SINS 
 
 UNIVERSITY OF CALIFORNIA 
 
 SAN FRANCISCO MEDICAL CENTER 
 
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COMPARATIVE ANATOMY 
 OF VERTEBRATES 
 
 KINGSLEY 
 
COMPARATIVE ANATOMY 
 
 OF 
 
 VERTEBRATES 
 
 BY 
 
 J. S.lKINGSLEY 
 
 PROFESSOR OF BIOLOGY IX TUFTS COLLEGE 
 
 WITH 346 ILLUSTRATIONS 
 LARGELY FROM ORIGINAL SOURCES 
 
 KC ~ 
 
 7 / (^ 
 
 PHILADELPHIA 
 
 P, BLAKISTON'S SON & CO 
 
 1012 WALNUT STREET 
 1912 
 
 ZllQlo 
 
 ! ! N 
 
Copyright, 1912, by P. Blakiston's Son & Co. 
 
 THE. MAPLE • PRESS- YORK • PA 
 
PREFACE. 
 
 Vertebrate anatomy is everywhere taught by the laboratory method. 
 The student studies and dissects representatives of several classes, thus 
 gaining an autoptic knowledge of the various organs and their positions 
 in these forms. These facts do not constitute a science until they are 
 properly compared and correlated with each other and with the condi- 
 tions in other animals. It is the purpose of the author to present a 
 volume of moderate size which may serve as a framework around which 
 these facts can be grouped so that their bearings may be readily recog- 
 nized and a broad conception of vertebrate structure may be obtained. 
 
 In order that this may be realized, embryology is made the basis, 
 the various structures being traced from the undifferentiated egg into 
 the adult condition. This renders it easy to compare the embryonic 
 stages of the higher vertebrates with the adults of the lower and to 
 recognize the resemblances and dijfferences between organs in the 
 separate classes. There has been no attempt to describe the structure 
 of any species in detail, but rather to outline the general morphology 
 of all vertebrates. To aid in the discrimination of the broader features 
 and the more minor details, two sizes of type have been used, the 
 larger for matter to be mastered by the student, the smaller for details 
 and modifications in the separate classes to which reference may need 
 to be made. 
 
 Considerable space has been given to the skull, as there is no 
 feature of vertebrate anatomy which lends itself more readily to 
 comparative study of the greatest value to the beginning student, 
 while the same specimens can be used in the laboratory year after year. 
 The skull also has a special interest since nowhere else is there the same 
 chance of tracing modifications in all groups since the first appearance 
 of vertebrates on the earth. To aid in this, extinct as well as recent 
 species have been included. 
 
 It was the desire of the author to adopt the nomenclature of the 
 German Anatomical Society ('BNA'), but this was often found im- 
 practicable. The BNA was based solely upon human anatomy and 
 it fails utterly in many respects when the attempt is made to transfer 
 its terms to other groups. The single example of ' transverse process' 
 
 v 
 
VI PREFACE. 
 
 is sufficient to illustrate this. To the writer another objection is that 
 the BNA strives to do away with all personal names. These, it would 
 seem, have a great value as they are indications of the history of 
 anatomical discovery and memorials of the great anatomists of the 
 past. Dorsal and ventral are used instead of the anterior and pos- 
 terior of human anatomy, while anterior indicates toward the head, 
 posterior toward the tail, these terms being readily applied to all ver- 
 tebrates, man only excepted. Cephalad and caudad, adopted by 
 some, lead to occasional peculiar phrases. The German word 
 ' anlage ' has been adopted bodily, and seems to call for no defense. 
 It implies the indifferent embryonic material from which a part or an 
 organ develops. 
 
 The illustrations have been drawn or redrawn expressly for this 
 work. Some of them are original, some based on figures in special 
 papers. Practically none have ever appeared in any text-book in the 
 English language. In selecting the objects to be figured especial 
 pains has been taken to avoid the forms usually studied in our 
 laboratories, thus relieving the student of the temptation of copying 
 the figure, instead of drawing from nature. Especial thanks are due 
 to Professor C. F. W. McClure, who allowed me to draw at will from 
 the splendid collection which he has built up at Princeton. These 
 figures are indicated by the word 'Princeton' followed by the num- 
 ber of the preparation in the museum of the University. 
 Tufts College, Mass. 
 
CONTENTS 
 
 Pagb 
 
 Introduction i 
 
 Introductory embryology 6 
 
 Histology i6 
 
 Epithelial tissues 17 
 
 Nervous tissues 19 
 
 Muscular tissues 20 
 
 Connective tissues 21 
 
 Comparative morphology of vertebrates 25 
 
 Integument 25 
 
 Skeleton 37 
 
 Dermal skeleton 39 
 
 Endoskeleton 42 
 
 Vertebral column 44 
 
 Ribs 53 
 
 Sternum 56 
 
 Epistemum 59 
 
 Skull 59 
 
 Skull of cyclostomes 75 
 
 Skull of elasmobranchs 76 
 
 Skull of teleostomes 77 
 
 Skull of amphibia 82 
 
 Skull of reptiles 87 
 
 Skull of birds 95 
 
 Skull of mammals 98 
 
 Appendicular skeleton 102 
 
 Median appendages 103 
 
 Paired appendages 103 
 
 Shoulder girdle 105 
 
 Pelvic girdle 109 
 
 Free appendages 114 
 
 Coelom (body cavities) 120 
 
 Muscular system 124 
 
 Parietal muscles 125 
 
 Visceral muscles 132 
 
 Dermal muscles 134 
 
 Diaphragm 135 
 
 Electrical organs 135 
 
 Nervous system 137 
 
 Central nervous system 138 
 
 Spinal cord 138 
 
 vii 
 
VIU CONTENTS, 
 
 Page 
 
 Brain 140 
 
 Brain of cyclostomes 152 
 
 Brain of elasmobranchs 153 
 
 Brain of teleostomes 153 
 
 Brain of dipnoi 155 
 
 Brain of amphibia 155 
 
 Brain of reptiles 157 
 
 Brain of birds 158 
 
 Brain of mammals 158 
 
 Peripheral nervous system 161 
 
 Spinal nerves 161 
 
 Sympathetic system 163 
 
 Cranial nerves 165 
 
 Sensory organs 177 
 
 Nerve-end apparatus 178 
 
 Lateral line organs " 179 
 
 Auditory organs 182 
 
 Organs of taste 189 
 
 Olfactory organs 189 
 
 Eyes 198 
 
 Digestive organs 205 
 
 Oral cavity 208 
 
 Teeth 208 
 
 Tongue 217 
 
 Oral glands 220 
 
 Pharynx 222 
 
 (Esophagus 222 
 
 Stomach 223 
 
 Intestine 227 
 
 Liver 231 
 
 Pancreas 234 
 
 Respiratory organs 235 
 
 Gills (branchiae) 236 
 
 Pharyngeal derivatives 245 
 
 Swim bladder 247 
 
 Lungs and air ducts 250 
 
 Air ducts 251 
 
 Lungs 255 
 
 Accessory respiratory structures 263 
 
 Organs of circulation 264 
 
 Blood and lymph 265 
 
 Blood-vascular system 266 
 
 Embryonic circulation 268 
 
 Heart 269 
 
 Arteries 273 
 
 Veins 276 
 
CONTENTS. IX 
 
 Page 
 
 Definitive circulation 280 
 
 Heart . . . . , 281 
 
 Aortic arches 282 
 
 Arteries 284 
 
 Veins 289 
 
 Foetal circulation 293 
 
 Circulation in the separate classes 294 
 
 Lymphatic system . ^02 
 
 Urogenital system ^07 
 
 Excretory organs 308 
 
 Reproductive organs 319 
 
 Reproductive ducts 321 
 
 Excretory organs of the separate groups 326 
 
 Reproductive organs of the separate groups 331 
 
 Copulatory organs 342 
 
 Hermaphroditism 346 
 
 Foetal envelopes 348 
 
 Adrenal organs 3^2 
 
 Bibliography 354 
 
 Definition of systematic names 381 
 
 Index 385 
 
INTRODUCTION. 
 
 Any animal or any plant may be studied from several different 
 points of view, four of which are concerned in the present volume. 
 We may study its structure, ascertaining the parts of which it is com- 
 posed and the way in which these parts are related to each other. This 
 is the field of Anatomy. If we go into the more minute structure, for 
 which the microscope has to be used, we are entering the special 
 anatomical field of Histology. When two or more different animals 
 are compared in points of structure, their resemblances and differences 
 being traced, the study is called Comparative Anatomy, and it is only 
 through such comparisons that we are able to arrive at the true meanings 
 of structure. Then it is of interest to see the way in which the structure 
 comes into existence in development from the comparatively simple egg 
 from which it arises — the province of Embryology or Ontogeny. Anat- 
 omy and ontogeny together give us a knowledge of the form and how 
 it has arisen and they are frequently grouped as Morphology. But mor- 
 phology merely deals with the parts of a machine and these are usually 
 studied in the dead organism; fully to appreciate the mechanism we 
 should know how the parts and the whole perform their work, the 
 study of function or Physiology. 
 
 In view of the foregoing the present volume is to be regarded as 
 rather a comparative morphology of vertebrates, with here and there 
 hints at the physiological side. Farther, there is an adaptation of 
 the organism to the conditions in which it has to live, and the inter- 
 actions of this environment upon the animal have to be considered, at 
 least to a slight extent. 
 
 Zoologists divide all animals into two great groups, the Protozoa, 
 in which the organism consists of a single cell, and the Metazoa, in 
 which the body is composed of many cells, which vary according to 
 the functions they have to perform. Of the Metazoa there are several 
 divisions — ^Porifera (sponges), Coelenterata (sea anemoiies, jelly fish), 
 Echinoderma (starfish, sea urchins), Platodes (flatworms), Rotifera, 
 Ccelhelminthes (ordinary worms), Mollusca, Arthropoda (crabs, 
 insects), and Chordata. 
 
2 INTRODUCTION. 
 
 The Chordata are bilaterally symmetrical animals with metameric 
 bodies, which agree in several features not found in the other groups. 
 These are (i) a central nervous system, entirely on one side of the di- 
 gestive tract; (2) the presence of gill slits in the young if not in the 
 adult; (3) an unsegmented axial rod, the notochord, between the 
 digestive tract and the nervous system. All of these features will be 
 described later. 
 
 There are three or four divisions of Chordata, the uncertainty 
 depending upon the position to be accorded the Enteropneusta. These 
 are worm-like animals, occurring in the sea and represented on our 
 shores by Balanoglossus, What has been described as a notochord is 
 a pocket from the digestive tract, lying in a curious proboscis above 
 the mouth. 
 
 The next division, the Tunicata, includes the (marine) ^sea-squirts.' 
 They were long regarded as molluscs, but the discovery that the young 
 have true gill slits, a nervous system on one side of the alimentary 
 canal, and, above all, a notochord, placed them in the present associa- 
 tion. Their young (larvae) are tadpole-like, the notochord is confined 
 to the tail, but later the tadpole features are lost and with them the 
 tail and notochord, and the adult is a sac-like animal with no re- 
 semblances to its former state, or to its allies. 
 
 The third division, the Leptocardii, embraces Amphioxus and 
 a few other marine, fish-like animals. They were long classed as fishes, 
 but are far more simple than any true fish. The body is markedly 
 segmented, the gill slits are very numerous and the excretory organs 
 open separately to the exterior and are vermian in character. Stomach, 
 vertebrae and heart are lacking and the brain and sense organs are 
 very rudimentary, while jaws and paired appendages are absent. 
 
 The last class, the Vertebrata, are most nearly related to the Lepto- 
 cardii, but differ in many important respects-. Thus there is always 
 a skull and vertebral column; the brain is larger than the spinal cord; 
 there are always nose, eyes and ears; a heart is present and the excre^ 
 tory organs open into a common duct on either side, with an external 
 opening near the anus. 
 
 Most of the characteristics of a vertebrate may be seen from' the 
 accompanying diagram. The body is bilaterally symmetrical, with 
 anterior and posterior ends, d6rsal and ventral sides well differentiated. 
 There is no external segmentation, since the muscles are not directly 
 attached to the skin, but a metameric arrangement of parts is notice- 
 
INTRODUCTION. 3 
 
 able in muscles, skeleton, nerves, blood-vessels, and, to a less extent, 
 in the excretory organs. There is no cuticular skeleton but the outer 
 layer of the skin may be cornified or the deeper layer may give rise 
 to ossifications (scales of fishes, etc.). 
 
 There is an internal axial skeleton, consisting of the notochord, 
 around which are developed rings of denser material, constituting a 
 backbone or vertebral column, while in front a skull encloses the brain 
 and organs of special sense, and gives support to the primitive respira- 
 tory organs (gills) , which are always connected with the digestive tract. 
 Typically there are two kinds of appendages, each with an internal 
 skeleton. These are the unpaired or median fins, dorsal and ventral, 
 which occur only in the Ichthyopsida, and the paired appendages, 
 of which there are two pairs, anterior and posterior in position. 
 
 Fig, I. — Diagram of a vertebrate, a, anus; &, brainy c, coelom; da, dorsal aorta; df^ 
 dorsal fin; g, gonad; gd, genital duct; h, heart; i, intestine; /, liver; m, mouth; n, notochord; 
 p, pancreas; pc, pericardium; pf, pectoral fin; ph, pharynx, with gill clefts; s, stomach; 
 sc, spinal cord; sp, spleen; u, ureter; va, ventral aorta; vc, vertebral column; rf, ventral fin. 
 
 The central nerv^ous system consists of brain and spinal cord which 
 lie dorsal to the notochord, and are usually protected by arches arising 
 from the vertebrae and by the roof of the skull. Eyes and ears are the 
 highest of the sense organs. The alimentary canal always has a 
 liver connected with it, and a portion of the canal just behind the mouth 
 is developed into a pharynx, from which, in the young of all, gill clefts 
 extend through to or toward the exterior. In the terrestrial vertebrates 
 these gill clefts are later replaced by lungs which develop from the 
 hinder part of the pharyngeal region. 
 
 The blood, which always contains two kinds of corpuscles, flows 
 through a closed system of vessels. A heart, ventral to the digestive 
 tract and lying in a special cavity, the pericardium, is always present. 
 
4 INTRODUCTION. 
 
 The heart consists of two successive chambers, an auricle (atrium) 
 and a ventricle, and in forms which respire by means of gills, contains 
 only venous blood. With aerial respiration both chambers may become 
 divided into arterial and venous halves. A dorsal aorta, lying above 
 the alimentary canal, is always present. 
 
 The sexes are usually separate. The reproductive and excretory 
 systems are closely related, giving rise to a urogenital system. The 
 excretory ducts usually carry ofif the reproductive products (eggs and 
 sperm). The urogenital ducts empty near the anus. Reproduction 
 is strictly sexual; parthenogenesis and reproduction by budding do not 
 occur and alternation of generations is unknown. The viscera are 
 enclosed in a large body cavity (coelom) which in the adult does not 
 extend into the head. Each viscus is supported by a fold (mesentery) 
 of the lining membrane of the cavity. 
 
 For details of the classification of vertebrates reference must be 
 made to special text-books of zoology, but as some of the larger groups 
 must be referred to frequently, so these with a slight definition and one 
 or two examples are given here. 
 
 Series I. CYCLOSTOMATA. 
 
 These are eel-like in form, breathe by gills, have but one nostril, 
 a circular mouth, incapable of closing, for no jaws are present. 
 The skeleton is poorly developed and there are no paired appendages. 
 — Lampreys and hagfishes. 
 
 Series II. GNATHOSTOMATA. 
 
 This includes all other vertebrates. They have usually two pairs 
 of appendages, true jaws and a well developed skeleton. 
 
 Grade I. Ichthyopsida. 
 
 Fish-like, breathe, at least while young, by gills, have paired ap- 
 pendages, in the shape of legs or fins. In development there are never 
 formed those structures to be described later as amnion and allantois. 
 
 Class I. Pisces. 
 
 Fishes respire permanently by gills developed in gill slits in the 
 sides of the pharynx, have median and paired fins unless the latter be 
 lost by degeneration. 
 
INTRODUCTION. 
 
 Sub-class I. Elasmohranchii. 
 
 Fishes with cartilaginous^ skeleton, mouth usually on the lower 
 side of the head, the gills usually opening separately on the neck, and the 
 tail with the upper lobe the larger (heterocercal) . Sharks and skates. 
 The Holocephali differ in having the gill slits covered wdth a fold of 
 skin, so that but a single external opening appears. 
 
 Sub-class II. Ganoidea. 
 
 Intermediate between elasmobranchs and teleosts. — Garpike, 
 sturgeon. ' 
 
 Sub-class III. Teleostei. 
 
 Fishes w^ith bony skeleton, mouth with true jaws at the tip of the 
 snout, gill openings concealed by an operculum or gill-cover supported 
 by bone. Tail with upper and lower lobes equal. — All common fishes. 
 
 Sub-class IV. Dipnoi. 
 
 The lung fishes are tropical forms in which the air bladder func- 
 tions as a lung, the gill openings are covered with an operculum, and 
 the tail is very primitive (diphycercal). 
 
 Class II. Amphibia. 
 
 Ichthyopsida with legs replacing the paired fins, lungs 'present and 
 replacing the gills in the adult, nostrils connecting with the mouth. 
 
 Sub-class I. Stegocephali. 
 Extinct amphibians with well developed tail. 
 
 Sub-class II. Urodela. 
 
 Amphibia with well developed tail, gills sometimes retained through 
 life. — Salamanders, Tritons, newts, efts. 
 
 Sub-class III. Anura. 
 
 Tailless as adults, the young a tadpole with external gills. — Frogs 
 and toads. 
 
 Sub- class IV. Gymnophiona. 
 
 Blind, burrowing, legless amphibians occurring in the tropics. — 
 Caecilians. 
 
INTRODUCTION. 
 
 Grade II. Amniota. 
 
 • Vertebrates in which there are never fins, never functional gills, 
 the respiration being by lungs. In development the embryo becomes 
 covered by an embryonic envelope called the amnion, while a second 
 outgrowth from the hinder end of the digestive tract is concerned in 
 the embryonic nutrition and is called the allantois. 
 
 Class I. Sauropsida. 
 Body, at least in part, with scales, eggs large. 
 
 Sub-class I, Reptilia. 
 
 Cold-blooded vertebrates, the whole body covered by scales or 
 horny plates. The living forms are turtles, lizards, snakes and alli- 
 gators (crocodiles) and a New Zealand species Sphenodon. The 
 fossil forms are more numerous and include Theromorphs, Plesiosaurs, 
 Ichthyosaurs, Dinosaurs, and Pterodactyls. 
 
 Sub-class II. Aves. 
 The birds are recognized by their warm blood and their feathers. 
 
 Class II. Mammalia. 
 
 The mammals are as sharply marked by their hair as are the birds 
 by their feathers. They have warm blood; except the monotremes 
 they bring forth living young which are nourished by milk secreted 
 by glands (mammae) in the mother. 
 
 There are a few other terms of convenience which may be defined 
 here as they will save much circumlocution. The term Teleostomes is 
 applied to ganoids and teleosts, from the fact that they have true jaws. 
 The amphibia and the amniotes are frequently united as Tetrapoda, 
 from their possessing feet, in contrast to the fishes with fins. 
 
 The geological history of these groups is important; their first 
 appearance and their geological range is indicated in the accompanying 
 table of the geological periods. 
 
 INTRODUCTORY EMBRYOLOGY. 
 
 The structure of an adult vertebrate can be fully appreciated and the 
 bearing of the facts recognized only by a knowledge of the develop- 
 ment of the parts concerned. It would often appear, for example, 
 that certain organs in different groups were exact equivalents of each 
 
INTRODUCTION. 
 
 g 2? 
 cr c 
 2. 2. 
 
 Ostracoderms 
 
 P alaeospondy lus 
 
 Elasmobranchs 
 
 Ganoids 
 
 Teleosts 
 
 Arthrodira 
 
 Dipnoi 
 
 Stegocephals 
 
 Gymnophiona 
 
 Urodela 
 
 Anura 
 
 Theromorphs 
 
 Plesiosaurs 
 
 Chelonia 
 
 Ichthyosaurs 
 
 Rhynchocephals 
 
 Dinosaurs 
 
 Squamata 
 
 Crocodiles 
 
 Pterodactyls 
 
 Birds 
 
 Monotremes 
 
 Marsupials 
 
 Edentata 
 
 Insectivores 
 
 Chiroptera 
 
 Rodentia 
 
 Ungulata 
 
 Sirenia 
 
 Cetacea 
 
 Carnivores 
 
 Primates 
 
 Table showing the geological distribution of the various groups of vertebrates. 
 
8 ' INTRODUCTION. 
 
 Other — duplicates in function and details of structure — while a knowl- 
 edge of their development may show that they have had entirely 
 different origins and different histories, and hence cannot be identical ; 
 they are examples of what the evolutionist calls convergent evolution. 
 Such cases are apt to lead one astray as to the relations of the forms 
 in which they occur. Farther, the development affords a framework 
 around which the details of organization may be arranged in a logical 
 manner, thus aiding in their remembrance. For' these reasons the 
 following pages are based on embryology. Not only are the histories 
 of the separate organs traced before an account is given of the adult 
 conditions, but this introductory chapter gives in the most generalized 
 form the earlier stages before the organs are outlined. 
 
 The enormously complicated 
 body of every vertebrate is derived 
 from a comparatively simple special- 
 ized cell, the egg or ovum. This 
 ovum, must be fertilized by a still 
 more specialized cell, the spermato- 
 zoon, derived from the male. After 
 Fig. 2.— Successive stages in the seg- this fertilization the egg goes through 
 
 mentation of an amphibian egg. 1-7, ^^Hprlv hut vprv (rrflHiinl c:prip«^ 
 
 Results of the corresponding cleavage ^^ oroeriy Dut very graouai serics 
 planes. of changes which bring it contin- 
 
 ually nearer the adult condition. The phases of this differ with 
 different animals; here only a generalized account will be given, which 
 is subject to modifications in the several groups, for an account of which 
 reference must be had to embryological text-books. 
 
 The Segmentation of the Egg. — The first steps of the process are 
 the segmentation or cleavage of the egg, in which it divides again and 
 again, until the single-celled egg is converted into a large number of cells 
 or blastomeres (fig. 2). The character of this segmentation is 
 modified accordingly as the egg is large or small, as it contains varying 
 amounts of nourishment — deutoplasm or food yolk stored up for the 
 growing embryo. These same variations also affect the later stages of 
 development; the description given here follows the simplest conditions. 
 
 As a result of segmentation the egg is converted into a spherical 
 mass of cells in which a cavity appears, called the segmentation 
 cavity because it is formed during segmentation. It also has the 
 name archicoele as it is the first or oldest space to appear in the 
 embryo. This stage of the embryo is called the blastula (fig. 3). 
 
EMBRYOLOGY. 9 
 
 Its cells at first show but little differentiation except in size. Next 
 follow processes which are to differentiate the cells into layers, charac- 
 terized by both position and fate. 
 
 Gastrulation. — In the simplest form this differentiation is brought 
 about by an inversion of one-half of the blastula into the other, thus 
 more or less completely obliterating the segmentation cavity, much as 
 one may push one side of a rubber ball into the other, forming a double- 
 walled cup (fig. 4). This stage is called the gastrula, and the process 
 of inpushing fs invagination. With this the first appearance of the 
 structures of the adult is seen. The outer wall of the cup is turned^to 
 the external world and thus act as a skin for the embryo. This layer 
 is called the ectoderm. The opening or mouth into the cup is the 
 
 Fig. 3. — Diagram of a typical 
 blastula with central segmentation 
 cavity. 
 
 b sc a en 
 
 ec 
 
 Fig. 4. — Diagram of a gastrula. 
 a, archenteron ; b, blastopore ; ec, ecto- 
 derm; en, entoderm; 5C, segmentation 
 cavitv. 
 
 blastopore. The inside of the cup is well fitted for the digestion of 
 food as it can be held together there and the digestive fluids are less 
 liable to waste. Hence the cavity is called the archenteron (primitive 
 stomach), and the layer of cells which line it is the entoderm. That 
 these comparisons are more than analogies of position is shown by. 
 their fates, the ectoderm forming part of the skin of the adult, the 
 entoderm the lining of the digestive tract. Between ectoderm and 
 entoderm are the remains of the segmentation cavity, filled with an 
 albuminous fluid. It will be convenient later to speak of the line where 
 ectoderm and entoderm meet at the blastopore as the ect-ental line. 
 Closure of the Blastopore. — Next, the blastopore closes, the 
 process beginning at what will be the head end of the embryo and pro- 
 
lO 
 
 INTRODUCTION. 
 
 ceeding gradually backward. Usually the closure is complete, but 
 occasionally the hinder part remains open and forms the anus. Where 
 it closes completely the vent is subsequently formed in the line of closure. 
 This union of the two lips of the blastopore in closing marks the middle 
 line of the back of the future animal, and is called at first the primitive 
 groove, the region on either side of it being known as the primitive 
 streak, terms of importance in understanding the gastrulation of the 
 higher vertebrates. 
 
 Mesoderm. — With the closure of the blastopore the embryo elon- 
 gates and the archenteron is converted into a tube. Next, from the 
 region of closure and from the entodermal tissue, a fold of cells grows 
 in on either side between ectoderm and entoderm, thus farther en- 
 croaching on the segmentation cavity. These cells form the middle 
 
 Fig. 5. Fig. 6. 
 
 Fig. 5. — Stereogram of the anterior end of a developing amphibian, showing the out 
 lining of the mesothelium, nervous system and notochord. a, anterior end ; ar, archenteron; 
 c, coelom; ch, notochordal cells; ec, ectoderm; mp, mesodermal pouch; ng, primitive groove; 
 np, neural plate; nr, neural folds; sc, segmentation cavity; so, somatic wall of coelom; sp^ 
 splanchnic wall of coelom. 
 
 Fig. 6. — Stereogram of the anterior end of a vertebrate, shov^dng the relation of the 
 coelomic pouches; c, coelom; d, digestive tract; e, ectoderm; nc, nervous system; «, notochord; 
 sc, segmentation cavity; so, somatic and sp, splanchnic walls. 
 
 layer or mesoderm. Inside this fold is a space, connected at first with 
 the archenteron, but soon the cavity of each side is cut off by a growing 
 together of the opening into the archenteron and is henceforth known 
 as a coelom^ or body cavity. Each coelomic space has two walls, one 
 toward the ectoderm, the somatic layer, the one toward the entoderm 
 being the splanchnic layer (figs. 5 and 6). 
 
 The mesoderm arising in this way and bounding the coelom is 
 called mesothelium to distinguish it from another kind — the mesen- 
 
 ^ A ccelom formed in this way is an enterocoele. Usually the coelomic walls arise as a 
 solid mass of cells from the corresponding region, which later splits internally, forming a 
 schizocoele. The two are readily compared. 
 
EMBRYOLOGY. 1 1 
 
 chyme — which also comes to lie in the segmentation cavity. This 
 mesenchyme arises as separate cells, coming largely from the mesothe- 
 lium, and to a less extent from the entoderm (see p. i6). Whether 
 any arises from the ectoderm is disputed. 
 
 The Germ Layers. — Ectoderm, entoderm and the two types of 
 mesoderm are called the germ layers, because in the animals first 
 studied they were arranged like layers one on the other. Each plays 
 its part in the ' formation of the adult and gives rise to its peculiar 
 structures. 
 
 The ectoderm forms the outer layer of the skin, hair, claws, feathers, 
 the outer layer of scales, enamel of teeth, and the essential or character- 
 istic part of all sensory and nervous structures. 
 
 The entoderm gives rise to the lining of the digestive tract, and the 
 various outgrowths — gills, lungs, liver, pancreas, etc. — connected with 
 it. The notochord is also entodermal and possibly the lining of the 
 blood-vessels is derived from this layer. 
 
 The mesothelium produces the lining of the coelomic cavities 
 — pericardial, pleural, peritoneal — the reproductive and excretory 
 organs and the voluntary muscles and those of the heart. 
 
 The mesenchyme develops the deeper layer (corium) of the skin 
 and of scales, the dentine of teeth, involuntary muscles (except those of 
 the heart) connective tissue, ligaments, cartilage, bone, and the corpus- 
 cles of blood and lymph. 
 
 In the development of the embryo several processes of differen- 
 tiation occur simultaneously, but in the written account one has to 
 follow another. Hence it must be understood that the modifications 
 described here may be taking place at the same time. 
 
 The Central Nervous System. — During the closure of the blasto- 
 pore the ectoderm in front and to either side of the blastoporal lips 
 becomes thickened, the cells elongating at right angles to the surface 
 and becoming cylindrical or fusiform. These cells form the neural or 
 medullary plate (fig. 5, w/>), sharply marked off from the surrounding 
 cells, which are more flattened, and which eventually are concerned in 
 the formation of the outer layer (epidermis) of the skin. The neural 
 plate is to develop into the brain and the spinal cord, and it is to be 
 noted that later it extends around the hinder end of the blastopore. 
 After it is outlined the plate is rolled into a tube, its front end and lateral 
 margins rising up, forming neural folds (nr), between which is the 
 medullary groove. Eventually the folds meet and fuse above so that 
 
12 
 
 INTRODUCTION. 
 
 the tube results (fig. 6, nc), the cavity of which persists throughout life 
 as the cavities (ventricles) of the brain and the central canal of the 
 spinal cord. From the cells of the walls of the canal the nervous tissue 
 arises. 
 
 This process of infolding progresses from in front backward. For 
 a time, in some vertebrates, a small opening, the anterior neuropore, 
 persists at the anterior end. The infolding extends back to the poste- 
 rior end of the neural plate so that, as will readily be understood, the 
 whole limits of the blastopore are included in the floor of the neural 
 canal. Occasionally the closure of the neural folds is completed before 
 that of the blastopore so that for a short time a short tube, the neuren- 
 teric canal (fig. 7), connects the archenteron with the neural canal. 
 Soon after the closure of the neural tube the fused tissue splits horizont- 
 ally, separating the nervous sys- 
 tem from the rest of the ectoderm. 
 Its subsequent history will be 
 traced in the section of the Ner- 
 vous System. 
 
 The Notochord. — Immediately 
 beneath the neural plate is an axial 
 strip of entoderm (fig. 5, ch)^ 
 
 Fig. 7.-Schematic section of the hinder bounded On either side of the OUt- 
 
 end of an amphibian embiyo, showing the growing mesothclium. When the 
 
 relations of the neurenteric canal, ac, alimen- . \ v' i. j 
 
 tary canal ;ec, ectoderm (black) ;«, notochord; latter separates (p. lO) thlS band 
 
 ne. neurenteric canal; nt neural tube; p, is momentarily rejoined tO the rCSt 
 proctodeum; pa, post-anal gut; y, yolk. •' •' 
 
 of the entoderm but is still recogniz- 
 able from its different cells. It soon rolls into a rod (a tube in some 
 amphibians and birds), is cut off from the rest (fig. 6, n) and lies 
 between the digestive tract and the nervous system where it forms an 
 axis around which the skull and vertebral column develop later. 
 
 The Digestive Tract. — After the separation of the notochord, the 
 entoderm forms a tube, closed in front and usually behind as well. 
 The anterior end of the tube abuts against the ectoderm of the ventral 
 side of the embryo. Later the ectoderm grows in at the point of con- 
 tact, carrying the entoderm before it and forming a pocket, the stomo- 
 deum, which gives rise to the cavity of the mouth. (In some the 
 stomodeal ingrowth is at first solid, the pocket being formed later by 
 splitting). Eventually the ectoderm and entoderm fuse at the bottom 
 of the cup, and then the fused area breaks through, placing the archen- 
 
EMBRYOLOGY. 
 
 13 
 
 teron in connexion with the exterior. A similar, but less well defined 
 proctodeum (fig. 7, p) arises at the hinder end of the digestive tract. 
 Thus the anterior and posterior ends of the alimentary canal a^e 
 ectodermal, the middle region entodermal, in origin. 
 
 Metamerism. — In the adult, various parts, essentially like each 
 other, are repeated one after another — are metameric. The list 
 includes, among others, muscles, nerves, blood-vessels, vertebrae, ribs, 
 etc. There is much evidence to show that metamerism had its origin 
 in the mesothelial structures and has been secondarily impressed on 
 other systems. 
 
 Fig. 8. — Stereogram of a later stage than fig. 6, showing the segmentation of the meso- 
 thelium. The approach of the walb of the coelom (c), dorsal and ventral to the alimentary 
 canal, to form the mesenteries is shown, d, alimentary canal; em, epimere;/6, forebrain; 
 /t6, hind brain; hm, hypomere; m, myotome; nth, midbrain; mm, mesomere; mc, metacoele; 
 myc, myocoele; n, nervous system; nc, notochord; s, stomodeal region; so, sp, somatic and 
 splanchnic layers; st, sclerotome. 
 
 The mesothelial coelomic pouches, as left above, are near the dorsal 
 side of the embryo. With growth they gradually extend downward 
 on either side and tend to enclose the whole archenteron, and upward 
 on either side of the notochord and spinal cord (fig. 8). The fates of 
 the different parts of the mesothelial walls warrants the recognition of 
 three horizontal regions or zones in the walls of each coelom. These 
 are a dorsal muscle-plate zone (epimere, em), a lower or lateral-plate 
 zone (hypomere, hm), and a middle-plate zone (mesomere, mm) 
 between them. All three of these occur in the trunk, but only the 
 epimere is well developed in the anterior part of the head. 
 
14 INTRODUCTION. 
 
 A series of vertical constrictions begins at the dorsal margin of each 
 coelomic pouch and cuts down through epimere and mesomere, so 
 that the whole may be compared to a glove with a large number of 
 fingers extending from its upper surface, each finger being hollow, and 
 all of the cavities connecting with that in the hypomere (palm). This 
 process begins at front and gradually extends backward. Viewed 
 from above in the transparent embryo, each of these fingers appears 
 like a square box and early students thought that they gave rise to the 
 vertebrae, and so they were called proto vertebrae. Next, the dorsal 
 part of each of these fingers is cut off from the rest, along the line 
 between mesomere and epimere, thus forming a series of hollow cubes, 
 known as myotomes, each with a part of the ccelom in its interior, the 
 myocoele. After the separation from the rest each myotome grows 
 upward and to a greater extent downward, insinuating itself between 
 the ectoderm and the somatic wall of the hypomere (fig. 9, in the 
 direction of the arrows). From these myotomes the body (somatic) 
 musculature arises. 
 
 From the medial mesomeral part of the fingers arises the mesen- 
 chyme that gives origin to the vertebrae while the rest furnishes the 
 material for the excretory organs. From their origin both of these 
 are metameric at first, the skeletogenous parts being called scler- 
 otomeis, the excretory parts, nephro tomes (fig. 8, mm, st). The 
 history of both will be followed in their proper places. 
 
 The Ccelom. — The parts of the coelom in the myotomes soon 
 disappears, that in the nephrotomes, of inconsiderable size, forms 
 the lumina of the excretory ducts. That in the hypomere (fig. 9, c) 
 forms the large body cavity (peritoneal cavity) surrounding the 
 chief viscera, and the smaller one (pericardial) around the heart. 
 In surrounding the archenteron the walls of the two coelomic cavities, 
 which at first are separate, tend to meet above and below the entoderm, 
 so that there is in both Regions a thin membrane supporting the digest- 
 ive tract above and below. Such supports are collectively called 
 mesenteries. Usually that below {v mes) largely disappears, but the 
 dorsal {d mes) one persists more or less completely. At first these 
 mesenteries are merely double membranes of mesothelium, but soon 
 mesenchyme grows in between them and extends around the digestive 
 tract, so that mesothelium and entoderm are bound together by the 
 invading tissue. In a similar way the somatic wall of the coelom is 
 bound to the muscles arising from the myotomes and these in turn to 
 
EMBRYOLOGY. 
 
 15 
 
 the ectoderm by the mesenchyme. In this way the ccelom comes to 
 have two thick walls. That on the outer side, consisting of ectoderm, 
 muscles and peritoneal lining, is called the somatopleure {so)f that of 
 peritoneum and digestive wall is the splanchnopleure (sp). 
 
 For convenience the different mesenterial structures have separate names. 
 As the digestive tract bfecomes coiled, the different parts of it are connected by 
 similar membranes which are called omenta (om). The dorsal mesentery is sub- 
 divided into regions supporting the different portions of the digestive tract. 
 
 Fig. 9. — Diagrammatic transverse section of a vertebrate to illustrate mesenteries, 
 omentum and downward growth of the myotomes, al, alimentary tract; ao, aorta; c, 
 coelom; ec, ectoderm; dmes, dorsal mesentery; my, myotome; nc, notochord; neph, nephro- 
 tome; o, omentum; sc, spinal cord; so, sp, somatic and splanchnic layers of mesothelium; 
 vmes, ventral mesentery. 
 
 Thus there is a mesogaster for the stomach, a mesentery proper for most of the 
 intestine, and mesocolon and mesorectum for colon and rectum respectively. 
 On the ventral side there is a mesohepar, bounding the liver to the ventral body 
 wall. In the same way the omenta are distributed into hepato-duodenal, gas- 
 tro-hepatic (small omentimi), etc., while in mammals there is a great omentum, 
 a double fold of mesogaster and mesocolon which connects the stomach with the 
 transverse colon. 
 
 Similar folds are formed in connection with other organs. Thus the heart 
 
1 6 INTRODUCTION. 
 
 for a time is bound to the pericardial walls by dorsal and ventral mesocardia; 
 there is, in mammals, a mediastinum between the two pleural cavities, connect- 
 ing the pericardium to the body wall, while frequently the ovaries and the testes 
 project into the ccelom, carrying the peritoneum with them, thus giving rise 
 to a mesovarium or a mesorchiimi, according to the sex. 
 
 The Mesenchyme has two chief places of origin. One is from 
 the splanchnic wall of the segments of the mesomere, each of which 
 is the centre of rapid cell proliferation and forms the sclerotome (fig. 
 8, St) J since some cells arising from it are concerned in the formation 
 of the axial skeleton. These cells pass in to surround the notochord, 
 and upward on either side of the central nervous system and downward 
 beside the alimentary canal, thus forming a partition between the 
 two sides of the body. A second source of the mesenchymatous cells 
 is from the somatic wall of each myotome, all of the cells of which are 
 transformed into this layer, and lie immediately beneath the ectoderm. 
 Thus there is a complete envelope of mesenchyme around the whole 
 body. From these and from other sources the mesenchyme extends 
 everywhere in the remains of the segmentation cavity — between the 
 muscles and around the various viscera — forming a framework in 
 which the products of all the other layers are enveloped (fig. 30). This 
 mesenchymatous framework has great importance in the development 
 of the skeleton and its general plan will be described in connection with 
 the skeletal structures. 
 
 HISTOLOGY. 
 
 In the gastrula the cells differ from each other chiefly in position, 
 and the same is true even when the germ layers are first differentiated. 
 As development goes on the differences between the various groups of 
 cells increase, each group becoming more specialized for some one 
 purpose and losing the power to do more than the one kind of work. 
 For community of work cells of the same kind become associated to- 
 gether, the result being tissues. A tissue then is a connected mass of 
 cells similar in appearance and function, together with a varying 
 amount of intercellular substance, usually formed by the cells them- 
 selves. The study of the minute structure of animals and especially 
 of the tissues is the province of histology. 
 
 There are many kinds of tissues, only a few of which need mention 
 here, but all may be grouped under four great heads: epithelial 
 
HISTOLOGY. 
 
 17 
 
 nervous, muscular and connective tissues; the members of each group 
 having certain fundamental points in common. 
 
 Epithelial Tissues. 
 
 Epithelia are the covering tissues, and occur on any free surface, 
 internal or external, of the body. Both comparative anatomy and 
 embryology show them to be the primitive tissues, for there are many 
 lower animals which are made up entirely of epithelia, while in the 
 vertebrates the embryo consists solely of epithelia until the mesenchyme 
 appears. Epithelia may come from any of the germ layers, in rare 
 cases (synovial cavities) even from mesenchyme. 
 
 Fig. 10. — Epithelia: A, cubical; B, squamous; C, cylindrical; D, stratified cylindrical, 
 ciUated at E; F, stratified squamous. 
 
 The character of epithelium varies according to the character of 
 the work it has to perform. That on the outside of the body is largely 
 protective, hence it is often thickened and strengthened in different 
 ways to afford resistance against external injuries. In other places, 
 as glands, it has to elaborate and to allow the passage outward of 
 material from within. In the body cavity and in the blood-vessels 
 it has merely to form the thinnest of coverings, while in the case of 
 sensory structures it is modified (sensory epithelium) to receive the 
 stimuli from without. 
 
 The usual classification of epithelia is based on the shapes and 
 arrangements of the cells. Thus in cubical epithelium (fig. 10, A) 
 the cells are about as high as broad; in columnar (C) their height 
 
i8 
 
 INTRODUCTION. 
 
 exceeds their diameter; while in squamous epithelium, the cells are 
 thin and fiat, covering the largest amount of surface with the least 
 amount of material (B). Sometimes the epithelial cells are in a single 
 layer, forming simple epithelium (A, B, C); in other places there are 
 several layers— the epithelium is stratified (D, E, F). 
 
 Frequently epithelia, usually of the columnar variety, are called upon 
 to move fluids slowly; then the free surface is covered with minute 
 vibratile hairs or cilia {E) which create currents. In glandular 
 epithelium the cells, usually cubical or columnar, are specialized for 
 the elaboration of secretions to be used by the animal or of waste prod- 
 ucts (excretions) to be voided from the body. 
 
 Fig. II. 
 
 -Different types of glands; A, to D, tubular; E, F, acinous; A, simple; B, coiled; 
 C-F, branched. 
 
 Glands. — The chief kinds of glands may be mentioned here. All have for 
 their function the extraction and elaboration of certain products from the blood, 
 consequently they have a good blood supply. Glands may be unicellular or multi- 
 cellular according as they consist of isolated cells or of many cells. In unicellular 
 glands (abundant in the digestive tract) each cell passes its own secretion directly 
 to the place where it is to be used (fig. 19, u). 
 
 Multicellular glands occur where a large amount of secretion is necessary in a 
 limited space, hence they are not on the surface but at some deeper point, and their 
 product is conveyed to the desired place by a duct. Multicellular glands are of two 
 structural kinds. In the tubular gland the whole is approximately of the same 
 diameter throughout, with little differentiation of gland and duct. It may be 
 simple (A) or coiled (B) or branched (C, D), these modifications serving to in- 
 crease the secreting surface. In acinous glands (Z?, E) there is a marked differ 
 ence between gland and duct, the glandular part forming an enlargement (acinus) 
 on the end of the duct. Both simple and compound acinous glands are common. 
 
 Still another type of gland, the ductless or 'internal secretion' gland occurs. 
 In this there is no duct, the secretion elaborated by the cells passing by osmose into 
 the blood-vessels. These secretions, collectively known as hormones, have 
 recently acquired great prominence from their influence on different organs. 
 
HISTOLOGY. 
 
 Nervous Tissues. 
 
 19 
 
 Nervous tissue has for its^unction the correlation of the animal with 
 its environment. In order to accomplish this it must provide for the 
 recognition of stimuli from without, the inauguration of other impulses 
 within itself and the transfer of both to other parts. The essential 
 constituent of the tissue is the nerve cell, ganglion cell or neuron, 
 to which are added others of a supportive (glia cells) or nutritive 
 character. As the parts to be connected by the nervous tissue are often 
 remote from each other the neuron is not compact like most other cells, 
 but gives off long processes from the central mass, these processes differ- 
 ing in their terminations. Some end in places where they can only 
 
 Fig. 12. — ^V^arious kinds of nerve cells. A, multiJ)olar cells; B, portion of nerve fibre 
 with sheaths; C, unipolar cell; D, pyramidal cell; a, axon; c, collateral; d, dendrites; cb, 
 cell body; m, medullary sheath; n, nucleus of cell of Schwann's sheath; s, sheath of Schwann; 
 t, telodendron. 
 
 receive stimuli, others where the stimuli can only cause parts to act. 
 Thus the processes are physiologically divisible into afferent and 
 efferent tracts, the body of the cell being the place for the regulation and 
 correlation of the impulses, and apparently in many cells for the inau- 
 guration of new impulses. 
 
 A nerve cell (fig. 12) is uni-, bi- or multipolar accordingly as it has 
 one, two or more of these processes. In the case of unipolar cells (C) 
 the single process sooner or later divides, so that the cell in reality is at 
 least bipolar. At the ends the processes may either break up in minute 
 twigs (dendrites, d) or may end, as in muscles and sense organs, in 
 special end organs. The part connecting the efferent termination and 
 the central cell body is called the axon (a). Axons and cell bodies are 
 gray in color, but usually the axons are surrounded by a medullary 
 sheath (m) of a peculiar white substance (myelin) rich in fat, which 
 
20 INTRODUCTION. 
 
 apparently acts as an insulator, preventing nervous impulses from pass- 
 ing from one axon to another. This sheath does not continue over the 
 dendrites. Frequently the dendrites of two neurons interlace for the 
 transference of stimuli from one to the other, but the present opinion is 
 that, at least in vertebrates, there is no actual continuity of substance 
 between neurons, only an interlacing of terminal twigs. The medullary 
 sheath is not cellular, but frequently fibres may be surrounded by a 
 sheath of Schwann (s), with scattered nuclei. This has been re- 
 garded as mesenchymatous, but recent researches tend -to show that 
 it is ectodermal, its cells coming from the nervous system. 
 
 Nervous tissue consists of these neurons plus connective tissue and 
 glia cells. A nerve, as found in dissection, consists of numbers of 
 axons, bound together by a connective-tissue envelope (perineureum) . 
 The myelin gives these nerves a white color. In the brain and spinal 
 cord there are tracts of meduUated fibres (white matter) while the 
 parts with abundant nerve cells are gray. When such gray matter is 
 aggregated in the course of a nerve, it causes an enlargement called a 
 ganglion. Interlacing among the neurons in brain and spinal cord is 
 the neuroglia, which is also derived from the ectoderm, and acts as a 
 support but has no nervous functions. Certain of these glia cells 
 develop many branches (mossy cells) which twine among nervx cells, 
 axons, and dendrites. 
 
 Muscular Tissues. 
 
 While several kinds of cells have the power of changing shape, 
 those composing muscular tissue possess it in a marked degree, acting 
 quickly and with force, so that these tissues are preeminently the tissues 
 of motion. The cells become elongate and develop on their interior a 
 large amount of contractile substance (myofibrillae), which on stimula- 
 tion, contracts, shortening the cell. In the vertebrates, muscular tissue 
 always arises from the mesoderm, yet two types are recognized, differing 
 markedly in origin, appearance and physiological action. 
 
 The smooth or involuntary muscles arise from the mesenchyme. 
 They consist of long and spindle-shaped cells (fig. 13, A), each with a 
 single nucleus, the protoplasm traversed by numerous myofibrillae, 
 which appear like fine longitudinal lines. In the vertebrates the 
 smooth muscle is not under control of the will; it contracts slowly. 
 
 In contrast to the smooth is the striped or voluntary muscular tis- 
 
HISTOLOGY. 2 1 
 
 sue, which arises from a modification of the mesothelium. Except in 
 the case of the muscles of theheart, the striped tissue is under control of 
 the will; it usually occurs in larger masses than does the smooth, and 
 is capable of rapid contraction. It differs structurally from smooth 
 muscle. Instead of distinct, uninucleate cells there are long cylindrical 
 elements (fig. 13, B), the primitive fibres, each with several nuclei in the 
 interior in lower vertebrates, on its periphery in the higher. Most of 
 the protoplasm of the fibre has been altered to minute contractile 
 fibrillae, each crossed by lighter and darker bands, and as these come 
 opposite each other in the different fibrillae, they give the fibre its 
 characteristic cross-banded appearance. 
 
 Fig. 13. — A, smooth muscle cell; B, striped muscle. 
 
 The primitive fibres rarely branch at their extremities. Each is 
 surrounded by a structureless envelope, the sarcolemma, while num- 
 bers of fibres are bound into bundles and muscles by connective 
 tissue (perimysiiun) which carries nersTS and blood-vessels. At the 
 ends of the bundles the perimysium continues into the tendons which 
 attach the muscles to other parts. 
 
 The heart muscle also arises from the mesothelium, is cross-banded, 
 but is removed from control of the will. The cells are usually short 
 (usually with a single nucleus) ; they branch, the branches connecting 
 adjacent muscle cells. 
 
 Connective Tissues. 
 
 The tissues grouped here arise from the mesenchyme and are 
 distinguished from all other tissues by the great amount of intercellular 
 substance produced by the cells themselves. This substance or matrix 
 varies in character and determines the variety of tissue. Frequently it 
 is dense and hence the connective tissues may give the body support, 
 and in fact they are sometimes called supportive tissues. 
 
22 
 
 INTRODUCTION. 
 
 In the earliest phase, known as embryonic connective tissue 
 (fig. 14, ^), the cells are scattered, with long radiating processes, and 
 between the cells a thin gelatinous matter. It is by increase of this 
 intercellular substance by taking up water that many embryos gain so in 
 size without taking food. The embryonic connective tissue may de- 
 velop in various directions. 
 
 ^^^ 
 
 Fig. 14. — Connective tissues. A, embryonic, from Amhly stoma; B, expanded and con- 
 tracted pigment cells from Amhly stoma; C, fibrous, from tendon. 
 
 Thus some of the cells may contain pigment granules, forming 
 pigment cells (J5), or oil globules may be deposited in them to such an 
 extent that the cells become spherical, while the intercellular substance 
 is reduced, thus affording fat or adipose tissue. Most common of 
 the connective tissues is fibrous tissue (white or non-elastic tissue) in 
 which the cells are branched or spindle-shaped while the matrix is 
 filled with fine fibrillae of considerable strength and little elasticity. 
 
 These fibrillae are parallel to each other in tendons (C), which have to 
 convey strains in one direction; or they maybe interlaced confusedly, the 
 tissue then forming sheets or membranes. Occasionally, as between 
 the skin and the muscles, the fibrous tissue may be loose (areolar 
 tissue). In elastic tissue fibres of another kind are mingled among 
 the non-elastic fibrils. These are yellow and elastic, and when abun- 
 dant give an elastic character to the whole. 
 
 In cartilage and bone the matrix is more solid and is abundant. 
 These are the skeleton-building tissues. In cartilage the matrix is 
 
HISTOLOGY. 
 
 23 
 
 firm and consists of a peculiar substance called chondrin. When the 
 chondrin is nearly pure it is milky in appearance (hyaline cartilage, 
 fig. 15), but it may be invaded by numerous strands of fibrous or 
 elastic tissue, resulting in 'fibrous or elastic cartilage. Cartilage in- 
 creases in size by additions to the exterior and also by divisions of 
 its cells and by increase in the amount of matrix. Externally it is 
 
 Fig. 15. — Hyaline cartilage. 
 
 bounded by an envelope of connective tissue (perichondrium) which 
 bears blood-vessels and may give attachment to muscles, etc. 
 
 Bone may arise directly from embryonic connective or fibrous tissue, 
 or by the ossification of cartilage. In either case the result is a strong 
 matrix composed of calcium phosphate and carbonate in a ground 
 
 
 • » - «. * \ 
 
 
 '^y)Pn\ 
 
 Fig. 16. — A, Stereogram of bone; B, cross-section of bone, more enlarged; c, canaliculi; 
 bl, bone lamellae; h, Haversian canal; /, lacvma. 
 
 substance of organic matter (ossein). Minute tubes (Haversian 
 canals) , bearing blood-vessels, etc., run through the matrix (fig. 16), 
 and parallel to these canals or to the external surface of the bone are 
 the cells arranged in layers. The space occupied by a cell is called 
 a lacuna, from which minute tubules or canaliculi penetrate the matrix. 
 There are small spaces in many bones occupied by the red marrow, 
 
24 INTRODUCTION. 
 
 which is especially noticeable as one of the places of formation of red 
 blood-corpuscles. Externally every bone is covered by a layer of 
 fibrous connective tissue, the periosteum. , 
 
 The dentine of teeth and placoid scales is closely allied to bone, 
 the chief difference in density, the bone-forming cells (odontoblasts) 
 not being enclosed in the matrix, while the canaliculi (here called den- 
 tinal canals) are parallel to each other. 
 
 Blood is sometimes regarded as a connective tissue, the corpuscles 
 being the cells and the fluid part (plasma) the matrix. It is here dealt 
 with in connection with the circulatory system. 
 
COMPARATIVE MORPHOLOGY OF 
 VERTEBRATES. 
 
 THE INTEGUMENT. 
 
 The integument is the covering of the body, the term including 
 the skin (cutis) and all structures derived from it. From its position 
 it is a protective coat. It comes into relation with the external 
 world and is modified in various w^ays, becoming hardened to ward 
 against mechanical injury, developing sensory structures to give in- 
 formation of untoward conditions and being impervious so as to prevent 
 loss of the body fluids or the entrance of others from without. Natur- 
 ally the habitat, aquatic or terrestrial, has great influence in the 
 character of the modifications. 
 
 In all vertebrates the integument consists of two layers, an outer 
 epidermis which consists of the ectoderm after the separation of the 
 nerv^ous system, and a deeper layer, the cerium (derma) of mesenchyme, 
 derived from the somatic wall of the myotomes, into which other struc- 
 tures (nen-es, blood-vessels, etc.) extend. Strictly speaking the 
 bony scales of fishes are integumental, but on account of their close 
 relations to the skeleton they are best treated in that connexion. 
 
 In the epidermis, again, two layers are always present. At the base, 
 next to the corium is theMalpighian layer (stratiun germinativum) , 
 the cells of which are nourished by the fluids of the corium. Hence 
 they can grow and divide, the new cells thus formed gradually passing 
 to the outside where they form the second layer, the stratum corneum, 
 the outer cells of which are usually worn away as fast as new ones are 
 added from below. Occasionally these outer cells come off in large 
 sheets, as when a salamander or a snake sloughs its *skin.' In the 
 development of the epidermis of the terrestrial vertebrates the first 
 layer of cells budded from the Malpighian stratum form a continuous 
 sheet which is later shed as a whole. This is the periderm (fig. 17), 
 the older name of epitrichivmi being inappropriate, since the layer is 
 found in reptiles and birds where no hair occurs. 
 
 25 
 
26 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 The Malpighian layer alone is concerned in the formation of the 
 glands connected with the skin, and the corresponding part of the 
 ectoderm contributes to the sensory structures like the nose and ear. 
 The corneum, on the other hand, is concerned in the formation of 
 protective structures like hair, nails, claws, feathers, and other cuticular 
 outgrowths. The epidermis is generally thicker in terrestrial than in 
 aquatic vertebrates, and in the latter, being constantly moist, shows 
 less of the horny consistency, than occurs in animals which live in the 
 air. 
 
 The corium lies immediately beneath the epidermis and is less 
 sharply separated from the deeper tissues by a looser layer of connective 
 tissue (subcutis, tela subjunctiva) in which fat is frequendy exten- 
 sively developed. The corium is largely composed of fibrous connec- 
 tive tissue, intermingled with elastic tissue, blood-vessels, nerves, 
 smooth muscle fibres, etc. It is usually thin in the lower vertebrates. 
 
 Fig. 17. — Section of developing scales of lizard, Sceleporus. c, papilla of corium; 
 e, outer layer of epidermis which later becomes cornified;/, fibrous layer of skin; m, Mal- 
 pighian layer; p, periderm; ts, tela subjunctiva. 
 
 but is much thicker in most mammals, and forms the whole of ordinary 
 leather. Pigment cells may occur in both epidermis and corium. These 
 are mesenchyme cells, loaded with pigment, which are frequently 
 under control of the nervous (sympathetic) system, and can be altered 
 in shape (chromatophores) , thus producing color changes, which, as 
 in the chameleons, may be very marked. 
 
 Horny scales, produced by a cornification of the epidermis, are found 
 in all groups of terrestrial vertebrates, but they are rare in amphibians 
 and mammals. The development is best seen in reptiles (fig. 17). 
 By a multiplication of the cells of both corium and epidermis in defi- 
 nite regions the skin becomes divided into thicker areas, separated 
 by thinner lines, each area corresponding to a future scale, which arises 
 by the conversion of the stratum corneum into horny material. In 
 snakes and lizards these scales, together with all of the stratum corneum 
 (even the covering of the eye) is periodically molted, the separation tak- 
 
INTEGUMENT. 
 
 27 
 
 ing place at the surface of the stratum Malpighii. In turtles and 
 alligators there is a gradual wearing away of the surface. 
 
 Closely allied to scales are claws, hoofs and nails (fig. 18). A 
 claw may be regarded as a cap of the tip of a digit, formed by two scales 
 one dorsal (unguis), the other ventral (subunguis). Of these the 
 unguis is the more important. It grows continually from a root, and in 
 mammals is forced forward over its bed. In the claw (B) the unguis 
 is curved both transversely and longitudinally, the subunguis forming 
 its lower surface. In the human nail (A) it is nearly flat in both direc- 
 tions and the subunguis is reduced to a narrow plate just beneath the- 
 
 Fig. 18. — Diagrams of (A) nails, (B) claws, and (C) hoofs, based on Boas, e, unmodified 
 epidermis; n, unguis; s, subunguis. 
 
 tip of the nail. In the hoof (C) the unguis is rolled around the tip of the 
 toe, while the subunguis forms the 'sole' inside it. The 'frog' is 
 the reduced ball of the toe which projects into the hoof from behind. 
 
 The integument presents many diJBFerent conditions in the separate 
 groups of vertebrates, and so details are best given under the special 
 heads. 
 
 FISHES. — The aquatic life renders the epidermis of fishes soft and 
 cornifications of it are comparatively rare, among them the peculiar 
 'pearl organs' which appear in the skin of some teleosts at the breed- 
 ing season. Glands, on the other hand, are abundant. These are 
 unicellular and multicellular mucus glands of different shapes in the 
 epidermis, the secretion of which furnishes the slime on the surface. 
 Some elasmobranchs and a number of teleosts have poison glands, 
 usually in close relation to the spines of the fins. The elasmobranchs 
 also have large glands in the 'claspers' of the males, but their purpose 
 is not well understood. 
 
28 
 
 COMPARATIVE MORPHOLOGY OF VERTEBR.4TES. 
 
 Possibly the most striking of the epidermal organs are the luminous 
 organs or photophores, which are most common in elasmobranchs and 
 teleosts from the deep seas, where sunlight does not exist. They are 
 apparently modified glands, and the development is known in Porich- 
 
 FiG. 19. — Section of skin of Protopterus. c, corium; e, epidermis; ^, multicellular gland; 
 
 M, unicellular gland. 
 
 thys. There is an involution of cells of the Malpighian layer into the 
 corium, where they become cut off from their point of origin, and are 
 differentiated into a deeper glandular layer and an outer rounded body, 
 the lens (fig. 21). Around this the corium forms a reflecting layer 
 
 Fig. 20. — A, head of Noturus flavus; B, section of poison gland of Schilheodes miurus 
 (after Reed), e, epidermis; p, pore of poison gland, pg; s, spine of pectoral fin. 
 
 enclosed in a pigment coat. The glandular layer is the seat of light 
 production. In other photophores either reflector or pigment may be 
 lacking, but in their highest development they so resemble an eye that 
 at first they were described as such. 
 
 In the myxinoids the skin contains numerous thread cells in pockets which may 
 extend into the underlying muscles. Each thread cell contains a long thread, which 
 
INTEGUMENT. 
 
 29 
 
 is discharged upon stimulation, the threads forming a network in which the mucus 
 secreted by the ordinary gland cells is entangled. 
 
 The corium is thin and consists of horizontal bands of fibrous tissue, crossed at 
 intervals by vertical strands. Fat is common in the tela subcutanea, and in some 
 fishes this layer contains numerous crystals of guanin which gives it a silvery 
 appearance. This guanin forms the base of 'essence of pearl' from which artificial 
 pearls are made. The scales of fishes, although formed in the skin, are con- 
 sidered in connection vsith the skeleton. 
 
 Fig. 21. — Section of luminous organ (photophore) of Porichthys, after Greene, e, 
 epidermis \\-ith mucous cells; gl, glandular layer of photophore; /, lens; r, reflector sur- 
 rounded by pigment. 
 
 AMPHIBIA. — The amphibia are remarkable in that the epidermis 
 of the larvae is ciliated in the early stages, and is two cells in thickness 
 from the first. The skin, in the larvae and the aquatic species, con- 
 tains numerous mucus glands and some for the production of poison, 
 some of the latter being prominent like the 'parotid glands' on the 
 neck'of the anura and the gland on the back near the base of the tail. 
 
 The corium is thin, and in the frogs is separated from the underlying parts by 
 large lymph spaces which render the skinning of these animals so easy. As the 
 amphibia respire largely by the skin (there are several lungless salamanders) the 
 corium is richly supplied with blood-vessels, and at the time of the metamorphosis 
 of the anura these penetrate even into the epidermis, as at that time the lungs are 
 not yet functional and the gills are absorbed. The stratum comeum is shed 
 periodically, either as a whole (urodeles) or in patches. The warts of toads are in 
 part cornifications of the epidermis, and a similar hardening of the skin on the 
 ends of the toes of some results in claws. In the mates of an African frog 
 (Astylosternus) the skin has the granules of the surface developed, at the breed- 
 
so 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 ing season into hair-like structures, supplied with nerves and apparently 
 sensory in character. 
 
 REPTILES. — ^All living reptiles are characterized by the extensive 
 development of horny scales and frequently of bony plates in the skin, 
 but some of the fossil groups (ichthyosaurs, pterodactyls, some dino- 
 saurs, possibly plesiosaurs) had a naked skin. Correlated with this 
 cornification of the epidermis, glands are rare. Some turtles have scent 
 glands beneath the lower jaw and along the line between carapace and 
 plastron; snakes and crocodilians have them connected with the cloaca, 
 while the latter have others, of unknown function, between the first and 
 second rows of plates along the back, as well as protrusible musk 
 glands on the lower jaw. These latter are not true glands as they 
 produce no secretion but cast out the lining cells. 
 
 The corium presents two layers, the outer rich in chromatophores, but, aside 
 from some snakes and lizards, the color changes are not remarkable. The femoral 
 pores of lizards are not connected with glands but with branching tubes filled with 
 cast cells. Claws are common on the toes. 
 
 BIRDS have both layers of the skin very thin, the epidermis develop- 
 ing both scales and feathers. Correlated with this extensive develop- 
 
 FiG. 22. — Diagram of base of contour feather, a, aftershaft; b, barbs; bl, barbules; 
 h, hooks on ends of barbules; lu, lower umbilicus; q, quill; s, shaft; u, umbilicus; v, vane. 
 A, portion of a barb showing the barbules and hooks. 
 
 ment of cornified structures is a striking paucity of glands. There are 
 none in the ostriches, but others have the familar oil (uropygial) glands 
 at the base of the tail, the secretion of which is used in dressing the 
 feathers. The only other dermal glands in birds are modified sebaceous 
 
INTEGUMENT. 
 
 31 
 
 glands near the ear in some rasores. The scales on the legs and the 
 claws on the feet and occasionally on the wings, are derivatives from 
 reptilian ancestors. The feathers are also derived from scales, but are 
 greatly modified. 
 
 Feathers. — There are several kinds of feathers but all may be 
 grouped under three heads: hair feathers (filoplumes), down feathers 
 (plumulae), and contour feathers (plumae). The latter have all of the 
 feather features (fig. 22) and in the typical form consist of shaft and 
 vane. The basal part of the shaft is the hollow quill,, in which is a 
 
 Fig. 23. — ^Feather tracts of Geococcyx cali/omianus, after Shufeldt. 
 
 small amount of loose pith. In the region of the vane the shaft, here 
 called rhachis, is solid, and running the length of its lower surface is a 
 groove, the umbilicus. The vane consists of lateral branches (barbs) 
 on either side, which have, in turn, smaller side branches (barbules), 
 these with small hooks at their sides and tips (B). Interlocking of 
 these hooks gives firmness and continuity to the whole vane. In down 
 feathers the barbs arise directly from the end of the quill, and as hooks 
 are lacking, the barbs do not interlock and no vane is formed. Hair 
 feathers are merely long and slender shafts with no barbs, the simplest, 
 if not the most primitive kind of feather. It is still a question as to 
 
32 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 the primitive type. The oldest fossil bird, ArchcBOpteryx, had well 
 developed contour feathers. 
 
 Except in the ostriches, penguins, and toucans, feathers are not 
 distributed everywhere on the surface of the body, but are gathered in 
 feather tracts (pterylae), separated by apteria in which no contour 
 feathers and but few down or hair feathers occur. These vary in their 
 arrangement in different groups of birds and are of systematic im- 
 portance (fig. 23). 
 
 Complicated as they are, feathers are probably derived from scales, and the 
 section of lizard skin (fig. 17) might well represent an early stage in the develop- 
 ment of a feather. A down feather begins as a thickening of the corium, pushing 
 the epidermis before it. By continued growth this forms a long, finger-like papilla. 
 
 Fig. 24. — Stereogram of developing down feather, hv, blood-vessels entering pulp; 
 c, corium; ep, epidermis; /, feather follicle; f, pulp (mesenchyme) of developing feather; 
 per, periderm; r, rods of epidermis, which later dry, separate, and form the down. 
 
 projecting from the skin. The corium extends into the outgrowth, carrying blood- 
 vessels with it, while an annular pit, the beginning of the feather follicle, forms 
 around the base of the papilla. Next, the corium or pulp of the distal part of the 
 papilla forms several longitudinal ridges (fig. 24) which gradually increase in 
 height, growing into the epidermis and pressing the Malpighian layer above them 
 against the periderm. As a result the stratum comeum is divided distally into a 
 number of slender rods arising from the base (quill), which at last are only held 
 together by the periderm. Then the pulp retracts, carrying with it the Mal- 
 pighian layer. With the blood supply removed, the epidermal parts dry rapidly, 
 the periderm ruptures, allowing the rods to separate to form the down. 
 
 A contour feather has much the same development, differing in details, for an 
 account of which reference must be made to special papers. The ridges of the 
 corium are no longer longitudinal, but beginning on the dorsal side of the papilla, 
 run obliquely outward and downward (fig. 25) until they meet below. Thus 
 
INTEGUMENT. 33 
 
 there are formed a series of rods set at an acute angle to the undivided dorsal strip, 
 the future shaft. When set free, as before, by the rupture of the periderm, these 
 rods straighten out, forming the vane. In the region of the shaft there are two 
 longitudinal ridges on the ventral side. These gradually roll together, thickening 
 and strengthening the shaft, the groove between them forming the umbilicus. As 
 will be understood, the dorsal and ventral sides of the feather were the outside and 
 inside of the stratum comeum of the papilla. 
 
 The corium is thin and consists of irregularly interlaced fibres; it is rich in sense 
 (tactile) organs and smooth muscle fibres, which are largely used in elevating the 
 feathers. The colors of feathers depend in part upon pigment — red, yellow, orange, 
 brown, and black — deposited in them, but the iridescent colors are due to interfer- 
 ence spectra. 
 
 .M .,-,.. 
 
 Fig. 25. — Stereogram of part of developing contour feather; compare with fig. 24. b 
 developing barbs; pc, pith cavity; per, periderm; s, rhachis. 
 
 MAMMALS have a skin relatively thicker than have other verte- 
 brates, both layers contributing to the thickness and the whole rather 
 loosely attached to the lower tissues. There are numerous glands, and 
 the hair, abundant in all orders except the whales and sirenians, is 
 found in no other class. Other cuticular structures as horn and claws 
 (p. 27) are widely distributed and scales occur in several forms. 
 
 The corium is thick and composed of irregularly interlaced fibres with muscles, 
 blood-vessels, etc. Its outer surface is frequently thrown into papillae or ridges, 
 especially on the palms and soles, these carrying the epidermis with them. In the 
 thick epidermis several strata may usually be recognized: at the base a thick 
 Malpighian layer; then a thin stratum lucidum in which distinct cells cannot be 
 recognized; and on the outside the stratum comeum. One or more others are 
 sometimes present. As w411 readily be understood a cell passes through all of 
 these layers before it is worn from the surface of the skin. 
 
 Hair. — The epidermis takes the initiative in the formation of hair. 
 
 It thickens in spots, the thickenings pushing into the corium and each 
 
 being cupped at the tip, blood-vessels extending into the cup. The basal 
 
 cells of the ingrowth, thus richly nourished, proliferate rapiidly and the 
 
 3 
 
34 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 new cells thus formed are forced outward, forming the hair. While 
 this is going on the ingrowth splits around the hair, forming the follicle, 
 while another ingrowth of the Malpighian layer forms the sebaceous 
 gland which oils the hair. 
 
 A section through a hair and its follicle gives the following layers (fig. 26). 
 Around all is the connective-tissue envelope, formed from the corium; next inside 
 is the outer root sheath formed of the Malpighian layer and extending to the cavity 
 of the follicle. Around the root of the hair is the inner root sheath, two cells in 
 thickness, the layers being known as Henle's and Huxley's layers. These do not 
 extend outside the follicle. In the hair itself there is a cortical layer surrounding 
 the central medulla, the hair not being hollow. 
 
 Fig. 26. — Diagram of structure of hair, h, blood-vessels; c/, cuticle of hair; ex, cortex g, 
 gland; h, hair; he, Henle's layer; hf, hair follicle; hx, Huxley's layer; m, medulla; p, papilla; 
 sg, stratum germinativum of epidermis. 
 
 Hair differs greatly in size, the spines of the porcupines forming one extreme, the 
 prenatal hair (lanugo) of man the other. Hair is shed at intervals. The old hair 
 ceases to grow, separates from its base, and later is pushed out when the root begins 
 again to proliferate. There are smooth muscle fibres connected with the roots of 
 the hairs, their function being to raise the hair from its usual inclined position under 
 influence of the sympathetic system. There are also usually nerves distributed 
 to the base of the hairs, making them to some extent sense organs, a condition 
 which reaches its greatest development in the facial hairs (vibrissae) of carnivores 
 and the hairs on the wings of bats. 
 
 Scales occur in several orders, being usually best developed on the 
 tail and feet. They may be rounded, quadrangular or hexagonal, the 
 square scales being arranged in rings around the part, the others in 
 quincunx. These are closely similar to the cuticular scales of reptiles 
 (p. 26). Recent investigations tend to show that there is a close rela- 
 
INTEGUMENT. 
 
 35 
 
 tion between scales and hairs, since in the mammals with scales the 
 hairs are usually arranged in groups of three or five behind each scale 
 (fig. 27) ; while in those without scales the hairs are frequently grouped 
 in the same manner. The illustration (fig. 28) is interesting as 
 showing the arrangement in man and the possible relation to ancestral 
 
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 Fig. 27. — A, arrangement of the two kinds of hair in Ornithorhynchus; B, Arrangement 
 of hair in Ptilocerus lori, with the probable relation of the hair to the ancestral scales; 
 both after Meijere. 
 
 Fig. 28. — Arrangement of the hairs in groups of threes and fives in the human embryo, 
 with the probable ancestral arrangement of the scales; after Stohr. 
 
 scales. The statement is also made that at first the hairs are arranged 
 in longitudinal rows and that the grouping comes later. 
 
 The mammalian skin is usually rich in glands which are of two 
 types, tubular and acinous (p. 18) . To the first belong the sweat glands, 
 which extend from the Malpighian layer, where they arise, down through 
 
36 
 
 COMPARA.TIVE MORPHOLOGY OF VERTEBRATES. 
 
 the corium and then are coiled in order to obtain greater length. The 
 acinous glands are represented by the sebaceous glands in connection 
 with each hair (fig. 26, g), and by the scent glands in the anal or in- 
 guinal region of many carnivores, rodents and edentates. Others 
 may occur in very diverse regions as on the face (bats, deer), in 
 the occipital (camel) or temporal region (elephant) or on the legs 
 (swine). 
 
 The mammary or milk glands are now known to be modified tubu- 
 lar glands possibly derived from sweat glands. In the monotremes the 
 simplest condition is found, numbers of glands opening into a pair of 
 sacs in the sides of the marsupium, or pouch where the young are kept, 
 
 Fig. 29. — Scheme of different kinds of nipples, based on figures by Weber. 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 condition in cow. B and C are true nipples, F a pseudo- 
 nipple (teat). 
 
 on the ventral side of the body. In the marsupials there is a slight nip- 
 ple developed from the bottom of the pocket. In the higher groups of 
 mammals the first appearance of the milk glands is the formation of a 
 ' milk line, ' a ridge on either side of the body from in front back to the 
 inguinal region. This is soon divided into *milk points ' from each of 
 which there is an ingrowth of epidermis into the corium, the interme- 
 diate parts of the line disappearing. Each of the points may develop 
 into a definitive mamma, but not all of them come to full development, 
 for the number in the adult is less then in the embryo, varying from a 
 single pair to eleven in Centetes, the number roughly corresponding to 
 
SKELETON. 
 
 37 
 
 the number of young at a birth. This method of formation explains 
 the varying position of the niammse and also the occasional occurrence of 
 more than the normal number (polymastism) in man and other mam- 
 mals. Each gland is provided with a nipple and of these there are two 
 kinds (fig. 29). In the one the whole surface on which the lacteal 
 ducts empty becomes elevated, in the other the region around the 
 openings of the ducts becomes drawn out into a tube with the ducts 
 opening at the bottom (ungulates). 
 
 THE SKELETON. 
 
 The term skeleton as used here is applied to any of the harder parts 
 of the body, developed from the mesoderm and serving for support, 
 
 Fig. 30. — Diagram of the skeletogenous tissue in the caudal region of a vertebrate. 
 bv, blood-vessels; epmu, epaxial muscles; hs, horizontal partition; hymy, hypaxial muscles; 
 msd, msv, dorsal and ventral median septa; mys, myosepta; n, spinal cord; nc, notochord. 
 
 for the attachment of muscles, for protection and the like. This ex- 
 cludes any purely epidermal hard parts, and these have been included 
 with the integument. 
 
 As the skeleton can only develop where there is mesenchyme, the 
 distribution of the chief skeletogenous parts, sometimes called the 
 membranous skeleton, may be given here, continuing the account from 
 page 16. First is the corium, immediately beneath the epidermis, 
 
38 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 forming an envelope around the internal structures. This connects 
 in the middle line, above and below, with a longitudinal partition which 
 separates the muscle masses of the two sides. This partition splits 
 to pass on either side of the central nervous system and the notochord, 
 and, just beneath the peritoneum, around the viscera. From the 
 median partition sheets of mesenchyme (myosepta) pass vertically 
 between the myotomes to the dermal layer, they being, like the myotomes, 
 metameric. Then there is a horizontal sheet on either side which lies 
 between the epaxial and hypaxial muscles (p. 127). Not all parts of 
 this membranous skeleton develop hard structures, but these are most 
 apt to arise at the intersection of the various planes. 
 
 The skeletal structures are divided into the dermal, arising in the 
 outer mesenchymatous envelope, and the endoskeleton, formed in 
 the other parts and lying deeper in the body. The dermal skeleton 
 includes the scales of fishes, the dermal armor of many reptiles and 
 fossil amphibians and the bony scales in the skin of crocodilians and 
 some mammals. In the strict sense the so-called membrane bones of 
 the skull and the cleithrum of fishes and the clavicle and episternum 
 of higher vertebrates should be included here, since they apparently 
 have been derived from dermal ossifications, but convenience of treat- 
 ment necessitates their consideration with the endoskeleton, with which 
 they are intimately associated. ; 
 
 It is a question whether the dermal or the endoskeleton is the older. The most 
 primitive of the living species, the cyclostomes, have no exoskeleton, but have 
 cartilage developed to some extent. In development, also, cartilage always ap- 
 pears before there is a trace of the exoskeleton. On the other hand, some of the 
 oldest fishes known have a well developed dermal armor, while the best preserved 
 ostracoderms show no trace of an internal skeleton. The external skeleton has 
 probably arisen as a means of protection, the internal as a result of muscular or 
 other strains. 
 
 Bones are connected (articulated) with each other in different ways* 
 They may be so articulated that one can move on the other (diar- 
 throsis) or there may be no motion possible (synarthrosis), each with 
 several varieties. Of the immovable joints there may be sutures, 
 where the two bones are connected by the interlocking of saw tooth-like 
 projections, or the two may be united by bony growth (anchylosed) 
 so that the line between the two disappears. In those cases of diar- 
 throdial joints where there is much motion there is usually a closed sac, 
 lined by a synovial membrane between the two bones. This mem- 
 brane secretes a fluid which lubricates the surfaces. 
 
SKELETON. 
 
 39 
 
 Cartilages and bones are covered on their outer surfaces by an 
 envelope of connective tissue, called respectively perichondrium or 
 periosteum. These membranes form the means by which muscles 
 are attached to the bones and by which blood-vessels obtain entrance to 
 them. The periosteum is also a seat of bone formation. 
 
 DERMAL SKELETON. 
 
 When present, the dermal skeleton arises by a marked prolifera- 
 tion of cells at definite points in the corium. These cells become 
 specialized (scleroblasts, odontoblasts or osteoblasts) for the 
 deposition of salts of lime plus a varying amount of organic matter 
 (ossein). Upon limy plates formed in this way other parts, also 
 calcareous, may be laid down by the basal surface of the epidermis, 
 so that the whole dermal element may be in part mesenchymatous, 
 in part ectodermal in origin. 
 
 -^^ -s^? 
 
 Fig. 31. — Cross-sections of developing scale of Acanthias. c, stratum corneum; J, dentine 
 of scale; ee, enamel organ; m, stratum Malpighii; />, pulp. 
 
 It is generally thought that the primitive dermal skeleton resembled 
 that of existing sharks, and that from the hypertrophy or fusion of 
 such scales the so-called membrane bones have arisen. Then the 
 scales of other vertebrates are to be traced back to an elasmobranch 
 ancestr)% while teeth are thought to be modified scales. Hence the 
 structure and development of the elasmobranch scale should be 
 understood. 
 
 At regular intervals in the skin of a shark there is a multiplication of 
 cells of the corium, each aggregation forming a small papilla which 
 projects above the surrounding corium, carrying with it the basal layer 
 of the epidermis. The surface cells of the papilla and the region 
 around it becomes converted into osteoblasts which secrete calcic 
 
40 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 salts on their outer ends, thus forming a small plate of dentine (p. 24) 
 with a central spine into which the papilla extends. The overlying 
 epidermal cells form an enamel organ, the lower surface of which 
 secretes an even harder layer of enameP upon the dentine base, this 
 being thickest on the tip of the spine. The mesenchyme in the papilla 
 is the so-called pulp. With continued growth the spine projects through 
 the epidermis, giving the skin of the shark its characteristic rough 
 (shagreen) condition. This is the placoid type of scale. 
 
 FISHES. — In the adult elasmobranchs the scales may be large and remote from 
 each other (skates) or small and closely set. In the torpedo scales are lacking, 
 while in the chimaeroids they occur only on the claspers, on the frontal horn, and 
 as extreme forms, in a great spine in front of the dorsal fin. 
 
 Fig. 32. — ^Ventral armor of Stegocephals (after Credner-Zittel). A, Branchiosaurus; B, 
 detail of same; C, detail of Archegosaurus; D, of Petr abates. 
 
 A few ganoids lack scales {Polyodon), while the sturgeon have minute granules 
 and five rows of large plates along the sides. Amia has scales of the cycloid type, 
 soon to be described. With these exceptions the ganoids have ganoid scales, which 
 are rhomboid in outline and joined to each other like parquetry. They consist of 
 two layers, the lower apparently homologous with the dentine of sharks, except that 
 it is formed in, not on, the corium. The outer layer of ganoin is formed by the 
 corium and consequently cannot be enamel as once was thought. 
 
 A few teleosts are scaleless (some eels), but elsewhere scales are formed in 
 
 pockets in the corium (fig. 181). At first they lie side by side, but with growth they 
 
 overlap like shingles. There is only one layer of bone mixed with a large amount 
 
 of ossein.- In cycloid scales the element is circular and is marked with concentric 
 
 and radiating lines. The ctenoid scales differ in having the posterior edge of 
 
 * There is some question whether this layer is really enamel; the usual statement as to 
 its nature is followed here. 
 
SKELETON. 
 
 41 
 
 each scale truncate and this edge and the surface toothed. Cycloid and ctenoid 
 scales intergrade and both kinds may occur on the same fish (many gobiids). 
 
 AMPHIBIA. — A dermal skdeton occurs in the recent amphibians only as rows 
 of plates in the cutaneous rings on the bodies of the caecilians and in the skin of the 
 back of a few exotic toads. In some fossil stegocephalans there was a ventral 
 armor and in others one protecting the whole body. The ventral exoskeleton, 
 sometimes of scales or plates, sometimes long bars, is arranged in oblique rows, 
 and is interesting as probably being the source of the gastralia found in many 
 reptiles {infra). Epistemum and clavicle were possibly dermal in these forms, 
 but they will be described in connection with the shoulder girdle. Apparently 
 certain of the gastralia of these fossils were modified into comb-like organs which 
 have been thought to have sexual significance. 
 
 REPTILES. — The dermal skeleton is best developed in the turtles of Hving 
 reptiles, though here it is closely associated with the endoskeleton. The dermal 
 plates form a box for the protection of the body. This consists of a dorsal carapace 
 and a ventral plastron, united to varying extents and each consisting of a number 
 of elements. In the carapace there is a middle line of neural plates (fused vAih. 
 
 F'G. ^^. — Section through developing vertebra, rib and exoskeleton of Chelone imbricata, 
 after Gotte. c, cutis; cs, primitive vertebral body, ep, epidermis; m, external oblique muscle; 
 p, perichondriiun; r, rib; sp, spinal process. 
 
 the vertebrae), marginal plates around the margin, and costal plates, fused to 
 
 the ribs, between neurals and marginals. The plastron (fig. 34) usually consists 
 of nine plates, wholly dermal, the names shown in the figure. The three posterior 
 pairs are regarded as the same as the gastralia of other reptiles, the anterior pair as 
 the clavicles, while the unpaired entoplastron is supposed to be homologous with 
 the epistemum of other tetrapoda. 
 
 Some of the extinct crocodilia were armored with closely applied scales and 
 these have been retained in the existing species in a reduced condition. They 
 also have well developed gastralia. These are of rods dermal bone in the ventral 
 body wall between the true ribs and the pelvis, and so closely resemble ribs that 
 they were called 'abdominal ribs.' They do not meet in the middle line; each, 
 except the first, consists of two distinct parts, and the pairs correspond to the 
 somites in number. In Sphenodon (fig. 35) the gastralia are more numerous than 
 the somites. 
 
 In a few lizards there are dermal scales, while the extinct stegosaurs had 
 
42 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 dermal ossicles, sometimes of great size (plates a yard across, spines half a yard long) 
 in the dorsal region. 
 
 BIRDS. — Recent birds lack all dermal ossifications, but Archcdopteryx had 
 gastralia. 
 
 Mammals rarely have dermal bones. They are known in the extinct zeuglo- 
 dont whales and in several fossil edentates, but in the living species they occur 
 
 Fig. 34. — ^Plastron of Trionyx. en, ento- 
 plastron; ep, epiplastron; hpp, hypoplastron; 
 hyp, hyoplastron; xp, xiphiplastron. 
 
 Fig. 35. — ^Ventral ends of ribs (r) 
 and gastralia (g) of Sphenodon. 
 
 only in the armadillos where they form a complete armor above, the plates 
 arranged in transverse rows, some of which are movable on each other. In the 
 extinct glyptodons they formed an inflexible case. It is uncertain whether these 
 are a new acquisition in the edentates or have been inherited from non-mammalian 
 ancestors. 
 
 THE ENDOSKELETON. 
 
 The endoskeleton may pass through three stages in its develop- 
 ment, including the membranous stage. From this it may pass through 
 a cartilage stage before becoming bone, or it may in part develop 
 directly into bone from membrane, or, lastly, it may never pass beyond 
 the cartilage stage. Thus only the membranous stage is constant. 
 These dijfferences in development are of great importance in tracing 
 homologies between bones in different groups, but the distinction be- 
 tween bones developing directly from membrane (membrane bones) 
 
SKELETON. 43 
 
 and those passing through a cartilage stage (cartilage bones) can only 
 be recognized by following the ontogeny of the element in question. 
 
 As stated above, there is much evidence to show that the membrane bones are 
 dermal bones which have sunk to a deeper position and have become secondarily 
 associated with the endoskeleton. This is especially evident in the skulls of some 
 of the lower ganoids. Ossification of cartilage takes place in two wa]^. In 
 ectochondrostosis the deposit of lime salts begins on the deeper surface of the 
 perichondrium and gradually invades the cartilage. In entochondrostosis the 
 cartilage becomes broken down in the interior, some of the cells becoming modified 
 into osteoblasts, and from these as centres of ossification, the process of bone forma- 
 tion extends in all directions. In ectochondrostosis at least, the centres of ossifica- 
 tion may have been derived, phylogenetically, from elements of the dermal skeleton. 
 
 In ossification the bone is developed in layers, between which the osteoblasts are 
 arranged. In the elasmobranchs the skeleton is frequently strengthened by 
 deposits of lime, but this calcified cartilage differs from bone in that the deposits 
 of lime take the form of polygonal plates and there are no lacunae. 
 
 © CO) (Q 
 
 Fig. 36. — Diagram of growth of bone. A, from an animal recently fed with madder 
 causing a layer of bone (black) colored by the dye; B, later, no madder fed for some time, 
 a deposit of colorless bone on outside of colored layer, internal layer thinner; C, still later, 
 outer layer thicker, inner layer absorbed. 
 
 Many bones increase in length by the addition of epiphyses at the ends. These 
 are separate ossifications which only unite with the main bone at the time the adult 
 condition is reached. The increase in diameter has some interesting features. In 
 animals fed with madder, the bone formed during the feeding is colored. In this 
 way it is found that the new bone (fig. 36, ^) is laid down on the outside of the 
 other, and that with growth {B and Q, the 'marrow cavity' on the inside is in- 
 creased in size by the resorption of the bone already formed. 
 
 For convenience of treatment the endoskeleton is divided into axial 
 and appendicular portions, the axial consisting of the vertebral column 
 (backbone) and the skull, together with the ribs and sternum which are 
 closely associated with the vertebrae. The appendicular skeleton in- 
 cludes the framework of the limbs and fins and the girdles to which 
 they may be attached. 
 
 Axial Skeleton. 
 
 Both the skull and the vertebral column surround and protect the 
 brain and spinal cord, and in this way the skull is an enlarged and 
 
44 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 specialized portion of a continuous axis, but it is not possible to carry 
 the comparison into details. The idea of Oken that the skull is a com- 
 plex of three or four vertebrae has long been overthrown. The skull 
 differs markedly from the vertebral column in the presence of numerous 
 membrane bones. 
 
 Vertebral Column. 
 
 The notochord (p. 12) is the foundation around which the verte- 
 brae and the posterior part of the skull are developed. It is a cylin- 
 
 FiG. 37. — Section of developing vertebra of 45 mm. Amhlystoma. c, cartilage of inter 
 centrum; cs^, outer chorda sheath; cs^, inner chorda sheath; dm, dura mater; e, epithelioid 
 layer of notochord (elastica interna) ; end, endorhachis, torn froni wall of vertebral canal; 
 np, neurapophysis (ossified); ns, neural spine of preceding vertebra; nt, notochord; sc, 
 spinal cord sd, subdmral space. 
 
 drical rod of entodermal origin, without segmentation,^ extending from 
 the infundibulum (see brain) to the posterior end of the body. Its cells 
 become vacuolated and at length most of the protoplasm, together 
 with the nuclei, migrate to the surface of the cord, where they appear 
 like an epithelium, which, together with its basal membrane, is called 
 the internal elastic membrane (elastica interna, fig. 37, e). 
 
 * Segmental undulations occur in the notochords of some mammals, but their significance 
 is not clear. 
 
SKELETON. 
 
 45 
 
 Next, mesenchymatous cells, derived from the sclerotomes, form 
 a notochordal sheath, boynded externally by an elastica externa. 
 The mode of formation and the history of the sheath vary in different 
 groups, for accounts of which reference must be made to special 
 papers. Other skeletogenous tissue extends outward from the sheath 
 toward the periphery, as described on a previous page (p. 38, fig. 30) 
 from which the ribs of all vertebrates are developed, the cyclostomes 
 passing but little beyond this membranous condition in the tnmk 
 region. 
 
 With the appearance of cartilage segmentation is introduced into 
 the skeleton. As cartilage is firm and comparatively unyielding, in 
 
 Fig. 38. Fig. 39. 
 
 Fig. 38. — ^Two caudal vertebrae of alligator, c, centrum; ha, haemapophysis; hs, 
 haemal spine; na, neurapophysis; ns, neural spine; poz, prz, post- and prezygapophyses; 
 t, transverse process. The arrow passes through the neural arch. 
 
 Fig, 39. — Diagrams of {A and B) fish vertebrae and (C) vertebra from higher groups. 
 b, basal stumps; c, capitular head of rib; ct, centnmi; d, diapophysis;/r, fish rib; ha, haemal 
 arch; na, neural arch; p, parapophysis; r, rib; t, tubercular head. 
 
 order that the trunk may bend, the cartilage becomes divided into 
 separate blocks, which, in order that they may be moved by the muscles 
 connected with them, must alternate with the myotomes. Hence the 
 metamerism of the vertebral column is the result of that of the muscular 
 system. 
 
 A typical vertebra, whether of cartilage or bone, consists of several 
 parts, the names of which are necessary for the understanding of the 
 following account. Surrounding the notochord is the body or centrum, 
 developed from the notochordal sheath or from tissue surrounding it. 
 A neural arch, enclosing the spinal cord, extends dorsally from the 
 centrum. It consists of a plate on either side (neurapophysis), the 
 
46 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 arch being completed by a neural spine as a keystone. Ventral 
 to the centrum is a similar haemal arch, composed, in like man- 
 ner, of haemapophyses and haemal spine, and enclosing, in the 
 caudal region, the caudal artery and vein, farther forward, the 
 coelom and viscera. This type of vertebra is common in many 
 fishes, and in the tails of some higher forms. In the lowest fishes 
 it is simplified by the omission of parts, while in the higher verte- 
 brates other structures are added. Among these are articular proc- 
 esses (zygapophyses) on the anterior and posterior faces of the neural 
 arch (distinguished by position as pre- and post-zygapophyses) 
 which lock the successive vertebrae together and strengthen the column 
 without interfering with its flexibility (fig. 38). 
 
 Fig. 40. — Diagrammatic sagittal sections of (^4) amphicoelous; (B), procoelus; (C), opistho 
 ccelous; and (D), amphplatyan vertebrae; the head supposed to be at the left. 
 
 In all vertebrates above fishes most of the vertebras bear transverse 
 processes (pleurapophyses), extending laterally on either side. Of 
 these there are two kinds, a parapophysis arising from the centrum, 
 and a diapophysis projecting from the neural arch. The ribs articu- 
 late with the ends of these, as will be explained later. The distinctions 
 are the most marked in the lower vertebrates, but careful comparisons 
 show them in the mammals. Other processes, of less frequent occur- 
 rence, will be mentioned below in connection with the groups in which 
 they occur. 
 
 The ends of the centra, where they articulate with each other, 
 may take five different shapes. They may be hollow at both ends 
 (amphicoelous); they may fit together with a ball and socket joint, 
 the hollow being sometimes in front (proccelous), sometimes behind 
 (opisthocoelous). In the mammals flat or amphyplatyan conditions 
 are common, while in birds saddle-shaped ends occur (figs. 40, 49). 
 
 In the history of vertebrae both comparative anatomy and embryology agree 
 that the process of vertebral formation began with the arches and extended thence 
 
SKELETON. 
 
 47 
 
 to the sheath of the notochord. In what must be considered the most primitive 
 condition the arches extend no further than the sheath and nothing comparable to 
 a centrum is found, even when ossification occurs. In the formation of centra two 
 methods of extension of cartilage to the chordal region are known. In the elasmo- 
 branchs immigrating cells from the arches break through the elastica externa and 
 distribute themselves through the sheath, converting 
 it into cartilages. In other vertebrates (fig. 43) the 
 immigrating cells extend around the elastica externa 
 so that the sheath eventually comes to lie inside the 
 centrum. 
 
 In many fishes and fossil amphibians 
 another element, the intercalare, enters into 
 the composition of the neural arch on either 
 side. The intercalaria lie above and behind „ ^ . _. u 
 
 Fig. 41. — ^Trunk vertebrae 
 
 the neurapophyses and may expand dorsally of Rhynchohatus, after Dum€- 
 so that the arch is completed by them above. tercakr^^te;^'r^meS; 
 The dorsal root of the spinal nerve usually «,. neural process; r, rib; s, 
 
 . spinous process. 
 
 passes through the mtercalare, the ventral 
 
 through the neurapophysis, but both roots may pass between them. 
 Similar intercalaria may occur in the haemal arclL In the trunk region 
 there may be separate elements of the centra; in each somite a trans- 
 verse cartilage (hypocentnun) across the under side of the neural 
 sheath, and a pleurocentrum on either side, behind the hypocentrum 
 (fig. 42). . 
 
 Fig. 42. — Stegocephalan vertebrae, after Zittel and Woodward. A, phyllospondylous; 
 B, rhachitomous of Chelydrosaurus; C, Callopterus; D, embolomerous of Eurycormus; hs^ 
 hypocentrum arcuale; hp, hypocentnim pleurale; np, neurapophysis; «5, neural spine. 
 
 Comparisons of different adult vertebrae show that these vertebral elements 
 may combine in different ways, though they have not been recognized in the on- 
 togeny of the higher forms. Apparently the phyllospondylous vertebra of some 
 stegocephals (fig. 42) are formed of hypocentrum and neural arch, both contribu- 
 ting to the hollow transverse process. In others haemal arch and hypocentrum 
 unite, while the pleurocentra meet and fuse above the notochord. Expansion of 
 these makes the vertebral column look like a series of interposed triangles (fig. 42 C). 
 
48 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 This is the rhachitomous or temnospondylous vertebra. Still farther expansion 
 of hypo- and pleurocentra causes the former to unite with the neural arch, while 
 the two pleurocentra meet below the notochord (fig. 42 D), the result being two rings 
 in each somite, the embolomerous vertebra, which occurs in some stegocephali, 
 some fossil ganoids and in the tail of Amia. Lastly these two rings (often called 
 centrum and intercentrum) may fuse, giving the typical centrum. 
 
 The neural and haemal spines which complete the arches are formed by seg- 
 mental chondrifications of the interspinous ligament which runs the length of the 
 body above and between the halves of the neural arches. 
 
 The vertebrae are outlined at an early stage of the embryo and their 
 number is not subsequently increased. Consequently increase in 
 
 Fig. 43. — Earlier and later stages of development of a vertebra of Amhly stoma, cc, 
 cartilage in centre of vertebra; ei, elastica interna; i, incisure cutting through ic, intercentral 
 cartilage; «, notochord; »5, notochordal sheath; v, vertebra (bone) black. 
 
 length of the vertebral column can only occur by growth of the vertebrae 
 themselves. When first formed each centrum encircles the notochord 
 and prevents its increase in diameter at this point, while between the 
 centra it can expand. As a result the notochord soon resembles a string 
 of beads (moniliform) with intervertebral enlargements. Then, as 
 additions are made to the centra to increase their length, the new parts 
 must form around the intervertebral enlargements and in this way the 
 ends of the centra become cup-shaped and th^ amphiccelous condition 
 (fig. 43, /) is produced. In some urodeles this stage is followed by 
 
SKELETON. 
 
 49 
 
 the deposition of cartilage in the cups (fig. 43, //) producing inter- 
 vertebral constrictions of the cord. As this progresses absorption of the 
 cartilage begins between the ends of the vertebrae (ic) and continues 
 in such a way that the result is a ball of cartilage attached to the hinder 
 vertebra and a corresponding cup in the one in front; in other words, 
 an opisthoccelous condition. 
 
 Several regions may be differentiated in the vertebral column, these 
 being the most numerous in the higher groups of vertebrates. These 
 are (i) the cervical, in the neck, with great reduction or even absence 
 of ribs; (2) the thoracic, following the cervical, with distinct ribs; (3) 
 
 Fig. 44. — Section through atlas 
 (at) and axis (ax) of fowl, cut sur- 
 faces lined, e, epistropheus; /, facet 
 for articulation with skull; /, trans- 
 verse ligament. 
 
 Fig. 45. — ^Proatlas, atlas 
 and axis of alligator, a, atlas ; 
 e, epistropheus (axis) ; p, pro- 
 atlas; r, rib of third vertebra; 
 ra, re, ribs of altas and epis- 
 tropheus. 
 
 lumbar, without ribs; (4) sacral, including one or more vertebrae with 
 which the pelvis is connected; (5) caudal, the tail, behind the sacrum. 
 Sometimes the ribs extend back to the sacrum so that thoracic and 
 lumbar cannot be distinguished, all being then grouped as dorsal. 
 Then in the fishes and some higher vertebrates (snakes, whales, etc.) 
 sacral vertebrae are not differentiated, and in the fishes there is no line 
 between cervicals and dorsals, so that only trunk or abdominal, and 
 caudal regions can be distinguished, the line being drawn (fishes) at the 
 point where haemal arches are transformed into ribs. 
 
 One or two of the anterior vertebrae are modified in the higher 
 (tetrapodous) vertebrates and have received names. The first, which 
 immediately adjoins the skull, is the atlas. It bears on its anterior face 
 an articular surface which receives the one or two condyles of the cra- 
 nium. In the amniotes the second vertebra, the axis or epistropheus is 
 also specialized. On the anterior face of its centrum is a pivot (the 
 
50 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 dens or odontoid process) on which the atlas turns. Development 
 shows that this dens is the centrum of the atlas which has separated 
 from its own verfebra and has fused to that of the axis. 
 
 In a few reptiles and possibly some mammals a so-called proatlas occurs as a 
 plate or pair of plates (fig. 45) of bone between the atlas and the skull, in the posi- 
 tion of a neural arch. It is not certain whether this is the remains of a vertebra 
 which once occupied this position, or is a new formation. Nor has it been settled 
 whether the atlas of the amphibians is homologous with that of mammals. 
 
 In cyclostomes, fishes and aquatic urodeles the posterior end of the 
 vertebral column is concerned in the formation of the caudal fin, 
 which presents three modifications. The most primitive is the diphy- 
 
 FlG. 46. — Tails of fishes. A, young Amia; skeleton (homocercal) ; B, diphycercal; C, 
 heterocercal; D, homocercal; h, hypurals; «, notochord; s, spinal cord. 
 
 cereal tail in which the vertebral column runs straight to the end of the 
 body, the fin being developed symmetrically above and below it. This 
 is found in the young of all fishes and in the adult cyclostomes, dipnoans, 
 many crossopterygians and urodeles. In the heterocercal tail, which 
 occurs in elasmobranchs and ganoids, the axis bends abruptly upward 
 near the tip, and while retaining the caudal fin of the diphycercal stage, 
 has a second, smaller lobe developed below, giving the whole an unsym- 
 metrical appearance. In the homocercal tail, which occurs in Amia 
 and all teleosts since the cretaceous, there is the same upward bend to 
 the Vertebral column, but symmetry is restored externally by the re- 
 duction of the neural arches and the development and fusion of the 
 haemals into larger plates (hypurals), while the lower lobe of the tail 
 grows out to equal the other. 
 
SKELETON. 
 
 51 
 
 CYCLOSTOMES have a persistent notochord, increasing in size -with the 
 growth of the animal, and lacking constrictions since no centra are developed. 
 In the myxinoids there are neurapophyses and intercalaria deYcloped in the caudal 
 region; in the lampreys they occur in the trunk as well. 
 
 FISHES. — In the elasmobranchs the typical vertebrae are developed in cartilage, 
 with intercalaria in connection with the arches. Usually the centra undergo more 
 or less calcification (p. 43), the lime being either deposited in concentric rings 
 around the notochord (cyclospondylous vertebrae) or in radiating plates (astero- 
 spondylous) . In the trunk region each centrum often bears a pair of transverse 
 processes with short ribs at their extremities. In a few forms (skates, etc.) 
 embolomerism (p. 48) occurs in the tail, and in the holocephali the centra are 
 replaced by numerous rings of cartilage. In skates and in Chimcera there is a true 
 joint between the skull and the column, but in the sharks the anterior vertebrae are 
 fused together and to the skull. 
 
 Fig. 47. 
 Fig. 47. — Diagrammatic sections of elasmobranch vertebrae 
 
 C, asterospondylous. 
 Fig. 48. — Cross-section of teleost vertebra; bone, black; cartilage, dotted 
 
 Fig. 48. 
 A, B, cyclospondylous; 
 
 The ganoids vary greatly in vertebral characters, some of the Chondrostei 
 having only cartilage and some of the fossil forms lacked centra. On the other 
 hand, nearly the whole vertebra is ossified in Amia and Lepidosteus, the latter having 
 opisthoccele vertebrae, a condition not reappearing until the amphibians, as all 
 other fishes in which centra are developed have amphicoelous vertebrae. 
 
 As the name implies, ossification of vertebrae and other parts is common in 
 teleosts. The arches are almost always ossified, while the centra may be, or those 
 parts directly connected with the arches may remain cartilaginous while the rest 
 ossifies (fig. 48), so that the section presents a radiate figure as in the asterospondy- 
 lous sharks. Some teleosts have zygapophyses and a few genera have transverse 
 processes on some of the vertebrae. 
 
 The dipnoans, so far as ossification of the vertebrae is concerned, are on a par 
 with the cartilaginous ganoids. There are no centra and the notochord grows 
 throughout life. 
 
 AMPHIBIA, except the legless forms, have caudal, sacral, trunk, and a single 
 cervical vertebra, the sacrals being single except in a family of extinct anurans. 
 Zygapophyses and both kinds of transverse processes may be present. 
 
52 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 The stegocephals had the greatest range of vertebral structure, rhachitomous, 
 embolomerous, and amphicoelous types occurring, the first two even in the same 
 individual. Phyllospondylous vertebrae (fig. 42) are found only in the fossil 
 Branchiosauridae. 
 
 The cascilians have a very large number (up to 275) of amphicoelous vertebrae 
 in correlation vdth the snake-like body form. The perennibranchs, derotremes and 
 many salamandrina are amphicoelous; the rest of the urodeles are opisthocoelous. 
 
 The anura, as a rule, have procoelous vertebrae, but a few genera have them 
 opisthoccele. All recent species have eight presacral vertebrae, but there were nine 
 in the tertiary forms. A striking feature is the 
 fusion, in the adult, of all of the caudal verte- 
 brae into the well-known rod, the coccyx or 
 urostyle. 
 
 REPTILES always have the vertebrae ossi- 
 fied, although remnants of the notochord may 
 persist in the centra, of which all types, amphi-, 
 pro-, opisthocoelous and flat occur in the group. 
 In lizards, snakes and dinosaurs the articulation 
 between the successive vertebrae is strengthened 
 by zygantra and zygosphenes, a cavity on one 
 vertebra into which a projection from the next 
 
 Fig. 49. Fig. 50. 
 
 Fig. 49. — Cervical vertebra of a bird showing the saddle-shaped articular surface {af) 
 on the centrum, c; cr, cervical rib; nc, neural canal; ns, neural spine; poz, prz, post- and 
 prezygapophyses. 
 
 Fig. 50. — Central view of synsacrum and pelvis of hawk (Buteo). il, ilium; m, 
 ischium; p, pubis; pp, pectineal process; s, sacral ribs. 
 
 fits. In the existing species there are never more than two sacral vertebrae, but 
 the pterosaurs had from three to seven, while in the dinosaurs there might be ten, 
 all being co-ossified when there were more than three. 
 
 Little is known of the theriomorph backbone, except that some had persistent 
 notochords, others amphicoelous centra. In the plesiosaurs they were flat, while 
 in the turtles the dorsals are fused and the neural spines are united with the neural 
 plates (p. 41). The other centra vary. Those of the rhynchocephals and most 
 dinosaurs are flat, while snakes and lizards, except the geckos have them procoelous. 
 In the earliest crocodiled they were amphicoelous, while later they are procoelous or 
 flat, and in the pterodactyls they are procoelous in front, amphicoelous in the tail. 
 
 BIRDS usually have saddle-shaped ends to the centra (the atlas procoelous). 
 
SKELETON. 
 
 53 
 
 Several of the dorsals are usually fused for strength, but the first presacral is free. 
 A characteristic feature is the sjrnsacrum, foreshadowed in the dinosaurs. As the 
 bird stands on two feet and holds the body obliquely, several of the dorsal and caudal 
 vertebrae (up to 20) fuse with the sacrals into a common mass, a large proportion 
 also uniting with the pelvis. The true sacrals (three in ostriches, two elsewhere) lie 
 just behind the pits occupied by the kidneys and may be recognized by their lower 
 articulation to the pelvis. A few of the caudals behind the synsacrum are free, as 
 all were in ArchcBopteryx, but the others in recent birds are united into an upturned 
 bone, the pygostyle. 
 
 MAMMALS, except whales where the sacrum is lacking, have all the five verte- 
 bral regions differentiated. With four exceptions the cervicals are seven in number 
 (Manatus australis and Choloepus hofmanni, six; Brady pus torquotus, eight; B. 
 tridactylus, nine). The dorsals (thoracics plus lumbars) vary between fourteen in 
 armadillos and thirty in Hyrax, but usually are nineteen or twenty, the number 
 of thoracics usually increasing at the expense of the lumbars. There are primi- 
 tively two sacrals, but others may unite until they amount to nine or ten in some 
 edentates. Usually the centra are amphiplatyan, but in the cervicals of ungulates 
 opisthocoele vertebrae are common. It is to be noted that the 'transverse proc- 
 esses ' of the cervical vertebrae are, as in birds, composed in part of reduced ribs, as 
 will be shown below. 
 
 Ribs. 
 
