QL ROS KY8 | N CORNELL UNIVERSITY THE Hlower Veterinary Library FOUNDED BY ROSWELL P. FLOWER for the use of the . Y. STATE VETERINARY COLLEGE 1897 J CORNELL UNIVERSITY LIBRARY wun DATE DUE GAYLORD A MANUAL OF THE ANATOMY OF VERTEBRATED ANIMALS. BY THOMAS H. HUXLEY, LL.D., F.R.S., AUTHOR OF “LAY SERMONS,” ‘‘MAN’S PLACE IN NATURE,”’ ‘‘ ORIGIN OF SPECIES,” ETC., ETC. NEW YORK: D. APPLETON AND COMPANY, 549 & 551 BROADWAY. 1872. fds B05 HIB PREFACE. Tue present work is intended to provide students of comparative anatomy with a condensed statement of the most important facts relating to the structure of verte- “brated animals, which have hitherto been ascertained. Except in a very few cases, I have intentionally abstained from burdening the text with references ; and, therefore, the reader, while he is justly entitled to hold me respon- sible for any errors he may detect, will do well to give me no credit for what may seem original, unless his knowledge is sufficient to render him a competent judge on that head. About two-thirds of the illustrations are original, the rest * are copied from figures given by Agassiz, Bischoff, Burmeister, Busch, Carus, Dugés, Flower, Gegenbaur, Hyrtl, Von Meyer, Miller, Pander and D’Alton, Parker, Quatrefages, and Traquair. A. considerable portion of the book has been in type for some years; and this circumstance must be my excuse for appearing to ignore the views of several valued con- temporaries. I refer more especially to those contained in recently-published works of Professors Flower and Gegenbaur. Lonpoy, September, 1871. * Namely, Figures 1, 6, 10, 11, 12, 18, 15, 18, 28, 26, 28, 29, 30, 81, 36, 39, 41, 42, 46, 50, 51, 54, 55, 56, 57, 58, 59, 60, 61, 75, 79, 82, 101, 107, 108, 109, 110. CONTENTS. PAGE Cuar, I.—A Generat VIEW OF THE ORGANIZATION OF THE VERTEBRATA— THE VERTEBRATE SKELETON, . 5 é : ‘ S II.—Tue Muscies AND THE ViscERA—A GENERAL VIEW OF THE ORGANIZATION OF THE VERTEBRATA, . . : . . 44 JJ.—Tue Provinces oF THE VERTEBRATA—THE Crass Pisces, . 100 IV.—Tue Crass AMPHIBIA, . : 3 ‘ é i . 149 V.—TuE CLASSIFICATION AND THE OSTEOLOGY OF THE RepriLia, . 167 VI.—Tue CLASSIFICATION AND THE OsTEOLOGY oF Brirps,_.. - 238 VII.—Tue Muscies aND THE VISCERA OF THE SAUROPSIDA, . . 256 VIII.—TuHE CLASSIFICATION AND ORGANIZATION OF THE MamMauia, . 273 THE ANATOMY or VERTEBRATED ANIMALS. CHAPTER I. A GENERAL VIEW OF THE ORGANIZATION OF THE VERTE- BRATA—THE VERTEBRATE SKELETON, The Distinctive Characters of the Vertebrata.—The Verte- brata are distinguished from all other animals by the circum- stance that a transverse and vertical section of the body exhibits two cavities, completely separated from one another by a partition. The dorsal cavity contains the cerebro-spinal nervous system; the ventral, the alimentary canal, the heart, and, usually, a double chain of ganglia, which passes under the name of the “sympathetic.” It is probable that’ this sympathetic nervous system represents, wholly or partially, the principal nervous system of the Annulosa and Mollusca. And, in any case, the central parts of the cerebro-spinal ner- vous system, viz., the brain and the spinal cord, would appear to be unrepresented among invertebrated animals. For these structures are the results of the metamorphosis of a part of the primitive epidermic covering of the germ, and only acquire their ultimate position, in the interior of the dorsal tube, by the development and union of outgrowths of the blastoderm, which are not formed in the Jnvertebrata.* Again, in the partition between the cerebro-spinal.and vis- * It is possible that an exception to this rule may be found in the Ascid- jans. The tails of the larve of these animals exhibit an axial structure, which has a certain resemblance to a vertebrate notochord; and the walls of the pharynx are perforated, much as in Amphioxus. 8 THE ANATOMY OF VERTEBRATED ANIMALS, ceral tubes, certain structures, which are not represented in invertebrated animals, are contained. During the embryonic condition of all vertebrates, the centre of the partition is occu- pied by an elongated, cellular, cylindroidal mass—the noto- chord, or chorda dorsalis. And this structure persists through- out life in some Vertebrata; but, in most, it is more or less completely replaced by a jointed, partly fibrous and cartilag- inous, and partly bony, vertebral column. In all Vertebrata, that part of the wall of the visceral tube which lies at the sides of, and immediately behind, the mouth, exhibits, at a certain stage of embryonic development, a series of thickenings, parallel with one another and _ trans- verse to the axis of the body, which may be five or more in number, and are termed the visceral arches. The intervals between these arches become clefts, which place the pharyn- gea] cavity, temporarily or permanently, in communication with the exterior. Nothing corresponding with these arches and clefts is known in the Invertebrata. A vertebrated animal may be devoid of articulated limbs, and it never possesses more than two pairs. These are always provided with an internal skeleton, to which the muscles mov- ing the limbs are attached. The limbs of invertebrated ani- mals are commonly more numerous, and their skeleton is always external. When invertebrated animals are provided with masticatory organs, the latter are either hard productions of the alimentary mucous membrane, or are modified limbs. Vertebrated ani- mals also commonly possess hard productions of the alimen- tary mucous membrane in the form of teeth; but their jaws are always parts of the walls of the parictes of the head, and have nothing to do with limbs. All vertebrated animals have a complete vascular system. In the thorax and abdomen, in place of a single peri-visceral cavity in communication with the vascular system, and serving as a blood-sinus, there are one or more serous sacs. These invest the principal viscera, and may or may not communicate with the exterior—recalling, in the latter case, the atrial cavi- ties of Mollusca. In all Vertebrata, except Amphioxus, there is a single valvular heart, and all possess an hepatic portal system ; the blood of the alimentary canal never being wholly returned di- rectly to the heart by the ordinary veins, but being more or less completely collected into a trunk—the portal vein, which ramifies through and supplies the liver. THE DEVELOPMENT OF THE VERTEBRATA. 9 The Development of the Vertebrata.—The ova of Verte- brata have the same primary composition as those of other animals, consisting of a germinal vesicle, containing one or many germinal spots, and included within a vitellus, upon the amount of which the very variable size of the vertebrate ovum chiefly depends. The vitellus is surrounded by a vitelline membrane, and this may receive additional investments in the form of layers of albumen, and of an outer, coriaceous, or cal- cified shell. The spermatozoa are always actively mobile, and, save in some rare and exceptional cases, are developed in distinct individuals from those which produce ova. Fie. 1.—Diagrammatic section of the pregnant uterus of a deciduate placental mammal eee : u, uterus; Z, Fallopian tube; ¢, neck of the uterus; dw, uterine decidua; de, ecidua serotina; dr, decidua refieca; #2,’ villi; ch, chorion; am amnion; nd, umbilical vesicle; a7, allantois. Impregnation may take place, either subsequently to the extrusion of the egg, when, of course, the whole development of the young goes on outside the body of the oviparous parent; or it may occur before the extrusion of the egg. In the latter case, the development of the egg in the interior of the bedy may go no further than the formation of a patch of primary tissue ; as in birds, where the so-called cicatricula, or “tread,” which is observable in the new-laid egg, is of this nature. Or, the development of the young may be completed 10 THE ANATOMY OF VERTEBRATED ANIMALS. while the egg remains in the interior of the body. of the parent, but quite free and unconnected with it; as in those vertebrates which are termed ovoviviparous. Or, the young may receive nourishment from its viviparous parent, before birth, by the close apposition of certain vascular appendages of its body to the walls ‘of the cavity in which it undergoes its development. The vascular appendages in question constitute the chief part of what is called the placenta, and may be developed from the umbilical vesicle (as in Mustelus among Sharks), or from the allantois and chorion (as in most mammals). At . birth, they may be either simply detached from the substance of the parental organism, or a part of the latter may be thrown off along with them and replaced by a new growth. In the highest vertebrates, the dependence of the young upon the parent for nutrition does not cease even at birth; but certain cutaneous glands secrete a fluid called milk, upon which the young is fed for a longer or shorter time. When development takes place outside the body, it may be independent of parental aid, as in ordinary fishes; but, among some reptiles and in most birds, the parent supplies the amount of heat, in excess of the ordinary temperature of the air, which is required, from its own body, by the process of incubation. The first step in the development of the embryo is the division of the vitelline substance into cleavage-masses, of which there are at first two, then four, then eight, and so on. The germinal vesicle is no longer seen, but each cleavage- mass contains a nucleus. The cleavage-masses eventually be- come very small, and are called embryo-cells, as the body of the embryo is built up out of them. The process of yelk- division may be either complete or partial. In the former case, it, from the first, affects the whole yelk; in the latter, it commences in part of the yelk, and gradually extends to the rest. The blastoderm, or embryogenic tissue in which it results, very early exhibits two distinguishable strata—an inner, the so-called mucous stratum (hypoblast), which gives rise to the epithelium of the alimentary tract; and an outer the serous stratum (epiblast), from which the epidermis and the cerebro-spinal nervous centres are evolved. Between these appears the intermediate stratum (mesoblast), which gives rise to all the structures (save the brain and spinal mar- * row) which, in the adult, are included between the epidermis 11 ¢), ? A stoderm, and ry tract ta 2 ? o imen groove (Fi e bla © £ th ve ium of the al t face o tmi ithel the pr ? THE DEVELOPMENT OF THE VERTEBRATA. ession tegument and the ep ar depr A line makes it appearance on the sur and its appendages. of the in *sadn0k ony any) Jodo] oan sapjo ony ‘aanyen um ‘yng “WySuol oynjosqe onus oT) Jo UMTAp 910 GOAIGIID OD, “OIQISIA 04” fSUJOA O[RAagaMIO|PYdtO aq} pune oquinu Ul pasvasony oAvY MAqoylaAojoud oy, "UyZUAL OOYA Toy} A[rou ynogSnosmy poyun Aujavy wujwey pessop ony “(aoyeqnouy yo Aup puovss 944) poourape saoyyany oAquia *“q— “uoldad [eulds Jo(soy08 ayy Uy oy1ON 04 Jajaujseq ote pure ‘uolsat aljeqdes oy} Jo yard Joyvosd ony qnoysAnosgy poyun savy [esdop ayy, ‘a1ojaq sv suayal ony ‘Q—aqaz19A0j0id any ‘9 Soul] e[ppiu aq} uy poyyun o4jnb you pup ‘nortes on Ajao padoaaap yak sv wuyurry (psiop any ‘p + asojoq se ‘a ‘gq ‘D * poourape soqyiny OAIqUIA ayy ‘q—9A0045 aAyyyeoyAd oy] ‘o ¢ pus ‘Wepnes sy g ‘oyeydoo sy, D t osIqUa O44 JO JUOUNI pH yoay ONY “Y— [MON BJO Apog ouy Jo yuoudojaAap 9uy Jo BodNYs AlAv9 OU —Z “DI 12 THE ANATOMY OF VERTEBRATED ANIMALS. the substance of the mesoblast along each side.of this groove grows up, carrying with it the superjacent epiblast. Thus are produced the two dorsal lamine, the free edges of which arch over toward one another, and eventually unite, so as to con- vert the primitive groove into the cerebro-spinal canal. The portion of the epiblast which lines this, cut off from the rest, becomes thickened, and takes on the structure of the brain, or Encephalon, in the region of the head; and of the spinal cord, or Myelon, in the region of the spine. The rest of the epiblast is converted into the epidermis. The part of the blastoderm which lies external to the dor- sal laminz forms the ventral lamine ; and these bend down- ward and inward, at a short distance on either side of the dorsal tube, to become the walls of a ventral, or visceral, tube. The ventral laminz carry the epiblast on their outer surfaces, and the hypoblast on their inner surfaces, and thus, in most cases, tend to constrict off the central from the peripheral portions of the blastoderm. The latter, extending over the yelk, encloses it ina kind of bag. This bag is the first-formed and the most constant of the temporary, or foetal, appendages of the young vertebrate, the umbilical vesicle. While these changes are occurring, the mesoblast splits, throughout the regions of the thorax and abdomen, from its ventral margin, nearly up to the notochord (which has been developed, in the mean while, by histological differentiation of the axial indifferent tissue, immediately under the floor of the primitive groove), into two lamelle. One of these, the visceral lamella, remains closely adherent to the hypoblast, forming with it the splanchnopleure, and eventually becomes the proper wall of the enteric canal ; while the other, the parietal lamella, follows the epiblast, forming with it the somatopleure, which is converted into the parietes of the thorax and abdomen. The point of the middle line of the abdomen at which the somatopleures eventually unite, is the umbilicus. The walls of the cavity formed by the splitting of the ventral laminz acquire an epithelial lining, and become the great pleuroperitoneal serous membranes. The Fotal Appendages of the Vertebrata—At its outer margin, that part of the somatopleure which is to be con- verted into the thoracic and abdominal wall of the embryo, grows up anteriorly, posteriorly, and laterally, over the body of the embryo. The free margins of this fold gradually ap- proach one another, and, ultimately uniting, the inner layer of the fold becomes converted into a sac filled with a clear THE FETAL APPENDAGES. 13 fluid, the Amnion ; while the outer layer either disappears or ite 3), with the vitelline membrane, to form the Chorion Fig. 3). Fia. 8.—Later stages of the development of the body of a Fowl than those represented in Fig. 2.—E, embryo at the third day of incubation; g, heart; /, eye; 7, ear; %, visceral arches and clefts ; 7, m, anterior and posterior folds of the amnion which have not yet united over the body; 1, 2, 3, first, second, and third cerebral vesicles; 1a, vesicle of the third ventricle.—F, embryo at the fifth day of incubation. The letters as before, except nm, 0, rudiments of the anterior and posterior extremities; Am, amnion; Add (the allan- tois, hanging down from its pedicle); Um, umbilical vesicle—G, under-view of the head of the foregoing, the first visceral arch being cut away. Thus the amnion encloses the body of the embryo, but not the umbilical sac. At most, as the constricted neck, which unites the umbilical sac with the cavity of the future intestine, becomes narrowed and elongated into the vitelline duct, and as the sac itself diminishes in relative size, the amnion, in- creasing in absolute and relative dimensions, and becoming distended with fluid, is reflected over it (Fig. 1). A third foetal appendage, the Al/antois, commences as a single or double outgrowth from the .under surface of the meso- 14 THE ANATOMY OF VERTEBRATED ANIMALS. blast, behind the alimentary tract; but soon takes the form of a vesicle, and receives the ducts. of the primordial kidneys, or Wolffian bodies. It is supplied with blood by two arteries, called hypogastric, which spring from the aorta; and it varies very much in its development. It may become so large as to invest all the rest of the embryo, in the respiratory, or nutri- tive, processes of which it then takes an important share. The splitting of the ventral laminz, and the formation of a pleuroperitoneal cavity, appear to take place in all Vertebrata. Usually, there is a more or less distinct umbilical sac; but in fishes and Amphibia there is no amnion; and the allantois, if it is developed at all, remains very small in these two groups. Reptiles, birds, and mammals have all these foetal append- ages. At birth, or when the egg is hatched, the amnion bursts and is thrown off, and so much of the allantois as lies outside the walls of the body is similarly exuviated; but that part of it which is situated within the body is very generally converted, behind and below, into the urinary bladder, and, in front and above, into a ligamentous cord, the wrachus, which connects the bladder with the front wall of the abdomen. The umbilical vesicle may either be cast off, or taken into the in- terior of the body and gradually absorbed. The majority of the visceral clefts of fishes and of many Amphibia remain open throughout life; and the visceral arches of all fishes (except Amphioxus), and of all Amphibia, throw out filamentous or lamellar processes, which receive branches from the aortic arches, and, as branchi@, subserve respiration. In other Vertebrata all the visceral clefts become closed and, with the frequent exception of the first, obliterated ; and no branchiz are developed upon any of the visceral arches. In all vertebrated animals, a system of relatively or abso- lutely hard parts affords protection, or support, to the softer tissues of the body. These, according as they are situated upon the surface of the body, or are deeper seated, are called exoskeleton, or endoskeleton, The Vertebrate Endoskeleton.—This consists of connective tissue, to which cartilage and bone may be added in various proportions ; together with the tissue of the notochord and its sheath, which cannot be classed under either of these heads, The endoskeleton is distinguishable into two independant por- tions—the one awial, or belonging to the head and trunk ; the other, appendicular, to the limbs. The axial endoskeleton usually consists of two systems of THE VERTEBRATE ENDOSKELETON. 15 skeletal parts, the spinal system, and the cranial system, the distinction between which arises im the following way in the higher Vertebrata : The primitive groove is, at first, a simple. straight depres- sion, of equal diameter throughout; but, as its sides rise and the dorsal lamin gradually close over (this process commen- cing in the anterior moiety of their length, in the future ce- phalic region), the one part becomes wider than the other, and indicates the cephalic region (Fig. 4, A). The notochord, which underlies the groove, terminates in‘a point at a little distance behind the anterior end of the cephalic enlargement, and indeed under the median of three dilatations which it presents. So much of the floor of the enlargement as lies in front of the end of the notochord, bends down at right angles to the rest ; so that the anterior enlargement, or anterior cere- bral vesicle, as it is now called, lies in front of the end of the notochord ; the median enlargement, or the middle cerebral vesicle, above its extremity ; and the hinder enlargement, or the posterior cerebral vesicle, behind that extremity (Fig. 4, D and EK), The under surface of the anterior vesicle lies in a kind of pit, in front of, and rather below, the apex of the noto- chord, and the pituitary gland is developed in connection with it. From the opposite upper surface of the same vesicle the pineal gland is evolved, and the part of the anterior cerebral vesicle in connection with which these remarkable bodies arise, is the future third ventricle. Behind, the posterior cerebral vesicle passes into the primi- tively tubular spinal cord (Fig. 4, A). Where it does so, the head ends, and the spinal column begins; but no line of de- marcation is at first visible between these two, the indifferent tissues which ensheath the notochord passing without inter- ruption from one region to the other, and retaining the same character throughout. The first essential differentiation between the skull and the vertebral column is effected by the appearance of the proto- vertebree. At regular intervals, commencing at the anterior part of the cervical region, and gradually extending backward, the indifferent tissue on each side of the notochord undergoes a histological change, and gives rise to more opaque, quadrate masses, on opposite sides of the notochord (Fig. 2, B, C). Each pair of these gradually unite above and below that struct- ure, and send arched prolongations into the walls of the spinal canal, so as to constitute a protovertebra. No protovertebre appear in the floor of the skull, so that, 16 THE ANATOMY OF VERTEBRATED ANIMALS. even in this early stage, a clear distinction is drawn between the skull and the spinal column. Fic. 4.—Successive stages of the development of the head of a Chick. I, II, IIL, first, sec- ond, and third cerebral vesicles; Za, vesicle of the cerebral hemisphere; Jd, vesicle of the third yentricle; a, rudiments of the eyes and optic nerves; }, of the ears; g, of the olfactory organs; d, the infundibulum; ¢, the pineal gland; ¢, protovertebre ; A, noto- chord; 1, 2, 3, 4, 5, visceral arches; V, VII, VIII, the trigeminal, portio dura, and ninth and tenth pairs of cranial nerves; %, the nasal process: 7, the maxillary process; @, the first visceral cleft. A, B, upper and under views of the head of a Chick at the end of the second day. C, side-view at the third day. D, side-view at seventy-five hours. E, side-view of the head of a Chick at the fifth day, which has been subjected to slight press- ure. F, head of a Chick at the sixth day, viewed from below. OSSIFICATION OF THE VERTEBRA. 17 The Spinal System.—The protovertebre consist at first of mere indifferent tissue; and it is by a process of histologi- cal differentiation within the protovertebral masses that, from its deeper parts, one of the spinal ganglia and a cartilaginous vertebral centrum—from its superficial layer, a segment of the dorsal muscles, are produced. Chondrification extends upward into the walls of the dorsal tube, to produce the neural arch and spine of each vertebra ; and, outward, into the wall of the thoracic and abdominal part of the ventral tube, to give rise to the transverse processes and ribs. In fishes, the latter remain distinct and separate from one another, at their distal ends; but, in most reptiles, in birds, and in mammals, the ends of some of the anterior ribs, on both sides, unite together, and then the united parts coa- lesce in the middle line to form a median subthoracic cartilage —the sternum. When ossification sets in, the centra of the vertebrae are usually ossified, in great measure, from ringlike deposits which closely invest the notochord ; the arches, from two lateral de- posits, which may extend more or less into the centrum. The vertebral and the sternal portions of a rib may each have a separate ossific centre, and become distinct bones; or the sternal parts may remain always cartilaginous. The sternum itself is variously ossified. Between the completely-ossified condition of the vertebral column and its earliest state, there are a multitude of grada- tions, most of which are more or less completely realized in the adult condition of certain vertebrated animals. The verte- bral column may be represented by nothing but a notochord with a structureless, or more or less fibrous, or cartilaginous sheath, with or without rudiments of cartilaginous arches and ribs. Or there may be bony rings, or ensheathing ossifications, in its walls; or it may have ossified neural arches and ribs only, without cartilaginous or osseous centra. The vertebra may be completely ossified, with very deeply biconcave bodies, the notochord remaining persistent in the doubly-conical inter- vertebral substance ; or, ossification may extend, so as to ren- der the centrum concave on one surface and convex on the other, or even convex at each end. Vertebrae which have centra concave at each end have been conveniently termed amphiccelus ; those with a cavity in front and a convexity behind, proccelus ; where the position of the concavity and convexity is reversed, they are opistho- coelous, 18 THE ANATOMY OF VERTEBRATED ANIMALS. In the Mammalia, the centra of the vertebre are usually flat at each end, the terminal faces being discoidal epiphyses, developed from centres of ossification distinct from that of the centrum itself, The centra of the vertebra may be united together by synovial joints, or by ligamentous fibres—the intervertebral ligaments. The arches are connected by ligaments, and gen- erally, in addition, by overlapping articular processes called zygapophyses, or oblique processes. In a great many Vertebrata, the first and second cervical, or atlas and awis, vertebre undergo a singular change; the central ossification of the body of the atlas not coalescing with its lateral and. inferior ossifications, but either persist- ing as a distinct os odontoideum, or anchylosing with the body of the axis, and becoming the so-called odontoid process of this vertebra. In Vertebrata with well-developed hind-limbs, one or more vertebra, situated at the posterior part of the trunk, usually become peculiarly modified, and give rise to a sacrum, with which the pelvic arch is connected by the intermediation of expanded and anchylosed ribs, In front of the sacrum the ver- tebree are artificially classed as cervical, dorsal, and lumbar. The first vertebra, the ribs of which are connected with the sternum, is dorsal, and all those which lie behind it, and kave distinct ribs, are dorsal. Vertebre without distinct ribs, between the last dorsal and the sacrum, are lumbar. Ver- tebree, with or withcut ribs, in front of the first dorsal are cervical. The vertebrae which lie behind the sacrum are caudal or coccygeal, Very frequently, downward processes of these vertebrz enclose the backward continuation of the aorta, and may be separately ossified as subcaudal, or chevron, bones..- A tolerably complete segment of the spinal skeleton may be studied in the anterior part of the thorax of a crocodile (Fig. 5). It presents a proccelous vertebral centrum (C), united with which by the neurocentral suture is the neural arch, which rises into the neural spine (NV. S.). Two pro- cesses, the prezygapophyses (Z), extend from the front part of the arch, and have flat articular surfaces turned dorsally. Two others of similar form, but having their articular surfaces turned ventrally, proceed from the posterior face of the neural arch, and are the postzygapophyses (Z'). By these, which are often called oblique, or articular, processes, the vey- tebra articulates with the corresponding processes ofits prede- A SEGMENT OF THE SKELETON. 19 cessor or successor in the series. The transverse processes are two on each side, one superior and one inferior. ‘he former (Zt) articulates with the tuberculum of the rib, the latter ( Cp.t) with its capitulum. They may, therefore, be called ca- pitular and tubercular transverse processes respectively. Hach St SEr Fra. 5.—A segment of the endoskeleton in the anterior thoracic region of the body of a croc- odile.—@, the centrum or body of the vertebra; N.S. the neural spine; Z, the prezy- gapophysis; 7, the postzygapophysis; Z:¢, the transverse process which articulates with the tuberculum of the rib (¢); Cp.t, that which articulates with the capitulum of the rib (Cp); V7, the ossified vertebral rib; V.r’, the part of the vertebral rib which remains cartilaginous; S¢7, the sternal rib; S?, an artificially-separated segment of the sternum; P.v, the uncinate process. 