Comparative Anatomy, the science which treats of the structure and relations of organs in the various branches of the animal kingdom, without a knowledge of which it is impossible to understand the beautifully progressive development of organization; necessary even for the full comprehension of the uses of many parts of the human body, which, apparently rudimentary and useless in man, are highly developed in other animals. This science is also the basis of physiology and the natural classification of animals. On a subject so vast as this, comprehending the whole range of animal life, it will be impossible here to give anything but a sketch; selecting a few only of the more important functions, instead of attempting an exhaustive treatment of the science. In order to simplify the subject, and to make this article especially referable to human anatomy, the whole division of the invertebrata will be left out, for consideration under its separate branches and classes. - Skeleton. In the vertebrata, the most striking character is the great preponderance of the nervous system, which impresses peculiar forms on the skeleton, and corresponding arrangements on the vital organs; everything in their structure seems destined for the protection and suitable exercise of the nervous system; this is less observable in the lower forms, but in all the nervous centres are largely increased and collected together, compared with the invertebrata, and the principal portions enclosed in the bony canal of the spine and skull, to which the limbs with their muscles are attached; their bony fabric, therefore, is properly called neuroskeleton, to distinguish it from the external or dermoskeleton of invertebrates, the former having a basis of phosphate of lime, the latter chiefly of carbonate of lime.
In the lowest vertebrates, as in eels among fishes, and serpents among reptiles, the spinal column and its cranial continuation constitute the principal and essential parts of the skeleton, the development of limbs being secondary; the skull is composed of the same elements as the spine, enclosing the cephalic ganglia and protecting the organs of special sense. The vertebral column is the first rudiment of the skeleton in the human embryo. The bones of fishes are comparatively soft, flexible, and elastic, in the lowest species without division into vertebrae, and in the sharks and rays cartilaginous, resembling the embryonic conditions of higher animals. Having no weight to support from the density of the medium in which they live, and being forced only to propel themselves through the water, which is effected by the lateral movements of the spine, the vertebras are very numerous, and united by biconcave surfaces enclosing a gelatinous substance which admits of easy motion of one bone on another when the vertically flattened tail strikes the water; in aquatic mammals, as the whales, the tail is flattened horizontally to enable them to rise to the surface to breathe air, which distinguishes them from fishes.
The number of vertebras varies from 25 in uranoscopus to more than 200 in sharks, and some are said to have only 13. The lateral surface of the fish is extended by large spinous processes and fin rays on the back, or what may be called the dorsal or abdominal vertebras, and to these are attached also the ribs when any are present; those which have spines below are called caudal vertebras, the last being triangular and flattened to support the fin rays of the tail; the dorsal and abdominal fins move only with the spine. Though requiring no chest for respiratory organs, many fishes have ribs, and a few a rudimentary sternum. The cranium in the cartilaginous fishes is very simple, but in the osseous tribes is composed of a great number of pieces whose homologies are not always well determined; the brain cavity forms but a small part of the head, and its component bones are easily recognized as direct continuations of the vertebras; the bones are thin and united by squamous sutures, which favors extension during growth; the lower jaw and operculum are supported on each side by a series of bones resembling the os quadratum of birds; the hyoid bone is greatly developed, supporting the branchial arches and organs of respiration.
The pectoral fins represent the anterior extremities and the ventrals the posterior; the latter are frequently absent; the former are attached to an osseous scapular arch, articulated to the skull; to this are attached an arm, forearm, and numerous carpal or wrist bones, from which the fin rays or fingers spring; the arm remains within the body, only the hand being external, consisting of a large number of fingers with many joints; no animal above fishes has more than five fingers, and some have only one (the middle finger), as the horse. The pectorals of the skates are wing-like, surrounding and even projecting in advance of the head; in the flying fishes they are so long that they serve for a species of flight; they vary in position, being sometimes under the throat and sometimes behind the ventrals; in a few species they are wanting. The posterior limbs, or ventral fins, are not articulated to the spine and do not form a bony arch as in the shoulder, but are suspended to the rib-like iliac bones at a distance between the head and anus varying in different families; in the jugular and thoracic fishes the bones supporting the ventral fins are attached to the arch which sustains the pectorals; there are small bones between the pelvic and the fin rays, which, if any, must represent the femur, tibia, fibula, and tarsus of the higher animals.
A singular peculiarity in the skeleton of fishes is its want of symmetry in some genera; in the soles (pleuronectes) and flounders (platessa), one side is turned upward instead of the back, both eyes are placed on the same side, and the cranial bones are distorted to allow this arrangement of the organs. In most osseous fishes there are many small forked bones interspersed among the muscles, having no connection with the skeleton, serving as points of support to the muscles. - In .the amphibia, which constitute a class of animals intermediate between fishes and true reptiles, there is an extraordinary difference of external form and internal structure during the metamorphosis which most of them undergo. In all, the spine consists of dorsal, sacral, and caudal portions; in the frog there are only 8 or 9 vertebras, in some of the salamanders more than 40, and in the siren 80 or 90; in the higher forms the vertebras have no ribs, but long transverse processes, and are united by a ball-and-socket joint, with anterior concavity and posterior convexity; in the tadpole and perennibran-chiate genera the spine is like that of fishes; in the frog, destitute of a tail, there is no coccyx, but in the salamander the coccygeal vertebrae are as numerous as those of the trunk, forming a powerful swimming organ.
The bones of the skull are less numerous and more united than those of fishes; the brain cavity is very small, and the facial bones much developed transversely; the os hyoides undergoes remarkable changes according to the persistence of the branchiae, as detailed in the article AMPHBIA; the tailed genera have cartilaginous appendages to the transverse processes, like rudimentary ribs. The scapular arch is well developed, consisting of the usual three bones, which unite to form the glenoid cavity, and in most genera has a distinct breast bone extending beyond the chest both anteriorly and posteriorly; the bones of the arm, forearm, and hand are easily recognized. The posterior extremities in the leaping batrachians consist of a long and cylindrical femur, a tibia and fibula consolidated into one bone, a tarsus and long metatarsus, and five toes; in the aquatic species the limbs are small and feeble, in some rudimentary, or even wanting, the powerful tail serving for rapid locomotion. - In serpents the skeleton is little more than a vertebral column and its ribs; there are no limbs (only the merest rudiments in some), and the motions of creeping, climbing, swimming, and springing are performed entirely by means of the flexible spine and the very numerous ribs; the number of the vertebrae is greater than in any other animals, being sometimes over 300, and their arrangement by ball-and-socket joints allows of very free lateral motion; the ribs, upon which they creep as upon so many feet, like an articulate, extend from the head to the anus, and are attached to no sternum; the bones of the head are very numerous, and movable upon each other; the brain cavity is small.
Snakes may be distinguished from lizards without feet by the separate movements of the two halves of the jaws, which allow the remarkable expansion of the mouth necessary to swallow their large prey. In lizards the skeleton is more perfect; in them are seen for the first time cervical vertebras; they have also distinct dorsals, bearing ribs, lumbar, sacral, and caudal vertebras, articulated by ball-and-socket joints; also a sternum, scapular and pelvic arch, with very distinct and normally divided limbs in most genera; they have what have been called cervical and abdominal ribs before and behind the true ribs, and the sternum is prolonged nearly to the pubis, giving origin to cartilaginous arches supporting the abdominal viscera, of which the homologues in man are the lineaa transversae of the rectus abdominis muscles. In the flying dragon, a small lizard, the ribs are elongated and covered with a thin membrane, by means, of which the animal sails, like the flying squirrel, from one point to another below, using this membranous expansion as a parachute. In the fossil pterodactyl, a flying lizard, there is a remarkable elongation of one of the anterior fingers, which supports a flying membrane similar to that of the bat.