 Two different structures are included under the common name of 
 rib, both connected at one end with a vertebra, the other supporting the 
 body walls around the viscera. In following forward the haemal arches 
 in the skeleton of a bony fish (fig. 39,^4, B) it is seen that when the 
 
 Fig. 51. — ^Vertebrae and ribs of (/) anterior and (//) posterior trunk region of Polypterus, 
 after Gegenbaur. p, pleiural rib; h, haemapophysial rib. 
 
 body cavity is reached the arch becomes incomplete below, the 
 two hasmapophyses separating and coming to lie just beneath the 
 peritoneum in the walls of the coelom. Above, it is either united 
 directly to the centrum or is jointed to a small process of it. More 
 careful study shows that this fish rib (haemapophysial rib) lies in 
 the intersection of a myoseptum with the median partition of the 
 skeletogenous tissue (p. 38) and is medial to the hypaxial muscles. 
 In the higher vertebrates the rib is formed in the intersection, of 
 
54 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 the myosepta with the horizontal plate, and thus is lateral to the 
 hypaxial muscles and between them and the epaxial series. This 
 is the true or pleural rib. Any vertebra may bear ribs of either 
 kind (including haemal arches) and the two kinds frequently coexist on 
 the same vertebra in the trunk of salmonids, clupeids and Polypterus, 
 and in the caudal region of urodeles and some reptiles. Their possible 
 occurrence in all parts of the body is explained by the existence of the 
 myosepta and other skeletogenous structures in all regions. 
 
 The haemapophysial ribs end freely below, never being connected with a sternum. 
 In some aberrant fishes they are lacking, while in the ostariophysi they play a part 
 in the 'Weberian apparatus' connecting the swim bladder with the ear (see ear). 
 The teleosts have, in addition, numerous rib-like structures which are not preformed 
 in cartilage (epineurals, epimerals, epipleurals) which are formed in the epaxial 
 or hypaxial regions or in the horizontal partition. 
 
 Fig. 52. — ^Front and side views of cervical vertebra of fowl, showing the cervical rib. 
 c, centrum; cs, spinal canal; d, diapophysis; p, parapophysis; r, rib; va, vertebrarterial 
 canal; the arrow in the side view passes through the canal. 
 
 The typical rib (it is not certain whether this is the primitive form) 
 has two heads for articulation with the vertebra, a capitular head 
 connecting with the parapophysis, a tubercular head joining the 
 diapophysis. Between the two heads and the centrum is a space, the 
 vertebrarterial canal, through which the vertebral artery passes 
 (fig. 39, C.) The true ribs, which are preformed in cartilage, have 
 various extents in the different regions of the body. In the thoracic 
 region, where they have the greatest extension, the ribs have to allow 
 for changes in size of the contained cavity, and hence parts of them 
 are frequently left unossified, or at least they are jointed, the two parts 
 being called vertebral and sternal ribs. 
 
 In the cervical region the true ribs are usually greatly reduced and are lacking 
 in the turtles. In many reptiles they clearly show their nature, being short, 
 bicipital and with their heads articulated to dia- and parapophyses (fig. 45). In 
 the birds they may be recognized (fig. 52), their distal ends being bent inward to 
 
SKELETON. 55 
 
 protect the carotid arteries. In the mammals they form the distal part of the 
 'transverse process' of human anatomy, the vertebrarterial canal and the develop- 
 ment revealing their true nature. 
 
 The dorsal ribs are very short in amphibians, never extending far 
 from the backbone. They are bicipital in most forms, except the 
 anura where they form small projections on the ends of the transverse 
 
 Fig. 53. — Skeleton of trunk of common goose, Anser domesticus. c, cuneiform; ca, 
 carina; co, coracoid;/, furcula (clavicle) ; /e, femur; h, humerus; il, ilium; is, ischium ; mc, 
 metacarpals; p, pubis; ph, phalanges; r, radius; s, scaphoid; sc, scapula; sr, sternal rib; st, 
 sternum; u, uncinate process; ul, ulna; vr, vertebral rib; 2, 3, 4, digits. 
 
 processes. In the amphibia the vertebral artery is ventral to the par- 
 apophysis. In all other vertebrates with a sternum at least a part of the 
 dorsal ribs reach that structure, encircling the viscera like the hoops of a 
 barrel. Those ribs which do not reach the sternum are called false 
 ribs. In most reptiles and some birds most of the thoracic ribs bear an 
 uncinate process directed upward and backward (fig. 53), overlapping 
 
56 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 the rib behind and strengthening the thorax. In the chelonia the ribs 
 are confined to the dorsal side of the body and are fused to the costal 
 plates (dermal skeleton) to form the carapace. Single- and double- 
 headed ribs often occur in the same individual of various groups, and 
 in the mammals the capitular head, instead of articulating with a 
 distinct parapophysis, may rest in a socket formed by two successive 
 vertebrae. 
 
 Fig. 54. — Sacral vertebrae, ribs and pelvis of Trionyx, obliquely from below. /, head and 
 rochanter of femur; il, ilium; is, ischium; p, pubis; sr, sacral ribs; sv, sacral vertebrae. 
 
 The pelvis is never directly united to the sacrum, but sacral ribs 
 intervene. These are distinct in the reptiles (fig. 54), but are fused to 
 the transverse processes in other groups. 
 
 The Sternum (Breastbone). 
 
 The sternum includes the skeletal parts on the ventral side of the 
 body, which are closely connected with the shoulder girdle and, except 
 in the amphibia, with the ribs. The fact that it occurs only in verte- 
 brates with legs (it is lacking in snakes and caecilians) shows that it has 
 arisen in adaptation to terrestrial locomotion. In man it consists of 
 three parts, a manubrium in front, a middle piece (gladiolus) > and a 
 xiphoid (ensiform) process behind, and these terms have been car- 
 ried into other groups. 
 
 In development the sternum arises in mammals by the formation of a longi- 
 tudinal bar of cartilage in the linea alba on either side, ventral (medial) to the ends 
 
SKELETON. 
 
 57 
 
 of the ribs, eventually connecting them together (fig. 55). With continued growth 
 these bars of the two sides meet atid fuse in the median line, forming a median plate, 
 the sternum. Later this separates from the ribs, and with the appearance of bone, 
 becomes a series of separate elements, the sternebrae (fig. 57), alternating with the 
 ribs; by fusion of sternebrse the parts in man arise. 
 
 In the amphibia the short ribs never extend to the sternum, but skeletal parts 
 occur near the mid-ventral line in a few forms, which may be ventral ribs as they 
 participate in the formation of the sternum. Nothing is known of a true sternum 
 in the stegocephals. In the urodeles it is a short cartilaginous plate, lying mostly 
 behind the girdle, with its sides grooved to receive the medial ends of the coracoids. 
 
 Fig. 55. 
 
 -Development of sternum in 30 mm, human embryo, after Ruge. cl, lower end 
 of clavicle; r, ribs; s, two halves of stemimi; ss, suprastemalia. 
 
 In the toads and their allies (arcifera) it has hardly passed beyond the urodele 
 condition, but the hinder angles are produced into long processes. In the frogs 
 (firmisternia) it consists of a slender thread between the medial ends of the girdles 
 (epicoracoids), but in front it expands into an omostemum, ossified behind; while 
 behind the girdle it forms a broad xiphistemiun, the anterior part of which is bone. 
 
 In the lizards the sternum is a large rhomboid plate, largely cartilag- 
 inous, sometimes perforated with two foramina and joined by a vary- 
 ing number of ribs (fig. 56). In the crocodilia there is an anterior 
 rhombic plate, joined by two pairs of ribs and followed by a second, 
 so-called abdominal sternum, connected with from five to seven pairs 
 of ribs. Ichthyosaurs, plesiosaurs and snakes have no sternum, 
 while it was imperfectly ossified in theriomorphs and dinosaurs. 
 
 In the birds (fig. 53) the sternum is ossified and at most is con- 
 nected with eight pairs of ribs. Behind it may be rounded, perforated. 
 
S8 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 notched, or prolonged into one or two long processes. In the ostriches 
 the ventral surface is smooth and this was formerly used as a character 
 separating these birds as a group of ratites, in contrast to all other 
 birds (carinatae) which either use their wings in flight or in swimming 
 (penguins) and in which there is a necessity for strong wing muscles. 
 For the attachment of these the ventral surface of the sternum is de- 
 veloped into a strong projecting keel (carina). It is to be noted that 
 a similar keel is developed in the bats and pterodactyls. 
 
 • Fig. 56. — Sternum, etc., of 
 Iguana tuber culata, after B Ian- 
 chard, c, coracoid;c/, clavicle; 
 e, epistemum; h, humerus; pc, 
 procoracoid; x, xiphisternum. 
 
 Fig. 57. — Sternum of guinea 
 pig. sr, sternal rib; st, sterne- 
 brae; vr, vertebral rib, x, xiphi- 
 sternum. 
 
 In the mammals the number of ribs connected with the sternum 
 is greater than in the lower classes. The sternebrae may remain dis- 
 tinct throughout life (fig. 57) or, as in man, they may fuse into fewer 
 elements, the xiphoid process being unconnected with the ribs. In the 
 edentates and rodents elements known as ossa suprasternalia and pro- 
 sternum occasionally occur in front of the sternum, the significance of 
 which is unknown. It is possible that traces of them persist in the 
 higher orders, even in man (fig. 55). 
 
SKELETON. 
 
 59 
 
 Episternum (Interciavicle). 
 
 In stegocephals and the oldest rhynchocephals there is a median 
 bone on the ventral surface, called the episternum (fig. 58). It 
 is rhomboid in front and may have a long posterior process, the medial 
 ends of the clavicles lying ventral to the broad anterior end. This 
 element is regarded as homologous with a T-shaped membrane bone 
 which occupies a similar position in lizards (fig. 56) and crocodilians, 
 where it acts as a brace between the shoulders. It arises by two centres 
 
 Fig. 58. — Shoulder girdles of (A) Melanerpeton and (B) diagram of Branchiosaurus, after 
 Credner, the determination of elements after Woodward, cl, clavicle; co^ coracoid; e, 
 episternum; s, scapula. 
 
 of ossification in membrane and hence cannot be the same as the su- 
 prasternalia of mammals. An episternum also occurs in theriomorphs, 
 pythonomorphs, ichthyosaurs, and plesiosaurs, and possibly the 
 entoplastron of the chelonians (fig. 34, p. 42) is the same structure. 
 It has not been recognized in birds, but it reappears in the monotremes 
 among mammals (fig. 113), with nearly the same relations as in the 
 lacertilians. 
 
 The Skull. 
 
 The skull is a complex structure and the last word concerning its 
 composition has yet to be said. A century ago Oken pointed out that 
 a series of parts could be distinguished in the mammalian skull, each 
 of which somewhat resemble a vertebra in its general relations, and 
 thus laid a foundation for a * vertebral theory of the skull ' which was 
 farther developed by Owen. Huxley showed that these were superficial 
 resemblances, that the three or four vertebrae they would recognize were 
 nothing of the sort, and that the skull shows no real metamerism 
 except in the occipital region and in the visceral arches. 
 
 In its development the skull, like the rest of the skeleton, passes 
 through two, and in the bony vertebrates, three stages: membranous, 
 
6o COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 cartilaginous and osseous, and in the early stages there is no trace of seg- 
 mentation or of vertebrae, the Okenian segments only appearing with 
 the appearance of bone. The skull may be divided into two portions, 
 a cranium, composed of a case for the brain, and sense capsules en- 
 closing the organs of special sense (ears, eyes and nose) ; and a visceral 
 skeleton, more or less intimately related to the anterior end of the 
 digestive tract. 
 
 Development of the Skull. 
 
 Little is known in detail of the development of the membranous 
 skull save that it envelops the brain and sense organs, extends into 
 the visceral region, and that it affords the substance in which the second, 
 or cartilaginous, skull is formed. 
 
 Fig. 59. — Early chondrocranium of Acanthias, after Sewertzoff. (The brain in outline.) 
 als, alisphenoid cartilage; ch, anterior end of notochord; h, hyoid arch; ma, mandibular 
 arch, not yet divided into pterygoquadrate and Meckelian; oc, otic capsule; /, trabecula; 
 1-5, branchial arches. 
 
 The cartilaginous envelope of the brain and sense organs is called 
 the chondrocranium. The notochord extends forward beneath 
 the brain as far as the infundibulum and a horizontal cartilage plate 
 forms on either side of it. These parachordal plates extend later- 
 ally as far as the ears, forward as far as the end of the notochord and 
 back to the exit of the tenth nerve. A little later a cartilaginous otic 
 capsule forms around each ear and joins the parachordals, thus form- 
 ing a trough in which the posterior part of the brain lies, its floor formed 
 of parachordals and notochord (basilar plate) and its sides of the 
 sense capsules. 
 
 From this posterior part two cartilages extend forward on either 
 side, forming a somewhat similar trough for the anterior part of the 
 
SKELETON. 
 
 6i 
 
 brain; the lower of these, the trabeculae cranii, join the anterior 
 margin of the basal plate while the dorsal bars, the alae temporales 
 or alisphenoid cartilages are eventually connected with the anterior 
 wall of the otic capsules. In most vertebrates the trabeculae and 
 alisphenoids develop as a continuum, but in some elasmobranchs they 
 are at first distinct (fig. 59). The two 
 trabeculae unite in front to form a 
 median ethmoid plate beneath the 
 olfactory lobes, beyond which they 
 diverge as two horns, the comua tra- 
 beculae, ventral to the nasal organs. 
 The floor of the trough is formed by 
 the ethmoid plate in front, while behind 
 it is usually of membrane, but in the 
 elasmobranchs cartilage gradually ex- 
 tends from one trabecula to the other, 
 closing last below the infundibulum 
 and hypophysis, these lying for a time 
 in an opening (fenestra, later fossa 
 hypophyseos), and after the closure, 
 in a pocket in the floor of the chon- 
 drocranium, one of the cranial land- 
 marks, the sella turcica. 
 
 Fig. 60. — Early (platybasic) chon- 
 drocranimn of an elasmobranch, 
 straightened out Compare with fig. 
 59. als, alisphenoid; ctr, comua tra- 
 beculae; ep, ethmoid plate; fhyp fenes- 
 tra h)rpophyseos; oc, otic capsule; ov, 
 occipital vertebrae; n, notochord; pc, 
 parachordal plate; tr, trabeculae. 
 
 In the elasmobranchs and amphibians 
 the trabeculae are widely separated until they 
 reach the ethmoid plate, a condition correla- 
 ted with the anterior extension of the brain. 
 This is the platybasic chondrocranium. In 
 the other classes the brain does not extend 
 so far forward and the two trabeculae meet just in front of the hypophysis (fig. 
 62) to continue forward as a trabecula communis to the ethmoid region. The 
 trabecula communis is usually compressed between the eyes to a vertical interor- 
 bital septum. This represents the tropibasic chondrocranium. 
 
 In the more primitive vertebrates the trough is converted into a 
 tube around the brain by the extension of cartilages between the ali- 
 sphenoid cartilages and the otic capsules of the two sides dorsal to the 
 brain. This roof or tegmen cranii is usually incomplete, having one 
 or more gaps or fontanelles, closed only by membrane. In the higher 
 vertebrates the cartilage roof is at most restricted to a mere arch, the 
 synotic tectiun, between the otic capsules of the two sides. Later 
 
62 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 a pair of nasal capsules develop around the olfactory organs. These 
 are usually fenestrated and become united to the cornua, alisphenoids, 
 and ethmoid plate. In a similar way a sclera (sclerotic coat) forms 
 
 
 Fig. 6i . — Diagram of early elasmobranch chondrocranium in side view, the brain out- 
 lined behind, al, alisphenoid plate; bp, basal plate; gc, gill clefts; h, hyoid; hm, hyomandib- 
 ular; I, upper labials; II, lower labials; nc, nasal capsule; oc, otic capsule; ov, occipital 
 vertebrae; ptgq, pterygoquadrate; si, suspensory liganents; sp, spiracle; tr, trabeculae; v, 
 vertebrae; 7-F/7, visceral arches; 1-5 branchial arches. 
 
 around each eye, but since the eye must move, this sense capsule never 
 unites with the rest of the cranium. Behind the otic capsules a vary- 
 ing number of (four in some sharks and most teleosts, in others three, 
 
 ft.. \ ' . . . 
 
 Fig. 62 . — ^\' antral view of (tropibasic) cranium of Lacerta agilis after Gaupp. aop, 
 antorbital plate; bpt, basipterygoid process; c, entrance to nasal conch; col, columella; 
 fh, fenestra hypophyseos; /^o, post-optic foramen; na, nasal capsule; nf, notochord; of, 
 optic foramen; pa, prominence of posterior ampulla; pt, pterygoid; g, articular process of 
 quadrate; tc, trabecula communis; tmg, taenia marginalis; tr, trabecula; VII, XII 
 seventh and twelfth nerves. 
 
 in amphibia two) occipital vertebrae are developed, which later fuse 
 with the rest of the chondrocranium. They alternate with myotomes 
 and nerves in this region as do the vertebrae of the vertebral column. 
 
SKELETON. . 63 
 
 The cartilaginous visceral skeleton arises in the pharyngeal region 
 which is weakened by the presence of the gill clefts. It consists of a 
 series of pairs of bars, the visceral arches (fig. 6i, I-VIT), lying in 
 the septa between the clefts, the bars of a pair being connected below the 
 pharynx. Each bar, at first, is a continuous structure, but to allow of 
 changes of size in the pharynx, each becomes divided into separate parts, 
 while the arches become connected in the mid-ventral line by unpaired 
 elements, the copulae. The two anterior arches are specialized and 
 have received special names, the first being the mandibular, the second 
 the hyoid arch, the others, in the region of the functional gills, being 
 called collectively gill or branchial arches. The number of these 
 last varies with the number of gill clefts, there being seven in the primi- 
 tive sharks, a smaller number in the higher groups, in which, with the 
 loss of branchial respiration, their form and functions may be altered. 
 At first all are clearly in the head region, but by the unequal growth of 
 cranium and pharynx the gill arches are carried back. All of the 
 arches are cartilaginous at first. 
 
 The mandibular arch lies in the region of the fifth nerve, behind the 
 mouth and between it and the first visceral cleft or pocket, the spiracle 
 or Eustachian tube. The arch is divided into dorsal and ventral 
 halves (fig. 61, /), known respectively as the pterygoquadrate (pala- 
 toquadrate ptgq), and Meckelian cartilages (w). In the elasmo- 
 branchs and chondrostei the pterygoquadrate forms the upper jaw, 
 lying parallel to and joined to the cranium by ligaments or (chimaeroids) 
 by continuous growth. With the appearance of bone a new upper jaw is 
 formed, as described below, and the pterygoquadrate becomes more or 
 less reduced, and ossifies as two or more bones with greatly modified 
 functions. Meckel's cartilage is the lower jaw of the lower vertebrates, 
 while in the higher it forms the axis around which the membrane bones 
 of the definitive jaw are arranged. 
 
 The hyoid arch lies between the spiracle and the first true gill cleft, 
 in the region of the seventh nerve. It divides into an upper element 
 the hyomandibular cartilage (fig. 6i, hm), and a ventral portion, the 
 hyoid proper, which may subdivide into several parts {infra). In the 
 lower elasmobranchs the hyomandibular and the rest of the hyoid arch 
 are closely connected, but in the higher fishes the hyomandibular be- 
 comes more separated from the ventral portion and tends to intervene 
 between the mandibular arch and the cranium, becoming a suspensor 
 of the jaws (fig. 63). Still higher it loses its suspensorial functions, 
 
64 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 becomes greatly reduced, and apparently is subsidiary to the sense of 
 hearing (see auditory ossicles), or it may be lost, the matter not 
 being decided. The hyoid proper becomes more or less intimately con- 
 nected with the arches behind and is largely concerned in affording a 
 support for the tongue. 
 
 The branchial arches are all similar to each other in the lower 
 vertebrates, but with the loss of branchial respiration in the higher 
 
 Fig. 63. — Ventral view of cranium and visceral arches of skate {Rata) after Gegenbaur. 
 cp, copula; h, hyoid; hm, hyomandibular; la, upper labials; mk, Meckelian cartilage; 
 nc, nasal capsule; pg, pterygoquadrate; r, rostrum. 
 
 groups, they tend to become reduced, the reduction beginning behind. 
 Some may entirely disappear, others give rise to the laryngeal cartilages 
 (see respiration) and the first may fuse with the hyoid. The first arch 
 is in the region of the ninth nerve; the others in that supplied by the 
 tenth. 
 
SKELETON. 
 
 6S 
 
 The elements of the branchial arches have the names, beginning above, pharyn- 
 gobranchial, epibranchial, cefatobranchial and hypobranchial, the copulae 
 being the basibranchials. The elements of the hyoid are correspondingly, epi-, 
 cerato-, and hypohyal. These parts lie in the medial ends of the gill septa, medial 
 to the aortic arches. 
 
 Other cartilages, which seem to be of less morphological importance, occur in 
 the same region. Among these are the labial cartilages (fig. 67, /), usually two 
 above and one below, which lie in front (outside) of the cartilages of the mandibular 
 arch of sharks, and in a modified form as high as some of the ganoids. By some they 
 are regarded as remnants of visceral arches of the preoral region. In the branchial 
 
 Fig. 64. — Branchial arches of {A) Heptanchus; (5), CMamydoselache; and (C) Cestracion; 
 A and C after Gegenbaur, B after Garman. c, ceratobranchial; e, epibranchial; h, hyoid; 
 hb, hyobranchial; he, hyoid copula; cbr, cardiobranchial (posterior copula); p, pharyngo- 
 branchial; 1-7, branchial arches. 
 
 region of the elasmobranchs a variable number of extrabranchial cartilages may 
 occur, small bars external and parallel to the upper and lower ends of the gill arches. 
 The foregoing sketch of the chondrocranium is based on conditions in the 
 gnathostomes, and ignores the peculiarities of the cyclostomes which are summar- 
 ized below. 
 
 In the elasmobranchs and cyclostomes the skull is cartilaginous 
 throughout life, or at most is calcified cartilage, without sharp division 
 into separate elements. In the higher vertebrates the cartilage is sup- 
 plemented or almost entirely replaced by bone which may be of the 
 two kinds, cartilage bone and membrane bone (p. 42), the distinctions 
 between which must constantly be kept in mind in tracing homol- 
 ogies in the different classes. The membrane bones are usually 
 derivatives of the deeper or dentinal layer of scales or teeth which have 
 fused together (fig. 65) and have sunk to a deeper position, coming 
 
66 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 into close connection with the elements derived from the cartilage 
 skull, in some cases replacing considerable of it. The cartilage bones 
 arise by an ossification of the cartilage. Even in the sturgeons the 
 chondrocranium is complete, the membrane bones being superficial 
 and not intimately connected with the deeper parts. 
 
 ,^^' 
 
 Fig. 65. — ^Vomer of 
 2 5 mm. A mbly stoma larva, 
 after Hertwig, showing 
 the bone developed by the 
 fusion of the bases of 
 teeth. 
 
 The names of the bones are largely based on the term- 
 inology of human anatomy. In many cases what appears 
 as a single bone in the human skull is represented by 
 several bones in the young and in the lower vertebrates. 
 In these cases the bones in the lower forms are usually 
 given names which indicate their relation to the human 
 bones or to the part or region in which they occur. 
 Dermal bones are apparently the older, phylogenetically, 
 but for convenience the cartilage bones are considered 
 first. 
 
 The chondrocranium shows several centres of ossification, but only 
 those giving rise to distinct bones are considered here.^ The bones of 
 
 Fig. 66. — Ventral view of schematic skull, chondrocranium dotted, cartilage bones 
 with lines and dots. ha.s\oc, basioccipital; hasisph, basisphenoid ; als, alisphenoid; exoc^ 
 exoccipital; ors, orbitosphenoid; presph, presphenoid; premax, premaxilla; qu,qua.<iia.te; 
 quju, quadratojugal; squamos, squamosal; zygom, zygomatic; other names in full. 
 
 the cartilaginous brain case may be arranged in four groups, beginning 
 behind and called respectively occipitalia, sphenoidalia and ethmoi- 
 
 ^ Basi- and presphenoid, for example, arise each from two centres, but in all vertebrates 
 the resulting bones are unpaired. 
 
SKELETON. 67 
 
 dalia, there being two sets of sphenoidalia. The occipitalia arise in the 
 occipital vertebrae and in the basilar plate. Of these there are four 
 (figs. 66, 67) : A supraoccipital above, an exoccipital on either side, 
 and a basioccipital below, the latter extending forward into the basilar 
 plate. These four form a ring around a central opening, the foramen 
 magnum, through which the spinal cord connects with the brain. 
 
 Just in front of the basioccipital the basilar plate ossifies to form the 
 basisphenoid, which extends forward to the sella turcica, and there is 
 succeeded by the presphenoid, arising from the trabeculae, and ex- 
 tending forward to the ethmoid plate. On either side a bone, the 
 alisphenoid) ossifies in the cartilage of the same name, and comes into 
 close relation with the basisphenoid. Father in front a second element, 
 the orbitosphenoid, arises in the alisphenoid cartilage and comes into 
 relation to the presphenoid. The alisphenoid bone is just in front of 
 the otic capsule, but there is always a large gap (sphenoidal fissure, 
 foramen lacenmi anterior) between it and the orbitosphenoid, through 
 which the third, fourth, and sixth and the ophthalmic branch of the 
 fifth nerve pass, the rest of the fifth nerve passing through the alisphe- 
 noid bone. The optic nerve usually perforates the orbitosphenoid, 
 but may pass through a notch in its margin. 
 
 The ethmoid plate may ossify into a median mesethmoid bone 
 bounded on either side by an ect ethmoid and in some there may be 
 added other bones included among the 'turbinal bones.' The ol- 
 factory nerves pass on either side of the mesethmoid, the ectethmoids 
 (below) in the mammals developing as perforated plates (cribiform 
 plate). 
 
 A series of otic or petrosal bones is developed in each otic cap- 
 sule. The most constant of these are a prootic in front, an opisthotic 
 behind, the two usually meeting below (fig. 66), and between them, 
 above, an epiotic, concerning which more evidence is needed. In 
 the teleosts and some other forms the lateral wall of the otic capsule 
 may develop in addition a sphenotic in front and a pterotic behind, 
 the latter overlying the horizontal semicircular canal of the ear. In the 
 higher groups the various otic bones fuse in the adult to a single petro- 
 sal bone, which is wedged in between the lateral parts of the basi- 
 occipital and basisphenoid. 
 
 In the stegocephals, reptiles and birds the sclera often gives rise to 
 a ring of sclerotic bones (fig. 67), which, however, never unite with 
 the other bones of the skull. The nasal capsules often develop a 
 
68 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 lateral ethmoid on the upper wall, and turbinals on the medial and 
 lateral walls. 
 
 To place these bones in the terms of human anatomy: the four occipitalia fuse 
 to form the single occipital of man; the six sphenoidalia similarly unite to form 
 the single sphenoid, the alisphenoids forming the greater wings, the orbitosphenoids 
 the lesser wings, while the ethmoidalia fuse to the ethmoid. 
 
 In all bony vertebrates the cranial walls are completed dorsally 
 by membrane bones, which in the lower fishes overly the tegmen cranii, 
 while in the higher groups they replace it, the cartilage failing to develop 
 
 Fig. 67. — Dorsal view of schematic skull, the chondrocranium dotted, cartilage bones 
 with lines and dots, premix, premaxilla; pref, prefrontal; postfr, postfrontal; postor, 
 postorbital; squamos, squamosal; quju, quadra to jugal; qu, quadrate; inp, interparietal; 
 exoc, exoccipital; supratem, supra temporal; other names in full. 
 
 in the roof. The number of these elements varies between wide limits, 
 the following being the most constant. 
 
 Beginning in front (fig. 67), there are, on either side of the median 
 line a pair of nasal bones covering the olfactory region; a pair of 
 f rentals between the orbits; a pair of parietals at the level of the 
 otic capsules, between which there is frequently a parietal foramen 
 for the connexion of the parietal eye with the brain; and an inter- 
 parietal, arising from paired centres, between the parietals and the 
 supraoccipital. 
 
 In the higher vertebrates (where the interparietal frequently fuses 
 
SKELETON. 69 
 
 with the supraoccipital) tjiese are practically all of the membrane 
 bones in the cranial roof of the adult. In the lower groups there are 
 several other bones, some of which may appear in the development of 
 the higher forms. Thus lateral to each parietal there may be a su- 
 pratemporal ; behind the orbit a postf rental may articulate with the 
 frontal, and lateral to this, and forming the rest of the posterior wall of 
 the orbit a postorbital. Occasionally the superior (or medial) wall of 
 the orbit is formed by one or more supraorbital bones, which, when 
 present, exclude the frontal from the orbit. The orbit may be bounded 
 in front by a prefrontal bone, adjoining the antero-lateral margin of 
 the frontal, and lateral to this there is usually a lacrimal bone. Less 
 constant are an intertemporal bone dorsal (medial) to the aUsphenoid, 
 a pair of postparietal bones between parietals and interparietals and 
 a so-called 'epiotic' above each otic capsule, which, since it is not a 
 cartilage bone and has no relation to the true epiotic, is better called 
 the tabulare. 
 
 In the ichthyopsida, and to a less extent in the sauropsida the 
 basilar plate and trabeculae may fail to ossify. In these cases the floor 
 of the cranium (roof of the mouth) is formed by a membrane bone, 
 the parasphenoid, which lies ventral to the cartilage in the sphenoid 
 region. Farther forward, in the nasal region, are an additional pair 
 of membrane bones, the vomers. Both vomers and parasphenoids 
 frequently bear teeth and their origin by fusion of the bases of teeth 
 is clearly seen in developing amphibia (fig. 65). 
 
 Some think the parasphenoid the homologue of the mammalian vomer, calling 
 the vomers of the non-mammals prevomers, their representatives being sought in 
 the 'dumb-bell bone' of the monotremes. More evidence is needed on these 
 points. 
 
 With the appearance of bone the mandibular arch undergoes the 
 greatest modifications of all the visceral arches. Its pterygoquadrate 
 half loses its function as the upper jaw and becomes more closely 
 connected with the cranium in front, its median portion disappearing, 
 even as cartilage, and being replaced by a pair of membrane bones, 
 the palatines (fig. 66), which lie between the pre- or parasphenoid and 
 the vomers. The rest of the arch ossifies as two bones on either side, 
 an anterior pterygoid and a posterior quadrate, which now becomes 
 the suspensor of the lower jaw. In the teleosts and reptiles there are 
 a series of pterygoid bones. 
 
 A second arch of membrane bones develops outside of the pterygo- 
 
70 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 quadrate to form the functional upper jaw (figs. 66, 67) in all bony 
 vertebrates. In its fullest development it consists of bones on either 
 side, beginning behind with a squamosal, which overlies the quadrate, 
 and followed by a quadrate jugal, a zygomatic (malar or jugal), 
 and a maxillary, which joins the premaxillary, the latter forming the 
 
 Fig. 68. — Dorsal and ventral views of skull of young Sphenodon, after Howes and 
 Swinnerton, Explanation of letters used in figvires of skulls (figs. 68 to 105) unless other- 
 wise stated, an, angulare; ao, antorbital; ap, antorbital process; ar, articulare; as, ali- 
 sphenoid; h, basale; hh, basibranchial; hh, basihyal; ho, basioccipital; hs, basisphenoid ; 
 ch, ceratobranchial; ch, ceratohyal; d, columella; co, coronoid; cp, copula; cr, cranial rib; 
 <r. eth, cribiform plate of ethmoid; d, dentary; de, dermal ethmoid; dee, dermal ectethmoid; 
 eb, epibranchial; ee, ectethmoid; eh, epihyal; enp, entopterygoid ; eo, exoccipital; ep, 
 ectoptery gold ; epo, epiotic; eth, ethmoid; ethpp; perpendicular plate of ethmoid; es, extra- 
 scapular; exh, extrabranchial;/, frontal;//*, frontoparietal; g, goniale; ^, hyoid;^&, hypohyal; 
 hm, hyomandibular; hr, hyoid rays; i, incus; if, infratemporal fossa; io, interoperculum; ip, 
 interparietal; j, jugal; I, lacrimal; la, labial; md, mandibular; me, mesethmoid; mk, Meck- 
 elian; ml, malleus; mm, mentomeckelian ; mpt, mspt, mesopterygoid ; mtp, metapterygoid ; 
 mx, maxillary; mxp, maxillopalatine; mxt, maxilloturbinal; n, nasal; na, neural arch; nc, 
 nasal capsule; no, notochord; o, occipital; oc, occipital condyle; 00, opisthotic; op, operculare; 
 OS, orbitosphenoid ; ot, otic bones; p, parietal; pd, predentary; pe, petrosal; pf, postfrontal; 
 pi, palatine; pm, premaxillary; po, preoperculare; poo, postorbital; pq, pterygoquadrate; 
 prf, prefrontal; pro, preorbital; prot, prootic; prs, presphenoid; ps, parasphenoid ; pt, ptery- 
 goid; ptc, pterygoid cartilage; pto, pterotic; q, quadrate; qj, quadra to jugal; r, rostral; rm, 
 rostrum; sa, suprangulare; sbo, suborbital; sc, sagittal crest; scl, sclerotic; se, sphenethmoid; 
 sf, supra temporal fossa; sh, stylohyal; so, supraoccipital; sop, subopereulare; sor, supra- 
 orbital; sp, sphenoid; spht, sphenoturbinal; spl, splenial; spo, sphenotic; spt, supra temporal; 
 sq, squamosal; ssc, suprascapular; st, stapes; sy, symplectic; t, temporal; tr, transversum; 
 tu, turbinal; ty, tympanic; v, vomer; vp, vomeropalatine. 
 
 tip of the jaw and meeting its fellow of the opposite side. Of these 
 only the maxillary and premaxillary bear teeth. 
 
 In the lower vertebrates the roof of the skull is continuous, its only 
 openings being those for the nares and the orbits. In the higher 
 
SKELETON. 7I 
 
 groups vacuities or fossae appear in the postero-lateral parts, these being 
 bounded by bars or arcade^ of bone. At most there may be three of 
 these fossae. The more lateral of these, the infratemporal fossa 
 (fig. 68), is bounded laterally by the zygomatic and quadratojugal, 
 while on the inner side it is separated from the supratemporal fossa 
 by a squamoso-postorbital arcade. The posttemporal fossa lies 
 between parietal, supratemporal and occipital bones. Occasionally 
 only the infratemporal fossa is present, or, by disappearance of the inter- 
 vening arcade, infra- and supratemporal fossae may unite in a single 
 temporal fossa. Lastly, by the breaking down of the zygomatic- 
 postorbital bar, the temporal fossa and the orbit may unite. 
 
 One or another of these bones may disappear in some groups, either by fusion 
 or by complete dropping out. Occasionally they may obtain different connexions 
 and relations as in the case of the quadrate in mammals (see ear bones) so that the 
 homologies are traced with difficulty. The complexity is increased by the fusion of 
 membrane bones and cartilage bones and by the union of cranial bones with those 
 of the visceral arches. 
 
 In the lower jaw there are no such extensive modifications as in the 
 upper. At most Meckel's cartilage gives rise by ossification to two 
 bones in either half. Behind, at the articulation of the jaw with the 
 quadrate, there is an artic- 
 ular bone, while at the 
 tip, at either side of the 
 union (symphysis) of the 
 two halves of the jaw, there 
 
 is rarely a mentO-Meck- Fig. 69. — Reconstruction of developing jaw of 5ce/e- 
 ,. , ,^, ^ porus, cartilage dotted; letters as in fig. 68. 
 
 elian bone. The rest of 
 
 Meckel's cartilage forms an axis around which the membrane bones 
 which form the definitive jaW are arranged. These are, at most, as 
 follows: (i) a dentary which surrounds the Meckelian in front and 
 usually bears teeth; (2) a splenial on the inner side, behind the 
 dentary and frequently bearing teeth; (3) an angulare on the lower 
 side, usually extending back to the hind end of the jaw; (4) a supran- 
 gulare on the outer posterior part of the jaw; (5) a coronoid on the 
 upper side, afi'ording attachment for the muscles which close the 
 jaws; and (6) a goniale (dermarticulare) on the medial and ven- 
 tral sides of the articulare, with which it usually fuses. This whole 
 series is present in few vertebrates, dentary, splenial and angulare 
 being the most constant. 
 
72 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 TABLE OF THE PRINCIPAL CRANIAL BONES. 
 
 Chondrocranium 
 
 Cranium 
 
 Membrane bones 
 
 Maxillary arch 
 
 Mandibular arch 
 
 Visceral 
 skeleton 
 
 Hyoid arch 
 
 Gill arches 
 
 Notochord 
 parachordals 
 
 Trabeculae 
 
 Ethmoid 
 plate 
 
 Sense cap- 
 sules 
 
 Lateral line 
 
 Membrane bones 
 
 Pterygoquadrate 
 cartilage 
 
 Membrane 
 
 Meckel's cartilage 
 
 Membrane 
 
 Basi-, ex-, and supra- 
 
 occipitals 
 
 Basi- and ali-, pre- and 
 
 orbitosphenoids 
 
 Mes- and ectethmoids 
 
 f Pro-, epi-, opisth-, pter- 
 Otic \ and sphenotics (petro- 
 
 [sal) 
 
 Optic (Sclerotics) 
 
 ^^ . / Lateral ethmoid, tur- 
 Nasal < L. , ' 
 
 (^ bmals 
 
 Parietals, frontals, na- 
 sals, pre- and post- 
 frontals, supra- and 
 postorbitals 
 Lacrimals, infraorbitals 
 
 (Premaxillary, maxilla- 
 ry, zygomatic, quad- 
 ratojugal, squamosal 
 1 Pterygoid (ect-, ent-, 
 epi-, mesoptery golds), 
 quadrate (incus) 
 Palatines, vomers 
 J Articulare (malleus), 
 I mento-Meckelian 
 
 Gill arches 
 
 Dentary, splenial, coro- 
 noid, angulare (tym- 
 panic), suprangulare, 
 goniale 
 
 Hyomandibulare (sta- 
 pes) symplectic, inter- 
 hyal, epi-, cerato-, 
 hypo-, and basihyal 
 (corpus, copula) (col- 
 umella) (lesser cornua) 
 Pharyngo-, epi-, cerate-, 
 hypo-, basi-, hypohyal, 
 (copula, greater cornua) 
 
SKELETON. 
 
 73 
 
 In the hyoid and branchial arches ossification occurs to a greater or 
 less extent, the resulting cartilage bones having the same names as the 
 corresponding cartilages. There are never any membrane bones in 
 this region. In the teleosts the hyomandibular ossifies as two bones, a 
 dorsal hyomandibular and a lower symplectic which connects with the 
 quadrate. There is, however, a considerable amount of union between 
 the various arches in the adults of all tetrapoda, where the branchial 
 respiration is lost and the arches have to assume other functions than 
 the support of gills. 
 
 The mode of suspension of the 
 jaws varies. In a few elasmobranchs 
 the pterygoquadrate articulates di- 
 rectly with the cranium (amphistylic) ; 
 in others it is suspended by ligament 
 and by the interposition of the 
 hyomandibular between the otic cap- 
 sule and the hinder end of the jaw 
 (hyostylic) ; while in all groups above 
 the fishes the pterygoquadrate is more 
 or less completely fused with the 
 cranium (autostylic). 
 
 The ear bones or ossicula audi- 
 tus are best treated together here, 
 although their consideration requires 
 the mention of structures not yet de- 
 scribed. The ear bones occur only 
 in the tetrapoda; they present several 
 modifications not readily homologized 
 with each other, though they all have 
 the same function of conveying sound waves across the tympanum 
 to the inner ear. In all there is an opening, the fenestra vestibuli 
 (f. ovale) in the lateral wall of the otic capsule, which is occu- 
 pied by a movable bone, the stapes, of uncertain homologies, but 
 probably representing the hyomandibular of the fishes, which otherwise 
 is lacking in all tetrapoda. This view is the more probable since in 
 some vertebrates the stapes is connected developmentally with the rest 
 of the hyoid arch. 
 
 In urodeles and caecilians a slender process extends from the quad- 
 rate across the poorly developed tympanic cavity to articulate with the 
 
 Fig. 70. — Diagram of the middle 
 ear of a lizard, after Versluys. a, 
 articulare;c, columella; ec, extracolu- 
 mella; h, hyoid; ie, inner ear; mpt, 
 pterygoid muscle; o, oral cavity; po, 
 parotic process; pt, pterygoid bone; 
 5, stapes; /, tympanic cavdty; tm, 
 tympanic membrane. 
 
74 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 .-••■■^N 
 
 stapes (fig. 82). In the anurans there is no connection of quadrate 
 with stapes, but there is a slender rod, the columella, extending from 
 the tympanic membrane to the stapes. This columella arises behind 
 the tympanic cavity but with growth is included in it, so that in the 
 adult it appears to run directly through it. In the sauropsida the 
 relations are much as in the anura, but when ossification sets in, the 
 columella may form several elements. In development the columella 
 in these forms is directly connected with the hyoid arch. 
 
 In the mammals there is 
 a chain of three bones to carry 
 the sound waves across the 
 tympanic cavity. In the 
 fenestra vestibuli is the stapes, 
 which connects with an incus 
 and lastly comes the malleus, 
 which has two long processes, 
 a manubrium which is in- 
 serted in the tympanic mem- 
 brane, and a processus an- 
 terior (Folian process) which 
 extends into the petrotym- 
 panic (Glaserian) filssure 
 of the temporal bone. That 
 these parts are not to be com- 
 pared to the columella of the 
 sauropsida and anura is 
 shown by the fact that they 
 invade the tympanic cavity 
 from in front and that they 
 are in front of the chorda 
 tympani nerve, the columella of the non-mammals lying behind it. 
 
 The homologies of these parts seem clear. In development the 
 malleus is the posterior end of Meckel's cartilage, being in the position of 
 the articulare of lower groups. It articulates with the incus, which in 
 turn at first articulates with the wall of the otic capsule, as well as with 
 the stapes, and thus corresponds with the quadrate. The stapes is 
 apparently the same throughout the whole of the tetrapoda. It is to be 
 noted that many paleontologists deny the homologies recognized here, 
 think that in the mammals the quadrate has been lost in the glenoid 
 
 Fig. 71. — Diagram of ear bones of embryo pig, 
 the tympanic cavity laid open, g, goniale; i, 
 incus; Ij, lower jaw; m, malleus; mk, Meckel's 
 cartilage; mm, manubrium of malleus; s, stapes; 
 sq, squamosal; z, zygomatic. The outlines of the 
 zygomatic arch and the hind end of the jaw are 
 dotted. 
 
SKELETON. 
 
 75 
 
 fossa, and find the malleus and incus in the columella. For this they 
 have no evidence except comparisons with certain theriomorph reptiles. 
 The literature, which is extensive, should be consulted for details. 
 
 The Skull in the Different Classes, 
 
 CYCLOSTOMES have only the cartilage skull, and this can be homologized 
 only in part with that of other vertebrates; indeed the skulls of the two groups of 
 cyclostomes are not easily compared. The peculiarities are in part due to the 
 development of a suctorial mouth with its necessary framework. The chondro- 
 cranium of the Ammocoete stage of Petromyzon 
 is readily understood. Parachordals, otic capsules 
 and trabeculae (fig. 72) are normal, but a pair of 
 ventral horns are problematical. Their position 
 in front of and below the otic capsule renders 
 doubtful the interpretation of hyoid or quadrate 
 sometimes given them. 
 
 The adult Petromyzon has a typical brain 
 trough, roofed by a slender synotic tectum and 
 fibrous tissue and closed in front by the unpaired 
 nasal capsule, bound to the rest by fibrous tissue. 
 The cranium is continued forward by a large plate 
 (mesethmoid ?) lying dorsal to the mouth, this 
 part being roofed by two 'dorsal cartilages,' the 
 anterior articulating with the annular cartilage 
 supporting the mouth. A subocular bar extends 
 forward from each otic region and an elongate 
 lingual cartilage extends from the mouth back 
 to the gill region. Several other elements occur, 
 the names and positions of which may be seen from the figures. 
 
 The myxinoid skull, the development of which is unknown, is readily inter- 
 preted so far as basilar plate, trabeculae and otic capsules are concerned. The large 
 nasal capsule is continued forward by a latticed framework for the naso-hypo- 
 physial canal and a bar (pterygoquadrate) joins the trabecula of either side and in 
 front is continued in a cornual cartilage. The lingual cartilage is enormous (is it 
 the lower jaw as has been suggested?), is divided into three segments and bears a 
 dental plate with teeth at its tip. There are cartilage axes to the tentacles around 
 the mouth. 
 
 The branchial skeleton of the lampreys consists of a gill basket of continuous 
 cartilage with fenestrae for the gills and above and below them as well. It cannot 
 be homologized with the branchial skeleton of other vertebrates as it lies imme- 
 diately beneath the skin and is lateral to gill pouches and aortic arches. It is more 
 easily compared to the extrabranchials (p. 65) of the elasmobranchs. The 
 branchial apparatus of the m)rxinoids is reduced, consisting of two true gill arches, 
 in front of which is another arch, usually interpreted as a hyoid. 
 
 nc 
 
 72. — Early chondro- 
 of Ammocoete stage 
 
 of Petromyzon, after Schneider. 
 
 h, hyoid; nc, notochord; oc, otic 
 
 capsule; tr, trabecula. 
 
 Fig. 
 cranium 
 
76 
 
 COMPARATIVE MORPHOLOGY OF VEGTEBRATES. 
 
 ELASMOBRANCHS have a nearly typical chondrocranium which is never 
 divided into separate elements and is never ossified. The floor is complete, the hypo- 
 physis resting in a sella turcica. Above there is an anterior fontanelle, closed by 
 membrane and a posterior fontanelle may occur. The occipital region typically 
 
 Fig. 73. — ^Ventral and lateral views of the skull of lamprey {Petromyzon marinus), after 
 Parker, ad, anterior dorsal cartilage, hb, branchial basket; gc, gill cleft; Ic, labial carti- 
 lage; Idm, lateral distal mandibular; Ig, lingual cartilage; nc, nasal capsule; oc, otic capsule; 
 on, optic nerve; pc, pericardial cartilage; pd, posterior dorsal cartilage. 
 
 Fig. 74. — Side view of cranium of Bdellostotna, after Ayers and Jackson, h, basal 
 plate; hr, branchial basket; c, cornual cartilage; d, dental plate; h, hyoid; /, lateral labial 
 cartilage; n, nasal tube; nc, notochord; 0, otic capsule, oc, olfactory capsule; pq, pterygo- 
 quadrate bar; sp, suprapharyngeal plate. 
 
 articulates with the vertebral column by a pair of prominences, the occipital con- 
 dyles, but in most species this joint is not functional, the skull being immovably 
 united to the backbone. In front the snout is supported by rostral cartilages, 
 usually three in number, but these are frequently fused to a single mass. 
 
SKELETON. 
 
 77 
 
 The pterygoquadrate and the Meckelian cartilages bear teeth and form the 
 functional jaws. Most species are hyostylic (p. 73), the pterygoquadrate being 
 supported in front of the orbit by a ethmopalatine ligament on either side; behind 
 by ligament and by the hyomandibular. The Notidanids are amphistylic, the 
 hyomandibular being connected with the rest of the hyoid and not acting as a 
 suspensor of the jaws, but the pterygoquadrate bears a strong process which ar- 
 ticulates with the postorbital process of the cranium. A third condition is found 
 in the holocephalans where the pterygoquadrate, free in the young, becomes auto- 
 stylic by fusion with the cranium. 
 
 The variations in the branchial skeleton (figs. 63, 64) are readily reducible to 
 the typical conditions. In living elasmobranchs the number of gill arches is five, 
 
 Fig. 7; 
 
 -Skull ofSquatina, after Gegenbaur. h, hyoid; hm, hyomandibular; i 
 cartilages; m, Meckel's cartilage; pq, pterygoquadrate; r, rostrum. 
 
 .1*, labial 
 
 except in Hexanchus and Chlamydoselache (six) and Heptanchus (seven). Hyoid 
 and branchial arches bear numerous branchial rays which support the gills and 
 the gill septa, while smaller cartilages on the inner surface of each arch extend into 
 the gill strainers. 
 
 TELEOSTOMES show a wide range of structure of skull, yet the series so inter- 
 grade that no sharp lines can be drawn. The chondrocranium persists to a consid- 
 erable extent, and numerous membrane bones are present, supplementing those of 
 cartilaginous origin. With few exceptions cartilage bones (the four occipitals, orbito- 
 and alisphenoids and prootics are the most constant) are developed, while the inner 
 wall of the otic capsule disappears, so that the cavity is connected with that for the 
 brain. Even more characteristic is the presence of skeletal structures supporting 
 the opercular fold that covers the external openings of the gill slits. This is in part 
 of membrane bones, in piart of cartilage or cartilage bones. There are two parts 
 to the opercular fold, a gill cover or operculum above and a branchiostegal 
 membrane below. The latter is supported by branchiostegal rays, comparable 
 to the hyoid branchial rays of the elasmobranchs, while the operculum contains 
 membrane bones, there being, at most, four of these: a preoperculum in front, 
 and behind this in a row from above downward, operculare, suboperculum and 
 interopercultun. The preoperculum, overlies hyomandibular, symplectic and 
 quadrate, and it is possible that the opercular bones have been developed in con- 
 
78 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 nexion with the hyomandibular rays of the elasmobranchs. There are five 
 branchial arches, the last more or less reduced. Often they bear teeth on their 
 inner surfaces, thus acting as accessory chewing organs. 
 
 Fig. 76. — Side view of skull of mackerel (Scomber) after Allis. For letters see fig. 68. 
 
 The chondrostei, the most shark-like of the Ganoids, have no cranial cartilage 
 bones. They are also primitive in the great development of the rostral cartilage 
 (enormous in Polyodon), which gives the mouth its ventral position, and in the 
 
 Fig. 77. — Chondrocranium of Polypterus, after Budgett. a, afferent artery to external 
 gills; b^*, branchials; e, efferent artery from external gills; lb, labial cartilage; 2, 5, 7, 
 nerve exits; other letters as in fig. 69. 
 
 extension of the cranial cavity into the ethmoid region. They have a few bones in 
 the visceral skeleton, while there are numerous membrane bones in the roof of 
 the skull, a few of them readily homologized with those of other vertebrates. 
 
SKELETON. 
 
 79 
 
 In other ganoids (holosteans and crossopterygians) the skull is much like that 
 of the teleosts, differing in the extension forward of the cranial cavity. There are 
 
 Fig. 78. — Median section of skull of mackerel (Scomber) after Allis. For letters see fig. 68. 
 
 one {Amia) or two (Polypterus) gular bones developed between the rami of the 
 lower jaw, and in Polypterus parietals, frontals and nasals fuse with age, and there 
 are numerous small bones in the cranial roof, developed along the lateral line 
 canals. Amia has several splenials in 
 the lower jaw. 
 
 Teleosts (fig. 76-80) have a consider- 
 able range of skull structure. In the lower 
 groups like siluroids and cyprinids, the 
 chondrocranium is largely persistent and 
 the cranial cavity extends into the eth- 
 moid region as in the higher ganoids. 
 In other teleosts the trabeculae are ap- 
 proximate between the orbits (tropibasic) 
 and develop a thin interorbital septum 
 which limits the anterior ends of the 
 cranial cavity. The cartilage bones are 
 more numerous. All four occipitalia are 
 present, the occipital condyle being formed 
 by basi- and exoccipitals. Basi-, all-, and 
 orbitophenoids occur, and besides ecteth- 
 moids a pair of mesethmoid ossifications. 
 In the otic capsule there are usually 
 pterotic and sphenotic ossifications. 
 
 The cranial roof is largely formed by 
 the frontals and parietals, the latter fre- 
 quently separated by a strong process of 
 the supraoccipital. Several of the car- 
 tilage bones are visible from above. The 
 roof of the mouth is formed by the large 
 parasphenoid and the vomers. Premaxil- 
 laries (rarely lacking) and maxillaries yig. 79.— Dorsal view of skull of mack- 
 form the upper jaw, both usually bearing erel. Scomber, after Allis; letters as in fig. 68 
 
8o 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 teeth, but occasionally, by overdevelopment of the premaxillary, the maxillary is 
 excluded from the margin of the jaw. 
 
 Instead of the single pterygoid of higher vertebrates there are three bones, an 
 entopterygoid adjoining the palatine, a mesopterygoid (ectopterygoid) which ex- 
 tends back to the quadrate, and a metapterygoid above the quadrate. When the 
 hyomandibular cartilage ossifies it forms a hyomandibular bone from its upper por- 
 tion and a sjmiplectic (an element not known outside the teleostomes), which sup- 
 ports the quadrate. A small bone, the interhyal, intervenes between the hyomanr 
 dibular and the rest of the hyoid. The hyoid copula consists of several elements, 
 the anterior, which supports the tongue being called the entoglossal, the posterior, 
 which connects with the branchial arches, the urohyal. The fifth gill arch consists 
 of a single element on either side, the hypopharyngeal bone, which usually bears 
 
 Fig. 
 
 80. — ^Pterygoids, suspensorium and operculum of mackerel {Scomber) after Allis. 
 For letters see fig. 68. 
 
 teeth, the two sides being fused in the plectognaths, forming a pharyngeal jaw. 
 The upper elements of the other arches are frequently expanded, bear teeth, and 
 are called epipharjmgeal bones. 
 
 Dipnoi. — In the three existing genera the skull is comparatively uniform, but 
 the fossils, beginning in the Devonian, have a wide range of structure. In the 
 former the cavity of the chondrocranium extends to the ethmoid region and the nasal 
 capsules have a second opening, corresponding to the inner nares (choanae) inside 
 the oral cavity. The pterygoid is fused with the cranium (autostylic) and there are 
 one (Protopterus) or two (Ceratodus) labial cartilages connected with the nasal 
 capsules. In Ceraiodus there are no cranial cartilage bones, but in the other 
 genera a plate composed of fused ex- and supraoccipitals occurs. 
 
 The membrane bones are few, but their homologies are not always certain.- 
 The roof is largely formed by an unpaired bone in the position of frontals and 
 parietals, in front of which is a median bone (supraethmoid or fused nasals) above 
 the nasal capsules. In Ceratodus a bone of uncertain homology occurs on either 
 side of the fronto-parietal, but it is lacking in the others, unless it be represented in 
 Protopterus by a pair of bones which abut against the supraethmoid and overlap 
 
SKELETON. 
 
 8l 
 
 the fronto-parietals. The otic capsule and quadrate are covered by a squamosal, 
 and the roof of the mouth is formed by a large parasphenoid, in front of which are 
 a pair of palatines. In advance of these last are a pair of large teeth resting directly 
 on cartilage, their bases representing the greatly reduced vomers. The lower jaw 
 has three bones on either side, a small dentary, a larger angulare, and an enormous 
 splenial, which alone bears teeth. 
 
 In Ceratodus there is a hyomandibular fused to the cranium behind the exit of 
 the seventh nerve, but elsewhere there is only the hyoid. The operculum has one 
 or two elements (operculare and interoperculum) the free edges of which bear 
 
 Fig. 8i. — Skull oi Lepidosiren, after Bridge, an, angulare; ap, antorbital process; ch 
 ceratohyal; cr, cranial rib; de, dermal ethmoid; dee, dermal ectethmoid; eo, exoccipital;/^ 
 frontoparietal; hr, hyoidean ribs; mk, Meckel's cartilage; na, first neural arch; nc, nasal 
 capsule; nsp, neural spine; pq, pterygoquadrate; sc, sagittal crest of frontoparietal; sp 
 splenial; sq, squamosal; i-io, nerve exits. 
 
 cartilaginous rays, and the gill arches are five in Ceratodus, six in the other genera. 
 A peculiar feature of Protopterus and Lepidosiren is the so-called head rib, a slender 
 cartilage bone articulated with the chondrocranium below the occipital plate, and 
 extending backward and downward across the shoulder girdle. 
 
 In those extinct Dipnoi which are united with the recent genera to form the order 
 Sirenoidea, the skull is much as in the existing forms, except for the more numer- 
 ous bones. In the Arthrodira (formerly called placoderms) the cranium is hinged 
 to a large plate which covers the anterior part of the trunk, and the skull is roofed 
 with a few large plates, some of which may be homologized with those of the siren- 
 oids, the others not being readily compared with the bones of other vertebrates. 
 The suggestion has been made that the problematic fossil Palceospondylns resembles, 
 in its skull, the larvae of the dipnoans, the adults of which were common in the same 
 seas. 
 
 6 
 
82 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 AMPHIBIA. — Several points distinguish the amphibian from other skulls. The 
 chondrocranium is platybasic (p. 6i) ; except for a small synotic tectum frequently 
 present, it is not roofed by cartilage; the otic capsule has a fenestra vestibuli occu- 
 pied by the stapes, a development connected with the power of hearing (p. 73) ; 
 there are two occipital condyles; and the quadrate is immovably united to the 
 cranium by two processes, an otic process, joining the otic capsule, and an * ascend- 
 ing process' which reaches the upper margin of the trabecula, and which, in many 
 reptiles, often ossifies as the epipterygoid bone. 
 
 Fig. 82. — Chondrocranium of Amphiuma, lateral and dorsal views, aop, antorbital 
 process; ap, ascending process of quadrate (epipterygoid) ; ct, cornua trabeculae; c?e, 
 foramen, for ductus endolymphaticus; ep, ethmoid plate;/o, fenestra vestibxili ; m, Meckel's 
 cartilage; n, notochord; oc, olfactory capsule; ov, occipital vertebrae; p, parachordal; 5, 
 quadrate; s, stapes; t, trabecula; 2-8, nerve exists. 
 
 The cartilage cranial bones are few. Usually only exoccipitals are developed 
 in the hinder region, while the rule is a single petrosal (prootic), but occasionally 
 epi-, opisth-, and pterotic occur. There is but a single pterygoid, while basi-, pre-, 
 and alisphenoids are not ossified. The membrane bones in existing amphibians 
 have separated from the integument and have sunk to a deeper position than in 
 fishes, but in the stegocephals the presence of grooves for the lateral line system 
 would indicate a close connexion between skin and bones. In the latter group the 
 membrane bones are numerous, but in existing species they are noticeably reduced. 
 Except in stegocephals and the caecilians there are large vacuities in both floor and 
 roof of the skull. The lower jaw also has a reduced number of bones, there being 
 at most five including the articulare and the mento-Meckelian. 
 
 The most primitive conditions occur in the stegocephals, where, as the name 
 
SKELETON. 
 
 83 
 
 indicates, the dorsal surface is covered, leaving only gaps for the eyes and nostrils. 
 In general the account of the skull given on page 67 ff will apply to these forms, and 
 so far as the dorsal surface is concerned little more needs to be said, aside from the 
 fact that the supratemporal is sometimes transversely divided, that an interparietal 
 foramen occurs (indicating the existence of a parietal eye), that the bones called 
 supraoccipital may be interparietal, and that the sclerotics are common. The 
 floor of the cranium is formed by a large parasphenoid, bordered in front by a pair 
 of (usually toothed) palatines, in front of which are the vomers. Of the carti- 
 laginous parts almost nothing is known; a few, clearly larval forms have well 
 developed branchial arches preserved. 
 
 Fig. 83 . — Skull of a stegocephalan {papitosaurus) after Zittell. Letters as in fig. 68. 
 
 Of the Gymnophiones (caecilians) the cartilage skull is known only in Ichthy- 
 ophis; its peculiarities are the reduced parachprdals, an ethmoidal nasal septum, 
 a stapes, perforated as in mammals, and alisphenoid and trabecular cartilages more 
 distinct than in most amphibia. Most noticeable of the cartilage bones is the eth- 
 moid, while otics and exoccipitals are fused as are quadrate and pterygoid. The 
 membrane bones form a complete roof to the skull, recalling the stegocephals, but 
 the number of bones is smaller, squamosal, supratemporal, jugal and quadrato- 
 jugal being absent, while a large prefrontal and a larger postfrontal (usually called 
 squamosal) occur. In the roof of the mouth maxillary and palatine are fused, the 
 vomers distinct, while the united parasphenoid and basioccipital form a large os 
 basale. In the lower jaw there are only dentary and angulare, the latter being 
 produced behind the articulare in a remarkable way. 
 
 In the cartilage skull of the Urodeles (fig. 82) the pterygoid does not usually 
 reach the anterior part of the skull but projects as a process from the quadrate, 
 which bears, besides the two processes already mentioned (p. 82), a palatobasal 
 
84 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 process joining the otic capsule in front of the otic process. Cartilage bones are 
 few; supra- and basioccipital, alisphenoid and ethmoids are lacking; the otics fuse 
 to a single petrosal; an orbitosphenoid occurs and quadrate and pterygoid are 
 continuous. 
 
 The roof of the adult skull is chiefly formed of parietals, frontals and nasals, 
 the latter being frequently separated by processes of the premaxillaries. Each 
 
 Fig. 84. — Skull of Amblystoma punciatum, after Wiedersheim. Letters as in fig. 68. 
 
 frontal has a ventral process which limits the cranial cavity in front; there is usually 
 a prefrontal and a septoiiiaxillary may be developed on the postero-lateral part of 
 the nasal capsule. A supratemporal is always lacking, the squamosal extending 
 up to the parietal. The upper jaw is composed of premaxillaries and (except some 
 perennibranchs, fig. 85) maxillaries; a jugal is always absent and the quadratojugal, 
 
 Fig. 85. — Skull of Proteus, after Wiedersheim. For letters see fig. 68. 
 
 formed in the larva, fuses with the squamosal. In the roof of the mouth are the 
 large parasphenoid, frequently with teeth, and a pair of vomero-palatines, the 
 choanae lying behind the vomerine portion, which is farther back than in 
 the dipnoi. 
 
 In the lower jaw Meckel's cartilage persists, its hinder end forming the articulare, 
 while in front it is surrounded by the dentary and splenial, each bearing teeth. In 
 the larvae the branchial skeleton is nearly typical, there being a hyoid and four gill 
 arches. In the adult, with the loss of aquatic respiration, the posterior arches are 
 
SKELETON. 
 
 85 
 
 reduced or even disappear, those remaining being connected by a one or two-jointed 
 copula. 
 
 The chondrocranium of the larval Anura (Rana, fig. 86) differs considerably 
 from that of other amphibia as well as from the adult conditions. Like all amphib- 
 ians it is platybasic. The pterygoquadrate has, besides the normal otic and 
 epipterygoid processes, a cranio-quadrate process connected with the nasal region, 
 in front of which is the articulation of the lower jaw. In front of the comua are a 
 pair of suprarostral cartilages and a similar pair of infrarostrals lie in front of the 
 
 Fig. 86. — Chondrocranium of tadpole of Rana before the metamorphosis; after Gaupp. 
 c.,ant, anterior canal; els, superior labial cartilage; dr, comu trabeculae; car, foramen for 
 carotid; exi. c, external canal; /<?, ethmoid fenestra; m, Meckel's cartilage; pc, posterior 
 canal; po, otic process of quadrate; pr. as.' ascending process of quadrate; q, quad- 
 rate; /w, tectmn medialis; Urn, taenia tecti marginalis; tsyn, tectum synoticum; I-V, nerves 
 and nerve exits. 
 
 Meckelian, from which they are apparently derived. These four rostrals form a 
 ring around the suctorial mouth and recall the labial cartilages of the elasmobranchs 
 and the annular cartilage of the cyclostome mouth. 
 
 At the time of metamorphosis the changes are great, and as the result is more 
 like the chondrocranium of other amphibia, the larval condition must be regarded 
 as adaptive rather than ancestral. The suprarostrals disappear and the jaw 
 shifts the hinge back to the normal position, this being accompanied by the elon- 
 gation of Meckel's cartilage, an absorption of the ascending process and a folding 
 of the pterygoquadrate bar. At the same time a pterygoid grows out in front to 
 join an antorbital process from the cranium. A stapes develops and connects 
 
86 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 with the columella, which meets the tympanic membrane. This membrane is 
 stretched on a cartilaginous tympanic annulus, derived from the pterygoquadrate. 
 (Annulus and columella are lacking in those genera, Bombinaior, etc., which have no 
 tympanum). There is no connexion between stapes and quadrate. 
 
 The chondrocranium largely persists, the only constant cartilage bones being 
 the exoccipitals and prootics. A supraoccipital rarely occurs and basioccipital and 
 
 t- trried 
 
 Fig. 87. — Chondrocranium of a frog after metamorphosis, from Gaupp. fov^ fenestra 
 ovalis; m, Meckel's cartilage; mtg, metapterygoid; wc, nasal capsule; ptgq, pterygoquadrate; 
 tnas, tectum nasalis; tsyn, tectum synoticum; ttmed, tsenia tecti medialis. 
 
 basisphenoid are unknown. In the ethmoid region, except in the aglossa, there is 
 a peculiar bone, the sphenethmoid, which arises as two bones on either side. 
 These fuse, forming a ring (*os en ceinture') around the olfactory nefves and the 
 anterior end of the brain. 
 
 The frontals and parietals of a side are fused and often the fronto-parietals are 
 continuous across the middle line. They may extend to the nasals or there may 
 
 Fig. 88.- 
 
 -Dorsal and ventral views of skull of toad, Bufo americanus. 
 
 fig. 68. 
 
 For letters see 
 
 be a gap between, leaving the sphenethmoid visible from above. A large squamosal 
 extends above the quadrate, from the otic region to the angle of the jaw. The upper 
 jaw consists of premaxillary and maxillary, and, except in the aglossa, of quadrato- 
 jugal. The pterygoid cartilage persists, but is overlaid by a membrane bone, also 
 called the pterygoid. Slender palatines, transverse to the axis of the skull, are 
 lacking only in the aglossa, while small vomers are almost always present. The 
 
SKELETON. 
 
 87 
 
 floor of the cranium is completed by a X-shaped parasphenoid, which extends to 
 the premaxillaries in the aglossa, elsewhere only to the sphenethmoid. 
 
 In the lower jaw there is a mento-Meckelian in front, followed by dentary and 
 angulare, Meckel's cartilage persisting through life. The larval branchial and 
 hyoid arches are typical, there being four gill arches. With the loss of gills the 
 posterior arches disappear, and the broad hyoid plate of the adult has four processes 
 which are new formations. 
 
 REPTILES. — The skull of existing reptiles is very different from that of amphib- 
 ians, but that of many theriomorphs is strikingly like that of the stegocephalans. 
 The principal differences alluded to in the first sentence have arisen by reduction 
 and disappearance of bones appearing in the more primitive types, but aside from 
 these there is little except the parasphenoid to separate the two groups. 
 
 Fig. 89. — Chondrocranium of Sphenodon, stage 'R,' after Howes and Swinnerton. 
 ep, epipterygoid; e$, ethmosphenoidal plate; «c, extranasal cartilage; exp, extranasal process; 
 h, hyoid; mk, Meckel's cartilage; nc, nasal capsule; oc, otic capsule; pt, pterygoid; g, quad- 
 rate; sb^ subnasal process; 1-5, exits of nerves. 
 
 The chondrocranium is known in but a few forms and these agree with other 
 amniotes in being tropibasic, except in snakes and amphisbaenans (see fig. 62). 
 In the adults cartilage largely disappears, except in the ethmoid region, more 
 persisting in Sphenodon (fig. 89) and the lizards than elsewhere. All four occipitalia 
 are ossified, but some may not participate in framing the foramen magnum, the 
 basioccipital being excluded in many chelonians, the supraoccipital in snakes, 
 crocodiles and theriomorphs. There is but a single occipital condyle (except in a 
 few theriomorphs), which is borne on the basioccipital as in the crocodiles, or on 
 this and the exoccipitals as in chelonians and squamata. Basi- and presphenoids 
 are present, orbito- and alisphenoids are but slightly ossified and the ethmoid region 
 is largely cartilaginous. Pro-, epi- and opisthotics are present, the epiotic fusing 
 with the supraoccipital, while the opisthotic in all recent forms except the turtles 
 unites with the exoccipital in the adult. 
 
 In all except the squamata, in which it is movable (streptostylic), the quad- 
 
88 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 rate is firmly united to the squamosal and sometimes to other bones (monimostylic). 
 The pterygoids extend forward to the palatines. In the squamata and the ichthyo- 
 saurs pterygoids and palatines are widely separated in the middle line, but else- 
 where they are closely approximate, the pterygoids even meeting the basisphenoid. 
 In all except chelonians, some dinosaurs and the typhlophida an ectopterygoid 
 (os transversum) extends from pterygoid to maxilla, while in plesiosaurs and 
 most lizards (kionocraniate) ossification of the ascending process of the quadrate 
 forms an epipterygoid bone between the pterygoid and the parietal. 
 
 Membrane bones are more numerous than in the amphibians. In many 
 theriomorphs there is a supratemporal fossa between parietal and supratemporal 
 bones and the same is true of plesiosaurs, ichthyosaurs and chelonians. In the 
 rhynchocephals, dinosaurs, pterodactyls and crocodiles there is in addition, an 
 infratemporal fossa, bounded laterally by an arcade in which squamosal, quad- 
 ratojugal and zygomatic participate in varying degrees. In the lizards the two 
 unite in a single temporal fossa by the disappearance of the upper arcade, and 
 lastly, in the snakes the lower arcade is lost and the fossa becomes a gap in the 
 side of the skull. 
 
 Parietals and frontals are usually paired, a parietal foramen being common; 
 pre- and postfrontals usually occur, sometimes excluding the frontal from the orbit. 
 Lacrimals are common and the margins of the upper jaw are formed in front by 
 premaxilla and maxillary, the latter connected with the squamosal, sometimes 
 by jugal and quadratojugal, or the jugal may drop out, or lastly the jaw may end 
 with the maxillary. Several membrane bones may aid in the formation of the 
 roof of the mouth. There is a small parasphenoid in ichthyosaurs, plesiosaurs, 
 many squamata, some rhynchocephals, and rarely in turtles. It is usually asso- 
 ciated with the basisphenoid and in ophidia it forms the base of the interorbital 
 septum. The vomers are paired except in the chelonia, and only in Sphenodon 
 of recent species do they bear teeth, and here but one on each bone. The maxil- 
 laries usually have broad palatal processes extending toward the middle line, causing 
 the choanae to open farther back, and in some, these, together with the palatines 
 and pterygoids, form a false palate, ventral to the nasal passages, so that, as in the 
 crocodiles, the choanae are carried far back in the mouth. In some dinosaurs 
 there is a rostral bone in front of the premaxillae. 
 
 The two halves of the lower jaw are united by ligament in most rhynchocephals, 
 snakes and pythonomorphs; by suture in crocodiles, rhynchocephals and lizards; 
 while they are fused in turtles and pterosaurs. All of the bones mentioned on 
 page 71 may occur in the lower jaw, usually with distinct sutures, while in croco- 
 diles, theriomorphs and some dinosaurs there are gaps or vacuities in its walls. 
 In many dinosaurs there is a predentary bone at the tip of the jaw. Except in 
 the chelonia and a few isolated forms, both jaws bear teeth, which may be restricted 
 to maxillaries and premaxillaries, or may also occur on palatines, vomers and 
 pterygoids. In their fixation three types are found: acrodont, when fused to the 
 margin of the bone; pleurodont, when fastened to the side of the bone; and the- 
 codont, when implanted in sockets. 
 
 The hyoid apparatus is much modified, but is adequately known only in recent 
 
SKELETON. 
 
 89 
 
 species. The branchial arches are usually better developed than the hyoid proper, 
 which is cartilaginous in most snakes and is lacking in the crocodiles. In the 
 chelonia (fig. 93) two branchial arches are usually present. 
 
 The Theriomorphs (fig. 90) have a short, broad skull with parietal foramen; 
 and that of the cotylosaurs was much like that of the stegocephals. In the more 
 differentiated groups the skull recalls that of mammals, especially in the partici- 
 pation of the squamosal in the hinge of the jaw. Lacrimals are occasionally 
 absent, sclerotics sometimes present. The palatal region is known in a few forms. 
 The pterygoids may meet only in front, lea\T[ng a vacuity between it and the 
 basisphenoid, or they may meet that bone. The choanae are in front of the pal- 
 atines but (theriodonts) may be displaced backward by palatine processes of the 
 maxillaries. 
 
 All four occipitalia are developed; the occipital condyle is tripartite, being formed 
 by basi- and exoccipitals, but in Cynognathus the recession of the basioccipital 
 results in a dicondyHc condition. The greatest variations occur in the temporal 
 
 Fig. 90. — Skull of Procolophan, after Woodward. For letters see fig. 68. 
 
 region. In the lower cotylosaurs the cranial roof is without fossae (Broom doubts 
 the infratemporal fossa of Procolophon). In other theromorphs quadratojugal 
 and supra temporal are lacking, the squamosal meeting the parietal. Placodus 
 has only the supratemporal fossa, but in the majority the upper arcade has dis- 
 appeared, leaving a large temporal vacuity, much as in mammals. 
 
 Little is known of the lower jaw. The bones are sometimes discrete, sometimes 
 extensively fused. The teeth are thecodont, and in the theriodonts are differentiated 
 into incisors, canines and molars, but in the anomodonts teeth are absent, or at most 
 there are a pair of large incisors in the upper jaw. 
 
 In the Plesiosaurs and their allies the skull is about a twelfth of the total length. 
 There is a parietal foramen between the parietals, which have a process for artic- 
 ulation with the squamosal, the supratemporal being absent. The large pre- 
 frontals intervene between the frontals and the orbits; lacrimals and usually 
 nasals are absent. The large temporal fossa in bounded externally by the jugal 
 which extends back to the quadrate. The choanae are in front of the palatines; an 
 OS transversum is present and there is frequently a parasphenoid in the inter- 
 pterygoid vacuity. All have a subtemporal vacuity and there is another in the 
 
90 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 Fig. 91. — Skull of Plesiosaurus macrocephalus, after Andrews, ang, angulare; art, 
 articulare; ch, choana;/r, frontal; orb, orbit; pa, parietal; pal, palatine; pas, parasphenoid; 
 po+pof, postorbital and postfrontal; sf, supratemporal fossa; /, transversvim. Other 
 letters as in fig. 68. 
 
 mx 
 
 Fig. 92. — Dorsal and ventral views of the skull of turtle, Trionyx ss, exoccipital; 
 m, maxillary; p, (behind naris), preorbital; pno, postorbital; s, supraoccipital; other 
 letters as in fig. 68. 
 
SKELETON. 
 
 91 
 
 plesiosaurs in the angle between palatine and transversum. The usual bones are 
 frequently distinct in the lower jaw. 
 
 In the Chelonians the cranial cavity extends forward between the eyes and the 
 mesethmoidal cartilage largely persists in the adult. Although the bones are 
 comparatively few, the skull is primitive and can only be derived from that of the 
 cotylosaurs. The bones are firmly united, but the sutures are evident. The 
 basioccipital is usually excluded from the foramen magnum, and it and the ex- 
 occipitals participate in the tripartite occipital condyle. The supraoccipital is 
 often prolonged into an occipital spine and is fused with the epiotics. The basi- 
 sphenoid is present, but pre-, ali- and orbitosphenoids are not ossified, a descending 
 plate of the parietal taking the place of the alisphenoid. The pterygoids meet the 
 
 Fig. 93. — Hyoid apparatus of Tryonyx. b^, b^, first and second branchial arches; bhy 
 basihyal (copula); h, reduced hyoid; cartilage dotted. 
 
 basisphenoid and may extend to the basioccipital. No ectopterygoid is present. 
 The monimostylic quadrate is large and expanded laterally to support the tympanic 
 membrane, and notched or perforate behind for the columella. 
 
 In the most primitive chelonians a complete false roof is formed by the expanded 
 postfrontals, parietals and squamosals. In most of the species the recession of the 
 parietals and squamosals causes a large gap, bounded in front by postfrontal and 
 jugal and exposing the otic bones. Laterally this gap is limited by an arcade of 
 squamosal and quadratojugal, but the latter may be reduced or {Cistudo) absent. 
 In front of the frontals are a pair of bones, which bound the single naris behind. 
 These occupy the position of lacrimals, nasals and prefrontals, and are called by 
 the latter name. The premaxillaries are usually fused; the maxillae have broad 
 palatal processes and trenchant margins. They, together with the zygomatics, 
 form the lower border of the orbit. 
 
 The vomer is a single vertical plate separating the two choanae. The palatines, 
 which bound the choanae behind are broad and are firmly united to pterygoids 
 and basisphenoid. A parasphenoid is known only in Dermochelys. In the lower 
 
92 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 jaw the bones are often fused, the two halves being united. Again the bones may 
 be distinct, the splenial being the least constant element. The hyoid apparatus 
 consists of a cartilaginous copula and two pairs of cornua which do not reach the 
 cranium. 
 
 IcHTHYOSAURS havc a short temporal region but elongate nasals and pre-max- 
 illaries form a long rostrum. There is a large supratemporal fossa and enor- 
 
 FiG. 94. — Dorsal (A), posterior (B), ventral (C), and lateral (D) views of the skull 
 of Ichthyosaurus longifrons, after Woodward, nar, naris; pas, parasphenoid ; pmx, pre 
 maxilla; ptf, postfrontal; pto, postorbital. Other letters as in fig. 68. 
 
 mous orbits, bounded above by pre- and postfrontals, below by an elongate jugal, 
 and containing a sclerotic ring. The nares are just in front of the orbits and the 
 parietal foramen is at the junction of frontals and parietals. All four occipitalia 
 bound the foramen magnum; the basisphenoid is short, the presphenoid long; and 
 the pterygoids are separated in front by the vomers, leaving large pterygoid vacui- 
 
 FiG. 95. 
 
 -Side and posterior views of skull of young Sphenodon, after Howes and Swinner- 
 ton. Compare with fig. 69. Cartilage dotted; letters as in fig. 68. 
 
 ties. The choanse are far forward. Teeth (sometimes absent) occur in grooves. 
 The lower jaw has five or six distinct bones, and a rib-like hyoid has been found in 
 some species. 
 
 The only living Rhynchocephalian is Sphenodon (Hatteria) of New Zea- 
 land. It is lizard-like, but its skull (figs. 68, 95) differs in the three temporal 
 fossae, the infratemporal arcade being osseous as in no lizard. Then the quadrate 
 is anchylosed to pterygoid, squamosal and quadratojugal. Premaxillae, maxillse and 
 
SKELETON. 
 
 93 
 
 palatines bear teeth; an epipterygoid is present and the lower margin of the orbit 
 is formed by the maxillary. In the extinct genera the jugal may bound the orbit 
 below (Palaeohatteria), and the vomer may bear teeth. 
 
 Dinosaurs have both supra- and infratemporal fossae and frequently a pre- 
 orbital vacuity as well. The rostral and predentary bones have been mentioned 
 (p. 88). The palatal region recalls that of Sphenodon, except that the teeth, in 
 grooves or sockets, never occur on the palatines. There are such variations in the 
 skulls that few general statements can be made. 
 
 Statements that will apply to all Squamata are few. Except in chamaeleons 
 the quadrate is movable, a quadratojugal is lacking, the boundary of the infra- 
 temporal fossa being completed by ligament. The external nares are separate, there 
 
 Fig. 96. — Skull of Gerrhonotus imhricattts, after Siebenrock. For letters see fig. 68. 
 
 are large vacuities in the floor of the skull and the choanae are forward. An 
 ectopterygoid occurs except in the typhlopids and all four occipitalia bound the 
 foramen magnum. 
 
 The chondrocranium of the Lizards (fig. 62), while much like the general type of 
 tropibasic, is very light and is fenestrated to an extent not seen in the ichthyopsids. 
 Among the peculiarities of the adult skull are the fusion of exoccipital and opisthotic 
 to form a 'parotic process' which, together with the squamosal, supports the quad- 
 rate. There is a looseness of connexion of the front of the skull with the occipito- 
 sphenoidal portion, these parts moving on each other. The hyoid apparatus 
 bears two cornua which either end freely in the neck or may reach the parotic 
 process. 
 
 In the Pythonomorphs the striking features are the large supratemporal fossae, 
 the quadrate recalling that of chelonians; and the joint in the lower jaw, between 
 dentary and angular regions. 
 
94 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 The Ophidia (snakes) lack parotic process, parietal foramen, temporal arcades 
 and epipterygoid, and have the squamosal excluded from the cranial wall. The 
 attachment of the visceral skeleton to the cranium is loose, the pterygoid being 
 connected to the other parts by a long bar, consisting of squamosal and quadrate 
 behind and by transversum and palatine in front, features related to the great 
 distensibility of the jaws. In the poisonous serpents the poison fangs are either 
 permanently erect, or they fold back when the mouth is closed. In the latter the 
 fangs are supported on the maxillaries, which are moved by a rod formed of quad- 
 rate, pterygoid and ectopterygoid. In the lower jaw distensibility is provided for 
 by the elastic ligament connecting the two halves in front. Some species have 
 remnants of the hyoid apparatus, but occasionally all are lost in the adult. 
 
 as 
 
 Fig. 97. — Skull of snake, Tropidonotus, after W. K. Parker. 
 
 When the whole series of Crocodilia, recent and extinct, is considered the 
 range of variation in the skull is considerable. In all, supra- and infratemporal 
 fossae are present, the quadrate is immovable, there is more or less of a secondary 
 palate, no parietal foramen, and the thecodont teeth are confined to the margins 
 of the jaws. In the complete series the gradual change of position of the choanae 
 can be traced from the oldest in which they are beside the vomers; then in the meso- 
 suchia the palatines meet in the middle line, carrying the choanae back as a single 
 opening behind these bones; while in the recent species the pterygoids have also 
 met, so that the choanae are between them and the basisphenoid. 
 
 Among the recent species the basioccipital is excluded from the foramen mag- 
 num, pre- and orbitosphenoids are imperfectly ossified, the nasals are long and the 
 
SKELETON. 
 
 95 
 
 premaxillaries short so that the nares are far in front; parietals and usually the 
 frontals are fused in the middle line. There are vacuities in both walls of the 
 lower jaw, which is also pneumatic. 
 
 Although there is no relation between the two, the skull of the Pterosaurs is 
 very bird-like in its length and in having its axis at right angles to that of the body, 
 while the elongate premaxillae form a bird-like beak. The sutures between the 
 bones are largely obliterated in the adult and the brain cavity recalls that of birds. 
 The resemblances are heightened in some by the lack of teeth, in others they are 
 in sockets. Both supra- and infratemporal fossae are present, as well as a large 
 preorbital vacuity, sometimes united with the naris. Squamosal and quadrate 
 are inclined forward so that the hinge of the jaw is often beneath the orbit. There 
 is no parietal foramen and all of the bones of the jaw are fused, including those of 
 the two halves. 
 
 Fig. 98. — Skull of Caiman laiirostris, based on a figure by Reynolds; the irregularitesi of 
 the surface omitted. Letters as in fig. 68. 
 
 AVES. — The skull of birds is similar in many respects to that of lizards. The 
 chondrocranium arises as two distinct parts, pre- and perichordal, which, on account 
 of the great head flexure, are at an angle of 100° to each other, later increased to 
 160°, which persists through life. There are three (or four?) occipital vertebrae be- 
 hind the ear, the last being the most prominent, and there is a small synotic tectum. 
 From the first the otic capsules are continuous with the basal plate and the fenestra 
 vestibuli is formed later by resorption of the cartilage. The trabeculae are at first 
 distinct from each other as well as from the perichordal part; later they fuse in 
 front of the hypophysis to give rise to the base of the interorbital septum. In 
 Tinnunculus the ethmoid plate arises early as an intertrabecular mass, from which, 
 later, the dorsal part of the interorbital septum arises as a backward growth of 
 cartilage. Large alisphenoid cartilages are connected with the otic capsules. 
 The nasal capsules are complicated and later give rise to several centres of ossi- 
 fication. The quadrate is free from the rest (streptostylic) and its pterygoid process, 
 the homologue of the pterygoid cartilage in other groups, is greatly reduced. The 
 other visceral arches are much as in the adult (infra). 
 
96 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 The bones are lighter than those of reptiles and are often pneumatic, that is, 
 are penetrated with canals connected with the respiratory system. The brain 
 cavity is larger than in reptiles; sutures between the bones largely disappear in the 
 adult, and the single occipital condyle (mostly basioccipital) is on the floor of the 
 skull so that the axis of the skull is at right angles to that of the body. There is 
 only a single temporal fossa, bounded laterally by an arcade of jugal and quad- 
 ratojugal, connecting quadrate and maxillary. There is a preorbital vacuity; 
 and the nares may have the posterior margin rounded (holorhinal) or slit-like 
 (schizorhinal). The premaxillaries are fused and sclerotic bones are common. 
 
 A peculiarity of the ventral surface is the union of the anterior part of the 
 
 Fig. 99. — Earlier and later stages of skull of bird (Tinnunculus) after Suschkin. a/, 
 alisphenoid cartilage; at, foramen for internal ophthalmic artery; b, basal plate; bpt, 
 basipterygoid ; ec, external semicircular canal; hm, 'hyomandibular;' iorb, interorbital 
 plate; it, intertrabecula; mc, middle concha of nose; ov, occipital vertebrae; pc, posterior 
 semicircular canal; sorb, supraorbital; str, supratrabecula; tr, trabecula. 
 
 parasphenoid to the basisphenoid to form a *rostrum sphenoidale* which projects 
 forward in the middle line. The rest of the parasphenoid forms a *basitemporal 
 plate' below the basisphenoid and basioccipital. Dorsal to the rostrum is a 
 small presphenoid (sometimes lacking in the adult) to which the orbitosphenoids are 
 attached as alae, while the alisphenoids become similar wings to the basisphenoid. 
 Ectethmoids are connected with the mesethmoid; they are sometimes large, appear- 
 ing ('prefrontals') on the top of the skull. Epi- and ectopterygoids are lacking. 
 The pterygoids, here membrane bones, extend from the quadrates to the palatines, 
 and the two either slide along the rostrum or the vomers intervene. This, together 
 with the hinging of the front part of the skull upon the rest, forms a mechanism by 
 which the upper jaw is raised when the mouth is opened, the temporal arcade aiding 
 in the motion. The vomers may be paired; usually they form a thin vertical plate 
 between the anterior ends of the pterygoids; occasionally they disappear. The 
 choanae are between the palatines and vomers. Some birds have an *os lincin- 
 
SKELETON. 
 
 97 
 
 atum,' a small bone connecting the lacrimal with the palatine or jugal bar. All 
 
 of the bones enumerated on page 71 may appear in the development of the lower 
 
 jaw. 
 
 Teeth occur only in a few fossil birds, where they are implanted in sockets; 
 
 several species are known to have a dental ridge in the embryo (see Development 
 , ' of Teeth). The hyoid apparatus (fig. loi) 
 
 consists of a pair of cornua (first branchials) 
 sometimes extremely long, connected by the 
 hyoid copula (os entoglossum), behind which 
 is a second copula (urohyal) while in front of 
 the entoglossum is a ^paraglossaP element 
 with a pair of small cornua. 
 
 The palatal structures have considerable 
 importance in classification. All living birds 
 can be arranged in two groups. In the *dro- 
 maeognathous' group the palatines and ptery- 
 
 FiG. 100. — ^\'entral view of skull 
 of a duck; letters as in fig. 68. 
 
 Fig. ioi. — Hyoid of hen, after Parker. 
 e, entoglossal; />, paraglossal; u, urohyal; 
 ///, posterior cornua. 
 
 golds do not articulate with the rostrum, the vomers usually intervening. In 
 the *euomithes' the articulation occurs. The latter are subdivided into the 
 desmognathous forms where the vomer is small or wanting, and the maxillo- 
 palatines meet in the middle line; the schizognathous in which the maxillo- 
 palatines do not meet the vomer or each other; the aegithognathous, like the 
 last except that the vomer is broad and truncate; and the saurognathous 
 with^deKcate, rod-like vomers and maxillopalatines scarcely extending inwards 
 from the maxillaries. 
 7 
 
98 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 The chondrocranium of the MAMMALS has several peculiarities. There are 
 four occipital vertebrae, the last only with a complete vertebral character, all event- 
 ually fusing with the synotic tectum. The dorsal part of the otic capsule chondrifies 
 first, owing to the late development of the cochlear part of the ear in the lower half; 
 and the capsules themselves have their axes inclined, so that the exit of the seventh 
 nerve is on the anterior rather than on the lateral face. The trabeculae soon join 
 the basal plate and from their sellar part an alary process is given off on either side 
 
 Fig. I02. — Chondrocranium of a pig, after Mead, as, alisphenoid; cl, posterior clinoid 
 process; cr, fenestra cribrosa; end, foramen for endolymph duct;/w, foramen magnum; h, 
 fossa h5rpophyseos; Isr, lateral superior recess; os, orbitosphenoid; pi, parietal lamina; sn, 
 septum nasi; <», tectum nasi; 2-12, exils of nerves. 
 
 which extends upward to join an alisphenoid (ala temporalis) which chondrifies 
 separately, but soon joins the otic capsule above, leaving between them the foramen 
 ovale for the third branch of the fifth nerve, the other branches passing forward 
 over the ala and then between it and the orbitosphenoid (ala orbitalis) through 
 the sphenoidal fissure (foramen lacerum anterior). The ala orbitalis joins the 
 trabecula by two processes, bar and processes sometimes forming a reduced inter- 
 orbital septum. Later a marginal band (taenia marginalis) extends back from 
 
SKELETON. 
 
 99 
 
 the orbitosphenoid to a cartilage plate developed on the otic capsule. The ethmoid 
 parts are complicated, consisting of the two nasal capsules, the septum between 
 them, and, on the inside, coiled turbinal cartilages to support the olfactory membrane. 
 
 Some of the visceral arches have been mentioned in speaking of the ear bones 
 (p. 74). The pterygoid cartilage is apparently lacking, and there is nothing that 
 can be interpreted as a quadrate except the incus. Meckel's cartilage extends for- 
 ward from the incus to the tip of the jaw. In the procartilage stage the hyoid is 
 continuous with the stapes; later it joins the otic capsule behind the fenestra ves- 
 tibuli, while ventrally it joins its fellow and is connected with the first branch- 
 ial arch by a median cartilage, probably the copula. 
 
 In the adult the so-called facial bones are more closely related to the cranium 
 than in the lower groups, and distinct bones are fewer than in lower vertebrates, 
 the reduction being due in part to actual loss, in part to the fusion of elements 
 
 Fig. 103. 
 
 -Diagram of the bones of the mammalian skull, altered from Flower, 
 bones dotted, membrane bones lined; 2-12, nerve exits. 
 
 Cartilage 
 
 which elsewhere remain distinct. The obliteration of sutures has gone farther in 
 the monotremes and some of the carnivores and apes than elsewhere. Connected 
 with the loss of bones is the absence of the supratemporal arcade, but the infra- 
 temporal bar consisting of processes from the squamosal and zygomatic (malar) 
 is always present, bounding the single temporal fossa. This may be separated 
 from the orbit by a bar formed by zygomatic and frontal, or the bar may be in- 
 complete or absent so that orbit and fossa are one. 
 
 Usually the bones fuse in such a way that the complexes named on page 66 are 
 readily recognized. The occipitalia are usually united into a single occipital bone, 
 though the sutures between them may persist for some time. The basioccipital 
 forms the so-called basilar process, while the exoccipitals bear the two occipital 
 condyles for articulation with the atlas. The exoccipitals may also bear strong, 
 ventrally directed, paramastoid processes (paroccipital). The membranous 
 interparietal is sometimes distinct, sometimes fused to the supraoccipital, though it 
 may unite with the parietals. 
 
lOO COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 The sphenoidalia form the sphenoid bone of human anatomy. Basi- and pre- 
 sphenoid form a 'body' from which two pairs of 'wings' arise, the alisphenoids 
 being the greater, the orbitosphenoids the lesser wings. A pair of pterygoid pro- 
 cesses are given off from the ventral side of the body and a part of these in some cases 
 persist as distinct pterygoid bones, but apparently are not homologous with the 
 elements of the same name in the lower vertebrates since they are membrane bones. 
 The equivalents of the pterygoids of the non-mammals occur in the monotremes. 
 A second pair of membrane bones, the intertemporals, also belong to the sphenoid 
 complex, fusing at an early date with the dorsal margin of the alisphenoids. 
 
 The ethmoid complex consists of a mesethmoid which ossifies in the septum 
 between the nasal organs, and an ectethmoid in the outer wall of each nasal capsule. 
 Mes- and ectethmoids are distinct for a time, the olfactory nerve passing between 
 them. Later bony strands passing between the nerve fibres unite them, producing 
 perforated cribiform plate, characteristic of the mammals. The part of the 
 mesethmoid projecting above the cribiform plates is the cristi galli, below them is 
 
 Fig. 104. — Median section of skull of young Erinaceus, after I'arkcr. For letters see 
 
 fig. 68. 
 
 the perpendicular plate. Two other centres in the lateral wall of each capsule 
 give rise to coiled bones (inferior and sphenoidal turbinal) on which the olfactory 
 membrane is spread, while two other turbinals (superior and middle) arise from 
 the ectethmoid. A few mammals have in addition, a prenasal bone, developed 
 in the septum in front of the mesethmoid. 
 
 The temporal complex consists of squamosal, otic bones and tympanic. On 
 the ventral side of the squamosal is the glenoid fossa for the articulation of the 
 lower jaw; in front the bone gives ofiF a zygomatic process for articulation with a 
 similar process of the zygomatic (malar) bone, the two forming the arcade bounding 
 the temporal fossa. The tympanic (apparently the angulare of the lower vertebrates) 
 curves below the auditory meatus, joining the squamosal on either side. In many 
 forms it expands to form a large capsule, the auditory bulla. The otic bones 
 (it is said that there are six centres of ossification in the otic capsule) unite early 
 to form a single petrosal bone, which, in turn (cetaceans excepted) fuses with squa- 
 mosal to form the temporal bone. Later, the posterior part of the otic region expands 
 to form the mastoid process, while the upper part of the hyoid, fused to the cap- 
 sule, forms a styloid process. 
 
 On account of the great size of the brain some parts of the skull are changed in 
 
SKELETON. lOI 
 
 position. Thus the petrosal, instead of forming part of the side wall, is carried to 
 the floor of the brain cavity and the squamosal forms part of the lateral wall. 
 The roof of the brain cavity is largely formed by parietals and frontals. (In some 
 whales, denticetes, the supraoccipital and interparietal extend to the frontal, pre- 
 venting the parietals from meeting.) The frontals may be distinct or they may 
 fuse. In many ungulates they bear horns or antlers. In catde, antelopes, sheep 
 and goats (cavicornia) a strong bony process or horn core is developed on each 
 frontal, and this is covered by a cornified epidermis and persists through life. The 
 antlers of the deer differ from horns. Each year there is an outgrowth of bony 
 material, covered by a richly vascular skin, from each frontal bone. This grows 
 with remarkable rapidity, and when its full extent is reached, the skin ('velvet') is 
 lost, leaving the core alone. After about a year resorption takes place at the base 
 so that the antler is soon lost, to be replaced by a similar but larger one in a few 
 weeks. 
 
 The nasals lie above and behind the nares. The margin of the upper jaw is 
 formed by premaxillaries followed by the maxillaries which ossify from several 
 centres, difficult to homologize with distinct bones in the lower vertebrates. The 
 inferior turbinals fuse to the inner surfaces of the maxillaries. Premaxillaries and 
 maxillaries may fuse or they may remain distinct. They have broad palatine 
 processes on the oral surface, these meeting in the middle line and forming the 
 anterior part of the hard palate, with frequently one or two incisive foramina 
 for the passage of the nasopalatine nerve between them. The choanae are usually 
 behind the palatine bones which form the rest of the hard palate, but in some eden- 
 tates and whales the pterygoids form part of the partition between the narial pas- 
 sages and the mouth cavity. 
 
 The ingrowth of the hard palate has forced the vomer from the roof of the mouth 
 to a position just ventral to the anterior part of the cartilage of the nasal septum. 
 In the monotremes there is a 'dumb-bell 
 bone* in front of the vomer (p. 69). A 
 lacrimal bone always occurs at the inner 
 side of the orbit and the zygomatic forms 
 the external wall of that cavity. 
 
 The lower jaw articulates directly with 
 the squamosal without the intervention of 
 a quadrate (see ear bones, p. 74). Its 
 halves may unite in front by ligament or ^^^ io5.-Hyoid of rhi^ceros (Ate- 
 
 by complete anchylosis. It is usually lodus). ac, anterior comu; b, body; c, 
 described as consisting of a pair of den- ceratohyal;er,epihyal:^<;, posterior comu 
 taries, but there are several centres of ossi- ^ >^ ^ -'• 
 
 fication and a splenial and possibly a coronoid may be recognized. The angulare 
 is apparently the tympanic, while the articulare of lower vertebrates is the malleus. 
 A remarkable feature in development is an enormous cartilage at the posterior 
 angle of the jaw, the dorsal side of which forms the condyle for articulation with 
 the glenoid fossa. 
 
 The hyoid apparatus varies. As described above, the hyoid is connected above 
 with the otic region, below with the first branchial. The part connected with the 
 
I02 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 otic capsule forms the styloid process (p. loo), while the rest may ossify as epi-, 
 cerato-, and hypohyals, or a part may change to a stylohyal ligament, connecting 
 the ventral parts with the skull. The hyoid of the adult consists of the copula 
 forming the body, a part of the hyoid the anterior cornua, while the first branchial 
 arch (of which at most but one or two HhyrohyaP elements are formed) give rise 
 to the posterior cornua. These are connected by ligament with the greatly modified 
 posterior branchial arches, described in connection with the larynx (see respiratory 
 organs). 
 
 Appendicular Skeleton. 
 
 The appendages fall in two categories, the median or azygos 
 (median fins) found only in aquatic vertebrates and the paired appen- 
 dages, which (cyclostomes excepted) are found in every class, although 
 here and there individual species or genera may lack them. Both 
 kinds have an internal skeleton. Opinions differ as to the origin of 
 these appendages. The two most prominent views are given below. 
 
 Fig. io6. — Diagram of the origin of median and paired appendages from lateral fin folds. 
 
 According to one view the two types have no relation to each other. The 
 paired appendages are derived from gill septa, all traces of which are otherwise 
 lost from these somites. The girdles which support the appendages are modified 
 gill arches, while the skeleton of the appendage itself is derived from the radialia 
 which support the gills, one radial forming an axis, the adjacent radials being 
 arranged on either side of this, and carried outward from the arch by the growth 
 of the septum to form the body of the appendage (fig 122). A somewhat similar 
 view is that the appendage itself is a modification of an external gill, such as is 
 found in larval amphibians. 
 
 Another view supposes an ancestor with two pairs of longitudinal folds running 
 the length of the body behind the head, each fold supported by a series of skeletal 
 rods (fig. 106). With farther development the upper folds on either side migrated 
 
SKELETON. IO3 
 
 dorsally until the two met and fused in the middle line of the back, thus producing 
 a continuous dorsal fin. The ventral folds migrated downward in the same way, 
 eventually meeting behind the vent, but that opening prevented their meeting 
 farther forward. From the fused part behind the vent the anal and the lower part 
 of the caudal fins were formed, while the paired appendages are differentiations of 
 the preanal parts of the ventral longitudinal folds. 
 
 It may be said that in development there is no such double origin of the dorsal 
 fin. In several sharks the paired fins arise from continuous folds, while in the 
 Japanese gold fish the anal fins are frequently paired and the caudal has a double 
 condition below, such as would result from the failure of folds to unite in this region. 
 In criticism of the gill-arch theory it may be said that the supports of the paired 
 appendages arise outside of the body musculature, while the visceral arches (p. 65) 
 are internal. 
 
 The Median Appendages. 
 
 The median or azygos appendages always have the form of fins, 
 and may be dorsal, terminal (caudal) or ventral (anal) in position. 
 Primitively, and in many species through life, they are continuous, but 
 usually gaps occur during development so that the fins of the adult are 
 separated by intervals from each other. They occur in practically all 
 fishes, in larval and tailed amphibians, and in isolated groups like the 
 ichthyosaurs and whales. In amphibians and higher groups the 
 median fins have no skeleton, but elsewhere it is of cartilage, bone, 
 or a horny substance (elastoidin) , the latter being the most constant 
 and occurring in connection with either of the others. 
 
 The simplest skeleton consists of a metameric series of cartilage or 
 osseous bars, each usually divided into a deeper basale and a more 
 distal radiale, the former frequently articulating or alternating with 
 the spinous processes of the vertebrae, while the latter support the fin 
 proper. The elastoidin elements consist of a number of slender rods 
 (actinotrichia) , outnumbering the somites, and arising from the 
 corium, immediately below the epidermis. Frequently they are 
 united into bundles (soft fin rays) and may replace the radialia. 
 
 Paired Appendages. 
 
 The paired appendages are not, as the gill-arch theory would demand, derived 
 from a single somite, but a varying number of segments participate in their forma- 
 tion. Apparently the simplest fin known is that of the extinct shark, Cladoselache 
 (fig. 107), in which it is a rounded lobe supported by a number of rods, like the 
 radialia in a median fin. These are attached proximally to a few larger plates, the 
 
I04 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 basalia, the basalia of the two sides being unconnected with each other. Greater 
 growth of the basalia would result in some of them meeting and fusing in the mid- 
 dle line, thus forming a bar across the ventral side of the body, giving additional 
 support to the fin. Then to compensate for the rigidity, the basals become jointed 
 
 on either side, leaving the medial bar 
 with an articular surface on either side 
 for the reduced basalia. The ventral 
 muscles of the fin would find firm at- 
 tachment to the bar, while the need for 
 a similar attachment for the dorsal 
 results in an extension of the bar dor- 
 sally above the articulation of the 
 limb, thus producing the typical girdle. 
 The derivation of the fin of any fish 
 from that of Cladoselache is easily 
 imagined, but no satisfactory compari- 
 son of the fin with the leg has yet been 
 made. 
 
 In the appendicular skeleton 
 the internal supports or girdles 
 and the skeleton of the free ap- 
 pendage are to be recognized. 
 Each girdle is an inverted arch 
 crossing the ventral side of the 
 body and extending up on either 
 side above the articulation of 
 the limb. The girdles, as well 
 as the skeleton of the free ap- 
 pendage, are aWays laid down 
 in cartilage, and in the latter, 
 aside from the actinotrichia, no 
 parts of other than cartilaginous 
 orgin occur. In the girdles mem 
 brane bones may be added as 
 
 Fig. 107.— Ventral surface of Cladoselache, will appear below. 
 
 after Jaeckel. j^^ -^^ typical State each girdle 
 
 consists of three elements, one dorsal and two ventral, meeting at the 
 point of attachment of the free appendage, all contributing to the 
 socket (glenoid fossa, acetabulum) which receives the basal element of 
 the skeleton of the limb. The limbs themselves are much alike in 
 their general structure, as may be seen from the adjacent diagram. 
 
SKELETON. 
 
 The Shoulder Girdle. 
 
 lO: 
 
 FISHES. — The pectoral or shoulder girdle in the elasmobranchs is 
 more or less U-shaped, the bottom of the arch crossing the ventral 
 surface between the skin and the peritoneal membrane, this ventral 
 portion being known as the coracoid region, which is limited dorsally 
 
 
 Fig. io8. — Diagram of girdles and appendages from the posterior side; upper letters, 
 fore limb; lower, hind limb, a, acetabulum; c, carpus; co, coracoid, /, femiu:; fi, fibula; 
 g, glenoid fossa; h, humerus; il, ilium; is, ischium; mc, nU, metacarpals, metatarsals; p, 
 pubis; pc, procoracoid; ph^-^^ phalanges; r, radius; s, scapula; m, ulna; 1-5 digits. 
 
 by the point of attachment (glenoid fossa) of the fin. Dorsal to the 
 fossa is the scapular region. Not infrequently the dorsal part of 
 the scapular region is segmented off as a separate suprascapula. 
 
 Fig. ioq. — Pectoral girdle and cartilaginous fin skeleton oiScyllium. c, coracoid region ; 
 gl; glenoid surface; ms, mesopterygium; mt, metapterygium; p, propterygium; r, radialia: 
 5, scapular region. 
 
 The girdle is usually free from the axial skeleton, but in the skates 
 (raiae) the suprascapula articulates with the adjacent vertebrae. 
 
 In the simpler teleostomes (some ganoids, dipnoans) the cartilagin- 
 ous girdle is reinforced by membrane bones derived from the skin. 
 
I06 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 Of these there are at least two on either side, a pair of clavicles which 
 overlie the coracoid region and meet in the middle line, and lateral to 
 each clavicle and extending to or above the glenoid fossa, a second 
 bone, the cleithrum. In some ganoids {Polypterus, fig. no) the 
 cleithra extend toward the middle line, and a little higher in the scale, 
 meet and take the strains. This assumption of stress by the membrane 
 bones results, in the higher forms, in the separation of the two halves 
 of the cartilaginous girdle. 
 
 In the higher ganoids and teleosts the cleithrum has increased 
 greatly, usurping the function of the clavicles, which have consequently 
 
 Fig. 1 10. — ^Pectoral girdles of (^1) Acipenser and (B) Polypterus, after Gegenbaur. ct, 
 cleithrum; cv, clavicula; rfr, dermal rays; g, glenoid surface. 
 
 disappeared. Dorsal to the cleithra other membrane bones frequently 
 occur. There may be one or two supracleithra (post- or supra- 
 temporals, fig. 79) which connect the girdle with the skull, and 
 occasionally others as postclavicle, infraclavicle, etc. As a result 
 of the great development of the cleithra the cartilaginous girdle has been 
 reduced, but it usually has at least two ossifications on either side, a 
 scapula dorsal to the glenoid fossa and a coracoid in the ventral region, 
 these contributing to the support of the appendage. 
 
 AMPHIBIA. — In the stegocephals the cartilage has not been 
 preserved and the bones are variously interpreted (fig. 58) . The bone 
 meeting the episternum is the clavicle, and lateral to this is an equally 
 slender bone, usually called scapula, but by some the cleithrum. A 
 
SKELETON. IO7 
 
 large round element is called the coracoid. In the recent amphibians 
 we are on firmer ground. The halves of the girdle develop separately, 
 and the cleithrum is lacking. In urodeles the coracoid region has two 
 processes diverging from the glenoid fossa, an anteriorly directed pro- 
 coracoid and a coracoid proper, directed toward its fellow of the 
 opposite side, the two meeting the sternum behind and overlapping in 
 front. Ossification sets in in the neighborhood of the glenoid fossa, 
 the resulting bone being called the scapula, although it invades the 
 coracoid region, the cartilage dorsal to it being the suprascapula. 
 