4 rib is divided by an articulation into a vertebral (V-r) and a sternal (Str) part. The former remains unossified for a con- siderable distance at its distal end ( V-r'); the latter is more or less converted into cartilage bone. The proximal end of the ver- tebral rib bifurcates into a tuberculum (t) and acapitulum ( Cp). The distal end of the sternal rib unites with the more or less os- sified but unsegmented cartilage, which forms the sternum (S¢). A cartilaginous, or partly ossified, uncinate process (P.u.) pro- jects from the posterior edge of the vertebral rib, over the in- tercostal space. The student will find it convenient to famil- iarize himself with the conception of such a spinal segment as this, as a type, and to consider the modifications hereafter described with reference to it. In the majority of the Vertebrata, the caudal vertebre gradually diminish in size toward the extremity of the body, and become reduced, by the non-development of osseous pro- cesses or arches, to mere centra. But, in many fishes, which possess well-ossified trunk-vertebree, no distinct centra are developed at the extremity of the caudal region, and the notochord, invested in a more or less thickened, fibrous, or cartilaginous sheath, persists. Notwithstanding this embry- 10Tr so as to form xtreme end of the spine same direction as the tion of the axis of the tail, the superior and infer arches, and the interspinous bones, may be completely formed i in cartilage or bone. THE ANATOMY OF VERTEBRATED ANIMALS. d Whatever the condition of the e of a fish, it occasionally retains the onic con trunk part, but is far more generally bent up, 20 ‘2 put ‘9 ‘p soyujd snoosso on} £q pasoaoo psoqoojon yuoysisiad & paw [avo £q ApWO poysaamt st paioyoojou au} pue ‘pay q 118 48 ATpavy Suyaq (yo) paoysojou ayy Jo Ayr V) snuajgdhjog jo soyyyu10.)x0 [epneo e7TL—'9 “OL ‘souoq yeinddq popuvdxo yr worod0.10}0y, ALS s ‘ I [suo01s st ‘Ayjwug ‘oupny ‘oF “om A[preq iv souog jeanday 043 yng bapieodeyen Ai@mnewaae st ne a quoi “WX Of} [BoreAydip AjAwou sf srwazdhjyog *(Q) ougny pue “(g) Duy “( UNAS NHS RO TX i aN CM x ey ny bY NR) fy =A \ HN \ aoe THE CRANIAL SKELETON. 21 an obtuse angle with the latter. In the former case, the ex- tremity of the spine divides the caudal fin-rays into two nearly equal moieties, an upper and a lower, and the fish is said to be diphycercal (Fig. 6, A). In the latter case, the upper di- vision of the caudal fin-rays is much smaller than the lower, and the fish is heterocercal (Fig. 6, B, C). In most osseous fishes the hypural bones which support the fin-rays of the inferior division become much expanded, and either remain separate, or coalesce into a wedge-shaped, nearly symmetrical bone, which becomes anchylosed with the last ossified vertebral centrum. The inferior fin-rays are now disposed in such a manner as to give the tail an appearance of symmetry with respect to the axis of the body, and such fishes have been called homocercal. Of these homocercal fish, some (as the Salmon, Fig. 6) have the notochord unossified, and protected only by bony plates developed at its sides. In others (as the Stickleback, Perch, etc.), the sheath of the no- tochord becomes completely ossified and united with the cen- trum of the last vertebra, which then appears to be prolonged into a bony urostyle, Fie. 7.—The cartilaginous cranium of a Fowl at the sixth day of incubation, viewed from be- low.—P, the pituitary space; tr, the trabecule, uniting in front, in the bifurcated eth- tmovomerine plate; Qu, the quadrate cartilage; Sc, the semicircular canals; Co, the cochlea; /, the notochord imbedded in the basilar plate, The Cranial System.—As has been stated, no protover- tebr appear on the floor of the skull; nor is there any cra- nium, nor any developmental stage of a cranium, in which sep- arate cartilaginous centres are known to occur in this region. On the contrary, when chondrification takes place, it ex- tends continuously forward, on each side of the notochord, 22 THE ANATOMY OF VERTEBRATED ANIMALS. and usually invests the anterior termination of that body, more or less completely, as a basilar plate. The basilar plate does not extend under the floor of the pituitary fossa, but the cartilage is continued forward on each side of this, in the form of two bars, the trabecule crantt. In front of the fossa, the trabecule reunite and end in a broad plate, usually bifurcated in the middle line—the ethmovome- rine plate. On each side of the posterior boundary of the skull, the basilar cartilage grows upward, and meets with its fellow in the middle line, thus circumscribing the occipital foramen, and furnishing the only cartilaginous part of the roof of the skull; for any cartilaginous upgrowths which may be devel- oped in the more anterior parts of the skull do not ordinarily reach its roof, but leave a wide, merely membranous space, or JSontanelle, over the greater part of the brain. Before the skull has attained this condition, the organs of the three higher senses have made their appearance in pairs at its sides; the olfactory being most anterior, the ocular next, the auditory posterior (Fig. 4). Each of these organs is, primitively, an involution, or sac, of the integument; and each acquires a particular skeleton, which, in the case of the nose, is furnished by the ethmovo- merine part of the skull; while, in that of the eyes, it apper- tains to the organ, is fibrous, cartilaginous, or osseous, and remains distinct from the skull. In the case of the ear, it is cartilaginous, and eventually osseous: whether primitively dis- tinct or not, it early forms one mass with the skull, immedi- ately in front of the occipital arch, and often constitutes a very important part of the walls of the fully-formed cranium. The ethmovomerine cartilages spread over the nasal sacs, roof them in, cover them externally, and send down a parti- tion between them. The partition is the proper ethmoid, the lamina perpendicularis of human anatomy; the posterolat- eral parts of the ethmovomerine cartilages, on each side of the partition, occupy the situation of the prefrontals, or lateral masses of the ethmoid of human anatomy. The ingrowths of the lateral walls, by which the nasal mucous membrane ac- quires a larger surface, are the turbdinals. Riblike cartilaginous rods appear in the first, second, and, more or fewer, of the succeeding, visceral arches in all but the lowest Vertebrata. The upper ends of the first and second of these become connected with the auditory capsule, which lies immediately above them. THE CRANIAL SKELETON. 23 The first visceral arch bounds the cavity of the mouth be- hind, and marks the position of the mandible or lower jaw. The cartilage which it contains is termed Meckel’s cartilage. The cartilaginous rod contained in the second visceral arch of each side is the rudiment of the hyoidean apparatus. Like Fic. 8.—Under-view of the head of a Fowl] at the seventh day of incubation.—ZJa, the cere- bral hemispheres causing the integument to bulge; a, the eyes; g, the olfactory sacs; &, the fronto-nasal process; 7, the maxillary process; 1, 2, the first and second viscera} arches; @, the remains of the first visceral cleft. the preceding, it unites with its fellow in the ventral median’ line, where the so-called “ body ” of the hyoid arises. A ridge, continued forward from the first visceral arch to the olfactory sac (Fig. 4, F; Fig. 8, 2), bounds the mouth on each side, and is called the mawillary process. A cartilaginous palato-pterygoid rod, developed in this process, becomes con- nected with Meckel’s cartilage behind, and with the prefrontal cartilage in front. The maxillary process is at first separated by a notch cor- responding with each nasal sac, from the boundary of the antero-median part of the mouth, which is formed by the free posterior edge of a fronto-nasal process (Fig. 4, F; Fig. 8, x). This separates the nasal sacs, and contains the cartilaginous, ethmovomerine, anterior termination of the skull. The notch is eventually obliterated by the union of the fronto-nasal and maxillary processes, externally ; but it may remain open in- ternally, and then gives rise to the posterior nasal aperture, by which the nasal cavity is placed in communication with that of the mouth, 24 THE ANATOMY OF VERTEBRATED ANIMALS. The General Modifications of the Vertebrate Skull.—The lowest vertebrated animal, Amphiowus, has no skull. In a great many fishes, the development of the skull carries 1t no further than to a condition which is substantially similar to one of the embryonic stages now described; that is to say, there is a cartilaginous primordial cranium, with or without superficial granular ossifications, but devoid of any proper cranial bones. The facial apparatus is either incompletely developed, as in the Lamprey; or, the upper jaw is repre- sented, on each side, by a cartilage answering to the palato-- pterygoid and part of Meckel’s cartilage, while the larger, distal portion of that cartilage becomes articulated with the rest, and forms the lower jaw. This condition is observable in the Sharks and Rays. In other fishes, and in all the higher Vertebrata, the cartilaginous cranium and facial arches may persist to a greater or less extent; but bones are added to them, which may be almost wholly membrane bones, as in the Sturgeon ; or may be the result of the ossification of the car- tilaginous cranium itself, from definite centres, as well as of the development of superimposed membrane bones. The Osseous Brain-case.—When the skull undergoes com- plete ossification, osseous matter is thrown down at not fewer than three points in the middle of its cartilaginous floor. The ossific deposit, nearest the occipital foramen, becomes the basi- occipital bone; that which takes place in the floor of the pitu- itary fossa becomes the basisphenoid ; that which appears in the reunited trabecule, in front of the fossa, gives rise to the presphenoid. Again, in front of, and outside, the cranial cav- ity, the ethmoid may be represented by one or more distinct ossifications. An ossific centre may appear in the cartilage on each side of the occipital foramen, and give rise to the ex-occtpital ; and above it, to form the supra-occipital, The four occipital ele- ments, uniting together more or less closely, compose the oc- cipital segment of the skull. In front of the auditory capsules and of the exit of the third division of the fifth nerve, a centre of ossification may appear on each side and give rise to the alisphenoid ; which, normally, becomes united below with the basisphenoid. In front of, or above, the exits of the optic nerves, the orbitosphenoidal ossifications may appear and unite below with the presphenoid. In front of the occipital segment, the roof of the skull is formed by membrane ; and the bones which complete the two THE TYPICAL BONY SKULL. 25 segments of which the basisphenoid and presphenoid form the basal parts, are membrane bones, and are disposed in two pairs. The posterior are the parietals, the anterior the fron- tals ; and the segments which they complete are respectively ( 4 4 4 g a g A yy & H 58 8 8 Branchial apparatus, z ° 2 . a . 4 a A, 4 B 8 4 A g a 4 3 Hyoidean apparatus. ZEs ) a : A. & &§ & mn 19 a a b z 7 & 8 3 Mandibular Suspensorium, - 5 B io é 8 : 2) es 4 e Bl ¢ : B Aelia A fl @ z a2 a4 8 9 i Ey EI to 4 4 a an ‘ _ AR le 8 4 4 A a & a 64 4 e ‘ Bs ag & E q 4H Aa. eB g a 8 fQ s a < io] g g r 8 E 4 S a nm *e i d : 5 3 ° a 3a ‘ a A 6 é j a q E—#—| eS Ho 3 a 44 4 B g 4 4 a a cy ‘ L 3° pa J e PREFRONTAL. . 4 g| a oy q B 4 d s gl# B&B q & #(3 34 a ae é Pa 26 THE ANATOMY OF VERTEBRATED ANIMALS. called parietal and frontal. Thus the walls of the cranial cav- ity in the typical ossified skull are divisible into three segments —I. Occipital, 12. Parietal, III. Frontal—the parts of which are arranged with reference to one another, the sensory organs and the exits of the first, second, fifth, and tenth pairs of cranial nerves (L, IL, V., and x.), in the manner shown in the diagram * on the preceding page. . The cartilaginous cases of the organs of hearing, or the periotic capsules, are, as has been said, incorporated with the skull between tke ex-occipitals and the alisphenoids—or, in other words, between the occipital and the parietal segments of the skull. Each of them may have three principal ossifi- cations of its own. ‘The one in front is the prodtic ; the one behind and below, the opisthotic ; and the one which lies above, and externally, the epiotic. The last is in especial re- lation with the posterior vertical semicircular canal; the first. with the anterior vertical semicircular canal, between which, and the exit of the third division of the fifth nerve, it lies, These three ossifications may coalesce into one, as when they constitute the petrosal and mastoid parts of the temporal bone of human anatomy; or the epiotic, or the opisthotic, or both, may coalesce with the adjacent supra-occipital and ex-occipi- tals, leaving the prodtic distinct. The prodtic is, in fact, one of the most constant bones of the skull in the lower Vertebra- ta, though it is commonly mistaken, on the one hand for the alisphenoid, and on the other for the entire petro-mastoid. Sometimes a fourth, pterotic ossification, is added to the three already mentioned. It lies on the upper and outer part of the ear-capsule between the prodtic and the epiotic (see the fig- ure of the cartilaginous cranium of the Pike, infra). In some Vertebrata the base of the skull exhibits a long and distinct splint-like membrane bone +—the parasphenoid, * The names of the purely membrane bones in this diagram are in large capitals, as PARIETAL; while those of the bones seitigh ane preformed i om are in ee type, as BAsIspHENoID. ones may be formed in two ways. They may be preceded by cartilag and the ossific deposit in the place of the future bose mee at first i eeed in the matrix of that cartilage, or the ossifie deposit may take place, from the first, in indifferent, or rudimentary connective, tissue. In this case the bone is not prefigured by cartilage. In the skulls of Elasmobraneh fishes, end in the sternum and epicoracoid of Lizards, the bony matter is simply ossified car- tilage, or cartilage bone. The parietal or frontal bones, on the other hand Ha always devoid of cartilaginous rudiznents, or, in other words, are membrane Ones, In the higher Vertebrata the cartilage bones rarely, if ever, remai . but the primitive ossified cartilage beeomes in great mascots abecakel mea replaced by membrane bone, derived from the perichondrium. ’ THE BONES OF THE FACE. 27 which underlies it from the basi-occipital to the pre-sphenoidal region. In ordinary fishes and Amphibia, this bone appears to replace the basisphenoid and presphenoid functionally, while in the higher Vertebrata it becomes confounded with the basisphenoid. The Vomer is a similar, splint-like, single or double, membrane bone, which, in like manner, underlies the ethmoid region of the skull. In addition to the bones already mentioned, a prefrontal bone may be developed in the prefrontal region of the nasal capsule, and bound the exit of the olfactory nerve externally. A postfrontal bone may appear behind the orbit above the alisphenoid. Sometimes it seems to be a mere dismember- ment of that bone; but, in most cases, the bone so named is a distinct membrane bone. Furthermore, on the outer and upper surface of the audi- tory capsule a membrane bone, the sguamosal, is very com- monly developed; and another pair of splint-bones, the nasals, cover the upper part of the ethmovomerine chambers, in which the olfactory organs are lodged. The Osseous Facial Apparatus——The bones of the face, which constitute the inferior arches of the skull, appear with- in the various processes and visceral arches which have been enumerated. Thus, the premazilic are two bones developed in the oral part of the naso-frontal process, one on each side of the middle line, between the external nasal apertures, or anterior nares, and the anterior boundary of the mouth. Ossification occurs in the palato-pterygoid cartilage at two chief points, one in front and one behind. The anterior gives rise to the palatine bone, the posterior to the pterygoid. Outside these, several membrane bones may make their ap- pearance in the same process. The chief of these is the maz- alla, which commonly unites, in front, with the premaxilla. Behind the maxilla there may be a second, the jugal ; and occasionally behind this lies a third, the guadratojugal. Between the maxilla, the prefrontal and the premaxilla, another membrane bone, called lachrymai, from its ordinary relation to the lachrymal canal, is very generally developed; and one or more supra-orbital and post-orbital ossifications may be connected with the bony boundaries of the orbit. When these and the postfrontal membrane bone are si- multaneously developed, they form two series of bony splints attached to the lateral wall of the skull, one set above and one below the orbit, which converge to the lachrymal. The 28 THE ANATOMY OF VERTEBRATED ANIMALS. upper series (lachrymal, supra-orbital, post-frontal, squamosal), terminates posteriorly over the proximal end of the guadrate bone, or mandibular suspensorium. The lower series (lachry- mal, maxillary, jugal, quadrato-jugal) ends over the distal end of that bone, with which the quadrato-jugal is connected, The two series are connected behind the orbit by the post- orbital (when it exists), but more commonly by the union of the jugal with the post-frontal and squamosal. The Ichthy- osauria, Chelonia, Crocodilia, and some Lacertilia, exhibit this double series of bones most completely. Each nasal passage, at first very short, passes between the premaxilla below, the ethmoid and vomer on the inner side, the prefrontal above and externally, and the palatine behind, to open into the forepart of the mouth. And, before the cleft between the outer posterior angle of the naso-frontal process and the maxillary process is closed, this passage communi- cates laterally, with the exterior, and, posteriorly, with the cavity of the orbit. When the maxillary and the naso-frontal processes unite, the direct external communication ceases; but’ the orbito-nasal passage, or dachrymal canal, as it is called, in consequence of its function of conveying away the secretion of the lachrymal gland, may persist, and the dachry- mal bone may be developed in especial relation with it. ‘ In the higher Vertebrata, the nasal passages no longer communicate with the forepart of the cavity of the mouth; for the maxillaries and palatines, regularly, and the pterygoid bones, occasionally, send processes downward and inward, which meet in the middle line, and shut off from the mouth a canal which receives the nasal passages in front, while it opens, behind, into the pharynx, by what are now the poste- rior nares. Two ossifications commonly appear near the proximal end of Meckel’s cartilage, and become bones movably articulated together. The proximal of these is the guadrate bone found in most vertebrates, the madleus of mammals; the distal is the os articutare of the lower jaw in most vertebrates, but does not seem to be represented in mammals. The remainder of Meckel’s cartilage usually persists for a longer or shorter time, but does not ossify. It becomes surrounded by bone, arising from one or several centres, in the adjacent membrane, and the ramus of the mandible thus formed articulates with the squamosal bone in mammals, but in other Vertebrata is ee ers oe) THE OSSEOUS MANDIBLE. 29 immovably united with the os articulare. Hence the complete ramus of the mandible articulates directly with the skull in mammals, but only indirectly, or through the intermediation of the quadrate, in other Vertebrata. In birds and reptiles, Fie. 9.—The head of a fetal Lamb dissected so as to show Meckel’s cartilage, Jf; the malleus, m,; the incus, 7; the tympanic, 7y; the hyoid, H; the squamosal, Sg; pers Pi; palatine, pi; lachrymal, Z; premaxilla, pma,; nasalsac, V; Tustachian tube, Lu. the proximal end of the quadrate bone articulates directly (with a merely apparent exception in Ophidia), and indepen- dently of the hyoidean apparatus, with the periotic capsule. In most, if not all fishes, the connection of the mandibular arch with the skull is effected indirectly, by its attachment to a single cartilage or bone, the hyomandibular, which repre- sents the proximal end of the hyoidean arch (see Fig. 24). The ossification of the hyoidean apparatus varies immense- ly in detail, but usually gives rise to bony lateral arches, and a median portion, bearing much the same relation to them as the sternum has to the ribs. When the lateral arches are com- plete, they are connected directly with the periotic capsule. The proximal end of the hyoidean arch is often united, more or less closely, with the outer extremity of the bone, called columella auris, or stapes, the inner end of which, in the higher Vertebrata, is attached to the membrane of the Senestra ovalis, Tn ordinary fishes, a fold of the integument extends back- ward from the second visceral arch over the persistent bran- 80 THE ANATOMY OF VERTEBRATED ANIMALS. chial clefts; within this is developed a series of raylike mem- brane bones, termed epercular and branchiostegal, which be- come closely connected with the hyoidean arch, A corre- sponding process of the skin is developed in the Batrachian Tadpole, and grows backward over the branchiz. Its posterior edge, at first free, eventually unites with the integument of the body, behind the branchial clefts, the union being com- pleted much earlier on the right side than on the left. ” In most mammals a similar fold of integument gives rise to the pinna, or external ear. The branchial skeleton bears the same relation to the posterior visceral arches that the hyoidean does to the second. When fully developed, it exhibits ossified lat- eral arches, connected by median pieces, and, frequently, provided with radiating appendages which give support to the branchial mu- cous membrane. It is only found in those Vertebrata which breathe by gills—the classes Pisces and Amphibia. In the higher Verte- brata, the posterior of the two pairs of cornua, with which the hy- oidean apparatus is generally pro- vided, are the only remains of the branchial skeleton. : The skull and face are usually symmetrical in reference to a me- dian vertical plane. But, in some Cetacea, the bones about the re- gion of the nose are unequally developed, and the skull becomes asymmetrical. In the Flatfishes (Pleuronectide), the skull be comes so completely distorted, that Mcapartieteget toe tone he the two eyes lie one side of Heal ate iseramazte. the body, which is, in some cases, sition of the two eyes in their orbits; the left, and, in others, the right teh Weaual Pot ae Pee Pa side. In certain of these fishes, ie SO, supra-occipital; £p.0, the rest of the skull and facial bones, the spine, and even the limbs, partake in this asymmetry. The base of the skull and THE CARPUS AND THE TARSUS. 31 its occipital region are comparatively little affected ; but, in the interorbital region, the frontal bones and the subjacent carti- laginous, or membranous, side-walls of the cranium are thrown over to one side; and, frequently, undergo a flexure, so that they become convex toward that side, and concave in the op- posite direction. The prefontal bone of the side from which the skull is twisted, sends back a great process above the eye of that side, which unites with the frontal bone, and thus en- closes this eye in a complete bony orbit. It is along this fronto-prefrontal bridge that the dorsal fin-rays are continued forward, just as if this bridge represented the morphological middle of the skull. (Fig. 10.) The embryonic Pleuronectidw have the eyes in their nor- mal places, upon opposite sides.of the head; and the cranial distortion commences only after the fish are hatched. The Appendicular Endoskeleton.—The limbs of all verte- brated animals make their appearance as buds on each side of the body. In all but fishes, these buds become divided by constrictions into three segments. Of these, the proximal is called brachium in the fore-limbs, femur in the hind; the middle is antebrachium, or crus; the distal is manus, or pes. Each of these divisions has its proper skeleton, composed of cartilage and bone. The proximal division, normally, con- tains only one bone, os humeri, or humerus, in the brachium, and os femoris, or femur, in the thigh; the middle, two bones, side by side, radius and ulna, or tibia and fibula ; the distal, many bones, so disposed as to form not more than five longi- tudinal series, except in the Ichthyosauria, where marginal bones are added, and some of the digits bifurcate. The skeletal elements of the manus and pes are divisible into a proximal set, constituting the carpus or tarsus; and a distal set, the digits, of which there are normally five, articu- lated with the distal bones of the carpus and tarsus. Hach digit has a proximal basi-digital (metacarpal or metatarsal) bone, upon which follows a linear series of phalanges. It is convenient always to count the digits in the same way, com- mencing from the radial or tibial side. Thus, the thumb is the first digit of the hand in man; and the great-toe the first digit of the foot. Adopting this system, the digits may be represented by the numbers i, ii, iii, iv, v. There is reason to believe that, when least modified, the carpus and the tarsus are composed of skeletal elements which are alike in number and inarrangement. One of these, primitively situated in the centre of the carpus or tarsus, is 32 THE ANATOMY OF VERTEBRATED ANIMALS. termed the centrale ; on the distal side of this are five car- palia, or tarsalia, which articulate with the several metacar- pal or metatarsal bones; while, on its proximal side, are three bones—one rudiale or tibiale, articulating with the radi- us or tibia; one ulnare or jfibulare, with the ulna or fibula; and one intermedium, situated between the foregoing. Car- pal and tarsal bones, or cartilages, thus disposed are to be met with in some Amphibia and Chelonia (Fig. 11), but, Fre. 11.—The right fore-foot of the Chelonian Chelydra, and the right hind-foot of the Am- phibian Sulamandra.—U, ulna; PR, radius; J, fibula; 7, tibia. Proximal carpa! bones: 7, radiale; 2, intermedium; w, ulnare; the centrale is the middle unlettered bone. Proximal tarsal bones; ¢, tibiale; ¢, intermedium; jf, fibulare; ce, centrale; 1, 2,3,4,5, distal carpalia and tarsalia; 1, 11, 111, tv, v, digits. commonly, the typical arrangement is disturbed by the sup- pression of some of these elements, or their coalescence with one another. Thus, in the carpus of man, the radiale, inter- medium, and ulnare are represented by the scaphoides, lunare, and cuneiforme respectively. The pisiforme is a sesamoid bone developed in the tendon of the flexor carpi ulnaris, which, has nothing to do with the primitive carpus. The centrale is not represented in a distinct shape, having proba- bly coalesced with one of the other elements of the carpus. The fourth and fifth carpalia have coalesced, and form the single unciforme. In the tarsus of man, the astragalus repre- sents the coalesced tibiale and intermedium; the caleaneum, the fibulare. The naviculare is the centrale.. Like the cor- THE POSITION OF THE LIMBS. 33 responding bones in the carpus, the fourth and fifth tarsalia have coalesced to form the cuboides. Lhe Position of the Limbs.