]B3B^BQ^^^^^^^^^E3E I Marine Biological Laboratory Library I ffl Woods Hole, Mass. H HI [0 (D ^---^ [0 li Presented by ]] H^ The estate of Dr. Herbert W, | ^^ Rand UJ I Jan, 9, 1964 B ID ID D) Dl 0] ra 83 B CHORDATE ANATOMY ^A(ecii AND 1^/n/ CHORDATE ANATOMY BY HERBERT V. NEAL PROFESSOR OF ZOOLOGY, TUFTS COI.LFX.E AND HERBERT W. RAND ASSOCIATE PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY WITH 378 ILLUSTRATIONS LIBRARY ^^\ MASS. XX^j PHILADELPHIA P. BLAKISTON'S SON & CO., Inc. 1012 WALNUT STREET Copyright, 1939, by P. Blakiston's Son & Co., Inc. PRINTED IN U. S. A. BY THE MAPLE PRESS COMPANY, YORK, PA. PREFACE Comparative anatomy has both practical and theoretical interest. Courses in it based, like other science courses, upon laboratory work give the student first-hand acquaintance with the structure of fishes, amphi- bians, reptiles, birds and mammals. If, as psychologists assert, all our ideas derive from sense impressions, all real knowledge of animal structure must be based upon such laboratory experience. "Starve the senses and you starve the soul." In the laboratory, as the student increases his acquaintance with animals, he gains also in resourcefulness and inde- pendence. Many biologists believe that comparative anatomy affords the best approach to the understanding of human structure and function. In many American colleges and universities the course in comparative anatomy is a prerequisite to advanced courses in histology, embryology and physiology. In this way the student passes in his analysis from the general to the particular, from the gross to the microscopic. To many persons, however, the theoretical interest of comparative anatomy makes stronger appeal than does practical familiarity with animals. The major problem which faces the student of comparative anatomy is that of the genesis of the human body. The clue which gives meaning to many of the details of anatomy is found in the evolution theory. Most teachers of biology are so convinced of the truth of that theory that the issue is no longer debated by them. Each generation, however, must examine for itself the evidence which has led to the general acceptance of evolution by experts. While the record of the rocks is probably the most convincing evidence of evolution, the facts of com- parative anatomy greatly strengthen the case for evolution. As a result of the researches of several generations of comparative anatomists, it is now possible to sketch in fairly firm outlines the hypothetical past history of the human body. One of the purposes of this book is to summarize some of this evidence. To meet the laboratory needs of students, a number of excellent labora- tory guides have been written, some dealing with the dissection of a single animal, others with several animals. Most of them, however, make no attempt to deal with animals comparatively. They let the student make his own comparisons, which is possibly more than may be expected of him. The present book is intended to help him in these comparisons and to correlate and interpret the facts gathered in the laboratory. The VI PREFACE material presented has been selected for the light it throws on the phylo- genesis (racial history) and the physiology of the human body. The present text has been written to meet the needs of students in semester courses in comparative anatomy. It is an abbreviation of the Comparative Anatomy written by the same authors and published in 1936. In the condensation, however, no material essential to the proper understanding of the physiology and phylogenesis of the human body has been omitted. Due acknowledgment of assistance in the preparation of the book has been made in the earlier work, to which the reader is referred. LIBRARY CONTENTS Page Preface v 1. The Animal Kingdom i The Linnaean System 2 The Animal Phyla 3 Summary of Classification 22 Sequence of Organisms in Geologic Time 25 2. Reproduction 26 The Germinal Bodies 26 Oviparity, Viviparity, Impregnation 30 Protection, Nutrition, and Respiration during Development. 31 Evolutionary Significance 3^ Development 3^ Cleavage and Blastula 3^ Gastrula 43 Third Layer, Mesoderm 49 Early Development in Placental Mammals 55 Organogenesis 5^ Organogenesis in Amphioxus 58 Organogenesis in Vertebrates 62 Relation of Yolk to Organogenesis 78 Embryonic and Fetal Membranes 80 3. Histology 85 Epithelial Tissues 86 Glands 91 Non-EpitheUal Tissues 93 Muscular Tissue 93 Nervous Tissue 96 Tissues Serving for Mechanical Support 100 Adipose Tissue 106 Blood 106 Histological Specificity 108 4. The Integumentary System 1 1 1 Evolution of the Skin m Structure of the Human Skin 113 vii ^^".^7 Vlll CONTENTS Page Development of the Skin 114 Finger-prints and Their Meaning 115 Appendages of the Integument 116 Horny Scales 117 Horns 118 Nails, Claws, and Hoofs 118 Feathers 119 Hairs 120 Pigment 123 Cutaneous Glands 125 5. Teeth 129 Evolution of Teeth 129 Evolution of Compound Teeth 133 Teeth of Mammals 134 Teeth of Man 139 Development of Teeth 141 6. The Skeletal System 145 The Axial Skeleton 146 The Appendicular Skeleton 177 Evolution of Paired Appendages 177 Summary of Skeletal Evolution . ■ 182 The Appendicular Skeleton in Man 186 Development of the Appendicular Skeleton 187 7. The Muscular System 191 Evolution of the Muscular System 193 Muscles in Man 204 Development of the Muscles 205 8. The Digestive System 215 Evolution of the Digestive System 215 The Human Digestive System 217 9. The Respiratory System 245 Introduction 245 The Branchial System 246 The Pulmonary System 253 10. The Vascular System 262 Evolution of the Blood Vessels 262 Evolution of the Heart 275 CONTENTS IX Page Evolution of the Aortic Arches 275 Evolution of Arteries 277 Evolution of Veins 277 Evolution of Lymphatics 278 Development of the Heart 279 Development of the Aortic Arches 282 The Heart in Man 283 Pulmonary Circulation in Man 288 Systemic Circulation in Man 288 The Lymphatic System in Man 288 11. The Urogenital System 290 Evolution of the Urogenital System 290 The Urogenital System of Man 304 Development of the Urogenital System 315 12. The Endocrinal Organs 324 The Pancreas 325 The Gonads 326 The Suprarenal Glands 327 The Thyroid Gland 329 The Parathyroid Glands 33^ The Ultimobranchial Bodies 333 The Thymus 333 The Pituitary Gland 334 13. The Nervous System 33^ The Elements of the Nervous System 338 The Organization of the Nervous System 345 Nervous System of Chordates 34^ Evolution of the Brain 362 Evolution of the Spinal Cord 3^7 Evolution of the Peripheral Nervous System 372 Evolution of the Cranial Nerves 374 Evolution of the Spinal Nerves 37^ The Autonomic Nervous System 377 Evolution of the Autonomic System 382 Development of the Brain 3^3 Development of the Spinal Cord 3^6 Development of Motor Nerves 3^^ Meninges 39 1 X CONTENTS Page 14. The Sense Organs 393 Evolution and Development of Sense Cells 393 Cutaneous Senses 395 Lateral Line Organs 397 Olfactory Organs 399 Taste Organs 404 Visual Organs 406 Static and Auditory Organs 41 S Muscle Spindles 426 15. The Ancestry of the Vertebrates 427 Glossary 433 Index 457 CHORDATE ANATOMY CHAPTER I THE ANIMAL KINGDOM Since some of the so-called lower animals, living or extinct, more or less resemble hypothetical ancestors of man, some knowledge of them is necessary for a proper understanding of the history of the human body. Moreover, certain highly complex and obscure organs of man are most easily understood in the light of the simpler conditions of lower forms. Even the plants, so unHke us in outward appearance, contribute something to our knowledge of ourselves. But the organic world is so enormously complex that no human mind can carry its detail adequately without some system by which facts are classified and summarized. Most useful of such systems are those based on natural relations which, therefore, exhibit the course of evolution of each species, and place it correctly in an evolutionary scheme. For evolution, nowadays, is the key to all genetic animal relationships. Such an evolutionary scheme begins by dividing all living things into plants and animals. Plants are creatures which contain chlorophyl, and therefore can produce or make their food directly out of inorganic mate- rials, or else they are, obviously, such creatures as have lost their chloro- phyl and adopted the feeding habits of the simpler animals. Animals may or may not have descended from plants; only rarely do they contain chlorophyl, hence all their structure and habits rest on other means of obtaining food. There are, however, many simple organisms, for example, the slime molds, which are as much one as the other, plants or animals indifferently. Even some of the higher plants, like the Venus's fly-trap, catch and devour insects; and also some of the unicellular algae feed like animals. . The animal kingdom as a whole is commonly divided into about a dozen phyla, the precise number and the precise definitions of which have not yet been agreed upon by taxonomists. These phyla, in turn, are spht into classes, the classes into orders, the orders into genera, and the genera into species. It is sometimes convenient also to recognize sub-orders and sub-classes, and to combine similar genera into families. Scientific naming is by genera and species, a scheme devised by the great naturaUst Linnaeus, or Linne, about the middle of the eighteenth I CHORDATE ANATOMY rentury, and called the Linnaean, or binomial, system. Thus all birches are called by their Latin name Betula, and that is their genus. White, 4AMMALS^ 17000 SPJ BIRDS 20.000 SP. INSECTS 500. 000 SP. AMPHIBIANS^ 180*0 SR :ptii iooosp; IRUSTACEAl 8000 SR Varachnidsi »,5ooosp; annelids 4boosf R0TIFERS1 850^ THREADWORMS, 1600 SR MOLLUSCS. 61.000 SR uroch6rds I40p SB |MOL^~^ LUSCOIDS l>00S /HE Ml /CHORDS FISHES. J2.000SR CYCLOSTOMES CEPHALOCHORDS 22 SR J. -SP^ECHINODERMS. 10,000 SR, FLAi;WORMS^ 4500 SR SPONGE 250 OSE (COELENTERATES V7000 SP.y >ROTOZOA] 10,000 SP.. A PHYLOGENETIC TREE OF THE ANIMAL KINGDOM. Fig. -A phylogenetic tree of the animal kingdom, showing the dichotomy animals into Prnterostomians and Deuterostomians. yellow, and black birches are therefore respectively, as species, Betula alba, Betula lutea, and Betula nigra; but the American white birch is Betula papjrrifera, that is, the paper birch; and the common gray birch THE ANIMAL KINCiDOM 3 is populifolia, because it has leaves that twinkle in the wind like those of a poplar tree. The common cat is Felis domestica; the lion, Felis leo; the tiger, Felis tigris; and there are, in all, some forty species more in the genus Felis. Linne called us Homo sapiens. We belong to the family Homi- nidae (of which we are the only living species), to the order Primates, the class Mammalia, the phylum Chordata, and the animal kingdom. In a general way, for the larger and more famiHar animals and plants, the vernacular name, such as pine or elephant, refers to the genus. On the basis of the number of cells in the body animals are divided into the two Sub-Kingdoms — (i) the unicellular Protozoa, (2) multicellular Metazoa. Since the Protozoa are the simpler organisms, it may be assumed that the first animals on earth were protozoans. The division of animals into the two phylogenetic series graphically represented in Fig. i is based upon differences in the fate of the embryonic mouth or "blastopore." The left-hand branch includes the Protero- stomians, which are the Metazoa in which the blastopore becomes the mouth or lies near the adult mouth. Most animal phyla are Protero- stomians, which include such diverse forms as the Porifera, Coelenterates, Platyhelminths, Molluscoids, Rotifers, Annelids, Molluscs and Arthro- pods. This branch of the animal kingdom reaches its climax in the arthropods and molluscs. The Deuterostomians are the animals in which the blastopore becomes the anus or lies near the anus. The group includes the Echinoderms and the Chordates. The branch reaches its cUmax in the vertebrates and man. Since our present interest centers in man and chordates, and since none of the non-chordate phyla are believed to lie in the direct line of human ancestry, the non-chordate phyla will be mentioned only when they possess structures resembUng those of the hypothetical ancestors of man. It may be assumed that students in a comparative anatomy course have some acquaintance with non-chordates. Consequently no detailed description of them is needed here. We may therefore turn our attention directly to the sub-phyla and classes of chordates. Phylum CHORDATA The chordates are animals which, at least early in life, have a supporting rod, the notochord or chorda dorsalis, between the alimentary canal and the central nervous system. In higher chordates the notochord is replaced during ontogenesis by a cartilaginous or bony vertebral column. All have a dorsal tubular nervous system. The heart is ventral, and the pharynx has functional or embryonic gill sUts. Most chordates are metameric in structure, although the metamerism may become greatly obscured in the adult. Segmental excretory organs are generally present. CHORDATE ANATOMY Nearly 50,000 species are known. Four sub-phyla are included in the phylum — Hemichorda, Urochorda, Cephalochorda, and Vertebrata. Sub-Phylum Hemichorda (Enteropneusta) The hemichordates or Enteropneusta hold a somewhat uncertain posi- tion in the animal kingdom. Morphologists are by no means agreed that their closest affinities are with the chordates. Some associate them with PENING OF PERIBRANCH.CAV. /ANUS DORS. NERVE CX«D /NOTOCHORD IBmnniiiiimiiiiiiiiiiiiiiiiimimrniiiiiiiiiniiMMiiririiiiiTiiiflfiininm A. LARVAL UROCHORDATE GILL SLfTS' /DORSAL NERVE ANUS POST OPENING OF PERIBRANCHIAL CAVITY B CEPHALOCHORDATE .DORS. NERVE CORD KCILL SUT5 'HEART LIVER *> INTESTINE ANUSi C. VERTEBRATE (CYCLDSTOME) Fig. 2. — Diagrams of A, larval urochordate, B, cephalochordate (Amphioxus), and C, vertebrate (Petromyzon), illustrating the fundamental characteristics of chordates; redrawn after Hesse- Doflein. the annelids, while the resemblance of their larval stage to that of echino- derms leads others to place them near that group. Their inclusion among the chordates rests on their possession of pharyngeal gill-sUts, enteric coelomic pouches, a notochord-Uke diverticulum of the fore-gut in the pre- oral lobe, and upon the relations of the blood-vessels and nerves. Seg- mental excretory organs are, however, absent. There are possibly 50 species. BALANOGLOSSUS, the best-known genus, may be taken as representa- tive. The body of Balanoglossus is worm-Hke, and is divided into five regions, proboscis, collar, gill region, "liver" region, and intestinal region. THE ANIMAL KINGDOM The proboscis is a hollow muscular organ with an opening, a pore, on the left dorsal side of the neck. The mouth lies on the ventral side between the proboscis and the collar. The collar, like the proboscis, contains a division of the coelom, which opens to the exterior by a pair of pores near the mid-dorsal line. Like the proboscis, the collar also is muscular, and used by the organism as a means of burrowing in the sand where it lives. PROBOSCIS COLLAR BALANOGLOSSUS. MOUTH gills PiG_ 3_ — Balanoglossus, a typical genus of the sub-phylum Hemichorda. (Redrawn after Bateson.) The pharynx is divided into a dorsal portion which contains the numer- ous gill-apertures and a ventral portion which functions as the digestive passage of the pharynx. Posterior to the pharynx, the body contains a series of gonadic sacs, each of which has a pore-like opening to the exterior. The sexes are separate. In the so-called liver region, the intestine shows a series of paired diverticula, each of which produces a corresponding bulging of the rela- LONG bUSCLE OF PROBOSCIS BLOOD VESSEL Fig. 4. — Balanoglossus, the typical genus of the hemichordates, seen in left lateral aspect. The possession of both dorsal and ventral nerve-cords links hemichordates on the one hand with invertebrates and on the other with vertebrates. (Redrawn after Stempell.) For the purposes of the diagram, the body of the animal is bent upon itself. tively thin body-wall. These diverticula are glandular and supposed to have a digestive function, hence their name. Behind the liver-region, the intestine passes without convolution directly to the posteriorly situated anus. The circulatory system resembles that of annelids, but is supplemented by a lacunar system of lymph spaces. The nervous system consists of dorsal and ventral nerve strands containing occasional giant nerve cells. There are no special sense organs. CHORDATE ANATOMY APICAL PLATE GILL SLITS^ P/IC (ANT ENDO. DIVERTICULA) COLLAR CAVITIES NOTOCHORD PROBOSCIS IRCULAR BAND OF CILIA ^TRUNK CAVITIES Fig. 5. — Balanoglossus embryos. A. A horizontal section of a young embryo, showing the origin of mesodermal pouches. McBride and others have noted the similarity of this section to that of a young Amphioxus embryo as evidence of the close affinity of these two forms. B. A young Balanoglossus larva with five pairs of gill-slits, viewed from the left side. The gill-slits of Balanoglossus bear a striking resemblance to those of Amphioxus. On the other hand the young larva of Balanoglossus is strik- ingly like the larva of echinoderms. (Redrawn after Bateson.) .GILL-SLIT A ESOPHAGEAL REGION - X.& B. HARRIMANIA - STEREOGRAM OF COLLAR REGION. Fig. 6. — Harrimania, a hemichordate. A. The dorsal portion of a cross section of Harrimania in the region of the esophag\is. The resemblance of this cross section to one of Amphioxus is striking and serves to demonstrate the close genetic affinities of these two chordates. In Harrimania the notochord is present not only in the preoral lobe as in other hemichordates but also in the collar and anterior pharyngeal regions. B. A stereogram of Harrimania in the collar and anterior pharyngeal region, showing the presence of the notochord in these regions. Such evidence tends to remove the doubt that a true notochord exists in hemichordates. (Redrawn after Ritter.) PROBOSCIS PORE ARMSr NERVE STRANO .GONAD 'PHARYNX IPHARYGEAL POUCH - STALK >^-Ji^ I 1 ; *^ " notochord' MOUTH' CEPHALOOISCUS. Fig. 7. — A diagram of Cephalodiscus viewed from the left side as if in median optical section. The presence of a notochord in the pre-oral lobe is one of the reasons for placing this animal among hemichordates. While not regarded as a form " ancestral to vertebrates, Cephalodiscus interests morphologists as a primitive chordate. (Redrawn after W. Patten.) THE ANIMAL KINGDOM 7 The so-called nolo(ln)r(l is a diverticulum of the intestine which extends with a narrow lumen into the proboscis from a point just behind the mouth. The larva of Balanoglossus, known as Tornaria, shows rather striking resemblances to the larva of echinoderms. As in echinoderms, the blastopore becomes the anus. The sub-phylum, therefore, is included in the group of Deuterostomia. Cephalodiscus and Rhabdopleura are genera which show resemblances to Balanoglossus but which have a U-shaped alimentary canal. Rhabdo- pleura is without gill apertures. Sub-Phylum Urochorda (Tunicata) The urochordates, the tunicates or sea-squirts, are so named because the notochord, absent in the sessile adult, is always limited to the tail TENTACUES CIUATED GBOOVt SUBNEURAL GLAND, BRAIN ^AMlFl, NERVE CORD ESOPHAGEAL ASCIDIA-A UROCHORDATE Fig. 8. — Ascidia, a urochordate. The animal is viewed as if cut in median longitudinal section and as seen from the right side. (Redrawn from Sewertzoff, after Boas.) region. Another character common to the group is the presence of a tunicin mantle which is secreted by the skin. Tunicin is a chemical substance that resembles cellulose. A coelom is sometimes present, but is limited to the region of the ventral heart. Nephridia or coelomoducts are wanting. The body is unsegmented, and the alimentary canal is bent on itself so that the anus lies near the mouth. The pharynx is perforated by gill-slits, the number of which varies greatly in the different species. The nervous system consists of a nerve ganglion dorsal to the pharynx, from which nerves extend to the various organs. In some forms both sexual and asexual methods of reproduction occur. Individuals, however, are usually hermaphroditic. Development generally involves metamor- 8 CHORDATE ANATOMY phosis. The sexually-produced tailed larva bears certain striking resem- blances to the larva of Amphioxus. Some systematists recognize 1400 species. ClONA is a sessile tunicate, three or four inches in length, which is attached by tunicin stolons to its substratum. A tunicin test or tunic, which is secreted by the skin, encloses the entire animal as a sac. Beneath the test and loosely connected with it, except in the region of the two apertures of the body, lies the body-wall or mantle. This consists of an external simple epithelial ectoderm, and, beneath this, connective tissue containing a network of muscle fibers which are more abundant in the BRAIN CILIATED FVJNNEC MOUTH GILL SLITS ■ATRIAL CAVITY/ SPINAL CORD \notochord intestine \ENDOsrYLE "^"^ B. METAMORPHOSIS. Fig. 9. — Diagrams of stages in the metamorphosis of a uruchordate larva. When the larva settles down and becomes fixed by its adhesive papillae, the tail is lost and the notochord disappears. Thus the chordate characters which are so evident in the larva are partly lost in the mature organism. (Redrawn from Korscheldt and Heider, after Seeliger.) region of the two apertures of the body, which they serve to close and open. Of the two external apertures, the more ventral is the inhalent or oral siphon and the other the exhalent or atrial siphon. The former leads directly to the mouth, which is surrounded by a circle of tentacles. The mouth leads into a greatly enlarged pharynx, which is perforated by numerous gill-slits or stigmata. The action of the cilia on the bars of these slits serves to maintain a current of water from the pharynx into the surrounding peribranchial or atrial cavity. Such relations resemble those of similar organs in Amphioxus. In the floor of the pharynx extends a longitudinal groove, the endostyle, which morphologists generally homologize with the thyroid gland of vertebrates. A somewhat similar groove extends also along the dorsal side of the pharynx. The alimentary THE ANIMAL KINGDOM 9 canal consists of a short esophagus, a spherical stomach, and an intestine which leads to an anus situated well forward in the atrial chamber. The heart lies ventral to the esophagus in the pericardial chamber. There are no closed blood-vessels, but the blood is pumped from the heart forward to the pharynx in lacunar spaces the relations of which resemble those of the afferent branchial vessels of vertebrates. The reproductive organs He in the loop of the intestine, posterior to the stomach. Their ducts extend forward and open into the atrial cavity near the anus. The gonads are hermaphroditic. The nervous system consists of a ganglion or brain, which lies in the body-wall between the two apertures of the body. Ventral to the brain is a neural gland which has been compared with the neural part of the pituitary gland of vertebrates. The unpaired eye and static organ con- tained in the brain vesicle of the larva degenerate in the metamorphosis. STATIC ORGANx ENOOOERM STRAND PHARYNX'' CILL SLITS OCELLUSf l^f^r Fig. 10.— Diagram of a larval urochordate. The similarity of the larval uro- chordate to the embryo of a cephalochordate (Amphioxus) suggests that a form like this lies near the main line of vertebrate ancestry. (Redrawn after von Beneden and Julin modified.) Ciona during its ontogenesis undergoes a striking metamorphosis, which indicates that the animal is a degenerate descendant of a primitive branch of the chordate tree. Of the four orders of urochordates the Larvacea are of special interest since they develop without metamorphosis, and hence show no sign of degeneration. Their caudal appendage contains a notochord and spinal cord. That they lie close to the main line of vertebrate ancestry seems not unlikely. Sub-Phylum Cephalochorda (Acrania) The cephalochordates are those chordates in which the notochord occurs not only in the head, as in Hemichorda, or in the tail, as in the Urochorda, but throughout the entire length of the body. The group is sometimes called the Acrania because, as the name suggests, a brain case is lacking. Metamerism is strikingly manifested in the muscles and nerves, which form an unbroken series from the tip of the snout to the tip of the tail. Segmental protonephridia are metamerically arranged, but lO CIIORDATE ANATOMY are limited to the gill region. As in urochordates, the gills open into a peribranchial cavity. Development involves metamorphosis. There are possibly 25 species. AMPHIOXUS. The lancelet, Amphioxus, the characteristic genus of the group and the so-called connecting link between vertebrates and invertebrates, interests morphologists because of its resemblance to the hypothetical ancestor of vertebrates. Its resemblance to the larva of cyclostomes is impressive. (Fig. 11) The Amphioxus is a lance-shaped animal, not more than two inches long, with a laterally compressed body and a median caudal fin. Its external orifices are an anterior ventrally placed mouth, an anus to the left of the caudal fin, and an atriopore somewhat behind the middle of the CIRRI ORAL HOOO! Fig. II. — Amphioxus, in ventral and side views. Metamerism, lacking in uro- chordates, and scarcely evident in hemichordates, is strikingly shown by Amphioxus. Whether this metamerism is inherited from annelid-like ancestors or is a convergent trait independently acquired, is a moot question in morphology. (Redrawn after Kirkaldy.) body. The atrial chamber which surrounds the elongated pharynx is formed by the union of paired lateral folds which meet in the mid-ventral line of the body. Such a structure seems to be an adaptation to the sand-burrowing habit of the adult animal. The atrial cavity ends blindly in front, and opens externally by the atriopore just behind the pharynx. In the region of the pharynx, a pair of ventro-lateral metapleural folds extend as far back as the atriopore. The body is covered by a thin external cuticula secreted by the simple epithelial epidermis. Beneath the skin and visible through it are sixty pairs of myotomes which alternate with one another along the two sides of the body. As in the vertebrates generally, these myotomes are greatly thickened along the dorsal side of the body. Each myotome is V-shaped with the apex of the V pointed forward. The mouth, surrounded by a circle of tentacles, leads directly into the elongated pharynx, the walls of which are perforated by numerous THE ANIMAL KINGDOM II gill-slits. A ciliated groove, which is similar in function and in relations to the endostyle of urochords, extends the entire length of the pharynx. NERVE CORD^. NOTOCHORO- -DORSAL ARTERY.~. , PHARYNX r-^ METAPLEURAL FOLD. , NERVE CORD PERIBRANCHIAL CAVITY GILL APERTURE ! VENTRAL ARTERY. GILL APERTURE VENTRAL ARTERY< DIGESTIVE GROOVE. A. AMPHIOXUS. B.PTYCHODERA. Fig. 12. — Diagrams illustrating divergent methods by which the peribranchial cavity is formed. In Amphioxus (A) the pleural folds are separated from the pharynx by paired folds which extend dorsally from the ventral side. In the hemichordate Ptychodera (B), on the other hand, the paired folds begin to form at the dorsal side of the worm and extend ventrally. The peribranchial cavity in urochordates arises in a similar manner. As frequently happens in animals, a similar end-result is attained by divergent means. (Redrawn after Gaskell.) LATERAL TRUNK MUSCLES ■SPINAL CORD NOTOCHORD DORSAL AORTA- PRECARDINALV EPIBRANCHIALGROOVE GILL LAMELLAE CARTILAGE BAR GILL-RODS ge?~;^::j — TRANSVERSE MUSCLE •VENTRAL AORTA )JS 'PERIBRANCHIAL CAVITY HYPOBRANCHIAL MUSCLE METAPLEURAL FOLD A. AMPHIOXUS B AMMOCOETES. Fig. 13. — Cross sections of A, Amphioxus and B, Ammocoetes (larval Petromyzon) through the pharyngeal region showing their fundamental resemblance. Opposite it, in the roof of the pharynx, is a somewhat similar epipharyn- geal groove. The liver is a hollow tubular sac, which opens into the 12 CHORDATE ANATOMY floor of the intestine just behind the pharynx and extends forward to the left below the pharynx. The intestine is straight. The coelom, considerably reduced in size in the region of the pharynx, extends posteriorly to the region of the anus. Ninety pairs of nephridia, limited to the gill-region, open into the atrial cavity. The solenocytes attached to the nephridia are specialized excretory cells which strikingly resemble those of annelids. Nephrostomes are absent. (Fig. 263) Sexes are separate. Two dozen or more gonadic sacs surrounded by the peritoneum project into the atrial cavity. Except for the absence of a heart, the circulatory system resembles that of fishes, but the blood contains few blood corpuscles. The nervous system, as in vertebrates, is tubular and dorsal. The brain is a simple vesicle, which may possibly be compared with the fore- brain vesicle of vertebrates. The nerves are of two kinds, dorsal (sensory and motor) and ventral (motor). The former pass directly to the skin and to visceral muscles by way of the myocommata. Dorsal and ventral nerves do not unite. Sympathetic cells and fibers are not segregated to form a sympathetic system. Sense organs comparable with those of vertebrates are wanting. A median dorsal pit at the anterior end of the brain is mistakenly spoken of as the olfactory pit. A pigment spot on the brain is likewise somewhat uncritically called the cerebral eye. Amphioxus is, however, very sensi- tive to light. There is no ear. Sub-Phylum Vertebrata (Craniota) The vertebrates or craniotes are chordates with a vertebral column and a brain-case. The evolution and perfection of a light and strong endoskeleton has been an important factor in making the vertebrates masters of the world. Exoskeletal structures also appear, as among the invertebrates, but only exceptionally are heavy enough to interfere with the activity of the animal. A many-layered epidermis with various appendages enables the vertebrates to withstand successfully the vicissi- tudes of weather met by land animals. In correlation with their activity, senses multiply and become acute and the brain is much enlarged. The original metamerism characteristic of lower vertebrates becomes much obscured in the higher. The heart is ventral and may be either two, three, or four-chambered. Some 25,000 species are known. Vertebrates may be divided into seven classes. Class Cyclostomata Cyclostomes are the round-mouthed lamprey eels and hag-fishes. They have a persistent notochord, lack a biting jaw, and the beginnings THE ANIMAL KINGDOM 13 of vertebrae appear in the form of cartilaginous neural arches. In the genus Bdellostoma there are often as many as fifteen pairs of gill-sHts. There are no scales in the skin, and the teeth are horny. Some species are hermaphroditic. Paired appendages are absent. BDELLOSTOMA C PETROMYZON Fig. 14. — Three characteristic genera of cyclostomes — Bdellostoma, Myxine, and Petromyzon. That they are the most primitive vertebrates is shown in many traits, such as a permanent notochord, absence of paired appendages and jaws, etc. (Redrawn after Dean.) The lamprey, Petromyzon, is a familiar genus which undergoes meta- morphosis during its development. Its larval stage is known as Ammo- coetes. Other genera are Myxine and Bdellostoma. CEPHALASPIS- AN OSTRACODERM Fig. 15. — Cephalaspis, an ostracoderni, appears to have affinities with cyclostomes and has been thought by W. Patten to connect vertebrates with arachnids. Class Ostracodermi The ostracoderms are fossil forms which, as Stensio and others have shown, resemble cyclostomes in some striking respects. Unlike the latter. 14 CHORDATE ANATOMY THE ANIMAL KINGDOM 15 however, their heads were covered by heavy bony armor. Like the lam- preys they lacked jaws and paired appendages. As in cyclostomes the nasal aperture was median and dorsal in position. It has been asserted but not demonstrated that the ostracoderms are the ancestors of carti- laginous fishes, which are consec^uently assumed to have lost their heavy body exoskeletons. Most morphologists, however, consider ostracoderms rather highly specialized types and not primitive ancestral forms. Cepha- laspis and Pterichthys are characteristic genera. HEPTANCHUS - AN ELASMOBRANCH „.Aii^,e^d,^_^__ Fig. 17. — Types of three sub-classes of fishes — Heptanchu,s, an elasmobranch; Polypterus, a crossopterygian ganoid; and Scomberomorus, a teleost. (Redrawn after Dean.) Class Pisces Fishes are vertebrates with usually scaly skins, permanent gills, and paired fins. The heart is two- or three-chambered. The skeletons may be cartilaginous or bony. Gill-apertures number four to seven pairs. Dorsal and ventral spinal nerves join to form mixed trunks. Sympathetic ganglia are differentiated. The liver has at least two lobes. Of special interest are the orders of fishes which are believed to be near the line of ancestry of land animals. Probably cartilaginous forms like the Elasmobranchs (sharks and skates) were the common stock from which the remaining orders of fishes were evolved. Their gills are not covered by an operculum ; their skull is devoid of covering membrane bones ; their intestine has a spiral valve. The dogfish Squalus (Fig. 16) is a familiar example. 1 6 CHORDATE ANATOMY Ganoids are "ray-finned" forms with either cartilaginous or bony skeletons. Their gills are covered with an operculum; they have a spiral valve; the tail is heterocercal; their air-bladder is connected with the pharynx or esophagus by means of an open duct; their skin has ganoid scales, or sometimes, bony scutes, or it may be naked. The order of Crossopterygii, "lobe-finned" ganoids, which make their first appearance in the Devonian period, were air-breathers and possibly the direct ancestors of land animals. The Nile " bichir, " Polypterus, is a Hving representative of this largely extinct group. The sturgeon is a famiUar example of the ganoid group. Teleosts are "ray-finned " forms with a wholly bony skeleton. Unlike the ganoids their tail is never heterocercal. They are usually scaly but may be scaleless. The air-bladder when present does not have an open duct; they lack a spiral valve. Teleosts are the most abundant of fishes. The cod and salmon are familiar types. Arthrodires are fossil fishes possibly related to the modern lung-fishes or Dipnoi. The Dipnoi are not believed to be the ancestors of land forms, but they are in many ways transitional in structure between fishes and amphibia. They may have either one or two lungs. The Dipnoi are represented by the Ceratodus of Australia. Class Amphibia Amphibians bridge the gap between land and water vertebrates. Either permanent or temporary gills occur. Lungs are usually present, NECTURUS. Fig. i8. — Necturus, a urodele amphibian, interests zoologists because more than any- other living amphibian it resembles the fossil Stegocephala, another " ancestral " group. but some are lungless. Except in some fossil forms, scales are lacking in the skin. The olfactory pits communicate with the mouth cavity by means of narial passages. The paired appendages are toed. The heart is three-chambered. A postcaval vein is present. The embryo develops without an amnion. Amphibia are subdivided into Urodela or tailed forms, the newts and salamanders, Anura or tailless forms, the frogs and toads, and the Gym- nophiona or limbless types. Besides these living orders of amphibia, the fossil order Stegocephala is important, since they appear to be the direct ancestors of reptiles. Fishes and Amphibia have been grouped together as Ichthyopsida in contrast with Sauropsida which includes reptiles and birds. The embryos THE ANIMAL KINGDOM 1 7 of the latter are protected by fetal membranes, while those of the former are without them. Class Reptilia Reptiles are horny-scaled vertebrates which breathe by lungs. The embryos develop in a liquid-filled sac, the amnion. The skull articulates with the atlas vertebra by means of a single occipital condyle. Arterial and venous blood are mixed in the dorsal aorta. Living reptiles are divided into Rhynchocephalia, Lacertilia, Ophidia (Serpentes), Chelonia, and Crocodilia. Among fossil orders, the Thero- morpha are important, since, especially in their dentition, they resemble mammals, and the dinosaurs because they are the ancestors of the birds. Rhynchocephalia are mostly fossil reptiles having very primitive char- acteristics. Like some primitive amphibians they have only two sacral SPHENODON Fig. 19. — Sphenodon has been characterized as a "living fossil." Asa "primi- tive" type of reptile it interests the student of phylogenesis. It belongs to the Order Rhynchocephalia. vertebrae. Sphenodon (Hatteria), the only living representative, lacks the external copulatory organs present in all other reptiles. The Lacertilia are the lizards. They usually have two pairs of limbs; the anus is a transverse slit; eyelids and an external ear opening are usually present. Ophidia (Serpentes) are limbless reptiles devoid of movable eyelids and external ear-opening; the tongue is forked; the scales along the ventral side of the body are specially modified to assist in locomotion. Snakes. Chelonia (Testudinata) are toothless reptiles, the broad bodies of which are enclosed by a "shell" which consists of a dorsal carapace and a ventral plastron. The eyes have lids and nictitating membrane. Turtles and tortoises. Crocodilia have their teeth set in alveoli; the anus is a longitudinal slit; the tail is laterally compressed; the bodies are large. Alligators and Crocodiles. Theromorpha are fossil reptiles which may have been the progenitors of mammals; in some the teeth are differentiated as in mammals; the quadrate 1 8 CHORDATE ANATOMY bone is attached to the cranium as in mammals; the zygomatic arch of the skull resembles that of mammals. Class Aves Birds differ from reptiles in having both feathers and scales, and in having the anterior appendages modified as wings. The heart is four- chambered, and the single aortic arch on the right. Teeth are wanting in modern forms. The body temperature is higher than in other animals. Two large divisions are recognized, the flying birds or Carinatae with a keeled sternum, and the running birds or Ratitae which have no keel on the sternum. Class Mammalia Mammals are vertebrates with hairs and mammary glands. A few, the monotremes, lay eggs, but all the rest bring forth their young well developed. Mammals have a pair of occipital condyles, a muscular diaphragm, and a chain of three ear bones. The heart is four-chambered, and the aortic arch is on the left. The jaw articulates between the dentary and squamosal bones. Two major divisions are recognized, Placentals, the embryos of which are attached to the mother by a vas- cular placenta; and the Non-Placen- tals, the monotremes and marsupials, most of which lack a placenta. ORNITHORHYNCHUS. Fig. 20. — Ornithorhynchus is a repre- sentative of the most primitive group of mammals, the monotremes. As an egg-laying mammal it bridges over the gulf separating reptiles and mammals. Fig. 21. — Opossum, the typical genus of didelphians. (Redrawn after Newman.) Sub -Class Monotremata (Prototheria) The monotremes or ornithodelphians are egg-laying mammals with a cloaca. Teats are lacking. Ornithorhynchus, the duck-bill of Australia, is the best-known genus; and there are two species of the spiny anteater. Echidna. There seem to be no more than a half-dozen species surviving for the entire sub-class.- THE ANIMAL KINGDOM 19 Sub-Class Marsi'itals The marsupials or didelphiansgive birth to their young in a most imma- ture state and nourish them for some time in an external marsupial pouch situated on the ventral side of the body of the female. The brain has no corpus callosum. A loose allantoic placenta occurs in some. Dasyurus has a yolk-sac placenta. Opossum and kangaroo are well-known examples. All the indigenous mammals of Australia are non-placental. Sub-Class Placentals The placentals or monodelphians have a placenta, a corpus callosum in the brain, and no marsupial bones. Urogenital and digestive outlets are separated. Placentals are subdivided into at least ten living orders. Order I. Insectivora. The insectivores include shrews, moles, and hedgehogs. They are flat-footed and five-toed, and their denti- tion is unspecialized, so that they are appar- ently nearest of surviving forms to the original placental. Order 2. Xenarthra. The Xenarthra include part of the group formerly included in the edentates such as the armadillos, sloths and anteaters. The teeth of adults are either absent or lack enamel and roots. Dentition is limited to a single set. Order 3. Rodentia. The rodents are gnawing animals, such as rats, rabbits, squirrels, guinea pigs, beavers, porcupines, gophers. Canine teeth are absent, and the incisor teeth in both jaws grow continuously Fig. throughout life. The cecum is very large. Order 4. Carnivora. The carnivora include the fossil creodonts, the cats, dogs, weasels, bears, raccoons, and seals. Each foot has four or five toes. The canine teeth are sharp and elongated. The clavicle is reduced or absent. Order 5. Artiodactyla. Artiodactyls are such hoofed forms as cattle, deer, swine, sheep, goats, camels, llamas, hippopotamuses, and giraffes, which usually have an even number of toes on each foot. The third and fourth toes are larger, and the second and fifth reduced or absent. The stornach is complex and the cecum reduced. 22. — Tupaia, the tree- shrew, an insectivore. 20 CHORDATE ANATOMY Order 6. Perissodactyla. The perissodactyls usually have an uneven number of hoofs on each foot. They include the horse, ass, zebra, tapir, and rhinoceros. The third toe is the largest and the only one func- tional in the horse. The enamel of the back teeth is complexly folded. Order 7. Subungulata. Hoofed forms usually with plantigrade feet. Subungulates are the elephants and mastodons, and the hyrax or cony. The proboscidians such as the elephants have on each foot five toes on which they walk. Their testes do not descend into a scrotum. Sireni- ans (Manatee and Dugong) are a suborder of this group. Order 8. Cetacea. The cetaceans include whales, porpoises and dolphins. They are aquatic mammals with fish-like bodies. Hairs and LEMUR CATTA. Fig. 23. — Lemur, a primitive Primate. (Redrawn after Shipley and McBride.) pelvic extremities are absent in the adult. There are two abdominal teats. Teeth may be replaced by whalebone. Order 9. Chiroptera. Chiroptera are the bats and flying foxes. Their anterior limbs are modified to support the wings, the fingers are joined by a web, and the sternum has a keel. Order 10. primates. The primates include lemurs, marmosets, monkeys, baboons, apes, and men. They are mostly arboreal in habit. Nearly all have five digits with flattened nails, and in aU except the lowest forms the thumb is freely opposable to the fingers. Mammary glands are usually a single pair and thoracic. Primates are divided into two sub-orders. Sub-order Lemuroidea. The lemuroids include the lemurs and tarsiers. They are arboreal and nocturnal, small and not especially monkey-like. Typical lemurs have a claw on the second digit of the THE ANIMAL KINGDOM 21 hind-foot. Thumb and great toe are not completely opposable to the other digits. The uterus is two-horned. Sub-order Anthropoidea. Anthropoids include the remainder of the primates. Hands and feet are differentiated, and either the thumb or the great toe is opposable. Finger and toe-nails are flat, except in the marmo- sets which have claws. VOUNG CHIMPANZEE. _ Pig. 24.— Young chimpanzee, a type of anthropoid. (From photograph by Fred Johnson.) Three chief sections of anthropoids are recognized : Platyrhina. The South American monkeys, with broad nasal septum, three premolar teeth in each half-iaw (except the marmosets which, like the Old World monkeys, have two), and a cUmbing foot. Catarrhina. The Old World monkeys and the great apes, with a narrow nasal septum, two premolar teeth in each half jaw, and a climbing foot. Bimana. Also with narrow nasal septum and two premolars, but with the great toe non-opposable and a walking foot. 2 2 CHORDATE ANATOMY CLASSIFICATION OF ANIMALS— SUMMARY Animal Phyla 1. Protozoa. Unicellular. Reproduce by fission. METAZOA, multicellular. 2. Porifera. Multicellular. Acoelomate. Pores in body-wall. 3. Coelenterata. Multicellular. Acoelomate. Radial symmetry may possibly disguise primitive bilateral symmetry. Two-layered body- wall. Nettling-cells. Enteron with single opening. 4. PL.A.TYHELMINTHES. Bilateral. Flat-bodied. Without coelom. Anus in a few genera. 5. Nemathelminthes. Pseudocoelomate. Cylindrical. Anus. 6. Molluscoida. Usually coelomate. U-shaped alimentary canal. Lophophore. 7. Rotifera. Pseudocoelomate. Trochophore-like worms. Cilia around mouth. 8. Echinodermata. Coelomate. Spiny-skinned. Water-vascular system. Bilateral symmetry disguised. 9. Annelida. Coelomate. Metameric. Appendages, when present, without joints. 10. MoLLUSCA. Coelomate. Non-metameric. Mantle, mantle-cav- ity, foot. 11. Arthropoda. Pseudocoelomate. Metameric. Jointed append- ages. Classes, Crustacea, Arachnida, Onychophora, Myriapoda, Insecta. 12. Chordata. Notochord. Dorsal tubular nervous system. Sub-Phyla of Chordates Hemichorda. Notochord limited to oral and pre-oral region. Worm- like. Body in three primary divisions. Balanoglossus. Urochorda. Notochord limited to tail region. Body-wall covered with cellulose sac. Appendicularia, Ascidia. Cephalochorda. Notochord in head, trunk, and tail throughout life. Metameric. Amphioxus. Hemichorda, Urochorda and Cephalochorda together are often called protochordates. Vertebrata. Chordates with brain case and vertebrae. Squalus. Classes of Vertebrates (Craniota) I. Cyclostom.\ta. Without paired appendages or biting jaw. Usu- ally hermaphroditic. Petromyzon, Myxine, Bdellostoma. THE ANIMAL KINGDOM 23 2. OsTRACODERMi. Fossil monorhine fishes related to the cyclostomes. 3. Pisces. With paired appendages and movable lower jaw. Skin usually scaly. Permanent gills. Fishes are subdivided into five subclasses : Elasmobranchii. Gills lack operculum. Skeleton cartilaginous. Sharks, skates, and rays. Crossopterygii. Fossil forms related to the ganoids. Ganoidei. With operculum. Cartilage skeleton largely replaced by bone. Garpike, sturgeon. Teleostei. With operculum and bony skeleton. Common bony fishes. Dipnoi. With gills and one or two lungs. All following are Tetrapods. 4. Amphibia. Living forms are without scales and usually have lungs. Toed appendages instead of fins. Claws or nails lacking. The Stegocephala are a group of fossil amphibians. Fishes and Amphibia grouped together as Ichthyopsida. All to this point are Anamnia. All that follow are Amniota, having the embryo protected by an amnion. 5. Reptilia. Adults scaly. Lungs only — no gills. Aortic arch on both sides. The Theromorpha are fossil reptiles. 6. AvES. Feathered. Modern forms toothless. Aortic arch on the right side only. Reptiles and birds grouped together as Sauropsida. 7. Mammalia. With mammary glands and hair. Sub-Classes of Mammals 1. Monotremata. Egg-laying mammals. 2. Marsupialia. Pouched mammals. 3. Placentalia. Mammals with a placenta. Orders of Placentalia 1. Insectivora. Insect eaters. 2. Xenarthra. Toothless or teeth without enamel. 3. Rodentia. Incisors specialized for gnawing. 4. Carnivora. Flesh eaters. 5. Artiodactyla. Generally even number of hoofs. 6. Perissodactyla. Generally odd number of hoofs. 7. SuBUNGULATA. Proboscidians, hyrax, and sirenians. 8. Cetacea. Whales. 9. Chiroptera. Winged mammals. 10. Primates. Usually a single pair of thoracic mammae. Thumb usually opposable. 24 CHORDATE ANATOMY Lemuroidea. "Half-apes." Thumb not fully opposable. Anthropoidea. Thumb or great toe, except in New World monkeys, opposable. Sub-Orders of Anthropoidea Platyrhina. With broad nasal septum. New World. Catarrhina. With narrow nasal septum. Old World. BiMANA. Great toe not opposable. Species and Genera of Bimana Pithecanthropus erectus, the Java man. EoANTHROPUS Dawsoni, the Sussex man. Sinanthropus Pekinensis, Pekin man. Homo Neanderthalensis, Neanderthal man. Homo Heidelbergensis, the Heidelberg man. Homo Rhodesiensis, the Rhodesian man. Homo sapiens, Cro-Magnon and modern man. (Negro, Mongolian, etc.) THE ANIMAL KINGDOM Sequence of Organisms in Geologic Time 25 Eras Periods Years (Barrell) Characteristic Organisms Recent I , 000 , 000 to I ,500,000 Modern races of men. Recent plants and animals. Cenozoic Pleistocene (Glacial) Early species of men and primates. Mam- mals dominant life. " Age of man." Tertiary 95 ,000,000 to 115, 000 , 000 "Age of mammals." Lemuroids and in- sectivores appear. First placentals. Cretaceous 116, 000 , 000 to 136,000,000 Mammals mostly marsupials. Reptiles highly specialized. Mesozoic C 0 m a n - chean 120,000,000 to 150,000,000 Bony fishes abundant. Flowering plants appear. Jurassic 155, 000 , 000 to 195,000,000 Diverse reptiles. Ganoid fishes. First birds. Triassic 190,000,000 to 240,000,000 Crocodiles and dinosaurs. Reptiles dom- inant. First mammals. Permian 215, 000 , 000 to 280,000,000 Mammal-like reptiles. Trilobites disappear. Pennsyl- vanian 250,000,000 to 330,000,000 Primitive amphibians and reptiles. Conifer- ous plants. Mississip- pian 300,000,000 to 3 70 , 000 , 000 Earliest amphibian fossils. Horse-tails and club-mosses. Paleozoic Devonian 360,000,000 to 420,000,000 Amphibian foot-prints. Lungfishes. Earli- est land plants. Silurian 390,000,000 to 460 , 000 , 000 Ostracoderm (armored) fishes. Elasmo- branchs. Land plants begin. Ordovician 480,000,000 to 590,000,000 Vertebrates appear. First fishes. First insects. Cambrian 550,000,000 to 700 , 000 , 000 Invertebrate phyla abundant. First tri- lobites. CHAPTER 2 REPRODUCTION Anatomy, broadly defined, includes embryology which deals with the progressively changing anatomy of the animal in the course of its develop- ment from egg to adult. Many anatomical peculiarities of animals are unintelligible so long as only the adult is studied. Embryology gives some reason for such facts as that the chief artery emerging from the heart turns to the right in a bird but to the left in a mammal and that the diaphragm of a mammal is supplied by nerves from the neck region instead of from the neighboring trunk region of the spinal cord. The theory of evolution rests to an important extent on facts derived from the com- parative embryology of vertebrates. Sexes. Reproduction in the vertebrates always involves gonads of two types, the ovary which produces eggs (ova) and the testis which produces sperm (spermatozoa). In some tunicates (Urochorda), pre- sumably remote allies of vertebrates, alternation of sexual and asexual generations occurs. A fertilized egg becomes an asexual individual from which arise buds. These become sexual adults which are her- maphrodite, that is, they produce both eggs and sperm. In all vertebrates except a few fishes the individual is either male or female — the dioecious condition. The eel-like hag, Myxine (a cyclo- stome), and several of the bony fishes (Teleostei) are normally her- maphrodite (monoecious). Among vertebrates which are normally dioecious many abnormal cases have been reported, especially in fishes and amphibians, in which germ cells of both sexes were found in one individual. The Germinal Bodies. The spermatozoa are derived from cells in the walls of delicate tubules which are the essential part of the testis. The ova come from primordial germ cells contained within the tissues of the usually solid ovary. (Fig. 274) The "head" of the spermatozoon (Fig. 25) consists of compacted nuclear material (chromatin) derived from the primordial germ cell. A locomotor "tail" is formed from the cytoplasm (extranuclear proto- plasm) of the original cell. The "ripe" spermatozoon is essentially a motile nucleus. The egg in the course of its differentiation acquires a greatly increased body of cytoplasm within which is deposited more or less food material, 26 REPRODIKTION 27 the yolk or deutoplasm. The egg may become invested by membranes or envelopes, either protective {e.g., the vitelline or yolk membrane; the hard calcareous shell of a bird's egg; see Fig. 28) or nutritive (e.g., the albumen or "white" of a bird's egg). Eggs differ most remarkably as to the amount of contained yolk and as to their outer coverings. The microscopic egg of a mammal and the gigantic ostrich egg encased in its hard shell would seem to be hardly Fig. 25. — Spermatozoa of dogfish (Squalus), frog (Rana), parrot (Psittacus), mouse (Mus) and ape (Inuus). H, head; M, middle piece; 7", tail. The spermatozoon of the frog is about o.i mm. long. (Redrawn from Retzius.) comparable objects. The thing referred to in kitchen and market as an "egg" consists of the egg in strict sense, or ovum, plus various extraneous substances and structures. The hen's ovum, corresponding to the small egg of some fish, is merely the yellow sphere commonly called the "yolk" of the "egg," enclosed in its vitelline membrane (Fig. 28). The following data illustrate the differences in eggs in regard to size and content of yolk: Egg Amphioxus Some frogs Domestic fowl (ovum or "yolk' Approximate diameter, mm. o. I 2.0 30.0 Relative volumes 8,000 2 7 , 000 , 000 The volume of an ostrich ovum would be hundreds of millions of times greater than that of a mouse egg whose diameter is about 0.06 mm. Size of eggs is correlated primarily with the method of development. Correlation with size of body may appear when the developmental methods of the animals are similar, e.g., in reptiles and birds. CHORDATE ANATOMY '- EGG-CASE The eggs of fishes are usually relatively small, less than 5 mm. in diameter. Eggs of sharks and skates, however, contain much yolk and rival in size the eggs of birds. These large eggs are enclosed in shells consisting of a horn-like material secreted by the anterior part of the oviduct. In oviparous sharks and skates the shell is usually flat and quadrangular and has long tendrils which serve to anchor it to seaweed or other objects. (Fig. 26) The eggs of amphibians, which always contain considerable yolk, are larger than the eggs of many fishes, but smaller than the average for reptiles and birds. Eggs of various frogs range from 1.5 to 3 mm. in diameter. Eggs of large salamanders (Necturus, Cryptobranchus) are 5 or 6 mm. in diameter. The amphibian oviduct deposits upon the egg a layer of gelatinous substance which, after the egg has been extruded into the water, swells to form a thick jelly-like envelope. (Fig. 27) Reptiles and birds produce eggs con- taining an enormous amount of yolk (Figs. 28, 36). The protoplasm in these great eggs is aggregated at one spot on the surface of the egg, marking the animal pole, while the remainder of the egg is yolk nearly, if not quite, devoid of protoplasm. The local- ized protoplasm (germ-disc; Fig. 36) appears as a small white fleck on the sur- face of the yellow yolk. Before the egg is fertilized the germ-disc contains a single nucleus. These large eggs are invested by a tough vitelline membrane external to which may be more or less nutritive albumen (the "white" of a hen's egg) and an outer shell which in most reptiles is of a leathery texture, but in crocodiles, alligators and birds is highly calcified and therefore hard and brittle. Eggs of mammals, with two exceptions, are minute, containing a minimum of yolk. The exceptions are the duck-bill (Ornithorhynchus) and the spiny ant-eater (Echidna) of the Australian region. These two mammals, presumably of primitive type, lay large eggs encased in tough shells. In general these mammals are reptilian in their methods Fig. 26. — Egg-case of small shark. T, tendrils coiled around branches of a horny (gorgonian) coral. About half actual size. (Drawn from specimen in the anatomical collection of the Biological Laboratories, Harvard University.) REPRODUCTION 29 of reproduction. Otherwise mammalian eggs are of microscopic dimen- sions (0.06 to 0.3 mm. in diameter). The egg (Fig. 29) is covered by a delicate membrane (zona pellucida) external to which may be a cellular membrane (corona radiata), both contributed by the ovary. G' 'OV Fig. 27. — Amphibian eggs. A, of frog, soon after laying; B, early larva of frog, just before hatching; C, of the salamander, Cryptobranchus allegheniensis. A and C, approximately actual size; B, enlarged. G, gelatinous layer; L, larva; OV, ovum. {A and B, redrawn from Marshall, "Vertebrate Embryology"; C, after A. M. Reese.) Fertilization. Development is initiated by the "fertilization" of the egg. A spermatozoon penetrates the egg (impregnation) and the sperm chromatin becomes joined with the chromatin of the egg nucleus. nucleus of Pander neck of latebra white yolk less dense albumen yellow yolk ^ Fig. 28. — Diagram representing a section of a hen's egg cut in a plane including the long axis of the egg and passing through the blastoderm. (From Patten, "Embryology of the Chick"; after Lillie.) The "maturation" process through which all germ cells pass reduces their chromatin to approximately half that contained in body cells, so that the union of sperm chromatin and egg chromatin provides the fertilized egg with a nucleus containing the full complement of chromatic bodies (chromosomes) characteristic of all body cells of the animal. 30 CHORDATE ANATOMY Therefore the fertilized egg, although the product of two cells, possesses the mechanism of a single cell. It possesses no visible structures which would adequately account for its development into a large complex animal like the parent animals. Compared to such cells as those of muscle and nervous tissue, it is strikingly devoid of visible special mechanism. Yolk is characteristic of eggs, but yolk is an inert food sub- stance, not a mechanism. The motile and aggressive sperma- tozoon might seem to be the essen- tially "animal" body in development while the relatively large unfertilized egg, burdened with inert yolk, would appear rather as a passive and vegeta- tive thing. But in normal develop- , , ment the spermatozoon merely imparts Fig. 29. — Human ovum surrounded ^ ... by follicular cells. Actual diameter of the Stimulus which mitiates develop- ovum about 0.25 mm. C. cytoplasm ^^^^ ^^^^ provides for inheritance containing some volk; CR, corona radiata; F, follicular cells; N, nucleus; from a male parent. Expcrmienta- ZP, zona pellucida. (After Nagel.) ^^j^ ^isi^ proved that the egg is fully capable of producing a characteristic adult without the assistance of a spermatozoon. Obviously, however, such an adult inherits only from a mother. Exit of Sperm and Eggs. The sperm is usually carried by ducts which lead from the testis to the exterior, but in cyclostomes and some bony fishes it is discharged from the testes into the body-cavity and finds exit through abdominal or genital pores which pierce the body-wall. Ova are usually liberated from the surface of a solid ovary (Fig. 274) into the body-cavity whence they pass into oviducts which lead to the exterior. In cyclostomes and some bony fishes the eggs pass out through abdominal pores. In other bony fishes the ovary is hollow, eggs are liberated into its lumen and pass out by way of a duct which is an extension of the wall of the ovary. The genital ducts are usually closely associated with the duct system of the kidneys. Exceptional conditions occur in bony fishes. Oviparity, Viviparity, Impregnation. The means whereb>- ovum and spermatozoon are brought together depends on whether the animal is oviparous or viviparous; also on whether the outer envelopes of the egg can be penetrated by a spermatozoon. In most oviparous fishes the eggs are impregnated after the genital products have been discharged into the water ("external fertilization"). But oviparous sharks and skates produce eggs whose shells are impene- trable by sperm. Therefore copulation must occur and the egg must be REPRODUCTION 31 reached by the sperm before the shell is deposited. Some sharks and a few teleosts are viviparous; copulation and "internal fertihzation" are therefore necessary. Among amphibians there is much diversity. In most frogs and toads impregnation is external. In tailed amphibians (Urodela) it is commonly internal, in oviparous as well as in viviparous species, and in many cases is effected by means of a spermatophore, a mass of sperm agglutinated together by a secretion from cloacal glands of the male. The spermato- phore may be introduced into the cloaca of the female or else attached to the external surface of the female. In some cases it is merely discharged and picked up later by the female. Some reptiles are viviparous. All birds are oviparous. But in all reptiles and birds the egg-shell necessitates copulation and internal impregnation. Modern mammals, except Ornithorhynchus and Echidna, are vivi- parous. The two exceptional animals lay eggs of reptilian sort. There- fore in all mammals impregnation must be internal. In general, eggs which acquire such envelopes as a layer of albumen or a hard shell must be impregnated while in the anterior region of the oviduct and before these external coverings have been deposited. Development begins immedi- ately after fertilization. Therefore, if fertiliza- tion has actually occurred, the "egg" which is "laid" by the reptile or bird contains not an ovum but an embryo at an early stage of develop- ment. Provisions for protection, nutrition and res- piration during the period of development are most diverse. In most fishes the eggs are abandoned to the hazards of the environment. Some fishes, especially those of fresh water, arrange crude nests in gravel, sand or mud. Some fishes guard their eggs. In the sea-horse (Fig. 30) and pipe-fish, the male carries the developing eggs in a brood-pouch on the ventral drawn after Boulenger in surface of the body or tail— an arrangement Jt'to^yT^""^^' ^^'"'^^ suggestive of the marsupial pouch of a female kangaroo. The smaller fish eggs, scantily endowed with yolk, develop rapidly and soon become free-living and self-supporting while still very minute. The miniature fish then enters upon a long period concerned mainly with feeding and growth. Eggs containing larger quantities of yolk pass through a longer period of development and the young fish attains relatively large size before it is obliged to obtain food from an BROOD-POUCH- FiG. 30. — Sea-horse (Hippocampus.); male, with brood-pouch. (Re- 32 ■ • CHORDATE ANATOMY external source. The embryo and young of the viviparous fish not only receive maximum protection, but may obtain from the mother some food in addition to the initial supply of yolk. In so-called "placental" sharks the wall of the oviduct develops highly vascular folds or processes and similar folds arise on the abdominal wall of the embryo. The two sets of projecting structures, maternal and embryonic, become closely approxi- mated, thus providing for diffusion of substances from the blood of one to that of the other. Among amphibians there is, in general, better provision for protection of eggs and young than in fishes. Nests and guarding of eggs are common. Among frogs and toads occur various peculiar ways of caring for eggs and young. The male of the European "obstetric" toad carries the long strings of eggs wound about his body and legs until the tadpoles emerge. In some cases eggs are carried in the mouth or vocal pouch of the male. In the South American "marsupial" frog the eggs develop in a capacious Fig. 31. — Necturus larva of about 25 mm. length. (After Eycleshymer.) pouch formed in the skin on the back of the female. The eggs of the toad, Pipa americana, develop in individual vesicles in the skin on the back of the mother. Viviparity, affording a maximum of protection, occurs in a few amphibians, including representatives of each of the three orders, Urodela, Anura, and Gymnophiona. The amphibian egg, whether laid in the open or enclosed in some protective way, develops rapidly into a highly characteristic larva, the tadpole or "poUiwog" (Fig. 31) which, with its functional gills and locomotor tail, as well as in many features of internal anatomy, is a dis- tinctly fish-like animal and, if its environment is external water, it lives the hfe of a fish. The larval period, ranging from a few weeks in some salamanders to a year or more in some frogs, is devoted mainly to feeding and growth. It terminates in a metamorphosis in the course of which the animal acquires. the adult characteristics. The transformation is most radical in frogs and toads; legs and lungs develop, tail and gills are absorbed, gill clefts close, and other changes occur. In certain excep- tional species of frog, especially large eggs are laid on land and develop to adult form without passing through a tadpole stage. In the Urodela the changes are less marked, the tail and sometimes also the gills being retained. Adult Necturus, with its tail and functional gills, is sometimes REPRODUCTION 7,7, called a ''permanent larva." Sexual maturity is ordinarily not attained during the larval state. But the Mexican axolotyl, the larva of the sala- mander, Ambystoma tigrinum, regularly breeds in the larval state. The diversity of reproductive arrangements in amphibians is in marked contrast to the uniformity which prevails in reptiles and birds. The large yolk-mass of the eggs of these animals makes possible a long develop- mental period during which the young can attain relatively great size. A newly hatched alligator is gigantic compared to a newly hatched salmon. These circumstances, together with the fact that development takes place within a thick shell, make necessary some special provision whereby food derived from the yolk may be made accessible to all parts of the large embryo and an adequate supply of oxygen obtained from some external source. The outstanding feature of the development of the reptile or bird appears when the embryo itself goes about the business of constructing a complex system of membranes so disposed and so equipped with blood- vessels as to serve very eflEiciently not only for respiration but for some other and secondary functions. Early in development, at a time when the main organs are in process of formation (Fig. 74), the outer layer of the embryo, representing the prospective body-wall of the animal, throws up a system of folds which arch over and ultimately enclose the whole of the definitive embryo — much as if an animal should enwrap itself in a highly exaggerated fold of its own skin. Thus are formed the investing membranes known as the amnion and the chorion (serosa). The amnion is derived from the inner layer of the fold, the chorion from the outer. The amnion does not fit the embryo snugly. The intervening space is occupied by a watery solution whose chemical constitution resembles that of blood — and also resembles that of sea water. Thus the embryo during its further develop- ment is bathed by a fluid whose chemical nature is compatible with that of the embryonic tissues. Further, immersion of the embryo in watery fluid affords the best possible protection from externally caused mechanical pressures and impacts. Meanwhile the enormous yolk-mass has been enclosed (Fig. 74) by cellular layers which are prospectively the wall of the digestive tube. Then from the hinder region of the embryonic digestive tube a sac bulges out ventrally (Fig. 74) and, like a great and growing hernia, pushes beyond the ventral body-wall. Having thus attained the exterior of the embryo proper, it becomes vastly expanded (by growth) and eventually spreads out so that the greater part of its outer surface is, in conjunction with the chorion, in close relation to an extensive area of the inner surface of the egg-shell. This sac is the allantois. It becomes highly vascular, its arteries and veins communicating with the main vessels of the embryo. 34 CHORDATE ANATOMY A considerable part of the blood of the embryo is diverted into the allantoic arteries and circulates vigorously through a rich system of small vessels lying close to the inner surface of the shell. The shell is porous. Thus ready interchange of respiratory gases between the blood and the external air is provided for. The allantoic sac serves also as a. receptacle for embryonic waste. The ducts from the kidneys open into the extreme hind end of the digestive tube whence the fluid excreted by the kidneys readily passes into the cavity of the allantois. The inner cellular layer (yolk-sac; Fig. 74) immediately enclosing the yolk-mass is highly vascular and its vessels, like those of the allantois, communicate with the main arteries and veins of the embryo. The blood circulating through these vitelline vessels picks up dissolved yolk materials which are conveyed to all parts of the embryo, thus making the yolk available everywhere for metabolism and growth. In viviparous reptiles, the amnion, the allantois with its vascular system, and the yolk-sac circulation are developed as in the embryos of oviparous reptiles. The oxygen obtained by the allantoic vessels, how- ever, must be derived from the maternal blood in the wall of the oviduct. In reptiles and birds building of nests and parental care of young are much more prevalent than in fishes and amphibians, reaching high specialization and efi&ciency in birds. Correlated with the greatly increased protection afforded during development, relatively few eggs are produced. Primitive mammals, as indicated by such surviving examples as Ornithorhynchus and Echidna, must have retained reptilian methods of reproduction. The duck-bill, a burrowing animal, deposits the eggs (usually two) in the burrow. Echidna, producing usually only one egg in a season, places the egg in a fold of abdominal skin, a temporary marsupium, where it is carried and incubated by the warmth of the body until the young hatches. The embryos of these two mammals develop amnion, chorion, allantois, and allantoic and yolk-sac circulations essen- tially as do reptiles. The one new thing which these animals do is to provide the young with a convenient source of food to serve for a time immediately after hatching. Milk produced by mammary glands (see page 127) developed in and by the abdominal skin serves to prolong the period of dependence on the maternal food. All known existing mammals except the duck-bill and spiny ant-eater are viviparous. The minute eggs contain so little yolk that they could never pass beyond the very early stages of development unless additional food material were somehow provided. In the great majority of mammals this is done by means of an organ which is one of the most characteristic features of a mammal. The egg, liberated from the ovary and fertilized, becomes caught and lodged in the superficial tissue of the uterine wall. REPRODUCTION 35 Here it passes into the early phases of development and very shortly gives rise to an amnion, a chorion and an allantois, essentially similar to those structures as developed in reptiles and birds. Curiously, in spite of the absence of any considerable amount of yolk, a yolk-sac also, although devoid of yolk, is formed. This is usually interpreted as a relic of reptilian ancestors. The allantoic sac becomes greatly expanded, more or less enwrapping itself around the embryo, and certain regions of it fuse with the adjacent chorion and enter into a very peculiar relation to the uterine wall (Fig. 75). From the conjoined allantoic and chorionic membranes grow out slender extensions (villi) which penetrate more or less deeply into the adjacent uterine wall. They may become more or less branched. These villi are highly vascular, fetal blood circulating in them under the drive of the fetal heart. The surrounding uterine tissue is likewise highly vascular. There is, however, no open communication between the blood- vessels of the villi and those of the uterine wall. But the fetal and the maternal vessels are so close together that materials readily diffuse from one blood to the other. Dissolved food substances and oxygen pass from the maternal to the fetal blood; waste materials and certain special fetal substances of hormone nature pass from the fetal to the maternal blood. By means of this placenta, intervening between mother and young, the nutrition and respiration of the young animal are provided for through the usually long period of intra-uterine development. Mammals show many variations in the mode of origin and details of structure of the placenta. The marsupial mammals (Metatheria; the kangaroo and its allies) produce only a weakly developed and briefly temporary placenta or none at all. Accordingly the development of the young cannot proceed beyond what is made possible by the initial small yolk supply plus what nutritive material may be absorbed by the embryo and its investing membranes directly from the neighboring uterine tissues and fluids. The young marsupial is therefore necessarily born at an early fetal stage and while very small. The deficiency of the intra-uterine arrangements is compensated for by the marsupium, a pouch formed by a fold of abdominal skin. The mammary glands are within this pouch. The very immature and quite helpless new-born young (in the great kangaroo, Macropus major, being only about one inch long) is trans- ferred to the marsupium by the mother. The young becomes attached to one of the mammary nipples and feeds passively, the milk being pumped in by contraction of muscle about the mammary gland. This "mam- mary fetus" inhabits the marsupium for a time which is usually much longer than its period of intra-uterine development For example, in the great kangaroo the period of intra-uterine gestation is between five and six weeks, but the young kangaroo is carried in the pouch and nour- ished by mammary glands for about eight months. 36 CHORD ATE ANATOMY In placental mammals, as compared to marsupials, the young are born at a relatively advanced stage of development and growth. The mammary organs, however, are in all cases an important post-natal provision for bringing the young animal along to a degree of size and strength favorable to ultimate success. They afford the great advantage, too, that the young animal is not thrown upon the world abruptly, but may acquire independence gradually. Evolutionary Significance Surveying the whole group of vertebrates, the great diversity in the conditions and arrangements attending reproduction is most impressive. It would be difficult to imagine any practicable reproductive expedient or condition which is not exhibited by some animal. There are micro- scopic eggs and there are ostrich eggs. The quantity of yolk may be vast or it may be next to nothing. The primary food supply, yolk, may in various ways be supplemented by secondary sources of nutriment — egg albumen, maternal blood, mammary milk, pigeon "milk." One egg or millions of them may be produced at a time. They may or may not have shells. Parental care of eggs or young ranges from nothing to the human maximum. Vertebrates may be oviparous or viviparous. A primary ovi- parity may be succeeded by a secondary substitute for viviparity, as when eggs develop within a fish's mouth, an amphibian vocal sac, or integumentary pouches of various sorts. Differentiation of organs may precede growth or it may be delayed until the embryo is relatively large. The newly hatched larva of so large a fish as the Atlantic salmon is about 0.65 inch long; a new-born whalebone whale is about twenty feet long. The embryo may develop directly to the adult form or there may be a larval period terminated by a metamorphosis. The embryo may or may not produce a complex set of temporarily functional membranes — amnion, chorion, allantois. The important point to be appreciated is that the association together of any two or more of these various alternatives in a single animal is not haphazard. If one circumstance is, in itself, inadequate for the success of reproduction, it is supplemented by something else. If a large fish were to produce one single microscopic egg annually and deposit it any- where in the Pacific Ocean, the species would soon become extinct. On the other hand, there is no unnecessary duplication of highly specialized arrangements. A placental mammal does not produce a large yolky egg. The entire complex of reproductive conditions occurring in any one animal comprises a consistent grouping of alternatives such that, as a whole, it is adequate. Des[)ite the great differences in methods of reproduction, the net results are equally good, or nearly so, and generation REPRODUCTION 37 after generation the life of the world goes on with at most only very slow change in the general biological balance and scheme of things. Fishes and amphibians show this reproductive diversity most mark- edly. Assuming a genetic series from fish to bird and mammal, the evolution of reproduction has not been a direct progress along one straight and narrow path. Instead, the animals within each class, especially the lower, have tried (so to speak) a variety of methods. From the many reproductive "experiments" of the lower vertebrates finally emerge two distinct types to which the higher vertebrates fairly closely adhere. Reptiles and birds exhibit one of these types, mammals the other. Yet certain distinctive features of these finally emergent types of reproduction are anticipated by some lower vertebrates. The enormous eggs of ovip- arous sharks and skates, encased in thick shells, resemble eggs of reptiles. Some viviparous sharks produce vascular uterine structures (see page 32) suggestive of the mammalian placenta. Certain vivi- parous lizards (genus Seps) develop what is practically a placenta. But there can hardly be any direct genetic connection between these structures in sharks and the somewhat similar structures in reptiles or mammals, nor between the "placenta " of a lizard and that of a higher mammal. The exaggerated filamentous gills of the intra-uterine larvae of some viviparous salamanders and the much expanded bell-shaped gills of the larvae of the "marsupial" frog, Gastrotheca, suggest that the larva may obtain nutriment as well as oxygen from neighboring maternal sources — prac- tically a "branchial placenta." The marsupial structures of vertebrates afford another example of convergence in evolution — that is, the independent origin of functionally similar but genetically unrelated things. Defining a marsupium as a brood-pouch developed on the external surface of the body-wall, there are marsupial fishes (sea-horse; pipe-fish), marsupial frogs and marsupial mammals. Viviparity is commonly thought of as something peculiarly mam- malian. Yet there are viviparous fishes, amphibians and reptiles. The only vertebrate class which contains no viviparous members is Aves. In view of the fact that all birds and the most primitive mammals that we know are oviparous and the further fact that oviparity predominates among the lower classes of vertebrates, it is highly probable that the earliest vertebrates were oviparous and that the animals which con- stituted the main trunk of the vertebrate genealogical tree were oviparous. But viviparity has appeared on twigs of various lower branches of the tree as well as at its mammalian top. The chordate ancestors of vertebrates must have been small animals and presumably produced small eggs with little yolk. It is likely that primitive vertebrates had small eggs and that large yolk masses have 38 CHORDATE ANATOMY been secondarily acquired. But even within a small group of vertebrates the yolk content of eggs may be highly variable, being apparently easil}- susceptible to evolutionary change. In point of size and yolk content the vertebrate egg has evidently had many ups and downs. In spite of the diversity of vertebrate methods of reproduction, an evolutionary trend is clearly to be seen. There is a certain extravagance about the primitive method — millions of eggs, perhaps, in a season, but only a small percentage of survival. The evolutionary tendency has been, by introduction of efBcient protective, nutritive and respiratory arrangements, together with parental care, toward the guarantee of the survival of every potential adult. This tendency bifurcates and culminates in two very differently specialized methods, one in birds, the other in mammals. Unquestionably the high degree of efficiency which has been attained by the sauropsidan method of reproduction and also by placental reproduction in mammals is some- how correlated with the necessity of adaptation to the circumstances of living on land and in air. The primitive fish methods would obviously be impracticable. An aquatic larval stage in the development of a horse or an elephant can hardly be imagined although, developing as it does in the fluid-filled amnion, the terrestrial descendant of ancient aquatic ancestors does spend its early life in a fluid medium. With increase in chance of survival there is reduction in number of eggs produced. This result has the appearance of achieving economy but there is perhaps room for question as to just how and where the economy comes in. Does it cost a cod any more to produce seven million eggs than it costs a viviparous dogfish to bear four or five large "pups"? By either method of reproduction the numerical status of the species may be maintained and so, as remarked above, the net results of the two methods are equally good. DEVELOPMENT Cleavage and Blastula Development involves great protoplasmic activity. There must be a building up of new protoplasm, rapid dividing of cells, movement and change of form. All of this calls for rapid metabolism. Metabolism requires inter-action of nuclear material and cytoplasm and exchange of materials between the protoplasm and the external medium. The area of the nuclear membrane and area of the external surface of the cell therefore impose a limit on metabolic rate. Two cells are capable of more rapid metabolism than one cell whose nuclear and cytoplasmic volumes are respectively equal to the combined volumes of the cor- responding parts of the two cells because the limiting membranes of the two cells have greater total area than those of the single cell. REPRODUCTION 39 'I'he smallest egg cells are large compared to most tissue cells of the animal to which the egg belongs. The metabolic rate in an egg before fertilization is relatively low. After fertilization the rate increases. Before entering upon a prolonged period of activity at high metabolic rate the bulky ovum increases its surfaces by dividing into small cells — the process called cleavage. The successive divisions of the original egg nucleus are, in fact, accompanied by absolute increase in the quantity of nuclear chromatin, a substance which ^^ undoubtedly plays an important part in determining the course of develop- ment. In Amphioxus. Amphioxus is not literally a vertebrate. But it is a chordate and in many respects obvi- ously primitive. The adult is a slender fish-like animal about 5 cm. long (Fig. 11). The egg is correspond- ingly small, about o.i mm. in diam- eter, and contains very little yolk. (Fig. 32) The plane of the first cleavage (Fig. 33) of the egg corresponds to the definitive median (sagittal) plane of the future adult. The two cells re- VEG. Fig. 32. — Median section of a ferti- lized egg of AMPHIOXUS. Diameter of egg about 0.1 mm. ^ TV, animal pole; suiting from the first cleavage there- iV, male and female pronuclei ;P, polar r J. J.I. • Ui J 1 rj. body; S, remnant of spermatozoon; fore represent the right and left y^^, vegetal pole; Y, region of cyto- halves of the body. The plane of the plasm occupied by coarse granules of J , . J- ^ J. yolk. (After Cerfontaine.) second cleavage is perpendicular to that of the first and the third cleavage plane is perpendicular to both the first and second. The second and third cleavages each divide the egg slightly unequally. Further cleavages follow one another in rapid succession, their planes adhering to a fairly rigidly determined order. Meanwhile the cells gradually shift their relative positions and surfaces of contact in such a way that a space opens out at the center of the whole mass. At the thirty-two cell stage the cells are disposed to form a hollow sphere whose wall is everywhere one cell in thickness. Thus every cell of the thirty-two is in direct relation to the exterior, a most favorable position for respiration and excretion. This hollow spherical shape is retained as cleavage continues (Fig. t^^ G-I) until between two hundred and three hundred cells have been formed. This stage of the embryo is called the blastula. The name, blastocoele, is applied to the cavity. The second and third cleavages introduce inequality of size among the resulting cells. This inequality persists as cleavage goes on. It is 40 CHORDATE ANATOMY correlated with the distribution of yolk in the protoplasm, the larger cells containing the more yolk. The cells of the blastula grade from PIGMENT NUCLEUS Fig. 33. — Cleavage of egg of AMPHIOXUS. A , undivided egg; B, in process of first cleavage; C, four-cell stage, lateral view; D, four-cell stage, polar view; E, eight-cell stage, lateral view; F, sixteen-cell stage, lateral view; G, eighty-eight cells, lateral view; H. same stage as G, median section; /, later stage, lateral view. P, polar body. (After Hatschek.) minimum size at one pole (animal) of the sphere to maximum size at the pQi_^ opposite pole (vegetal). This polarity is established in the egg before cleavage begins. In Amphibians. Some amphi- bian eggs (not including the gela- tinous envelope) are about 2 mm. in diameter. Such an egg would possess a volume about eight thousand times that of an egg of Amphioxus. The greater part of the increased bulk is yolk. The egg (Fig. 34) is strongly polarized with reference to the distribution The of the volk in the protoplasm. Development of the Frog's Egg"; The From the animal polc where yolk is Macmiiian Co.) ^^ ^ minimum the quantity in- creases toward the opposite vegetal pole where the maximum occurs. VEGETAL POLE Fig. 34. — Ovarian egg of frog; median section. (Redrawn from Morgan REPRODUCTION 41 Yolk is a non-living, quite inert substance. The active material in development is protoplasm. The developmental behavior of eggs containing much yolk shows quite clearly that the yolk is an impediment to the free carrying out of developmental operations — just as the necessity of carrying a heavy burden of supplies may impede the progress of a company of explorers. Figure 35 represents the cleavage stages of a frog's egg. The successive divisions follow the same general order as in Amphioxus. Cleavages ANIMAL POLE BLASTOCOELE Fig. 35. — Cleavage of the frog's egg. A, first cleavage in process; B, two cells; C, eight cells; D, fourth cleavage complete in animal hemisphere but just beginning in the four cells at the vegetal pole; E, early blastula, median section; F, G, successively- later stages, lateral view. {D, F, G, redrawn from Morgan, "The Development of the Frog's Egg"; E, redrawn from Marshall, "Vertebrate Embryology.") succeed one another at intervals of about an hour, but the period varies with temperature. The yolk evidently hinders cleavage, especially in the vegetal hemisphere. The second cleavage begins at the animal pole before the first is completed at the vegetal pole. In fact, the third cleavage may begin while both first and second are still incomplete in the region of the vegetal pole. Further, the inequality in size of cells at animal and vegetal poles is much greater than in Amphioxus, another consequence of the greater yolk mass. After the third cleavage a cavity appears in the midst of the group of eight cells. As cleavages proceed this cavity enlarges and the embryo, 42 CHORDATE ANATOMY as in Amphioxus, becomes a hollow sphere or blastula (Fig. 35£). Its cavity (blastocoele) is excentric, occupying approximately the animal hemisphere only. Its wall is more than one cell thick. The great thick- ^,T-,^,«*w,-., ness of the wall of the vegetal hemi- Y' sphere and the consequent excen- tricity of the blastocele are obviously due to the yolk. In Reptiles and Birds. In eggs whose yolk-mass greatly exceeds that of the amphibian egg all the proto- plasm is segregated into a thin plate, the germ-disc, lying on the surface of the relatively enormous mass of yolk (Fig. 36). In such an egg, obviously, there is no mechanism for dividing the yolk. Cleavage is Fig. 36. — Cleavage of the germ-disc of the egg of a turtle (Glyptemys insculpta) ; eight-cell stage. The egg-shell is not shown. About twice natural size. A, r 1 ^ ^i ^1 r xi. albumen; C, the eight-cell blastoderm; Confined tO the protoplasm Ot the F, yolk. (Redrawn from Louis Agassiz, ggrm-disc which, following fertiliza- " Embryology of the Turtle.") ... , ,.^ .,, tion of Its nucleus, sphts up rapidly and soon consists of hundreds of small cells forming what is then called the blastoderm lying as a thin plate of cells on the surface of the yolk (Figs. 36 and 37). But there is continuity of blastoderm with yolk only around the periphery of the blastoderm. Elsewhere a thin space, the sub- germinal cavity, intervenes between blastoderm and yolk (Fig. 37). lY" lY' Fig. 37. — Early blastoderm of chick; plane of section passes through center of egg. B, blastocoele (subgerminal or cleavage cavity); C, cells of blastoderm; V, fluid- filled vesicles; F^ yellow yolk, F^ white yolk. Magnified nearly twenty diameters- (Redrawn from Duval, "Atlas d'Embryologie.") Comparing this embryo with the blastula stages of Amphioxus and frog, it seems reasonable to interpret it as a blastula whose blastocoele is the subgerminal cavity, while its blastoderm is the animal region and the yolk-mass is the vegetal region of the embryo. This recognition of a blastula stage, comparable to that of Amphioxus, in the development of a reptile or bird would hardly have been possible but for the intermediate condition exhibited by the amphibian with its moderate yolk-mass and total cleavage. REPRODUCTION 43 The blastula is an essentially one-layer stage of the embryo, the "layer" being the wall of the blastula, whether one cell thick or more than one cell thick. This stage has two-fold significance. Its immediate importance is that it gives the embryonic material increased superficial contact with the environment, thus favoring metaboHsm. Its prospective significance lies in the fact that further development is to consist, to a large extent, in the manipulation of layers of embryonic material. The adult is hollow. It has a body-cavity and other cavities. Most of its organs are hollow. The walls of the hollow structures are constituted of layers — skin, epithelium, endothelium, peritoneum, muscle layers, connective tissue layers. For the construction of such a many-layered thing, the embryo naturally proceeds as early as possible to dispose its building material in the form of layers. Gastrula In Amphioxus. The blastula stage is briefly transitory. At once changes set in which transform it to a two-layered embryo. In Amphioxus the two-layered gastrula form is attained in a very simple way (Fig. 38). The vegetal hemisphere first flattens, then becomes curved inward. The infolding (invagination) continues until the material of the original vegetal hemisphere comes into close relation with the inner surface of the wall of the animal hemisphere. The spherical blastula thus becomes an approx- imately hemispherical embryo whose wall is two layers thick (Fig. 38C). As the process goes on the blastocoele is reduced and finally obliterated. The gastrula is hollow. Its cavity, resulting from the invagination process, at first opens widely to the exterior but the width of the opening is rapidly diminished by inbending of the wall about it and it is soon reduced to a narrow blastopore. In consequence of this contraction of the wall around the blastopore, the form of the entire gastrula tends at first to become spherical, but before the contraction is completed the gastrula begins to elongate in the direction of the axis which passes through the blastopore. An important accessory activity attends this process of narrowing the blastopore. The blastoporal rim is a region of transition from the outer to the inner layer. This region is marked by very rapid proliferation of cells, especially at the dorsal edge of the blastopore (Fig. 38!)) . Cells produced within this growth zone or germ-ring are added, some to the outer layer and some to the inner layer. This growth process, then, is concerned both in the narrowing of the blastopore and the elongating of the embryo. A direct consequence of it is that the material of a certain region of the inner layer immediately adjoining the blastopore attained its internal position not as result of the primary invagination but by the secondary growth process. 