 In the toads and allied anura (arcifera) the halves of the girdle 
 overlap as in the urodeles, but the procoracoids extend toward the 
 middle line, each being joined to its coracoid by longitudinal cartilage 
 plate, the epicoracoid, leaving a gap between them. With the ap- 
 pearance of bone, scapula and coracoid ossify, while a clavicle of mem- 
 
 FlG. III. — Arciferous girdle of Ceratophrys ornatus. cl, clavicle; co, coracoid; e, epicora- 
 coid; h, head of humerus; s, scapula; 55, suprascapula; cartilage dotted. 
 
 branous origin overlies the procoracoid cartilage. In the frogs (firmi- 
 sternia) the relations are much the same, except that the epicoracoids, 
 instead of overlapping, abut against each other, and the clavicles nearly 
 or quite replace the procoracoid, while sternum and omostemum join 
 the girdle in front and behind. Girdles are lacking in the gymnophiones. 
 REPTILES. — With the development of a considerable neck in the 
 reptiles the pectoral girdle is removed farther from the head; it shows 
 considerable differences in the various groups. In the fossil rhyn- 
 chocephals it is much as in the stegocephals, except that the scapula 
 is large. In the turtles it occupies a peculiar position, being inside 
 the carapace, i.e., internal to the ribs; but this is explained by the de- 
 velopment; the girdle arises in front of the ribs and later sinks to the 
 definitive position. Scapula, procoracoid and coracoid are well 
 developed, the medial ends of the latter two being connected by a cartil- 
 aginous epicoracoid. Elsewhere in the reptiles the procoracoid tends 
 to reduction, the clavicle taking its place, though it is retained in the 
 lizards in a reduced condition (fig. 112). The clavicle in turn is 
 
io8 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 lost in chamaeleons and crocodiles, and if present in the chelonians, it is 
 represented by the epiplastron (p. 41), an element of the carapace. 
 The girdles are greatly reduced in the limbless lizards and have van- 
 ished in the ophidians. 
 
 In the BIRDS (fig. 53) the scapula is a sword-shaped bar overlying 
 the ribs, while the coracoid extends from its junction with the scapula 
 at the glenoid fossa to the anterior end of the sternum. The clavicles 
 of the two sides are united at their medial or ventral ends to form the 
 well-known furcula (wishbone) which may articulate with the sternum 
 between the two coracoids, or, with diminishing powers of flight, may 
 end freely below. 
 
 Fig. 112. Fig. 113 
 
 Fig. 1X2. — Sternum and pectoral girdle of Amblyrhynchus, after Steindacher. c, 
 
 coracoid; cl, clavicle; e, epicoracoid; es, epistemum; h, humerus; m, mesocoracoid ; ms, 
 
 mesoscapula; p, procoracoid; sc, scapula; s, sternum. 
 
 Fig. 113. — Shoulder girdle of Ornithorhynchus . cl, clavicle; co, coracoid; e, epister- 
 
 num; g, glenoid fossa; pc, procoracoid; s, scapula; st, sternum. 
 
 MAMMALS. — The shoulder girdle of the monotremes is strikingly 
 like that of lizards, the coracoids acting as a brace between sternum and 
 glenoid fossa, while the resemblance is strengthened by the presence of 
 the episternum. This same large development of the coracoids occurs 
 in the young of some marsupials, but in the adults, as in the rest of the 
 mammals, the coracoid is greatly reduced, persisting only as a small 
 projection, the coracoid process, anchylosed to the ventral end of the 
 
SKELETON. IO9 
 
 scapula, where it often forms a part of the glenoid fossa. The scapula 
 is always well developed, and in the placental mammals bears a strong 
 crest (spina scapulae) on its external surface, terminating ventrally in 
 an acromion process. The clavicle varies with the freedom of motion 
 of the limb. Thus in rodents, insectivores, bats, some marsupials and 
 the higher primates it forms a strong brace between shoulder and ster- 
 num. In ungulates, whales, and a few carnivores it has entirely dis- 
 appeared, while in other mammals it persists as a rudiment without 
 functional value. In development two small elements frequently 
 intervene between the clavicles and the sternum (fig. 55). They 
 are preformed in cartilage but eventually fuse with the sternum. 
 Their homology is very uncertain. They have been called episternalia, 
 suprasternalia, etc. 
 
 The Pelvic Girdle {Pelvis). 
 
 In its broader features the pelvis {cf. fig. 108) is much like the 
 shoulder girdle, and in its full development, may be compared, part by 
 part, with the anterior arch. Thus the acetabulum or socket where the 
 appendage is attached, is comparable to the glenoid fossa. Dorsal 
 to this is the ilium in the position of the scapula, while ventral and 
 medial to the acetabulum are, on either side, an os pubis in front, an 
 ischiimi behind, with a gap (ischio-pubic fenestra) between them, just 
 as between coracoid and procoracoid. An important landmark is 
 the point of passage of the obturator nenx through the pelvis. This 
 may have its own (obturator) foramen, though the pubic portion or 
 the foramen may unite with the fenestra, the condition in the mammals 
 where the common opening is called the obturator foramen. 
 
 The phylogenetic history of the pelvis is more clearly indicated than 
 is that of the pectoral girdle, for in many fossils, as well as in the 
 sturgeon, there is little advance over Cladoselache (p. 104). The 
 basalia of a side have fused to a single basal, often perforated for the 
 obturator nerve, and bearing the radialia on its distal surface. The 
 basalia of the two sides have not met, but there is frequently between 
 them a pair of small cartilage plates, possibly the homologues of the 
 epipubis of the tetrapoda (infra). There is no acetabular joint. In 
 the other ganoids and in teleosts there is little advance, aside from ossifi- 
 cation of parts, while no epipubic elements occur. A noticeable 
 feature in many acanthopterygians is the forward migration of the pelvic 
 
no 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 fins SO that they come to lie in front of the pectorals (the old group 
 of 'jugulares'). 
 
 The elasmobranchs have a true girdle, but without separate ele- 
 ments as it does not pass beyond the cartilage stage. It consists of a 
 continuous ischio-pubic bar, extending from one acetabulum to the 
 other, and usually prolonged dorsally above the acetabulum by an 
 iliac process. 
 
 In all fishes the pelvic girdle is free from the vertebral column, but 
 in the tetrapoda, where the limbs have to support the body weight, the 
 girdle becomes connected with the sacrum by 
 the intervention of one or more sacral ribs (p. 
 56). In the interpretation of some of the 
 pelvic elements there is some uncertainty. 
 
 In the stegocephals ischium and ilium and 
 usually pubis were distinct bones with appar- 
 ently considerable cartilage between them. In 
 
 Fig. 114. Fig. 115. 
 
 Fig. 114. — ^Pelvis of Discosaurus, after Credner. il, ilium; is, ischium; p, pubis. 
 
 Fig. 115. — ^Ventral view of pelvis and ypsiloid cartilage of Cryptohranchus, after Wieder 
 sheim. a, acetabulum; il, ilium; is, ischium; 0, obturator foramen; p, conjoined pubes; 
 y, jrpsiloid cartilage. 
 
 the urodeles the two ischio-pubic cartilages are usually united in the 
 median line, but the ossifications vary in extent, the pubic region lagging 
 behind the ischium and being at times indistinguishably fused with it. 
 In some cases there is, as in Necturus, an extension of the median 
 cartilage forward in an epipubic process, and frequently a pectineal 
 process from the antero-lateral of each pubis. An interesting feature 
 is furnished by the ypsiloid cartilage (fig. 115) formed independently 
 of the pubis and extending forward in the linea alba through two or 
 three somites. This occurs only in salamanders with functional lungs, 
 where it furnishes attachment for muscles connected with respiration. 
 In the anura all three pelvic bones are present, and all participate in 
 the formation of the acetabulum. Correlated with the leaping habits 
 
SKELETON. 
 
 Ill 
 
 the ilium is very long and the ischio-pubis is strongly compressed, 
 obturator foramen and ischio-pubic fenestra being absent. 
 
 Omitting the extinct rhynchocephals, whose pelvis resembles that of 
 the stegocephals, the reptiles have the pelvic bones more solid and dis- 
 tinct than do the ichthyopsida; the ilium is strong, with its dorsal end 
 frequently expanded; the ischio-pubic fenestra is large; and ischium 
 and pubis are united to their fellows directly, or by the intervention of 
 the epipubic cartilage, or its modification, the ligamentum medium 
 pelvis. As a rule all three bones meet in the acetabulum and there 
 are large prepubic processes, though these are small in the lizards and 
 are lacking in crocodiles. 
 
 Fig. ii6. Fig. 117. 
 
 Fig. 116. — Pelvis of snapping turtle {Chelydr a) {rom below, e, epipubis;/, femur; 
 hy hypoischium; /, ligamentum medium pelvis; p, pubis; pp, pectineal process. 
 
 Fig. 117. — ^Pelvis of Iguana tuberculaia, after Blanchard. a, acetabulum; e, epipubic 
 cartilage;/, femur; U, ilium; is, ischium; of, obturator foramen; p, pubis; pp, prepubis; 
 J,* s^, first and second sacral vertebrae. 
 
 Many theriomorphs have the pelvic bones fused much as in mam- 
 mals. In Sphenodon and turtles the epipubic cartilage bounds the 
 fenestra on the median side, and Sphenodon and the plesiosaurs have 
 a separate obturator foramen, but the two are merged in the chelonians. 
 Most lizards have slender pubic bones, perforated by the foramen, and 
 the part of the epipubis between the fenestras reduced to a ligament, 
 while the posterior part of this, behind the ischium, may ossify as a 
 distinct bone (os cloacaB or hypoischium). In the footless lizards the 
 pehds is reduced, being represented in the amphisbaenans by rudiments 
 of ischium and pubis, while all traces of the pelvis are lost in snakes, 
 except the boas and some opoterodonts. The obturator foramen is 
 very large in the crocodiles, the result of the oblique position of the 
 
112 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 pubes, which do not unite with each other; each is tipped with car- 
 tilage (? separate epipubes). All three bones meet at the acetabulm 
 which is perforate in recent species. The lower end of the ilium sepa- 
 rates as a distinct bone (pars acetabularis). 
 
 The pelvis of the dinosaurs has the same great extension of the ilium 
 forward and back as is seen in the birds and a corresponding in- 
 crease of the sacrum (p. 53), the result of the partially upright position. 
 
 Fig. 118. 
 
 -Pelvis and hind limb of Cantptosaurus, after Marsh. /, femur; fb, fibula; il, 
 ilium; is, ischium; p, pubes; pp, postpubis; t, tibia; I-IV, digits. 
 
 The ischia are greatly elongate and are directed backward, being fre- 
 quently united below. The pubic bones are remarkable in being 
 directed forward and downward and in ^having strong postpubic 
 processes which are parallel to the ischium. Frequently the ilium 
 gives off an iliac spine near the acetabulum. 
 
 The pterodactyls had the same elongate ilium as the dinosaurs, the 
 ischium being fused to it so as to exclude the pubis from the acetab- 
 ulum, the latter^ being usually loosely articulated to the ischium and 
 
 ^ This pubis is sometimes regarded as a prepubis, the ischium being called an ischio- 
 pubis. 
 
SKELETON. 
 
 113 
 
 meeting its fellow in the median line below. The pehac opening was 
 very small. The pelvic bones of the ichthyosaurs were weak, long and 
 slender, and apparently were imbedded in the muscles. 
 
 In recent birds (figs. 50, 53) the pelvic bones are fused. The ilium 
 is greatly elongate and usually fused with the synsacrum (p. 53) ; ischium 
 
 Fig. 
 
 119. 
 
 Fig. 120. 
 A, chick of 6 days. 
 
 Fig. 119. — Development of pelvis of chick, after Miss Johnson. 
 B, older; C, 20 days; cartilage dotted, bone white, a, acetabulum; il, ilium; is, ischium; 
 in, ischiadic nerves; on, obturator nerve; p, pubis; pp, pectineal process. 
 
 Fig. 120. — Pelvis of Galeopithecus, after Leche. ah, acetabular bone; i, ischium; i/, 
 ilium; p, pubis; cartilage dotted. 
 
 and pubis directed backward. The pubes, lying in the position of the 
 postpubes of the dinosaurs, never meet below except in the ostriches. 
 In the embryo (fig. 119) they are at first directed forward and only 
 attain the final position later. A pec- 
 tineal process arises from the aceta- 
 bular region and extends forward, simu- 
 lating the dinosaur pubis. 
 
 In the mammals, obturator foramen 
 and ischio-pubic fenestra are united, 
 the opening being bounded on the 
 medial side by processes from ischium 
 and pubis. All three bones may meet in 
 the acetabulum, but more often the ex- 
 tension of ilium and ischium excludes 
 the pubis from the fossa. A peculiarity is the common occurrence of an 
 additional bone in the formation of the acetabulum (acetabular or coty- 
 loid bone) . This lies between ilium and pubic bone and may fuse with 
 any of the elements. In marsupials and monotremes the interpubic car- 
 
 FiG. 121. — Left side of pelvis of 
 duck-bill, Ornithorhynchus, a, ace- 
 tabulum; il, ilium; is ischiimi; w, 
 marsupial bone; of, obturator fora- 
 men; />, OS pubis; sv, sacral vertebra. 
 
114 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 tilage persists for some time, or through life, but elsewhere it disap- 
 pears and the elements unite by symphysis. The same groups of non- 
 placental mammals are characterized by the presence of marsupial 
 bones (fig. 121). These are preformed in cartilage and extend for- 
 ward from either pubis in the ventral abdominal wall. Their homol- 
 ogy is very uncertain; but they are not the ypsiloid of the urodeles. 
 
 ///////Ac, 
 
 D 
 
 Fig. 122. — Diagrams illustrating theories of origin of appendages. A,'B,C, origin of 
 biserial appendage (C) from gill arch {A)\D, biserial appendage (archipterygium) ; £, F, 
 evolution of elasmobranch fin; G, dotted lines indicate parts involved in origin of leg from 
 fin; iJ, dotted parts show another view of origin of elements of leg. 
 
 The Free Appendages. 
 
 These are of two kinds, the paired fins (ichthyopterygia) of the 
 fishes and the legs or their modifications (chiropterygia) found in all 
 classes of tetrapoda. The former is merely a mechanism for altering 
 the position of the body in the water, and requires a small amount of 
 flexibility, being moved as a whole. The assumption of terestrial 
 habits necessitates the support of the body above the ground and its 
 propulsion. Hence the chiropterygium must have a firmer skeleton, 
 with at the same time joints for motion and intrinsic muscles to move the 
 parts on each other. The chiropterygium was undoubtedly derived 
 from the fish fin, but the problem of how the change was made has not 
 been solved. Only paleontology can give the answer. 
 
 There are two views as to the origin of the chiropterygium, both based upon the 
 loss of certain parts and the persistence of others in a modified form. One view 
 assumes the persistence of a basal as the framework (humerus or radius) of the 
 
SKELETON. 
 
 115 
 
 upper limb. Two proximal radials as that of the next limb segment, while the 
 skeleton of ankle and foot is derived from a corresponding number of distal radials 
 on the anterior side of the fin. The 'archipterygial theory' of Gegenbaur assumes 
 an appendage like that of Ceratodus (the * archipterygium *) as the type from 
 which all legs and other fins have been derived, by a 
 shortening of the axis and a loss of radials, chiefly on 
 the preaxial side. The two views are illustrated in the 
 adjacent sketches. No known facts of either embry- 
 ology or paleontology throw any certain light on the 
 matter. 
 
 »/ 
 
 Cladoselache (fig. 107) and the lower ganoids 
 have what is apparently the most primitive type 
 of fin with a large number of basalia which 
 support a large number of radialia. From these, 
 as we go upward in the scale, there is a reduction 
 in the number of basalia, either by disappear- 
 ance or fusion, while the other parts are variously 
 modified. Thus in recent elasmobranchs the 
 characteristic number of basalia is three in the 
 pectoral, two in the pelvic fin. These are known, 
 from in front backward as the pro-, meso-, and 
 metapterygium, the middle one being absent 
 
 Fig. 123. Fig. 124. 
 
 Fig. 123. — ^Pelvic fin and part of girdle of Ceratodus, after Davidoff.a, axial skeleton 
 of fin; pil, iliac process; pirn, processus impar; r, radialia. 
 
 Fig. 124. — Skeleton of pectoral fin of Xenacanthus, after Fritsch. 
 
 from the hind limb. The numerous radials are jointed transversely 
 (fig. 109), permitting more flexibility, and these may be arranged 
 entirely on one side of the basalia (uniserial), or the metapterygium 
 may be prolonged as an axis, and while most of the radialia are 
 on the preaxial side, some may occur on the postaxial side (biserial) 
 as seen in the carboniferous shark, Xenacanthus (fig. 124). In the 
 recent species the skeleton of the fin is continued by actinotrichia. 
 
ii6 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 In the male elasmobranchs the pelvic fin is divided into two lobes, 
 the medial, the so-called clasper (mixipterygium) being the longer and 
 narrower. This is used in copulation and is supported by thespecialized 
 terminal radialia of the metapterygium. 
 
 In other ganoids and in teleosts the skeletal parts are more or less 
 ossified, the basalia more numerous than in the higher elasmobranchs 
 and are shortened and more closely associated with the girdles, while 
 
 the numerous radii form most of the 
 skeleton of the fin itself. It is not un- 
 common for the anterior element of the 
 pectoral fin to form a strong defensive 
 spine, not infrequently connected with a 
 poison gland. In some teleosts, e.g., 
 eels, the pelvic fin may be lacking. 
 The fins of the dipnoi are easily under- 
 stood by comparison with a biserial fin 
 like that of Xenacanthus (fig. 1 24) . The 
 axial part has been elongated and in 
 Ceratodus it bears biscerial radialia, 
 while in Protopteriis and Lepidosiren 
 only the axis persists. 
 
 Embryology tells little as to the primitive 
 
 Fig. 125. — Cartilage skeleton of 
 shoulder girdle and left pectoral fin of ,. . r ^ • ^ ^ • e • ^ 
 
 larval Polypterus, after Budgett, bvf, condition of the ichthyopterygium, for in the 
 foramina for blood-vessels; c, cora- precartilage stage the condensation of mesen- 
 
 ^r^^.Lft'J;^'^':tt r^^fT""' •'^''' chyme for the skeleton of the fin forms a con- 
 mesopteryguim; WCT, metapterygium; -' 
 
 pro, protopteryigum; r, developing tinuum which later becomes broken into the 
 radialia; s, scapula. separate parts (fig. 125). 
 
 The legs (chiropterygia) of all tetrapoda are essentially alike (fig. 
 108). Each consists of several regions, comparable in detail with each 
 other. The proximal is the upper arm (brachium) or thigh (femur) 
 containing a single bone, the humerus or femur in the fore and hind 
 limb respectively. The next region, the forearm (antebrachium) 
 or shank (crus), contains two bones, a radius or tibia on the preaxial 
 and an ulna or fibula on the postaxial side. Next follows the podium, 
 the hand (manus) in front, the foot (pes) behind, each consisting of 
 three portions. The basal podial region, the wrist (carpus) or ankle 
 (tarsus) consists of several small bones; the second division (metapo- 
 dium) is the palm (metacarpus) or instep (metatarsus) and lastly 
 come the fingers or toes (digits) , each digit consisting of several bones. 
 
SKELETON. 
 
 117 
 
 the phalanges. These separate parts are included in the accompany- 
 ing table, in which the terms given to the separate elements of the wrist 
 and ankle of man are included. 
 
 Fore Limb 
 Upper arm (Branchixim) 
 
 Fore arm (Antebrachium 
 
 Naviculare 
 
 (Scaphoid) 
 Lunatum 
 Triquetrum 
 
 Basi- 
 podium 
 
 Wrist 
 (Carpus) 
 
 Palm 
 
 (Metapo- 
 
 dium) 
 
 Fingers (Phalanges) 
 
 Pisiforme 
 
 Multangulum 
 
 majus 
 (Trapezium) 
 Multangulum 
 
 minus 
 (Trapezoides) 
 Capitatum 
 
 Hamatum 
 
 (arm) 
 
 Humerus = Femur 
 / Radius = Tibia 
 
 \ Ulna=Fibula 
 
 Radiale =Tibiale 
 
 Intermedium = Intermedium 
 
 Ulnare = Tibiale 
 
 Centrale » + 2 = Centrale ' + 2 
 
 Carpale ' = Tarsa le ' 
 
 Carpale2 = Tarsale» 
 
 Carpale' =Tarsale' 
 j Carpale* = Tarsale* \ 
 \ Carpale* = Tarsale5 / 
 
 Metacarpale ^— * = Metatarsale ' 
 Digits *-** = Digits'-' 
 
 Hind Limb (leg) 
 
 Astragalus 
 (Talus) 
 
 Calcaneus 
 Naviculare 
 
 pedis 
 (Scaphoid) 
 
 Cuneiform* 
 
 Cuneiform* 
 
 Cuneiform' 
 Cuboides 
 
 Thigh 
 \ Shank 
 / (Cms) 
 
 Basi- 
 podium 
 
 Ankle 
 (Tarsus) 
 
 (Metapo- 
 dium) 
 
 Instep 
 (Phalanges) Toes 
 
 The basal podial region, which is nearly typical in some reptiles, 
 urodeles and man, consists of three rows of bones, a proximal of three 
 bones, a radiale or tibiale on the anterior side, an ulnare or fibulare 
 on the other, and an intermedium between them. The distal row 
 consists of five carpales or tarsales, numbered from the anterior side. 
 The third row is composed of one or two centrales between the other 
 rows. The metapodials and the digits, also numbered from one to 
 five, have, in some cases special names, the thumb (digit I) being 
 the poUex, the corresponding great toe being the hallux, the fifth 
 digit being called minimus. 
 
 From this typical condition all forms — legs, arms, wings — are 
 derived by modification, fusion and disappearance of parts. The 
 more distal a part the more variable it is; reduction takes place on the 
 margins of the appendage, the axial portions being the last to disappear. 
 Occasionally in various groups (amphibia, mammals) there occur what 
 
Il8 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 are interpreted as rudimentary additional digits — prehallux, prepol- 
 lex, postminimus — but their status is uncertain. There are also certain 
 membrane bones developed in the appendages including the patella 
 (knee-pan) in some reptiles, birds and many mammals, in the tendon 
 that passes over the knee-joint, the fabellae in the angle of the knee of 
 a few mammals, and the pisif orme in the carpus of man and some other 
 mammals. 
 
 In the ancestral limb, as exemplified in the urodeles, the basal 
 joint was directed horizontally at right angles to the axis of the body, but 
 higher in the scale it approaches the sagittal plane and in such a way 
 that the angles of the fore and hind limbs open in opposite directions. 
 Besides there is frequently a torsion of the bones 
 of the forearm (fig. 127) or shank on each other. 
 The lower amphibians have nearly typical legs, 
 although, as in Siren and Amphiuma, they may be 
 greatly reduced, while in some stegocephals and 
 
 Fig. 126. Fig. 127. 
 
 Fig. 126. — 'Tarsus of Geotriton,. after Wiedersheim, showing the arrangement of the 
 metapodial elements, c, centrale; /, fibulare; F, fibula, i, intermedium; t, tibiale; T 
 tibia; 1-5, tarsales. 
 
 Fig. 127. — Torsion in developing human arm, after Braus. m, r, ulna and radius; 
 dotted line, course of radial nerve. 
 
 the gymnophiones they are entirely lacking. In the anura the radius 
 and ulna or tibia and fibula are frequently fused and the tarsals 
 elongated. 
 
 The most marked feature of the reptilian limb is the occurrence of 
 an intratarsal joint, the motion of the foot upon the leg being largely 
 between the two rows of tarsal bones, instead of between tarsus and the 
 bones of the shank (fig. 128). There is also a greater range of form 
 than in the amphibia. Limbs are lacking in snakes and some lizards; 
 on the other hand there is a great increase in the number of phalanges, 
 correlated with a shortening of the proximal bones in the plesiosaurs, 
 which reaches its extreme in the ichthyosaurs where there may be a 
 hundred phalanges in a digit. The wings of the pterodactyls are re- 
 
SKELETON. 
 
 119 
 
 markable for the great development of the fifth digit (elongation of the 
 phalanges) as a support for the wing; the other digits are more normal. 
 
 Fig. 128. 
 
 -Hind leg of snapping turtle {Chelydra) showing intratarsal joint at i. h, 
 humerus, r, radius; u, ulna; I-V, digits. 
 
 The wings of birds (fig. 55) are even more modified. Until the 
 carpus is reached the structure is approximately normal, but the carpal 
 bones are greatly reduced by fusion, while the metacarpals and digits, 
 extensively modified, number only 
 three. Development shows that the 
 first digit is entirely lost and that the 
 fifth metacarpal, which is present in 
 the embryo, fuses early with the 
 fourth, so that the digital formula 
 is II, III, IV. There is also an ex- 
 tensive fusion of the bones of the 
 tarsus and pes. The ankle-joint is 
 markedly intratarsal, the basal row 
 of tarsal bones fusing with the tibia 
 (the fibula is reduced) to form a 
 *tibiotarsus,' while the tarsales 
 have united in the same way with 
 the fused metatarsals, forming a 
 *tarso-metatarsus' (fig. 129). 
 The toes are rarely more than four 
 in number, the first apparently lack- 
 ing, and as a rule the number of 
 phalanges increases from two in 
 digit II to five in digit V. Many 
 birds have the toes reduced to 
 three and in the true ostriches to two. 
 
 In the mammals the limbs, especially the fore limbs, exhibit a con- 
 siderable range of modification. Thus in the primates the skeleton is 
 
 Fig. 129. — ^Foot of parrot {Psittacus 
 amazanicus),/, femur; fb, fibula; p, patella; 
 tm, tarsometatarsus; tt, tibiotarsus; //-F, 
 digits. 
 
I20 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 nearly typical, but there is a marked power of rotation of the foot and 
 especially of the hand by the motion of the lower end of the radius 
 around the ulna. There also the appendages may form grasping 
 organs, both features being found to a less extent in several lower groups. 
 In the bats digits II to V are greatly elongated (either metacarpals or 
 phalanges may be lengthened) to support the wing, the first digit remain- 
 ing normal. In the whales and sirenians the basal parts of the fore 
 limb are greatly shortened, while there is a multiplication of the pha- 
 langes, recalling that of the plesiosaurs. The hind limb is entirely 
 lacking in the sirenians and some of the whales; in other whales there 
 are two vestigial bones ( Pfemur and tibia) imbedded in the muscles of 
 the trunk. 
 
 The mammalian humerus is frequently perforated by a (supra- or 
 entepicondylar) foramen passing through the inner lower end, a 
 feature found elsewhere only in some theriomorphs. In many un- 
 gulates the ulna is reduced and may be fused with the radius; elsewhere 
 it is well developed. Even where reduced it always bears on its prox- 
 imal end a strong olecranon process, extending beyond the elbow- 
 joint for the attachment of the extensor muscles of the lower limb. 
 The femur bears a varying number (up to three) of prominences or 
 trochanters for the attachment of muscles. The fibula resembles the 
 ulna in its tendency to reduction. The patella (p. ii8) at the knee- 
 joint is analogous to the olecranon process, though it never joins the 
 other bones. 
 
 The details of the modification of the feet cannot be described here. 
 The ankle-joint is never intratarsal but always between tarsal and 
 crural bones. There is considerable variety in the extent to which the 
 bones of the feet rest upon the ground. In the plantigrade foot, as in 
 the bear and man, the sole of the foot includes the metapodial bones; in 
 the digitigrade forms, like the dog and cat, the sole includes only the 
 distal phalanges, while in unguligrades (cow, horse) the weight of the 
 body is supported on the hoofs (p. 27) developed on the upper (ante- 
 rior) surface of the distal phalanges. There is frequently a reduction 
 of the digits, reaching its extreme in the horse where only digit III 
 persists in a functional condition. 
 
 THE CCELOM (BODY CAVITIES). 
 
 The ccelom includes all of the primitive cavities, right and left, 
 enveloped by the mesothelium (p. 10). With the division of the 
 
CCELOM. 
 
 121 
 
 walls into epimere, mesomere and hypomere, the coelom undergoes a 
 corresponding division. That portion in the epimere is divided into 
 a series of cavities in the myotomes (myocoeles), which are eventually 
 obliterated (p. 126); the portions in the mesomere persist only as the 
 lumina of the excretory organs and their ducts, described under the 
 urogenital system; while that part of the original coelom in the hypomere 
 gives rise to all of the permanent body cavities of the adult. 
 
 The hypomeres gradually descend between the ectoderm and the 
 entoderm (fig. 130) until their lower margins meet, ventral to the diges- 
 tive tract. In this way the latter 
 becomes surrounded by a pair of 
 cavities, the splanchnocoeles or body 
 cavities of the adult. Each is 
 bounded by epithelium, the tunica 
 serosa, in which an outer or somatic 
 wall is turned toward the ectoderm, 
 while the inner or splanchnic wall 
 adjoins the alimentary canal. Later, 
 when the muscle plates extend down- 
 ward (fig. 135), they unite ectoderm 
 and serosa into the outer body wall, 
 the somatopleure, while the invasion 
 of mesenchyme unites the splanchnic 
 serosa with the entoderm into a similar 
 splanchnopleure. 
 
 Mesenteries. — As has just been 
 stated the walls of the two ccelomic 
 cavities meet below the digestive tract, thus forming a double membrane 
 running lengthwise of the body and binding the alimentan^ canal to the 
 ventral body wall. This membrane is called the ventral mesentery. 
 In a similar way the splanchnic walls meet above the digestive tract 
 forming a dorsal mesentery. These mesenteries are eventually more 
 than double serosal \valls, since mesenchyme comes in between, uniting 
 them and affording a tissue through which blood-vessels, lymphatic 
 vessels and nerves can reach the digestive organs. 
 
 For convenience of reference different parts of these mesenteries have received 
 special names, according to the organs supported. The ventral mesentery 
 usually almost entirely disappears, only a small portion persisting in the region 
 of the liver, the mesohepar, which, in the ichthyopsida may carry blood-vessels 
 
 Fig. 130. — Diagram of early meso- 
 derm, showing the zones, e, h, mm, epi-, 
 hypo-, and mesomeres, the walls of the 
 coelom, dm, vm, to form the dorsal 
 and ventral mesenteries. A, alimen- 
 tary canal; e, ectoderm; so, sp, soma- 
 topleure and splanchnopleure. 
 
122 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 to that organ. The dorsal mesentery is usually more complete, but it is 
 interrupted in various groups. Its regions are called mesogaster, mesentery 
 proper, mesocolon, mesorectum, etc., accordingly as they support stomach, 
 small intestine, colon and rectum. Except in the cyclostomes the alimentary 
 canal is bent on itself and the bends are connected by similar membranes, here 
 called omenta. These also have special names. Thus the gastrohepatic 
 omenttmi (small omentum) connects stomach and liver; then there are gastro- 
 splenic, doudeno-hepatic omenta, etc., while in mammals there is a great 
 omentimi, a double fold of mesogaster and mesocolon which connects the stomach 
 
 Fig. 131. — Diagrammatic section of a vertebrate to show the relation of the body walls, 
 etc. av, aorta; c, coelom; e, ectoderm; ep, epaxial muscles; ^, gonads; ^a, haemal arch; /^/), 
 hypaxial miiscles; i, intestine; mes, mesentery; w, nephridium; 0, omentum; r, rib; jo, 
 somatopleure; sp^ splanchnopleure; v, vertebra. 
 
 with the colon. This forms a large sac, the bursa omentalis, which opens into 
 the rest of the body cavity by a small foramen of Winslow (foramen epiploicum) 
 near the hinder end of the liver. 
 
 Homologous structures are formed in connection with other organs. Thus 
 in the formation of the heart there are formed temporary membranes, the meso- 
 cardia, connecting it with the walls of the pericardium; while in the mammals a 
 mediastinum, between the two pleural cavities binds the pericardium to the ventral 
 body wall. Frequently the reproductive organs project so far into the body cavity 
 that the serosa meets behind them, forming similar supports, mesovaria for the 
 ovaries, mesorchia for the testes. 
 
 The primitive body cavity extends from a point just behind the head 
 back to the vent. It soon becomes divided into two cavities. Just 
 in front of the liver a pair of blood-vessels, the Cuverian ducts, enter 
 the heart from the sides. These arise in the ventral body wall but soon 
 ascend, carrying the serosa before them. In this way they form a 
 
CCELOM. 
 
 123 
 
 transverse partition, the septum transversum, attached to the anterior 
 wall of the liver, which cuts off an anterior pericardial cavity, con- 
 taining the heart, from the posterior part (metacoele) of the body cavity. 
 In many lower vertebrates the septum is not complete, but one or more 
 openings (pericardio-peritoneal canals) connect the pericardium 
 with the metacoele. 
 
 In the mammals a second partition, the diaphragm (p. 135), cuts 
 off another pair of (pleural) cavities from the metacoele. Traces of 
 similar structures occur as low as the amphibia; their homology with 
 the mammalian diaphragm is not always certain, but in some cases the 
 
 Fig. 132. — Diagram showing the relations of the coelomic cavities (black) in A, fishes, 
 B, amphibians and sauropsida; and C, in mammals; H, heart in pericardial coelom; 
 L, liver; P, lungs in C in pleural ccelom; S, septum transversum; D, diaphragm. 
 
 parts concerned have the same nerve supply. The development of the 
 diaphragm is very complicated and can be stated here only in outline. 
 It involves in part the septum transversum, in part is a new formation. 
 At first a part of the metacoele extends forward, dorsal to the pericardial 
 cavity and alimentary canal, and into this the lungs protrude as they 
 are developed. Then a pair of muscular folds arise from the dorsal 
 surface of the metacoele, posterior to the lungs; these grow downward 
 until they meet the septum adjacent to the attachment of the liver, 
 cutting off a pair of pleural cavities containing the lungs, from the 
 rest of the metacoele, now known as the peritoneal cavity. With 
 increase of the lungs in size the pleural cavities increase, insinuating 
 themselves laterally beween the pericardium and the body wall, and 
 eventually reaching the ventral side, where the two are separated by 
 their two walls, the ventral mediastinimi. From the original folds 
 the dorsal muscles of the diaphragm are derived; the ventral come 
 from the rectus muscles of the ventral abdominal wall. The dia- 
 
124 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 phragm undergoes many shiftings of position before reaching its final 
 place. 
 
 The tunica serosa lining the various divisions of the splanchnocoele has special 
 names in each. Thus the pericardial and pleural cavities are lined by peri- 
 carditun and pleura respectively, that portion of the pericardium covering the 
 heart being sometimes called the epicardium. The metacoele or peritoneal cavity 
 is lined by the peritoneum. 
 
 The metacoele is not always cut off completely from the external 
 world. In the lower vertebrates the urinary ducts frequently open into 
 the body cavity by the nephrostomes (fig. 133), and in these and even 
 in the mammals the oviducts of the female connect the cavity with the 
 
 'y'^y'y'^yy^^ ^^yyyy'yy^^^^^^':^^ 
 
 Fig, 133. — Diagram of possible connection of coelom with the exterior, modified from 
 Bles. c, ccelom; c/, cloaca; g, glomerulus of kidney; i, intestine; «, nephrostome; pa, 
 porus abdominalis. 
 
 exterior. In many fishes there are pori abdominales leading from the 
 metacoele to the outside near the vent. These may be single or paired 
 and are found in cyclostomes, many elasmobranchs and teleosts, 
 ganoids, and dipnoi. None are known in amphibia, birds or mammals, 
 but in turtles and crocodilians so-called peritoneal canals occur, 
 usually ending blindly in chelonians, but emptying into the cloaca 
 in the crocodiles. These may be homologous with the abdominal 
 pores, but only the development can settle the question. In some 
 fishes the pores serve for the escape of the genital products; in other 
 animals their function is uncertain. 
 
 THE MUSCULAR SYSTEM. 
 
 Practically all motion in vertebrates is caused by muscles arising from 
 the mesoderm. While other cells may have a certain power of chang- 
 ing shape, the muscle cells possess this in a marked degree, and so 
 that they may cause the greatest amount of motion in the parts to which 
 they are attached, they are very long, stimulation causing them to 
 contract in length and at the same time to increase in diameter. 
 
MUSCULAR SYSTEM. I25 
 
 There are two kinds of muscles which differ in origin, histological 
 appearance, physiological action and distribution. The smooth 
 muscles, the appearance of which has been described (p. 20), arise 
 from the mesenchyme and are not under control of the will, but are 
 innervated by the sympathetic nerv^ous system. Their action is much 
 slower than that of the other type. They are found in the skin, in the 
 walls of blood-vessels and of the alimentary canal, and in the urogenital 
 system. Occasionally they occur as isolated fibres, but frequently 
 they form sheets or bands, sometimes of considerable thickness. 
 
 In the alimentary tract they are arranged in two layers in the straight 
 parts of the tube, an outer layer of fibres which run longitudinally, 
 and inside this a layer of circular muscles. In enlargements of the 
 tube this regularity is interrupted and the course of the fibres is more 
 irregular. The circular muscles, by their contraction, lessen the diam- 
 eter of the canal, at the same time causing it to elongate, while the 
 longitudinal fibres shorten it and cause it to increase in diameter. In 
 the blood-vessels there are only circular fibres, the enlargement of the 
 lumen being caused by the internal blood pressure. 
 
 The striped muscles are derived from the walls of the coelom and 
 hence are of mesothelial origin. Excepting those of the heart (to be 
 mentioned below) and some of those at the anterior end of the alimentary 
 canal, they are under control of the will and are supplied by the motor 
 nerves of the brain and spinal cord. They are also able to contract, 
 more rapidly than the smooth muscles. The striped muscles make up 
 the great mass, of the musculature — the 'flesh' — of the body. They 
 occur in the body walls, organs of locomotion, the head, diaphragm 
 and the anterior part of the digestive canal. 
 
 The voluntary muscles are derived in part from the somites (myo- 
 tomes), in part from the lateral plates, the latter furnishing the vis- 
 ceral muscles, including those of the head (except the eye muscles and 
 the sternohyoid and its derivatives in the higher vertebrates) and those 
 of the heart. The heart muscles, the development of which is traced in 
 the account of the circulatory system, differ from the other striped 
 muscles in the uninucleate condition of their short and usually branched 
 cells, while, physiologically, they are involuntary in character. 
 
 THE PARIETAL MUSCLES. 
 
 After the myotomes are cut off from the rest of the coelomic walls 
 (p. 14) each consists of a closed sac, containing a part of the coelom 
 
126 
 
 COMPARATIVE MORPHOLGY OF VERTEBRATES. 
 
 (myocoele) and an inner (splanchnic) and an outer (somatic) wall 
 The cells of the splanchnic wall rapidly increase in number and size, 
 thus tending to obliterate the myocoele. At the same time they be- 
 come rearranged, so that, instead of forming a cubical or columnar 
 epithelium, they have their long axis parallel to the long axis of the body 
 
 Fig. 134. — Myotomes of Amhlystoma developing into muscle fibres, ec, ectoderm; wy, 
 myocoele; ms, mesenchyme; so, somatic layer which will form corium. 
 
 (fig. 134), each becoming multinucleate. Gradually the mass of the 
 protoplasm becomes converted into contractile substance and the cell 
 is converted into a muscle fibre, the nuclei being in the interior in the 
 lower vertebrates, on the surface of the fibres in the mammals. In this 
 way the splanchnic wall of each myotome is converted into a muscle; 
 
 Fig. 135, — ^Diagram of descending myotomes, c, coelom; g, gonad; m, splanchnic wall 
 of myotome developing into muscles; mc, myocoele; p, peritoneum; pd, pronephric duct; 
 so, somatic wall of myotome; v, ventral border of myotome. 
 
 hence there are as many pairs of these primitive muscles as there were 
 of myotomes. The somatic wall of the myotome does not participate 
 in the muscle formation, but is gradually changed into mesenchyme 
 and eventually gives rise to the corium of the skin. Mesenchyme also 
 invades the spaces between the successive myotomes, develops into 
 
MUSCULAR SYSTEM. 
 
 127 
 
 fibrous connective tissue, and forms the ligamentous connections 
 (myosepta, myocommata) between the muscles of a side. This 
 primitive condition is readily recognized in the trunk and tail of the 
 lower vertebrates, and even in the adults of the more modified birds 
 and mammals the original segmentation can be traced in the inter- 
 costal and rectus abdominis muscles. At first the myotomes lie 
 at about the level of the notochord and spinal cord, but with growth 
 they extend upward and to a greater extent downward, insinuating 
 themselves between the skin and the w^alls of the ccelom and thus 
 
 Fig. 136. — Head of embryo dogfish {Acanthias) seen as a transparent object, showing 
 the preotic mesodermal somites, with dotted outlines, as a, i, 2, and 3. b^-b*, gill clefts, 
 the fifth not yet open; e, eye; oc, otic capsule; p, epiphysial outgrowth; s, spirade; V, tri 
 geminal, VII, facial-acustic; IX, glossopharyngeal; X, vagus nerves. 
 
 forming part of the somatopleure. The downward growth continues 
 imtil the muscles of the two sides all but meet in the mid-ventral line, 
 the intervening space being occupied by connective tissue, the linea 
 alba of the adult. 
 
 In the fishes the trunk and tail muscles formed in this way become 
 divided horizontally into dorsal and ventral portions, the epaxial and 
 hypaxial muscles, the line of division which follows more or less 
 closely the lateral line, bein g marked by a partition of connective tissue 
 already mentioned (figs. 30, 131). These plates of muscle do not retain 
 their flat ends in the adult, but one end becomes conical and fits into a 
 corresponding hollow in the next plate. In the tail of the amphibia 
 
128 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES, 
 
 epaxial and hypaxial muscles are clearly recognizable, but farther 
 forward the hypaxials are greatly reduced, and in the amniotes the 
 reduction is carried so far that the hypaxial muscles, greatly modified, 
 can only be recognized in the cervical and pelvic regions. 
 
 In the head the developmental conditions are more complicated 
 than in the trunk, our information being most complete with regard to 
 the ichthyopsida. Here, in the region which develops into the head, 
 ten coelomic pouches are developed (in amniotes the number is appar- 
 ently twelve) . These are known by number, except that the most anter- 
 ior, which was not known when the numbers were applied is called A. 
 These coelomic cavities (also known as head cavities) differ from the 
 
 myotomes farther back in having no undivi- 
 ded portion of the coelom below, correspond- 
 ing to the hypomeral zone, a difference possi- 
 bly due to the existence of visceral clefts in 
 this region (fig. 136). 
 
 Four of these cavities lie in front of the ear. 
 Of these A disappears completely, its cells 
 joining the mesenchyme, while the other three 
 give rise to the 'eye muscles' which move 
 the eye-ball. Without going into all of the 
 details, i, which lies in front of the mouth, 
 gives rise to the superior oblique muscle ; 2, 
 which lies in the region of the jaws, forms four 
 muscles, the inferior oblique and three of 
 the rectus muscles (in some forms also a 
 retractor bulbi), while 2, in the hyoid region, develops the external 
 rectus. This method of origin explains the distribution of the eye- 
 muscle nerves to be described later, each nerve supplying only the 
 derivatives of a single myotome. Several of the other head myo- 
 tomes disappear in development, while the posterior form the so-called 
 hypoglossal musculature (fig. 138). 
 
 In the above account there is given merely the origin of the con- 
 tractile tissue of the muscles. To this other parts of connective tissue 
 are added. Mesenchyme cells invade the masses of muscle fibres, 
 forming envelopes (perimysium) which bind the fibres into bundles 
 (fasciculi) which, in turn, are united by other envelopes, the fascia. 
 These connective-tissue envelopes are extended beyond the contractile 
 tissue and form the cords or tendons by which the muscle is attached to 
 
 Fig. 137. — Diagram of 
 the eye muscles of the right 
 eye, seen from the medial 
 side, er, external rectus ; ?/>, 
 inferior rectus; io, inferior 
 obhque; itr, internal rectus; 
 so, superior oblique ; sr, supe- 
 rior rectus; ///, coulomotor; 
 IV, trochlearis; VI, abducens 
 nerves. 
 
MUSCULAR SYSTEM. 1 29 
 
 other parts. One point of attachment, the origin, is fixed, that 
 to the part to be moved is called the insertion. Tendons may be long and 
 slender, allowing the muscle to lie in or near the trunk, while the part 
 to be moved is in the appendage. Again they may form broad flat 
 sheets (aponeuroses), and these may occur not only at the ends but in 
 the middle of a muscle. Not infrequently parts of tendons may ossify, 
 as in the patella or in the 'drum-stick' of the turkey. Small rounded 
 ossifications of this kind are called sesamoid bones. In a few cases 
 the parietal muscles are without attachment, but form rings which are 
 used to diminish the size of an opening (sphincter muscles). 
 
 Muscles vary greatly in shape. They are usually short and flat in the trunk, 
 prismatic or spindle-shaped in the appendages. They may be simple or they 
 may have several 'heads' or points of origin (biceps, triceps, etc.), or several 
 points of insertion as in pinnate or serrate muscles. Again, there may be two 
 or more contractile portions (bellies) in a muscle, separated by a tendon or 
 aponeurosis. 
 
 Usually muscles are arranged in antagonistic groups, the action of one being 
 the opposite of its antagonist. Thus there are flexors to bend a limb, extensors to 
 straighten it; elevators to close the jaw, depressors to open it; sphincters working 
 against dilators, etc. 
 
 Only a few points in the progressive modifications of the primitive 
 musculature described above can be mentioned here, partly from lack of 
 space, partly from deficient knowledge. There are great difl&culties in 
 tracing exact homologies through the different groups of vertebrates, 
 on account of their very different fimctions in the separate classes and 
 their great variability, even in the same family. The best test of 
 homology is nerve supply, every muscle derived from any one myotome 
 being innervated by branches of the nerve originally connected with 
 the segment, as is beautifully illustrated in the case of the eye muscles 
 as mentioned above. Next in importance are origin and insertion of 
 the muscles, while the work done by the muscles is of little value. 
 Differentiations from the primitive condition may take place in various 
 ways. A single muscle may split into layers or it may divide longi- 
 tudinally into two or more distinct muscles, or transversely into two 
 successive portions. On the other hand, two muscles, different in 
 origin, may fuse, while with loss of function of a part, its muscles may 
 degenerate or entirely disappear. Muscles may wander far from their 
 point of ontogenetic origin and become connected with parts widely 
 remote, a condition strikingly illustrated in the facial muscles of the 
 9 
 
I30 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 higher mammals, where nerve supply still shows the original history. 
 In the ichthyopsida the trunk muscles clearly show their myotomic 
 origin, but even here there are tendencies to division and specialization. 
 The ventral muscles on either side of the body cavity of the amphibia 
 (fig. 140) are divided into a lateral oblique and a medial rectus sys- 
 tem, the rectus muscles of the two sides being separated by the linea 
 alba already referred to. The rectus muscles, in turn, become divided 
 into successive groups, a rectus abdominis in the abdominal region, 
 extending from the pelvis to the sternum; a sternohyoid from the 
 
 hyp.n 
 
 Fig. 138. — Diagram of muscle segments in head of embryo vertebrate, based upon a 
 shark, after Neal. The anterior myotomes tend to divide into dorsal and ventral moieties; 
 persistent myotomes lined, transient writh broken lines; central nervous system dotted, 
 nerves black, a, premandibular somite; ah, abducens; nerve, hyp, hypoglossal musculature; 
 hypn, hypoglossal nerves; om, oculomotor nerve; sp, spiracle; 1-6, first six somites (4, 5, 6, 
 functional in Petromyzon); I-VIII neuromeres. 
 
 Sternum to the hyoid bone, and a geniohyoid from the hyoid to the tip 
 of the lower jaw. The oblique region is also divided into three layers 
 (obliques and transversus) characterized by the direction of the fibres. 
 In the higher vertebrates, with the appearance of well developed ribs, 
 the oblique muscles furnish the two layers of intercostal muscles, 
 extending from rib to rib, and in front of the ribs they form the scalene 
 muscles, extending from the ribs along the side of the neck, and the 
 sternocleidomastoid, from the breast bone and clavicle to the skull. 
 In the non-placental mammals a strong pyramidalis muscle extends, 
 ventral to the rectus, from the inner side of the marsupial bones to the 
 sternum, but disappears with these bones. 
 
 The dorsal muscles are more conservative, undergo less modifica- 
 tion than those just mentioned, and always show, more or less clearly, 
 their metameric nature. They become connected with various parts 
 of the vertebrae and with the ribs, and are correspondingly divided into 
 
MUSCULAR SYSTEM. I3I 
 
 different groups. Thus the spinales connect the spinous processes, the 
 transversales the transverse processes of the successive vertebrae, 
 while the transverse -spinales extend from the transverse process of 
 one vertebra to the spinous process of the next. In the higher verte- 
 brates the anterior spinaUs, connecting the first vertebra with the skull, 
 is divided into several rectus capitis muscles. The longissimus 
 dor si group extends from the pelvis to the head, lying on either side in 
 the angle between spinous and transverse processes. It may be differen- 
 tiated into separate muscles — a longissimus dorsi proper in the lumbar 
 region, an ileo-costalis inserted on the dorsal part of the ribs, and a 
 longissimus capitis along the side of the neck to the temporal region 
 of the skull. 
 
 The muscles which move the appendages are divided into the 
 intrinsic, which are located in the limb itself and have their origin 
 either from the bones of the Umb or from the supporting girdle, and the 
 extrinsic, which have their origin on the trunk and are inserted on the 
 girdle or the base of the limb. The latter move the limb as a whole, 
 
 Fig. 139. — Budding of muscles of appendage from myotomes in Pristiunis^ after Rabl 
 6, muscle buds; wy, myotomes. 
 
 while the intrinsic bend the limb on itself. As would be expected from 
 the motions of the fins, the intrinsic muscles are hardly noticeable in the 
 fishes, the various movements being accomplished by the extrinsic 
 group. These latter are divided into protractors which draw the 
 member forward; retractors which pull it back against the body; 
 levators which lift it and depressors which pull it down. 
 
 In those vertebrates which are sufficiently known the first traces of the develop- 
 ment of the musculature of the appendages are the appearance of two buds (fig. 
 139) from the ventral border of a varpng number of myotomes in the region of the 
 developing limb. These buds proliferate cords of cells which soon lose their 
 distinctness and form a blastema from which the intrinsic muscles arise, the 
 definitive muscles being innervated by as many spinal nerves as there are contribut- 
 ing myotomes. The extrinsic muscles arise directly from the myotomes. 
 
132 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 With the development of the paired appendages into organs for the 
 support of the body (tetrapoda) the skeleton of the leg is converted into 
 a series of levers, and the intrinsic muscles are correspondingly dif- 
 ferentiated and developed. Details cannot be given here as there are 
 so many modifications, but they may be grouped as flexors, which 
 bend the limb or its parts; extensors which straighten it, and rotators 
 which turn it on its axis. These undergo the most modification in the 
 peripheral regions, the muscles of the upper arm and thigh being more 
 constant in character and position. Even more constant are the ex- 
 trinsic muscles, which may be grouped as in fishes. Most prominent 
 
 ^^C" 
 
 Fig. 140. — Superficial muscles of anterior part of Salamandra ntaculata, after Fiir- 
 bringer. a, anconeus; bi, humero-branchialis inferior (biceps); bs, levator scapulae; cmc, 
 cucularis; dtr, dorso-trachealis; dg, digastric; ds, dorsalis scapulae; eo, external oblique; 
 Id, latissimus dorsi; m, petro-tympano-maxillaris (masseter); mh, mylohyoid; pc, pectoralis; 
 ph, procoraco-humeralis; ra, rectus abdominis; spc, supracoracoid. 
 
 of the levators of the fore limb are the trapezius and levator scapulae 
 muscles, while the pectoralis and serratus anterior act as depressors; 
 the sternocleidomastoid and the levator scapulae anterior act as 
 protractors, the pectoralis minor and the latissimus dorsi are their 
 antagonists. In the pelvic region the extrinsic muscles are less dif- 
 ferentiated in function. The pectineus and adductors act as pro- 
 tractors, the pyriformis counteracts them; the limb is drawn toward 
 the middle line by a pubofemoralis, while the gluteus muscle acts as 
 a retractor and elevator. 
 
 THE VISCERAL MUSCLES. 
 
 In the gill-bearing vertebrates a special system of muscles is devel- 
 oped in connection with the visceral arches, which have to open and 
 close the visceral clefts, including the mouth. With the loss of the gills 
 some of these muscles are lost while others become changed in function, 
 several retaining their connection with the hyoid. These visceral 
 muscles may be divided into two sets according as they are derived 
 
MUSCULAR SYSTEM. 
 
 ^33 
 
 from muscles which originally ran in a transverse (circular) or in a 
 longitudinal direction. 
 
 To the first category belong the epibranchial muscles, the sub- 
 spinales and interbasales, which lie in the dorsal part of the branchial 
 region, while the coraco-arcuales are in the ventral or hypobranchial 
 half. The most anterior of the circular group are those which open 
 (digastric or depressor mandibulae) or close (adductors) the mouth, 
 and the mylohyoid which extends between the two rami of the lower 
 
 Fig. 141. — Dorsal and ventral head muscles of the skate {Raid), after Marion; the dorsal 
 muscles more deeply dissected on the left side, the ventral on the right and, lateral man- 
 dibular adductors; amm, medial mandibular adductors; csd, csv, dorsal and ventral con- 
 strictors; cm, coraco-mandibularis; chy, coraco hyoideus; chm, coraco-hyomandibularis; 
 cbr, coraco-brachialis; cac, common coraco- arcual; intbr, interbranchials; Us, superior 
 labial levators; Imi, levator of lower jaw; Ihm, hyomandibular levator; /r, levator of rostrum; 
 <r, trapezius; VII, seventh nerve; dm, depressor mandibulae (digastric). 
 
 jaw. Usually there are several adductors, known as masseter, tem- 
 poralis, pterygoideus, accordingly as they have their origin from 
 different parts of the skull. The longitudinal muscles are largely con- 
 fined to small slips which pass from one arch to the next. In the 
 amphibians these various muscles undergo considerable differentia- 
 tion, while in the amniotes this is in part carried farther, in part is re- 
 duced on account of the loss of branchial respiration and the degenera- 
 tion of the parts connected mth it. Hence the most noticeable of the 
 visceral muscles are those connected with opening and closing the 
 mouth. 
 
134 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 THE DERMAL MUSCLES. 
 
 The muscles already mentioned are connected with the skeleton, 
 but in the higher vertebrates a dermal musculature appears in which 
 the muscles are inserted in the skin, although they are derived from the 
 skeletal muscles. This system is poorly developed in the amphibia, 
 and increases in the reptiles and birds, where it serves to move the 
 scales, scutes and feathers. It is especially noticeable in the snakes, 
 where it is largely concerned in the movement of the scutes in 
 creeping. 
 
 The system acquires its greatest development in mammals. In the 
 marsupials, for instance, there is an extensive dermal musculature, the 
 panniculus carnosus, covering a large part of the body and the ap- 
 pendages. It is by means of this that various mammals twitch the 
 
 Fig. 142. — ^Principal dermal muscles of head of man. aa, as, auriculares anterior and 
 superior;/, frontalis; m, ma,sseter; oc, occipitalis; 00, orbicularis oris; op, orbicularis pal- 
 pebrarum; jfrw, platysma myoides; s, sternocleidomastoid; t, trapezius. 
 
 skin to dislodge insects, etc., while armadillos and hedgehogs roll them- 
 selves into a ball by means of a part of the layer. In the primates the 
 dermal muscles are restricted to the neck (platysma myoides) and the 
 head, all parts of them being supplied by the facial nerve belonging 
 primitively to the hyoid region. The platysma extends forward from 
 the neck and by growth and division gives rise to the muscles of ex- 
 pression — the orbiculares which close the lips and eyelids, the muscles 
 
MUSCULAR SYSTEM. I35 
 
 which lift lips, nose and lids and those which move the ears — muscles 
 which as a whole have their highest development in man (fig. 142). 
 
 THE DIAPHRAGM. 
 
 The diaphragm is a transverse voluntary muscle which crosses the 
 body cavity of the mammals just behind the pericardium and lungs. 
 Its muscles are in part derived from those of the back, in part from the 
 rectus muscles of the lower surface. Various attempts have been made 
 to recognize similar muscles in the lower vertebrates, in some cases 
 with considerable success. Its development is outlined in the section 
 on the coelom (p. 123). The diaphragm is dome-shaped and is attached 
 to the vertebral column and to the ribs. It is traversed by the 
 oesophagus and the large arterial and venous trunks. In some verte- 
 brates the muscular portion is confined to the margin, the centre being 
 membranous; in others the muscle fibres extend across it. Contrac- 
 tion of the muscles flatten it, thus enlarging the pleural cavities and 
 drawing air into the lungs, thus aiding in respiration. It is supplied by 
 the phrenic nerve. 
 
 ELECTRICAL ORGANS. 
 
 It is well known that the contraction of a muscle causes the dis- 
 charge of a minute amount of electrical energy, so it is not surprising 
 that in certain cases muscles are modified into electrical organs. The 
 known cases occur only in elasmobranchs and teleosts. The discharge 
 is w^eak in most species, but is strong in Torpedo and Gymnotus. In all 
 but Malapterurus the electrical organs are clearly modified muscles, situ- 
 ated in the head in Torpedo and Asiroscopus, in the trunk of Gymnotus , 
 and in the tail of Raia, the nerve supply being correspondingly varied. 
 Thus in Torpedo the seventh, ninth and tenth cranial nerves are con- 
 cerned, while in Gymnotus and the skates the supply is from the spinal 
 nerves. Malapterurus is peculiar in that the organ is in the integu- 
 ment and has been supposed by some to arise from modified glands. 
 It is more probable that here as elsewhere it is derived from the muscles, 
 as the organ is under control of the will; the development has yet to be 
 studied. This diversity of origin clearly shows that the electrical 
 functions have been separately acquired in the different species. 
 
 The organs are composed of a large number of electrical plates 
 
136 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 (electroplaxes) arranged at right angles to the axis of the primitive 
 muscle, each derived, where the history has been traced {Torpedo, 
 Rata), from a primitive muscle cell. In the typical condition each 
 plate consists of an outer electric layer, differentiated into a nervous side 
 and a so-called nutritive side, with a middle striated layer between them, 
 the latter in a few cases being weakly developed or absent. Nervous 
 stimulation is always by motor roots leading to the nervous layer, the 
 connexion corresponding to the nerve-end of a muscle cell. Numbers 
 
 Fig. 143. — Head of Astroscopus y-grcecum, after Dahlgren and Silvester. The dotted 
 line on right shows extent of electric organ, on the left the eye- muscles, and nerves as forced 
 out of place by the electric organ, ah, abducens; cil, ciliary nerve; e, eye; en, electric nerve; 
 n, xiaris; olf, olfactory nerve; om, oculomotor; op, optic nerve; re, rif, rini, rs, external, in- 
 ferior, internal and superior rectus muscles; rp, palatine nerve; so, superior oblique muscle; 
 //, trigeminal facial nerve. 
 
 of these electroplaxes are included in a connective-tissue compartment 
 with a gelatinous substance between them and all with their nervous 
 layer turned in the same direction. 
 
 In Torpedo the organ apparently is derived from part of the jaw 
 muscles and the prisms of plates are arranged vertically. In Astro- 
 scopus (fig. 143) it is supposed that the tissue comes from one of the eye 
 muscles, while in Gymnotus the ventral trunk muscles are concerned 
 and the columns of electroplaxes are horizontal. In the same fish the 
 discharge is always in the same direction, e.g,^ in Torpedo from below 
 upward. 
 
NERVOUS SYSTEM. I37 
 
 THE NERVOUS SYSTEM. 
 
 Nervous and sensory structures are closely related to each other, 
 
 ► and their distinction in the higher animals is the result of differentiation 
 
 among cells which were originally both nervous and sensory in character, 
 
 and it is in this broader sense that the term nervous structures is used 
 
 in these introductory paragraphs. 
 
 The nervous system primarily has to inform the animal of the con- 
 ditions, good and bad, in the environment, to correlate this information 
 and to regulate the motions so that advantage may be had of this knowl- 
 edge. These facts have determined several features of the nervous 
 system. Thus they have determined its origin in the ectoderm, the 
 outer layer of the body, which comes into relation with the external 
 world. Since this information has to be carried to internal parts, con- 
 ducting tracts or nerves have arisen, while the correlating fimction 
 has been localized in the body of the cells where incoming and out- 
 going tracts meet. 
 
 Most important of the primitive functions was the determination 
 of the character of the food, which would lead to the greater aggregation 
 of the nervous tissue around the mouth. As we have seen (p. ii) 
 the anlage of the central nervous system of the vertebrates occupies 
 such a position. aroimd the blastopore, or mouth of the gastrula, in the 
 form of the neural plate. As the external surface of the body is naost 
 exposed to injury, the nervous structures, with the closure of the blasto- 
 pore, have been protected by removal to a deeper position, through the 
 rolling of the plate into a tube. The closure of the blastopore brings 
 the two halves of the plate into close association with each other, making 
 it a bilateral structure. With bilaterality comes the tendency of one 
 end of the animal to take the lead, resulting in the concentration of 
 nervous and sensory structures at the anterior end, which first comes 
 in contact with foreign objects. In this way a brain has been special- 
 ized apart from the rest of the nervous system. 
 
 With the appearance of metamerism in the mesothelium and the 
 development of muscles from the myotomes there results a serial 
 repetition of motor nerves going to these, since each muscle must have 
 its own ner\^e supply, while sensory nerves are the result of the sinking 
 of the neural plate to a deeper position, as the sensory organs must be 
 largely in the skin. 
 
138 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 The close association of sensory and motor nerves in the trunk region of verte- 
 brates is not yet satisfactorily explained. The fact that in Amphioocus the two 
 kinds of nerves are independent of each other throughout their course shows that 
 the vertebrate condition is not primitive. 
 
 The infolding of the nervous plate has been described (p. 11) 
 and with that stage the present account begins. As the plate is broad- 
 est in front, the result is a larger anterior portion of the tube, the brain, 
 while the rest of the tube gives rise to the spinal cord. Brain and 
 cord constitute the central nervous system, while the nerves arising 
 from the brain and cord form the peripheral nervous system. 
 
 CENTRAL NERVOUS SYSTEM. 
 
 The two halves of the neural plate are separated by a median band 
 of non-nervous tissue, hence, when it is rolled into a tube, the mid- 
 ventral line — the floor plate — is thinner and differs from the side walls. 
 With the closure of the tube (fig. 144, A) a similar roof plate appears, 
 
 Fig. 144. — A, diagram of early spinal cord; B, later, showing increase in size and con- 
 sequent ventral fissure, c, central canal; e, ectoderm;/, floor plate; g, anlage of spinal 
 ganglion; nc, neural crest; r, roof plate; s, sulcus of Monro; v, ventral fissure. 
 
 while the lumen of the tube, the central canal, is oval in section, its 
 side walls, consisting of embryonic nervous tissue, being thicker than 
 roof or floor. 
 
 From the method of its formation it will be seen that the inner surface of the 
 neural tube is homologous with the outer surface of the general epidermis of the 
 body. The account given above is not exact for cyclostomes, teleosts and some 
 ganoids, where the neural plate forms a keel extending below the surface, in 
 which a central canal appears later, so that the final result is closely similar to 
 the typical condition. 
 
 The Spinal Cord. 
 
 From this simple tube the spinal cord of the adult is developed by 
 several modifications. The cells of the side walls proliferate rapidly, 
 
NERVOUS SYSTEM. 1 39 
 
 while those of roof and floor plates do not. As a result the sides soon 
 extend downward on either side beyond the floor plate, thus forming a 
 longitudinal groove, the anterior or ventral fissure of the cord, ex- 
 tending its whole length (fig. 144, B). The roof plate, on the other 
 hand, is at first carried upward by the growth, thus increasing the ver- 
 tical diameter of the central canal. Then the dorsal portion of the 
 tube closes up — the exact steps are uncertain — and later the tissue 
 along the line of closure is invaded by connective tissue and 
 blood-vessels, the result being the dorsal or posterior fissure of the 
 cord. 
 
 Besides the increase in the number of cells, the sides of the cord are 
 modified in other ways. Those cells which line the cavity — ^floor, roof 
 and sides — retain their epithelial character, never develop nervous struc- 
 tures, and are known as the ependjmia. The remaining cells become 
 differentiated in two directions. Some develop processes which sur- 
 round and support the others, these forming the neuroglia (*glia'), 
 while the others form the true nervous tissue — ganglion or nerve cells. 
 In the primitive condition the primitive nervT cells have no connexion 
 with distant points and hence cannot function. These connexions are 
 established by protoplasmic outgrowths from each cell, these forming 
 the fibres (dendrites or axons). Some of these extend directly out- 
 ward from the cord as nenxs (see below) , but others run for a greater 
 or less distance on the external surface of the cord, and since these 
 have medullary sheaths (p. 20) and are consequently white, these 
 tracts constitute the white matter of the cord, in contrast to the gray 
 matter formed by the cell bodies and neuroglia. 
 
 In sections of the adult cord the gray matter has something of the 
 shape of the letter |— | , its uprights forming the anterior and posterior 
 horns or cornua, while the cross-bar extends above and below the 
 central canal, from one side to the other. Physiological phenomena 
 and matters of nerve origin lead to the recognition of a lateral cornu 
 on either side, in the lateral prominence of gray matter. Since both 
 dorsal and ventral cornua approach the surface of the cord to connect 
 with the nerve roots described below, they divide the white matter into 
 three tracts on either side, known as the anterior, lateral and pos- 
 terior columns of the cord, each subdivided in the higher vertebrates 
 into several bundles. As this white matter is composed of nerve fibres, 
 it follows that these columns are the tracts by which nervous impulses 
 are carried to and from the brain, the anterior columns leading from, 
 
I40 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 the dorsal to the brain (ascending and descending tracts), while 
 impulses travel in both directions in the lateral columns. 
 
 In fishes the cord tapers regularly to the tip, but with the develop- 
 ment of legs in the terrestrial vertebrates the cord is considerably en- 
 larged where the nerves to the limbs are given off, the enlargements 
 bearing some ratio to the size of the limb. In the early stages the 
 spinal nerves leave the cord at right angles to its axis. With' growth 
 
 Fig. 145. — Diagram of spinal cord and nerve roots; gray matter shaded. A, L, P; 
 anterior, lateral and posterior columns; Ap, anterior pyramidal tract; B, column of Burdach, 
 cc, central canal; ca, cl, cp, anterior, lateral and posterior cornua; Dr, dorsal root; Fa, Fp, 
 anterior and posterior fissures; G, column of Goll; g, ganglion of dorsal root; /c, lateral 
 cerebellar tract; Ip, lateral pyramidal tract; 5«, spinal nerve. 
 
 the angle changes since the peripheral parts increase more in length 
 than does the cord. The result is that the posterior nerves are very 
 oblique and in the hinder part of the spinal canal they form a bimdle 
 of parallel nerves, the cauda equina. Another result of the unequal 
 growth may be the drawing out of the hinder end of the cord into a 
 slender non-nervous thread, the filum terminale. 
 
 The Brain. 
 
 The spinal cord throws light on the extremely complex brain. 
 Here, as in the cord, there is primitively a tubular structure, with roof, 
 floor and sides, and with nerves connected with it w^hich recall those 
 of the cord. In its development, as stated above, the brain from the first 
 is larger than the cord. It early has three enlargements separated by 
 two constrictions, the third enlargement passing gradually into the cord. 
 These enlargements are called, from in front backward, fore-brain, 
 
BRAIN. 
 
 141 
 
 mid-brain and hind-brain, the constriction between mid- and hind- 
 brains being the isthmus. In the sides, as in the cord, two zones 
 may be recognized, dorsal and ventral, separated internally by a 
 groove, the sulcus of Monro, which lies at about the middle of the 
 tube. At the extreme anterior end a small region, the optic recess, 
 is" wedged in between the two zones on either side, the end of the tube 
 
 Fig. 146. — Diagrams of (i) primitive brain. (2) an intermediate stage, and'(3) with 
 the definitive parts. (Compare 3 with fig. 147). AQ^ aqueduct; AC^ anterior commissure; 
 C, cerebral region; CB, cerebeUimi; CS, corpus striatum; HC^ habenular commissure; 
 7, infundibulum; LT, lamina terminalis; MO, medulla oblongata; O, olfactory region; 
 P, epiphysial region; PC, posterior commissure; RO, optic recess; 52", subthalamica; T, 
 tegmentum; TH, thalamus. Dorsal zone plain, ventral zone dotted. 
 
 just above the recesses being the lamina terminalis. The most 
 marked modifications in converting the primitive into the adult brain 
 take place in the dorsal zone. 
 
 In the fore-brain the anterior part of the dorsal zone on either side 
 forms an outgrowth which rapidly increases in size, the two eventually 
 forming a pair of hollow vesicles, the cerebral hemispheres (telen- 
 cephalon, prosencephalon) which extend far beyond the lamina 
 terminalis. In the wall of each hemisphere may be recognized a basal 
 ganglionic portion, the corpus striatum, while the rest of the wall 
 is the pallium or mantle. An olfactory lobe (rhinencephalon) 
 grows out from the lower anterior part of each hemisphere to meet 
 
142 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 the olfactory epithelium (see sense organs), and into this a portion of 
 the cavity (ventricle) of the hemisphere may extend (fig. 146). 
 
 Considerable differences exist in the olfactory lobes. In some cases they are 
 directly continuous with the hemispheres, but they may be prolonged, each having 
 two portions, a narrower stalk, the tractus olfactorius, and a distal enlargement, 
 the bulbus olfactorius. The true olfactory nerve takes its origin from the end 
 of the bulb or its homologue (for details see cranial nerves). 
 
 The ventral zone and posterior part of the dorsal zone of the fore- 
 brain, after the differentiation of the telencephalon, forms the thala- 
 mencephalon ('twixt-brain, diencephalon). Its sides, the optic 
 
 Fig. 147. — Half of model of brain of embryo pig, 15 mm. long. (Compare with fig. 
 146, 3.) c, cerebrum; cb, cerebellum; cs, corpus striatum; i, infundibulum; is, isthmus; 
 fi, interventricular foramen; m, mesencephalon; mo, medulla oblongata; t, thalamus. 
 
 thalami, remain without marked modification, but its floor and roof 
 form median outgrowths, corpus pineale, epiphysis, etc., above, in- 
 fundibulum below — to which reference will be made again later. 
 
 In the mid-brain there is little modification except a thickening of 
 the walls forming a pair of prominences, the optic lobes, or corpora 
 bigemina (in mammals two pairs of lobes, corpora quadrigemina) 
 on the dorsal surface. The mid-brain of the adult is also called the 
 mesencephalon (fig. 146). 
 
 In the hind-brain the great modifications occur again in the dorsal 
 zone. Its dorsal portion extends itself upward and backward as a 
 broad lobe which tends to arch over the rest of the hind-brain. This 
 outgrowth forms the cerebellum or metencephalon, while the 
 remainder of the hind-brain constitutes the medulla oblongata or 
 myelencephalon (fig. 146). 
 
 Thus there arise in the adult brain five regions — telencephalon, 
 thalamencephalon, mesencephalon, metencephalon and myelencephalon 
 — derived from the primitive three. These usually retain in the interior 
 
BRAIN. 
 
 143 
 
 the cavity of the primitive three (continuation of the central canal of 
 the spinal cord), but modified in different ways. The cavity in the 
 primitive fore-brain is divided with the outgrowth of the hemispheres 
 into three chambers known at ventricles, a pair of cerebral ventricles 
 in the hemispheres and a third ventricle in the thalamencephalon. 
 The paired ventricles are connected with the third by a pair of narrower 
 passages, the foramina of Monro (for. interventriculares). In the 
 higher vertebrates the cavity of the mid-brain becomes reduced to a 
 narrow tube, the aqueduct (or iter), but in the lower classes (fig. 156) 
 this expands dorsally into a cavity, the epicoele, in the upper part of the 
 optic lobes. The aqueduct terminates behind in the fourth ventricle 
 which lies in the hind-brain, extending forward beneath the cerebellum 
 and gradually diminishing in the medulla to the central canal of the 
 spinal cord. Sometimes there is a prolongation of the fourth ventricle 
 into the cerebellum (metacoele, fig. 156). 
 
 Fig. 148. — Median section of brain of pig 15.5 mm. long, showing flexures of the brain 
 C, principal flexure; cs, corpus striaUmi; CP, chorioid plexus of fourth ventricle; h, hypo- 
 physis; «, infundibulum; M, mid-brain; A^, nuchal flexure; P, pontal flexure; RO, optic 
 recess; T, 'twixt-brain. 
 
 So far the brain has been treated as if it were a continuation of the 
 spinal cord in a straight line. In reality, by unequal growth in dorsal 
 and ventral zones, it becomes flexed in the vertical plane. In the lower 
 vertebrates, these flexures never attain great prominence and largely 
 disappear in the adult. They are more developed in the higher groups 
 and persist throughout life. Most constant is the primary flexure 
 in the mid-brain, by which the derivatives of the fore-brain are bent 
 downward at a right angle (or more) to the axis of the rest. Second 
 to appear is the nuchal flexure in the hinder part of the medulla ob- 
 
144 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 longata, which also bends in the same direction. The pontal flexure, 
 beneath the cerebellum, bends in the opposite direction and thus 
 tends to counteract the other two. Nuchal .and pontal flexures are at 
 best but weakly developed in the ichthyopsida and all are practically 
 obliterated in the adult, but in the amniotes they are increasingly 
 developed and persist through life (fig. 148). 
 
 The brain, like the spinal cord, is composed of nerve cells (gray 
 matter) and fibres (white matter), but their arrangement is exceedingly 
 complicated and but the slightest outline of their distribution can be 
 attempted here, in connection with the general account of the regions 
 of the brain. 
 
 Fig. 149. — Cross-section of medulla of Acanthias embryo, 60 mm. long, showing the 
 greatly broadened roof plate and, below, a bit of the meninx of the nervous system, c, 
 cartilage of basal plate; e, ependyma; mp, meninx primitiva; pc, perichondrium (endo- 
 rhachis); r, roof plate. 
 
 The myelencephalon is most nearly like the spinal cord of any part 
 of the brain. It is triangular in outline, viewed from above, and is 
 widest anteriorly, due in part to the separation of the side walls by the 
 great development of the roof plate over the fourth ventricle. Blood- 
 vessels press against the roof, carrying parts of it before them into the 
 ventricle, thus forming the chorioid plexus of the fourth ventricle, a 
 means of introducing nourishment into the brain. (Usually in dis- 
 sections this roof is torn away, leaving a triangular or rhomboid opening 
 into the fourth ventricle — fossa rhomboidea). The floor plate in 
 this region is obliterated by the development of numerous nerve centres 
 — 'nuclei' or ganglia — in the walls, some closely connected with the 
 
BRAIN. 
 
 145 
 
 fibre tracts soon to be mentioned, some with nerves arising from this 
 region. 
 
 Most noticeable of these ganglia are the olivary bodies (oliva) near the roots of 
 the hypoglossal or first spinal nerves; the nuclei of the cuneate and slender funiculi 
 connected with the posterior columns of the cord; the eminentia medialis in the 
 floor of the fourth ventricle, connected with the anterior and lateral columns; and 
 the tuber acusticum, an enlargement connected with the eighth nerve; its anterior 
 end in the ichthyopsida is specialized as the lobe of the lateral line system 
 
 The cerebellum is developed from the dorsal zones and the roof 
 plate, the latter invaded by nerve cells from the sides. In front it dips 
 deeply into the fourth ventricle, its anterior portion being vertical and 
 together with part of the roof of the isthmus, forming the valve of 
 
 Fig. 150. — Diagrammatic longitudinal section of brain, ac, anterior commissure in 
 lamina terminalis; aq, aqueduct; c, cerebrum; ch, cerebellum; cp, chorioid plexus; cs, corpus 
 striatum; cv, cerebellar ventricle; A, hypophysis; he, habenular commissure; «/>, inferior 
 chorioid plexus; m, mesencephalon; ml, myelencephalon; p, pinealis; pa, paraphysis; /»c, 
 posterior commissure; pe, parietal eye; v, valve of Vieussens; vt, velum transversum with 
 aberrant commissure. 
 
 Vieussens (velum medullare anterius, fig. 150). In the ichthyopsida 
 and lower reptiles there is no special differentiation of parts in the cere- 
 bellum, but in the higher reptiles and in the birds a central portion, the 
 vermis, and a pair of lateral lobes, the flocculi (fig. 161) occur. In the 
 mammals the cerebellum is still farther enlarged, chiefly by the develop- 
 ment of large cerebellar hemispheres between vermis and flocculi, the 
 latter being forced by them to the lower side of the cerebellum. In the 
 walls of each hemisphere, besides others, there is a large nerve centre, 
 the nucleus dentatus, connected with the posterior peduncle of the 
 cerebellum to be mentioned shortly, and with the fibres which go 
 farther forward in the brain. 
 
 The mesencephalon is relatively largest in the lower vertebrates, 
 less conspicuous and tending to be covered by cerebrum and cerebellum 
 in the higher groups. On its dorsal surface are the two optic lobes 
 (transversely divided in the mammals) each connected with an optic 
 
146 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 tract leading to the eye of the opposite side. In the lower groups the 
 lobes contain an epicoele (p. 143), but in the higher they are solid, the 
 cavity being reduced to the aqueduct. The floor of the mid-brain is 
 formed of large fibre tracts (see below), the floor plate having been 
 invaded by their fibres. 
 
 In the thalamencephalon ('twixt-brain) the lateral walls are thick- 
 ened, the dorsal zones developing a nerve centre, the optic thalamus, 
 
 Fig. 151. — ^Parietal, eye of Anguis fragilis, after Nowikoff. ct, connective tissue cells 
 around nerve; gc, ganglion cells; /, lens; w, nerve fibres; pn, parietal nerve; pc, pigment cells; 
 r, retinal cells; vh, vitreous body. 
 
 on either side. These are ganglionic and are closely related to the 
 corpora striata. Frequently the thalami of the two sides touch or 
 even unite above, forming the so-called soft commissure (commis- 
 sura mollis, fig. 152) — really not commissural in character. Still 
 more dorsal is a small habenular ganglion on either side, in front 
 of the pinealis to be described in a moment. 
 
 Under the head of epiphysial structures are several parts devel- 
 oped in the roof plate of the primitive fore-brain. At the junction of 
 cerebral hemispheres and twixt-brain (fig. 150) there is an internal epi- 
 thelial fold, the veltmi transversum, depending from the cerebral 
 roof. In front of this an outgrowth, the paraphysis, arises on the top 
 of the brain in nearly all vertebrates. It is non-nervous and apparently 
 is an extra-ventricular chorioid plexus with secretory functions. The 
 other epiphysial structures belong to the 'twixt-brain and consist of a 
 parietal organ and a pinealis. Both arise from the roof between the 
 
BRAIN. 147 
 
 habenular ganglion and the posterior commissure, at the boundary 
 between 'twixt- and mid-brains, sometimes as two distinct structures, 
 sometimes as the result of division of a single outgrowth of the roof. 
 The anterior of these is the parietal organ or eye; the other the 
 pinealis or epiphysis proper. The two vary in development in dif- 
 ferent vertebrates, the parietal eye being well-marked only in cyclos- 
 tomes, Amia, teleosts and most lizards (fig. 151), while the pinealis 
 is almost invariably present. 
 
 In its fullest development in lizards and Sphenodon the parietal 
 organ extends as a slender stalk, hollow at first, through the parietal 
 foramen of the skull, expanding beneath the skin to a vesicle, above 
 which the integument is usually thin and transparent, forming a physi- 
 ological cornea. The distal wall of the vesicle is thickened in the 
 middle, forming a lens, while the cells of the proximal side elongate, 
 each becoming differentiated into a distal, rod-like end and a proximal 
 portion w^hich contains the nucleus and is connected with a nerve fibre. 
 Pigment is deposited between these cells so that the whole forms a 
 retina. An important point, to be better appreciated after the con- 
 sideration of the paired eyes, is the fact that these parietal eyes are 
 like those of most invertebrates in having no inversion of the retina. 
 How far these eyes are actually fimctional is not settled. Even in 
 Sphenodon, where it is best developed, experiments have resulted in no 
 decided reactions. 
 
 In other vertebrates the parietal organ does not pass outside the 
 skull, and even may not appear transitorily in development. The 
 pinealis to some extent may take its place and often shows, as in certain 
 lizards, traces of a visual structure. In the anura its tip approaches 
 the skin and later is cut off from the brain by the development of the 
 skull, forming the so-called frontal organ, visible from the exterior. 
 Pineal and parietal organs differ in their nervx supply, the parietal being 
 connected with the superior commissure of the ^twixt-brain, the 
 pinealis and its derivatives with the posterior commissure. In the 
 higher vertebrates the epiphysial structures are completely covered by 
 the backward growth of the cerebrum. The large parietal foramina 
 in many extinct reptiles would seem to indicate that they had well 
 developed parietal or pineal organs. The roof of the brain in this 
 region, behind the lamina terminalis, also gives rise to a chorioid plexus 
 like that of the fourth ventricle, a part of which invades the third 
 ventricle and another portion, the inferior plexus, sends branches 
 
148 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 through the foramina interventriculares into the ventricles of the hemi- 
 spheres, thus providing for a blood supply on the interior of these 
 structures (fig. 150), 
 
 The floor of the diencephalon remains thinner behind the optic 
 recess, a portion of it becoming funnel-shaped and pushing out from the 
 ventral surface toward the roof of the mouth. This is the infundib- 
 ulum which meets an ectodermal diverticulum, the hypophysis. 
 This arises, in the cyclostomes from the ectoderm between the nostril 
 and the mouth; in other vertebrates from the roof of the oral cavity. 
 It retains its connection with the parent epithelium for a time, the point 
 of ingrowth being known as Rathke's pocket. Later the stalk dis- 
 appears and the infundibulum and hypophysis, closely associated, lie 
 just beneath the brain in the sella turcica on the floor of the skull 
 (p. 61). In the hypophysis (pituitary body) two parts are distin- 
 guished, rich in blood- and lymph-vessels and forming a gland of internal 
 secretion whose action is connected with the fat-storing powers of the 
 animal. The infundibulum may be a simple pit, as in most vertebrates, 
 or its lateral walls may become enlarged and folded, blood-vessels 
 lying in the folds, and the whole forming the so-called saccus vascu- 
 losus. The paired eyes are also connected with the 'twixt-brain, both 
 in origin and in the adult; they are described with the other sense 
 organs. 
 
 The cerebrum (telencephalon) consists of a pair of hemispheres, 
 separated in front by an inter cerebral fissure, slight in fishes, well 
 marked in other vertebrates. Each hemisphere typically contains a 
 ventricle, the walls of which are formed by the corpus striatum below 
 and elsewhere by a thinner portion, the pallium or mantle. To the 
 roof belong the paraphysis and the inferior chorioid plexus, already men- 
 tioned. In some vertebrates, like the teleosts, the whole of the pallium 
 remains thin and epithelial throughout life; elsewhere it is invaded to a 
 greater or less extent by nervous matter. In the amphibia and reptiles, 
 where the olfactory lobes are merged in the hemispheres, the medial 
 wall of each hemisphere as far back as the interventricular foramen is 
 dalled the septum, while the part above the foramen, together with the 
 posterior dorsal and lateral walls, is to be regarded as homologous with 
 a region, long recognized only in mammals, the hippocampus, connected 
 with the olfactory sense. In the mammals a new element, the neopal- 
 litmi, appears in the cerebrum. In the lower groups it is on the outer 
 wall, behind the olfactory tract, and, increasing in extent in the higher 
 
BRAIN. 
 
 149 
 
 groups, forces the hippocampus to the medial side of the hemisphere. 
 Other modifications are better imderstood after a consideration of 
 the commissures of the brain. 
 
 The amount of gray matter in the pallium is evidently correlated with the mental 
 powers of the animal, being greatest in the mammals. Here the nerve cells form 
 a layer (cortex) on the surface of the neopallium. Increase in the number of 
 these cells can be accommodated to some extent by increase in the size of the 
 cerebrum, but the extent of this increase is limited, and in the higher mammals the 
 amount of surface is increased by folding, so that the cerebrum is marked extern- 
 ally by numerous fissures or sulci separating convolutions or gyri, as will be 
 mentioned in the paragraphs on the mammalian brain. 
 
 In order that the two sides of the body may work in harmony it is 
 necessary that the right and left side of the central nervous system be 
 
 Fig. 152. — Medial plane of brain of Ornithorhynchus, after G. Elliot Smith, ac, anterior 
 commissure; bo, bulbus olfactorius; cm, commissura mollis; d, cerebellum; e, epiphysis; 
 fd, fasciculus dentatus;/*, interventricular foramen; h, hypophysis; he, habenular commis- 
 sure; Ip, lobus pyriformis; mc, corpus mamillare; md, medulla oblongata; n, nodulus; ol, 
 olfactory lobes; oi, olfactory tubercle; pal, pallium; pc, posterior commissure; ic, tuber 
 cinereum; v, velum medullare; vmo, motor root of fifth nerve; vm, maxillary of fifth. 
 
 connected. This is accomplished in the spinal cord by nerve fibres 
 which pass above and below the central canal from one side to the other. 
 In the brain these commissures are more localized. Then there are 
 longitudinal fibre tracts in the brain, some of which are continuous with 
 the columns of the cord already mentioned. Only a few of these connex- 
 ions, which are more numerous in the higher than in the lower verte- 
 brates, can be mentioned here. 
 
 Most constant and important of the commissures are the following: 
 
150 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 In the lamina terminalis, a little below the interventricular foramen, 
 an anterior commissure, connecting the two hemispheres; a poster- 
 ior commissure in the roof at the junction of di- and mesencephalon; 
 and a superior or habenular commissure associated with the habenular 
 ganglia and lying between the epiphysial structures and the velum 
 transversum. In the amphibia, with the differentiation of the hip- 
 pocampal region, a dorsal or hippocampal commissure appears in 
 the lamina terminalis, just dorsal to the anterior commissure, connecting 
 the hippocampi of the two sides. This persists, with slight modifica- 
 tions, through the sauropsida and monotremes, but in the higher 
 mammals it is subdivided into the hippocampal commissure proper and 
 a more anterior portion, the corpus callosum. This corpus callosum 
 is only in part the result of the division, but is more largely formed by 
 new fibres, anterior to the hippocampal portion, connecting the neopal- 
 lium of the two sides. The result is a broad band (the largest com- 
 missure in the brain of man) which invades the intercerebral fissure 
 from behind. In the lower vertebrates a few fibres pass downward from 
 either side of the cerebellum beneath the fibre tracts of the medullary 
 region and so to the other side of the cerebellum. In the mammals 
 these are greatly increased in number, forming a marked projection on 
 the lower surface, the pons (Varolii), the prominence of which is in- 
 creased by the great development of ' nuclei ' in the medullary floor. 
 
 The longitudinal tracts are more numerous and more complex. 
 As will be recalled, there are dorsal, lateral and ventral columns in 
 the spinal cord. These extend into the medulla oblongata and there 
 pursue different courses. 
 
 Some of the fibres of the dorsal columns end in connection with the nuclei of 
 the medulla (p. 144), while others unite with fibres from the lateral column and with 
 some from the oliva to form an enlargement, the corpus restifonne, and then 
 bend upward (posterior peduncle) to enter the cerebellum. Other fibres from 
 the lateral column, together with some from the dentate nucleus, enter the cere- 
 bellum farther in front as the anterior pedimcle, those from the dentate nucleus 
 pass forward to the roof of the mid-brain, some terminating in the optic lobes, 
 others continuing to the cerebrum. In this forward course, after leaving the cere- 
 bellum, the fibres cross (decussate), those from the right side passing to the left 
 side of the brain farther forward and vice versa. In the dorsal region of the medulla 
 there is a short tractus solitarius (fasciculus communis) derived from fibres 
 from the seventh to tenth nerves and extending no farther forward than the seventh. 
 
 In the higher vertebrates there are the crossed and the direct pyramidal tracts 
 on the ventral side of the medulla, the direct being continuations of part of the ven- 
 tral columns, the crossed of the deeper lateral columns. In the medulla these en- 
 
BRAIN. 
 
 151 
 
 large and become somewhat pyramidal, the enlargement being due in part to the 
 decussation of the crossed tracts. The tracts pass forward from the decussation 
 and in the mid-brain region they diverge to pass the hypophysial structures farther 
 in front, the diverging portions being called the crura cerebri. The fibres of the 
 crura enter the corpora striata and in the mammals, the cerebral cortex. 
 
 The direct pyramidal tracts have no decussation in the medullary region, but 
 pass to the hemisphere of the same side; the fibres, however, do cross in the spinal 
 cord. Recently attention has been called to Reissner's fibres which occur in all 
 vertebrates, but are relatively largest in fishes. They arise from the roof of the 
 mid- brain, descend to the aqueduct and pass through the fourth ventricle and into 
 the central canal to terminate at various points in the region of the spinal nerves. 
 It has been suggested that they afford a short cut for visual reflexes. Another 
 supposition is that they regulate the flexion of the body. 
 
 Of the numerous longitudinal tracts in the anterior part of the brain 
 the fornix must be mentioned. It appears first in the amphibia and 
 is well developed in the mammals. Its fibres are connected in front 
 with the hippocampus, pass downward through the lamina terminalis 
 to the floor of the third ventricle, where they produce a marked swelling 
 (corpus albicans) on either side of the ventral surface of the dien- 
 cephalon. They ascend from this point to the optic thalami. The 
 passage of the tracts of the fornix through the lamina terminalis and 
 the forward growth of the corpus callosum stretch the lamina into a 
 thin triangular area, the septum pellucidimi, and at the same time the 
 callosum causes the lamina to split, the enclosed cavity being called 
 the * fifth ventricle' though it has no relation, physical or mor- 
 phological, with the true ventricles of the brain. 
 
 ENVELOPES (MENINGES) OF THE CENTRAL NERVOUS 
 
 SYSTEM. 
 
 Both brain and spinal cord are surrounded by envelopes (meninges) 
 of connective tissue which support and protect them, and also, by 
 carrying blood-vessels, provide for their nourishment. These meninges 
 become more complicated with ascent in the vertebrate series. The 
 canal of the vertebral column and the cavity of the skull are lined with 
 a layer of connective tissue, the endorhachis, which is really the perios- 
 teum or perichondrium of the skeletal parts and hence not a true meninx. 
 In the fishes (fig. 149) there is a single envelope, the meninx primi- 
 tiva, which bears the blood-vessels and lies close upon the spinal cord. 
 Between it and the endorhachis is a perimeningeal space, somewhat 
 broken by strands of tissue passing from meninx to endorhachis, and 
 
152 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 filled, like all meningeal spaces, with an albumen-containing cerebro- 
 spinal fluid. 
 
 From the urodeles upward there is an increasing division of the 
 meninx primitiva into two layers, a pia mater bearing the blood- 
 vessels and lying close to the cord, and a dura spinalis, separated from 
 the pia by a subdural space, the perimeningeal space now being known 
 as the peridural. In the mammals the pia becomes invaded by cavities 
 separating a delicate arachnoid membrane from its outer surface, so 
 that there is another space, the subarachnoid, in these forms. 
 
 There may be slight differences in the region of the brain in the 
 higher groups where the dura presses against and finally unites with the 
 endorhachis, forming the dura mater of human anatomy, thus obliterat- 
 ing the subdural space. In the mammals and to 
 fa less extent in birds the dura mater forms two 
 strong folds. One of these is longitudinal and 
 presses in between the two cerebral hemispheres as 
 a firm membrane, the falx cerebri. The other 
 fold, the tentorium, is transverse, and is inserted 
 between cerebrum and cerebellum. It is occasion- 
 ally ossified and united to the skull. 
 The Brain in the Separate Classes. 
 CYCLOSTOMES.— The brain is very different in the 
 two classes of cyclostomes. All parts lie in the same hori- 
 zontal plane, the flexures having disappeared, and the 
 whole presents a primitive, almost embryonic appearance. 
 In the lampreys the somewhat slender brain is elongate 
 and its roof is largely epithelial, this extending to the mid- 
 brain, of which only the hinder part is nervous in the middle 
 line. The small cerebral hemispheres are largely com- 
 FiG it:^— Brain of P°^^^ ^^ ^^^ corpora striata and the dorsal part of the 
 Bdellostoma (Princeton, pallium is purely epithelial, the ventricles being well de- 
 2204). o, skeleton of yd oped and extending into the olfactory lobes. The 
 brain behind tiiis- V-X ^P^^^ lobes and the medulla are relatively broad, but 
 nerves. the cerebellum is reduced to an inconspicuous fold in front 
 
 of the fossa rhomboidea. 
 Authors do not agree regarding the interpretation of some parts of the myxinoid 
 brain. The whole is much broader and shorter than in the other class and is 
 marked dorsally by a groove running the whole length. According to Retzius, the 
 'twixt-brain of Myxine is invisible from above and the cerebellum is large, com- 
 pletely covering the fossa rhomboidea. The cavities are greatly reduced, the 
 
BRAIN. 
 
 153 
 
 aqueduct ending blindly in the mid-brain, in front of which is only the third ven- 
 tricle, completely cut ofif from the rest. The brain of Bdellostoma (fig. 153) 
 differs from this in several respects. 
 
 ELASMOBRANCHS (figs. 154, 167) usu- 
 ally have the brain somewhat compact, but in 
 a few it is long and slender. The more strik- 
 ing features are the slight development of the 
 intercerebral fissure, the large hemispheres be- 
 ing lateral expansions just in front of the dien- 
 cephalon. The optic lobes are large and the 
 large cerebellum overlaps both lobes and the 
 fossa rhomboidea. The olfactory lobes arise 
 from the antero-lateral angle of each hemi- 
 sphere; their length varies between wide limits. 
 The epithelial roof of the 'twixt-brain is wide 
 and bears a pinealis which often reaches the roof 
 of the skull, but the parietal organ is lacking. 
 The hypophysis and infundibulum are pro- 
 vided with large inferior lobes and a well devel- 
 oped saccus vasculosus. The cerebellum has a 
 longitudinal groove and usually one or more 
 transverse grooves, dividing the upper surface 
 into paired lobes. The medulla dififers in the 
 sharks and the skates, being very short in the 
 latter, much longer in the former. In both the 
 corpora restiformia are large folds on either ^^x, 
 side of the cerebellum, in front of and lateral 
 to the fossa rhomboidea. 
 
 In most elasmobranchs-the ventricular sys- 
 tem is well developed, but in some the paired 
 and third ventricles are not well separated, 
 while in the Myliobatidae there is no cavity in 
 the cerebrum. There is a large epicoele ex- 
 tending upward from the aqueduct into the 
 optic lobes and a similar cavity usually enters //, 
 the cerebellum. 
 
 TELEOSTOMES.—There is a wide range 
 of form in the brain of ganoids and teleosts. It 
 is usually small in proportion to the size of the 
 animal and is noticeable for the small size of 
 the telencephalon and the usually non-nervous 
 character of the pallium, which in the teleosts is 
 purely epithelial. Consequently the cerebrum 
 
 consists largely of the corpora striata and the intercerebral fissure is slightly de- 
 veloped. The paired ventricles are small, but they extend into the olfactory lobes. 
 The 'twixt-brain, at a lower level than the rest, has a large infundibulum, saccus 
 
 Fig. 154. — Brain of Heptanchus, 
 after Gegenbaur. ho, bulbus olfac- 
 torius; c, cerebrum; ch, cerebellum; 
 em, eminentla teretes; /, inftmdibu- 
 lum; m, mesencephalon; 00, olfactory 
 organ; ot, olfactory tract; my, myelen- 
 cephalon; /, 'twixt-brain; II-X, cra- 
 nial nerves. 
 
154 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 Fig. 155. — Dorsal and side views of brain of buffalo fish (Carpiodes tumidus) after 
 Herrick. c, cerebrum; cl, cerebellum; cs, corpus striatum; h, hypophysis; i, infundibulum; 
 ol, olfactory lobes; p, pallium; vl, vagus lobes; II~X, nerves. 
 
 cl 
 
 M 
 
 
 V Jfn, D 
 
 bo r 
 
 df sv I'i h Ch <^i 
 
 ca. CS 
 
 Fig. 156. — Sagittal section of brain of trout, after Rabl-Ruckhard. aq, aqueduct; bo, 
 bulbus olfactorius; ca, ch, ci, cp, anterior, horizontal, inferior and posterior commissures; 
 CO, central canal; cl, cerebellum; h, hypophysis; i, infundibulum; oc, optic chiasma; p, 
 pallium; pi, pinealis; sv, saccus vasculosus; tl, torus longitudinalis; to, tectum of optic 
 lobes; v^, v* ventricles; vc, valvula cerebelli. 
 
BRAIN. 155 
 
 vasculosus and inferior lobes. On its roof is a large pinealis which reaches the skull in 
 a few ganoids. The parietal organ appears in the embryo and soon degenerates; the 
 paraphysis is usually well developed. The optic lobes are large and are usually 
 divided into two hemispheres by a median groove, but this occasionally is scarcely 
 noticeable. The cerebellum is large, much larger than appears from the surface, 
 since a considerable part, the valvula, projects into the ventricle of the mid-brain. 
 In the cerebellar region there is sometimes an enormous development of the lobes 
 of the vagus (fig. 155). 
 
 The brain of Polypterus differs from that of other ganoids in several respects. 
 There is no dififerentiation of cerebral hemispheres; the optic lobes and the cerebel- 
 lum are moderate, the latter being thin in the median line and the valvula smaller. 
 The medulla oblongata has thin walls and the ventricle is large. The brain has a 
 primitive appearance, but it shows little resemblance to those of the amphibia or 
 of the dipnoi. 
 
 DIPNOI. — The brains of Lepidosiren and Protopierus differ considerably from 
 that of Ceratodus. In all the cerebrum is larger than the optic lobes and the 
 
 Fig. 157. — Brain of Protopterus, after Burckhardt. cb, cerebellum; e, epiphysial 
 structiures; h, hypophysis; i, infundibulum; m, mid brain; 5e, sacois endolymphaticus; sp, 
 spinal nerves; t, cerebrum; 1-12, cranial nerves. 
 
 olfactory bulb is separated from the cerebrum by a long olfactory tract. In Cer- 
 atodus the hemispheres are united above by a part of the chorioid plexus, while 
 internally they are separated from the diencephalon by a well marked velum. The 
 pinealis is long and rests upon a large 'zirbelpolster' developed as an outgrowth 
 of the roof of the third ventricle in front of the superior commissure. The optic 
 lobes are separated into two hemispheres^ while the cerebellum is scarcely more than 
 a transverse plate and is, together with the fossa rhomboidea, covered with a com- 
 plicated chorioid plexus. In Protopterus (fig. 157) the elongate hemispheres are 
 parallel, the pinealis and its 'polster' are smaller and the mid-brain has but a 
 single rounded lobe. 
 
 AMPHIBIA. — ^The parts of the amphibian brain are more distinct from each 
 other than is usual in vertebrates, and, except in the gymnophiones, the flexures 
 have largely disappeared in the adult. There is a deep intercerebral fissure 
 between the hemispheres, but in the anura the two halves of the cerebrum are 
 connected by a transverse band just behind the olfactory lobes. The telencephalon 
 is relatively larger than in fishes, the increase being due to the invasion of the pal- 
 lium by nervous matter, while the corpora striata are relatively smaller than in other 
 ichthyopsida. In the pallium the inner part is largely composed of nerve cells, 
 the outer layer consisting of nerve fibres. 
 
 The diencephalon, broad in the anura, narrower in the urodeles and caecilians, 
 
156 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 is visible from above. The infundibulum and hypophysis are well developed but 
 the saccus vasculosus and inferior lobes are smaller than in fishes. In the gymno- 
 phiones, owing to the pontal flexure the hypophysis is carried back beneath the 
 medulla oblongata. Both paraphysis and pinealis are present, the latter not reach- 
 
 Fig. 158. — Dorsal and ventral views and sagittal section of brain of DesmogncUhus, after 
 Fish, a, anterior commissure and rudimentary corpus callosum; c, cerebrum; d, cere- 
 bellum; e, epiphysis; h, hypophysis; z, infundibulum; oc, optic chiasma; ol, optic lobes; p, 
 paraphysis; pc, posterior commissure; cp, chorioidplexuses ; sc, superior commissure; I-X, 
 nerves. 
 
 ing the cranial roof except in the anura, the conditions in this group having already 
 been mentioned (p. 147). The cerebellum is very small, a mere transverse fold 
 on the anterior border of the fossa rhomboidea. The gymnophione brain is notice- 
 able for the pontal flexure already alluded to, which carries the hemispheres so far 
 
BRAIN. 
 
 157 
 
 back that they almost touch the sides of the medulla, and for the double roots of the 
 olfactory nerves. 
 
 REPTILES. — There is considerable range in the brain of the reptiles, all show- 
 ing an advance over the amphibians in having the cerebrum larger than the optic 
 lobes; in having, in the pallium, besides the basal layer of gray matter, a distinct 
 cortical layer of nerve cells; the well developed hippocampus; while the olfactory 
 lobes may either be sessile upon the hemispheres or differentiated into tracts and 
 bulbs. 
 
 Fig. 159. Fig. 160. 
 
 Fig. 159. — Brain of Iguana tuherctdata (Princeton, 2293). Compare fig. 172. 
 
 Fig. 160. — Side and dorsal views of brain of young aL&gator, after Herrick. c, cere- 
 brum; cl, cerebellum; e, epiphysial structures; h, hypophysis; », infundibulum; ol, olfactory 
 lobes; II-XII, cranial nerves. 
 
 The greater size of the cerebrum and the large optic lobes result in covering the 
 diencephalon so that it is scarcely visible from above (figs. 159, 160). Infundibulum 
 and hypophysis are well developed, but the sacci vasculosi are rudimentary and the 
 inferior lobes are inconspicuous. The epiphysial structures reach their highest 
 development in this group. In most species the parietal organ is rudimentary, but 
 in many lizards and especially in Sphenodon it penetrates the roof of the skull and 
 
158 
 
 COMPARA.TIVE MORPHOLOGY OF VERTEBRATES. 
 
 forms a well-developed eye (fig. 151), lying Just beneath the skin and connected 
 with the brain by more or less rudimentary nerves. In some the pinealis also 
 shows eye-like features. 
 
 The optic lobes are distinct from each other. The cerebellum is usually 
 small (fig. 159), but in the crocodilia (fig. 151), it attains considerable size. In 
 all reptiles there is a thicker central portion and thinner lateral parts, an approach 
 to the differentiation into vermis and flocculi found in birds. There are no 
 special features in the medulla calling for notice. 
 
 AVES. — ^The bird's brain (fig. 161) is short, broad and highly specialized. The 
 smooth cerebral hemispheres are large, their size being due more to the enormous 
 corpora striata than to enlargement of the pallium, which is comparatively thin, 
 while the olfactory lobes are very slightly developed, in correlation with the deficient 
 powers of smell. The large cerebellum extends forward between the hinder ends 
 
 C 
 
 A 
 
 
 Fig. 161. — Brain of golden eagle, Aquila chryscBtos, after Herrick. c, cerebrum; cl, cere- 
 bellum;/, flocculus; mo, medulla oblongata; ol, optic lobes; an, optic nerve. 
 
 of the cerebrum, thus forcing the optic lobes into a lateral position and completely 
 covering the 'twixt-brain. The epiphysial structures are large but rudimentary in 
 character, the pinealis extending up in the angle between cerebrum and cere- 
 bellum. Below, the hypophysis completely hides the infundibulum. The large 
 cerebellum has its median portion transversely furrowed, this constituting the 
 vermis, while the smaller lateral lobes, which vary in extent, form the flocculi. 
 The myelencephalon is very short and the fossa rhomboidea is covered by the 
 cerebellum. 
 
 MAMMALS. — The brain in the mammals becomes exceedingly complex. Only 
 the most important features and those of general occurrence will be noted here. 
 Most marked are the large size of the cerebellum and the still greater development 
 of the cerebrum, correlated with the great increase in mental powers. The cere- 
 
BIL\IN. 159 
 
 brum covers the di- and mesencephalon, and in the primates even the whole of the 
 cerebellum from above. This increase of the cerebrum is largely an increase of 
 the nervous matter of the pallium, a portion — the neopallium — developing on the 
 lateral side of each hemisphere between the hippocampus and the basal structures 
 (pyriform lobes). This increase in cerebrum is limited in forward and backward 
 growth by the limitations of skull development. Hence it overlaps the olfactory 
 
 
 T^ip 
 
 \ 
 
 Fig. 162. — ^Ventral surface of brain of Ornithorhynchus, after G. Elliot Smith, bo, 
 bulbus olfactorius; c^, first cervical nerve; cl, cerebellum; cm, corpus mamillare;/, floc- 
 culus; Ip, lobus pyrifonnis; op, olfactory peduncle; rf, rhinal fissure; ic, tuber dnereimi; to, 
 olfactory tubercle; tV, tuberculum quinti; Vm, Vmd, Vmx, motor root and maxillaris and 
 mandibularis roots of trigeminal nerve; I-XII, cranial nerves. See ako fig. 152. 
 
 lobes in front, so that they appear to rise from its ventral surface, while behind it 
 extends backward, then turns downward and lastly extends forward along the sides 
 of the mid- and 'twixt-brains, even overlapping a part of the cerebrum itself. In 
 this way the cerebrum becomes marked off into a series of regions called the frontal 
 lobes in front, the parietal above, the occipital behind, while the reflexed ventral 
 portion of either side makes a temporal lobe. 
 
 This folding and overgrowth causes grooves or fissures in the surface of the 
 cerebrum, the most constant being a rhinal fissure between olfactory and frontal 
 lobes, a Sylvian fissure between the temporal lobe and the lower surface of the 
 
i6o 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 cerebrum against which it is folded. In the bottom of the Sylvian fissure is a part 
 of the side wall of the cerebrum which has received the |j^ame of insula (island of 
 Riel), while a hippocampal fissure causes the hippocampus to appear as a pro- 
 nounced swelling on the floor of each ventricle. In the lower mammals these are 
 the only fissures present, the rest of the cerebral surface being smooth. In the 
 higher mammals other grooves (sulci) separating convolutions (gyri) appear. 
 These convolutions increase the extent of cerebral surface and as a consequence 
 they permit of more cortical gray matter upon which mentality depends. The 
 number of gyri increases in the primates and reaches its extreme in man. The 
 folding of the cerebrum also affects the cavities of the cerebrum as well as the course 
 of the fibre tracts, especially of the fornix which becomes greatly bent on itself. 
 In the ventricles distinct regions or 'horns' are recognized, an anterior comu in 
 
 Fig. 163. Fig. 164. 
 