—In their primitive position, the limbs are straight, and are directed outward, at right angles to the axis of the body; but, as development proceeds, they become bent in such a manner that, in the first place, the middle division of each limb is flexed downward and toward the middle line, upon the proximal division; while the distal division takes an opposite bend upon the middle division. Thus the ventral aspects of the antebrachium and erus come to look inwardly, and the dorsal aspects outwardly ; while the ventral aspects of the manus and pes look downward and their dorsal aspects look upward. When the position of the limbs has been no further altered than this, the radius in the antebrachium, and the tibia in the crus, are turned for- ward, or toward the head; the ulna and the fibula backward, or toward the caudal extremity. On looking at these parts with respect to the axis of the limb itself, the radius and the tibia are pre-axial, or in front of the axis; while the ulna and fibula are post-axial, or behind it. The same axis traverses the centre of the middle digit, and there are therefore two pre-axial, or radial, or tibial digits; and two post-axial, or ulnar, or fibular digits, in each limb. The most anterior of the digits (i) is called pollex, in the manus; and Aallwx in the pes. The second digit (ii) is the index, the third (iii) the medius ; the fourth (iv) the annularis ; and the fifth (v) the minimus. In many Amphibia and Reptilia, the limbs of the adult do not greatly depart from this primitive position; but, in birds and in mammals, further changes occur. Thus, in all ordi- nary quadrupeds, the brachium is turned backward and the thigh forward, so that both elbow and knee lie close to the sides of the body. At the same time, the forearm is flexed upon the arm, and the leg upon the thigh. Iv Man a still greater change occurs. In the natural erect posture, the axes of both arm and leg are parallel with that of the body, in- stead of being perpendicular to it. The proper ventral sur- face of the brachium looks forward, and that of the thigh backward, while the dorsal surface of the latter looks forward. The dorsal surface of the antebrachium looks outward and backward, that of the leg directly forward. The dorsal surface of the manus is external, that of the pes, superior. Thus, speaking broadly, the back of the arm corresponds with the front of the leg, and the outer side of the leg with the inner side of the arm, in the erect. position. 34 THE ANATOMY OF VERTEBRATED ANIMALS. In Bats, a line drawn from the acetabulum to the foot is also, in the natural position, nearly parallel with the long axis of the body. But, in attaining this position, the leg is bent at the knee and turned backward; the proper dorsal surface of the thigh looking upward and forward, while the corre- sponding surface of the leg looks backward and upward, and the ungual phalanges are turned backward. The chief modifications of the manus and pes arise from the excess, or defect, in the development of particular digits, and from the manner in which the digits are connected wita one another, and with the carpus or tarsus. In the Ichthyo- sauria and Plesiosauria, the Turtles, the Cetacea and Sirenia, and, in a less degree, in the Seals, the digits are bound together and cased in a common sheath of integument, so as to form paddles, in which the several digits have little or no motion on one another. The fourth digit of the manus in the Prerosauria, and the four ulnar digits in the Bats, are vastly elongated, to support the web which enables these animals to fly. In existing birds the two ulnar, or post-axial, digits are aborted, the metacarpals of the second and third are anchylosed together, and the digits themselves are enclosed in a common integu- mentary sheath; the third invariably, and the second usually, is devoid of a claw. The metacarpal of the pollex is anchy- losed with the others, but the rest of that digit is free, and frequently provided with a claw. Among terrestrial mammals, the most striking changes of the manus and pes arise from the gradual reduction in the number of the perfect digits from the normal number of five to four (Sus), three (hinoceros), two (most Ruminantia), or one (Lyuidee), The Pectoral and Pelwie Arches.—The proximal skeletal elements of each pair of limbs (humeri or femora) are sup- ported by a primitively cartilaginous, pectoral, or pelvic girdle, which lies external to the costal elements of the verte- bral skeleton, This girdle may consist of a simple cartilagi- nous arc (as in the Sharks and Rays), or it may be complicated by subdivisions and additions. The pectoral arch may be connected with the skull, or with the vertebral column, by muscles, ligaments, or dermal ossifications, though, primitively, it is perfectly free from, and independent of, both; but it is never united with the verte- bree by the intermediation of ribs. At first, it consists of one THE PECTORAL ARCH. 35 continuous cartilage, on each side of the body, distinguish- able only into regions and processes, and affording an articular surface to the bones or cartilages of the limb. But ossifica- tion usually sets up in the cartilage, in such a way as to give rise to a dorsal bone, called the scapula, or shoulder-blade, which meets, in the articular, glenoidal cavity for the hu- merus, with a ventral ossification, termed the coracoid, By differences in the mode of ossification of the various parts, and by other changes, that region of the primitively Fic. 12.—Side-view of the pectoral arch and sternum of a Lizard (Iguana tuberculata),— Se, scapula; 8.8c, supra-scapula; cr, coracoid; gi, glenoidal cavity; S¢, sternum; @.s¢, xiphisternum; .8c, mesoscapula; p.er, precoracoid; m.cr, mesocoracoid; e.cr, epi- coracoid ; ¢/, clavicle; é.cd, interclavicle. cartilaginous pectoral arch which lies above the glenoidal cavity may be ultimately divided into a scapula and a supra- scapula ; while that. which lies on the ventral side may pre- sent not only a coracoid, but a precoracoid and an epicora- coid. In the great majority of the Vertebrata above fishes, the coracoids are large, and articulate with the antero-external margins of the primitively cartilaginous sternwm, or breast- bone. But, in most mammals, they do not reach the sternum, and, becoming anchylosed with the scapula, they appear, in adult life, as mere processes of that bone. Numerous Vertebrates possess a clavicula, or collar-bone, which is connected with the pre-axial margin of the scapula and coracoid, but takes no part in the formation of the glenoid cavity, and is usually, if not always, a membrane bone. In many Vertebrata, the inner ends of the clavicles 36 THE ANATOMY OF VERTEBRATED ANIMALS. are connected with, and supported by, a median membrane bone which is closely connected with the ventral face of the sternum. This is the interclavicula, frequently called epister- ~ num, hj Fie, 18,—Ventral view of the sternum and pectoral arches of Iguana tuberculata. The letters as in Fig. 12. The pelvic, like the pectoral, arch at first consists of a simple continuous cartilage on each side, which, in Vertebrata higher than fishes, is divided by the acetabulum, or articular cavity for the reception of the head of the femur, into a dorsal and a ventral moiety. Three separate ossifications usually take place in this car- tilage—one in the dorsal, and two in the ventral, moiety. Hence, the pelvic arch eventually consists of a dorsal portion, called the iliwm, and of two ventral elements, the pubis ante- riorly, and the ¢schiwm posteriorly. All these generally enter into the composition of the acetabulum. The ilium corresponds with the scapula. In the higher Vertebrata the outer surface of the latter bone becomes di- vided by a ridge into two fosse. The ridge, called the spine of the scapula, frequently ends in a prominent process termed the aecromion, and with this, in Mammalia, the clavicle artic- ulates. In like manner, the outer surface of the ilium be- THE PELVIC ARCH. 87 comes divided by a ridge which grows out into a great crest in Man and other Mammalia, and gives attachment to mus- cles and ligaments. The ischium corresponds very nearly with the coracoid in the pectoral arch; the pubis with the precoracoid, and more or less of the epicoracoid. The pelvis possesses no osseous element corresponding with the clavicle, but a strong ligament, the so-called Pou- part's ligament, stretches from the ilium to the pubis in many Vertebrata and takes its place. (Fig. 14, Pp.) Fia. 14.—Side-view of the left Os innominatum of Man: J/, ilium; Js,ischium; Pp, pubis; A, acetabulum; Pp, Poupart’s ligament, On the other hand, the marsupial bones of certain mam- mals, which are ossifications of the tendons of the external oblique muscles, seem to be unrepresented in the pectoral arch; while there appears to be nothing clearly corresponding with a sternum in the pelvic arch, though the precloacal car- tilage, or ossicle, of Lizards has much the same relation to the ischia as the sternum has‘to the coracoids. Very generally, though not universally, the ilia are closely articulated with the modified ribs of the sacrum. The pubes and ischia of opposite sides usually meet in a median ventral symphysis; but in all birds, except the Ostrich, this union does not take place. The Limbs of Fishes.—The limbs of Fishes have an endo- skeleton which only imperfectly corresponds with that of the higher Vertebrates. For while homologues of the cartilagi- > 38 THE ANATOMY OF VERTEBRATED ANIMALS. nous, and even of the bony, constituents of the pectoral and pelvic arches of the latter are traceable in Fishes, the cartila- ginous, or ossified, basal and radial supports of the fins them- selves cannot be identified, unless in the most general way, with the limb-bones, or cartilages, of the other Vertebrata, In its least modified form, as in ZLepidosiren, the endo- skeleton of the fish’s fin is a simple cartilaginous rod, divided into many joints; and articulated, by its proximal end, with the pectoral arch. The Elasmobranchii possess three basal cartilages which articulate with the pectoral arch, and are called, respectively, from before backward—propterygial, me- sopterygial, and metapterygial basalia. With these are artic- ulated linear series of radial cartilages, upon which osseous, or horny, dermal fin-rays are superimposed. (Fig. 15.) Among the Ganoid fishes, the fins of Polypterus are, fun- damentally, like those of the Elasmobranchit ; but the pro- pterygial, mesopterygial, and metapterygial basalia, are more or less ossified, and are succeeded by a series of elongated radialia, which are also, for the most part, ossified. Beyond Fic, 15.—The right pectoral member of the Monkfish (Squatina); a mesopterygium; 24, aoe Droplenyalann os THE LIMBS OF FISHES. 39 these follow some small additional radialia, which remain car- tilaginous, and are embraced by the bases of the fin-rays. In the other Ganoids the propterygial basale disappears, and some of the radialia, pushing themselves between the meso- pterygial and metapterygial basalia, articulate directly with the pectoral arch, ‘The mesopterygial basale is embraced by, and becomes more or less incorporated with, the large ante- rior fin-ray. From these Ganoids the passage is easy to the Zeéleostei, in which, also, the mesopterygial basale always becomes fused with the anterior fin-ray, whence the latter seems to articulate directly with the shoulder-girdle. Four bones, of very similar general form, usually articulate with the pectdral arch, be- neath and hehind the mesopterygial basale and its fin-ray. At their distal ends small cartilaginous nodules may lie, and these are embraced by the fin-rays. Of these four bones, or partially-ossified cartilages, the lowermost and hindermost answers to the metapterygial basale of the Shark; the others seem to be radialia, (See the figure of the Pike’s pectoral fin, injra.) The ventral fins have basal and radial cartilages and fin- rays, more or less resembling those of the fore-limbs. In most Ganoids and Teleosteans the pectoral and pelvic arches are, in part, or completely, ossified; the former fre- quently presenting distinct scapular and coracoid bones. To these, in all Ganoids and Teleosteans, membrane bones, rep- resenting a clavicle, with supra-clavicular and post-clavicular ossifications, are added, In all Elasmobranchs and Ganoids, and in a large propor- tion of the Teleosteans, the pelvic fins are situated far back on the under side of the body, and are said to be “ ventral” in position; but, in other Teleosteans, the ventral fins may move forward, so as to be placed immediately behind, or even in front of, the pectoral fins. In the former case they are said to be “ thoracic,” in the latter “ jugular.” The Vertebrate Hcoskeleton.—The Hxoskeleton never at- tains, in vertebrated animals, the functional importance which it so frequently possesses among the Invertebrata, and it va- ries very greatly in the degree of its development. _ The integument consists of two layers—a superficial, non- vascular substance, the epidermis, composed of cells, which _are constantly growing and multiplying in the deeper, and being thrown off in the superficial, layers; and a deep vascu- lar tissue, the dermis, composed of more or less completely- 40 THE ANATOMY OF VERTEBRATED ANIMALS. formed connective tissue. An exoskeleton may be developed by the hardening of either the epidermis, or the dermis. The epidermal exoskeleton results from the conversion into horny matter of the superficial cells of the epidermis. The horny plates thus formed are moulded upon, and follow the configuration of, arex, or processes, of the dermis. When the latter are overlapping folds, the horny epidermic investment is called a scale, sgwama. When the dermic process is papilli- form, and sunk in a pit of the dermis, the conical cap of modi- fied epidermis which coats it is either a hair or a feather. To become a hair, the horny cone simply elongates by continual addition of new cells to its base; but, in a feather, the horny cone, which also elongates by addition to its base, splits up, for a greater or less distance along the middle line of its under surface, and then spreads out into a flat vane, subdivided into barbs, barbules, etc., by a further process of splitting of the primary horny cone. : The epidermis remains soft and delicate in Fishes and Amphibia. In Reptilia it sometimes takes the form of plates, which attain a great size in many Chelonia ; sometimes, that of overlapping scales, as in Ophidia and many Lacertilia ; but, sometimes, it remains soft, as in some Chelonia and in the Chameleons. LEpidermic plates in the form of nails appear upon the terminal phalanges of the limbs. : All Aves possess feathers. In addition, the beak is partly or completely ensheathed in horn, as in some Reptilia. Corni- fied epidermic tubercles or plates are developed on the tarsi and toes, the terminal phalanges of which (and sometimes those of the wing) havé nails. Besides these, some birds pos- sess spurs, which are ensheathed in horn, on the legs or wings. In Mammalia, the horny exoskeleton may take all the forms already mentioned, except that of feathers. In some Cetacea it is almost absent, being reduced to a few hairs, pres- ent only in the foetal state. The Pangolin (Manis), on the other hand, is almost completely covered with scales, the Armadillos with plates, and most terrestrial mammals with a thick coat of hair. The greater part of the mass of the horns of Oxen, Sheep, and Antelopes, is due to the epidermic sheath which covers the bony core. Where the horny epidermis be- comes very thick, as in the hoof of the Horse, and in the horn of the Rhinoceros, numerous long papille of the dermis extend into it. These papillw, however, are comparable to the ridges of the bed of the nail, not to the papillz of the hairs, THE EXOSKELETON. : 41 The dermal exoskeleton arises from the hardening of the dermis ; in the majority of cases by the deposit of bone-earth, in more or less completely-formed connective tissue, though the resulting hard tissue has by no means always the struct- ure of bone. It may happen that cartilage is developed in the dermis ; and, either in its primary state or ossified, gives rise to exoskeletal parts. Fie. 16.—A, outline of a Pike (Esow), to show the fins: P, pectoral ; V, ventral; A, anal; C, caudal; D, dorsal, fins. Op., operculum i P.Op., breoperewiurs Br, branchiostegal rays.—B, scales of the dermal exoskeleton of the same fis: No dermal exoskeleton (except that of the fin-rays) is found in the lowest fishes, Amphioxus and the Marsipobranchii. In most Zelcoste/, the integument is raised up into overlapping folds ; and, in these, calcification takes place in laminz, of which the oldest is the most superficial, and lies immediately beneath the epidermis. As a general rule, the calcified tissue of the “scale” thus formed, does not possess the structure of true bone in the Zéleoste’, But, in other fishes, the dermal calcification may consist of true bone (as in the Sturgeon) ; or, as in the Sharks and Rays, may take on the structure of teeth, and consist mainly of a tissue exactly comparable to dentine, capped with enamel, and continuous by its base with a mass of true bone, which takes the place of the cresta petrosa, or cement of the teeth. A form of dermal exoskeleton, which is peculiar to and highly characteristic of fishes, is found in the jin-rays. These are developed in the integument either of the median line of the body, or in that of the limbs. In the former case, they usually enter into, or support, folds of the integument which are termed dorsal, caudal, or anal fins—according as they lie 42 THE ANATOMY OF VERTEBRATED ANIMALS. in the dorsal region, or at the extremity of the body, or on the ventral aspect, behind the anus. Ordinary fin-rays are com- posed of a hornlike, or more or less calcified, substance, and are simple at the base, but become jointed transversely, and split up longitudinally, toward their extremities (Fig. 6). Each fin-ray consists of two nearly equal and similar parts, which cohere by their applied faces for the greater part of their extent; but, at the base of the rays, the halves commonly diverge, to embrace, or more or less completely coalesce with, cartilaginous or osseous elements of the exoskeleton, In the median fins, these are the interspinous cartilages, or bones, which lie between the fin-rays and the superior or inferior spines of the vertebre. In the paired fins, they are radial or basal, cartilaginous or osseous, elements of the endoskeleton. The Amphibia in general are devoid of dermal exoskeleton, but the Cecilie have scales like those of fishes. Ceratophrys has plates of bone developed in the dorsal integument, which seem to foreshadow the plates of the carapace of the Chelonia,; and the extinct Labyrinthodonts possessed a very remarkable ventral exoskeleton. The Ophidia have no dermal exoskeleton. Many Lizards. have bony dermal plates corresponding in form and size with the epidermal scales. All Crocodiléa have such bony plates in the dorsal region of the body and tail; and in some, such as the Jacares and Caimans, and the extinct Teleosauria, they are also developed in the ventral region. In these animals there is a certain correspondence between the segments of the exoskeleton and those of the endoskeleton. But the dermal exoskeleton attains its greatest development in the Chelonia, and will be particularly described under the head of that order. In the Mammalia the development of a dermal exoskeleton - is exceptional, and occurs only in the loricated Edentata, in which the dorsal region of the head and body, and the whole of the tail, may be covered with shields of dermal bone. In connection with the dermis and epidermis, the glandu- lar and pigmentary organs of the integument may be men- tioned. Integumentary glands’ do not appear to exist in Fishes, but they attain an immense development in some of the Amphibia, as the Frog. Among Reptilia, Lizards fre- quently present such glands in the femoral and cloacal regions ; and, in Crocodiles, integumentary glands, which secrete a musky substance, lie beneath the jaw. In Birds they attain a considerable size in the uropygial gland; and, in Mammalia, acquire a large development in connection with the sacs of the THE EXOSKELETON. 43 hairs, or as independent organs, in the form of sweat-glands, musk-glands, or mammary glands, The color of the integument may arise from pigment- granules, deposited either in the epidermis or in the dermis; and, in the latter case, it is sometimes contained in distinct chromatophores, as in the Chameleon. CHAPTER II. THE MUSCLES AND THE VISCERA—A GENERAL VIEW OF THE ORGANIZATION OF THE VERTEBRATA. Tux muscular system of the Vertebrata consists of muscles related partly to the exoskeleton, partly to the endoskeleton, and partly to the viscera, and formed both of striated and un- striated muscular fibre. The latter is confined to the vessels, the viscera, and the integument; the parts of the endoskele- ton being moved upon one another exclusively by striated mus- cular fibre. The muscles of the endoskeleton may be divided, like the endoskeleton itself, into one system appertaining to the trunk and head, and another belonging to the limbs. The Muscular System of the Trunk and Head.—This con- sists of two portions, which differ fundamentally in their origin, and in their relations to the endoskeleton. The one takes its origin in the protovertebre ; each protovertebra be- coming differentiated, as we have seen, into three parts; 2 spinal ganglion and a segment of the vertebral endoskeleton, in the same plane, and a more superficial sheet of muscular fibres. These muscular fibres are consequently situated above the endoskeleton, or are episkeletal. Other muscular fibres are developed below the endoskeleton, and may be termed hypo- skeletal muscles. The hyposkeletal muscles are separated from the episkeletal, not only by the endoskeleton of the trunk (or the vertebrae and their prolongations, the ribs), but by the ventral branches of the spinal nerves. As the episkeletal muscles are developed out of the proto- vertebrae, they necessarily, at first, present as many segments as there are vertebra, the interspaces between them appearing as intermuscular septa. The development of the hyposkeletal muscles has not been worked out, but it appears to take place much later than that of the episkeletal set. EPISKELETAL AND HYPOSKELETAL MUSCLES. 45 In the lowest Vertebrata—as, for example, in ordinary fishes—the chief muscular system of the trunk consists of the episkéletal muscles, which form thick lateral masses of longitu- dinal fibres, divided by transverse intermuscular septa into segments (or Myotomes) corresponding with the vertebra. The lateral muscles meet in the middle line below, and divide, in front, into a dorso-lateral mass connected with the skull, and a ventro-lateral attached, in part, to the pectoral arch, and, in part, continued forward to the skull, to the hyoidean appa- ratus, and to the mandible. Posteriorly, the lateral muscles are continued to the extremity of the tail. The hyposkeletal muscular system appears to be undeveloped. In the higher Vertebrata, both the episkeletal and hypo- skeletal muscular systems are represented by considerable numbers of more or less distinct muscles. The dorso-lateral division of the lateral muscle of the fish is represented by the superior caudal muscles, and by the erector spine ; which, as it splits up, anteriorly, and becomes attached to the vertebre, and to the ribs, and to the skull, acquires the names of spi- nalis, semispinalis, longissimus dorsi, sacrolumbalis, inter- transversalis, levatores costarum, complexus, splenius, recti postict, and recti laterales. The ventro-lateral division of the fish’s lateral muscle is represented, in the middle line of the trunk and head, by a series of longitudinal muscles; and, at the sides, by obliquely- directed muscles. The former are the recti abdominis, extend- ing from the pelvis to the sternum—the sterno-hyoidei, be- tween the sternum and the hyoidean apparatus—the genio- hyoidei, which pass from the hyoid to the symphysis of the mandible, The latter are the obliqui externi of the abdomen —the external intercostales of the thorax—the subclavius stretching from the first rib to the clavicle; the scaleni from the anterior dorsal ribs to the cervical ribs and transverse processes, and the sterno- and cleido-mastoidei from the ster- num and clavicle to the skull. The fibres of all these oblique muscles take a direction, from parts which are dorsal and anterior, to others which are ventral and posterior. ; The trunk muscles of the lower Amphibia exhibit arrange- ments which are transitional between those observed in Fishes and that which has been described in Man, and which substan- tially obtains in all abranchiate Vertebrata. _ The muscles of the jaws and of the hyoidean apparatus appear to be, in part, episkeletal, and, in part, hyposkeletal. 46 THE ANATOMY OF VERTEBRATED ANIMALS. The mandible is depressed by a muscle, the digastric, arising from the skull, and supplied by a branch of the seventh nerve : it is raised by a muscular mass, which is separable into mas- seter, temporal, and pterygoid muscles, according to its con- nection with the maxillo-jugal bones, the sides of the skull, or the palato-pterygoid bones, and is supplied by the fifth nerve. The proper facial muscles belong to the system of cutane- ous muscles, and receive branches from the seventh nerve. The hyposkeletal system is formed, partly, of longitudinal muscles which underlie the vertebral column; and partly, of more or less oblique, or even transverse fibres, which form the innermost muscular walls of the thorax and of the abdomen. The former are the subcaudal intrinsic flexors of the tail ; the pyrifermis, psoas, and other muscles proceeding from the inferior faces of the vertebrae to the hind-limb; the longus colli, or intrinsic flexor of the anterior part of the vertebral column; and the recti capitis antici, or flexors of the head upon the vertebral column. The latter are the obliquus in- ternus of the abdomen, the fibres of which take a direction crossing that of the external oblique muscle; and the ¢rans- versalis, which lies innermost of the abdominal muscles, and has its fibres transverse. In the thorax, the intercostales interni continue the direction of the internal oblique, and the triangu- laris sterni that of the transversalis. The diaphragm and the levator ani must also be enumerated among the hyposkeletal muscles. The hyposkeletal muscles of the posterior moiety of the body attain a great development in those Vertebrata which have no hind-limbs, such as Ophidia and Cetacea. The Muscular System of the Limbs.—The muscles of the limbs of Fishes are very simple, consisting, on each face of the limb, of bundles of fibres, which proceed (usually in two layers) obliquely, from the clavicle and supraclavicle to the finrays. The pectoral and pelvic arches themselves are im- bedded in the lateral muscles, In the Amphibia and all the higher Vertebrata, the muscles of the limbs are divisible into—intrinsic, or those which take their origin within the anatomical limits of the limb (including the pectoral or pelvic arch); and extrinsic, or those which arise outside the limb. Supposing the limb to be extended at right angles to the spine (its primitive position), it will present a dorsal aspect and a ventral aspect, with an anterior, or pre-axial, and a pos- terior, or post-azxial, side. THE MUSCLES OF THE LIMBS. 47 In the Vertebrata above fishes, the following muscles, which occur in Man, are very generally represented : Extrinsic muscles attached to the pectoral and pelvic arches, on the dorsal aspect.