In the rhizodonts, including the crocodiles and the large extinct fossil genera, the skeleton is still further developed, approaching more nearly to the mammalian type, especially in the extremities. The bones of the cranium are firmly united to each other, and there is no lateral movement of the jaws, as in snakes; the brain cavity is small; the whole number of vertebras is decidedly less, and locomotion is proportionally performed by the feet, though the tail in the aquatic genera is still largely developed and very muscular. In tortoises, the highest reptiles, it would be difficult at first sight to recognize the usual structure of the class, but a closer examination betrays no essential difference. The spine consists of 7 or 8 cervical vertebrae, 8 to 14 dorsal, 3 sacral, and 20 to 30 caudal; the broad flat bones under the scales are a series of ribs, as may be seen by examining their connection with the spine from the inside, and the lower cuirass or plastron is a series of sternal bones, in which the ends of the ribs unite; the aquatic species have these ribs united only in the portions nearest to the spine; this immovable box of ribs and sternum, overlaid with the scales of the dermoskeleton, is admirably adapted to resist pressure.
The bones of the skull are firmly united, and form large cavities and fossas for the protection of the muscles of the jaws; the brain cavity is still small. The anterior limbs are attached to the inside of the chest; the scapula below the viscera, and close to the plastron, is united to the collar bone by suture; the humerus is arched and twisted, the forearm short, broad, and permanently pronated; three rows of carpal bones, metacarpus, and phalanges. In the pelvic arch the ilium and the pubis seem to change places, the latter being broad and flat, and uniting below with the ischium, while the former is long and narrow, and projects backward, the whole pelvis being movable on the spine; the femur presents a trochanter, and has quite a mammalian aspect; the bones of the leg are separate, and nearly equal; the aquatic species have longer and more slender limbs. - In birds the number of vertebrae is quite variable in the different regions of the spine in the neck, according to Cuvier, from 23 in the swan to 9 in the sparrow; in the back, from 11 in the cassowary and swan to 6 in the bullfinch; in the sacrum, from 20 in the ostrich to 7 in the coot; in the tail, from 9 to 5; these proportions being connected with the habits of the species.
Though so very different from the lower classes, birds differ from each other only slightly in their skeletons; the bones of the neck, back, chest, and extremities are remarkably similar in all; they vary chiefly in the shape and size of their bills, the form of the feet, and the proportions of their bodies. Their bones are white, compact, fragile, and hollow for the introduction of air. The cervical vertebrae are the most numerous; their bodies lock into each other so as to allow a forward motion in the upper and lower ones, and a backward motion in the middle ones; the transverse processes are long, and have rudimentary ribs, especially developed in birds of prey. The bodies of the dorsal vertebra) are short and compressed laterally, and have large transverse processes; they are generally immovable in birds of powerful flight. The sacral vertebrae are firmly united together, and with the pelvis, in order to give support to the superior parts of the body in flight, and a sufficient base for the lower extremities. The bones of the tail are broad and short, penetrated by the spinal cord except the last, which is compressed, supporting the tail feathers and an oil gland.
The skull is united into a single box at an early period; the brain cavity is very much larger than in the reptiles and fishes, and is occupied fully by the nervous centre; the brain of a sparrow is 100 times greater than that of a large marine turtle, in proportion to the size of the animals. The most striking characteristic of the class of birds is that the anterior and posterior extremities are entirely dissimilar in appearance and function, though the anatomical structure of the wings and legs is the same. The perfect condition of the chest indicates the energy of the respiratory system, and the consequent muscular activity. The ribs are strong, attached to the sternum in front by bony continuations instead of elastic cartilages, movable only at each end, articulated to the sides of the vertebrae in the flyers, but to the intervertebral cartilages in the runners; in those which require great solidity of the chest for powerful flight, each rib is strengthened by bony splints running obliquely upward and backward to the succeeding bone, to which it is attached by strong ligaments.
The sternum is provided with a keel or crest in front for the attachment of the muscles of the wings, large in proportion to the flying power; in the ostrich, which does not use its wings to rise in the air, this bone is flat as in man; in some aquatic birds the breast bone contains several convolutions of the windpipe within its cavity. The shoulder blade is parallel to the spine, long and narrow, at the glenoid cavity articulating with the clavicles and cora-coid bones. The coracoid bones, or posterior clavicles, are strong, extending from the glenoid cavity to a transverse groove in the anterior portion of the sternum; these are called the collar bones by some of the older authors. The true clavicles, the "merrythought" or "wishbone," vary considerably in size, being sometimes quite rudimentary, and in other species strong arches reaching the sternum. The wing consists of a humerus, radius, and ulna, carpus and metacarpus of two bones each, a' small single-jointed ulnar finger, a two-jointed radial finger, and a rudimentary thumb.
Prof. J. Wyman has shown in the anas acuta, or pin- ' tailed duck, an arrangement of the bones and ligaments by which all the segments of the extended wing are retained in a fixed position independently of muscular action, and the flexion or extension of the hand on the forearm when the latter is flexed or extended on the arm; during flexion and extension the radius advances and recedes upon the ulna, carrying with it the upper bone of the carpus, and with this last the hand; when the upper carpal bone, to which the lower is attached firmly, is drawn over the end of the ulna as the radius recedes, the lower is drawn up between the hand and the end of the ulna, maintaining the hand extended, like a wedge, until it is displaced by the reversed action of the radius. ("Proceedings of the Boston Society of Natural History," vol. v., p. 169.) In the lower extremity the iliac bones correspond to the shoulder, the thigh bone to the arm, the leg to the forearm, and the foot to the hand. Though one be covered with feathers and the other bare, the wings and legs present the same analogies that may be traced between the fore and hind legs in all the vertebrata. - Form. In the mammalia, or animals bringing forth living young and nourishing them with milk, there is a great variety of form.
Man, the horse, the whale, the seal, and the bat, notwithstanding their dissimilar external appearance, exhibit in their skeletons and internal structure homologies which show that they belong to the same class; the whale is nearer to man than to the fish, and the bat is more human than bird-like. The form of the skull varies much in mammals, according to the development of the brain cavity, and the proportions of the face. A direct relation is found to exist between the size of the cranium and face and the intelligence of an animal. In man the cranium, or brain cavity, is directly over the face; and in proportion as the former retreats behind the latter, and as the face is prolonged forward, the animal propensities preponderate over the intelligence. The differences between human crania in this respect early attracted the attention of anatomists, and Camper measured them by what is called the facial angle; this is formed between a line drawn horizontally from the opening of the ear to the upper teeth, and a line drawn vertically from the forehead to the same incisors.
In the white races this angle is from 85° to 80°, thence decreasing in the other races as low as 65°. In some of the antique statues this angle is made 90° and in one case 100°, which last never existed unless in disease. In children, the forehead is more prominent, than in adults, and their facial angle is usually 90°; this explains their generally pleasing countenances as well as the diminution of their innocent beauty as age advances. Those animals which have the lowest heads and the longest snouts are always considered the most stupid and gluttonous, as the hog among quadrupeds; as we descend to reptiles and fishes, it has been seen that the jaws constitute almost all the head, and these are known to be the most voracious of creatures, apparently living only to eat; on the contrary, a great degree of intelligence is attributed to the elephant from his well marked forehead, and the perpendicular-visaged owl is made the companion of the goddess of wisdom; though in the last instance these semblances do not depend on any greater development of brain, but are mere bony expansions.
Even among men, we instinctively regard him as stupid and sensual whose face is very prominent and whose forehead is receding; the advancement of the forehead toward the line of the face is always understood by artists as representing a noble and elevated character. As we recede from man the brain cavity diminishes, the jaws and nasal fossae are lengthened, the orbits are directed more externally and are less distinct from the temporal fossae; the occipital foramen and its two condyles gradually fall behind the middle of the base of the skull, and finally occupy its posterior face, so that the jaws, instead of being at right angles to the spine, become parallel to the axis of the body. The eight cranial bones of man may be recognized in the mammal skull, though they are variously subdivided in the different families, and in some are united together; the two parietals are united in the car-nivora, while the frontals remain separate, and the temporal tympanum is divided from the rest of the bone by a suture; in the elephant the frontals and parietals are early united with the other bones into a solid box.
