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rv.v - ■:.::■. 

Marine Biological Laboratory 

1W IVP H May 25 j 19 49 : 

Accession No. 65066 

Given By Dr » n L * p * Sayles 

City College of New York 










From a group in the American Museum of Natural History in New York City. 
Reproduced by permission. 




Herbert E. Walter 



Leonard P. Sayles 





19 4 9 

Third Edition Copyright, 1949, by The Macmillan Company. 

All rights reserved — no part of this book may be reproduced in 
any form without permission in writing from the publisher, except 
by a reviewer who wishes to quote brief passages in connection 
with a review written for inclusion in magazine or newspaper. 


First and second editions copyright, 192S, 1939, by The Macmillan Company. 


"A textbook is an attempt to establish a happy relationship between 
teacher and pupil. Of the trinity of text, teacher, and pupil, the pupil is 
without doubt the most important, but even after the teacher is labeled 
'emeritus,' and direct classroom contact with the pupil is a thing of the past, 
there still remains a possible mission for the old textbook to accomplish. 

"Hence this Indian summer revision, after many years of trying to tell 
the fascinating story of vertebrate life to more or less attentive students. It 
has been fun " 

These words, jotted down for possible use in the preface to a third edi- 
tion of this book, were the last to come from the hand of Professor Walter. 
The fun which he had in teaching his classes and in preparing the first two 
editions of this text, the humor which he wove into this fabric, have made 
the study of Comparative Anatomy more interesting to thousands of us. 

For the opportunity to assume the responsibility for the future of the 
Biology of the Vertebrates, I am indebted to Mrs. Walter, who has done 
everything within her power to make the preparation of this third edition 
the enjoyable undertaking which it has been. 

In the present edition major portions of the chapters on vertebrate types, 
embryology, skeleton, nervous system, and sense organs have been rewritten, 
while less extensive changes have been made in other sections. I hope that in 
so doing I have still retained something of the flavor of previous editions. 

Through the generous cooperation of The Macmillan Company, pub- 
lishers, it has been possible to use new copy for over four hundred illustra- 
tions, some of which are redrawings of figures previously used, but many of 
which are new to the text. About half of this new illustrative material was 
prepared by Dr. Margaret Esther Potzger. Of the remainder many were 
drawn by Dr. C. J. Hylander. A number of illustrations were borrowed 
from a variety of sources. In this connection I wish to express my apprecia- 
tion to The Blakiston Company, Henry Holt and Company, Houghton 
Mifflin Company, W. B. Saunders Company, and The Williams and Wilkins 
Company, as well as The Macmillan Company, for their permissions to use 


vi Preface 

illustrations from books for which they hold the copyrights, as duly noted 
wherever such illustrations appear in the text. 

In the preparation of this edition I have been greatly aided by the advice 
and assistance given me by many who have used the earlier editions in their 
courses, or who, for other reasons, have read them critically. While it is not 
possible to enumerate all who have been helpful, I should like to make special 
mention of several of my colleagues in the Department of Biology at The 
City College to whom I am especially indebted, namely : Professors H. Her- 
bert Johnson, Percy L. Bailey, Jr., Raymond W. Root, James I. Kendall, 
Herman T. Spieth, and Donald W. Farquhar. But my greatest indebtedness 
is to the librarians at the American Museum of Natural History who expe- 
dited my work by their cheerful and efficient help in my search for references, 
regardless of the difficulties involved. 

Floral Park, N.Y. 

Table of Contents 


CHAPTER I. Chordate Characters 

I. Type Study, 3 
II. Comparative Study, 4 

III. Essential Features of Every Animal Type, 5 

IV. Symmetry, 6 
V. Metamerism, 9 

VI. Coelom, 9 
VII. Chordate Characteristics, 9 

1. Notochord, 9 

2. Dorsal Hollow Central Nervous System, 10 

3. Pharyngeal Breathing Device, 10 

4. Ventral Heart, 1 1 

5. Closed Blood System, 11 

6. Hepatic Portal System, 1 1 

7. A Post- Anal Tail, 11 

8. Red Blood Cells, 12 

VIII. Comparative Diagrams of Chordates and Non-Chordates, 12 

CHAPTER II. Kinds of Vertebrates (Classification) 

I. Taxonomy, 13 
II. Scientific Names, 15 
III. A Roll Call of Chordata, 17 

A. Borderline Chordates, 17 

1. Subphylum Hemichordata, 17 

2. Subphylum Urochordata, 18 

3. Subphylum Cephalochordata, 21 

B. Subphylum Vertebrata (Craniata), 25 

1. Class Cyclostomata, 25 

2. Class Pisces, 27 

a. Subclass Chondrichthyes, 29 

( 1 ) Order Cladoselachii, 29 

(2) Order Elasmobranchii, 30 

(3) Order Holocephali, 31 

b. Subclass Osteichthyes, 32 

( 1 ) Order Crossopterygii, 32 

(2) Order Dipnoi, 33 v 

(3) Order Actinopterygii, 33 

(a) Suborder Chondrostei, 34 

[ viz 



Table of Contents 

(b) Suborder Holostei, 35 

(c) Suborder Teleostei, 35 
Class Amphibia, 35 

( 1 ) Order Stegocephalia, 39 

(2) Order Gymnophiona, 40 

(3) Order Urodela (Caudata), 40 

(4) Order Anura (Salientia), 42 
Class Reptilia, 43 

(1) Extinct Reptiles, 45 

(2) Order Rhynchocephalia, 48 

(3) Order Chelonia, 48 

(4) Order Squamata, 49 

(5) Order Crocodilia, 52 
Class Aves, 52 

a. Subclass Archaeornithes, 54 

b. Subclass Neornithes, 55 
Class Mammalia, 57 

a. Subclass Prototheria, 58 
Order Monotremata, 58 

b. Subclass Metatheria, 59 
Order Marsupialia, 59 

c. Subclass Eutheria, 60 


Order Insectivora, 62 


Order Dermoptera, 63 


Order Chiroptera, 63 ' 


Order Carnivora, 65 


Order Rodentia, 65 


Order Primates, 67 


Order Edentata, 70 


Order Pholidota, 71 


Order Tubulidentata, 72 


Order Perissodactyla, 72 


Order Artiodactyla, 73 


Order Proboscidea, 75 


Order Hyracoidea, 77 


Order Sirenia, 77 


Order Cetacea, 78 

CHAPTER III. The Distribution of Animals in Space (Chorology 
and Ecology) 

The Point of View, 81 

Ecology and Chorology, 82 

Habitats, 82 

The Laws of Distribution, 83 

Malthus' Law of Overpopulation, 84 

Factors Inducing Expansion, 85 

1. The Food Problem, 85 

2. Change of Habitat, 85 
Means of Dispersal, 86 
Factors of Repression, 89 

1. Inadequate Means of Dispersal, 89 

2. Non-Adaptability to New Conditions, 89 








Table of Contents 



3. Barriers, 90 

Classification of Life Realms, 91 

CHAPTER IV. The Distribution of Animals in Time 

Vanishing Species, 95 
Fossils, 96 

1. Former Ideas about Fossils, 97 

2. Conditions of Fossilization, 97 

3. Uses of Fossils, 99 

4. Kinds of Fossils, 100 
Imperfections in the Record, 101 
A Geologic Time Scale, 102 
Pictet's Palaeontological Laws, 104 







CHAPTER V. Man in the Making (Anthropology) 

I. The Ancient History of Man, 106 

II. Tradition and Evidence, 107 
III. The Habitable Earth, 108 
IV The Time Required, 108 

V. Archaeological Chronometers, 110 

1. Kitchen Middens, 110 

2. Pile Dwellings, 110 

3. Painted Grottoes, 1 1 1 

4. Large Stone Monuments, 113 

5. Tools and Weapons, 114 
VI. Human Fossils, 120 

1. Java 'Ape-man" (Pithecanthropus) , 120 

2. Peking Man (Sinanthropus), 122 

3. Heidelberg Man, 122 

4. Piltdown Man, 123 

5. Neanderthal Man, 124 

6. Cro-Magnon Man, 126 
VII. Time Scales, 127 

VIII. Conclusions, 128 


CHAPTER VI. Units of Structure (Cytology) 

I. The Cell Theory, 129 

II. A Typical Cell, 129 

III. Cytomorphosis, 130 

IV Cell Differentiation, 131 

V. Chromosomes, 133 

VI. Mitosis, 133 

VII. Fertilization, 136 

VIII. The Determination of Sex, 138 

IX. A World of Billions, 140 

CHAPTER VII. Division of Labor in Tissues (Histology) 

I. Tissues, 141 

II. Fluid Tissues, 142 

III. Epithelial Tissue*, 142 

IV. Connective and Supporting Tissues, 143 



Table of Contents 

1. Connective Tissues, 143 

2. Supporting Tissues. 145 

(a) Cartilage, 145 

(b) Bone, 147 
V. Muscle Tissue, 152 

1. Smooth Muscle, 153 

2. Striated Muscle, 153 

3. Cardiac Muscle, 154 
VI. Nerve Tissue, 154 



CHAPTER VIII. The Development of the Individual 

The Starting Point, 156 
The Necessary Partners, 157 

1. Differentiation of the Germ Cells, 157 

2. Kinds of Sperm, 157 

3. Kinds of Eggs, 158 
Development of a Chordate, 159 

1. Cleavage, 160 

2. The Blastula, 160 

3. The Gastrula, 160 

4. Formation of the Nervous System, 162 

5. Formation of Notochord and Mesoderm, 164 

6. Differentiation of the Mesoderm, 164 

7. Emigration of the Mesenchyme, 165' 

8. Assembling of the Digestive Tube, 1 66 

9. The Fate of the Germ Layers, 167 
IV. Early Development of Telolecithal Eggs, 168 

V. Early Development of Mammals, 173 
VI. The Major Cavities, 176 
VII. Organization Centers, 178 
VIII. Soma and the Germ Line, 180 
IX. The Succession of Generations, 181 


CHAPTER IX. Biological Discords (Pathology) 

I. The Point of View, 182 
II. Deviations from the Normal, 183 

III. Disease, 183 

IV. Disturbances That Work 111, 184 

1. Internal Disturbances, 184 

(a) Formative Disturbances, 184 

(b) Mechanical Interferences, 185 

(c) Responsive Maladjustments, 185 

(d) Hereditary Handicaps, 186 

2. External Disturbances, 186 

(a) Thermal Factors, 186 

(b) Chemical Factors, 187 

(c) Barometric Factors, 187 

(d) Mechanical Factors, 187 

(e) Biological Factors, 187 

V. Sources of Pathological Knowledge, 18E 
VI. The Control of Disease, 191 


Table of Contents 



CHAPTER X. A Jack of All Trades (The Integument) 195 

I. In General, 195 ' 

II. Uses of the Integument, 196 

1. Protection, 196 

2. Reserve Storage of Food, 197 

3. Regulation of Heat, 198 

4. Sensation, 199 

5. Excretion, 200 

6. Secretion, 200 

7. Respiration, 200 

8. Locomotion, 201 

III. The Human Skin, 201 

1. Macroscopic, 201 

2. Microscopic, 203 

3. Embryonic, 206 

IV. Comparative Anatomy of the Integument, 207 

1. Invertebrate Integuments, 207 

2. Tunicates, 208 

3. Amphioxus, 208 

4. Cyclostomes, 209 

5. Amphibians, 210 

6. Scaly Forms, 21 1 

7. Birds, 213 

8. Mammals, 213 

V. Derivatives of the Integument, 214 
1. Glands, 214 

(a) In General, 214 

(b) Invertebrate Skin Glands, 215 

(c) Vertebrate Glands, 216 

(d) Sweat Glands, 219 

(e) Sebaceous Glands, 220 

(f) Other Alveolar Integumentary Glands, 221 

(g) Mammary Organs, 222 

2. Scales, 229 

(a) Fishes, 229 

(b) Amphibians, 232 

(c) Reptiles, 232 

(d) Birds, 233 

(e) Mammals, 233 

3. Horns, 233 

(a) Keratin-Fiber Horns, 234 

(b) Antlers, 234 

(c) Pronghorns, 236 

(d) Hollow Horns, 236 

4. Digital Tips, 237 

(a) Claws, 237 

(b) Hoofs, 239 

(c) Nails, 240 

5. Miscellaneous Corneal Structures, 241 

6. Feathers, 243 

xii Table of Contents 

7. Hair, 249 

8. Friction Ridges, 257 

CHAPTER XL Intake Apparatus (Digestive System) 261 

I. In General, 261 

1. The Whirlpool of Life, 261 

2. Rate of Living, 261 

3. Hunger and Thirst, 262 

4. The Intake Mechanism of Animals and Plants Contrasted, 262 

5. The Mission of the Food Tube, 264 

6. Kinds of Feeders, 264 
II. The Food Tube, 265 

1. Its Evolution, 265 

2. Increase in Digestive Surface, 266 

(a) Increase in Diameter, 266 

(b) Increase in Length, 267 

(c) Internal Folds, 268 

(d) Supplementary Diverticula, 268 

3. Development, 270 

4. Histology, 270 

5. Regions of the Tube, 271 

III. Ingressive Zone, 272 

1. Food Capture and Prehension, 272 

2. The Mouth Aperture and Lips, 274 

3. Buccal Cavity, 275 

4. Oral Cavity, 277 

5. Tongue, 280 

6. Teeth, 287 

(a) Structure, 287 

(b) Development, 287 

(c) Number, 290 

(d) Succession, 291 

(e) Situation, 292 

(f) Attachment, 293 

(g) Movement, 294 
(h) Differentiation, 294 
(i) Dental Formulae, 296 

(j) Origin of the Molars, 298 

(k) Unusual Teeth, 299 

(1) The Trend of Human Teeth, 300 

IV. Progressive Zone, 301 

1. Pharynx, 301 

2. Esophagus, 304 

3. Stomach, 306 

V. Degressive Zone, 3 1 1 

1. The Small Intestine, 311 

2. Glands, 314 

(a) Liver, 315 

(b) Pancreas, 319 

(c) Intestinal Glands, 322 
VI. Egressive Zone, 324 

VII. The Essentials of Digestion, 326 

Table of Contents x m 

VIII. Mesenteries, 326 

CHAPTER XII. Internal Transportation (Circulatory System) 329 
I. In General, 329 
II. Blood and Lymph (The Carriers), 330 
1. Uses of the Blood, 330 

(a) Equilibration of Water Content, 330 

(b) Liberation of Energy, 331 

(c) Distribution of Food, 331 

(d) Regulation of Temperature, 331 

(e) Transmission of Chemical Substances, 331 

(f) Defence against Parasitic Invasion, 331 

(g) Disposal of Cell Wreckage, 331 
(h) Chemical Elaboration, 332 

(i) Clinical Diagnosis, 332 

2. Amount of Blood, 332 

3. Erythrocytes, 332 

4. Leucocytes, 335 

5. Thrombocytes, 338 

6. Plasma, 339 

III. Blood Channels In General, 340 

1. The Evolution of Organic Irrigation, 340 

2. General Plans of Circulation, 341 

(a) Annelid Plan, 341 

(b) Amphioxus Plan, 342 

(c) Gill Plan of Fishes, 342 

(d) Lung Plan of Mammals, 343 

3. Structure of Blood Vessels, 344 

4. The Role of Capillaries, 346 

IV. Origin of Circulatory Systems, 348 
V. The Heart, 351 

1. In General, 351 

2. Embryonic Development, 352 

3. Evolution, 353 

(a) Single-Circuit Hearts, 354 

(b) Transitional Hearts, 356 

(c) Double Hearts, 359 

4. Size and Position, 359 

5. Valves, 360 

6. The Work of the Heart, 361 

VI. Arteries and Their Transformations, 364 
VII. Venous Routes, 368 

1. In General, 368 

2. Development of Veins in Elasmobranchs, 370 

3. Evolution, 372 
VIII. Lymphatics, 377 

CHAPTER XIII. The Release of Energy (Respiratory System) 382 

I. In General, 382 

1. The Respiratory Environment, 382 

2. The Exchange of Gases, 383 

3. The Essentials for Any Respiratory Device, 384 

xiv Table of Contents 

4. Different Kinds of Respiratory Mechanisms, 385 

II. Gills, 387 

1. Invertebrate Gills, 389 

2. Primitive Gills of Amphioxus and Tunicates, 389 

3. Cyclostome Gills, 390 

4. Elasmobranch and Holocephalian Gills, 391 

5. Ganoid and Teleost Gills, 393 

6. Dipnoan Gills, 395 

7. Amphibian Gills, 395 

8. Gill Structures in Land Vertebrates, 397 

III. Swim Bladder, 397 

IV. Lungs, 401 

1. General Plan, 401 

2. Air Passages, 403 

(a) Nasal Chamber and Pharynx, 403 

(b) Trachea, 403 

(c) Bronchi, 405 

(d) Bronchioles, 405 

(e) Alveoli, 405 

3. Phylogeny, 406 

(a) Dipnoans, 406 

(b) Amphibians, 406 

(c) Reptiles, 408 

(d) Birds, 408 

(e) Mammals, 412 

4. Pleural Envelopes, 413 

5. Origin of the Lungs, 413 
V. Devices for Securing Air, 414 

1. Fishes, 415 

2. Amphibians, 416 

3. Reptiles, 417 

4. Birds, 417 

5. Mammals, 418 
VI. Voice Apparatus, 420 

1. Amphibians, 421 

2. Reptiles and Birds, 422 

3. Mammals, 423 

CHAPTER XIV. Outgo Apparatus (Excretory System) 427 

I. Excretion, 427 
II. Kidneys, 429 

1. Forms, 429 

2. Position, 430 

3. Gross Structure of the Mammalian Kidney, 431 

4. A Urinary Unit, 432 

5. Urine, 435 

III. Urinary Ducts, 436 

IV. Bladders, 437 

V. The Succession of Kidneys, 440 

1. In General, 440 

2. The Ncphridial Apparatus of Amphioxus, 440 

3. Pronephros, 1 i 1 

Table of Contents 


4. Mesoncphros, 443 

5. Metanephros, 445 

CHAPTER XV. The Preservation of Species (Reproductive 

System) 44.7 

I. The Significance of Reproduction, 447 
II. Methods of Reproduction, 448 

III. The Essential Reproductive Cells, 449 

1. Sperm Cells, 449 

2. Eggs, 451 

IV. Secondary Reproductive Apparatus, 454 

1. Gonads, 454 

(a) Testes, 456 

(b) Ovaries, 460 

(c) Hermaphroditism, 463 

(d) Gametogenesis, 463 

2. Ducts, 466 

(a) Disposal of Gametes, 466 

(b) Sperm Ducts, 467 

(c) Oviducts, 470 

3. Apparatus for Effecting Fertilization, 472 

(a) Fertilization of Animals and Plants Contrasted, 472 

(b) Holdfast Mechanisms, 472 

(c) Male Copulatory Organs, 473 

(d) Female Genitalia, 478 

4. Accessory Glands, 480 

(a) Originating in the Sperm Duct or Oviduct, 481 

(b) Originating from the Urogenital Canal, 482 

(c) Originating in the Integument, 483 

5. Devices for the Care of Eggs and Young, 483 

(a) Uterus, 483 

(b) Brood Sacs, 485 

(c) Nidamental Glands, 485 

(d) Placenta in Mammals, 486 

6. Degenerate and Rudimentary Organs, 486 

V. Determination and Differentiation of Sex, 489 
VI. Periodicity in Reproduction, 491 

VII Care of the Young, 494 

CHAPTER XVI. Involuntary Regulation (Glands of Internal 



I. In General, 497 
II. Pharyngeal Glands, 499 

1. Thyroid, 499 

2. Parathyroids, 502 

3. Thymus, 503 

4. Ultimobranchials, 503 
III. Cranial Glands, 504 

1. Pituitary, 504 

(a) Posterior Lobe, 505 

(b) Intermediate Lobe, 506 

(c) Anterior Lobe. 506 

xvi Table of Contents 

2. Pineal Body, 507 
IV. Sexual Glands, 508 

1. Male Gonads, 508 

2. Female Gonads, 508 
V. Abdominal Glands, 509 

1. Adrenals, 509 

2. The Islands of Langerhans, 512 


CHAPTER XVII. The Skeleton 517 

I. The Role of the Skeleton, 517 
II. Joints, 520 

III. Divisions of the Skeleton, 521 

IV. The Main Skeletal Axis, 522 

1. The Parts of a Vertebra, 522 

2. The Notochord and Its Sheaths, 524 

3. The Formation of Vertebral Arches, 525 

4. The Embryonic Development of Vertebrae, 526 

5. Types of Articulation between Centra, 529 

6. History of the Vertebral Column, 531 

7. The Entire Backbone, 537 
V. The Thoracic Basket, 539 

1. In General, 539 

2. Ribs, 542 

3. The Sternum, or Breastbone, 548 
VI. Gastralia, 554 

VII. The Skull, 554 

1. The Development of the Brain Case, 555 

2. The Splanchnocranium, 562 

3. Types of Jaw Suspension, 564 

4. Comparative Anatomy of the Skull Bones, 565 

5. Middle Ear Ossicles, 582 

6. Temporal Fossae, 583 

7. Vertebrate Skulls, 584 

(a) Cyclostomes, 584 

(b) Elasmobranchs, 585 

(c) Ganoids, 585 

(d) Teleosts, 585 

(e) Stegocephals, 586 

(f) Modern Amphibians, 586 

(g) Reptiles, 587 
(h) Birds, 589 

(i) Mammals, with Special Reference to Man, 590 
VIII. The Locomotor Skeleton, 600 

1. The Necessity for Animal Locomotion, 600 

2. Evolution of Locomotor Devices, 601 

3. Kinds of Appendages, 603 

4. Origin of the Girdles and Appendages, 604 

5. Homology and Adaptation, 606 

6. The Comparative Anatomy of Girdles, 608 

Table of Contents xvii 

(a) Girdles in General, 608 

(b) Pelvic Girdle, 609 

(c) Pectoral Girdle, 618 
7. The Free Appendages, 623 

(a) Unpaired Fins, 623 

(b) Lateral Appendages in General, 625 

(c) Different Pectoral Appendages, 626 

(d) Different Pelvic Appendages, 633 

(e) The Human Hand, 640 

CHAPTER XVIII. Production of Motion and Locomotion 

(Muscles) 642 

I. In General, 642 
II. Muscular Activity, 643 

III. Muscles as Tissues, 645 

IV. Muscles as Organs, 646 

V. Embryology of Muscles, 650 
VI. Kinds of Voluntary Muscles, 651 

1. Metameric Muscles, 652 

( 1 ) Axial Muscles, 652 

(a) Head, 652 

(b) Trunk, 654 

(c) Diaphragm, 657 

(2) Appendicular Muscles, 657 

(a) Extrinsic Muscles, 657 

(b) Intrinsic Muscles, 658 

2. Branchiomeric Muscles, 658 

3. Integumental Muscles, 659 
VII. Electric Organs in Fishes, 663 

CHAPTER XIX. The Dominating Factor (Nervous System) 665 

I. General Character, 665 
II. The Structural Units, 666 

1. Polarity, 667 

2. Nerve Fibers and Their Sheaths, 668 

3. Microscopic Structure, 669 

III. Reflex Arcs, 670 

IV. Primitive Nervous Systems, 672 

V. The Embryonic Rise of the Nervous System, 674 
VI. Subdivisions of the Nervous System, 676 
VII. The Cord and Spinal Nerves, 677 

1. Form of the Cord, 677 

2. Extent of the Cord, 678 

3. Spinal Nerves, 679 

(a) Roots, 682 

(b) Branches, 683 

(c) Plexuses, 684 

4. Development of the Spinal Cord, 685 

5. Internal Architecture of the Adult Cord, 688 

(a) Ascending Sensory Tracts, 690 

(b) Descending Motor Tracts, 692 

(c) Association Ground Bundles, 693 

6. Comparative Anatomy of the Cord, 695 

xviii Table of Cojitents 

VIII. Meninges, 698 
IX. Brain, ^700 

1. In General, 700 

2. Comparison of Brain and Cord, 701 

3. Differentiation of the Brain, 703 

(a) Constriction, 703 

(b) Unequal Thickening of the Walls of the Brain, 704 

(c) Invaginations and Evaginations, 704 

(d) Bendings, 705 

4. Ventricles, 706 

5. General Topography of the Mammalian Brain, 706 

6. Cerebral Localization, 715 

7. Craniospinal Nervous Pathways, 718 

(a) Fasciculi gracilis and cuneatus, 718 

(b) Spinothalamic Tracts, 719 

(c) Spinocerebellar Tracts, 719 

(d) Corticospinal Tracts, 720 

8. The Comparative Anatomy of the Brain, 723 

(a) Cyclostomes, 723 

(b) Elasmobranchs, 725 

(c) Other Fishes, 727 

(d) Amphibians, 727 

(e) Reptiles, 728 

(f) Birds, 729 

(g) Mammals, 729 

9. The Control of the Body by the Brain, 730 
X. Cranial Nerves, 731 

1. Terminal Nerve, 737 

2. Olfactory Nerve, 738 

3. Optic Nerve, 738 

4. Eye-Muscle Nerves, 740 

5. Trigeminal Nerve, 741 

6. Facial Nerve, 742 

7. Acoustic Nerve, 742 

8. Glossopharyngeal Nerve, 743 

9. Vagus Nerve, 743 

10. Spinal Accessory Nerve, 744 

11. Hypoglossal Nerve, 744 

XI. The Autonomic Nervous System, 744 

1. Visceral Reflexes, 745 

2. Autonomic Outflows, 746 

3. Thoracico-lumbar Division, 748 

4. Double Autonomic Supply, 750 

5. Comparative Anatomy of Autonomic System, 751 

CHAPTER XX. Ports of Entry (Sense Organs) 753 

I. In General, 753 

1. Stimuli, 754 

2. Receptors, 754 

3. Response, 756 

4. Sensory Range in Animals, 756 

5. Accessory Structures, 757 

Table of Contents 








Classification of Receptors, 758 
Cutaneous Sense Organs, 760 

1. Tangoreceptors, 760 

2. Thermoreceptors, 764 

3. Algesireceptors, 767 

4. Rheoreceptors, 769 
Chemical Sense Organs, 772 

1. Olfactoreceptors, 772 

(a) Odors, 773 

(b) Structure, 774 

(c) Accessory Parts, 777 

(d) Evolution, 778 

2. Gustoreceptors, 780 
(a) Taste Buds, 780 

fb) Comparative Anatomy, 781 
(c) Classification, 782 

3. Irritoreceptors, 783 
Gravity Organs, 784 

1. Equilibrium, 784 

2. Statoreceptors, 785 

3. Semicircular Canals, 786 

4. Weber's Organ, 787 
Phonoreceptors, 788 

1. The External Ear, 788 

2. The Middle Ear, 791 

3. The Internal Ear, 795 

(a) Development of the Membranous Labyrinth, 795 

(b) Structure in Mammals, 796 

(c) Physiology of Hearing, 800 

(d) Comparative Anatomy, 802 
Photoreceptors, 804 

1. In General, 804 

2. Photoreceptors That Are Not Eyes, 805 

3. Eyes, 805 

(a) Direction Eyes, 805 

(b) Image-Forming Eyes, 806 

4. Human Eyes, 806 

(a) Development, 808 

(b) Structure of the Human Eye, 809 

(c) Accessory Parts, 817 

5. Median Eyes, 820 

6. Comparative Anatomy, 821 

(a) Amphioxus, 821 fe) Reptiles. 825 

(b) Cyclostomes, 821 (f) Birds, 826 

(c) Fishes, 821 (g) Mammals, 828 

(d) Amphibians, 824 
Internal Sense Organs, 830 

1. Proprioceptors, 830 

2. Interoceptors, 831 






Chordate Characters 


No one knows how many different kinds of animals there are living 
today. When Aristotle (384-322 B.C.) wrote the first History of Animals he 
succeeded in rounding up only about 500 species in spite of the fact that 
Alexander the Great, his famous pupil, gave special instructions to his con- 
quering armies to aid in collecting from the ends of the earth information 
about foreign animals which his old master so eagerly desired. 

Since Aristotle's day explorers have stretched the horizon that then shut 
in the Mediterranean world, until now even Darkest Africa has been 
entirely criss-crossed, both poles have been trampled upon, and no consider- 
able corner of the globe anywhere, on land or sea, is left from which 
authentic tales of animal life have not been brought back. 

According to recent estimates upwards of 1,000,000 species of living 
animals are known to science. Of these probably 65,000 are chordates. In 
addition there are fossil remains of many more extinct animals that have no 
living representatives. As long ago as 1890, according to Ward, the manu- 
script catalogue of known plants at the Kew Gardens weighed over a ton. 
The inquirer who would be informed about the different kinds of living 
things might well be appalled at the prospect of passing in review within a 
single lifetime of study even a tithe of this wealth of animal and plant life. 

John Malpet, who in 1567 wrote one of the first "natural histories" in 
the English language, started his treatise with the hopeful sentence, "Let us 
begin alphabetically with the adder." There is an easier way out of the 
situation, however, than by John Malpet's alphabetical method. Even 
Aristotle recognized in the make-up of animals a unity of plan by which they 
could be placed in natural groups so that acquaintance with a single 
representative of a group would give a considerable working knowledge of 
all other kinds within that particular group. Familiarity with the mechanism 
and behavior of a house cat, for example, gives one a good idea of all other 
kinds of cats, such as lions, tigers, lynxes, leopards, ocelots, jaguars, wildcats, 


4 Biology of the Vertebrates 

pumas, cheetahs, and panthers. In fact much of the fascination that goes 
with the study of biology lies in recognizing resemblances and differences 
between various sorts of plants and animals. 

Although the number of kinds of plants and animals is very great, the 
different general types or plans of structure are relatively few, so that the 
student, by using the type-study method of sampling, may set out confidently 
and with a brave heart upon the ambitious quest of intellectually conquer- 
ing all creation. 

Limiting the survey solely to animal life, a list of the chief types of 
animals comprises: protozoa, coelenterata, platyhelminthes, nema- 



Of all animal types the chordate type is of most immediate interest, 
since it includes man. Many of the riddles connected with that much studied 
animal find their solution in the lower forms. 

' For instance, the parietal body, a conical projection about the size of a 
cherry stone, is buried between the lobes of the human brain. Its origin and 

use baffled anatomists for centuries 

< ^^?ne? s ^ix^. until Baldwin Spencer in 1886 dis- 

^^*^? ^-2^$^^^^^ sected a New Zealand lizard, Spheno- 

^ A^ rj£^^2^^^^^^^' don ( Fig. 1 ) , by some called a "living 

_.. ., . . . . , T r, , , ,. fossil." He discovered that in the chor- 
rig. 1. A primitive JNew Zealand liz- 
ard, Sphenodon. (After Berg.) date type, the "parietal body, ' or a 

part arising from it, is simply a de- 
generate median eye since in this curious primitive reptile it reaches, with 
retina and nerve complete, all the way to a transparent window in the roof 
of the skull and, in early life at least, may function as a third eye. 

It is entirely true that often more may be learned of human development 
and structure by the intelligent examination of a dogfish, or some other 
lowly vertebrate, than by the direct study of the human body itself. This 
is due not so much to the greater availability of lower animals for dissec- 
tion and experimentation, as to the fact that they furnish sidelight stages 
through which the human body has passed in arriving at its present degree 
of complexity, and thus give a clue for interpreting the why and wherefore 
of the "fearfully and wonderfully made" human mechanism. Herein lies 
the value of the study of comparative biology. 

The indirect path has thus been often the shortest cut to unexpected 
attainment in the history of science. Inquisitive Ben Franklin, out in the 

Chordate Characters 

thunder-storm with his key and kite, took the first step towards harnessing 
electricity ; the Frenchman Daguerre, trying to discover some way to clean 
tarnished silver, blazed a path which has become a broad highway in 
photography and the colossal motion-picture industry; Alexander Graham 
Bell, attempting to aid the deaf to hear, led to the invention of the tele- 
phone ; Joseph Cushman, with insatiable curiosity about the variety of forms 
of microscopic shelled protozoans, hit upon a way to tell those who bore for 
oil when they were on the right track ; while Pasteur, a thinking chemist 
interested primarily in the apparently remote subject of the shape of crystals, 
laid firm foundations for the far-reaching developments of bacteriology and 
modern medicine. 

When such facts as these are recalled, nothing about the structure or 
activities of any animal, however familiar or strange, becomes insignificant 
or trivial to the seeker after truth concerning man. 


Every form of life, whether plant or animal, must possess machinery of 
some sort for accomplishing two fundamental processes, namely, metabo- 
lism and reproduction. 

Metabolism includes all activities that concern the upkeep of the indi- 
vidual, such as the intake of energy by way 
of food, its release in the form of action which 
constitutes "living," and the disposal of waste 
products incident thereto. 

Reproduction provides for the continu- 
ation of the species upon the earth, often at 
the cost of the individual life. 

The former function may be designated 
as selfish and egoistic, the latter as unselfish 
and altruistic. 

A typical insect, for example, is made up 
of three easily distinguishable regions, in 
order of relative importance, the abdomen, 
thorax, and head (Fig. 2). In the large ab- 
domen is lodged the principal machinery for 

metabolism and reproduction, that is, most of the digestive apparatus, the res- 
piratory, excretory, and circulatory machinery, and the reproductive organs. 

The thorax is devoted primarily to locomotion, furnished as it is with 
three pairs of legs and usually with two pairs of wings together with the 
muscles necessary to work them, and is thus enabled to transport the 

Fig. 2. A typical insect, show- 
ing the abdomen, all important 
as the chief region of metabolism 
and reproduction, the locomotor 
thorax, and the directive head. 

6 Biology of the Vertebrates 

important abdomen to places where it can selfishly procure energy-produc- 
ing food and unselfishly provide for the next generation. Finally, there is 
the head, with its battery of directive sense organs and a controlling brain, 
which tells the thorax and abdomen where to go and what to do upon 
arrival. Many animals get along comfortably without a head or locomotor 
devices, but none can dispense with the all-important abdomen or some- 
thing corresponding to it. Even in man that crowning glory, the head, 
which he is quite apt to regard as important, as well as the locomotor 
legs, becomes quite subsidiary when the trunk, that corresponds to the 
insect's abdomen, sends out the imperious call of hunger or of sex. 

The function of metabolism is usually accomplished in a different way 
by animals than it is in plants, with the result that most plants remain sta- 
tionary, while most animals move about. The reason for this difference lies 
in the fact that green plants possess the power, in the presence of sunlight, 
of building up their foods out of universally distributed materials, such as 
carbon dioxide in the air, water, and various inorganic compounds in the 
soil. No animal can do this, so it comes about that all animals must find 
their energy, either directly or indirectly, in the stored supply already 
captured by green plants from the sun. This is why most animals are forever 
fated, like the Wandering Jew, to be travelers, a condition which necessi- 
tates in animal types some adequate device for locomotion, and consequently 
an accompanying directive sensory equipment. The fact that certain animals 
like oysters and corals are sedentary is the exception to the rule. "Their 
strength is to sit still." Even in the case of these animals the indirect depend- 
ence on green plants is quite as complete as among locomotor forms, since 
they feed upon microscopic green plants that form the floating meadows of 
the ocean, and in consequence have developed secondary devices for bring- 
ing this floating food to them. Most animals digest their food in an 
alimentary canal, which begins in a mouth near the anterior end of the 
body and terminates posteriorly in an anus. 


The science of the form and shape of organisms is called Morphology, a 
term coined by the many-sided Goethe in 1817. It is closely related to the 
mathematical science of Solid Geometry, with the difference that the mathe- 
matician has little occasion to inquire why one figure is a cube and another 
a sphere except to determine the relation of the different dimensions to 
each other, while the biologist is constantly being challenged to explain why 
each organism is shaped as it is, in relation to the particular life that it 
leads. Moreover, the shapes and forms with which the geometrician deals 

Chorddte Characters 7 

are arbitrary creations of the human mind, not particularly related to the 
environment, having no modifying past and no forward look to a future 
in which modifications may take place. The forms of animals and plants 
which a biologist considers are the products of an actual historical sequence 
that has taken place, of ancestral shapes that have in succession all left their 
determining impress. 

There are no animals with less than three dimensions, although some of 
the lower forms are so small and thin as to necessitate very delicate instru- 
ments to determine their length, breadth, and thickness. 

Three fundamental shapes and forms are recognized and, as a result, 
three general types of symmetry, namely, spherical, radial, and bilateral. 
Each of these types may be camouflaged in various ways by secondary 
modifying qualifications. 

Spherical symmetry in organisms is rare. It is to be found only among 
microscopic animals, such as the Heliozoa, or "sun animalcules" of the 
protozoan type which float without contact with anything solid, surrounded 
by water on all sides. Many floating animals, on the other hand, become 
attached, during a part of their life at least, and lead a sedentary plantlike 
existence. Such anchored animals are usually headless, and frequently 
develop a crown of radiating arms or tentacles that enable them to reach 
out in every direction to explore as far as possible their immediate neighbor- 
hood. This headless plan is the radial type of symmetry, which in general is 
characteristic of trees and other stationary plants, as well as of attached or 
sessile animals, whose food is brought to them floating in water. In all of 
these organisms the body possesses polarity, being organized along a longi- 
tudinal axis with an attached end and a free end. 

On land, where food does not float in a transporting medium, animals 
have to travel to obtain it when they are hungry. This has made necessary 
a directive head. Although a head end is characteristic of certain water 
animals such as fishes, it becomes an absolute necessity for locomotor land 
animals. Whenever an animal moves persistently in one direction with refer- 
ence to its own body, in other words whenever a true head end is estab- 
lished, bilateral symmetry results, and a stagnant life of watchful waiting 
ceases. With the appearance of this type of symmetry animals usually 
develop the habit of keeping one particular side of the body either in contact 
with the substrate or facing downwards. This undersurface is the ventral 
side of the animal, while the upper surface is the dorsal side. The body 
presenting this type of symmetry may be divided into halves by means of 
three planes which can be arranged with reference to length, breadth, and 

8 Biology of the Vertebrates 

In radial symmetry, on the other hand, the number of planes dividing 
the animal into similar halves is practically infinite, like the number of 
ways in which a cylinder may be split lengthwise into two equal parts. 




I I 


Fig. 3. The planes of symmetry in bilaterally symmetrical animals, with 
the resulting regions, s.p, sagittal plane; f.p, frontal plane; t.p, trans- 
verse plane; r, right; l, left; d, dorsal; v, ventral; a, anterior; p, posterior. 

The three planes (Fig. 3) bisecting length, breadth, and thickness 
divide any bilaterally symmetrical animal into definite regions, very useful 
as landmarks in description, as follows: 

Sagittal plane dividing the body into right and left halves, mirror images 
of one another; 

Transverse plane dividing the body into anterior and posterior halves ; 

Frontal plane dividing the body into dorsal and ventral halves. 

The sagittal and frontal planes are so named because of certain sutures 
in the human skull with which they coincide. It is obvious that upright 
man moves forward with the ventral body-half in front, instead of the 
anterior body-half, because he is a bilaterally symmetrical animal tipped 
up on end. 

Although the comparative anatomist uses the above terms, the student of 
human anatomy sometimes uses a different set. The head-end may be 
known as the superior portion in man, while the lower part of the body is 

Chordate Characters g 

the inferior portion. The terms anterior and posterior are then used as the 
synonyms of "ventral" and "dorsal," respectively, of comparative anatomy. 


The body of an annelid worm, an arthropod, or an embryonic chordate 
consists, in basic plan, of a series of similar, repeated divisions (metameres 
or segments) arranged one behind the other. An adult annelid (e.g., 
Nereis) very closely approximates this basic plan, with the the metameres 
clearly marked off externally. Each metamere, except the first and last, 
possesses a pair of appendages. Internally, at the level of each external 
constriction, there is found a cross-partition {septum). Other internal 
structures, such as nephridia, tend to be repeated in each metamere. Chord- 
ates, however, never have external constrictions, although most of them 
exhibit internal segmentation of a number of embryonic organs, for exam- 
ple, the skeletal muscles. The axial skeleton and the nervous system show 
a modified metameric organization. 


The chordates, in common with annelids, echinoderms, and some other 
invertebrate phyla, possess a coelom (the so-called true body cavity), lined 
with mesodermal tissue and lying between the digestive tract and the body 
wall. Thus great freedom is permitted for the development and activity of 
embryonic organs as well as for the movement of such adult parts as the 
heart, stomach and intestine. 


The backboned animals (Vertebrata) , together with a few closely 
related animals which do not possess a backbone, are ordinarily included in 
the Phylum Chordata. In the preceding sections certain chordate features, 
also commonly possessed by members of other animal phyla, have been 
considered. These primitive characters of chordates include : ( 1 ) reproduc- 
tive glands; (2) alimentary canal ; (3) polarity; (4) bilateral symmetry; 
(5) metamerism; and (6) coelom. 

Chordates also possess certain other features, which distinguish them 
from all other animals. The chief diagnostic characters of chordates are the 

1. Notochord 

During the embryonic development of every chordate there appears a 
supporting rod, the notochord, which lies immediately dorsal to the digestive 

io Biology of the Vertebrates 

tract. In some chordates this structure persists throughout life ; in others it is 
partially or completely replaced by a skull and a "backbone" made up of 
separate bony elements, or vertebrae, as the name "vertebrate" indicates. 
Essentially the notochord consists of a tough connective tissue sheath in 
which soft cells are packed so tightly that the whole structure possesses a 
certain turgor, somewhat like that when sausage meat is crowded into a 
casing (Fig- 98). 

2. Dorsal, Hollow, Central Nervous System 

The chordate nervous system develops on 
Sgg^^^gjsS the dorsal side of the body by a process known 
as invagination ( Fig. 4 ) . In this structure, even 
sft\ /?&. SassHssS in adult animals, there is a cavity which is con- 

w^ tinuous from near the anterior end of the brain 

gs^^rjs to the posterior end of the nerve cord. The cen- 
tral nervous system of non-chordates, on the 
SaS-g-? other hand, is formed on the ventral side of the 
body and is solid. In the vertebrate members of 

_. . „ . . the chordate phylum, the anterior end of the 

Fig. 4. Successive stages in ' . 

the migration of outside tis- central nervous system is much enlarged into a 

sues to the inside, a, by in- brain with which there are associated three pairs 
vagination; b, by delamina- f ma jor sense organs: olfactory (nose); optic 
tion. . , i • / \ 

(eyes) ; and otic (ears). 

3. Pharyngeal Breathing Device 

Fishes have several porthole-like passage-ways, or gill slits, penetrating 
through the lateral walls of the food tube on either side of its anterior end. 
Within these gill slits in water-dwelling chordates hang feathery tufts of 
capillaries, or gills, which rob the circulating water of some of its dissolved 
air, thus accomplishing the function of breathing. 

Gill slits, or traces of them, are present, at least in embryonic life, in all 
chordates, whether dwelling in water or out of it, and even in reptiles, birds, 
or mammals which never breathe by means of gills. Whenever breathing is 
accomplished by lungs, such organs develop as side alleys from this same 
anterior pharyngeal region of the food tube where the gills originate. No 
non-chordate breathes in this way, although many kinds of animals employ 
"gills" of various sorts. Pharyngeal gills and gill slits, or traces of them, are 
peculiar to chordates. 

These first three chordate characteristics are present at some time during 
the life of every individual of the chordate phylum. 

Chordate Characters 11 

4. Ventral Heart 

The heart, which is the headquarters of the circulatory system, is 
ventrally located in chordates. In other animals, when a heart is present, it 
is on the dorsal side of the body. 

5. Closed Blood System 

In chordates the blood courses through a continuous system of tubes 
from heart, to arteries, to capillaries in the various tissues, to veins, and back 
to the heart again. Most non-chordates, on the other hand, have an open 
blood system, that is, one in which the blood may pass freely back and 
forth between the blood vessels and surrounding spaces or sinuses. The con- 
trast is remotely like that between the waterworks of a modern city with 
water and sewage confined to pipes and mains, and the open ponds and 
streams of the countryside. 

6. Hepatic Portal System 

Although venous systems, beginning in capillaries in the tissues of the 
body, ordinarily terminate at the heart, there are places where veins not 
only begin but also end in capillaries. Such a group of veins is known as a 
portal system. In most chordates, the food-laden blood from the digestive 
tract passes through a strainer-like capillary network, the liver, before it 
arrives at the heart to be sent over the hungry body. The group of veins 
beginning in capillaries in the digestive tract and ending in hepatic (he pat-, 
liver) capillaries is known as the hepatic portal system. 

Although other animals have organs that are called "livers" by courtesy, 
only chordates have a true liver, or clearing house, 
where the strained blood is reorganized by addi- 
tion and subtraction of various substances before 
being distributed to different parts of the body. 

7. A Post-Anal Tail 

A true tail may be defined as a continuation \r\ ^.y Umbilical 

of the body axis posterior to the anal exit of the 

food tube. That part of a lobster, for example, 

which is sometimes erroneously called the "tail," 

is not a true tail at all, but the abdomen, since Flff< 5 ; Lateral view of a 
, . young human embryo show- 

the anus opens at the end ol it. Each vertebrate - ing ta Q (After Ecker.) 

has a true tail, either throughout life or em- 

bryonically and ancestrally. Even tailless man has in his early fetal stages 

an unmistakable tail (Fig. 5), and there are numerous well-authenticated 

Biology of the Vertebrates 

J. — 

cases reported in medical literature of human tails that persist beyond em- 
bryonic life. 

8. Red Blood Cells 

The red respiratory pigment of the blood may be dissolved in the liquid 
part of the blood or may be confined in red blood cells. In the chordates the 
pigment is always in cells. In non-chordates it is usually in the plasma, but 
in a number of species, scattered among various invertebrate phyla, the red 
pigment is in cells. Among the non-chordates possessing red blood cells are: 
Area a bivalve mollusc; Glycera, a polychaete annelid; and Thyone, a sea 
cucumber of the echinoderm phylum. These particular examples are chosen 
because they are relatively common animals representative of three different 
phyla. No evolutionary significance is to be attached to the occurrence of 
this chordate feature in widely scattered species of invertebrates. 


A visualized diagrammatic summary of some of the outstanding points 
of contrast between a generalized chordate and a corresponding non- 
chordate is presented in Fig. 6. 


Exoskeleton Pulsating Dorsal Blood Vessel 


Nerve Cord Digestive Tube'' ^^ 


v Post-Anal Tail 

Gill Slits*' 


Heart Liver 

Fig. 6. Comparative diagrams of the fundamental plans of a non- 
chordate (above) and a chordate (below). 


Kinds of Vertebrates 


In nature one encounters all sorts of different animals intermingling 
without any apparent law or order. It is necessary, therefore, with all this 
diversity to invent some workable system that will bring cosmos out of chaos 
and make "type study" possible, otherwise confusion is inevitable and the 
effort to become familiar with all living things is hopeless. 

First of all it is essential to become acquainted with as many kinds of 
animals as possible, not alone through pictures and names of animals that 
live so dreary a life in textbooks, but through actual acquaintance with real 
animals. Taxonomy, or classification of animals and plants, is dull and with- 
out point until one has gained a personal acquaintance with enough organ- 
isms to make it worth while. This chapter consequently should be referred 
to only as a last resort after various kinds of animals encountered begin 
to be familiar and interesting, and there is something to classify. 

Classifications, it should be noted, are more than arbitrary sets of 
pigeon-holes labeled with forbidding technical names, in which to file away 
and forget our animal associates, for they involve a compact summary of 
knowledge concerning the origin and derivation of different organisms. 

In mentally putting together animals of a kind, the ideal criterion to 
employ is hereditary relationship rather than external resemblance. It is the 
particular province of comparative morphology to discover such relation- 
ships. A whale, for example, is properly classified with mammals rather than 
with fishes, which it superficially resembles and with which it associates, 
because its common origin with animals of the mammalian type is indicated 
by the fundamental fact that, along with many other mammalian peculiar- 
ities, its young are born alive and fed at first upon milk. 

Superficial features, like the transparency of many open-sea forms as 
diverse as jellyfish, shrimp, pteropod mollusks, worms, and larval fishes, or 
the power of aerial flight on the part of such plainly unrelated creatures as 
birds, bees, and bats, tell us where the animal has been spending its life, 


14 Biology of the Vertebrates 

while animals as unlike in appearance as whales and bats, herons and hum- 
mingbirds, eels and flatfishes, butterflies and bedbugs, or lobsters and 
barnacles, belong together in any scientific classification, because each pair 
is built on the same fundamental plan and has a blood relationship, one 
with the other. 

Owing to the incompleteness of our present knowledge about the evolu- 
tion and blood relationship of animals there is still considerable uncertainty 
and controversy among taxonomists as to "who's who" in any classification, 
and as a result several different arrangements are current in books dealing 
with the subject. The same scientist, as his store of knowledge grows, may 
change his original classification. For instance, David Starr Jordan, Amer- 
ica's foremost authority on the group of fishes, in a classification of North 
American fishes (Jordan and Copeland) in 1876, named 670 species. 
Twenty-two years later, in 1898, he published a new list (Jordan and Ever- 
man) including in it only 585 species in spite of the fact that meanwhile 
130 new species had been brought to light. 

Ward refers to two kinds of taxonomists, namely, "hair-splitters" and 
"lumpers," and we are free to choose between them, for there is no indis- 
putable hard and fast classification that we are bound to accept to the 
exclusion of all others. 

Although opinions differ with regard to the details of systems of classi- 
fication, there is substantial agreement with regard to the sequence of the 
following groups in which 

like individuals make up species 








The manner of employing these groups may be illustrated by classifying a 
particular individual house cat, named "Tom" (Fig. 7). It will be seen 
that this cat finds itself admitted successively into more and more inclusive 
groups, until finally, as a member of the vast animal kingdom, it has quite 
lost its individual importance. If now we retrace our steps in the diagram 
from the all-inclusive animal kingdom, we see the individuality of "Tom" 
gradually emerging until it may be concluded that this cat at least possesses 
not only the general characteristics listed in the preceding chapter as verte- 
brate characteristics, but also that it has the special equipment that makes 

Kinds of Vertebrates 


it -a mammal and a carnivore, like the lions, tigers, and their kind, and last 
of all it has an individuality that distinguishes it from all other domestic 
cats which are well known in their several households. 

Fig. 7. The taxonomy of a cat named "Tom." 

Although Darwin, who wrote The Origin of Species, was unable to 
define just what are the limits of a species, which is a concept that has no 
exact counterpart in nature, he nevertheless made it clear that a species is a 
real entity that outlives the separate individuals composing it. The species 
concept of "cats" will remain long after the individual Tom has lived out 
the traditional nine lives of cats and turned to dust. 


In a serious study of animals it is necessary to employ scientific names. 
Common names which, like nicknames or pet names, may have only a 

16 Biology of the Vertebrates 

limited local application, do not invariably lead to accuracy in identifica- 
tion. Sailors are not initiated into life on the deep until they can command 
a vocabulary of technical terms that are strange to the landsman. Even 
baseball fans have a lingo all their own which corresponds to the scientific 
terminology that the biologist finds not only useful but indispensable. 

It is noteworthy that the first recorded task ever done by man is 
reported in Genesis 2 : 20, "and Adam gave names to all cattle, and to the 
fowl of the air, and to every living beast of the field." So Taxonomy is the 
first and most ancient of all sciences ! 

The great Swedish naturalist Linne (1707-1778), who introduced into 
biology a complete system of nomenclature, thereby raised biology from an 
inferior position as an adjunct of medicine to the dignity of a separate 
science simply by paving the way with an adequate biological terminology. 
He employed Latin mostly in making his scientific names. This was advis- 
able since Latin is a "dead language," no longer subjected to the changes in 
form and meaning to which any spoken language is liable. Latin, moreover, 
came the nearest to being the fundamental universal language of educated 
peoples of all tongues. Faithfully christening all the animals and plants known 
to science in his day with a scientific name, Linne even included himself so 
that he is generally known by the Latin name of Linnaeus. 

A complete scientific name consists of three parts, as follows : the name 
of the genus to which the animal belongs; the name of the species; and the 
name of the namer, or godfather, who does the christening. In all languages, 
therefore, Felis domestica Linn, is the proper scientific name for every 
common house cat, because these cats belong to the genus Felis, to the 
species domestica, and were so named in the first place by Linne. 

When the same kind of an animal is given two or more scientific names 
independently and unawaredly, as frequently occurs, the confusion is 
remedied by adopting the first name assigned, if it can be determined, in 
accordance with the "law of priority." 

In any scientific name the genus is invariably written with a capital 
letter and the species with a small letter, although it is permissible some- 
times, when a species is named in honor of a person or place, to employ a 
capital letter. Both generic and specific names are always printed in italics. 
According to common practice the name of the namer, which is principally 
useful in determining priority in doubtful cases, is frequently omitted. 

Every student of biology who sets out in earnest to excel, must conquer 
any childish aversion he may have for the imaginary terrors of unfamiliar 
scientific names and should acquire, as soon as possible, facility in the use 
of these indispensable tools of his trade. 

Kinds of Vertebrates 17 


Before proceeding further with a consideration of the comparative 
biology of the vertebrates, it is necessary to pass in brief review the different 
kinds of vertebrates between which comparisons are to be made. 
The phylum Chordata may be lined up in the following array: 

Subphylum I hemichordata 

Subphylum II urochordata 

Subphylum III cephalochordata 

Subphylum IV vertebrata (craniata) 

Class I Cyclostomata 

Class II Pisces 

Class III Amphibia 

Class IV Reptilia 

Class V Aves 

Class VI Mammalia 


There are certain animals which have difficulties in qualifying in all 
particulars as vertebrates but are, nevertheless, classified as chordates. These 
interesting connecting links between vertebrates and invertebrates may be 
called "borderline chordates," or Protochordates. They comprise three 
subphyla, namely: hemichordata, urochordata, and cephalochordata. 

1. Subphylum HEMICHORDATA 

Dolichoglossus kowalevskyi, found on the Atlantic coast (Fig. 8), may 
be taken as a representative of the Hemichordata, of which there are only 
a few genera. This small, fragile, wormlike animal has no common name, 
because it is not commonly known, an addi- 
tional reason for resorting to the use of a sci- 
entific name. Its body is divided into three 

general regions: (1) a proboscis used to push Mouthy | ^ 

aside the sand as the animal works its way Collar, 

along; (2) a short collar; and (3) the trunk, ^ Gill slits 

making up by far the greater part of the body. //$/ /rv _ .3>i2 

It lives buried in mud at various localities 
along the Atlantic seashore where at low tide 
its burrows may be identified by peculiar Fig. 8. Dolichoglossus, a bor- 
coiled piles of castings similar to those de- derIin e chordate. (After Bate- 
posited by earthworms (Fig. 9). S ° 

The hemichord is of special interest to the biologist since, unlike any true 
worm, it has the three major chordate characteristics. Numerous, U-shaped, 

Biology of the Vertebrates 

Fig. 9. Dolichoglossus in its tube 
30-60 cm deep in sand. (After 

pharyngeal gill slits are present in the anterior trunk region. There is a 
dorsal nerve cord which usually possesses merely a few isolated small cavities 
in the collar region only. A ventral nerve strand, more or less comparable 

to that of invertebrates, is connected with 
the dorsal cord by a nerve ring in the collar 
region. Finally, an outgrowth from the 
dorsal side of the digestive tract of the 
collar region, extending forward into the 
proboscis, may represent the notochord. 

Although hemichordates are wormlike 
in many structural details, the few chor- 
date features just mentioned suffice to in- 
dicate their problematical position between 
invertebrates and vertebrates. 

The Tornaria larvae of hemichords 
resemble corresponding stages of echino- 
derms so much that the Tornaria was for 
a long time thought to belong to some 
member of that phylum. The similarities of . these two larval types have 
served as a basis for considering the echinoderms as the invertebrate group 
most closely related to the chordates. 

Different species are found widespread in similar habitats the world 
over. For example, in 1896 during the Harriman Expedition, Dr. W. E. 
Ritter, an authority on this group of primitive animals, discovered among 
other kinds along the shore of Alaska a new genus of hemichords, probably 
the most primitive of them all, which he christened Harrimania in honor of 
the expedition. 

2. Subphylum UROCHORDATA 

The Urochordata are tunicates or ascidians, so called not only from a 
peculiar baglike outer lifeless envelope, or "tunic," with two openings but 
also because their general appearance suggests an "askidion," the Greek 
name for a primitive wine sac made of goatskin. 

These degenerate representatives of the chordates are all marine organ- 
isms, living for the most part a sedentary life. Molgula manhattensis, 
DeKay, a common "sea squirt" found along the Atlantic shore attached to 
piles and other objects, may be taken as a typical species. 

The only structures conspicuous externally are two projections each of 
which bears a terminal opening (Fig. 10). Water is continuously taken into 
the body through one of these openings (the mouth) and discharged 

Kinds of Vertebrates 




through the other (the atriopore). Internally, the mouth leads into a 
pharynx, an enlargement which occupies a considerable portion of the body 
cavity of the animal. The pharynx is permeated by numerous gill slits which 
open into the atrial cavity, a space almost 
completely surrounding the pharynx and 
opening to the outside through the atri- 
opore. Beyond the pharynx the diges- 
tive tract continues for a short distance 
as a small tube, which usually makes one 
loop and then opens into the atrial 
cavity. Located on the dorsal side of the 
pharynx, between mouth and atriopore 
regions, is a solid mass of nerve tissue, the 
neural ganglion. There is no notochord. 
Thus only one of the three main chor- 
date characteristics is present in the adult. 

It is the early life of the tunicate, 
however, that gives the real clue to its 
unmistakable relationship to true chor- 
dates, which it so little resembles when 
superficially examined. The egg develops 
into a free-swimming tadpole-like larva 
( Fig. 11), showing in its locomotor tail 
an unmistakable notochord and a single tubular dorsal nerve cord. This is 
the reason for the name of the group, "Urochordata" (uro, tail; chorda, 
notochord) . 

After the larva swims about for a time and increases in size, it settles 
down on a suitable support and enters upon its lifetime of stationary exist- 
ence. The tail, no longer needed for locomotion, is absorbed into other 
tissues of the body. The central nervous system, reduced to a simple dorsal 
ganglion, permits only an over-all contraction of the entire body in response 
to any stimulation. The pharynx, on the other hand, enlarges greatly and 
becomes the only one of the three chief chordate characteristics to remain 
well developed in the adult. The tunicate thus sacrifices most of its 
birthright of chordate characteristics, that is, notochord and tubular nerve 

Some tunicates as the result of budding are colonial in habit, living con- 
nected together in more or less dependence upon each other, a state of 
affairs not uncommon among invertebrates but which does not occur else- 
where among chordates. The compound or colonial manner of life is shown, 

Fig. 10. Internal organization of a 
simple tunicate. (After Haller.) 

Biology of the Vertebrates 

Dorsal Nerve Cord 


Stomach /** 



ii^s^Cerebral Vesicle 

-> Adhesive 

/' Heart J \ 'Atrium 
Epicardium Gill Slit Endostyle 


Mouth Gill Slit 



b Atrial Opening C 

Fig. 11. Diagrams of the metamorphosis of a tunicate larva, a, at time 
of attachment; b, at mid-point of metamorphosis; c, metamorphosis com- 
pleted. (From Hegner, College <W/ogy, copyright 1942, by permission 
of The Macmillan Company, publishers.) 

for example, by Botryllus ( Fig. 1 2 ) , a small tunicate that grows in starlike 
slippery patches over the surface of seaweeds floating in the shallow waters 
of the seashore. The incurrent openings of the several individuals in the 

Fig. 12. Botryllus, a compound tunicate. Left, section of a colony, show- 
ing the common exit; Right, surface view of two colonies surrounded 
by a gelatinous mass, growing upon the flat surface of a bit of seaweed. 

colony are separate and arranged in a circle around a common excurrent 

Other colonial tunicates, such as the beautiful transparent "chain 
salpas," are pelagic, or free-swimming in habit, forming elongated rafts of 

Kinds of Vertebrates 21 

barrel-shaped glassy-clear individuals which float near the surface of the 
ocean, usually many miles off shore. 

Most primitive of all these humble cousins of the vertebrates are the 
microscopic Appendicularia, tiny ghostlike creatures of the vast ocean waters, 
that live a life of complete freedom and do not relapse, like other tunicates, 
into unambitious sedentary degeneration, but remain larval "tadpoles" 
throughout life. Animals such as these, which never "grow up" but become 
sexually mature while in the larval stage, are said to be paedogenic. 


The Cephalochordata, so named because they have a notochord extend- 
ing into the head region, include only two genera, Branchiostoma and 
Asymmetron. This subphylum is also called Acraniota because of the absence 
of a cranium, or brain case. If these animals had a cranium they would have 
nothing to put in it, for they are brainless little creatures whose nerve cord 
fails to enlarge at its anterior end into anything like a brain. 

Amphioxus {Branchiostoma) , the most widely known member of the 
group, has had enough written about it to fill more than one ponderous 
tome. For a century or more it was looked upon as a most primitive chor- 
date, a "fish in the making," illustrating the beginnings of many great 
things. More recent work has cast considerable doubt upon this point of 
view, especially when some of the highly specialized features of amphioxus 
are taken into consideration. Yet this animal has apparently retained a 
great many relatively simple features, despite the fact that it has probably 
evolved a long way from the actual ancestors of the chordates. If we direct 
our attention away from the few specialized structures of amphioxus and 
toward its many important basic features, as we must always do in any 
type study, we find that this animal shows us the chordate plan reduced 
to almost its lowest terms. 

Amphioxus dwells in the shallow waters of tropical and semi-tropical 
seas in locations as far apart as the Bay of Naples, the coast of Peru, Japan, 
the Indian Ocean, California, the Philippines, West Indies, Australia, North 
Carolina, Hawaii, Maldive Islands, and China. This wide distribution 
suggests the great antiquity of the type which, in spite of its restricted means 
of locomotion, has had time to spread to the uttermost tropical corners of 
the earth. 

Off the coast of southern China, north of the Island of Amoy, amphioxus 
is so abundant that it forms an important food fishery which has been 
worked by the Chinese for centuries. Professor S. F. Light writes in Science 
for July 27, 1923: "Here on this little strip of coast about 400 fishermen, 

22 Biology of the Vertebrates 

using 200 small boats, are engaged for from two to four hours on the ebb 
tide of every calm day during the nine months from August to April of 
each year in dredging for amphioxus for the market. The catch averages 
about 2600 pounds, well over a ton for each calm day during the nine 
months of the fishing season and a total of hundreds of tons of amphioxus 
are taken during the year!" 

Rostrum — = 

\\ Buccal Hood 

Buccal Cirri 
Dorsal Fin Rays Photoreceptors in Nerve Cord 

Atriopore \ Ventral Fin 

\ Gonads r 

\ V 

x Metapleural Fold Ventral Fin Rays 


Dorsal Fin 

• Intestine 

\ Velur,, 
d N v v Branchial Bars _. 

Wheel Organ branchial Clefts j PharynX 

, Atriopore 


Caudal Fin 

Fig. 13. Diagrams of amphioxus. a, side view of the entire animal; b, 
side view showing internal features, (a, after Kirkaldy.) 

It is not as a source of Chinese food, however, that the chief interest in 
amphioxus lies, but because in its development and structure this little 
animal points the way to the rise of the complicated conditions found in 
higher vertebrates. It will be necessary later on, when tracing the origin 
of various vertebrate organs, to go back repeatedly to the stage presented 
by amphioxus. 

Amphioxus, or the "lancelet" as it is commonly called, is an elongated, 
semi-transparent, fishlike animal, two or three inches in length when fully 
grown, and somewhat pointed at either end, as its Greek name (amphi, 
both; oxus, sharp) indicates. In habit it is largely sedentary, lying buried 
in the sandy bottoms with its anterior end projecting. It is a poor swimmer, 
coming to rest by lying on its side when not burrowing into the sand. 

At the anterior end, guarding the mouth, is a circle of bristle-like cirri 
attached to the edge of the buccal funnel within which is a whirlpool of 
cilia, the "wheel organ," which helps to direct the microscopic food into the 
mouth (Fig. 13). 

Kinds of Vertebrates 

Running along the entire dorsal side of the body, then around the tail 
end and forward on the ventral side is a continuous median fin. This fold 
of the skin, as it passes around the tail end, expands to form a conspicuous 
caudal fin. As the fold reaches the re- 
gion of the atrial pore, anterior to the 
anal opening, it divides like a letter Y, 
and extends forward in ventro-lateral 
folds on either side of the body. It is 
out of persisting parts of similar folds, 
which are laid down temporarily in the 
embryos of fishes, that fins are formed 
(Fig. 14). 

Of all known lower chordates, am- 
phioxus shows the best development of 
diagnostic chordate features. Extending 
the entire length of the body just dorsal 
to the digestive tract is a notochord 
( Fig. 15 ) . Above the latter is the dorsal, 

hollow central nervous system which, instead of enlarging anteriorly into a 
brain, actually is more slender near its anterior end than throughout most of 
its length. Not far from the mouth, the digestive tract becomes greatly en- 

Fig. 14. Diagrams of the phylogene- 
tic development of unpaired and paired 
fins according to the fin-fold theory. 
a, primitive stage, with continuous fins; 
B, differentiated stage, with fins remain- 
ing after partial absorption of the 
primitive continuous fins. (After Wie- 
dersheim. ) 

Myotomes «- 

Ventral , 

Nerve Root 

Dorsal Fin Ray 
!}„•-■ Dorsal Nerve Root 
^^•- Nerve Cord 
V.-- Notochord 

•g<£f Dorsal Aorta 

l^C" EpipharyngealGroove 

Dorso-Lateral Coelom 
~r — Atrial Cavity 
g^^m- Liver 
"hHL-- Ovary 

SS^r — Cardinal Vein 

?A^ — Transverse Muscle 
^-liKp&l-Branchial Bars 


—Ventral Aorta 

Metapleural Fold 
Fig. 15. Cross section of amphioxus through posterior part of pharynx. 

2 4 Biology of the Vertebrates 

larged into a pharynx, the wall of which is penetrated by 26 pairs of long, 
narrow gill slits. Beyond the pharynx an uncoiled intestine of small diameter 
leads to an anus located on the left side of the mid-ventral region a short 
distance from the posterior end of the body. Thus, a post-anal tail is 

Cilia of the wheel organ and pharyngeal wall ordinarily maintain a 
continuous flow of water through the pharynx and gill slits. This water not 
only acts as a respiratory current but also brings in numerous small organ- 
isms which serve as food. 

Extending the entire length of the pharynx are two grooves, a ventral 
endostyle and a dorsal epipharyngeal groove. Both are heavily ciliated. In 
the endostyle numerous glandular cells secrete mucus which is carried 
anteriorly by ciliary action. Other cilia transport small streams of the mucus 
dorsally along the inner surfaces of the gill-bars into the epipharyngeal 
groove. In the latter the cilia carry the mucus posteriorly into the intestine. 
Microscopic organisms brought into the body by the so-called respiratory 
current are trapped by the mucus and eventually carried into the intestine. 
The lining of the latter is also ciliated (an invertebrate characteristic). 

As in tunicates an atrial cavity, a specialized feature probably associated 
with the animal's particular habits of life, surrounds the pharynx. Water, 
passing from the gill slits into this atrial cavity, is discharged to the outside 
through a relatively small atriopore located about two-thirds of the way 
back on the ventral side of the body. Thus the delicate respiratory mem- 
branes are removed from the surface of the body where they would be seri- 
ously damaged, if not destroyed, as the animal pushes its way through the 

Growing out ventrally from the intestine is a blind sac, the liver diver- 
ticulum, lined with glandular epithelium and supplied with a network of 
blood vessels that represents the beginnings of a vertebrate hepatic portal 
system, since the blood from the food tube has to pass through this capillary 
strainer in the liver diverticulum before going forward to the gills and 
thence over the body. 

There is no heart present, but a pulsating ventral blood vessel, lying 
below the pharynx, is larger than the other blood vessels and is prophetic 
of the future ventral heart of vertebrates. 

The gonads are segmental and can be seen from the outside through the 
transparent body wall. The germ cells are discharged directly into the atrial 
cavity from which they are carried to the outside by the respiratory current. 
The sexes are separate. 

Unlike urochordates and hcmichordates, cephalochordates probably 

Kinds of Vertebrates 25 

represent simple advancing forms, and not ones whose simplicity is the result 
of degradation. 


All vertebrates exhibit at least traces of a backbone, or vertebral column, 
which begins its development around the embryonic notochord and nerve 
cord. They also have a distinct head, including a brain, olfactory organs, 
eyes, internal ears, and a skull {cranium) associated both with these struc- 
tures and with the anterior portion of the digestive tract. In contrast with 
the situation in amphioxus, the notochord of vertebrates extends forward 
only as far as the level of the mid-brain. The pumping organ of the circula- 
tory system is a ventrally located heart divided into several chambers, at 
least one of which is highly muscular. 

A parapod of Nereis, a leg of a lobster, or any other invertebrate ap- 
pendage arises from a single segment. Vertebrate appendages, however, are 
formed from several segments, sometimes many. The arm of man receives 
muscles and nerves from seven different segments. Some of the fishes (notably 
the skates and rays ) have anterior paired fins arising from a score or more of 
segments. Many invertebrates have a pair of appendages on almost every 
segment of the body, while others have only a few pairs, for example, four 
in spiders and three in insects. Vertebrates alone are limited to two pairs. 
This is a reasonable and logical arrangement because four legs furnish the 
most efficient and economical support for any sort of an elongated bilaterally 
symmetrical object, such as a table-top, a bedstead, or a horse. Certain 
Devonian sharks are an exception to this rule of never more than two pairs 
of paired appendages. Some vertebrates, it is true, have lost one or the other 
of their pairs of appendages. Snakes, for example, have lost both pairs. 


The lamprey eels and the hagfishes show a great advance in the verte- 
brate series over the forms thus far considered, but are so different from 
other fishes that they have been placed in a class by themselves (Fig. 16). 
They are called "cyclostomes" {cyclo, round; stoma, mouth) for the reason 
that, instead of typical vertebrate jaws, they have round jawless sucker-like 
mouths by which they attach themselves to the sides of fishes or to other 
objects. They are more committed to a life of parisitism than any other 
vertebrate, and when one fastens to a fish, it may rasp a hole with its filelike 
horny teeth that are attached to its muscular "tongue," quite through the 
skin of the unfortunate host whose death eventually results. 

Cyclostomes are distinguished chiefly by the absence of certain customary 

26 Biology of the Vertebrates 

fishlike structures. They are not only jawless, but are also without paired 
fins, scales, swim bladder, cloaca, oviducts, true mesodermal teeth, vertebral 
centra, ribs, or bones of any kind. They have only one external nasal open- 
ing instead of the pair present in most vertebrates. They breathe by means 
of internal gills. All are eel-like in shape, but are not to be confused with 
true eels, which have a bony skeleton including jaws, instead of a skeleton 
consisting principally of a persistent notochord. 

Fig. 16. CYCLOSTOMES. a, Bdellostoma. Light apertures along entire 
length are mucous pits; larger, dark apertures are branchial openings. 
b, Myxine. Left common branchial aperture is at *. c, Petromyzon. (From 
Hegner, College ^oology, copyright 1942, and Dean, Fishes Living and 
Fossil, copyright 1895, by permission of The Macmillan Company, pub- 
lishers of both books.) 

The larval lamprey is so different from the adult that it was formerly 
assigned to a distinct genus and named Ammocoetes before its whole life 
history was known. In its early stages an endostyle-like groove is present in 
the pharynx. Later this groove pinches off and gives rise to a subpharyngeal 
gland believed by some to be homologous with the thyroid gland of other 

Cyclostomes are usually marine in habitat although they frequent fresh 
waters to breed, and some species are permanent fresh-water inhabitants. 

Lampreys scoop out a nest in the sandy bottom of a flowing stream in 
which to deposit their eggs, meanwhile fastening themselves by means of 
their suctorial mouths to a stone in order not to be carried down stream. 
This habit has given rise to their genus name of Petromyzon {petros, rock; 
myzon, sucker) . 

Hagfishes are particularly slippery creatures, often producing so much 
mucus when uncomfortably confined in a bucket of stagnant water that the 
water is thickened into a gluey mass. Linnaeus describes Myxine glutinosa, a 

Kinds of Vertebrates 27 

European hagfish, in compact Latin : "Intrat et devorat pisces; aquam 
in gluten mutat." 

Lampreys have for a long time been used for food, particularly in 
Europe. Cicero in one of his orations bewailed the tendency of young spend- 
thrift Romans of his day, who as in every generation were regarded by their 
elders as going to the dogs, because they spent their time reveling at night 
and "feasting upon such delicacies as lampreys." 

Only a few genera are reported as belonging to North America. The 
best known American and European genera are: the hagfishes Myxine 
(Atlantic) and Bdellostoma (Pacific) ; the lampreys Petromyzon, found in 
both salt and fresh water, Ichthyomyzon, of the Great Lakes and Mississippi 
Valley; and the brook lamprey, Lampetra. 

Closely related to the Cyclostomes are the small jawless armored fishes 
(Fig. 17) known collectively as the Ostracoderms {ostraco, shell; derm, 
skin). Because of the absence of jaws these 
two groups are sometimes classified together 
as the Agnatha (a, without; gnath, jaw). 
In the ostracoderms, long since extinct in- 
habitants of fresh waters, thick bony plates 

developed in the connective tissue mem- Fi §- 17 - A restoratlon of an ar - 
, r 1 1 ■ mi i i mored ostracoderm Ptcrichthys, 

branes of the skin. These plates were large from the Devonian in Scotland. 

and immovably joined to one another in (After Traquair.) 
the head region to form a shell-like struc- 
ture over this part of the fish. On the rest of the body the scales were smaller, 
thus permitting the freedom of movement necessary for locomotion. 

2. Class PISCES 

When Pliny (23-79 a.d.) wrote his Historia Naturalis, he enumerated 
94 kinds of fishes then known to the Roman world. In 1735 Linnaeus listed 
478. Today it is estimated that at least 25,000 species are known. 

The importance of this class of vertebrates is brought out by the fact 
that the waters of the earth, of which fishes are the dominant inhabitants, 
occupy four times the area of the land masses combined. Although primeval 
fishes lived in fresh water before the oceans became salty, only approximately 
one species out of twelve now lives in inland fresh waters, while ten are 
distributed in open oceans or coastal regions and one is a dweller in the 
darkness of the deep sea. 

Fishes vary enormously in size all the way from the dainty Mistichthys 
luzonensis of the Philippines, which is about half an inch long when full 
grown, to the colossal shark, Rhinodon typicum that, according to Haempel, 

2 g Biology of the Vertebrates 

has been known to attain a length of 65 feet. There is likewise a remarkable 
range in the body form of fishes, as indicated by representatives shown in 
outline in Fig. 18. 

Fig. 18. Some unusual styles of fishes, a, Hippocampus; b, Stcrnoptyx; 
c, Serrivomer; d, Sebastopristis; E, Tetraodon; f, Chactodon; g, Anti- 
gonia. (a, after Hilzheimer; b and c, after Goode and Bean; d to o, 
Hawaiian fishes.) 

Kinds of Vertebrates 29 

The key for understanding the fish plan is to be sought in the adaptation 
of these animals to life in water. For example, their chief respiratory organs 
are internal gills. The tail, which frequently is more extensive than all the 
rest of the body, is the main organ of propulsion, entirely effective in scull- 
ing through the resistant medium of water, but quite useless in thin air. 
The other unpaired fins as well as the paired ones, which are homologous 
with our arms and legs, are used for steering and to prevent rotation of the 
body on its long axis, which would otherwise occur as the result of the 
sweeping strokes of the tail. 

As in all higher vertebrates, skeletal rings, the beginnings of the centra 
of vertebrae, encircle the embryonic notochord of most fishes. During devel- 
opment, these rings gradually fill in to constrict the enclosed notochord con- 
siderably in fishes, and obliterate it almost completely in higher forms. 
Well-developed skulls, consisting of elements associated with both brain and 
gills, first occur in fishes. In some of the lowest fishes (Cladoselache, for 
example) it can be seen that biting jaws arise as modifications of the most 
anterior of the gill supports. Because of the presence of jaws, fishes and all 
higher vertebrates may be grouped together as Gnathostomata (gnath, jaw; 
stoma, mouth). 

As far as living fishes are concerned, the Pisces may be divided into two 
subclasses : 

a. Chondrichthyes (chondro, cartilage; ichthy, fish) 

b. Osteichthyes {osteo, bone) 


The cartilaginous fishes are so called because their skeletons are entirely 
of cartilage, the material commonly called gristle. There is considerable 
evidence that the absence of bone from these animals may be a degenerate 
condition. Many believe that the ancestors of all the fishes were forms simi- 
lar to the Ostracoderms. 

None of the cartilaginous fishes has an air bladder. The intestinal sur- 
face available for both secretion and absorption is considerably increased 
by a spiral valve (Fig. 212), a tightly coiled fold of the inner part of the 
intestinal wall. 

The Orders of cartilaginous fishes include: (1) cladoselachii; (2) 


( 1 ) claloselachians.- — The long extinct, primitive sharks belonging 
to the genus Cladoselache possessed many interesting features (Fig. 19). 
The jaws of these ancient sharks were clearly in line with the gill arches 


Biology of the Vertebrates 

which lay immediately behind them. In these animals it is even more obvious 
than in modern sharks that the jaws represent the first pair of gill arches. 
The ventral halves of the first arches have become the lower jaw while the 
dorsal halves, tilted forward, have become the upper jaw. A rudimentary 
vertebral column, consisting of an unconstricted notochord onto which were 
laced vertebral arches, was present. 

Fig. 19. Restoration of Cladoselache fyleri, lateral and ventral views. 
(From Parker" and Haswell, A Text-Book of Z°°l°gy> copyright 1940, 
by permission of The Macmillan Company, publishers.) 

The fins of Cladoselache had broad attachments to the body, a condi- 
tion which may be cited in support of the fin-fold theory of origin of 
appendages. According to this theory (Fig. 14) the first appendages were a 
set of fin folds, including : ( 1 ) a continuous median fin extending along 
the dorsal side of the body, around the tail and forward along the ventral 
side of the body as far as the anus; (2) a pair of ventro-lateral folds extend- 
ing from the anal region forward to points immediately behind the last gill 
openings. Individual fins may have been derived from the dropping out of 
portions of these folds and the retention of other portions. The primitive 
individual fins would therefore presumably have broad attachments, such 
as those of Cladoselache. Then a later development would be the narrowing 
of the base from which the fins fan out, as in many modern fishes. 

(2) elasmobranciis (gristle fishes). — Most living cartilaginous fishes 
are elasmobranchs, including the sharks (Suborder Selachii) and the skates 
and rays (Suborder Batoidea). The sharks, including dogfishes and their 

Kinds of Vertebrates 

3 1 

allies (Fig. 20a and c), all graceful, elongated, streamlined animals, actively 
prey upon other fishes. The rays, with the skates, torpedoes, guitar fishes, 
and their allies, on the contrary, are flattened sluggish bottom feeders (Fig. 
20b). In the Batoidea the pectoral fins, considerably enlarged, are the 
organs of propulsion. 

A. Squalus, 
Spiny Dogfish 

B. Raia, 
A Skafe 

Fig. 20. ELASMOBRANCHS. (a, after Dean; b, after Goode and 
Bean; c, after Boas.) 

An elasmobranch may be distinguished by the following characteristics : 
( 1 ) a large mouth ventral in position rather than terminal ; ( 2 ) separate 
openings of the gill slits (usually five pairs) not concealed as in other fishes 
behind a gill cover; (3) dorso-ventrally flattened head; (4) placoid scales, 
resembling tiny thumb tacks embedded, point up, in the skin without shin- 
gling over each other like ordinary fish scales; (5) tail heterocercal, that is, 
with the vertebral column extending into the dorsal part of the caudal fin ; 
(6) paired pelvic fins modified into "clasping organs" in the male, permit- 
ting internal fertilization of the egg; and (7) the production of a relatively 
small number of eggs that in some species are covered with shells and then 
laid, while in others they develop into some size within the oviduct of the 
female before the young are born alive. 

(3) holocephalians. — Biological interest in the uncommon and 
bizarre "elephant fishes," or "spook fishes," centers in their intermediate 
anatomical position between elasmobranchs and other fishes. They differ 
from elasmobranchs in a number of respects among which are : ( 1 ) small 
size of mouth ; ( 2 ) scarcity of scales ; and ( 3 ) presence of a gill cover, or 
operculum, a fold of the body wall extending over the gill slits but with a 
free posterior margin under which the respiratory current leaves the body. 

32 Biology of the Vertebrates 

The name holocephali (holos, whole; cephalon, head) is given to them 
because the upper jaw is immovably fused with the cranium after the man- 
ner of higher forms, instead of being indirectly suspended by means of 
ligaments and cartilages as in elasmobranchs. 

Of the three living genera, Chimaera (Fig. 21) is found on the Pacific 
coast of North America, as well as on the coasts of Europe and Japan and 
at the Cape of Good Hope. 

Fig. 21. Chimaera, a holocephalian fish. (From Newman, The Phylum 
Chordata, copyright 1939, by permission of The Macmillan Company, 
publishers. After Bridge.) 


The modern bony fishes are grouped into the subclass Osteichthyes. At 
least a part of the skeleton is ossified. Over the skull and pectoral girdle 
region, investing bony plates are laid down in the dermal portion of the 
skin. The gills are protected by an opercular fold which has investing bony 
supports. No claspers are present. Air bladders, outgrowths from the 
pharynx, are present in all bony fish except a few teleosts. 

There are three orders of bony fishes: ( 1 ) crossopterygii; (2) dipnoi; 
and (3) actinopterygii. 

(1) crossopterygians.— The Crossopterygii are commonly called 
lobe-finned fishes because each of their paired fins has a thick, fleshy basal 
lobe. The skeletal support of this fin consists of a single basal element fol- 
lowed by two bones beyond which are a number of irregular small bones. 
This condition suggests the plan of the land type of appendage. Other fea- 
tures of this group are: ( 1 ) air bladders serving as lungs; (2) a spiral valve 
in the intestine; and (3) nasal cavities opening into the roof of the mouth 
cavity. These fish are believed to be the ancestors of land animals. 

All members of this group are extinct with the exception of Latimeria 
which is represented by one specimen, about 5 feet long, brought up in a 
net off the coast of South Africa in 1939. Unfortunately the internal parts 
of this specimen were destroyed before its zoological significance became 

Kinds of Vertebrates 


(2) dipnoans. — As the word dipnoi (di, two; pneum, air) suggests, 
the "lungfishes" have two ways of breathing, that is, by means of gills and 
by a modified swim bladder, or lung (Fig. 330). 

They are semitropical, freshwater, primi- 
tive fishes dwelling only in countries where 
wet and dry seasons alternate, instead of win- 
ter and summer. During the dry season the 
African and South American species bury 
themselves in muddy pits (Fig. 22) and 
breathe air like land animals, but when the 
rainy season supplies an abundance of water 
they swim about fish-fashion, breathing 
through gills. This passive method of bridg- 
ing over a season of unfavorable dryness is 
termed aestivation, corresponding to hiber- 
nation, or the habit of animals like bears, 
bats, and woodchucks, which retire from 
activity during the cold winter season. 

Other interesting features of these fishes 
are: (1) a spiral valve; (2) nasal passages 
opening into the roof of the mouth cavity; 
and (3) fleshy portions in the paired fins. 

There are no lungfishes in North America, and only three living genera 
are known anywhere. These are Protopterus in Africa; Neoceratodus in 
Australia (Fig. 23); and Lepidosiren in Brazil (Fig. 24). They possess 

Fig. 22. The African lungfish, 
Protopterus, undergoing aestiva- 
tion in the mud during the dry 
season. (After Hilzheimer.) 

Fig. 23. Australian lungfish, Neoceratodus. (After Bridge.) 

much interest for the zoologist not only by reason of their peculiar habits 
and rarity, but also because of the intermediate combination of their ana- 
tomical characters which puts them in a class by themselves. 

Fig. 24. Brazilian lungfish, Lepidosiren. (After Lankester.) 

(3) ACTiNOPTERYGiANS. — Included among the ray-finned fishes {actin, 
ray; pteryg, fin) are the several groups possessing fins with no basal lobes 


Biology of the Vertebrates 

but with their entire free parts membranous and supported by slender 
dermal rays. Their nasal pits do not communicate with the mouth cavity. 
This group includes 3 sub-orders: Chondrostei, Holostei and Teleostei. 

(a) Chondrosteans. — Those ray-finned fishes which have superficial, 
dermal, bony plates but an inner skeleton composed essentially of cartilage 
are called Chondrostei (chondro, cartilage; osteo, bone). These animals 
also possess a spiral valve and ganoid scales. Among the living members of 
this group are Polypterus, Acipenser, Scaphirhynchus, and Polyodon 
(Fig. 25). 

A, Polyodon 

B, Acipenser 

C, Polypterus 

Fig. 25. CHONDROSTEI. a, Polyodon; b, Acipenser \ c, Polypterus. (a, 
after Boas; b and c after Bridge.) 

Polypterus {poly, many; pter, fin), so named because it has a row of 
small dorsal fins, is found in the tropical fresh waters of Africa. A well- 
developed pair of swim bladders is connected with the ventral side of the 
pharynx. A secondarily acquired basal lobe in the pectoral fins has led some 
taxonomists to include this animal among the Crossopterygii. 

Acipenser and Scaphirhynchus are the sturgeons of the rivers and lakes 
of the Northern Hemisphere. Caviar, the delicacy made famous by the 
Russians, is the eggs of sturgeons. Like the sharks these animals have a ros- 
trum, a ventral mouth, and a heterocercal tail. The scales are reduced in 
number. In Polyodon, the Mississippi spoon-bill, the rostrum is greatly 
enlarged into a dorso-ventrally flattened, paddle-like structure and the skin 
is almost devoid of scales. The sturgeons and spoon-bills have a persistent, 
unconstricted notochord onto which cartilaginous arches are laced by con- 
nective tissue. 

Kinds of Vertebrates 


A, Lepidosteus 

B, Amia 
Fig. 26. HOLOSTEI. a, Lepidosteus; 
Amia. (a, after Goode; b, after Bridge.) 

(b) Holosteans. — Those ray-finned fishes which have superficial, der- 
mal, bony plates and also a rather completely ossified inner replacing skele- 
ton are called Holostei (holo, 

complete; osteo, bone). Other 
features are : ( 1 ) a reduced spi- 
ral valve; (2) bony vertebrae; 
and (3) ganoid scales. There are 
only two living genera of this 
group. Lepidosteus (Fig. 26a), 
the garpike of the fresh waters of 
North America, possesses heavy, 
shiny, ganoid scales which fit to- 
gether edge to edge, rarely over- 
lapping, like the tiles around a 
fireplace. Amia (Fig. 26b), the 

freshwater dogfish of the United States, has overlapping, cycloid scales on 
the trunk and tail regions, ganoid scales being limited to the head region. 

The Chondrostei and Holostei are frequently referred to as the Ganoid 
fishes, with the former being called the cartilaginous ganoids and the latter, 
the bony ganoids. 

( c ) Teleosts. — The Teleostei ( tele, entire ; osteo, bone ) are the true bony 
fishes. They constitute probably 90 per cent of all known kinds of fishes. 
They have an almost completely bony skeleton ; no spiral valve ; thin, round 
overlapping scales on most species; and a homocercal tail. A few unusual 
representatives out of the great variety of teleosts are suggested by outline 
sketches in Figure 18. Some of the best known teleosts are herring, salmon, 
trout, eels, catfish, mackerel, carps, pickerel, perch, flounders, and cod. Of 
the carps alone there are approximately 1000 species. 

3. Class AMPHIBIA 

The clumsy amphibians (amphi, both; bios, life), like Dr. Jekyl and 
Mr. Hyde, typically lead a double life, that is, first in the water and then on 
the land. As a class they bridge one of the greatest gaps in vertebrate evolu- 
tion. The result of this ambitious attempt is that they present a medley of 
makeshift adaptations, which, while leaving them still a long way from 
vertebrate perfection, nevertheless make them of particular interest to the 
student of comparative biology. 

. Along with access to two different habitats they must encounter a double 
set of enemies, but at the same time they have two avenues of escape, land 
and water. 

■$6 Biology of the Vertebrates 

The earliest record of any walking vertebrate is a single illuminating 

footprint, as unique as that which Robinson Crusoe found in the sand of his 

island beach, left in the Upper Devonian shales of Pennsylvania, and now 

to be seen in the Peabody Museum of Yale University. The three-toed fossil 

ancestor of the amphibians that made this famous footprint 

has been christened Thinopus (Fig. 27). 

Among the dual adjustments that any animal living a part 
of the time submerged in water and a part of the time upon 
land must make, are those associated with locomotion and pro- 
tection against desiccation. 
Fie. 27. Thi- ^ n wat er an elongated fishlike body, propelled by a muscu- 

nopus, the lar tail, has proved to be the most efficient mechanism for 
oldest known locomotion. On land such an arrangement would be out of the 
(After question, because in thin air a propeller that could develop 
Thorn.) power enough to move the body at all would necessitate so 
great an addition of heavy muscles as to defeat the possibility of 
aerial or terrestrial locomotion. When the weight of the body is no longer 
supported by a surrounding medium of water, the two pairs of appendages 
become modified into legs which act as levers ,to lift the body away from 
frictional contact with the ground. It is quite possible to equip such levers 
with adequate muscles without adding excessively to the entire weight to be 

Amphibians, like all land animals, are therefore tetrapods (tetra, four; 
pod, foot ) . At best, however, they are not particularly successful at locomo- 
tion on land. 

The legs of salamanders, ridiculously small and inadequate, cannot even 
lift the body from the ground, for instead of being directed ventrally as 
supports, they project laterally, and can be used only slightly in poking the 
wriggling body along over the ground. Even in frogs and toads, where 
amphibian legs reach their highest development, such locomotor appendages 
are so inefficiently anchored to a single vertebra of the supporting backbone 
that these animals cannot bear their weight upon them in the sustained man- 
ner necessary for standing or walking, and can progress only by the momen- 
tary exertion of hopping or jumping. When not locomoting they never stand 
but always sit. 

The problem of protection against desiccation arises from the fact that 
the air, usually far from saturated with moisture, takes up water rapidly 
from any moist surface. The cells of the body, as complex chemical labora- 
tories, must contain considerable water to permit metabolic processes to go 
on. One phase of this problem is that of breathing. 

Kinds of Vertebrates yj 

The essential feature of every breathing device is a delicate wall, or 
membrane, separating blood from the oxygen-containing medium. In sub- 
merged animals, gills, thin-walled structures containing blood and hanging 
in water in which oxygen is dissolved, fulfil this condition. When exposed 
for any considerable time to free air air, however, thin-walled gills dry up 
and collapse, making gaseous exchange no longer possible. In animals 
breathing air, lungs are developed. These are enclosed sacs in which an 
enormous expanse of capillary blood vessels, behind very thin moist walls, 
is exposed to air. The drying up of this kind of wall is prevented because 
openings from the lungs to the outside through which evaporation can occur 
are relatively small, and also the air on the way to the lungs may be mois- 
tened in the respiratory passages. 

Amphibians not only utilize gills and primitive lungs in respiration but 
they also exchange gases to a very large extent directly through the skin, 
which, so long as it is kept moist, may remain thin enough to serve as the 
membrane separating the blood from the surrounding air. Consequently 
these animals can live only in moist places. On the other hand, higher land 
animals, in which an efficient pulmonary system is formed, are not restricted, 
because they develop a thick, relatively dry integument which is resistant 
to desiccation. Relatively inefficient respiratory organs, together with vari- 
ous other anatomical handicaps, prevent amphibians from maintaining a 
body temperature independent of that of their surroundings. Since they can 
never be active when it is cold, they are excluded entirely from frigid 
regions, while in temperate zones, where winter condemns them to hiberna- 
tion, they are able to exercise seasonal activity for only a part of the time. 

The problem of desiccation is also involved in the breeding habits of 
amphibians because they have not made the changes required of true land 
animals. In the case of reptiles, birds, and mammals, the embryo very early 
becomes enclosed in an amnion (Fig. 34), a liquid-filled sac produced by 
the embryo itself. As this amnion persists until hatching or birth, the embryo 
is protected in liquid throughout its development. No amnion is produced 
by embryos of lower vertebrates including the Amphibia. The latter must, 
therefore, go back to the water to breed in most cases. A few avoid this 
requirement by various means (Fig. 28). Some tree frogs lay their eggs in 
rain-filled holes in trees or pouches formed by folding leaves. Others carry 
their eggs about in various ways in pouches or pits on the body. A few lay 
their eggs in very moist places, beneath logs or stones. 

Furthermore, the metamorphosis of such an amphibian as a frog or a 
toad, necessitated by its emergence from water to land, works profound 
changes both in its structure and in its feeding habits. During its lifetime the 


Biology of the Vertebrates 

Fig. 28. Care of young among Amphibia, a, nests of Hyla faber built of 
mud. (After Wiedersheim.) b, Rhacophorus schlegeli of Japan in am- 
plexation within a hole in a muddy bank of a stream. The eggs are 
deposited in a mass of foamy mucus, and washed out into the stream 
below by the rain. (After Wiedersheim.) c, a Gymnophiona, Ichthyophis 
glutinosa, guarding its eggs. (After P. & F. Sarasin.) d, the "nurse frog" 
of Europe, Alytes obstetricans. The male carries strings of eggs attached 
to its hind legs. (After Cope.) e, Hyla goeldii, with eggs glued to back 
of female. (After Ihring.) f, Nototrema pygmaeum, female with dorsal 
brood pouch containing only a few large eggs. (After Brandes and 
Schoenichen.) g, South American toad, Pipa dorsigera, the eggs of which 
are deposited in pits upon the spongy back of the female by means of 
the everted cloaca, that serves as ovipositor. They remain in the back 
until the metamorphosis of the tadpoles into tiny toads. (After Bartlett.) 
h, Rhinoderma darwini, section of head region showing eggs carried 
within the vocal sac, the position of which when inflated is represented 
by the dotted circle. (After Wiedersheim.) i, Arthroleptis seychellensis, 
the tadpoles of which are transported to fresh pools by being attached 
to the back of the male. (After Brauer.) 

toad changes its diet six times. While in the egg it absorbs the yolk stored 
within; then, upon hatching, it develops a temporary mouth and eats its 
way out through the jelly of the egg envelopes ; next it becomes a free tad- 
pole, swimming about by means of a fishlike tail, and feeding mainly upon 
vegetation found in the water. With the growing pains of its coming trans- 
formation it loses its temporary mouth and along with it the appetite for 
vegetable food. Tiny legs and arms now sprout out through slits in the skin, 

Kinds of Vertebrates 39 

and instead of swimming about much the little toad sits quietly in its shirt- 
sleeves and devotes itself introspectively to the task of making its tail sub- 
stance over into more useful parts of the body. By this time cold weather is 
approaching and it goes into a long winter retirement during which its only 
food is a pair of fat bodies, peculiar nutritive storage organs attached near 
the gonads in the body cavity and provided to meet the intervening demands 
of hibernation. With the warmth of returning spring the young toad, mean- 
while equipped with a new mouth and a marvelous lassoing tongue, emerges 
into a life of carnivorous activity upon land, catching slugs and insects for 
a living. 

It is not very difficult to recognize amphibians, although by the uncriti- 
cal they are sometimes confused with reptiles. It was Brogniart who in 1 804 
separated the Amphibia from the Reptilia as independent classes, because 
the former have fingers and toes without claws; a scaleless skin; two occipi- 
tal condyles on which the skull articulates with the first vertebra; hind legs 
attached to the vertebral column by a single sacral vertebra; and young 
which breathe by means of gills. Reptiles, on the contrary, have claws; scaly 
skin ; a single occipital condyle ; two sacral vertebrae ; and young which 
never resort to gill-breathing. In common with the reptiles, amphibians 
exhibit certain features not found in fishes, including: (1) modification of 
paired appendages into legs ; ( 2 ) modification of swim-bladder region into 
lungs; (3) two completely separate auricles in the heart; (4) development 
of a middle ear cavity with a bone to transmit vibrations from external tym- 
panic membranes to internal ear. 

Living amphibians of approximately 2000 species may be disposed of 
in three orders: gymnophiona, urodela, and anura. To these should be 
added the extinct stegocephali, or "Labyrinthodonts," including some 
200 species so far discovered. 

( 1 ) stegocephalians.— The stegocephalians, whose ancestry has been 
traced by some biologists back to the lobe-finned crossopterygians, bear a 
resemblance to living amphibians, although they disappeared from the earth 
before any known representatives of modern amphibians made their appear- 
ance. This fact, as pointed out by Jaekel, is embarrassing when one seeks 
to establish them as the undoubted ancestors of the amphibians of today. 
The gap separating these similar groups of animals may sometime be filled 
by the discovery of intermediate fossil forms. 

The stegocephals flourished in the swampy Carboniferous Period, along 
with giant rushes, mosses, and tree ferns, before there were any birds, insects, 
or flowers, and when the warm steamy sluggish atmosphere was probably 


Biology of the Vertebrates 

heavily charged with an abundance of carbon dioxide. They have the dis- 
tinction of being the earliest four-footed air-breathers on the earth, large 
awkward creatures with an armor of scales on the head ( Fig. 29 ) , and with 
a brain so small that it could have been easily pulled out through the 
foramen magnum at the back of the skull. The reason for the name "steg- 
ocephali" (stegos, roof; cephalon, head) is that the size of the skull by no 
means indicates the cranial capacity of these stupid beasts, there being a 
large attic-like space roofed in above the brain-case itself. 

C ' D 

Fig. 29. Stegocephalians, extinct primitive amphibians. A, Amphiba- 
mus; b, Diplocaulus; c, Cacops; d, Eryops. (From Newman, The Phy- 
lum Chordata, copyright 1939, by permission of The Macmillan Com- 
pany, publishers. After Osborn. ) 

(2) gymnophiona. — Of modern amphibians the naked, legless Gym- 
nophiona are the least familiar. They include about 50 tropical species from 

Africa, South America, Ceylon, and In- 
dia, which burrow in the ground. No 
fossils are known in this order. 

In appearance these animals resemble 
worms (Figs. 30 and 28c), although pos- 
sessing a vertebral column of as many as 
250 vertebrae, and numerous other char- 
acteristics which place them unmistak- 
ably with the Amphibia. 
As examples of the order may be cited the "blind caecilian," Caecilia, 
of West Africa, and Ichthyophis, of Ceylon. 

(3) urodela (caudata) . — Urodeles are newts, mud puppies, and sala- 
manders. They retain their tadpole-like tails throughout life, and many of 
them never emerge from existence in water, although some do so, living 

Fig. 30. A tropical wormlike am- 
phibian, Caecilia, partly out of its 
burrow. (After P. & F. Sarasin.) 

Kinds of Vertebrates 

4 1 

under stones, rotten logs, and in damp situations generally. Most of them 
undergo a metamorphosis during which the gills are lost but the tail retained. 
Some, however, retain their gills and spend their entire lives in the water. 
Necturus ( Fig. 31), the mud puppy of the Mississippi River drainage sys- 
tem, is one of these perennibranchs, so named because of their persistent 

Fig. 31. Urodeles. A, Cryptobranchus; b, Amphiuma; c, Necturus. 
(From Newman, The Phylum Chordata, copyright 1939, by permission 
of The Macmillan Company, publishers. After Lydekker.) 

gills (perenni, lasting through the year; branch, gill). Other urodeles are: 
Amphiuma, the so-called "Congo Snake" of the Southern United States; 
Cryptobranchus, the "hellbender" of the Ohio River Valley; Ambly- 
stoma, the commonest American salamander and one which has been exten- 
sively used for research in experimental embryology; and Triton (Fig. 32), 
which includes both European and American species. 

The "black salamander," Salamandra atra, of Switzerland, is particu- 
larly adapted to life in the cold tumultuous waters of the high glacial streams 

4 2 

Biology of the Vertebrates 

where it lives. Its eggs, only two of which develop at one time, are protected 
and prevented from being washed away by remaining within the oviduct of 
the mother, where they hatch and pass through their entire tadpole-hood, 
reaching a size large enough to insure their safety as independent animals 
before they are born into the world. 

Fig. 32. Triton cristatus, a urodele. a, female; b, male in nuptial dress. 
(From Newman, The Phylum Chordata, copyright 1939, by permission 
of The Macmillan Company, publishers. After Gadow.) 

(4) anura (salientia). — Anurans are frogs, toads, and hylas, that 
lose their tails before becoming adults. They are the first truly vocal verte- 
brates. Other amphibians as well as fishes, with the exception of those which 
make sounds by means of their air-bladders, are silent. These quaint and 
cheerful singers, moreover, are the first animals with a lacrimal gland, and 
so are enabled to wink and to shed tears. This does not mean that trouble 
enters the world for the first time with them, since tears and blinking are 
primary adaptations for keeping the eyes of land animals clean, rather than 
serving as machinery for the expression of the emotions. 

The most populous genus of toads in North America is Bufo. There are 
many genera in other parts of the world, particularly in South America 
which has a greater variety of amphibians than any other continent. 

Some years ago the Department of Agriculture in Washington, in a 
pamphlet on the economic value of the common toad, Bufo americanus, 
estimated that a single individual in a garden was worth $19.44 as an insect 
destroyer. With the changed value of the dollar and the added cost of living, 
this precise governmental figure should no doubt now be increased. Scaphio- 
pus is the American spadefoot toad. Bombinator of Europe is a famous 

Kinds of Vertebrates ai 

scarlet-bellied toad that escapes attacks of storks because its warning color 
is associated with a bad taste, as storks have discovered. 

The commonest genus of frogs is Rana, sev- 
eral species of which are found in Europe and 
North America. Xenopus of Africa, and Pipa, of 
South America, are anurans of particular ana- 
tomical interest, as will appear later. 

The little tree frogs have adhesive discs at the 

ends of their fingers and toes that enable them 

when they leave the water to climb trees where 

they conceal themselves, persistently sending out 

their ventriloquistic calls. American genera are: 

the "cricket frog," Acris; the "swamp tree frog," 

Pseudacris ; and the common tree frog, Hyla „. „„ „ , 

' &' / Fig. 33. A tree frog, Hyla. 

(rig- 33). (After Dickerson.) 

Many amphibians are remarkable for the 
ways in which they care for their eggs and young. Some examples are illus- 
trated by the sketches in Figure 28. 

4. Class REPTILIA 

There are about 5500 species of living reptiles, of which over 300 are 
found in the United States. 

Although, as compared with amphibians, their legs lengthen and 
strengthen, "reptiles" (repere, to crawl) are named with an eye to the leg- 
less crawling snakes. The group includes not only snakes but also lizards, 
turtles, and alligators, as well as Sphenodon, a New Zealand genus contain- 
ing a single species. Also included is a vast company of forms now extinct, 
many of which were gigantic, that dominated the Mesozoic world through- 
out a dynasty that endured for ages. 

Reptiles are the first true land vertebrates freed from the necessity of 
returning to the water to breed. "Things that before swam in the water now 
went upon the ground" (Wisdom of Solomon). This saying is true even of 
alligators, certain turtles, and water snakes which, although they spend 
much of the time in water, come out upon the land to lay their eggs. Each 
egg is usually fertilized and then covered with a shell while still inside the 
body of the female. 

As mentioned in the discussion of amphibians, each embryo of a reptile, 
bird or mammal is surrounded by an amnion during most of its development 
(Fig. 34). The astonishingly rapid growth of any developing embryo neces- 
sitates a protective covering for the extremely delicate cells and tissues that 


Biology of the Vertebrates 

does not involve hampering adaptations for withstanding exposure to dry 
air or mechanical shocks during their tumultuous multiplication. Growing 
embryos of fishes and amphibians have such a provision in the surrounding 
medium of water in which they are immersed, but reptiles and all other con- 
querors of the land who cannot cradle their growing youngsters in open 

Fig. 34. Diagrams of -the embryonic membranes, amnion, allantois, 
yolk-sac of amniotes. a, Sauropsida (reptiles and birds); b, mammal with 
primitive allantoic placenta. (From Newman, The Phylum Chordata, 
copyright 1939, by permission of The Macmillan Company, -publishers. 
After Wilder.) 

water, depend upon a protective antenatal robe that the young embryo 
forms about itself. This is the amnion, a thin enveloping sac filled with a 
secreted watery fluid in which the embryo floats. It may be trulysaid, there- 
fore, that in a certain sense every vertebrate passes its early life submerged 
in water. Because all reptiles, birds and mammals have an embryonic 
amnion, they are collectively known. as Amniota', while all lower vertebrates 
in which no amnion develops, are called Ana/nnia. 

In the closed amnion sac neither gills nor lungs, nor even body surface, 
can serve as respiratory organs. Hence, contemporary with the amnion is 
an allantois, an outgrowth from the posterior part of the digestive tract 
which serves as a temporary breathing and excreting organ. This emergency 
organ, bearing a rich network of blood vessels, grows out into the space 
between the amnion and the thin, inner egg-shell membrane, or chorion, so 
that through the latter and the porous egg-shell there is effected the inter- 
change of gases between the blood and the outside world essential to breath- 
ing and excretion. In the case of mammals, the c^ has no shell, but develops 

Kinds of Vertebrates 


into a fetus that is parasitically attached to the uterine walls of the mother. 
The capillary-laden allantois, coming into intimate contact with the richly 
vascularized wall of the uterus, by interdigitations, forms the placenta. It is 
through this organ that the young animal breathes and excretes until it is 
born into independence. 

Reptiles have a rather thick integument with few glands and many 
scales. The lungs are usually well developed in adults. The excretory organs 
(metanephroi) are of a more advanced type than those (mesonephroi) of 
fishes and amphibians. On the ends of the digits are claws. The skull bears 
only one occipital condyle for articulation with the first vertebra. In modern 
species, the pelvic appendages are connected with two sacral vertebrae. A 
partial or complete, partition develops between the right and left sides of the 
ventricle of the heart. Like the lower vertebrates, however, they are cold- 

C D 

Fig. 35. Palaeozoic reptiles. A, Seymouria; b, Labidosaurus; c, Cynog- 
nathus, a mammal-like reptile; d, head of Scymnognathus, a South- 
African "dog-toothed" reptile. (From Newman, The Phylum Chordata, 
copyright 1939, by permission of The Macmillan Company, publishers. 
After Osborn.) 

( 1 ) extinct reptiles. — Extinct reptiles with their fossil remains wrote 
a long and dramatic chapter in the history of living things upon the earth 
for modern man to read. Several entire orders, the flying pterosaurs, aquatic 
ichthyosaurs, and long-necked plesiosaurs, for example, have, so far as is 
known, left no living descendants, but others have been the ancestors of not 
only recent reptiles but also birds and mammals. 

There are more than a dozen orders of reptiles of which only four 
include living species. During the Golden Mesozoic Age of Reptiles, which 
lasted according to some geologists, from 125 to 150 million years, these 

4 6 

Biology of the Vertebrates 

ruling animals attained a great diversity of form and adaptation, enabling 
them to live in a variety of habitats, such as forests, water, swamps, dry 
land, and air. The imagination is thrilled by a picture of the Mesozoic land- 
scape with its weird reptilian population. Some of these strange creatures of 
past precamera days are suggested by the sketches in Figure 35. 

Fig. 36. Various extinct reptiles, selected to indicate diversity of form. 
They arc not drawn to a common scale, but are mostly gigantic in size. 
A, restoration of Diplodocus. (After Smit.) b, restoration of Igudnodon. 
(After Heilman.) c, Ichthyosaurus. (After Conybeare.) d, Stegosaurus. 
(After Marsh.) e, Pterodactylus. (After Sceley, but according to Abel 
in incorrect quadrupedal attitude.) f, TriceratopS skeleton. (After 
Marsh.) g, Brontosaurus. (After Marsh.) h, restoration of TriceratopS. 
(After Nuhn.) 

Kinds of Vertebrates aj 

The cotylosaurs, or stem-reptiles, were the earliest members of the group 
(Fig. 35a). Structurally they closely resembled the most primitive of the 
Amphibia. In external appearance they probably looked like the larger of 
our lizards. 