—In the fore-limb, the cleidomastoideus, from the posterolateral region of the skull to the clavicle; the trapezius, from the skull and spines of many of the vertebra to the scapula and clavicle; the rhomboidet, from the spines of vertebree to the vertebral edge of the scapula, beneath the foregoing. Sometimes there is a tracheloacromialis, from the transverse processes of the cervical vertebra to the scapula. On the ventral aspect, the subclavius, which passes from the anterior rib to the clavicle, may be regarded as, in part, a mus- cle of the limb; the pectoralis minor, from the ribs to the coracoid. Between the dorsal and the ventral aspects muscular fibres arise from the cervical and dorsal ribs, and pass to the inner aspect of the vertebral end of the scapula: anteriorly, these are called levator anguli scapule ; posteriorly, serratus magnus. An omohyoid muscle frequently connects the scapula with the hyoidean arch. The posterior limb does not seem to offer any muscles ex- actly homologous with the foregoing. Sovfar, however, as the rectt abdominnis, the obliquus externus, and the fibres of the erector sping, are attached to the pelvic girdle, they cor- respond in a general way with the pre-axial, or protractor, mus- cles of the pectoral arch; and the ischio-coccygeal muscles, when they are developed, are, in relation to the pelvic arch, retractors, though, owing to the relative fixity of the pelvis, they act in protracting, or flexing, the caudal region. The psoas minor, proceeding from the under surfaces of posterior dorsal (or lumbar) vertebree to the ilium, or pubis, is a protractor of the pelvis, but, as a hyposkeletal muscle, has no homologue in the fore-limb. Extrinsic muscles attached to the humerus or femur, on the dorsal aspect.—In the fore-limb there is the post-axial latis- simus dorsi passing from spines of dorsal vertebrae to the humerus. On the ventral aspect, the pectoralis major extends from the sternum and ribs to the humerus. In the hindlimb, the gluteus maximus, so far as it arises from the sacral and coccygeal vertebra, and is inserted into the femur, repeats the relations of the latisstmus dorst. In the absence of any thing corresponding with the sternum, or the ribs, no exact homologue of the pectoralis major can be said to exist, though the pectineus comes near it. The psoas 48 THE ANATOMY OF VERTEBRATED ANIMALS. major, passing from posterior dorsal or lumbar vertebree—the pyriformis from sacral vertebree—the femoro-coccygeus (when it exists) from caudal vertebrae—to the femur, are all hypo- skeletal muscles, without homologues in the anterior extremity. All the other muscles of the limbs are intrinsic, taking their origins from the pectoral or petvic arches, or from some of the more proximal segments of the limb-skeleton, and hav- ing their insertion in the more distal segments. They are thus arranged in Man and the higher Mammatia: Intrinsic muscles proceeding from the pectoral or pelvic arches to the humerus or femur, on the dorsal aspect.—In the fore-limb, the deltoides proceeds from the clavicle and scapula to the humerus. This superficial shoulder-muscle continues the direction of the fibres of the trapezius ; and, when the clavicle is rudimentary, the adjacent portions of the two mus- cles coalesce into a cephalo-humeralis muscle. Beneath the deltoid the supra-spinatus, on the pre-axial side of the spine of the scapula; the in/ra-spinatus, and the teres major and minor, on its post-axial side, run from the dorsal aspect of the scapula to that of the head of the humerus. In the hind-limb, the tensor vagine femoris, which passes from that part of the ilium which corresponds with the spine and acromion of the scapula, to the femur, appears to answer better to the deltoid than does the gluteus maximus, which, at first sight, would seem to be the homologue of that muscle. The ¢iacus, proceeding from the inner surface of the crest of the ilium to the smaller trochanter, answers to the supra- spinatus ; the gluteus medius and minimus, which arise from the outer surface of the ilium, to the infra-spinatus and teres. In the fore-limb, a muscle, the subscapularis, is attached to the inner face of the scapula, and is inserted into the hu- merus. No muscle exactly corresponding with this appears to exist in the hind-limb. On the ventral aspect in the fore-limb, the coracobrachialis passes from the coracoid to the humerus. In the hind-limb, a number of muscles proceed from the corresponding (ischio- pubic) part of the pelvic arch to the femur. These are, from the outer surface of the pubis, the pectineus, and the great ab- ductors of the femur; with the obturator externus, from the outer side of the ischiopubic fontanelle, or obturator membrane. The gemelli and the quadratus femoris take their origin from the ischium. No muscle is attached to the proper inner surface of the ilium, so that there is no homologue of the subscapularis in THE MUSCLES OF THE LIMBS. 49 the hind-limb. On the other hand, a muscle, the odturator internus, attached to the inner surface of the ischiopubic fon- tanelle, and winding round to the femur, has no homologue in ‘the upper extremity of the higher Vertedrata, unless it be the so-called corasobrachialis, which arises from the inner surface of the coracoid in many Sauropsida. Muscles of the Antebruchium and Crus.—On the dorsal aspect of the fore-limb, as of the hind-limb, certain muscles arise in part from the arch, and, in part, from the bone of the proximal segment of the limb, and go to be inserted into the two bones of the second segment. These are, in the fore- limb, the triceps extensor and the supinator brevis ; in the hind-limb, the guadriceps extensor, There is this difference between these two homologous groups of muscles—that in the fore-limb, the principal mass of the muscular fibres goes, as the triceps, to be inserted into the post-axial bone (ulna), and the less portion, as supinator brevis, into the pre-axial bone (radius); whereas, in the hind- limb, it is the other way, almost the whole of the muscular fibres passing, as the guadriceps, to the pre-axial bone (tibia), the tendon éommonly developing a sesamoid patella ; while only a few fibres of that division of the quadriceps which is called the “vastus externus” pass to the post-axial bone (fibula). On the ventral aspect, the fore-limb presents three mus- cles, arising either from the pectoral arch, or from the hume- rus, and inserted into the two bones of the forearm. On the pre-axial side are two muscles ; one double-headed, the biceps, arising from the scapula and the coracoid, and inserted into the radius. A second, the supinator longus, passes from the humerus to the radius. On the post-axial side, the brachialis- anticus arises from the humerus, and is inserted into the ulna. The hind-limb has two muscles, the sartorius, arising from the ilium, and the gracilis, from the pubis, in place of the biceps brachii, and inserted into the pre-axial bone, the tibia, which corresponds with the radius. _Two other muscles, the semi- membranosus and semi-tendinosus, pass from the ischium to the tibia, and replace, without exactly representing, the sw- pinator longus. Corresponding with the brachialis anticus is the short head of the biceps femoris, arising from the femur, and inserted into the post-axial bone of the leg, the fibula. The long head of the biceps femoris, which proceeds from the ischinm, appears to have no representative in the fore-limb. In the fore-limb, a muscle, the pronator teres, passes ob- 3 50 THE ANATOMY OF VERTEBRATED ANIMALS. liquely from the post-axial condyle of the humerus to the radi-. us. In the hind-limb, a corresponding muscle, the popliteus, proceeds from the post-axial condyle of the femur to the tibia. The pronator quadratus, which passes from the ulna to the radius, has its analogue, in some Marsupialia and Reptilia, in muscles which extend from the fibula to the tibia. The Muscles of the Digits.—The remaining muscles of the two limbs are, primarily, muscles of the digits, and are at- tached either to the basi-digital (metacarpal or metatarsal) bones, or to the phalanges, though they may acquire second- ary connections with bones of the tarsus or carpus. The plan upon which they are arranged, when they are most com- pletely developed, will be best understood by commencing with the study of their insertion in any one of those digits which possesses a complete set; such, for example, as the fifth digit of the manus, or little finger, in Man and the higher Primates. On the dorsal aspect this digit presents; first, attached to the base of its metacarpal bone, the tendon of a distinct mus- cle, the extensor carpi ulnaris. Secondly, spreading out over the phalanges into an aponeurosis, which is principally at- tached to the first and second, is a tendon belonging to another muscle, the extensor minimi digiti. Thirdly, entering the same expansion is one tendon of the extensor communis digitorum, On the ventral aspect there are: first, attached to the base of the metacarpal, the tendon of a distinct muscle, the flexor carpi ulnaris ; secondly, arising from the sides and ventral face of the metacarpal, and inserted into either side of the base of the proximal phalanx, two muscles, the interossei ; thirdly, inserted into the sides of the middle phalanx by two slips, a tendon of the flexor perforatus ; and fourthly, passing be- tween these two slips, and inserted into the base of the distal phalanx, a tendon of the flexor perforans. Thus there are — special depressors, or flexors, for each segment of the digit. There appear, at first, to be but three elevators, or extensors, but, practically, each segment has its elevator. For the ten- dons of the extensor communis and extensor minimi digiti are attached to the middle and the proximal phalanges; and the distal phalanx is specially elevated by the tendons of two lit- tle muscles, which, in Man, are usually mere subdivisions of the interossei, and pass upward, joining the extensor sheath, to be finally inserted into the distal phalanx. “ . The fifth digit of the pes, or little toe, sometimes presents the same disposition of muscles, namely: . ° THE MUSCLES OF THE LIMBS. 51 On the dorsal aspect: first, the peronwus tertius for the metatarsal bone; secondly, one tendon from the extensor digi- torum brevis, but this last is commonly absent in Man; third- ly, one tendon from the extensor digitorum longus. Fic. 17.—Part of the middle digit of the manus of an Orang with the flexors and extensors of the phalanges: mcp., metacarpal bone; Ph, 1, Ph. 2, Ph. 8, the three phalanges; Fat. 1, the deep long extensor tendon from the eatensor indicis; Hat. 2, the superfi- cial long extensor tendon from the extensor communis ; J. e., the interosseous short ex- tensor; Z.7!, the interosseous short flexor; /. pns., the deep long flexor (perforans); F. pts., the superficial long flexor (perforatus). On the ventral aspect: first, the peronceus brevis, attached to the base of the metatarsal; secondly, two interosseé ; thirdly, a perforated flexor ;-and fourthly, a perforating flexor, like those of the manus. The divisions of the dnterossei, which send tendons to the extensor sheath on the dorsum of the digits of the foot in Man, are hardly distinct from the ven- tral divisions of those muscles. In addition to the muscles which have been mentioned, the fifth digit has an abductor and an adductor, which may be regarded as subdivisions of the interosset, arising within the manus or pes, and inserted into opposite sides of the proximal phalanx; and an opponens, a muscle attached to the ventral face of the carpus or the tarsus, and inserted into the post- axial edge of the shaft of the metacarpal or metatarsal. Finally, a dumbricalis muscle proceeds from the tendon of the -perforating flexor, on the pre-axial side of the digit, to the extensor sheath. None of the other digits of the manus, or of the pes, has a greater number of muscles than this; in fact, all the others have fewer muscles, some of those . enumerated being sup- 52 THE ANATOMY OF VERTEBRATED ANIMALS. a pressed. What are often regarded as muscles special to man, such as the extensor proprius indicis and extensor minims digiti, are only remains of muscles which are more fully de- veloped in lower mammals, and send tendons to all four of the ulnar digits. Only the pollex has an opponens.* Only the pollex and hallux have adductors and abductors. Some of the digits lack one or more of the ventral, or of the dorsal, muscles. The correspondence between the muscles which have heen mentioned, at their insertion in the digits, is clear enough, but some difficulties present themselves when the muscles are traced to their origins. In Man, the flexors and extensors of the digits (except the interossei) of the fore-limb arise in part from the humerus, and in part from the bones off#the forearm, but not within the manus. On the contrary, none of the flexors and extensors of the digits of the pes arise from the femur, while some of them arise within the pes itself. The origins of the muscles seem to be, as it were, higher up in the fore-limb than in the hind-limb. Nevertheless, several of the muscles correspond very closely. Thus, on the dorsal aspect, the extensor ossis metacarpt pollicis passes from the post-axial side of the proxi- mal region of the antebrachium obliquely to the trapezium and the metacarpal of the pollex, just as its homologue, the tibialis anticus, passes from the post-axial side of the upper part of the leg to the entocuneiform and the base of the me- tatarsal of the hallux; the two muscles correspond exactly. But the extensors of the phalanges of the pollex, and the deep extensors of the other digits of the manus, arise on the same side of the antebrachium, below the extensor ossis metacarpt pollicis ; while, in the leg, one of the deep extensors of the hallux, and all those of the other digits, arise still lower down, viz., from the calcaneum. Not less remarkable is the contrast between the more superficial sets of extensors in the two limbs. In the fore limb, proceeding from the pre-axial to the post-axial side, the following extensor muscles arise from the external or pre- axial condyle of the humerus: the extensor carpi radialis lon- gus to the base of the second metacarpal; the extensor carpr radialis brevis to the base of the third metacarpal ; the exten- sor communis digitorum to the four ulnar digits; the exten- sor minimi digiti to the fifth digit; the extensor carpi ul- * I have seen an opponens in the hallux of an Orang, THE MUSCLES OF THE LIMBS. 53 naris to the base of the fifth metacarpal. In the hindlimb, there are no homologues of the first two of these muscles. The homologue of the extensor communis is the long extensor,. which arises, not from the femur, but from the fibula. The peroneus tertius,* passing from the dorsal face of the fibula to the fifth metatarsal, is the only representative of the exten- sor carpi ulnaris, On the ventral aspect of the human fore-limb, two deep flexors arise from the radius, ulna, and interosseous membrane, and run parallel with one another, though disconnected, to the digits. These are, on the pre-axial side—the flexor polli- cis longus, to the distal phalanx of the pollex; and the flexor digitorum perforans, to the distal phalanges of the other - digits. In the hind-limb, two homologous muscles, the jlewor hal- lucis longus and the flecor digitorum perforans, arise from the tibia and fibula and interosseous membrane, and their ten- dons are distributed to the distal phalanges of the digits. But, before they divide, the tendons become connected to- gether in such a way that many of the digits receive tendi- nous fibres from both sources. : . In the fore-limb, there are no other deep flexors, but the internal, or post-axial, condyle of the humerus gives origin to a number of muscles, These, proceeding from the pre-axial to the post-axial side, are the flezor carpi radialis to the base of the second metacarpal; the palmaris longus to the fascia of the palm; the flexor perforatus digitorum to the middle, phalanges of the four ulnar digits; the flexor carpi ulnaris to the base of the fifth metacarpal. The sesamoid, pisiform bone is developed in the tendon of the last muscle. The only muscle which exactly corresponds with any of these, in the hind-limb, is the plantaris ; which, in Man, is a slender and insignificant muscle proceeding from the outer (post-axial) condyle of the femur to the plantar fascia—and answers to the palmaris longus. In many quadrupeds, as the Rabbit and Pig, the plantaris is a large muscle, the tendon of which passes over the end of the calcaneal process en- sheathed in the tendo achillis, and divides into slips, which become the perforated tendons. of more or fewer of the digits, * This muscle, which lies altogether on the dorsal face of the hind-limb, and which ] have seen only in Man, should not be confounded, as it often is, with one or more muscles, the peronad 8tit, 4ti, et 5ti digitt, which are very often developed in other Mammalia, but arise on the ventral face of the fibula, and send their tendons below the external malleolus to the extensor sheaths of the fifth, fourth, and even third digits. 54 THE ANATOMY OF VERTEBRATED ANIMALS. The flexor carpi radialis is also roughly represented by the tibialis posticus—a muscle which passes from the tibia and interosseous membrane to the entocuneiform, and therefore differs in insertion, as well as in origin, from its analogue in the forelimb. The fleror perforatus digitorum. of the foot takes its origin sometimes from the calcaneum ; sometimes, in part from the calcaneum, and in part from the perforating flexor ; or it may be closely connected with the tendons of the plantaris, The peroneus brevis represents the flexor carpi alnaris by its insertion, but it arises no higher than the fibula, and has no sesamoid. Two most important muscles yet remain to be considered in the leg. The one of these is that which is inserted by the tendo achillis into the calcaneum, and arises by four heads, two from the condyles of the femur (called gastrocnemzus), and two from the tibia and fibula (called soleus). The other muscle is the peroneus longus, arising from the fibula, pass- ing behind the external malleolus, and then crossing the foot to the base of the metatarsal of the hallux. The latter muscle does not appear to have any representa- tive in the fore-limb. The gastrocnemius*and soleus may pos- sibly represent the crural part of the perforated flexor, since, in many of the Vertebrata, the tendo achillis is but loosely connected with the calcaneum, and passes over it into the plantar fascia and the perforated tendons, A peculiar adduc- tor muscle of the hallux in Man and Apes is the transversalis pedis, which is inserted into the basal phalanx of the hallux, and arises from the distal ends of .the metatarsals of the other digits. The muscle sometimes has an analogue in the manus. Electrical Organs.—Certain fishes belonging to the gen- era Torpedo (among the Hlasmobranchii), Gymnotus, Ma- lapterurus, and Mormyrus (among the Teleoste?), posses organs which convert nervous energy into electricity, just as muscles convert the same energy into ordinary motion, and therefore may well be mentioned in connection with the ner- vous system, The “electrical organ” is always composed of nearly parallel lamella of connective tissue, enclosing small chambers, in which lie what are termed the electrical plates. These are cellular structures, in one face of which the final ramifications of the nerves, which are supplied to the organ by one or many trunks, are distributed. The face on which ‘ the nerves ramify is in all the plates the same, being inferior in Torpedo, where the lamelle are disposed parallel to the THE ELECTRICAL ORGANS. 55 upper and under surfaces of the body; posterior in Gymno- tus, and anterior in Malapterurus, the lamelle being disposed perpendicularly to the axis in these two fishes. And this sur- face, when the discharge takes place, is always negative to the other. Fie. 18.—The Torpedo, with its electrical apparatus displayed.—b, branchie; ¢, brain; 4, electric organ; g, cranium; me, spinal cord; 7, nerves to the pectoral fins; nl, nervé laterales ; 1p, branches of the pneumogastric nerves going to the electric organ; 0, eye. In Zorpedo the nerves of the electrical organs proceed from the fifth pair, and from the “electric lobe” of the medulla oblongata, which appears to be developed at the origin of the pneumogastrics. In the other electrical fishes the organs are supplied by spinal nerves; and, in Malapte- rurus, the nerve consists of a single gigantic primitive fibre, which subdivides in the electrical organ. The ordinary Rays possess organs of much the same structure as the electrical apparatus, at the sides of the tail. The Nervous System: the Encephalon.—In all verte- brated animals except Amphioxus, the brain exhibits that separation into a fore-brain, mid-brain, and hind-brain, which 56 THE ANATOMY OF VERTEBRATED ANIMALS, results from its embryonic division, by two constrictions, into the three thin-walled vesicles—the anterior, middle, and pos- terior cerebral vesicles—already mentioned. The cavities of these vesicles—the primitive ventricles of the brain—freely communicate at first, but become gradually diminished by the thickening of their sides and floors. The cavity of the ante- rior vesicle is, in the adult human brain, represented by the so-called third ventricle ; that of the middle vesicle, bv the iter a tertio ad quartum ventriculum ; that of the posterior vesicle, by the fourth ventricle. The floor and sides of the posterior vesicle, in fact, thicken and become the medulla oblongata, together with the pons varoliz, in those animals which possess the latter structure. SSIS Fie. 19.—Diagrammatic horizontal section of a Vertebrate brain. The following letters serve for both this figure and Fig. 20: Jb, Mid-brain. What lies in front of this is the fore-brain, and what lies behind, the hind-brain. ZZ. ¢., the lamina terminalis: OZf, the olfactory lobes; //mp, the hemispheres; 7. £, the thalamencephalon; Pn, the pineal gland; Py, the pituitary body; Jf, the foramen of Munro; (CS, the corpus striatum ; Th, the optic thalamus; CQ, the corpora quadricemina; OC. the crura cerebri: Cb, the cerebellum; PV, the pons varolii; JO, the medulla oblongata; J, olfactorii; TT, optici; ITI, point of exit from the brain of the motores oculorum ; JV, of the putheticit; V7, of the abducentes; V-XVZ, origins of the other cerebral nerves. 1, olfactory ventricle; 2, lateral ventricle; 3, third ventricle; 4, fourth ventricle; +, iter @ tertio ad quartum ventriculum. THE ENCEPHALON. 57 The posterior part of the roof is not converted into nervous matter, but remains thin and attenuated; the ependyima, or lining of the cerebral cavity, and the arachnoid, or serous membrane which covers the brain externally, coming nearly into contact, and forming, to all appearance, a single thin membrane, which tears with great readiness, and lays open the cavity of the fourth ventricle. Anteriorly, on the other hand, the roof becomes converted into nervous matter, and may enlarge into a complex mass, which overhangs the posterior division, and is called the cerebellum. The pons varolii, when it exists, is the expression of commissural fibres, which are developed in the sides and floor of the anterior part of the posterior cerebral vesicle, and connect one half of the cerebellum with the other. Thus, the Aind-brain differs from the posterior cerebral vesicle in being differentiated into the medulla oblongata (or myelencephalon) behind, and the cerebellum with the pons varolii (which together constitute the metencephalon) in front. The floor of the middle cerebral vesicle thickens and becomes converted into two great bundles of longiiudinal fibres, the crura cerebri, Its roof, divided into two, or four, convexities by a single longitudinal, or a crucial, depression, is converted into the “optic lobes,” corpora bigemina or quadrigemina. And these parts, the optic lobes, the crura cerebri, and the interposed cavity, which either retains the form of a ventricle, or is reduced to a mere canal (the iter a Fie. 20.—A longitudinal and vertical section of a Vertebrate brain.—The letters as before. The lamina ter minaiis is represented by the strong black line between F2f and 3, tertio ad quartum ventriculum), are the components of the mid-brain or mesencephalon. The anterior cerebral vesicle undergoes much greater 58 THE ANATOMY OF VERTEBRATED ANIMALS. changes than either of the foregoing; for, in the first place, it throws out from. its anterior lateral parietes two hollow prolongations, the hemispheres (or prosencephala), and each of these again protrudes from its anterior end a smaller hollow process, the olfactory lobe (or rhinencephalon). By the development of these processes the anterior vesicle becomes divided into five parts—one median and posterior, and four anterior and paired. The median and posterior, which remains as the representative of the greater part of the original anterior cerebral vesicle, is the vesicle of the third ventricle (or thalamencephalon). Its floor is produced into a conical process, the infundibulum, the blind end of which is connected with the pituitary body, or hypophysis cerebri. Its sides thicken greatly, acquire a ganglionic structure, and become the optic thalami. Its roof, on the other hand, resembles that of the fourth ventricle, in remaining very thin, and, indeed, a mere membrane. The pineal gland, or epiphy- sis cerebri, is developed in connection with the upper wall of the third ventricle; and, at the sides of its roof, are two ner- vous bands, which run to the pineal gland, and are called its peduncles. The front wall of the vesicle, in part, becomes the so-called lamina terminalis, which is the delicate anterior boundary of the third ventricle. In certain directions, however, it thickens and gives rise to three sets of fibres, one transverse and two vertical—the former lying in front of the latter. The trans- verse fibres pass on either side into the corpora striata, and constitute the anterior commissure which connects those bodies. The vertical fibres are the anterior pillars of the fornia, and they pass below into the floor of the third ventricle, and into the corpora mammillaria, when those structures are de- veloped. : , The outer and under wall of each cerebral hemisphere thickens and becomes the corpus striatum, a ganglionic struct- ure which, from its origin, necessarily abuts against the outer and interior part of the optic thalamus. The line of demar- cation between the two corresponds with the lower lip (tenia semicircularis) of the aperture of communication (called the foramen of Munro) between the third ventricle and the cavity of the cerebral hemisphere, which is now termed the lateral ventricle. In the higher Vertebrata, the upper lip of the foramen of Munro thickens, and becomes converted iuto a bundle of longitudinal fibres, which is continuous, anteriorly, with the anterior pillars of the fornix before mentioried. Pos- THE MODIFICATION OF THE BRAIN. 