Though the skull of the highest apes resembles that of man in shape, the bones are differently connected; the wing of the sphenoid does not reach the parietal and barely touches the frontal, and the temporal suture is serrated rather than squamous. From the position of the occipital foramen it is evident that the head of mammals is not balanced on the spine, but is suspended from the neck and back by the ligamentum nuchae. The bones of the face differ from the human in the greater number of pieces and in their horizontal extent. In man the upper jaw bones contain all the upper teeth; but in the lower animals the incisors are contained in an intermediate bone, the intermaxillary, a persistence of a separation which may be detected in the human foetus. The palate bones are small in the carnivora, and large in the rodents; the upper jaw is elongated in all. The peculiarities of the bones forming the orbits will be given under the heads of the different families. No animal but man has a chin; in all below him the anterior arch of the lower jaw is convex vertically and retreating at its lower margin; in the whale it resembles two immense ribs, united at the points, without any ascending branches, the articular surface being directed backward; in the hare and rabbit the coronoid process, to which the elevating muscles are attached, is small, but in the squirrel and rat it is large, and the condyle or articular process is compressed laterally and largest in front; in carnivora the condyle is transverse, and admits only of a hinge-like movement; in the ruminants the flat articular surface allows considerable lateral motion; in cetacea and edentata there is neither ascending ramus nor coronoid process, and the angle formed by the body and ramus is gradually reduced until it becomes on a line with the axis of the jaw; in carnivora, rodents, and ruminants, the two sides of the lower jaw are never firmly united at the symphysis.
Many mammals have the head surmounted with horns; that of the rhinoceros belongs to the skin, and is only an assemblage of closely united hairs, but the horns of ruminants have a bony axis springing from the frontal bone. The bony prominence in the giraffe is covered by the skin permanently; in the stags the bony core is at first under the skin, but soon becomes exposed, and after a certain time falls off, to give place to another similar growth; in the ox, sheep, goat, and antelope, the osseous axis grows during life, and never falls, being covered by a sheet of horn, growing by layers; these bony cores generally communicate with the frontal sinuses, and thus receive air into their interior. The species with falling horns have generally only the males thus armed; the reindeer, however, is an exception. The comparative anatomy of the brain has been sufficiently given under the title Beain. - Nervous System. The vertebrate nervous system is not homologous with the invertebrate; the spinal cord of the former is enclosed in a vertebral canal, and its vesicular substance is continuous throughout, while the ganglionic chain of the latter is always in the general cavity with the viscera, forms a ring through which the oesophagus passes, and its vesicular substance is frequently interrupted.
Though the vertebrate spinal cord cannot be considered as a chain of ganglia, it may be regarded as a series of segments arranged in a linear direction, having a distinct enlargement in many animals at the origin of the spinal nerves, and particularly of those sent to the extremities. The spinal cord of fishes terminates near the end of the spine. Owen says that in typical fishes it gradually tapers to a point in the heterocercal or unequal-tailed species, but swells again into a terminal ganglion in most equal-tailed species; in describing that of the angler (lophius), he has evidently fallen into an error in regard to its length, as shown by the researches of Prof. J. Wyman (see "Proceedings of the Boston Society of Natural History," vol. iv., pp. 150, 151); the latter found the cord extending as usual quite near to the tail, where it ended in a ganglion, presenting the striking peculiarity of being sheathed in a great part of its extent by an immense number of bundles of nervous fibres, of a vastly-greater bulk than the cord they surrounded.
Where the pectoral fins are much developed, as in the rays and flying fishes, the cord is enlarged at the origins of the nerves; in the torpedo and electrical eel, nerves are freely distributed upon the thin membranous laminae which constitute the electric apparatus, and act as conductors, if not as generators, of this force. The cord is composed of an external white or tubular portion, and an internal gray or vesicular matter, the reverse being the arrangement in the brain. In the amphioxus, the simplest vertebrate, the cord, with its nerves on each side, forms the whole nervous system; but in fishes generally the cerebral ganglia, with the nerves of special sense, constitute a distinct brain. The olfactory lobes, by some considered the representatives of the cerebral hemispheres in man, from which the nerves of smell arise, are well developed, and in the sharks are four instead of two, the usual number. The optic lobes, behind these, homologous with the tubercula quadrigemina, are larger than the other parts of the brain, and are in proportion to the development of the optic nerves which arise from them and the perfection of the sense of sight.
In the blind fish of the Mammoth cave of Kentucky (am-blyopsis spelmus), in which the eyes are rudimentary, there appears to be an exception to this law; though he could detect no optic nerves, Prof. Wyman found the optic lobes of good size, though less than in the allied fishes. Between the olfactory and optic lobes are the true cerebral hemispheres, largest in the sharks; behind the optic lobes is the cerebellum, which comparative anatomy shows to preside over the coordination of the movements, largest in the active sharks. In the perennibranchiate amphibia the brain and nervous system are very much like those of fishes; in the genera which undergo metamorphosis the changes from the fish-like to the reptilian brain are rapid and remarkable, the hemispheres becoming enlarged and the spinal cord shorter as the tail disappears. In frogs, there are found attached to each spinal nerve, before the division into sensory and motor roots, vesicles containing a white, chalky, crystalline substance; from their constant presence, they are considered essential parts of the nervous system; nervous filaments have been traced into the interior of these vesicles; the chalky matter resembles that found in the vestibule of the ear.
In true reptiles the brain is more developed and nearly fills the cranial cavity; the hemispheres are increased in size, and the cerebellum is large in proportion to the activity and complexity of the movements; the nerves are large compared with the centres, and the sympathetic system is more closely connected with the blood vessels. In birds the parts of the brain are no longer on the same plane, are larger, more complicated, indicating more intelligence and more activity.
In the spinal cord there is an enlargement where the nerves are given off to the wings, and another where those of the legs take their origin; that these are not proportional to muscular force, but rather to sensibility, is shown by the fact that the latter is the larger; though the wings are more muscular, they are, from their feather covering, less sensitive to external impressions than the legs; this is more clearly shown in the mammalian bat, where the enlargement corresponding to the exceedingly sensitive wings is by far the greatest. In mammals the cerebral and cerebellar hemispheres reach their highest development, the former gradually covering over the latter, and the convolutions becoming more numerous and complicated up to man; the commissures more intimately connect their different portions; the spinal cord is larger in proportion to the size of the body, but smaller when compared with the brain; and the sympathetic system is more extensively distributed. - Organs of Special Sense. By means of the organs of special sense, placed at the extremities of the cerebro-spinal nerves, and generally near the entrance of the alimentary canal, animals are brought into relation with external objects; an impression communicated to the outer surface is transmitted to the sen-sorium along the sensory fibres of nerves, and there causes the phenomena of smell, vision, taste, hearing, or general sensation, with their resultant motor acts from the transmission of the nervous influence along the motor fibres; in all the vertebrates there are organs set apart for this purpose, the nose, the eyes, the ears, the tongue, and the skin.
The sense of touch, residing in the skin, is the most universally diffused, and is capable of answering most practical purposes of the other special organs; and indeed the senses of taste and hearing seem to be little else than modifications of the sense of touch; the senses of smell and vision depend upon the influence of such delicate changes in the surrounding air that we can hardly comprehend them except through their effects. The skin consists essentially of two layers, the cuticle and the true skin; the surface of the latter is the seat of sensibility, and is provided with numerous papillae into which the nervous loops enter; the cuticle is made up of nucleated cells, becoming dry externally and falling off in the form of thin flakes. In fishes the body is generally covered with scales, not fit for receiving tactile impressions; some of the siluroids, as the horn-pout, have fleshy barbels attached to the lips, into which nervous filaments may be traced; their skin is lubricated with a viscid mucus poured out through numerous tubes. In the amphibia the skin is soft and yielding, and quite sensitive; it is well known to be in this class an important accessory organ of respiration. In serpents and lizards the tongue and the tail are the principal organs of touch.