44 CHORDATE ANATOMY At the close of the gastrula period (Fig. 38D) the embryo is an elongated ovoid, the slightly larger end being anterior while the now very narrow blastopore marks the posterior end of the long axis. So rapid is development that this stage is attained about seven hours after fertilization. Significance of the Gastrula. The gastrula is the animal in its bare essentials. The outer layer, ectoderm, is potentially protective and ANIMAL POLE I VEGETAL POLE EC\ Fig. 38. — Gastrulation in AMPHIOXUS. The figures represent sections through the polar axis of the embryo. A, blastula with vegetal region flattened; B and C, earlier and later stages of invagination of vegetal hemisphere; D, gastrulation completed; with elongation of the gastrula, its long axis becomes the horizontal antero-posterior axis of the embryo. A, archenteron; B, blastocoele; BP, blastopore- EC, ectoderm; EN, endoderm; P, polar body. (After Cerfontaine.) nervous. It gives rise to the essential outer part of the adult skin, which produces so many important protective structures, and to the whole nervous system, both peripheral and central. The inner layer, endoderm, is nutritive. The cavity within it is the primary digestive cavity or archenteron. It is significant that the wall of the archenteron is derived from the vegetal hemisphere of the blastula. Thus, appropriately, the greater quantity of yolk comes to lie in the lining of the embryonic diges- tive cavity. In the vertebrates the blastopore never becomes mouth and REPRODUCTION 45 rarely becomes anus. The future motor mechanism, muscle, is derived indirectly from the gastrula layers. The gastrula is strongly suggestive of the two-layer body plan of a coelenterate. A simple coelenterate such as Hydra, two-layered through- out, including even the tentacles, can be regarded as a somewhat elab- orated gastrula, the Hydra "mouth" corresponding to the blastopore (Fig. 39). This resemblance, together with the fact that a gastrula stage, modified in one way or another, occurs nearly universally in the develop- ment of metazoan animals, gave rise to Ernst Haeckel's "gastraea" theory which proposed that gastrula-like animals (essentially coelen- terates) must have been the ancestors of all Metazoa. According to this theory, the occurrence of the gastrula form in the ontogeny of a Fig. 39. — Diagrams showing structural similarity of a coelenterate and a gastrula. A, Hydra, longitudinal section; B, gastrula, axial section. A, archenteron, prospective digestive cavity; BP, blastopore'; E, enteric (digestive) cavity; EC, ectoderm; £ A'', endoderm; M, mouth; T, tentacle. vertebrate is a "repetition" of the coelenterate stage in phylogeny. This may very well be true but it is not necessary to hold this view in order to account for the gastrula stage in ontogeny for some such form as the gastrula is the necessary precursor of any adult metazoan which has a skin (ectoderm) and a digestive tube (endoderm). In Amphibians. In the amphibian the vegetal wall of the blastula (Fig. 7,$E-G) is so thick that the vegetal hemisphere is, in. effect, solid. It consists of large cells heavily laden with inert yolk. Such a wall cannot readily bend inward as does the corresponding thin and labile layer of the Amphioxus blastula. In the amphibian three processes going on simultaneously effect gastrulation. The beginning of gastrulation is seen when a crescent- shaped groove (Fig. 40 A, /) forms at a certain place on the surface of the blastula. It lies just on the vegetal side of the equator determined by the animal and vegetal poles and extends transversely to the median 46 CHORDATE ANATOMY VEG Fig. 40. — Gastrulation in the frog, stages; viewed toward the vegetal pole. A, B, C, the whole embryo at successively later A', B' and C represent, in somewhat diagram- matic way, sections of corresponding stages cut in the plane including the polar axis and bisecting the gastrular invagination, /; this plane corresponds to the median plane of the adult. During the latter part of the period of gastrulation, as result of shifting of the heavy yolk (compare B' and C), the embryo rotates so that the axis passing through the blastopore {BP) becomes horizontal (see Fig. 44-4). A, arohenteron; AN, animal pole; B, blastocoele; BP, blastopore; EC, ectoderm, EN, endoderm; 1, invagination; NP, neural plate; VEG, vegetal pole; Y, yolk; YP, yolk plug. REPRODUCTION 47 plane determined by the first cleavage. The equator and a zone extending superficially somewhat into the vegetal hemisj)her(' are marked by espe- cially rapid cell-proliferation. It is in this ])articularly active region, the germ-ring, that the groove appears. Figure 40.1' represents a section in the median plane of an embryo at this stage. The groove (/) is the result of an invagination which occurs near where the upper thin wall and lower thick wall of the blastula join. The outer layer bounding the invagination consists of smaller cells which have moved inward from the superficial germ-ring region; the deeper wall of the invagination consists of yolk cells. The groove, initiated as a slight invagination, rapidly deepens (Fig. 40B-B'), not by continued invagination, but by active growth of the upper (for later events prove it to be dorsal) lip of the groove —that is, the lip resulting from the infolding of germ-ring material. This growth process serves to build out the dorsal lip of the original invagination so that the fold is caused to extend farther and farther downward over the yolk cells. Meanwhile the groove, originally a short crescent as seen on the surface of the blastula, lengthens laterally or in the direction of the curve of the crescent (Fig. 40-B) until it describes a semicircle and, con- tinuing, finally completes a circle. As the groove progressively lengthens, the newly arisen region of its outer fold, continuous with the "dorsal lip " of the initial region of the groove, grows centripetally over the surface of the yolk cells. Therefore the radius of the curve described by the groove is ever decreasing. The groove is obviously deepest at the region where it began to form and shallower in the successively newer parts of it. Having completed the circle, the centripetal growth of the outer fold of the groove continues until the original vegetal hemisphere is completely covered except for a small aperture through which bulges a mass of yolk cells, the so-called yolk plug (Fig. 40C-C'). The result of the processes just described is the formation of a new cavity in the embryo. This cavity is bounded externally by the two layers of the overgrowing fold, internally by the yolk cells. It potentially opens to the exterior but its actual opening is partly blocked by the yolk plug. If no process other than those already mentioned were involved the cavity would be exceedingly thin. It is, in fact, greatly enlarged by another process. During the progress of the overgrowth of the vegetal hemisphere, the large yolk cells become extensively rearranged. They move into the blastocoele, finally practically obliterating it. They carry out this movement in such a way that the space left vacant by them is added to the cavity formed by invagination and overgrowth. Figure 40C' represents a median section of a frog embryo at the close of gastrulation. The embryo is two-layered throughout. The outer layer, ectoderm, is uniformly thin. The inner layer, endoderm, is very thin over approximately the dorsal half of the embryo but thick in the 48 CHORDATE ANATOMY ventral region where the greater part of the original mass of yolk cells persists. The endoderm surrounds a capacious cavity, the archenteron, whose external opening, the blastopore, is occupied by the yolk plug. The blastopore marks the posterior end of the embryo. The greater part of the original yolk is now in the endoderm. The difference between gastrulation in Amphioxus and that in the amphibian is essentially this: in Amphioxus the vegetal hemisphere (prospective endoderm) of the blastula actively moves into the interior of the embryo; in the amphibian the eventual interior position of the endoderm material is due mainly to the enclosing of the yolk-mass by overgrowth (epiboly) carried out by the fold which was initiated by invagination. In Amphioxus the endoderm goes inside; in the amphibian it is put inside by being covered over. Quite clearly the difference is the necessary consequence of the presence of the great mass of inert yolk in the amphibian blastula. Fig. 41. — Gastrulation in the pigeon. Section approximately median, showing formation of endoderm by invagination at posterior edge of blastoderm. A, archen- teron; B, blastocoele (cleavage cavity); BP, blastopore; EC, ectoderm; EN, endo- derm; V, vitelline membrane; Y, yolk. Magnified about 100 diameters. (After J. T. Patterson.) In Reptiles and Birds. A reptilian or avian embryo whose yolk-mass may be millions of times that of Amphioxus could hardly be expected to carry out a process of gastrulation similar to that of Amphioxus — if, indeed, anything comparable to gastrulation were to be recognized at all. Yet the original single layer of the blastoderm, formed by cleavage (Figs. 36, 37), must somehow give rise to additional layers. The fact is that the blastoderm does at an early period become two-layered. The details of the mode of origin of the second layer differ considerably in various members of the Sauropsida. The significant fact is that the deeper layer (endoderm) results, in part if not entirely, from an inward movement of blastoderm cells at the median region of what proves to be the posterior edge of the blastoderm (Fig. 41). This inward movement may consist in the formation of a small pit, an actual invagination, from whose bottom cells move forward and laterally underneath the original blastodermic layer to become the endoderm. In other cases there is merely an in-turning of the mid-posterior edge of the blastoderm without formation of a complete pocket or invagination. In either case the process is confined to the mid-posterior region of the edge of the blastoderm. REPRODUCTION 49 The endoderm, thus initiated, rapidly spreads over the yolk-mass and under the original layer which is now identified as the ectoderm. The growth of the endoderm may be augmented by cells which become detached from the under surface of the outer layer. It is noteworthy that the place of origin of the endoderm in the saurop- sidan embryo is always at the posterior edge of the blastoderm. If the primary blastoderm is to be regarded as corresponding to the animal hemisphere and the yolk-mass to the vegetal hemisphere of the amphibian embryo, then the formation of endoderm in the sauropsidan embryo begins at a point which corresponds very closely to the position of the primary gastrular invagination in the amphibian (Fig. 40,!', /). This fact, together with later events in the sauropsidan embryo, justifies the application of the term, blastopore, to the aperture of the little invagina- tion or the slit formed by infolding of the hind edge of the blastoderm. Comparisons. Comparison of the early development of Amphioxus, amphibian and reptile or bird compels the conclusion that, were it not for difference in volume of yolk, the several embryos would be practically alike in form, at least through the gastrula stage. It is as if the embryo with the larger yolk mass " tried " to behave like the embryo of Amphioxus but is compelled by the yolk to modify its behavior. Amphioxus with total and nearly equal cleavage; the amphibian with total but very unequal cleavage; the reptile or bird with partial cleavage; the several embryos at corresponding stages exhibiting radical differences in the configuration of their materials — ^yet analysis of the processes concerned in the develop- ment of all these animals reveals a basic similarity. The actual animal is the protoplasm. Developmental processes are its dynamic expression. Yolk, although necessary, is mere inert luggage. In all these animals its composition is essentially the same. The similarities which exist in spite of variation in yolk volume are certainly much more significant than the differences which exist because of variation in yolk volume. The method whereby the sauropsidan embryo achieves a two-layered condition is not the simplest imaginable. The easy and direct way would consist in the splitting of the original blastoderm to form two layers, an inner and an outer. Such splitting or " delamination " of layers commonly occurs at other stages in development. The fact that the sauropsidan embryo initiates endoderm formation by invagination or infolding at the posterior edge of the blastoderm is open to no better explanation than that there is some necessity of adhering as closely as possible to the developmental methods employed by amphibians and Amphioxus. Such necessity can come only through inheritance. The Third Layer, Mesoderm The greater extent of the ectoderm of the embryo persists as the essential layer, epidermis, of the adult skin. The endoderm gives rise 50 CHORDATE ANATOMY directly to the lining epithelium of the adult digestive tube. But in the adult animal a great complex of structures — muscle, skeleton, central nervous organs, lungs, liver, and the reproductive, excretory and cir- culatory organs, making up the greater part of the bulk and weight of the animal — intervenes between the epidermis and the endodermal digestive epithelium. Some of these intermediate organs take origin directly and independently from the primary ectoderm or endoderm. For example, before the close of the gastrula stage the central nervous organs begin to differentiate from the dorsal ectoderm. Later, lungs, liver and pancreas arise as separate localized outgrowths from the endoderm of the early digestive tube. Others of the intermediate organs have an indirect relation to the primary layers of the gastrula. The close of the gastrula stage is marked by the formation of a layer, or system of layers, of embry- onic material which comes to be interpolated between the outer and inner layers of the gastrula. This middle and third layer, the mesoderm, spreads extensively between the primary layers and at first appears to be quite undifferentiated throughout. Later it undergoes local differentia- tion to form muscle, skeleton, kidneys, circulatory organs and various other structures. In Amphioxus. At the close of the gastrula stage the Amphioxus embryo is approximately ovoidal, the long axis antero-posterior with the blastopore at its posterior end. The dorsal surface of the embryo is somewhat flattened. Figure 38Z) shows a sagittal section of the embryo at this stage. Figure 42.4 shows a section cutting the embryo trans- versely and within the anterior third of its length. Except for the dorsal flattening, the configuration of layers is as simple as possible. Figures B-G show transverse sections at stages successively later than that of Fig. 42/I. Several things are happening simultaneously. A broad band of dorsal ectoderm {NP), slightly thicker than the adjacent regions of the layer, becomes separated, along its right and left edges, from the neighbor- ing ectoderm. This process involves the mid-dorsal ectoderm con- tinuously from the blastopore almost to the anterior end of the embryo. The median ectoderm thus delimited from the lateral ectoderm is the material of the prospective central nervous organ, the neural tube. In this initial stage it is called the neural (or medullary) plate. The dorsal endoderm is at first flattened in conformity with the neural ectoderm but later (Fig. 42D-F) it becomes convoluted along three lines extending lengthwise of the embryo. Its median slightly thicker region becomes sharply folded upward. On either side of this median fold a longitudinal groove appears on the inner surface of the endo- derm. Then the endoderm in the region of each of these grooves assumes the form of a fold extending outward dorso-laterally. Thus arise three folds, one median and a lateral pair, all convex outward, and extending REPRODUCTION 51 .NP Pig. 42. — AMPHIOXUS. Transverse sections of embryos at successively later stages, showing origin of notochord, neural tube and mesoderm. A, section somewhat anterior to the middle of the length of an embryo slightly older than that represented in Fig. 38Z?. E, from embryo having two pairs of mesodermal pouches. G, section near the middle of the length of an embryo having nine pairs of mesodermal pouches. A, archenteron; EC, ectoderm; EN, endoderm; MES, mesoderm; NC, notochord; NP, neural plate; NT, neural tube. (After Cerfontaine.) 52 CHORDATE ANATOMY nearly the whole length of the embryo. As time goes on these folds become more emphasized, but soon a difiference arises between the median fold and the lateral folds. The median fold remains continuous through- out its entire length. The lateral folds, however, become interrupted by the formation of sharp deep transverse folds which cut from above downward through each lateral fold. This process of subdivision or segmentation begins near the anterior ends of the lateral folds. Its immediate result is a pair of approximately globular pouches lying sym- metrically either side of the median fold, each pouch having a small central cavity opening by a narrow passage into the archenteron. Later this passage is closed and then the pouch becomes detached (Fig. 42F) ^MES 1-6^ EC Fig. 43. — AMPHIOXUS. Frontal (horizontal) section of an embryo having six pairs of mesodermal somites. The section is through the notochord and just below the blastopore. At the posterior end of the section may be seen a region where the notochord, endoderm and mesoderm merge indistinguishably. A, archenteron near the blastopore; EC, ectoderm; EN, endoderm; MES i-6, mesodermal somites; NC, notochord. (After Cerfontaine.) from the archenteric wall which, at the place where the pouch had formed, closes so that nothing is left to mark the spot. Immediately behind each pouch of the first pair another similar pouch forms exactly as the first did. At this stage of development, marked by the presence of two pairs of these pouches, the embryo escapes from the egg membrane ("hatches"). The period between fertilization and hatching varies considerably, its average being probably not far from twelve hours. These two pairs of pouches derived from the dorso-lateral endodermal wall of the archenteron constitute the first definitely delimited mesodermal material. The remainder of the dorso-lateral folds, extending back to the blastoporal region, is destined to give rise, after hatching, to additional mesodermal pouches. The median endodermal fold, which has remained intact during this process of segmentation of the lateral folds, is the material of the future notochord. (Fig. 42, .VC) REPRODUCTION 55 At the time of hatching, then, the embryo has made important progress beyond the gastrula stage. ' Not only has the segregation of mesoderm begun but two important organs, the central nerve tube and the notochord are indicated. After hatching, additional pairs of mesodermal segments are cut off from the lateral mesodermal folds, the addition taking place progressively from anterior to posterior, until a total of ordinarily fourteen pairs have been produced. In several of the more posterior segments cavities do not occur, the mesodermal folds merely breaking up into a succession of solid blocks of cells. (Fig. 42G) In the formation of these fourteen pairs of mesodermal pouches the material of the original mesodermal folds is completely utilized. NP\ ,NC Fig. 44. — Sectiuns of an amphibian embryo at an early stage in the development of the notochord and mesoderm. Semi-diagrammatic. A , median longitudinal section. B, transverse section near the middle of the longitudinal axis. A, archenteron; BP. blastopore; EC, ectoderm; EN, endoderm; MES, mesoderm; NC, notochord; NP, neural plate. Later the series of segments is extended backward by addition of successive solid blocks of cells which become detached from the growth zone encircling the blastopore (Fig. 43). By this means the number of pairs of mesoder- mal segments is increased to the adult total, usually sixty-one. In Amphibians. In amphibians, as in Amphioxus, the blastoporal rim or germ-ring is the all-important source of mesoderm. The amphib- ian, however, gives little evidence of anything comparable to the paired mesodermal pouches which push out from the dorso-lateral endoderm of Amphioxus. During the process of gastrulation in the amphibian the material destined to become mesoderm lies within the advancing edge of the over- growing fold (Fig. 40) which is the chief agency in the enclosing of the yolk. As the edge of this fold, the narrowing blastoporal rim, advances, it (in effect) leaves behind it — "behind" being anterior because the fold advances posteriorly — a trail of potential mesoderm which, however, is at first in no way distinguishable from other material destined to be 54 CHORDATE ANATOMY permanently endoderm (Fig. 4.0C, EN). That is, the two materials together and in no way delimited from one another constitute the deeper layer of the overgrowing fold. Later this layer virtually splits (the process called delamination) to form two layers, an inner one abutting on the archenteric cavity and an outer one which is then recognizable as a definite mesoderm (Fig. 44B). This layer, although now distinct from the endoderm which parallels it, for a time retains continuity with its source, the proliferation zone about the blastopore (Fig. 44-4). Initiated in this way, the mesoderm extends into the lateral and anterior regions of the embryo partly by growth within itself, partly by continued con- tributions from the blastoporal growth zone and possibly augmented by the detachment of cells from neighboring surfaces of the endoderm. The mesoderm of Amphioxus is segmented at the time of its detach- ment from the primary gastrular layers and some of the more anterior segments are hollow. The amphibian mesoderm is primarily unseg- mented and solid. In view of the fact that it later acquires segmentation and hollow- ness these initial differences are outweighed by the essential similarity in the relations to the blastoporal region. In Reptiles and Birds. In reptiles and birds endoderm is initiated by a small invagination or infolding at the posterior edge of the early blastoderm (see page 48). The abortive blastopore thus produced exhibits the usual feature of a blastopore in that, in terms of germ layers, it is an in- different region where ectoderm and endoderm merge together without sharp demarcation (Fig. 41). Following gastru- lation the blastodermal lavers continue to ■t~-o ^A-V V , iit2c p -y^_^ *■ ■J' Fig 45 — Surface view of blastoderm of chick after 15 hours incubation. C, "anterior cres- cent," occasioned by an irregular fold of underlying endoderm; M, region occupied by mesoderm; spread rapidly over the surface of the yolk. the blastoderm; P, area pellucida — transparent in absence of ad- hering yolk (see Fig. 41); PS, primitive streak. X 14. (After Duval, "Atlas d'Embryologie.") O, area opaca whose opacity is , • 1 1 • 1 caused by adherence of yolk to In SO domg, the growth posteriorly causes the somewhat thickened region of the blas- toporal rim to become drawn out into a long streak, the primitive streak, lying in the median line of the blastoderm (Fig. 45). Along the whole extent of this modified blastoporal region the ectoderm and endoderm merge without sharp demarcation just as they did in the earlier blastoporal walls (Fig. 46). This primitive streak is the primary seat of mesoderm formation. Rapid proliferation of cells within the substance of the thickened streak gives rise to masses of cells which move out into the space between ecto- REPRODUCTION 55 derm and endoderm (Fig. 46, MES). These masses of cells increase by continued contribution from the streak and by growth within themselves and soon become arranged in a layer which rapidly grows laterally and forward from the primitive streak and always in the space between ectoderm and endoderm. This layer, like the early mesoderm of amphib- ians, is at first unsegmented and devoid of cavity. Y- -^.'r^r^,**,' Fig. 46. — Section transverse to the primitive streak of a chick embryo of about 15 hours incubation. The section is taken near the middle of the length of the streak. EC, ectoderm; EN, endoderm; MES, mesoderm; PG, primitive groove of primitive streak; y, yolk at inner margin of area opaca. X 100. (After Duval, "Atlas d'Embryologie.") In the sauropsidan embryo, then, as in the amphibian, rapid growth and cell proliferation within the blastoporal rim is the primary source of mesoderm. Early Development in Placental Mammals The early development of placental mammals exhibits features peculiar to the group and more or less difficult of comparison with any- thing in the development of lower vertebrates. The minute egg (Fig. 29) CV_- A. Fig. 47. — Early stages in development of a rabbit. A, morula stage, 47 hours after coitus; B, early blastodermic vesicle, 80 hours; C, blastodermic vesicle at 83 hours. The investing layers of the einbryo are not shown. CV, cavity of blastodermic vesicle; 7, inner cell-mass; T, trophoblast. Magnified about 285 diameters. (After Assheton.) contains a bare minimum of yolk. Cleavage is total, more or less unequal and often very irregular in respect of planes and sizes of cells (Fig. 47.-I). The cells resulting from cleavage remain in a solid cluster, the morula, until as many as sixty or seventy cells are present. Then, as the number increases further, a cavity appears within the morula (Fig. ^jB-C). Most of the cells remain in a solid group at one side of the cavity whose wall elsewhere is only one cell thick. At this stage the embryo looks like a blastula, but further development proves that the stage is not the CHORDATE ANATOMY equivalent of a blastula of a lower vertebrate. The term, blastodermic vesicle, is applied to this stage of the mammalian embryo. The definitive embryo is developed entirely from the thick cell-mass of the vesicle. The thin region (trophoblast Fig. 47, T) of the wall of the vesicle becomes concerned with the early attachment of the embryo to the wall of the uterus. The fluid-filled cavity of the blastodermic vesicle rapidly enlarges and meanwhile the thick cell-mass splits off a thin layer adjoining the cavity (Fig. 48). This inner sheet of the thick mass then extends over the inner surface of the thin wall of the vesicle and ordinarily completely lines it. The vesicle as a whole thereby becomes two-layered throughout, a condition which characterizes a gastrula stage. The fur- S - Fig. 48. — Early stage of the Fig. 49. — Embryonic area or "shield" of the blastodermic vesicle of the hedge- blastodermic , vesicle of the rabbit after about hog. EC, ectoderm; EN, endo- 172 hours development. PS, primitive streak- derm; L, lacunae, spaces occupied 5-5. position of section represented in Fig. 50. by maternal blood; T, trophoblast (After Assheton.) (trophoderm). (After Hubrecht.) ther history of the two layers identifies them as embryonic ectoderm and endoderm. However, both in mode of origin and in further history the mammalian embryo at this stage shows perplexing discrepancies as com- pared to the gastrula of a lower vertebrate. As stated above, the material which constitutes the definitive embryo is within the thick and solid cell-mass (Fig. 47, /) of the early blastodermic vesicle. As development proceeds the behavior of this cell-mass is very much like that of the blastoderm of the embryo of a reptile or bird. If the cavity of the vesicle were occupied by yolk instead of b>- a watery fiuid the whole embryonic complex would resemble closely an early reptilian embryo. The thick cell-mass, lying in relation to the vesicular cavity much as the reptilian blastoderm lies upon the surface of the yolk, flattens and thins out to form the embryonic shield (Fig. 4q) in the axis of which appears an elongated thickening similar to the primitive streak KEPKUDUCTION 57 of a sauropsidan embryo. At the anterior end of this mammalian streak is usually found a small pit or even a perforation extending through the shield into the cavity of the blastodermic vesicle — very suggestive of an abortive blastopore. It is along this mammalian primitive streak, as in the similar sauropsidan structure, that rapid proliferation of cells produces a mesoderm (Fig. 50) which progressively interpolates itself between the already separated ectoderm and endoderm and spreads eventually into all regions of the embryo. The mesoderm is at first a continuous layer — unsegmented — and devoid of cavity. VMES 'en Fig. 50. — Transverse section of the embryonic shield of a rabbit at the stage repre- sented in Fig. 49. The section is taken at the position indicated by the line 5-5 in Fig. 49. EC, ectoderm; EN, endoderm; MES, mesoderm; PG, primitive groove of primitive streak. X175. (After Assheton.) In a rabbit embryo the embryonic shield is established ordinarily by the fifth day of development, the entire blastodermic vesicle then having a diameter of about 1.5 mm. The early development of the placental mammal presents many perplexing features. It could be expected that the minute egg, unem- barrassed by yolk, would revert to the relatively simple and direct methods of early development which, for the most part, characterize Amphioxus. But it does not. Mammalian stages precisely comparable to the blastula and gastrula of Amphioxus or amphibians cannot be recognized. When it comes to the formation of mesoderm, the laying out of the germ layers, and the early shaping up of the embryo, the behavior of the mammal is closely similar to that of a reptile or bird. This similarity exists in spite of the absence of a large yolk-mass in the mammal. These facts point to the conclusion that the developmental behavior of the reptilian embryo had become so strongly established in the protoplasm of ancestral reptiles and primitive mammals that it persisted even though the reduction of yolk had removed the immediate necessity for many of its peculiarities. The many millions of years of primitive mammalian and of reptilian lineage constituted a barrier quite impassable by any tendency for rever- sion to the indefinitely more remote developmental methods of primitive Amphioxus-like chordates. Unquestionably the yolk content of the chordate egg is much more readily subject to evolutionary change than is the developmental mecha- nism of the germinal protoplasm. That mechanism can be changed. 50 CHORD ATE ANATOMY but there is a high degree of inertia about it. The initiation of evolu- tionary change is evidently not within the embryo itself. Its inertia is such that it tends always to follow the old methods and it changes only as it must. Organogenesis The earlier period of development is concerned with laying out the building materials, the embryonic or "germ" layers. In the later and longer period these layers are shaped into organs. The formation of the central nervous organs and the notochord may begin, however, before the mesoderm is fully established. Amphioxus, partly because it is so small and partly because it is in so many respects primitive, affords what may be regarded as a simplified and diagrammatic view of the early relations of the organs in chordates. Organogenesis in Amphioxus In the preceding account of the early development of Amphioxus the embryo has been followed to a stage where the mid-dorsal ectoderm has become delimited from the lateral ectoderm to form the neural plate, the mid-dorsal endoderm has given rise to a sharp thick upward fold which is the prospective notochord, and paired mesodermal pouches are in process of formation from the dorsal endoderm either side of the noto- chordal fold, the pouches increasing in number by addition of new pouches in successively more posterior positions. (Figs. 42 and 43) In the course of further development the thickened ectodermal neural plate becomes depressed slightly below the level of the neighboring lateral ectoderm (Fig. ^2B-D). Along the line of demarcation between neural plate and lateral ectoderm separation occurs following which the lateral ectoderm extends progressively over toward the median plane and external to the neural plate. Eventually the edges of the right and left sheets of ectoderm meet in the median plane and coalesce to form a con- tinuous layer above the neural plate (Fig. ^2E), Meanwhile the neural plate transforms itself into a tube by bending its lateral regions upward and inward until the edges meet in the median plane where they become joined. (Fig. 42F-G) The neural plate originally extends back to the blastopore. The over-arching process whereby the neural plate is covered proceeds back- ward and around the posterior margin of the blastopore. Thus neural plate and blastopore come to lie under a common roof of ectoderm and the blastopore, no longer opening directly to the exterior, opens into the small space between the neural plate and its newly acquired ectodermal roof. The resulting relation of layers and cavities are shown in Fig. 51, a sagittal section of an embryo at this stage. Upon conversion of the REPRODUCTION 59 plate into a tube, the blastopore is left in communication with the lumen of the tube. At its anterior end the closure of the neural tube is delayed so that for a time its lumen is open to the exterior by a small aperture, the neuropore. The extraordinary result of these changes is an embryo whose prospective digestive cavity, still devoid of definitive mouth and anus, communicates via the neurenteric canal (the former blastopore) with the hind end of the cavity of the prospective spinal cord and thence to the outside by the anterior neuropore (Fig. 5i,P). These relations, however, are merely temporary. Eventually neuro- pore and neurenteric canal close. The definitive enteric apertures, mouth, gill clefts and anus, arise by very similar processes. At the UP ,NC Fig. 51. — AMPHIOXUS. Median longitudinal section of an embryo having two mesodermal pouches, a stage approximately like that of the transverse section in Fig. 42E. The blastopore, roofed over by ectoderm, has become the neurenteric canal. .4, archenteron; EC, ectoderm; EN, endoderm; NC, endoderm destined to become notochord; NE, neurenteric canal; NP, neural plate; P, neuropore. X350. (Based on a figure by Hatschek.) appropriate locality enteric endoderm and superficial ectoderm approach one another and coalesce. The resulting double layer then thins out until perforation occurs. The notochord, whose development is initiated by an upward folding of mid-dorsal endoderm (Fig. 42D-F), early becomes detached from the enteric endoderm and acquires its characteristic cylindrical form. The enteric endoderm meanwhile closes in beneath the notochord and restores the integrity of the dorsal wall of the enteron (Fig. 42G). As the embryo increases in length the notochord grows within itself and receives accessions from the active blastoporal region with which its posterior end remains for some time connected (Fig. 43). The more anterior mesodermal pouches (or somites), soon after their formation and long before the more posterior somites have been developed, begin to acquire their characteristic differentiation. The pouch expands, especially ventralwards, and its cavity is correspondingly enlarged. That part of its wall lying against the notochord becomes much thickened 6o CHORDATE ANATOMY -EC -NC -M -MC -EN while elsewhere the wall remains relatively thin. The expansion of the pouches continues until the walls of right and left pouches meet in the median plane beneath the enteric endoderm. At this stage three regions of the mesoderm may be distinguished: the thickened part lying alongside the notochord; an outer thin layer contiguous to the ectoderm; and an inner thin layer similarly contiguous to the endoderm. The thick part is destined to form a segment of body-muscle and is therefore called the myotome (Fig. 52,M). The outer layer, being, in conjunction with the ectoderm, the body-wall of the em- bryo, is called the somatic or parietal layer. The inner layer, associated with the wall of the enteron, is called visceral or splanchnic. The now capacious cavity resulting from expansion of the pouch is a segment of the embryonic body-cavity or coelom. The myotome rapidly thickens and also increases its dorso-ventral extent. As it thickens, the adjacent upper portion of the coelomic space is correspondingly reduced. Pig. 72*X^MPHI0XUS. Eventually the somatic and visceral layers Transverse section midway of become joined by a horizontal septum formed the length of the body of a . ^„. x r-^^ larva with five gill clefts, just below the myotome (Fig. 52). Con- C, coelom; EC, ectoderm; sequently a lower major part of the original EN, endoderm; I, intestine; . . . j r ^^ M, myotome; MC, myocoei; coelomic space IS Separated from an upper NC, notochord; NT, neural remnant of it, the myocoele (MC) which, with tube; V, subintestinal vein. . • r .^i ^ • £ „ii, (Modified from a figure by contmued expansion of the myotome, IS hnaliy Hatschek.) obliterated, while only the lower cavity partici- pates in forming the definitive coelom (C). The thin portion of the wall of the myocoele later gives rise to connective tissue including the myoconmias which intervene between and tie together successive seg- ments of muscle. As a result of the general expansion of the mesodermal layers, not only, as stated above, are the walls of right and left pouches brought together in the mid-ventral region, but the adjacent walls of successive pouches on the same side of the embryo become closely pressed together. At this stage, then, the paired coelomic spaces of the several pouches are separated from one another by thin partitions, some transverse and others median, each consisting of two layers of cells. These partitions become progressively thinner until they perforate and finally completely disappear except that remnants of the median ventral wall may persist in connection with the development of blood-vessels. With the oblitera- tion of these partitions, the several segmentally developed coelomic REPRODUCTION 6i cavities are all thrown into free communication to form one large space, the definitive coelom, which finally shows no trace of its segmental origin. An embryo of Amphioxus, at a stage when fourteen or fifteen pairs of mesodermal pouches are present, is a delicate, colorless, transparent animal having a length of about one millimeter and a diameter of one- eighth that except at the somewhat enlarged anterior end (Fig. 53). It has a straight digestive tube (enteron, /) extending from an anterior mouth to a posterior anus. There is a single gill cleft, opening from the right side of the anterior region of the digestive tube. The mouth also is unsymmetrical at this stage, opening on the left side. Later, as numer- ous additional gill clefts are formed, they shift their positions so as to become ultimately a series of symmetrically placed paired apertures. Meanwhile the mouth shifts from its original left to a median position. Just above the digestive tube lies the median rod-like notochord (NC) extending the entire length of the animal. Immediately above the Fig. 53. — Amphioxus at beginning of larval period; 14 or 15 pairs of mesodermal somites. Actual length of larva about i.o mm. CG, club-shaped gland; /, intestine; MES, mesodermal somites; NC, notochord; NE, neurenteric canal; NP, neuropore; NT, neural tube; P, pigment spot in neural tube. (After Hatschek.) notochord is the neural tube (A^T), its somewhat enlarged anterior region suggesting a brain. At the anterior end of the neural tube the dorsal neuropore (AP) is still open. The neurenteric canal (NE), at this stage, has ordinarily become closed. In the anterior region, where the differen- tiation of the mesoderm is most advanced, a coelom intervenes between the enteric tube and the outer body-wall (Fig. 52, C). The body-wall (somatopleure) consists of the ectoderm and the somatic layer of meso- derm. The enteric endoderm together with the contiguous visceral or splanchnic layer of mesoderm constitute the wall (splanchnopleure) of the digestive tube. The somatic and visceral sheets of mesoderm provide the coelom with a continuous and complete lining, the peritoneum. The superficial ectoderm is a skin. The more anterior myotomes contain partially differentiated muscle tissue capable of feeble contraction. The animal is free-swimming but the locomotor mechanism consists merely of long cilia produced by the ectodermal layer. In its main features this young Amphioxus is like a vertebrate. If its true origin and nature were not known, it might reasonably be expected to 62 CHORDATE ANATOMY proceed to develop directly into a tvpical vertebrate. But it does not. It acquires no vertebral column; the notochord serves as definitive axial skeleton. It develops no structures morphologically similar to the heart, kidneys, specialized sense organs, or paired appendages of a vertebrate. Further, in later development it acquires, especially in the head region, a variety of unique structures which adapt the adult to its peculiar mode of living but make it conspicuously unlike any adult vertebrate. Nevertheless Amphioxus is "verte- brate" in too many features to make it credible that they could have arisen otherwise than in gene- tic relationship with those of the vertebrates. Herein, then, lies in part the justification for describing the early development of Amphi- oxus to illustrate the main features of the corresponding stages of vertebrates. Further justification is derived, as already stated, from the fact that the paucity of yolk in the egg of Amphioxus relieves the embryo of the factor which in- troduces varying degrees of com- plication into the development of vertebrates and occasions much difficulty in the study and inter- pretation of the processes. Organogenesis in the Vertebrates In the late embryo of Amphi- oxus the main lines of the body plan of a vertebrate are drawn. Brief statements concerning the embry- D V Fig. 54. — Diagrams illustrating method of origin of the neural tube of vertebrates. Transverse sections in the mid-trunk region of embryos at successively (A to D) later stages. C, neural crest; CC, canalis centralis of neural tube; EC, ectoderm; EN, endoderm; MES, mesoderm; NC, notochord; NG, neural groove; NP, neural plate; NT, neural tube; V, blood-vessel onic Origin of the major Organs of (paired dorsal aorta). vertebrates follow. Neural Tube. In Amphioxus the neural plate becomes detached from the adjacent lateral ectoderm (Fig. 42) and transforms itself into a tube not until after it has been covered by the lateral ectoderm. In vertebrates a longitudinal folding of the neural plate and adjoining ecto- derm occurs in such a way that the movement of the neural material REPRODUCTION 63 into a deep position, its conversion into a tube, and the covering of it by lateral ectoderm take place simultaneously (Fig. 54). Not until the tubular form is attained does the neural ectoderm of vertebrates become detached from the overlying superficial ectoderm. Figure 55 shows, in a diagrammatic way, the characteristic appearance of a recently formed neural tube with its neural crests, dorso-lateral extensions of ectodermal material on each side of the tube. Later the neural crest becomes detached from the tube, undergoes segmentation corre- sponding to that of the myotomes, and gives rise to spinal ganglia (Fig. 345). Cells of the crest become ganglion cells whence grow out nerve fibers which constitute the dorsal sensory root of a spinal nerve. The fibers of the other Fig. 55.— Stereogram of embryonic constituent root of a spinal nerve, the "''^'■^} ^^^^ showing the segmenting neural crest, e, superficial ectoderm; ventral motor root, grow out from cells nc, neural crest; s, central canal. within the neural tube. Some cells of (^^°"^ Kingsley, " Comparative Anat- omy of Vertebrates.") the neural crests migrate mto various visceral localities and give rise to ganglia ("sympathetic"; Fig. 345) and nerves of the autonomic system. The anterior region of the tube expands to form the brain. Three enlargements, the primary brain vesicles — fore-brain, mid-brain and hind-brain (Figs. 57, 58) — characterize the cephalic part of the tube in all vertebrate embryos. Later subdivision of the first and third vesicles results in the five brain regions universally characteristic of adult verte- brates. The nervous structures (retina and optic nerve) of the paired eye grow out from the second (numbered from the front) region but the lens of the eye is derived from neighboring superficial ectoderm (Fig. 56). The receptor (that is, stimulus-receiving) nervous structures of the ear and olfactory organ originate not from the neural tube but from super- ficial ectoderm. Notochord. The notochord in the several classes of vertebrates exhibits many variations in details of its mode of origin. The essential fact is that, in vertebrates as in Amphioxus, its material is derived from mid-dorsal endoderm and from the actively growing region about the blastopore. In amniotes the origin of the notochord is closely related to that of the mesoderm. Its material, like that of the mesoderm, usually seems to be derived from the primitive streak (see page 54), a region where ectoderm and endoderm merge indistinguishably. As cells proliferated from the streak laterally give rise to mesoderm, so proliferation forward from the anterior end of the streak produces a median cord of cells which form the notochord. It may, however, receive 64 CHORDATE ANATOMY accessions from the endoderm with which it is usually in close relation. The Enteron. Gastrulation produces a two-layered embryo whose endoderm surrounds a cavity opening to the exterior by the blastopore. This archenteric cavity is the prospective digestive cavity. As the embryo elongates, the cavity is correspondingly elongated and in later develop- ment the enteric tube increases in length faster than the embryo with result that the tube becomes bent or even coiled to adapt itself to the coelomic space. In the early embryo the ectoderm at a median anteroventral position gives rise to a shallow depression or pit, the stomo- deum, whose deeper wall meets the forward-growing endoderm to form tem- porarily a two-layered oral membrane (Figs. 57, O and -720) separating the external stomodeal cavity from the enteric cavity. Soon a perforation appears at the center of the membrane and its peripheral remnant is rapidly obliterated. The per- foration and obliteration of the membrane apparently result from progressive centrif- ugal flow or movement of its cellular sub- stance. Thus is formed the mouth. The posterior enteric aperture or embryonic "anus" develops usually by a similar process. The blastopore rarely persists as a definitive posterior aperture although it does so in cyclostomes and possibly in some urodele amphibians. Otherwise, exactly as in Amphioxus, it becomes roofed over by the neural folds and thus converted temporarily into a neurenteric canal (Fig. 57) connecting the hind ends of neural tube and enteric cavity. An ectodermal pit, the proctodeum, situated just below the neurenteric canal, perforates into the hind end of the enteric cavity to form the definitive hind aperture, either anal or cloacal (Fig. 57). As result of the mode of development of the enteric apertures, the lining of more or less of the mouth cavity is derived from stomodeal ectoderm and that of the posterior region from proctodeal ectoderm. The remaining and by far greater part of the adult enteric tube is lined by endoderm which constitutes the digestive epi- thelium, the essential secreting and absorbing layer of the tube. It is a noteworthy fact that various organs which have nothing directly to do with digestion have their origin in the enteric endoderm. The Fig. 56. — Stereogram of the developing eye. The head of the embryo is cut transversely in the region of the fore-brain. cf, choroid fissure; fb, wall of fore-brain; I, ectodermal thicken- ing which invaginates to form lens; oc, optic cup; os, optic stalk; p, outer thin wall of optic cup, becoming the pigmented epithe- lium which lies behind the defini- tive retina; r, inner thick wall of optic cup, becoming the sensory retina of the eye. (From Kings- ley, "Comparative Anatomy of Vertebrates.") REPRODUCTION 65 anterior region of the embryonic enteron — the part becoming the pharynx of the adult — is concerned particularly with the organs of respiration. Gills of fishes and amphibians develop in relation to paired apertures, the pharyngeal or visceral clefts, which pierce the lateral walls of the enteron and the ectoderm and open to the exterior. A pharyngeal cleft is developed as follows. A deep lateral pouch or furrow of the endoderm bulges outward and meets a similar but shallower pouch or furrow which EC NT NC EN Fig. 57. — Frog: median longitudinal sections of embryos; .4, just before conversion of blastopore into neurenteric canal; B, just after formation of neurenteric canal and perforation of proctodeum to form cloacal aperture. B, brain; BP, blastopore; C, cloacal aperture; EC, ectoderm; EN, endoderm; H, hypophysis; HT, heart; MES, mesoderm; NC, notochord; A'^jE, neurenteric canal; AT, neural tube; O, region where mouth will perforate; P, proctodeum; PH, pharynx; R, rectal region of enteron; Y, yolk cells of endoderm; i, fore-brain; 2, mid-brain; 3, hind-brain. A, X24; B, XiQ. (Redrawn from Marshall, "Vertebrate Embryology.") the ectoderm pushes inward. The resulting two-layered membrane is then obliterated by the same process which removes the oral membrane, leaving a free passage between the pharynx cavity and the exterior. Vascular complications of the endodermal lining of these clefts produce internal gills — although it is possible that some so-called internal gills are derived from ingrowing ectoderm. External gills are ectodermal structures developed in close relation to the external apertures of pharyn- geal clefts. In amniotes the pharyngeal pouches are merely temporary 66 CHORDATE ANATOMY embryonic features except as those of the first pair are, in a modified way, represented in the auditory passages. Lungs develop by outgrowth from the endoderm of the pharynx (Figs. 235, 238). The entire epithelial lining, being the essential respi- ratory membrane, of the adult lung, is endodermal and continuous, byway of the lining of bronchi and trachea, with the lining of the digestive tube. The air bladders (swim-bladders) of fishes are endodermal sacs which grow out from an anterior region of the embryonic enteron. They are usually dorsal, rarely lateral, or ventral as in the ganoid Polypterus. The important endocrine glands, thyroid, parathyroid and th3mius, and various gland-like bodies mostly of dubious nature and function, arise as outgrowths of the endoderm of the pharyngeal pouches or the wall of the pharynx. (Fig. 235) More posterior regions of the enteric endoderm give rise to various accessory digestive organs, most important of which are the liver and pancreas. The liver develops as a mid-ventral outgrowth, sometimes more than one, from the anterior region of the prospective intestinal portion of the enteron. The pancreas arises similarly and in close relation to the liver. Vascular and connective tissues make up a large part of the bulk of the adult organs but the essential hepatic cells and the secretory tissue of the pancreas are endodermal. The position of the opening of the bile duct into the intestine marks the point of origin of the embryonic liver. The cloaca of the adult vertebrate is a superficial chamber situated at the hind end of the body-cavity and opening ventrally to the exterior. Into it open the intestine and the ducts of the kidneys and genital organs. It is commonly present in vertebrates below mammals except in Teleostei. It is derived from the extreme hind end of the embryonic enteron. Mammalian embryos develop a cloaca but only those primitive mammals, Ornithorhynchus and Echidna, retain it in the adult. In other mammals the embryonic cloaca becomes subdivided into a dorsal part connected with the intestine and a ventral part which receives the urinogenital ducts. In course of further development these two divisions of the cloaca are separated and carried apart and acquire independent openings to the exterior, the latter being the more ventral. Therefore the more distal portion of the urinogenital passage of the adult, both male and female, is a remnant of the cloaca while another remnant of it persists in the posterior region of the rectum. The Mesoderm. The vertebrate mesoderm is at first devoid of seg- mentation and ordinarily contains no definite cavity (Fig. 44). At an early embryonic stage the mesoderm upon either side splits into two layers; an outer, lying against the ectoderm, and an inner lying against the endo- derm. The two layers remain connected, however, at the upper edge REPRODUCTION 67 of the original sheet (Figs. 58, 59.! J. At about the same time the dorsal and thicker part of the mesoderm develops transverse fissures which divide it into a series of paired blocks (somites) lying symmetrically either side of the neural tube (Fig. 58). This segmentation begins in the anterior part of the embryo and progresses backwards just as, in Amphi- oxus, the mesodermal pouches are formed successively from anterior to posterior. The process of segmentation involves only the upper part of the mesoderm. As segmentation goes on, the space between the lower thin and unsegmented layers on either side becomes wider — a space already Fig. 58. — Stereogram of the anterior region of a vertebrate embryo showing the segmentation of the mesoderm. The ectoderm has been removed from the left side of the embryo, al, endoderm of alimentary tube; c, coelom; etn, epimere;/6, fore-brain; hb, hind-brain; km, hypomere; m, myotome; mb, mid-brain; mm, mesomere; n, neural tube; nc, notochord; s, stomodeal region; sk, sclerotome; so, sp, somatic and splanchnic walls of coelom. (From Kingsley.) recognizable as the coelom bounded externally by a somatopleure con- sisting of ectoderm and the outer sheet of mesoderm, and internally by a splanchnopleure consisting of endoderm and the adjacent layer of meso- derm. The mesodermal layers upon either side grow down to the mid- ventral region, carrying with them the coelom, and meet mid-ventrally to form a double vertical layer, a ventral mesentery, extending from the enteron to the outer body wall and separating right and left coelomic cavities. (Fig. 58) The splitting of the original sheet of mesoderm extends so far dorsally as to involve the somite which accordingly contains a more or less definite cavity, the myocoele — "myo-" because the somite is mainly muscle- forming. Shortly the somites become detached from the lower somatic 68 CHORDATE ANATOMY and visceral sheets of mesoderm and the myocoeles lose continuity with the permanent coelom (Fig, 59^). Eventually, as the somite differen- tiates, the myocoele is obliterated. Fig. 59. — Diagrams, (transverse sections) showing embryonic origin of pronephric tubules. A, earlier stage; B, later, c, coelom; d, pronephric tubule and duct; e, epimere; h, hypomere; m, mesomere (cross-lined); my, myotome; n, nephrostome; so, somatic layer of hypomere; sp, visceral (splanchnic) layer of hypomere. (From Kings- .ey, after Felix.) The differentiation of the vertebrate mesoderm is more elaborate than that of Amphioxus, especially in the prospective trunk region. Here, upon each side, early arise three zones of differentiation: the epi- FiG. 60. — Diagrammatic transverse section of the body of a vertebrate embryo at an advanced stage. The muscle-forming myotome is beginning to extend into the ventral body-wall of the embryo, c, coelom; g, genital ridge; jh, muscle derived from myotome; 7nc, myocoele; p, peritoneum; pd, pronephric duct; so, somatic layer (dermatome) of somite; v, advancing ventral border of myotome; the finely dotted areas are occupied by mesenchyme. (From Kingsley.) mere, a dorsal mainly muscle-forming part; the mesomere, a kidney- forming zone situated just below the epimere; and the h5rpomere, the most ventral zone, constituting the somatic and visceral layers of peri- toneum (Figs. 58 and 59^4). REPRODUCTION 69 The epimere undergoes three kinds of diflferentiation. Its heavier inner wall is mainly converted into striated body-muscle, not only the dorsal but the ventral muscle. The myotome material grows ventral- wards, pushing its way between the ectoderm and the somatic meso- derm, until it reaches the mid- ventral plane (compare Figs. 60 and 61). The medial region of the epimere gives rise to loosely aggre- gated cellular masses (mesen- ch5mae) surrounding the notochord and neural tube (Figs. 58, 60). This material produces such supporting structures — connective tissue, carti- lage and bone — as may later be developed around these two axial organs. The thin outer wall of the epimere breaks up to form loose cellular masses, mesenchyme, which give rise to the dermis, the deeper fibrous and vascular layer of the skin. The terms myotome, sclerotome and dermatome are applied respec- tively to the muscle-forming, skeleton-forming and dermis-forming regions of the epimere. (Fig. 58) The mesomeres give rise to the tubular structures of the kidneys. The process begins in the more anterior mesomeres and progresses pos- teriorly. Certain differences in mode of development and in eventual structure compel the distinction between an earlier and more anterior system of tubules, the pronephros (Figs. 59, 62), and a later more posterior and more extensive system, the mesonephros. In anamnia the meso- nephros becomes the adult kidney and the pronephros disappears except that in a few fishes it is the definitive and only kidney. In amniotes, following development of a pronephros and a mesonephros, the tubule- forming process continues backward, but with some modifications, to form a third kidney, the metanephros, which becomes the adult kidney. The tubular epididjnnis, associated with the testis of the adult amniote, is a part of the embryonic mesonephros which otherwise disappears except for certain vestiges which are apparently of little functional importance. Fig. 61.— Diagrammatic transverse section of the body of a vertebrate. av, aorta; c, coelom; e, ectoderm; ep, epaxial (dorsal) muscle; g, gonad; ha, hemal rib; hp, hypaxial (ventral) muscle; i, intestine; mes, mesentery; n, nephrid- ium; o, omentum; r, rib; p, somatopleure; sp, splanchnopleure; v, centrum of verte- bra and, above it, neural arch containing spinal cord. (From Kingsley.) 70 CHORDATE ANATOMY SPINAL CORD SPINAL GANGLION^.. NEPHROSTOME j GLOMERULUS POSTCARDINAL VEIN ■ , ^^ 1 MESONEPHRIC TUBULES PERITONEUM \// MESENTERY PRONEPHRIC TUBULES PRIMITIVE DUCT Fig. 62. — Stereogram of the developing pronephros and mesonephros. (After Kingsley modified.) MESONEPHRIC TUBULE t lYOTOM^ DERMATOME MESONEPHRIC TUBULES ^EURAL TUBE j ', Gl Of 1ERULUS NEPHROSTOME PERITONEUM POSTCARDINAL VEIN I GENfTALRIDGE i PRIMITIVE DUCT MESENTERY COELOM Fig. 63. — Stereogram of the developing mesonephros; stage later than that of Fig. 62. (After Kingsley modified.) REPRODUCTION 71 Meanwhile, as the pronephric tubules form, the mesomere material on each side of the embryo gives rise to a longitudinal tube (Fig. 59) which extends from the pronephric region to the cloaca into which it finally opens. The pronephric tubules of each side join the corresponding longitudinal pronephric duct (Fig. 62) thus putting the coelom into com- munication with the exterior by way of the cloaca. The coelomic open- ings or nephrostomes (Figs. 59^, n and 62) of the pronephros are ciliated. The arrangement apparently serves for drainage from the coelom to the exterior. The mesonephric tubules acc|uire connection with the already-formed longitudinal duct which, as the pronephros degenerates, then serves, at least in part, as the mesonephric or Wolffian duct. In Anamnia usually each mesonephric tubule has a ciliated nephrostome opening into the coelom. In the kidneys of amniotes, nephrostomes rarely appear. Mesonephric and metanephric tubules usually form specialized excre- tory structures. The tubule (Figs. 63, 64) gives rise to a cup-shaped expansion (Bowman's capsule). The hollow of the cup is occasioned by ingrowth of a dense network of fine blood-vessels, the glomerulus. The capsule and glomerulus together constitute a renal (or malpighian) corpuscle. The part of the tubule between the corpuscle and the mes- onephric duct eventually becomes much elongated, coiled and locally differentiated. Fig. 64. — Diagram of renal (Malpighian) corpuscle, a, artery; b. Bowman's capsule; gl, glomerulus; n, nephrostome; t, nephridial tubule; v, vein. (From Kingsley, "Comparative Anatomy of Vertebrates.") In the absence of nephrostomes drainage of waste from the coelom does not occur and the function of excretion must be confined to the renal corpuscle, where the glomerulus brings blood-vessels into close relation to the lumen of a kidney tubule, and to other vascular regions of the tubule. The amniote metanephros has outlet by way of a duct, the ureter, which develops as a forward-growing branch from the cloacal end of the mesonephric duct of the same side of the embryo. The tubular struc- tures of the metanephros are formed largely by outgrowth from the anterior end of the ureter. 72 CHORDATE ANATOMY The adult kidney (Fig. 271) consists of the entire system of tubules — mesonephric or metanephric — of one side of the embryo, increased to great number by formation of secondary tubules from the primary tubules, each tubule tremendously elongated and much coiled, the tubules bound together by connective tissue with blood-vessels richly interspersed, and the whole complex ensheathed by connective tissue and thereby delimited from adjacent tissues of the body-wall. The hypomere mesoderm, later backed up by a layer of connective tissue, becomes the definitive peritoneum. Its somatic layer completely MVJ Fig. 65. — Diagrammatic transverse section of the body of a vertebrate showing relations of organs to the peritoneum and coelom. A, dorsal aorta; C, coelom; EN, endodermal epithelium of digestive tube; G, gonad; /, integument; K, kidney ;L, liver; M, musclelayer of digestive tube; MD, dorsal muscle of body-wall; MV, ventral muscle of body-wall; NC, position of embryonic notochord; NT, neural tube (spinal cord); PP, parietal peritoneum; PV, visceral peritoneum; R, rib; VC, vertebral column. lines the body-wall. Its visceral layer covers the coelomic surfaces of the digestive tube and of all other organs which occupy the coelom. In the median plane at all regions not occupied by median organs (Fig. 58) the right and left visceral layers of the hypomere meet one another to coalesce and become membranes or mesenteries which connect and support the viscera. In later stages of development the mesenteries undergo considerable reduction, especially those between the digestive tube and the ventral body wall (Fig. 60). Figure 65 shows the ideal relations of REPRODUCTION 73 the peritoneum and mesenteries to the coelomic organs. It is clear that no organ can be said to lie in the coelom except as the peritoneum investing that organ is regarded as a part of the organ. In strict sense, median organs lie between the peritoneal sheets of the right and left halves of the body. The peritoneum plays a part in the development of the gonads although it is not necessarily the source of the germ cells. The prospective gonads first appear as longitudinal thickenings or genital ridges in the dorsal peritoneum, one on each side and between the dorsal mesentery and the mesonephros (Figs. 60, 61 and 66). The earlier belief that the germ cells Fig. 66. — Section of genital ridge of a chick of five days incubation, e, peritoneal epithelium of ridge; c, genital cords; o, primordial germ cells. (From Kingsley, after Semon.) • are derived from the peritoneal layer has been shaken by evidence that the primordial germ cells first appear in the mid-dorsal enteric endoderm whence they migrate into the genital ridge. The deeper substance of the definitive gonad is derived either from the thickened peritoneum of the genital ridge or, especially in the male, from the mesoderm of the closely adjacent mesonephros. The gonads find outlet by way of ducts which arise in relation to the kidneys. The seminiferous tubules of the testis acquire connection with the neighboring mesonephric tubules and thereby gain exit by way of the Wolffian duct which therefore, in Anamnia, serves as a urinogenital duct. In amniotes the adult male retains, in the epididymis, that part of the embryonic mesonephros which provided connection between the testis and the Wolffian duct. With metanephros and ureter serving the urinary function,' the Wolffian duct is left as a vas deferens or sperm duct only. 74 CHORDATE ANATOMY Fig. 67. — Transverse sec- tion through the urinogeni- tal region of a four-day chick embryo, g, mesoder- mal epithelium (peritoneum) of genital ridge; w, infolding The oviducts in elasmobranchs and probably some amphibians arise by longitudinal splitting of the pronephric duct, one portion of it serving thereafter as the mesonephric duct while the other portion acquires, by fusion of several pronephric nephrostomes, a wide anterior opening into the coelom in the vicinity of the ovary. In other vertebrates, the oviduct develops as a fold of peritoneum (Fig. 67, m) closely parallel to the Wolffian duct but independent of it. The Mesenchyme. Reference has been made (page 69) to the fact that certain regions of the mesodermal somite, the sclerotome and the dermatome, are the source of cellular material which becomes detached from the somite and aggregates in the spaces between the somite and neighboring organs or layers where it produces skeletal, connective and integumentary tissues. This secondary meso- derm ("derm" implying a sheet or layer), being usually not disposed in definite layers, is called mesenchyme. But the somite is not the only of peritoneum to form Miil- source of mesenchyme. Quantities of it are lerian duct; ms, mesentery; Q^UCed in all rCgionS of the CmbryO. s, mesenchyme cells which i^ ° ■' give rise to the stroma (non-genital tissue) of gonad; t, mesonephric tubules; W, Wolffian duct. (From visceral layers of the hypomere are a prolific Kingsiey, after Waideyer.) ^^^^^^ ^^ -^^ numcrous cells becoming detached from the outer (next the ectoderm) surface of the parietal layer and from the inner (next the endoderm) surface of the visceral layer. Also the endoderm contributes to the mesenchyme which accumulates between the enteric wall and the adjacent layers of mesoderm. The ectoderm plays a minor part but evidence has been found indicating that mesenchyme of ectodermal origin, "mesectoderm," participates in the development of parts of the skeleton of the pharyngeal region. Mesenchyme spreads from its place of origin and eventually is found in all parts of the embryo. Although late in origin, its importance is by no means secondary. Chief among its derivatives are the following materials and structures. Fibrous connective tissue is omnipresent in the adult vertebrate. It invests, supports, connects, separates or cushions parts of the body. Every location where cartilage or bone is destined to develop is occu- pied by mesenchyme. The deeper parts of the skull, the vertebral column, ribs, sternum and the skeleton of the paired appendages are first constructed of cartilage. The entire endoskeleton is permanently Beyond question, most of the mesenchyme comes from the mesoderm. The parietal and REPRODUCTION 75 cartilaginous in elasmobranchs. Cartilage is a direct product of mesen- chyme. Cells of the mesenchyme become cartilage cells (Fig. 68) and deposit the ground substance or matrix of the cartilage. In the great majority of vertebrates the primary cartilaginous skeletal structures are, Mes. PrcCart. Cart. A'- m. 0': '"^^^^^j|^ Fig. 68. — Diagrams illustrating formation of cartilage by mesenchyme. A, in fishes, according to Studnicka; B, in mammals, according to Mall. Cart., cartilage; Mes., mesenchyme; Pre. Cart., precartilage. (From Bremer, "Text-book of Histology.") in later development, more or less completely replaced by bone. The process of replacement (Fig. 178) involves the destruction of the greater part of the cartilage. The remnants of the cartilage are in form of a spongy meshwork whose strands become calcified and serve as a frame- work upon which bone-producing cells, osteoblasts, build up bone. Osteoblasts. Calcifying connective-tissue bundles. Bone matrix. Bone cells. Fig. 69. — Development of dermal (secondary) bone from mesenchyme. From a section of the mandible of a human embryo of four months. X240. (From Bremer, "Text-book of Histology.") In the development of certain of the more superficial bones of the cranium, the outer bones of the jaw skeleton and some parts of the shoulder girdle, no cartilage is formed. Mesenchyme cells, becoming osteoblasts, build up bone directly on the surfaces of strands of calcified connective 76 CHORDATE ANATOMY tissue (Fig. 69). Most of the bones which develop in this manner are derived from the embryonic mesenchyme of that same general superficial layer which otherwise gives rise to the dermis of the skin. They are accordingly called dermal bones. Bone resulting from replacement of cartilage is called cartilage bone. Mesenchyme is the source of nearly all unstriated or "smooth" muscle, whether in the walls of viscera or in the body-wall. Most visceral organs are hollow. In their early embryonic stages their primary and essential walls are either endoderm as in the case of the digestive tube, lung or urinary bladder; or mesoderm as in the urinogenital ducts. The outer surfaces of these primary walls are always adjacent to regions occupied by mesenchvme. The unstriated muscle fibers of these organs are differen- FiG. 70. — Diagrammatic transverse sections of developing heart. In A the descend- ing right and left mesodermal hypomeres have nearly met; mesenchyme cells appear between them. In B the layers have met ventrally forming the ventral mesocardium; the enclosed mesenchyme has formed the endocardium. In C the layers have met dorsally to form a dorsal mesocardium; meanwhile the ventral mesocardium has dis- appeared and the right and left coelomic spaces have become the pericardial cavity. c, coelom; ec, ectoderm; en, endoderm; end, endocardium; m, ventral wall of hypomere; p, pericardial cavity; v, mesenchyme cells. (From Kingsley, "Comparative Anatomy of Vertebrates.") tiated from cells of the adjacent mesenchyme. Unstriated muscle fibers occur in the walls of larger blood-vessels and of some integumentary glands where they serve to expel the contents of the gland. Hairs and feathers are erected by contraction of delicate muscles, usually unstriated. The dilators fibers in the iris of the human eye, however, are apparently of ectodermal origin. The statement that blood-vessels are derived from mesenchyme is probably admissible although some vessels seem to arise fairly directly from the mesoderm. They may arise as solid cords of cells, later becoming hollow, or may be hollow from the beginning. The essential wall or endothelium having been established, the outer layers of connective tissue and unstriated muscle are provided by adjacent mesenchyme. The heart develops in the region just behind that where the pharyngeal clefts are forming. The right and left hypomeres of the mesoderm push ventralwards and in the median ventral space between them (Fig. 70) accumulate cells derived from the adjacent hypomeres, therefore essen- tially mesenchymal. These cells arrange themselves to form a very thin layer which becomes the endothelial lining or endocardium of the prospec- REPRODUCTION 77 tive heart. In some cases, at first two endothelial tubes are formed, lying side by side, later coalescing into one. The thick muscular layer (myocardium) and the outer layer (epicardiimi) of the wall of the heart, also the pericardium lining the pericardial cavity, are derived from the adjacent hypomeric mesoderm. The heart muscle, however, unlike that of blood-vessels, is striated. The transverse septum, separating pericardial from abdominal cavity, consists of pericardium in front and peritoneum behind, with connective tissue between. The diaphragm of the mammal is not the exact equivalent of the transverse septum of other vertebrates (Fig. 71). That part of the coelomic space lying on the cephalic side of the diaphragm is subdivided into three cavities, the pericardial and the right and left pleural cavity containing the corresponding lobes of the lungs. The Fig. 71. — Diagrams showing the relations of the coelomic cavities (black) in fishes (^4), amphibians and sauropsida (B), and mammals (C). L, liver; P, lungs; S, septum transversum; D, diaphragm. In B the lungs lie in the peritoneal (or pleuroperitoneal) cavity; in C they occupy special pleural subdivisions of the coelom. (From Kingsley.) diaphragm is muscular. Its muscle is striated and, like body-wall muscle, is derived from epimere mesoderm. Strangely, however, it is mesoderm which shifts backward from somites of the neck region. This accounts for the innervation of the diaphragm by cervical spinal nerves. Head, Neck, Tail. The mesoderm of the head is less definitely seg- mented than that of the trunk. The six muscles, consisting of striated fibers, which effect the movements of the eyeball in its orbit are developed from head mesoderm which is probably the equivalent of three somites or epimeres of the trunk. There is nothing corresponding to the mesomere of trunk mesoderm. The neck region, whether or not differentiated externally, corresponds approximately to that of the embryonic pharyngeal pouches. In this region the dorsal mesoderm forms epimeres which give rise to neck muscles. The lateral mesoderm, remaining unsegmented,* corresponds to the hypomere of the trunk. Whereas the trunk hypomere forms onl}- 78 CHOKDATE ANATOMY the unstriated muscle of the digestive tube and other visceral parts, the pharyngeal h>^omeric mesoderm produces striated muscle which dififeren- tiates into an elaborate system of muscles (branchiomeric muscles) related to the skeleton of the jaws and gill region (Fig. 189). The tail is produced by growth of ectodermal and mesodermal parts backward from the region of the blastopore. Growth of the mesoderm keeps pace with that of the neural tube and notochord. The mesoderm forms somites which produce the segmental striated caudal muscle and the mesenchyme which gives rise to skeletal, vascular and connective- tissue structures of the tail. Relation of Yolk to Organogenesis Cleavage, gastrulation and the mode of origin of the mesoderm and the notochord are necessarily much affected by the presence of the bulky and inert yolk. Once the germ layers have been established, however, the development of organs proceeds in vertebrates of all classes with only minor differences in details of the processes. Apparently each germ layer is capable of producing certain structures and no others and those particular structures arise from that layer in all vertebrates, whether fish or man. Yet at early stages of development the embryonic material may not be so rigidly determined. By appropriate operations at suffi- ciently early stages of embryos, both vertebrate and invertebrate, it has been proved that a certain region of germ material may be caused to produce structures other than those which it would have produced normally. Yolk is food. The appropriate place for food is in the enteron. In an amphibian embryo the yolk is contained within cells. Gastrulation having established the enteron, the greater part of the embryonic food is then present, not in the enteric cavity but, even better than that, within the cells which constitute the wall of the enteron where it may be directly acted upon by the endodermal protoplasm and made available, as the blood system develops, for transportation to all parts of the growing embryo. The enormous yolk of the egg of a shark, reptile or bird is mor- phologically a part of the original ovum. But by the time cleavage of the germ-disc has progressed so far as to produce a many-celled blastoderm spreading out thin and flat on the surface of the yolk, the cells of the blastoderm can be regarded as, at most, merely joint proprietors of the food supply and the yolk is essentially e.xtra-cellular. As development proceeds, the blastoderm differentiates into the typical germ layers, the mesoderm splits to form somatic and visceral sheets with coelomic space between them, and all -of these layers progressively spread over and around the non-living yolk until eventually it is entirely enclosed (Fig. 74) by REPRODUCTION 79 splanchnopleure and somatopleure with coelom between them. The embryo is put to the necessity of building not only its enteron but its body-wall around its prospective food. ectodsrm o( neural plat ctodcrm of blastoderm fore-gut illantoic bud yolk- stalk Fig. 72. — Diagrams representing median longitudinal sections of chick embryos after incubation for approximately one day, A ; two days, B; three days, C; four days, D. The four stages show progressive differentiation of the regions of the enteron and pro- gressive constriction between the yolk-sac and the shaping body of the embryo. (From Patten, "Embryology of the Chick.") In course of development the yolk is assimilated and utilized in the building of new protoplasm. It therefore steadily decreases in bulk both relatively and absolutely. As the body of the embryo begins to 8o CHORDATE ANATOMY take form, a constriction involving both somatopleure and splanchno- pleure (Figs. 72, 74) appears between the yolk-sac and the remainder of the embryo. The constriction deepens until the embryo presents the appearance of a small animal having a narrow-necked globular sac sus- pended from the under side of the body (Figs. 72Z), 73). In amniotes the amnion is concerned in this constriction (Fig. 74). As the embryo increases in size the shrinking yolk-sac is drawn up into the body. The inner wall (splanchnopleure) of the sac finally constitutes a small region of the wall of the intestine. In elasmobranchs the somatopleure of the yolk-sac finally flattens out and persists as a part of the abdominal wall. In reptiles and birds at the time of hatching the somatopleure is ruptured Fig. 73.- -Young dogfish shortly before birth. The yolk-sac, containing a remnant of the yolk of the egg, protrvides from the ventral body-wall. at the constriction between the definitive body and the extra-embryonic structures and everything external to the rupture is abandoned. Embryonic and Fetal Membranes In the description (pages 33-36) of the reproductive arrangements in vertebrates a general account of the embryonic membranes, amnion, chorion and allantois, of reptiles, birds and mammals was given. The foregoing account of the origin of the germ layers and the shaping up of the embryonic body now makes it possible to appreciate the manner of formation of these membranes in terms of germ layers. All eggs are invested by protective coverings which are either pro- duced by the ovum itself or are secreted about the egg by the oviduct. Such membranes consist of material which is not cellular and not in any sense living. They have merely passive functions. The amnion, chorion and allantois are produced by the germ layers at a relatively advanced stage of the embryo. They are constituted of living cellular material and they are actively concerned with such important functions as nutrition, respiration, excretion and circulation. REPRODUCTION 8l SOhMTOFLEURE AMNION CAVITY\ EXTRA-EMBRYON, COELOM SEROSA ^CHORION I AMNION .COELOM A. AMNION CAVITX AMNION, ENTERON BRAIN SEROSA FOETAL PLACEfNfTA ALLANTOIS COELOM YOLK-STALK YOLK SPLANCHNOPLEURE Fig. 74. — Diagrams illustrating the development of the amnion and allantois. Upper figure, earlier stage; section transverse to long axis of embryo. Lower figure, later stage; longitudinal section of embryo. (After Kingsley, modified). CHORDATE ANATOMY The amnion and chorion are simultaneously produced by an up-rising fold of the somatopleure (ectoderm accompanied by mesoderm) or embryonic body-wall (Fig. 74). The embryo becomes completely sur- rounded by such a fold which then grows in centripetally from all direc- tions and finally encloses the embryo. Where opposite edges of the fold meet above the embryo they coalesce. Reference to Figs. 74 and 75 will serve better than description to make clear the resulting relations of layers and spaces. Fig. 75. — Diagram of the fetal structures of a mammal. (The broken lines represent mesoderm.) A, amnion; AL, cavity of allantois; B, brain; C, chorion; E, enteron; EX, extra-embryonic coelom; H, heart; NC, notochord; NT, neural tube; P, placental region of allantois and chorion; SM, somatopleure; SP, splanchnopleure; V, chorionic villi; YS, cavity of yolk-sac. The somatopleural folds which give rise to the amnion and chorion are, at the time of their formation, a living part of the embryo. The statement that the folds eventually enclose the embryo anticipates the fact that the amnion and chorion do not become any part of the adult. Therefore "the embryo" which the folds enclose is the definitive body region of the embryo. Everything else is conveniently referred to as extra-embryonic. The allantois, an outgrowth from the hind region of the enteron (Figs. 72Z), 74, 75), is a product of the splanchnopleure and is lined by endoderm. In the region of its fusion with the chorion the apposed meso- dermal layers of the two membranes develop a rich network of fine blood- vessels which are connected bv the allantoic arteries and veins to the main REPRODUCTION 83 blood-vessels of the embryo. This allantoic circulation in a reptile or bird provides for respiration (see pages 33, 34). Before the time of hatching the shrinking yolk-sac is drawn up into the growing body. The umbilical stalk — that is, the whole complex of connexions between the definitive body of the embryo and the extra- embryonic membranes — becomes narrowly constricted. At time of hatching the amnion and the slender neck of the allantois are ruptured at the umbilicus. As the young animal emerges, the amnion and chorion and the extra-embryonic part of the allantois are abandoned. The proximal portion of the allantois, remaining within the body, becomes enlarged and serves as the urinary bladder of such adult reptiles as possesses that organ. In birds, the adult having no urinary bladder, the proximal remnant of the allantois degenerates. Among mammals there is some diversity as to the manner of origin of the amnion and chorion. Once established, however, these membranes possess the same relations to the germ layers and to the definitive body of the embryo as in reptiles. The main facts concerning the development of a placenta, by the chorio-allantoic membrane have already been stated (see page 35). The highly vascular vilU produced by the chorio-allantoic membrane (Fig. 75) may be merely lodged in depressions in the uterine wall or they may pierce more or less deeply into its tissues. In extreme cases (e.g., in man) there is destruction of walls of uterine blood-vessels and the extravasated blood fills large sinuses in the uterine wall. The villi project into these sinuses so that the villous surfaces are directly bathed by maternal blood, an arrangement providing maximum efficiency in the exchange of materials between fetal and maternal blood. Mammals exhibit various types of placenta, depending on the dis- tribution of villi in the chorionic surface. When the villi are uniformly distributed over the chorion, as in the horse, pig and other ungulates, the placenta is called diffuse. In most ruminant ungulates, such as cattle, the villi are localized in numerous patches or clusters of varying sizes — the cotyledonary placenta. In carnivores the placenta usually takes the form of a broad band or zone encircling the chorion at a position about midway between head and tail of fetus — the zonary placenta (Fig. 76). A discoidal placenta, in which villi are restricted to a single relatively large area of the chorion, occurs in insectivores, bats, rodents and higher primates including man. A fetal placenta whose villi do not penetrate deeply into the uterine wall separates from it readily and without loss of uterine material. Such a placenta, called non-deciduate, occurs in most ungulates, in the whale and dugong, and in lemurs. When, however, the fetal villi are deeply imbedded in the uterine wall, at time of birth the involved layer of the 84 CHORDATE ANATOMY Uterus is split ofif and discharged with the fetal placenta. This deciduate condition occurs in carnivores, in the elephant, and commonly in animals having a discoidal placenta. In certain marsupials (Dasyurus) it is the splanchnopleure of the yolk-sac which joins the chorion and forms a placenta-like vascular area which is apposed against the uterine wall. Possibly in early mammals both the yolk-sac and the allantois were potentially placenta-forming. In higher primates, the allantoic sac is rudimentary and the fetal portion Fig. 76. — Fetus of cat, removed from uterus without rupturing chorionic sac (C), showing zonary distribution of placental villi. of the placenta is of chorionic origin only; yet the allantois develops far enough to bring its blood-vessels into connexion with the chorionic vessels of the placenta. The umbilical cord is the much elongated and attenuated connexion between the body of the fetus and the extra-fetal membranes. At time of birth the amnion and chorion are ruptured and the young mammal is expelled, along with the amnionic fluid, by muscular contrac- tion of the uterine walls. The amnion, chorion, allantois, fetal placenta, and more or less uterine tissue in a placenta of the deciduous t>^e are dis- charged later as the "after-birth." The umbilical cord is severed. That portion of the allantois remaining within the body becomes the urinary bladder. CHAPTER 3 HISTOLOGY Animals are constituted of ''living substance" or protoplasm together with various non-living materials which are produced by protoplasm. It is chemically complex and possesses a definite, elaborate and minute physical structure. Its basic activities as "living" substance are nutri- tion, respiration and excretion. For the adequate carrying on of these processes, every particle of protoplasm must be in close relation to an environment containing food and oxygen and providing for removal of wastes. Therefore protoplasm cannot exist in indefinitely large con- tinuous masses. The protoplasm of larger animals is subdivided into minute (usually microscopic) structural and physiological units called cells. Circulation of fluid in intercellular spaces provides for the meta- bolic requirements of the individual cell. Animals, e.g., most of the Protozoa, may be so small as to be organized as single cells. The body of a large animal is locally differentiated for the carrying on of various functions. The specialized regions, more or less definitely delimited from one another and each characterized by a configuration which is consistent with its special function, we call organs. These organs, in contrast to the organs of a protozoan, comprise many cells, and the cells of any one organ, so far as they are concerned in carrying on one common function, all exhibit intracellular differentiation of the same kind. Such a group or system of cells, coordinated in one common function and alike in their internal differentiation, constitutes a tissue. An ideally simple organ would consist of only one tissue. As a matter of fact, nearly all organs are concerned with more than one function. An organ's primary function usually demands certain accessory functions, and a corresponding diversity of tissues enters into the constitution of the organ. In a stomach the primary tissue is the lining layer or digestive epithelium. Muscular, nervous, vascular and connective tissues play accessory but nevertheless necessary roles. Vascular and connective tissues enter into the constitution of all major organs. Anatomy deals with organs as such. Histology concerns itself with the internal and specific structure and organization of tissues. Since the tissue is constituted of cells, histology is necessarily concerned with them. Cytology, narrowly defined, deals with cells as such — that is, with that fundamental cell mechanism which is common to all cells and inde- pendent of tissue specialization. 85 86 CHORD ATE ANATOMY Most vital functions involve the surface between protoplasm and the medium immediately external to it. Food enters from without. Respiratory gases pass in and out. Waste is expelled from the surface. Special secretions are produced at the surface. External forces impinge upon the surface. Further, most of the organs of the adult animal are hollow. They contain something or they convey something — food, air, blood. Even such organs as the liver and pancreas, upon casual inspection apparently quite solid, are minutely hollow. Muscles, how- ever, are solid. Connective and skeletal tissues may form bulky solid masses — solid, that is, except insofar as they are penetrated by blood vessels. Bone may contain cavities, but these cavities have a merely passive mechanical significance. The occupation of bone cavities by a blood-forming marrow makes advantageous use of what might otherwise be mere waste space in the animal, but this marrow tissue has no direct relation to the skeletal function of the bone. Such nervous organs as brains, ganglia, central nerve cords, and nerves need not be hollow and ordinarily are not. Every surface of the animal, whether apposed directly to the external medium or to some internal cavity, is a critical region. It is a surface on the one side of which is living substance while on the other side of it may be food, water, air, blood or something else between which and the protoplasm is being carried on some vitally necessary activity — digestion, respiration, absorption, secretion, excretion, diffusion. Or it may be a surface at which the underlying protoplasm deposits a protective non- living substance. Provision for the adequate carr3dng on of these essential and diverse surface activities can be afforded only by the presence of a superficial membrane constituted of living material and specialized appropriately for the functional requirements of the particular surface. Consequently, with very rare exceptions, every free surface of an animal, external or internal, is the surface of a more or less specialized cellular la_ver, an EPITHELIUM. EPITHELIAL TISSUES Epithelia are tissues of primary importance. They are, in double sense, the most primitive of tissues. The smaller simpler coelenterates consist merely of an outer and an inner epithelium. The gastrula of animal embryos consists of two epitheha. It is evident, then, that epithelium provides for all animal needs, and therefore all-epithelial animals may and do exist. The outer layer of the vertebrate gastrula, while it is the source of various structures which attain a deeper position, otherwise persists as the epidermis which is the external epithelium of the adult body. The HISTOLOGY 87 inner layer of the gastrula, giving rise to various organs such as the liver, pancreas and lungs which grow outward from the enteron, otherwise persists as the lining of the digestive tube, the digestive epithelium, which is the innermost epithelium of the adult body. By far the greater Term. bar. CS^ M iii*!"*^*?.*" ^r-l Top plat -^-'- 3 7-, © ., ^.& ai"^ Conn, tissue. Squam.epith. Fig 77. — At left, section of the allantois and amnion of a pig embryo at a region where the mesodermal layers of the two membranes have coalesced. The section is perpendicular to the surfaces of the allantois (above) and the amnion (below). At right, surface view of allantois. The allantoic epithelium is cuboidal, the amnionic epithelium is squamous. The "top plate" is a superficial denser layer of the cell; "terminal bars" are thickenings of intercellular substance just beneath the surface of the epithelium. (From Bremer, "Text-book of Histology.") part of the massive adult has been inserted between the two primary layers. The term endothelium is commonly applied to the lining layer of blood-vessels and lymphatics. Mesothelium may be used for the peri- toneal epithelium. Cells may form a layer resembling an epithe- lium but not abutting upon a cavity. The tissues of some endo- crine glands are of this nature. To such tissues is applied the adjec- tive epithelioid. Fig. 78. — Types of epithelia. B. simple squamous; C, simple columnar; D, strati- fied columnar, ciliated a.t E; F, stratified polyhedral, upper cells squamous. (From Kingsley.) Epithelia carry on functions of most diverse kinds. The diversity is reflected in the structure of epithelia. Only a few of the more general features of structure can be mentioned here. s& CHORDATE ANATOMY Simple Epithelium. An epithelium only one cell in thickness is termed simple. There is, however, great variation in the thickness of ^/wM noTz.^ (»44« Fig. 79. — Columnar ciliated epithelium from human trachea. Most of the cells are slender, with axes more or less curved, and extend from the basement membrane to the free surface of the epithelium. Occasional short cells, basal cells, lie at or near the basement membrane and do not extend to the free surface. Several swollen mucous cells ("goblet" cells) are shown. (From Bremer, "Text-book of Histology.") simple epithelia. The cells, seen in sections perpendicular to the surface, may be approximately square in outline. Such an epithelium is called cuboidal (Fig. 77), but incorrectly for the cells are usually hexagonal prisms. A simple epithelium consisting of tall prismatic cells (Fig. 78C) is called columnar. At the extreme of thinness are epithelia (flat or squamous) each of whose cells is a broad flat plate, hexag- onal in outline. (Figs. 77, 78S) Stratified Epithelium. On Amphi- oxus, a slender marine animal only four or five centimeters long, an epidermis one cell thick afi'ords adequate protection. On an elephant it would not. Surfaces of large heavy animals are exposed to excessive mechanical friction and impact. Loss of material at the surface is best compensated for by a stratified epithelium whose lower layers persistently grow to replace the loss. A stratified epithelium may be two or several or many cells in thickness (Fig. 