which, of course, have a structural basis of some kind) are color blind- 
ness, shortsightedness, night blindness, degeneration of the optic nerve 
(Leber's disease), degeneration of the retina, and paralysis of the eye 
muscles. The last defect has appeared as a recessive character in the 
offspring of cousin marriages. Undoubtedly the most data have been 
collected bearing on the inheritance of the shortsighted (myopic) 
condition, night blindness, and color blindness. 

In connection with myopia, which is one of the least serious of the 
various eye defects, the study in Berlin, a few years ago, of over 900 
family histories showed that the character behaved in all cases as a 
Mendelian recessive but that more than one pair of genes were involved 
in producing the defect. Night blindness, which is due to a defect 
in the visual cells of the retina, has been traced through 10 generations 
of the Nougaret family in France by the study of more than 2,000 case 
histories. The results clearly show that the defect was inherited in 
this family as a simple Mendelian dominant. Much less complete 
records of other families in the United States give evidence that the 
defect may be sex-linked. It is apparent from the examples cited that 
no general statement can be made to cover the inheritance of human 



eye defects, for they may be recessive or dominant or sex-linked, and 
also considerable evidence indicates that inheritance of the same type 
of defect is not uniform in different families. 

A B 

FIG. 214. Portion of the skeleton of the cat's forelimb. A, normal condition ; #, 
polydactylous condition as a result of mutation producing twinning of certain digits. 
(Coe, "Evolution of Earth and MOM" Yale University Press.) 

Skeletal Characteristics. The inheritance of defects in skeletal 
structures has been mostly studied in the bones of the hands and feet. 
The departures from the normal gene complex may result in the occur- 
rence of extra digits (polydactyly) or the opposite extreme in which a 
complete absence of hands and feet occurs, as has been recorded in the 
members of one Brazilian family. In addition to polydactyly, fairly 
frequent examples are found in which the number of digits has been 
reduced through a fusion of the bones (syndactyly). In other cases, a 
webbed condition in the hands or feet is inherited. This latter defect 
is due to the persistence of a web of skin tissue between the digits, 



4 B 

FIG. 215. Illustrating human polydactyly. A, heritable mutation producing 
twinning of thumb; J?, twinned little finger produced apparently as a defect in develop- 
ment and, therefore, not heritable. (Coe, "Evolution of Earth and Man," Yale Univer- 
sity Press. After MUles.) 

FIG. 216. Drawings showing external structure (left) and also the skeleton of the 
foot of the "mule-footed" pig. This condition is produced by a mutation which pro- 
duces a fusion of the terminal phalanges and the hoof covering this region. (Coe, 
"Evolution of Earth and Man" Yale University Press.) 


usually the second and third, and is not a skeletal defect. Finally, 
an abnormally short type of digit (brachydactyly) occurs, in which one 
joint is missing from each digit, that appears as a hereditary character. 
The data from a large number of individuals show that skeletal defects, 
with certain exceptions, behave as simple Mendelian dominants. The 
presence of a skeletal mutation in an individual is, therefore, very good 
evidence that the defect will be transmitted to the next generation. 
(Figs. 213 to 216.) 


The studies of Gal ton on human inheritance are probably the most 
thorough of any of the pre-Mendelian studies, and his "Laws of 
Ancestral Heredity/' published in 1897, just a few years before the 
rediscovery of Mendel's results, were considered of the highest impor- 
tance by biologists in the early years of the present century. But 
Gal toil's laws have not continued to be of major importance because he 
failed to recognize the participate nature of inheritance based upon the 
gene mechanism (page 355). Nevertheless, Galton has a great deal to 
his credit. He was the first to distinguish between alternative and 
blending inheritance. An example of the former was found in the 
inheritance of hair, color in Basset hounds and of the latter in human 
stature. From his extended and thorough studies on the inheritance 
of stature, or tallness, in man, Galton established the principles of 
biometry, the measurement of living things. 

Biometrical data, correlated in accordance with mathematical 
formulae developed by Pearson, have proved to be of the greatest 
importance in statistical studies of variation in heredity. Since, as 
shown earlier, the inheritance of various characters in which multiple 
genes are concerned results in a graded series of variations in the off- 
spring, the answers to the problems involved require the determination 
of the characteristics of a large population rather than of an individual 
(page 376). Biontetrical methods must be used. The inheritance of 
stature is a very; good example. If a large number of individuals are 
measured for height, it will be found that a small percentage are very 
short, a corresponding number are very tall, but the heights of most of 
those measured fall between these two extremes. Thus, for example, 
in a certain population, the extremes for height might be found to be 
58 and 76 in., with the greatest number measuring around 67 in. From 
the data thus secured, a curve can be constructed that will show the 
results graphically. The same methods were applied in determining 
the inheritance of color in the brown-seeded oats, described in the 
previous chapter, through the examination of large numbers of seeds 



and establishing the percentages with respect to the amount of color 
present. In this case, the two extremes were very dark-colored seeds 

Number of 

























166 169 


165 168 171 174 177 180 L83 186 189 192 195 198 
Height in Centimeters 

FIG. 217. Illustrating variation in heights of 1,000 Harvard students, ages 18 to 26, 
The curve (dotted line) is computed from the number of individuals at a particulai 
height. (Castle, "Genetics and Eugenics," Harvard University Press.} 

and seeds with no color. Each of these was found to comprise about 
6 per cent of the total population. The greatest percentage of seeds 
was found to be intermediate in color, from which they graded toward 



the dark and toward the light-colored. The percentages obtained 
gave the clue to the number of genes involved (page 372). (Figs. 217, 

There is every evidence that height, general body build, shape of 
head, and various other body characters are determined by multiple 
genes. But in some instances, there is also evidence of alternative 




























4ft 48 51 64 67 60 63 66 69 72 76 78 81 84 87 90 93 96 99 102 10ft 

Weight in Kilogram* 

Fio. 218. Illustrating variation in weights of 1,000 Harvard students, ages 18 to 25. 
Cf. Fig. 217. (Castle, "Genetics and Eugenics," Harvard University Press.) 

inheritance. In the inheritance of stature, for example, some data 
show that the progeny from matings of tall and short individuals tend 
to be below the average in height, which indicates that the genes for 
shortness are dominant, to some extent at least. But the problem is 
even more complicated because the size of the vertebrate body is 
markedly affected by hormonal action. This is well shown in the 
gigantism resulting from hyperactivity of the pituitary gland or in 


dwarfing that results from other hormonal factors (page 113). Hor- 
monal action, in turn, is directly influenced by environmental condi- 
tions. The failure of the environment to provide iodine in the food 
will produce a cretinous condition, no matter what the gene complex 
of the individual happens to be (page 106). 