In birds the tactile organs are the bill, the cere, and various appendages in the shape of combs, wattles, bristly hairs, and caruncles; in the swimmers and waders the bills are very sensitive; in the snipe and woodcock, for example, the fifth pair of nerves is freely distributed to the long bill, making it an exceedingly delicate organ for the explorations necessary in their search for food. In mammals, most of which are covered with hair or thick hides, the sense of touch is most acute in the neighborhood of the mouth; the lips of the horse are very sensitive; those of the carnivora and rodents are provided with long whiskers, into which nerves enter, forming exquisite organs of touch; the complicated appendages to the nose of bats, and also their delicate wings, are very sensitive tactile organs. With the exception of the tongue, which is sensitive in all, there is no approach to the perfect development of touch in the human subject until we reach the quad-rumana, in which the hand is the chief tactile organ.
The fore foot of the ruminant, ending in a hoof, serves only for support and locomotion; in the feline tribes it is in addition prehensile, and to a certain extent tactile; in the monkey the palmar cuticle is thin and sensitive, though the hand is still used in locomotion; in man the sense of touch is very acute in the tips of the fingers, though all parts of the skin are more or less sensitive. The sense of touch may be educated to a degree almost substitutive for the sense of sight; what the bat's wing is naturally, the blind man's fingers may become by education. The sense of taste, even in man, must be considered as principally a modification of the sense of touch; as it is impossible to draw the line between the mucous membrane and the skin, which are also composed of the same elements, and as the sensory branches of the same fifth nerve are distributed to the face, lips, mouth, and tongue, it is naturally inferred that a great part of what is called taste is really touch, applied to a special locality for special purposes, as in the wing of the bat and the tips of the fingers; and some are of opinion that the sepse of smell supplies all of taste which is not derived from touch.
The papillas of the tongue are more highly developed than those of the skin, and are scattered over its surface in all parts likely to come in contact with matters in a state of solution. The tongue of fishes can rarely be an organ of taste, being of a hard consistence, and frequently armed with teeth; their voracity and the element in which they live are also against their possessing in the tongue anything more than an organ of touch. In all the vertebrates the tongue answers for other purposes than that of taste; in the toad it is darted with great quickness and usually unerring aim against the insects upon which it feeds; in the chameleon it is capable of considerable protrusion, enlarged at the end, and covered with a glutinous secretion which entraps its insect prey when projected against them. In birds it is chiefly an organ of prehension, rarely possessing papillae, and generally sheathed in front with horn; the tongue of the woodpecker is a kind of barbed spear, protruded by a muscle and long bony tendon from the top of the head, for the transfixion of grubs and insects.
In mammals generally the tongue is only an organ of taste; but in the giraffe it forms a long flexible process, which can take up small bodies almost like a hand, ahd by which the leaves of high trees are stripped off into the mouth; the tongue of the ant-eater is covered with a viscid secretion, to which the ants adhere in great numbers; in the cats it is beset with strong spines for purposes of prehension and of tearing the small fibres of the flesh on which they feed; in man it is also of the first importance in the articulation of words. - The sense of smell doubtless exists in many invertebrates, at the entrance of the respiratory passages. In fishes the organ of smell consists of a sac containing a folded membrane largely supplied with nerves, and not communicating with the mouth or pharynx; that this sense is acute in the cod and other deep swimmers is probable from the fact, well known to fishermen, that these may be attracted from great distances by peculiar kinds of bait, while the top-water and mid-water species select their food by the sight, also often showing what seems to be a decided sense of taste; the sharks, from their large olfactory nervous centres, are believed to have an acute sense of smell.
In the higher amphibia the nostrils become reptilian, being partly osseous and opening into the anterior part of the mouth; in saurians and chelonians the nostrils open posteriorly into the pharynx. In birds the external nostrils vary much in size, shape, and situation, but are generally freely open; their sense of smell is not so acute as had been supposed from some of the habits of the vultures, as Audubon has satisfactorily shown that these birds detect their carrion prey from great heights by the sense of sight and not by that of smell. In mammals the olfactory lobes are much more fully developed than in man, as also are the nasal cavities in which the sensitive membrane is spread out, and the external nose; the nostrils are valvular in the cetaceans, beavers, seals, etc.; in the hog the nose is enlarged into a gristly ring, in the tapir into a short proboscis, and in the elephant into a long prehensile trunk. The sense of smell is most acute in the carnivora and ruminants, as would be expected from the extent and convolutions of the turbinated bones. From the communication of the nose with the pharynx, mammals are able to breathe with the mouth shut. - An organ of hearing may be detected in many invertebrates, as in the lobster, insects, and cephalopods.
Its simplest form in the vertebrates is in fishes, where it is a sac filled with fluid, in which are distributed the nervous filaments on small particles of calcareous matter; in connection with this sac are two or three semicircular canals opening into its cavity; the auditory nerve arises from the side of the spinal bulb, without any distinct ganglion; in such an ear as this the vibrations of the water are communicated to the skin, then to the fluid, and finally to the nerves. Sonorous impressions are conveyed with greater intensity in liquids than in air, so that the sense of hearing in fishes may be tolerably acute. In the sharks the semicircular canals are quite large, and the vestibule of the carcharias ob-scurus would contain the whole internal human ear; of course in such an organ there is no tympanic cavity, and no external ear. In the aquatic amphibia the organ of hearing is like that of fishes; but in the frog and salamander there is a membrane of the tympanum. In the reptiles there is added a cochlea with its own nervous filaments, the tympanic cavity is larger, and a bone, the columella, makes a communication between the vibrating membrane and the fluid of the auditory sac; a Eustachian tube communicates also with the throat, and the tympanum is either bare on the level of the skin or just underneath it.
In birds there is no external cartilaginous auricle, as in mammals, and only a portion of the external auditory canal; but in many, especially in the rapacious families, the feathers are erectile around the meatus, and arranged to catch distant sounds; the bone-surrounded tympanum has also its columella, and its cavity communicates with the fauces by a Eustachian tube, and with the air cells of the skull; the cochlea is more developed than in reptiles, but, like the rest of the internal ear, does not reach the perfection of the mammalian type. In the owl there is a crescentic valvular fold of integument around the external ear. In mammals there is a perfect cochlea, a chain of three tympanic bones, an external canal, and an external movable ear; in cetaceans and other aquatic families, the external ear is either wanting, very small, or provided with a valve; in ruminants and the timid rodents it is large and directed backward; in the carnivora it is small and inclined forward. The use of the external ear in man is not exactly determined; its small size and direction would make it of but little use in collecting sounds and transmitting them to the tympanum.
The experiments of Savart go to show that it acts not only in reflecting sounds, but as a conductor, by virtue of the elasticity of its cartilage. In general the ear of mammals resembles that of man. In all animals living in air the vibrations of sonorous bodies are transmitted to a tense membrane, the tympanum or drum; the tympanic vibrations are transmitted to the fluid of the internal ear, in which the nerve floats, by the chain of bones; and in order that the membrane may vibrate freely, the cavity of the tympanum communicates with the throat, the tension being equal on both sides. - The most complicated of the organs of special sense is the eye. In all vertebrates the eyes are two in number, and with few exceptions symmetrically arranged on the sides of the head; they are essentially on the same general plan, the differences being chiefly in relation to the density of the medium in which the animals reside. Aquatic animals, whether fishes, reptiles, birds, or mammals, have the lens nearly spherical to compensate for the similarity of the densities of the humors of the eye and the surrounding water.
In fishes the eyes are generally large and on the sides of the head, though they are small in the eel, directed upward in uranoscopus, and both on one side in the flounder; the cornea flattened, the sclerotic thick and sometimes partly ossified, the pupil large and round, the lids rudimentary, and the lachrymal gland wanting. The blind fish (amblyopsis) has been found to possess a sclerotic and choroid coat, a layer of cells beneath the latter resembling a retina, a rudimentary lens, and a nerve; still there is no trace of eye dots on the skin, though such were found in the mass of areolar tissue occupying the usual situation of the orbit; such an eye cannot be regarded as an organ of vision, as the skin and tissues beneath prevent the passage of light except in a very faint degree; but such as it is, it corresponds to the vertebrate, and not to the invertebrate eye, with the last of which it has been sometimes compared; the deficiency of vision in this species is made up by the largely developed organ of hearing, and by the remarkably sensitive papillae on the head supplied by the fifth pair of nerves.