 Fig. 163. — Brain of Chrysothryx sciureus, after Weber. /, frontal lobe; i, interparietal 
 fissure; 0, occipital lobe; p, parietal lobe; s, Sylvian fissure; t, temporal lobe; ts, sulcus 
 temporalis. 
 
 Fig. 164. — Brain of Manis javanica, after Weber, ch, cerebellar hemispheres; h^ 
 hippocampal lobe; 0, olfactory lobe; pSy presylvian fissure; s, Sylvian fissure; ss, sulcus 
 sagittalis; v, vermis; II, optic nerve. 
 
 the frontal lobe, a posterior in the occipital lobe and an inferior comu in the 
 temporal lobe. Associated with the cortical gray matter are nerve fibres (compara- 
 tively few in the lower, extremely numerous in the higher mammals) which form 
 a corona radiata and connect the cortex with the more posterior regions of the 
 brain. In the non-placental mammals the anterior commissure is very large, 
 forming the chief association tract between the two hemispheres, but in the higher 
 groups the corpus callosum becomes greatly developed and largely replaces it. 
 
 The diencephalon is greatly reduced, the hypophysis and infundibulum being 
 small, the latter showing traces of the saccus vasculosus and inferior lobes so 
 prominent in the lower vertebrates. The parietal organ is lacking, but the pinealis 
 is relatively large. It is separated from the roof of the skull by the occipital 
 lobes of the cerebrum. It is connected with the roof of the brain by two bands 
 or peduncles and its cavity contains a quantity of so-called 'brain sand.' A 
 transverse groove divides the optic lobes so that they consist of four lobes (corpora 
 quadrigemina). 
 
SPINAL NERVES. l6l 
 
 The cerebellum is divided into a median vermis and a pair of lateral portions, 
 each consisting of a large cerebellar hemisphere, ventral (morphologically lateral) 
 to which is a flocculus (fig. 162), homologous to that of the sauropsida. The 
 surface of the hemispheres is convoluted and this results in the arrangement of 
 the white and gray matter in such a way that they have a markedly dendritic ap- 
 pearance (arbor vitae, fig. 152) when seen in longitudinal section. The pons, 
 characteristic of the mammalian brain, has already been mentioned (p. 150). 
 
 THE PERIPHERAL NERVOUS SYSTEM. 
 
 The Spinal Nerves. 
 
 The spinal nerves are metameric structures, connected with the 
 spinal cord by two separate portions or roots which differ greatly from 
 each other in development, structure and function. At the time of 
 the closure of the neural tube a band of cells occurs on either side of the 
 neural plate at the junction of neural and epidermal areas. With the 
 closure of the tube these form two bands, the neural crests, one on 
 either side of the dorsal surface of the cord (fig. 144). By unequal 
 growth each crest soon develops a series of metameric enlargements, the 
 portions of the crest between these gradually disappearing, while the en- 
 largements form the ganglia of the dorsal roots of the nerves. Each of 
 its cells, like those of the cord, sends out processes, one of which grows 
 medially and enters the cord in the region of the posterior cornu, while 
 the other extends peripherally to the skin or viscera, these processes 
 constituting the dorsal root of the nen^e, the ganglion forming an 
 enlargement upon it, near its connection with the cord. The other or 
 ventral root is formed by fibres which grow out in a similar way from 
 cells in the ventral horn of the cord itself and leave it between the an- 
 terior and lateral columns, to extend to the muscles, glands, etc. As the 
 ganglion cells are inside the cord, there is no ganglion on the ventral 
 root. Except in the cyclostomes the dorsal and ventral roots unite 
 soon after leaving the cord, the combined trunk being a t}^pical spinal 
 nerve (figs. 145, 166). 
 
 Physiologically the roots differ in that the dorsal roots are mainly 
 composed of sensory fibres, while the ventral roots contain only motor 
 fibres. That is, on stimulation of the parts to which they are distrib- 
 uted the dorsal roots and their fibres carry nervous impulses to the cord 
 — they are afferent — while the impulses in the ventral roots are carried 
 in the opposite direction by efferent fibres. In their case stimulation 
 arises in the central nervous system and the impulse is carried outward 
 
l62 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 to the parts to which the fibres are distributed, causing these to act — 
 muscles to contract, glands to secrete, etc. Hence the ventral roots are 
 called motor roots. Their fibres are without sensory functions, while 
 sensory fibres are equally unable to cause action in any peripheral 
 part (Bell's law). 
 
 After a longer or shorter course, each spinal nerve, formed by the 
 union of dorsal and ventral roots, divides into three branches, each of 
 which receives both sensory and motor fibres. These are known as 
 
 ^^ I 
 
 JT 
 
 Fig. 165. — A, diagram of collector nerve; B, of a nerve plexus, after Braus; C, branchial 
 plexus of Salamandra maculata, after Fiirb ringer. 
 
 the ramus dorsalis, ramus ventralis and ramus visceralis or in- 
 testinalis. The first goes to the skin and muscles of the dorsal region; 
 the second to those of the sides and ventral parts of the body; while the 
 visceral branch descends to the roof of the coelom, near the insertion of 
 the mesentery, where it connects with the sympathetic nervous system 
 to be described below (fig. 166). 
 
 Recent physiological and histological analysis shows the existence 
 of two groups of nervous elements in both sensory and motor nerves. 
 There are somatic sensory and motor fibres, distributed to the skin 
 and most of the external sense organs and to the voluntary muscles, and 
 
SYMPATHETIC SYSTEM. 1 63 
 
 there are also visceral fibres of both kinds, supplying the viscera 
 (alimentary canal, excretory and reproductive organs) and the circula- 
 tory system. The dorsal and ventral rami contain mostly somatic 
 fibres with a few of the visceral type, while the visceral rami are com- 
 posed of visceral fibres alone. The farther subdivision of these nerves 
 will be considered later. 
 
 To the statement that the dorsal roots are purely sensory the exception must be 
 made that in the lower vertebrates some of the visceral motor fibres, arising in 
 the neighborhood of the lateral cornu, pass out from the cord through the dorsal 
 root. In the mammals they are said to leave by the ventral roots like all other 
 motor fibres. 
 
 In the regions of the appendages the spinal nerves usually form 
 networks or plexuses, branches of a varying number of ventral rami 
 interlacing in a complicated manner before entering the appendage. 
 Plexuses are poorly developed in the fishes, but here many spinal nerves 
 are united before entering a limb by means of a longitudinal *col- 
 lector' nerv^e, there being no exchange of fibres such as occurs in a 
 plexus. In the amphibia there are two plexuses, a cervico-brachial 
 near the fore limb, and a lumbo-sacral for the hind limb. In the 
 higher groups there may be four plexuses: cervical, brachial, lum- 
 bar and sacral, the positions of which are indicated by their names. 
 
 The Sympathetic System. 
 
 The function of the sympathetic system is the control of the viscera, 
 various glands, the smooth muscles, and through the latter, of the size 
 of the blood-vessels and the supply of blood to the various parts. 
 The system is connected with the spinal nerves by the visceral rami 
 (rami communicantes) already mentioned. As has just been said, 
 these visceral rami contain both motor and sensory fibres. As these 
 rami extend downward in their development, they carry with them 
 ganglion cells derived from the ganglia of the dorsal roots of the 
 spinal nerves, and these give rise to the sympathetic ganglia. Of 
 these there are three groups. Nearest to the spinal nerves on either 
 side are a series of the sympathetic trunk (chain ganglia), usually 
 connected with each other by a longitudinal sympathetic trunk. 
 Nerves run from these chain ganglia to the prevertebral ganglia, 
 some of which, like the cardiac, pelvic, hypogastric and solar 
 (plexuses) are of considerable size. From these nerves go to the 
 
164 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 peripheral ganglia, situated at various points along the viscera, some at 
 some distance from the sympathetic centres. 
 
 In the sympathetic system four kinds of nervous elements are to be 
 distinguished. The original trunk that grows out (the ramus visceralis) 
 consists of motor and sensory fibres. The latter arise from ganglion 
 cells in the ganglia of the dorsal roots. The motor fibres have their 
 cell bodies in the cord at about the level of the lateral cornu, and pass 
 
 visceral motor 
 somatic motor 
 visceral sensory 
 somatic sensory 
 sympathetic 
 
 Fig. 166. — Diagram of the relations of the sympathetic system, based on Huber. The 
 character of the different fibres is shown by conventional Unes. bv, blood-vessel; eg, chain 
 ganglion; d, dorsal ramus; dr, dorsal root; g, gland; gr, gray ramus; pg, peripheral ganglion; 
 pvg, prevertebral ganglion; st, sympathetic trunk; v, ventral ramus; vi, visceral ramus; vr, 
 ventral root; wr, white ramus. 
 
 out, in the lower vertebrates by the dorsal, in the mammals by the 
 ventral root. In the sympathetic system itself there are sensory and 
 motor (excitatory) cells, derived from the ganglion cells carried down 
 by the growing nerves. These develop their dendrites and axons, 
 and some of these run up the rami communicantes to the dorsal and 
 ventral rami, and follow along them to the peripheral glands and 
 blood-vessels of the body. Others grow into the various viscera. These 
 purely sympathetic fibers are not medullated and hence are gray in 
 
CRANIAL NERVES. 1 65 
 
 color, and form gray rami communicantes for a part of their course to 
 the spinal nerves. 
 
 The sympathetic system is best developed in the trunk, but it 
 extends forward into the head, where a series of sympathetic ganglia 
 (ciliary, sphenopalatine, etc.) is connected with the cranial nerves 
 as far forward as the fifth. The sympathetic trunk in this region is 
 usually closely connected with other nerves, but occasionally (Vidian 
 nerve from the sphenopalatine to the facial ganglion, Jacobson's 
 commissure from the seventh to the ninth, fig. 170) it is distinct. 
 
 A few words may be added to this general account. In the elas- 
 mobranchs there is no sympathetic trunk, this first appearing in the 
 teleosts. The system is more highly developed in the aquatic than 
 in the terrestrial urodeles or in the anura. In the sauropsida the 
 trunk is usually double on either side in the neck region, one branch 
 running through the vertebrarterial canal of the vertebrae. In the 
 mammals the cervical part of the trunk is usually closely associated 
 with the pneumogastric nerv^e. In the development certain ganglion 
 cells migrate from the developing sympathetic system and pass to various 
 parts of the body, being usually closely associated with the glands of 
 so-called internal secretion — hypophysis, carotid gland, suprarenals, 
 etc. They possess a peculiar ajQ&nity for chromic salts and are 
 known as chromaffine cells. Little is known of their function. 
 
 The Cranial Nerves. 
 
 The nervxs which arise from the brain and pass out through the 
 foramina in the skull are known as the cranial nerves. While in a 
 general way they resemble the spinal nerves, they have been specialized 
 and modified in many respects in correspondence with the specialization 
 of the head itself, some consisting of sensory fibres alone, some of only 
 motor fibres, while others are mixed, that is, contain both kinds of 
 fibres. There can also be recognized somatic and visceral nervxs as in 
 the trunk, while the somatic sensory fibres may be arranged in dif- 
 ferent groups, differing in their connexions inside the brain and in the 
 sense organs to which they are distributed. Thus six different kinds of 
 fibres may occur in the cranial nerv^es, as follows: 
 
 1. The somatic motor, which go to the muscles derived from the 
 myotomes of the head. 
 
 2. The visceral motor, distributed to the muscles of the gill region, 
 
1 66 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 Fig. 167. — Brain and cranial nerves of Carcharias littoralis (Princeton 310) , natural size. 
 60, olfactory bulb; 6r^-*, branchial nerves; cr, corpus restiforme; d, diencephalon; e, epi- 
 physis; ec, external canal of ear; er, external rectus; eo, external oblique; Aw, hyomandibular 
 nerve; /, lateralis nerve; md, mandibularis nerve; ms, mesencephalon, also, maxillaris 
 superior; 00, olfactory organ; os, ophthalmicus superficialis nerve; ot, olfactory tract; pal, 
 palatine nerve; pc, posterior canal; po, post-trematic branch; pr, pretrematic branch; so, 
 superior oblique; sr, superior rectus, t, telencephalon; m, utriculus; v, visceral branch of X; 
 I-X, cranial nerves. 
 
CRANIAL NERVES. 1 67 
 
 and their homologues in the higher vertebrates, arising from the lateral 
 plate region of the embryo. 
 
 3. The visceral sensory nerves are connected inside the brain 
 with the communis tract of the medulla (fascicularis solitarius of human 
 anatomy), while they terminate in special taste organs, usually within 
 the mouth, but in many teleostomes distributed on the sides of the body 
 as well. 
 
 4. The general cutaneous sensory nerves, corresponding to the 
 somatic sensory of the trunk. Internally they are connected with the 
 dorsal horns of the spinal cord and the homologous parts of the myelen- 
 cephalon, while distally they terminate either as free nerves or in special 
 tactile organs in the skin. 
 
 5. The acustico-lateralis nerves, the centre of which is in the 
 cerebellum and in the tuberculum acusticum of the myelencephalon. 
 Distally the fibres terminate in peculiar collections of sense cells known 
 as sense' hillocks or neuromasts occurring in the inner ear and in the 
 lateral line organs of the ichthyopsida. 
 
 6. The nerves of special sense (olfactory and optic). 
 
 The first four of these groups occur in the spinal nerv^es; the last two 
 are confined to the head. While each spinal nerve contains all four 
 components, the same is not true of most of the cranial nen^es, some 
 having but a single kind of fibre. On this and other accounts it is 
 necessary to review each nerve in some detail. In the lower verte- 
 brates (ichthyopsida) there are ten of these cranial nerves; in the am- 
 niotes there are twelve. These are known by both name and number. 
 I. The Olfactory Nerve differs considerably in the various groups 
 of vertebrates. The term strictly includes only the fibres extending 
 between the olfactory lobe of the brain and the olfactory epithelium, 
 the fibres terminating in the rhinencephalon by dendrites which, in- 
 terlacing with dendrites of cerebral neurons, form oval bodies, the 
 glomeruli. The olfactory nerve differs from all others in that it arises 
 from cells of the epidermis. In some vertebrates (elasmobranchs, 
 some teleosts, ganoids, snakes, some lizards, fig. 168, ^),the nerve proper 
 is very short, while the olfactory lobe is developed into an elongate 
 structure in which separate regions may be distinguished, a part of the 
 lobe remaining in connexion with the cerebrum, next a narrower stalk, 
 the tractus, and lastly a larger bulbus olfactorius, containing the 
 glomeruli, close to the nasal organ. In other vertebrates (some 
 teleosts, amphibia, some lizards, turtles, fig. i68, B) the nerve is more 
 
1 68 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 elongate and the lobe is not differentiated into bulb and tract. The 
 nerve in all forms consists solely of special sensory fibres, and its ap- 
 parent origin from either the tip or the ventral surface of the cerebrum 
 
 Fig. i68. — Diagrams of the different kinds of oKactory bulb, tract, and nerve, bo, ol- 
 factory bulb; ^, glomeruli; ol, olfactory lobe; on, olfactory nerve; to, olfactory tract. 
 
 Fig. 169. — Brain and olfactory and {nt) terminalis nerves of Raia, after Locy. 
 
 is to be explained by the varying development of the hemispheres as 
 given above (p. 159). 
 
 In some fishes (several elasmobranchs, dipnoi, Amia) a small 
 nerve arises dorsally (some elasmobranchs) or ventrally from the cere- 
 brum, has a distinct ganglion and, following somewhat closely the 
 
CRANIAL NERVES. 1 69 
 
 olfactory nen^e, is distributed to the olfactory epithelium. Apparently 
 the same nerve occurs in the human embryo and it may be looked for 
 elsewhere. It is called the terminalis nerve and probably belongs to 
 the general cutaneous system. 
 
 II. The Optic Nerve arises from the floor of the diencephalon and 
 extends to the eye where it spreads over the inner surface of the retina. 
 Together with the olfactory nerv^e it is usually stated to differ from the 
 other cranial nerves in being an outgrowth from the brain. In its 
 history, which is closely connected with that of the eye, there is first 
 formed the optic stalk with the optic vesicle at its tip (see eye for details). 
 The stalk grows out from the recessus opticus and hence is clearly 
 dorsal in position. Soon after the involution of the optic cup, nerve 
 cells are proliferated from the distal surface of the retina, which pass 
 through the chorioid fissure and along the groove on the ventral side of 
 the optic stalk. These fibres and not the cells of the stalk form the 
 definitive optic nerve of the adult, and the cells from which they arise 
 form the optic ganglion, which, to a certain extent, is comparable to the 
 ganglion of a dorsal root. This view also lessens the differences be- 
 tween the optic and other cranial nerves, a view which was natural 
 before the history of the nerve was known and when it was thought 
 that the stalk itself was transformed into the nerve. 
 
 The nerve fibres, in their centripetal growth, do not stop on reaching 
 the diencephalon, but continue across its ventral surface and become 
 connected with the opposite side of the brain. There is thus a crossing 
 or chiasma of the optic nerves, that from the left eye going to the right 
 side of the brain and vice versa. In most vertebrates the chiasma is 
 plainly seen from the surface, but in cyclostomes and dipnoans it may 
 occur in the substance of the brain itself. In the lower vertebrates the 
 chiasma is complete and the nerves from the two sides may simply over- 
 lap or they may interlace with varying degrees of complexity. In the 
 mammals, on the other hand, the chiasma can be analyzed only by 
 microscopic methods, so intimately are the fibres interwoven, while here 
 some of the fibres ('lateral fibres'), instead of crossing, enter the cor- 
 responding side of the brain. The internal connections of the optic 
 nerves are not with the 'twixt-brain, but the fibres, after passing the 
 chiasma, grow dorsally and posteriorly and become connected with the 
 dorsal part of the mid-brain, hence called the optic lobes. 
 
 There has been described in the embryo elasmobranch, under the name thal- 
 amic nerve a small strand arising between the di- and mesencephalon. It disap- 
 
lyo COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 pears without leaving a trace, unless it contribute to the ciliary ganglion. Its status 
 as a nerve is very uncertain. 
 
 The Eye Muscle Nerves (fig. 137). — The III (oculomotorius), 
 IV (trochlearis) and VI (abducens) nerves are distributed to the 
 muscles which control the movements of the eye and hence are treated 
 together. The oculomotor supplies the superior, inferior and internal 
 rectus and inferior oblique muscles; the trochlearis goes to the superior 
 oblique, while the abducens innervates the external rectus muscle. 
 
 laletaUs 
 vibUTol ynoloT 
 visceral 5en*or\j 
 ^■=«:«=» QcneroV cuUjvtus 
 
 Fig. 170. — Diagram of branches and components of the fifth or trigeminal nerve in a 
 shark, gg, Gasserian ganglion; ;', Jacobson's commissure, connecting with glossophar}m- 
 geal; md, mandibularis nerve; mx, maxillaris nerve; op, os, ophthalmicus profundus and 
 superficialis nerves. 
 
 These peculiarities of distribution are explained by the development 
 of the muscles (p. 128), the derivatives of each somite having a common 
 nerve supply. The oculomotor nerve springs from the ventral surface 
 of the mid-brain, the fourth from the dorsal surface at the hinder margin 
 of the mesencephalon, while the sixth comes from the ventral surface 
 of the myelencephalon. Inside the brain the trochlearis is traced to 
 its nucleus in a ventral position. 
 
 In the majority of vertebrates these nerves are readily traced from 
 the brain to the muscles they supply, but not infrequently the abducens 
 (lacking in Petromyzon) is united proximally with the fifth nerve, 
 while in a few forms the trochlearis has not been recognized, and it is 
 said that in the adult Bdellostoma all eye-muscle nerves are lacking. 
 The ciliary ganglion is closely associated with the oculomotor nerve. 
 All three eye-muscle nerves belong to the somatic motor group. 
 
 V. The Trigeminal, one of the largest of the cranial nervxs, arises 
 
CRANIAL NERVES. 171 
 
 from the anterio-lateral angle of the myencephalon, and its fibres pass 
 almost immediately into the semilunar (Gasserian) ganglion, 
 which may lie either within or without the skull. In the higher verte- 
 brates the nerve divides beyond the ganglion into three main trunks — 
 ophthalmic, maxillary, and mandibular — whence the name. In 
 the lower vertebrates the maxillary and mandibular pursue a common 
 course for some distance before separating. 
 
 In the fishes the ophthalmic is represented by two branches, an 
 ophthalmicus superficialis (not to be confused with the similarly 
 named branch of the seventh with which it is closely associated) and 
 an ophthalmicus profundus, which passes between the eye muscles 
 on its way to the tip of the head. Both are purely sensory-, in most 
 vertebrates general cutaneous, but in the teleosts they, together with 
 the maxillary, supply also the taste organs (visceral sensory) of the 
 surface of the head. The superficialis innervates the skin above and 
 in front of the eye; the profundus goes to the eyelids, conjunctiva, 
 snout and the mucous membrane of the nose, passing through the 
 ciliary ganglion in its course. In the urodeles, where the maxillaris 
 is reduced, the profundus supplies its region. 
 
 The maxillaris, with components similar to those of the ophthalmic, 
 runs beneath the eye, passing the sphenopalatine ganglion (p. 165) 
 in its course, supplying much the same territory as the ophthalmic and, 
 in addition, the roof of the palate and the teeth of the upper jaw. 
 
 The mandibularis ramus is a mixed nerve. The motor components 
 (visceral) innerv^ate the muscles of the jaws, some muscles of the floor 
 of the mouth and in mammals the tensor tympani muscle. The sensory 
 component (general cutaneous) divides into two parts, the lingualis 
 going to the tongue and the mandibularis to the skin of the lower 
 jaw, chin and lower lip, and to the lower teeth. In mammals there is 
 a weak auricularis superficialis nerve arising from the mandibularis 
 and going to the temporal region and to the conch of the ear. 
 
 Two ganglia of the sympathetic system are associated with the trigeminal : the 
 otic ganglion near the exit from the skull and the submaxillary where the lingualis 
 bends to enter the tongue. From the otic a trunk (Jacobson*s commissure, p. 165) 
 runs back to connect \\-ith the ninth nerve. 
 
 The fifth nerve is usually compared to a post-otic nerve (vide infra) in that the 
 mouth is regarded as the homologue of a pair of gill clefts, the maxillary being 
 the pre- and the mandibularis the post-trematic nerves (see nerve IX, below). 
 The homologies of the ophthalmicus are less certain. Some facts seem to point 
 
172 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 to the fifth being a compound nerve, this branch being the remnant of a somite 
 otherwise lost. Others would view the ophthalmic as the representative of the 
 dorsal ramus of a spinal nerve. 
 
 VII. The Facial Nerve, the hindmost of the preotic nerves, differs 
 greatly in the branchiate and the pulmonate vertebrates. In all there 
 is a close association with the eighth nerve, and in the anura, some 
 teleosts and ganoids, and in the holocephals the fifth and the seventh 
 are so closely related that their ganglia are fused. 
 
 Fig. 171. — Dagram of seventh (facial) nerve; for components see fig. 170. b, buc- 
 calis nerve; d, chorda tympani; gg, geniculate ganglion; h, hyoid nerve; hm, hyomandib- 
 ular nerve; Ig, lateralis ganglion; mxe, maxillaris externus nerve; os, ophthalmicus 
 superfidalis nerve; pal, palatine nerve; sp, spiracle. 
 
 In the aquatic ichthyopsida the several roots by which the seventh 
 nerve leaves the medulla unite in a compound ganglion, the upper 
 element being the ganglion of the lateralis component, the lower the 
 geniculate, the true ganglion of the seventh nerve. Beyond the 
 ganglion the nerve divides into five trunks, as follows : 
 
 A. The ophthalmicus superficialis, which runs forward near 
 the dorsal surface of the head; B. the buccalis, which courses nearly 
 parallel to the maxillaris of the trigeminal and is often bound up with it; 
 C. the mandibularis externus, which usually divides into two branches, 
 usually follows much the same course as the mandibularis trigemini, 
 and supplies the lower jaw and the spiracular and hyoid region; D. 
 the palatinus which goes to the mucoUs membrane of the oral cavity, 
 E. the hyoideus (usually united with the mandibularis for some distance 
 as a hyomandibular nerve), which goes ventrally and supplies the 
 mucosa of the mouth and the muscles of the hyoid region. In cases, 
 like many elasmobranchs, where a spiracle is present, the hyomandib- 
 
CRANIAL NERVES. 1 73 
 
 ularis passes behind it and hence is a post-trematic ramus. In some 
 cases a small twig bends down from the palatine and represents the 
 pretrematic branch. 
 
 Three of these — ophthalmicus superficialis, buccalis and mandib- 
 ularis externus — belong to the lateralis system, which is unrepre- 
 sented in the spinal nerves. This has its own ganglion, which may unite 
 with geniculate or semilunar, and it supplies the lateral line system of 
 the head (see sense organs). The superficial ophthalmic innervates the 
 supraorbital line of these organs, and in the elasmobranchs, breaks 
 up distally to go to the modified organs (ampullae of Savi and Loren- 
 zini) at the tip of the snout. In the same way the buccalis supplies 
 the infraorbital line and the mandibularis externus those of the lower 
 jaw and the hyoid and spiracular regions. 
 
 As there are no myotomic muscles in the region supplied by the 
 facial nerve, there are no somatic motor components The general 
 cutaneous elements run in the hyoideus to the skin of the hyoiji region, 
 but in other vertebrates this territory is supplied by branches of the fifth, 
 which spread to the operculum and to the dorsal surface of the head. 
 The visceral motor components occur in the hyoid nerve and, in the 
 aquatic forms, they supply the muscles of the hyoid region and the 
 posterior belly of the depressor mandibulae. In the mammals, with 
 the development of the muscles of expression (p. 134), the same 
 branch (known in human anatomy as the main branch of the facial) has 
 a much greater extension, the result of the migration of the muscles 
 from the hyoid region to their definitive position. 
 
 The geniculate ganglion belongs to the visceral sensory system, 
 the fibres of which run in the hyoid and palatine nerves to reach the 
 sense organs in the oral cavity and in the spiracular gill when this is 
 present. In those fishes where there are taste organs on the outer 
 surface of the body there is a *nerve of Weber' (ramus lateralis ac- 
 cessorius) which goes to the dorsal surface of the head and then to back, 
 fins and tail, wherever the gustatory organs occur. It is frequently 
 accompanied by fibres of the tenth nerve. In the mammals the visce- 
 ral sensory fibres occur in the main trunk of the seventh, in the great 
 superficial petrosal and in the chorda tympani nerves. This last is a 
 post-trematic nerve which passes through the middle ear and thence on 
 the medial side of the lower jaw to join the lingualis. 
 
 In the adult anura and in the amniotes, where gills and lateral line 
 organs are lacking, the facialis undergoes a corresponding reduction, 
 
174 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES 
 
 the lateralis nerves being lost, while, as stated above, the motor por- 
 tions are increased in the mammals, in correlation with the greater 
 development of the facial muscles. 
 
 Fig. 172. — ^Ventral view of brain and cranial nerves of Iguana, after Fischer. I-XII, 
 cranial nerves; 1-3, first three cervical nerves; gp, petrosal ganglion; i, Jacobson's commis- 
 sure; h, hypoglossal; n, nasalis ramus of V; rf, ramus frontalis of V; sy, sympathetic. 
 
 VIII. The Acustic (Auditory) Nerve is closely associated with the 
 seventh, but microscopic analysis shows that it has its own roots and 
 ganglion. It is purely sensory, its branches going to the sensory areas 
 of the inner ear. Its connections inside the brain and the development 
 of the ear itself (see sense organs) show that the nerve belongs to the 
 lateralis system, the ear being a group of modified lateral line organs. 
 Beyond the ganglion the nerve divides into a vestibular branch, 
 supplying the utriculus and semicircular canals, and a cochlear branch, 
 going to the lagena and to its homologue in the mammals, the cochlea. 
 
CRANIAL NERVES. 
 
 175 
 
 IX. The Glossopharyngeal, the first of the post-otic nerves, has its 
 typical development in the branchiate vertebrates. Its roots, both 
 motor and sensory, pass into the petrosal ganglion, beyond which a 
 dorsal ramus is given off to the top of the head, while the main trunk, 
 passing outward and backward, leaves the skull, either by its own fora- 
 men (most branchiates) or together with the tenth nerve (anura and 
 amniotes) . It divides, just above the first gill cleft, into pre- and post- 
 trematic nerves, which run in the anterior and posterior walls of the 
 cleft to the ventral wall of the pharynx, the pretrematic giving off a 
 nerv-e to the mucous membrane of the palate. Ninth and tenth nerves 
 
 Fig. 173. — Diagram of ninth (glossophan-ngeal) and tenth (vagus) nerves of a shark; 
 for components see fig. 170. d, dorsal ramus; g, gastric nene; h, to heart; ;, Jacobson's 
 commissure; /, lateralis nerve; Ig, lateralis ganglion; po, pr, post- and pretrematic branches; 
 5/>, spiracle; st, to supratemporal lateral line organs. 
 
 are usually closely associated (their ganglia may fuse) , while ninth and 
 fifth are frequently connected by Jacobson's commissure and the pala- 
 tine branch may connect with the geniculate ganglion. The glos- 
 sopharyngeal may contain three kinds of components, the somatic 
 motor fibres being absent because of the failure of the myotomic muscles 
 to develop. The general cutaneous is usually represented by the dorsal 
 ramus alone, but in Petromyzon its fibres reach the ventral skin through 
 the post-trematic nervx. The visceral sensory fibres reach the taste 
 organs by way of the pharyngeal and lingual nerves, while the visceral 
 motor elements go to the muscles of the gill region by way of the post- 
 trematic ramus. 
 
 X. The Vagus is a complex of nen^es, each similar to the ninth, 
 with the addition, in the branchiates, of lateralis components, and sup- 
 plying in them the remaining gill clefts. Numerous rootlets pass from 
 
176 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 the medulla to the ganglion jugularis, beyond which the dorsal rami 
 arise and then the main trunk runs backward, giving off as many 
 branchial nerves as there are gill clefts, each with an epibranchial 
 ganglion and each dividing into pre- and post-trematic rami. To 
 this extent the tenth is a polymeric nerve with coalesced proximal 
 portions. 
 
 Near the last cleft the main trunk divides into two nerves. One of 
 these, the ramus lateralis, continues back, just beneath the skin, to 
 innervate the lateral line organs of the trunk and tail. The other, the 
 
 Fig. 174. — Diagram of cranial nerves of a cat, the lower jaw reflected, after Mivart. 
 II-XII, cranial nerves; c/, chorda tympani; d, dentary nerve; g, Gasserian ganglion; io, 
 infraorbital nerve; /, lingual nerve; li, Is, laryngeus inferior and superior; md, mandibularis 
 nerve; mx, maxillaris nerve; o, ophthalmic nerve; t, tongue. 
 
 ramus intestinalis, goes inward and backward to supply the oesoph- 
 agus, stomach, heart and other viscera (in air-breathing vertebrates the 
 lungs also, whence the name pnexmaogastric nerve). In the dorsal 
 rami and the branchial nerves the components are about the same as in 
 the ninth nerve. The most caudal of the motor roots of the vagus 
 furnish visceral motor fibres which go to some of the muscles connected 
 with the pectoralarch and appendages, while others pass, by way of the 
 intestinalis, to the viscera. In the same way visceral sensory fibres go 
 through the same nerve to the taste buds of the pharynx, and in the 
 
SENSORY ORGANS. 177 
 
 higher vertebrates the same components occur in the pharyngeal, 
 laryngeal, oesophageal, and gastric branches of the intestinalis. The 
 distribution of the vagus shows that the parts supplied are to be re- 
 garded as morphologically derived from the head, though (heart, 
 lungs and stomach) they may be far removed from it in the adult. 
 Although details have been mentioned, some differences between 
 air- and water-breathing vertebrates may be summarized. The lateral 
 line organs are associated with an aquatic life, occurring in the branchiate 
 forms, even of the amphibia. With the assumption of a pulmonate 
 respiration the lateral line organs are lost and with them go the lateralis 
 elements of the seventh and tenth nerves. In the amniotes neither the 
 organs nor the nerves appear, even in development. Also the loss of 
 gills, and the closure of the clefts results in a modification of the nerves 
 of the ventral regions. 
 
 XI. The Accessory Nerve appears as a distinct nerve in the am- 
 niotes, though traces of it appear in the ichthyopsida where the poster- 
 ior roots of the vagus furnish fibres which go to muscles in the pectoral 
 region. In the amniotes the number of these roots is increased (up to 
 seven in mammals) , the additions being made to the posterior end of the 
 series. The fibres run forward between the dorsal and ventral roots of 
 the cervical nerves and unite to form a trunk, distinct from the vagus, 
 which bends back to supply muscles connected with the pectoral arch. 
 The components of this accessory nerve belong to the visceral motor 
 system, and the explanation of muscles connected with locomotion being 
 supplied by visceral ner\^es is not easy. 
 
 XII. The Hypoglossal Nerve of the adult contains only somatic 
 motor fibres, but in the young of several forms, both anmiote and 
 ichthyopsidan, two or more ganglionated roots are formed which soon 
 disappear. The roots of the nen^e lie at the junction of brain and 
 spinal cord, and hence the nerve lies outside the skull in the lower, in- 
 side it in the higher forms. The nerve contributes to the innervation 
 of the tongue, the trunk and the brachial plexus in the lower vertebrates, 
 while in the higher groups it is more restricted to the tongue and the 
 sternohyoid muscle. 
 
 THE SENSORY ORGANS. 
 
 The sensory organs are to receive information from without and 
 to transform it into stimuli to be carried by the nerves to the ganglia, 
 usually those of the central nervous system. This information varies 
 
178 
 
 COMPARATIVE MORPHOLOGY OF. VERTEBRATES. 
 
 in character and the organs consequently differ in structure according 
 to the impressions they are to receive. 
 
 With very few exceptions the characteristic portions of the organs, 
 the sensory cells, arise from the ectoderm, but accessory parts, chiefly 
 of mesodermal origin, may be so abundant as to form the bulk of the 
 organ. In some cases the organs may remain in connexion with the 
 surface of the body (the parent ectoderm) throughout life, but frequently 
 they sink to a deeper position and become surrounded with a protective 
 sense capsule, while those connected with the sympathetic system may 
 be scattered throughout almost the entire body. 
 
 B 
 
 Fig. 175. — Free nerve termina- 
 tions in the skin of Salamandra, 
 freely after Retzius. 
 
 Fig. 176. — Sensory cells, after Fiirbringsr. 
 a, crista cell of ear; b, rod cell of eye; c, ol- 
 factory cell. 
 
 The recipient structures may be of two kinds. In the one (fig. 
 175) the ends of the nerve receive the impressions from without, often 
 aided by various accessory structures. In the other there are specialized 
 sense cells (fig. 176), the peripheral ends of which bear different kinds 
 of cuticular percipient parts — hairs, bristles, rods, cones, etc. — while 
 the basal ends of the cells are connected with the terminations of nerve 
 cells which act as the conducting elements. The distinction between 
 the two is one of convenience rather than one of physiological or mor- 
 phological importance, for the 'nerves' of the first are in reality but 
 the prolongations of sensory cells. 
 
 Nerve-end Apparatus. 
 
 In many cases — skin, alimentary tract, muscles, etc. — the ends of 
 the sensory nerves lose their medullary sheath and break up into fine 
 fibrillae which terminate, without special accessory structures, among the 
 cells of the tissue to which they are distributed (free nerve termina- 
 tions). On the other hand, there are numerous end organs, espe- 
 
SENSORY ORGANS. 
 
 179 
 
 cially among the terrestrial vertebrates, in which accessory parts are 
 present. For details of these reference must be made to histological 
 text-books; only a mention of some of the kinds can be made here. 
 In the simple tactile corpuscle the nerve terminates with a cup 
 in which is seated a lenticular tactile cell (fig. 177, A). Somewhat 
 allied are Grandry's (MerkePs) corpuscles in which two or more 
 tactile cells are enclosed in a connective tissue sheath, while the nervx, 
 losing its medullary sheath as it reaches the capsule, expands into 
 plates which are inserted between each two tactile cells (fig. 177, ^). 
 
 Fig 
 
 -A, tactile corpuscle; 
 B, Grandry's corpuscle. 
 
 Fig, 178. — Vater-Pacinian corpuscle. 
 
 In another series of sensory structures the end of the nerv-e is club- 
 shaped and is surrounded by a connective-tissue sheath, either simple 
 (cylindrical corpuscles), or in Pacini*s (Vater's, fig. 178) and 
 Herbst's corpuscles, the sheath is formed of layers of cells, recalling 
 the coats of an onion, while immediately around the club is a layer 
 of cubical cells. Still another variant is found in Krausse*s (corpus- 
 culiun bulboideiun) and Meissner*s corpuscles, where the nerv^e, 
 on entering the corpuscle, breaks up into numerous branches which 
 surround an axial core of large cells. 
 
 It is impossible at present to state with certainty the function of 
 each of these and other nerve-end apparatuses and to say which are 
 connected with the different senses — tactile, pressure, pain, heat and 
 cold, muscular, etc. — which are commonly confused under the term 
 'touch.' 
 
 Lateral Line Organs. 
 
 The lateral line organs occur only in the ichthyopsida and here 
 only during the branchiate stages. They arise as thickenings of the 
 ectoderm on either side of the head in the neighborhood of the ear. 
 From here the thickenings extend in definite lines which determine the 
 
i8o 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 series of organs in the adult. At points on these lines the sensory- 
 areas are developed by the differentiation of two kind of cells, the 
 supporting cells which extend through the epidermis from the corium 
 to the free surface, and the sensory cells which reach from the surface 
 only part way to the base. The latter are pear-shaped and bear 
 cuticular hairs or bristles on their free ends (fig. 179), while the deeper 
 ends are embraced by the non-medullated fibrils of the lateralis system 
 of nerves, which follow the lines of organs, and in development keep 
 pace with their extension. These sensory areas are the nerve hillocks 
 or neuromasts alreadv referred to. 
 
 Fig. 179. Fig. 180. 
 
 Fig. 179. — Sense organ of lateral line of Diemyctylus (aquatic form) freely after Kings- 
 bury, C, cone cells; s, spindle cells. 
 
 Fig. 180. — Developing lateral line organ on one side of head of Amia, showing method 
 of closure of grooves to canals, after Allis. an, anterior naris; io, so, infra- and supraorbital 
 lines; pn, posterior naris. 
 
 In the cyclostomes and aquatic amphibia each sensory patch 
 sinks into a separate pit (fig. 179), but in all other itchhyopsida the 
 lines of organs sink in the same way, the patches being connected by 
 grooves. In ChimcBra these grooves remain open, but in all others they 
 are closed except at certain points where pores connect the canals 
 formed by the closed grooves with the exterior. In this way the sensory 
 areas come to lie in canals beneath the surface, water obtaining access 
 to them through the pores. In many teleosts (fig. 181) the pores 
 pass through notches or openings in the scales, while on the head the 
 canals themselves frequently run through some of the cranial bones. 
 
 Of considerable morpholggical importance, especially in connection with the 
 morphology of the ear, are the facts that the sensory areas multiply by elongation, 
 followed by division, and that the pores themselves increase in the same way 
 
SENSORY ORGANS. 
 
 l8l 
 
 (fig. 1 80); the pore elongates and then its margins meet in the middle, thus 
 producing two pores. There has been much discussion as to the development of 
 the lateralis nerves, especially that of the trunk, some thinking that it increases by- 
 additions from the ectoderm of the skin. It appears more probable that all of its 
 material is derived from the nerve and that there are no additions from other sources. 
 
 Fig. 181. — Stereogram of lateral line organs of a fish, c, lateral line canal; /n, lateralis 
 nerve; p, pores connecting with the exterior; s, scales in skin; so, sense organs of lateral line. 
 
 Fig. 182 . — Head of pollack, showing lateral line canals and nerves of the lateralis system, 
 after Cole. Lateralis nerves black, canals and brain dotted. &, buccalis ramus of VII 
 nerve; dl, dorsal ramus of lateralis of X nerve; h, hyomandibularis nerve; hm, hyomandib- 
 ular line of organs; io, infraorbital Hne; I, lateral line canal; n, nares; 0, olfactory lobe; op, 
 operculum; os, ophthalmicus superficialis nerve; soc, conunissure connecting lines of the 
 two sides; so, supraorbital line of organs; st, supra temporal part of lateral line; id, 
 ventral ramus of lateralis of X nerve; x, visceralis part of X nerve. 
 
 The distribution of these organs and their canals varies considerably. 
 The most constant lines are the following (fig. 182): A supraorbital 
 line running forward from the region of the ear, above the eye, to the 
 
1 82 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 tip of the snout and innervated by the ophthalmicus superficialis 
 branch of the seventh nerve; an infraorbital line running in the same 
 way beneath the eye and supplied by the buccalis nerve; a hyomandib- 
 ular line extending along the lower jaw (and the operculum when 
 present), and innervated by the mandibularis externus; and lastly 
 the lateral line proper (sometimes double) which runs back on either 
 side to the tail and is supplied by the lateralis of the tenth nerve. 
 Frequently the systems of the two sides are connected by a supra - 
 temporal line extending across the hinder part of the skull, from one 
 side to the other. 
 
 The lateral line organs appear in the larvae of all amphibia, but on 
 the assumption of a terrestrial life they sink beneath the skin and 
 usually degenerate, all traces of them and the lateralis ner\xs being lost 
 in the adult. In a few cases {Triton, Amhly stoma, etc.) they are said 
 not to be entirely lost, but to reappear at the surface when the animals 
 return to the water for oviposition. Various functions have been assigned 
 to the lateral line organs. Since they contain much mucus they were 
 long called slime organs. Then they were recognized as sensory and a 
 * sixth sense ' was attributed to them. Recently it has been made very 
 probable that they are to recognize vibrations of a slow rate in the 
 water and thus, among other things, to determine currents, etc. 
 
 Closely allied to the lateral line organs in nerve supply are the 
 ampullae of Savi and Lorenzini which occur on the head of elasmo- 
 branchs. Each consists of a long tube, opening by a pore at the surface 
 of the skin and ending with a chambered enlargement, the ampulla, 
 at the deeper end. The tube is filled with a crystal mucus and the 
 ampulla is embraced by fibres of the lateralis nerve. The organs 
 have been supposed to be connected with a pressure sense. The 
 statement is made that when they are removed the fish is unable to 
 sink; this may throw some light on their functions. 
 
 The Auditory Organs. 
 
 Both in character of innervation and in certain peculiarities of 
 development the sensory parts of the vertebrate ears are closely related 
 to the lateral line organs. In their most complete expression three 
 parts are recognized in the auditory organs, the outer, middle and 
 inner ears. Of these the last is the essential portion and occurs in all 
 vertebrates, the middle ear first appearing as such in the amphibia 
 
AUDITORY ORGANS. 
 
 183 
 
 and the outer ear, more or less completely developed, is found only in 
 the amniotes. 
 
 The Inner Ear arises as a circular area of thickened ectoderm on 
 either side of the head, between the seventh and ninth nerves (fig. 136). 
 This soon becomes cup-shaped and then the cup closes in to form an 
 auditory vesicle (fig. 183), the cavity of which is connected wuth the 
 exterior by a slender tube, the endoljrmph duct, the result of incomplete 
 
 Fig. 183. — Diagram of developing human labyrinth from 6 to 30 mm. long, after 
 Streeter. a, ampulla; c, cochlear region and cochlea; au, ampullo-utricular region; d, 
 endolymph duct; e, endolymph region; sc, semicircular canal; se, endolymph sac; s, sac- 
 culus: u, utriculus; us, utriculo-saccular canal; v, vestibule. 
 
 closure. As one portion of the medial wall of the vesicle develops an 
 area of sensory epithelium like that of the lateral line system, this 
 stage may be compared to an isolated canal organ with a single pore. 
 
 In the amphibia and some of the ganoids, where there is a two-layered ectoderm 
 from the early stages, there is never an open auditory cup. The lower, so-called 
 nervous layer of the ectoderm is alone concerned in the formation of the auditory 
 vesicle, while the outer layer extends as an unbroken sheet across the cup. In 
 the elasmobranchs the endolymph duct opens to the exterior throughout life, the 
 external pores being recognizable on the top of the head. Elsewhere they later lose 
 their external openings, and the distal end of each usually expands into an enlarge- 
 ment, the sacculus endolymphaticus ; but in the amphibia the ducts of the two 
 sides may unite dorsal to the brain, while other parts may branch and grow in a 
 root-like manner, in the canal of the spinal cord, sending diverticula (frog) into the 
 so-called calcareous glands, which surround the basal parts of the spinal nerves. 
 
 The next stage in the auditory vesicle is its differentiation by a 
 constriction into two chambers, an upper vestibulum or utriculus 
 
i84 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 and a lower sacculus (fig. 183), the two connected by a narrow sac- 
 culo-utricular canal. The sensory area becomes divided between 
 the two, but the endolymph duct is connected with the sacculus alone. 
 The anterior, posterior and lateral walls of the utriculus now produce 
 flattened outgrowths, the lateral in the horizontal, the others in vertical 
 planes, and parts of the sensory areas extend into each. Next, the 
 walls of these diverticula become pinched together so that each pocket 
 
 Fig. 184. — Diagram of the membranous labyrinth of a vertebrate, the sensory areas 
 dotted, ac, anterior semicircular canal; ap, ampullae; ca, cristae acusticae in the ampullae; 
 de, ductus endolymphaticus; he, horizontal (external) canal; /, lagena; ml, mn, ms, mu, 
 maculae of lagena (neglecta, sacculi and utriculi); pc, posterior semicircular canal; s, 
 sacculus; 5e,saccus endolymphaticus; 5MC,sacculo-utricuar canal; u, utriculus. 
 
 is converted into a tube or canal, open at either end into the utriculus, 
 and hence approximately semicircular in outline. In one end of each 
 of these semicircular canals there is a patch of sensory epithelium 
 and the wall expands around this into an ampulla, the ampullae of the 
 anterior and external canals being side by side, that of the posterior 
 canal at its lower end. 
 
 In the lower ichthyopsida there is little differentiation in the sac- 
 culus, but in the higher a pocket, the lagena, is given off from its poster- 
 ior side, a portion of the sensory epithelium extending into it. With in- 
 
AUDITORY ORGANS. 
 
 i8s 
 
 creasing powers of hearing the lagena becomes greatly elongate, until 
 in the mammals it acquires a peculiar development and is known as the 
 scala media, the structure and relations of which are described below. 
 
 In the cyclostomes utriculus and sacculus are not differentiated. In the 
 myxinoids there is but a single semicircular canal, with, however, an ampulla at 
 either end. In the lampreys there are two canals, both in the vertical plane, and 
 each with an ampulla at its lower end. 
 
 Fig. i86. 
 
 Fig. 185.^ — Labyrinth of human embryo, 30 mm. long, after Streeter. a, ampulla; ac, 
 anterior canal; c, cochlea; cr, cms; de, endolymph canal; nc, cochlear nerve; s, sacculus; se, 
 endolymph sac; u, utriculus; v, vestibular nerve. 
 
 Fig. 186. — Section through one of the coils of cochlea of guinea pig, after Schneider. 
 Bone lined; Is, spiral ligament; r, Reissner's membrane; sg, spiral ganglion; snt, st, sv, 
 scalae media (ductus cochlearis), tympani and vestibuli. 
 
 These parts of the internal ear form the membranous labyrinth. 
 With the formation of canals, lagena, etc., the sensory epithelium 
 divides into separate areas (fig. 184), some of which (maculae 
 acusticae) have sensory cells with short hairs or bristles, while others 
 (cristae acusticae), characteristic of the ampullae, have cells with longer 
 hairs. The membranous labyrinth is filled with a fluid, the endo- 
 lymph, in which are solid particles, the otoliths. These are usually 
 microscopic crystals of calcium carbonate which give the endolymph 
 
1 86 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 a milky appearance, but in the teleosts the lime is aggregated into one 
 or more ' ear stones ' of considerable size. 
 
 With the appearance of cartilage the membranous labyrinth be- 
 comes enclosed in a protecting otic capsule (p. 60), which usually 
 follows pretty closely the divisions and canals of the epithelial parts, 
 thus forming the skeletal labyrinth, separated from the membranous 
 labyrinth by a slight gap filled with fluid (the perilymph). When 
 ossification occurs the skeletal labyrinth is converted into the several 
 otic bones. Sometimes the perilymph space is separated from the 
 brain cavity by membrane alone, but usually firmer structures inter- 
 vene, interrupted only by foramina for the passage of nerves and blood- 
 vessels, for the endolymph duct and for a similar perilymph duct 
 which extends downward. On the other hand, in all vertebrates in 
 which the middle ear is developed the lateral part of the skeletal wall 
 has two openings into the middle ear. The lower of these (fig. 188), 
 the fenestra tympani (f. rotunda), is closed by membrane. In the 
 upper (fenestra ovale or vestibuli) the membrane supports a small 
 bone, the stapes (p. 73). 
 
 One part of this compound skeletal and membranous labyrinth of 
 the mammals becomes very complicated. The lagena becomes greatly 
 elongated and in order to accommodate its length it is coiled in a 
 spiral, its sides reaching the walls of the skeletal labyrinth on either 
 side. In this way the perilymph space is divided into two spiral tubes 
 (fig. 186), called scalae, from their resemblance to spiral stairways. 
 The upper of these is the scala vestibuli, the lower the scala tympani, 
 while the scala media is formed by the lagena. This whole part of 
 the inner ear is the cochlea, so-called from its resemblance to a spiral 
 shell. 
 
 The sense organ of the scala media is very specialized and is known 
 as the organ of Corti (fig. 187). In general it may be said that the 
 scala diminishes in width from base to apex of the cochlea, and is accom- 
 panied in its coils by a branch (cochlear) of the acustic nerve. The 
 sensory structures consist of hair cells and Deiter's cells, regularly 
 arranged, and a series of pillar cells, inclined to each other like the 
 rafters of a roof, in an A-like manner (fig. 187). As the A's diminish 
 in width from base to apex of the cochlea, this part has been thought 
 to play a part in the recognition of pitch. There is also a cuticular 
 structure, the membrana tectoria, which extends from the medial wall out 
 over the hair cells, and this maybe the intermediate organ of stimulation 
 
AUDITORY ORGANS. 
 
 187 
 
 and may have to do with the recognition of sound waves of different 
 rapidity. It has recently been shown that the membrana tectoria is 
 connected with the hairs of the hair cells. The fact that in birds, where 
 pitch is certainly recognized, there is no organ of Corti, renders 
 all speculation doubtful. 
 
 Fig. 187. — Organ of Corti of guinea pig, after Schneider, d, Deiter's cells; he, Henson's 
 cells; ih, inner hair cells; ip, inner pillar cells; Is, limbus spiralis; mt^ membrana tectoria; w, 
 nerve fibres; oh, outer hair cells; op, outer pillar cells; si, inner sulcus; st, scala tympani; /, 
 tunnel; tn, tunnel nerve. 
 
 The Middle Ear or tympanum first appears in the anura. It con- 
 sists of a cavity (cavum tympani) in front of and below the otic 
 capsule, connected by a slender duct, the Eustachian tube, with the 
 phan-nx. Externally it is separated from the outer world by a thin 
 partition, the tympanic membrane, from which a chain of bones, the 
 ossicula auditus (p. 73), extends across the cavity to the fenestra ovale, 
 and sen-es to transmit the sound waves to the inner ear. The tympanic 
 cavity is the homologue of the spiracular cleft of the elasmobranchs 
 (see respiration), which never breaks through. The tympanic mem- 
 brane, covered externally with ectoderm, on the inner surface with 
 entoderm, represents the imperforate w^all of the cleft, while the Eusta- 
 chian tube is the narrowed internal end of the spiracle. The chain of 
 ear bones has already been described. It is to be noted that the 
 chain consists of columella and stapes in anura and sauropsida, 
 while in the mammals columella is replaced by incus and malleus. 
 In the urodeles and gymnophiones, where no tympanic cavity is devel- 
 oped, the quadrate articulates with the stapes. 
 
 The External Ear. — In the anura and in many reptiles the tym- 
 panic membrane is flush with the surface of the head, but in other rep- 
 tiles and in birds it is at the bottom of a canal, the external auditory 
 meatus, the simplest expression of an external ear. In the mammals 
 
i88 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 (whales, sirenians and some seals are exceptions) an external conch is 
 developed behind the meatus to collect the sound waves and to direct 
 them to the inner parts. In some birds the feathers are arranged 
 around the meatus so as to play the same part. The conch is strength- 
 ened by cartilage and is moved by muscles (fig. 142). There is evi- 
 dence which points to the conch being homologous with either the 
 operculum of fishes or with the first external gill of amphibians. 
 
 Fig. 188. — Diagram of mammalian ear. a, ampullae of semicircular canals ; an, acustic 
 nerve; en, cochlear nerve; em, external auditory meatus; eu, Eustachian tube;//, fenestra 
 tympani; i, incus; nt, malleus; p, perilymph space (black) ; pd, perilymph duct; ph, pharynx; 
 s, stapes; sc, sacculus; sm, st, sv, scalae media, tympani et vestibuH; sg, spiral ganglion; t, 
 tympanic cavity; tm, tympanic membrane; u, utriculus; v, vestibular nerve. 
 
 Functions. — The vertebrate ear is primarily an organ of equi- 
 libration by which the animal recognizes all changes of position. 
 Though the purposes of the various parts are not accurately known, 
 the following conclusions seem warranted. Every movement of the 
 head afifects the endolymph and the contained otoliths, causing them 
 to move (by gravity or by momentum, or by both) over the cristae 
 acusticae in the ampullae and thus to stimulate the sense cells and nerves. 
 The position of the semicircular canals in approximately the three 
 dimensions of space would seem to afford a means for the recognition 
 of the directions and amounts of the components of any motion. The 
 maculae, and especially that of the lagena, are probably concerned in the 
 recognition of sound. In the fishes the lagena is poorly developed, 
 
OLFACTORY ORGANS. ^89 
 
 and while some fishes have been proved to hear, others have given 
 negative results. With the terrestrial vertebrates the sound percipient 
 functions of the ear are beyond a doubt, while they still retain their 
 equilibrational use. The sound waves strike the tympanic membrane, 
 are carried across the middle ear by the auditory ossicles, and set the 
 perilymph in motion and thus affect the parts of the membranous 
 labyrinth. 
 
 Organs of Taste. 
 
 The sense of taste is resident in groups of cells known as taste buds. 
 These differ morphologically from the lateral line organs in having 
 each sensory cell extend the depth of the bud, ending at the basal mem- 
 brane, while the majority of the supporting cells are on the outer side of 
 the bud. Each sense cell bears a short, bristle-like percipient struc- 
 ture on its free end, while the basal end is embraced by the fibrillae of the 
 nerve. According to the accounts of the development the taste buds 
 are derived from the entoderm, the only case apparently established 
 for the origin of sense organs except from the ectoderm. In the higher 
 vertebrates the organs are restricted to the cavity of the mouth where 
 (mammals) they occur on the tongue, especially on and near the cir- 
 cumvallate papillae, on the soft palate and on the epiglottis. In the 
 fishes the distribution is much wider, for they are found in the pharynx, 
 on the gills, and in many species on the surface of the body, even upon 
 the tail. The barbels about the mouth of many forms are richly 
 supplied with these organs. 
 
 The taste organs are supplied by different nerves. Apparently 
 those of mammals are supplied by the chorda tympani and the lingual 
 branch of the ninth nerve. In the fishes those of the pharyngeal 
 region are supplied by the post-trematic branches of the glossopharyn- 
 geal and vagus; those of the mouth by the palatine and mandibular 
 branch of the seventh; while those on the head of teleostomes are 
 supplied by the ophthalmic and maxillary branches of the fifth; and 
 those of the trunk by the nerve of Weber (p. 173), formed by fibres 
 from the seventh and sometimes of the tenth nerves. 
 
 Olfactory Organs. 
 
 While the senses of smell and taste are closely associated physiolog- 
 ically, being what might be called the chemical senses, the organs con- 
 
190 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 cerned differ considerably in structure and relations. The olfactory 
 epithelium is always restricted to one or two patches at the anterior end 
 of the head and differs from the taste buds in histological structure. 
 Both sensory and supporting cells of the olfactory organs are variously 
 constituted. The supporting cells are the stouter, some being ciliated, 
 some muciparous at their free ends. The sense cells (fig. 176, C) are 
 thread-like or rod-like, being greatly expanded around the spherical 
 nucleus, while the basal end of each contracts to a nerve fibre which 
 extends back to the olfactory tract (p. 168), where the dendrites, inter- 
 lacing with those of the olfactory lobe, form the glomeruli. In the 
 higher vertebrates a third kind of cislls, the basal cells, occur at the base 
 of the olfactory epithelium. 
 
 The olfactory epithelium arises as part 
 of the surface ectoderm of the top of the 
 head, but with growth it changes its position. 
 For protection it sinks beneath the surface as 
 an olfactory sac, connected with the external 
 world by (usually) a pair of openings, the ex- 
 ternal nares. The growth of the dorsal side 
 of the head carries the nares toward the tip 
 of the snout and, in the elasmobranchs, to 
 the ventral side of the head. 
 
 The accessory parts of the olfactory 
 „ „ ^^ , , organs are the skeletal nasal capsules (p. 
 
 Fig. 189.— Nasal organ of , ° . r \r 
 
 caEcilian(£/>imMw), after Sara- 62), which are always present; in the tetra- 
 t»bsfn^,trriraldu??/ft PO^ous forms gknds to keep the epithelium 
 lateral cavity; mp, middle pas- moist, and the organ of Tacobson. The in- 
 sage; OS, olfactory sac. , . p ^ ^ 
 
 volution of the nasal sacs necessitates some 
 mechanism for bringing the external medium (water or air) to the sen- 
 sory cells. These will be described in connection with the several 
 groups below. The organ of Jacobson is a kind of accessory 
 olfactory organ, first appearing in the amphibia, supplied by the first 
 and fifth nerves and apparently serving to test the character of the food 
 while in the mouth. The position of the organ near the internal 
 nostrils lends probability to this view of the function. 
 
 The cyclostomes differ markedly from the other vertebrates in their olfactory 
 organs. The unpaired area of olfactory epithelium develops in the region of the 
 anterior neuropore (p. 12) and becomes involved v^^ith the involution for the 
 hypophysis (fig. 190) so that there is but a single external opening, serving for both 
 
OLFACTORY ORGANS. 
 
 191 
 
 olfactory organ and hypophysis. Hence cyclostomes, having but a single nostril, 
 are called monorhinal, in comparison with all other vertebrates which have two 
 nostrils (amphirhinal). The median opening or naris of the cyclostomes connects 
 with a naro-hypophysial duct, on the upper, posterior wall of which is the ol- 
 factory sac, formed of pairs of lateral folds (fig. 191) covered with the olfactory 
 
 Fig. 190. — Longitudinal section of head of 19 day Petromyzon embrgyo. ch, optic 
 chiasma; ep, epiphysial outgrowth; h, hypophysial ingrowth; mes, mesenteron; n, nasal 
 epithelium; nc, notochord; oc, oral ca\dty; op, oral plate; sc, canal of spinal cord; th, 
 thjTcoid. 
 
 epithelium and supplied by a pair of olfactory nerves. The lower part of the duct, 
 now purely hypophysial, descends to the hypophysis on the ventral side of the brain, 
 where it either ends blindly (petromyzons) or opens into the dorsal part of the oral 
 cavity (myxinoids). In the latter group the olfactory organ is surrounded by a 
 complicated nasal capsule of enormous size (fig. 153). 
 
 on 
 
 Fig. 191 Fig. 192. 
 
 Fig. 191. — Xario-hypophysial region oi Petromyzon, iromaboxe. c, cartilage of nasal 
 capsule; hd, hypophysial duct; of, folds of olfactory membrane; on, olfactory nerve. 
 Fig. 192.— Head of Murcena, after Jordan and Evermann, showing double nostrils. 
 
 All Other vertebrates have paired olfactory areas and paired nostrils 
 (nares) are developed in connection with them, and they have at no 
 time any relation to the hypophysis. The mechanism for bringing the 
 water or air to be tested to the olfactory surface differs accordingly as 
 the animals are air or water breathers. In all fishes, with the exception 
 of the dipnoi, the sensory surface is at the bottom of a pit with no 
 connection with the alimentary canal. In the elasmobranchs, in 
 
192 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 order that water may flow more readily through this pit, a fold is 
 developed on one side of each naris, which practically divides it into 
 two. In many teleosts there is an actual division of each primitive 
 nostril into two, which may be at some distance from each other, 
 
 Fig. 193. — Section through the nasal labyrinth of Polypterus. The nerve runs through 
 
 the centre. 
 
 '~^i-vy 
 
 Fig. 194. Fig. 195. 
 
 Fig, 194. — ^Head of chick of 5^ days, showing development of oro-nasal canal 
 after Keibel. cf, chorioid fissure; /, thickening for lacrimal duct; n, nasal pit; on, oro- 
 nasal groove. 
 
 Fig. 195. — Model of mouth of Echidna embryo, after Seydel, showing method of in- 
 growth of palatal folds {pf) to cut ofif secondary nasal passages, ch, primitive choanae; et, 
 egg tooth; 7, opening of Jacobson's organ. 
 
 often at the ends of prominent tubes (fig. 192). Inside the nasal 
 capsule the olfactory epithelium is variously folded in order to increase 
 the sensory surface, often forming a labyrinth of considerable complex- 
 ity (fig._ 193). 
 
OLFACTORY ORGANS. 
 
 193 
 
 In air-breathing vertebrates, beginning with the dipnoi, a means is 
 developed for drawing air over the sensory surface, the first traces of 
 which are seen in the elasmobranchs. These frequently have an 
 oro-nasal groove, leading from each naris to the angle of the mouth. 
 In some species this groove is practically converted into a tube by the 
 meeting of the walls below. Beginning with the dipnoi and continuing 
 with the amphibia and amniotes (fig. 194) a similar groove is formed 
 on either side before the formation of skeletal parts. This closes in, 
 the edges of each groove uniting, so that a tube or duct is formed, lead- 
 ing from the naris into the oral cavity, where an internal naris or 
 choana occurs. Later maxillary and premaxillary bones arise ventral 
 to the narial passage, so that the ducts appear to run through the skull. 
 The position of the choanae varies considerably, being just inside the 
 jaws in the amphibia and lower reptiles, farther back in the higher 
 reptiles and the birds and mammals, the nasal passages being cut off 
 from the roof of the primitive mouth by the ingrowth of the palatal 
 processes of the maxillary bones and higher, by similar extensions of 
 the palatines, and in some cases, of the pterygoids (fig. 195). 
 
 Incomplete closure of the oronasal groove results in the deformity known as 
 'hare-lip' externally, while 'cleft palate' is the result of failure of palatines, and 
 sometimes of maxillaries to meet below the nasal passages. 
 
 Fig. 196. Fig. 197. 
 
 Fig. 196. — Section through the nasal region of Siren, after Seydel. en, nasal cavity, 
 jg\ Jacobson's gland; jo, organ of Jacobson; v, vomer. 
 
 Fig. 197. — Section of nose oi Chelonia cauana, aitei Gegenbaur. c, concha; ch, choana; 
 i, inner olfactory groove; n, projection of naris between dotted lines. 
 
 In the dipnoi the olfactory membrane forms a few large folds on 
 the dorsal side of the respiratory duct formed from the oronasal tube. 
 In the amphibia the sensory surface has a similar position on the upper 
 medial surface (fig. 196), with frequently a lateral pocket lined with 
 sensory epithelium, the beginnings of an organ of Jacobson. In the 
 same group glands (inner and outer Jacobson's glands) occur for 
 
194 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 moistening the olfactory epithelium. Usually there is little complica- 
 tion of the olfactory surface, but in a few urodeles (Plethodon) there 
 is a projection from the lateral wall, the first indication of the conchae 
 which acquire such development in the higher groups. There is fre- 
 quently a differentiation of the nasal passage into a ventral respiratory 
 duct lined with ordinary and a more dorsal olfactory duct lined with 
 sensory epithelium. In the lower urodeles the diverticulum repre- 
 senting the organ of Jacobson is on the medial side of the nasal cavity; 
 a little higher it is ventral, while in the highest urodeles it has rotated 
 to the lateral side. It may be noted that some of the amphibia have 
 smooth muscles to close the external nares. 
 
 Aside from the varying position of the choanae the changes from 
 amphibia to reptiles in the olfactory organs are comparatively slight. 
 
 Fig. 198. — Longitudinal section of nasal region of alligator, after Gegenbaur. c, concha; 
 ms, maxillary sinus; «, naris; p, pseudoconcha. 
 
 The olfactory region becomes more distinct from the respiratory tract 
 and the latter shows a tendency to be differentiated into an anterior 
 atrium or vestibule, a middle area connected with the olfactory region, 
 and a posterior naso-pharyngeal duct between the basis cranii and 
 the roof of the mouth. This latter duct varies in length accordingly 
 as the choanae are anterior or posterior in position, the extreme being 
 reached in the crocodiles, where by ingrowth of palatines and ptery- 
 goids, the internal nares are carried back nearly to the hinder end of 
 the skull. A single concha, supported by bone, is developed in the 
 lateral wall of the reptilian nose. It is weak in the turtles (fig. 197), 
 but is larger elsewhere, and in the crocodiles (fig. 198) it becomes 
 divided in front, while a 'pseudoconch' (its homology with the supe- 
 rior concha of birds is uncertain) is developed above and behind the 
 true concha. Jacobson's organ occurs only in the squamata, where 
 it forms a simple pocket in the primitive position, ventral and medial 
 to the nasal cavity, near the nasal septum. 
 
OLFACTORY ORGANS. 
 
 195 
 
 There are three folds developed on the wall of each nasal cavity in 
 birds, an anterior and inferior concha vestibuli, a middle and a superior 
 fold, the middle supported by the maxillo-turbinal, the superior by the 
 naso-turbinal bones. The vestibular conch lacks olfactory epithelium 
 at all times, while it disappears from the middle one after hatching, 
 
 Fig. 199. — Olfactory region of hen in longitudinal and transverse section, after Gegen- 
 baur. c, middle concha; ch, choana; i, inferior (anterior) concha; o, connection of air 
 cavity of head; p, septum of nose; s, superior concha. 
 
 leaving the upper conch as the sole seat of smell in the adult, which 
 corresponds with the limited sense of smell in these animals. Jacob- 
 son's organ is never developed in the adult, though traces of it appear 
 in the embryos. 
 
 With the great increase of the sense of smell in the mammals the 
 
 Fig. 200. Fig. 201. 
 
 Fig. 200. — Model of the nasal cavity of a rabbit embryo, 13^ mm. head length, after 
 Peter, ch, choana; el, first ethmoturbinal; j, organ of Jacobson; oj, opening of same; mt, 
 maxilloturbinal; rU, nasoturbinal. 
 
 Fig. 201. — Nasal cavity of Erinaceus, after Paulli, showing the foldings of the maxUlo- 
 turbinals (mt) and the nasoturbinals (n/). 
 
 nasal labyrinth undergoes a corresponding complication, and is farther 
 characterized by the great length of the naso-pharyngeal duct, and by 
 the position of the olfactory area below a part of the brain cavity. The 
 folds of the labyrinth may be supported by processes, more or less com- 
 plicated, of three bones or cartilages, the ethmo-turbinals, the naso- 
 
196 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 turbinals and the maxillo-turbinals (fig. 200), the purpose of these 
 folds being to increase the amount of sensory surface, while the skeletal 
 supports keep the folds from touching each other. With diminution 
 of the powers of smell the folds are correspondingly reduced, even to 
 a loss of the turbination of the bones concerned. 
 
 The maxillo-turbinals and naso-turbinals arise from the lateral wall 
 of the nasal cavity (the former as a distinct turbinal bone) , the ethmo- 
 turbinals as outgrowths from the ethmoid bone, 
 appearing first at the upper hinder part of the 
 septal wall and extending to the lateral wall. 
 The result is that the ethmo-turbinal tends to 
 insinuate itself between the hinder ends of the 
 other two (figs. 200, 201). Each of these may be 
 subdivided, with corresponding subdivision of 
 the epithelial covering, and in the case of the 
 ethmo-turbinals the subdivisions may be of 
 varying heights (fig. 202), the ecto- and ento- 
 turbinals. The nasoturbinals often disappear in 
 the adult, while the epithelium of the maxillo- 
 turbinals is not sensory in character, this part 
 of the nose being apparently to warm and 
 moisten the air in its passage to the lungs. 
 The homologies of the various parts of the nasal labyrinth in dif- 
 ferent amniotes are thus stated (Peter). 
 
 I. Concha of the anterior epthelium: concha vestibuli (birds). 
 
 II. Conchge of the primitive sensory epithelium : 
 
 1. Arising from the lateral wall (conchge laterales). 
 
 A. Anterior: 
 
 a. Primary, ventral: concha of reptiles; middle concha 
 of birds; maxillo-turbinals of mammals. 
 
 b. Secondary, dorsal: Upper or posterior of birds; naso- 
 turbinals of mammals (? pseudoconch of crocodiles). 
 
 B. Arising from the posterior part: conchae obtectae of mam- 
 mals. 
 
 2. Arising from the primitively median wall: ethmo-turbinals 
 
 of mammals, numbered from in front backward. 
 Jacobson's organ (vomero-nasal organ) is laid down in the embryo 
 of most mammals as a groove or pocket on the lower medial side of 
 each nasal cavity, opening in rodents and in man near the duct of 
 
 Fig. 202 . — S e c t i o n 
 through the nasal cavity 
 ofj'a new bom dog, after 
 Paulli, I-IV, entoturbi- 
 nals; 1-5, first to fifth ec- 
 to turbinals. 
 
OLFACTORY ORGANS. 
 
 197 
 
 Stenson's gland; in other mammals, so far as known, its duct becomes 
 cut off from the nasal cavity and opens into the naso-palatal canal. Its 
 medial wall is covered with sensory epithelium, supplied by a branch 
 of the olfactory nen^e. In the primates the organ is more or less de- 
 generate in the adult. 
 
 There are two kinds of glands in the nasal cavity, the smaller and 
 scattered Bowman's glands and the larger Stenson's gland lying in 
 the lateral ventral wall and opening into the vestibule. There are 
 usually several sinuses in the bones of the skull, connected with the 
 
 Fig, 203, — Lateral wall of nasal cavity of man, after Coming, c^, crista gaUi; «', cm, 
 cs, inferior, middle and superior conchae; fpm, foramen palatinum ma.jus;fsp, sphenopala- 
 tine foramen; ic, incisive canal; osm, opening of maxillary sinus; sf, frontal sinus; ss^ 
 sphenoidal sinus. 
 
 nasal cavities by foramina. Chief of these are the maxillary sinuses 
 (antra of Highmore), the frontal and sphenoidal sinuses in the 
 
 corresponding bones, the relations of which may be seen in fig. 203. 
 Others may occur in other bones of the face. 
 
 Mammals are characterized by an external fleshy nose, supported 
 by the nasal bones and by cartilages, developed in part from the eth- 
 moid cartilage of the embryo, in part from paired cartilages, a new 
 acquisition of the mammals. Beyond these skeletal parts is the fleshy 
 portion which may form a proboscis of considerable size (swine, 
 elephant shrew, elephant). 
 
198 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 In most mammals the sense of smell is well developed, but is comparatively 
 slight in the seals, whalebone whales and in the primates, while it is completely lost 
 in most of the toothed whales where even the olfactory nerve may disappear. 
 
 The Eyes. 
 
 The sensory part of the eyes comes from the ectoderm of the neural 
 plate, and in several embryos the regions which are thus destined may 
 
 be recognized on its dorsal surface 
 before it is infolded to form the vesi- 
 cles of the brain. The accessory parts 
 of the eye are derived in part from the 
 general ectoderm, in part from meso- 
 derm of both kinds. 
 
 As the neural plate closes up to form 
 the brain (p. 11), the optic areas begin 
 to grow outward from the fore-brain 
 toward the sides of the head, each form- 
 ing at first a hollow outgrowth, the optic 
 vesicle, connected with the brain by a 
 hollow optic stalk. The next phase is 
 the involution or invagination of the 
 distal side of the vesicle so that it is 
 converted into a double walled optic 
 cup (fig. 204). There thus results a differentiation of parts in the 
 optic outgrowth and a partial obliteration of the cavity of the vesicle. 
 The distal wall which forms the inside of the cup is called the retinal 
 
 Fig. 204. — Stereogram of de- 
 veloping eye. c/", chorioid fissure ;/6, 
 cut wall of fore-brain; /, anlage of 
 lens; oc, optic cup; os, optic stalk; p, 
 layer for pigmented epithelium; r, 
 retinal layer. 
 
 Fig. 205. — Sections of successive stages in the development of the lens of the eye from the 
 first thickening of the ectoderm {ec) to the complete separation ot the lens, l. 
 
 layer; the outer wall the pigment layer, in anticipation of their de- 
 velopment into the corresponding parts of the adult. 
 
 The involution of the retina is not easily described, but may be 
 
EYES. 
 
 199 
 
 understood from figure 204. It occurs on the lower distal side so that 
 the cup is not complete but is interrupted by a deep notch, the chorioid 
 fissure, below, and this is extended as a groove on the ventral side of 
 the optic stalk. Later the fissure closes (fig. 194), but not until 
 some of the changes described below have occurred. 
 
 Opposite the distal part of each optic vesicle the ectoderm of the 
 side of the head thickens, then becomes invaginated (fig. 205), the 
 mouth of the invagination closes, and the hollow ball thus formed is 
 cut off from the rest of the ectoderm and sinks into the mouth of the 
 optic cup, where it forms the lens of the eye. From the first the cells 
 of the two sides of the lens differ in size, those of the outer wall being 
 cubical, those of the other being elongate, while the cavity is a narrow 
 cleft. Later the cavity is obliterated, while the lens is increased in 
 size by the addition of new cells, like the coats of an onion, by budding 
 from the equatorial zone of the lens. 
 
 Fig. 206. — Mammalian retina; above the general appearance, below the diagrammatic 
 relations; the lens toward the left, c, cone; cc, cone cell; g, ganglion cells; ig, inner granular 
 layer; im, inner molecular layer; m, basal membrane; «/", nerve fibres; og, outer granular 
 layer; om, outer molecular layer; r, rod; re, rod cell. 
 
 The Retina consists of several layers which constitute the ganglion 
 and the sensory cells, the latter being on the outer surface, i.e., that 
 which is turned away from the lens. Each sensory cell bears on its 
 outer end the percipient structure, rod or cone, which has given these 
 the name of rod and cone cells. These rods and cones project 
 through the basal membrane which encloses the retina into the 
 pigment layer to be described shortly. The bodies of the cells with 
 their nuclei are inside the basal membrane, where they form the so- 
 
200 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 called outer granular (nuclear) layer, separated by an outer 
 'molecular' (reticular) layer of interlacing dendrites from the 
 inner granular layer. This is ganglionic in character and is con- 
 nected by the inner molecular layer with the rest of the ganglionic layer 
 which lines the inside of the retinal cup. 
 
 In order to understand the latter layer and the relations of the optic 
 nerve, an account of the development is necessary. At first the retinal 
 layer is comparatively thin, but it increases in thickness, in part by a 
 multiplication of cells, in part by their increase in length and the devel- 
 opment of the dendrites of the molecular layers. Each cell of the inner 
 layer (the one turned toward the lens) also develops an axon which 
 runs over the free surface of the cells to the chorioid fissure, passes 
 through this and along the ventral groove of the optic stalk to the 
 diencephalon. 
 
 As will readily be understood, it is these fibres and not the optic 
 stalk which form the optic nerve (p. 169). When the chorioid fissure 
 closes, the nerve appears to leave through the centre of the retina, 
 and as this part contains no sense cells, the point of exit constitutes 
 the 'blind spot' of physiological works. Besides the cells already 
 mentioned the retina contains supporting or radial cells, like other 
 sense organs or like the brain itself (neuroglia). These extend through 
 from the nerve fibres to the basal membrane. Either rods or cones 
 may be absent in isolated groups of vertebrates. Usually there is a 
 spot, the macula lutea (yellow spot) or fovea centralis at the centre 
 of the retina where vision is most distinct. Here the rod and cone cells 
 are shorter and more crowded than elsewhere. 
 
 Here may be mentioned a point of morphological importance. It will be 
 recalled (p. 138) that the ependymal surface of the brain corresponds to the external 
 surface of the ectoderm of the rest of the body. Therefore, as a glance at fig. 204 
 will show, the rods and cones are on the primitively outer and the ganglion cells and 
 nerve fibres are on the deeper surface of the ectoderm. Hence rods and cones 
 correspond to the percipient cuticular structures of other sensory organs like the 
 lateral line, taste buds and the like. Before it can affect the sensory cells the light 
 has to traverse the whole of the retina and then the nervous impulses have to 
 pass back through the same layers to reach the optic nerve. This constitutes an 
 'inverted eye' and, with the exception of a few molluscs, it is unknown, except in 
 the vertebrates. A comparison with the parietal eye of reptiles (fig. 151) is very 
 instructive. 
 
 The cavity between lens and retina is filled with a semisolid vitre- 
 ous body, the origin of which is in dispute. In mammals blood-vessels 
 
EYES. 
 
 20I 
 
 and mesenchymatous cells enter the optic cup through the chorioid 
 fissure before its closure. Some suppose that the vitreous body arises 
 from a modification of these cells, some regard it as an exudate from the 
 blood-vessels, and others think it a retinal secretion. The fact that 
 the blood-vessels mentioned do not occur in birds is of interest in this 
 connection. In mammals, when the chorioid fissure closes, the vessels 
 appear to enter through the centre of the optic nerve (central retinal 
 artery and vein — fig. 207). In the early stages the retinal artery 
 
 Fig. 207. — Diagrammatic section of half a mammalian eye. ac, anterior chamber; 
 ca, ciliary arteries; c, eyelash (cilium); cj, conjunctiva; co, cornea; cp, ciliary process; cr, 
 central retinal artery and vein; cs, conjunctival sac; ct, chorioid tunic; d, dura of optic 
 nerve; i, iris; on, optic nerve; os, oia. serrata; pc, posterior chamber; pe, pigmented epithel- 
 ium; r, retina; sc, sclera; tg, tarsal gland; w, vorticose vein; cc, zonula zinii. 
 
 divides inside the cup, one branch (hyaloid artery) going through the 
 vitreous body to the neighborhood of the lens, the other being distrib- 
 uted over the inner surface of the retina. Later the hyaloid artery 
 disappears, while retinal arteries are rare except in mammals. 
 
 The outer wall of the optic cup forms the pigmented epithelium 
 of the eye, developing a large amount of black pigment which eventu- 
 ally surrounds and isolates the rods and cones, so that each can be 
 affected only by the light which falls directly upon it. As will readily be 
 understood the side of the pigment layer away from the retina corre- 
 sponds to the deeper surface of the skin and so comes into relation with 
 the connective tissue. From this is developed the envelopes of the 
 eye — tunica vasculosa, sclera, etc. 
 
 Surrounding the retina and pigmented epithelium and extending 
 forward over the lateral parts of the lens is the tunica vasculosa, 
 in which two parts are recognized, the iris and the chorioid. The 
 
202 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 whole is richly vascular, and the chorioid, supplied by the ciliary 
 arteries which enter at the side, is the chief source of nourishment 
 for the rod and cone cells. To the vascular part certain other portions 
 are added in some groups. Thus just outside the blood-vessels there 
 may be a large lymph space, and outside of this, in most fishes and 
 some amphibia and turtles, there is an argenteal layer containing 
 calcic crystals which give the layer a whitish appearance. On the 
 other hand, the side toward the retina frequently develops a somewhat 
 similar tapetum lucidum, with a metallic lustre, which reflects 
 light strongly and is the cause of the apparent shining at night of the 
 eyes of many selachians and some other fishes and carnivore mammals. 
 In a few teleostomes (those with a pseudobranch) there is a so-called 
 chorioid gland just oustide the vascular layer, near the entrance of 
 the optic nerve. It partakes of the nature of a rete mirabile. 
 
 The chorioid extends as far forward as does the retina, when its 
 anterior edge is produced into a circular ciliary process, which is best 
 developed in the amniotes, though appearing here and there in the 
 ichthyopsida. This process is muscular (ciliary muscles) at its base and 
 is connected at its margin with the delicate capsule surrounding the 
 lens by a double fenestrated membrane, the zonula ciliaris (Zinnii). 
 By the action of the muscles the lens is moved toward or away from the 
 retina, while variations in tension may slightly alter its shape, thus 
 changing its focal point (accommodation of the eye). 
 
 Beyond the ciliary process the vascular tunic continues in front 
 of the lens as the iris, a circular curtain with a central opening, the 
 pupil. Pigment in the posterior layer (uvea) of the iris renders it 
 opaque, while in many fishes the outer surface is silvery owing to the 
 continuation of the argentea into this region. The rest of the iris is 
 muscular, the muscles increasing in extent from the lower to the 
 higher forms. They are arranged in two groups. The circular muscles 
 (sphincter pupillae), by their contraction, diminish the size of the pupil; 
 the radial (dilator pupillae) are antagonistic and efifect an enlarge- 
 ment of the opening in the iris. In the sauropsida these iridial muscles 
 are cross banded, in amphibia and mammals of the smooth variety. 
 
 Surrounding all of the structures of the eye so far described is the 
 sense capsule, which differs from all other sense capsules (p. 62) in 
 not being connected with the rest of the skull, as a result of its neces- 
 sity for movement. In the capsule two parts are distinguished, the 
 sclera which covers the proximal side of the eye, and the cornea, 
 
EYES. 203 
 
 perfectly transparent, through which light passes to reach the lens. 
 The cornea, covered externally by the conjunctiva, the modified 
 epidermis of the front of the eye, consists of connective tissue; the 
 sclera is usually white. In most of the lower vertebrates and in the mono- 
 tremes it is partly or wholly cartilaginous, but in other mammls and in 
 the lampreys it consists of fibrous tissue. In the stegocephals and in 
 many reptiles and birds portions of the sclera ossify as a ring of scle- 
 rotic bones (p. 67). 
 
 Sclerotic bones are lacking in snakes, plesiosaurs and crocodiles. In the 
 sturgeon and many teleosts two or more dermal bones develop upon the sclera, but 
 neither these nor the calcifications to be found in some sharks and teleosts are to 
 be confused wdth true sclerotics. 
 
 Between cornea and lens is a cavity which is partially divided by 
 the iris into anterior and posterior chambers which connect with 
 each other through the pupil. These are filled with a refracting fluid, 
 the aqueous hiunor. 
 
 The parts so far described form the eye-ball (bulbus oculi) which 
 is more or less freely movable in its socket in the side of the head. It 
 is moved by the six muscles (p. 128) which are constantly present. 
 Others may occur here and there. Thus in the amphibia a distinct 
 muscle (retractor bulbi) is developed from the external rectus to 
 pull the ball back into the socket, while portions of the jaw muscles 
 may be set apart as elevators and depressors of the ball. In the elasmo- 
 branchs a cartilaginous rod, the optic pedicel, extends from the ball 
 to the skull. This is replaced in the teleosts by a fibrous band, the 
 tenaculum, but its equivalent is not found in the higher groups. 
 
 Among the accessory parts of the eye are the lids, of which there 
 may be three, the upper and the lower lids so familiar in the higher 
 vertebrates and the third lid, the nictitating membrane, a transparent 
 sheet which may be drawn horizontally across the front of the eye 
 from the inner (anterior) angle of the eye or from beneath the lower lid. 
 All three lids are folds of the skin. The upper and lower are poorly 
 developed in the ichthyopsida, but appear in the amniotes. They 
 are lined on the side next the eye by a continuation of the conjunctiva, 
 which continues beyond the edge of the lid as the epidermis. The 
 nictitating membrane appears in some sharks, again in the amphibia, 
 and receives its highest development in the sauropsida, while in the 
 mammals it is reduced to a rudimentary fold, the plica semilunaris, 
 at the inner angle of the eye. 
 
204 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 There are no glands connected with the eyes in cyclostomes or 
 fishes, but in the urodeles a series of glands is developed from the con- 
 junctival lining of the lower lid. In the amphibia they show little 
 differentiation, but in all sauropsida (glands are lacking from a few 
 reptiles — crocodile tears are non-existent) they become divided into 
 two groups. One becomes aggregated near the inner angle and forms 
 what is known as Harder*s glands (glandula membrana nictitans) ; 
 the others migrate toward the outer angle of the eye and constitute the 
 true lacrimal or tear gland. In the mammals the migration continues 
 until the gland comes to lie beneath the upper lid, where it shows its 
 multiple nature by the numbers of ducts by which it pours its secretions 
 into the conjunctival sac. In most mammals Harder's gland degener- 
 ates. The tears secreted by the glands pass over the conjunctiva and are 
 collected at the inner angle of the eye, where they are drained by the 
 lacrimal duct into the cavity of the nose. This duct is formed as a 
 thickening of the epidermis which later becomes perforated. It 
 follows the course of an earlier groove (fig. 194) leading from the orbit 
 to the nasal invagination and which was formerly thought to form the 
 duct. 
 
 The eyes of the cyclostomes are degenerate. In the larval (Ammocoetes) stage 
 of Petromyzon the eye is buried under a thick skin, but this thins out in the adult. 
 In the myxinoids the lens and eye muscles are lacking, and iris, cornea and sclera 
 are not dififerentiated. 
 
 Fishes have eyes with a very flattened cornea, a spherical lens and very long 
 retinal rods. A peculiar feature in many fishes is the falciform process, a vascular 
 and muscular structure which enters the retinal cup through the chorioid fissure and 
 extends to the lens where it bears an expansion, the campanula Halleri. The 
 whole is supposed to act as a means of accommodation, there being no ciliary 
 muscles. In most fishes the eyes are so placed on the sides of the head that there 
 must be monocular vision. In the flat fishes (Heterosomata) one of the eyes mi- 
 grates during development, so that both eyes come to lie on one side of the head. 
 
 Most sauropsida are characterized by the development of a process from the 
 inner retinal surface which reaches its extreme in the pecten of the birds. In the 
 reptiles it is a small conical process arising from the point of entrance of the optic 
 nerve, but in the birds this expands distally into a quadrangular plate, folded like a 
 fan, to which various functions have been ascribed. It has been recently shown 
 to be rich in sense cells. The shape of the eye of the bird is peculiar, but is not 
 easily described. It consists of a hemispherical posterior part, followed by a 
 conical portion, and this surmounted by a hemispherical corneal region, the whole 
 being somewhat telescopic in shape. The whole is very large in proportion to the 
 size of the animal. 
 
 The pecten is said to be outlined in the foetal stages of some mammals. The 
 
DIGESTIVE ORGANS. 205 
 
 pupil of the mammals is not always circular, but is a vertical slit in the cats, a 
 horizontal opening in the whales, many ungulates, etc. During development the 
 lids fuse for a time, separating in some cases, only after birth. The edges of the 
 lids are fringed with short hairs, the eye-lashes or cilia, and internal to these are 
 the ducts of sebaceous glands (tarsal or Meiobomian glands), the glands them- 
 selves being in the substance of the lids. The whales have an enormously thick 
 sclera which, here as elsewhere, appears as a continuation of the dural sheath of 
 the optic nerve. 
 
 THE DIGESTIVE ORGANS. 
 
 Few articles of food, as they come to a vertebrate, are in shape to be 
 taken immediately into the organism and to be used, without modifica- 
 tion, as a source of energy or as material for the construction of new 
 tissue or the repair of old. They have to be altered so that they are 
 soluble and so able to pass by osmosis into the blood-vessels (proteids, 
 carbohydrates), or they must be broken up (hydrocarbons) so as to be 
 taken up by the absorbtive vessels (lacteals) of the lymphatic system. 
 These changes in the food, which are the result of the action of the 
 secretions of the digestive glands, constitute the process of digestion. 
 The digestive tract or alimentary canal, where these changes take 
 place, also has to provide for the passage of the digested food into the 
 blood-vessels, to be carried by them to all parts of the body. It is there- 
 fore richly supplied with blood- and lymph-vessels. 
 
 The alimentary canal, which is complete (i.e., has both mouth and 
 vent), is largely entodermal in origin, but small portions at either end 
 are derived from the ectoderm. The entodermal portion, the mesen- 
 teron, consists of the wall of the archenteron (p. 12) after the separa- 
 tion of the notochord, the mesothelium, and a few less prominent struc- 
 tures. The ectodermal parts are a stomodeum at the cephalic end and 
 a proctodeum behind. 
 
 In the early stages of all vertebrates the mouth is lacking, the ce- 
 phalic end of the archenteron abutting directly against the ectoderm of 
 the ventral side of the head, so that an oral (pharyngeal) plate is 
 formed, consisting of both ectoderm and entoderm. Next this plate is 
 pushed inward, either as a pocket (fig. 190) or as a solid plug, carrying 
 the entoderm before it. This ingrowth constitutes the stomodemn, 
 and the site of its ingrowth forms the mouth opening of the adult. 
 Later the oral plate breaks through, placing the stomodeum and 
 mesenteron in communication. 
 
2o6 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 In the majority of vertebrates the blastopore closes behind, so that 
 the anus is a new formation, although it arises 
 in the line of closure. In the amniotes this 
 opening is preceded by the formation of a 
 pocket, the proctodeum, similar to the 
 stomodeum, and opening later into the 
 mesenteron in the same way. In the adult it 
 is impossible to find any lines separating the 
 three regions, stomodeum, mesenteron and 
 proctodeum. 
 
 The proctodeum lies wholly behind the entrance 
 of the urogenital ducts into the cloaca. The ecto- 
 derm of the stomodeum extends inward as far as the 
 posterior teeth, following the outline of the jaws. On 
 the dorsal side of the oral cavity two pits persist for 
 some time, the limits of ectoderm and entoderm pass- 
 ing between them. The posterior of these, SeessePs 
 pocket, is of unknown significance. The other, 
 Rathke's pocket (fig. 253), lies just in front of the 
 oral plate. It marks the point of invagination of the 
 hypophysis (p. 148) and remains open for a time as 
 the hypophysial duct (fig. 148). 
 
 In some teleosts, where the stomodeal ingrowth is 
 slight, the mouth appears at first as a pair of per- 
 forations in the oral plate, these later coalescing to 
 form the permanent mouth. This condition lends 
 plausibility to the view that the vertebrate mouth has 
 arisen from the coalescence of a pair of gill clefts. 
 
 Except in the higher mammals the ento- 
 dermal part of the alimentary canal contains a 
 large amount of food yolk in the early stages. 
 In the sauropsida this is so abundant that the 
 whole cannot be contained in the body walls, 
 and hence it causes the ventral side of the 
 canal to protrude as a yolk-sac, which is 
 gradually absorbed with the digestion and re- 
 moval of the yolk by the blood-vessels. 
 
 The first differentiation in the mesenteron 
 is the development of a ventral diverticulum, 
 the anlage of the liver, which arises just caudal 
 to the head. This divides the alimentary canal into pre- and post- 
 
 F I G. 208. — Reconstruc- 
 tion of alimentary canal of 
 human embryo, after His. 
 al, allantois stalk; cl, cloaca; 
 g, glottis; h, hyoid arch; li, 
 liver; lu, lung; md, mx, man- 
 dibular and maxillary arches ; 
 n, nasal pit; o, omphalomesa- 
 raicvein; 5, stomach; -y, vis- 
 ceral arches; vi, vitelline 
 stalk; w, Wolffian body. 
 
DIGESTIVE ORGANS. 207 
 
 hepatic portions (fig. 209). From the anterior of these is formed 
 part of the cavity of the mouth with the salivary glands, the pharynx, 
 oesophagus, stomach, and duodenum; the post-hepatic portion gives 
 rise to large and small intestines, rectum and cloaca, as well as to the 
 urinary bladder. Of these parts the pharynx will be considered in 
 connection with the respiratory organs, the bladder with the urogenital 
 system. Mouth and pharynx belong primitively to the head, but by 
 unequal growth the pharynx may be carried apparently to some 
 distance behind the brain and other characteristically cephalic 
 structures. 
 
 Fig. 209. — Diagrams of the alimentary canal in embryos of 6 and 8 days of Gymnarchus 
 nitoticus, after Assheton. ab, air bladder; 6, early diverticxilum for air bladder; gb, gall 
 bladder; /, liver; pa, pancreas; pb, posterior part of air bladder; pc, pyloric caeca; ph, 
 phar}'nx; s, stomach. 
 
 In the following account stress is laid upon the epithelial lining 
 (entoderm), the characteristic tissue of the digestive tract, but it must 
 not be forgotten that the wall contains other tissues of mesenchymatous 
 origin. That part of the canal which runs through the body cavity 
 has the following layers. The lining epithelium is supported by a 
 layer of connective tissue, containing the capillary absorb tive vessels; 
 outside of this are two layers of smooth (involuntary) muscles, the inner 
 with the fibres running in a circular, the other in a longitudinal direc- 
 tion. By the action of these antagonistic muscles the peristalsis or 
 movement of the digestive tract is effected, by which the food undergoing 
 digestion is churned and thoroughly mixed with the digestive fluids, 
 and all parts of it are brought into contact with the absorbtive surfaces. 
 The outer surface of stomach, intestine and associated glands is covered 
 with the serous coat, the lining of the peritoneal cavity, but this is 
 lacking from those parts (pharynx, oesophagus, etc.) which are outside 
 the region of the coelom. 
 
208 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 THE ORAL CAVITY. 
 
 The cavity of the mouth is limited anteriorly by the line of the stom- 
 odeal involution and extends back to the pharynx. It is lined in part 
 by ectoderm, in part by entoderm, the line between the two, as stated 
 above, not being recognizable in the adult. In the amphibia the lining 
 is ciliated, the cilia extending back to the stomach. In the cyclostomes 
 the oral cavity is funnel-shaped, with a circular or quadrangular open- 
 ing supported by a cartilaginous ring and has the name of oral hood. 
 It is permanently open, there being no jaws capable of closure (see 
 skeleton, p. 73) thus furnishing a marked contrast to all other verte- 
 brates in which there are jaws and which are consequently known 
 as gnathostomes. (Development gives little support to the view that 
 the cyclostome tongue is the homologue of the lower jaw of the 
 gnathostomes.) 
 
 In development the mouth arises on the ventral side of the head, 
 some distance from the anterior end of the body. This position 
 is retained throughout life in most elasmobranchs and in the sturgeons; 
 but elsewhere, by the development of the bony upper jaw in front of 
 the pterygoquadrate (p. 69) and the concomitant extension of Meckel's 
 cartilage, the mouth opening is gradually transferred to the anterior 
 end and becomes terminal. 
 
 In most lower gnathostomes (the holocephali and other isolated 
 forms are exceptions) the mouth opening is bounded by folds of epithe- 
 lium which meet when the mouth is closed. Usually these folds are 
 soft and are supported below by connective tissue, but in birds, turtles 
 and monotremes they are comified. It is only in the mammals that 
 true lips occur. These are fleshy folds around the mouth and their 
 development in this group is correlated with the presence of the dermal 
 facial muscles (p. 134), by which they are moved. With the develop- 
 ment of lips there is formed a space between lips and teeth, the vesti- 
 bule of the mouth, which sometimes (e. g., some rodents) forms cheek 
 pouches, lined with hair, of considerable size. 
 
 Teeth. 
 
 The primitive function of the teeth was apparently to hold the prey 
 taken into the mouth and this is their sole use in many forms. In 
 other species they have become efficient organs for the comminution of 
 food, either by cutting or by crushing it. 
 
DIGESTIVE ORGANS. 209 
 
 There are two types of teeth, much alike in function, but differing 
 markedly in structure and development and without genetic relation- 
 ships. The typical vertebrate teeth are comparable to placoid scales; 
 they arise as a calcareous secretion at the junction of ectoderm and 
 mesenchyme and are a product of both layers. The other type contains 
 purely cuticular teeth, formed by a comification of the epithelium and 
 have their analogues in many invertebrates. 
 
 True Teeth. — The ability to form scales is characteristic of the 
 skin of many vertebrates. The primitive type of these scales is the 
 placoid (p. 40), consisting of a basal portion of dentine capped with 
 enamel and the apex projecting through the integument as a spine. 
 When invaginated to form the stomodeum the skin retains this capacity 
 of forming hard structures and hence any portion of the stomodeal 
 walls may secrete scale-like plates. In fact, in the teeth of some 
 elasmobranchs (Rata, Mustelus, Trygon, etc.) the placoid scale can be 
 recognized with scarcely a modification. In the ichthyopsida teeth 
 may form anywhere in the oral cavity where there are skeletal parts 
 — cartilage or bone — to support them. Thus they may occur, not only 
 on the margins of the jaws, but on vomers, palatines and parasphenoid, 
 and in some teleosts on the tongue, where they are attached to the 
 hyoid. In the amniotes -(some squamata excepted) teeth occur only 
 on the margins of the jaws. Teeth are lacking, here and there, in 
 various families of vertebrates as well as from all turtles and living 
 birds, but some extinct birds had teeth. In the embryos of both 
 chelonians and aves the dental ridge is formed (vide infra), but it soon 
 completely disappears. 
 
 In the development of a tooth, as of a placoid scale, there is 
 first a thickening of the ectoderm, the basal layer of which pushes into 
 the cutis, and at the same time the mesenchyme cells of the latter layer 
 multiply beneath the centre of the ectodermal ingrowth, pushing it 
 outward, so that the basal layer forms a cup with the opening toward 
 the deeper tissues (fig. 210). The mesenchyme within the cup forms 
 the dental papilla, while the ectoderm cells lining the cup form the 
 enamel organ. With farther development the outer cells of the papilla 
 are converted into odontoblasts, so-called from their function of form- 
 ing a bone-like substance, the dentine or ivory of the tooth. This, 
 in accordance with the method of its formation by secretion from the ends 
 of the odontoblasts, has a prismatic structure. The basal surface of 
 the enamel organ secretes a denser substance, the enamel, which lies like 
 14 
 
2IO 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 a cap, firmly united to the top and sides of the dentine. By continued 
 additions to the deeper portions of the dentine the tooth is gradually 
 forced up through the epithelium so that its tip or crown comes into 
 position for use (eruption of the tooth). 
 
 Fig, 2IO. — Section of developing tooth germ of Amblystoma. e, epidermis; co, enamel 
 organ; m, Malpighian layer; wc, mesenchyme; />, pulp of tooth. 
 
 In the lower vertebrates there may be a separate invagination of the ectoderm 
 for each tooth, but in the mammals there is a continuous ingrowth, the dental ridge 
 (fig. 21 1) along the margin of the jaw. Later this becomes differentiated into 
 separate enamel organs and dental papillae, the separate teeth developing much as 
 
 Fig. 
 
 211. — Model of ectodermal parts of jaw of human embryo 40 mm. long, after Rose, 
 showing the dental ridge with the enamel organs for the first teeth. 
 
 in other groups. From the posterior side of this dental ridge there arises a continu- 
 ous projection, the dental shelf (fig. 212) which later gives rise to the enamel 
 organs for the second or permanent dentition {infra). 
 
 The dental papilla persists throughout life as the pulp of the tooth, 
 continuing to occupy the space (pulp cavity) in which it first appeared. 
 
DIGESTIVE ORGANS. 
 
 211 
 
 Nerv^es (branches of the trigeminal) and blood-vessels enter the cavity 
 through the base of the tooth. Usually, when the tooth is fully formed, 
 the odontoblasts cease to act, but exceptionally, even in mammals 
 (tusks of elephants, incisors of rodents) they function through life and 
 the tooth continues to grow. In the mammals an additional layer of 
 modified bone, the cement, is formed around the root of the tooth 
 and may extend on to the crown. 
 
 Just as the scales are arranged in quincunx on the surface of the 
 body, so are the teeth in the mouths of skates and some other elasmo- 
 
 FiG. 212. Fig. 213. 
 
 Fig, 2 12 . — Diagram of germs of milk and peimanent dentitions in a mammal, based on 
 Rose, b, basal layer of e, ectoderm; dr, dental ridge; ds, dental shelf; eo, enamel organ of 
 milk tooth; m, mesenchyme; p, pulp of milk tooth; pg, germ of permanent tooth. 
 
 Fig. 213. — Diagrammatic section of indsor tooth, c, cement; (f, dentine; e, enamel; 
 p, pulp cavity. 
 
 branchs, where they form a tessellated pavement above and below, the 
 teeth being flattened and used for crushing the molluscs on which these 
 animals feed. More commonly the teeth are flattened in the antero- 
 posterior direction and have sharp cutting edges. In such cases, as 
 a rule, only the anterior row of teeth is functional, the others lying folded 
 down behind, ready to come into use when one of the first row is lost. 
 Most vertebrates have a succession of teeth (polyphyodont dentition) 
 and the elasmobranchs show how this has come about. The second 
 arises on the (morphologically) posterior side of the first and so on. 
 In the non-mammalian classes the number of such dentitions is in- 
 definite (polyphyodont), but in the great majority of mammals there 
 are two, the first or milk dentition and the second or permanent 
 dentition (diphyodont condition). 
 
212 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 In a few mammals only one dentition has been retained (monophyodont) ; among 
 these may be mentioned the monotremes, sirenians and cetacea. In the marsupial 
 Myrrecobius, where the permanent dentition is greatly reduced, and in some of the 
 insectivores and rodents, a prelacteal dentition has been observed in the embryo, 
 while Rose has described traces of a prelacteal and a post-permanent dentition in 
 man. In a number of mammals (guinea pigs, many bats, etc.) the milk dentition is 
 lost before birth. 
 
 Fig. 214. — Jaws of a six month lion, after Weber. Milk teeth white, permanent dotted. 
 i, incisors; c, canines; m, molars; p, premolars. 
 
 Only a few fishes (adult Acipenser, Coregonus, etc.) lack teeth, while 
 in most they extend to the lining bones of the mouth and in some to 
 the hyoid and branchial arches (pharyngeal bones) . Usually they are 
 conical, but they may be flattened and pavement-like or even form 
 large plates, apparently by the coalescence of numbers of primitive 
 teeth (dipnoi). In the amphibians the teeth are not so widely distrib- 
 uted in the mouth, occurring on the margins of the jaws and on the 
 palatines and vomers, rarely on the parasphenoid, while they are 
 entirely lacking in Bufo and Pipa. 
 
 Among the reptiles the turtles and some of the pterodactyls are 
 toothless; most of the others have the teeth confined to the margin of 
 the jaws, though they occur on the palatines and pterygoids in the 
 snakes and lizards, and rarely (Sphenodon) on the vomer. While the 
 conical shape prevails, the teeth present a great variety of forms, some of 
 the theriomorphs closely simulating the mammals in their heterodont 
 dentition. The teeth may be anchylosed to the summit of the jaws 
 
DIGESTIVE ORGANS. 213 
 
 (acrodont); applied to their inner side (pleurodont, fig. 97, d); or 
 have their roots implanted in grooves or sockets or alveoli (thecodont). 
 Mention must also be made of the poison fangs of certain serpents. 
 These are specialized teeth borne on the maxillary bones and are either 
 permanently erect (proteroglypha) or the bone may turn, as on a pivot, 
 so that when the mouth is closed the teeth lie along the roof of the 
 mouth, but when it is opened, they are brought into position for striking 
 the prey (vipers, rattlesnakes — solenoglypha) . Correlated with the 
 fixed or movable condition is a modification in the teeth themselves. In 
 the proteroglypha a groove runs along the anterior side of the fang by 
 
 Fig. 215. — Poison gland and fang of rattlesnake, Crotalus horridus. (Princeton 1404) 
 p, poison gland; /, labial glands. 
 
 which the poison is conducted from the poison gland into the wound. 
 In the solenoglypha the groove is rolled into a tube with openings near 
 the base and apex of the tooth (fig. 215). In these solenoglyphous 
 snakes only a pair of fangs are functional at a time, but there are 
 reserve teeth which can come into use on the loss of the first. 
 
 The greatest variation is found in the teeth of mammals, the heter- 
 odont dentition being the rule. Four kinds of teeth are recognized. 
 These are the incisors in the premaxillary bones, followed by a single 
 canine at the anterior end of each maxillary bone. This resembles 
 the incisors and differs from the other maxillary teeth in its conical shape 
 and single root. Behind the canines come the premolars (the bicus- 
 pids of the dentists) which have two roots and complicated crowns and 
 appear in both milk and permanent dentitions. Lastly are the molars, 
 like the premolars in form, with several roots, but appearing only in the 
 permanent dentition. The corresponding teeth in the lower jaw have 
 the same names. 
 
 In a few mammals, like the whales, all of the teeth are of a simple 
 conical shape, but in the majority the crown of the molars is marked 
 by projections — cones, tubercles, crests, etc. — which are variously 
 
214 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 arranged. When the teeth are adapted for cutting they are called 
 secodont (cats, fig. 214); for crushing, bunodont (man); when mark- 
 ed by transverse ridges, lophodont (elephants) ; when there are longitu- 
 dinal crests, more or less crescentic in outline, they are selenodont 
 (horse, fig. 216). 
 
 In the triconodont tooth there are three prominences in the crown 
 arranged in a straight line, parallel to the axis of the jaw. The middle 
 and more prominent of these in the upper jaw is the protocone, with 
 a smaller paracone in front and a metacone behind. In the lower 
 jaw the corresponding terms are proto-, para-, and metaconid. In 
 
 Fig. 216. — A, triconodont tooth of Dromatherium; B, tri tubercular tooth oiSpalaco- 
 therium; C, interlocking of upper (dark) and lower (light) tritubercular molar teeth (after 
 Osborn); t>, molar of Erinaceus; E, of horse (selenodont type); c, cingulum; m, metacone 
 (metaconid) ; p, paracone (paraconid) ; pr, protocone (protoconid) ; t, talon. 
 
 a tritubercular tooth the three cones are arranged in a triangle, in 
 such a way that they alternate in the two jaws, the protocone being on 
 the inner side, the protoconid on the outer. Tritubercular teeth may 
 have a lower projection (talon) on the hinder side. When this devel- 
 ops into a prominent tubercle (hypocone, hypoconid) the tooth 
 becomes quadritubercular. Then crests or lophs may develop, 
 connecting the cones, so that the crown becomes ridged rather than 
 tubercular. 
 