59 teriorly, these longitudinal fibres are continued backward and downward along the inner wall of the cerebral hemisphere, following the junction of the corpora striata and optic thalami, and pass into a thickening of the wall of the hemisphere, which projects into the lateral ventricle, and is called the hippocampus major. Thus a longitudinal commissural band of nervous fibres, extending from the floor of the third ven- tricle to that of the lateral ventricle, and arching over the fora- men of Munro, is produced. The fibres of opposite sides unite over the roof of the third ventricle, and constitute what is called the body of the fornix. Behind this union the bands receive the name of the posterior pillars of the fornia. The optic thalami may be connected by a gray soft com- missure ; and a posterior commissure, consisting of transverse nerve-fibres, is generally developed between the posterior ends of the two thalami. In the Mammalia, a structure, which is absent in other Vertebrata, makes its appearance; and, in the higher members of that class, this corpus callosum is the greatest and most im- portant mass of commissural fibres. It is a series of trans- verse fibres, which extends from the roof of one lateral ventr- cle to that of the other, across the interval which separates the inner wall of one hemisphere from that of the other. When the corpus callosum is largely developed, its ante- rior part crosses the interspace between the hemispheres con- siderably above the level of the fornix; so that between the fornix and it, a certain portion of the inner wall of each hemisphere, with the intervening space, is intercepted. The portion of the two inner walls and their interspace, thus isolated from the rest, constitutes the septum lucidum, with its ‘contained fifth ventricle. The Modifications of the Brain.—The chief modifications in the general form of the brain arise from the development of the hemispheres relatively to the other parts. In the lower vertebrates the hemispheres remain small, or of so moderate a size as not to hide, by overlapping, the other divisions of the brain. But, in the higher Mammalia, they extend forward over the olfactory lobes, and backward over the optic lobes and cerebellum, so as completely to cover these parts ; and, in addition, they are enlarged downward toward the base of the brain. The cerebral hemisphere is thus, as it were, bent round its carpus striatum, and it becomes distinguished into regions, or dobes, which are not separated by any very sharp lines of demarcation. These regions are named the frontal, parietal, 60 THE ANATOMY OF VERTEBRATED ANIMALS. occipital, and temporal lobes—while, on the outer side of the corpus striatum, a central lobe (the insela of Reil) lies in the midst of these. The lateral ventricles are prolonged into the frontal, occipital, and temporal lobes, and acquire what are termed their anterior, posterior, and descending cornua. Furthermore, while, in the lower vertebrates, the surface of the cerebral hemispheres is smooth; in the higher, it be- comes complicated by ridges and furrows, the gyri and sulci, which follow particular patterns. The superficial vascular lay- er of connective tissue which covers the brain, and is called pia mater, dips into these sulci: but the arachnoid, or delicate serous membrane, which, on the one hand, covers the brain, and, on the other, lines the. cranium, passes from convolution to convolution without entering the sulci. The dense perios- teal membrane which lines the interior of the skull, and is itself lined by the parietal layer of the arachnoid, goes by the name of the dura mater. / The general nature of the modifications observable in the brain as we pass from the lower to the higher mammalia is very well shown by the accompanying figures of the brain of a Rabbit, a Pig, and a Chimpanzee (Iigs. 21 and 22). In the Rabbit, the cerebral hemispheres leave the cerebel- lum completely exposed when the brain is viewed from above. There is but a mere rudiment of the Sylvian fissure at Sy, and the three principal lobes, frontal (A), occipital (B), and tem- poral (C'), are only indicated. The olfactory nerves are enor- mous, and pass by a broad .smooth tract, which occupies a great space in the lateral aspect of the brain, into the natiform protuberance of the temporal lobe (C). In the Pig, the olfactory nerves and tract are hardly less conspicuous; but the natiform protuberance is more sharply notched off, and begins to resemble the unciform gyrus in the higher Mammalia, of which it is the homologue. The tem- poral gyri (C’), though still very small, begin to enlarge down- ward and forward over this. The upper part of the cerebral hemisphere is much enlarged, not only in the frontal, but also in the occipital region, and to a great extent hides the cere- bellum when the brain is viewed from above. What in the Rabbit was a mere angulation at Sy, in the Pig has become a long sulcus—the Sylvian. fissure, the lips of which are formed by a gyrus, the Sylvian, or angular, gyrus. Two other sets of gyri, more or less parallel with this, are visible upon the outer surface of the hemisphere; and at the entrance of the THE MODIFICATION OF THE BRAIN. 61 Fia. 21.—Lateral views of the brains of a Rabbit, a Pig, and a Chimpanzee, drawn of nearly the same absolute size. The Rabbit's brain is at the top; the Pig's, in the middle, the Chimpanzee’s, lowest.—(/, the olfactory lobe; A., the frontal lobe; &., the occipital lobe; ., the temporal lobe; Sv., the Sylvian fissure; Jn., the insula; S.Or., supra- orbital; S77, ILF., 7.F., superior, middie, and inferior frontal gyri; A.P., antero-pari- etal; P.P., postero-parietal gyri; . sulcus of Rolando; P.P, postero-parietal lobule ; O.Pf., external perpendicular or occipito-temporal suleus; An, angular gyrus; 2, 3, 4, annectent gyri; 4. hr MT, P.T, the three temporal, and SOc, I. Oc» I.Oc., tho three occipital gyrl. 2 62 THE ANATOMY OF VERTEBRATED ANIMALS. Sylvian fissure, at Jn, there is an elevation which answers to the insula, or central lobe. In the Chimpanzee, the olfactory nerves, or rather lobes, are, relatively, very small, and the tracts which connect them with the-uncinate gyri (substantw perforatic) are completely hidden by the temporal gyri (C’). The Sylvian fissure is very long and deep, and begins to hide the insula, on which a few fan-shaped gyri are developed. The frontal lobes are very large, and overlap the olfactory nerves for a long distance ; while the occipital lobes completely cover and extend beyond the cerebellum, so as to hide it completely from an eye placed above. The gyri and sulci have now attained an arrangement which is characteristic of all the highest Mammalia. The fissure of Rolando (2) divides the antero-parietal gyrus (A. P) from the postero-parietal (P,P). These two gyri, with the postero-parietal lobule (P.f/.), and part of the angular gyrus (An), constitute the Parietal lobe. The frontal lobe, which lies anterior to this, the occipital lobe, which lies behind it, and the ¢emporal lobe, which lies below it, each present three tiers of gyri, which, in the case of the frontal and occipital lobes, are called superior, middle, and inferior—in that of the temporal lobe, anterior, middle, and posterior. The inferior surface of the frontal lobe, which lies on the roof of the orbit (S. Or.), presents many small sulci and gyri. On the inner face of the cerebral hemisphere (Fig. 22) the oly sulcus presented by the Rabbit’s brain is that deep and broad depression (ZZ) which runs parallel with the posterior pillar of the fornix, and gives rise, in the interior of the de- scending cornu of the lateral ventricle, to the projection which is termed the hippocampus major, In the Pig, this hippocam- pal sulcus (ZZ) is much narrower and less conspicuous; and a marginal (M) and a calossal (C) gyrus are separated by a well-marked calloso-marginal sulcus. As in the Rabbit, the uncinate gyrus forms the inferior boundary of the hemisphere. In the Chimpanzee, the marginal and callosal gyri are still better marked. There is a deep internal perpendicular, or occipito-parietal, sulcus (Ip). The calcarine sulcus (Ca) causes a projection into the floor of the posterior cornu, which is the hippocampus minor ; while the collateral sulcus (Coll) gives rise to the eminence of that name in both the posterior and descending cornua. The hippocampal suleus (ZZ) is relatively insignificant, and the lower edge of the tem- poral lobe is formed by the posterior temporal gyrus. In the Rabbit, the corpus callosum is relatively small, much THE MODIFICATION OF THE BRAIN. 63 Fig. 22.—Inner views of the cerebral hemispheres of the Rabbit, Pig, and Chimpanzee, drawn as before, and placed in the same order. 21., olfactory lobe; C.c., corpus callo- sum; A.c.. anterior commissure; //, hippocampal sulcus; Un., uncinate; Jf, mar- ginal; (“, callosal gyri; J.p., internal perpenuicular; Ca., calcarine; Coll., collateral sulci; #7, fornix. . inclined upward and backward; and its anterior extremity is ‘but slightly bent downward, so that the so-called genw and rostrum are inconspicuous. The Pig’s corpus callosum is 64° THE ANATOMY OF VERTEBRATED ANIMALS. larger, more horizontal, and possesses more of a rostrum: in the Chimpanzee, it is still larger, somewhat deflexed, and very thick posteriorly; and has a large rostrum. In proportion to the hemispheres, the anterior commissure is largest in the Rabbit and smallest in the Chimpanzee. The Rabbit and the Pig have a single corpus mammillare, the Chimpanzee has two. The cerebellum of the Rabbit is very large in proportion to the hemispheres, and is left completely uncovered by them in the dorsal view. Its median division, or vermis, is straight, symmetrical, and large in proportion to the lateral lobes. The jflocculi, or accessory lobules developed from the latter, are large, and project far beyond the margins of the lateral lobes. The ventral face of the metencephalon presents on each side, -behind the posterior margin of the pons varolii, flattened rec- tangular arez, the so-called corpora trapezoidea, In the Pig, the cerebellum is relatively smaller, and is par- tially covered by the hemispheres ; the lateral lobes are larger in proportion to the vermis and the flocculi, and extend over the latter. The corpora trapezoidea are smaller. In the Chim- panzee, the relatively still smaller cerebellum is completely covered ; the vermis is very small in relation to the lateral lobes, which cover and hide the insignificant flocculi. There are no corpora trapezoidea. ° In all the characters now mentioned, the brain of Man differs far less from that of the Chimpanzee than that of the latter does from the Pig’s brain. The Myelon.—The spinal canal, and the cord which it con- tains, are lined by continuations of the three membranes which protect the encephalon. The cord is sub-cylindrical, and con- tains a median longitudinal canal, the canalis centralis, the remains of the primitive groove. It is divided by anterior and posterior median fissures into two lateral halves, which are, usually, connected only by the comparatively narrow isthmus, which immediately surrounds the canalis centralis. The cord may, in the adult, extend through the whole spinal canal, or it may come to an end at any point between the caudal extrem- ity and the anterior thoracic region. The distribution of the two essential constituents of ner- vous tissue, ganglionic corpuscles and nerve-fibres, is very defi- nite in the spinal cord, ganglionic corpuscles being confined to the so-called “ gray matter” which constitutes the isthmus, and spreads out into two masses, each of which ends in an an- terior (or ventral) and a posterior (or dorsal) horn, Nerve- THE MYELON. 65. Fic. 23.—A diagrammatic view of the Chief Trunks of the Cerebro-spinal and Sympathetic Nervous Systems of Rana esculenta scen from below (twice the size of nature)—I. The olfactory nerves. N. 'The olfactory sac. II. The optic nerve. O. The eye. JZ. op. The optic lobes. 7v. Optic tracts pissing from the optic lobes to the chiasma, behind which lies the pituitary body. ID]. Oculomotorius. IV. Patheticus. VY The tri- geminal, with which the abducens (VL), facialis (VIL), and the upper end of the sym- pathetic (17S), are closely connected. Branches of this nervous plexus are Tia, the nasal and ophthalmic branches of the fifth and the abducens. Tb, 0, d, the palatine, maxillary, and mandibular branches of the fifth. FP. 4 the tympanic branch into which the proper facial nerve (VII.) enters, and, with a branch of the vagus, forms the so- called ficial nerve of the Frog, # VIII. The auditory nerve. -¥., with its branches 1, ¥2, 43, V4, represents the glossopharyngeal and the vagus. The medulla ob- longata (ilyetencephaton) ends, and the medulla spinalis (.ve/on) begins. about the region narked by the letter V2 3/1-10, the spinal nerves. .4 2. the brachial nerves, Af 7, 8, 9, the ischiatic plexus, from which proceed the crural (N. ¢.) and ischiatic (N. ¢.) norves. S. The trunk of the sympathetic. Sf The communicating branches with tho spinal ganglia, S1-10. The sympathetic ganglia. 66 THE ANATOMY OF VERTEBRATED ANIMALS. fibres also abound in the gray matter; but the so-called “ white matter,” which constitutes the external substance of the cord, contains only the fibrous nervous matter, and has no gangli- onic corpuscles, The spinal nerves arise in opposite pairs from the two halves of the cord, and usually correspond in number with the vertebre through, or between, which they pass out (Fig. 23). Each nerve has two roots, one from the dorsal, and one from the ventral, region of its half of the cord. The former root has a ganglionic enlargement, and only contains sensory fibres; the latter has no ganglion, and exclusively contains motor fibres.* After leaving the vertebral canal, each spinal nerve usually divides into a dorsal and a ventral branch; but, in the Ganoid fishes, each of these branches is a distinct nerve, arising by its own proper roots. The Cerebral Nerves——The greatest number of pairs of nerves ever given off from the vertebrate brain is twelve, in- cluding the so-called olfactory nerves, and the optic nerves, which, as has been seen, are more properly diverticula of the brain, than nerves in the proper sense of the word. The olfactory “nerves” (olfactorti) constitute the first pair of cerebral nerves. They always retain their primary connection with the cerebral hemispheres, and frequently con- tain, throughout life, a cavity, the olfactory ventricle, which communicates with the lateral ventricle. The optic “nerves” (optict) are the second pair of cere- bral nerves. In the Lampreys and Hags (Marsipobranchii) these nerves retain their embryonic origin from the thalam- encephalon, and each goes to the eye of its own side. In other Vertebrata, the nerves cross one another at the base of the brain (Zeleostet), or are fused together into a chiasma (Ganoidet, Elasmobranchii, and all the higher Vertebrata). In the higher Vertebrata, again, the fibres of the optic nerves become connected chiefly with the mesencephalon. All the other cerebral nerves differ from these in arising, not as diverticula of any of the cerebral vesicles, but by histo- logical differentiation of the primitive brain-case, or lamine dorsales of the skull. The third (motores oculorum) and fourth (pathetict) pairs of nerves are distributed to the muscles of the eye; the third to the majority of these muscles, the fourth to the superior * Amphioxus appears to be an exception to this, as to most other, rules of Vertebrate anatomy. THE CEREBRAL NERVES. 67 oblique muscles. The third pair of nerves issues from the crura cerebri, or inferior division of the metencephalon, upon the base of the brain; the fourth pair, from the fore-part of the upper division of the metencephalon, immediately be- hind the optic lobes, upon the superior surface of the brain. This region is known as the Valve of Vieussens in the Mam- malia, All the other cerebral nerves originate in the posterior di- vision of the Aind-brain—the myelencephalon. The great Jifth pair (trigemin/) passes out from the sides of the meten- cephalon, and supplies sensory nerves to the integument of the head, and motor nerves to most of the muscles of the jaws, by its three divisions—the ophthalmic, the superior masillary, and the inferior maxillary, nerves. Of these divisions the two latter are, very generally, closely connected together, while the ophthalmic division remains distinct. The ophthalmic division passes to the cleft between the trabecula and the maxillary process (which nearly corre- sponds with the orbit, and might be termed the orbito-nasal cleft), and is distributed to the inner and the outer side of that cleft. Hence its main branches are nasal and lachrymal. The two maxillary nerves, on the other hand, are distributed to the inner and outer sides, or anterior and posterior boundaries, of the buccal cleft. Hence the superior maxillary belongs to the posterior, or outer, side of the maxillary process, while the in- ferior maxillary appertains to the anterior region of the first visceral arch. The superior maxillary commonly unites with the outer, or lachrymal, division of the ophthalmic; the in- ferior maxillary with the anterior division of the facial. In the higher Vertebrata, the trigeminal nerve usually has two very distinct roots, a dorsal sensory, provided with a gan- glion (the Casserian ganglion), and a ventral motor, non-gan- glionated. The fibres of the latter pass almost exclusively into the inferior maxillary division. In addition, the ophthalmic division may have a ganglion (céJiary) ; the superior maxillary another Ciphenapalarine or Meckelian), and the inferior maxil- lary a third (otic). The stath pair (abducentes) issues from the inferior surface of the brain, at the junction of the myelencephalon with the metencephalon. It supplies the external straight muscles of the eye; with the muscles of the nictitating membrane, and the retractor bulbi, or musculus choanoides, when such mus- cles exist. The seventh pair ( faciales) supplies the superficial facial 68 THE ANATOMY OF VERTEBRATED ANIMALS, muscles, and ultimately divides into two branches, one of which is in relation with the mandibular, and the other with the hyoidean arch. The five nerves which have just been mentioned are often intimately connected together. Thus, in the Lepidosiren, the . three motor nerves of the eyeball are completely fused with the ophthalmic division of the fifth.* In the Myxinoid fishes there are no motor nerves of the eyeball ; but, in the Lamprey, the rectus externus and inferior, and the obliguus inferior, are supplied by the ophthalmic, while the oculomotor and the pa- thetic unite into a common trunk, which gives branches to the rectus superior and internus, and obliquus superior. The ocu- lomotor, the pathetic, and the abducens, are more or less con- founded with the ophthalmic in the Amphibia ; but in Tele- ostei, Ganoidei, Hlasmobranchii, and in all the higher Verte- brata, the nerves of the muscles of the eye are distinct from the fifth pair, except where the oculomotor unites with the ophthalmic into the ciliary ganglion. : The facial and the trigeminal nerves have common roots in fishes. In wtmphibia, though the roots are distinct, the facial may be completely united with the ganglion of the tri- geminal, as in the Frog. In all abranchiate Vertebrata the two nerves are quite distinct. Whether the nerves are distinct or not, a palatine, or vidi- an, nerve (which, in the higher Vertebrata, is especially con- nected with the facial), runs through, or beneath, the base of the skull, parallel with its long axis; and, after uniting with the superior maxillary, and usually contributing to form the sphenopalatine, or Meckelian, ganglion, is distributed to the mucous membrane of the roof of the mouth ; and the mandib- ular division of the seventh, or chorda tympani, unites with the inferior maxillary division of the fifth nerve. The eighth pair (auditorii) is formed by the nerves of the organ of hearing. The ninth pair (glossopharynge?) is especially distributed to the pharyngeal and lingual regions of the alimentary canal, o primarily, supplies the boundaries of the second visceral cleft. The tenth pair (pneumogastrici or vag?) consists of very *Tam greatly disposed to think that the motor nerves of the eye more nearly retain their primary relations in Lepédosiren than in any other verte- brated animal; and that they are really the motor portions of the nerves of the orbito-nasal cleft, the third and fourth appertaining to the inner division of the ophthalmic, the sixth to its outer division. THE EXITS OF THE CEREBRAL NERVES. 69 remarkable nerves, which pass to the gullet and stomack, the respiratory and vocal organs, to some parts of the integument of the body, and to the heart. In the Ichthyopsida they give off, in addition, long lateral nerves to the integuments of the sides of the body. In the higher Vertebrata, these lateral nerves are represented only by small branches distributed chiefly to the occipital region. The ninth and tenth pairs are both motor and sensory in function, and are often so inti- mately connected as to form almost one nerve. The eleventh pair (accessorii) are cerebral only by courtesy, as these nerves take their origin from the spinal cord, by roots which issue between the proper anterior and posterior roots of the spinal nerves, and, joining together, form, on each side, a nerve which passes out with the pneumogastric, partly joining it, and partly going to muscles which arise from the head and anterior vertebre, and are inserted into the pec- toral arch. The spinal accessory exists in no Ichthyopsid vertebrate, but is found in all Sauropsida, with the exception of the Ophidia, and in the Mammalia. The ¢welth and last pair (hypogtoss?) are the motor nerves of the tongue, and of some retractor muscles of the hyoidean apparatus, In the Ichthyopsida the first cervical nerve supplies the distributional area of the hypoglossal; but in all the abran- chiate Vertebrata there is a hypoglossal, which traverses a foramen in the ex-occipital, though it oitens remains closely connected with the first cervical, and may rather be regarded as a subdivision of that nerve, than as a proper cerebral nerve. Thus the nerves arising from the hind-brain, in all the higher Vertebrata, fall into three groups: 1st, a sensori-motor, pre-auditory, set (3d, 4th, 5th, 6th, 7th); 2d, the purely sen- sory auditory nerve (8th); 3d, the sensori-motor, post-audi- tory, set (9th, 10th, 12th). The apertures by which several of these nerves leave the skull, retain a very constant relation to certain elements of the cranium on each side. Thus: a. The filaments of the olfactory nerve always leave the cranium between the lamina perpendicularis, or body of the ethmoid, and its lateral or prefrontal portion. b. The optic nerve constantly passes out behind the cen- tre of the orbitosphenoid and in front of that of the alisphe- noid. 10 THE ANATOMY OF VERTEBRATED ANIMALS. c. The third division of the trigeminal, or fifth nerve, al- ways leaves the skull behind the centre of the alisphenoid and in front of the prodtic. d. The glossopharyngeal and pneumogastric always make their exit behind the centre of the opisthotic, and in front of the centre of the ex-occipital. The apertures for the exit of the cranial nerves denoted in the, paragraphs a, 6, e, d, when surrounded by bone, and well defined, are called respectively : a, the olfactory foramen ; b, the optic foramen ; c, the foramen ovale ; d, the foramen lacerum posterius. The adjacent bones may take equal shares in bounding these foramina, or the foramina may be alto- getherin one bone; but their positions, as here defined, never change. Another point to be especially considered respecting the general disposition of the cranial nerves, is the relation which some of them bear to the visceral arches and clefts, and which has already been incidentally mentioned. Thus, the seventh nerve is distributed to the posterior part of the first visceral arch, and to the anterior part of the second visceral arch, its two branches enclosing the first visceral cleft. In like man- ner, the ninth (glossopharyngeal) uerve is distributed to the hinder part of the second arch and to the front part.of the third, its branches enclosing the second visceral cleft. The first branch of the pneumogastric has similar relations to the third and fourth arches and to the third cleft; and, in bran- chiate Vertebrata, the other anterior branches of the pneumo- gastric are similarly distributed to the successive branchial arches, the two divisions of each branch enclosing a branchial cleft. The second and the third divisions of the trigeminal are distributed, in an analogous manner, to the anterior region of the first visceral arch, and to the posterior or outer region of the maxillo-palatine process—the gape of the mouth repre- senting a visceral cleft between thetwo. The inner and outer portions of the first division of the trigeminal are similarly related to the inner, or anterior, region of the maxillo-palatine process, and the outer side of the trabecula cranii—the orbito- nasal fissure representing the cleft between the two. Considerations of this kind suggest that the trabeculz and the maxillo-palatine processes may represent pre-oral’ visceral arches, which are bent forward; and, in the case of the tra- becule, coalesce with one another. Such an hypothesis would enable us to understand the signification of the naso-palatine THE SYMPATHETIC NERVES. "1 canal of the Myxinoid fishes, which would be simply the in- terspace, or passage, between the trabeculz (which must have originally existed if ever they were distinct visceral arches) not yet filled up; and the anomalous process of the roof of the oral cavity, which extends toward the pituitary body in the embryos of the Vertebrata in general, might be regarded as the remains of this passage. On this hypothesis, six pair of inferior arches belong to the skull—namely, the trabecular and maxillo-palatine, in front of the mouth; the mandibular, the hyoidean, and two others (first and second branchial), behind it. For, as there are three cranial nerves embracing the first three visceral clefts which lie behind the mouth, there must be four post-oral, cra- nial, visceral arches. Supposing that the occipital segment in the brain-case an- swers to the hindermost, or second branchial, cranial, visceral arch, the invariable attachment of the proximal ends of the mandibular and hyoidean arches to the auditory capsule leads me to assign the parietal and the frontal segments to the max- illo-palatine and trabecular visceral arches, And thus the os- sifications of the auditory capsule, alone, are left as possible representatives of the neural arches of the three anterior post- oral visceral arches. But these speculations upon the primitive composition of the skull, however interesting, must not, as yet, be placed upon the same footing as the doctrine of its segmentation, which is simply a generalization of anatomical facts. The Sympathetic—A Sympathetic Nervous System has been observed in all the Vertebrata except Amphioxus and the Marsipobranchii. It consists, essentially, of two longi- tudinal cords, placed one upon each side of the inferior face of the cranio-spinal axis. Each cord receives communicating fibres from the spinal nerves of its own side, and, when com- plete, from all the cranial nerves except those of the special senses of hearing, sight, and smell—the Vidian nerves consti- tuting the anterior terminations of the sympathetic cords. At the points of communication ganglia are developed, and the nerves which emerge from these ganglia are distributed to the rouscles of the heart and vessels, and to those of the viscera. These peripheral nerves of the sympathetic system frequently present small ganglionic enlargements. * In the Marsipobranchii, the place of the sympathetic ap- pears to be taken, to a great extent, by the pneumogastric; 72 THE ANATOMY OF VERTEBRATED ANIMALS. and, in Myzxine, the two pneumogastrics unite upon the intes- tine, and follow it, as a single trunk, to the anus. The Sensory Organs.