In the aquatic amphibia the eyes are like those of fishes; in the higher genera, and in reptiles, except ophidians, there are lids moving vertically, a lachrymal apparatus, and a movable iris; in the snakes there are no lids, but the skin passes directly over the cornea, as in the eel. In birds the eyes are always well developed; from the anterior convexity and lateral location, their sphere of vision is very extensive; the retina is quite thick, and apparently gives origin to a fan-shaped dark membrane, the pecten plica-turn, which extends from the entrance of the optic nerve toward the lens; from its being composed chiefly of vessels, some think it a process of the choroid, and accordingly its use may be either to absorb superabundant rays of light to extend the visual surface, or perhaps, as Owen has suggested, to push forward the lens by its erectile property. The pupil is round, the iris very contractile, the cornea large and transparent, and the sclerotic strengthened in front by a series of bony plates; there are two horizontal lids, the lower largest and most movable, and a third vertical nictitating membrane, semi-transparent, which may be drawn over the cornea from the internal angle; lachrymal glands are also present.
There is some apparatus in the eyes of birds, either the pecten or the muscles, by which these organs can adapt themselves to the very different distances at which it is necessary for them to have distinct sight, by which the curvature of the lens and the focus of vision may be changed. In mammals which seek their food by night, the eyes, like the ears, are proportionally larger than in the day feeders, and their pupil, when contracted, assumes the form of a vertical slit instead of a circular aperture; in the moles and subterranean species the eyes are extremely small, and sometimes quite rudimentary; in the aquatic genera the lens is more spherical, as in fishes, and in cetacea the lids are imperfectly developed and the whole organ comparatively small; in carnivora especially there is at the bottom of the eye a brilliant tapetum, which shines at night with metallic reflections; the lids are generally formed as in man, the upper being the largest and the most movable; except in man and monkeys, there is usually a third nictitating membrane.
The direction of the eye in man and monkeys is forward; but as we descend in the scale it becomes lateral, so that the animal cannot see directly before him, and the sphere of vision becomes different for each eye. - Organs of Alimentation and Digestion. There is no organ so characteristic of the animal, as distinguished from the vegetable, as an internal digestive cavity for the conversion of organic substances into nutritive material. In the sac-like polyps the food is introduced into the simple stomach, and dissolved without any mechanical division; in the echinoderms there is a complicated apparatus of teeth, and the digestive cavity is arranged in a radiating manner; in the higher invertebrates and all the vertebrates there is a distinct mouth, an apparatus for mastication, a stomach for digestion, and an intestine from which the nutrient matters are absorbed and the useless materials are expelled. Accessory salivary, biliary, and pancreatic organs are found from the higher radiates up to man; in vertebrates the teeth are confined to the cavity of the mouth, and generally to the jaws, none being found in the stomach.
Fish are very voracious, and most of them live upon animal food, swallowing whatever small prey they can obtain; a few have no teeth, but they are usually well provided with them, the sharks having several rows; the teeth are found not only in the jaws, but on the palate and vomer, the tongue, the branchial arches, and pharyngeal bones; they are numerous, without roots, united to the bone which supports them, deciduous and replaced by others growing under or by the side of the old ones, and thinly covered with enamel; the form is generally conical, as they serve only to retain or tear their food, rarely to crush and grind it; they are numerous and sharp in the pike and salmon, serrated in some sharks, fiat and pavement-like in the rays, strong fangs in the wolf-fish, in others soft and velvety, tuberculated, or sharp-edged, and absent in the sturgeon and the sucking genera: to fit them better for their prehensile office, they are placed alternately, and not opposed to each other as in masticating animals.
The salivary glands are absent, or very rudimentary; the gullet is short and wide, with its membrane folded longitudinally; the stomach varies in shape from globular to long and tapering, with both orifices near together and guarded by constrictor muscles; the intestine is short as in all carnivorous feeders, and not divided distinctly into large and small; to compensate for its shortness, the intestine in the sharks is provided with folds arranged in a spiral or longitudinal direction, which delay the passage of the food and greatly increase the absorbing surface; the anus varies in position from under the throat to the base of the tail. The liver is soft, light-colored, of large size, and many-lobed, discharging its bile into the commencement of the intestine, while the pancreas pours in its secretion on the other side; the latter organ is a large gland in the shark, but it is more commonly a series of tubes or csecal appendages, the simplest form of a gland, placed around the pylorus; digestion is rapidly performed, and the chyle is taken up by numerous lacteals which end in the venous system near the heart. The spleen is small, of various forms, attached to the stomach, generally simple, but lobulated in the sharks.
The amphibia resemble fishes in their digestive apparatus, in their prehensile teeth in the palate and jaws, in the absence or rudimentary condition of the salivary glands, in the short and wide gullet, narrow stomach, and short and simple intestine; in some of the higher forms they approach the reptiles, in the less numerous teeth, elongated tongue, and distinct small and large intestine. In snakes and saurians, mostly carnivorous, the intestinal canal is shorter than in the herbivorous testu-dinata. In serpents, which feed on living prey, the sharp conical teeth are directed backward, and the bones to which they are attached are freely movable, enabling them to swallow animals considerably larger than themselves; the venomous genera have in front of the teeth of the upper jaw two long curved fangs, communicating by a canal or a groove with the poison gland behind and below the orbit; the muscles which close the jaws press the venom into the wound made by the teeth; in the rattlesnake these fangs are movable, and may be bent backward in a fold of the gum when not in use; behind the ones actually employed, there are the rudiments of others which soon complete the terrible armature if one fang happens to get broken.
The tongue is long, sheathed, and bifurcated; salivary glands are present; the gullet is long and very extensible; the stomach capacious, simple, and capable of great distention, separated from the intestine by a distinct valve; the duodenum receives the biliary and pancreatic secretions, and begins to present a villous surface; the large intestine is distinguishable from the small by its size, and ends in the cloaca with the ureters and genital openings; the liver, spleen, and pancreas are elongated to conform to the shape of the body. In the carnivorous saurians the arrangement is equally simple, though the teeth are fewer, chiefly in the jaws, the stomach short and rounder; in the iguana and other vegetable feeders, the intestine is the longest. In testu-dinata, instead of teeth the jaws are armed with sharp edges of horn; the tongue and gullet are provided with long papillae, sharp in the marine species; the salivary glands are tolerably developed; the gullet is long, wide, and muscular, and the stomach wide and fleshy; the intestine is about six times the length of the body, and the colon has a short, wide coacum; the canal opens into a general cloaca.
The food of birds is so various that their digestive apparatus would be expected to present considerable differences, and in no part is there greater variety than in the bill, which in most species is the principal organ of prehension, whether the food be seeds, insects, fish, or the flesh of animals. The bill furnishes to the zoologist as good characters for the classification of birds as do the teeth for that of mammals; its exterior and the sharp edges are covered with solid horn, but it never has any true teeth, so that there is no proper mastication in this class; the birds of prey have the upper mandible short, strong, curved, and terminating in a sharp point, and in the falcons with a tooth-like process on each side, indicating by these characters the more or less carnivorous propensities of the genus; the tooth-billed hawks are the boldest, while the vultures, with their more elongated beaks, rarely attack living animals; in the parrots and granivorous birds it is broad and powerful to break hard shells and seeds; in aquatic genera it is broad for obtaining worms and insects from water and mud; in insect-eaters it is long and slender, or short and broad, according as the prey is taken on the wing or not, as in the bee-eater in the first case, and the whippoorwill in the second; it is long and straight in the kingfisher and heron for seizing small fish or reptiles; in the pelican the under mandible is provided with a large pouch for holding fish, and in the hornbill the upper is surmounted by a large and hollow casque.