 In the homodont dentition the number of teeth may be very large, varying 
 from 100 to 200. With the heterodont dentition the number is smaller, the full 
 dentition in the placental mammals including 44 teeth. From this number reduc- 
 tions may occur by the loss of teeth of any kind. The number of teeth and of 
 those of each kind is important in systematic work, and a dental fonnula has been 
 devised to express this. As the number of teeth in the two sides of each jaw is the 
 same, only one side is represented in the formula, while the teeth of the upper and 
 lower jaws are represented as fractions. The number of incisors, canines, pre- 
 molars and molars of man are represented by 
 
 •21 2 -l . ^, ,.■?! ^4 
 
 1 ~ > c - , pm - , m -; that of the opossum by 1 - , c - , pm - , m -. 
 2123 4134 
 
DIGESTIVE ORGANS. 
 
 215 
 
 Not infrequently the enamel is lacking from the teeth of mammals, as in whales, 
 dugongs and edentates, or it may be restricted to one side of a tooth, as in the 
 incisors of rodents. Sexual differentiations occasionally occur in mammals, certain 
 teeth (usually canines or incisors, more rarely premolars) being better developed 
 in the males than in the females of the same species. 
 
 There are two views as to the way in which the complicated molars of the mam- 
 mals have arisen. Both start with the conical tooth as the primitive condition. 
 One theory is that the fusion of such simple teeth is sufficient to account for the 
 multiplication of roots and tubercles in all of their varying forms (figs. 217, 218). 
 
 A^^ 
 
 /AAA 
 'A AAA 
 
 A/\AA 
 AAAA 
 AAA A 
 AAAA 
 
 « 
 
 « 
 
 ^XA 
 
 ? 
 
 AA 
 
 f? 
 
 m^ 
 
 ? 
 
 Fig. 217. — Diagram of Ae relation of the human 
 
 Rose. 
 
 teeth to the primitive dentition, after 
 
 The other hypothesis is that parts have been developed on the primitive cone, 
 giving, first, the triconodont shape. Next these three cones have been shifted to 
 the tritubercular position; and later other parts — h)rpocone, lophs, etc. — have 
 been added and these have been modified in different directions. Each view has 
 much in its favor. Embryology is not at all decisive, while paleontology favors the 
 latter view. 
 
 Epidermal Teeth occur in cyclostomes and in larval amphibia and 
 in embryonic monotremes. In the cyclostomes they are cones of 
 cornified epithelium covering an underlying core of the integument; they 
 are differently arranged in the lampreys and myxinoids. In the latter 
 they are few, there being a single tooth on the 'palate' and two chev- 
 ron-shaped rows on the tongue. In the lampreys nearly the whole 
 inner surface of the oral hood is lined with these teeth of varying shape, 
 
2l6 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 and there are a varying number upon the tongue. These teeth are 
 used as a means of fastening the animals to their prey, and those of the 
 myxinoid tongue are used for boring into the fishes on which these 
 animals feed. 
 
 In the larval anura (the larval Siren is said to resemble them) the 
 edges of the jaws are armed with cornified papillae, serving as teeth, the 
 arrangement of which varies in different genera. They are frequently 
 aggregated in dental plates, used in scraping the algae from submerged 
 objects. They are not related to the teeth of cyclostomes. 
 
 In the embryo monotreme teeth are formed as in 
 other mammals, of a multituberculate type, with a 
 normal enamel organ (fig. 219), but these are lost 
 before birth. During their eruption the adjacent epi- 
 dermis becomes cornified, gradually extends beneath 
 
 v,w^;;3v™w;mw ,,^^^1^^.^ ^^^©2x 
 <^^^. |(^^/> ^£v__^,^ 
 
 Fig. 218. Fig. 219. 
 
 Fig. 218. — Teeth of Chlamydoselache (after Rose)^ showing a triconodont tooth arising 
 from the fusion of three simple teeth. 
 
 Fig. 219. — Diagram of development of teeth in Ornithorhynchus, after Thomas and 
 Poulton. a, tooth covered with enamel organ, beneath oral epitheUum; h, just before 
 eruption; c, tooth erupted; d, edges of epithelium cornified; e, horny plate formed, contains 
 the tooth;/, tooth lost, plate separated from its surroundings. 
 
 each tooth and after the loss of the true tooth this forms a horny 
 plate, used, like those of many birds, in holding and crushing the 
 food. 
 
 In this connection mention may be made of the baleen or 'whale- 
 bone' of the balenid whales. This takes the form of large plates of 
 horny material, attached in series to the margins of the upper jaw, so 
 that with their fringed ends and edges they serve as strainers to extract 
 the plankton (minute floating life) from the sea. This baleen is formed 
 by the agglutination of enormously developed cornified papillae. 
 
 Egg Teeth. — In the embryos of certain lizards and snakes one of the median 
 teeth of the first dentition of the premaxillary region projects from the mouth and 
 is used for the rupture of the egg shell, thus allowing the escape of the young. 
 In the turtles, Sphenodon, crocodiles, birds, and monotremes an egg tooth is formed 
 on the upper surface of the beak which is used for the same purpose. However, it 
 differs greatly as it is but a thickening, often calcified, of the epidermis (Fig. 195).- 
 
DIGESTIVE ORGANS. 217 
 
 The Tongue. 
 
 The tongue as it occurs in its more primitive condition in the fishes 
 is merely a fleshy fold developed from the floor of the mouth between the 
 hyoid and mandibular arches, the hyoid frequently extending into and 
 supporting it. It is incapable of motion, except as moved by the sup- 
 porting skeleton, for it lacks intrinsic muscles. It is sensory, having 
 both tactile and gustatory functions. It is often papillose, and in a 
 few teleosts it bears teeth (p. 209). 
 
 The tongue in the cyclostomes is considerably different. Here it 
 is thick and fleshy and is supported by a cartilaginous skeleton (p. 75) 
 and is moved by appropriate protractor and retractor muscles at the 
 base, developed from the postotic myotomes and innervated by the 
 hypoglossal nerve. With its terminal armament of epidermal teeth 
 it ser\' es as the boring organ with which the myxinoids obtain entrance 
 into their prey, while in the lampreys it serves as a rasping organ and 
 also as part of the sucking apparatus. 
 
 In the amphibia there is a greater range of structure. In a few 
 anura (aglossa) the tongue is practically absent; in the perennibranchs 
 it is scarcely more advanced than in the fishes, but elsewhere it contains 
 intrinsic muscles and is extremely mobile. It consists of a small basal 
 portion corresponding to the tongue of the fish, to which is added a 
 large glandular part arising between the copula and the lower jaw. 
 This secretes the slime, so useful in capturing the prey. In the anura 
 the tongue is attached at the margin of the jaw, its free end, when at 
 rest, being folded back on the floor of the mouth. In urodeles the base 
 of attachment is more extensive and embraces the anterior margin of 
 the tongue and part of the ventral surface as well. The supporting 
 skeleton (fig. 85) consists of the median portion (copula) with usually 
 two pairs of cornua, largely formed from the ventral ends of the hyoid 
 and first branchial arches (see p. 64). 
 
 The reptilian tongue includes not only the parts found in the am- 
 phibia (the fold above the basihyal), but also a median growth, the 
 tuberculum impar, arising between the basihyal and the lower jaw, 
 and also a pair of lateral folds lying above the first visceral arch 
 (Lacerta). In the turtles and crocodiles the tongue lies on the floor 
 of the mouth and is not protrusible. In the squamata it can be extended 
 from the mouth, and in snakes and many lizards there is a sheath into 
 which it is withdrawn. In many snakes the tongue is two-pointed at the 
 
2l8 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 tip; in the lizards its shape varies greatly, the differences being used in 
 classifying these animals. In the reptiles (fig. 220) with retractile 
 
 tongue the hyoid apparatus extends into 
 the tongue, its unpaired anterior portion 
 being called the os entoglossum (copula 
 or basihyal), while the two cornua (usually, 
 hyoid and first branchial) afford attachment 
 for the retractor muscles. In addition to 
 the usual lingual nerve (glossopharyngeal) 
 the tongue also receives a lingual twig from 
 the mandibular branch of the fifth nerve. 
 
 In birds the tongue has lost the lateral 
 parts of the reptilian tongue and with this 
 the trigeminal branch. It contains no in- 
 trinsic muscles. In its form it varies greatly, 
 but usually it is slender and is covered with 
 retrorse papillae. Its skeleton is also re- 
 
 FiG. 220.-Hyoid apparatus of ^^^^^ (^g- ^^l) ^^^ COnsistS of an OS en- 
 
 Heloderma, after Cope, b, first toglossum, bearing in front a pair of ele- 
 
 branchial; c, copula: h. hyoid. , , . . , i . i 
 
 ments (paraglossae) and on the sides a 
 pair of cornua (first branchials) and in the median line behind, a 
 urohyal portion. This skeleton has a marked development in the 
 woodpeckers, where the cornua curve around the base of the skull 
 
 Fig. 221. — ^Two stages in developing tongue and pharyngeal floor of man, after His. 
 c, copula (basihyal element); cs, cervical sinus; ep, epiglottis; g, glottis; h, hyoid arch; m, 
 mandibular arch; mth, median anlage of thyreoid; t, tuberculum impar; tg, tongue. 
 
 and over its dorsal side to the neighborhood of the nostril, a condi- 
 tion correlated with the use of the tongue in these animals. 
 
DIGESTIVE ORGANS. 
 
 219 
 
 In the whales the tongue has little power of motion, but elsewhere 
 in the mammals it is very mobile, reaching the extreme in the ant-eaters. 
 This mobility is largely due to the extensive intrinsic musculature. 
 The tongue is developed from the tuberculum impar, which furnishes 
 the larger anterior part (fig. 221), the rest arising from the fleshy ridges 
 above the hyoid arch. In the adult the line between these parts is largely 
 obliterated, but it lies near the line of circumvallate papillae (p. 189) 
 and the foramen caecum, a blind tube connected with the development 
 of the thyreoid gland. Arising in this .,__ S 
 way from the tubercle and the lateral supra- 
 hyoid parts, the tongue of the amphibia is 
 
 Fig. 222. Fig. 223. 
 
 Fig. 222. — \^entral and side views of tongue of Stenops gracilis, after Weber. /, 
 lateral margin of sublingua; m, plica mediana. 
 
 Fig. 223. — Section through lyssa of late dog embryo, after Nussbaum. c, cartilage of 
 lyssa, cl, capsule of lyssa; m, muscles of tongue; ml, longitudinal and transverse muscles 
 of lyssa; s, septum of tongue. 
 
 unrepresented in that of most mammals, unless it be in the sublingua, 
 a fleshy fold developed beneath the functional tongue in the marsupials 
 and lemurs (fig. 222). Traces of this are to be found in other mam- 
 mals, even in man, as folds (plicse fimbriatae) beneath the tongue. 
 In some cases {Stetwps) this sublingua is supported by a cartilage 
 which is regarded as an entoglossal part. Others think that the 
 tongue of the lower vertebrates is represented in the mammalian 
 tongue and regard the lyssa as the os entoglossum. The lyssa is 
 a vermiform structure of cartilage, muscle and connective tissue (fig. 
 223) lying ventral to the septum of the tongue. 
 
 The tongue varies considerably in shape in the different mammalian 
 orders, but the differences are of little morphological importance. 
 
220 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 The dorsal surface is usually covered with a soft epithelium, developed 
 into papillae of varying shapes, some being sensory in character, and 
 some are occasionally (monotremes, felidae) cornified. 
 
 The skeleton of the mammalian tongue (hyoid apparatus) varies 
 considerably. In its most complete development it consists of a body 
 (copula of the hyoid and first branchial) in the median line, which bears 
 two pairs of cornua. The anterior pair (lesser horns of human 
 anatomy) are usually elongate, and consist of a series of ossicles (p. loi) 
 connecting the body with the otic region of the skull. The second 
 pair (greater cornua of man) are occasionally absent. In man the 
 greater part of the anterior cornua is represented by the stylohyoid 
 ligament, the proximal portion being fused to the skull as the styloid 
 process. 
 
 Oral Glands. 
 
 In the cyclostomes there is a large, so-called 'salivar}' gland' of 
 unknown function, opening into the mouth on either side below the 
 tongue. With this exception, glands are lacking from the mouths of 
 aquatic ichthyopsida. With the assumption of pulmonate respiration 
 and more terrestrial habits, the mouth is no longer constantly bathed 
 
 with water and so glands appear, 
 increasing in number and com- 
 plexity in the higher forms. The 
 secretion of these glands aids in 
 moistening the food, and not in- 
 frequently it is adhesive and is 
 used in capturing the prey. In the 
 mammals true salivary glands ap- 
 pear. The saliva secreted by them 
 contains not only mucus, but also a 
 digestive ferment (ptyalin) which 
 changes starch into sugar. The 
 names of the various oral glands 
 (labial, buccal, lingual, retrolingual, etc.) are roughly indicative of 
 their position. 
 
 In the terrestrial amphibia, snakes (fig. 215) and lizards there are 
 labial glands, opening at the bases of the teeth, and an intermaxil- 
 lary or internasal gland in the septum between the nasal cavities, as 
 well as palatal glands near the choanae (the internasal gland is lacking 
 
 Fig. 224. — Transverse section of tongue 
 and lower jaw of Lacerta, after Gegenbaur. 
 d, tooth ;/j, hyoid cartilage; /, labial glands; 
 w, muscles; si, sublingual gland; t, tongue. 
 
DIGESTIVE ORGANS. 
 
 221 
 
 in the caecilians) . Many reptiles also have a sublingual gland on either 
 
 side (fig. 224). In many snakes a pair of the labial glands are greatly 
 
 developed and have migrated into the zygomatic ligament, where they 
 
 have become modified into the well-known poison glands (fig. 215), 
 
 the ducts of which connect with the poison fangs 
 
 (p. 213). In the only known poisonous lizards 
 
 {Heloderma) the sublingual glands furnish the 
 
 poison. Oral glands are poorly developed in 
 
 the sea turtles and the crocodilians. 
 
 ►'• Birds lack the labial and internasal glands, 
 
 but they have numerous other glands opening 
 
 separately into the roof of the mouth (fig. 225) 
 
 as well as anterior and posterior sublinguals and 
 
 frequently an 'angle gland' at the angle of the 
 
 mouth, which may be the last remnant of the 
 
 labial glands of the other Sauropsida. 
 
 Besides numerous smaller glands (labials, 
 buccals, Unguals, palatines) imbedded in the 
 mucous membrane and opening separately into 
 the mammalian mouth, the salivary glands, 
 though absent from the cetacea, form a distin- 
 guishing feature of the group. These salivary 
 glands are usually in the neighborhood of the 
 mouth, but one or more of them may be carried 
 back into the neck (fig. 226), but in all cases the 
 homologies are decided by the openings of the 
 ducts. The salivary glands include the sub- 
 maxillary and sublingual of the lower groups, and in addition the 
 parotid gland, apparently a development within the class. The sub- 
 maxillary normally lies in the lower jaw beneath the mylohyoid 
 muscle, and its duct (Wharton's duct) opens near the lower incisor 
 teeth. Near this is frequently a retrolingual gland, its duct open- 
 ing near the former. The sublingual gland occurs between the tongue 
 and the alveolar margin of the lower jaw and usually empties by 
 numerous duct. The parotid gland has its normal position near the 
 ear and its ducts (Stenon's duct) pours the secretion out near the 
 molars of the upper jaw. Other oral glands are occasionally present, 
 like the molar glands of ungulates and the orbital glands of dogs, 
 both of which have ducts leading into the mouth. 
 
 
 Fig. 225. — ^Palatal sur- 
 face of hen, after Heid- 
 rich. ch, anterior end of 
 choana; gs, openings of 
 sphenopterygoid glands; 
 in, infundibular opening; 
 Ip, mp, openings of lat- 
 eral and medial palatine 
 glands; m, opening of 
 gl. maxillaris monosto- 
 matica. 
 
222 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 PHARYNX. 
 
 The pharynx is the division of the alimentary canal intervening 
 between the cavity of the mouth and the oesophagus and is characterized 
 by being at once alimentary and respiratory. From its walls are devel- 
 oped the gill clefts and lungs as well as a number of derivatives of 
 these, and it also receives the internal openings of the nasal passages. 
 Hence it is best described in connection with the respiratory system. 
 
 Fig. 226. — Salivary glands of fruit bat, Pteropus cotnpicillatus (Princeton, 2065). 
 P, pd, parotid gland and duct; rl, rid, retrolingual gland and duct; sm, smd, submaxillary 
 gland and duct. 
 
 THE (ESOPHAGUS. 
 
 That part of the digestive tract between the pharynx and the 
 entrance of the bile duct (fig. 209) develops into oesophagus, stomach 
 and that part of the intestine known as the duodenum. Stomach and 
 duodenum are separated by the pyloric valve described below, but it 
 is difficult to draw a clear line between oesophagus and stomach. In 
 general it may be said that the oesophagus is the tract immediately 
 succeeding the pharynx, lying in front of the body cavity and thus 
 lacking a serous coat; that it is smaller than the stomach, and that 
 there are no digestive glands in its walls; but all of these statements 
 have exceptions. 
 
DIGESTIVE ORGANS. 223 
 
 The oesophagus varies in length with the length of the neck of the 
 animal, being short in the ichthyopsida, longer in the reptiles, and 
 reaching its extreme in the birds. In some its internal lining epithelium 
 is smooth, but more commonly it bears longitudinal folds, while in the 
 chelonians it is provided with comified papillae, pointing backward. 
 Outside of the epithelium its walls contain muscles, those at the 
 cephalic end being striped and these may extend back, in some in- 
 stances, even on to the stomach. They are apparently derivatives 
 of the pharyngeal region. Usually the oesophagus is of the same di- 
 ameter throughout, but frequently in birds it has a marked dilatation, 
 the ingluvies or crop. This may be an expansion of one side of the 
 tube, or, as in pigeons, it may consist of a median and a pair of lateral 
 chambers. The extreme of development of the crop occurs in Opis- 
 thocomusj where the organ is extremely muscular and has numerous 
 longitudinal folds. 
 
 The crop, which is usually supported by the furcula, may be either 
 a resen^oir for food, or it may be a glandular organ, its secretions 
 sending to moisten the food or even to initiate its digestion. In the 
 pigeons at the breeding season the secretion is a milky fluid and is 
 used in feeding the young. 
 
 THE STOMACH. 
 
 The stomach is apparently a new acquisition in the vertebrates, 
 possibly arising as a place for the storage of food. This view is sup- 
 ported by several facts. In the embryo vertebrate and in the adult of 
 Amphioxus the duct from the liver immediately follows the pharynx, 
 opening just behind the last gill cleft; while the innervation from the 
 tenth nerve shows that both stomach and oesophagus are parts of the 
 pharynx greatly drawn out (fig. 209). 
 
 The pylorus, which limits the stomach behind, is a fold of the 
 lining mucous membrane projecting into the interior and reinforced 
 by a circular (sphincter) muscle, which by its contraction, closes the 
 tube so that no food can pass from the stomach until it is properly 
 acted upon by the gastric fluids. The anterior end of the stomach is 
 not so well marked. Usually it is differentiated from the oesophagus 
 by its greater diameter, but in some of the fishes (fig. 227, a) there 
 is no distinction in size. The stomach lies in the coelom and hence is 
 covered externally by the serous membrane (peritoneum), but the 
 
224 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 oesophagus usually extends a short distance into the body cavity and 
 then its lower end has the same coat. 
 
 The true stomach is characterized by the presence of glands, de- 
 veloped from the mucous layer and emptying into the lumen. Of 
 these glands there are at most (mammals) three kinds: cardiac, near 
 the* entrance of the oesophagus, which secrete an albuminoid fluid; 
 
 Fig. 227. — Dififerent shapes of stomachs, mostly after Nuhn (Keibel). a, Belone; h, 
 Proteus; c, Tropidonotus natrix; d, Gobius; e, shark; f, Phoca vitulina; g, Polypterus; h, 
 Fulica atra; i, Testudo grosca; k, land tortoise; /, rabbit; w, pig; n, owl; 0, crocodile; p, 
 Delphinus; q, Halmaturus. 
 
 pyloric, near the pylorus, which form mucus; and the most character- 
 istic, the fundus glands, which secrete a digestive ferment, pepsin. 
 (For the structure of these glands reference should be made to histological 
 text-books.) Tested by glands, many vertebrates (dipnoi, cyprinoids) 
 lack a true stomach, while the sturgeons have the gastric glands extend- 
 ing into the oesophagus. On the other hand, a part of the enlargement 
 called the stomach in mammals often includes a part of the oesophagus 
 (fig. 228, A, E), 
 
DIGESTIVE ORGANS. 225 
 
 The shape of the stomach is to some extent dependent upon the 
 shape of the body. In the elongate species it lies in the axis of the 
 trunk, especially in the lower vertebrates (fig. 227, a), but with increase 
 in the body width it becomes more transverse. This involves a bending 
 and a torsion of the tube, always to the right, and results in two faces 
 or * curvatures,' a lesser or anterior, and a greater or posterior, the 
 greater curvature often expanding into a so-called fundus region. 
 The end of the stomach which connects with the oesophagus is nearest 
 the heart and hence is called the cardiac end. 
 
 In the fishes the stomach may be either straight or saccular, often assuming the 
 form of a blind sac (fig. 22 y, g). The line between oesophagus and stomach is not 
 well marked, as the oesophageal folds may continue into the stomach. The teleosts 
 exhibit the greatest variety in shape, in correlation to the differences in food. All 
 gastric glands are lacking in the cyprinoids, while Amia has both cardiac and pyloric 
 glands, and, like many teleosts, the stomach is ciliated. In the amphibians and 
 reptiles the distinctions between oesophagus and stomach are more marked, most 
 in the crocodiles. In the amphibians the ciliation of the mouth is continued into 
 the stomach. 
 
 In the birds there is a differentiation of the gastric region into two 
 regions, an anterior glandular stomach or proventriculus, and a pos- 
 terior muscular gizzard. The proven tricular glands secrete a diges- 
 tive fluid, and the food, mixed with this, is passed on to the gizzard. 
 The walls of the latter have their muscles developed into a pair of discs 
 with tendinous centres, while the glands of the gizzard form a secretion 
 which hardens into a horny (keratoid) lining, sometimes developing 
 into tubercular structures, of great use in grinding the food, thus in part 
 making good the absence of teeth. In the grain-eating birds small 
 pebbles are taken into the gizzard and are used in triturating the food. 
 (In the fossil pterodactyls small clusters of stones are sometimes found 
 in such a position as to lead to' the supposition that these reptiles also 
 had a gizzard.) The gizzard is best developed in the grain-eating 
 birds and is weakest in the birds of prey. In one species of pigeon 
 part of the wall of the gizzard is ossified. 
 
 The mammalian stomach shows the greatest range of form (figs. 
 227, 228) and the greatest development of different kind of glands. 
 It may be a simple sac or it may be subdivided into a series of chambers. 
 It may be almost wholly oesophageal in character {Ornithorhynchus, 
 fig. 228, A). Occasionally the cardiac glands may be absent. It 
 may be a simple sac, longitudinal or transverse in position, or it may be 
 IS 
 
226 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 Fig. 228. — Outlines of the stomachs of various mammals (various authors), after 
 Oppel, to show the distribution of the different glandular regions. Horizontal lines, 
 oesophageal; oblique, cardiac; dots, fundus; crosses, pyloric; A, Ornithorhynchus; B, 
 gray rat; C, tapir; Z), seal; E, whale (Lagenorhynchus) ; F, mouse; G, dog; H, kangaroo 
 (Macropus). 
 
 Fig. 229. — Diagram of ruminant stomach, the dotted line showing the course of the 
 food, a, abomasum; oe, oesophagus; p, pylorus; fs, psalterium (omasus, manyplies); tr^ 
 reticulum (honeycomb); ru, rumen (paunch). 
 
DIGESTIVE ORGANS. 227 
 
 divided into chambers, the division reaching its extreme in the rumi- 
 nants (fig. 229) and the cetacea (fig. 228, E) where four compart- 
 ments can be recognized. In the ruminants two of these, the mmen 
 or paunch and the reticulum or honey-comb are expansions of the 
 oesophagus and serve as reservoirs for food lDef ore its complete mastica- 
 tion, after which it follows the course of the dotted lines to the 
 psalteriimi, omasus or manyplies and the abomasus or rennet 
 stomach for gastric digestion. 
 
 INTESTINE. 
 
 The remainder of the pre-hepatic portion of the alimentary canal, 
 the duodenum, extending from the pylorus to the entrance of the bile 
 duct, is considered as part of the intestine. It is especially noticeable 
 
 Fig. 230. — Digestive tube of garpike, Lepidosteus (after Gegenbaur). i, small intestine; 
 oe, cesophagus; pc, pyloric caeca; pg, pylorus; r, rectum; s, stomach; sv, spiral valve. 
 
 in many ganoids and teleosts (figs. 230, 233) where it may bear from 
 one to two hundred blind digestive tubes, the pyloric caeca. The 
 same region in a few elasmobranchs may have a pair of these caeca or 
 (Galeus) it may be expanded into a pouch ('bursa Entiana'). 
 
 The post-hepatic intestine is the seat of most of the digestive pro- 
 cesses and of absorption of the products of digestion. Here the food, 
 coming from the stomach, is mixed with the bile from the liver and 
 with the pancreatic juice and with the secretions of numerous small 
 glands in the intestinal wall. The increase of surface needed for ade- 
 quate digestion and absorption is provided in several ways. There 
 may be an elongation of the tube which results in its becoming coiled in 
 the body cavity; the mucous lining may develop folds, both longitudinal 
 and circular; or the folds may break up into numerous minute, finger- 
 like processes (villi) which give the surface a velvety appearance. The 
 food undergoing digestion is moved back and forth (peristaltic mo- 
 tion) by the antagonistic action of the muscles of the intestinal wall 
 (p. 207), bringing all of it in contact with the absorb tive surface. 
 
228 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 The length of the intestine is roughly related to the food, being 
 longer in the plant-eating than in the carnivorous species. This is 
 strikingly shown in the frogs, where the tadpole (larva) has a very 
 long intestine, correlated with the vegetable food, while the adult 
 flesh-eating frog has a canal hardly longer than that of the tadpole of 
 half the size. 
 
 In the intestine there are two divisions, an anterior small intestine 
 and a posterior large intestine, terms adapted from the digestive tract 
 of man, though not always appropriate in the lower groups. The 
 line between the two may be marked externally by the development of 
 
 Fig. 231.- — Spiral valve of Raia, after Mayer. 
 
 one or two blind pouches or caeca at their junction or by a circular 
 fold or a pair of internal folds of the lining, constituting an ileo-colic 
 (ileo-caecal) valve, both valve and caeca coexisting in many cases. 
 Both large and small intestines may be subdivided, chiefly by differ- 
 ences in their walls. Thus in the small intestine 
 there may be recognized in different groups a 
 jejunum, a spiral valve region and an ileum, 
 while the large intestine may furnish a colon, a 
 rectmn and a cloaca. 
 
 In the cyclostomes but two regions occur, 
 the intestine and the rectum, differentiated ex- 
 ternally by the larger size of the latter. In the 
 petromyzonts there is an internal fold of the in- 
 testine which pursues a slightly spiral course, 
 constituting a spiral valve, a structure which 
 reaches its highest development in the elasmobranchs. 
 
 In the elasmobranchs the intestine is nearly straight, but its dif- 
 ferentiation has proceeded farther. At the junction of small and large 
 intestine is a dorsal blind sac, the rectal gland. Its function is un- 
 
 FiG. 232. — Diagram 
 of spiral valve of Carcha- 
 rias. 
 
DIGESTIVE ORGANS. 
 
 229 
 
 known, but it apparently corresponds to the caeca of the higher groups. 
 In the ' small ' intestine is the spiral valve which has two forms, both 
 leading to increase of surface. In most species a fold, carrying blood- 
 and lymph-vessels, arises in a spiral line from the wall of the tube, and 
 its free edge projects into the lumen like a spiral stairway (fig. 231). 
 In a few forms (Carchariidae, Galeocerdo) the 
 line of origin of the fold is straight and its free 
 margin is coiled like a roll of paper (fig. 232). 
 In the large intestine rectum and cloaca are 
 recognized, the cloaca being that part which 
 receives the ends of the excretory and repro- 
 ductive ducts and thus is both digestive and 
 urogenital in character. 
 
 Ganoids and dipnoi (figs. 230, 233) also have the 
 intestine nearly straight and a spiral valve, least 
 developed in Lepidosteus. In the teleosts the canal 
 may be straight (fig. 227) or may make more or fewer 
 coils, the predaceous species being simplest, while in 
 the mullet (MugiT) there may be 13 or 14 turns. In 
 the teleosts the line between small and large intestine 
 is often marked by an ileo-colic valve and a few species 
 have a caecum or rectal gland. A spiral valve rarely 
 occurs in teleosts and a cloaca is never found. In a 
 few teleosts, in correlation with the translation of the 
 ventral fins, the anus may lie in front of the pectoral 
 girdle. 
 
 The intestine is straight in the caecilians, has a 
 few coils in the perennibranchs and more in the sala- 
 manders, while the anura have a greatly convoluted 
 intestine. (Reference has already been made to the 
 differences between the intestines of the larval and 
 adult frogs (p. 228). The line between small and 
 large intestine is frequently marked in the amphi- 
 bians by an ileo-colic valve and in a few forms 
 (Rana, Salamandra) there is a rudimentary caecum. 
 The rectum is larger than the rest of the intestine and a cloaca is always present 
 in the amphibia. 
 
 The reptiles have the intestine coiled (nearly straight in amphisbaenans) and 
 usually of about the same diameter throughout. Small and large intestine are 
 separated by an ileo-colic valve, and except in crocodiles a caecum is usually present, 
 while a cloaca constandy occurs. The spirally twisted coprolites of the ichthyo- 
 saurs have been supposed to indicate the existence of a spiral valve, but since in 
 other groups the faeces are formed in the rectum, this is not conclusive. 
 
 Fig. 233. — Digestive tract 
 of soup {Stenostomus chrysops 
 — ^Princeton 296). bd, bile 
 duct; gb, gall bladder; I, liv- 
 er; It, ' large intestine; pc, 
 pyloric caeca; si, small in- 
 testine. 
 
230 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 The intestine is longer in the birds than in the reptiles, but there is considerable 
 difference in the group in this respect. The great increase comes in the colon which 
 is coiled in different ways, which may be reduced to seven plans or combinations 
 of loops and spirals (fig. 234). In a few forms (woodpeckers, parrots, etc.) there 
 
 Fig. 234. — Types of coiling of the intestines of birds, after Gadow. A, isocoelous; 
 B, anticoelous; C, antipericoelous ; D, isopericoelous; E, cyclocoelous; F, plagiocoelous ; G, 
 telogyrous; p, pylorus. 
 
 is no caecum, but usually the junction of large and small intestine is marked by one 
 or two caeca (fig. 235). In some cases these caeca are lined with villi, or portions 
 may be ciliated, while the very large caecum of the ostrich is spirally coiled. Many 
 birds have a pocket, the bursa Fabricii, of unknown functions, developed from the 
 
 Fig. 235. — Alimentary canal of Chauna, after Mitchell, c, caeca; /, large intestine; p 
 proventriculus; pv, portal vein; rv, rectal vein; s, small intestine; v, remnant of vitelline 
 duct. 
 
 dorsal part of the cloaca. It arises from the ectodermal (proctodeal) portion and 
 extends forward, dorsal to the rectum (fig. 236). In some cases it degenerates in 
 the adult. 
 
 The limits of large and small intestine in the mammals are usually marked by an 
 ileo-colic valve and a single caecum, but there are two cajca in some edentates, 
 while some edentates, bats, carnivorous mammals and many whales lack either 
 caecum or valve. The caecum is larger in the herbivorous forms and frequently 
 
DIGESTIVE ORGANS. 
 
 231 
 
 there is a relation between the development of caecum and stomach. The caecum 
 becomes enormous in certain rodents and marsupials (sometimes longer than the 
 body) and plays an important part in digestion, being sometimes lobulated or 
 furnished with internal folds, those of the rabbits being arranged in a spiral manner. 
 In man and the anthropoids and some other forms, as is well known, the distal part 
 of the caecum degenerates to a rudiment, the vermiform appendix, which tends to 
 become obliterated with increasing age. 
 
 Fig. 236. Fig. 237. 
 
 Fig. 236. — Diagrammatic longitudinal section of the cloacal region of a duck embryo 
 at the twenty-second day of incubation, after Polndyer. ag, anal groove; c, cloaca; cp, 
 cloacal plate; /, bursa Fabricii; p, phallus, with caecal duct; sp, stercoral pouch of rectum. 
 
 Fig. 237. — Semidiagrammatic course of intestine of new-born deer Cenms canadensis, 
 after Weber, c, caecum; d, duodenum; co, colon; j, jejunum; m, mesentery. 
 
 Both small intestine and colon are at first straight, but with growth they become 
 longer, involving convolutions varying in pattern and extent in different groups, 
 the patterns of the colon being of some systematic value. The full history has 
 been worked out only for man, two stages being represented in figure 238. The 
 genus Hyrax is remarkable for a pair of caecal diverticula arising from the colon 
 (fig. 239). In the monotremes the rectum terminates in a cloaca as in the saurop- 
 sida, and the same condition occurs in the young of all higher mammalia. In the 
 later stages, however, the urogenital and digestive openings become separated by 
 the formation of a perineal fold between the two. 
 
 THE LIVER (HEPAR). 
 
 The liver, the largest gland in the body, has several functions. It 
 secretes the bile (gall) and forms several internal products such as 
 glycogen, urea and uric acid, of great importance in the animal economy. 
 
232 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 Fig. 238. — Scheme of alimentary canal and mesenteries in human embryos, 30 and 50 
 mm. long, after Klaatsch. c, caecum; co, colon; d^ duodenum; ^, kidney; r, rectum; rd^ 
 recto-duodenal ligament; rl, recto-lienal ligament; rrd, recto-duodenal recess; 5, stomach; 
 50, spleen. 
 
 Fig. 239. — ^Alimentary canal of Hyrax capensis after Flower, c, caecum ; d, blind diverticula 
 of colon; i, ileum; r, rectum; 5, stomach; si, small intestine. 
 
DIGESTIVE ORGANS. 
 
 233 
 
 The bile is passed to the intestine by the bile duct (choledochal or 
 hepatic duct), but the other products are carried away by the blood 
 
 (internal secretion). 
 
 The anlage of the liver is a ventral diverticulum from the archenteron 
 (p. 206), which grows forward from its point of origin, branches again 
 and again, the ultimate branches forming the glandular part of the 
 organ, the proximal parts of the outgrowth giving rise to the bile duct 
 (ocasionally multiple) which empties into the intestine. As a result of 
 this method of formation the liver is to be regarded as a compound 
 tubular gland, the lumens of the tubules forming the gall capillaries 
 which eventually empty into the duct. This tubular condition is 
 readily recognized in the ichthyopsida, but it is masked in the amniotes 
 and especially in the mammals, in part by the anastomosis of the 
 tubules, in part by the interrelation of the bile and blood-vessels. 
 
 With development the liver grows cephalad from its point of origin, 
 but this forward growth is limited by the presence of the blood-vessels 
 which develop the sinus venosus and the hepatic veins and also contrib- 
 ute to the septum transversum (hepatic 
 veins — see circulation), and so its later 
 increase must cause it to grow in the op- 
 posite direction. As it increases in size 
 there is an immigration of mesenchyme 
 between the lobules and with these the 
 
 Fig. 241. 
 b, gall bladder; ch, choledochar duct; 
 
 Fig. 240. 
 
 Fig. 240. — Diagram of two tj-pes of bile ducts. 
 h, hepatic ducts; i, intestine. 
 
 Fig. 241 . — Liver and pancreas of American ostrich (Rhea) after Gegenbaur. d, duo- 
 denum; rfA, bile ducts; /, liver; oe, oesophagus; p, pancreas; pd, pancreatic duct; s, stomach. 
 
 blood-vessels enter. At the same time the liver grows away from the 
 alimentary canal, carrying the peritoneum before it so that it receives 
 an outer serous coat. 
 
 Usually the bile duct (when there are several ducts only one is con- 
 cerned) forms a lateral diverticulum, the gall bladder, which serves as 
 a reservoir for the bile. This is usually placed on the dorsal side of the 
 
234 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 liver, but it may be immersed in the substance of the gland. In some 
 cases, even in mammals, the gall bladder may be lacking. When a 
 gall bladder is present, three regions may be recognized in the bile 
 ducts. Those parts which lead from the liver to the connexion with 
 the bladder are called hepatic ducts ; these are met by the cystic duct 
 leading from the bladder, and the common duct, formed by the two and 
 which empties into the intestine is the choledochal duct (fig. 240). 
 The shape of the gland is in part determined by the shape of the body, 
 being long in elongate species, sometimes consisting of two consecutive 
 lobes. Another modifying factor is the shape and size of the adjacent 
 organs, stomach, etc. Usually the liver is divided into right and left 
 halves, these corresponding to the first division of the anlage, but these 
 halves are hardly indicated in some of the teleosts. Frequently, and 
 especially in mammals, the halves become subdivided into lobes of 
 varying size, which are arranged in various ways. The liver is rela- 
 tively larger in the ichthyopsida than in the amniotes, but the cyclo- 
 stomes have a small liver, that of the myxinoids being in two parts. It 
 is larger, too, in the flesh-eating than in the herbivorous species. The 
 blood supply, chiefly through the portal vein and to a less extent by the 
 hepatic artery (see circulation) is very large. The color of the gland 
 is very variable, especially in teleosts, where it may be brown, yellow, 
 purple, green and even vermilion. 
 
 THE PANCREAS. 
 
 The second largest of the digestive glands, the pancreas, secretes 
 digestive ferments of great strength (trypsin, steapsin, amylopsin), 
 which digest both proteids and carbohydrates. In some respects it 
 resembles the salivary glands and so compensates in part for the 
 absence of them in the lower vertebrates (p. 220). The pancreas 
 arises by diverticula from the wall of the intestine close to the liver. 
 There are usually three of these diverticula, one dorsal and two ventral, 
 the ventral soon uniting (fig. 242), but in the sharks there is only a 
 single dorsal, diverticulum, while in the sturgeon there are two dorsal and 
 two ventral. In a general way these develop much like theliver, the distal 
 portions of the divisions forming the glands, which are of the acinous 
 type; the proximal portions form the ducts. Of these ducts all may 
 persist; all but one may disappear, while in the lampreys all may be 
 lost. In many mammals two ducts persist, the ventral forming the 
 
RESPIRATORY ORGANS. 235 
 
 main pancreatic duct (Wirsung's duct), the dorsal, the accessory 
 or Santorini*s duct. The ducts may remain distinct; they may unite 
 before entering the intestine or one of them may unite with the bile 
 duct. 
 
 For a long time it was supposed that a pancreas was lacking in 
 certain vertebrates (some teleosts, dipnoi, cyclostomes) , but recent 
 studies have shown its presence in many of these. In the case of some 
 
 ccj> 
 
 Fig. 242. — Diagram of developing pancreas of cat, after Thyng. c, ductus coledo- 
 chus; d, duodenum; dp, dorsal pancreas; ddpy its duct; », small intestine; s, stomach; vp, 
 ventral pancreas. 
 
 teleosts it occurs as a slender tube in the mesentery; in the dipnoi it is 
 outside of the muscles in the intestinal wall, while in the cyclostomes 
 it is partly concealed at the insertion of the spiral valve, partly (myxi- 
 noids) in the liver. In these forms, owing to the complete disappearance 
 of the duct it becomes a gland of internal secretion. The pancreas 
 may be elongate, compact, or sometimes extremely lobulated. Usually 
 (fig. 241) it lies in a loop of the duodenum. From certain peculiarities 
 of structure the queston has arisen as to whether two distinct structures 
 are included in the pancreas. 
 
 THE RESPIRATORY ORGANS. 
 
 The respiratory organs have for their purpose the exchange of gases 
 between the blood and the surrounding medium — water or air — 
 carbonic dioxide being given off and oxygen being absorbed by the 
 circulating fluid. In order that the exchange be readily effected it is 
 necessary that the organs be richly vascular, that the walls between the 
 blood and the surrounding medium be extremely thin so as to permit 
 rapid osmosis, and that the osmotic surface be as great as possible. 
 Further, there must be an adequate mechanism for passing the oxygen- 
 containing medium over the respiratory surfaces. 
 
236 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 In the vertebrates the organs of respiration are developed in more 
 or Jess intimate connection with the cephalic portion of the digestive 
 tract, just behind the cavity of the mouth. This part of the alimentary 
 
 canal, which thus serves for the pass- 
 age of food and for the performance 
 of respiratory functions is called the 
 pharynx. The organs themselves 
 may take the form of gills or branchiae, 
 adapted for aquatic respiration, or of 
 lungs (puhnones) fitted for breathing 
 air. In this connection must be con- 
 sidered the cases of certain fishes, 
 amphibia, and turtles where respiration 
 is effected in part by the skin, the 
 pharyngeal epithelium, or the diges- 
 tive tract. There are also a number 
 of other structures — air bladder, thy- 
 mus and thyreoid glands, etc., which 
 are derived from the pharynx, though 
 they are without respiratory functions. 
 
 GILLS OR BRANCHI^. 
 
 The typical gills or branchiae are 
 developed on the walls of some of the 
 visceral clefts (gill or branchial 
 clefts) which are formed in the sides 
 of the pharynx. These clefts arise as 
 paired pouches or grooves of the en- 
 toderm of the pharynx (fig. 208). 
 They extend laterally, pushing aside 
 the mesoderm, until they reach the 
 ectoderm, ectoderm and entoderm 
 then fusing to a plate. This in most 
 cases becomes perforated, so that the 
 cavity of the pharynx is connected with the exterior by a series of 
 openings (fig. 243), the clefts developing in succession from the 
 cephalic end backward. 
 
 , ;^ These visceral pouches develop in all vertebrates, but in the mam- 
 mals only a few or even none of them break through to the exterior. In 
 
 Fig. 243. — Pharyngeal region of a 
 young Acanthias embryo, h, blood- 
 vessels; c, coelomic cavities of gill arches; 
 gy developing gills; gc, gill clefts; h, 
 hypophysis; w, mouth; n, notochord; o, 
 oculomotor nerve; oe, oesophagus; />, 
 peritoneal cavity; s, spiracular cleft; 
 I-III, first to third head cavities. 
 
RESPIRATORY ORGANS. 237 
 
 the adult amniotes the pouches may completely disappear without 
 leaving a trace, aside from the Eustachian tube (p. 187) and the thymus 
 glands to be mentioned below. The number of clefts or pouches 
 varies between considerable limits. The largest number in any true 
 vertebrate (there are more in Amphioxus and the enteropneusts) is 
 fourteen pairs in some specimens of Bdellostoma. In other cyclo- 
 stomes there are seven, eight to seven in the notidanid sharks, six in 
 other elasmobranchs, five or six in teleostomes, amphibia and reptiles 
 and five in mammals. In this numbering the oral cleft is not included, 
 although there is some evidence that the mouth has arisen by the 
 coalesence of a pair of gill clefts (p. 206). 
 
 The serial repetition of the visceral clefts does not strictly correspond to the 
 other segmentation of the body, their number and position being at variance with 
 those of the myotomes. There is a branchiomerism or serial repetition of the 
 gill clefts, apparently distinct from the true metamerism of the head. The ap- 
 pearance of these clefts or pouches and the relation of aortic and branchial arches 
 in the amniotes, where gills are never developed, can best be explained by the 
 assumption that these forms have descended from branchiate ancestors. 
 
 Between each two successive gill clefts there is an interbranchial 
 septxim, covered externally with ectoderm, internally with entoderm, 
 and with an axis of mesoderm, the latter in the earlier stages carrying 
 with it a diverticulum of the coelom (fig. 243, c). Later blood-vessels 
 (aortic arches) and skeletal elements (visceral arches, p. 63), are devel- 
 oped in each septum, the visceral arches appearing on the splanchnic 
 side of the coelom and hence not being comparable to ribs or girdles. 
 
 In the cyclostomes and fishes the gills are developed from the an- 
 terior and posterior walls of the typical interbranchial septa. They 
 were long regarded as of entodermal origin, but in recent years con- 
 siderable doubt has been thrown on this, at least for the fishes, and 
 there is some evidence for their ectodermal origin. The matter cannot 
 yet be regarded as settled. These gills are either filamentous or la- 
 mellate outgrowths of epithelium, each carrying a loop of a blood- 
 vessel. Thus each typical cleft is bounded in front and behind by gill 
 plates or filaments (fig. 246) , those on a side constituting a demibranch, 
 the two demibranchs of a septum constituting a gill, while a cleft is 
 bounded by demibranchs belonging to two gills. In the young 
 elasmobranchs and in the young of a few teleosts (before birth) the 
 gill filaments protrude from the clefts as long threads, but later they 
 are withdrawn. 
 
238 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 In the cyclostomes and notidanid sharks the first cleft (between the 
 mandibular and hyoid arches) bears gills like the rest, but elsewhere 
 it dififers. In most elasmobranchs and in a few ganoids {Acipenser^ 
 
 Fig. 244. — Diagram of relations of oesophagus and respiratory tracts in {A) Myxine and 
 Ammocoetes, and {B) Petromyzon, b, bronchus; oe, oesophagus; i, thyreoid gland. 
 
 Polyodon, Polypterus) it becomes reduced in size in the adult, the 
 closure beginning ventrally (fig. 136) so that the persistent part of the 
 opening is on the upper side of the head. This opening is called the 
 spiracle. In other vertebrates, including the 
 chimaeroid sharks and many true sharks, the 
 spiracle is closed in the adult, but in the anura and 
 the amniotes its inner portion persists as the 
 Eustachian tube and the tympanic cavity of the 
 ear (p. 187). 
 
 Usually the series of gills begins wdth the 
 demibranch on the caudal side of the hyoid arch, 
 while none ever appears on the caudal side of the 
 last cleft. In the teleosts the series of gills is still 
 further reduced, the reduction reaching its ex- 
 treme in Amphipnous, where there are no demi- 
 branchs on the first and fourth branchial arches 
 and only one on the second. 
 
 In the cyclostomes the gill clefts occur at a consider- 
 able distance behind the mouth, partly the result of the 
 great development of the lingual apparatus. In the larvae 
 of Petromyzon (Ammocoetes) the seven gill clefts are 
 nearly typical, the gill extending inward nearly to the 
 pharyngeal wall, each cleft having a short efferent duct 
 leading to the exterior, and the oesophagus beginning at 
 the hinder end of the pharynx (fig. 244, A). In the meta- 
 morphosis to the adult the oesophagus grows forward, 
 dorsal to the gill clefts, to the cephalic end of the 
 
 pharynx, thus cutting ofif a ventral respiratory tube, the so-called bronchus (fig. 
 
 244, B). At the same time the gill-bearing region of each cleft becomes separated 
 
 Fig. 245. — Gill 
 pouches and blood-vessels 
 of Myxine, after Miiller. 
 b, gill pouches, ed, effer- 
 ent ducts; eo, external gill 
 opening; h, heart; oe, 
 oesophageo- cutaneous 
 duct; ph, pharynx. 
 
RESPIRATORY ORGANS. 
 
 239 
 
 from the bronchus by the development of a short afferent duct, while the demi- 
 branchs come to lie in oval pouches (much as in Myxine, fig. 245), in allusion to 
 which the cyclostomes are sometimes called marsipobranchs (pouched gills). 
 
 In the myxinoids the tract between the mouth opening and the pharynx is 
 more elongated arid the pharyngeal region (fig. 244, A) is not differentiated into 
 oesophagus and bronchus, as in the adult lampreys. In Myxine there are six pairs 
 of gills; in Bdellosioma the number ranges from seven to fourteen, varying even on 
 the two sides of our Pacific species, B. dombeyi. In the petromyzons and in 
 
 Fig. 246.- — Diagram of gill clefts in {A) elasmobranchs and {B) teleosts. A^ and B\ 
 a single gill of each, a, artery; br, branchial ray; d, demibranchs; gc, gill chamber; gr, 
 gill raker; 0, operculum; oe, oesophagus; 00, opercular opening; s, spiracle; v, veins. 
 
 Bdellostoma the efferent ducts of the gill pouches open separately to the exterior; in 
 Myxine (fig. 245) they unite into a common duct on either side, the left also receiv- 
 ing an oesophago-cutaneous duct, behind the last gill. This duct, which leads 
 from the oesophagus to the exterior, resembles a gill cleft, but lacks gills. A similar 
 duct occurs in the same position in Bdellostoma. 
 
 In the fishes there are two types of gills and associated structures. 
 In the elasmobranchs (the chimaeroids excepted) the interbranchial 
 septum is greatly developed (fig. 246, A'), extending some distance 
 laj:erally beyond the gill folds so that the distal part of the cleft forms an 
 excurrent canal. This prolongation of the septum extends to the ex- 
 terior and then turns backward, thus protecting the delicate gills from 
 
240 FOMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 injury (fig. 246, A). In other fishes the posterior margin of the hyoid 
 septum grows back as a broad fold over the clefts behind, thus forming 
 a gill cover or operculum (fig. 246, B, 0) , enclosing an extrabran- 
 chial or atrial chamber into which all of the clefts empty and which 
 in turn opens to the exterior by a single slit (po) behind the operculum. 
 This opercular opening is usually broad, but it is reduced to a circular 
 opening on either side in a few teleosts, while in the symbranchii the 
 openings of the two sides are united to a single one in the mid-ventral 
 line. Correlated with this protection of the gills by the operculum is 
 the reduction of the interbranchial septum (fig. 246, 5'), which forms 
 only a slender bar, from which the demibranchs project far into the 
 gill chamber. 
 
 Fig. 247. — Head of Chlamydoselache, after Garman;/, opercular fold. 
 
 Usually the two opercular folds are continuous beneath the pharynx, 
 which points to the beginnings of an operculum in the shark, Chlamy- 
 doselache (fig. 247). In the chimaeroids the operculum is farther 
 developed and is supported by cartilaginous rays. In the teleostomes 
 two parts may be recognized in the operculum, the operculum or gill 
 cover proper, supported by a series of large bones (p. 77), and a more 
 ventral part, the branchiostegal membrane, which is very flexible 
 and has a skeleton of slender (branchiostegal) rays, connected with 
 the hyoid. 
 
 In the sea horses and pipe fishes (lophobranchs) the gills form small rounded 
 tufts. In the labyrinthine fishes there is a complicated bony structure in the bran- 
 chial chamber, covered by a folded membrane which is used in aerial respiration. 
 In the young crossopterygians (Polypterus, Calamoichthys) bipinnate external gills 
 persist for some time. In Amphipnous, just referred to, a sac opening between 
 the hyoid and the first branchial arch is developed on either side of the head. 
 Its walls are very vascular thin vessels being connected with both the branchial 
 arteries and the dorsal aorta. 
 
 The gills are so placed that there can be an almost continuous stream 
 of water over them, thus bringing the oxygen needed by the blood. As 
 a rule, this water is drawn in through the mouth by the enlargement of 
 the oral cavity, and by its contraction is forced out through the clefts. 
 
RESPIRATORY ORGANS. 
 
 241 
 
 In the myxinoids the oesophago-cutaneous duct is supposed to act as the 
 incurrent opening when these animals burrow into fishes. In the lam- 
 preys the water is said to pass both in and out through the gill clefts 
 when these animals are attached to some object. In at least some of 
 the elasmobranchs water passes in through the spiracle which regularly 
 opens and closes. 
 
 Many, if not all of the teleosts have breathing valves. There are two pairs of 
 these, an anterior pair attached to the margins of the jaws, which permit the ingress 
 of the water but prevent its outflow. The other pair is formed by the branchiostegal 
 membrane, which closes the opercular opening and only allows -the water to pass 
 out. The action of both pairs can be easily seen from fig. 248. 
 
 Fig. 248. — Breathing valves of teleosts, after Dahlgren. A, schematic figure, the 
 anterior half in the vertical, the posterior in the horizontal plane; B, mouth of sunfish 
 {Eupomotis) ; b, branchiostegal valve; mn, mx, mandibular and maxillary valves; v, oral 
 valves. 
 
 In certain fishes with an operculum (Acipenser, Lepidosteus, many teleosts) a 
 gill is developed as a series of lamellae on the inner surface of the operculum. This 
 opercular gill has respiratory functions. The pseudobranchs are homologous 
 with the true gills. They are developed in some elasmobranchs as vertical folds on 
 the anterior wall of the spiracular cleft, occurring in some cases, even where the 
 spiracle is closed externally. They, however, receive arterial blood and so cannot 
 be respiratory in function. The blood, still arterial in character, passes from them 
 to the chorioid coat of the eye and in some cases to the brain. From their position 
 they must be interpreted as the demibranch of the posterior side of the mandibular 
 arch. 
 
 Pseudobranchs are common in teleosts, usually lying on the medial side of the 
 hyomandibular bone. When free, they are gill-like in appearance, but in some 
 species (fig. 249) they are covered by muscles and connective tissue, when they 
 have a blood-red, glandular appearance. Pseudobranchs also occur in Lepidosteus, 
 most sturgeons and Ceratodus; they are lacking in Amia and Protopterus. Polyp 
 terus and Polyodon have opercular gills. 
 16 
 
242 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 In the amphibia the gill clefts are formed in the same way as in the 
 fishes, but the first and fifth never break through, and all are usually 
 closed in the adult, the exceptions being in the perennibranchs and 
 derotremes where from one to three clefts remain open through life. In 
 the urodeles and caecilians there is a reduced operculum which never 
 becomes prominent, being merely a fold of the integument in front of 
 the gill area. In the larval anura it is well developed, though skeletal 
 
 Fig. 249. — Dissection of pseudobranchs (j>5) and cephalic circle in pike {Esox), after 
 Maurer. cc, cephalic circle e, vessels to eyes; g, gills; n, vessels to palate and nose; 
 I-IV, efferent branchial arteries. 
 
 Supports are lacking, as in all amphibia. Before the time of metamor- 
 phosis it grows backward over the gills, gill clefts, and the anlagen of 
 the fore limbs, and fuses with the sides of the body behind the latter. 
 In this way these parts are enclosed in an extrabranchial or atrial 
 chamber, the chambers of the two sides being in communication below. 
 During larval life the branchial chambers usually communicate with 
 the exterior by a single excurrent pore, usually on the left side, but in 
 the larval aglossa right and left excurrent pores are found. 
 
 The gills of the amphibia are certainly of ectodermal origin (cf. p. 
 237). First to appear are the external gills, covered with ciliated epi- 
 
RESPIRATORY ORGANS. 
 
 243 
 
 thelium. Three pairs of these usually arise, before the gill clefts break 
 through, on the outer surface of the third, fourth and fifth arches, and 
 they are supplied by the corresponding (aortic) arches of the blood 
 system. They are without any skeletal support and are of varying form 
 — pectinate, bipinnate, dendritic, etc. (fig. 250)— and in one species 
 
 Fig. 250. — External gills of young Amphiuma, partially covered by opercular fold. 
 
 of caecilians, where but a single pair occurs, they are large leaf-like 
 lobes. When the gill clefts break through there is an ingrowth of ecto- 
 derm into each cleft, from which (except in perennibranchs) gill fila- 
 ments are developed on the sides of the septa,' so that for a time there 
 may be both external and internal gills (fig. 251, right side). In the 
 
 Fig. 251. — Diagram of the relations of external and internal gills in the anuran tad- 
 pole, ifter Maurer. ab, eb, afferent and efferent branchial arteries; A, heart; 0, ear cavity; 
 ph\ pharynx; ra, radix aortae. 
 
 perennibranchs the external gills persist through life (they are said to 
 be absorbed and reformed in Siren) , but in other urodeles and in caecil- 
 ians they are absorbed at the time of metamorphosis. In the anura 
 (fig. 251), as the operculum grows back over the clefts, the external 
 gills, which are so prominent in the earlier stages, become folded into 
 the extrabranchial chamber, where they are gradually reduced, while 
 
244 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 those belonging to the cleft become the functional organs, the water 
 taken in through the mouth passing over them in its way to the exterior 
 via the extrabranchial chamber. Then, with the completion of the 
 metamorphosis, the lungs become functional, the gill clefts are closed 
 and the gills absorbed, the legs are developed and the anterior pair 
 released from the extrabranchial chamber, the tail is absorbed, and the 
 tadpole (larva) becomes the adult. 
 
 Fig. 252. — Cast of oropharyngeal region of pig embryo, 17 mm. long, after Fox. alf, 
 alveo-lingual fold; ctm, cervical cord of thymus; dp^, dp'^, dorsal apex of first and second 
 pharyngeal pouches; dptm, dorsal plate of thymus;/, filiform appendix of second pouch; 
 ilr, lateral thyreoid; stt, sulcus tubo-tympanicus; im, thymus; vf, vestibular fold of mouth. 
 
 Little is known of the gills in the stegocephals, but the presence of well developed 
 branchial arches in the larvae of some species (p. 83) would imply the existence 
 of functional gills. 
 
 For some time it was thought that the fish gills were of entodermal origin, and 
 those of the amphibia were derived from the ectoderm. Hence the conclusion was 
 that the two had no genetic connexion, the gills of the amphibia being a new 
 acquisition, developed within the group or arising from the external gills of some 
 form like Polypterus. Lately the doubts thrown upon the entodermal origin of the 
 gills of fishes (p. 237) render it possible that all vertebrate gills are homologous. 
 
 Gills are never developed in the amniotes, but in the embryos the 
 paired visceral pouches are formed (figs. 208, 252) — five in the saurop- 
 sida, four in mammals — in the same way as in the fish-like forms. 
 Few, if any, of them break through to the exterior, although their 
 position is indicated by grooves on the outside of the neck. The proc- 
 ess of obliteration of these e^Kternal grooves is interesting. The ante- 
 rior arches enlarge and slide back over the posterior, so that at least 
 the external branchial grooves lie in the wall of a pocket, the cervical 
 sinus, on either side of the neck (fig. 253). Later a process of the 
 anterior (hyoid) arch extends over and closes the sinus, a process re- 
 
RESPIRATORY ORGANS. 
 
 245 
 
 calling the history in the anura. Internally the entodermal branchial 
 pouches, with the exception of the first, disappear, but the first persists 
 as the tympanic cavity and Eustachian tube described in connexion 
 with the ear. 
 
 Fig. 253. — Head of human embryo with phar3mgeal floor removed, after Hertwig. 
 Cut surfaces lined. Compare with fig. 221. cs, cervical sinus; e, eye; h, hyoid arch; hd, 
 hypophysial duct (Rathke's pocket); I, lung; Ig, lacrimal duct; w, naris; md, mandible; 
 an, oronasal groove; tr, trachea. 
 
 Pharyngeal Derivatives. 
 
 Several structures arise in the pharyngeal region — some developed 
 from gill clefts, some from other parts — which, while not respiratory 
 in character, naturally come for mention here. 
 
 Among these are the thymus glands. These arise from the ento- 
 dermal epithelium at the dorsal angle of a varying number of visceral 
 clefts (elasmobranchs, clefts 2-6 and possibly the spiracle; teleosts and 
 caecilians, 2-6; urodeles, 1-5, i and 2 degenerating; anura, i and 2, 
 the latter only persisting; amniotes 3 and 4). 
 
 The organ which results has varying positions and shapes in the 
 different groups. It becomes richly vascular, and by the intrusion of 
 connective tissue, assumes an acinous form. In Myxine a number of 
 lobules behind the gill region have been regarded as a thymus, but now 
 are interpreted as pronenephric. In some cases (fishes, etc.) the thymus 
 retains its primitive position dorsal to the gill clefts (usually above the 
 
246 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 fourth in teleostomes), and it maintains its branchiomeric character in 
 snakes and gymnophiones. It may lie above and behind the angle of the 
 jaw (most amphibians), close to the carotid arteries (most sauropsida), 
 sometimes extending along the neck (crocodiles and birds). In the 
 young mammals the thymus (sold in the markets as 'throat sweet- 
 breads'), which arises from a single pair of clefts, is largely behind the 
 sternum, extending forward along the neck. Later it gradually grows 
 smaller, the extreme development being reached in man between the 
 fourteenth and sixteenth years, but retaining its functional structure 
 
 Fig. 254. — Schemes of the origin of several pharyngeal derivatives in (A) Rata, (B) 
 anuran and (C) chick, after Verdun, cd, carotid gland ; e, epithelial body ; gr^ gill remnants ; 
 p, postbranchial body; tm, thymus; tr, thyreoid; 7-77, gill pouches or clefts. 
 
 until middle life. The function of the thymus glands is as yet unknown ; 
 though leucocytes are abundant in them, they are not lymphoidal in 
 character. 
 
 Other structures arising in the pharynx, either from the gill clefts 
 or from the pharyngeal walls, are the ' epithelial bodies,' post-branchial 
 bodies, suprapericardial bodies, gill remnants, etc., concerning which 
 little is known. The carotid glands of the same region are referred to 
 elsewhere. 
 
 The thyreoid gland cannot be dismissed in such a summary 
 manner. This is a ductless gland in the pharyngeal region of all 
 vertebrates, ventral to the alimentary tract. In the lower vertebrates 
 it arises as an unpaired pocket in the floor of the pharynx (fig. 254), 
 this retaining its connexion with the parent tube in the ammocoete 
 stage of the lamprey (fig. 190), but at the time of metamorphosis it 
 loses its duct (as is early the case in all other vertebrates) and eventu- 
 
RESPIRATORY ORGANS. 247 
 
 ally becomes follicular. In most vertebrates, the anlage, after separa- 
 tion, forms a network of epithelial tubes before becoming follicular. 
 Usually it exhibits a marked bilaterality, and in amphibia and birds 
 it becomes divided into two glands. 
 
 In the elasmobranchs the thyreoid lies between the end of the ventral 
 aorta and the symphysis of the lower jaw; in teleosts the groups of 
 follicles lie around the ventral aorta, extending out on the anterior aortic 
 arches. In the urodeles the gland lies just behind the second arch and 
 in the anura on the hinder margin of the thyreoid process of the hyoid 
 plate. In reptiles it is ventral to the trachea (at about its middle in 
 lizards, nearer its division in other groups), while in the birds the two 
 glands occur at the base of the bronchi. In the mammals it is usually 
 near the larynx, and while generally two-lobed, it is here and there 
 (monotremes, some marsupials, lemurs, etc.) paired. 
 
 Like the other ductless glands, the thyreoid supplies the blood with 
 substances necessary to the well-being of the organism, in the case of 
 mammals at least, an iodine-containing albumen. Degeneration or 
 extirpation of the thyreoid result in cerebral trouble. In the ancestral 
 vertebrate the thyreoid apparently had to do with some part of the 
 digestive work, as is shown by its late connexion with the pharynx 
 in the ammoccete. 
 
 In the pharynx and at the entrance of the mouth into the pharyn- 
 geal cavity (isthmus of the fauces) occur certain lymphatic structures 
 called tonsils, concerning which our knowledge is yet very deficient. 
 One account says they arise from inwandering epithelial cells, the 
 other maintains that they are formed from the sub-epithelial meso- 
 derm. Two different groups of organs are included under this name, 
 the true tonsils at the isthmus of the fauces, and the pharyngeal 
 tonsils. The latter may be represented by lymphoid structures in the 
 floor or roof of the pharynx of urodeles and anura. They are well 
 developed in reptiles and birds, occurring in the latter behind the 
 choanae. In mammals they are inconstant structures. The true 
 tonsils of mammals lie one on either side of the isthmus. Both 
 types of tonsils consist of an adenoid ground substance containing 
 numerous lymph cells, and become follicular after birth. 
 
 THE SWIM BLADDER. 
 
 While the air or swim bladder (pnenmatocyst) is not respiratory, 
 it is included here from its possible connexion with the lungs. It 
 
248 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 occurs only in teleostomes, and while found in most species (frequently 
 absent from bottom-feeding forms — pleuronectids, etc.), it is lacking 
 here and there, even among species classed as physostomous {Lori- 
 caria, etc.). In the young of a few sharks {e.g.^ Scyllium) there is a 
 pouch on the dorsal side of the oesophagus which suggests the possible 
 origin of the organ. 
 
 The swim bladder lies dorsal to the alimentary tract, outside of the 
 peritoneum (which frequently covers only its ventral surface) immedi- 
 
 FiG. 255. — Air bladder of Megahps cyprinoides, after de Beaufort, a, anus; h, air 
 bladder; d, pneumatic dust leading from the oesophagus; /, ligament; p, anterior part of 
 bladder extending to skull. 
 
 ately below the vertebrae and excretory organs (mesonephroi). In 
 some instances it extends the whole length of the body cavity and 
 (clupeids) may even send diverticula into the head. In other species 
 it may be much shorter. In development it arises as a diverticulum of 
 the a;limentary canal (fig. 209), and in the ganoids and one group of 
 teleosts (physostomi) it is connected with the digestive tract throughout 
 
 Fig. 256. — Swim-bladders of physostomous fishes; A, pickerel {Esox); B, carp (Cypri- 
 nus); and C, eel {Anguilla) after Tracy. 6, swim-bladder; ^, duct; g, red gland; oe, 
 oesophagus. 
 
 life by the pneumatic duct. This usually empties into the oesophagus, 
 but it may connect with the stomach. In most teleosts, however, the 
 duct becomes closed at an early date and the bladder loses its connex- 
 ion with the digestive tract (physoclisti) . 
 
 The swim bladder is usually unpaired (paired in most ganoids) and 
 may be simple or divided into two (rarely three) connecting sacs (fig. 
 256). It is usually regular in outline, but diverticula of all kinds are 
 
RESPIRATORY ORGANS. 
 
 249 
 
 common, the form being most varied in the physoclistous species. In- 
 ternally the walls may be smooth and the cavity simple, or it may be sub- 
 divided by septa (fig. 257), or, as in Amia and Lepidosteus, it may be 
 alveolar, recalling the condition in the lungs of higher vertebrates. 
 The walls sometimes contain striated muscle, and in some siluroids and 
 cyprinoids they are more or less calcified, partly by the inclusion of 
 processes from the vertebrae. 
 
 Fig. 257. — \"entral view of opened air bladder and Weberian apparatus of Macrones, 
 
 combined from Bridge and Haddon. a, atrial cavity; ac, anterior chamber of air bladder, 
 the arrows showing the connexion with the posterior chamber; de, endoljonph duct; s, 
 sacculus; sc, scaphium; sk, sub vertebral keel; tra, trc, anterior and crescentic processes of 
 tripos; M, utri cuius. 
 
 The blood supply is arterial, coming from either the aorta or the 
 coeliac axis, in some instances dififerent portions receiving blood from 
 both. In the walls the arteries break up into networks of minute 
 vessels (*rete mirabile'), these frequently making *red spots' on the 
 inner surface. From the retia the blood passes to the body veins, (post- 
 cardinal, hepatic or vertebral) . In the ganoids and phystomous species, 
 especially those with a wide pneumatic duct, the gases contained in the 
 swim bladder may be obtained directly from the air or water, but in the 
 physoclists this is impossible and the red spots may be the place of its 
 
250 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 secretion and possibly of its absorption, the probability being increased 
 by the greater abundance of the spots in species with closed ducts. 
 
 While the pneumatic duct usually connects with the dorsal side of the alimentary 
 canal, it enters the left side in Eryihrinus, and in the mid-ventral line in Polypierus 
 and in Calamoichthys, In Polypterus the bladder arises from the ventral side and 
 there are paired swim bladders, the right being the longer. The blood in this genus 
 comes from the efferent branchial arteries and hence is arterial. 
 
 The swim bladder is supposed to have hydrostatic functions, aiding 
 in the recognition of differences of pressure due to changes in depth. 
 In the clupeids the air bladder sends a diverticulum into the head, 
 there giving a branch to each ear. In some physostomes (siluroids, 
 cyprinids, gymnonoti) parts of the anterior vertebrae are modified into a 
 chain of bones — the Weberian apparatus — adapted to convey dif- 
 ferences of bladder pressure to the internal ears. One pair of bones 
 is connected with the dorsal wall of the air bladder, a second with a 
 diverticulum (sinus impar) of the internal ear, while others are in- 
 tercalated between these extremes (fig. 257) . Changes in the distention 
 of the bladder are thus conveyed to the inner jear and probably affect 
 the sense organs. 
 
 LUNGS AND AIR DUCTS. 
 
 Lungs arise as a diverticulum from the ventral side of the pharynx* 
 immediately behind the last gill pouch. The diverticulum divides 
 almost as soon as outlined into right and left halves, each the anlage of 
 the corresponding lung. As development proceeds, the two grow in a 
 caudal direction into the trunk, carrying the peritoneum with them as 
 they protrude into the ccelom, so that they eventually have an entodermal 
 lining, derived from the epithelium of the pharynx; an outer serous 
 layer of peritoneum, with mesenchyme carrying blood- and lymph- 
 vessels, nerve and smooth-muscle fibres between the two. In this 
 development two parts are differentiated, the lungs, the actual seat of 
 the exchange of gases, and the air ducts leading from the pharynx to 
 •them. The ducts may consist of an anterior unpaired portion, the 
 wind-pipe or trachea, connecting with the pharynx, and usually divid- 
 ing at its lower or posterior end into two tubes, the bronchi, leading to 
 the two lungs. In most air-breathing vertebrates the anterior part of 
 the trachea is specialized and forms a larynx. In addition to these 
 parts, the mechanism by which air is drawn into and expelled from the 
 lungs forms a part of the respiratory apparatus. 
 
RESPIRATORY ORGANS. 
 
 251 
 
 THE AIR DUCTS. 
 
 The opening from the pharynx into the air ducts is known as the 
 glottis, usually an elongate slit capable of being closed and opened 
 by appropriate muscles. This is immediately succeeded by the ducts, 
 which, except in the dipnoi, are more or less differentiated into regions 
 and have skeletal supports in their walls. 
 
 In the dipnoi the glottis is either in the mid-ventral line (Protopterus) or a little 
 to one side (Lepidosiren, Ceratodus) and the air duct passes up on the right side 
 of the oesophagus to reach the lungs which are dorsal to the alimentary canal. The 
 tube is without skeletal supports and connects directly with both lungs without any 
 division into bronchi. 
 
 Larynx.^The beginnings of the larynx are seen in the amphibia, 
 where in the lower types (Necturus) a pair of cartilages are developed on 
 the sides of the glottis, in the position of 
 a reduced visceral arch, each cartilage 
 extending posteriorly a short distance 
 along the air ducts. In other genera of 
 urodeles the anterior end of each lateral 
 cartilage separates from the rest as an 
 arytenoid, the first of the laryngeal carti- 
 lages, imbedded in the walls of the glottis. 
 The rest of the lateral cartilages may 
 remain entire (fig. 258) or they may 
 separate into a number of pieces, extend- 
 ing along the lateral walls of the trachea 
 and bronchi. Usually the anterior pair 
 of these pieces fuse in the mid-ventral line, ^i^- 258.— Trachea, etc., of 
 
 . Amphtuma, after Wilder. a, 
 
 thus formmg the second (criCOia) ele- arytenoid cartilages; 6*, fourth bran- 
 
 ment of the pharyngeal framework. LtdeTtj.tyoph^a^gertS 
 
 These parts are moved by antagonistic cle; tr, trachea with cartilages in its 
 
 muscles. One set of these, extending to 
 
 the persistent branchial arches, serves as dilatators of the glottis; the 
 others, connected with the laryngeal cartilages themselves, constrict 
 the opening. In the anura the cricoid is converted into a ring, with 
 the arytenoid hinged w^ithin and anterior to it, the whole larynx moving 
 anteriorly to a position between the hinder processes of the hyoid plate. 
 Inside of the short larynx thus framed by these cartilages are a pair 
 of folds of the laryngeal lining, the vocal cords, extending parallel to 
 
252 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 the margins of the glottis. These may be tightened or relaxed, and 
 by their vibration of their edges under influence of the breath the 
 voice is produced. 
 
 The larynx is scarcely more developed in reptiles. The cricoid is usually an 
 incomplete ring, to which the arytenoids are attached, and the whole is placed just 
 ventral to the median part of the hyoid, with which it is closely associated (fig. 259). 
 In several reptiles there is a fold of the mucous membrane just in front of the glottis 
 which is supposed to represent the beginnings of an epiglottis (infra), while in 
 geckos and chameleons a pair of folds, running dorso-ventrally in the larynx, 
 serve as vocal cords. The larynx is also rudimentary in the birds, its place as a 
 vocal organ being taken by the syrinx to be described below, in connexion with the 
 trachea. The arytenoids are frequently ossified in birds. 
 
 Fig. 259. Fig. 260. 
 
 Fig. 259. — Laryngeal apparatus of Chelone, after Goppert. a, arytenoid; b^--, first and 
 second branchial arches; cr, cricoid; d, dilator laryngis muscle; g, glottis; h, hyoid; he, 
 hyoid cornua; sph, sphincter laryngis; tr, trachea; cartilage dotted, bone black. 
 
 Fig. 260. — Ventral and side views of monotreme larynx, after Gegenbaur. c, cri- 
 coid; h, hyoid; th, thyreoid; tr, trachea. 
 
 In the mammals the larynx reaches its highest development. Its 
 framework is formed by the arytenoid and cricoid cartilages, homol- 
 ogous with those of the lower groups, and in addition, a thyreoid 
 cartilage (or cartilages) on the dorsal side anterior to the arytenoids 
 and cricoids. The origin of the thyreoid is best seen in the monotremes 
 where the hyoid apparatus enters into close relations with the larynx 
 (fig. 260), while the second and third branchial cartilages form two 
 plates, the lateral elements of the thyreoid on either side, the median 
 
RESPIRATORY ORGANS. 253 
 
 element of the hyoid forming a copula. In the higher mammals the 
 association of hyoid and larynx is not so intimate, even in the embryo, 
 but the thyreoid shows its double origin in its development. 
 
 In the higher mammals the thyreoid cartilage forms a half ring on 
 the ventral side of the anterior end of the larynx, its anterior dorsal 
 angles being produced into cornua connected by ligament with the 
 hyoid (fig. 261). Dorsal to the thyreoid is the glottis with the aryte- 
 noids in its walls. Posterior to it is the ring-shaped cricoid, following 
 which is the trachea. Anterior to the glot'tis is a fold of the mucous 
 
 Fig. 261. — Dorsal and side views of larynx of opossum, Didelphys virginianus (Prince- 
 ton 1739) cartilages dotted, a, arytenoid; c, cricoid; e, epiglottis; g, glottis; h, hyoid; 
 t, trachea; th, thyreoid. 
 
 membrane of the pharynx, the epiglottis, supported by an internal car- 
 tilage (possibly the fourth branchial arch) which articulates with the 
 anterior margin of the thyreoid. The epiglottis usually stands erect, 
 leaving the glottis open for respiration, but during deglutition it folds 
 back over the glottis, thus preventing the entrance of food into the 
 trachea. 
 
 Internally the cavity of the larynx bears a vocal cord on either side. 
 These are folds of the mucous membrane, extending from the thyreoid 
 to the arytenoids, and by movements of these latter cartilages they can 
 be tightened or relaxed, thus altering the pitch of the note caused by 
 their vibration. Anterior to these cords is a pocket, the laryngeal 
 ventricle (sinus of Morgagni) on either side, small in most mammals, 
 but developed in the anthropoid apes to large vocal sacs (in some 
 there is a median vocal sac in addition), which act as resonators, 
 adding to the strength of the voice. 
 
254 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 In the whales and young marsupials the larynx is prolonged so that it projects 
 into the choana behind the soft palate. In the whales (fig. 262) this is an adaptation 
 to the manner of taking food from the water and breathing at the same time. In 
 the young marsupials the milk is forced into the mouth by the muscles of the mam- 
 m£e of the mother and this arrangement prevents strangulation. 
 
 Trachea. — In the tetrapoda the trachea is strengthened by the 
 formation of cartilage in its walls, the beginnings of which are seen 
 in the urodeles where the fifth branchial arch gives rise to these ele- 
 ments (p. 251). Their arrangement varies considerably in the urodeles 
 and caecilians, being sometimes scattered pieces, 
 sometimes regularly arranged and even united in 
 the lateral walls (fig. 258). Corresponding to the 
 posterior position of the lungs the trachea is long 
 in these groups, but in the anura it can scarcely 
 be said to exist, the lungs succeeding almost 
 immediately to the larynx. 
 
 In the reptiles the trachea varies in length, 
 being shortest in lizards (except amphisbaenas) , 
 longer in snakes, tortoises and crocodiles, divid- 
 FiG. 262.— Larynx of ing into bronchi at varying distances from the 
 Xiphius cavirostris (after j^ngs. It is frequently bent in turdes. In many 
 
 Gegenbaur) from side ® ^ j ^ j 
 
 showing the prolongation reptiles the cartilage rings of the trachea are in- 
 eloir'w wtLhlr^^c't complete, but in Sfhenoion, lizards and some 
 into the choana; c, cricoid; snakes some Cartilages (usually the more anter- 
 
 th, thyreoid. . . , , . , 1 
 
 lor) form complete rmgs, the others being com- 
 pleted dorsally by membrane. In snakes the successive rings are 
 often united, especially on the sides. 
 
 The trachea is greatly elongate in birds in correlation with the 
 length of the neck and the position of the lungs within the thorax. 
 The rings, which are usually complete, are frequently ossified. The 
 trachea is occasionally (male ducks, etc.) widened in the middle and 
 in various groups becomes greatly convoluted so that its length from the 
 glottis to the lungs exceeds that of the neck. In some these convolu- 
 tions occur beneath the integument of the thorax; in some between the 
 sternum and the muscles; and in the cranes and swans within the 
 keel of the sternum. 
 
 The larynx is never the organ of voice in the birds, its place being 
 taken by a somewhat similar structure, the syrinx, at the division of 
 the trachea into the bronchi. The sound-producing elements are 
 
RESPIRATORY ORGANS. 
 
 255 
 
 membranes which vibrate by the passage of air, as do the vocal 
 cords of mammals. Most common is the broncho-tracheal syrinx, 
 in which the last rings of the trachea are united to form a reso- 
 nating chamber, the tympanum, while folds of membrane, internal 
 and external t)mipanic membranes (not to be confused with the simi- 
 larly named structure in the ear, p. 187), extend into the cavity from the 
 median and lateral wall of each bronchus. In some cases there is also 
 an internal skeletal element (pessulus) which bears a semilunar mem- 
 brane on its lower surface. In many birds this type of syrinx is 
 often asymmetrical (fig. 263) and is ex- 
 panded into a (usually) bony resonat- 
 ing vesicle. In the tracheal type of 
 syrinx the lateral port ons of the last 
 tracheal rings disappear and the mem- 
 brane which closes the gap forms the 
 vibratile part. In the bronchial syrinx 
 the membranes occur between two suc- 
 cessive rings of each bronchus, each ring 
 being concave toward its fellow. By a 
 shortening of the bronchial wall these 
 membranes are forced as folds into the 
 tube. In all types of syrinx there are 
 muscles attached to trachea and bronchi, 
 which, by moving these parts, alter 
 the tension of the folds, thus changing 
 the note. 
 
 In the mammals the trachea is elon- 
 gate (shortest in the whales and sire- 
 nians, dividing in the latter immedia- 
 tely behind the cricoid into the two ^^"^' ^>'' tympanum. 
 bronchi), and the cartilage rings are usually incomplete dorsally, 
 the gaps being closed by membrane. This structure allows the tube to 
 remain open under ordinary conditions and yet allows it to give when 
 food is passing down the oesophagus, just dorsal to it. In the cetacea 
 and sirenia the tracheal cartilages are sometimes spirally arranged. 
 
 Lungs. 
 
 The morphology of the lungs may be ujiderstood by following their 
 development in the mammals and then describing their modifications 
 
 Fig. 263. — Syrinx of canvas-back 
 duck, Ayihya, laid open (Princeton 
 915). b, bronchi; p, pessulus; t, tra- 
 
256 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 in the various classes of vertebrates. As stated above the lungs arise as 
 a diverticulum (fig. 264, A) on the ventral side of the pharynx which 
 quickly divides into two sacs, the anlagen of the two lungs. These are 
 gradually pushed posteriorly toward the body cavity, still retaining 
 their connexion with the pharynx by the air duct, and each consisting 
 of an enlarged terminal vesicle connected by a slender portion (the 
 beginning of the primary bronchus) with the undivided tracheal portion. 
 With continued growth each terminal vesicle divides again and again, 
 the result being a number of rounded vesicles connected with the pri- 
 mary bronchi by slender tubes, the secondary bronchi (fig. 264, B). 
 
 Fig, 264. Fig. 265. 
 
 Fig. 264. — Two stages in the development of the lung of the pig, ventral views, after 
 Flint. A, pig 5 mm. long; B, 18.5 mm. long, b, gill pouch; d, /, v, dorsal, lateral and 
 ventral bronchi; oe, oesophagus; i, trachea. 
 
 Fig. 265. — Scheme of mammalian lung structure, ad, alveolar duct; b, bronchus; 
 //, bronchiole; i, infundibulum lined with alveoli. 
 
 By a continuation of this process tertiary and other bronchi are out- 
 lined, and also slender tubes, the bronchioles, to be described later, 
 which connect the terminal vesicles with the ultimate bronchi. Next, 
 the inner wall of each vesicle becomes divided into small chambers, the 
 alveoli, the whole vesicle now being known as an infundibulum. 
 The result of these many divisions is an enormous amount of internal 
 respiratory surface without great increase in the size of the whole 
 organ. It is to be noticed that in this subdivision the entodermal li- 
 ning takes the initiative, the outer (serous) surface showing but slight 
 signs of the internal modifications. 
 
 Each infundibulum has its own duct which, when smooth internally, 
 is called a bronchiole, when lined with alveoli, an alveolar duct. 
 
RESPIRATORY ORGANS. 
 
 257 
 
 The alveoli of infundibulum and duct are lined with squamous 
 epithelium, and in the walls is an extensive network of capillary blood- 
 vessels. The lining cells of the bronchioles are cubical and those of the 
 bronchi ciliated columnar. There are no skeletal elements in the bron- 
 chioles, but the bronchi have small cartilages in the walls, these ex- 
 hibiting a tendency in the larger tubes to approximate the rings or 
 semi-rings of the trachea. 
 
 In their backward growth into the coelomic region the lungs either 
 insinuate themselves dorsal to the lining of the dorsal side of the 
 body cavity (dipnoi and a few scattered forms) so that only their ventral 
 surface has a serous coat; or they grow out as free structures, covered 
 on all sides by the coelomic epithelium, and are bound to the dorsal wall 
 by a mesenterial-like fold of varying extent. This outer coat of epithe- 
 lium has received the name of pleura, the term being extended in the 
 case of the mammals to include the 
 whole lining of the pleural cavity, 
 separated from the rest of the coelom 
 by the diaphragm (p. 135). 
 
 DIPNOI, — In Ceratodus there is a single 
 lung sac; Protopterus and Lepodosiren have 
 paired lungs, the two being united in front at 
 the entrance of the air-duct. In all three the 
 inner surface is divided more or less regu- 
 larly into groups of alveoli, separated by 
 more prominent partitions. The pulmonary 
 arteries arise from the last efferent branchial 
 artery of either side, and hence the blood 
 supply, under normal conditions, is arterial 
 and the lungs cannot act as respiratory 
 organs. In times of drought (Protopterus) 
 or of foul water (Ceratodus) the gills no longer function and the pulmonary arteries 
 bring venous blood to the lungs. 
 
 Fig. 266. — Different types of am- 
 phibian lungs. A, Necturus, without 
 alveoli ; B, alveoli in the proximal por- 
 tion; C, frog, alveoli throughout. 
 
 AMPHIBIA. — In the lower urodeles the two lungs are elongate (the 
 left the longer) and are united at their bases, true bronchi being absent. 
 Internally they may be entirely smooth as in Necturus, or there may be 
 alveoli in the basal portion (fig. 266), the whole representing a terminal 
 vesicle either connected directly with the trachea (A) or by the interven- 
 tion of an alveolar duct (B). In the caecilians the left lung is very short; 
 the other elongates, with alveoli developed throughout. In the frogs 
 (fig. 266, C) the two lungs are distinct, and their walls are divided into 
 17 
 
258 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 a series of sacs or infundibula lined with alveoli. The infundibula 
 open into a central chamber, which, since it is ciliated and has numerous 
 glands in its walls, may be compared to a bronchiole. In the toads and 
 aglossa the alveoli are more extensively developed in correlation with 
 the more terrestrial habits. 
 
 It has recently been shown that a number of terrestrial urodeles are lungless in 
 all stages of development, and that no traces of larynx or trachea occur, even after 
 the gills are absorbed. In these species there is a great development of capillaries 
 in the skin and in the walls of the mouth and pharynx, the respiratory functions 
 being transferred to these parts. In the frogs the skin is also respiratory and it is 
 largely supplied by the cutaneous arteries which arise from the same arch as the 
 pulmonary arteries. 
 
 In the amphibia the air ducts enter the anterior end of the lungs, 
 but in the amniotes the lungs extend anteriorly to the entrance of the 
 bronchi which is on the medial side. This change is in part the result 
 of the transfer of the heart into the thorax, the position of the pulmonary 
 arteries forcing the bronchi toward the centre of the lungs. In the 
 amniotes, also, the ducts are characterized by the presence of cartilage 
 in their walls, so that they are true bronchi. These bronchi may also 
 extend inside of the lungs, often dividing into secondary and tertiary 
 bronchi inside them. 
 
 REPTILES. — In many reptiles (snakes, amphisbaenans, many 
 skinks) the lungs are asymmetrical (left usually larger in snakes, right in 
 lizards) and exceptionally one may be absent in snakes. The internal 
 structure shows considerable variation. The simplest conditions are 
 found in the snakes and in Sphenodon (fig. 267), where the lungs consist 
 of a single sac lined with infundibula in the basal portion (snakes) or 
 throughout (Sphenodon). In the lizards (fig. 268) one or more par- 
 titions or septa extend from the distal wall of the lung nearly to the en- 
 trance of the bronchus, thus dividing the lung into chambers lined with 
 alveoli; while a part of the bronchus may extend (main bronchus, 
 fig. 268, B) to the extremity of the lung. In the chameleons the septa 
 do not reach the distal wall so that the chambers communicate here as 
 well as at the proximal side, the result being that the bronchus enters a 
 cavity, the atrium, which connects with the chambers separated by the 
 septa, and these in turn open into a terminal vesicle, a condition recall- 
 ing the parabronchi of the birds, soon to be described. This resem- 
 blance is heightened by the development in these same lizards of long, 
 thin-walled sacs from the posterior part of the lung which extend among 
 
RESPIRATORY ORGANS. 
 
 259 
 
 the viscera, even into the pelvic region. These air sacs, which are 
 used to inflate the body, foreshadow the similarly named structures in 
 birds. In the higher lizards {Varanus, fig. 268, B) and the turtles and 
 crocodiles there is no atrium, the bronchus, on entering the lung, 
 breaking up into several tubes. As these connect with smaller tubes 
 which lead to the infundibula, the whole lung has a spongy texture. 
 
 Fig. 267. Fig. 268. 
 
 Fig. 267. — Lungs of Sphenodon, after Gegenbaur; the left lung opened to show the 
 alveoli.; 
 
 Fig. 268. — A, left lung of Iguana; B, right lung of Varanus, after Meckel, b, bronchus 
 c, connection between dorsal and ventral chambers; cb, chief bronchus; d, dorsal chamber; 
 lb, lateral bronchi; s, septa; sb, secondary bronchus; v, ventral chamber. 
 
 BIRDS. — In the birds the lungs are closely united to the ribs and 
 vertebral column and hence undergo less considerable changes of 
 shape than those of other groups. Each bronchus enters the meso- 
 ventral surface of the lung, immediately expanding into a sac, the 
 atrium or ventricle, and then continues as a main trunk, the meso- 
 bronchus, near the ventral side of the organ (fig. 269). In this course 
 it gives rise to the secondary bronchi (usually eight lateral ectobronchi 
 and from five to six dorsal entobronchi) and these in turn connect with 
 very numerous small tubes, the lung pipes or parabronchi. These 
 run approximately parallel to each other and connect with another 
 bronchus at the other end. Each parabronchus bears a number of 
 elongate diverticula radiately arranged (fig. 270), these having a nar- 
 rower basal portion and being branched and lobulated distally. The 
 
26o COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 PL 
 
 Fig. 269. Fig. 270. 
 
 Fig. 269. — Diagram of structure of bird's lung, a, connexions of bronchi with air 
 sacs; 6, bronchus; e, entobronchi; ec, ectobronchi; i, infundibula; m, mesobronchus; />, 
 parabronchi. 
 
 Fig. 270. — A, lung pipes of bird from a corrosion preparation; B, section of lung pipe 
 with radiating infundibula, after Schulze. 
 
 Fig. 271. — Diagram of the relations of the chief air sacs in a bird, lung tissue shaded, 
 a, axillary sac; ab, abdominal sac; ai, anterior intermediate sac; 6, bronchus; pb, pre- 
 bronchial sac; pi, posterior intermediate; sb, subbronchial sac; t, trachea. 
 
RESPIRATORY ORGANS. 
 
 261 
 
 parabronchi are to be compared to bronchioles, the diverticula to 
 infundibula. 
 
 The mesobronchus and usually four other bronchi do not stop at 
 the lung wall, but are continued as thin walled vesicles, the air sacs, 
 
 Fig. 272. — Air sacs of pigeon, after Bruno Muller. c\ c', intertransverse canal; 
 da^, da^, axillary diverticulum and its ventral outgrowth; dCy diverticulum costale; d/a, 
 dfp, divert, femorale anterior et posterior; dot, divert, oesophago-tracheale; ds^ div. sub- 
 scapulare; dst^ div. stemale; pc, preacetabular canal; sad, sas, saccus abdominalis dexter et 
 sinister; sc, saccus cervicalis; sia, sip, saccus intermedins, anterior et posterior. 
 
 structures peculiar to birds (and in a slight extent to chameleons) and 
 occurring in all recent species. Each sac (figs. 271, 272) has received 
 several names. The sub-bronchial, anterior to the furcula, is usually 
 unpaired. The cervical, lateral to the first, lies at the base of the 
 
262 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 neck, and gives off a branch which forms an axillary sac in the axillary 
 region. Other sacs lie in the abdomen, lateral to the viscera, and are 
 called the anterior intermediate, posterior intermediate and 
 abdominal, the latter extending into the pelvis. From these air sacs 
 slender diverticula, not shown in the figures, extend among the viscera 
 and into certain of the bones. The pelvis, humerus, coracoid, sternum 
 and ribs most frequently contain prolongations of the air sacs — are 
 pneumatic — less frequently the femur, furcula and scapula. 
 
 The functions of the air sacs are not certainly known. The fact that the walls 
 are supplied with blood by branches from the aorta negatives the idea that they are 
 respiratory. It has been suggested that they are concerned with the maintenance 
 of the equilibrium of the body during flight and that they also lessen the specific 
 gravity of the body. More plausible is the view that by the motion of the parts 
 about them they aid in the inspiration and expiration of air, especially during flight, 
 thus allowing the thoracic framework to remain rigid as an attachment of the 
 muscles, and at the same time causing the air to pass twice over the respiratory 
 surfaces of the lungs. The bones of the fossil bird Archeceopteryx were not pneu- 
 matic but those of some of the dinosaurian reptiles were. 
 
 MAMMALS. — The general structure of the mammalian lung was 
 outlined above (p. 256), The external shape is largely due to the 
 position in the pleural cavity, where it has to fit itself around the peri- 
 cardium, while it is flattened or truncate behind as a result of the 
 presence of the diaphragm. In a number of mammals (cetacea, sirenia, 
 horse, rhinoceros, Hyrax^ etc.) both lungs are undivided, but usually 
 one or both are subdivided into lobes (the larger number in the right 
 lung), there being as many as five or six lobes in some species. In- 
 ternally there is a main bronchus from which dorsal and ventral 
 secondary bronchi arise, the ventral being the stronger. The bronchi 
 are supported and kept open by cartilages, rings in the larger, scattered 
 pieces in the smaller trunks. Frequently the bronchi are grouped as 
 eparterial and hyparterial (fig. 264), accordingly as they lie above 
 or below the pulmonary artery, but the distinction has little morpho- 
 logical value. Eparterial bronchi may be lacking or there may be 
 one or two in each lung. 
 
 The phylogenetic history of the lungs is uncertain, one view being that they 
 have arisen from the air bladder of the fishes, the other being that they are modified 
 gill pouches, which, instead of growing laterally and fusing with the ectoderm, 
 have extended caudally and have encroached upon the ccelom. In favor of the 
 former view are the double condition of the bladder in some ganoids, with alveolar 
 walls like those of the lungs of higher vertebrates, and the peculiarities of the pneu- 
 
RESPIRATORY ORGANS. 263 
 
 matic duct and the blood supply in Polypterus. On the other hand the dorsal 
 position of the opening of the duct into the oesophagus and the arterial supply from 
 the aorta in fishes are difficult to reconcile with the conditions obtaining in the 
 tetrapoda. Favoring the gill-pouch theory are the following facts. The lungs are 
 paired outgrowths from the pharynx immediately behind the kist gill cleft; the 
 blood supply can readily be derived from the branchiate condition; while the skeletal 
 supports of the larynx have the appearance of rudimentary visceral arches, and the 
 muscles of the region are modified from those of the gill arches. 
 
 The mechanisms by which air is caused to enter the lungs (in- 
 spiration) or is expelled from them (expiration) differ considerably 
 in the various classes. In the amphibia air is drawn into the mouth via 
 the nares by depressing the floor of the oral cavity. Then, the nares 
 being closed by small muscles, the contraction of the mylohyoid muscle 
 forces the air into the lungs. Expiration is affected in part by the 
 elasticity of the lungs, in part by the muscles of the body wall. In 
 most reptiles the position of the ribs is altered by the action of the 
 intercostal muscles, thus altering the size of the pleuro-peritoneal 
 cavity, to accommodate which air is drawn into and expelled from the 
 lungs. It is difficult to understand how inspiration is effected in the 
 chelonia, but transverse muscles run ventral to the lungs, and these by 
 their contraction, expel the air. In the birds the lungs are attached to 
 the ribs and vertebrae, so that any motion of the latter necessitates a 
 change in shape and size of the lungs. In addition the air sacs, as 
 noted above, may play a part in the movement of the air. 
 
 In the mammals the ribs are hinged at an oblique angle to the verte- 
 bral column, the angle being changed accordingly as the intercostal 
 muscles are contracted or relaxed, and thus the size of the thoracic 
 cavity is increased or dimininshed. Then the diaphragm (p. 135) 
 also plays an important part in this alteration in size. This transverse 
 muscle forms a complete partition between pleural and peritoneal 
 cavities, projecting into the former like a dome when relaxed. When 
 it contracts it flattens, thus increasing the size of the pleural cavity 
 and drawing air in through the trachea. The abdominal muscles 
 also have their effect. Expiration is caused in part by the action of the 
 intercostal and abdominal muscles, in part by the elastic tissue and 
 smooth muscles in the lungs themselves. 
 
 ACCESSORY RESPIRATORY STRUCTURES. 
 
 Allusion has already been made to the pharyngeal and dermal 
 respiration of the amphibia (p. 258). There are several fishes in which 
 
264 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 the hinder part of the alimentary tract is also respiratory. Thus in 
 Cohitis water is drawn in and expelled from the anus, and the posterior 
 half of the digestive canal is richly vascular and is the seat of consider- 
 able respiration. 
 
 Before hatching or birth the lungs of the amniotes are unable to 
 function, while a certain amount of oxygen is necessary for the devel- 
 opment and the carbon dioxide formed must be carried away. This 
 respiratory function is assumed by the allantois. The allantois is 
 a ventral diverticulum from the hinder part of the alimentary canal, 
 which during foetal or embryonic life, acquires a relatively enormous 
 development. It extends beyond the body limits and in reptiles and 
 birds comes into close relations with the porous egg shell, while in the 
 mammals it plays an important part in the formation of the placenta. 
 In all these the allantois is extremely vascular, developing a rich net- 
 work of blood-vessels close to the shell (sauropsida and monotremes) 
 or to the walls of the maternal uterus, (mammals) which serves for 
 the rather limited exchange of gases necessary for the young. After 
 free life begins the allantois is either absorbed (sauropsida) or is lost with 
 the rest of the placenta (mammals), only the basal part persisting as the 
 urinary bladder, described in connection with the urogenital system. 
 
 ORGANS OF CIRCULATION. 
 
 The functions of the circulation are two-fold: to carry food and 
 oxygen to the tissues and organs of the body and to remove the waste 
 from them. In addition it has been made probable that every activity 
 of the body results in the formation of peculiar substances — activators — 
 which have fixed and definite effects upon the various organs. These 
 activators pass into the blood and form the stimulus which may cause 
 other organs or cells, remote from the place where the activator is formed, 
 to act. This subject is a new one and much may be expected from it in 
 the future. 
 
 The structures concerned in the circulation are two fluids, the blood 
 and the lymph; and the vessels (vascular system) in which the fluids 
 circulate, certain parts of the vessels being specialized (hearts) for the 
 propulsion of the blood and lymph. A blood heart occurs in all verte- 
 brates in connexion with the blood circulation; most vertebrates have 
 lymph hearts in connexion with the lymph vessels, but in the higher 
 groups the flow of the lymph is due to the blood pressure and also to the 
 motion of the parts through which the lymph vessels course. 
 
CIRCULATORY ORGANS. 265 
 
 BLOOD AND LYMPH. 
 
 The two circulating fluids, blood and lymph, are much alike. 
 Each consists of a fluid portion, the plasma, in which float numer- 
 ous solid particles, the corpuscles. The plasma is colorless or 
 slightly yellow and can be separated by clotting into a solid part, 
 fibrin, and a fluid, the serum, which is, under ordinary circum- 
 stances, incapable of clotting again. The lymph plasma contains 
 less of the fibrin-forming substances (fibrinogen) than does the blood 
 plasma. The composition of the plasma is very complex. Besides 
 water it contains proteids, extractives, salts, and a number of less- 
 known substances, internal secretions, enzymes, etc. The plasma 
 can also absorb a considerable amount of carbon dioxide. It serves 
 to carry nourishment to the tissues and takes away from them the 
 waste of metabolism. 
 
 The corpuscles are of three kinds, erythrocytes, leucocytes and 
 blood plates. Only the leucocytes occur in the lymph while the 
 blood contains all three. 
 
 The erythrocytes, or red corpuscles give the blood its color. 
 They have fixed outlines and are flattened oval discs in the non- 
 mammals and the camels, circular biconcave discs in the other mam- 
 mals, and in all except the mammals they are nucleated throughout 
 their existence. They owe their color to an iron-containing proteid, 
 haemoglobin, which readily combines with oxygen and carbon dioxide 
 and as readily gives up these gases in places where they are scanty. 
 This renders the erythrocytes the respiratory elements of the blood. 
 
 It has recently been stated that the erythrocytes of the mammals are hat- 
 shaped, (hollow cones) while inside the blood-vessels and that they assume the 
 biconcave shape after leaving them. This account has been disputed. 
 
 The size of the erythrocytes varies in different vertebrates, being the largest 
 in the amphibia {Amphiuma) and smallest in the vertebrates (musk deer). A 
 few measurements are giving here in microns (o.ooi mm.). Where two dimen- 
 sions are given they are the length and breadth of the oval corpuscles. Musk 
 deer, 2.5//; man, 7.7/z; hen, 7x12;^; carp, 9x15/^; frog, 16X25/X; Necturus, 
 3ix58.5/t; Amphiuma, ?xysfi. 
 
 In the higher vertebrates the red corpuscles arise by division of giant cells 
 (erythroblasts) in the red bone marrow, but in the young and at times of great 
 depletion of the blood new red corpuscles may be formed in the spleen and the 
 liver. At first all are nucleated but in the mammals the nucleus is soon lost. 
 
 The leucocytes or white corpuscles (divided accordingly as they 
 
266 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 occur in blood or lymph into leucocytes and lymphocytes) are very 
 variable in shape (amoeboid) and may be uni- or polynucleate. By 
 their amoeboid motions they are able to pass through the endothelial 
 walls of the capillaries and to pass among the cells of the different 
 tissues, hence they are often called wandering cells. They have the 
 power of ingesting foreign bodies which renders them of value in 
 combating pathogenic organisms; and they also aid in the absorb tion 
 of fats and peptones. 
 
 The blood plates are very little known. Their size is less than 
 that of the red corpuscles and they rapidly degenerate when drawn 
 from the vessels. They are circular or elliptical in outline. 
 
 THE BLOOD-VASCULAR SYSTEM. 
 
 The blood-vessels include the arteries, which carry the blood from 
 the heart to all parts of the body; the veins, which bring it back, and the 
 
 Fig. 273. — Embryonic circulation of snapping turtle, Chelydra, showing relations of 
 allantois, after Agassiz and Clarke, a, right auricle; al. allantois; av, allantoic vessels; c, 
 caudal vein; da, dorsal aorta; h, hypogastric artery; ;', jugular; /, liver; oa, ov, omphalo- 
 mesenteric artery and vein; pc, post-cardinal; sc, subcardinal vein; uv, umbilical vein; w, 
 Wolffian body; y, yolk sac. 
 
 capillaries which connect the ends of the arteries and veins, for the 
 system is closed, and there is a complete circulation. 
 
 Since all transfer of gases and nourishment takes place through the 
 capillaries, these vessels have extremely thin walls, consisting of a 
 single layer of squamous epithelium, the so-called intima. Usually, as 
 
CIRCULATORY ORGANS. 267 
 
 the name implies, the capillaries are very small in diameter, but atten- 
 tion has recently been called to the sinusoids, vessels with similar walls 
 but larger in diameter, which are noticeable in some developing organs, 
 especially the liver. Here also must be mentioned the retia mirabilia, 
 places where an artery or vein suddenly breaks up into a network of 
 small vessels (often capillary) which unite again, as in the glomeruli of 
 the kidney, to form a vessel as large as before. In the lymph nodes 
 there are similar networks of the lymph vessels. 
 
 Arteries and veins (fig. 274) are larger than the capillaries and they 
 have their walls strengthened outside of the intima by layers of smooth 
 
 Fig. 274. — Diagram of artery or vein. At the left the intima alone; covered in the middle 
 by the muscularis, and at the right with the adventitia added. 
 
 muscle fibres (muscle wall) and connective tissue, mostly elastic (ad- 
 ventitial wall). Since the arteries are subjected to greater pressure 
 than the veins their walls are relatively much thicker, but in other re- 
 spects the two are much alike, except that valves to prevent the back- 
 flow of the blood, may occur in the veins, especially those that are 
 vertical in the normal position of the animal (legs). 
 
 It has been suggested, with much plausibility, that the main blood-vessels are 
 the remnants of the segmentation cavity, which elsewhere has been obliterated by 
 the increase of the mesoderm. As will be recalled (p. 121) the mesothelium grows 
 toward the middle line above and below the digestive tract, thus tending to narrow 
 the segmentation cavity in these regions into two longitudinal tubes. The epimeral 
 part of the mesothelium divides into somites, and of course the segmentation cavity 
 extends between these, and as these somites grow downward, these lateral exten- 
 sions of the segmentation cavity are carried ventrally, so that at last they form a 
 series of pairs of transverse vessels connecting the longitudinal trunks, thus forming 
 the vessels of the somatic wall. Other tubes, connecting the dorsal and ventral 
 trunks, would form between the two walls of the mesentery and between the 
 splanchnic mesoderm and the entoderm, thus outlining the vessels of the alimentary 
 tract. 
 
 Even more speculative is the suggestion that the original circulation was lymph- 
 oidal and that the blood circulation is a specialization of a part of this, the definitive 
 lymph vessels being the unmodified part of the primitive system of vessels. 
 
268 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 An appreciation of this probable ancestral condition makes the 
 actual structures more easily understood. In development much of 
 this phylogenetic history has been lost, while other parts have been 
 masked by the development of additional vessels. Many vessels, 
 which theoretically should arise as spaces between other tissues, are 
 actually formed as solid cords of cells, which are later canalized and 
 converted into tubes. Again, separate vessels of the embryo may fuse 
 during development into a single vessel of the adult. 
 
 The chief features of the theoretically primitive condition may be 
 summarized here (fig. 275). A dorsal tube carries the blood toward 
 the tail. From this transverse vessels — right and left, somatic and 
 splanchnic — arise, which connect with two ventral longitudinal tubes. 
 
 Fig. 275. — Diagram of the primitive vertebrate circulation, a, anus; al, alimentary 
 canal; av, abdominal vein; ca, cv, caudal artery and vein; da, dorsal aorta; h, heart; ic, 
 intercostal (somatic) transverse vessels; iv, intestinal vessels; m, mouth; si, subintestinal 
 vein; va, ventral aorta. 
 
 one in the wall of the alimentary tract and extending forward to its junc- 
 tion with the second which runs in the ventral body wall, a single tube 
 coursing from the point of union to the anterior end of the body. 
 In Amphioxus various parts of this system develop muscular walls and 
 act as pumping organs. In the vertebrates, so far as the blood system 
 is concerned, there is a single pumping organ, the heart (the portal 
 heart of the myxinoids may be ignored in this general statement). 
 The heart arises in the ventral tube beneath the pharynx and anterior 
 to the junction of the two tubes. It marks the line of division of the 
 transverse tubes into ascending and descending, those in front of the 
 heart carrying the blood upward while those behind return it to the 
 ventral vessels which carry it forward. The transverse vessels are not 
 continuous, but capillaries intervene between their dorsal and ventral 
 moieties. 
 
 The Embryonic Circulation. 
 
 In all vertebrates a series of blood-vessels is laid down in the early 
 stages, forming a framework around which the rest of the circulation is 
 
CIRCULATORY ORGANS. 
 
 269 
 
 arranged. Hence these parts are first described, the additions and 
 modifications being taken up later. 
 
 The Heart. 
 