The therapsids, another early group, possessed certain features which 
indicate they were ancestral to the mammals (Fig. 35). Their teeth were 
differentiated into the three major types : cutting incisors ; large, pointed 
canines; and grinding molariform teeth. Their jaws also showed a trend 
toward the mammalian plan. 

The dinosaurs (Fig. 36b, d, and h), including many species which 
varied widely in appearance, ranged in size from tiny forms about the size 
of a hen to the well-known enormous species which were the largest animals 
ever to walk on the face of the earth. Many of these reptiles were probably 
bipeds, capable of raising themselves up on their hind legs for more rapid 
running. To counterbalance the front parts of the body there was a long, 
rather heavy tail. The front legs, shorter than the hind legs, were presum- 
ably used when the animal was resting or walking 
slowly. These bipedal forms may have been close 
cousins of the earliest birds, both groups coming 
from the same immediate ancestral stock. The 
largest of the dinosaurs were undoubtedly quad- 
rupeds (four-footed). Apparently many of them 
developed amphibious habits, spending much of 
their time in lagoons and swamps where the water 

reduced the weight which it was necessary for their * i g " 37, _. A P teros ^ur- 

J (from JNewman, The 

legs to bear. Phylum Chordata, copy- 

Of the reptiles which returned to the water to right 1939, by permission 

live, the ichthyosaurs (ichthyo, fish; saur, lizard, of The Macmillan Com- 
., N , . , . . ... ,_. pany, publishers. After 

reptile) were the best adapted to aquatic life (Fig. Osborn.) 

36c). Their paired appendages were flippers, mod- 
ified from typical tetrapod legs. In general appearance they were fish-like. 
Without doubt they breathed by means of lungs. As amniotes, each of their 
embryos presumably developed an amnion and an allantois and was 
incapable of surviving if the egg was laid in the water. A number of 
specimens have been found with small complete skeletons inside the 
body of the adult. It is believed, therefore, that these animals were 

Some of the bipedal early reptiles apparently evolved into flying species 
(Fig. 37). These were pterosaurs (ptero, wing). The fourth digit of each 
front appendage became strong and considerably elongated to support the 

4 S 

Biology of the Vertebrates 

wing, a fold of the skin. Just how much of their activity was true flying and 
how much was gliding is still problematical. 

Modern surviving reptiles may be grouped into the following orders: 


(2) rhynchocephalia. — The rhynchocephals, which include many 
fossil kinds, are represented today by a single surviving genus, Sphenodon, 
or the "tuatara" of New Zealand (Fig. 1). This is -a long-tailed, lizard-like 
animal usually somewhat less than two feet in length as an adult. In com- 
mon with the many extinct species of this Order, Sphenodon shows many 
primitive reptilian features. It probably owes its survival to its isolation in an 
area where it did not -have to compete with mammals. Reference has already 
been made to the median eye of this interesting "old curiosity shop" of 
ancestral peculiarities, and there will be future occasion to rummage further 
in this anatomical «attic for sidelights of the vertebrate past. 

(3) chelonia. — The chelonians, or turtles and tortoises, modified "rep- 
tiles in a box," are unusual in many respects. Their internal organs, in both 
shape and arrangement, «are 'adapted to fit into a short, broad box formed 
by the ventral, relatively flat plastron and the -dorsal, more or less curved 
carapace. This shell consists of bony plates covered over with large, -thin, 

A B 

Fig. 38. Chelonia. A, Dermochcelys (Sphargis) coriacea, leather-back 
turtle; b, Chelydra serpentina, snapping turtle. (From Newman, The Phy- 
lum Chordata, copyright 1939, 'by permission of The Macmillan Com- 
pany, publishers. After Lydekker.) 

horny scales. The trunk vertebrae are attached to a row of these plates run- 
ning down the middle of the carapace. Consequently the only flexible por- 
tions of the vertebral column are the neck and tail regions. These two 
regions, together with the four legs, -may be withdrawn beneath the shell 
into a place of protection. Turtles possess toothless jaws encased in horny 

A few of the genera of turtles are: the "leatherback," Dermochelys 
(Fig. 38); the "loggerhead," Thala'ssochelys, which cruises about in salt 

Kinds of Vertebrates 


water and may attain a weight of several hundred pounds; the "green tur- 
tle," Chelonia mydas, also a seagoing animal, prized as food ; the "snap- 
pers," Chelydra (Fig. 38) and Macrochelys, the latter of which has a bite 
powerful enough to amputate a foot; the "box turtle," Terrapene, that is 
able to withdraw its head entirely within its shell and to close the door with 
a hinged lid; and finally, the small beautifully decorated "painted turtle," 

America of all the continents is particularly rich in chelonial inhabitants. 

Fig. 39. Wall gecko, Tarentola 
mauritanica, a lacertilian. (From 
Newman, The Phylum Chordata, 
copyright 1939, by permission of 
The Macmillan Company, pub- 
lishers. After Lydekker.) 

Fig. 40. An Indian lizard, 
Draco, the "flying dragon." 
(After Hilzheimer.) 

(4) squamata. — The squamates are reptiles clothed with a great num- 
ber of regularly placed scales which cannot be separately detached like 
the scales of bony fishes but are connected together into a continuous armor. 
They comprise two suborders : the lacertilia, or lizards, and the ophidia, 
or snakes, distinguished from each other by the fact that the "former 
have movable eyelids, visible earpits and usually legs, while the latter 
do not. 

The lizards are typically sun worshippers, dwelling in regions of much 
sunshine, and for the most part avoiding water. A notable exception is the 
water lizard, Amblyrhynchus, which is an inhabitant of the rocky shores of 
the Galapagos Islands. 

The "geckos," of the Malay region and the Mediterranean countries, 
have adhesive toes that enable them to clamber about with great agility 
after flies and other insects in trees and upon the walls inside of houses (Fig. 
39). The "flying dragon" of India, Draco (Fig. 40), is able to volplane 


Biology of the Vertebrates 

from branch to branch of the trees that it inhabits, by means of a capelike 
expansion of the skin down the sides. 

Iguana is a large arboreal Mexican lizard of fierce aspect but harmless 
habit which is regarded as good to eat. The giant of them all is the rare 
Varanus komodoensis, the "dragon" of the East Indies, which may attain a 
length of twelve feet. 

Fig. 41. An African lizard, Chameleon, which has a long extensile 
tongue and is famous for its power of changing color. (After Hilz- 
heimer. ) 

The "chameleon," Chameleon (Fig. 41 ) , has a prehensile tail and. grasp- 
ing feet and flaunts a Joseph's coat of many colors. It is a native of Africa, 
although its name is sometimes erroneously applied to the little American 
Anolis (Fig. 42) of changeable colors that inhabits the cane fields of the 
South and preys upon the insects which are attracted by the sweet juice 
that oozes from the cane. 

Fig. 42. An American "chameleon," Anolis. (After Ditmars.) 

The "glass snake," Ophisaurus of the Old World, and the "slow worm," 
Amphisbaena, are legless lizards. 

In the desert region of the southwestern United States are found the 
grotesque "horned toad," Phrynosoma (Fig. 43), that is called a toad only 

Kinds of Vertebrates 

S 1 

by the undiscriminating, and the "Gila monster," Heloderma (Fig. 44), an 
ugly black and orange beast, with a large round stubby tail, which is the 
only lizard whose bite is venomous. 

The serpents are the legless snakes, described by Ruskin as "a wave but 
without wind, a current but with no fall." They walk upon their numerous 
ribs, or "jerk themselves forward 
by a rapid straightening of their 
sinuous curves" (Thomson) . The 
curious arrangement of the in- 
ternal organs of these creatures 
has a direct connection with their 
external architecture. 

Most snakes have few human 
friends, in spite of the fact that 
most of them are beneficial ani- 
mals, feeding largely upon in- 
jurious insects and small rodents. 

They have suffered vicariously from an unsavory reputation ever since one 
of their number was reported to have taken part in the original eternal 
triangle play in the Garden of Eden. 

Fig. 43. The "horned toad," Phrynosoma, of 
the desert region of southwestern United States. 
It is not a "toad" but a lizard. (From Hegner, 
College Zoology, copyright 1942, by permis- 
sion of The Macmillan Company, publishers. 
After Gadovv.) 

Fig. 44. "Gila monster," Heloderma, the only known venomous lizard. 

Of approximately 110 species in the United States, less than 20 are ven- 
omous. The most dangerous of these, so far as man is concerned, are the 
several species of rattlesnakes in the genus Crotalus (Fig. 45) ; the "copper- 
head," Agkistrodon mokasen, and the "water moccasin," Agkistrodon pis- 
civorus, of the South ; and the "coral snakes," Micrurus, also of the South. 

The "black snake," Coluber; the "puff adder," Heterodon; the "milk 
snake," Lampropeltis ; and the "garter snake," Thamnophis, are among the 
common harmless varieties. 

S 2 

Biology of the Vertebrates 

(5) crocodilia. — The crocodiles are, in a number of respects, the most 
advanced of the reptiles. Their lungs are very efficient organs. Their heart 
has two completely separate ventricles, a condition found also in birds and 
mammals. The brain has large cerebral hemispheres. The Crocodilia include 

the crocodiles proper of India, China, 
Africa, the Malay Archipelago, Central 
America, Mexico, and the southeastern 
United States; the broad-snouted alligators 
of the Mississippi Basin, Florida, and China 
(Fig. 46) ; the caimans of Central and 
South America ; and the narrow-snouted 
gavials of the Ganges in India. All are in- 
habitants of tropical or semi-tropical coun- 
tries and, though clumsy and stiff-necked on 
land, are quite at home in shallow water, 
where their powerful laterally compressed 
tails enable them both to swim forward and 
to strike powerful side blows. 

5. Class AVES 

Fig. 45. Texas rattlesnake, Cro 
talus. (After Stejneger.) 

All birds, of which there are perhaps 
15,000 species, have feathers. This one con- 
spicuous characteristic suffices to identify a 
bird, even to a child, for no other animals have feathers. The vertebrate type 
probably reaches its highest differentiation, in certain directions at least, in 
birds, and for this reason it is not at all difficult to find many other dis- 
tinguishing characteristics, aside from feathers, in this familiar and much 
studied class of animals. 

Fig. 46. Alligator. 

The secret of the anatomical peculiarities of birds lies in their adapta- 
tion to flight. Speed of animal locomotion culminates in birds. The same 
combination of organs which converts a fish into a living submarine and 
adapts a reptile to a life of continuous contact with the earth, transforms a 
bird into a flying machine heavier than air. 

The skeletal framework of a bird, comparable bone by bone with that 
of other vertebrates, is compacted together, thus affording the smallest pos- 

Kinds of Vertebrates 53 

sible bulk to pass through the air, although the surfaces of individual bones 
remain relatively expansive for the attachment of greatly developed muscles 
of flight. The surfaces of the breastbone, the humeral heads, and the sacro- 
pelvic complex particularly, are increased beyond those of other vertebrates. 
Every possible part of a bird is transferred from the anatomical suburbs into 
the compact urban district of the body. The heaviest parts hang beneath the 
line of support joining the wing-sockets where the power is applied in flight. 
Considerable weight is shifted from the periphery to the center by means of 
the replacement of heavy dense teeth, commonly found in the head of other 
vertebrates, by the light horny beak, while a tough muscular centralized 
gizzard, containing powerful grinding stones, does the work which teeth once 
did in ancestral birds. 

The cumbersome trailing reptilian tail is telescoped into a degenerate 
skeletal stub, thus centralizing weight. In place of it a secondary tail of light 
air-resisting feathers is added as a rudder in flight. The presence in birds 
of a bony tail, composed of several foreshortened vertebrae instead of a sin- 
gle bone which might better have served as a support for the tail feathers, 
is one of the numerous evidences of reptilian ancestry. Indeed someone has 
happily described birds as "glorified reptiles." 

The bones of a bird are not only compact but are also lightened and 
adapted as parts of a flying machine, by being hollowed out to the limit of 
mechanical safety. Furthermore, bodily weight is particularly counterbal- 
anced by the development of numerous air sacs that grow out from the 
lungs, occupying all available spaces between the internal organs and 
extending even to the cavities of the hollow bones. Feathers, which clothe a 
bird, hold a blanket of enveloping air next to the body, that, since it is 
warmed by the body and is consequently lighter than the surrounding air, 
adds somewhat to the bird's buoyancy. 

In addition, the large intestine, particularly the rectum where the feces 
are carried, is very much reduced in length. Since flying animals can ill 
afford to be weighted down with any excess fecal baggage, birds, having no 
suitable provision for its storage, promptly get rid of it. 

The entire support of a bird's body devolves upon the hind legs alone, 
leaving the fore legs free to serve as wings. The wing is composed largely 
of feathers attached to an arm terminating in three reduced and partially 
fused fingers. As a consequence the forelegs, or arms, which are modified 
and entirely given over to flight, cannot be used for the capture and manipu- 
lation of food. The head, therefore, is necessarily not only periscopic but 
prehensile, being mounted upon an extremely flexible neck and equipped 
with a forceps-like beak for picking up food. As a result the remarkablv 


Biology of the Vertebrates 

developed eyes, located on either side of a bird's beak, are much nearer their 
objective than the eyes of any other vertebrate which reaches for its food. 

The excessive activity involved in flight is provided for in birds by a 
relatively larger heart than other animals possess, as well as by a particularly 
effective respiratory apparatus, which so increases the warmth of the body 
as to render it constant, regardless of the surrounding temperature. Birds 
are active, therefore, the year around, in cold as in warm weather, never 

becoming sluggish or obliged to hibernate as 
"cold-blooded" animals do. Enabled ordi- 
narily to rise above the handicaps of tem- 
perature and climate, when occasion de- 
mands they resort by migration to distant 
and more congenial localities. 

There are two subclasses of the Aves : 
(a) archaeornithes (archae, old, primi- 
tive; ornithos, bird) ; (b) neornithes 
(neo, new). 

a. Subclass ARCHAEORNITHES (Primitive 

On account of their light bones and the 
rapid disintegration of their bodies after 
death, birds are not subject to fossilization 
except under the most favorable conditions, 
and do not, therefore, present so extensively 
recorded a story of past achievements as 

The earliest known trace of bird life is 
the imprint of a single tail feather, discov- 
ered in the Jurassic slate quarries of Bavaria. 
This unmistakable fragment dates back to 
the middle of the long Age of Reptiles, eons before mammals had arisen 
to become a power upon the earth. In splitting up the fine-grained 
lithographic stone of the Solenhofen deposit in Bavaria from which 
this priceless feather came, there were found also at different times later 
two entire skeletons, crushed flat and embedded in the slate, of the same 
land of birds that doubtless produced this famous tail feather. From these 
slight but convincing remains, the species of this oldest of all known birds, 
was named Archaeopteryx lithographica (Figs. 47 and 48). It was about as 
large as a crow, had lizard-like teeth set in sockets in the elongate jaws, a 
long uncentralized bony tail bearing two oblique rows of feathers, a flat 

Fig. 47. The oldest known bird, 
Archaeopteryx, showing teeth, 
three fingers, feathers, and a lizard- 
like tail. The Berlin specimen. 
(After Parker and Haswell.) 

Kinds of Vertebrates 


sternum, three fingers with claws terminating each wing instead of one claw- 
less finger as modern birds have, and feathers. They resembled the small 
dinosaurs so closely that only the presence of 
feathers has prevented them from being placed 
among the reptiles. 

b. Subclass NEORNITHES (Modern Birds) 

The neornithes include all present-day birds 
in addition to a number of extinct species which 
resemble modern birds in most respects. The 
chalk beds of western Kansas, which were laid 
down at a much later date than the Bavarian 
slates of Jurassic times, have yielded the fossil re- 
mains of extinct birds with teeth, for example, 
Ichthyornis, in form resembling a tern, and Hes- 
perornis (Fig. 49), a flightless loon-like water 
bird, of which over a hundred specimens have 
been found. Excluding the toothed ancestral birds 
of Kansas, modern birds may be divided, according to flying ability, into 
two unequal subclasses, ratitae and carinatae. 

Fig. 48. Restoration of 
Archaeopteryx, after Heil- 
mann. (From Newman, The 
Phylum Chordata, copyright 
1939, by permission of The 
Macmillan Company, pub- 
lishers. After Osborn. ) 

Fig. 49. A toothed loon-like bird, Hes- 
perornis, without a keel on the breast- 
bone. From Kansas chalk beds. (After 
Marsh. ) 

Fig. 50. A primitive wingless burrow- 
ing bird, Apteryx, from New Zea- 
land. (Drawn from a specimen in 
the collection at Brown University.) 


Biology of the Vertebrates 

Fig. 51. A giant "moa," Dinor- 
nis, from a mounted specimen 
eighteen feet in height in the ex- 
hibit of the Government of New 
Zealand at the Panama-Pacific 

The ratites, none of which are indigen- 
ous to North America, are flightless running 
birds that have powerful legs and small 
wings. They include ostriches, cassowaries, 
emus, rheas, and the curious wingless Ap- 
teryx, or "kiwi" of New Zealand (Fig. 50), 
that, in the absence of ability to escape by 
flight, has survived the perils of a hostile 
world by burrowing in the ground. 

In New Zealand also, a land of special 
interest to the biologist, have been found 
abundant fossil remains of Dinornis, the 
largest of all known birds, commonly called 
the "moa" (Fig. 51), which reached a 
height of at least 1 8 feet. It is likely that this 
species of gigantic ostrich-like bird has be- 
come extinct within the memory of man for 
when the whites first came into contact with 
the native Maoris of New Zealand they had 
legends about these birds that had been 
handed down to them from their fathers. 

The carinates are flying birds whose 
wide breastbone has developed an expansive 
keel, or carina, for the attachment of mus- 
cles of flight. Ribs made up entirely of bone 
hold the breastbone firmly in place. 

According to Gadow of the British Mu- 
seum the carinates comprise thirteen orders 
as named below. A few more or less fa- 
miliar examples are given to represent each 

1. colymbiformes : loons, grebes. 

2. sphenisciformes : penguins. 

3. procellariiformes: petrels, albatrosses. 

4. ciconiiformes: pelicans, herons, cor- 

morants, flamingos, storks. 

5. anseriformes: geese, ducks, swans. 

6. falconiformes : vultures, hawks, eagles, 


Kinds of Vertebrates 5j 

7. tinamiformes : tinamous. 

- 8. galliformes: turkeys, fowls, quail. 

9. gruiformes : rails, and other marsh birds. 

10. charadriiformes: plover, sandpipers, and other shore birds, gulls, terns, 

auks, pigeons, doves. 

11. cuculiformes: cuckoos, parrots. 

12. coraciiformes : kingfishers, owls, whip-poor-wills, swifts, humming birds, 


13. passeriformes : flycatchers, sparrows, swallows, vireos, wrens, nuthatches, 

kinglets, thrushes. 

6. Class MAMMALIA 

Mammals (mamma, breast), of which there are about 15,000 species, 
are perhaps less spectacular anatomically than reptiles or birds, but in spite 
of this fact they have come to occupy a dominant place among the verte- 
brates. Mammals emphasize a new note of cooperation not discoverable to 
any great extent in the universal competition that elsewhere characterizes 

The first mammals to appear on the earth were small and insignificant 
contemporaries of the gigantic reptiles of the Mesozoic Age. As long as huge 
carnivorous dinosaurs held sway, the small ancestral mammals, which were 
probably largely arboreal in habit, kept out of the way and bided their time. 
Perhaps they hastened that time somewhat by feeding upon the eggs of 
their terrifying enemies while eluding capture themselves. At any rate it is 
certain that in the long struggle for a "place in the sun," it has been wits 
rather than brute force that has enabled mammals to out-distance com- 

No doubt the mechanism which insures "warm-bloodedness," that is, a 
constant bodily temperature independent of changes in the surrounding 
atmosphere, has much to do with the conquest of the earth by mammals. By 
reason of this important characteristic they have been able to establish them- 
selves not only throughout temperate areas but even in dry deserts and 
frigid polar regions. 

While the highly specialized birds have sacrificed everything to develop- 
ing the power of locomotion by flight in air, mammals have chosen the 
better part of improvement along the line of the nervous system, especially 
in the considerable enlargement of the very important cerebral hemispheres. 
Achievement in this direction has undoubtedly been the greatest of all fac- 
tors in determining the present supremacy of the mammalian type. An 
unusual amount of plasticity and versatility is exhibited among mammals. 
For example, they vary in size from a field mouse scarcely more than an 

5<5 Biology of the Vertebrates 

inch in length, to whales which may attain a length of over 100 feet, or well 
over 1000 inches. 

Mammals are variously fitted for successful life in such diverse habitats 
as on the land (deer) ; in water (otters) ; in burrows (gophers) ; under- 
ground (moles) ; in open oceans (whales) ; in forests (monkeys) ; and in the 
air (bats). 

The most important characteristics of mammals are: 

( 1 ) Milk Glands, providing food for the more or less helpless young for 

some time after birth. Masses of milk glands (breasts) have given 
us the name Mammalia. 

( 2 ) Hair, almost as typical of mammals as feathers are of birds. Occurs at 

least in embryonic life. 

(3) Sweat Glands. One interpretation is that certain sweat glands became 

modified into milk glands. 

(4) Enucleate Erythrocytes. The red blood cells are without nuclei when 

freed into the blood stream. Nuclei, present as the individual cells 
form, are extruded from them while they are still in the blood- 
forming tissues. 

(5) Seven Cervical (Neck) Vertebrae, in all' except a few species. 

(6) Muscular Diaphragm, separating the thoracic cavity, containing lungs 

and heart, from abdominal cavity. 

(7) Differentiation of the Teeth into three major types (cutting incisors, 

piercing canines, and grinding molars and premolars), a condition 
foreshadowed in the therapsids, the early reptiles from which mam- 
mals arose. 

Additional characteristics will be considered during the discussions of 
the various organ systems. 

Living mammals may be arranged in seventeen orders, which fall into 
three subclasses : prototheria, metatheria, and eutheria. 

a. Subclass PROTOTHERIA 

Order Monotremata. — The prototheria (proto, first; ther, beast) com- 
prise a single order, monotremata, of which only three genera are living 
today, namely, Echidna, Proechidna, and Ornithorhynchus. The digestive, 
urinary, and genital systems empty into a cloaca through which all of them 
communicate with the outside by a single opening {mono, one; trema, 
opening) . 

Monotremes are curious exceptional mammals that lay relatively large 
yolk-laden eggs, from which the young are hatched instead of being born 

Kinds of Vertebrates 


alive in ordinary mammalian fashion. Ornithorhynchus incubates its leath- 
ery-shelled eggs in a shallow nest of grasses, while Echidna forms a tempo- 
rary pouch from a fold of the skin upon its belly. In this portable nest the 
newly-laid egg is placed and incubated until hatched and the helpless off- 
spring kept through the precarious days of its early growth and development. 
The young Echidna is fed upon a nutritious substitute for true milk, secreted 
by the mother from modified sweat glands, which it licks up with its long 
tongue from tufts of hair on the belly of the mother. No nipples are present 
and if they were the baby monotreme would not be able to suck, since its 
lips are prolonged into a horny toothless beak, not at all fitted for the muscu- 
lar operation of sucking, but useful later for poking into ant-hills after food. 

Fig. 52. The "duckbill," Ornithorhyn- 
chus, an Australian monotreme with 
webbed toes and a ducklike bill. (After 

Fig. 53. The spiny anteater, Ech- 
idna, a monotreme. (From New- 
man, The Phylum Chordata, copy- 
right 1939, by permission of The 
Macmillan Company, publishers. 
After Vogt and Specht.) 

In Ornithorhynchus (Fig. 52) the beak is large and flattened, giving 
rise to the name of "duckbill" for this creature, a name all the more appro- 
priate because it lives much of the time in water and has feet with webbed 
toes like those of a duck. The "incredible duckbill" is a native of South 
Australia and Tasmania. 

Echidna, the spiny anteater (Fig. 53), is found in Australia, Tasmania, 
and New Guinea. 

Proechidna, distinguished from Echidna by an unusually long snout that 
gives it a "ridiculous resemblance to a miniature elephant," is confined to 
New Guinea. 

b. Subclass METATHERIA 

Order Marsupialia. — The metatheria (meta, after; ther, beast), com- 
prising the single order marsupialia, are primitive, or possibly degenerate, 
mammals whose young, born prematurely in an extremely helpless condi- 
tion, are fed upon true milk and carried about in a permanent brood pouch, 
or marsupium. 


Biology of the Vertebrates 

At first the young vestpocketed marsupials are unable to exercise the 
necessary muscular effort involved in sucking, and are securely attached in 
a passive way to the nipple by means of a sphincter-like mouth ( Fig. 54 ) , 
while the female expresses milk from her mammary 
glands down the throat of the helpless fetus by the con- 
traction of the abdominal muscles. Later on, as develop- 
ment advances, the young marsupial draws its milk in 
the orthodox way. 

Osborn, in The Age of Mammals, catalogues 76 
genera of marsupials of which 37 are extinct. The living 
ones, excepting the opossums Didelphys of North, Cen- 
tral, and South America, and Caenolestes of Central 
America, are confined to the Australian region. Extinct 
Eocene genera ranged over what is now Europe, as well 
as both Americas and Australasia. 

It is considered probable that the origin and spread 
of marsupials occurred before the ancient land bridge 
that joined Australia to South America had disappeared. 
Those forms which became isolated at that time in the 
marsupial Ark of Australia were afterwards able to con- 
tinue their handicapped existence with comparative 
success, since they were not brought into competi- 
tion with the true mammals that developed later on 
the other great continental areas. Man's introduction of cats, dogs, and 
especially rabbits into Australia has probably doomed many, if not all, 
species of monotremes and marsupials of that continent to extinction. As in 
America, a few species may be able to survive. 

It is a striking fact that not only all the native mammals of Australia 
were monotremes and marsupials, but also that the latter became diversified 
in much the same way as true mammals into different types adapted to 
various habitats. The "koala" is a bear-like form. Species resembling wolves, 
hyenas, cats, rabbits, jumping mice, woodchucks, moles, flying squirrels, 
and mice are also found in this Order. The kangaroos, Macro pus; bandi- 
coots, Perameles ; and opossums, Didelphys and Coenolestes, are marsupials 
that suggest cousins among the true mammals to a less degree. 

Some of these marsupials are represented in sketches in Figure 55. 

c. Subclass EUTHERIA 

The eutheria (eu, true; theria, beasts) include all the other mammals, 
frequently termed "placentals" because they are characterized by the pres- 

Fig. 54. A young 
marsupial, Srnintho- 
pis, with its snout 
gripped by the cup- 
like marsupial 
pocket, and the nip- 
ple (represented in 
black) crowded far 
down the throat. 
(After Bresslau.) 

Kinds of Vertebrates 


Fig. 55. Marsupials. A, Virginia opossum, Didelphys virginiana; b, rock 
wallaby, Haimaturus, a kangaroo; c, native cat, Dasyurus; d, marsupial 
mole, Notoryctes; E, koala, or marsupial bear, Phascolarctus, carrying 
young on back; f, Tasmanian wolf, Thylacinus; G, wombat, or mar- 
supial woodchuck, Phascoloynys. (All from Newman, The Phylum Chor- 
data, copyright 1939, by permission of The Macmillan Company, pub- 
lishers, a and b, after Vogt and Specht; c, e, and f, after Brehm; d, after 
Beddard; g, after Lydekker.) 

ence of a placenta, formed where the capillary-laden allantois comes into 
intimate contact with the richly vascularized wall of the uterus (Fig. 34). 
Here interdigitations of the allantois and uterine wall bring the embryonic 
and maternal blood streams close together. Across the thin membranes 


Biology of the Vertebrates 

which separate the two blood streams oxygen and food pass into the blood 
of the embryo, and waste products from the blood of the embryo to that of 
the mother. In this manner the placenta forms a functional connection 
between the mother and offspring throughout the long preparatory life 
before birth. 

Fig. 56. Insectivores. a, Tupaia, the tree shrew, considered by Osborn 
as near the prototype form of all high placental mammals; b, golden 
mole, Chrysochloris. (From Newman The Phylum Chordata, copyright 
1939, by permission of The Macmillan Company, publishers. A, after 
Osborn; b, after GiAnther.) 

Arranged according to the degree of specialization which they exhibit 
from the most generalized to the most aberrant forms, the orders of living 
placentals are : insectivora, dermoptera, chiroptera, carnivora, ro- 


tyla, artiodactyla, proboscidea, hyracoidea, sirenia, and cetacea. 

A word of identification and comment about each of these orders of true 
mammals, with examples of a few representative genera, is essential in 
rounding out a roll call of the vertebrates. 

(1) insectivora. — The insectivores (vorare, to eat) subsist largely 
upon insects, hence their name. They are mostly small, sharp-snouted 

Kinds of Vertebrates 


Fig. 57. Flying lemur of Mada- 
gascar, Galeopithecus. (After Vogt 
and Specht.) 

animals with leanings towards nocturnal or subterranean life. They include 
among other genera: the European hedgehog, Erinaceus; the moles (Fig. 
56b) of which the common mole of the eastern United States, Scalopus, 
and the peculiar star-nosed mole, Condy- 
lura, as well as Scapanus of the Pacific 
Coast, are American genera ; also the 
shrews, Sorex, and the short-tailed Blarina, 
both North American representatives. A 
few, the tree shrews (Fig. 56a), have re- 
tained the primitive mammalian habit of 
life in the trees. In a number of respects the 
anatomy of these tree shrews indicates that 
this group is not far removed from the in- 
sectivore stock which was probably ancestral 
to the Primates. 

Osborn names a total of 45 fossil and 34 
living genera of insectivores. 

(2) dermoptera. — The dermopterans, 

which are without fossil representation, are set aside in an independent 
Order, although it consists of but a single Genus, Galeopithecus (Fig. 57), 
the "flying lemur" of the Malay region that is an anatomical connecting link 
between insectivores and bats. 

(3) chiroptera. — The chiropterans (chir, hand; pter, wing), or bats, 
are mammalian aviators, rather helpless when not on the wing, which fly 

about at twilight by means of enormously 
elongated webbed fingers. While the er- 
ratic flight of bats is by no means as sus- 
tained as the more powerful flight of 
birds, yet, aided by extremely responsive 
sense organs, these creatures are unsur- 
passed in avoiding obstacles while hawk- 
ing insects in the crepuscular traffic of 
The food of bats in general, and of North American bats in particular, 
consists practically of insects caught on the wing (Fig. 58). In the Old 
World tropics the habit of eating fruit has developed on the part of certain 
large bats called "flying foxes," that live upon figs, guavas, and similar soft 

Another aberrant adaptation in the chiropteran type is presented by the 
blood-sucking vampires of Central and South America, that have a highly 

Fig. 58. An insectivorous bat, Syno- 
tus. (After Vogt and Specht.) 

6 4 

Biology of the Vertebrates 

Fie 59 Fissipede carnivores, a, Canada lynx, Felis canadensis; B, civet 
cat Viverra civetta; c, spotted hyaena, Crocuta rnaculata; d, gray or 
timber wolf, Canis nubilus; E, raccoon, Procyon lotor; f, badger, 1 ax- 
idea taxus; G, otter, Lutra canadensis; H, Alaska brown bear, Ursus gyas, 
latest of the bears. (All from Newman, The Phylum Chordata, copy- 
right 1939, by permission of The Macmillan Company, publishers, b and 
c, after Beddard; all others after Fuertes.) 

Kinds of Vertebrates 6$ 

modified saclike stomach for the storage of blood which they gorge from 
some unwilling host. Aside from the unpleasantness of their blood-sucking 
habits, these bats are under suspicion because they may be agents for the 
transfer of vicious blood parasites. 

Myotis is the common harmless brown bat of cosmopolitan distribution. 
Of 63 genera of bats in Osborn's list only three are extinct. 

(4) carnivora. — The carnivores (cam, flesh) have specialized in alert- 
ness and brains. They are keen, swift, athletic, gladiatorial killers, feeding 
by preference upon the flesh of other animals. Their place in the general 
scheme of things seems to be to keep within limits the prolific rodents that 
in their absence tend to overrun everything. They also have by their aggres- 
sive ways served in the course of evolution as schoolmasters for other mam- 
mals, particularly the primates to which man belongs, by stimulating self 
reliance and resourcefulness. The carnivores are not particularly modified 
except that the clavicles (collar bones) are reduced or missing and the teeth 
are somewhat specialized. 

They are represented by 73 living and 113 extinct genera in Osborn's 
list. The suborder fissipedia (fissi, split; ped, foot) is made up principally 
of land animals, while the suborder pinnipedia (pinna, fin) comprise fish- 
eating carnivores that have become secondarily modified for aquatic life. 

Among the better known fissipede carnivores (Fig. 59) are the follow- 
ing: Canis, dogs, wolves, and coyotes; Ursus and Thalassarctos, bears; 
Felis, cats, lions, and their kind ; Cynaelurus, the "cheetah" of India, a kind 
of cat without retractile claws ; and the hyenas, foxes, raccoons, weasels, 
minks, ermines, skunks, and others. 

Among pinnipede carnivora (Fig. 60) are the sea-lions, seals, and 

(5) rodentia. — The rodents, or gnawing animals, with 101 living and 
61 fossil genera, are the most numerous of all living mammals, particularly 
as they make up in number of individuals what they lack in size. Their 
strong, deep-set incisor teeth, which grow continuously throughout life, are 
self-sharpening chisels. The hard enamel of this type of tooth, confined to 
the anterior surface, does not wear away as readily as the softer dentine 
which makes up the bulk of the tooth. Thus a chisel-like, sharp cutting 
edge of enamel is always maintained. They are prevailingly plant eaters 
and form an important link in nature's chain, since they hand on the sun's 
energy, stored by green plants, to the carnivores by which they are devoured. 

The rodent bloc (Fig. 61) in the Congress of Mammals is represented 
by the following: Lepus, hares; Sciurus, squirrels; Cavia, guinea pigs; 
Sciuropterus, flying squirrels; Mus, rats and mice; and the beavers, musk- 


Biology of the Vertebrates 

Fig. 60. Pinnipede carnivores. A, Pacific walrus, Odobenus obesus; B, 
male, and c, female, of Steller sea-lion, Eumetopias jubata; d, Green- 
land seal, Phoca groenlandica. (All from Newman, The Phylum Chor- 
data, copyright 1939, by permission of The Macmillan Company, pub- 
lishers. After Fuertes.) 

rats, porcupines, prairie dogs and woodchucks. By some authorities the 
rabbits and hares are placed in a separate Order because of their extra pair 
of incisors and other evidence that they have independently acquired the 
gnawing habit. 

This group is of great economic importance. On the negative side are 
their destructive habits and possibly disease transmission, such as the carry- 
ing of bubonic plague by fleas which may inhabit the fur of rats. On the 
other hand, several species furnish us with fur or meat and several have been 
of great value for laboratory experimentation and tests of serums and other 

Kinds of Vertebrates 


Fig. 61. Representative rodents. A, squirrel, Sciurus; B, prairie dog, 
Cynomys. (Both after Dugmore.) c, flying squirrel, Sciuropterus. (After 
Lydekker. ) d, beaver, Castor. (After Dugmore.) e, muskrat, Fiber. (After 
Carlin.) f, jumping jerboa, Dipus, of Europe. (After Beddard.) g, porcu- 
pine, Erethizon. (After Dugmore.) 

(6) primates. — The primates, including 39 living genera of lemurs, 
monkeys, apes, and mankind, while not so highly specialized in many ways 
with regard to bodily structure as the several orders yet to be mentioned, 
stand first in the vertebrate class with respect to brain development. Most of 
the 23 fossil genera of this order are lemurs, which probably dwelt in trees 
just as their modern representatives in the forests of Madagascar do to this 
day. There is considerable evidence that the primates arose from a primi- 
tive, tree-dwelling Insectivora stock. Practically all species living today are 
arboreal, with skeletons not greatly modified from the condition of primitive 
mammals. Their teeth, also, are comparatively unspecialized but tend to be 
reduced in number from the 1 1 pairs of teeth on each jaw of primitive 


Biology of the Vertebrates 


Fig. 62. Smith's dwarf lemur, Mi- 
crocebus srnithii. (From Newman, 
The Phylum Chordata, copyright 
1939, by permission of The Macmil- 
lan Company, publishers. After Bed- 

placentals. The brain and eyes become highly developed, the cerebral 
hemispheres increasing in size until they completely cover the rest of the 
brain in higher primates. 

Although many travel on all fours, their arboreal habit has resulted in 
an upright sitting posture that made possible the development of a pair of 
handy hands and admission to the Manual Training School of the Treetops. 

This was the beginning of a wedgelike 
vista of possibilities at the broad end of 
which lies the intellectual life. Organs of 
defense, like scales, claws, horns, and 
hoofs, are not needed by primates since 
wits take their place. 

The three suborders are: lemuro- 


The lemurs (Fig. 62) , very primitive, 
arboreal primates not greatly different 
from the tree shrews, are found chiefly 
in Madagascar but with some species in 

Africa and Southern Asia. These small animals have an elongate snout and 

pointed ears. They are covered with a heavy coating of hair. Their big 

toes and thumbs are set apart from the other digits. Their long tail is not 


Tarsius (Fig. 63), the only living tarsioid genus, is intermediate between 

the other two primate suborders. A reduction of the olfactory organs and 

shortening of the snout have permitted a shifting 

of the eyes forward into a position for binocular 

vision, although neither eyes nor cerebrum is as 

well developed as in the monkeys. Like the lemurs 

they have pointed ears. 

In the Anthropoids (Fig. 64) the cerebral 

hemispheres are greatly enlarged and richly 

convoluted. The eyes are highly developed for 

increased clarity of vision. The New World 

monkeys, usually called Platyrrhines (platy, flat; 

rhin, nose) have broad, flat noses with the ex- 
ternal openings directed somewhat laterally. 

Their thumbs are usually reduced. Their tails are 

in most cases long and prehensile. Probably the 

best known member of this group is Cebus, the Capuchin monkey, famous as 

the companion of the hand-organ man. 

Fig. 63. Tarsius spectrum. 
(After Haacke.) 

Kinds of Vertebrates 

6 9 

The Old World Anthropoids, known as the Catarrhines because the 
external nares are close together and usually open downward (cata, down), 
include three general types: the various African monkeys, macaques, and 
baboons; the anthropoid apes (belonging to the Family Simiidae) ; and 
man. These animals have opposable thumbs. Their teeth are reduced in 
number to 32. The tail is non-prehensile and frequently greatly reduced. 
Macacus is the lively little monkey so often seen serving time behind the 
bars in zoological gardens and menageries. The rhesus monkey, Macacus 
rhesus, is used extensively in experimental work. The importance of this 
animal in work on the so-called Rh factor of the blood led to the use of the 
first two letters of Rhesus as the symbol for the factor. 

Fig. 64. Anthropoid Primates, a, long-tailed monkey, Aluatta; b, gibbon, 
Hylobates; c, chimpanzee, Anthropopithecus; d, orang-utan, Simla; E, 
gorilla, Gorilla, (e, after Beddard; others, after Schmid.) 

The tailless Simians include four genera (Fig. 64). The gibbon {Hylo- 
bates) is a long-armed, arboreal form of small size, found in the Malay 
region and neighboring islands. The orang-utan (Simia) , a native of 
Sumatra and Borneo, is also a long-armed arboreal type but is considerably 
larger than the gibbons. The chimpanzee (Anthropopithecus) and the 
gorilla ( Gorilla ) , both African forms, have relatively shorter arms than 
the gibbon or orang. The chimpanzee, primarily an arboreal form, 

jo Biology of the Vertebrates 

spends some of its time on the ground. The gorilla, essentially a ground 
dweller, is the largest and most powerful of all apes. Both chimpanzee and 
gorilla exhibit some degree of intelligence and reasoning power of the 
human type. Of the attempts to educate these apes, the most successful 
ones have been those using chimpanzees. They can be taught to wear 
clothing, dine at a table, ride a bicycle, act in moving pictures, smoke a pipe, 
expectorate with precision, and perform many other acts characteristically 

Modern man of whatever race or color belongs zoologically to a single 
genus and species, Homo sapiens, Linn., although the name "wise man" is 
more appropriate for some individuals than for others. Although similar to 
Simiidae in most respects, man differs from them in some structural details. 
He is more erect, has shorter arms and relatively larger thumbs. The big 
toes of the apes are opposable but in man these toes are in line with the 
others. While the simians have powerful teeth set in heavy jaws, man has 
smaller front teeth and tooth-bearing regions of the jaws. Consequently man 
has a chin. The supraorbital ridges over the eyes are much less pronounced 
in man. The human cerebrum is distinctly larger, in part due to the con- 
siderable development of the frontal lobes .where the speech centers are 
located. Room for these anterior lobes is provided by a raised front part of 
the skull which gives the higher forehead region. These differences are 
chiefly associated with man's mental development, speech and ground- 
dwelling habits. 

The fossil representatives of man will be considered in a later chapter. 
Together with modern man all are included in the family Hominidae. 

(7) Edentata. — The edendates (Fig. 65c and d) are rather de- 
generate mammals, either toothless as the name implies, or with poor chalky 
teeth. Their center of distribution is South America, for all of the 15 living 
genera are found there, although the anteaters, Tamandua, Myrmecophaga, 
and Cyclopes, extend as far north as Central America and Mexico, and the 
nine-banded armadillo, Dasypus novemcincta, even into Texas. Of the 34 
genera of fossil edentates, four are from North America, three from both 
North and South America, and twenty-five exclusively from South America. 
Thus it is evident that the center of distribution of this order is South 

The topsy-turvy sloths, Brady pus (three-toed), and Choelepus (two- 
toed), are well named because of their sluggish habits. They are awkward 
defenseless creatures clothed with coarse gray hair and equipped with long 
hooklike claws which enable them to hang upside down in the branches 
of tropical trees, where they depend upon their resemblance to motionless 

Kinds of Vertebrates 


masses of "gray-beard mosses" for protection from their carnivorous foes. 
Sidney Smith said of them: "Sloths live suspended, and sleep suspended, 
and in fact, pass their whole lives in a state of suspense, like a young curate 
when he is distantly related to a bishop." 

Of the armadillos, besides the nine-banded one already mentioned, there 
is the six-banded Dasypus sexcinctus of Paraguay and Brazil, and the three- 
banded Tolypeutes. Some of these can roll up in their scaly armor like "pill 
bugs" when danger threatens, presenting a hard nut for any predaceous 
foe to crack. 

Fig. 65. Representatives of the "edentate" Orders, a, scaly anteater of 
Africa, Manis, belonging to the Pholidota; b, aardvark of Africa, Orycter- 
opus, belonging to the Tubulidentata; c, three-toed sloth of South 
America, Bradypus, and d, six-banded armadillo of South America, 
Dasypus, both belonging to the Edentata, (a, after Beddard; b and c, 
after Schmid; d, after Brehm.) 

The extinct giant armadillo, Glyptodon, which was encased in an armor 
without bands that would enable it to roll up, was, however, so well pro- 
tected that, like a war tank, it did not need to imitate a pill bug. In spite 
of their armor these giants became extinct and their fossil remains are an 
eloquent memorial to the fact that something more than passive resistance 
is necessary in order to maintain a species on the earth. 

(8) pholidota. — The pholidotes include two genera, Manis and 
Pholidotus, the pangolins or scaly anteaters (Fig. 65a) of Africa and 
Southern Asia. They have elongate heads with long, very protrusible 
tongues with which they pick up termites. No teeth are present. Their bodies 
are covered with large, horny, overlapping scales. Powerful, sharp claws 
are present, especially on the front feet. Although long grouped with the 

7 2 

Biology of the Vertebrates 

edentates, these animals are more properly placed in a separate order for 
geographical as well as anatomical reasons. 

(9) tubulidentata. — The tubulidendates, formerly placed among the 
edentates, include a single species of the genus Orycteropus, the practically 
hairless aard-vark of the South African Boers (Fig. 65b). This animal has 
the long snout, long tongue and powerful claws of a termite eater. Its teeth, 
without enamel, have a perforated dentine. 

Fig. 66. Perissodactyl ungulates. A, wild ass of Syria, Equus onanger; 
B, tapir of South America, Tapirus; c, two-horned rhinoceros of Africa, 
Rhinoceros bicornis; d, one-horned rhinoceros of India, Rhinoceros 
indicus. (All after Beddard.) 

(10) peris sodactyla. — The perissodactyls (perissos, odd-numbered; 
dactyl, finger, toe) are large, herbivorous ungulates (ungula, hoof) which 
walk on the hoofed tips of an odd number of digits. Their molariform teeth 
have large chewing surfaces usually bearing high cross-ridges which greatly 
increase the effectiveness of the teeth in grinding grass, leaves and other 
vegetable matter. In all of these animals, the main axis of each appendage 
passes through the third digit, which therefore carries most of the weight. 

The tapirs (Fig. 66b) of South America and the Malay region have 
probably retained more of the general features of the ancestral stock of the 
Order than have any other members. They have four toes on the front feet 
(thumbs missing) and three on the hind feet (big and little toes missing). 
Their practically complete dentition includes relatively generalized molari- 
form teeth. Among their specializations is a short proboscis formed by a 
slight elongation of upper lip and nose. 

Kinds of Vertebrates 


The rhinoceros (rhino, nose; ceros, horn) group (Fig. 66c and d) 
includes several species of large, awkward animals with three toes on the 
hind feet and three or four on the front ones. On the top of the snout, in 
the mid-line, they have either a single horn or two, one behind the other. 
Each horn is really a mass of hairs stuck together. 

The horses (Fig. 66a) are characterized by the reduction of their 
digits until only one (the third) is in contact with the ground while two 
others ( the second and fourth ) are small splints at the base of the functional 
toe. All living species of horses, including their close relatives the asses and 
zebras, belong to the genus Equus. These animals have perhaps the most 
definitely traced pedigree of all mammals. Their record goes all the way 
back by successive links to Eohippus, a small Eocene ancestor about the 
size of a fox that had four toes on the front feet and three on the hind 
feet, as do present-day tapirs and some rhinoceroses. 

Fig. 67. Non-ruminant artiodactyls. A, peccary of South America, 
Pecari; b, wart hog of Africa, Phacochoerus; c, Hippopotamus of Africa. 
(All after Beddard.) 

(11) artiodactyla. — The artiodactyls (artios, even-numbered) are 
large, herbivorous ungulates with molariform teeth similar to those of 
perissodactyls. In these animals, however, the main axis of the leg passes 
between the third and fourth digits which usually bear equally the weight 
of the animal. The first digit (thumb or big toe) has disappeared and 
usually the second and fifth digits are considerably reduced. 

The pigs (genus, Sus), peccaries, wart hogs of Africa, and hippopotami 
use all four toes and have simple stomachs and no horns (Fig. 67). They 
are not limited to a diet of plants but will also eat flesh. 

All the other artiodactyla are ruminants (ruminare, to chew over 
again), so called because they first swallow their food whole, then regurgi- 


Biology of the Vertebrates 

tate it at their leisure, and thoroughly chew it ( Figs. 68 and 69 ) . The small 
balls of food returned to the mouth are known as cuds. Strictly herbivorous, 
they have three- or four-chambered stomachs adapted to their cud-chewing 
habits. They use only their third and fourth digits. Many are provided 
with defensive horns, either hollow and permanent, like those of the cow, or 
solid and periodically shed and renewed like the antlers of a stag. Among 
the better known members of this group are: the camels and dromedaries 
{Camelus) ; the South American llamas; the giraffes (Giraffa) ; the deer, 

Fig. 68. Representative ruminants, a, dromedary, Camelus drome darius. 
(After Schmid.) b, camel, Camelus bactrianus. (After Beddard.) c, llama 
of South America, Auchenia. (After Beddard.) d, chevrotain of India, 
Tragulus. (After Beddard.) e, giraffe, Giraffa; F, okapi, Okapia. (After 

Kinds of Vertebrates 


Fig. 69. Horned ruminants. A, reindeer, Rangifer. (After Beddard.) 
b, arctic musk ox, Ovibos. (After Schmid.) c, American buffalo, Bison; 
d, moose, Alces. (After Beddard.) e, antelope, Antilopa. (After Schmid.) 
f, pronghorn antelope, Antilocapra. (After Dugmore.) 

elks, moose, and similar forms; sheep (Ovis) ; goats (Capra) ; and domes- 
tic cattle (Bos). 

Ogden Nash has described the common cow as follows: 

"The cow is of the bovine ilk, 
One end is moo, the other milk." 

The articdactyls and perissodactyls are of great economic importance 
to man. They furnish us with portions of our clothing, transportation, food 
— including milk and much of our meat — and serums so important in 
fighting disease. 

(12) proboscidea. — The proboscideans are the elephants, largest of 
living land animals, which are so bulky that they are obliged to walk stiff- 

7 6 

Biology of the Vertebrates 

legged in order to support their tremendous weight (Fig. 70). The heavy 
head is sustained horizontally by a short stout neck, and the rigidity brought 
about by this arrangement, as well as by the stiff uncompromising pillar-like 
legs, is compensated by the development of a "trunk," a combination of the 
nose and the upper lip enormously drawn out into a flexible prehensile 
organ (proboscis). Each of their five digits terminates in a hoof -like struc- 
ture. Their molariform teeth reach the extreme of development of the 
grinding cross-ridges. Their upper incisors are greatly enlarged into long 

Fig. 70. Proboscidea. a, extinct mammoth, Elcphas primigenius. (After 
Schmid.) b, restoration of an extinct dinothere, Dinotherium giganteum. 
(After Abel.) c, African elephant, Loxodonta. (After Schmid.) 

Some proboscideans, such as the mastodons and the hairy mammoths 
of the Ice Ages, became extinct in comparatively recent times, geologically 
speaking, while other less specialized ancestors, as Dinotherium of Europe 
and Asia, and Palaeomastodon of Egypt, are considerably more ancient. 

There are two genera of living proboscideans : Elephas, the small-eared 
Asiatic elephant; and Loxodonta, the large-eared African variety. The 
former species has been domesticated in India for a long time. One famous 
individual, "Jumbo," weighed 6/ 2 tons and was 1 1 feet high. For years this 
gigantic beast entertained thousands of children, young and old, under 
Barnum as impressario. Jumbo's monumental skeleton stands in the 
American Museum of Natural History in New York. 

Kinds of Vertebrates 77 

(13) HYRACOiDEA. — The hyracoideans are coneys or "rock rabbits," 
which, according to the Book of Proverbs "are but a feeble folk, yet they 
make their homes in the rocks." They are small cud-chewing animals, 
superficially resembling guinea pigs, with hooflike tips to their toes, of which 
there are four on each front foot and three on each hind foot. They were 
probably derived from the same basic stock as the proboscideans and the 
sirenians. They include two genera: Dendrohy- 
rax (Fig. 71) of Africa, and Procavia of Syria 
and Arabia. 

Because of certain anatomical similarities the 
last four Orders just mentioned ( Perissodactyla, 
Artiodactyla, Proboscidea, and Hyracoidea) Y\z.l\. African coney, Den- 
may be artificially grouped together as the drohyrax. (After Beddard.) 
ungulates or hoofed mammals. Hoofs have prob- 
ably appeared independently in the different orders. For the most part they 
are large, rather stolid, plant-feeding creatures, most at ease when standing 
up on their highly specialized feet which are adapted for bearing continuous 
weight by being encased in shoelike hoofs. Unlike the soft-footed carnivores 
that collapse into a reposeful recumbent posture at every opportunity only 
to spring into alert activity upon the slightest incentive, ungulates never 
sit down and do not lie down without considerable deliberation and 

The 73 genera of ungulates include many kinds of great utility to man. 
They have also played a notable role in the past history of the world, as 
evidenced by the fact that 204 genera of fossil ungulates are known, many 
more than of any other order of mammals. These numbers are no doubt due 
in part to the readiness with which this group of animals has left fossil 
evidence of a former existence. 

(14) sirenia. — The sirenians, although of very different external ap- 
pearance, have certain unmistakable anatomical affiliations with elephants 
and vegetarian ungulates. They are large clumsy water animals having a 
broad snout covered with sparse coarse bristles and an otherwise hairless 
skin. The anterior legs are modified into swimming flippers, while the hind 
legs are entirely absent. 

They are perhaps the animals that have furnished the slender basis of 
fact from which imaginative sailors from time immemorial have spun tales of 
mythical mermaids and sirens. A less romantic but more apt common name 
for them is "sea cows." 

7 S 

Biology of the Vertebrates 

Of- this order only two genera (Figs. 72 and 73) are represented by 
living animals, that are separated from each other on the globe about as far 
as it is possible, since "manatees" representing the genus Trichechus inhabit 
the rivers of the northeastern coast of South America and beyond as far 
north as the Everglades of Florida; while the "dugong," Halicore, lives in 
the Red Sea and Indian Ocean. 

Fig. 72. Atlantic sea cow or manatee, Fig. 73. 
Trichechus. (After Dugmore.) Halicore. 

Dugong, or Indian Ocean sea cow, 

(After Schniid.) 

Of the 7 fossil genera, one, Rhytina stelleri, or Steller's sea cow, has been 
extinct less than 200 years. This species first became known in 1741 when 
Steller, a Russian whaler, was shipwrecked upon a small group of islands 
in Bering Sea. He was saved from starvation by finding a rookery of these 
large sea cows upon which he and his crew fed until rescued. During the 
following quarter of a century Russian whalers with human greed and 
stupidity hunted these valuable food animals to extinction, for Nordenskiold, 
who visited the islands in 1768, reported that the last individual of the 
colony had been killed. This species of sea cow has never been found 

(15) cetacea. — The cetaceans, or whales and their allies, among 
which are to be found the largest animals that ever lived, include the levia- 
thans of the oceans. The ancestry of 
the cetacea is a puzzle for the solution 
of which fossils give scanty aid. Compar- 
ative anatomy shows that they bear 
unmistakable hall-marks of mammalian 
forbears, such as breathing air by means 
of lungs and feeding the young upon milk. 
Since the mammalian plan undoubtedly 
originated with land forms, cetaceans 
must have undergone profound modifi- 
cation in order to become adapted secondarily to a marine existence where 
their great weight could be supported in water. By reason of their warm- 
bloodedness and a thick blanket of heat-retaining blubber under the skin, 

Fig. 74. Humpback whale stickling 
her young. (After Scammon. ) 

Kinds of Vertebrates 


these gigantic animals are able to pursue their activities even in Arctic 

While in the act of nursing, which is obviously accomplished under 
difficulties, the young whale presents a curious resemblance to a small tug 
attached to the side of an ocean liner (Fig. 74). 

Whales may be grouped into two suborders; odontoceti, or toothed 
whales that feed primarily upon fishes, and mystacoceti, or whalebone 
whales which, by means of a peculiar brushlike device of "whalebone" in the 
cavernous mouth cavity, strain out and swallow countless myriads of micro- 
scopic ocean inhabitants, that constitute for them a nutritious "sea soup" of 
unlimited supply. 

Fig. 75. Odontoceti, or toothed whales, a, killer whale, Orca; b, por- 
poise, Phocacna; c, narwhal, Monodun. (a, after True; b and c, after 
Schmid. ) 

The toothed whales ( Fig. 75 ) are usually not of extraordinary size and 
frequently forage about in their watery hunting grounds in schools. Some of 
them are: Delphinus, the dolphin; Phocaena, the porpoise; Grampus, the 
grampus; Orca, the killer; Monodon, the narwhal, with a single enormous 
twisted tooth projecting horizontally in front like a pikestaff ; Physeter, the 
sperm whale, with teeth only in the lower jaw ; and Hyperoodon, the bottle- 
nosed whale. The last two attain considerable size. 

Examples of the giant whalebone whales are: Rachianectes, the gray 
whale; Balaenoptera, the blue or sulphur-bottomed whale; Megaptera, the 
hump-backed whale; and Balaena, the right whale (Fig. 76). The blue 
whale is reputed to reach a length of 100 feet and a weight of perhaps 
150 tons. 

There are 9 genera of fossil cetaceans, and 27 genera of living ones, 
some of which are becoming scarce because they have been so relentlessly 
hunted by man. 

8o Biology of the Vertebrates 

Speaking of whales, John Godman a century ago wrote the following 
perfect apology for those travelers who return from foreign lands with tall 
stories to tell to those who stay at home: "Large as the size of the whale 

Fig. 76. Right whale, Balaena. (After Schmid.) 

certainly is, it has been much over-rated ; for such is the avidity with which 
the human mind receives communications of the marvellous, and such the 
interest attached to those researches which describe any remote and extraor- 
dinary production of nature, that the judgment of the traveller receives a 
bias, which, in case of doubt, induces him to fix upon that extreme point 
in his opinion which is calculated to afford the greatest surprise and interest." 


The Distribution of Animals in Space- 

Chorology and Ecology 


An observant traveler going from home in any direction gradually leaves 
behind a familiar world of animals and plants and, if his travels are suffi- 
ciently extensive, arrives in a land of strange organisms for the most part 
quite unlike those he already 
knows. He discovers that no one 
kind is to be found everywhere, 
but that each kind has its own 
home territory beyond which it 
does not ordinarily venture. In 
imagination he might map out 
upon the globe the home patch, 
with all its irregular boundaries, 
which each of the 600,000 or more 
species of living animals occupies. 
Such a map would be exceedingly 
complex because the areas thus de- 
limited would not only be very 
unequal in size but would also 
overlap each other in a great variety of ways like a gigantic palimpsest. A 
diagram to express this idea, in which the areas of only six instead of 
600,000 species are involved, is shown in Figure 77. 

Such a picture, moreover, if truly represented, would not be definite and 
fixed but would be a motion picture, presenting constant change like a 
rotating kaleidoscope, since the frontiers established by living things can 
never remain constant. It is well known that animals in the past occupied 
territory from which they are absent today, and that the contrary is equally 


Fig. 77. Hypothetical limits of the distribu- 
tion of six different species of animals, ar- 
ranged in superimposed areas. 

82 Biology of the Vertebrates 

Evidence from fossils, for example, shows that at one time tropical 
parrots and, at another, arctic reindeer were natives of what is now 
temperate France; that elephants formerly roamed over the United States; 
and antarctic albatrosses flew over England. Just as history records a suc- 
cession of civilizations, so plant and animal life, past, present, and future, 
presents a shifting scene. 

The locality where any species of animal is found is as much a diagnostic 
characteristic of the kind of animal in question as its peculiarities of struc- 
ture or behavior. It follows that any kind of fossil or living form loses much 
of its value for the scientist if the place it comes from is unknown. 


The comparatively new biological science of Ecology (oikos, home; 
logos, discourse) deals with the intimate arrangement and behavior of 
organisms within their respective habitats. Ecology may be defined as "scien- 
tific natural history." 

The province of the more inclusive science of C horology (choros, place; 
logos, discourse) is to determine the general distribution of animals and 
plants over the earth and to discover the why and wherefore of their occur- 
rence. Chorology is a science for the traveler, while ecology is the science 
for the stay-at-home. 


The immediate surroundings in which any animal is "at home" are 
called its habitat. In general animals are said to occupy either a land or 
a water habitat. Some of the more specific descriptive terms applied to 
habitats are: desert; forest; mountain; subterranean; prairie; meadow; 
marsh; pelagic; abyssal; pond; marine; fluviatile; and estuarial. This list 
of descriptive local areas may be almost indefinitely extended according to 
the minuteness with which the details are scrutinized. 

The arrangement of these various kinds of habitats over -the surface of 
the globe determines to -a large extent the distribution of living forms. It is 
obvious, for instance, that arboreal animals are not to be expected in the 
open ocean, which constitutes about three fourths of the entire surface of 
trie globe, and much less are fishes to be discovered in waterless deserts. 

Animals found living successfully in any habitat must be measurably 
adapted for life conditions there, although there are many cases in nature of 
imperfect adaptation where a square peg is attempting to fill a round hole, 
and vice versa. The usual result in such a misfit is that the peg either 
explores until it finds its proper hole, or gradually changes to fit the hole 

The Distribution of Animals in Space 83 

that it is in. Both hole and peg are changeable things but the initiative of 
change belongs not to the hole but to the peg. 

Any habitat is not occupied by all the animals and plants adapted to 
live in it. The prevalent idea, for example, that climate determines the 
distribution of organisms is largely erroneous. There are no grizzly bears in 
Switzerland, no birds-of-paradise in California, and no "snakes in Ireland," 
although the climate in each case is suitable for the absentees. The equa- 
torial forests of Africa and South America have practically the same 
climate, yet the former region is characterized by elephants, apes, leopards, 
giraffes, and guinea fowl, while the latter has none of these animals but does 
support tapirs, long-tailed monkeys, jaguars, and toucans, which are never 
found in Africa. 

So long as mankind was satisfied with the naive supposition that the 
earth has been arbitrarily populated by independent acts of special creation, 
much as a person might arrange chessmen upon a board, there was no 
sense nor object in developing a science of chorology. There was nothing 
to explain. Leopards, for example, were in Africa and jaguars in South 
America because they were placed there, newly made, in the beginning. 
The two kinds of large cats were entirely independent in origin and without 
any relation to each other. When men developed the conception, culminat- 
ing with Darwin, that all organisms are more or less related to each other 
as descendants of common ancestors, and that every species arose in the 
course of -time by modification from some other species, then the manner 
of distribution over the earth became full of significance, seriously challeng- 
ing the attention of thinking people. 


Three laws governing the distribution of animals were formulated by 
Jordan and Kellogg in Animal Life. These laws may be stated as follows: 
Every species is found everywhere unless (1) it was unable to get there; 
(2) having "got there," it was unable to stay; or (3) having arrived, it 
became modified into 'another species. It will be profitable to consider these 
laws briefly. 

First, it is not the suitability of a habitat so much as its accessibility from 
a place of origin, that determines the presence of an inhabitant. For in- 
stance, there are no hummingbirds in Africa, while there are over 450 
species in South and Central America, not because Africa itself is unfavor- 
able to hummingbird occupation, but because these tiny fairylike creatures 
have never been able to cross the wide oceans separating their ancestral 
American home from farawav Africa. 

#4 Biology of the Vertebrates 

Second, there are many instances of animals and plants that have suc- 
ceeded in invading new territory, but have been unable to hold their own 
there. Not all pioneers become settlers. At one time our federal govern- 
ment introduced a herd of camels after an adventurous sea voyage into the 
semi-arid region of the Southwest, and allowed them to run wild in the hope 
that they would multiply, spread, and eventually form a valuable addition 
to a region inhospitable to most large animals. The environment was very 
like that from which the animals came and the experiment might have 
proved successful but, as has been asserted, for the unfortunate fact that 
local cowboys, with little regard for consequences, had so much sport 
periodically rounding them up and putting them through their paces, that 
the strange incongruous beasts were literally worried to death. 

Third, successful invaders may win out in occupying new territory at 
the expense of their own specific individuality. They are the adaptable 
round pegs thrown into new habitats of square holes, that nevertheless 
remain and square themselves to fit the new holes. They are the immigrants 
that have deserted the ways of their mother country and become naturalized 
in the land of their adoption. A classical illustration of cases of this kind, 
cited by Darwin in The Origin of Species, is that of animals upon the 
Galapagos Islands off the northwest coast of South America. Of 26 species 
of land birds found upon these islands, 23 species are similar to, but still 
specifically different from, those inhabiting continental land a few hundred 
miles away. The interpretation give by Darwin is that when the Galapagos 
group was separated from the mainland in recent geological times, a new 
habitat was formed in which various individuals of continental species were 
isolated. Each of these 23 species, changing gradually under the molding 
influence of isolation, grew to be sufficiently different from its original main- 
land ancestors and cousins to rank as a different and distinct species. These 
facts so impressed Darwin that he began to think about the origin of species, 
with the fortunate result that subsequently a great many other people were 
induced to think about the same subject. 

To these three laws of distribution may be added a fourth, namely: 
Each species originated historically from some preceding species at some 
definite place, and its present distribution is the result of two opposing forces, 
expansion and repression. 


It would be as impossible for an unrestrained gas to remain in one place, 
as for any species of animals or plants to forego the attempt to occupy unoc- 

The Distribution of Animals in Space 8c 

cupied territory to which it has access. The reason for this is the enormous 
possibilities of expansion inherent in the reproductive processes of all organ- 
isms, a condition formulated by Malthus (1766-1834) in his "Law of 
Overpopulation." For example, when a single codfish produces 9,000,000 
eggs in one season, it is obvious that infant mortality must come to the 
rescue, else in a few generations every available inch of space in the ocean 
would be preempted by codfish. Even slow-breeding animals like elephants, 
which produce perhaps six young in a lifetime of a hundred years, would, 
according to Darwin, require less than 800 years to produce from a single 
pair nearly 19,000,000 elephants. Allowing 20 feet of space for each ele- 
phant, this would make a continuous parade, which would have delighted 
Barnum, reaching nearly three times around the world at the equator. Ele- 
phants and codfish, however, do not multiply out of all bounds as the above 
theoretical figures suggest, for the expansive forces of reproduction are kept 
in control, year in and year out, by opposing repressive factors which main- 
tain a balance in nature. 


1. The Food Problem 

Somewhere in one of his delightful essays, Dr. Samuel McChord Croth- 
ers presents the illuminating statement that "the haps and mishaps of the 
hungry make up natural history." There is no doubt that the insistent need 
for food, as expressed by hunger, is a mainspring of animal activity that, 
like a centrifugal force, compels animals to go forth in the quest of what 
they may devour. Even among higher animals which exercise parental care, 
there comes a time when the young may expect no longer to share food 
with their parents but must seek fresh pastures. To illustrate with a botanical 
instance, it would be disastrous if the acorns produced by an oak tree all 
remained to grow up within the parental circle. 

Not only is there competition for food and place among animals and 
plants of a kind, but there is also severe rivalry between different kinds of 
creatures for the same food supply. The miscellaneous company which at 
any time sits at Mother Nature's table, does not always, or even often, 
observe the restrained table manners of polite society, so that there is every 
inducement to go elsewhere. 

2. Change of Habitat 

Another general factor that causes organisms to spread, is change in the 
habitat occupied. Such a change may be temporary, like the drying up of 

86 Biology of the Vertebrates 

ditches and streams that affects aquatic organisms, or it may be permanent, 
like deforestation at man's hands, which leaves arboreal animals homeless 
and leads to flood conditions. 

It may be sudden and catastrophic, like a prairie fire or a flood, forcing 
all sorts of animals to flee at once for their lives ; it may be gradual like the 
change of seasons, when winter succeeds summer; or it may be so very slow 
that it extends over generations in time, like the relentless dawn of a glacial 

In all cases, however, when an environment becomes unfavorable, there 
are at least four alternatives open to the inhabitants : ( 1 ) organisms may 
simply succumb to the environmental change, completing their normal life 
cycles before the unfavorable conditions befall, as in the case of annual 
plants and most insects; (2) they may retire from active life and mark time 
while temporary unfavorable conditions last, as do hibernating animals and 
encysting protozoans, or trees that shed their leaves in winter and "hold 
their breath" until spring; (3) they may remain plastic enough to change 
themselves as the environment changes, thus keeping pace by adaptation to 
new conditions; or (4) they may forsake unlivable surroundings and seek a 
more favorable place to carry on, like migrating birds, grasshoppers, and 
emigrants of all kinds. This latter alternative of migration, brought about 
by change in the habitat, plays an important role in the distribution of 
animals and plants. 


The ways and means, direct and indirect, that are employed by organ- 
isms for dispersal, furnish a fascinating chapter in natural history. Only a 
few of the most common agents may be mentioned here. 

Among plants wind is an important agent. In many instances seeds are 
rigged with ballooning or parachuting devices, or are so light as to be easily 
borne upon currents of air. The tiny dustlike seeds of certain orchids, for 
instance, have been known to float in air from Holland across the North 
Sea, while molds testify to the efficiency of air movements in scattering 
spores of these ubiquitous organisms everywhere. 

Over 60 species of North American birds have been reported, which 
have reached Europe and become established there by being borne out of 
their migratory routes by winds, while flying insects like grasshoppers an' 
frequently assisted in their widespread movements by air currents. 

Water furnishes another highway for travel. The uneasy tides keep the 
congested inhabitants of the seashore constantly stirred up and on the move, 

'ihe Distribution of Animals in Space S~* 

while flowing streams and ocean currents act continually as agents in the 
involuntary transfer of all sorts of organisms from one place to another. 
Even floating icebergs are precarious rafts upon which stray arctic animals 
are sometimes borne some distance into new regions. 

Animals themselves assist each other in dispersal in a multitude of ways, 
as stowaways, kidnappers, and "thumbies." The larval glochidia of certain 
sluggish fresh-water clams of the genera Unio and Anodonta, fasten them- 
selves to the gills of swiftly moving fishes, thus stealing a ride to some distant 
point in the stream where they detach themselves and set up their semi- 
stationary housekeeping in a new place. Parasites naturally go wherever their 
hosts go, and so are introduced into the society of new hosts. Animals are 
particularly useful agents in scattering the seeds of plants. "Sticktights" and 
burrs of all sorts are makeshifts on the part of plants to steal a ride by 
attaching to passing animals. Seeds of various kinds too are embedded in 
attractive fruits with the result that they are eaten by animals and so depos- 
ited in some new locality after passing unscathed through the digestive tube 
of the traveling host. Thus cherry bushes are planted beneath a wood 
thrush's nest, and fences along the waysides are draped with poison ivy by 
feathered conservationists. 

The mistletoe, which grows parasitically as an air plant attached to the 
bark of trees, presents an extreme instance of distribution through animal 
agency. Doves eat the seeds of the mistletoe, because they are encased in 
alluring sticky berries. Frequently it happens, much as when the traditional 
small boy emerges from the jam closet, that remains of the feast adhere 
around the margin of the mouth. The dove flies away to another tree where 
it performs its toilet by wiping a sticky beak, with the adhering seeds, upon 
a branch. The seeds are wiped off in this way and stuck to a fresh branch 
in the exact location favorable for the growth of a new epiphytic plant in a 
new place. 

Of all animals man, however, has probably done more than any other 
in furthering the spread of organisms. In many instances this has been done 
intelligently and to the ultimate benefit of man himself, as in the case of 
cultivated plants and domesticated animals. The biological landscape has 
been changed almost everywhere by the transforming hand of man. Crops 
of various kinds dot the surface of the globe where wilderness once flour- 
ished, while introduced flocks and herds roam in safety over territory once 
the possession and battleground of native wild animals. 

Frequently man has made serious mistakes from the human standpoint 
in meddling with the balance of nature. The introduction of that over- 
successful "avian rat," the English sparrow, into the society of American 

88 Biology of the Vertebrates 

birds has been many times regretted by man on account of injury to native 
birds with which it comes into competition. The bloodthirsty mongoose, that 
was brought to Jamaica and also to Hawaii to kill rats infesting sugar cane 
fields, proved to be an efficient rat exterminator, but it went further and 
destroyed other animals, particularly chickens. As a result poultry-raising in 
these islands has been seriously interfered with, and now a price has been 
set on the head of every mongoose. 

Several years ago a gentleman in Medford, Massachusetts, who con- 
ceived the idea that some more hardy insect than the silkworm might be 
found to spin silk, and at the same time feed upon less restricted food than 
mulberry leaves, brought back from Europe a few gipsy moths, Porthetria 
dispar, to use in experiments. The box in which they were contained, so the 
story goes, was accidentally knocked out of an open window and some of 
the moths escaped, but for the time the incident was forgotten. This was in 
1869. By 1889 the descendants of these chance immigrants had prospered 
to so great an extent that the people around Medford became alarmed and 
a town meeting was held at which $300 was appropriated to fight the pests. 
"In that summer," the record shows, "the numbers were so enormous that 
the trees were completely stripped of their leaves, the crawling caterpillars 
covered the sidewalks, the trunks of the shade trees, the fences, and the sides 
of the houses, getting into the food and into the beds." Dr. Lutz in his Field 
Book of Insects published in 1921 wrote: "Millions of dollars have been 
spent in an effort, so far unsuccessful, to free us from the invader, and the 
most that has been done has been to confine it to New England." 

The white cabbage butterfly, Pieris rapae, first came to America from 
Holland in a sloop load of wormy cabbages landed at Quebec in 1861. 
Twenty years later, according to the Department of Agriculture at Wash- 
ington, it had colonized America on the Atlantic Coast from Hudson's Bay 
to Florida. In 1886 it had arrived at Denver, and in 1900 had reached the 
Pacific Coast, having accomplished the conquest of the entire United States 
and a part of Canada in less than thirty years. 

Shipworms, Teredo, on the outside of the hulls of vessels, and rats on 
the inside have spread themselves the world over wherever shipping has 
gone. In 1827 mosquitoes, traveling as "wigglers" in the bilge water of a 
sailing vessel, arrived at the Hawaiian Islands. They have prospered since 
then and made the Islands their own. 

Several years ago a marble statue, made by the sculptor Thorwaldsen in 
Italy, was set up in Copenhagen. It is said that, as an accidental result, 
twenty-five species of Italian weeds, the seeds of which were in the straw 
packing around the statue, made their appearance in the immediate vicinity. 

The Distribution of Animals in Space 89 

Similar instances of the effects of the interference of meddlesome man 
with the natural arrangement of organisms in space could be indefinitely 


Among various factors which hinder world conquest on the part of any 
single species of animals or plants, are: (1) inadequate means of dispersal; 
(2) non-adaptability to new conditions; and (3) barriers of different kinds. 

1. Inadequate Means of Dispersal 

The difficulties of "getting there" are not especially apparent in the case 
of free-moving animals like birds and insects. They become very real, how- 
ever, as well as serious for many organisms whose structure is not particu- 
larly adapted for locomotion over considerable distances, yet the race is by 
no means always to the swift. The story of the tortoise and the hare finds 
plenty of parallels in nature. 

When Gould, with his searching eye for mollusks of all kinds, wrote 
A Report on the Invertebrata of Massachusetts in 1841, he made no men- 
tion of Littorina litorea, a small familiar periwinkle that at present is one 
of the most abundant species of snails along the Atlantic Coast. In 1855 
Morse found a few of these animals in the Bay of Chaleur at the mouth of 
the Saint Lawrence river, which had been accidentally brought over in 
ballast from their original home in Europe. By 1875 Verrill reported two 
as a rare find at Woods Hole, Massachusetts, several hundred miles to the 
southward, and in 1880 Smith found the first one to be noted as far south 
as New Haven, Connecticut. 

Another example of the surprising spread of an animal handicapped by 
poor methods of locomotion is that of Sagartia luciae, a small semi-trans- 
parent sea-anemone that lives attached to stones in the tidal zone. It was 
discovered at New Haven by Verrill in 1892, who named it luciae in honor 
of his daughter Lucy. In 1895 it was reported at Newport, R. I.; in 1898 
at Woods Hole, Mass.; in 1899 at Nahant, Mass.; in 1901 at Salem, Mass. ; 
and in 1903 at Cold Spring Harbor, Long Island, N. Y., where it is now 
abundant. Thus this "stationary" animal spread itself over several miles of 
coast line within a single decade. 

2. Non-adaptability to New Conditions 

The non-adaptability of organisms to new habitats, which they may 
have invaded, is doubtless much greater than appears on the surface, for it 


Biology of the Vertebrates 

surely acts as a deterrent to their spread. Successful invaders that gain a 
new foothold and retain it catch the eye and claim attention, while unsuc- 
cessful ones which reach the Promised Land but are unable to establish 
themselves there, escape attention and pass unnoticed. 

Many plants that thrive under cultivation, like maize or Indian corn, 
appear to be unable to maintain themselves in nature when by chance they 
are allowed to run wild. 

The yellow fever mosquito, Stegomyia fasciata, fortunately does not 
succeed north of a certain dead-line, although no doubt it has repeatedly 
crossed this invisible limit, like the English ivy that clothes the walls of 
southern buildings in luxuriance, but fails to grow well in more northern 
situations, in spite of being repeatedly planted and nurtured there. 

3. Barriers 

Barriers which check or stop organisms on all sides are at least three in 
kind: physical, geographical, and biological. 

Temperature is a widespread physical barrier. The exclusion of "cold- 
blooded" animals, such as amphibians and reptiles, from the occupation of 


Fig. 78. Diagram of the general parallel sequence of organisms in alti- 
tude and longitude. 

lands of prevailingly low temperatures, is quite evident. In general tempera- 
ture zones extend not only in latitude north and south from the equator, 
but also in altitude in a parallel succession from tropical sea level to high 
mountain peaks (Fig. 78). 

In the ocean, pressure acts as a barrier that stratifies the inhabitants 
within certain limits to which they have become specifically adapted. Deep- 
sea fishes cannot pass freely from abysses to surface waters, nor can pelagic 

1'lie Distribution of Animals in Space 91 

forms sink far below and survive. Similarly in the air there is an upper alti- 
tude limit beyond which flying birds cannot rise. 

Humidity sets up a barrier which, according to its degree, is largely 
impassable to exploring organisms, dependent upon a certain optimum of 

Light, another physical barrier, halts the traffic of nocturnal darkness 
lovers, although it usually has more of an ecological than chorological bear- 
ing. Green plants, on the other hand, do not live in the abyssal regions of 
the ocean because photosynthesis cannot be carried on there in the absence 
of light. 

Geographical barriers are such features of the earth's surface as oceans, 
land masses, rivers, mountains, waterfalls, deserts, forests, and the like. A 
barrier to one organism, however, may be a highway to another. Thus, a 
desert would form an impassable barrier to a squirrel but not to a camel, 
while a forest in which a squirrel would revel would prove an effectual 
barrier to a camel. 

Biological barriers are bound up in the first place with the eternal food 
problem, since absence of food of a particular kind in a region may prevent 
the advance of invading animals, while poverty of soil discourages occupa- 
tion by plants dependent upon the missing soil constituents. 

Secondly, biological barriers often take the form of other animals, by 
habit predaceous or parasitic, which hinder or forbid advance in certain 

Thirdly, the greatest biological barrier of all is man, since he is able to 
control the forces of nature far more than any of his animal allies. It should 
also be pointed out that limiting biological barriers may exist within animals 
themselves in the form of scanty wits, lack of initiative or adaptability, 
causing failure to enter into new areas even though the door of opportunity 
swings wide open. 

If the factors of expansion and repression were equal in all directions, 
the area occupied by each species would remain constant as a perfect circle, 
but such a condition is unknown. The irregular shapes and boundaries of 
claims actually staked out in nature by various organisms proclaim the com- 
plex interaction of the fundamental opposing forces that determine distri- 


An attempt has been made by chorologists to divide the land masses 
of the world into life realms, according to the distribution of animals and 

9 2 

Biology of the Vertebrates 

plants. Such realms in no way coincide with the familiar political bound- 
aries that separate nations from each other, being much more indefinite 
in their limits. 

It is evident that life realms must vary according to the kind of animal 
or plant inhabitants selected to serve as determinants. Perhaps the first 
serious attempt to divide the earth into zoological realms was made in 1851 
by Sclater, who based his conclusions upon the distribution of birds. There 
are, however, very apparent objections to utilizing vagrant and barrier- 
defying creatures like birds for this purpose. Accordingly Murray in 1866, 
and more in detail Alfred Russell Wallace in 1876, divided the surface of 
the earth into zoological regions based chiefly on the distribution of mam- 
mals. Reptiles, amphibians, fresh-water fishes, insects, and spiders have each 
in turn been used as the foundation for zoological map making, as well as 
various combinations of animals, but mammals undoubtedly present the 
most advantages for this purpose. The reason for this lies in the fact that 

TABLE I. Number of Mammalian Families 

Represented in Each of Wallace's Subregions 

























Orders w 






















































































































































































































































































































'['he Distribution of Animals in Space 


mammals are warm-blooded, capable of occupying a great range of habitats, 
and being the most recently evolved large group of animals on the earth, 
have not had as much time as other types of animals to radiate from their 
centers of origin, with consequent confusion as to which species are native 
or endemic, and which introduced. 

Fig. 79. Mercator map of the world, divided into zoological regions and 
sub-regions. Compare with Table I. (According to Wallace.) 

Wallace's classification consists of six large regions, each divided into 
four subregions, as indicated in Table I. This table shows also the number 
of different families of mammals represented in each of the 24 subregions. 
It will be seen that bats (Chiroptera) are most generally represented, there 
being no subregion that does not have at least one of the five families of 
bats within its borders, while whales (Cetacea) do not appear at all, 
because they are not definitely associated with any land masses. 

The richest subregion so far as numbers of mammalian families goes is 
the South African, although the East African, West African, Mediterranean, 
and Indo-Chinese are likewise conspicuously populous. The poorest is the 
New Zealand subregion that has no native mammals with the exception of 
two families of bats. 

The mercator map of the world ( Fig. 79 ) shows roughly the extent of 
each of Wallace's regions and subregions. 

94 Biology of the Vertebrates 

The study of chorology helps to solve such puzzles as ( 1 ) why related 
species, like the edentates in South America, are found in the same conti- 
nental areas; (2) why areas physically alike, as Africa and South America, 
have different kinds of inhabitants ; ( 3 ) why an original center for a species, 
like Wyoming for lemurs, may have no living representatives today ; and 
(4) why regions near together, for example, Florida and the Bahamas, may 
have quite diverse faunas and floras, while regions remote from each other, 
like North America and Eurasia may have many similar forms. 


The Distribution of Animals in Time- 


It is quite as essential to an intelligent understanding of living organ- 
isms upon the earth today, to have some vision of the long pageant of 
preliminary life in the past, as it is for a statesman to be well versed in the 
history of events leading up to the state of affairs with which his present 
problems are involved. 

Species of animals and plants, like individuals, pass through successive 
stages that resemble the phases of a single life. Expanding childhood, vig- 
orous youth, sustained maturity, and decrepit old age succeed each other 
only to end inevitably in death or extinction. Sometimes a species like an 
individual may complete its life without leaving any issue behind, but 
oftener, in the long course of its existence, it somehow gives rise to species 
different from itself, a process which has brought about the infinite diversity 
of living forms that connect monad with man. 

Certain conservative kinds of organisms that are well adapted to their 
niches in nature persist, retaining their characteristics without significant 
evolutionary advance for unthinkably long periods of time, while other 
species, exhibiting a wider range of variability, live a faster, more diversified 
life and advance more rapidly along the transforming highway of evolution, 
only to meet extinction sooner. The brachiopods, Lingula and Terebratula, 
for example, the modern living representatives of which are hardly to be 
distinguished from remote fossil ancestors found buried in the most ancient 
sedimentary rocks, are instances of conservative species that have shown 
almost no progress, while trilobites, ammonites, pterosaurs, and dinosaurs 
are large representative groups of more ambitious animals, of astonishing 
diversity of form and structuial detail, which have long since paid the death 
penalty for their high degree of specialization. 


g6 Biology of the Vertebrates 

Examples of all stages of the process of coming to an end on the part of 
a species may be cited. For instance, among birds the ivory-billed wood- 
pecker and California condor are probably marked for extinction in the 
near future. This is not so much because they are being crowded off the 
earth by dominant man as because they are in the biological blind alley of 
overspecialization with a consequent lack of ability to adapt themselves to 
changing conditions, which means that they are nearing the end of their 
organic resources. In fact, birds taken as a group are so highly specialized 
that they have no future evolutionary escape, since that is possible only in 
generalized types having capacity for further adaptation. 

There are people now living who remember the hordes of passenger 
pigeons that formerly darkened the skies, but the last individual of this 
species died in captivity only a few years ago, while the passing of the dodo, 
the great auk, and Steller's sea cow are matters of recently recorded history. 
The hairy mammoth, New Zealand moa, sabre-toothed tiger, and woolly 
rhinoceros came to their end just before the beginnings of recorded hu- 
man history. Back of these recent antiquities stretches a long interminable 
line of various species whose chapter of existence closed so long ago 
that our ordinary measures of time entirely fail to express the fact ade- 

There is no doubt that living species number but a small fraction as 
compared with vanished ones formerly peopling the globe, whose race has 
long since been run. 

The dawn of life is unknown, for the oldest sedimentary rocks in which 
the first known evidences of life appear yield a wide variety of forms, such 
as protozoans, sponges, corals, jellyfishes, echinoderms, worms, brachiopods, 
mollusks, and trilobites. This means that the great Canterbury Pilgrimage of 
organisms had already been traveling for some time along the evolutionary 
road, before we catch our first glimpse of the pageant. 


Fossils are nature's hieroglyphics. They include the sum total of our 
actual documentary evidence of organic evolution, and besides form the 
alphabet in which the language of biological history is written. Sir Charles 
Lyell, the eminent geologist who did so much to influence young Charles 
Darwin at the beginning of his career, defines a fossil as "any body or traces 
of body, animal or vegetable, buried and preserved by natural causes." 
Every fossil is cither ancestral to some living thing, or is representative of 
an extinct line. 

The Distribution of Animals in Time 07 

The science of fossils, or the ancient history of animals and plants, is 
called Palaeontology. 

1. Former Ideas About Fossils 

Fossil remains of animals and plants, although known for a long time, 
have been variously misunderstood in the past. To Aristotle and the ancients 
they were artificial results of spontaneous combustion, or abortive attempts 
of inorganic matter to take on the form of life. Empedocles, who found fossil 
hippopotamus bones in Sicily, thought he had discovered a battle-ground 
where gods and titans fought. Henrion, in 1718, regarded fossils as molds 
and casts left over in the creation of animals and plants. He was the cock- 
sure writer who reported that the height of Adam was 123 feet and 9 inches, 
but since he carelessly neglected to specify whether or not the measurement 
was taken in his "stocking feet," and as he did not make clear how he 
arrived at his result, his opinion is regarded with some suspicion by modern 

As late as 1823 William Buckland of Oxford wrote learnedly of fossils 
under the title, On Observations on Organic Remains attesting the Action 
of a Universal Deluge. Lyell states that it took a hundred and fifty years of 
dispute and argument to persuade scholars that fossils were really remains 
of what were once living organisms, and a hundred and fifty years more to 
convince them that they were not the results of Noah's flood. 

Today a vast number of fossils have been recovered from oblivion from 
many parts of the world, and together they present a most illuminating and 
convincing mass of evidence concerning the ancient inhabitants of the earth. 
Even when fragmentary and imperfect, as most of them are, they furnish 
irrefutable proof of vanished life. The only questions that arise about fossils 
today concern the restoration of missing parts, the period or geological hori- 
zon when they lived, and their place in the evolutionary series. Dr. Lull of 
the Peabody Museum at Yale University, whose wide knowledge of fossils 
gives weight to his opinion, declares that "of the finally established facts 
which the fossils proclaim, we are as certain as we are of anything in this 

2. Conditions of Fossilization 

Various factors are involved in the process of fossilization. There is no 
reason to believe that these factors which have been effective in the past are 
not at work today. The great majority of individual animals and plants do 
not become fossils, but return at death to their inorganic origins along the 
route of decay, or by being devoured by animals. 

9^ Biology of the Vertebrates 

It is usually essential that hard parts like bones, teeth, shells, scales, or 
chitin be present, and that the conditions for natural burial and the exclu- 
sion of air be such as to aid in the preservation of these parts. 

However, Dr. C. D. Walcott has published a book of unexpected facts 
concerning Fossil Medusae, in which are pictured a great variety of these 
fragile creatures which succeeded in leaving a fossil record of themselves in 
spite of the fact that their jellylike bodies had no hard parts, and were over 
95 per cent water. 

The manner of burial in fossilization may be sudden and catastrophic, 
as by landslide, earthquake, devastating flood, overwhelming sand storm, 
or by a rain of volcanic ashes such as fossilized the entire cities of Hercu- 
laneum and Pompeii, or it may be exceedingly slow, as in the formation of 
sedimentary rock under water, the incrustations resulting from immersion 
in mineral-impregnated hot springs, or by the drip of limy water which 
forms stalactites and stalagmites in limestone caverns. 

Quicksands, swamps, and bogs may engulf animals also and thus favor 
fossil formation by preventing rapid decay through the exclusion of air. As 
a matter of fact "bog water" is said to possess antiseptic properties to a 
remarkable degree. 

Amber, which is fossilized pitch, or the solidified juice of resinous plants, 
furnishes another kind of burial place. Insects crawling on the trunks of 
ancient conifers, that became entangled in the sticky exudations there, have 
succeeded far better than any ancient mummified Egyptian, dreaming of 
immortality, in perpetuating their mortal bodies intact in a world of uni- 
versal decay. 

At Rancho la Brea, near Los Angeles, California, there are famous 
asphalt beds in which at some time long ago a great variety of animals, 
horses, tapirs, llamas, elephants, mastodons, giant sloths, huge wolves, lions, 
and sabre-toothed tigers, were not only entrapped and killed but were also 
preserved as fossils. 

In detritus-filled caverns where dying animals have retreated, fossils are 
frequently found. 

Sixty miles north of the Arctic Circle at Beresovka, in Siberia, a mam- 
moth was discovered in a pit, frozen and so perfectly preserved in ice that 
some of the flesh was eaten by the discoverers, many thousand years after 
it was accidentally placed there in cold storage. This was not an isolated 
case. Many other instances of frozen carcasses of mammoths have been 
reported in northern Siberia. 

On oceanic islands, such as the Chincha Islands off the coast of Peru, 
where for long periods of time sea birds have resorted to nest and where 

The Distribution of Animals in Time 


there is scarcely any rainfall, the dried excreta of birds, commercially known 
as "guano," is deposited, frequently to a depth of several hundred feet, foim- 
ing a natural burial place for organic remains. 

In the Peabody Museum at Yale University is the skeleton of an extinct 
species of ground sloth, Nototherium, that was recovered from a cave in 
New Mexico where it was buried and preserved in bat guano. The preserva- 
tion was so complete that it was possible to determine by the stomach con- 
tents that it died in the spring of the year, and that the vegetation of Pleis- 
tocene times was practically like that of today. 

3. Uses of Fossils 

Fossils, as Dr. Joseph Leidy many years ago quaintly said of the Pro- 
tozoa, are chiefly useful as "food for the intellect." 

Among various intellectual uses to which fossils are put, not the least 
is that of "faith testers," so called by good people alarmed at the silent evi- 

























(North and South America, Asia, Europe and Africa) 

I I 

(North America, Asia, Europe and North Africa) 

(Sooth America) 

(South America.) 

(North America) 

(North America! 

(North America) 

(North America) 

(North America. Asia and Europe) 

(Europe and North America) 

r* x 


(North Ameri 
(North America) 
(North America) 
(Europe and North America) 

Fig. 80. The evolution of the modern horse, Equus. (After Matthew. 

dence thus presented of the great antiquity of the earth which they had been 
taught to believe had been created only a few thousand years ago. To the 
scientist these "medallions of creation" show first of all something of the 
racial history of animals and plants. In the absence of direct evidence, the 
past history of most animals and plants must remain largely a matter ol 
conjecture, but there are some authentic instances of modern animals whose 

ioo Biology of the Vertebrates 

ancestral modifications are written very legibly in the fossils that have been 
found. For example, the horse has a well established family tree extending 
backward without serious gaps for at least three million years to the little 
four-toed ancestor, Eohippus, of Eocene days. The actual fossil evidence for 
this remarkable pedigree may be seen by any visitor at the American 
Museum of Natural History in New York City, or at the Peabody Museum 
of Yale University in New Haven (Fig. 80). 

Fossils are furthermore useful as indicators of past climatic conditions 
on the earth. The discovery of fossil palms in Wyoming, breadfruit in Cali- 
fornia; ferns in Greenland; reindeer in France; and musk oxen in Ken- 
tucky, records the indisputable fact that profound changes in climatic con- 
ditions have occurred in all of these places in the past. 

Fossils also serve as measures of geologic time. Just as the date upon the 
corner stone indicates the year when the building was dedicated, so the 
presence of certain types of fossils in a particular stratum of sedimentary 
rock indicates the approximate time when those rocks were laid down. Or 
to state the value of a time measure by a further comparison, just as the 
character / instead of s on the page of an old book measures the limit of its 
publication by the year 1800, about which time the character s replaced / 
in general use, so the presence of a time-dating fossil on a geologic page 
measures the limits of its formation. 

4. Kinds of Fossils 

The following classification of different kinds of fossils is modified from 
that given by Professor R. M. Field in Science for June 25, 1920. 

I. Those furnishing direct evidence: 

1. Actual remains, such as insects in amber, and mammoths in ice; 

2. Minute replacements, molecule by molecule of the original organic mat- 
ter by mineral salts, resulting in petrifaction', 

3. Coarse replacements, secondhand copies of originals by means of molds 
and casts; 

4. Prints and impressions, of leaves, jellyfish, etc. 

II. Those furnishing indirect evidence: 

1. Coprolites, that is, solidified excreta or casts of the same; 

2. Artifacts, such as ant-hills or prehistoric fashioned flints; 

3. Tracks, trails, and burrows, all autographs of living animals; 

4. Geologic formations, originating from organic sources, such as graphite, 
limestone, flint, coal, and petroleum. 

The Distribution of Animals in Time toi 


Huxley said that the whole geologic record of fossils is "only the skim- 
mings of the pot of life." Although incomplete it is nevertheless the most 
convincing evidence of the story of the past. 

The absence of suitable conditions for fossilization which surrounds the 
passing of the vast majority of animals and plants, as well as the inacces- 
sibility to man of most of the fossils that actually succeed in being formed, 
make the task of the palaeontologist a particularly difficult one. The pages 
of the Great Stone Book on which the buried dead have written their own 
autographs cannot be freely shuffled over in order to read the story con- 
tained therein, because they are firmly stuck together. The fossil writing is, 
therefore, quite inaccessible except as lucky chance reveals enticing frag- 
ments of it, as when slow erosion bevels down the margin of the page expos- 
ing some few organic syllables, or when, by the puny engineering feats of 
man, the surface of the earth is somewhere scratched open, accidentally 
uncovering part of its buried treasures. 

In many instances the natural sequence of rock stratification has been 
so confused that the student finds the pages of his book misplaced, by dis- 
tortion, faulting, or folding, as in mountain formation. The more recent 
strata sometimes even come to lie beneath the older ones. The irregular and 
fragmentary character of the fossiliferous strata thus greatly increases the 
difficulties that confront the student who would correctly read the story of 
the past. 

Sedimentary rocks of the earth's crust containing fossils are not arranged 
in uniform continuous strata that envelop the entire globe like the layers of 
an onion, but form in patches of unequal thickness and extent, according to 
the distribution of the water areas at the time of their deposition. There is 
no doubt that the earlier records of life in the form of fossils have in many 
cases been entirely obliterated by the action of heat and pressure during the 
metamorphosis of rocks into gneiss, marble, and granite, while the fossils 
that are buried in sedimentary rocks of the ocean floor are "forever hidden 
from hammer and mind." 

According to the Bureau of Mines, Department of the Interior * the 
deepest hole that man has ever made down into the undisturbed fossil- 
bearing epidermis of the earth is in West Virginia, where borings to the 
depth of 7579 feet were made in search for natural gas. The deepest mine 
in the world is said to be the St. John del Rey mine in Brazil, while the 
"Village Deep" workings of the Transvaal gold mines of South Africa take 

* Science, N. S., LX, No. 1541. 

i()2 Biology of the Vertebrates 

second rank with a depth of 6263 feet. In the United States the deepest 
mine workings are those of the "Calumet and Hecla" in Michigan which 
are reported to have reached 5990 feet below the surface. This is a distance 
of about a mile and is the nearest approach that man has ever made to the 
center of the earth. These extraordinary depths when compared with the 
total diameter of the earth, or even with the known thickness of fossiliferous 
rocks, are so insignificant that it is doubtful if they could be graphically 
represented to scale even by a shallow scratch on the surface of a four-foot 
globe. David Starr Jordan has truly said that the case of the palaeontologist 
is much like that of a traveler who, landing for five minutes on some remote 
corner of Australia, forthwith attempts a description of the entire continent 
from the observations made. The wonder is not that so little is known of the 
fossil record of animals and plants, but that, in the face of so many diffi- 
culties, so complete and connected a story of ancient life has been unearthed. 


The fragment of eternity that comes within the vision of the geologist 
has been divided into unequal eras of time, beginning after the earth had 
cooled down enough to be clothed with an atmosphere and to have its 
surface diversified into areas of land and water. See Table II. The succeed- 
ing eras are measured by the time taken to form stratified rocks through the 
erosion and disintegration of the original fire-fused rocks, and the subse- 
quent rearrangement of their component particles as sediment under water. 
Such sedimentary rocks afford sanctuary to organic remains and form the 
happy hunting grounds of palaeontologists. 

Eras from ancient to modern times are: archaeozoic, proterozoic, 
palaeozoic, mesozoic, and CENOZOIC. 

The Archaeozoic Era is characterized principally by igneous and meta- 
morphosed rocks without proved fossils, although traces of graphite indicate 
that plant life, probably in the form of primitive seaweeds, must have been 
in existence. The fiery furnace that fashioned the archaeozoic rocks, how- 
ever, was no suitable place for the preservation of whatever organic remains 
existed in those formative days. 

The Proterozoic Era saw the slow rise of the lower plants and most of 
the main general types of invertebrate animals. Together with the Archaeo- 
zoic Era, according to Professor Schuchert, it constitutes over one half of 
the total column of known sedimentary rocks, which reaches- all together a 
maximum thickness of 114 miles in North America, although he qualifies 
this statement by saying: "In no one place, however, can be seen more than 

The Distribution of Animals in Time 


a small part of this record, for usually the local thickness is under one mile, 
though there are limited regions where as much as twenty miles of it are 

TABLE II. A Geologic Time Scale (After Schuchert and Dunbar) 








Periodic glaciation. Evolving of man. 
Mammals evolve rapidly. Man diverges 
from apes. 





First placentals. Extinction of dinosaurs, 
pterodactyls and toothed birds. 

Rise of teleosts, birds, and flying reptiles. 

Rise of dinosaurs. First primitive mam- 






Rise of reptiles. Another great ice age. 
Rapid evolution. Many extinctions. 
First known reptiles. Rise of insects. 

Accumulation of coal. 
First known amphibians. Rise of fishes. 
First known land flora. 
First fishes (ostracoderms). 
Abundance of marine invertebrates. 



Algae. Protozoa. Lower worms. 

Early and late glacial periods (ice ages) . 



Origin of protoplasm and simplest life. 
Little evidence of life. No fossils. 

* Millions of years estimated to have elapsed since beginning of era or period. 

The Palaeozoic Era has been called the "Age of Fishes" because these 
animals became dominant during this time. The actual interval which 
elapsed in the Palaeozoic Era has been estimated as about 300 million years, 
surely sufficient time for many dynasties of plants and animals to have had 
their day. 

Following the Palaeozoic, the Mesozoic Era witnessed the "Golden Age 
of Reptiles," some 150 million years long. 

Finally, the Cenozoic Era, or "Age of Mammals," probably represents 
a little more than 60 million years. The geologists usually subdivide the 
Cenozoic into seven epochs so that beginning with the oldest, the Tertiary 

io<l Biology of the Vertebrates 

is made up of the Paleocene, Eocene, Oligocene, Miocene and Pliocene 
which are then followed by the two Ouarternary epochs, the Pleistocene and 
Recent. During the Pleistocene there were great climatic changes in north- 
ern Europe and North America. Possibly as many as four different times a 
gradual fall in the temperature of the northern United States and southern 
Canada resulted in a southward advance of the ice sheet to cover these areas 
for tens of thousands of years, only to be followed by a sufficient rise in 
temperature to bring about a retreat of the glaciers and return of these areas 
to temperate or even tropical conditions. Thus periods of glaciation alter- 
nated with interglacial periods. Similarly, areas of northern Europe were 
subjected alternately to long periods of glaciation and temperate climates. 
The most recent episode in all this great moving spectacle of earth trans- 
formation is the story of human evolution, extending over only a few hun- 
dred thousand years at the outside, which in comparison with the stretches 
of time under consideration is but the thinnest surface film on the face of 
an abyssal ocean. Our actual fossil records of man are limited to Pleistocene 
and Recent times. 


A summary of some of the more important conclusions which follow 
from a study of fossils is embodied in the six "laws" adapted from Traite 
elementaire de palaeontologie by Jules Francois Pictet (1809-1872), as 
follows : 

( 1 ) All stratified rocks may contain fossils, therefore, life has been present 
on the earth at least since the beginning of the Palaeozoic Era. 

(2) The oldest strata contain extinct species and largely extinct genera, 
while more recent strata contain forms like the living, therefore, the 
deeper the stratum the more divergent from those now living are the 
forms found therein. 

(3 ) Different fossil faunas and floras follow each other in the same sequence 
everywhere, the layers nearest together stratigraphically contain forms 
most alike, therefore, fossils show the evolution of forms from one 

(4) Constant change is the inevitable law of life. Species characteristic of 
one level or time are partly or completely replaced later by other 
species, therefore, species are not permanent or unchanging but are 
constantly giving way to modified forms that are presumably better 
qualified to occupy their place in nature. 

The Distribution of Animals in Tunc 



Species, as well as individuals, pass regularly through a cycle, including 
infancy, youth, maturity, and senility, therefore, many groups of 
organisms (as graptolites, trilobites, and ammonites), have died out 
entirely and do not reappear, having completed their cycle. 
(6) The approximate age of any stratum may be determined by the degree 
of similarity of its fossils to living forms. Similar fossils in different 
regions are indications of geologic strata of contemporaneous forma- 
tion, therefore, fossils serve to determine the age of rocks in which they 
are found. 


Man in the Making-Anthropology 


One of the riddles that perennially charms and challenges us, is the 
origin of mankind on this earth, for the farther back we go the more vague 
is our knowledge about man. As a matter of fact it would be much easier 
to collect data about the iniquity of man than about his antiquity, because 
then we would have no lack of material for our discourse. 

The subject of the antiquity of man must always remain more or less 
shrouded in mystery. The reality of human antiquity, however, even in 
the absence of specific details, is beyond question. 

Twenty years ago Professor Walter drew the following personal illus- 
tration. "The writer was born sometime in the nineteenth century. In the 
eyes of children of the twentieth century he must seem to be quite ancient. 
He can remember when there was not a single automobile in existence. He 
has lived through the entire Golden Age of the Bicycle and participated in 
its rise and fall. He recalls when there was no radio, no telephone, no phono- 
graph, no electric lights, no X-rays, no typewriters, no motion pictures, and 
when Darius Green and his Flying Machine, by John T. Trowbridge, rep- 
resented the final word in aviation. He remembers his grandparents as very 
old people, associated with ox teams and candlelight, for they were babes in 
arms when the war of 1812 was being fought. Their grandparents in turn 
lived before the Revolutionary War and even traditions about them are now 
vague and hazy. Back of them there must have been many other genera- 
tions, but they lived so long ago that the present day has entirely lost sight 
of them." Beyond this personal survey, it is possible to resort to pages of 
history, going back in imagination to hoary landmarks of time such as the 
discovery of America, the Norman Invasion, the dramatic beginnings of the 
Christian Era, and far beyond these milestones to remote semi-mythical 
days when the Ten Thousand beat their famous retreat, or the Children of 
Israel passed dry-shod through the Red Sea, or when Tutankhamen was 
living flesh and blood instead of a celebrated mummy. 

1 06] 

Man in the Making 107 

The palaeontologist laughs in his sleeve at anyone who pauses to con- 
sider such contemporary events as these, while the astronomer, dreaming of 
the majestic march of worlds other than ours, pities the short-sighted palae- 
ontologist who is content to dwell on fragments of time as slight as Geologic 

How far back.into the shadowy past can the flickering torch of humanity 
be followed? What are the facts about the antiquity of man? Was there 
ever a time so remote that man was not man but something else? The 
sciences of Anthropology, and Prehistoric Archaeology are concerned with 
questions such as these. To confine ourselves to the events of our own day 
and generation, absorbing though they be, is like trying to breathe in a small, 
closed, stuffy room. 


Various traditions of human origins are a part of the folklore of every 
racial stock. One legend of the sudden inorganic origin of mankind is that 
of Deucalion and Pyrrha who, at the suggestion of Jupiter, peopled the 
earth by simply throwing stones over their shoulders, the stones becoming 
full-grown men and women according to which one of the celestial pair did 
the throwing. If these wonder-workers had operated upon a glacial hillside 
of New England, instead of the summit of Mount Parnassus where stones 
are rather scarce, no doubt the overpopulation problem would have become 
acute much earlier. 

The Greek and Roman classics are full of naive tales of dryads born of 
trees, and of Galateas coming to life from cold marble or lifeless ivory. 
Such stories and traditions, however, are in no sense evidences of the actual 
origin and antiquity of mankind on the earth. These evidences must be 
sought for in less romantic records of written history, in human fossils, and 
in persisting works of vanished hands, or indirect testimony of various kinds 
from other sources. 

In America historical records of man practically date from the discovery 
by the whites only a few centuries ago, although there are abundant archi- 
tectural remains in Mexico, Central America, and South America, that 
mark the presence of earlier, highly advanced civilizations, now van- 

In Europe man was in a condition of illiterate savagery long after he 
had attained a high degree of development elsewhere. Recorded human his- 
tory goes back with undoubted assurance only about 5000 years, continuing 
in Egypt and Mesopotamia with halting steps for perhaps 2000 years more. 

io8 Biology of the Vertebrates 

after which the historical record fades, and it becomes necessary in tracing 
the antiquity of man to resort to the unwritten evidences of prehistory. 

The prehistoric evidences of human antiquity may be grouped in five 
categories, as follows: 

( 1 ) Indirect evidence from the length of time during which the earth has 
been habitable by man; 

(2) Indirect evidence from the amount of time which must have elapsed 
in order to allow mankind to reach his present degree of development; 

(3) Indirect evidence from telltale fragments of extinct animals found asso- 
ciated with human remains; 

(4) Direct evidence from actual prehistoric human bones; 

(5) Direct evidence from the enduring handiwork of man. 

Some brief explanation and elaboration of these different lines of evi- 
dence is necessary to make their content clear. 


Astronomers, physicists, and geologists- all testify to an unthinkably 
remote period of time since the stage has been set for human life upon the 
earth. It does not necessarily follow that man appeared as soon as the earth 
was ready for human occupancy, but this testimony definitely removes any 
objections on the score of possible geological unpreparedness with regard to 
his abiding place. 

Scientists have made various estimates of the age of the earth, using for 
their calculations such yardsticks as the rate of radioactive transformations, 
the rate of heat loss from the cooling earth, the time required for the 
weathering of rocks and their subsequent deposition as sedimentary strata, 
or the time necessary to allow for the leaching out by water of the earth's 
crust enough to make the oceans as salty as they are today. The most recent 
estimates, based upon radioactivity, place the age of the earth at near 2000 
million years. Hurst says: "One of the most remarkable features of modern 
science is the rapid expansion in recent years of the scientific estimates of 
the age of man, life, the earth, and the universe." 


Anatomically the human body is a collection of parts, assembled in vary- 
ing degrees of perfection. There is every indication that the process of 
adaptation and modifn ation is still going on, and that the human body as 

A Jem in the Making 109 

wc see it today is the result of repeated changes which have taken place in 
the past. 

There is no structural detail in the human body not foreshadowed in the 
lower animals. The tracing out of resemblances and sequences in structure 
and organization between different animals and man is the peculiar province 
of Comparative Anatomy, and the mass of facts which constitutes the work- 
ing basis of this biological science furnishes undeniable evidence bearing 
upon the antiquity of man. Just as a modern ocean liner, with its luxurious 
appointments and efficient intricate machinery, gives evidence of years of 
invention and experimentation with preliminary boats of a lesser order of 
elaboration, so the four-chambered heart, the larynx, or the brain of man, 
to anyone who knows something of the detail and complexity of these 
organs, tells a long story of preparatory variation and adaptation that must 
have required an enormous length of time for its accomplishment. 

The science of Chorology, or the geographical distribution of animals 
and plants over the face of the earth, furnishes abundant evidence, of an 
undeniable kind, of the antiquity of man. The spread of human beings to 
the uttermost corners of the earth, which we recognize as an accomplished 
fact, could never have occurred by any series of migrations from common 
centers of origin without involving considerable lapses of time. 