Blood Groups. Nearly forty years ago, it was discovered that a 
very serious reaction, which resulted in the sticking together, or 
agglutination, of red blood corpuscles, took place when blood from 
certain individuals was mixed. Later it was shown that the agglutina- 
tion of the red cells was not a haphazard phenomenon, that there were 
four types of blood found in man, now known as Groups A, B, AB, 
and 0, and that the agglutination reaction always occurred when 
certain groups were mixed. Extensive investigation has shown that 
agglutination depends upon the presence of two blood substances: 
the antigen, carried in the red blood cells; and the antibody, carried 
in the serum (page 166). Agglutination requires the presence of the 
antigen and its specific antibody. Both antigen and antibody cannot, 
of course, be present in the blood of the same individual for, if they 
were, agglutination would occur. The condition of the various types 
of blood may be summarized as follows: 

Group A carries antigen A and antibodies for Groups B and AB. 

Group B carries antigen B and antibodies for Groups A and AB. 

Group AB carries antigen for the three other groups but lacks anti- 
bodies. It will be agglutinated by any of the other groups. Group 
AB individuals are undesirable as blood donors. 

Group lacks antigen but carries antibodies for the three other 
groups. It will not be agglutinated by any of the other three groups. 
Group individuals are important blood donors. (Fig. 219.) 

Genetical studies have brought the data from the inheritance of 
the different blood groups into line with the Mendelian principles by 
the establishment through experimental breeding of a method of 
inheritance that involves an extension of the Mendelian laws beyond 
those considered in the previous chapter. This condition may be 
explained in a few words by the statement that the paired genes 
(allelomorphs) for a certain character may be present in different 
varieties or forms in the various individuals of a population. Instances 
of this genie variation have been demonstrated, for example, in the 
genes for certain eye colors in Drosophila and also in hair color in the 
rabbit. The data there obtained fit with that of human blood group 
inheritance which can now be stated. 

In the transmission to offspring, one mky say that the genes for 
Group A act as a dominant, Group as a recessive, and Groups B and 



AB are intermediate. If the dominant gene associated with the blood 
groups is designated as A and the recessive designated as a, then, just 
as in a typical monohybrid, Group A individuals may occur either as 
pure dominants with the genotype A A or as hybrids with the genotype 

Serum of 
Group A 

Serum of 
Group B 



FIG. 219. Illustrating the technique for determining the blood group to which the 
blood obtained from a particular donor belongs. "Two drops of serum, one of Group 
A and one of Group B, are put on a glass slide and a bit of the unknown blood placed 
in each. If the red blood cells agglutinate in one (A or B) or both (AB) or neither (0), 
the group of the unknown blood is determined in accordance with the scheme illus- 
trated." (Skull, modified from Snyder, "Blood Grouping" The Williams & Wilkins 

A a. All Group individuals act as pure recessives with the genotype 
aa. Now in the individuals belonging to Group B and AB, the paired 
genes determining the blood group are present in a different form 
which, for convenience, may be indicated as a'. The homozygous 



Group B individual has the genotype a'a', and the hybrid Group B has 
the genotype a'a. All Group AB individuals are hybrid and have the 
genotype Aa f . If now the groups and genotypes are arranged in 
tabular form, it will be possible to note the genotypes and the corre- 
sponding types of gametes produced. 


Blood group 





















In addition to their basic importance in making the highly valuable 
blood transfusions possible, the methods used in determining blood 
groups are sometimes important in medicolegal work for establishing 
parentage. In approximately one-third of the cases involving the 
question of parentage it is possible to speak with authority. 

Using the various gametes in the square (XXI) will show the vari- 
ous possibilities with regard to inheritance. Let us analyze two or 
three of the possibilities. Thus it can be shown that, if one parent 
belongs to Group A and the other to Group #, it is possible for a child 
to belong to any of the four groups. Obviously both of the parents in 
such a case must be hybrids, for the mating of a homozygous A A and a 
homozygous a'a' could give only children with the genotype A a' and 
belonging, therefore, to Group AB as shown in the square: 




Aa 1 

With heterozygous A and B parents, each producing two types of 
gametes as shown above, the children may belong to any one of the four 
groups as follows: 








Again it is evident that, if the mother belongs to Group A and the 
child belongs to Group J3, then the father cannot belong to Group 0. 
Thus, if the father belongs to Group and the mother is homozygous, 
as AA, then the child will have to be Aa, or a member of the A group 
as indicated: 






If the mother is heterozygous, or A a, the children will have to be either 
Group A (Ad) or Group (ad) : 





Thus it is shown that the association of a Group A parent and a 
Group B child excludes the other parent from Group 0. Likewise, it 
can be established that the other parent cannot be a member of Group 

Numerous other patterns for blood-group inheritance are estab- 
lished that we cannot take the space to analyze, but a few words should 
be said with reference to the possibility of establishing a relationship 
between blood type of child and father. The blood-group methods 
cannot, of course, definitely establish that a particular individual is 
a parent of the child in question. All that can be said is that certain 
combinations of the blood groups of one parent and child make it either 
possible or impossible for a member of a particular blood group to 
have been the other parent. To cite a few more instances, if both the 
parents have Group as their blood group, it is impossible for the 
children to belong to any other group. With one Group parent and* 
one Group A parent, the child cannot belong to either the B or the AB 
groups; If the mother and child are both type 0, then the father 
cannot belong to group AB. Conversely, type A B in both mother and 


child excludes type as the male parent. Eveii uiuoci A c dictions of 
relationship have been brought about by the discovery of two addi- 
tional antigens and the establishment of the M and N blood groups. 