 The heart, the central organ for the propulsion of the blood, lies in a 
 sac, the pericardium, a part of the coelom, which is ventral to the 
 pharynx or oesophagus and is partially filled with a serum, the per- 
 
 FiG. 276. Fig. 277. 
 
 Fig. 276. — Diagram of the formation of the heart tube, showing the descending meso- 
 thelial plates from above, c, coelom; cd, first appearance of the Cuvierian ducts; A, grooves 
 to form heart and ventral aorta; I, liver; m, mouth; ma, mandibular artery; om, omphalo- 
 mesenteric veins; so, sp, somatic and splanchnic walls of coelom. 
 
 Fig. 277. — Early stage of the heart; the descending plates of fig. 276 have met, forming 
 the heart and ventral aorta, c, peritoneal coelom; ^, pericardial coelom; ppc, pericardio- 
 peritoneal canals; other letters as in fig. 276. 
 
 icardial fluid. In the heart we have to consider its epithelial lining 
 (endocardium), its muscular walls (myocardium) and its covering 
 epithelium and connective tissue (epicardium). 
 
 The development of the heart is simplest in the vertebrates with 
 relatively small yolk. It is more modified in the elasmobranchs, 
 where the head is early completed below, and is most modified in the 
 large yolked eggs of the sauropsida and in the mammals where the yolk 
 sac is large, though the yolk is small. The following account is based 
 upon the development in the amphibia: 
 
 From just behind the point where the first or spiracular gill cleft 
 is to form, backward to the region just in front of the anlage of the 
 
270 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 iver the hypomeral portions (lateral plates) of the coelomic walls grow 
 ventrally beneath the alimentary canal, in much the same way as 
 farther back (p. 121). In these descending plates splanchnic, mesen- 
 terial and somatic walls, as well as the coelomic cavity can be recognized. 
 As they descend, cells which have received the name of vascular 
 cells appear between the coelomic walls and the entoderm. The 
 origin of these has been in dispute, but the present evidence favors 
 their origin from the mesothelium. Some of these vascular cells are 
 more dorsal and aid in the formation of the dorsal blood-vessels, while 
 the ventral (fig. 278, A) contribute to the heart and the ventral trunks. 
 
 Fig. 278. — Diagrammatic cross sections of developing heart. Compare with figs. 
 276 and 277. In A the descending mesothelial plates have nearly met, a number of 
 vascular cells between them. In B the plates have met ventrally, forming the ventral 
 mesocardium ; most of vascular cells utilized in forming the endocardium. In C the plates 
 have met dorsally, with the resulting dorsal mesocardium; the ventral mesocardium has 
 disappeared, placing the two coelomic cavities, now the pericardium, in communication, 
 c, coelom; ec, ectoderm; en, entoderm; end, endocardium; m, edges of descending meso- 
 thelium; p, pericardium; v, vascular cells. 
 
 The descent of the lateral plates continues until their lower edges 
 meet just dorsal to the ventral ectoderm and the ventral parts of the 
 mesenterial regions of the two sides fuse to a vertical plate, the ventral 
 mesocardiixm (fig. 278, B), above which is a groove in which the 
 ventral vascular cells lie. Next, the edges of the plates crowd in above 
 the groove and meet to form a dorsal mesocardium, the result being 
 that groove is converted into a tube. The mesocardia disappear early, 
 the ventral usually being lost before the dorsal is formed (fig. 278, 
 C). The walls of the tube, which are to form the muscular and epicar- 
 dial walls of the heart, are called the myoepicardial mantle.^ The 
 vascular cells, which are enclosed in this mantle, gradually arrange 
 themselves as a continuous sheet, the endocardium, which lines the 
 future heart. 
 
 With the disappearance of the mesocardia the coelomic spaces on 
 the two sides communicate with each other so that the myoepicardial 
 mantle lies free on all sides in a coelomic sac, being bound to the walls 
 only at the two ends. This cavity or sac is the pericardial cavity, 
 
 ^ The fact that the heart muscles arise from this layer — mesothelial and yet not myotomic 
 — partly explains the differences between cardiac and other muscle. 
 
CIRCULATORY ORGANS. 27 1 
 
 the extent of which is decreased by the fusion laterally of the somatic 
 and splanchnic walls (j5g. 277). 
 
 In front of and behind this tube the descending lateral plates are 
 kept from meeting in the middle line by the projections for the mouth 
 and liver (fig. 276). Vascular cells, however, are formed in these 
 regions and these furnish the lining of tubes on either side, arising 
 in the edges of the lateral plates. These tubes consequently diverge 
 from the myoepicardium in front and behind and form the first stages of 
 the vessels connected with the heart, the anterior pair giving rise to the 
 mandibular arteries, the posterior to the omphalomesenteric veins. 
 At about the same time a transverse tube appears on either side, which 
 connects with the heart tube, just in front of the division into omphalo- 
 mesenterics (fig. 276). These transverse vessels continue laterally 
 between the lateral plate and the ectoderm, forming the venous trunks 
 known as the ducts of Cuvier (trunci transversi), the other rela- 
 tions of which will be described later. The ccelom on either side of 
 the heart is restricted behind by the ridge formed by the Cuvierian 
 ducts (fig. 277); with growth this interruption grows larger, the result 
 being a transverse partition, the septtim transverstmi, which bounds 
 the pericardial cavity behind and separates it from the rest of the ccelom, 
 the peritoneal cavity. At first this septum is incomplete, and in the 
 elasmobranchs it never closes dorsally to the omphalomesenterics, but 
 leaves two openings, the pericardio-peritoneal canals (fig. 277). 
 Elsewhere the pericardial and peritoneal cavities are entirely separate 
 in the adult. 
 
 In teleosts and amniotes, where the eariy embryo is closely appressed to the 
 very large yolk sac, the development of the heart is modified. At first the pharynx 
 is not complete below but communicates ventrally with the yolk. Hence the two 
 hypomeres are prevented, for a time, from meeting ventrally. Each, however, is 
 accompanied by its vascular cells; its edge becomes grooved and the grooves are 
 rolled into a pair of tubes, lined with endocardium, so that for a time the anlage of 
 the heart consists of two vessels, each connected in front and behind with its own 
 mandibular artery and omphalomesenteric vein, and is surrounded with its 
 pericardial sac. Later the two tubes approach and fuse, with the formation of 
 mesocardia as before: these latter soon disappearing, leaving the whole much as 
 in the small yolked forms. 
 
 In the early stages the pericardium is relatively large, but it does 
 not keep pace with the growth of the other parts, until finally in the adult 
 it is only large enough to accommodate the changes in size and shape 
 
272 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 of the heart, due to its alternating enlargement (diastole) and contrac- 
 tion (systole). 
 
 While the mesocardia are present the cardiac tube is a straight 
 canal, lying in the pericardial sac and connected with its walls in front 
 and behind. With their disappearance the tube increases in length 
 more rapidly than the pericardium, the result beng the flexure of the 
 tube on itself, something like the letter 00 , the flexures being largely 
 in the vertical plane. At the middle point of the flexure the tube re- 
 mains small, forming the atrio-ventricular canal, but in front of 
 and behind this the walls become thickened and the lumen enlarged. 
 The posterior and dorsal of the chambers thus formed becomes the 
 atrium (auricle), the ventral and anterior the ventricle of the heart. 
 
 The atrium is bounded posteriorly by a constriction, behind which 
 the tube expands into another chamber, the sinus venosus, which 
 extends back to the posterior wall of the pericardium and receives the 
 ducts of Cuvier and the omphalomesenteric veins. The ventricle, also, 
 does not reach the anterior wall of the pericardium, but the anterior 
 part of the heart tube forms a smaller trunk, the truncus arteriosus, 
 while from the pericardium to the mandibular arteries is an arterial 
 vessel, the ventral aorta. 
 
 Muscles, as stated above, are developed in the wall of the heart, 
 but to an unequal extent in the different parts, being scanty in the 
 sinus venosus, and most abundant in the ventricle. Folds or valves of 
 the endocardium appear in places at an early date and are so arranged 
 that they permit the blood to flow forward but prevent any backflow. 
 In the base of the truncus these valves take the form of pockets on the 
 walls, there being several (3-5) rows with several valves in a row in the 
 elasmobranchs (fig. 287, A) and ganoids. This valvular part of the 
 truncus is called the conus arteriosus. In other vertebrates the conus 
 is reduced to a single row of valves. 
 
 Valves also occur in the atrio-ventricular canal (fig. 279) but here 
 the pocket-like condition is impossible. The folds extend from the 
 canal into the ventricle and are prevented from folding back into the 
 atrium, under the heavy ventricular pressure, by ligaments — chordae 
 tendineae — which extend from the edges of the valves to the opposite 
 wall of the ventricle, and are kept taut during systole by short muscles 
 (columnae carnea) at the base. Othervalves, more simple in character, 
 occur around the opening from the sinus into the atrium and, in some 
 vertebrates, where the hepatic veins empty into the sinus. 
 
CIRCULATORY ORGANS. 
 
 273 
 
 In many fishes the conus arteriosus is followed by a strongly muscu- 
 lar region, the bulbus arteriosus (fig. 287, B) which has muscles like 
 those of the heart (p. 125), while the tnmcus in front of this has smooth 
 muscles, like the rest of the blood-vessels. Hence conus and bulbus 
 are to be regarded as a part of the heart, while the region in front is a 
 part of ventral aorta to be described below. 
 
 When first formed, the heart lies 
 close behind the mandibular artery (first 
 aortic arch to be described below), but 
 as other vessels are formed it is forced 
 farther back into a position, in the lower 
 vertebrates, ventral to and a little behind 
 the pharynx, but in the adult tetrapoda 
 it is carried back, as a result of unequal 
 growth even into the thorax, the extreme 
 of migration being seen in the giraffe 
 and the long-necked birds. 
 
 Although all of the blood of the body 
 passes through the heart at short inter- 
 vals, this is not sufficient for the nourish- 
 ment of that organ. Therefore its mus- 
 cles are usually supplied with blood 
 
 through coronary arteries which arise from the aortic arches and 
 run back along the truncus arteriosus to reach the atrium and ventricle. 
 
 Fig. 279. — Diagrammatic cross 
 section of heart showing atrio- 
 ventricular valves; a, atriimi; ct, 
 chorda tendinea; m, muscula pap- 
 illosa; v, ventricle; vl, atrio- ven- 
 tricular valves. 
 
 The Arteries. 
 
 Aorta and Aortic Arches. — The ventral aorta is the trunk in front 
 of the pericardium, extending from the truncus arteriosus to the mandib- 
 ular artery (first aortic arch) . It runs, not through a cavity, but be- 
 tween muscles and through connective tissue. The mandibular arter- 
 ies continue dorsally on either side of the pharynx until they reach its 
 dorsal surface. With development, the ventral aorta elongates and at 
 the same time other aortic arches arise between the mandibular arteries 
 and the pericardium, these extending dorsally until they meet the back- 
 ward prolongations of the first, thus forming a pair of longitudinal 
 tubes, dorsal to the alimentary tract, the radices aortae. 
 
 The number of pairs of aortic arches varies with the number of gill 
 clefts, the vessels coursing between the clefts. The number of arches 
 18 
 
274 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 is greatest in the myxinoids, where the number of clefts varies (p. 239); 
 seven or eight in the notidanid sharks; and, as recent investigations 
 tend to show, probably six in the embryos of all other vertebrates. The 
 history of these arches differs greatly in the different classes (fig. 280), 
 there usually being a reduction in number by the more or less complete 
 
 ^c 
 
 ic 
 
 m 
 
 ma 
 
 Fig. 280. — Modifications of the aortic arches in different vertebrates, after Boas. 
 A, primitive scheme; B, dipnoan; C, urodele; D, frog; E, snake; F, lizard; G, bird; H, 
 mammal, c, coeliac artery; da, dorsal aorta; db, ductus Botallii; ec, ic, external and internal 
 carotids; p, pulmonary artery; s, subclavian; va, ventral aorta. Vessels carrying venous 
 blood black, those which disappear, dotted. 
 
 abortion of one or more pairs as well as a modification of those that per- 
 sist, accompanying changes in the respiratory system. 
 
 With the development of gills (ichthyopsida) each aortic arch be- 
 comes divided into two portions, an afferent branchial artery convey 
 ing blood from the ventral aorta to the gills and an efferent branchial 
 artery (sometimes called a branchial vein) carrying it from the gills 
 
CIRCULATORY ORGANS. 275 
 
 to the radix aortae (fig. 281). These two vessels parallel each other for 
 a pa rt of their course and are connected with each other by numerous 
 capillary loops which run through the gill filaments. In passing 
 through the gills the blood loses its carbon dioxide and takes up oxygen, 
 and thus becomes converted from venous to arterial blood. In the am- 
 nio tes afferent and efferent branchial arteries are never differentiated, the 
 aortic arches being continuous from ventral aorta to the radices aortae. 
 The first of these arches (the mandibular arteries) never forms 
 afferent and efferent portions since no gills are ever developed in their 
 region. From each half of this arch an artery, the external carotid, 
 
 av 
 
 Fig. 281. — Scheme of branchial circulation in elasmobranchs. a, atrium; aa, afferent 
 branchial arteries; av, abdominal vein; c, gill clefts; cc, common carotid; da, dorsal aorta; 
 ea, efferent branchial arteries; hv, hepatic vein; ic, internal carotid; ec, external carotid 
 artery; i, jugular vein; /, liver; pc, postcardinal vein; sc, subclavian vein; sv^ sinus venosus; 
 tr, truncus arteriosus. 
 
 extends forward to supply the lower and a part of a upper jaw, while an 
 internal carotid artery forms an extension forward of each radix and 
 supplies the brain and face. Later their relations are such that the 
 carotids appear to arise from the first of the functional arches. 
 
 The radices aortae of the two sides meet and fuse behind the last 
 aortic arch, forming a single tube, the dorsal aorta, w^hich runs in the 
 middle line, dorsal to the alimentary tract, to the end of the body. The 
 fusion may also extend forw^ard from the last aortic arch, involving the 
 whole of the radices. 
 
 From the dorsal aorta segmental arteries extend laterally between 
 the somites, these forming the upper halves of the transverse somatic 
 vessels alluded to on page 268. To these the name of intercos- 
 tal arteries, derived from human anatomy, is given. Ventral to them 
 the aorta also gives off other arteries (nephridial arteries) to the excre- 
 
276 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 tory organs. Other arteries, arising from the dorsal aorta, run ventrally 
 into the mesenterial structures and supply the alimentary canal and 
 other viscera. Two pairs of these, the omphalomesenteric (omphalo- 
 mesaraic) and the hypogastric arteries, may be mentioned at present. 
 The first of these arise in the trunk region, pass on either side of the 
 intestine, and finally empty on the lower side of the body into the om- 
 
 FiG. 282. — 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, Cuverian duct; 
 ec, external carotid; h, heart; ha, hypogastric artery; i, intestine; ic, internal carotid; ij, 
 inferior jugular; j, superior jugular; /, liver; m, mouth; oma, omv, omphalomesenteric 
 artery and vein; pc, postcardinal vein; si, subintestinal vein; 1-6, aortic arches. 
 
 phalomesenteric veins, soon to be described. The hypogastric arteries 
 arise from the dorsal aorta at the junction of trunk and tail and pass on 
 either side of the intestine, to meet posterior continuations of the 
 omphalomesenteric veins, here known as the subintestinal veins. 
 Behind the origin of the hypogastric arteries the dorsal aorta is called 
 the caudal artery (figs. 275, 282). 
 
 Veins. 
 
 Behind the pericardium the edges of the descending lateral plates 
 (p. 270) are kept from meeting by the anlage of the liver (figs. 276, 
 277) . The edges of the plates become grooved just as in front and each 
 groove becomes rolled into a tube, lined with vascular cells, so that two 
 vessels, the omphalomesenteric veins, extend backward from the heart, 
 around the liver, to meet the omphalomesenteric arteries already de- 
 scribed. Behind the connection of the omphalomesenteric arteries and 
 veins the pair of vessels continue back, ventral to the alimentary canal 
 as the subintestinal veins, until just behind the anus they fuse into a 
 median tube, the caudal vein, which extends the length of the tail. 
 
 The two subintestinal veins soon fuse to a single median vessel (fig. 
 283, B) save for a loop around the anus connecting it with the caudal 
 vein. The right omphalomesenteric vein disappears except for a short 
 distance between the sinus venosus and the liver, leaving the left as the 
 
CIRCULATORY ORGANS. 
 
 277 
 
 trunk connecting the posterior parts with the heart, this passing along 
 the left side of the liver (fig. 283, B). 
 
 Portal Circulation. — As the liver develops from the simple sac it is 
 at first, into the compound tubular condition (p. 233), the left omphalo- 
 mesenteric breaks up into a sort of rete mirabile of sinusoids, which 
 ramify among the liver tubules, finally connecting with both omphalo- 
 mesenterics on the anterior side of the liver (fig. 283, B). As the liver 
 increases in size the network of sinusoids increases in complexity, 
 supplying all of the tubules. For a time the left omphalomesenteric 
 
 Fig. 283 . — Three stages in the development of the hepatic portal system. A , primitive; 
 B, liver tubules beginning to develop, right omphalomesenteric interrupted ; C, definitive 
 condition, liver not indicated, dc, Cuverian ducts, hp, hepatic portal vein; hv, hepatic 
 vein; /, liver; lo, ro, left and right omphalomesenteric veins; si, subintestinal veins; sv, sinus 
 venosus. 
 
 retains its primitive importance on the side of the liver and is known 
 as the ductus venosus (Arantii), but soon this preeminence is lost 
 and all blood coming from behind passes through the network of cap- 
 illaries in the liver before it enters the heart (fig. 283, C). Such a 
 capillary circulation occurring in the course of a vein is known as a 
 portal system, and this one occurring in the liver is the hepatic portal 
 circulation. It consists of the vessels bringing the blood to the liver 
 (portal vein) — a part of the original omphalomesenteric — the capil- 
 lary vessels and the bases of both omphalomesenterics, now known as 
 the hepatic veins, which convey the blood from the. liver to the heart. 
 
 In eggs with a large yolk (elasmobranchs, sauropsida) the presence of this 
 large food supply exercises a modifying influence on these ventral veins (fig. 284). 
 From the junction of the omphalomesenteric and the subintestinal veins a pair of 
 large vitelline veins run out into the yolk sac, over the yolk, and play a large part 
 in the transfer of material to the growing embryo. The distal parts of these veins 
 follow the margin of the yolk sac, forming a tube (sinus tenninalis) into which 
 
278 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 smaller veins empty. Blood is brought to the yolk by the omphalomesenteric 
 arteries, which are also distributed to the yolk sac, dividing up distally into a net- 
 work of capillaries connecting distally with the vitelline veins. By these the blood 
 is carried to the liver and through the portal circulation to the heart. In the mam- 
 mals a similar vitelline circulation is developed, but as the yolk sac contains no 
 yolk, it is of minor importance. 
 
 In the amnio tes an outgrowth, the allantois (p. 318), arises as a diverticulum 
 from the hinder end of the alimentary canal, increases in extent, growing downward 
 and carrying the ventral body wall before it. Branches of the hypogastric arteries, 
 known as the allantoic arteries, extend into it and are connected by capillaries 
 
 Fig. 284. — Diagram of embryonic circulation in a large-yolked vertebrate; compare 
 with fig. 282. aa, aortic arches; al, allantois; an, anus; ca, cv, caudal artery and vein; da, 
 dorsal aorta; dc, Cuverian duct; h, heart; ha, hypogastric (allantoic) artery; i, jugular vein; 
 /, liver; oma, omv, omphalomesenteric artery and vein; pc, postcardinal vein; si, subintes- 
 tinal vein; st, sinus terminalis; va, ventral aorta; y, yolk; ys, yolk stalk. 
 
 with umbilical veins which arise from the subintestinal vein behind the vitelline 
 veins. There thus is formed an allantoic circulation which is both respiratory 
 and nutritive in character. In the reptiles both of the umbilical veins persist 
 through the foetal life (only one shown in fig. 273), but in birds and mammals one 
 aborts, leaving the other as the efferent vessel of the allantois. With the end of 
 foetal life (at hatching or at birth) both the vitelline and the allantoic circulations 
 disappear, leaving only inconspicuous rudiments. 
 
 The entrance of the Cuverian ducts into the heart was mentioned 
 on page 271. These ducts are a pair of transverse vessels which enter 
 the sinus venosus, one from either side, and, together with the hepatic 
 veins, mark the posterior limit of the heart. Each develops outside 
 of the somatic wall of the hypomere and extends dorsally until it reaches 
 the level of the top of the coelom (fig. 282). In this course, in the 
 fishes, each receives an inferior jugular vein which comes from the 
 head, bringing back blood from the muscles of the lateral and ventral 
 branchial regions. At its dorsal end each Cuverian duct divides into 
 
CIRCULATORY ORGANS, 
 
 279 
 
 the two cardinal veins, an anterior cardinal (superior jugular) and 
 a postcardinal vein (fig. 285), which belong to the dorsal half of 
 the body. The superior jugular comes from the head, dorsal to the 
 gill clefts and brings blood from the more dorsal regions. Since the 
 inferior jugulars are found only in fishes and salamanders, the anterior 
 cardinal is usually called simply the jugular and that usage will be 
 followed here. 
 
 The postcardinals are closely related in development to the nephric 
 system, and keep pace with its development backward, so that they 
 eventually reach the loop which the caudal and subintestinal vein 
 
 Fig. 285. — Developing anterior veins of Scyllium embryo, 26 mm. long; after Grosser. 
 h ^-®, veins of the visceral arches; cd, Cuverian duct; h, vein of hyoid arch; */, inferior jugu- 
 lar; w, vein of mandibular arch; os, orbital sinus; sv, segmental veins; vca, vcp, pre- 'and 
 post-cavas; III-X, cranial nerves; 2-8, spinal nerves. 
 
 makes in passing around the anus. They run just above the dorsal 
 side of the coelom and dorsal to the nephridial arteries (p. 275). They 
 are preeminently the blood-drainage system of the early excretory 
 organs and they retain that function throughout life in the lower 
 vertebrates. 
 
 Closely associated with the postcardinals are the subcardinals. 
 As the mesonephroi (see Excretory Organs) reach the hinder end of the 
 ccelom, the caudal vein loses its primitive connection with the subintesti- 
 nal vein and becomes connected with a pair of vessels, the subcardinal 
 veins, which develop between the mesonephroi and ventral to the nephrid- 
 ial arteries (fig. 286, B). The blood from the tail now goes through 
 the subcardinals and from them into the excretory organs, passing 
 through a system of capillaries, to be gathered again in the postcardinals 
 
28o 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 and by them to be returned to the heart. Here, then, there is 
 another portal system (p. 277), the first renal-portal system, 
 which may be modified later as will be described below. 
 
 Fig. 286. — Scheme of development of the principal veins, a, anus; az, azygos major; 
 c, coronary vein; ca, caudal vein; cd, Cuvierian duct; ei, external iliac; g, gonads; ge^ 
 genital (spermatic, ovarian) vein; h, hepatic veins; ht, heart; i, ischiadic; j, jugular; /»', 
 left innominate; mn, mtn, meso- and metanephroi; om, omphalomesenterics; p, postcava; 
 pc, postcardinal; pn, pronephros; pr, precava; r, renal; ri, right innominate; s, subclavian; 
 sc, subcardinal; si, subintestinal; sic, superior intercostal. 
 
 In A the early condition with paired omphalomesenterics and subintestinals, the post- 
 cardinals extending back as far as the pronephroi. B, mesonephroi developed and with 
 them the subcardinals and the beginning of the postcava; one omphalomesenteric lost and 
 subintestinals and caudals beginning to fuse; the intestinal vessels omitted in the later 
 figures. C, postcava has joined sinus and postcardinals have reached caudals; D, amniote, 
 appearance of metanephroi (true kidneys) with obsolescence of mesonephroi; the post- 
 cardinals lose connexion with caudal, their place being taken by the backward extension 
 of the subcardinals; formation of cross connexions between jugulars and between post- 
 cardinals of the two sides. E, breaking up of postcardinals and disappearance of left 
 Cuvierian duct, the other being called the precava. 
 
 Postcaval elements crosslined, subcardinal, dotted, other veins black. 
 
 The Definitive Circulation. 
 
 It is impossible here to follow in detail the development of all parts 
 of the circulatory system, or even to mention all of the vessels in all 
 of the groups. All that can be attempted is an account of the more 
 important parts and their modifications, with here and there references 
 to their history which will render their peculiarities more intelligible. 
 Most of the major trunks are now known to appear at first as lines 
 of vascular cells, similar to and arising in the same way as those de- 
 scribed in connexion with the heart (p. 271), and it seems possible that 
 
CIRCULATORY ORGANS. 
 
 281 
 
 the intima of all of the blood-vessels is in genetic relations to such lines 
 of cells. It should be remembered that the vascular system is ex- 
 tremely variable, even within the limits of the species. 
 
 The Heart. 
 
 The heart, as it was left on page 273, was a venous or branchial 
 heart, in that all of the blood which enters it is venous blood and is 
 all pumped directly to the gills to lose its carbon dioxide and to take 
 up oxygen, before being distributed to the various parts of the body. 
 
 ^^•' ->;c ->i?' '^P^ yirls- "nk 
 
 Fig. 287. — DifiFerent stages in the differentiation of the parts of the heart. A, elasmo. 
 branch; B, teleosts; C, amphibia; D, lower reptiles; E, alligator; F, birds and mammals- 
 a, atrium; ao, aorta; b, bulb us arteriosus; c, conus; cd, Cuvierian duct; h, hepatic veins; pa, 
 pulmonary artery; pc, pre- and postcaval veins; pv, pulmonary vein; pa, pulmonary artery; 
 s, sinus venosus; sa, septum atriorum. 
 
 In its course through the body it passes but once through the heart in 
 order to make the complete circuit. Such, in general, is the heart in 
 the cyclostomes and fishes (fig. 287, A, B), 
 
 When, however, lungs are formed (dipnoi and amphibia) to share in 
 the respiratory processes, the heart begins to divide into arterial or 
 systemic, and venous or respiratory halves. This division is brought 
 about by the formation of a septum or partition in the atrium, partially 
 or completely dividing the chamber, the pulmonary vein (infra) open- 
 ing into the left half, which thus becomes arterial, while the sinus, 
 with its veins, is connected with the right alone (fig. 287, C). 
 
 Still higher in the scale the partition or septum extends through the 
 atrio-ventricular canal, dividing its valves into two groups (tricuspid 
 valves on the right side, mitral on the left) and partially dividing the 
 ventricle (most reptiles fig. 287, D). In the crocodilia (fig. 287, E) 
 
282 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 the division of the ventricle is completed by the extension of the 
 septum to the anterior end, but there is an opening (foramen 
 Pannizae) between the two sides of the aortic trunk, so that some 
 admixture of arterial and venous blood can occur. In the birds 
 and mammals (fig. 287, F) there is complete internal separation 
 of the two sides of the heart, though externally it shows but slight 
 signs of the division. As a result of this division blood must pass twice 
 through the heart (once through the venous, once through the arterial 
 half) in order to make a complete circuit of the body. Venous blood 
 enters the right atrium, passes to the right ventricle, by which it is 
 forced to the lungs (puhnonary or respiratory circulation). Re- 
 turning to the heart by the pulmonary veins, it passes through the left 
 atrium and ventricle and thence through the systemic circulation, by 
 which all parts of the body are supplied. Details of the modifications 
 of the heart in the different classes of vertebrates are given at the end of 
 this chapter. 
 
 Aortic Arches. 
 
 As was said above, the typical number of aortic arches is six pairs, 
 this number being but rarely exceeded!. In all groups except cyclos- 
 tomes and fishes they undergo considerable modification, and in the 
 fishes they are frequently more or less reduced in correlation with the 
 reduction of the gills (p. 238). The modifications may be outlined as 
 they occur in the successive pairs of arches. 
 
 In many fishes and all tetrapoda the first arch on either side dis- 
 appears beyond the point where the external carotid arises, while, 
 correlated with the reduction of the spiracular gill, the second pair of 
 arches is partially or completely lost in the adult. The third pair is 
 always persistent and through them flows the blood for the internal 
 carotids and, in the fishes, gymnophiona and a few urodeles (fig. 280, 
 C) and reptiles, (E) blood for the radices aortae as well. In all other 
 tetrapoda the radix disappears between the third and fourth arches (fig. 
 280, D) and consequently here the third arch is purely carotid in char- 
 acter. When this occurs the portion of the ventral aorta between the 
 third and fourth arches carries blood for the carotids alone and hence 
 forms a common carotid trunk, usually divided into right and left 
 common carotid arteries. 
 
 The fourth pair of arches are the systemic trunks in all tetrapoda, 
 
CIRCULATORY ORGANS. 
 
 283 
 
 carrying blood from the Ventral to the dorsal aortae, while the fifth, re- 
 duced in size, perform a similar function in a few urodeles (fig. 280, C), 
 but elsewhere they entirely disappear. The fourth arches show a dif- 
 ferentiation between the two sides in many reptiles. That on the left 
 side becomes separated from the rest of the ventral aorta (fig. 280, £, F) 
 and has its own tnmk connecting with the right side of the partially 
 divided ventricle, and, as will be understood from the relations of the 
 heart (p. 281), it may carry a mixture of arterial and venous blood. 
 From the dorsal side, this blood of the left fourth arch is largely dis- 
 tributed to the digestive tract, the coeliac axis arising from its radix, 
 while the part connecting it with the dorsal aorta is reduced in size. 
 The right arch and the carotids are connected with the left side of the 
 
 Fig. 288. — Aortic arches of amniotes, after Hochstetter. A, Varanus; B, snake; C, 
 alligator; D, bird; E, mammal, b, basilar artery; cc, common carotid; ci, ce, internal and 
 external carotids; da, dorsal aorta; p, pulmonary; s, subclavian. 
 
 heart and hence are purely arterial, the arch forming the main trunk 
 connecting the heart with the dorsal aorta. In the birds (fig. 280, G) 
 the radix of the left side of the adult disappears distal to the origin of 
 the subclavian artery, so that this arch supplies only the fore limb of 
 that side, while the right arch is purely aortic in character. In the 
 mammals (fig. 280, H) these relations are exactly reversed, the right 
 arch being subclavian, the left supplying the dorsal aorta and the 
 subclavian of that side. 
 
 With the development of lungs (dipnoi, tetrapoda) a pair of pul- 
 monary arteries are developed from the sixth pair of arches on the 
 ventral side of the pharynx. These grow back into the lungs, while the 
 rest of the arch, dorsal to their origin, becomes reduced to a small vessel 
 the ductus arteriosus (d. Botallii) in some urodeles, and persists 
 occasionally vestigially in higher vertebrates. Elsewhere it entirely dis- 
 appears. In the dipnoi and amphibia, where the ventricle remains 
 
284 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 undivided, the pulmonary arteries are connected with the same trunk 
 (ventral aorta) as are the other aortic arches (fig. 280, C, D). In the 
 amniotes (£), F, G, H) with partial or complete division of the ventricle, 
 the truncus and the ventral aorta are divided in such a manner that 
 derivatives of the sixth arch are connected with the right side of the heart, 
 while the rest of the ventral aorta, save for the exception noted in the 
 reptiles above, receives its blood from the left side of the heart. 
 
 In connexion with the almost complete obliteration of the fifth arch, and in 
 most pulmonate vertebrates, the separation of the sixth from the rest, it is interesting 
 to note that in the lower vertebrates (elasmobranchs) there is already a dififerentia- 
 tion of these two arches from the rest of the series (fig. 281). 
 
 Arteries. 
 
 The dorsal aorta arises from the fusion of two primitive trunks 
 running approximately parallel to the notochord, and extends as a me- 
 dian vessel, usually lying just dorsal to the origin of the mesentery, 
 from the point of union of the radices back nearly to the posterior end 
 of the body. 
 
 In human anatomy the different parts of the aortic vessels have names different 
 from those adopted here. The persistent portion of the ventral aorta is called the 
 ascending aorta, the persistent fourth arch is the arch of the aorta, and the 
 adjacent part of the dorsal aorta is the descending aorta. The rest of the dorsal 
 aorta is divided into the thoracic and abdominal aortae, accordingly as they lie 
 in the regions of the corresponding cavities. These terms are inapplicable in 
 comparative anatomy. 
 
 The arteries arising from the dorsal aorta may be grouped under the 
 two categories, visceral and somatic (p. 268). To the former belong 
 the vessels running through the mesenterial-like structures (mesen- 
 teries, omenta, mesorchium, etc.) to supply the digestive tract and the 
 excretory and reproductive organs. In the primitive condition those 
 going to the alimentary canal are numerous but they do not show a meta- 
 meric character. In the majority of vertebrates they become united 
 into a smaller number of main trunks from which branches go to the 
 various regions. The principal of these trunks are the following: 
 There is usually present a coeliac artery, arising from the radix or 
 from the dorsal aorta near it, and dividing in the mesogaster into 
 gastric, splenic and hepatic arteries, distributed to stomach, spleen 
 and liver. The superior mesenteric artery is connected in develop- 
 
CIRCULATORY ORGANS. 
 
 285 
 
 ment with the omphalomesenteric arteries (p. 276) and goes to the ante- 
 rior part of the intestine; while frequently an inferior mesenteric artery- 
 is distributed to the posterior part of the digestive tract. The superior 
 mesenteric may fuse with the cceliac to form the coeliac axis while not 
 infrequently other mesenteric arteries may be developed. 
 
 The hypogastric arteries, already mentioned, need further notice. 
 These primitively connect the dorsal aorta with the subintestinal vein 
 in the neighborhood of the anus, and later give off vessels to the region 
 of the rectum. When, as in all classes, from the amphibia upward, a 
 urinary bladder is developed from the rectal (cloacal) region, the 
 
 Fig, 289. — Diagram of vertebrate circulation based on a urodele. Arteries cross- 
 lined; veins black except the pulmonary vein, white, av, abdominal vein; a, ccEliac arter}'; 
 ca, cv, caudal artery and vein; d, dorsal aorta; ec, external carotid; g, gonad; h, hepatic 
 vein; ha, hepatic artery; hy, hjqpogastric artery; ic, internal carotid; il, iliac artery and vein; 
 ;, jugular; Iv, Uver; m, mv, mesenteric artery and vein; pa, pulmonary artery; pcd, post- 
 cardinal; pcv, postcava; pv, hepatic portal vein; r, rectal artery; ra, renal advehent vein; 
 sc, subclavian artery and vein. 
 
 hypogastrics form its blood supply, these vessels being the vesical 
 arteries. In the amnio tes the distal end of the anlage of the bladder 
 forms a foetal structure known as the allantois, described in another 
 section (p. 318), and parts of the vesical arteries are carried out as 
 allantoic arteries (fig. 273), into the new formation. Since these 
 pass through the umbilicus, they are also known as the umbilical 
 arteries. Later, when the umbilicus disappears, the allantoic arteries 
 are lost and only the rectal and vesical arteries remain of the hypo- 
 gastric trunks. 
 
 The arteries going to the excretory and reproductiye organs are 
 paired and, in the more primitive vertebrates show a marked metamer- 
 ism. They are best described in details along with the urogenital 
 structures in a subsequent section. It may be mentioned here that the 
 metamerism is well shown in the nephridial or renal arteries going to 
 
286 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 the pro- and mesonephroi, while there is usually but a single pair of 
 renal arteries to supply the metanephroi (true kidneys) of the amni- 
 otes. The arteries to the gonads may be included under the single 
 head of genital arteries, though they are usually subdivided into the 
 spermatic and ovarian arteries according to the sex. Like the neph- 
 ridial, the genital arteries are 
 more numerous in the lower 
 and are reduced in number in 
 the higher forms. 
 
 The somatic arteries are 
 more numerous and are meta- 
 merically arranged. In the 
 
 ^a cjcr 
 
 Fig. 290. Fig. 291. 
 
 Fig. 290. — Diagram of early relations of vertebral arteries in an amniote. av, vertebral 
 artery; da, dorsal aorta; ec, ic, external and internal carotids; pa, pulmonary artery; ra, 
 radix aortae; sa, subclavian. 
 
 Fig. 291. — A, side view of developing anterior arteries oiLacerta, after van Bemmeln; 
 the vertebral artery not developed behind; B, ventral view of the relations of the arteries at 
 the base of the vertebrate brain, av, vertebral artery; h, basilar artery; cw, circle of Willis; 
 da, dorsal aorta; ec, ic, external and internal carotids; pa, pulmonary artery; ra, radix 
 aortae; sa, segmental arteries; sc, subclavian; 2-6, aortic arches. 
 
 early stages they are given off in pairs from the radices and the 
 dorsal aorta, an artery on either side, extending laterally between 
 each two successive myotomes (fig. 275). Many of these remain 
 in a slightly modified condition and are called intercostal arteries 
 (including lumbar and sacral arteries, etc., according to position). 
 These usually become connected on either side (fig. 290), near their 
 
CIRCULATORY ORGANS. 
 
 287 
 
 origin, by a longitudinal vessel, the vertebral artery, which, in the 
 higher vertebrates, runs through the vertebraterial canal (p. 54) of 
 the vertebrae. 
 
 In the region of the aortic roots, after the formation of the vertebral 
 artery, all of the segmental arteries except the last of the series lose 
 their connexion with the radix and 
 henceforth are supplied by way of the 
 posterior segmental and the vertebral 
 (fig. 291). Anteriorly the vertebral 
 arteries pass to the ventral side of the 
 spinal cord (or medulla oblongata) 
 dividing there into two branches, one 
 of which, joining its fellow of the 
 opposite side, runs back beneath the 
 spinal cord as a spinal artery, while 
 the anterior branches unite in the same 
 way to form a basilar artery, running 
 forward beneath the medulla (fig. 291, 
 B). At the point just behind the 
 hypophysis the basilar divides, one-half passing on either side of 
 that structure and receiving the internal carotid of that side. The 
 trunks thus formed unite in front in the region of the optic chiasma. 
 There is thus formed an arterial ring, the circle of Willis, round 
 the hypophysis. 
 
 Fig. 292. — Diagram of origin of 
 blood supply of vertebrate appendage. 
 V, abdominal vein; da, dorsal aorta; 
 si, subintestinal vein; so, somatic (seg- 
 mental) vascular arch. 
 
 P'iG. 293 . — Three stages in the development of the arteries of the forelimb of the white 
 mouse, after Goppert. A, 8 days; B, 9 days; C, 10 days; a, aorta; b, brachial plexus. 
 •(The vessels are extremely variable, not agreeing even on the two sides of a single 
 individual.) 
 
 As the limbs grow out, segmental arteries, corresponding in number 
 to the somites concerned in the appendages, grow out into the member. 
 Distally these arteries become connected with each other and with the 
 veins of the limb by a network of small vessels. By enlargement of 
 
288 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 parts of these main trunks and of the connecting network, and the 
 partial or complete atrophy of other portions the definitive circulation 
 of the limb is established. This explains the numerous variations in 
 the blood supply of the limbs, both in the distal parts and in the origin 
 of the main trunks, which may arise from the dorsal aorta or from the 
 radices as far forward as the third aortic arch. 
 
 The main trunk of the fore limb may have different names in differ- 
 ent parts of its course. It is the subclavian artery as it leaves the 
 dorsal aorta, the axillary as it enters the limb, and the brachial in the 
 upper arm. It divides near the elbow into radial and ulnar arteries, 
 which run near the corresponding bones into the podium. 
 
 There are some additional elements of complexity in the develop- 
 ment of the arteries of the hind leg. As in front several somatic vessels 
 are concerned and there is the same formation of a capillary network. 
 Two of the arteries attain special prominence. In front is the epigas- 
 tric artery, which descends from the aorta to the ventral side of the 
 body and runs forward to supply the lower portion of the myotomes, 
 becoming connected at first with the epigastric veins, although later 
 they may anastomose with the hinder ends of the cutaneous arteries 
 (infra) . When the hind limb grows out, the epigastric sends a branch, 
 the external iliac or femoral artery, into its anterior side. As the 
 leg increases in size this may surpass the parent epigastric in size, the 
 latter now appearing as a side branch. 
 
 The second pair of somatic arteries are the sciatic (ischiadic) 
 arteries. These descend into the posterior side of the leg, the name 
 changing at the angle of the knee to popliteal artery, and farther 
 down it divides into peroneal and anterior and posterior tibial 
 arteries, the peroneal supplying the calf of the leg, the others continuing 
 into the foot. 
 
 The arrangement of vessels thus outlined is characteristic of the 
 lower tetrapoda where the femoral artery is small. It is also character- 
 istic of the embryos of the mammals, but in the latter, before birth, the 
 femoral artery grows down, joins the popliteal, and thus becomes the 
 chief supply of the limb. These trunks and the hypogastric do not 
 always remain distinct, but may fuse in different ways at the base. 
 Epigastric and hypogastric arteries are distinct in many reptiles and in 
 birds, but elsewhere they fuse to form the common iliac artery, so 
 called since the proximal portion of the femoral is often called the 
 external, the hypogastric the internal iliac artery. The sciatic, too, 
 
CIRCUXATORY ORGANS. 289 
 
 may remain distinct or it may fuse with the others at the base, and 
 then its independent portion appears as a branch of the common 
 iliac artery. 
 
 The dorsal aorta, which continues into the tail, is called the caudal 
 artery behind the point where the sciatics (common iliacs) arise. 
 
 A cutaneus artery, arising from either the subclavian or the 
 pulmonary artery of either side (both conditions occur in the amphibia), 
 runs backward in the skin of the trunk, and may extend back and unite 
 with the epigastric artery. When, as in the amphibia, these arise 
 from the pulmonary they contain venous blood and the skin acts as 
 a subsidiary respiratory organ (p. 258). 
 
 Veins. 
 
 The position and development of the chief longitudinal venous 
 trunks have already been outlined. Both these and other veins yet 
 to be mentioned frequently undergo shiftings of position and other 
 modifications during growth, but before describing these changes some 
 other vessels must be described. 
 
 With the development of the limbs corresponding veins arise (fig. 
 294), a subclavian vein for each fore limb, a common iliac for the 
 hind leg, these bringing the blood from the appendage to the trunk. 
 In the young each subclavian empties into the postcardinal of the same 
 side, but in the adult the opening may shift to the Cuvierian duct. 
 The common iliac vein likewise empties into a vein, the epigastric 
 or lateral abdominal, which runs forward in the body wall to connect 
 with either the postcardinal or the duct of Cuvier (fig. 294, A). This 
 condition obtains throughout life in some elasmobranchs, but higher 
 in the scale the iliac vein, while retaining its connexion w4th the 
 epigastric, grows toward the middle line and joins the postcardinal of 
 the same side, a condition which is permanent in amphibia and reptiles 
 (fig. 294, B, C), where blood coming from the hind limb has two 
 routes to the heart. 
 
 The epigastric veins of the two sides may fuse in the median line 
 in front (amphibia, some reptiles, birds), forming an anterior ab- 
 dominal vein (fig. 294, C) which reaches the heart by passing through 
 the remains of the ventral mesentery (ligamentnm teres) to the liver 
 and thence forward. A similar anterior abdominal vein has been 
 described in Echidna but is unknown elsewhere in the mammals. 
 19 
 
290 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 In the fishes the vessels of the appendages are but slightly developed, 
 there being a subclavian vein entering the Cuvierian duct, and occa- 
 sionally a brachial vein which may empty into the sinus venosus. In 
 the amphibia a cutaneus magnus vein (fig. 302), coming from the 
 skin of the trunk, may enter the subclavian, while in all tetrapoda the 
 subclavian, after leaving the limb, receives a superficial cephalic and an 
 axillary vein, the latter changing its name in the appendage to the 
 
 Fig. 294. — Relations and modifications of the post- and subcardinal, abdominal and 
 postcaval veins in different stages ot the amphibia. In A the veins (il) from the hind Hmb 
 return directly to the heart by the lateral abdominal veins {la), while the blood from the 
 tail (c) passes by way of the subcardinals (sc) through the mesonephroi to the postcardinals 
 (pc). In B the lateral abdominals have united in front to form the anterior abdominal 
 vein (aa) ; the iHacs have sent a branch to the postcardinals, which have grown back to join 
 the caudals, while the subcardinals have lost their connexion with the caudal and have 
 acquired one with the postcava (p), a backward growth from the sinus venosus. In C the 
 postcardinals have been interrupted, the posterior half of each now forming an advehent 
 vein while the subcardinals, as in B, form the revehent veins (r). 
 
 brachial vein. In the hind limb the common iliac vein is formed by 
 the union of the femoral and sciatic (ischiadic) veins, as well as the 
 hypogastric (internal iliac) vein already referred to. 
 
 In the classes above fishes (dipnoi, amphibia and amniotes) a new 
 vein, the postcava (vena cava inferior) appears. This arises in 
 part from scattered spaces, in part as a diverticulum of the sinus 
 venosus and the hepatic veins, and grows backward, dorsal to the liver, 
 until it meets and fuses with the right subcardinal vein (fig. 295), a 
 
CIRCULATORY ORGANS. 
 
 291 
 
 portion of which now forms a new trunk, carrying blood from the 
 posterior part of the body to the heart (figs. 294, 295). 
 
 With the appearance of the postcava changes are introduced in the 
 embryonic renal portal circulation ( p. 280) which may be summarized 
 as follows : The subcardinals lose their connexion with the caudal vein 
 and become connected with each other by transverse vessels (interrenal 
 veins) while parts of the postcardinals adjacent to the nephridial 
 organs separate from the parts in front, while they grow backward 
 
 om 
 
 Fig. 295. — Development of postcaval system in birds {A, B, sparrow; C, D, chick), 
 schematized after A. M. Miller. In A the postcardinals have extended nearly to the 
 pelvic region and the subcardinals are appearing as isolated spaces. In B the 
 subcardinal spaces are uniting and the capillary system connecting with the postcardinals 
 is developing, while the postcava is arising. In C the postcava has united with the 
 subcardinal of the right side, ai, ischiadic artery; ate, external iUac artery; au, imibilical 
 (hypogastric) artery; da, dorsal aorta; m, mesonephric veins; om, omphalomesenteric 
 artery; p, postcava and its anlagen; sc, subcardinal and its elements; vei, external iliac 
 vein; vi, ischiadic vein. 
 
 and connect with the caudal vein (fig. 295). These posterior parts 
 of the postcardinals now become the advehent veins of a second 
 renal portal system, bringing blood from the tail and hind limbs to the 
 excretory organs (mesonephroi) . The subcardinals of the two sides 
 usually fuse in the middle line, a process initiated by the appearance 
 of the interrenal veins, and now act as a revehent vessel, carrying 
 blood from the excretory organs to the postcava and the anterior 
 
292 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 portion of the postcardinals which have joined the anterior ends of the 
 subcardinals (fig. 294, C). The changes in the postcardinals and the 
 renal portal system of mammals will be described below. 
 
 In Ceratodus (dipnoi, fig. 296, A) there are some differences from the above 
 account. Thus the anterior portion of the right postcardinal (not shown in the 
 figure) loses its connexion with the vessels behind and acts as a vertebral vein, 
 taking the blood from the intercostal veins of that side back to the heart. The 
 
 Fig. 296. — A, venous system of Ceratodus, dorsal view, after Spencer; B, of a urodele, 
 ventral view, ab, abdominal vein; av, venae advehentes; b, brachial; c, caudal; cd, Cuvierian 
 duct; ej, external jugular; h, heart; hp, hepatic portal; ij, inferior jugular; j, jugular; il, 
 iliac; /, Hver; Ic, lateral cutaneus; m, mesonephros; p, ppstcava; pc, postcardinal; r, venae 
 revehentes; s, subclavian; /, testes. 
 
 caudal and the subcardinals form a continuous trunk, the revehent vessels forming 
 side branches. The posterior portions of the postcardinals grow back into the 
 tail as paired vessels, forming no connexion with the caudal vein. In Protopterus 
 the vertebral vein is lacking, the subcardinals are not fused behind while the 
 advehent veins are connected with the caudal. 
 
 The development of lungs brings about the appearance of one or 
 more pairs of pulmonary veins which bring the (arterial) blood from 
 these organs to the heart. These arise as an outgrowth from the 
 
CIRCULATORY ORGANS. 293 
 
 left atrial portion of the heart, dividing farther back to reach the two 
 lungs. At no time do the pulmonary veins connect with the sinus 
 venosus, but they, always empty into the left atrium (fig. 285). 
 
 The Foetal Circulation. 
 
 Some features of the foetal circulation of the amniotes have already 
 been alluded to, but the whole may be summarized here. In 
 the amniotes, with the development of a large yolk sac and of the 
 allantois, the vessels on the ventral side of the body become corre- 
 spondingly modified. The processes involved may be readily under- 
 stood from a comparison of figs. 282 and 284. The yolk sac is to be 
 regarded as a diverticulum of the intestine while the allantois is a 
 similar outgrowth from the urinary bladder, itself a process of the ali- 
 mentary canal. These outgrowths naturally carry with them the blood- 
 vessels distributed to the parts from which they arise. Hence the 
 omphalomesenteric artery and the vitelline veins (derivatives of the 
 omphalomesenteric veins) extend out ever the yolk, increasing in 
 number as well as in extent of their branches as the yolk sac spreads 
 over the yolk. 
 
 In the same way the hypogastric arteries are carried out with the 
 allantois, these portions being called the allantoic or umbilical 
 arteries, the blood being carried back to the trunk by a single allan- 
 toic vein. These two kinds of vessels — arteries and veins — are con- 
 nected in the distal part of the allantois by a rich network of capillary 
 vessels. It is by these that the allantois is able (p. 264) to act in the 
 sauropsida as an organ of respiration. In the mammals, by means of 
 osmosis through the placenta, it is not only respiratory, exchanging 
 gases with the uterine walls (there is no exchange of blood with the 
 mother), but they serve as recipients of nourishment by the passage 
 of plasma from the maternal tissues. 
 
 From the foregoing statements it will be seen that in the sauropsida 
 five vessels — three arteries and two veins — pass out through the um- 
 bilicus to the foetal adnexa, but in the mammals, where the yolk is 
 wanting and the yolk sac reduced and transitory in character, the 
 omphalomesenteric artery and the vitelline vein disappear early, leav- 
 ing but three vessels in the umbilical cord. In the elasmobranchs, 
 where there is a large yolk sac but no allantois, only the yolk sac cir- 
 culation is found. 
 
294 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 Circulation in the Separate Classes. 
 
 CYCLOSTOMES present marked differences in the circulation of the two 
 groups, the petromyzons being nearly normal, the myxinoids- decidedly aberrant. 
 The aortic arches vary in number with the number of gill pouches (p. 239). In the 
 myxinoids the common carotid is connected with all of the efferent branchials by a 
 
 Fig. 297. — Oblique ventral view of venous system of Petromyzon, drawn from a corro- 
 sion preparation (Princeton, 669); ac, precardinal; c, caudal; gs, genital sinus; hv, hepatic 
 vein; ij, inferior jugular; pc, postcardinal; sv^ sinus venosus; va, ventral aorta. 
 
 trunk running parallel to the body axis, just dorsal to the gill pouches. The inter- 
 segmental arteries of the dorsal region are irregular, sometimes alternating, some- 
 times appearing in pairs on the two sides of the median line. In the myxinoids 
 (fig. 297) the subcardinals are united behind, the postcardinals in front, these 
 latter uniting' with the single inferior jugular of the left side to form the unpaired 
 Cuverian duct, the presence of which renders the sinus venosus asymmetrical and 
 
 Fig. 298. — ^Anterior arterial vessels of the tile fish (Lopholatilus), after Silvester, a, 
 auricle; ab, to air bladder; am, to angle of mouth; c, coeHac axis; d, dorsal arteries; da, 
 dorsal aorta; ec, external carotid; g, genital artery; gs, gastrosplenic; h, hyoid artery; ha, 
 hepatic; I, lingual; Ig, left gastric; m, mesenteric; mh, middle hypobranchial; 0, ophthalmic; 
 pa, parietal; po, postOrbital; ps, pseudobranch; rg, right genital; so, supraorbital; v, ven- 
 tricle; va, ventral aorta. 
 
 forces the hepatic veins to empty into the right side. The hepatic portal receives 
 a vein from the head, and then passes back to a contractile portal heart, just before 
 it enters the liver. 
 
 FISHES. — In the fishes, the dipnoi excepted, the circulation corresponds rather 
 closely in its main features with the primitive condition described above. The 
 
CIRCULATORY ORGANS. 
 
 295 
 
 heart is purely venous and the only peculiarities to be mentioned are the following: 
 In the elasmobranchs and ganoids the valves of the conus are arranged in several 
 (3-8) rows, but in the teleosts (Buiyrinus excepted) they are reduced to a single row, 
 apparently corresponding to the first of the lower forms. In the latter group the 
 bulbus is especially well developed. The aortic arches correspond in number to 
 the functional gill slits — six or seven in the notidanid sharks, five in other elasmo- 
 branchs and at most four in ganoids and teleosts. Paired inferior jugulars are 
 usually present, but they are lacking in Polypterus, while in Lepidosteus and many 
 teleosts they are united into a single trunk emptying direcdy into the sinus venosus. 
 Epigastric veins are usually present and paired but are absent from many bony 
 fishes. 
 
 Fig. 299. — ^ Anterior venous system and heart oiLopholaiiltis, after Silvester, a, auricle 
 ab, veins from air bladder; b, bulbus; bv, brachial vein; c, cerebral vein; cd, Cuvierian duct; 
 cv, caudal vein; d, dorsal branches of parietal veins;/, facial vein; g, gastric veins; hp, 
 hepatic portal; hv, hepatic veins; ij, inferior jugular; in, is, veins from intestine and spleen; 
 /, Uver; pc, postcardinal; pd, postcloacal; per, peritoneal; ph, pharyngeal; po, postorbital; 
 re, anterior revehentes; s, sinus venosus; si, veins from stomach and intestine; th, thyreoid; 
 tnt, thymus; v, ventricle; va, ventral aorta; vf, vein from ventral fin; w, outline of Wolfl&an 
 body. 
 
 DIPNOI. — In this group the atrium, in correlation with the development of 
 lungs, becomes partially divided as described above. No true atrio-ventricular 
 valves occur, their place being taken by a strong ridge which, in systole, closes the 
 canal and at the same time partially divides the ventricle into arterial and venous 
 halves. The conus has eight rows of valves and in Ceraiodus the tnincus shows the 
 beginning of a division (completed in Protopterus) separating the arterial from the 
 venous arches. For veins, see fig. 296. 
 
 AMPHIBIA. — In the amphibia the division of the atrium by a septum atriorum 
 into right (venous) and left (arterial) halves is carried farther. This septum is 
 fenestrate in urodeles and gymnophiones, entire in anura, but in none is it carried 
 clear to the atrio-ventricular wall. In systole the edge of the septum is forced for- 
 ward, completely separating the two atria. No corresponding septum is developed 
 in the ventricle, but numerous muscular bands extending through its cavity tend 
 to prevent the mingling of arterial and venous blood. In Proteus, Cryptobranchus 
 and the caecilians the bulbus is simple but in the other urodeles and the anura a 
 spiral septum (possibly representing fused valves) is developed in it, separating it 
 
296 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 into two tubes. This is continued in the anterior part of the truncus by a horizontal 
 septum (short in urodeles, longer in anura) separating aortic and pulmonary trunks, 
 the former subdivided in a similar way a little farther forward into carotid and aortic 
 portions. 
 
 In the early larvae of the amphibia each fully developed aortic arch except the 
 last extends into the gills, but as the branchiae begin to be absorbed, a small vessel 
 connecting the afferent and efferent arteries at the base of each gill enlarges and 
 
 Fig. 300. — Heart and adjacent parts of Protopterus, after Rose a, atrium; aoe, 
 oesophageal artery; I, air bladder (lung); c, conus; h, hepatic vein; ji, is, superior and 
 inferior jugular veins; oe, oesophagus; pa, pulmonary artery; pc, postcardinal vein; ph, 
 pharyngeal artery; s, sinus venosus; sc, subclavian vein; 1-4, afferent branchial (aortic) 
 arteries. 
 
 becomes the path of the main blood stream and a part of the arch of the adult (fig. 
 304). Of these four arches — ^3, 4, 5, and 6 of the primitive scheme — the fifth is 
 lost in the adults of all except a few urodeles and caecilians. The fourth connects 
 with the dorsal aorta and the sixth with the pulmonary arteries. These last, which 
 often have a ductus Botallii, are noticeable for the large cutaneus arteries — anterior 
 and posterior — which arise from them and which play an important part in respira- 
 
CIRCULATORY ORGANS. 
 
 297 
 
 tion. Connected with the carotid arteries are the carotid glands (fig. 304). In 
 the larval stage each consists of a network of blood-vessels — a rete mirabile — 
 between the afferent branchial and the carotid artery, but in the adult this degener- 
 
 Tnaxsup 
 maxtif 
 intjuy 
 
 Fig. 301, 
 
 Fig. 302. 
 
 ates into a small muscular organ containing sympathetic cells (p. 165), at the 
 base of the carotid. 
 
 The postcava is well developed and the epigastric veins unite to form an anterior 
 
298 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 maxinf 
 \ extmax 
 
 vahd 
 
 Fig. 303. 
 Figs. 301, 302, 303. — Circulatory system of Desmognathus fuscus after Miss Seelye; 
 fig. 301, superficial vessels; fig. 302, deeper vessels; fig. 303, vessels of the dorsal body 
 wall; all from the ventral surface, aames, mesenteric arteries; acut, cutaneus artery; aduo, 
 duodenal artery; aepig, epigastric artery; ag, artery to anal gland; agas, gastric arteries; 
 ahep, hepatic artery; ail, iliac artery; aintcom, communis intestinal artery; ao, aorta; aoc, 
 ocular artery; aph, pharyngeal artery; an, anus; apul, pulmonary artery; asc, subclavian 
 artery; asp, splenic artery; hi, urinary bladder; ca, caudal artery; cutmag, cutaneus major 
 vein; cutp, cutaneus parva vein; cv, caudal vein; ec, external carotid; extmax, external 
 maxillary; ic, internal carotid; ilv, iliac vein; intcom, conmion intestinal; intjug, internal 
 jugular; intv, intestinal vein; ling, Ungual; liv, Uver; maxinf, maxsup, inferior and superior 
 maxillaries; mn, mesonephros; ce, oesophagus; pc, postcava; r, rectum; spl, spleen; st, 
 stomach; sv, sinus venosus; vabd, abdominal vein; vcut, cutaneus vein; vhp, hepatic vein; 
 vert, vertebral artery; vmes, mesenteric vein; vp, portal vein; vra, vena renalis advehentis: 
 vsp, splenic vein; vves, vein from bladder. 
 
CIRCULATORY ORGANS. 
 
 299 
 
 abdominal vein (fig. 294), while the blood from the hind limbs may return to the 
 heart through either the anterior abdominal or the renal portal system. 
 
 In the lungless salamanders (p. 258) the heart and blood-vessels show corre- 
 sponding modifications. There is no septum atriorum and the pulmonary arteries 
 and veins fail to develop. The cutaneus arteries and the smaller vessels supplying 
 the pharyngeal region are greatly enlarged, respiration taking place through the 
 skin and the mucous membrane of the throat. 
 
 The action of the anuran heart may be out- 
 lined here. The two atria contract at the same 
 time, forcing arterial and venous blood into the 
 ventricle, but it is kept from mixing by the mus- 
 cular bands already alluded to. At the systole 
 of the ventricle the venous blood, which is near- 
 est the truncus, is first forced forward. This 
 takes the most direct course through the wide 
 and shorter pulmonary arteries, which are prac- 
 
 FiG. 304. Fig. 305. 
 
 Fig. 304. — Diagram of the aortic arches in amphibia. Arterial blood cross lined, 
 venous black. The gill circulation omitted, its course indicated by arrows; the permanent 
 circulation after the absorption of gills shown, eg, carotid gland; da, dorsal aorta; d, 
 ductus BotalU; pa, pulmonary artery; va, ventral aorta; 3-6, aortic arches. 
 
 Fig. 305. — Heart of snapping turtle, Chelydra serpentina (Princeton, 479). aa, aortic 
 arch; c, cceliac artery; da, dorsal aorta; dh, Botall's duct; ec, ic, external and internal 
 carotids; la, left auricle; p, pulmonary artery; ra, right auricle; sc, subclavian artery; v, 
 ventricle; m, mesenteric artery. 
 
 tically empty at the time. The next portion of the blood, containing both arterial 
 and venous, follows the next easiest course through the aortic arches, while the 
 last to leave the ventricle, consisting of pure arterial blood, can only go into the 
 carotids, where the resistance is greater on account of the small size of the vessels 
 and the obstacles presented by the carotid glands. 
 
 REPTILES. — In the reptiles the division of the heart (fig. 287) is carried still 
 farther and the sinus venosus tends to be merged in the right atrium. The atrial 
 septum is complete and is continued forward as a ventricular septum, partially 
 
300 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 (Sphenodon, turtles, squamata) or completely (crocodiles) separating the two 
 ventricles. The peculiar relations of the aortic arches have been mentioned (p. 
 283). Correlated with the dififerences between the aortic (fourth) arches of the 
 two sides in the majority of reptiles are certain features in the origin of the arteries. 
 Thus both of the subclavian arteries (lacking in snakes) arise from the right radix, 
 while the left gives rise to the coeliac artery. In many reptiles the anterior parts of 
 the postcardinals are replaced by vertebral veins. The renal portal system is 
 developed in the embryo and persists (much as in the amphibia) to a greater or less 
 extent in the adult. Usually paired 'anterior abdominal veins are present. 
 
 BIRDS. — The peculiarities of the heart and aortic arches were mentioned on 
 page 283. Birds have the same reduction of the postcardinals as is found in reptiles. 
 The renal portal system is formed in the embryo, but the only blood received by 
 the adult kidney comes through renal arteries like those of mammals. The iliac 
 veins extend to the postcava and lose all connexion with the anterior abdominal 
 veins. The paired epigastric veins persist only in front. 
 
 MAMMALS. — In the mammals the four chambers of the heart are completely 
 separated and the sinus venosus has been completely merged in the right atriumt 
 The persistent left fourth aortic arch forms the sole connexion between the hear. 
 
 Fig. 306. — Modifications of the origin of the carotid and subclavian arteries in 
 
 mammals. 
 
 and the dorsal aorta and from it arise the carotid and subclavian arteries, the 
 arrangement of these representing almost every possible condition (fig. 306). In 
 the lower groups {e.g., rodents) both Cuvierian ducts persist, but in the higher orders 
 a cross connexion (the innominate vein) arises between the trunks formed from 
 the jugulars and subclavian veins of the two sides (fig. 308) so that the blood from 
 the left side of the head, neck and fore limb joins that of the left side in a common 
 trunk, the precava (anterior vena cava) which enters the right atrium. With 
 this development the left Cuvierian duct, as such, disappears. 
 
 The renal portal system has but a transitory existence in the embryo (best 
 developed in the monotremes) and early disappears with the degeneration of the 
 Wolffian bodies (mesonephroi). As these organs disappear a part of the capillary 
 system of the Wolffian bodies enlarges and forms a main trunk connecting the 
 postcava with the posterior parts of the postcardinal veins (fig. 307, C) which bring 
 the blood from the tail, the iliacs and the permanent kidneys. With farther develop- 
 ment (Z>, E) the left postcardinal is largely lost (except the part connecting with the 
 suprarenal and gonad of that side) and all the blood from the posterior part of the 
 body is returned by the right postcardinal and the postcava, which appear (fig. 
 308, A) as if they arose from a union of the iliac veins. Correlated with these 
 changes in the venous system and the impossibility of venous blood entering the 
 excretory organs, there is developed a renal artery from the aorta for each of the 
 permanent kidneys. 
 
CIRCULATORY ORGANS. 
 
 301 
 
 Fig. 307. — Development of posterior veins of rabbit, after Hochstetter. C and D repre- 
 sent only the hinder part of the whole shown in A to C In B the veins for the postcaval- 
 subcardinal system have tapped the postcardinal veins, which in C have lost their connec- 
 tion with the anterior part and empty now through the postcava exclusively. In E the 
 left posterior postcardinal is entirely lost, i, ischiadic vein; ie, external iliac; i, jugular; 
 nU, metanephros (kidney); p, postcava; pc, postcardinal; s, subclavian; sc, subcardinal: 
 sr, suprarenal; m, ureter. 
 
 Fig. 308. — Development of the anterior veins of a mammal. A, earlier stage, to be 
 compared with fig. 307 C; B, definitive condition of adult, a, azygos; c, coronary; e, i, 
 external and internal jugular; ha, hemiazygos; il, iliac; in, innominate; p, postcava; pc 
 postcardinal; pre, precava (superior vena cava); si, superior intercostal veins. 
 
302 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 The anterior parts of both postcardinals have separated from the posterior por 
 tion and receive only blood coming from the intercostal veins (fig. 308). A 
 cross vessel now connects the posterior parts of the postcardinals of the two sides, 
 after which the left vessel separates into two portions. The anterior of these 
 (fig. 308, B) connects with the heart by way of the jugular and innominate vein and 
 forms the superior intercostal vein of human anatomy. The rest of the left 
 postcardinal is now known as the hemiazygos vein and it returns blood from the 
 trunk by way of a cross connexion and the anterior part of the right postcardinal 
 (now called the azygos vein), to the precava and so to the heart. 
 
 THE LYMPHATIC SYSTEM. 
 
 The lymphatic system consists of (i) a series of lymph vessels which 
 penetrate all parts of the body; (2) of pulsating portions of these vessels, 
 the lymph hearts; and (3) peculiar aggregates of connective tissue, 
 leucocytes and lymph vessels which are grouped under the general 
 head of Ijrmph glands. 
 
 There are different views as to the morphology of the blood and lymph systems. 
 According to one (Marcus) the lymph vessels were primitively connected with the 
 coelom and have only secondarily come into relations with the blood-vascular 
 system. Others think that both blood and lymph vessels have arisen from 
 extraccelomic spaces, from which, by modification and specialization, the two 
 systems have been differentiated. The fact that in many invertebrates there is 
 but a single system, best compared with the lymph system of the vertebrates, and 
 that, even in the Crustacea, lymphatic and blood systems are but partially differ- 
 entiated, is of interest in this connexion. 
 
 The lymph vessels are, in part, capillary in character with walls of 
 endothelium alone. The larger ducts and the still larger sinuses are 
 strengthened by smooth muscle fibres and by elastic and fibrous tissue. 
 The capillaries have numerous anastomoses, but the vessels are said 
 to terminate blindly, while, at least in the higher vertebrates, some may 
 connect with the coelom by minute openings (stomata) in the peritoneal 
 lining. The larger vessels have valves at intervals to prevent back- 
 flow of the lymph, these often giving the vessels a lobulated appearance. 
 Proximally the vessels open at two or more points into the veins. The 
 fluid portion of the lymph is derived in part by osmose from the walls of 
 the blood capillaries, in part from the alimentary canal. 
 
 The development of the lymph vessels has been traced mainly 
 in birds and mammals (chiefly in the latter), with fewer observations on 
 amphibia and other classes. Many points remain to be worked out, 
 there being considerable differences in the various accounts. Appar- 
 ently the process in its main features is as follows: 
 
CIRCULATORY SYSTEM. 
 
 303 
 
 Near the junction of pre- and postcardinals on either side numerous 
 small diverticula are given off from the lateral side of these veins (fig. 
 309, A). These diverticula unite with each other, forming small 
 tubes parallel to the parent vessels and united to them for a time at 
 numerous points where the budding took place. Later these connex- 
 ions are lost and the tubes are separated from the veins (fig. 309, B) 
 forming an anterior cephalic duct, extending forward, parallel to the 
 jugular vein; an ulnar lymphatic duct destined to grow into the fore 
 limb; and, a little later, a thoracic duct grows back, parallel to the 
 
 Fig. 309. — Early development of the lymph vessels in the cat, after McCliire and 
 Huntington. A, in 6.5 mm. embryo; B, in 10.5 mm. embryo; C, definitive stage; D, 
 diagram of developing diverticula of chick which are to form lymph heart, based on Sala. 
 ac, anterior cardinal vein; c ^-^, coccygeal veins; cd, Cuverian duct; cv, cephalic vein; dls, 
 dorsal veno-lymphatic sinus; ej, ij, external and internal jugulars; pre, precava; th, thoracic 
 duct; ul, primitive ulnar lymphatic; uva, anlage of ulnar vein; vis, ventral veno-lymphatic 
 sinus; 1-7, segmental vessels; lymphatic-forming tissue stippled. 
 
 postcardinal vein. All of these vessels are united near their point of 
 origin by a large sinus, the jugular lymph sac (fig. 309, C). Later 
 the lymph sac reestablishes communication at one or two points in the 
 subclavian-jugular region with the vein. 
 
 The conditions at the posterior part of the body are less certainly 
 known (fig. 309, D). In this region a cistern of chyle (a mesenterial 
 lymph sac) and a posterior lymph sac develop in close connexion 
 with the postcava in the region of the nephridial organs, and it is pos- 
 sible that a portion of the thoracic duct grows forward from the cis- 
 tern of chyle, while other vessels grow into other regions. Later the 
 primitive trunks thus outlined give off branches which gradually ex- 
 
304 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 tend into all parts of the body, but of their development little is known. 
 Anastomoses occur between the vessels of the two sides of the body 
 and not infrequently the thoracic duct of one side shows more or less 
 degeneration, resulting in a lack of symmetry in the adult. 
 
 Not enough is known of the distribution of the lymphatic trunks to 
 render broad generalizations possible, but it may be said that the sys- 
 tem is most extensively developed in the subcutaneous tissue, in the 
 corresponding envelopes (meninges) of the central nervous system, 
 in the intermuscular connective tissue, in the walls 
 of the alimentary canal, and, as a network, in close 
 connexion with the blood-vessels of the body. 
 
 The l3miph hearts are enlarged and contrac- 
 tile portions of the lymph vessels, provided with 
 valves to prevent backflow of the fluid (fig. 310). 
 Usually these contract by means of the intrinsic 
 muscles of the walls, but in some urodeles {Am- 
 hly stoma) there is an unpaired lymph heart beneath 
 the truncus arteriosus which enlarges and con- 
 tracts with the systole and diastole of the blood 
 heart. 
 
 As was intimated above there is a constant 
 osmosis of fluid from the blood capillaries into 
 the surrounding tissues. This finally passes into 
 the distal capillaries of the lymph system, while 
 in the walls of the alimentary canal there are, in 
 addition, the results of the digestive processes 
 added to the fluid in the lymph vessels. As this latter portion has a 
 milky appearance, due to the contained fat, it is called chyle and 
 the lymphatics which contain it are called lacteals and chyle 
 ducts. All of these additions to the contents of the lymph vessels 
 make a current in the larger lymph trunks, and finally the whole 
 of the lymph is returned to the veins by the several connexions already 
 mentioned. In addition to the propelling force of the lymph hearts and 
 the pressure due to absorption and osmosis, the lymph is also carried 
 along by the motions of the parts in which the vessels ramify, their 
 pressure being supplemented by the action of the valves. 
 
 In those fishes which have been accurately studied the lymph system is well 
 developed and opens into the veins in the cardiac and caudal regions. The vessels 
 are especially developed in the tail, where (myxinoids, teleosts) lymph hearts occur. 
 
 Fig. 310. — Scheme 
 of caudal lymph heart 
 of teleost, after Favaro. 
 a, atrium; /, lymph ves- 
 sels; Is, lymph sinus; v^ 
 ventricle; vs, venous 
 sinus of caudal vein. 
 
CIRCULATORY ORGANS. 
 
 305 
 
 There is also a large lymph sinus in the scapular region into which the trunks from 
 head and body empty. Frequently there is also a large caudal sinus (physostomes) 
 connected with a lymph heart (fig. 310) which forces the lymph into the caudal 
 vein. 
 
 The urodeles have the thoracic ducts united behind but separate in front, a 
 cephalic trunk emptying into each, and each duct opening into the corresponding 
 subclavian vein, while a series of from fourteen to twenty lymph hfearts occur in 
 connexion with the trunk accompanying the lateral line. The anura are noticeable 
 for the complete disappearance of the thoracic ducts, their place being taken by a 
 
 Fig. 311. — Deeper anterior lymphatics (stippled) of Scorpenichthys, after Allen, a, 
 auricle; ahs, abdominal sinus; h, brachial sinus; hr, brain; cs, cephalic sinus; d, dorsal trunk; 
 fm, facialis-mandibularis vein; hs, hyoid sinus; ij, inferior jugular vein; ips, inner pectoral 
 fin sinus; ;', jugular vein; /, lateral trunk; on, orbito-nasal vein; p, pericardial sinus; pf^ 
 profundus facialis lateral trunk; pv, profimdus ventral trunk; sf, superficial lateral trunk; 
 55/, superior spinal longitudinal trunk; v, ventricle; va, ventral aorta; vfs, ventral fin sinus; 
 vp, ventral pericardial sinus; vt, ventral abdominal trunk. 
 
 pair of trunks between the dorsal myotomes and those of the lateral body wall. 
 They have also enormous subcutaneous lymph spaces, separated from each other 
 by narrow partitions. It is the presence of these large spaces that makes the skin- 
 ning of a frog such an easy matter. Two pairs of lymph hearts are present, one 
 pair in the neighborhood of the extremity of the urostyle, the other between the 
 transverse processes of the third and fourth vertebrae. In the caecilians there is a 
 pair of lymph hearts for each segment of the trunk. 
 
 Reptiles have two cephalic lymph trunks and one (lizards) or two thoracic 
 
306 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 ducts, the one of the lizards being divided in front so as to empty into either sub- 
 clavian vein. There is a single lymph heart at the junction of trunk and tail. In 
 the birds both thoracic ducts occur and there is a pair of lymph hearts present in 
 the young in the position occupied by the single heart of the reptiles. 
 
 In the mammals the primitively paired thoracic ducts are sometime retained 
 throughout life, but usually only one persists. This begins at the cistern of chyle 
 in the lumbar region and empties into the left brachiocephalic vein near the entrance 
 of the single cephalic duct. The thoracic duct receives the lymph vessels from the 
 limbs and those (lacteals) from the alimentary canal. In those cases where there 
 
 Fig. 312. — Early lymph system (black) of 10 mm. rabbit embryo, after F. T. Lewis. 
 Sit, anterior tibial; c, caudal;/j, primitive fibular; ej, ij, external and internal jugular; em, 
 external mammary; pc, postcardinal; ul, primitive ulnar veins. 
 
 is but a single thoracic duct in front, its representative on the right side is a much 
 smaller vessel connected with the right side of the venous system. No lymph 
 hearts are known in the mammals. The jugular lymph sacs of the embryo have 
 been regarded as such, but the absence of valves and muscles in the walls renders 
 such an interpretation doubtful. 
 
 Lymph Glands. — In connexion with the lymph vessels are numer- 
 ous structures included under the heads of lymph glands, lymph 
 nodules and hlood-lymph glands. These are most abundant in the 
 walls of the coelom (mesenteries) and of the digestive tract, although 
 they may be found at remote points. They consist of aggregates of 
 
UROGENITAL SYSTEM. 307 
 
 adenoid tissue (reticular connective tissue crowded with leucocytes). 
 Well-known among these structures are the so-called fat bodies (cor- 
 pora adiposa) connected with the gonads of the amphibia, and the 
 'hibernating glands' of some rodents and insectivores, which con- 
 sist of richly vascularized masses of fat. In the lymph nodules this 
 adenoid tissue is enmeshed in a rete mirabile of lymph vessels. In 
 the blood-ljrmph glands there is a somewhat similar relation to blood- 
 vessels as well, for the details of which reference must be made to 
 histological text-books. These lymph structures, which occur at 
 various points of the body, are apparently places for the formation of 
 leucocytes (lymphocytes) . 
 
 The spleen, attached to the mesentery near the stomach and pan- 
 creas, is intermediate in some respects between lymph and blood- 
 lymph glands and is the largest lymph structure in vertebrates. It 
 is developed in the walls of the alimentary canal and is said to have 
 an entodermal origin. Later it separates from the stomach and 
 assumes its definitive position. It serves, apparently, as a place for 
 the disintegration of the red blood corpuscles in addition to functioning 
 as a leucocyte-forming organ. 
 
 The tonsils (p. 247) belong to the category of adenoid glands. 
 There are two kinds of these, the pharyngeal and the palatine tonsils, 
 the latter occurring between the inner ends of the Eustachian tubes of 
 amniotes, the palatine (best developed in mammals) are paired struc- 
 tures on either side of the pillars of the fauces. Other tonsil-like 
 structures occur at different points of the floor and roof of the mouth 
 of the tetrapoda. 
 
 THE UROGENITAL SYSTEM. 
 
 In several phyla of the animal kingdom there is an intimate relation 
 between the reproductive and excretory organs, the ducts of the latter 
 sendng either to carry the products of the gonads directly to the ex- 
 terior or acting as brood organs where a portion of the development 
 of the egg takes place. This close association of the two systems is 
 especially marked in most vertebrates and hence this section is headed 
 Urogenital System, because of the diflBiculty of treating the two com- 
 ponents separately. 
 
 The urinary or excretory organs have for their purpose the elimina- 
 tion of the nitrogenous waste (and occasionally other products) from 
 
308 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 the system. They are paired organs which consist of glandular por- 
 tions, the nephridia (kidneys), and their ducts. The reproductive 
 organs include the gonads or sexual 'glands,' which (ovaries) pro- 
 duce the eggs or (testes) the spermatozoa, and the passages by which 
 these products are carried to the external world. To these are fre- 
 quently to be added accessory reproductive structures by which, in 
 certain cases, the sperm is transferred to the female. 
 
 Fig. 313. — Urogenital organs of Emys europea, after Bojanus. b, urinary bladder; 
 g, opening of vas deferens into the urogenital sinus; k, kidney; t, testis; u, ureter; vd, vas 
 deferens. 
 
 THE EXCRETORY ORGANS. 
 
 The nephridia consist of a series of excretory tubules, specialized 
 in different ways, and of the ducts into which the tubules empty. As 
 the function of the nephridia is the elimination of the nitrogenous 
 waste (uric acid, urea, etc.) which accumulates in the blood, they have 
 an abundant blood supply, entirely derived, in the younger stages of 
 all vertebrates and in the adults of the higher groups from the dorsal 
 aorta, while in the later developmental stages and in the adults of most 
 anamniotes the aortic blood is supplemented by blood coming from 
 the tail and hind limbs by way of the caudal and iliac veins (fig. 303). 
 
 In its extreme development one of the excretory tubules may con- 
 sist of the following parts (fig. 314): At the proximal end the tubule 
 opens into the ccelom (metacoele) by a ciliated funnel, the nephro- 
 
UROGENITAL SYSTEM. 
 
 309 
 
 stome; the cilia, which may continue for some distance along the 
 inside of the tubule, serving to create a current which carries the 
 coelomic fluid into the tubule and thence outward. Farther along 
 the tubule expands into a Malpighian or renal corpuscle (fig. 315). 
 This consists of a vesicle (Bowman's capsule), one side of which 
 
 Fig. 314. — Diagram of conventionalized excretory tubule. a, ascending limb of 
 Henle's loop; h, Bowman's capsule of Malpighian body; c' — c^, first and second con- 
 voluted tubules; ct, collecting tubule; d, descending limb of Henle's loop; g, glomerulus 
 of Malpighian body; wnth artery and vein; h, Henle's loop; n, nephrostome opening into 
 coelom; x, entrance of other tubules into collecting duct. 
 
 projects into the other, nearly filling the cavity. This intumed portion 
 is the glomerulus. It consists of a network of capillary blood-vessels, 
 supplied by an artery and drained by a vein. Beyond the connexion 
 of the Malpighian body the tubule becomes contorted or convoluted 
 and its cells are strongly glandular in character. This first convoluted 
 tubule is succeeded by a nearly straight tract, folded once on itself into 
 
 Fig. 315. 
 
 -Diagram of renal (Malpighian) corpuscle, a, artery; h. Bowman's capsule; 
 gl, glomerulus; «, nephrostome; /, nephridial tubule; v, vein. 
 
 the descending and ascending limbs of Henle's loop. Next follows 
 the second convoluted tubule, which passes by means of a short 
 connecting tubule into a non-glandular collecting tubule into which 
 several other systems of excretory tubules enter, and which leads 
 more or less directly into the urinary duct which conveys the waste 
 from the excretory organ to the exterior. 
 
3IO COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 One or another of these typical parts may be lacking in certain 
 groups. Thus in the amniotes the nephrostomes are never formed, 
 though they occur in most ichthyopsida. In the pronephros the 
 Malpighian corpuscle is rudimentary or lacking at all stages while 
 there is no differentiation of convoluted tubules and Henle's loop. 
 
 The function of the various parts of the nephridial tubule is in 
 outline as follows: Theoretically it would appear that in the primitive 
 condition the nitrogenous waste, which is elaborated in the liver, 
 collected in the coelom and, together with the coelomic fluid, was 
 passed outward through the nephrostomes and the tubules, which 
 acted merely as ducts. Higher in the scale the parts become more 
 differentiated and specialized. The renal corpuscles form a filtering 
 apparatus by which water is passed from the blood-vessels of the glom- 
 erulus into the tubules near their beginning, and this serves to carry 
 out the urea, uric acid, etc., secreted by the glandular portions of the 
 walls of the tubules (convoluted tubules, ascending limb of Henle's 
 loop). 
 
 In development there may be three successive series of nephridial 
 structures, the higher number occurring only in the amniotes. These 
 are known as the pronephros (head kidney), mesonephros (Wolffian 
 body), and the metanephros (permanent kidney of the amniotes). 
 All three are closely related in development and structure but are 
 distinguished by differences in origin and in the final details. Three 
 views are held as to their relations one to another. According to one 
 they are parts of an originally continuous excretory organ (holone- 
 phros) which extended the length of the body cavity. This has be- 
 come broken up into the separate parts which differ merely in time 
 of development and function, with minor modifications in details. A 
 second view is that they are three separate organs, while a third regards 
 them as superimposed structures which occasionally overlap (birds, 
 gymnophiona) and thus are not, strictly speaking, homologous but 
 rather homodynamous. The first view has the most in its support, 
 but for convenience the three structures are kept distinct here. All 
 arise from the mesomeric somites or from the Wolffian ridge which 
 appears on either side of the median line where the mesomeres separate 
 from the rest of the wall of the body cavity, the mesomeric cells furnish- 
 ing the nephrogenous tissue from which the definitive organs develop. 
 
 Pronephros. — ^The pronephros is the first to appear in develop- 
 ment. As will be recalled (p. 14) the mesomere, like the epimere, 
 
UROGENITAL SYSTEM. 
 
 311 
 
 becomes segmented, and later, when the epimere separates to form 
 the myotome, the dorsal end of each mesomere becomes closed, the 
 whole then forming a sac, opening below into the ventral, undivided 
 coelom (metacoele). A varying number of these nephrotomes (as 
 they are called) lying a little behind the head are concerned in the 
 
 Fig. 316. — Scheme of origin of pronephric tubules after Felix. .4, earlier, B, later 
 stage, c, coelom; d, pronephric tubule and duct; e, epimere; h, hypomere; w, mesomere 
 (lined); n, nephrostome; my, myotome; so, sp, somato- and splanchnopleure. 
 
 formation of the pronephros (two in most urodeles and amniotes; 
 three in lampreys, anura, some sharks and some anmiotes; four or 
 five in some sharks £ind Lepidosteus; seven or eight in skates; eight to 
 eleven in Amia; and a dozen in some caecilians; while it is claimed that 
 the whole series of nephridial tubules of Bdellostoma is pronephric). 
 The somatic wall of these nephrotomes (fig. 316) grow out toward 
 
 Fig. 317. — Reconstruction from longitudinal sections of pronephros of Hypogecphis 
 (caecilian), after Brauer. Pronephric duct {pd) and primary pronephric tubules light; 
 the rest of the somites (nephrotomes) black; glomeruli between tubules 2-8. The three 
 trunk somites in front of i develop no tubules. 
 
 the ectoderm, thus forming slender pronephric tubules (or solid cords 
 which later become canalized), the proximal end of each communica- 
 ting freely with the metacoele by way of the cavity of the nephrotome, 
 the opening of the latter into the metacoele being the nephrostome. 
 As will be understood, these tubules, like the nephrotomes, are meta- 
 meric in character, equalling the somites in number. The distal ends 
 
312 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 grow outward until they are just beneath the ectoderm, when they 
 bend toward the posterior end of the body, the anterior tubules fusing 
 with those behind. From the junction a tube, the pronephric or 
 archinephric duct, gradually grows backward just beneath the 
 ectoderm (figs. 317, 318) until it reaches the posterior end of the meta- 
 coele, when it fuses with the hinder end of the digestive tract (cloaca) 
 or with the ectoderm in the vicinity of the anus. An opening now 
 breaks through, thus putting the coelom indirectly in communication 
 with the outer world. 
 
 At first the pronephric duct lies closely below the ectoderm and is 
 almost equally near the lining of the metacoele. As the myotomes 
 grow downward they come to lie between the ducts and the ectoderm 
 so that eventually the ducts are just beneath the lining of the definitive 
 body cavity. 
 
 There has been considerable dispute as to the origin of the cells which form the 
 pronephric duct. They were long thought to be solely of mesothelial character, 
 arising by proliferation from the tube itself. Then it was noticed that the back- 
 ward-growing tube fused at its tip with the ectoderm and it was thought that there 
 was an actual contribution of ectodermal cells at this point. This view received 
 considerable support from its agreement with certain theoretical views. The 
 matter is not yet decided. The writer is convinced, from the study of perfectly 
 preserved material in which cell boundaries are clearly shown, that in the sharks 
 (Acanthias) which were thought most strongly to support the view of ectodermal 
 contribution, that the whole duct is of mesothelial origin. 
 
 In the teleosts the dorsal end of the nephrotome grows out to form the pro- 
 nephric tubule, to which both somatic and splanchnic walls thus contribute. In 
 the amphibia the nephrotome is not distinctly separated from the lateral plates 
 (hypomere) and the pronephric tubules are formed from the common area. 
 
 The pronephros is functional for a time in the embryos of some 
 lower vertebrates; in other groups it is a rudimentary and transitory 
 structure, save for its participation in the oviducts and the ostium 
 tubae abdominale (see below). When functional it takes the nitro- 
 genous waste from the body cavity, while its filtering apparatus con- 
 sists either of separate glomeruli (one for each tubule) or the glomeruli 
 of the separate somites may run together, forming a glomus. These 
 glomeruli or the glomus of the pronephros do not project into a Bow- 
 man's capsule, but lie immediately above the dorsal wall of the coelom^ 
 between the mesentery and the nephrostomes (fig. 318), pushing the 
 epithelium before them. Later, as in the csecilians, they and the 
 nephrostomes may be enclosed in a cavity cut off from the coelom, 
 
UROGENITAL SYSTEM. 
 
 313 
 
 SO that the whole resembles a renal corpuscle, but is dijfferent in origin. 
 In either case the exuding fluid passes into the metacoele from which 
 it is drawn by the cilia of the nephrostomes and passed into the tubules. 
 The blood is brought to the glomus or glomeruli by short segmental 
 arteries arising from the dorsal aorta (fig. 318) and, after passing 
 through the capillaries, it is carried away by the postcardinal veins 
 of the corresponding side to the heart, these veins keeping pace in 
 their backward development with the development of the nephridial 
 tubules. 
 
 Fig. 318. — Stereogram of developing pro- and mesonephros. a, aorta; g, glomus or 
 glomerulus; m, mesenter>'; m/, mesonephric tubule; n, notochord; nc, cavity of (tit) nephro- 
 tome; ns, nephrostome; pc, postcardinal vein; pd, pronephric duct; pt, pronephric tubule; 
 ptm, peritoneal membrane. 
 
 There is much that goes to show that the pronephros formerly had a much 
 greater extension than at present, including a larger number of somites. It has, 
 however, been replaced in the adults of all vertebrates (with the possible exception 
 of Bdellostonm) by the mesonephros, and later, in the amniotes, by the metanephros 
 as described below. 
 
 Mesonephros. — The mesonephros or Wolffian body is the second 
 excretory organ to arise. It arises after the pronephros and its 
 duct are formed, by the development of a series of mesonephric tubules, 
 which grow out from the nephrotomes behind those concerned in the 
 formation of the pronephros. These tubules extend laterally until 
 they meet and fuse with the pronephric duct, w^hich now acts as the 
 excretory canal of the new gland. In some cases the point of origin 
 
314 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 of the mesonephric tubules is clearly dorsal to that of the pronephric 
 tubules (fig. 318), and in some cases (birds, caecilians) pro- and meso- 
 nephric tubules have been described as arising from the same nephro- 
 tome, one above the other. In most ichthyopsida the opening of the 
 nephrotome into the metacoele forms the nephrostome, but in the 
 amniotes this opening is closed before the tubules are formed and 
 consequently nephrostomes are lacking in the latter group. 
 
 Fig. 319. — Stereogram of mesonephros. a, aorta; cv, postcardinal vein; g, genital 
 ridge; gl, glomerulus; m, mesentery; mc, Malpighian corpuscle; mi, mesonephric tubules; 
 my, myotome; n, nephrostome; nc, notochord; p, peritoneal lining; w. Wolffian duct. 
 
 Segmental arteries grow out from the aorta to the splanchnic wall 
 of each nephrotome, forming there a network of capillaries at a higher 
 level than the pronephric glomeruli (fig. 319). The glomerulus thus 
 formed presses the wall before it, while the rest of the nephrotome 
 closes around it as a Bowman's capsule, the whole forming a Mal- 
 pighian body (in some rodents the glomeruli are rudimentary or absent). 
 In most ichthyopsida the Malpighian body is connected on one side 
 with the metacoele by the nephrostome, and on the other with the 
 mesonephric tubule. ^ 
 
UROGENITAL SYSTEM. 315 
 
 Thus at first the mesonephros is a metameric structure, extending 
 over a much larger number of somites than does the pronephros and 
 reaching nearly to the posterior limits of the metacoele. As the devel- 
 opment of the embryo proceeds the number of tubules increases by 
 budding in a manner not readily described (fig. 320). These tubules 
 unite with the distal ends of those first formed, so that the distal part 
 of these form collecting tubules. Each of these secondary tubules 
 forms its own Malpighian body and all of the tubules elongate, be- 
 come convoluted, and the mesonephros loses its primitive metameric 
 character. 
 
 a»»;»i^«s^iai^caE^*^3^%s^ai*s*r^««; 
 
 Fig. 320. — Reconstruction of three somites of the Wolfl5an body (mesonephros) of 
 Hypogeophis, after Brauer. a, aorta; w^-w^, primary and secondary Malpighian bodies; 
 n^-n^, corresponding nephrostomes; s, tertiary segments of mesonephros; t^-t^, primary 
 and secondary mesonephric tubules; w. Wolffian duct. 
 
 At the same time changes are introduced into the mesonephric 
 circulation. The veins emerging from the renal corpuscles extend 
 out into the region of the tubules, each breaking up there into a second 
 system of capillaries which envelop the tubules before returning the 
 blood to the postcardinal vein. The subcardinal vein (p. 279) brings 
 the blood from the caudal region (and usually from the hind limbs) 
 to the Wolfl&an body and this is also returned via the postcardinals to 
 the heart. (For details of the modifications of the mesonephric circu- 
 lation see pages 290-292.) 
 
 The Mesonephric Ducts. — The conditions in the elasmobranchs 
 have been regarded as very primitive. In them (and to some extent 
 in some of the amphibia), when the mesonephros develops, the pro- 
 nephric duct divides longitudinally from its hinder end as far forward 
 as the anterior end of the Wolffian body. Of the two ducts thus 
 formed (fig. 321, A)j one, the Wolffian (Leydig's) duct, remains 
 connected with the tubules of the mesonephros and forms its excretory 
 canal. The other, the MUllerian duct, is similarly related to the 
 
3l6 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 pronephros and its derivatives, and in the female forms the tube 
 (oviduct) by which the eggs are carried to the exterior. In other 
 amphibia and in the amniotes the pronephric duct does not divide, 
 but remains solely in the service of the mesonephros and forms the 
 Wolffian duct, while the oviduct arises in another manner, to be de- 
 scribed in connexion with the reproductive organs. In the teleosts 
 also there is no division of the pronephric duct. 
 
 Fig. 321. — Diagrams of urogenital structures in (.4) indifferent and in female elas- 
 mobranchs and amphibians; (B) male elasmobranchs and amphibians; (C) male amniote 
 (mammal); (D) female amniote (mammal), b, urinary bladder; c, cloaca; e, epididymis; 
 k, kidney (metanephros);/, Fallopian tube; g, gonad; h, 'stalked hydatid'; /, longitudinal 
 tubule; m, MuUerian duct (oviduct), rudimentary in B andC; mn, mesonephros; o, ovary; 
 ot, ostium tubae abdominale; pd, paradidymis; po, paroophoron; pv, parovarium; r, rectum; 
 t, testis; u, uterus; ua, urethra; ur, ureter; va, vas aberrans; vd, vas deferens; ve, vasa effer- 
 entia; w, Wolfl5an duct, urinary in A, urogenital in B, genital in C and rudimentary in D. 
 
 Metanephros. — The mesonephros is functional in the embryos of 
 all vertebrates and throughout life in the ichthyopsida. It also func- 
 tions for a short time after birth in certain reptiles (lizards) and in the 
 lowest mammals {Echidna, opossum). It becomes replaced in the 
 adults of all amniotes by the mesonephroi, the only structures to which 
 the name kidneys is strictly applicable. Each metanephros arises 
 behind the mesonephros of the same side. From the dorsal hinder 
 end of the Wolffian duct, near its entrance into the cloaca, a tube, the 
 
UROGENITAL SYSTEM. 
 
 317 
 
 Tris 
 
 Fig. 322. — Profile reconstructions of lizard {Lacerta agilis) {A) 16 mm. long; (JB) 20 mm. 
 long; and (C) human embr\-o 115 mm. long, after Schreiner. a, allantois stalk; c, cloaca; 
 cc, cranial collecting tubule ; cd, caudal collecting tubule ; k, permanent kidney (metanephros) ; 
 met, median collecting tubule; mt, metanephric (nephrogenous) tisssue; mtb, mesonephric 
 tubules; pet, primary collecting tubule; pii, Wolffian duct (primitive ureter); r, rectimi; 
 5, secondar}- collecting tubule; u, ureter; cm, u and pu, common portion of primitive and 
 permanent ureters. 
 
 Fig. 323. — Models of two stages in the development of tubules of kidney (metanephros) 
 of man, after Stoerk. b, Bowman's capsule; e, collecting tubule; en, connecting tubule; 
 cv, convoluted tubule; h, Henle's loop; /, intercalar)' tubule; /, lower arch; m, middle piece. 
 
3l8 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 ureter (fig. 322, k) grows forward, parallel to the parent duct, into 
 the tissue posterior and dorsal to the mesonephros. This nephro- 
 genous tissue is apparently serially homologous with that from which 
 the mesonephric tubules have arisen, but all traces of metamerism 
 have disappeared from it. In this nephrogenous tissue the anterior 
 end of the ureter gives off a varying number of branches (fig. 322), 
 each of which expands at its tip, thus forming a primary renal vesicle, 
 and a little later the place where the branches and the ureter unite 
 expands, the enlargement forming the pelvis of the definitive kidney. 
 The cells of the nephrogenous tissue form a number of aggregates 
 around each primary vesicle; each aggregate soon becomes hollow, 
 and develops into an S-shaped tubule (fig. 323, left), one end of which 
 joins the primary renal vesicle, while a glomerulus arises at the other 
 end, but no nephrostomes are formed. Later there is a great mul- 
 tiplication of these tubules and an extension of the capillary system 
 of the glomeruli around them, much as in the mesonephros. The 
 differentiation of each tubule into convoluted, collecting and Henle's 
 regions occurs early (fig. 323, right). 
 
 Urinary Bladder. — At or near the hinder ends of the excretory 
 ducts there is frequently a reservoir for the urine, the urinary bladder 
 or urocyst. Of these there may be three kinds. In most fishes the 
 bladder arises by a fusion of the hinder ends of the Wolffian ducts 
 plus a part derived from the hinder end of the digestive tract (cloaca), 
 the Wolffian ducts emptying into it and the whole opening to the 
 exterior, usually dorsal and posterior to the anus. In the dipnoi there 
 is a diverticulum from the dorsal wall of the cloaca, anterior to the 
 openings of the Wolffian ducts. This is usually called the urinary 
 bladder (fig. 325, Z)), but it may be homologous with the rectal gland 
 of the elasmobranchs. 
 
 The third type, the allantoic bladder, occurs in all tetrapoda. 
 This arises as a ventral diverticulum from the cloaca. In the amphibia 
 the whole of the outgrowth forms the bladder and its walls are sup- 
 plied by the hypogastric arteries. In the amniotes the proximal 
 portion alone is converted into the urinary bladder, while the more 
 distal portion, in the embryo becomes the respiratory organ of the 
 growing young, the allantois. This part extends far beyond the body 
 wall, carrying with it branches of the hypogastric arteries (allantoic 
 arteries), and in the mammals forms a part of the placenta. The 
 allantois becomes reduced in the later stages and at the beginning of 
 
UROGENITAL SYSTEM. 319 
 
 free life is entirely absorbed or is lost with the placenta. In the 
 amphibia the urine finds its way into the urinary bladder via the cloaca, 
 as the urinary ducts (Wolffian ducts) do not open into it. In those 
 amniotes in which a bladder is present the ureters open into it, and 
 the urine is conveyed to the exterior by a single tube, the urethra. 
 In many sauropsida there is no urinary bladder, though the allantois 
 is formed in development. 
 
 There is great dijficulty in comparing the excretory system of the vertebrates 
 with anything known in the invertebrates. In general the nephridial tubules may ' 
 be compared with those of the annelids. Both have nephrostomes opening into 
 the coelom, convoluted tubules, enveloped in a network of capillary blood-vessels, 
 but in the annelid each tubule opens separately to the exterior in the somite behind 
 that in which the nephrostome lies, while in the vertebrate the series of tubules 
 empty into a common duct. When it was thought (p 312) that the ectoderm con- 
 tributed to the pronephric duct, the homologies appeared easy. The duct was 
 originally a groove on the outer surface into which the separate tubules opened. 
 Then the groove was rolled into a tube which continued backward to the vicinity 
 of the anus By the downgrowth of the myotomes the duct became cut off from 
 its primitive position and came to lie just outside the peritoneal lining When, 
 however, it is considered that in all probability the pronephric duct is formed solely 
 from the mesoderm the homology falls to the ground and an explanation is still a 
 desideratum. 
 
 THE REPRODUCTIVE ORGANS. 
 
 The tissue which is to form the ovaries and testes early forms a 
 pair of genital ridges, one on either side of the mesentery and between 
 it and the Wolffian ridge (fig. 319). At one time it was thought that 
 the anlage of the gonad was segmental in character and ' gonotomes,' 
 comparable to nephro tomes and myotomes, were described. It has 
 since been shown that no metamerism exists and that the primary 
 germ cells, which alone characterize the gonads, arise in several groups 
 of vertebrates (possibly in all) from the entoderm, which is never 
 metameric. At about the time of the differentiation of the somites 
 they migrate through the developing mesoderm to their definitive posi- 
 tion in the epithelium of the genital ridges, the primitive or primordial 
 ova (whether to form eggs or sperm) being recognizable from their 
 size and their reaction to microscopic stains (fig. 324, 0). In the 
 adults of many vertebrates the gonads at maturity project far into 
 the coelom and are often suspended by a fold of peritoneum which is 
 called a mesorchitmi in the male, a mesoarium in the female. 
 
320 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 Ovaries. — In the ovarian epithelium the primitive ova multiply, 
 and the products, accompanied by some of the epithelial cells, sink 
 into the deeper stroma of connective tissue, thus forming ovarial cords 
 each containing a number of ova. Then the cords break up and each 
 egg becomes surrounded with a layer of epithelial cells, the whole 
 forming a Graafian follicle, the follicle cells supplying nourishment 
 to the contained ovum. In the higher vertebrates there is a great 
 increase in the number of follicle cells, which become arranged in 
 several layers. Then a split arises in the follicle, the cavity becoming 
 
 
 Fig. 324. — Section of genital ridge of chick of five days' incubation, after Semon. e, epithel- 
 ium of ridge (coelomic wall) ; c, medullary cords; 0, primordial ova. 
 
 filled with a follicular liquor, while the ovum, surrounded by several 
 layers of cells, adheres to one side of the cavity, this part being called 
 the discus proligerus. 
 
 When the eggs have attained their full size and the proper time 
 has arrived some of the follicles migrate to the surface of the ovary, 
 then the follicles rupture and the contained ova escape into the coelom. 
 Their history from this point will be outlined in connection with the 
 genital ducts. Each ruptured follicle (at least in elasmobranchs, 
 amphibians and amniotes leaves a scar on the surface of the ovary — 
 the corpus luteum — characterized by the presence of peculiar (' lutein ') 
 cells. 
 
 Testes. — In the gonads of the male (testes) there is a somewhat 
 similar insinking of the primordial ova and epithelial cells into the 
 stroma of the genital ridge, but, instead of breaking up into separate 
 follicles, each sexual cord develops a lumen and becomes converted 
 
UROGENITAL SYSTEM. 32 1 
 
 into a seminiferous tubule, in the walls of which both the epithelial 
 cells and the primordial ova are recognizable, as well as a third kind 
 of cell, called Sertoli's cell, concerning which accounts are some- 
 what at variance, some regarding them as derivatives of the epithelial 
 cells, others as coming from the primitive germ cells. They play no 
 part in the actual formation of the spermatozoa, but act rather as 
 nutritive or 'nurse cells' for the developing spermatozoa. For the 
 differentiation of the germ cells into spermatozoa reference must be 
 made to the text-books of embryology and histology. In most verte- 
 brates the testes continue in the position where they first appear, but 
 in most mammals they eventually descend to a position outside of the 
 body cavity and are enclosed in a special pouch, the scrotum. This 
 descent of the testes is described in connection with the reproductive 
 organs of the mammals, below. 
 
 THE REPRODUCTIVE DUCTS. 
 
 The reproductive products formed in the gonads have to be carried 
 to the exterior, either as spermatozoa, or as eggs or young in different 
 stages of development, the ducts in the male being called vasa def er- 
 entia, those of the female being oviducts. The former are usually 
 the Wolffian ducts, the latter may be either the Mullerian ducts or 
 tubes developed for the special purpose, or lastly, the abdominal pores. 
 
 Male Ducts. — In elasmobranchs, amphibia and amniotes the 
 Wolffian ducts (fig. 321) serve as the outlet for the sperm. While 
 the seminiferous tubules are developing, there occurs a proliferation 
 of cells from the wall of the Bowman's capsules in the anterior end 
 of the mesonephros. These medullary cords extend through the 
 adjacent connective tissue and into the genital ridge where they come 
 into close connexion with the developing seminiferous tubules (fig. 
 324). When the latter acquire their lumen the medullary cords also 
 become canalized, so that both form a continuous transverse tubule 
 (vas efferens) leading from the genital cells to the Malpighian cor- 
 puscles, and thence by the mesonephric tubules to the Wolfl&an duct 
 (fig. 325, .4). These vasa efferentia become connected by a longi- 
 tudinal canal before entering the Wolfl&an body, while usually there 
 is another longitudinal canal connecting them in the body of the testis 
 (fig. 321, B). Usually this connexion of testis and Wolfl&an body 
 takes place at the anterior end of the mesonephros, but in some dipnoi 
 
322 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 the posterior end of the mesonephros alone is involved. This is fre- 
 quently accompanied by a degeneration of the glomeruli of the tubules 
 concerned, so that this part of the mesonephros loses its excretory 
 character and becomes subsidiary to reproduction. With this forma- 
 tion of vasa efferentia the sperm never enters the coelom except as this 
 is represented in the cavities of the mesonephric tubules. 
 
 As a farther result the anterior end of the Wolffian duct becomes 
 purely reproductive in the male and is usually greatly coiled, this 
 portion being called the epididymis. In the amniotes, where the 
 hinder portion of the mesonephros is supplanted by the true kidney 
 (metanephros) , the whole Wolffian duct is a sperm duct (vas deferens) 
 in the male, while in the female it largely or completely degenerates. 
 In the amphibia and elasmobranchs the hinder end of the duct is both 
 reproductive and excretory in the male; in the female it is purely 
 excretory. 
 
 In the ichthyopsida, other than elasmobranchs and amphibia, the 
 sperm is carried to the exterior in other ways, and there is no connexion 
 of the testes with the excretory organs. In the cyclostomes the sperm 
 escapes from the testes into the coelom and then is passed to the exterior 
 by way of the abdominal pores (p. 124) which in the lampreys open 
 into a cavity (sinus urogenitalis) which also receives the hinder ends 
 of the Wolffian ducts. In the myxinoids the pores are united and 
 open to the exterior behind the anus and between it and the urinary 
 openings. 
 
 The conditions found in the sturgeons (fig. 325, A) and in Polyp- 
 terus give a possible explanation to the aberrant structures of the tele- 
 osts. In the first group can be made out the vasa efferentia and the 
 two longitudinal canals connecting them, these extending the whole 
 length of the testis. In Polypterus (fig. 325, C) the connexion between 
 the testis and mesonephros is confined to the hinder portion of 
 organs, the anterior vasa efferentia and the longitudinal canal 
 disappearing in front, the longitudinal testicular canal taking the 
 sperm from the anterior end of the testis and carrying it farther back 
 for passage through the mesonephros. Here the anterior end of the 
 Wolffian duct is purely excretory. A farther concentration of the 
 efferent functions to the last vas efferens would give, with a few other 
 modifications, the conditions of the teleosts (fig. 325, -S). In all of 
 this group there is no connexion of testes with mesonephroi. The 
 seminiferous tubules are connected by a longitudinal canal (apparently 
 
UROGENITAL SYSTEM. 
 
 323 
 
 the longitudinal testicular canal of other vertebrates) which runs in 
 the membrane (mesorchium) supporting the testis, back to the external 
 opening, which is either directly to the exterior between the urinary 
 opening and the anus (fig. 328) or into a urogenital sinus (fig. 321, 5). 
 