Two other fields of science, Ethnology and Philology, also prove the 
necessity of postulating an extended period of past time for the existence of 
man, in order to account for the present development of customs and 

Ethnology deals with the customs and institutions of the various races 
of man, while philology is concerned with human language and its evolu- 
tion. In both of these fields man has attained a high degree of specializa- 
tion. When one attempts to disentangle the various steps that must have 
preceded it, he is carried back so far that there can be no doubt about the 
antiquity of man. Moreover, as Physical Anthropology shows, the great race 
divisions of mankind into those dressed in integumental uniforms of black, 
brown, yellow, and white are of no recent growth but were already distinct 
long before the beginning of the historic period. 

As for language which makes possible the oral transfer of experience, 
Linguistic Palaeontology shows that each of the branches of the so-called 
Aryan group of primitive languages, Persian, Indian, Semitic, Romance, 
Hellenic, Slavonic, Teutonic, and Keltic, has its roots buried in antiquity. 
The Hebrew and Arabic tongues are both ancient languages and neither the 
original of the other, and, therefore they are derived from still more remote 
ancestral sources. 

no Biology of the Vertebrates 


In some instances the handiwork of man has survived for a longer time 
than his own bones. The enduring touch of the vanished hand is particu- 
larly apparent in the case of the "indestructible flint" tools and weapons 
which he fashioned. The more important evidences of human antiquity 
that fall within the field of Prehistoric Archaeology are kitchen middens, 
pile dwellings, painted grottoes, monuments of various kinds, and fashioned 

1. Kitchen Middens 

Kitchen middens are ancient garbage dumps where prehistoric man evi- 
dently congregated and feasted. Attention was first called to them by 
Thomsen in 1836 who described them from Denmark. Since then they have 
been noted in many other localities as widely separated as Japan, Spain, 
Brazil, Oregon, California, Maine, Denmark, the Aleutian Islands, Terra 
del Fuego, the north coast of Africa, and the shores of the Baltic Sea. 

Kitchen middens consist principally of enormous masses of shells that 
could not have been collected together by any natural agency. They usually 
occur near the seashore, or where the seashore once was, because there 
primitive man had easy access to a natural food supply of shellfish. Mingled 
with shellfish remains are significant archaeological treasures of various 
kinds such as teeth, scales, bones of animals eaten, fragments of crude pot- 
tery, anvil stones, hammers, implements, and ornaments of different sorts 
made out of stone, obsidian, and flint, while pieces of charred wood and flat 
stones blackened with fire indicate, even to an amateur archaeological Sher- 
lock Holmes, the use of fire by the people who left these extensive piles 
of refuse. 

Some shell heaps, like those on the coast of Maine, for example, do not 
bear the earmarks of great antiquity, probably dating no further back than 
precolonial Indian days. Other kitchen middens, however, as those of the 
Baltic region, contain internal evidence of great age, for they consist largely 
of shells of salt-water mollusks, which cannot grow in brackish or fresh waters 
that characterize the Baltic Sea today, but must have flourished long ago 
when there was an open communication between the Baltic Sea and the 
salt ocean. 

2. Pile Dwellings 

The Greek historian Herodotus gives a detailed account of Thracian 
aborigines who dwelt in rude huts built upon piles out in the waters of Lake 

Man in the Making 111 

Prasias in the land that is now modern Rumclia. The most interesting expo- 
nents of this style of semi-aquatic architecture are the still older lake dwell- 
ers who lived and died probably some 6000 years B.C., or just beyond the 
outer halo of written history. They represent a primitive type of vanished 
civilization that came to flower particularly in the general region centering 
in Switzerland, where they erected their pile dwellings along the margins 
of numerous Alpine lakes. During a year of great drouth and low water in 
1854, the submerged ruins of some of these curious pile dwellings came to 
light on the shores of Lake Zurich, and subsequently a large number of 
sunken remains of pile-built settlements have been discovered, while from 
the surrounding mud a great number and variety of relics have been recov- 
ered that make possible a fairly complete picture of the kind of life these 
ancient lake dwellers lived. The sites of over 200 of these prehistoric villages 
have now been located in the Swiss lakes alone. 

The pile-dwellers represent a decided advance over the precarious 
nomadic life of the cave dwellers who preceded them. Building together 
upon piles out over the water enabled them to establish a haven of com- 
parative safety from the assaults of hostile marauders and ferocious beasts, 
at the same time furnishing the security and leisure necessary in taking initial 
steps in invention, the arts of peace, and of organized warfare. 

The pile dwellers made dugout canoes and crude pottery. Primitive agri- 
culture and food storage were no doubt stimulated when pottery containers 
were invented. The ancestral fig-leaf had long since been replaced by furry 
skins and bark clothing on the part of the shivering cave dwellers of the 
icy Pleistocene times, but the pile dwellers went further and supplemented 
their wardrobes by fashioning coarse textiles, fragments of which were pre- 
served buried in the mud. These may be seen in the Antiquarisches Museum 
in Zurich together with many other specimens of the prehistoric handiwork 
of the vanished race of lake dwellers. Living over the water doubtless 
insured some degree of primitive sanitation unknown, or at least unlikely, 
among those who pitched their camps in caves and forests on land. It can- 
not be doubted that the consequences following in the wake of ignorance 
of sanitation must have been quite as inevitable then as in these latter days 
when the bacteria of disease have been discovered and domesticated by 
modern man. 

3. Painted Grottoes 

About the time of the reindeer and wild horse occupation of what is 
now France and northern Spain, there flourished a remarkable period of 
prehistoric art, represented chiefly by crude drawings and paintings limned 


Biology of the Vertebrates 

upon the protected walls and ceilings of caverns. There have been cata- 
logued from such troglodytic art galleries nearly 3000 different pictures in 
outline, monochrome and polychrome. In the Dordogne region of France, 
one single cavern, the "Combarelles," is a veritable prehistoric Louvre, 
which contains 109 wall pictures covering an area of over 2000 square feet. 
The pictures are mostly outlines of animals, such as bison, reindeer, 
mammoths, wild horses, woolly rhinoceroses, and others, that were contem- 
porary in Europe with the cave-dwelling, flint-using folk who drew them. 
Usually they are well enough done to be unmistakable. The most ancient 
of them, apparently the work of the Aurignacian wild horse hunters, are 
bare outlines roughly engraved on the cavern walls by means of flint tools, 
and probably by the light of flickering torches. 

The best of these old animal pictures, as well as the majority of them, 
were evidently made later by the Cro-Magnon reindeer-hunting people, and 
are for the most part flat surfaces chipped into the solid rock and colored 
with various substances such as chalk, charcoal, red and yellow ochre, and 
other mineral pigments. The famous polychrome frescoes of the Altimira 
caverns in northwest Spain near Santander, which would be a credit to 
artists of a much later time, mark the highest point of excellence in glyp- 
tic art. 

It is quite likely that the painted grottoes were not decorated as an 
expression of "art for art's sake," but as part of a magic ritual to aid the 
hunters in successful pursuit of their prey. Since the grotto artists or necro- 
mancers drew what they saw, a study of 
their pictures throws considerable light upon 
the state of affairs in their part of the world 
some 15,000 to 30,000 years ago. As a single 
example, the skyline of the hairy mammoth 
is represented with a depression in the neck 
region (Fig. 81 ) which is absent from all 
modern proboscideans and is not evident on 
the fossil mammoth skeletons that have been 
assembled. This means that the living mam- 
moth had a fat lump, not to be seen in the 
skeleton, posterior to the notch of the neck. 
The fat lump indicates storage of food which 
enabled these great beasts to live through 
seasons of scarcity. The outline of the hairy mammoth as depicted by the 
Cro-Magnon artists in this way informs us that the painted grottoes were 
decorated when France had an arctic climate. 

Fig. 81. A prehistoric sketch from 
a cavern in Dordogne, France, 
showing a hairy mammoth with a 
hump of stored fat on its back, 
suggesting that it lived in an arctic 

Man in the Making 


The approximate age of the painted grottoes is also determined in part 
by the kind of bones and flint implements found in the rubbish that filled 
the caverns, thus protecting the pictures throughout thousands of years 
from exposure and destruction. Certain other time marks, such as paintings 
of different ages superimposed one above the other, tell the story of different 
hands that wrought them. All in all these early attempts at artistic expres- 
sion are direct evidences of the great antiquity of man, for of all creatures 
only man could have left such signs of the times. 

4. Large Stone Monuments 

The ancient Egyptians who built pyramids, sphinxes, and obelisks in the 
attempt to outwit the devastating tooth of time, were not the first to leave 
some enduring memorial of themselves to succeeding generations. Prehistory, 
as well as written history, bears witness to the same human desire for 
impressing posterity. This desire has found expression not only in the form 
of mounds or earthworks of unmistakable human workmanship, but also 
of large stones, or megaliths, arranged and set up in various unnatural ways. 

Fig. 82. Megaliths of Stonehenge. (After Quennell.) 

Large columnar stones set up on end are called menhirs. Of these over 
700 have been located in Brittany alone. Primitive man must have exercised 
a good deal of engineering skill, probably by digging pits, building tempo- 
rary inclined planes, and using pulleys of some sort, in order to jockey these 
huge stones into position. Their size and shape preclude the possibility of 
their placement by any natural agency. 

Frequently menhirs were set up in parallel rows, termed alignments, or 
in circular arrangement, designated as cromlechs. A flat stone resting upon 
two uprights is called a trilith, but when several uprights support a top 
stone, like a rude table or altar, the structure is called a dolmen. Nearly 

ii -j. Biology of the Vertebrates 

5000 dolmens have been found in France, while at Stonehenge and Ave- 
bury (Fig. 82) in England there are two very famous and much described 
collections of megaliths, arranged as alignments, cromlechs, dolmens, and 
single menhirs. The collection of megaliths at Avebury is more extensive 
than that at Stonehenge, but it is not as well preserved because many of the 
stones were removed to build the modern village of Avebury by people not 
interested in antiquities. 

One cromlech 1200 feet in diameter, and made up of 100 separate 
stones each seventeen to twenty feet high, forms a part of the Avebury 

Some of the most curious and mysterious evidences of ancient human 
activities are the stone images of Easter Island. On this isolated island in the 
south Pacific, 2000 miles west of South America, there are over 600 stone 
statues, hewn out of volcanic tufa and weighing up to thirty tons each. They 
are all patterned alike, regardless of size, and represent a half-length human 
figure with hands placed across the front of the body. Most of the statues 
when discovered were found overthrown and the present scanty inhabitants 
of Easter Island have no traditions concerning how the statues came to be 
there. Their origin is one of the most puzzling of archaeological enigmas. 

In addition to megalithic witnesses of the distant past there are mounds, 
tumuli, and earthworks of various sorts, that tell the same story of the 
antiquity of man. Some of these structures were no doubt connected with 
ancient burial customs or religious ceremonies, while others were probably 
once places of refuge, or fortresses, the ruins of which remain to remind us 
of the gray days during which our distant ancestors kept alive on the earth 
the precious spark of humanity. 

5. Tools and Weapons 

Man, of all animals, is the only one fitted to grasp tools and weapons. 
The tools and weapons of other animals, such as horns, teeth, tails, claws, 
and hoofs, are a part of the permanent equipment of their possessors, built 
into the body, and may be improved or substituted for other tools only in 
the slow age-long workshop of adaptive evolution. After all it is the brain 
that makes the grasping of tools or weapons effective. Apes pound a stone 
with a nut, but man discovered that he got better results if he pounded the 
nut with a stone. 

Some of the earliest tools and implements fashioned by man furnish 
direct evidence of human antiquity that antedates his oldest fossil remains. 
This kind of evidence is far more accurate and reliable than any historical 
chronicle whatsoever that has been colored by human judgment on the 

Man in the Making n 


part of the historian. Sir W. R. Wilde has emphasized this point by saying, 
"Men are liars, stones are not." 

The materials employed in outfitting the grasping hand of man have 
been principally flint, which is composed of the finely crystallized remains 
of silicious sponges and other marine organisms, dissolved and redeposited 
in the form of irregular lumps. Quartz, and obsidian or "volcanic glass," 
which like flint fractures in flakes from cores by pressure or by percussion, 
were also employed, as well as horn, bone, shells, ivory, wood, and later 
on, metals. Of these various materials wood is the least enduring and so 
furnishes very little evidence today of the uses to which in all probability 
it was formerly put by prehistoric man. 

Of the metals, copper occurs in comparatively free form in nature and 
was the first metal utilized by man. Malleable bronze, which is an alloy of 
copper and tin, was discovered or invented long before iron, "the great 
lever of civilization," was successfully smelted from the ore, beginning prob- 
ably about 1300 B.C., on the shores of the Black Sea. 

Archaeologists speak of successive stages of human culture based upon 
the materials employed in the manufacture of tools and weapons. They are 
the Stone Age, Bronze Age, Copper Age, and Iron Age. To them might be 
added a fifth, the Steel Age, in which we live today. 

These ages are not uniform in origin or duration in different parts of 
the world. Egypt, for example, had already reached a high point in the 
Bronze Age at the time when Europe still lingered in the Stone Age of cul- 
ture ( Fig. 83 ) , while in North America and Australia the Stone Age was in 
full swing when these countries were discovered by white men in recent his- 
torical times. Some races of man still living today, that have been isolated 
from contact with other races, such as the Hottentots of Africa, Veddahs of 
Ceylon, Botacudos of Brazil, Andaman Islanders, Fuegans, and Eskimos, 
are still in a very primitive stage of tool culture. 

The Stone Age, before iron was applied to the follies of war or the arts 
of peace, offers a most fascinating field for study, since it furnishes the 
earliest direct evidence of human activity upon the earth. It is a field that 
has attracted many scholars. As late as 1938 there was opened in the Ohio 
State Museum at Columbus, a "Lithic Laboratory" in which to specialize 
in types of stone tools, showing the materials used, and the technics 
employed in making them. 

The stone instruments of primitive man were fitted to a variety of uses. 
They include arrow-points, lance-heads, knives, axes, hammers, saws, chop- 
pers, borers, etchers, scrapers, punches, polishers, spark-producers, and orna- 
ments. Their evolution tells the story of human progress through many cen- 


Biology of the Vertebrates 



Historical Landmarks 

\^f 50 Conquest of Gaul 
200 Homer 

1180 Trojan War 
1400 RsmesesiL(Eoypi) 








9000 s 


"*" OAOOO ? 

Fig. 83. A chart to show that "cultural ages," while following the same 
sequence in different regions, were not of equal duration. (Modified from 

turies of time. The interpretation and significance of this story got its initial 
start when M. Boucher de Perthes in 1838 discovered the first known 
authentic flint hatchet in a sand bed near Abbeville, France, together with 
rhinoceros and mammoth bones. Since then in the last century a very great 
number of flint tools and implements of different degrees of perfection in 
workmanship have been found and carefully studied. France has been 

A Ian in the Making 

i i 



particularly fortunate, not only in unearthing these records of early man, 
but also in having distinguished scholars who have collected and described 

According to prescientific interpreta- 
tions the relics of the Stone Age, chiefly 
flints, were described variously under the 
name of "fairy darts" and "thunder- 
bolts," and their origin assigned to the 
Druids, Romans, or the Devil, a con- 
venient trinity which has been made 
responsible for other strange things as 

Wherever fashioned flints have been 
found, even in such diverse regions as the 
Nile Valley, Algeria, Europe, England, 
Somaliland, and America, they exhibit 
the same universal sequence of patterns, 
and a parallel succession of evolutionary 
differences or degrees of refinement. It is 
thus possible to subdivide the Stone Age 
into four unequal successive divisions of 
workmanship, representing stages of hu- 
man culture, namely, eolithic, palaeo- 
lithic, mesolithic, and neolithic. See 
Figure 84. Experts have further arranged 
the successive cultures of the Stone Age 
into subdivisions, the names of which for 
the most part are derived from localities 
in France near which the typical char- 
acteristic flints were first discovered. 

Reading downward from the most ancient to the most recent, they are as 
follows : 


Fig. 84. Flints, representing three 
periods of the Stone Age. (Drawn 
from specimens in the collection at 
Brown University.) 

I. Eolithic Period 

II. Palaeolithic Period 

A. Lower Palaeolithic 

1. Abbevillian (Chellean) 

2. Acheulian 

3. Mousterian 



Biology of the Vertebrates 

B. Upper Palaeolithic 

1. Aurignacian 

2. Solutrean 

3. Magdelenian 

Mesolithic Period 

1. Azilian 

2. Tardenoisian 

IV. Neolithic Period 

Stone implements of the oldest type, representing the eolithic culture, are 
terms eoliths (Fig. 84). They mark the "great extent of time that has 
elapsed between the picking up of the first stones with an intelligent pur- 
pose, and the acquirement of sufficient knowledge to shape them into the 
crudest form of palaeoliths" (Wilder). Eoliths, nowhere abundant, are 
always more or less problematical objects, since they are not undeniable 
artifacts in the sense of purposeful manufacture, but are merely pieces of 
stone of convenient size and shape to fit ( the hand and lend persuasive 
weight to the fist. They are distinguishable by showing the effects of use. 
The uncertainty connected with them depends upon whether they were 
bruised and fashioned by man or nature. Professor G. F. Scott-Elliot says 
of them, "Surely if there is little to prove that eoliths were made by man, 
there is even less to convince us that they were formed in any other way." 

The flints of the Palaeolithic Period, as contrasted with eoliths that were 
perhaps accidentally scarred, are unmistakably the result of human manip- 
ulation. They are all definitely chipped and fashioned (Fig. 84), exhibiting 
an evolution of workmanship from crude lower palaeolithic forms, imper- 
fectly worked, to exquisitely fashioned tools and weapons of later time in 
the "golden age" of the grotto painters. 

The Abbevillian flints are almost exclusively the cores of nodules 
rather than flakes or fragments that have been chipped off. These flints are 
mostly oval hand-axes ("coup-de-poing" of the French), coarsely chipped. 
The idea of fastening the ax to any sort of a handle was yet to come. 

Characteristic Acheulian tools were hand-axes more carefully sharpened 
and with straighter cutting edges than those of the Abbevillian. In both of 
these cultures hearths are occasionally found, indicating the use of fire by 
these early Palaeolithic nun. 

The flints of the Mousterian, chipped on one side only, were of con- 
siderably better workmanship than anything that had gone before. For 

Man in the Making ng 

example, flakes from stone nodules were used as scrapers, knives, and with 
notched edges as saws, while the cores of the nodules were made into 
crude fist-axes. 

The three subdivisions just enumerated make up the Lower Palaeolithic 
Group, as contrasted with the three following subdivisions, or Upper 
Palaeolithic Group, which brought in a "new kit of tools," showing the 
highest development of flint work. 

The Aurignacians, preceding the Cro-Magnons, lasted some 7000 to 
8000 years in Europe, and added bone and ivory harpoons to their equip- 
ment. Among other things they also invented needles, or slender splinters of 
bone with a hole in one end so that thongs of sinew could be threaded 
through and skins sewed together by means of them. One of the greatest of 
human inventions is the needle. No animal, even an ape with busy exploring 
fingers, ever conceived such an idea, much less put it into effect. 

The most beautifully chipped of all flints are the thin "laurel-leaf" 
lance-heads of the Solutrean culture. From this time on the art of fashioning 
flints declined. 

The Magdelenian subdivision is characterized more by the use of bone, 
horn, and ivory, and by polychrome paintings in caverns. 

The Azilian and Tardenoisian subdivisions of the transitional Mesolithic 
Period, which followed the last glaciation, witnessed a further decline of the 
art of working flint, although it is apparent that handles were employed 
for ax-heads and arrow- or spear-points were bound by thongs to wooden 
shafts at that period. These two latter subdivisions are distinguished from 
each other by the fact that the Tardenoisian subdivision was characterized 
by the occurrence of painted pebbles of unknown use, not present in the 
Azilian culture. 

The comparatively short Neolithic Period is distinguished from the 
Palaeolithic subdivisions not only because neolithic tools and weapons were 
fashioned into a larger variety of definite shapes for obvious uses, but also 
for the reason that they were polished smooth (Fig. 84). All evidence thus 
far gained shows that flints were chipped for thousands of years before man 
learned to polish them. The most important features of this period are, 
however, the beginning of the manufacture of pottery and the development 
of agriculture and domestication of animals. 

Many flint instruments, particularly those of the neolithic type, con- 
tinued to be manufactured and used long after metals were employed in the 
Bronze and Iron Ages, just as modern means of locomotion, such as ox-carts, 
horses, bicycles, automobiles, and airplanes, while tending to supersede what 
has gone before, still persist long after they have lost their dominance. 

120 Biology of the Vertebrates 


The actual bones of prehistoric man furnish the best direct evidence of 
human antiquity, but unfortunately they are very scarce. This is due in 
part to the arboreal life of human ancestors whose dead bodies, left without 
burial, were liable to be devoured on the spot or subjected to immediate 
decay and disintegration. 

Moreover, a large part of the earth, including many localities where 
human remains might have been overwhelmed and fossilized, has not as yet 
been thoroughly explored by competent scientists. Evidence of human 
antiquity from fossils has been acquired only within the last century. As 
late as 1852 the eminent French palaeontologist Cuvier, gave the famous 
opinion, backed by his extensive knowledge of what was known in his day, 
"L'homme fossile n'existe pas." All the supposed discoveries of prehistoric 
human remains up to that time were shown to be of comparatively recent 
origin or not human at all. One often cited case is that of Scheuchzer, 
who found and described in 1732 what he called a fossil man, to which he 
gave the name Homo diluvii testis, or "man, witness of the flood," with the 
pious comment, "Rare relic of the accursed race of the primitive world. 
Melancholy sinner preserved to convert the hearts of modern reprobates!" 
This famous specimen, which now reposes in a museum in Haarlem, turned 
out, however, to be not a human skeleton at all but the bones of a giant 

1. Java "Ape-Man" 

Pleistocene deposits have yielded not only tools and weapons but many 
human fossils, the oldest of which bear resemblances to the early Pleistocene 
and Pliocene apes. Klaatsch, who speaks from a profound knowledge of 
comparative anatomy, describes apes as "unsuccessful attempts to compass 
the road to mankind from prehuman stock." 

The German zoologist Haeckel was so confident that species interme- 
diate between apes and man once lived that in 1866, before many dis- 
coveries of human fossils had been made, he assigned the tentative name 
of Pithecanthropus (ape-man) to the unknown, and prophesied its future 

In 1891, however, Dubois, then an officer in the Dutch army stationed 
in Java, found fossil remains after extended search on one of the banks of 
the Solo River near Trinil, thus fulfilling Haeckel's prophecy. These remains 
were a skullcap, or calvarium, and a molar tooth, to which were added 
from the same locality later a left thigh bone and two other teeth, together 

Man in the Making 


with a fragment of a lower jaw. These fragments of the "ape-man" were 
recovered some twenty yards apart, making the likelihood of intrusive burial 
improbable and casting doubt on the probability that the several bones did 
come from one individual. 

Companion bones of various ani- 
mals were unearthed that filled 400 
packing cases. This material was 
brought back to Holland and sub- 
jected to painstaking study. The spe- 
cimens included bones of the extinct 
proboscidian, Stenodon ; the ungu- 
lates Leptobos and Hippopotamus; 
and the giant pangolin, species no 
longer inhabitants of that part of the 
world, as well as tapirs now found 
only in South America on the other 
side of the globe. Altogether 24 
species of Pleistocene animals under 
45 feet of undisturbed stratified de- 
posits have been identified among 
these remains, fixing the time when 
Pithecanthropus lived as approxi- 
mately 500,000 years ago (Fig. 85). 
Despite attempts of others to ob- 
tain additional specimens of the Java "ape man," none was found until 
von Koenigswald, between 1936 and 1939, unearthed portions of three 

skulls and a lower jaw, also in the Solo River 

valley but near Sangiran, above Trinil. From the 

combined evidence of all available material it 

has been determined that Pithecanthropus had 

a somewhat apelike brain case with low forehead 

and heavy supraorbital ridges but with a capacity 

of slightly more than 900 cubic centimeters ( Fig. 

86). The brain would therefore be intermediate 

in size between that of the great apes, which 

ordinarily do not exceed 600 cubic centimeters, 

and modern man whose cranial capacity varies 

between 1200 and 1500. Although no tools have been discovered with any of 

the skeletal remains, primitive stone instruments have been found in Java in 

the same strata as those to which the specimens of ape man belonged. 

Fig. 85. Diagrammatic cross section 
through the fossiliferous strata in Java 
where the Pithecanthropus bones were 
found. A, surface soil; b, undisturbed sand- 
stone; c, volcanic layer; d, level of the 
Pithecanthropus bones and of various ex- 
tinct Tertiary animals. (After Dubois.) 

Fig. 86. A comparison of 
the skull capacities of vari- 
ous primates. (After Boule. ) 

122 Biology of the Vertebrates 

From all of the evidence now available it is clear that Pithecanthropus 
was an erect-walking primate with the ability to use tools, and with features 
of the skull and brain which are more human than simian. He may have 
been a stage in the evolution of man or of at least a portion of the human 
race. As he apparently had European contemporaries it is quite possible 
that he represents but one branch of a human stock which had originated 

2. Peking Man 

At Choukoutien, some thirty miles southwest of Peiping, China, the 
investigation of a group of caves revealed middle Pleistocene deposits which 
included a number of skeletal remains of various animals. Dr. Davidson 
Black, of Peking Union Medical College, becoming interested in two teeth 
which were found, supervised further excavations which uncovered a third 
tooth, a lower molar, in 1927. He was so certain that the new find came 
from a primitive man that he established the new genus Sinanthropus for 
it. That he was justified was shown by the discovery, in 1929, of an almost 
complete brain case embedded in limestone under 1 10 feet of cave deposits. 
After Black's death, Dr. Franz Weidenreich continued the work and by 
1938 nearly 40 Sinanthropus individuals were represented in the collection 
of teeth, jaw fragments, portions of skulls and a few parts of the appendic- 
ular skeleton. 

Sinanthropus resembled Pithecanthropus in such features as a receding 
chin, large supraorbital ridges, and low forehead, but had a somewhat larger 
cranial capacity, the average of the capacities reported being 1075 cubic 
centimeters. Several other details indicate that Java man was slightly more 
primitive than Peking man who may have been an intermediate stage 
between Java man and Neanderthal man. 

With the skeletal remains there have been discovered palaeolithic tools 
and hearths, good evidence that Peking man was a fire-user. As all of the 
skulls had the bases smashed in, it is believed that he was a cannibal who 
ate the brains and probably other parts of the body, even as Homo of recent 
years has been wont to do. 

3. Heidelberg Man 

The "Heidelberg man," Homo heidelbergensis, lived among animals 
that characterized the first interglacial period of perhaps 450,000 years ago. 
He was therefore an European contemporary of Pithecanthropus. This race 
is represented only by a lower jaw (Fig. 87), found in 1907 buried in a 
gravel pit near the mouth of the Neckar River valley about six miles from 

Man in the Making 

2 3 

Heidelberg, Germany. Over eighty feet of undisturbed sand and sedimentary 
rock had been deposited over this interesting ancestor. For subsequent 
centuries the slow eroding action of the Neckar river carved out the valley, 
before this famous human fragment was 
eventually exposed to modern view ( Fig. 88 ) . 
The jaw is heavy, massive, and chinless. 
It is apelike in character but the teeth are 
comparatively small and have the shortened 
roots and dilated crowns that distinguish 
human teeth. With our knowledge limited to 
these interesting features of the lower jaw it is 
difficult to tell what may have been Heidel- 
berg man's position with relation to other 
fossil men. 


Fig. 87. Outline of the Heidel- 
berg jaw, contrasted with that of 
the chimpanzee and man. (After 


Fig. 88. Diagram of the sand pit where the Heidelberg jaw was dis- 
covered, a-b, layer of "Newer Loess"; b-d, "Older Loess"; c-d, Mauer 
sands; e, clay. The cross indicates the spot under seventy-nine feet of 
undisturbed strata where the fossil jaw was found. (After Schoetensack, 
in Osborn, Men of the Old Stone Age. Charles Scribner's Sons.) 

4. Piltdown Man 

Skull fragments, found by Charles Dawson in 1908 and 1911 near Pilt- 
down Common, Sussex, England, so interested Sir Arthur Smith-Woodward 

i^4 Biology of the Vertebrates 

that he joined in the search for human fossils in that area. In 1912 these 
workers discovered a larger piece of a skull, and in 1915, about two miles 
from the first discoveries, part of a second skull. To this type of primitive 
man Smith- Woodward gave the name Eoanthropus dawsoni ("Dawson's 
dawn-man"). Early Pleistocene animal skeletons found in these same 
Piltdown gravel beds show that this man was probably a first interglacial 
contemporary of Heidelberg and Pithecanthropus. 

The lower jaw of Eoanthropus is almost completely apelike but the 
brain case is of a quite advanced type. The supraorbital ridges are reduced, 
the forehead high, and the cranial capacity near 1350 cubic centimeters. 
Except for the fact that the bones are about twice as thick as those of 
modern man, in this respect resembling Pithecanthropus, the brain case is 
quite modern. 

Further evidence of primitive man in England was discovered by 
A. T. Marston in 1935 and 1936 at Swanscombe in the lower Thames 
valley. In the same stratum with Acheulian implements and middle Pleis- 
tocene fauna an occipital bone and then a parietal bone were uncovered. 
They closely resemble corresponding parts of the Piltdown skulls and 
indicate a cranial capacity of somewhat over 1300 cubic centimeters but 
apparently belong to a later period, probably the second interglacial. Sir 
Arthur Keith, who has been cited as "perhaps the greatest authority on 
fossil man," believes these parts belonged to a later member of the Piltdown 

5. Neanderthal Man 

In 1856, not far from the time that Darwin's Origin of Species (1859) 
appeared, throwing both the scientific and the theological worlds into 
intellectual convulsions, there came to light the original Neanderthal man, 
the first primitive fossil man to be discovered. This is an incomplete human 
skeleton unearthed by workmen in a detritus-filled cavern near Diisseldorf, 
Germany, high up on the precipitous side of a ravine about 60 feet above a 
stream and 100 feet below the top of a cliff. Together with the skeleton 
were found, embedded in hard loam, the bones of animals long extinct, the 
cave bear, woolly rhinoceros, mammoth, cave hyena, and other Arctic 
forms. Man is known by the company he keeps, and these bones with super- 
imposed detritus covered over by undisturbed strata of sedimentary rock, 
put the seal of unquestionable antiquity upon the Neanderthaler. 

The bones themselves, which have undergone more expert scrutiny than 
perhaps any other set of bones, indicate a burly, squat, bow-legged indi- 
vidual, with thick skull, projecting brows, low retreating forehead, and 

Man in the Making 

12 5 

receding chin, characters distinctly unlike those of modern man. That this 
individual was not a unique prehistoric hermit has been unquestionably 
demonstrated by subsequent discoveries in various localities in Europe, as 
well as in Palestine, of about 70 individuals represented by bony fragments 
of one kind or another, all agreeing essentially with the original find. The 
existence of this species of human beings, Homo neanderthalensis, whom 
the late G. Elliot Smith refers to as "weird caricaiures of mankind roaming 
far and wide to satisfy their appetites and avoid extinction," is now no more 
in doubt than the existence of ancient Egyptians. 

Accompanying Mousterian implements and Arctic, or glacial-period, 
animals indicate that the Neanderthalers flourished between 150,000 and 
115,000 years ago during the difficult last days of the great Pleistocene 
period. By the latter part of the fourth glaciation, 50,000 to 70,000 years 


Existing apes 
and man 

Gibbon Man Chimpanzee 

(Asia) ( Homo sapiens) (Africa) 


Unknown ancestral stock 
of Old World Primates 

Fig. 89. Ancestral tree of the anthropoid apes and man, according to 
the conclusions of Osborn. 


Biology of the Vertebrates 

ago, they had disappeared and the caves were being occupied by men of our 
own species. 

6. Cro-Magnon Man 

The fossils thus far considered are conceded to be evidently human, 
although of species different from our own, Homo sapiens. There is con- 
siderable evidence that the men of modern types who superseded the 
Neanderthalers were probably not their descendants but invaders from other 
regions, who had progressed to the modern type while Neanderthalers had 





Fig. 90. Phylogenetic tree of the anthropoid apes, primitive man, and 
modern man, illustrated by drawings of heads some of which are recon- 
structed. (From Hegner, College Z°°l°gy, copyright 1942, by permission 
of The Macmillan Company, publishers. Drawings of the heads courtesy 
Amor. Mus. Nat. Hist.) 

Man in the Making 


remained as "palaeontological hang-overs, outmoded survivors of an earlier 
stage of human evolution," to use the words of Professor E. A. Hooton. 

One of the finest races physically that ever existed, of which many fossils 
have been recovered, was the Cro-Magnon race of reindeer hunters that 
lived 15,000 to 30,000 years ago near the close of the last ice age. These 
nomads of frigid days not only left some of their bones behind, that showed 
them to be tall finely built specimens of humanity with large capacious 
skulls, but they also left mural pictures of contemporary animals which they 
painted on the walls of their caverns. The bones of wild horses, wild boars, 
and reindeer, abundant all over that part of Europe where most of the 
fossil remains of man have been found, are eloquent witnesses of an ancient 
regime when man lived in a different world than that of today. In southern 
France alone there were eighteen species of large animals which were 
formerly common that have now migrated to more congenial climes. Of 
these, thirteen, like the reindeer, have gone north, and five, like the 
chamois and mountain goat, have retreated to cool mountain tops. 

Figures 89 and 90 are attempts to indicate the probable relationship 
of the better known primates. 


There is no doubt that, to our myopic vision at least, contemporary 
events of our individual lifetimes are the most interesting part of all human 
history, but a single lifetime of even "three score years and ten" is only one 

TABLE III. Time Scale 

(Based on the unit in which a human lifetime of "three score years and ten" 

is equal to one minute.) 




Years ago 

A single lifetime . 







162/ 3 




Columbus discovered America .... 
Christian Era began 


Written history began 


Cro-Mae;non race lived 


Neanderthal race lived 


Piltdown race and Pithecanthropus 


Cenozoic Era began 






Mesozoic Era began . 


Palaeozoic Era began 


128 Biology of the Vertebrates 

one-hundredth of the approximate time involved in written history. His- 
torical events that seem very ancient in contrast to the times in which we 
now live, appear quite recent against the background of the "Cultural 
Ages" of the archaeologist, while in turn the cultural ages crowd magically 
forward into what seems like the immediate present when their position in 
geologic time is considered. It is a good thing occasionally to ignore the 
fleeting news events of the day and to stretch one's sense of time by probing 
into the abysmal past. Man is the only animal who has the capacity to look 
both ways and to be aware of the past and future. 

Table III is an attempt to visualize the reaches of past time. 


The continuing wonder of mankind is man. His achievements are like 
a rapidly widening wedge, awakening "undiminished interest in every man 
born into the world" (Huxley) . The fascinating theme of the anthropologist 
and the archaeologist, however, is particularly liable to fall a prey to 
premature generalizations. Nevertheless certain conclusions from "spade 
history," as contrasted with written or legendary history, may be stated with 
considerable confidence. 

( 1 ) There are many converging lines of evidence which point unmistakably 
to the great antiquity of man when measured by any historical time 

(2) Man appeared in Europe at a remote time when climatic conditions 
were different from those at present, and when he was the contem- 
porary of many kinds of animals now extinct. 

(3) In America the evidences of human antiquity, as yet discovered, by 
no means go back as far as in Europe and Asia, although the recently 
discovered flint culture of New Mexico, typified by unique fluted 
so-called "Folsom points," two of which were discovered embedded in 
the backbone of an ancestral bison in Texas, indicate the presence of 
man in America as far back as 15,000 to 25,000 years ago when bisons 
roamed the western plains. 

(4) The direct descent of Homo sapiens is not through any species of living 
primates, but is to be traced back to arboreal ancestors of very remote 
common ancestry. 

(5) Man is a part of the general evolutionary scheme that includes all life. 
As Dr. W. W. Keen once said in pointing out the reasonableness of 
this conclusion, "The laws of mathematics do not hold up to 1 ,000,000 
and then give way to something else." 


The Units of Structure— Cytology 


It is quite essential in constructing any house of knowledge to know the 
units out of which the intellectual edifice is built. Thus, the chemist must 
know the elements from which his compounds are made, the writer must 
be acquainted with the alphabet, the mathematician with numerals, while 
the biologist must know the units that combine to form the diversities of 
living bodies. 

The structural units with which the biologist deals are termed cells, and 
the science of cells is called Cytology. 

Robert Hooke first suggested the word "cell" in 1665, when he de- 
scribed "little boxes or cells distinguished from one another" that he saw 
in thin slices of cork. The word is not a fortunate choice, however, suggest- 
ing as it does prison walls, for although "walls do not a prison make" 
neither do they adequately describe the biological unit. Nevertheless the 
term has come to stay and its use is now extended to indicate units of bio- 
logical structure, regardless of whether the walls of a cell are in evidence or 

The conception that all living creatures are made up of organic units, 
or cells, dates from 1838-1839, when Schleiden and Schwann, botanist and 
zoologist respectively, published important investigations on the subject. 
The essential conclusions of the "cell theory" propounded by them, as now 
modified, are: 

(1 ) Every living thing is composed of organic units (cells), or products of 
their activity; 

(2) Every living thing begins life as a single cell; 

( 3 ) Every cell is derived by a process of division from some preceding cell. 


A generalized diagram of a cell is represented in Figure 91. Within the 
cell is the nucleus, surrounded by a nuclear membrane. Outside of the nu- 


130 Biology of the Vertebrates 

cleus is the cytosome, or body of the cell, enclosed in a cell membrane. 
Within the cytosome may be embedded pigment granules, chondriosomes, 
crystals, oil-droplets, vacuoles, plastids, or other substances. Frequently 
there may also be identified in the cytosome a tiny body called the centro- 
some, from which delicate lines radiate in every direction. 

, ■_'. 'i^f^^J- yCentriole 

SSsS^^X '■'"', 7 vv 5 ^/ -^Golgi bodies 
Nucleus -^^^T/ r • Wy^^y : '"--^Vl Chondriosomes 



Fig. 91. Diagram of a generalized tissue cell. (From Huettner, Funda- 
mentals of Comparative Embryology of the Vertebrates, copyright 1941, 
by permission of The Macmillan Company, publishers.) 

The nucleus is the headquarters of the whole organized unit, since 
changes which the cell undergoes are initiated there. It is made up of more 
than one substance, a fact that is revealed by applying certain stains which, 
through chemical union, affect parts but not the whole of the nuclear sub- 
stance equally. That part most readily stained by certain dyes is called 
chromatin, or "colored material," and during certain phases of cell life the 
chromatin material masses together into visibly definite structures called 

When cells crowd together during the formation of tissues, they become 
subjected to mutual pressure, so that their typical spherical shape becomes 
modified. Any of the several parts of a cell may undergo extreme modifica- 
tion, but most of the fundamental features outlined above, which make a 
cell an organized unit of living substance, characterize every cell. 


Cells are subject to the inevitable laws of change common to living 
things generally. The succession of changes through which each individual 
cell passes during its lifetime is termed cytomorphosis. 

The Units of Structure 131 

The term may be extended to include also the transformations through 
which successive generations of cells pass in the process of differentiation. 
Thus, in Lewis and Stohr's Histology the following definition is given: 
"Cytomorphosis is a comparative term for the structural modification which 
cells, or successive generations of cells, undergo from their origin to their 
final dissolution. In the course of their transformation, cells divide repeat- 
edly, but the new cells begin development where the parent cells left off." 

The initial phase of cytomorphosis is characterized by a lack of speciali- 
zation. This is followed by a series of progressive changes in which the cell 
becomes finally fitted for its life work, whatever it may be. After a varying 
period of usefulness, signs of old age appear; eventually the cell goes the 
way of all flesh and its dead remains are removed from the society of its 

Some kinds of cells, like red blood corpuscles or epidermal cells, live a 
strenuous life, completing their entire cytomorphosis in a comparatively 
brief time, while others, like germ cells, may remain dormant for years in 
an undifferentiated embryonic condition before they finally move forward 
to fulfil their destiny. 

There is much similarity between the life of a cell and of an individual. 
Both begin with a primitive generalized stage; both pass through expand- 
ing infancy and differentiating youth; both arrive at specialized maturity 
and usefulness; both wear out and die. In one particular, however, there is 
a striking difference. An individual reproduces its successors only when 
mature, that is, after it has become differentiated or specialized. A cell, on 
the other hand, reproduces its kind only while it is still in the comparatively 
undifferentiated embryonic phase of its life cyle, losing almost entirely the 
power to do this after it has attained specialization. The result is that manv 
cell units of the body, for example nerve cells, having once gone through 
to the extreme end phases of their differentiation, lose the power to replace 
themselves with daughter cells. They have passed beyond the embryonic 
stage of cytomorphosis when replacement is possible, and after wearing out 
are unable to leave successors. 


The path of specialization that a cell follows in cytomorphosis is de- 
pendent upon the work it has to perform in the cell community of which it 
is a part. 

A typical embryonic cell, uncrowded by neighbors, tends to be spherical 
in form but rarely has opportunity to remain so. Red blood cells of the lower 
vertebrates perhaps come nearest to retaining the original form of embry- 

1 3 2 

Biology of the Vertebrates 

onic cells, because the work they do of rolling about through the blood chan- 
nels of the body is facilitated by their round shape. Leucocytes, or white 
blood cells, frequently become irregular or amoeboid in form and have the 
ability upon occasion to change their shape. They are thus able to escape 
from the blood vessels by squeezing through between the cells of the cap- 
illary walls into surrounding tissues, where they nose their way between 
other cells to remove dead ones or to devour invading bacteria in their 
mission of sanitation. 

Fig. 92. Various types of cells. A, smooth muscle cells; b, striated muscle 
fiber; c, cartilage cells; d, amphibian red blood cell; e, amoeboid white 
blood cell; f, nerve cell; g, ciliated epithelial cell; h, spermatozoan; i, 
squamous epithelial cells, surface view; j, fat cells; k, bone cell. 

Squamous epithelial cells, whose business it is to cover surfaces, become 
flattened like shingles, while muscle cells, specialists in contraction, assume 
a much elongated form by which their function of shortening is best ac- 

Detached cells like spermatozoa, that need to acquire the ability to 
travel in a fluid without having the propelling power of the heart back of 
them, as do blood cells, differentiate from the embryonic spherical form 
into a tadpole-like shape, becoming equipped with a powerful locomotor 
tail that drives them forward to their destiny. 

The Units of Structure 133 

Skeletal cells, the service of which is to furnish support or protection, 
develop an interstitial substance; while some secreting cells exhaust them- 
selves at the expense of the cytoplasm in the production of their secretions. 
Extreme modification in form is seen in nerve cells where specialization has 
gone so far that it is hopeless to expect such cells ever to perform any other 
function than that to which they have become committed. 

These examples of differentiation are only a few of the many guises in 
which the building blocks of organic structure appear. 

Some type forms are diagrammatically indicated in Figure 92. 


With the development of the compound microscope, the invention and 
utilization of aniline dyes and improved cytological technic generally, the 
presence of chromosomes within the cell nuclei became known, and the far- 
reaching importance of their peculiar structure unquestionably established. 

It has been found ( 1 ) that chromosomes are probably always constant 
in number and shape in every cell of every individual of any particular spe- 
cies; (2) that they behave in a predictable way throughout all the vicissi- 
tudes of cytomorphosis; (3) that every cell not only comes from a preceding 
cell but every chromosome in the nucleus comes from a preceding chromo- 
some like itself; (4) that during the changes of cytomorphosis when chromo- 
somes apparently break up into indefinite masses, losing their characteristic 
appearance, they later reappear in the identical size, shape, and sequence 
of units that they formerly had; (5) that this behavior is evidence that their 
individuality is maintained and that they are not simply chance masses of 
unorganized stuff; and (6) that the various chromosomes of any cell not 
only assume characteristic shapes and sizes, but that they also occur in 
pairs, two of each kind in every cell excepting the sex chromosomes of many 


The usual behavior of a cell during the period of increase when two cells 
form out of one, is called mitosis. The astonishing and intricate details of 
mitosis are generally unsuspected and quite strange to the uninitiated, 
although the process is occurring continuously in all living creatures, with 
countless repetitions. Upon its orderly performance depends every step that 
serves to differentiate any animal or plant, starting from the fertilized egg 
and including all growth and organic repairs. The founders of the cell 

i?4 Biology of the Vertebrates 

theory surely could not have imagined the extent of the vistas which their 
preliminary generalizations were destined to open up, for mitosis is a far 
more detailed and complicated performance than simply pinching the origi- 
nal cell in two. 

The essential thing in cell division seems to be the equipment of each 
new cell unit with a complete set of chromosomes in its nucleus, duplicating 
in number, form, and size those of the parent cell. In the process of cell 
formation, which consists of a parent cell dividing into two daughter cells, 
the chromosomes play the leading part. A very brief and general descrip- 
tion of the phases of a typical mitosis, with a series of explanatory diagrams 
follows, but it should be kept in mind that the stages described as distinct 
really merge into each other continuously, like a moving picture, and that 
the actual process of mitosis from start to finish will have repeatedly taken 
place somewhere within the bodily frame of the reader many times over 
during the studious reading of this paragraph. 

Four general phases of mitosis are recognized as sufficiently distinct to 
mark well-defined changes from the "resting cell." These are known as 
the prophase, metaphase, anaphase, and telophase. 

The resting cell (Fig. 93a) is characterized by the presence of a nuclear 
membrane and a chromatin network within the nucleus, as well as often- 
times also by a pair of centrosomes. In the beginning of the prophase (Fig. 
93b-d) delicate spindle fibers appear between the centrosomes, and the 
chromatin is in the form of a threadlike spireme. As the prophase progresses 
the centrosomes move apart and the chromatin thread gives rise to separate 
chromosomes each more or less completely split into two. 

In the metaphase (Fig. 93e) the chromosomes lie at the equator of the 
cell, being connected by spindle fibers with the centrosomes, each of which 
now occupies a polar position. 

In the anaphase (Fig. 93f) these split chromosomes, each containing 
a sample of every different kind of substance that was distributed along the 
length of the parent chromosome, begin to separate from each other and 
to move towards the poles. 

During the telophase (Fig. 93g) the elongated cell body begins to pinch 
in two, while the migration of the chromosomes towards the poles is com- 

Finally the division of the cell body into two parts enclosed in separate 
cell membranes becomes complete. The spindle fibers have disappeared and 
a nuclear membrane has re-formed around the chromatin network, derived 
from the chromosomes. Two resting cells take the place of the single one 
with which the quadrille of mitosis began (Fig. 93n). 

The Units of Structure 


/Centriole Spindle^ 

Aster - 

Fig. 93. Mitosis of an animal cell, a, interphase; b-d prophase- e 
metaphase; /, anaphase; g, telophase; h, interphase. (From Huetmer' 
fundamentals of Comparative Embryology of the Vertebrates, copyright 
1941, by permission of The Macmillan Company, publishers ) 


Biology of the Vertebrates 


Mitosis makes clear how cells are derived from preceding cells. It does 
not account, however, for the process of vertebrate reproduction involved 
in the formation of a new individual from two parents. 

<f ? 

Primordial Sex Cells 

Fig. 94. Diagram to show typical maturation and fertilization. 

Sexual reproduction, which is the prevailing means of numerical in- 
crease among higher animals and plants, involves something more than an 
unbroken succession of mitoses. The essential feature of it is the combina- 
tion of the chromosomal resources of two cells, egg an d sperm, to form an 
independent starting point for a new series of mitoses. 

The Units of Structure 


If mitosis can be defined as making two cells out of one, then sexual re- 
production may be described as making one cell, that is, a fertilized egg, 
out of two cells (Fig. 94). 

a. Entry of Sperm 

b. Loss of Sperm Tail 

c. Division of 

d. Approach of Sperm 

e. Increase of Sperm 

f. Formal ion of 

g. Splitting of Chromosomes 

h. Anaphase 

i. Two-celled Stage 
Fig. 95. Fertilization. 

As has been shown, mitosis is an elaborate process by means of which 
the same number of chromosomes is maintained throughout all the succes- 
sive generations of cells that make up an individual. When cells from two 
such individuals unite, some provision must be made for the reduction in 
the number of their chromosomes, else the cells of the new series of mitoses 
in the new individual will have double the typical number of chromosomes 

1^8 Biology of the Vertebrates 

for the species. This situation is met by a preparatory process of maturation, 
or "maturing," whereby the chromosomal material in each cell is cut in 
half. This process of the disposal of half of the germ-cell chromosomes is 
termed meiosis. 

Meiosis includes two mitotic divisions, of a special type, following one 
another in rapid succession. Let us take as an example a species in which 
the general body cells contain four chromosomes. During ordinary mitoses 
of these cells the number of chromosomes is temporarily increased to eight; 
but, with the distribution of half to each daughter cell, the number imme- 
diately returns to four. In the same species the two meiotic divisions, which 
are always accompanied by only one period of chromosome-splitting, also 
have only eight chromosomes to distribute. The two successive cell divisions 
during meiosis, however, result in the formation of four cells and, with the 
equality of distribution typical of ordinary mitoses, each of these four 
granddaughter cells receives its share of the eight chromosomes, that is, two, 
the reduced number. 

The union of the two germ cells, each equipped with the reduced num- 
ber of chromosomes, results in the formation of a fertilized egg with the 
full complement of chromosomes, half of which were contributed by the 
mature egg and half by the sperm ( Fig. 95 ) . In basic plan, therefore, there 
is for each chromosome derived from the mature egg a comparable one 
from the sperm. Thus the chromosomes may be said to exist in pairs, each 
pair consisting of one member contributed by the male parent and one by 
the female. 


In man there are 23 pairs of mated chromosomes (autosomes) in each 
cell of the body and in addition in the male a mismated pair of sex chromo- 
somes (allosomes) , sometimes known as x and y, making a total of 48 
all together (Fig. 96). In the female besides the 23 pairs of autosomes there 
is a pair of allosomes, both of which are of the x type, likewise making a 
total of 48. 

As a result of this inequality, when the sex cells undergo meiosis and re- 
duce to one-half their equipment of chromosomes, the mature eggs are all 
alike (23 + x), while the mature sperm cells are of two sorts (23 -\- x) 
and (23 -j- y). The sex of the future individual is consequently dependent 
upon which kind of sperm unites with the egg, as follows: 

egg sperm fertilized egg 

(23 + x) + (23 + x) r= (46 + 2*) 9 (female) 

(23 + x) 4- (23 +y) = (46 + x + y) $ (male) 

The Units of Structure 


There is one outstanding exception to the rule that all individuals of 
any particular species have the same number of chromosomes in every one 
of their structural units. Among many species there is found to be one more 
chromosome in each cell of the female than of the male, although curiously 
in birds as well as in butterflies and moths (Lepidoptera) the reverse is 




Fig. 96. Chromosomes of human male. At the left is shown the typical 
arrangement of 48 chromosomes on the equatorial plate of a spermato- 
gonium. The smaller chromosomes are near the center. At the right the 
chromosomes from a somatic cell are sorted out and arranged in pairs. 
(After Evans and Swezy. ) 

true, the male showing one more chromosome than the female in each com- 
ponent cell. This sexual difference in chromosomal count occurs because 
the allosomes, instead of being represented in one sex by a mismated pair, 
are present in that sex as an odd chromosome. In this case they are labelled 
x and o, the o indicating absence of an allosome. Determination of the sex 
of the resulting individual, however, comes out exactly as in the case of the 
mismated allosomes, that is: 

egg sperm fertilized egg 

(autosomes -J- x) -f- (autosomes -f- x) = (double autosomes -\- 2x) = ?, 
(autosomes -f- x) -f- (autosomes -f- o) = (double autosomes -j- \x) =. $. 

Still other combinations of allosomes have been observed, but although 
the number of allosomes and autosomes varies in different species of ani- 
mals and plants, all cases agree in producing either one kind of mature eggs, 
and two kinds of mature sperm, or the reverse, so that the determination of 
sex in a new individual is referable to a definite combination of the chromo- 
somes, making the chances fifty to fifty that either sex results, which agrees 
approximately with observed findings. 

140 Biology of the Vertebrates 


The total number of cellular units taking part in the structure of the 
human body is beyond all imagination. Dr. Keen notes that the hair of a 
man's beard grows one millimeter in twenty-four hours. The constituent 
cells in the make-up of a millimeter of hair are, by count and computation, 
roughly 10,000, so that seven or eight new cells per minute are formed for 
every hair. Multiplying this number by the total estimated hairs of the head, 
one arrives at figures that even a mathematician has difficulty in compre- 

The example given concerns but one of the many kinds of organic units 
which take part in the formation and repair of the human frame. When it 
is remembered that each one of these cells arises from a preceding cell by 
the elaborate machinery of mitosis, the laziest person may feel well assured 
that he has accomplished something at the close of every day. 


Division of Labor in Tissues-Histology 


Histology, or the science that deals with tissues, includes Cytology which 
was considered in the preceding chapter. The cells formed by successive 
mitoses from the fertilized egg differentiate into various tissues that consti- 
tute the body. A tissue is an association of similar cells which have under- 
gone specialization for some particular purpose, and intercellular material, 
the amount of which varies greatly in different tissues. Thus, bone tissue in- 
cludes bone cells, that are very much alike, and a considerable amount of 
hard intercellular material, while epithelial tissue is an association of epi- 
thelial cells, that resemble each other, but contain a minimum amount of 
intercellular substance. 

The similar cells constituting a tissue may be connected with each other 
either by delicate strands of cytoplasm that penetrate the enclosing cell 
walls, or by an intercellular ground substance of some sort either secreted by 
the cells themselves in the form of exaggerated cell walls, or formed out of 
intruded interstitial material arising extraneously, like mortar between 

Combinations of tissues make organs in much the same way that differ- 
ent textiles are combined into garments, and in turn organs make systems, 
just as different garments together make costumes. For example, the stom- 
ach is an organ that is assembled out of muscle tissue, blood tissue, gland 
tissue, nerve tissue, and the like, which together with other organs like the 
teeth, intestine, and pancreas, forms the digestive system. 

For purposes of general description tissues may be classified as follows: 

I. Fluid tissues 
II. Stationary tissues 

1. Epithelial 

2. Connective 

3. Muscle 

4. Nerve 



Biology of the Vertebrates 


The fluid tissues are blood and lymph. Their cellular components are 
disconnected and are, therefore, constantly rearranging themselves with ref- 
erence to each other, unlike the cells of other tissues which maintain a com- 
paratively stable spatial relationship with each other. 

In the lower invertebrates, such as the coelenterates and flatworms, the 
body fluid has no cells. Many invertebrates have only amoeboid white blood 
cells, but the blood of vertebrates generally is characterized by the presence 
of both white and red blood cells, and is consequently an elaborated fluid 

Fluid tissues permeate the spaces which separate other tissues, and even 
the interstices between the cells of these tissues. They also occupy larger 
spaces, like the cavities of the joints, for example, and particularly circu- 
late through special channels, called blood vessels and lymph spaces, that 
extend to almost every part of the body. In a later chapter, further consid- 
eration of the blood will be given. 

Jo o o o 0000 


o Y o \\ o V°/v^V 

To Viol o ToLV\° 

C D E F 

Fig. 97. Epithelial tissues, a, one-layered squamous (flat) epithelium; 
B, one-layered cuboidal epithelium, three cells with cuticle and three with 
cilia; c, columnar epithelium; d, pseudo-stratified ciliated columnar epi- 
thelium; e, stratified squamous epithelium; f, stratified columnar epi- 


Epithelial tissues (Fig. 97) are the most primitive of all tissues. They 
come into contact with other stationary tissues on one surface only, since 
they clothe the outer surfaces of the body and line various cavities and pas- 
sage-ways, including the blood vessels. They produce both cells that receive 
stimuli from the outside world and those that secrete and excrete different 
substances, as well as giving rise to the highly important sex cells. 

There is usually a minimum amount of intercellular material in epi- 
thelial tissues. The eells composing them may assume a squamous, cubical. 

Division of Labor in Tissues 143 

or columnar form, and may be arranged in a single layer (simple epi- 
thelium) or a succession of layers (stratified epithelium). 

Glandular epithelial tissues have a great variety of functions in the econ- 
omy of the organism, namely, digestive in the salivary, gastric, and pancre- 
atic glands; defensive in poison glands of snakes, and stink glands of skunks 
and other carnivores; protective in the mucous glands of fishes and am- 
phibians, the shell-producing glands of mollusks, and the ink glands of 
squids ; lubricative in the oil glands of the hair, and in mucous glands gen- 
erally; nutritive in the mammary glands and in the albuminous glands of 
birds; constructive in the spinning glands of spiders and cocoon-forming 
insects; cleansing in the lacrimal glands of the eyeball; and temperature- 
regulating in the sweat glands of the mammalian skin. 

When substances produced by gland cells are utilized for the benefit of 
the organism as a whole, they are defined as secreting glands, but if the sub- 
stances produced are waste products that are cast out of the body, they are 
termed excreting glands. 

If a gland is supplied with a duct whereby its products may reach the 
outside or be poured into some internal cavity or passage-way, it is termed 
an exocrine gland, but if no duct is present and the products of glandular 
activity must be transferred to the blood in order to be distributed, then it 
is known as an endocrine gland, and the substance which it produces, as a 

The morphology and behavior of these various glands will receive more 
attention later. It is sufficient here merely to assign them to their proper 
place among the epithelial tissues. 


Connective and supporting tissues of vertebrates lie usually inside of the 
integument that clothes the body. The component cells of these tissues do 
not form layers, as epithelial tissues tend to do, but are massed together 
with more irregularity. Their intercellular substances are usually much more 
in evidence, particularly in cartilage and bone. 

1. Connective Tissues 

Included among connective tissues, whose mission is filling space be- 
tween organs and parts of organs, are at least five different sorts that may 
in some instances merge into each other, namely, (1) gelatinous; (2) noto- 
chordal; (3) reticular; (4) adipose; and (5) fibrillar. 