That the genes for a particular character, which necessarily lie in 
the same chromosome position, may be 'found in more than one form, 
as indicated by a and a', is not surprising from the chemical standpoint 
when it is realized that a very slight change in the position of an atom 
or atoms in a molecule may result in the production of a different 
substance with distinctive characteristics. Various results have 
shown that a gene for a particular character may exhibit four or even 
more varieties. Technically, this condition of gene variety is known 
as multiple allelomorphs (multiple alleles). Careful distinction must be 
made between the multiple genes condition, which involves separate 
pairs of genes as in seed color, and the multiple allelomorphs, which are 
concerned with different varieties of a single pair of genes as just shown 
in the blood groups. Furthermore, it must be understood that an 
individual can never form gametes that carry more than one variety of 
a particular allelomorph. Thus the gametes determining the blood 
groups carry either A or a or a' but never any combination of these. 
Accordingly, not more than two allelomorphic varieties are ever 
present in the resulting zygote and the mature individual. The latter, 
for example, may be A A, A a, or A a', as shown in the table above, 
but never Aaa'. (XXI.) 

Human Hybridization. The various races of Man inhabiting the 
world today belong to one genus, Homo, and one species, sapiens. 
The most widely diverse human types interbreed and produce fertile 
offspring. The ability to interbreed and to produce fertile offspring 
has long been recognized as one of the most decisive characteristics in 
the determination of a distinct species. Modern knowledge of the 
specificity of the chromatin complex, as shown in the previous discus- 
sions, has added additional prestige to interbreeding as a species 
limitation (page 334). In general, it is found that two individuals 
belonging to widely separated species cannot produce offspring; 
individuals from species that are more closely related may produce 
offspring, but the latter are usually infertile as sfyown by the mule. 
The production of fertile offspring represents an even closer relation- 
ship, and the individuals participating must possess homologous 
chromosomes bearing the same gene complex. Accordingly they may 
be assigned to membership in a common group, the species. Member- 
ship in a species does not mean absolute uniformity by any means; 
different varieties, or races, are included within the species limits, and 
even smaller subdivisions with recognizable differences continue down 



to individual differences. In the final analysis, as has often been said, 
no two individuals in any species are exactly alike. It may be of 
interest in this connection to call attention to the results obtained when 
two closely related species of Ungulates, the horse and the ass, are 
mated. The two species are fertile when mated and produce a charac- 
teristic hybrid offspring, the mule, when the male ass (jack) is mated 
with the female horse (mare). 1 But the hybrid mule is practically 
always sterile, and the reason lies in the fact that the chromatin 
complex is different in the two species. Gametogeriesis in the mule 
cannot occur because homologous mates for the chromosomes received 
from the diverse parents are lacking; this prevents synapsis and the 
formation of functional gametes the mule is sterile. However, even 

FIG. 220. Illustrating the basis of sterility in species hybrids, as described on page 
414. Chromosomes of the mule in the developing germ cells. A, early stage showing 
the maternal chromosomes of the horse (large) and the paternal chromosomes of the 
jack (small) ; J5, later stage of germ cell formation showing degeneration of chromatin 
due to the inability of the chromosomes to pair in synapsis with homologous mates. 
(Jennings, "Genetics," W. W. Norton & Company, Inc. After Wodsedalek.) 

though sterile, the mule has been found to be a very desirable hybrid 
type because of its strength and high resistance to unfavorable 
conditions, (Fig. 220.) 

The human species, Homo sapiens, as seen in the world today, 
includes three great primary subdivisions or races : the whites (Cauca- 
sian), the yellow-browns (Mongolian), and the blacks (Negroid). 
Chief subdivisions of the Caucasians include the Nordic, Alpine, and 
Mediterranean peoples; subdivisions of the Mongolians include the 
Mongolic (Chinese and the Japanese), the Malay, (Hawaiians and 
other South Sea Islanders, American Indians, and Eskimos) ; subdivi- 
sions of the Negroid race include the Negroes proper and a diverse 
group, the Pygmies. These various peoples have long since ceased to 
exist as completely segregated groups. Almost every conceivable 
racial mixture has occurred at one time or another during the thousands 

1 Essentially the same condition obtains with the reciprocal cross between the 
male horse (stallion) and the female (jenny). 



1, Nordic 


3 . Medi 


5. Negroid 6. Half-breed (page 417.) 

PLATE XVII. Representatives of various human races. (Baur, Fischer, and Lenz, 
"Human Heredity" George Allen & Unwin, London.) 



of years that Homo sapiens has roamed the earth, and, so far as the 
scientist is aware, there are no barriers to the production of fertile 
offspring from the union of individuals from even the most diverse 
races. Well-authenticated examples of human hybridization, involv- 
ing considerable numbers of individuals and extending over many 
years, have been studied by the experts in this field. A few of the 
more important of these may be noted. (Plate XVII; Fig. 221.) 

Ewing Galloway 
FIG. 221. Photograph of the African pygmies. 

The Pitcairn' Islanders represent a mixture of British and Polyne- 
sian stock, which was instigated in 1790 following the mutiny on a 
British ship, the Bounty. Some of the mutineers, in order to escape 
punishment, made their way to the then unknown Pitcairn Island and 
took with them twelve native Polynesian women and six men. The 
descendants of the motley group, now numbering a thousand or so, 
occupy Pitcairn Island and the neighboring Norfolk Island as well. 
The racial mixture appears to have established a healthy, vigorous 



stock. Many other examples of racial mixtures involving the Polyne- 
sian peoples are known, particularly in Hawaii. Thus, offspring from 
Hawaiian-Chinese unions give notable evidence of the establishment 
of a very favorable hybrid type. 

Hybridization between Dutch colonists and Hottentots in South- 
west Africa has resulted in the establishment of a distinct group which 
has held together and developed certain distinctive and, on the whole, 
favorable features (page 415, 6). In many of the alternative parental 
characters exhibited by the very diverse parental types, it is clear that 

FIG. 222. Drawings illustrating hybrids in the Fi and Fz generations produced by 
mating the French bulldog with the Dachshund. (Stockard, "Physical Basis of Per- 
sonality," W. W. Norton & Company, Inc.) 

the hybrids are intermediate. Hybridization of the native Filipino, on 
the contrary, produces offspring that are often notably inferior to the 
parental types in various respects. This hybrid degeneration is not 
so apparent in the physical characteristics as it is in the mental trails. 
Considerable data are available relative to the hereditary pattern in 
the offspring from crosses between the White and Negro races in North 
America. The hybrid individual, or mulatto, shows a blend of certain 
characters as in the skin pigmentation and various facial features, 
whereas in their general body build, the hybrids tend more toward the 
Negro ancestry. There appears to be no scientific evidence that 
sterility appears in the offspring from crosses involving less of the negro 
ancestry as sometimes stated. 