 This view is farther supported by the relations in the dipnoi. In 
 Ceratodus there are numerous vasa efferentia which extend from the 
 testis into the mesonephros. In Lepidosiren the efferent ductules are 
 
 Fig. 325. — Diagrams of urogenital organs of male fishes, after Goodrich. Ay Acipenser 
 (Lepidosteus and Amia similar, but lack the oviduct); B, teleosts; C, Polypterus; D, Pro- 
 topterus; E, urogenital openings of female salmon, a, anus; ap, abdominal pore; c6, 
 cloacal ('urinary') bladder; e, vasa efferentia; gp, genital pore (papilla); m, mesonephros; 
 md, Mullerian(?) duct; r, rectum; re, renal corpuscle; s, urogenital sinus; t, testis; u, up, 
 urinary pore; ugp, urogenital pore; v, vas deferens; w, Wolfl&an duct. 
 
 fewer in number and they arise from a posterior degenerate portion 
 of the testis, while in Protopterus (fig. 325, D) there is but a single 
 vas efferens on either side and this passes through the posterior end 
 of the Wolffian body. 
 
 Oviducts. — In the elasmobranchs the Miillerian duct, which, as 
 described above, arises by a splitting of the pronephric duct, serves 
 as the oviduct. After separation from the Wolffian duct this opens 
 in front into the coelom by means of the pronephric tubules and their 
 
324 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 nephrostomes. Then these flow together, forming a large opening, 
 the ostium tubae abdominale, on either side (in elasmobranchs the 
 ostia of the two sides are usually united ventral to the liver) through 
 which the eggs, which pass from the ovary into the coelom are carried 
 into the oviduct. 
 
 In some amphibia (Salamandra) the pronephric tubules and neph- 
 rostomes take a part in the formation of the ostium tubae and the 
 beginning of the oviduct, while in Amhly stoma the ostium develops 
 in close connection with the pronephric nephrostomes. Here, as in 
 all other tetrapoda, the rest of the oviduct arises by the formation of a 
 groove of the peritoneal membrane close beside the Wolffian duct. 
 This becomes rolled into a tube, the Mullerian duct. In the amniotes 
 the anterior end of the groove does not close, but remains open as the 
 ostium tubae (fig. 321, ^). 
 
 Usually the condition in the elasmobranchs has been regarded as 
 the primitive one, a supposition which renders it difficult to homologize 
 the Mullerian ducts (oviducts) of elasmobranchs with those of other 
 forms. Still, when the adult conditions are considered — similar ostia, 
 similarity of position, and of external openings — it is hardly possible 
 to believe them as merely analogous, as examples of convergence. 
 The facts in the amphibia, referred to in the preceding paragraph 
 are additional evidence of homology. If, however, it be assumed 
 that the more common type of development, by the infolding of coe- 
 lomic epithelium, be the primitive condition, the difficulties are less, 
 though not entirely solved. Then, if it be that the homologous tissue 
 in the elasmobranchs was at first included in the tissue of the pro- 
 nephric duct and that the splitting is a secondary operation to separate 
 parts which elsewhere are always distinct, the similarities are more 
 apparent. 
 
 In the females, as in the males, of cyclostomes and teleosts the 
 reproductive ducts are not easily brought into harmony with those 
 of other vertebrates, and an answer to all questions cannot be had until 
 the development of the parts has been studied in more forms, and 
 especially the ganoids and dipnoi. In the cyclostomes the eggs are 
 shed from the ovaries into the coelom and are thence passed outward 
 through the abdominal pores. 
 
 In the teleosts there are several conditions. The ovaries may be 
 simple and solid bands or saccular in character with an internal lumen 
 (fig. 326, £). In the first the eggs pass into the coelom and thence 
 
UROGENITAL SYSTEM. 
 
 325 
 
 to the exterior by abdominal pores or by oviducts of varying lengths 
 (fig. 326, F). Concerning the nature of these ducts there is uncer- 
 tainty. They may be true Miillerian ducts or new formations within 
 the group. The fact that similar tubes occur, with permanently open 
 ostia in both sexes of the sturgeons (fig. 325), and that these open 
 
 Fig, 326. — Diagrams of urogenital organs of female fishes, after Goodrich. A, Pro- 
 topterus; B, Polypterus; C, Amia; D, Lepidosteus; E, most teleosts; F, salmonid. ap, ab- 
 dominal pore; c6, cloacal bladder; d, cloaca;/, funnel of oviduct; gp, genital pore or papilla; 
 w, mesonephros; o, ovary; od, oviduct; r, rectum; s, urogenital sinus; up, urinary pore, 
 (papilla) ; ugp, urogenital pore (papilla) ; w, Wolffian ducts. 
 
 behind into the Wolffian ducts, lends probability to the view that the 
 ducts of the ordinary teleosts are Miillerian in character, but greatly 
 modified. 
 
 The saccular condition of the ovaries appears to arise in two ways. 
 In the one the primitively free edge of the ovary bends laterally and 
 fuses with the coelomic wall, thus enclosing a cavity, the parovarial 
 
326 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 canal, closed in front. In the other type a groove of the covering 
 epithelium forms on the surface of the ovary. This closes over and 
 sinks inward, forming what is termed as an entovarial canal. Either 
 canal may extend backward to the hinder end of the body cavity, thus 
 forming an oviduct, or the oviduct may be formed from both kinds of 
 canals, one in front, the other behind. From this it would appear 
 that the ovary originally extended back to the hinder end of the ccelom 
 (as it does in Cyclopterus) or that the par- or entovarial canal had 
 united with a Miillerian duct which has otherwise been entirely lost. 
 The oviducts thus formed usually unite before opening to the exterior, 
 either directly ox via a urogenital sinus. The oviducts in the dipnoi 
 (fig. 326, A) are much like those of the selachians, emptying inde- 
 pendently into the cloaca. They persist, though of small size, in the 
 males (fig. 325, Z)). 
 
 EXCRETORY ORGANS IN THE SEPARATE GROUPS. 
 
 CYCLOSTOMES. — ^In the lampreys the pronephros extends over thirteen 
 ' somites, but only the anterior five form complete tubules, the remainder, however, 
 join the pronephric duct. The pronephros is best developed in the Ammocoete, 
 10 mm. long, and in this stage the mesonephros is also developed and both are 
 functional. With increase in size there is a degeneration of the mesonephric tubules 
 in front and a formation of new ones behind, the definitive organ extending over 
 about two-fifths of the body length. Each pronephros projects into the ccelom as 
 a band supported by a fold of the peritoneal membrane. The two pronephric ducts 
 unite a little in front of the hinder end, forming a urogenital sinus into which the 
 abdominal pores empty, and which, in turn, opens at the tip of a urogenital papilla 
 just behind the anus. 
 
 In the myxinoids the nephridial tubules develop as a continuous series, the 
 organ in the earliest stage known extending over somites 11-80. Later the organ 
 becomes divided into two parts by the degeneration of the intermediate tubules. 
 The anterior part projects into the body cavity and is provided with nephrostomes, 
 while the posterior part, reaching through some twenty or thirty somites, has its 
 tubules strictly segmental, each with a Malphigian body. This is the functional 
 excretory organ. 
 
 ELASMOBRANCHS. — ^The pronephros is never functional as an excretory 
 organ. The Wolffian bodies of the two sides are somewhat influenced in form by the 
 other viscera, and are sometimes asymmetrical. Usually the nephrostomes are closed 
 in the adult, but they persist in several genera, among them Acanthias, while they are 
 lacking in Scyllium and Rata. The anterior end of each mesonephros is narrowed 
 and serves as the connexion with the testes in the male, while the anterior end of 
 the Wolffian duct forms a much-coiled epidymis in the same sex. A urinary blad- 
 der is formed by the union of the ducts of the two sides. In the female the blad- 
 
UROGENITAL SYSTEM. 327 
 
 der opens to the exterior at the tip of a genital papilla, but in the male it connects 
 with a urogenital sinus, into which a pair of reservoirs of sperm empty. The duct 
 from the urogenital sinus opens into the cloaca at the tip of a urogenital papilla. 
 In Chimara the anterior end of the mesonephros lacks Malphigian bodies and forms 
 a large (Leydig's) gland, the secretion of which may possibly be used in dissolving 
 the spermatophores (fig. 331). 
 
 GANOIDS. — In Polypterus the pronephric tubules are two in number, belonging 
 to the second and fifth post-otic somites; in Lepidosteus there are five or six; sturgeon 
 six; and Amia eight to eleven. The large size of the pronephros in Polypterus is 
 due to the extensive coiling of the anterior end of the duct. In the sturgeon a part 
 of the excretory organ is separated from the rest but it is not certain that this is 
 really a pronephros. 
 
 The mesonephros is markedly segmental, the glands of the two sides being en- 
 larged and united behind in the sturgeon. Nephrostomes are late in appearance, not 
 being formed until after the tubules have joined the duct. The urinary bladder 
 differs from that of teleosts in that the Miillerian ducts enter it. 
 
 TELEOSTS (fig. 327) have a pronephros which extends over from one to five 
 somites. It is usually transitory in character, but it persists through life in several 
 species and functions during the larval stages in many more. The mesonephros 
 varies considerably in shape. Where there is an air bladder this covers some or all 
 of the ventral surface of the mesonephroi. Frequently the organs of the two sides 
 are united behind, while lobes may extend forward from the main mass, or back 
 into the tail. The duct is sometimes visible from below, sometimes it is immersed 
 in the mass of the organ. There is no sexual part to the mesonephros and there 
 are no nephrostomes in the adult. The urinary ducts of the two sides unite behind 
 and from the united portion and from the ventral wall of the cloaca the urinary 
 bladder is formed. Later the opening of the bladder separates from the cloaca and 
 usually comes to lie behind the anus, sometimes united with the sexual openings. 
 
 DIPNOI. — In Ceratodus there are two pronephric tubules, that of the third 
 somite being complete, that of the fourth rudimentary. The glomerulus lies beside 
 the open nephrostome. The mesonephros is at first strongly metameric. There 
 are no nephrostomes in the adult and none appear at any time in Lepidosiren. The 
 adult mesonephros is widest behind, but the relations of the efferent ductules of the 
 male are differently arranged in the separate genera, as mentioned above. 
 
 AMPHIBIA. — The pronephros (developing from two somites in the urodeles, 
 three in anura and twelve or more in gymnophiones) retains its functions in uro- 
 deles and anura until the metamorphosis, when its tubules degenerate. At first 
 the mesonephros consists of a tubule with nephrostome and renal corpuscle for each 
 somite, but in the adult this metamerism is lost, except at the anterior end, by the 
 development of secondary tubules, each complete like the original ones, the nephro- 
 stomes sometimes amounting to over a thousand on the ventral surface of each 
 Wolffian body. In the adult anura the nephrostomes lose their connexion with 
 the excretory system and join branches of the renal arteries, thus placing the coelom 
 in connexion with the circulatory system.. 
 
 In the urodeles the mesonephroi form a pair of ridges on the dorsal wall of the 
 coelom, but they occasionally project as folds. Their length is somewhat propor- 
 
328 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 tional to the total body length. The anterior end of each loses its excretory char- 
 acter and in the male becomes accessory to reproduction, as described above (p. 
 522). In the anura the organs are more compact and the differentiated anterior 
 end is lacking, though the efferent ductules of the testes pass through the organ. 
 The caecilians (fig. 334) resemble the urodeles, except in having the mesonephroi 
 more lobulated, the result of aggregates of tubules around the collecting tubules. 
 
 pes 
 
 pcd 
 
 Fig. 327 . — Urinary organs of teleosts, after Haller. A, pronephros and ducts of young 
 Salmo fario; B, excretory organs of adult perch, Percafiuviatilis; C, of carp, Cyprinus carpio; 
 a, aorta; cv, caudal vein; d, urinary duct; m, mn, mesonephros; pcd, pes, right and left 
 postcardinal veins; p, pn, pronephros; r, rectum; u, urinary bladder; w, ivd, Wolffian duct. 
 
 The Wolffian ducts are excretory in both sexes and are also reproductive in the 
 male. The ducts of the two sides open separately into the cloaca, with, usually in 
 the male, an enlargement, the seminal vesicle, which in the breeding season serves 
 as a reservoir for the sperm. The urinary bladder differs from that of the ichthy- 
 opsida in being ventral to the cloaca; it is of the allantoic type (p. 318). It is very 
 
UROGENITAL SYSTEM. 
 
 329 
 
 long in the caecilians (fig. 334) and Amphiuma, saccular in most urodeles, and bifid 
 at the tip in most anura, being even divided into two sacs, connected only at the 
 opening into the cloaca in some species. 
 
 SAUROPSIDA. — In reptiles and birds, as in all amniotes, the pronephros is 
 rudimentary at all stages and never functions as 
 an excretory organ. The mesonephros takes its 
 place in fcetal life, and in some it continues to 
 function for some time after hatching, but in all it 
 is eventually replaced by the metanephros, though 
 its degenerate remains persist in the reptiles (better 
 preserved in the female) forming the so-called 
 'golden yellow body.' Another part is retained 
 in the male as a part of the efferent ductules of 
 the testes, somewhat as in mammals. 
 
 The metanephros (fig. 328) never has the ex- 
 tent of the mesonephros of the ichthyopsida, but 
 it is usually restricted to the posterior half of the 
 body cavdty, often to the pelvic region. It is usu- 
 ally small and compact (snakes form an exception) 
 or somewhat lobulated, in the snakes the lobulation 
 sometimes being so extensive that the lobules are 
 only connected by the ureter. In the lizards the 
 
 sS?P^ 
 
 Fig. 
 
 Fig. 329. 
 
 Fig. 328. — Urogenital organs of Monitor, after Gegenbaur. d, opening of digestive 
 tract into cloaca; e, epididymis; k, kidney; />, papillae of urogenital system; r, rectum; 
 t, testes; u, ureter; vd, vas deferens. 
 
 Fig. 329. — Urogenital organs in pig embryo 67 mm. long, after Klaatsch. a, allantois; 
 g, gonad; ms, mt, meso- and metanephroi; sr, adrenal. 
 
 organs of the two sides may be connected behind. In the birds there are usually 
 three lobes in each mesonephros, these lying in cavities in the pelvis between 
 the sacral vertebrae and the transverse processes. 
 
330 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 The Wolffian ducts persist only as the ducts of the testes (vasa deferentia) and 
 the ureters take their place as carriers of the nitrogenous waste. These latter 
 tubes open separately into the cloaca. An (allantoic) urinary bladder is found 
 only in lizards and turtles (fig. 313). The urine is semisolid and consists largely 
 of uric acid. 
 
 MAMMALS. — In the mammals but two tubules are outlined in the 
 pronephros and these never become functional. The pronephric duct 
 is formed as a solid cord on the surface of the nephrotomic segments 
 which later becomes canalized. Of the fate of the pronephros 
 nothing certain is known. The mesonephros (fig. 329), on the other 
 
 hand, is an important structure in foetal 
 life, and in the monotremes and mar- 
 supials it continues to function in the im- 
 mature stages. Later it largely disap- 
 pears in all, with the exception of the 
 parts concerned in the formation of the 
 efferent ductules of the testes and some 
 inconsiderable remnants in both sexes. 
 Only in Echidna are nephrostomes 
 formed and in some rodents there is no 
 formation of glomeruli. 
 
 The peculiar development of the 
 mammalian metanephros (p. 316) results 
 in the kidney of the young stages having 
 a lobulated appearance, the lobules cor- 
 responding to the ducts given off from 
 the end of the ureter, so that each has 
 its own duct. This condition is retained 
 in the adult elephants, some ungulates, 
 carnivores (fig. 330) and primates, and especially in the aquatic 
 species (whales, seals), the lobules being most numerous in some of 
 the whales. In all other forms the ducts fuse later and the lobules 
 unite into a compact mass lying in the lumbar region near the last 
 rib. Each kidney has a peculiar shape (giving rise to the adjec 
 tive reniform), convex on the lateral, concave on the medial sur- 
 face, the latter being called the hiliim and receiving the excretory 
 duct (ureter) and the blood-vessels of the organ (hepatic artery 
 and vein). Just inside the hilum is a cavity, the pelvis of the 
 kidney, into which one or several papillae project, each bearing the 
 
 Fig. 330. — Lobulated kidney 
 (metanephros) of otter, Lutra cana- 
 densis (Princeton, 2234). a, aorta; 
 w, ureter; v, postcava. 
 
UROGENITAL SYSTEM. 33 1 
 
 openings of numerous collecting tubules (p. 309). In section the 
 substance of the kidney shows two different textures, recognizable 
 to the naked eye. There is an outer cortical and an inner medul- 
 lary substance, the two interlocking as a series of pyramids. These 
 different appearances are due to the fact that the cortex contains the 
 renal corpuscles and convoluted tubules, while the medulla is com- 
 posed of the straight tubules of Henle's loops and of the collecting 
 system. 
 
 The ureters are free for most of their course from the kidney to the 
 urinary bladder, into which they enter instead of the cloaca. The 
 bladder, in the monotremes and marsupials, is solely allantoic in 
 nature, but in the placental mammals a portion of the cloaca is also 
 included in it. From the bladder a single tube, the urethra, leads to 
 the exterior. The mammalian urine contains urea instead of uric 
 acid, a resemblance to the amphibia and a contrast to the sauropsida. 
 
 REPRODUCTIVE ORGANS OF THE SEPARATE GROUPS. 
 
 CYCLOSTOMES. — The gonads, which are usually unpaired, are supported by 
 a fold of the peritoneal membrane (mesorchium or mesovarium, p. 122). The eggs 
 and sperm escape into the coelom and are carried thence by way of the abdominal 
 pores. The myxinoids have hermaphroditic gonads, the anterior part being female, 
 the posterior testicular; but one sex predominates. Nansen believes that the sexes 
 alternate in function (proterandric hermaphroditism). The eggs of the petromy- 
 zonts are small, those of the myxinoids are larger and are enclosed in a horny shell, 
 with anchoring hooks at either end. 
 
 ELASMOBRANCHS. — In the elasmobranchs, as in all other vertebrates, the 
 gonads are at first paired and symmetrical, though occasionally one side or the other 
 may be reduced or become degenerate or those of the two sides may fuse. Thus in 
 some skates only the left gonad may be functional. Elsewhere in the group they are 
 paired and lie far forward, attached to the dorsal wall of the coelom. The Miillerian 
 ducts of the two sides in the female meet in front in a common opening (ostium 
 tubae), the derivative of the pronephric nephrostomes. This receives the eggs, 
 which pass from the ovaries into the coelom. The diflferent parts of the duct are 
 specialized, the upper part serving as a shell gland, forming the capsule for the 
 eggs. This is horny and in most species is provided with tendril prolongations at 
 the four corners, by which the eggs ('skate barrows') are attached to submerged 
 objects. Some species of both sharks and skates are viviparous. In these the 
 lower part of the Miillerian duct (oviduct) serves as a kind of uterus. In some 
 species the lining of this uterus is covered by vascular villi, by which nourishment 
 and oxygen are conveyed to the growing young which escapes in approximately the 
 perfect shape. The eggs of elasmobranchs are very large, those of some 
 species exceeding even those of the ostrich in size. 
 
332 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 The testes, supported by mesorchia, He at various levels in the coelom. The 
 relations of their ducts to the mesonephros are typical (p. 521). The vasa deferentia 
 of the two side unite just before entrance into the cloaca to form a urogenital sinus, 
 with which an oval sperm sac is connected on either side. In Chimara the genital 
 portion of the mesonephros (fig. 331) is 
 widely separated from the functional por- 
 tion, the two being connected by the 
 Wolffian duct. In the male the Miillerian 
 duct is rudimentary and frequently is with- 
 out a lumen. 
 
 GANOIDS.— Nothing is known of the 
 development of the sexual organs of the 
 ganoids, except as to the origin of the germ 
 cells in two species. In most species the 
 ovary is band-like and the oviducts open by 
 broad funnels into the coelom, but in Lepi- 
 dosteus the ovary is saccular, the eggs pass- 
 ing into the central cavity, the duct being 
 apparently a sterile, backward prolongation 
 of the ovary. In the male the testes are 
 frequently lobulated and a system of effer- 
 ent ductules, connected by a longitudinal 
 canal, pass from the testes into the meso- 
 nephros (fig. 325) and thence separately 
 or by a single tubule into the Wolffian 
 duct. In the males of all hut Lepidosteus 
 there are short tubes with funnels, appar- 
 ently the homologues of the oviducts of the 
 females. 
 
 TELEOSTS. — In some of the lower 
 teleosts (salmonids, etc.) the elongate ovary 
 is solid and the eggs pass from it into the 
 coelom and are carried thence to the exterior 
 by short peritoneal funnels (fig. 332), or the 
 tubes and funnels may be absent and 
 the eggs then pass out by abdominal pores, 
 is a closed sac (like that of Lepidosteus, fig. 326) continued behind by a slender 
 oviduct. The ducts of the two sides may open separately, but usually their hinder 
 ends are united and open by a single genital pore between the anus and the rectum 
 In some instances (fig. 325, E), the urinary and genital pores are on a urogenital 
 papilla. In the male the elongate testes are either simple or lobulated. Internally 
 each consists of radial chambers of varying shape which are connected with a 
 complicated system of tubules which lead to a vas deferens running back to open 
 into the hinder end of the Wolffian duct, or separately to the exterior (fig. 333, go). 
 
 In most teleosts the number of eggs produced in a season is very large, sometimes 
 numbering millions. Usually, after passing from the oviducts, they are left to the 
 
 Fig. 331. — Testis and anterior end 
 of mesonephros of Chimcera, after Par- 
 ker and Burland. bv, blood-vessel; 
 cvl, longitudinal tubule; m, MuUerian 
 duct; ms, anterior end of mesonephros 
 (Leydig's gland); spd, sperm duct; ve, 
 vet, vasa efferentia; vs, seminal vesicle. 
 
 In most teleosts, however, each ovary 
 
UROGENITAL SYSTEM. 
 
 333 
 
 mercy of the water, but a number of species (Embiotocids, Gambusia, several 
 Cyprinodonts, etc.) are viviparous, the development of the eggs taking place in the 
 ovary, which sometimes provides nourishment for the growing young. In the 
 lophobranchs the eggs are received in a pouch between the ventral fins of the male 
 and are incubated there. Other peculiar breeding habits are known. 
 
 Fig. 332. — Relations of oviducts and peri abdominales in Coregonus, after Weber. 
 a, anus; i, intestine; n, nephridial opening; o, ovar>'; p, pore of right side; r, opening of 
 oviduct. 
 
 DIPNOI.— In the dipnoi more normal conditions occur. There are oviducts 
 with inner ostia, resembling in structure, at least, the Miillerian ducts, and especially 
 those of the amphibia, like them secreting a gelatinous substance around the eggs. 
 These same ducts are also retained in the male Ceratodus and to a less extent in 
 the other genera {Lcpidosiren and Protopterus). The gonads are long and are cov- 
 
 FiG. 7,T,T,. — Hinder part of urogenital organs of male pike, Esox lucius, after Goodrich 
 a, anus; ab, air bladder; ao, aorta; d. Wolffian duct; c, cardinal vein; g, genital duct; go, 
 genital opening; i, intestine; pc, postcardinalvein;M6, urinary bladder; uo, luinary opening. 
 
 ered on the ventral side with lymphoid tissue. The testes in Protopterus and 
 Lepidosirm contain numerous alveoli lined with sperm-forming cells. The sperm is 
 carried into a longitudinal tubule (fig. 325) and from thence by one (Protopterus) 
 or several efferent ductules to the Malpighian bodies of the posterior end 
 of the mesonephros, the epididymis thus being posterior in position. In Ceratodus, 
 which is imperfectly known, the ductules are more numerous and the epididymis 
 is anterior. 
 
 AMPHIBIA. — The amphibians are the most typical of the anamnia, the elasmo- 
 
334 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 branchs excepted. The gonads are roughly correlated in form to the shape of the 
 body, being shortest in the anura, longest in the caecilians and urodeles. The 
 ovaries are saccular (a single long sac in urodeles, a number of short ones in anura) 
 and the eggs pass into the cavity and then break into the ccelom. The oviducts 
 are Miillerian ducts with ostia far forward. In the adults 
 they are greatly coiled and are glandular, their walls se- 
 creting the gelatinous substance which envelops the eggs. 
 Usually the oviducts of the two sides open separately into 
 the cloaca, but the two unite behind in Bufo. 
 
 The testes have both the longitudinal and the testicular 
 canals connecting the efferent ductules. In the gymno- 
 phiona (fig. 334) the testes resemble a string of beads, each 
 bead consisting of a number of seminiferous sacs, the 
 string being united by the testicular canal. The efferent 
 ducts pass through the mesonephros, sometimes utilizing 
 the nephridial tubules, sometimes pursuing a separate course, 
 the two conditions being found in different species of frog 
 (Rana) in Europe. Our species have not been studied in 
 this respect. 
 
 The cloaca of the urodeles has a glandular lining and in 
 the females it contains tubules which act as reservoirs of 
 sperm. In the male the glands secrete a substance binding 
 the spermatozoa together In many urodeles fertilization 
 is internal, though there is no intermittent organ save the 
 somewhat protrusible cloacal opening. 
 
 There are many interesting accessory reproductive rela- 
 tions among the amphibia. Thus the cajcilians and Am- 
 phiuma lay their eggs in long strings in the soil and the 
 female incubates them. The male often takes charge of the 
 eggs. In Pipa each egg undergoes development in a pit in 
 the skin of the back of the female and in Nototrema and 
 Opisthodelphys (South America tree-toads) there is a large 
 pocket in the skin of the back, opening near the coccyx, 
 where the eggs are carried until partially {Nototrema) or 
 entirely developed. Salamandra maculosa and S. atra bring 
 forth living young, the former being born with gills, the latter 
 in the perfect condition. Oviposition usually occurs in the 
 spring in colder climates (in the autumn with Cryptohranchus 
 of America) and as the drain on the system is very consider- 
 able immediately after hibernation, the substance of the 
 fat body, which always is closely connected with the gonads, 
 is utilized at this time. 
 SAUROPSIDA. — The birds and reptiles agree in the broader features of the 
 amniote urogenital system as outlined in the general account above. There is a 
 general correlation between the shape of the body and that of the gonads, and often 
 there is a lack of symmetry between the organs of the two sides Thus in snakes 
 
 fcJ 
 
 Fig. 334. — Male 
 urogenital organs of 
 Epicrium, after 
 Spengel. a, anus; b, 
 urinary bladder; cl, 
 cloaca;/, fat bodies; 
 m, Miillerian ducts; 
 mg, glandular part 
 of same; t, testes;//, 
 longitudinal testi- 
 cular canal; w, 
 Wolfl5an body. 
 
UROGENITAL SYSTEM. 
 
 335 
 
 and lizards the gonad of one side is in advance of the other, while in forms with 
 large eggs there is a marked tendency for one ovary to degenerate (right in birds) 
 the other alone being functional. 
 
 The oviducts, which are Miillerian ducts, are modified in accordance with the 
 peculiarites of the eggs. The upper portion is usually much coiled and glandular, 
 this part of the tube secreting the white, while parts farther toward the external 
 opening form the shell membrane and the shell. The walls are also somewhat 
 muscular, the muscles acting like constrictors to force the eggs along. The 
 
 Fig. 335. — Model of cloacal region of human embryo, 6.5 mm. long, after Keibel 
 a, allantois; c, cloaca; cm, cloacal membrane; k, outgrowth to form kidney and ureter; 
 r, rectum; u, where bladder will develop; wd, Wolffian duct. 
 
 mesonephros and the Wolfl5an duct are largely degenerate in the female, being 
 represented by rudiments between the oviduct and the vertebral column, best 
 developed in turtles and snakes. 
 
 The testes (figs. 313, 328) are short, round or oval in outline, and in birds one 
 is usually the larger, though both increase in size at the breeding season. The 
 Wolffian duct is solely reproductive (vas deferens), and its anterior, greatly coiled 
 end, together with the vasa ejGFerentia form the epididymis. Traces of the Miillerian 
 duct persist in the male sauropsida. There are several accessory reproductive 
 glands in the reptiles but little is known of their function. 
 
 MAMMALS. — In considering the urogenital structures of the mam- 
 mals the following parts are to be kept in mind : They are composed of 
 
336 COMPARATIVE MORIHOLOGY OF VERTEBRATES. 
 
 the embryonic excretory organs (mesonephroi) and their (Wolffian) 
 ducts; the permanent kidneys (metanephroi) and the ureter; the gonads; 
 the Mullerian ducts; the cloaca and the anlagen of the external genitalia, 
 which arise in the anterior or ventral wall of the urogenital sinus. 
 
 In the embryonic stages the Wolffian and Mullerian ducts and the 
 ureters open into the cloaca (fig. 335). Then a part of the latter, with 
 the openings of these ducts, is cut off to form the allantois, a portion 
 of which becomes the urinary bladder, this part receiving the ureters 
 
 Fig. 336. — Model of pelvic region of human embryo 25 mm. long, after Keibel. (Com- 
 pare with fig. 335.) a, anal opening; /, lateral ligament of uterus; w, Mullerian duct; o, 
 ovary; pu, primitive ureter (Wolffian duct); r, rectum; s, symphysis pubis; sg, septum of 
 genital protuberance; sug, urogenital sinus; w, ureter; uh, urinary bladder; ur, recto-uterine 
 excavation. 
 
 (except in monotremes) while the Wolffian and Mullerian ducts open 
 into the basal part of the allantoic outgrowth which is separated from 
 the bladder by a narrower stalk which becomes the urethra. This 
 part, into which the two pairs of ducts and the urethra empty, forms 
 the urogenital sinus (fig. 336, sng). With the formation of the per- 
 manent kidneys the mesonephros largely disappears (see p. 341) and 
 the same fate extends to one or the other pair of ducts, the Mullerian 
 largely disappearing in the male, the Wolffian in the female. The 
 parts which persist are more specialized than in any other group of 
 vertebrates, this being in part due to the fact that usually a large part 
 
UROGENITAL SYSTEM. 
 
 337 
 
 of the development of the young is passed inside the body of the 
 mother. 
 
 In their early stages the gonads arise anteriorly to the permanent 
 kidneys and they retain this position in the adult monotremes (fig. 
 337). In all others they are gradually carried farther posterior in the 
 abdominal cavity, so that they lie on the caudal side of the kidneys. 
 
 Fig. 337. — Urogenital organs of male Ornithorhynchus, after Gegenbaur. 6, bladder; 
 ep, epididymis; k, kidney opened, showing ends of collecting tubules; sr^ adrenal; sug 
 urogenital sinus; t, testis; ur, ureter; vd, vas deferens. 
 
 This transfer of position is effected by a rather complicated apparatus, 
 only the broader features of which can be outlined here. In the early 
 stages the membranes supporting the gonads (mesorchia, mesoaria) 
 are attached to the medial side of the double fold of the serous mem- 
 brane around the mesonephros. When the latter organ degenerates 
 the fold becomes the broad ligament of the female, while another 
 
^^S COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 fold continues down the genital ducts forming the ligament of the 
 ovary or testis. In the male broad ligament and ligamentum testis 
 together form the gubernaculum. Unequal growth of body and 
 these ligaments draws the gonads (except in the monotremes) farther 
 back into the pelvic region. 
 
 There is some variation in the ovaries. In the monotremes the left is larger 
 (cf. birds) and it is interesting to note that eggs have been found only in the left 
 oviduct. There is also some variation in shape in the marsupials. Elsewhere 
 the ovaries are relatively small (sometimes increasing in size at the breeding season), 
 rounded or oval and with the surface smooth or furrowed. 
 
 In male whales, elephants, some edentates, etc., the testes remain 
 permanently in the abdominal cavity. In all others a descent of the 
 testes occurs. By the same relative difference of growth of body 
 and gubernaculum the testes are drawn out of the abdomen into a 
 pouch (scrotum) — ^really a part of the body wall into which a part 
 of the coelom (bursa inguinalis) extends. The wall of this is formed 
 in part from the genital folds (see copulatory organs) which surround 
 the genital prominence. This scrotum is in front of the penis in the 
 marsupials, behind it in all placentals. When the canal connecting 
 the cavity of the bursa with the rest of the coelom remains open (mar- 
 supials, insectivores, rodents, bats, etc.) the descent is temporary, the 
 testes being withdrawn into the coelom at the close of the breeding 
 season by a ^cremaster muscle.' In other mammals the descent 
 is permanent, though in some species it does not occur until the time 
 of sexual maturity. 
 
 In the oviducts (Miillerian ducts) two regions can be recognized 
 in monotremes (figs. 338, 339, A), three in all other forms. The two 
 are the Fallopian tube, which opens into the body cavity by a broad, 
 fringed ostium tubae, and second the uterus, in which the egg is retained 
 for a part of its development. In the other mammals Fallopian tube 
 and uterus are retained, the latter being specialized for the longer 
 development of the young, and the third region is added — the vagina, 
 which receives the copulatory organ of the male. The vagina opens 
 into the urogenital sinus (fig. 339, B), but in the monotremes the 
 vagina is lacking and the uterus and the sinus are directly connected. 
 In the marsupials a vagina is developed for each Miillerian duct, and 
 in some there is a peculiar fusion of the ducts distal to the vaginae so 
 that a caecal pocket results, and in a few this pocket also connects with 
 the urogenital sinus, thus forming a third vagina (fig. 339, B). 
 
UROGENITAL SYSTEM. 
 
 339 
 
 In the placental mammals the posterior (vaginal) ends of the two 
 Miillerian ducts fuse in the median line, thus forming a single vagina. 
 In some the two uteri remain distinct, each having its own opening 
 (os uteri) into the vagina. This forms the uterus duplex (figs. 339, J5, 
 340, //), found in most rodents. In carnivores, ruminants, horse and 
 
 '*-•-.;.-. ,_i--- 
 
 FiG. 338. — ^Female genitalia of Echidna, after Owen, a, openings of ureters into, 
 ug, urogenital sinus; b, bladder, a bristle passing into urogenital sinus; c, cloaca; d, opening 
 of rectum into cloaca; o, ovar>', od, oviduct, the lower part uterine, r, rectum; w, ureters. 
 
 pig the fusion has been carried farther so that there is a single os uteri 
 and the two uteri are almost completely separated (uterus bipartitus, 
 fig. 340, ///) or the fusion is carried farther, the result being the' 
 uterus bicornis (fig. 339, C) in which the double nature is still shown 
 by the two pouches at the upper (anterior) end. Lastly, in the pri- 
 mates, the fusion of the two primitive uteri is complete, the result 
 being the uterus simplex (figs. 339, D; 340, III-VI), in which the 
 
340 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 double nature is shown only by the separate openings of the two 
 Fallopian tubes. 
 
 In the monotremes the primitive relation of urogenital sinus and 
 rectum — both emptying into the cloaca (figs. 338, 339, A) — persists 
 through. life, the result being a single external opening for the digestive 
 
 Fig. 339. — Uteri of {A) Ornithorhynchus; (B) Halmaturus; (C) sheep and (D) Inuus, 
 after Gegenbaur. b, bladder; bo, bursa ovarica; c, cornua uteri; d, cloaca; /, ligament of 
 ovary; o, ovary; od, oviduct (Fallopian tube); pi', processus vaginalis; sus, sug, urogenital 
 sinus; u, uterus; ur, ureter; v, vagina; vc, vaginal canals. 
 
 tract and the urogenital ducts, whence the name monotreme. In 
 all other mammals the cloaca becomes divided by a partition, the 
 perinaeum, between the urogenital and the rectal portions, there thus 
 being formed two external openings. However, in certain mammals, 
 as in marsupials and some rodents, both may be enclosed in a common 
 
UROGENITAL SYSTEM. 
 
 341 
 
 fold of integument (fig. 341) and in the former group may be provided 
 with a common sphincter muscle. 
 
 The testes are relatively small and the outer surface is smooth as 
 the result of the development around them of a fibrous envelope, the 
 tunica albuginea. This sends inward partitions (trabeculae) which 
 separate groups of seminiferous tubules into lobules. From the 
 
 Fig. 340. — Modifications of female urogenital structures in /, monotreme; //, 
 Orycteropus (uterus duplex); ///, many monodelphs (uterus bipartitus); /F, most mono- 
 delphs; V, Bradypus; VI, Dasypus; b, bladder; c, urinary canal, cu, urogenital sinus; g, 
 genital sinus; o, oviduct, u, uterus; v, vagina. 
 
 lobules the sperm is carried outward by numbers of small tubules, the 
 homologues of the efferent ductules of the lower vertebrates, and 
 like them connected together by vessels which correspond to the longi- 
 tudinal canals. The ductules empty into the anterior end of the 
 Wolffian duct, the upper end of which is greatly coiled, the coiled por- 
 tion and the ductules forming the epididymis. From the entrance 
 of the ductules to its entrance into the urogenital sinus or canal the 
 duct is called the vas deferens. From this point the urogenital canal 
 is provided with muscular walls and forms an ejaculatory duct. 
 
 In the female the Wolffian duct and the mesonephros are largely lost in the 
 adult, the mesonephros forming a small collection of tubules near the anterior 
 end of the ovary which are known as the parovarium. In the male the Miillerian 
 
342 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 duct is also largely lost, the lower portion sometimes persisting as a small blind 
 tubule imbedded in the prostate gland and known as the uterus masculinus. 
 
 In the testes between the tubules are small aggregates of cells known as inter- 
 stitial cells, which have recently been shown to be glands with internal secretion. 
 In man their products, which pass into the blood, apparently cause the assumption 
 of the secondary male characters — growth of hair on the face, change of voice, etc. 
 — at the time of puberty. There would also seem to be some analogous structure 
 in the ovary governing the development of female characteristics and controlling 
 some of the features of menstruation. 
 
 There are a number of accessory glands connected with the genital ducts, these 
 being usually better developed in the male than in the female. Only the more 
 
 Fig. 341 . — Diagram of male genitalia of beaver, Castor canadensis, after Weber, a, 
 anus; ag, anal gland; b, urinary bladder; gv, gland of vas deferens; oa, opening of anal gland; 
 op, OS penis; p, prostate; pp, preputial gland; r, rectum; u, ureter; vd, vas deferens. 
 
 prominent are mentioned here. The seminal vesicles (present in some rodents, 
 bats, insectivores and in ungulates and primates) are a pair of tubular or saccular 
 glands opening into the vasa deferentia just before their entrance into the urogenital 
 canal. The prostate glands, which occur in all placental mammals with the 
 exceptions of edentates and whales, are connected with the urogenital canal. 
 Farther along the canal are Cowper's glands which occur in almost all mammals 
 as scattered bodies or aggregated into larger masses, and surrounded by smooth 
 muscle. 
 
 Concerning the functions of these glands considerable uncertainty exists. 
 From the fact that removal of the prostate and the seminal vesicle in rats prevented 
 fertilization, and the further fact that the secretion of the seminal vesicles increases 
 the activity of the spermatozoa, it seems probable that they are of great importance 
 in connexion with fertilization. Then it has been shown that in some instances 
 the coagulation of the secretion of these glands closes the vagina after copulation 
 has occurred, thus preventing the exit of the sperm. 
 
 COPULATORY ORGANS. 
 
 In many vertebrates the eggs are fertilized after passing from the 
 oviducts. This is the case with the cyclostomes, most fishes, with 
 the exception of the elasmobranchs, and with many amphibians. In 
 
UROGENITAL SYSTEM. 
 
 343 
 
 Other groups fertilization is internal. In some cases the transfer of 
 the sperm from the male to the female is effected by the apposition 
 of the cloacae of the two sexes, but in others copulatory organs of an 
 intromittent character occur. These are formed on several plans and 
 are not- homologous throughout. 
 
 Fig, 342. — Hemipenes of Crotalus horridus, after J. Miiller. One hemipenis is 
 exserted, the other retracted but laid open, cl, cloaca; g, seminal groove; p, hemipenis; 
 r, rectum, rp, retractor muscle of hemipenis; u, ureter; vd, vas deferens (Wolffian duct)."*^ 
 
 In the male elasmobranchs the posterior or inner side of the 
 pelvic fins are specialized for this purpose. The metapterygium 
 (p. 116) and the basalia connected with it are more or less completely 
 separated from the rest and form the so-called clasper (*mixip- 
 terygium'). Each of these is grooved along its medial surface and 
 
344 
 
 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 when the two are inserted in the cloaca the grooves unite to form a 
 tube for the passage of the sperm. There is a large gland in the 
 clasper but its relation to copulation and fertilization is unknown. 
 
 In the snakes and lizards a second kind of structures occurs. In 
 the young there are developed behind the vent a pair of sacs presenting 
 the appearance of appendages. With farther growth these two 
 hemipenes are withdrawn into a sac opening into the hinder side of 
 
 Fig. 343. — Cloacal region of adult turtle {Emys lutaria), after von Moller. The 
 rectum and cloaca have been laid open from the dorsal surface and the urogenital sinus 
 exposed. From the opening of the sinus into the cloaca a seminal groove extends along 
 the ventral cloacal surface and can be cut off by a pair of folds (plicce urorectales) from the 
 cloacal cavity, av, anal vesicle; h, urinary bladder; o, opening of anal vesicle into cloaca; 
 p, penis, exserted; pu, plicae urorectales; r, rectum; sg, seminal groove; ug, urogenital 
 groove. 
 
 the cloaca. Each hemipenis bears a spiral groove for the passage of 
 the sperm. At the time of copulation these are everted through the 
 anus (fig. 342). 
 
 In all other aminotes the copulatory organs are formed from the 
 same anlage. The lower anterior wall of the cloaca is largely con- 
 cerned in this, the anterior cloacal lip being produced into a genital 
 prominence (fig. 336) which can be traced in many forms as the 
 clitoris of the female and the glans penis of the male. In the embryos 
 of the higher mammals it is surrounded by a pair of integumental 
 
UROGENITAL SYSTEM. 
 
 345 
 
 folds which develop into the labia of the genital opening in the female 
 while in the male they furnish a part of the scrotal envelope. 
 
 The most primitive type of the cloacal penis is found in the chel- 
 onians (fig. 343) and crocodiles, and slightly more developed in the 
 
 Fig. 344. — Ventral cloacal wall and penis of Rhea (schematized), after Boas, b, 
 blind sac;/, corpus fibrosum; g, seminal groove; g', its continuation along blind sac;o, 
 opening of blind sac. Mucous membrane dotted, seminal groove black. 
 
 ostriches and some of the aquatic birds. In these the ventral or 
 anterior wall of the cloaca and its lip become specialized by the develop- 
 ment in it of a longitudinal band of fibrous tissue, covered on the 
 cloacal side by cavernous tissue (containing large spaces, which on 
 
 Fig. 345. — Diagrams of male urogenitalia in /, monotreme; II, marsupials; and III; 
 monodelphs, after Weber, a, anus; b, bladder; c, cloaca; cc, corpus cavern osus urethra, 
 cp, Corp. cav. penis; cd, Cowper's gland; p, perinasum; pg, prostate gland; r, rectum; s, 
 symphysis pubis; /, testis; u, ureter; v, vas deferens; vg, vesicular gland; i/m, ventral muscles. 
 
 being filled with blood render the whole firm and enlarged — erectile 
 tissue). The cavernous tissue is marked by a longitudinal groove 
 through which the seminal fluid from the urogenital sinus runs. Be- 
 sides the enlargement caused by the filling of the cavernous tissue with 
 
346 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 blood, the whole structure, the distal end of which is free, can be 
 protruded from the cloaca and retracted by suitable muscles (fig. 344) . 
 In the monotremes (fig. 345, /) the penis is still cloacal in position 
 and the urogenital sinus still communicates with the cloacal cavity. 
 But the advance" is made that the groove of the sauropsida has been 
 converted into a tube which carries the urine as well as the sperm. 
 The whole structure can be protruded and retracted again into a 
 sheath formed from the loose mucous membrane of the cloaca. In 
 the other mammals the connection of the urogenital ducts with the 
 alimentary tract is lost and the cloaca disappears. In the lower 
 mammals (figs. 341, 345, //) the retractile condition is retained but 
 in the higher the organ is permanently external (fig. 345, ///). In 
 the marsupials the tip of the penis is frequently bifurcate, corresponding 
 to the two vaginae of the female. In many rodents (fig. 341, op)j 
 bats, many carnivores, whales and some of the primates a penis bone 
 is developed in the middle line of the intromittent organ. 
 
 HERMAPHRODITISM. 
 
 Individuals of either sex which have assumed some of the external 
 or secondary sexual characters of the other sex are sometimes spoken 
 of as hermaphrodites, especially in the case of mammals if the copu- 
 latory organs be concerned. This is not true hermaphroditism, which 
 consists in having both ovarian and testicular organs or tissues in the 
 same individual and as a consequence the ability to produce both eggs 
 and spermatozoa. There may be both kinds of tissue in the different 
 parts of the same gonad, or the two may be intermingled (ovotestis) 
 or the gonads of the two sides of the body may be of different sexes. 
 Both ovaries and testes may be functional at the same time, or one 
 may be functional at one time and the other at another (proterandric 
 hermaphroditism) . 
 
 There is an enormous literature dealing with the problem of the 
 determination of sex. Almost every conceivable possibility has been 
 invoked to account for fact that one individual is male and another 
 female — chance, multiple impregnation, difference in age of parents 
 or of eggs and spermatozoon, matters of temperature and nutrition, 
 etc. Within the last few years there has been a strong tendency to 
 regard the matter as determined at the time of impregnation of the 
 egg and to depend upon differences in chromosomes. 
 
UROGENITAL SYSTEM. 347 
 
 In the formation and maturation of spermatozoa and eggs a peculiar substance in 
 the nucleus— chromatin— becomes aggregated in small bodies called chromosomes, 
 the number of which in the mature genital products is half of that occurring in 
 the other cells of the body. In most species the number in the body cells is always 
 even and is therefore exactly divisible, but it was found that in certain insects there 
 were differences between the sexes, the male having an odd, the female an even 
 number. When the reduction division occurs, by which the chromosomes are 
 divided between the mature eggs or the spermatozoa (for details see cytological 
 works), the eggs would all have the same number of chromosomes while the 
 spermatozoa would be dimorphic, some having an odd and some an even 
 number of chromosomes. In other cases there is frequently one or more 
 chromosomes (idiochromosomes) which differ from the rest, and these are dis- 
 tributed in the same way at the reduction division. At the fertilization of the 
 egg there is an addition of the chromosomes of the spermatozoa to those of the 
 egg, consequently some of the eggs will have the odd number and some the 
 even number of chromosomes, this being perpetuated in all of the cells of the 
 resulting organism until the next reduction division. It would thus follow that 
 sex was determined at the time of fertilization of the egg. But this is difficult 
 to reconcile with the existence of hermaphroditism. 
 
 Another view, which better accords with the facts, is that sex is a matter of 
 Mendelian inheritance, the females in some instances being heterozygous, the 
 males homozygous; or these relations may be reversed In the first condition 
 the element of 'femaleness' dominates over the recessive 'maleness'. In such 
 cases it seems reasonable to suppose that the hermaphrodites are really 
 heterozygous females in which the normally recessive 'maleness' has become 
 equally potent with the female, while under ordinary conditions the matter of 
 sex is dependent upon the character of the chromosomes combined with the 
 Mendelian inheritance. 
 
 Among the cyclostomes there are occasional specimens of lam- 
 preys which have been regarded as hermaphroditic, but in the myx- 
 inoids this is the regular occurrence, the anterior end of the gonad 
 is male and the posterior female. One or the other of these is func- 
 tional, the animal being predominantly either male or female, and 
 some individuals are regarded as sterile. Nansen regards this as a 
 case of proterandric hermaphroditism. In the teleosts several species 
 of Serranus are regularly hermaphroditic as is Chrysophrys aurata, 
 while in several other species it is an occasional occurrence. Triton 
 tcBfiiatus is the only urodele in which it is reported, but in the anura it 
 is more common. Thus it is frequent in the frogs and occasional in 
 other genera. In the toads (Bufo) there is frequently a * Bidder's 
 organ' in front of the gonads which contains immature ova in the 
 male. Among the birds the phenomenon has been reported in the 
 chaflSnch. (The assumption of male plumage by female birds at the 
 
348 COMPARATIVE MOGPHOLOGY OF VERTEBRATES. 
 
 close of sexual life is not a case of hermaphroditism.) Among the 
 mammals the cases are extremely rare, but cases, apparently well 
 authenticated, have been reported in the goat, pig and man. 
 
 NUTRITION AND RESPIRATION OF THE EMBRYO— FOETAL 
 
 ENVELOPES. 
 
 In all vertebrates except the mammals there is enough nourish- 
 ment stored in the egg to carry the young through its development 
 up to the point where it hatches and shifts for itself. In the cyclo 
 stomes, dipnoi and amphibia this nourishment (food-yolk or deuto- 
 plasm) is soon enclosed in the body wall. In ganoids and teleosts, 
 where it is relatively larger in amount, it forms for a time a projecting 
 mass enclosed in a yolk sac, and this condition reaches its extreme in 
 the elasmobranchs and sauropsida. The yolk sac, in the fishes, is an 
 extension of the intestine and the body wall and is richly supplied by 
 vitelline arteries and veins which are derivatives of the omphalo- 
 mesenteric vessels (p. 276). In the sauropsida, owing to the develop- 
 ment of the amnion and the consequent separation of the non- 
 embryonic somatopleure from the yolk, the yolk sac is composed of 
 the splanchnopleure alone, but it has homologous blood-vessels. In 
 the mammals (monotremes excepted) the yolk is greatly reduced and 
 the yolk sac (here often called the umbilical vesicle) is vestigial in 
 character. 
 
 The vitelline vessels take the yolk and carry it into the body where 
 it is utilized in building the embryo, all of it being eventually metabo- 
 lized and used by the cells. The rich supply of capillary vessels in the 
 sac also forms an efficient respiratory apparatus. In the viviparous 
 sharks villi are developed on the oviducal lining and these afford a 
 means of exchange of gases with the embryo, and for getting rid of the 
 nitrogenous waste. It is a question how far there is a transfer of food 
 by the same means. In some species of Mustelus and Carcharias 
 the villi fit into depressions in the yolk sac, thus forming an analogue 
 to the placenta of the mammals — a vitelline placenta — though formed 
 in a greatly different manner. 
 
 The viviparous teleosts have saccular ovaries and the development 
 of the egg takes place in the cavity, the walls of which at the breeding 
 season become villous. In the viviparous Salamandra atra only one 
 egg develops and this leaves the mother in the adult shape. The 
 other eggs degenerate and are used as food by the one. There is also 
 
UROGENITAL SYSTEM. 349 
 
 a modification of the lining of the oviduct in this species which allows 
 some blood to escape and this gives additional nourishment. 
 
 In the amniotes the yolk sac reappears and there are in addition 
 
 Fig. 346. — Diagrams of the development of amnion and allantois. Upper figure earlier, 
 transverse section; lower later, longitudinal, a, amnion; al, alimentarj' canal; am. cav, 
 amniotic cavity; ch, beginning of chorion; 5, serosa; so, somatopleure ; ys, yolk stalk. 
 
350 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 two other embryonic structures which are peculiarly characteristic, 
 the aUantois and the amnion, to which reference has been made before. 
 The amnion arises as a fold of the somatic wall of the coelom in 
 front of and on. either side of the embryo. These folds extend upward 
 and then inward until they finally meet above the embryo, thus en- 
 closing it in an amniotic cavity. The folds fuse in the middle line and 
 then the two sides break through so that above the wall of the amniotic 
 cavity — the true amnion — there is a second cavity directly continuous 
 with the coelom, and this is bounded externally by the rest of the 
 amniotic fold, this part being called the serosa or false amnion. This 
 lies immediately beneath the vitelline membrane of the egg or its 
 equivalent, to which many different names have been given. 
 
 Little is known as to the phylogeny of the amnion, a structure without parallel 
 in the animal kingdom except in the scorpions, where one is formed in the same 
 way. Of course there is no genetic connexion between the two. It has been sug- 
 gested that in both groups there is a tendency for the embryo to sink into the yolk 
 and that the amnion is to prevent its being completely covered with this substance. 
 
 The homologue of the aUantois is found in the urinary bladder of 
 the amphibia. It is an outgrowth from the hinder end of the alimentary 
 tract and consists of a lining of entoderm, covered externally with the 
 splanchnic layer of the mesoderm — is purely splanchnopleuric — and 
 projects into the coelom. In its outgrowth it carries with it branches 
 of the hypogastric blood-vessels, now known as the allantoic arteries 
 and veins (usually but a single vein). As it develops, the distal 
 end of the aUantois swells into a large vesicle, connected with the 
 digestive tract by a slender stalk. The vesicle extends into the coelom 
 between the amnion and serosa and soon fuses with the serosa. The 
 terminal sac flattens and gradually extends until it encloses the whole 
 embryo and amniotic sac. 
 
 In the sauropsida the aUantois (and serosa) comes eventually to 
 lie just beneath the shell, and as the latter is porous and the aUantois 
 is very vascular, the latter is in position to act as the respiratory 
 apparatus of the growing young. The cavity of the aUantois, con- 
 nected by its stalk with the cloacal region, serves as the reservoir for 
 the urine. 
 
 While the embryo is increasing in other respects, the side walls of 
 the body gradually close in ventral to the embryo until they reach the 
 stalks of the yolk sac and the aUantois. In this way these structures 
 come to be connected with the body by a narrow cord, called in mam- 
 
UROGENITAL SYSTEM. 35 1 
 
 mals the umbilical ornavel cord, in which the blood-vessels run. In 
 the mammals there are several variations from the above account of the 
 development of the allantois, but they can be reconciled with the 
 typical condition in the sauropsida. There are also several other 
 variations and the relations of allantois to the other structures is more 
 complicated, but details and the many modifications must be ignored 
 here, only an outline of the broader features being given. 
 
 In the mammals there is the same fusion of allantois and serosa as 
 in the sauropsida, the fused area here being called the chorion. On 
 arrival in the uterus by way of the Fallopian tube, the egg becomes 
 implanted in the uterine wall, and a little later, with the development 
 of the chorion, villi are formed on the outer surface of the egg. These 
 are invaded by the chorionic blood-vessels and they branch and extend 
 into depressions or crypts in the walls of the uterus. The latter become 
 very vascular, the blood spaces of the maternal tissue enveloping the 
 villi with only the thinnest of walls between the vessels of the mother 
 and those of the young. (There is never any actual connexion between 
 the blood-vessel of parent and embryo and so blood corpuscles cannot 
 pass from one to the other. All that takes place is largely of the nature 
 of osmosis — solutions of gases, of nourishing substances and of nitrog- 
 enous waste passing from one to the other. There is difiSculty in 
 explaining the passage of proteids and fats.) This structure, consisting 
 of the allantoic derivatives of the embryo and the mucous lining of the 
 uterus, is known as the placenta. 
 
 In the monotremes and in most marsupials no placenta is formed, 
 but it has been recently shown that a true placenta occurs in a few of the 
 latter group. In other mammals a placenta always occurs, the struc- 
 tures presenting many forms, but these may be grouped under a few 
 heads. (It must be borne in mind that this classification is purely 
 morphological and does not necessarily imply close relations of the 
 species included or identity of method of formation.) 
 
 In many mammals, at the time of birth, the maternal and embryonic 
 parts of the placenta simply separate, only the latter passing away 
 with the young. These are called non-deciduate placentae. In 
 the others the union of the foetal and the maternal tissues is so intimate 
 that the inner surface of the uterus is included in the afterbirth. These 
 form the deciduate type. The non-deciduata include two divisions. 
 In the diffuse placentae (edentates, whales, perissodactyls, many 
 artiodactyls) the villi are distributed over the entire surface of the 
 
352 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
 
 chorion. In the cotyledonary placenta the villi are grouped in small 
 areas (cotyledons) with spaces of naked chorion between them. This 
 form is characteristic of the ruminants. The deciduate type includes 
 the zonary and the discoidal forms. In the zonary placenta (eden- 
 tates, sirenians, elephants, hyracoids and carnivores) the villi form a 
 girdle around the placental sac, the ends of the chorion being free from 
 them. In the discoidal forms (insectivores, rodents, bats, edentates, 
 primates) the villi are restricted to one side of the chorion. 
 
 ADRENAL ORGANS. 
 
 Under this heading are included two sets of structures, interrenals 
 and suprarenals, of uncertain morphology and function. The names 
 are given in allusion to the fact that they are usually closely associated 
 in position with the nephridial structures, though they have no other 
 relation to them. The two differ in structure and probably in function 
 and are very distinct in the lower vertebrates but in amphibia and 
 amniotes they are united in a common structure, the interrenals forming 
 the cortex, the suprarenals the medulla of the mammalian adrenals. 
 
 The interrenals arise from the coelomic epithelium but it is as yet 
 uncertain as to the details, some thinking that they are connected with 
 the pronephros, others with the mesonephric structures, while still others 
 regard them as distinct in origin. They are at first either isolated 
 clusters of cells or longer bands of cells near the dorsal margin of the 
 mesentery, sometimes bilaterally symmetrical and in the lower verte- 
 brates extending through the length of the cpelom. 
 
 The suprarenals find their anlage in the sympathetic ganglia, from 
 which certain cells early separate. Among these are peculiar cells 
 which are called chromafifin cells (chromaphile or phaeochrome 
 cells) because of their staining brown or yellow with chromic acid 
 salts. These usually are closely associated with the blood-vessels, 
 either the dorsal branches of the segmental arteries or the postcardinal 
 veins. 
 
 In the fishes the two organs are separate, the suprarenals often 
 being more or less metameric in character, and in close relations to 
 the vessels of the mesonephros. The interrenals form more compact 
 organs between the nephridia of the two sides. In all tetrapoda the 
 two organs are more closely associated, the tissues of the two being 
 mixed in the adults of the amphibia and reptiles, while in the mammals 
 
UROGENITAL SYSTEM. 353 
 
 the interrenal tissue is on the outer side of the adrenal organ, the su- 
 prarenal forming the inner portion. In the amphibia the adrenals are 
 closely connected with the mesonephroi, being attached to their 
 inner margins (urodeles) or to the ventral surface (anura). In the 
 reptiles they are lobulated structures near the gonads. In the mammals 
 they are more compact (often called suprarenals) and are placed at the 
 anterior end of the kidneys, often unsymmetrically. 
 
 Both organs are regarded as glands of internal secretion, their 
 product being passed directly into the blood. The secretion of the 
 medullary portion (suprarenal) of the mammals is adrenalin, an acti- 
 vator or hormone, which by its action on the muscular system causes 
 an increase in the blood pressure. Even less is known of the function 
 of the interrenal. Certain observ^ations render it probable that the 
 secretion of this is of value in destroying certain products of metabolism 
 which otherwise might be injurious to the organism. 
 
 2Z 
 
BIBLIOGRAPHY. 
 
 In this list of books and articles dealing with vertebrate morphology there have been in- 
 cluded only such titles as are likely to be accessible in the majority of the laboratories of 
 the countr}'. Hence citations are largely from the periodicals and society publications of 
 America and England and from the leading journals of the Continent. The student who 
 wishes to go farther into any subject will find additional references in the papers quoted 
 here and also in the works of Wiedersheim, Gegenbaur, Hertwig and others, while the cur- 
 rent papers are listed in the Anatomischer and Zoologischer Anzeigers. For economy of 
 space the titles have been abbreviated, but in such a way as to indicate something of the 
 character and contents of the work. 
 
 JOURNALS Ain) TRANSACTIONS. 
 
 Academy of Natural Sciences, Philadelphia, Proceedings. 
 
 American Naturalist. 
 
 American Journal of -Anatomy. 
 
 American Academy of Arts and Sciences, Proceedings. 
 
 Anatomical Record. 
 
 Anatomischer Anzeiger. 
 
 Anatomische Hefte. 
 
 Archiv fiir Anatomie und Physiologic, Anatomische Abtheilung. 
 
 Archiv fiir mikroscopische Anatomie. 
 
 Biological Bulletin. 
 
 Boston Society of Natural Histor}', Memoirs and Proceedings. 
 
 Ergebnisse der Anatomie und Entwicklxmgsgeschichte. 
 
 Jenaische Zeitschrift fiir Naturwissenschaften. 
 
 Journal of Anatomy and Physiology. 
 
 Journal of Comparative Neurology. 
 
 Journal of Morphologv'. 
 
 Mittheilungen aus der zoologischen Station zu Neapel. 
 
 Morphologische Arbeiten. 
 
 Morphologisches Jahrbuch. 
 
 Museum of Comparative Zoology. Bulletin. 
 
 Quarterly Journal of Microscopical Science. 
 
 Royal Society- of London, Philosophical Transactions. 
 
 Zeitschrift fiir wissenschaftliche Zoologie. 
 
 Zoologischer Anzeiger. 
 
 Zoologischer Jahrbiicher, Abteilung fiir Anatomie und Entwicklungsgeschichte. 
 
 Zoological Society of London, Proceedings and Transactions. 
 
 TEXT -BOOKS, MANUALS AND GENERAL WORKS. 
 
 Balfour: Treatise on comparative embr}-ology. 2 vols., London, 1880-82. 
 
 Barker: Anatomical Terminology. Philadelphia, 1907. (Contains nomenclature of 
 Basel Commission — 'BNA.'). 
 
 Bohm und Davidofif: Histolog\% trans, by Huber, Philadelphia. 
 
 Bronn's Klassen und Ordnungen des Thierreichs. 
 
 Works of John Samuel Budgett. Cambridge, 1907. (Mostly teleosts, dipnoi and am- 
 phibia.) 
 
 355 
 
356 BIBLIOGRAPHY. 
 
 Cambridge Natural History. 10 vols., London, 1895-1909. 
 
 Choronshitzky: Entstehung der Milz, Leber, Gallenblase, Pankreas, und Pfortader- 
 
 system bei verschiededen Wirbelthiere. Anat, Hefte, 13, 1910. 
 Dahlgren and Kepner: Principles of animal histology. N. Y., 1908. 
 Dohrn: Studien zur Urgeschichte des Wirbelthierkorpers. Mitth. zool. Sta. Neapel, 
 
 3—17, 1881-1904. 
 Festschrift zu yosten Geburtstage Rudolf Leuckarts. Leipzig, 1892. 
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 Huxley: Manual of the anatomy of vertebrated animals. N. Y., 1872. 
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 Lankester: A treatise on zoology. London, 1900 — (9 vols, published). 
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 1892. 
 McMurrich: Development of the human body. Philadelphia, 1904. 
 Oppel: Lehrbuch der vergleichenden mikroscopischen Anatomie der Wirbelthiere. 
 
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 Owen: Anatomy of vertebrates. 3 vols. London, 1866-68. 
 Parker: Course of instruction in zootomy. London, 1895. 
 Parker and Haswell: Text-book of Zoology. 2 vols. London, 1897. 
 Schneider: Lehrbuch der vergleichenden Histologic. Jena, 1902. 
 Stannius: Handbuch der Zootomie, zweiter Theil, Wirbelthiere. (Only fishes, amphibia 
 
 and reptiles published) Berlin, 1856. (Invaluable as summary of older work.) 
 Stohr: Text-book of histology, trans, by Lewis. Philadelphia, 1910. 
 Wiedersheim: Vergleichende Anatomie der Wirbeltiere; 7th edition, Jena, 1909. 
 
 (Abridgment trans, by Parker, London, 1908.) 
 Wilder: History of the human body. N. Y., 1909. 
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 Cyclostomes. 
 
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 ton and muscles. Jour. Morph., 17, 1901. 
 
 Cole: Papers on anatomy of Myxine: Trans. Roy. Socy. Edinburg, 1905-12. 
 
 Dean: Development of Bdellostoma. Quar. Jour. Micr. Sci., 40, 1897. 
 
 Howes: Afiinities, interrelationships and systematic position of marsipobranchs, Trans 
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GENERAL. 357 
 
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 1903. 
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 Balfour and Parker: Structure and development of Lepidosteus. Phil. Trans., 1882. 
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 Pollard: Anatomy and position of Polypterus. Zool. Jahrbuch, Abth. Anat., 5, 1892. 
 Rauther: Panzerwelse. Zool. Jahrb. Abt. Anat., 31, 1911. 
 
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 Amphibia. 
 
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 Abth. Anat., 10, 12, 16, 1897-1902. 
 Cope: Batrachia of North America. Bull. U. S. National Museum, 34, 1889. 
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 Holmes: Biolog}- of frog. N. Y., 1906, 
 
 Hoffmann: Amphibien, in Bronn's Klassen und Ordnung das Thierreiches. 
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358 BIBLIOGRAPHY. 
 
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 Reptiles. 
 
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 Birds. 
 
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 Mammals. 
 
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INTEGUMENT. 359 
 
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 Piersol: Human Anatomy, Philadelphia, 1907. 
 
 Reighard and Jennings: Anatomy of the cat. N. Y., 1901. 
 
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 CCELOM, ABDOMINAL PORES. 
 
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 Morph. Jahrb., 12, 1887. 
 
 INTEGUMENT. 
 
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 Goppert: Phylogenie der Wirbelthierkralle. Morph. Jahrb., 25, 1898. 
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360 BIBLIOGRAPHY. 
 
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 DERMAL SKELETON. 
 
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SKELETON. 361 
 
 SKELETON— GENERAL. 
 
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 1900. 
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 1867. 
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 Fishes. 
 
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362 BIBLIOGRAPHY. 
 
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 1892. 
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 Pollard: Suspension of jaws in fishes. Anat. Anz., 10, 1894. 
 
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 1884. 
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SKELETON. 363 
 
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 Amphibia. 
 
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 1896. 
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 Reptilia. 
 
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364 BIBLIOGRAPHY. 
 
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 Birds. 
 
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 Voit: Primordialcranium des Kaninchens. Anat. Hefte, 38, 1909. 
 
MUSCULAR SYSTEM. 365 
 
 Weiss: Entwicklung der Wirbelsaule der weissen Ratte. Zeitsch. wiss., Zool., 69, 1901. 
 Whitehead and Waddell: Development of human sternum. Am. Jour. Anat., 12, 191 1. 
 
 MUSCULAR SYSTEM. 
 
 AlUs: Cranial muscles of Amia. Jour. Morph., 12, 1897. 
 
 Avers and Jackson: Myolog\' of myxinoids. Jour. Morph., 17, 1901. 
 
 Blum: Schwanzmuskulatur des Menschen. Anat. Hefte, 4, 1894. 
 
 Braus: Entwicklung der Musculatur und periph. Nervensystem der Selachier. Morph. 
 
 Jahrb., 26, 27, 1898-9. 
 Bruner: Smooth facial muscles of Amphibia. Morph. Jahrb., 29, 1901. 
 Byrnes: Develop, limb muscles in amphibia. Jour. Morph., 14, 1897. 
 Chamock: Muscles of mastication and movements of skull in lacertilia. Zool. Jahrb., 
 
 Anat. .\bth., 18, 1903. 
 Coming: Entwicklung Kopf- und Extremitaten-Muskulatur bei ReptiUen. Morph. 
 
 Jahrb.. 2S, 1899. 
 Coming: Vergl. Anat. der Augenmuskulatur. Morph. Jahrb., 29, 1900. 
 Davidoff: ^'e^gl. Anat. der hinteren Gliedmassen der Fische. Morph. Jahrb., 5-6, 
 
 1879-So. 
 Driiner: Zungenbein-, Kiemenbogen- und Kehlkopfmuskeln der Urodelen. Zool. Jahrb. 
 
 Abth. Anat., 15, 1901; 19, 1904. 
 Edgeworth: Development of head muscles in Gallus. Quar. Jour. Micr. Sci., 51, 1907. 
 Edgeworth: Morpholog)^ of the cranial muscles of some vertebrates. Quar. Jour. Micr. 
 
 Sci. J 56, 191 1. 
 Fewkes: Myolog)' of Echidna. Bull. Essex Inst., 9, 1877. 
 Fiirbringer: Muskeln der schlangenahnlichen Saurien. Leipzig, 1870. 
 Fiirbringer: \ergl. Anat. der Schultermu skein. Urodeles, Jena. Zeitsch., 7, 1873; Anura, 
 
 1. c, 8, 1S74; Birds, 1. c, 36, 1902; Reptiles. Morph. Jahrb., i, 1876. 
 Fiirbringer: Muskulatur des Kopf der Cyclostomen. Jena. Zeitsch., 9, 1875. 
 Fiirbringer: Muskulatur des Vogelfliigels. Morph. Jahrb., 6, 1885. 
 Gadow: Bauchmuskeln der Reptilien. Morph. Jahrb., 7, 1881. 
 Gadow: Myologie der Extremitaten der Reptilien. Morph. Jahrb., 7, 1881. 
 Humphrey: Several papers on muscles of sharks, dipnoi and urodeles. Jour. Anat. and 
 
 Phys.', 1873. 
 Humphrey: Muscles of Lepidosiren (Protopterus) . Jour. Anat. and Phys., 6, 1872. 
 
 Muscles of Ceratodus, same vol. 
 Lamb: Eye-muscles in Acanthias. Am. Jour. Anat., i, 1902. 
 MacDowell: Myology of Anthropopithecus. Am. Jour. Anat., 10, 19 10. 
 McMurrich: Phylogeny of forearm flexors. Am. Jour. Anat., 2, 1903; of palmar mus- 
 culature, same vol. 
 Mall: Development of human diaphragm. Jour. Morph., 12, 1897. 
 Mall: Development of human diaphragm. Johns Hopkins Hosp. Bull., 12, 1901. 
 Marion: Mandibular and branchial muscles of elasmobranchs. Am. Nat., 39, 1905; 
 
 Tufts College Studies, 2, 1905. 
 Maurer: \'entral Rumpfmuskulatur der Urodelen. Morph. Jahrb., 17, 1892. 
 Mivart: Myology of Menopoma, Menobranchus, Chameleon. Proc. Zool. Socy. London, 
 
 1869-70. 
 
 Xeal: Development of hypoglossal musculature in Petromyzon and Squalus. Anat. Anz., 
 13, 1S97. 
 
 Ribbing: Armmuskulatur der Amphibien, Reptilien und Saugetiere. Zool. Jahrb. Abth. 
 
 Anat., II, 1907. 
 Ruge: Geschichtsmuskeln der HalbafiFen. Morph. Jahrb., 11, 1885. 
 Schufeldt: Myology of the raven. London, 1890. 
 Shufeldt: Anatomy (mostly muscles) of Geococcyx. Proc. Zool. Socy. London, 1886. 
 
366 BIBLIOGRAPHY. 
 
 Uskow: Entwicklung des Zwergfells, u. s. w. Arch. mikr. Anat., 32, 1883. 
 Wilder: Appendicular muscles of Necturus. Zool. Jahrb., Suppl. 15, 2 Ed., 1912. 
 
 ELECTRICAL ORGANS. 
 
 Ballowitz: Anatomic des Zitteraales. Arch. mikr. Anat., 50, 1897. 
 
 Ballowitz: Elektrischen Organe von Torpedo. Arch. mikr. Anat., 42, 1893 (Large 
 
 bibliography.) 
 Dahlgren and Silvester: Electric organ of Astroscopus. Anat. Anz., 29, 1906. 
 Ewart: Development of electric organ in skate. Phil. Trans., 179B, 1889. 
 
 NERVOUS SYSTEM. 
 
 Barker: The nervous system and its constituent neurones. N. Y. , 1899. 
 
 Edinger: Vorlesungen iiber den Bau der nervosen Centralorgane des Menschen und der 
 
 Tiere. 7th edit., 2 vols., 1904-8. 
 Johnston: Nervous system of vertebrates. Philadelphia, 1906. 
 Johnston: Morphology of vertebrate head from viewpoint of functional divisions of 
 
 nervous system. Jour. Comp. Neurol., 15, 1905. 
 Johnston: Central nervous system of Vertebrates. Ergebnisse und Fortschritte der 
 
 Zoologie, 2, 1910. 
 
 BRAIN AND SPINAL CORD. 
 
 Barnes: Development of posterior fissure of spinal cord. Proc. Am. Acad. A. and Sci., 
 
 1883-4. 
 Dejerine: Anatomic des centres nerveux. Paris, 1895. 
 Herrick: Morphology of forebrain in amphibia and reptiles. Jour. Comp. Neurol., 20, 
 
 1910. 
 Hill: Primary segments of vertebrate head. Zool. Jahrb., 13, 1899. 
 
 His: Allgemein. Morphologic des Gehirn. Arch. Anat. und Phys., Abth. Anat., 1892. 
 Johnston: Morphology of forebrain vesicle in vertebrates. Jour. Comp. Neurol., 19, 1909. 
 Johnston: Morphology of vert, head from point of division of nervous system. Jour. 
 
 Comp. Neurol. 15, 1905. 
 Johnston: Gehirn und Cranialnerven der Anamnier. Ergebnisse, 11, 1901. 
 McClure: Segmentation of primitive brain. Jour. Morph., 4, 1890. 
 Nakagawa: Origin of cerebral cortex and homology of optic lobe layers. Jour. Morph., 
 
 4, 1890. 
 Osborn: Origin of corpus callosum. Morph. Jahrb., 12, 1886. 
 Smith: Origin of corpus callosum. Trans. Linn. Socy., 7, 1897. 
 Tilney: Hypophysis cerebri. Memoirs Wistar Inst., 2, 191 1. 
 
 Cyclostomes and Fishes. 
 
 Ayers and Worthington: Finer anatomy of brain of Bdellostoma. Am. Jour. Anat., 8, 
 
 1908. 
 Bing und Burckhardt: Zentralnervensystem von Ceratodus. Anat. Anz., 25, 1904. 
 Burckhardt: Centralnervensystem von Protopterus. Berlin, 1892. 
 Chandler: Lymphoid structure above myelencephalon of Lepidosteus. • Univ. Calif. Pub. 
 
 Zool., 9, 191 1. 
 Cole: Cranial nerves of Chimaera. Trans. Roy. Socy. Edinb., 38, 1896. 
 Dammerman: Der Saccus vasculosus der Fische ein Tieforgane. Zeit. wiss. Zool., 96, 
 
 1910. 
 Franz: Das Mormyriden (brain). Zool. Jahrb., Abt. Anat., 32. 191 1. 
 Franz: Kleinhirn der Knochenfisrhe. Zool. Jahrb., Abt. Anat., 32, 191 1. 
 
NERVOUS SYSTEM. 367 
 
 Goronowitsch: Gehim und Cranialnerven von Acipenser. Morph. Jahrb., 13, 1888. 
 
 Haller: Bau der Wirbeltiergehixns. I, Salmo und Scyllium. Morph. Jahrb., 26, 1898. 
 
 Herrick: Brains of some American fresh water fishes. Jour. Comp. Neurol., i, 1891. 
 
 Herrick: Brain of certain ganoids. Jour. Comp. Neurol., i, 1891. 
 
 Johnston: Brain of Acipenser. Zool. Jahrb., 15, 1901, 
 
 Johnston: Brain of Petromyzon. Jour. Comp. Neurol,, 12, 1902. 
 
 Johnston: Telencephalon of selachians. Jour. Comp. Neurol., 21, 191 1. 
 
 Johnston: Olfactory lobes, forebrain and habenular tracts of Acipenser. Zool. Bull., i. 
 
 1898. 
 Johnston: Telencephalon of selachians. Jour. Comp. Neurol,, 21, 1911. 
 Johnston: Telencephalon of ganoids and teleosts. Jour. Comp. Neurol,, 21. 1911 
 Kappers: Teleost and selachian brain. Jour. Comp. Neurol., 16, 1906, 
 Kingsbury, Oblongata in fishes. Jour. Comp. Neurol. 7, 1897, 
 Locy: Contribution to structure and development of vertebrate head. Jour. Morph., 11, 
 
 1895, 
 
 Mayer: Gehim der Knochenfische. Arch. Anat. und Phys., 1882. 
 Mayser: Gehim der Knochenfische (Cyprinoids) . Zeit. wiss. Zool., 36, i88r. 
 Neal: Segmentation of nervous system in Acanthias. Bull, Mus. Comp. Zool., 31, 1898. 
 Nicholls: Reissner's Fibre. Anat. Anz., 40, 1912. 
 Sargent: Reissner's fibre. Bull. Mus, Comp. Zool., 45, 1904. 
 Sargent: Toms longitudinalis of teleost brain. Mark Anniv. Vol., 1904. 
 Waldschmidt : Centralnervensystem \md Gemchsorgane von Polyterus. Anat. Anz., 2, 
 1887. 
 
 Worthington: Brain and cranial nerves of Bdellostoma. Quar. Jour Micr. Sci., 49, 
 1905. 
 
 Amphibia. 
 
 Burckhardt: Him und Gemchsorgan von Triton und Ichthyophis. Zeitsch. wiss. Zool., 
 52, 1891. • 
 
 Fish: Central nervous system of Desmognathus. Jour. Morph., 5, 1895. 
 Fischer: Amphibiorum nudorum neurologiae specimen primus. Berlin, 1843. 
 Gage: Brain of Diemyctylus compared with Amia and Petromyzon. Wilder quarter- 
 century book, 1893. 
 
 Griggs: Early development of central nervous system in Amblystoma. Jour. Morph., 
 21, 1910. 
 
 Kingsbury: Brain of Necturus. Jour. Comp, Neurol., 5, 1895. 
 
 Kingsley and Thyng: Hypophysis in Amblystoma. Tufts Coll. Studies, i, 1904. 
 
 Osbom: Brain of Amphiuma. Proc. Acad. Nat. Sci., Philadelphia, 1883. 
 
 Osbom: Intemal stmcture of amphibian bra'n. Jour. Morph,, 2, 1888, 
 
 Waldschmidt: Nervensystem der Gymnophionen. Jena. Zeitsch,, 20, 18S6. 
 
 Reptilia. 
 
 Gisi: Gehirn von Hatteria [Sphenodon], Zool. Jahrb., Abth, Anat,, 25, 1907. 
 Haller: Bau des Wirbeltiergehims. II, Emys. Morph. Jahrb,, 28, 1900. 
 Herrick: Brain of certain reptiles. Jour, Comp. Neurol., i, 1891; see also vol. 3. 
 Herrick: Brain of alligator. Jour. Cincinnati Soc}-, Nat. Hist., 12, i8qo. 
 Humphrey: Brain of Chelydra. Jour, Comp. Neurol., 4, 1894. 
 Koppen: Anatomic des Eidechsensgehirn. Morph. Arbeiten, i. 
 Rabl-Riickhard: Centralnervensystem des Alligator. Zeitsch. wiss. Zoo!., 30. 
 Rabl-Riickhard: Gehim des Riesenschlange. Zeitsch. wiss. Zool., 58, 1894. 
 
368 BIBLIOGRAPHY. 
 
 Birds. 
 
 Bumm: Grosshirn der Vogel. Zeitsch. wiss. Zool., 38, 1893. 
 
 Kamon: Entwicklung des Gehirns des Hiinchens. Anat. Hefte, 30, 1906. 
 
 Streeter: Spinal cord of ostrich. Am. Jour. Anat., 3, 1903. 
 
 Turner: Avian brain. Jour. Comp. Neurol., i, 1891. 
 
 Mammals. 
 
 Bechterew: Leitungsbahnen im Gehirn und Riickenmark. Leipzig, 1898. 
 Gronberg: Untersuchungen an Gehirn von Erinaceus. Zool. Jahrb., 15, 1891. 
 Haller: Bau des Gehirn von Mus und Echidna. Morph. Jahrb., 28, 1900. 
 Herrick: Brain of rodents. Bull. Denison Univ., 6, 1891. 
 Landau: Das Katzenshirns. Morph. Jahrb., 38, 1908. 
 Sab in: Atlas of medulla and mid brain. Baltimore, 19 10, 
 Smith: Brain of foetal Omithorhynchus. Quar. Jour, Micr. Sci., 39, 1896. 
 Smith: Morphology of brain in mammals. Trans. Linn. Socy. London, Zool., 8, 1903. 
 Symington: Commissures in marsupialia and monotremes. Jour. Anat. and Phys., 27, 
 1892. 
 
 EPIPHYSIAL STRUCTURES. 
 
 Beard: Parietal eye of cyclostomes. Quar. Jour. Micr. Sci., 29, 1888. 
 
 Dendy: Devel. parietal eye, etc., in Sphenodon. Quar, Jour. Micr. Sci., 42, 1899. 
 
 Dexter: Development of paraphysis in fowl. Am. Jour. Anat., 2, 1902. 
 
 Eycleshymer: Paraphysis and epiphysis in Amblystoma. Anat. Anz., 7, 1892. 
 
 Gaupp: Zirbel, Parietalorgan und Paraphysis. Ergebnisse, 7, 1897. 
 
 Hill: Develop, of epiphysis in Coregonus. Jour, Morph., 5, 1891; of teleosts and Amia, 
 
 idem, 9, 1894, 
 Kingsbury: Encephalic evaginations in ganoids. Jour. Comp. Neurol., .7, 1897, 
 Minot: Morpholog}' of pineal region based on Acanthias, Am. Jour, Anat., i, 1901. 
 Nowikoff: Parietalauge von Saurien. Zeit, wiss. Zool,, 96, 19 10, 
 Reese: Develop, of paraphysis and epiphysis in alligator, Smithson. Misc. Coll., 
 
 54, 1910- 
 Ritter: Parietal eye in some lizards. Bull. Mus. Comp. Zkx)l., 20, 1891. 
 Spencer: Pineal eye in Lacertilia, Quar. Micr. Sci., 27, 1886. 
 Warren: Pineal region in Necturus. Am. Jour. Anat., 5, 1905. 
 Warren: Pineal region in reptiles. Am, Jour. Anat., 11, 191 1, 
 
 PERIPHERAL NERVES. 
 
 Allis: Cranial muscles and nerves of Amia. Jour, Morph., 12, 1897. 
 
 AUis: Cranial nerves in Scomber. Jour, Morph., 18, 1903. 
 
 Beard: Branchial sense organs and associated ganglia in ichthyopsida. Quar. Jour. 
 
 Micr. Sci., 25, 1885. 
 Bowers: Cranial nerves of Spelerpes. Proc. Amer. Acad., 36, 1901. 
 Braus: Innervation der paarigen Extremitaten bei Selachiern und Dipnoer. Jena. 
 
 Zeitsch., 31, 1898. 
 Brook: Bau des sympathet. Nervensystems der Saugetiere. Jour. Morph., 37, 1907; 
 
 38, 1908. 
 Brookover: Olfactory nerve, terminalis nerve and preoptic sympathetic in Amia. Jour. 
 
 Comp. Neurol,, 20, 1910; olfact. and terminalis in Amiurus, Idem, 21, 1911, 
 Carpenter: Develop, oculomotor and abducens nerves and ciliary ganglion in chick. 
 
 Bull. Mus. Comp. Zool., 48, 1906. - 
 
SENSE ORGANS. 369 
 
 Coghill: Cranial nerves of Amblystoma. Jour. Comp, Neurol., 12, 1902. 
 
 Cole: Cranial nerves of Chimaera. Trans.Roy. Socy., Edinburg, 38, 1896. 
 
 Cole: Cranial nerves of Gadus. Trans. Linn. Socy. London, ZooL, 7, 1898. 
 
 Fischer: Anat. Abhandl. iiber Perrennibranchiaten und Derotremen. Hamburg, 1854. 
 
 See also Amphib. Nudorum, etc., under Brain. 
 Gegenbaur: Kopfnerv-en von Hexanchus. Jena. Zeitsch., 6, 187 1. 
 
 Hammersten: Innervation der Bauchflossen bei Teleostiem. Morph. Jahrb., 42, 191 1. 
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 Herrick: Criteria of homology in peripheral nerv^ous system. Jour. Comp. Neurol., 19, 
 
 1909. 
 Herrick: Peripheral nervous system of bony fishes. Bull. U. S. Fish Commiss. for 1898. 
 Herrick: Nervus terminalis in frog. Jour. Comp. Neurol., 19, 1909. 
 Huber: Sympathetic nervous system. Jour. Comp. Neurol., 7, 1897. 
 Huber: Minute anat. of sympathetic ganglia. Jour. Morph., 16, 1899. ^ 
 
 Johnston: Cranial nerves of Petromyzonts. Jour. Comp. Neurol., 18, 1908. 
 Johnston: Cranial nerve components of Petromyzon. Morph. Jahrb., 34, 1905. 
 Kunz: Development of sympathetic in turties. Am. Jour. Anat., 11, 191 1; mammals and 
 
 birds. Jour. Comp. Neurol., 20, 1910; of amphibia, idem, 21, 1911; Evolution symp. 
 
 syst. in vertebrates, idem, 21, 1911. 
 Kupffer: Entwicklimgsgeschichte des Kopfes. Ergebnisse, 1895. 
 Kupffer: Development of cranial ner\'es. Jour. Comp. Neurol., i, 1891. 
 Landacre: Cranial ganglia in Amiurus. Jour, Comp. Neurol., 20, 1910. 
 Landacre: Epibranchial placodes of Lepidosteus and their relation to the cerebral ganglia. 
 
 Jour. Comp. Neurol., 22, 1912. 
 Locy: New cranial nerve in selachians. Mark Anniv. Vol., 1903. See also Anat. Anz., 
 
 26, 1905. 
 Lubosch: Nervus accesorius Willisii. Arch. mikr. Anat., 54, 1899. 
 
 Mayhoff: 'Monomorphe' Chiasma opticum der Pleuronectiden. Zool. Anz., 39, 1912. 
 McKebben: Nervous terminalis in Amphibia. Jour. Comp. Neurol., 21, 1911. 
 Neal: Development of ventral nerves in selachii. Mark Anniv. Vol., 1903. 
 Norris: Cranial nerves of Amphiuma. Jour. Comp. Neurol., 18, 1908. 
 Parker: Optic chia§ma in teleosts. Bull. Mus. Comp. Zool., 40, 1903. 
 Pinkus: Hirnnerven des Protopterus. Morph. Arbeiten, 4, 1894. 
 
 Prentiss: Development of hypoglossal ganglion in pig. Jour. Comp. Neurol., 20, 1910. 
 Punnett: Pelvic plexus and nervus collector in Mustelus. Phil. Trans. 192, B, 1900. 
 Sheldon: Nervus terminalis in carp. Jour. Comp. Neurol., 19, 1909. 
 Stannius: Peripherische Nervensystem der Fische. Rostock, 1849. 
 Streeter: Development of cranial and spinal nerves in occipital region of man. Am. Jour. 
 
 Anat., 4, 1904. 
 Strong: Cranial nerves of amphibia. Jour. Morph., 10, 1895. 
 
 SENSE ORGANS. 
 
 Okajima: Sinnesorgane von Onychodactylus. Zeit. wiss. Zool., 94, 1909. 
 Osawa: Sinnesorgane der Hatteria [Sphenodon]. Arch. mikr. Anat., 52, 1898. 
 Schwalbe: Lehrbuch der Anatomie der Sinnesorgane. Erlangen, 1883. 
 
 Dermal and Lateral Line Organs. 
 
 Allis: Lateral line system in Amia. Jour. Morph., 2, 1889. 
 Allis: Lateral sensory canals of Mustelus. Quar. Jour. Micr. Sci., 45, 1902. 
 Allis: Lateral canals of Polyodon. Zool. Jahrb., Abth. Anat., 17, 1903. 
 Ayers and Worthington: Skin end organs of trigeminal and lateralis nerves of Bdellos- 
 stoma. Am. Jour. Anat., 7, 1907. 
 
 24 
 
370 
 
 BIBLIOGRAPHY. 
 
 Beard: Segmental sense organs and associated ganglia in ichthyopsida. Quar. Jour. Micr. 
 
 Sci., 25, 1885. 
 Boll: Savi'schen Blaschen von Torpedo. Arch. Anat. und Phys., 1S75. 
 Clapp: Lateral line system of Batrachus. Jour. Morph., 15, 1899. 
 Collinge: Sensory canal system of ganoids. Quar. Jour. Micr. Sci., 36, 1894. 
 Ewart and Mitchell: Lateral sense organs of Elasmobranchs. Trans. Roy. Soc. Edinburg, 
 
 38, 1892, (See Garman, Bull. Mus. Comp. ZooL, 17, 1888.) 
 Ewart: Sensory canal of Laemargus. Trans. R. Socy., Edinb., 37, 1893. 
 Harrison: Entwicklung der Sinnesorgane der Seitenlinie der Amphibien. Arch. mikr. 
 
 Anat., 63, 1903. 
 Kingsbury: Lateral line system of American amphibia and comparisons with dipnoi. 
 
 Proc. Amer. Micros. Socy., 17, 1895. 
 Maurer: Die Epidermis and Hire Abkommlinge. Leipzig, 1895. 
 Moodie: Lateral line system in extinct amphibia. Jour. Morph., 19, 190S. 
 Morrill: Pectoral appendages of Prionotus. Jour. Morph., 11, 1895. 
 Munkert: Lorenzini'schen Ampullen. Anat. Anz., 19, 1901. 
 
 Parker: Function of lateral line system in fishes. Bull. Bureau of Fisheries, 24, 1905. 
 Pollard: Lateral line system in siluroids. Zool. Jahrb., 5, 1893. 
 Ritter: Eyes, integumental sense papillae and skin of Typhlogobius. Bull. Mus. Comp. 
 
 Zool., 24, 1893. 
 
 Taste. 
 
 Herrick: Phylogeny and morphol. position of terminal buds of fishes. Jour. Comp. 
 
 Neurol., 13, 1903. 
 Herrick: Organs and sense of taste in fishes. Bull. U. S. Fish Comm., 22, 1903. 
 Herrick: Terminal buds of fishes. Jour. Comp. Neurol., 13, 1903. 
 Schwalbe: Geschmacksorgane der Saugetiere. Arch. mikr. Anat., 4, 1868 
 Tuckerman: Gustatory organs of mammals. Jour. Morph., 2, 1888; 4, 1S90: 7, 1892. 
 
 SmelL 
 
 Anton: Jacobson'schen Organ und Nasenhohle der Cryptobranchiaten. Morph. Jahrb., 
 
 38, 1908. 
 Bawden: Nose and Jacobson's organ with reference to amphibia. Jour. Cqmp. Neurol., 
 
 4, 1894. 
 Berliner: Entwicklung des Geruchsorgane der Selachier. Arch, mikr, Anat., 60, 1902. 
 Blaue: Nasenschleimhaut bei Fischen und Amphibien. Arch. Anat. und Phys., Anat. 
 
 Abth., 1884. 
 Born: Nasenhohle und Thranennasengang der Amphibien. Morph. Jahrb., 2, 1876; der 
 
 Amnioten, ibid., 5, 1879; 7> 1882, 
 Disse: Riechschleimhaut und Riechnerv bei Wirbeltiere. Ergebnisse, 11, 1901. 
 Dogiel: Geruchsorgane bei Ganoiden, Knochenfische, und Amphibien. Arch. mikr. 
 
 Anat., 29, 1887. 
 Fischer: Nasenhohle und Tranengange der Amphisbaenen. Arch. mikr. Anat., 55, 1900. 
 Frets: Entwicklung der Nase bei Affen, Sangern und Menschen. Morph. Jahrb., 44, 
 
 1912. 
 Gaupp: Nervenversorgen der Mund- und Nasenhohlendriisen der Wirbeltiere. Morph. 
 
 Jahrb., 14, 1888. 
 Gegenbaur: Nasenmuscheln der Vogel. Jena. Zeitsch., 7, 1873. 
 Holm: Develop. Olfactory organ in Teleosts. Morph. Jahrb., 21, 1894. 
 McCallum: Nasal region in Eutasnia. Proc. Canadian Inst., i, 1883. 
 Peter: Entwicklung d. Geruchsorgane. Ergebnisse, 20, 191 1. 
 Read: Olfactory apparatus in dog, cat and man. Am. Jour. Anat., 8, 190S. 
 Schaeffer: Lateral wall of cavum nasi in man. Jour. Morph., 21, 1910. 
 
SENSE ORGANS. 37 1 
 
 Seydel: Nasenhohle und Jacobson'sche Organ der Amphibien. Morph. Jahrb., 23, 
 
 1895. 
 Strong: Olfactory organ and smell in birds. Jour. Morph., 22, 191 1. 
 Wilder: Xasengegend von Menopoma [Cryptobranchus] und Amphiuma. Zool. 
 
 Jahrb., Abth. Anat., 5, 1892. 
 Wilder: Lateral nasal glands of Amphiuma. Jour. Morph., 20, 1909. 
 Zukerkandl: Jacobson'sche Organs. Ergebnisse, 18, 1910. 
 Zukerkandl: Jacobsonsorgane und Riechlappen der Amphibien. Anat. Hefte, 41, 1910. 
 
 Eyes. 
 
 Bage: Retina of lateral eyes of Sphenodon. Quar. Jour. Mic. Sci., 57, 1912. 
 
 Berger: Sehorgane der Fische. Morph. Jahrb., 8, 1882. 
 
 Brauer: Augen der Tiefseefische. Verhandl. deutsch. zool. Gesellsch., 1902. 
 
 Carriere: Sehorgane der Thiere. Mlinchen, 1885. 
 
 Coming: Anatomie der Augenmuskulatur. Morph. Jahrb., 29, 1900, 
 
 Eggeling: Augenlider der Saugetiere. Jena. Zeitsch., 39, 1904. 
 
 Eigenmann: Eyes of blind vertebrates. Biol. Bull., 2, 1900; 5, 1903. 
 
 Eigenmann: Eyes of Amblyopsidae. Arch. Entw. Mechan., 7, 1899. 
 
 Eycleshymer: Development of optic vesicles in amphibia. Jour. Morph., 8, 1893. 
 
 Eycleshymer: Development of optic vesicles in Amphibia. Jour. Morph., 8, 1890. 
 
 Jelgersma: Ursprung des Wirbeltierauges, Morph. Jahrb., 35, 1906. 
 
 Lamb: Development of eye muscles of Acanthias. Am. Jour. Anat., i, 1901. 
 
 Locy: Optic vesicles of elasmobranchs and tiieir relations to other structures. Jour. 
 
 Morph., 9, 1894. 
 Mall: Histogenesis of retina. Jour. Morph., 8, 1893. 
 Peters: Harder'schen Driise. Arch. mikr. Anat., 36, 1890. 
 Rabl: Bau und Entwicklung der Linse. Zeitsch. wiss. Zool., 63, 1898; 65, 1898; 67, 
 
 1899. 
 
 Robinson: Formation and structure of optic ner^e and its relation to optic stalk. Jour. 
 
 Anat. and Phys., 30, 1896. 
 Schaeffer: Develop, nasolacrimal passages in man. Am. Jour. Anat., 13, 1912. 
 Studnicka: Sehnerven der Wirbeltiere. Jena. Zeitsch., 31, 1897, 
 W>ysse: Histogenesis of retina. Am. Nat., 40, 1906. 
 Williams: Migration of eye in Pseudopleuronecetes. Bull. Mus. Comp. Zool., 40, 1902. 
 
 Ears. 
 
 Ayers: Vertebrate cephalogenesis (large bibliography). Jour. Morph., 6, 1892. 
 
 Ayers: Relations of hair cells of ear. Jour. Morph., 8, 1893. 
 
 Bridge and Haddon: Air bladder and Weberian ossicles of siluroids. Phil. Trans., 184, 
 1893. 
 
 Druner: Anatomie und Entwicklung des Mittelohres beim Menschen und Maus. Anat. 
 Anz., 24, 1904. 
 
 Gaupp: Schalleitenden Apparatus bei Wirbeltiere. Ergebnisse, 8, 1898. 
 
 Kingsbury: Columella auris and nervous facialis. Jour. Comp. Neurol., 13, 1903. 
 
 Kingsley: Ossicula auditus. Tufts College Studies, i, 1900. 
 
 Mall: Development Eustachian tube, middle ear, etc., of chick. Studies Biol. Lab. 
 
 Johns Hopkins, 4, 1887. 
 Norris: Development of auditory vesicle in Amblystoma. Jour. Morph., 7, 1892. 
 Okajima: Entwicklung d. Gehororganes von Hydnobius. Anat. Hefte, 45, 1911. 
 Parker: Hearing and allied senses in fishes. Bull. U. S. Fish. Comm. for 1902, 1903. 
 
 See also Am. Nat., 37, 1903. 
 Streeter: Development of labyrinth and acoustic and facial nerves in human embr}'o. 
 
 Am. Jour. Anat., 6, 1907. 
 
372 BIBLIOGRAPHY. 
 
 Versluys: Mittlere und aussere Ohrsphare der Lacertilier. ZodI. Jahrb. Abth. Anat., 12, 
 
 1898. 
 Willy: Development of ear and accessory organs in frog. Quar. Jour. Micros. Sci., 30, 
 
 1890. 
 
 ALIMENTARY CANAL. 
 Teeth. 
 
 Beard: Teeth of marsipobranchs. Zool. Jahrb., 3, 1889, 
 
 Burckhardt: Gebiss der Sauropsiden. Morph. Arbeiten, 5, 1895. 
 
 Cope: Tritubercular molar in human dentition. Jour. Morph., 2, 1888. 
 
 Harrison: Development and succession of teeth in Hatteria [Sphenodon]. Quart. Jour, 
 
 Micr. Sci., 44, 1901. 
 Hertw^ig: Zahnsystem der Amphibien und seine Bedeutung fiir den Genese des Skelettes 
 
 der Mundhohle. Arch. mikr. Anat., 9, 1874. 
 Kiikenthal: Urspring und Entwicklung der Saugetierzahne. Jena. Zeitsch., 26, 1892. 
 Laaser: Entw. der Zahnleiste der Selachier. Anat. Anz., 17, 1900. 
 Leche: Entwicklung des Zahnsystem der Sauger. Morph. Jahrb., 19, 1892, 
 Oppel: Verdauungsapparat. Ergebnisse, 13, 1903: 14, 1904: 16, 1906. 
 Osborn: Evolution of mammalian molars to and from tritubercular type. Am. Nat., 22, 
 
 1888. 
 Osborn: Succession of teeth in mammals. Am. Nat., 27, 1893. 
 Osborn: Trituberculy. Am. Nat., 31, 1897. 
 
 Poulton: Teeth and horny plates of Ornithorhynchus. Quar, Jour. Micr. Sci., 29, 1888. 
 Rose: Entwicklung der Zahne des Menschen. Arch, mikr. Anat,, 38, 1891. 
 Rose: Zahnleiste und Eischwiele der Sauropsiden. Anat. Anz., 7, 1892, 
 Rose: Phylogenese des Saugetiergebisses, Biol. Centralblatt., 12, 1892. 
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 Tomes: Manual of Dental Anatomy. Philadelphia, 1898. 
 de Terra: Vergl. Anatomie menschlichen Gebisses und der Zahne der Vertebraten. 
 
 Jena, 1911. 
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 Wilson: Tooth development of Ornithorhynchus. Quar. Jour, Micr. Sci., 51, 1907. 
 
 Mouth and Tongue. 
 
 Flint: Submaxillary gland. Am. Jour, Anat., 2, 1903. 
 Gegenbaur: Unterzunge der Saugethiere, Morph. Jahrb., 9, 1884. 
 Gegenbaur: Phylogenese der Zunge. Morph. Jahrb., 11, 1886; 21, 1894. 
 Gegenbaur: Gaumenfalten des Menschen. Morph, Jahrb., 4, 1878. 
 Hammar: Entwicklung der Zunge und Speicheldriisen. Anat. Anz., 19, 1901. 
 Heidrich: Mund und Schlundkopf hohle der Vogel und ihre Driisen. Morph. Jahrb., 37, 
 
 1907. 
 Kallius: Entwicklung der Zunge. Anat. Hefte, 16, 1901; 28, 1905. 
 Kallius: Entwicklung der Zunge. Anat. Hefte, 41, 1910. 
 Maurer: Blutgefasse im Epithel. Morph, Jahrb., 25, 1887, 
 Oeder: Munddriisen und Zahnleiste der Anuren, Jena, Zeitsch,, 41, 1906. 
 Pawlowsky: Giftdriisen einiger Scorpaeniden. Zool. Jahrb,, Abt. Anat., 31, 191 1. 
 Poulton: Tongue of Perameles. Quar. Jour. Micr. Sci., 23, 1883, 
 Reichel: Mundhohldriisen der Wirbeltiere, Morph. Jahrb., 7, 1882. 
 Wiedersheim: Kopfdriisen der Amphibien. Zeit. wiss. Zool., 28, 1877, 
 
 Thyreoid Glands, Etc. 
 
 Erdheim: Kiemenderivate bei Ratte, Kaninchen und Igel, Anat. Anz., 29, 1906. 
 
ALIMENTARY CANAL. 373 
 
 Greil: Kiemendarmderivate von Ceratodus. Anat. Anz. Erganz. Hefte, 29, 1906. 
 Ferguson: Th\Teoid in Elasmobranchs. Am. Jour. Anat., 11, 1911. 
 Gudematsch: Thyreoid of Teleosts. Jour. Morph., 21, 1911. 
 Hammar: Elasmobranch Thymus. Zool. Jahrb., Abt. Anat., 32, 1911. 
 Johnstone: Thymus in marsupials. Jour. Linn. Socy., London, Zool., 26, 1898. 
 Kastschenko: Schicksal d. embryon. Schlundspalten bei Saugetieren. Arch. mikr. 
 
 Anat., 30, 1887. 
 Marcus: Schlundspaltgebiet der Gymnophionen. Arch. mikr. Anat., 71, 1908. 
 Maurer: Schilddruse, Thymus und Kiemenreste der Amphibien. Morph. Jahrb., 13, 
 
 1887. 
 Xorris: Ventraler Kiemenreste and Corpus propericardiale of the frog. Anat. Anz., 21, 
 
 1902. - 
 Piatt: Development of Thyroid and suprapericardial bodies in Necturus. Anat. Anz., 
 
 II, 1896. 
 Rabl: Anlage der ultimobranchialen Korper bei Vogel. Arch. mikr. Anat., 70, 1907. 
 SchaflFer: Schilddruse von Myxine. Anat. Anz., 28, 1906. 
 
 Soderlund imd Bachman: Studien liber Thymusinvolution. Arch. mikr. Anat., 73, 1909. 
 Stockard: Development of thyreoid in Bdellostoma. Anat. Anz., 29, 1906. 
 Zuckerkandl: Entwicklung der Schilddruse und Thymus bei der Ratte. Anat. Hefte, 21, 
 
 1903. 
 
 Digestive Tract. 
 
 Boas: Magen der Cameliden. Morph. Jahrb., 16, 1890. 
 
 Brachet: Developpement du foie et pancreas de TAmmocoetes. Anat. Anz,, 13, 1897. 
 
 Bensley: Pancreas of guinea pig. Am. Jour. Anat., 12, 1911. 
 
 Braun: Pancreas bei Alytes. Morph. Jahrb., 36, 1906. 
 
 Claypole: Enteron of lamprey. Proc. Am. Micros. Socy., 1894. 
 
 Choronshitzky: Entstehimg der Milz, Leber, GaUenblase, Bauchspeicheldriise und 
 
 Pfortadersystem bei verschieden. Wirbeltiere. Anat. Hefte, 13, 1900. 
 Eggeling: Dunndarmrelief und Emahrung bei Knochenfischen. Jena. Zeitsch., 43, 1907. 
 Goppert: Entwicklung des Pancreas bei Knochenfischen. Morph. Jahrb., 20, 1893. 
 Gadow: Verdauungssystems der Vogel. Jena. Zeitsch., 13, 1879. 
 Helbling: Darm einiger Selachier. Anat. Anz., 22, 1903. 
 Helly: Pancreasentwicklung der Saugetiere. Arch. mikr. Anat., 67, 1901. 
 Howes: Intestinal canal of Ichthyopsida. Jour. Linn. Socy. London, Zool., 23, 1890. 
 Johnston: Limit between ectoderm and entoderm in mouth of amphibia. Am. Jour. 
 
 Anat., 10, 1910. 
 Jungklaus: Magen der Cetaceen. Jena. Zeitsch., 32, 1898. 
 
 Kerr: Development of alimentary tract in Lepidosiren. Quar. Jour. Micr. Sci., 54, 1910. 
 Killian: Bursa und Tonsilla phar}'ngea. Morph. Jahrb., 14, 1888. 
 Kingsbury: Enteron of Necturus. Proc. Am. Micros., 1894. 
 Lewis and Thyng: Intestinal diverticula in embr}-o5 of pig, rabbit and man. Am. Jour. 
 
 Anat., 7, 1908. 
 
 Mayr: Entwicklung des Pancreas bei Selachier. Anat. Hefte, 8, 1897. 
 
 Mayer: Spiraldarm der Selachier. Neap. Mittheil., 12, 1897. 
 
 Oppel: Verdauungsapparat. Ergebnisse, 7, 1897. 
 
 Osawa: Eingeweiden der Hatteria [Sphenodon]. Arch. mikr. Anat., 49, 1897. 
 
 Parker: Spiral valve in Raia. Trans. Zool. Socy. London, 11, 1880. 
 
 Piper: Entwicklung von Magen, Duodenum, Sch'w-immblase, Leber, Pancreas und ^lilz 
 
 bei Amia. Arch. Anat. und Physiol., 1902. 
 Pohlman: Development of cloaca in human embryos. Am. Jour. Anat., 12, 1911. 
 Rex: Morphologie der Saugerleber. Morph, Jahrb., 14, 1888. 
 
374 BIBLIOGRAPHY. 
 
 Rlickert: Entwicklung des Spiraldarmes bei Selachiern. Arch. f. Entwick. mechan. 
 
 4, 1896. 
 Segeratsrale: Teleostierleber. Anat. Hefte, 41, 1910. 
 
 Stieda: Bau und Entwicklung der Bursa Fabricii. Zeit. wiss. Zool., 34, 1880. 
 Stohr: Entwicklung der Hypochorda und dorsal Pancreas bei Rana. Morph. Jahrb., 23, 
 
 1895. 
 Teichmann: Kropf der Tauben. Arch. mikr. Anat., 34, 1889. 
 
 Thyng: Pancreas in embryos of pig, rabbit, cat and man. Am. Jour. Anat., 7, 1908. 
 Volker: Entwicklung des Pancreas bei den Amnioten. Arch. mikr. Anat., 59, 1901. 
 
 RESPIRATORY ORGANS. 
 
 General. 
 
 Clemenz: Aussere Kiemen der Wirbeltiere. Anat. Hefte, 5, 1904. 
 