Gelatinous tissue reaches its most characteristic expression ill sponges, 


Biology of the Vertebrates 

and semi-transparent pelagic animals, such as medusae and ctenophores, 
in which the jelly-like bulk of the body is composed of secreted inter-cellu- 
lar material throughout which are scattered a few cells, frequently joined 
together in a very open meshwork by delicate cytoplasmic bridges. This type 
of tissue does not commonly appear in the bodies of adult vertebrates, al- 
though during embryonic development the so-called mesenchyme passes 
through a gelatinous tissue phase. 

~~~S"~ Nerve Cord 

^» Connective Tissue 

Z-m Elastica Externa (Primary Sheath) 

^Xv^iil — Fibrous (Secondary) Sheath 
'---^ — Peripheral Notochordal Cells 
Vacuolated Notochordal Cells 

*■ Dorsal Aorta 

Fig. 98. Cross section through the notochord and its sheaths, from a 
young dogfish. 

In notochordal tissue, on the contrary, there is a great reduction of in- 
tercellular material, so that the thin-walled cells lie closely pressed together 
( Figs. 98 and 211). Whatever rigidity is attained by this tissue is due largely 
to the fact that the cells are so tightly packed within a tough sheath that 
a certain turgor results like that when sausage meat is crowded into a casing. 

Reticular connective tissues (Fig. 99) form the meshlike supports that 
characterize many of the softer organs, like the liver, spleen, and lymph 
nodes, which are ordinarily thought of as being without internal skeletal 
devices of anv kind. 

Division of Labor in Tissues 


Adipose tissue is somewhat similar to the ordinary reticular tissue that 
forms the skeletal matrix of soft parts, for in this tissue groups of cells that 
specialize in fat storage lie enmeshed in a loose reticulum. When the fat 
cells are melted out of a piece of fat pork by frying, for example, there is 
left behind a residual network which is the skeletal, or reticular, part of the 
adipose tissue. 

Fig. 99. Reticular con- 
nective tissue from a lymph 
gland of a cat, to show the 
supportive skeleton of a 
soft organ. (After Krause- 

Fig. 100. Fibrillar tis- 
sue in the form of elas- 
tic cartilage from the 
external ear of man. 
(After Bohm, Davidoff 
and Huber.) 

Like other connective tissues, fibrillar tissue (Fig. 100) consists of cells 
but it is distinguished principally by fibers that interlace among the cells. 
These fibers, themselves the product of cellular activity, are of two sorts, 
white non-elastic fibers, and yellow elastic fibers. The yellow fibers are pe- 
culiar to vertebrates. They occur in such parts of the body as the walls of 
the blood vessels, valves of the heart, the lining of the alveolae of the lungs, 
and in intervertebral ligaments. Both yellow and white fibers may be 
densely compacted together, as in fascia and sheaths of muscles, in peri- 
chondrium and periosteum, around cartilage and bone respectively, or they 
may be arranged in the form of looser texture, such as is found in the walls 
of blood vessels and the dermal part of the mammalian skin where they form 
the substance that is manufactured into leather. 

In the sclera of the eyeball and in tendons, between muscles and bones, 
the fibers are mostly white. Fibrillar tissue plays an indispensable part in 
holding things together and is probably the most widespread tissue in the 
vertebrate body. 

2. Supporting Tissues 

(a) Cartilage. — Cartilage, or "gristle," is a nerveless, bloodless, rela- 
tively flexible tissue that enters into the skeleton of vertebrates. Its texture 

246 Biology of the Vertebrates 

is not as firm and unyielding as bone, and consequently it is better adapted 
as scaffolding for water animals, such as fishes, where the surrounding 
medium helps to support the body, than for such use in land animals whose 
weight is held up in thin air. 

There may be distinguished at least five kinds of cartilage, namely, 
( 1 ) precartilage ; ( 2 ) hyaline ; ( 3 ) fibrous ; ( 4 ) elastic ; and ( 5 ) calcified 

Precartilage is a temporary embryonic type that precedes the formation 
of other kinds, but may sometimes endure in the adult organism, as in the 
fin rays of certain fishes. It consists of cells (chondroblasts) that have the 
power to secrete a thickened cell wall, or an intercellular matrix, at the 
expense of their own cytoplasm. 


% XW 

@jfp : :--*Cartilage Cells 
-;-— Hyaline Matrix 


Interstitial Substance-' 

Fig. 101. The differentiation of hyaline cartilage, earliest stage at the 
left. (After Maximow and Bloom.) 

When this process has continued until the diminishing cells have isolated 
themselves from their neighbors in a seat of surrounding matrix, which is 
somewhat firm and translucent, the hyaline stage has been reached (Fig. 
101). Hyaline cartilage is found in the bendable and projecting part of 
the human nose, at the ends of the ribs joining the breastbone, in the stiff 
incomplete rings that keep the tracheal and bronchial tubes from collapsing, 
and in other parts of the bodily structure. 

The matrix between the cartilage cells may be interwoven with fibers, 
either white or yellow, as in fibrillar connective tissue, in which case either 
fibrous or elastic cartilage is the result. Fibrous cartilage is typically repre- 
sented by the padlike intervertebral discs separating the centra of the verte- 
brae in the backbone, while elastic cartilage is found in such places as the 
epiglottis, and the pinna of the external ear which fortunately springs readily 
back into its original shape when distorted. 

Sometimes the intercellular matrix of hyaline cartilage becomes infil- 
trated with limy salts, when it is designated as calcified cartilage. In adult 
cartilaginous fishes much of the cartilage is of the calcified type. 

Division of Labor in Tissues i^j 

( b ) Bone.— Bone is the best known of the skeletal tissues of vertebrates. 
As contrasted with cartilage it is supplied with nerves and blood vessels, 
and is considerably more rigid. It includes at least two kinds of cells, 
osteoblasts and osteoclasts. The first of these are bone-forming cells which 
produce the limy intercellular matrix that characterizes bone. The osteo- 
clasts, on the other hand, are bone-wrecking cells, that tear down bone 
tissue and make possible the rearrangement of material necessary to the 
accomplishment of growth among such unyielding building materials as 
bony plates. 

Bone consists of two essential substances : ( 1 ) , an organic base of 
living cells, and (2), in the excessively developed matrix surrounding these 
cells, an infiltrated mass of inorganic limy salts. These two components are 
so intimately joined that there is no visual way of separating them, yet each 
alone is sufficient to give characteristic contour to a bone, for when the 
organic part is burnt out by fire, or the inorganic component is dissolved 
away by acid, the part remaining in each instance preserves the original 
form of the bone. 

In relative weight the inorganic, or mineral, part of bone is about three 
fourths of the whole, although the ratio of inorganic to organic varies with 
age, ordinarily becoming greater the longer the bone lives. According to 
Heintz an analysis of the mineral constituents of a human femur resulted 
as follows : 

Per Cent 

Calcium carbonate 9.06 

Calcium phosphate 85.62 

Magnesium phosphate 1.75 

Calcium fluoride 3.57 


The embryonically active osteoblasts are responsible for the formation 
of bone tissue. By their rapid multiplication living bone cells are formed 
which in turn secrete the hard parts, or lamellae. As bone grows, however, 
it becomes necessary not only to add new tissue but also to remove that 
which has already been formed. Comparing the lower jaw of an infant with 
that of an adult (Fig. 102), it is evident that no single cell of the former 
structure can persist unchanged throughout the process of growth. The jaw 
of an infant is not simply added to as it becomes larger, but all the building 
material composing it must be broken down bit by bit and reassembled, and 
supplemented many times before the adult bone is fashioned. It is as if a 


Biology of the Vertebrates 

stone building were enlarged not simply by adding to the outside of it as it 
stands, but by tearing it down and reassembling it with additional stones in 
order to enclose a larger area. 


v -„..- 

Fig. 102. Diagram compar- 
ing the jaw of an infant with 
that of an adult. (After 

Fig. 103. The condition of the 
jaw in old age, showing the acute 
projection of the chin resulting 
from the loss of teeth and the ab- 
sorption of the sockets for the 

Fig. 104. In 

toothless old age 
the chin and the 
nose tend to hob- 
nob together. 
(After Camper.) 

This wrecking of bone tissue already formed in order to make way for 
rearrangement and enlargement is accomplished by the rather large defi- 
nitely identified cells called osteoclasts. The destructive work of these cells 
is not always followed by equally constructive reorganization, however, for 
when the work of the osteoclasts exceeds that of the osteoblasts, a bone 
decreases in size. Thus, in toothless old age (Fig. 103), the lower jaw not 
only becomes smaller through the loss of teeth, but the bony sockets in 
which the teeth were set also decrease in size through the removal of tissue 
by osteoclasts, with the result that the chin and nose tend to hobnob 
together (Fig. 104). 

^!^___- — Spongy Bone 

;^t — Compact Bone 

i1?T^M- Marrow Cavity 

«-•'•! -~* Willi 

Haversian Canals----^:^^^: 

!^^^;^C;"0;^>^;;^j^T^V7 — I nterstitia I 
W^^^!^')'- £• ^W La m e I lae 

Concentric Lamellae -^skt^ 

Circumferential Lamellae-' ^-^^^^ 
Fig. 105. Stereogram of bony tissue. 

Division of Labor in Tissues 


When a thin slice of bone is taken from a cross section through the shaft 
of the femur, for instance, and ground down to translucent thinness, if 
examined under the miscroscope, it is seen to be made up of innumerable 
small bony plates, or lamellae (Fig. 105). Even to the naked eye there are 
two kinds of bone, namely, spongy and compact. In spongy bone the 
lamellae are arranged in an open framework leaving many spaces filled 
with bone marrow. Compact bone appears solid but actually includes many 
minute spaces, in which cells lie or through which blood vessels and nerves 
run. The lamellae are arranged in at least three different ways, concentri- 
cally, interstitially, and circumferentially. 

Concentric lamellae, somewhat like rings of growth around the pith of 
a woody stem, envelop small tubelike branching passage-ways, the Haver- 
sian canals, which permeate the bone lengthwise, 
forming the conduits for the passage throughout 
the bony tissue of capillaries, lymphatics, and 
nerves. The Haversian canals, surrounded by 
concentric lamellae, are best seen in the dense 
tissue of the cylindrical shafts of the long bones 
in the appendages, where they communicate 
both with the periosteal coverings of the bone on 
the outside and, through the entire bony tissue, 
with the marrow cavity inside. They constitute 
the subways for organic traffic throughout the 
bone tissue, making this part of the skeleton a 
living adjustable structure. 

Interstitial lamellae are necessarily irregular 
since they fill in spaces between neighboring 
Haversian systems. 

Finally, circumferential lamellae are arranged 
either around the outside margin of the whole 
cross section of the bone shaft, just beneath 
the periosteum, or as an internal layer grading 

over into the spongy tissue that borders the marrow cavity within the 

The Haversian canals are not the only spaces between the lamellae that 
help to make the living bone porous. Between the hard lamellae separating 
them from each other are tiny spaces, called lacunae, or "little lakes," in 
which lie imprisoned the living bone cells (Fig. 106). Lacunal spaces com- 
municate with each other through canaliculi, microscopic passageways 
through the w-alls of the limy lamellae. 

Fig. 106. A diagram show- 
ing a fragment of bone tis- 
sue at the edge of an Haver- 
sian canal, through which 
blood vessels and nerves 
penetrate the substance of 
the bone. Eight lacunae, 
each containing a bone cell, 
are indicated and also their 
connection by canaliculi. 
The bony lamellae are rep- 
resented in black. 

i S o 

Biology of the Vertebrates 

Most hard parts of the skeleton are laid down first as connective tissue, 
followed by cartilage which is later slowly replaced by bone. These are 
replacing or cartilage bones. A few parts, notably the flat bones of the skull, 
are laid down directly in connective tissue membranes without a cartilage 
intermediary. To these the name investing or membrane bones is applied. 
In either case the hard parts are produced by connective tissue cells which 




■s.'--' _ 'Y': 

; , x : 

. .-; ■ £«&!> ') 



■•■ ■ . f ">':.• 


. i-j- -•£*%■*.: •]■ 

• ':'.^. ■■'.■• •■';/' r 


:'.'"': '!■?&" 

">■•■!• -:''",* 

'^^ Cartilage Cells in 

■ ^^■r^ Successive Stages 

/ of Degeneration 

Blood f--sf -^-* 
Cells ■*!§§& 

Osteoblasts t--- ^ 

-"Giant Cells or 
4 Osteoclasts 

— Young Bone 

Fig. 107. Reconstruction of cartilage into bone. Beginning at the top of 
the figure, the cartilage cells are in successive stages of degeneration. 
( From Dahlgren and Kepnoi . ) 

Division of Labor in Tissues 

1 5 1 

acquire the ability to produce cartilage or bone. Although at first all of the 
bony tissue is of the spongy type, a thick region on each surface is later con- 
verted into compact bone, leaving inside a filler of spongy tissue in which 
the spaces are occupied by bone marrow. The connective tissue membrane 
which remains as a covering of the bones is called the periosteum. 

In the case of a replacing bone like the tibia of the lower leg (shank), 
a cartilaginous rod is laid down in the connective tissue. Later this cartilage 
is replaced by bone in somewhat the following manner. Some of the peri- 
chondrial cells and others in- 
side of the cartilage near the 
middle of the rod become bone 
builders (osteoblasts) and lay 
down a ring of bone at that 
point. Adjacent to this ring the 
cartilaginous matrix begins to dis- 
solve, probably under the influ- 
ence of certain cells which have 
been called chondroclasts. As fast 
as the cartilage is destroyed, 
spongy bone takes its place (Fig. 
107). From the middle of the bar 
these changes spread toward the 
ends until the entire shaft (dia- 
physis) has ossified leaving only 
the ends (epiphyses) of the tibia 
as cartilage. This is the condition 
in man at about the time of birth. 
Soon after birth a center of ossi- 
fication appears in each epiphysis 
and enlarges until the only cartilage left here is a cap on the articular 
surface and a plate between diaphysis and epiphysis (Fig. 108). The 
articular caps remain throughout life but the plate is a temporary device 
to permit elongation of the bone. The cartilage in the middle of the plate 
continues to grow and expand. At the same time the process of minute 
replacement goes on at the two surfaces where it is in contact with diaphysis 
and epiphysis. At an age of 17-25 years the cartilage of the plate ceases 
regeneration and the invasion of bone completely replaces the plate. The 
bone has then completed its elongation. During this developmental period 
there is also an increase in the diameter of the bone, by the addition of 
material on the surface. At about the time of birth osteoclasts commence to 

Fig. 108. Ossification and growth in a long 
bone of mammals, a, cartilaginous stage; b 
and c, laying down of endochondral bone 
(dotted) and perichondral bone (black); d, 
appearance of an ossification center for an 
epiphysis at each end; e, beginning of for- 
mation of marrow cavity (lighter dotted); f, 
fusion of epiphyses with shaft, leaving carti- 
lage only at articular ends. (After Arey.) 

i$2 Biology of the Vertebrates 

destroy the bony plates in the center of the shaft leaving only the soft 
marrow which, from the beginning, is in between the plates. A large marrow 
cavity, running the entire length of the shaft, thus develops. This cavity 
slowly enlarges as the bone increases in length and diameter. During this 
same period the rest of the spongy bone of the diaphysis, but not of the 
epiphysis, is reorganized into compact bone laid down in concentric rings 
around the numerous small blood vessels which run through the bony tissue, 
thus forming Haversian systems. 

In this manner the adult condition of the bone is reached. In the Ions; 
bones the diaphysis consists of compact bone surrounding a large elongate 
marrow cavity. The epiphyses remain as spongy bone except for a very thin, 
outer layer of compact bone, which may not completely cover these parts. 
The spaces in this spongy bone are continuous with the marrow cavity and, 
as previously mentioned, filled with marrow. The connective tissue covering, 
which was called perichondrium in the cartilage stage, becomes known as 
periosteum where bone is present beneath it. 


One of the commonest manifestations of life is movement. Even in sta- 
tionary plants living cytoplasm streams about within the cells, and fluids are 
passed from one part to another. Within the animal body certain members 
of the cell community, like blood cells, shift about with much freedom, 
while other kinds of cells, leucocytes for example, are liable to change their 
shapes. The well-nigh universal ability of living cells to move or change 
shape, culminates in muscle cells, whose conspicuous contractility not only 
causes internal movement but exercises an influence for motion also in more 
or less distant parts of the body to which they are directly or indirectly 

The cytoplasm of the elongated muscle cells is differentiated into sarco- 
plasm and myofibrils, as well as sheaths which clothe the sarcoplasm like a 
thin rubber glove. Myofibrils are embedded in the sarcoplasm and are the 
particular mechanism of contractility. They are peculiar in that they effect 
contraction in only one direction instead of in any direction, as in the case 
with ordinary contractile cytoplasm. Muscular movement is always brought 
about by the pull of muscles, while restorative movements are in turn 
effected by the pull of antagonistic muscles, and not by the relaxation that 
follows contraction. Muscle tissue is, therefore, simply specialized tissue in 
which the general function of contractility is carried out more effectually 
than elsewhere. 

Division of Labor in Tissues 1 53 

Dr. A. E. Shipley has vividly emphasized the power that may be stored 
in muscle tissue by citing the performance of a jumping flea. Some patient 
person who succeeded in weighing nine fleas found their average weight to 
be 0.38 milligrams. Fleas can leap from 8 to 13 inches. If a man who weighs 
70 kilograms, or about 150 pounds, made a corresponding leap, he "could 
leap to the moon in about ten jumps." 

There are three kinds of muscle tissue that differ in the degree or 
manner of differentiation, namely, smooth, striated, and cardiac. 

1. Smooth Muscle 

Smooth muscle cells have a single nucleus near the center of the cell, 
are usually spindle-shaped, and rarely forked at the ends (Fig. 92a). In 
man they vary in size from 15 micra (15/1000 of a millimeter) in blood 
vessels, to 200 micra in the digestive tube, while in the walls of the uterus 
during pregnancy they may reach 600 micra in length. In the pliant walls 
of the bladder they are more or less interlaced or felted together, lying in 
every direction, so that the bladder when it is emptied contracts like a toy 
balloon rather than collapsing like an empty hot-water bag. 

Smooth muscle cells are often isolated or in thin layers, or more rarely 
massed together into bulky tissues. They are widely distributed throughout 
the body, and are found for example in the skin where they act as hair- 
raisers, feather-fluffers, or around the openings of glands where they act as 
doorkeepers. They also form a large part of the contractile walls of various 
tubes and passage-ways, such as blood and lymph vessels, the digestive tube 
(except the upper part of the esophagus), the trachea and bronchi, the 
reproductive ducts, and the ureters. 

2. Striated Muscle 

Striated muscle tissue is "flesh," and in man it constitutes approximately 
fifty per cent of the weight of the entire body (Fig. 92b). It is found not 
only in the bulky body wall, and the muscles of the limbs where it effects 
locomotion, but also, at least in the higher vertebrates, in the diaphragm, 
tongue, esophagus, pharynx, larynx, and the muscles of the eyeball. 

The component fibers of straited muscle tissue, which are quite evident 
in cooked corned beef, are commonly large, elongated cells that are no 
longer able to be served, like smooth muscle cells, by a single nucleus. In 
consequence scattered along the fiber many nuclei are present, like sub- 

The descriptive term "striated" refers to the fact that the embedded 
elastic myofibrils, which extend throughout the length of the fibers, are dif- 

J 54 

Biology of the Vertebrates 

ferentiated into alternate beadlike bands lying side by side across the 
bundles of fibers in such a fashion as to produce a striated effect. These 
beadlike parts of myofibrils are physically and chemically unlike the con- 
necting parts between the "beads," because they stain differentially with 
aniline dyes and refract light differently, the dark beads, or anisotropic 
bands, being doubly refractive in polarized light, while the isotropic bands, 
or parts between the beads, are singly refractive in polarized light. 

Moreover there is a physiological difference, as well as physical and 
chemical, in these parts of the contractile fibrils within a muscle fiber, since 
anisotropic bands shorten more than isotropic bands during contraction. 

In birds the "white meat" of the breast is characterized by an excess of 
myofibrils, while the "dark meat" has more sarcoplasm and less myofibrillar 
substance in its fibers. 

In general the striated muscles effect quick movements of comparatively 
short duration and are voluntary, that is, under the control of the will, while 
smooth muscle tissue is involuntary and much slower in action. There are 
certain notable exceptions to this generalization among invertebrates, for 
the body muscles of some mollusks are smooth and voluntary, while the 
visceral muscles of insects and crustaceans are typically striated and in- 

3. Cardiac Muscle 

The tissue of the muscular vertebrate heart is intermediate in char- 
acter between smooth and striated muscle, in that the component cells are 

comparatively short, branching, and in- 
voluntary in action, although striated 
in appearance and multinuclear (Fig. 

The enormous dynamic force exer- 
cised by any kind of muscle tissue is 
seldom realized. The tireless heart of 
man, for example, knows no rest, as one 
ordinarily thinks of rest, but throbs faith- 
fully day and night without skipping a 
beat throughout a long lifetime. 

Fig. 109. Cardiac muscle cells. 
(After Szymonovicz. ) 


Nerve tissue is characteristic of animals rather than plants, although 
the nervous function of sensitivity is a fundamental property of cytoplasm, 
by no means absent from plant life. This tissue consists of specialized nerve 

Division of Labor in Tissues 155 

cells, or neurons, accompanied by nutritive components of various sorts, con- 
nective tissue, and non-nervous supporting neuroglia cells of ectodermal 
origin. The cytoplasm of neurons is differentiated by the presence of neuro- 
fibrils, that differ from the rest of the cell in chemical composition, as shown 
by staining methods, and which are particularly fitted for reception of 
stimuli and the transmission of impulses, just as myofibrils are specialized 
instruments of contractility in muscle cells. 

Neurons exhibit extreme modification from the characteristic spherical 
embryonic form, the cytoplasm being drawn out into extremely elongated 
processes or fibers of two kinds, called respectively dendrites and neurites. 
Dendrites are numerous and branch freely like a tree, as their name indi- 
cates, while there is only a single neurite to each cell (Fig. 92f). 

When impulses travel through a neuron along the neurofibrils, they do 
not go at random in any direction but always enter through dendrites and 
pass out through the neurite. Impulses are relayed from cell to cell by chain- 
like contact {synapse) between the neurite of one neuron and the dendrite 
of the next. 

"Nerves" are bundles of neurites and dendrites that extend like cables 
outside of the central nervous system. They are enclosed in a common sheath 
of connective tissue. Ganglia, also outside of the central nervous system, are 
aggregates of cell bodies of neurons. 

Nervous tissue accomplishes a double mission : first, that of relating the 
organism to its environment through sense organs; and second, that of regu- 
lating and correlating the bodily activities by means of the central and 
autonomic nervous apparatus. 


The Development of the Individual- 


No animal or plant is an orphan in the sense that it has no parents. In 
protozoans produced by fission the parent perishes by division into progeny. 
The spontaneous origin, even of the most minute organisms, at least under 
present conditions on this globe, has been effectually disproved. Modern 
control of bacterial diseases, together with the incalculable boon of aseptic 
surgery and antiseptic practice, depends upon the clear understanding of 
this fact. 

A new organism may be introduced into the brotherhood of living things 
by one parent or by two, but no animal or plant comes into the world of 
today unsponsored by preceding life. 

When there is only one parent the new individual is said to arise by 
asexual reproduction. The method of sexual reproduction, however, that 
involves a double source, is by far the commoner way for higher animals 
and plants to begin their separate existence. 

The starting point of a sexually produced individual is a zygote, or 
fertilized egg, which is a combination of two parental gametes, or mature 
sex cells. It is the purpose of this chapter to trace some of the more impor- 
tant episodes in the "miraculous pageant of transformations" that take place 
between the setting up of the zygote and the establishment of the adult 
organism. A fascinating part of biology is the particular province of Em- 
bryology, some familiarity with which is essential to the understanding of 
the structure and functions of adult animals and plants. 

Ever since Aristotle described what he saw when he opened hen's eggs 
at various stages of incubation, embryology has been occupied with record- 
ing what changes take place during development, with the result that a 
considerable body of detailed information has been accumulated. In recent 


The Development of the Individual icy 

years embryologists have not been content simply to find out what happens 
in a transforming embryo, and how the changes occur, but have sought 
more and more to explain why such changes take place. This attempt has 
led to Experimental Embryology, an aspect of biological science with alluring 
vistas that is claiming much attention today. 

Why does the power to regenerate lost parts decrease as we go up the 
vertebrate scale? Why does the orderly sequence of development sometimes 
become upset so that a monstrous organism results? Why do groups of cells 
sometimes go on a rampage and form unorganized cancerous growth that 
the animal cannot control? 


1. Differentiation of the Germ Cells 

Two germ cells, sperm and egg in animals, are necessary partners in 
the enterprise of a fertilized egg. The fact that different terms, namely, 
pollen grains and ovules, are commonly employed to designate the repro- 
ductive units in sexual plants does not indicate any essential difference in 
the germ cells of plants and animals. A pollen grain is a spore which pro- 
duces an organism, the pollen tube, from which a fertilizing element or cell, 
homologous with the animal sperm, passes to the ovule. 

Fertilization is the union of two diverse germ cells, and consequently 
provision has to be made for getting them together. 

The egg, which is ordinarily loaded with first-aid nutriment for the 
future organism during the critical early stages of its development, tends to 
become relatively heavy and stationary, thus throwing upon the sperm the 
responsibility of doing the traveling in the necessary process of getting to- 
gether. The egg does not meet the sperm half way. The sperm has to travel 
the entire distance. This circumstance has brought about a high degree of 
morphological difference in the germ cells of the two sexes. 

2. Kinds of Sperm 

The result of the physiological necessity of a union of germ cells to effect 
fertilization is that the sperm cells of animals, and the male cell in the pollen 
grains of plants, become specialized into structures adapted particularly for 
locomotion. In animals this involves a fluid medium in which to travel. 
In plants the traveling male germ cell within the pollen grain is more often 
adapted for transport through the air by the wind or through the agency 
of insects. 

A typical sperm cell adapted for locomotion along a fluid highway is 

158 Biology of the Vertebrates 

pictured in Figure 92h. The human sperm cell, according to Waddington, 
can travel at the rate of about an inch in three minutes. 

The differentiated head of the sperm, which is principally made up of 
the nucleus, carries the chromosomes that are freighted with the hereditary 
determiners, while the middle piece and the locomotor tail represent trans- 
formed cytoplasm, modified for particular uses. An animal sperm is thus 
adapted for sculling forward through a fluid medium by means of the 
vibratile tail. Animals never employ aerial routes for this purpose as do 

Among vertebrates, bony fishes and anurans usually broadcast their eggs 
and sperm in water, and the sperm cells travel in this medium to reach the 
egg. Among land forms like reptiles, birds, and mammals, internal fertiliza- 
tion occurs, so that the sperm travel up the oviduct to the egg in a 
secreted fluid medium that serves the same purpose as water in the case 
of aquatic animals. Copulation, which occurs in all land animals, is simply a 
device for insuring placement of locomotor sperm cells in a suitable high- 
way leading to the waiting egg. 

An exception to the almost universal type of sperm with a locomotor 
tail is found among certain worms, Ascaris fpr example, and in crabs, where 
an amoeboid or angular form is assumed by the sperm cell. The approach 
to the egg in this instance is accomplished by much slower movements, the 
creeping sperm being in contact with solid objects. 

3. Kinds of Eggs 

The eggs of animals differ specifically with reference to the load of 
nutritive yolk which they carry. Curiously those with a minimum amount of 
stored food are found at the two extremes of the chordate scale, namely, 
the eggs of amphioxus and of mammals. The eggs of amphioxus probably 
represent a primitive condition in the matter of the acquisition of yolk. The 
poverty of yolk in the small eggs of mammals has doubtless come about 
through a different chain of causes, correlated with the fact that not much 
stored food is required in the eggs of this group, since they early become 
implanted like parasites in the uterine wall of the mother from whom they 
derive their necessary nutritive start in life. As a consequence of the scarcity 
of yolk, the mammalian egg is remarkably small, that of man measuring 
only about 1/125 of an inch in diameter (Fig. 110). As the yolk is scattered 
equally through these eggs, they are described as isolecithal (iso, equal; 
lecithin, yolk). 

In cyclostomes, fishes, and amphibians, the abundant lifeless yolk is 
massed in the lower or "vegetal" half of the egg while the nucleus and 

The Development of the Individual 

2 59 

most of the living cytoplasm, constituting the embryogenic or "animal" pole, 
appears on the upper side. Such eggs are therefore telolecithal ("end- 

So much yolk is present in the telolecithal eggs of reptiles and birds that 
the nucleus of the egg cell, together with its tiny halo of active cytoplasm, 
forms only a small area, or germinal disc, at the animal pole. When un- 
hindered the heavy yolk invariably rotates so that the germinal disc comes 
to lie uppermost. 

— Follicle Cells of 

m $jk Corona Rodiata 

Zona Pellucida 

_, — Nucleus 
MfiSj^ Cytoplasm 

1 Vitelline 


A B 

Fig. 110. a, human ovum, approaching maturity, much enlarged, b, 
human sperm, correspondingly enlarged. (After Arey.) 

In addition, birds have a reserve food supply of nutritive albumen or 
"white," packed around the egg within the protective shell. 

If one stretches the point to include such accessory food material, the 
bird's egg must be regarded as the largest kind of all animal cells. The egg 
of the wingless Apteryx of the Antipodes weighs nearly one fourth as much 
as the entire bird. An ostrich's egg is equivalent in bulk to about a dozen 
hen's eggs, while that of the gigantic "moa," now extinct (Fig. 51), had 
twelve times the content of an ostrich's egg and might, therefore, easily 
hold the palm as being the largest animal "cell" ever known. 


The manner of development of an egg is dependent upon the amount 
of inert yolk that is present. In amphioxus, where there is very little yolk to 
hinder the process of orderly cell division, the entire egg mass is equally 
involved in cell formation, and early development is less complicated than 

160 Biology of the Vertebrates 

in most vertebrates. Before considering specific points for several representa- 
tive chordates, let us first examine the fundamental plan of chordate 

1. Cleavage (Segmentation of the egg) 

After the union of the sperm and egg, the first of a long series of proc- 
esses that transform the fertilized egg into an adult individual occurs. This 
first process, called cleavage, consists of a rapid succession of mitoses in 
which the initial cell becomes divided in turn into two, four, eight, and so 
on (Fig. Ill), until a small mass of cells results, without appreciable in- 
crease in total weight over that of the fertilized egg. Each one of these small 
cells contains a complete double set of chromosomes, bearing hereditary 
potentialities from two parents, thus duplicating the original complement 
in the fertilized egg. 

Segmentation Cavity 


E ;/ F 

Fig. 111. Stages in the segmentation of the egg of amphioxus, termi- 
nating in the formation of the blastula (e) which is shown in hemisec- 
tion at f. (After Hatschek.) 

The result of these rapid preliminary cleavage divisions is the breaking 
up of the original cell into many separate working units. A very funda- 
mental principle underlying differentiation is division of labor, and this is 
facilitated when there are different nuclear centers present for the initiation 
of different cellular enterprises. 

2. The Blastula 

As the small cells, of essentially uniform size, become more numerous, 
they arrange themselves in the form of a hollow sphere, or blastula, the 
cavity within being known as the segmentation cavity and the individual 
cells, blastomeres. 

3. The Gastrula 

The cells at one pole of the hollow sphere divide oftener and become 
more numerous and crowded than at the other pole. Since they remain in 
contact with each other without changing their relative positions to any 

The Development of the Individual 161 

great extent, they tend to form a continuous layer that is more than sufficient 
to make up the original surface of the sphere in this region. They are, there- 
fore, forced to find more standing room which is accomplished by their 
pushing into the segmentation cavity, with the result that a double cup, or 
gastrula, is formed (Fig. 112). 




Fig. 112. Gastrulation in the development of amphioxus. (From Huett- 
ner, Fundamentals of Comparative Embryology of the Vertebrates, 
copyright 1941, by permission of The Macmillan Company, publishers.) 

The outer layer of this cup is the ectoderm, the inner layer the endo- 
derm (or "entoderm" of some authors). The new cavity within the cup is 
termed the archenteron, or primitive digestive cavity, and its opening to 
the exterior, the blastopore. The latter is at the posterior end of the embryo. 

The inpushing (invagination) of the endoderm continues until the seg- 
mentation cavity is almost obliterated. Meanwhile the addition of new 
material in a growth region near the posterior end of the gastrula, around 
the blastopore, brings about an elongation of the embryo and a marked 
decrease in the size of the blastopore. As this elongation is more rapid 


Biology of the Vertebrates 

ventrally than elsewhere, the reduced blastopore is moved to a somewhat 
dorsal position instead of remaining strictly terminal. This more rapid elon- 
gation of the ventral side of the gastrula is a good illustration of the far- 
reaching principle of unequal growth in the process of differentiation. In 
fact "unequal growth," that is, unequal in quantity or rate, lies at the very 
foundation of many processes of morphogenesis, constituting much of the 
subject matter of embryology. 

4. Formation of the Nervous System 

One of the earliest organ systems to appear is the nervous system. Along 
the dorsal side of the gastrula, ectoderm cells begin to multiply at a greater 
rate than in the surrounding region, thus forming a thickened area, the 
neural plate. Further increased mitotic activity in these cells soon results in 


Medullary Plate 

Somatic Mesoderm i^/p/P 

Splanchnic Mesoderm '&~f&~ ' 

Cavity of Archenteron isi-fl^T' 

Endoderm §-W 

Neural Fold 
vriz — Notochord 
* Coelom 
— Ectoderm 

Neural Tube 


Cavity of 

Medullary Plate 

Mesodermal, \ I Caviry G f Archenteron 

Somatic Mesoderm 



Ectoderm * 

A ^^M^ -*mm^ c 

Fig. 113. Formation of the neural (medullary) plate, neural tube, 
mesoderm, notochord, and coelom in chordates, based on amphibians. 
(a, after Hertwig and Mark; b and c, after Hyman.) 

the formation of a groove along the middle of the neural plate, as the cells 
in the mid-dorsal region push down into the underlying segmentation cavity 
and those on either side simultaneously are folded up into longitudinal 
ridges, neural folds ( Fig. 113). The furrow between the ridges is the neural 
groove, the walls of which are the forerunner of the central nervous system. 
The neural folds continue to approach one another until they meet above 

The Development of the Individual 


the invaginated groove to form the neural tube which soon splits off from 
the surface ectoderm. This process, beginning in the head region, proceeds 
posteriorly. Consequently when the groove has been completely closed over 
near its anterior end, the neural folds may be merely beginning to appear 
in the posterior part of the embryo. The extreme anterior end of the groove 
remains open for a time, however, as the neuropore which leads into the 
cavity of the neural tube, the neurocoele. 

Neural Fold- 
Blastopore — 


D Endoderm 

Fig. 114. Development of neural folds and formation of neurenteric 
canal in the development of amphioxus. (From Huettner, Fundamentals 
of Comparative Embryology of the Vertebrates, copyright 1941, by per- 
mission of The Macmillan Company, publishers.) 

Gradually the neural folds extend farther and farther toward the pos- 
terior end of the embryo until they reach points on either side of the tiny 
blastopore which, as a result, then lies in the bottom of the neural groove. 
As soon as the two folds have extended beyond the blastopore they turn 
toward the mid-line where they meet (Fig. 114). As at other levels, the 
folds in this posterior region eventually meet over the top of the groove and 


Biology of the Vertebrates 

complete the formation of a tube. Because the neuropore was on the floor 
of the groove, there is now a passageway [new enteric canal) between the 
neurocoele and the primitive gut cavity. This connection is later obliterated, 
leaving the two systems completely separate in this region. 

5. Formation of Notochord and Mesoderm 

Concurrently with the appearance of the central nervous system, three 
groups of cells grow into the dorsal part of the segmentation cavity, either 
from the dorsal endoderm or from the zone of proliferation just in front of 
the blastopore. One of these groups, lying in the mid-line, forms an un- 
paired strand of cells which later separates from the parent tissue to become 
the notochord. The other two groups of cells, one on each side of the noto- 
chord, are the paired beginnings of the third germ layer, the mesoderm 
(Fig. 113). 

Through rapid cell proliferation these mesodermal sheets spread laterally 
and ventrally around the endoderm. Meanwhile each sheet has split into an 
outer somatic layer, adjacent to the ectoderm, and an inner splanchnic layer, 
next to the endoderm. The new cavity thus formed, the coelom, lying wholly 
within the mesoderm, is a single continuous space on each side of the body. 

6. Differentiation of the Mesoderm 

As the mesodermal sheets spread and split, each undergoes differenti- 
ation into three regions, namely : ( 1 ) a dorsal epimere, near the neural 
tube; (2) a small mesomere; and (3) a ventral hypomere, enclosing a large 
part of the coelomic cavity (Fig. 115). The epimere, beginning first near 
the anterior end, becomes gradually divided transversely into parts known 


Notochord -- 
Endoderm — P 
Ectoderm - 

""* Coelom 
"Somatic Mesoderm 
Splanchnic Mesoderm 

Fig. 115. Differentiation of the mesoderm in a chordate with complete 


The Development of the Individual 165 

as somites, which soon become separated from the mesomere. Only weak 
and temporary segmentation ever appears in the mesomere, while the hypo- 
mere never shows evidence of segmentation or of separation from the 

The thin-walled hypomeres of the two sides grow toward the mid-line, 
both dorsal and ventral to the archenteron, until they come in contact with 
one another to form the two-layered dorsal and ventral mesenteries (Fig. 
116). Although the dorsal mesentery persists in the adult animal, nearly 
all of the ventral mesentery soon disappears so that the two coelomic cavities 
become continuous. Between the thin, approximated walls of the dorsal 
mesentery, blood vessels and nerves extend to and from the digestive tube. 

Nerve Cord —zvffi£i 

, Sclerotome 

Dermatome--'^;' /••'w?--' -r x >-*A ■ . 

&& 'i*\%iW >&-- Dorsal Aorta 

Myotome-^^r.;.;^.0 -^'A 
Sclerotome 'f^^f^M ^ ^ 
Mesomere -§ // --""VPfl- \\VVQ 
Dorsal Mesentery II" A, W&^°™ 
Endoderm -14 Infill Wf -.Dermatome 

Mesenchyme A— ~mM M" S ° ma | iC Mesoderm 
Coelom-^£--^|| ~^Jvf -Splanchnic Mesoderm 

^§^Jfc^§^~ Ventral Mesenter y 


Fig. 116. Differentiation of the epimere into sclerotome, myotome, and 
dermatome. Sclerotome and dermatome are composed of mesenchyme. 
a, earlier stage; b, later stage. 

7. Emigration of the Mesenchyme 

From the splanchnic part of each hypomere and both portions of each 
epimere, cells, collectively known as mesenchyme, migrate into the segmen- 
tation cavity. By amoeboid movement, they may wander anywhere within 
this cavity. Many cells from the median part of each epimere mass along- 
side the notochord and nerve cord to form a sclerotome ("skeletal seg- 
ment" ) , while most of those from the lateral part of the epimere gather just 
beneath the ectoderm as the dermatome ("dermal segment"). The portion 
of the epimere remaining after these mesenchymal cells have been given off 
is known as the myotome ("muscle segment"). See Fig. 1 16. 

Some of the mesenchymal cells from the hypomere group about the 
endoderm to form the smooth muscles, blood vessels and connective tissue 
of the wall of the digestive tract. Other mesenchymal cells, from both hypo- 
mere and epimere, migrate throughout the segmentation cavity to form, at 


Biology of the Vertebrates 

appropriate places, connective tissue, cartilage, bone, smooth muscles, blood 
cells and blood vessels. 

After mesenchymal migration has begun, the dermatomes and thin por- 
tions of the myotomes grow ventrally into the region between the somatic 
mesoderm of the hypomere and the ectoderm. With the filling in of dermal 
cells in the mid-line, both dorsally and ventrally, a continuous sheet of 
material, which differentiates into the derma, is laid down. Although the 
myotomes also grow to the mid-line, dorsal to the nerve cord and ventral 
to the body cavity, the muscles of the two sides of the body never fuse, but 
remain separated by a thin partition of connective tissue. The ventral, thin 
portions of the myotomes give rise to the several thin sheets of voluntary 
muscle tissue of the ventral and lateral parts of the body wall. 

8. Assembling of the Digestive Tube 

With the closing over of the blastopore, no direct opening into the 
archenteron remains. Later a digestive tube, with an inlet at one end and 
an outlet at the other, is established. The larger part of this food tube is 
made up of the archenteron, lined with endoderm, already in use. The 
original archenteron is supplemented at either end by ectodermal invagina- 

Pharynx— -^f^ff 
Heart — 
Stomodaeum--B77- _- 7? r - is 

Spinal Cord 

^k-.-- Cloaca 

Allantoic Bud 

Rarhke's Pouch--^\ 

^Yolk Stalk 
Yolk Sac 
Fig. 117. Sagittal section of a four-day chick embryo. (After Patten.) 

tions that come in contact with it and finally break through, thus forming 
a continuous canal through the body of the embryo. The inpushing of the 
ectoderm at the anterior end, which marks the region of the future mouth, 
is called the stomodaeum, while the corresponding invagination at the pos- 
terior end, that forms the anal exit of the food tube, is called the procto- 
daeum ( Fig. 117). Thus it comes about that food passing through the 
alimentary tract first rubs against walls of ectodermal origin, then follows 
along the major distance in contact with endodermal walls where much 
of it is absorbed, and finally the residue passes out through ectodermal 

The Development of the Individual 


9. The Fate of the Germ Layers 

With gastrulation and differentiation of the primitive germ layers there 
begins to be an increase in the size of the embryo, or growth, accompanied 
by diversification and establishment of the organs and systems that consti- 
tute the mechanisms of the adult animal. It is the task of Organology, or 
Descriptive Embryology, to follow out the changes that take place. Obvi- 
ously within the confines of a brief introductory chapter it is necessary to 
avoid many alluring side alleys that entice one from the main highway, 
and to be content with a brief resume of the structures formed by the several 
regions just described (Fig. 118). 

Sperm Ovu 

\ / 
Fertilized Egg 

by cleavage 






' Epimeres- 





Epidermis and its 

Nervous System 
Sense Receptors 

(Voluntary Muscles 
Derma (in Part) 
Skeleton (in Part) 

_^ f Excretory System 
1 Gonads 

Involuntary Muscles 
of Digestive Tube 
Branchial Muscles 

Skeleton (in Part) 
Derma (in Part) 
Circulatory System 


Digestive Epithelium 
Lining of Tubules of 

Pancreas and Liver 
Lining of Respiratory 




Fig. 118. The fate of the germ layers. 

The ectoderm gives rise to: ( 1 ) the nervous system, including the brain, 
spinal cord, nerves, and receptor endings; (2) the lining epithelium of the 
mouth and nasal cavities (from stomodaeum) and of the last part of the 
rectum (from proctodaeum) ; and (3) the epidermis and all of its deriva- 
tives, including feathers, hairs, nails, claws, scales (except in fishes), integu- 
mentary glands, enamel of the teeth, and lens of the eye. 

The endoderm forms not only the lining of almost the entire digestive 

1 68 Biology of the Vertebrates 

tract but also the epithelial layer in the tubules of such outpocketings of the 
tract as the lungs, liver, and pancreas. 

Of the mesodermal regions the epimeres give rise to : ( 1 ) mesenchymal 
sclerotomes which develop into the vertebral column; (2) mesenchymal 
dermatomes which form most of the dermal part of the integument ; ( 3 ) myo- 
tomes from which nearly all of the voluntary muscles arise; and (4) other 
mesenchymal cells which contribute to the formation of skeletal, dermal, 
and circulatory structures as well as smooth muscles. 

The mesomeres are the source of the excretory system and the gonads 
which harbor the germ cells. 

The hypomeres, in addition to forming mesenteries and linings of the 
body cavities, also give rise to some of the voluntary muscles of the head 
and neck regions and to part of the mesenchymal aggregate from which 
develop portions of the skeletal, dermal, and circulatory organs, and the 
smooth muscles. 

The question of how these structures arise from their embryonic ante- 
cedents awaits our attention in later chapters. 


The preceding section has concerned itself with a simplified plan of 
development which might be followed by an isolecital egg of a lower verte- 
brate. In many respects this plan also applies to the development of telo- 
lecithal eggs but it is modified, particularly in early stages, by the presence 
of yolk. 

Fig. 119. Cleavage of an Amblystoma egg. (After Eyclesheimer.) 

When the yolk is disposed polar fashion, as in an amphibian's egg, the 
mitoses at the embryogenic animal pole, in the neighborhood of the original 
nucleus, go forward at an accelerated rate, while cell division is retarded at 
the opposite vegetal pole where the inert yolk is particularly in evidence 
(Fig. 119). A blastula is evidently formed but the segmentation cavity 
within the hollow sphere is eccentric, its walls being of very unequal thick- 
ness, because the blastomeres at the animal pole are considerably smaller 
and more active than those at the opposite or vegetal pole (Fig. 120). 

The Development of the Individual 


Segmentation Cavity 

In reptiles and birds the results of segmentation are still further modified 
by the relatively enormous amount of yolk present. The nucleus of the ferti- 
lized egg undergoes the usual mitoses, but the new cell boundaries fail to be 
extended so as to include the great sphere of yolk material. The result is a 
patch or disc of crowded blasto- 

^dS@S&£&z~ Animal Pole 

meres of unequal size at the ani- 
mal pole, the larger cells with 
incomplete boundaries being at 
the periphery ( Fig. 121). Al- 
though earlier even the central 
cells were not separated from the 
yolk below, they have by this time 
split off leaving a space, the seg- 
mentation cavity, between them 
and the yolk (Fig. 122). 

In the amphibian egg, in 
which the segmentation cavity 
has walls of unequal thickness, a 
slight invagination to form a groove appears on one side at the point 
where the upper thin wall passes over into the lower thick wall of the 
vegetal pole. The small cells of the upper lip (on the animal pole side) 
of the groove now proliferate rapidly, and the lip begins to grow down 

m— Yolk 

Vegetal Pole 

Fig. 120. A hemisected blastula of the frog. 
(From Huettner, Fundamentals of Compara- 
tive Embryology of the Vertebrates, copy- 
right 1941, by permission of The Macmillan 
Company, publishers.) 

Fig. 121. The segmenting disc of a hen's egg. (After Coste.) 

Segmentation Cavity, 

peripheral Cells 

Vi ^V^Vv^^XN «!g$r*7.*8 


Zone of Junction 
Fig. 122. Section through the blastula of a chick. (From Huettner, 
Fundamentals of Comparative Embryology of the Vertebrates, copyright 
1941, by permission of The Macmillan Company, publishers.) 


Biology of the Vertebrates 

over the large yolk-filled cells (Fig. 123). The outer layer of the lip is 
ectoderm, the inner layer endoderm, and the cavity between endoderm and 
yolk cells is the archenteron. Aided by some growth of the small cells over 
the large ones all around the equator of the blastula, the dorsal lip eventu- 


Overgrowth of 

Endoderm and 

Dorsal Lip 
of Blastopore 

. Ectoderm 

Archenteron ESfoysfcTOJ* 




■. ,.i„-» Dorsal Lip 

-Yolk Plug 

Ventral Lip 
of Blastopore 

Fig. 123. Gastrulation in the development of the frog. (From Huettner, 
Fundamentals of Comparative Embryology of the Vertebrates, copyright 
1941, by permission of The Macmillan Company, publishers.) 

Zone of Junction 

Archenteron ,Endoderm 

Dorsal Lip 
of Blastopore 

Margin of Overgrowth \«. : 

Germ Wall Segmentation Blastopore' 


Fig. 124. Gastrula of the chick. (From Huettner, Fundamentals of 
Comparative Embryology of the Vertebrates, copyright 1941, by permis- 
sion of The Macmillan Company, publishers.) 

The Development of the Individual 


ally covers all but a small yolk plug, composed of the large cells. This plug 
occupies what is actually the blastopore. Meanwhile the yolk cells them- 
selves have been gradually shifting away from the small archenteron into 

A — 

}'° • - S • p . ° » ■ 

Primitive Streak' 

»- Ectoderm 
a -~f -.- — Endoderm 

•-.-^ Archenteron 





9 ." ° ° - • ■ °° ' o > ° o ° .• • o° fi • ° . ° i '•; v ♦ "7° *-!>? '~~- Archenteron 

Fig. 125. Schematic transverse sections through primitive streak of the 
developing chick, showing formation of endoderm and then mesoderm 
from the streak. (After Patten.) 

the segmentation cavity, with the result that the archenteron has increased 
considerably in size, mainly at the expense of the segmentation cavity. As 
this gastrulation is taking place, the notochord and two mesodermal sheets 
are arising from the dorsal endo- 
dermal cells and the zone of pro- 
liferation in the region of the 
dorsal lip of the blastopore. The 
neural tube has also been devel- 
oping in a manner similar to that 
described above, though differing 
in some details. In most respects 
further development of the vari- 
ous embryonic tissues follows the 
general chordate plan. 

In reptiles and birds, a cre- 
scentic fold of tissue forms on one 
side at the edge of the disc of 
blastomeres, sending cells under- 
neath between the yolk and the 
disc ( Fig. 124). The proliferation 
of cells, becoming most marked 

in the mid-part of the fold, soon becomes evident in the disc cells in a 
narrow line extending forward from the edge toward the center of the disc 
until there is a longitudinal streak of cells, primitive streak, in what is really 
the posterior part of the now elongating disc (Figs. 125 and 126). Cells 

Neural Plate 
ti~ Notochord 
^S^""^ — Primitive Streak 

Fig. 126. Surface view of a chick embryo of 
about 18 hours, showing formation of meso- 
derm from the primitive streak region (ar- 
rows indicate direction of growth) and for- 
ward growth of the notochord from a point 
near the anterior end of the streak. (After 

2 7 2 

Biology of the Vertebrates 

arising in great numbers from the primitive streak are added to those 
derived from the infolding to form an endodermal layer between the yolk 
and those disc cells which remain on the surface as the ectoderm. The space 
between endoderm and yolk is a greatly modified archenteron, the floor of 
which is the non-cellular yolk mass. 
Neural Plate 
Endoderm """■-- 


Neural Fold 
Somatic Mesoderm 


, Coelom 


■ Primitive Gut 

Post-cardinal Vein 

/ Intra-embryonic Coelom 

, Extra-embryonic 


Dorsal Aorta 

Mesonephros J~~ 

■jj Dorsal Mesentery :rf 

Gut- ~*V 

Coelom ^s,. 

Ventral Mesentery 

Yolk Sac 

„,. Notochord ..^ 

Mesenchyme o_ 'W. ;: '.Vr; 

Oorsal Mesentery / jP'O) 

~~"Veniral Ligament 
of Liver 

Fig. 127. A scries of cross section diagrams showing differentiation of 
the mesoderm in the chick. (After Patten.) 

The Development of the Individual 

2 73 

Further cell proliferation from the primitive streak region gives rise to 
three parts: an unpaired cord of cells, the notochord, extending forward 
from the streak; paired sheets of mesoderm, growing laterally from the 
streak and spreading forward alongside the notochord. Meanwhile the 
ectoderm above the notochord, in front of the streak, is forming the neural, 
plate and folds which develop into the central nervous system as in most 
chordates (Fig. 127). 

In this manner the original patch of blastomeres on top of the big yolk 
mass have given rise to the nervous system, notochord, and the three germ 
layers. The cells on the outside have become the ectoderm, those under- 
neath next to the yolk, the endoderm, while the mesodermal cells are pro- 
liferating between them. These pioneer 
cells and their descendants then set out 
to spread over and enwrap the entire 
yolk. As these layers spread out the meso- 
derm differentiates into epimere, meso- 
mere, and hypomere, while mesenchyme 
cells appear and organize various parts 
in the same general manner described for 
most chordates. 

Eventually the embryonic tissues 
grow entirely around and enclose the 
yolk in a yolk sac, as in a bag (Fig. 
128). At the same time the embryo 
proper becomes raised up and separated 
from the yolk mass except for a slender 
yolk stalk, through which the cavity of 

the gut is continuous with that of the sac. Despite this continuity, the transfer 
of nutritive material from the sac to the embryo is apparently entirely 
through vitelline blood vessels which spread over the yolk sac with the ad- 
vancing embryonic layers. 

Once the three primary germ layers become established, as described 
above, the further development of a reptile or bird follows the general 
chordate plan. 

Fig. 128. Three stages in the process 
of enveloping the yolk by embryonic 
blood vessels (vitelline arteries and 
veins). (After von Lenhossek.) 


The mammalian egg does not behave in segmentation like amphioxus, 
which it resembles in its small supply of yolk. The reason for the difference 
in development is probably that mammals have inherited developmental 


Biology of the Vertebrates 

traditions from a series of ancestors which amphioxus never had. In mam- 
malian cleavage the entire egg is equally divided into blastomeres but with- 
out the regularity characteristic of amphioxus, so that, instead of a hollow 
blastula, an irregular solid mass of cells is formed. 

Later the outer blastomeres of the germinal mass make a somewhat dis- 
tinctive layer, enclosing more spherical central cells (Figs. 129 and 130). 

Fluid collects within this mass and 
a hollow sphere results. The outer 
enveloping layer of cells, the tropho- 
blast, comes into intimate contact 
with the inner wall of «the uterus at 
the point where' the developing em- 
bryo is implanted. Within the sphere 
is an eccentrically located inner cell mass, which is destined to give rise 
to all the cells that are to take part directly in the formation of the 

Fig. 129. Stages in the segmentation of 
the egg of a rabbit. (After van Beneden.) 

Outer Layer v 

Cell Mass 

Cell Mass 


Fig. 130. Formation of trophoblast and inner cell mass in the develop- 
ment of a mammal. (After Keith.) 

Some of the cells of the eccentric inner cell mass migrate and spread 
out to form a layer, the endoderm, lining the fluid- filled cavity (Fig. 131). 
This cavity now corresponds to the combined archenteron and cavity of the 
yolk sac, only here the entire space is filled with fluid, while in reptiles and 
birds there is a small liquid-filled archenteron and a large yolk-filled sac. 
The remaining cells of the inner mass, now the ectoderm, next spread out 
into a flat "embryonic plate" which corresponds to the ectodermal disc of 
the reptile or bird. The portion of the endodermal layer which is imme- 
diately beneath the "plate" belongs to the embryo proper, while the rest of 
this inner layer represents the yolk sac. Thus the parts present at this time 
may be homologized with those of the gastrula of a bird or reptile. Further 
development of the mammalian embryo follows, in many ways, the an- 
cestral pattern cut out for it by the reptiles, including the appearance of 
a primitive streak, in the embryonic plate, from which cells grow out to 
produce notochord and mesoderm (Fig. 132). 

The Development of the Individual 



Endoderm -- 


- Trophoblast 




Fig. 131. Establishment of endoderm and ectoderm in mammalian 
development. In a embryonic and extra-embryonic portions of the 
endoderm have arisen from the inner cell mass. In b the ectoderm has 
broken through the trophoblast and begun to spread out in a layer joined 
peripherally to the trophoblast. (After Parker and Haswell.) 

T. — r,j-"~ — Primitive 


Primitive Groove of Primitive Streak 

Fig. 132. The "embryonic plate," or "shield," of a rabbit of about 172 
hours, a, surface view; b, transverse section at level of line b-b in A. 
(From Neal and Rand, Comparative Anatomy, copyright 1936, by per- 
mission of P. Blakiston's Son and Company, publishers. After Assheton.) 


Biology of the Vertebrates 

From these few brief statements concerning the early stages in the de- 
velopment of various vertebrates it is clear that gastrulation and the forma- 
tion of mesoderm and notochord vary considerably among the several verte- 
brate classes. It should be emphasized, however, that once the three primary 
germ layers become established they give rise to the principal organ systems 
with great uniformity, as shown in Figure 118. 


A coelomic cavity usually develops on each side of the body in the 
manner we have described, by the splitting of the mesodermal sheet. Hence 
it is a schizocoele (schizo, split). In amphioxus, however, the anteriormost 
mesoderm arises as a series of outpocketings, or pouches, from the dorso- 

Neural Groove 


^Mesodermal Pouch 

^rr- 1 ^Archenteron 

Segmentation Cavity 
Fig. 133. Formation of mesodermal pouches in amphioxus. (After Wilder.) 

lateral regions of the archenteron (Fig. 133) . After the mesodermal pouches 
separate from the archenteron the cavities of all those on each side of the 
body combine to form a continuous coelom, usually known as an entero- 
coele because of its origin from the archenteron. This method of mesoderm 
formation does not occur in the vertebrates, with the probable exception of 
some amphibia, but is typical of hemichords and echinoderms. This forma- 
tion of enterocoeles in echinoderms and some chordates as well as the pos- 
terior position of the blastopore in echinoderms and all chordates indicates 
that these two phyla may be closely related. 

Mesenteries and other parts of the mesoderm arise in much the same 
manner whatever the method of formation of their germ layer may be. In 

The Development of the Individual 

i 77 

either case the paired coelomic cavities, confined mainly to the hypomeric 
region, unite into a single cavity upon the disappearance of most of the 
ventral mesentery. In birds and mammals the embryonic coelomic cavity 
becomes divided into three different sorts of spaces, namely : the pericardial 
cavity enclosing the heart, the two pleural cavities surrounding the lungs 
and the peritoneal cavity housing chiefly the major part of the digestive 
tract and the urogenital organs (Fig. 134). 

Pericardial Cavity Pericardial Cavity v 


/ \ / v 


Transversum v / \ 



A / ung 
/' V\, Diaphragm 

~ Liver 

Peritoneal Cavity Pleuroperitoneal Cavity 

A B 

Peritoneal Cavity 


Fig. 134. Diagram showing relations of the coelomic cavities in fishes 
(a); amphibians, reptiles and birds (b); and mammals (c). (After 

As the first evolutionary step in lower vertebrates, the coelom becomes 
divided by a double transverse mesodermal wall, the transverse septum, into 
a small anterior pericardial cavity and a large posterior peritoneal or ab- 
dominal cavity. In some fishes (e.g. Squalus) this septum is not quite com- 
plete, so that a communication between the two cavities persists throughout 
life in the form of the so-called pericardio-peritoneal canal lying along the 
ventral side of the esophagus. 

With the appearance of lungs, which grow back into the anterior part 
of the abdominal cavity, the name pleuroperitoneal cavity is more properly 
applied to this region of amphibia and reptiles. Usually the two lungs are 
in more or less individual forward extensions of the main cavity. In most 
of these animals the heart shifts posteriorly to lie ventral to the anterior 
part of the abdominal cavity, with the result that the ventral part of the 
transverse septum is pushed posteriorly while the dorsal part keeps its more 
anterior attachment. 

i 7 S 

Biology of the Vertebrates 

In birds and mammals a new partition develops, extending from the 
ventral part of the transverse septum to the dorsal body wall. Although 
membranous in birds, it is invaded by myotomic muscle tissue in mammals 
to become the muscular diaphragm. In mammals the cavity anterior to this 
new partition is known as the thoracic cavity, while the one posterior to it 
becomes once more the peritoneal cavity, for the lungs are no longer in- 
cluded here. The thoracic cavity contains a pericardial cavity, ventrally in 
the center, and two pleural cavities, one on each side, lateral and dorsal to 
the heart. 

Dorsal Septum 
Spinal Cord 

Epaxial Muscle — 'hM/1~&1^% o(IIxj VV&- Notochord 

Vertebra """tfl^^^^^n^ffl^SS^^^ : ^. Horizontal Septum 

Rib S)v^^^3f^^®^^^f&"" Kidne y 

Dorsal Aorta ^^^^^^^^^^^..(^^ 

Hypaxial Musde '''fll^ S< 

Dorsal Mesentery-'B I, #^M ^PfilJVf B 
Visceral Peritoneum--^]]- J & 

Lumen of Intestine-" "^VvSi ^^^^^//^^ IB$h" Integument 
Parietal Peritoneum— -^-r?\ IL5J M 

Ventral Septum 
Fig. 135. Diagrammatic section through a vertebrate. (After Kingsley.) 

In connection with this discussion it should be borne in mind that no 
organs are actually in the coelom but all, instead, are in the segmentation 
cavity. An organ may push into the coelom but it always carries ahead of it 
a fold of the peritoneal wall (Fig. 135). 


When an egg is fertilized the entering sperm is something added to it 
from without, starting a series of internal changes that finally result in the 
adult body. Other outside agents, like a pin- prick or contact with certain 
chemicals, may also, in certain cases, start up the cleavage of an egg as if it 
had been fertilized by a sperm. If the sequence of internal events is inter- 
rupted, or fails to occur in the nick of time, the whole subsequent proce- 
dure is upset. What is the internal mechanism that regulates this marvelous 
performance, once it is initiated? 

The Development of the Individual 


It has been discovered that it is possible to transplant a bit of one em- 
bryo to an unnatural position in another embryo, and that the transplant 
carries out its original structural tradition even in the unnatural surround- 
ings of its host's body. For example, the embryonic bud of a tadpole 
destined normally to grow into a leg, when transplanted to the back of 
another tadpole, will still carry out its original design and form a leg, even 
in so bizarre a location. 

If one blastomere of a frog's egg, when it is in the two-cell stage of 
cleavage, is killed by stabbing the nucleus with a hot needle, the other 
blastomere will carry on and develop a hemi-embryo which may eventually 
restore the missing half embryo and complete the pattern of the entire 
embryo (Fig. 136). 

Fig. 136. Experimental hemi-embryos from frog's eggs, a, an early stage; 
B, a tadpole with its missing half partly restored; c, cross section of b, 
showing two notochords. (After Walter.) 

The ability to perform such a recovery or to develop an organ from an 
extirpated embryonic bud lasts for only a critical brief period. Once this 
time is past, if the sequence of normal events is interrupted, the internal 
mechanism is unable to carry out the original structural design. The par- 
ticular region of a developing organism that possesses this magical power of 
directing internal operations is called an organization center. 

One of the earliest organization centers is around the dorsal lip of the 
blastopore where the mesoderm is organized. This primary center is suc- 
ceeded by other formative centers, secondary, tertiary, and so on, each of 
which is dependent upon the successful operation of preceding centers. The 
discovery of such formative centers by Spemann, Harrison, and others 
through experiments upon developing embryos, is a promising beginning 
towards solving the problem of why the architectural plan of a particular 
species is carried out successfully in the innumerable individuals which grow 
to maturity. 

180 Biology of the Vertebrates 


In the long series of mitoses that follow the initial fertilized egg, there 
comes a time when the two daughter cells resulting from some particular 
cell division are no longer identical twins in their differentiation. They 
may still have the same kind of chromosomal equipment as the result of a 
preceding mitosis, and may be indistinguishable in appearance, but, as their 
future behavior shows, they have come to a fundamental parting of the 
ways, for one of the pair goes on as one of the ancestors of all the myriad 
cells that differentiate into various tissues and organs to form the growing 
individual (soma), while the other becomes the ancestor of all eggs or 
sperm (germ-line), and so is charged with the necessary business of repro- 
ducing the species. 

.A Soma 

Germ Line 

Fig. 137. Diagram to show how the continuous germ-line gives rise to 
successive somas, or individuals. 

There are pronounced differences in these two streams of differentiating 
cells. The soma becomes the conspicuous thing which is known as the 
animal or plant body, and is biologically the guardian of the inconspicuous 
and less commonly known germ-line. The soma is mortal, for after a time 
it inevitably breaks down and dies either a natural or a violent death. The 
cells of the germ-line, on the other hand, although they may perish with 
the dying soma, are potentially immortal, since they form the only biological 
bridge in vertebrate animals across which the spark of life may be borne 
from one generation to another. 

It is quite possible to go backward in imagination step by step without 
a break in the life-line of living cells, from any particular individual cell of 
an adult organism to the fertilized egg from which it came, and to see how 
the material in that fertilized egg was in turn a part of the unbroken series 
of cells of the germ-line that were housed in preceding generations of somas, 
and so on to the very remotest ancestral source. 

The Development of the Individual 181 

The soma within limits can maintain and repair itself. The germ-line 
can not only do that but it can also give rise to new somas (Fig. 137). This 
is its mission, to reproduce new individual organisms, while it is the business 
of the soma, or the individual body, to nourish, protect, transport, and unite 
germ-lines. Otherwise inevitable death ends all. 


The science that deals with the germ-line is Genetics. The resemblance 
everywhere so apparent between individuals of successive generations of a 
species has its explanation in the fact that both parent and offspring are 
somatic expressions of the same germ-line. That is why "pigs is pigs," and 
chickens hatch out of hen's eggs. The laws of heredity are fundamentally 
concerned, therefore, with the behavior of the germ-line and its expression 
in the soma. 

There are various ways to get at the matter. In the past the approach 
to the problems of heredity has been made usually by comparing points of 
likeness and difference in individuals of succeeding generations of a species. 
This somatic method is facilitated by the experimental breeding of plants and 
animals. During the last forty years great advance has been made in such 
breeding by resort to the fundamental principles known as "Mendelism." 

Another line of approach is the direct study of the germ-line, which has 
given rise to an increasing army of biological specialists, who are concerned 
with the intimate behavior of hereditary units, or genes, located in chromo- 
somes, particularly those of the germ cells. 

To these investigators we are indebted for an expanding body of knowl- 
edge about spermatogenesis and oogenesis in animals and plants, as well 
as for the facts and laws which concern germplasmal origins. 


Biological Discords-Pathology 


One of the chief concerns not only of medical practice and surgery but 
of daily life as well, is the repair of biological machinery that has gone 
wrong. Although the human machine, unlike any man-made device, has 
the marvelous ability to adjust the interaction of its parts and to take care 
of routine repairs without outside aid, there is one obvious difference 
between the biological and the mechanical apparatus. In the case of the 
human machine extra parts to replace those worn or injured, if false teeth 
and wooden legs are excepted, are not procurable. 

Much of the success in medical practice depends upon the restorative 
power inherent in the patient without outside assistance, or even in spite of 
outside interference. 

From time immemorial "medicine men" and quacks, with vendors of 
cure-alls and patent medicines, have thrived upon the ignorance, credulity, 
and fears of their victims, but there is another more reassuring side to the 
picture. From Hippocrates and iEsculapius down there have arisen medi- 
cine men of a different stripe who have unselfishly sought the truth about 
the whence and why of bodily ills, to the great advantage of mankind. 

The scientific study of the causes underlying biological disharmonies, or 
disease, is the field of Aetiology, of comparatively recent origin. To succeed 
in such investigations it is necessary to know what it is that has gone wrong, 
and this is the concern of Pathology which forms the basis of every system 
of medicine worthy of consideration. 

Pathology, or the study of the abnormal, goes hand in hand with 
Physiology, the science that deals with the normal activities of organisms. 
To understand the abnormal it is indispensable to first know the normal. 
Both physiology and pathology in turn depend upon a knowledge of 
Morphology, or the science of form and structure, since normal as well as 
abnormal function is referable to a structural basis. 


Biological Discords 183 


The "normal" is the prevailing type. If house cats with few exceptions 
were of the tailless Manx variety, a cat with a tail would appear abnormal, 
just as the unusual condition of six-fingeredness in man is regarded as 
abnormal simply because most people have only five digits on each hand. 

Deviations from the normal frequently turn out to be a handicap to 
their possessors. The very fact that normality is only another way of saying 
that success in some particular has been gained by a majority of individuals, 
implies that variations from the standard have been less successful. 

Deviations, however, are not always unfortunate. Lefthandedness, for 
example, is exceptional but it is not necessarily a handicap. Deviations from 
the normal that do handicap the possessor may take the form of deformities, 
misplacements, or disturbances, external as well as internal, that work ill 
to the organism. Disease, which is the particular province of pathology, may 
be broadly defined as any departure from the normal standard of structure 
or function of a tissue or organ. 

There are at least three elementary activities of organisms, namely, ( 1 ) 
formative, that result in the growth and establishment of structural parts ; 

(2) metabolic, having to do with the maintenance of the organism; and 

(3) responsive, which concern the interplay between the organism and the 
stimuli affecting it. Under normal conditions there is an optimum relation 
in each of these three lines of activity. An injury or disease may upset this 
optimum balance of health and well-being and cause either a cessation of 
these activities (death), or a quantitative or qualitative modification of one 
or all of them. 


For centuries man's greatest obstacle to advancement has been disease. 
It has turned back armies and caused the downfall of empires. 

In 1792 the battalions of Prussia were halted in their attack upon the 
French revolutionary forces by an epidemic of dysentery. Disease decimated 
Napoleon's horde of 500,000 soldiers, reducing their number to 3000 sur- 
vivors during his march on Moscow. It was disease that reduced the cru- 
saders from 300,000 to 20,000 in three years around 1100 a.d., while Haiti 
was lost to France in 1803 because yellow fever killed all but 3000 of the 
25,000 soldiers sent by Napoleon to subdue the natives. 

The construction of the Panama Canal was prevented until American 
engineers destroyed the breeding grounds of the mosquito that transmits 

184 Biology of the Vertebrates 

yellow fever. Smallpox killed more than 60,000,000 people in Europe 
during the 18th century and maimed many others. In Mexico, where this 
scourge was introduced by the Spanish, 3,000,000 Indians succumbed to it, 
so that the conquest of Mexico by Cortez was due to disease rather than 
arms. Records show that at the time of the First World War more than 
150,000 soldiers and prisoners died of typhus fever during the first six 
months. In 1664, bubonic plague claimed 24,000 out of a population of 
200,000 in Amsterdam alone. 

The spectacular statistics of epidemics are hardly more appalling than 
the daily occurrence of preventable deaths and disabilities, happening on all 
sides of us, to which we have become accustomed. Future generations 
should be educated to appreciate the valiant attack of pathologists who seek 
to lessen these disasters and to alleviate human suffering and to forestall 
the sacrifice of lives. 

1. Internal Disturbances 

Disturbances that work ill to an organism by upsetting the optimum 
balance may be internal or external in their origin, although it is not always 
easy to determine to which of the two categories a particular case belongs. 

Although an outline analysis might be carried to much greater length, 
only four kinds of probable internal disturbances are here mentioned, 
namely, (1) formative disturbances; (2) mechanical interferences; (3) 
responsive maladjustments; and (4) hereditary handicaps. 

(a) Formative Disturbances. — When the complicated activities of 
growth and differentiation, to which attention was called in Chapter VIII, 
are passed in review, one wonders that so few structural mistakes or acci- 
dents actually occur. 

The successful outcome of all embryonic development depends con- 
stantly upon the precise timing and infallible performance of each step, 
because every change and advance is conditioned upon what precedes and 
surrounds it. In the orchestra of developing parts a group of cells or an 
organ that is out of rhythm, like a blundering kettle-drummer, may throw 
all the other performers into confusion and change a symphony into discord. 

The malformations and disharmonies which result from disharmony in 
growth and differentiation are termed terata, and the somber science con- 
cerned with such morphological misfits is called Teratology. 

Terata may involve the entire individual, as in the case of "Siamese 
twins" of various kinds, or affect only parts of individuals or organs, as in 

Biological Discords 185 

such deformities as club foot, cleft palate, or hunchback. Other abnormal- 
ities may be simply groups of cells, like tumors, that have somehow lost step 
with the advancing host of correlating parts, and so fallen into disharmony. 
Tumors of this nature are uncoordinated members of the cellular state, and 
are termed benign or malign, according to the degree and manner in which 
they encroach upon or injure surrounding tissues. Malign tumors, like 
"cancers," constitute one of the most disastrous disharmonies to which 
mankind is subject. Much study and exhaustive research is being directed 
towards the understanding and control of these troublesome formative 

Under the heading of formative disturbances there should also be 
included modifications in growth, which are evidently associated with some- 
thing wrong in the behavior of certain regulatory endocrine glands, as for 
example, dwarfism and giantism. 

(b) Mechanical Interferences. — Obstructional disturbances in the nutri- 
tional mechanism or the excretory apparatus may also work ill to an organ- 
ism. The circulatory system, for example, through which the individual 
needs of the cellular structures are supplied, may suffer from local obstruc- 
tions, blood deficiency, or interference with nerve supply. When for any 
considerable time a part of the body is deprived of blood by hemorrhage, 
or by such local obstructions as may result from congestion, pressure, 
wounds, or blood clots, a nutritive unbalance results. If the siege is not raised 
eventually, local starvation and death of the isolated tissues is the outcome, 
or, following exposure to ubiquitous putrefactive organisms, gangrene may 
set in with serious consequences to the neighboring living tissues. If there is 
protection from such foreign invasion, and the dead tissues are not too 
extensive, they finally become absorbed or are sloughed off, and normal 
conditions are restored. 

Interruptions of the stimulative service from the nerve supply, by 
paralysis, shock, or any other interference, are also the immediate cause of a 
myriad of nutritional woes. 

(c) Responsive Maladjustments. — By responsive maladjustments are 
meant such functional disturbances as follow in the wake of internal dis- 
harmonies of one kind or another that interfere with physical performance. 
The response to overwork, for example, may cause an increase in the 
number of the component cells in an organ and result in hypertrophy, or 
excessive growth. If this response is called forth to meet a normal physio- 
logical emergency, as in the hypertrophy of the mammary glands during 
lactation, or of the uterus in pregnancy, then it is normal and lies outside 
the field of pathology, but if it works ill to its possessor, like hypertrophy of 

186 Biology of the Vertebrates 

the heart valves or the walls of the so-called "athlete's heart," then it 
becomes pathological. 

Atrophy, either degeneration or arrest of growth, is an instance of nutri- 
tive disturbance that causes irregularity in the responsive activities. It 
usually follows a cessation of function, as when the optic nerve atrophies 
after the loss of an eye, or when a paralyzed leg or arm wastes away. 

(d) Hereditary Handicaps. — Deviations from the normal that lead to 
disease may be of two kinds. First, they may be acquired in a great variety 
of ways during the lifetime of the individual ; or second, they may be 
germinal, that is, inherited from ancestral streams of germplasm. Blindness, 
for example, may be acquired by accident any time before or after birth, 
or it may be germinal, as in the case of certain types of "congenital" 
cataract that "run in the family" and are inborn. 

In the miscellaneous collection of germinal heirlooms that constitute 
our heritage there are bound to be some things that we wish we did not 
have. Every person in this imperfect world has at least one such hereditary 
"skeleton in the closet." Frequently the skeleton cannot be suppressed and 
kept concealed in a closet, but must be painfully carried about in plain 
sight, like the burden on the back of Bunyan's immortal pilgrim. 

Diseases as such, particularly bacterial diseases, do not cross over the 
tenuous bridge of germ cells that connects one generation biologically with 
another. Constitutions and tendencies, however, that insure the eventual 
sequence of disease, are a part of the hereditary equipment of everyone. 

The philosophy of the comparative anatomist and the pathologist con- 
sists in recognizing the fact that the most successful life does not depend upon 
anatomical and physiological perfection, but in making the best of imperfec- 

2. External Disturbances 

Many of the causes of disturbance that put the "pathos" into pathology 
have their origin outside of the individual in the form of various environ- 
mental factors, of which those described as (1) thermal; (2) chemical; 
(3) barometric; (4) mechanical; and (5) biological, are representative. 

(a) Thermal Factors. Extreme variations from the normal limits of 
temperature to which an organism has become adapted may result in 
thermal disturbances that work ill. Here belong the disastrous effects of 
scalds, burns, and sunstrokes at one extreme, and frostbite and freezing at 
the other. The harm done in these disturbances may take the form of 
nervous shock, hemorrhage, or necrosis of the part involved, with sub- 
sequent invasion and infection by destructive bacteria. 

Biological Discords i8y 

(b) Chemical Factors. Injurious chemical contacts, as in the case of 
ptomaines and various poisons introduced into the digestive tube and the 
blood, cause a variety of troubles. Painters and lead-workers frequently 
suffer from lead poisoning. The consequences of handling phosphorus or 
other deleterious chemical substances are also particularly unfortunate for 
those who are continually engaged in their use. 

(c) Barometric Factors. Deep-sea divers, mountain climbers, and avia- 
tors, who depart from the barometric environment to which they are 
normally attuned, harvest a crop of pathological protests as a result. Men in 
deep mines, or those engaged in tunnel construction who are forced to work 
for hours under abnormal atmospheric pressure, may acquire "caisson dis- 
ease," which manifests itself in paralysis of the legs, profuse bleeding from 
the nose, ears, and mouth, or even in apoplexy. 

(d) Mechanical Factors. Outside mechanical agencies may bring about 
sudden injuries of varying degrees of seriousness from mere scratches to 
extensive wounds that include the destruction of so much of the body as to 
imperil life itself. The power to repair such damage varies greatly in young 
and old. Comparatively undifferentiated tissues exhibit greater recuperative 
power than those that have attained considerable differentiation. The 
response of the body in repairing wounds, involving as it does the behavior 
of the cellular units concerned, is of particular interest to the pathologist. 

Sometimes mechanical factors instead of resulting in sudden wounds, 
may take the form of irritants that work more slowly and insidiously. Cancer 
of the lip, for instance, is said to be more frequent in the case of pipe- 
smokers, who have subjected themselves for a considerable time to local 
mechanical irritation of a pipestem, than among non-smokers. 

Many occupations that involve inhaling irritating dust particles, like 
handling coal, threshing grain, cutting stone, and polishing metals with 
abrasives, induce manifestation of diseases as a consequence of mechanical 
irritants acting upon the respiratory surfaces. 

(e) Biological Factors. There are three general kinds of parasites that 
may attack other organisms and upset their normal course of living. They 
are (1) pathogenic bacteria ; (2) pathogenic protozoans; and (3) 'a hetero- 
geneous group of larger parasites, including certain worms, insects, fungi, 
and other harassing forms that prey upon their betters. 

Pathogenic bacteria are microscopic plants that cause such diseases as 
tuberculosis, cholera, tetanus, anthrax, and typhoid fever. The harm they do 
is usually the result of toxins, or poisons, which they set free in the tissues 
of their hosts during the course of their own metabolic processes, or when 
they die. They -may, however, by sheer force of numbers resulting from their 

188 Biology of the Vertebrates 

prodigious powers of multiplication, either plug up the capillaries in which 
they swarm so that the circulation of blood is impeded or prevented, as in 
anthrax, or they may induce the formation of a bulky mass that acts as a 
strangling gag upon their victim, as in diphtheria, or "membranous croup," 
as it was formerly called. 

Beginning with Pasteur and Lister within the memory of people now 
living, the science of Bacteriology, which has to do with these minute foes 
of mankind, has so increased in importance and achievement that it has 
become indispensable in all modern medicine and surgery. Without doubt 
the future will see still greater triumphs and conquests in this fertile field of 
human endeavor. 

The science of Protozoology, with its wide application to the control of 
diseases induced by pathogenic one-celled animals, has lagged somewhat 
behind the twin science of bacteriology, partly because the technic involved 
in obtaining pure cultures of organisms for accurate experimentation is 
more difficult. Nevertheless much has been learned already and greater 
discoveries and successes in this field surely await the investigator just 
around the corner. 

Many diseases are connected with protozoan parasites which infest 
the blood of their hosts, particularly in the tropics, such as malaria and 
African sleeping sickness, while the troubles following in the train of amoebic 
dysentery are an example of the consequences caused by protozoan high- 
waymen that infest the digestive tract. 

Parasitology in general, out of which the flourishing young sciences of 
bacteriology and pathogenic protozoology have sprung, is now usually asso- 
ciated with larger parasites, like tapeworms, flukes, hookworms, and other 
worms, that take up their domicile in the bodies of their hosts, or such 
external visitors as ticks, lice and fleas, which Mark Twain said keep a dog 
from "thinking about being a dog." 


A knowledge of the facts of pathology, that is daily contributing so much 
to the alleviation of abnormal conditions, is gained principally through 
clinics, autopsies, physiological and micropathological research, comparative 
pathology, and animal experimentation. 

Clinics include bedside experiences gained by actual observation of the 
abnormal conditions exhibited by the patient. 

Autopsies are post-mortem examinations in order to find out what has 
gone wrong with the machinery of biological clocks that have stopped. 

Biological Discords 189 

Micro pathological research is directed towards an intimate understand- 
ing of the behavior and appearance of cellular units under abnormal condi- 
tions. It includes not only Pathological Histology, but also Bacteriology and 
Pathological Protozoology. 

Comparative Pathology, also with human pathology as an objective, 
gains added facts by the indirect method of approach through other mem- 
bers of the animal kingdom. Man is too complex a mechanism to be under- 
stood at once without some preliminary acquaintance with simpler mecha- 
nisms of animal life. Moreover, pathology in general is much more than the 
science of human ills. It is a field of study fertile enough to promise rewards 
to the student of pure science whose eyes are not necessarily fixed upon 
immediate utility to man. 

Animal experimentation has made possible not only a knowledge of the 
facts and principles of pathology, but has also cleared the way for the 
diagnosis and control of the pathological disharmonies that beset mankind. 
Without recourse to animal experimentation the triumphs of modern medi- 
cine could never have come about. 

It is unfortunate that the word "vivisection" in this connection has 
become such a bogey, for it has caused many people to remain uninformed 
or misinformed about a very important matter. The truth is well stated by 
Dr. W. W. Keen in a pamphlet entitled, What Vivisection Has Done for 
Humanity published in 1910, the concluding paragraphs of which are here 
quoted : 

"The alleged atrocities so vividly described in antivivisection literature 
are fine instances of 'yellow journalism,' and the quotations from medical 
men are often misleading. Thus, Sir Frederick Treves, the eminent English 
surgeon, is quoted as an opponent of vivisection in general. In spite of a 
denial published seven years ago the quotation still does frequent duty. I 
know personally and intimately Horsley, Ferrier, Carrel, Flexner, Crile, 
Cushing, and others, and I do not know men who are kinder or more 
lovable. That they would be guilty of deliberate cruelty I would no more 
believe than that my own brother would have been. 

"Moreover, I have seen their experiments, and can vouch personally 
for the fact that they give to these animals exactly the same care that I do 
to a human being. Were it otherwise their experiments would fail and 
utterly discredit them. Whenever an operation would be painful, an anes- 
thetic is always given. This is dictated not only by humanity, but by two 
other valid considerations: first, long and delicate operations cannot be 
done properly on a struggling, fighting animal any more than they could 
be done on a struggling, fighting human being, and so again their experi- 

190 Biology of the Vertebrates 

ments would be failures; and second, should any one try an experiment 
without giving ether he would soon discover that dogs have teeth and cats 
have claws. 

"Moreover, it will surprise many of my readers to learn that of the total 
number of experiments done in one year in England 97 per cent were 
hypodermic injections and only 3 per cent could be called painful ! 

"If anyone will read the report of the recent British Royal Commission 
on Vivisection he would find, says Lord Cromer, 'that there was not a 
single case of extreme and unnecessary cruelty brought forward by the Anti- 
vivisection Society which did not hopelessly break down under cross 

"In view of what I have written above — and many times as much 
could be added— is it any wonder that I believs it to be a common-sense, a 
scientific, a moral, and a Christian duty to promote experimental research? 
To hinder it, and, still more to stop it would be a crime against the human 
race itself, and also against animals, which have benefited almost as much 
as man from these experiments. 

"What do our antivivisection friends propose as a substitute? Nothing 
except clinical — that is, bedside — and post-mortem observations. These have 
been in use for two thousand years and have not given us results to be com- 
pared for a moment with the results gained by experimental research in the 
last fifty, or even the last twenty-five years. 

"Finally, compare what the friends and foes of research have done 
within my own professional lifetime. The friends of research have given us 
antiseptic surgery and its wonderful results in every region and organ of 
the body; have abolished, or nearly abolished, lockjaw, blood poisoning, 
erysipelas, hydrophobia, yellow fever; have taught us how to make mater- 
nity almost absolutely safe ; how to reduce the mortality of diphtheria and 
cerebrospinal meningitis to one-fourth and one-third of their former death- 
rate, and have saved thousands of the lower animals from their own special 

"What have the foes of research done for humanity? Held meetings, 
called the friends of research many hard names and spread many false and 
misleading statements. Not one disease has been abolished, not one has had 
its mortality lessened, not a single human life has been saved by anything 
they have done. On the contrary, had they had their way, puerperal fever 
and other hideous diseases named above, and many others, would still be 
stalking through the world, slaying young and old, right and left — and the 
antivivisectionists would rightly be charged with this cruel result." 

Biological Discords 101 


In earlier days of human ignorance, disease was regarded as due to the 
presence of evil spirits, and cures were supposed to be effected when these 
malign visitors were properly exorcised by some conjurer or medicine man. 
Although the conjurer in various guises still trades upon superstition and 
ignorance, the modern controller of disease has come to recognize that all 
methods of healing, almost without exception, resolve themselves simply into 
extensions of the natural phenomena of growth and repair that are inherent 
in the patient. For example, it has been found that by injecting dead cul- 
tures of the causal agents into subjects infected by a pathogenic organism, 
there is produced in the body fluids a substance (opsonin) which apparently 
in favorable conditions unites with the living causal pathogenic bacteria 
and so sensitizes them that they are readily taken up and destroyed by the 
phagocytic cells of the blood. The afflicted body, therefore, cures itself when- 
ever a cure is effected, and frequently nearly all that the modern medicine 
man can do is to direct intelligently the efforts of the body in its task of 
restoring normal conditions. 

Three general directions are followed in modern attempts to control 
disease, namely, by curative, preventive, and creative medicine. 

Curative medicine, finding itself in a world of disease and disaster, sets 
out to heal the sick and bind up the wounds of the injured. It has assumed a 
colossal task and, like the Good Samaritan that it is, has gone about the 
business with noble devotion and increasing success. 

Preventive medicine, on the other hand, seeks to forestall trouble. Dis- 
eases like smallpox are prevented by vaccination, while by means of anti- 
toxins the poison of invading germs, like that in diphtheria, is counter- 
balanced and rendered innocuous. Protective immunity against disease is 
thus accomplished by using vaccines, antitoxins, opsonins, endocrine extracts, 
and other resources of the bacteriologist and physiological chemist. 

Creative medicine, which at present is hardly more than a dream for 
the future, takes a long look ahead and attempts to prevent the abnormal 
with all its disastrous chain of consequences, by seeing to it that, so far as 
possible, only the normal are born into the world. This is the hopeful field 
of Eugenics, which seeks to lessen and prevent disease by providing an 
hereditary equipment that is able to maintain itself triumphantly harmo- 
nious in the face of besetting discords. 

All of these lines of possible betterment must advance through the 
frontier of pathology, hence the importance of this field of biology. 





A Jack of All Trades-The Integument 


In making a study of the structures of the body it is fitting to begin with 
the integument because, like the wrappings around a parcel, it is the first 
part to be encountered in the examination of any animal. It is, however, 
much more than a mere wrapping encasing organs within, for it is itself an 
"organ," just as definitely as the liver, brain, or heart are organs. 

The vertebrate integument consists of the skin and its derivatives. Exposed 
on its outer surface to the hazards of a varying external environment, on 
its inner face it abuts intimately upon a closed universe of organs bathed and 
permeated by blood and lymph. While acting as a protective barrier, the 
integument not only completely envelops the entire outside of the body, in- 
cluding even the surface of the eyeballs, but it also passes over continuously 
at the nose, mouth, anus, urinary and genital openings into a related envelop- 
ing tissue, the mucous membrane, which lines the internal passage-ways. 
Thus all the organs of the body, except the integument on the outside and 
the mucous membrane on the inside, are completely shut off from the out- 
side world, as if in a closed sac with no direct opening into it. Paradoxical as 
it sounds, food within the digestive tract is still on the outside of the body 

The integument may be considered a compound organ. Morphologically 
it is compound because it is structurally double, being made up in all verte- 
brates of epidermis and derma, or corium. Embryonically its compound 
character is indicated by its derivation from two separate germ layers, 
namely, the ectoderm, from which the epidermis arises, and the mesoderm, 
from the dermatome of which the derma arises (Fig. 116). 

finally, physiologically the integument is also a very versatile organ, 
since it performs so wide a range of functions that it may be quite appro- 
priately termed a "Jack of all trades." 


ig6 Biology of the Vertebrates 


Among the various uses for which in different animals the structure of 
the integument is adapted, are the following : ( 1 ) protection ; ( 2 ) reserve 
food storage; (3) heat regulation ; (4) sensation; (5) excretion; (6) secre- 
tion ; ( 7 ) respiration ; ( 8 ) locomotion ; ( 9 ) sexual selection ; and (10) 

1. Protection 

The skin is inevitably a protective organ. Four aspects of its protective 
function may be mentioned. First, it shields the animal body against 
mechanical injuries that may result from pressure, friction, or blows of 
various sorts. Like any other wrapper, the primary function of which is to 
protect the enclosed parts, the vertebrate integument is admirably adapted 
for this purpose, since typically it is closely woven in texture, resistant, and 
at the same time so pliable that it tends to "give" under mechanical stress 
rather than to rupture or break away. Although many of the individual 
cells that compose it are soft and delicate, for example in the skin stretched 
over the knuckles, they are crushed only by a hard blow against a solid 

In addition to the enveloping skin itself, most animals are equipped with 
protective integumentary modifications, such as scales, bony plates, feathers, 
hair, or cushions of fat, which aid in minimizing the effects of blows or 
injurious contact of any kind. 

Invertebrates, such as crustaceans and mollusks, are conspicuously forti- 
fied by exoskeletons against an unfriendly world, while turtles, armadillos, 
alligators, and porcupines are noteworthy instances among vertebrates of 
animals that go forth, like armored knights of old, well clad to resist the 
blows and harassments of their adversaries. 

Second, the integument protects the body against foreign substances. 
Whenever skin infection from any outside source occurs, it is usually through 
some break, however slight, in the enveloping integument. Since the skin is 
practically germproof so long as it remains whole, the internal realm of the 
body is protected from invasion through it by foreign immigrants. 

Cleaning the skin of whatever undesirable substances may stick to it is 
usually accelerated in the case of civilized man by the application of soap 
and water. However, in the absence of these aids the human skin auto- 
matically cleans itself by surface renewal. Among the unsoaped relatives of 
man the same result is accomplished in a variety of ways. The production of 
mucus over the skin of certain slippery fishes and amphibians, for example, 

A Jack of All Trades 10- 

makes a constantly renewable jacket of slime, which in sloughing off carries 
foreign accretions with it. In various other ways also the outermost dead 
layers of the skin, with such epidermal structures as hair and feathers, are, 
like soiled clothes, periodically cast off. When a snake "sheds its skin," 
although only the outermost part of the epidermis is involved, it emerges 
bodily clean from the gauzy corneal envelope, which may have become 

Third, the skin protects the body tissues from excessive loss of moisture. 
This is a very important function, since in living tissues water plays a major 
role. Both terrestrial and aquatic organisms are equally dependent upon 
water. The protoplasm in every cell must maintain a certain degree of 
fluidity, otherwise it dies. The enclosed universe of the body tissues contains 
a considerable percentage of fluids. These tissues cannot successfully be 
subjected to unrestrained evaporation and still carry on the life processes. 
Moreover, water is the basis for all internal transport of materials, as well 
as being the great chemical solvent of the substances to be transported. It 
is also the indispensable agency by means of osmosis for transferring food 
substances and liquid wastes in and out of the closed body. These precious 
underground waters of the body are conserved very largely by means of 
the water-proof blanket of the integument. Its impervious character serves 
not only to retain the moisture within but also to keep out an unregulated 
amount of water in the case of submerged animals, such as fishes, whose 
more delicate underlying tissues would become water-soaked without such 

Fourth, the skin or integument acts as an organ of protection in all 
those animals exhibiting protective coloration, whereby some degree of 
invisibility, and consequent escape from enemies, is secured by close resem- 
blance to the surroundings. Similarly, so-called warning colors, like the con- 
spicuous black and white markings of the skunk, which serve as "hands-off" 
signals from its possessor, are integumental modifications, protective in 
function. Thus "it pays to advertise," and it is the integument that pro- 
vides the most available billboards. 

It is obvious too that the poisonous skin glands of toads, the eluding 
slipperiness of certain water animals, and the embarrassing armor and 
spines of several well-known vertebrates, are all integumental protective 
devices against the attack of enemies. 

2. Reserve Storage of Food 

In the deeper subcutaneous layers of the skin reserve food in the form of 
fat is stored to be drawn upon, like a savings bank account, in times of 

ig8 Biology of the Vertebrates 

need. The fat stored temporarily in the liver and in the muscles is for 
immediate daily use, whereas what is laid down in the subcutaneous part 
of the skin may be retained for weeks and months. 

The manner of the irregular distribution of fat in cushions and pads 
forms the basis of those contours in "the outward form and feature" that 
have pleased the eye of the artist from time immemorial, while providing 
an anatomical reason for the familiar phrase, "beauty is only skin 

In man stored subdermal fat may constitute as much as twenty per cent 
of the entire weight of the body. In whales and seals it forms an extensive 
blanket of considerable thickness, called blubber, that not only serves as 
food storage, but also acts as a non-conducting retainer of body heat. 

The characteristic sexual differences in human body form and contour 
are largely dependent upon the distribution and manner of the dermal fat 

3. Regulation of Heat 

Heat is being constantly generated by the oxidation of tissues within the 
animal body. Coming more abundantly from soft parts like muscles than 
from hard parts like skeletal organs, it is distributed and equalized by the 
flowing blood which permeates nearly every part of the body, so that in 
"warm-blooded" animals a practically constant temperature is maintained. 
From such an animal heat is lost in three ways : ( 1 ) with the expired warm 
breath; (2) with the expelled excreta; and (3) from the skin. 

Every breath of warmed air carries away a certain amount of body 
heat, for cold air that is drawn into the lungs is warmed at the expense of 
the body before it is expired. 

The excreta, both urine and feces, are kept at body temperature until 
expulsion, when a loss of heat occurs. Probably nine-tenths of the heat 
loss of a warm-blooded animal like a mammal, however, is through the 
skin. Regulation of bodily heat consequently is very largely an integumental 

The skin effects the regulation of the loss of heat in two ways, one 
physiological and the other physical. Physiological regulation is brought 
about by the expansion, or relaxation, and the contraction of the skin and 
the walls of the capillaries contained therein. When exposed to cold air 
the skin tends to contract, sometimes to the point of forming "goose flesh," 
with the result that the capillaries carrying the warm blood are reduced to 
a smaller size and buried somewhat from the surface. The amount and 
degree of cold to which the circulating blood is exposed is thus diminished, 

A Jack of All Trades ion 

and there is a lessened loss of heat. In warm air the skin relaxes, affording 
the capillaries, now more exposed and with walls more expanded, an oppor- 
tunity to permit greater loss of heat from the blood. 

Physical regulation is accomplished by the evaporation of sweat which is 
constantly being excreted from the mammalian skin, even though it does 
not always appear in visible drops. Heat is universally required for the 
physical process of changing a liquid into a gas. During the evaporation of 
sweat the necessary heat is abstracted from the body through the skin and 
is thus eliminated. 

Loss of heat from the body is further controlled by the fact that parts of 
the integument, like the blubber of a whale, serve as a non-conducting 
blanket to hold in the generated heat, making life endurable even in icy 
waters. "Cold-blooded" animals do not specialize in dermal fat. 

A film of oil produced by the sebaceous glands in the mammalian integu- 
ment serves the same heat-retaining function. When Gertrude Ederle swam 
the English Channel, she was generously greased all over in order to meet 
the unusual thermal conditions of that famous adventure. 

In birds the dissipation of bodily heat is regulated through a covering of 
adjustable feathers by means of which a blanket of warmed air is retained 
next to the skin. The thickness of this blanket of warm air can be adjusted 
with meticulous nicety and almost instantaneously by fluffing the feathers. 
Thus, an English sparrow on a hot summer's day is streamlined with its 
feathers hugged tight down close to the body, but in cold weather it assumes 
adequate underwear by fluffing out its feathers and so surrounding itself 
with a more generous layer of body-insulating air. 

The same result is accomplished more awkwardly in the case of civilized 
man by means of clothing, which in reality is nothing more than extra layers 
of non-conducting artificial integument added to that which nature has 
provided. Skin and clothes in themselves are not warm. They are simply 
devices for retaining heat generated within. Even the finest sealskin cloak 
thrown over the marble shoulders of the famous statue of Venus de Medici 
would not warm up that attractive work of art, whatever may have been 
the probable temperature of the original Venus. 

4. Sensation 

The most universal of all the senses, the great confirmatory sense of 
touch, has its receptors located in the skin. The allied senses of pressure, 
temperature, and pain are also referable for the most part to integumentary 
nerve endings. Even the chemical senses of taste and smell, which occupy the 
neighboring mucous membrances of the nose and mouth cavity of higher 

200 Biology of the Vertebrates 

forms, are still found on the outside of the body in the skin of the lower 
aquatic vertebrates. 

5. Excretion 

Among mammals the sweat glands supplement the kidneys in removing 
waste products from the blood. The mammalian skin has been referred to 
quite appropriately as an "unrolled kidney," since each sweat gland in the 
skin, with its accompanying capillaries, is a complete kidney apparatus in 
miniature. Whenever the activity of the sweat glands is accelerated by exer- 
cise, heat, or diuretic and diaphoretic drugs, such as aspirin, caffein, or 
pilocarpin, there is less work for the kidneys to do. 

The constant shedding of corneal material from the surface of the ver- 
tebrate skin also may be regarded as a kind of excretion from the integ- 

6. Secretion 

The most notable example of the integument functioning as an organ of 
secretion is found in mammals, whose mammary glands, which are a very 
specialized form of the integument, develop' as an indispenable part of the 
reproductive apparatus of these vertebrates. 

There are also present in the mammalian skin, associated with hairs, a 
great number of sebaceous glands which secrete an oily substance that tends 
to spread over the skin, rendering it supple and more or less resistant to 
soaking by water and to loss of heat. In fishes and amphibians the mucous 
glands, already mentioned, are also important organs of secretion. 

Other instances of the integument functioning as an organ of secretion 
could be cited, particularly among invertebrates, as, for example, the "crust" 
of the crustaceans, which is a product of the hypodermis, or invertebrate 

7. Respiration 

The moist skin of the amphibians accomplishes to a remarkable degree 
the exchange of gases which constitutes the process of respiration. Cutaneous 
arteries, for example, supplying the skin of a frog, are larger than the pul- 
monary arteries that go to the lungs. Even in man the skin supplements the 
work of the lungs. 

The gills of water-dwelling animals may be regarded morphologically 
as extensions of the skin, as are also the tracheae, or breathing tubes of 

A Jack of All Trades 201 

8. Locomotion 

The cilia and flagella by means of which microscopic aquatic forms 
move about are derivatives of the outer envelope of these skinless animals, 
while in the diversified group of the arthropods, which include much over 
half of all known kinds of animals, locomotion is accomplished by lever-like 
appendages, the actuating muscles of which are attached to the inside of 
the integumentary exoskeleton. The wings of insects are entirely integu- 

Among vertebrates the fins of fishes and the wings and tail feathers of 
birds that are essential to locomotion, are also integumentary in origin. The 
skin takes a conspicuous part too in the wings of bats, and in the flying 
mechanism of all gliding animals, such as flying squirrels, flying lemurs, and 
the "flying dragon" (Draco) of India, as well as in the wings of the extinct 
pterodactyls, which had a web of skin stretched between the fourth finger 
and the sides of the body. Web-footed animals like ducks and frogs depend 
upon the skin between the toes to enable them to paddle in the water. 

1. Macroscopic 

The human skin as a whole conforms to the underlying parts of the body 
as a continuous organ. A baby, which at first may easily be held in the 
palms of two hands, grows in three dimensions, but the skin keeps pace with 
the change in size, always fitting the enlarging body perfectly without any 
bursting at the seams. The clothes, or adventitious skin, in which the child 
is encased by its parents are frequently too large, because of the hopeful 
expectation that he will grow to fit them eventually. Not so the marvelously 
pliable skin. Its smooth expanse is diversified by a few noteworthy eleva- 
tions and depressions, as when it is stretched over the cartilaginous frame- 
work of the external ear, or descends into the ear passage itself. The innu- 
merable tiny pits, appearing wherever there are emerging hairs or openings 
of the miniature volcano-like sweat glands, are microscopic depressions that 
do not entirely penetrate the skin or in any way interrupt its continuity. 

Wrinkles and creases around the joints aid in accommodating the elastic 
integument to changing contours. In old age the skin frequently exhibits 
wrinkles, because it does not shrink as rapidly as the underlying muscles, 
in the process of diminishing repair attendant upon advancing years. 

The skin is thinnest where it passes over the exposed part of the eyeball. 
It is so thin and translucent here, as well as in the double layer of the eyelids, 

202 Biology of the Vertebrates 

that it is possible to perceive light through three layers of skin at once, a 
fact easily demonstrated by turning the closed eyes toward a brilliant light, 
when the difference between light and darkness is distinguishable through 
the double skin fold that is the eyelid, and the continuous conjunctiva which 
constitutes the front face of the cornea ( Fig. 111). 

The thickest region of the skin is found on the soles of the feet. Corns, 
callouses, and other local thickenings wherever there is continued excessive 
friction or pressure, are evidences of increased thickness by use and are 
particularly pronounced on the soles of the habitually unshod. 

According to Lamarck the thickened skin on the soles of a baby's feet 
before they have been subjected to use is inherited from ancestors who 
acquired it while walking up and down the earth. This is by no means, 
however, the most plausible explanation. Mudpuppies (Necturus) likewise 
have the thickest skin on the soles of the feet. Since these primitive amphi- 
bians have their bodies always supported by the surrounding water, they do 
not use the soles of their feet, nor is it likely that any of their ancestors did 
so. Obviously there must be another reason for the differentiation in skin 
thickness on the soles of the feet. 

According to Rauber the area of a typical human skin is about 1.6 
meters square, or approximately five feet square. This fact is instructive 
when it is remembered that certain functions of the skin, as an organ of 
excretion or respiration, for example, depend upon its expanse. 

The weight of the human skin with the subcutaneous fat removed, as 
determined in autopsies, is stated by Bischoff to be 3175 grams for a thirty- 
two year old female, and 4850 grams for a male thirty-three years old, or 
approximately 7 and 10.7 pounds. 

The color of the skin depends upon two factors, namely, translucency, 
which permits the underlying capillaries to show through as in blushing, 
and the presence of pigments, of which there are several kinds, white, yel- 
low, black, and red. Excepting in albinos, these pigments are all present in 
varying proportions in the different races of mankind. They are unequal in 
distribution, even in a single individual, being heavier on exposed parts of 
the skin, and around the axillae, nipples, and genitalia. The general color 
of the skin of so-called white people varies also with age, from pink baby- 
hood to yellow senescence. 

Characteristic blue-gray birth marks in the sacral region of newly-born 
mongoloid people, which fade out in the course of two or three years, are 
due to brown pigment granules located in the deeper translucent layers of 
the skin. 

Nerve endings, except those of the most undifferentiated character, do 

A Jack of All Trades 


not ordinarily extend into the intercellular spaces of the epidermis. Conse- 
quently stimuli which affect the body must reach the deeper-lying nerve 
endings of the corium through the protective barrier of the epidermis. 

2. Microscopic 

The cells of the epidermis are arranged in stratified layers, like the 
leaves of a book, with the most important and indispensable layer next to 
the corium. From it the other, more superficial, layers are derived, together 
with such accompanying modifications as hair,' feathers, and nails. This 
remarkable life-giving restorative layer of germinative cells is called the 
Malpighian layer (Fig. 138), in honor of Marcello Malpighi (1628-1694) 
who first pointed out its significance, thereby erecting to his name a memo- 
rial far more enduring than an isolated mausoleum or a marble shaft. 

Epidermis ' 


Subcutaneous — Jtr^s 



Q Malpighian 

Fig. 138. Diagram of the skin, showing how the Malpighian layer gives 
rise to the superimposed layers of the epidermis. If drawn in proportion, 
the derma would be several times as thick as the epidermis. 

The cell progeny arising from the germinative Malpighian layer are 
gradually modified while they are being crowded toward the exposed sur- 
face of the skin. Their walls become thicker at the expense of the cytoplasm, 
while the breakdown of their nuclei is accompanied by a sequence of chem- 
ical changes in the cell substance. Finally, each cell flattens until eventually 
only a dead scalelike remnant remains, like the collapsed skin of a grape 
after the pulp has been squeezed out. The squamous husks of the outermost 
dead cells thus formed are constantly breaking free from the underlying 
layers, being shed with no interruption of function of the skin as a whole, 
while at the same time a continuous renewal from the Malpighian layer 

204 Biology of the Vertebrates 

below is maintained. Dandruff is formed of the matted masses of the outer- 
most dead epidermal cells. 

It has been estimated that a person who has attained three score years 
and ten has, quite unawares and painlessly, gotten rid of over forty-five 
pounds of dead and discarded epidermal cells. Due to the eternal youth of 
the Malpighian layer, the skin in this way is cleaned over and over without 
wearing thin the way clothes do that are repeatedly scrubbed. The dead 
outermost layer is the corneum. The region between the outer corneum and 
the living Malpighian cells below is characterized in certain areas of the 
human body by the presence of two transitional layers, called the stratum 
granulosum and the stratum lucidum. The former is best seen in cross sec- 
tions of skin taken from the soles of the feet or the palms of the hands. It is 
several cells in thickness next to the Malpighian layer, and is called "granu- 
losum" because, upon the breakdown of the Malpighian nuclei, kerato- 
hyalin granules (Waldeyer) are formed, which give it an appearance of 
greater density. 

The apparently homogenous stratum lucidum, which lies just outside the 
stratum granulosum and is derived from it, owes its semi-transparency and 
comparative resistance to all ordinary histological stains to the fact that the 
kerato-hyalin of the stratum granulosum becomes changed at this point into 
a different chemical compound, called eleidin (Ranvier). This layer is 
usually wanting except where the skin is particularly thick, but it reaches a 
conspicuous development in the nails, which it principally composes. 

Skin pigment is usually located in the Malpighian layer of the epidermis, 
although in some vertebrates it is distributed among the deeper-lying cells 
of the corium. 

The corium, or derma, is the distinctive part of the vertebrate skin, being 
unrepresented in invertebrate integuments. It is a network of connective 
tissues, consisting of cells and fibers produced by cells, felted together. It 
underlies the superficial epidermis and is many times thicker. When leather 
is made it is the corium that is tanned to produce it, the epidermis being 
discarded. The corium of the human skin as well as that of different animals 
can be made into excellent leather. As a matter of historical fact, during the 
French Revolution, shoes were made from the tanned skin of guillotined 
persons. It is related of one Johann Ziska, a fire-eating German patriot of 
olden days, that he stipulated in his will that his skin be tanned and made 
into a drumhead, the martial resonance of which should incite those who 
heard it to fight as valiantly as if his own voice were urging them on. 

Among the many structures embedded in the corium are: capillaries 
and lymph vessels in abundance, nerve endings, sense organs, migrating 

A Jack of All Trades 


pigment cells, deposits of glycogen and fat, smooth muscle cells, sweat 
glands, sebaceous glands, and hairs, the last three being downgrowths from 
the epidermis. 

The deeper parts of the corium form the subcutaneous layer, character- 
ized by the inclusion of masses of soft fat cells and by the looser weave of 
the felted reticulum, which allows greater freedom of motion to the under- 
lying muscles. Its blood supply may include a large fraction of the total 
amount of blood in the entire body. Some of the fibers of the subcutaneous 
region interlace with the fibers composing the connective tissue sheaths that 
envelop the muscles, thus fastening the skin down, as it were, more firmly. 
This is demonstrated better in the palm than over the back of the hand 
where the skin is looser. 


Corneal Layer 

Malpighian Layer 

Sweat Gland — 

Fig. 139. Diagram showing some of the details of friction skin. The 
ridges on the surface, matched by corresponding epidermal projections 
into the derma, are penetrated by the ducts of the sweat glands which 
lie coiled up in the derma below. Two sensory papillae are shown. 
(After Wilder and Wentworth.) 

In regions of the body such as the finger tips that are much in contact 
with things, the outer part of the corium just under the epidermis is thrown 
up into rows of tiny projections, or papillae, that form ridges (Fig. 139). 
It is customary consequently to speak of a papillary layer of the corium, 
although stratification of the corium is not as pronounced as stratification 
of the epidermis. The roughened papillary layer helps possibly to hold the 
corium and epidermis together at points on the skin where friction or pres- 
sure is frequently applied, for the epidermis dovetails intimately into the 
minute hills and valleys formed by the dermal papillae. 

There are two sorts of papillae in the papillary layer, namely, nutritive 
and sensory, the former containing a capillary knot, the latter occupied by 
a sensory nerve ending. It is possible to demonstrate these two kinds of 

206 Biology of the Vertebrates 

papillae experimentally in the finger tips by patient manipulation with a 
very fine needle. When a nutritive papilla is punctured there is no particu- 
lar pain, although a tiny drop of blood may appear, but when a sensory 
papilla is pricked no blood flows and pain is felt. Both kinds are so close 
together, and any needle point is relatively so large, that it requires nice dis- 
crimination to perform the experiment successfully. 

Wherever papillae are present, three layers, which shade imperceptibly 
into each other, may be distinguished in the corium, namely, papillary, 
reticular, and subcutaneous. 

3. Embryonic 

As already indicated, the human skin as well as the vertebrate skin in 
general has a double embryonic origin. The epidermis, which is the primary 
component, arises from that part of the ectoderm remaining after the med- 
ullary tube, which forms the central nervous system, has migrated in from 
the outside by invagination. It consists at first of a single layer of ectodermal 
cells that soon gives rise to a temporary skin, the epitrichium (Fig. 140), a 

— ^/— v^l Epidermis 
' (Ectoderm) 

i Derma 
>**** I (Mesoderm) 

Fig. 140. Skin from the head of a human embryo of 2/2 months. (From 
Bremer and Weatherford, Lewis and Stohr's Histology, copyright 1944, 
by permission of P. Blakiston's Son and Company, publishers. After 
Bowen. ) 

delicate outer layer of somewhat swollen cells which take certain stains dis- 
tinctively, thus showing a specific chemical character. Corneal cells, derived 
from the Malpighian layer, soon appear under the epitrichium until, at the 
age of about three months in the human embryo, the epidermis of the 
fetus has acquired a thickness of three or four cells deep. About the fifth 
month of fetal life, when embryonic hairs begin to emerge from the skin, 
the gauzy epitrichium is shed from the entire body, excepting the palms and 
soles, into the amnionic fluid, and is never replaced in kind. The name 
epitrichium (epi, upon ; trichium, hair) signifies that this layer temporarily 
rests upon the tips of the budding hairs. A more inclusive term for this 
evanescent embryonic mantle is periderm (peri, around; derm, skin), since 
it is present as a part of the embryonic skin of reptiles and birds also, where 
there are no hairs upon which it can rest. 

The corium is derived from cells of thr .somatic mesoderm and the mes- 

A Jack of All Trades 207 

enchyme. It is secondarily wedded to the overlying epidermis, which it 
eventually exceeds many times in thickness. 


1. Invertebrate Integuments 

The microscopic bodies of protozoans are without a true integument, 
although in Amoeba there is a clearer marginal area, the ectosarc, which is 
different from the more granular inner part, or endosarc of the cell. 

In other invertebrates that expose a cellular covering to the outside 
world, the integument is entirely ectodermal in origin, the mesodermal com- 
ponent being absent. No one has ever heard of leather being made from 
any backboneless animal, for leather is manufactured from the mesodermal 
part of vertebrate skin, and there -is no such, thing as invertebrate corium. 

— Cuticle 

~ Hypodermis 

— Muscles 

Fig. 141. A section through the integument of an earthworm. The 
cuticle is a secretion of the hypodermis. (After Schneider.) 

The simple invertebrate skin is called hypodermis, in distinction to the 
epidermis and corium of the compound vertebrate skin. The hypodermis 
may consist of a single layer of flat epithelial cells, as in sponges and many 
coelenterates ; of columnar epithelium, as in worms generally (Fig. 141); 
or of ciliated epithelium, as in flatworms and various larvae. Sensory and 
gland cells of various kinds may be interspersed between other cells of the 
hypodermis, and thus be in a favorable position to come into relation with 
the environment. 

Frequently the hypodermis secretes a more resistant outer coat of chitin, 
lime, or other substance, that is not in itself cellular but which comes to 
constitute an integumental exoskeleton. This is particularly the case with 
arthropods and mollusks. As the body increases in size within the unyielding 
integumentary armor, it becomes necessary periodically for the hypodermis 
to loosen, and to cast off the lifeless, unaccommodating secreted envelope in 
order to renew it on a larger scale. Reminders of this process of ecdysis, 
or "molting the skin," which is typical of arthropods particularly during the 
growing stages of metamorphosis, still persist even among vertebrates in 
the various ways by which dead corneal cells are sloughed off from the 


Biology of the Vertebrates 

Unlike arthropods, most mollusks do not undergo general ecdysis but 
retain, with unfortunate parsimony, the exoskeletal limy shells secreted by 
the hypodermis, until they become so weighted down by adding layer after 

layer that locomotion is made difficult and 
sensation largely superfluous. Eventually 
sedentary contentment and accompanying 
degeneration take the place of the natural 
progressive evolutionary consequences that 
follow upon a more active and exploratory 




~~~>s>r—-s/~ %? ^Vacuoles 

2. Tunicates 

Among tunicates or ascidians, which 
occupy a borderland position between in- 
vertebrates and vertebrates, the epidermis 
is much like the hypodermis of lower forms, 
because of its power to secrete an external 
tunic of non-cellular material (Fig. 142). 
The peculiar substance secreted is called 
tunicin, which is not encountered elsewhere 
in the animal kingdom, although a chemi- 
cally similar substance, cellulose, is a widespread constituent of plant tissues. 
Not only blood vessels and nerves but wandering irregular mesenchyme cells 
also penetrate into the tunicin matrix thus secreted, adding to the protec- 
tive toughness of the mantle or tunic which 
gives these animals their general name of 


Fig. 142. Section through the 
mantle of a tunicate, Phallusia. 
The wandering mesenchyme cells 
secrete the intercellular tunicin. 
(After O. Hertwig.) 

=r- Cuticle 

<.<Agj— Epidermis 
'- Gland Cell 


'yfi \ Derma 

W%— Muscle 

Fig. 143. A section through the 
integument of amphioxus. (After 


In amphioxus the typical compound in- 
tegument of the vertebrates is reduced to its 
simplest expression. The epidermis consists 
of a single layer of columnar cells, which in 
the larval stage are ciliated as in certain 
worms, and which later produce a thin non-cellular cuticle that is reminiscent 
at least of the exoskeletal structures secreted by the hypodermis of invertebrate 
forebears. Thus amphioxus, in assuming the dignity of a vertebrate, does 
not entirely burn all its invertebrate bridges behind itself. The corium in the 
skin of amphioxus is represented by a thin layer of gelatinous connective 
tissue overlying the musculature (Fig. 143). 

A Jack of All r l'rades 


4. Cyclostomes 

The slippery lampreys and hagfishes specialize in a highly glandular skin 
(Fig. 144). There are no scales present to restrict or modify the abundant 




iT->Granular Gland Cell 
— Beaker Gland Cell 
Malpighian Layer 

Fig. 144. Diagrammatic section through the integument of a lamprey 
eel, Petromyzon. (After Haller.) 

and characteristic glands of various kinds, principally mucous, that are dis- 
tributed among the cells of the thick many-layered epidermis. Epidermal 
cells in the skin of cyclostomes, from the deep-lying germinative Malpighian 
region to the surface, do not exhibit the 
same sort of progressive degeneration toward 
a lifeless corneal condition that is character- 
istic of the mammalian skin. The outermost 
cells even retain their youthful cytoplasmic 
character and are active enough to secrete a 
thin cuticle over their exposed surfaces, a 
lingering trace perhaps of long-vanished in- 
vertebrate days. 

The horny teeth (Fig. 145), upon the 
surface of the pistonlike fleshy tongue and 
the wall of the buccal funnel, are the only 
epidermal cornifications in these animals. 
Periodically shed and renewed in the ortho- 
dox fashion of other corneal structures, they are thus to be regarded as 
corneal modifications of the epidermis. 

The corium, which is thinner than the epidermis in these primitive aber- 

^ Buccal Funnel 
^S^ Mouth 
„"Hr- Papillae 

fi-S-s* Teeth of 
Buccal Funnel 
s Teeth of Tongue 
— Eye 

Fig. 145. Ventral view of head 
of Petromyzon, showing oral 
sucker with horny "teeth" and 
piston-like "tongue." (After 


Biology of the Vertebrates 

rant fishes, is an interwoven network of vertical and horizontal connective 
tissue fibers, practically undifferentiated into strata. 

5. Amphibians 

The amphibian skin has much in common with that of cyclostomes, 
being highly glandular, scaleless, and with a relatively thin corium (Fig. 
146). The epidermis, although consisting of several layers, is nevertheless 
thinner than that of cyclostomes. The glands, however, are of a more com- 
plicated type, being composed of several cells each, instead of a single cell 
as in cyclostomes. Although arising in the Malpighian layer of the epidermis, 
the compound integumental glands of amphibians do not remain in an epi- 
dermal position, as do the skin glands of cyclostomes, but push deeper down 
into the corium. Since amphibians are transitional animals, in and out of 
a water habitat, their plentiful glands help to keep the skin moist and suffi- 
ciently permeable for respiratory service. The vascularization of the amphib- 
ian skin is particularly pronounced during the critical period of metamor- 
phosis, when, in some cases, the unusual vertebrate condition of penetration 
of the epidermis by capillaries takes place. 


! — Mucous Gland 

"Poison" Gland 

•Loose Connective Tissue 
of Derma 

i r ~ Pigment Cell 

_..- Subcutaneous 
Connective Tissue 

— Muscle 

Fig. 146. Section through the integument of a frog. (After Haller.) 

Among the higher amphibians which spend much of their time out of 
water, the corneum is differentiated in the epidermis with the result that 
ecdysis occurs, the dead outer layer sloughing off, sometimes in fragmentary 
rags and tatters. The corneum, however, is especially characteristic of land 
animals, not being as evident in aquatic forms. 

A secreted invertebrate-like cuticle, such as amphioxus and cyclostomes 
have, is transiently present in some larval amphibians, much to the delight 
of the comparative anatomist, although it no longer appears in adult forms. 

Pigment cells of the amphibian skin are located mostly in the corium, 
where they come under the control of the nervous system so that certain 
species, tree frogs for example, are able to adapt themselves with consider- 

A Jack of All Trades 211 

able success to the color of the background on which they find themselves, 
thus escaping detection. It should be noted that the skin glands of "warty" 
toads take on an irritating or even poisonous function, which discourages 
the advances of molesting enemies. 

6. Scaly Forms 

In many vertebrate species scales form a conspicuous modification of the 
integument. The character of the different kinds of scales will be considered 
later. In this connection attention will be directed simply to some of the 
characteristic integumental features of vertebrates with scaly skins. 

Most fishes possess scaly skins. Aside from scales the integument of fishes 
is generally marked by the snugness with which it fits the underlying 
muscles. There is a tailored nicety about the skin of a fish that is not appar- 
ent in the baggy jacket of a frog, the loose integument of a bird, or the com- 
fortable elasticity and wrinkles of the mammalian skin. 

—Sense Organ 
%- Epidermis 

Fig. 147. Long section through the skin of a teleost, Barbus. (After 
Maurer. ) 

The epidermis of fishes is highly glandular. Usually the epidermal glands 
are superficial one-celled structures outside the scales (Fig. 147), which 
serve to anoint the body with mucus. Although prophetic indications of a 
corneum are found in some instances among fishes, in general the epidermis, 
as in cyclostomes, does not differentiate a definite external corneal layer, 
for a dead corneum is an adaptation to life on land and exposure to 
dry air. 

The corium of fishes is a typical meshwork of connective tissue, more 
stratified in its deeper parts, and bearing the embedded scales to which it 
gives rise. Frequently the corium as well as the epidermis displays pigment 
of different kinds that decorates the body with an endless variety of pat- 
terns and colors, particularly in brilliant bizarre tropical fishes. 

In the evolution of amphibians it appears that multicellular glands have 
displaced scales as the most characteristic features of the skin. These two 
structures are to a considerable degree mutually exclusive. A truly glandular 
skin would be hampered by the presence of scales, while a scaly skin is in 
no wise a convenient place for glands. The tiny one-celled mucous glands 


Biology of the Vertebrates 

over the surface of the scales of a fish are not to be compared in this con- 
nection with the dominant many-celled glands that characterize the amphib- 
ian skin. 

Extinct stegocephals of the Carboniferous Age were as scaly as any of 
their contemporary fishlike neighbors. Many of them were large creatures 
resembling salamanders in form though greater in size (Fig. 29). They 

were conspicuously clothed with a cumbrous plate- 
like armor quite in style, for they lived in the days 
of scales when defensive knighthood was in flower 
among the animals of the earth. 

Of modern amphibians only the degenerate 
tropical caecilians (Gymnophiona) have any sug- 
gestive trace of scaliness. The cylindrical bodies of 
these small wormlike animals are encircled by 
bands of tiny scales embedded in the skin, alter- 
nating with areas of a typically glandular char- 
acter (Fig. 148). In the skin of these lowly 
inconspicuous bearers of the amphibian name, is 
written the final episode of the evolutionary story 
of the rout of scales by glands. 

The high water mark in completeness and 
elaboration of a scaly skin is reached by reptiles. 
One has only to examine with care the pattern, 
sculpture, and arrangement of the scales on a 
snake or a lizard, to be impressed with their ex- 
quisite perfection. 
As a group, reptiles are definitely committed to life on land, in spite of 
certain backsliding exceptions. This fact has left its modifying impress on 
the skin, which is no longer thin, moist, and respiratory, but thick and 
cornified against exposure to dry air. The struggle for a place in the sun 
between scales and glands has had quite a different issue in reptiles than in 
amphibians, since the former habitually rub much against the dry ground, 
thus having use for a corneal skin to safeguard them against frictional con- 
tact as well as desiccation in dry air. In consequence ecdysis is necessary for 
the removal of the dead outer layer of epidermis. Integumentary glands, 
which are superfluous in a highly cornified skin, are found only in excep- 
tional cases as relics of the days before the ascendency of scales. 

Some extinct reptiles, for example ichthyosaurs and pterodactyls, appar- 
ently had a scaleless skin, but most of the dinosaurs and their mesozoic 
relatives were burdened with an enormously developed integumentary armor 

Fig. 148. Section through 
the skin of a caecilian am- 
phibian, Ichthyophis. The 
scales, embedded in the 
corium, are in black, be- 
tween two giant gland 
cells. Two smaller epider- 
mal glands projecting into 
the corium, are represented 
near the surface. (After 
P. & F. Sarasin.) 

A Jack of All Trades 


made up of large dermal plates (Fig. 36d), which were usually embossed 
in bas relief, and sometimes bore along the back formidable spines project- 
ing upward two feet or more. 

The corium in modern reptiles plays a secondary role, while the epi- 
dermis reaches perhaps a greater elaboration than in 
any other group of vertebrates. 

Among birds and mammals scaliness is of ex- 
ceptional occurrence. The scaly legs and feet of 
feathered birds (Fig. 149) reveal their reptilian an- 
cestry, while there are a few scale-specialists among 

7. Birds 

Anyone who has ever attempted taxidermy 

i u i t i- i -i * • 4.u i • Fie. 149. Scaly foot of 

knows how loose, thin, and easilv torn is the skin & ' . . . 

J . an osprcy. (After 

of a bird. Those parts not covered by feathers, like Schaff.) 

the shanks and the bare areas around the base of 

the beak, exhibit a thickened corneal layer of epidermis, but everywhere 

else not only the epidermis but also the corium is reduced to a delicate 


The typical looseness of a bird's skin, so unlike the 
tightly fitting integument of the fishes, is an advantage 
in flight, enabling the muscles, unhampered by a binding 
integumentary covering, to contract freely and to change 
their shape easily. The looseness of the skin on the belly 
of penguins serves a special purpose adapted to icy an- 
tarctic conditions. During incubation the single egg is 
lifted off the frozen ground to a secure position on top of 
the webbed feet of the parent bird and a generous apron 
of loose skin from the region of the belly is snugly 
wrapped around the egg to keep it warm. 

Exoskeletal structures of birds, such as feathers, beaks, 
leg-scales and claws, are entirely epidermal, since dermal 
elements like the scales of fishes or the bony plates of 
certain reptiles are absent in this group. 

Fig. 150. Embryo 
of Erinaccus, the 
European hedge- 
hog, 4.5 cm in 
length, showing a 
temporarily scaly 
skin. (After 

8. Mammals 

Among the few cases of real scaliness in mammals 
are the armored armadillos of America and the pangolins or scaly anteaters 
of Africa. The skin of the fetal brown bear and European hedgehog (Fig. 

214 Biology of the Vertebrates 

150) too are scaly all over with hairs interspersed. Rats, opossums, and 
beavers have scaly tails that are conspicuous emblems of ancient allegiances 
which the comparative anatomist who runs may read. 

The essential features of the mammalian integument have already been 
described in the previous section on the human skin. It may be emphasized 
here, however, that among mammals the corium reaches its greatest devel- 
opment, becoming many times thicker than the epidermis. 

The conspicuous modifications of the mammalian epidermis are hairs 
and glands. These structures with others will be considered in the next 

1. Glands 

(a) In General. — Glands are cellular structures that produce either a 
secretion or an excretion. In addition to the integumentary glands consid- 
ered in this chapter, there are other glands that open on the mucous mem- 
branes lining the internal passage-ways, and still others within the body 
which, in consequence of having lost their ducts, depend upon blood vessels 
for the disposal of their products. 

All of the integumentary glands of vertebrates take their origin in the 
Malpighian layer of the epidermis. They may consist of single cells which 
have gone farther than their ectodermal neighbors in glandular specializa- 
tion, or they may be composed of groups of similar cells that join in the 
common enterprise of producing some kind of substance which is, or is not, 
of use to the organism. In the former case it is a secretion, and in the latter, 
an excretion. 

Glands such as the sebaceous hair glands are called holocrine because 
in the production of secretions individual cells are used up, extruded with 
their secretions, and replaced by new cells. Another type, like sweat glands 
for example, is merocrine in character, that is, the glands continue to elabo- 
rate secretions without fatal results to their structural units. 

The simple one-celled glands of lower forms, such as the mucous glands 
in the hypodermis of an earthworm, have at first a surface exposure to the 
outside world. As glandular needs increase with the enlargement of the 
body, and the amount of available outside surface becomes inadequate, they 
push down into the underlying corium, thus adding enormously to the total 
secreting area without taking up any more room at the surface, just as large 
bays and inlets increase greatly the actual extent of the coast line between 
two points as a crow Hies. 

A Jack of All Trades 

21 S 

Many-celled epidermal glands which occur in land forms higher up in 
the scale are either tubular or alveolar (Fig. 151), and may be either sim- 
ple, branched, or compound. The amount of space that a compound gland 
occupies at the surface is relatively small, being represented simply by a tiny 
pore for the escape of the secretion produced. 

Fig. 151. Diagrams of various types of glands, shown as invaginations 
from a layer of indifferent epithelium, a, primitive unicellular glands; 
B, simple tubular gland; c, coiled tubular gland; d, branched tubular 
gland; e, simple alveolar gland; f, compound alveolar gland, with a 
single duct; g, more highly differentiated alveolar glands, with com- 
pound ducts. (Modified from Wilder.) 

( b ) Invertebrate Skin Glands. — Representatives from nearly every phy- 
lum of invertebrates exhibit integumentary glands of various sorts that serve 
a variety of purposes. The Cnidaria among coelenterates receive their name 
from the wide-spread occurrence of glandular stinging cells, or nematocysts, 
in the ectoderm, by means of which small prey is paralyzed, and the attacks 
of enemies probably are warded off. 

Sedentary animals in some instances may gain anchorage by glandular 
activity. Thus, the cement glands of barnacles enable these curious crus- 
tacean cousins of the crabs to stand on their heads, securely fastened within 
their protective shells, in which position they can tranquilly kick food into 
their mouths in safety. 

Many mollusks also, for example mussels, attach themselves to some 
solid foundation by the secretion of tough byssal threads from a byssal gland. 
Even microscopic rotifers, as they inch along, manipulate their tiny bodies 
by the aid of a sticky tail gland, while the lacquered cocoon in which an 
earthworm deposits its underground eggs is secreted by the glandular 

216 Biology of the Vertebrates 

Many insects produce glandular secretions. The defensive odor of "stink 
bugs," the protective millinery of woolly aphids, the poisoning or irritating 
power of myriapods, spiders, and brown-tail moths, as well as the thread and 
web spinning of caterpillars, are all due to the activity of hypodermal glands. 
Anyone who has picked up a fat-bodied blister beetle (Meloe) will remem- 
ber the acrid yellow "elbow grease" that exudes glandular unfriendliness 
from its joints. Bee's wax is another product of invertebrate integumentary 

(c) Vertebrate Glands. — The almost universal epidermal glands of 
fishes are superficial one-celled mucous glands, which are widespread both 
over the surface of scales, and wherever naked skin occurs. They are supple- 
mented by two kinds of less common glandular cells, namely, granular 
gland cells, which are especially abundant throughout the epidermis of 
cyclostomes (Fig. 144), and more deeply lying beaker cells that frequently 
extend from the Malpighian layer all the way to the surface. All three of 
these kinds of glands contribute to render fishes slippery and hard to grasp. 
Doubtless too by lubrication they may facilitate to a certain extent the pas- 
sage of these submarines through the water, also effecting the constant 
removal of foreign substances that may adhere to their bodies. 

The African lungfish, Protopterus, has skin glands that secrete a varnish- 
like cocoon in which the animal aestivates, buried in the mud, thus surviv- 
ing the dry season. 

Pterygopodial glands, associated with the pelvic "claspers" of male dog- 
fish and other selachians, are multicellular mucous glands having to do with 


Fig. 152. A deep-sea teleosf, Chauliodus, with a double row of lumi- 
nescent organs on either side of the body. (After Lendenfeld.) 

Deep-sea fishes, that live in a world of darkness where no ray of sun- 
light can penetrate, are in many instances equipped with glandular integu- 
mentary organs of considerable complexity, which produce light. These 
luminescent organs (Fig. 152) are practically the only many-celled glands 
in the skin of fishes. They are usually accompanied in those species possess- 

A Jack of All Trades 


ing them by enormously large eyes adapted for catching the faintest glimmer 
of luminescence, so that when Diogenes of the Deep Sea fares forth, his 
lantern may not pass by unnoticed. Other deep-sea fishes, without light- 
producing organs, are usually entirely blind or have only very degen- 
erate eyes. 

Certain fishes, as well as amphibians and reptiles, also have integumen- 
tary cells of a glandular nature, called chromatophores, by means of which 
the color of the body may be modified to conform to the color of the 
environment in which the animal temporarily finds itself. It has been dem- 
onstrated by Parker that the operation of these color changes depends not 
so much on direct stimulation through the nervous system as upon certain 
"neurohumors" or hormones produced by ductless glands within the body. 

With the exception of the so-called Leydig's glands found in the larvae 
of some anurans, one-celled epidermal glands, so characteristic of the fish 
skin, do not appear in amphibians, being replaced by many-celled alveolar 
glands which also provide mucus. 

One of the functions of the skin glands of both fishes and amphibians, 
that does not recur as an integumental activity in higher vertebrates, is the 
production of irritating or poisonous substances as a means of defense 
against enemies. In fishes such poison glands are usually at the base of punc- 
turing spines or sharp fin rays, but in 
amphibians they are more generally dis- 
tributed over the body. Toads, for ex- 
ample, are usually left alone on account 
of the noxious secretions from their skin 
glands. The glandular rings that alter- 
nate with the tiny embedded scales of 
the blind caecilians are equipped not 
only with many-celled mucous glands, 
characteristic of amphibians, but also 
with peculiar giant poison glands (Fig. 

Another function of epidermal 
glands is shown by tree frogs (Fig. 153 ) 
and certain salamanders which have 
glandular feet that enable them to stick 
to vertical surfaces, and by some male frogs that are unique in having 
glandular thumbs swollen during the breeding season, making it possible 
for them to saddle on to the slippery backs of the females until the extrusion 
of the sperm and eggs is accomplished (Fig. 154). 

Fig. 153. Tree frog, showing glandu- 
lar sucking discs at the ends of the 
toes. (From Newman, The Phylum 
Chordata, copyright 1939, by permis- 
sion of The Macmillan Company, pub- 


Biology of the Vertebrates 

Epidermal glands are much reduced in reptiles and birds, and when- 
ever they do appear are quite localized. For instance, from neck to tail 
down the long back of an alligator there is a 
crowded row of degenerate glands between the first 
and second row of scales on either side of the mid- 
line, the use of which has not been determined. 
On the underside of the lower jaw also there is a 
pair of evertible glandular structures that during 
the mating season give forth a strong musky odor 
which probably has something to do with the sexual 
psychology of these animals. 

Similar odoriferous glands occur in other rep- 
tiles. They are a most notable possession, for ex- 
ample, of the "stink-pot" turtle, whose scientific 
name, Aromochelys odor at a, is almost as descrip- 
tive as its common name. Odor glands are located 
particularly about the cloacal opening of copperheads and certain other 

The so-called femoral "glands" of male lizards, extending in a row along 
the inside of each hind leg from knee to cloaca like a row of tiny portholes 
(Fig. 155), produce a dry gummy secretion which hardens into short spines, 
or "teeth" (Fig. 156), that are useful as a holdfast gripping device during 

Fig. 154. Right fore foot 
of a male frog, Rana es- 
culenta, showing the epi- 
dermal swelling on the 
radial side, which ap- 
pears temporarily during 
the breeding season, and 
is an aid in grasping the 
female securely during 
amplexation. (After Ley- 

Femoral Glands 

Cloacal Opening 

Fig. 155. Femoral "glands" of male La- 
certa, probably useful in grasping the fe- 
male during copulation. (After Maurer.) 

Fig. 156. Section through a sin- 
gle femoral pore (Fig. 155) of a 
lizand, Lacerta, showing projecting 
plug of dry cells that may help to 
prevent slipping during copulation. 
(After Butschli.) 

The uropygial, or preening glands, are best developed in water birds, 
and are reported as being odoriferous during sexual activity, which suggests 
that their ancestral function was sexual allurement, although their chief use 
now has come to be that of supplying pomatum for use in preening. They 
are paired structures usually with a single outlet from which the bird 

A Jack of All Trades 219 

squeezes out the greasy secretion with its beak when dressing the feathers. 
In ducks and pelicans there are several ducts, instead of a single opening, 
that allow the oily secretion to escape. 

Aside from this curious uropygial gland at the base of the tail, the only 
other integumental glands found in birds are oil-glands in the external ear 
passages of certain gallinaceous birds, like the European capercaillie ( Tetrao 
urogallus), and the American turkey. 

Integumental glands reach their greatest variety and differentiation in 
the mammalian skin. Never unicellular, they are either tubular or alveolar 
in character. 

(d) Sweat Glands. — Sweat glands are the most common and generally 

distributed of mammalian tubular glands. Dr. Oliver Wendell Holmes, in 

his delightful lectures to Harvard medical students, likened sweat glands 

to "fairies' intestines." Each one is an elongated tube, the walls of which are 

composed of cells (Fig. 157) . The deeper glandular portion is usually coiled 

up to occupy a minimum of space, while the outermost 

part, that serves as a duct and opens at the surface with a 

funnel-shaped pore, often spirals like a corkscrew as if it 

found difficulty in penetrating the compacted outer corneal 

layer of the skin. Although originating in the epidermis 

like all other integumentary glands of vertebrates, sweat 

glands by a process of growth push deep down into the 

corium where their terminal coiled parts come into inti- Fig- 157. A sweat 

mate contact with the capillaries, making possible the S land - A > a net ~ 

, , , , work of capil- 

extraction 01 sweat from the blood. laries inside of 

In a healthy man the fluid sweat, visible and invisible, which lies the 

amounts to a daily loss of from one to five pints, and in c0 ^ e< ^ end °* a 

, , , sweat eland, b. 

extreme instances to as much as two per cent 01 the en- ,., 5. , , 

tire body weight. It has been estimated that there are 

two and one half million sweat tubules in an average human skin each with 

a separate pore just at the limit of visibility to the naked eye. Stated more 

graphically, there are about four hundred printed words on this page, and 

in the entire book approximately one eighth as many words as sweat glands 

in the author's skin, all of which it might be added have been exercised in 

laboriously arranging the letters as they stand. 

Sweat glands in the human skin are not equally distributed, being more 
numerous on the palms and soles than elsewhere, and attaining a notably 
greater size under the arm pits. 

Racial differences in the abundance of sweat glands have been observed, 
as shown in the following counts per square centimeter on the finger tips: 

■220 Biology of the Vertebrates 

American, 558; Filipino, 653; Negrito, 709; Hindu, 738.* Negroes can 
endure the tropics better than the whites because they are more generously 
supplied with sweat glands. There is so much individual variation in whites 
with respect to the ability and ease to perspire, that army authorities of 
England and the Netherlands take cognizance of this fact and do not detail 
for service in their tropical colonies those men who are unable to sweat 

In mammals that are abundantly clothed with hair, the sweat glands 
become crowded out or localized in restricted areas. Thus, in cats, rats, and 
mice these glands are confined to the soles of the feet; in bats, to the sides 
of the head; in rabbits, to an area around the lips; in deer, to the region at 
the base of the tail; in shrews, to a line down either side of the body; in 
ruminants, to the muzzle and the skin between the toes ; while in the hippo- 
potamus sweat glands occur only on the ears, which are the parts of the body 
of these semi-aquatic monsters most exposed to air. Sweat glands are want- 
ing in Echidna, some insectivores and the water-dwelling sirenia and 

The male of the giant kangaroo is named Macro pus rufus because its 
sweat is reddish in color, and the African antelope, Cephalophus pygmaeus, 
is said to produce albuminous sweat that forms a bluish lather. It will be 
remembered that in the "horse and buggy days," when a harness chafed an 
overheated horse, white lather appeared because of albumin present in the 

The ciliary (cilium, eyelash) glands of Moll, that are the center of 
trouble whenever a sty is formed, are modified sweat glands. 

(e) Sebaceous Glands. — While tubular glands are confined to mam- 
mals, alveolar glands of various kinds occur not only in the mammalian 
skin but also in the skin of other land vertebrates as has already been noted. 

The most universally distributed of the mammalian alveolar glands are 
sebaceous glands which produce an oily secretion {sebum), usually in con- 
nection with hairs (Fig. 158), although they are also found independent 
of hairs at the edge of the lips and about the genitalia, where the skin passes 
over into the mucous membrane. On the tip of the nose, particularly the bul- 
bous noses of the indulgent, middle-aged type, the openings of the free seba- 
ceous glands may be seen as tiny pits, marking the locality of ancestral hairs 
that have been lost in the evolutionary shuffle. 

Sebaceous hair-follicle glands number frequently two or three to each 
hair, opening into the pocket from which the hair shaft projects, rather than 
directly upon the surface. The size of sebaceous glands is not in relative 

* Anat. Rec, 1917, 1. 

A Jack of All Trades 


proportion to the size of the hairs with which they are associated. They fre- 
quently become enlarged in the absence of hairs, which suggests that their 
primary function is not so much concerned with oiling the dry hair, as is 
commonly assumed, as with providing the surface of the skin with a filmy 
coating of oil. 



External — 
Root Sheath 

Internal — f 
Root Sheath l 

Hair Papilla J ( 

/Corneal Layer 


Hair Shaft 

-Arrector Pili 


■ Hair Follicle 

in Tangential 



Fig. 158. Relation of the various parts of a hair and its follicle to the 
parts of the integument. 

The two-toed sloth, Choloepus; the Cape mole of South Africa, Chry- 
sochloris; the scaly anteater, or "pangolin," Manis; and the water-inhabit- 
ing sirenians and cetaceans, already cited for their lack of sweat glands, are 
equally deficient in sebaceous glands, although the first two are abundantly 
hairy animals. 

(/) Other Alveolar Integumentary Glands. — Along the edge of each 
eyelid there is a line of modified sebaceous glands, called tarsal or Meibo- 
mian glands ( Fig. 717), which produce an oily film across the exposed part 
of the eyeball between the edges of the eyelids and the eyeball itself, a film 
that retreats and advances with every wink. This oil seal ordinarily retains 
a film of tears which constantly moistens the surface of the eyeball. In the 
case of weeping the oily dam is broken by the flood pressing from behind, 
and tears trickle down the cheeks (Fig. 159). 

Another kind of integumentary alveolar glands is associated with sexual 
activity in various mammals. These structures should not be confused in any 


Biology of the Vertebrates 

Fig. 159. Diagram of the evo- 
lution of the lacrimal glands. 
a, position in amphibians; b, 
in reptiles and birds; c, in 
man. Sometimes in man the 
lacrimal glands are found lo- 
cated in the "b" position. 
(After Wiedersheim.) 

way with the so-called primary "sex glands" which produce eggs and sperm, 
since they are derivatives of the epidermis having usually only a lubricating 
function in connection with the genital organs. Examples of such glands 

are the preputial and vulval glands in the male 
and female respectively, and scent glands which 
act as an allurement to the opposite sex. These 
latter glands are usually located near the anus, 
as in the musk deer, beaver, civet cat, dog, fox, 
and skunk. The scent glands between the toes 
of goats, whatever their effect on humankind, 
may have a meaning for the goats themselves. 
In the external ear passages of most verte- 
brates are found the ceruminous or wax glands, 
which in form show affinities with the tubular 
type but in function resemble sebaceous glands, 
since they produce a gummy or waxy secre- 
tion more like oil than sweat. They serve to 
arrest dust particles, and to discourage adven- 
turous crawling insects that might otherwise be 
tempted to invade the sacred precincts of the ear, a function not so appar- 
ently needful in the case of man as of a dog sleeping in the sunshine with a 
halo of busy insects buzzing around its head. 

(g) Mammary Organs. — Of paramount importance in the life of mam- 
mals are the milk glands which characterize this order of vertebrates. The 
mammary glands, although resembling the necrobiotic sebaceous glands in 
structure, are intermediate in method of secretion between sweat and seba- 
ceous glands. Their derivation in all probability should be traced not to 
sweat or sebaceous glands, but to some common ancestral type less differen- 
tiated than either. Their activity is periodic instead of continuous and, for 
the most part, finds expression only in the female. 

The mammary apparatus includes not only the mammary glands them- 
selves, but also the elevated nipples, that furnish an outlet for the glands, 
and breasts, or mammae, which are integumentary swellings produced by 
the localized presence of the enlarged mammary glands in the skin (Fig. 

The normal number of nipples varies from two in the horse, bat, whale, 
elephant, and man, to twenty-five in the opossum, Didelphys henseli (Fig. 
161 ). Carnivores usually have six or eight; rodents, two to ten; pigs, eight 
to ten ; and ruminants, four. In those species where several young are born 
in a litter there is a corresponding provision in the number of nipples. 

A Jack of All Trades 





The number of ducts per nipple that drain the glands is also subject to 
considerable variation. In mice, ruminants, and insectivores, there is only 
one ; in the pig, two or three ; and in carnivores, three to six. In man there is 
a cluster of about twenty separate ducts opening into each nipple. 

Milk, which is secreted by these glands, is the natural food of young 
mammals. It is composed of water derived from the blood stream, butter- 
fat, milk-sugar, albumin, and certain salts in varying proportions. Albumin 
in milk favors rapid growth of the young. The milk of a reindeer, which 
lives in a habitat where it is desir- 
able for the young to attain enough Clavicle,, 
maturity to care for themselves as 
soon as possible, has a large albumin 
content. The guinea pig, whose milk 
contains approximately ten per cent 
of albumin, doubles its weight after 
birth in six days, while the human 
infant feeding upon milk with less 
than two per cent of albumin, re- 
quires 180 to 200 days in which to 
double its weight. Other factors, in 
addition to the kind of milk, enter 
into this difference in rapidity of 
growth, but the fact is apparent that 
different kinds of milk are adapted 
in nature to different requirements. 

Mammary glands may develop 
in various places on the mammalian 
skin. Instances are recorded in medi- 
cal literature of the abnormal occur- 
rence in human beings of mammae 
under the arm pits, on the shoulders, 

and even upon the hips. Their normal distribution in different species of 
mammals, however, holds a definite relation to the accessibility of the nip- 
ples to the suckling young. Thus in carnivores and swine, which attend to 
their nurslings while lying flat on the side, the nipples are arranged in two 
rows along the ventral side of the body. Those quadrupeds which habitually 
stand while nursing their young usually have the nipples in a protected sit- 
uation between the legs, either anterior as in elephants, or posterior as in 
cattle and horses, while the nurslings brace themselves on stiltlike legs as 
they drain the maternal udders. Arboreal animals that hold their "babes in 

Diagram of a human mam- 
mary gland. (From Woodruff, Animal 
Biology, copyright 1941, by permission 
of The Macmillan Company, publishers. 
After Gerrish.) 

224 Biology of the Vertebrates 

arms" have conveniently located pectoral nipples. Mankind, with a prob- 
able arboreal ancestry, also has pectoral nipples. The grotesque sea-cows, 
which enfold their single offspring between their anterior flippers and 
"stand" with the head elevated out of the water, likewise have pectoral nip- 
ples. This circumstance has no doubt contributed to the mermaid myths 
among sailors who have chanced to glimpse at a distance the intimate family 
life of these rare strange creatures. 

Fig. 161. Arrange- 
ment of nipples 
in Didelphys hen- 
seli. (After O. 

Fig. 162. The flying lemur, Galeo- 
pithecus, "whose offspring literally 
cling for dear life to the breasts of 
their floppy mothers." (After Hilz- 
heimer. ) 

Unlike the young of the sea-cow, the baby whale (Fig. 74) is a marine 
"trailer," for the maternal nipples from which it secures milk while navi- 
gating the high seas are situated far posterior on either side of the sexual 
orifice entirely out of the mother's sight, in pockets which fit over the snout 
of the baby whale in such a way as to minimize the chance of the milk 
becoming too much diluted with salt water. 

The opposite extreme to the position of nipples in the cetaceans is found 
in the topsy-turvy bats and flying lemurs (Fig. 162) , whose offspring literally 
"cling for dear life" to the breasts of their aerial mothers, the accessible 
nipples of which are axillary in location, or under the arm pits. 

The development of the mammary apparatus is initiated by the forma- 
tion of an epidermal ridge down either side of the belly from axilla to groin, 
called the milk-line stage (Fig. 163). It appears in man near the beginning 

A Jack of All Trades 225 

of the second fetal month when the embryo is still less than half an inch in 
length. The milk-line stage is succeeded by the milk-hill stage (Fig. 163), 
which results when the epidermal ridge of the milk-line becomes absorbed 
except for a beadlike row of remnants, each one of which marks the pos- 
sible location of a future mammary gland. These tiny milk-hills are compact 
masses of cells that later sink down into the underlying tissue, leaving no 

Anterior Limb-bud -«. JqC/'-"\ Vs, ^"cV^ Ff > "^\ 
Milk Line S-IM A Milk Hill **£? ' 1 

J 1A 4. %\ SSQ 

Umbilical Cora "«£ V ^//» vcK \ /*lY 

Posterior Limb-bud-"'"" ^cQiEffi^ ^v£/2tt£/ 

Fig. 163. Early stages in the development of the mammary apparatus. 
a, milk-line stage in a pig embryo of 1.5 cm; b, milk-hill stage in a pig 
embryo of 1.9 cm. (After O. Schultze.) 

visible trace of the developing mammary apparatus. The double row of 
depressed "hills," thus embedded in the corium, becomes the milk-field stage. 
As the leveled hills of the milk-field stage sink deeper and become valleys, 
there is formed where the hills formerly were a double row of pits along the 
lateral walls of the belly, converging posteriorly from the anterior region. 
This represents the milk-pocket stage (Fig. 164c). It is the cells that line 
the sides and bottom of these milk pockets which directly give rise to the 
mammary glands. 

In forms that do not have two complete rows of nipples, some of the 
pockets fail to develop. In man, for example, it is the fourth pair of embry- 
onic milk pockets at the anterior end that become the permanent mammae. 

The final differentiation of the mammary apparatus takes place when 
the milk-pocket stage is succeeded by the nipple stage. According to the 
two ways of their formation true and false nipples are distinguished. Among 
marsupials, rodents, and primates the floor of the milk pocket, into which 
the ducts of the mammary glands open, elevates, carrying the elongated 
ducts of the milk glands with it thus causing them to open at the tips of the 
true nipples. In the case of false nipples, which characterize pigs, carnivores, 
horses, and ruminants, the floor of the milk pocket with its ducts remains 
unelevated, while the margins of the pocket pull up all around to form a 


Biology of the Vertebrates 

hollow nipple. There is thus formed a secondary tube or elongation upward 
of the milk pocket itself, called the milk canal, into which the mammary 
glands pour their secretion, to be pumped to the tip of the false nipple. 

The mammary apparatus develops equally in both sexes up to the time 
of puberty, when it degenerates in the male and becomes potentially func- 
tional in the female. The male may produce milk, as in the primitive mono- 
treme Echidna, and also in exceptional instances among higher mammals, 
even in man. Such abnormal behavior is termed gynecomastism. 

E F • 

Fig. 164. Development of the mammary glands. A, diagrammatic cross 
section through the milk-line (Fig. 163a); b, the epidermal milk-line, 
after breaking up into a chain of isolated milk-hills (Fig. 163b) has sunk 
down into the corium (dotted) and is no longer apparent externally, 
thus forming the milk-field stage; the levelled area, where the milk-hills 
were, becomes depressed, c, forming the milk-pocket stage. The sunken 
epidermal plug penetrates still deeper into the corium, giving rise to the 
mammary glands, d, preliminary indifferent milk-pocket stage, with the 
two longer arrows indicating the direction of epidermal growth that 
results in the formation of a "false nipple," while the short arrow shows 
how the "true nipple" forms; e, false nipple, with ducts of the mammary 
gland opening at the bottom of a milk-canal; f, true nipple, with the 
mammary ducts opening directly at the tip. 

In man as well as other mammals, extra nipples (hyperthelism) (Fig. 
165) not infrequently occur, as also do extra breasts (hypermastism) (Fig. 
1 66 ) . Such persistent embryonic relics, particularly in the case of hyper- 
thelism, occur quite as often in males as in females. Usually these super- 
numerary parts are arranged along the vanished embryonic milk-line. 

The mammary apparatus of monotremes presents many exceptions to 
that of other mammals. Instead of being alveolar in form the mammary 

A Jack of All Trades 


Fig. 165. Hyperthelism. 
Three supernumerary nip- 
ples are shown. (After Mar- 
tin and Mollison.) 

glands are branched-tubular, producing a sort of nutritious sweat instead 
of the usual milk (Fig. 167). No nipples are present, tufts of hair serving in 
their stead. The young monotreme does not have muscular lips and is fur- 
ther handicapped by a horny beak. In conse- 
quence it is quite unable to suckle, so it licks the 
nutritious sweat from the makeshift tufts of 
hair on the mother's breast, with its protrusible 
tongue. The skin on the belly of Echidna forms 
a temporary pouch, or incubatorium, that sur- 
rounds the mammary area while the young are 
being cared for. Into this pouch is deposited the 
single leathery-shelled egg, which soon hatches 
into a very premature helpless embryo, there to 
undergo the preliminary perils of early develop- 
ment which other mammals accomplish in 
greater safety within the protective uterus of the 

Even an incubatorium is lacking in Ornithor- 
hynchus, which broods its egg in a hole in the 
ground that serves as a nest. The ventral mam- 
mary area is depressed, as in the milk-pocket stage 

of development, and from the depression tufts of hair project, which serve 
as nipples. It is probable that gynecomastism occurs in both Echidna and 
Ornithorhynchus with both parents sharing in the feeding of the young. 

Most marsupials regularly possess a permanent pouch for carrying the 
immature young (Fig. 55c), although between incubations it may decrease 
somewhat in size. In the Didelphyidae to which the opossum belongs, the 

marsupium, or pouch, is mostly wanting or 
represented by two insignificant folds of 
skin. True nipples are present within the 
marsupium and typical milk glands supply 
real milk. The nipples project, however, 
only during lactation. At other times, like a 
disappearing gun, they retract within a sur- 
rounding pit in the skin. The young marsu- 
pial retains its hold on the nipple within 
the enveloping edge of the marsupium by 
means of powerful muscles around its mouth ( Fig. 54 ) . At first the young 
animal is so helpless that it can only stay passively attached to the nipple, 
and it is necessary for the mother to pump the milk down its waiting throat 

Fig. 166. A case of hypermast 

ism. (After Hansemann.) 


Biology of the Vertebrates 

by means of the contraction of breast muscles. Later, as it becomes able to 
use its own muscles and nerves, it feeds itself as ordinary mammalian young- 
sters do, by its own efforts. 

Young marsupials, even after they attain considerable size and have 
gained some degree of independence, are glad to 
retreat into the maternal marsupium on the ap- 
proach of danger. 

In placental mammals the marsupial pouch 
disappears, since the fetus is cradled in greater safety 
within the uterus until at birth a stage of develop- 
ment has been reached that makes bodily protection 
on the part of the mother less imperative. There 
are, however, certain dim vanishing reminders of 
a marsupial pocket around the nipple even in the 
human embryo, for at about the beginning of the 
second fetal month, when the future mammary 
apparatus is being set up, there develop around the 
milk-hills transitory epithelial thickenings that pos- 
sibly represent the last remnants of an ancestral 
marsupium (Fig. 168). 

The long period of obligatory milk-feeding 
among the higher mammals not only allows ample 
time for more extended development of the young but is also a necessary 
preliminary to the invaluable process of learning through prolonged asso- 
ciation with the parent. This opportunity is denied to all those unmothered 
kinds of animals that are born equipped with instincts which make it un- 
necessary for them to learn how to live. The dominant mammals are fortu- 

Fig. 167. Ventral view 
of Echidna with the skin 
loosened on one side 
to show the mammary 
glands. (After Semon.) 

/f?K?ht /) r/n ; 

J, Left flrm\ 

Fig. 1 68. Reconstruction of transitory epithelial structures around the 
mammary glands in the skin of a human fetus 56 days old. The dotted 
circles represent the area where the budding arms joined the body. The 
two large black dots are the epidermal "milk-hills" that are to give rise 
to the mammary glands, and which locate the position of the future 
nipples. The twenty-one smaller black dots are epithelial thickenings 
around the nipples, which may be the vanishing remains of ancestral 
marsupial pockets. (After Walter, in Anat. Ariz. XXII, 1902.) 

A Jack of All Trades 229 

nate because they must work out their salvation by learning how, and have 
been endowed with the capacity to do it. 

2. Scales 

(a) Fishes. While every class of vertebrates except cyclostomes has some 
representatives with a scaly skin, the presence of scales may be regarded as 
the most notable modification of the integument of fishes and reptiles. 
Whenever fishes lack scales, as for example many Siluridae and certain 
bottom-feeding forms, it is to be regarded as a secondary modification and 
not the primary ancestral condition. Even in the case of the apparently 
naked eels, tiny vanishing scales appear for a time in the embryonic stages. 

There are at least four general kinds of fish scales of particular interest 
to the comparative anatomist, namely, placoid, ganoid, cycloid, and ctenoid, 
not including bony dermal plates that reached a high degree of elaboration 
in extinct ostracoderms ( Fig. 17), and other armored fishes of early geologic 

The most primitive fish scales are placoid, appearing first in the ancestral 
sharks of the Upper Devonian times, and found today among selachians 
generally. In structure a placoid scale consists of a somewhat flat, basal plate 
originating from the corium and embedded in it, and usually carrying a 
spiny projection of toothlike dentine capped with a harder substance con- 
sidered by some observers to be enamel, formed by epidermal cells, but 
regarded by others as a special type of dentine, produced by dermal cells. 

The obvious transition in both structure and position from placoid scales 
of the skin on the outside of the head, to the rows of teeth within the inner 
margin of the shark's jaws is so 
continuous and unmistakable that _ 7- 

teeth may be regarded as modified 
placoid scales ( Fig. 169). The 
basal dermal plate of the scale cor- 
responds to the root of a tooth, 
while the projection of dentine and 
its enamel cap are quite like similar 
familiar parts of a typical tooth 
(Fig. 240). These relationships are 
not surprising to one who remem- 
bers that the lining of the anterior part of the digestive tract, formed from a 
stomodaeal invagination, is really modified epidermis. 

Placoid scales in dogfishes and sharks are usually small and closely set 
without actually overlapping, although their backward-projecting enameled 


Fig. 169. Diagram of the edge of a shark's 
jaw, to show the relation of placoid scales 
and teeth. Ti, tooth in service at the edge 
of the jaw; t^, T3, T4, reserve teeth. 

230 Biology of the Vertebrates 

spines aid in effecting protection of the spaces of skin between the embedded 
scales. Pavement scales of various shapes may give added protection to the 
most exposed areas. Before the invention of sandpaper and emery cloth, the 
rasping "shagreen" skin of dogfishes and sharks, which is covered with 
sharp, thick-set placoid scales, was frequently used by cabinet makers for 
putting the final smooth finish on wooden surfaces. Quite large placoid 
scales equipped with jagged spines appear in skates and rays and are often 
localized in certain exposed areas, as down the median line of the back, 
leaving scaleless patches of skin unprotected. 

Ganoid scales of considerable diversity are the common characteristic 
feature that stamps the ganoid fishes. The few genera of ganoids living in 
fresh waters today are the last survivors of a large populous and diversified 
order of fishes which once ruled the Devonian seas. Their scales furnish a 
variety of form and structure out of all proportion 
to the number of species involved. In the sturgeon, 
Acipenser, for example, they are large isolated bony 
scutes, not entirely covering the skin but located in 
exposed situations on the body where there is the 
greatest wear and tear, like the rows of brass-headed 
^ ^ nails decorating the edges of great-grandfather's 

Fig. 170. Ganoid scales chest, 
of Lepidosteus. Three of I n the garpike, Lepidostens (Fig. 170), on the 

em aie per ora e y q^^ nanc j as we Q as m the related forms Cala- 
openings 01 the lateral 

line organs. moichthys and Polypterus, the scales are hard pol- 

ished rhombic plates fitting edge to edge, or very 
slightly imbricated one over the other, thus forming a complete armor. The 
skin of the spoonbill sturgeon Polyodon is almost entirely without scales, while 
in Amia modified ganoid scales occur on the head but only cycloids in the 
trunk and tail regions. 