—The organs of the three higher senses—Smell, Sight, and Hearing—are situated, as has been already described, in pairs, upon each side of the skull, in all vertebrate animals except the lowest fishes; and, in their earliest condition, they are alike involutions of the integu- ment. The Olfactory Apparatus acquires no higher complication than this, being either a single sac (Amphiorus (?) Marsipo- branchit), or, more commonly, two, the surfaces of which are increased by plaiting, or by the development of turbinal carti- lages, or bones, from the lateral portions of the ethmoid. Upon these, nervous filaments arising from the olfactory lobe of the brain are distributed. The cavities of the olfactory sacs may be placed in communication with that of the mouth by the nasal passages; or, as in the great majority of fishes, they may have only an external aperture, or apertures. In Reptiles, Birds, and Mammals, a peculiar nasal gland is frequently connected with, and pours its secretion into, each olfactory chamber. The foramina incisiva, left between the premaxillaries and the palatine plates of the maxillaries in Mammalia, are sometimes closed by the mucous membranes of the nasal and oral cavities, and sometimes not. In the latter case they are the canals of Stenson, and place these two cavities in com- munication. Glandular diverticula of the mucous membrane, supplied with nervous filaments from both the olfactory and the fifth pair, may open into these canals. They are called, after their discoverer, the “ organs of Jacobson.” The Eye is formed by the coalescence of two sets of struct- ures, one furnished by involution of the integument, the other by an outgrowth of the brain. The opening of the integumentary depression which is pri- marily formed on each side of the head in the ocular region becomes ciosed, and a shut sac is the result. The outer wall of this sac becomes the transparent cornea of the ‘eye; the epidermis of its floor thickens, and is metamorphosed into the crystalline lens ; the cavity fills with the agueous humor. A vascular and muscular ingrowth taking place round the cir- cumference of the sac, and, dividing its cavity into two seg- ments, gives rise to the iris. The integument around the cor- THE EYE. "3 nea, growing out into a fold above and below, results in the formation of the eyelids, and the segregation of the integu- ment which they enclose, as the soft and vascular conjunctiva. The pouch of the conjunctiva very generally communicates, by the lachrymal duct, with the cavity of the nose. It may be raised, on its inner side, into a broad fold, the néctituting membr ane, moved by a proper muscle or muscles. Special glands—the lachrymal externally, and the Harderian on the inner side of the eyeball—may be developed in connectiou with, and pour their secretion on to, the conjunctival mucous membrane. The posterior chamber of the eye has a totally distinct ori- gin. Very early, that part of the anterior cerebral vesicle which eventually becomes the vesicle of the third ventricle, throws out a diverticulum, broad at its outer, and narrow at its inner end, which applies itself to the base of the integu- mentary sac. The posterior, or outer, wall of the diverticulum then becomes, as it were, thrust in, and forced toward the op- posite wall, by an ingrowth of the adjacent connective tissue ; so that the primitive cavity of the diverticulum, which, of course, communicates freely with that of the anterior cerebral vesicle, is obliterated. The broad end of the diverticulum ac- quiring a spheroidal shape, while its pedicle narrows and elon- gates, the latter becomes the optic nerve, while the former, surrounding itself with a strong fibrous sclerotic coat, remains as the posterior chamber of the eye. The double envelope, resulting from the folding of the wall of the cerebral optic ves- icle upon itself, gives rise to the retina and the choroid coat: the plug, or ingrowth of connective tissue, gelatinizes and passes into the vitreous humor, the cleft by which it entered becoming obliterated. Even in the higher Vertebrata the optic nerve is, at first, connected exclusively with the vesicle of the third ventricle, and makes no decussation with its fellow. But by degrees the roots of origin of each nerve extend over to the opposite side of the brain, and round the thalamus, to the mesencepha- lon on that side, and the trunks of the two nerves become in- termixed below the third ventricle, in a close and complicated manner, to form a chiasma, In Amphioxzus and Myzxine, the eyes are very imperfectly developed, appearing to consist of little more than a rudimen- tary lens imbedded in the pigment, which encloses the tormi- nation of the optic nerve ; and, in Myzine, this rudimentary eye is hidden by muscles and integument. It appears doubtful 4 74 THE ANATOMY OF VERTEBRATED ANIMALS. whether in these fishes, and in the Lampreys, the eye is ¢ veloped in the same way as in other Vertebrata. In all other Vertebrata, the eyes have the typical structm though sometimes, as in the Blind-fish (Amblyopsis) and t Mole, they have no functional importance. In the Jchth opsida and Sauropsida, but not in Mcimmalia, the scleroi is often partially ossified, the essification usually forming ring around its anterior moiety. It becomes enormous thickened in the Cetacea. , Except in Amphioxus and the Myxinoid fishes, the ey ball is moved by six muscles; of these, four, proceeding fro the interior of the orbit to the periphery of the eyeball, a1 surrounding the optic nerve, are termed superior, inferior, i ternal, and external recti. The other two are connected wi the upper and the lower margins of the orbit respectively, az pass thence to the outer side of the bulb. These are the suz rior and the inferior obligui, In many Reptiles and Mar mals a continuous funnel-shaped sheet of muscle, the muse lus choanoides, lies within the four recti, and is attached 1 the circumference of the posterior moiety of the ball ofthe ey It would appear, from the distribution of the nerves, whic has already been described, that the musculus choanaide the external rectus, and the nictitating muscle, constitu! a group of eye-muscles morphologically distinct from the oth three recti, the obligui, and the levator palpebre superiori In many Reptiles, and in the higher Vertebrata, the eyelic are closed by circular muscular fibres, constituting an orbici laris palpebrarum, and are separated by straight fibres pri ceeding from the back of the orbit, usually to the upper ey: lid only, as the levator palpebre superioris ; but sometimes t both lids, when the lower muscle is a depressor palpebre inf vioris, The Harderian and lachrymal glands are not found i fishes ; but the former is met with in the Batrachia, and bot are of common occurrence in the Sauropsida and Mammali: In Lacertilia, Crocodilia, Aves, and many Fishes, a pect liar vascular membrane, covered with pigment, like the chi roid, projects from near the entrance of the optic nerve, on th outer side of the globe of the eye, into the vitreous humo and usually becomes connected with the capsule of the len This is the pecten, or marsupium. The Har.—tThe first rudiment of the internal ear is an ii volution of the integument into a small sac, which is situate THE EAR. "5 on each side of the posterior cerebral vesicle, just above the end of the second visceral cleft. The mouth of the involution soon closes, and a shut sac results. The sac enlarges, and, by a remarkable series of changes, its upper part becomes (ordi- narily) converted into three semicircular canals—the anterior and posterior vertical, and the external or horizontal canals of the membranous labyrinth. The body of the sac remains, for the most part, as the vestibule ; but a cecal process, which eventually becomes shut off from the vestibule, is given off downward and inward, toward the base of the skull, and is the rudiment of the scala media of the cochlea. This may be called the membranous cochlea. In the anomalous vertebrate, Amphiowus, no ear has yet been discovered. The Hag (Myzwine) has only one, and in the Lampreys (Petromyzon) there are only two, semicircular ca- nals; but, in fishes in general, all three are developed, and it is a question whether the cochlea is not also represented. In fishes, the periotic cartilage and its ossifications enclose this membranous labyrinth, externally, and present no merely membranous gaps, or fenestr@, toward the first visceral cleft, or the space which represents it. But in higher Vertebrata (Amphibia, Sauropsida, Mam- malia), in which the membranous labyrinth is always enclosed within a complete bony periotic capsule, the outer wall of this capsule invariably remains unossified over one cr two small oval area, which consequently appear like windows with membranous panes, and are termed the fenestra ovalis and the Senestra rotunda. The fenestra ovalis is situated in that part of the periotic mass which bounds the chamber containing the membranous vestibule externally ; and it is always found that, when both the proitic and the opisthotic bones exist, they contribute nearly equal shares to the formation of its boundaries. In fact, the JSenestra ovalis is situated in the line of junction of these two bones. The fenestra rotunda, on the other hand, is below the fenestra ovalis, and lies altogether in the opisthotic. It forms part of the outer wall of the cavity in which the mem- branous cochlea is lodged. In the Sauropsida and Mammalia, this membranous ccck- lea, become flattened and bandlike, and its communication with the vestibule obliterated, is lodged in a conical cavity, in such a manner as to divide that cavity into two portions, called scale, which only communicate at their apices. The base of the one scala, called scala vestiluli, opens into the 76 THE ANATOMY OF VERTEBRATED ANIMALS. cavity which contains the membranous vestibule: that of the other, scala tympani, abuts against, and is as it were stopped by, the membrane of the fenestra rotunda. The cavity of the membranous cochlea stretched between, and helping to divide, these two scale, is called the scala media. In Reptiles, Birds, and Ornithodelphous Mammals, the cochlea is only slightly bent or twisted upon itself. But, in the higher Mammalia, it becomes coiled in a flat or conical spiral of one and a half (Cetacea, Erinaceus) to five (Cwlo- genys Paca) turns. The membranous labyrinth is filled with a clear fluid, the endolymph, and usually contains otolithes of various kinds. Between the membranous labyrinth and the walls of the cav- ity of the periotic mass in which it is contained, lies another clear fluid, the perilymph, which extends thence into the scale vestibuli and tympani. In all animals which possess a fenestra ovalis, its mem- brane gives attachment to a disk, whence an ossified rod, or arch, proceeds. Where the former structure obtains, as in Birds, most Reptiles, and some Amphibia, the bone is com- monly called columella auris ; when the latter, as in most Mammals, stapes. But there is really no difference of impor- tance between stapes and columella, and it is advisable to use the former name for the bone under all its forms. In the majority of Vertebrata of higher organization than fishes, the first visceral cleft does not become wholly obliter- ated, but its upper part remains as a transversely elongated cavity, by means of which the pharynx would be placed in communication with the exterior, were it not that the oppo- site sides of the canal grow together into a membranous par- tition—the membrana tympani. So much of the canal as lies external to this is the external auditory meatus ; while what lies internal to it, is the tympanum, or drum of the ear, and the Hustachian tube, which places the tympanum in communi- cation with the pharynx. While the outer wall of the tym- panum is the tympanic membrane, its inner wall is the periotic mass with its fenestrae ; and, in all Vertebrata below Mamn- mals, the outer end of the stapes is either free, or, more com- monly, is fixed to the tympanic membrane, and thus fhe latter and the membrane of the fenestra ovalis become mechanically connected. In all these animals the mandible is connected with the skull by the intermediation of an os guadratum. But, in the Mammalia, the mandible is articulated directly with the squamosal, and the guadratum is converted into one THE EAR. ne of the so-called osstcula auditds, and named the malleus. The malleus becomes attached to the membrana tympani, by a special process ; while its other extremity, which was continu- ous with Meckel’s cartilage in the embryo, is converted into the processus gracilis, or Folianus, and lies between the tym- panic, the squamosal, and the periotic bones. In the singular lizard Sphenodon (A, Fig. 24), the anterior cornu of the hyoid is continuous with the distal end of the stapes, and the latter sends a cartilaginous process upward, which passes into the wall of the periotic capsule, just behind the proximal end of the os guadratum. Thus the stapes stands out at right angles to the hyoid cornu, and the latter becomes divisible into a supra-stapedial part, and a part which lies below the stapes, and answers to the styloid process, or stylohyal, of the Mammalia. The supra-stapedial part is rep- resented by cartilage, or ligament, in other Sauropsida, but seems not to ossify. In the Ja@nimatia (B, Fig. 24), the su- pra-stapedial part ossifies, becomes the ireus, and its proximal end is usually articulated by a synovial joint with the malleus (= quadratum). A distinct ossification, the os orbiculare, usually arises at that part of the hyoidean cartilage in which the stapes and the trcus unite. That part of the hyoidean cartilage which is converted into the styloid process is gen- erally connected with the orbiculare by muscular tibres, which constitute the stapedius muscle. On the other hand, the pos Fie. 24.—Diagram of the skeleton of the first and second visceral arches in a Lizard (A), Mammal (B), and an Osseous Fish (C). The skeleton of the first visceral arch is shaded, that of the second is left nearly unshaded. I. First visceral arch. Ifck. Meckel's cartilage. Av. Articulare, Qt. Quadratum. Mpt. Metapterygoid ; 1/. Malleus; p.q., Processus gracilis, J/. Second visceral arch. JTy. Wyoidean cornu. St. H. Stylohyal. 8. Stapedius, Stp. Stapes. & Stp. Supra- stapedial. /74f, Hyomandibular. The arrow indicates the first visceral cleft. Pe. Tha periotic capsule. . The pterygoid. terior, or short process of the incus, is connected by ligament with that part of the periotic mass into which the styloid pro- 8 THE ANATOMY OF VERTEBRATED ANIMALS. cess is directly continued, and it is hard to say whether the styloid part of the hyoid is continued into the incus by these ligaments or by the stapedius. But, however this may be, the malleus and the incus are the proximal ends of the mandibular and hyoidean arches respectively. In osseous fishes (C, Fig. 24), which have no fenestra ova- lis or stapes, the supra-stapedial part of the hyoid becomes a large bone—the hyomandibwar. On the other hand, the proximal extremity of the quadrate cartilage atrophies, loses its direct connection with the periotic capsule, and becomes distinctly ossified, as the metapterygoid. In the Sharks, even the ascending, metapterygoid, part of the quadrate, is lost. The quadrate and supra-stapedial portions of the first and secoud visceral arches coalesce in the Chimera, Dipnot, aud many Amphibia, into a single cartilaginous plate. In the Mammalia, and to some extent in Aves, osseous matter is deposited in the fibrous tissue which surrounds the sides and base of the tympanic membrane, and gives rise to a special tympanic bone. In most Mammalia, ossification ex- tends into the sides and floor of the tympanum and external meatus ; and a process of integument, chiefly derived from the second visceral arch, is converted into a concha, or external ear. The Organ of Taste is the mucous membrane which covers the tongue, especially its posterior region, and probably also a part of that lining the fauces. When the sense is well de- veloped, the mucous membrane is raised into numerous papille of various forms, and is well supplied with filaments from the glossopharyngeal nerve. The sense of Touch is diffused over the integument and over the mucous membrane of the buccal cavity, which is, strictly speaking, a part of the integument. As special organs of touch in the higher Vertebrata, the nervous papill, containing “ tactile corpuscles,” and the long facial hairs, the papillze of which are well supplied with nerves, termed vibrissee, may be mentioned. In most, if not all Fishes, the integument of the body and of the head contains a series of sacs, or canals, usually disposed symmetrically on each side of the middle line, and filled with a clear gelatinous substance, The walls of the sacs, or canals, are abundantly supplied with nerves, ard the terminations of the latter enter rounded papillse, which project into the gelati- nous contents, These sensory organs are known as the “ or THE LIVER AND THE TEETH. "9 gans of the lateral line,” or “mucous canals ;” and they were formerly supposed to be the secretory glands of the slimy matter which coats the bodies of fishes, and which is really modified epidermis. The Alimentary Canal.—This part of vertebrate organi- zation always exhibits a differentiation into mouth, pharynx, cesophagus, stomach, and intestine; and the last has always a median, or nearly median, aperture on the ventral surface of the body. It may open by itself; or into a cloaca, or cham- ber common to it, the urinary and the genital organs. The intestine is generally distinguishable into small and large ; and, at the junction of the two, one or two cuca are frequently developed trom the former. The stomach and intestine are invested by a peritoneal membrane, and connected, by mesogastric and mesenteric folds of that membrane, with the median dorsal wall of the abdomi- nal cavity. Glands appertaining to the lymphatic system frequently abound in the mesenteric folds, and a highly-vas- cular gland of this system,.the spleen, is always (except in Amphioxus, Myxine, and the Leptocephalide) developed in close proximity to the stomach. A pancreatic gland very generally pours its secretion into the anterior end of the intes- tine. Salivary glands very commonly open into the mouth; and, in the higher Vertebrata, anal glands are not unusually developed in connection with the termination of the rectum. The structures connected with the alimentary canal of ver- tebrate animals, which are most characteristic and peculiar, are the liver and the teeth. The Liver.—In invertebrate animals this organ is always ultimately resolvable into cecal tubes, the ends of the hepatic ducts, which are lined with an epithelium, and not reticulated ; and it has no receptacle for the bile. In most Vertebrata the ends of the hepatic ducts have not been satisfactorily traced, nor is it certain that the immense proportional mass of hepatic corpuscles is contained in tubes continuous with them; if such be the case, the tubes must be reticulated. The ducts of the vertebrate liver very frequently pour the bile, directly or in- directly, into a receptacle, the gall-bladder. Amphioxus stands alone among vertebrated animals, in having a cacal diverticu- lum of the intestine for a liver. The Teeth.—Teeth, in Mollusca and Annulosa, are always 80 THE ANATOMY OF VERTEBRATED ANIMALS. “ ecderonic,” cuticular, or epithelial structures. In Vertebrata true teeth are invariably “ enderonic,” or developed, not from the epithelium of the mucous membrane of the alimentary canal, but from a layer between this and the vascular deep substance of the enderon, which answers to the dermis in the integument. The horny “teeth” of the Lampreys, and of Ornithorhynchus, appear to be ecderonic structures, homolo- gous with the “baleen” of the Cetacea, with the palatal plates of the Sirenia, or the beaks of Birds and Reptiles, and not with true teeth. The dense calcified tissue called dentine, characterized by the close-set parallel tubuli which radiate through it, branch- ing as they go, constitutes the chief mass of true teeth; but the dentine may be coated with ordinary bony tissue, which then receives the name of cementum, and its crown may be capped with imperforate, prismatically fibrous, enamel. The teeth are moulded upon papille of the mucous mem- brane, which may be exposed, but are more usually sunk in a fold or pit, the roof of which may close in so as to form a dental sac. And there may be one set of teeth, or several; the sacs of the new teeth, in the latter case, being developed either as diverticula of the old ones, or independently of them, In the majority of the Mammalia the teeth are limited in number, as well as definite in their forms and their mode of succession. There are two sets of teeth, forming a first, dectdu- ous, or milk dentition, and a second, or permanent dentition, The deciduous dentition, when most completely developed, con- sists of tncisor, canine, and molar teeth. The incisors are distinguished from the rest by the lodgment of the upper set in the premaxille, and the correspondence of the lower set with the upper. Their number and form vary. The distinc- tion between canines and molars is one of form and position in regard to the remaining teeth; the most anterior of the teeth behind the premaxillo-maxillary suture, if it is sharp and projecting, receiving the name of canine. There are never more than four canines. The other teeth are molars, and ordinarily do not exceed four upon each side, above and below. What is called a dental formula is a convenient combination of letters and figures for making the number and disposition of the teeth obvious. Thus, let di, de, dm, represent, respec- tively, the deciduous or milk set of incisors, canines, and molars. Then, by placing after each of these symbols figures arranged so as to show the number of the teeth of the kind symbolized, on each side of each jaw, we shall have the dental DENTAL FORMULA. 8 formula of a given animal. The dental formula of a chil : . 22, 1—1 2.2 over two years of age is thus—di. 23 de. 77 dm. a 20 which means that the child should have two incisors, on canine, and two molars on each side of each jaw. The neck of the sac of each deciduous tooth gives off diverticulum, in which one of the permanent teeth is de veloped; as it grows, it causes the absorption of the fan, of the corresponding deciduous tooth, which thus become shed, and is replaced from below by the permanent tootl The same letters, but without the prefix d, are used for th permanent incisors and canines; but the permanent teetl which replace the deciduous molars, are called premolar: and have the symbol pm. Furthermore, three or, it may be four permanent grinding teeth, on each side of each jaw, ar developed altogether bebind the milk molars, and thus com into place without replacing any other tooth from below These are called molars, and have the symbol m. Thus th formula of the permanent dentition in Man is written a a = mn fey Bid s_99% there being two incisors ia" aa ea a oe one canine, two premolars, and three molars on each sid above and below. It is a rule of very general applicatio among the Mammalia, that the most anterior molar come into place and use before the deciduous molars are shec Hence, when the hindermost premolar, which immediatel. precedes the first molar, comes into use by the shedding o: the last milk molar, the crown of the first molar is already | little ground down; and this excess of wear of the first mola over the adjacent premolar long remains obvious. The fac that, in the permanent dentition, the last premolar is les worn than the first molar which immediately follows it, i often a valuable aid in distinguishing the premolar from th molar series, No vertebrate animal has teeth in any part of the alimen tary canal save the mouth and pharynx—except a snak (Rachiodon), which has a series of what must be terme: teeth, formed by the projection of the inferior spinous prc cesses of numerous anterior vertebra into the cesophagus And, in the highest Vertebrata, teeth are confined to the pre maxill, maxillee, and mandible. The Circulatory Organs.—The heart of the vertebrat: embryo is at first a simple tube, the anterior end of whic! 82 THE ANATOMY OF VERTEBRATED ANIMALS. passes into a cardiac aortic trunk, while the posterior end is continuous with the great veins which bring back blood from the umbilical vesicle—the omphalomeseraic veins. The cardiuc aorta immediately divides into two branches, each of which ascends, in the first visceral arch, in the form of a forwardly convex aortic arch, to the under side of the rudimentary spinal column, and then runs, parallel with its fellow, to the hinder part of the body, as a primitive subverte- bral aorta. The two primitive aorte very soon coalesce throughout the greater part of their length into one trunk, the definitive subvertebral aorta ; but the aortic arches, sepa- rated by the alimentary tract, remain distinct. Additional arterial trunks, to the number of four in the higher Verte- brata, and more in the lower, are successively developed, behind the first, in the other visceral arches, and further con- nect the cardiac and subvertebral aorte. In the permanently branchiate Vertebrata, the majority of these aortic arches persist, giving off vessels to the branchial tufts, and becoming converted into afferent and efferent trunks, which carry the blood to and take it from these tufts. (Fig. 25, A, B, C, D, E.) In the higher Amphibia, which, though branchiate in the young state, become entirely air-breathers in the adult con- Gition, such as the Batrachia (Fig. 25, F) and Cecilia, the permeable aortic arches are reduced to two (the middle pair of the three which supply the external gills, and the fourth pair of embryonic aortic arches) by the obliteration of the cavities of the dorsal ends of the others. Of the posterior arches, the remains of the fifth and sixth become the trunks which give off the pulmonary arteries, and, in the Batrachia, cutaneous branches. The anterior, or third, primitive aortic arch becomes the common carotid trunk, and ends in the carotid gland, whence the internal and external carotids arise. In those Vertebrata which never possess gills, the arches become reduced either to two pair, as in some Lacer- tilia ; or to one pair, as in other Reptilia; or to a single arch, as in Aves and Mammalia. The aortic arches thus retained are, in the Lizards in question, the third and the fourth pairs in order from before backward; but the fourth pair only, in other Reptiles; in Birds, the right arch only of the fourth pair; and in Mammals, the left arch only of the fourth pair. The fifth pair of arches give off the pulmonary arteries, the so-called “ductus arteriosus” representing the remains of the primitive connéction of these arches with the MODIFICATIONS OF THE AORTIC ARCHES. é fourth pair and the subvertebral aorta. The dorsal ends c the first, second, and third arches become obliterated; bi their cardiac ends, and the branches which they give off, b come the arteries of the head and upper extremities. des Hy. Bre Br? Brs Beene Br® Br? I eller a [Tc rd W@W V Vi Vi wm KK eit ~_|Cle fy I wW wv owt‘w ww kK Fie. 25.—A diagram intended to show the manner in which the aortic arches become modi: fied in the series of the Vertebrata. — ‘ ; ; : A. A hypothetically perfect series of aortic arches, corresponding with the nine postoral vis- ceral arches, of which evidence is to be found in some Sharks and Mursipobranchii. AC, Cardiac aorta; AD. Dorsal or subvertebral aorta, I-1x the aortic arches corre. sponding with Jfm., the mandibular; Hy., the hyoidean, and Br.'—Br.7, the seven branchial visceral arches. 