'jSD-F). In all vertebrates the epidermis is stratified (Fig. 80). Fig. 80. — Skin of lung-fish, Proto- pterus; section perpendicular to sur- face; much enlarged, c, dermis (corium); e, epidermis; g, multi- cellular gland; u, unicellular gland. (From Kingsley.) HISTOLOGY 89 Its thickness varies with the size and habits of the animal, and, in a particular animal, it varies locally depending upon the degree of exposure to mechanical wear. In a thick stratified epithelium the cells of the bottom layer are usually columnar and those of the outer layers are more or less flattened. The intermediate cells have a form such as would result from crowding tightly together a mass of compressible spheres, that is, polyhedral. Yet the cells are not actually packed tightly together. They are separated by excessively thin intercellular lymph spaces through which seeps lymph Stratum comeum. Stratum germinativum. Corium (Tunica pronria.) Fig. 81. — Epidermis from the sole of the foot of an adult man. Section perpendicu- lar to surface of skin. External to the stratum germinativum, the strata show successive stages in the production of the stratum corneum. X360. (From Bremer, "Text-book of Histology.") derived from underlying blood-vessels and serving to provide for the metabolic needs of the individual cells. Cells on opposite sides of the intercellular space are connected by delicate strands of solid, or at least dense, substance. Presumably protoplasmic, the strands are called protoplasmic bridges or plasmodesms. Many epithelia, although "simple" in the sense of being only one cell thick, are not the ideally simple tissue of the definition (page 85), con- stituted of cells all "alike in their internal differentiation." Among the special functions of an epithelium are the following: (i) production of a 90 CHORDATE ANATOMY superficial covering of non-living, mechanically protective substance; (2) production of special secretions such as mucus; (3) reception of external stimuH; (4) provision for motile activity. Two or more of these functions may be carried on by one "simple" epithelium or by a stratified epi- FiG. 82. — Developing scales of dogfish, Squalus; sections perpendicular to surface of skin; much enlarged, c, upper layers of epidermis; d, dentine of scale, deposited by dermal cells beneath it; ee, enamel-forming organ of scale — a specialized region of the germinative layer (w) of the epidermis; p, "pulp", the dentine-forming organ. (From Kingsley.) thelium. Within the epithelium, then, cells will exhibit differentiation of as many types as there are functions. (i) Most epithelia produce a protective covering at the free surface. A cuticula is a dense, tough or hard nitrogenous material deposited on the exposed surface of an epi- thelium. The cells which produce it and underlie it remain alive. Keratin is a nitrogenous organic substance which is formed within some epithelial cells. It is the basis of the homy structures of the vertebrate skin. Tha "horny layer" (stratum comeum; Fig. 81) developed on the skin of vertebrates other than fishes, consists of one or more of the outer strata of the epidermis, the cells more or less filled with keratin and strongly adherent to one another so that the whole layer acquires a high degree of mechanical resistance. Completely keratinized cells are dead. Hair, feathers, reptilian scales, claws, nails and hoofs are horny structures. Calcareous material may be deposited by an epithelium, either at its outer surface {e.g., shell of a mollusk) or, exceptionally, at its inner surface {e.g., enamel of teeth; Figs. 82, 129). Fig. 83. — Sensory cells. A, cell from the sense organ (crista acustica) of an ampulla of the ear; B, rod cell from the retina; C, cell from the olfactory epithelium. (From Kingsley, After Furbringer.) HISTOLOGY 91 (2) A glandular epitheliiun is one in which secreting cells are scattered more or less abundantly throughout the layer. (Fig. 80, 11) Gustatory Pore Sustentacular cell canal cell Pore canal Sustentacular Pore Gustatory (cut obliquely) cell canal cell Connective tissue of tunica propria Fig. 84. — Taste-buds from a vallate papilla of the human tongue; as seen in section perpendicular to the surface of the epithelium. S is a diagrammatic representation of the structure of one "bud." X475. (From Morris, "Human Anatomy.") (3) In a sensory (or neuro-) epithelium certain cells are specialized for reception of stimulation by some agency in the cell's environment (Fig. 83). EpitheHal sensory cells may be grouped in clusters to form sense organs (Fig. 84). An epithelium may be rendered sensory by free nerve termina- tion, that is, the terminal twigs of a nerve fiber ramifying amongst the epithe- lial cells (Fig. 85). These nerve fibers, however, are not produced by the epithe- lium itself but invade it from adjacent tissue. (4) Cilia are extremely delicate motile ' filaments borne by the free ends of epitheHal ation in the epiderm^rof^'^S^la- Cells. A single cell mav carry from one ^^ndra. (From Kingsley. after ' ... Retzius.) to over a hundred. A cmated epithelium is one in which some or all of the cells carry cilia. (Figs. ^SE, 79) Cilia and mucous glands commonly occur in the same epithelium. The simple external epithelium of an earthworm and the stratified epider- mis of a fish combine cuticular, glandular and sensory specializations. Glands " Glands " whose products are as different as are sweat, eggs and blood- cells hardly merit the same name. Accepting the name, it is necessary 92 CHORDATE ANATOMY to distinguish different types of gland: (i) secretory glands whose products are retained at least temporarily and serve some useful purpose — e.g., mucous, salivary and thyroid glands; (2) excretory glands which eliminate waste — e.g., kidneys; (3) cytogenic glands which produce living cells— e.g., reproductive glands producing eggs or sperm, various lymph and blood glands in which white blood-cells are produced. Secretory glands may be unicellular (Figs. 79, 80, u) or multicellular (Fig. 80, g). Nearly all multicellular glands develop directly from epithelia and retain their epithelial character. Some endocrine glands are epithelioid. Most secretory glands develop from either the ectodermal or the endo- dermal epithelium and discharge at the surface of their native epithelium. Such are the many kinds of skin glands and digestive glands. The Fig. 86. — Types of multicellular glands. A-D, tubular; E, F, alveolar or acinous. A, simple; B, coiled; C-F, branched. The duct pierces the epithelium from which the gland has been produced. (From Kingsley.) mesoderm gives rise to some secretory glands, especially in connexion with the reproductive system — e.g., the albumen glands and shell glands of oviducts and the mucous glands of the mammalian uterus. Multicellular glands may be tubular (Fig, S6 A-D) , or alveolar (acinous; Fig. S6E,F). Glands of either type, complicated by branch- ing, are called compound (Fig. S6C-F). The larger multicellular glands, and especially those which are com- pound, require certain accessory structures. A good blood supply must be provided. Therefore the gland may have an outer investment of connective tissue containing blood-vessels and lymphatics. A thin layer of unstriated muscle fibers may be present on the wall of a gland which discharges its contents abruptly. The muscle would be accom- panied by nerve fibers and in some glands nerves may be traced to the secretory cells. Secretory glands in vertebrates range from unicellular mucous glands in the skin of fishes and amphibians and in the digestive epithelium of all vertebrates to such massive compound multicellular glands as the mammary glands and the liver. HISTOLOGY NON-EPITHELIAL TISSUES 93 The primarily essential parts of a metazoan animal are the epidermal epithelium and the enteric epithelium. Certain of the organs which, in the adult, lie between these two layers consist of tissues which do not retain the epithelial character of the embryonic tissues from which they are derived but give rise to more or less bulky and solid masses of material. The important types of adult non-epithelial tissues are the following: (i) muscular; (2) nervous, exclusive of neuro-epithelial structures; (3) tissues serving for mechanical support — the connective and skeletal tissues; (4) adipose tissue or fat; (5) blood. Muscular Tissue Locomotion in some protozoans is effected by beating of ciHa. The movements of large animals depend on contractile mechanisms. Con- tractility is inherent in protoplasm. The least specialized protoplasm is apparently able to contract in the direction of any of its axes. When protoplasmic mechanism for effecting vigorous, quick or long continued contracting is established, the ability to contract becomes restricted to one axis. The protoplasmic structures which seem to be somehow immediately concerned with contraction are exceedingly fine fibrils, the myofibrils, which extend through the cell parallel to the axis of contraction. Fig. 87. — A, unstriated ("smooth") muscle cell with single nucleus; B shows a small portion of the length of a multinucleate striated fiber. (From Kingsley.) Among invertebrates the usual type of muscle element is a much elongated cell having a single nucleus, more or less numerous myofibrils extending through the protoplasm lengthwise of the cell, and having the usual cell-wall devoid of any special membranous covering. Such cells, associated together to form layers, bundles or masses, constitute the muscles of the body-wall and the viscera. Certain invertebrates, how- ever, whose muscles are, in one way or another, especially efficient have muscle cells or more complex sort. The myofibrils become strongly developed and each fibril exhibits an alternation of darker and lighter zones. The zones of either type lie exactly alongside one another on adjacent fibrils so that they give the impression of transverse bands or 94 CHORDATE ANATOMY striations extending continuously across the cell. Muscle cells of this sort are called striated. Uninucleate striated fibers occur in the heart of some moUusks. In arthropods, especially insects, striated fibers Fig. 88. — Striated muscle; human. Above, in longitudinal section, showing small portions of several fibers; below, section transverse to the length of the fibers. Nuclei lie at the surface of the fiber. (From Bremer, "Text-book of Histology.") attain great length, are multinucleate, and exhibit a complex system of transverse striations. Vertebrates possess both striated and unstriated (or "smooth") muscle (Fig. 87). In general, the muscle of the bodv-wall is striated and HISTOLOGY 95 from A Fig. 89. — Motor guinea-pig; surface view of muscle fiber: B, from hedgehog; section perpendicular to surface of muscle fiber, g, granular substance of the motor plate; m, striated muscle; n, nerve fiber; t.r., terminal rami- fication of the nerve fiber. (From Bremer, "Text-book of Histology"; after Bohm and Davidoff.) visceral muscle is unstriated. But unstriated muscle occurs in the walls of blood-vessels which lie in the body-wall, in connexion with some skin structures such as hair and certain glands, and also in the iris of the eye. The muscles in the walls of the mouth, pharynx and at least the upper part of the esophagus are striated, and it is said that striated muscle occurs in the wall of the stomach of some fishes. Also the external anal muscle is striated. The muscular part of the diaphragm is derived from the embryonic body- wall and its muscle is accordingly striated. And in all vertebrates the muscle of the wall of the heart is striated. Unstriated muscle fibers in vertebrates are much like those of inver- tebrates. They are ordinarily not over a fraction of a millimeter in length and, in man, much less than a hundredth of a millimeter in diameter. ^^^^»„^^^ They are usually spindle-shaped (Fig. 87^!) ^'"^'^ , lying in the tissue with their tapering ends li,--'" ^ overlapping. B,.- " The somatic striated fibers of vertebrates are enormously larger than unstriated fibers (Fig. 87^, 88). Their diameter may approach a millimeter and their length, not accurately known, doubtless reaches several or many millimeters. But these great fibers are not, in strict sense, single cells. They contain scores or hundreds of nuclei. The myofibrils of striated fibers are much coarser than those of unstriated fibers. They are imbedded in a peculiar fluid sarcoplasm which is probably a nutrient medium rather than ordinary cytoplasm. The wall of the fiber, much more prominent than an ordinary cell-wall, is called the sarcolemma. The alternate dark and light bands on the individual fibril are due to physical differences such that, in polarized light, the dark bands are doubly refractive (anisotropic) while the lighter bands are singly refractive (isotropic). Both the dark and the light bands are traversed by finer markings seen only under high magnification. Fig. 90. — Human car- diac muscle; a very small portion seen under high magnification, d, intercal- ated disc; Z, Krause's mem- branes which lie transversely at regular intervals along each myofibril, bisecting each light band. The dis- tinction between light and dark bands does not appear in the figure. (From Bremer, "Text-book of Histology"; after Heidenhain.) 96 CHORDATE ANATOMY The relation of an unstriated fiber to its nerve is apparently of the simplest sort. A terminal twig of nerve merely attaches to the surface of the fiber, the end of the nerve often showing a knot-like enlargement. Presumably every striated fiber has a nerve connected to it. The nerve, however, enters a small flat plate of nucleated protoplasm lying super- ficially on the muscle fiber. Within this motor plate (Fig. 89) the nerve ramifies into fine twigs which seem to terminate in the substance of the plate. Striated fibers are bound together in bundles enwrapped by a con- nective-tissue perimysium. Thick muscles consist of several or many such bundles wrapped together. Cardiac muscle has striations which resemble those of somatic muscle but the fibers are relatively short and are branched. The sarcolemma is less strongly developed than in somatic fibers. A peculiar feature of the cardiac fiber is the presence of conspicuous transverse bands, the inter- calated discs (Fig. 90) which are quite distinct from the ordinary stria- tions. Their significance is not known. Nervous Tissue All nervous functions are carried on by protoplasm organized, as always, in cells. To say, as is often done, that nervous tissues consist of nerve cells and nerve fibers is inaccurate. So far as known, every fiber which conducts nervous impulses is developed as an outgrowth from a cell and can function and survive only so long as it remains in physical and physiological continuity with the nucleated region of the cell of which it is an integral part. Any cell engaged in nervous operations, together with all conducting fibers which have grown out from it, is called a neuron. A central nervous organ is a more or less complex system of physiologi- cally related neurons serving for the proper association, coordination and integration of nervous impulses. A ganglion is a minor localized nerve center consisting of the cell-bodies of neurons together with the adjacent regions of their nerve processes. Neurons are of various types depending on the form of the cell-body and the number of nerve processes (Figs. 91, 92). Unipolar cells, of comparatively rate occurrence, have a single process; bipolar neurons are usually spindle-shaped and have a process at each end; multipolar cells have several processes of which one, the neuraxon (axon or neurite), is relatively long, while the short dendrites branch out into fine twigs which end within a short distance of the cell-body. The neuraxon may give off lateral branches (collaterals) and its distal extremity breaks up into fine branches forming the terminal arborization. HISTOLOGY 97 Most types of receptor neurons are epithelial. In some of these the receptor cell itself produces a nerve fiber which conducts to the central Fig. 91. — Types of nerve cells. A, multipolar cell; B, portion of nerve fiber with sheaths; C, unipolar cell (such a cell may arise by modification of a bipolar cell as shown in Fig. 93); D, pyramidal cell (from cerebral cortex), a, axon; c, collateral; cb. cell- body; d, dendrites; m, medullary sheath; n, nucleus of cell of Schwann's sheath; r, node of Ranvier; s, sheath of Schwann; t, telodendron. (From Kingsley.) Fig. 92. Cell-bodies of neurons showing arrangement of neurofibrils. .4, from human spinal ganglion; two cut fragments of the neuraxon lie near the cell-body. B, "giant pyramidal cell" from human cerebral cortex. Highly magnified, a, neuraxon. (From Morris, "Human Anatomy.") organ— e.g., an olfactory cell and its fiber (Figs. 83C, 3485). In such cases, one neuron serves as both receptor and conductor. In other 98 CHORD ATE ANATOMY cases, as in the auditory organ and taste-buds (Figs. S4B, 348Z)), the epithelial receptors do not produce nerve fibers but are intimately related to the terminal twigs of afferent nerve fibers whose cell-bodies lie in some deep ganglion such as the acoustic ganglion or a spinal ganglion. Nerve cells vary greatly in size, but in general are relatively large. They are often the largest cells in the body exclusive of eggs. The most striking characteristic of the body of a neuron is the presence of large masses of a granular substance which has a strong affinity for the anilin dye, methylen blue. These Nissl's bodies (Fig. 94) have been shown to become reduced in neurons which have been excessively active, indicating that the bodies contain something which is a source of energy for nervous activity. Less conspicuous are the neurofibrils (Fig. 92), Bipolar cells. Fig. 93. — Diagram showing how an embryonic bipolar nerve cell is transformed into a unipolar cell ("T-cell") such as occurs in ganglia of the dersal roots of spinal nerves. (From Bremer, "Text-book of Histology.") Fig. 94. — Nerve cell, with processes cut short; from human spinal cord. X430. (From Bremer, "Text-book of Histology.") extremely fine fibrils which are ordinarily seen only after use of special staining methods. Such neurofibrils may form an elaborate system within the body of the neuron and may be traced into the neuraxon and larger dendrites. The appearance and arrangement of these neurofibrils strongly suggest that they are specialized avenues for conduction of impulses. The neuraxon is a delicate thread consisting of a probably modified protoplasm in which, as just mentioned, neurofibrils may be demonstrated. It may be surrounded by one or two special ensheathing layers. The medullary or myelin sheath is a relatively thick layer of fat-like substance, myelin, fitting the neuraxon closely. The neurilemma or sheath of Schwann is an exceedingly thin cellular layer wrapped around the neu- raxon. (Figs. 91, 304) A neuraxon may possess either, both, or neither of these two sheaths. When both are present the myelin sheath is always next the nerve fiber and, at fairly regular intervals (in man averaging about 0.5 mm.) along HISTOLOGY 99 the fiber, it seems to be nearly or quite interrupted so that the neurilemma there comes into close relation with the nerve fiber (Fig. giB). The neuraxon therefore presents a segmented appearance due to these nodes of Ranvier. Nerves whose individual fibers possess the myelin sheath appear more nearly white than do non-medullated nerves. The so-called " white " parts of the brain and spinal cord consist mainly of medullated nerves. Non-medullated fibers and the cell-bodies of neurons are the chief con- stituents of "gray matter." The sheaths doubtless serve for the protection, insulation and nutrition of the nerve fiber. The source of the myelin is not definitely known. Bundles of nerve fibers Epineurium. Perineurium. Endoneurium. Fig. 95. — Structure of a nerve. The figure represents a small part of a transverse section of a large nerve constituted of many bundles of medullated fibers. X20. (From Bremer, "Text-book of Histology.") A nervous organ is constituted of neurons supported by connective tissues accompanied by vascular tissues. In the brain and spinal cord of vertebrates occurs not only the usual mesenchymal connective tissue but another which is unique in that its cells have ectodermal origin in common with the nerve cells. Some of the cells of this neuroglia possess branched processes which make them confusingly similar in appearance to nerve cells. The neuroglia cells form, by means of their processes, a supporting network for the nerve cells. A nerve is a bundle of neuraxons, each of which may be ensheathed as described above, and all wrapped together within a sheet of connective tissue, the perineurium (Fig. 95) extensions of which (endoneurium) may penetrate into the bundle. Larger nerves consist of several or many bundles all tied together by connective tissue and enwrapped by a rela- tively thick epineurium. Small blood-vessels traverse the connective- tissue lavers of the nerve. loo chord ate anatomy Tissues Serving for Mechanical Support Protoplasm is a substance of semi-fluid or gelatinous consistenc\-. An elephant constituted of protoplasm only is a mechanical impossibility. Large animals, especially if they are land animals, require mechanical support. Protoplasm provides such support by appropriating various materials from the environment and building them into non-living struc- tures which are external to the cells and physically adapted to the mechanical needs of the animal as a whole and of its parts. The basis of the material of these supporting structures consists of various nitrogenous or protein substances. By impregnation of the /. f \ \ ^ A B Fig. 96. — -.4, mesenchymal tissue from, embryo of the amphibian, Ambystoma. B, pigment cells from Ambystoma; below, a cell with pigment dispersed in numerous branched processes of the cell; above, a "contracted" cell with pigment concentrated, the transparent processes not shown. (From Kingsley.) material with inorganic salts, chiefly those of calcium, hard or rigid supporting structures are produced. The protoplasmic or cellular agencies concerned in building the supporting tissues are mesenchyme cells, except in the cases of the notochord and the ectodermal neuroglia (page 99) of nervous organs. The embryonic precursor of supporting tissues other than the excep- tions mentioned is a more or less spongy mesenchyme (Fig. 96.4) whose individual cells have branching processes by means of which the cells are joined together The spaces within the meshwork of cells is filled by a homogeneous fluid substance, the matrix. Presumably the cells are the source of the matrix. Connective Tissue The essential mechanical structures in connective tissue are relatively coarse white fibers consisting of an albuminoid substance, collagen, HISTOLOGY lOI the source of gelatin and glue. These collagenous fibers are only slightly elastic. They may be branched. Each fiber is a bundle of very delicate fibrils. Exceedingly flattened cells with flat nuclei appear as if clinging closely to the surface of a fiber. These connective-tissue cells or fibro- cytes are presumably the agencies which have brought about the produc- tion of the fiber in the intercellular matrix. Elastic fibers are much finer than collagenous fibers and difi"er from them chemically in being composed of elastin which is not a source of gelatin. An occasional elongated fibrocyte may be seen stretching along the surface of a fiber (Fig. 97). Elastic fibers commonly occur inter-mingled with collagenous fibers. Connective tissue forming a loose open mesh-work, as does the subcutaneous tissue lying between the skin and the muscle of the body, is called areolar tissue. Tendons and ligaments are connective- tissue structures highly adapted to resisting tensile strain. They consist of coarse collagenous fibers arranged in compact bundles. Tendons are inelastic. Chroma tophores, pigment cells (Fig. 96S), may occur in connective tissue, especially in the dermal layer of the skin. The specific pigment appears as granules lying in the cytoplasm. Black pigment (melanin) is most common and cells con- taining it are called melanophores. Chro- matophores are usually richly branched. The pigment may at one time be distributed throughout the processes ("expanded" phase), at another time densely massed in the central part of the cell ("contracted" phase). Some pigment cells are migratory. Fig. 97. — A, elastic fibers of the subcutaneous areolar tissue of a rabbit. B, cells related to elastic fibers, as seen after treat- ment with acetic acid; from sub- cutaneous tissue of a pig embryo. (From Bremer, Text-book of Histology: A, after Schafer; B, after Mall.) Skeletal Tissues Notochord. The essential notochord material consists of cells each of which contains a relatively enormous vacuole occupied by a substance of fluid, or possibly gelatinous, consistency. The cytoplasm of the dis- tended cell is so stretched that it appears as the thinnest possible layer surrounding the vacuole. The very flat nucleus occasions a bulge in the contour of one side of the cell (Fig. 98). The outer cell-membrane, while very thin, is probably of semi-rigid consistency. Seen under the microscope, this tissue looks like a mass of soap bubbles crowded closely I02 CHORDATE ANATOMY together, the cytoplasm and cell-membrane of each cell being the wall of a bubble. The vacuolated notochord tissue is enclosed by sheaths which differ in number and nature in various animals. There is commonly an inner elastic sheath (Fig. 98, ei) composed of material secreted by an outer epithelioid layer of the notochord tissue, and a thick outer sheath of dense fibrous connective tissue. Mechanically, the notochord resembles a length of rubber tubing, closed at the ends, and filled with liquid under pressure. Fig. 98. — Developing vertebrae of the amphibian, Ambystoma; I, earlier; II, later. Longitudinal sections. Cartilage and bone are forming around the notochord. cc, cartilage in center of vertebra; ci, epithelioid internal elastic sheath of notochord; /, incisure cutting through ic, intercentral (intervertebral) cartilage; n, notochord; ns, outer notochordal sheath; v, developing bone (black) of centrum of a vertebra. (From Kingsley.) Cartilage. In development of cartilage, mesenchyme cells become densely massed and then produce an abundant intercellular substance whose accumulation causes the cells to become more or less widely sepa- rated from one another (Figs. 68, 99). The intercellular matrix becomes solid and acquires a firm or even hard consistency. Chemically it is a complex of collagenous, albuminoid and other protein substances. The cartilage cells remain imbedded in the matrix, each occupying a close- fitting space, a lacuna. In some cartilages have been described exceed- ingly fine canals penetrating the matrix and putting any one lacuna into communication with neighboring lacunae. HISTOLOGY 103 The external surface of cartilage is invested by a connective-tissue membrane, the perichondrium (Fig. 99) which contains blood-vessels but they do not penetrate into the cartilage. Hence cartilage cannot occur in thick masses. In growing cartilage, cells from the perichondrium become cartilage cells and add cartilage to the exterior of the mass already formed. At »• -"S^ Capsule. o? :*■-. <^;^. . Matrix. '^erichond. -Bl.v. ^ Fig. 99. — Hyaline cartilage, with perichondrium; from human trachea. Bl.v., blood-vessel; x, cartilage cell whose nucleus is not in section; y, new matrix forming between two cells resulting from a recent division of a cartilage cell. (From Bremer, "Te.xt-book of Histology.") the same time deep cartilage cells divide. The resulting cells secrete matrix substance whereby they become separated, each to lie in a lacuna of its own. Hyaline cartilage (Fig. 99), usually bluish and clear, is nearly devoid of fibrous material. In fibrocartilage the matrix contains fibers similar to those of ordinary connective tissue. Elastic cartilage contains numer- ous elastic fibers. Calcified cartilage is rendered white and relatively hard by deposit of calcium salts in the matrix. I04 CHORDATE ANATOMY Bone. Cartilage and bone are similar in that their essential skeletal material is a non-living matrix within which are imbedded living cells. Bone differs from cartilage in that the matrix is highly calcified and cor- respondingly hard and also in that it never exhibits the apparent homo- geneity of the matrix of hyaline cartilage but is disposed in very thin -'BLOOD VESSEL ^COMPACT BONE ^MARROW ARTICULAR UGAMENT Fig. 100. — Diagram of the structure of a long bone. (Redrawn from Kahn's " Der Mensch," Albert Miiller, Zurich.) parallel layers. Usually the deeper substance of a bone (Fig. loo) is of a porous or spongy texture (cancellous bone) while the outer region is dense or solid (compact bone). A section of fully developed compact bone, seen under high magnifica- tion, shows the matrix layers or lamellae arranged in parallel or concentric order (Fig. loiB and 102). Between adjacent lamellae are minute Fig. loi.- — A, stereogram representing a sector of the shaft of a long bone. B, transverse section, much more enlarged, showing part of one Haversian system, bl, bone lamellae; c, canaliculi; /;, Haversian canal; I, lacuna. (From Kingsley.) cavities, the lacunae. Exceedingly fine canals, the canaliculi, extend between each lacuna and neighboring lacunae, piercing the intervening lamellae. In bone of a living animal each lacuna is occupied by a living bone-cell (osteoblast) from which processes extend into the adjoining canaliculi. All external surfaces of bone are covered by a membrane, the perios- teum (Fig. 100), of dense fibrous connective tissue well supplied with blood-vessels which enter the bone and branch throughout it. Most HISTOLOGY 105 bones, notably the long bones of the appendages, have internal cavities (Fig. 100) occupied by a more or less vascular soft tissue, the marrow. The "yellow marrow" of long bones contains much fat. "Red marrow" is highly vascular, contains little fat and may be a source of blood cells of various types. In long bones the larger blood-vessels lie approximatel}- parallel to the long axis of the bone. Around such vessels the bone lamellae are arranged in concentric order (Figs. loi and 102) forming so-called Haver- FiG. 102. — Section, highly magnified, of compact bone from the shaft of the human humerus. The section, cut transversely to the long axis of the bone, shows four Haversian systems with their central canals, concentric lamellae of bone, lacunae between adjacent lamellae, and canaliculi extending between lacunae. (From Bremer, "Text-book of Histology"; after Sharpey.) sian systems. These concentric systems are much less prominently developed in fiat bones. The matrix of bone consists of commingled organic and inorganic materials. Collagenous and other protein substances constitute the organic part while various salts of calcium, mostly the phosphate and carbonate, are the most important inorganic ingredients. Bone, because of the rigidity of its calcified matrix, is incapable of such interstitial growth as occurs in cartilage. A further difference between cartilage and bone lies in the fact that the cartilage cell produces matrix in all directions and thus surrounds itself by its own product, whereas the osteoblast produces matrix only at such part of its surface as is adjacent to the already- formed bone. A layer of bone cells building up lamella upon lamella io6 CHORDATE ANATOMY of bone may be likened to a group of masons laying course upon course of stone at the unfinished top of a wall. But, in the case of the bone, every now and then one of the masons, an osteoblast, is left behind and buried between successive courses of the wall, remaining there in his little lacuna as a permanent bone cell. Adipose Tissue Adipose tissue or "fat" consists of cells each of which contains a globule or vacuole of oil so large that the cytoplasm appears as merely an bl.v. i bl.v. Fig. 103. — Fat cells in subcutaneous tissue of a human embryo of four months. bl.v., blood-vessel; c.t., white connective-tissue fibers; fib., young fibrocyte; mes., mesen- chymal cell ; X, young fat cell, nucleus not in section; 1,2,3, developing fat cells. (From Bremer, "Text-book of Histology.") exceedingly thin layer surrounding the vacuole (Fig. 103). The flat nucleus lies in the peripheral layer of cytoplasm. The irregular poly- hedral form of the cells is doubtless the result of their mutual pressures. Blood The circulatory function of blood requires that it be fluid but various special services are rendered by cells suspended in the fluid, some of them passively carried by it, others capable of independent motion somewhat like that exhibited by an ameba. HISTOLOGY 107 The fluid part of blood, the plasma, is water containing all the other substances which enter into the constitution of protoplasm together with various hormones and the waste products of metabolism. In its inorganic chemical constitution, the plasma resembles sea water. In the coagulation of blood, on exposure to air or under some other circumstances, a nitrogenous substance, fibrinogen, carried by the plasma in solution, becomes transformed into fine solid filaments of fibrin (Fig. 104). The uncoagulated portion of the plasma is called serum. The "clot" is a mass of fibrin with blood cells caught in its meshes. Fig. 104. — Coagulated blodd. Biconcave red corpuscles arranged in "rouleaux"; filaments of fibrin radiating from minute blood plates. (From Bremer, "Textbook of Histology"; after Da Costa.) Blood cells are of two main kinds, red corpuscles or er5rthroc)rtes and white corpuscles or leucocytes. The red cells are much more numerous. In human blood the red cells outnumber the white in the ratio of five or six hundred to one. Erythrocytes (Figs. 104 and 105) are relatively small and usually have the form of flat discs with elliptical outlines. These blood cells are the oxygen-carriers, being heavily loaded with hemoglobin, a complex protein substance containing iron and having a strong afl^nity for oxygen which the cells pick up at the respiratory surfaces of the animal. Their color is due to the hemoglobin. The mature erythrocytes of all verte- brates except mammals are nucleated. In adult mammals, the red cells in course of their differentiation lose their nuclei, thereby acquiring the form of biconcave discs. (Fig. 104) Io8 CHORD ATE ANATOMY Leucocytes are permanently nucleated and do not carry hemoglobin. Several types are recognized. (Fig. 105) L5rmphocyte: usually small, cytoplasm scanty and usually non- granular, nucleus spherical. Large mononuclear leucocyte (monocyte) : more abundant and non- granular cytoplasm, nucleus excentrically placed. Polymorphonuclear leucocyte : large, with conspicuous granules in cytoplasm, nucleus indented, lobulated, irregular or separated into two or more parts. Several kinds are distinguished on the basis of the reaction of their granules to anilin dyes. Basophiles have granules which take basic stains; eosinophiles have an afl&nity for eosin, an acid dye; the granules of neutrophiles take both basic and acid dyes. Most leucocytes are capable of active ameboid motion. Many are phagocytic. Blood plates (Fig. 105) are minute bodies which seem to be proto- plasmic and yet are not nucleated. They probably result from fragmen- tation of cells in bone marrow or elsewhere. They seem to have some relation to the clotting of blood as indicated by the fact that the filaments of fibrin (Fig. 104) tend to radiate from blood plates. Lymph resembles blood but lacks erythrocytes and is therefore color- less. The fluids occupying the coelomic spaces and the cavities of brain and spinal cord, the aqueous humor of the eye and the amnionic fluid are all of the general nature of lymph but contain relatively few cells and differ from one another in details of chemical constitution. HISTOLOGICAL SPECIFICITY In general, histological differences are less conspicuous than the corresponding anatomical differences. Unstriated muscle fibers appear much the same whether they are in the wall of a stomach or of a lung. Nevertheless tissues and cells usually exhibit characteristics which mark them as belonging to a particular organ or animal. The nerve cells of a spinal ganglion differ from the motor nerve cells in the spinal cord of the same animal. Epidermal tissue of a fish differs from that of a reptile. It follows, therefore, that the individual tissue cell may, in its visible structure, exhibit characteristics reflecting as many as four grades of Fig. 105. — Cells from smear preparation of normal human blood; Wright's stain. In the center: adult red blood corpuscles, blood platelets and a polymorphonuclear neutrophile. At left above: two polymorphonuclear basophiles and two polymorpho- nuclear eosinophiles. At right above: three large and four small lymphocytes. At left below: polymorphonuclear neutrophiles; two of these cells, the uppermost and lower- most of the group, are young, with merely crooked nuclei; the mature cells have multi- lobed nuclei. At right belov/: six monocytes; in the younger cells the nuclei tend to be rounded, in the adult cells they arc horseshoe-shaped, indented or lobed. (From Bremer, "Text-book of Histology'.') HISTOLOGY -m ^\ 109 :.-J> «* *^ £: T^oTtr Fig. 105. — ("See page ro8 for description.') no CHORDATE ANATOMY organization. First there are those cell organs, such as nucleus and chromatin bodies, which represent the fundamental organization of protoplasm as cells. Then there are those intracellular structures such as myofibrils or neurofibrils which mark the cell as belonging to a particular tissue — muscular or nervous. Thirdly, there may be features which identify the tissue as that of a certain organ; for example, the inter- calated discs in the heart muscle of vertebrates. Finally, the individual tissue element may have peculiarities which are specific for animals of a certain group; for example, the striated muscle fiber of an insect differs in details of structure from that of a vertebrate. Behind all of this visible dilTerentiation and specificity of structure must be chemical specificity. CHAPTER 4 THE INTEGUMENTARY SYSTEM EVOLUTION OF THE INTEGUMENT Since all life involves continual adjustment of processes within the organism to conditions outside, the skin and its appendages which mediate this relation are highly important organs. Even among the Protozoa, an external semipermeable membrane separates the living protoplasm from the surrounding medium. Most Protozoa have in addition an outer differentiated layer of clearer cytoplasm, the ectosarc, analogous in function to the skin of the higher animals though without genetic relation. A true multicellular skin appears first in sponges and coelenterates, the ectodermal layer of Hydra being a familiar example. Even in so simple a skin as this, there is some differentiation among cells. Most are epithelial covering cells, each commonly prolonged at its base into a contractile thread. But among these are gland cells, which by their various secretions in different coelenterates indicate a wide difference in metabolic processes. The secretion of lime salts by the skin of coelenterates may be regarded as the beginnings of the exoskeleton of many higher invertebrates. Most invertebrates retain essentially unaltered the simple epitheUal ectoderm of coelenterates. Some have a ciliated epidermis which aids locomotion. Many secrete an external cuticula, in which lime may or may not be present. The evolution of a simple epithelium into a stratified epidermis, such as occurs in vertebrates, results, presumably, from a change in the direction of cleavage planes during cell multiplication. So long as cell walls form perpendicular to the surface, a simple epithehum results. When, however, cleavage planes form parallel to the surface, the membrane becomes stratified. The outer layers of cells serve to protect the lower layer where growth and cell multiplication take place. In animals exposed to dry air, an outer layer of dead cells is obviously adaptive. Yet the beginnings of a protective outer layer appear in the exoskeletons of water-dwelling invertebrates. Among invertebrates appears also, though exceptionally, a connective-tissue layer or corium beneath the epidermis. The lowest chordates (Balanoglossus, Ciona, Amphioxus) have both an outer epidermis and an inner corium; but the epidermis is only a single I 12 CHORDATE ANATOMY layer of cells. Gland cells are numerous in the epidermis of Amphioxus. The layer secretes a thin cuticle like that of annelids. The corium in Amphioxus is gelatinous. Although the epidermis is stratified in all vertebrates, such low forms as cyclostomes do not have the outer dead horny layer, and they do have the thin cuticular layer of the invertebrates and Amphioxus. The skin of fishes is like that of cyclostomes, except for differences in gland secre- tions. See Fig. io6. A. AMPHIOXUS a PETROMYZON C SQUALUS. 0. RANA. Fig. io6. — Cross sections of the skin of four chordates, Amphioxus, Petromyzon, Squalus, and Rana, showing the fundamental differentiation of the skin into corium and epidermis. The difTerentiation of the epidermis into a dead outer layer and an inner liv- ing layer began in aquatic animals. (Redrawn mainly after Plate and Schimkewitsch.) The outer dead horny layer of the epidermis, the corneum, appears first in Amphibia, correlated apparently with the land habit, since most land animals have it. As the amphibian skin is fundamentally like that of higher vertebrates, the evolution of the skin beyond the amphibians presents no serious difficulties. The striking differences are in the secre- tions of the glands. It is indeed difficult to imagine how the skin mucus of amphibians could have evolved into the milk of mammals. It should, however, be remembered that sUght chemical differences often result in striking differences in properties; so that we should not be surprised to find chemical differences in the skin secretions of vertebrates that are far greater than any morphological differences in the glands themselves. THE INTEGUMENTARY SYSTEM 113 STRUCTURE OF THE HUMAN SKIN The skin of man, together with its appendages, hair, nails, teeth, mem- brane bones, and glands, is only about four per cent of the body weight. Like that of other mammals, it consists of two tissues, an outer epidermis and an inner connective-tissue corium. A cross section of the epidermis shows under the microscope a many- layered epithelium, which varies greatly in thickness in different parts of GERMINATIVUM :\ «^ SWEAT GLANO'^ Fig. 107. — A cross section of the thickened skin of the sole. The stratum corneum is especially thickened on the sole and on the palm of the hand. the body. Even where it is thinnest, as for example on the back, at least two layers of cells are distinguishable, an inner, growing stratum germina- tivum and an outer, horny stratum corneum. The cells of the stratimi germinativimi are columnar in shape; those of the stratum corneum are flattened and scale-like. The former are alive and, by their constant proliferation on division planes parallel to the surface of the skin, they make continual additions to the stratum corneum. The living cells in their turn, as by the wearing off of the outer layers they come nearer and 114 CHORD ATE ANATOMY nearer the surface, replace their Uving protoplasm by keratin, and become the horny scales of the outer epidermis. In man, as in most mammals, the stratum corneum wears away as rapidly as it is formed and never becomes greatly thickened on most parts of the body. Amphibia, however, shed the stratum corneum in sheets, sometimes sloughing off the entire covering of the body at once. Serpents do the same thing, scales and all. Sections of the thick epidermis of the palms and soles show between the stratum germinativum and the stratxim corneum, two intermediate layers, a stratum granulosum and a stratum lucidum. These, however, are merely transitions between the inner growing layer and the outer lifeless horn. See Fig. 107. Coriimi. The deeper layer of the skin, the corium, cutis, or dermis, is connective tissue, with a much greater variety of cell elements than the epidermis, and, unUke the epidermis, richly supplied with blood-vessels. Where it touches the epidermis, especially on the palms and soles, the corium is thrown up into many fine papillae, the capillaries of which feed the cells of the stratum germinativum. In some of these papillae are tactile corpuscles and other nerve terminations. Cutaneous glands and the roots of hairs, both derived from the epidermis, become embedded in the corium, and from it they are fed. Fat cells are numerous, especially in the lower layers. The greater portion of the corium is made up of connective-tissue libers, both elastic and non-elastic. Most of these lie parallel to the surface, interwoven like the fibers of felt; but some bundles are perpendicu- lar to the surface. The fibers are more compactly set in the outer parts of the corium than in the inner. The deepest layer is the loose or areolar connective tissue by which the entire skin is attached to the underlying muscle or bone. Skin muscles are few, and are mostly connected with the bases of the hairs. The elasticity of the skin decreases with age. Leather is made from the outer, compact layer of the corium of animals. The epidermis is removed by maceration, and the connective-tissue fibers are toughened by tanning. DEVELOPMENT OF THE SKIN Notwithstanding the close connexion between the two main layers of the skin, their origin in the embryo is diverse, the epidermis developing from the ectoderm, the corium from mesenchyma. Since the mesen- chyma is derived chiefly from the mesoderm, this contrast in origin is fundamental. Epidermis. The embryonic epidermis arises directly from the ectoderm, and is at first a simple cuboidal epithelium. By the end of the first month, as the result of cell divisions in a plane parallel to the surface, THE INTEGUMENTARY SYSTEM II5 this epithelium becomes two-layered, the outer and thinner layer being the periderm. By the continued multiplication of the basal cells, the number of layers gradually increases until, by the fourth month, all four layers of the thicker parts of the adult skin have appeared. The cells of the stratum comeum contain a fatty or waxy substance, which helps to form the pasty vemix caseosa which covers the body of the new-born infant. Developing hairs, instead of penetrating this layer, lift it as a continuous sheet, the epitrichial layer. Coriimi. In most parts of the body, the mesenchyma which produces the corium is derived from cells which have migrated from the parietal layer of the mesoderm. For this reason, that part of the epimere which forms the corium is called the dermatome. In some vertebrate embryos, if not in all, the ectoderm also contributes to the mesenchyma of the head and possibly, therefore, to the corium. Embryonic mesenchyma consists of scattered, stellate cells, separated by wide spaces. It becomes the connective tissue of the corium by secret- ing intercellular fibers, both elastic and non-elastic. By the fourth month, the compact fibrous layer of the corium is distinguishable from the loose areolar tissue under it. Blood-vessels and nerves invade the corium from below, hairs and glands grow into it from the epidermis. Abnormalities in the distribution of blood-vessels cause birthmarks. FINGER-PRINTS In all primates, the entire surface of the palms and soles, but no other portion of the body, is marked with fine parallel ridges separated by equally fine grooves. At definite places on hands and feet, these ridges form concentric lines or loops. Eleven distinct "friction-ridge patterns" have been distinguished, five on the finger tips, four at the base of the fingers, two near the wrist or ankle. See Fig. 108. Those of the finger tips are most familiar, since they are used for identification. Since no function has ever been proved for these designs, their presence and their constant position has stimulated much interest and discussion. To useless organs, the hypothesis of special creation gives no clue. Are they, then, rudiments of structures functional in the lower animals? The significant fact about these patterns is that they match precisely, both in number and position, the concentric rows of horny scales on the foot-pads of insectivores, a group which, for various reasons, is thought to be near the direct line of man's ancestry. In the insectivores, the posi- tion and arrangement of the scaly ridges is clearly adaptive and the best possible one to prevent slipping in any direction. These finger-print pat- ii6 CHORDATE ANATOMY terns, therefore, serve to convict men of animal ancestry, as they have on occasion served to convict them of crime. On the sides of the fingers, the friction-ridges merge into rows of wart- like elevations. This has been interpreted as confirming the opinion that the ridges are remnants of rows of horny scales. That the ancestors ^ FRICTION ^DIGITAL PADS^ _^^RIDGES m TRIRADIl TRIRADII MNTERDIGITALjl, PADS ^1- Y THENAR.< PAD \ .WPOTHENAR N, PAD HYPOTHENAR ^ PAD A. INSECTIVORE B. MONKEY C. MAN Fig. io8. — Friction-ridge patterns in three mammals — insectivore, monkey, and man. The presence of such useless and rudimentary concentrically arranged ridges in the human hand receives its only reasonable interpretation in the light of the evolution theory. (Redrawn after Wilder.) of the mammals were scaly is, however, supported by more convincing evidence than this. APPENDAGES OF THE INTEGUMENT Throughout almost the entire animal kingdom the skin tissues form various calcareous, chitinous, or horny structures — shells, spines, teeth. SPINE OENTI IE EriAriEL ORCAN COMPACT CONNECTIVE TISSUE LOOSE 'connective TISSUE BASAL PLATE B. D. Fig. 109. — A vertical section of the skin of an elasmobranch, showing five stages in the development of a placoid scale. The development of a placoid scale is essentially like that of a tooth. This fact taken together with the similarity of their structure sug- gests that teeth may have evolved from placoid scales. (Redrawn after Schimkewitsch, modified.) bones, scales, hair, feathers, horns — which serve for defense, support of tissues, or attachment of muscles. The limy shells of molluscs and the chitinous exoskeletons of arthropods serve all three purposes. Among vertebrates, the placoid scales, which first appear in certain sharks of the Upper Devonian, are especially important because of their THE INTEGUMENTARY SYSTEM 117 further evolution. Each of these scales has a flat basal plate of dentine embedded in the skin, and each has commonly also a projecting spine coated, like a tooth, with hard enamel. From these minute placoid scales of ancient sharks have evolved all the multiform teeth of all the higher vertebrates. From these and other types of scale have evolved also, by simple enlargement, the heavy continuous dermal armor of ganoids and other fishes. These same bony plates survive also in man and the higher Fig. 110.— The scales are scales of the li -ENAMEL SPINE -The imbricated pattern of placoid scale arrangement in elasmobranchs. arranged in rows and usually each scale is in line with the interval between nes in front and behind. (Redrawn after Klaatsch.) vertebrates as " membrane bones " which, unlike most parts of the skeleton, are not pre-formed in cartilage but develop directly in connective tissue. HORNY SCALES Vertebrates, besides bony scales, have also horny; but these have played a much less important part in evolution, and are confined to amniotes, more especially reptiles. In reptiles, the stratum corneum forms a continuous scaly layer over the entire body, the separate scales being local thickenings which con- tinue to grow by the addition of new keratin from underneath. Serpents commonly shed this scaly coat twice a year. But the rattlesnake retains bits of the old skin at the tip of the tail. These become the rattle, which therefore grows two rings a year. Il8 CHORD ATE ANATOMY Most reptiles have substituted horny scales for the bony scales charac- teristic of fishes. But crocodiles have both sorts on the same individual. On the ventral side of some snakes, large horny scales are attached to muscles and become organs of locomotion. The largest reptilian scales are those of Chelonia, in which horny scales fuse with the bony carapace and plastron. In birds horny scales cover the feet. Among mammals, the East Indian Manis, and the tails of rats and mice are scaled. Fig. III. — Section of developing scales of lizard, Sceloporus. c, papilla of corium; f, outer layer of epidermis which later becomes cornified; /, fibrous layer of skin; ni. Malpighian (stratum germinativum) layer; p, periderm; ts, tela subjunctiva. (From Kingsley's "Comparative Anatomy of Vertebrates.") It is a curious fact that, while horny scales are purely epidermal structures, their development is initiated, like that of bony scales, by the corium. HORNS To produce such diverse structures as hairs, feathers, scales, nails, and hoofs, demands most exceptional evolutionary potentiaUties on the part of the horny layer of the skin. Among the surprising developments of keratin-forming tissues are the horns of ruminants and rhinoceros. Those of the rhinoceros are formed wholly of keratin produced by the stra- tum corneum on the snout. The hollow horns of cattle have, in addition to external keratin, a bony base and core, which extends from the frontal bone into the cavity of the horn. The antlers of the deer tribe are bony out- growths with no covering of horn, but only the skin or "velvet" which is soon lost. Horns are best interpreted as weapons of defense and offense. NAILS, CLAWS, AND HOOFS Nails are scale-like thickenings of the stratum corneum at the ends of the fingers and toes, formed of homogeneous keratin identical with that of the stratum lucidum from which they develop. Nails and claws are strikingly alike except in form. Both develop from a matrix at the base, which in man appears as the whitish "lunula." Both have their bases surrounded by a fold of skin, the "nail wall." In both, a convex "outer plate" on the upper side of the digit may be dis- THE INTEGUMENTARY SYSTEM 119 tinguished from a concave "ventral plate" on the under side, each being morphologically a reptihan scale. The ventral plate in man is reduced to a narrow fold of skin between the nail and the finger pad. Claws appear first in urodele Amphibia. In some Anura, they are limited to certain hind toes. But certain male frogs, at mating time, develop horny papillae on their thumbs, which serve to hold a slippery female. Reptiles have claws on all toes. Those of mammals are like those of reptiles, except where the mammalian claw has altered into a hoof, or become retractile, as in cats, which walk on foot-pads and keep their claws sharp by raising them off the ground. Claws of mammals intergrade with nails, so that it is difficult to draw a line between the two. Some primates have both claws and nails on the same foot. Nails are Fig. 112. — Diagrams of (A) nails, (B) claws, and (C) hoofs, e, unmodified epidermis; M, unguis (outer plate); s, subunguis (ventral plate). (From Kingsley, after Boas.) then rudimentary claws, modified to correlate with the increased sensi- bility of the ends of the digits and their use as organs of touch. Some mammals, such as the horse and deer, which run on their toes, have hoofs instead of claws. The structure, development, and relations of hoofs, however, prove that they are nothing more than enlarged and modified claws. Both have dorsal and ventral plates. The attempt to divide mammals into hoofed and clawed types encounters the difficulty that at least one animal, Hyrax, has both claws and hoofs. FEATHERS Feathers, which are characteristic of birds, are modified scales, and their early development is the same. A corium papilla initiates both; but the feather anlage, instead of flattening to a scale, becomes an elon- gated cylinder, which spUts into the barbs and barbules of the developed feather (Fig. 113). A down feather, in fact, suggests an elongated and frayed-out scale. I20 CHORDATE ANATOMY Birds, which have descended from reptihan ancestors, still have scales on their feet, and even their bills and claws are presumably enlarged and modified scales. HAIRS Hairs, which are characteristic of mammals, are not comparable morph- ologically with either feathers or scales, since the development of hairs is CORIUM PAPILLA^ BARBULES P[JLP.^s:^=C^^ — ^PAPILLA C. "FOLLICLE' D. Fig. 113. — Four stages in the development of a feather. A, B, and C represent stages in the development of a down feather. D shows a contour feather in the feather- sheath. A-B and C-D are sections of a young contour feather at the levels indicated in D. In contrast with a hair the development of a feather is initiated by a corium papilla. (Redrawn from Ihle, after Biitschli.) initiated by the epidermis and not by the corium. When, as in Manis, hairs and scales occur together, the hairs are at the apices of the scales. That scales are older phylogenetically than hairs, is indicated by the fact that scales develop earUer in the embryo; and fossil evidence demon- strates that mammals have evolved from some scaly stegocephalan-Hke cotylosaurian. But since neither the skin nor its non-bony appendages are commonly fossiUzed, their history has to be made out chiefly from embryology and comparative anatomy. All mammals have hair; and man's relative hairlessness is by no means a distinctive human trait, since some mammals, for example the whales, have less hair than man. It is well known that changes in the secretion of the endocrine glands affect profoundly the growth of hair, and man's loss of hair may have been thus brought about. THE INTEGUMENTARY SYSTEM 121 Hair Structure. The hair of all mammals is essentially similar. There are, however, such differences of detail as enable an expert to identify different species. Each hair consists of a "root" buried in the skin, and an external shaft. Microscopic examination shows a multicellular structure, with the cells in three layers, an outer cuticle, a cortex, and a central medulla. The cells of the cuticle are scale-like, overlapping one another like shingles on a roof. Cortex cells, greatly elongated, make up the greater portion of each hair. The medulla occurs only in the "contour hairs" and is wanting in the finer hairs. It is made up of cuboidal cells usually in a double row. Fig. 114. — Diagram of structure of hair, b, blood-vessels; ct, cuticle of hair; ex, cortex; g, gland; h, hair; he, Henle's layer; hf, hair follicle; hx, Huxley's layer; w, medulla; p, papilla; sg, stratum germinativum of epidermis. (From Kingsley's "Comparative Anatomy of Vertebrates.") The root is surrounded by epithelial and connective-tissue sheaths. It ends in a swollen "bulb," from which it grows and which contains a connective-tissue papilla, with capillaries which feed the hair. Hairs of different human races differ in cross section. In general, the rounder the hair, the straighter it is; the more compressed, the curlier. It has not been shown that these differences have been developed either by natural or by sexual selection. Hair Direction. Hairs, instead of projecting vertically from the skin, emerge at an acute angle, have a slant in some special direction, and thus form streams in various parts of the body. Where such currents meet, either "rhomboids" or "vortices" may form, the latter being commonly called "cowlicks." The fact that such rhomboids and vortices appear on the human body in regions where the hair is short, has been interpreted to mean that man's hairy covering was once longer than at present. Although in general the direction of hair growth is such as to make gravity the determining influence, it is a curious fact that the hair on 122 CHORDATE ANATOMY the human forearm suggests his animal ancestry. The hair of the fore- arm slants from the wrist toward the elbow, in the reverse direction to the slant on the upper arm. Man shares this pecuHarity with the apes alone. All other mammals have the same hair direction on both parts of the limb. Why this resemblance of man to the apes unless they share a common ancestry? The pecuharity is not adaptive, and it is not easy to see why, if man and apes were independently created, they should resemble one another in this detail. Hair Arrangement. That the arrangement of hairs on the human body has any evolutionary meaning is, to say the least, surprising. Indeed, since such patterns can have no use, we should hardly expect to find them at all. No less surprising is an arrangement of hair in mammals that indicates descent from scaly ancestors. In most mammals, the hairs occur in Fig. II';. — Arrangement of the , rn, hairs in groups of threes and fives groups of three.or more. These groups in the human embryo, with the g^j-g arranged in parallel rows in such wise l^t:^L'"Trot KlSsTe^'aft:' that each cluster Ues opposite an interval Stohr.) in the rows in front and behind. In short, the arrangement is imbricated, Hke the universal arrangement of scales. This arrangement, though quite useless, is precisely what we should expect if mammals have descended from scaly ancestors. See Fig. 115. Histogenesis of Hairs. Hairs are, in origin, epidermal, and therefore ectodermal. Each begins as a minute epidermal papilla, which has arisen by local cell proliferation in the stratum germinativimi. See Fig. 116. Continued proUferation gradually converts this papilla into a cellular column, which extends obliquely downward into the underlying mesen- chyma which is to become the corium. The growing end swells into a bulb, in which later develops the corium papilla from which the hair is to grow. Cellular differentiation of the hair column results in an inner sheath and the hair-shaft, all surrounded by an outer sheath. From the bulb to the point in the hair column where the sebaceous gland develops, the cells of the hair-shaft become cornified. Above this point the central cells degenerate to form a canal in which the hair-shaft grows towards the surface. Continued cell multiplication of the stratimi germinativum of the papilla elongates the central hair-shaft to extend beyond the skin. Each hair thus formed continues to elongate throughout its life of several months or years, the rate of growth varying greatly in different THE INTEGUMENTARY SYSTEM I 2- parts of the body. But finally growth ceases, the hair dies, and is shed. If the hair papilla retains its stratum germinativum, a new hair grows. Each hair column, in addition to producing a hair, may form as lateral outgrowths one or more sweat or sebaceous glands. Muscle cells devel- oped from the mesenchyma of the corium attach themselves to the hair- roots and become arrectores pilorum. The human foetus before birth has a hairy covering, the ''lanugo," which is shed shortly before or soon after birth. The coat persists, SEBACEOUS GLAND EPITHELIAL BED ROOT OF HAIR Fig. ii6. — A vertical section of skin of a five month human embryo, showing four early stages in the development of a hair. The growth of a hair is initiated by the forma- tion of an epidermal papilla projecting (down) into the underlying corium. (Redrawn from Bremer after Stohr.) however, in certain types of "hairy men." The evolution theory affords the only rational explanation of the lanugo. PIGMENT Skin color in man is due in part to the blood in the capillaries of the corium. In addition, there are two pigments in the skin and hair, a brown, sometimes darkened to a black, both in granules; and a yellow, that may strengthen to a red, both diffused in the tissues. All are prod- ucts of cell metaboUsm.^ The pigments of the hair are confined to the cortex. The epidermis and the outer parts of the corium are both pigmented. Not until shortly after birth do pigment granules appear in the stratum germinativum, so that even Negroes are born white. Moles and freckles involve excessive local pigmentation. Freckles are small local patches of excess pigmentation, which are more likely to occur in light-skinned individuals who have been exposed to strong sun- light. A mole or nevus is an elevation of the skin due to local prolifera- T24 CHORDATE ANATOMY tion of epidermis and corium, and is usually excessively pigmented. When a mole is congenital and involves blood capillaries, it forms a "birthmark." Since pigments like those of vertebrates are found also in invertebrates, there is no reason to question their common origin. Many animals below the mammals have their pigments in special cells, the chromato- phores, which "expand" or "contract" (see page loi) under the influence of hormones and thus alter the color of the skin. The colors of lizards, which are often brilliant, are not in their scales, but in chromatophores of the underlying corium. Widely among vertebrates, pigments of scales, skin, hair, or feathers often show striking and elaborate patterns that serve for protection, warning, recognition, or sexual allure; but in man chiefly the region of the nipples and the external genitals are slightly darker than the rest of the body. In man and some other hairless mammals, such as the elephant, the function of the skin pigment is to check ultraviolet light before it penetrates to living cells. Everyone has observed the effect of the sun's rays upon unwonted skin, and the promptness with which the skin responds by tanning. Lacking skin pigment, men could not live in some parts of the earth. Color in Races and Individuals. The blue of the iris of human children and new-born kittens is an interference color, like the blue of the sky or the "eyes" of a peacock's tail. Later, as the iris fibers thicken, the inter- ference is less perfect, and the eye is gray. Brown pigment in some fibers only, gives hazel. Brown eyes are evenly pigmented. Dark brown eyes are called black. There is also the yellow pigment which, nearly free from brown, gives the amber eyes of some blondes. The same color intensified makes the red iris that sometimes accompanies red hair. The interference blue slightly masked by yellow, gives that rarest of all eye colors, green. In general, among Europeans, the eyes are less pigmented than the hair, so that dark hair with gray-blue eyes is common. But some blondes have a striking color scheme, eyes darker than the hair. Hair is colored by the same two pigments, both usually present, with the brown-black, masking the red-yellow, except in strong light. But some dark hair lacks the red and is blue-black. Some blondes have no brown pigment, and little yellow. Most have brown also, along with varying amounts of yellow. The tow head with a touch of dark, is the ash blond. Yellow with some red is golden; and starting from this, the red may strengthen to a rather unadmired carrot or orange. More brown carries the red over into auburn; still more gives bronze. Hair that has lost its pigment is white, for the same reason that snow is; the crystal faces of the one and the cell walls of the other scatter the light. THE INTEGUMENTARY SYSTEM 12 5 Skin color is like hair color except that the blood color below the pig- ment may show through, and that sunUght, which fades the Hfeless hair, stimulates the living skin to turn dark. Primitive man was dark as the ape ancestor was, and as most races are still. Reducing the black pigment, with the yellow retained, gives the Mongolian skin color. The stronger yellow, together with a good deal of brown, is the traditional hue of the Red Man, though as a matter of fact, most Indians are brown, like most White Men. The blond White Man is a local race that originated in some region near the Baltic, apparently since the last Ice Age. Being highly energetic and uncommonly well endowed, the descendants of these blond giants have made their way all over the world, and, much diluted with darker blood, still appear in most civilized countries of the world. Why their blondness, nobody knows. It cannot be due to chmate, for the Eskimos, Samoyads of Siberia, the Patagonians, and the people of northern China are all dark. Naturally, a blond race could hardly survive in the tropics; but a white skin is no obvious advantage anywhere. Yet certain studies of Chicago children show that the highly pigmented Italians are more Uable than lighter stocks to rickets, which is a sunlight- deficiency disease. So it may be that in high latitudes, in a wooded coun- try or one that has much cloud and fog, a fair skin that is still able to tan may have a selective value and be accounted for on Darwinian principles. Skin color plays queer tricks. Any two parents, even two Negroes, may have an albino child; two dark-haired parents may somehow miss with the brown pigment and have red-haired offspring. The most that anyone can say is that "Nordic" man probably began as a mutant from a dark stock. Possibly, after the mutation appeared, it was admired and selective mating kept it to the fore. CUTANEOUS GLANDS Since among invertebrates most glands are unicellular, it has generally been assumed that the multicellular glands of vertebrates have evolved from such beginnings, an increase in the size of the secreting cells tending to carry them into the underlying corium, where groups of such epidermal cells become multicellular organs. Be this as it may, cutaneous glands develop, much as hairs do, from solid cords, which are proliferated from the stratum germinativum and grow downward into the underlying corium. The lumen of the gland forms later, to connect with the exterior, and the gland anlage differen- tiates into a secretory portion and a lining for the duct. The secretory cells become intimately associated with blood-vessels and nerves. Sweat Glands. In man sweat glands occur in most regions of the body, and are especially abundant on the palms and soles. They are, for the most part, of the simple tubular type, much coiled to increase the 126 CHORDATE ANATOMY secreting area; but those of the axillae are branched and greatly enlarged. They are of the "vitally secretory" type, that is, the cell protoplasm merely produces the secretion, but is not converted into it, and the cell continues alive indefinitely. The sweat is usually oily but, in man, becomes watery under the influence of the nerves. See Fig. 117. ASSOCIATION NEURONE EFFERENT NERVE SWEAT GLAND CAPILLARIES Fig. 117. — A diagram illustrating the nervous mechanism of temperature regulation in man. The quantity of secretion of tubular glands (and consequently the amount of sweat which may evaporate to cool the body) depends upon the quantity of blood in the capillaries associated with the glands and dermal papillae. Through a reflex arc the circulation is regulated by the temperature of the skin. (Redrawn after Hough and Sedgwick.) Sebaceous Glands. Sebaceous glands in man occur on most parts of the body, but are wanting on the palms and soles. Most hairs have sebaceous glands connected with their follicles. They are of the acinous type, and necrobiotic, that is, their protoplasm forms the fatty secretion, which the cell extrudes, and then dies. Other Glands. Besides the sebaceous and sweat glands, there are other highly specialized cutaneous glands, the lacrimal glands of the eye and the Meibomian of the eyehds, the wax glands of the auditory meatus, besides preputial, vaginal and anal glands, which occur in most mammals, and mammary glands in all mammals. Of the organs which have evolved from glands, none are more sur- prising than the luminous organs or photophores of deep-sea fishes. These are true dark lanterns since they have a condensing lens and a reflecting membrane. The light is produced by the oxidation of luciferin secreted by the gland. No carbonic acid and little heat are evolved in the process. THE INTEGUMENTARY SYSTEM 127 Mammary Glands. Mammary glands first appear in monotremes as a pair of milk-secreting organs on the ventral side of the body. They are without nipples, and in Echidna they pour their secretion into a CUT LIMB BUD I POSITION OF DEFINITIVE GLAND 'I?^|f1"^^^ MILK LINE GLAND ANLAGE NIPPLE EPIDERMIS V u-^' SMOOTH MUSCLE.ltj_ CORIUM- ■ MUSCLE 13 5 MM. HUMAN EMBRYO B MUSCLE C FAT Fig. 118. — A figure illustrating the development of the mammary gland in man. A 13.5 mm. embryo shows the "milk line", a ridge which extends from the a.xillary region to the groin. The definitive gland develops only from the anterior portion of this line. Taken with the evidence of supernumerary teats in man, the line is interpreted as proof that the ancestors of man had more than a single pair of mammary glands. (Redrawn from Arey, after Kollmann.) A, B, and C are sections of the definitive mammary gland in successive stages of ontogenesis. A is from a two-months embryo, B from a four-months embryo, and C from a seven-months embryo. From its development the mammary gland is seen to be a compound tubtilar gland. (Redrawn from Arey, after Tourneux.) Fig. 119. — Scheme of different kinds of nipples. Single line, ordinary integument; double line, that of primary mammary pocket. A, primitive condition, found in Echidna; B, human nipple; D, Didelphys before lactation; C, same at lactation; E, embryonic, F. adult conditions in cow. B and C are true nipples, F, a pseudo-nipple (teat). (Based on figures by Weber from Kingsley.) depression, the "mammary pocket", surrounded by a fold of skin. From this condition in monotremes, the teats of the higher mammals have evolved, either by elevating the milk-field at the bottom of the pocket 128 CHORDATE ANATOMY into a "true teat," as in man, or else by elevating the surrounding ridge to form the "false teat" of ruminants. The number of teats corresponds roughly to the number of young in a litter. Certain abnormalities in the milk glands of man, however, confirm strongly the theory of the animal origin of the human body. Super- numerary nipples appear in man with a certain statistical frequency. But these extra teats, instead of being placed at random, are usually set Fig. 120. — The presence of supernumerary teats (polymastism) in man supports the theory of the animal origin of the human body. Their repeated occurrence has received no other rational explanation. They are reversions or atavisms. (Redrawn after Wiedersheim.) in two ventral rows, precisely like two rows of nipples which form the milk lines of lower mammals. They are, then, best interpreted as rever- sions to an animal ancestor. The theory of special creation gives no clue whatever to their occurrence in human beings. See Fig. 120. Functional differences among the glands of vertebrates are much greater than morphological, and their physiological evolution is a difficult problem in biochemistry. CHAPTER 5 TEETH No invertebrate has teeth at all comparable, save in function, position, and material, with the teeth of vertebrates. Some annelids, like Nereis, have horny pharyngeal teeth that act like pincers. A circle of calcareous teeth surrounds the mouth of the vegetarian sea-urchin to form the "lantern of x\ristotle," but each tooth has its own muscles and there is no jaw. Some snails have a radula with which they rasp their food. Many arthropods, notably the biting insects, have their appendages modified into hard mouth-parts that are both jaws and teeth. But a series of independent teeth operated by a movable jaw is peculiar to chordates. Nor do all chordates possess such teeth. The protochordates have no teeth of any sort. Cyclostomes have within the oral hood horny ecto- dermal teeth, with which they cling to their prey or bore their way into its flesh. In this absence of calcareous teeth attached to jaws, as in so many other characteristics, the cyclostomes exhibit their primitive nature. The larvae of some amphibia have upon their jaws horny teeth in the form of papillae. Most reptiles have true teeth; but the Chelonia have replaced those of their ancestors with horny beaks. So, too, have all modern birds, although their ancestors of the Jurassic and Cretaceous had typical reptihan teeth. The embryo of the duck-billed platypus has rudimentary teeth which it does not use. Even among the placental mammals, the edentates either have no teeth or have them without enamel. In the toothless whales, teeth are present in the embryo, but the adult has only whalebone strainers. EVOLUTION OF TEETH Since the protochordates are without teeth, the cyclostomes have horny ones, and all attempts to discover any sort of rudimentary cal- careous teeth in cyclostomes have proved unsuccessful, it seems clear that teeth of the vertebrate type are a new acquisition with no homologs anywhere among invertebrates. Typical or "true" vertebrate teeth have their beginning in the innu- merable, minute placoid scales which so roughen the skin of sharks that in former time shark skin, under the name shagreen, was widely used as an abrasive. 129 I30 CHORDATE ANATOMY In elasmobranchs, on the edges of the jaws, these minute scales become enlarged into formidable biting teeth or sometimes, inside the mouth, into bony pavements that are used for grinding the food. That the biting tooth of elasmobranchs is a modified placoid scale is obvious from inspection, since the two look alike, and there is a transition in size and form between them. This identity is further borne out by OLFACTORY PIT -- TASTE PAPILLA L:_.uPHAGtAL VILLUS Fig. 121. — The pharynx of an elasmobranch (Squalus) laid open to show the double row of teeth in both upper and lower jaws. Such teeth differ only in size from the placoid scales of the pharynx and skin. Elasmobranch teeth, like scales, are fastened in the skin and are not attached to the jaw cartilages. (After Cook.) other evidence. Like true teeth, placoid scales have a base of dentine, which contains a pulp-cavity filled with connective tissue. Both scales and teeth have a spinous process, covered with enamel, which protrudes through the skin. Moreover, their development is similar in that, in both, the enamel is secreted by the ectoderm and the dentine by mes- enchyme, and both arise in that portion of the mouth where the ectoderm has invaginated to line the digestive tube. See Fig. 122. TEETH 13 r Originally, in vertebrates, the teeth were for seizing and holding prey. Grinding and cutting teeth, tusks, and fangs, are all modifications of the primitive mouth trap. The number of these holding teeth is indefinite in elasmobranchs, which may have as many as one hundred. They are not attached to the jaws, but merely imbedded in the skin of the mouth. They are all about aUke; and when one is lost, another moves forward into its place. In teleosts, the number of teeth is somewhat reduced, although all parts of the mouth and even the pharynx may carry them. The special advance made by the teleosts is to set the teeth more firmly by fusing their bases with the membrane bones of the mouth. Fig. 122. — Comparison in development and structure between a placoid scale and a tooth, a, b, and c represent the scale; d, e, and / the tooth. In all the figures the epidermis is dotted, but its stratmn germinativiitn is represented by a layer of large cells with nuclei; and the cutis is presented as composed of fibers with scattered cells. X, enamel membrane; y, cutis papilla; e, enamel; d, dentine; p, pulp cavity. (From Wilder's "History of the Human Body," Henry Holt & Co.) Amphibia still farther reduce the number of teeth, but retain them on premaxilla, maxilla, mandible, vomer, and palatine bones, and more rarely on the parasphenoid. But toads have no teeth whatever. A strik- ing feature of certain ancient and long extinct amphibians, the labyrin- thodonts, which arose in the Coal Period and survived into the Triassic, was the enormously complicated folding of the tooth enamel and dentine, which anticipated, yet went far beyond, the similar arrangement in some mammals. Reptiles make two important advances toward the condition in mammals. Some of them, like some of the amphibians, have their teeth set on a ledge on the inner side of the jaw— pleurodont dentition. Or they may have the tooth set directly on the bone, acrodont dentition. But the crocodiles and some fossil reptiles attain to a thecodont dentition, in 1,3 • CHORX)ATE ANATOMY which each tooth is fixed in a separate socket, as in mammals. In addi- tion, some lizards and numerous fossil reptiles abandon the original homodont dentition, with all teeth about alike, and have their teeth more or less differentiated into incisors, canines, and molars, as in mam- mals, a heterodont dentition. The differentiation of teeth is obviously, an adaptive division of labor, the incisors acting as cutters or chisels, the canines as daggers, and the molars as grinders. The reptiles, more- over, limit their teeth to the two jaws. An especially elongated tooth occurs in the lower jaw of lizard and snake embryos, which is used to break through the tough membranous shell. A hardened tip of the horny beak of birds is used for the same pur- pose. The two structures are, however, morphologically quite different. A C Fig. 123. — Jaws of some primitive Jurassic mammals. The resemblance of these jaws and teeth to those of the theriodont reptiles of the same period suggests a similar genetic origin. (Redrawn from Romer, after Simpson.) Especially remarkable in reptiles are the highly specialized poison fangs of certain snakes. These are modified from the ordinary conical tooth, first by a folding of the tooth to form a groove along which the venom from a modified salivary gland flows into the wound. In other snakes, by a still further folding the edges of the groove unite, and the tooth becomes a hollow needle. One pair only is functional at any one time, others up to nearly a dozen pairs being held in reserve to take the place of the large fangs when these are lost. All the vipers, including the rattlesnakes, fold back the functional pair of fangs when the mouth is closed, and only in the act of striking pull them erect by special muscles. Mammals, besides having nearly always a heterodont dentition, with incisors, canines, and molars well differentiated, have acquired also a definite succession of sets of teeth, a set of "milk teeth" developed in early life being later replaced by a "permanent" set. The elasmobranchs, indeed, do have a certain succession of teeth, but only one at a time as single teeth are lost. Lower vertebrates, reptiles conspicuously, have a somewhat indefinite number of sets, and are said therefore to be polyphyodont. Only the mammals have two definite TEETH 133 sets, and are therefore diphyodont. But monotremes, sirenians, and toothed whales retain their milk teeth throughout life, have no second set, and are said to be monophyodont. In general, then, the course of evolution has been from a large and indefinite number of simple teeth all alike, not fixed firmly in place, and borne by any part of the mouth, to a reduced and definite number, set firmly in alveoU, confined to the jaws only, and differentiated into three sorts. Along with this has gone a shortening of the jaws and a change of food habits. But whether the change in diet caused the change in teeth, or the change in teeth made possible the change in foods, is still an unsolved problem. EVOLUTION OF COMPOUND TEETH Compound teeth resembling the molars of mammals first appear in certain late Permian and early Triassic reptiles, the theromorphs. Since amphibians and the earlier reptiles had simple conical teeth, the conclusion has been drawn that compound teeth are derived from conical teeth, and morphologists have advanced two theories as to how this evolution came about. The differentiation theory of Cope and Osborn assumes that the teeth of vertebrates were originally of the simple conical type found in most reptiles. Such were the teeth of the premammalian Stegocephala and of primitive theromorph reptiles. The first multitubercular type of molars of modern mammals appears in such a Triassic mammal as Dromatherium, the teeth of which had a large median cone or protocone in line with two smaller cones, a paracone in front and a metacone behind. Corresponding parts in the teeth of the lower jaw are called protoconid, paraconid and metaconid. Teeth of this sort are known as triconodont. Besides the three cones, triconodont teeth have a basal rim, the cingulum, which forms part of the crown. Marsupial-Uke mammals of the Tertiary had teeth of this triconodont sort. See Fig. 124. The secondary tubercles of such teeth show a tendency to enlarge to the size of the protocone. A further advance occurs when the three cones assume a triangular relation to one another, the secondary cones of the upper jaw migrating inwards, those of the lower jaw outwards. Teeth of this tritubercular sort occur in Amphitherium of the Jurassic period. Later, in mammals, appeared a posterior projection or talon, and a fourth tubercle, the hypocone and hypoconid. With these additions, the molar teeth assumed more and more the modern form with six cusps. It took many million years to accomplish these changes, which were 134 CHORDATE ANATOMY naturally based upon change in the form of the tooth-germ and involved budding of that organ. The concrescence theory accounts for the multitubercular molar teeth of mammals by supposing a fusion of the anlagen of conical teeth, the number of cusps corresponding with the number of conical teeth involved. Some observers claim to have found evidence of fusion of tooth-germs in vertebrate embryos, but most investigators are sceptical. It must be said, however, that tooth fusion is known to occur in the case of the massive pavement teeth of dipnoi. At the present time the con- crescence theory seems to have less factual support than does the differen- tiation theory. According to Bolk, in a modified form of the concrescence theory, compound teeth are formed by the fusion of the germs of successive sets. A B C D E Fig. 124. — A, triconodont tooth of Dromatherium; B, tritubercular tooth of Spalaco- therium; C, interlocking of upper (dark) and lower (light) tritubercular molar teeth (after Osborn) ; D, molar of Erinaceus; E, of horse (selenodont type); c, cingulum; m, metacone (metaconid) ; pa, paracone (paraconid) ; pr, protocone (protoconid) ; I, talon. (From Kingsley's "Comparative Anatomy of Vertebrates.") His theory assumes that the ancestors of mammals had more than two generations of teeth Hke the milk and permanent sets, that is, their denti- tion was polyphyodont. Under these conditions, the germs of successive sets might fuse with one another. The factual foundations of the theory, however, are weak. TEETH OF MAMMALS Teeth of mammals are especially important for the paleontologist, partly because they are hard and therefore Hkely to be preserved, but more because mammalian teeth are closely correlated with feeding habits. But feeding habits, in their turn, are correlated with the entire bodily structure, so that teeth are a key to the whole organism. Moreover, mammalian teeth are so highly specialized and so diverse in size and structure, that a single one is often sufficient to identify a species. In general, the tendency has been to reduce the number and to do away with the division into two sets, and at the same time to specialize and elaborate individual teeth. An ideally complete set for a placental TEETH 135 mammal would consist of three incisors, one canine, four premolars, and four molars in each half-jaw. The distinction between premolars and molars is that the premolars replace milk teeth, and are therefore of the second set, like the teeth in front of them ; but the molars have no predeces- sors, and are therefore really of the first set. Functionally, however, and often in size and shape, there is little difference, and the two groups are conveniently lumped together as cheek teeth. But while 3-1-4-4, 48 teeth in all, is ideal (conceptual), no placental mammal conforms to it, 44 being the usual limit in any actual animal and 3-1-4-3 a common formula. Nevertheless, it is convenient to think of each actual tooth as one particular member of the ideal set. Thus can the history of each tooth be followed throughout all the placental mammals, and each be identified wherever it occurs. But the marsupials are aber- rant, opossums, for example, having four and five incisors, so that their teeth cannot always be homologized with those of placentals. Starting, then, with the ideal dental formula: ^'3, c\, p\, m\, the little hyrax or cony, allied to the ungulates, is one of the few mammals to retain the full eight cheek teeth. But its incisors are reduced to one in each half-jaw and it has no canines. On the other hand, the ungulates tend to have the typical four front teeth, but to lack one molar and sometimes a premolar also. They usually have the canine like the incisors and so have practically, though not morphologically, four incisors. But all hollow-horned ungulates lack upper canines, and many, like domesticated sheep, have lost all four incisor-form teeth from the upper jaw. Their 0-0-^-3 dental formula is, therefore, in brief form: The pigs, with forty-four teeth in all, are peculiar in having the canines in both jaws grow throughout life as fast as they wear away. They are kept sharp by whetting against one another. The walrus makes tusks of the upper pair only, which also are unrooted. The narwhal, for a like pur- pose, uses one incisor, its mate remaining rudimentary, and has no other teeth. The Carnivora make the canines, especially the upper pair, into long curved daggers, which reach their extreme development in the extinct saber-toothed tiger of the Pliocene but are noteworthy even in the domestic cat. With each canine, in the flesh eaters, goes a "carnassial" tooth, especially developed in the cats, a premolar above and a molar below. Other cheek teeth, especially in the cats, tend to be reduced almost to rudiments. Moreover, the Carnivora, though uniform as to incisors and canines, 3-1-4-2 differ somewhat widely in the cheek teeth. Thus, while the dog is 3-1-4-3 the cat is reduced to The lynx is made a separate genus from 136 CHORDATE ANATOMY the cats because it has lost the minute first premolar of the upper jaw and ■2-J-2-I brought its dentition down to On the other hand, some of the ° 3-1-2-1 whales have gone back to primitive conical teeth used only for holding, are virtually or quite homodont, and have fifty or more pegs in each jaw. Characteristic of rodents is the complete absence of canines, and the reduction of the incisors to one functional pair in each jaw. The single pair, however, is a remarkable tool. Each tooth grows from a permanent germ that is set far back in the jaw, so that each passes under all the cheek teeth before it emerges at the front of the mouth. Enamel coats the front surface only, so that as the tooth wears, the dentine wears most, and the thin plate of enamel remains always sharp. Since these teeth grow throughout life, if they are not worn away by gnawing they become too long and the animal cannot feed. No rodent has more than six cheek teeth, many have only four, and an Australian mouse so far depends on its incisors, that it has brought its dentition down to But the hares and rabbits, and some other 1-0-0-2 rodents, as if to exhibit their affinities with other mammals, have two more incisors, very minute, behind the large pair in the upper jaw. Proboscidians. The most specialized of all teeth are those of elephants. Incisors and canines are completely lacking in the lower jaw. In the upper jaw, one pair only of incisors become the tusks, but the other two pairs have so completely vanished that it is not known certainly which pair remains. The tusks are rootless, and grow from far up in the skull. They elongate throughout fife, growing faster than they wear away, until in some instances they have reached a length of eight feet and a weight of more than 150 pounds each. Certain extinct elephants had tusks even larger, up to twelve feet and two hundred pounds. In the Indian ele- phant, only the males have tusks. But the larger African species, which uses the tusks for digging roots, has them in both sexes. The famous African elephant Jumbo, in a fit of rage, broke off both tusks inside his cheeks. When they grew out again, they made new holes through the flesh, but the original holes remained for the rest of the animal's life. Two extinct proboscidians, Tetrabelodon and Dinotherium, had tusks on the lower jaw also, those of Tetrabelodon nearly parallel with the upper pair, those of the Dinotherium turned downward like those of a walrus. Curiously, the small milk tusks of the young elephants, which are shed early, are rooted like ordinary teeth — another illustration of Von Baer's law that the young in a speciaUzed group tend to resemble general- ized ancestors. The cheek teeth of proboscidians, less conspicuous than the tusks, are even more remarkable. There are six in each half jaw, i.e., twenty- TEETH 137 eight teeth in all including the tusks and a pair of evanescent incisors. But of the six grinders not all are in use at one time. As the foremost wears down and is shed, a second and larger moves into its place, only to be followed by the remaining teeth in succession. Thus an old animal, since there are no canines, may have only the two tusks and four grinders. The grinders themselves are remarkable for their enormous size, the largest being more than a foot from front to rear and four inches wide. Each tooth is highly complex, with intricate folding of enamel and dentine, so that as the softer dentine wears away faster, the tooth keeps always its sharp grinding ridges. The same arrangement on a smaller scale appears in various other vegetarian mammals, notably in the horse. The -PALATINE A. CAUCASIAN B. NEGRO C. ORANG OUTAN Fig. 125. — Dental arcades of ape, Negro and Caucasian. The fcjrm of the Negro arcade is transitional between that of the ape and white man. With the shortening of the human jaw the diastema between incisor and canine teeth seen in the ape jaw is lacking. The refinement of the face is one of the most striking results of primate evolution. (Redrawn after Wiedersheim.) huge single teeth coming successively into use are a device for prolonging the life of the animal long after any set of teeth all functional at once would wear away. Apparently, as a result, the elephants are among the longest lived of mammals. Primates. The primates, except for their hands, feet, and brains, are a somewhat unspecialized group, and their teeth, though reduced in number to conform to the shortened jaws, are little differentiated and the enamel is not folded. The dental formula for the Old World monkeys is 2-1-2-3 ill both jaws. But the New World monkeys have another premolar, and sometimes lack one of the three molars. It is a curious fact, which no special creationist has attempted to explain, that man, also an Old World primate, has exactly the dental formula of the others. The canines in monkeys are somewhat longer than the other teeth, and in the male gorilla are much like those of the less specialized carnivores. 138 CHORDATE ANATOMY Significantly, in man, although even the upper canines are hardly larger than incisors, they have nevertheless the long roots of the animal tusk. That general tendency to shorten the mammalian face, which has brought down the cats to three and four cheek teeth and the higher primates to five, continues in man by a general reduction in size of all the teeth and by closing the diastema, the open space next the canines. Consequently, human teeth are a continuous series and no tooth is very much larger than another, for the canines ceased to be tusks at the begin- fCANINE INCISORS I PREMOLARS A^^^^^l— v .--" 'MOLARS \ "PREMOLARS INCISORS '^CANINE Fig. 126. — Human teeth viewed from the left side. The human dental formula is: is, c'l, pmS, mh. As a result of the shortening of the human jaws the third molars fre- quently do not erupt. The elongated root of the canine tooth suggests that as in lower primates the ancestors of man may have had fangs. (Redrawn after Braus.) ning of human evolution. Along with these, has gone a change in the direction of the incisors, correlated with the appearance of a chin. In the apes the incisors protrude, in man they stand upright. Incidentally, the human "bite" becomes horseshoe-shaped, with the rows of cheek teeth no longer parallel, as in all lower forms, even the apes. In addition, the triangular upper molars of the apes, with three cusps, become in man quadrangular with four, and, correlated with the reduced size of the single teeth, their pulp cavities become relatively still farther reduced, not to sacrifice unduly the thickness of the tooth wall. Some fossil human molar teeth, however, are taurodont, having a relatively large pulp cavity as in Neanderthal man. All these differences between apes and men are, however, bridged by various fossil creatures, on the whole human, some of the genus Homo TEETH I3Q but not our species, others quite outside the genus, but still within the family. TEETH OF MAN Human teeth are in structure substantially like those of most other mammals, and very like indeed to those of other primates. In each tooth three parts are distinguishable, an external enamel- capped crown, a root buried in a bony socket or alveolus, and a neck or constricted region between root and crown. The number of cusps or tubercles on the crown varies in the different teeth. The incisor and canine teeth have a single cusp, the premolars have two, and hence are known as bicuspids, and the molar teeth may have as many as five. The number of roots also varies in the different teeth. Incisors, canines, and premolars have but one, although the roots of the premolars are some- times divided into two. The lower molars have two roots, and the upper molars three. The finer structure of a tooth may be best seen in a thin longitudinal section. See Fig. 127. The central portion, the pulp cavity, is filled with connective tissue containing blood capillaries and nerve fibers, which enter the tooth through a minute foramen at the end of the root. The larger mass of the tooth is formed by a bone-like substance, the dentine or ivory. Unlike bone, however, dentine is devoid of cells. In section, the dentine takes on a somewhat fibrous appearance from the presence of parallel tubes, the dental canaliculi, which radiate from the pulp cavity through the dentine. At their peripheral terminations in the dentine, the canaliculi branch profusely. The sensitivity of the dentine to the dentist's drill is probably due to the living protoplasm in these canaliculi, which acts in the manner of nerve fibers. The larger part of the dentine, approximately 75%, consists of inorganic mineral salts such as calcium phosphate and calcium carbonate. The remaining 25% is organic material. At no place on the tooth does the dentine reach the surface, since the crown and neck are covered with enamel, while the root is surrounded by a heavy cement. Enamel is the hardest substance in the human body, since it contains only three and a half per cent of organic substance. It is thickest at. the apex of the crown, and thins out towards the neck and root. High magnification shows that the enamel consists of minute parallel hexagonal prisms which rest on the dentine and extend to the outer surface of the crown. Increase in the amount of enamel toward the outside of the crown is effected by means of increase in the number of enamel prisms and not by their enlargement or branching. In this way, the solidity of the enamel is maintained throughout the crown of the tooth. The mineral constituents of enamel are identical with those of dentine. 