The scales of fossil ganoids are quite large and platelike, just as in living 
survivors of the group where scales that cover the head sometimes become 
enlarged into dermal scutes which take part in the formation of the investing 
skull bones. 

The outer surface of scales of this type is composed of ganoin, a hard 
shiny substance secreted by the corium, not at all homologous with the ecto- 
dermal enamel that caps placoid scales. This material is not present, how- 
ever, on the ganoid scales of Acipenser, Scaphirhynchus, and Amia. The 
main underlying part of ganoid scales is made up of isopedin, a connective 
tissue substance in which bone cells are embedded and which like bone is 
penetrated by Haversian canals containing capillaries. 

A Jack of All Trades 


Fig. 171 

scale of 
growth. (After Hesse. 

Louis Agassiz distinguished two kinds of scales in teleosts, which he 
named, cycloid and ctenoid. Cycloid scales, as the name indicates, are 
rounded in shape (Fig. 171 ) and thicker in the center, thinning out towards 
the margin. They overlap like shingles and if spread over a surface like 
ganoid scales edge to edge would much more than 
cover the body. They are, however, embedded in 
pockets of the corium with only a part of the outer 
smooth margin exposed. Since they project diagon- 
ally at an acute angle with the surface of the skin, and 
overlap their neighbors, the entire body is protected 
by at least a double thickness of scaly armor at every 
point. Present-day bony dipnoans as well as Amia and 
some teleosts have cycloid scales. 

Ctenoid ("comblike") scales, also rounded in form, 
have in addition projecting teeth on the surface of the 
exposed areas (Fig. 172). All intermediate types be- 
tween the cycloid and the ctenoid stages are to be found. 
In some fishes, as for example certain flounders, the scales on the upper 
side are ctenoid, while those on the under side are cycloid. Of these two 
types the cycloid scale is the more primitive, occurring first in the fishes of 
the Jurassic Period, while the ctenoid type did not appear until Cretaceous 

Both cycloid and ctenoid scales are entirely dermal in their origin. The 
scleroblasts, or scale-forming cells in the corium, lay down two layers of 
different substance in the formation of a scale. The outer 
layer is homogeneous and bony, while the under side is 
fibrillar and contains calcareous deposits. Such scales in- 
crease in thickness and area by the activity of the sclero- 
blasts, successive additions being indicated by concentric 
lines of growth like similar rings of growth exhibited in the 
cross section of a tree trunk. Inasmuch as periods of growth 
alternate with comparative inactivity in the case of most 
fishes, according to the seasonal variation of their food supply, it is possible 
to estimate the age of a fish by an examination of the diary-like lines on 
its scales. 

In addition to marginal lines of growth, certain radial grooves are also 
present, caused by the failure of the outer homogeneous layer of the scale 
to be deposited in these places. Such radial grooves add somewhat to the 
flexibility of the individual scale, a very desirable feature since teleost scales, 
although thinner than the edge-to-edge ganoid type, by reason of their 

Fig. 172. Cten- 
oid scale. (After 
O. Hertwig.) 

21,2 Biology of the Vertebrates 

shingling arrangement, form a double envelope over the underlying muscles 
that might hamper free movement. 

Some teleost fishes, for example, the pipefish Syngnathus, and its curious 
relative, the seahorse Hippocampus, do not have overlapping scales but 
instead are encased in a cuirass of bony plates. 

(b) Amphibians. Scales in all modern amphibians are absent, except for 
the tiny bands of scutes embedded in the skin of the tropical legless caecilians 
(Fig. 148). 

The extinct stegocephalans that flourished in Devonian times were char- 
acterized by bony plates in the skin, particularly on the ventral side of the 

Scale Corneal Layer Scale 

y S ^^ 1 ^ -^^ \ = =^~z^^ 02^-- Bony Plate 

Derma \ ,' ; • , ' ''.... .""." — Derma 

v Malpighian Layer 

Fig. 173. Diagrammatic longitudinal sections through reptilian integu- 
ment. Scales occur as localized thickenings of the corneal layer of the 
epidermis. (After Boas.) 

(c) Reptiles. Reptiles are represented today by only a few divergent 
specialized types, namely, lizards, snakes, turtles, crocodiles, alligators, and 
Sphenodon. Superficially quite unlike each other these survivors agree, so 
far as scales are concerned, in the predominant part that the epidermis plays 
in their formation. Small dermal plates, or ossicles, sunken in the corium 
and spaced with much regularity, are present in most reptilian species, 
although many lizards and snakes lack them. When present they are covered 
over by a continuous layer of epidermis in which the dry outer corneal 
part becomes thickened or embossed wherever it covers an ossicle. It is 
thinner and more flexible between the thickened parts or scales of the 
epidermis, thus allowing some freedom of movement (Fig. 173). Even in a 
skin without ossicles the corneum thickens into epidermal scales that hang 
together in a continuous sheet. Thus they form an unbroken armor in which 
the scales cannot be scraped loose, as may the separate independent dermal 
scales of teleost fishes. 

As already mentioned the entire dry corneal part of the skin of snakes 
periodically loosens and is cast off. Such is not the procedure with turtles 
and alligators, for in these animals the corneal scales that overspread the 
dermal plates in the skin are discontinuous and the demands of increasing 
size are met by concentric marginal increments of growth which are added 

A Jack of All Trades 233 

to each scale. The highest development of this underpinning of ossicles is 
reached in the armored turtles, although some of the extinct dinosaurs were 
"fearfully and wonderfully made" with respect to dermal scales. 

Many reptiles, like the "horned toad" Phrynosoma, and Sphenodon 
and the alligators and crocodiles, exhibit spines or embossed patterns on the 
epidermal scales, while the head of a snake, which has to poke about 
between obstacles, is entirely ensheathed by large platelike scales, with the 
sense of touch that is thereby excluded from the surface of the head trans- 
ferred to the delicate protrusible tongue. 

(d) Birds. There is very little to say about scales in the skin of birds, 
since aviation has no use for such heavy clumsy structures. Only on un- 
feathered areas, such as the shanks and around the base of the beak, is the 
epidermis thickened and cornified into a semblance of reptilian scales, and 
nowhere do dermal ossicles occur under corneal thickenings. 

(e) Mammals. Scales, although generally replaced by hairs in mam- 
mals, still persist in a number of instances. Mammalian scales are epidermal 
like those of reptiles. 

They are large horny and imbricated in the scaly anteater, Manis 
(Fig. 65a), although absent on the ventral side of the body. During ecdysis 
they are shed and renewed singly. In armadillos no ecdysis occurs but 
growth is accommodated within the shieldlike armor by marginal accretion 
of the separate scales as in turtles. Areas of bare skin between the rows of 
scales permit these animals to roll up into a ball for the purpose of defence. 
Their fossil forebears, the giant glyptodons of South America, could not 
roll up, but their thick fused scales made such an impenetrable barrier, like 
that of a war tank, that they must have been practically carnivore-proof. 
The fact that telltale hairs project between the scales of armadillos pro- 
claims them to be mammals, although bearing a "wolf's clothing" of scales. 

A great variety of mammals have scaly tails reminiscent of earlier fore- 
bears, the beaver, rat, mouse, opossum, mole, shrew, and certain lemurs. 
Scales even appear on the back of the paws of moles and shrews, and it will 
be remembered that the fetuses of the brown bear and European hedge- 
hog (Fig. 150) testify to some sort of a scaly pattern in the past by develop- 
ing transient useless embryonic scales over the back. Only the armadillos 
have bony plates beneath their scales. 

3. Horns 

The earliest known horns appear as bony projections on the heads of 
certain ceratopsid dinosaurs whose skeletons have been recovered from the 
Cretaceous beds of North America. For instance, Triceratops, as its name 

2 34 

Biology of the Vertebrates 

Fig. 174. Head of 
a male lizard, Cera- 
tophora stoddaerti, 
with a horn on its 
snout. (After Dar- 

indicates, sported a horn over each eye and a third one on its nose (Fig. 

36f and h). Even among modern reptiles there are a few rare bizarre 
lizards with nose horns, for example Chamaeleon oweni 
and Ceratophora stoddaerti (Fig. 174). The "horned 
toad," Phrynosoma, of our southwestern desert region, 
also a lizard and not an amphibian as its name would 
imply, is another reptilian specialist in horns ( Fig. 43 ) . 
Aside from these few cases of horned lizards, horns are 
peculiar to mammals and are coupled with hoofs that 
characterize the ungulates. 

In early Triassic times certain ungulates that lived 
in North America outdistanced dinosaurs in the num- 
ber of their horns. Dinoceras, for example, had six large 
horns on its head. It is probable that these conspicuous 

bony projections were capped over by horny sheaths, although positive 

evidence is not furnished by the fossils of these animals. 

There are four general kinds of ungulate horns known today, namely, 

keratin-fiber horns, antlers, pronghorns, and hollow horns. 

(a) Keratin-Fiber Horns. Keratin-fiber horns are made up of hairlike 
keratin fibers produced from the corneal layer of the epidermis and cemented 
together in a hard compact mass. They are entirely epidermal and have no 
bony core. The Indian rhinoceros carries one of these horns on its "nose," 
as its name indicates ( rhino, nose ; ceros, horn ) , while the African rhinoc- 
eros has two, arranged tandem-fashion instead of side by side in the con- 
ventional way of paired structures (Fig. 66c and d) . It is reported that Bos 
triceros, one of the kinds of native African cattle, also has a median horn of 
this curious type, as its species name indicates. 

( b ) Antlers. Antlers are commonly borne by the various representatives 
of the prolific and diversified deer family ( Cervidae ) , ordinarily only by the 

Fig. 175. The growth of antlers, a, April 2; b, April 20. (After Stone 
and Cram.) 

A Jack of All Trades 


males, but in the case of the reindeer and the caribou by both sexes. They 
consist of bony outgrowths from the skull, which at first are entirely covered 
over by hairy skin. While in that condition a stag is said to be "in velvet" 
(Fig. 175). Later the skin dries and becomes rubbed off, leaving the antlers 
as unadorned bone, at which time it is incorrect to include them among 

4 5 6 7 

Fig. 176. Diagrams showing how antlers are shed. (After Nitsche.) 

integumental structures. At the end of the second year, before the mating 
season, the antlers weaken at the base next to the skull by the breaking 
down and absorption of some of the bony tissue, and are broken off. The 
surrounding skin grows over the wound thus made and a new pair of antlers 
"in velvet" grows out (Fig. 176); this time with an additional prong. 
Thereafter each successive breeding 
season is celebrated by new antlers, 
usually with a regular increase in the 
number of prongs (Fig. 177). This 
physiologically expensive process of 
shedding and renewing the antlers 
does not occur in castrated bucks, 
which is evidence that it is determined 
by the secretion of sex hormones. The 
antlers of the fossil Irish elk that once 
roamed the boglands of Ireland grew 

to an enormous expanse, as if they started and could not stop. Doubt- 
less their excessive size was a factor in the extinction of this ancient 

That "fantastic deer," the giraffe, has a stubby pair of single antlers that 
are permanent, and remain in velvet throughout life (Fig. 68f. ). 

Fig. 177. Big antlers of a moose. (After 
Stone and Cram.) 


Biology of the Vertebrates 

The lateral prongs of the reindeer's antlers (Fig. 69a) are greatly flat- 
tened, serving as snow-shovels to aid these arctic dwellers in getting at the 
snow-covered "reindeer mosses" on which they feed in winter. 

(c) Pronghorns. There are two species of ungulates with pronghorns. 
They are the pronghorn antelope, Antilocapra americana (Figs. 178 and 
69f), and the "saiga antelope" of the Russian steppes, Colus tatarica, both 
of which possess permanent bony horns covered with a thimble-like sheath 
of horny integument that is periodically shed and renewed without the 
loss of the bony core. 


Bony Core 


Fig. 178. Horn formation in the pronghorn antelope, Antilocapra. A, 
appearance shortly after shedding the cornified integumentary "thimble"; 
B, later stage with a new epidermal "thimble" forming with an extra 
prong; c, old thimble tip ready to be cast off, with new horny thimble 
already formed within. (After Nitsche.) 

(d) Hollow horns. Finally, the most familiar kind of horns are the 
hollow horns of domestic and wild cattle, sheep, goats, and antelopes, which 
are usually present in both sexes. 

Unlike pronghorns, hollow horns are not periodically shed. As the horn 
wears away it is renewed from the Malpighian layer of the epidermis, just 
as any other dead corneal structure is restored from within, the hollow 
corneal layer fitting over a core of living bone attached to the frontal region 
of the skull. 

Hollow horns do not branch like antlers and pronghorns, but they 
assume a considerable variety of forms all the way from the majestic graceful 
spread characteristic of the Texas steer of the range, to the "cow with the 
(rumpled horn." Polled, or hornless cattle were known long before cattle 
were domesticated by mankind, as shown by rude drawings of Palaeolithic 
polled cattle, depicted on the walls of prehistoric caverns of the cave dwellers 
in France and Spain. 

A Jack of All Trades 

2 37 

4. Digital Tips 

With the exception of the amphibians, the tips of the digits in those 
vertebrates that have fingers and toes are reinforced by hard integumentary 
structures, either claws, nails, or hoofs. Although amphibians do not have 
true claws, a thickening of the epidermal corneal layer at the ends of the 




Unguis / 


Unguis ^ubunguis 


Unguiss , 



Unguis Subunguis 






Fig. 179. Diagrams of hard digital tips. (After Biitschli. ) 

fingers and toes is a prophecy of claws to come later in the vertebrate series. 
In the African toad Xenopus, and the Japanase salamander Onychodac- 
tylus, these epidermal thickenings assume the definiteness of actual claws on 
some of the digits. 

(a) Claws. The typical claw of a reptile (Fig. 179, a) may be re- 
garded as being made up of two scalelike horny plates, dorsal and ventral, 


Biology of the Vertebrates 

Fig. 180. Young hoac- 
t z i n , Opisthocomus, 
climbing a tree by means 
of claws on wings. (After 

so placed as to converge to a point at the end of the digit. The convex dorsal 
plate, unguis, is rounded in two directions, towards the tip and towards the 
lateral margins. The smaller ventral plate, subunguis, which is pinched in 
between the lateral edges of the unguis, is more 
flattened and of a less dense texture. Both structures 
are produced entirely by the Malpighian layer of 
the epidermis. 

In most cases the claws of birds are confined 
to the toes, although Archaeopteryx, the oldest 
known bird, had three finger-claws on each wing, 
while some existing types of running birds (Rati- 
tae), have claws on the degenerate first and second 
fingers. The young hoactzin, of British Guiana, also 
has claws on its wings which enable it to venture 
from the nest and scramble about in trees on "all 
fours" like a lizard (Fig. 180) 

In general pattern the claws of a bird are rep- 
tilian, although assuming a wide variety of forms adapted to correspondingly 
different functions. The sharp, slender claws of a woodpecker or chimney 
swift are designed for clinging to rough surfaces; the blunt stout claws of 
the domestic fowl for scratching; the 
hook-like talons of hawks and owls for 
grasping prey ( Fig. 1 49 ) ; while the 
long straight claws of the Mexican 
jacana ( Fig. 1 8 1 ) , on the ends of elon- 
gated toes, enable this tropical species, 
in pursuit of its insect prey, to ski over 
unstable lily pads that float on the 
surface of the water. 

Certain species of the grouse 
family (Tetraonidae) undergo a pe- 
riodic ecdysis and renewal of the en- 
tire claw, a reminder of the changes 
undergone by other epidermal struc- 
tures, such as the thimble-like caps on 
the horns of pronghorn antelopes, or 
the corneal layer of a snake's skin. 

Mammalian claws cover the terminal bony phalanx of each digit. They 
consist (Fig. 179, b) of the unguis and subunguis of the reptilian claw, and 
in addition a terminal pud, or cushion, just behind the claw on the ventral 

Fig. 181. Mexican jacana, Parra, with 
long straight claws which increase its 
ability to run over lily pads without sink- 
ing into the water. The male also has 
wing-spurs with which to fight. (After 

A Jack of All Trades 

2 39 

Fig. 182. Retractile 
cat. (After Hesse.) 

side of the digital tip. Since the animal bears its weight on these cushions, 

the corneal layer that clothes them is considerably thickened in consequence. 

Usually the dominant unguis becomes laterally compressed and curved 

down to a point, with the result that the subunguis is much reduced. 
The claws of a cat are sharp and retractile within a protective sheath 

(Fig. 182), thus being kept unworn to be 

extended for use only in emergencies. The 

claws of a dog are less pointed and more 

exposed at all times, particularly so when it 

runs, for then they may come into contact 

with the ground, despite the presence of 


Bats and sloths (Fig. 65c) have claws 

developed into elongated hooks which, al- 
though making locomotion on the ground 

awkward and difficult, are very useful when these animals hang themselves 

upside down from branches in trees, as is their habit. 

There are two striking modifications of mammalian claws, namely, 

hoofs and nails. In the first case the unguis thickens enormously into the 

shoelike hoof, which is so convex that its edges reach all the way around to 
the ventral side and come into contact with the ground. 
In the other case the unguis becomes flattened into the 
conspicuous nail, and the subunguis shrinks to a narrow 
insignificant rudiment under the projecting eaves of the 
nail, while the terminal pad becomes transformed into a 
sensitive ball, occupying the entire ventral aspect of the 
digital tip. 

(b) Hoofs. In a typical hoof (Fig. 179, e) such 
as that of a horse, the subunguis fills in ventrally the 
space between the lateral edges of the unguis, and the 
pad forms into a tough mass of material behind it, 
called the "frog," that serves somewhat the purpose of 
a rubber heel on a shoe. Hoofs, like the thick-soled shoes 
of a traffic policeman, are useful in supporting the heavy 

weight of the body in such animals as stand for long periods of time and 

so need a firm foundation to bear up their weight. The heavy elephant, 
which has a hoof on each toe, is particularly well provided with "rubber 
heels" (Fig. 183). 

The surefootedness of hoofed animals, like the donkey and mountain 
goat, is in part due to the fact that the softer subunguis wears away faster 

Fig. 183. Foot of 
an elephant show- 
ing separate «hoof 
for each toe and a 
"rubber heel." (Af- 
ter Schmeil.) 


Biology of the Vertebrates 

than the harder outer edge of the unguis, thus insuring a constantly well- 
shod foot with a sharp hard edge, in spite of destructive contact with rocky 

(c) Nails. Nails occur in man and other primates where they reinforce 
and protect the sensitive finger pads which play such an incalculable role 
in life. A person who attempts to pick up a pin, for example, with the fingers 
encased in gloves quickly realizes how much these struc- 
tures can aid. 

The human nail, which corresponds to the unguinal 
part of the claw flattened out, is made up from closely 
compacted epidermal cells of the stratum lucidum, that is, 
the lifeless remains of what were once Malpighian cells. 
During its growth the distal part of the nail is continually 
advanced toward the tip of the finger or toe by addi- 
tions from a thickened germinal matrix of Malpighian 
cells at its base, the position of which, particularly on 
the thumb, is marked by a white half moon, or lunula 
(Fig. 184). 
The pinkish color of the nail, aside from the lunula, is due to its trans- 
lucency which allows the blood beneath to show through. The lunula is 
white because the mass of Malpighian cells forming the nail-bed is so thick 
that the blood does not show through. Upon pressure the blood in the 

Fig. 184. Tip of 

finger, n, nail; 
l, lunula; c, cor- 
neum (epony- 
chium) encroach- 
ing upon the nail 
at its base. 

Corneal Layer- -fgg~ 

Terminal Phalanx-- 7 —: 

Nail (Lucidum Layer) 

-Hyponychium (Subunguis) 
— Malpighian Layer 
Corneal Layer 
W^ Dermal Papilla 

Fig. 185. Diagram of a longitudinal section through a finger tip, show- 
ing relation of epidermal layers to the nail. Malpighian cells are most 
active in the region of the lunula, indicated by small arrow. (After 

underlying capillaries can be made to retreat, leaving the entire nail white 
like the lunula so long as the pressure is maintained. Transient white flecks 
sometimes appear in fingernails, due to accidental air-spaces imprisoned 
between the dead scalelike remains of the stratum lucidum. 

The whole nail pushes out through a superficial corneal layer of the 
epidermis, leaving a ragged margin of corneum, the eponychium (Fig. 

A Jack of All Trades 


185), that may be seen encroaching upon the lunula and along the sides of 
the unguis. Under the free outer edge of the nail, where the continuity of 
the corneum is again broken, there is a narrow transitional region which is 
all that remains of the subunguis of the reptilian claw. Dirt collects here. 
The refinement of manicuring consists largely in attending to the ragged 
frame of corneum through which the nail projects. 

Bone (Terminal Phalanx). 


Corneal Layer-- 

Nail (Lucidum Layer) 
Malpighian Layer 

Dermal Papi 

■5- Derma 

Fig. 186. Cross section through the finger tip of a child, showing rela- 
tion of epidermal layers to nail. The eponychium is the part of the 
corneal layer that encroaches upon the nail. The stratum lucidum forms 
the nail. (After Lewis.) 

The rate of growth of human nails is roughly an inch in six months, the 
rate varying with the general condition of health. A recent illness may be 
recorded by transverse depressions on the nails. 

If human nails were never trimmed or broken, they ought theoretically 
to attain a length of over ten feet by the time one 
reaches the allotted age of three score years and 

In the human fetus the twenty nails first appear 
as terminal amphibian-like epidermal thickenings, at 
about the ninth week. By the twelfth week they are 
perfectly formed, but it is considerably later when they 
finally migrate into their dorsal position (Fig. 186). 

The transition from the laterally compressed claws 
of most mammals to the flattened nails of primates 

is strikingly illustrated in certain lemurs which have claws on some of their 
digits and rounded nails on others (Fig. 187). 

5. Miscellaneous Corneal Structures 

Many fishes have horny, epidermal, supporting rays, actinotrichia (Fig. 
548 ) as well as bony or cartilaginous elements between the folds of skin that 
constitute the fins. 

The rattle on the tail of a rattlesnake (Fig. 188) is a unique corneal 

Fig. 187. Left foot 
of a lemur, Perodic- 
ticus, showing four 
nails and one claw. 
(After Huxley.) 


Biology of the Vertebrates 

apparatus. Each time, as the snake molts the outer layer of epidermis, a 
button, or ring of corneum, remains behind to record the fact. These rings 
are dry and loose enough to make a rattling noise when the thrill felt by an 
excited snake reaches the tip of the tail. 



Fig. 188. The rattle of a 

rattlesnake, Crotalus, with 

eleven rattles. The lower fig- 

^J&ftSjl ^3-s) 

ure is a long section, showing 

the vertebrae in black with 

the horny rattles fitting loosely 

one over the other. (After 

Fig. 189. 

Spur of a fight 


ing cock. 

Horny beaks are epidermal structures characteristic of the toothless 
turtles, birds, and monotremes. Among birds particularly they exhibit a 
great variety of form, and serve a wide range of uses. 

Some male birds, such as game cocks for example, also develop horny 
spurs upon the legs with which they settle questions of supremacy upon the 

avian field of honor (Fig. 189). The male 
jacana that strikes at its rival with out- 
spread wings is armed with effective wing 
spurs (Fig. 181). 

The great sheets of "whalebone" (Fig. 
190) with their frazzled edges that fill the 
mouth cavity of toothless whales, in the 
form of an elaborate mechanism for strain- 
ing the myriads of small marine organisms 
upon which these giants feed, are not bone 
at all, but horny epidermal structures. Thick 
at the base, each plate thins out rapidly and breaks up into a long fringe 
of slender, closely set processes like the teeth of giant combs. 

Camels and dromedaries are provided with thick corneal knee-pads to 
protect these heavy animals when they collapse to a kneeling posture before 
lying down upon the sands. Similar though smaller corneal pads of less 
obvious function, called "chestnuts," are found on the inside of a horse's leg. 

Fig. 190. Jaws of a whale, Ba- 
laena, with plates of "whale- 
bone" hanging from upper jaw 
(After Nuhn.) 

A Jack of All Trades 243 

The astonishingly conspicuous crimson and lilac callosities upon the seats 
of such monkeys as the African mandrill are still another manifestation of 
the epidermis, serving these interesting animals, which sit much of the 
time perched precariously on the branches of trees, as peripatetic sofa 

6. Feathers 

Feathers are integumentary structures characteristic of birds. Strong, 
light, elastic, and waterproof, these extraordinary modifications of the 
epidermis are particularly fitted to the needs of animated aeroplanes. During 
the Jurassic Period, when, so far as is known, birds made their debut in 
the form of Archaeopteryx (Figs. 47 and 48), they were already clothed 
with unmistakable feathers. It is probable that these unique epidermal 
structures are homologous with reptilian scales, although evidence for this 
supposition is mostly embryological, since neither comparative anatomy nor 
palaeontology shows any unmistakable transitional structures between scales 
and feathers. The apparent scales on the flipper-like wings of the seagoing 
antarctic penguins are not true scales but instead are miniature flattened 
feathers, as a close examination at once reveals. 

Fig. 191. A typical quill feather from a turkey. (From Sayles, Manual 
for Comparative Anatomy, copyright 1938, by permission of The Mac- 
millan Company, publishers.) 

A typical feather (Fig. 191) is an elaboration of the lifeless corneal 
layer of the epidermis. Its shaft, hollow at the inserted end or quill, bears on 
its sides lateral barbs from which barbules extend. The barbules on the side 
of each barb farther from the quill bear hooks, like microscopic crochet 
hooks, which interlock with neighboring barbules (Fig. 192) forming a 
continuous expanse called the vane, that makes a fan-like surface resistant 
to air. 

The germ of a developing feather appears as a papilla of dermal cells 
pushing the overlying epidermis up ahead of it. The base of the outgrowth, 
including derma and epidermal coat, gradually sinks into the skin (Fig. 
193). There is thus a plug of dermal cells filling the epidermal covering, 

2 44 

Biology of the Vertebrates 

j Dermal Papilla 

Fig. 192. Detail of a feather. (After Mascha.) 

- Epidermis Forming Feather 



Line of Longitudinal Split 

Fig. 193. Feather development. A, b, c, successive early stages, showing 
dermal papilla initiating development; d, later stage of a down feather 
cross sections of which, at levels e-e and f-f, are shown in e and f, 
respectively; g, a down feather, after rupture of periderm has permitted 
spreading of barbs; h, stereogram of part of a quill, or contour, feather. 
After periderm ruptures, germ splits lengthwise along lower side. The 
barbs, thus freed, spread upward and outward to form the vane. 

but as the epidermis, hardening and cornifying, is pushed out from the 
papilla region, the dermal plug withdraws little by little, leaving a hollow 
lifeless quill inserted in the skin (Fig. 194). The embryonic feather is, 
therefore, at first a tube of cornified epidermis, set in a pit of the corium. 
Within the tubular embryonic feather the wall down one side is consider- 
ably thickened and later becomes the shaft from which the slanting barbs 

A Jack of All Trades 


^Corneal Cups 

^- Epidermis 


Involuntary Muscle' 

Fig. 194. Developing feather of a pigeon. The corneal cups are left in 
the quill by the periodic withdrawal of the papilla. (After Krause.) 

of the developed feather extend on either side (Fig. 193). The wall of the 
tube opposite the shaft, the region of the distal tips of future barbs, is very 
thin. It is along this thin region that the rolled-up feather ruptures before it 
spreads out flat to assume its final definite form. 

There are three general kinds of 
feathers, namely, quill, down, and pin 
feathers. Quill feathers may be further 
classified as tail, wing, and contour 
feathers, devoted respectively to the func- 
tions of steering, flying, and thatching. 
Tail and wing quills are larger and less 
flexible than the lighter and more delicate 
contour feathers that serve to fill out the 
unevennesses of the surface of the body, 
giving grace of curving outlines to the 
living bird. The part that contour feathers 
play in streamlining a bird is very ap- 
parent when one observes the scrawny 
body of a dead bird from which the 
feathers have been plucked. 

There are two kinds of down feathers, namely, powder-down (Fig. 
195), and nestling-down. Powder-down feathers are characteristic of certain 
adult birds. They are interspersed and concealed among the contour 
feathers, and are more abundant on the breast and abdomen of herons 

A. powder-down feather. 

2^6 Biology of the Vertebrates 

and birds of prey than elsewhere. The heat-retaining quality of powder- 
down feathers aids in the incubation of the eggs and the protection of the 
semi-naked nestlings. The shedding of powdery fragments, dropped from 
time to time, effects a sort of sanitary cleaning of the plumage of birds 
whose nests are particularly liable to become daubed with excreta and 
the remains of animal food. 

Pin feathers, although superficially resembling hairs, are complemental 
in structure to down feathers. They have practically no barbs, consisting 
instead almost entirely of the shaft which is missing in down feathers that 
are made up of barbs and barbules. Pin feathers are scattered quite generally 
over the body among the contour feathers, although in certain birds, such as 
flycatchers and whip-poor-wills, they become localized about the mouth 
opening, serving as a "barbed wire" entanglement in the capture of insects 
on the wing. In common use, the term pin feather is also applied to the 
young quill feathers present, in addition to true pin feathers, after the so- 
called plucking of the bird. 

In a quill feather interlocking of the barbules occurs only in the exposed 
part of the vane which is not overlapped by other feathers. During the effec- 
tive downward stroke of the wing in flight, the vanes of neighboring feathers 
close up together, presenting to the air a common continuous impervious 
surface, while upon the return upstroke they separate somewhat, thus let- 
ting the air through with less resistance. An entirely different irregular 
arrangement is characteristic of down feathers and pin feathers, which have 
nothing to do with locomotion. 

The plumage of a bird consists of all the feathers taken together. The 
first plumage of young birds is the transient nestling-down, which appears 
as fluffy tufts on the tips of the emerging contour feathers. In this first 
plumage it is the tip of the epidermal tube that frays out like a brush 
(Fig. 193) to form the nestling-down feather, which is fated to wear off 
after temporary service and be replaced by the unfolding quill feathers. 

The nestling-down of the first plumage is thus replaced by the so-called 
juvenal plumage, which is made up of the first coat of true quill feathers. 
This lasts the young bird through its first winter when, in most cases, it is 
replaced* by the nuptial plumage that heralds the first love affair in the 
spring. In the following autumn, after the adventure of raising the first 
family has been accomplished, the nuptial plumage, now faded and shabby, 
is exchanged for a post-nuptial plumage. Every year thereafter that the 
bird lives there is a new post-nuptial plumage after the breeding season and 
in the case of many birds an additional nuptial plumage in the spring. 

This process of ecdysis by means of which one coat of feathers is 

A Jack of All Trades 2aj 

exchanged for another is called molting. When a dead feather loosens from 
its socket in the skin and is lost in molting, the living epidermal Malpighian 
cells at the bottom of the pit, backed up by nutritive resources of the blood 
vessels from the underlying corium, grow out into a new embryonic feather 
tube, which in turn unrolls to take the place of the dead feather that was lost. 
Water birds, gallinaceous birds, and some birds of prey are said to be 
precocial, because at hatching they are quite well clothed with nestling- 
down, while certain other birds, such as kingfishers and woodpeckers, are 
described as altricial, because they are hatched almost naked, only sub- 
sequently acquiring their first coat of feathers. 

Fig. 196 Pterylae, or feather tracts, on the body of a cock. Ventral view 
at the left, dorsal at the right. (After Nitsche.) 

Although the feather coat forms a remarkably complete covering over 
the body, the insertion of individual feathers in the skin is by no means 
equally spaced. Feathers are attached in localized patches called pterylae 
(Fig. 196), between which there are naked areas, apteria, covered by over- 
lapping feathers from neighboring pterylae. No doubt apteria in such areas 
as the "armpits" and the inguinal region facilitate freedom of locomotion 
in much the same way as do loose running trunks on the legs of a sprinter. 
Apteria on the abdomen of a bird may also be useful during incubation, 
because the eggs are thereby snuggled into more direct contact with the 
warm body of the brooding mother. 

The constancy and orderly arrangement of the various pterylae has been 
used by systematists in determining the relationships of different kinds of 
birds for purposes of classification. Ostriches, toucans, and modern penguins 
are apparently exceptional in that they do not in adult life show a pattern 

2^8 Biology of the Vertebrates 

of feathers in pterylae and apteria. That this is a secondary acquisition and 
not a primary condition is indicated by the fact that fossil (Tertiary) pen- 
guins, and embryonic ostriches, show distinct pterylae. 

Local deviations in feather arrangements, usually associated with sec- 
ondary sexual characters, are of frequent occurrence, such as the crests and 
ruffs of various birds, and the spectacular tails of peacocks, fantail pigeons, 
and lyre birds. In a strain of fancy poultry known as "frizzles," the plumage 
has departed from nature's approved style by reason of the twisting of the 
feather shafts, but it is doubtful whether these curious frowzy birds could 
successfully maintain themselves out of domestication. 

The shingle-like lay of the feathers is directed from the head toward the 
tail, thus reducing to a minimum the air resistance offered by the plumage. 
This orientation of the feathers also makes possible the retention under the 
feathers of a layer of warmed air next to the skin during rapid flight, 
which would be blown away if the feathers were arranged in any other 

The remarkably varied colors of feathers are due to one or both of two 
factors, namely, chemical pigments and the physical arrangement of the ele- 
ments making up the feathers. The reds, yellows, and blacks are due to 
pigments. The whites, blues, and iridescent colors are structural colors. The 
greens are usually due to a structural blue combined with a pigment yellow. 

Pigments are deposited mostly in the exposed parts of feathers and only 
during the period of their growth. After feathers have become differentiated 
lifeless structures there is no way to add pigment granules to them, so that 
further change in color of plumage can then only occur in one of three 
ways, by fading of the pigment already in the feathers, by the wearing away 
of the particolored feather tips, or by complete ecdysis of old feathers 
and their replacement by new ones. 

A feather appears white if no pigment is present and the polygonal cells 
of the barbs and barbules break up the light and reflect all wave lengths 

The blue color of feathers is of the type known as Tyndall blue. Many 
years ago Tyndall showed that the sky appears blue because minute sus- 
pended particles of dust, water, and the like scatter the short, blue waves 
of light while permitting the longer, red waves to pass through. Therefore, 
when we see light reflected from these particles or from similar suspended 
particles in any turbid medium (e.g., skimmed milk), we see the blue wave 
lengths. On the other hand, if we look at the turbid medium using only 
light which is transmitted through it, the material appears reddish because 
the short, blue waves have been scattered and the longer, red waves trans- 

A Jack of All Trades 249 

mitted. The blue color of feathers is localized in the layer of cells which is 
just beneath the outer sheath of the barbs. Minute pores in the walls of 
these cells bring about the scattering of the blue wave lengths while trans- 
mitting the longer ones. As we ordinarily see feathers by reflected light, any 
area which brings about this scattering will appear blue. 

In almost every case green feathers exhibit the same structural features 
as blue ones but have, in addition, a yellow pigment in the outer sheath 
of the barbs, the combination of the two colors producing green. A green 
copper pigment has, however, been extracted from the feathers of the West 
African "turacou," Turacus. 

Frequently contour feathers present complicated variegations of colors 
which combine to form patterns, involving the matching of parts of several 
neighboring feathers. Thus, a white wing-bar, or a spot on the breast, is in 
reality a baffling mosaic, made up of unequal fragmentary contributions of 
color from many separate overlapping feathers, which have grown inde- 
pendently into harmonious positions with relation to each other. No wonder 
that Darwin is said to have exclaimed that trying to think out how the 
"eye" on the dorsal feathers that constitute the peacock's tail came about, 
made him actually sick! 

In the process of molting, feathers in the centers of the separate pterylae 
are the first to fall out, and this loss with its subsequent replacement extends 
from these centers to the margins of the different feather islands. 

Sometimes a molt is incomplete not involving each feather, but simply 
the wearing away of a different colored tip. This may be quite effective, 
however, in accomplishing a change in general appearance, as for example, 
in the case of the male bobolink, Dolichonyx, which changes from a distinc- 
tive coat of black and buff in patches of color in the spring to an incon- 
spicuous streaked sparrow-like plumage in the fall. 

7. Hair 

Just as feathers characterize birds, so hairs are integumental hall-marks 
of mammals. Such apparently hairless animals as whales and sea-cows even 
are clothed in part before birth with embryonic hair, while the bare thick- 
skinned rhinoceros and the hippopotamus have sparse bristly hairs about the 
snout, and the big, apparently naked elephant, whose skin upon close 
examination is about as forbidding as a chestnut burr, has a supplementary 
mammalian passport in the form of a tuft of hair at the end of its ridiculous 

Hair serves a variety of uses besides its obvious benefit of affording 
general protection. The air-imprisoning pelt of a fur-bearing animal retains 


Biology of the Vertebrates 

Fig. 197. Vibrissae of 
a cat. They form a 
sensory halo that de- 
termines a hole large 
enough for the body 
of the cat to pass 

the bodily heat and sheds the rain ; the thick mane of a wild horse is a 
specially placed buffer against carnivorous enemies that would pounce 
upon its otherwise unprotected neck; the squirrel's 
frisky tail is a portable blanket which conveniently 
enwraps the owner when at rest; the anteater's bushy 
tail is a diverting and confusing barrier to the armies 
of ants that swarm forth in defense when the ant- 
eater makes a foray upon their citadel; the long 
hairs of the horse's tail form an effective brush to 
ward off pestering insects; the stiff sinus hairs, or 
vibrissae, that supply the snouts of many mammals, 
are sensitive "feelers" (Fig. 197) ; and lastly, the color 
schemes carried out on the bodies of mammals, what- 
ever uses they may serve, are due principally to hairs 
as color bearers. Transient sinus hairs on the inner or contact side of the 
forearm in the human embryo (Fig. 198) hark back to arboreal lemur- 
like life. 

In structure a single hair (Fig. 158) is an epidermal shaft 
projecting, usually at an acute angle like an exaggerated lean- 
ing tower of Pisa, from a pit or depression in the skin. The 
projecting dead part of the shaft is typically cylindrical, with 
the root concealed at the base in the pit expanding into a 
club-shaped bulb that derives nourishment from the corium 
through the living Malpighian cells producing it from below. 
Directly beneath the bulb and in intimate contact with it, 
is an upward-projecting dermal papilla containing capillaries 
and nerve endings which supply the hair root. Corneal cells, 
surrounding the root of the hair, constitute the inner root 
sheath. Other epidermal cells that form the outer root sheath 
line the walls of the pit. The root of the hair with its sheaths 
make up the hair follicle. 

Opening into the pit from the sides are sebaceous glands, 
which produce an oily secretion that renders the dead hair 
shaft less dry and brittle. 

In cross section a hair shaft ordinarily shows three kinds 
of cells, namely, those of an inside core, or medulla (absent in human body 
hair) ; a surrounding ring, the cortex, making up the bulk of the hair; and 
a thin outer single layer of shingling cells, the cuticle. 

Although the root of the hair may be embedded deep in the corium, the 
entire structure is epidermal in origin except the papilla, which is dermal. 

Fig. 198. Tran- 
sient sensitive 
hairs on the 
inner, or con- 
tact side, of 
the fore-arm 
in a human 
embryo. (Aft- 
er Broman.) 

A Jack of All Trades 


The shaft of the hair usually tapers towards the tip and does not branch, 
although bristles sometimes split distally. Frequently hairs taper also towards 
the root end, particularly near the point where they emerge from the skin, 
so that they tend to bend easily when stiff or give way instead of breaking 
off upon contact with external objects. The exquisite softness of the fur 
upon a mole skin when it is stroked either way is due to this adaptation of 
hairs thinned down at the surface of the skin, which enables the animal 
to go forward or backward in its burrow with a minimum of frictional 
injury to the pelt. 





1 s-y'ii'"- 


$$>*P,; \ 

Fig. 199. Diagram showing, A, the more usual hair currents upon the 
front or ventral aspect of the trunk, and b, on the back or dorsal aspect 
of the trunk. (After Kidd.) 

Each follicle is supplied with an involuntary muscle, arrector pill, run- 
ning from near its base diagonally to the superficial region of the corium, 
on the side towards which the hair slants. When this smooth muscle 
shortens, it pulls upon the base of the follicle causing the hair to "stand on 
end" (Fig. 158). The action of the arrectores pilorum is particularly 
noticeable upon the scruff of an angry dog's neck, or upon the tail of a 
frightened cat, when these animals take on a more terrifying aspect as the 
result of this reflex. The ghostly remains of this apparatus in man is the 
cause of "goose flesh," which appears when the skin contracts somewhat 
upon exposure to cold. "Then a spirit passed before my face; the hair of 
my flesh stood up." (Job 4: 15.) 

2 $2 Biology of the Vertebrates 

The slant at which hairs emerge from the skin varies in such a way 
that in their direction the hairs taken together form vortices and streams 
as they lie over the surface of the body (Fig. 199). This is particularly 
apparent on a horse or short-haired dog. 

Convergent vortices form around the base of projecting structures, such 
as horns, the tail, and the umbilical cord. These hair whirlpools persist even 
after the structure around which they converge has disappeared, for example 

in man about the umbilicus, and at the focus in the 

coccygeal region where the vanished embryonic tail was 

formerly located (Fig. 200). 

Perhaps the most familiar instance of divergent 

whirlpools is on the human scalp at the vertex of the 
F' ^00 T*~ft f crown 5 where the hairs are centrifugally arranged. Other 
coccygeal hairs on divergent vortices appear in the axillae. The coarse hair 
a human embryo of the sloth is divergently parted along the midline of 
suggesting an ances- the be ji i nsteac } f c j own t h e back as in most mammals, 
tral tail. (After _ . \ 
Ed^ .) This adaptation, as in other divergent streams and 

whirlpools wherever found, is useful for shedding rain. 
The unusual arrangement in the case of the sloth is due to the fact that this 
mammal customarily hangs suspended upside down from the horizontal 
branches of trees. 

Although hairs are not arranged in definite patches like the pterylae ol 
feathers, they do emerge from the skin embryonically in orderly array with 
reference to each other (Fig. 201 ). In man they appear in groups of twos, 
threes, and fours with the largest hairs in the middle of each row, these rows 
in turn being spaced in such a way as to suggest that each one is homologous 
with an interscale area. This hypothesis is further borne out by the arrange- 
ment of hairs in similar groups in other mammalian skins, particularly those 
of the armadillo and scaly anteater where scales are actually present with 
a definite group of hairs behind each scale. 

In mammals other than man, localized masses of hair appear as fetlocks, 
tufts, manes, and modifications of the tail. A horse is enabled to brush away 
annoying flies with a "swish" of long hairs on the tail, while a cow accom- 
plishes the same result, as every farm boy who has ever milked a cow knows, 
with a "flip" of the terminal tassel. 

Hairs occur in various shapes and forms all the way from hard rigid 
spines, like those of the porcupine, European hedgehog, spiny mouse, and 
spiny anteater, to the soft delicate wool of sheep and goats. The bristles of 
swine are stiff elastic hairs, sometimes with split ends, in which the outer 
laver of cuticle predominates. They are more numerous on the dorsal side of 

A Jack of All Trades 253 

a hog than elsewhere on the body and tend to make the wild animal look 
somewhat larger and more formidable. 

Fur is composed of dense soft hairs, frequently lacking the medulla, with 
a few long coarser hairs interspersed. In the process of transferring the skin 
of a seal to milady's back in the form of a sealskin coat, the long stiffer 
guard hairs are carefully removed, leaving the soft thick-set fur-hairs mak- 
ing a uniform surface. 

Tail Scale 

Myopotamus ^JJ *• •...„ ;••.:'/'•" Loncheres 

• m • 

Midas • • • £\) • O Auchenia 

Cercopithecus • • .' • (v) © O Canis 

Coelogenys . 

m ® • © Ornith 


• • • 

@ © © Q© 

T , © ® * ^ (S) © Castor 

Tragulus . . • • @ <^ @ © © ^ w 

^ ® © 

Dasypracta •...;.••• ' § Q Q Lutra 

Fig. 201. Hair groups of different mammals. (After Meijere. ) 

The cuticle of wool hairs is usually rough and scaly, and since the hair 
shafts are somewhat twisted they spin well into yarn because the separate 
hairs interlock easily. 

Sinus hairs, or "feelers" (Fig. 197), that radiate from the inquisitive 
noses of nocturnal prowlers, such as cats, rats, and weasels, are each seated 
in a large papilla especially well provided with nerve endings, so that any 
chance contact which disturbs the stiff outstanding dead shaft is com- 
municated at once to sensory headquarters through the mechanical agitation 
of the basal papilla. 

The unusual beard on the faces of goats and men is the very latest evolu- 
tionary style in hair decoration. That the human beard is not so much a 
relic of the past as a prophecy of the future, is evident not only by its sharp 
differentiation in the male sex, and its delayed appearance in the individual, 
but also by the fact that it is much less apparent in the more primitive races. 


Biology of the Vertebrates 

Sexual differentiation of human hair is largely controlled by hormone 
action as demonstrated when sexual hormones are prevented from normal 
occurrence by the removal of ovaries or testes. The distribution of adult 
female hair over the body is intermediate between that of the embryonic or 
infantile condition and the arrangement in the adult male. 

Although man is one of the least hairy of the mammals, with the excep- 
tion of the aberrant whales and sea-cows, an examination of his embryonic 
development shows his close relationship to other members of the order of 

Ma I pig h ia n Layer s 
/ Corneal Layers 


Root Sheaths 
— External-- 
"-- Internal-"''! 

■ I 



Dermal Papilla --^g 

Fig. 202. Six stages in the development of a hair. A and B, from 
embryo of a sheep. (After Schimkewitsch. ) c and d, from a mole. 
(After Maurer.) e and f, from human embryo. (After Hertwig.) 

The first evidence of hairs in the mammalian skin is found in the form 
of concentrations of epidermal cells which, because of displacement resulting 
from their rapid multiplication, grow down like plugs into the corium 
(Fig. 202), and become hair follicles. The bulb of each follicle, with its 
surrounding inner and outer root sheaths, soon differentiates, and the newly 
formed lengthening shaft pushes out towards the surface, loosening the 
temporary epitrichium which at this stage covers the body like a gauzy 
envelope. The hairs in man first emerge at about the fifth fetal month, in 

A Jack of All Trades 

2 SS 

the region of the forehead and eyebrows, eventually becoming a transient 
coat of delicate embryonic fur called lanugo (Fig. 203), which clothes the 
entire body with the exception of the lips, palms, soles, nails, and spaces 
around the apertures of the external genitalia. The lanugo usually reaches 
its highest development during the eighth fetal month, when it begins to be 
shed into the amnionic fluid that surrounds the embryo, and is replaced by 
the permanent hair, at least over cer- 
tain parts of the body. It remains 
longest on the shoulders and in many 
instances is still in evidence at birth. 

The permanent hair in attaining 
its growth becomes localized in distri- 
bution, and differentiated for various 
uses, as already pointed out. It is 
thickest on the top of the scalp, since 
it was originally adapted to shed the 
rain which fell alike on our just and 
unjust hatless arboreal ancestors. In 
apes, which assume a semi-erect pos- 
ture with the crown of the head pro- 
jecting somewhat forward instead of 
upward, the hair, as would be ex- 
pected, is thicker on the scruff of the 
neck than on the top of the head. 

Hairs are also conspicuously spe- 
cialized in man in the form of eye- 
brows, eyelashes, and as guardians of nasal and external ear passages against 
dust invasion. At the pubes and axillae cushions of hair that perhaps tend to 
lessen friction develop at puberty, while the remainder of the human body, 
which normally appears to be comparatively bare, is supplied in varying 
degree with hairy reminders of other days. 

Hairs of the head are straight, wavy, curly, or kinky. In cross section the 
series varies from nearly round in the straight head hair of Indians and 
Mongolians, to elliptical in the kinky hair of the Hottentot. The shaft of 
curly or kinky hair, growing more rapidly on one side than on the other, 
emerges from the skin in a curve. Pubic and axillary hairs usually curl, 
even in straight-haired people, and straight hair tends to curl in wet weather 
while curly hair tends to straighten. True "permanent waves," like poets, 
are born not made. 

Growth of hair varies individually, in health and sickness, seasonally, 

Fig. 203. Face of an embryo five 
months old with lanugo, or temporary 
hair covering. (After Ecker.) 

2 $6 

Biology of the Vertebrates 



with quantity and quality of food, with climate, and with the region of 
the body on which it occurs. The beard hairs may easily grow a millimeter 
in twenty-four hours. Dr. W. W. Keen reckoned that with the production 
per millimeter of approximately 500 cells of hair, there would need to be 
only 5000 hairs on the head to produce 40,000 hair cells per minute. This is 
what keeps barbers busy. 

According to the manner of growth, hair is either definitive or angora in 
character. Definitive hair grows until a certain length is attained, when 
it becomes pinched off from its base of supplies in the papilla of the bulb 

(Fig. 204), and the lifeless hair shaft loosens and 
11 1. ^P-A Is shed. A new hair then starts to grow. The inter- 

^_^„ ruption in growth at the root of an angora hair 

does not occur either as often or as completely as 
in definitive hair, so that the shaft continues to 
lengthen as long as the follicle remains intact. In 
man the body hairs are definitive, while those of 
the scalp are angora in character. In apes those 
of the scalp are also definitive. 

RThe color of hair is due to pigment deposited 
IJ ; during growth in the intercellular spaces of the 
Follicle-.. /'/-\'\ cortex. When hairs "turn gray" there is a reduc- 

/ ('(./) t * on * n tne amount °f pigment present and an in- 

J \j Ls crease in the number and size of the light-reflecting 

air-spaces between the cells. Gray hair in man 
appears first at the "temples," situated over the 
temporal bones, so-called because here the flight 
of time is marked. In dogs the graying of hair 
usually begins on the snout, while in mice and rats 
it may be anywhere on the body. Some animals 
such as the varying hare, Lepus americanus, for 
example, show a seasonal whitening of the hair coat, that brings them into 
harmony with their snowy habitat, thus insuring them a degree of protection 
against their enemies. 

Data on particular differences in human hair have been gathered in 
certain cases. For instance the head hair of blondes is usually finer, longer 
and more dense than that of brunettes. Someone has made an estimated 
census, after a partial count, of the number of head hairs on four females 
with the following result: blonde, 140,000; brown, 109,000; black, 
102,000; red, 88,000. A mathematical moment with a pencil and a pad of 
paper reveals the fact that if the blonde lady in question should have her 

A B 

Fig. 204. A, base of hair, 
fully grown, of definitive 
growth type, b, base of an- 
gora hair of indefinitive 
growth type. (After Castle 
and Forbes.) 

A Jack of All Trades 257 

hair bobbed, supposing that it was originally two feet long, she might 
thereby dispose of something over fifty linear miles of hair. 

Ecdysis, or molting, which is so universal a phenomenon of other epi- 
dermal structures, occurs at intervals also in the hair coat. With most mam- 
mals shedding the hair is more pronounced in spring and early summer than 
at other seasons, but with man it is a continuous process, involving a normal 
daily loss which may be increased under pathological conditions. A single 
head hair, according to Lewis, ordinarily lasts from four to five years, 
while eyelashes are normally replaced in as many months. Failure in the 
replacement of hairs of the scalp results in baldness of which there are two 
general types, both evidently hereditary. In one the divergent whirlpool of 
hair about the vertex of the crown is the first to go, when the subject comes 
to resemble a tonsured monk. In the other case the hair retreats from the 
forehead with the passing of the years, leaving an increasing expanse of 
apparent intellectuality. When both types of baldness descend upon the 
same individual the polished dome of the skull may be as bereft of hair as a 
billiard ball. Baldness, even if the truth be told, is very much more common 
in men than in women. 

Under pathological conditions, unusual abundance of hair, hypertri- 
chosis, or abnormal absence of hair, atrichosis, may occur. The latter 
condition is frequently associated with defective development of the 

W^hen embryonic lanugo persists it is spoken of as pseudo-hypertrichosis, 
as distinguished from hypertrichosis vera. The latter is exemplified by the 
presence of superfluous hair in the case of bearded women and shaggy men. 

8. Friction Ridges 

Upon the tips of human fingers one can easily see with the naked eye 
peculiar fine ridges, called friction ridges because they aid to a certain extent 
in preventing the fingers from slipping when brought into contact with 
objects. They are arranged mostly at right angles to the direction in which 
there is the greatest tendency to slip. Sweat glands that open upon them, 
like craters along the peaks of tiny volcanic mountain chains (Fig. 139), 
provide moisture, bringing about much the same result as when a workman 
"spits on his hands" to secure a better grasp. 

Since friction ridges appear only on those areas that come habitually 
into contact with objects, they are particularly developed on the palmar 
and plantar surfaces of the hands and feet of man and other primates, and 
also on the concave side of the prehensile tail of the long-tailed American 
monkeys ( Fig. 205 ) . They are absent from the middle of the back, fore- 


Biology of the Vertebrates 

head, and rim of the ear, and other regions not employed in taking hold of 


A histological examination reveals the fact that the ridges and furrows 

of the epidermis in the friction areas match corresponding downgrowths 

into the underlying derma. Furrows between papillary ridges should not be 

confused with the many wrinkles and folds that 
beset the skin all over the body. 

Friction areas are particularly associated with 
padlike epidermal elevations, or tori, that originally 
appear on the palms and soles. Typically there are 
ten of these elevated tori on each hand or foot, 
namely, five digital areas forming the balls of the 
fingers and toes; three inter digital areas on the 
palm or sole near the base of the digits; one 
thenar •; and one hypothenar area at the posterior 
part of the palm or sole on the side of the big digit 
and the little digit respectively (Fig. 206). Al- 
though present as distinct elevations throughout 
life on the feet of certain mammals, the mouse for 
example, and also on the hands and feet of the 

human embryo, tori as such disappear in adult man, since, as the human 

embryo grows older, these elevations or pads become less pronounced and 

are eventually flattened to form the friction areas. 

The various minute patterns which the ridges of the friction areas 

assume are all definitely established before birth and retain their individ- 

Fig. 205. Prehensile tail 
of a monkey, Atcles, 
showing friction ridges 
in region of contact. 
(After Journal of Hered- 
ity. April, 1918.) 

m — 4& 

^■--"•"Digital Tori 

Interdigital Tori-^j--\^--j^§ 


— Thenar 

Fig. 206. Arrangement of the tori, or elevations which become the fric- 
tion areas on the palmar surface of the hand. A, diagram of typical ar- 
rangement. (After Wilder.) b, hand of a human embryo of 22 mm in 
which corresponding tori are seen. (After Retzius. ) 

A Jack of All Trades 


uality, except for slight increase in size, throughout life. It has been demon- 
strated that when friction ridge patterns are destroyed by searing or by 
sandpapering the finger tips, the old patterns are restored upon subsequent 
growth of new epidermis. 

Since the details of the patterns are unlike, not only in different persons 
but also on the twenty fingers and toes of the same person, they furnish 
an excellent means for personal identification. Just as primitive peoples 
in the past have frequently employed indelible tattoo marks in order to dis- 
tinguish themselves from their fellows, so friction ridge patterns, which 
have been called "nature's tattoo marks," are made to serve a like purpose. 



Fig. 207. Diagrams of the four main types of finger patterns. The whorl 
and the composite have two deltas (A's); the loop, one, and the arch, 
none, The loop may be a radial loop, or an ulnar loop according to 
whether it opens outward toward the thumb (radial), or toward the little 
finger (ulnar.) (After Wilder and Wentworth.) 

The patterns may be roughly classified in general types, namely, whorls, 
loops, composites, and arches, as indicated in Figure 207. Loops may be 
ulnar or radial according to whether they stream outward toward the ulnar 
(little finger) or the radial (thumb) side, while arches may be simple, as 
in the figure, or if more pronounced, tented arches. Combinations of these 
types upon the fingers of both hands taken together, and the infinite variety 
in the minutiae that each type reveals upon careful scrutiny, make possible 
an almost unlimited subdivision and classification. Thus, it has come about 
that finger-print codes have been worked out, which may even be tele- 
graphed or radioed from one part of the world to another in the interests 
of personal identification. 

By reason of the fact that finger prints are easily made and kept on file, 
they may be utilized conveniently in a great variety of ways. Upon a bank 
cheque, passport, or non-transferrable documents of any kind, for example, 
such a personal imprint furnishes a unique signature which cannot be 
forged. In the case of soldiers, sailors, the personnel of large industrial 
plants, voters, babies at maternity hospitals, inmates of institutions, crim- 
inals, undesirable immigrants once rejected, dead bodies recovered from 
disastrous catastrophes or accidents, aphasia victims, and in many other 


Biology of the Vertebrates 

instances, finger prints offer a simple and invaluable means of establishing 
identity (Fig. 208). 


/M' -^ \\^ 

"///^•■= l: :.".'?v-.-V- , <;\>^- 

Fig. 208. Two sets of finger prints, superficially alike but quite dif- 
ferent in detail, a, print of the middle right finger of J. C. (Magnified 
two diameters.) The area enclosed in the square is shown below in an 
enlargement of 1 1 diameters, b, print of the right middle finger of J. W. 
(Magnified two diameters.) This was selected from several hundred 
prints of middle right fingers in the endeavor to get the nearest match 
to a. The corresponding enlarged square below shows distinct differences 
that are not evident upon superficial examination. (From Wilder and 

Since Galton's pioneer work in England,* and the appearance of Mark 
Twain's whimsical classic,! in which the imagination of the story teller 
anticipated the later applications of science, the serious study and utilization 
of the ineffaceable friction ridges has developed into a real science by itself 
(Dactyloscopy) with a considerable and growing bibliography. It is now 
known that two widely separated peoples, the Chinese and the Babylonians, 
in very early times made use of finger-print signatures. 

* Finger Prints, 1892. 

t The Tragedy of Pudd'nhead Wilson, 1894. 


Intake Apparatus-Digestive System 


1. The Whirlpool of Life 

Life is manifested as a process of release of energy, involving continuous 
death or destruction, since it is only by the breakdown of cells and tissues 
in which energy from food has been stored that the phenomenon of life can 
appear or continue. Thus, the paradox that we live by dying. There is, how- 
ever, more than one kind of death. The kind referred to in this connection 
is the local death of cells and tissues, which is usually accompanied by 
regeneration and recovery, while what may be called general death is that 
in which the correlation of functions depending upon the brain, heart, and 
lungs is interrupted so that it cannot again be resumed. Even in this latter 
case the component tissues may live on for some time after correlation is 
no longer possible, as shown, for example, by the excitability of the muscles 
of a frog's leg under electrical stimulation after the frog has been irrevocably 
killed by the complete removal of its brain and heart. 

Huxley likened an organism to the whirlpool below Niagara Falls. At 
no two moments of time is it made up of the same mass of water, yet its 
identity remains, and if photographed on succeeding days from the same 
point, the pictures would appear alike. In a similar manner all living things 
may be conceived as whirlpools of living matter and energy, which never- 
theless maintain a continuous individuality throughout the duration of life. 

The digestive system is the mechanism that makes good the constant 
losses which are inevitable in the mortal expense of living. It is with the 
intake aspects of the organic whirlpool that this chapter is concerned. 

2. Rate of Living 

The rate at which the metabolic waters of life flow through the organic 
whirlpool varies greatly with the age of the individual. During the first 
part of life while growth is taking place, the intake, like a spring freshet, 


262 Biology of the Vertebrates 

is greatly in excess of the outgo, but later there follows a prolonged period 
of balance during which losses of energy are simply made good, then the 
stream of life flows more slowly and becomes less and less in volume, and 
eventually ceases entirely as the head waters gradually dry up. 

It is not at all easy to realize the abounding life of animals during the 
onset of growth. A human baby normally doubles its weight in 200 days. A 
new-born mouse quadruples its weight in twenty-four hours, and a silkworm 
increases its size 500 times during the first day's intake of mulberry leaves. 
Dr. Keen says: "Were the same rule to hold, a baby weighing seven pounds 
at birth would weigh thirty-five hundred pounds the very next day, and 
when a month old would weigh one hundred and five thousand pounds, or 
over fifty 'short tons,' which, however, could hardly be called 'short 
weight.' " 

3. Hunger and Thirst 

Food, water, and oxygen are the necessary materials of subsistence 
taken into a going organism. Food carries energy to be stored up in the 
tissues for later use. Water is the universal solvent and fluid necessary for 
manipulating and shifting about materials within the organism, while oxy- 
gen effects the breakdown of tissues and the liberation of imprisoned 

The essential concern of every animal is the securing of these three pri- 
mary prerequisites for continued activity. This fact is so obvious that it 
escapes our attention. Anyone who has tried to follow the incessant activi- 
ties of a wild bird, for example, during the daylight hours will realize in 
part the imperious demands of hunger and thirst. It may be observed that 
most animals rarely succeed in overtaking their appetites. 

Even in the highly specialized routine of human society, the daily pro- 
gram of business, pleasure, education, religious activities, politics, philan- 
thropy, and all the rest, is secondarily tucked in between meals around 
which the day's activities are arranged, and any serious deviation from the 
periodic exercise of the sacred rites of intake are likely to border on the dis- 

4. The Intake Mechanism of Animals and Plants Contrasted 

Most plants are restricted to a diet. The food they use is monotonous 
in the extreme, yet there is no complaint. It is made up in the synthetic 
laboratories of the green cells of leaves or stem out of uniformly distributed 
raw materials, such as carbon dioxide and oxygen from the air, and water 
impregnated with dissolved salts of the soil. 

Intake Apparatus 


Liquid intake from soil-water is soaked up by osmosis through the 
delicate walls of root hairs (Fig. 209), which would quickly collapse if 
exposed to dry air. This does not ordinarily happen, however, as root 
hairs remain constantly protected in damp soil, since the plant is not forced 
to travel about seeking water and what it may devour. 

Animals, on the other hand, do not have the power of synthesizing 
foods out of air, water, and inorganic salts of the soil, so ordinarily they 
cannot remain anchored in one spot, manufacturing their foods out of raw 
materials at hand, but are obliged to forage for food already made. 

Fig. 209. Intake mechanism of plants, a, young seedling, showing root 
hairs; b, part of a section through a young root, showing some of the 
superficial cells growing into root hairs. A thin layer of cytoplasm 
(dotted) lines the cell wall and encloses the cell sap. (After Brown.) 

Like plants, animals depend upon osmotic intake through thin cell 
membranes, cellular middlemen between indispensable food and the animal 
body, which cannot remain without harm on the outside of bodies of ad- 
venturous locomotor organisms. The intake cells of animals, as well as of 
plants, must be protected from mechanical injury and from drying up, 
while their possessors are seeking food. This explains the evolution in loco- 
motor animals of the digestive tube, an enclosed passage-way arranged for 
one-way traffic and paved with thin-walled absorbing cells that correspond 
to the osmotic root hairs of plants. In one sense the digestive tube is 
simply an infolding of the integument, making a protected subway where 
food admitted at the entrance is exposed to intake cells, which proceed 
to do their osmotic duty in security without drying up while being trans- 
ported to fresh fields of food supply. Thus, in a way, an animal may be 
regarded as a plant turned outside-in. 

264 Biology of the Vertebrates 

5. The Mission of the Food Tube 

In the process of living, while energy is being released by the oxidation 
of the tissues, it becomes imperative that replacements be made from out- 
side sources, or in other words, that food be obtained. It is not enough, 
however, simply to get food, since energy-containing substances cannot be 
utilized until they are so liquefied and transformed that they may be taken 
into the blood, to be forwarded to the needy tissues where the actual feed- 
ing, or incorporation of food materials, occurs. To accomplish these trans- 
formations is the mission of the digestive tube with its accompanying con- 
tributory devices. 

The everyday miracle of a cat taking a captured mouse and changing it 
over into more cat, or of human flesh and blood, endowed with personal 
idiosyncrasies, made out of the hodge-podge of materials that appear on 
daily bills of fare, is so ordinary and familiar that these marvels have ceased 
to excite wonder. 

6. Kinds of Feeders 

Animals may be classified according to the prevailing character of their 
intake into herbivores, carnivores, omnivores, parasites, symbionts, and 

Herbivores are direct plant feeders. Carnivores feed upon animals, but 
in reality are plant feeders at least once removed, since the ultimate food 
of all animals is plants. Omnivores feed directly upon both animals and 
plants. Parasites feed at the expense of living organisms which "entertain" 
them as "hosts" without necessarily fatal results. Symbionts, such as green 
hydras and certain green worms, live vicariously at the expense of micro- 
scopic green plants embedded in their bodies, which have the ability com- 
mon to green plants of synthesizing food on the spot; while saprozoa, like 
certain flagellates and infusorians, are scavengers, specializing upon dead 
organisms in the last stages of their reduction into inorganic materials. Most 
vertebrates belong in the first three groups. 

Animals with a wide range of foods have a better chance in the strug- 
gle for existence than those that have become specialized for a single source 
of nutrition, such as the pronuba moth, which feeds only on the pollen of 
the yucca flower; termites with a diet of woody cellulose; boll-weevils that 
spurn everything except cotton "squares" before they bloom; and coproph- 
agous beetles that revel only in feces. 

There are a few curious carnivorous plants, like the Venus fly-trap, 
bladderwort, pitcher plants, and sundews, that have deviated far from 

Intake Apparatus 265 

the self-reliance of most green plants, which manufacture their own food. 
These isolated plants by various cunning devices have augmented the usual 
source of food of green plants by capturing small animals which they de- 
vour. A greater variety of plant forms, including bacteria and fungi that 
have no chlorophyll, live saprophytically on the dead organic remains of 
other plants. 

Animals in satisfying their demands of hunger from all sorts of sources 
are quite unaware that the chemist finds only three fundamental kinds of 
food in the world, with certain necessary additional inorganic trimmings 
in the form of water and salts. These three basic food substances, which 
occur in an infinite number of guises in the bill of fare of animals and 
plants, are proteins, that furnish building materials for growth, main- 
tenance, and reproduction, and fats, and carbohydrates, which supply the 
immediate energy indispensable to the business of living. 

1. Its Evolution 

In the lowest unicellular forms of animal life, the osmotic process of 
taking in food substances is performed by the outside of the body, some- 
what after the fashion of plants, as most simply demonstrated by Amoeba. 

Among sponges, which take the first step in the great adventure of cell 
associations, the method of intake is hardly different, although there is a 
a prophecy of an internal digestive tube in the ciliated passage-ways that 
honeycomb the loosely connected sponge mass, through which the food- 
laden water is made to stream. 

Hydras, corals, and sea-anemones, as well as all other typical coelen- 
terates, have a digestive sac open only at one end, and little else. This 
single external opening serves as both mouth and anus. In these pioneer 
animals everything is sacrified to securing a suitable place for the bestowal 
of food. The very shape of the body is determined by the food sac, for the 
whole animal is simply an animated food bag, decorated around the intake 
opening with a fringe of subservient tentacles. The importance of the food 
cavity is thus clearly emphasized by its early establishment before most 
other structural refinements peculiar to the animal organism. 

Even in echinoderms, although an anus is nominally present it plays 
only an occasional role, since these devastating, devouring creatures, of 
which starfishes and sea-urchins are typical examples, dispose so effectually 
of the food entering their maw, that there is very little waste left over for 
expulsion at the exit. As a matter of fact in the case of the starfish, most 


Biology of the Vertebrates 

of the food waste is not even taken into the mouth. Instead the stomach 
is everted from the mouth in feeding and enwraps the food or prey so com- 
pletely that the indigestible parts are left behind when the stomach is 
withdrawn, leaving no residue to be passed out at the anus. 

Worms and caterpillars may well be described as perambulating diges- 
tive tubes, with the important mouth end pointed toward a food-contain- 
ing world. Directive sense organs cluster around this exploratory end of 
the food tube, informing it where to go. 

A vertebrate in reality is a double tube. The outer tube is the protective 
body wall, and the inner tube, the digestive canal. Between the two tubes 
is the body cavity, which makes possible within a limited space the storage 
of a digestive canal much longer and more efficient than the exterior of the 
animal would lead one to suspect. Thus, the knapsack for carrying the ra- 
tions is bestowed within the body instead of being carried outside. 

2. Increase in Digestive Surface 

So long as the bulk of an animal's body remains small a straight diges- 
tive tube has an adequate internal surface to meet all alimentary demands. 
It is mathematically demonstrable, however, that while the surfaces of two 

homologous solids are to each other as -the 
squares, the masses are to each other as the 
cubes of their homologous dimensions. This 
means that the bulk of a growing animal 
increases more rapidly than its surface, with 
the inevitable result that a straight unmodi- 
fied digestive tube becomes inadequate to 
take care of the accompanying mass. This 
is particularly true in the case of herbivores, 
whose food is less concentrated than that of 
carnivores, and who consequently need di- 
gestive machinery adequate for handling a 
larger quantity of food in a given time. 

There are four general ways in which 
this need for increase of digestive surface 
has been met in various animals, namely, 
(a) by increase in diameter; (b) by increase in length; (c) by internal folds 
and elevations of various kinds; and (d) by the addition of supplementary 

(a) Increase in Diameter. — This method is not extensively employed, 
because of the limitations of space in the body cavity. If the inner tube 

Fig. 210. Comparison of the tad- 
pole and the young frog, Alytes, 
just after metamorphosis, to show 
the great difference in the digestive 
tract with the change from plant 
to animal diet. (After Reuter.) 

Intake Apparatus 


increases in diameter the outer tube of the body wall must also enlarge, 
which tends to defeat the object to be gained. Certain regions of nearly 
every digestive tube, such as the stomach and large intestine, are frequently, 
nevertheless, of greater diameter than the remainder of the tube. 

(b) Increase in Length. — Increase in length is a universal device among 
vertebrates for adding to the available digestive surface, since the body 
cavity furnishes possible space for stowing away coils and loops of the tube. 
The body cavity not only makes a place for an intestine longer than the 
body itself, but it also frees the intestinal tube from the muscular control 
of surrounding tissues, permitting it freedom to exercise peristaltic move- 
ments of its own. 

The characteristic swollen shape of a tadpole, resembling an animated 
head with a tail attached, is due to the enormously lengthened digestive 
tube which, is coiled about many times, packing the body cavity full. Just 

Integument ^ 


Cardinal Vein 

Dorsal Aorta -i|3 




Fig. 211. Cross section through an ammocoetes larva of a lamprey eel, 
showing typhlosole which increases the internal surface of the digestive 


Biology of the Vertebrates 

before metamorphosis, when the tadpole gut is adapted for plant food, it 
may measure eight to ten centimeters in length, whereas after metamor- 
phosis, when the young frog switches over to insect food thus requiring 
less digestive surface, the tube shortens to three or four centimeters in 
length, although the body itself is now considerably longer than before 
(Fig. 210). 

In man the entire digestive tube is between twenty-five and thirty feet 
in length, although the entrance and exit are only about two feet apart. 

(c) Internal Folds. — Increases both in diameter and length of the diges- 

tive tube make demands that soon encroach upon limits 
of possible space within the body cavity. Internal folds 
within the food tube itself avoid this difficulty by add- 
ing to the expanse of surface to which the food is 
exposed without adding to the external size of the 

A longitudinal fold extending into the cavity of the 
tube is termed a typhlosole ( Fig. 211). Such an arrange- 
ment is present in the cyclostomes. In dipnoans, as well 
as elasmobranchs and ganoid fishes, the intestinal part 
of the food tube is supplied with a spiral valve (Fig. 
212), a typhlosole so much longer than the tube in 
which it is placed that it must coil around like a spiral 
stairway, with one edge attached while the other is 

Certain invading transverse folds, called plicae cir- 
give a washboard effect to the inner surface of the an- 
terior part of the human intestine, while countless tiny elevations, or villi, 
projecting like the nap of velvet from the inner surface of the small intestine, 
particularly in the higher vertebrates, pro- 
duce, in a minimum of space, an enormous 
increase of absorbing area for contact with 
passing food. 

( d ) Supplementary Diverticula. — Side 
alleys, or diverticula, from the main tube 
occur in many instances. These are particu- 
larly abundant in fishes at the junction of 
the stomach with the small intestine, where 
they are called pyloric caeca (Fig. 214). 

They vary in number from one in the ganoid Polyptcrus and (he sand 
lance Ammodytes to over 200 in the mackerel Scomber. 

Fig. 212. Spiral 
valve of a dogfish. 
(After Roule.) 

culares (Fig. 213' 

Fig. 213. Transverse rugae, plicae 
circulates, lining the intestine. 
(After Cunningham.) 

Intake Apparatus 


Other diverticula, called colic caeca, are found at the junction of the 
small and large intestines in amniotes. The colic caecum of a turtle is only 
a slight enlargement (Fig. 215), but in rabbits and some rodents it may 
become an enormously enlarged tube with an internal capacity nearly 
equal to that of the rest of the digestive canal to which it is attached. 

Gall Bladder 
i* Pyloric Caeca 

Fig. 214. Pyloric caeca of a tele- 
ost fish, Mcrlucius. (After Krup- 
ski and Schimkewitsch.) 

\^-- Esophagus 
G=^ ))3— Stomach 

"•*""" Gall Bladder 
2r— Small Intestine 


j- -Large Intestine 

— Cloaca 

— Cloacal Opening 

Fig. 215. Digestive tube 
of a turtle. (After But- 

In man the colic caecum, with its troublesome shriveled prolongation, 
the processus vermiforrnis, or "vermiform appendix" (Fig. 216) , has outlived 
its usefulness and bears an unsavory reputation. Birds have typically two 
colic caeca (Fig. 217). 

1|> „ Large 
I *.~"iill Intestine -.-.J!] 

Caecum — -—3 
Processus Vermiforrnis- 

.- Processus Vermiforrnis 

Fig. 216. Caecum and processus vermiforrnis (vermiform appendix) in 
man. a, in the embryo; b, in an adult. (After Wiedersheim. ) 

270 Biology of the Vertebrates 

The large intestine of man, as well as of several other mammals, is 
pushed out into a series of bay window-like enlargements ( Fig. 218), which 
are diverticula of a sort called haustra. Connected with the rectal region 
throughout the vertebrate series are various proble- 
matical outpushings, such as the rectal gland of elas- 
mobranchs; the urinary bladder of amphibians; the 
bursa of Fabricius in birds, and the anal glands of cer- 
tain mammals, all of which have been made to serve 
different uses, although not necessarily connected with 
the process of digestion. 

Fig. 217. 

caeca of 

Two colic 
an owl. 

(After Pyecraft.) 


/ V^y r y^ r Y ' 

3. Development 

Mouth and anus are not essential during em- 
bryonic development when the body secures its nour- 
ishment either from the yolk mass or, in the case of 
mammals, from the maternal blood stream. There 
comes a time, however, when provision must be made 
to admit food into the digestive cavity from other 
sources. This necessity is met by the formation of two 
ectodermal invaginations, one near each end of the elongated archenteron, 
which break through to make a continuous open passage-way, the digestive 
tube (Fig. 117). 

The anterior ectodermal ingrowth is 
called the stomodaeum, and the posterior 
ectodermal part, the proctodaeum, while 
the endodermal region between them, 
which was originally the archenteron, is 
now termed the mesodaeum. The embry- 
onic stomodaeum stakes out the claim for 
the future mouth region; the proctodaeum 
locates the anus. The food tube thus con- 
sists of three embryonic components, al- 
though the landmarks that separate them 
from each other are obliterated in the adult. 

4. Histology 

A cross section of the digestive tube within the body cavity shows it to 
be made up of several concentrie layers of cells (Fig. 219). 

The innermost layer, or mucosa, the original embryonic endoderm, is 
supported by mesodermal connective tissue, the submucosa. The mucosa is 

Glandula Epiploica 

Fig. 218. Haustra of the large 
intestine of man, with small per- 
itoneal pockets, glandulae epi- 
ploicae, attached to the outside 
of the intestinal wall. (After 

Intake Apparatus 271 

only one cell-layer thick, except in the anterior esophageal region. It per- 
forms not only the "root hair" function of absorption, but also gives rise to 
various digestive glands whose secretions bring about chemical transforma- 
tion of the food taken in. All the other layers aside from the mucosa are 
secondary and are subsequently added to this most important absorptive 
primary lining of the food tube. 

The submucosa, next to the mucosa, is largely devoted to supporting 
a rich network of capillaries and lymphatics which bear away over the 
body the materials absorbed by the mucosa. 

Blood vessels _ . £ , ,. , ,, 
Epithelial cell 

Goblet cell 

Lumen of 



Circular muscles 
Longitudinal muscles 

Fig. 219. Part of a cross section through a frog's intestine. (From 
Mavor, General Biology, copyright 1947, by permission of The Mac- 
millan Company, publishers. After Holmes.) 

Outside of the submucosa there is a double layer, the muscularis, com- 
posed of circular muscles on the inside and longitudinal muscles on the 
outside. These muscles are involuntary in their action, except for a short 
distance at either end of the tube in the stomodaeal and proctodaeal re- 
gions, where they are under the control of the will. They effect movement 
of the food through alternate contractions by processes of segmentation 
and peristalsis. Segmentation churns the contents of the tube back and 
forth, while peristalsis forwards it. 

Protecting the muscular layers on the outside is a sustentative layer of 
tissue called the serosa, which is continuous with the mesenteries and with 
the peritoneum that lines the body cavity. In that part of the tube lying 
outside of the abdominal cavity, no serosa is present. 

5. Regions of the Tube 

Since food undergoes progressive modification as it passes through the 
digestive tube, the tube itself, as would be expected, shows structural adap- 
tations for the performance of these various tasks. Of necessity there has 


Biology of the Vertebrates 

evolved a physiological division of labor, or specialization, which has left 
its mark on the morphological features that characterize the alimentary 
tract in different regions. For purposes of description the entire tube may 
be divided into four zones, or regions, namely, ingressive, progressive, de- 
gressive, and egressive. 


Fig. 220. Silhouettes of the digestive system. (After Roule.) 

The ingressive zone is the intake region of prehension and mastication. 
It involves the lips and mouth with the teeth, tongue, and various other 
structures contained therein. The progressive zone, embracing the pharynx, 
esophagus, and stomach, is the region of forwarding the food-intake and 
passing it through the preliminary stages of modification. The degressive 
zone, coincident with the small intestine, is not only, the most extensive but 
also in a sense the most important part of all the zones, for here occurs the 
chemical preparation of the food stuffs, and their ultimate selection and 
absorption into the blood. Finally, the egressive zone, which is confined 
to the large intestine, is the region for the expulsion of the unusable residue 
that cannot be diverted into the blood and applied to the uses of the body. 
These regions are shown diagrammatically in silhouette for fishes, am- 
phibians, birds and mammals, in Figure 220. With this introduction we 
may now proceed upon an imaginary tour of inspection through the entire 
alimentary tract, with our eyes open for the anatomical scenery along the 


1. Food Capture and Prehension 

Before food can travel along the digestive highway, it must be captured 
and placed inside the entrance of the tube. This process, which may call 
for expert performance, occupies a large part of the waking hours of most 

Intake Apparatus 

2 73 

animals, and even in the case of intellectual man is the actuating motive 
behind much of his daily behavior. It is no concern of plants. 

Probably in the majority of cases the capture of food involves some sort 
of a chase, since the animal as well as its food may be in motion. Herbi- 
vores have the advantage of depending upon food that is generally sta- 
tionary, so they simply need to seek it out. Sedentary feeders, on the other 
hand, remain in one spot, catching motile food that comes their way. 
Devices of various kinds, therefore, like ciliary whirlpools or stretching ten- 
tacles, are employed by stationary animals to bring food within range. 
Many aquatic animals that are not sessile also use cilia to sweep micro- 
scopic food particles their way. The ciliated fraternity includes protozoans, 
sponges, anthozoans, bryozoans, rotifers, brachiopods, sessile annelids, 
brittle-stars, bivalves, pteropods, entomostracans, tunicates, amphioxus, and 
many larval forms. 

Many animals that are anatomically able to go in pursuit of food, suc- 
ceed better by lying in wait for passing food than by bestirring themselves 
in open chase. They have their breakfast, so to speak, served to them in 
bed. Such animals frequently develop 
camouflaging coloration, or, like 
spiders, construct elaborate snares and 
traps for their prey. Mucous threads 
are employed by certain coelenterates 
and mollusks to entangle food par- 
ticles that are then engulfed. With 
the evolution of bilateral symmetry 
and increased powers of locomotion, 
"watchful waiting" goes more and 
more into the discard and pursuit of 
daily bread becomes the more uni- 
versal method. 

When food is finally within reach- 
ing distance, there are many diverse 
organs of prehension (Fig. 221), which come into play for seizing it and 
placing it within the mouth. These adaptations range all the way from the 
slow pseudopod of an Amoeba to the reaching "boarding house arm" of 
modern man. 

Birds, possessing neither arms nor hands for taking hold of food, have 
the edge of the mouth opening drawn out into a point, forming a horny 
beak which is used as a pair of forceps in picking up things. 

The prehensile tongue of such diverse animals as toads, anteaters, and 

Fig. 221. A fish with a protrusible 
mouth, pulling an insect larva out of 
the muddy bottom. (After Hesse.) 

2-]<\ Biology of the Vertebrates 

cattle, becomes a very effective substitute for a grasping hand, while mus- 
cular lips, particularly of herbivores, serve a similar purpose in food pre- 

Some snakes, with no means for killing their prey when it is overtaken, 
seize it with their backward-projecting teeth and swallow it alive. When 
once within the mouth it cannot easily be ejected or escape, but is forced 
to inch its way down the gullet by the propalinal motion of the jaws. 

Many animals, as for example swans and giraffes, have an elongated 
flexible neek as an accessory organ of prehension, to aid in bringing the 
mouth into the immediate neighborhood of food. The trunk of an ele- 
phant, which is a drawn-out nose and upper lip combined, is a unique 
device for reaching food without the necessity of lowering the heavy head. 

Certain annelids and starfishes prehend their food by everting the phar- 
ynx, or the stomach, as the case may be, which enwraps the food and may 
even digest it outside the body. 

2. The Mouth Aperture and Lips 

The mouth is the architectural centerpiece of the face (Fig. 222). The 
shape and extent of the mouth opening varies greatly in different animals, 

depending largely upon the different kinds 
of food utilized. 

The limits of the oral slit in mammals 
are set by the fleshy cheeks. An animal 
without cheeks, like an alligator or a nest- 
ling bird, can open up the mouth to a sur- 
prising extent. 

Amphioxus and cyclostomes (Fig. 145) 

keep the mouth always open of necessity, 

1§ * " . ea ° a W ? " since structurally it cannot be closed. In 
amus, showing the projecting _ J 

face. (After Hilzheimer.) man the slit of the mouth normally extends 

from about the region of the first premolar 
teeth on one side to those on the other side, although there is considerable 
range of individual variation, as may be commonly observed. 

The puffed cheeks and rosebud mouth of infancy are muscular adapta- 
tions for sucking, mammalian characteristics which are largely lost in adult 
life (Fig. 223). The evolution of cheeks in the adult is closely connected 
with the muscular equipment for mastication, so it comes about that ani- 
mals with relatively small mouth openings are usually better able to chew 
their food than those with an expansive opening. Cheeks and chewing go 
together, for cheeks make possible the retention of food between the grind- 

Intake Apparatus 

2 7S 

ers. The retaining cheeks of cattle enable them even to chew "up hill" (Fig. 
224). The refinement of chewing food, with all its train of anatomical con- 
sequences, is a mammalian peculiarity, for it will be recalled that fishes, 
amphibians, birds, reptiles, and even many of the lower mammals, swallow 
their food without chewing it. 

Fig. 223. The 

profile of in- 
fancy. (After 

Fig. 224. The mastication 
plane of a cow which makes 
it necessary to chew "up 

In the higher vertebrates the lips are two movable folds at the edge 
of the mouth aperture. They are covered by skin on the outside and moist 
mucous membrane on the inside. The red part of a lip, an exposed zone 
of transition between skin and mucous membrane in man, is extremely 
sensitive to touch because of an abundant supply of nerve endings. The 
lower lip is more movable than the upper one. Attention to the form and 
shape of these portals to the digestive tube is shared alike by the compara- 
tive anatomist and the poet. 

3. Buccal Cavity 

Immediately within the mouth aperture of mammals is the vestibule, 
or buccal cavity, bounded outwardly by the lips and cheeks, and inwardly 
by the external face of the teeth and gums. When the mouth is closed and 
the teeth are in contact, this cavity becomes practically obliterated, but 
behind the back teeth, and between the closed teeth, there is still direct 
communication with the larger oral cavity within. 

Various glands open inside the buccal cavity. Along the inner* surface 
of the lips are numerous small labial glatids that secrete mucus. These glands 
may be easily identified by rubbing the point of the tongue back and forth 
against the inner surface of the lips, when they will be felt as tiny bunches. 
Other mucus-producing glands, the molar glands, open from the cheeks 


Biology of the Vertebrates 

into the buccal cavity opposite the back teeth, while opposite the second 
upper molar tooth on either side, is the exit of Stenson's duct that drains 
the large parotid gland (Fig. 225), from which saliva flows. It is not diffi- 
cult to locate the openings of these important ducts, for if one sticks the 
tongue into the cheek, and psychologically aids the flow of saliva by looking 

at a freshly sliced lemon, or something that 
"makes the mouth water," a tiny stream 
of saliva may be felt spurting into the 
buccal cavity. 

Birds, turtles, and monotremes with 
beaks, have dry cornified buccal cavities 
nearly devoid of glands. No one ever saw 
a bird "spit." 

Saliva, containing a digestive enzyme, 
ptyalin, is produced at the rate of as much 
as three pints a day, for the most part dur- 
ing the intake of food. Since saliva is not 
stored, the glands need periods of rest and 
recuperation between times of accelerated 
activity. The reader can draw his own con- 
clusions about the physiological results of 
the gum-chewing habit. 
On the inner face of the upper lip in the middle line, demonstrable 
by the exploring tip of the tongue, is a vertical fold of mucous membrane 
which tends to hold the lip close against the gums. This is called a labial 
frenulum. A second one occupies 

a similar median position with ref- (P\t^% IJMl-fr^-nr Jaw Bone 

erence to the lower lip. 

In some animals, such as the 
duckbill, Old World monkeys, 
apes (Fig. 226), gophers, squir- 
rels, and other rodents, the buccal 
cavity can be stretched into dis- 
tinct cheek pouches, which are 
used for the temporary storage of 

food when its collection occurs under circumstances of competition such 
as to make grabbing as much as possible in a minimum of time desirable. 
Sometimes greedy little children demonstrate their probable rise from ani- 
mal ancestry by reverting to the cheek-pouch method of excess disposal 
of food. 

Fig. 225. Salivary glands and 
their ducts. (After Cunningham.) 


Masseter Muscle 
"""•Cheek Pouch 

Fig. 226. Lower jaw of ape, showing lat- 
eral cheek pouches. (After Nuhn.) 

Intake Apparatus 


4. Oral Cavity 

Behind the mammalian buccal cavity and merging into it, is the oral 
cavity. The roof of this cavity, in higher vertebrates generally, is the arch- 
ing palate which has a skeletal foundation of bone, the hard palate, in the 
front part of it, and is supplemented behind by a flexible addition of con- 
nective tissue, the soft palate. 

The hard palate lies within the upper dental arch and is continuous 
with the gums (gingivae), that are rich in blood vessels but poor in nerves. 
The soft palate blending with the lateral walls behind the teeth, presents 
a free posterior border hanging like a curtain, in the region of the fauces, 
or the gateway leading to the pharynx. 

The posterior border of the soft palate in man is still further prolonged 
in the median line into a soft, pointed, dangling flap called the uvula, that 
projects downward and backward, and which may easily be seen hanging 
down in the back part of a wide-open mouth (Fig. 227). 

Fig. 227. Open mouth 
showing uvula, tonsils, and 
median raphe. (After Cun- 

Fig. 228. Palatine 
ridges in the roof 
of a dog's mouth. 
(After Wieder- 

Along the median line of the human hard palate, from a point near 
the upper median incisor teeth and fading out toward the region of the 
soft palate, is a faint ridge, the raphe, which indicates that the hard palate 
is formed by the union of two lateral components. It may be felt, in those 
individuals who still have it present in the roof of the mouth, by means of 
the tip of the tongue. 

In many instances there may also be similarly demonstrated a st-ries of 
transverse folds or ridges at right angles to the raphe, the palatine rugae, 

2 7 8 

Biology of the Vertebrates 

diminishing in size from the region of the teeth backward. The rugae are 
more in evidence in human embryos than in adults, although they not in- 
frequently persist throughout life. They are wash-board like in character 
and find their highest development in such carnivores as cats and dogs 
(Fig. 228), where no doubt they aid in securing a surer grip upon any 
struggling victim that has been seized in the jaws. 

The surface of the entire palate, particularly of the soft palate and 
the uvula, is beset with numerous palatine glands, whose secretion of mucus 
helps to keep the mouth cavity moist. 

Upper Lip 

~ — Tip of Tongue 

of Tongue 

--Sublingual Ridge 

Opening of 

Wharton's Duct 

-Frenulum of 
Lower Lip 

Lower Lip 

Fig. 229. The mouth, widely opened, with the tip of the tongue drawn 
upwards, to show the frenula of the tongue and lower lip, the openings 
of Wharton's ducts, and the sublingual ridges. (After Toldt.) 

The sides of the oral cavity posterior to the back teeth blend with the 
buccal cavity into a common space, while the floor is largely occupied by 
the bulky tongue, which fills practically the entire cavity when the mouth is 
closed. When the mouth is opened wide and the tongue is raised and curled 
back, the frenulum linguae may be seen in the shape of a fold of connective 
tissue along the midventral region, that tends to hold the tongue down to 
the floor of the oral cavity (Fig. 229). Occasionally, when the lingual 
frenulum is overdeveloped in a human infant, such an individual is said 
to be "tongue-tied," and a slight surgical operation is necessary before the 
tongue can acquire the freedom of movement essential for clear articulation 
in speech. 

Extending on either side of the frenulum linguae in man, and parallel 
to the lower teeth, is a crescentic fold of tissue, called the sublingual ridge. 
Along this ridge open the several ducts of Rivinus from the sublingual sali- 

Intake Apparatus 279 

vary glands, while at the widest part of the frenulum linguae, near the 
lower median incisor teeth on either side, are the openings of Wharton's 
ducts, that drain the submaxillary salivary glands. Thus, three sets of sali- 
vary glands, the parotid, sublingual, and submaxillary, pour their digestive 
and lubricating secretions of saliva into the buccal and oral cavities. 

This differentiation of mouth glands into various mucous and salivary 
glands common to mammals does not appear among the lower vertebrates. 
Fishes, which bolt their food without chewing, do not have digestive sali- 
vary glands, while mucous glands, the mission of which is to moisten the 
food in the oral cavity preparatory to swallowing it, are also unnecessary 
and practically absent. 

Among amphibians, living on the border line between submergence 
in water and life on land, scattered mucous glands, termed intermaxillary 
glands from their generalized location, make their appearance in some in- 
stances, while the protrusible tongue, particularly in frogs and toads, is 
supplied with lingual glands, secreting a viscous mucus that aids in the 
capture of insects and other moving prey. 

In reptiles the mouth glands are more grouped and localized, so that 
it is possible to speak of palatine, lingual, sublingual, and labial glands, 
according to their location. All of these glands produce fluid that moistens 
the food and renders the act of swallowing easier, although it is doubtful 
if they aid appreciably in digestion. 

Fig. 230. Poison gland of a rattlesnake, with duct passing into fang. 
(After Kingsley.) 

Poison glands in the mouth of certain snakes (Fig. 230), are trans- 
formed parotid glands, while those of the only lizard known to be poison- 
ous, the "gila monster," Heloderma, of the southwestern United States, 
are modified sublingual glands. 

Birds, as noted, have a paucity of oral glands. 

In the case of mammals, which usually chew their food to some extent, 
mouth glands of two general sorts are universally developed, mucous and 


Biology of the Vertebrates 

salivary, for the double purpose of lubrication, and of liquefaction and 
chemical modification. Mucous glands are especially essential for herbi- 
vores that consume large quantities of comparatively dry, bulky food. The 
action of the salivary glands, which is both chemical and mechanical, will 
be referred to later in the consideration of digestive glands in general (see 
Fig. 271). 

5. Tongue 

What passes under the name of "tongue" in the vertebrate series is not 
always strictly comparable to the "unruly member" in man (or woman), 
which must be regarded as the outcome of a long sequence of adaptations. 

v*.f i Rostrum 

^ — Nasal Capsule 

^.-Orbital Cavity 
.- Pterygoquadrate 
_ — Labial Cartilage 

**=*- Hypobranchial 
^-Ceratobranchial HI 
-Gill Rays 

Pharyngobranchial V 


Fig. 231. Diagrammatic ventral view of the splanchnocranium and 
neurocranium of Squalus acanthias. The basihyal cartilage, lying between 
the two sides of the lower jaw, is the skeletal basis of the fish's tongue. 

Amphioxus has no tongue at all, and the muscular piston-like tongue 
of cyclostomes is such an aberrant, highly specialized structure that it gives 
no safe clue to the true beginnings of this organ among vertebrates. 

In fishes, however, a primary tongue makes its definite appearance. It 
is a non-muscular elevation from the floor of the mouth cavity, consisting 
of a covering of mucous membrane, stretched over a skeletal support of 

Intake Apparatus 


cartilage or bone, derived from the framework of the gills ( Fig. 231). A 
projecting basihyal cartilage, that lies between the lower jaws of the man- 
dibular arch, is the skeletal basis of this kind of a tongue. Whatever move- 
ment it is capable of is due to extrinsic muscles that act upon the skeletal 
support in such a way as to enable it to change position but not shape, 
rather than upon intrinsic muscles that modify both shape and position. 
It is also not protrusible, although motile enough to aid somewhat in forc- 
ing back a mouthful of food to be swallowed, and, in some cases, is beset 
with prehensile teeth. 

Primary Tongue- 
' Field 

Glandular Field 




Fig. 232. Median section through the floor of the mouth, showing the 
formation of the glandular tongue, a, Triton alpestris; b, Salamandra 
maculosa. In the latter the glandular field is encroaching upon the pri- 
mary tongue to form the secondary tongue. (After Haller. ) 

The lower amphibians, such as the perennibranchiate urodeles, have 
fishlike tongues of mucous membrane with cartilaginous support. In the 
higher salamanders the horseshoe-shaped groove between the primitive 
tongue and the lower jaw becomes elevated, particularly in front, into a 
glandular field (Fig. 232), in which a glutinous mucus, useful in entang- 
ling captured insects, is secreted. This glandular field gradually rises, thus 
obliterating the original groove around the under edge of the primary 
tongue, until finally it becomes incorporated with the latter as an anterior 
projection, forming the so-called secondary tongue. 

In the median line at the junction of the primary and secondary 
tongues, and originally connected with the thyroid gland, there is a tubular 
down-growth, the thyroglossal duct that persists in mammals as the foramen 
caecum (Fig. 233). 

The secondary tongue soon becomes invaded by intrinsic muscles, 
which greatly increase the range of its movements, and make changes in 
its shape possible. Of these muscles the genioglossals act as protractors, and 
the hypoglossals, as retractors. In the American salamander Eurycea, they 
become so efficient that the sticky tongue may be shot out a considerable 
distance and retrieved with incredible speed in the capture of insect prey. 


Biology of the Vertebrates 

Epiglottis — 'MiMmf' 

Root of Tongue ^sr. 

Filiform Papillae 
Fungiform Papilla -If 

;„ ,11 

ILjSw-"- Lingual Tonsils 

f$&%. — Palatine Tonsil 
BUS (in Section) 

Foramen Caecum 

Vallate Papilla— a 

Foliate Papilla jffl*4fyjjjj^g^E§ Median Raphe 

Vl .» /t^b> : ^&^f ""^Sulcus Terminalis 

^';-M /: i^^0--- Body of Tongue 

(Upper Surface 
or Dorsum) 

Fig. 233. Dorsal view of the human tongue. (After Toldt.) 

The secondary tongue of most frogs and toads, which is attached far 
forward on the floor of the mouth cavity, is retroflexed when at rest, so 
that its point lies backward down the throat. When it is flipped out after 
an insect (Fig. 234) or a slug, it is "swallowed" upon its return, along with 
the captured food, and thus restored to its original position. One family of 

toads, including the genera Pipa and 
Xenopus, is named Aglossidae, because 
in these exceptional animals, the tongue 
is either absent or very poorly developed. 
Reptiles embryonically possess a 
double tongue, like that evolved by am- 
phibians, although with considerable 
modification. In turtles and alligators it 
is thick and only slightly protrusible, 
whereas in snakes and lizards it may 
become extremely extensible. The little 
wall lizards, or "geckos," for example, 
can easily lick the outside of their transparent eyelids with their tongues, 
while snakes can protrude their delicate sensitive forked tongues for some 
distance through a median notch in the edge of the lower jaw, without 
opening the mouth. 

The chameleon (Fig. 41), an arboreal African lizard famous for its 
kaleidoscopic color changes, while grasping the twig of a tree uses its long 
tongue like a lasso in entangling its elusive prey, in much the same way 
as the salamander Eurycea from a position on the ground shoots out its 

Fig. 234. 
a fly. 

Tongue of a toad catching 

Intake Apparatus 



tongue. The mechanism in the two cases is somewhat different. In Cham- 
eleon the bony framework of the primary tongue acts as a system of exten- 
sible levers to supplement the secondary muscular component of the tongue 
in its protrusion, which is not the case with Eurycea. 

In birds the bony framework of the primary tongue, which supports the 
secondary tongue, is especially well developed. This framework consists 
typically of a median bone or bones, the copula 
(Fig. 235), and two pairs of lateral bones, the 
small hyoids, and the first branchials, all of 
which are relics of ancestral gill arches. Its 
movement is facilitated by means of extrinsic 
muscles attached to these bones, the intrinsic 
muscles of the secondary tongue being reduced 
or absent. 

A woodpecker, whose horny spearlike 
tongue can be projected out of the long beak 
when impaling a grub in the bark of a tree, 
possesses an elaborate skeletal hyoid apparatus 
attached at the base of the tongue, and with 
long posterior horns (first branchials) lying just 
beneath the skin. When at rest each of these 
horns extends from the tongue into the neck, 
then dorsally and forward over the top of the 
skull, reaching even into the base of the beak 
(Fig. 236). As the tongue is extended the 
springy supporting hyoid coils are straightened 
out through the action of muscles, while the 
withdrawal of the tongue to its original posi- 
tion within the beak is accomplished by the 
elasticity of the hyoids which snap back 

into place like released watch-springs that have been temporarily straight- 
ened out. 

The mammalian tongue, like that of reptiles, is made up of two parts. 
The anterior region, somewhat rough and covered with numerous small 
elevations (papillae) of various shapes, is separated from the posterior part, 
bumpy in appearance due to masses of lymphoid tissue (lingual tonsils), 
by a V-shaped groove, the sulcus terminalis (Fig. 233). In the mid-line, at 
the posteriorly directed apex of the sulcus, is a small invagination, the fora- 
men caecum, the remains of the thyroglossal duct by which the embryonic 
thyroid gland communicated with the oral cavity. 

Fig. 235. Tongue appara- 
tus of a pigeon. Basihyal cor- 
responds to primary tongue. 
To the branchial arch ele- 
ments (basibranchials and 
posterior horns of hyoid ap- 
paratus) are attached the 
extrinsic muscles of the 
tongue. Cartilaginous parts 
are stippled. (After Parker 
and Haswell.) 

28 4 

Biology of the Vertebrates 

The papillae on the anterior section of the tongue, usually associated 
with taste buds, are of four types, namely, filiform, fungiform, foliate, and 

[ Posterior Horn of Hyoid N 

(Branchial I) 

Body of Hyoid 



Fig. 236. Schematic representation of the position of the hyoid appara- 
tus in the woodpecker, a, with the tongue withdrawn; b, with the tongue 
extended. (After Leiber.) 

Filiform papillae are tiny threadlike or conical projections that are 
largely responsible for the velvety appearance of the surface of the tongue. 
They are not particularly associated with taste buds, although they serve to 
retain food solutions temporarily. In many mammals the filiform papillae 
become capped over with corneal material, taking on a mechanical rasp- 
like character, as shown by the tongues of cats and cattle, who use this 
device not only in eating but also as a hair brush. 

Fungiform papillae (Fig. 237) are elevations from the surface of the 
mucous membrane that suggest the shape of a mushroom, hence their 
name. They are beset with taste buds and serve to bring these chemical 
receptors into contact with food solutions in the mouth cavity. Over the 
surface of the human tongue there may be as many as three or four hun- 
dred of these papillae, but they are always better developed in children 
than in adults. They are more numerous along the sides of the tongue than 
elsewhere, and have the appearance of small red spots. 

The foliate papillae, which are usually located near the base of the 
tongue, are tiny ridges bearing taste buds. In man there are only three to 
eight of these ridges, but in rodents their number and size is greater. 

The most elaborated of all the modifications for the display of taste 
buds are the vallate papillae. These resemble projecting knobs, surrounded 
by deep grooves, like the moat around a mediaeval castle, that serve to re- 
tain dissolved food substances. In the human fetus taste buds are distrib- 

Intake Apparatus 


uted even over the tops of the knobs, but in adults they are confined to the 
sunken walls of the moats, where they are not only in direct contact with 
solutions to be tested, but are also protected from mechanical injury to 
which they would be liable at the surface. 

Fungiform Papillae 

^-Epithelium x'^'^yii 

r~--Taste Buds 'V^ 

Von Ebner's Glands -\W&J'(f 

Fig. 237. Portions of the tongue of man showing lingual papillae in 
surface view and in section. Anterior toward the left in each drawing. 
(After Braus.) 

Serous glands, called Von Ebner's glands, open at the bottom of the 
moats and aid in keeping them filled with fluid. 

Vallate papillae are usually arranged in rows at the back of the tongue. 
There are two rows in monotremes, moles, bats, hares, pigs, horses, and 
edentates; three rows in marsupials, squirrels, many insectivores, and apes; 
four rows in the monkeys, Macacus and Cercopithecus ; and a single row, 
arranged in a V-shaped formation in front of the sulcus, in the dog and man. 
They are missing in guinea pigs and coneys. 

The posterior part of the tongue is derived from the bases of the hyoid 
and first two branchial arches, while the anterior or secondary tongue 
arises embryonically from a median and a pair of lateral swellings. In the 
human embryo of about four weeks of age, the secondary tongue first ap- 
pears as an elevation from the floor of the mouth cavity just anterior to the 
landmark of the ductus thyroglossus. This elevation, which is homologous 
with the "glandular field" of the amphibians, is called the tuberculum im- 
par (Fig. 238). On either side of it are lateral lingual swellings from the 
inner surfaces of the two sides of the skeletal mandibular arch, which meet 
at this point. These swellings soon increase until they completely surround 
the tuberculum impar, eventually forming the bulk of the anterior part of the 
tongue. In somewhat similar fashion the copula, the region enveloping the 
basihyal skeletal part that forms the foundation of the primary tongue lying 

2 86 

Biology of the Vertebrates 

A & B 

Fig. 238. Stages in the development of the human tongue. A, 6 mm 
embryo; b, 15 mm embryo. Contributions from the first three branchial 
arches (/, 77, III ) are indicated respectively by horizontal parallel lines, 
dots, and crosses; the tuberculum impar is marked by circles. (After 
Arey. ) 

behind the thyroglossal duct, is augmented by additions from the neighbor- 
ing hyoid and anterior branchial arches, to form the "root" of the tongue, 
the part lying in the pharyngeal cavity (Fig. 239). 

Anlage of Tongue 

.-•V'-.r Tuberculum 
\>j;/ Impar 

Fig. 239. Floor of the mouth and pharynx of a 7.5 mm human embryo, 
from a reconstruction. The swellings on either side, indicated by the 
halo of dots on the first branchial arch, give rise to the body of the 
tongue. (After McMurrich.) 

The tongue of mammals serves many purposes and in consequence of 
the detachment of its anterior portion from skeletal elements is capable of 
great freedom of movement. It keeps the food between the teeth during 
the process of chewing, and starts it on its way when it is ready to be swal- 
lowed. It is also decidedly prehensile in many herbivores as already men- 
tioned. Cows, for example, can grasp a tuft of grass with the tongue, to be 
sickled off against the lower incisors. It is a universal toothbrush giving 

Intake Apparatus 

28 7 

point to the phrase "as clean as a hound's tooth," and it also serves as a 
currycomb for fur-bearers, while animals like cats and dogs that lap up 
liquids use it as a spoon. Finally, its dorsal surface is thickly beset with sense 
organs of touch and taste, which stand in readiness to receive the password 
of admittance from entering food. In the human female it measures about 
three and a half inches in length — when at rest. 

6. Teeth 

Teeth are primarily devoted to the manipulation of food within the 
mouth cavity, to purposes of grasping, cutting or grinding, although in 
some instances they secondarily assume other functions, such as prehension 
of food, defence, offence, or even as aids in locomotion, as in the case of 
the walrus which uses its tusks in dragging its slippery body out of arctic 
water on to ice (Fig. 60). 

The extreme diversity of teeth, adapted to their many uses, affords the 
comparative anatomist much insight into the manner of life of different 
animals, while to the palaeontologist they 
are preserved tokens which, like hiero- 
glyphics, aid in reconstructing the story of 
the long-vanished past. 

Teeth are the first hard structures of 
the body to put in an appearance during 
vertebrate development, even before any 
part of the bony skeleton. Although they 
eventually come into intimate secondary 
relation with the skeleton, they are in real- 
ity derivatives of the stomodaeal region of 
the alimentary tract. Thus these integu- 
mentary derivatives, homologous with pla- 
coid scales, become morphologically, as 
well as physiologically, a part of the diges- 
tive system. 

(a) Structure. — In structure a typical 
mammalian tooth (Fig. 240) consists of a 
crown which projects beyond the gums; 
roots that are embedded in a socket of the 

jaw ; and the neck, which is the transitional region between the crown and 
roots. Inside the hollow tooth is the pulp cavity, harboring blood vessels and 
nerves that gain access through a passage-way usually remaining open at the 
base. So long as this opening is unobstructed the tooth can continue to grow, 

A Enamel 

L r Dentine 

-t — Cement 


Fig. 240. Diagrammatic long sec- 
tion through a typical canine tooth. 

288 Biology of the Vertebrates 

as gnawing teeth of rodents do, by means of inside additions of tooth mate- 
rial. In most of the teeth of higher vertebrates, however, the opening of the 
pulp cavity becomes so constricted that after a certain size is attained growth 
ceases and, as the tooth wears away by attrition on the outside, there is no 

The solid part of the tooth is three-fold in character. The bulk of it is 
dentine, or "ivory," a tissue denser than bone but, like it, permeated by tiny 
radiating canals, due to the fact that the dentine material is secreted 
around the branches of embryonic cells, the odontoblasts. 

Over the crown, wherever exposed to wear, the dentine is usually pro- 
tected by a layer of enamel, likewise penetrated by very minute canals in 
the lower forms but solid and prismatic in structure in higher vertebrates. 
Although not cellular in itself, enamel is the product of cellular activity 
and is the hardest, densest, most enduring part of the vertebrate body. 

Outside of the dentine around the roots of the tooth, in those cases 
where the tooth is set in a socket, there is a bonelike substance, cement, 
that anchors the tooth firmly to the jaw. In ungulates the cement extends 
over the crown. 

The composition of the dentine and enamel in the human tooth is given 
by Owen as follows: 

Dentine Enamel 

Calcium phosphate and fluoride . . . 66.72 89.82 

Calcium carbonate 3.36 4.37 

Magnesium phosphate 1.08 1.34 

Other salts .83 .88 

Organic matter 28.01 3.59 

100.00 100.00 

(b) Development. — In the development of teeth six steps may be rec- 
ognized, namely, dental lamina, enamel organ, dental papilla, crown for- 
mation, root formation, eruption. 

About the seventh week in the development of the human embryo, 
certain Malpighian cells of the epidermis along the edge of the jaws, where 
the future teeth are to be, start into accelerated activity, pushing down into 
the underlying dermal tissue in the form of the so-called dental lamina 
(Fig. 241). Along this lamina at intervals wherever a tooth is destined 
later to appear, groups of these Malpighian cells proliferate into spherical 
enamel organs, which later lose their connection with the dental lamina. 
Under each enamel organ a tubercle of mesenchymal cells constituting a 

Intake Apparatus 


dental papilla is formed, and presses the enamel organ into the form of a 
double-walled cup. 