As the food does not undergo mastication in the mouth, the salivary glands are small; the gullet is wide and muscular, and capable of great distention in birds of prey; at the lower part of the neck it communicates with a membranous pouch, called the crop, in which the food undergoes a softening preparatory to stomachal digestion; the crop is largest in the granivorous birds, but it is found in the rapacious orders, though absent in the ostrich and the fish-eaters. Below this the gullet becomes smaller, but shortly dilates again into a second digestive cavity, or proven-triculus, whose internal surface is studded with numerous follicles, generally of small size, sometimes hardly perceptible, but large in birds which have no crop; it secretes a fluid analogous to the gastric juice. This second stomach opens into a third, the gizzard, where chy-mification is completed, of variable size and structure; in carnivorous birds the gizzard is thin and membranous, while in the granivorous it is thick and muscular for compressing and crushing their hard food, performing the office of teeth; the lining membrane assumes a hard cartilaginous character, just as the skin of the palm and heel of man does; when circumstances favor or require it, mucous membrane may thus change into skin, as far as its dense cuticular layer is concerned; the power of the gizzard in the ostrich is enormous, wearing down the hardest substances; in gallinaceous birds its grinding action is assisted by the swallowing of pebbles, which serve the purpose of the gastric teeth of crustaceans and other invertebrates; the food of a bird may be known by the simple inspection of its gizzard, so close is the relation between its muscular power and the substances to be reduced by it.
The intestine is much shorter than in most mammals, and consists of small and large, the latter having two tubular appendages, or cceca, at its junction with the former; these cseca are very small in the birds of prey, and largest in the gallinaceous order; the rectum is dilated near the anus, forming a cloaca, in which the ureters, oviducts, and male genitals end. The liver is large, filling a considerable part of the thorax and abdomen, usually two-lobed, with a gall bladder and hepatic ducts; the pancreas is long and narrow, lodged in the first convolution of the small intestine; the spleen is small, variously shaped, and situated, beneath the liver. - The teeth are most perfectly developed in mammals, in which they serve not only for mastication, but for defence, attack, and locomotion. The structure of teeth presents three different substances: a central portion, forming the principal part of the bulk, characterized by minute canals radiating from the pulp cavity, the ivory or dentine; the enamel, investing the exterior and crown with a thin layer of extreme hardness; and the cementum, or crusta petrosa, covering the roots and sometimes around the crown with a thin lamina like bone.
These three substances are well seen in the grinder of an elephant, in which the central part is made up of ivory, in a series of ridges covered with enamel, and this last, except on the surface, concealed by crusta petrosa; the hard enamel leaves a projecting grinding surface, while the other two substances are worn away. The teeth in vertebrates are shed at least once, and in some many times; those of fishes and reptiles are continually undergoing this process; in most fishes and snakes the teeth shed have between them others which remain attached to the jaw or the soft parts, until new ones are formed, to be shed in their turn; in some fishes, the crocodile, and most mammals, the new teeth are developed below the old, which they push out, after the roots are absorbed and the crown drops off"; in the elephant and mastodon the new teeth are formed behind the old ones, gradually sliding forward as the latter are worn away. In man and in most mammals there are three kinds of teeth: the incisors, in front, with thin and cutting edges; the canines, conical, next to the incisors, four in number, in all animals except man longer than the other teeth, destined to tear and to cnt; arid the molars or grinders, with a wide and irregular surface, for crushing and bruising the food.
These different kinds of teeth are arranged in mammals according to the nature of their intended nourishment; and a simple inspection of the dentition indicates the kind of food, the habits, and even the structure of most animals. In the carnivora, or flesh-eaters, the canines are long, and the molars are compressed and sharp-edged, shutting like the blades of scissors; in the insect-eaters the molars are beset with conical points, meeting accurately in each other's interstices; in the fruit-eaters the surface of the molars is provided with rounded tubercles; in those which feed on hard grains, the grinding surface is flat and rough like a millstone; in the gnawing animals the incisors are greatly developed, and the enamel is so arranged that the wearing of the tooth keeps its edge sharp, and chisel-shaped. Of these different forms of teeth the molars are the most constant and important, bearing the strictest relation to the habits of the animal. Between the carnivora and the herbivora, the flesh-eaters and the vegetable-feeders, stands man: he has all the kinds of teeth equally developed, of the same length, and in an uninterrupted series, and of course his natural food is a mixture of animal and vegetable substances; placed in unnatural circumstances and in extremes of heat and cold, the animal or the vegetable element must predominate for the preservation of health, according to latitude.
Teeth are sometimes used wholly as weapons of attack or defence, as the long incisors or tusks of the mastodon and elephant; or as the single incisor of the narwhal, developed in the male, generally on the left side, into a long spirally twisted horn, that of the right side being rudimentary and concealed in the jaw, and both being rudimentary in the female. The tusks of the walrus serve also in locomotion, for pulling its unwieldy body up steep banks or blocks of ice. The teeth are wanting in the ant-eaters and pangolins; in the Greenland whale they are replaced by large, flexible, horny plates, called whalebone, in the upper jaw, the lower one having neither teeth nor plates; the upper incisors are wanting in ruminants, the lower in the walrus; the canines are absent in rodents, some ruminants, and most female soli-peds; in the ornithorhynchus of New Holland the muzzle is prolonged into a wide, horny beak, flattened like that of a duck, and like it furnished with transverse lamellae on the edges. The salivary glands are largest in herbivora. The gullet is wide and dilatable in carnivora, narrow in herbivora, and its fibres are arranged generally very much as in man. - The shape of the stomach varies much according to the food; it is usually simple, but multiple in the ruminants.
The animal food of the carnivora requires a simple stomach and a short alimentary canal; in the quadrumana and less carnivorous families, where the food is more mixed, the organ becomes larger transversely; in the lion the intestine is not more than three times the length of the body, and in some others of the family it is difficult to distinguish the large from the small. In the ruminants the stomach consists of four distinct cavities: the first, or paunch, receives the crude, unmas-ticated food while the animal is grazing; this occupies a large part of the abdomen, especially on the left side; the second, or honeycomb, is small, on the right of the gullet and in front of the paunch, of which it seems a mere appendage; its mucous membrane presents the polygonal cellular, structure noticed in tripe; the third, from its numerous folds, is called the manyplies; and the fourth, at the right of the last, the abomasum or rennet bag; this secretes the gastric juice, and is the proper digestive cavity. The first three stomachs communicate directly with the gullet, which opens almost equally into the first and second, and by a narrower canal into the third.
It may appear strange that while the coarse food enters the first cavity, after it has been brought back to the mouth and masticated it should enter the third; the experiments of Flourens show that this is the necessary result of the anatomical disposition of the parts, as follows: the unchewed food, when it arrives at the part of the gullet which is continued in the form of a tube, mechanically separates its borders and falls into the paunch; but when drinks or fine semi-fluid substances present themselves, the canal is not dilated, and they naturally pass chiefly into the third cavity; it is consequently the occlusion or dilatation of this canal which determines the entrance of the food into the first or third stomach, and it is the volume of the substances swallowed which opens or not this canal. The act of regurgitation from the paunch to the mouth is supposed by some to commence in the second cavity, which seizes a portion of the alimentary mass, compresses it into a rounded form, and forces it into the gullet, by whose vermicular contractions it reaches the mouth; others believe, with Flourens, that the two stomachs force the mass into the oesophageal canal, which detaches a portion and forces it upward.
Of these cavities the first is by far the largest, and the third the smallest; in early life the fourth is the only one developed for the reception of milk. In the camel and dromedary the paunch is fitted to receive large amounts of water in one of its compartments, the first answering to a paunch, and the second, the water cavity, to the honeycomb. As a general rule we find the stomach most complicated in herbivora, and the simplest in carnivora; yet there are many exceptions to this; for instance, in the horse, whose food differs but little from that of the ox, the stomach is simple; in some carnivorous cetaceans it is very complicated. The intestines are generally long, large, and sacculated in proportion to the vegetable nature of the food, but to this also there are remarkable exceptions; in frugiv-orous mice they are only three times the length of the body, while in the carnivorous seal they are twenty-eight times, the anomaly being probably explicable by the greater development of the caecum in the former.