Controlled breeding experiments with dogs have yielded interesting 
and important results in recent years which should be considered in 

connection with the problems of 
human hybridization. The many 
varieties of dogs distributed in 
every land are assigned to one 
species, Canis familiaris. All va- 
rieties interbreed freely and prod- 
uce hybrid offspring in which 
many of the parental characters 
are clearly inherited in accordance 
with Mendelian laws. But the 
hybrid offspring from certain 
crosses do not appear to have assembled a very satisfactory set 
of characters in their composite inheritance. The original parental 
types are much better. There is a certain structural disharmony in the 

FIG. 223. Illustrating hybrid off- 
spring from the mating of the St. Bernard 
and Dachshund. (Jennings, "Genetics," 
W. W. Norton & Company, Inc. After 

FIG. 224. Drawings illustrating F\ hybrids produced by the mating of the giant 
St. Bernard with the Great Dane. The hybrids are vigorous for the first three months, 
but later develop varying degrees of overgrowth, and all become paralyzed in the hind 
legs. (Stockard, "Physical Basis of Personality," W. W. Norton & Company, Inc.) 

offspring of diverse races that gives, to say the least, an unprepossessing 
appearance. Thus, when the short-legged Dachshund is crossed with 
the longer legged French bulldog, the F t offspring are moro or less 


intermediate. The latter, when interbred, produce several bizarre 
types with ears, legs, and bodies that are far from harmonious. The 
segregation of these characters in the F2 animals is in accord with 
Mendelian laws. Even more unsuitable are the offspring produced 
from matings of the very large St. Bernard and the small short-legged 
Dachshuud. The hybrids inherit the dominant stump legs of the 
Dachshund in association with the long, heavy St. Bernard body. 
The latter hangs so low that it may even drag on the ground. An even 
more serious hybrid defect appears when the St. Bernard and the 
Great Dane are mated. A defective gene becomes apparent in this 
mating, which results in the partial paralysis of the offspring when 
about three months old. (Figs. 222 to 224.) 

Ancl so the results of the breeding experiments with a highly 
developed mammal, like the dog, indicates with considerable clarity 
that the mixing of established diverse races within the species is not 
always helpful and may, in fact, be decidedly harmful. Possibly the 
same condition applies in the case of the human organism. Various 
authorities are convinced that the matings of diverse types give oppor- 
tunity for the production of hybrid progeny that tend to be badly 
assembled, as it were. The different parts of the hybrid body may not 
harmonize; there is lack of a unifying life architecture. This condition 
may not appear so serious in the first generation when many of the 
diverging parental characters show a blending inheritance with the 
production of an intermediate type. But the succeeding generations, 
produced by matings between hybrids, or between hybrids and either 
of the diverse parental types, are apt to result in the production of 
poorly adapted offspring as the result of the segregation of the diverse 
genes and the random union of the gametes. 


Quite the opposite of the condition just considered, with hybridiza- 
tion of diverse races, is of common occurrence in various plant and 
animal types, including man. This is inbreeding by the union of 
related individuals such as occurs in cousin marriages. It might be 
thought that, if mating between individuals belonging to diverse races 
has its dangers, inbreeding would be helpful. It is clear, however, that 
this is not always the case. Inbreeding may be helpful or harmful 
depending upon the genotype of the individuals concerned. If 
recessive genes for harmful characters lurk in the genotypes of a 
particular family, then the union with a related individual carrying 
this defective genotype is liable to end disastrously, since it will give 
the opportunity for the paired recessive genes coming from both 


Controlled breeding experiments with dogs have yielded interesting 
ad important results in recent years which should be considered in 
duced well-endowed offspring lor generations, with no hint ol any 
undesirable characters, offers very little danger from inbreeding. This 
assumption, however, is not based on solid ground, for it has been 
shown in Drosophila that a recessive character may remain concealed 
in the genotype for many generations only to appear in full force in the 
phenotype of a certain stock when the homozygous condition is 
attained by the union of two gametes both carrying the recessive gene. 
The gist of the matter may be given in the statement that inbreeding 
d^es not produce harmful characters; it only gives an opportunity for 
them to be shown as somatic characters in the offspring if they are 
present in the parental germ plasm. 

Inbreeding involving varying degrees of relationship is exhibited 
in the living world. The closest inbreeding occurs in self-fertilizing 
types of plant and animal that produce both the male and the female 
gametes in the same individual. This is commonly found in the plant 
kingdom and is not unusual among animals, as in the hermaphroditic 
earthworm and the parasitic flat worms. Self-fertilization may or may 
not be practiced in the hermaphroditic types. Earthworms go to 
great lengths to mate with another individual and thus to secure 
foreign sperm for the fertilization of their own eggs. The parasitic 
flatworms, on the other hand, depend upon self-fertilization. 

A similar condition is found in the plant kingdom. Darwin made 
an extensive study of the effects of self-fertilization and cross-fertiliza- 
tion in a wide variety of plants and found great divergence in the 
different types. Some plants will not tolerate self-fertilization; that is, 
they are self-sterile; others utilize self-fertilization exclusively and do 
not appear to receive any benefit when they are artificially cross- 
fertilized. Wide variation in fertilization requirements exists among 
relatively close plant groups, as in the domesticated grains, or cereals. 
For example, in wheat and oats self-fertilization occurs. In fact, it is 
quite difficult to carry on artificial cross-fertilization in these species 
because the flowers are structurally adapted for self-fertilization. In 
corn, quite the contrary condition is found; for iu this plant, cross- 
fertilization is essential for the production of normal progeny. The 
self-fertilization of corn may be forced artificially, but the plants so 
produced are inferior. Mendel, in his original experiments, used peas 
that normally were self-fertilized by pollen coming from the same 
flower. Self-fertilization occurring in the FI hybrids was a very impor- 
tant factor in enabling Mendel to interpret the results of his experi- 
ments correctly. 