Crown formation follows through the interaction of cells of both papilla 
and enamel organ. The cells of the dental papilla are odontoblasts which 
secrete the dentine on their outer surfaces, thus producing the bulk of the 
tooth. The cells of the enamel organ next to the odontoblasts are amelo- 
blasts which secrete a cap of enamel on the dentine. Meanwhile, capillaries 
and nerve endings invade the dental papilla and occupy the beginning of 
the pulp cavity. 

— Mafpighian 

*• Dental 

^f-.-j-r'rf-r-r-i — Dental 




Side of Jaw 

— -Ameloblasts 

^^T-^T - "" Odontoblasts 


-Dental Papilla 
'Dental Lamina 

S>?^uP^C^ Ameloblasts 

rakoEKSdf-- Enamel ) °{ 



I Milk 
--Dentine J Tooth 

Germ of 
Permanent Tooth 

Fig. 241. Development of teeth. A, dental lamina stage; b, enamel organ 
stage; c, D, early and later dental papilla stage; e, crown formation. 
(a-c, after Maximow and Bloom; d and E, after Parker and Haswell.) 

Root formation occurs sometime later, beginning just prior to the 
eruption of the tooth. The cells of the edge of the invaginated cup have 
continued to extend deeper into the tissue of the jaw but these deepest cells 
do not secrete enamel. Consequently the root, formed entirely by odonto- 
blasts, is composed solely of dentine. As the root elongates it pushes the 

290 Biology of the Vertebrates 

completed crown through the enamel organ and more superficial tissues 
until the crown emerges and becomes almost entirely exposed. This process 
of eruption is known as "cutting the teeth." Around the dentine of the em- 
bedded roots of each tooth is deposited, through the activity of neighboring 
mesenchymal cells from the derma, the cement tissue, which aids in fixing 
the tooth in the jaw. 

Permanent teeth of man are formed in much the same manner as de- 
scribed above for the milk teeth. The enamel organ arises, however, from 
the original dental lamina, on the lingual side of the first tooth germ in the 
case of teeth which are to fill in places vacated by "first teeth." 

Like the placoid scales of elasmobranch fishes with which they are 
homologous, teeth are compound structures of diverse origin, arising from 
ectodermal ameloblasts, mesodermal odontoblasts, and mesenchymal cells. 

The horny, rasplike "teeth" of the jawless cyclostomes are entirely ecto- 
dermal structures composed of cornified cells and not homologous with the 
teeth of other vertebrates. 

(c) Number. — Lower vertebrates generally have an indefinite number 
of teeth, but in mammals the number becomes definite and limited. A re- 
duction in the number of teeth is a mark of evolutionary advance associ- 
ated with terrestrial life, less food, more chewing, shorter jaws, and stronger 
muscles of mastication, whereas an increase in the number of teeth, such 
as occurs secondarily in dolphins and other toothed whales, may be re- 
garded as a reversion to ancestral conditions in connection with aquatic 
life, more abundant food, and less need for mastication. 

There are some toothless species representing every class of vertebrates. 
Among fishes may be mentioned the sturgeon, Acipenser, and the seahorses 
and pipefishes. Coregonus wartmanni, a whitefish native to Lake Con- 
stance in Switzerland, is a toothless member of a large family of toothed 
fishes (Salmonidae), although this aberrant species has transient embryonic 

Toads, and among urodeles Siren at least, are toothless, while frogs 
have no teeth on the lower jaw. Among reptiles the entire order of Chel- 
onia, which includes turtles and tortoises, are without teeth, although in 
Chelonia and Trionyx a reminiscent dental lamina develops temporarily 
in the embryo, only to fade away as the horny beak becomes ascendant. 
Several extinct fossil reptiles, for example, Oudenodon, Baptanodon, and 
Pteranodon, are likewise known to have possessed beaks instead of 

All modern birds are toothless. That this condition was not always the 
case, however, is shown by the presence of well-developed teeth in Archae- 

Intake Apparatus 291 

opteryx, and in the Cretaceous birds of Kansas, Ichthyornis and Hesperor- 
nis (Fig. 49). Embryonic teeth, of which there is ordinarily no trace in 
birds, have been found in the tern, Sterna. 

Among mammals, monotremes are without teeth, also the edentate 
Myrmecophaga and the pholidote Manis, and the large whales (Mysta- 
coceti) . 

A curious instance of hereditary toothlessness in man is reported by 
Thadani * from Hyderabad Sind in India, where there is an inbred com- 
munity in which the males never have any teeth. They are called "Bhudas," 
which means "toothless." This abnormality is accompanied by baldness 
and extreme sensitivity to heat, and the peculiarity follows the well-known 
laws of Mendelian inheritance, being a recessive sex-linked character. 

All of these widely different toothless mammals, however, furnish em- 
bryonic evidence that, with respect to this characteristic, they are degen- 
erate descendants of ancestors with teeth. 

(d) Succession. — Most of the lower vertebrates are polyphyodont, that 
is, they have a continuous succession of teeth throughout life. This is exem- 
plified particularly in sharks and dogfishes, where the reserve "understudy" 
teeth may be seen arranged in diminishing rows behind the line in active 
service at the edge of the jaw. The continuous gradation over the margin 
of the jaw that separates the serried rows of elasmobranch teeth from 
the placoid scales of the skin, points unmistakably to a common plan of 
structure and accounts for vertebrate teeth as modified scales (Fig. 169). 

Mammals are typically diphyodont, that is, they have a replacement 
of so-called permanent teeth following the first temporary milk dentition, 
which allows the young to chew their food at a time when the jaws are too 
small to accommodate permanent teeth. 

Certain marsupial embryos show traces of a still earlier dentition lo- 
cated in the arch between the milk teeth and the lips. Sometimes in ex- 
ceptional cases mammals produce an additional partial replacement of the 
"permanent" teeth in late life, making a total of four possible successions, 
namely, prelacteal, lacteal, definitive, and post-definitive, all of which sug- 
gests that typical diphyodontism of mammals has been derived from the poly- 
phyodont condition of lower forms. Bolk has pointed out that in diphyodont 
dentition the replacement comes from a different rudiment than that which 
gives rise to the first lacteal dentition, so that it is possible to have represent- 
atives of both dentitions present and on duty at the same time, whereas in 
polyphyodontism of the lower forms the succeeding tooth in each case 
arises from the same germ as its predecessor, thus preventing the intercala- 

* Journal of Heredity, Feb., 1921. 

292 Biology of the Vertebrates 

tion of one active generation of teeth with those of another succeeding 

There is, moreover, a tendency among mammals toward a still further 
reduction to a monophyodont condition. Marsupials, for instance, retain 
all their milk teeth except the last premolars, while certain insectivores, 
like the moles, Scalopus and Condylura, never cut their permanent teeth. 
The toothed cetaceans (Odontocoeti), and some rodents, as well as the 
reptile Sphenodon, may also be described as monophyodont. Bats and 
guinea pigs have so far foreshortened the normal procedure of tooth suc- 
cession as to shed their lacteal teeth in utero, coming into the world with 
their definitive teeth already established. 

It is related of Mirabeau, the great orator of the French Revolution, 
that he was born with teeth already cut, which if true must have been hard 
on his nurse. Such an abnormality is said to occur as rarely as once out of 
15,000 times. 

Ordinarily the eruption of milk teeth in man is accomplished in about 
two years, although it is not unusual for the second milk-molars to come 
a half year later. The appearance of the permanent dentition begins with 
the eruption, during the sixth year, of the first molars, just posterior to the 
last milk teeth. The next year the milk teeth' begin to drop out and ordinar- 
ily all of them are lost by the end of the twelfth year although an individual 
may carry some representatives of the lacteal dentition until much later 
in life. As rapidly as the milk teeth are lost, permanent ones take their 
places. After the completion of this gradual replacement, two additional 
molar teeth erupt behind each sixth-year molar, with the last permanent 
tooth, "wisdom tooth," being fully formed by the twentieth year in most 

Milk teeth differ from permanent teeth by their smaller size, whiter 
color, and by their shape, being more constricted in the neck region and 
having a greater spread of roots in the case of the back teeth. 

(e) Situation. — While the teeth in fishes and other aquatic animals 
occur attached to various skeletal foundations within the mouth cavity, 
such as the vomer, palatine, pterygoid, parasphenoid, and even on the 
tongue, on the hyoid and gill arches, in reptiles and mammals they are us- 
ually confined to the jaws, although in some snakes, and in Sphenodon, 
they occur also in the roof of the mouth on the vomer and palatine 

Teeth of the upper jaw are interspaced with reference to those of the 
lower jaw. In man the large median upper incisors bite against not only 
the median but also the lateral incisors of the lower jaw, and every other 

Intake Apparatus 


tooth of the upper jaw, except the last molars, bites against the correspond- 
ing tooth of the lower jaw and also the tooth behind it. 

(/) Attachment. — The manner in which teeth are attached to their 
skeletal support is dependent upon the degree to which the roots are de- 

The simplest type of attachment, termed acrodont (Fig. 242), occurs 
in teeth essentially without roots that 
are held to the edge of the jaw or 
other skeletal foundation either by fib- 
rous membrane, or ankylosed directly 
to the bone in shallow pits. Such teeth, 
which are broken of! easily, are poly- 
phyodont. In some cases they are 
hinged on by a ligamentous base and 
may be folded down when not in use, 
as in the pike and hake among fishes, as well as in many kinds of snakes. 
Fishes as well as amphibians are generally acrodont. 

An improvement over the acrodont method is seen in certain urodeles 
(e.g. Necturus) and lizards, where not only the base but one side of the 
tooth is involved in attachment to a shelflike ledge along the inner margin 
of the jaw (Figs. 242 and 243). By this method, which is called pleuro- 
dont, the blood and nerve supply enters at the side, as in acrodont teeth, 
instead of at the tip of the root. 

Fig. 242. 

Pleurodont Thecodont 

Types of attachment of 

teeth to jaws. (After Wiedersheim.' 

Fig. 243. Medial view of a lizard jaw bearing pleurodont teeth. (After 
Hilzheimer. ) 

The highest and most efficient type of tooth has well-developed roots 
set in bony sockets in the jaw, a method of attachment known as thecodont 
(Fig. 242), by which the capillaries and nerves enter the pulp cavity 
through the open tips of the hollow roots. 

Some reptiles are thecodont, alligators and crocodiles particularly, but 
this type of tooth attachment is more characteristic of mammals, in some 
of which the teeth have progressed much beyond the primitive grasping 
function, and consequently require a stronger anchorage than is afforded 
by either the acrodont or pleurodont methods. 


Biology of the Vertebrates 

Fig. 244. Teeth of a rodent, Geomys, 
showing diastema, or toothless space in jaw 
between incisors and molars. (After Bailey.) 

The incisor teeth of gnawing rodents are so deeply set in bony sockets 
of the jaws that they become very effective tools, as for example the in- 
cisors of the gopher Geomys ( Fig. 244 ) . The beaver Castor, in its engineer- 
ing operations, can cut down large trees with such teeth. 

(g) Movement. — Various types of movement for teeth set in jaws are 
made possible by the muscles of mastication. The commonest type is verti- 
cal, or orthal, movement, which 
consists in lifting up the lower jaw. 
Just as in a nutcracker, the farther 
back toward the angle of the jaw 
the work is done, the more power- 
ful is the effect. 

In carnivores the back teeth cut 
past each other like the blades of a 
pair of scissors. Ungulates which 
chew the cud with a sidewise mo- 
tion have a lateral method of jaw 
movement, while horses, elephants, 
rodents, and some other herbivorous 
animals practice a "fore and aft" 
movement. In all of these movements the effective use of the jaw involves 
the teeth on one side at a time, those of the opposite side being temporarily 
not in contact. 

Snakes with their sharp backward-projecting prehensile teeth use the 
fore and aft movement to advantage in relentlessly passing along struggling 
prey down the throat. In fact it works so automatically that a snake finds 
it difficult to eject a mouthful, once started in the proral-palinal mill. 

In the higher vertebrates still other modifications in the movement of 
the jaws may be noted. Dr. Hooton observes that "with the shortening 
of the canines the human stock developed certain rotary movements of 
mastication which may be observed in any gum-chewing stenographer." 

(h) Differentiation. — According to their degree of differentiation, teeth 
are described as homodont and heterodont. Teeth when practically all alike 
are called homodont, but if they are differentiated to serve a variety of 
uses, such as gripping, tearing, cutting, or crushing, they are known as 

The teeth of primitive water-dwelling vertebrates are commonly homo- 
dont, since aquatic animals do not chew. They are usually pointed or cone- 
shaped and adapted to serve as prehensile organs. Ordinary vertebrates 
with homodont teeth gulp their food whole. 

Intake Apparatus 

! 95 


Fig. 245. A human jaw, showing by ar- 
rows the two general types of differentiation 
in teeth from the primitive pointed canine 
teeth represented in black. 


In evolutionary history, hetero- 
dontism arose along with experi- 
menting upon a variety of foods and 
with the consequent occasion for 
chewing. The mammal-like therap- 
sid reptiles and the mammals them- 
selves are heterodont. The back 
teeth near the hinges of the jaws 
where the leverage is greatest be- 
come modified into grinding pre- 
molars and crushing molars, or 
"cheek teeth," while the front teeth, 
notably in the case of rodents, be- 
come specialized into cutting chisels, or incisors, to divide the food into mor- 
sels of convenient size for the grinding mill of the back teeth. Probably the 

most ancestral and least changed of 
all heterodont teeth are the cone- 
shaped canines, between the incisors 
and premolars, which resemble the 
pointed grasping teeth of the homo- 
dont type. On either side of the ca- 
nines, as a point of departure, modifi- 
cation has taken place progressively 
and in divergent fashion, as indicated 
by the arrows (Fig. 245), on the one 
hand toward the flattened, more 
chisel-like type of the incisors, and on 
the other, toward that of the flat- 
topped premolars and molars. 

Homodontism in the dolphin and 
other toothed whales is shown to be 
secondary by the fact that a fossil an- 
cestral whale, Zeuglodon, was hetero- 

The heterodontism of carnivores 
(Fig. 246) is characteristically differ- 
ent from that of herbivores. In the 
former case the sharp edges of certain grinders fit past each other like 
shears, for cutting up animal food, the grasping canines are prominent, 
and the back molars tend to become degenerate. It is the fourth premolars 








Fig. 246. Teeth of the upper left jaw 
of various carnivores. The vertical line 
passes through the fourth premolars, or 
carnassial teeth. (After Boas.) 



Biology of the Vertebrates 

Fig. 247. Selcnodont dentition, or molars with crescentic surfaces of 
hard enamel. 

on the upper jaws and the first molars on the lower jaws which have devel- 
oped this special tearing or shearing ability, for which reason they are 
called carnassial teeth. In the herbivore type of heterodont dentition the 
more anterior cheek teeth show degeneration, the canines 
being suppressed, while the posterior grinders near the 
hinge of the jaw become flattened and enlarged so as to 
crush seeds, fruits, nuts, and herbage of all sorts success- 

The molars of ruminants present a flat grinding sur- 
face further diversified by crescentic ridges of projecting 
enamel, alternating with softer dentine. Since the dentine 
wears away more rapidly than the enamel ridges, the 
enamel is constantly kept with sharp edges, and at the 
same time a rasp-like abrasive surface on the grinding teeth 
is maintained. Such crescentic-surfaced teeth are said to 
be selenodont (Fig. 247). Similar enamel ridges are present 
on the molars of elephants, the arrangement of which in 
transverse lines instead of in crescents makes a washboard- 
like pattern, described as lophodont (Fig. 248), that is par- 
ticularly effective in connection with the palinal or from 
behind forward movement of the jaws. 

In man and some other mammals, the grinding surface 
of the molars is raised slightly into separate rounded tubercles 
and, being entirely covered with enamel, wears away more evenly. This is 
described as the bunodont type of teeth. 

It is illuminating to know that some of the ancestral elephants, Palaeo- 
mastodon for example, were bunodont, while their more specialized de- 
scendants of today have become lophodont. 

Finally, to add two more "donts" to this descriptive vocabulary of the 
teeth, the term brachydont applies to teeth with short crowns and compara- 
tively long roots, as in man, while the term hypsodont characterizes teeth 

Fig. 248. 

Grinding sur- 
face, partly 
worn, of right 
upper molar 
of African el- 
ephant. (After 
Owen. ) 

Intake Apparatus 


with short open roots and long crowns, such as are found in the dentition 
of the horse, in the tusks (incisors) of elephants, and the canines of 

(z) Dental Formulae. — In the case of different species that have hetero- 
dont teeth, it is useful to express the degree of their diversity in some con- 
venient and compact form. This is accomplished by means of dental 
formulae. For example, the permanent dentition of man may be expressed 

2 12.3.. 
as follows: ' ,, ' ' , in which the figures above the horizontal line indi-' fe 

cate in order from left to. right the number of incisors, canines, premolars, 

and molars on the right side of the upper jaw, while the figures below the 

line stand for the corresponding teeth in the lower jaw. It is unnecessary 

of course to indicate the teeth on the left side, which are like those on the 

right side except in reverse order. 

The short-tailed monkeys (Catarrhini) of the Old World have the same 

dental formula as man, but the long-tailed monkeys (Platyrhini) of the 

New World have an additional premolar all around, making the formula 

2 13 3 

' . ' ' , with a total of thirty-six. 

Some other dental formulae are as follows: 



basic 3.1.4. 3 
placental 3.1.4. 3 ' 



3.1.0. 5 '' 






3.1.4. 2 


. 1 



3 . 

. 1 . 




. 1 




, 1 . 



3 . 

. 1 

. 3 , 


1 , 




1 , 


, 1 . 





. 3 





Plg 3. 

, 1 



3 , 

, 1 


. 3 

3 . 

. 1 





, 3 

3 . 

, 1 , 

,3 . 




. 3 . 


In herbivores the canine teeth are missing or much reduced, leaving a tooth- 
less space, the diastema ( Fig. 244 ) , between the incisors and the premolars. 
The canines are relatively so small in the horse that a practical diastema 
exists, furnishing the space where the bits of the bridle are held. 


Biology of the Vertebrates 

(j) Origin of the Molars. — There are at least two theories to account 
for the origin of the molar teeth in mammals. 

First, the concrescence theory of Rose and others assumes that they are 
the products of the fusion of separate primitive cone-shaped teeth. The 
posterior teeth in the jaw of Sphenodon offer evidence in support of this 
point of view. 

The other and more widely accepted explanation is the differentiation 
theory of Cope and Osborn, which postulates the budding out and growth 
of additional contact surfaces, or cusps, upon the crown of an originally 
conical tooth (Fig. 249). This theory is based largely upon evidence pre- 
sented by the ancestral teeth of fossil mammals. It is quite possible that both 
theories will be of use, since they are not mutually exclusive, in reaching 
a satisfactory conclusion in the matter. 


Protoconid B ., .. . . , 

\ Paraconid ^Metacomd 

H — Protoconus 


^^feCjvir N/^^\— *■ — / ^Metaconus 

#Nf Clonic. / 

"T 7'- * Roots A^/~ " v Roots AJ w 


Lower Upper 
Molar Molar 



Fig. 249. Plan of molar teeth. Anterior is to the left in each case. 
(After Osborn.) 

The addition of two such cusps gives rise to the tritubercular tooth 
which is the typical molar of mammals generally from the earliest repre- 
sentatives down to Eocene times. Even today the mole Chrysochloris and 
certain other insectivores, as well as the opossum Didelphys, and some 
lemurs, exhibit this ancestral tritubercular type, which is well adapted to 
the business of crushing insects. 

The three cusps of a tritubercular molar are arranged in the form of a 
triangle. Molars of the lower jaw have a more lateral cusp, the protoconid, 
medial to which are the two secondary cusps, the paraconid in the anterior 
position and the more posterior metaconid. The corresponding cusps on 
molars of the upper jaw are indicated by the termination — us. Thus in a 
molar of the upper jaw there are a medial protoconus and two more lateral 
cusps, an anterior paraconus and a posterior metaconus. This means that the 
orientation of the triangle of cusps on the upper jaw is the mirror image 

Intake Apparatus 299 

of that on the lower jaw. The seemingly elaborate terminology here em- 
ployed is indispensable to the student who would make a study of the 
story of mammalian teeth. The addition of extra cusps on more highly de- 
veloped molars may bring the total number to a maximum of six. Molars 
of the lower jaw, for example, elongate somewhat through the development 
of a posterior extension, the talonid or "heel," which may bear two or three 
extra cusps. Because the teeth of the upper jaw alternate with those of the 
lower jaw in most cases, the protoconus usually strikes against the talonid 
of the corresponding lower-jaw molar. 

(k) Unusual Teeth. — Sometimes a pair of teeth develop excessively, 
forming tusks. These may be either incisors or canines and are more likely 
to appear in the male than in the female, although both sexes of elephants 
and walruses have tusks. 

The largest known tooth is the tusk of an extinct mammoth, Archidisko- 
don, that is in the American Museum of Natural History in New York 
City. It weighs over 250 pounds and is more than sixteen feet in length. 

The wild boar with tusks formed from modified canines of the lower 
jaw, strikes upward, while the male "dugong," Halicore, or sea-cow of the 
Red Sea, makes the effective blow from above downward with tusks evolved 
from the upper incisors. In both sexes of Phacochoerus, the wart hog of 
Africa (Fig. 67b), there are four upward curving tusks, which are the 
transformed canines of both jaws, those of the upper jaw bending sharply 
to pierce the upper lip. 

In general, tusks, as well as the prominent cutting incisors of rodents, 
retain at their base a large opening into the pulp cavity, thus insuring an 
abundant blood supply and consequent continued growth to compensate 
for the wearing away of the crown to which these exposed teeth are sub- 
jected. Such teeth in a way may be likened to angora hair in their manner 
of continuous growth. 

The male narwhal, Monodon (Fig. 75c) has lost all its adult teeth 
except an upper left one which is prolonged enormously into a formidable 
twisted pikestaff that may reach seven to nine feet in length. The saw-fish, 
Pristis (Fig. 20c), which is not a mammal but a selachian, carries a similar 
weapon in the form of an elongated snout, or rostrum, with laterally pro- 
jecting teeth along its sides. 

In rodents the chisel-like incisors are faced with enamel only on the 
anterior surface. Because the enamel is much harder than the dentine that 
is posterior to it, these incisors wear away more rapidly behind than in 
front, constantly leaving sharp cutting beveled edges of enamel. When a 
rodent is so unfortunate as to lose an incisor of either jaw, leaving the 

joo Biology of the Vertebrates 

incisor of the other jaw with no tooth to wear against, the animal usually 
meets eventual death by starvation because the surviving tooth, unhindered 
in growth, often reaches so great a length that the mouth can no longer be 
properly closed and feeding becomes impossible. 

Among poisonous snakes a pair of anterior teeth may develop into fangs, 
which are teeth that are either grooved or hollow. Whenever a fang is 
struck into another animal the secretion of the poison gland at the base of 
the fang is pressed out through the hollow or groove into the wound 
(Fig. 230). 

A so-called egg tooth, composed largely of dentine, is present as a transi- 
tory structure in the embryos of snakes and lizards which are imprisoned 
within an eggshell. It is situated in a median position and projects forward 
at the tip of the upper jaw. The young reptile uses it like a can-opener to 
hatch itself out of the imprisoning shell. According to Rose a pair of egg 
teeth are present at first in the embryo of the viper, Vipera, but only one 
becomes developed sufficiently to be of service, and this is shed soon after 

There is a corneal egg "tooth" of horny texture on the tip of the beak 
of many unhatched birds. Although not homologous with the egg tooth of 

sriakes and lizards, it nevertheless 
/ ^ serves the same purpose. It may 
sometimes be seen still adhering 
to the tip of the beak of young 
chicks which have just hatched 
Fig. 250. Transitory corneal egg-tooth which into the world ( Fig. 250b ) . A 

is used as a "can opener" in hatchinsr out of • •■> 1 . 

. . ,. ,,. ,. , , A f tt-, similar horny temporary emer- 

the shell. A, alligator; b, bird. (After Hilz- ' . 

heimer.) gency tool is present in Spheno- 

don, the crocodiles, and turtles, as 
well as in the monotremes, which are the only mammals that hatch out of an 

(/) The Trend of Human Teeth. — The teeth of ancient man show cer- 
tain differences from those of man today, which possibly give some sugges- 
tion as to the direction of the future evolution of human dentition. The 
jaws in which the teeth are set are becoming shorter and less prognathous, 
with the result that the teeth of modern man are more crowded and less 
regular in eruption. Also decay, or caries, is more common in the teeth of 
modern civilized man than in the teeth of his prehistoric ancestors, where it 
was practically unknown. Wiedersheim reports on evidences of decay in 
teeth after an examination of a large number of skulls from various exten- 
sive museum collections, as follows: Eskimos, 2.5) per cent; Indians, 3-10 

Intake Apparatus 301 

per cent; Malays, 3-20 per cent; Chinese, 40 per cent; Europeans, 80-100 
per cent. 

In primitive man the upper incisors came into opposition, edge to edge, 
with the lower incisors, and were frequently worn flat in consequence, while 
in modern man there is a tendency for them to shut past each other like the 
blades of a pair of shears, and thus to maintain a cutting edge. 

The "wisdom teeth," or the third molars, so-named by Hippocrates, 
the Father of Medicine, are apparently doomed teeth. They are the last to 
appear and the first to go. Frequently they remain uncut, or do not develop 
a grinding surface. In prehistoric man, however, they were plainly in evi- 
dence, and they are unusually well developed in negroes, mongols, and 
aboriginal Australians. 

The upper lateral incisors and the second molars also show evidences 
of being degenerate structures, failing to appear in a considerable number 
of cases. 

Davenport * writes of human teeth as follows : "At birth the front teeth 
are always formed with enamel and dentine and so the trouble that is asso- 
ciated with these marvelous organs begins. . . . The teeth are cut in dis- 
comfort, they decay and treatment brings pain, they are pulled out with 
pain. . . . Let us be happy that we have not so many teeth as the sharks." 

1. Pharynx 

The pharynx includes the stretch of digestive highway from the posterior 
part of the mouth cavity to the beginning of the esophagus. While its actual 
extent is relatively small, its diversity of function, and consequently the 
degree to which it is modified in various vertebrates, is very great. 

The pharynx serves as a point of departure for describing the respirator)' 
system, since among fishes and amphibians it is the region of the gills, as 
well as the point of origin for the swim bladder of fishes and the lungs of 
land animals. This aspect of the pharynx will be taken up in the chapter 
on Respiration. The description of the numerous pharyngeal glands, thy- 
roid, parathyroids, thymus, epithelial corpuscles, and ultimobranchials, will 
also be deferred until the chapter on endocrinal glands. 

Traveling through the gateway of the pharynx are two quite different 
streams of material, namely food and oxygen. In fishes they enter the mouth 
together (Fig. 251a), and proceed in parallel course without mutual inter- 

* How We Carrie by Our Bodies, Henry Holt and Co., 19?>6. 


Biology of the Vertebrates 

ference, food passing straight to the esophagus where it continues on its 
way, while oxygen, dissolved in water, passes out over the gills hanging in 
the lateral gill slits, which like portholes pierce the sides of the pharynx. 
The paired nasal pits on the snout of a fish do not open into the mouth 
cavity, and have nothing to do with the pharynx or with breathing. 

In amphibians, the first land forms that possess lungs and breathe free 
air, the nasal pits deepen until they break through into the mouth cavity, 
thereby forming a pair of respiratory passage-ways. The openings into the 
mouth cavity are the internal nares, or choanae (Fig. 251b). These allow 
air to pass to the lungs without the necessity of opening the mouth, thereby 
exposing its mucous lining to disastrous drying up. Free air is taken into 
the mouth cavity through the choanae with the mouth closed. After valves 
in the external nares have closed, the floor of the mouth is raised. Thus 
the air is forced back into the lungs, a process which would be quite im- 
possible with the mouth open, as the air could then escape in the wrong 

Nasal Pit 

Internal Narls 

Food "~ir^^"iZr^~c7777777Zl~ • 





Fig. 251. Diagrams of the evolution of the pharyngeal chiasma. For 
man refer to Fig. 252. (After Wiedersheim.) 

Embryonically the lungs are ventral outgrowths from the floor of the 
pharynx, and thus, while the food takes a straight course from mouth to 
esophagus as in fishes, air, entering the nostrils dorsally, crosses the path 
of the food and is forced ventrally into the lungs. 

Vertebrates above the amphibians, that is, reptiles, birds, and mammals, 
have developed a hard palate, or secondary roof of the mouth, which forces 
the choanae backward so that the crossing of the ways is transferred from 
the oral cavity to the pharynx. This crossing is known as the pharyngeal 

In mammals this arrangement has been accompanied by the establish- 
ment of various anatomical modifications (Fig. 252) that, like traffic 
officers at a busy street crossing, regulate the traffic and prevent confusion, 
although adding materially to the expense of maintenance. The epiglottis. 

Intake Apparatus 


for example, is introduced as a trapdoor device to guard the entrance into 
the trachea so that food passing by shall not go the wrong way. 

It may be pointed out that if in man air had been made to pass through 
the chin instead of the nose, thus avoiding the pharyngeal chiasma entirely, 
at least some present troubles would 
have been eliminated, although with 
such a drastic change other difficul- 
ties might have been introduced. In 
any case the pharyngeal chiasma 
clearly stresses the fact that our ana- 
tomical machinery is the inevitable 
result of many successive modifica- 
tions rather than being formed de 
novo from blueprint specifications. 
A moral to be gained from compara- 
tive anatomy, as well as from other 
aspects of life, is that one should seek 
to make the best of his inheritance, 
whatever it is, rather than vainly to 
regret not having been endowed with 
perfection in the beginning. The ac- 
quisition of adaptations by successive 
stages, each of which is always de- 
pendent upon what has gone before, 
is not only the method invariably 
followed in the phylogenetic estab- 
lishment of species, but it is also the only way that any anatomical structure 
in an individual comes into being embryonically. 

In man the pharynx is shaped somewhat like a funnel about five inches 
deep. It extends from the base of the skull to the level of the sixth cervical 
vertebra, where it narrows into the esophagus. Its three merging irregular 
cavities, one below the other, may, for purposes of description, be designated 
as the nasopharynx, oropharynx, and laryngopharynx. 

The upper nasopharynx (Fig. 252), which is not concerned with ali- 
mentary traffic but is entirely respiratory in function, lies dorsal to the soft 
palate. In general it retains a definite contour, since its bony walls are prac- 
tically inflexible. On either side of the nasopharynx opens an Eustachian 
tube from the air chamber of the middle ear. These tubes are the morpho- 
logical successors of the second pair of gill slits, or the spiracles, between the 
mandibular and hyoid arches of ancestral aquatic forms. 


noidal Sinus 
V.__ -Nasal Cavity 
j«!L- Hard Palate 
,fo^S!L- Oral Cavity 
ftjpq.' Nasopharynx 
vC-- -Soft Palate 
iTlj|--Air Highway 
J~\f Tongue 
' X-^\""" "Oropharynx 
f-Food Highway 
]" "Vocal Cords 

Fig. 252. Sagittal section through the 
head and throat of man, to show the 
pharyngeal chiasma. The position of the 
chiasma here is pushed back as a result 
of the development of the hard palate. 

304 Biology of the Vertebrates 

The oropharynx, posterior to the nasopharynx, communicates through 
the isthmus of the fauces, or the posterior exit of the oral cavity, directly 
with the cavity itself. Forming the ventral wall of the oropharynx is the 
base of the tongue that here assumes a vertical position, practically reducing 
the oropharynx to a transverse slit when the mouth is closed. 

On the sides of the oropharynx are two masses of glandular or lymphoid 
tissue, the palatine tonsils, while upon the posterior wall of the nasopharynx 
is still more of this peculiar tissue, the pharyngeal tonsils, commonly known 
when enlarged as adenoids. On the vertical face of the base of the tongue, 
behind the sulcus terminalis, are the lingual tonsils (Fig. 233). An incom- 
plete ring of tonsillar tissue, therefore, surrounds the pharyngeal passage- 
way, of which the part made up of palatine tonsils is the most prominent. 

Along with the tonsils there is developed throughout the entire pharyn- 
geal region a variety of adaptive glandular and lymphoid structures having 
a wide range of functions, and forming the seat of so many complications 
and troubles, both structural and physiological, that physicians and sur- 
geons specializing in this field alone, have their busy hands full. 

The laryngo pharynx, continuous with the oropharynx, is the indefinite 
lower part of the pharynx between the soft palate and the esophagus. It 
includes the critical region of the pharyngeal chiasma, and surrounds the 
larynx, or voice box. Except during the passage of food, which slips past 
down either side of the closed glottis, the laryngopharynx is collapsed into a 
narrow slit. 

Thus it will be seen that the pharynx as a whole, like a colonial kitchen, 
opens into several adjoining spaces. It has communication in fact through 
seven different openings, namely, two choanae, and two Eustachian tubes 
in the nasopharynx; the isthmus of the fauces in the oropharynx; and in 
the laryngopharynx, the glottis and the esophageal opening. 

2. Esophagus 

Over the entrance of the esophagus might well be written Dante's im- 
mortal line: "All hope abandon, ye who enter here," for in its course the 
muscles of its walls pass from voluntary to involuntary nerve control. Of 
all the vertebrates, some birds and the ruminant mammals are exceptional 
in that they have voluntary muscle fibers extending the whole length of the 
esophagus and are, therefore, able to regurgitate the food at will. The 
so-called "milk" which some birds regurgitate is used to feed their young. 
Ruminants, on the other hand, regurgitate their hastily swallowed food for 
more prolonged chewing. 

The esophagus, a short comparatively unmodified part of the digestive 

Intake Apparatw 


tube between the pharynx and the stomach (Fig. 253), is primarily a 
sphincter, the office of which is to forward food by peristalsis along its 
course to a point beyond normal control. 
The peristaltic action of the walls of the 
esophagus is well shown by a horse drink- 
ing at a brook, for the gulps of water 
taken in have to travel up hill along the 
neck and their passage is externally vis- 
ible. In the case of a snake the violent 
peristalsis necessary in swallowing a com- 
paratively large morsel of food, such as 
a frog, is supplemented by the muscles of 
the body wall. 

When not in use the esophagus col- 
lapses to modest dimensions, but upon 
occasion it is capable of great temporary 
distension. There are certain fishes that 
can even swallow another fish larger than 
themselves (Fig. 254). In many verte- 
brates the inner lining of the esophagus 
is characterized by expansive longitudi- 
nal folds that allow for a sudden increase 
in diameter during the act of swallow- 
ing, but at other times contract so that 
the tube may occupy a minimum of space. 

The inner lining of the esophagus of 
marine turtles is beset with backward- 
projecting horny papillae, which enable 
them more easily to swallow the slippery 
seaweeds upon which they habitually 

The length of the esophagus is de- 
pendent largely upon the presence or ab- 
sence of a neck. In frogs and toads the 
neck is reduced to a minimum so that a 
fly entering the mouth of one of these 
animals finds itself almost immediately 

landed in the stomach, whereas in long-necked animals, such as the 
giraffe for example, the esophageal adventures of food are much more 
extended. In adult man the length of the esophagus is approximately 

Fig. 253. General diagrammatic view 
of the digestive system in man. The 
liver has been tipped anteriorly to 
show the gall bladder and common 
bile duct. The transverse colon has 
been cut to show the duodenum (the 
beginning of the small intestine), but 
its course is indicated by dotted lines. 
(After Cunningham.) 


Biology of the Vertebrates 

Fig. 254. A fish that has swallowed 
another fish larger than itself. 

fourteen inches, the lower end piercing the diaphragm to enter the body 
cavity before joining the stomach. It is only this short portion within the 

body cavity that is provided with a 
serosa layer of tissue outside the muscular 
and mucosa layers. 

A noteworthy differentiation of the 
esophagus in birds is a lateral enlarge- 
ment known, as the crop (Fig. 255), 
which may serve simply as a convenience 
for the temporary storage of food hastily 
secuied in the presence of enemies or 
competitors, as in the case of seed-eaters generally, or may be supplied 
with glands which act chemically upon the food eaten. Pigeons produce 
a cheesy nutritious substance from 
the lining of the crop, called "pigeon's 
milk," that is fed to nestlings by 
regurgitation. The tropical hoactzin, 
Opisthocomus, has a muscular crop 
which works mechanically upon the 
food that finds lodgment in it. A 
chicken going to roost with its crop 
filled with corn, falls asleep unham- 
pered by the continuous effort of pick- 
ing up food and "feeds" all night long 
while resting, as the crop, like the 
hopper of a gristmill, releases its con- 
tents automatically and periodically 
to the glandular stomach and grinding 
gizzard as needed. 

Among the lower vertebrates any 
external line of demarcation between 
the esophagus and stomach is either 
absent or vague, but in birds and 

mammals there is usually a definite point of transition. In many cases it is 
easier to gain entrance to the stomach from the esophagus than to escape 
from the stomach into the intestine. 

£s? Pancreas 


~ -Liver 


=^7~Small Intestine 

Colic Caeca 

"Cloacal Opening 
Fig. 255. Digestive tract of a hen. 

3. Stomach 

The stomach is a conspicuous enlargement of the digestive tract lying 
between the esophagus and the intestine (Fig. 256). Originally, as in some 

Intake Apparatus 


fishes and salamanders, it is spindle-shaped and arranged to conform with 
the general contour of an elongated body, but in higher vertebrates it 
becomes saclike in shape, assuming a somewhat transverse position in the 
body cavity. Between these extremes may be found many gradations of 
form and position. 

-j#- Pyloric Portion 
Greater Curvature 



Fig. 256. Different stomachs. 

The stomach of the dogfish, for example, instead of being a primitive 
straight spindle-shaped enlargement with the entrance and exit at opposite 
ends, is doubled back in the form of a J-shaped tube. Stomachs of similar 
shape occur also in the flounder, haddock, salmon, carp, sturgeon, sole, and 
many other fishes. In fishes such as the perch, smelt, herring, bullhead, and 
whiting, the loop becomes fused along its inner bend in such a way that a 
bag-shaped pouch, or fundus, is formed with the entrance and exit brought 

308 Biology of the Vertebrates 

near together at one side. This type of a stomach, when shifted into a trans- 
verse position, is much like that of man, with a lesser curvature on the upper 
side between the entrance and the exit, and a greater curvature forming the 
larger contour around the outer margin or elbow of the stomach. 

The entrance to the human stomach is somewhat larger than the exit 
and is less distinctly marked off, although the lining of the digestive tract 
itself in the region of the esophagus is easily distinguished from that of the 
stomach, even when the external transition from one part to the other is 
extremely vague and indefinite. 

The exit from the stomach is closed by the pylorus, or pyloric valve, a 
fold of mucous membrane reinforced by a sphincter muscle, which relaxes 
temporarily for the release of food into the intestine only when the stimula- 
tive password of proper acidulation is given. 

The walls of the stomach are muscular enough to insure the active 
movement of the food-mass around and around by peristalsis until it has 
been reduced, through mixture with glandular secretions, to a suitable con- 
sistency and degree of acidity. In other words, the food is kneaded and 
mixed under muscular pressure. Then, as it is presented at the closed pylorus, 
the sphincter muscle relaxes, allowing small successive amounts of the mix- 
ture, called chyme, to slip through into the intestine. 

Amphioxus and larval cyclostomes, which have not gone far enough in 
evolution to develop peristaltic muscles, have, according to Schimkewitsch, 
the entire digestive tube lined with cilia, the mission 
of which is to keep the food moving along. In the 
human fetus also, as a possible reminder of whence 
man came, the posterior part of the stomach lining is 
clothed with cilia. 

There is a tendency for the stomach to become differ- 
ing. 257. So-called entiated into two or more regions, distinguished from 
"hour-glass stom- . . . , . , . _..,,. 

ach" in man (After ea other by location and function, lhus, in the J- 

Wiedersheim.) shaped stomach of the dogfish which involves half the 

entire length of the digestive tract one speaks of a 
cardiac limb and a pyloric limb, while in certain mammals, the mouse for 
example, a constriction in the middle part of the stomach marks off a car- 
diac chamber from a pyloric chamber. 

Medical literature contains references to the occasional occurrence of 
so-called "hour-glass stomachs" in man (Fig. 257), which bear a strong 
resemblance to the two-chambered stomachs of mice. Certain monkeys 
(Hylobates and Semnopithecus) show the same feature. Whether such 
unusual structures in m?n are pathological or ancestral is uncertain. 

Intake Apparatus 309 

An extreme subdivision of the stomach is reached by the ruminants, 
which have four "stomachs" (Fig. 258). The first in order is the "paunch," 
or rumen, which is a spacious storage bag for the temporary reception and 
fermentation of grass or herbage upon which ruminants feed. Micro- 
organisms present in the rumen of domestic cattle, and possibly other rumi- 
nants, act upon simple nitrogenous compounds to synthesize proteins as 
well as sufficient quantities of the B-complex vitamins to supply the dietary 
needs of these vertebrates. From the rumen the food is passed over into the 
"honeycomb stomach," or reticulum, that, as its name indicates, is lined 
with many shallow pits. When 

leisure from prehensile feeding ;l:^~/75t masf ' ca /- IO j,M 

comes, food which was hurriedly Initial Swallowing -4[* l ewi " g . u 

swallowed with little mastication Regurgitation— lj,,/ Esophagus 

is regurgitated into the mouth for Omasum, 

rechewing. This material, corning 
in part from the rumen and in part Intestine 
from the reticulum, is known as 
the "cud." It includes roughage, 

such grains as happen to be Rumen— 

trapped in the roughage, and a Fig. 258. Diagram of a ruminant stomach, 
considerable quantity of water with broken lines showing the course of the 
which facilitates the passage of the 

cud up the esophagus. During the first few chewing movements the animal 
swallows most of the liquid brought up. After remastication and a thorough 
mixing with saliva, the food is again swallowed and passes once more into the 
rumen. Then another cud is regurgitated, thus beginning a new cycle of ru- 
mination. Most of the food that has been thoroughly chewed and mixed with 
liquid soon passes into the reticulum and then shortly into the omasum, or 
"manyplies stomach." This third chamber is lined with numerous folds and 
communicates directly with the true "glandular stomach," or abomasum, 
where the food is mixed with gastric juices and chemically modified before 
being forwarded into the intestine. In "water cells" in the walls of the rumen 
and reticulum, camels are able to store reserve water which enables these 
desert animals to endure prolonged periods of dryness. 

The vampire bat, Desmodus, exhibits a peculiar adaptation with refer- 
ence to its blood-sucking habits, the fundus of the stomach being drawn 
down into a deep elastic pouch (Fig. 256). When a vampire fastens on 
to a warm-blooded victim it can fill this spacious reservoir with blood until 
the entire body is swollen in consequence. 

The cardiac and pyloric regions of the stomach in birds have become 


Biology of the Vertebrates 

If a.m 

separated into chambers very unlike each other in character (Fig. 255). 

The cardiac chamber, or proventriculus, which opens into the gizzard, 

becomes a glandular stomach, where the food undergoes some preliminary 

maceration and chemical modification before reaching the 

gizzard through which it passes to the intestine. The gizzard 

has a thick muscular wall and is lined with a hard secreted 

layer. In this muscular mill food is ground up, instead of by 

w fffi S* means of teeth as in mammals. Gravel, or "gizzard stones," 

i£m retained temporarily within the gizzard cavity, aids in the 

^^P process of food attrition. The whole device is a part of the 

^^ general program of centralization of organs which the birds, 

a ^g| as adapted flying machines, have evolved. The highest dif- 

Q^^ fcrentiation of the gizzard is reached in seed-eating birds, and 

p,n the least in birds of prey. 

\^r Among reptiles the crocodilian stomach approaches that 

of birds in its differentiation, since a gizzard-like pyloric 
chamber receives the food after it passes over from the 
glandular cardiac sac, which corresponds to the chemically 
functioning stomach of other animals. 

There are at least three general functions performed by 
the vertebrate stomach, namely, storage, mechanical manipu- 
lation, and chemical modification. 

The advantages of food storage are obvious. Among 
lower sedentary creatures, like sponges and clams for exam- 
ple, there is no provision for food storage, in consequence 
of which feeding is practically a continuous process. With 
the necessity of hunting for food, rivalry for daily bread and 
adventurous escape from devouring enemies become more 
and more the constant daily program of animal existence. 
The necessity for seizing a sufficient supply of food, when 
it is available, in a minimum of time and then retiring 
to safety or engaging in other activities, is apparent. By 
periodic voluntary filling of a storage chamber like the 
stomach with food, opportunity is left for other activities 
at the same time that the involuntary machinery of the body 
is faithfully attending to the digestive processes with meticu- 
lous deliberation and care. 
The function of mechanical manipulation, or peristalsis, has already 
been mentioned. By this means the muscular walls of the stomach knead 
the food-mass around, mixing it with digestive secretions. The movement 


^•.30 p.m 





X • 

6 p.m. 

Fig. 259. 

X-ray of the 
contents of a 
cat's stomach 
at successive 
intervals after 
being fed with 
bread and 
milk and bis- 
muth subni- 
trate, which is 
opaque to the 
X-ray. (After 

Intake Apparatus 311 

may actually be seen upon a fluorescent screen when an animal like a cat, 
whose food has been mixed with barium sulphate or some other substance 
that is opaque to X-rays, is exposed to their action (Fig. 259). 

The function of chemical modification is dependent upon the presence 
of glands in the lining of the stomach, which produce secretions of various 
kinds. In the region of the fundus, gastric glands are most numerous. These 
produce hydrochloric acid and three kinds of enzymes, namely, pepsin, 
rennin, and gastric lipase, which do preliminary service in the chemical 
reduction of proteins and fats. Pepsin, acting only in an acidulated medium, 
breaks down protein foods to simpler compounds; rennin coagulates the 
protein casein, a constituent of cheese, out of milk, rendering it capable of 
being changed by pepsin into simpler substances that are prepared to 
undergo other necessary changes farther along in the digestive tract; finally, 
gastric lipase begins the work of splitting up fats into soluble fatty acids and 

It is the hydrochloric acid generously produced by the gastric glands of 
dogs that enables them to dissolve bones which they crunch and swallow. 

The protein lining of the stomach itself is not digested by its own secre- 
tions because its component cells are living and thus resistant to enzymatic 
action. It is only when a cell is dead that it yields to destruction by gastric 
juices. This explains how a tapeworm can live and prosper while bathed 
in the digestive gastric secretions of its host. 

Man is the only animal that hastens the reduction of food to soluble 
form by cooking it. 


The intestinal stretch of the digestive tract is a long lane with many 
turnings. In higher vertebrates it is differentiated into the small intestine, or 
degressive zone, in which food substances passing through are diverted into 
the blood stream, and a shortened part, usually of somewhat greater diam- 
eter, known as the large intestine, constituting the egressive zone from which 
the unutilized residue of the food-mass is ejected. 

1. The Small Intestine 

All other regions of the digestive tract are subsidiary in function to the 
small intestine. It is here that the food-mass, which has undergone chemical 
and mechanical modification on the way, is finally converted into suffi- 
ciently soluble form to be passed over into the blood stream by diffusion, 
whence it is finally distributed to the uttermost needy cells of the body. 

3 12 Biology of the Vertebrates 

The straight, comparatively short intestine of cyclostomes, undifferenti' 
ated into small and large regions, has its absorbing surface increased by a 
typhlosole, an internal fold including mucosa and submucosa, which makes 
only a few spiral turns in the entire length of the intestine ( Fig. 211). 

In elasmobranch fishes the typhlosole, making numerous spiral turns, 
is much longer than the intestine, with the result that it becomes twisted into 
a spiral valve ( Fig. 212). This makes an enlarged surface within a very 
compact space for the diversion of food, since the intestine is no longer than 
the J-shaped stomach and is not bent. 

A spiral valve is also present in the intestine of dipnoans, certain ganoids, 
and at least one exceptional teleost (Chenocentrus) . Twisted coprolites 
(fossil feces), found with the bones of ichthyosaurs, indicate that these 
extinct reptiles might also have been equipped with a spiral valve device 
that moulded the feces into a twisted shape. 

The ganoids show a different method of increasing the intestinal surface, 
by means of pyloric caeca which are saclike diverticula at the beginning of 
the small intestine (Fig. 214). Both spiral valve and pyloric caeca are pres- 
ent in the sturgeon, Acipenser, although poorly developed. 

The next step in the evolution of the small intestine is found in the 
teleost fishes, which have given up the spiral valve idea and gone over 
entirely to the elaboration of pyloric caeca. In some of the bony fishes these 
structures form a large tuft of tubules, occupying considerable space within 
the constricted body cavity (Fig. 214). They vary in number from one in 
the ganoid Polypterus, and the sand-lance, Ammodytes, to over two hun- 
dred in the mackerel, Scomber. 

The distinction between the small and large intestine begins with am- 
phibians, also the diversification of the inner surface of the small intestine by 
villosities, which reach their greatest differentiation in the small intestine of 
mammals. In amphibians the entire lining of the digestive tract is composed 
of potentially absorbing cells, corresponding in function to the small intes- 
tine of higher forms. 

The sluggish reptiles as a class have a definite large intestine marked off 
from the small intestine that joins the stomach. At the junction between the 
small and large intestines a new diverticulum, the colic caecum, appears 
(Fig. 215). The colic caecum of a turtle is hardly more than a slight 
enlargement, but higher up among rabbits and some other rodents it may 
become an enormously enlarged tube with an internal capacity nearly 
equal to that of the rest of the digestive canal to which it is attached. 

Birds, which have evolved a long way from their reptilian forebears, 
have a much coiled small intestine, two colic caeca, and a large intestine 

Intake Apparatus 323 

that is decidedly foreshortened, since it is incompatible with their strenuous 
aerial life to carry about the ballast of unnecessarily retained feces. The colic 
caeca are short in pigeons and comparatively long in owls and turkeys. In 
ostriches they are sometimes reduced to a single caecum, which is as capa- 
cious as all the rest of the small intestine and is made even more effective by 
the presence of a spiral valve. 

The small intestine of mammals is usually easily distinguishable from 
the large intestine, a single colic caecum marking the transition from one 
region to the other. Exceptions are Trichechus, Hyrax, and the edentates 
Dasypus and Myrmecophaga, which have two colic caeca. Monotremes, 
flesh-eating marsupials, edentates, insectivores, bats, carnivores, and toothed 
whales, either lack or have only a small colic caecum, but in herbivores it 
is so large that it may even exceed the body in length. Herbivores also have 
a noticeably longer intestine than carnivores. 

The degenerate free end of the colic caecum forms the processus vermi- 
formis in certain rodents, civets, monkeys, and man (Fig. 216) . According to 
Wiedersheim the processus vermiformis, or vermiform appendix, in man, 
which has outlived its usefulness and bears an unsavory reputation, varies 
in length from two to twenty-five centimeters, with an average of about 
eight and one half centimeters. It tends to shorten with age and to become 
closed in later life. Statistics on the closure in 1005 observed cases are given 
by Miiller in percentages as shown in Table IV. 

TABLE. IV. Closure of Human Vermiform Appendix 

Age (in years) (Percent) 

$ 9 

1 to 10 2.0 0.0 

11 " 20 5.1 5.4 

21 " 30 6.4 8.7 

31 " 40 12.7 23.8 

41 " 50 26.2 34.8 

51 " 60 20.5 30.0 

61 " 70 29.3 50.0 

71 " 80 38.7 26.0 

81 " 90 53.3 52.0 

The small intestine is divided more or lees arbitrarily into duodenum, 
jejunum, and ileum, a distinction which, though first made out in man, 
applies to most other mammals. 

The duodenum, or part next to the stomach, is comparatively short. The 
jejunum, which follows, and the more posteriorly located ileum, forming 


Biology of the Vertebrates 

about equal lengths of the remainder 
of the small intestine, are not distinctly 
marked off. The jejunum is richer in 
blood vessels as well as having a some- 
what thicker wall and wider lumen than 
the ileum. 

The characteristic modification of 
the lining of the mammalian small in- 
testine is the presence of innumerable 
tiny thickset velvety projections, or villi, 
which enormously increase, in a mini- 
mum of space, the absorbing surface ex- 
posed to the dissolved food. In fact these 
are the definite organs of absorption. 

Each villus consists of a thimble-like 
projection whose thin wall of cells en- 
closes a capillary loop and a micro- 
scopic lacteal, or terminal element of 
the intestinal lymphatics (Fig. 260). 
Fat that has been reduced to lower 

terms in the intestine passes into the lacteals and thence to the lymphatics, 

eventually emptying into the venous system by way of the thoracic duct. 

Digested proteins and carbohydrates are collected by the venous capillaries 

of the villi and carried to the liver by way 

of the portal vein. 

In the ileum particularly, the forest of 

microscopic villi is frequently interrupted 

by irregular bare patches from half an 

inch to three or four inches in extent, 

which show like worn places in the nap of 

a Brussels carpet. These "intestinal ton- 
sils," or Peyer's patches (Fig. 261), are 

lymphoid in character. It is important to 

remember that in typhoid fever the chief 

lesions occur in these areas. Smaller 

lymph nodes are also interspersed among 

the villi. 

Fig. 260. Diagram of a villus and a 
gland, or crypt, of Lieberkiihn. 

Peter's Lft 



Li/ mph 

Fig. 261. Surface view of a portion 
of the mucous membrane of the 
ileum, showing a Peyer's patch, 
and solitary lymph nodes. (After 
Piersol. ) 

2. Glands 

Two conspicuous glands differing greatly in appearance, intimate struc- 
ture, and function, but which are alike in being endodermal derivatives of 

Intake Apparatus 315 

the mesodaeum, are connected with the duodenum just posterior to the 
pylorus. These are the liver and the pancreas. They are so large that, 
although they arise from the lining of the small intestine, they push through 
to extend entirely outside the digestive tube itself and come to occupy space 
within the body cavity. 

(a) Liver. — The liver is an older organ, both ontogenetically and phylo- 
genetically, than the pancreas. It should not be confused with the so-called 
"liver" of starfishes, crabs, mollusks, or other invertebrates, since it is in no 
way either homologous with or analogous to these structures. 

Contrary to the popular impression, the vertebrate liver is not primarily 
an organ of enzymatic digestion, although the bile which it provides aids 
materially in the digestive processes by the stimulative action of the bile salts 
upon the enzymes of the pancreatic juice, by the emulsifying action of these 
salts, and by furnishing a favorable alkaline medium without which the 
digestive enzymes produced by the pancreas fail to act. In the bile itself no 
enzymes are present. 

The liver, which has been characterized as the "busiest port on the whole 
river of life," is so voluminous that in man it may easily contain one fifth 
of all the blood, while several- times an hour the entire blood supply of the 
body passes through its myriad capillaries, undergoing there profound modi- 
fications by way of additions and subtractions of constituent substances in 
the blood. Thus it acts like a strainer of the blood, storing sugar-fuel (glyco- 
gen) and then restoring it to the circulation when the muscles need it; 
balancing the circulating food ration generally by withdrawing and restoring 
certain constituents; eliminating bacteria; turning poisons into harmless 
wastes; abstracting nitrogenous by-products from protein compounds to be 
disposed of through the kidneys ; and by drenching the intestine with the 
indispensable alkaline bile. No wonder Dr. Woods Hutchinson said of the 
liver: "It is all together the most useful and desirable citizen, and withall 
a cheerful and even convivial one, mixing our drinks, putting the stick into 
our vitamin cocktails, and the sugar and cream into life's coffee." 

The bile is a bitter alkaline fluid, about ninety per cent water, tinged 
with pigments from the wreckage of red blood corpuscles and containing 
salts, both organic and inorganic, besides waste materials of different kinds. 
It is formed continuously in the liver and is poured at food-taking intervals 
into the duodenum where it mingles with the food-mass upon the escape of 
the latter through the pylorus into the intestine. The bile, which may 
amount to a pint or a pint and a half daily, contributes materially to the 
excreta that pass out of the alimentary canal. When an excess of bile is pro- 
duced it may temporarily be stored between periods of digestive activity, 
by backing up into the gall bladder, a reservoir-like enlargement of the bile 

?!() Biology of the Vertebrates 

duct (Fig. 266). That part of the bile duct that drains the liver is called 
the hepatic duct as distinguished from the cystic duct that comes from the 
gall bladder (Fig. 262). Whenever these two ducts join to empty into the 
intestine the common bile duct thus formed is called the ductus choledochus, 
a name that has the same root as ''melancholy" and "choleric," words 
descriptive of conditions for which the misunderstood liver has been held 
responsible by the popular mind in the past. 

Cystic Duct >, -^Jf Hepatic Duct 

Gall Bladder--^£^^^ = ^^^_^^#--Duodenum 

^^^ /'~7<M ('iff 

_, . . ,' MA-^n—v^— Pancreatic Duct 
Ductus Choledochus IP vw v ^ 

Fig. 262. Diagram of the liver ducts in the human adult. (After 
Bremer. ) 

The choledochal duct and the duct of the pancreas open together into 
the duodenum of the horse, dog, cat, ape, and man, but separately in the 
pig, ox, rabbit, and guinea pig. 

The gall bladder, which seems to be an emergency device for animals 
that digest a considerable amount of fatty food, is absent in many plant- 
eaters. Its total absence, as well as the abnormal presence of two gall blad- 
ders, has been noted in man. Its normal capacity in man is about an ounce 
and a half. 

"Gall stones," which are found in the gall bladder in about 6 to 1 2 per 
cent of autopsies, are concretions composed chiefly of cholesterol but usually 
including bile pigments and calcium salts. Gall stones containing limy 
deposits may be detected by the X-ray. If large they are comparatively 
harmless, but while small they may block the ducts, thus causing the bile to 
be resorbed and passed back by way of the lymphatics into the blood stream, 
a condition that results in jaundice. The cystic duct especially lends itself to 
such obstruction, since it is modified within by mucous folds forming the 
Heisterian valve, which makes blocking the passage-way easier than would 
be possible if the lumen were entirely open. 

The liver is an adaptive space-filler, weighing about one fortieth of the 
total body weight, and consisting usually of two or more lobes. Its shape and 
size are conditioned first by the abundant blood vessels, nerves and liga- 
ments, or connective tissue attachments; second, by the neighboring organs 
which crowd it; and third, by the confining walls of the body cavity. With 
every breath that expands the neighboring lungs, and because of the uneasy 

Intake Apparatus 

\ l 7 

peristalsis of the stomach and intestine pressing upon it, the flexible lobes of 
the liver are constantly slipping slightly over each other, changing mean- 
while somewhat in shape in accommodation and adjustment to the varying 
conditions of available space. 

In mammals the two larger lobes are separated by the umbilical fissure, 
as determined by the round ligament representing the atrophied remains of 
the umbilical vein. A large portal fissure marks the gateway for the blood 
vessels, ducts, and nerves that pass to and from the liver in adult life. 

In histological structure the mammalian liver is made up of cords of 
glandular cells in close contact with capillaries (Fig. 263) and with an elab- 
orate system of drainage ducts and an adequate nerve supply. The whole 
mass of cellular cords and capillaries is embedded and encapsuled in a sup- 
porting network of connective tissue. 

liver cells 
bile duct 

Fig. 263. Termination of a bile 
duct between liver cells. (After 

L-» Capillaries 

_— Blood 

"r- Nucleus 

Fig. 264. Four adjacent liver 
cells, showing bile capillaries, 
or canaliculi. (After Jordan 
and Ferguson, and Merkel.) 

Along the approximated sides of neighboring polyhedral gland cells 
that form the cords, are tiny intercellular spaces or grooves ( Fig. 264 ) , like 
the spaces between the fingers and knuckles when two fists are placed 
together. These bile canaliculi, formed as indenting intercellular grooves 
permeating the entire liver mass, empty into drainage ducts which com- 
pound with others in an ever enlarging array until they finally emerge as 
the single large outlet of the hepatic duct. The system of capillaries that 
enmeshes the gland cells is independent of the meshwork of canaliculi and 
drainage ducts. 

The blood supply of the liver is unlike that of most other organs in that 
there are two sources from which it is derived, namely, the hepatic artery 
which brings blood from the heart in the same way as all organs of the body 
are supplied, and the portal vein, that comes freshly laden with dissolved 
food from the intestine. The capillaries derived from both of these sources 
hopelessly lose their identity as they gradually anastomose within the liver 
to form the hepatic veins, which drain the blood of the liver into the heart. 

Biology of the Vertebrates 


Rathke's Pouch 

-- Location of 
Mandibular Arch 

Embryonically the liver is a hollow ventral outgrowth near the beginning 
of the intestine just anterior to the attachment of the embryonic yolk sac 
(Fig. 265). It lies at first between two layers of ventral mesentery in the 
transverse septum, but eventually it becomes so large that it projects some 

distance into the body cavity, push- 
ing a covering of serosa, or visceral 
peritoneum, with it. The endoder- 
mal outgrowth from the gut itself 
becomes the secretory glandular 
part of the liver, and this soon be- 
comes enmeshed with the vascular 
mesenchyme from the transverse 
septum and neighboring blood ves- 
sels, including the vitelline veins 
leading into the body from the yolk 
sac, to form the liver tissue. As the 
mass of this tissue grows into the 
transverse septum and the ventral 
mesentery, the material which is at 
the primary point of origin develops 
into the ducts and gall bladder. 

In amphioxus the liver remains 
a simple, single sac projecting 
forward ( Fig. 13), that is beset with capillaries which bring food from 
the intestine, much as in the early embryonic phase of higher forms. It 
is lined with ciliated glandular epithelium and probably secretes a digestive 

Typically the liver of most vertebrates has two lobes, although lampreys 
and snakes, perhaps on account of their elongated body form, have only 
one lobe. The liver is relatively larger in carnivores than in herbivores. In 
Anamnia, or fishes and amphibians, it is larger than in Amniota, or rep- 
tiles, birds, and mammals. This fact no doubt is connected with the presence 
of more fat in the diet of the former in each case. In certain carnivores, dogs 
and weasels for example, there are as many as seven lobes present. 

In man four lobes are described (Fig. 266) . The right lobe is the largest, 
constituting about four-fifths of the entire mass, while the wedge-shaped 
left lobe is next in size. These lobes are separated from one another by the 
falciform ligament. On the posterior aspect between these two major lobes, 
an oblong quadrate lobe lies near the gall bladder, while the small caudate 
lobe spreads between the postcaval and portal fissures. 

^--Allantoic Stalk 

——Hind Gut 

Wolffian Duct 

Fig. 265. Reconstruction of the alimentary 
canal of a 4.2 mm human embryo. (After 

Intake Apparatus ?io 

Posterior Vena Cava 
Caudate Lobe } 

Groove tor ^^^^a^ \ ^i^&WT^Z^r \W 

Esophagus --/^ ^k X ^Sp^ "fW" 

Hepatic Portal -JMf? ~~~7#W^ ^0 J^^^sJ^*^^- Common Bile Duct 

Hepatic --\1»^C lfe^^y:^ L T---^#^-W u *• n , 

Left Lobe -' ^^^^^« f#> //' )M§^ ^ 

Jtp^M Sjm^z^- Gal1 Bladder 

Round Ligament d ^"^5=8^ 

Fig. 266. Posterior surface of the human liver. (After Gray. 

(b) Pancreas. — The second largest vertebrate gland, the pancreas, is a 
compound alveolar gland of irregular shape lying in the fold between the 
stomach and the duodenum, and projecting into the body cavity from the 
point of its embryonic connection with the digestive tube, although in the 
lamprey, Petromyzon, and in certain teleosts, it may remain embedded in 
the wall of the intestine. 

--~ Dorsal Mesentery-^---- 
Dorsal Pancreas — 


Ventral Pancreas"---' 

/ Liver ____ 

Ventral Mesentery -^^ 
ess Ectoderm * 

Fig. 267. Origin of pancreas and liver. (After Schimkewitsch.) 

It arises as one or more endodermal outgrowths from the embryonic gut 
just posterior to the liver. These outgrowths are ordinarily three in number, 
of which one is dorsal and two are ventral in position (Fig. 267). The two 
ventral parts fuse together into a common gland, while the ducts formed 
at each point of outgrowth may either persist, or as is more often the case 
disappear with the exception of one (Fig. 268). The ducts of the ventral 
components are called Wirsung's ducts, while that of the dorsal pancreas is 
named the duct of Santorini. In some forms they unite either with each other 
to make a common duct, or with the bile duct. In lampreys all of the ducts 
are lost, the secretion of the pancreas consequently becoming entirely endo- 


Biology of the Vertebrates 

crine, that is, distributed by the blood rather than poured directly to the 
outside or into some passage-way leading to the outside. 

„ Gall Bladder 

Hepatic Duct 

Duct J 

Bile Duct. 

of Ducts 
"Dorsal Pancreas B 

Ventral Pancreas Pancrea 




Fig. 268. Development of the human pancreas, showing origin from 
dorsal and ventral endodermal outpocketings. a, in 12 mm embryo; b, 
in 16 mm embryo; c, in child at birth. (After Arey.) 

A curious modification of the pancreatic apparatus which not infre- 
quently appears, particularly in the cat, is a pancreatic bladder, or reservoir- 
like enlargement of the pancreatic duct for the temporary storage of exces- 
sive secretion, an emergency organ quite coniparable with the gall bladder 
that serves similarly as a storage reservoir for the bile of the liver (Fig. 269) . 

Pancreatic __ 

Gall Bladd 
Right Hepat 

Hepatic Duct 
Cystic Duct 

Pancreatic Duct — —^ 

Fig. 269. Pancreatic bladder from a female cat. (After Boyden.) 

In general it may be said that the pancreas is a gland of dual character 
since, in addition to its production of pancreatic "juice" which is poured 
into the intestine through ducts, there are present in the pancreatic tissue 
certain distinct interlobular cell aggregates, also endodermal in origin, called 
the islands of Langerhans (Fig. 270). These secrete substances (hormones) 
of a character quite different from the pancreatic juice itself, which are 
carried to all parts of the body through the circulating blood. The islands 

Intake Apparatus 


\Js/ond of 
,■■' Langerhans 

•it>-.^... Epithelium of 
pancreatic duct 

of Langerhans vary from single interstitial cells to masses of several hundred. 
They are more abundant late in life than in youth, appearing in man when 
the human embryo is only about 54 mm. in length. It is estimated that in 
the pancreas of the guinea pig there may be present as many as 25,000 
islands of Langerhans. 

Which of these two kinds of secreting glandular cells, the endodermal 
outgrowths from the embryonic gut or the islands of Langerhans, represents 
the original pancreatic tissue, and whether one has or has not been derived 
from the other, is still a matter of controversy. It may be pointed out, how- 
ever, that the islands of Langer- 
hans are invariably present in 
all true vertebrates, and are 
undoubtedly early settlers if not 
the first inhabitants, whereas 
enzymatic secreting cells are 
wanting in certain vertebrates. 

There is no pancreas in am- 
phioxus. In sturgeons the pan- 
creas is made up of two dorsal 
and two ventral components 
with only the right ventral duc- 
tus Wirsungianus remaining. 

The pancreas of bony fishes and lower vertebrates generally is primitive in 
character, being widely diffused and irregular in form. Among mammals 
there is also a great variety in the form, position and size of this important 
gland. A single surviving embryonic pancreatic duct is found in man, Wir- 
sung's duct, connecting with one of the ventral embryonic components. 

The activating substance, or hormone, that is diverted from the islands 
of Langerhans into the blood has to do with the utilization of sugar in the 
tissues. Its failure to be produced in sufficient quantity results in diabetes, or 
the condition in which sugar is eliminated unused through the kidneys. 

The substances secreted by the pancreas proper and eliminated through 
the ducts into the intestine are enzymes, which are essential in speeding up 
the chemical action begun in the intestine. 

The principal enzymes in the pancreatic juice that aid in the digestive 
process are three, namely, amylopsin, which like the ptyalin of the saliva acts 
in making carbohydrates soluble; the inactive precursor of trypsin, which 
modifies proteins ; and steapsin, which breaks down fats into simpler fatty 
acids and glycerine. These three important digestive enzymes, therefore, are 
prepared not only to transform chemically the three fundamental kinds of 

Fig. 270. Section through pancreas of rabbit. 

(After Krause.) 

3 22 Biology of the Vertebrates 

foods, carbohydrates, proteins, and fats, but also to render them fit for 
transference through the blood to all parts of the body. 

In man about a pint and a half of digestive pancreatic juice is poured 
daily into the intestine. 

(c) Intestinal Glands.- — In addition to the liver and pancreas, the secre- 
tions of which are mixed with food materials in the duodenum, there are 
numerous smaller glands, occupying the walls of the intestinal tract, that 
likewise make chemical contributions essential to digestion. An intestinal 
juice that has been given the blanket name of succus entericus combines the 
products of these small, intestinal glands. Among mammals there are at 
least two kinds of intestinal glands that contribute to this juice, namely, 
Brunnefs glands in the anterior end of the duodenum, and the glands of 
Lieberkiihn, which are vastly more numerous, being embedded throughout 
the entire length of the small intestine in its walls. The multitudinous glands 
of Lieberkiihn are in the form of sunken pits, or crypts, which are inter- 
spersed among the villi like deep gorges among steep mountains (Fig. 260). 

Upon chemical stimulation by the chyme, which is the acidulated food 
mass that passes through the pylorus from the stomach into the duodenum, 
the epithelium of the small intestine produces a hormone called pancreatic 
secretin, which is carried by the blood to the pancreas where it excites that 
gland into activity. This reaction occurs only when chyme enters the intes- 
tine; otherwise, if continuously produced, the pancreatic juice much of the 
time would be wasted, not having any food upon which to act. 

The succus entericus also includes several enzymes. Enterokinase con- 
verts the inactive trypsinogen of the pancreatic juice into the active trypsin. 
Erepsin completes the work of protein digestion begun by pepsin and tryp- 
sin. Other enzymes aid in carbohydrate digestion. Thus the intestinal con- 
tents include a variety of enzymes, derived from both pancreatic juice and 
succus entericus, which play a part in the complicated chemical preparation 
of the chyme for its transfer to the blood. 

Most food material owing to its colloidal nature is inert and insoluble 
when taken into the digestive tract and cannot be directly transferred by 
osmosis to the circulating blood. Even water and inorganic soluble salts, that 
undergo no metabolic alteration in passing through the body, temporarily 
join hands with other substances in various combinations. 

The army of enzymes, which are "substances of indefinite composition 
whose existence is known to us only by their action on other substances," 
play an indispensable role in making foods diffusible and available for use. 
Energy-producing carbohydrates, for example, if not taken in primarily in 
the form of soluble monosaccharids, must be made soluble by enzymatic 

Intake Apparatus 

3 2 3 

action. Such enzymes as ptyalin in saliva and amylopsin in the pancreatic 
juice change insoluble starches into soluble sugars. 

The complex proteins have two fundamental uses, namely, production 
of energy and restoration of tissues. As with carbohydrates, proteins must 
first be reduced to soluble form by the action of enzymes before they can 
be oxidized to produce energy, while to restore worn-out tissues the diverse 
complex molecules that make up proteins must be broken down and reas- 

TABLE V. Enzymes and Their Actions 







Amylolyt,c and Invertase 

inverting Maltose 


starch ) Lactase 

Lactic acid fer- 


Pancreatic juice 
Liver and muscles 
Small intestine 
Saliva and small 

Small intestine 


Starch to maltose 
Starch to maltose 
Glycogen to dextrose 
Cane-sugar to dextrose 
Maltose to dextrose 

Lactose to dextrose and 

Glucose to lactic acid 


Pancreatic juice 

Neutral fats to fatty acids 


and glycerines 

(Fat splitting) 


Liver, etc. 

Neutral fats to fatty acids 
and glycerines 


Gastric juice 

Proteins to peptones and 


Pancreatic juice 

Proteins to peptones and 



( Protein 


Small intestine 

Proteoses to amino-acids 



Pancreas, spleen, 

Nucleic acid to purin 

thymus, etc. 



Small intestine 

Trypsinogen to trypsin 


Thymus, adrenals, 

Guanin to xanthin 


Spleen, pancreas, 

Adenin to hypoxanthin 





Amino-acids to oxyacids 


Liver and spleen 

Arginin to urea 

324 Biology of the Vertebrates 

sembled into the specific kind of molecules that are the building blocks 
which go to form the different body tissues. This process, like wrecking an 
old house to build a new one, calls for a succession of breaking-down opera- 
tions, each dependent on a different enzyme. 

In the disposition of insoluble fats two things happen: they are emulsi- 
fied and then broken up into soluble fractions. Fats are said to be emulsified 
when mechanically broken up into particles of oil so small that each hangs 
suspended in a watery medium. Fats are chemically broken down into solu- 
ble fatty acids and glycerine, by such enzymes as steapsin in the pancreatic 
juice. These products are picked up by the epithelial cells of the intestinal 
lining in which they are reconverted into fats, which then pass into the 
lymphatic cores of the villi in emulsified condition, to arrive eventually in 
the blood stream. The emulsified fat, known as chyle, is milky in appearance 
as it is carried away by the lymphatics, which are therefore known as lac- 
teals (lact-, milk) . Of course milk itself is such an emulsion, too. 

Enzymes and what they do, a subject so appalling to the layman, fur- 
nishes the happy hunting ground for the physiologist. A list of the principal 
enzymes concerned with digestion, prepared by Burton-Opitz, is shown 
in Table V. 


The large intestine constitutes the egressive portion of the digestive tract 
through which the residue of the food mass is forwarded for expulsion after 
the usable part has been diverted through the walls of the small intestine 
into the circulating blood. Its diminished importance in the essential work 
of the digestive tract may account in part for the relative absence of struc- 
tural modifications designed to increase its inner surface. 

Except in the embryo it is without the villi which characterize the small 
intestine, and although Lieberkuhn glands are present in lessening abund- 
ance along its lining, whatever glandular secretions are present are not so 
much concerned with chemical digestion as with mechanical lubrication of 
the feces from which the water content has been largely withdrawn. 

The large intestine in man, which is about five feet in length, is differ- 
entiated into the colon, with ascending, transverse (from right to left), and 
descending portions, and finally the rectum, ending at the anal opening 
(Fig. 253). The colic caecum and the processus vermiformis, already men- 
tioned, properly belong to the large intestine, although they occur at the 
junction between the small and the large intestine. The same parts charac- 
terize the large intestine of many mammals but are not so distinctive in 

Intake Apparatus 125 

other vertebrates, since only the rectum of mammals is homologous with the 
large intestine of lower vertebrates. In the lower fishes as well as in the pipe- 
fish, Syngnathus, the stickleback, Gasterosteus, and others, the "large intes- 
tine" is actually smaller than the small intestine. 

In man the colon, which is looped around the small intestine (Fig. 253), 
is characterized by three bands of longitudinal muscles, the teniae coli, that 
pull this part of the intestine together so as to form three rows of pouches, 
or haustra, along its entire length, a modification present in varying degree 
in the large intestine of other mammals. Attached to the external wall of 
the haustra there are also numerous small processes of connective tissue, 
often distended with fat, called the glandulae epiploicae (Fig. 218), but 
neither the haustra nor the glandulae epiploicae extend to the rectum. 

In birds the rectal region of the large intestine is notably reduced. Since 
it is a disadvantage for these aerial creatures to carry about an unnecessary 
weight of useless fecal material, the provision for its temporary retention in 
a large intestine is curtailed, the excreta being disposed of as rapidly as 

In many vertebrates, including monotremes among mammals, the uri- 
nary and genital ducts enter the posterior part of the large intestine, which 
thus receives not only the feces from the digestive tube but also the products 
of the excretory and reproductive organs. This region, the common exit for 
all three systems, is known as the cloaca. In most mammals the cloaca splits 
longitudinally during embryonic development, thereby separating the more 
ventral urinary and genital pathways from the more dorsal rectal portion 
concerned solely with the feces. 

The rectum ends with the anus, which is kept closed by a sphincter 
muscle under the control of the will, unlike other circular muscles of the 
digestive tract that accomplish segmentation and peristalsis under involun- 
tary control. The inner walls of the rectum in man are modified by two or 
three transverse crescentic shelflike folds, the anal valves, which are doubt- 
less adaptations to erect posture, since they are absent in quadrupeds where 
the feces do not weigh against the anal sphincter. 

Metchnikof, as well as others, has pointed out that the large intestine in 
man, particularly in the rectal region, is a danger zone, a veritable "sewage 
swamp," because bacteria of various kinds flourish upon the undigested 
residue of the food retained there, contaminating the body constantly with 
the fermentive and toxic by-products of their metabolism. In one tabulation 
of 1 148 cases of cancer of the alimentary tract, eighty-nine per cent were 
located in the susceptible region of the rectum. 

There are some obvious advantages to mankind in possessing a volumi- 

326 Biology of the Vertebrates 

nous large intestine and rectum where excreta may be held temporarily so 
that their evacuation may be timed and controlled more readily than in the 
case of flying birds, although the disadvantages are far from negligible. It 
is somewhat difficult to see how so unfortunate an evolutionary acquisition 
could have been fostered by natural selection, but the fact that it reaches its 
greatest elaboration in herbivorous animals, where it is still useful in caring 
for plant foods rich in cellulose, may give some hint of why such a danger 
zone came to be acquired and preserved in man. 


A summary of the principal chemical adventures of food during its pas- 
sage through the digestive tube is shown in Figure 271. The blood stream 
receives the food from the small intestine after it has been reduced to absorb- 
able liquid form. This reduction, although partly mechanical, most of all 
involves a chemical breakdown into less complex substances, a process actu- 
ated by enzymes. 

The irregular lines in the diagram indicate the particular glands from 
which each enzyme arises, and, at the wavy interval on the line, the general 
locality in the digestive tract where it is effective, and also the kind of food 
involved in each instance. 

In conclusion it may be observed that when a hungry person sits down to 
a dinner table and surveys the viands spread before him, he rarely takes 
thought of all the necessary preliminary work that has made the occasion 
possible. Producers, handlers of raw materials, and an army of middlemen, 
as well as the manipulations of a cook, must at least be taken into account, 
yet in spite of all the detailed processes of preparation that have been 
involved, they are not a circumstance to what happens to the dinner in the 
twenty-five feet or so that it travels after it has been swallowed. 


In the chapter on Embryology we have seen how the dorsal and ventral 
mesenteries are formed by the meeting of the hypomeres dorsal and ventral 
to the digestive tract. These mesenteries consist therefore of two layers of 
mesoderm continuous with the parietal peritoneum, lining the body cavity, 
and the visceral peritoneum (serosa), covering the digestive tract. Thus they 
serve both as attachments of the digestive tract to the body wall and as 
bridges for the passage of blood vessels and nerves, which run between the 
two otherwise closely applied layers. During embryonic development the 
mesenteries undergo many changes including the dropping out of most of 
the ventral mesentery and modifications due to the great elongation of the 



"^ - J"~~Vl-- 






Biology of the Vertebrates 

intestine as well as the invasion of the mesenteries by such structures as 
liver, pancreas, and spleen. 

With the differentiation of the abdominal digestive tract into stomach, 
small intestine, colon, and rectum, the corresponding parts of the dorsal 
mesentery become known as mesogaster, mesentery proper, mesocolon, and 
mesorectum, respectively. When the liver grows into the anterior remnant 
of the ventral mesentery, the part of the latter between liver and ventral 
body wall is called the falciform ligament (ventral ligament of the liver), 
while the part between the liver and the digestive tract is the lesser omentum 
in which run the bile duct, hepatic portal vein, and other smaller blood 
vessels. Where the liver has pushed the two mesenteric layers apart to lie 
between them they become the serosa, peritoneal covering, of this organ. A 
posterior remnant of the ventral mesentery also persists as the ventral liga- 
ment of the bladder, extending from that organ to the ventral body wall. 


\ff Foramen ■*[//. 

L.>- Omental-.., 

~- Pancreas 


Duodenum- — \ 



A B 

Fig. 272. Longitudinal diagrams showing development of the great 
omentum in man. a, at four months; b, in adult. (After Arey and 

In mammals the mesogaster, which it will be remembered is composed 
of two layers of peritoneal tissue, becomes greatly elongated until it extends 
from the stomach posteriorly, in between ventral body wall and the coils of 
the intestine, to serve as a sort of ventral protective blanket for the viscera 
(Fig. 272). This folded portion of the mesogaster, known as the greater 
omentum, is a bag the walls of which are two layers thick. Where these walls 
become closely applied to one another in the posterior part of the sac, this 
omentum is an apron four layers thick. The omentum is also a storage organ 
for extensive fat deposits, laid down between what were originally the two 
mesenteric layers. 


Internal Transportation- 
Circulatory System 


The circulatory system is the mechanism that attends to transportation 
within the organism of various substances needed for its sustenance. 

Not only are food, water, and air taken into the animal body, but these 
indispensable substances must be distributed to every living cell wherever 
located, otherwise life would cease. Each living cell is a laboratory in which 
the chemical processes necessary for its existence are performed. Moreover, 
waste products, inevitable in the metabolic processes common to all living 
things, must be collected from every nook and cranny of the body for elim- 
ination, and transported either directly to the outside or indirectly by pas- 
sages leading to the outside. 

In microscopic unicellular animals internal transportation is accom- 
plished by the movement of the cytoplasm within the cell itself. It is easy to 
see how small particles of food materials, once introduced into a protozoan 
like Amoeba for example, whose cell contents flow around like cold molasses, 
may shift their position within the cell body. This intracellular cytoplasmic 
movement, which is the simplest kind of a circulatory system, is entirely 
adequate for bodies of microscopic mass. It persists, moreover, as the final 
essential phase of circulation even in organisms built up of countless cellular 
units, since intake materials must not only reach each individual cell, but 
must also become incorporated and circulated therein as well. 

In multicellular forms most of the component cells do not have direct 
surface exposure to the environment. The only way, therefore, that food, 
water, and air can enter the deeper cells of such organisms and penetrate to 
their ultimate parts, is either through or between the cells that form the 
body. The passing of materials through by osmosis from one cell to another 
is a slow and indirect method; also the ultimate cell in the series receives 
only what is left after all the intervening ones have taken their toll. Conse- 
quently it is a great improvement when passage-ways are opened up 

\ 3 2 9 

33° Biology of the Vertebrates 

between the cells so that materials may be forwarded freely, thus arriving 
at the ultimate cells directly and easily without repeated osmotic manipula- 
tion. The same sort of an advance in efficiency is seen when fire-fighters sub- 
stitute a hose line for a makeshift hand-to-hand "bucket brigade." The 
circulatory system proper may be briefly described as consisting of a network 
of pipelines throughout the body, together with a fluid vehicle in the form 
of blood and lymph for transporting the various materials necessary for the 
life of the organism, and lastly, a mechanism for propelling these fluids 
through the channels of distribution. 


1. Uses of the Blood 

Mephistopheles, in Goethe's Faust, is biologically quite right when he 
exclaims: "Blut ist ein ganz besonderer Saft!" (Blood is a very peculiar 

In the first place it is not a lifeless fluid incapable of metabolic changes, 
but a circulating tissue, made up of detached living cells floating in a liquid 
plasma. The fact that it moves about marks it off distinctly from all other 
tissues. It is conspicuously kaleidoscopic in character, changing constantly 
in its intimate composition as it passes through the different parts of the 
body, for it is not only the common carrier to the various tissues of every- 
thing needful for their maintenance, but also the collector from the tissues 
of the products of metabolism, such as carbon dioxide and soluble nitrogen 

In the last analysis cells of every kind within the animal body sooner or 
later contribute something to the blood and receive something in return, so 
that the blood reflects the entire metabolism of the body. In spite of this fact 
its component parts are normally kept at nearly a constant level. When seri- 
ous deviations occur, as in the excessive loss of water from the blood during 
cholera, or the diminution of the number of blood cells in anaemia or 
hemorrhage, pathological consequences are sure to follow. 

In general the following functions may be performed by the blood. 

(a) Equilibration of Water Content. — Water is the fluid universal that 
facilitates the internal transport of materials, making good losses by evapo- 
ration and otherwise, as well as preventing the drying up of tissues. It is also 
the great solvent of all sorts of substances in the food and throughout the 
tissues. The degree of activity exhibited by any tissue is directly dependent 
upon the fluidity of the cytoplasm within its cellular units, which in turn is 
ultimately a matter of the amount of water supplied by the blood. 

Internal Transportation 331 

(b) Liberation of Energy. — Tissues "burn" in the presence of oxygen, 
thus releasing energy, which is what constitutes "living." Certain softer tis- 
sues, like the muscles for example, lend themselves particularly to this oxi- 
dative process, while others, such as skeletal tissues, resemble more the iron 
girders of a "fire-proof" building, and do not burn as readily. The oxygen 
necessary for the release of stored energy is delivered to the tissues of verte- 
brates by the haemoglobin in the red corpuscles of the blood. 

(c) Distribution of Food. — From one point of view blood is a solution 
or an emulsion of food substances carried in the plasma. To some extent 
also it transports food as solid undissolved particles engulfed in the white 
blood cells. It is thus the grocery delivery boy for the cellular community. 

(d) Regulation of Temperature. — Body temperature, which results 
from the oxidation of the tissues, is equalized by means of the circulation of 
the blood, much as is the temperature of a building by the hot-water pipes 
of its heating plant. Such equalization is necessary because of the unequal 
production of heat-energy by different tissues of the body. 

In so-called "warm-blooded" animals the body temperature is main- 
tained at a practically constant level, regardless of the temperature of the 
environment, thus enhancing the animal's independence. "Cold-blooded" 
animals, on the other hand, owing partly to the low oxygen-carrying capac- 
ity of their blood, have a body temperature which fluctuates in response to 
that of the surroundings. A cold-blooded animal is consequently a thermal 
slave to the environment in which it finds itself. 

It has been demonstrated, however, that even cold-blooded fishes when 
ill may show fever-like fluctuations in temperature. 

(e) Transmission of Chemical Substances. — Hormones, the chemical 
messengers from endocrinal glands, frequently perform metabolic feats at 
some distance from their point of origin, after traveling along the blood 
highways. Drugs and poisons introduced into the organism likewise gain 
ready disposal over the body by means of the circulating blood. This is why 
a person with malaria, for example, feels "sick all over," since the blood 
carrying the organisms producing malarial poison literally goes over the 
whole body. 

(/) Defense against Parasitic Invasion. — Troublesome foreign invaders, 
such as infective bacteria and protozoans, are regularly combatted by white 
blood cells which capture and devour them. The cures of most "catching" 
diseases depend upon the successful outcome of this function of the blood. 

(g) Disposal of Cell Wreckage. — The blood is a continual funeral cor- 
tege, in which are being borne away the "ashes" of dead cells, foreign 
bodies, bacterial products, and wastes of metabolism generally. 

S3 2 Biology of the Vertebrates 

(h) Chemical Elaboration. — Furthermore, blood may be regarded as 
a peripatetic laboratory in which chemical transformations of wide variety 
are constantly going forward, as for example, the formation of "antibodies" 
of various sorts, oxidation and reduction of haemoglobin, elaboration of 
fibrin, and changes of fats and sugars to and from soluble states. 

(i) Clinical Diagnosis. — All of the functions of the blood thus far indi- 
cated have to do with personal biological benefits to the animal itself. Blood 
may also be useful, outside of the individual who elaborates it, to the physi- 
cian in identifying disease. No other tissue gives to the diagnostician so true 
a flashlight picture of the present state of the varying metabolism of the 
body as does the blood. The ease with which a sample of blood may be 
obtained for examination without injury to the patient, and the readiness 
with which deviations from the normal are revealed therein, have resulted 
in an increasing dependence upon it as a means of clinical diagnosis. For 
example, in a suspected case of either typhoid fever or appendicitis, the 
examination of the blood furnishes an immediate differential diagnosis, since 
in typhoid fever the number of white blood cells is below, while in appendi- 
citis it is above normal. There is no doubt that dependence upon blood 
examination in future medical practice will increase as technic is further 
perfected and new approaches to the study of blood are developed. 

2. Amount of Blood 

In adult man the amount of blood is estimated to vary from about one 
twentieth to one fifteenth of the total body weight, that is from seven and 
a half to ten pounds for a person weighing one hundred and fifty pounds. 
This is approximately six quarts. In a new-born child the percentage of 
blood to body weight is less than in the adult, while in lower vertebrates the 
relative amount of blood is less than in mammals. Haempel gives the quan- 
tity of blood in fishes as less than two per cent of the total body weight. 

The blood supply is temporarily increased in those regions that are 
active, as for example, in the wall of the stomach immediately after eating, 
or in that of the small intestine during digestion. 

As to specific gravity, which is dependent mainly upon the amount of 
haemoglobin present, "blood is thicker than water." For man the figures 
have been given as 1.035 to 1.067 (with distilled water at 1.000). 

3. Erythrocytes 

Our sanguinary forebears, as well as our contemporaries, were well 
acquainted with the general appearance of blood, for the pages of history 
are copiously stained with it. Not until 1696, however, about two centuries 

Internal Transportation 353 

after Columbus had discovered America and his adventurous bones had 
turned to dust, did the Hollander, Anthony van Leeuwenhoek, find, with 
his primitive lenses, that blood is "composed of exceeding small particles." 
These he named globules which he said "in most animals are of a red color, 
swimming in a liquor, called by physicians the serum," and further, that "by 
means of these globules the motion of the blood becomes visible, which 
otherwise would not be discoverable by sight." 

Red corpuscles are for the most part peculiar to vertebrates, although a 
few invertebrates, including the worms Glycera and Phoronis, the "blood 
clam" Area, and the holothurian Thyone, also possess red blood cells. Ordi- 
narily whenever invertebrates show red blood, as for example the pond 
snail Planorbis, the earthworm Lumbricus, or the larvae of the midge 
Chironomus, the haemoglobin producing the color is not located in red cor- 
puscles but is dissolved in the plasma. 

Red corpuscles, or erythrocytes as they are technically known, are 
directly concerned with respiration, the exchange of gases involved being 
facilitated by means of the respiratory pigment haemoglobin, which is 
present inside of the corpuscles. This pigment, which has a very complex 
molecule 68,000 times as heavy as the molecule of hydrogen, is a compound 
of iron and globulin, and possesses the ability to take on and give off oxygen 
readily. It is thus a sort of a shuttle device between outside oxygen and cells 
in the body that are in need of oxygen. Haemoglobin in the red blood cells 
is said to have the power of taking on seventy times as much oxygen as an 
equal volume of blood plasma, which can carry oxygen only in solution. The 
haemoglobin molecule thus loaded with oxygen assumes a brighter red color 
and becomes a very unstable substance, called oxyhaemoglobin. The pecul- 
iar ability of this molecular structure to transport the amount of oxygen 
necessary in breathing, may be destroyed by the action of certain "poison 
gases," such as carbon monoxide, for which haemoglobin has 250 times the 
affinity that it has for oxygen. When this gas, escaping from the exhaust of 
a running automobile engine in a closed garage, is inhaled, the oxygen of 
the oxyhaemoglobin molecule may be supplanted by carbon monoxide with 
the result that survival and recovery from suffocation will follow only if 
the person is hastily removed to fresh air where the oxygen will slowly 
replace the carbon monoxide. Haste is important to minimize the destruc- 
tion of cells, particularly irreplaceable neurons. (See U. S. Pub. Health 
Bull. Vol. 195; 1930, No. 211.) 

Erythrocytes of most vertebrates are oval discs that appear to bulge in 
the center on account of the presence of a nucleus. Among mammals ( and 
lampreys ! ) erythrocytes are more circular in outline, with a single exception 


Biology of the Vertebrates 


Fig. 273. Successive stages in the 
loss of a nucleus by an erythrocyte. 
(After Jordan and Ferguson.) 

in the case of camels and llamas which have oval erythrocytes resembling 
non-mammalian red blood cells in form. The nucleus in mammalian red 
blood cells disappears by extrusion, leaving the cell a degenerate sac, or 
stroma capsule, having an internal structure imperfectly understood but con- 
taining a concentrated solution of haemo- 
globin. The original name "corpuscle" 
(small body) fits the erythrocyte with 
more accuracy than "red blood cell," 
since nuclei are always associated with 
cells. Diagrams of the method of nuclear 
loss are shown in Fig. 273. 

In size erythrocytes, measured from 
dried smears of blood, range from 75 
micra (a micron is one one-thousandth 
of a millimeter) in the caudate Amphi- 
uma to 2.5 micra in the musk deer Trag- 
ulus. Other measurements in micra for the longest diameter of blood cor- 
puscles from dried smears have been reported as follows: Proteus, 58; frog, 
22 ; turtle, 20; carp, 17; pigeon, 14 ; lamprey, 13 ; chick, 12; conger eel, 1 1 ; 
elephant, 9.4; man, 7.5; dog, 7.3; rabbit, 6.9; cat, 6.5; horse, 4.6; goat, 4.1. 
In man the measurement is from 7.1 to 8.3, with an average of 7.5. In 
all these cases fresh corpuscles are slightly larger than dried smear specimens. 
In general the smaller the corpuscles the more surface they expose for 
oxygen transfer in a given volume of blood, just as there is more "skin" in 
a bushel of small crab apples than in a bushel of big apples. One of the fac- 
tors determining the "cold-bloodedness" of lower vertebrates as contrasted 
with mammals is the larger size of their erythrocytes with correspondingly 
smaller total surface of exposure for oxygen intake. 

The number of erythrocytes in the blood is of considerable clinical 
importance. In man it is normally about 5,500,000 per cubic millimeter in 
the male and 5,000,000 for the somewhat less metabolic female, while in 
the highly metabolic infant the count is greater than in adults of either sex. 
The total number of red corpuscles in the average human being has been 
estimated at 30 trillions representing a respiratory surface 1500 times as 
large as the surface of the body itself. The number of erythrocytes varies 
with regard to the time when taken and other factors. According to Vierrodt 
the number of red blood cells of hibernating animals drops to as little as 
one third of the normal count during their "winter sleep." The red blood 
cells are more numerous during the day than at night and directly after 
rating or violent exer< ise, as well as in high altitudes when mountain climb- 

Internal 1'ramportdtion 335 

ing or in airplane flights. Patients who are combating tuberculosis some- 
times resort to high altitudes where the lessened pressure of rarified air is 
supposed to demand a greater surface exposure to haemoglobin to accom- 
plish the normal amount of respiration, thus forcing the body to self-cura- 
tive effort by producing more red blood corpuscles in compensation. 

Haemolysis, that is the wearing out or destruction of erythrocytes, is the 
inevitable outcome of their strenuous travels through the blood channels. 
Physiologists have attempted to estimate the length of life of a single aver- 
age erythrocyte by measuring the density of the pigments in the bile, since 
these are due principally to the haemoglobin released from the broken- 
down erythrocytes that are being eliminated. When capillaries of the skin 
are ruptured so that blood oozes out into the surrounding tissues, the liber- 
ated haemoglobin from the erythrocytes breaks down with a display of 
pigments, a good example of which is a "black eye" with all its rainbow- 
like variegations. The amount of haemoglobin necessary to produce a known 
degree of color in a measurable quantity of bile eliminated during a known 
interval of time, gives a rough idea of the rate at which new blood cells must 
be manufactured in order to maintain a comparatively constant level of 
erythrocytes throughout the entire body. Those who have ventured to specu- 
late on this problem place the life span of a red blood corpuscle from ten to 
seventy days, which means that at the outside the erythrocyte population is 
completely renewed several times each year. Since the total number is esti- 
mated to be 30 trillion cells it means that the continuous production of red 
blood cells within the average human body, if reckoned on the most con- 
servative basis, must go forward at the rate of several thousand every second. 

The process of the formation of red blood cells, which is termed haemo- 
poiesis, is accomplished before birth in the mesenchymatous tissues and also 
in the liver and spleen. In fishes and amphibians the spleen forms red cells 
even in the adult animal. In other adult vertebrates, particularly in mam- 
mals, haemopoietic tissue is mostly confined to the red marrow in the hollow 
bones, in which factory the major part of the astonishing output of eryth- 
rocytes takes place. 

4. Leucocytes 

Intermingled with red corpuscles in the blood are "white blood cells," 
or leucocytes, best described as wandering cells that are not always confined 
to the blood channels, and which are independent of the nervous system. 
Unlike the erythrocytes of mammals, these are detached cells that not only 
retain their nuclei throughout life, but also possess other characteristic fea- 
tures of true cells. 

33 6 

Biology of the Vertebrates 


Within the same organism leucocytes show considerable differences with 
respect to the character of their nuclei, general size, shape, and function, 
differences that make possible their classification into three general cate- 
gories, namely, lymphocytes, granulocytes, and monocytes. It should be 
noted that this classification is based on the leucocytes of human blood 
which have been most studied. 

Lymphocytes (Fig. 274) constitute normally something like 22 to 25 
per cent of all leucocytes. They are roughly spherical, with a single large 

nucleus, and are about the size of ery- 
throcytes, being from 6 to 8 micra in 
diameter. No granules are present in 
their cytoplasm. 

Monocytes are giant amoeboid 
mononuclear leucocytes, twelve or 
more micra in diameter, constituting 
normally from 2 to 10 per cent of all 
leucocytes. Their nuclei are relatively 
small while their abundant cytoplasm 
is without granules. 

Granulocytes, which are somewhat 
larger than lymphocytes, being from 
7 to 10 micra in diameter, are char- 
acteristically amoeboid in changing 
shape and are chemotactic in behavior. 
They are often referred to as "poly- 
morphonuclear leucocytes" from the 
fact that their nuclei generally assume 
a variety of shapes. The granular cytoplasm, which gives the name "gran- 
ulocyte" to these cells has differential staining properties that, according to 
Ehrlich, serve to classify them chemically still further into neutrophils, eosin- 
ophils, and basophils, depending upon whether the granules have an affinity 
for (philos, love) neutral, acid, or basic dyes, respectively. The two latter 
kinds are comparatively rare, forming only about 3 and 0.5 per cent respec- 
tively of the total number of leucocytes, while the neutrophils furnish in 
the neighborhood of 70 per cent. As these percentages vary within quite 
wide limits pathologically, they make an extremely valuable indicator of 
abnormal conditions for the clinician. 

Dana and Carlson have pointed out that the number of new leucocytes 
contributed to the blood stream daily may be greater than the total average 
number present at any one time in the blood. In man the number of 


Fig. 274. Blood cells. 1, red blood cor- 
puscles in rouleau formation; 2, red 
blood corpuscles, surface view; 3, lym- 
phocyte; 4, polymorphonuclear leuco- 
cyte; 5, basophil leucocyte; 6, large mon- 
onuclear leucocyte; 7, blood platelets; 8, 
nucleated erythrocyte. (After Jordan and 

Internal Transportation -->- 

leucocytes of all kinds varies from 2000 to 13,000 per cubic millimeter with 
a normal average of around 7000. The numerical variation is proportion- 
ately much greater than that of erythrocytes. There is a normal increase in 
the total number of leucocytes for instance after vigorous exercise or eating, 
upon exposure to cold, in infections, and during pregnancy. When the 
number rises over 10,000, a pathological condition is indicated. 

With regard to their origin different kinds of leucocytes, like erythrocytes, 
may be produced in different parts of the body. In mammals certain 
embryonic cells in the marrow, haemoblasts, that do not circulate within the 
blood stream, are no doubt the busiest agencies for the manufacture of 
granulocytes as well as of red blood cells. Lymphocytes and monocytes 
are formed in lymphoid tissue throughout the body, in the lymph nodes and 
lymph "glands," of which the spleen is the largest representative. 

The three different kinds of leucocytes accomplish a variety of functions. 
For example, the wandering granulocytic neutrophils, as well as mono- 
cytes, remove worn-out tissue cells and invading bacteria by engulfing and 
digesting them in true amoeboid fashion, when they are known as phago- 
cytes (phag, eat). 

Fig. 275. Diapedesis, the passage of a leucocyte through the wall of a 
capillary. (After Clark, Clark and Rex.) ■ 

Owing to their amoeboid facility in assuming a variety of shapes they 
are able to squeeze between even the irregular edge-to-edge margins of the 
flat endothelial cells forming the thin walls of the capillaries (Fig. 275), 
thus escaping entirely from the closed blood system into the interstices 
between the cells of tissues everywhere in their phagocytic forays. This 
escape of the phagocytes through the capillary walls is termed diapedesis. 
There is indeed hardly a nook or cranny within the body that cannot be 
sought out and penetrated by these nomadic benefactors in the course of 
their sanitary and curative peregrinations. 

The mobilizing of phagocytes is particularly well demonstrated when a 
wound becomes infected by bacteria. The "inflammation" that results in 
swelling and pain is due to the crowded ranks of phagocytes assembled for 
a battle royal with the invading army of bacteria. If the bacteria win, "blood 
poisoning" with its gruesome consequences results. If the phagocytes win, 

33^ Biology of the Vertebrates 

health is restored, "Pus" is largely made up of dead phagocytes that have 
fallen in battle. Inflammation of the mucous membranes, or "catarrh," does 
not result in pus formation ordinarily, but in the production of a local excess 
of lymphatic fluid. 

The holes in the capillary walls through which the phagocytes escape 
are immediately closed, like a puncture in an automobile tire, so that the red 
blood cells are kept within the blood vessels. Vagrant phagocytes, however, 
like the prodigal son, return to the blood stream. They do not reenter the 
capillaries from which they have escaped but are picked up by the lymphatic 
vessels that permeate everywhere between the cells of the tissues. By means 
of a system of valves these lymph-containing vessels forward their cargo by 
one-way traffic towards veins that enter the heart, and thus restore the 
runaway phagocytes to the general circulation. 

Lymphocytes are neither amoeboid nor phagocytic, but monocytes and 
granulocytic neutrophils are both. Moreover, the non-phagocytic lymph- 
ocytes retain their spherical contour, never by diapedesis joining the 
phagocytes as "free lances of our corporeal militia" (Slosson). Instead they 
collect in the villi of the small intestine to engage in the transfer of fat 
globules, enmeshed in their cytoplasm, by way of the lacteals to the blood 

5. Thrombocytes 

In addition to erythrocytes and leucocytes there are present in the 
vertebrate blood less well-known bodies, generally called thrombocytes. 

In the frog these have been described as "spindle cells," intermediate 
in character between red and white blood cells, and able possibly upon occa- 
sion to be transformed into either. 

Although true spindle cells with nuclei have been found in the blood of 
certain fishes, amphibians, reptiles and birds, they are not present in mam- 
malian blood, their place being taken apparently by small bodies which 
Bizzozero has named blood platelets (Fig. 274). 

The unstable character of the thrombocytes is shown by the fact that 
they tend to mass together and disintegrate as soon as the blood is shed and 
exposed to air, which makes careful detailed observation of them difficult. 

The term "thrombocyte" (thrombus, clot ; cyte, cell) is not a very happy 
one to apply to blood platelets since it is doubtful if they are true cells but 
more likely small enucleated fragments of cells, with slight amoeboid 
motility. They have to do, however, with forming the "thrombus," or clot, 
that prevents excessive hemorrhage in case of wounds. 

The different kinds of thrombocytes in human blood vary in size from 

Internal Transportation 339 

0.5 to 4 micra in diameter, thus being considerably smaller than erythro- 
cytes. They have been estimated to number from 200,000 to 778,000 per 
cubic millimeter, with 500,000 given by Howell as an average for human 
blood. According to Wright blood platelets have their origin as fragments 
of the giant cells of bone marrow from which they are constricted off. 

6. Plasma 

Two thirds of the blood is fluid plasma in which the different kinds 
of cellular elements are borne along through blood vessels. 

Plasma is about 90 per cent water, 9 per cent organic substances such as 
fibrinogen, paraglobulin, and serum albumin, and about one per cent 
organic salts, which brings it up to approximately the same density as sea 
water. Animals with blood of balanced density living submerged in sea water 
do not suffer from upsetting osmotic exchange, which can be dangerous 
or even fatal when it occurs suddenly as in the case of salt-water fishes that 
are transferred to fresh water or vice versa. The salts dissolved in human 
blood plasma are reminiscent of osmotic conditions long ago when our 
remote ancestors had not yet emerged from an aquatic habitat. 

The plasma is a non-living fluid of much more chemical complexity 
than appears in the test tube. It contains a constantly changing variety of 
substances in solution, chief among which are dissolved food materials on 
the way to cellular delivery, and waste products that are being collected 
for elimination. There are also present enzymes of divers sorts which activate 
chemical changes: opsonins, that prepare trespassing bacteria for phago- 
cytosis; hormones, the chemical messengers from endocrine glands on their 
way to the performance of tasks of internal regulation ; antibodies and 
other problematical substances engaged in constant warfare against harmful 
invasion; and finally, fibrinogen, which although ordinarily free-flowing 
like other dissolved substances, can be turned when necessary into insoluble 
fibrin, that forms an entangling mesh like a barbed wire barrier through 
which cells do not easily pass. This is the clot which acts as an emergency 
plug to prevent the escape of blood from wounds while organic repairs are 
being made. 

According to Howell, a substance in the plasma known as prothrombin, 
together with calcium salts, may form thrombin upon exposure to a rough 
or ragged surface, such as the edges of a wound that are unlike the smooth 
inner walls of blood vessels. Thrombin has the power of transforming 
soluble fibrinogen into insoluble fibrin which in turn entangles the blood 
cells and forms a clot. 

Sometimes a blood clot forms around a solid bodv or breaks free from 

34° Biology of the Vertebrates 

a wound, when it becomes an embolus. Such a clot circulating within blood 
vessels may obstruct a capillary or a larger vessel and cause trouble. For 
example, if it blocks the first branching of the lung artery, it holds up the 
entire circulation and may cause sudden death. An embolus may arise from 
other causes than an outside wound, as in the case of "hardening of the 
arteries," when the wall of the blood vessel may become ruptured. If a 
traveling embolus is caught in the capillaries of the brain, it may give rise 
through pressure to a "shock," or apoplexy, recovery from which is depend- 
ent upon the removal or absorption of the embolus within a reasonable 

If it were not for the mechanism of the blood clot, loss of blood from 
even slight wounds, or breaks in the walls of the blood vessels allowing leak- 
age, would prove to be much more serious than it is. 

Haemophilia is a hereditary condition in which some link in the chain 
that normally results in clot formation is missing so that the inability to 
stop blood leakage from even a small wound may result fatally. Persons 
thus afflicted are known as "bleeders." Males are more susceptible to 
haemophilia than females, since it requires inheritance of the trait from 
both sides of the house to make a female haemophilic, while inheritance 
through one parent is enough to cause a male bleeder. 

1. The Evolution of Organic Irrigation 

Blood channels may be regarded as a device for increasing the inner 
surface area of an organism with reference to exposure to adequate nutritive 
and respiratory factors. In most invertebrates blood channels are largely 
open lacunar, or perivisceral spaces. 

The first evolutionary stages in the development of a circulators' 
apparatus are perhaps to be seen in the porous sponges, whose "blood," that 
is, the surrounding water, carries a random load of microscopic food and 
dissolved air past the loosely organized colonial cells within the sponge body. 

In flatworms, and some medusae also, there are neither true blood 
channels nor any specialized food-carrying medium of blood, since in these 
lowly creatures the digestive tube itself branches out like the twigs of a 
tree, extending between the cells of the body in such a way as to effect a 
direct delivery of needful nutriment without the mediation of a blood 

Among vertebrates two general types of channels appear, namely, a 
haemal system of closed tubes carrying blood, and an auxiliary lymphatic 

Internal Transportation ^ 2 

system carrying lymph, which is practically blood without respiratory red 
corpuscles. In general these channels form an irrigation system of flexible 
plumbing, consisting of a continuous series of cavities, lined throughout with 
flat endothelial cells, in which the blood circulates. 