140 CHORDATE ANATOMY growth lines in enamel pulp chamber growth lines in dentine root canal oral epithelium osteoblasts of periosteum of alveolus connective- tissue fibers cementoblasts ( from dental sac) bone of mandible blood vessels and nerves Fig. 127. — Schematic diagram showing the topography of a tooth and its relations to the bone of the jaw. The numbered zones indicate empirically the sequence of deposition of the dentine and enamel. The so-called growth lines in the dentine and enamel follow the general contours indicated by the dotted lines in the figure but are much more numerous. (From Patten's "Embryology of the Pig.") /DENTINE CANAL ODONTOBLASTS Fig. 128. — Diagrams illustrating the difference in the secretion of dentine, .4; and of bone, B. The functional polarization of the odontoblasts and osteoblasts is, how- ever, similar. (Redrawn after Braus.) TEETH 141 Cement is a bone-like substance covering the root of the tooth as a thin layer which becomes thickest at the apex. Like other bone, the cement contains lacunae connected with one another by canaliculi. The mineral constituents are identical with those of bone. Surrounding the cement is a connective-tissue dental sac or membrane continuous with the periosteum of the alveolus and at the neck connected with the covering of the gum, gingiva. Development of Teeth When the human embryo has attained a length of about 11 mm., that is, by the end of the sixth week, the ectodermal epitheHum covering the upper and lower jaws grows rapidly down into the underlying con- ERDERMIS CORIUM BONY ALVEOLU LINGUAL LAMINA ENAMEL ORGAN OF PERMANENT TOOTH Fig. 129. — Diagrams of three stages in the development of a mammalian tooth as seen in sections of the jaw. The anlage of the permanent tooth Mes on the lingual side of that of the milk-tooth. (Redrawn after O. Hertwig and Arey.) nective tissue to form a horseshoe-shaped ridge or lamina extending along the edge of the jaw. As growth continues, the lamina divides into an outer labial lamina and an inner lingual lamina. The two ingrowths, however, soon separate, one growing in labially, the other lingually. The latter forms the dental ridge or lamina. As in development of a hair, the dental ridge is formed by cell multiplication in the stratum germinativum of the epidermis. Early in the development of the dental lamina, a series of bell-shaped enlargements, ten in each jaw, appear along its labial border (Fig. 129). These are known as enamel organs since they secrete the enamel covering of the crowns of the teeth. Each of the twenty milk teeth has a separate enamel organ, and all of them are present in a 2^^ months embryo. Each enamel organ contains a mesenchymatous dental papilla, the outer cells of which, the odontoblasts, secrete the dentine of the tooth. The remaining cells of the papilla become the pulp of the tooth. As develop- 142 CHORDATE ANATOMY ment proceeds, each enamel organ recedes from the dental lamina with which it retains a transient connexion by means of a "neck" or cord of cells. The free edge of the dental lamina, losing connexion with the anlagen of the milk teeth, forms a second set of enamel organs lying on the lingual side of the primary set. In this way, the anlagen of the thirty-two permanent teeth come to lie embedded in the connective tissue of the jaws on the hngual side of the primary set. The permanent teeth are, however, relatively slow in development, the third molar usually not forming in the jaw before the fifth year. blood-vessel \ in pulp dentine enamel blood-vessel in mesenchyme odontoblast ' dentinal Tbme's ameloblast outer epithelium fiber process layer of enamel organ Fig. 130. — Projection drawing of small segment of developing incisor from 130 mm. pig embryo to show formation of enamel and dentine. X350. (From Patten's "Embryology of the Pig.") Soon after the enamel organs emerge from the dental lamina, they become differentiated into three layers, an inner ameloblast layer which secretes the enamel, a mesenchyme-like enamel pulp, and a layer of outer enamel cells. The ameloblast cells which line the enamel organs are columnar epitheHal cells derived directly from the stratum germinativum of the epidermis. Viewed from the inner surface, each ameloblast cell is hexagonal and each secretes a simple hexagonal prism of enamel. As the enamel increases in thickness, the multiphcation of ameloblast cells results in an increase in the number of enamel prisms. The twisting and curvature of the prisms in the developed tooth are a consequence of the torsion of the ameloblast layer during active secretion. While the enamel grows by addition from the outside, the dentine increases in thickness from within. Consequently as the tooth is formed the amelo- TEETH 143 blast and odontoblast layers are pushed farther and farther apart. During the secretion of the dentine, protoplasmic strands from the odonto- blasts are retained within the dentine thus forming the dental canaliculi. The odontoblast cells persist throughout life, and by their continued secretion may in old age entirely obliterate the pulp cavity of the tooth. The crown of the tooth is the first to develop, and for a while the tooth resembles a silver-plated thimble, the thin enamel coating cor- ENAMEL PULP PERMANEhq:, TOOTH PAPILLA' ■»^p?e!i. Fig. 131. — A section of the jaw of a nine-months human embryo, showing the anlage of a canine tooth. The enamel organ of the permanent incisor is seen on the lingual side of the milk-tooth. (Redrawn after Corning.) responding to the silver plate, the dentine to the underlying metal. As the tooth grows, it increases in length as well as in thickness, adding first a neck and later a root. The opening into the inner pulp cavity becomes more and more restricted as the root elongates until finally only a minute foramen remains to admit blood-vessels and nerves. The nerves grow into the pulp and acquire free terminations among the odontoblast cells. The cement layer is the last to be added. Cement is secreted by bone-cells which penetrate the connective-tissue sac enclosing the tooth. 144 CHORDATE ANATOMY Membrane bone is formed around the root of the teeth to form the alveoli of the jaw-bone and to hold the teeth firmly in place. The mechanics of the eruption of teeth is a problem which needs further elucidation. Among the factors which operate is the elongation of the root, although teeth erupt before the root has completed its growth. The eruption of the deciduous teeth begins during the seventh month after birth, and is usually completed by the end of the second year. Of PERMANENT- INCISORS DEC IDUOUS — c — r^3 INCISORS. \ \V SECOND PERMANENT- MOLAR. PERMANENT- PREMOLARS PERMANENT- CANINE PERNMNENT MOLAR. '^^^f^^y^ PERMANENT INCISORS- FiG. 132. — The teeth of a five-year-old child. Portions of the jaws have been removed so as to expose the roots of the milk teeth and the anlagen of the permanent teeth. The latter are stippled in the figure. (Redrawn after Sobotta.) the permanent set, the first to erupt are the first molars which appear during the sixth year. The last to erupt are the third molars, which frequently become impacted in the jaw-bone so that eruption is impossible. The shape of a tooth is determined by that of the tooth-germ. If the layer of ameloblasts is folded, the enamel is correspondingly modified, and teeth such as those of ruminants and elephants, which become ridged by wear as the result of the difference in hardness of enamel and dentine, owe this adaptive characteristic to the folding of the ameloblast and odontoblast layers. The multipHcation of roots as in molar teeth is produced by the budding of the odontoblast layer of the dental papilla. CHAPTER 6 THE SKELETAL SYSTEM Some creatures, jelly fish for example, have no skeleton. In some, as in many molluscs and more conspicuously the corals, the skeleton is heavier than all the soft parts combined. In man, the bones make up about a fifth of the entire weight of the body, and this is not far from the average for active air-breathing vertebrates that do not have dermal armor. All skeletons support or protect the softer parts. Supporting skeletons occur even in such lowly creatures as protozoans and sponges. Protective skeletons are conspicuous in echinoderms and molluscs, are universal among the arthropods, and are found among vertebrates in such diverse groups as the Paleozoic ostracoderms, ancient and modern ganoids, dinosaurs, turtles, and armadillos. Skeletal parts, which are also jointed levers used in locomotion, occur in arthropods and vertebrates alone. Arthropods solve the problem of locomotion by means of a chitinous exoskeleton with the muscles inside it. Such a skeleton is highly efficient as attachment for muscles, and it has the further advantage of providing armor at the same time. Its disadvantage is that it cannot grow, so that all the arthropods, by one device or another, shed their exoskeletons as their bodies enlarge. This leaves them for a time helpless. Furthermore, since the tissues of the molting arthropod are unsupported, no arthropod can attain any considerable size. Among arthropods the largest air- dwellers are foot-long centipedes; and although among water-dwellers the euripterids of the lower Paleozoic and earlier were more than a yard long, a twenty-pound lobster is about the limit for a modern form. The typical arthropod is a tiny insect. The endoskeleton of vertebrates, light and strong, and capable of indefinite growth, has the single disadvantage that skeletal armor must be developed independently. But vertebrates have for the most part abandoned armor. Their success as a group has depended not a little on their admirable endoskeleton. To its usual functions, the vertebrates add the production of blood cells by the marrow, especially in the long bones. The Two Parts of the Skeleton. Historically, the vertebrate skeleton consists of two parts, which began independently, have evolved separately, 145 146 CHORDATE ANATOMY and not even in the 'higher forms have become completely integrated. These are the appendicular skeleton of the four limbs with their girdles; and the axial skeleton, which includes the skull with the jaws, and the vertebral column, the sternum, and the ribs. The individual bones number, in man, sixty-four for the shoulder girdle and the arms, sixty-two in the pelvic girdle and the legs, twenty-three in the skull, twenty-six in the backbone, and twenty-five for ribs and sternum, with six ear bones besides, over two hundred in all. TARSALS METATARSALS PHALANGES Fig. 133. — A diagram of the vertebrate skeleton, showing the division of the skeleton into axial, visceral, and appendicular. Membrane bones are shown in black, cartilage bones stippled. THE AXIAL SKELETON Evolution of the Vertebral Colunin. Nothing like a vertebral column appears in any invertebrate, so that the earlier portions of its history are unknown; though, if Amphioxus gives the clue, it was once no more than a medial dorsal fold of the ahmentary canal. Its first certain beginnings are the notochord of the lower chordates, the Hemichorda, Urochorda, and Cephalochorda. In the cyclostomes, the notochord is still the main part of the axial skeleton. Since the cyclostomes have cartilaginous neural arches, it is probable that neural arches are the earhest vertebral elements. Elasmobranchs, both fossil and modern, show a considerable advance over the cyclostomes. Cartilaginous haemal arches and centra appear, with both neural and haemal spinous processes. The anterior trunk vertebrae of elasmobranchs have short lateral or "costal" processes which extend between the myotomes and which suggest the future ribs of mammals. Since in fossil and living forms two centra may occur in each body segment, and since each centrum usually develops in ontogenesis by the fusion of antero-posterior anlagen, it is possible that two centra in each segment (diplospondyly) may have been the original arrange- ment in vertebrates. Elasmobranchs, moreover, begin the long process of vertebral differentiation, the vertebrae of the tail being unlike those THE SKELETAL SYSTEM 147 of the trunk, the difference correlated with a difference in the relation of the coelom to the vertebrae. In the trunk region, where the body-cavity Fig. 134. — Diagram of the skeletogenous tissue in the caudal region of a vertebrate. bv, blood-vessels; d, corium; epmu, epaxial muscles; hs, horizontal septum; hytny, hypaxial muscles; msd, msv, dorsal and ventral median septa; mys, myosepta; n, spinal cord; nc, notochord. (From Kingsley's " Comparative Anatomy of Vertebrates.") . CARnL>CE3 /NEURAL PROCESS B CESTRACION, A SHARK. Fig. 135. — A, The skeleton of a cyclostome. Petromyzon; B, The skeleton of an elasmobranch, Cestracion. Elasmobranchs were the first animals to invent paired appendages and the skeletal elements to support them. Marked differences in the axial and branchial skeletons of cyclostomes and elasmobranchs also appear. (Redrawn after Dean.) lies, the haemal arch of each vertebra is incomplete, while in the caudal region each arch is complete with a median spinous process. The noto- chord persists intervertebrally and the centra are biconcave. The 148 CHORDATE ANATOMY skeleton is still cartilaginous, but the cartilage is often hardened with lime. Bony vertebrae make their appearance in ganoid fishes, some of which however retain a cartilagin- ous vertebral column. Ball-and- ^^^^^^ socket joints between the centra are developed in Lepidosteus (gar- pike) as in some Amphibia. Am- phicoelous or biconcave vertebrae, however, predominate in all groups of fishes. Centra are wanting in Fig. 136. — Sagittal section of Sqaulas the Dipnoi, vertebrae, cut surfaces obliquely lined. ^-^j^ ^-^^ amphibians, bone c, calcifications of centra; cd, caudmeurals; _ ^ cdh, caudihemals; cr{i), cranineurals SUCCecds cartilage; and the VCrte- (intercalaria) ; d exits of dorsal nerve roots; ^^^^ ^^^ differentiated intO Cervical, crh, cranihemals; n, notochord; v, exits or ventral nerve roots. (From Kingsley's trunk, Sacral, and caudal. The "Comparative Anatomy of Vertebrates.") ^-^g^^ ^^^^^^ vertebra is but slightly modified for attachment to the pelvic girdle. A single atlas represents the cervical series of higher forms. Zygapophyses, for articulating each vertebra with its two neighbors, first appear in this group. Articulation Fig. 137. Fig. 138. Fig. 137. — Diagrams of (.4 and B) fish vertebrae and (C) vertebra from higher groups, b, basal stumps; c, centrum; ch, capitular head of rib; rf, diapophysis; ha, hemal arch; hr, hemal rib; n, notochord; na, neural arch; p, parapophysis; r, rib; /, tubercular head. (From Kingsley's "Comparative Anatomy of Vertebrates.") Fig. 138. — Two caudal vertebrae of alligator, c, centrum; ha, hemapophysis; hs, hemal spine; na, neurapophysis; ns, neural spine; poz, prz, post- and prezygapophyses; t, transverse process. The arrow passes through the neural arch. (From Kingsley's "Comparative Anatomy of Vertebrates.") with the ribs is effected by two sorts of processes, diapophyses from the neural arches and parapophyses from the centra. Lumbar vertebrae are first differentiated in reptiles, which also have two sacral vertebrae. Here also appear vertebrae with centra flattened THE SKELETAL SYSTEM 149 on both anterior and posterior sides and with the centrum of the atlas fused with the axis, as in mammals. The vertebral column of mammals shows little advance beyond that of reptiles. A few Insectivora have intercentra in the lumbar region — a diplospondylous condition reminiscent of elasmobranchs. Parapophyses are reduced to shallow pits for articulating the heads of the ribs. The human spine differs little from that of other mammals, except that the tail is reduced to a COCC50C with a few variable muscles attached. Man's only distinctive feature is the sigmoidal curve, which bends his spine in two directions, instead of one only as in other creatures. In addition to the two main spinal curvatures, thoracic and lumbar, man has two lesser curvatures, cervical and sacral, in the region of the neck and sacrum respectively. The Vertebral Column in Man. In the backbone of a child there are thirty-three vertebral elements. During growth the last nine fuse to form Fig. 139. — Diagrammatic sagittal sections of (A) amphicoelous; (B) procoelous; (C) opisthocoelous; and (D) amphiplatyan vertebrae. The head is supposed to be at the left. Cut surfaces obliquely lined. (After Kingsley modified.) two adult bones, the sacrum and the coccyx. The other twenty-four vertebrae remain separate throughout life and become differentiated into seven cervical vertebrae, twelve dorsal or thoracic, and five lumbar. These are sometimes called "true" vertebrae in contra-distinction from those of the sacrum and coccyx which are called "false" vertebrae. Although the vertebrae are separate bones, they are nevertheless so firmly fastened together by ligaments and fibrous cartilages as to make the backbone a fairly rigid column. Four curvatures appear in the adult — cervical, thoracic, lumbar, and sacral. The Structure of a Vertebra. A typical vertebra consists of a cylin- drical body, the centrum, which is flattened on its superior (cranial) and inferior (caudal) surfaces. A neural arch arises from the dorsal side of the centrum and surrounds a vertebral canal. That part of the neural arch which connects with the centrum is the pedicle. A spinous process extends backwards and downwards from the mid-dorsal side of the neural arch. That part of the neural arch between the spinous process and the pedicle is the lamina. Anterior and posterior notches or incisures con- strict the pedicles so that the incisures of two successive vertebrae form 15° CHORDATE ANATOMY EPISTROPHEUS CERVICAL ,' CURVATURE^ }'CERV1CAL the foramina for the spinal nerves which pass out between the vertebrae. Articular processes or zygapophyses project forwards and backwards from the neural arches. A postzygapophysis of one vertebra overlaps a prezygapophysis of the next vertebra and the two are bound together by ligaments: thus the backbone is strengthened, but at the same time made less flexible. On each side a transverse process projects from the neural arch laterally into the muscles of the body wall. The Kinds of Vertebrae. There are five kinds of vertebrae, cervical, thoracic, lumbar, sacral, and caudal or coccygeal. A distinguishing fea- ture of cervical vertebrae is a trans- verse foramen which in the upper six vertebrae transmits the vertebral artery. The lateral border of this foramen is formed by the fusion of a rudimentary rib with the vertebra. The first two cervical vertebrae are the atlas and the axis or epistro- pheus. A pecuUarity of the two is that the centrum of the atlas fuses with that of the axis to form the odontoid process upon which the atlas rotates. The forms and arrangement of the cervical verte- brae permit greater freedom of move- ment than is possible in other parts of the column. The spinous process of each cervical vertebra except the last is forked or bifid. Only the twelve thoracic verte- brae carry ribs. A pit in the cen- trum articulates with the head of the rib and a similar pit at the extremity of the transverse process articulates with the tubercle of the rib. The head of most ribs articulates with two adjacent centra. The five lumbar vertebrae are the largest. Short ribs fuse with them to form conspicuous transverse processes. The neural arches of these vertebrae have mammillary and accessory processes in addition to articular. Coccygeal. Fig. 140. — The human vertebral col- umn viewed from the left side. (Redrawn after Sobotta.) THE SKELETAL SYSTEM 151 The sacrum is a spade-shaped bone formed by the fusion of five vertebrae. Its lateral wings are modified ribs fused together and articulated with the hip bones. Spinous processes are much reduced. Between the costal processes four pairs of sacral foramina provide exit for nerves and blood-vessels. The sacral canal is the continuation of the vertebral canal. The coccyx consists of four fused centra which lack neural arches and processes. Frequently the first of these vertebrae fuses with the sacrum, and only the last three form the coccyx. Successive vertebrae are connected to form a continuous column by intervertebral discs of fibrous cartilage. Interconnexions are further strengthened by numerous vertebral ligaments. — EPIDERMIS I— SCLEROTOME NOTOCHORD -MYOTOME DERMATOME Fig. 141. — A horizontal section of an elasmobranch embryo, showing the differentia- tion of the mesoderm (epimere) into sclerotome, myotome and dermatome. The sclerotome surrounding the notochord gives rise to the centrum of the vertebra. Development of the Vertebral Column. In man as in other verte- brates the primary axial skeleton is the notochord. Around this the definitive axial skeleton is built; and the notochord disappears, slight traces only being left as nuclei pulposi of the intervertebral cartilages. The processes involved in this replacement are complicated, beginning with the appearance of mesenchyma cells around the notochord and the neural tube. In this mesenchymal matrix, cartilage develops only to be destroyed in its turn and replaced by bony vertebrae. The mesenchyma from which the vertebrae arise is produced by proliferation of the sclerotome, the cells of which migrate into the space between the mesoderm and the notochord. Later, by a continuation of the same migration, the neural tube becomes completely surrounded by mesenchyma. Before cartilage is secreted in the mesenchyma, the sclerotome median to each myotome becomes differentiated into a denser posterior portion 15^ CHORDATE ANATOMY and a less dense anterior half. As the definitive vertebrae are formed, the posterior half of each vertebral anlage fuses with the anterior half of the following one. By this process the definitive vertebrae come to lie intersegmentally, alternating with the myotomes. The result is obvi- ously adaptive, since only by this arrangement could each myotome become connected with two vertebrae and with two successive ribs. The Ribs. Man has twelve pairs of ribs, which form a basket sur- rounding the thoracic cavity. Each rib is a curved flat bone ending ventrally in a costal cartilage. By means of these costal cartilages the first seven pairs connect directly with the sternum and are therefore EPIDERMIS DERMATOME MYOTOME -SCLEROTOME NOTOCHORD -SCLEROTOME °o"0 MYOTOME DERMATOME .Ssils^^^^^s^j^SsSCsSs^i^^SjS^is^^s^^s^tfR^^J^EPIDERMIS Fig. 142. — A diagram showing the relations of myotome and sclerotome as seen in a horizontal section of a vertebrate embryo. The upper half of the figure shows the relations in an earlier .stage of development, while the lower half represents a later stage. The posterior half of each sclerotome unites with the anterior half of the following sclerotome to form a centrum which thus alternates in position with the adjacent myotome. Thus each myotome becomes attached to two vertebrae. called sternal or true ribs while the five remaining pairs are distinguished as "false" ribs. The last two pairs, the eleventh and twelfth, do not connect with others, and are known as floating ribs. Each rib has a head or capitulum, which articulates with the vertebral centrum, and a tuber- culum, which articulates with the transverse process. As the rib basket rises and falls in breathing, each rib rotates on an a.xis running through the tuberculum and the capitulum. Each rib has a costal groove extending along its lower or posterior border. To the ridges which border this groove are attached the external and internal intercostal muscles. The Development of Ribs. Ribs develop in the embryo as costal processes of the vertebrae in the intermuscular septa or myocommata. Primarily, the cartilaginous anlagen of the ribs are continuous with the THE SKELETAL SYSTEM 153 cartilaginous vertebrae. The short costal processes in the cervical, lumbar, and sacral regions unite with the transverse processes and are indistinguishable from them in the adult. In the thoracic region, separate centers of ossification in the ribs are formed and articulations with the vertebrae develop. Epiphyses at the capitulum and tuberculum make possible the elongation of thoracic ribs. The ventral extremities of the ribs do not ossify but remain throughout Kfe as the costal cartilages. The Evolution of Ribs. Ribs are wanting in chordates below the elasmobranchs, and even in elasmo- branchs they occur only in the anterior trunk region as short cartilaginous processes lying in the horizontal septum separating epaxial and hypaxial muscles. Such true ribs should not be con- fused with the hemal arches of fishes which are median to the lateral trunk muscles and adjacent to the peritoneal lining of the body cavity. See Fig. 143. The ribs of modern Amphibia show little advance above those of the elasmobranchs, and in many Anura continue as short cartilagin- ous processes of the vertebrae. But bony ribs are present in urodeles such as Necturus and the attachment to the vertebrae is, as in the higher vertebrates, by means of tubercular and capitular proc- esses. In some fossil Amphibia the ribs were elongated and extended around the body to the ventral side. Abdominal ribs were also present, as in some modern reptiles. In reptiles, ribs increase in number, and in some forms encircle the abdominal cavity. Abdominal ribs are common. The ribs of snakes are especially numerous. In mammals and man, ribs which articulate with the vertebrae and extend around the body-cavity are limited to the thoracic region. The Sternum. The sternum is a flat, dagger-shaped bone lying mid- ventrally of the chest. Three parts are distinguished, i. The manubrium or presternum, triangular, the widest portion and the most anterior. It articulates with the clavicle. 2. The gladiolus or mesosternum, the longest portion. Fig. 143. — Diagrammatic section of a vertebrate to show the relation of ribs to the muscles of the body wall, av, aorta; c, coelom; e, ectoderm", ep, epaxial muscles; g, gonads; -ha, hemal rib; hp, hypaxial muscles; i, intestine; mes, mesentery; n, nephridium; o, omentum; r, true rib; p, somatopleure; sp, splanchno- pleure; v, vertebra. (From Kingsley's ' ' Comparative Anatomy of Vertebrates. ' ' ) 154 CHORDATE ANATOMY formed by the fusion of four sternal elements or sternebrae. 3. The posterior metasternum, xiphoid or ensiform process. The xiphoid PHALANGES . -METACARPALS. CARPALS . CERVICAL VERTEBRAE ACROMION, PROCESS PHALANGES Fig. 144. — Human skeleton viewed from behind. (Reproduced in modified form from "The Human Body" by Dr. Logan Clendening, (Copyright ig27, 1030 by Alfred A. Knopf, Inc.) by permission of and special arrangement with Alfred A. Knopf, Inc., authorized publishers.) process is sometimes perforated by a foramen and is sometimes forked posteriorly. THE SKELETAL SYSTEM 155 Development of the Sternum. The sternum arises from connective tissue which is afterwards chondrified to become a pair of cartilaginous PHALANGES.-j^ METACARPALS, - MANDIBLE. /CLAVICLE. ACROMION PROCESS. CORACOID. MANUBRIUM. STERNUM. XIPHOID PROCESS. -HUMERUS. RADIUS. Stt-carpals. PHALANGES. Fig. 145. — Human skeleton viewed from in front. (Reproduced in modified form from "The Human Body" by Dr. Logan Clendening, (Copyright 1927, 1930 by Alfred A. Knopf, Inc.) by permission of and special arrangement with Alfred A. Knopf, Inc., authorized publishers.) bars, which secondarily unite in the mid-ventral line and only later connect with the costal cartilages. Ossilication begins in a series of paired 156 CHORDATE ANATOMY centers, but the manubrium usually has one center only. Ossification of the metasternum or xiphoid process remains incomplete until very late in life. Evolution of the Sternum. Opinion is divided as to the beginnings of the sternum. Some morphologists take the median portion of the elasmobranch pectoral girdle to be the homologue of the mammalian presternum, notwithstanding the fact that in some urodeles the sternum is a midventral plate of cartilage quite unconnected with the pectoral girdle. Since, however, the median ventral portion of the elasmobranch pectoral girdle is limited to a single intersegment, while the sternum of higher vertebrates extends through several segments and in mammals is I A Fig. 146. — Scheme of development of mammalian sternum. A, early stage; B, cartilage, the halves uniting; c, coracoid(?) procartilage; cl, clavicle; co, centers of ossification; w, mesosternal parts; mn, manubrium; p, presternum; st, sternebrae; x, xiphisternum. (From Kingsley's "Comparative Anatomy of Vertebrates.") clearly metameric, this hypothesis leaves the metamerism of the sternum unexplained. To meet this difficulty, it would be necessary to assume an antero-posterior extension of the sternum along the mid-ventral line, and a secondary segmentation. But the fact that in urodeles, where the sternum makes its first appearance in the vertebrate series, the sternum is independent of the pectoral girdle, and the additional fact that the sternum develops in ontogenesis independently of the pectoral girdle, make it difficult to accept this hypothesis. See Fig. 147. A second and more plausible hypothesis assumes that the sternum arose by the fusion of the ventral ends of a series of ribs. In favor of this opinion it is pointed out that in such a primitive amphibian as Necturus the sternum is represented by a series of four or five pairs of cartilages near the mid-ventral line. Like ribs these cartilages are intermyotomic. While in Necturus ribs do not extend from the vertebrae to the ventral side of the body, it is believed that there were primitive amphibians THE SKELETAL SYSTEM 1 57 in which the ribs were so extensive. The hypothesis that the sternum is a rib-sternum has at least so much foundation. The facts of mammalian ontogenesis, however, do not appear to support this view. As stated above, the mammalian sternum arises independently of the ribs by the union of a pair of longitudinal cartilages which arise near the mid-ventral line. The connexion of these cartilages with the ribs is secondary. If an opinion were to be based upon the relation of the sternum in Necturus and of the ontogenesis of the mammalian sternum alone, we NTERCUkVICLE XIPHOID PROCESS Fig. 147. — Types of vertebrate sterna. A, Squalus; B, Salamandra; C, Necturus; D, Rana; E, Felis; F, Crocodilus; G, Homo. The sternum is shown in black. While there is no doubt of the homology of the various amniote sterna, their homology with those of anamnia is in dispute. should have to conclude that the sternum arose from paired segmented cartilages formed near the mid-ventral line independently both of the girdle and of the ribs. Under the circumstances, and until more decisive evidence is discovered, suspension of judgment is necessary. In the reptiles the sternum is converted into a metameric structure composed of a series of sternebrae and connected with the ribs as in mammals. The mammahan sternum differs little from that of reptiles. It is divided into the same three elements as those of reptiles and man, pro- meso-, and meta-sternum. 158 CHORDATE ANATOMY The Skull. There are two chief parts to the skull, which have different origins and a different history. One of these is the cranium or brain-case, together with the bones of the face except the two jaws. The other is the visceral skeleton, that is to say, the two jaws, the hyoid bone, the ear bones, and the cartilages of the larynx. 3ASISPHENOI0 ORBITOSPHENOID \ ALISPHE>JOID PRESPHENOID \ \ \ PARIETAL FRONTAL VOMER PREFRONTAL \ \ J^>Cpp@>W NASAL ^ ^ \ ^< 3 < a t3 Quadrate Pterygoid*t Palatine Quadrato-jugal Jugal Maxillary Premaxillary X X X ? X X X X X X X X X X X X X X X 0 0 OT 0 0-3. t: rt c 5 SftOJ OJ (OX- Incus X X Malar X X INCUS Pterygoid processes on sphe- noid X MALAR 1 Fused to form ( MAXILLA < < W M 0! >— 1 is 0 ►J Articular Dentary Splenial Coronoid Supra-angular A na,ular X X X X X X X X X X X X X X X X X Malleus X ? ? Tympanic MALLEUS X Adult lower jaw is a single dermal bone, the — MANDIBLE Tympanic fused with temporal < w 0 > 9^ 0 Hyomandibular Symplectic Interhyal Epihyal X X X X Columella Columella ossify in_ doi embryonic ' Stapes and stapes sal part of lyoid arch. STAPES Intermediate part of hyoid arch becomes STYLOID PROCESS of temporal bone, and STYLO-HYOID LIGAMENT HYOID BONE m. v. Ceratohyal Hypohyal Basihyal m, v X X X A variable ventral hyoic number of elements BRANCHIAL ARCHES Maximum number in adult 7 421 Additional cartilages of I and more pc larynx and ] I sterior arches are represented in perhaps of trachea. OPERCULUM OF FISHES Operculum Preoperculum I nleroperculum Suboperculum X X X X Probably nc jt represente d in verteVjrates above fishes Cartilage bones are in bold-face type; dermal bones in italics. d, dorsal; v, ventral; 1, bones which are lateral and paired; m, a median bone. All bones not otherwise designated are lateral and paired. * Cartilage bone more or less augmented by addition of dermal bone. t A name which (with appropriate prefixes) applies to two or more bones which are closely related in origin and position: e.g., mesethmoid, ectethmoid; entopterygoid, metapterygoid, etc. X Homology of alisphenoid and vomer of mammals with alisphenoid and vomer, respectively, of lower vertebrates is questionable. (Modified from Rand, Comparative Anatomy of Vertebrates, Harvard University Press, 1929.) 1 68 CHORDATE ANATOMY 1400. The discovery of the cranium of the Java man with a brain capacity of between 800 and 900 cc. helps to reduce this contrast. The striking fact revealed by fossil human skulls is that the characteristics which distinguish them from the skulls of modern man tend to bridge over the gap between man and apes. In other words, all fossil skulls, except that of the Cro-Magnons which is like that of modern man, are more ape-like than those of modern races. The dental arch of Negroid races is intermediate between that of apes and Europeans. The chin which is such a striking feature of the modern human jaw is lacking in some fossil men as in the great apes. See Fig. 158. Another contrast between the skull of man and apes is in the relation of the skull to the backbone. The skull of modern man is poised on the occipital condyles at about its center of gravity, but the condyles of apes lie far behind the center of gravity of the head. Therefore are the neck muscles of modern man relatively weak, those of the ape massive. It is an interesting fact from the evolutionary point of view that the skull of Neanderthal man shows an intermediate condition. The Human Skull. The human skull consists of twenty bones of which eight form the cranium or brain-case and the remaining twelve make the facial skeleton. The eight bones of the cranium are the frontal, occipital, ethmoid, sphenoid and the paired parietals and temporals. The facial skeleton includes the mandible and vomer, and the paired maxillaries, zygomatics (malars), nasals, lacrimals, and palatines. Since the turbinal bones of the nose are extensions of the ethmoid they are not counted as separate bones. A comparison of the mammal skull with that of man reveals that the bones are homologous. Development of the Cranium. The history of the vertebrate skull revealed by the study of its comparative anatomy is amply supported by its embryology. Both prove it to be a complex formed by the union of diverse elements, capsules that contain the sense organs, supports for the gills, and underpinning and protective covering for the brain, while in the occipital region vertebrae appear to have fused with the brain case. The primordial cartilaginous cranium of the human embryo arises as a pair of parachordal cartilages and a pair of prechordal cartilages or trabeculae. These fuse together; later they combine with two pairs of sensory capsules, the olfactory and the auditory. This formation of the cartilaginous basis of the cranium, the chondrocranium, takes place during the second month of intra-uterine Ufe. Ossification from separate centers begins with the third. See Fig. 149. Evidence from comparative anatomy proves that the bones of the human skull correspond to a much larger number of separate bones which appear in the fishes and have been progressively reduced by fusion with THE SKELETAL SYSTEM 169 one another during the course of evolution. This also is borne out by the development of the human cranium. Ossification of the occipital bone, for example, begins in four centers corresponding with the basioccipital, the paired exoccipitals with their condyles, and a supraoccipital of lower vertebrates. To these are later added a membranous interparietal. Ossification of the occipital begins in the third month but is not completed until the seventh year. NASAL LACRIMAL MAXILLA ZYGOMATIC VECKEL'S CART MANDIBLE NASAL CONCHA (fR; >J^s3jr- OCCIPITAL - -A^-i^jf TEMPORAL MASTOID PROCESS 'AUDITORY MEATUS ZYGOMATIC ARCH Fig. 159. — The human skull, embryonic (A) and adult (B-D). In the fetal skull (14 weeks) membrane bones are black, and the cartilage cranium (chondrocranium) stippled. Figure B shows the adult skull in basal aspect, figure C in frontal aspect, and figure D in left lateral aspect, approximately one quarter natural size. (Redrawn after Sobotta.) The development of the sphenoid bone is even more complex; no fewer than ten centers of ossification are recognized. Six of these arise in the body of the bone and four more in the two paired wings. Membrane bone is added both to the pterygoid processes and to the great wings. Fusion of the separate elements is completed before the second year. Ossification of the ethmoid remains throughout life incomplete. Three centers of ossification corresponding to the pro-, epi-, and opisthotic 170 CHORDATE ANATOMY bones of lower vertebrates develop in the otic capsule and help to form the petrosal and mastoid portions of the temporal bone. The styloid process of the temporal is an ossified portion of the hyoid cartilage which fuses with the temporal. The squamous portion of the temporal is mem- branous in origin. An outgrowth of the epitheUum of the middle ear penetrates the mastoid process to form a cavity or antrum. The rest of the cranium is membrane bone. Because the roofing bones of the brain case ossify slowly and expand from centers, uncovered regions or fontanelles persist for some months after birth as "soft-spots" between the frontal, parietal, and occipital bones. lo Idm ^° ^9 INCUSCQUADRATEJ— MALLEUS CARTICULAR) MECKEL'S CARTILAGE CMANDIBULAR)^ HYOID DENTARY- STYLOHYOID LIGAMENT CHYOID) - MANDIBULAR A ELASMOBRANCH HYOMANDIBULAR (COLUMELLA) QUADRATE ARTICULAR DENTARX. MANDIBULAR - C AMPHIBIAN AND REPTILil. D. MAMMAL. Fig. 164. — Diagrams of the first and second visceral arches in A, Elasmobranch; B, Teleost; C, Amphibian and Reptile; and D, Mammal, illustrating the transformation of the hinge of the jaw of lower vertebrates into the malleus and incus of the mammal. The third earbone, the stapes, comes from the hyomandibular. (Redrawn after Gegenbaur and Stempell.) The three ear bones are named malleus, incus, and stapes from their fancied resemblance to hammer, anvil, and stirrup. Within the cavity of the middle ear they extend in the order given from the ear drum or tympanum to the oval window or fenestra vestibuli of the internal ear. Thus they serve to carry vibrations from the ear drum to the liquids of the internal ear. See Fig. 369. 176 CHORDATE ANATOMY Development of the Visceral Skeleton. In the human embryo a series of visceral arches separated by pharyngeal pouches appear in relations corresponding to those of aquatic vertebrates. In the first of those arches the maxilla of the upper jaw and the mandible of the lower jaw develop Fig. 165. — Early chondrocranium of Squalus. (The brain in outline.) als, alisphenoid cartilage; ch, anterior end of notochord; h, hyoid arch; ?na, mandibular arch, not yet divided into pterygoquadrate and Meckelian; oc, otic capsule; t, trabecula; 1-5, branchial arches; cartilages dotted. (From Kingsley's " Comparative Anatomy of Vertebrates," after Sewertzoff.) as membrane bones. The mandible, however, surrounds a cartilage, Meckel's cartilage, which corresponds to the mandibular cartilage of the lower jaw of elasmobranchs. While most of Meckel's cartilage disappears during ontogenesis, that portion which extends into the cavity of the middle ear ossifies in two centers, one of which forms the malleus and the other Fig. 166. — Diagram of early elasmobranch skull, bp, basal plate; c, trabecular cornu;^, foramen lacerum; ga^-\ gill arches; gc, gill cleft; /;, hyale; lim, hyomandibular; ii, i^, upper labials; II, lower labials; m, Meckel's cartilage; nc. nasal capsule; oc, otic capsule; of, orbital foramen; ov, occipital vertebrae; pq, pterygoquadrate; s, suspensor ligament; sp. spiracle; si, sphenolateral; t, trabecula; v, vertebrae; I-VII, visceral arches; i-io, cranial nerves. (From Kingsley's "Comparative Anatomy of 'Vertebrates.") the incus. The so-called Meckel's cartilage of the mammalian embryo appears therefore to correspond not only to the mandibular or Meckel's cartilage of lower vertebrates, but with their cjuadrate element also. The quadrate element develops into the incus while the articular portion of the mandibular ossifies as the malleus. THE SKELETAL SYSTEM 177 The cartilages of ihe remaining visceral arches in the human embryo have a diversified fate. The dorsal part of the second, the hyoid, ossifies to form the stapes of the middle ear, while the ventral portion forms the lesser horn and a part of the body of the hyoid bone. The intermediate portion of the hyoid cartilage forms the stylohyoid ligament by which the hyoid bone is suspended from the petrosal bone of the cranium. The cartilage of the third visceral arch fuses with that of the second, and later ossifies to form the greater horn and part of the body of the hyoid bone. The cartilages of the fourth and fifth arch persist as the thyroid and aryte- noid cartilages of the larynx, and form also the cuneiform and corniculate cartilages. The cartilage of the fifth arch is said to form the cricoid cartilage also. Other observers claim that the cartilage of the sixth arch contributes to the formation of the cricoid. See Fig. 163. II. THE APPENDICULAR SKELETON Evolution of Paired Appendages. The cyclostomes have no paired appendages and, so far as the evidence goes, never have had them. We are, consequently, forced to conclude that the paired appendages of vertebrates have no genetic connexion with those of invertebrates, but have arisen independently as vertebrate novelties. Unfortunately for the speculative morphologist, the beginnings of these appendages are obscure. Those of elasmobranchs are the simplest in living vertebrates, but even these are highly differentiated. The most promising attempt to solve the problem of the origin of paired fins is the so-called fin-fold theory. See Fig. 167. According to this, paired fins began as paired folds of skin extending from the region posterior to the gills back to the anus. The paired metapleural folds of Amphioxus are often mentioned, with dubious justification, as structures which suggest how the fin-folds may have had their origin. Pectoral and pelvic fins are supposed to be formed by enlarging the end portions of these folds and suppressing the intermediate region. In favor of this hypothesis is the presence of longitudinal paired "Wolffian" folds in vertebrate embryos, and the fact that the anlagen of the appendages extend through more segments of the embryonic body than do the appendages of the adult, the bases of the appendages becoming constricted dming ontogenesis. Some morphologists have used the continuous fin-folds of skates as evi- dence supporting the fin-fold theory, while others doubt their significance. The second step in this evolution was the invasion of connective tissue and muscle into the fin-folds. A similar migration actually occurs in ontogenesis. In elasmobranchs the muscle buds which invade the fin- folds are metamerically arranged. The third step in this evolution was the appearance of a series of inter- myotomic cartilage rays like those which support both median and paired lyS CHORDATE ANATOMY fins of elasmobranchs. The universal occurrence of skeletal material in connexion with muscle, and indeed wherever in an organism stresses occur, may possibly be taken as explaining these radial cartilages. The position between the myotomes is obviously adaptive, as is also their position between the dorsal extensor muscles of the appendage and the ventral flexor muscles. m'////my//////////y^^ MEDIAN FIN-FOLD PECTORAL FIN ^^,.T PELVIC FIN ANUS PRIMITIVE RADIALS PELVIC GIRDLf Fig. 167. — Diagram illustrating the hypothetical evolution of the paired fins and their skeletal supports. A represents the primitive stage of continuous fin-folds. The dorsal fin and the ventral fin posterior to the anus are median and unpaired. B is the definitive elasmobranch stage. The paired fin-folds persist only in the region of pectoral and pelvic fins. The median fins also become discontinuous. C-E illustrate hypo- thetical stages in the evolution of the skeleton of the pelvic fins of elasmobranchs. The right side of C and E represents a later stage in phylogenesis than the left. In E the skeletons of the girdle and extremity are differentiated. (After Wiedersheim.) Further steps in the evolution of the appendicular skeleton involve the thickening and fusion of the basal or proximal portions of the radial cartilages and the extension of the basal cartilages thus formed into the body-wall and towards the mid-ventral hne. The result of this appears today in the pelvic fin of elasmobranchs. The beginnings of a girdle are seen in a ventral cartilaginous plate, the ischio-pubis. A doubtful begin- ning of the ilium may be seen in the so-called iliac process. Evolution in thp: skeletal system 179 the pectoral girdle seems to have been more rapid than in the pelvic girdle, if we may base our conclusion on the fact that in elasmobranchs the scapula or dorsal arm of the pectoral girdle is already well developed when there is little, if any, indication of a dorsal arm, the ilium, of the pelvic girdle. In both girdles in the elasmobranch, however, a ball-and-socket articulation between girdle and free extremity has already made its appearance. An advance towards the pectoral girdle of higher vertebrates appears in living and fossil ganoids in which a membrane bone, the clavicle, is added to the pectoral girdle. There is no structure in the pelvic girdle homologous with the clavicle. PREAXIAL BORDER TO RIGHT ABCD HYPOTHETICAL STAGES Fig. 168. — Diagrams illustrating the hypothetical evolution of the extremities of dipnoan (/), ganoid (H), and elasmobranch (G) from a fin-fold supported by a series of similar radial cartilages. By fusion basal elements are differentiated. The skeletal supports of fins eventually differ in the relations of the basal elements to the radialia. (Redrawn after A. Brazier Howell.) A tripartite pectoral girdle makes its first appearance in amphibians. The ventral arm, which in fishes was single and undivided, becomes in amphibians differentiated into posterior and anterior moieties, the coracoid and precoracoid. The dermal clavicle becomes closely apposed to the precoracoid . The dorsal scapula and suprascapula remain undivided as in fishes. The dorsal arm of the pelvic girdle, the ilium, articulates with the transverse process of a single sacral vertebra. In its most primitive form in amphibians, the ventral portion of the pelvic girdle resembles that of ganoid fishes, and consists of a broad cartilaginous plate with which the femur articulates. See Fig. 169. Centers of ossification corresponding with the ischium and pubis of reptiles arise successively in this plate. i8o CHORDATE ANATOMY The girdles of reptiles are essentially like those of amphibians. In the turtle they become definitely Y-shaped. The clavicle fuses with the pre- coracoid and becomes indistinguishable from it. The ilium connects with JO-PUBO-lSOflAWC CARTCLAGE- E. DACTYLETHRA a NECTURUS. Fig. 169. — A series of six appendicular skeletons illustrating the gradual emergence of the elements of the pelvic girdle found in reptiles and mammals. They probably represent fairly well stages in the evolution of the human pelvis. First came the separa- tion of girdle and extremity (A and B) ; then the fusion of the paired elements of the girdle into a median ventral cartilaginous plate (C and D) ; the differentiation of bony ischium, pubis, and ischium (D and £); and finally the appearance of the obturator foramen (F). There is no essential difference between the reptile and mammal girdle. (Redrawn after Wilder, "History of the Human Body"; Henry Holt & Co.) SUPRASCAPULA| SCAPULA' -OMOSTERNUM CLAVICLE^ ACROMION- EPIPHYSIS OF CLAVICLEi INTER- \ .clavicular " 'ligament HUMERUS ■'^PRECORACOID 'HUMERUS -i--rnDarr,in COSTOCOR ACO I D/ y-CORACOID LIGAMENT ^ --EPICORACOID CARTILAGES'^ LIGAMENT -STERNUM 'sternum NSCAPULA /OSSA SUPRASTERNALIA B Fig. 170. — Diagrams illustrating the fundamental similarity of the human (B) and amphibian (A) pectoral girdle. In man the coracoid element has degenerated into a process (coracoid) and a connective-tissue ligament containing occasional cartilage nodules. (Redrawn after Huntington.) two sacral vertebrae. In pythons, a rudimentary hip girdle connects with a pair of rudimentary claws in the anal region. Both are useless; both go to prove the descent of serpents from tetrapod ancestors. THE SKELETAL SYSTEM i8i In mammals, the coracoid is reduced to a process fused to the scapula. In man, in addition to the coracoid process, a remnant of the coracoid bone survives in the coracoid ligament which extends from the coracoid process to the sternum, and in which occasional pieces of cartilage are found as rudiments of the coracoid. The clavicle has supplanted the precoracoid, remnants of which, however, usually occur within the clavicle. See Fig. 170. The mammalian hip bone differs Httle from that of reptiles. The number of sacral vertebrae to which the coxal bone is attached increases in mammals. In man there are five sacral vertebrae, to three of which the hip bone is attached. Evolution of the Free Extremities. Two contrasting types of free extremity appear in vertebrates, the fins characteristic of fishes and the VLRTEBRAL MARGIt 5-STERNAL EXTREMITY Fig. 171. — Human pelvic and pectoral girdles in lateral aspect. A is the pelvic girdle of the right side and B-C the pectoral girdle of the same side. toed appendages such as are found in the remaining classes from amphibians to man. The conversion of the one into the other continues to be a vexed cjuestion of vertebrate morphology. Technically stated, the problem has been to determine how the evolution of the ichthyopterygium into the cheiropterygium has occurred. Interest has centered especially in the transformation of the skeleton. Primarily the fish fin, like that of the fossil shark Cladoselache, was supported by radial cartilages which articulated with basalia, of which one or more articulated with the girdle. In the pectoral fin of modern elasmobranchs three basalia, propterygium, mesopterygium, and meta- pterygium (Fig. 172^), connect the fin with the girdle. Morphologists, however, disagree as to the skeleton of the primitive extremity, the archipterygium. While some suppose it to have been uniserial, i.e., the radial cartilages were limited to one side of the basal cartilages or axis as l82 CHORDATE ANATOMY in elasmobranchs, other morphologists regard the biserial fin skeleton of Dipnoi as the more primitive. Conclusions in regard to the evolution of the skeleton of the extremity differ, therefore, as one or other of these two types of fish-fin skeleton is assumed as more primitive. Summary of Skeletal Evolution. Most animal phyla, even the Protozoa, have some sort of skeletal structures. But there seems to be no genetic connexion between the skeletons of invertebrates and those of vertebrates. In the evolution of a skeleton, vertebrates have been given RADIAUA A. ICHTHYOPTERYGIUM PHALANGES l\ n METTACARPALS , / I ICMETATARSALS5 "^ Ji RADIUS ^ffl!»->. MS>> |C TIBIA 5Gt^ is a nine-week human embryo in left lateral view. hand. In the lower hmb they are hip, thigh, leg or shank, and foot. A comparison of the human skeleton (Figs. 144 and 145) with that of the gorilla (Fig. 175) shows that, bone for bone, the two correspond. The dif- THE SKELETAL SYSTEM 187 ferences are those of pro{)ortion only. The facts are in harmony with the assumption that the two have evolved from a common ancestry. Homologies of the Limb Bones. The striking similarity of the bones of the upper and lower limbs, notwithstanding their great diversity of function, is interpreted by morphologists as indicating a primary similarity in use. Their present differences in form, size, and function have arisen secondarily and adaptively. Development of the Appendicular Skeleton. The paired appendages of vertebrates arise from two Wolffian folds, which extend along the sides of the embryo at approximately the level where the hypomere NCURAL PROC£33. 1 1 MM. EMBRYO Fig. 177. — Stages in the development of the appendicular skeleton of man. A, left lateral aspect of arm in 11 mm. embryo; B, left lateral aspect of arm in 16 mm. embryo; C, left lateral aspect of arm in 20 mm. embryo; D, left lateral aspect of leg in 11 mm. embryo; E, left lateral aspect of leg in 14 mm. embryo; F, left lateral aspect of leg in 20 mm. embryo. (Redrawn after Bardeen and W. H. Lewis from Keibel and Mall.) connects wath the mesomere. Only the end portions of these folds, how- ever, go to form the Umbs; the intermediate region atrophies and disap- pears. The e\ddence accords with expectation from the standpoint of the fin-fold theory of the origin of the extremities. See p. 177. The Wolffian folds consist of an external covering of ectoderm and a core of mesenchyma, which in the human embryo is of uncertain origin. In their early development, both arms and legs take the form of shovel- shaped outgrowths (Fig. 176), which gradually elongate. The cartilagi- nous anlagen of the bones arise in the mesenchyma (Fig. 176.4) and are slowly converted into bone through complex processes partly suggested in CHORDATE ANATOMY Fig. 178. In the human embryo fingers and toes make their appearance at the ends of the extremities as early as the second month. (Fig. 176) Four stages in the development of a long bone are shown in Fig. 179. The connective-tissue membrane or perichondrium which surrounds the cartilage anlage of the bone secretes a cylinder of bone around the shaft of CARTILAGE S<>> CELLS SUCCESSIVE STAGES BLOOD. CELLS ' vM'^^^^iT^''^^ ■'^ l"^? DEGENERATION OSTEOBLAST OSTEOCLASTS BONE Fig. 178. — Endochondral bone formation at the end of a long bone. Destruction of cartilage is followed by the secretion of lime in the form of thin lamellae. Osteoblasts then lay down bone upon these lamellae. In this way cancellous bone replaces cartilage. (Redrawn after Dahlgren and Kepner.) the anlage. Thus the perichondrium is converted into a periosteum which persists throughout the life of the bone. Other perichondria! cells penetrate the cartilage and destroy it. Eventually a fatty marrow takes the place of the cartilage within the bony cylinder. The cylinder becomes the diaphysis of the adult bone. Since the diaphysis is formed THE SKELETAL SYSTEM 189 outside and around ihe original cartilage this mode of bone formation is known as perichondral bone formation. The diaphysis of the infant is GROWTH DISC DIAPHYSIS—^ PERICHONDRIAL BONE EPIPHYSIS-J GROWTH DISC EPIPHYSIS DIAPHYSIS - MARROW a_ PERICHONDRIAL BONE i ENDOCHONDRAL BONE GROWTH DISC Fig. 179. — Diagrams illustrating four stages in the development of a long bone. Perichondral bone cross-hatched; endochondral bone stippled; maiTow black; cartilage unshaded. (Redrawn from Coming's "Human Embryology," after Duval.) --'BLOOD VESSEL >COMPACT BONE »MARROW CAVITY ARTICULAR LIGAMENT Pig. 180. — Diagram of the structure of a long bone. (Redrawn after Fritz Kahn, "Der Mensch," Albert Muller, Zurich.) converted into that of the adult by the continuous addition of new bone on the outside of the shaft. At the same time the marrow cavity of the shaft is enlarged through the destruction of bone on the inside of the shaft. In igO CHORDATE ANATOMY this way the marrow cavity of an adult bone becomes large enough to contain the entire bone of the infant. The long bone grows in length at the two ends. Through endo- chondral bone formation centers of ossification are formed within the cartilage at the ends of the shaft. The bony centers are known as epiphy- ses. During growth new bone is added between epiphysis and diaphysis in a cartilaginous ''growth-disc." When adult size is attained the growth- disc is converted into bone, the epiphyses fuse with the diaphysis, and growth ceases. The regulation of size is influenced through the action of endocrinal secretions of the pituitary and thyroid glands. CHAPTER 7 THE MUSCULAR SYSTEM The muscular system of an active vertebrate makes up nearly half the entire body-weight, in man slightly more than forty per cent. A muscle can do only one thing — contract. It cannot expand and, having once contracted, must be pulled out to its resting length by one or more antagonistic muscles. Each skeletal muscle consists of a fleshy part or belly, each end of which is attached to a bone or cartilage, either directly to the periosteum or indirectly by means of a tendon. The attachment TENDON- BELLY- -- 'ARTERY VEIN NERVE TENDON Fig. i8i. — A diagram of the biceps muscle taken as a typical muscle, showing its nervous and vascular relations. Each skeletal muscle is attached to a bone either directly to the periosteum or indirectly — as in the case of the biceps — by means of tendons. (Redrawn after Keith.) which moves most when the muscle contracts is its insertion; the other is its origin. Each muscle is surrounded by a connective-tissue membrane or perimysium, from which septa may grow into the muscle and divide it into several muscle shps, each of which has a separate function. Muscles vary greatly in shape according to the arrangement of their fibers and the way these are attached. Muscles may be segmented into a series of similar units such as appear in the body muscles of fishes. igi 192 CHORDATE ANATOMY Fig. 182. — Diagrammatic outlines to illustrate various types of muscle architecture and the relations of the main nerve branches to the fiber-bundles of the muscle, a. Two segments of the rectus abdominis muscle of a small mammal, b. Portion of sheet-like muscle with two nerve-branches and intramuscular nerve plexus, c. Typical quadri- lateral muscle with nerve passing across the muscle about midway between the tendons. d and e. Two triangular muscles with different types of innervation. /, Long ribbon-like muscle with interdigitating fiber-bundles, g, Unipenniform muscle, h, Bipenniform muscle, i. Typical fusiform muscle. The main intramuscular nerve branches are distributed to the fiber-bundles about midway between their origins and insertions. (From Morris' "Human Anatomy.") THE MUSCULAR SYSTEM IQ3 They may spread out in thin sheets that are ribbon-Hke, triangular, pinnate, or fan-like. Appendicular muscles are more frequently spindle- shaped and massive. Each muscle is well supplied with capillaries and with both motor and sensory nerves. As to origin, muscles are sharply divided into two kinds: skeletal (epimeric or myotomic), derived from the dorsal or epimeric portion of the mesoderm; and visceral (hypomeric) derived from the hypomere. In the trunk region, in contrast with the head, visceral muscles arise from the splanchnic layer of mesoderm only. Smooth visceral muscle fibers are found not only in the wall of the intestines, but also in the walls of blood-vessels, in the lungs, the bladder, the genital organs, and the skin. Skeletal muscles are composed of striped fibers whose response to stimulation is a rapid contraction. Most visceral muscles, on the other hand, consist of slow-acting smooth or non-striped fibers. The former are voluntary (in man, "under control of the will"), the latter are usually involuntary. Exceptions are found in the heart muscle which is visceral and involuntary, but formed of striped fibers, and in the chewing and facial muscles which are visceral and at the same time striped and voluntary. EVOLUTION OF THE MUSCULAR SYSTEM The muscles of man and other mammals are the last term in the series of transformations of the mechanism of contraction, the evolution of which it is now possible to sketch in fairly firm outlines. Contractility appears to be one of the original properties of living cells. Touch an amoeba and it responds by drawing together into a sphere. There is no single axis, but contraction takes place from all directions towards a center. In some Protozoa, however, progressive advance in the function appears in the differentiation of contractile fibrils. A cluster of such fibrils in the stalk of Vorticella is so arranged as to contract in one direction only, like a muscle fiber. True muscle cells first appear in the animal series in the sponges. The primary independence of muscle and nerve is indicated by the presence of muscle cells in this group which lacks nerves altogether. The epithelio-muscular cells of coelenterates are essentially similar to those of sponges. The next step in the evolution of muscles appears in the flatworms, in which muscle cells are aggregated into clusters. The bilateral sym- metry characteristic of the muscles of higher animals also appears in this group. Transitional evolutionary stages between flatworms and chordates are, it must be admitted, highly speculative. Even if we accept the assump- tion that annelids resemble the ancestors of vertebrates, there still remains 194 CHORDATE ANATOMY a wide gulf to be filled between flatworms and annelids. In certain characteristics, the muscles of annelids, it is true, strikingly resemble those of vertebrates. Among these are the segmentation of the muscles, and their separation by a body-cavity into somatic and visceral divisions. It is impossible, however, to be sure that these similarities are not cases of convergence. The eyes of cuttle-fish and of man are similar in many respects, but this does not prove a genetic connexion. While the pre-chordate history of muscles is obscure, the evolutionary changes of muscles in chordates are fairly clear. Since the lower chor- / MOUTH A AMPHIOXUS. METAPLEURAL FOLD V LATERAL TRUNK CSOMATIC) MUSCLES GONADS QLL APERTURES A B. PETROMYZON. HYPOBRANCHIAL MUSCLE LATERAL TRUNK MUSCLES ANUS XJRSAL CONSTRICTORS LEVATOR WAXILLAE f^ _^ SPIRACLE. I ^P^""^ pAXIAL TRUNK MUSCLES HYPAXIAL MUSCLES CLOACAL-ANAL ORIFICE Fig. 183. — The lateral trunk muscles of a cephalochordate, a cyclostome, and an elasmobranch, showing their striking metamerism, and fundamental similarity. A, Amphioxus; B, Petromyzon; C, Squalus. dates, the Hemichorda and Urochorda, are non-metameric, we must assume that the metamerism of Amphioxus and vertebrates is a new acquisition in the group. The trunk muscles of Amphioxus form an unbroken series of segments extending throughout the entire length of the animal. Each muscle segment or myotome is a mass of muscle tissues which extends around the side of the body nearly to the mid-dorsal and mid-ventral line. Each myotome terminates anteriorly and poste- riorly in connective-tissue septa, the myocommata, which separate suc- cessive myotomes. A sharp bend near the middle of each myotome gives it in side view the shape of a letter V. All alike are innervated by somatic motor nerves. THE MUSCULAR SYSTEM 195 The viscera] muscles in the wall of the intestine are non-metameric, and are differentiated into an inner circular and an outer longitudinal MYOTOME I MYOTOME 10 ANTERIOR/. CAVITIES ENDOSTYLE' A.AMPHIOXUS EMBRYO. isT perm.gill-slit FACIALIS GANGLION LENS OTIC CAPSULE MYOTOME 4- myotome 10 SPIRACULAR POUCH 1ST GILL-SUT HYPOBRANCHIAL B. CYCLOSTOME EMBRYO. ^^^^""^ OTIC CAPSULE LATERAL TRUNK MUSCLE MYOTOME 4 IST GILL -SLIT HYPOBRANCHIAL MUSCLE C. ADULT CYCLOSTOME. Fig. 184. — Diagrams illustrating the origin of the hypobranchial muscles of verte- brates. Lacking in cephalochordates (Amphioxus), hypobranchial muscles make their first appearance in cyclostomes in the form of muscle buds from post-branchial myo- tomes. They become the tongue muscles of tetrapods and are innervated by the hypoglossal (XII) nerve. In cyclostomes as in higher vertebrates myotomes i, 2, and 3 form eye muscles. layer. In the region of the gills, the visceral muscles are connected with the gill cartilages, and are dififerentiated into levators, depressors, and constrictors of the gills. 196 CHORDATE ANATOMY The lateral trunk muscles of cyclostomes strikingly resemble those of Amphioxus. In the region of the body-cavity, on the ventral side, an external layer of oblique muscles is differentiated. The most important evolutionary advance, however, appears in the differentiation of six eye muscles. Paired eyes first appear in this group, and with them six eye muscles like those found in all vertebrates up to man. All six are formed from the first three embryonic myotomes. Like the eye muscles of higher vertebrates, they are innervated by the 3rd, 4th, and 6th cranial nerves. Since in cyclostomes the fourth myotome of the embryo forms the first permanent trunk myotome, all the myotomes of the embryo persist in the adult. Of none of the higher vertebrates is this true. (Figs. 184, 185, 186) Hypobranchial muscles, lacking in Amphioxus, first appear in cyclo- stomes. They arise from postbranchial myotomes which send myotomic SOP. RECTUS^ 0CUU3M0T0RN. N. TBOCHLCARIS^ EXT RECTUS (CUT) M. 0BLI0UU5 SUR. Cxr. RECTUS — NF OBLfQUE INF RECTUS Fig. 185. — Diagrams of the eye muscles of man. A shows the left eye-ball and associated muscles viewed from the outer side. B is the left eye-ball with associated muscles and nerves viewed from the median side. (Redrawn after Warren and Car- michael. Courtesy of Houghton Mifflin & Co.) buds ventrally and anteriorly below the gills as far forward as the mouth. The development and nerve relations of this hypobranchial musculature prove that it is the homologue of the tongue and throat muscles which, in higher vertebrates, are innervated by the twelfth nerve, the hypoglossal. Cyclostomes, however, have no true tongue. The hypobranchial muscles function as a part of the lateral trunk muscles. (Fig. 184, C) The embryos of elasmobranchs provide a clue to the history of the eye muscles, by demonstrating that the differentiation of the three anterior myotomes into the six eye muscles involves primarily a longitudinal splitting of the myotomes into dorsal and ventral moieties such as happens also in the first and second post-otic myotomes of cyclostomes. The facts suggest that the spUtting occurred along the series of lateral-line sense organs, which at one time may have included the lens of the eye and the ear vesicle. Each of the two divisions of the first myotome splits again lengthwise, thus making the four eye muscles innervated by the oculo- motor nerve. The dorsal of the two moieties of the second myotome forms the superior oblique muscle innervated by the trochlear nerve, THE MUSCULAR SYSTEM 197 FACIALIS GANGLION 0^,^ CAPSULE MYOTOME 3 MYOTOME 2 MYOTOME I. MYOTOME MYOTOME 2 A. PETROMYZON. HYPOBRANCHIAL MUSCLE 1ST PERM. GILL-SLIT FACIALIS GANGLION MYOTOME 3V MYOTOME 2 MYOTOME OTIC CAPSULE ST PERM. META-OTIC MYOTOME THYROID ANTERIOR CAV. HYPOBRANCHIAL MUSCLE MYOTOME B. SQUALUS. SUP. RECTUS SUP. OBLIQUE INT. RECTUS EXT. RECTUS 1ST META-OTIC MYOTOME OLF. PIT INF. OBLIQUE INF. RECTUS C. ADULT SQUALUS. HYPOBRANCHIAL MUSCLE Fig. 186. — Diagrams based upon cyclostome and elasniobranch embryos illustrating the phylogenesis of the six eye muscles. The eye muscles develop from the first three embryonic myotomes. Myotomes are cross-hatched. Those which degenerate in ontogenesis are cross-hatched with broken lines. In C the eye muscles are shown as if viewed from the median side of the eye. 198 CHORDATE ANATOMY while the ventral portion unites with the third myotome to form the external rectus muscle innervated by the abducent nerve. The dorsal division of the third somite breaks up into loose mesenchyme to form connective tissue. The myotomes of the fourth, fifth, and sixth somites also break up into connective tissue, so that the first persistent trunk myotome is the seventh. In this way, a hiatus occurs in the series of myotomes, and the eye muscles are left as an isolated group which owe their persistence to the fact that they become functionally connected with the eyeball. (Fig. 186) If we may draw phylogenetic conclusions from these facts of onto- genesis, we must consider the eye muscles not as relatively young muscles LATERAL UNE ADDUCTOR OF MANDIBLE' Fig. 187. — The superficial muscles in the shoulder region of Squalus. From such relatively simple beginnings have evolved the complex muscles of the arm and shoulder of man. The flexor protractor muscle which corresponds to the deltoid muscle in mammals is covered in the figure by the posterior gill constrictor. (Redrawn after A. Brazier Howell.) or as post-otic muscles which have migrated into the pre-otic region, but as the first three myotomes of the vertebrate body. Their present isola- tion may be interpreted as a consequence of the enlargement of the otic capsules. The ontogenesis of cyclostomes and elasmobranchs supports the assumption that in the ancestors of vertebrates, as in Amphioxus today, the myotomes formed an unbroken series extending throughout the entire length of the body. The history of the eye muscles sums up as the transformation of the first three myotomes of an Amphioxus-like ancestor into the six eye muscles of the vertebrates. In elasmobranchs the metamerism of the body muscles, which is such a characteristic feature of the musculature of cyclostomes, is retained with slight modification. A more elaborate folding of the myo- tomes of elasmobranchs, however, greatly compUcates their form. The cause of this folding is unknown. The total amount of muscle remains the same; and, although the myocommata are folded along with the THE MUSCULAR SYSTEM 199 i|i^ ^^^--^ FiM-rniX"^ MYOTOMIC buds' \ ^'^-"-^ h, FIN-FOLD "•" "Sypogldssus muscle buds B. \ COELOM,/;- Fig. 188. — Diagram of bvidding of hypoglossal and pectoral fin muscles from trunk myotomes in an elasmobranch embryo. A. Lateral view after Braus. 2-6, visceral arches. B. Cross section in region of pectoral fin-fold. LEVATORES ARCUORUM C(-7) VISCERAL SKELETAL ARCHES Cl-7) \ DEPRESSORES ARCUORUM Cl-7^ LEVATORES [-4 DIGASTRICUS MASSETER TEMPORALIS DORSO-LARYNGIS AND DORSO- TRACHEALIS B. INTERMANDIBULARES HYO-PHARYNGEI Fig. 189. — Diagrams illustrating the hypothetical evolution of the branchiomeric muscles. A. Hypothetical ancestral form. B. Branchiomeric muscles in urodele amphibian. (Redrawn after Wilder's "History of the Human Body," Henry Holt & Co.) 200 CHORDATE ANATOMY myotome so that the surface of attachment of the muscles is increased, it has not been proved that this increase is adaptive. See Fig. i86, C. A novelty first appearing in this group is the division of the lateral trunk myotomes by a horizontal connective-tissue septum into epaxial D NECTURUS. OCPffESSOft MANOIBUl>e SPHINCTER COLLI TRAPEZIUS DOnSAUS SCAPULAE EPAXIN. MUSCLES HYRUUAL UUSXCS Fig. 190. — Superficial lateral trunk muscles in an amphibian, a reptile, and a mam- mal. D, Necturus. E, Sphenodon. F, Felis. The metamerism of the lateral trunk muscles which is such a striking feature of the lower vertebrates is retained in urodeles and reptiles, but disappears in mammals. The factors in this change are chiefly the increasing dominance of the appendicular muscles and the fusion of the primarily metameric embryonic trunk muscles. The primitive metamerism, however, appears in mammalian embryos and hypaxial groups of muscles. Five post-branchial myotomes send buds anteriorly into the floor of the pharynx to form the hypobranchial musculature innervated by the hypoglossal nerve. THE MUSCULAR SYSTEM 20I ,. The most important advance, however, made by the elasmobranchs is the first appearance in vertebrates of the muscles of pectoral and pelvic fins. As the myotomes extend ventrally in the body- wall, hollow epithe- lial buds branch off laterally into the fin anlagen. See Fig. i88. The appendicular muscles are thus seen to be derivatives of lateral trunk muscles. Differentiation of the muscles thus formed takes place in two directions in elasmobranchs and higher animals. First, the appendicular muscles are subdivided into intrinsic muscles which lie within the fin and extrinsic muscles which are connected with the fin but He within the body-wall. Both groups are subdivided into levators and depressors. On the anterior side of the fin, a muscle is formed which pulls the fin forward M. TRANSVERSOSPI NALIS, TRA^4SVERSE PFOCESS- --M. LONGISSIMUS DORSl. M. RHOMBOIDEUS. /M. SERRATUS POSTERIOR. SCAPULA. M. L4TISSIMUS DORSl GLENOID CAVITY. ^^ 'ACROMION. M EXTERNAL'yO^^S^f''''^'?''' ~"^^i^( ^^"^ " '| ' ' " ^,e^^J<^-^^ tORACDID. M. RECTUS ABDOMINIS^ ^ / ^ M BRACHIALIS MANUBRIUM STEBNl! ^~~^^ ' '^^ PECTDRALIS MAJOR. INFERIOR. CL^VVICLE' ^M. TRAhJSVERSUS THORACIS. Fig. 191. — Thoracic and lumbar muscles of man as seen in cross section. Thoracic muscles on the right, lumbar on the left. The muscle arrangement is fundamentally like that of any mammal. (Redrawn after Braus.) towards the head. No special antagonistic muscle is differentiated in elasmobranchs, the adduction of the fin being effected by the combined action of the posterior part of the levator and depressor groups acting together. The extension of the extrinsic muscles of the fins in fan-like form over the lateral trunk muscles tends to obscure the metamerism of these in the region of the appendages. The trapezius muscle, which extends from the scapula anteriorly above the gills, makes its first appear- ance in this group. In the head region, the visceral muscles become specialized in relation to the jaws. The levators of the first two visceral arches form the jaw muscles, including the masseter, temporalis, and pterygoids, while the depres- sors of these arches become the intermandibularis muscles. The muscles of the remaining visceral arches remain relatively unmodified. (Fig. 189) In the urodeles, the metamerism of the lateral trunk musculature persists as a striking characteristic. The extrinsic muscles of the append- 202 CHORDATE ANATOMY ages, however, become widely extended anterior and posterior to the legs. Such definitive muscles as the pectoralis and the latissimus dorsi now appear, and the intrinsic muscles subdivide into those of the arm and thigh, the forearm and shank, and the feet. By further spHtting of the original muscle mass within the limb, many new muscles arise, some of which may be homologized with those of man. On the sides of the body, the lateral trunk muscles become delaminated into layers, some amphibians -M. PIRIFORMIS 'M. ABDUCTOR CAODAE VENT M. EXTENSOR CAUOAE LATERALIS M ABDUCTOR CAUDAE DORSAL IS DORSAL Fig. 192. — Human caudal muscles viewed from A. ventral and B. dorsal side. These rudimentary muscles are the last remnants of the powerful caudal muscles of the lower vertebrates. The presence of such useless rudiments receives its best interpreta- tion in the evolution theory. (Redrawn from Wilder's " History of the Human Body," Henry Holt & Co.; after Lartschneider.) having as many as four. The epaxial muscles of the trunk divide into longitudinal bundles connected with the head. A further novelty in amphibians is a movable tongue. Its intrinsic muscles are those which, as we have seen, grow from occipital myotomes into the floor of the throat and are innervated by the hypoglossal nerve. In this group also we find differentiated sternohyoid and geniohyoid muscles, which connect sternum and lower jaw respectively with the hyoid. No very striking developments affect the muscles of reptiles. The three sets of epaxial muscles of the trunk, — trans verso-spinalis, lumbo- costalis, and ilio-costalis, appear. The fusion of the lateral trunk THE MUSCULAR SYSTEM 203 myotomes and the consequent loss of metamerism leads towards the con- ditions in mammals. An extreme degree of delamination affects the lateral trunk muscles, some reptiles having as many as eight layers in the body-wall. FRONTAL- M. ORBICULARIS OCULI NASAL- ZYGOMATIC AUR I CULO- LABIAL SUR AURICULO-LABIAL INF. TRIANGULAR A. ATELES. SUR AURICUI-AR OCCIPITAL POST. AURICULAR 1-. / M. ANTITRAGICUS ANT. AURICULAR M. PLATYSMA- ANT. AURICULAR M. ORBICULARIS OCULI M. OUADRATUS LAB 1 1 SUR CANINE- M. ORBICULARIS ORIS M. RISORIUS M. QUADRATUS LABI! INF M. MENTAL IS SUR AURICULAR POST. AURICULAR TRIANGULAR' B. HOMO. Fig. 193. — Mimetic muscles in monkey (Ateles) and man. A, Ateles (redrawn from Wilder after Ruge) and B, Homo. The similarity of these muscles both in function and relations attests their similar genetic derivation. With the great enlargement of the appendages of mammals, there appears a corresponding increase in the appendicular musculature and trunk muscles become relatively reduced. Subdivision and migration of muscles increase. Caudal muscles dwindle with the reduction of the tail. In the trunk region, metamerism is preserved only in the intercostals, the rectus abdominis, and the intervertebral muscles. 204 CHORDATE ANATOMY Integumental (dermal or cutaneous) muscles in the form of a panni- culus carnosus group appear suddenly in monotremes and marsupials only to disappear in the higher primates except as rudiments. In the head and neck region, however, the platysma and facial muscles persist in man and apes. In the trunk region, these integumental muscles are outgrowths of the pectoralis minor complex. In the head region, however, they are visceral in origin. The most important muscular novelty contributed by mammals is the diaphragm. Its innervation by branches of cervical spinal nerves proves that it is a derivative of cervical myotomes. Muscles in Man There is no essential difference between the muscles of man and those of other mammals. The presence in man of such useless muscle rudi- ments as the sacro-coccygeal and ear muscles suggests a mammalian derivation. The evolutionary process of subdivision, fusion, migration, and sphtting of muscles reaches its cUmax in primates, forearm and hand being especially noteworthy. The human body has nearly four hundred paired or bilaterally sym- metrical muscles, of which forty-seven pairs are visceral and the rest skeletal. In addition to these, four unpaired muscles are recognized. Each part of the body — head, neck, back, abdomen, thorax, diaphragm, shoulder and chest, upper arm, forearm and hand, hip, thigh, lower leg and foot, pelvis — has its intrinsic set of muscles. Space does not permit the description of all these muscles. There is no question that in fundamental pattern the muscles of vertebrates and of man are alike. (See Fig. 190.) Comparison of the superficial muscles of man (Figs. 194 and 195) with those of the cat (Fig. 190) reveals a surprising degree of resemblance. On account of their exact homology many of these muscles in the two forms are given identical names. The same is true of many of the deeper muscles. When the muscles of man are compared with those of another primate, the similarity is much greater. There is no reason to doubt that the similarity of the mimetic muscles in man and monkey (Fig. 193) has genetic signifi- cance. Few, if any, muscles in man are without homologues among primates. The evolution theory in its appHcation to the human body derives much support from the comparative anatomy of the muscles. The presence in man of useless muscle rudiments such as those of the coccyx and ear mentioned above (page 204) receives its only adequate interpretation in this theory. Pointing in the same direction is the existence in man of inconstant and variable muscles, the homologues of which are functional in THE MUSCULAR SYSTEM 205 lower animals. The pyramidalis abdominis muscle is an example. When present in man the pyramidalis arises from the pubic bone anterior (ven- trad) to the rectus abdominis muscle. Its length varies greatly in individu- als. It may occur on one or both sides or may be wanting. In non-placental mammals the pyramidalis is powerfully developed in con- nexion with the marsupial bones which it serves to support. Even in insectivores in which the marsupium has disappeared the pyramidalis muscle is well developed. The presence in man of such a useless rudiment suggests the animal origin of the human body. Rudimentary integumentary muscles occasionally appear in individu- als. Among these are the stemalis muscle of the chest and the axillary muscle connected with the pectoralis in the axillary region. They are normally present in apes, but occur in human individuals only excep- tionally. They are interpreted as remnants of the panniculus camosus of lower mammals. Of similar significance is the fact that, although metamerism is evident in few adult mammalian muscles (intercostals, intervertebrals and the rectus abdominis), nevertheless all the skeletal muscles arise from the metameric somites of the embryo. Why are embryonic myotomes meta- meric when the muscles which develop from them are not metameric? The primary metamerism corresponds with the muscular metamerism of the skeletal muscles of the lower vertebrates. Does this fact not give the clue to the muscular metamerism of the human embryo? Developm.ent of the Muscles. Classified on the basis of their onto- genetic development, muscles are of two kinds: (i) Somatic Muscles, derived from the mesodermal epimere, and (2) Visceral Muscles, which develop from the hypomere. With the exception of the smooth muscles of the eyeball, which are of ectodermal origin, all muscles are mesodermal. We may then describe first the development of the Somatic Muscles, derived from the Epimere or Somite. Very early in ontogenesis the mesoderm becomes divided into a metameric series of "somites." In all chordates above the cephalo- chordates (Amphioxus) the metamerism affects only the dorsal portion of the mesoderm, that is, the portion known as the epimere. In the trunk region — but not in the head — of vertebrate embryos the adjacent mesomere becomes segmented as the nephrotome. The epimere becomes later differentiated into (i) myotome which forms muscle, (2) sclerotome which forms skeletal material, and (3) dermatome which gives rise to loose connective tissue. In the embryos of the lower vertebrates, as shown in Fig. 197, the somites extend in an unbroken series throughout head and trunk. In embryos of the higher vertebrates, however, the metamerism of the mesoderm in the head region disappears. Only in the embryos of Amphioxus and cyclo- 2o6 CHORDATE ANATOMY stomes (Petromyzon) do all the somites produce myotomes. In the higher forms the series is broken in the ear region. FLEX. CARPI ULNARIS FLEX. CARPI RAD. LOXB.s BRACHIORADIAL.' Da CARPI RAD LONGUS- PALMARIS LONGUS. BRACHIALIS TRICEPS •■'- CORACO BRACHIALIS-' TERES MAJOR' LATISSIMUS DORS I SERRATUS ANTERIOR LINE A ALBA RECTUS ABDOMINIS EXTERNAL OBLIQUE ~ ILIUM TENSOR FASCIA LATA~ ILIOPSOAS RECTI NEUS ■ PUBIS '■ RECTUS FEMORIS- SARTORIUS ~Mljl ADDUCTOR LONGUS - -"" "' ADDUCTOR MAGNUS - VASTUS LATERALIS PERONEUS LONGUS GASTROCNEMIUS^-^- PERONEUS BREVIS- EXT. HALLUCIS LONG.- TRANS. CRURAL LIGAMENT. FRONTAL . ORBICULARIS OCULI. -AURICULAR .' ^ZYGOMATIC. -MASSETER. OMOHYOID. STERNOCLEIDOMASTOID. SCAUENES h^c-z /^^-3^-T RA PEZ I US . ::^^^=\ -CLAVICLE ^■""■"^PECTORALIS MAJOR. ' ■■ -DELTOID- TRICEPS . ••^/BICEPS . ' it-BRACHIAL- EXT CARPI RADIALIS. BRACHIORADIAL. SUPER. FLEXORS. FLEXOR CARPI ULNARIS. -PATELLAR LIGAMENT. J r(,jj,i,t> INSERTION OF SARTORIUS. ■'JJ- TIBIALIS ANTERIOR. _ 'ML., 40-TiBIA wt-^-EXT OIGITORUM COM. LONG. -FLEX. DIGITORUM COM. LONG. CRUCIATE LIGAMENT. Fig. 194. — Superficial nniscles of man; front view. (Reproduced in modified form from "The Human Body" by Dr. Logan Clcndening (Copyright 1927, 1930 by Alfred A. Knopf, Inc.) by permission of and special arrangement with Alfred A. Knopf, Inc., authorized publishers.) The serial homology of the head cavities or somites with trunk somites, which for many years was a controverted problem, has now been demon- THE MUSCULAR SYSTEM 207 strated by the fact that, in the embryos of lower vertebrates, the head somites form a series of mesodermal segments continuous wilh the trunk TEMPORAL -f- - OCCIPITAL^t^"-*' STERNOCLEIDOMASTOID -'*•- TRAPEZIUS^:;^ SPINE OF SCAPULA >^^-\ \ DELTOID -i^-.> ~0, , EXT.POLLICIS BREVIS. ABD. POLLICIS LONGUS. GASTROCNEMIUS CRUCIATE LIGAMENT. ACHILLES TENDONt Fig. 195. — Superficial muscles of human body ; back view. (Reproduced in modified form from "The Human Body" by Dr. Logan Clendening (Copyright 1927, 1930 by Alfred A. Knopf, Inc.) by permission of and special arrangement with Alfred A. Knopf, Inc., authorized publishers.) somites. Like the latter, they become differentiated into myotome and sclerotome, are innervated by somatic motor nerves, and are dorsal 208 CHORDATE ANATOMY to notochord and dorsal aorta. Furthermore, their segmentation is independent of that of the visceral arches. Another point of resemblance is that the first and second head cavities divide during ontogenesis into dorsal and ventral moieties precisely as do the first and second post-otic myotomes in Petromyzon. The fusion of portions of two myotomes, the second and the third, to form the external rectus muscle of the eye resem- bles the fusion of trunk myotomes such as occurs in the formation of the tongue muscles. THE MUSCULAR SYSTEM I5T SOMITE ANTERIOR CAVITY ENDOSTYLE CLUB-SHAPED GLAND A. AMPHIOXUS EMBRYO OMfTE 10 1ST PERM. GILL-SLIT 209 FACIALIS GANGLION OTIC CAPSULE 1ST SOMITE LENS, SOMITE 10 THYROID ST PERM.GILL-SUT B. PETROMYZON VI FACIALIS GANGLION 1ST SOMITE SOMITE 10 ANTERIOR CAVITY HEART C.SQUALUS Pig. 197. — Diagrams of the mesodermal (somatic) segmentation in the head region of embryos of lower chordates as viewed from the left side. In embryos of these lower vertebrates, just as in the adult Amphioxus, the somites (myotomes) extend in unbroken succession throughout head and trunk. Roman numerals number the brain " neur omeres . " 2IO CHORDATE ANATOMY Lateral Trunk Muscles. The lateral trunk muscles of man develop from myotomic segments which first appear in the fourth week of onto- genesis, and by the end of the second month have increased to nearly SCLEROTOME- HYPOCHORDA NEURAL CREST NEURAL TUBE ■ jil^EPIMERE /wiL 'SOMATIC MOTOR NERVE— r^^^\^~--— NOTOCHORD-''" MESOMERE-y^' ECTODERM j ^ — ENDODERM CRMATOME MYOTOME SCLEROTOME HYPOCHORDA HYPOMERE COELOM SUBINTESTINAL, BLOOD VESSEL A. HEAD B. TRUNK Fig. 198. — Diagrams of cross sections in A, head and B, trunk regions of an elasmo- branch embryo showing the fundamental similarity of the two regions. The discovery that the coelom of elasmobranch embryos extends throughout head and trunk and that in this respect the two regions are alike was made by the English embryologist, Francis Balfour. EVTREMITV SOMATIC MUSCLE INTESTINE ENDODERM COELOM NEURAL CREST SOMATIC MOTOR NERVE SPINAL CORD SPINAL GANGLION RAMUS DORSALIS NOTOCHORD CENTRUM NEPHROTOME CORIUM VISCERAL MUSCLE RAMUS VENTRALIS •r /" DERMATOME MYOTOME SCLEROTOME PARIETAL MESODERM ISCERAL MESODERM Fig. 199. — A stereogram of the trunk region of a vertebrate embryo, based upon elasmobranch embryos. The figure shows an earlier stage of development on the right side, a later stage on the left. The extension of the myotome to form the lateral trunk musculature is shown. The lateral trunk musculature of the ventral half of the body- wall thus arises as a secondary invasion. (Redrawn after Braus.) forty pairs. The original metamerism of the myotomes, which persists even in the adults of the lower vertebrates, becomes largely lost in adult man and mammals as the result of a number of processes among which THE MUSCULAR SYSTEM 211 fusion is the most important. As the myotomes grow in size and thick- ness through cell multiplication, the connective-tissue septa between them disappear. In this way are formed such elongated muscles as the spinalis and iliocostalis. Among the processes tending to obscure the original metamerism is the degeneration of myotomes into connective-tissue fasciae and aponeuroses, which may be very extensive. Migration of muscles may accompany their fusion. Among the other ontogenetic changes in trunk myotomes is tangential splitting of muscles into sheets. One of the most characteristic ontogenetic processes affecting the trunk muscles SOMITES lO-l-f 'MCXJTH I I MliOTOMIC BUDS YPOPHYSIS HYPOGLOSSUS' VISCERAL ARCH A CYCLOSTOKC NERVE '■" 'OLR«CTORY PIT VISCERAL ARCH 4 B ELASMOBRANCH C. REPTILE D. MAMMAL Fig. 200. — Diagrams illustrating the mode of origin of hypoglossal (hypobranchial) muscles in A. Cyclostome, £. Elasmobranch, C. Reptile, and/?. Mammal. \t\A,B, and C cervical myotomes send myotomic buds into the hypobranchial region. In mammals such buds are not formed but a migration of mesenchyme cells from cervical myotomes provides material for these muscles. The number of myotomes which participate is usually four or five. is the subdivision of a muscle mass into a number of bellies each of which acquires an independent origin or insertion, or both. The original seg- mentation of the trunk myotomes is, however, retained in such muscles as the transversospinalis, intercostalis, and rectus abdominis. By the growth of a horizontal connective-tissue septum which extends laterally from the transverse processes of the vertebrae, the lateral trunk muscles become divided into epaxial and hypaxial portions, of which the former are innervated by dorsal rami of the spinal nerves, the latter by ventral rami. The muscles of the diaphragm, which are peculiar to man and mammals, migrate into the chest from the neck, as is evidenced by the fact that they are innervated by branches of the third, fourth, and fifth cervical nerves. 