1, U1, UL, IV, V, V1, Vu, the seven branchial clefts. The tirst 84 THE ANATOMY OF VERTEBRATED ANIMALS. visceral cleft is left unnumbered, and one must be added to the number of each bran- chial cleft to give its number in the series of visceral clefts, 7 B. Hypothetical diagram of the aortic arches in the Shark Heptanchus, which has seven branchial clefts. Sp. The remains of the first visceral cleft as the spiracle. Branchiz are developed on all the arches. . C. Lepidosiren.—The first arch has disappeared as such, and the first visceral cleft is ob- literated. Internal branchie are developed in connection with the second, fifth, sixth, and seventh aortic arches; external branchiw in connection with the fourth. fifth, and sixth. A.—The pulmonary artery. The posterior two visceral clefts are obliterated. D. A Teleostean Fish—The first aortic arch and first visceral cleft are obliterated as before. The second aortic arch bears the pseudo-branchia (Ps.B.), whence issues the ophthalmic artery, to terminate in the choroid gland (Ch.). The next four arches bear gills. The seventh and eighth arches have been observed in the embryo, but not the ninth, and the included clefts are absent. in the adult. E. The Axolotl (Siredon), a perennibranchiate amphibian. The third, fourth, fiftb, and sixth aortic arches, and the anterior four branchial clefts, persist. The first visceral cleft is obliterated. F. The Frog.—The three anterior aortic arches are obliterated in the adult. The place of the third, which is connected with the anterior external gill in the Tadpole, is occupied by the common carotid and the rete mirubile (carotid gland, Ca. G.) which terminates it. The fourth pair of aortic arches persist. The fifth and sixth pair lose their connec- tions with the subvertebral aortic trunk, and become the roots of the cutaneous and pulmonary arteries. The first visceral cleft becomes the tympanum, but all the others are obliterated in the adult, The embryonic aorta gives off omphalomeseraic branches (Fig. 26, 0) to the umbilical vesicle ; and ends, at first, in the hypogastric arteries (which are distributed to the allantois in the abranchiate Vertebrata), and a median caudal continuation. The blood from the umbilical vesicle is brought back, as before mentioned, by the omphalomeseraic veins (Fig. 26, 0’), which unite in a dilatation close to the head; the dilatation (sinus - venosus) receives, on each side, a short transverse venous trunk, the ductus Cuviert (Fig. 26, DC), which is itself formed, upon each side, by the junction of the anterior and posterior cardinal veins, which run backward and forward, parallel with the spine, and bring back the blood of the head and of the trunk. ; The blood of the allantois is returned by the umbilical vein, or veins (Fig. 26, 2’), which are formed in the anterior wall of the abdomen, and open into the venous sinus before mentioned. The blood of the posterior extremities and kid- neys is, after a while, brought to the same point by a special median vein, the vena cava inferior (Fig. 26, cv). The development of the liver effects the first great change in the arrangements now described. It, as it were, interrupts the course of the omphalomeseraic vein, which is not only the vein of the umbilical sac but also that of the intestine, ard converts it into a meshwork of canals, which communicate, on one side, with the cardiac part of the vein, and, on the other side, with its intestinal part. The latter is thus converted into the vena porte (Fig. 26, vp), distributing the blood of the stomach and intestines to the liver ;“while the former becomes THE DEVELOPMENT OF THE VASCULAR SYSTEM. € the hepatic vein (vh), carrying the hepatic blood to the i: ferior cava, and thence to the heart. The umbilical vein further gives a branch to the liver while, on the other hand, it communicates directly with tt venous sinus (now almost merged in the vena cava inferio: by a trunk called ductus venosus (Fig. 26, Dv). Fig. 26.—Diagram of the arrangement of the principal vessels in a human fetus.—/, tt heart; 7A, the aortic trunk or cardiac aorta; c, the common carotid; c’, the extern carotid; c/’, the internal carotid; s, subclavian; v, vertebral artery; 1, 2, 3, 4, 5, tk aortic arches—the persistent left aortic arch is hidden. A’, subvertebral aorta; 0, on phalomeseraic artery, going to the umbilical vesicle », with its vitelline duct dv; 0, om phalomeseraic vein; vp, the vena porte; Z, the liver; ww, the hypogastric or umbilic arteries, with their placental ramifications, w/” w’; w/, the umbilical vein; Dv, the ductt “yenosus; h, the hepatic vein; cv, the vena cava inferior; vé/, the iliac veins; az, a ven azygos; ve’, a vena cardinalis posterior; DC, a ductus Cuviert; the anterior cardin vein is seen commencing in the head and running down to the ductus Cuvieri on th under side of the numbers 1, 2, 3, 4,5; P, the lungs. When the umbilical vesicle and allantois cease to have an: further import, as at birth, or before, the omphalomeseraic ar teries have become intestinal arteries, and the omphalomeserai vein, the vena porte. The hypogastric arteries are obliter ated, except so much of them as is converted into the commor iliac arteries. The umbilical vein, or veins, also disappear, o are represented by mere ligaments. 86 THE ANATOMY OF VERTEBRATED ANIMALS, OF the three veins which open into the venous sac—viz,, the inferior cava, and the right and left ductus Cuvieri—all may persist, the latter receiving the title of right and left su- perior cave. Or, as very often happens in the higher Verte- brata, the left ductus Cuviert becomes more or less obliterated ; the veins which properly open into it acquiring a connection with the right ductus, which then remains as the sole supertor cava. The posterior cardinal veins give off anastomosing branches, which are converted into the venw azygos ; the an- terior cardinal veins become metamorphosed into the external jugular veins and vene innominate. In Fishes, the s¢nus venosus and the cardinal veins persist throughout life; but the anterior cardinal veins, which bring back the blood from the head and from the anterior extremi- ties, are called venw jugulares. The caudal veins are either directly continued into the cardinal veins, as in Marsipobranchii and Elasmobranchit, or branch out into the kidneys, as in many Zéleoste?. In either case the efferent renal veins open into the cardinal veins. The portal veins, conveying the blood of the chylopoietic viscera, and sometimes that of other organs and of the abdomi- nal walls, may be one or many. In Amphioxus and Myzxine the vein is rhythmically contractile, and forms a portal heart. In most Amphibia and Reptilia the sinus venosus persists, and is rhythmically contractile, valves being placed at its opening into the right auricle, The anterior cardinal veins are represented by jugular veins, the posterior cardinal by vertebral veins ; these, and the veins of the anterior extremities, when they are present, pour their blood into the ductus Cuvieri, which are now termed an- terior vene cave. The vena cava inferior takes its origin chiefly by the coa- lescence of the efferent veins of the kidneys and reproductive organs, and does not always receive the whole of the hepatic veins—more or fewer of the latter opening independently into the sinus venosus. The blood which leaves the kidneys by its efferent veins is supplied, not only by the renal arteries, but by the veins of the caudal region, and of the hinder extremities, which branch out like a vena porte in the substance of the kidneys. This renal portal system is less developed in Reptilia than in_Am- phibia, All the blood of the posterior extremities and caudal region does not traverse the kidneys, however, more or less of it being led away by great branches of the iliac veins, which THE MODIFICATIONS OF THE VASCULAR SYSTEM. € run along the anterior wall of the abdominal cavity, either < two trunks, or united into one. These ven abdominales a teriores are eventually distributed to the liver, along with th branches of the proper vene porte. In Birds, the sinus venosus is not distinct from the righ auricle, and there are two anterior venew cave. The ven cava inferior arises, as in Mammals, by the union of the tw common iliac veins. It receives both the right and the le: hepatic veins, and, in addition, the anterior abdominal vein n longer enters the portal system, but passes up the anteric wall of the abdomen and through the hepatic fissure to joi the inferior cava. : The caudal and pelvic veins unite into three princip: trunks, of which one is median and two are lateral. Th median enters into the portal system. The lateral branche pass along and through the kidney, receiving veins from i: but giving none to it; and eventually, after receiving th ischiatic veins, unite with the crural veins to form the commo iliacs. Thus there is no renal portal system in birds. In Mammalia, the sinus venosus is not distinct from th right auricle. The anterior cave are frequently reduced t: one, the right. The vena cava inferior commences in th caudal ‘region, and receives all the blocd of the posterio moiety of the body, except so much as is carried away by th: azygous veins. The anterior abdominal veins are representec only during foetal life, by the umbilical vein or veins, Th efferent veins of the kidneys open directly into the trunk oi the inferior vena cava, and the portal vein is compose exclusively of radicles proceeding from the chylopoieti viscera. Many of the veins of Amphioxus, the portal vein of Mya ine, dilatations of the caudal vein in the Eel, the venz cava and the iliac and axillary veins of many Amphibia, the veins of the wing of Bats, possess a rhythmical contractility, which in combination with the disposition of their valves, assists the circulation of the blood. In Vertebrata of all classes, and in very diverse parts of the body, both veins and arteries occasionally break up intc numerous branches of nearly equal size, which may or may not unite again into larger trunks. These are called retic mirabilia, Modifications of the Heart.—Great changes go on in the structure of the heart, pari passu with the modifications of the 88 THE ANATOMY OF VERTEBRATED ANIMALS. rest of the circulatory system, in the development of the highest Vertebrata. The primitively simple tube becomes bent upon itself, and divided from before backward into an aortic, or ventricular, and a venous, or auricular, portion, A median septum then grows inward, dividing the auricular and ventricular chambers into two, so that a right auricle and right ventricle become separated from a left auricle and left ventricle. A similar longitudinal division is effected in the cardiac aorta. The septa are so disposed in the auriculo-ven- tricular chamber that the right auricle communicates with the venous sac and the trunks of the visceral and body veins, while only the veins from the lungs enter into the left auricle. And the cardiac aorta is so divided that the left ventricle com- municates with the chief aortic trunk, the right with the pul- monary artery. Valves are developed at the auriculo-ventric- ular apertures and at the origins of the aortic and pulmonary trunks, and thus the course of the circulation is determined, The septum between the auricles remains incomplete for a much longer period than that between the ventricles—and the aperture by which the auricles communicate is called the Soramen ovale. In the adult state of Aves and Mammalia, the foramen ovale is closed; there is no direct communication between the arterial and venous cavities or trunks ; there is only one aortic arch; and the pulmonary artery alone arises from the right ventricle. In the Crocodilia, the auricles and ventricles of opposite sides are completely separate; but there are two aortic arches, and one of these, the left, arises from the right ventricle along with the pulmonary artery. In all Reptilia, except Crocodiles, there is but one ventricular cavity, though it may be divided more or less distinctly into a cavum veno- sum anda cavum arteriosum. The auricles are completely separated (except in some Chelonia), and the blood of the left auricle dows directly into the cavum arteriosum, while that of the right passes immediately into the cavum venosum, The aortic arches and the pulmonary artery all arise from the cavum venosum (or a special subdivision of that cavity called the cavum pulmonale) ; the ostium of the pulmonary artery being farthest from, and that of the right aortic arch nearest to, the cavum arteriosum. In all Amphibia, the spongy interior of the ventricle is undivided, and the heart is trilocular, though the auricular septum is sometimes small and incomplete. In all Pisces, ex- cept Lepidosiren, there is no auricular septum. In Amphi- THE MODIFICATIONS OF THE HEART. 8 oxus the heart remains in its primitive state of a simple, cor tractile, undivided tube. In the Ganoidei, the Elasmobranchii, and the Amphibic the walls of the enlarged commencement of the cardiac aortz called the bulbus aortce, contain striped muscular fibre, an are rhythmically contractile. The Ganoidet and Elasmobranchii possess, not merely th ordinary semilunar valves, at the junction between the ventr: cle and the cardiac aorta, but a variable number of additiona valves, set, in transverse rows, upon the inner wall of th aortic bulb. The change of position which the heart and the great ves sels of the highest Vertebrata undergo during embryonic lif is exceedingly remarkable, and is repeated as we ascend in th series of adult vertebrates. At first, the heart of a mammal lies under the middle oi the head, immediately behind the first visceral arches, in whicl the first pair of aortic arches ascends. As the other pairs o! aortic arches are developed the heart moves backward; bu the fourth pair of aortic arches, by the modification of one oi! which the persistent aorta is formed, lies, at first, no farthe back than the occipital region of the skull, to which, as wi have seen above, the fourth pair of visceral arches belongs As the two pairs of cornua of the hyoid belong to the secouc and the third visceral arches, the larynx is probably develope within the region of the fourth and fifth visceral arches; hence the branches of the pneumogastric, with which it is supplied must, originally, pass directly to their destination. But, a: development proceeds, the aortic arches and the heart becom: altogether detached from the visceral arches and move back until, at length, they are lodged deep in the thorax. Henc the elongation of the carotid arteries; hence also, as th larynx remains relatively stationary, the singular course, ir the adult, of that branch of the pneumogastric, the recurren laryngeal, which primitively passed to the laryngeal region behind the fourth aortic arch, and consequently become: drawn out into a long loop—the middle of it being, as i were, pulled back, by the retrogression of the aortic arch int¢ the thorax. The Blood- Corpuscles.—Corpuscles are contained in the blood of all Vertebrata. In Amphioxus they are all of onc kind, colorless and nucleated. The genus Leptocephalus among the Zvicoste?, is said to possess the same peculiarity 90 THE ANATOMY OF VERTEBRATED ANIMALS. but, in all other known Vertebrata, the blood contains corpus- cles of two kinds. In Ichthyopsida and Sauropsida, both kinds are nucleated ; but one set are colorless, and exhibit amzeboid movements, while the others are red, and do not display contractility. Except in the Marsipobranchit, which have round blood-cor- puscles, the red corpuscles are oval. They attain a larger size in tae perennibranchiate Amphibia than in any other Vertebrates. In Mammalia, the blood-corpuscles are also of two kinds, colorless and red, the colorless possessing, and the réd being devoid of, nuclei. It is but very rarely that a nucleated cor- puscle, with a red color especially developed about the ru- cleus, is seen in Mammalian blood; but such cases do occur ; and, from this and other circumstances, it is probable that the Mammalian red corpuscle is a free-colored nucleus. The colorless corpuscles of Mammalia are spheroidal, and exhibit ameeboid movements; the red corpuscles are flattened, usually circular, but sometimes oval (Camelide) disks, devoid of contractility. The Lymphatic System.—This system of vessels consists, chiefly, of one or two principal trunks, the thoracic duct, or ducts, which underlie the vertebral column, and communicate, anteriorly, with the superior vene cave, or with the veins which open into them. ; From these trunks, branches are given off, which ramify through all parts of the body, except the bulb of the eye, the cartilages, and the bones. In the. higher Vertebrata, the larger branches are like small veins, provided with definite coats, and with valves opening toward the larger trunks, while their terminal ramifications form a capillary net-work ; but, in the lower Vertebrates, the lymphatic channels assume the form of large and irregular sinuses, which not unfrequently com- pletely surround the great vessels of the blood-system. The lymphatics open into other parts of the venous sys- tem besides the affluents of the superior cave. In Fishes there are, usually, two caudal lymphatic sinuses which open into the commencement of the caudal vein. In the Frog, four such sinuses communicate with the veins, two in the coccy- geal, and two in the scapular, region. The walls of these si- nuses are muscular, and contract rhythmically, so that they re- ceive the name of Lymphatic hearts, The posterior pair of these hearts, or non-pulsating sinuses corresponding with them, are met with in Reptilia and Aves, THE RESPIRATORY ORGANS. 9 Accumulations of indifferent tissue in the walls of some o the lymphatic sinuses are to be met with in Fishes; but it : only in the Crocodilia, among Reptilia, that an accumulatio of such tissue, traversed by lymphatic canals and blood-vessel is apparent, as a Lymphatic gland, in the mesentery. Birc possess a few glands in the cervical region; and, in Man malia, they are found, not only in the mesentery, but in man parts of the body. The Spleen is substantially a lymphatic gland. The Z/y mus—a glandular mass with an internal cavity, but devoid o any duct—-which is found inall Vertebrata except Amphioxu appears to belong to the same category. It is developed i the neighborhood of the primitive aortic arches, and is doub] in most of the lower Vertebrata, but single in Mammalia. The nature of two other “ductless glands,” the Zhyrot gland and the Suprarenal capsules, which occur very widel among the Vertebrata, is by no means well understood. The thyroid gland is a single or multiple organ, formed o closed follicles, and is situated near the root of the aorta, c the great lingual, or cervical, vessels which issue from it. The suprarenal capsules are follicular organs, often abut ‘dantly supplied with nerves, which appear to occur in Fishe: and are very constant in the higher Vertebrata, at the anteric ends of the true kidneys. The Lymph Corpuseles, which float in the plasma of th lymphatic fluid, always resemble the colorless corpuscles o the blood. The Respiratory Organs.—Vertebrated animals may pos sess either branchie for breathing the air contained in wate! or lungs for atmospheric respiration; or they may posses both kinds of respiratory organs in combination. Except in Amphioxus, the branchie are always lamellai or filamentous, appendages of more or fewer of the viscera arches; being sometimes developed only on the proper brat chial arches, sometimes extending to the hyoidean arch, o (as would appear to be the case with the spiracular brar chiz of some fishes) even to the mandibular arch. The brar chiz are always supplied with blood by the divisions of th cardiac aorta; and the different trunks which carry the aérate: blood away, unite to form the subvertebral aorta, so that al vertebrated animals with exclusively branchial respiration have the heart filled with venous blood. | In the early life of many branchiated Vertebrata, the bran 92 THE ANATOMY OF VERTEBRATED ANIMALS. chie project freely from the visceral arches to which they are attached, on the exterior of the body; and in some Amphibia, such as the Axolotl (Siredon), they retain their form of exter- nal plume-like appendages of the neck throughout life, But in the adult life of most Fishes, and in the more advanced con- dition of the Tadpoles of the higher Amphibia, the branchiz are internal, being composed of shorter processes, or ridges, which do not project beyond the outer edges of the branchial clefts; and, generally, become covered by an operculum developed from the second visceral arch. The lungs of vertebrated animals are sacs, capable of being filled with air, and developed from the ventral wall of the pharynx, with which they remain connected by a shorter or longer tube, the ¢rachea, the division of this for each luug being a bronchus. Venous blood is conveyed to them directly from the heart by the pulmonary arteries, and some* or all of the blood which they receive goes back, no less directly, to the same organ by the pulmonary veins. The vascular distribution thus described constitutes an es- sential part of the definition of a lung, as many fishes possess hollow sacs filled with air; and these sacs are developed, oc- casionally, from the ventral, though more commonly from the dorsal, wall of the pharynx, cesophagus, or stomach. But such air-sacs—even when they remain permanently connected with the exterior by an open passage or pneumatic duct—are air-bladders, and not lungs, because they receive their blood from the adjacent arteries of the body, and not direct from the heart, while their efferent vessels are connected only with the veins of the general circulation. The wall of each pulmonic air-sac is at first quite simple, but it soon becomes cellular by the sacculation of its parietes. In the lower pulmonated Vertebrata, the sacculation is more marked near the entrance of the bronchus ; and when the lung- sac is long, as in many Amphibia and in Snakes, the walls of the posterior end may retain the smooth condition of the em- bryonic lung. In Chelonia and Crocodilia, the lung is com- pletely cellular throughout, but the bronchi do not give off branches in the lungs. In Birds, branches are given off at right angles; and, from these, secondary branches, which lie parallel with one another, and eventually anastomose. In Mammalia, the bronchi divide dichotomously into finer and finer bronchial tubes, which end in sacculated air-cells, * Generally all, but in some Amphibia, such as Proteus, part of the blood . supplied to the lungs enters the general circulation, THE ORGANS OF VOICE. 93 Blind air-sacs are given off from the surfaces of the lungs in the Chameceleonide, and the principal bronchial tubes termi- nate in large air-sacs in Aves. The Larynx and the Syrine.—The trachea is commonly kept open by complete, or incomplete, rings of cartilage, and the uppermost of these undergo special modifications, which convert them into a Larynx, an organ which, under certain circumstances, becomes an instrument of voice. When completely developed, the larynx presents a ring- like cartilage called ericoid, which lies at the summit of the trachea, With the anterior and dorsal edge of this, two aryt- enotd cartilages are movably articulated, and a thyroid car- tilage of a V-shape, open behind, is articulated movably with its sides. Folds of the mucous membrane, containing elastic tissue, termed the vocal cords, stretch from the arytenoid car- tilages to the reéntering angle of the thyroid cartilage, and between them lies a slit-like passage, the glottis. This is cov- ered by a cartilage, the epiglottis, attached to the reéntering angle of the thyroid, and to the base of the tongue. Folds of mucous membrane, extending from ihe epiglottis to the arytenoid cartilages, are the aryepiglottic ligaments. The in- ner surfaces of these end below in the false vocal cords, be- tween which and the true chord vocales lie recesses of the mucous membrane, the ventricles of the larynx. The chief accessory cartilages are the cartilages of San- torini, attached to the summits of the arytenoid cartilages, and the cartilages of Wrisberg, which lie within the aryepi- glottic ligaments. Birds possess a larynx in the ordinary position; but it is another apparatus, the lower larynx or syrinzx, developed either at the end of the trachea, or at the commencement of each bronchus, which is their great vocal organ. The Mechanisin of Respiration.—The mechanism by which the aérating medium is renewed in these different respiratory organs is very various. Among branchiated Vertebrata, Ai- phioaus stands alone in having ciliated branchial organs, which form a net-work very similar to the perforated pharyngeal wall of the Ascidians. Most Fishes breathe by taking aérated wa- ter in at the mouth, and then shutting the oral aperture, and forcing the water through the branchial clefts, when it flows over the branchial filaments. Pulmonated Vertebrata, which have the thoracic skeleton incomplete (as the Amphibia), breathe by distending their pharyngeal cavity with air; and then, the mouth and nostrils 94 THE ANATOMY OF VERTEBRATED ANIMALS. being shut, pumping it, by the elevation of the hyoidean ap- paratus and floor of the pharynx, into the lungs. A Frog, there- fore, cannot breathe properly if its mouth is kept wide open. In most Reptilia, and in all Aves and Mammalia, the ster- num and ribs are capable of moving in such a way as alter- nately to increase and diminish the capacity of the thoracico- abdominal cavity, and thereby to give rise to an inspiratory and expiratory flow of air. In the Reptilia, the elastic lungs dilate with the inspira- tory, and contract with the expiratory, act; but, in Aves, the air rushes through the principal bronchial passages of the fixed and little distensible lungs, into the very dilatable and com- pressible air-sacs. From these the act of expiration expels it back through the principal bronchial passages to the trachea, and so out of the body. Both in Reptilia (e. g., Chelonia) and in Aves, muscular fibres pass from the ribs to the surface of the lungs beneath the pleuroperitoneal membrane, and this rudimentary dia- phragm acquires a very considerable development in the Ra- tite, or struthious birds. So far as the contraction of these fibres tends to remove the ventral from the dorsal walls of the lungs, they must assist inspiration. But this diaphragmatic in- spiration remains far weaker than the sterno-costal inspiration. Finally, in the Mammalia, there are two equally-important respiratory pumps, the one sterno-costal, the other diapnrag- matic. The diaphragm, tuough it makes its appearance in Sauropsida, only becomes a complete partition between the thorax and the abdomen in mammals; and, as its form is such that, in a state of rest, it is concave toward the abdominal cavity, and convex toward the thorax, the result of its con- traction, and consequent flattening, necessarily is to increase the capacity of the thorax, and thus pump the air into the elastic lungs, which occupy a large part of the thoracic cavity. When the diaphragm ceases to contract, the elasticity of the lungs is sufficient to expel the air taken in. Thus, mammals have two kinds of respiratory mechanism, either of which is efficient by itself, and may be carried on in- dependently of the other. The Renal Organs.—The higher Vertebrata are all pro- vided with two sets of renal organs, the one existing only dur- ing the early foetal state, the other persisting throughout life, ‘The former are the Wolffian bodies, the latter the true Kidneys. THE REPRODUCTIVE ORGANS. 