 Goppert: Kehlkopf der Amphibien und Reptilien. Morph. Jahrb., 22, 1904; 26, 1898; 
 
 28, 1899. 
 Gotte: Ursprung der Lunge. Zool. Jahrb., Anat. Abth., 21, 1905. 
 Miller: Structure of the lung. Jour. Morph., 8, 1893. 
 
 Moser: Entwicklungsgeschichte der Wirbeltierlunge. Arch. mikr. Anat., 60, 1902. 
 Oppel: Athmungsapparat. Ergebnisse, 13, 1903; 14, 1904; 16, 1906. 
 Schmidt: Kehlhugel der Amnioten. Morph. Jahrb., 43, 191 1. 
 Spengel: Schwimmblasen, Lungen und Kiementaschen der Wirbeltiere. Zool. Jahrb. 
 
 Suppl., 7, 1904. 
 
 Cyclostomes and Fishes. 
 
 Babak: Darmathmung der Cobiten. Biol. Centralb., 27, 1907. 
 
 Beaufort: Schwimmblase der Malacopterygii. Morph. Jahrb., 39, 1909. 
 
 Braus: Embryonal Kiemenapparat von Heptanchus. Anat. Anz., 29, 1906. 
 
 Bridge and Haddon: Air-bladder and Weberian ossicles of Siluridae. Phil. Trans., 184, 
 
 1893. 
 Corning: Wundernetzbildes in Schwimmblase der Teleostier. Morph. Jahrb., 14, 1888. 
 Dahlgren: Breathing valves of teleosts. Zool. Bull., 2, 1898. 
 Dohrn: Urgeschichte, u.s.w. Spritzlochkieme der Selachier, Opercularkieme d. 
 
 Ganoiden, Pseudobranchie der Teleostier. Neapel. Mitth., 7, 1886. 
 Greil: Homologie der Anamnierkiemen. Anat. Anz., 28, 1906. 
 Jaeger: Physiologie der Schwimmblase. Biol. Centralbl., 24, 1904. 
 Kellicott: Develop, vase, and respiratory systems of Ceratodus. Mem. N. Y. Acad. 
 
 Sci., 2, 1904. 
 Mauer: Pseudobranchien der Knochenfisches. Morph. Jahrb., 9, 1883. 
 Moroff: Entwicklung der Kiemen der Knochenfischen. Arch. mikr. Anat., 60, 1902. 
 Moser: Entwicklung der Schwimmblase. Arch. mikr. Anat., 63, 1904. 
 Muller: Entwicklung und Bedeutung der Pseudobranchie bei Lepidosteus. Arch. mikr. 
 
 Anat., 49, 1897. 
 Nusbaum: Gasdriise in Schwimmblase. Anat. Anz., 31, 1907. 
 Rand: Functions of spiracle in skate. Am. Nat., 41, 1907. See also Darbyshire, Jour. 
 
 Linn. Socy., Zool., 30, 1907. 
 Stockard: Development of mouth and gills in Bdellostoma. Am. Jour. Anat., 5, 1906. 
 Thilo: Luftsacke bei Kugelfische. Anat. Anz., 16, 1899. 
 Wiedersheim: Ein Kehlkopf bei Ganoiden und Dipnoern. Zool. Jahrb. Suppl. 7, 
 
 1904- 
 Zograff: Labyrinthine apparatus of labyrinthine fishes. Quar. Jour. Micr. Sci., 28, 1889. 
 
 Amphibia. 
 
 Bruner: Smooth facial muscles of anura and salamandrina (respiratory mechanism). 
 Morph. Jahrb., 29, 1901. 
 
CIRCULATION. 375 
 
 Greil: Anlage der Lungen und Ultimobranchialkorper. Anat. Hefte, 29, 1905. 
 
 Fox: Tympano-Eustachian passage in toad. Proc. Acad. Nat. Sci., Phila., 1901 
 
 Martens: Entwicklung der Kehlkopfknorpel bei Anuren. Anat. Hefte, 9, 1897. 
 
 Ochsner: Lung of Necturus. Bull. Univ. Wisconsin, ;^;^, 1900. 
 
 Seelyee: Circulator)^ and respirator}- systems of Desmognathus. Proc. Boston Socy., 
 
 Nat. Hist., 32, 1906. 
 Whipple: Ypsiloid apparatus of Urodeles. Biol. Bull., 10, 1906. 
 Wilder: Phylogenesis of larynx. Anat. Anz., 7, 1892. 
 Whipple: Xaso-labial groove of salamanders. Biol. Bull., 11, 1906. 
 Wilder: Amphibian larynx. Zool. Jahrb. Abth. Anat., 9, 1896. 
 Wilder: Lungless salamanders. Anat. Anz., 9, 1894; 12, 1896. 
 Wilder: PharAngeo-oesophageal lung of Desmognathus. Am. Nat., 35, 1901. 
 
 Sauropsida. 
 
 Cope: Lungs of ophidia. Proc. Am. Phil. Socy., $;i, 1904. 
 Gage: Aquatic respiration in soft-shelled turtles. Am. Nat.,' 20, 1886. 
 Hacker: Unter Kehlkopf der Singvogel. Anat. Anz., 14, 1898. 
 Heidrich: Mund-Schlundhohle der Vogel. Morph. Jahrb., 37, 1907. 
 Huxley: Respirator}- organs of Apter}'x. Proc. Zool. Socy. London, 1882. 
 Milani: Reptilienlungen. Zool. Jahrb. Abth. Anat., 8, 1894; 10, 1897. 
 Miiller: Air sacs of pigeon. Smithsonian Misc. Coll., 50, 1907. 
 Sappey: Recherches sur I'apparaeil respiratoire des oiseaux. Paris, 1847, 
 Strasser: Luftsacke der Vogel. Morph. Jahrb., 3, 1877. 
 
 Mammals. 
 
 Bremer: Lungs of opossum. Am. Jour. Anat., 3, 1904. 
 
 Dubois: Morphologie des Larynx. Anat. Anz., i, 1886. 
 
 Fox: Phar\-ngeal pouches and their derivatives. Am. Jour. Anat., 8, 1908. 
 
 Goppert: Herkunft des Wrisberg'schen Knorpels. Morph. Jahrb., 21, 1894. 
 
 His: Bildungsgeschichte der Lungen bei mensch. Embr}-onen. Arch. Anat. und Phvs., 
 
 1887. 
 Justesen: Entwicklung und Verzweigtmg des Bronchialbaumes der Saugetierlunge. Arch. 
 
 mikr. Anat., 56, 1900. 
 Mall: Branchial clefts and thymus of dog. Johns Hopkins Studies Biol. Lab., 4, 1888. 
 Shaeffer: Sinus maxillaris in Man. Am. Jour. Anat., 10, 1910. 
 Schaeffer: Lateral walls of cavum nasi in man. Jour. Morph., 21, 1910. 
 Symington: The marsupial lar}-nx. Jour. Anat. and Physiol., ;^^, 1898; 35, 1899. 
 
 CIRCULATION. 
 
 General. 
 
 AUis: Pseudobranchial and carotid arteries in gnathostomes. Zool. Jahrb., Abth. Anat.. 27, 
 
 1908. 
 Ayers: Morpholog}- of the carotids. Bull. Mus. Comp. Zool., 17, 1889. 
 Boas: Arterienbogen der Wirbelthiere. Morph. Jahrb., 13, 1887. 
 Broman: Entwicklung, 'Wanderung' und Variation der Bauchaortenzweige bei Wirbel- 
 
 tieren. Ergebnisse, 16, 1906. 
 Greil: Anatomic und Entwicklung des Herzens und Truncus arteriosus der Wirbelthiere. 
 
 Morph. Jahrb., 31, 1903. 
 Greil: Entwicklung des Truncus arteriosus der Anamnier. Verhandl. Anat. Gesellsch., 
 
 17, 1903. 
 Grosser: Kopfvenensystem der Wirbeltiere. Verh. Anat. Gesellsch., 21, 1907. 
 Hochstetter: ^'ergl. Anat, und Entwicklung des Venensystem der Amphibien und Fische, 
 
 Morph. Jahrb.. 13, 1888. 
 
376 BIBLIOGRAPHY. 
 
 Hochstetter: Entwicklungsgeschichte des Gefasssystem. Ergebnisse, i, 1S92. 
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 Lewis: Development of the vena cava inferior. Am. Jour. Anat., i, 1902. 
 Lewis: Sinusoids. Anat, Anz., 25, 1904. 
 
 Rose: Vergl. Anat. des Herzens der Wirbeltiere. Morph. Jahrb., 16, 1890. 
 Weidenreich: Die roten Blutkorperchen. Ergebnisse, 13, 1903. 
 Weidenreich: Morphologic der Blutzellen. Anat. R,ecord, 4, 1910. 
 Wright: Histogenesis of blood platelets. Jour. Morph., 21, 1910. 
 
 Weidenreich: Blut und Blutbildenden und -zerstorenden Organe. Arch. mikr. Anat. 
 65-72, 1904-8. 
 
 Fishes. 
 
 Allen: Blood-vascular system of Loricati. Proc. Washington Acad. Sci., 7, 1905. 
 
 Allis: Pseudobranchial and carotid arteries in Polypterus and Amiurus. Anat. Anz., ^;^, 
 
 1908; in Esox, Salmo, Gadus and Amia, 1. c, 41, 1912. 
 Allen: Subcutaneous vessels in head of Polyodon and Lepidosteus. Proc. ^^'a5hington 
 
 Acad. Sci., 9, 1907. 
 Carazzi: Sistema arteriosa di Squalidi. Anat. Anz., 36, 1905. 
 Danforth: Heart and arteries of Polyodon. Jour. Morph., 23, 1912. 
 Hofifmann: Entwicklung des Herzens und Blutgefasse bei Selachiern. Morph. Jahrb., 
 
 19, 1893. Venensystem, idem, 20, 1893. 
 Holbrook: Origin of endocardium in bony fishes. Bull. Mus. Comp. Zool., 25, 1894. 
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 Allen: Subcutaneous vessels in tail of Lepidosteus. Am. Jour. Anat., 8, 1908. 
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 Acad. Sci., 2, 1905. 
 
 Mayer: Entwicklung des Herzens u. d. grossen Gef^ssstamme bei Selachier. Mittheil. 
 
 zool. Sta. Neapel, 7, 1887; see also 8, 1888. 
 Parker and Davis: Blood-vessels of heart of Carcharias, Raia and Amia. Proc. Boston 
 
 Socy. Nat. Hist., 29, 1899. 
 Parker: Blood-vessels of heart of Orthagoriscus. Anat. Anz., 17, 1900. 
 Rand: Posterior connections of lateral vein in skates. Am. Nat., 39, 1905. 
 Rex: Hirnvenen der Elasmobranchier. Morph. Jahrb., 17, 1891. 
 Senior: Conus arteriosus in Tarpon and Megalops. Biol. Bull., 12, 1907. 
 Senior: Development of heart in shad. Am. Jour. Anat., 9, 1909. 
 Silvester: Blood-vascular system of Lopholatilus. Bull. Bureau of Fisheries, 24, 1904. 
 Sobotta: Entwicklung des Blut, Herzens und grossen Gefassstamme der Salmoniden. 
 
 Anat. Hefte, 19, 1902. 
 
 Amphibia. 
 
 Bethge: Blutgefasssystem von Salamandra, Triton und Spelerpes. Zeit. wiss. Zool., 6^, 
 1898. 
 
 Bruner: Heart of lungless salamanders. Jour. Morph., 16, 1900. 
 
 Huxley: Skull and heart of Menobranchus [Necturus]. Proc. Zool. Socy. London, 1874. 
 Hopkins: Heart of lungless salamanders. Am. Nat., 30, 1896. 
 
 Marshall and Bles: Development of blood-vessels in frog. Studies Biol. Lab. Owens' 
 College, 2, 1890. 
 
 Maurer: Kiemen und ihre Gefasse bei Amphibien. Morph. Jahrb., 14, 18S8. 
 Miller: Blood- and lymph-vessels of lung of Necturus. Am. Jour. Anat., 4, 1905. 
 Parker: Persistence of left postcardinal vein in frog; homologies of veins in Dipnoi. Proc. 
 
 Zool. Socy. London, 1889. 
 Rabl: Bildung des Herzens der Amphibien. Morph. Jahrb., 12, 1887. 
 Rex: Hirnvenen der Amphibien. Morph. Jahrb., 19, 1892. 
 
CIRCULALION. 377 
 
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 Santhoff and van \'orhis: Vascular system of Necturus. Bull. Univ. Wise, s^, 1900. 
 Seelye: Circulatory and respirator}' systems of Desmognathus. Proc. Boston Socy. Nat. 
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 Sauropsida. 
 
 Bruner: Cephalic veins and sinuses of reptiles. Am. Jour. Anat., 7, 1907. 
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 Evans: Earliest blood-vessels in anterior limbs of birds. Am. Jour. Anat., 9, 1909. 
 Grosser and Brezina: Entwicklung der Venen des Kopfes und Raises der Reptilian. 
 
 Morph. Jahrb., 23, 1895. 
 Hochstetter: Ent%\-icklungsgeschichte des Venensystems der Amnio ten. Reptilian. Morph. 
 
 Jahrb., 19, 1892. 
 Hochstetter: Arterien des Darmcanals der Saurier. Morph. Jahrb., 16, 1898. 
 Mackay: Development of branchial arches in birds. Trans. Roy. Socy. London, 179, 
 
 1888. 
 Miller: Development of postcava in birds. Am. Jour. Anat., 2, 1903. 
 Stromsten: Anat. and develop, venous system of Chelonia. Am. Jour. Anat., 4, 1905. 
 
 Mammals. 
 
 Baddard: Az}gos veins in mammals. Proc. Zool. Socy. London, 1907. 
 Bom: Entwicklungsgeschichte des Saugetierherzens. Arch. mikr. Anat., s^, 1889. 
 Davis: Chief veins in early pig embryos. Am. Jour. Anat., 10, 1910. 
 Dexter: Vitelline veins of cat. Am. Jour. Anat., i, 1902. 
 
 Goppert: Entwicklung von Varietaten im Arteriensystem der weissen Maus. Morph. 
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 Hochstetter: Entwicklungsgeschichte das Vanensystams der Anmiotan. Mammalia. 
 
 Morph. Jahrb., 20, 1893. 
 Hochstetter: Venensystem dar Edantaten. Morph. Jahrb., 25, 1897. 
 Lewis: Development of vena cava inferior. Am. Jour. Anat., i, 1902. 
 Lewis: Development of veins in limbs of rabbit. Am. Jour. Anat., 5, 1905. 
 McClure: Abnormalities in postcava of cat. Am. Nat., 34, 1900. 
 McClure: Anatomy and development of venous system of Didelphys. Am. Jour. Anat., 
 
 2, 1903; 5, 1905- 
 Minot: Veins of Wolffian body of pig. Proc. Boston Socy. Nat. Hist., 28, 1898. 
 Parker and Tozier: Thoracic derivatives of postcardinals in swine. Bull. Mus. Comp. 
 
 Zool., 31, 1898. 
 Reagan: Fifth aortic arch in mammals. Am. Jour. Anat., 12, 1912. 
 Rose: Ent^sdcklung des Saugetierherzens. Morph. Jahrb., 15, 1890. 
 Salzar: Entwicklung der Kopfvanen des ^leerschweinnchens. Morph. Jahrb., 23, 1895 
 Sichar: Entwicklung dar Kopfarterien von Talpa. Morph. Jahrb., 44, 1912 
 Tandler: Anatomie der Kopfarterien bei Mammalia. Anat. Hafta, 18, .1901. 
 
 Lymphatics. 
 
 Allen: Lymphatics of Scorpaenichthys. Proc. Washington Acad. Sci., 8, 1906. Am. Jour. 
 Anat., II, 1911. 
 
 Allen: Subcutaneous vassals in tail of Lapidosteus. Anat. Record, 3, 1908. 
 Baetjar: Mesenteric lymph sac in pig. Anat. Record, 2, 1908. 
 Budge: Lymphherzen bei Hiihnerembr}'onen. Arch. Anat. u. Physiol., 1887. 
 Helly: Hamolymphdriisen. Ergebnissa, 12, 1902. 
 
 Hopkins: Lvmphatics and enteric epitheilum of Amia. Wilder Quarter Century Book, 
 1893. 
 
 Hoyer und Udziela: Lymphgefasssystem von Salamanderlarv^en. Morph. Jahrb 
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378 BIBLIOGRAPHY. 
 
 Huntington: Anatomy and development of systematic hmphatic vessels of cat. Memoirs 
 
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 Huntington and McClure: Numerous papers on lymph system of mammals in Am. Jour. 
 
 Anat. and Anat. Record. 
 Killian: Bursa und tonsilla pharyngea. Morph. Jahrb., 14, 1888. 
 Knower: Development of lymph hearts and lymph sacs in frog. Anat. Record, 2, 
 
 1908. 
 Lewis: Development of lymphatics in rabbit. Am. Jour. Anat., 5, 1905. 
 McClure: Development of lymphatics in cat. Anat. Anz., 32, 1908. 
 Marcus: Intersegmentale Lymphherzen der Gymnophionen. Morph. Jahrb., 38, 1908. 
 Maurer: Anlage der Milz und lymphat. Zellen bei Amphibien. Morph. Jahrb., 16, 1890. 
 Meyer: Heemolymph glands of sheep. Anat. Record, 2, 1908. 
 Miller: Development of jugular lymph sac of birds. Am. Jour. Anat., 12, 1912. 
 Miiller: Lymphherzenz Chelonier. Abhandl. Berlin Acad., 1839. 
 
 Sabin: Origin of lymphatic system in pig. Am. Jour. Anat., i, 1902; 3, 1904; 4, 1905. 
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 UROGENITAL ORGANS. 
 
 GeneraL 
 
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UROGENITAL ORGANS. 379 
 
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 London, 1901. 
 Krall: Mannliche Beckenflosse von Hexanchus. Morph. Jahrb., 37, 1908. 
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 Ruckert: Entstehung der Exkretionsorgane bei Selachiem. Arch. Anat. u. Physiol., 
 
 188S. 
 Schreiner: Generationsorgane von Myxine. Biol. Centralbl., 24, 1904. 
 WTieeler: Development of urogenital organs of Lamprey. Zool. Jahrb. Abth. Anat., 13, 
 
 1899. 
 Wijhe: Entwicklung des Exkretionssystemes und andere Organe bei Selachiern. Anat. 
 
 Anz., 2, 1SS8. 
 Woods: Origin and migration of germ-cells in Acanthias. Am. Jour. Anat., i, 1902. 
 
 Amphibia. 
 
 Field: Development of pronephros and segmental duct in amphibia. Bull. Mus. Comp. 
 
 Zool., 21, 1891. 
 Field: Morphologie der Hamblase bei Amphibien. Morph. Arbeiten, 4, 1894. 
 Hall: Development of mesonephros and Miillerian ducts in amphibia. Bull. Mus. Comp. 
 
 Zool., 45, 1904. 
 King: Bidder's organ in Bufo. Jour. Morph., 19, 1908. 
 King: Anomalies in genital organs of Bufo. Am. Jour. Anat., 10, 1910. 
 Marshall and Bles: Development of kidneys and fat bodies in frog. Studies Biol. Lab. 
 
 Owens Coll., 2, 1890. 
 Mollendorf: Entwicklung der Darmarterien und Vornieren Glomerulus bei Bombinator. 
 
 Morph. Jahrb., 43, 191 1. 
 Semon: Bauplan der Urogenitalsystems, dargelegt an Ichthyophis. Jena. Zeitschr., 26, 
 
 1891. 
 Spengel: Urogenitalsystem der Amphibien. Arbeit, zool. zoot. Inst. Wiirzburg, 3, 1886. 
 
 Sauropsida. 
 
 Boas: Begattungsorgane der Amnioten. Morph. Jahrb., 17, 1891. 
 Cope: Hemipenes of the sauria. Proc. Acad. Nat. Sci., Philadelphia, 1896. 
 Fleck: Entwicklung des Urogenitalsystem beim Gecko. Anat. Hefte, 41, 1910. 
 Gasser: Entstehung der Kloacaloffnung der Huhnerembr\'onen. Arch. Anat. und Physiol., 
 
 1880. 
 Gregor}': Development of excretory system in turtles. Zool. Jahrb., Abth. Anat., 13, 
 
 1900. 
 Hoffmann: Entwicklung der Urogenitalorgane bei Reptilien. Zeit. wiss. Zool., 48, 1889. 
 Rabl: Entwicklung der Vorniere bei Vogel. Arch. mikr. Anat., 72, 1908. 
 Schreiner: Entwicklung der Amniotenniere. Zeitsch. wiss. Zool., 71, 1902. 
 Wiedersheim: Entwicklung des Urogenitalapparates bei Krokodilien und Schildkroten. 
 
 Arch. mikr. Anat., 36, 1890. 
 
 Mammals. 
 
 Beiling: Anatomie der Vagina und Uterus der Saugetiere. Arch. mikr. Anat., 67, 1906. 
 Broek: Urgenital-apparates der Beutler, Morph. Jahrb., 41, 1910. 
 Boas: Begattungsorgane der Amnioten. Morph. Jahrb., 17, 1891. 
 
380 BIBLIOGRAPHY. 
 
 Bremer: Morphology of tubules of testis and epididymis. Am. Jour. Anat., 11, 1911. 
 
 Broek: Mannlichen Geschlechtsorgane der Beuteltiere. Morph. Jahrb., 41, 1910. 
 
 Cole: Intromittent sac of male guinea pig. Jour. Anat. and Physiol., 32, 1897. 
 
 Courant: Preputialdriise des Kaninchens. Arch. mikr. Anat., 62, 1903. 
 
 Daudt: Urogenitalapparates der Cetaceen. Jena. Zeitschr., 32, 1898. 
 
 Gerhardt: Entwicklung der bleibenden Niere. Arch. mikr. Anat., 57, 1901. 
 
 Gudernatsch: Hermaphroditismus verus in man. Am. Jour, Anat. 11, 191 1. 
 
 Gilbert: Os Priapi der Sanger. Morph. Jahrb., 8, 1892. 
 
 Gerhardt: Kopulationsorgane der Saugetiere. Jena. Zeitsch., 39, 1904. 
 
 Kaudem: Mannl. Geschlectsorgane von Insectivoren und Lemuriden. Zool. Jahrb., 
 
 Abt. Anat., 31, 1910. 
 Keibel: Entwicklung der Harnblase. Anat. Anz., 6, 1891. 
 Keibel: Entwicklung des menschlichen Urogenitalapparates. Arch. Anat. und Physiol. 
 
 Anat. Abth., 1896. 
 Klaatsch. Descensus testiculorum. Morph. Jahrb., 16, 1890. 
 MacCallum. Wolffian body of higher animals. Am. Jour. Anat., i, 1892. 
 Montgomery: Human cells of Sertoli. Biol. Bulletin, 21, 191 1. 
 Miiller: Prostate der Haussaugetiere. Anat. Hefte, 26, 1904. 
 Poulton: Structures connected with ovarial ovum of marsupials and monotremes. Quart. 
 
 Jour. Micros. Sci., 24, 1884. 
 
 Robinson: Position and peritoneal relations of mammalian ovum. Jour. Anat. and 
 Physiol., 1887. 
 
 Schreiner: Entwicklung der Amniotenniere. Zeitsch. wiss. Zool., 71, 1902. 
 
 Sobotta: Entstehung des Corpus luteum. Ergebnisse, 8, 1898; 11, 1901. 
 
 Weber: Entwicklung des uropoetisches Apparats. bei Saugern. Morph. Arbeiten, 7, 1897. 
 
 SUPRARENALS. 
 
 Aichel: Entwicklungsgeschichte und Stammesgeschichte der Nebennieren. Arch, mikr. 
 
 Anat., 56, 1900. 
 Collinge and Vincent: So-called suprarenals in cyclostomes. Anat. Anz., 12, 1896, 
 Flint: The Adrenal, Johns Hopkins Hospital Report, 1900. 
 Kohn: Nebennieren der Selachier. Arch. mikr. Anat., 53, 1898. 
 Kimz: Develop, adrenals in turtle. Am, Jour. Anat,, 13, 1912. 
 Srdinko: Nebennieren bei Anuren. Anat. Anz., 18, 1900, 
 Srdinko: Nebennieren der Knochenfischen, Arch, mikr, Anat., 71, 1908. 
 Vincent: Discussion of suprarenals. Jour. Anat. and Phys., 38, 1903, 
 Weldon: Suprarenals of vertebrates. Quar. Jour. Micr, Sci., 24, 1884; 25, 1SS5. 
 
 FCETAL ENVELOPES, PLACENTA, ETC. 
 
 Corning: Erste Anlage der Allantois bei Reptilien. Morph. Jahrb., 23, 1895. 
 
 Hill: Placentation of Perameles. Quar. Jour, Micr. Sci,, 40, 1898, 
 
 Hubrecht: Placentation of Erinaceus, Quar. Jour, Micr. Sci., 30, 1889. Of Sorex. 
 
 idem, 35, 1893. Spolia Nemoris (lemurs and edentates), idem, 36, 1904. 
 Minot: Uterus and embryo. Jour. Morph., 2, 1889. 
 Minot: Theorj' of the structure of the placenta. Anat, Anz., 6, 1891. 
 Osbom: Foetal membranes of marsupials. Jour. Morph., i, 1887. 
 Robinson: Segmentation cavity, archenteron, germ layers and amnion of mammals. 
 
 Quar. Jour. Micr, Sci,, ^^, 1892. 
 Turner: Lectures on the anatomy of the placenta. Edinburgh, 1876. 
 Van Beneden et Julin: Formation des annexes foetales chez les Mammiferes. Archives 
 
 de Biol., 5, 1884. 
 
DEFINITIONS OF SYSTEMATIC NAMES. 
 
 Acanthias, genus of sharks including com- 
 mon dogfish. 
 
 Acipenser, genus of ganoids; sturgeon. 
 
 Aglossa, tongueless toads from Africa and 
 South America. 
 
 Aliantoidea, the higher vertebrates with 
 allantois; reptiles, birds and mammals. 
 
 Amblystoma, genus of tailed amphibians, 
 largely American. 
 
 Amia, genus of ganoid fishes peculiar to 
 America. 
 
 Ammocoetes, the larval stage of the lam- 
 preys. 
 
 Amniotes, division of vertebrates with 
 amnion and allantois in development; 
 reptiles, birds and mammal. 
 
 Amphibia, class of vertebrates, yoimg with 
 gOls, adults with lungs; frogs, toads and 
 salamanders. 
 
 Amphioxus, fish-like form without verte- 
 brae, type of group of Leptocardii. 
 
 Amphipnous, eel-like fishes from India. 
 
 Amphisbaenans, legless lizards. 
 
 Amphiuma, genus of tailed amphibians 
 with rudimentary legs and gill slits; 
 southern U. S. 
 
 Anallantoidea, vertebrates without an 
 allantois; ichthyopsida. 
 
 Anamnia, vertebrates without an amnion; 
 ichthyopsida. 
 
 Anguis, genus of foodess lizards. 
 
 Anser, genus of birds including geese. 
 
 Anthropoids; sub order of primates includ- 
 ing the higher apes and man. 
 
 Anura, order containing the tailless amphib- 
 ians; frogs and toads. 
 
 Aquila, genus of birds including eagles. 
 
 Archaeopteryx, a fossil bird with teeth and 
 a reptilian tail 
 
 Archegosaurus, genus of extinct stego- 
 cephal amphibians 
 
 Arcifera, group including toads and tree 
 toads. 
 
 Arthrodira, order of extinct dipnoi (lung- 
 fishes) some ver}'- large, 
 
 Artiodactyla, ungulate mammals with even 
 number of toes; cattle, sheep, deer. 
 
 Astroscopus, genus of electric fishes; 
 marine. 
 
 Atelodus, genus of two-toed rhinoceros. 
 
 Aves, the class of birds. 
 
 Bdellostoma, genus of myxinoids; hag 
 fishes of the Pacific. 
 
 381 
 
 Belone, genus of fishes; bony gars. 
 Bombinator, genus of European toads, 
 
 unke. 
 Brady pus, genus of edentate sloths. 
 Branchiosaurus, genus of extinct stego- 
 
 cephal amphibia. 
 Bufo, genus of amphibians, toads. 
 Buteo, genus of raptorial birds, hawks. 
 Butyrinus, genus of herring-like fishes. 
 
 Caecilians, a group of legless tropical am- 
 phibians. 
 Caiman, genus of crocodiles. 
 Calamoichthys, genus of ganoid fishes from 
 
 Africa. 
 Callopterus, genus of extinct ganoid fishes. 
 Camptosaurus, genus of extinct dinosaur 
 
 reptiles. 
 Capitosaunis, genus of extinct stegocepha- 
 
 lous amphibia. 
 Carcharias, genus of sharks; sand shark. 
 Carinatae, birds with a keel to the sternum, 
 
 includes all living birds except ostriches. 
 Carnivores, order of flesh-eating mammals; 
 
 cats, dogs, bears, weasels, seals. 
 Castor, genus of rodents, beaver. 
 Ceratodus, genus of dipnoi (lung-fishes) 
 
 from Australia. 
 Ceratophiys, genus of So. American toads. 
 Cervus, genus of Ungulates, common deer. 
 Cestracion, genus of sharks from the 
 
 Pacific. 
 Cetacea, order of mammals, whales. 
 Chauna, genus of So. America crane-like 
 
 birds; hooded screamers. 
 Chelonia, order of reptile turtles. 
 Chelone, genus of turtles, greec turde. 
 Chelydra, genus of turtles, snapping turtle. 
 Chelydrosaurus, genus of extinct stego- 
 
 cephalous amphibia. 
 Chimaera, genus of peculiar deep-water 
 
 sharks. 
 Chimaeroids, order of shark-like fishes; 
 
 Holocephali. 
 Chiroptera, order of mammal bats. 
 Chlamydoselache, genus of primitive deep- 
 sea sharks from Japan. 
 Choloepus, genus of edentates, sloths. 
 Chondrostei, order of ganoid fishes, stur 
 
 geon. 
 Chn-sophr\-s, genus of fishes; sea bream of 
 
 Europe. 
 Chr>'sothrix, a genus of So. American 
 
 monkeys. 
 
382 
 
 DEFINITIONS OF SYSTEMATIC NAMES. 
 
 Cistudo, genus of chelonia; box turtles. 
 Cladoselache, genus of extinct sharks. 
 Clupeidae, family of fishes including herring, 
 
 shad, ale wives and menhaden. 
 Cobitis, genus of fishes; loaches. 
 Coregonus, genus of fresh-water fishes; 
 
 white fish. 
 Crocodilia, order of reptiles including the 
 
 alligator. 
 Crotalus, genus of snakes, rattlesnakes. 
 Cryptobranchus, genus of tailed amphibians 
 
 with permanent gill slits; hellbender of 
 
 No. America. 
 Cyclostomes; class of vertebrates without 
 
 jaws, including lampreys and hag fishes. 
 Cynognathus, genus of extinct theromorph 
 
 reptiles. 
 Cyprinids, family of freshwater fishes, 
 
 carp, minnows. 
 
 Delphinus, genus of whales; dolphins. 
 
 Derotremes, tailed amphibia with perma- 
 nent gill slits. 
 
 Desmognathus, genus of salamanders. 
 
 Didelphys, genus of marsupials, opossums. 
 
 Diemyctylus, genus of small spotted sala- 
 manders. 
 
 Dinosaurs, extinct terrestrial reptiles, some 
 of enormous size. 
 
 Dipnoi, sub-class of fishes with gills and 
 lungs, lung-fishes. 
 
 Discosaurus, genus of stegocephalous 
 amphibians. 
 
 Dromatherium, genus of extinct, primitive 
 mammals. 
 
 Echidna, genus of monotremes, spiny ant- 
 eaters of Australia. 
 
 Edentates, order of mammals including 
 sloths, armadillos, etc. 
 
 Elasmobranchs, a sub-class of vertebrates 
 including the sharks and skates. 
 
 Embiotocids, family of fishes from the 
 Pacific which bear living young; surf 
 perches. 
 
 Epicrium, genus of caecilians. 
 
 Erinaceus, genus of insectivorous mammals; 
 hedgehogs. 
 
 Er>'thrinus, genus of tropical fishes. 
 
 Euornithes, a name given to all recent birds. 
 
 Eupomatus, fresh-water sunfish. 
 
 Eurycormus, genus of fossil ganoid fishes. 
 
 Firmisternia, anurous amphibia with the 
 halves of the sternum united to each 
 other; frogs. 
 
 Fulica, genus of water bird; coots. 
 
 Galeocerdo, genus of selachians; tiger 
 
 sharks. 
 Galeopithecus, a flying mammal from Asia 
 
 of uncertain position. 
 Galeus, genus of sharks; dogfish. 
 
 Gallus, genus of birds including the com- 
 mon fowl. 
 
 Gambusia, genus of fishes; top-minnow. 
 
 Ganoids, subclass of fishes intermediate 
 between sharks and bony fishes; stur- 
 geon, garpike, etc. 
 
 Geococcyx, a genus of cuckoos. 
 
 Geotrition, a genus of European salaman- 
 ders. 
 
 Gerrhonotus, genus of lizards. 
 
 Glyptodon, genus of edentates allied to 
 armadillos. 
 
 Gnathostomes, vertebrates, which have jaws; 
 includes all except cyclostomes. 
 
 Gobiids, family of small fishes, mostly 
 marine; gobies. 
 
 Gymnophiona, order of amphibia without 
 tail or legs; tropical; caecilians. 
 
 Gymnotus, electric eel of So. America. 
 
 Halmaturus, genus of kangaroos. 
 Hatteria, another name for Sphenodon. 
 Heloderma, poisonous lizard from Arizona; 
 
 Gila monster. * 
 Heptanchus, primitive shark with seven 
 
 gill slits. 
 Hexanchus, primitive shark with six gill 
 
 slits. 
 Holocephali, order of shark-like fishes; 
 
 Chimaera. 
 Hypogeophis, genus of Caecilians. 
 Hyracoidea, order of mammals including 
 
 Hyrax. 
 
 Ichthyophis, genus of caecilians from Ceylon. 
 Ichthyopsida, group of vertebrates which 
 
 have gills; fishes, amphibia. 
 Ichthyosaurs, extinct aquatic reptiles. 
 Iguana, genus of tropical American lizards. 
 Insectivores, order of small mammals; 
 
 moles, shrews, etc. 
 Inuus, genus of macaques including the 
 
 Barbarj' ape. 
 
 Lacerta, genus including the common 
 lizards of Europe. 
 
 Lacertilia, sub-order of reptiles including 
 all lizards. 
 
 Lagenorhynchus, a genus of dolphins. 
 
 Lepidosiren, genus of lung fishes (dipnoi) 
 from South America. 
 
 Lepidosteus, genus of ganoid fishes, gar- 
 pike. 
 
 Lopholatilus, genus of teleosts from Gulf 
 Stream; tile fish. 
 
 Macropus, genus of marsupials; kangaroos. 
 Mammals, class of vertebrates, with hair, 
 
 nourishing the young with milk. 
 Manatus, genus of sirenians. manatees. 
 Manis, genus of old-world edentates; scaly 
 
 ant-eaters. 
 
DEFINITIONS OF SYSTEMATIC NAMES. 
 
 383 
 
 Marsupialia, subclass of mammals with 
 pouch for young, opossums, kangaroos, 
 etc. 
 
 Megalops, genus of fishes including the tar 
 pon. 
 • Melanerpeton, genus of extinct stegocephal 
 amphibians. 
 
 Monodelphia, subclass of mammals, in- 
 cluding all except monotremes and 
 marsupials. 
 
 Monotremata, subclass of mammals with 
 cloaca; includes duckbill and Echidna 
 of Australia. 
 
 Morones, genus of catfishes. 
 
 Mugil, genus of fishes, mullets. 
 
 Mustelus, genus of small sharks; dogfish. 
 
 Myrmecobius, genus of Australian mar- 
 supials. 
 
 Myxine, genus of cyclostomes; hag fishes, 
 
 Myxinoids, group of Cylostomes; hag 
 fishes. 
 
 Necturus, genus of aquatic amphibians 
 with tail and external gills, central U, S. 
 
 Notidanids, sub-order of sharks with more 
 than five gill clefts. 
 
 Nototrema, genus of South American toads 
 with dorsal brood sac. 
 
 Ophidia, sub-order of reptiles; snakes. 
 
 Opisthocomus, South American bird, type 
 of a separate sub-order. 
 
 Opisthodelphys, genus of tropical American 
 tree- toads. 
 
 Omithorhynchus, genus of monotremes; 
 duckbill of Australia. 
 
 Ostariophysi, bony fishes with Weberian 
 apparatus. 
 
 Ostracoderms, a group of extinct verte- 
 brates of very uncertain position. 
 
 Palaeohatteria, a fossil reptile allied to 
 Sphenodon. 
 
 Palaeospondylus, a problematical fossil 
 vertebrate from Scotland. 
 
 Perennibranchs, tailed amphibia which 
 retain the gills through life. 
 
 Perissodactyls, sub order of mammals with 
 odd number of toes; horses, rhinoceros, 
 tapirs. 
 
 Petrobates, genus of extinct theromorph 
 reptiles. 
 
 Petromyzonts, subclass of cyclostomes, 
 lampreys. 
 
 Phoca, genus of carnivores including com- 
 mon seals. 
 
 Physoclisti, fishes in which the air-bladder 
 is closed. 
 
 Physostomi, group of fishes in which the air- 
 bladder has a duct; mostly fresh water. 
 
 Pipa, tongueless toad from South America. 
 
 Pisces, the class of fishes. 
 
 Placentalia, all mammals (except marsupials 
 and monotremes) in which a placenta 
 occurs. 
 
 Placodus, genus of extinct theriomorph 
 reptiles 
 
 Plesiosaurs, order of extinct, long-necked 
 swimming reptiles 
 
 Polyodon, genus of ganoid fishes, paddle 
 fish. 
 
 Polypterus, genus of ganoids from Africa. 
 
 Porichthys, genus of fishes from Pacific; 
 midshipman. 
 
 Primates, highest order of mammals, 
 including monkeys, apes and man. 
 
 Pristiurus, genus of European dogfish. 
 
 Proboscidea. order of mammals, including 
 elephants. 
 
 Procolophon, genus of extinct theromorph 
 reptiles. 
 
 Proteus, genus of tailed amphibians from 
 caves of Austria, allied to Necturus. 
 
 Protopterus, genus of dipnoi from Africa. 
 
 Psittacus, genus of parrots. 
 
 Pterodactyls, extinct flying reptiles. ' 
 
 Pterosaurs, extinct flying reptiles, ptero- 
 dactyls. 
 
 Pythonomorphs, a group of extinct swim- 
 ming reptiles. 
 
 Raia, genus of elasmobranchs, including 
 the skates. 
 
 Rana, genus of amphibia, frogs. 
 
 Ratitae, birds without keel to sternum, 
 ostriches. 
 
 Rhea, three-toed South American ostrich. 
 
 Rhynchobatus, genus of tropical skates. 
 
 Rhynchocephalia, order of lizard-like rep- 
 tiles; Sphenodon of New Zealand only 
 living species. 
 
 Rodentia, order of mammals with gnawing 
 teeth, rats, rabbits, beaver. 
 
 Ruminants, group of ungulate mammals 
 which chew the cud. 
 
 Salamandra, genus of tailed amphibia from 
 
 Europe. 
 Salamandrina, order of tailed amphibians 
 
 without gills. 
 Salmonids, family of fishes including trout 
 
 and salmon. 
 Sauropsida, class of vertebrates including 
 
 reptiles and birds. 
 Sceleporus, genus of lizards of eastern 
 
 United States. 
 Scomber, genus of fishes; mackerel. 
 Scorpsenichthys, genus of sculpins. 
 Selachii, order of elasmobranchs; sharks. 
 Serranidae, family of marine, perch-like 
 
 fishes. 
 Siluroids, order of fishes containing the 
 
 cat-fishes. 
 Siren, genus of tailed amphibian from U.S. 
 
 with external gills. 
 
3^4 
 
 DEFINITIONS OF SYSTEMATIC NAMES. 
 
 Sirenia, order of marine mammals; 
 
 manatees and dugongs 
 Sirenoidea; order of lung-fishes, containing 
 
 the living species. 
 Spalacotherium, genus of extinct mammals. 
 Sphenodon, genus of lizard-like reptiles 
 
 from New Zealand; order Rhyncho- 
 
 cephalia, 
 Squamata, order of reptiles including snakes 
 
 and lizards. 
 Stegocephala, order of extinct amphibians. 
 Stegosaurs, family of extinct dinosaur 
 
 reptiles, some very large. 
 Stenops, genus of lemurs. 
 Stenostomus, genus of fishes; scup. 
 
 Teleostomes, fishes with true jaw, includes 
 
 ganoids and teleosts. 
 Teleosts, order of fishes with bony skeleton, 
 
 including all common fishes. 
 Testudo, genus of land turtles. 
 Testudinata, turtles, same as a Chelonia. 
 Tetrapoda, term to include amphibia, 
 
 reptiles, birds, and mammals, which 
 
 have feet in place of fins. 
 Theromorpha, extinct reptiles forming the 
 
 lowest order of the class. 
 Tinnunculus, genus of hawk-like birds; 
 
 kestrel. 
 
 Torpedo,' genus of skates with remarkable 
 
 electric powers. 
 Trionyx, genus of fresh-water turtles. 
 Triton, genus of tailed amphibian, aquatic, 
 
 European. 
 Tropidonotus, genus of snakes, including 
 
 our water snake. 
 Trygon, genus of skates, string-rays. 
 Typhlopidae, family of peculiar tropical 
 
 serpents. 
 
 Ungulates, order of mammals which walk 
 on the tips of the toes; horse, cattle, deer, 
 antelope, etc. 
 
 Urodeles, order of tailed amphibia. 
 
 Varanus, genils of lizards from Africa. 
 
 Xenarthra, sub-order of American edentates, 
 
 ant-eaters and armadillos. 
 Xenopus, genus of tongueless toads from 
 
 Africa. 
 
 Zeuglodon a genus of extinct whales 
 
 (Cetacea) . 
 Ziphius, genus of toothed whales. 
 
INDEX. 
 
 Abdominal aorta, 284 
 
 pores, 124, 322 
 
 ribs, 41 
 
 sternum, 57 
 
 vein, 289 
 
 vertebras, 49 
 Abducens nerve, 170 
 Abomasum, 227 
 Accessor}' nerve, 177 
 Acetabular bone, 112, 113 
 x\cetabulum, 104, 109 
 Acinous glands, 18 
 Acrodont, 88, 213 
 
 dentition, 213 
 Acromion process, 109 
 Actinotrichia, 103 
 Activators, 264, 353 
 Acustico-Iateralis nerves, 167 
 Acustic nerve, 174 
 Adductor muscles, 132 
 Adenoid tissue, 307 
 Adipose tissue, 22 
 Adrenalin, 353 
 Adrenal organs, 352 
 Advehent vein, 291 
 .^githognathous, 97 
 Afferent branchial artery, 274 
 
 duct of gills, 239 
 
 nerve root, 161 
 Air-bladder, 247 
 
 ducts, 251 
 
 sacs, 261 
 Ala orbitalis, 98 
 
 temporalis, 61, 98 
 Alimentarv' canal, 205 
 Alisphenoid, 67 
 
 cartilage, 61 
 Allantoic arteries, 278, 285, 293 
 
 bladder, 318 
 
 veins, 350 
 Allan tois, 264, 278, 350 
 Alveolar ducts, 256 
 Alveoli of jaws, 213 
 
 of lung, 256 
 Amnion, 350 
 Amniotes, copulatory organs, 344 
 
 development of heart. 271 
 Amniotic cavity, ^50 
 Amphibia, brain, 155 
 
 circulation, 295 
 
 dermal skeleton, 41 
 
 excretory organs, 327 
 
 gills, 242 
 
 Amphibia, girdles, 106, 110 
 
 glands, 29 
 
 intestine, 229 
 
 larjTix, 251 
 
 lateral line organs, 180 
 
 limbs, 118 
 
 lungless, 258 
 
 lungs, 257 
 
 reproductive organs, ^t,^ 
 
 skin, 29 
 
 skuU, 82 
 
 teeth, 212 
 
 thymus, 246 
 
 thyreoid, 247 
 
 tongue, 217 
 
 vertebral column, 51 
 Amphicoelous, 46 
 Amphiplatyan, 46 
 Amphiohinal, 191 • 
 Amphistviic, 73 
 Ampullae of ear, 184 
 
 of Lorenzini, 182 
 
 of Savi, 182 
 Amylopsin, 234 
 Anchylosis, 38 
 Angvdare, 71 
 Anlage, vi. 
 Antibrachium, 116 
 Anterior abdominal vein, 289 
 
 cardinal vein, 279 
 
 cephalic duct, 303 
 
 chamber of eye, 203 
 
 comua, 139 
 
 process, 74 
 
 vena cava, 300 
 Anthers, loi 
 
 Antrum of Highmore, 197 
 Aorta, 273, 284 
 Aortic arches, 273 
 
 arches, modifications of, 282 
 Aponeurosis, 129 
 Appendages, 102, 114 
 Appendicular skeleton, 102 
 Apteria, 32 
 Aqueduct, 143 
 Aqueous humor, 203 
 Arachnoid membrane, 152 
 Arbor vitae, 161 
 Arcades, 11 
 Archenteron, 9 
 Archiccele, 8 
 Archinepteric duct, 312 
 Archipter}gium, 115 
 
 385 
 
SS6 
 
 INDEX. 
 
 Areolar tissue, 22 
 Argential layer of eye, 202 
 Arterial ring, 287 
 Arteries, 266, 284 
 
 afferent branchial, 274 
 
 allantoic, 278, 285, 293, 350 
 
 axillary, 288 
 
 basilar, 287 
 
 brachial, 288 
 
 branchial, 274 
 
 carotid, 275 
 
 caudal, 276 
 
 central retinal, 201 
 
 ciliary, 202 
 
 cceliac, 284 
 
 common carotid, 282 
 
 coronary, 273 
 
 cutaneus, 289 
 
 development of, 273 
 
 efferent branchial, 274 
 
 epigastric, 288 
 
 external iliac, 288 
 
 femoral, 288 
 
 gastric, 284 
 
 genital, 286 
 
 hepatic, 284 
 
 hyaloid, 201 
 
 hypogastric, 276, 285 
 
 iliac, 288 
 
 intercostal, 275, 286 
 
 ischiadic, 288 
 
 lumbar, 286 
 
 mandibular, 271 
 
 mesenteric, 284 
 
 nephridial, 275, 285 
 
 omphalomesaraic, 276 
 
 omphalomesenteric, 276 
 
 ovarian, 286 
 
 peroneal, 288 
 
 popliteal, 288 
 
 pulmonary, 283 
 
 radial, 288 
 
 renal, 286, 300 
 
 sacral, 286 
 
 sciatic, 288 
 
 somatic, 284 
 
 spermatic, 286 
 
 spinal, 287 
 
 splenic, 284 
 
 subclavian, 288 
 
 tibial, 288 
 
 ulnar, 288 
 
 umbilical, 285 
 
 vertebral, 287 
 
 vesical, 285 
 
 visceral, 284 
 
 vitelline, 293 
 Articular bone, 71 
 Articular process, 46 
 Articulare, 74 
 Articulations, 38 
 Arytenoid cartilage, 251 
 Ascending aorta, 284 
 
 Ascending process, 82 
 
 tracts, 140 
 Asterospondylous, 51 
 Astragalus, 117 
 Atlas, 49 
 
 Atrial chamber of gills, 240 
 Atrioventricular canal, 272 
 Atrium of heart, 272 
 
 lungs, 258 
 
 of nose, 194 
 Auditory, bulla, 100 
 
 meatus, 187 
 
 nerves, 174 
 
 organs, 182 
 
 vesicle, 183 
 Auricle of heart. 272 
 Aiu"icularis superficialis nerve, 171 
 Autostylic, 73 
 Axial skeleton, 43 
 Axillary artery, 288 
 
 vein, 290 
 Axis, 49 
 Axon, 19,139 
 Azygos appendages, 103 
 Azygos vein, 302 
 
 Baleen, 216 
 Barbs, 31 
 Barbules, 31 
 Basalia, 103 
 Basibranchial, 65 
 Basilar artery, 287 
 
 plate, 60 
 Basioccipital, 67 
 Basisphenoid, 67 
 Basitemporal plate, 96 
 Bicuspids, 213 
 Bidder's organ, 347 
 Bile, 231 
 
 duct, 233 
 Birds, see also Amniotes, Sauropsida. 
 
 air- sacs, 261 
 
 brain, 158 
 
 circulation, 300 
 
 gill pouches, 244 
 
 girdles of, 108, 113 
 
 intestine, 230 
 
 limbs, 119 
 
 lungs, 259 
 
 scales, 31 
 
 skin, 30 
 
 skull, 95 
 
 stomach, 225 
 
 thymus, 246 
 
 thyreoid, 247 
 
 tongue of, 218 
 
 vertebral column of, 52 
 Biserial fins, 115 
 Bladder, air, 247 
 
 allantoic, 318 
 
 smm, 247 
 
 urinary, 318 
 Blastomeres, 8 
 
INDEX. 
 
 387 
 
 Blastopore, 9 
 
 Blastula, 8 
 
 Blood, 24, 265 
 
 Blood circulation, embrj'onic, 268 
 
 phylogeny of, 267 
 
 primitive, 268 
 Blood corpuscles, 265 
 
 -lymph glands, 307 
 
 plasma, 265 
 
 plates, 266 
 
 vascular system, 266 
 
 vessels, structure of, 267 
 Body ca\^t}% 120 
 Bone, 23 
 
 development of, 43 
 
 of ear, 73 
 Botall's duct, 283 
 Bowman's capsule, 309, 314 
 
 glands, 197 
 Brachial artery, 288 
 
 plexus, 163 
 
 vein, 290 
 Brachium, 116 
 Brain, 140 
 
 flexures of, 143 
 
 sand, 160 
 
 ventricles of, 143 
 Branchiae, 236 
 Branchial arteries, 274 
 
 arches, 63 
 
 clefts, 236 
 
 vein, 274 
 Branchiomerism, 237 
 Branchiostegal membrane, 77, 240 
 
 rays, 77, 240 
 Breathing valves of teleosts, 241 
 Breast bone, 56 
 Broad ligament, 337 
 Bronchi, 250, 256 
 Bronchioles, 256 
 Bronchus of lampreys, 238 
 Buccal glands, 221 
 Buccalis nerve, 172 
 Bulbus arteriosus, 273 
 
 oculi, 203 
 
 olfactorius, 142, 167 
 Bunodont, 214 
 Bursa Entiana, 227 
 
 inguinalis, 338 
 
 omen talis, 122 
 
 Caeca, intestinal, 228 
 
 pyloric, 227 
 Calcaneus, 117 
 Calcareous glands, 183 
 Calcified cartilage, 43 
 Campanula HaUeri, 204 
 Canaliculi, 23 
 Canines, 213 
 Capillaries, 267 
 Capitatum, 117 
 Capitular head of rib, 54 
 Capsules, sense, 60 
 
 Carapace, 41 
 Cardiac glands, 224 
 
 plexus, 163 
 Cardinal veins, 279 
 Carotid arteries, 275 
 Carotid glands, 246, 297 
 Carpale, 117 
 Carpus, 116 
 Cartilage, 22 
 
 bones, 43, 66 
 
 calcified, 43 
 
 lingual, 75 
 
 rostral, 76 
 Cauda equina, 140 
 Caudal artery, 276 
 
 vein, 276 
 
 vertebrae, 49 
 Cavum tympani, 187 
 Cement, 211 
 
 Central canal of nervous system, 138 
 Centrale, 117 
 
 Central nervous system, 11, 137 
 Centrum, 45 
 Cephalic vein, 290 
 Ceratobranchial, 65 
 Cerebellar hemispheres, 161 
 Cerebellum, 142, 145 
 Cerebral hemispheres, 141 
 Cerebrospinal fluid, 152 
 Cerebrum, 148 
 Cervical plexus, 163 
 
 sinus, 244 
 
 vertebrae, 49 
 Chain ganglia, 163 
 Chambers of eye, 203 
 Chiasma, 169 
 Chiropterygia, 114 
 Choanae, 80, 193 
 Choledochar duct, 233 
 Chondrin, 23 
 Chondrocranium, 60 
 Chorda tympani, 173 
 Chordae tendiniae, 272 
 Chordata, 2 
 Chorioid coat, 202 
 
 fissure, 199 
 
 gland, 202 
 
 plexus, 144, 147 
 Chorion, 351 
 Chromaffine cells, 352 
 Chroma phile cells, 352 
 Chromatophores, 26 
 Chyle, 304 
 Chyle ducts, 304 
 Cilia, 205 
 
 Ciliary- arteries, 202 
 Ciliary ganglion, 165, 170 
 
 muscles, 202 
 
 process, 202 
 Ciliated epithelium, 18 
 Circle of Willis, 287 
 Circulation, allantoic, 278 
 
 foetal, 293 
 
388 
 
 INDEX. 
 
 Circulation, hepatic- portai, 277 
 
 portal, 277 
 
 pulmonary, 282 
 
 renal-portal, 280 
 
 respiratory, 282 
 
 systemic, 282 
 Circulatory organs, 264 
 Cistern of chyle, 303 
 Claspers, 27, 116, 343 
 Clavicles, 106 
 Claws, 27 
 Cleft palate, 193 
 Cleithrum, 106 
 Cloaca, 228 
 Coccyx, 52 
 Cochlea, 186 
 Cochlear nerve, 174, 186 
 Coeliac artery, 284 
 
 axis, 285 
 Coelom, 10, 14, 120 
 Collecting tubule, 309 
 Collector nerves, 163 
 Colon, 228 
 Columella auris, 74 
 Columnae carnea, 272 
 Columnar epithelium, 17 
 Columns of cord, 139 
 Commissura mollis, 146 
 Common carotid artery, 282 
 
 iliac vein, 289 
 Concha of ear, 188 
 
 of nose, 194 
 Cones of eye, 199 
 Conjunctiva, 203 
 Connective tissues, 21 
 Contour feathers, 3 1 
 Conus arteriosus, 272 
 Convoluted tubule, 309 
 Convolutions of brain, 149, 160 
 Copulae, 63 
 
 Copulatory organs, 342 
 Coraco-arcual muscles, 133 
 Coracoid bone, 107 
 
 process, 108 
 
 region, 105 
 Corium, 25 
 Cornea, 202 
 Cornua of cord, 139 
 
 trabeculae, 61 
 Cornua radiata, 160 
 Coronary arteries, 273 
 Coronoid bone, 71 
 Corpora adiposa, 307 
 
 bigemina, 142 
 
 quadrigemina, 142, 160 
 Corpus albicans, 151 
 
 callosum, 150 
 
 luteum, 320 
 
 restiforme, 150 
 
 striatum, 141 
 Corpusculum bulboideum, 179 
 Cortex of cerebrum, 149 
 Corti's organ, 186 
 
 Cotyloid bone, 113 
 Cowper's glands, 342 
 Cranial bones, table of, 72 
 
 nerves, 165 
 Cranio-quadrate process, 82 
 Cranium, 60 
 Cremaster muscle, 338 
 Cribriform plate, 67, 100 
 Cricoid cartilage, 251 
 Cristae acusticae, 185 
 Crista galli, 100 
 Crop, 223 
 Crura cerebri, 151 
 Crus, 116 
 Ctenoid scales, 40 
 Cubical epithelium, 17 
 Cuboides, 117 
 Cuneiform, 117 
 Cutaneus artery, 289 
 
 magnus vein, 290 
 Cutis, 25 
 
 Cuverian ducts, 271, 278 
 Cycloid scales, 40 
 Cyclospondylous, 51 
 Cyclostomes, brain, 152 
 
 circulation, 294 
 
 ear, 185 
 
 excretory organs, 321 
 
 eyes, 204 
 
 gills, 238 
 
 intestine, 228 
 
 lateral line organs, 180 
 
 mouth, 208 
 
 nasal organs, 190 
 
 reproductive organs, 33 1 
 
 skull, 75 
 
 teeth, 215 
 
 thymus, 245 
 
 thyreoid, 246 
 
 tongue, 217 
 
 vertebral column, 51 
 Cylindrical corpusle, 179 
 Cystic duct, 234 
 
 Decussation of fibres, 150 
 Deiter's cells, 186 
 Demibranch, 237 
 Dendrites, 19, 139 
 Dens, 50 
 Dental formula, 2 14 
 
 papilla, 209 
 
 ridge, 210 
 
 shelf, 210 
 Dentary bone, 71 
 Dentinal canals, 24 
 Dentine, 24, 209 
 Dentitions, 211 
 Depressor mandibulae, 133 
 
 muscles, 131 
 Derma, 25 
 Dermal muscles, 134 
 
 skeleton, 38, 39 
 Dermarticulare, 71 
 
INDEX. 
 
 389 
 
 Descending aorta, 284 
 
 tracts, 40 
 Desmognathous, 97 
 Deutoplasm, 8 
 Diaphragm, 123, 135 
 Diapophysis, 46 
 Diarthrosis, 38 
 Diastole, 272 
 Diencephalon, 142 
 Digastric muscle, 133 
 Digestive tract, 12, 205 
 Digitigrade, 120 
 Digits, 116 
 Dilator pupillae, 202 
 Diphycercal, 50 
 Diphyodont dentition, 211 
 Dipnoi, brain, 154 
 
 circulation, 292, 295 
 
 excretory organs, 327 
 
 lungs, 257 
 
 reproductive organs, 333 
 
 skull of, 80 
 Discus proligeru§, 320 
 Dorsal aorta, 275 
 
 fissure of cord, 139 
 
 nerve root, 161 
 
 vertebrae, 49 
 Down feathers, 31 
 Dromaeognathous, 97 
 Ductless glands, 18 
 Ductus arantii, 277 
 
 arteriosus, 283 
 
 Botalii, 283 
 
 Cuverii, 271, 278 
 
 venosus, 277 
 Dumb-bell bone, 69, loi 
 Duodenum, 227 
 Dura spinalis, 152 
 
 Ear, 182 
 
 bones, 73 
 
 external, 187 
 
 fimctions of, 188 
 
 inner, 183 
 
 middle, 187 
 
 stones, 186 
 Ect-ental line, 9 
 Ectethmoid, 67 
 Ectobronchus, 259 
 Ectochondrostosis, 43 
 Ectoderm, 9 
 Ectopterygoid, 80, 88 
 Ectoturbinals, 196 
 Efferent branchial artery, 274 
 
 duct of gills, 238 
 
 nerve root, 161 
 Egg, segmentation of, 8 
 
 teeth, 216 
 Ejaculator>' duct, 341 
 Elasmobranchs, brain, 153 
 
 copulator}' organs, 343 
 
 excretor}' organs, 326 
 
 gills, 239 
 
 Elasmobranchs, girdles of, 105 
 
 intestine, 228 
 
 reproductive organs, 331 
 
 skull, 76 
 Elastica externa, 45 
 
 interna, 44 
 Elastic tissue, 22 
 Electrical organs, 135 
 Electric plates, 135 
 Electroplax, 136 
 Embolomerous, 48 
 Embryology, i, 6 
 Embryonic tissue, 22 
 Eminentia medialis, 145 
 Enamel, 40 
 
 organ, 40, 209 
 Endocardium, 269 
 Endolymph, 185 
 
 duct, 183 
 
 sac, 183 
 End organs, 178 
 Endorhachis, 151 
 Endoskeleton, 38, 42 
 Ensiform process, 56 
 Entepicondylar foramen, 120 
 Enteroccele, 10 
 Enteropneusta, 2 
 Entobronchus, 259 
 Entochondrostosis, 43 
 Entoderm, 9 
 Entoglossal, 80, 97, 218 
 Entoplastron, 42, 159 
 Entopterygoid, 80 
 Entoturbinals, 196 
 Entovarial canal, 326 
 Envelopes of nervous system, 15] 
 Eparterial bronchi, 262 
 Epaxial muscles, 127 
 Ependyma, 139 
 Epibranchial cartilage, 65 
 Epibranchial ganglia, 176 
 
 muscles, 133 
 Epicardium, 124, 269 
 Epicoele, 143 
 Epicoracoid, 107 
 Epidermis, 25 
 Epididymis, 322 
 Epigastric artery, 288 
 
 vein, 289 
 Epiglottis, 252, 253 
 Epimerals, 54 
 Epimere, 13 
 Epineurals, 54 
 Epiotic, 67, 69 
 Epipharyngeal bones, 80 
 Epiphyses, 43 
 Epiphysial structures, 146 
 Epiphysis, 147 
 Epiplastron, 42, 108 
 Epipleurals, 54 
 Epipter\-goid, 82 
 Epipubis, III 
 Episternalia 109 
 
390 
 
 INDEX. 
 
 Episternum, 59 
 Epistropheus, 49 
 Epithelial bodies, 246 
 
 pigmented, of eye, 201 
 Epithelium, 17 
 Epitrichium, 25 
 Erectile tissue, 345 
 Erythrocytes, 265 
 Essence of pearl, 29 
 Ethmoidalia, 67 
 Ethmoid bone, 68 
 
 plate, 61 
 Ethmopalatine ligament, 77 
 Ethmo-turbinals, 195 
 Eustachian tube, 187, 237 
 Excitatory cells, 164 
 Excretory organs, 307 
 
 development of, 310 
 Exoccipital, 67 
 Extensor muscles, 132 
 External carotid artery, 275 
 
 ear, 187 
 
 gills, development of, 242 
 
 iliac artery, 288 
 Extrabranchial cartilages, 65 
 
 chamber, 240 
 Extrinsic muscles, 131 
 Eyelashes, 205 
 Eyelids, 203 
 Eye muscles, 128, 203 
 Eye- muscle nerves, 170 
 Eye, parietal, 147 
 Eyes, 198 
 
 Fabellae, 118 
 Facialis nerve, 172 • 
 Falciform process, 204 
 Fallopian tube, 338 
 False amnion, 350 
 
 rib, 55 
 Falx cerebri, 152 
 Fascia, 128 
 Fasciculi, 128 
 Fasciculus communis, 150 
 Fat, 22 
 
 bodies, 307 
 Fauces, isthmus of, 247 
 Feather tracts, 32 
 Feathers, 31 
 Femoral artery, 288 
 
 pores, 30 
 
 vein, 290 
 Fenestra hypophyseos, 61 
 
 ovale, 73, 186 
 
 rotunda, 186 
 
 tympani, 186 
 
 vestibuli, 73, 186 
 Fibrous tissue, 22 
 Fibula, 116 
 Fibulare, 117 
 Fifth ventricle, 151 
 Filoplumes, 31 
 Filum terminale, 140 
 
 Fins, 102 
 
 anal, 103 
 
 biserial, 115 
 
 caudal, 103 
 
 dorsal, 103 
 
 paired, 114 
 
 uniserial, 115 
 Fishes, circulation, 294 
 
 eyes, 204 
 
 fins, 115 
 
 gills, 238 
 
 girdles, no 
 
 glands, 27 
 
 intestine, 229 
 
 lateral line organs, 180 
 
 scales, 40 
 
 skin, 27 
 
 skull, 77 
 
 tails of, 50 
 
 teeth, 212 
 
 thymus, 245 
 
 thyreoid, 247 
 
 tongue, 217 
 
 vertebral column, 51 
 Fissures of brain, 149, 159 
 Flexor muscles, 132 
 Flocculi, 145 
 Flexures of brain, 143 
 Floor plate, 138 
 Foetal circulation, 293 
 
 envelopes, 348 
 Folian process, 74 
 Fontanelles, 61 
 Forebrain, 140 
 Foramen caecum, 219 
 
 epiploicum, 122 
 
 incisorum, loi 
 
 interventriculares, 143 
 
 lacerum anterior, 67 
 
 magnum, 67 
 
 of Monro, 143 
 
 of Panniza, 282 
 
 of Winslow, 122 
 Fornix, 151 
 Fossa hypophyseos, 61 
 
 rhomboidea, 144 
 Fossae of skull, 71 
 Fovea centralis, 200 
 Free appendages, 114 
 
 nerve terminations, 178 
 Frontal bones, 68 
 
 lobes, 159 
 
 organs, 147 
 Fundus glands, 224 
 Furcula, 108 
 
 Gall. 231 
 
 bladder, 233 
 
 capillaries, 233 
 Ganglia of dorsal roots, 161 
 Ganglion, 20 
 
 cell, 19 
 
 of retina, 200 
 
 i 
 
INDEX. 
 
 391 
 
 Ganoids, excretory organs, 327 
 
 reproductive ducts, 322 
 
 scales, 40 
 
 skull of, 78 
 Ganoin, 40 
 
 Gasserian ganglion, 171 
 Gastralia, 41 
 Gastric artery, 284 
 Gastrula, 9 
 Gastrulation, 9 
 Geniculate ganglion, 172 
 General cutaneus nerves, 167 
 Geniohyoid muscle, 130 
 Genital artery, 286 
 
 prominence, 344 
 Geological distribution, 7 
 Germ layers, 11 
 Gill arches, 63 
 
 basket, 75 
 
 clefts, 236 
 
 cover, 77, 240 
 
 pouches, 239 
 
 remnants, 246 
 Gills, 236 
 Girdles, 104 
 Gizzard, 225 
 Gladiolus, 56 
 Glands, 18 
 
 of amphibia, 29 
 
 of birds, 30 
 
 buccal, 221 
 
 cardiac, 224 
 
 carotid, 297 
 
 chorioid, 202 
 
 Cowper's, 342 
 
 excretory, 307 
 
 of fishes, 27 
 
 fundus, 224 
 
 Harder' s, 204 
 
 hibernating, 307 
 
 intermaxillary, 220 
 
 intemasal, 220 
 
 labial, 22 
 
 lacrimal, 204 
 
 lingual, 221 
 
 of mammals, 35 
 
 mammary, 36 
 
 meiobomian, 205 
 
 milk, 36 
 
 molar, 221 
 
 oral, 220 
 
 orbital, 221 
 
 palatal, 220 221 
 
 parotid, 221 
 
 prostate, 342 
 
 poison, 27, 221 
 
 pyloric, 224 
 
 rectal, 228 
 
 of reptiles, 30 
 
 retrolingual, 221 
 
 sexual, 308 
 
 sublingual, 221 
 
 submaxillary, 221 
 
 Glands, tarsal, 205 
 
 tear, 204 
 
 uropygial, 30 
 Glandula membrana nictitans, 204 
 Glandular epithelium, 18 
 Glaserian fissure, 74 
 Glenoid fossa, 100, 104 
 Glia, 139 
 
 cells, 19, 20 
 Glomerulus, 309 
 
 Glomeruli of olfactory nerve, 167 
 Glomus, 312 
 
 Glossopharyngeal nerve, 175 
 Glottis, 251 
 Gluteus muscle, 132 
 Gonads, 308, 319 
 Goniale, 71 
 Gonotomes, 319 
 Graafian follicle, 320 
 Grandry's corpuscle, 179 
 Gray matter, 20 
 
 of cord, 139 
 Great omentum, 122 
 Guanin, 29 
 Gubemaculum, 338 
 Gular bones, 79 
 Gyri, 149, 160 
 
 Habenular ganglion, 146 
 Haemopophysial ribs, 53 
 Haemal spine, 46 
 Haemapophysis, 46 
 Hair, 33 
 Hair cells, 186 
 Hallux, 117 
 Hamatum, 117 
 Harder's glands, 204 
 Hare-lip, 193 
 Haversian canals, 23 
 Head cavities, 128 
 
 kidney, 310 
 
 rib, 81 
 Heart, 281 
 
 branchial, 281 
 
 development, 269 
 
 division of, 281 
 
 muscles, 125 
 
 portal, 294 
 
 structvu-e, 269 
 
 venous, 281 
 Hemiazygos vein, 302 
 Hemipenes, 344 
 Hemispheres, cerebellar, 161 
 Hemispheres, cerebral, 141 
 Henle's loop, 309 
 Hepar, 231 
 Hepatic artery, 284 
 
 duct, 233 
 
 -portal system, 277 
 
 veins, 277 
 Herbst's corpuscle, 179 
 Hermaphroditism, 346 
 Heterocercal, 50 
 
392 
 
 INDEX. 
 
 Hibernating glands, 307 
 Highmore, antrum of, 197 
 Hilum of kidney, 330 
 Hippocampus, 148 
 Hlndbrain, 141 
 Histology, I, 16 
 Holonephros, 310 
 Holorhinal, 96 
 Homocercal, 50 
 Honey comb, 227 
 Hoofs, 27 
 
 Hormones, 18, 353 
 Horns, 10 1 
 Humerus, 116 
 Humors of eye, 203 
 Hyoid apparatus, 220 
 Hyoid arch, 63 
 Hyoideus nerve, 172 
 Hyomandibular bone, 73 
 
 cartilage, 6^ 
 
 nerve, 172 
 Hyoplastron, 42 
 Hyostylic, 73 
 Hyparterial bronchi, 262 
 Hypaxial muscles, 127 
 Hypobranchial, 65 
 Hypocentrum, 47 
 Hypocone, 214 
 Hypoconid, 214 
 Hypogastric artery, 276, 285 
 
 plexus, 163 
 
 vein, 290 
 Hypoglossal muscles, 128 
 
 nerve, 177 
 Hypoischium, iii 
 Hypomere, 13 
 Hypopharyngeals, 80 
 Hypophysial duct, 191 
 Hypophysis, 148 
 Hypoplastron, 42 
 Hypurals, 50 
 
 Ichthyopterygia, 114 
 Ileum, 228 
 Ileo-caecal valve, 228 
 Ileo-colic valve, 228 
 Ileocostal muscle, 131 
 Iliac artery, 288 
 
 vein, 289 
 Ilium, 109 
 
 Incisive foramina, 10 1 
 Incisors, 213 
 Incus, 74 
 Inferior jugular vein, 278 
 
 mesenteric artery, 285 
 
 oblique muscle, 128 
 
 turbinal, 100 
 Infraclavicle, 106 
 Infratemporal fossa, 71 
 Infundibulum, 148, 256 
 Ingluvies, 223 
 Inner ear, 183 
 Innominate vein, 300 
 
 Insertion of muscles, 129 
 Insula 160 
 Integument, 25 
 Interbranchial septum, 237 
 Intercalare, 47 
 Intercellular substance, 21 
 Intercentrum, 48 
 Intercerebral fissure, 148 
 Interclavicle, 59 
 Intercostal arteries, 275, 286 
 
 muscles, 130 
 Interhyal, 80 
 
 Intermaxillary glands, 220 
 Intermedium, 117 
 Internal iliac artery, 288 
 
 vein, 290 
 Internal secretion, 18 
 Internasal gland, 220 
 Interoperculura, 77 
 Interorbital septum, 61 
 Interparietal bone, 68 
 Interrenal organs, 352 
 
 veins, 291 
 Interspinous ligament, 48 
 Interstitial cells, 342 
 Intertemporal bones, 100 
 Intestine, 227 
 Intratarsal joint, 118 
 Intrinsic muscles, 131 
 Invagination, 9 
 Inverted eye, 200 
 Involuntary muscles, 20, 125 
 Iris, 202 
 Ischiadic artery, 288 
 
 vein, 290 
 Ischio-pubic fenestra, 109 
 Ischio-pubis, 112 
 Ischium, 109 
 Island of Riel, 160 
 Isthmus, 141 
 Iter, 143 
 Ivory, 209 
 
 Jacobson's commissure, 165, 171 
 
 gland, 194 
 
 organ, 190, 196 
 Jaws, 63 
 Jejunum, 228 
 Jugal bone, 70 
 Jugular ganglion, 176 
 
 lymph sac, 303 
 
 vein, 279 
 
 Kidney, 310 
 
 development of, 316 
 Krausse's corpuscle, 179 
 
 Labial cartilages, 65 
 
 glands, 220, 221 
 Labyrinth of ear, 185 
 
 nasal, 192 
 Lacrimal bone, 69 
 
 duct, 204 
 
INDEX. 
 
 393 
 
 Lacrimal gland, 204 
 
 Lacteals, 304 
 
 Lacunae, 23 
 
 Lagena, 184 
 
 Lamina terminalb, 141 
 
 LarjTigeal cartilages, 64 
 
 Laryngeal ventricle, 253 
 
 Larynx, 250, 251 
 
 Lateral abdominal vein, 289 
 
 column of cord, 139 
 
 comu, 139 
 
 ethmoid, 67 
 
 line lobe, 145 
 
 line organs, 173, 179 
 
 plate, 13 
 Lateralis nervous system, 173 
 Latissimus dorsi, 132 
 Legs, 116 
 Lens of eye, 199 
 Leptocardii, 2 
 Leucocytes, 265 
 Levator muscles, 131 
 
 scapulae, 132 
 Leydig's duct, 315 
 Lids of eye, 203 
 Ligament, interspinous, 48 
 
 of ovar}', 338 
 
 of testis, 338 
 Ligamentum medium pelvis, in 
 
 teres, 289 
 Linea alba, 127 
 Lingual glands, 221 
 Lingualis nerve, 171 
 Lips, 208 
 Liver, 231 
 Longissimus capitis muscle, 13 1 
 
 dorsi muscle, 131 
 Lophodont, 214 
 Lophs, 214 
 
 Lorenzini's ampullae, 182 
 Lower jaw, 71 
 Lumbar arterj-, 2 86 
 
 plexus, 163 
 
 vertebrae, 49 
 Luminous organs, 28 
 Lunatum, 117 
 Lungs, 250, 255 
 Lungs, phylogeny of, 262 
 Lung pipes, 259 
 Lungless salamanders, 299 
 Lutein cells, 320 
 Lymph, 265 
 
 glands, 302, 306 
 
 hearts, 302, 304 
 
 nodules, 306 
 
 sacs, 303 
 •stomata, 302 
 
 vessels, development of, 302 
 Lymphatic system, 302 
 Lymphocytes, 266 
 Lyssa, 219 
 
 Macula lutea, 200 
 
 Maculae acusticae, 185 
 
 Malar bone, 70 
 
 Male ducts, 321 
 
 Malleus, 74 
 
 Malpighian corpuscle, 309, 314 
 
 Malpighian layer, 25 
 
 Mammals, brain, 158 
 
 circulation, 300 
 
 dermal skeleton, 41 
 
 excretory organs, 328 
 
 foetal envelopes, 348 
 
 gill pouches, 237, 244 
 
 girdles of, 108, 113 
 
 glands of, 35 
 
 intestine, 230 
 
 larynx, 254 
 
 limbs, 119 
 
 lungs, 262 
 
 reproductive organs, 335 
 
 salivary glands, 221 
 
 skin of, 33 
 
 skull of, 98 
 
 stomach, 225 
 
 teeth, 213 
 
 thymus, 246 
 
 thyreoid, 247 
 
 tongue, 219 
 
 vertebral column of, 53 
 Mammary glands, 36 
 Mandibular arch, 63 
 
 arteries, 271 
 
 nerve, 171 
 Mantle of cerebrum, 141 
 Manubrium, 56 
 
 mallei, 74 
 Manus, 116 
 Manyplies, 227 
 Marsupial bones, 114 
 Masseter muscle, 133 
 Mastoid process, 100 
 Matrix, 21 
 
 Maxillaris extemus nerve, 172 
 Maxillar)' bone, 70 
 
 nerve, 171 
 Maxillo-turbinals, 196 
 Meatus, external auditory, 187 
 Meckelian cartilage, 63, 71 
 Mediastinum, 16, 122 
 Medullary cords, 321 
 
 groove, II 
 
 plate, II 
 
 sheath, 19 
 Medulla oblongata, 142 
 Meibomian glands, 205 
 Meissner's corpuscle, 179 
 Membrana tectoria, 186 
 Membrane bones, 42, 65 
 
 bones of skull, 68 
 Membranous lab}Tinth, 185 
 
 skeleton, 37 
 Meninges, 151 
 Meninx primitiva, 151 
 Mento-Meckelian bone, 71 
 
394 
 
 INDEX. 
 
 Merkel's corpuscle, 179 
 
 Mesencephalon, 142, 145 
 
 Mesenchyme, 10, 16 
 
 Mesenteric arteries, 284 
 
 Mesenteries, 14, 121 
 
 Mesenteron, 205 
 
 Mesethmoid, 67 
 
 Mesobronchus, 259 • 
 
 Mesocardia, 16, 122, 270 
 
 Mesocolon, 122 
 
 Mesoderm, 10 
 
 Mesogaster, 15, 122 
 
 Mesohepar, 15, 121 
 
 Mesomere, 13 
 
 Mesonephric tubules, 313 
 ducts, 313, 315 
 
 Mesonephros, 310, 313 
 
 Mesopterygium, 115 
 
 Mesopterygoid, 80 
 
 Mesorchium, 16, 122, 319 
 
 Mesothelium, 10 
 
 Mesorectum, 15, 122 
 
 Mesovaria, 16, 122, 319 
 
 Metacarpals, 117 
 
 Metacarpus, 116 
 
 Metacoecle, 123, 143 
 
 Metacone, 214 
 
 Metaconid, 214 
 
 Metamerism, 13 
 
 Metanephros, 310 
 .Metapodium, 116 
 
 Metapterygium, 115 
 
 Metapterygoid, 80 
 Metatarsale, 117 
 Metatarsus, 116 
 
 Metazoa, i 
 
 Metencephalon, 142 
 
 Midbrain, 141, 145 
 
 Middle ear, 187 
 plate, 13 
 turbinal, 100 
 Milk dentition, 211 
 glands, 36 
 line, 36 
 points, 36 
 Minimus, 117 
 Mitral valve, 281 
 Mixipterygium, 116, 343 
 Molar gland, 221 
 Molars, 213 
 Monimostylic, 88 
 Monophyodont dentition, 212 
 Monorhinal, 191 
 Monro, foramen of, 143 
 
 sulcus of, 141 
 Morphology, i 
 Mossy cells, 20 
 Motor-nerve root, 162 
 Mouth, 208 
 Miillerian duct, 315 
 Multangulum, 117 
 Multicellular glands, 18 
 Muscle plate, 13 
 Muscles of appendages, development of, 131 
 
 Muscles of, dermal, 134 
 
 visceral, 132 
 Muscular system, 124 
 
 tissue, 20 
 Myelencephalon, 142 
 Myocardium, 269 
 Myocoele, 14, 121, 126 
 Myocommata, 127 
 Myoepicardial mantle^ 270 
 Myofibrillae, 20 
 Mylohyoid muscle, 133 
 Myosepta, 38, 127 
 Myotomes, 14, 121, 127 
 
 Nails, 27 
 
 Nares, external, 190 
 
 internal, 193 
 Naro-hypophysial duct, 191 
 Nasal bones, 68 
 
 capsules, 190 
 Naso-palatal canal, 197 
 Naso-pharyngeal duct, 194 
 Naso-turbinals, 195 
 Navel cord, 351 
 Naviculare, 117 
 
 pedis, 117 
 Nepopallium, 148 
 Nephridia, 308 
 Nephridial arteries, 275, 285 
 Nephrotomes, 14, 308, 311 
 Nerve cell, 19 
 Nerve-end apparatus, 178 
 Nerve of Weber, 173, 189 
 Nervous system, 137 
 
 central, 138 
 
 development of, 137 
 
 tissue, 19 
 Neural arch, 45 
 
 crest, 161 
 
 folds, II 
 
 plate, II 
 
 spine, 46 
 Neurapophysis, 45 
 Neurenteric canal, 12 
 Neuroglia, 20, 139 
 Neuromasts, 167 
 Neuron, 19 
 Neuropore, 12 
 Nictitating membrane, 203 
 Non-elastic tissue, 22 
 Nose, 197 
 Notochord, 12, 44 
 
 sheath of, 45 
 Nuchal flexure of brain, 143 
 Nuclei of brain, 144 
 Nucleus dentatus, 145 
 
 Oblique muscles, 130 
 
 of eye, 128 
 Obturator foramen, 109 
 Occipitalia, 66 
 Occipital bone, 68 
 lobes, 159 
 
INDEX. 
 
 395 
 
 Occipital vertebrae, 62 
 Oculomotor nerve, 170 
 Odontoblasts, 39, 209 
 Odontoid process, 50 
 (Esophago-cutaneus duct, 239 
 (Esophagus, 222 
 Olecranon process, 120 
 Olfactory bulb, 142, 167 
 
 duct, 194 
 
 lobe, 141 
 
 nerve, 167 
 
 organs, 189 
 
 sac, 190 
 
 tract, 142 
 Oliva, 145 
 Olivar}' bodies, 145 
 Omasum, 227 
 Omentum, 15, 122 
 Omostemum, 57 
 Omphalomesaraic artery, 276 
 
 vein, 271 
 Omphalomesenteric artery, 276 
 
 vein, 271 
 Ontogeny, i 
 Operculare, 77 
 Opercular gill, 241 
 Operculum, 77, 240 
 Ophthalmic nerve, 171 
 Ophthalmicus profundus nerve, 171 
 
 superficialis nerve of seventh, 172 
 of fifth, 171 
 Opisthocielous, 46 
 Opisthotic, 67 
 Optic capsule, 202 
 
 chiasma, 169 
 
 cup, 198 
 
 ganglion, 169 
 
 lobes, 142, 169 
 
 nerve, 169, 200 
 
 pedicel, 203 
 
 recess, 141 
 
 stalk, 198 
 
 thalami, 142, 146 
 
 tract, 146 
 
 vesicle, 198 
 Oral cavity, 208 
 
 glands, 220 
 
 hood, 208 
 
 plate, 205 
 Orbicular muscles, 134 
 Orbital gland, 221 
 Orbitosphenoid, 67 
 Organ of Corti, 186 
 
 of Jacobson, 190, 196 
 Origin of muscles, 129 
 Oronasal groove, 193 
 Os cloacae, in 
 
 en ceinture, 86 
 
 entoglossum, 97, 218 
 
 pubis, 109 
 
 transversum, 88 
 
 uncinatum, 96 
 Ossa suprastemalia, 58 
 
 Ossein, 23, 39 
 
 Ossicula auditus, 73, 187 
 
 Ossification, 43 
 
 Osteoblasts, 39 
 
 Ostium tubae abdominale, 324 
 
 Otic bones, 67 
 
 capsule, 60, 67 
 
 ganglion, 171 
 Otoliths, 185 
 Ovarial cords, 320 
 Ovarian artery, 286 
 Ovaries, 308, 320 
 Oviduct, 316, 321, 323 
 Ovo testis, 346 
 Ovum, 8 
 
 Pacini's corpuscle, 179 
 Paired appendages, 103 
 
 fins, 114 
 Palatal glands, 220, 221 
 Palatine bones, 69 
 
 nerve, 172 
 Palatoquadrate cartilage, 63 
 Pallium, 141, 148 
 Pancreas, 234 
 Pancreatic duct, 235 
 Panniculus camosus, 134 
 Parabronchi, 259 
 Parachordal plates, 60 
 Paracone, 214 
 Paraconid, 214 
 Paraglossae, 218 
 Paraglossal, 97 
 Paramastoid process, 99 
 Paraphysis, 146 
 Parapophysis, 46 
 Parasphenoid, 69 
 Parietal bones, 68 
 
 eye, 147 
 
 foramen, 68 
 
 lobes, 159 
 
 muscles, 125 
 
 organ, 147 
 Paroccipital process, 99 
 Parotic process, 93 
 Parotid gland, 221 
 
 of Amphibia, 29 
 Parovarial canal, 325 
 Parovarium, 341 
 PateUa, 118, 120 
 Paunch, 227 
 Pearl organs, 27 
 Pecten of eye, 204 
 Pectineal process, no, 113 
 Pectineus muscle, 132 
 Pectoral girdle, 105 
 Pectoralis muscle, 132 
 Pedimcles of cerebellum, 150 
 Pelvic girdle, 109 
 
 plexus, 163 
 Pelvis, 109 
 
 of kidney, 330 
 Penis, 345 
 
396 
 
 INDEX. 
 
 Pepsin, 224 
 
 Pericardial cavity, 14, 123 
 
 fluid, 269 
 Pericardio-peritoneal canals, 123, 271 
 Pericardium, 124, 269 
 Perichondrium, 39 
 Periderm, 25 
 Peridural space, 152 
 Perilymph, 186 
 
 duct, 186 
 Perimeningeal space, 151 
 Perimysium, 21, 128 
 Perineurium, 20 
 Periosteum, 24, 39 
 Peripheral nervous system, 161 
 Peristalsis, 207, 227 
 Peritoneal canals, 124 
 
 cavity, 14, 123 
 Peritoneum, 124 
 Permanent dentition, 211 
 Peroneal arter}% 288 
 Perpendicular plate of ethmoid, 100 
 Pes, 116 
 Pessulus, 255 
 Petrosal bones, 67 
 
 ganglion, 175 
 Petrotympanic fissure, 74 
 Phasochrome cells, 352 
 Phalanges, 116 
 Pharyngeal bones, 80 
 
 derivatives, 245 
 
 plate, 205 
 
 tonsils, 247 
 Pharyngobranchial, 65 
 Pharynx, 207, 222, 236 
 Phosphorescent organs, 28 
 Photophores, 28 
 Phyllospondylous, 47 
 Physiology, i 
 Physoclistous, 248 
 Physostomous, 248 
 Pia mater, 152 
 Pigment cells, 22, 26 
 
 layer of eye, 198 
 Pigmented epithelium of eye 20 r 
 Pillar cells, 186 
 Pinealis, 147 
 Pisiforme, 117 
 Pituitary body, 148 
 Placenta, 318, 351 
 
 vitelline, 348 
 Placoid scale, 40 
 Plantigrade, 120 
 Plasma, blood, 265 
 Plastron, 41 
 Platybasic skull, 61 
 Platysma myoides, 134 
 Pleura, 124 
 Pleural cavities, 123 
 
 rib, 53 
 Pleurapophysis, 46 
 Pleurocentrum, 47 
 Pleurodont. 88 
 
 Pleurodont dentition, 2 13 
 Plexus, chorioid, 144 
 Plexuses, 163 
 Plica fimbriata, 219 
 
 semilunaris, 203 
 Plumae, 31 
 Plumulae, 31 
 Pneumatic bones, 96 
 
 duct, 248 
 Pneumatocyst, 247 
 Pneumogastric nerve, 176 
 Podium, 116 
 Poison glands, 27, 221 
 Pollex, 117 
 Polymastism, 37 
 Polyphyodont denition, 211 
 Pons (Varolii), 150 
 Pontal flexure of brain, 144 
 Popliteal artery, 288 
 Pori abdominales, 124, 322 
 Portal circulation, 277 
 
 heart, 294 
 
 vein, 277 
 Postbranchial bodies, 246 
 Postcardinal vein, 279 
 Postcava, 290 
 Postclavicle, 106 
 Posterior chamber of eye, 203 
 
 column of cord, 139 
 
 cornua, 139 
 
 fissure of cord, 139 
 
 horns of cord, 139 
 
 lymph sac, 303 
 Postfrontal bone, 69 
 Posthepatic digestive tract, 206 
 Postminimus, 118 
 Postorbital bone, 69 
 Postotic nerves, 175 
 Postparietal bone, 69 
 Postpermanent dentition, 212 
 Postpubic process, 112 
 Post-temporal bone, 106 
 
 fossa, 71 
 Post-trematic nerves, 175 
 Postzygapophysis, 46 
 Precava, 300 
 Predentary bone, 88 
 Prefrontal bones, 69 
 Prefrontals of birds, 96 
 Prehallux, 118 
 
 Prehepatic digestive tract, 206 
 Prelacteal dentition, 212 
 Premaxillary bone, 70 
 Premolars, 213 
 Prenasal bone, 100 
 Preoperculum, 77 
 Prepollex, 118 
 Prepubic process, iii 
 Presphenoid, 67 
 Presternalia, 109 
 Pretrematic nerves, 175 
 Prevertebral ganglia, 163 
 Prevomer, 69 
 
INDEX. 
 
 397 
 
 Prezygapophysis, 46 
 Primilive groove, to 
 
 ova, 319 
 
 streak, 10 
 Primordial ova, ^nj 
 Process, articular, 46 
 
 transverse, 46 
 Proccelous, 46 
 Procoracoid, 107 
 Proctodeum, 13, ?o6 
 Pronephric duct, 312 
 
 tubules, 311 
 Pronephros, 310 
 Prootic, 67 
 Propterygium, 115 
 Prosencephalon, 141 
 Prostate glands, 342 
 Prostemum, 58 
 
 Proterandric hermaphroditism, 346 
 Proteroglypha, 213 
 Protocone, 214 
 Protoconid, 214 
 Pro to vertebrae, 14 
 Protractor muscles, 131 
 Proventriculus, 225 
 Psalterium, 227 
 Pseudobranch, 241 
 Pseudoconch, 194 
 Pterotic, 67 
 Pterygoid bones, 69, 80 
 
 muscle, 133 
 Pterygoquadrate, 63, 69 
 Pterylae, 32 
 Ptj-alin, 220 
 
 Pubofemoralis muscle, 132 
 Pubis, 109 
 Pulmonar>- arteries, 283 
 
 circulation, 282 
 
 veins, 292 
 Pulmones, 236 
 Pulp of tooth, 210 
 Pupil, 202 
 Pygostyle, 53 
 Pyloric caeca, 227 
 
 gland, 224 
 Pylorus, 223 
 P}Tamidalis musde, 130 
 Pyramidal tracts, 150 
 PjTiform lobes, 159 
 Pyriformis muscle, 132 
 
 Quadrate bone, 69, 74 
 Quadratogugal bone, 70 
 Quadritubercular, 214 
 
 Radial arter)-, 288 
 
 cells, 200 
 Radiale, 117 
 Radialia, 103 
 Radius, 116 
 Radix aortae, 273 
 Rami communicantes, 163 
 Ramus dorsalis, 162 
 
 Ramus intestinalis, 162 
 
 of tenth nerve, 176 
 
 lateralis accessorius, 173 
 of tenth nerve, 176 
 
 ventralis, 162 
 
 visceralis, 162 
 Rathke's pocket, 148, 206 
 Rectal gland, 228 
 Rectxun, 228 
 Rectus abdominis muscle, 130 
 
 capitis muscles, 131 
 
 muscles, 128, 130 
 of eye, 128 
 Red spots, 249 
 Reissner's fibres, 151 
 Renal artery, 286, 300 
 
 corpuscle, 309 
 
 portal system, 280 
 Rennet, 227 
 Respiration, accessor)' structures, 263 
 
 mechanism of, 263 
 Respiratory circulation, 282 
 
 duct, 194 
 
 organs, 235 
 Reproductive ducts, 321 
 
 organs, 308, 319 
 Reptiles, see also Sauropsida, Amniotes. 
 
 aortic arches, 283 
 
 brain, 157 
 
 circulation, 299 
 
 copulatory organs, 344 
 
 dermal skeleton, 41 
 
 gill pouches, 244 
 
 girdles, 107, iii 
 
 glands, 30 
 
 intestine, 229 
 
 larynx, 251 
 
 limbs, 118 
 
 limgs, 258 
 
 scales, 30 
 
 skull, 87 
 
 thymus, 246 
 
 thyreoid, 247 
 
 tongue, 217 
 
 vertebral column, 52 
 Rete mirabile, 249, 267 
 Reticulum, 227 
 Retina, 199 
 Retinal arter>', 201 
 
 ganglion, 200 
 
 layer of eye, 198 
 
 vein, 201 
 Retractor bulbi, 128, 203 
 
 muscles, 131 
 Retrolingual gland, 221 
 Revehent vein, 291 
 Rhachitomous, 48 
 Rhinencephalon, 141 
 Ribs, 53 
 
 abdominal, 41 
 Rods of eye, 199 
 Roof plate, 138 
 Roots of spinal nerves 161 
 
398 
 
 INDEX. 
 
 Rostral bone, 88 
 
 cartilage, 76, 85 
 Rostrum sphenoidale, 96 
 Rotator muscles, 132 
 Rumen, 227 
 
 Saccule- ventricular canal, 184 
 Sacculus endolymphaticus, 183 
 
 of ear, 184 
 Saccus vasculosus, 148 
 Sacral artery, 286 
 
 plexus, 163 
 
 vertebrae, 49 
 Sacrum, 49 
 Salivary glands, 220 
 Santorini's duct, 235 
 Sarcolemma, 21 
 Saurognathous, 97 
 Sauropsida, eyes, 204 
 
 excretory organs, 327 
 
 foetal envelopes, 348 
 
 reproductive organs, 334 
 
 teeth, 212 
 Savi's ampullae, 182 
 Scala media, 185 
 Scalae of ear, 186 
 Scalene muscles, 130 
 Scales, 26, 39 
 
 ctenoid, 40 
 
 cycloid, 40 
 
 development of, 39 
 
 ganoid, 40 
 
 of mammals, 34 
 
 placoid, 40 
 Scaphoid, 117 
 Scapular region, 105 
 Schizocoele, 10 
 Schizorhinal, 96 
 Sciatic artery, 288 
 
 vein, 290 
 Sclera, 62, 202 
 Scleroblasts, 39 
 Sclerotic bones, 67, 203 
 
 coat, 62 
 Sclerotomes, 14 
 Scrotum, 321, 338 
 Secodont, 214 
 Seessel's pocket, 206 
 Segmentation cavity, 8 
 
 of egg, 8 
 Selenodont, 214 
 Sella turcica, 61 
 Semicircular canals, 184 
 Semilunar fold, 203 
 
 ganglion, 171 
 Seminal vesicles, 342 
 Seminiferous tubule, 321 
 Sense hillocks, 167 
 Sensory epithelium, 17 
 
 nerve root, 162 
 
 organs, 177 
 Septum, interorbital, 61 
 
 of cerebrum, 148 
 
 Septum pellucidum, 151 
 
 transversum, 123, 271 
 Serosa, 350 
 
 Serous coat of digestive tract, 207 
 Serratus anterior muscle, 132 
 Sertoli's cells, 321 
 Serum, 265 
 Sesamoid bones, 129 
 Sheath of Schwann, 20 
 Shoulder girdle, 105 
 Sex, determination of, 347 
 Sexual cords, 320 
 
 organs, 308 
 Sight, organs of, 198 
 Sinus, cervical, 244 
 
 frontal, 197 
 
 impar, 250 
 
 maxillary, 197 
 
 of Morgagni, 253 
 
 sphenoidal, 197 
 
 terminalis, 277 
 
 urogenitalis, 322 
 
 venosus, 272 
 Sinusoids, 267 
 Sixth sense, 182 
 Skeletal labyrinth, 186 
 Skeleton, 37 
 
 appendicular, 102 
 
 axial, 43 
 
 dermal, 38 
 
 membranous, 37 
 
 visceral, 63 
 Skin, 25 
 
 of mammals, ^^ 
 Skull, 59 
 
 development of, 59 
 
 table of bones of, 72 
 Small omentum, 122 
 Smell, organs of, r89 
 Smooth muscles, 20 124 
 Soft commissure, 146 
 Solar plexus, 163 
 Solenoglypha, 213 
 Somatic layer, 10 
 
 motor nerves, 165 
 
 nerves, 162 
 
 wall, 121 
 Somatopleure, 15, 121 
 Spermatic artery, 286 
 Spermatozoon, 8 
 Sphenethmoid, 86 
 Sphenoid bone, 68 
 Sphenoidalia, 67 
 Sphenoidal fissure, 67 
 
 turbinal, 100 
 Sphenopalatine ganglion, 165, 171 
 Sphenotic, 67 
 
 ganglion, 165 
 Sphincter muscles, 129 
 
 pupillae, 202 
 Spina scapulae, 109 
 Spinal artery, 287 
 
 cord, 138 
 
INDEX. 
 
 399 
 
 Spinal muscles, 131 
 
 nerves, 161 
 Spiracle, 238 
 Spiral valve, 228 
 Splanchnic layer, 10 
 
 wall, 121 
 Splanchnoccele, 121 
 Splanchnopleure, 15, 121 
 Spleen, 307 
 Splenial bone, 71 
 Splenic artery, 284 
 Squamosal bone, 70 
 Squamous epithelium, 18 
 Squatina, genus of sharks. 
 Stapes, 73, 186 
 Steapsin, 234 
 Stenon's duct, 221 
 Stenson's gland, 197 
 Sternal rib, 54 
 Stemebrae, 57 
 
 Sternocleidomastoid muscle, 130 
 Sternohyoid muscle, 130 
 Sternum, 56 
 
 abdominal, 57 
 Stomach, 223 
 
 Stomata of lymph system, 302 
 Stomodeum, 12, 205 
 Stratified epithelium, 18 
 Stratum comeum, 25 
 
 germinativum, 25 
 Streptostylic, 87 
 Striped muscles, 20, 125 
 Styloid process, 100 
 Subarachnoid space, 152 
 Subcardinal veins, 279 
 Subclavian artery, 288 
 
 vein, 289 
 Subcutis, 26 
 Subdural space, 152 
 Sub intestinal vein, 276 
 Sublingua, 219 
 Sublingual gland, 221 
 Submaxillary ganglion, 171 
 
 gland, 221 
 Suboperculum, 77 
 Subspinal muscles, 133 
 Subunguis, 27 
 Sulci, 160 
 
 Sulcus of Monro, 141 
 Superficial petrosal nerve, 173 
 Superior intercostal vein, 302 
 
 jugular vein, 279 
 
 mesenteric artery, 284 
 
 oblique muscle, 128 
 
 turbinal, 100 
 Supracleithra, 106 
 Supracondylar foramen, 120 
 Supraethmoid, 80 
 Suprangulare, 71 
 Supraoccipital, 67 
 Supraorbital bones, 69 
 Suprapericardial bodies, 246 
 Suprarenal organs, 352 
 
 Suprascapula, 105, 107 
 Suprasternalia, 58 
 Supratemporal bone, 69, 106 
 
 fossa, 71 
 Suspensor, 63, 69, 73 
 Sutures, 38 
 Sweetbreads, 246 
 Swim bladder, 247 
 Sylvian fissure, 159 
 Sympathetic ganglia, 165 
 
 system, 163 
 
 trunk, 163 
 Symplectic, 73, 80 
 Synarthrosis, 38 
 Synotic tectum, 61 
 Synovial membrane, 38 
 Synsacrum, 53 
 Syrinx, 254 
 
 Systemic circulation, 282 
 Systole, 272 
 
 Tabulare, 69 
 Tactile corpuscles, 179 
 Taenia marginalis, 98 
 Tails of fishes, 50 
 Talon, 214 
 Talus/ 117 
 
 Tapetum lucidum, 202 
 Tarsale, 117 
 Tarsal glands, 205 
 Tarso-meta tarsus, 119 
 Tarsus, 116 
 Taste buds, 189 
 
 organs of, 189 
 Tear gland, 204 
 Tectorial membrane, 186 
 Teeth, 208 
 
 development of, 209 
 
 epidermal, 215 
 
 phylogeny, 215 
 Tegmen cranii, 61 
 Tela subjunctiva, 26 
 Telencephalon, 141, 148 
 
 brain, 153 
 
 breathing valves^ 241 
 
 excretory organs, 327 
 
 girdles, 105 
 
 skull, 77 
 
 reproductive organs, 332 
 Temnospondylous, 48 
 Temporal fossa, 7 1 
 
 lobes, 159 
 Temporalis muscle, 133 
 Tenaculum, 203 
 Tendons, 129 
 Tentorium, 152 
 Terminalis nerve, 169 
 Testes, 308, 320 
 
 descent of, 338 
 Thalamencephalon, 142 
 Thalamic nerve, 169 
 Thalamus, 142, 146 
 Thecodont, 88, 213 
 
400 
 
 INDEX. 
 
 Thoracic aorta, 284 
 
 duct, 303 
 
 vertebrae, 49 
 Thread cells, 29 
 Thymus glands, 245 
 Thyreoid cartilage, 252 
 
 gland, 246 
 Thyrohyals, 102 
 Tibia, 116 
 Tibiale, 117 
 Tibial artery, 288 
 Tibio-tarsus, 119 
 Tissue, 16 
 Tongue, 217 
 Tonsils, 247, 307 
 Trabecula cranii, 61 
 
 communis, 61 
 Trachea, 250, 254 
 Tractus oKactorius, 142, 167 
 
 solitarius, 150 
 Transverse bone, 88 
 
 muscles, 130 
 
 process, 46, 55 
 Transverso-spinal muscles, 131 
 Trapezium, 117 
 Trapezius muscle, 132 
 Trapezoides, 117 
 Triconodont, 214 
 Tricuspid valve, 281 
 Trigeminal nerve, 170 
 Triquetrum, 117 
 Tritubercular, 214 
 Trochanter, 120 
 Trochlearis nerve, 170 
 Tropibasic skull, 61 
 Truncus arteriosus, 272 
 
 transversus, 271 
 Trypsin, 234 
 Tuber acusticum, 145 
 Tubular glands, 18 
 Tubercular head of rib, 54 
 Tuberculum impar, 217 
 Tunica albuginea, 341 
 
 serosa, 121 
 
 vasculosa of eye, 201 
 Tunicata, 2 
 
 Turbinal bones, 67, 100, 195 
 Turtles, armor of, 41 
 'Twixt-brain, 142, 148 
 Tympanic annulus, 82 
 
 bone, 100 
 
 membrane, 187 
 Tympanum of ear, 187 
 
 of syrinx, 255 
 
 Ulna, 116 
 
 Ulnare, 117 
 Ulnar artery, 288 
 
 lymph duct, 303 
 Umbilical artery, 285 
 
 cord, 351 
 
 veins, 278 
 
 vesicle, 348 
 
 Umbilicus of feather, 31 
 Unguis, 27 
 Unguligrade, 120 
 Uncinate bone, 97 
 Unicellular glands, 18 
 Uniserial fin, 115 
 Upper jaw, 70 
 Ureter, 318 
 Urethra, 319, 331 
 Urinary bladder, 318 
 
 organs, 307 
 Urocyst, 318 
 Urogenital sinus, 322 
 
 system, 307 
 Urohyal, 80, 97, 218 
 Uropygial glands, 30 
 Urostyle, 52 
 Uterus, 338 
 
 masculinus, 342 
 Utriculus, 183 
 Uvea, 202 
 
 Vagina, 338 
 Vagus nerve, 175 
 Valve, ileocascal, 228 
 
 ileo-colic, 228 
 
 of Vieussens, 145 
 
 spiral, 228 
 Vas eflferens, 321 
 Vasa deferentia, 321 
 Vascular cells, 270 
 Vater's corpuscle, 179 
 Veins, 266, 276, 289 
 
 abdominal, 289 
 
 advehent, 291 
 
 allantoic, 350 
 
 anterior abdominal, 289 
 cardinal, 279 
 
 axillary, 290 
 
 azygos, 302 
 
 brachial, 290 
 
 branchial, 274 
 
 caudal, 276 
 
 central retinal, 201 
 
 cephalic, 290 
 
 common iliac, 289 
 
 cutaneus magnus, 290 
 
 epigastric, 289 
 
 femoral, 290 
 
 hemiazygos, 302 
 
 hepatic, 277 
 
 hypogastric, 290 
 
 iliac, 289 
 
 inferior jugular, 278 
 
 innominate, 300 
 
 internal iliac, 290 
 
 interrenal, 291 
 
 ischiadic, 290 
 
 jugular, 279 
 
 lateral abdominal, 289 
 
 omphalomesaraic, 271 
 
 omphalomesenteric, 271 
 
 portal, 277 
 
 i 
 
 i 
 
INDEX. 
 
 401 
 
 Veins, postcardinal, 279 
 
 pulmonary, 292 
 
 revehent, 291 
 
 sciatic, 290 
 
 subcardinal, 279 
 
 subclavian, 289 
 
 sub intestinal, 276 
 
 superior intercostal, 302 
 jugular, 279 
 
 umbilical, 278 
 
 vitelline, 277 
 
 vertebral, 292 
 Velum medullare anterius, 145 
 
 transversum, 146 
 Vena cava, anterior 300 
 
 inferior, 290 
 Ventral aorta, 273 
 
 nerve root, 161 
 Ventricles, cornua of, 160 
 
 fifth, 151 
 
 lar>Tigeal, 253 
 
 of brain, 12, 142 
 
 of heart, 272 
 
 of lungs, 259 
 \'ermis, 145 
 
 Vertebra, development of, 48 
 Vertebrae, 45 
 
 occipital, 62 
 Vertebral artery, 287 
 
 column, 44 
 
 rib, 54 
 
 vein, 292 
 Vertebraterial canal, 54 
 Vertebrata, 2 
 Vesical arteries, 285 
 Vestibular nerve, 174 
 Vestibule of mouth, 208 
 
 of nose, 194 
 Vestibulum, 183 
 Mdian nerve, 165 
 Vieussens, valve of, 145 
 Villi, 227 
 Visceral arches, 63 
 
 clefts, 236 
 
 Visceral motor nerves, 165 
 
 muscles, 132 
 
 nerves, 163 
 
 pouches, 236 
 
 sensory nerves, 167 
 
 skeleton, 63 
 Vitelline veins, 277 
 Vitreous body, 200 
 Vocal cords, 251, 253 
 
 sacs, 253 
 Volimtarj' muscles, 20, 125 
 Vomer, 69 
 Vomero-nasal organ, 196 
 
 Weberian apparatus, 54, 250 
 Weber's nerve, 189 
 Whalebone, 216 
 WTiariion's duct, 221 
 \Miite matter, 20 
 
 of cord, 139 
 
 tissue, 22 
 Willis, circle of, 287 
 Winslow's foramen, 122 
 Wirsung's duct, 235 
 Wishbone, 108 
 Wolffian body, 310, 313 
 
 duct, 315 
 
 ridge, 311 
 
 Xiphioid process, 56 
 Xiphistemum, 57 
 
 Yellow spot, 200 
 Yolk, 8 
 
 sac, 206, 277, 348 
 Ypsiloid cartilage, no 
 
 2k)nes of nervous system, 1 4 r 
 Zonula ciliaris, 202 
 
 Ziimii, 202 
 Zygantra, 52 
 Zygapophysis, 46 
 Zygomatic bone, 70 
 Zygosphenes, 52 
 
BOOKS FOR STUDENTS OF BIOLOGY. 
 
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 engraved especially for this book, largely from original sources. Octavo. 
 
 Cloth, $4.50. 
 
 P. BLAKISTON'S SON & CO., PHILADELPHIA. 
 
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