The fact that blood is not subject to ebb and flow but is constantly 
in motion always in one direction, and that it repeatedly during life makes 
the entire circuit of the blood vessels, was established in 1619 by William 
Harvey (1578-1657), long before anyone actually saw the blood pass 
through the smaller connecting channels. It was Malpighi who in 1661 dis- 
covered the capillaries by which the out-going and in-coming blood vessels 
are connected. In 1696 Leeuwenhoek charmingly described the capillary 
circulation in a bat's wing as follows: "I perceived in many places an artery 
and a vein placed close beside each other and of a size large enough to 
admit the passage of ten or twelve globules at the same time ; and in this 
artery the blood was protruded or driven forward with great swiftness, and 
flowed back through the vein, which was a most pleasing spectacle to 

Although the circulatory system penetrates to nearly every part of the 
living organism, there are a few regions of the highly differentiated verte- 
brate body that are not invaded by blood vessels of any kind, namely, the 
cornea of the eye, cartilage tissue, and the epidermis together with its 
derivatives, hair, nails, feathers, horns, claws, and the enamel of teeth. 

2. General Plans of Circulation 

Blood vessels are related to each other as shown in Figure 276. 

Arteries (centrifugal) 

Arterial capillaries 

Venous capillaries 
Veins (centripetal) 
Fig. 27G. The relation of blood vessels to one another. 

(a) Annelid Plan. — Two longitudinal blood vessels, one dorsal and one 
ventral, with collateral connections, and joined at either end by capillary 
networks, make up the main circulatory system of the practically heartless 
annelids. The blood flows forward through the dorsal vessel and backward 
through the ventral vessel in the simplest manner (Fig. 277). 

3-t 2 

Biology of the Vertebrates 

( b ) Amphioxus Plan. — The circulatory system of amphioxus is in many 
respects a preview of the basic vertebrate plan. In the first place the blood 
flows around in the opposite direction from that of the circuit in the annelid 
plan. By rotating an annelid 180° on its long axis, thus shifting its dorsal 
and ventral sides, the course of its general circulation may be brought into 
agreement with that of amphioxus. 

As in the lower vertebrates, a large part of the anterior capillary system 
is associated with the pharynx region, which makes up about half the length 
of the digestive tube in amphioxus. Many of the posterior capillaries enwrap 
the remainder of the digestive tract. In addition there is introduced a new 
feature consisting of an extra capillary network spreading over the liver 
diverticulum to interrupt the vessel leading from the intestine to the gills. 
Thus an hepatic portal system is set off from the rest of the veins (Fig. 277 ) . 

Dorsal Blood Vessel 

Body Tissues 


Dorsal Ao rta 

X Ventral 


Intestine v 

Hepatic Porte I body Tissues 
Fig. 277. Plan of circulation of annelid and amphioxus contrasted. 

At the posterior end of the pharynx the hepatic vein, from the liver, is 
joined by cardinal veins, from the body wall, to form the ventral aorta, 
which carries blood to the pharyngeal wall. It is by means of peristaltic- 
waves, which pass anteriorly along the ventral aorta, that the blood is 
pumped through the various parts of the circulatory system of amphioxus. 
At least the posterior part of this ventral aorta is probably homologous with 
the heart, the great pumping organ of vertebrates. 

(c) Gill Plan of Fishes. — The amphioxus plan becomes further elabo- 
rated in fishes (Fig. 278) by the development of a heart, or blood pump, 
and by the introduction of an additional capillary complex involving the 
excretory organs, thereby establishing the renal portal system. 

The heart is simply a muscular enlargement and modification of a part 
of the main ventral blood vessel, lying between the hepatic capillaries and 

Internal I 'ransportation 


the gills, through which the blood flows forward. The development of the 
vertebrate heart from this ventral vessel has its possible homology in the 
rotated annelid plan, since in the latter it is the dorsal vessel which becomes 
muscular enough to pulsate and serve as a pumping organ. 

Dorsal Aorta 

Renal Portal 


Aorta i 



Body Tissues] \& 

Cardinal Vein 
Fig. 278. Plan of circulation in a fish. 

The return of blood from the region of the large propeller-like tail, char- 
acteristic of fishes, is effected by means of the rental portal system which 
carries the blood through a capillary network in the kidneys, whence it joins 
the main blood stream. In fishes, therefore, besides the capillaries which 
unite the outward-bound distributing system of blood vessels (arteries) with 
the inward-bound collecting system (veins), there are two major strainer- 
like complexes of capillaries within the kidneys and the liver respectively, 
that interrupt the large vessels and modify the stream of blood returning 
to the heart. 


Dorscl Aorta 


\% Pulmonary Artery 
Auricles: Is N £C 

D- Le u!~>5?V X Pulmonary! 
Right'/ rA^L VeinJ; 

Hepatic Portal ^ ~ Intestine y 

v / S" X 

Lacteals/' ,^"' Lymphatics 

Vena Cava 
Fig. 279. Plan of circulation in a mammal. 

(d) Lung Plan of Mammals. — The general plan of circulation among 
higher vertebrates, when reduced to the simplest terms, may be repre- 
sented by a diagram (Fig. 279). The dotted lines, which are connected at 
only one end with the closed haemal circulatory system, show the relation 
of the auxiliary lymphatic vessels by means of which lymph is collected 


Biology of the Vertebrates 

from all regions of the body, together with the white blood cells that have 
escaped by diapedesis from the capillaries, returning them to the venous 
system just before reaching the heart. 

The change from branchial respiration by means of gills to pulmonary 
respiration through lungs makes necessary the introduction of a double 
blood circuit, namely, the systemic circulation over the body and the pul- 
monary circuit to the lungs, with two central clearing houses, or hearts, 
instead of a single one as in fishes. These two hearts are placed so intimately 
together, however, that when looked at superficially they have the appear- 
ance of a single heart. 

With the diminishing importance of the tail upon emergence from the 
water and the evolution of locomotion on land by means of legs, the renal 
portal system becomes discontinued. Thus it will be seen that the changing 
methods of respiration as well as of locomotion have modified the circula- 
tory plan in land animals. 

3. Structure of Blood Vessels 

The walls of blood vessels show certain differences that serve to dis- 
tinguish arteries, veins, capillaries, and lymphatics from each other. 

Arteries and veins of the same size externally are not easily confused, 
since veins have thinner walls and a larger bore inside, and consequently 
are more liable to collapse than arteries when emptied of blood. The walls 
of both arteries and veins are made up of three layers of tissue (Fig. 280) 

Tunica Adventitia 

Tunica Media 

^3§£\\\ Tunica Intima 



Fig. 280. Transverse section of an artery and a vein of corresponding 
size, showing the three layers. (After Huxley.) 

known as tunica intima, tunica media, and tunica adventitia. The inside 
layer, or tunica intima, is invariably composed of a lining of flat endothelial 
cells, continuous and universal throughout all the blood vessels including 
the heart itself. This lining, except in the capillaries, is wrapped about by 
reinforcing connective tissue. The middle layer, or tunica media, is largely 

Internal Transportation 


composed of smooth muscle cells, arranged mostly in circular fashion and 
interspersed with connective tissue fibers, while the outer layer, or tunica 
adventitia, is principally connective tissue, more or less elastic and pene- 
trated by lymphatics as well as vaso-motor nerve fibers that control the 
changing caliber of the blood vessel. 

There are certain large veins in man, such as the umbilical, iliac, splenic, 
renal, and superior mesenteric, which are noteworthy because longitudinal 
muscle fibers also are found in the outside layer of the wall. 

Arteries are thick-walled conduits carrying blood away from the heart, 
and are characterized by a well-developed elastic tunica media that is thick 
enough to maintain the shape of the blood vessel without collapse. The 
tunica adventitia in arteries is relatively thin. 

As arteries follow their course throughout the body away from the heart, 
they gradually decrease in size, at various stages being called arterioles and 
arterial capillaries, until eventually they become true capillaries with very 
thin walls and minute bore, making it necessary for blood cells to pass in 
"Indian file" and even to assume distorted shapes in order to squeeze through. 

The true capillaries, which lack both a tunica media and a tunica ad- 
ventitia, form innumerable anastomoses and networks between the arterial 
capillaries on the one hand and the venous capillaries on the other. 
Arterial and venous capillaries, therefore, 
are transitional in location as well as in size 
and in thickness of their walls between the 
capillaries proper and arterioles and ven- 
ules respectively. 

Veins, which always take their rise 
from capillaries, are relatively thin-walled 
and collapsible. Since their walls are 
largely deficient in elastic tissue and muscle 
cells of the tunica media, the tunica ad- 
ventitia becomes the most highly developed 
of the three layers in the walls of veins. 

Pocket-like valves that prevent or hin- 
der the back-flow of the blood (Fig. 281 ) 
are present in the larger veins but not in 
arteries, except in the immediate neighbor- 
hood of the heart of some gill-breathers. Valves are not present in all veins, 
being largely absent from the veins of the brain, cord, meninges, bones, and 
the umbilical vein, as well as most visceral veins, excepting branches of the 
hepatic portal system. The thin-walled veins are much more likely to anasto- 

A B C 

Fig. 281. Valves in veins. A, swol- 
len vein from the outside indicating 
the presence of a valve; b, valve 
open allowing the blood to flow in 
the direction of the arrows; c, valve 
closed preventing the backflow of 
blood in the direction of the arrows. 

2^6 Biology of the Vertebrates 

mose, become varicose, or to enlarge into sinuses, than the thick-walled 

Lymphatics are typically varicose as well as capable of great distension. 
Ordinarily they do not acquire thick muscular or elastic walls and are very 
collapsible, although the larger lymphatics nearer the heart develop a definite 
tunica media with muscle cells that even enable them to pulsate. Lymphatics 
entwine around the other blood vessels in the most intimate fashion, yet do 
not communicate with them except at one or two definite openings near 
the heart where the lymph may be returned to the general blood stream. 
Lymph capillaries, although never as small as haemal capillaries, have the 
same sort of thin endothelial walls. 

Physiologically, if not morphologically, the large serous cavities, such 
as the body cavity, and the pericardial and pleural cavities, as well as the 
synovial spaces around joints, belong to the lymphatic system, although 
their walls have a somewhat different origin and structure from those of 
ordinary lymphatic vessels. 

Like veins, lymphatics possess valves along their course in the form of 
crescentic folds of the tunica intima which act like sluice-gates preventing 
the retreat of the fluid to any great extent in the direction away from the 
heart. Thus they establish one-way traffic in the lymphatic system. 

4. The Role of the Capillaries 

The first blood vessels to form in the embryo and the last to be dis- 
covered on account of their size are capillaries ( eapilla, little hair ) . Physi- 
ologically they are the most important part of the whole intricate system 
of blood channels in the vertebrate body, because in them the final transfers 
of the circulatory system are made. If the entire circulatory system be com- 
pared to a railroad system, the capillaries correspond to stations where pas- 
sengers and freight are entrained and detrained, while the more con- 
spicuous arteries and veins are simply lines of track connecting the stations. 

Anatomists have always been more concerned with following out and 
homologizing veins and arteries, which it is possible to trace and describe, 
than with the nameless capillaries that defy isolation and cataloguing. 
When one considers Krogh's estimate that there may be at least 2000 
capillaries permeating a cubic millimeter of human muscle, no one of 
which is over a millimeter in length, and that the total length of all the 
capillaries of the human body, if untangled and placed end to end, would 
be as much as 100,000 kilometers, that is, equal to two and a half times 
around the earth at the equator, it is small wonder that anatomists are 
forced to describe them in the most general terms. 

hitermil J ransportation 


Capillaries*--!.! ^^~ 

Unlike the twigs of a tree that come to an end, capillaries are con- 
tinuous and keep right on, forming anastomosing networks which have 
a larger total carrying capacity than the blood vessels they immediately 
connect (Fig. 282). The result is 
that the rate of flow of the blood 
stream slows down as it goes 
through the capillary networks, 
just as a swiftly flowing river that 
spreads out upon entering a lake 
loses iis momentum. Blood cells in 
capillaries may be said to "crawl," 
but as the size of the blood vessels 

Circular Muscle Fibers 

Fig. 282. A terminal arteriole, surrounded 
by a "stopcock" cuff of circular muscle fibers, 
which is supplied by a nerve ending for regu- 
lating the flow of the blood. The capillary 
they are passing through enlarges, network having greater internal expanse than 
they "hustle" more and more. the arteriole shows why the blood flows 

T -ii • i r more slowly thrpugh the capillaries than in 

In capillaries the rate of move- w . . , , A( ; F .., x 

the arterioles. (Alter Keith.) 

ment, which varies within wide 

limits, has been given as one twentieth of an inch per second, while in the 

highway of the aorta it is three hundred times as rapid. 

Capillaries may measures from 0.003 mm. to 0.01 mm. in diameter, 
while the largest human arteries and veins sometimes attain a diameter of 

Fig. 283. Diagram to illustrate 
the behavior of red blood cor- 
puscles in the capillaries. The 
arrows mark the course of the 
blood, a, a "saddlebag" cor- 
puscle; b, corpuscle bending it- 
self as it enters a side branch; c, 
deformity in a narrow capillary. 
(After Howell.) 

Fig. 284. Rouget muscle cells that 
control the caliber of the capillaries in 
the frosr. (After Krosih.) 

3 cm. This is a difference of 10,000 diameters or 100,000,000 times in 
carrying capacity. Single capillaries may be so small that, when undilated, 
blood cells that can penetrate a fine filter readily are unable to pass through 

34# Biology of the Vertebrates 

even in single file, or can only squeeze through with difficulty by tempo- 
rarily distorting their shape (Fig. 283). 

Although capillaries generally are intermediaries between veins and 
arteries, they may sometimes connect veins and veins, when they constitute 
a "portal system," or arteries and arteries, as in the "red gland" within the 
swim bladder of fishes, or in the glomeruli of the kidneys. 

Outside the single layer of endothelial cells that form the walls of 
capillaries some vertebrates, but probably not mammals, have at intervals 
flat branching involuntary muscle cells (Rouget cells) that control the 
caliber of these minute vessels ( Fig. 284 ) . Resistance to blood flow is fur- 
thermore exerted by cufflike circular muscles around the arterioles and 
arterial capillaries under the control of vasomotor nerves. When the bore 
of the arteriole is lessened by the contraction of these circular muscles, 
blood cells pass into the capillaries at a slower rate or are temporarily 
excluded. The operation of these neuromuscular stopcocks of the arterioles 
is also influenced by mental states as reflected when a person is "pale with 
anger" or "flushed with joy." 


The beginnings of the circulatory apparatus appear very early, since 
the transport of food stuffs to the region where the new animal is destined 
to materialize is a primary necessity. In the chick the beginnings of the 
formation of the circulatory apparatus are as follows. As soon as the fertilized 
egg has undergone preliminary cleavage, and the potential pioneer cells 
have arranged themselves into the primary germ layers, certain marginal 
cells of the spreading splanchnic mesoderm become clumped together, form- 
ing so-called blood islands (Fig. 285). These consist of haemopoietic or 
blood-forming cells surrounded by flat endothelial elements which are 
destined to become the lining of future capillaries (Fig. 286). The blood 
islands thus form a halo around the embryo on the surface of the yolk 
between the endoderm and the splanchnic mesoderm. The developing capil- 
laries that permeate the blood islands coalesce to form eventually a pair of 
definite blood vessels, the vitelline veins, one on either side. These lead 
directly into the growing body of the embryo where they unite into a com- 
mon trunk which is the beginning of the future heart and ventral aorta 
(Fig. 287). Anteriorly the ventral aorta splits into two parts from which a 
pair of vessels, the first aortic loops, extends around onto the dorsal side of 
the primitive gut and leads into the paired dorsal aortae. The aortae com- 
municate with vitelline arteries which emerge and continue outward to 

Internal 'Transportation 


the capillary field of the blood islands. This primary circuit, the first to 
be established in vertebrate embryos with well-developed yolk, is called the 
vitelline circulation and has to do with the transportation of food from the 
yolk sac into the body. 

Prosencephalon (Fore-Brain) 


Border of Fore-Gut \ 

Subcephalic Pocket' 
Mesenchyme »»,*; 

Mesencephalon (Mid-Brain) 


Vitelline Vein ~-^— __\V' D^ 

Neural Tube — -——-——- — r\ 

Lateral -- 1^- l rf ^-'~ 

Mesoderm ; h ■■V'W' *•>£.'; '. .•"* ' -' Y 

— Rhombencephalon 

Margin of Anterior 
ntestinal Portal 

Epimere (Somite) 


*iV- — - — — — — -'-r, —.-^'-%rt Rhom boidalis 

Blood Island-y^-C^— -- 

Border of — — — — $sg§£j 

-Hensen's Node 
•Primitive Streak 



— Extra- Embryonic 
Vascular Plexus 

Fig. 285. Dorsal view of a chick embryo of about 28 hours. (From 
Patten, Early Embryology of the Chick, copyright 1929, by permission 
of P. Blakiston's Son and Company, publishers.) 

Budding off from the vitelline circuit within the enlarging body of the 
embryo are secondary trunk lines which extend to and from both anterior 
and posterior regions of the body. These, together with relics salvaged from 
the temporary vitelline circuit, later make up the permanent systemic cir- 
culation (Fig. 2G8). 

Still a third circulation, the allantoic, is temporarily necessary during 
the development of higher vertebrates to meet the conditions imposed by 
embryonic life within an amnion. It consists of a pair of allantoic arteries 
arising from the posterior region of the aorta, which extend out to supply 
the temporary saclike respiratory allantois. The returning allantoic veins 
enter the systemic circulation close to the entrance of the vitelline veins. 

In mammals the allantoic arteries and veins become the umbilical 


Biology of the Vertebrates 


Blood Island 

Endoderm Cell 

Yolk — 


i ^ p >-®^ 


■Central Cells of 
Blood Island 

■Peripheral Cell 
of Blood Island 


Splanchnic #w*j 

Mesoderm "^1^£. ^j^ggf*! 
Blood Cells — '4 ' 

^*5r»s-^ -Mesoderm 

^fe w;^ c ° elom 

"-Endothelial Cell 

Endoderm Cell --^^-^^^^^^^-^ Lu 



Fig. 286. Drawings of cross sections through the extraembryonic vas- 
cular area of a chick embryo, to show the organization of blood islands. 
(From Patten, Early Embryology of the Chick, copyright 1929, by per- 
mission of P. Blakiston's Son and Company, publishers.) 





Fig. 287. Early circulation of vertebrate embryos. A, appearance of 
vitelline circulation; b, later stage, during formation of aortic arches. 
(From Wilder, History of the Human Body, copyright 1923, by permis- 
sion of Henry Holt and Company, publishers.) 

Internal Transportation 


arteries and veins that form the respiratory bridge through the placenta 
between the fetus and mother. The placenta itself is a compound capillary 
mammalian organ of double derivation, the part from the allantois of the 
embryo interdigitating into the uterine wall of the mother so that by osmosis 
there can occur a transfer of materials between the blood of the two. In 
the earliest known human embryo in which the circulation is described, the 
allantoic component seems to take precedence over the vitelline circulation, 
a state of affairs not unexpected since in mammals, as contrasted with 

Common Cardinal Vein Dorsal Aorta 

Jugular Vein 
Aortic Arches 

Posterior Cardinal Vein 
/ Liver Diverticulum „Subintestinal Vein 

-Caudal Vein 

Mouth / Heart 
Ventral Aorta ,«-' 


Vitelline Vein Vitelline Artery 

Fig. 288. Diagram of embryonic circulation in a large-yolked vertebrate. 
(After Kingsley.) 

reptiles and birds, the yolk sac and the accompanying vitelline blood vessels 
play a role of ever decreasing importance. The need of a vitelline circuit 
wanes with the vanishing yolk, while the allantoic circuit becomes useless 
upon hatching or at birth. Thereafter the systemic circuit takes up its con- 
stant functions and maintains them throughout life. The precarious tran- 
sition from embryonic to permanent circulatory devices involves profound 
modifications. In mammals, where the umbilical cord is severed at birth, 
it is very abrupt. 


1. In General 

The vertebrate heart, which is essentially a modified blood vessel half 
artery and half vein, consists of two kinds of chambers, a thin-walled venous 
receiving chamber, where the returning blood collects, and a thick-walled 
arterial muscular forwarding chamber, separated from the former by valves 

3S 2 

Biology of the Vertebrates 

which prevent the retreat of the blood when the muscular walls contract 
(Fig. 289). 

The walls of the heart are composed of the three histological layers 
common to blood vessels, with the exception that the involuntary muscle 

cells of the tunica media are of a 


/' Reservoir 







Fig. 289. The various parts of a force 
pump (a), compared with the corre- 
sponding parts of the left ventricular 
pump of the heart (b). (After Keith!) 

peculiar striated branching type 
which is particularly effective for the 
enormous and unremitting work that 
the heart has to do. 

Dr. Keen apostrophizes the heart 
as follows: "The heart is one of the 
most wonderful pieces of mechanism 
in the world, more powerful in pro- 
portion to its weight than any Bald- 
win locomotive, more delicately con- 
structed than the finest watch, an 
organ which must do and — mirabile 
dictu! — does do its own repairs 
while, busy at its work. It knows 
no Fourth of July or Christmas or 
Easter holiday, never can even know 
the joy and relief of sleep, 'tired 
nature's sweet restorer.' It begins its orderly reiterated contractions and 
relaxations long before birth, and they cease only at death. It must con- 
tinue them in health and in sickness, when its function is often sadly dis- 
turbed. In mid career let it stop for but a few moments and death comes 
swiftly, almost instantly." 

2. Embryonic Development 

As the ventral parts of the hypomeres approach one another to form 
the ventral mesentery in the region of the pharynx, they thicken. Between 
them mesenchymal cells establish two thin-walled endothelial tubes (Fig. 
290). These vessels soon fuse into a single endocardial tube surrounded by 
thickened ventral mesentery the layers of which meet both above and below 
the tube as dorsal and ventral mesocardia, respectively. While the meso- 
cardia disappear almost immediately, the rest of the ventral mesentery, 
adjacent to the endocardial tube, continues to thicken, especially in the 
region where the ventricle is to form. The wall of the original endocardial 
tube becomes the endocardium, the lining layer of the adult heart, while 
the surrounding material develops into the myocardium, the muscular layer 

Internal Transportation 


^ EndoderrrK 

^-Splanchnic^^ v ^>-,^J.umen of Pharyn 
y/ Mesoderm ^S^rfi; 
^Coelomic Cavity-^-- 

"*•- Somatic """"" \j>o^ 
Mesoderm ^^X^^/ 

|eI . af Pericardial Cavity^ 

Endocardial Tube 

/ \ Plate Dorsal / 

Segmentation Cavity Mesenchymal Cells Mesocardium 

_, . , Pericardial Cavity 

My oca rd i u m - Avj 

Ectoderm — 

!>&&&&* Endocardial _ . 

Ventral MesocaTdium Tubes 

Fig. 290. Diagrammatic cross sections showing the formation of the 
heart. In A the descending mesothelial plates have nearly met, with a 
number of mesenchymal cells between them. In b the plates have met 
ventrally; most of the mesenchymal cells have been utilized in forming 
a pair of endocardial tubes. In c the endocardial tubes have united; the 
plates have met dorsally; the ventral mesocardium has disappeared; addi- 
tional mesodermal cells are grouping about the single endocardial tube; 
and the pericardial cavity has been established. In u the dorsal meso- 
cardium has disappeared, and the primordium of the heart is attached 
to the wall of the cavity only at each end, as in Figure 291b. Three layers 
of the cardiac wall are now evident, namely, the endocardium, the lining 
layer; the myocardium, or muscle layer; and the epicardium, or visceral 
pericardium, the covering layer. 

of the heart, and a thin covering layer, the visceral pericardium. With the 
disappearance of the mesocardia, the heart lies free in the pericardial cavity 
attached only posteriorly, at the transverse septum, and anteriorly at its exit 
where it passes over into the ventral aorta ( Fig. 29 1 ) . 

Although the heart is at first a relatively simple tube, it soon enlarges 
and becomes modified. Included among the changes which occur are : con- 
strictions into several chambers; differential thickening of the myocardium, 
resulting in the establishment of the thin-walled receiving part and the 
thick-walled forwarding part of the heart; and a kinking of the chambers, 
necessitated by rapid growth within crowded quarters, so that they no 
longer lie in a straight line. 

3. Evolution 

In amphioxus, in which the circulatory apparatus is so primitive that 
red blood corpuscles are scarce, an accessory lymphatic system is not yet 


Biology of the Vertebrates 

present and there is no heart at all. The ventral blood vessel which extends 
between the liver diverticulum and the gills is contractile enough to send 
the blood forward. It is this part of the ventral aorta that marks the location 
of the future vertebrate heart. From such a beginning the evolving verte- 
brate heart passes through a series of modifications of increasing complexity, 
until eventually there is developed the four-chambered mammalian heart. 


Mesocardial -/~*j/t 


Duct of Cuvier 
Hepatic Vein 


Conus Arteriosus 

Pericardial Cavity 


i Peritoneal Cavity 

/ Atrium 

Cavity '{/]$/] 

Conus -- 


Ventral Body Wall' 

Sinus Venosus 


- Sinus Venosus 

--Septum Transversum 


Fig. 291. Diagrams illustrating relation of heart chambers to one 
another and to pericardial cavity, a, hypothetical primitive condition; 
B, selachian stage. (After Goodrich.) 

The first step in the differentiation of the vertebrate heart is encountered 
in the larval ammocoetes stage of the lamprey eel in which the prophetic 
ventral aorta lying between the liver and the gills becomes somewhat en- 
larged and modified by constrictions, unequal thickening, and kinking. 
Although the ammocoetes stage of the lamprey shows the heart still in the 
common body cavity with the liver, intestine, and other visceral organs, 
among fishes generally a transverse septum forms by a proliferation of the 
peritoneal walls, and ever after the heart is housed within the privacy 
of an enveloping space of its own, the pericardial cavity. 

In elasmobranch fishes the pericardio-peritoneal canal, a slitlike open- 
ing between the pericardial cavity and the peritoneal cavity, represents the 
last step before the establishment of pericardial independence. 

(a) Single-circuit Hearts. — The relatively small heart of fishes consists 
typically of a series of four chambers through which only non-aerated blood 
passes, since the spent blood sent forward from the heart to the gills for 

Internal Transportation 


Fig. 292. Evolution of the heart, a, elasmobranchs; b, teleosts; c, amphj 
bians; d, lower reptiles; e, alligators; f, birds and mammals; a, atrium 
or auricle; v, ventricle; a.o., aorta; b, bulbus arteriosus; c, conus; d.c, 
duct of Cuvier; h, hepatic veins; p. a,, pulmonary artery; p.c, post-caval 
vein; p.v., pulmonary vein; s, sinus venosus; t, truncus arteriosus. (After 
Kingsley. ) 

aeration must make the grand tour of the body and become again non- 
aerated before it is returned to its starting point in the heart. 

Beginning posteriorly the four chambers in the heart of elasmobranch 
fishes are the sinus venosus, atrium, ventricle, and conus arteriosus (Fig. 
291b). The first two belong to the re- 
ceiving region of the heart and are 
reservoirs with elastic rather than thick 
muscular walls, while the ventricle is 
thick-walled and muscular, as befits 
the forwarding pump of the blood. 
The conus has a muscular wall of 
moderate thickness, which by its elas- 
ticity aids in regulating the back pres- 
sure of the blood as it is forced through 
the ventral aorta into the nearby capil- 
laries of the gills. 

In elasmobranchs the single atrium 
receives the blood that has been 
poured into the sinus venosus from the 
ducts of Cuvier and the hepatic veins 
(Fig. 292). A row of cup-like valves, 
with their concavities in front, guard 

the atrio-ventricular opening permitting the blood to go into the ventricle 
but filling with blood to block the opening when the ventricle contracts to 
force the blood through the conus (Fig. 293). Several rows of semilunai 

Ventral Aorta 

--Afferent Branchial 
Artery 3 

— Conus Arteriosus 


Sinu-Atrial Cpenina 

Fig. 293. Heart and ventral aorta of 
Squalus acanthias, from the dorsal side. 
with the atrium cut open. (After Rose.) 

3S 6 

Biology of the Vertebrates 

valves in the conus allow this blood to flow forward but, when the ventricle 
begins to relax, they block the conus by filling with blood as soon as it 
starts to flow backwards (Fig. 294a). 

Ventral Aorta 



•^r- Bulbus 



A B C 

Fig. 294. Schematic long section through the hearts of various fishes 
to show the relation of the conus and bulbus. a, elasmobranch; B, 
ganoid; c, teleost. (After Boas.) 

In bony fishes the conus and ventricle gradually telescope together until 
in most teleosts the conus is represented by a very short region including 
a single row of semilunar valves (Fig. 294)., With this shortening, the pos- 
terior part of the ventral aorta is drawn back into the pericardial cavity 
where it develops a thick muscular wall and becomes known as the bulbus 

( b ) Transitional Hearts. — With the introduction of land life and lungs 
a new secondary shorter circuit is initiated by means of which aerated blood 
from the lungs is returned directly to the heart before making the excursion 
around the body. The pulmonary blood is poured into the left side of the 
atrium, a partition having developed that divides the original receiving 
chamber longitudinally into two chambers, a right and a left auricle. This 
inter auricular septum develops to the left of the opening from the sinus 
which therefore sends all of its blood into the right auricle (Fig. 292c). 

In dipnoans and amphibians, which accomplish the precarious transfer 
from gills to lungs, there is thus developed what may be regarded as a heart 
and a half, or a heart with one ventricle and two atria or auricles. The 
auricular partition is incomplete in dipnoans so that a mixture of aerated 
and non-aerated blood results within the auricles of the heart through the 
so-called foramen ovale. However, as this mixture is passed on through the 
ventricle, a twisted partition in the conus, which has not yet become in- 
corporated in the ventricle, tends to shunt the mixed blood two ways, that 
is, to the lungs and over the body. 

In amphibians while the auricular partition is complete and the foramen 

Internal Transportation 


ovale is obliterated, there is a mixture of aerated and non-aerated blood in 
the common cavity of the ventricle. When sent over the body without 
having first been revivified by a trip to the source of oxygen in the lungs, 
there results a condition comparable with burning coal that is half burned- 
out ashes and "slag." It burns poorly. This is one reason why these animals 
are "cold-blooded," since the only pure blood in an amphibian is in the 
short pulmonary veins. 

Common Carotid - = - 

Systemic Exit —f+ 

Systemic Arch — ~£: 

Septum of Conus r 

Right Auricle - = ^§J 

Right Auriculo- 



Ventricle " 

^E5§\ vTTA Septum 

I t;AU \\-t4 Entrance of 

pLIiS. \\ j Pulmonary Vein 

Entrance of 
Sinus Venosus 

^Pulmonary Artery 
"Left Auricle 

Left Auriculo- 
Ventricular Opening 

Dorsal Aorta 

Fig. 295. The transitional heart of a frog showing the beginnings of 
differential distribution of blood despite the retention of a single ven- 
tricle. (Heart modified from Kerr.) 

This handicap of mixed blood within the ventricle of the amphibian 
heart is partially avoided by the rapidity of the heart-beat which does 
not allow time for a thorough mixing of the two kinds of auricular blood 
that enter the ventricle from the two auricles, and by the spongy reticular 
structure of the ventricular chamber. Every time that the ventricle is filled, 
the mass of blood occupying the ventricular cavity may be thought of as 
momentarily of three kinds, arranged in a sort of temporary stratification, 
with non-aerated blood from the right auricle placed nearest the exit of the 
ventricle (Fig. 295), the aerated blood from the lungs farthest away from 
this exit, and the inevitable mixture of the two somewhere in between. As 

■$$8 Biology of the Vertebrates 

the ventricle expels its contents before these relations have time or oppor- 
tunity to change, the result is that the non-aerated blood nearest the exit 
is directed by the septum of the conus into the first possible avenues of 
escape, which are the pulmonary arteries leading to the lungs. The inter- 
mediate mixed blood, unable to enter the already filled arteries to the lungs, 
is forced along into the next available blood vessels, which are the systemic 
aortae distributing blood over the body generally. The best aerated blood 
of all at the bottom of the ventricle, being the last to emerge and finding 
all other passage-ways crowded full, passes on to the carotid arteries that 
supply the brain. Thus the brain, that always needs the best available 
aerated blood, is in the way of obtaining it, even in such unintellectual 
ancestors as frogs and toads. 

The transition from a single to a double heart is further shown in the 
reptiles, which have come to forsake entirely the gill method of respiration, 
but, with the exception of the Crocodilia, have not yet arrived at the estate 
of a complete double heart. 

, ^flpi'! ,h < w IM|l |1 

Systemic Aorta---^^pL_ Pu | monary Aorta 

>ramen Panizzae^ V^p \gt&*K\S' L ?, ft Auricula- 
/if^ ; ' Mft " \ Ventricular 

I # ^ fk/i «*C'!\ \ Foramen 

Right Auriculo— j-§r--Wfe | 

Ventricular i| : j|f §|h. --#4- Left Ventricle 

Foramen \ %r "/ 'A ^^^ %/ A 

^^T^^^^^TT^^^"^ Interventricular 

Right Ventricle / ■ ~* ' ^ Septum 

Fig. 296. Human heart of a 7.5 mm embryo, cut open. (After Kollmann.) 

Among reptiles an interventricular septum forms which tends to keep 
separate the aerated blood, returning from the lungs by way of the left 
auricle to the left side of the ventricle, and the non-aerated blood of the 
body entering the right side from the right auricle (Fig. 292d). This par- 
tition is incomplete in most reptiles, so that there still exists some degree of 
mixture between the right and left ventricles through the foramen Panizzae 
(Fig. 296), which represents the last gap in the uncompleted ventricular 
septum. Non-aerated blood from the right ventricle goes out not only to 
the lungs but in part also to the dorsal aorta, the main distributing trunk 
of the body. The result is that in reptiles, although the blood is kept un- 
mixed as far as the dorsal aorta, from that point on it is mixed blood, 

Internal Transportation 559 

being distributed over the body with a corresponding inevitable sluggish- 
ness of behavior. 

(c) Double Hearts. — In the Crocodilia (Fig. 292e) the foramen Paniz- 
zae finally becomes obliterated and two complete hearts, superficially 
incorporated into one, are established. One of these hearts, made up of the 
left auricle and powerful left ventricle, constitutes the pump for the major 
circuit over the body, while the other, the right auricle and right ventricle, 
takes care of the blood coming back from the general body tissues and 
being sent to the lungs. 

Aerated and non-aerated blood, which are mixed within the single 
ventricle of the heart in amphibians and in the dorsal aorta of reptiles, 
are kept completely separate among birds and mammals. As in Crocodilia 
the blood of birds and mammals passes alternately through two circuits, 
the pulmonary and the systemic. Blood entering the right auricle is sent 
into the right ventricle, then out to the lungs, back to the left auricle, to 
left ventricle, out to the general body tissues, and back once more to the 
right auricle. In birds and mammals the telescoping of the receiving cham- 
bers reaches the point where the sinus venosus is virtually eliminated, the 
chief systemic veins emptying directly into the right auricle. 

4. Size and Position 

The heart of a bird is proportionately larger than that of any other 
vertebrate, for the reason that an especially efficient pumping apparatus is 
required to keep the machinery of strenuous aerial locomotion going. Among 
mammals small species have relatively larger hearts than large forms. The 
proportionate size of the heart also decreases with the relative decrease of 
the heat-dispensing body surface that accompanies growth. For example, 
the weight of a newly-born rabbit's heart has been found to be 5.9 per cent 
of the total weight, while that of an adult rabbit is 2.8 per cent. 

The position of the vertebrate heart is always ventral to the digestive 
tube, and in gill-breathing vertebrates, far anterior. When the head of a 
fish or a salamander is cut squarely off, the heart is usually included with 
it. With the development of a neck there is a backward migration of the 
heart in higher vertebrates, until in such long-necked forms as swans and 
giraffes, it comes to lie a long distance from its original location, being 
much more centralized with reference to the body. To have the "heart in 
the mouth" is, therefore, a sort of ancestral sensation that should in no wav 
disquiet a comparative anatomist. 

The adult human heart weighs not far from ten ounces, and is approxi- 
mately the size of the clenched fist. It is median in position between the 


Biology of the Vertebrates 

lungs (Fig. 297), and not on the left side where it is popularly located by 
tragic actors and sentimental lovers. The reason it seems to be on the left 
side is because the throbbing tip of the cone-shaped ventricular part nor- 
mally projects from behind the sternum 
towards the left side, where its kick is 
most readily felt. 

There are many misconceptions cen- 
tering around the human heart. For 
instance, it is never "heart-shaped" ac- 
cording to the conventional Saint Valen- 
tine's outline, but instead is a flattened 

5. Valves 

The most constant valves of the heart 
in the vertebrate series are the auriculo- 
ventricular valves, which separate the re- 
ceiving auricle from the forwarding ven- 
tricle. They are present in all vertebrates 
and, in higher forms, are kept from re- 
versing under the pressure of the con- 
tracting ventricle by tendon-like guys, 
chordae tendineae, that are anchored in 
the muscular walls of the ventricle (Fig. 298) . There are only two such valves 
in the heart of fishes but in the double heart of mammals there are five 
present, two between the auricle and ventricle of the left side {bicuspid 
valves) and three (tricuspid valves) on the right side. The bicuspid valves 
are commonly known as mitral valves from a fancied resemblance to a 
bishop's miter. It was Huxley who once humorously said that he could 
always easily remember the location of the mitral valves on the left 
side of the heart because he "never knew a bishop to be on the right 

The semilunar valves of the conus region are best seen in elasmo- 
branchs and ganoids, where as many as eight rows may appear in some 
species (Fig. 294a and b). They are cuplike pockets, lying flat against the 
inner wall as the blood passes out over them, but filling immediately to 
block the passage-way when the blood attempts to retreat. Similar valves 
guard the exits from the heart to the pulmonary arteries and to the systemic 
aorta in the higher forms. 

The primitive heart of fishes in which the sinus venosus still persists as 

Fig. 297. Diagram showing median 
position of human heart, in dotted 
outline. (After Spalteholz. ) 

Internal Transportation 

3 6. 

a distinct chamber has a pair of sinu-auricular valves between the sinus 
venosus and the atrium that, like swinging doors, allow the blood to pass to 
either way. This does no harm since both sinus and atrium have the com- 
mon function of acting as reservoirs for returning blood. 

Vena Cava 

Right 1 

Pulmonary — ^ 

Systemic— -Wpr^Mr 
Aorta J J 

Pulmona ry - -jf?- 4-.-^- 
Aorta /|V; 

Semilunar--/? 1 - '■$£■-- 

Valves pi v ••"??; j- 

Right— fe 

Tricuspid - 

Vena Cava 
Posterior-- 1 ^ — 
(Postcava) v. 

Papillary Muscle 

^,_ Systemic Arch 




•Left Auricle 

'^§^ r -Semilunar 


Bicuspid Valve 



i— Left 

Right Ventricle" 

Dorsal Aorta 

- Interventricular 

Fig. 298. Heart showing valves. (After Jammes.) 

Between the atrium, or auricle, and the ventricle, on the other hand, 
one-way traffic must be maintained when the muscular ventricle forces out 
the blood, consequently mitral and tricuspid valves with their chordae ten- 
dineae, swing only so far and no farther. 

6. The Work of the Heart 

While compressing muscular movements of the body are largely respon- 
sible for the propulsion of lymph through the lymphatic channels, the heart 
is the indispensable pump by means of which the circulation of the blood 
is accomplished. 

362 Biology of the Vertebrates 

The blood must be kept in constant motion. That this is done is shown 
by the familiar fact that from any wound, however slight, which makes 
a break in the circulatory channels, blood immediately flows out. 

In amphioxus and certain annelid worms a constant circulation is 
brought about simply by the contraction of arterial blood vessels, but in 
vertebrates generally, owing to the enormous expanse of the capillaries de- 
veloped, contraction of the arterial walls is not sufficient to keep the blood 
in motion, and a heart becomes necessary. As has been indicated, the heart 
acts both as a force pump ( Fig. 289 ) , filling the arteries from the ventricles, 
and as a suction pump, drawing venous blood into the auricles. 

The rate of flow of the blood is faster of course when an animal is active 
than when quiet. The contractions of the heart of a hibernating fish, for 
example, may fall from over 100 a minute to two or three, while that of a 
mouse, whose normal heart-beat is about 175 per minute, may go up to 600 
per minute under the sudden stimulus of fright. 

When a person is sitting quietly, about five pints of blood per minute 
are forced into the aorta, an amount which upon violent exercise may rise 
to an output of thirty-five pints per minute. Since the total amount of blood 
in a human adult is only ten to fourteen pints, it is evident that, while un- 
dergoing moderate exercise, all of the blood of the body passes through 
the heart at least twice every minute. Thus, by the most conservative esti- 
mates, the strenuous red blood corpuscles in their brief lifetime travel many 
miles, while accompanying leucocytes that detour constantly from the main 
path, like an active exploring dog on a country ramble with his master, 
have still more extensive locomotor adventures. Though ranging wider, 
they do much of it more slowly and therefore do not maintain the average 
rate of the red corpuscles. 

Another way of reckoning the marvelous work performed normally and 
continuously by the human heart is to recall that with 72 beats per minute 
two ounces of blood are squeezed out at each beat, making the total daily 
output approximate 13,000 pounds. Even the heart of delicate Juliet, sigh- 
ing in her balcony, did that. When a husky stevedore is forced to handle 
13,000 pounds of freight in his day's work, he is honestly weary at nightfall 
and quite in the frame of mind to strike for shorter hours. 

The constancy of the flow is aided not only by frictional resistance of 
the moving blood against the inside walls of blood vessels, but also by ad- 
justable variations in pressure upon the blood stream exerted by the con- 
tractile walls of the blood vessels under the regulatory stimulus of involun- 
tary vasomotor nerve endings, that act as "stopcocks" (Fig. 282). "As these 
terminal arteries number tens of thousands, and each of them is regulated 

Internal 1 reimportation 


and controlled, one can conceive how complex the stopcock system of the 
human machine is" (Keith). If the varying work of the heart were not 
regulated by some kind of automatic device for adjusting the blood pres- 
sure and controlling the flow, disaster would inevitably follow whenever 
in the countless exigencies of life, a sudden extra load is thrown upon this 
faithful pump. 

The tireless beat of the heart itself is initiated and regulated at the sinu- 
auricular node (Fig. 299). This "pace setter of the vertebrate heart" is a 
narrow zone of tissue that marks the transitional region between the sinus 
venosus and the atrium in the fish heart, and which becomes incorporated 

Ventral Aorta 

Sinu-Auricular Junction-, 
Sinus Venosus - 

— it*— Con us Arteriosus 

die of His 


Fig. 299. Diagrammatic section of a heart to show location of timing 
mechanism. (After Keith.) 

as a part of the auricle in higher vertebrates. Another indispensable part 
of the mechanism of the throbbing heart is the auriculo-ventricular node, a 
dense network of cardiac muscle fibers connecting the auricular and ven- 
tricular walls, and acting somewhat like the "timer" in an automobile. 
Across this bridge the initiator)' stimulus, originating in the sinu-auricular 
node, is transmitted to the ventricle completing the heart-beat. The auric- 
ulo-ventricular node was discovered in the human heart by His in 1893, 
and is consequently known as the bundle of His. 

Although the heart beats in successive throbs, a constant flow of blood 
is maintained because the elastic arteries, stretched by the pressure gen- 
erated by the ventricular contraction, gradually contract until they air 
suddenly distended again by the next "beat" of the ventricle. There is thus 
a < (instant flow of blood from arteries into capillaries. 

3 6 4 

Biology of the Vertebrates 


In arteries, as already indicated, the blood flow is away from the heart, 
making the outward delivery trip of the things needful from the source 
of supplies to the capillaries. This object is attained in all vertebrates by 
means of one fundamental scheme of pathways, although the general plan 
is modified to meet the demands of different types of vertebrates. Once the 
main trunk line of the dorsal aorta emerges from the complexities of the 
arterial arrangement in the head and gill region, further distribution to the 
various organs of the body of arteries branching off from the aorta is fairly 
uncomplicated and uniform throughout the vertebrate series. The anterior 
arteries of the gill region that bear the brunt of the transformation from 
water to land life, however, show greater diversity. 

First Aortic Arch 

■Ventral Aorta 

Heart & 

h — Vitelline Vein 
Dorsal Aorta 
Vitelline Artery 

Fig. 300. Diagrams illustrating the arrangement of the primitive heart 
and aortic arches, a, early stage; b, later stage. (After Heisler. ) 

In amphioxus the ventral aorta, or the main blood vessel anterior to the 
liver diverticulum (Fig. 277), is connected with the dorsal aorta by a num- 
ber of pairs of lateral loops that encircle the anterior part of the digestive 
tube. There may be as many as sixty pairs of these loops in the adult ani- 
mal, occupying a large portion of the anterior part of the body. Each pair 
is interrupted by the insertion of gill capillaries in which the blood takes 
on oxygen from the surrounding water. 

In true vertebrates the number of pairs of branchial loops, although less 
in the adult, is typically six during embryonic development, with the ex- 
ception of certain primitive sharks, for example Hexanchus which has 
seven, and some cyclostomes which exceed the typical number. The first 
pair is laid down as part of the original vitelline circulation (Fig. 287). 
Subsequently five additional pairs are laid down one after the other, be- 
ginning with the most anterior one (Fig. 300). 

Internal I 'ransportation 


The usual embryonic arrangement of these branchial arterial vessels, 
as indicated in Figure 301, may be taken as a point of departure for the 
adaptations to follow in the different vertebrate classes. 

It will be seen that the branchial loops do not connect directly with the 
main dorsal aorta but first join with two smaller blood vessels, the radices 
aortae, which secondarily join, like the converging arms of the letter T, to 
make the single dorsal aorta. 



Dorsal Aorta— 

Fig. 301. Theoretical plan of 
embryonic arterial loops. (After 



-Gill Region 

Ventral Aorta 
-- Radix Aortae 


Dorsal Aorta 

Fig. 302. Plan of arterial loops 
in fishes. 

In fishes (Fig. 302), the two most anterior pairs of loops, that are sup- 
ported by the mandibular and hyoid arches of the splanchnocranium, are 
reduced to branches of the third in adult life, leaving the remaining four 
to become the branchial arches, interrupted by the capillaries of the internal 
gills as they pass from the ventral side to the radices aortae. 

In urodele amphibians (Fig. 303) external gills are introduced which, 
unlike the internal gills of fishes, do not directly interrupt the branchial 
loops but are established on a detour from the loops (Fig. 328), so that it 
is possible for the blood to pass from the ventral to the dorsal aorta by 
either of two routes, one through the uninterrupted branchial loop in which 
no capillaries are present, and the other by way of a side line through the 
capillaries of the external gills. 

Three pairs of such external gills, situated on loops IV, V, and VI. 
may be present in urodeles. Thus in those salamanders that discard their 

366 Biology of the Vertebrates 

gills during metamorphosis, it is possible for the blood to progress without 
interruption by way of the branchial loop direct, avoiding the disastrous 
consequences which would inevitably result if a single unavoidable route 
through the internal gills, as in fishes, were put out of commission. 

In urodeles, it should be noticed that the last and most posterior loop 
(VI) has its ventral portion pressed into the service of the pulmonary artery 
which goes to the newly-established lungs, for the oxygen-supply, instead 
of directly to the dorsal aorta. The dorsal part of this loop is reduced to a 
ductus arteriosus, or duct of Botallus. In all amniotes this duct functions 
until hatching or birth and its ghostly remains still haunt the arterial com- 
plexes of higher vertebrates, even of man, serving as a reminder of emer- 
gence from water to land life. Furthermore, in urodeles loop V and that 
part of each radix aortae between loops III and IV, in anticipation of their 
later obliteration in anurans, become much attenuated. 

The anurans, represented by the frog ( Fig. 295 ) , pass through a youth- 
ful tadpole stage in which their branchial arteries resemble those of uro- 
deles, but they go a step further in burning their ancestral arterial bridges 
behind them, since only three pairs (III, IV, and VI) of the six original 
embryonic loops survive in the adult. Loop ,IV becomes the large graceful 
paired systemic arch, while loop III is entirely devoted to supplying the 
head region. Since the connectives of the radices aortae that run between 
loops III and IV are suppressed, the blood in the third loop can no longer 
pass backward directly into the dorsal aorta. Loop V, already showing 
signs of degeneration in the perennibranchiate urodeles, disappears entirely 
in adult frogs, as does the dorsal part of loop VI, the ventral part of which 
persists as the pulmocutaneous artery. 

In reptiles three pairs of loops, namely, III, IV, and VI, survive (Fig. 
304). With the beginning of a separation of the ventricle into two parts, 
the ventral aorta near the heart splits, not into two but into three parts, 
two of which go with the right ventricle and the third with the left one. 
Of the portions draining the right ventricle, one connects with only loops 
VI to become the pulmonary aorta while the other leads into the left aortic 
•arch IV as the left systemic aorta. The division of the ventral aorta draining 
the left ventricle sends blood forward into the carotids and also through the 
right fourth loop, right systemic aorta. 

In amphibians the mixture of aerated and non-aerated blood occurs in 
the single ventricle of the heart before it is sent out over the lVth arterial 
loop, but in reptiles, which have at least a partial partition established be- 
tween the ventricular chambers of the heart, the mixing of "pure" and 
"impure" blood may be postponed until the right and left branches oi 

Internal Transportation 

5 6 7 

loop IV pour their diverse contributions into the common dorsal aorta. 
Reptiles as well as amphibians are "cold-blooded," one contributing reason 
being that in both cases some of the blood that has not been oxygenated is 
poured back into the dorsal aorta and sent again "unpurified" over the 
body. The result, like mixing clinkers with coal, is that the fires do not bum 
any too brightly and cold-bloodedness follows. 

c-c — Subclavian 

Fig. 303. Plan of arterial 
loops in urodeles. (After 



Left Systemic 

Right Systemic 

- Pulmonary 


Fig. 304. Plan of arterial loops 
in reptiles. (After Boas.) 

Birds, which arose from an advanced reptilian stock, follow the general 
reptilian pattern but they discard the left systemic aorta ( Fig. 305 ) . Thus 
the right fourth loop and accompanying radix aortae form the single sys- 
temic arch. This important change results in all of the blood pumped by 
the right ventricle going to the lungs, none of it becoming mixed with the 
blood which is sent out from the left ventricle to all of the other organs of 
the body. 

Mammals, derived from a primitive reptilian stock before major modi- 
fications in the direction of the modern reptilian plan, have both right and 
left fourth loops connected with the portion of the ventral aorta leading 
from the left ventricle (Fig. 306). In mammals it is the left fourth loop and 
accompanying radix aortae which persist as the systemic arch while the 
right fourth loop becomes the beginning of the right subclavian artery. 

Thus in man a single large aortic loop, like a shepherd's crook, arises 

3 68 

Biology of the Vertebrates 

from the heart, arches over to the left, and passes backward to supply the 
body and its various organs. This loop is the combined product of ( 1 ) the 
embryonic ventral aorta; (2) the left side of the IVth branchial loop; and 
(3) the left arm of the radices aortae. 



; Subclavian 

j! '; Aorta 

V; Left 

\-j I Artery 

;/ Right 
"ft— Pulmonary 
'/' Artery 

Pulmonary Aorta 

Fig. 305. Plan of arterial loops 
in birds. (After Boas.) 



^ Left 




monary Aorta 

Fig. 306. Plan of arterial loops 
in mammals. (After Boas.) 

Occasional rare cases, reported in medical literature, of double aortic 
arches in man, or of the aortic arch on the right side as in birds, find a 
ready interpretation in the light of comparative anatomy. 

1. In General 

The channels by means of which the blood is collected and returned to 
the heart have undergone a greater degree of evolutionary adaptation in 
the vertebrate series than the corresponding arteries that distribute the 
blood over the body from the heart. This is due in part to the elaborate 
complexes of capillaries inserted in the course of the veins which form the 
"portal systems," and in part to the accessory services of the lymphatic 
system of channels. 

As would be expected the least complicated arrangement of veins is 
found in amphioxus (Fig. 307). Blood from the body wall is picked up by 





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Biology of the Vertebrates 

paired anterior and posterior cardinal veins which communicate with the 
ventral aorta by means of the common cardinal veins; or ducts of Cuvier. 
An unpaired caudal vein picks up blood from the postanal region of the 
body and, near the anus, joins both the right postcardinal and the subintes- 
tinal vein, which continues forward beneath the intestine, not only receiv- 
ing contributions from the capillaries that encircle the alimentary canal, 
but also deriving food from the canal itself. 

Upon reaching the liver diverticulum the subintestinal vein breaks up 
into capillaries, thus establishing a primitive hepatic portal system. The 
hepatic vein, from the liver, is soon joined by the common cardinals to 
form the ventral aorta, which carries blood to the gills and is destined to 
evolve into the heart. 

2. Development of Veins in Elasmobranchs 

A brief survey of the main steps in the development of the veins in elas- 
mobranchs will aid us in a better understanding of the adult plans of not 
only the lower fishes but vertebrates in general. At least seven of these 
major steps may be recognized, namely, ( 1 ) vitelline stage, ( 2 ) subintes- 
tinal stage, (3) common cardinal stage, (4) hepatic portal stage, (5) sub- 
clavian stage, (6) subcardinal stage, and (7) renal portal stage (Fig. 308). 

A pair of vitelline veins entering the body from the yolk sac, as de- 
scribed in section IV, is the first evidence of the venous system. They run 
forward ventral to the digestive tract, passing first along the duodenal region 
where the liver is soon to grow out ventrally (Fig. 265). 

Soon a subintestinal vein, usually connecting with the left vitelline vein, 
develops along the ventral side of the posterior part of the intestine and 
into the tail, after looping around the cloacal region. 

Paired common cardinal veins, or ducts of Cuvier, each formed by the 
union of an anterior cardinal from the head region and a posterior cardinal 
from the body wall posterior to the heart, next appear. At this time anasto- 
moses between the vitelline veins form two venous rings, the anterior one 
looping around the small liver diverticulum. 

As the liver outgrowth increases in size it pushes into the anterior vitel- 
line loop, gradually breaking up the vessels here until they are reduced to 
capillary size. In this manner there is established an hepatic portal system 
which in this stage begins in capillaries in the tail region as well as in the 
wall of the digestive system. The anterior parts of the vitellines, which 
carry blood from the liver to the sinus venosus, now become known as the 

The next new vessels to develop are the subclavian veins, leading from 

Internal Transportation 

37 1 

the anterior fin buds to the common cardinals, and the ventral abdominal 
veins running along the ventral abdominal wall to empty into the sub- 
clavians. Meanwhile the posterior cardinals have grown back until they 
have joined with the cloacal loop and the subintestinal vein has subse- 
quently broken away from this loop. The hepatic portal system is now lim- 
ited to the draining of the digestive system, as in adult animals. 

tf _ f Sinus 1 


E — Vitelline 

Subintestinal — 

Cloacal 1 

Caudal - 



Liver 1 

Subintestinal \ 

Hepatic,, | 

Liver -^ J 

Capillaries 1^| 



Hepatic! ~\~" 
Portal J 




- Hepatic 

"1 Portal j~ 






Anterior Cardinal -- 
Common Cardinal «.„ 

— Subclavian s 

Hepatic -~ 

"""* Liver-""" 

Hepatic Portal -"f 

J Ventral 


Capillaries J 


Portion of Adult 

Posterior Cardina 

Renal Portal -■ 


Fig. 308. Development of veins in elasmobranchs, diagrammatic, a, 
vitelline stage; b, subintestinal stage; c, common cardinal stage; d, hepatic 
portal stage; e, subclavian stage; f, subcardinal stage; <;, renal portal 
stage. (After Hochstetter and Hvman.) 


Biology of the Vertebrates 

Between the developing mesonephroi there next appears a pair of sub- 
cardinal veins, each with numerous lateral connections between it and the 
corresponding posterior cardinal. Gradually portions of all except the most 
anterior pair of these connections are broken up into capillaries in the mes- 

Finally, the posterior parts of the posterior cardinals become the renal 
portal veins as they break away from their anterior parts. As a result of this 
separation the adult plan of veins associated with the mesonephroi is estab- 
lished. The renal portal system, composed of the caudal vein and two renal 
portal veins, takes blood from the tail to the mesonephroi. Blood from the 
mesonephroi is carried to the heart by the veins known in adult animals 
as the posterior cardinals, despite their embryonic origin from several 
sources, including a pair of subcardinals and a pair of lateral connections 
as well as the anterior parts only of the embryonic posterior cardinals. 
Meanwhile the ventral abdominal veins, growing posteriorly, have extended 
into the posterior fin buds as the iliac veins. 

By these various steps the adult plan of elasmobranchs has been reached 
(Fig. 308g). Two pairs of large veins enter the heart, namely the hepatic 
veins from the liver and the common cardinal veins, into which empty the 
posterior cardinals, anterior cardinals and subclavians. From the tail region 
the renal portal system carries blood to the mesonephroi where it passes 
through capillaries and is picked up by the posterior cardinals which also 
drain the dorsal part of the trunk region. The anterior cardinals, aided by 
much smaller jugulars, return blood to the heart from the head. The sub- 
clavians drain not only the pectoral appendages and neighboring body 
wall, but also, through the ventral abdominals, the ventral body wall and, 
through the iliacs, the pelvic appendages. Blood from the digestive system 
is picked up by the hepatic portal system, carried to the liver where it is 
strained through capillaries, and picked up by the hepatic veins. 

3. Evolution 

In the larger group of bony fishes the venous arrangement is much like 
that of the cartilaginous elasmobranchs, except that the ventral abdominal 
veins disappear, and the traffic from the body wall and pelvic fins is 
shunted over to the posterior cardinals. 

Salamanders and frogs, as representative amphibians, present further 
stages in the evolution of vertebrate veins, which can be interpreted by 
comparison with the more generalized arrangement already described for 
elasmobranch fishes. 

The plan of the principal veins in salamanders, as shown diagrammat- 

Internal 'Vrcmsportation 


ically in Figure 309a, presents three striking innovations. First, there ap- 
pears a new vein, the postcava or vena cava posterior, that rivals the an- 
terior portions of the posterior cardinals, collecting their blood from the 
mesonephroi. Arising embryonically from the hepatic veins, this important 
blood channel grows back through the liver and then along the dorsal body 
wall near the dorsal aorta to join the postcardinals where they fuse near 
their junction with the subcardinals, at the anterior ends of the mesone- 
phroi. It increases in size until it takes over most of the transportation from 
the abdominal cavity to the heart. 

W, _T~- External Jugular 
nj\~ Internal Jugular 
\ Subclavian 
v Hepatics 

>^ Posterior Cardinal 
&.£""* Posterior Vena Cava 
" Hepatic Portal 

Ventral Abdominal 

-U — Renal Portal 

41 Left Mesonephros 

"— Posterior Vena Cava 

i — "Pelvics 
- Caudal 


d I 




'External Jugular 
-Internal Jugular 
-Left Anterior Vena Cava 

Sinus Venosus 


' Left Hepatic 
^Hepatic Portal 
"Posterior Vena Cava 
"Renal Portal 
-Ventral Abdominal 

A B 

Fig. 309. Diagrams of the venous systems of (a) a urodele and 
frog. Left auricle and pulmonary veins not shown. 


Secondly, the iliac veins fork, each sending one branch to its renal 
portal, while the other branch, representing the ventral abdominal vein of 
the elasmobranch, fuses with its fellow to form a median ventral abdominal 
vein that empties anteriorly into the hepatic portal vein. Blood returning 
from the hind legs of an amphibian, therefore, may pass through either the 
renal portal or the hepatic portal capillary strainer before reaching the 
heart, whereas in fishes the iliac blood goes directly to the heart by way of 
the ventral abdominal veins without portal interference of any kind. 

Thirdly, amphibians, as lung breathers, develop a pair of pulmonary 
veins. The pulmonary veins are not represented in these figures which show 

y]4 Biology of the Vertebrates 

only the veins emptying into the sinus venosus or right auricle. Because 
amphibians use the skin to a considerable extent as a supplementary breath- 
ing organ, they have in addition a pair of well-developed cutaneous veins, 
from the skin, which join the subclavians. Diagrammatically these bear a 
superficial resemblance to the lateral veins of elasmobranchs but should not 
be confused with them. 

The arrangement of the veins in a frog embryo, or tadpole, is like that 
of a salamander, except that the rivalry between the newly established 
postcava and the diminishing posterior cardinals culminates in the case of 
the frog tadpole in the successful monopoly of the circulatory blood traffic 
by the former and the disappearance of the latter (Fig. 309b). With the 
elimination of the posterior cardinals, the anterior cardinals become single, 
continuous channels with the ducts of Cuvier, forming veins now called 
precavas {venae cavae anterior es) into which the jugular veins empty to 
return blood from the head directly into the right auricle of the heart. In 
the frog the iliacs are represented by femoral and sciatic veins, the femoral 
splitting to enter the renal portal (postcardinal) and abdominal, and the 
sciatic entering the renal portal. Though the tail is lost, there is no conse- 
quent loss of the renal portal. Such drastic, changes as these, designed to 
meet the difficult conditions accompanying the precarious transitional 
method of their life, are typical of the many bodily makeshifts which this 
small struggling group of vertebrates has had to resort to in order to accom- 
plish the great evolutionary feat of emerging from water to land. 

The degenerating posterior cardinals become replaced in reptiles by a 
pair of longitudinal vertebral veins (Fig. 310a), that involve anastomoses 
of intersegmental and intercostal veins. 

In the head region of lizards, snakes, and turtles, as pointed out by 
Bruner,* the venous system is characterized by an abundance of sinuses or 
blood-filled enlargements of the veins, both inside and outside of the cra- 
nium. Through a modification of the blood pressure in the superficial sinuses 
that extend over the skull beneath the skin, the molting (ecdysis) of the 
corneal layer over the head is facilitated. Recourse to such a loosening de- 
vice as these sinuses is advantageous in the case of these reptiles because 
their thick dry integument, which is particularly tight over the head, does 
not easily allow for ecdysis. In Phrynosoma, the "horned toad" of the 
cactus regions of southwestern United States, the venous sinuses, together 
with associated muscles, form a curious protective mechanism whereby 
these grotesque animals under excitement are able to squirt blood from 
their eyes by way of ruptured sinuses of the orbital veins. Modifications of 

* Airier. Jour. v. VII, 1907. 

Internal 1 reimportation 


the head veins are much less in evidence in the Crocodilia than in the three 
lower groups of reptiles just mentioned. In fact the Crocodilia deviate 
markedly in many particulars from all other living reptiles and may pos- 
sibly be regarded as anatomically the evolutionary advance-guard of the 
reptilian army. 

„ --Jugular 
)\^-''' *S )\T ^Subclavian 

""\ nr^\r"" Precava 

,—)■ -V- U-i- . _VL _ _ Left Hepatic 
A\^--V-4f- Right Hepatic 
■IK- Hepatic Portal 

f/V ^-Vertebral 
s Intercostal 


Ventral Abdomina 

—Renal Portal 
** ^Kidney 


Fig. 310. Diagrams of the venous systems of (a] 
bird. Left auricle and pulmonary veins not shown. 


?• Pectoral 

— bclavian 
-«./ ~"~~Precava 

,_l"x _(.-:'..,, """-^ Right Auricle 
^4=Rf j >r- Liver 

^r- Hepatic Portal 
'Us Intestine 
\\T~~ Postcava 

*C^$T~ Left lliac 

). /vJsJiAV- Femoral 
-||- Kidney 

Renal Portal" 
Internal Iliac 

/,- Epigastric 



a reptile and (b) a 

The renal portal system, although possibly persisting in modified form 
in reptiles, disappears in birds, with the loss of a muscular tail, while the 
abdominal vein of amphibians and reptiles merges in birds into an epigas- 
tric vein (Fig. 310b), which is possibly homologous with the embryonic 
umbilical vein of mammals. 

The lost renal portal system is not recovered in the higher vertebrates, 
even in those species that possess well-developed tails (Fig. 311). 

The venous system of mammals is further characterized by the intro- 
duction of certain novelties. The persisting anterior end of the right pos- 
terior cardinal vein, together with remnants of the transient supracardinal 
and subcardinal parallel to it, becomes the azygos vein, while a fragment of 
this complex on the left may join forces as the hemiazygos vein in the more 
posterior part of the body. The azygos and the hemiazygos veins are con- 
nected by transverse anastomosing bridges into the azygos system. 


Biology of the Vertebrates 

In marsupials, rodents, insectivores, and many artiodactyls, the azygos 
system is about equally developed on the two sides, while in the head re- 
gion two precavas still persist to return blood to the heart in balanced fash- 
ion (Fig. 311a). In edentates, carnivores, and primates, on the other hand, 


Left Jugular 
L^L. Subclavian 
fT-Left Precava 
|„, Right Auricle 
)fj "'yi'- Coronary 
f ^ r & ij— L. Postcardinal 
Vf~ — Hepatic 


\ mi>ti \ 


t'un\ — Postcava 



fes§z>?- Hepatic Portal 

/ __-L. Adrenolumbar 
•^jL— L. Renal 


-^Sf L. Iliolumbar 

_ — L Common Iliac 
-" Caudal 

Anastomosis of 
iL ^Brachiocephalic 

i^-^^oi- Right Precava 

T~ ., \i Left Precava 

|p"\ ,oD\-- Coronary 


i Azygos Anastomosis 

— Right Postcardinal 
\ Left Postcardinal 

^Sjr— L. Internal Jugular 
■..External Jugular 

^p^^~ S L. Subclavian 
7^7."!^^ ^""1- Brachiocephalic 
r "*^ .\ ^R. Brachiocephalic 
JJn^— Precava 
fr\ ^Coronary 
;SSo£> v Azygos 
■W^\ Hepatic 
>'^ N Hepatic Portal 
"^^ Postcava 
t>l "L. Adrenolumbar 
N L Renal 
J^\ [""*VC~ L. Iliolumbar 

_-- L. External Iliac 
L. Internal Iliac 


r\r"^r~ Coronary 

t — U j! — Azygos 

— Hemiazygos 


Fig. 311. Diagrams of the venous systems in mammals, a, lower mam- 
malian plan; b, embryonic stage of anterior vessels in higher mammals; 
c, anterior vessels of those higher mammals which retain part of left 
postcardinal, showing in dotted lines the parts which disappear; d, venous 
plan of those higher mammals which lose the hemiazygos. 

a reduction of the azygos system on the left side results in an asymmetrical 
shifting of most of the blood from this area to the right side for delivery to 
the heart, while in the head region the right precava becomes dominant in 
the following manner. A cross-vein, the left brachiocephalic, is laid down 
from the right precava diagonally across and forward to the left precava 

Internal Transportation 377 

at the point where the latter is formed by the union of jugular and sub- 
clavian ( Fig. 311b). The left precava between this cross-vein and the heart 
then degenerates except for a small part, the region of the original duct 
of Cuvier, which persists as the coronary sinus through which blood from 
the wall of the heart is emptied into the right auricle. The part of the right 
precava in front of the cross-vein is now known as the right brachiocephalic 
while the posterior part remains as the single precava which empties into 
the right auricle after receiving the azygos vein, the original right postcar- 
dinal (Fig. 311c). 

Not only the heart but all of the larger blood vessels, arteries, veins, and 
lymphatics, are supplied in their outer walls (tunica adventitia) with a 
ramifying system of nutrient blood vessels, called vasa vasorum, or "vessels 
of the vessels," for just as "shoemaker's children must also have shoes," so 
blood vessels need to be provided with a supply mechanism of their own. 


The essential points to find out about the elusive ghostly lymphatics are 
their extent and relation to the tissue spaces of the body, which are not 
empty at all but instead are filled with various fluids, the more important 
of which are ( 1 ) plasma, confined within the closed haemal system of ar- 
teries, veins, and capillaries; (2) lymph, carried within the lymphatic ves- 
sels, which is much like plasma but is without red corpuscles; (3) tissue 
fluid, which is plasma that has seeped out of the main blood stream and 
bathes the cells of most of the tissues much as the waters of a swamp sur- 
round soil particles and keep them moist; (4) cerebrospinal fluid in the 
subarachnoid spaces around the central nervous system which contains 
certain products of internal secretion and is different from the lymph in 
the lymphatic channels; and finally (5) the fluid in the aqueous chambers 
of the eye, and that in the inner ear which, although described by the 
terms endolymph and perilymph, has no direct connection with the lym- 
phatic system. 

The anastomosing network of the lymph channels, which are crocheted 
all around the other blood vessels (Fig. 312), is anatomically and physio- 
logically distinct from the much more easily seen arteries, veins, and cap- 
illaries of the haemal system through which the red blood flows. They prob- 
ably equal if they do not exceed the haemal system in total carrying capac- 
ity although they do not extend everywhere throughout the body. 

To quote Dr. Sabin who is an outstanding authority in this phase of 
anatomy: "Lymphatics have not been demonstrated in the adult liver 

37 8 

Biology of the Vertebrates 




beyond the capsule and the connective tissue septa, nor in the spleen beyond 
the capsule. It is well known that lymphatics are abundant in tendons; but 
that they have not been demonstrated in striated muscle. On the other 

hand, it has been definitely shown, both in 
the embryo and in the adult, that there are 
no lymphatics in the central nervous sys- 

The lymphatic vessels of the digestive 
tract, which project into the innumerable 
villi of the small intestine, are known as lac- 
teals because the absorption by them of the 
fatty products of digestion results in an 
emulsion that gives to them a white milk- 
like appearance. 

Lymphatic channels vary in size from 
minute varicose capillaries where transfer of 
materials takes place, to large ducts and 
sinuses. Under the loose skin of a frog there 
are extensive lymph reservoirs in the form of 
subcutaneous sacs (Fig. 313) which, like a 
wet blanket, protect the underlying tissues 
from excessive loss of water during the 
temporary excursions of this amphibian 
from water to land. 

Unlike red blood, lymph always travels 
toward the heart, backsliding being pre- 
vented by numerous pocket-like valves like 
those in certain veins, which allow forward 
movement but prevent retreat. While the 
flow of lymph is accomplished in some degree by gravity, its advance is prin- 
cipally brought about by the muscular movements of the exercising body, by 
means of which it is squeezed along its one-way course toward the heart. 

Since the lymphatic channels in the adult are not incorporated in the 
closed circuit of the haemal system, the pumping power of the heart, being 
an integral part of the haemal system, can have no effect upon lymph 

In the lower vertebrates the larger lymph channels may acquire mus- 
cular walls and become pulsating lymph hearts, thus supplementing by 
their contraction the body muscles in the forward movement of the lymph. 
Among amphibians the caecilians (Apoda) have a pair of lymph hearts 

Fig. 312. Lung of Necturus with 
the blood and lymph vessels in- 
jected. The lung was cut open, 
spread out flat, and mounted in 
balsam. Only the main branches 
of the artery and vein are shown. 
The blood and lymph vessels were 
drawn by means of the camera 
lucida and show the exact rela- 
tions of both sets of vessels. (After 

Internal Transportation 


for each of the numerous segments of their elongated wormlike bodies, 
while urodeles likewise possess a double row of several lymph hearts, situ- 
ated along the posterior part of the lateral line on either side. Frogs, and 
their relatives, typically have four lymph hearts in adult life, situated at 

Fig. 313. Subcutaneous lymph sacs of a frog. The skin is represented as 
removed and the sacs are outlined by lines of connective tissue that 
join the skin to the underlying muscles. (After Gaupp.) 

four busy centers with reference to the legs and arms (Fig. 314). The an- 
terior pair are at the level of the third vertebra near the junction of the 
subclavian and jugular veins at the base of the arms. The posterior pair are 
at the base of the iliac veins in association with the blood supply of the 
legs on the dorsal side to the right and the left of the urostyle and can easily 
be seen throbbing under the skin of a live frog. 



Fig. 314. Lymph hearts in a frog, Rana. A, antero-dorsal region; B, 
postero-ventral region. (After Schimkewitsch.) 

In reptiles only the posterior pair of lymph hearts typically persist, while 
in birds and mammals even these vanish after a reminiscent embryonic 

Added to the regular lymphatic channels and their modifications in 
the form of enlarged sinuses and pulsating hearts, there are present through- 

^80 Biology of the Vertebrates 

out the lymphatic system, particularly in mammals, numerous lymph 
nodes, as well as certain localized tissues and organs of a predominantly 
lymphoid character, such as the bone marrow, the "fat bodies" of hibernat- 
ing animals, tonsils, Peyer's patches in the small intestine, and the spleen. 

Lymph nodes are usually encapsuled, with an internal mesh of con- 
nective tissue in which leucocytes are lodged. They are supplied with in- 
current and excurrent lymph vessels, the former being more numerous than 
the latter. In these nodular substations not only is the filtering of solid ma- 
terials in the lymph accomplished, but also tarrying leucocytes may there 
undergo dissolution and removal, as well as renewal by mitosis. 

The largest and most constant of the lymphatic organs is the spleen, 
which lies encapsuled in the body cavity of vertebrates. It attains an aver- 
age weight of seven ounces in man and upon occasion may be extirpated 
without fatal results. The exact function of the spleen is so problematical 
that no statement with reference to it has ever gone uncontradicted. The 
ancients were quite at sea regarding it. Hippocrates, the Father of Medi- 
cine, says that "it draws the watery part of the food from the stomach." 
The great Aristotle could only guess that it is a "prop for the stomach," 
while Galen, who was the prime authority in anatomy for centuries, oracu- 
larly declared that its function is "to keep the body warm." In more recent 
times, with almost equal vagueness, word goes forth that "it is generally 
believed that this organ shares with the lymph tissues in the formation of 
lymphocytes, and is also concerned in the destruction of waste red blood 
corpuscles." The ambitious student with an itch for discovery may be grati- 
fied to know that there are still left many little known regions awaiting ex- 
ploration beyond the horizon. 

The largest of the lymphatic vessels is the thoracic duct, into which 
the others posterior to the diaphragm and on the left side of the anterior 
part of the body empty. It opens into the venous system at the junction of 
the left jugular and left subclavian veins. The right lymphatic duct, which 
receives all lymphatics from the right anterior part of the body, empties into 
the right subclavian vein. In mammals, although embryonically paired, only 
the left thoracic duct persists. 

The lymphatic channels arise from the haemal blood vessels, as has 
been demonstrated by Dr. Sabin, and not independently in the interstices 
of the tissues as formerly thought. Endothelial buds from the walls of the 
capillaries grow out and make a centrifugal invasion of the entire body, 
eventually becoming disconnected from the closed haemal system except 
at the points where the thoracic duct and the right lymphatic duct connect 
with the veins in the neighborhood of the heart. 

Internal Transportation 382 

The lymphatic capillaries forming as outgrowths of the endothelial buds 
are thus closed at their distal ends and do not open freely into tissue spaces. 
The return of the lymph from these spaces to the thin-walled lymphatic 
capillaries, therefore, like its escape from the haemal capillaries, is by the 
process of seepage and diapedesis and not by direct entry into open ends 
of lymphatic vessels. 


The Release of Energy-Respiratory 



1. The Respiratory Environment 

Every living thing of which we have any knowledge exists on the 
planet earth at the bottom of a vast atmospheric ocean. Air envelops not 
only all land surfaces but extends also to the uttermost depths of every 
body of water, large or small, so that aquatic as well as terrestrial animals 
and plants find themselves bottom-dwellers with respect to the all-inclusive 

Air not only presses on all the external surfaces of the body at approxi- 
mately fourteen pounds to the square inch, but also envelops the internal 
surfaces of the lungs. The oxygen contained in the air forms the indispens- 
able setting for the drama of life. Ordinarily the amount present is about 
21 per cent by volume, while approximately 78 per cent is free nitrogen, an 
inert gas which dilutes oxygen to livable proportions. 

Although the large amount of free nitrogen in the air plays no direct 
part in respiration, it is a most important chemical component of the pro- 
tein compounds that make up living matter. It is not, however, available 
for" use as protoplasm-building material in its abundant free form, but must 
undergo a sequence of chemical combinations through the agency of plant 
life before it can finally be incorporated in the animal body. 

The mixture of oxygen and nitrogen which we call air is essential to 
life. It is said that a dog can live three months without food, three days 
without water, but only three minutes without air. Of all known organisms 
only the extremely specialized group of anaerobic bacteria seem to be able 
to live without the oxygen of the air, and even they obtain it by the chem- 
ical break-up of their own bodily structure. 

Although the atmospheric envelope is enormously extensive in every 


The Release of Energy 383 

outward direction, the only part of it occupied by living things is compar- 
atively the merest film in thickness, where the atmosphere comes in con- 
tact with the solid earth. The greatest vertical distance from the atmospheric 
floor reached by any organism has been attained by modern human aviators, 
who in order to outstrip soaring birds have had to include in their equipment 
a supplementary supply of stored oxygen. The exceptional altitudes gained 
by human pioneers of the air must be regarded as insignificant in extent 
when compared with the distances involved in the horizontal exploration 
of the earth's surface. 

Probably eighty per cent of all known animals breathe free air. This 
category includes mammals, birds, reptiles, many amphibians, some fishes, 
the great fraternity of tracheate arthropods, besides certain gastropods 
among the mollusks, and some annelids. Strictly, however, no animal 
breathes "free air," since a water film is necessary for every respiratory 
surface. In the minority innumerable aquatic invertebrates, fishes, and per- 
ennibranchiate amphibians habitually breathe air that has been dissolved 
in water, that is, air which occupies the invisible interstices between mole- 
cules of water. Some animals, for example whales and pulmonate snails, 
live habitually in water but come periodically to the surface for free air, 
while a few exceptional land animals, such as the terrestrial isopods and 
land crabs, still retain at an obvious disadvantage the primitive aquatic 
method of taking air in water by keeping their gills moist, although they 
have deserted water as a medium in which to live. 

Since there is considerably more oxygen in free air than is dissolved in 
water, free-air breathers in general exhibit more energy than aquatic forms, 
living as they do in a more favorable respiratory environment. As a matter 
of comparison sea water contains five to seven cubic centimeters of oxygen 
per liter; flowing fresh water, six to eight cubic centimeters; and free air 
over 200 cubic centimeters per liter. 

2. The Exchange of Gases 

When the Declaration of Independence was signed no one knew that 
life processes are due to a form of slow combustion, dependent upon a 
component in air called oxygen. Lavoisier made this clear for the first time 
in 1777. In 1794 he was guillotined by his unappreciative fellow country- 
men as "one of the enemies of the country." Thus politics dominates 

The exchange of gases which we term "breathing" is primarily a physi- 
cal rather than a biological phenomenon. The taking in of oxygen is a 
process of passive diffusion that ceases as soon as the oxygen within the 

384 Biology of the Vertebrates 

cells concerned balances with that outside. As a result of taking in oxygen, 
tissues are slowly broken down, while the energy used to build them up is 
released, much as "stored sunshine" from plants of the Carboniferous Per- 
iod is recovered in the form of heat energy whenever these fossils which 
we call "coal" are burned under a draft of air. In both burning coal and 
living body the most conspicuous product of combustion is carbon dioxide. 
This is given off directly, since it acts as a poison when retained. In fact 
the removal of carbon dioxide is so urgent a matter that no animal can 
"hold its breath" very long without being compelled by an imperative 
stimulus quite beyond its control, to resume breathing movements. This 
powerful stimulus, which insures the continuous working of the mechanism 
of respiration, is due to an excess of carbon dioxide in the blood acting 
upon a reflex center in the medulla of the brain. 

Unlike food, neither the carbon dioxide of metabolism nor the 
oxygen of the air can be stored within the body. Consequently, although 
respiration can be reduced to a minimum during times of exceptional in- 
activity, it cannot entirely cease during life, a fact that distinguishes a living 
energy-producing organism from dead things. 

Respiration, with reference to carbon dioxide, is an excretory function, 
for in the maintenance of life it seems to be of more importance to get rid 
of this deadening gas than to acquire more oxygen, although the two proc- 
esses go hand in hand and are both indispensable. Aquatic animals are 
easily killed in carbonized water, even when oxygen is present in sufficient 
amount for breathing. The excretory phase of respiration is clearly dem- 
onstrated by the fact that hydrogen sulphide, injected into the blood, is 
eliminated through the lungs. 

Physiologists distinguish between external and internal respiration. 
The former is concerned with the gaseous exchange of oxygen and carbon 
d.oxide between blood and air. The latter has to do with the essential trans- 
fer between blood and other tissues, or ultimate cells of the body, that 
constitutes the effective part of respiration, and brings about the release 
of energy characteristic of life. The distinction between external and in- 
ternal respiration disappears in small animals which have not elaborated 
a circulatory system, the transfer of gases taking place directly through the 
undifferentiated surface of the organism in contact with its atmospheric 

3. The Essentials for Any Respiratory Device 

In order to utilize the oxygen of the air, any living mechanism that has 
evolved far enough to have a true circulatory system must meet the fol- 

The Release of Energy 


lowing conditions: (1) the blood that is to receive oxygen must be sepa- 
rated from the air by a retaining cellular wall; (2) the wall must be suffi- 
ciently permeable to permit easy osmosis of gases; (3) the wall must be kept 
moist in order to permit thinness and permeability without drying up upon 
exposure; (4) the total walls or respiratory surfaces must be extensive 
enough in area to insure an adequate osmosis of oxygen for the organism 
concerned; and (5) a current of fresh air must be made to pass repeatedly 
or continuously across the respiratory surface. These conditions are met in 
a variety of ways by different animals. 

4. Different Kinds of Respiratory Mechanisms 

In the more primitive aquatic forms, diffuse breathing through the sur- 
face of the body precedes localized breathing through specific respiratory 
organs, such as gills or lungs, although both methods may be employed 
simultaneously, as in amphibians. Diffuse breathing is a decided handicap, 
however, since the necessarily delicate integument in animals that employ 
this method is not only subject to mechanical injuries, but its possessor 
must remain under water in order to escape the disastrous 
effects of exposure to drying air. 

The two most successful breathing mechanisms among 
terrestrial animals are tracheal tubes and lungs. Tracheal 
tubes, which have been elaborated by the great special- 
ized host of insects, consist essentially of ramifying tubes 
of inturned integument that admit air to the immediate 
neighborhood of the blood within the body cavity 
(Fig. 315). 

Lungs are air sacs in intimate proximity to vascular 
networks, with elaborate modifications for increasing the 
respiratory surface without harmful exposure to the desic- 
cating effects of the outside air. 

In addition to the gills, skin, and lungs of verte- 
brates, the tracheal tubes of insects, and the more 
primitive ectoplasmic devices of protozoans, a museum 
of respiratory inventions would be bound to contain a long array of de- 
vices that different members of the animal kingdom use in solving the 
universal biological problem of gas exchange. To cite only a few of the 
more familiar of these devices among invertebrates there may be mentioned 
the aboral tentacles of starfishes ; the respiratory tree safe within the di- 
gestive cavity of the mud-inhabiting sea cucumbers; the curious respiratory 
bladder of rotifers; the integument of the sinuously waving annelids; the 

Fig. 315. Re- 
spiratory tracheal 
tubes of a honey 
bee. (After HerU 
wisr. ) 


Biology of the Vertebrates 

Fig. 316. Branching re- 
spiratory barbules of the 
larva of an African toad, 
Xenopus. (After Bles.) 

expansive mantle of mollusks; and the compact lung-books of spiders. 
Among vertebrates may be added the remarkable respiratory tail of the 
goggle-eyed Periophthalmus of the Indo-Pacific mangrove swamps, a fish 

that can remain for hours out on land with only 
its highly vascular tail submerged in water. 
Finally there is an Antillean frog, Hylodes mar- 
tinensis, which undergoes its entire metamorpho- 
sis within the egg, likewise accomplishing breath- 
ing during this critical period by means of a 
broad respiratory allantois-like tail. 

It must not be forgotten, moreover, that in all 
reptiles, birds, and mammals, the allantois is 
provided as a temporary breathing organ during embryonic life. This highly 
vascular device for gas exchange is absorbed before hatching in the case of 
reptiles and birds, and in mammals is lost at birth with the placenta. 

The lungless salamanders which swallow air have a pharyngo-esopha- 
geal network that acts as an accessory respiratory contrivance to supple- 
ment the integument and gills, while the larva of the South American toad 
Xenopus (Fig. 316), possesses a kind of integumentary chin whiskers which, 
according to Bles, are respiratory in function. 

Certain fishes, Callichthys, Hypostomus, Doras, 
Misgurnus, and Cobitis, breathe by means of a vascu- 
lar rectum, alternately sucking in and squirting out 
water through the anus. Turtles in similar fashion uti- 
lize a pair of lateral cloacal sacs with capillary 

The air-breathing labyrinthine fishes, Polycanthus, 
Osphromenus, Trichogaster, Macropodus, Ophioce- 
phalus, Clarias, and the East Indian climbing perch, 
Anabas (Fig. 317), have a peculiar enlargement of 
the gill cavity, behind the eyes and dorsal to the first 
and second gill arches, in which pocket-like space 
there is a much folded vascular structure, the labyrinth, 
that meets all the requirements of a respiratory organ, 
under the difficult conditions of enveloping mud. 

The glimmering flying fishes, Exocoetes, that enliven the surface of 
tropical waters, may "hold their breath" for the brief intervals during 
which they forsake the water, since their gills do not seem to be supple- 
mented by any peculiar additional breathing organs, although moist gills 
for a brief time in air would be admirable breathing organs. 

Fig. 317. The gills 
of the climbing perch, 
Anabas, exposed to 
show how they are 
protected beneath the 
operculum so as to be 
kept from drying up 
during the excursions 
out of water which 
this curious fish 
makes. (After Hilz- 

The Release of Energy 387 

The swim bladder of fishes, as well as the accessory air sacs of birds 
and of some reptiles, which will be more fully described later, are both 
special devices connected with the function of respiration. 


The problematical ancestral vertebrates, amphioxus and the tunicates, 
that are largely sedentary in habit, obtain their microscopic food through 
ciliary action by creating a current of water which flows into the mouth. 
Since the current bf water also contains oxygen essential to respiration, it 
is obvious that respiratory organs placed in its pathway will be most favor- 
ably located for obtaining oxygen. Gills, lungs, and even more uncommon 
respiratory devices, such as swim bladders in fishes and pharyngoesophageal 
capillaries in lungless salamanders, therefore, are found colonizing at the 
anterior end of the alimentary canal. There is in fact an intimate relation- 
ship between the respiratory organs of vertebrates and the anterior end of 
the digestive tube. 

Originally perhaps the entire digestive tube shared in the respiratory 
function, as suggested in the rectal breathing of certain fishes. Surely in 
amphioxus as much as the anterior half of the tube is concerned with res- 
piration (Fig. 13). In most vertebrates, however, the apparatus for breath- 
ing is more restricted, although still closely connected with the pharyngeal 

Aquatic vertebrates like fishes have a series of paired lateral openings, 
the gill slits, in the sides of the pharynx, making possible a dual disposal of 
the water taken into the mouth. This can either pass straight on through 
the esophagus to the stomach and intestine, in which case its food content 
is utilized, or it can stream out through the porthole-like gill slits, when the 
oxygen that it contains is transferred to the delicate respiratory gills, or 
branchiae, hanging suspended within the space of the gill slits. In this way 
both respiration and alimentation are effected. 

Between the gill slits, embedded in the walls of the pharynx, are skel- 
etal parts of the splanchnocranium that form the gill arches, which furnish 
support for the vascular gills. 

During the early embryonic development of all vertebrates, a series of 
internal pockets, or pharyngeal pouches (Fig. 318a), push outward from 
the sides of the pharynx. These are lined with thr same sort of endodermal 
tissue found throughout the alimentary canal. 

Meanwhile on the outside, corresponding indentations or depressions in 
the ectoderm, called visceral furrows (Fig. 318b), make an appearance. 

3 88 

Biology of the Vertebrates 

Later the pouches and the furrows meet, separated only by a thin sheet 
of tissue which in fishes breaks through to form the visceral clefts, or gill 
slits (Fig. 318c), thus completing the passage-way from the pharyngeal 
cavity to the outside. 

In addition, along the margins of the clefts, delicate thin-walled evagi- 
nations containing capillaries develop as gill filaments, which are the essen- 
tial organs of respiration. Each gill septum, or wall between successive vis- 
ceral clefts, together with all of the filaments on both of its surfaces makes 
up a holobranch, or gill. 

Pharyngeal Pouch 
'"TSyPharynx^ \ 

Visceral Furrow 


-Q- Middle Ear 

O. 1 Cavity 

^~ @"5gS -Thyroid Gland 

S» y- Giiisii * 

CP^ <£££)-- Gill Septum 
CUD — ^S^-Pharynx 



Fig. 318. Diagrams to illustrate the formation of the gill slits. A, early 
stage showing the outpocketings of the pharynx that form the pharyngeal 
pouches; b, intermediate stage, showing formation of visceral furrows by 
inpushings of the ectoderm; c, later stage, in which pouches and furrows 
have met and formed passage-ways, or gill slits, from the pharynx to the 
outside. (After Hyman.) 

The number of pairs of gill slits varies from fourteen in the cyclostome 
Bdellostoma polytrema of the Pacific Coast, to only one pair, between the 
third and fourth gill arches, in certain salamanders. 

The complicated branchial basketwork of tunicates (Fig. 10) forms a 
remarkable exception, as well as the sixty or more pairs of gill slits in the 
elongated pharynx of amphioxus (Fig. 13). 

Among reptiles, birds, and mammals gill slits do not normally break 
through, although both pouches and furrows occur in the embryo. When- 
ever a pharyngeal fistula, as occasionally reported in medical literature, 
turns up in a human being, the comparative anatomist, if not himself the 
possessor of the strange anomaly, is delighted with this reminder of past 

The Release of Energy 389 

1. Invertebrate Gills 

The gills of invertebrates are not associated with pharyngeal gill slits, 
as in vertebrates, for there are no gill slits present. Gills may be located any- 
where along the outside of the body within easy access of the water. For 
example, in free-swimming annelids, such as Nereis, large leaflike para- 
podial appendages extending along either side of the body function as 
""gills." In mud-dwelling or tube-inhabiting annelids gills when present 
crowd together in beautiful feathery tufts at the exposed anterior end, since 
they would be at a disadvantage buried within the tube or the mud burrow. 

The ciliary "gills" of lamellibranch mollusks are concerned primarily 
with directing a stream of microscopic food toward the mouth opening, 
and they are of doubtful respiratory service, this function being performed 
instead by the expanded surface of the "mantle." 

The thoracic gills of crustaceans and the abdominal gills of larval aqua- 
tic insects are simply thin feathery expansions of the chitinous exoskeleton, 
enclosing tracheal tubes which in this way extend outside of the body rather 
than being turned inside, as in land tracheates. 

2. Primitive Gills of Amphioxus and Tunicates 

The fact that tunicates are enveloped in a thick non-respiratory cellu- 
lose tunic makes necessary an enlarged pharyngeal basket, that forms a 
respiratory structure often exceeding in size all the other organs of the body. 
The tunicate Phallusia (Fig. 10), for example, has a branchial basket per- 
forated with several hundred gill slits, while the ghostly transparent Ap- 
pendicularia, which are microscopic tunicates without a cellulose mantle 
and consequently with greater capacity for diffuse integumental breathing, 
possess only a single pair of gill slits. 

Amphioxus is characterized by an exceptionally generous number of 
gill slits, and these are bordered by flat primitive vascular tissue that pre- 
sents much less respiratory surface than is common to the more elaborate 
gills of fishes and amphibians. No expansive gill filaments, such as appear 
in higher aquatic vertebrates, are found hanging in the gill slits, so this 
ancestral animal is obliged to make up in number what it lacks in area of 
individual gills. The entire pharyngeal region is enclosed in an atrial cavity, 
so that the pharynx opens indirectly to the outside through a single ventral 
atrial pore, instead of directly through the gill slits. The walls of this atrial 
chamber protect the delicate gill surfaces from the sand in which amphi- 
oxus burrows, while allowing the unobstructed passage of water through 
the gill slits. 


Biology of the Vertebrates 

Fig. 319. Diagrammatic cross 
section through the endostyle 
of Clavelina, showing glandu- 
lar walls and ciliary bands in 
the groove. (After Seeliger.) 

Since the numerous gill slits of both amphioxus and the tunicates in- 
crease the hazard of the escape of food through them, the development 
of a device for directing bits of food past these lateral openings to the 

esophagus is necessary. The apparatus ac- 
complishing this end consists of two ciliated 
glandular grooves, the ventral endostyle (Fig. 
319), and the dorsal epibranchial groove, in 
which food particles become collected to- 
gether into a sort of mucous rope that spins 
along continuously into the gullet. Later in 
vertebrate evolution, when the endostyle is 
no longer needed for the purpose of collect- 
ing and steering food, this pharyngeal groove 
becomes transformed into the thyroid gland 
with an entirely different function. 
The student of comparative anatomy often finds things like this in the 
morphological junk pile, that have become utilized for purposes entirely 
different from those to which they were originally put. 

3. Cyclostome Gills 

The arrangement of gills in cyclostomes is somewhat different from 
that in typical fishes. In Petromyzon a forward extension of the esophagus 
forms a partition between the alimentary and 
respiratory parts of the pharynx, leaving the 
branchial apparatus in a blind pocket ventral 
to the esophagus (Fig. 320). Water in its pas- 
sage over the gills consequently may enter 
through the anterior gill slits instead of the 
mouth, which is frequently otherwise occu- 
pied on account of the suctorial habits of these 
animals. The gill slits are also modified by the 
narrowing of each end of the visceral cleft so 
that the filaments are enclosed in a rounded sac 
with a slender duct leading to it from the 
pharynx and another from it to the surface of 
the body (Fig. 321). Since the gills are lo- 
cated in these pockets, cyclostomes are fre- 
quently referred to as Marsipobranchs, mean- 
ing "pouched gills." In this group different species of Bdellostoma have from 
ten to fourteen pairs of gill slits; Petromyzon, eight embryonic pairs but only 

Fig. 320. Diagram of the re- 
lation of the esophagus and 
respiratory tract in (a) Myxine 
and in (b) Petromyzon. b, 
branchial duct; e, esophagus; 
/, thyroid gland. (After Kings- 

The Release of Energy 


seven in the adult ; and Myxine, six pairs, with all the external passage-ways 
on each side of the body uniting into a common canal opening by a single 
pore to the outside (Fig. 321b). 

"t-^r* Gullet 

Gill Sac 

■Outer Duct 
from Sac 


Opening of 

Branchial Ducts 

Fig. 321. Diagrams of the heads of (a) Bdellostoma and (b) Myxine. 
Dorsal views partly dissected to show arrangement of gills. (After Dean.) 

4. Elasmobranch and Holocephalan Gills 

Gills of elasmobranch fishes are lateral in position in sharks and dog- 
fishes but ventral in the flattened skates and rays. They open independently 
to the outside and are separated from each other not only by skeletal gill 
arches of cartilage, but also by primitive partitions attached to these arches, 
called branchial septa, on either side of which is located a set of filaments, 
or demibranch. A septum and its two demibranchs together make up a 
gill, or holobranch. Along the inner margins of the gill arches are finger- 
like projections, the gill rakers, which not only keep food from entering the 
slits but also aid in directing it along the straight and narrow esophageal 
path in which it should go (Fig. 322a). 

The most anterior pair of the pharyngeal portholes in elasmobranchs, 
between the mandibular and the hyoid arches, develops into the spiracles, 

39 2 

Biology of the Vertebrates 

not far posterior to the eyes in position. The walls of the spiracles are not 
provided with true gills, but in some cases may support, on one side at least, 
a "false gill," or pseud obranch, so called because its blood supply is not 
derived directly like that of true gills from an afferent branchial artery 
bearing "impure" blood, but from the efferent branchial artery of the fol- 
lowing gill arch which, having already given up its load of carbon dioxide 
and taken in oxygen, carries "pure" blood. 

— Demibranch 

\\ Branchial 




Fig. 322. Relation of gills to branchial chamber, a, elasmobranch, with 
long gill septa; b, teleost, without gill septa. (After Schimkewitsch.) 

In bottom-feeding skates and rays the spiracles open dorsally instead of 
ventrally as the other pharyngeal gill slits do. They are useful, therefore, in 
taking the water of respiration into the branchial cavity when the mouth 
is otherwise occupied grubbing for food in the mud. No doubt sharks and 
dogfishes, which swim about freely and gracefully in water, likewise use 
the spiracles upon occasion, instead of the mouth, as an accessory port 
of entry for the water of respiration. The cub shark, Carcharias, and the 
mackerel shark, Lamna, are without spiracles. 

The primitive shark, Heptanchus, has seven pairs of gill slits, and the 
frilled shark, Chlamydoselachus, as well as Hexanchus, six pairs, while for 
most elasmobranchs the typical number, aside from the spiracles, is five 
pairs. On each face of each gill slit, except the posterior side of the last slit, 
there is a demibranch, making a total of nine pairs of demibranchs in a 
fish with five pairs of slits. 

The Release of Energy 393 

Certain larval elasmobranchs which undergo considerable development 
within the eggshell before hatching have gill filaments so long that they 
hang out of the gill slits as temporary "external gills" (Fig. 323). These 
unusual structures may serve not only for respiration but also as absorbing 
organs in connection with the enormous yolks present in these eggs. 

Fig. 323. Embryonic "external gills" of dogfish. (After Schimkewitsch.) 

Fig. 324. A primitive shark, 
Chlamydoselachus, with an ex- 
ternal flap for each separate 
srill slit. (After Schimkewitsch.) 

The holocephalans, or strange elephant fishes, which have much in com- 
mon with elasmobranchs, possess only four pairs of gill slits and are further 
differentiated from them by an operculum. This is a flap of the integument 
developed on either side and extending back- 
ward from the margin of the hyoid arch, until 
it covers the external openings of the three 
anterior pairs of gill slits, leaving only the last 
pair open directly to the outside after the 
elasmobranch fashion. Possibly the forerunner 
of this opercular flap is seen in the elasmo- 
branch, Chlamydoselachus, where the skin on 
the anterior margin of each gill slit extends 
backward as a small independent protective 

fold covering the opening of each gill slit separately (Fig. 324), a feature 
which gives Chlamydoselachus the common name of "frilled shark." 

5. Ganoid and Teleost Gills 

The gill system of ganoids in some ways represents a connecting link 
between that of elasmobranchs and teleost fishes. Most Chondrostei still 
have nine pairs of demibranchs but nearly all Holostei lose the most anterior 
pair so that they are limited to eight pairs of demibranchs, i.e., four pairs 
of complete gills. In all bony fishes the gills are covered by an operculum 
that is stiffened by flat skeletal plates between the two surfaces of folded 
integument. Outside and anterior to the operculum on either side, there is 
a degenerate spiracle in some of the ganoids, while on the inner surface of 
the operculum there is attached a small opercular gill which is not homol- 
ogous either with true gills or with the pseudobranchs of the hyoid arch. 

394 Biology of the Vertebrates 

The interbranchial septa in ganoids are reduced so that the demibranchs 
placed upon them back to back are no longer in separate individual cham- 
bers, but occupy a common branchial cavity (Fig. 325c). The reduction 
of the interbranchial septa becomes complete in teleost fishes, so that the 
gills all lie compacted closely together in a common chamber covered by 
the operculum (Figs. 322b and 325e). 


E D B C A 

Fig. 325. The progressive reduction of gill septa in different fishes, a, elasmo- 
branch; b, holocephalan; c, ganoid; d and e, teleosts. (After Boas.) 

The number of gill arches in both ganoids and teleosts is usually four 
or five pairs, although they may be reduced to three, or even two pairs in 
some of the bony fishes. Spiracles are not characteristic of teleosts. 

The opercular opening becomes 
much diminished in such fishes as 
eels, which are thus enabled to 
retain water in the branchial cham- 
Capillaries in ber under unfavorable conditions. 
Gill Filament Probably the immediate reason 
why fishes suffocate when removed 
from a water environment is not 
because the gills dry up at once 

JE^L Efferent Artery with a collapse of the capillaries in 

- Gill Arch tne S^ filaments, but because when 
out of water the gills adhere to 
Fig. 326. Structure of a typical gill, show- each other, leaving the exposable 
ing blood vessels. (After Cuvier.) respiratory surface reduced beyond 

the danger point. 
The structure of a typical teleost gill, with its relatively great expanse 
of respiratory surface within a small compass, and the arrangement of its 
capillaries are indicated in Fig. 326. 


1 'he Release of Energy 


In ganoids and teleosts there are additional devices, besides the oper- 
cular lid, for protecting the delicate gills. Along the posterior and ventral 
margins of the operculum beyond the part stiffened by the flat opercular 
bones, a bordering fringelike flap is sometimes developed, which is sup- 
ported by fanlike skeletal elements, the branchiostegal rays, that help in con- 
trolling the passage of water out of the branchial cavity. 

On the inner pharyngeal side of the gill slits also there are present in 
varying degree, a series of stiff comblike projections along the inner margins 
of the gill arches, the gill rakers. 

,- External Gill 

s Pelvic Fin 
Fig. 327. External gills in a larval stage of Lepidosiren. (After Kerr.) 

6. Dipnoan Gills 

Of the three genera of living lungfishes that are found respectively in 
Australia, South America, and Africa, Neoceratodus has four pairs of gills, 
Lepidosiren, three, and Protopterus, two. Spiracles are present in the 
embryos of these fishes, although not 
in the adults. In addition to the 
pharyngeal gills common to this 
group, four pairs of supplemental 
true external gills of the pinnate 
(pinna, feather) type are present in 
the larval stages of Lepidosiren and 
Protopterus (Fig. 327). 


-r- Brain 


Efferent N 





Amphibian Gills 

The external gills of amphibians 
are attached as capillary detours 
upon the aortic loops in such a way 
that blood can go either directly 
around the vascular loops or round- 
about through the gills (Fig. 328). This is quite different from the arrange- 
ment of the internal gills of fishes, which offers no alternative for the circu- 
lating blood except to pass through the gills themselves. 

Fig. 328. Section through branchial re- 
gion of a 6 mm larva of Rana esculenta, 
showing vascular loop„ in external gills and 
anastomosis between afferent and efferent 
branchial arteries. (After Maurer.) 

39 6 

Biology of the Vertebrates 

As compared with internal gills, external gills are only weakly supported 
by a skeletal framework, and although always present during the tadpole 
stages of amphibians, they persist throughout life only in the perennibran- 
chiate urodeles, as the name indicates. Such external gills occur in the larval 
forms of a few fishes, for example, in the dipnoans as already mentioned, in 
the cartilaginous ganoids, Polypterus and Calamoichthys, and in the teleosts, 
Gymnarchus and Heterotis. 

The urodele amphibians have been separated into three groups accord- 
ing to the character of their gills, as follows : ( 1 ) Perennibranchiata, retain- 
ing both gills and gill slits throughout life; (2) Derotremata, losing gills 
and all the gill slits except one pair; and (3) Myctodera, or true sala- 
manders, having neither gills nor gill slits when adult. 

Although perennibranchiates preserve throughout life their tadpole-like 
external gills with two or three pairs of gill slits, these pharyngeal openings 
are no longer useful for their original purpose, since the water of respiration 
does not pass through the pharynx. Instead a fresh supply of dissolved 
oxygen is brought into contact with the gill filaments as the gills wave to 
and fro in the water, by means of muscles attached to the base of each gill. 
Five pairs of gill pouches form embryonically in the pharyngeal cavity, but 
the first and the fifth no longer break through. Necturus and Proteus belong 
in this group. 

In derotremes only the gill slit between the third and the fourth gill 
arches becomes complete, while the external gills vanish during larval life. 

Amphiuma, Cryptobranchus, 
and Siren are derotremes. 

The myctodere salamanders, 
as well as frogs and toads, have 
no gill slits, although embryonic 
pouches and furrows develop. 
The temporary external gills of 
these forms are sacrificed with 
the development of pulmonary 
and cutaneous respiration. The 
newts and true salamanders, in- 
cluding Amblystoma and Triton, 
are myctoderes. 

The external gills of frog tad- 
poles become enclosed during metamorphosis by a fold of the skin without 
skeletal support, while the protective peribranchial chamber thus formed 
usually has a single opening to the outside, more rarely two, as in Pipa and 

Fig. 329. External leaflike gills of a larval 
caccilian. (After P. & F. Sarasin.) 

The Release of Energy 


Xenopus, corresponding physiologically to the atrial pore in the peribran- 
chial chamber of amphioxus (Fig. 13). 

One of the tropical limbless amphibians, Caecilia, is exceptional, having 
larval external gills of peculiar crumpled leaflike structure, with a relatively 
large respiratory surface (Fig. 329). 

8. Gill Structures in Land Vertebrates 

The persistence of branchial remains in land forms, that have no use 
for gills even in embryonic life, is striking evidence of the common ancestry 
of all vertebrates. 

Although gills are never present in reptiles, birds, or mammals, there are 
five pairs of embryonic visceral pouches and furrows in reptiles and mam- 
mals and four pairs in birds. Ordinarily these break through only briefly 
in reptiles and birds but not at all in most mammals. Only the anteriormost, 
or hyomandibular, pair remains well developed in adult mammals. The 
hyomandibular pouches become the Eustachian tubes and the middle ear 
cavity, which ordinarily remain separated by the ear drum from the external 
ear canal, derived from the ectodermal furrows (Fig. 692). 


A swim bladder, or air bladder, is found in most fishes. It is a derivative 
of the anterior region of the digestive tube and, if not primarily respiratory 
in function, is at least found in suspi- 
cious intimacy with respiratory organs. 

Located dorsally in the body cavity 
just outside of the peritoneum under 
the vertebral column (Fig. 330), it is 
ordinarily a single elongated struc- 
ture, although it may be bifurcated or 
paired, as for example in Polypterus 
and Calamoichthys among ganoids and in the swellfish, Sphacroides, the 
curious headfish, Alola, and the sea robin, Prionotus, among teleosts. 

Great variation in the shape, form, and size of the swim bladder is con- 
nected with its hydrostatic function, as well as with variation in the center 
of gravity in different shaped fishes. 

In amphioxus, cyclostomes, elasmobranchs and a few of the higher 
fishes, particularly bottom-feeders and deep-sea forms, the swim bladder is 
absent. The flatfishes (Pleuronectidae) possess a swim bladder only in early 
life during the period when they maintain a vertical position. As they tip 

Fig. 330. Location of the swim bladder. 
(After Hesse.) 

39^ Biology of the Vertebrates 

over on one side in the course of development and assume their lazy adult 
habit of life, the swim bladder degenerates. 

Fishes which possess swim bladders in the adult form may be divided 
into two groups according to the character of the swim bladder, namely, 
the physostomi, having a pneumatic duct leading from the swim bladder 
to the alimentary tract ; and the physoclisti, having the outlet duct closed 
or atrophied. To the first group belong the bony ganoids, the dipnoans, and 
the soft-rayed teleosts. In the latter group are included all the spiny-rayed 

The pickerel, Esox, represents a simple type of physostomous fish having 
a single sac for a swim bladder, with a pneumatic duct at the anterior end 
opening into the esophagus (Fig. 331a). In some fishes the single swim 
bladder is made up of two connecting sacs (Fig. 331b). 

Pneumatic Duct Swim ^^Swlm Bladder^ 

\ Red Gland Bladder / /^- >< ^ / /.Pneumertic Duct 


Esophagus Esophagus Esophagus 


Fig. 331. Diagrams of swim bladders. The eel shows the posterior 
chamber forming by the enlargement of the pneumatic duct. (After 
Tracy. ) 

In physoclistous fishes, upon the degeneration of the pneumatic duct, 
the swim bladder becomes a closed sac having two chambers separated by 
a partition through which there is a sphincter-like opening, regulated in size 
by both circular and radiating muscles similar in arrangement to those in 
the iris of the eye. The posterior chamber is formed by an enlargement of 
the pneumatic duct that is no longer needed for its original function. This 
is quite apparent in the eel, Anguilla, a physostomous fish on the verge of 
becoming physoclistous, in which the duct is caught in the very act of 
enlargement into a separate chamber (Fig. 331c). The formation of the 
posterior chamber by the enlargement of the pneumatic duct has been 
clearly indicated by Tracy in a series of diagrams based upon wax recon- 
structions of serial sections, showing stages in the development of the swim 
bladder in the pipefish, Siphostoma (Fig. 332). The manner in which the 
anterior chamber is produced by the forward growth of the budding swim 
bladder has also been demonstrated by Tracy in the early stages of the toad- 
fish, Opsanus (Fig. 333), which later develops a typical closed swim blad- 
der, with the same three histological layers of tissue that characterize the 

The Release of Energy 

Pneumatic Duct 

Swim Bladder 

Anterior Chamber 


Posterior Chamber 

Esophagus'" "^s&iii 


Fig. 332. Diagrams to show the formation of the posterior chamber 
of the swim bladder from the pneumatic duct in the pipefish, Sipho- 
stoma. a, at the time of the release of the larva from the brood sac; 

b, stump of pneumatic duct growing backward to form U-shaped tube; 

c, pneumatic duct expanded to form posterior chamber. (After Tracy.) 

alimentary tract from which it was derived, shown in section in Figure 

An important modification in the epithelial lining of the anterior cham- 
ber of the swim bladder results in a structure unique among animal tissues, 
the red gland, which produces free oxygen (O2) by the reduction of oxy- 
haemoglobin in the red blood corpuscles when brought into close contact 
with secreting epithelial cells. No other gland is capable of isolating pure 
molecular oxygen. As Tower has demonstrated, this oxygen constitutes a 
large part of the gas that distends the swim bladder. 