212 CHORD ATE ANATOMY Tongue Muscles. The origin of the h\'poglossal muscles in the human embrvo is somewhat uncertain. Since, however, they have the same innervation as in lower vertebrates, it is generally assumed that their development is essentially similar. In all vertebrates below mammals, muscle buds grow from four or five occipital myotomes ventrally into the floor of the throat. From the mass of cells thus formed arise the intrinsic muscles of the tongue, innervated by the twelfth nerve, the h>-poglossus. In man and mammals e\adence is lacking of muscle buds in the formation of the hypoglossal muscles. It may be assumed that cell migration takes the place of bud formation and extension. Appendicular Muscles, In the embryos of lower vertebrates, elasmo- branchs to reptiles, as the myotomes grow ventrally in the body-wall and reach the level of the lateral folds from which the appendages develop, they give off lateral buds into the appendicular folds. After they have entered the folds, these buds lose their connexion \\dth the trunk muscles, although they still retain their epithelial character. Within the anlage of the appendage, the appendicular muscle buds subdi\dde into dorsal and ventral moieties, from which develop respectively the levator and depressor muscles of the appendage. The appendicular muscles of man and mammals, on the contrary, do not develop from myotomic buds, but arise by cell migration. The two methods are after all not radically different. In fishes, for example, where most of the appendicular muscles arise from myotomic buds, some muscles which develop later than the others come from migrant mesen- chymatous cells as they do in mammals. Similarity of innervation, however, attests the homology of the appendicular muscles throughout the vertebrate series. The fact that the arm muscles of man are innervated by the last four cer\'ical and the first thoracic nerves further justifies the assumption that they are derived from the myotomes of these segments. To the group of muscles derived from this source, are added others, such as the trapezius, sterno-cleido-mastoid, and levator scapulae. The pectoralis and latissi- mus dorsi muscles spread out from the arm. Most of the muscles of the shoulder, chest, and arm appear early in the second month, and are differentiated by the beginning of the third. From the connexion of the muscles of the lower leg with spinal nerves, including the last four lumbar and first three sacral, it may be assumed that their cellular anlagen are derived from the corresponding myotomes. In all essentials their development resembles that of the muscles of the arm. A common mass of cells within the Hmb-bud differentiates into dorsal and ventral muscle anlagen. The muscles from the ventral group become innervated by the femoral nerve while the dorsal group are con- nected with the obturator. The subdivision of the primary muscle THE MUSCULAR SYSTEM 213 mass into the separate muscles of the adult limb is mostly completed by the end of the second month. Visceral Muscles, Derived from the Hypomere. The visceral or hypomeric muscles include those of the heart and main blood vessels as well as those associated with the alimentary canal. While most of them consist of smooth muscle fibers, the visceral muscles of the head and heart are striped. The muscles of the wall of the alimentary canal are formed from mesenchymatous cells proliferated from the visceral layer of the hypomere. Such cells fill the space between the mucous epithehum lining the aU- miwmmniiimimmrjffi •/MYOTOMES 1-4 1ST. CERVICAL MYOTOME ANL M TRAPEZIUS* "M. STERNOCLEIDO- MASTOID ^SPINAL /GANGLIA MANDIBULAR MUSCLES' ANLACE DIAPHRAGM 1ST. THORACIC MYOTOME' _ Fig. 201. — The anlagen of the cranial muscles with their nerve relations as seen in a 7 mm. human embryo. (Redrawn from Keibel and Mall, after W. H. Lewis.) mentary canal and the adjacent hypomere. They also differentiate into both the connective tissues and the blood-vessels of the wall of the ali- mentary canal and into its circular and longitudinal muscles. The circular layer of muscles is formed before the longitudinal layer. The fate of the hv-pomere in the head is much more complex than in the trunk. Besides forming the heart and pericardium, the head hypo- mere gives rise to the chewing muscles, the muscles of expression, and the pharyngeal and laryngeal muscles. In general, the processes involved are similar in lower and higher vertebrates. In embryos of lower vertebrates, e.g., elasmobranchs, the coelom extends throughout head and trunk. In the head region, as a result of the outpocketing of pharyngeal pouches, the h^-pomere becomes divided into a series of pouches each of which lies in a visceral arch. This hypo- meric segmentation (branchiomerism) is independent of the segmentation 214 CHORDATE ANATOMY of the epimere (mesomerism) , and should not be confused with this, although it is possible that the two types of segmentation may originally have coincided. From the mesoderm of the visceral arches arise the muscles, connective tissues, and blood-vessels of the arches. In the fishes, these muscles are differentiated into levators, depressors, and constrictors of the gills. In the process of conversion the epithelium of the hypomere breaks up into mesenchyme and the coelomic cavity disappears. In mammals and man, the coelom is absent in the visceral arches and the muscles are formed from masses of mesenchymatous cells. From the first visceral arch arise the muscles innervated by the mandibular branch of the fifth nerve, the masseter, temporalis, pterygoid, mylohyoid, and tensor veH palatini. (Fig. 201) From the same source come the tensor tympani of the ear and the anterior belly of the digastricus. The muscles innervated by the facial nerve are derived from the second visceral arch, the hyoid. They include the muscles of expression, the stylo-hyoid, stapedius, and the posterior belly of the digastricus. From the third visceral arch arise the stylopharyngeus muscle innervated by the glossopharyngeal nerve, and the constrictors of the pharynx innervated by the vagus nerve. The laryngeal muscles, innervated by the vago- accessory nerve, originate from the fourth and fifth visceral arches. As ah-eady explained, the muscles of the tongue and throat innervated by the hypoglossal nerve are myotomic, not visceral, in origin. CHAPTER 8 THE DIGESTIVE SYSTEM Life depends upon an unceasing intake and outgo of matter. Each living thing takes in food or the raw materials for food, assimilates this into its own peculiar sorts of protoplasm, and after forming these chemical substances, promptly burns them up into simpler chemical substances, which finally leave the body as the wastes and ashes of life. Upon this fundamental chemical process of metabolism, all other vital functions depend. The foundations of life are chemical. The products of plant metabolism, on their way back to the inorganic world, become, directly or indirectly, the food of animals. Thus all animals are parasites on the green plants. But their feeding habits are varied. Some marine organisms live on the mud as well as in it; earth- worms pass through their digestive tract enormous quantities of soil for the sake of the organic matter which they extract from it. But leeches live chiefly on blood. Oysters sweep bacteria into their mouths by ciliary action. Barnacles kick food into their mouths by means of their six pairs of legs. Some insect larvae feed on cellulose, some on fur and wool. Some whales eat minute swimming crustaceans, which they strain out by means of the whalebone. Others live chiefly on gigantic cuttle-fish. Some mammals are herbivorous; some are carnivorous; others, like man, are omnivorous. Man alone cooks his food. Digestion. The first chemical change which ingested foods undergo is a process by which insoluble substances are made soluble, so that they may be absorbed through the lining membranes of the small intestine. The agents in this chemical process are certain remarkable enzymes which, like other and inorganic catalyzers, are able to bring about chemical changes without appreciable effect on themselves. During digestion, these enzymes split up the huge molecules of colloids into simpler mole- cules, small enough to pass through animal membranes. Their composi- tion is unknown; but they are thought to be rather simple colloids derived from proteins. The specificity of their action is remarkable, each enzyme affecting only one food substance. All are secreted by glands connected with the alimentary canal. EVOLUTION OF THE DIGESTIVE SYSTEM The Protozoa have no digestive system. The single cefl merely engulfs the food particle, surrounds it with protoplasm, digests and assim- 215 2l6 CHORDATE ANATOMY ilates it, and extrudes what remains. The Porifera, though they have a cloacal cavity, do their feeding essentially like Protozoa, each cell for itself. The first real step in evolving a proper digestive system is taken by the coelenterates. These, as their name affirms, have a cavity or enteron which is the digestive tract. This has but one opening to the exterior, which serves both as mouth and anus. See Fig. 375. Fig. 202. — Diagram of a vertebrate, a, anus; h, brain; df, dorsal fin; h, heart; i, intestine; /, liver; m, mouth; n, nephridia; p, pancreas; pc, pericardium; pf, pectoral fin; 5, stomach; sc, spinal cord; sp, spleen; vf, ventral fin. (From Kingsley's "Com- parative Anatomy of Vertebrates.") Most flatworms, like coelenterates, have a single opening to the digestive cavity (enteron), and this opening serves as both mouth and anus. A few species of flatworms, however, possess an anus — some indeed have two ani — the invention of which therefore should be credited to flatworms. Threadworms, with few exceptions, have both mouth and Fig. 203. — Spiral valve of Raia. Cartilaginous fishes increase the absorbing surface of their intestine not by elongation, as is done by higher animals, but by a spiral fold in theintestine. (From Kingsley's " Comparative Anatomy of Vertebrates," after Mayer.) anus, and their alimentary canal is separated from the muscular body- wall by a space, a false body-cavity or pseudocoelom. The digestive tube in threadworms is purely epithelial and non-muscular. A muscular digestive tube, one of the important steps in animal evolu- tion, is contributed by the annelids. In these for the first time in the phylogenesis of animals an epithelium-lined coelom or ''body-cavity" THE DIGESTIVE SYSTEM 217 NASAL PHARYNX proper separates the alimentary canal from the body-wall. In annelids, as in all the higher animals, there is no connexion between the two cavities, enteron and coelom. The single tube that forms the body of lower forms has become double, and the muscular activities of the alimentary canal are carried on independently of those of the body-wall. Among the forms which lie near the main line of human ancestry, pharynx, esophagus, and stomach are first differentiated in urochor- dates. A liver arises in the cepha- lochordates. The cyclostomes contribute a pancreas and a bilobed liver. Elasmobranchs, utilizing dermal scales as teeth and transforming a visceral arch into a jaw, convert the sucking mouth into a biting one. To increase surface for ab- sorbing digested food they develop a spiral fold or "valve" in the intestine. (Fig. 203) They de- velop also a new cavity, the cloaca, to receive the wastes and secretions of the urogenital and digestive systems. The amphibians fasten their teeth in a groove in the jaw bone, VFJiMIFORM PROCESS Fig. 204. — Diagram of the alimentary canal. (From Morris' "Human Anatomy.") invent salivary glands, utilize hypobranchial muscles to make a mobile tongue, and differentiate small from large intestine. Mammals greatly elongate the intestine and, by suppressing the cloaca, separate the rectum from the urogenital sinus. The result is a muscular, epithelium-lined alimentary canal, differentiated into nearly a dozen different organs, and having about the same number of different glands associated with it. THE HUMAN DIGESTIVE SYSTEM Mouth The mouth cavity is divided into an anterior vestibule or labial cavity lying between the lips and the teeth, and a posterior mouth cavity proper or buccal cavity underlaid by the tongue and extending to the posterior margin of the soft palate. The roof of the mouth cavity proper is formed 2l8 CHORDATE ANATOMY by the hard and soft palates, which separate the mouth cavity from the nasal passage above. Development. At a relatively late state of ontogenesis, at the anterior end of the fore-gut where the mouth is to break through, the ectoderm invaginates to form the stomodeum. At the bottom of the stomodeum, ectoderm and endoderm are in contact as a two-layered membrane, which ruptures and disappears leaving no trace in the adult. The covering of the lips and gums is derived from the ectodermal stomodeum, while that of the rest of the mouth is endodermal. Evolution. There is no doubt that the mouths of all vertebrates are homologous, the sucking mouth of cyclostomes being no exception. Cyclo- epiphysis lateral telencephalic vesicle hyoid arch visceral arch III Fig. 205. — Drawing to show the external appearance of the structures in the oral region of a four-day chick. Ventral aspect. (From Patten's "Embryology of the Chick.") stome and gnathostome mouths have the same fundamental structure, development, and relations to other parts, and must therefore be considered homologous. Beard and Kupffer, however, are persuaded that vertebrates have had two mouths — an old paleostoma and new neostoma. The paleostoma, in their opinion, may be represented by the hypophysis, which in some cyclostomes, e.g. Bdellostoma, opens directly into the pharynx. (See Fig. 206, A) According to Kupffer, the hypophysis of vertebrates repre- sents a paleostoma which functioned as a mouth in prechordates, following their abandonment of the original blastoporic mouth. In support of this assumption, he points out that the definitive mouth of vertebrates arises late in ontogenesis in such relation to the series of gill-slits that it might have been formed from a pair of coalesced gill-sUts; that the presence of a THE DIGESTIVE SYSTEM 219 pre-oral gut in vertebrate embryos suggests that the alimentary canal formerly extended anterior to the present mouth; and, finally, that in the myxinoids and the embryonic sturgeon the hypophysis actually opens into the pharynx and, like the mouth of urochordate larvae, has a dorsal external opening. (Fig. 208) Whatever view is held of the origin of vertebrates, we must believe that there have been at least two mouths in the course of vertebrate phylogene- sis. The reason for this conclusion is that the original coelenterate mouth becomes the mouth in no vertebrate, while only in cyclostomes, dipnoans, and possibly some amphibians does it become the anus. The coelenterate HYPOPHYSIAL DUCT A.BDELLOSTOMA. PHARYNX/ ,1 ST GILL APERTURE HYPOBRANCHIAL MUSCLE HYPOPHYSIAL ^ DUCT ^=-^ (RESPIRATORY TUBE NOTOCHORD I ST GILL APERTURE B. PETROMYZON. HYPOBRANCHIAL MUSCLE Fig. 206. — Diagrams of median longitudinal sections of the heads of Bdellostoma and Petromyzon, showing the relations of the hypophysial ducts in the two forms. In the former the hypophysial duct opens posteriorly into the pharynx, suggesting the possi- bility that it may once have served as a mouth (paleostoma). In Petromyzon the hypo- physis fails to open into the pharynx and is converted into a pipette-like organ into which the olfactory pits open. On the basis of this difference cyclostomes are divided into two sub-classes, Hyperotreta and Hyperoartia. mouth becomes the blastopore of chordate embryos. And the blastopore of chordates lies at the posterior end of the body and forms the neurenteric canal, which connects the neural tube with the enteron, while the chordate mouth develops at the anterior end of the enteron. Consequently, it seems indisputable that there have been at least two mouths in the history of vertebrates. While, however, all agree that the vertebrate mouth is not the primary animal mouth, and that at least two mouths have successively appeared, some rnorphologists believe that there have been at least three mouths, Delsman (1922), reviving an earlier suggestion of Kowalevsky (1877), claims that " in the ontogeny of vertebrates we see three successive mouths appear in the same succession as they appeared in phylogeny, viz., the blastopore (Urmund), the neuropore (the annelidan mouth), and finally 220 CHORDATE ANATOMY .BLASTOPORE NEUROPORE. ^""^<^^"' A. B. C. Fig. 207. — A diagram illustrating the way in which, according to Delsman, the blasto- poric mouth of coelenterates is in chordates converted into the neurenteric canal. Delsman homologizes the chordate neural tube with the ectodermal f oregut of annelids. NEURAL NOTOCHORD BLASTOPORE NEUROPORE TUBE | BLASTOPORE ENDODERM A.GASTRULA GILL POUCHES B.AMPHIOXUS EMBRYO ENTERON MOUTI NEUROPORE NOTOCHORD GILL POUCHES C.UROCHORDATE LARVA ENDODERM STRAND NEUROPORE OTIC CAPSULE notocho^Jd'^'^nte^ic^canal^^ POST-ANAL GUT ANUS GILL POUCHES DEFINITIVE MOUTH HYPOPHYSIS D. VERTEBRATE Fig. 208. — Diagrains illustrating the hypothetical phylogenesis of the vertebrate mouth. The primitive animal mouth, the blastopore, is converted in vertebrates either into an anus or a neurenteric canal. The definitive mouth of vertebrates therefore is a secondary mouth. But the relations of the neuropore are such that at one time in the ancestry of chordates this may have served as a mouth and the neural tube as a f oregut. It is also possible that the mouth of urochordates is not homologous with the definitive mouth of vertebrates. The evidence of a paleostoma or hypophysial opening suggests that this may once have been a functional mouth. Thus the definitive mouth may have been the last in a series of four mouths. THE DIGESTIVE SYSTEM 221 the definitive mouth." According to this view, the neural tube was for- merly a part of the digestive system, and its anterior embryonic external opening, the neuropore, once functioned as a mouth. For a part of the digestive system to become nervous in function is indeed a surprising change, which is no greater, however, than others which have occurred in phylogenesis. If we add to the three mouths mentioned by Delsman the hypophysial "paleostoma " mentioned by Beard and Kupffer, then there have been four mouths in the phylogenesis of vertebrates, the present mouth being the fourth and last. Diagrams showing the position of the four mouths mentioned are shown in Fig. 208. The objection to this idea that there have been a series of mouths in the course of animal phylogenesis, on the ground that the chances are against the appearance of more than one ingestive opening into the enteron, loses much of its weight in view of the fact that many openings into the aUmentary canal, such as the gill-slits, have made their appearance in the course of phylogenesis. The phylogenesis of the vertebrate mouth remains, therefore, an unsolved problem. That there have been at least two mouths in the course of animal evolution, all morphologists agree. These are the coelenterate mouth, w^hich is the blastopore, and the definitive vertebrate mouth. Evidence is, however, not wanting that the embryonic neuropore and the hypophysis may have served as mouths. But such assumptions are considered to have a relatively insecure foundation. The Salivary Glands in Man As food enters the mouth, it is moistened by the secretion of a number of salivary glands, in addition to which are lingual, labial, buccal, palatine and molar mucus-secreting glands. Besides moistening the food, the chief salivary glands contain serous cells which secrete the starch-splitting enzyme ptyalin and the sugar-spUtting enzyme maltase. The sublingual and submaxillary glands secrete mucus also. The largest of the salivary glands is the parotid, which lies below the ear. It is a serous tubulo-acinous gland, and empties by Stenon's duct into the vestibule of the mouth opposite the second upper molar tooth. The submaxillary is a mixed (mucous and serous) tubulo-acinous gland located in the floor of the mouth near the angle of the lower jaw. Its secretions are carried by Wharton's duct which opens near the frenulum at the front margin of the tongue. The sublingual is also a mixed tubulo-acinous gland lying below the tongue in the front of the mouth near the median line. Mucus and serous cells are about evenly distributed. The openings of the sublingual ducts lie in front of the tongue near those of Wharton's ducts. 222 CHORDATE ANATOMY Development. From their position and the relations of their ducts, it is generally assumed that the chief saHvary glands are of ectodermal origin. The numerous glands of the tongue, however, are formed by the local prolification of the stratum germinativum of the endodermal mucous lining of the mouth. History of Salivary Glands. Salivary glands are not unknown among the invertebrates. Multicellular mucus glands connected with the mouth are present in molluscs. Malaria is transmitted by the saliva of mosquitoes. It is doubtful, however, if the saUvary glands of inverte- brates have any genetic relation with those of vertebrates. Part of an excretory duct A crescent consisting of eight serous cells. Lumen Fig. 209. — Section of a human sublingual gland, X252. Histology.") Tangential section of serous cells. Mucous cells and thick membrana propria . Connective tissue. (From Bremer's "Text Book of Lower chordates have no salivary glands, and fishes only unicellular mucus glands. It has generally been assumed that the multicellular glands of the higher vertebrates have their beginnings in such unicellular glands. Multicellular oral glands appear in Amphibia. Besides the mucus- secreting cells of the tongue, most amphibians have an intermaxillary gland, the duct of which opens between the intermaxillary bones. In some amphibians, e.g., Rana, mucus glands are located also in the posterior nasal passages. That enzymes are secreted by the mucus cells of fishes and amphibians has, however, not been demonstrated. In the reptiles, there are serous cells in the oral glands, and lingual, sublingual, and palatine glands occur. Glands connected with the teeth are differentiated as the poison glands of some snakes. THE DIGESTIVE SYSTEM 223 True salivary glands secreting enzymes are limited to mammals. There seems no good reason to doubt, however, that the salivary glands of mammals are derived from the oral glands of reptiles. Labial and buccal glands become abundant in mammals, and possibly the parotid is an enlarged buccal gland. In addition to the Ungual and palatine glands, TUBULAR GLANO> TUBULAR GLAND iCOMPOUND TUBULAR GLAND Fig. 210. — Various types of digestive and endocrinal glands which develop from the endodermal (mucous) lining of the alimentary canal. The endocrine glands are duct- less. The digestive glands may be simple or compound, tubular or alveolar (acinous). (Redrawn after Braus.) the sublingual and submaxillary glands are present; and in general, the glands of man resemble those of other primates. The Tongue The tongue is a muscular organ of miscellaneous functions — digestive, sensory, conversational — lying in the floor of the mouth cavity and attached to the hyoid bone. It consists of an apex or body directed towards the teeth of the lower jaw, a root or muscular attachment, a dorsum divided by the sulcus terminalis into an anterior papillated por- tion and a posterior tonsillar and glandular portion, and an inferior surface below the apex. The sulcus terminahs is a V-shaped groove with the apex of the V pointing backwards and marking the position of the foramen coecum. See Fig. 211. The dorsum of the tongue anterior to the sulcus is covered with numerous papillae which give the tongue its characteristic rough appear- ance. Four kinds of papillae are distinguished, vallate, filiform, foliate, and fungiform. The vallate papillae are the largest, and are distinguished also by the deep depression or fossa which surrounds each of them. On their sides they bear numerous taste-buds. Their number varies from six to twelve, and they occur in a V-shaped row just in front of the sulcus terminahs. Of the various forms of papillae on the tongue the filiform papillae are the most numerous. Each filiform papilla is covered with filamentous processes. Foliate papillae are three to eight parallel folds on each side of the tongue. Like the vallate papillae, the fohate papillae have taste-buds. The fungiform papillae are scattered over the entire 224 CHORDATE ANATOMY dorsum of the tongue, and are distinguished by their reddish color and their globular mushroom shape. They also bear taste-buds. No papillae occur on the posterior and inferior surfaces of the tongue. (Fig. 211) Most of the mass of the tongue consists of striated muscle. In the connective-tissue corium of the tongue, both mucus and serous glands are abundant. The lingual tonsils lie on the posterior dorsum. Development of the Tongue. The apex and root of the tongue, which develop from separate anlagen, remain throughout life divided by the EPIGLOTTIS. 'v . A^ FORAMEN CAECUM \/ Fig. 211. — The dorsal surface of the tongue. The sulcus terminalis divides the body or apex of the tongue from the root. The two regions have a different embryonic origin. (Redrawn after Sobotta.) sulcus terminalis. The apex of the tongue is formed by the union of a median tuberculum impar with the basal portions of the two halves of the mandibular arch. (Fig. 212) The root of the tongue arises from por- tions of the second, third and fourth visceral arches. The tongue muscles, however, are not formed from those of the visceral arches, but from post- occipital myotomes which send buds downward and forwards into the tongue. History of the Tongue. None of the lower chordates has a tongue, so that the vertebrate tongue seems to be an emergent organ like the notochord. The so-called tongue of cyclostomes is a muscular piston THE DIGESTIVE SYSTEM 225 associated with the sucking mouth and cannot be compared with the tongue of higher vertebrates since the hypobranchial muscles which form the mass of tongue muscles in higher vertebrates, though present in cyclostomes, have no connexion with the so-called tongue. Gnathostome fishes have an immovable tongue, which .forms a swelling in the floor of the mouth and is supported by the basihyal or os ento- glossum. Although it lacks muscles, this fish tongue is generally regarded as homologous with the root of the tongue of tetrapods. The tongue of tetrapods, beginning with amphibians, consists of an apex and root as in man. While the root is derived from the tongue of BODY ''sft. TUBERCULUM TUBERCULUM' \ IV IMPAR •■ EPIGLOTTIS Fig. 2 12. — Two stages in the development of tongue and pharyngeal floor of man. The body of the tongue comes from paired and unpaired anlagen of the mandibular arch ; the root from second and third visceral arches. That the fourth arch is involved is doubtful. (After Kallius.) fishes, the body is a new formation derived from the mandibular arch united with a median outgrowth from the floor of the mouth. The tetrapod tongue is further modified by the ingrowth of hypo- branchial muscles by which it attains a high degree of mobility. Con- sequently, in addition to its other functions of moving food in mouth and swallowing, it serves as a means of capturing food. Its gustatory function continues throughout the entire vertebrate series. Some have assumed that the primary function of the tongue muscles was that of squeezing secretions out of the lingual glands. Papillae appear first in amphibians, but become more highly diiTerentiated in mammals. The Pharynx The pharynx is that part of the alimentary canal where the respiratory and digestive passages cross one another. It is bordered by the soft palate above, the tongue below, and the glossopalatine arch on each side. The glossopalatine arch partially covers the palatine tonsil, a mass of 226 CHORDATE ANATOMY adenoid tissue pitted with numerous crypts which tend to be a source of infection. The hypertrophy of adenoid tissue, especially that of the soft palate in childhood, interferes with breathing and often requires surgical treatment. The soft palate is a muscular partition separating digestive and respiratory portions of the pharynx. From its posterior border hangs the uvula. (Fig. 213) Seven cavities open into the pharynx — the mouth, the two nasal passages, the two Eustachian tubes, the larynx, and the esophagus. PARIETAL BONP GYRUS CINGULI PITUITARY, SUBDURAL CAVITY- FRONTAL LOBE- FRONTAL BONE- RIGHT CEREBRAL HEMISPHERE ORPUS CALUDSUM NASAL BONE SPHENOID BONE- NASAL CONCHAE EUSTACHIAN TUBE MAXILLA MOUTH CAVITY- PALATINE BONE- VESTIBULE- SOFT PALATE M. GENIOGLOSSUS MANDIBLE- M. GENIOHYOID M. MYLOHYOID FORNIX PINEAL GLAND DURA MATER -OCCIPITAL BONE EREBELLUM PHARYNX- EPIGLOTTIS- LARYNX>==^ SPINAL CORD SPLENIUS TRAPEZIUS SEMISPINALIS CERVICIS ESOPHAGUS' Fig. 213. — A median longitudinal section of the human head sho-wing the relations between digestive and respiratory passages in the pharyngeal region. (Redrawn after Braus.) Three divisions may be distinguished, oral, nasal, and laryngeal. The palatine tonsils lie in the oral portion, the nasal passages and Eustachian tubes open into the nasal portion, while the larynx opens into the laryngeal portion. When food enters the pharynx the entire pharynx is raised by the contraction of the stylo-pharyngeal muscles while the constrictor muscles of the pharynx squeeze the bolus towards the esophagus. The nerve supply of the pharynx comes chiefly from the glossopharyngeal. Since the pharyngeal region is closely associated with the respiratory organs of vertebrates, the description of the evolution and development of the pharynx is omitted here and will be found in the following chapter. THE DIGESTIVE SYSTEM 227 The Esophagus The esophagus is that portion of the alimentary canal which extends from the pharynx to the stomach. It is nearly ten inches in length and is the narrowest part of the digestive tract. From the pharynx it passes just beneath the backbone through the mediastinum and diaphragm to the cardiac region of the stomach. The wall of the esophagus consists of the four layers characteristic of the digestive tract, tunica mucosa, tunica submucosa, tunica muscularis, MUCOSA > i-»SUBMUCOSA MUSCULARIS MUCOSAE MUCOUS GLAND CIRCULAR MUSCLES ,_ '(^LONGITUDINAL i,'' MUSCLES ^ADVENTITIA VAGUS NERVE Fig. 214. — The esophagus as seen in cross section. A is a section of the entire esophagus. J5 is a small portion much enlarged. The layers of tissue characteristic of the entire alimentary canal are found in the esophagus. (Redrawn after Braus.) and tunica adventitia ; but the serous layer which covers the stomach and intestine is wanting in the esophagus, since the body-cavity Uned by the serosa does not extend into the neck. The tunica mucosa includes not only the stratified squamous epithelium which lines the esophagus, but also a connective-tissue tunica propria and a muscularis mucosae, a thin layer of longitudinal muscle fibers. The muscular coat of the esophagus consists of striped fibers in the upper third, while those of the lower two- thirds are smooth. (Fig. 214) 2 28 CHORDATE ANATOMY The submucosa is a layer of loose connective tissue containing glands and many blood and lymph vessels. The tunica muscularis consists of an inner layer of circular muscles and an outer longitudinal layer. The connective tissue between them contains a plexus of sympathetic nerve fibers. By the wave-like peristalsis of the circular muscles food is con- veyed from the pharynx to the stomach. Development of the Esophagus. Beginning with the fourth week, the esophagus develops as an elongation of the fore-gut between pharynx and stomach. Its single-layered columnar epithelium becomes gradually converted into a stratified squamous epithelium like that which lines the pharynx. History of the Esophagus. There is little to distinguish the esophagus of a fish from its stomach, except the relative scarcity of glands, and the fact that its muscle fibers, like those of the pharynx, are striated, while those of the stomach are smooth. In amphibians, the esophagus becomes slightly elongated. Its considerable elongation in reptiles and mammals is correlated with the elongation of the neck. In these groups, it becomes constricted in diameter and most of its muscle fibers become smooth. The Stomach The stomach, lying between the esophagus and small intestine, is the most expanded part of the alimentary canal. Its shape in man varies greatly, depending upon the quantity of food contained. The human stomach lies almost transversely across the abdominal cavity with a greater curvature on the left side of the body and a lesser curvature to the right. The opening of the esophagus into the stomach is the cardiac orifice, that into the small intestine is the pylorus. The anterior more enlarged portion of the stomach is the cardiac portion, the posterior more constricted region is the pyloric portion. The pyloric portion of the stomach diminishes in size towards the pylorus, which is reduced to a small aperture by a local ring-like thickening of the mucous lining and of the layer of circular muscle. The wall of the stomach contains the same four layers of tissue as are seen in the esopha- gus, plus an external serous layer. The tunica muscularis contains three layers of muscle, longitudinal, circular, and obUque. By their combined action under the stimulus of the sympathetic nerves, the stomach maintains a peristaltic churning action as long as food is present. The simple mucous epithelium which lines the stomach joins abruptly the stratified epithelium of the esophagus. Viewed with a hand-lens, the inner surface of the stomach appears to be filled with minute pores, which are the apertures of the ducts of the gastric glands. Three kinds of stomach glands are distinguished, cardiac, gastric, and pyloric. The THE DIGESTIVE SYSTEM 229 '■greater CURVATUIte PYLORIC STOMACH Fig. 215. — The right half of the human stomach, viewed from within. (Redrawn from Braus, after Elze.) MUCOUS EPITHELIUM- ^OASTRIC PIT<.' MUCUS GLANDS<' ^MUCUS GLANDS ETAL CELLS MUSCULAR I Sc Fig. 216. — Cross sections of the wall of the human stomach, showing A, the struc- ture of the gastric (fundus) glands, and B, that of the pyloric glands. While the secre- tions of gastric glands are chiefly digestive (gastric juice), the pyloric glands secrete mucus chiefly. (Redrawn after Braus.) 230 CHORDATE ANATOMY cardiac glands occupy a relatively small area near the cardiac orifice and resemble closely the glands of the esophagus. Each cardiac gland consists of a group of parallel tubules opening into a single duct or pit. The walls of the tubules are formed of cells which secrete zymogen or pepsinogen granules, of parietal cells which secrete the chemical precursor of hydrochloric acid, and of mucus-secreting cells. Most of the glands of the stomach are gastric each of which, Hke the cardiac glands, consists of a duct or pit connected with a group of straight pORSAL PANCREAS ^•VENTRAL PANCREAS INTESTINE -CAECUM PHARYNX- PERITDNEAL CAVITV ^MESONEPHRIC PANCREAS VENTRAL PANCREAS URO SINUS UMBIUCAL CORO' Fig. 217. — Stages A-F in the ontogenesis of the alimentary canal and associated structures in the human embryo. A, early embryo; B, three-weeks embryo; C, three to four weeks embryo; D, four weeks embryo; E, five weeks embryo; F, seven weeks embryo. Notable among the changes represented are the great elongation of the canal, the outgrowth of numerous appendages, and in the cloacal region the separation of the organs of excretion and digestion. (Redrawn after Thompson, Ingalls, F. T. Lewis and Arey.) or slightly curved tubules. The pits are relatively short and are Hned with mucous gland cells like those which cover the inner surface of the stomach, while the tubular glands are relatively elongated and their walls are formed of granular chief cells and of peripheral parietal cells. The chief cells secrete two kinds of zymogen granules— pepsinogen and prochymosin. When mixed with hydrochloric acid secreted by the parietal cells, pepsinogen becomes pepsin, which spUts the molecules of albumen into peptones, and the prochymosin becomes chymosin or rennin, which changes casein into paracasein. It is also asserted that the gastric glands secrete lipase, a fat-splitting enzyme. THE DIGESTIVE SYSTEM 23 I The pyloric glands are limited to the pyloric portion of the stomach. Their chief secretion is mucus, but the presence of some chief and parietal cells suggests that they may also secrete some gastric juice. They differ from gastric glands also in having relatively long pits and short, branched and twisted tubules. Thus they resemble duodenal glands. Development of the Stomach. During ontogenesis, beginning with the fifth week, the stomach arises as a local enlargement of the fore-gut. Its lining, therefore, together with the glands derived from it, is endo- dermal. The external peritoneal membrane is mesodermal ; the remainder of the stomach wall, including the submucous and muscularis layers, is mesenchymatous. The more rapid growth of the dorsal wall produces the greater curvature of the stomach. The lesser curvature develops from the ventral side. The original dorsal side shifts to the left side of the body, while the primitive ventral side comes to lie towards the right. Gastric glands begin to appear as local proliferations of the lining epi- thelium during the seventh week. History of the Stomach. Since stomachs are not unknown among invertebrates, it might be assumed that the stomach of vertebrates is derived directly from that of invertebrates. However, among the proto- chordates, the hemichordates and some urochordates possess a stomach, while the cephalochordates do not, the pharynx passing immediately into the intestine. The liver of Amphioxus develops as a ventral outgrowth a short distance behind the pharynx. Consequently, if we consider Amphioxus as an ancestral type, the stomach of vertebrates must have arisen from the short portion of the alimentary canal which in cephalo- chordates lies between pharynx and liver. The esophagus must likewise have developed from this region. In the cyclostomes, the stomach is a slight enlargement of the ali- mentary canal. As in the Dipnoi, there is no flexure. In most fishes, however, the stomach becomes J-shaped by the bending of the pyloric region, and this curvature persists throughout the vertebrate series. The complications of stomachs such as are found in ruminants are of considerable importance and interest. The stomach of the cow, for example, is divided into four functional divisions, rumen, reticulum, omasum (psalterium) , and abomasum. Since, however, such adaptations to a special diet throw no light on the problem of human phylogenesis, detailed description is omitted. The Intestine The intestine is the portion of the alimentary canal from the pylorus to the anus. Its length averages about thirty feet, of which five feet are included in the large intestine and the remainder in the small intestine. 232 CHORDATE ANATOMY Small Intestine. The small intestine extends, gradually diminishing in diameter, from the pylorus to the ileocolic valve of the colon. The small intestine is distinguished not only by its smaller diameter but also by the presence of numerous villi which cover its inner surface and give it a velvety appearance. Somewhat arbitrarily three regions are dis- tinguished, duodenum, jejunum, and ileum. The duodenum, the anterior portion of the small intestine, averages about nine inches in length, and "VILLI ---PLICA CIRCULARIS S^- LYMPHATIC BLOOD VESSELS ■VILLUS CRYPT MUSCULARIS MUCOSAE ■SUBMUCOSA CIRC. MUSCLE "LONG. MUSCLE -SEROSA Fig. 2i8. — A longitudinal section of the human jejunum, showing in cross section one of the circular plicae (valvulae conniventes). XiS- is characterized by the presence of tubulo-acinous glands located in the submucosa and known as duodenal or Brunner's glands. The duodenal glands secrete an alkaline mucus which neutralizes the acidity of the food which enters the duodenum from the stomach. Zymogenic cells are also found in the duodenal mucosa. The jejunum, which forms two-fifths of the remainder of the small intestine, contains numerous transverse crescentic folds, the plicae or valvulae conniventes, covered with large villi. See Fig. 218. These plicae serve to retard the passage of food and also to increase the absorptive THE DIGESTIVE SYSTEM 233 surface. In ihe ileum, Ihe crescentic folds disappear, and villi become smaller and more scattered. The four layers of tissue characteristic of the alimentary canal are present in the small intestine. Throughout the entire length of the intestine are numerous tubular mucus-secreting glands, perpendicular to the surface of the intestine, the intestinal glands or crypts of Lieberkiihn. Goblet-shaped cells distended with mucus are abundant in the walls of these glands. The secretions of these glands are said to stimulate peristal- sis of the intestine as well as lubricate its surface. (Figs. 218-219) r -MUCOUS EPITHELIUM "CRYPT -LYMPHOID NODULE -_ MUSCULAR! S MUCOSAE *^r^4^<*;^t*:?:^s^-SUBMUC0SA ■CIRCULAR MUSCLE i- LONGITUDINAL MUSCLE "^ "" "^ — SEROSA Fig. 219. — A longitudinal section of the human colon. Xi5- Each villus is covered with a mucus epithelium containing numerous goblet-cells, and each villus has a core of connective tissue filled with capillaries and lymph vessels. A single lymphatic or lacteal occupies the center of each villus, and a network of capillaries lies just below the base- ment membrane of the mucous epithelium. Each villus is therefore a mechanism admirably adapted for absorbing the digested food which bathes it. Besides the peristaltic waves which pass along the intestine squeezing the food backwards towards the large intestine, divisive or churning movements are also carried on, bringing the digested food into contact with the villi. 234 CHORDATE ANATOMY Absorption takes place in the small intestine in accordance with the law of osmosis. The dissolved foods pass through the lining membranes, are taken up by the blood capillaries and the lymphatics, enter the general circulation, and are absorbed into the cells of the various tissues. Large Intestine. The large intestine or colon differs from the small not only in its great diameter but also in the absence of villi in the adult. The walls of the large intestine are sacculated, and they bear externally numerous fatty appendages, the appendices epiploicae. The longitudinal muscles do not form a continuous layer as in the small #ffi>r- .^^„ LONGITUDINAL MUSCLE ~'^f'^:;7r'^~" CIRCULAR MUSCLE - <5--v «< -iS "* %. ^^^^^ /' >\^ ^ ^^^'^'plx w:t^?^;:^~^T'^^ CRYPT •i^jh #A. "^ :'^^"T1GERM. CENTER vV% X . '-~---;|^;^^^-SUBMUCOSA . ^^* -^^''?;:; ;- serosa Pig. 220. — A cross section of the human vermiform appendix. Xi5« intestine, but are arranged in three longitudinal bands, the teniae. Trans- verse crescentic folds, the plicae semilunares, are abundant. Between these the wall of the colon bulges out to form haustra. The large intestine is divided into cecum, vermiform appendix, colon, rectum, and anus. The cecum is a blind sac, about two and a half inches in length, lying near the ileocolic valve in the right ihac fossa. The vermiform appendix of the cecum is an elongated worm-shaped tube between three and four inches in length, attached to the apex of the cecum. The structure of the appendix is similar to that of the large intestine in having numerous Lieberkiihn's glands and lymph nodules. In the majority of persons, the lumen becomes occluded in later life. The THE DIGESTIVE SYSTEM 235 appendix appears to be a rudiment of a more extended cecum functional in the ancestors of man. The colon is divided into four regions, ascending, transverse, descend- ing and sigmoid colon. The ascending colon passes up the right side of the abdominal cavity as far as the liver, where it bends to the left to form the transverse colon. Reaching the lower end of the spleen on the left side, it curves sharply downward, to become the descending colon. Passing down the left side to a point below the kidney, the descending colon bends toward the median plane of the body and enters the pelvic cavity, where it forms the sigmoid flexure. The rectum is continuous with the sigmoid colon and extends to the anus. In the rectum, a number of transverse folds of the wall tend to prevent fecal matter from pressing into the anal canal. In the anal region, the layer of circular muscles is thickened to form the sphincter ani, which, unlike that of the lower rectum, is non- striated and not under control of the will. The external sphincter of the anus, however, is striated and voluntary. Development of the Intestine. Except in the mouth and anal regions, the mucous lining of the alimentary canal and the secretory epithelium of the glands connected with it develop from the endoderm. Primarily, the endoderm of the embryonic area is continuous with that which lines the yolk-sac, (Fig. 221) In correlation with the development of head- fold and tail-fold, a fore-gut and hind-gut are formed in connexion with the yolk-sac by means of anterior and posterior intestinal portals (Fig. 72). From the fore-gut develop pharynx, esophagus, stomach, and the anterior part of the small intestine; from the hind-gut the remainder of the intestine. Early in development, an allantois arises as a ventral outpocketing of the hind-gut, with which it retains connexion by an allantoic stalk. The cloaca is the posterior portion of the hind-gut into which allantois and intestine open, and which is closed to the exterior by the cloacal membrane (Fig. 72Z}). The later development of the intestine involves its elongation and twisting. The opening into the yolk-sac becomes reduced to a slender vitelline duct, which disappears during the second month. Becoming at first too long for the body-cavity, a loop of the intestine pushes down into the umbilical cord. In a six weeks' embryo, the beginning of a cecum is indicated by a swelling posterior to the vitelline duct. Later a horizontal septum grows backward to divide the cloaca into a dorsal rectum and a ventral urogenital sinus. The septum forms the perineum of the adult. During the second month, an anal canal is formed by the invagination of an ectodermal proctodeum and the rupture of the cloacal membrane. (Fig. 217) The four layers of the intestinal wall develop as has been described for the stomach. History of the Intestine. The intestine as a region for the digestion and absorption of food is present in the great majority of animals from 236 CHORDATE ANATOMY flatworms to man. An anal aperture makes its first appearance in flat- worms. Vertebrate morphologists generally regard the anus of verte- brates as homologous throughout the group notwithstanding differences in ontogenetic development in different groups. The post-anal gut may be interpreted as a special modification correlated with the elongation of the tail, and not as a primitive trait. The assumption of a partial homology of the vertebrate anus with the blastoporic mouth of invertebrates seems to be in harmony with all known facts. The uncertainty of pre-chordate homologies will explain why most vertebrate morphologists take the intestine of Amphioxus as the starting point for intestinal evolution. The intestine of Amphioxus extends as a straight tube from the region of the liver directly to the left-sided anus. The intestine of cyclostomes is almost as simple. A spiral fold projecting into the cyclostome intestine, however, suggests the beginning of intestinal differentiation. The intestine of elasmobranchs contains a more elaborate spiral valve. Intestinal elongation has its inception in the sigmoid flexure of elasmobranchs. Increase of intestinal surface is effected in elasmo- branchs and ganoids mainly by the development of a spiral valve. A finger-hke rectal gland makes its appearance in elasmobranchs near the anus. A cloaca also makes its first appearance in this group. A further step in advance is seen in the teleosts, which have a convoluted small intestine, intestinal ceca, and a somewhat enlarged colon. Most amphib- ians except Gymnophiona differentiate smaU and large intestines. All have a cloaca. Some have, in their small intestine, intestinal glands, valvulae conniventes, and villi. The intestine of reptiles is relatively short. Their large intestine is short, and they retain a cloaca. In mammals, the small intestine becomes greatly elongated and differ- entiated into duodenum, jejunum, and ileum. Valvulae conniventes, vflh, and intestinal glands become very numerous. Duodenal glands make their appearance. Colon and rectum are differentiated. In many mammals, especially herbivorous forms, the cecum becomes much elon- gated and forms an important organ of absorption. In others, as in man, it degenerates in size and serves as an adenoid organ. Mesenteries and Omenta The peritoneum lining the abdominal cavity is a serous membrane formed from the embryonic hypomere. It not only lines the body-wall, but is reflected over the viscera, so that parietal and visceral portions are distinguishable. The complex relations of the peritoneum are due chiefly to the complications of the ahmentary canal with which it is connected. These are best understood by tracing their development in the embryo. In the region of the pharynx the splanchnic layers of meso- THE DIGESTIVE SYSTEM 237 derm unite in the median plane to form the tubular heart and the meso- cardial membranes in which the embryonic heart is suspended. In the abdominal part of the coelom the splanchnic layers of mesoderm unite dorsal mesoderm intermediate mesoderm lateral mesoderm Fig. 221. Schematic diagrams of cross sections at various stages to show the establishment of the coelom and mesenteries. (From Patten's "Embryology of the Chick.") above and below the alimentary canal to form dorsal and ventral mesen- teries. (Fig. 221) The dorsal mesentery persists throughout life, but the greater part of the ventral mesentery disappears in ontogenesis. Only the anterior portion which connects stomach, liver and ventral body- wall 238 CHORDATE ANATOMY is retained. With the differentiation of the successive regions of the aHmen- tary canal, corresponding portions of the dorsal mesentery are recognized as mesogaster, mesentery, mesocolon, and mesorectum. The mesenteries serve not only as means of attachment of the intestine to the body-wall, but also as a passage for the blood-vessels of the ahmentary canal. In the adult the mesenteries become very complex in relations as the result of the elongation of the intestine, formation of omenta, and local adhesions. As the stomach develops its greater curvature, it rotates on its long axis so that its left side becomes ventral and the right side dorsal. As a result the dorsal mesogaster is stretched to the left and a pouch or bursa between the mesogaster and the right side of the stomach is formed. /MESOGASTER 'GREATER OMENTUM < ^1^^^ |t~-CAECUM-^ j/ APPENDIX--^ j SMALL INTESTINE '/-COLON •YOLK STALK ESOPHAGUS - CURVATURE STOMACH '— GREATER OMENTUM COLON CAECUM ^j VPPENDIX -■!^ SMALL INTESTINE' Fig. 222. — Diagrams illustrating the development of the mesenteries and omentum in the human embryo. An arrow marks the opening (foramen of Winslow) of the greater omentum. (Redrawn after Hertwig.) As the sacculation of the mesogaster progresses, dorsal and ventral layers become distinguishable. The two-layered sac thus formed grows ventrally and posteriorly between the viscera and the ventral wall of the abdomen as an apron-like membrane, the greater omentum. Much of the original cavity of the omentum is lost through the fusion of dorsal and ventral layers. In the region of the stomach, however, the cavity persists as the bursa omentalis, which opens by the foramen epiploicum into the coelom of the right side. The omentum becomes the seat of deposit of con- siderable fat and serves as a blanket to keep the viscera warm. The Liver The functions of the Hver are diverse. During early ontogenesis, it forms red blood corpuscles. Later in life, it becomes an agent in the eUmination of blood cells. It transforms both sugar and protein into a polysaccharid, glycogen, which it stores in its cells for later use. It also THE DIGESTIVE SYSTEM 239 secretes bile, which aids in the emulsification of fats, in the activation of lipase secreted by the pancreas, and in the stimulation of peristalsis of the intestine. The Hver is a reddish-brown organ l>ing between the stomach and the diaphragm and is the largest gland in the body, weighing between two and three pounds. It is wedge-shaped, and divided into a smaller left lobe DIAPHRAGM ' ESOPHAGUS ■SPINAL CORD DORSAL AORTA LIVER FORAMEN EPIPLaCUM PANCREAS STOMACH ?. MESENTERrCART. ^_rri_LTRANSVERSE COLON ili=irriI\OL.i J.INR MESENTERIC ART. — ^ |5~r5 1 — MESENTERY rV^— --J- A_0MENTAL BURSA BLADDER SYMPHYSIS UTERUS RECTUM Fig. 223. — A median section of the abdominal cavity, showing the relations of the omental bursa. The diaphragm is cross-hatched. The course of the duodenum- jejunum and of the colon is shown by means of arrows. (Redrawn after Braus.) and a larger right lobe. The two lobes are separated by the falciform ligament, which is developed from the ventral mesentery and attaches the liver to the diaphragm and the ventral body-wall. Two smaller lobes, the caudate and quadrate, He between the right and left lobes on their inferior surface. The gall bladder lies below the right lobe near the duo- denum. The postcaval vein passes through the right lobe. 240 CHORDATE ANATOMY Secretions pass from each lateral lobe by a single duct, the two uniting to form the hepatic duct. Nearer the intestine, the hepatic duct joins the cystic duct from the gall bladder to form the common bile duct or ductus choledochus, which opens into the duodenum at a point about three or four inches from the pylorus. The liver is a compound tubular gland, the tubules of which are arranged radially around branches of the hepatic vein. Each cluster of tubules around a central intralobular vein forms a lobule. (Fig. 225) The numerous lobules of the Hver are bound together by interlobular connective tissue containing interlobular veins which are branches of the POST CAVA VEIN CUT EDGE OF PERITONEUM HEPATIC ARTERY RIGHT LOBE QUADRATE LOBE/ GALL BLADDER/ \BILE DUCT Fig. 224. — The human liver viewed from below. (Redrawn after Sobotta.) portal vein, interlobular ducts carrying bile, and branches of the hepatic artery. Connexions between intralobular and interlobular veins are effected by means of intralobular capillaries or sinusoids, which bathe the liver tubules and supply them with the materials for secreting the bile. The relations may best be understood by examination of the diagram (Fig. 226). While branches of the vagus nerve reach the liver, most of its nerves belong to the sympathetic system. The gall bladder is a pear-shaped muscular sac between three and four inches in length, holding about 30 cc. Its inner surface is lined by a mucous epithelium which is thrown into folds. Crescentic folds in the neck of the bladder and in the common bile duct form a sort of spiral valve. When food enters the duodenum from the stomach, the muscles of the gall bladder squeeze the bile into the intestine. Development of the Liver. The anlage of the liver appears in a 2.5 mm. human embryo as a ventral outpocketing of the fore-gut near the anterior intestinal portal, between the two vitelline veins. See Fig. 217. THE DIGESTIVE SYSTEM 241 b o rl C Fig. 225. — Liver of a pig. The lobules have artiticially shrunken from the inter- lobular tissue, a; b, bile duct; c, hepatic artery; d, interlobular vein (a branch of the portal) ;f, trabeculae;/, central vein. Highly magnified. (From Bremer's "Text Book of Histology," after Radasch.) fr--_PORTAL VEIN- LOBULE-- -HEPATIC VEIN A B C Fig. 226. — A, B, and C — diagrams of successive stages in the development of the lobules of the liver. The subdivision of the liver anlage into lobules is correlated with branching of the portal and hepatic veins. The branches of the hepatic vein are intralobular, and those of the hepatic portal vein — shown in black — are interlobular. (Redrawn after Mall.) 242 CHORDATE ANATOMY The liver diverticulum projects into the ventral mesentery and the meso- derm of the septum transversum which separates the pericardial cavity from the abdominal cavity. The outgrowth soon becomes differentiated into an anterior mass of branching cords surrounded by branches of the vitelline veins, and a posterior hollow sac which later becomes the gall bladder. The multiphcation of the cords, correlated with that of the blood capillaries associated with them, produces the lobules. Mesenchyme cells form the interlobular connective tissue. Bile capillaries appear within the cell cords, which thus become hepatic tubules, and the blood capillaries acquire endothelial walls. As a result of this, the lumen of each bile capillary is separated from that of each blood capillary by a layer of gland cells and a layer of endothelial cells. (Figs. 217, 221, 226) The multiplication of tubular cords and of blood spaces results in a rapid enlargement of the liver, which begins to bulge out from the septum transversum and the ventral mesentery and to push into the abdominal cavity between the septum and the stomach. In this way the liver becomes covered by the peritoneum. Meanwhile it acquires its two chief lobes. The ventral mesentery into which it originally grew forms the falciform ligament. (Fig. 221) History of the Liver. The vertebrate liver has no homolog among invertebrates, though many of these have organs which are called livers. The liver of Amphioxus is generally regarded as representing the beginning of that of vertebrates. This is a ventral outpocketing of the intestine immediately behind the pharynx. It grows ventrally and forwards beneath the pharynx, and remains a hollow sac throughout life. Its rela- tions to the blood-vessels resemble those of the liver of vertebrates. The liver becomes bilobed in cyclostomes and elasmobranchs, and a gall bladder is differentiated. In the higher vertebrates and man no important morphological changes occur. The form, however, varies with the shape of the abdominal cavity and the pressure of surrounding organs. The Pancreas The pancreas is a light pinkish organ about five inches in length, extending across the abdominal cavity from a loop of the duodenum on the right side to the left colic flexure. In man the pancreas usually has two functional ducts. One of these, the pancreatic or Wirsung's duct, generally opens into the common bile duct; the other, the accessory or Santorini's duct, opens into the duodenum about an inch above the opening of the bile duct. The pancreas secretes trypsinogen, which is converted into trypsin through the action of enterokinase secreted by the intestinal glands. Tryj)sin splits proteins into amino-acids. The enzyme amylopsin secreted THE DIGESTIVE SYSTEM 243 by the pancreas splits starch into monosaccharids. Another enzyme, lipase or steapsin, when activated by enterokinase breaks fats into fatty acids and glycerine. Another enzyme, ereptose or erepsin, splits pro- teoses and peptones. The digestive activity of the pancreas is stimulated through the endocrinal effect of secretions poured into the blood by the intestinal glands when the acid chyme enters the intestine from the stomach. Besides this digestive function, the pancreas acting as an endocrine gland regulates the sugar metabolism of the body by means of the hormone insulin. The histological structure of the pancreas strikingly resembles that of the parotid gland, both being compound acinous glands divided into lobes and lobules by connective-tissue septa which contain interlobular ducts, ..--STOMACH DUCT OF SANTORIN RSAL PANCREIAS DORSAL PANCREAS "VENTRAL R^NCREAS' DUODENUM BILE DUCT WIRS UNO'S DUCT A. EARLIER STAGE. B. LATER STAGE. Fig. 227. — A and B, two stages in the development of the pancreas. The duct of the dorsal pancreas, Santorini's duct, may degenerate in ontogenesis. The two gland anlagen unite into a single organ in the adult. (Redrawn after Broman.) blood-vessels, and nerves. The acini of the pancreas, instead of being hollow, contain central cells. Scattered irregularly among the acini of the pancreas are clusters of lightly-staining cells. The area of each cluster in section is considerably greater than that of a single acinus. These are the islands of Langerhans, endocrinal organs which secrete insulin. (Fig. 289) Development of the Pancreas. Like the liver, the pancreas develops from the endoderm. It is formed by the fusion of two separate out- growths of the intestine, a ventral bilobed outpocketing from the bile- duct, and a dorsal evagination of the intestine slightly anterior to that of the liver. By the proHferation of the cells of these anlagen, two pan- creases are formed. They secondarily unite, but retain usually the two primary connexions with the intestine, the ventral becoming Wirsung's duct and the dorsal Santorini's, the two connecting within the body of the gland. The dorsal pancreas grows much faster than the ventral, and forms the body and tail of the gland and part of the head. 244 CHORDATE ANATOMY History of the Pancreas. The pancreas seems to be an emergent trait of vertebrates, since no comparable structure is found in the invertebrates or even in the lower chordates. In cyclostomes, the pancreatic tissue remains buried in the substance of the Uver or in the wall of the small intestine. Since no duct appears in these forms, it is assumed that the pancreas was primarily endocrinal and not digestive. Other verte- brates, beginning with the elasmobranchs, have both dorsal and ventral pancreases. CHAPTER 9 THE RESPIRATORY SYSTEM Introduction. Living protoplasm, that is to say a living organism, burns slowly and continually. When oxidation ceases, life ceases also. Galen in the second century saw the similarity between respiration and burning. But it was many centuries before Lavoisier (1771-1780) proved its chemical nature. Breathing is but a subordinate part of respiration. Respiration is the process of gaseous exchange which occurs in a living body through the oxidation of carbon compounds. This exchange involves an intake of oxygen and an outgo of carbon dioxide. The process requires uncombined oxygen, which forms one fifth of the air. Aquatic organisms obtain their oxygen from air dissolved in water. Two kinds of respiration may be distinguished, external and internal. In external respiration, animals make use either of a moist skin or of specialized respiratory organs such as lungs and gills in which blood capillaries are brought into intimate relation with moist membranes. Under these conditions, intake of oxygen goes on in accordance with the law of diffusion of gases separated by semipermeable membranes. In inter- nal respiration, in accordance with the same law, gaseous exchange takes place within all the tissues of the body which are bathed with blood or lymph. Cells draw on the oxygen in these just as a burning match gets its oxygen from the air. The living cell, however, unUke the match, is the master of the oxidative process and not its servant. The necessity for two kinds of respiratory organs, one adapted to aquatic and the other to land and aerial life, has produced in chordates two distinct but possibly not entirely independent respiratory systems to complicate evolutionary history. These are the pharyngeal gills of the lower and the lungs of the higher classes. Chordates have not inherited their respiratory system from their invertebrate forbears, but have invented new ones of their own. Fortunately for the land vertebrates, their fish ancestors were already prepared for the transition from water to land life before the event occurred. By a change of function and some modifications of structure and relation, the bilobed air bladder of the crossopterygian fishes was made to serve as a lung. Furthermore the advantage of nasal passages in air breathing was probably already anticipated by the fish ancestors of amphibians. This assumption seems justified by the fact that some fishes, such as the 245 246 CHORDATE ANATOMY Dipnoi, have narial passages. But it is not generally believed that the Dipnoi are in the direct line of amphibian ancestry. The story of gills is one of great multiplication in number in forms like the protochordates which use the pharynx both for obtaining food and for gaseous exchange. In the fishes and amphibians, however, the gills are considerably modified, are reduced in number and finally in higher verte- brates disappear. Startling changes of function occur. Supporting skeletal elements are converted into a sound-conducting apparatus. Gill-slits degenerate into blind pharyngeal pouches, which in higher verte- brates become endocrinal glands. The transformation of a ventral air bladder into lungs is sufficiently well attested to be plausible. The chief evolutionary change which lungs undergo is an enormous increase of respiratory surface so that, even within the limits of the mammahan chest, they expose many square yards of moist surface for gaseous exchange. To meet respiratory needs, two sorts of organs have emerged in animals, branchial organs or gills found in aquatic animals and pulmonary organs characteristic of land forms. A. The Branchial System. The fact that lungs are wanting in all classes of protochordates, as well as in the more primitive groups of verte- brates, proves that the primary respiratory system is the series of paired pharyngeal gills which form the branchial system of chordates. Rem- nants of this system persist in all higher vertebrates. The transition between gilled and lunged forms occurs in the amphibians most of which, at least at some time in their individual development, have both gills and lungs and which thus bridge the gap between aquatic and terrestrial life. Gills, like lungs, function as respiratory organs by bringing a network of blood capillaries in close contact with moistened membranes through which gaseous exchange takes place. Their efficiency is increased either by the activity of cilia which cover the surface of the gills or by the con- traction of muscles which pump a stream of water through the pharynx, or by waving the gills to and fro as in Necturus. Gills are not the pharyngeal openings through which water passes in respiration; these are gill-slits or gill-clefts. Two sorts may be distin- guished, internal gills within the body-wall and external gills. Those of most animals are internal; a few fishes and amphibians have external. The gills of elasmobranchs may be taken as typical. They are modifica- tions of the branchial bars or arches which alternate with the gill-slits and serve to keep them open. Each branchial arch consists of an inter- branchial septum of connective tissue which is covered on the surface of the body by skin, and which includes near the pharyngeal lining a car- tilaginous arch as a support. Within the septum are branches of the dorsal and ventral aortae which supply the gills with blood. The septa THE RESPIRATORY SYSTEM 247 are further supported by skeletal gill-rays extending from the skeletal branchial arch laterally towards the skin. Each interbranchial septum bears on each surface a half-gill or hemi- branch, which together constitute a holobranch. Each hemibranch is a mucous membrane folded into minute parallel lamellae or branchial filaments, each of which has parallel secondary folds containing a capillary network. Between the capillaries and separating them are pilaster cells peculiar to the gill filaments. In the ganoids and teleosts the inter- FiG. 228. — Diagram of gill clefts in (A) elasmobranchs and (B) teleosts. A' and B', a single gill of each, a, artery; ba, branchial arch; br, branchial ray; d, demibranchs; gc, atrial chamber; gr, gill raker; o, operculum; oe, esophagus; 00, opercular opening; 5, spiracle, in A', septum; v, veins. (From Kingsley's "Comparative Anatomy of Vertebrates.") branchial septum becomes reduced and tends to disappear, leaving only the portion containing the skeletal arch and branchial blood-vessels. In these forms, the gill-slits do not open separately to the exterior as in elasmobranchs but are covered by an operculum formed by the backward growth of the septum of the hyoid arch. (Fig. 228) The mechanism of breathing diflfers considerably in fishes which, like the elasmobranchs, have modified the first gill-slits into spiracles, and those which have not. In all fishes, through the action of antagonistic pharyngeal muscles, the cavity of the pharynx is alternately expanded and contracted, so that water is sucked in through the mouth or the spiracles and forced out through the gill-slits. In forms with an opercu- 248 CHORDATE ANATOMY lum, this functions as a valve, and prevents the entrance of water through the gill-shts. Gaseous exchange takes place through the thin mucous epithelium which covers the gill lamellae. The gills of fishes function also as excretory organs, excreting nitrogenous waste as do the kidneys. Fig. 229. — Diagram of the reUitiuns of external and internal gills in the anuran tadpole, ab, eb, afferent and efferent branchial arteries; h, heart; o, ear cavity; ph, pharynx; ra, radix aortae. (From Kingsley's " Comparative Anatomy of Vertebrates," after Maurer.) External gills are of two sorts, external gill filaments such as occur in elasmobranch embryos as prolongations of the posterior gill lamellae, and external gills which characterize some adult urodeles and the larvae of some fishes and amphibians. The evidence on the whole supports the opinion that they are secondary derivatives of the gill system, developed DIENCEPHALON LENS ESOPHAGUS ECTODERM SPINAL CORD PRONEPHROS NOTOCHORD MYOTOMES DORSAL AORTA PETROMYZON. 16-DAY EMBRYO -FRONTAL SECTION. Fig. 230.- — Frontal (horizontal) section of a 16-day Petromyzon embryo, showing seven pairs of gill pouches (1-7) formed as lateral diverticula of the pharynx. Slight invaginations of the ectoderm to meet the gill pouches are seen. By the rupture of the double (ectoderm-endoderm) membrane each gill pouch is converted into a gill cleft. Between the successive gill pouches the mesoderm is divided into a series of branchiomeric segments, from which the muscles and skeletal arches of the gills develop. in adaptation to special conditions. They have no genetic relation to any human structure. Development of Gills. Gill-slits develop from a series of paired endo- dermic diverticula of the pharynx which meet corresponding invaginations of the ectoderm. (Fig. 230) By the disappearance of the double mem- THE RESPIRATORY SYSTEM 249 BRAIN , MYOTOME brane thus formed the pouches are converted into gill-shts. The branchial arches develop from the regions between the gill-slits. Each arch has an endodermal pharyngeal lining and an external ectodermal covering. The core of each arch is mesodermal. The levator and depressor muscles of the gills are developed from the hypomeric mest)derm enclosed in each branchial arch. The connective tissue, the cartilage or bone, and the blood-vessels of each arch are derived from the mesenchyma. History of the Gills. Pharyn- geal gills are peculiar to chordates and are one of the most constant characteristics of the group. This should not be understood to imply that invertebrates are without structures from which gills might have evolved. The origin of gills from endodermic diverticula sug- gests the possibility that their be- ginnings may be seen in the intestinal diverticula of fiat worms. Were these diverticula to meet the skin and become perforate, aper- tures similar to gill-slits would be formed. Gill-slits first appear in the hemichordates. Rhabdopleura has none, but Cephalodiscus has a single pair. In most hemichordates the number is considerable and in- creases throughout life. Early in their development, their number is doubled by the growth of "tongue- bars" which extend from the dorsal side of the gill aperture to the ventral side. Later the gill bars thus formed become intercon- nected by cross rods or sjmapticulae such as occur also in urochordates and cephalochordates. In urochordates, the number of gill-slits varies from a single pair in Appendicularia to the many characteristic of most genera. The gill-slits of this group open into an atrial cavity developed by an ectodermal ingrowth along the dorsal side of the body. COELOM"-- ESOPHAGUS Fig. 231. — The pharyngeal regio 1 of a young Squalus embryo, showing the visceral arches and clefts. !5o CHORDATE ANATOMY SOMITES S0M3 MIDBRAIN SOMITE 7 I rOREGUT SOMITE A \ » SOMITE I HEART PERICARDIAL CAVITY POST -ANAL GUT NEURENTERIC CANAL' Fig. 232. — A 7 mm. Squalus embryo viewed as a cleared specimen from the left side. The yolk-sac has been mostly removed. Two gill-slits are open. Cranial nerve anlagen are indicated by Roman numerals. ANTERIOR CAVITY MYOTOME 2 CHORDA ENTERON MOUT ENDOSTYLE / LEFT 1ST (TRANSIENT)GILL-POUCH CLUB-SHAPED GLAND A. EARLIER LARVA. ENDOSTYLE RIGHTIST PERMANENT GILL-SLIT PREORAL PIT CLUB-SHAPED GLAND' TRANSIENT GILL-SLIT LE FT 1ST PERMANENT GILL-SLIT B. LATER LARVA. MEDIAN VENTRAL BLOODVESSEL Fig. 233. — A, yottng Amphioxus larva viewed from the left side as a translucent object. (Redrawn after Hatschek.) 5, later larva. (Redrawn after van Wijhe.) The mouth of Amphioxus becomes enormously enlarged and, by its growth backward on the left side, interferes with the symmetry of development of the gill-slits. The gill-slits of the left side of the body develop before those of the right side. The median line of the ventral side of the pharynx is indicated by the median ventral blood-vessel. Modifi- cation of function and degeneration affect the anterior three pairs of gill-slits. The first pair become the endostyle. The second pair form the transient larval club- shaped gland. The left third slit has no mate and soon disappears. The fourth pair of slits form the first permanent pair. THE RESPIRATORY SYSTEM 2!; I Amphioxus, the typical genus of cephalochordates, has as many as one hundred and eighty paired openings or stigmata. As in hemichor- dates, the original number is doubled by formation of secondary gill-slits. Before metamorphosis the number of primary gill-slits in the larva of Amphioxus is nineteen pairs. The large number of gill-slits in the proto- chordates is apparently an adaptation, since these organisms use their gills not only for respiration but also as a mechanism for obtaining food by ciliary action. The history of gills in vertebrates is one of continuous reduction in number and modification in function. The transformation of their skeletal supports has been described in the history of the skeletal system. Even in Amphioxus, some of the gill pouches of the embryo are modified or lost. (Fig. 233) The first pair become the endostyle while the second pair form the larval club-shaped gland. The third left slit never has a corresponding right slit and disappears early in ontogenesis. The first Fig. 234. — Diagram of relations of esophagus and respiratory tracts in (A) Myxine and Ammocoetes, and {B) Petromyzon; b, branchial duct ("bronchus"); oe, esophagus; t, thyroid gland. (From Kingsley's " Comparative Anatomy of Vertebrates.") permanent gill-slit of Amphioxus is therefore the fourth of the ontogenetic series. There are cogent reasons for homologizing this sHt with the spiracle of elasmobranchs, but there is httle agreement among morpholo- gists in regard to the exact homology of serial organs in chordates. The popular belief among morphologists that the vertebrate mouth has been formed by the coalescence of a pair of gill-sHts is supported by the mode of development of the mouth in Amphioxus. The endoderm takes the initiative in the development of the mouth of Amphioxus, as would be the case if it were a gill-slit. In this respect, the mouth of Amphioxus differs from that of vertebrates, in which the ectoderm initiates development. The question whether or not gills are metamenc structures nas been an open one. The metamerism of chordates is manifested primarily in the mesodermal somites. Since there are none of these in hemichordates and urochordates, it is impossible to demonstrate in these forms a cor- respondence between mesomerism and branchiomerism, and thus to estabHsh the metamerism of the latter. The case is different, however, in Amphioxus, where the mesodermal segmentation is one of the most striking features. In the adult animal, there is no correspondence 252 CHORDATE ANATOMY between gills and myotomes. But in the larva, the gill-slits not only take an intermetameric position in relation to the myotomes, but also are innervated by metameric nerves. A similar metameric correspondence is strikingly shown in the embryos of cyclostomes. The conclusion drawn is that mesomerism and branchiomerism correspond. The number of gills varies greatly in different cyclostomes. In the genus Bdellostoma, the number ranges from fourteen to six pairs. The number in Myxine and Petromyzon is respectively six and seven, or one more counting the spiracular pouch which does not become perforate. By the backward growth of the hyoid septum, the external apertures in Myxine becomes reduced to a single pair, a condition not unlike that in bony fishes. Among elasmobranchs, Heptanchus has seven pairs of gill-slits in addition to the spiracles, which are evidently modified gill-slits since they bear rudimentary hemibranchs. Hexanchus and PHotrema have six pairs of gill-slits. Most elasmobranchs have five pairs of gill-slits plus spiracles. In bony fishes the number is reduced to four pairs and the spiracle is absent. Gill-slits disappear in adult tailless amphibians, but are present in some aquatic urodeles. The number however is reduced. Some adult urodeles have three pairs of gill-slits, some two, and some only one. In the newts they disappear entirely. Cutaneous respiration is common in the group, and some respire by means of a highly vascular pharynx. Nevertheless, even in those adult forms which are devoid of functional gills, gill pouches occur in the embryo, and the embryos of Gymnophiona may have as many as six such pouches, suggesting a corresponding number of functional gills in their ancestors. Most amphibian larvae have functional gills. Functional gills are lacking in amniotes, but rudiments of gills are represented by transient embryonic pharyngeal pouches and their inter- mediate visceral arches. In the embryos of reptiles, some of the gill-slits usually become perforate and later close. The perforation of gill-slits in mammals is abnormal. Pharyngeal pouches are, however, always formed in the human embryo, and when these become perforate fistulae in the throat, they may persist and require surgical treatment. The presence of five pharyngeal pouches and six visceral arches alternating with them in the human embryo receives its most reasonable interpretation in the evolution theory. As has already been explained, the disappearance of the visceral arches in man and mammals is incomplete. The skeletal elements are converted into ear bones, attachment for the tongue, and support of the larynx. Three of the aortic arches also persist, as will be shown in the next chapter. Moreover, in addition to these rudiments there are certain derivatives of the gill pouches which require special discussion. THE RESPIRATORY SYSTEM 253 Pharyngeal Derivatives. From the epithelial lining of the embryonic pharyngeal pouches arise some of the important endocrinal glands, thyroid, parathyroid, thymus, and the ultimobranchial bodies. In addition to these which occur in man, some vertebrates have also epithelial bodies and suprapericardial bodies. From the second pair of pharyngeal pouches come the palatine tonsils. The history and development of these m. HYPOPHYSIS -MOUTH -,POUCH 1 . iTeustachian tube) 1 -thyroid gland -palatine tonsil -POUCH 2 PARATHYROID 1 ^ -POUCH 3 _j%-THYMUS 1 gp-PARATHYROID 2 ^"THYMUS 2 POSTBRANCHIAL BODY POUCH 4: POUCH 5--^ TRACHEA" ^.-LUNG LOBE ESOPHAGUS Fig. 235. — Ventral view of pharyngeal region of a human embryo showing the pharyn- geal pouches and their glandular derivatives; semidiagrammatic. pharyngeal derivatives will be taken up in the chapter on endocrinal organs. B. The Pulmonary System. The respiratory system of man and mammals includes lungs, larynx, trachea, bronchial tubes, nasal passages, and diaphragm. Lungs. Lungs are the essential respiratory organs of land vertebrates. Man, like virtually all land animals except snakes, has two, the left having two lobes and the right three. The lungs lie within the rib basket, and when expanded obliterate the potential pleural cavities. They are separated from one another by the mediastinum or interpleural space, 254 CHORDATE ANATOMY which contains the heart, esophagus, and the great blood-vessels which leave the heart. In childhood, the color of the lungs is pinkish, but may become slaty grey in the adult as the result of the accumulation of soot. The structure of the lungs is admirably adapted to the need of exposing to the air a large amount of surface, estimated to equal that of a balloon S-ALVEOLAR SAC Fig. 236. — Diagram of a lung lobule showing the subdivision of a bronchiolus into alveolar ducts, sacs and alveoli. Respiratory epithelium may extend into the bronchioli. (Redrawn after Bremer.) ten feet in diameter, and a section of the lungs shows that the volume of air space greatly exceeds that of solid tissue. The required moisture is supplied by mucous glands. The trachea or wind-pipe subdivides into bronchi, both structures having cartilaginous supports. The bronchi divide into bronchioli, the THE RESPIRATORY SYSTEM 255 bronchioli into alveolar ducts, the alveolar ducts into atria, alveolar sacs, and alveoli which form the ultimate subdivisions (Fig. 236). Exchange of gases occurs chiefly in the alveoU, although the thin respira- tory epithelium is found also in the atria and alveolar sacs and may extend even into the bronchioli, which in general are lined with a simple cuboidal non-respiratory epithelium. There is an elaborate network of capillaries in the walls of the alveoli, so that only two extremely thin membranes separate the blood in the capillaries from the air in the alveoK. Lungs are very elastic, and their elasticity is increased by the smooth muscle fibers which extend into the connective tissue of the lungs as far as the alveolar sacs but not into the walls of the alveoli. The respiratory blood-vessels of the lung are branches of the pulmonary arteries and veins. The bronchial artery and vein supply the connective tissues of the lungs. The innervation of the lung is through branches of the vagus and of the sympathetic. On the outside of the lung the pleura, corresponding to the peritoneal lining of the abdominal cavity, consists of a subserous connective tissue which extends into the walls of the lobules of the lung, and an external epithelial serosa. The pulmonary pleura is reflected back on the inside of the chest as the parietal pleura. Larynx. The larynx or voice-box lies between the root of the tongue and the trachea, and opens into the pharynx by the glottis. Nine carti- lages support it, the unpaired epiglottic, thyroid, and cricoid cartilages, and the paired arytenoids, corniculate, and cuneiform cartilages. Small paired triticeous cartilages also sometimes are found. Numerous muscles are attached, some extrinsic and some intrinsic. The extrinsic muscles are chiefly to lift the larynx in swallowing. Among the intrinsic muscles are the thyro-arytenoid or vocalis and the cricothyroid, which affect the pitch of the voice. At puberty in the male, the larynx becomes enlarged and the vocal cords within it elongated so that the voice is deepened. The epiglottis and vocal cords are covered with the same kind of squamous stratified epitheUum as that which lines the pharynx, but the rest of the larynx is lined with ciliated columnar epithelium similar to that of the trachea. The action of the cilia is such as to carry the secretions of the mucous glands of the lungs, together with particles of dust, out into the pharynx. Mucous glands are numerous. The nerve supply is from the vagus and the sympathetic. Trachea and Bronchi. The human trachea or wind-pipe is a membra- nous tube, four to five inches long, supported by fibrous connective tissue and incomplete U-shaped rings of cartilage. It carries air to and from the lungs. The cartilages vary in number from sixteen to twenty and are incomplete on the side next to the esophagus. The trachea divides to form the right and left bronchi. The lining of the trachea is a mucous 2^6 CHORDATE ANATOMY PARIETAL BONE' GYRUS CINGULI PITUITARY, SUBDURAL CAVITY- FRONTAL LOBE FRONTAL BONE- yRIGHT CEREBRAL HEMISPHERE iRPUS CALU3SUM NASAL BONE SPHENOID BONE NASAL CONCHAE EUSTACHIAN TUBE MAXILLA MOUTH CAVITY- PALATINE BONE VESTIBULE SOFT PALATE M. GENIOGLOSSUS MANDIBLE M. GENIOHYOID M. MYLOHYOID •FORNIX PINEAL GLAND CORPORA QUADRIGEMINA dura mater -occipital bone :erebellum ESOPHAGUS' '\:ENTRA Fig. 237.- — A median longitudinal section of the human head showing the relations between digestive and respiratory passages in the pharyngeal region. (Redrawn after Braus.) MCXTTH INVAGINATION ILL POUCHES LUNG SAC anlage of lung PHARYNX pronephros Fig. 238. — Stages in the development of lungs in vertebrates. /I is a hori- zontal section of a salamander embryo showing the series of paired pouches which form the gill-slits; after Goette. The last pair of pharyngeal pouches are the anlagen of the lungs. Such evidence suggests that lungs may have arisen in phylogenesis from a pair of gill pouches which failed to reach the surface. B and C are earlier and later stages in the development of the lungs in an amphibian. D is a cross section of the lung anlage in a reptile; after Wiedersheim. (Redrawn from Ihle.) THE RESPIRATORY SYSTEM 257 ciliated stratified columnar epithelium. Below this is a submucous connective tissue containing many mucous glands derived from the mucous layer. Between the cartilage and the mucosa is a layer of circular muscle fibers. Nasal Passages. Air is taken in and expired through the nasal passages. The external orifices are the external nares and the openings into the pharynx are the choanae. The paired nasal passages are sepa- rated from one another by the nasal septum and the median plates of the maxillary and vomer bones, and from the cavity of the mouth by maxillary and palatine bones. They are hned with a ciliated columnar epithelium containing many mucus-secreting goblet cells. Diaphragm. Air is drawn into the lungs under atmospheric pressure as the result of the contraction of the muscles of the diaphragm and ribs. Their contraction raises the rib-basket and flattens the dome-shaped dia- phragm. As a result, the size of the pleuroperitoneal cavity is increased. To fill the enlarged space thus formed, air enters the lungs and inflates them to the size of the chest cavity. The diaphragm is a -muscular partition which divides the cavity of the chest from that of the abdomen and which occurs only in man and other mammals. Lacking a diaphragm, the amphibians must swallow their air. The phrenic nerve, a branch of the cervical plexus of nerves, innervates the diaphragm. Development of the Lungs. During the fourth week of the human embryo a laryngo-tracheal groove is formed in the floor of the pharynx immediately behind the fourth gill pouch. Externally this groove appears as a ridge which is bordered on either side by a groove or furrow. By the approximation of these paired lateral grooves and their union in the median plane, the lung anlage is separated from the pharynx, except anteriorly where connexion with the pharynx is retained. The posterior blind end of the diverticulum swells to form the lung anlage while the less expanded anterior portion becomes the larynx and trachea (Fig. 235). The lung anlage later divides into two lateral buds which, by successive subdivision, gradually assume the adult structure. (Fig. 239) The cartilages which support the larynx correspond exactly with those which in aquatic vertebrates support the fourth and fifth branchial arches. The muscles of these arches form the laryngeal muscles. Vocal cords appear during the eleventh week. Beginning with the fifth week, the paired lung-buds branch in the manner of a compound tubular gland. In this way, the entire lining of the lungs is derived from the pharyngeal endoderm. The connective tissue develops from the surrounding mesenchyma. The splanchnic mesoderm forms the serosa, which covers the lungs and lines the chest cavity. As the two lungs enlarge, they push laterally into the body-cavity and by their ventral extension nearly surround the heart, from which they 258 CHORDATE ANATOMY remain separated by the pericardium. With the development of the diaphragm, the pleural cavity containing the lungs becomes separated from the more posterior peritoneal cavity. According to Broman the diaphragm arises from four sources, the septum transversum anterior to the liver, the pleuroperitoneal membranes, the body-wall, and the dorsal mesentery. The nasal passages of lower vertebrates, such as the Dipnoi and Amphibia, develop from nasobuccal grooves similar to those seen in some adult elasmobranchs. In the embryo an ectodermal groove extends from each olfactory pit to the corner of the mouth. Later the groove deepens, Fig. 239.^ — Stages in the development of the trachea, bronchi and lungs in the pig. The pulmonary arteries are shown in black; the veins are cross hatched. Ep, bud of eparterial bronchus. (From Patten's "Embryology of the Pig," after Flint.) its edges meet and fuse together and convert the groove into a tubular passage which connects the pit with the mouth cavity (Fig. 352). The development of the nasal passage in the human embryo is slightly different. In the month-old embryo a similar nasobuccal groove makes its appearance. The nasal passage, however, is not formed by the closure of this groove, but by the backward extension of the epithehum of the olfactory pit, which thus acquires a secondary connexion with the mouth (Fig. 353). History of the Pulmonary System. Invertebrates have no organs comparable with the human pulmonary system, the so-called lungs of pulmonate molluscs being modifications of the mantle and not out- growths from the alimentary canal. Opinions are divided as to the origin of the lungs. According to some, a pair of gill pouches which failed to reach the skin have been converted into lungs. Others suppose that lungs have evolved from the air bladder of fishes. Some seek to reconcile these THE RESPIRATORY SYSTEM 259 two divergent opinions by asserting that the air bladder is itself derived from a pair of modified gill pouches. Goette (1875) was the first to suggest that lungs are modified gill pouches, on the ground that in some amphibian embryos the lungs develop from a pair of posterior endodermal pouches in series with the gill pouches. (Fig. 238) A number of observers have confirmed this observation and reached the same conclusion. In support of Goette's hypothesis is the fact that the pulmonary arteries develop from the sixth pair of aortic arches. Furthermore, it is obvious that, if a gill pouch were to fail to reach the skin and were to grow backwards into the body-cavity, it would assume the relations of a lung. On the other hand, supporters of the air-bladder hypothesis emphasize the fact that the air bladder of such a fish as the Nile bichir (Polypterus) develops, like the lung, as a median ventral outgrowth of the pharynx. Its bilobed adult form is secondary, as is also its vascular connexion with the sixth aortic arch. Basing the homology of air bladder and lung upon their similar development as median ventral outgrowths from the pharynx, the supporters of this view are skeptical of the attempt to com- pare a median organ with paired structures such as gill pouches. To meet this difficulty, it may be pointed out that the transformation of a paired organ into a median one is not unknown. For example, the thyroid gland in all vertebrates develops as a median ventral outpocketing of the pharynx, yet all morphologists agree in homologizing the thyroid with the endostyle of Amphioxus. The endostyle, however, in Amphioxus develops from a pair of gill pouches. It may be doubted whether we have any adequate explanation of the substitution of lungs for gills as respiratory organs. The fact that lungs are much better adapted to the needs of land animals than gills, which tend to dry in air, does not explain their origin. It is to be noted, however, that in this change of life animals have "played safe." Even before they Fig. 240. — Diagrams of air bladder in fishes. A, Physostomous fishes; B, Lepidosteus and Amia; C, Erythrinus; D, Ceratodus. The air bladder of the crossopterygian fish, Polypterus, is, like the lungs of amphibians, bilobed and connected with the floor of the pharynx. (From Kingsley's "Comparative Anat- omy of Vertebrates," after Dean.) 26o CHORDATE ANATOMY abandoned the water for a land life, they had acquired an organ, the air bladder, which would serve as a substitute for gills. While some uncertainty remains in regard to the origin of the lungs, the facts on the whole seem to accord with the gill-pouch hypothesis. If it is assumed that the crossopterygian air bladder is a pair of modified gill pouches, the rest of the problem of the history of the lungs is easily solved, since there are among living vertebrates all intergradations in Fig. 241. — Diagrams of stages in the phylogenesis of the lungs. The respiratory surfaces are stippled, and conductory passages cross-hatched. Embryological stages corresponding with the comparative anatomical series shown in A-E occur in the ontogenesis of lungs in mammals. See Fig. 239. (Redrawn after Huntingrton.) complexity between the simple air bladder of Polypterus and the mam- malian lung. (Fig. 240) The evolutionary changes which occur involve chiefly a great increase in the lung surface effected through the branching and subdivision of the primary lobes. The facts of embryology and com- parative anatomy are in complete agreement. An evolutionary series based upon evidence from comparative anatomy is shown in Fig. 241. In birds a special modification of the lungs occurs. (Fig. 242) Air sacs grow from the bronchi into the abdominal cavity, the thorax, the neck, and even into some bones. Since they have very few blood-vessels in THE RESPIRATORY SYSTEM 261 their walls, they function only slightly in gaseous exchange. They serve as a reservoir of non- vitiated air which is used when the vitiated air is forced Fig. 242. — Air sacs and canals of pigeon, c'-c^, intertransverse canals; da^-da"^, axillary sac and its ventral diverticulum; dc, canal for ribs; dot, infraclavicular canal; ds, subscapular sac; dst, sternal canal; pc, preacetabular canal; sad, sas, right and left abdominal sacs; sc, cervical sac; sia, sip, anterior and posterior intermediate sacs. (From Kingsley's "Comparative Anatomy of Vertebrates," after Bruno Miiller.) from the lungs. Thus there is a double tide of fresh air through the lungs. Wetmore suggests that they assist in the regulation of body tem- perature, which is higher in birds than in mammals. CHAPTER lo THE VASCULAR SYSTEM The Vascular System. While the blood of many invertebrates fills intercellular spaces without specialized walls, the circulation in verte- brates is a closed system, the essential components of which are a circulat- ing fluid, a heart with receiving and propulsive chambers and valves so arranged as to permit the blood to flow in one direction only, arteries to carry blood away from the heart, veins to bring blood back to the heart, and microscopic capillaries connecting arteries and veins. The walls of the capillaries are so thin that they permit passage of plasma from the blood into the tissues. This fluid, in the form of lymph, is restored to the DORSAL AORTA PRECARDINAL V I POSTCARDINAL V CAUDAL ARTERY 'CAUDAL VEIN VENTRAL AORTA 1 L.COMMON CARDINAL V. AORTIC ARCH 6 Pig. 243. — Diagram of the primitive chordate circulation. The arteries are shown in black, the veins are stippled. The similarity of this circulatory system to that of annelids has suggested to morphologists a common genetic origin. (After Kingsley, modified.) veins by way of special vessels, the lymphatics which, Hke veins, permit flow in only one direction. It was William Harvey (1616) who first demonstrated the circulation of the blood. Before Harvey's day it had been assumed that the blood ebbs and flows in the arteries and veins hke water in tidal streams. Har- vey was able to demonstrate that the valves in the heart and blood-vessels permit a one-way movement only; that a cut artery spurts blood from the cut end nearer the heart, while a cut vein bleeds most from the end farther from the heart; that pressure of a finger on a vein results in distension on the side farther from the heart; and that in a dead body Uquid injected into an artery may pass to the heart by a vein, while the liquid injected into a vein will not enter the heart from an artery. Later, following the inven- tion of the microscope, Malpighi (1661) discovered the interconnexion of arteries and veins by way of the capiUaries. 