In the elephant the total length is about 60 feet, in the camel 130, in man from 25 to 30; the intestine is 10 times the length of the body in the horse, and 28 times in the sheep; in an ornithorhyn-chus 17 1/2 inches long, the intestinal canal was 5 2/3 feet, ending in a cloaca as in birds. The liver is large in cetacea, smaller in herbivora, and least in carnivora. There seems to be no general law for the presence or absence of the gall bladder; it is said that all the mammalia in which it is absent, except the porpoise, are vegetable-feeders. The spleen, pancreas, peritoneum, and other appendages of the digestive cavity, are much like those of man, from which they differ principally in shape; the biliary and pancreatic fluids are received into the duodenum. Absorption is effected in the lowest animals by simple membranous surfaces, without the aid of vessels or tubes; the latter are gradually added, and in man and the higher animals like cylindrical processes in immense numbers are developed in the intestinal mucous membrane, the villi, by which the nutrient materials are absorbed and conveyed into the circulation.
In all vertebrates there are special vessels, the lacteals, in the coats of the intestine, for the absorption of chyle, which convey it to the thoracic duct, from which it is poured into the venous blood near the heart; in fishes they are destitute of complete valves, which exist in reptiles, and in both these classes their convoluted arrangement supplies the place of glands; the fluid contained is limpid in the fish and milky in the reptile; in birds, glands appear in connection with the lymphatics, but not with the lacteals, and the valves are more numerous and distinct; in mammals the chyle assumes more of the characters of blood, the valves are increased in number, the glands are numerous in the mesentery, and the thoracic duct becomes a distinct vessel, occasionally double. - External Covering. The skin of fishes is generally covered with scales, varying from rough grains to large flattened tubercles or thick plates, and thin lamella overlapping each other like the tiles on a roof; they are held by folds of the skin, very much like the nails, which they resemble except in being more calcareous. Carpenter considers the scales of fishes as developed in the dermis, and those of reptiles as mere epidermic appendages, like nails, hoofs, feathers, and hairs.
They are ornamented with the most beautiful and varied colors, presenting all the metallic reflections. The most recent chemical researches have established that fish scales have the constitution and many of the peculiarities of structure and growth of bone; the arrangement of the la-cunae and their forms are of importance in studying the resemblances of allied fishes; they are composed chiefly of phosphate of lime, and contain a little magnesia and fluorine. The order of development of the scales in osseous fishes is not well ascertained; bat in young garpikes (lepidosteus), according to Agassiz, a row of scales is first formed along the middle line of the body; as the age advances, other rows appear above and below the median line, and the scales are crowded together and of a rhomboidal form toward the tail; the same is true of the sturgeon and other ganoid fishes. From the study of fossil species, Agassiz was led to recognize and to classify fishes by the structure of the scales; he found that all living species resembling the ancient types had scales of a peculiar structure.
A common scale is composed of successive layers of horny or bony matter, the oldest layer being the lowest; over this bony layer in fossil fishes is a covering of enamel-like substance as hard as that of the teeth; by this character the affinities of many modern species have been determined. The sharks and rays have scales which consist only of enamel, forming rough points, known in the former as shagreen; these he called placoids. Finding singular coincidences between the structure of scales and the general form and internal organization, he united the sharks and rays into one order, characterized by a cartilaginous skeleton, by a separation of the vertebral column from any upper or lower appendages, by teeth without sockets, attached to the jaw by skin and movable, and by the gills being covered only by strips of skin, forming as many openings on the sides or undor , part of the head as there are gills. The fishes whose scales are covered by enamel he called ganoids, embracing among living species the sturgeon and the garpike of North America, and among fossils some of the most remarkable forms, well described in the works of Hugh Miller; the scales in this order are extremely hard, and more or less smooth, and with the reptilian character of vertebras united by ball-and-socket joint.
These two orders were numerous at the remote geological epoch before reptiles, and consisted of many genera, with species of great size; the types of these ancient fishes are now reduced to a very few genera. These first created vertebrates he considers, in his "Essay on Classification," as classes of the animal kingdom distinct from fishes proper. In ordinary bony fishes he found two types of scales: in one they consisted of simple layers with regular outlines, and in the other the edges were serrated posteriorly, the serrations becoming more numerous and the surface more rough as the scales increased in size; the former he called cycloids, the latter ctenoids. Ctenoids have bony spines in the anterior portion of the dorsal fin, and serrations or spines in the opercular bones; this order includes the perch family, and others with spiny dorsals and rough scales; the flat fishes (pleuronectes), characterized by their want of symmetry on the two sides, are an exception in having rough scales with soft fin rays.
Cycloids, embracing fishes with smooth scales, like the cod, herring, trout, and eel, are the most numerous types in the present epoch; the rays are soft, and the bones of the head are smooth and simple; the mackerel has hard rays, and both serrated and smooth scales, and seems intermediate between cycloids and ctenoids. It appears, therefore, that this division into four orders according to scales is not perfect, and it is not now much insisted upon by its author, except for the first three. So intimate is the relation between the scales of fishes and their general organization, that Agassiz was enabled to restore a fossil fish from isolated scales, as is shown in his work on "Fossil Fishes;" in like manner Cuvier repeatedly restored fossil genera of mammals, giving the entire skeleton and outline of the form, from single bones found in the gypsum near Paris. In testudinata, the corneous integument is applied directly to the bony box which encloses their soft parts; the epidermis is covered with large scales, adherent by all their lower surface, except in the species which produces the tortoise shell of commerce, in which they overlap each other; these plates grow by all their adherent surface and at their circumference, as indicated by the concentric lines of increase, and become larger in proportion to the body; on the neck, tail, and limbs the epidermis is like that of mammals, except in being thicker and rougher.
The scales of other reptiles are implanted on the dermis, and vary from horny in most genera to bony in the crocodile; contiguous by the borders on the head, and generally imbricated on other parts of the body; of various shapes and sizes, and arranged in serpents in long bands, moved by muscles, serving the purpose of limbs by their contact with the ground at their free posterior edges; the epidermic covers even the scales, and at certain seasons of the year is shed; in serpents the change is so complete that the cast skin comes off in a single piece, including even the covering of the eyes; the coloring matter, often of the most brilliant hues, is placed immediately under the epidermis. In batrachians, the skin has no corneous appendage, except the nail-like processes of the limbs in a few species. - The feathers of birds are analogous to the hairs of mammals, but more complicated in structure; they are believed to be not the simple product of a secretion, as is frequently maintained, but developed from bulbs formed from cells and supplied with vessels; when the feather is formed the vessels disappear, and it gradually becomes dry and dead from the summit to the base, and finally is not susceptible of further living changes, resembling in this respect the horns of the stag.
Each principal feather may be moved by means of the greatly developed cutaneous muscles, which send slips to them. A feather usually presents at its lower extremity a corneous tube open at the end, continuous with the shaft, which is webbed on each side, with fringed barbules. According to F. Cuvier, the capsule which forms the feather grows during the whole development of the latter;the bulb, after it has fulfilled its office, forms in drying a series of membranous cones in the tube, generally called the pith. The new feather is at first covered by the investing capsule, which often extends several inches from the skin; it gradually becomes free, and the barbs, at first rolled up, spread laterally; the end of the tube is implanted lightly in the skin, and at every moulting season is displaced for a new feather; moulting takes place generally every year after the season of incubation, and sometimes twice a year, and is a period during which the bird loses its voice, and appears more or less unwell.
Some feathers resemble the quills of the porcupine, as four or five of those in the wing of the cassowary; in the eagle the barbs are stiff and united by hooked barbules into a broad lamina for retaining better hold on the air; in the ostrich the plumes of the tail and wings are of great softness and lightness, and in the marabout the feathers resemble the softest down; in the turkey's breast they are transformed into bristly hairs, and in some cuckoos into corneous plates at the ends. It is unnecessary to more than allude to the magnificent and varied colors of these appendages, which are usually the finest in the males and in adults. - In a few mammals the skin is naked, but in the greatest number it is protected by hairs, characteristic of this class. Hairs are produced, like feathers, from cutaneous follicles or capsules lined with a cell membrane, and containing at the bottom a conical bulb supplied with blood, the soft interior constituting the pulp. Various as are the forms of hairs, they consist essentially of an external corneous cortical substance, and a medullary matter in the interior; the quills of the porcupine are only magnified and modified hairs, their cortical substance being very dense, and the medullary matter a pithy aggregation of very large cells, without any evident fluid portion in the perfect quill; so in birds, the cortical substance is found alone in the quill, while the cellular pith is confined to the shaft; in some animals the cortical substance is strongly imbricated, and the medulla made up of rounded cells.