In organisms in which male and female individuals occur, as in all 
the higher animals, it is obvious that self-fertilization is impossible. 
The closest possible inbreeding occurs in matings between brother and 
sister or between parent and offspring. A considerable amount of 
experimental breeding has been directed in an endeavor to discover the 
results from long continued, close inbreeding. Drosophila has been 
inbred between brother and sister for 59 generations without producing 
degeneration, provided care is taken to select vigorous individuals from 
each generation for propagation. Likewise, inbreeding involving 
brother and sister matings have been studied in mammals, particularly 
in rats and guinea pigs, for some 25 generations and has given essen- 
tially the same result as in Drosophila. The crucial point of these 
inbreeding experiments lies in the selection of vigorous normal indi- 
viduals in each generation to carry along the line. If parental selec- 
tions are not made, defective types will increase in the population. 
This result is not due to the inherent harmf ulness of the inbreeding for, 
as we have seen, self-fertilization involving the closest possible inbreed- 
ing is the accepted method in various types of plants and animals. 
Defective types which appear in the progeny of inbred animals result 
from the outcropping of recessive genes present in the closely related 
chromatin of the two parents. In the controlled breeding experiments, 
the defective individuals, homozygous for the undesirable character, 
are discarded and not allowed to propagate. Thus, under the experi- 
mental conditions, inbreeding aided by the selection of the most 
desirable individuals for reproduction tends to rid the germ plasm of 
undesirable recessive genes and to produce a homozygous condition 
carrying only desirable characters. 

Suppose now that a vigorous individual from an inbred race, 
selected over a considerable number of generations, and therefore 
highly homozygous, is crossed with an individual from another homo- 
zygous race, which is not closely related. Considerable evidence 
exists that offspring from matings of such individuals will be more 
vigorous and, in general, more desirable than those obtained from 
continued inbreeding in either of the parental lines. This is the phe- 
nomenon of heterosis or, more commonly, hybrid vigor which is most 
strikingly shown in the experimental breeding of the corn plant. 
Strains of this plant, which have been developed by artificial self- 
fertilization through several generations until homozygosity is well 
established, will continue to produce normal progeny indefinitely, but 
the ears formed will be small, and the individual plants lacking in size 
and vigor. When two of these homozygous lines are crossed, the 
hybrid FI plants will be larger and more vigorous. And in the next 


generation, grown from the Fi seeds, a strikingly superior type of corn 
will always be produced. Of course, it is apparent that the unre- 
lated homozygous parents must be desirable types and also that they 
should not belong to widely diverse races within the species. Thus, as 
noted above, the progeny of the St. Bernard-Great Dane cross are not 
going to be satisfactory even though each of the parents is homozygous 
(page 418). 

In the human race, it is evident that the peoples of the various 
nations carry a high degree of hybridization in their gene complex as 
the result of racial mixtures following migrations to other lands at 
various times in the past. This is particularly true in a melting pot 
of the races such as is found in the United States. But even a well- 
established people, like the English, contains additions to the genotype 
from the Mediterranean peoples brought in by the Roman invaders. 
At various times, the Germanic, Norman, French, and other nationali- 
ties have added to the racial mixture that is more or less stabilized in 
the British type of today. Everywhere among the Caucasian peoples, 
the story is much the same, whether they live in Germany or France 
or Italy or Spain. Evidence that these established nationalities have 
developed from marked racial mixtures in the past may be used as an 
argument for the belief that the final result of the racial mixture in this 
country will not necessarily be unsatisfactory. It is, in fact, impossible 
to draw any definite conclusions as to the future, for no one knows 
what the blending of the diverse genotypes will bring forth in the 
generations that lie ahead. 

In addition to lack of knowledge of the individual human genotypes, 
the question of mutations is to be considered, because the appearance 
of some unfavorable character in the progeny may be due to a sudden 
chromosomal aberration, in one or the other of the parental gametes, 
that has never been in the germ plasm of either parent previously. 
The classic example of a mammalian mutation occurring naturally 
was noted in 1791 when a mutant type of sheep was born to normal 
parents. The animals belonged to a farmer, Seth Wright, of the 
Massachusetts colony. This mutation produced a short-legged, or 
ancon, type of sheep which was highly regarded for a time because it 
lacked the fence-jumping ability of its long-legged relatives. But the 
ancon sheep were lacking in other ways and, all things considered, did 
not measure up to the standards of the normal animals. Accordingly, 
the mutant type was propagated for some years until the advantages 
and disadvantages became apparent and better methods of fence 
building were devised and then was discarded in favor of the generally 
more desirable long-legged varieties. This particular mutation in the 


germ plasm, producing short logs, appeared without warning and was 
easily propagated because it was dominant over the normal gene 
complex responsible for length of leg. Most mutations, which have 
been studied, are found to be recessive in nature, and, therefore, they 
are unable to alter a particular character unless it is present in the 
zygote in a homozygous condition. (Fig. 225.) 

The upshot of the matter is apparent; it is impossible to determine 
when the recessive gene for an. unfavorable character became estab- 
lished in a particular human gene complex. It may have been during 
gamete formation in the previous generation, or the recessive gene may 

FIG. 225. Photograph showing the sho^-legged ancon sheep (left) in compari- 
son with the normal condition (right). (From photograph by Dr. W. Landauer, Univer- 
sity of Connecticut.} 

have been there for untold generations before getting the opportunity 
to be present in a zygote in a homozygous condition. But it is always 
true that the chances of an undesirable recessive gene finding a homo- 
zygous mate are much greater when the parental chromatin is related 
than when the gametes have a diverse ancestry. Hence it seems wise 
to accept the established belief, which in various regions has crystal- 
lized into law, that marriage between cousins is not desirable. That 
is not to say that the progeny of cousin marriages are always below 
grade. Quite the contrary is the case, as can be seen in examples from 
various distinguished families. It may be that statistics would show 
no higher percentage of defectives in the children from cousin mar- 
riages than from the union of unrelated persons. Nevertheless, our 
present-day knowledge of the heredity mechanism makes clear the 
nherent dangers. 