262 THE VASCULAR SYSTEM 263 Blood. The human body contains about one gallon of blood, one twen- tieth of the weight of the body. Blood has two constituents, a fluid plasma and blood corpuscles which are cells. Blood, therefore, may be regarded as a tissue composed of cells (corpuscles) and a Hquid intercellular mate- rial. The plasma consists of a liquid serum and a coagulable material, fibrinogen. By stirring blood it is possible to separate these two elements. Blood corpuscles are of two sorts, red erythrocytes and white leucocjrtes. To every cubic milHmeter of blood there are from four and one-half to five million erythrocytes, and from five to seven thousand white corpuscles. Variations from this proportion are of diagnostic value in disease. Functions of the Blood. The blood-vessels and blood constitute an organic transportation system. Blood is a common carrier of foods and wastes to and from all parts of the body. Among its numerous functions are equaUzing the temperature of the body, regulating the water content Pig, 244. — Diagram of the circulation in an early stage of a small-yolked vertebrate (amphibian), a, anus; ca, cv, caudal artery and vein; da, dorsal aorta; dc, Cuvierian duct; ec, external carotid; h, heart; ha, hypogastric artery; i, intestine; ic, internal carotid; ij, inferior jugular; j, superior jugular; I, liver; m, mouth; oma, omv, omphalo- mesenteric artery and vein; pc, postcardinal vein; si, subintestinal vein; 1-6, aortic arches. (From Kingsley's "Comparative Anatomy of Vertebrates.") of the various tissues, distributing hormones and thus assisting in the integration of the body. The red corpuscles carry oxygen, the white act as scavengers of the blood. The color of red corpuscles is due to hemo- globin, a nitrogenous substance with an afhnity for oxygen. Most, if not all, white corpuscles are amoeboid, and, like amoebae, surround and engulf bacteria. Foods and wastes are carried in the plasma and not by the corpuscles. The plasma contains also mineral salts of the same sorts and in nearly the same proportions as those of somewhat dilute sea water. Among other constituents of the blood are enzymes, antibodies, antitoxins, antithrombin, etc. Antibodies are substances produced in the tissues in response to the poisons caused by bacteria. They are regulatory in their action and help to preserve the normal chemical balance of the blood. Antithrombin is a substance, normally present in blood, which prevents the clotting of the blood by preventing the action of thrombin on fibrinogen. Evolution of the Blood-Vessels. Two types of circulation may be dis- tinguished in animals, intracellular and intercellular. The former occurs in all cells alike as foods enter the cell and are distributed to the various cell organs and as the cell wastes are excreted. Intracellular circulation is 264 CHORDATE ANATOMY similar in Protozoa and Metazoa. A true vascular system, however, is multicellular and therefore limited to the Metazoa. The simpler metazoans, such as the sponges and coelenterates, are devoid of a vascular system. In an animal such as Hydra, in which the body-wall forms the alimentary canal, a vascular system is unnecessary. With only two layers of cells in the body-wall, the diffusion of food stuffs from the digestive cavity into the cells may take place by osmosis. The excretion of wastes is likewise direct and requires no special system of transportation. A circulatory system is necessary and is present in all animals in which the body-wall is separated from the Hning of the aUmentary canal either by a mass of mesoglea or by a body-cavity. In other words, the emer- gence of a circulatory system in animals is conditioned by increase in size of body and in mass of tissue, as well as by the separation of the body- wall from the alimentary canal by a coelom. Stages in the evolution of blood- vessels are represented in hving invertebrates. Metazoa have two kinds of vascular systems, an open lacunar system such as occurs in most invertebrates and a closed system Hke that of vertebrates. The facts support the assumption that the lacunar system is the more primitive. A lacunar system is well represented in flatworms, in which a fluid plasma fills the spaces between loose mesenchymatous cells. No heart is present and no true circulation occurs. The contraction of the muscles of the body-wall and the movements of the worm bring about more or less irregular currents in the plasma. In many flatworms, numerous diverticula of the intestine bring digested food near most parts of the body so that a vascular system is unnecessary. The beginnings of blood-vessels, however, make their appearance in nemerteans which are sometimes classified with flatworms. Nemerteans have, in addition to lacunar spaces in the mesenchyma, three longitudinal blood-vessels, two lateral and one dorsal. Interconnexions between these vessels occur at the anterior end of the worm. The fluid contained in these vessels is a sort of lymph, without blood corpuscles and without hemoglobin. It may be assumed that the walls of these vessels are formed directly from the surrounding mesenchyma and that the vessels therefore are evolved from lacunar spaces. In the nematodes, a pseudocoelom provides an adequate mechanism of circulation in these animals which have no thick masses of tissue to nourish. Most of the invertebrate phyla above the nematodes have composite circulatory systems, partly lacunar and partly closed. Before Malpighi (1661) discovered the capillary circulation in vertebrates and thereby demonstrated in them a closed circulation, it was assumed that the verte- brate circulatory system was likewise partly lacunar and partly closed. That the lacunar system of invertebrates is comparable with the lymphatic THE VASCULAR SYSTEM 265 system of vertebrates has been more recently suggested. Such a sug- gestion, however, is obviously based upon erroneous assumptions. The facts accord better with the assumption that both blood-vascular and lymphatic systems have had a common origin from the primitive lacunar systems of invertebrates. While in the invertebrates the circulatory system has remained partly open, in the vertebrates on the other hand the circulatory system, both blood-vascular and lymphatic, has become wholly closed. The vascular system of annelids is fundamentally like that of chordates. In both groups occur two main longitudinal vascular trunks, one above and one below the aHmentary canal, connected with one another by aortic arches around the pharynx. In the earthworm there are five pairs of aortic arches. It is true that the direction of flow of blood in annelids is the reverse of that in vertebrates. But this difference in blood flow is nullified if the dorsal and ventral sides of the worm are reversed. In spite of the necessity of inverting the worm, proponents of the annelid hypothesis of vertebrate ancestry have stressed the similarity of the vascular systems of the groups as strong support of their views. The absence of a heart in annelids and its presence in vertebrates is not a serious objection to this view, since the dorsal blood-vessel of annelids is contractile through- out its length and it may be reasonably assumed that this contractile function is concentrated and localized in the vertebrate heart. Such a diagram of the hypothetical primitive vertebrate blood- vascular system as is shown in Fig. 243 is based, however, not on the assumption of an anneUd ancestry of chordates but upon evidence from comparative anatomy and embryology. While the circulation of blood in the ancestral chordate was probably due, as in annelids, to the contractility of the walls of the blood-vessels, a contractile heart such as is found in all vertebrates is added to the diagram. Blood is pumped by the heart towards the head through a median ventral truncus arteriosus from which pass the series of aortic arches, which connect around the pharynx with the median dorsal aorta. In fishes and in Amphioxus, the aortic arches are divided, by a network of capillaries in the gills, into ventral afferent and dorsal efferent arteries. The carotid arteries carry blood forward to the head while the dorsal aorta carries it posteriorly to the trunk and tail, giving off metameric intersegmental arteries to the body-wall, and median unpaired splanchnic vessels to the alimentary canal. In the tail, venous blood is carried towards the heart by the caudal vein. In the primitive circulation, the caudal vein is assumed to be connected by intersegmental vessels with the caudal artery. When the caudal vein reaches the region of the anus it encircles the alimentary canal. From this point blood may return to the heart either by a subintestinal vein (which also collects blood from the intestine) or by an abdominal vein which extends along the median ventral 266 CHORDATE ANATOMY body-wall. These two vessels parallel one another and both connect anteriorly with the heart. Venous blood from the body-wall is returned to the heart by the cardinal veins, anterior and posterior. > !ti Amphioxus. In most details the vascular system of Amphioxus resembles the hypothetical ancestral system just described. But Amphi- oxus is heartless, and the circulation of its blood is dependent upon the contractility of the walls of its main blood trunks. Blood is carried for- ward beneath the pharynx by a contractile ventral blood-vessel, the THE VASCULAR SYSTEM 267 truncus arteriosus, and distributed to the numerous gills by a series of paired afferent vessels, the aortic arches. Contractile enlargements or bulbils of these vessels aid in the propulsion of blood to the gills. Neph- ridia similar to those of annelids are associated with the gills and pre- sumably assist in the eUmination of nitrogenous wastes. From the efferent vessels the blood passes to the paired dorsal aortae, the anterior extensions of which correspond to the internal carotid arteries of verte- brates. Posterior to the gills the paired aortae unite to form the median 268 CHORDATE ANATOMY dorsal aorta of the trunk region. From the dorsal aorta paired inter- segmental vessels are given off to the body-wall and a series of median unpaired vessels to the aUmentary canal. In the tail region the caudal artery has intersegmental connexions with the caudal vein. Beginning at the anal region blood from the caudal vein may be returned to the heart either through the right postcardinal vein or by the subintestinal vein. Like the hepatic portal vein of vertebrates, the subintestinal vein of Amphi- oxus breaks up in capillaries within the liver. Anteriorly the hepatic capillaries unite to form the hepatic vein which carries blood to the sinus venosus and the truncus arteriosus. Amphioxus has no renal portal system such as occurs in fishes and amphibians. Venous blood is brought SINUS VENOSUS SEMILUNAR* VALVES ""■ — Fig. 246. — A diagram of the primitive (fish) heart, as seen in a median longitudinal section. Anterior is to the right. The course of blood in the heart — indicated by arrows — takes the form of a letter S. (Redrawn after Keith.) from the anterior part of the body by the precardinal veins and from the posterior body- wall by the postcardinal veins. These veins unite with the sinus venosus by means of paired common cardinal veins or ductus Cuvieri. The vascular system of Amphioxus thus resembles that of cyclos- tomes and elasmobranchs. The blood, however, lacks hemoglobin and is colorless. Blood cells are scarce. (Fig. 245, A) Cyclostomes. The vascular system of cyclostomes shows little advance above that of Amphioxus. An S-shaped heart with three cham- bers, sinus venosus, atrium, and ventricle, is a novelty in this group. A conus with valves makes its appearance at the root of the truncus arterio- sus. The common cardinals drain into a thin-walled sinus venosus. In the adult the left common cardinal degenerates and both precardinals connect with the right common cardinal or precava as in some mammals. THE VASCULAR SYSTEM 269 The two posterior cardinals unite into a common cardinal which passes to the left side of the body and unites with the sinus venosus. In the heart atrio-ventricular valves prevent a reverse flow of blood. With the development of a septum transversum the coelom becomes divided into an anterior pericardial cavity and a posterior abdominal cavity. Red blood corpuscles make their appearance in this phylum and the blood is consequently red. A renal portal system is wanting in cyclostomes, the caudal vein draining directly into the postcardinal veins. Elasmobranchs. The blood-vascular system of elasmobranchs differs little from that of cyclostomes and with slight changes may easily be derived from the latter. Associated with the appearance of paired fins, the subclavian and ihac arteries and veins are present. The arteries are connected with the dorsal aorta. The subclavian vein is a branch of the Fig. 247. — Diagram of vertebrate circulation based on a urodele. Arteries cross- lined; veins black except the pulmonary vein, white, av, abdominal vein; c, celiac artery; ca, cv, caudal artery and vein; d, dorsal aorta; ec, external carotid; g, gonad; h, hepatic vein; ha, hepatic artery; hy, hypogastric artery; ic, internal carotid; il, iliac artery and vein; j, jugular; Iv, liver; m, mv, mesenteric artery and vein; pa, pulmonary artery; pcd, postcardinal; pcv, postcava; pv, hepatic portal vein; r, rectal artery; ra, renal advehent (portal) vein; sc, subclavian artery and vein. (From Kingsley's "Comparative Anatomy of Vertebrates.") precardinal, while each iliac vein drains into a lateral abdominal vein. The latter is believed to have been developed from the median ventral abdominal vein of the primitive chordates. Internal jugular veins paral- leHng the precardinal veins are a novelty in this group. The caudal vein of elasmobranchs divides anteriorly into the paired renal portal veins which break up into capillaries within the mesonephroi. The heart resembles that of cyclostomes. The branches of the dorsal aorta are subclavians, intersegmentals, celiac, anterior and posterior mesenteries, spermatics and ovarians, and renals. (Fig. 245, B) Dipnoi. In the Dipnoi, with the emergence of lungs, some advances towards the mammalian circulation are seen. In the heart of lung fishes both atrium and sinus venosus become partly divided by incomplete longitudinal septa. Impure blood from the veins enters the right atrium while aerated blood from the lungs flows into the left atrium. Atrio- ventricular valves are lacking but the conus contains a series of valves. 270 CHORD ATE ANATOMY Immediately in front of the conus the truncus divides into four pairs of aortic arches, the third to the sixth of the original series. In the Dipnoi pulmonary arteries make their first appearance in the vertebrate series as posterior branches of the last pair of aortic arches. As another novelty in fishes, the right postcardinal vein degenerates and a new vein, the postcava, drains most of the posterior part of the body. The caudal vein in this group bifurcates into the left postcardinal and the postcaval veins. The ihac veins, as they leave the pelvic fins, divide into pelvic and renal portal veins. The two pelvic veins unite to form a median abdominal vein. Venous blood from the fins may thus reach the heart either by way of a capillary network in the mesonephroi or by the abdominal vein. The efferent renal veins drain into the postcava and into the left post- cardinal veins. Thus in the appearance of an atrial septum, pulmonary arteries and veins, and a postcaval vein, the Dipnoi make notable advances towards the circulatory system of the higher vertebrates. The differences between the dipnoan and amphibian circulation are slight. Amphibia. In the amphibians the connexion of the sinus venosus is shifted to the right atrium while the pulmonary veins connect with the left atrium. The two atria are divided by a septum which is usually perforate in urodeles. There is, however, little mixing of impure and pure blood in the atria. In the undivided ventricle some mixing of the two kinds of blood does occur. A spiral septum in the truncus arteriosus shunts the venous blood from the right side of the ventricle chiefly into the pulmonary arteries while that which passes to the dorsal aorta and systemic arteries is mostly aerated blood. Of the six original aortic arches of the embryo, the last four persist in some adult amphibians while in others only the third and fourth arches persist. One of the most important changes in circulation which occur within the group is the abandonment by the higher amphibians of the capillary branchial network character- istic of fishes. In the perennibranch amphibians most of the blood in the aortic arches short-circuits the gills, and with the loss of gills in the Anura the aortic arches form direct connexions between ventral and dorsal aortae. In the Anura as in most amniotes that portion of the dorsal aortae between the carotid (third) and systemic (fourth) arches degener- ates. In urodeles as in Dipnoi the pulmonary arteries form posterior branches of the sixth aortic arch while in the Anura the connexion by a ductus arteriosus with the dorsal aortae is lost as in mammals. Several splanchnic arteries convey blood from the dorsal aorta to the intestine. In Anura, however, they are reduced to three, celiac, anterior, and posterior mesenteric arteries. In urodeles three veins drain the mesonephroi, the right and left postcardinal veins and the postcava. The connexion of the iliac veins with the renal portal or advehent veins, which made its appear- ance in Dipnoi, is also present in amphibians. Impure blood from the THE VASCULAR SYSTEM 27 1 hindlegs may thus return to the heart either through the mesonephroi, or by the abdominal vein. The median ventral abdominal vein, which made its first appearance in Dipnoi, takes the place of the lateral abdominal veins of elasmobranchs. The increased flexure of the heart brings the atria anterior as well as dorsal to the ventricle. (Fig. 246) Reptiles. The reptilian vascular system strikingly resembles that of amphibians. The main arteries and veins are homologous in the two groups. The chief differences appear in the heart and truncus arteriosus. The ventricle is partly divided by a septum in lower reptiles and more or less completely divided in the crocodiles and alligators. Conse- quently pure and impure blood are separated in the two sides of the heart as in mammals. A peculiarity of the reptiUan circulation, however, is manifested in the triple splitting of the truncus arteriosus. Three arteries, instead of the two characteristic of mammals, leave the heart. One of these is the pulmonary artery carrying venous blood from the right ven- tricle to the lungs. The remaining two vessels are the systemic arteries, one of which comes from the right, and the other from the left, ventricle. Soon after leaving the heart each artery crosses to the opposite side of the body. Thus the right aortic arch comes from the left ventricle and con- veys pure blood to the dorsal aorta and the head. The left aortic arch comes from the right ventricle and carries mixed blood into the dorsal aorta. Consequently, the dorsal aorta of reptiles contains mixed, and not pure, blood. Since the celiac and mesenteric arteries are given off from the left aortic arch which carries mixed blood, they carry mixed blood to the stomach and intestine. In some reptiles a foramen Panizzae con- nects the blood streams in the two ventral aortae so that some mixing of the blood in the two vessels may take place. In the lower reptiles systemic and carotid arches are connected with one another, as in urodeles, by the dorsal aortae. In the crocodiles this connexion is lost, as in mammals. The connexion between the postcava and the postcardinals is lost in reptiles and the blood from the kidneys returns to the heart by the postcava as in mammals. Both right and left common cardinals (ductus Cuvieri) persist and bring blood from the head and anterior limbs into the sinus venosus. Thence it passes to the right atrium. Blood from the hind legs, as in amphibians, may return to the heart either by the renal portal veins or by the abdominal vein. Mammals. The complete division of the heart into a right venous half and a left arterial half, which was attained by reptiles, is retained by mammals. In mammals, however, the sinus venosus merges into the right atrium. In this region is located the sinu-auricular node, a bundle of muscular and connective tissue richly supplied with nerve fibers, which is said to be the "pace-maker" of the heart-beat. Mammals have a single ventral aorta. Of the paired systemic arches of amphibians and 272 CHORDATE ANATOMY THE VASCULAR SYSTEM 273 < 'O >. 0 06S " ^ a >>^ IN a t-i ■ o'^it 274 CHORDATE ANATOMY reptiles only the left one persists. The renal portal system has disappeared and with it the abdominal veins. The latter, however, form the transient umbilical veins of the fetal circulation. The right and left iliac veins estabUsh connexions with the postcava by way of the posterior cardinals and their transverse anastomosis in the lumbar region. The postcava of mammals appears to be only in part homologous with that of lower vertebrates. Four distinct embryonic vessels unite to form the mammahan postcava. These are the anterior hepatic portion, the subcardinal anastomosis, the supracardinal veins (in part), and the pos- terior portion of the right postcardinal vein. The supracardinal veins seem to be mammalian novelties, arising in the embryo dorsal to the post- and subcardinal veins. There is difference of opinion in regard to the first ysVZ' Fig. 249. — DiflFerent stages in the differentiation of the parts of the heart, ventral view. A, elasmobranch ; B. teleosts; C, amphibia; D, lower reptiles; E, alligator; F. birds and mammals, a, atritim; ao. aorta; b, bulbus arteriosus; c, conus; cd, Cuvierian duct; h, hepatic veins; pa, pulmonary artery; pc, pre- and postcaval veins; pv, pulmonary vein; s, sinus venosus; 5a, septum atriorum; v, ventricles. (From Kingsley's "Com- parative Anatomy of Vertebrates.") appearance of subcardinal veins, whether they are new in mammals or present in vertebrates from amphibians to man. According to McClure subcardinal veins are present in all vertebrates. The origin of the azygos and hemiazygos veins is also in doubt. Most textbooks describe these vessels as persistent remnants of the postcardinal veins together with their transverse anastomosis. The researches of Huntington and McClure, however, indicate that the azygos and hemi- azygos veins are hmited to mammals and that they are, in chief part, per- sistent portions of the supracardinal veins which appear to be mammalian novelties. The renal veins of mammals are not the homologs of the renal veins of lower vertebrates but are new vessels formed from the inter- subcardinal anastomosis. In the lower mammals both common cardinals occur as in lower vertebrates. In the higher mammals and man, however, a transverse THE VASCULAR SYSTEM 275 anastomosis between the precardinals or jugulars is converted into the left innominate vein which brings venous blood from the left arm and left side of the head across to the right jugular vein. The left common cardinal consequently degenerates but persists in part as the coronary vein. Evolution of the Heart. The chief changes which the heart has under- gone in phylogenesis may be briefly summarized as follows. The verte- brate heart is a differentiated portion of a median ventral blood-vessel. The contractile function which originally extended throughout the length of this vessel became locaUzed and concentrated in the subpharyngeal region. Primarily the heart had neither valves nor chambers but con- sisted of a two-layered tube with a muscular wall and an endothelial lining. The first subdivision of the heart was into a receiving chamber or atrium and an anterior propulsive division, the ventricle. Later were added a posterior sinus venosus and an anterior conus. Atrioventricular and semilunar valves in turn made their appearance, thus ensuring a one-way flow of blood. With the elongation of the heart in confined space, a sigmoid flexure was formed and the atrium consequently came to lie dorsal to the ventricle. Fishes added a muscular bulbus anterior to the conus. In the Dipnoi and Amphibia the connexion of the sinus venosus was shifted to the right atrium while aerated blood from the lungs entered the left atrium. In the Dipnoi and Amphibia, while the atrium became divided by an incomplete septum into right and left atria, the ventricle remained undivided, so that some mixing of aerated and impure blood occurs. The increased flexure of the heart brings the atria in Amphibia anterior to the ventricle. In the crocodilian reptiles the complete division of the heart into arterial and venous halves is effected, but the beneficial effects of this separation are partly neutralized by the mixing of the two kinds of blood in the dorsal aorta. In mammals the sinus venosus becomes merged with the walls of the right auricle. Evolution of the Aortic Arches. The device of oxygenating blood in pharyngeal gills is peculiar to chordates. Nevertheless aortic arches connecting ventral and dorsal aortae in the pharyngeal region occur in annelids. It is a matter of opinion whether this point of resemblance between anneUds and chordates has a phylogenetic significance or is simply a case of convergence. In chordates the number of aortic arches is correlated with the number of visceral arches. Amphioxus has the largest number, nineteen pairs, of primary visceral arches among chor- dates. The aortic arches are correspondingly numerous. The largest number of aortic arches in vertebrates occur in some species of cyclostomes, fifteen pairs in Bdellostoma stouti. Although some elasmobranchs have more, six pairs are usually the maximum number of aortic arches in gnathostomes. Of these the first two pairs, belonging to the mandibular and hyoid arches, partly lose their respiratory function and 276 CHORDATE ANATOMY consequently the aortic arches are reduced. In fishes the persistent aortic arches are broken up into a capillary net-work in the gills. In urodeles the last four pairs of aortic arches persist, the blood from the ventral aorta largely short-circuiting the gills. In Anura gills are lost and blood passes directly through the aortic arches to the dorsal aorta. In the Anura the fifth pair of aortic arches degenerate. That portion, the ductus arteriosus, Fig. 250. — Modifications of the aortic arches in difTerent vertebrates. A, primi- tive scheme; B, dipnoan; C, urodele; D, frog; £, snake; F, lizard; G, bird; H, mam- mal, c, celiac artery; da, dorsal aorta; dh. ductus Botalli; ec, ic, external and internal carotids; p, pulmonary artery; s, subclavian; va, ventral aorta. Vessels carrying venous blood, black; those with mixed blood, shaded; those which disappear, dotted outlines. (From Kingsley's "Comparative Anatomy of Vertebrates," after Boas.) which connects the pulmonary branches of the sixth aortic arch with the dorsal aorta also atrophies. The third pair of arches persist as the roots of the carotid arteries. In reptiles as in Anura three pairs of aortic arches persist, the 3rd, 4th, and 6th in part as pulmonary. In reptiles the com- mon carotid arteries connect with the right and not with the left systemic or fourth aortic arch. In mammals portions of three aortic arches persist in the adult, the 3rd, 4th, and 6th as in reptiles. The systemic arch of the THE VASCULAR SYSTEM 277 right side, however, forms the right subclavian artery while that of the left side becomes the arch of the aorta. Since this is connected directly with the left ventricle it carries only aerated blood to the dorsal aorta. The aortic arches in man resemble those of other mammals. (Fig. 250) The Evolution of Arteries. Aside from the transformation of the aortic arches, the phylogenetic changes in the arterial portion of the vascu- lar system have not been profound. The main trunks persist throughout the series (Fig. 248). The appearance of the subclavians and iliacs is correlated with that of the paired fins, but once invented these vessels persist in man to supply the arms and legs. In the Dipnoi pulmonary arteries make their debut as branches of the sixth pair of aortic arches when the lungs which they supply emerge from air bladders. The connexion of the sixth aortic arch with lungs persists throughout the vertebrate series. With the substitution of a metanephros for the mesonephros of lower vertebrates new renal arteries are formed. The history of the caudal artery is one of degeneration until in man it becomes the rudimentary median sacral artery. The number of splanchnic arteries supplying the intestine in man remains the same — three — as in elasmobranchs. The Evolution of Veins. The phylogenetic alterations of the veins are much more radical than those of the arteries just described. Few veins persist throughout the entire chordate series. Included among such persistent veins are the jugulars, the precava (the right common cardinal or right ductus Cuvieri) and subintestinal, a part of the portal vein. The usual assumption that the postcardinals persist as the azygos and hemiazygos, except in part, is not supported by the evidence from embryology. The primary veins are paired, e.g., the precardinals, postcardinals, lateral abdominals or umbilicals, vitelUnes, and even the subintestinal vein which is paired at the time of its first appearance in vertebrates. The portal vein is one of the primitive veins of Amphioxus and the modi- fications of its development in connexion with the right vitelline vein appear to be recent adaptations. The postcava appears as a new vein in Dipnoi and undergoes considerable reconstruction in mammals through the addition of parts of the subcardinal anastomosis, right postcardinal, and supracardinal veins. The views of investigators in regard to the origin of the subcardinal veins are divergent. The evidence seems to support the opinion that the subcardinals make their appearance with that of the renal portal veins and that they persist throughout the verte- brate series and in part are incorporated in the postcava of mammals. In man, and to a lesser degree in other mammals, veins become differenti- ated into a superficial set which drain the skin and outer organs, and a deep set which carry blood away from the deeper organs of the body. 278 CHORDATE ANATOMY Evolution of the Lymphatic System. Little is known of the relations of chordate lymphatics to those of prechordates. The contrast between lymphatics and blood-vessels is less marked in invertebrates than in vertebrates. It seems not unreasonable to assume that primarily there was no distinction between blood-vessels and lymphatics and that the two systems have had a common origin. As in the case of most blood- vessels it is impossible at the present time to homologize particular lymphatic vessels in chordates and pre-chordates. In Amphioxus the lymphatics surround the blood-vessels and occur also in the metapleural folds, dorsal fin, and around the central nervous Fig. 251. — The lymphatics of the scrotum. Showing the transition of the capillaries to the vessels with valves (a, a, a). (From Morris, after Teichmann.) system. In vertebrates the distribution of lymphatics corresponds roughly with that of veins, although lymphatics are far more variable in position than are veins. Like the veins the lymphatics are divided into superficial and deep systems. The deep system develops in close relation to the cardinal veins and acquires connexions with them and with the superficial system. Lymphatic vessels occur in cyclostomes and fishes. ' They surround the veins in elasmobranchs, while their relations to the veins are less intimate in other fishes. One or two main trunks may parallel the dorsal aorta in this group and therefore are comparable to the paired thoracic ducts of mammals. Lymph sinuses surround the heart and nervous THE VASCULAR SYSTEM 79 system. Some fishes have lymph hearts serving to assist the circulation of the lymph. But fishes do not have lymph glands. A thoracic duct is present in .Amphibia. Larger subcutaneous sinuses occur in this group possibly as an adaptive arrangement which prevents dessication. Lymph hearts may occur in various parts of the body. Reptiles have large paired lymphatic trunks. Lymph hearts also are found in this group. Most mammals have paired thoracic ducts. In man the left duct persists throughout life in connexion with the left subclavian vein, while EXT. JUGULAR EXT JUGULAR_ A ^^B ' C D Fig. 252 . — Diagrams illustrating the chief 1 ymphatic trunks and their relations to the veins in mammals and man. A, South American monkeys; B, Hiammals (Lepus) in which postcaval-renal communications are wanting; C, mammals in general; D, man. In all mammals lymph enters the veins at the point of junction between the jugular and the subclavian veins. In most mammals there is also communication between the lymph vessels and the postcaval and renal veins. In man the right thoracic duct degenerates in part and the only communication with veins is at the root of the jugulars. the right duct is rudimentary, having a length of only a few centimeters. One lymphatic sinus, the cistema or receptaculum chyli, also persists in man. The lymph is returned to the veins at the point of least pressure, where subclavian and jugular veins meet. Lymph glands are numerous in man and mammals and generally occur in clusters in the axillary region and in the groin and neck. Lymph hearts are wanting in mammals and man. Lymphoid or adenoid tissue is found in all vertebrates. Lymph nodes however make their first appearance in reptiles. Development of the Heart. The heart makes its appearance as a two-layered tube ventral to the pharynx, so that the early embryo has its heart in its throat. Of the two layers, the inner becomes the endothelial lining of the heart, while the outer forms the epicardium and the muscular 28o CHORDATE ANATOMY myocardium. The right and left halves of the heart begin as longitudinal folds of the splanchnic mesoderm. Between these mesodermal folds and the adjacent endoderm, scattered mesenchyme cells appear, and soon become arranged as a thin-walled endothelial tube in each of the folds. The paired mesodermal folds with their enclosed endothelial tubes arise before the ventral wall of the pharynx is formed. The union of the two halves occurs in correlation with the formation of the floor of the pharynx from the endoderm. Successive stages in the process are shown in Fig. 253. c d" Fig. 253. — Sections cut transversely through the cardiac region of pig embryos of various ages to show the origin of the heart from paired primordia. A, 5-somite embryo; B, 7-somite embryo; C, lo-somite embryo; D, 13 somite embryo. (Projection diagrams X50, from series in the Carnegie Collection.) (From Patten's "Embryology of the Pig.") Soon after the tubular heart forms below the pharynx, its wall becomes three-layered by proliferation of cells from the outer or epimyocardial layer. In this way, a thick muscular layer is formed between the endo- thelial lining and the outer serosa. By the union of the visceral layer of mesoderm above and below the heart, dorsal and ventral mesocardia are formed in a manner resembling the formation of mesenteries in relation to the intestine. When the human embryo is a month old, the ventral mesocardial' membrane disappears and the right and left halves of the pericardial cavity are thus brought into direct connexion with one another. By the formation of septum transversum and diaphragm, the pericardial cavity becomes separated from the abdominal cavity. THE VASCULAR SYSTEM 281 Soon after the two halves of the heart are united in the mid-ventral line, the heart itself becomes S-shaped as a consequence of its elongation in a confined space. The dorsal curve is posterior and connects directly with the paired viteUine (omphalomesenteric) and umbilical veins. The ventral curve is anterior and extends forward beneath the pharynx as the truncus arteriosus. Circulation has already begun when the heart G H K 76 HOURS M somites 100 HOURS 45 somites Fig. 254. — Ventral views of the heart of chick embryos at successive stages to show- its changes of shape and its regional differentiation. Abbreviations: a. v., constriction between atrium and ventricle; i. v., interventricular groove. (From Patten's "Embryol- ogy of the Chick.") is in this tubular condition. By the time the human embryo is two months old, the heart, although its size is minute, has reached its adult form and structure. (Fig. 254) The processes involved in converting a tubular heart into a four- chambered one include: i. The increased flexion of the heart so that the posterior atrial portion becomes anterior, while the morphologically anterior ventricular portion hes posteriorly. 2. The formation of a 282 CHORDATE ANATOMY longitudinal septum which divides the heart into right and left chambers. 3. The relative hypertrophy of the two atria, that of the right side enlarg- ing more rapidly. 4. The separation of atria and ventricles by growth of atrio-ventricular valves. 5. The inclusion (in amniotes) of the posterior division of the heart, the sinus venosus, within the right atrium. 6. The division of the anterior portion of the heart, the conus, into aorta and cervical intersegmental branches of dorsal aortic root subclavian a (right) arterial circle (of Willis; hypophysis ophth. a ant. cerebral a. mid. cer. a. post. cer. a int. carotid artery subclavian a. (left) ^ thoracic intersegmental arteries subclavian a internal mammary Fig. 255. — Diagrams illustrating the changes which occur in the aortic arches of mammalian embryos. A, ground plan of complete set of aortic arches; B, early stage in modification of arches; C, derivatives of aortic arches. Abbreviations: br. ceph., brachiocephalic artery; cer. a., cerebral artery; lin., lingual artery; max., maxillary artery; ophth. a., ophthalmic artery; stap. a., stapedial artery; thy., thyroid artery. (From Patten's "Embryology of the Pig," adapted from several sources.) pulmonary artery. The changes thus briefly summarized are best understood from diagrams of the successive stages. (Figs. 249, 254) Development of the Aortic Arches. In early stages of human ontogene- sis, in 1.5 mm. to 2.00 mm. embryos, before the two halves of the heart are completely united in the mid-ventral line, connexion between ventral and dorsal aortae is established around the pharyn.x by means of a single aortic THE VASCULAR SYSTEM 283 arch, the first. In a 2.6 mm. embryo, a second aortic arch, the hyoid, is added. Others are added in succession until, in a 4. 2 embryo, there are five aortic arches. But the fifth or last of these is really the sixth arch, the true fifth aortic arch being a rudimentary vessel which appears only transiently in the 7.0 mm. embryo. By this stage, however, the first two aortic arches have degenerated. Consequently, while all six aortic arches arise, they are not present simultaneously in the human embryo. Only three of the six embryonic aortic arches are represented in the adult, the third, fourth, and part of sixth. The right and left third aortic arches become respectively the root of the right and left internal carotid arteries, the external carotids coming from the ventral aorta. The left fourth aortic arch becomes the arch of the aorta of the adult, while the right forms the right subclavian artery. The dorsal aorta between the LUNG ANLAGE PHARYNGEAL POUCHES MYELENCEPHALON METENCEPH ECTODERM SPINAL CORD ■STOMACH NCrroCHORD DORSAL AORTA 'LIVER YOLK STALK' ALLANTOIS' OPTIC VESICLE '' ^^5ai^5?\T-ELENCEPHALON CLOACA' Fig. 256. — A diagram of a 4.2 mm. human embryo showing five aortic arches. (Redrawn after W. His.) Why does the human embryo have six aortic arches of which only three persist in the adult, unless man's ancestors had six func<-.ional arches? third and fourth arches degenerates and disappears. Posterior branches of the sixth pair of aortic arches connect with the lungs and form the pulmonary arteries. Until birth, the remainder of the left sixth aortic arch persists as the ductus arteriosus connecting the pulmonary artery with the dorsal aorta. The vessel which connects the right pulmonary artery with the right dorsal aorta degenerates. The fifth pair of arches degenerate soon after their appearance. The Heart in Man. The human heart is a hollow muscular organ about the size of the closed fist, weighing from nine to eleven ounces. It is shaped like a truncated cone with its apex pointing downwards to the left. It is placed asymmetrically behind the sternum, with its apex just above the left fifth rib. A muscular partition extending from apex to base divides it into right and left cavities, each of which is subdivided into an anterior atrium or auricle and a posterior ventricle. Externally the division of the heart into atria and ventricles is indicated by a groove, the sulcus coronarius, and the two are separated internally by atrio- 284 CHORDATE ANATOMY ventricular valves. Two large veins, the precava and postcava, enter the right atrium, while four pulmonary veins return blood from the lungs to the left atrium. The pulmonary artery connects with the right, the aorta with the left ventricle. The walls of the ventricles are thicker than those of the atria, and the wall of the left ventricle is thicker than that of the right, for the right ^LEFT SUBCLAVIAN ART. ARCH OF AORTA ^PULMONARY ARTERY :z^, PULMONARY VEINS POST CAVA TRICUSPID' VALVE Fig. 257. — A diagram of the chambers of the mammalian heart and their associated vessels and valves. The walls of the ventricles are shown in black, those of the auricles are stippled. The direction of flow of blood is indicated by arrows. (Redrawn after Jammes.) ventricle pumps blood only to the lungs, while the left ventricle forces blood to all other parts of the body and is correspondingly more muscular. The heart wall has three layers, endocardium, myocardium, and epicardium. The endocardium consists of a thin layer of connective tissue and an endothelial layer continuous with that of the blood-vessels. The myocardium is the thick muscular layer. The fibers of cardiac muscle are striped, and are peculiar in having anastomosing connexions with one another. The epicardium is a thin layer of connective tissue THE VASCULAR SYSTEM 285 covered with the serous membrane which lines the pericardial cavity. A similar serous epithelium is reflected on the outer side of the pericardial cavity. The space between contains a small amount of fluid. The muscles of the myocardium are wound circularly around the heart and arranged in layers. The fibers of the outer layers run at approximately right angles to those of the inner layers, thus insuring a maximum contraction of the heart cavities during contraction or systole. The muscle of the atria is mostly independent of that of the ventricles and the two are separated by a connective-tissue septum. There is, however, an atrio -ventricular bundle of specialized muscle fibers which extends from the atrial septum to the ventricular septum and serves to convey to the ventricles the rhythm of contraction of the atria. The atrio-ventricular valves are attached by chordae tendineae to the walls of the ventricles in such a manner as to open freely into the ventricles but to prevent the return of blood when the ventricles contract. The attachment of the chordae tendineae to the heart wall is by means of special papillary muscles, anterior and posterior, in each ventricle. The right valve is partially divided into three "cusps" and the left into two. Hence they are known respectively as tricuspid and bicuspid or mitral valves. (Fig. 257) At the opening of the aorta and of the pulmonary artery crescentic semilimar valves prevent the return of blood into the ventricles. Each artery contains three of these valves so arranged that under pressure of the blood they meet together and occlude the lumen completely. Near the semilunar valves He the openings of the coronary arteries which supply blood to the wall of the heart. This blood is returned to the coronary sinus of the right atrium by coronary veins which parallel the coronary arteries. Course of Blood in the Heart. The blood from the systemic veins, the precava and the postcava, enters the right atrium and by the con- traction of the atrium is pumped through the tricuspid valve into the right ventricle. From the right ventricle the blood is carried by the pulmonary artery to the lungs, returning from the lungs by the four pulmonary veins to the left atrium. Forced by the contraction of the atrium through the bicuspid valve into the left ventricle, it is pumped into the aorta and to all parts of the body. Arteries. Arteries convey blood away from the heart and, because they are subjected to considerable pressure when the heart contracts, their walls are correspondingly thick and elastic. A cross section shows three layers, an intima, a relatively thin layer consisting of the fining endotheUum and a connective-tissue layer with elastic fibers; a media, a relatively thick layer of muscle and elastic fibers; and an externa, a layer of loose connective tissue consisting largely of white inelastic fibers. 286 CHORDATE ANATOMY SUPERFICIAL VOLAR ARCH DEEP VOLAR ARCH VOLAR INTEROSSEOUS ULNAR COM. INTEROSSEOUS RADIAL RECURRENT SUPERIOR ULNAR COLLATERAL DEEP BRACHIAL THORACODORSAL! S SUBSCAPULAR LATERAL THORAC IC INNOMINATE VENA CAVA RIGHT AURICLE RIGHT GASTRIC GASTRODUODENAL HEPATIC COELIAC RIGHT COLIC INTESTINALS INFERIOR MESENTERIC ILIOLUMBAR COMMON ILIAC MIDDLE SACRAL HYPOGASTRIC EXTERNAL ILIAC SUPERIOR GLUTEAL CIRCUMFLEX lUUM PROF INFERIOR VESICALIS HEMORRHOIDAL INTERNAL PUDENDAL RAM. FRONTAL TEMPORAL TRANSVERSE FACIAL OCCIPITAL INFERIOR ALVEOLAR EXTERNAL CARCTID LI NGUAL THYREOCERVICAL TRANSVERSE COLLI AXILLARY THORACOACROMIAL SUBCLAVIAN LEFT COMMON CAROTID - INTERNAL MAMMARY •y^i-^^r T- CIRCUMFLEX HUMERAL ■* w 4— ARCH OF AORTA BRACHIAL PULMONARY LEFT AURICLE DEEP BRACHIAL i— VENTRICLE SPLENIC SUPERIOR MESENTERIC INTERCOSTAL RENAL SPERMATIC COVARIAN) RADIAL RECURRENT ULNAR RECURRENT ULNAR DORSAL INTEROSSEOUS RADIAL WLAR INTEROSSEOUS V^DORSAL RETE METACARPAL ■PRINCEPS POLLICIS LATERAL CIRCUMFLEX RAM. MUSCULAR GENU SUPERIOR LAT. POPLITEAL RAM. PERFORATING PERONEAL TARSAL (lateral) ARCUATE plantar arch ARTICULAR RETE GENU SUPERIOR MED. ANTERIOR TIBIAL PERONEAL POSTERIOR TIBIAL ANT LATERAL MALLEOLAR LATERAL TARSAL ARCUATE METATARSAL DIGITAL (plantar) Fig. 258. — The chief arteries of the human body viewed fr(jm in front. THE VASCULAR SYSTEM 287 ^SUP. OPT HALM I C "^^ — SUP CEREBRAL MEDIAN ANTEBRACH, MED CUBITI BASILIC BRACHIAL- 'ERIOR CAVA -lAV;r.c., posterior centrosome. (From of efferent tubulCS, the fCte teStlS. Bremer's "Text Book of Histology," after Yvom the rete testis paSS the efferent M^GVCS ) ducts which convey the sperm from the testis into the ductus epididjnnidis. These relations are shown in Fig. 278. The seminiferous tubules are Uned with an irregular many-layered epithehum from which the spermatozoa are proliferated, the multiplica- tion of cells beginning in the basal layers. During this process a reduc- tional or maturation division takes place previous to the metamorphosis of the cells into mature spermatozoa. All transitional stages in the conversion of epithelial cells into spermatozoa may be seen in a cross section of the seminiferous tubules. The Sertoli cells which occur among the germ-cells are usually assumed to have a nutritive function. As the spermatozoa lose connexion with the epithelium, they pass into the lumen of the tubules, and thus find their way to the epididymis, in which they may be retained for some time. They are contained in a mucous alkaline liquid, also secreted by the epithelium of the seminifer- C E F Fig. 277. — Diagrams of the develop- ment of spermatozoa, a.c, anterior centrosome; a.f., axial filament; c.p., con- THE UROGENITAL SYSTEM 313 ous tubules. Diminution in the activity of the tubules occurs in old age, and may begin as early as thirty-five years. The interstitial tissue between the seminiferous tubules is believed to have an endocrinal function and to influence the development of secondary sex trails and the vigor of the individual. Also associated with the testes are certain rudimentary organs (ductuli aberrantes, paradidymis, hydatid of Morgagni) the significance of which will be better understood after the description of their develop- ment. Each testis is attached to the scrotum by a connective-tissue cord, the gubernaculum. SPERMATIC- CORD. PUBIC SYMPHYSIS CORPUS CAVERNOSUM PENIS. urogen ital "Diaphragm, •bulbourethral GLAND. BULBUS. --/-RECTUM EXTERNAL -SPHINCTER MUSCLE. •ANUS. Fig. 278. — The male urogenital system. The glandular complications of the male urogenital system appear to be in part an adaptation to the double function of the urethra — excretory and reproductive. The alkaline secretions of the glands serve to neutralize the acidity of the urethra caused by the acid urine. (Sobotta.) Ductus Deferens. By the efferent ducts of the testis, sperm is carried to the ductus epididymidis, a much convoluted tube twenty or more feet in length, which together with the efferent ducts forms the head of the epididymis. The ductus epididymidis is the beginning of the ductus deferens and, like it, Hned with ciliated columnar epitheHum. The ciUa beat towards the urethra and carry the spermatozoa to the seminal vesicles where they may be temporarily stored. Layers of circular and longitudinal muscles are present in the wall of the duct. At the lower end of the testis the ductus loops back along the tail or Cauda of the epididymis, and then leaves the epididymis to join the spermatic cord. As a component of the spermatic cord it passes through the inguinal canal. Entering the body cavity, the ductus leaves the spermatic cord and 314 CHORD ATE ANATOMY passes medially to the ureter to enter the prostate gland where it becomes the ductus ejaculatorius. As it approaches the prostate, it enlarges into an ampulla and is joined by the duct of the seminal vesicle. Each seminal vesicle is about two inches in length and three quarters of an inch in diameter, formed by an elongated tube four to five inches long coiled within a connective-tissue capsule. It secretes continuously an alkaline mucous fluid. (Fig. 278) Prostate and Bulbo-urethral Glands. At the point where the ductus ejaculatorius opens into the urethra, this passage is surrounded by a conical mass of glandular and muscular tissue, the prostate gland. The glandular portion of the prostate is formed by fifteen to thirty branched tubular glands embedded in connective tissue containing compact masses of smooth muscle fibers. The development of the prostate shows that it is a modified portion of the wall of the urethra. Its alkaline mucous secretion, produced at times of sexual excitement, has a stimulating effect upon the movement of spermatozoa. Characteristic albuminoid concre- tions are formed in the alveoli of the gland. In later years of life these concretions increase in size and number and become calcified, so that the lumen of the urethra tends to become occluded by the pressure of the prostate. Embedded in the prostate is a median pouch, the prostatic utriculus or uterus masculinus, which opens by a median aperture near the openings of the ejaculatory ducts. The utriculus is a rudiment of the embryonic Mlillerian ducts which in the female become the uterus. The bulbo-urethral glands or Cowper's glands are tubulo-alveolar glands less than half an inch in diameter, embedded in the connective tissue of the urogenital diaphragm near the bulbus urethrae. Their ducts open into the cavernous portion of the urethra. At times of sexual excitement they secrete an alkaline mucous liquid. The Penis. The male urethra extends into the intromittent organ, the penis. Thus in the male the urethra serves both as an excretory and as a reproductive outlet. Three portions of the urethra are recognized, prostatic, membranous, and cavernous portions. (Fig. 278) The body of the penis consists of three masses of erectile tissue, paired corpora cavernosa penis and the unpaired corpus cavemosimi urethrae. The latter enlarges at the root of the penis into a bulbus urethrae and terminates at the extremity of the penis as a swollen mass of erectile tissue, the glans penis. In its flaccid condition the glans is covered by the foreskin or prepuce. The paired corpora cavernosa are prolonged into the peritoneal region as far as the tuberosity of the ischium. In this way they form the fixed portion of the penis, the crura penis. To each criis is attached an erector muscle, the ischio-cavemosus. The nerves of the penis are several. Branches of the second, third and fourth sacral (spinal) nerves are known as erector nerves since their THE UROGENITAL SYSTEM 315 stimulation causes the erection of the penis. Pressure upon the sym- pathetic centers of the hypogastric plexus also stimulates erection. In a llaccid penis the arterial blood supply is reduced through occlusion of the lumen of the vessels by the contraction of local thickenings of their walls. When, however, the artery is dilated, a free flow of blood into the venous spaces of the corpora cavernosa causes them to become engorged with blood and the penis consec^uently erected. DEVELOPMENT OF THE UROGENITAL SYSTEM OF MAN A. The Urinary System The urinary organs, except the lining of the bladder, are mesodermal in origin, both urinary tubules and ureter being formed from the nephrotome. INAL CORD ::^^MYOTOME CEPIMERO -PRIMITIVE DUCT MVOCOEL MYOTOME PRIMITIVE UCT PRINCIPAL DIVISION PRIMITIVE DUCT SUPPLEMENTARY DIVISION NEPHROSTOME HY POME RE ^OUTER GLOMERULUS Fig. 279. — A diagram showing three stages, A-C, in the development of the primitive duct and a pronephric tubule. The duct and each tubule connected with it arise from the mesomere. Two types of glomeruli — outer and inner — become associated with the pronephric tubules. (Redrawn after Felix.) Strangely enough, three renal organs, pronephros, mesonephros, and metanephros, develop in succession in the human embryo as well as in all amnio te embryos. The pronephros in man is a functionless rudiment. The mesonephros probably functions during foetal life. The metanephros is the definitive kidney. The occurrence of three kidneys in the embryos of amniotes is best interpreted by the evolution theory. Pronephros. The pronephros or ''head kidney" is the most anterior of the three, and in the human embryo develops from the nephrotomes of 3i6 CHORDATE ANATOMY segments seven to fourteen. The first of the pronephric tubules makes its appearance in a 1.7 mm. embryo, and all eight tubules are formed by the time the embryo has reached a length of 2.5 mm. Degeneration begins soon and the anterior tubules disappear before the posterior ones are differentiated. The development of pronephric tubules is initiated by the parietal layer of the nephrotome or intermediate cell mass from which cells are proliferated towards the ectoderm. The nephrotome loses its connexion with the epimere above, and, together with the lateral outgrowth just mentioned, forms a pronephric tubule. In most vertebrate embryos, pronephric tubules are primarily solid, but become hollow later. As they SPINAL CORD. SPINAL GANGLION-/. MESONEPHRIC TUBULES PRIMITIVE DUCT GLOMERULUS POSTCARDINAL VEIN MESENTERY PRONEPHRIC TUBULES Fig. 280. — Stereogram of the developing pronephros and inesonephros. (After Kingsley, modified.) grow laterally towards the ectoderm, the pronephric tubules also grow posteriorly and unite to form a mass of rapidly dividing cells which con- tinue to extend posteriorly as a cellular rod until they reach the cloaca. Connexion with the cloaca is established and a lumen forms. In this way is produced the primitive or pronephric duct. Although the proneph- ric tubules which produced the primitive duct degenerate soon after their appearance, the duct itself persists as the Wolffian or mesonephric duct, so-called because it forms the outlet of the tubules of the mesonephros. In some vertebrates, but apparently not in the human embryo, the pronephric tubules open into the body cavity and, at least in part, persist in the adult as the ostium tubae (the anterior opening of the oviduct). THE UROGENITAL SYSTEM 317 Fig. 28 1. — StereoRram of the deveLiiiinK mesonephros at a stage later than that of Fig. 280. (After Kingsley modified.) neural tube notochord dorsal aorta afferent vessel glomerulus effefent vessel somite posterior cardinal vein collecting vein connecting sub-and post- cardinals capsule mesonephric duct ,. subcardinal 1 Fig. 282. — Diagrams showing the relations of the blood vessels to a mesonephric tubule. (From Patten's "Embryology of the Pig," based on figures by McCallum.) 3i8 CHORDATE ANATOMY Mesonephros. The tubules of the mesonephros are formed from nephrotomes posterior to those which form the pronephros. At its full development in the human embryo, the mesonephros extends from the sixth cervical to the third lumbar segment. While the anterior nephrotomes are segmented, the more posterior mesonephric tubules are derived from a continuous unsegmented nephrogenic cord or intermediate cell mass. At their first appearance, the anlagen of the mesonephric tubules are budded off as solid spherical cell masses, which secondarily attain connexion with the primitive duct, now become the mesonephric duct. Most of the mesonephric tubules degenerate, only twenty-six pairs remaining in a 20 mm. embryo. In the male, some of these are converted into the efferent ductules of the testes, and in this sex the mesonephric or Wolffian ducts become the ductus deferentes. ^URETER Fig. 283. — A diagram illustrating the repeated branching of the collecting tubules in a nine-weeks (30 mm.) human embryo. The diagram shows also the origin of secretory tubules from the nephrogenic tissue. (Redrawn from Braus, after Kampmeier.) Metanephros. The definitive kidney of man and other amniotes, the metanephros, is the last to appear in ontogenesis. Like the meso- nephros, it has a double origin. The collecting tubules and the ureter are derived from an outgrowth of the mesonephric duct. The cortex of the kidney, on the other hand, arises from the posterior portion of the nephrogenic cord in the lumbar region. In a 6 mm. human embryo, the ureter appears in the form of a hollow outgrowth from the mesonephric duct near its posterior end. At its anterior end, this outgrowth expands into a vesicular enlargement. Growing dorsally, the vesicle comes in contact with the nephrogenic cord which covers it as a cap. As the ureter elongates, the nephrogenic cap is pushed anteriorly, and takes a position dorsal to the posterior portion of the mesonephros. The vesicular enlargement of the ureter becomes the pelvis. Two outgrowths from it, one anterior and one posterior, form two major calyces. (Fig. 271) Two more are added later between the first two. THE UROGENITAL SYSTEM 319 Minor calyces arise by the continued subdivision and branching of the major calyces. Further branching to the twelfth generation produces the collecting tubules and the medullary portion of the kidney. Those of the fifth generation to the number of twenty to eighty for each renal calyx become the papillary ducts which convey urine from the collecting tubules and which open directly into each of the renal calyces. (Fig. 283) Fig. 284. — Schematic diagrams to show the relations of pronephros, mesonephros, and metanephros at various stages of development. In the adult male the mesonephric (Wolffian) duct is retained as the ductus deferens. (From Patten's " Embryology of the Chick.") Each of the collecting tubules terminates in a slight swelling or vesicu- lated enlargement, upon which a mass of nephrogenic tissue rests as a cap. From this mass arise excretory tubules, which subsequently acquire connexion with the collecting tubule. (Fig. 283) Differentiation of the excretory tubules begins with the formation of vesicular cell-clusters which separate from the remaining nephrogenic tissue. These vesicles elongate into excretory tubules and their ends enlarge to form Bowman's capsules. 320 CHORDATE ANATOMY Subsequent changes involve the attachment of the tubule to the adjacent collecting tubule, the elongation of the excretory tubule, and the ingrowth of a glomerulus into the Bowman's capsule. Arterial and venous con- nexions are subsequently estabhshed similar to those of the mesonephros. (Fig. 282) These changes occur in the later months of intra-uterine hfe. Connexions with the Bladder. During the earher stages of its develop- ment, each ureter shares with a mesonephric duct a common lateral opening into the cloaca. Between 10 mm. and 17 mm. stages, the cloaca becomes divided by a septum into a dorsal rectum and a ventral urogenital sinus. The septum, which is completed during the seventh week, becomes the perineum of the adult. After the septum is formed the mesonephric ducts and ureters retain their connexion with the urogenital sinus. Even before the development of the septum is completed, in an 11 mm. embryo, the urogenital sinus becomes subdivided into a vesico-urethral portion into which the ureter and mesonephric ducts enter, and a phalUc portion which extends into the genital tubercle. By the time the embryo attains a length of 25 mm. (2 months), the ureters and mesonephric ducts are separated, the ureters opening into the bladder and the mesonephric ducts into the urethra. The bladder, therefore, arises not from the allantois but from the cloaca. The bladder anlage, however, is continued ventrally as the allantoic stalk, which subsequently atrophies to form the middle umbiUcal ligament. B. Reproductive Organs The human embryo is usually characterized as sexually indifferent. This popular view is based upon the similarity of the anlagen of male and female reproductive glands in the early embryo, and upon the occasional appearance of hermaphroditic adult individuals. According to modern genetical opinion, however, sex is definitely predetermined in the fertilized egg and is only exceptionally modifiable. Nevertheless, ovaries and testes develop from morphologically similar genital folds, located between the mesonephroi and the mesentery. At their first appearance, the genital folds are elongated masses of epitheUal cells which become differentiated into an external many-layered epithelium and an inner epithelial mass derived from the peritoneum. Together with the mesonephros, the gonad forms a urogenital ridge, the prominence of which is increased by the growth of underlying adrenal tissue. (Figs. 66, 67, 281) As the genital folds increase in size, longitudinal grooves develop separating them from the lateral mesonephros and the median mesentery. Connexion of the gonad with the mesonephros and with the body-wall is finally reduced to a thin mesentery-like membrane. In the male, this membrane forms the mesorchium and in the female the mesovarium through which blood and nervous connexions are retained. THE UROGENITAL SYSTEM ;2i Descensus of Gonads. A comparison of earlier and later stages leveals the fact that the gonads shift their position posteriorly in the body- cavity. The prime factor in this backward migration is the continued growth of the posterior portion of the gonads and the associated atrophy of the anterior portion. These processes result in the change of the gonads from an abdominal to a pelvic position. The ovaries retain this position throughout life but the testes migrate into the scrotal sac. . .AN 0UCT\ TESTIS C. SEX CORU B OVARY C&WTNElOs DUCT) Fig. 285. — Diagrams of the testis (A) and ovary (B) showing the homologies of their components. (Redrawn after Mall, modified.) The testis is originally an abdominal organ like the ovary, and its position in the scrotum is the result of a migration or descensus in which it drags with it blood-vessels, lymphatics, nerves, and the cremaster externus and internus muscles which, together with the ductus, constitute the spermatic cord. During the third to the sixth month of development, paired out- pocketings of the body-cavity, vaginal sacs, extend ventral to the pubic Tes*M Gcbe Pig. 286. — Schematic diagrams illustrating the descent of the testis as seen from the side. d. def., ductus deferens; Proc. Vag., processus vaginalis (the diverticulum of the peritoneum pushed into the scrotal sac). (From Patten's "Embryology of the Pig.") bones into the scrotal sacs. During the seventh to the ninth month, the testes descend into the scrotal sacs. This "descensus" occurs not into the vaginal sacs but beneath the peritoneum dorsal to the vaginal sacs. Normally, the passage between the body-cavity and the vaginal sac is obliterated soon after the migration of the testis (7th to 9th month). Failure to close results in liability to inguinal hernia. The condition of undescended testes is known as cryptorchism and is accompanied by 322 CHORDATE ANATOMY Sterility since spermatozoa are unable to survive the normal temperature of the body. The scrotal sac appears to act as a thermoregulator. The factors involved in the descensus of the testes are complex. Chief among them appears to be the contraction of the connective-tissue gubemaculum testis which extends from the testis to the posterior wall of the scrotum. The gubernaculum contracts to one quarter of its original length and, after the descensus, almost completely atrophies. During the course of its descent, each testis rotates through an arc of 1 80° so that its anterior and posterior ends are reversed. Rarely, the ovaries undergo a similar descent into the labia majora. Normally, however, the enlargement and relations of the uterus prevent this migration. Reproductive Ducts. The mesonephric or Wolffian ducts are utihzed by the male as reproductive ducts, the ductus deferentes. Mliller's ducts are also developed in both sexes. Each Miiller's duct arises from a longitudinal groove on the lateral side of the mesonephros. The peritoneal epitheUum sinks into the underlying mesenchyma (Fig. 67). Except at its anterior end, where it remains open as the ostium tubae, the groove closes over and grows posteriorly as far as the cloaca into which it acquires an opening. Male Reproductive Ducts. In a three-month embryo, the anterior and posterior regions of the mesonephros differ. In the anterior region, which consists of five to twelve renal tubules, the collecting portions of the tubules separate from the excretory portions and acquire connexion with the tubules of the rete testis. In this manner, anterior mesonephric tubules form the ductuli deferentes and serve as outlets for the external products of the testis. The remaining, posterior, portion of the meso- nephros mostly degenerates, remnants persisting as the paradidymis and ductuli aberrantes. The cranial portion of the mesonephric duct increases greatly in length to form the ductus epididymidis. The posterior portion becomes the ductus deferens. In the male, Miiller's ducts begin to atrophy in the third month. In the adult, remnants of the anterior and posterior extremities may persist as functionless rudiments. The former is the appendix testis and the latter the uterus masculinus. Remnants of the mesonephros persist also in the female. Some anterior mesonephric tubules unite with the rete ovarii to form the epoophoron. The posterior part of the mesonephros becomes the rudi- mentary paroophoron. The functionless remnant of the Wolffian duct in the adult is known as Gartner's duct. Female Reproductive Ducts. Miiller's ducts parallel the meso- nephric ducts and open into the urogenital sinus median to them. The position of the primary opening marks the place where later the hymen is located. During the fourth month, the posterior portions of the paired THE UROGENITAL SYSTEM 323 MuUerian ducts unite to form uterus and vagina. The anterior portions form the uterine tubes. External Genitals. The external genitals of the two sexes, like the gonads, have similar beginnings. Slight differences, however, quickly make their appearance. In an 8 mm. embryo, a rounded eminence, the genital tubercle, develops between the tail and the umbilical cord. Along its caudal surface extends a shallow urethral groove bordered by urethral folds, the inner genital folds. Labial or scrotal swellings, the outer genital folds, border the urethral folds laterally. When the embryo has reached a length of 15 mm., the urethral groove is perceptibly longer in the male. In both sexes the tubercle elongates to form a phallus, the termination of which enlarges as the glans penis or the glans clitoridis. x;v^^<^ \ CORPUS FOLLICLE "^ ./;--gr^^«»*- -^^^=» ••wJ, LUTEUM Fig. 290. — A diagram of the interactions of pituitary, follicular and luteal hormones. (Redrawn after Dickinson's "Sex Anatomy," Williams & Wilkins Co.) plenty of evidence that hormones influence the secondary sex traits, the first striking evidence appearing at puberty when the boy assumes some of his adult male characteristics. Crew cites the case of a fowl which was successively a mother and a father as a result of the destruction of the ovary by disease and subsequent growth of a testis in place of the ovary. Female Sex Glands. In the ovary, as in the testis, interstitial and germ cells may be distinguished. That the interstitial cells have an endocrinal function is suggested by the fact that they increase during pregnancy. The argument from analogy with the testis has less weight. Observed facts convince physiologists that at least two hormones, the follicular and the luteal, are secreted by the ovary. The causal THE ENDOCRINAL ORGANS 327 relation between follicular secretion and menstruation is demonstrated by the fact that menstruation follows the discharge of the follicular secretion and ceases when the ovaries are removed. The follicular hormone, theelin, has the chemical formula C18H24O3 and is a fatty substance, soluble in alcohol and other hpoid solvents. It is used hypodermically to induce puberty and menstruation. One of the names for the hormone of the corpus luteum is progestin. Its action is antagonistic in some effects to that of theelin, for, while theelin acts as a sex stimulant, progestin prepares the uterus for the reception of the ovum. Suprarenal Glands. The suprarenals or adrenals of man are two small glands, each averaging only four to five grams in weight, and each, as the HEP. PORTAL V BILE DUCT R. SUPRARENAL VES .ESOPHAGUS L INF PHRENIC A. DORSAL AORTA R. RENAL VES R. KIDNEY VENA CAVA LEFT SUPRARENAL GLAND L. KIDNEY -L SUPRARENAL VES .^=1.. RENAL VESSELS SUP MESENTERIC A. Up „, ^_ up^T£(, SPERMATIC COVARIAN) VES'/ SnF. MESE^^■ER!C A Fig. 291. — Shows the suprarenals in relation to the kidneys as seen when the fat which normally encloses them is removed. The relations of the blood vessels which supply them are also shown. names suggest, lying like a cap upon the upper end of a kidney, embedded usually in the same mass of fat. Accessory adrenals also occur not infrequently near the kidneys or the gonads. The adrenal gland has a rich blood supply. Arterial blood comes from three sources, phrenic, aortic, and renal vessels. The gland is drained by the right and left suprarenal veins, the former connected with the post- cava, the latter with the left renal. Lymphatic vessels are abundant. An adrenal has two kinds of tissue, an outer yellowish cortex and an inner brownish medulla. The central or medullary portion of the adrenal is characterized by relatively large "chromaffin" cells, so-called because they have a strong affinity for chromic salts which stain them brown. They are arranged in clusters separated by numerous lacunar blood spaces. A compact connective-tissue capsule encloses the glandular tissues. (Fig. 292) The adrenals have a double function corresponding with their histo- logical differentiation into two tissues. The cortex secretes an endocrine, 328 CHORDATE ANATOMY cortin, of unknown chemical composition, which is essential to life. Destruction of the cortex is followed by Addison's disease, which is char- acterized by a deep pigmentation of the skin and great weakness. Death comes rapidly unless cortin is administered. The cortex of the adrenal is, relatively to the medulla, largest during foetal life, and there is Kttle doubt that its secretions have a strong influence upon growth. The endocrine secreted by the medullary tissue of the adrenal is epinephrine or adrenin, its empirical chemical formula being C9H13O3N. Like the sympathetic nerves, adrenin has a stimulating effect upon smooth muscle. It is so potent a drug that its physiological effects appear even when it is diluted to one part in 400,000,000 of blood. ITRESSIN AND PITOCIN) PANCREATOTROPIC. KETOGENIC . PARATHYREOTROPIC 4TH DAY OF MENSTRUAL CYCLE WTH DAY 28TH DAY 4TH DAY Fig. 297. — A diagram illustrating the complex interrelations of the endocrine organs. The multiplicity of pituitary influences is only partially indicated. For the sake of clearness the diagram over simplifies the relations. The pancreatropic, keto- genic, and parathyreotropic factors are not thoroughly understood. (Adapted from Therapeutic Notes through courtesy of Parke, Davis & Company.) The pituitary gland has a double origin. The anterior lobe comes from an ectodermal sac, Rathke's pouch (hypophysis), in the roof of the mouth, while the posterior lobe is formed as an outgrowth of the base of the diencephalon. The posterior end of Rathke's pouch comes into contact with the infundibular outgrowth from the base of the brain. As 53^ CHORDATE ANATOMY development proceeds, Rathke's pouch loses connexion with the ectoderm and breaks up into vesicles, the cavities of which are remnants of the pouch cavity. The ventral end of the infundibulum becomes thickened as the anlage of the posterior lobe. The cells which later become differentiated as pars intermedia and pars tuberalis are derived from the anlage of the anterior lobe. The pituitary gland occurs only in vertebrates. The elements of the pituitary make their first appearance in cyclostomes. In myxinoids (Myxine, Bdellostoma) the neural (posterior) lobe is represented by the epithelial ventral termination of the infundibulum. The anterior lobe is represented by the hypophysial duct, an ectoderm-lined tube which opens ' INFUNDIBULUM-. INTERMEDIATE LOBE Z.- PHARYNGEAL ^ APERTURE ANTERIOR LOBE-'' INFUNDIBULUM 'HYPOPHYSIAL DUCT B. PETROMYZON INTERMEDIATE C. HEPTANCHUS ^^^ (POSTERIOR LOBE INFUNDIBULUM INTERMEDIATE — ANTERIOR LOBE — PARS TUBERALISC POSTERIOR ^^ ANTERIOR LOBE INFUNDIBULUM POSTERIOR LOBE' D. RAMA E. REPTILE F. HOMO Fig. 298. — A series of diagrams showing conditions in six different vertebrates, which are believed to represent stages in the evolution of the pituitary gland. The complexity of origin of this gland is correlated with the complexity of its endocrinal functions. The posterior lobe is cross-hatched, intermediate lobe stippled, anterior (hypophysial) lobe piebald, and the pars tuberalis solid black. (Redrawn from Oppel, after Stendell.) in front of the mouth anteriorly and into the pharynx posteriorly. (Fig. 298, .1) The only element of the myxinoid pituitary which is glandular is the intermediate lobe, represented by clusters of cells lying between the infundibulum and the hypophysial duct. These cells are proliferated from the hypophysial duct. Since the epithelium of the infundibulum and of the hypophysial duct is not thickened but remains single-layered in myxinoids, there is no evidence that these elements in this group have an endocrinal function. Stendell, therefore, seems justified in the conclusion that the intermediate lobe is the first part of the vertebrate pituitary which is differentiated as an endocrinal organ. (Fig. 298) An advance in the evolution of the pituitary is found in Petromyzon. In this animal, as in all higher vertebrates, connexion of the hypophysial THE ENDOCRINAL ORGANS 337 duct with the pharynx is lost and the organ ends bUndly at its posterior end. From it, however, are proliferated cells which form the inter- mediate lobe. Other clusters of cells nearer the hypophysial duct produce hollow vesicles which are believed to represent the beginning of a glandular anterior lobe. Since the remainder of the embryonic hypophysis persists in the adult as a blind pouch and is not, as in higher vertebrates, converted into an anterior lobe, it is evident that the anterior lobe of the pituitary of Petromyzon is only partially homologous with that of higher vertebrates. The nervous lobe of the pituitary can hardly be said to exist in Petromyzon. The evolution of these elements which have their inception in cyclo- stomes may be briefly summarized. In elasmobranchs the nervous portion of the gland is only slightly indicated. As one passes through the verte- brate series from fishes to man, all three elements seen in cyclostomes are present. The anterior lobe steadily increases in relative size while the intermediate lobe shrinks. An increase takes place in size and differentia- tion of the posterior lobe. The presence of colloidal material in the pituitary in all vertebrate groups justifies the assumption that the gland has an endocrinal function throughout the series. According to the view just expressed, the evolution of the hypophysis involves the metamorphosis of a tubular hypophysial duct into an endo- crine organ. Another view is that the hypophysis was in the beginning a gland which opened into the mouth, but for this opinion there seems to be less evidence. CHAPTER 13 THE NERVOUS SYSTEM Of the two agencies which integrate the various functions of the body, the nervous system is the more important. In addition, however, to this integrative function the nervous system, with the intermediation of the sense organs, serves to bring the organism into relation with its environment. The general protoplasmic properties upon which the actions of nerves depend are merely the irritability and conduction which are characteristic of all cells. An Amoeba, for example, responds to a stimulus by contract- ing. If one of its pseudopodia is touched, all pseudopodia withdraw. Obviously, both irritability and conduction are involved in this reaction. FOOD VACUOLE PSEUDOPODIUM AMOEBA. Fig. 299. — Amoeba. Amoeba shows primitive responsiveness to stimulation. All cells of higher animals presumably retain these two powers, but they become the special functions of nerve and sense cells. ELEMENTS OF THE NERVOUS SYSTEM In simple colonial animals such as Volvox, nerves are wanting and impulses are transmitted from cell to cell by means of intercellular bridges or plasmodesms. Special nervous cells first appear in coelenterates, in the form of neurosensory cells located in the skin. Each neurosensory or receptor cell is connected with deeper tissues, such as muscle fibers, by an elongated process or neurite, which carries nervous impulses to the effector cell. In a characteristic reflex action in worms, a ganglion or transmittor cell, comparable with the motor cell of vertebrates, is interpolated between the receptor and effector cells. A similar reaction in vertebrates usually involves four cells: — i a receptor cell in the skin or sense organ; 2, an 338 THE NERVOUS SYSTEM 339 afferent or sensory cell; 3, an efferent or motor nerve cell, which is con- nected with 4, an effector cell by a neurite. Further complications arise CEREBRUM CEREBELLUM- CERVICAL PLEXUS BRACHIAL PLEXUS Fig. 300. — A general view of the nervous system of man as seen from behind. by the chaining together of additional nervous units within a central nervous system until, in vertebrates, so few cells are devoid of nervous 340 CHORDATE ANATOMY connexions that, if all were destroyed except the nervous tissues, the general form of the body would still be preserved. The steps in the evolution of a reflex nervous system such as that of worms and vertebrates involve, first, the differentiation of a neurosensory cell in the ectoderm. The body of such a cell remains in the ectoderm, and one or more protoplasmic hairs may extend above the surface. The most characteristic feature of such a cell, however, is the neurite which grows away from the surface towards the underlying muscles. By branch- ing into terminal telodendria, such a neurite may increase the number of its connexions. EPITHELIAL CELL MUSCLE CELL TOioi4^ioioH=iirTnwniFE --SENSOIW CELL- miMoioKi-iiiiiiiTrrn METAMERIC MUSCLES Fig. 301. — A diagram showing hypothetical stages in the evolution of the reflex arc of the higher animals. In A the series begins with the neuromuscular cell of coelenterates. In B the neurosensory cell becomes differentiated from the muscle fiber. In C the body of the neurosensory cell recedes from the surface. In D the neurosensory cell becomes a sensory neuron and a secondary sensory cell conveys external stimuli to the sensory neuron. In £ a motor neuron is interpolated between the sensory neuron and the muscle fiber. Finally, in F, by means of several motor neurons, the connexion of a sensory neuron with several muscle fibers is effected. (Redrawn after Fritz Kahn, "Der Mensch," Albert Miiller. Zurich.) A second evolutionary step is taken when the body of the neurosensory cell sinks below the surface into the underlying connective tissue, but retains connexion with the superficial epithelium by means of a process with branched terminations or dendrites attached to the skin. Further advance appears when the dendrites, instead of ending freely among the epithelial cells, become connected with special receptor or secondary sense cells in the skin. The somatic sensory cells of verte- brates are at this evolutionary stage. The so-called primitive ganglion cells of coelenterates exhibit another line of differentiation. That these cells derive from neurosensory cells is a conclusion supported by considerable evidence. That they are more differentiated than neurosensory cells is indicated by the fact that they THE NERVOUS SYSTEM 341 contain tigroid substance and neurofibrillae characteristic of the nerve cells or neurons of the higher animals. Physiologically, however, they are simpler than neurosensory cells, since they transmit nervous impulses in any direction, while neurosensory cells are definitely polarized and transmit impulses in a single direction only. They are, therefore, inter- preted as neurosensory cells which have lost both their primary connexion with the skin and their functional polarity. (Fig. 303) NEUROSENSORY CELL BIPOLAR NEURONE ' ' UNIPOLAR NEURONE Fig. 302. — A diagram illustrating hypothetical stages in the phylogenesis of the characteristic sensory (afferent) neuron of vertebrates. Earlier stages at the left, final stage at the right. Arrows suggest that the direction of growth of the neurite is away from the source of stimulus. The diagram assumes that the primitive neurosensory cell becomes the definitive sensory neuron and that the definitive sense receptor is secondary. It is quite possible, however, that the neuron is secondary and that the neurosensory cell becomes the definitive receptor. (Redrawn after Ariens-Kappers, modified.) In some coelenterates, the primitive ganglion cells form a loose subcu- taneous network or plexus in which, as experiments show, nervous impulses may be carried in any direction. Morphologists incUne to the opinion that the nerve-net of coelenterates becomes the central nervous system of higher animals. A final step in the evolution of the neuron is taken by the flatworms in which the neurons, like the neurosensory cells of coelenterates, trans- 342 CHORDATE ANATOMY mit nervous impulses in only one direction. In contrast with neurosensory cells, however, a neuron always has at least two nervous processes, of which the dendrite carries impulses toward the cell body, while the neurite (neuraxon) carries impulses away from the cell body. Each neuron has but one neurite, and may have one or many dendrites. In ;> NERVE FIBERS *^OANCUON CELLS Fig. 303. — Plexus of ganglion cells and fibers in the tentacle of a coelenterate. From such a nerve-net as this it is believed that the nervous systems of higher animals have evolved. (Redrawn from Ariens-Kappers, after Wolff.) ontogenesis the neurite grows away from the source of stimulation and the dendrites towards the stimulus. The differentiation of the neuron is accompanied by the appearance of two sheaths, neurilemma and medullary or myelin, which cover the EPINEURIUM NODE OF RANVIER NEURILEMMA MYELIN SHEATH NEURITE (AXONE) ENDONEURIUM NUCLEUS OF NEURILEMMA'^ A LONGIT SECTION B. CROSS SECTION Fig. 304. — A portion of (A) longitudinal and {B) cross section of a nerve prepared by the vom Rath method. A nerve is a bundle of axons (neurites) covered by an epi- neurium. Each axon is surrounded by an inner myelin (fatty) sheath and an outer cellular neurilemma sheath. nervous processes and serve to insulate and nourish them. Each neurite may be covered by a chain of neurilemma cells, or it may not; each may or may not have a fatty medullary sheath. The familiar distinction between white and gray matter in the nervous system rests on the presence or absence of medullary sheaths, Ganglia and non-meduHated fibers THE NERVOUS SYSTEM 343 form gray matter; meduUated fibers appear white. That the medullary sheath serves at least for insulation is indicated by the fact that nervous impulses are conveyed more rapidly in meduUated than in non-medullated nerves. Within the central nervous system of vertebrates, neurites lack the neurilemma sheath, but are usually meduUated. The presence of a neurilemma is, therefore, not essential to the secretion of this fatty myelin sheath. Most peripheral nerves are meduUated, but the medullary sheath does not appear until after the neurilemma is differentiated. The nerves of Amphioxus and of cyclostomes are not meduUated. This primitive MOTOR GANGUON CELL A. DIAGRAM OF A REFLEX ARC Fig. 305. — Diagram of a nervous arc. In ,4 three neurons — afferent, intercalary, and efferent — are shown in their relations to one another and to the skin and muscle. The intercalary neuron is located in the gray matter of the spinal cord. B is an enlarged section of a nerve fiber. condition is retained by the sympathetic nerves and plexuses of higher forms. Primitive ganglion ceUs like those of coelenterates occur, in vertebrates, only in the parasympathetic plexuses associated with the alimentary canal. The forms assumed by neurons in vertebrates are varied and, in general, the more complex the "animal, the more complex its neurons. Complication in form usually involves an increase in the number of dendrites, and denotes a multipUcation in the number of possible func- tional relations. In ontogenesis, as in phylogenesis, aU the processes of neurons are formed as processes of primarily simple neuroblasts. Nerve cells manifest a tendency not only to spin out elongated proto- plasmic processes so as to connect with various parts of the body and with one another, but also to form plexuses and gangUonic masses. In this way the complex nervous systems of higher animals have been built up. 344 CHORDATE ANATOMY The first nervous connexions appear to have been between skin and muscle, by means of neurosensory cells, so that a motor response to external stimulus is made possible. So simple an arrangement as this, however, is rarely found, even in worms and molluscs. Usually at least two nerve cells are involved in a reflex action, a neurosensory receptor and a motor ganglion cell which connects with muscle or gland. Even more frequently, a third or association cell is interpolated between the receptor and the motor cells. These association cells may multiply to form a chain of neurons within the central nervous system. Such complications were made possible by the genesis of the central nervous systems of higher animals from the nerve-net of coelenterates. (Fig. 305) In the primitive nerve plexuses of lower animals such as the coelen- terates, and also in those of the vertebrate aUmentary canal, the ganglion cells are in protoplasmic continuity with one another, and nerve impulses are carried directly from cell to cell. The differentiated neurons of higher animals appear not to be so interconnected, but have greater individuality, for the telodendria of one neuron are brought into relation with the dendrites of another only indirectly through the so-called sjmapse. A synapse is the region where the fine telodendria of one neuron are brought into physiological relation with the dendrites of another neuron. Nerve impulses which involve the activity of two neurons must pass through such a synapse. The transmission of a nervous impulse through the synapse is beUeved to involve a semipermeable membrane through which impulses pass from one neuron to another. The physical process by which such a transfer is effected has been compared to the jump-spark action of a gasoline engine. It should not, however, be understood that this synaptic membrane has been demonstrated beyond a reasonable doubt. Its presence is inferred chiefly because refined neurological technique has not been able to demonstrate the continuity of the neurofibrillae of adjacent neurons. The fact that each neuron arises from a neuroblastic cell which is primarily independent of other cells, and that the termination of a growing neurite is free, further strengthens this conclusion. Some physiological experi- mental evidence points in the same direction. With the differentiation of a neuron having neurite and dendrites which normally convey nerve impulses in one direction only, and with tigroid bodies and neurofibrillae in its cytoplasm, the evolution of the nervous unit or neuron reaches its climax. The steps in the phylogenesis of the nervous system are, therefore, the differentiation of the neurosensory cell, the attainment of functional con- nexion with muscle or gland cells, the recession of the neurosensory cell from the external epithelium to form a primitive ganglion cell, the forma- tion of an interconnected nerve-net containing association cells, the union THE NERVOUS SYSTEM 345 of afferent and efferent neurites into bundles or nerves, and the concen- tration of nerve-cell bodies to form ganglia. ORGANIZATION OF THE NERVOUS SYSTEM When nervous units convey impulses towards and away from a subcu- taneous nerve-net, as in the coelenterates, there are the beginnings of a nervous system. The nerve-net forms the central nervous system, the afferent and efferent neurites the peripheral nervous system. The primi- tive and characteristic function of such a system is the nervous reflex. A nervous reflex, or reflex action, is a simple motor response to stimulus involving sensory and motor neurons and their interconnexions within a nerve-net or nerve center. If we take the subcutaneous nerve-net of coelenterates as an early stage in the evolution of the central nervous system and the neurosensory A. LOWER njVTWORM B HIGHER FLATWORM C. ARTHROPOD 0. VERTEBRATE Fig. 306. — Diagrams of the nervous system of ^, lower flatworm, B, higher flatworm, C, arthropod, and D, vertebrates. The higher nerve centers are cross- hatched. The sympathetic cords are indicated by dotted Hnes. Many morphologists assume that the figures represent a phylogenetic series. (Redrawn after Stempell.) cells as the beginnings of sensory nerves, the primitive ganglion cells are the original association and motor cells. From such beginnings it is not difficult to derive the complex nervous systems of the higher animals. The flatworms show a distinct advance above the coelenterate stage. In them, the nerve-net is partly aggregated into two or more paired longi- tudinal cords or connectives, which unite at the anterior end of the worm, in close association with pigmented eye-spots, to form the beginnings of a brain. (Fig. 306) Morphologists are inclined to derive the paired lateral nerve cords of flatworms directly from the subumbrellar gangUonic ring of a medusa. Like the subumbrellar ring, the nerve cords of flatworms consist of nerve fibers associated with clusters of primitive ganglion cells. (Fig. 307) Kappers explains the concentration of nervous material in the anterior brain as a result of the great exposure of the head to stimuli. Unfortu- nately, such an hypothesis is unsupported by experimental evidence. 346 CHOEDATE ANATOMY In the simple flatworm Planocera, there is a single pair of nerve cords. The number increases in other forms, and the cords may be dorsal and ventral as well as lateral. This fact is important in its bearing upon the development of the nervous systems of higher animals which, in general, are assumed to have evolved from flatworm-like ancestors. For the presence of both dorsal and ventral nerve cords in flatworms makes it possible to derive annelids and arthropods from flatworms in which the ventral cords become the dominant nervous centers, and to derive verte- NERVE CORD FLATWORM Fig. 307. — A diagram illustrating the hypothetical evolution of the nerve-net of a Hydra into the subumbrellar nerve-ring of a medusa on the one hand and into the paired nerve cords of flatworms and annelids on the other. The nerve cords are formed, it is assumed, by the concentration of the fibrillar and cellular elements of the nerve-net. (Redrawn after Fritz Kahn, " Der Mensch," Albert Miiller, Zurich.) brates from flatworms in which the dorsal cords become predominant. The dorsal nerve cord of vertebrates, therefore, need not have been derived from the ventral cord of annelids by the inversion of the worm. It has been asserted that the ventral nerve cords are larger in flatworms which crawl, while the dorsal cords are larger in free-swimming types. As we pass from the flatworms to higher groups, two contrasting trends are noticeable. In annelids and arthropods the nerve cords become markedly metameric and are non-tubular, while that of chordates is tubular and j)rimarily non-metameric, as in protochordates. THE NERVOUS SYSTEM 347 The nerve cord of the primitive annelids consists of a chain of paired ganglia Hnked together both by longitudinal connectives and by trans- verse commissures. The longitudinal connectives pass around the eso- phagus to connect the supra- and sub-esophageal ganglia. In the higher annehds the paired ganglia tend to unite in the mid-ventral line and to lose the primitive rope-ladder arrangement. Concentration and fusion greatly reduce the number of ganglia, especially in arthropods. (Fig. 308 ) These profound changes in the form of the nervous system of articu- lates are accompanied by histological and physiological differences. Most of the nerve cells become definitely polarized to transmit impulses in one direction only, either towards or away from the central nerve cord. Thus ; GANGLION CELLS. ^EPITHELIUM MOTOR NEURONE. NERVE FIBERS. /SENSORY NEURONE GANGLION SENSORY /CELL NEURONE ~ MUSCLE . 'EPITHELIUM. ^MUSCLE. Fig. 308. — Invertebrate (annelid) and vertebrate nervous systems compared. In both sub-kingdoms the nervous system is derived from the ectoderm. The central nervous system of Sigalion (.4) retains its original connexion with the skin. In AUo- lobophora (B), however, as in most annelids and in vertebrates (C) the nerve cord separates from the skin. In figure C the vertebrate spinal cord is shown in reversed position with dorsal side down. In A, B, and C the axon processes of the ganglion cells within the nerve cord are similarly directed away from the surface. D and E show the relations of afferent and efferent neurons in an annelid (D) and a vertebrate (E). (Redrawn after Parker.) the neurons become in all essentials like those of vertebrates, being differentiated either as sensory or motor, while those within the nerve cord become association cells. A reflex-arc pattern is thus estabUshed in which the motor and sensory cells involved in the reflex may be either homolateral or heterolateral according as the neurons involved belong to one or to both sides of the body. In insects separate motor and sensory nerve roots appear, reversed in position as compared with vertebrates. Motor roots are dorsal and the sensory roots ventral, while in vertebrates motor roots are ventral and sensory roots dorsal. It will be noted, how- ever, that if the insect were turned over on its back so as to bring the nerve cord on the upper side of the body as in vertebrates, the relations of the nerves would be similar to those of vertebrates. Giant ganglion cells and fibers, resembling those of fishes, appear in the nerve cord of annelids. 