95 The Wolffian bodies make their appearance very early, on each side of the ventral aspect of the spinal region of the em- bryo, as small transversely-disposed tubuli, opening into a duct which lies upon their outer side, and enters, posteriorly, into the base of the allantois, and thence into the primitive cloaca with which that structure is connected. The Wolffian duct is one of the first-formed structures in the embryo, and precedes the tubuli. The Avdneys appear behind the Wolffian bodies, and, ap- parently, independently of them; their ducts, the wreters, are also distinct, but likewise terminate in the pelvic part of the allantois. Thus the urinary secretion passes into the allantois, and it is that portion of this organ which lies within the abdo- men, and becomes shut off from the rest by the constriction and obliteration of the cavity of an intermediate part, and its conversion into the wrachus, that gives rise to the urinary bladder. The ultimate secreting tubuli of both the Wolffian body and the kidney, are alike remarkable for ending in dila- tations which embrace convoluted capillaries—the so-called Malpighian tufts. Neither Wolffian bodies nor kidneys have been observed in Amphioxwus. It is doubtful whether true kidneys are developed in Jchthyopsida, or whether the so- called kidneys of these animals are not, rather, persistent Wolf fian bodies. The Reproductive Organs.—These, in vertebrated animals, are primitively similar in both sexes, and arise on the inner side of the Wolffian bodies, and in front of the kidneys, in the abdominal cavity. In the female the organ becomes an ovari- um. This, in some few fishes, sheds its ova, as soon as they are ripened, into the peritoneal cavity, whence they escape by abdominal pores, which place that cavity in direct communi- cation with the exterior. In many fishes, the ovaries become tubular glands, provided with continuous ducts, which open externally, above and behind the anus. But, in all other Ver- tebrata, the ovaries are glands without continuous ducts, and which discharge their ova from sacs, the Graafian follicles, successively Geveloped in their solid substance. Nevertheless, these ova do not fall into the peritoneal cavity, but are con- veyed away by a special apparatus, consisting of the Fallopian tubes, which result from the modification of certain embryonic structures called the Afilleriun ducts. The Miillerian ducts are canals which make their appear- ance alongside the ducts of the Wolffian bodies, but, through- 96 THE ANATOMY OF VERTEBRATED ANIMALS. out their whole extent, remain distinct from them. Their proximal ends lie close to the ovary, and become open and dilated to form the so-called ostia. Beyond these ostia they generally remain narrow for a space, but, toward their hinder openings into the genito-urinary part of the cloaca, they com- monly dilate again. In all animals but the didelpbous and monodelphous Mammalia, the Miillerian ducts undergo no further modification of any great morphological importance ; but, in the monodelphous AZammalia, they become united, at a short distance in front of their posterior ends; and then the segments between the latter and the point of union, or still farther forward, coalesce into one. By this process of conflu- ence the Miullerian ducts are primarily converted into a single vagina with two werd opening into it; but, in most of the Monodelphia, the two uteri also more or less completely coa- lesce, until both Miullerian ducts are represented by a single vagina, a single uterus, and two Fallopian tubes. The didel- phous Mammalia have two vaginz which may, or may not, coalesce anteriorly for a short extent; but the two uteri re- main perfectly distinct. So that what takes place in them is, probably, a differentiation of each Miillerian duct into Fallo- pian tube, uterus, and vagina, with or without the union of the two latter, to the extent to which it is effected in the ear-’ lier stages of development in Monodelphia. The Wolffian ducts of the female either persist as canals, the so-called ca- nals of Gaertner, which open into the vagina, or disappear altogether. Remains of the Wolffian bodies constitute the parovaria, observable in certain female mammals. In the male vertebrate embryo, the testis, or essential re- productive organ, occupies the same position, in front of the ‘Wolffian body, as the ovary ; and, like the latter, is composed of indifferent tissue. In Amphioxus and in the Marsipo- branchit, this tissue appears to pass directly into spermatozoa ; but, in most Vertebrata, it acquires a saccular or tubular struct- ure, and from the epithelium of the sacs, or tubuli, the sperma- tozoa are developed. At first, the testis is as completely de- void of any excretory canal as the ovary; but, in the higher vertebrates, this want is speedily supplied by the Wolfhan body, certain of the tubuli of which become continuous with the tubuli seminiferi, and constitute the vasa recta, while the rest abort. The Wolffian duct thus becomes the vas deferens, or excretory duct of the testis; and its anterior end, coiling on itself, gives rise to the epididymis, A vesicula seminalis is a THE REPRODUCTIVE ORGANS. 97 diverticulum of the vas deferens, near its posterior end, which serves as a receptacle for the semen, W wa Fic. 27.—Diagram exhibiting the relations of the female (the left-hand figure, ¢) and of the male (the right-hand figure, $) reproductive organs to the general plan (the middle fig- ure) of these orgaus in the higher Vertebrata. C7, the cloaca; 2, the rectum; B/, the urinary bladder; JU, the ureter; KX, the kid- ney Uh, the urethra; G, the genital gland, ovary, or testis; W, the Wolffian body; ‘d, the Wolffian duct; J£ the Miillerian duct; Pst, prostate gland; Cp, Cowper's gland; Csp, the corpus spongiosum; Ce, the corpus cavernosum. In the female, ?. the vagina; Ué, uterus; Fp, the Fallopian tube; Gt, Gaertner’s duct; P.v, the parovarium; A, the ants; Ce, C.sp, the clitoris. In the male, Csp, Ce, the penis; U?, the uterus masculinus; Vs, vesicula seminalis; Vd, the vas deferens. If the Wolffian bodies, the genitalia, and the alimentary canal of a vertebrate embryo, communicated with the exterior by apertures having the same relative position as the organs themselves, the anus would be in front and lowest, the Wolf- fian apertures behind and highest, and the genital apertures would lie between the two. But the anal, genital, and uri- nary apertures are found thus related only among certain groups of fishes, such as the Zéleostei. In all other Vertebrata there is either a cloaca, or common chamber, into which the rectum, genital, and urinary organs open; or, the anus isa 5 98 THE ANATOMY OF VERTEBRATED ANIMALS. distinct posterior and superior aperture, and the opening of a genito-urinary sinus, common to the urinary and reproductive organs, lies in front of it, separated by a more or less consid- erable perinceum. These conditions of adult Vertebrata repeat the states through which the embryo of the highest vertebrates pass. At a very early stage, an involution of the external integu- ment gives rise to a cloaca, which receives the allantois, the ureters, the Wolffian and Millerian ducts, in front, and the rectum behind. But, as development advances, the rectal di- vision of the cloaca becomes shut off from the other, and opens by a separate aperture—the definitive anus, which thus ap- pears to be distinct, morphologically, from the anus of an osse- ous fish, Fora time, the anterior, or genito-urinary part of the cloaca, is, to a certain extent, distinct from the rectal di- vision, though the two have a common termination; and this condition is repeated in Aves, and in ornithodelphous Mam- malia, where the bladder, the genital ducts, and the ureters, all open separately from the rectum into a genito-urinary sinus. In the male sex, as development advances, this genito- urinary sinus becomes elongated, muscular, and surrounded, where the bladder passes into it, by a peculiar gland, the pros- tate. It thus becomes converted into what are termed the JSundus, and neck of the bladder, with the prostatic and mem- branous portions of the urethra. Concomitantly with these changes, a process of the ventral wall of the cloaca makes its appearance, and is the rudiment of the intromittent organ, or penis. FPeculiar erectile vascular tissue, developed within this body, gives rise to the median corpus spongiosum and the lateral corpora cavernosa. The penis gradually protrudes from the cloaca; and, while the corpus spongiosum terminates the anterior end of it, as the gldnds, the corpora cavernosa at- tach themselves, posteriorly, to the ischta. The under, or pos- terior, surface of the penis is, at first, simply grooved; by’ de- grees the two sides of the groove unite, and form a complete tube embraced by the corpus spongiosum. The penial urethra is the result. Into the posterior part of this penial urethra, which is frequently dilated into the so-called budbus urethree, glands, called Cowper's glands, commonly pour their secretion; and the penial, membranous, and prostatic portions of the urethra (genito-urinary sinus) uniting into one tube, the male definitive urethra is finally formed. In sundry birds and reptiles, the penis remains in the con- MODIFICATIONS OF THE REPRODUCTIVE ORGANS. 99 dition of a process of the ventral wall of the cloaca, grooved on one face. In ornithodelphous mammals the penial urethra is complete, but open behind, and distinct from the genito- urinary sinus. In the Didélphia the penial urethra and gen- ito-urinary sinus are united into one tube, but the corpora cavernosa are not directly attached to the ischium. Certain Reptilia possess a pair of eversible copulatory or- gans situated in integumentary sacs, one on each side of the cloaca, but it does not appear in what manner these penes are morphologically related to those of the higher Vertebrata. In the female sex, the homologue of a penis frequently makes its appearance as a clitoris, but rarely passes beyond the stage of a grooved process with corpora cavernosa and corpus spongiosum—the former attached to the ischium, and the lat- ter developing a glans. But, in some few mammals (e. g., the Lemuride), the clitoris is traversed by a urethral canal. In no vertebrated animal do the ovaries normally leave the abdominal cavity, though they commonly forsake their primi- tive position, and may descend into the pelvis. But, in many mammals, the testes pass out of the abdomen through the inguinal canal, between the inner and outer tendcns of the external oblique muscle, and, covered by a fold of peritoneum, descend temporarily or permanently into a pouch of the integ- ument—the serotum. In their course they become invested with looped muscular fibres, which constitute the cremaster, The cremaster retracts the testis into the abdominal cavity, or toward it, when, as in the higher mammals, the inguinal canal becomes very much narrowed or altogether obliterated. In most mammals the scrotal sacs lie at the sides of, or behind, the root of the penis, but in the Didelphia the scrotum is sus- pended by a narrow neck in front of the root of the penis. In most mammals the penis is enclosed in a sheath of in- tegument, the preputium ; and, in many, the septum of the corpora cavernosa is ossified, and gives rise to an 0s penis. In the female the so-called labia majora represent the scro- tal, the labia minora the preputial, part of the male organ of copulation. Organs not directly connected with reproduction, but in various modes accessory to it, are met with in many Verte- brata. Among these may be reckoned the integumentary pouches, in which the young are sheltered during their devel- opment in the male Pipefish (Syngnathus), in some female Amphibia (Notodelphys, Pipa), and Marsupialia ; together with the mammary glands.of the Mammalia. CHAPTER TI. THE PROVINCES OF THE VERTEBRATA—THE CLASS PISCES. Tux Vertebrata are divisible into three primary groups or provinces: the Ichthyopsida, the Sauropsida, and the Mam- malia. I.—tThe Ichthyopsida 1, Have the epidermic exoskeleton either absent, or very slightly represented. 2. The spinal column may persist as a notochord with a membranous sheath, or it may exhibit various degrees of chondrification or ossification. When the vertebre are dis- tinct, their centra have no epiphyses. 3. The skull may be incomplete and membranous, more or less cartilaginous, or osseous. When membrane bones are developed in connection with it, there is a large parasphenoid. The basisphenoid is always small, if it be not absent. 4, The occipital condyle may be absent, or single, or double. When there are two occipital condyles they belong to the ex-occipital region, and the basi-occipital region is un- ossified or very imperfectly ossified, 5. The mandible may be absent, or be represented only by cartilage. If membrane bones are developed in connection with it, there is usually more than one on each side. The articular element may be ossified or not, and may be con- nected with the skull by the intermediation of a quadrate and a hyomandibular element, or by a single fixed plate of carti- lage representing both these and the pterygo-palatine arch. A stapes may be present or absent. 6. The alimentary canal may or may not terminate in a cloaca. When there is no cloaca, the rectum opens in front of the urinary organs. 7%. The blood-corpuscles are always nucleated, and the heart may be tubular, bilocular, or trilocular. THE SAUROPSIDA. 101 8. There are never fewer than two aortic arches in the adult. 9. Respiration takes place by branchie during part, or the whole, of life. 10. There is no thoracic diaphragm. 11. The urinary organs are permanent Wolffian bodies. 12. The cerebral hemispheres may be absent, and are never united by a corpus callosum. 13. The embryo has no amnion, and, at most, a rudimen- tary allantois, 14. There are no mammary glands, Ii.—The Sauropsida 1. Almost always possess an epidermic exoskeleton in the form of scales or feathers. 2. The centra of the vertebre are ossified, but have no terminal epiphyses. 3. The skull has a completely ossified occipital segment, and a large basisphenoid. No separate parasphenoid exists in the adult. The prodtic is always ossified, and either remains distinct from the epiotic and opisthotic throughout life, or unites with them only after they have anchylosed with adjacent bones. 4, There is always a single, convex, occipital condyle, into which the ossified ex-occipitals and basi-occiptal enter in vari- ous proportions. 5. The mandible is always present, and each ramus con- sists of an articular ossification, as well as of several mem- brane bones. The articular ossification is connected with the skull by a quadrate bone. The apparent “ ankle-joint” is situ- ated,not between the tibia and the astragalus, as in all Mam- malia, but between the proximal and the distal divisions of the tarsus. 6. The alimentary canal terminates in a cloaca. %. The heart is trilocular or quadrilocular. Some of the blood-corpuscles are always red, oval, and nucleated. 8. The aortic arches are usually two or more, but may be reduced to one, which then belongs to the right side. 9. Respiration is never effected by means of branchie, but, after birth, is performed by lungs. The bronchi do not branch dichotomously in the lungs. 10. A thoracic diaphragm may exist, but it never forms a complete partition between the thoracic and the abdominal viscera. 102 THE ANATOMY, OF VERTEBRATED ANIMALS. 11. The Wolffian bodies are replaced, functionally, by per- manent kidneys. 12. The cerebral hemispheres are never united by a corpus callosum. 13. The reproductive organs open into the cloaca, and the oviduct is a Fallopian tube, which presents a uterine dilata- tion in the lower part of its course. 14, All are oviparous, or ovoviviparous. 15. The embryo has an amnion, and a large respiratory ~ allantois, and is developed at the expense of the massive vitellus of the egg. 16. There are no mammary glands. T.—The Mammalia 1, Always possess an epidermic exoskeleton in the form of hairs. 2. The vertebre are ossified, and (except in the ornitho- delphia) their centra have terminal epiphyses. 3. All the segments of the brain-case are completely ossi- fied. No distinct parasphenoid exists in the adult. The prodtic ossifies, and unites with the epiotic and opisthotic before these coalesce with any other bone. 4, There are always two occipital condyles, and the basi- occipital is well ossified. 5. The mandible is always present, and each ramus con- sists (at any rate, in the adult) of a single membrane bone, which articulates with the squamosal. The quadrate kone, and the supra-stapedial element of the hyoidean arch, are con- verted into a malleus and an incus, so that, with the stapes, there are, at fewest, three ossicula auditis. 6. The alimentary canal may, or may not, terminate in a cloaca. When it does not, the rectum opens behind the genito-urinary organs. %. The heart is quadrilocular. Some of the blood-cor- puscles are always red and non-nucleated. 8. There is only one aortic arch which lies on the left side. 9. Respiration is never effected by means of branchis, but, after birth, is performed by lungs. 10. There is a complete diaphragm. 11. The Wolffian bodies are replaced by permanent kidneys. 12. The cerebral hemispheres are united by a corpus cal- losum. 13. The reproductive organs may, or may not, open into a cloaca. The oviduct is a Fallopian tube. THE CLASS PISCES. 103 14, The embryo has an amnion and allantois. 15. Mammary glands supply the young with nourishment. The Ichthyopsida.—Class I.—Pisczs, The class of Fishes contains animals which vary so much in their grade of organization, and in their higher forms so closely approach the Amphibia, that it is difficult to draw up any definition which shall be at once characteristic and diag- ‘nostic of them. But they are the only vertebrated animals which possess median fins supported by fin-rays; and in which the limbs, when present, do not exhibit that division into brachium, antebrachium, and manus, which is found in all other Vertebrata. The presence of the peculiar integmentary organs con- stituting what is known as the system of mucous canals and the organs of the lateral line (supra, p.79 ), is highly charac- teristic of Fishes, though these organs cannot be said to exist in the entire class. The class Pisces is divisible into the following primary groups: A. The notochord extends to the anterior end of the body. There are no skull, brain, auditory, or renal organs, such as exist in the higher Verte- brata, The heart is a simple tube, and the liver is saccular. © (Lepro- carpia. Haeckel.) I.—Pharyngobranchii. B. The notochord ends behind the pituitary fossa. A skull, brain, auditory, and renal organs are developed. The heart is divided into auricular and ventricular chambers. The liver has the ordinary structure. (Pacuy- carpia. Hek.) z u. The nasal sac is single, and has a median external aperture. Neither mandibles nor limb arches are developed. (Monorhina. Hck.) IL—WMarsipobranchii. b. There are two nasal sacs with separate apertures. Mandibles and limb arches are developed. (Asphirhina. Hck.) : a. The nasal passages do not communicate with the cavity of the mouth. There are no lungs, and the heart has but one auricle. a, The skull is devoid of membrane bones. Ill.— Hlasmobranchii. 8. Membrane bones are developed in relation with the skull. 1. The optic nerves form a chiasma, and there are several rows of valves in the aortic bulb. IV.— Ganoidei. 2. The optic nerves simply cross, and there is only one row of valves in the aortic bulb. V.— Teleostei. 8. The nasal passages communicate with the oral cavity. There are lungs, and the heart has two auricles. ‘ VL—Dipnoi. 104 THE ANATOMY OF VERTEBRATED ANIMALS. I. The Poaryncosrancuu.—This order contains but one species of fish, the remarkable Lancelet, or Amphioxus lanceo- latus, which lives in sand, at moderate depths in the sea, in many parts of the world. It is a small, semitransparent crea- ture, pointed at beth ends, as its name implies, and possessing no limbs, nor any hard epidermic or dermal covering. The dorsal and caudal regions of the body present a low median fold of integument, which is the sole representative of the system of the median fins of other fishes. The mouth (Fig. 28, A, a) is a proportionally large oval aperture, which lies behind, as well as below, the anterior termination of the body, and has its long axis directed longitudinally. Its mar- gins are produced into delicate ciliated tentacles, supported by semi-cartilaginous filaments, which are attached to a hoop of the same texture placed around the margins of the mouth (Fig. 29, f, g). These probably represent the labial cartilages of other fishes. The oral aperture leads into a large and dilated pharynx, the walls of which are perforated by numerous PEER RECE lie aan Fie. 28.—Amphiorus lanceolatus,—a, mouth; 6, pharyngobranchial chamber; ¢, anus; d, liver; e, abdominal pore.—B, the head enlarged ; a, the notochord; 0, the represent- atives of neural spines, or fin-rays; ¢, the jointed oralring; @,the filamentary append- ages of the mouth; ¢, the ciliated lobes of the pharynx; 7, g, part of the branchial sac; h, the spinal cord. clefts, and richly ciliated, so that it resembles the pharynx of an Ascidian (Fig. 28, B, fg). This great pharynx is con- nected with a simple gastric cavity which passes into a THE PHARYNGOBRANCHI. 105 straight intestine, ending in the anal aperture, which is situ- ated at the root of the tail at a little to the left of the me- dian line (Fig. 28, A, ¢c). The mucous membrane of the in- testine is ciliated. An aperture called the abdominal pore (Fig. 28, A, e), placed in front of the anus, leads into a relatively spacious cavity, which is continued forward, on each side of the pharynx, to near the oral aperture. The water which is con- stantly propelled into the pharynx by its cilia, and those of the tentacles, is driven out through the branchial clefts, and makes its exit by the abdominal pore. The liver (Fig. 28, A, d) is a saccular diverticulum of the intestine, the apex of which is turned forward, = in i \ \ i l BGNes Fie. 29.—Anterior end of the body of Amphioatis.—Ch, notochord; My, myelon, or spinal chord; a, position of olfactory (?) sac; 6, optic nerve ; c, fifth (2) pair; @, spinal nerves; é, representatives of neural spines, or fin-rays; 7, g, oral skeleton. The lighter and darker shading represents the muscular segments and their interspaces. The existence of distinct kidneys is doubtful; and the re- productive organs are simply quadrate glandular masses, attached in a row, on each side of the walls of the visceral cavity, into which, when ripe, they pour their contents. The heart retains the tubular condition which it possesses in the earliest embryonic stage only, in other Vertebrata. The blood brought back from the body and from the ali- 106 THE ANATOMY OF VERTEBRATED ANIMALS. mentary canal enters a pulsatile cardiac trunk, which runs along the middle of the base of the pharynx, and sends branches up on each side. The two most anterior of these pass directly to the dorsal aorta; the others enter into the ciliated bars which separate the branchial slits, and, therefore, are so many branchial arteries. Contractile dilatations are placed at the bases of these branchial arteries. On the dorsal side of the pharynx the blood is poured, by the two anterior trunks, and by the branchial veins which carry away the aérated blood from the branchial bars, into a great longi- tudinal trunk, or dorsal aorta, by which it is distributed throughout the body. Notwithstanding the extremely rudimentary condition of the liver, it is interesting to observe that a contractile truak, which brings back the blood of the intestine, is distributed on the hepatic sac after the manner of a portal vein. The blood is collected again into another contractile trunk, which repre- sents the hepatic vein, and is continued into the cardiac trunk at the base of the branchial sac. The corpuscles of the blood are all colorless and nucleated. The skeleton is in an extremely rudimentary condition, the spinal column being represented by a notochord, which extends throughout the whole length of the body, and terminates, at each extremity, in a point (Fig. 28). The investment of the notochord is wholly membranous, as are the boundary-walls of the neural and visceral chambers, so that there is no appearance of vertebral centra, arches, or ribs. A longitudinal series of small semi-cartilaginous rod-like bodies, which lie above the neural canal, represent either neural spines or fin-rays (Fig. 28, B, 6). Neither is there a trace of any distinct skull, jaws, or hyoidean apparatus; and, indeed, the neural chamber, which occupies the place of the skull, has a somewhat smaller capacity than a segment of the spinal canal of equal length. There are no auditory organs, and it is doubtful if a ciliated sac, which exists in the middle line, at the front part of the cephalic region (Fig. 29, a), ought to be considered as an olfac- tory organ. The myelon traverses the whole length of the spinal canal, and ends anteriorly without enlarging into a brain. From its rounded termination nerves are given off to the oral region, and to the rudimentary eye or eyes (Fig. 26, 0, ¢c). According to M. Kowalewsky,* who has recently studied : “Mémoires de l’Académie Impériale des Sciences de St. Petersburg,” 1867. ’ THE PHARYNGOBRANCHII. 107 the development of Amphiowus, the vitellus undergoes com- plete sermentation, and is converted into a hollow sphere, the walls of which are formed of a single layer of nucleated cells. The wall of the one moiety of the sphere is next pushed in, as it were, until it comes into contact with the other, thus re- ducing the primitive cavity to nothing, but giving rise to a secondary cavity, surrounded by a double membrane. The operation is, in substance, just the same as that by which a double nightcap is made fit to receive the head. The blasto- derm now acquires cilia, and becomes nearly spherical again, the opening into the secondary cavity being reduced to a small aperture at one pole, which eventually becomes the anus. M. Kowalewsky points out the resemblance, amounting almost to identity, of the embryo at this stage with that of many Invertebrata. One face of the spheroidal blastoderm becomes flattened, and gives rise to lamin dorsales, which unite in the charac- teristically vertebrate fashion; and the notochord appears between and below them, and very early extends forward be- yond the termination of the neural canal. The neural canal remains in communication with the exterior, for a long time, by a minute pore at its anterior extremity. The mouth arises as a circular aperture, developed upon the right side of the anterior end of the body, by the coalescence of the two layers of the blastoderm, and the subsequent perforation of the disk formed by this coalescence. The branchial apertures arise by a similar process which takes place behind the mouth ; and they are, ut first, completely exposed on the surface of the body. But, before long, a longitudinal fold is developed upon each side, and grows over the branchial apertures. The two folds eventually coalesce on the ventral side, leaving only the abdominal pore open. One cannot but be struck with the resemblance of these folds to the processes of integument which grow over the bran- chiz of the amphibian larva ; and, in like manner, enclose a cavi- ty which communicates with the exterior only by a single pore. In a great many of the characters which have been enu- merated—as, for example, in the entire absence of a distinct skull and brain, of auditory organs, of kidneys, of a cham- bered heart; in the presence of a saccular liver, of ciliated branchie and alimentary canal; and in the extension of the notochord forward to the anterior end of the body—