Fortunately the history of a family over a number of generations 
usually reveals with considerable accuracy the desirability or undesir- 
ability of the gene complex even though all the family pedigrees have 
not been studied with scientific exactness. The extreme examples of 
an undesirable gene complex with respect to mental characteristics, 
which after all is the final consideration, are afforded by the studies that 
have been made of members of the " Jukes" and "Kallikak" families 
through several generations. Possibly these examples have been held 
up to view so frequently in the past twenty-five years that they have 
lost their effectiveness. And perhaps a great deal of the trouble in 
these families was the result of very unfavorable environmental condi- 
tions producing individual modifications. But it would seem that an 
unprejudiced observer on the side lines would have to conclude that 
something was wrong with the family genotypes when more than 40 
per cent of the individuals in successive generations are mentally 

That the genotype is responsible for mental inadequacy and other 
departures from the normal human pattern is possibly even more 
strikingly shown by the careful studies that have been made on the 
behavior of , twins with respect to criminal tendencies. It should be 
emphasized that two types of human twins are recognized, namely, 
fraternal, or dizygotic, twins; and identical, or monozygotic, twins. 
Dizygotic twins, as the name indicates, develop from two zygotes; that 
is, two eggs were fertilized at the same time. Accordingly, except for 
the fact that they are of the same age, dizygotic twins are no more 
alike than other members of the family. Monozygotic twins develop 
from the same zygote and, therefore, have identical genotypes. Such 
a condition is believed to arise by the separation of the two daughter 
cells, following the first cleavage of the fertilized egg, and the independ- 
ent development of the two cells thereafter so that each cell forms a 
twin. Identical twins are always of the same sex and so nearly alike in 
appearance that it is usually impossible for strangers to tell them 

An examination of the prisons in Bavaria some years ago revealed 
that one or both members of 30 pairs of twins were imprisoned or had 
prison records. Very complete information was secured with respect 
to these individuals. Of the 30 pairs of twins with criminal records, 
it was found that 17 pairs were fraternal and 13 pairs were identical. 
Examination of the prison records of the 17 pairs of fraternal. twins 
showed that criminal tendencies in one member of the pair gave no 
evidence that the other twin would likewise be a burden to society, for 
it was found that in only two cases had both members of the pair been 


imprisoned. Quite the reverse condition was found * the criminality 
of the 13 pairs of identical twins. The investigation showed that, in 
10 cases, both members of the pair had prison records, and in only 
3 cases was imprisonment confined to one twin. The numbers of 
cases used as a basis of these investigations were necessarily small, but 
the trend is so decisive that it appears safe to conclude that the 
behavior of an individual as well as his structural pattern is largely the 
outgrowth of the gene complex received at the time of fertilization. 


Society has recognized more and more that some individuals are 
inherently burdened by an undesirable gene complex, though that term 
may not have been used to express the situation, and accordingly has 
taken measures to protect future generations against further trans- 
mission of the undesirable germ plasm. The traditional method of 
accomplishing this desirable aim has been through the segregation of 
the afflicted individuals in government institutions of one kind and 
another. The trouble with segregation has been, so far, that there are 
far too many afflicted with, for example, an estimated 2 to 5 per cent 
of the population feeble-minded to make segregation effective. 
Furthermore, great pressure is continually brought to bear to bring 
about the release or parole of individuals who are lightly afflicted but, 
nevertheless, potentially dangerous individuals from the standpoint of 
heredity. Laws with regard to the requirements for marriage differ 
widely, with the result that a license denied in one locality can usually 
be secured in another. Accordingly, many cases are found in which 
afflicted individuals, who should be permanently segregated, find it 
possible to marry and produce offspring. And the alarming fact for 
the future is that the rate of reproduction of mentally deficient couples 
is probably twice that of couples with high mentality. 

Increasingly, the tendency in the United States during the last 
twenty-five years has been to pass laws requiring or permitting the 
sterilization of certain classes of defectives of both sexes in such a way 
that the production of offspring is impossible, though the normal 
sexual relations of the married state are in no way disturbed. In both 
sexes, the sterilization operation consists of cutting the ducts from the 
gonads so that the germ cells cannot pass through them. In the male, 
the operation is a very simple one, since the testes lie outside the 
abdominal cavity and the connecting ducts are easily exposed. In the 
female, an abdominal operation is involved which may fairly be 
compared in severity with an operation for appendicitis. In 1909, 
only four states had sterilization laws; in 1934, the number had 


increased to 27. These laws have been opposed chiefly on three 
grounds. It has been argued that sterilization would tend to increase 
sexual immorality, that it conflicted with the Constitution in that it 
constituted " cruel and unusual punishment/' and that it represented 
a dangerous infringement of personal liberty, The legality of the 
sterilization laws of one state, Virginia, was carried to the Supreme 
Court in 1927 and there upheld, a decision that carried the famous 
remark of Justice Holmes that " three generations of imbeciles are 

The advisability of sterilization is a question that cannot be settled 
in a few years, but a careful study of the results in California, based 
on nearly 10,000 legal sterilizations, indicates a much more favorable 
result than might have been expected. The application of sterilization 
laws throughout the past ages would undoubtedly have prevented the 
appearance of many unfortunates, but, at the same time, it is also 
possible that some of the geniuses of the first rank, who have greatly 
enriched civilization, would never have been known. The incompre- 
hensibly complex chromatin of the human race can never be analyzed 
to the extent that all of the possibilities inherent in the offspring of 
two individuals can be determined previous to their appearance. The 
offspring of a particular marriage will always be a gamble. But on the 
other hand, it should be remembered that the principles of selective 
breeding, which man has rigorously applied in order to obtain desirable 
domesticated plant and animal types, have been tremendously effective 
in establishing new varieties far superior to the original stocks. The 
biologist, knowing that the same hereditary mechanism is at work in 
the human organism, is certain that the same methods, if it were 
possible to apply them, would be effective with the heritable qualities 
in man. 

The discussion so far has dealt with the prevention of the trans- 
mission of defective chromatin to another generation. To some 
authorities this is " negative eugenics," which may be helpful to some 
extent but should be augmented by a policy of " positive eugenics" in 
which selected human stocks would be encouraged to transmit the 
desirable genotype to an increasing number of progeny. In other 
words, every possible measure should be taken to increase the birth rate 
among the better endowed families rather than to let it continue to sink 
to lower levels, as appears to be the case at present. Admittedly, if this 
were possible, much could be accomplished in improving the human 
race, but just how it could be effected seems to test the limits of human 
intelligence. Some countries, notably France, have in recent ^years 
been experimenting with a general family allowance plan for additional 


children, but it will be a long time "before any conclusion can be safely 
drawn as to the desirability of such* a plan. It is one thing to adopt a 
plan that will give an allowance to all families with a large number of 
children, but quite another to select families that are thought to have a 
more desirable gene complex and to reward them for increased numbers 
of progeny, while at the same time restricting other families and requir- 
ing them to share in the increased expense of maintaining a subsidy for 
the selected families. 