348 CHORDATE ANATOMY NERVOUS SYSTEM OF CHORDATES Hemichordates. Hemichordates have a dorsal tubular nerve cord limited to the collar region, but a ventral invertebrate type of nerve cord in the gill and trunk region. (Fig. 4) The tubular portion of the dorsal cord remains open at both ends throughout life. It contains neuro- sensory cells of a primitive type and some giant ganglion cells, is sur- rounded by an outer fibrous layer as in vertebrates, and continues forward into the proboscis and backwards into the body as a non-tubular strand of nervous tissue. A circumesophageal ring connects this dorsal cord with a ventral median strand. The ventral nerve strand is simply a local thick- ening of a layer of nerve fibers which forms a continuous network beneath the skin. The association in hemichordates of invertebrate and vertebrate types of nervous system helps to bridge over the gulf between the two groups. Urochordates. The nervous system of larval urochordates shows an advance towards that of vertebrates, for the ventral invertebrate nervous system has disappeared, and the nerve cord is tubular throughout. Three divisions may be distinguished: an expanded anterior brain or sense vesicle which encloses an unpaired eye and static organ; a short trunk portion; and, behind, a slender cord which in the larva extends into the tail. Paired nerves connect the cord with the caudal muscle. The brain is anterior to the notochord, and has therefore been considered as the homologue of the forebrain of vertebrates. (Fig. 2, A) In most urochordates, the tail is lost during metamorphosis, and with it the associated nerves, so that only in Appendicularia do these persist throughout life. Cephalochordates. Cephalochordates have a nervous system with many vertebrate characteristics. The cord extends through the entire length of the body as a tube with a slit-like lumen which is expanded anteriorly in the region of the so-called brain. It also resembles that of vertebrates in its origin from a thickened placode of ectoderm on the dorsal side of the embryo. The neuropore persists in the larva, but closes in the adult animal to form the so-called olfactory pit. (Fig. 2 , B) Two divisions of the brain are recognized, an anterior prosencephalon and a posterior deuterencephalon. The prosencephalon is lined with cihated columnar epitheUum which shows little if any nervous differentia- tion. In its anterior wall is a pigment spot which, with scant justification, is called an eye. From the prosencephalon a pair of sensory nerves, the terminal nerves, extend forward towards the snout. The posterior boundary of the brain is marked by a cluster of ciliated sense cells, the infundibular organ. THE NERVOUS SYSTEM 349 The deuterencephalon is possibly homologous with the mid- and hind-brain of vertebrates. It differs from the spinal cord in having in its dorsal wall large neurosensory cells known as cells of Joseph. Two paired dorsal sensory nerves — numbered II and III — connect with the deuterencephalon. The first pair of motor nerves connect with its ventral wall. (Fig. 331) In Amphioxus dorsal and ventral nerves alternate with one another throughout the length of the body. Except the two anterior pairs, which are wholly sensory, the dorsal nerves of Amphioxus are mixed in function. They extend between the myotomes to the skin where they divide into dorsal and ventral rami. Ventral nerves, on the other hand, pass from the cord directly to the myotomes opposite. Con- sequently, in cephalochordates, dor- sal and ventral nerves do not unite. The ganglion cells of the dorsal nerves lie either in the dorsal waU of the cord or are embedded in the nerves. The motor ganglion cells, as in vertebrates, lie within the ventro-lateral wall of the cord. (Fig. 326, A) On the basis of their peripheral distribution, four kinds of nerve fibers are distinguished, somatic motor and somatic sensory, visceral motor and visceral sensory. Each somatic motor nerve innervates three successive myotomes, but most of its fibers pass to the middle one. Giant ganglion cells occur in the mid-dorsal line of the cord at the anterior and posterior parts of the body, but are wanting in the intermediate region. Since these connect with the sensory nerves, they are probably elements in a reflex system. Sympathetic nerve fibers connect with the blood-vessels and the viscera, but there is no chain of sympathetic gangUa. Cyclostomes. Compared with cephalochordates, the cyclostomes show a marked advance in the complexity of their nervous system Fig. 309. — Dorsal and lateral views of the brain of Petromyzon planeri. The telae chorioideae removed, and the epiphysial structures not shown in the side view, c, olfactory lobes; e, epiphysial structures; is, saccular part of infundibulum; la, acoustic lobe; Ih, left habenular ganglion; //, lobular part of infundibulum; Iv, lobe of vagus; m, mid-brain; of, olfactory bulb; pc, posterior commissure; rh, right habenular ganglion; s, first spinal nerve; th, thalamus; 1-12, cranial nerves. (From Kingsley's "Comparative Anatomy of Vertebrates," after Ahlbom.) 350 CHORDATE ANATOMY Instead of only two brain divisions, cyclostomes have five, telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon. Ontogenesis reveals, however, that these five vesicles of cyclostomes and higher vertebrates develop from the original three which are correlated with the three major senses, smell, sight, and hearing. The primitive fore-brain, which in the opinion of most morphologists corresponds to TELEN- MYELENCEPHAUON epiphysis' parietal organ habenular ganglion dorsal sac' paraphysis Monroe's foramen HEMISPHEREv OLFACTORY /^ LOBE BASAL ganglion' ANT COMMISSURE CHORIOID PLEXUS' .POST. COMMISSURE /TECTUM OPTICUM SPINAL [GANGLION POST RECESS INFUNDIBULUM \ HYPOPHYSIS ^OPTIC THALAMUS LATERAL VENTRICLE, BASAL GANGLION HEMISPHEREi OPTIC THALAMUS EPIPHYSIS VENT IE I I /HABENULAR GANG. ANT. COMMISSURE' 'VELUM TRANS PARAPHYSIS CHORIOIDEA RHOMBOIDAL FOSSA Fig. 310. — Diagrams of the vertebrate brain, based upon the brain of a cyclostome. A shows the brain in median longitudinal section, with nerves as if projected upon the median plane. B is the brain viewed from above. (Redrawn from Plate, after Biit.schli.) the prosencephalon of Amphioxus, becomes the telencephalon and dien- cephalon, the mid-brain continuing as the mesencephalon, and the original hind-brain divides into metencephalon and myelencephalon. Since the cyclostome brain may be taken as the complete prototype of that of all vertebrates, and since most of its features persist in higher forms, these are worthy of mention in some detail. (Fig. 309) The telencephalon is paired, in correlation with the development of paired olfactory lobes. Although paired eyes develop from the fore-brain, THE NERVOUS SYSTEM 35I they acquire sensory centers within the optic lobes of the mesencephalon. Olfactory centers arise in the diencephalon and effect connexion with spinal motor centers to form a mechanism for olfactory reflexes. Also involved in these reflexes are paired habenular ganglia in the roof of the diencephalon, and the interpeduncular nucleus in the base of the mesencephalon. Paired epiphyses project from the roof of the diencepha- lon, and its lateral walls are thickened as the ganglionic masses of the thalami. A funnel-shaped infundibulum projects from the floor towards the roof of the mouth. (Fig. 310) The roof of the mesencephalon of cyclostomes is peculiar in having a chorioid plexus which serves as a means of nourishing the brain. Lateral to this plexus are the conspicuous paired swellings of the optic lobes, the centers of vision. The thickened lateral wall of the mid-brain is largely fibrous and is known as the tegmentum. The floor of the mid- brain contains the motor center of the oculomotor nerve, which innervates four of the eye muscles. The functions of the mid-brain are predominantly locomotor and somatic rather than visceral. From the roof of the metencephalon arises an inconspicuous cerebel- lum, which is the anterior continuation of the lateral-line centers of the myelencephalon. In it are located the complex Purkinje cells, which have fiber connexions with motor cells in the medulla and cord, and are found in the cerebellum of all vertebrates. The lateral walls and base of the metencephalon consist largely of fiber tracts, most of which are ascending and descending fibers which connect brain and spinal cord. No pons, which is so prominent a feature of the base of the mammalian metencephalon, is present in cyclostomes or in any of the lower vertebrates. The myelencephalon or medulla oblongata is a transitional region between brain and spinal cord. Its roof is largely differentiated as a chorioid plexus. The lateral walls contain the sensory centers of the lateral-line nerves as well as those of other cranial nerves. The motor centers of the trigeminal, facial, glossopharyngeal, and vagus nerves are located in the medulla. The lumen of the brain expands into four large ventricles, two of which lie in the paired divisions of the telencephalon, the third is in the dienceph- alon, and the last in the myelencephalon. Three fiber tracts or com- missures connect the right and left halves of the brain, the anterior in the wall of the telencephalon, the habenular in the roof of the dien- cephalon and the posterior in the roof of the mesencephalon. All three persist throughout the vertebrate series to man, and serve as important landmarks by which to determine homologies. In addition to the numerous nervous structures which emerge in the cyclostome brain, the hypophysis acquires intimate relations with the infundibulum, and shows the first stages in the formation of the 352 CHORDATE ANATOMY pituitary gland, which is represented by a cluster of vesicles derived from the hypophysis. The nourishment of the brain is effected chiefly through the three chorioid plexuses in the roof of diencephalon, mesencephalon, and myelencephalon. The spinal cord in cyclostomes has become a thick-walled tube in which three layers are differentiated, an outer marginal layer of fibers, a middle mantle layer of gray matter, and a central ependymal layer which lines the central canal. The gray matter has only two lateral wings or columns instead of the four characteristic of higher vertebrates. A number of giant nerve fibers like those of Amphioxus extend along Fig. 311. — Diagram of cranial nerves of lower vertebrate. Eye-muscle nerves omitted; central nervous system dotted, fifth nerve represented as composed of two nerves; lateralis nerves separated from the ninth and tenth nerves. I-X, cranial nerves; 1-5, gill clefts; b, buccalis nerves; c, chorda tympani; g, geniculate ganglion; h, hyoid nerve; i, intestinal (pneumogastric) nerve; J, jugal ganglion, /, lateral-line nerve of X; m, mouth; ind, mandibular nerve; mt, mentalis nerve; tnx, maxillary nerve; op, ophthalmicus profundus nerve; osV, osVII, superficial ophthalmic nerves of V and VII; p, palatine nerve; po, posttrematic nerves; pr, pretrematic nerves; pt, petrosal ganglion; s, semilunar (Gasserian) ganglion; sp, spiracle. (From Kingsley's "Com- parative Anatomy of Vertebrates.") the spinal cord, but they do not decussate (i.e., cross to the opposite side of the cord), and they carry impulses caudad only, an indication of the growing dominance of the anterior portion of the nervous system. A primitive trait appears in the location of sensory ganglion cells \vithin the wall of the neural tube. The anterior ten pairs of nerves in clycostomes have their exit through foramina in the cranium and hence are known as cranial nerves. It is not unlikely that all correspond to anterior nerves of Amphioxus, e.xcept the optic which is a fiber tract of the brain and not a true peripheral nerve. The ten cranial nerves are the olfactory (I), optic (II), oculo- motor (III), trochlearis (IV), trigeminal (V), abducens (VI), facialis (VII), auditory (VIII), glossopharyngeus (IX), and vagus (X). In lower vertebrates the hypoglossus and spinal accessory nerves are not cranial but spinal. Of the ten cranial nerves, I, II, and VTII are sensory. III, IV, and VI somatic motor, and the others mixed sensorv and motor. THE NERVOUS SYSTEM 353 The so-called nervus terminalis appears not to be an independent nerve, but a component of the olfactory. The remaining neurites of the olfactory are processes of neurosensory cells in the olfactory epithelium. The optic nerve develops in correlation with the eye, the retina, from which the optic nerve fibers arise, being a segregated part of the wall of the telencephalon. Some of its fibers cross below the brain in front of the infundibulum to form the optic chiasma. The optic nerves, after entering the wall of the diencephalon, pass by way of the optic thalami to their reflex centers in the roof of the mesencephalon. The oculomotor, a somatic motor nerve with its nucleus or motor center in the base of the mid-brain, innervates four eye muscles, the superior, inferior, and anterior recti and the inferior obUque. Sympathetic non-medullated fibers presumably occur in the oculomotor, but no distinct sympathetic ganglion is formed. The trochlearis arises from a motor nucleus in the floor of the meten- cephalon posterior to that of the oculomotor and supplies the superior oblique eye muscle. Its fibers emerge from the medulla near the root of the profundus. The dorsal chiasma of the trochlearis appears to be absent in cyclostomes. The trigeminus is so named because, in all vertebrates, it has three chief branches, a sensory ophthalmicus profundus which extends above the eye to the skin on the upper side of the snout, a sensory maxillaris branch to the skin on the side of the snout and the region which cor- responds to the upper jaw of fishes, and a mixed mandibularis branch which supplies the skin and muscles of the first visceral arch. Cyclostomes are the only vertebrates in which the profundus branch arises by an independent root. This fact supports the conclusion that the profundus was once an independent segmental nerve and that its union with the trigeminus in all vertebrates above cyclostomes is secondary. The motor center of the trigeminus is in the lateral column of the medulla. A unique feature of the trigeminus is a sensory nucleus in the roof of the mesencephalon, the fibers of which bring nerve impulses from mandibular muscles. Most of the sensory fibers of the trigeminus arise from ganglion cells in the large Gasserian ganglion near the nerve's root of origin from the medulla. With few exceptions, the sensory nerves of all vertebrates have similar ganglia near their roots of origin. The abducens is a somatic motor nerve which emerges from the medulla ventral and posterior to the root of the trigeminus and innervates the posterior or external rectus eye muscle. The facialis carries both special and general somatic sensory fibers, and also visceral sensory and motor fibers. The motor fibers arise from an elongated nucleus in the lateral column of the medulla, and supply muscles of the hyoid arch. There are four major branches. The Sensory 354 CHORDATE ANATOMY lahralis visctrol molot viicerol scvibori^ QeneraX culaivtoui Fig. 312.— Diagrams of the branches and components of (A) the trigeminal, (B) facial, and (C) glossopharyngeal and vagus nerves of a lower vertebrate, b. buccalis nerve; cl. chorda tympani; d, dorsal rami of IX and X; g, gastric nerve; gg. in .4. Gas- serian ganglion, in B, geniculate ganglion; h, hyoid nerve; hm, hyomandibular trunk; J, Jacobson's connective; jg, jugular ganglion; I, lateralis nerve; 7nxe, maxillaris externus nerve; op, ophthalmicus profundus nerve; os, superficial ophthalmic nerve; pal, palatine nerve; po, pr, post- and pretrematic rami; sp, spiracle; st, nerve to supratemporal lateral hne organs; 1-5, gill clefts. (From Kingsley's" Comparative Anatomy of Vertebrates.") THE NERVOUS SYSTEM 355 ramus ophthalmicus superficialis innervates the supraorbital series of lateral-line organs. The buccalis supplies the infraorbital series. The deep palatme branch is distributed to the skin of the roof of the mouth. The hyomandibular innervates the muscles of the hyoid arch and the skin of the mandibular region. The auditory supplies the otic vesicle which in cyclostomes is chiefly an organ of equilibration. As might be expected from its origin as a branch of the facial nerve, its roots are closely associated with those of the facial. Since the otic vesicle is a modified lateral-line organ, the fibers of the auditory nerve belong to the group of lateralis or special somatic sensory components. The glossopharyngeal is the mixed nerve which supplies the third visceral arch. It forks over the first gill-sht and a pretrematic branch is distributed to the posterior wall of the hyoid arch. The post-trematic branch contains sensory fibers from the floor of the pharynx and motor fibers which innervate the muscles of the third arch. The vagus is a mixed nerve formed of fibers which are distributed to the muscles and skin of the posterior visceral arches. This suggests that a number of segmental nerves are united in the vagus. A lateralis branch is the nerve of the posterior series of lateral-line organs. A visceral branch goes to heart, stomach, and intestine, and carries sympathetic fibers connected with these organs. Each branchial branch divides into pre- and post-trematic rami, which contain both visceral sensory and visceral motor fibers. In the trunk region there is a general correspondence between the number of myotomes and of spinal nerves, since for each myotome there are usually a sensory and a motor nerve. In Petromyzon these are not united; but in myxinoids they join to form a mixed spinal nerve with two roots, a dorsal ganglionated sensory root and a ventral motor root, each neurite of which arises from a multipolar ganglion cell located in the gray matter of the cord. Synaptic connexions between sensory and motor neurons take place within the gray matter. Peripherally, each spinal nerve divides into dorsal and ventral rami which supply skin and muscles. This simple one-to-one metameric correspondence of spinal nerves and myotomes is, however, somewhat modified in the occipital region of Petromyzon where the first five post-otic myotomes are innervated by the nerves of the fourth and fifth myotomes, and the nerves of the three anterior myotomes have disappeared, at least as independent roots. The nerves of post-otic myotomes 6 to 1 2 unite to form the hypoglossal nerve which supplies the hypobranchial muscles. In this fusion of occipital nerves may be seen the beginnings of the cervical plexus which persists throughout the vertebrate series. Since paired appendages are wanting in cyclostomes, no thoracic or lumbar plexuses are formed. 356 CHORDATE ANATOMY Little can be said concerning the sympathetic nervous system of cyclostomes. The nerves of cyclostomes, like those of Amphioxus, are not medullated, so that this means of distinguishing sympathetic from other fibers cannot be used. A number of observers claim to have found Cerebellum Olfcictury bulh Olfactory tract Vagus nerve N. X Glossopharyngeal nerve N. IX / / Acoustic nerve N. VIII / Abducens nerve N. VI ! I I optic nerve N. II Inferior lobe I i Oculomotor nerve N. Ill ' Saccus vasculosus Trochlearnerye N. IV Trigeminal and facial nerves Nn. V, VII Fig. 313. — The brain of the dogfish, Squalus acanthias, lateral view. (From Ranson's "The Anatomy of the Nervous System," courtesy of W. B. Saunders Company.) clusters of sympathetic ganglion cells associated with the vagus. Also a plexus comparable to Auerbach's plexus of higher vertebrates is found in the intestinal wall. Chromaffin cells, which have an origin in common with sympathetic cells, are found in the trunk region and have a segmental distribution as in mammals. Optic lobe Epiphysis' Mesocwle Paraphysis Cerebral hemisphere \ j Olfactory tract \ " ' ' Olfactory bulb Cerebellum i Metacxle Tubercuhtm acusticum chorioidea Fourth ventricle ccral lobe Telencephalon / i ! i j '. Mctencephalon Mydenccphalon Preoptic recess • 'it >.i f rj , ,1 Mesencephalon Velum transver sum , [ Saccus vasculosus Optic chiasma Xhird ventricle Fig. 314. — The brain of the dogfish, Squalus acanthias, medial sagittal section. (From Ranson's "The Anatomy of the Nervous System," courtesy of W. B. Saunders Company.) Elasmobranchs. The nervous system of elasmobranchs shows some advances above that of cyclostomes. The roof or pallium of the telen- cephalon has thickened and expanded. To the corpus striatum is added an epistriatum, both connected with olfactory fibers. The telencephalon THE NERVOUS SYSTEM 357 remains predominantly an olfactory center. Elongated olfactory tracts are differentiated. A saccus vasculosus, which possibly functions as a pressure organ, is appended to the infundibulum. The mid-brain has lost its chorioid plexus and its roof has become thickened and wholly nervous. With the increased importance of the lateral-Hne organs, their Nasal capsule /Olfactory nerve A'. I / Rhinocoele — Olfactory bulh Nervus terminalis Olfactory tract - - ■ Cerebral hemisphere- Interventricular for Epiphysis Optic verve N. II Thalamu: Optic lobes Trochlear nerve N. IV i — Cerebellum ■Lobus lineae lateralis ■ Facial nerve N. VII - ,^^ -J Medulla oblongata Glossopharyngeal nerve N. IX — Acoustic nerve N. VIII Tuberculiim acusiicum ■Medial longitudinal fasc. Visceral lobe- ^._. Vagus nerve N.X Third ventricle Diencephalon Mesoccele Mesencephalon ■ Spinal cord ^Fourth ventricle Metencephalon / \Cerebellum '^ {caudal part) /Fourth ventricle I Myelencephalon Fig. 315. — The brain of the dogfish, Squalus acanthias, dorsal view. (From Ranson's "The Anatomy of the Nervous System," courtesy of W. B. Saunders Company.) Fig. 316. — The brain of the dogfish. Squalus acanthias, with the ventricles opened, dorsal view. (From Ranson's "The Anatomy of the Nervous System," courtesy W. B. Saunders Company.) centers in the lateral lobes of the medulla are more developed. Possibly for the same reason, the cerebellum, which is a static center, is greatly enlarged. In the spinal cord, dorsal and ventral columns of gray matter are differentiated. The dorsal column, however, remains unpaired. The cranial nerves are identical with those of cyclostomes but rela- tively enlarged both in correlation with the increased size of sense organs 358 CHORDATE ANATOMY and muscles, and by the addition to the nerve fibers of medullary and neurilemma sheaths. A cranial sympathetic ganglion, the ciliary, has developed in association with the oculomotor and profundus nerves. The profundus has now become a branch of the trigeminal. Some of the supraorbital series of lateral-line organs are innervated by fibers of the superficial ophthalmic branch of the fifth nerve, while the remainder are supplied by the superficial ophthalmic of the facialis. The somatic motor nerves of five post-otic myotomes unite to form the hypoglossal, which supplies hypobranchial muscles. A thoracic plexus is formed by the union of the nerves immediately posterior to those of the cervical plexus. But the number of nerves which participate varies greatly in different elasmobranchs. In many species the fibers of the cervical and thoracic plexuses unite as a cervico-thoracic plexus. In the region of the pelvic fin is a similar but smaller lumbo-sacral plexus. Fig. 317. — Brain of Protopterus, a dipnoan fish, ch, cerebellum; e, epiphysial structures; h, hypophysis; i, infundibulum; tn, mid-brain; se, saccus endolymphaticus; sp, spinal nerves; /, cerebral hemisphere; I— 12, cranial nerves. (From Kingsley's "Comparative Anatomy of Vertebrates," after Burckhardt.) Well developed sympathetic ganglia appear in the trunk region, in the vicinity of the dorsal aorta. Their arrangement is metameric, but the anterior and largest is formed by the union of primarily separate ganglia. Each is connected by a ramus commuiiicaiis with a spinal nerve. A longitudinal sympathetic cord or connective is only imper- fectly developed. An intestinal plexus occurs, as in all vertebrates. Amphibia. Amphibia have a relatively simple brain like that of cyclostomes and dipnoans. Olfactory lobes are relatively large and merge without constriction into the cerebral hemispheres. The pallium is thick, and cells have migrated from the gray matter into the external marginal zone of white matter. The lumen of each hemisphere is reduced by the thickening of its median and lateral walls. An inner longitudinal sulcus or groove divides these walls into dorsal and ventral halves. The dorsal half of the lateral wall is the paleocortex, ventral to it is the epis- triatum, and below the epistriatum is the paleostriatum. In the median wall, the dorsal half is the archicortex or primordium hippocampi. The medio- ventral wall forms the septum by which fibers pass to and from the hippocampus (Fig. 324). The hemispheres are interconnected by anterior, anterior pallial and posterior pallial commissures located in the lamina THE NERVOUS SYSTEM 359 terminalis. Habenular and posterior commissures persist in the roof of the diencephalon. The epiphysis forms a pineal gland. A chorioid plexus invaginates into the third ventricle. The saccus vasculosus of fishes has disappeared. The thickened walls of the mid-brain reduce the lumen to a narrow passage, the aqueduct. The cerebellum is rudimentary like that of cvclostomes. Fig. 318. — Side and dorsal views of brain of young alligator, c, cerebrum; cl, cerebellum; e, epiphysial structures; /;, hypophysis; i, infundibulum; ol, optic lobes; II-XII, cranial nerves. (From Kingsley's "Comparative Anatomy of Vertebrates," after Herrick.) The ten cranial nerves of fishes persist in Amphibia. Urodeles have lateralis nerves, but these disappear in the Anura in correlation with the loss of lateral-line organs. With the loss of gills, the number of branches of the vagus is reduced. Cervical and lumbar enlargements of the spinal cord appear in cor- relation with the enlargement of the appendages. Sympathetic nerve cords or connectives unite the series of sympathetic ganglia. Reptiles. The cerebral hemispheres of reptiles are larger than those of Amphibia and by extension caudad have partially overgrown the i6o CHORDATE ANATOMY diencephalon. The paired ventricles are nearly obliterated by the enlarge- ment of the striate bodies, archistriatum and neostriatum. For the first time in the vertebrate series, a cortical layer of pyramidal cells appears in the pallium, having nervous connexions with fibers of the olfactory tract. Septum and hippocampus appear in the medial wall much as in Amphibia. The dorsal wall of each hemisphere is homologized with the gyrus dentatus of the mammalian brain. The transitional Fig. 319. — Brain of goose, ac, anterior commissure; cb, cerebellum; e, epiphysis; /, flocculus; h, hypophysis; hs, hyperstriatum; i, infundibuluni; /, lateral ventricle; m, medulla oblongata; ms, mesostriatum; ob, olfactory bulb; ol, optic lobe; s, striatum; i, temporal lobe; III, third ventricle; x, plane of section E. (From Kingsley's "Com- parative Anatomy of Vertebrates," after Butschli.) region between this dorsal pallium and the neostriatum has important potentialities, since it is unconnected with olfactory fibers, and since in mammals it becomes the neopallium from which develops the greater part of the cerebral cortex on which the higher psychic activities of man depend. (Figs. 318, 324) In the region of the diencephalon of lizards the anterior epiphysial outgrowth, the parietal organ, develops a lens, retinal and pigment layers, and nerve fibers which are connected with centers in the brain wall. It is therefore, so far as its structure goes, an eye. The thalamic thicken- ings of the lateral wall divide the third ventricle into a dorsal and ventral cavity connected by a narrow slit-like passage. In snakes the optic lobes THE NERVOUS SYSTEM ,^(5i become corpora quadrigemina by the division of each optic lobe into anterior and posterior moieties. The cerebellum is slightly larger in reptiles than in amphibians. The gray matter of the spinal cord as seen in cross section assumes the form of a capital H with dorsal and ventral columns, as in mammals. In reptiles occipital vertebrae fuse with the cranium. Consequently, two nerves, the spinal accessory which innervates shoulder and neck muscles and the hypoglossus which supplies the tongue, both being spinal in lower vetebrates, now become cranial. Each arises by a series of segmentally arranged roots, and is therefore believed to be formed by Fig. 320. — Median section of brain of calf, a, aqueduct; ac, anterior commissure; cc, corpus callosum;/, fornix; h, habenula; hy, hypophysis; i, infundibulum; itn, inter- mediate mass ("soft commissure"); -mb, mammillary body; ob, olfactory bulb; oc, optic chiasma; ol, optic lobes; p, pinealis; pc, posterior commissure; r, recessus suprapinealis; s, septum pellucidum; III, IV, third and fourth ventricles. (From Kingsley's "Com- parative Anatomy of Vertebrates," based on a figure by Biitschli.) the union of a number of spinal nerves. Other changes in the nervous system in reptiles are relatively unimportant. Mammals. The brains of lower mammals differ little from those of reptiles. Within the mammalian group, from monotremes to man, there is an enormous enlargement of the cerebral hemispheres and of the cerebellum. The expansion of the hemispheres affects chiefly the neopal- lium, the beginnings of which were noted in reptiles. The archipallium of reptiles, which serves chiefly as an olfactory center, becomes in mam- mals the hippocampal lobe. As a result of the growth of the neopallium, the hippocampus is crowded to the lower part of the brain. Increase in the size of mammalian brains is accompanied by com- pHcation in form and structure. The cortex becomes mainly cellular, and consequently gray. The amount of cortical material increases many-fold, so that if the human cortex were spread out flat it would cover 362 CHORDATE ANATOMY a surface eighteen inches square. The number of neurons runs into the biUions, and five layers of cells may be distinguished. The increase in the mass of the cerebral hemispheres as we pass from lower to higher mammals is the result, not of multiplication of layers of neurons in the cerebral cortex, but of folding of the cortex. A notable development is that of the anterior pallial commissure, which enlarges enormously to form the corpus callosum, interconnecting the two hemispheres. The olfactory lobe degenerates and is covered by the hemispheres. The corpora striata elongate caudally and rest MASSA INTERMEDIA THALAMI SULCUS CINGUL GYRUS CINGULI SUPERIOR FRONTAL GYRUS FORNIX frontal pole ' anterior commissure- terminal lamina optic nerve- optic chiasma'' infunoibulum hypophysis' mammillary body oculomotor nerve THALAMUS 'CENTTRAL SULCUS TELA CHORIOIDEA C3RD VEKTT.) CORPUS CALLOSUM , POSTERIOR COMMISSURE , PINEAL BODY SUBPARIETAL SULCUS ' LAMINA QUADRIGEMINA OCCIPITAL LOBE CALCARINA FISSURE LINGUAL GYRUS OCCIPITAL POLE PONS' FOURTH VENTRICLE' MEDULLA ' CENTRAL CANAL ' ^'CEREBELLUM TELA CHORIOIDEA t4TH VENT.) \^-SPINAL CORD Fig. 321. — The human brain in median section and aspect. (Redrawn after Sobotta.) Besides the enormous enlargement of the cerebral hemispheres and cerebellum in the human brain, the great size of the corpus callosum and the pons Varolii is noteworthy. upon the thalami. The epiphysis forms a gland, the pineal. Cerebellum and pons enlarge. The pons is a bridge of nerve fibers, present only in birds and mammals, which extends around the brain-stem ventral to the cerebellum, and which connects the two halves of the cerebellum. EVOLUTION OF THE BRAIN Comparison of vertebrate brains from cyclostomes to man reveals a gradual and progressive change such as would be expected if the higher forms have evolved from the lower. The cerebral hemispheres are the least conservative region. Although the hemispheres are enormously enlarged in man, the differences between man and apes are quantitative rather than qualitative. Even the speech center in the frontal lobes, which is said to be peculiar to man, is but an enlargement of regions already THE NERVOUS SYSTEM 363 developed in apes. The brains of such fossil types as the Java and Peking man are transitional between those of modern man and apes. Nor is the gap between the brain size of mammals and that of reptiles formidable. In dorsal view, all five divisions of the primitive brain are visible alike in monotremes and alligators. The overgrowth of the hemi- spheres, begun in reptiles, reaches its climax in man, whose domination in the animal world may be ascribed to the enlargement of his conscious control centers in the cerebral hemispheres. The cortical enlargement in mammals, however, involves more than an increase of gray matter. Correlated with the multiplication of cells C. TARSIER. D. MARMOSET Fig. 322. — Diagrams of the brains of insectivores and of lower primates viewed from the left side. The figures show the increasing dominance of the centers of vision over those of smell. A, brain of jumping shrew. B, brain of tree shrew. C, brain of the primate Tarsius. D, brain of the marmoset. (Redrawn after G. Elliot Smith.) is an increase in the number of nervous interconnexions. Association fiber tracts connect all parts of the enlarging brain so that all regions, however remote from one another, are interconnected. The brain pro- duced by these evolutionary changes is an organized and integrated whole, no part of which appears to function independently of the rest. In primates a marked retrogression of the olfactory lobes accompanies the enlargement of the hemispheres. The olfactory centers in the hip- pocampus persist, but other regions of the cortex enlarge disproportion- ately. Vision in primates is more important than smell, and changes in the brain express relative functional values. Cortex. Two parts of the primitive vertebrate brain participate especially in the great enlargement of the cerebral hemispheres, the striate 264 CHORDATE ANATOMY body and the neopallium. These two regions are not clearly differentiated in cyclostomes, but are distinguishable in fishes. Of the two, the pallium changes more. The hemispheres of teleost fishes have a thin epithelial pallium or mantle. Compared with the mantle of teleosts, that of elasmo- branchs and dipnoans, which are more dkectly in the fine of mammaUan ancestry, is relatively thick. Homologies with the palHum of higher vertebrates are difficult on account of the lack of differentiation. In the palHum of fishes the cellular gray matter is adjacent to the ventricle, while the external layer is fibrillar. Even in the palUum of fishes, however, some cells migrate from the gray into the fibrillar zone. (Fig. 323) The palHum of Amphibia, taking Rana as a type, is thick, and is differ- entiated into a median archicortex and a lateral paleocortex, both asso- A ' ^ B C ' ■ D Fig. 323. — Horizontal diagrams of ichthyopsid brains. A, sturgeon; B, elasmo- branch; C, teleost; D, amphibian. Primitive fore-brain wall stippled; telencephalic evaginations horizontally lined; thalamus vertical lines, c, common ventricle, /, inter- ventricular foramen; /, lateral ventricle; m, mid-brain; o, olfactory bulb; t, terminal lamina; II, optic nerve; 3, third ventricle. (From Kingsley's " Comparative Anatomy of Vertebrates," after Herrick.) ciated with olfactory fibers. In reptiles the number of cell layers in the pallium increases to three. The medio-dorsal region of each hemisphere forms an archicortex or hippocampus. In the lateral pallium, dorsal to the striate body, are possibly the beginnings of a neocortex. (Fig. 324) A true many-layered neocortex appears in all mammals and enlarges so much that the paleocortex is crowded into a ventral position and the archicortex pushed dorsally toward the median plane. The number of cell layers has increased until five are distinguished in most, if not all, mammals. The evolution of the cortex is accompanied by cellular changes. In the pallium of lower vertebrates, cell bodies lie close together. The thickening of the cortex in mammals is correlated with separation of the cells, which, however, retain connexion with one another by means of elongated dendritic processes, the number of interconnexions with adjacent THE NERVOUS SYSTEM 36: neurons increasing with the multiplication of dendrites. In general, the higher the animal, • the longer and more numerous the dendrites, and consequently the greater the possible number of interrelationships between cellular elements. A correlated increase appears in the number of asso- ciational fiber tracts which connect the gyri or folds of the cortex. NEOCORTEX MEDIAL AREA EPISTRIATUM A LEPIDOSIREN A UPIOOSIREN .CORTEX OF g \ HIPPOCAMPUS EPISTRIATUM B. RANA. VENTRAL C. LACERTA. Fig. 324. — Cross sections of the left cerebral hemisphere of. A— A', a fish (Lepidosiren), B, an amphibian (Rana), and C, a reptile (Lacerta), showing the increasing relative importance of the epistriatum. The epistriatum arises dorsal to the striatum as a local thickening of the ventro-lateral wall of the hemisphere. (Redrawn from Plate, after Kuhlenbeck.) In the millions of years which elapsed during the Tertiary period, there was a marked increase in the size of mammalian brains. This increase affected all parts, and was accompanied by a corresponding increase in the size of the cranium. The growth of the cerebral cortex was, however, out of all proportion to the enlargement of the rest of the brain. This increase was made possible by the complex folding of the outer layers of the brain and resulted in the formation of the gyri and 366 CHORDATE ANATOMY sulci which are such a characteristic feature of the surface of the human brain. Changes in the striate body (basal gangUon) are summarized in Fig. ^24. In mammals each striate body extends posteriorly and rests upon the thalamus — Uke a sack of flour on a horse's back. Brain Coimnissures. Commissures are fiber tracts which cross the median plane of the body and bring lateral halves of the nervous system into relation with one another. Some of those in the brain persist through- \^y/^\ MYELENCEPHALDN ■pic,. 325. — A diagram of the brain of a four-months fetus as seen in median longi- tudinal section. The figure shows the location of the more important brain commis- sures. (Redrawn from Coming's "Human Embryology," after Burckhardt.) out the vertebrate series, and serve as important topographic landmarks to determine homologous regions. (Fig. 325) Commissures have not been demonstrated in Amphioxus. In the terminal lamina of the telencephalon of cyclostomes are two, a more ventral anterior and a more dorsal pallial. Both connect the olfactory lobes with the hippocampi of the opposite side. Two habenular ganglia in the roof of the diencephalon are connected by the habenular commis- sure. Fibers from the hemispheres and also from the hypothalamus are contained in this commissure. Another commissure in the roof of THE NERVOUS SYSTEM 367 the brain, the posterior commissure, marks the boundary between (Hen- cephalon and mesencephalon. A dorsal commissure in the roof of the mid-brain connects the optic lobes. In addition to the commissures, the fibers of two cranial nerves, the optic and the trochlearis, cross the median plane to form chiasmas. The optic chiasma is ventral and just anterior to the infundibulum. The trochlearis chiasma, which occurs in all vertebrates except cyclostomes, lies in the dorsal constriction which separates mes- and metencephalon. The dorsal commissure is lacking in fishes. In amphibians and reptiles a dorsal pallial or hippocampal commissure, connecting right and left hippocampi, adds a third to those located in the terminal lamina of the brain. In tnonotremes and marsupials both anterior and posterior pallial or hippocampal commissures occur. The corpus callosum of placental mammals is a new commissure — possibly derived from the anterior pallial — which connects the two halves of the neocortex. Its enlargement is correlated with that of the cerebral cortex. EVOLUTION OF THE SPINAL CORD The spinal cord is a much more conservative portion of the central nervous system than the brain. Consequently, although the contrast between the so-called brain of Amphioxus and that of man is so very great that their homology may be doubted, the spinal cords of these chordate extremes are recognizably similar. Both are tubular and both have a central mass of gray matter and an external layer of fibrous tissue. The relations of dorsal and ventral nerve roots are similar. The differences are bridged over by intermediate conditions in the vertebrate series. The fact that the brain of Amphioxus differs from its spinal cord, not in greater size but chiefly in the expansion of its lumen, appears to support the inference that spinal cord and brain were originally undifferentiated from one another. The same columns of gray matter are recognizable in both, although, because of the absence of myeUn nerve sheaths, Amphi- oxus lacks the color contrast between white and gray matter. This conclusion is substantiated by evidence from ontogenesis, which shows that the distribution of nervous matter is essentially similar in cord and brain. The spinal cord of Amphioxus is somewhat triangular in cross section, with the apex of the triangle dorsal, the base resting upon the notochord. The small amount of gray matter lies close to the sHt-like central canal. The lateral walls are much thicker than the dorsal and ventral, since fibrillar material which makes up most of the substance of the cord is wholly lateral in position. Some cellular differentiation into sensory and motor ganglion cells is visible in the gray matter, ependymal cells being the most abundant. Sensory ganglion cells connect with the dorsal 368 CHORDATE ANATOMY nerves, and giant neurochord cells extend across the central canal. The fibers of the giant nerve cells, after decussation, extend lengthwise of the cord, some caudad and some cephalad. Since these do not form nerve 'CENTTRAL CA^aAL Fig. 326. — Cross sections of the spinal cord of various vertebrates: — A, Amphioxus; B, Petromyzon; C, Squalus; D, Rana; E, Alligator and F, Homo. The magnification is not to the same scale. The section of the cord of Amphioxus is enlarged four times as much as that of Petromyzon. In these two forms the axons are non-meduUated. The striking differences between the cord of Amphioxus and that of man are bridged over by intermediate conditions in lower vertebrates. DORSAL NUCLEUS ,FASC GRACiLIS (CLARKE^ COLUMN}^ GROUND BUNOLD DORSAL ROOT SUBSTANTIA CELATINOSA^ POSTERIOR COLUMN LATERAL CELLS LAT COLUMN CELLS - MIDDLE CELLS - !'DORSO-LATERAL- FASC. CUNEATUS INTERMEDIATE ^yENTRO- LATERAL ANT. \ VENTROMEDIAL COLUMN '-^ CELLS SENSORY TRACT lissauer's marginal ZONE "^ASC CEREBELLO -SPINALIS - FASC CEREBRO-SPINALIS LAT ventral root/-- ^^ dorsomedial' fasc anterolateral superfic spino-thalamic tract "helweg's bundle fasc anterior pr0priu3 anterior marginal bundle Fig. 327. — A diagram of a cross section of the spinal cord, showing the fiber tracts or fasciculi, and the arrangement of nuclei in the gray matter. (Redrawn after Sobotta.) fibers which leave the cord, it is supposed that the giant fibers give off collaterals to the motor centers along the cord, and serve to correlate their activities. (Fig. 326) THE NERVOUS SYSTEM 369 The spinal cord of cyclostomes is much flattened, with the cellular matter distributed in a pair of lateral wings. Ventral to the central lumen, a number of giant or Muller's fibers extend lengthwise and carry from the brain impulses thought to be chiefly static. Dorsal to the lumen, are sensory ganglion cells like those of Amphioxus. The outer fibrous or marginal layer of the cord is divided into longitudinal bundles of fibers, the funiculi. Medullary sheaths are lacking, so that sympathetic flbers can not be distinguished from others. Relations of neuron in the cord indicate that it is a reflex center and a pathway for intersegmental nervous connexions. Increase in descending fiber tracts demonstrates the increas- ing dominance of the brain. Polynuclear gland cells, possibly of endocrinal function, occur in the caudal region of the cord. (Fig. 326) In elasmobranchs nerve fibers are myelinated, so that white and gray matter show the same contrast as in the cord of higher vertebrates. Dorsal and ventral columns of gray matter are differentiated, but the dorsal columns merge together in the median plane. Somatic motor cells of the ventral column are very large, as in other fishes, amphibians, and reptiles; and the dendrites extend into the dorsal column. Sensory ganglion cells, except the embryonic and transient Rohon-Beard ceUs, have migrated into the spinal ganglia. Dorsal, lateral, and ventral funiculi have relations similar to those of higher vertebrates. The enlarge- ment of the lateral walls of the cord results in the formation of a deep ventral fissure. The suggestions of the formation of a dorsal septum are, however, slight. (Fig. 326) The spinal cord of Amphibia resembles in fundamental characters that of elasmobranchs. The dorsal columns of gray matter become more distinctly paired, so that the gray matter assumes in cross section the form of a capital H characteristic of all higher animals. In the gray matter the nerve cells retain their central position surrounded by a network of fibers and their synaptic connexions. A dorsal septum has developed in connexion with the increased thickness of the dorsal portion of the lateral wall of the cord. In the cervical and lumbar regions the diameter of the cord is considerably increased in correlation with the enlargement of the appendages. (Fig. 326) The spinal cord of reptiles difi"ers in no essentials from that of mammals. The increase in thickness of the marginal layer of longitudinal fibers indicates an increased integration of the body. The fibers which ramify through the gray matter are non-medullated, and their color is gray in contrast with the white of the medullated fibers of the marginal zone. Within the gray matter of the cord, sensory and motor nerves of reflex arcs usually effect their synaptic connexions by the intermediation of association neurons located in the gray matter. The intermedio-lateral column, which throughout the vertebrate series contains the ganglion 370 CHORDATE ANATOMY cells of the motor nerves which supply visceral muscles, becomes more distinctly demarked than in lower vertebrates. The gray matter crosses the median plane of the cord as the gray commissure which surrounds the central canal. A considerable increase in the amount of white matter in mammals indicates a further dominance of the brain and a greater integration of the body. The relative amount of white matter diminishes from the medulla to the filum terminale in which the spinal cord ends. In the region of the arms and legs, at the cervical and lumbar enlargements, both gray and white matter increase in quantity. The division of the white PARIETAL LOBE INTERNAL CAPSULE CEREBRAL PEDUNCLE TEMPORAL LOBE CORTICAL MOTOR AREA CAUDATE NUCLEUS LENTICULAR NUCLEUS VEtTTRAL CEREBRO-SPIMAL TRACT U^TERAL CEREBRO-SPIMAL TRACT, (-VENTRAL ROOT OF SPINAL NERVE SPINAL CORD Fig. 328. — Diagram of the descending (pyramidal) conduction paths. (Redrawn after Morris.) matter into funiculi, begun in cyclostomes, reaches its climax in man. Dorsal, lateral, and ventral funiculi are separated from one another not only by dorsal and ventral nerve roots, but also by external dorso-lateral and ventro-lateral grooves or sulci, which extend lengthwise of the cord. The medullary sheaths of the fibers which compose the funiculi develop at different times in ontogenesis as the fibers come into functional activity. By the study of the time of myelination of fibers and their degeneration after they are cut, it has been learned that fibers of similar origin and function occur in bundles or tracts. Each funiculus consists of a number of such tracts, together with groups of tracts or fasciculi. A tract or a fasciculus may contain either ascending sensory fibers or THE NERVOUS SYSTEM 37i 372 CHORDATE ANATOMY descending motor fibers. A fasciculus may contain both kinds. Fibers may have their origin or termination at any level and relatively few extend the entire length of the cord. (Fig. 329) Neuron Relations in the Cord. The spinal cord is both the center for reflexes and the pathway for impulses towards and away from the brain. In the simplest possible reflex action where only two neurons are involved, the synaptic connexions between the two lie in the gray matter of the cord. The cell body of the afferent neurone is in the sensory gangHon of the afferent nerve. That of the effector neuron is in the central column of the spinal cord, and its neurite extends by way of the ventral root to a muscle fiber or a gland. Usually, however, more than two neurons are chained together in a reflex act, for there may be one or more association neurons, located in the gray matter of the cord, which carry the impulse from the receptor neuron to the effector neuron. But it should not be understood that a somatic motor nerve cell located in the ventral column has synaptic connexions with the telodendria of only a single neurite. On the contrar}-, many neurites may have synaptic relations with the dendrites of each somatic motor neuron. The motor neuron is simply "the final common path." Within the gray matter of the spinal cord, the central connexions of the neurites of an afferent neuron may be of various kinds. The telodendria may connect directly with the dendrites of a somatic motor cell. This is the simplest relation. They may connect with a neuron in Clark's column near the median line at the base of the dorsal column, the nervous impulse being carried in the dorsal cerebellar tract; or they may pass to a commissural neuron, the impulse being carried to the opposite side of the spinal cord. There are also connexions with sym- pathetic neurons. The neurites of receptor neurons, having entered the spinal cord, immediately dichotomize to give off long ascending and short descending branches and thus add to the fibers of the fasciculus cuneatus. As these fibers pass towards the brain, they are displaced inwards by the fibers which are added from higher levels. The result of this process is that, in the neck region, the neurites which enter the cord in the lower trunk region come to lie in a median fasciculus, the fasciculus gracilis. Most of the descending short fibers end in the gray matter of the cord. From both ascending and descending neurites, fine collateral branches pass into the dorsal column of gray matter, and come into synaptic connexions with dendrites of the same or of the opposite side. The fibers which enter the spinal cord by the dorsal root are somatic afferent or visceral afferent, depending upon their peripheral connexions. THE NERVOUS SYSTEM 373 The visceral motor fibers which appear in the dorsal roots of lower vertebrates are not found in the dorsal spinal nerves of mammals. The neurites of the ventral roots are purely efiferent. Of these there are two kinds, somatic motor and visceral motor. The somatic innervate skeletal muscles derived from the mesodermal somites, the visceral are Docsal Root Venirat Roai Ramus dor^alis Fig. 330. — Diagram of the nerve components of a spinal nerve. Somatic motor fibers are indicated by continuous lines; visceral motor in long broken lines; somatic sensory in short broken lines; visceral sensoi»y by fine dotted lines. (From B. Patten, after Froriep.) connected through the sympathetic ganglia with visceral involuntary muscles or with glands. The ganglion cells of both types lie within the gray matter of the spinal cord; but those of the somatic motor nerves lie in the ventral column, while the visceral motor fibers come from ganglion cells in the intermedio-lateral column. See Fig. 330. 374 CHORDATE ANATOMY EVOLUTION OF THE CRANIAL NERVES The two anterior pairs of nerves of Amphioxus are purely sensory, while all posterior dorsal nerves are mixed. The second pair of sensory nerves of Amphioxus may be the homologues of the ophthalmic nerves of craniotes. The four mixed nerves posterior to the ophthalmic have been compared respectively with the trigeminal, facial, glossopharyngeal, and vagus. It is probable, however, that at least three segmental nerves are represented in the vagus nerve of vertebrates. Of the somatic motor CIRRI AMPHIOXUS. VELUM Fig. 331. — A diagram showing the distribution of the anterior nerves of Amphioxus. The two most anterior nerves of Amphioxus lie anterior to the myotomes and are purely sensory. The remaining nerves are either mixed like those shown in the figure or are purely motor. The former alternate, the diagram shows, with the myotomes, while the latter are metameric in position. (Redrawn after Hatschek, slightly modified.) nerves of vertebrates, the oculomotor is possibly represented in the first ventral root of Amphioxus. The number of cranial nerves varies in craniotes. Fishes and amphib- ians have ten pairs; reptiles, birds, and mammals have twelve. These, beginning with the most anterior, are: I. Olfactory. Special afferent. II. Optic. Sensory. A specialized fiber tract of the brain. III. Oculomotor. Somatic efferent, with some visceral efferent fibers. rV. Trochlearis. Somatic efferent. V. Trigeminus, (ieneral somatic afferent and visceral efferent. VI. Abducens. Somatic efferent. THE NERVOUS SYSTEM 375 VII. Facialis. Mixed visceral afferent and efferent. Special somatic afferent. VIII. Acoustic. Special somatic afferent. IX. Glossopharyngeal. General and special visceral mixed. Somatic sensory (?). X. Vagus. General and special visceral mixed. Special somatic afferent. XI. Accessory. Somatic efferent (?), and general and special visceral efferent. XII. Hypoglossal. Somatic efferent. From the evolutionary standpoint it is a significant fact that man has the same ten cranial nerves as are found in fishes and amphibians. To HYPOPHYSIS ANTERIOR PERFORATED SUBSTANCE SEMILUNAR GANGLION OPTIC CHIASMA TUBER CINEREUM MAMMILLARY BODIES LATERAL GENICULATE BODY MASTICATOR NERVE ■V CEREBELLUM Fig. 332. SPINAL CORD -Human brain stem, showing nerve conne.xions. after Allen Thomson.) (Redrawn from Morris, these, two nerves are added in amniotes as a result of the addition of vertebrae to the occipital region of the skull, making twelve cranial nerves altogether in amniotes. Between fish and man in the course of phylogenesis few changes in individual cranial nerves have occurred. In all forms above cyclostomes the trochlearis has a dorsal chiasma which is lacking in cyclostomes. The factors in its formation are unknown. The ophthalmicus profundus, which is an independent nerve in cyclo- stomes, becomes united with the trigeminal in remaining vertebrates beginning with the elasmobranchs. With the disappearance of lateral- 376 CHORDATE ANATOMY line organs in land animals the lateralis components of cranial nerves disappear. In mammals the facial nerve becomes largely motor. But these are all minor changes. Cranial nerves appear to be among the most conservative of vertebrate organs. EVOLUTION OF SPINAL NERVES Phylogenetic changes in the spinal nerves are not very great. Man has thirty-one pairs of nerves connected with the spinal cord. These are Fig. 333. — The twelve cranial nerves shown as if projected upon a median section of the head. I. Olfactory lobe. II. Optic. III. Oculomotor. IV. Trochlearis. V. Trigeminus. VI.Abducens. VII. Facialis. VIII. Acusticus. IX. Glossopharyngeus. X. Vagus. XI. Accessorius. XII. Hypoglossus. metamerically arranged just as are the nerves of Amphioxus. In Amphio.xus nerves are of two sorts, dorsal (sensory and motor) and ven- tral (somatic motor). The two kinds alternate with one another and do not unite. Among vertebrates, however, the spinal nerves of Petro- myzon alone show this primary independence. In all other vertebrates THE NERVOUS SYSTEM 377 the dorsal and the ventral nerve of each trunk segment unite to form a mixed nerve. In this way every spinal nerve possesses two "roots," a dorsal sensory and a ventral motor root. Another phylogenetic change is the loss of motor fibers in the dorsal roots. This takes place in the vertebrate series. Anamnia retain in their dorsal spinal roots the visceral motor fibers characteristic of the dorsal nerves of Amphioxus. In amniotes, however, these visceral motor fibers have their exit from the spinal cord by way of the ventral roots. The meaning of this shift is not clear. ixbcd. FiG- 334- — -4, diagram of collector nerve; B, of a nerve plexus. (After Braus.) C, brachial plexus of Salamandra maculata. (From Kingsley's "Comparative Anatomy of Vertebrates," after Fiirbringer.) Nerve plexuses are not found in Amphioxus. A cervical plexus appears in cyclostomes. In higher vertebrates four plexuses arise — cervical, brachial, lumbar and sacral. To form a plexus, the fibers of a number motor nerves unite in an interlaced network. In this way any muscle may be innervated by more than a single nerve and thus a summation of stimulation is effected. THE AUTONOMIC NERVOUS SYSTEM In the higher chordates a special system of nerves, the autonomic, is distributed to the smooth muscles of the digestive and circulatory systems 378 CHORDATE ANATOMY OLFACTORY BULB- OCCIPITAL LOBE- SYMPATHETIC GANGLIA' RAMI COMMUNICANTES- FROfTTAL LOBE SYMPAT«TIC TRUNK SPLANCHNIC NERVE- TEMPORAL LOBE ST CERVICAL NERVE (•- BRACHIAL PLEXUS ST THORACIC NERVE NAL GANGLION LUMBAR NERVE LUMBAR PLEXUS ST I SACRAL NERVE FILUM TERMINALS'' ' XCOCCYCEAL NERVE Fig. 335. — The brain and spinal cord of man, in ventral aspect, shown in relation to nerve roots and the chief autonomic ganglia. (Redrawn from Morris, after Allen Thomson.) THE NERVOUS SYSTEM 379 and to many other organs. This autonomic system is most evident in the chain of sympathetic ganglia which lie along the dorsal aorta from the neck to the sacrum. These, however, are only a portion of the autonomic system, which is distinguished by functional rather than anatomical characteristics, for the cerebro-spinal system is so intimately connected with autonomic nerve fibers that the two systems cannot be separated SKIN SOMATIC, <^v.,-H MOTOR GANGLION CELLS tLSOMATlC ^ MUSCLE SYMPATHETIC GANGLION VISCERAL MOTOR ^,<^. FIBERS ^^' Fig. 336. — A diagram of neurons of the spinal cord and spinal nerves shown in their relations to one another and to their end-organs. Sjmatic sensory fibers are shown by continuous lines, somatic motor fibers by fine dots. Visceral sensory fibers are indi- cated by short broken lines, visceral motor by long broken lines. (Redrawn after Plate.) anatomically. The vagus nerve, for example, which seems to be a part of the cerebro-spinal system, contains many autonomic fibers connected with the nervous plexuses of the viscera. Moreover, each sympathetic ganglion of the trunk has fiber connexions with a spinal nerve and with the plexuses of the intestine. (Figs. 335, 336, 337) Autonomic nerves are connected not only with the digestive and circulatory systems but also with respiratory and urogenital systems, 38o CHORDATE ANATOMY STOM -^^ PAN '"v>'>"-^ fPREGANG. testis' UTERUS' I UTERINE flex! nUUM TERMINALE SYMPATHETIC . p^^ FIBERS^ p^; IHAEMORHOID PLEX. PARASYMPATHETIC i I POST '^ Fig. 337. — The autonomic nervous system in man. Autonomic ganglia are lettered, autonomic nerves given Arabic numerals, cranio-spinal nerves are indicated by Roman numerals. Relations to brain and spinal cord are shown to the right by a series of cross sections taken at various levels, i, lacrimal nerve; 2, oculomotor nerve; 3, nasociliary nerve; 4, Gasserian ganglion; 5, ramus maxillaris; 6, posterior nasal nerve; 7, vidian nerve; 8, superficial petrosal n.; 9, deep petrosal n.; 10, chorda tympani n.; II, minor superficial petrosal n.; 12, lingual nerve; 13, vagus nerve; 14, inhibitor cordis nerve; 15, broncho-dilator nerves; 16, accelerator cordis n.; 17, vertebral nerve; 18, major splanchnic nerve; 19, minor splanchnic nerve; 20, hypogastric nerve; 21, pelvic nerve; 22, nervus erigens. A, ciliary ganglion; B, spheno-palatine gang.; C, otic gang- lion; D, carotid ganglion; E, sublingual ganglion; F, superior cervical ganglion; G, stellate ganglion; H, celiac ganglion; /, inferior mesenteric ganglion. (Redrawn from Ariens Kappcrs, after L. R. Miiller.) THE NERVOUS SYSTEM 38 1 cndocrinal and other glands, and the skin, so that there are few parts of the body which autonomic fibers do not reach. Except possibly the autonomic fibers connected with the ciUary muscle of the eye, autonomic nerves, although markedly influenced by the emotions, are not under the control of the will. Two kinds of autonomic nerves, sjnnpathetic and parasympathetic, may be distinguished on the basis of their antagonistic action and their different response to drugs. A sharp distinction between the sympathetic and parasympathetic fibers cannot be drawn on the basis of function. The sympathetic fibers are usually excitatory; the parasympathetic are usually inhibitory. Most organs of the body have this double innervation and the action of the two kinds of nerves is antagonistic. But some parasympathetic fibers, as for example those in the vagus nerve, are excitatory. There are three distinct groups of autonomic nerve fibers, a cranial group, a thoracico- lumbar group, and a sacral or pelvic group. Of these, the cranial and sacral elements are parasympathetic, and hence are grouped together as the cranio-sacral division of the autonomic system. The thoracico-lumbar division constitutes the sympathetic portion of the autonomic system. Autonomic nerves may be classified also, on the basis of their distribu- tion, into somatic fibers which innervate the blood-vessels of the body- wall and the smooth muscles of skin and sweat glands, and visceral fibers which supply the glands and smooth muscles of the viscera. The somatic fibers act upon the hairs to stimulate their erection and cause " goose flesh." They also serve the important function of regulating temperature by influencing the tonus of the capillaries in the skin and thus, by changing the rate of blood flow, altering the amount of secretion of the sweat glands. (Fig. 117) The course followed by sympathetic and parasympathetic fibers within the central nervous system is almost unknown. Evidence is not lacking that stimulation of the cerebral cortex may be followed by reactions of the viscera. Three kinds of autonomic fibers connect with sympathetic ganglia. Preganglionic fibers are visceral efferent fibers which come from ganglion cells located in the lateral column of the spinal cord and have their ter- minations in sympathetic ganglia. Postganglionic fibers are also visceral efferent and have their cell bodies within sympathetic ganglia and their telodendria upon smooth muscles of the intestine and of the blood-vessels. The preganglionic fibers are medullated and form the white rami com- municantes which connect spinal nerves with sympathetic ganglia. The postganglionic fibers are rarely medullated. They pass either to the viscera by way of sympathetic nerves or to the body-wall and skin by way 382 CHORDATE ANATOMY of the spinal nerves with which they are connected through the gray rami communicantes. Visceral afferent fibers, the third type, with cell bodies in the dorsal ganglia, carry impulses directly from visceral parts to the gray matter of the cord. Visceral afferent fibers having cell bodies in the sympathetic ganglia have not been demonstrated, the cells of sympathetic ganglia being exclusively motor. Most, if not all, actions of the autonomic system are reflexes mediated through the brain or cord. Some intestinal reactions, however, may occur after all nerve connexions with the cord and brain have been severed. It is possible, therefore, that some visceral reflexes pass through the intes- tinal plexuses only. Three kinds of autonomic ganglia may be distinguished, ganglia of the S5mipathetic trunk, collateral ganglia such as the celiac and mesenteric located in the wall of stomach and intestine, and terminal ganglia like the ciliary and cardiac located in the organs which they innervate. Evolution of the Autonomic System The nervous system of coelenterates is a plexus of primitive ganglion cells connected with neurosensory cells and smooth muscle fibers and located between the two primary body layers. This persists as an intes- tinal plexus in other invertebrates from flatworms to molluscs and insects. That the intestinal plexus of vertebrates is homologous with that of invertebrates has not been demonstrated beyond a reasonable doubt, but may be assumed in the absence of evidence to the contrary. The late ontogenetic appearance of the plexus in vertebrates does not, however, harmonize with this assumption, since, in the light of the fundamental law of biogenesis, we should hardly expect the most ancient part of the nervous system to be one of the last to appear in the embryo. On the other hand, we may have here another example of retarded development, of which there are numerous examples in ontogenesis. Moreover, the rela- tions of the myenteric and submucous plexuses of the walls of the stomach and intestine resemble those of the invertebrate intestinal plexuses, and they are equally autonomic in their functions. Finally, in elasmobranchs, sensory cells in the wall of the alimentary canal form a part of the system as in invertebrates. Evidence of similar cells in mammals is wanting. In invertebrates and vertebrates alike the evolution of the autonomic system keeps pace with that of the digestive and circulatory systems. Sympathetic and parasympathetic systems are recognized in arthropods; but no structures are homologous with the autonomic ganglia of vertebrates. As a system, therefore, the sympathetic of vertebrates is a new addi- tion which arises late both ontogenetically and phylogenetically, Kap- THE NERVOUS SYSTEM 383 pers, however, calls attenlion to the fact that in arthropods the intestinal plexuses, like the parasympathetic system of vertebrates, are limited to the cerebral and caudal part of the intestine. So far as sympathetic ganglia are concerned, chordates appear to start with a clean slate, since there is no evidence of sympathetic ganglia in any of the protochordates. The autonomic system is, however, represented in protochordates, as in vertebrates, by the visceral nerves, motor and sensory. In the myxinoids the intestinal plexuses develop exclusively from the brain region, mostly from the vagus nerve. Tretjakoff has found in Petromyzon autonomic fibers in all spinal nerves. Some sympathetic and parasympathetic ganglia are found in elasmobranchs where, as in the higher vertebrates, the ciliary ganglion is parasympathetic and connected with the oculomotor and ophthalmic nerves. Several sympathetic ganglia occur in the trunk in connexion with a limited number of spinal nerves. The metameric arrangement seen in the embryos is modified in the adult through fusion. No longitudinal connectives are found in elasmo- branchs. But Allis has described, in the head region of teleosts, segmental autonomic gangha chained together by connectives. Nothing similar has been found in other vertebrates. Fig. 338. — Sympa- —,, . . , . ... thetic system of right i he autonomic system m tetrapods IS essentially side of a frog. Somatic similar to that of man. A shift in the relations of "^rves dotted, sym- pathetic black, a, atlas; at, common in- testinal artery; ao, aorta; c, coccyx; cr, crural nerve; j, jugal ganglion; i, sciatic nerve; r, radices aortae; s, base of skull; sp, splanchnic nerve; st, sympathetic trunk; ill, ilio-hypogastric nerve; II-XI, second to elev- enth trunk nerves. (From Kingsley's "Comparative Anat- . omy of Vertebrates," Ihe central nervous system of vertebrates arises after Gaupp.) as a thickened placode of dorsal ectoderm anterior to the blastopore. This placode is known as the neural plate. Next to the notochord, the nervous system is the first organ to develop. By the elevation of its edges, the neural plate is converted into a neural groove the autonomic fibers occurs in phylogenesis. In Anamnia, visceral motor fibers have their cell bodies in the lateral horn and have their exit from the tube by way of the dorsal roots; those in the thoracico- lumbar and sacral region of amniotes, on the other hand, enter the ventral or somatic motor roots. In the head region, the connexion with dorsal roots is maintained throughout the vertebrate series. DEVELOPMENT OF THE BRAIN 384 CHORD ATE ANATOMY bordered by neural folds. The anterior more widely expanded portion forms the brain, and the narrower posterior portion the spinal cord. The transition between the two is, however, in most animals, gradual rather than abrupt. MID- HINDBRAIN Ialar pl>te ibasal plate '""' ^_1__1 jfloor plate - •' — -» MESENCEPHALON EPIPHYSIS METENCEPHALOM NOTOCHORD MYELENCEPHALON TELENCEPHALON PREOPTIC RECE Fig. 339. — Diagrams of the development of the brain. A, early neural plate before closure, with zones marked; B, longitudinal section of early brain tube; C, later stage with parts differentiated. The dorsal zone (alar plate) is finely stippled; the ventral zone (basal plate) is coarsely stippled; the floor plate is cross-hatched. Cer., cere- bellum; c.str., corpus striatum; inf., infundibulum; mam., mammillary recess; olf., olfactory lobe; pal., pallium; thai., thalamus. (After Kingsbury, modified.) CEPHALIC FLEXURE DIENCEPHALON TELENCEPHALON I. FOREBRAIN ^ CEREBELLUM OPTIC VESICLE OLFACTORY LOBE IKIFUNOIBULUM MAMMILLARY BODY OTIC CAPSULE MESENCEPHALON EPIPHYSIS DIENCEPHALON TELENCEPHALON METENCEPHALON YELENCEPHALON OLFACTORY LOBE PONTINE FLEXURE CERVICAL FLEXURE SPINAL CORD A. B. C. Fig. 340. — Three stages — A, B, and C — in the development of the human brain, showing the brain vesicles and flexures. A is an early stage, dorsal aspect, B the brain of a three-weeks embryo in lateral aspect. C that of an eight-weeks embryo in lateral aspect. (Redrawn after His and Hardesty.) As the neural folds rise, they bend towards the median plane and finally unite to form a neural tube with an anterior enlarged brain and a posterior constricted spinal cord. The closure of the neural tube begins in the neck region and proceeds craniad and caudad. But even before the neural folds in the cephalic region unite, a series of three expansions THE NERVOUS SYSTEM 385 appear, corresponding with fore-brain, mid-brain, and hind-brain, the fore- brain differing from the other two in being anterior to the notochord. From these, by processes of local unequal growth, all the parts of the definitive brain are differentiated. Experiments demonstrate that the position of the three brain divisions is predetermined in the open neural plate. Soon after their closure and expansion as mid-brain and hind-brain vesicles, the lateral walls become divided by a longitudinal sulcus into a ventral basal plate and a dorsal alar plate. Less clearly seen is a narrow floor plate in the mid-ventral line, and a roof plate in the mid-dorsal line. Since this sulcus does not develop in the fore-brain, and ends where the notochord ends, it appears that the fore-brain consists of alar and roof plates only. By the time a human embryo is a month old, the primitive fore-brain vesicle has begun to divide into the anterior telencephalon and posterior diencephalon. In a five-weeks embryo, the hind-brain has begun to divide into the anterior metencephalon and posterior myelen- cephalon. The undivided mid-brain is the mesencephalon. These five Brain Vesicles and Their Derivatives (After Keibel and Mall) Primary vesicles Secondary vesicles Derivatives Ventricles Telencephalon Olfactory lobes Corpora striata Cerebral cortex Optic thalami Lateral (ist & 2nd) ventricles Anterior part of 3rd ventricle Fore-brain Diencephalon Epithalamus (pineal gland, etc.) Thalamus Hypothalamus Infundibulum Tuber cinereum Mammillary bodies Posterior lobe of pi- tuitary Posterior part of 3rd ventricle Mid-brain Mesencephalon Corpora quadrigemina Tegmentum Crura cerebri (pe- duncles) Aqueduct Hind-brain Metencephalon Cerebellum Pons I'ourth ventricle Myelencephalon Medulla 386 CHORDATE ANATOMY brain regions occur in all vertebrates, and from them all the parts of the adult brain are formed. Brain Flexures. While the subdivision of the primary vesicles is taking place in ontogenesis, the brain undergoes, in amniote embryos, three successive flexures, the cephalic or primary, the pontine, and nuchal or cervical. The cephalic flexure occurs in the mid-brain region, the other two in the region of the hind-brain. All three flexures are in a vertical plane, but the bend of the pontine is the reverse of the other two. The bending is presumably the result of the elongation of the brain in limited space, since the brain elongates more rapidly than does the head itself. The cephalic flexure is well marked in the embryos of Anamhia, but the pontine and nuchal flexures scarcely appear. They become increasingly evident as we pass from lower to higher amniotes. (Fig. 340) DEVELOPMENT OF THE SPINAL CORD When the neural plate becomes the neural tube, its wall is a simple columnar epithelium. In consequence of more rapid cell proHferation, erve root D. 100mm. E. Adult Fig. 341. — -Transverse sections through spinal cord of the pig at various ages. Note especially the parts of the adult cord derived from the ependymal, mantle, and marginal layers of the embryonic neural tube. (From Patten's "Embryology of the Pig.") limited to a layer of germinal cells which lie near the lumen of the cord, the lateral walls become greatly thickened, while the mid-dorsal and mid- ventral portions remain as thin roof and floor plates. This thickened lateral wall becomes divided by a median longitudinal sulcus into a dorsal alar plate and a ventral basal plate. Three layers are differentiated in the lateral walls of the cord, an ependymal layer next to the lumen, a thick mantle layer of spindle-shaped THE NERVOUS SYSTEM 387 cells, and an outer marginal layer of fibers free from cells. The marginal layer increases in thickness by the addition of fibers which grow lengthwise of the cord. By the addition of myelin sheaths to these fibers, the marginal zone is converted into the white matter of the cord. As the lateral walls increase in thickness, the lumen of the cord is narrowed down so that in a cross section it appears as a slit which, how- ever, widens out slightly at the level of the longitudinal sulcus. Finally, the lateral walls fuse together, except in the ventral region, where a portion of the lumen is left as the central canal of the cord. The plane of fusion of the ependymal cells persists in the adult as the dorsal septum. On the ventral side of the cord, however, a median fissure is formed as a result of the increase in thickness of the lateral walls and the failure of the floor plate to grow. The mantle layer becomes the gray matter of the cord. As a result of unequal growth of this layer, dorsal, lateral and ventral gray columns develop. By the time the human embryo is three months old, the gray matter has assumed in cross section its charac- teristic H-shape. In the early stages of the spinal cord, two kinds of cells are differ- entiated, germinal cells which be- come neuroblasts, and non-nervous spongioblast cells. Following the the bundle of neurites passes a neuroblast, a stimulofugal process is formed. Later by a reverse or stimulopetal process the neuroblast is drawn towards the activating bundle and dendritic processes grow towards the bundle. Successive stages in the proc- ess are shown beginning at the top of the diagram. (Redrawn after Ariens-Kappers.) Fig. 342. — A diagram illustrating the theory of neurobiotaxis of Ariens-Kappers. The diagram represents a series of motor neuroblast cells, lying in the wall of the spinal cord, as activt^ted to form neurites by a longitudinal bundle of neurites. As outgrowth of the neurite from a neuroblast, a number of dendrites grow in the opposite direction. According to the theory of "neuro- biotaxis" of Kappers, the neurite is stimulofugal, that is, grows away from the source of stimulus, and the dendrites are stimulopetal, that is, they grow towards the stimulus. Nerve cells of two kinds arise, motor, which are Hmited to the anterior and lateral gray columns, and association cells, which may lie in the dorsal column. Two types of supporting cells develop from the spongioblasts, ependy- mal cells with elongated processes which extend radially from the central canal to the periphery of the cord, and neuroglia cells which have shorter and more numerous processes and which do not extend through the entire thickness of the wall. Some of the spongioblasts form transient 388 CHORDATE ANATOMY neurilemma cells which enclose the neurites while the myehn sheaths are formed. Most of these disappear in the adult cord. Some of the ependymal cells persist as the epithelial lining of the lumen of the cord. The marginal layer gradually thickens by the addition of fibers, some of which grow craniad and some caudad, and most of which soon acquire a myehn sheath. With the appearance of dorsal and ventral nerve roots in connexion with the spinal cord, the marginal zone of white fibers becomes divided into dorsal, lateral, and ventral funiculi. The time of myelination of fibers differs in the several tracts, depending upon the time when they begin to function. Some, such as the pyramidal, become medullated only after birth. neural ■^r — ectoderm neural tube Fig. 343. — Drawing showing closure of the neural tube and formation of the neural crest. From pig embryos of : — A, 8 somites; S, 10 somites; C, 11 somites; D, 13 somites. X135. (From Patten's "Embryology of the Pig.") Development of Motor Nerves. How nerve and muscle become con- nected with one another has been a much discussed problem in biology. According to Hensen and Kerr the connexion is primary and not second- ary. Nerve and muscle are assumed by them to be, like all the other cells of the body, in protoplasmic continuity which is never completely broken when cells divide. It has, however, been the general opinion of neurologists that the connexion of nerve and muscle is acquired second- arily. According to Francis Balfour chains of cells derived from the central nervous system form the motor nerves. Kupffer, however, claimed that a neurite grows out as a protoplasmic process from a motor ganglion cell and that neuro-muscular connexions are therefore secondary. Many years later Harrison demonstrated experimentally that Kupflfer's assumption is correct. It is this process theory of neurogenesis which underlies Kappers' theory of neurobiotaxis. THE NERVOUS SYSTEM 3S9 Development of Sensory Nerves. While the neurites of cranio-spinal motor nerves are formed as outgrowths of neuroblastic cells located in C NODOSUM\ C PETHOSUM- OTIC C /ACCESSORY NERVE i;;;7CERVICAL GANGLIA /AORTA Fig. 344. — The autonomic system of a 16 mm. human embryo. The sympathetic trunk is shown in solid black. The intestine is stippled. (Redrawn from Bremer, after Streeter.) SPINAL CORD DERMATOME SOMATIC MOTOR NERVE MYOTOME ENTERON SYMPATHETIC GANGLION PRIMITIVE DUCT- COELOM SENSORY GANGUON NOTOCHORD HYPOCHORDA "DORSAL AORTA POSTCARDINAL VEIN SPINAL NERVE VITELUNE VEIN MESENCHYME Fig. 345.- — A cross section of a 1 7 mm. cla^mobranch (Squalus) embryo, in the trunk region, showing an early stage in the formation of sympathetic ganglia. The yolk-sac to which the embryo is attached has been removed. the wall of the central nervous system, those of the sensory nerves are formed as processes of ganglion cells located in the sensory ganglia. 390 CHORDATE ANATOMY From each of these cells protoplasmic processes extend in two directions — one towards the central nervous system and one towards the periphery. The olfactory nerve is peculiar in its derivation from cells in the olfactory pit which extend their neurites towards the brain and hence are unipolar. The neural crest is formed of cells left between the neural tube and the skin when the neural tube separates from the skin. The crest, which is primarily continuous, becomes secondarily broken up into the series of cranial and spinal ganglia. Development of S3nnpathetic Ganglia. The sympathetic ganglia of vertebrates are derived, like the neurilemma cells, from the dorsal (sensory) ganglia by the migration of cells ventrally along the nerves toward PLEXUS OF AUERBACH^^ PLtXUS OF UElSSNERIk jLONGITUDINAL -PERITONEUM Fig. 346. — A stereogram of a portion of tlie small intestine, showing the arrangement of sympathetic neurons in the plexuses of Meissner and Auerbach. Motor cells are shown in black, sensory cells with white nuclei. (Redrawn after Kahn.) the dorsal aorta. They first appear as clusters of cells, each cluster connected with the nerve from which it arose, at the level of the aorta. In the head the ciliary, sphenopalatine, otic, and submaxillary ganglia are formed in this way. In the trunk the superior and inferior cervical ganglia, and the series of vertebral and prevertebral ganglia belonging to the sympathetic are derived from the neural crest by the prolonged migration of nerve cells. In the sympathetic ganglia the nerve cells "spin" the postganglionic fibers to the blood-vessels and viscera. Con- nexions with the nerves from which the sympathetic ganglia arise appear as rami commimicantes. The metameric ganglia become secondarily connected by a sympathetic cord which runs parallel with the dorsal aorta. The prevertebral plexuses, cardiac, celiac, and hypogastric, arise by the more extensive migration of cells from the ganglia of the vagus nerve. THE NERVOUS SYSTEM 391 The most extensive cellular migration leads to the formation of the myen- teric and submucous plexuses (the plexuses of Auerbach and of Meissner). But, however remote from their source such sympathetic cells may be, they retain fibrous connection with the rest of the nervous system. (Fig. 346) The whole sympathetic system is well established in a three-months human embryo. MENINGES The spinal cord of Amphioxus is surrounded by loose connective tissue. In cyclostomes this tissue shows the beginnings of differentiation CORPUS CALLOSUM TaXNCEPHALON THIRD VENTRICLE OPTIC NERVE- LATERAL VEirmiCLE ISTERNA SUPERIOR NTORIUM REBaj-l SUR+ INF. COLLICUU CEREBELLUM CHORIOID PLEXUS CI STERNA SUBDURAL CAVITY ARACHNOI dura mater :entral canal subarachnoidal space subdural cavity filum terminaie Fig. 347. — Diagram showing the relations of the meninges to the central nervous system, as shown in median longitudinal section and in cross section. (Redrawn after Rasmussen's "Principal Nervous Pathways," The Macmillan Co.) into compact outer and inner layers with loose tissue between, the three representing possibly the three meningeal layers of higher vertebrates. In fishes the cranium and the vertebrae are lined by compact periosteum or perichondrium, between which and the brain or cord the connective tissue is loose, except where the connective tissue comes in contact with the central nervous organs. There it becomes the highly vascular meninx primitiva. Such connective-tissue membranes or meninges 392 CHORDATE ANATOMY surrounding the central nervous organs (brain and cord) serve both for protection and for nourishment. Two such meningies surrounding brain and cord occur in Amphibia, a pia mater primitiva next to the brain and cord and, outside this, a dura mater. The wide space between dura mater and periosteum is bridged by connective-tissue trabeculae. In mammals three meninges are dififerentiated. Innermost is the pia mater, thin and highly vascular, from which connective-tissue processes grow into brain and cord carrying in blood-vessels, and supporting the nervous tissue. Outside this is the arachnoid which, as its name sug- gests, is a delicate web-like tissue. Only its outer layer is organized into a membrane. Outermost of the three is the dura mater, thickest and toughest of all, and more or less closely attached to the periosteum which lines the cranium and the vertebral canal, so that this periosteum is sometimes reckoned as a part of the dura mater. Where the dura mater of mammals penetrates between the cerebral hemispheres it forms the falx cerebri. A similar fold of the dura mater grows between the hemispheres and the cerebellum to form a tentorium cerebelli. AH three meninges develop from the loose mesenchyma which sur- rounds the embrvonic neural tube. CHAPTER 14 THE SENSE ORGANS The sense organs of animals consist of sensory cells or groups of cells adapted to respond to stimuli and transmit an impulse to the nerves. The essential property of sense cells is, therefore, irritability. Like nerve cells they are able to transmit impulses caused by physical and chemical changes, either in the environment or within the organism. The receptor organ may be either the neurosensory cell itself, or a secondary sensory cell which transmits a stimulus to the nerve. In higher animals, moreover, sense organs, in addition to receptor cells, have various mechanisms for protecting and supporting the sensory cells or conveying stimuli to them, so that great diversity of receptor cells and of sensory nerve terminations has arisen during the course of evolution. Vertebrates possess a considerable number of special senses. They may be classified as major or minor, according to their importance. Major senses are touch, taste, smell, hearing, and sight. Minor, in addi- tion to the muscular and visceral senses, include heat, cold, pain, hunger, thirst, fatigue, sex, and equilibrium. It is evident that this division of the senses rests on their conscious accompaniments. On the basis of the source of stimulus, senses are exterior — sight, hearing, taste, smell, pressure, heat, cold; or interior — pain, hunger, thirst, the muscular and visceral senses, equilibrium, lust. Some physiologists recognize as many as thirty-six special senses, separated for the most part by differences in sensation. Certain it is that we have more than the traditional five. EVOLUTION OF SENSE CELLS Sense organs and sensory nerves alike begin in responsiveness to stimuli, such as is manifested by Amoeba which reacts to changes in pressure, light, heat, chemical substances, and electricity. The first sense cells differentiated are the neurosensory cells of coelen- terates, which establish nervous connexion with underlying muscle cells by means of protoplasmic processes, while the body of the cell remains in the external epithelium. Each cell may have a stiff sensory bristle or hair. An advance towards conditions in higher animals is taken when the body of the cell recedes from the surface, retaining connexion by means 393 394 CHORDATE ANATOMY of an elongated process. Usually the outer termination of such a cell is beset with one or more hair-like sensory processes. Neurosensory cells of this sort have a wide distribution in the animal kingdom. Those of the olfactory epithelium of vertebrates are at this stage. A third stage is represented by sensory neurons which have lost their connexion with the external surface, but retain "free" nerve termina- tions in the epidermis. Such a cell is bipolar in form with both cutaneous and central connexions. Similar neurons may terminate within the underlying corium, where they may branch freely in the connective tissue, A. NEUROSENSORY CELLS OF AN^EUD. B NEUROSENSORY CELLS OF OLFACTORY MEM- BRANE OF MAN. C. PRIMARY SENSORY CELL D. SECONDARY SENSORY WITH FREE NERVE CELLS CONVEY IMPULSES TERMINATION. TO THE PRIMARY SENSORY CELL. Fig. 348. — Stages in the hypothetical evolution of secondary sense cells. A and B, diagrams of neurosensory cells in invertebrate (A) and vertebrate (B). C, a sensory cell (neuron) with free nerve termination. D, secondary sense cells convey impulses to the primary sensory cell. The series assumes that the definitive receptor cells are secondary. The possibility that the neurosensory cells become the definitive sense cells and that the sensory nerve is secondary must be admitted. (Redrawn after Fritz Kahn, "Der Mensch," Albert Miiller, Zurich.) end between tactile cells, or become surrounded by a connective-tissue capsule. A definitive evolutionary stage is attained when a secondary sensory cell becomes the receptor element by means of which a stimulus is transferred to the dendrites of a neuron. The receptor cells of the taste-buds and the hair-cells of the cochlea represent a final stage of this sort. The substitution of a secondary sensory cell for the primary neurosensory one presents a problem which has never been satisfactorily solved. That such an evolutionary change has occurred seems indisput- able, but transitional stages are wholly conjectural. Among the factors which have led to the formation of the sense organs of higher animals may be noted the tendency of the neurosensory THE SENSE ORGANS 395 cells, which were primarily scaltered and separate, to become concentrated in clusters to form the sense organs. Secondary sense cells show the same tendency, and the cephalization of the nervous system is correlated with the concentration of the sense organs in the head region. Attention has already been called to the fact that the three primary divisions of the brain are associated with three major sense organs, the olfactory, the eye, and the ear. CUTANEOUS SENSES At least four human senses, pressure or touch, pain, warmth, and cold, are based upon nerve terminations in the skin. The experiments of Goldschneider and others demonstrate that, corresponding with these four sensations, there are four different sorts of nerve terminations. These may be either free or encapsuled. Usually the free nerve terminations are located in the basal layers of the epidermis, and are therefore more superficial, while the encapsuled terminations lie in the corium below. Evolution of Cutaneous Sense Organs It may be assumed that the various cutaneous sense organs were derived from neurosensory cells of the epidermis, which by the outgrowth of neurites became connected with the nerve-net or cord. When the body of the cell gradually migrated into the underlying connective tissue and the epidermis became many-layered, all connexion with the surface was lost, and the neurosensory cell was converted into a sensory neuron, with free nerve terminations in the lower layers of the epidermis and central connexions with the nerve cord. As a last step, connexion with the epidermis was lost, and the peripheral termination of the sensory cell was buried in the corium. (Fig. 349, A, C) All these stages are represented in chordates. The skin of Amphioxus, for example, is beset with many neurosensory cells, both single and in clusters, many of which possess a stiff terminal bristle which projects above the general surface. Amphioxus also has sensory nerves with free nerve terminations branching among the epithelial cells. The encapsuled nerve terminations have apparently followed two independent lines of evolution. On the one hand, free nerve terminations in the corium have become encapsuled by concentric layers of connective- tissue cells, as represented in the corpuscles of Pacini, Krause, and Golgi- Mazzoni. On the other hand, some nerve terminations are associated with tactile cells, which primarily were located in the basal layer of the epidermis, but which later migrated into the corium. In some instances, a single lenticular tactile cell may rest upon a cup-shaped termination of a sensory neurite, or the nerve may branch among a cluster of such cells. As a final evolutionary stage, a cluster of tactile cells connected 396 CHORDATE ANATOMY with the dendrites of a sensory nerve may become encapsuled by con- nective tissue to form a Meissner's corpuscle. Free nerve terminations occur in the skin of all classes of chordates, usually in the form of multiple arborizations or. dendrites. These may lie in the epidermis or in the corium, in either case being located where they may respond to changes in pressure. Such free nerve terminations in the skin are found in all classes of vertebrates, and are believed to be CLUB-SHAPED TACTILE CORP (KRAUSE) ENCAPSULED NERVE- KNOT- (GOLGI-MAZZONI)' <;?^r^5— EPI DERM I S FREE NERVE - ENDING IN EPIDERMIS FREE NERVE- ENCAPSULED GROUP OF TACTILE CELLS (MEISSNER) DIAGRAMS B.-D. CUTANEOUS SENSE ORGANS IN CRANIOTES WITH SEODNDARY SENSE CELLS. Fig. 349. — Varieties of cutaneous sense organs in chordates. A, B, C show sensory terminations in the skin of Acrania. B' to D' show varieties of terminations in craniotes. C to C* and D' to Z?'* respectively represent stages in the hypothetical evolution of encapsuled nerve terminations. (Redrawn after Plate.) the sensory mechanism of painful sensations arising in the skin. The sense of touch apparently depends chiefly upon the tactile cells or cor- puscles in the corium, of which various forms occur. Meissner's corpuscles, present only in primates, are located in the corium papillae of the palms and soles, and in the external genital organs. Each corpuscle consists of a group of tactile cells surrounded by a rela- tively thin envelope of connective tissue and connected with one or more nerve fibers. The non-medullated nerve fiber twists spirally among the tactile cells, each of which is in contact with a reticular nerve termination. In birds and reptiles nerve endings are connected with tactile cells of Merkel but without a connective-tissue capsule. These tactile cells THE SENSE ORGANS 397 are sometimes solitary, sometimes clustered. On the other hand, the Grandry's corpuscles of birds are encapsuled, and the nerve termination lies between two tactile cells. In the relatively small corpuscles of Krause and in the large one of Pacini, both found in mammals, the nerve termination is club-shaped and encapsuled. In a Pacini corpuscle are as many as eight concentri- cally arranged lamellae. A secondary or adjunct nerve fiber penetrates the capsule and forms a varicose network within the inner lamella. Pacini's corpuscles are located not only in the deeper layers of the skin, hut also in the mesenteries, tendons, and periosteum. Those present in tendons give rise to sensations which serve to indicate the position of a limb. Those in the deeper parts of the body probably give rise to painful sensations associated with disease. The Golgi-Mazzoni corpuscles are spherical or club-shaped tactile corpuscles in which the coiled nerve termination is enclosed by, but not in contact with, the surrounding capsule. In them, both chief and adjunct nerve fibers are present. These corpuscles occur in the corium, the peritoneum, and in the conjunctiva of the eye. The genital cor- puscles found in the corium of the glans penis are supposed to be the sensory mechanism associated with sexual desire. Free nerve terminations are formed by the outgrowth of sensory nerve fibers from the sensory ganglia. Of these, some remain free, some effect connexion with tactile cells, some become enveloped by connective- tissue capsules. LATERAL-LINE ORGANS Lateral-Une organs are a specialized type of cutaneous sense organs limited to fishes and water-dwelling amphibians. It is believed that they respond to currents of water and to sudden changes in pressure. Although , among cutaneous sense organs, the arrangement in rows is peculiar to the lateral-hne organs, this linear arrangement is presumably second- ary, since both invertebrates and vertebrates have, scattered over the surface of the body, sensory papillae or neuromasts similar to those of the lateral-line organs. Dorsal, lateral, and ventral rows of lateral-Une organs occur in verte- brates on each side of the body. Usually all three are present only in embryonic and larval stages. The lateral rows persist in the trunk region of adult fishes and urodeles. Three fines, however, are present in the head, supraorbital, infraorbital, and mandibular, innervated by branches of the facial and vagus nerves and exceptionally by a branch of the glossopharyngeal nerve. A supratemporal line may connect the systems of the two sides across the posterior part of the skull. 