<C^> Co 

Fig. 333. Diagrams of the early stages in the formation of the closed 
swim bladder in the toadfish, Opsanus. l.d., liver duct; ant.c, anterior 
chamber; p.c, posterior chamber. (After Tracy.) 

Excess gas produced by the red gland escapes through the pneumatic 
duct in all physostomous fishes. Since the two-chambered swim bladders 
of physoclistous fishes have the red gland located in the anterior chamber, 
the mechanism for the removal of excess gas is of necessity different from 
that in fishes with a pneumatic duct. The entire posterior chamber is lined 
with a thin epithelium beneath which is a capillary network, the rete mira- 
bile, through which excess gas generated in the red gland is absorbed directly 
into the blood. By enlarging the opening in the partition between the cham- 
bers, more gas is admitted to the posterior chamber for disposal through the 
blood, while by restricting it, the gas is retained. In this way the degree of 


Biology of the Vertebrates 

distention of the swim bladder is automatically regulated by the interaction 
of the red gland and the rete mirabile. 

A further modification of the closed swim bladder sometimes appears, 
for example in the squeteague, Cynoscion (Fig. 334), when the posterior 

chamber with the rete mirabile be- 

Sphincter-like Opening 

Rete Mirabile 


Anterior Chamber s Red Gland 

Fig. 334. Closed swim bladder of sque- 
teague, Cynoscion, showing oval, the re- 
duced posterior chamber. (After Tracy.) 

comes flattened almost to oblitera- 
tion and is designated as the oval. 
There are various uses for the 
swim bladder in fishes. Although 
primarily respiratory, it has become 
in most instances a hydrostatic' or- 
gan, or "float," for the purpose of 
maintaining a certain level in water 
without muscular effort. When its 
gaseous content is increased, the 
fish rises to higher levels, or if di- 

minished, sinks deeper in the water. By shifting the volume of gas from 
one end of the swim bladder to the other through muscular compression, 
changes in the center of balance in the body also occur, which enable the 
fish to make a variety of movements easily at the same level. 

In some fishes, particularly Siluroids, Cyprinoids, and Gymnoti, anterior 
prolongations of the swim bladder are present that come into intimate rela- 
tion with the inner ear either directly or through a chain of bones derived 
from parts of the first three vertebrae, forming the so-called Weber's organ. 
Variations in the distention of the swim bladder are conveyed to the inner 
ear by means of this device, that probably acts as .a regulatory sense organ 
either after the fashion of a manometer or a barometer. Whether Weber's 
organ aids in any way as an organ of hearing is doubtful. 

Another use for the swim bladder is that of respiration, for which reason 
its description is included in the present chapter. This function applies par- 
ticularly to lungfishes, whose swim bladder becomes alveolar inside like the 
lungs of amphibians and the lower reptiles, being usually paired as well as 
taking on all the essential features of simple lungs. It even derives, after the 
fashion of true lungs, a supply of venous blood from the last pair of aortic 
loops, whereas the typical hydrostatic swim bladder receives arterial blood 
only, and gives off venous blood. The swim bladder of lungfishes, therefore, 
apparently is a more efficient breathing organ than the primitive lungs of 
the perennibranchiate urodeles. 

A third incidental use of the swim bladder is the production of sound. 
Drum fishes (Sciaenidae), "grunters" (Haemulonidae), and a few other 

The Release of Energy 


forms, such as the sea-robin, Prionotus, and the toadfish, Opsanus, are 
exceptional noise-producers in a modest way among the otherwise mostly 
silent brotherhood of fishes. According to Tower, who has carefully investi- 
gated the matter, the chief source of the drumming noise in drum fishes is 
the contraction of a "drumming muscle," musculus sonificus (Fig. 335), 
which, being superficially attached to the 
swim bladder, "produces a vibration of the 
abdominal walls and organs, and espe- 
cially of the swim bladder." D. S. Jordan 
says that the "grunting" of the Haemu- 
lonidae is caused "by forcing air from part 
to part of the complex swim bladder." 

1. General Plan 

Lateral Horn 



Lungs are the typical breathing organs 
of the higher verte'brates. Physiologically 
they represent an apparatus interposed be- 
tween the two parts of a double heart and 
in which air and blood are brought to- 
gether. Morphologically they consist of a 
much elaborated' respiratory surface of 
maximum exposure within a minimum 
space, together with a system of non- 
collapsible passage-ways for admitting air 
from the outside, that passes over these 
respiratory surfaces in intimate juxtaposi- 
tion with capillaries. 

The passage-ways to the lungs begin 


Fig. 335. Swim, bladder of male 
squeteague, Cyno scion. The dou- 
ble musculus sonificus is shown 
laterally displaced. The central 
tendon of the musculus sonificus 
lies free in the mid-dorsal line just 
above the swim bladder and below 
the kidneys. The double musculus 
sonificus is inserted laterally in the 
common fascia of the rectus abdo- 
minis muscle, and by its contrac- 
tion in connection with the dis- 
tended swim bladder gives rise to 
the drumming sound. (After 

either with the nasal chamber or oral cav- 
ity, leading through the pharynx to the trachea, bronchi, and bronchioles, 
and eventually reaching the innumerable terminal alveolar sacs that constitute 
the true respiratory area where the gaseous exchange of respiration is effected. 

Gills have one place for the intake of the oxygen-containing water (the 
mouth) and another for the outgo (gill slits), so that the respiratory proces- 
sion is continuous. In animals with lungs the same part is employed for the 
entry and departure of oxygen-containing air, with the result that inspira- 
tion and expiration become alternating processes. 

It is estimated that in man the respiratory alveolar surface enmeshed in 


Biology of the Vertebrates 

capillaries makes a total expanse of a hundred times the area of the entire 
skin, or, if inflated into a single sac, one that would form a balloon ten feet 
in diameter, yet this extensive structure is packed away in a relatively small 
space, the contour of which is determined largely by neighboring organs and 
general form of the body. 

Alveoli in contact with capillaries are lined with thin pavement epithe- 
lium, but the trachea, bronchi, and in part the bronchioles leading to the 
alveoli, are lined with ciliated cells, the activity of which, so long as they 
remain moist, tends to keep the air passages free from dust and other foreign 


\ pleura 

Mediastinum Somatic pleura 
Fig. 336. The lungs in the thoracic cavity, a, air expired by raised diaphragm 
and compressed ribs; b, air drawn in by lowered diaphragm and 
expanded ribs. 

The lungs as a whole are highly elastic and, although encapsuled in a 
double pleural sac in the higher vertebrates, are freely movable within the 
sac, except at the point of their attachment near the base of the bronchi 
(Fig. 336). Here they are joined by the trachea, a stalklike tube connecting 
with the pharynx, and are penetrated by the pulmonary artery and vein 
which hook up with the heart and the circulatory apparatus. Otherwise the 
lungs are free and unattached, thus enabling them to glide easily over the 
inner surface of the thorax with every breath while filling all the available 

The whole apparatus, which somewhat resembles in form a compound 
sebaceous gland, might be compared to a luxuriant tree, entirely hollow in 
all its parts, that has been pulled up by the roots and crowded top first into 
a bag. The root region corresponds to the nasal chamber, the oral cavity, 
and the pharynx; the main trunk of the tree to the trachea; the larger 
branches to the bronchi that subdivide into lesser branches and twigs, or 
bronchioles, terminating in the leafy foliage, or alveoli, that are crowded 
together in such a way as to occupy all available space within the envelop- 
ing sac, or pleural envelope. 

The Release of Energy 403 

2. Air Passages 

(a) Nasal Chamber and Pharynx. — The entrance to the pulmonary 
system is usually through the air-conditioning nasal chamber (Fig. 252), 
although in higher vertebrates the oral cavity may also serve in an emer- 
gency as an entrance. The nasal chamber is lined with a mucous mem- 
brane, known as the Schneiderian membrane. The tissue beneath this lining 
layer is richly supplied with capillaries, thus providing moisture and a cer- 
tain degree of warmth for incoming air. A moistened surface is further 
insured by the fact that the lacrimal ducts, from the constantly operating 
tear glands in the bony orbits of the eyes, drain into this chamber. The 
ciliated cells of the Schneiderian membrane maintain a continuous flow of 
mucus, and adhering particles of foreign matter, from the nasal cavity into 
the oropharynx. 

The walls of the nasal chamber are variously enlarged in different ver- 
tebrates by scroll-like turbinate bones, which not only increase the moist 
vascular surface, but also prevent an easy entrance of undesirable objects by 
making the passage-way tortuous. This latter purpose is also furthered by a 
forest, more or less dense, of outward-projecting hairs guarding the 
entrances, or nostrils, of the nasal chambers. This part of the air passages is 
cleared of undesirable accumulations by "blowing the nose." Man is the 
only mammal that performs this feat in any way acceptable to his neighbors. 

(b) Trachea. — From the nasal cavity air passes back through the 
nasopharynx to the oropharynx, where it crosses the food route on the way 
to the esophagus, thence entering the trachea through a slit-like opening, 
the glottis. 

The trachea, or "windpipe," is a rigid tube, very short in frogs and toads 
whose lungs are far anterior in the body cavity. It is somewhat longer but 
still insignificant in the urodeles. In lizards it is relatively shorter than in 
other reptiles, although unmistakably present, while in turtles and croco- 
diles it is frequently so long that it becomes convoluted or even spiral in 
form. The lengthened trachea in the chelonians is an accommodation to the 
accordion-like movements of the head and neck. 

Birds with long necks are of course provided with a long trachea, but 
frequently the trachea is even longer than the neck itself, so that it cannot 
remain straight but loops about. In swans these extra loops are stored within 
the hollow breastbone (Fig. 337), while in some birds they lie coiled under 
the skin, or may even extend into the body cavity. Extra long tracheae make 
it possible for their possessors to stretch out the neck without pulling the 
lungs out "by the roots." 

4°4 Biology of the Vertebrates 

Usually the windpipe is nearly cylindrical, but sometimes, as in the little 
vocal wall lizards, or "geckos," and also certain ducks, it may show a bulb- 
like enlargement that acts as a resonance sac when air is expelled. 

In mammals the trachea is practically straight, with a length directly- 
dependent upon that of the neck, except in the three-toed sloth, Brady pus, 
whose trachea is so elongated that it extends down as far as the diaphragm 
and back before entering the lungs. It will be remembered that the upside- 
down sloth, while hanging from the limbs of tropical trees, feeds lazily upon 
leaves without scrambling about. It therefore has a very stretchable neck. 

Trachea v 

Clavicle — 

Carina (CutOpen) — 

Fig. 337. Trachea of swan, embedded in sternum. (After Schim- 
kevvitsch. ) 

At all times the elastic walls of the trachea are kept mechanically dis- 
tended for the passage of air by encircling rings of cartilage, resembling the 
metal rings embedded in a garden hose to give it flexibility and durability, 
and at the same time to keep it uncollapsed and open. In the case of mam- 
mals these skeletal tracheal rings are usually incomplete on the dorsal side, 
that is, on the side liable to press against the esophagus that lies parallel to 
it, thus minimizing the "corduroy road" effect that might otherwise be 
encountered by a bolus of food when swallowed. 

Among reptiles, birds, and pinniped mammals, the tracheal rings are 
entire, while cetaceans present the unusual case of tracheal rings incomplete 
on the ventral rather than the dorsal side. 

Camels and giraffes are noteworthy in having upwards of one hundred 
separate tracheal rings, and whales and sea-cows in having these skeletal 
structures spirally arranged. 

Although usually of hyaline cartilage, tracheal rings become bony in the 
python Agama, and also in many birds. 

The Release of Energy aoc 

(c) Bronchi. — The trachea usually branches into two bronchi, that 
resemble it in structure with the exception of being smaller in size and in 
having weaker skeletal rings. 

There are three bronchi in certain ruminants, pigs, and whales, but in 
most snakes, with the degeneration of one lung as an accommodation to the 
extraordinarily elongated shape of the body, there remains only one 

(d) Bronchioles. — Bronchioles, which continue and multiply the air 
passages from the bronchi, have only limited cartilaginous supports which 
are in the form of rings. These supports become progressively smaller, and 
the mucous cells of the linings of the bronchioles fewer, until both are com- 
pletely absent from terminal bronchioles. The latter serve simply as ducts 
leading the way into the ultimate air chambers, or alveoli, in which respi- 
ration occurs. 

In mammals generally the bronchioles arise like the twigs of a tree and 
diverge from each other, but in crocodiles and birds they run together, 
forming intercommunicating loops instead of terminal twigs, from the sides 
of which alveoli are given off. 

- Endothelium 

Blood in 



Air in ^s> Epithelium 


Fig. 338. Diagram showing relation of capillary, carrying blood, to 
alveolus, containing air. 

(e) Alveoli. — The alveolar sacs, or the respiratory part of the whole 
system of air passage-ways, are hemispherical enlargements at the ends of 
the bronchioles. They have exceedingly thin delicate highly elastic walls 
over the outside of which, like vines over a trellis, extends a closely woven 
maze of capillaries (Fig. 338) . It is estimated that in a pair of human lungs 
there may be more than six million of these tiny chambers, all in ultimate 
communication with the outside atmosphere through the air-passages which 
unite in the trachea. 

The amount of air admitted to the alveoli is automatically regulated by 
means of nerve endings, the headquarters of which are located in the 
medulla of the brain. These nerve endings are inserted into tiny cufTs of 
circular muscle fibers that encircle the walls of the innumerable bronchioles, 
causing them to constrict or relax, as occasion demands. 


Biology of the Vertebrates 

jjr- — Esophagus 

3. Phylogeny 

The phylogeny of the lungs is a story of internal modification for the 
increase and efficiency of the respiratory surface, and also for adaptation to 
the shape of the body. The storing of lungs, for example, within the body 
of a squat toad, a lithe cat, a capacious cow, a box turtle, or an elongated 
snake, presents in each instance a different problem. 

(a) Dipnoans. — A transition between the swim bladder and true lungs 
is found in the dipnoans which, although not ancestral to the land verte- 
brates, show many of the features probably possessed by the ancestors of the 
latter. During the aestivation of these lungfishes the gills are not used for 
respiratory purposes, but instead branches of the sixth aortic loops, the pul- 
monary arteries, bring venous or "impure" blood to the swim bladder, 
which then functions as a lung. Pulmonary veins re- 
turn the blood to the left side of the auricle. Polyp- 
terus and Amia are the only other fishes with similar 
pulmonary arteries but their veins send the blood into 
the sinus venosus. 

In Neoceratodus, the lung or swim bladder is a 
single wide sac, resembling the swim bladder of phy- 
sostomous fishes, but in Protopterus and Lepidosiren, 
the sac is bilobed, its inner surface being increased 
somewhat by its coarse spongy alveolar structure. 

(b) Amphibians. — Amphibians in general carry 
on the pulmonary plan of the dipnoans although the 
primitive lungs of perennibranchs are less elaborated 
than those of the lungfishes. The lungs of Necturus 
(Fig. 339), for instance, are two long simple sacs, 
enmeshed on the outside by arterial and venous 
capillaries and opening directly through a slitlike 
glottis without the intervention of either trachea or bronchi. The inner 
surface is not increased by folds. The whole apparatus, resembling a pair of 
enlarged elongated alveolar sacs, is probably more hydrostatic than respira- 
tory in function. 

Amphiuma goes a step further, in that the proximal half of each lung 
has the inside surface considerably increased by the elaboration of folds. 

Owing to the form of their bodies, frogs and toads have more spherical 
lungs than salamanders. The folds within the frog's lungs extend from the 
inner walls in such a way as to divide the entire cavity into marginal stall- 
like spaces or compartments, all opening freely into a common central 

Fig. 339. Ventral view 
of the lungs of Nectu- 
rus, showing arrange- 
ment of blood vessels 
on them. (After Miller.) 

The Release of Energy 


cavity. The double walls of the "stalls," formed by the invagination of the 
outside wall, each carry a capillary network which increases the available 
blood supply over the surface of the lungs (Fig. 340). In toads the stalls 
become partially shut off from the central cavity of the lung by right angle 
additions along the inner margin of the partition walls, so that a secondary 
internal wall is formed that is perforated on all sides with openings between 
the central cavity and the air chambers, or stalls, which resemble semi- 
private luncheon niches around the margin of a common dining room. 




Fig. 340. Diagrams to show the evolution of lung structure. The plan 
of the lung of Sphenodon may be regarded as like a cluster of toads' 
lungs; in turn the alligator's lung may be considered as composed of 
several lungs of the sphenodon type. 

The very primitive tracheal and bronchial tubes of amphibians enter the 
lungs at the extreme anterior end. With an increased development of the 
anterior part of the lungs, the bronchi acquire a more lateral entrance in 
higher forms. 

As a result of a migratory invasion into the body cavity, the lungs of 
amphibians become invested on the outside by a single layer of peritoneum, 
which is pushed ahead of them into the body cavity during development. 
They do not, therefore, have a separate pleural cavity of their own, but 
instead lie freely in a common body cavity. 

Usually the left lung in amphibians is larger than the right one, but in 
tropical legless forms (Apoda) the reverse is true, as the left lung is rudi- 
mentary. Some salamanders, for example Eurycea and Salamandrina, are 
lungless, respiration being accomplished through the integument and the 
bucco-pharyngeal epithelium. 


Biology of the Vertebrates 

(c) Reptiles. — Integumental breathing is eliminated in reptiles, whose 
lungs, also abdominal in position, are much sacculated within and whose 
trachea and bronchi are developed into definite structures. The air-transfer- 
ring passage-ways and the air-absorbing mechanism of the lungs are dis- 
tinctly differentiated in reptiles. 

The primitive New Zealand lizard, Sphenodon, has spongy lungs that 
might be compared to a cluster of lungs like those of a toad, opening into 
a common passage, or atrium (Fig. 340), while the lungs of crocodiles go 
a step further in elaboration, corresponding to a bouquet 
of Sphenodon lungs placed together, the atria of which 
now open into a common bronchus. 

In snakes the left lung usually becomes aborted, only 
the right lung remaining to occupy the narrow quarters 
that are available. Boa constrictors and pythons are the 
most ancestral in this respect, having both lungs present 
with the left one somewhat shorter than the right. 

The different levels of the long single lung of the 
snake are unequally elaborated, recapitulating from dis- 
tal tip to the entrance at the glottis the early phylogeny 
of vertebrate lungs. Thus, the distal tip is a smooth sac 
like the lung of Necturus, but advancing toward the 
proximal or anterior end there develops a gradual evolu- 
tion of internal folds, at first resembling the lungs of 
Amphiuma, and later the chambered structure of the 
lungs of frogs and toads. Finally, at the base, these cham- 
bers become compounded and open into an atrium, sug- 
gesting the degree of complexity arrived at in the lung of 

Certain lizards, particularly Chameleon (Fig. 341), 
have peculiar lungs with saclike diverticula, which enable 
them to swell up to a certain extent, a device used perhaps to frighten their 
enemies. The inflated lungs of sea turtles, on the other hand, probably serve 
as floats, or life preservers, in maintaining a position at the surface of the 

(d) Birds. — The lungs of all modern birds are highly modified by the 
presence of supplementary air sacs, cellulae aereae, which facilitate the cir- 
culation of air through the lungs, but in themselves are not directly respira- 
tory in function, as shown by the paucity of capillaries over their surfaces. 

The bronchioles, instead of ending blindly in alveolar sacs, form a system 
of communicating loops, and open eventually into the reservoir-like air sacs 

Fig. 341. Lungs of 
Chameleon, show- 
ing supplementary 
air sacs. (After 

The Release of Energy 


Left Lung N 
Cervical Air Sac 


.Recurrent Bronchus 

A-r \<%uL\i «^# \ ^^ // s Abdominal 

/4^/^W^ , \ ^ \ ^^-S/ Air Sac 
Trachea V / ^CT^ 

/ \^^^'^ 0% ' l '^ Q '? Q)iX(: Septum 


Air Sac -===i=== i s^ Posterior Thoracic Air Sac 


Heart Anterior Thoracic Air Sac 

Fig. 342. Diagram of respiratory organs of a bird, left-side view. (After 

(Fig. 342) . Surrounding the smallest bronchioles and opening into them are 
elaborate meshworks of minute air tubules surrounded by capillaries so that 
their walls serve as the actual respiratory surfaces (Fig. 343). It is possible, 
therefore, for the air to be drawn back and forth entirely through the air 
tubules of the lungs, with gaseous exchange taking place both on the way 
through the lungs to the air sacs as well as on the return. The air in the 
lungs of other animals is never entirely renewed with each respiration, as in 

Large ^:C^A''7Q\^Svr :{1^<JUA)\ \/7 

Vessel ^ 1 Mffi-'-£^ S ^m\ 
Par abronchus-~ife%08^ I 

(Cut Across) \A^7 P%yg^&^Ph \ ' N \ 

Biocd Spaces ^^|§g» « 

Ai, Co pi l, arie s^-^^W 5 <es,ib u ,e S 

Fig. 343. Diagrammatic thick section of a small portion of a bird's 
lung, much enlarged. Vestibules are parabronchial outpushings into 
which air-capillaries open. Spaces between air-capillaries have an 
abundant supply of blood capillaries. (After Goodrich.) 


Biology of the Vertebrates 

birds, since some of it regularly remains stagnated in the alveolar ter- 

Embryonically air sacs sprout out from the lungs at various points and 
extend into the body cavity, occupying spaces between the viscera; beneath 
the skin (in pelicans) ; between the muscles, supporting and connective 
tissues: between and around the joints of the cervical vertebrae; and pene- 
trating even into the pneumatic cavities of the hollow bones (Fig. 344). 

Cervica I ~r ilt^"S^:^ 
Air Sac 

Air Sac 

Air Sac 

Thoracic Air Sacs'' 

Air Sacs 

Fig. 344. The injected air sacs of a pigeon, showing how completely 
they occupy all available spaces. (After Miiller.) 

The primitive Aptcryx of New Zealand alone has much reduced air sacs 
that do not enter the bones or penetrate the transverse septum to invade the 
body cavity. 

Although the air sacs of the bird's lung are not supplied to any great 
extent with a capillary network, and consequently are not directly respira- 
tory in function, yet they have several different uses, acting as bellows, bal- 

The Release of Energy mi 

loons, ballast, friction pads, heat regulators, reservoirs, and resonance aids 
to the voice. 

On account of the elasticity of the bird's lung, which is hampered by 
being anchored fast to the dorsal wall of the thoracic basket, some mechani- 
cal aid for effecting an efficient circulation of air through the lungs becomes 
all the more necessary. Such a mechanism is supplied by air sacs acting as 
bellows, enabling the air to be forced back and forth entirely through the 
lungs proper. In the capacity of balloons, the air sacs when inflated cause 
the specific gravity of the bird to be lessened owing to the intake and reten- 
tion of heated air. Without this storage of warmed air it would require 
considerably more muscular effort to sustain a body heavier than air in 
suspension for considerable periods of time. Possibly inflated air sacs may 
also by their turgor aid mechanically in maintaining the wings in an 
extended position during soaring or volplaning. 

As ballast the arrangement of the air sacs is such that a proper center of 
gravity may be established for balanced flight, and equilibrium easily main- 
tained by shifting the air content of the sacs from one part of the body to 

The insertion of air sacs like pads between the muscles lessens friction, 
thereby giving flexibility and grace to the aerial movements of birds. Because 
they are filled with warm moist air the air sacs help to maintain and regu- 
late the body temperature, for the skin of birds in the absence of evaporating 
sweat is of little service for this purpose. 

As containers of reserve air, the air sacs are undoubtedly useful. The 
muscular mechanism by which a resting bird causes air to enter the lungs, 
like that of mammals, involves the alternate elevation and depression of 
the breastbone through the activity of the intercostal muscles. It is necessary, 
however, during flight for the breastbone and the entire thoracic basket to 
remain firm, in order to insure substantial anchorage of the powerful flying 
muscles. To do this the intercostal respiratory muscles are held in tension 
and are for the time being not pumping fresh air into the lungs. Therefore, 
an internal reservoir of air is indispensable, while the flying muscles which 
ventilate the lungs by acting upon air sacs as bellows also control respiratory 
movements during flight. The more rapid the flight, the greater the auto- 
matic supply of air drawn through the lungs to and from the pneumatic 
chambers by the flight muscles. Violent action in mammals interferes with 
respiration, but with birds it enhances it. This is why fast flying birds do not 
"get out of breath," or probably suffer from "mountain sickness" in the air 
of high altitudes because the necessary increased wing strokes bring in a 
compensatory supply of rarefied air. The frigate bird, Fregata, that easily 


1 2 

Biology of the Vertebrates 

maintains a rate of one hundred miles an hour, has about the best develop- 
ment of air sacs to be found in any bird. 

The pneumatic diverticula of the lungs of Chameleon, already men- 
tioned (Fig. 341), and those of certain other lizards may perhaps be 
regarded as prophetic of the air sacs of birds. 

(e) Mammals. — The lungs of mammals are usually characterized in 
two ways: first, by being subdivided externally into two lobes; and secondly, 

by showing some degree of asym- 
Anterior Lobe . , 

metry in accommodation to sur- 
rounding organs. When asymmet- 
rical the lobes are more numerous 
on the right than on the left side. 
Thus, in man (Fig. 345), there 
are three lobes in the right lung 
and two in the left. The upper- 
most odd lobe of the right lung 
lies behind the right pulmonary 
artery, while the absence of a cor- 
responding lobe on the left side 
permits the presence of the large 
left aortic arch. 

Certain mammals, as for example Cetacea, Sirenia, Proboscidea, Hyra- 
coidea, and most Perissodactyla, resemble other vertebrates in the absence 
of pulmonary lobes, while Monotremata are transitional, since they possess 
lobes only in the right lung. 

The lungs of whales, which are located rather posteriorly in the hulls of 
these seagoing leviathans, are probably hydrostatic as well as respiratory in 
function. Whales have a unique breathing apparatus that enables them, 
during a plunge into the ocean depths, to imprison air in the capacious nasal 
chamber which is several times larger than the brain case. In fact the nasal 
chamber occupies the major part of a whale's head and is capable of storing 
a generous supply of air that would otherwise be forced out of the lungs 
by the enormous pressure of the water. The apertures leading from the nasal 
passages to the lungs can be shut off by two plugs of tissue which function 
like the stopper in a bathtub. As the air in the lungs becomes stale the plugs 
open long enough to exchange the stale air with pure air in the nasal reser- 
voir, and when the whale finally comes to the surface the reservoir is emp- 
tied with considerable violence accompanied by confined water vapor, caus- 
ing the excited whalers who witness the performance to shout, "Thar she 

Posterior Lobe 


Fig. 345. Lobes of human lungs. Outlines 

of the outer, or costal, surface. (After Toldt. ) 

The Release of Energy 


4. Pleural Envelopes 

The lungs of higher vertebrates are enclosed in compartments called 
pleural cavities, separated from the abdominal cavity which is the storehouse 
of most of the internal organs. 

The establishment in mammals of exclusive chambers for the lungs has 
been a gradual evolutionary process. The primitive lungs of amphibians 
push down into the general body cavity, carrying with them a thin covering 
of serosa continuous with the peritoneum that lines this common cavity but 
without the formation of independent pleural chambers. 

In reptiles, along with the formation of a transverse septum formed by 
the invasion of peritoneal folds and the assurance of privacy for the heart 
by the partitioning off of a pericardial chamber, there is formed around the 
lungs a second envelope, also derived from the peritoneal serosa, that con- 
stitutes the outer, or parietal, wall of the pleural cavity (Fig. 336). The 
inner, or visceral, wall is the original derivative of the peritoneum already 
mentioned, and this intimately invests the lungs like a tight-fitting garment. 

The space between the parietal and visceral walls, that is, the pleural 
cavity, is filled with a serous lubricating fluid which allows freedom of 
movement on the part of the extensible lungs within the pleural space. 

5. Origin of the Lungs 

The lungs, like the swim bladder, probably come from rudimentary gill 
pouches. While the swim bladder is usually a dorsal outgrowth from the 
floor of the foregut, the first evidence of lungs in man, which may be seen 
about the third week of fetal life, is a ventral groove on the floor of the same 
region from which the swim bladder sprouts out dorsally. As this groove 

Lobe of Lung — --, 

i — Trachea 

c- __ Bronchus 

Fig. 346. Early stages in the development of lungs, trachea, and bronchi 
of human embryos. Ventral views, a, 4 mm embryo; b, 5 mm embryo; 
c, 7 mm embryo; d, 10 mm. embryo. (After Arey; a, after Grosser and 
Heiss; b, c, d, after Heiss and Merkcl.) 


Biology of the Vertebrates 

pushes deeper down, it forms a single bud that soon becomes a bilobed sac 
representing the future lungs (Fig. 346) . Soon after, a common stem or duct 
is formed by a further outgrowth of the lung sacs. This is the trachea, whose 
appearance is followed by the branching bronchi, and last of all by the 
elaboration of subdivisions within the lung sacs and the establishment of 
the alveoli. 

During the differentiation of the lung sacs'the endodermal lining invades 
the surrounding mesoderm, as shown by Moser in a series of illuminating 
diagrams (Fig. 347), with the end result that a maximum surface of re- 
spiratory endodermal tissue is brought into intimate contact, back to back, 
with vascular mesodermal tissue carrying blood-filled capillaries. 




Primary Alveolar_ 

Mesoderm v v 

Fig. 347. Diagram of the formation of the lungs in turtles, a, intrapul- 
monary bronchus with several primary alveolar chambers invading the 
surrounding mesoderm; b, intermediate stage; c, the adult condition. The 
chambers have become subdivided into alveoli by ingrowth of their walls. 
Mesoderm reduced to partitions between the chambers. (After Moser.) 

Human lungs assume definite shape before the end of the third month, 
although they do not take on their respiratory function as long as the embry- 
onic placenta is active and are not entirely inflated for three or four days 
after birth. The alveoli are laid down by the seventh month and thereafter 
merely undergo enlargement. 


The process of breathing demands more than a mechanism that simply 
allows blood and air to get together within osmotic distance of each other, 

The Release of Energy 


since there must also be present means for securing a continuous circulation 
of fresh air across the respiratory surfaces. 

1. Fishes 

In the case of submerged fishes, water charged with various gases, 
including the essential oxygen, enters either the mouth or the spiracles and 
passes out through the gill slits. It is forwarded and directed in its course, 
not only by muscular movements which alternately expand and contract the 
walls of the orobranchial chamber, but also by a system of valves that pre- 
vents the water from going the wrong way. 

Maxillary Valve 


Mandibular Valve 


Fig. 348. Diagrams illustrating the mechanism of respiration in teleosts. 

a, phase of inspiration; b, phase of expiration; c, anterior view of the 
mouth valves. In a and b the anterior part (in front of break in wall of 
oral cavity) represents a vertical section, and the posterior part (in the 
vicinity of the gills) a horizontal section. Arrows indicate direction of 
water current and pressure, and those passing through walls of oral cavity 
the expansion and contraction of the opercular apparatus. In a, the 
respiratory valves (maxillary and mandibular) are open, and the 
branchiostegal membranes closed. In b, this condition is reversed, (a and 

b, after Dahlgren, from Sayles, Manual for Comparative Anatomy, copy- 
right 1938, by permission of The Macmillan Company, publishers; c, 
after Kingsley.) 

The anterior set of these valves are collapsible folds along the inner edge 
of the mouth opening, those of the upper edge being called maxillary, and 
of the lower, mandibular. They reach their greatest differentiation in teleost 
fishes which have a well-developed opercular apparatus. The posterior, or 
branchiostegal, set of valves are membranes along the free margins of the 
opercular flaps (Fig. 348) . A freely moving current of water is produced in 

416 Biology of the Vertebrates 

the following way. First, the mouth remains open as a narrow slit, while the 
anterior maxillary and mandibular valves lie flat, or open, and the posterior 
branchiostegal valves close. Next the walls of the orobranchial chamber 
spread apart by muscular action, thus pulling water into the mouth to 
occupy the increased space. Then the valves reverse, that is the anterior ones 
close the slitlike mouth aperture and the posterior ones open the opercular 
slit, while the walls of the orobranchial cavity squeeze together, forcing the 
water backward over the gills and out of the opercular openings. 

In some teleosts, particularly those that feed on microscopic plankton, 
the branchiostegal valves play the major role in this process, but in others, 
for example the Percidae and the Sciaeridae, the opercular flaps take the 
most prominent part. 

2. Amphibians 

Amphibians never breathe through the open mouth but instead inspire 
air through the nostrils and the choanae, as the newly-established passage- 
ways into the mouth cavity are called. They are not even able to exercise 
emergency breathing through the mouth, as mammals do, for as long as the 
mouth remains open there is no way to compel the air to enter the lungs. 
Necturus and other perennibranchiate urodeles sometimes come to the 
surface of the water and gulp air through the mouth, which may soon be 
seen escaping in the form of bubbles through the gill slits, although it is 
doubtful whether much of it reaches the lungs. This occasional air-gulping 
behavior does not furnish fresh air for the external gills that hang outside 

the gill slits, since by means of muscles 
that cause them to wave back and 
forth, these animals obtain their supply 
of oxygen dissolved in the water. 
1 The intake of air in the frog, which 

„.„.„„,. , f may be taken as a representative of 

rig. J49. Breathing mechanism 01 a . 

£ rog the anurans, is accomplished by a com- 

bination of pumplike throat muscles 
and nostril valves (Fig. 349). It will be seen that when the nostril valves 

l 5- 

open in the manner of lids and the throat muscles draw down, the oral 
cavity within is enlarged and air is necessarily inhaled. With the closure of 
the nostril valves and the contraction of the throat muscles, the lungs auto- 
matically become filled by the mouthful of air that is forced backward. The 
expiration of air alternates with inhalation and is accomplished by means 
of the contraction of body muscles. 

The Release of Energy 

4 l 7 

Water Line 

3. Reptiles 

The problem of getting air into the lungs of reptiles is much like that in 
the case of amphibians, although some improvement is seen since ribs and 
rib muscles furnish a mechanical means for admitting air that is not present 
in the practically ribless amphibians. This improvement, however, is inef- 
fective in turtles, whose ribs form a boxlike armor of uncompromising rig- 
idity. These animals still resort to the am- 
phibian method of utilizing throat muscles 
and nostril valves, swallowing the air by 
"working the throat." No doubt the in-and- 
out movements of the turtle's head and 
neck aid in pumping air into the lungs 
while the pectoral muscles, which are inside 
of the ribs in these bizarre reptiles instead 
of outside as in other vertebrates, are 
aided by the abdominal muscles in bring- 
ing about the expulsion of air from the 
lungs. The usefulness of rib muscles in 
pumping air in and out of the lungs is 
very apparent in the panting snakes, liz- 
ards, and alligators. 

The latter have an exceptionally elon- 
gated nasal passage-way with a curtain, or 
velum, that closes off the inner choanal 

openings from the mouth cavity. This device makes it possible for the alli- 
gator to breathe with the mouth open under water while holding the drown- 
ing prey between the cavernous jaws, only the tip of the snout with the 
openings of the external nares being above the water line (Fig. 350). 

4. Birds 

So long as a bird is not in flight it breathes by means of its rib muscles 
after the typical reptilian manner. When a bird flies, however, as already 
explained, the powerful pectoral muscles, on which flight depends, require 
and secure anchorage upon a rigid thoracic basket that does not change 
shape with every breath. The bellows-like air sacs, which are filled and 
emptied by the action of the flying muscles rather than the rib muscles, fur- 
nish an effective means for irrigating the lungs of a flying bird with air, 
while the inactive rib muscles remain temporarily fixed and rigid. 

Fig. 350. Diagram to show respira 
tory passage (dotted) in an alliga- 
tor. It leads to the trachea behind 
a flaplike velum in the back part of 
the mouth cavity, which enables the 
animal to breathe under water and 
at the same time to drown its prey 
held between the jaws, so long as 
the external nostrils are above the 
water line. 

418 Biology of the Vertebrates 

5. Mammals 

In mammals both nasal and oral breathing are made possible by the 
backward migration of the glottis to a position in the posterior region in the 
throat. Nasal breathing, however, with the greater facilities thus provided 
for warming and moistening the inhaled air, and the added advantage of 
testing its quality by means of its passage over the sensitive olfactory sur- 
faces in the nasal chamber, is the better and more favored method among 
mammals generally. 

The outstanding advance in the breathing mechanism of mammals is 
furnished by the muscular diaphragm, which in lower vertebrates is fore- 
shadowed by the transverse septum that separates pleural and pericardial 
chambers from the body cavity. The diaphragm consists of a central tendi- 
nous component, from which extend radiating muscle fibers derived from 
mid-cervical myotomes. The diaphragm when relaxed is shaped somewhat 
like an arched vault (Figs. 336 and 351), and is perforated by the dorsal 
aorta, esophagus, azygos vein, thoracic duct, postcava, and vagus nerve. As 
its radiating fibers shorten by contraction, the vault of the cuplike dia- 
phragm lowers or flattens, thus increasing the space within the thoracic 
cavity. Consequently the atmospheric pressure from without forces air into 
the lungs. At the same time the viscera within the body cavity are crowded 
down so that the abdominal wall bulges out. The muscular opponents of 
the diaphragm are the strong walls of the abdomen. 

In addition to abdominal diaphragmic breathing, mammals also utilize 
the reptilian method of rib muscles to enlarge the thoracic cavity when 
inspiring air. The ribs are bent, like jointed levers at an oblique angle to the 
vertebral column, and if acted upon by the intercostal muscles the movable 
sternum, to which they are attached ventrally, moves farther away from the 
relatively stationary backbone, thus enlarging the thoracic cavity in which 
the lungs are located. So it comes about that inspiration is effected not only 
by the depression of the diaphragm but also by the elevation of the ribs, 
both efforts calling for muscular activity. 

Expiration, on the other hand, is to a large extent automatic through the 
elasticity of the stretched body walls, the taut cartilaginous ends of the bent 
ribs, and the tensity of the expanded lung tissues. 

In big heavy animals, abdominal or diaphragmic breathing predomi- 
nates over rib breathing. Jumping animals, like kangaroos and monkeys, 
utilize rib muscles rather more than the diaphragm in respiration. 

Breathing by means of the ribs is also more pronounced in human 
females than in males in whom abdominal breathing predominates. The 

The Release of Energy 


reason for the sexual difference in the respiratory mechanism may be an 
evolutionary adaptation brought about in connection with pregnancy, dur- 
ing which period the presence of a growing fetus interferes somewhat with 
freedom of movement of the diaphragm. 

Trachea - 



*■ Expiration 

S --V Expiration 

, —Inspiration 

■ Inspiration 
* Expiration 


Fig. 351. Abdominal and thoracic contours during respiration. A, the 
relative positions of the chest wall, the diaphragm, and the abdominal 
wall at the end of maximal inspiration and expiration; b, diaphragm at 
end of normal inspiration and expiration. In quiet breathing there is no 
marked change in the position of the central tendon; in forced inspira- 
tion it may be drawn posteriorly, pulling the heart down with it. (From 
Howell, Fulton, Human Physiology, copyright 1946, by permission of 
W. B. Saunders Company, publishers.) 

The amount of air required daily by a human being varies within wide 
limits but may average over 10,000 liters, including over 2000 liters of 
oxygen. The lungs of an adult may have an average capacity of 4000 cc 
of which some 500 cc, called tidal air, is renewed with every breath. 
Of the remainder, about 2500 cc is com pie mental air that can be inspired 
on occasion by deep breathing at the end of a normal inspiration of tidal air. 
The other 1000 cc is supplemental air, or the amount which can theoreti- 
cally be forcibly expired after a normal expiration. After the deepest pos- 
sible expiration there is still about a liter of air left in the lungs and respira- 
tory tract. This residual air is not included in the data for lung capacity, as 
given above. 

^zo Biology of the Vertebrates 


The emotions of animals are frequently expressed in various ways by 
characteristic movements and noises. Male rabbits sound an alarm by 
pounding the ground with their hind feet, while bucks and bulls send forth 
a resounding challenge by stamping with their front hoofs. Rattlesnakes 
shake their caudal castanets and many stridulating insects express them- 
selves audibly by rubbing hard parts together. 

Among vertebrates that produce a distinctive noise, the voice apparatus 
is generally a wind instrument, and consequently a direct part of the respira- 
tory system. 

The majority of vertebrates are dumb. Fishes, which outnumber all other 
vertebrates in species as well as individuals, have only a few representatives, 
such as drumfishes and "grunters," that break the piscine vow of silence. 

Amphibians, excluding the musical frogs and toads, are practically voice- 
less, and reptiles also, if a few unusual cases such as hissing snakes, guttural 
geckos, and bellowing bull alligators are omitted, are prevailingly silent. 

When King Solomon ecstatically sang in a springtime of long ago, "The 
flowers appear on the earth ; the time of the singing of birds is come and the 
voice of the turtle is heard in our land," he did not refer to the silent reptile 
of that name, which is quite unable to celebrate the changing seasons 
vocally, but to the plaintive notes of the turtle dove. 

Some moralist has pointed out the fact that probably the first vocal 
words spoken on this earth were the words of the croaking frogs, which 
sound like "work, work, work." It should be noted in passing that the 
sedentary frogs, like many other armchair givers of advice, pay no heed to 
their own exhortations. 

Birds as a rule are notably vociferous but there are numerous species, 
such as the stately storks, that preserve a dignified silence, while it is a 
curious fact that among mammals gigantic whales have relatively less voice 
than tiny squeaking mice. 

The words in common use to describe sounds produced by mammals 
indicate a wide variety of distinctive "voices," with corresponding diversity 
in the wind instruments involved. For example, the horse "whinnies"; the 
cow "moos"; the donkey "brays"; the pig "squeals"; the sheep "bleats"; 
the elephant "trumpets" ; the porcupine "grunts" ; the lion "roars" ; the cat 
"purrs"; the wolf "howls"; the dog "barks"; the rat "squeaks"; the lemur 
"wails"; the monkey "chatters"; and some human beings "sing." 

Since most voices are dependent upon the expulsion of air from the 
lungs, the vocal apparatus, or larynx, is advantageously located around the 

'Hie Release of Energy 


glottis, that is, the slitlike entrance to the trachea. Different sounds are pro- 
duced by modifying the shape of the aperture and the contour of the cavity 
through which expelled air escapes. The cartilages, membranes, walls, and 
muscles of the larynx constitute a mechanism for effecting this result. 

The mouth cavity in man, as well as the pliable cheeks and flexible 
tongue, aid greatly in altering the character of the chamber through which 
the column of air from the lungs is forced, thus changing the sounds pro- 
duced. This can be easily demonstrated by pronouncing the vowels, A, E, 
I, O, U, in succession and mentally noting the changes that meanwhile 
result in the position of the lips, tongue, and cheeks, and the consequent 
alteration in the contour of the mouth cavity while executing these distinc- 
tive sounds. 

In this connection it is a suggestive fact that the evolution of voice has 
a close dependence upon emergence from water to life in air. It is obvious 
that the traditional episode of the Tower of Babel could only have been 
staged very late in the evolutionary story, after the human larynx had come 
into its own. 

1. Amphibians 

There is no true larynx in fishes, but in voiceless salamanders the larynx 
consists of two tiny triangular guardian cartilages, lateral cartilages (Fig. 
352), embedded one on either side of the glottis. In some cases when there 
is enough trachea to permit, as for example in Siren, there are additional 
fragmentary cartilaginous rings below the laterals. These prophetic carti- 



TracheaK " </> 

Cartilages \ " ^ 

o o 
Fig. 352. Laryngeal car- 
tilages of urodeles. a, Nec- 
turus; B, Siren. (After 
Wiedersheim. ) 


Fig. 353. Laryngeal apparatus of 
a frog. (After Wiedersheim.) 

4 22 

Biology of the Vertebrates 

lages of urodeles become developed into elongated laryngeal cartilages in 
the musical frogs, toads, and hylas. The "brek-ek-ek-kex, ko-ax, ko-ax" of 
Aristophanes' famous frogs, imitated in one of the modern college yells, is 
a characteristic bit of virile vocalization familiar to everyone whose experi- 
ences include a frog pond in springtime. The mechanism by which these 
haunting nocturnes are produced consists of a pair of arytenoid cartilages, 
and in addition, of a new cartilage, the cricoid, 
which is an elaboration of the first tracheal ring or 
rings (Fig. 353). Dilator and adductor muscles 
operate these skeletal elements. 

Two folds on the inner walls of the laryngotra- 
cheal chamber, the vocal cords, lie parallel with the 
slitlike glottis. The Anura, especially the males of the 
species, have besides, internal vocal sacs in the throat 
region which are apparent externally when inflated. These serve as chambers 
of resonance for increasing the carrying quality of their vocalizations. A single 
median vocal sac is common to all hylas (Fig. 354), as well as the toad, 
Bufo, while two lateral sacs show at the shoulders of frogs, being especially 
pronounced in the male bullfrog, Rana catesbiana, when its twanging 
"iug-o'-rum" note is being broadcast. 

As a matter of fact the lungs of frogs and toads are largely organs for 
producing sound after the manner of bagpipes, the respiratory function 
being taken care of mostly by the skin, 
as shown by the fact that the cutaneous 
arteries exceed the pulmonary arteries 
in size. 

Fig. 354. Hyla, showing 
inflated resonance sac. 
(After Boulenger.) 

2. Reptiles and Birds 

Reptiles and birds have less larynx 
and more trachea than Anura. Compen- 
sating this deficiency, birds have evolved 
an additional secondary "larynx," called 
the syrinx, which is located at the lower 
end of the trachea at its junction with 
the bronchi, instead of near the glot- 
tis (Fig. 355). The position of this unique voicebox is in line with the ex- 
treme structural modifications of birds whereby all possible weight is central- 
ized for purposes of equilibrium in flight. 

There are no vocal cords in the reduced larynx of birds. Instead sounds 
are due to vibrations of membranes in the syrinx. The median wall of the 

A B 

Fig. 355. Different syringes. A, mal- 
lard; b, goosander; c, velvet scoter; d, 
eider duck. (After Pyecraft.) 

The Release of Energy 

4 2 3 

beginning of each bronchus is a thin membrana tympaniformis interna (Fig. 
356). As there is a projection of the interclavicular air sac which lies 
between the bases of the bronchi, the tympaniform membranes can vibrate 
as air is expelled past them. In singing birds there is also an unpaired vibra- 
tory membrana semilunaris, extending dorso-ventrally near the junction of 
the bronchi and trachea. A bony ridge, the pessulus, supports the semilunar 
membrane. The cartilages of the syrinx, which are modifications of tracheal 
and bronchial rings, are combined with intercartilages, membranous walls, 
and a variety of muscles, into an efficient vocal mechanism. By means of 
this apparatus the bird is enabled to change the shape of the tracheo- 
bronchial chamber, thus producing a variety of different sounds. 



Bronchia kzif 
Rings i-j 

IIUI- — '"If .\::)l a^ — '- 




\ — Semilunar 

Air Sac 

'"*— Infernal 

Fig. 356. Longitudinal section through the syrinx of a thrush. (After 

3. Mammals 

The cartilages of the mammalian larynx include, in addition to a pair 
of triangular arytenoids around the glottis, and a bandlike cricoid just 
below, a relatively large quadrilateral shieldshaped ventral thyroid cartilage, 
that in man forms the movable prominence in the neck known in academic 
circles as the pomum Adami, and elsewhere as "Adam's apple." 

The thyroid cartilage is originally paired, as shown by the fact that in 
monotremes it is made up of two separate lateral plates instead of a single 
piece, and in all mammals it is derived embryonically from the remains of 
paired branchial arches. 

Still another structure developed in mammals is a cartilaginous lid above 
the glottis, called the epiglottis. This aids in closing off the tracheal tube 


Biology of the Vertebrates 




from food that is passing down to the esophagus, a process accomplished not 
so much by the closing down of the epiglottis as by the elevation of the 
tracheal tube to fit against the overhanging lid. That the larynx is tempo- 
rarily elevated during the act of swallowing is easily demonstrated by plac- 
ing the thumb and finger lightly against the Adam's apple, at the same time 
imitating the transit of food by swallowing. 

Furthermore, in mammals the horseshoe-shaped hyoid bone, situated 
above the glottis region, is made a part of the laryngeal complex by con- 
necting ligaments which suspend the 
larynx. In addition certain minor laryn- 
geal cartilages are described in human 
anatomy, for example, two minute rod- 
like cuneiform cartilages of Wrisberg, 
in the fold between the epiglottis and 
the arytenoids; two small conical nod- 
ules, the cartilages of Santorini, sur- 
mounting the apices of the arytenoids; 
and the triticeous cartilages, embedded 
in the ligaments connecting the hyoid 
bone on either side with the thyroid 
cartilage, so called because of a resem- 
blance to grains of wheat (Triticum) . 
The location and relationships of the 
most important cartilages that make up 
the laryngeal voicebox of man are in- 
dicated in Figure 357. 
The vocal cords, which reach their greatest differentiation in mammals, 
are two pairs of bandlike folds on the inner wall of the larynx, one above the 
other, extending between the arytenoids and the thyroid cartilage (Fig. 
358). Of these the upper pair are called "false," and the lower pair "true" 
vocal cords. Made up of dense bands of elastic fibers covered over by 
mucous membrane, their position and tension may be altered by means of 
accompanying muscles. During ordinary breathing they do not vibrate suffi- 
ciently to produce sound, but when desired the column of air that is forced 
over their surface is modified through their activity into audible vibrations. 
In most cases sound is produced by expiration of air over the vocal cords, 
although in exceptional instances, a familiar example of which is the "hee- 
haw" of the donkey, the air does vocal service both going in and corn- 
ing out. 

Between the true and false vocal cords a groove or concavity, known as 



Fig. 357. Front view of human larynx 
(After Cunningham.) 

The Release of Energy 




Cut Surface of 
Thyroid Cartilage 


Crico-Thyroid— - 

Cavity of Trachea 

Dorsal Face of 

J — Hyoid Bone 

\ Muscle 

-False Vocal 

jj— True Vocal Cord 


-^f^— Ventral Part of 
fyf/ Cricoid Cartilage 

" Cut Surface of 
Cricoid Cartilage 

v Cut Surface of 
Tracheal Cartilage 

Fig. 358. Vertical section, of human larynx. (After Testut.) 

the ventriculus laryngis Morgagni, which is 
certain howling monkeys and vociferous 
apes, has the capacity of swelling out on 
either side into air-filled resonance sacs that 
function much as do similar structures in 
frogs, by adding intensity to the sounds pro- 
duced. Curiously elephants are without false 
vocal cords, while hippopotami have no 

In young marsupials that remain at- 
tached to the nipples of the mother for pro- 
tracted periods by means of an automatic 
sphincter muscle around the mouth, the 
larynx becomes so elongated that it extends 
up into the nasopharynx behind the soft 
palate, enabling these young animals to 
breathe and take in milk at the same time. 

particularly well-developed in 



Fig. 359. Larynx of Ziphius, a 
whale. Anterior ends of elongate 
epiglottis and arytenoids extend 
forward into nasopharynx. (After 

A26 Biology of the Vertebrates 

This device eliminates complications usually attendant upon the double 
traffic of air and food in the pharyngeal crossing of the ways. 

Whales have an elongated larynx which extends into the nasopharynx 
so far that the glottis can be enwrapped by the soft palate, an arrangement 
that mitigates some of the difficulties to which these aberrant mammals are 
subjected by their marine existence (Fig. 359). 


Outgo Apparatus-Excretory System 


The inevitable consequence of the metabolic processes characterizing 
living creatures is that various by-products are formed in the body that must 
be got rid of for the reason that they not only are useless to the organism but 
may become decidedly harmful if retained. 

Since there are many kinds of animals, there is a variety of devices for 
accomplishing this universal function. Excretions should not be confused 
with glandular secretions, such as saliva, milk, tears, mucus, enzymes, and 
hormones, which are of sendee to the organism. The mechanism of sewage 
disposal is the excretory apparatus. 

The substances eliminated by excretion may be in the form of gases, 
solids, or liquids. Lungs and gills furnish the principal mechanism for the 
excretion of the gas carbon dioxide, resulting from the respiratory oxidation 
of carbohydrate and fat foods, while the digestive tube is the avenue of 
escape for the solid refuse from ingested food. Although the latter may not 
be regarded as true excretion, since the solids evacuated have never been 
incorporated as a part of the body, it is nevertheless an indispensable part 
in the process of the disposal of waste. Both of these methods of elimination 
have already been considered in the foregoing chapters upon the digestive 
and respiratory apparatus. 

In addition to these two methods of disposal of the unusable products 
of the body, there is also a constant excretion, or sloughing off, of cellular 
material from the epithelial surfaces of the body, both from the outer exposed 
surface, and also from the lining of various tubes and ducts which have 
access directly or indirectly to the outside. 

The present chapter is concerned primarily with the disposal of liquid 
waste by means of the urinary apparatus which is ordinarily referred to as 
the "excretory system." 

As a matter of fact liquid, or water in various guises, is disposed of in 
the animal organism through several different channels. It is thrown off 

U 2 7 

428 Biology of the Vertebrates 

from the lungs and the sweating skin of mammals as vapor ; from the diges- 
tive tract as the fluid component of the feces; and above all from the 
kidneys in the form of urine. It comes to the kidneys from the blood, 
charged with salts in solution, both organic and inorganic, together with a 
variety of other chemical substances, as well as cell wreckage of various sorts. 
Probably the simplest urinary apparatus of excretion is the contractile 
vacuole in protozoans, which periodically expels its liquid contents, accumu- 
lated from the surrounding substance of the cell to the outside. 

In the bloodless flatworms (planarians) excretion is accomplished 
through a system of branching ducts that ramify throughout the body and 
join before emptying their excretory contents to the outside. The numerous 
extreme tips of this hollow branching system end blindly in swollen knobs, 
called flame cells, because in the cavity within them there is a tuft of cilia 
whose flamelike flickering motion forwards the collected waste liquid on its 
outward way from the surrounding tissues. 

In animals having a body cavity, drainage tubes, or nephridia, are intro- 
duced connecting the cavity with the outside. When nephridia occur in 
invertebrates they are typically paired and independent of each other, but 
in vertebrates they are more or less massed together into definite organs of 
excretion, known as kidneys. Primarily nephridia open at one end into the 
body cavity and at the other, either independently or indirectly, through a 
common connecting duct to the outside. This furnishes a means of escape 
for the coelomic fluid in the body cavity, which receives contributions by 
way of the blood from all parts of the body. Such an arrangement is of 
particular significance in many invertebrates, for example annelid worms, 
although decreasing in importance among vertebrates. On account of the 
lessening usefulness of the coelomic fluid in the absence of an open blood 
system and the elaboration of a closed blood system, the liquid wastes of 
vertebrates are collected directly by the blood stream rather than after 
finding temporary sanctuary in the coelom. 

The excretory system of vertebrates may be described, therefore, as 
fundamentally made up of nephridia, more or less completely emancipated 
from the original direct connection with the body cavity, but nevertheless in 
intimate osmotic contact with blood capillaries by means of which waste 
materials are collected from the blood stream and transferred to the outside 
through ducts of exit. 

The nephridia] tubes of the kidneys have structurally much in common 
with the sweat glands of the mammalian skin that have been described as 
minute supplementary kidneys. Both are tubes with walls of excreting cells 
in (lose juxtaposition to capillaries, and have the power of abstracting waste 

Outgo Apparatus 429 

materials from the blood stream. When it is remembered that there are over 
2,000,000 sweat glands in the skin of an ordinary human being, and that 
end to end they constitute over twenty-five miles of glandular tubing, 
according to Macfie, it will be realized that these microscopic structures 
are by no means insignificant understudies to the kidneys in the disposal of 
liquid waste materials from the body. 

For purposes of general description the urinary apparatus of a typical 
vertebrate may be considered under three headings : ( 1 ) kidneys ; ( 2 ) uri- 
nary ducts ; and ( 3 ) bladders. 

I. Forms 

With respect to the shape of the paired kidneys the evolutionary tend- 
ency is towards compactness and consequently there is a certain parallel 
between their form and the general contour of the body. The kidneys of the 
primitive eel-like cyclostomes are long straplike bands, while in fishes gen- 
erally they extend not only throughout the length of the body cavity but 
also they may penetrate even beyond into the tail muscu- 
lature. Frequently too, the typical shape of the kidneys of 
fishes is modified to conform to the presence of the swim 
bladder, which in fishes is a bedfellow of these organs. 

Among amphibians the wormlike Gymnophiona and 
the long-bodied urodeles have correspondingly elongated 
kidneys, narrower anteriorly and widening posteriorly, 
while in the squat anurans these organs become much more 
compact and rounded in shape. Fig. 360. Left 

Although lizards and alligators somewhat resemble lobulated kidney 

urodeles superficially, the relation between the shape of the ,. ^ r ' ff r , * 

r ; ' . l dissected. (After 

kidneys and the form of the body is less marked. The Henle.) 
kidneys, however, are still somewhat elongated, but in 
turtles become decidedly compact, conforming to the rigid requirements 
of space imposed by the shell. The opposite extreme is shown by snakes 
that have the kidneys not only attenuated like the body but also entirely 
crowded out of the typical side by side position, so that they lie tandem- 
fashion one behind the other. 

The concavities of the elaborate pelvis of birds, in which the kidneys are 
for the most part packed, form a restricting bony casket that determines 
their lobulated form. The highest degree of compactness is found among 

43° Biology of the Vertebrates 

The bear (Fig. 360), ox, seal, walrus, and porpoise have lobed kidneys, 
a condition appearing also in human embryos but which becomes obliterated 
soon after birth. 

Fusion of the two kidneys occurs frequently in fishes (Fig. 396), and in 
some lizards, at least at the posterior end, as well as in many birds. Posterior 
fusion of the kidneys may exceptionally appear even in man, when a 
so-called "horseshoe kidney" results. 

If for any reason one of two kidneys is put out of commission, the other 
usually enlarges into a "compensating kidney," taking over the work of its 
incapacitated mate in addition to its own. 

2. Position 

The kidneys are closely associated with the dorsal wall of the body 
cavity where they lie outside of the peritoneum. 

In fishes and birds they fit with intimate snugness along either side of 
the backbone, but in amphibians, reptiles, and mammals are less closely 
attached to the body wall, sometimes projecting into the body cavity (Fig. 
361). In all of these classes the kidneys are usually retroperitoneal, behind 
the peritoneum, although in some mammals they may even hang free, 
enclosed in a peritoneal envelope which entirely surrounds them. 

„I^k Dorsal Mesogaster 

— Kidney 

if w7iir _Peritoneum 

If // i^.^L-r'^Greater Omentum 

it \ll 

//_ 4tPa — Cesser Peritoneal Cavity 


fesj-Greater Peritoneal Cavity 


Fig. 361. Diagram showing position of kidney behind the peritoneum. 
(After Huntington.) 

Symmetry of position is quite fixed in birds whose kidneys are rigidly- 
held side by side in depressions of the pelvis, but it is less apparent in mam- 
mals where, being less restricted, one kidney is usually not exactly opposite 
the other. The left kidney in man is ordinarily situated at a somewhat 
higher level than the right, although many exceptions to this arrangement 
have been observed. 

Outgo Apparatus 




Renal artery 

Renal vein 

Each human kidney weighs about four and one-half ounces, and in its 
three major dimensions measures slightly more than four by two by one 
inches. It is shaped like a "kidney bean," with a convex lateral and a 
concave medial margin. The depression of the concave margin is the hilus, 
where the renal artery and the nerves enter and the renal vein and ureter 
make their exit (Fig. 362). 

3. Gross Structure of the Mammalian Kidney 

When split lengthwise in the frontal plane the mammalian kidney is 
seen to be a hollow organ with walls of very unequal thickness on the con- 
vex and the concave sides. The excentric cavity within is known as the renal 
pelvis. This cavity is really a funnel-shaped expansion of the urinary duct, 
or ureter. The thicker walls are the solid substance of the kidney itself, made 
up of a mass of nephridia, blood vessels, 
and connective tissue, which even to 
the naked eye appears to be differen- 
tiated into a narrow, outer, rather 
uniform cortical zone, and a wider, 
inner, more diversified medullary zone, 
bordering immediately on the renal pel- 
vis. The entire structure is surrounded 
by a tightly fitting capsule of connective 
tissue, the tunica fibrosa. 

The medullary zone presents a 
number of cone-shaped segments, the 
Malpighian pyramids, the bases of 
which rest against the cortical layer while 
the apices, papillae, project into the 
cavity of the pelvis. There are seven to 
twenty pyramids in the human kidney. 
Each cup-shaped part of the pelvis 
that surrounds a papilla is called a calyx. Between the Malpighian pyramids 
are masses of blood vessels, forming the columns of Bertini, that are made 
up of subdivisions of the renal arteries and veins on their way to and from 
the renal tubules of excretion which are crowded together in the Malpighian 
pyramids. The narrow outer layer of cortex upon which the bases of the 
Malpighian pyramids rest, lies just underneath the tunica fibrosa. Due to the 
presence of the cortical rays, it has a striated appearance. 

The kidneys of many animals, for example, Echidna, marsupials, insec- 
tivores, rodents, carnivores, perissodactyls, and apes, that are unipyramidal 



Pyramid of medullary 

Fig. 362. Frontal section of human 
kidney. (From Woodruff, Animal Biol- 
ogy, copyright 1941, by permission of 
The Macmillan Company, publishers.) 

a?2 Biology of the Vertebrates 

with only a single papilla (Fig. 363), are never lobed, since the number of 
pyramids determines the number of lobes. The true multilobed condition of 
the human kidneys, which is most plainly apparent at about the fourth fetal 
month, is masked by the growth of parts which eventually fill in the inter- 
stices between the lobes. The relation of all these parts is described in the 
following section dealing with the microscopic structure of a single urinary 
unit or nephridial apparatus, and its position in the Malpighian pyramid. 

Fig. 363. Evolution of pyramids in different kidneys. At the left is the 
unipyramidal type; in the center a iobulate kidney with several pyramids; 
and at the right, a kidney composed of several pyramids but tissue filled 
in between them so that it is no longer externally Iobulate. (After 

4. A Urinary Unit 

A urinary unit is a transformed nephridium that has gained an intimate 
connection with the blood system and established an avenue of drainage to 
the outside. The various parts of such a unit are pictured in Figure 364. 

The junction where the nephridial tube makes contact with the blood 
stream is called the renal corpuscle (Fig. 365) . It consists of a spherical tuft 
or knot of arterial capillaries, the glomerulus, enveloped by a double cup of 
epithelial cells, Bowman's capsule, between the double layers of which is 
the cavity of the renal tubule, or nephridium. 

The formation of Bowman's capsule around the glomerulus at the tip of 
the renal tubule is brought about when the glomerulus comes into contact 
with the tip of the tubule and pushes it in from the outside, like the finger tip 
of an empty glove. The delicate inner cup of Bowman's capsule is thus 
closely adherent to the glomerulus so that the blood in the glomerular capil- 
laries is separated from the cavity at the end of the renal tubule only by two 
exceedingly thin cell layers, that is the wall of the inner cup of Bowman's 
capsule and the wall of the glomerular capillary itself. Thus filtration of the 
liquids to be excreted from the blood into the nephridial tube is made easily 
possible. Once through the inner wall of Bowman's capsule, the excretory 
filtrate passes down the neck into a thick-walled, kinked-up glandular por- 

Outgo Apparatus 


tion of the tube, known as the proximal convoluted tubule, whence it con- 
tinues around a non-glandular hairpin curve, Henle's loop, into a second 
thick-walled, kinked-up glandular part, the distal convoluted tubule, which 
opens in turn into a collecting tubule (Fig. 364). Eventually the collecting 

Connecting Tubule 

- — Proximal Convoluted 

Distal Convoluted 

Bowman's Capsule 
j — Interlobular Artery 

Collecting Tubule 

Ascending Limb of 
Henle's Loop 

Descending Limb of 
Henle's Loop 

Duct of Bellini 

Fig. 364. Diagram of a urinary unit. (After Piersol.) 

tubules of neighboring units join into larger common channels, called the 
ducts of Bellini, that finally open into the renal pelvis at the tips of the 
papillae. Thus the entire urinary unit from glomerulus to pelvis is a con- 
tinuous canal, the walls of which vary much in character and function. As 
many as ten to twenty-four ducts of Bellini, or papillary ducts, may empty 
into the calyx of the pelvis from a single papilla of the human kidney. It is 


Biology of the Vertebrates 

'^/f """"Bo w m a n 's 
££^ Capsule 


Fig. 365. Arrangement of capillaries 
in a glomerulus. Each branch of 
afferent arteriole continues, without 
anastomoses with the others, until all 
join to form efferent vessel. Afferent 
arteriole is larger than efferent one. 
(After Vimtrup. ) 

the pelvis, or enlarged end of the ureter, which, by way of the bladder and 

urethra, establishes a highway of communication with the outside world. 

The blood stream passes through the kidney in the following manner 

(Fig. 366). It enters at the hilus by way of the renal artery that subdivides 

in the columns of Bertini into arterioles 
and finally into arterial capillaries. The 
latter knot up into the glomeruli, which 
are entirely arterial, forming a rete mir- 
abile, rather than an arteriovenous cap- 
illary transition. The afferent twig enter- 
ing the glomerulus is larger than the 
efferent twig which makes its exit near 
by, on the same side that the afferent 
twig enters. The inequality in the size of 
these twigs tends to build a pressure 
within the glomerulus that is probably 
higher than in any other capillaries. The 
emerging efferent twig soon breaks up 
into a capillary network which entangles 
the convoluted tubules. Here in this cap- 
illary network the arterio-venous transition occurs, since at this point intimate 
contact with the glandular walls of the convoluted tubules occurs, which 
makes possible the change in the blood from arterial to venous character. 

The venules emerging from the capillary network anastomose with 
neighboring venules from other units in the columns of Bertini between 
the Malpighian pyramids, finally joining to form the renal vein that emerges 
from the kidney at the hilus. 

The cortical region presents to the naked eye a granular appearance 
because in it are embedded the glomeruli and the convoluted tubules of the 
urinary units, while the striated appearance of the Malpighian pyramids is 
due to the presence of the parallel Henle's loops and the collecting tubules 
(Fig. 364). 

It is estimated that the human kidneys may include as many as 
2,000,000 urinary units that establish continuous open channels from the 
glomeruli to the pelvis, through which some of the components of the 
onward-rushing blood stream are diverted and eventually discarded. 

The secretion of the urine probably involves two processes : ( 1 ) filtra- 
tion at the glomerulus and (2) reabsorption at the convoluted tubules. 
Water and all other non-protein constituents of the blood plasma are filtered 
into the tubule at the glomerulus. This very dilute fluid is then concentrated 

Outgo Apparatus 


at the convoluted tubules by active secretion back into the blood stream of: 
99 per cent of the water; all of such useful substances as glucose and amino 
acids; most of the inorganic ions such as sodium, chlorine, and potassium; 
little or none of the urea, uric acid, creatinin, and sulphate. It is possible 
that under certain conditions substances may be added to the urine in the 


i[=S=gl^§=s" Fibrous Capsule 
of Kidney 


7mW N Distal Convoluted 
W Tubule 

l^li>/liMo|/| bowman's Capsule 

iljTDescending LimbJHenle's 
/^Ascending Limb J Lo °P 
/ ^Collecting Tubule 

f£!$&if=* Papillary Ducts 

Fig. 366. Diagram showing the relation of urinary units to the circula- 
tory system. (After Bailey.) 

5. Urine 

The composition of urine varies enormously in different animals and at 
different times in the same animal. This is because the blood from which 
the urinary excretion is obtained by the kidneys is such a kaleidoscopic 
modifiable fluid tissue that it reflects constantly different states of metabol- 
ism within the body. Furthermore, under pathological conditions still other 
variations from the normal in the composition of the urine appear. Con- 
sequently urine analysis is an important aid to the diagnostician in finding 
out what is going on within the body. This fact was realized even by the 

436 Biology of the Vertebrates 

medical practitioners of former centuries to whom the refined chemical 
technic of modern urine analysis was not available. At least certain abnor- 
malities of urine could be discovered by simple visual inspection and this 
was very generally, and no doubt more or less oracularly, done. In the 
crude woodcuts of old medical books of the sixteenth and seventeenth cen- 
turies there recurs over and over again, like a motif in architecture or music, 
the urine motif in the form of a urine flask, which plainly tells the story of 
the importance given to the examination of samples of urine in the medical 
diagnosis of those days. 

Carnivores generally have an acid urine, while that of herbivores tends 
to be alkaline, except when they are feeding largely upon milk. It is usually 
more concentrated in animals, such as turtles and birds, which drink spar- 
ingly. In man its specific gravity varies from 1.016 to 1.020, and normally 
about a liter and a half is produced every twenty-four hours. 

Urine is characterized by nitrogenous waste products, such as urea, 
creatinin, hippuric acid, ammonia, and uric acid, although nitrogen-free 
constituents and inorganic substances, such as sodium chloride, and sulphates 
and phosphates of sodium, potassium, calcium, and magnesium are also 

In mammals, amphibians, and fishes, urea, formed from the blood in 
the liver, is the prominent nitrogenous compound present, while in reptiles 
and birds, with a minimum of water as a component, it is uric acid. 

Mitchell gives a table that accounts for about 99 per cent of the con- 
stituents in most human urines, in which 95.1 per cent is water; 2.55 per 
cent nitrogen-containing constituents; 1.26 per cent inorganic materials; 
and .052 per cent nitrogen-free substances. 


The ducts that drain the paired kidneys are usually two, although some 
fishes may have six or eight supplementary ducts when the nephridial organs 
extend into the extensive tail region. In higher vertebrates the urinary ducts 
are termed ureters. These tubes with muscular walls and a comparatively 
small bore forward the continuous products of the kidneys, not by gravity 
alone, but by peristalsis regardless of the position of the body. 

The length of the ureters depends upon the position of the kidneys 
within the body cavity. They are very long in snakes and extremely short in 
birds. In adult man they average from eleven to fourteen inches in length. 

Urinary ducts terminate at the outside in various ways. In bony fishes, 
male amphibians, and monotremes, they unite with the sexual ducts into a 

Outgo Apparatus 


common channel, or urogenital canal, opening at the genital aperture. In 
elasmobranchs, most reptiles, and birds, they debouch into the cloaca, while 
in mammals they open into a reservoir, the bladder, whence by a second 
duct, the urethra, the outside is finally reached (Fig. 367). 

Fallopian Tube\ 

Ostium Tubae %ln 

Vagina'" if™ Urethra 

Fig. 367. Urogenital apparatus of female mammal. 


The continuous excretion of liquids from the kidneys has given rise to 
the necessity for a temporary storage sac which may be emptied at suitable 
intervals. There are three general types of urinary sacs, or bladders, namely, 
tubal, cloacal, and allantoic. 

Tubal bladders, which are present in most fishes from the ganoids on, 

Urinary Duct 

V j!|L -- Ductus^ 
siu/ Deferens 

if uererens \\] 

r — Urogenital — h 
I Duct l ! 



\-f, Urogenital 



Fig. 368. Various types of urinary bladders in fishes. A, duplex type in 
Gadus; b, bicornis type in Lepidosteus; c and d, simplex type with united 
urinary ducts. (After Nuhn.) 

43 8 

Biology of the Vertebrates 

— Urinary 



are formed by the widening or enlargement of the urinary ducts. In many 
fishes two independent bladders may form, vesica duplex (Fig. 368a), one 
near the end of each urinary duct, with the two ducts afterwards uniting 
into a common passage-way of exit; or the two may run together into a 
common bilobed bladder vesica bicornis (Fig. 368b), as in Lepidosteus and 
some other ganoids; or finally, the two excretory ducts may first join and 
then expand into a single bladder, vesica simplex (Fig. 368c), as for exam- 
ple, in the pike, Esox. In all of these cases the urinary ducts enter at one 

end of the bladder, while the exit is made at 
the opposite pole. The tubal bladder, which is 
frequently larger in the female than in the 
male, is most common in certain teleost fishes 
like the Pleuronectidae that have no swim 
bladder. It is somewhat difficult to account for 
the presence in fishes of these structures that 
are rarely absent yet of doubtful utility. 

The cloacal bladder occurs in dipnoans, 
amphibians (Fig. 369), and monotremes. It 
is a diverticulum of the cloacal wall opposite 
the point where the urinary ducts, with which 
it has no direct connection, enter. It is located 
dorsally in lungfishes and ventrally in amphi- 
bians. In the perennibranchiate amphibians it 
is considerably elongated, but rounded and 
broadened in frogs and toads. Frequently it is bilobed while in some urodeles, 
for example Salamandra, Triton, and Eurycea, the lobes are prolonged into 
hornlike processes. 

Cloacal bladders are filled by the closure of the outer cloacal sphincter 
and the backing up of the urinary secretion into them. 

The allantoic bladder, according to most embryologists, arises from the 
enlargement of the proximal or basal end of the embryonic allantoic stalk. 
It is characteristic of mammals and of such reptiles as turtles and certain 
lizards that have a bladder. In the case of other amniotes, like snakes, croco- 
diles, some lizards, and birds, the whole allantois degenerates without devel- 
oping a bladder. In mammals that part of the allantoic stalk left within the 
body wall, when the umbilical cord connected with the placenta is severed 
at birth, enlarges to form the bladder, and also the urachus, or vesico- 
umbilical ligament which anchors the bladder to the inner body wall at the 
umbilicus. Thus the proximal end of the allantois stalk enlarges into the 
hollow sac of the bladder while the distal part within the body wall under- 

J v° 

Fig. 369. Ventral view of 
cloacal bladder of an amphi- 

Outgo Apparatus 


goes quite a different fate in being transformed into a solid ligament utilized 
as a guy rope to support the bladder. 

The occurrence of an occasional urachal cyst in man, with urine escaping 
through the umbilicus by a fistula, apparently demonstrates the embryonic 
derivation of the urachus and bladder from the common origin of the 
allantoic stalk. 

Arey * says, however, "Contrary to earlier views, the allantois con- 
tributes nothing to the bladder or urachus." According to his view the 
mammalian bladder is a derivative of the embryonic cloaca. Future studies 
on this question will be welcome. 

The mammalian bladder is lined with mucous membrane and coated on 
the outside with peritoneum. It has a highly muscular wall abundantly 
supplied with nerves and blood vessels, the involuntary muscle fibers being 
diverted from their originally regular, longitudinal 
and circular arrangement so that they interweave 
like felt in many directions. Upon contraction the 
cavity of the bladder becomes smaller, therefore, in 
all dimensions like a leaking toy balloon, rather than 
collapsing like a hot-water bag from which the water 
has been emptied. 

The exit from the bladder is by way of a single 
duct, the urethra, the entrance to which is kept 
closed by a muscular sphincter, except periodically, 
during micturition, when, upon the relaxation of the 
sphincter, the urine is expelled by the contraction of 
the muscular walls of the bladder. 

There is considerable variation in the location 
of the points where the ureters enter the bladder. 
Only rarely, as in Lepus and Hyrax, do they come 
in at the opposite pole from the urethral exit as is 
the case among fishes. In most mammals they enter 
low down near the urethra by an oblique pas- 
sage through the wall of the bladder (Fig. 370). This arrangement 
makes the backflow of urine into the ureters difficult, particularly when the 
bladder is full, because the pressure from distention tends to close the bore 
of the ureters. 

Both allantoic and cloacal bladders may be attached to the ventral body 
wall by a remnant of the ventral mesentery, the ventral ligament of the 
bladder, continuous with the visceral peritoneum covering this organ. 

* Developmental Anatomy, W. B. Saunders Co., pp. 147-8. 

Fig. 370. Diagram of 
the diagonal passage of 
the ureter through the 
wall of the bladder. As 
the bladder becomes dis- 
tended the pressure in 
the direction of the ar- 
row tends to close the 

44° Biology of the Vertebrates 


1. In general 

Although all kidneys are fundamentally nephridial tubes that extract 
liquid waste from the blood, the kidneys of different animals are by no 
means homologous structures. 

Among vertebrates there are three kinds of nephridial structures serv- 
ing as kidneys that differ from each other not only in structure and position 
in relation to the blood system and the excretory tubes, but also in their 
embryonic history. 

The three kinds of kidneys, as named by the English embryologist, 
Balfour, are the pronephros of a few cyclostomes; the mesonephros of fishes 
and amphibians ; and the metanephros of reptiles, birds, and mammals. 

The higher vertebrates, whose kidneys are of the metanephric type, pass 
through preliminary pronephric and mesonephric stages before the per- 
manent metanephric stage is 
^ ^mm m-vm-vvm^m }-n»t Wnll reached. As is frequently the case, 

~~jf v,^_„_^„___-^ Blood Vessels comparative anatomy and embry- 

f|^7f3ig|si^|^j| Glandular Portion ology ' have supplementary and 

ii/7^^ Nephrostome confirmatory stories to tell from 
til f i 

sfejll/ loeiom different angles about the same 

S^^sajsS^K^} Body Wall ,. & 

i — ^== fc ^r — thing. 

e P um ep n lo " The nearest approach among 

Fig. 371. Diagram of a nephridium of an invertebrates to the vertebrate ne- 
annelid worm. 1 he anterior end ot the 

animal is toward the left. phridial apparatus is found in the 

nephridia of many annelid worms 
(Fig. 371), which, however, are not connected together by common excre- 
tory ducts to form excretory organs, like the nephridia of all vertebrates, but 
rather consist of metamerically arranged pairs of independent tubes. 

2. The Nephridial Apparatus of Amphioxus 

The nephridial apparatus of amphioxus is very much like that of certain 
marine annelids (Fig. 372), but instead of extending practically the entire 
length of the body with a pair of protonephridia in every segment, as in the 
annelid worms, the primitive protonephridia of amphioxus are localized in 
the anterior part of the body throughout the region of the gill slits. They 
lie somewhat above the pharynx near the dorsal region and may equal the 
gill slits in number. Each nephridium is open externally, emptying inde- 
pendently into the peribranchial chamber (atrial cavity) surrounding the 

Outgo Apparatus 


gills, and terminating internally in flame cells of the flagellate type. There 
are present no common excretory ducts for carrying away the excretion from 
the nephridia of amphioxus, but the peribranchial chamber, with its atrial 
opening, accomplishes the same purpose while serving at the same time as 
the avenue of escape for the water of respiration. 

A B 

Fig. 372. Excretory tubules of amphioxus. A, a single protonephridium 
showing several -processes, which lie in the coelom, and the single open- 
ing, which empties into the atrial cavity; b, one nephridial process en- 
larged to show the group of flame-cell-like solenocytes, each with a collar 
and flagellum. (From Newman, The Phylum Chordata, copyright 1939, 
by permission of The Macmillan Company, publishers. Modified from 
Boveri and Goodrich.) 

Thus the apparatus for urinary excretion in amphioxus is primarily 
concerned with coelomic drainage rather than with direct extraction of 
urinary waste from the blood, and it consists not of a single pair of organs, 
or nephroi, with their ducts, as in all true vertebrates, but of a series of 
independent paired excretory tubules of the protonephridial type, resem- 
bling those of some annelid worms. 

3. Pronephros 

The pronephric tubules, prone phridia, are few in number and metameri- 
cally arranged in the anterior part of the trunk region. Originating as 
evaginations of the coelomic epithelium along the lateral portions of the 
mesomeres, or nephrotomies, each retains a ciliated mouth, or nephrostome, 
which opens into the nephrocoele, the coelomic cavity of this region (Fig. 
373) . The outer extremities of the pronephridia, ending blindly at first, soon 
turn posteriorly, growing until they come in contact with one another and 

44 2 

Biology of the Vertebrates 

Myo to m e — ~2*?t 7 y 

Nephrostome — iy^"^- 
Segmental Duct — T/o"" 
Nephrocoele — ~)f~~~ 

Coelom — eK'St 

, Nerve Cord 


" — Dorsal Aorta 
^"Cardinal Vein 

Fig. 373. Cross section diagram showing pronephridial stage of the 
excretory system. Successive glomeruli frequently fuse into a single, 
elongate capillary-mass, known as a glomus (See Fig. 374). 

join together, down each side of the body, to form a common segmental, or 
pronephric, duct. This duct is eventually extended posteriorly, mainly by 
additions from the coelomic wall, to open into the cloaca. 

On the opposite side of the nephrocoele from the nephrostomes a capil- 
lary ridge, the glomus, forms along the coelomic wall 
(Figs 373 and 374), so that there are two methods of 
obtaining excretory products from the blood. They can 
pass first by diffusion into the general body cavity and 
then into the nephrocoeles or they can go directly from 
the capillaries of the glomus into the nephrocoele. In 
either instance the liquid excretory material is passed on 
through the ciliated nephrostomes and pronephridia to 
the segmental ducts, which dispose of the waste to the 

No encapsuling connective tissue, like the tunica fib- 
rosa of the human kidney, surrounds and unifies the 
pronephridia into a definite organ. 

The pronephroi are best developed in cyclostomes, 
where in some species they persist throughout life, al- 
though replaced functionally in most cases by mesone- 
phroi or kidneys of the second order. 
It is probable that in some myxinoids, Polistotrema or Bdellostoma, for 
example, they remain as the lifelong functional kidneys. They also persist 

Fig. 374 





of the 



aorta; c, coelom; 
d, pronephric duct; 
g, glomus; 1, 2, 3, 
pronephridia. (Af- 
ter Kingsley; Se- 

Outgo Apparatus aa? 

structurally in some telcosts. In other vertebrates, particularly types like 
elasmobranchs and amphibians that have a larval development, they put in 
a temporary embryonic appearance and later vanish. 

In the shark Pristiurus there are four pairs of pronephric tubules; in the 
elasmobranch Torpedo, six ; while in the legless amphibian Caecilia, ten 
pairs of pronephridia hold the stage for a time during early development. 
Transient traces of pronephridia in mammals have been described, one or 
more pairs even having been identified in early human embryos where their 
maximum growth is attained in embryos of about 3.5 mm. in length. 

Although the downfall of the pronephridia seems to be universal, with 
the possible exception of certain cyclostomes already mentioned, the seg- 
mental ducts are more persistent and, as will be seen later, are retained to 
play an important part in the succeeding dynasty of the mesonephros, 
which reaches its maximum in human embryos of about 10 mm. in length. 

4. Mesonephros 

The second type of kidney in the vertebrate succession is the meso- 
nephros, sometimes known as the Wolffian body (Fig. 396). Like other 
kidneys this structure is made up of nephridial tubules, mesonephridia in 
this instance, that develop in the embryo from nephrotomes posterior to 
those which form pronephridia and at a later time. They are distinct from 
pronephridia as shown by their relation to the excretory ducts and by the 
fact that both pronephridia and mesonephridia may be present at the same 

Mesonephridia, which are much more numerous than pronephridia, do 
not generally show the primitive metameric arrangement. The most anterior 
mesonephridia are the oldest, and subsequent additions arise posteriorly. 
They originate independently and connect secondarily with the paired seg- 
mental ducts that hold over from the former regime (Fig. 375). 

Each mesonephridium primarily forks at its inner end. One branch 
terminates with a nephrostome opening into the coelome, while the other 
ends in a Bowman's capsule associated with an independent glomerulus 
from the blood system (Fig. 376). Additional capillaries develop about the 
tube proper somewhat in the same manner as in the case of the mammalian 
urinary unit previously described. Thus there are established two avenues 
for excretory collection, namely, the ciliated nephrostome for drawing 
whatever fluid collects in the body cavity, and the renal corpuscle and 
tubule proper for direct abstraction from the blood. 

Although the nephrostomes of some of the more anterior mesonephridia 
remain permanently open, particularly in the elasmobranchs and ganoids. 


Biology of the Vertebrates 

Anlage of 

Mesonephric — 



*■■-— Mesonephric - PiL"~ 




Bowman's Capsule' 

C D 

Fig. 375. Diagrams showing the differentiation of a mesophric tubule 
(mesonephridium). (After Arey and Felrx-Prentiss.) 

while in cyclostomes they are retained throughout the entire length of the 
mesonephros, they are for the most part obliterated. 

When the nephrostomes all become closed, as in amphibians and 
amniotes generally, the body cavity virtually becomes a closed sinus so far 
as the blood system is concerned, and the peritoneal fluid, with whatever 
excretory products may be present in it, can escape only like other lymphatic 
fluids through the blood channels. 

The mesonephros functions as the kidney throughout life not only in 

Bowman's Capsule 

Mesonephridium ^^^gf 

T^'F "Nerve Cord 

~m,: — Notochord 

Dorsal Aorta 
Cardinal Vein 


Fig. 376. Cross section diagram showing early mesonephridial stage of 
the excretory system. 

Peritonea I Funnel — — •r-'vr t--ii3^ - 

Outgo Apparatus -M$ 

cyclostomes, with the exception of the hagfishes already mentioned that 
retain a pronephros, but also in fishes and amphibians. It also serves 
temporarily as the kidney for practically all other vertebrates until it is 
superseded in turn by the metanephros. 

In reptiles, as well as in Echidna and Didelphys among mammals, the 
mesonephros endures until some time after birth, and in the case of the 
lizard Lacerta, even until after the first hibernation. 

In most mammals the functional mesonephros is confined to the embry- 
onic period, although occasionally, as in the guinea pig and mouse, degen- 
erating so early that it probably never functions as a kidney at all. 

In man the mesonephros, or Wolffian body, reaches its high-water mark 
during the second month of fetal life, when it appears as a slightly project- 
ing ridge on either side along the dorsal part of the coelom from the 
posterior cervical region to the pelvis, where the two ridges fuse into one. 

The drainage ducts of the mesonephroi are the persisting segmental 
ducts of the vanished pronephroi, known in their new role as Wolffian ducts. 
Thus, if the whole mesonephros, whose secretion is delivered through a duct, 
is compared to a gland, which it certainly resembles in a superficial way, 
there is one striking difference to be noted between it and an ordinary gland, 
namely, the duct is formed before the secreting part is developed and inde- 
pendently of it. 

When in reptiles, birds, and mammals the mesonephros degenerates and 
gives way to the metanephros, many of its component parts, in particular 
the ducts of excretion, are salvaged and utilized for other purposes in con- 
nection with the reproductive system. 

The whole mesonephros, a much more compact and unified organ than 
its pronephric predecessor, is retroperitoneal, usually fitting snugly against 
the dorsal body wall, although in amphibia it projects somewhat into the 
abdominal cavity. 

5. Metanephros 

The third and last type of vertebrate kidney, which replaces the meso- 
nephros in reptiles, birds, and mammals, is the metanephros. 

Its nephridial tubules, the metanephridia, already described as "urinary- 
units, " take their origin in the nephrogenic tissue surrounding the posterior 
part of the Wolffian ducts in the sacral region of the body, ventral to the 
sacrum and dorsal to the cloaca when this is present. 

The metanephridia are very numerous, elongated, and highly modified 
histologically, as already indicated. They are not arranged metamerically at 
any stage, and are massed together inside of the tunica fibrosa so that they 

446 Biology of the Vertebrates 

form organs, particularly in mammals, more independent of the coelomic 
wall than either pronephroi or mesonephroi. 

In position the metanephridia are posterior to the last mesonephridia, 
developing at a later time. A nephrostome is never present, and conse- 
quently at no stage do the metanephridia communicate with the coelom. 
As a result of the absence of the nephrostome branch of the nephridial tube, 
the connection with the blood system, that is the renal corpuscle, appears 
to be at the blind end of the tubule and not on a branch like one arm of 
a Y, as in the mesonephridium. 

Wolffian Duct---_^=^glSa^fe^^f||=|P^|^. — —Intestine 

r'^^^W^ ,! |«Fll" Bud ° ( Ure ' er 

Allantoic Duct''"^^^^^\ 


vuMWJm- Neural Tube 

Urinary Vesicle S^J^MM^L— -Post-Anal Gut 

Fig. 377. The cloaca of a human embryo of about twenty-six days, 
showing beginnings of ureter. (After model by Keibel.) 

The true ureters, which sprout out from the bases of the Wolffian ducts 
(Fig. 377 ) , are not made over from something that has gone before, but are 
an entirely new pair of excretory ducts. The free end of each ureter enlarges 
and pushes into the metanephric tissue to form the renal pelvis and main 
collecting tubules of the metanephric kidney. Here again the duct is formed 
independent of the nephridia but in this case the duct grows to the nephridia 
instead of the reverse situation which is found in the development of the 
mesonephros. Meanwhile the Wolffian ducts that have already served more 
than one master, being shorn of their former excretory mission, are turned 
over to the reproductive system to take on still another function as sperm 
ducts in the male, while in the female they degenerate and pass out of the 

In man the metanephros, or permanent kidney, assumes dominance dur- 
ing the third month of fetal life. The remains of the mesonephros, as well as 
the mesonephric ducts, become accessory to the reproductive apparatus 

In the chapter on the Reproductive System more complete consideration 
will be given to the fates of all of these ducts and tubules which, although 
at first urinary in function, become in many cases associated with the genital 
portions of what is commonly known as the Urogenital System. 


The Preservation of Species- 
Reproductive System 


The individual is the triumphant outcome of the age-long interaction of 
all the mighty and intricate forces of evolution. 

The various mechanisms of metabolism thus far considered, including 
the integumentary, digestive, circulatory, respiratory, and excretory systems, 
as well as the mechanisms of motion and sensation which are to be described 
in Part III, all contribute directly to the development and maintenance of 
the individual. There is indeed only one other concern in nature of greater 
moment than the up-keep and well-being of the individual and that is the 
continuation on the face of the earth of those precious products of evolution- 
ary travail which have "won a place in the sun." Life must go on. Such is 
nature's ultimatum, although no single individual, even of the genus Homo, 
is of so supreme importance that it cannot be spared. Individuals die only 
to be replaced by others, and in the long run this is a fortunate provision, 
since it is the fate of every organism, like any other delicate piece of machin- 
ery, to wear out and become useless eventually in the natural course of 

To provide for the life of the species by replacement is the part of 

Filling the gaps caused by the death of individuals in the rank and file 
of organisms may be regarded as a matter of extra growth beyond individual 
requirements at the expense of non-living materials. The surplus thus gained 
may become detached from the original organism to form a new and inde- 
pendent individual. Excess growth material endowed with the capacity to 
reconstruct a new individual is termed germ plasm, while the body from 
which it is detached is composed of somatoplasm. 

Organisms generally, which are made up of these two kinds of materials, 


448 Biology of the Vertebrates 

consequently lead a dual life. The mortal somatoplasm is inevitably 
headed towards eventual death, while the potentially immortal germplasm 
has opened up before it the possibility of escape from death through rein- 
carnation in a new individual life. 

The germplasm thus forms a continuum which joins generations 
together. Any particular organism represents not something new, like a 
manufactured article, but the ultimate link in an unbroken chain extending 
back into the evolutionary past farther than the imagination can follow it. 
In this way the torch of life is not extinguished but is handed on. 

Maintenance of the species, as contrasted with maintenance of the indi- 
vidual, is an unselfish altruistic function, frequently accomplished at the 
expense of individual comfort, or even at the sacrifice of the individual life. 

The effective operation of the function of reproduction on the part of 
animals lacking the ability to reason and uncontrolled by altruistic motives 
is insured by being grounded firmly in fundamental urges and universal 
desires which carry the reward of selfish satisfaction while at the same time 
accomplishing the altruistic end of providing for others. 

That flower of creation, moral man, in spite of the fact that he is by 
no means entirely emancipated from the effective laws of the jungle, has in 
comparatively recent evolutionary times set up certain ethical rules to govern 
the operation of the indispensable function of reproduction that are some- 
what at variance with a life of selfishness. Moral responsibility does not 
worry animals. 

It is perhaps biologically fortunate that man, although subjected to an 
overlay of social restraints, is still bombarded by the same universal com- 
pelling physiological urges with their rewards of selfish gratification which 
serve to safeguard and insure the altruistic and sacrificial ends that result in 
the perpetuation of the species. 


Sex, in spite of the popular impression to the contrary, is not essential to 
reproduction. Many organisms reproduce their kind asexually by processes 
of unequal fragmentation, such as budding and spore formation, or by 
more or less equal division into two, as in the fission of many protozoans. 

The first event in the life of any vertebrate, however, is the union of two 
highly endowed cells, called gametes, furnished by two different individuals, 
male and female. The undifferentiated cell thus formed is a "fertilized egg," 
or zygote, than which no other cell has so great expectations. 

This is sexual reproduction and in describing the mechanism involved 

The Preservation of Species 


one should first of all distinguish the essentially "immortal germplasm" 
(Weismann) that bridges the generations, from the accessory structures 
which minister thereto, but are destined to perish with the life of the indi- 
vidual of which they form a part. 


Germplasm, the essential material concerned in reproduction, consists of 
sperm cells and eggs in sexual animals. 

1. Sperm Cells 

The detachable germinal units de- 
rived from the male individual are 
sperm cells. They are frequently called 
spermatozoa (sperma, seed; zobn, ani- 
mal), an awkward and misleading 
word, embalming the historical fact 
that when these cells were first discov- 
ered by early microscopists, they were 
thought to be tiny independent parasitic 

A sperm cell in order to produce a 
new individual must always join forces 
with an egg cell. It can never become 
an individual unassisted. Such union, 
however, is not indispensable in the 
case of the egg cell, which among cer- 
tain invertebrates may develop parthen- 
ogenetically, that is, without the assist- 
ance of a male sperm cell. 

The sperm cells of different species 
have a chemical specificity for the eggs 
of their own kind, and animals do not 
bastardize under ordinary conditions 
even though their germ cells may have 
free access to each other, as in sea-water 
for example, where a variety of different kinds of eggs and sperm are pres- 
ent. If this were not true, untold confusion would result. 

The motility which enables the active sperm to seek out the compara- 
tively stationary egg is accomplished among vertebrates by the development 

C E T 

Fig. 378. Diagrams of the develop- 
ment of spermatozoa, a.c, anterior 
centrosome; a.f. axial filament; c.p., 
connecting pieces; ch.p., chief piece; 
g.c, galea capitis; n., nucleus; nk., 
neck; p., protoplasm; p.c, posterior 
centrosome. (From Bremer and Wea- 
therford, Lewis and Stohr's Histology, 
copyright 1944, by permission of P. 
Blakiston's Son and Company, pub- 
lishers. After Meves.) 

45° Biology of the Vertebrates 

of a vibratile "tail," that sculls the cell body of the sperm forward through 
a liquid medium. Certain invertebrates, however, such as some of the nema- 
tode worms, arachnids, mites, myriapods, and many crabs, have amoeboid 
sperm cells which creep to their destination in contact with a substrate 
instead of swimming freely through an intervening fluid. Fish sperm do not 
take on an independent motility until they are expelled into the water. 

The details of structure of a typical sperm, together with an indication 
of the sequence of events by which it attains a highly specialized locomotor 
form from its generalized embryonic shape, are shown in Figure 378. Cock- 
erell has happily described a mature sperm cell as stripped "like a Roman 
soldier without impedimenta." Its entire cytoplasm, indeed, is sacrificed to 
forming a structure devoted to carrying forward the "head" of the sperm, 
which is practically only the nucleus containing the chromosomal bearers 
of heredity. 

The numbers of sperm cells produced by male animals of different 
species is greatly in excess of the number of eggs furnished by corresponding 
females. It has been estimated by Shipley that in man, for instance, the total 
number of sperm cells produced during the sexual life may be 340 billion, 
while the eggs that come to maturity during the lifetime of a human female 
will hardly exceed 400. This makes the ratio of possible sperm to eggs in 
humankind something like 850,000,000 to 1. 

There is a corresponding discrepancy in size between the male and 
female germ cells, yet it is a demonstrated fact that the egg and sperm are 
essentially equal partners with respect to hereditary chromosomal deter- 
miners which each sex contributes to the mutual enterprise of a new 

The viability of sperm cells after detachment from the male likewise 
shows great variation. Haempel gives the duration of independent life in 
water of the sperm of certain fishes, as follows : 

Trutta fario 23 seconds 

Trutta viridea 40 

Trutta salar 45 " 

Salmo hucho 45 " 

Barbus nuviatilis 120 

Esox lucius 3^4- minutes 

Cyprinus carpio 5 " 

According to Lewis, human sperm may retain activity for three days 
after the death of the male, and if deposited in the female genital tract, for 
a week or more. 

The sperm of some bats remain alive and efficient from the time of 

The Preservation of Species 4S 1 

pairing in autumn until the following spring, when the eggs are ready for 
fertilization. Among invertebrates may be cited the remarkable case of the 
honey bee, where the sperm from the drone may live in the body of the 
queen for over a year. 

2. Eggs 

The ova, or germplasmal cells of the female, are less independent of the 
individual which produces them than the sperm cells of the male. In many 
instances they tarry within the protective body for a considerable time after 
attaining potential independence and may even undergo extensive develop- 
ment into a new organism, as for instance in mammals, before forsaking 
the maternal body in which they originated. 

The fact that an egg is "fertilized" by sperm and not vice versa has 
entailed the necessity for providing various additions to the egg cell itself in 
the form of stored nutrition and protective envelopes for the forthcoming 
individual that are entirely unnecessary in sperm cells. The 
chief emergency ration stored in the egg is yolk, which repre- 
sents one of the earliest instances of "preparedness" on record. 
The quantity of such food in the eggs of different species of 
animals varies all the way from a meager fat droplet in certain 
protoplasmic eggs to the relatively enormous supply making 
up the familiar yellow sphere in a hen's egg. The large size of 
the latter is due to the generous supply of stored food material 
and not to the amount of living cytoplasm which is little differ- 
ent from that of much smaller eggs. 

In addition to yolk, the eggs of turtles, crocodiles, and birds 
have a supply of albumen, or "white," wrapped around the 
yolk that adds variety to the embryonic bill of fare. 

The eggs of amphibians also are covered with a glairy al- 
buminous coating, which has the property of swelling up into 
a thick protective jelly-like envelope upon exposure to water 
(Fig. 379) . This explains why the total quantity of eggs which 
a submerged frog or toad lays all at once, enlarges and floats to the surface, 
forming a mass considerably greater than that of the entire body whence it 
came. The eggs of toads are strung together like pearls, while those of frogs 
and salamanders are in gelatinous clumps. 

Eggs that are not shed directly into water, or do not undergo preliminary 
development into embryos within the sheltering body of the female, are pro- 
vided with some sort of a protective shell. This may be leathery or of a 
texture like parchment, as in many reptiles, but it is usually calcified, being 

Fig. 379. A 
bunch of 
frog's eggs 
attached to 
a stick. 
(After G. H. 


Biology of the Vertebrates 

perforated by innumerable tiny air holes through which respiration takes 
place. The calcareous-shelled egg of the warm-blooded bird differs from 
that of the cold-blooded reptile by having: ( 1 ) a heavier firmer shell; (2) 
an air chamber at one end within the shell (Fig. 380) ; and (3) a greater 
amount of supplementary albumen, a part of which develops into the 
chalaza, that anchors the yolk at either pole like a twisted guy rope, pre- 
venting undue mechanical disturbance. 


Less Dense Albumen 
Dense Albumen 

Blastoderm — y/7 t.- 

Yellow Yolk- 
White Yolk 

~I3^ :: ~ 

j^ — —Nucleus of Pander 
-^i- Neck of Latebra 
spinner Shell Membrane 
NT "Air Space 

— Outer Shell Membrane 
"""-Vitelline Membrane 

Fig. 380. Egg of a bird (After Schimkewitsch.] 

The chalaza also allows the yolk, bearing its precious protoplasmic disc, 
to rotate within the shell so that the disc is always on top and not pressed 
against the shell, regardless of the position of the egg as a whole. All of these 
additional modifications of the avian female germ cell are devices called 
forth by the necessity of egg-laying on land and subsequent incubation. 

After an egg has taken in a sperm and is "fertilized," it requires a period 
of enhanced temperature in order to begin development into an embryo. 
This is the period of incubation. Fishes usually resort to warmer waters to 
spawn and turtles deposit their eggs in sand where the heat of the sun 
has access to them. Sea turtles, whose young are liable to greater hazards 
than land turtles, come ashore and abandon on the friendly doorstep of 
sand and sun as many as 150 or 200 eggs, while alligators pile up a swampy 
nest of rotting vegetation in which to leave their eggs, the fermenting mass 
engendering the amount of added heat requisite for bringing the eggs to 
the hatching point. Frogs' eggs, deposited in the shallow water along the 
margins of ponds in spring, receive the sun's rays through their transparent 
spherical jelly-like envelopes which act like a lens in focusing the heat, thus 
providing the necessary increase in temperature. Birds build incubators in 
the form of nests where the temperature of their eggs is raised by means of 
contact with the warm bodies of the parents. 

In shape the eggs of vertebrates are typically spherical, particularly those 

The Preservation of Species 


of fishes and amphibians that are deposited in water, as well as those of 
mammals which do not require a shell because they are not exposed. The 
eggs of reptiles are usually elongated and elliptical, while those of birds are 
prevailingly oval with one end more pointed than the other so that they 
pack economically within the confines of a nest. Most sea birds build shallow 
nests, depositing their eggs either in perilous crevices or in flat exposed 
situations, consequently their eggs taper so much that they do not roll away 
when disturbed, but simply pivot about in a circle, remaining safely in the 

As already pointed out, the egg by reason of the presence of yolk and 
albumen exceeds the sperm many times in size. The human egg (Fig. 110) 
is very small, but although only about %25 of an inch in diameter, it is 
nevertheless 50,000 times larger in volume than a single sperm, which 
measures scarcely 5 micra in diameter. 

The eggs of marine fishes are usually smaller and more numerous than 
those of fresh-water fishes, although elasmobranchs form a notable excep- 
tion, as they have the largest eggs not only of any 
fishes, but also probably of any animal. Braus re- 
ports a specimen of the shark Hexanchus griseus 
that measured 4.2 meters in length and weighed 
400 kilograms, from which he took out of a single 
oviduct 53 eggs of approximately the same size, 
each measuring 9 by 1 1 centimeters and weighing 
about 500 grams. The eggs of certain Japanese 
carcharid sharks are known to attain the size of 
14 by 22 centimeters, dimensions considerably 
exceeding the average of 12 by 15 centimeters 
common to ostrich eggs. 

The number of eggs produced bears a direct 
relation to the chances for attaining maturity. 
Elasmobranch fishes, the young of which are 
born alive and well advanced at birth towards a 
stage when they can fend for themselves, produce 
only a few eggs (Fig. 381). Prevost, for instance, 
gives four to fourteen ova as the seasonal output 
of the elasmobranch Torpedo marmorata. The 
stickleback, Gasterosteus, which makes a nest that 

is guarded by the male, lays less than a hundred eggs. On the other hand 
the codfish, Gadus, whose unprotected eggs are exposed to the countless 
perils of the open ocean, broadcasts several million eggs during a single 

Fig. 381. Egg case of 
Scyllium, a dogfish, cut 
open to show the embryo 
surrounding the yolk with- 
in. The egg case is pro- 
longed into entangling 
tendrils. (After Home.) 

454 Biology of the Vertebrates 

breeding season. This enormous output in turn is numerically low when 
compared to that of certain termites among the insects whose queen lays 
eggs continuously at the rate of one per second for a year at a stretch, 
making a total of some 30 millions from a single female. 

Ascending the vertebrate scale from fishes through amphibians (Fig. 
28), reptiles, and birds to mammals, there is an increasing provision for 
parental care with a corresponding falling off in the number of eggs pro- 
duced that suggests the mathematical computations and conclusions of an 
expert life insurance actuary. 

When eggs are supplied with a large amount of yolk, the embryos have 
the opportunity to reach a more advanced stage of development before 
hatching, but in the case of poorly provisioned eggs they must embark on 
their worldly adventures with less preparation. Mammals, whose eggs are 
practically devoid of stored nourishment, attain the advantages of advanced 
preliminary development by the device of gestation. 


As already indicated, eggs and sperm are the essential reproductive cells. 
They preserve and carry forward the architecture and traditions of each 
species, but in doing this they require a place of abode, devices for 
bringing the germ cells together in effective union, and provision for the 
safe development of the fertilized egg into a new individual. These aids to 
the germplasm are a part of the somatoplasm (Weismann), or the non- 
germinal body of the individual. From the viewpoint of the species the 
body, or somatoplasm, may be regarded simply as a mortal vehicle for 
temporarily maintaining and transmitting the immortal germplasm. 

The parts of the body that are in the direct service of the germ cells 
are ( 1 ) gonads, or organs that house eggs and sperm ; ( 2 ) ducts, or passage- 
ways that transmit the germ cells ; ( 3 ) apparatus of various sorts for facili- 
tating the union of the sex cells; (4) accessory glands; (5) devices for the 
care of the eggs before and after fertilization; (6) certain rudimentary 
organs of doubtful utility; and (7) modifications of the body that differen- 
tiate male from female, and which may possibly influence sexual be- 

1. Gonads 

The gonads are paired masses of mesodermal tissue (Fig. 382) that 
develop on either side of the mesentery in the antero-lateral part of the 
vertebrate body cavity. They become invaded by potential germ cells which, 

The Preservation of Species 


Dorsal Aorta 

Wolffian Duct 

Mullerian Duct 

Genital Ridge 

Body Wall-I 
Nephrostome— f. ...^ 

_,£- j-Glomerulus 
h"]~ Wolffian Duct 
<"~Mullerian Duct 
Genital Ridge 
-M— j-Coelom 

—■—Dorsal Mesentery 

Fig. 382. Diagrammatic cross section across the Wolffian and genital 
ridges, to show the origin and relations of the Mullerian ducts and 
tubules of the Wolffian body. (After Pasteau.) 

like all other cells, are the direct lineal descendants by mitosis of the original 
fertilized egg from which the organism arose (Fig. 383). They differ from 
other cells of the body in retaining their undifferentiated condition for a 
longer time. 

Gonads that harbor future sperm cells are called testes, and those in 
which egg cells are embedded, ovaries. According to an almost universal rule 
among vertebrates, the testes and 
ovaries develop in different indi- 
viduals, that is, the sexes are distinct 
from each other. 

Usually gonads are massive struc- 
tures that do not show metamerism, 
but in amphioxus they are arranged 
metamerically along the gill region 
in as many as twenty-six pairs. The 
largest pairs are in the middle with 
others decreasing in size at either end. 
Although superficially alike, the sexes 
are distinct. 

In most vertebrates, however, there 

Fig. 383. Separation of somatic and 
germplasmal cells in five stages in the 

is not more than a single pair of develo P in S embl 7° of the nematode 

. worm, Ascaris. The darkened cells rep- 

gonads, and consequently metamerism 

or segmentation, so characteristic of 
many organs, disappears. In a few 
vertebrates only a single gonad is pres- 
ent, either as the result of the fusion 
of a pair as in the lamprey eel, or of unilateral degeneration, as in the hag- 
fish, and the single left ovary of most birds, while frequently one of a pair 
of gonads will be smaller than its mate. 

resent the germplasm, becoming in e, 
the ancestral cells from which the eggs 
or sperm arise. All other cells by re- 
peated division become the body of the 
worm. (After Meisenheimer.) 

456 Biology of the Vertebrates 

The germ cells, the distinctive features of gonads, arise from embryonic 
endoderm and migrate into the genital ridges. 

The shape of gonads is in general influenced by the body form of the