In the previous chapters, attention has been primarily centered on 
the structural and functional features of the human organism. It is 
now time to broaden this viewpoint somewhat and give attention to 
man's relationships to other members of the living world, infinite in 
number, which surround him on every side and with which he is indis- 
solubly linked in a complex living fabric, aptly termed the web of life. 
Man is a part of, not apart from, the living world. His basic require- 
ments for food and clothing are supplied by materials produced by 
other living organisms. And all plants and animals are necessarily 
dependent upon the constructive photosynthetic activities of green 
plants for the formation of the essential foodstuffs carrying abundant 
supplies of potential energy which may be utilized in maintaining the 
varied life activities and for the construction and maintenance of 
protoplasm itself. 

In the final analysis, whether or not an organism is successful, as 
evidenced by its ability to survive and to propagate its kind, depends 
directly upon its ability to secure adequate food supplies in the particu- 
lar environment to which it is permanently adapted. The abundance 
of life and the relative scarcity of suitable energy-supplying, proto- 
plasm-building foodstuffs make it necessary for organisms to compete 
for their nutritive requirements. The innumerable living organisms 
surviving today are adapted for every possible environment in which 
energy-containing substances are to be found. As the English 
biologist Dendy has well said: 

At the present day we see the surface of the earth teeming with hosts of 
living things, incalculable in number and of endless diversity in form and 
sftucture. Every situation where life is possible is occupied by plants or 
animals of some kind or other, all specially adapted in bodily organization to 
the conditions under which they have to maintain their existence. From the 
bleak and inhospitable summits of high mountain ranges to ocean depths 
which can be measured in miles, from the perpetually frozen circumpolar 
regions to the torrid zone on either side of the equator, living things abound. 
Seas, rivers, lakes, dry land, and air have all alike been taken possession of 
by representatives of the animal and vegetable kingdoms. 1 

1 Dendy, "Outlines of Evolutionary Biology," D. Appleton-Century Com- 
pany, Inc. 




Thus it is clear that the world of life, as seen today, presents a 
bewildering array of species that are able to supply their specific 
nutritive requirements under very different environmental conditions. 
Basically, however, all living organisms may be regarded as either 
autotrophic or heterotrophic in their nutrition. Autotrophic organ- 
isms are those which possess the ability to construct, or synthesize, the 
essential nutritive substances from the abundant inorganic elements 
and compqunds in their environment and are, therefore, independent 
in their nutrition. Heterotrophic organisms require complex organic 

FIG. 226. Diagram illustrating formation of carbon compounds by photosynthesis 
in the green plants and their destruction by bacteria and other colorless plants. (Re- 
drawn from Lutman; slightly modified.) 

compounds as the basis for their food supply and consequently are 
dependent in their nutrition upon the synthetic activities of the auto- 
trophic forms. It is apparent, therefore, that the autotrophic organ- 
isms manufacture food materials for themselves and also for the 
heterotrophic forms. (Fig. 226.) 


Autotrophic organisms consist almost entirely of the green, chloro- 
phyll-bearing plants, equipped for photosynthesis. There is, however, 
another group of autotrophic plants which though inconspicuous are, 
nevertheless, of considerable importance, namely, the autotrophic 
bacteria. These unicellular colorless plants we able to disrupt various 
highly stable inorganic substances through the action of powerful 



intracellular enzymes and to utilize the energy thus released for the 
synthesis of the complex carbon compounds which are essential for the 
repair and growth of their protoplasm. Possibly the autotrophic 
bacteria are to be regarded as the most primitive of all forms of life. 
Presumably, they were the first type to appear on this earth. The 
development of chlorophyll and the associated processes of photo- 
synthetic food formation apparently represent later stages of proto- 
plasmic phenomena. The nutritive activities of two important groups 
of autotrophic bacteria may now be described. 

Sulphur Bacteria. Sulphur is one of the essential elements of 
living tissues, and it is through the activities of a large and diverse 

B c 

FIG. 227. Sulphur bacteria. A, (Spirillum granulatum) , with dividing cell; 
B, C, D, giant sulphur bacterium (HUlhousia mirabilis); B, normal cell with sulphur 
bodies filling the entire cell; (7, an individual in which the sulphur globules have been 
used in respiration after being kept in tap water for a week; >, sulphur crystals obtained 
when animals are dried. (Lutman, B, C, D after West and Griffiths.) 

group of sulphur bacteria that suitable compounds of sulphur are 
supplied for animal and plant nutrition. Thus, in the formation of 
plant proteins, the green plants utilize the supplies of sulphur obtained 
from certain soluble sulphates dissolved in the soil waters. The auto- 
trophic sulphur bacteria are able by enzyme action to oxidize the 
hydrogen sulphide gas (H 2 S), released into the air during the decay of 
organic compounds, to form water and sulphur. In so doing, energy is 
obtained for the vital activities of the organism. Then the sulphur 
may be combined with water and oxygen to form sulphuric acid. The 
latter is released into the soil where it combines with various mineral 



Nucleus - 

elements to form the soluble sulphates, noted abdve, which are 
absorbed by the root tissues of the green plants and utilized in protein 
formation. Sulphur bacteria are unable to survive without an 
adequate supply of sulphur compounds for their energy requirements. 
(Fig. 227.) 

Nitrifying Bacteria. The soil- 
living nitrifying bacteria constitute 
another important group of auto- 
trophic bacteria that make their liv- 
ing by salvaging the nitrogen in the 
residues resulting from the decay of 
plant and animal tissues. There 
are various species which can be 
separated into two groups: the ni- 
trite bacteria and the nitrate bacte- Chloroplas-r -~j 
ria. During the decay of proteins, 
ammonia gas (NH 3 ) is formed. 
From the latter, the nitrite bacteria 
are able to form nitrous acid (HN02) 
by oxidative processes. The ni- 
trate bacteria find the nitrous acid 
suitable for their metabolic activi- 
ties and add additional oxygen to 
form nitric acid (HNO 3 ). The lat- 
ter is released into the soil where 
it combines with mineral elements 
to form soluble nitrates which are 
in time absorbed by the green plants 
and utilized in protein synthesis. 
(Fig. 226.) 