398 CHORDATE ANATOMY A sense organ of the lateral line consists of a cluster of pear-shaped hair cells, each of which is connected with a branch of the lateral-line nerve. Usually the cluster of sense cells is encircled by a ring of columnar epi- thelial cells. The sensory cells of neuromasts, unlike those of taste-buds, do not extend to the base of the epithelium. It is generally assumed that neuromasts have evolved from clusters of neurosensory cells like those of lower invertebrates. Similar clusters occur in the oral tentacles of Amphioxus. Later in phylogenesis, however, sensory cells of the secondary type become the sensory elements of the neuromasts. How this change occurred is problematic. Fig. 350. — Head of pollack, showing lateral-line canals and nerves of the lateralis system. Lateralis nerves black, canals and brain dotted, b, buccalis ramus of VII nerve; dl, dorsal ramus of lateralis of X nerve; h, hyomandibularis nerve; hm, hyo- mandibular line of organs; io, infraorbital line; I, lateral-line canal; n, nares; o, olfactory lobe; op, operculum; os, ophthalmicus superficialis nerve; soc, commissure connecting lines of the two sides; so, supraorbital line of organs; st, supratemporal part of lateral line; vl, ventral ramus of lateralis of X nerve; x, visceralis part of X nerve. (From Kingsley's "Comparative Anatomy of Vertebrates," after Cole.) The stages in the evolution of neuromasts are repeated in ontogenesis and also represented in the lateral-line organs of adult craniotes. Pri- marily, the neuromasts are scattered over the surface of the body, but appear first in the head region. Those which become components of the lateral-hne organs sink into grooves, which extend from the ear region both craniad and caudad. There is evidence that neuromasts originally had a metameric arrangement, but this metamerism is usually lost through the subdivision and multiplication of the primary clusters of sensory cells. In cyclostomes and tailed amphibians each neuromast sinks into a separate pit. In the fishes lines of neuromasts sink below the surface. In Chimaera the grooves remain open through life. In other groups of THE SENSE ORGANS 399 fishes, this being the definitive condition, the grooves are converted into mucus-filled canals with occasional pores opening to the surface. Lateral-line organs, although limited to the Ichthyopsida, are of special interest to morphologists, since the ear appears to be derived from one or more lateral-line organs. Fig. 351. — Stereogram of lateral-line organs of a fish, c, lateral-line canal; In, lateralis nerve; p, pores connecting with the exterior; s, scales in skin; so, sense organs of lateral line. (From Kingsley's "Comparative Anatomy of Vertebrates.") OLFACTORY ORGANS There are two chemical senses, smell and taste, but distinction between the two is difficult to draw in lower animals in which differentiated organs are wanting. For smell and taste aUke, chemical substances, in order to affect sense receptors, must be dissolved in water. This response to dis- solved chemicals is a fundamental property of organisms, as is shown by such facts as that an Amoeba will engulf a protein particle but not a piece of glass, and that injurious substances swept into the gullet of a Para- mecium cause a reversal of ciliary action. A similar sensitivity is shown by multicellular forms. A sea anemone responds differently to pieces of meat and of blotting paper, yet it has no specialized olfactory or gustatory organs. Beginning with this fundamental property of organisms, evolutionary changes in the chemical sense have followed two paths, one leading to the differentiation of an olfactory epithelium and the other to the forma- tion of taste-buds. Both types of sense organ occur in aquatic animals, and they become still further differentiated in land animals, one respond- ing to chemicals suspended in the air and the other to substances dissolved in water. Of the two, smell is far more delicate. In us the two senses are much confused in experience, since much that we eat is not only tasted but also smelled. In general we do not taste flavors such as those of onion or coffee, but smell them. We may infer that smells and tastes are not distinguished by coelen- terates, since thev are not attracted to food at a distance. Actinians 400 CHORDATE ANATOMY react to food placed on the tentacles, but not to food placed near the mouth. The chemical sense receptors are therefore probably neurosensory cells located in the tentacles. Some flatworms search for bait placed at a distance and are therefore credited with an olfactory sense. Annelids are believed to have two sorts of chemical sense organs, paired ciliated "olfactory organs" near the anterior end of the body, and sense buds formed of clusters STOPORECM0UTH) a MOLLUSC BLASTOPORE CANUSJ BLASTOPORE (ANUS) CMeurenterk: canal) E ECHINOOERM, F HEMICHORDATE. G UROCHORDATE, H. CEPHALOCHORDATE. Fig. 375. — Diagrams of embryonic stages illustrating the contrast in the fate of the blastopore in various groups of animals. The forms in which the embryonic blastopore becomes the mouth were grouped together by Grobben as PROTEROSTOMIA. The DEUTEROSTOMIA include those animals in which the blastopore becomes the anus or lies in the anal region. The coelenterates, flatworms, annelids, and molluscs are Proterostomians, while echinoderms and chordates are Deuterostomians. A somewhat similar view is that of Grobben who divides animals into two phylogenetic series based upon differences in the fate of the primitive mouth or blastopore. The forms in which the blastopore becomes the adult mouth (or cardiopore) reach their evolutionary climax in arthropods and molluscs. Those, on the other hand, in which the blastopore becomes APICAL PLATE CARDIOPORE STOMODEXWi PBEORAL CIUATID ntOTOCPHRIDIUM * MUOOERM Fig. 376. — A diagram of a trochophore larva According to Delsman's theory of the origin of vertebrates, the cardiopore (blastopore) of the trochophore larva is homologous with the neurenteric canal of vertebrates. In other words, the relatively short stomo- deum of the annelid is stretched to become the neural tvibe of vertebrates. (Redrawn after Hatschek.) the anus or lies near the anus lead to the chordates and vertebrates. (I^'ig- 375) A third attempt to solve the phylogenetic problem on the basis of embryological evidence is that of Delsman, who derives chordates, annelids and molluscs from a form like that of the trochophore larva characteristic 430 CHORDATE ANATOMY of free-swimming annelids. (Figs. 376, 377) According to Delsman, the fundamental differences between the adults of these three groups are due to alteration in the growth centers of the embryos of the three groups. The ectodermal foregut of the annelid is, he thinks, the homologue of the neural tube of the chordate. If any one of these hypotheses is correct, it follows that none of the several extant types of invertebrates may be regarded as vertebrate "ancestors." Whatever resemblances to vertebrates they show do not prove that they are in the direct line of vertebrate ancestry, but simply illustrate the principle of parallelism or that of convergence. That such resemblances should occur in forms which started with similar potencies is not surprising. CHORDATE TROCHOPHORE ANNELID BLASTOPORE, ENTERON NEURAL TUBEv FOREGUT \ /MOUTH MOLUUSC Fig. 377. — Diagram illustrating the theory of Delsman that, by change in the loca- tion of the growth center, a trochophore (A) may be converted into either a chordate (B), or a mollusc (C), or an annelid (D). That the protochordates in many ways represent the ancestors of vertebrates seems highly probable. Cyclostomes, more especially their larval forms, lead us in the direction of Amphioxus which, "if it hadn't existed, would have had to be invented." The similarity of Amphioxus embryos to larval tunicates strongly suggests their common origin. Since this seems the most reasonable interpretation of the facts, we may conclude that metamerism has been attained de novo by chordates and not inherited from metameric invertebrates. A "family tree" which expresses these conclusions in regard to the phylogenesis of man is given in Fig. 378. The diagram assumes the dichotomy of Metazoa into Protero- and Deuterostomians. Chordates belong to the latter branch of the animal kingdom. There is general agreement among morphologists as to the phylogenetic series, from cyclo- stomes to man, shown in the Figure. THE ANCESTRY OF VERTEBRATES 431 THREADWORM 1600 SR A PHYLOGENETIC TREE OF THE ANIMAL KINGDOM. Fig. 378. — A diagram of the phylogenesis of man, based on the assumption that the Protochordates resemble vertebrate ancestors more closely than do the annelids. Annelids and other Proterostomians have, it is assumed, diverged from the main line of vertebrate ancestry very early in animal phylogenesis. Before man emerged in the late Tertiary, transitional forms between man and the lower Primates made their appearance. Among these the best known are Propliopithecus, Dryopithecus and Australopithecus, which are true "connecting links." GLOSSARY abdomen. The portion of the body between thorax and pelvis. abducens. The sixth cranial nerve. abduction. The withdrawal of a part from the median plane. aberrant. Wandering from the usual. abnormality. Deviation from the normal. abomasum. The fourth stomach of a ruminant. aboral. Opposite to the mouth. abortion. Expulsion of a fetus before it can live. acetabulum. The socket in the co.xal bone in which the head of the femur articulates. acidophilic. Easily stained with acid dyes. acidosis. Reduction of alkali reserves in the body. acinus. A grape-like terminal subdivision of a gland or lung. acoelomate. Without a body cavity (coelom). acrania. Chordate animals without a brain-case.* acrodont. With teeth fastened to edge of jaw and not lodged in sockets. acromegaly. A disease which involves an enlargement of bones due to over-functioning of pituitary gland. acromion. The lateral extension of the spine of the scapula. adduction. The act of drawing a part towards the median plane, adenoid tissue. Lymphocyte-forming tissue. adrenal (suprarenal) gland. An endocrine organ located near the kidney. adrenin. The suprarenal medullary hormone, afferent. Centripetal, conveying towards the center. after-birth. Extra-embryonic membranes discharged from uterus after the child is born. air bladder (sac). Respiratory or hydrostatic organ in fishes. ala cinerea. The vagal eminence which projects into the fourth ventricle. alar plate. The dorso-lateral portion of the embryonic neural tube. alisphenoid. That part of the embryonic cartilage cranium which forms most of the great wing of the sphenoid. allantois. A hollow outgrowth of the embryonic hind-gut. alveolus, i. A tooth-socket. 2. A terminal acinus of a gland. 3. A respiratory sac of the lung. ameloblast (adamantoblast) . A cell which secretes enamel, ammocoetes. The larval stage of Petromyzon. amnion. The liquid-filled sac which encloses the embryos of reptUes, birds and mammals. amniota. The animals the embryos of which are enclosed in an amnion, amphicoelous. Biconcave like the centrum of a fish vertebra. ampulla. A flask-like dilatation. amylopsin. A starch-splitting enzyme secreted by the pancreas. analogy. Resemblance based on similarity of function. anamnia. Vertebrates the embryos of which lack an amnion. anastomosis. The communication of two vessels or connexion of two nerves. angioblast. One of the cells from which blood and blood vessels develop. 433 434 CHORD ATE ANATOMY angular (e). A membrane bone of the lower jaw. anisotropic. Doubly refracting or polarizing. ankylosis. Consolidation or fusion of the bones of a joint or suture. anlage. The embryonic fundament of an organ. anterior. Toward the head. In human anatomy toward the ventral side. anthropoids. The man-like apes. antibodies. Chemical substances formed by a body in reaction to foreign substances introduced. antiheUx. The inner curved edge of the external ear. antithrombin. A substance in the blood which prevents clotting. antitoxin. A substance in the blood serum antagonistic to a poisonous substance or toxin, antitragus. An ear prominence opposite the tragus. antrum. A bone cavity. anus. The egestive opening of the intestine. aorta. The chief artery which leaves the heart. aponeurosis. A connective tissue membrane or fascia which surrounds or attaches a muscle. appendices epiploicae. Fatty pouches attached to the colon. appendicularia. A free-swimming pelagic genus of urochordates. aqueous humor. The refractive liquid between cornea and lens of the eye. arachnoid. A web-like membrane between the dura mater and pia mater of brain and spinal cord. arbor vitae. The tree-like arrangement of fiber tracts seen in section of the cerebellum. arch. A bent or curved structure. archenteron. The primitive digestive cavity. archencephalon. The primitive forebrain of chordates. archetype. An ideal original form. archicortex (archipallium). The olfactory cerebral cortex including the hippocampus. archinephros. The primitive kidney or mesonephros (Wolffian body). archipterygium. The original appendicular skeleton of vertebrates. areolar tissue. A fibrous tissue containing minute interspaces. arrectores pilorum. Cutaneous muscles attached to hairs. articulare. The bony articular element of the lower jaw of lower vertebrates. arytenoid. A pitcher-like cartilage of the larynx or voice-box. assimilation. Constructive metabolism. astragalus. The ankle bone which articulates with the tibia. ateliotic dwarf. A normal dwarf or midget. atlas. The first cervical vertebra. atrium, i. An auricle of the heart. 2. The chamber surrounding the gills of lower chordates. atrioventricular bundle. A muscle which connects auricle and ventricle. atrophy. The wasting of a part or organ. atropine. A poisonous alkaloid which stimulates the sympathetic. auditory bulla. A capsule-like portion of the tympanic bone. auditory meatus. The external meatus extends from the drum to the outside. auditory tube. A passage from the middle ear to the pharynx. Auerbach's plexus. A sympathetic plexus in the wall of the intestine. auricle, external. The pinna of the ear. auricularis magnus nerve. A sensory nerve distributed to face, ear, and neck. Australopithecus. A South African fossil anthropoid. autonomic system. The sympathetic and parasympathetic nervous systems. GLOSSARY 435 axilla. The arm-pit. axis (epistropheus). The second cervical vertebra. axolotyl. The larval, and sometimes permanent, stage of the salamander Ambystoma. Bartholin, glands of. The vulvo-vaginal glands. basalia. The pro.ximal elements of the cartilaginous skeleton of the e.xtremity. basihyal. The ventral element of the hyoid skeletal arch. basilar membrane. The basal membrane of the organ of Corti. basilar plexus. A venous plexus in the dura mater which lines the occipital bone. basioccipital. The basal portion of the occipital ring. basophilic. With affinity for basic dyes. Bell's law. The dorsal roots of spinal nerves are sensory, the ventral motor. biceps brachii. The arm muscle which flexes the forearm. bicuspid. A tooth (premolar) with two cusps. The left atrioventricular valve of the heart. bilateral. A type of symmetry such that one plane, and only one, will divide a body into equal halves. biogenesis. Life comes from life, not from the lifeless. biogenesis, fundamental law of. Ontogenesis repeats phylogenesis. blastocoele. The cavity of the blastula. blastoderm. The membrane from which the embryo develops. blastopore. The external orifice of the gastrula. blastula. The one-layered stage of ontogenesis. body-staJk. The mesodermal bridge which connects the embryo with the chorion. Bowman's capsule. The globular dilatation of an uriniferous tubule enclosing a glomerulus. brachium conjunctivum. The superior peduncle of the cerebellum. brachiimi pontis. The middle peduncle of the cerebellum. branchial bars. The gill or visceral skeletal arches. branchiomerism. The metamerism represented in the visceral arches and pharyngeal pouches. broad ligament. The peritoneal fold which supports uterus and ovary. bronchiolus. One of the branches of a bronchus. bronchus. One of the two branches of the trachea. Bnmner's glands. Submucous glands of the duodenum. bulbus. The enlarged origin of the aorta. bulbo-urethral glands. Co\vper's glands of the urethra. bulbus urethrae. An elongated swelling of the urethra. bursa. A sac-like cavity. buttock. The prominence formed by the gluteus muscle. calcaneimi. The heel bone. calcareous. Composed of lime (calcium) salts. callosity. A local thickening of the horny layer of the skin. calyx (pi. calyces). One of the recesses of the renal pelvis which encloses the pyramids. canaliculus. One of the fine canals which connects bone lacunae. canalis reuniens. The duct which connects the cochlear duct with the sacculus. cancellous bone. Spongy bone. canine tooth. The single cuspid tooth between lateral incisor and first premolar. capillaries. The minute vessels which connect arteries and veins. capitate. The os magnum of the carpus. capitulum costae. The head of a rib. cardinal veins. The paired veins which drain head and trunk in lower vertebrates. 436 (THORDATE ANATOMY camassial teeth. The last upper premolar and the first lower molar tooth of carnivora. carnivorous. Flesh-eating animals. carpus. The wrist. cartilage. An elastic connective tissue with cells embedded in a homogeneous matrix. castrate. To remove the testes. catalyzer. A substance which by its presence changes the velocity of a chemical reaction. caudad. Toward the tail. cecum. That diverticulum of the colon into which the vermiform appendix opens, cenogenetic. Of recent, as contrasted with ancestral, origin. centrum. The body of a vertebra. cephalic. Pertaining to the head. cerebellxmi. The little brain, a coordinating center above the fourth brain ventricle, cerebrum. The chief division of the brain. cheiropterygium. The skeleton of the fingered appendage. chemoreceptor. A sensory cell which responds to chemical stimulation. chiasma. A decussation or x-shaped crossing of nerve fibers within the central nervous system. chief cells. The pepsinogen secreting cells of the gastric glands. chitin. The hornynitrogenous substance which forms mostof the skeletonsof arthropods, choana. A funnel-shaped opening. cholesterol. A fat-like substance which forms the usual type of gallstones. chondrin. A gelatin-like protein found in cartilage. chorda dorsalis. The notochord, the primary chordate axial skeleton. chorda tympani. A mixed branch of the facial nerve. chordae tendineae. The tendons of the heart-valves. chorio-allantois. The fused membranes of the allantois and chorion. chorioid (choroid). A chorion-like vascular membrane. In the eye a layer between retina and sclera. chorioid fissure. A defect in the optic cup and a groove along the optic stalk. chorion. The outer protective extra-embryonic membrane of amniotes. chromaffin cells. Cells of sympathetic origin having an affinity for chrome salts, chromatophore. A pigment cell. chyme. The partly digested material which passes from the stomach to the duodenum, ciliary body. A local thickening of the choroid layer of the eye to which the lens is attached. cingulvmi. An association fiber tract which encircles the corpus callosum near the median plane. cistema chyli. A receptacle or enlargement at the lower extremity of the thoracic duct. Clarke's column. A tract of nerve cells in the dorsal column of gray matter of the cord, cleft palate. Congenital fissure in the roof of the mouth. cleithrum. A membrane bone of the shoulder girdle of fishes and amphibia, clitoris. An erectile organ of the female homologous with the penis. cloaca. The cavity into which digestive and urogenital organs open. cochlea. The spiral tube of the inner sensory ear. celiac (solar) plexus. A large sympathetic plexus in the epigastric region. coelom. The chordate body-cavity. coelomoducts. Paired segmental reproductive ducts of annelids. collagen. The chief organic constituent of bone and connective tissue. collateral. A side branch of an axon. GLOSSARY 437 colliculus. One of the divisions of the corpora quadrigemina. colloid. Glue-like. colon. The large intestine to the rectum. columella auris. The ear bone of amphibia and sauropsida. commissure. A bundle of nerve fibers which connect right and left halves of central nervous sj'stem. Concrescence Theory, i. The theory that separate primordia unite in the median plane to form the right and left halves of the embryo. 2. The theory that com- pound teeth are formed by the fusion of simple conical teeth. condyle. A rounded articular surface of a bone. congenital. Existing at or before birth. conjunctiva. The delicate membrane which covers the eyeball and lines the eyelid. conus arteriosus, i. The conical portion of the right ventricle which Joins the pulmo- nary artery. 2. The valvular region of the ventral aorta. copulation. Sexual congress. coracoid. The posterior of the two ventral elements of the pectoral girdle. corium. The deeper mesodermal layer of the skin. cornea. The translucent anterior portion of the sclera of the eyeball. comified. Converted into horny tissue. corona radiata. The projection fibers which radiate from the internal capsule to the cerebral cortex. coronary. Encircling like a crown. coronoid bone. A membrane bone of the lower jaw. corpora bigemina (quadrigemina). The midbrain centers of optic reflexes. corpora cavernosa penis. The paired masses of erectile tissue of the penis. corpus albicans. The fibrous tissue formed after the discharge of the ovum. corpus callosum. A group of commissural fibers which connect the two cerebral hemispheres. corpus cavemosmn urethrae. The spongy tissue surrounding the urethra. corpus luteimi. The yellowish tissue formed in the Graafian follicle when an ovum is discharged and fertilized. corpus striatum. The basal ganglion of the cerebral hemisphere, cortin. The hormone secreted by the adrenal cortex, coxal bone. The hip bone. cranium. The brain case. cretin. A type of idiotic dwarf supposedly caused by deficient thyroxin secretion, cribriform plate. A sieve-like portion of the ethmoid bone. cricoid. One of the laryngeal cartilages which resembles a seal ring. crista. A crest or ridge of sensory hair-cells. crura cerebri. The brain peduncles formed by descending fiber tracts from the hemispheres, crypt. A pit or follicle. cryptorchism. The condition of undescended testicles. cuboid. The most lateral distal tarsal bone of the foot. cuneiform bones. Three wedge-shaped bones of the distal tarsals, cusp. A conical projection of a tooth, cutaneous. Pertaining to the skin. cuticle. The epidermis or outer layer of the skin, cystic duct. The duct from the gall bladder to the common bile duct. deciduous placenta. The primitive placenta in which chorion and uterine mucosa are loosely associated. 438 CHORDATE ANATOMY decussation. A decussation of nerve fibers occurs when they cross the median plane to connect unlike centers on the two sides. Deiter's cells. The cells in Corti's organ which support the outer hair cells. delamination. The splitting of a cell layer into two or more layers of cells. demersal eggs. Eggs (of fishes) which sink to the bottom of the water. demilunes. Crescent-shaped cells of mucus-secreting acini, dendrite. One of the branched processes of a neuron which carries impulses towards the cell body. dentary. The membrane bone of the lower jaw to which teeth are attached, dentine. The bone-like substance which forms most of the material of a tooth. dentition. The kind, number, and arrangement of the teeth. depressor. A muscle which lowers an organ. A nerve which inhibits action. dermatome. That portion of the epimere which forms corium. deuterencephalon. That part of the embryonic brain which includes mid- and hind- brain. deuterostomia. Animals in which the blastopore forms the anus or lies near the anus. deutoplasm. The passive nutritive portion of the germ-cell. development. The process by which an egg changes into an adult. diabetes. A disease which is marked by excessive excretion of urine. diaphragm. The muscular partition which in mammals separates thorax and abdomen, diapophysis. A process of the neural arch which articulates with the tubercle of a rib. diastema. A space between the teeth, especially between the canine and the lateral teeth. diencephalon. The second of the five successive divisions of the brain. differentiation theory. The theory that compound teeth have evolved by differentia-' tion from simple conical teeth. digestion. The process by which foods are made soluble. dioecious. Having sexes separated in two individuals. diphyodont. Having two sets of teeth. diploe. The cancellous bone between the two layers of compact bone of the cranium. diploid. The double number of chromosomes characteristic of body cells. diplospondyly. The condition of double centra in vertebrae. distal. Opposed to proximal. dorsal. Pertaining to the back or dorsum. duodenum. The anterior portion of the small intestine. dura mater. The tough outermost membrane which surrounds the central nervous system. ectoderm. The outermost germ layer. , ectosarc. The outer layer of the Protozoon cell. effector. The end-organ, muscle or gland, which responds to nervous stimulation. efferent. Away from a center or organ. egg. The animal ovum. ejaculatory duct. The duct which unites the seminal vesicle and ductus deferens with the urethra. "elan vital." Bergson's name for the "vital factor." endobranchiate. Forms with endodermal gills. endocardium. The epithelial membrane which lines the heart, endochondral bone. Bone which develops within cartilage. endocranium. The dura mater of the brain. endocrine gland. A gland which secretes internally into the blood. GLOSSARY 439 endoderm. The innermost germ layer. endolymph. The liquid contained in the membranous sac of the inner ear. endometrium. The mucous lining of the uterus. endoneurium. The connective tissue which subdivides a nerve into funiculi. endoskeleton. The internal skeleton as distinguished from the dermal skeleton. endostyle. A ciliated groove in the floor of the pharynx of lower chordates. endothelium. The thin membrane which lines blood vessels and lymphatics. end-plate. The expanded termination of a motor nerve on a muscle fiber. ensiform. Shaped like a sword. enterocoele. A mesodermal cavity formed as an outpocketing of the endoderm. enteron. The cavity of the alimentary canal. enterokinase. An enzyme secreted by the small intestine which converts trypsinogen into tripsin. enzyme. A catalytic organic compound which facilitates chemical changes such as the splitting of foods into simpler substances in the alimentary canal. eosinophile. A leucocyte with an affinity for eosin. epaxial. Dorsal to the vertebrate axis. ependyma. The lining membrane of the central nervous system. epibranchial organs (ganglia or placodes) . Ectodermal masses above the gill-pouches of vertebrate embryos. epicardium. The outer covering of the heart. epidermis. The outer ectodermal layer of the skin. epididymis. The mass of convoluted tubules and duct attached to the testis. epiglottis. The lidlike structure which covers the entrance to the larynx. epimere. The dorsal portion of the mesoderm. epinephrine (adrenin). The hormone secreted by the medulla of the suprarenal. epineurium. The connective tissue covering of a nerve. epipharynx. The nasal portion of the pharynx. epiphysis. The terminal portion of a long bone. The pineal organ. epithalamus. The dorsal portion of the diencephalon. epithelium. The tissue which covers a surface. epitrichium. The outer layer of the fetal epidermis. epoophoron. A rudiment of the mesonephros near the ovary. ereptose ferepsin). A peptone-splitting enzyme secreted by the intestinal mucosa, erythrocytopoietic tissue. Tissue forming red blood corpuscles. esophagus. The portion of the alimentary canal between pharynx and stomach. ethmoturbinals. The superior and middle turbinated bones, ethmoid. The sieve-like bone at the front of the skull. eunuch. A castrated individual. eustachian tube. The passage which connects the tympanic cavity with the pharynx excretion. The elimination of liquid wastes. exoskeleton. That part of the skeleton which is derived from the skin. exteroceptive. Receptive to external stimuli. extirpation. The complete removal of a part. extravasation. The escape of blood from a vessel into surrounding tissues. extremity. A limb. extrinsic. Originating outside an organ. falciform ligament. A fold of peritoneum between liver and diaphragm. Fallopian tube. The uterine tube or anterior portion of Muellerian duct. falx cerebri. That portion of the dura mater between the hemispheres. fasciculate layer. The middle layer of the suprarenal cortex. 440 CHORDATE ANATOMY fasciculus. One of the divisions of a funiculus of the spinal cord- femur. The thigh bone. fenestra cochleae. The round window of the internal ear. fenestra vestibuli. The oval window to which the stapes bone is attached, fertilization. The union of sperm and egg nuclei within the ovum. fetus. The child in the womb after the end of the third month. ' fibril. A cytoplasmic thread. fibrin. The clot-forming substance of the blood. fibrocj^e. A flat elongated connective tissue cell. fibula. The outer shin bone. filum termlnale. The thread-like termination of the spinal cord. fimbriae tubae. The fringe-like end of the uterine tube. fission. Asexual division into equal parts. fissure. A deep fold of the cerebral corte.x which involves the entire thickness of the brain wall. fisttila. A deep ulcer-like opening, usually into a hollow organ. flabellum. A fan-like set of radiating fibers in the corpus striatum. flagellate. Having whip-like processes. A protozoon with flagellae. fiagellum. A whip-like protoplasmic process of a cell. flame-cell. An excretory cell having one or more flagellae. flexion. The condition of being bent. flexure. A bend or fold. flocculus. A small lobe on the lower side of the cerebellum. floor plate. The floor of the neural tube. foliate papillae. Leaf-like folds on the sides of the tongue. follicle. A small secretory sac or gland. The integumentary sac enclosing the base of a hair or feather. fontanelle. One of the unossified regions in the skull of the infant. foramen. A small opening, usually in a bone. fornix. A band of commissural fibers ventral to the corpus callosum. fossa. A pit or depression. fovea centralis. A pit in the macula lutea of the retina where the layer of nerve fibers is lacking. freemartin. The sterile female twin of a male calf. frenulum linguae. A median fold between tongue and mandible. friction-ridge pattern. A concentric arrangement of fine ridges on the hands and feet. frontal. The bone of the forehead. A sectional plane which divides a bilateral body into dorsal and ventral divisions. function. The normal activity of an organ. fundus. The part of a hollow organ opposite its opening. fungiform. Mushroom-shaped. funiculus. One of the three chief divisions of white matter on the sides of the cord. gall bladder. The pear-shaped sac which stores the bile secreted by the liver. gamete. A sexual cell — ovum or spermatozoon. ganglion. A functional group of nerve cells outside of the central nervous system. Gartner's duct. The rudiment of the Wolffian duct in the female mammal. Gasserian ganglion. The semilunar ganglion of the trigeminal nerve. "gastraea" theory. The theory that the gastrula stage represents a coelenterate stage in vertebrate phylogenesis. gastric glands. The glands of the stomach wall. GLOSSARY 441 gastrula. The two-layered stage of ontogenesis. gelatin. The translucent protein which forms jelly and glue. genetic. Pertaining to origin or birth. geniculate body. A tubercle-like body in the lower part of the optic thalamus. geniculate ganglion. The ganglion of the facial nerve. genital. Pertaining to the organs of reproduction. gestation. Pregnancy. The condition of being with child. gigantism. Excessive growth sometimes due to disease of the anterior lobe of the pituitary. gingiva. The gum. The tissue which covers the jaw and surrounds the necks of the teeth. gizzard. The muscular grinding stomach of birds and reptiles. gladiolus. The middle chief portion of the sternum. glans penis. The swollen terminal portion of the penis. glenoid. The concave depression in the scapula in which the head of the humerus articulates. globus pallidus. The pale interior of the lenticular nucleus of the corpus striatum. glomerulus. The knot of capillaries of a renal corpuscle, glottis. The pharyngeal opening of the larynx. glycogen, .\nimal starch, a carbohydrate stored in the liver and other tissues. gnathostomes. Fishes with biting jaws in contrast with cyclostomes. goblet-cells. INIucus-secreting cells of the intestine. goiter. Enlarged thyroid gland. Golgi-Mazzoni corpuscles. Special tactile corpuscles of the finger-tips. gonad. A gamete-producing gland. gonotome. That part of the mesoderm which forms the gonad or germ-gland. goose flesh. The formation of skin papillae due to the action of cutaneous muscles on hairs. Graafian follicle. An ovarian vesicular sac containing an ovum and secreting an hormone (estrin). Grandry's corpuscles. Special tactile corpuscles of tongue and mouth. The same as Merkel's corpuscles. granulocyte. A leucocyte which contains coarse granules. gray matter. That portion, mostly cellular, of the central nervous system which lacks the white myelin. groin. The part of the abdominal wall adjacent to the thigh. gubemaculum testis. The fetal cord which attaches the testis to the scrotal sac. gullet. The esophagus. gustatory. Pertaining to the sense of taste. gynandromorph. An organism having both male and female characteristics. gyrus. A fold or convolution of the brain cortex. hagfish. The cyclostome myxine. habenular commissure. The "superior" commissure which connects the habenular ganglia in the roof of the diencephalon. habenular ganglia. Olfactorj' centers anterior to the pineal body. hamate. The distal carpal bone adjacent to the fifth metacarpal. haploid. The reduced number of chromosomes found in mature germ-cells. harelip. A congenital cleft in the upper lip, rarely double. Harrimania. A genus of hemichordates. harvest men. Long-legged spiders or "daddy-long-legs." Hassall's corpuscles. Clusters of concentrically arranged epithelial cells of the thymus. 442 CHORDATE ANATOMY Hatschek's pit. A preoral pore in amphioxus which opens into the left anterior cavity. haustra. Sacculations of the wall of the colon. Haversian canal. A bone canal which contains a blood vessel and nerves. "heat." Estrus or sex ardor in animals. helix. The margin of the pinna of the ear. hemal. Pertaining to the blood. hemibranch. The anterior or posterior half of the respiratory portion of a visceral arch. hemoblast. A primitive blood cell. hemoglobin. The coloring matter of a red blood cell. hemopoietic. Blood-forming. Henle's loop. A loop of a uriniferous tubule. Hensen theory. The theory that nerve fibers are enlarged plasmodesms. hepatic. Pertaining to the liver. hermaphrodite. An individual with both male and female characteristics. hernia. The protrusion of an organ through an abnormal opening. heterodont. Having specialized kinds of teeth. heterogamy. Reproduction which involves the union of unlike gametes. heterolateral. Relating to opposite sides. Highmore's antrum. A cavity or sinus of the maxillary bone. hilus fhilum). The pit of an organ where blood vessels and nerves enter. hindbrain. The posterior of the three primary embryonic brain vesicles. hippocampus. A gyrus in the floor of the lateral ventricle which constitutes the greater portion of the olfactory centers. histogenesis. The ontogenetic differentiation of a tissue. holobranch. An entire fish gill. homodont. Having teeth all alike, homolateral. On the same side. homologous. Having the same structure, development, and relations. hormone. The secretion of an endocrine gland which affects the activity of one or more other organs. hyaline. Translucent. hydatid of Morgagni. The rudiment of the M ullerian duct attached to the oviduct or testis. hydrostatic organ. An organ of aquatic organisms which serves to adjust internal to external pressure. hymen. The membrane which more or less completely closes the external opening of the vagina. hyoid. The second visceral arch of vertebrates. The hyoid bone of mammals develops from skeletal elements of the second and third visceral arches. hyomandibula. The dorsal element of the hyoid skeletal arch. hypaxial. Ventral to the chief axis. hypertrophy. The abnormal enlargement of an organ, hypobranchial muscles. The muscles ventral to the gills. hypochorda. A transient cord of cells ventral to the notochord. hypocone. A fourth tubercle of a compound tooth, hypodermic. Administered beneath the skin. hypomere. The ventral portion of the mesoderm. hypoparathyroidism. Insufficient secretion of the parathyroid glands. hypophysis. Usually identified with the pituitary gland but strictly forming only the anterior lobe of the latter. hypothalamus. The ventral portion of the diencephalon. hypothenar. The ridge on the ulnar side of the palm. GLOSSARY 443 hypothesis. An attempted explanation. ichthyopsida. The sub-phylum which includes the fish-like forms — Fishes and Amphibia. ichthyopterygimn. The skeleton of the extremity of fishes. ileum. The posterior portion of the small intestine. ilium. The dorsal element of the pelvic girdle. imbricate. A tile- or shingle-like arrangement of alternating parts. immunity. The condition of resistance to infection. impulse, nervous. The change transmitted along an active nerve fiber. incisive canal. The passage in the maxillary bone from the nasal fossa to the mouth cavity. incisor. One of the four front teeth of either jaw. incus. The anvil-like middle earbone. infundibulum. The funnel-like portion of the hypothalamus which connects with the pituitary gland. inguinal canal. The passage in the groin which contains the spermatic cord, inscriptiones tendineae. Fibrous bands which partially divide the belly of a muscle. insertion. The attachment of a muscle to the bone which it moves. insula. A triangular portion of the cerebral hemisphere covered by the temporal lobe. insulin. The hormone secreted by the islands of Langerhans in the pancreas, integration. The coordination of functions. intercalated. Placed between or interposed, intercostal. Between the ribs. intergrade, sex. An individual with characteristics intermediate between those of the sexes. interoceptors. Sense organs in the lining of the alimentary canal. interrenal body. The tissue which in higher vertebrates forms the adrenal cortex, interstitial tissue. The endocrinal tissue of the testis. intestinal portal. The transient embryonic anterior opening of the hindgut and the posterior opening of the foregut which appear during the formation of the floor of the intestine. intima. The innermost lining of an artery. intrinsic. Situated within a part or organ. invaginate. The enclosure of a part by another portion of the same thing. iris. The circular pigmented membrane between the cornea and lens of the eye. ischium. The posterior of the two ventral elements of the pelvic girdle. islands of Langerhans. Endocrinal organs embedded in the substance of the pancreas which secrete the hormone insulin. isogamy. The union of similar gametes. Jacobson's organ. An accessory olfactory organ which opens into the nasal cavity. jejunum. The middle of the three divisions of the small intestine. jugular ganglion. The "root" ganglion of the vagus nerve. keratin. An insoluble protein which forms the base of horny structures. kidney. The chief excretory organ of amniotes. Koelliker's pit. The rudiment of the embryonic neuropore of Amphioxus. Krause's corpuscles. Rounded sensory corpuscles located in mucous membranes. labial cartilages. Rudimentary cartilages of elasmobranchs. labium. A lip or lip-like structure. 444 CHORDATE ANATOMY labyrinth. The internal ear of vertebrates. lacrimal. Pertaining to the tears. lacteal. Pertaining to milk. An intestinal lymphatic. lacuna. A pit or hollow. In bone the cavity filled by a bone cell. lagena. An outgrowth of the sacculus of the ear. lamella. A thin leaf or plate as of bone. lamina terminalis. The thin membrane which forms the anterior wall of the third brain ventricle. lanugo. The fine hairy covering of the fetus of man. larva. An immature but active stage in the development of an organism. larynx. The cartilaginous organ which encloses the vocal cords. lateral line. A series of sense organs which extends along the sides of the body of fishes and some amphibia. lenunatochord or bothriochord. An arthropod structure which W. Patten has com- pared with the notochord. leucocyte. A "white" blood corpuscle. Lieberkuehn's glands or crypts. Tubular mucous glands of the intestine. linea alba. The tendon in the median line of the abdomen. lingual. Pertaining to the tongue. lipase (steapsin). A fat-splitting enzyme secreted by the pancreas and some other digestive organs. lipoids. Fat-like cell constituents soluble in alcohol and ether. lobule. A normal small division of a lobe. Lorenzini's ampullae. Tubular sensory organs of Elasmobranchs. luciferin. A substance which combined with luciferase in luminous animals produces light. lumen. The cavity of a hollow organ. lunula. The whitish crescent at the root of a nail. luteal hormone. The hormone progestin secreted by a corpus luteum. lymph. The coagulable liquid of the lymphatic vessels. maculae acusticae. The sensory patches of the sacculus and utriculus. macula lutea. The point of clearest vision at the center of the retina. malar bone. The zygomatic or cheek bone. malleolus. The hammer-headed process of a bone (tibia and fibula). malleus. The ear ossicle which is attached to the drum. maltase. The enzyme which splits maltose into dextrose. manuna. The breast or mammary gland. mammillary bodies. Paired rounded bodies posterior to the tuber cinereum. mandible. The horseshoe-shaped lower jaw. mandibular arch. The anteriormost visceral arch. mantle. The body wall of urochordates. The shell-secreting organ of molluscs. manubrium. The anterior division of the sternum. marsupial pouch. The abdominal pouch of marsupials in which the young are carried after birth. mastoid. A process of the temporal bone. maturation. The process by which homologous chromosomes are segregated. maxilla. The upper jaw bone, maxillo-turbinals. The inferior turbinated bones. meatus. A passage or opening. Meckel's cartilage. The lower jaw of cartilaginous fishes. mediastinum. The thick partition which divides the two pleural cavities. GLOSSARY 445 medulla. The marrow or core of an organ. medulla oblongata. The posterior brain division which contains the fourth ventricle. Meibomian glands. Sebaceous glands of the eye lids. Meissner's corpuscles. Tactile corpuscles with thick capsule and brush of nerve terminations. melanophore. A pigment cell containing melanin. membrane bones. Bones which are not preformed in cartilage. meninges. The three membranes which enclose the brain and spinal cord. menopause. The time in life when menstruation normally ends. menstruation. The monthly How of women. mentum. The chin. Merkel's corpuscles. Tactile corpuscles of the tongue and mouth. mesencephalon. The third or mid-brain vesicle. mesenchyme. The embryonic connective tissue. mesentery. The peritoneal membrane which attaches the intestine to the body wall. mesethmoid. The median cribriform portion of the ethmoid bone. mesocardium. The membrane which connects the embryonic heart with the body wall. mesoderm. The middle germ layer. mesogaster. The peritoneal membrane which attaches the stomach to the body wall. mesoglea. The gelatinous middle tissue of sponges and coelenterates. mesomere (nephrotome). That portion of the trunk mesoderm which connects each somite fepimere) with the ventral mesoderm or hypomere. mesomerism. The segmentation of the epimeric or dorsal mesoderm into somites. mesonephros. The middle kidney or Wolffian body, the functional kidney of anamnia. mesopterygium. The middle basal cartilage of the elasmobranch pectoral fin. mesorchium. The peritoneal membrane which attaches the testis to the dorsal body wall. mesorectum. The mesentery of the rectum. mesothelium. The cellular layer which encloses the coelom. mesovarium. The peritoneal membrane which connects the ovary to the body wall. Mesozoic. The middle life era of geologic time. metabolism. The chemical cycle of matter in living organisms. metacarpus. The group of five bones between the wrist and the fingers. metacone. The postero-external cusp of the upper molar teeth of mammals. metaconid. The postero-internal cusp of the lower molar teeth of mammals. metamere. One of the serial divisions of the body of a segmented animal. metamorphosis. A striking change of form during development, as seen in the trans- formation of the tadpole into a frog. metanephros. The definitive kidney of the amniotes. metaotic. Posterior to the ear vesicle. metapleural folds. Paired folds on the ventral side of the body of Amphioxus. metapterygium. The posterior basal cartilage of the pectoral fin of elasmobranchs and some ganoids. metastemum. The posterior or xiphoid process of the breast bone. metatarsus. The five bones between the ankle and the toes. metazoa. Many-celled animals. metencephalon. The fourth brain division which forms cerebellum and pons. midbrain. The third or mesencephalon division of the brain. mitral valve. The left atrioventricular valve of the heart. moa. An extinct running bird of New Zealand. molar teeth. The three grinding teeth of each half jaw. 446 CHORDATE ANATOMY monocytes. Large mononuclear leucocytes with kidney-shaped nuclei. monoecious. Having male and female flowers separated on the same plant. monophyletic theory. The theory that all forms of blood cells arise from a common primitive type of cell or hemoblast. monophyodont. Having a single permanent set of teeth. monorhine. Having a single narial aperture. monosaccharid. A sugar the molecules of which have only six carbon atoms. mons pubis. The pubic eminence or mons veneris. morphology. The science which treats of form and structure of organisms. morula. The mulberry stage of segmentation of the egg. motor (efferent) neuron. A nerve cell which conveys impulses away from a nervous center. mucosa. The lining of the intestine. Muellerian duct. The oviduct or uterine tube. multitubercular. Having many cusps as the molar teeth do. muscularis mucosae. The layer of smooth muscle fibers in the mucosa. myelencephalon. The fifth or posterior division of the brain — the medulla oblongata. myelin. The fat-like substance which forms the Schwann's sheath of nerve fibers. myelocytes. Cells of red bone marrow usually with granular cytoplasm. myenteric plexus. The network of sympathetic fibers on the muscles of intestine. myocardium. The muscular layer of the heart. myocoele. The coelom of the somite. myocomma. The connective tissue between two myotomes. myofibril. One of the fine fibrils within a muscle fiber. myotome. The muscular portion of a somite. myxine. The hagfish, a genus of cyclostomes. nail wall. The skin which covers the base of the nail. nares. The nostrils. navicular. The scaphoid bone of the carpus and tarsus. Neanderthal. A gorilla-like type of fossil man. necrobiotic. Dying as a result of functional activity. neocortex. The major portion of the cerebral hemispheres. neostoma. The definitive mouth of vertebrates. neostriatum. An olfactory center, near the corpus striatum, which becomes in mammals the nucleus amygdalae. nephridiimi. The ectodermal excretory tubule of annelids and amphioxus. nephrostome. The opening of a nephridium into the body cavity. nephrotome. The intermediate portion of the mesodermal somite which gives rise to a renal tubule. nerve. A bundle of nerve fibers which connect the central nervous system with some part of the body. nerve fiber. The axon process of a neuron plus sheaths when such occur. nerve nucleus (nidulus). A group of nerve cells within the central nervous system. nerve tract. A bundle of nerve fibers of similar origin and function within the central nervous system. nervus terminalis. A sensory nerve associated with the olfactory." neuraxon. The nerve fiber or axon process of a nerve cell. neurenteric canal. The blastoporic opening which in chordate embryos connects the neural tube with the enteron at the posterior end of the body. neurolemma. 'J'he cellular sheath of an axon. neurite. The axon process of a neuron which carries impulses away from the cell body. GLOSSARY 447 neurobiotaxis. The migration of the cell bodies of neurons towards the source of stimulation. neuroblast. The embryonic neural cell which forms an axon process. neuro-epithelial cell. A sensory receptor cell the body of which is located in an epithelium. neurofibrillae. The fine fibrils within a neuron. neurogenesis. The differentiation of nervous tissue in ontogenesis, neuroglia. The ectodermal supporting tissue of the central nervous system. neurohumor. A nerve secretion which stimulates an effector cell, neuromast. A cluster of sensory cells in the skin such as is represented in a lateral line organ. neuromere. An embryonic segmental division of the central nervous system. neuromuscular spindle. Specialized sense organs located in muscles. neuron. The functional unit of the nervous system. neuropore. The anterior opening of the embryonic neural tube. neurosensory cell. A sensory or receptor cell the cell body of which lies in an epithelium. neurostoma. The hypothetical mouth (Delsman) of vertebrate ancestors, which is represented in the neuropore of chordate embryos. neurotendinous spindle. A special type of sensory nerve termination connected with a tendon. neutrophile. A blood cell with an affinity for neutral stains. Nissl bodies. Large protein granules with an affinity for basic dyes found in nerve cells. nodosum ganglion. The ganglion of the vagus below the jugular ganglion. non-deciduate placenta. The tynpe of mammalian placenta which does not involve the uterine mucosa at birth. non-medullated. Devoid of myelin sheath. Nordic. The blond teutonic type of man of northern Europe and Africa. normoblast. A nucleated stage in the histogenesis of a red blood corpuscle. notochord. The axial rod between the chordate nervous system and the dorsal aorta, nuchal. Pertaining to the nape of the neck. obturator foramen. The opening between the pubis and ischium. occipital lobe. The posterior lobe of the cerebral hemispheres. occlusion. The state of being closed. odontoblast. One of the dentine-secreting cells. odontoid process. The tooth-like process of the axis (epistropheus) vertebra. oestrus (estnis). The period of "heat" or receptivity in the female. olecranon process. The process of the ulna at the elbow. oliva. A prominence of the medulla oblongata lateral to the pyramid. omasum (psalterium). The third division of the ruminant stomach. omentum. A sac formed by the doubling of the mesentery. ontogenesis. The development of the individual from the egg. oocyte. The immature egg. operculum. The gill-cover of fishes. opisthonephros. The posterior metanephros-like portion of the mesonephros of anamnia. optic vesicle. The hollow lateral outpocketing of the forebrain which forms the retinal and pigment layers of the eye. oral hood. The funnel-like membrane which in Amphioxus bears the tentacles. orbit. The bony socket which surrounds the eye. OS uteri. The orifice of the uterus. 4^8 CHORDATE ANATOMY osculum. A minute aperture. osmosis. The passage of dissolved substances of different concentration through a semipermeable membrane which separates them. ossification. The formation of bone. osteoblast. A bone-secreting cell. ostium tubae. The opening of the oviduct or uterine tube into the body cavity. otic. Pertaining to the ear. otoconia (or otoliths). Crystals of calcium carbonate contained in the endolymph of the membranous ear. ovariotomize. To remove an ovary or ovarian tumor. oviduct. The egg duct or uterine tube. oviparous. Egg-laying. ovulation. The discharge of an egg from the ovary. oxytocin. A pituitary hormone which affects the uterine muscle. pabulum. Food. Pacini's corpuscles. Large tactile corpuscles with single dendrite and many-layered capsule. palate. The roof of the mouth — hard and soft. palatine. The membrane bone which forms the posterior part of the hard palate. paleocortex. The primitive corte.x, or olfactory portion of the hemispheres. paleontology. The science which treats of fossils. paleostoma. The hj-pothetical primitive mouth of vertebrates — possibly represented in the hypophysial duct. paleostriatum. The primitive corpus striatum (basal ganglion) as distinguished from the Epi- and Neostriatum. paleozoic. The ancient life era from the Cambrian to the Permian period. pallium. The cerebral cortex or layer of gray matter which covers the cerebral hemispheres. pancreas. A combined digestive and endocrinal gland between stomach and duodenum. panniculus camosus. A layer of integumentary muscles represented in man by the platysma of the neck. papilla. A small nipple-shaped elevation. parachordal. A cartilage which lies along the anterior end of the notochord. paracone. The anterior external cusp of an upper molar tooth. paraconid. The anterior cusp of a lower molar tooth, paradidymis. A rudiment of the mesonephros near the testis. paraphysis. A hollow outgrowth of the anterior dorsal roof of the diencephalon. parapodium. One of the paired appendages of annelids. parapophysis. A transverse process from the centrum of a vertebra. parasite. An organism which depends upon another for its living without paying board. parasphenoid. A membrane bone of the roof of the mouth of fishes and amphibia, parasympathetic. The craniosacral portion of the autonomic nervous system. parathyroid glands. Ductless glands, usually four, which lie near the thyroids. parietal organ (eye). The anterior epiphysial outgrowth from the diencephalon. paroophoron. A rudiment of the mesonephros near the mammalian ovary. parotid. The serous salivary gland below the ear. patella. The knee-pan. pecten. A comb-like structure in the vitreous body of the eyes of reptiles and birds. pectoral. Pertaining to the chest. pedal. Pertaining to the foot. GLOSSARY 449 pedicle. The bony connexion between thie lamina and centrum of a vertebra. peduncle. A iiber tract which connects the cerebellum with the brain stem. Peking man. A fossil species of man intermediate between the Java man and the Neanderthal. pelvis. The basin-shaped ring of bone which connects back and leg bones. penis. The male intromittent organ. pepsinogen. A zymogen which when combined with hydrochloric acid forms pepsin. peptone. A soluble derived protein formed by the hydrolysis of protein, perennibranch. With persistent gills. pericardium. The membranous sac which encloses the heart. perichondrium. The connective tissue membrane which surrounds cartilage. periderm. The transient external layer of the mammalian embryonic epidermis, perilymph. The liquid in the space between the membranous and the skeletal labyrihth of the internal ear. perimysium. The delicate connective tissue membrane which surrounds a bundle of muscle fibers. perineum. The floor of the pelvic outlet. perineurium. The connective tissue covering a nerve cord. periosteum. The connective tissue membrane which surrounds a bone, peristalsis. The wave of contraction which passes along the intestine. peritoneum. The serous membrane which lines the body cavity and covers th.3 viscera. petrosal. The petrous portion of the temporal bone. Pflueger's egg tubes. Cords of peritoneal cells which are said to grow into the stroma of the ovary. phagocyte. A cell which ingests bacteria. phalanx. One of the finger or toe bones. phallus. The penis. pharynx. That part of the alimentary canal which connects mouth and esophagus. photophore. A luminous organ. photoreceptor. A sensory cell sensitive to light. phylogenesis. Racial history. The evolution of higher from lower animals. pia mater. The innermost and most vascular of the three coverings of the central nervous system. pilaster cells. Columnar supporting cells peculiar to fish gills. pillar cells. Special columnar cells of Corti's organ located between the inner and outer rows of hair cells. pineal body. The posterior epiphysis of the diencephalon. pinna. The projecting portion of the external ear. pisiform. A pea-shaped proximal carpal bone on the ulnar side of the wrist. pit organs. A pair of sensory pits anterior to the eyes of vipers, pituitary gland. An endocrine organ attached to the infundibulum. pituitrin. Extract of posterior lobe of pituitary gland, used to stimulate contraction of smooth muscle (uterus, etc.). placenta. The mammalian organ of attachment and nutrition of the embryo. placode. A local disc-like thickening formed as an anlage of an organ. placoid scale. The typical elasmobranch scale with enamel and dentine layers. plankton. Floating organisms which may be collected with a tow-net. plantigrade. Flat-footed. plasma. The liquid portion of the blood, the serum and fibrinogen. plasmodesm. A fine protoplasmic thread which connects two cells. plastron. The ventral bony shield of the turtle. 450 CHORDATE ANATOMY platysma. The integumentary muscle of the neck. pleura. The serous membrane which lines the chest and covers the lungs, plexus. A network of nerves or blood vessels. plica. A fold or pleat. polymerization. The chemical synthesis of two or more molecules to form a new com- pound without the production of a secondary compound. polymorphonuclear leucocyte. A white blood corpuscle which has a nucleus with irregular constrictions. polyp. The sessile or attached stage of a coelenterate. polyphyletic theory. The theory that the various blood cells have had a multiple origin. polyphyodont. Having more than two sets of teeth. pons. A bridge of fibers below the cerebellum which connects cerebrum, cerebellum and medulla oblongata. postanal gut. That part of the embryonic digestive tract posterior to the anus, postfrontal. A roofing bone of the skull posterior to the frontal. posttrematic branch. That division of a cranial nerve which forks behind a gill-slit. precoracoid. The anterior of the two ventral elements of the pectoral girdle, pregnancy. The condition of being with child. Gestation. premandibular cavity. The second somite which forms the superior oblique eye muscle and — it is asserted — a part of the external rectus. premolar. A bicuspid tooth. preoral gut. That part of the embryonic intestine which is anterior to the mouth, prepuce. The skin fold which covers the glans penis or the clitoris. pretrematic branch. The nerve branch which forks in front of a gill-slit. primitive duct. The pronephric duct. primitive streak. The elongated and closed blastopore of amniote embryos. process theory. The theory that nerve fibers (axons) develop as processes of neuroblast cells. proctodeum. That portion of the hindgut which is lined by ectoderm. progestin. A luteal hormone which affects the endometrium of the uterus, progynon. A proprietary name for the female sex hormone, pronation. The act of turning the palm of the hand downwards. pronephroi. The primitive vertebrate kidneys. proprioceptors. The mechanisms for receiving stimuli from within the body, propterygium. The anterior basal element of the fish extremity. prosencephalon. The anterior division of the embryonic brain which forms the cerebral hemispheres. prosimian. Pertaining to primitive apes. prostate gland. A muscular gland which surrounds the urethra where it leaves the bladder. prostomium. The preoral lobe of Annelids. proterostomia. Those animals in which the blastopore becomes the mouth or lies near the mouth. proteose. A soluble protein formed by hydrolytic cleavage of a protein. protochordate. The primitive chordates which do not acquire a vertebral column. protocone. The inner cusp of an upper molar. protoconid. The outer cusp of a lower molar. protonephridia. Primitive excretory tubules without nephrostomes and with soleno- cytes. prototype. The original form or type from which others evolve. proximal. Towards the body. pseudocoelom. A false body cavity not lined by periloneum. GLOSSARY 451 pterygoid. A portion of the mandibular arch which doubtfully becomes the pterygoid process of the mammalian sphenoid. ptyalin. The starch-splitting enzyme found in tlie saliva. puberty. The age at which reproductive organs start to function, pubis. The anterior of the two ventral arms of the peMc girdle. Purkinje cells. Large much-branched neurons of the cerebellar cortex. putamen. The outer darker portion of the lenticular nucleus of the corpus striatum. pylorus. The aperture at the posterior end of the stomach. pyramids, renal. The conical masses present in the medulla of the kidney. pyramids of the medulla. Paired eminences on the ventral side of the medulla. pyramidal cells. Neurons with pyramid-shaped cell-bodies in the cerebral corle.x. Rathke's pouch. The hypophysis of amniote embrj'os. ray-finned fishes. Fishes with bony and horny rays — includes most fishes. recapitulation theory. The theory that individual development repeats briefly the evolutionary history of the race. receptaculum (cisterna) chyli. A chamber for the storage of lymph at the lower end of the thoracic duct. receptor. A sensory cell. rectiun. The lower six to eight inches of the large intestine. red nucleus. A nervous center in the tegmentum of the midbrain. Its cells contain a red pigment. reduction. Meiosis. The process by which the hapluid number of chromosomes is attained. reflex action. An action which involves a reflex arc, i.e., a sensory and a motor neuron connected within a central nervous system. renal corpuscle. The expanded termination of an excretory tubule containing a glomerulus. respiration. The regulated burning of carbon compounds within living cells. restiform bodies (inferior peduncles). A fiber tract which connects sensory spinal nerves with the cerebellum. rete testis. A network of fine ductules between the seminiferous tubules and the ductuli efferentes. retina. The sensory innermost layer of the eye. Rhodesian man. A prehistoric type of man having affinities with the Neanderthal man. "rhomboid." A kite-shaped area where four hair currents meet. rhombomere. A hindbrain neuromere. roof plate. The median dorsal wall of the embryonic neural tube. rods and cones. The receptor cells of the retina. rubrospinal tract. A longitudinal fiber tract which connects the red nucleus with the somatic motor cells of the spinal cord. nunen. The anteriormost stomach of a ruminant. sacculus. The membranous sac connected with the cochlear duct of the ear. sacnun. The five fused vertebrae to which the ilium is attached. sagittal plane. A median longitudinal vertical section. Santorini's duct. The accessory duct of the pancreas. sarcolemma. The delicate elastic membrane which surrounds a muscle fiber. sarcoplasm. The interfibrillar substance of a striped muscle fiber. Savi's vesicles. Cranial sensory vesicles of Torpedo without external openings. scala media. The cochlear duct. 452 CHORDATE ANATOMY scala tympani. The ventral or descending staircase of the cochlea. scala vestibuli. The dorsal or ascending staircase of the cochlea. scapula. The shoulder blade. The dorsal arm of the pectoral girdle. Schwann's sheath. The neurilemma of a nerve fiber. sclerotic (sclera). The tough white outer covering of the eye-ball. sclerotome. The part of the somite which forms the vertebral column. scrotum. The integumentar}^ sac in which the testes of mammals are lodged. scute. A scale-like skeletal structure. sebaceous gland. A gland which produces a greasy secretion. secretin. An intestinal hormone which activates the pancreas. secretion. A product of cell metabolism which is thrown out as waste or is used by the organism in its normal activity. sella turcica. The pituitary fossa in the sphenoid bone, semen. The external secretion produced by the male in coitus. semicircular canal. One of the ducts of the membranous ear used in equilibration, semilunar valves. Crescentic valves at the root of the aorta and pulmonary artery which prevent blood from returning to the ventricles. seminal vesicle. A secretory gland and reservoir for spermatozoa connected with the ductus deferens. seminiferous tubule. One of the secretory tubules of the testis. septula testis. The connective tissue partitions which divide the testes into compart- ments. septimi pellucidtun. A double membrane which lies in the median plane of the brain, between the corpus callosum and the fornix. septum transversmn. A transverse partition which separates pericardial and peritoneal cavities of vertebrates. serous gland. A gland which secretes a thin watery secretion (serum). Sertoli cells. Columnar cells of the seminiferous tubules, which serve as a base of attachment for spermatozoa, serum. The clear liquid portion of the blood devoid of cells and of fibrinogen. sessile. Fixed or attached, sex. The differentiation of two kinds of reproductive cells and of the individuals which produce them. shell membrane. A tough double membrane attached to the shell of the hen's egg. sigmoid flexure. The bent lower part of the colon to the rectum. Sinanthropus. The Peking species of fossil man. sino-auricular node. A group of fibers at the root of the precava which in contraction serves as "pace-maker of the heart" beat. sinus. A cavity or hollow space. sinusoid. Relatively large anastomosing blood spaces lined by endothelium without adventitia. solenocyte. An elongated flagellated excretory cell associated with the protonephridia of annelids. somatic muscle. Derived from the somite or epimere in contrast with the visceral muscle which is derived from the hypomere. somatopleure. The combined layers of ectoderm and parietal mesoderm. somite. A segment of the dorsal or epimeric mesoderm. spermatogonia. Immature germ-cells which by enlargement form the primary sperma- tocytes. sphenopalatine ganglion. Meckel's ganglion of the autonomic and facial nerves. splanchnopleiu-e. The double embryonic layer formed by the visceral layer of mesoderm and the endoderm. GLOSSARY 453 spongioblast cells. The cells of the neural tube which form ependyma and glia cells. squamosal. The bone which forms the thin vertical element of the temporal. stapes. The innermost earbone which is attached to the foramen vestibuli. static organ. .\n organ of equilibration, a statocyst. status thymicolymphaticus. A edematous condition associated with enlarged thymus. steapsin (lipase). A fat-splitting enzyme secreted by the pancreas. Steno's (Stensen's) duct. The duct of the parotid gland. steraebra. One of the segments of the sternum. stimulofugal. Reacting by motion or growth away from a stimulus. stomodeum. The ectodermal invagination which leads to the formation of the mouth. stratum comeum. The outer horny layer of the epidermis. stratum germinativum. The lowest layer of the epidermis from which the other layers are derived. stroma. The connective tissue matri.x of an organ, styloid. Long and pointed. sublingual gland. A salivary gland below the tongue. submaxillary gland. A mixed salivary gland below the angle of the lower jaw. submucosa. The connective tissue layer below the mucous lining of the intestine. sulcus. A groove or fissure especially of the brain, superciliary. Pertaining to the region of the eyebrows. superior colliculi. The anterior larger pair of swellings of the corpora quadrigemina. supination. Turning the palm upwards. Sussex man. A prehistoric type of man the remains of which were found at Piltdown, Sussex, England. suture. The line of junction of two cranial bones. sympathin. A neurohumor secreted by sympathetic nerves. symphysis. The line of junction of primarily separate bones, synapse. The histological connexion between two neurons, synarthrosis. The fusion of two bones with resultant elimination of a joint, syncytium. A multinucleate mass of protoplasm, systole. The contraction of the heart. talus (astragalus). The ankle bone which articulates with the tibia. tarsal (Meibomian) glands. Sebaceous glands of the eyelid. tarsus. The ankle or instep Avith its seven bones. taste-bud. A cluster of chemoreceptors which form a taste-organ. Taungs man-ape. A fossil ape skull discovered in South Africa. taurodont. With extended pulp-cavity as in some fossil human molar teeth — doubtfully primitive. tectorial membrane. The colloidal membrane which covers Corti's organ of the cochlear duct. tectum. The roof of the midbrain. tegmentum. The gray matter which covers each of the brain peduncles. telencephalon. The anteriormost of the five brain vesicles which forms the cerebral hemispheres. teloblast cells. The posterior pair of cells which in annelids forms the mesodermal bands. telodendria. The motor end-plates or terminations of a neurite. tendon. The fibrous attachment which connects a muscle with a bone or other struc- ture. teniae coli. Three longitudinal muscle bands of the colon. tentorium cerebelli. The dura mater partition between cerebrum and cerebellum.. terminal nerve. A ganglionated portion of the olfactory nerve. 454 CHORDATE ANATOMY tetany. Painful muscular spasms associated with calcium deficiency. thalamus. Gray matter located in the lateral walls of the diencephalon. thecodont. That type of dentition in which the teeth are lodged in sockets. theelin. The female sex hormone. thoracic duct. The main lymph channel which conveys lymph from the lower half of the body to the left jugular vein. thrombin (fibrinogen). A hypothetical enzyme in clotted blood which converts fibrinogen into fibrin. th3mius. A ductless gland, largest in infancy, which lies below the sternum in the mediastinum. thyroglossal duct. A rudiment of the connexion of the thyroid gland with the pharynx. thyroid gland. An endocrine organ located on the sides of the trachea. thyroxine. The iodine-containing hormone secreted by the thyroid gland. tibia. The shin bone which carries the weight of the body to the ankle. tigroid substance. The deeply-staining granular substance which forms the Nissl bodies. tonus. The normal condition of tension of a muscle. Tomaria. The free-swimming larva of Balanoglossus (Hemichordate) . trabeculae. The embryonic prechordal cartilages which enter into the forrnation of the basicranium. trachea. The cartilaginous tube which connects the larynx with bronchial tubes. tragus. The cartilaginous projection before the opening of the auditory meatus. transverse septum. The connective tissue partition which separates the pericardial cavity from the abdominal cavity. triconodont. A primitive type of dentition in which three cusps in line occur. tricuspid valve. The right atrio-ventricular valve. trigone. A triangular area as of the bladder. trilobite. A fossil crustacean extinct since the Carboniferous. triquetrum. A proximal carpal bone between the lunate and the pisiform. trochanter. One of the two processes below the head of the femur. trochlea. A pulley-like structure. trochophore. The swimming larval stage of some annelids. trophoblast. The ectodermal portion of the fetal villi of mammalian embryos, trjrpsin. The protein-splitting enzyme secreted by the pancreas. tuber cineretim. A conical process of the subthalamus between the mammillary bodies and the infundibulum. tuberculum impar. The anlage of the apex of the tongue. tuberosity. A tuber-like process of a bone. tunica. A covering tissue or coat. ultimobranchial (postbranchial) bodies. Glands of unknown function derived from the last pair of pharyngeal pouches. umbilicus. The scar which marks the abdominal connexion of the umbilical cord. uncinate. Hooked. imcus. The backward-bent anterior portion of the hjppocampal gyrus. tmgulate. Hoofed, urachus. The canal which leads to the allantois and which becomes the median umbilical ligament. ureter. The duct which connects the metanephros (kidney) with the bladder. urethra. The outlet of the bladder. In the male the urinogenital passage. uterus. The "womb" in which the fetus develops. utriculus. That part of the static organ with which the semicircular canals connect. GLOSSARY 455 uvula. The median posterior pendulous process of the soft palate. vagina. The tubular passage from the uterus to the outside. valvulae conniventes. Transverse folds of the lining of the small intestine. varicose. Tortuous and swollen. vas deferens. The efferent duct of the testis. vasa efferentia. The ductules which connect the testis with the ductus (vas) deferens. vasopressin. An hormone secreted by the posterior lobe of the pituitary, which stimulates contraction of smooth muscle, veliger. The trochophore larva of molluscs. ventricle. A small cavity such as is found in the brain and heart, vermis. The median lobe of the cerebellum. vemix caseosa. The waxy covering of the fetal skin. vesicle. A liquid-filled sac or cavity. vitamin. A food substance of unknown composition necessary for growth and health, vitelline. Pertaining to vitellus or yolk. vitreous humor. The translucent semisolid substance between retina and lens, viviparous. Giving birth to living young. volvox. A genus of flagellate algae. Wharton's duct. The duct of the submaxillary. Wirsung's duct. The duct of the pancreas. Wolffian ducts. The ducts of the mesonephros. In the male the ductus deferentes. Wolffian folds or ridges. Paired longitudinal ridges of the embryo from which the paired appendages are differentiated. xiphistemum. The xiphoid cartilage of the sternum. zygapophysis. An articular process of a vertebra. zygomatic bone. The malar bone, zymogen. An enzyme-forming substance. INDEX Bold numbers refer to figures. Abdomen, 215-244 Abducens nerve, 352, 353, 374 Abomasum, 231 Absorption, 234 Acrania, 293 Acromegaly, 335 Adenoid, 226 Adipose tissue, 106 Adrenals, 327, 329 Adrenin, 328 Air bladder, 66, 246, 259, 260 Alar plate, 385, 386 Alimentary canal, 213, 218, 230, 343 Allantois, 87, 235 Amnion, ^;^, 87 Amoeba, 338, 399, 406 Amphibia(n), 16, 23, 29, 31, 32, 40, 45, 49, 53, 129, 131, 148, 153, 162, 167, 171, 172, 180, 184, 200, 217, 222, 225, 228, 236, 252, 258, 259, 270, 279, 297, 300, 329, 358, 359, 364, 367, 369, 375, 397, 398, 403. 40s, 417 Amphioxus, 10, 11, 39-45, 48-54, 58-62, 170, 196, 231, 242, 251, 259, 265- 268, 278, 331, 343, 349, 350, 356, 366, 367, 376, 378, 391, 400, 404- 407 Ampulla, 420 Amylopsin, 242 Anal glands, 126 Anatomy, 85 Ancestry of vertebrates, 427 Animal kingdom, i, 2 Animals, classification of, 22 Angle, facial, 164 Annelids(a), 22, 265, 291, 299, 347, 406 Anthropoids, 21, 24, 94 Antitragus, 419 Anus, 234, 235 Aortic arches, 275, 282, 283 Appendages, 203 paired, 147, 177 to skin, 116 Appendices epiploicae, 234 Appendicular muscles, 212 skeleton, 146, 186, 187 Appendix, 234 Aqueous humor, 409 Arches, 167, 275 branchial, 171 glossopalatine, 225 neural, 149 visceral, 175, 177 Archinephros, 295 Arcurate arteries, 306 Arm muscles, 2 1 2 upper, 186 Arteries, 262, 277, 285, 286, 288, 306, 412 Arthropod(a), 22, 145 Artiodactyla, 19, 23 Ascending colon, 2^5 Atlas, 150 Atrium, 283 Auditory centers of brain, 423 nerve, 352, 355, 375 organs, 415-425 Autonomic nervous system, 377 Aves, 18, 23 Axial skeleton, 146 B Balanoglossus, 4-6 Basal plate, 385, 386 Bile, 239, 240 Bimana, 21, 24 Birds, 18, 31, s,^, 42, 48, 49, 54, 78, 120, 129, 162, 167, 260, 261, 329, 402 eggs, 28 Bladder, 296, 297, 304, 307, 320 Blastocoele, 39, 42 457 458 INDEX Blastoderm, 42, 56 Blastopore, 43, 48, 54, 219 Blastula, 38, 39, 42, 43, 45, 56 Blood, 106, 109, 234, 263-266 plates, 108 vessels, 76, 262, 277, 285-288, 306, 412 Bone, 104, 105, 116, 146, 167 cartilage, 164, 167 centers, 190 homologies of, 187 membrane, 161, 170 Brain, 63, 183, 349, 350, 357, 358, 360, 361, 362-367, 378, 383-38^- 4' -' case, 158 flexures, 386 proportion of, 165 vesicles, 385 Branchial system, 246-252 • Broad ligament, 310 Bronchial tubes, 253, 255 Brunner's glands, 232 Bulbo-urethral glands, 314 Bursa omentalis, 238 Concha, 419 Concrescence theory, 134 Contractile, vacuole, 290 Convergence, 37 Cope's Theory, 133 Coracoid, 181 Cord spinal, 357, 367-373, 387 umbilical, 235 Corium, 114, 115 Corpora cavernosa penis, 303, 314 Corpus albicans, 308 callosum, 362 cavernosum urethrae, 303, 314 luteum, 308 Corpuscles, 107, 238, 263, 396, 397 Cortex, 304, 363 Cortical rays, 304 Costal cartilages, 153 groove, 152 processes, 152 Cow's stomach, 231 Cranial nerves, 353-357- 374, 375, 376 Craniotes(a), 12, 293 Cranium, 158, 167 Cremaster muscle, 312 Crista, 420 Crossopterygii, 23 Cntptochism, 321 Crypts of Lieberkiihn, 233 Cutaneous glands, 125 senses, 395-397 Cyclostomes, 12, 22, 217, 231, 268, 278, 299, 343, 349, 350, 352, 369, 39S Cytogenic glands, 92 Cytology, 85 D Dendrites, 340, 387 Dental arcades, 137 canaliculi, 139 formulas, 135, 137 papilla, 141 ridge, 141 Dentine, 140, 141 Descending colon, 235 Deuterencephalon, 348 Deuterostomia, 3 Deutoplasm, 27 Development, 38, 49, 57 aortic arches, 282 brain, 383-386 cranium, 168 ear, 423 esophagus, 228 eye, 412 gills, 248 heart, 279 intestine, 235 liver, 240 lungs, 256, 257 motor nerves, 388 mouth, 218 muscles, 205 pancreas, 243 ribs, 152 sahvary glands, 222 sensory nerves, 389 skeleton, 187 skin, 1 14 skull, 159 spinal cord, 386-391 sternum, 155 stomach, 231 sympathetic ganglia, 390 teeth, 141 tongue, 224 INDEX 459 Development, urogenital system, 290-323 vertebral column, 151 visceral skeleton, 176 Diaphragm, 77, 204, 253, 257 Diaphysis, 188 Diencephalon, 350, 351, 385 Differentiation theory of Cope and Osborn, 133 Digestion, 244 Digestive epithelium, 87 system, 215-244 Dioecious, 26 Dipnoi, 23, 231, 269, 270, 401 Dogfish, 80, 90, 356, 357 Ducts, female, 322 hepatic, 240 interlobular, 240 male, 322 Miillerian, 301, 302, 322, 323 papillary, 319 renal, 307 reproductive, ^22 Santorini's, 242 Stenoris, 221 thoracic, 279, 289 Wharton's, 221 Wirsung's, 242 Wolffian's, 301, 322 Ductus deferens, 313 ejaculatorius, 314 Duodenum, 232 Dura mater, 392 Dwarf, 334 Ear, 415-425 bones, 425 Echinoderms, 3, 22 Egg, 26, 27, 28, 29, 30, 37, 39, 40, 41, 55, 57,78 Ejaculatory duct, 314 Elasmobranchii, 23, 131, 132, 146, 160, 162, 171, 176, 196, 198, 217, 252, 269, 300, 356, 369, 401, 403, 417, 419 Elastic fibers, loi Embryo, ss, 43, 47-49, 51-53. 55. 57, 5^, 62-64, 67, 68, 74, 79, 80, 82, 87, 177, 235 Embryonic membranes, 80 nerve cell, 98 Embryonic shield, 56 Enamel, 139, 141 Endocardium, 284 Endocrine, 328 glands, 66, 120, 253, 324-337 Endolymph, 420 Endoskeleton, 145 Endothelium, 87 Ensiform process, 154 Enteron, 64 Enteropneusta, 4 Enzymes, 215, 221, 230, 242, 243 Eoanthropus Dawsoni, 24 Ependymal layer, 386, 387 Epicardium, 284 Epidermis, 86, 89, 112, 114 Epididymis, 312 Epinephrine, 328 Epiphyses, 190, 351 Epithelial bodies, 253 tissues, 86 Epithelium, 88 ciliated, 88 columnar, 88 digestive, 87 evolution of, 1 11 glandular, 91 neuro-, 91 sensory, 91 stratified, 88 I':pitheloid, 87 P^pitrichial layer, 115 Equihbration, 420 Erythrocytes, 107, 263 Esophagus, 217, 226, 227, 228, 231 Ethmoid, 169 Eustachian tubes, 226, 419, 425 Evolution, 37 aortic arches, 275 arteries, 277 autonomic system, 382 blood vessels, 263 brain, 362-367 cortex, 364 cranial nerves, 374 cranium, 158 cutaneous sense organs, 395 digestive system, 215 extremities, 181 heart, 275 integument, 11 1 lymphatic system, 278 460 INDEX Evolution, mouth, 218 muscular system, 193 paired appendages, 177 ribs, 153 sense cells, 393 skeleton, 182 skin, III skull, 162, 164 spinal cord, 367-373 nerves, 376, 377 sternum, 156 teeth, 129 urogenitary system, 290-297 veins, 277 vertebral column, 146 visceral skeleton, 170 Excretory organs, 50, 92, 248, 290-297 Exoskeleton, 145 Exterior senses, 393 Externa, 285 External ear, 419 genitals, 302, 323 gills, 248 respiration, 245, 246-249 sphincter, 235 Extremities, 181 Eyeball, 412, 413 Eyelids, 412 Eyes, 64, 408-415 coloring, 124 unpaired, 415 Facial angle, 164 Facialis nerve, 352, 353, 354, 375 Falciform ligament, 239 Falx cerebri, 392 Feathers, 116, 119, 120 Felis, 3 Female reproductive organs, 308-311,322, 326 Fertilization, 29 Fetal membranes, 80 structures, 82 Fibrin, 107 Fibrinogen, 107, 263 Filiform papillae, 223 Fin-fold theory, 177 Finger-prints, 115 Fins, 177, 181 Fishes, 15, 77, 126, 148, 159, 160, 162, 167, 225, 231, 245-249, 259, 265, 272, 276, 278, 329, 331, 359, 367, 369. 375, 397, 398, 403 Flame cells, 291 Flatworms, 193, 216, 236, 290, 291, 298, 299, 341, 345, 346, 406 Floating ribs, 152 Floor plate, 385, 386 Foliate papillae, 223 Follicular layer, 308 Fontanelles, 170 Foot, 186 Foramen epiploicum, 238 Forearm, 186 Fore-brain, 385 Fossil skulls, 165, 166 Fovea centralis, 411 Freckles, 123 Friction-ridge patterns, 115, 1 16 Frog, 32, 40, 41, 42, 46, 47 Fungiform papillae, 223 Gall bladder, 239, 240 Ganglia, 96, 340, 342, 412 habenular, 351 sympathetic, 358, 379, 381, 382 Ganoidei, 23, 162 Gastric gland, 228, 230 Gastrula, 44, 45 Genitals, external, 302, 323 folds, 320 Geologic time, 25 Germ disc, 28, 42 ring, 43, 47 Germinal bodies, 26 cells, 387 Gestation, 35 Gills, 65, 184, 246-249, 251, 252, 417 Girdles, 146, 178, 179, 181 Gladiolus, 153 Glands, 66, 91, 112, 124-127, 221 adrenals, 327, 329 anal, 126 Brunner's, 232 buccal, 221, 223 bulbo-urethral, 314 cardiac, 228, 230 Cowper's, 314 endocrine, 66, 120, 253, 324-337 INDEX 461 Glands, female sex, 326 gastric, 228, 230 greater vestibular, 311 intestinal, 233 labial, 221, 223 lacrimal, 126,412,413 Lieberkiihn's, 234 lingual, 221, 223 male sex, 326 mammary, 126, 127 Meibomian, 126, 412 molar, 221 multicellular, 92 palantine, 221, 223 parathyroid, 253, 331, 332, 33;^ pineal, 362 pituitary, 334-336 preputial, 126 prostate, 314 pyloric, 228, 231 salivary, 221-223 sublingual, 221, 223 submaxillary, 221, 223 suprarenal, 327 tarsal, 413 thymus, 253, 7,33 thyroid, 253, 329, 331, 332 tubulo-acinous, 221 Glans, 302 clitoridis, 304, 323 penis, 314,323 Glossopharyngeus nerve, 352, 354, 355, 375 Glottis, 255 Glycogen, 238 Goethe-Oken theory, 158 Gonads, 321 "Goose flesh," 381 Gorilla, 185 Gray matter, 357, 369, 372 Greater curvature of stomach, 228, 231 omentum, 236 vestibular glands, 311 Groove, costal, 152 Gubernaculum testis, 322 H Hair, 116, 120-123 arrangement, 12: "cowlicks," 121 cross section, 121 Hair, direction, 121 histogenesis, 122 pigments, 123, 124 "rhomboids," 121 root, 121 slant, 121 structure, 12 r "vortices," 121 Hare-lip, 403 Hatching, 53 Haversian systems, 105 Hearing, 393, 415-425 Heart, 76, 183, 262, 275, 279, 281, 283-285 HeUx, 419 Hemichordates, 4, 5, 22, 231, 348 Hilum, 304 Hind brain, 385 Histology, 85 History of esophagus, 228 gills, 249, 251 intestine, 235 liver, 242 pancreas, 244 pulmonary system, 258 saHvary glands, 222 stomach, 231 tongue, 224 Homo Heidelbergensis, 24 Neanderthalensis, 24 Rhodesiensis, 24 sapiens, 24 Hoofs, 119 Hormone, 325 Horns, 118 "Horny layer," 90 scales, 117, 118 Human arteries, 286 bones, 146 brain, 378, 380, 384 circulation, 288 digestive system, 217-244 ear, 419-425 eye, 409 heart, 283-285 lymphatic system, 288 muscles, 202, 203, 204-214 nervous system, 339 olfactory organ, 402 ovum, 30 skeleton, 154, 155, 167, 173, 174, 186 skin, 113 skull, 164, 165, 168, 169 462 INDEX Human spinal cord, 378, 380 spine, 149, 150 stomach, 228, 229 teeth, 139-144 urogenital system, 304-315 veins, 287 Humors, 409 Hymen, 310, 322 Hyoid, 173 Hypocone, 133 Hypoconid, 133 Hypoglossus nerves, 352, 375 Hypomere, 213 Hypophysis, 351 Ichthyopsida, 23 Ileocolic valve, 232, 234 Ileum, 232, 233 Impregnation, 30 Incisures, 149 Inferior concha, 173 Infundibulum, 308, 348 Inguinal canals, 312 Insectivora, 19, 23, 149 InsuUn, 243, 325 Integumentary system, 1 1 1 Interior senses, 393 Interlobular arteries, 306 Internal ear, 420 respiration, 245, 253-261 Interstitial tissue, 312, 313 Intestine, 231-236 Intima, 285 Intra-uterine development, 35 Invertebrates, stomachs of, 231 teeth of, 129 Iris, 410 Islands of Langerhans, 243, 325 Jaws, 146 Jejunum, 232 Jelly fish, 145 K Kangaroo, 35 Keratin, 90 Kidneys, 72, 183, 304, 305, 306, 307 Labia majora, 311, 323 minora, 311, 323 Labial swellings, 323 Lacrimal glands, 1 26 Lamina, 149 labial, 141 lingual, 141 Land animals, 183, 184 Large intestine, 231, 234-236 Larva, 32, 36 Larvacea, 9 Laryngeal cavity, 226 Larynx, 226, 253, 255 Lateral-Hne organs, 397-399 Leg, 183, 186, 212 Lemuroid(s) (ae), 20, 24 Lens, 410 Lesser curvature of stomach, 228, 231 Leucocytes, 108, 263 Ligaments, loi falciform, 239 Limb, upper, 186 Lining of stomach, 228 Lipase, 230, 243 Liver, 50, 66, 217, 238-242 "Living substance," 85 Long bones, 188-190 Lungs, 50, 66, 246, 253-255 Lymph, 108, 262 Lymphatics, 234, 262, 278, 279, 288, M Macula lutea, 411 Major senses, 393 Male reproductive organs, 295, 303, 311, 313, 322, 326 Malpighian corpuscle, 71 Mammal(ia), 18, 23, 31, 34, 120, 122, 132, 134, 157, 162, 167, 172, 176, 181, 183, 203, 217, 223, 236, 271, 273, 279, 284, 301, 329, 361, 402, 418 Mammal eggs, 28, 31 Mammary glands, 126, 127 Mandible, 173 Mantle layer, 386, 387 Manubrium, 153 Marginal layer, 387 Marsupials(ia), 23, 35, 84, 303 Maxilla, 175 INDEX 46^ Meatus, 419 Media, 285 Mediastinum, 253 Medulla, 304 Medullary sheath, 98, 342, 370 Meibomian glands, 126 Meissner's corpuscles, 396 Melanophores, loi Membrane bones, 161, 170 cloacal, 235 Meninges, 391 Mentum, 173 Mesencephalon, 350, 351, 385 Mesenchyme, 74, 100, 151, 413, 414 Mesenteries, 236, 238 Mesoderm, 49, 54, 59, 66 Mesonephros, 294, 315, 317, 318 Mesosternum, 153 Mesothelium, 87 Mesovarium, 308 Mesozoic, 25 Metabolism, 38, 290 Metacone, 133 Metaconid, 133 Metamorphosis, 32 Metanephridia, 291, 292 Metanephros, 296, 315, 318 Metasternum, 154 Metazoa, 3, 45, 264, 291 Metencephalon, 350, 351, 385 Mid-brain, 385 Middle ear, 419, 425 Midget, 334 Minor senses, 393 Moles, 123 Mollusca, 22 Molluscoida, 22 Monodelphians, ig Monoecious, 26 Monotremes, 18, 23 Mons pubis, 311 Motor nerves, 352, 388 plates, 93, 94 Mouth, 226 cavity proper, 2 1 7 Muscles, 50, 93, 191-214 alimentary canal, 213 appendicular, 212 architecture, 192 arm, 212 belly, 191 cardiac, 96 Muscles, extrinsic, 201 eye, 196, 197, 414 heart, 193 hypobranchial, 196 integumental, 204 intrinsic, 201 laryngeal, 214 lateral trunk, 210 leg, 212 origin, 193 paired, 204 smooth, 93, 95, 193, 227, 228 somatic, 205 spindles, 426 striated, 94, 193, 224, 228 striped, 94, 193, 227 superficial in man, 206, 207 tendon, 191 tongue, 212 unstriated, 93, 95, 193 visceral, 195, 205, 212 Muscular digestive tube, 216 system, 191-214 Myelencephalon, 350, 351, 385 Myocardium, 284 Myocommata, 194 Myotome, 194 N Nails, 118 Nasal cavity, 226 passages, 226, 253, 257, 258 Nemathelminthes, 22 Neocortex, 364 Neostoma, 218 Nephrostome, 292 Nerves, 99, 288 cells, 98, 99, 343 types of, 97 cranial, 352, 357, 374, 376 spinal, 212 terminals, 91 Nervous system, 50, 96, 338-392 Neural arch, 149 crest, 390 groove, 383 plate, 383 tube, 62 Neuraxon, 98 Neurilemma, 98 Neurite, 340 464 INDEX Neuroblast cells, 487 Neuroglia, 99 Neuromasts, 397 Neuron, 96, 97, 341, 342, 344 relations in spinal cord, 372 Neurosensory cells, 338, 393, 394 Nodes of Ranvier, 99 Non-epithelial tissues, 93 Notochord, 52, 59, 63, loi, 102, 146, 185 224 Nuclei pulposi, 151 O Oculomotor nerves, 352, 353, 374 Olfactory nerves, 352, 353, 374 organs, 399-403 Omasum, 231 Omenta, 236, 238 Ontogenesis, 157 olfactory organ, 403 Optic nerve, 352, 353, 374, 413 vesicles, 413 Oral cavity, 226 Organisms, sequence of, 25 Organogenesis, 58, 62, 78 Organs, 85 Orifice, cardiac, 228 Ornithodelphians, 18 Os uteri, 310 Osborn's Theory, 133 Ostium tubae, 308 Ostracoderm(ata)(i), 13, 23 Ova, 308 Ovarian ligament, 308 Ovaries, 308, 309 Oviparity, 30, 31 Ovum(a), 30 Pain, 395 Paired appendages, 147 eyes, 196, 407 fins, 178 gills, 246 muscles, 204 Palantine tonsil, 225, 226, 253 Palate, soft, 225, 226 Paleostoma, 218 Paleozoic, 25 Pancreas, 66, 217, 242-244, 325 Papillae, 223, 225 Paracasein, 230 Paracone, 133 Paraconid, 133 Parasympathetic nerves, 381, 383 Parathyroid, 66, 331-333 Parietal cells, 230 eyes, 415 Parotid, 221, 223 Pectoral girdle, 179, 180, 181, t86 Pedicle, 149 Pelvic girdle, 148 Pelvis, 318 Penis, 314 Pepsin, 230 Perichondrium, 188 Perilymph, 420 Perimysium, 191 Perineum, 235, 320 Periosteum, 188 Perissodactyls(a) , 20, 23 Peristaltic waves, 233, 239 Peritoneum, 236 Pharyngeal cleft, 65 derivatives, 253 Pharynx, 217, 225 Phylogenesis, nervous system, 344 reproductive system, 297-303 urinary system, 290-297 vertebrate mouth, 221 Phylogenetic series, 3 tree, 2, 431 Pia mater, 392 Pigmentation, 123, 124 Pineal, 362 eyes, 415 Pinna, 419 Pisces, 15 Pithecanthropus, 24 Pituitary, 334-33^ Placentals, 19, 23, 36, 37, 55, 57, 83, 303 Plasma, 107, 263 Platyhelminthes, 22 Platyrhina, 21, 24 Pleural cavity, 253-261 Plexus, 341 Polyphyodont, 134 Porifera, 22 Postcava, 284 Postganglionic fibers, 381 Precava, 284 Preganglionic fibers, 381 Prepuce, 314, 323 INDEX 465 Preputial glands, 126 sac, 303 Pressure, 395 Presternum, 153 Primates, 20, 23, 34, 137 Primitive streak, 54, 55 Proboscidians, 136 Process articular, 150 costal, 152 ensiform, 154 spinous, 149 Progestin, 327 Pronephros, 294, 315 Prosencephalon, 348 Prostate gland, 314 Protection of eggs, 31 Proterostomians, 3 Protochordates, 129, 231, 246, 299, 430 Protocone, 133 Protoconid, 133 Protonephridia, 291 Protoplasm, 85 Protozoa, 3, 22, 215, 290, 297 Psalterium, 231 Pulmonary system, 253-261 veins, 284 Pulp cavity, 139 Pyloric gland, 228, 231 Pylorus, 228 R Rami, 173 Ranvier, nodes of, 99 Raphe, 312, 323 Receptor neurons, 97 Rectum, 234, 235 Renal columns, 304 corpuscle, 71 ducts, 307 organs, 315 pyramids, 304 sinus, 304 tubules, 304 Rennin, 230 Reproduction, 26, 36, 38 Reproductive organs, 50, 320-323 Reptile(ia), 17, 23, 31, 33, 42, 48, 49, 54, 78, 117, 118, 129, 131, 132, 148, 153, 157, 162, 164, 167, 172, 180. 181, 202, 222, 228, 236, 271, 273, 297, 300, 331, 359, 361, 367, 369, 402, 403, 418 Reptile eggs, 28 Respiratory s>'stem, 170, 245-261 Reticulum, 231 Retina, 411 Ribs, 146, 152, 153 Rodent(ia), 19, 23, 136 Roof plate, 385, 386 Rotifera, 22 Rumen, 231 Ruminants, 231 Sacrum, 151 Salivary glands, 221 Scala media, 421 Scales, 116, 117 Schwaun, sheath of, 98 Scrotal swellings, 323 Scrotum, 311 Sebaceous glands, 126 Secondary sense cells, 340 Secretin, 325 Secretory glands, 92 Segmentation, 52 Seminal vesicles, 313 Sense organs, 393-426 Sensory nerves, 352, 389 neurons, 393 processes, 394 Sertoli cells, 312 Serum, 107, 263 Sexes, 5, 12, 26, 296-323 Sexual maturity, ^^ reproduction, 297 Shank, 186 Sheath, medullary, 98 Schwann's, 98 Shells, 116 Shoulder girdle, 146 Sight, 393, 406-415 Sigmoidal curve, 149 Sinanthropus Pekinensis, 24 Sigmoid colon, 235 Single cell organisms, 215 Skeleton, 50, loi, 145-190 appendicular, 146 axial, 146 parts, 145 system, 145 tissue, 147 visceral, 158 466 INDEX Skin, 49, 86, 89, iii-ii6 color, in man, 123, 125 Skull, 146, 158, 162, 167 fossils, 165, 166 proportion of, 164 Smallintestine, 231-233 Smell, 393, 399-400 Soft palate, 225, 226 Somatic fibers, 381 motor nerves, 352, 358 muscles, 205 Sperm, 30 Spermatophore, 31 Spermatozoon(oa), 26, 27 Sphenoid bone, 169 Sphincter ani, 235 Spinal accessory nerves, 352, 375 cord, 357, 367-373. 3^7 nerves, 212 Spines, 116, 149 Spinous process, 149 Spiral ligament, 421 Sponge(s), 193 Spongioblast cells, 387 Static organs, 415- 425 Stratum germinativum, 122, 123, 125 Stegocephala, 23 Stenon's duct, 221 Sternum, 146, 153, 155, 156 Stomach, 217, 228-231 Stomodeum, 218 Stroma, 308 Structure of hair, 1 2 1 Subgerminal cavity, 42 Submucosa, 227, 228 Subungulates(a), 20, 23 Suprapericardial bodies, 253 Suprarenals, 327 Surface of stomach, inner, 228 Survival, 38 Suspensory ligament, 30S Sweat glands, 125 Sympathetic ganglia, 358, 379, 381, 383, 390 Systemic circulation, 28S Tactile cells, 39s Tadpoles, 32 Tails, 183 Taste, 393, 404-406 buds, 91, 224, 404 Teat, 128 Teeth, 116, 129-144 carnivora, 136 compound, 133 horny, 129 human, 139-144 mammals, 134 Neanderthal man, 138 permanent, 142 proboscidians, 136 rodents, 136 Telencephalon, 350, 385 Teleostei, 23, 131, 162, 171, 417 Telodendria, 340 Tendons, loi, 191 Teniae, 234 Tentorium cerebelli, 392 Testes, 311 Tetrapods, 23, 225, 383 Theelin, 327 Theory of evolution, 204 vertebral, of skull, 158 Thermorpha, 23 Thigh, 186 Thymus, 66, 333 Thyroid, 66, 173, 329, 331, 332 Thyroxine, 330 Time, 25 Tissue, 85 adipose. 106 areolar, loi connective, 100 mechanical support, 100 skeletal, loi subcutaneous, 106 Tongue, 202, 212, 223-225 Tonsil, 225, 226, 253 Touch, 393, 395 Trachea, 253-255 Tragus, 419 Transverse colon, 235 foramen, 150 septum, 77 Tributercular, 133 Triconodont, 133 Trigeminal nerve, 352, 353. 354. 374 Trochlearis nerve, 352, 353, 374 Trypsin, 242 Trypsinogen, 242 Tuberculum, 152 INDEX 467 Tunica adventitia, 227 albuginea, 312 mucosa, 227 muscularis, 227, 228 submucosa, 227, 228 Tympanic membrane, 419 U Ultimobranchial bodies, 253, ^^^ Umbilical cord, 235 stalk, 83 Ureters, 304, 307 Urethra, 303, 308, 323 Urinary canal, 303 organs, 304-308 sj'stem, 290 Urochordates, 7, 9, 22, 231, 249, 348, 406, 416 Urodeles, 201 Urogenital system, 290-323 Uterine tubes, 308 Uterus, 310, 323 Uvula, 226 Vagina, 310, 32^ Vaginal glands, 1 26 sacs, 321 Vagus nerve, 352, 354, 355, 375 Vallate papillae, 223 Vascular system, 262-289 Vasa vasorum, 288 Vasoconstrictor nerves, 288 Vasodilator nerves, 288 Veins, 262, 277, 287, 412 Ventricle, 283 Vermiform appendix, 234 Vertebrae, 102, 148 cervical, 149 dorsal, 149 kinds of, 150 lumbar, 149, 150 sacral, 181 structure, 149 thoracic, 149, 150 Vertebrae, "true," 149 Vertebral canal, 149 column, 146, 149, 151 Vertebrates, 12, 22, 62, 67-69, 72, 94, 117, 129, 131, 146, 167, 176, 182, 205, 216, 218, 219, 220, 224, 225, 228, 231, 242, 251, 256, 265, 296, 298, 343, 347, 349, 352, 376, 377, 382, 400, 416, 425 ancestory of, 427 eye, 412 Vestibular nerve, 424 Vestibule, 217, 311, 323 Visceral fibers, 381, 382 muscles, 195, 205, 213 skeleton, 158, 167, 170, 172, 173,174, 177 Vitelline duct, 235 membrane, 28 Vitreous humor 409 Viviparity, 30, 31, 34, 37 W Walk of digestive tract, 227 Warmth, 395 Waves, peristaltic, 233, 239 Wax glands, 1 26 Wharton's duct, 221 White matter, 370 rami communicantes, 381 Wolffian folds, 187- X Xenarthra, 19, 23 Xiphvid. 154 Yolk, 27, 34, 41, 57, 78 plug, 47, 48 sac, 34 Zygapophyses, 150