The Photosynthetic Organisms. 
It is the presence of chlorophyll in 
the cells of green plants that is 
responsible for photosynthesis. 
Unquestionably, chlorophyll is the 
most important pigment known to 
man, for it is essential to the 
formation of the foodstuffs required by every living organism 
with the exception of the autotrophic bacteria as just noted. 
Chlorophyll is also responsible for the liberation of free oxygen 
into the atmosphere during the photosynthietic processes. Respira- 
tion, involving the utilization of oxygen, is essential for every 

FIG. 228. Drawing of an active 
photosynthetic cell (palisade cell) from a 
leaf. The chloroplasts lie embedded in 
a thin, transparent layer of cytoplasm 
(not shown) which also surrounds the 
nucleus. The center of the typical plant 
cell is largely occupied by the fluid-filled 
cell vacuole. (Sinnott.) 


living cell, for ho other method is available to release the potential 
chemical energy stored in the complex organic molecules. Oxygen is 
an active element and combines readily with other elements, so that 
the free oxygen in the atmosphere would quickly disappear were it not 
for its continuous release during photosynthesis. (Figs. 226, 228.) 

The analyses of chlorophyll 1 show it to be a very complex substance 
in which two chlorophyll compounds are associated. These are known 
as chlorophyll a (CbsH^Os^Mg) and chlorophyll 6 (C55H7o0 6 N 4 Mg). 
Chemical analyses, however, shed no light on the basic problem, 
namely, why this particular assemblage of common elements is the 
only one of all the innumerable compounds known to the chemist able 
to bring about the photosynthetic reaction. Of particular interest, as 
previously noted, is the fact that the chemical composition of hemo- 
globin, the essential oxygen-carrying pigment present in the red blood 
cells of man and the vertebrates generally, is closely related to that of 
chlorophyll. Two other yellowish pigments, carotene and xantho- 
phyll, of doubtful function, are associated with chlorophyll. 

In the earlier discussion of retinal function, consideration was given 
to the physical characteristics of the energy-bearing light waves with 
particular reference to their wave lengths and associated colors in the 
visible spectrum (page 238). In the utilization of the radiant energy 
by the green plant cells, the function of absorption of the light rays of 
the proper wave length is of primary importance. As is well known, 
an object appears of a certain color because it reflects that particular 
color of the spectrum and absorbs the other colors. An object that 
appears black absorbs all the colors of the spectrum and reflects none. 
The reverse condition obtains with white objects, which absorb none 
and reflect all the spectral colors equally, thus producing the sensation of 
white. It is obvious, therefore, that chlorophyll appears green because 
it reflects the light rays from the green portion of the spectrum and 
absorbs the rays from the not-green portions, the latter containing the 
energy-bearing rays essential to the photosynthetic reactions. (Fig. 

The absorption of these rays can be demonstrated by examining 
the spectrum obtained when the rays of sunlight are passed through a 
chlorophyll solution. Under such conditions, it will be found that the 
resulting spectrum is incomplete, for the red and orange rays have 
been absorbed by the chlorophyll from one eftd of the spectrum and 
violet rays from the other. Accordingly, it is evident that the red- 
orange and the blue-violet rays absorbed by the chlorophyll are the 
ones that function in photosynthesis. The radiant energy actually 
1 Consult Appendix: Chlorophyll. 


used by leaves, under the most favorable condition of photosynthesis, 
probably never exceeds 3 per cent of the amount available and usually 
is considerably less than that. Thus the sun continuously supplies an 
incredible amount of radiant energy, only a very small portion of which 
is utilized by the green plants for photosynthesis and stored as poten- 
tial chemical energy in the compounds associated with the plant tissues. 
Protoplasm has a great capacity to do work so long as it is supplied 
with the energy-containing foodstuffs. Life is characterized by a 







FIG. 229. Diagram showing tho colors produced in the spectrum when a ray of 
light is passed through the prism of a spectroscope (above) ; (below) diagram showing 
the bands absorbed from the spectrum when the ray of light is first passed through 
chlorophyll solution. Described 011 page 432. (Sinnott). 

continuous supply of energy. Living organisms have no method 
for creating energy but only for the transformation of radiant energy 
received from the sun. Furthermore, as just stated, the ability to 
utilize radiant energy is limited to the green plants. They perform 
this essential function through the synthesis of simple inorganic sub- 
stances to form complex organic compounds which are suitable for food, 
and thus available to keep the wheels of life turning. 

The physicist defines energy 1 as the capacity to do work and sees 
that it may be manifested as energy of position, shown in gravitation, 
motion, etc. ; as chemical energy, which is evidenced in molecular and 

'-Consult Appendix: Energy. 


heat phenomena; and as radiant energy, illustrated in the phcnomen 
associated with light and electricity. Energy is accumulated a 
potential energy and later released as active, or kinetic, energy. Th 
biologist is particularly interested in chemical energy and in the radian 
energy present in the sun's rays, for the chloroplasts in the green plan 
cells have discovered the secret of transforming radiant energy int 
potential chemical energy and storing the latter in complex organi 
nutritive compounds, the carbohydrates, fats, and proteins ; synthese 
that result from the essential, but poorly understood, process o 

The conventional equation for photosynthesis, namely, 

6H 2 + 6CO = C 6 H 12 6 + 60 2 

does not show the energy relations that are basic for the maintenanc< 
of the life functions. Each molecule of glucose that is formed fy 
photosynthesis requires 677.2 calories 1 of radiant energy. Accord 
ingly, the equation for the photosynthetic reaction will read : 

6H 2 O + 6CO 2 + 677.2 calories = C 6 H 12 O 6 + 6O 2 

The oxidation of the carbohydrate molecule in the living tissues during 
respiration results in the liberation of this amount of heat energy fo 
the maintenance of the life functions, as shown in the following 

C 6 Hi 2 O 6 + 6O 2 = 6CO 2 + 6H 2 O + 677.2 calories 

Thus, in the living tissues, the potential chemical energy of glucose is 
transformed into kinetic energy and used to maintain the life activities 

But the living organism requires more from the foods that are taker 
in than the mere release of energy for materials must also be suppliec 
for the repair and growth of the tissues. Our previous consideratior 
of human nutrition has made it evident that the tissue requirements 
are supplied in full only when an adequate assortment of proteins i* 
secured from the utilization of various plant and animal tissues (page 
56). In a word, it is recognized that universal food requirement* 
operate throughout the world of life and that the food supply of al 
organisms rests finally upon the photosynthetic activities of the greer 
plants. Here, then, is a basic interdependence binding together al 
living organisms. 

Furthermore, the materials accumulated in the tissues of ever} 
living organism, together with the wastes continually formed during 
life, must be returned to the great storehouses of nature for latei 

1 Small calories: see footnot