Pigmentary matter is developed in the central portion. Hair is, therefore, the product of epidermic cells, developed in abundance in the follicles, and it grows by the addition of new matter at the base. Hairs may become spines, bristles, wool, or down, according to softness and fineness; their color varies much, though less than that of feathers, being generally some modification of white, black, reddish brown, or yellowish. Recent observations seem to connect the supra-renal capsules with the regulation of the amount of pigmentary matter in man and animals. Hairs, like feathers, are usually shed once a year, with or without change of color and character; in winter the fur is finer and thicker than in summer, being intermixed with more downy fibres. Hairs sometimes are so closely approximated as to form horns or solid plates; the horn on the nose of the rhinoceros is made up of firmly united hairs; the shields of the pangolins seem to be the product of hairs intimately consolidated, and the latter are seen projecting from the former in various parts of the body.
The whiskers on the nose of carnivorous and rodent animals have the bulb projecting far into them, and are freely supplied with vessels and nerves, forming very sensitive organs of touch. - In this imperfect sketch of comparative anatomy, it must be evident to the most superficial observer that there is a general plan of structure in the animal kingdom, varying in its details, but always pointing to man as the head of creation, as the most perfect of the inhabitants of the earth. The contemplation of this vast picture of animal life is most exalting and ennobling to the devout student of nature; following the endless varieties and marvellous adaptations of created types, the mind at last must rest on the infinite wisdom and power and goodness of the supreme Architect of the universe. Those who wish to pursue this vast subject in its details are referred to the writings of Cuvier, John Hunter, Home, Oarus, Miiller, Meckel, Bell, Oken, Owen, Grant, De Blainville, Saint-Hilaire, Carpenter, Siebold and Stannius, Flourens, Strauss-Durckheim, and the various articles in Todd's " Cyclopaedia of Anatomy and Physiology." - History of Comparative Anatomy. Though the philosophers of Greece had some idea of the internal structure of the animals offered as sacrifices to their gods, it was not until the time of Aristotle, or the 4th century before the Christian era, that we find any scientific treatise on comparative anatomy; the first chapter of his "History of Animals," though very imperfect, and even erroneous, in anatomical details, may be regarded as the first work written on this science.
He was an exact observer, a patient collector of facts, and studied successfully many laws of nature not before recognized. After him came Theophrastus and Erasistratus. Galen, in the last half of the 2d century A. D., made many dissections of animals, of the anthropoid monkeys, and it is believed of man himself. During the middle ages the science of comparative anatomy fell into oblivion, from which it did not emerge until the 14th century; in the 16th and 17th appeared the writings of Berengario, Vesalius, Rondelet, Aldrovandus, Riolan, Harvey, and C. V. Schneider. Up to this time the science had only been studied in its separate details; the Neapolitan M. Aure-lio Severino, in his Zootomia Democritea, first united the scattered fragments in a general treatise on comparative anatomy in 1645; after him Collins, in England, pursued the subject into the domain of natural history and pathology. The more minute organisms of the invertebrates began now to attract attention, and were well studied by Ruysch, Steno, Willis, Malpighi, Swammerdam, and Reaumur. In France, the academy of sciences early occupied itself with this science. In the latter half of the 17th century Perrault, Duverney, and Mery made exact observations on the structure of reptiles and fishes.
About this time Needham in England, Redi in Italy, and Leeuwenhoeck in Holland, were pursuing their researches with the microscope on the minute animals, which Raspail and Ehrenberg have since so successfully illustrated. Up to this period a great multitude of scattered facts were brought together by the industrious compilers Blaes and Valen-tini, and in Manget's Bibliotheca Anatomiea. With Boerhaave, in the early part of the 18th century, the science received a check from which it did not recover for nearly 50 years. He was a skilful botanist, but a poor zoologist, and dogmatically maintained that the study of comparative anatomy could in no way advance the knowledge of the functions of the human organism. Notwithstanding this high authority, the great Haller, Spallanzani, and Ch. Bonnet continued their valuable observations on the general and comparative structure of man and animals. Physicians now had begun to consider this science as quite foreign to the art of medicine, but naturalists had already conceived the happy idea of making it the basis of a natural classification in zoology. Buffon was the first to perceive the full importance of the relation of comparative anatomy to natural history, and Daubenton made it the basis of a zoological classification.
If Linnaeus and his followers had been more familiar with it, they would have made less erroneous divisions of the animal kingdom, especially in the class of worms. Encouraged by Daubenton, Vicq d'A-zyr, famous for his discoveries in myology, in the anatomy of birds, his researches on incubation, and his description of the brain, conceived a vast plan for the illustration of comparative anatomy, which was frustrated by his early death, but was nearly realized in the beginning of the present century by the great Cuvier in his Lepons d'anatomie comparee. In the last 100 years, among the noted cultivators of this science were Barthez in France; J. Hunter and Everard Home in England; Pallas, O. F. Miiller, Merrem, Schneider, Kielmeyer, and Blumenbach in Germany; Camper in Holland; Scarpa and Poli in Italy. The time of Cuvier marks the opening of a new epoch in comparative anatomy; he applied this science to natural history, physiology, and to the study of fossils. During this epoch have flourished Geoffroy Saint-Hilaire, Meckel, Oken, Cams, Panizza, Grant, and Owen, besides a host of writers of monographs on the various classes of the animal kingdom, and on special systems and organs.
The first edition of the Legons appeared about the beginning of the present century, and the second was the last work upon which Cuvier labored; for more than 30 years he had collected an immense amount of facts and materials, which are partly embodied in this book; it is a monument of patient industry, a model in arrangement, and a mine of knowledge of which all observers since have availed themselves; many of its unavoidable deficiencies have been supplied by later writers, especially by Meckel; in it are laid down not only the analogies, but the differences in the structure of organized beings. His Ossemens fossiles, published in the first quarter of the century, is invaluable for its scientific and minute descriptions of the bones and teeth of extinct and living vertebrates. Contemporary with Cuvier was Geoffrey Saint-Hilaire, whose writings on comparative and philosophic anatomy gave great impetus to the study of natural history. At the head of the German school of philosophical anatomy stands Lorenz Oken, who extended to natural science the principles which Kant had applied to mental and moral science.
In 1802 he divided the animal kingdom into five classes, according to the predominance of the special senses; in 1805 he maintained that all organic beings originate from and consist of cells; in 1806, while walking in the Hartz forest, he picked up the blanched skull of a roebuck, and, after contemplating the partially separated bones, exclaimed: "It is a vertebral column!" In the following year he delivered the famous discourse on the " Signification of the Bones of the Skull," renewing the idea of the cranial and vertebral homologies, which has since been modified, extended, and perfected by Richard Owen. Carus, in his " Treatise on Comparative Anatomy," of which the second edition was published in 1834, devotes about half to philosophical anatomy and the geometrical construction of the skeleton, carrying out the idea of Oken, Saint-Hilaire, and Spix, that the whole bony fabric is nothing but a vertebra repeated. According to Oken, the head is a repetition of the whole trunk, with all its systems. Cuvier ridiculed the idea of these transcendental homologies, and at his death the vertebral theory, of the skull had apparently fallen into oblivion.
In Owen's "Archetype and Homologies of the Vertebrate Skeleton," this theory is briefly stated as follows: "The head is not a virtual equivalent of the trunk, but is only a portion, i. e., .certain modified segments, of the whole body. The jaws are the 'haemal arches of the first two segments; they are not limbs of the head." The head is not a repetition of all the rest of the body; the skull is a region in itself, consisting of a series of segments or vertebra) essentially the same as those constituting the rest of the skeleton. The endoskeleton of the cranium, according to Owen, consists of the occipital, parietal, frontal, and nasal vertebras; the ribs of the first are the shoulder blades, and the divergent appendages of the anterior extremities; the divergent appendages of the second are the branchiostegal rays, of the third the operculum, of the fourth the pterygoid and the zygoma. The different modifications of the type skeleton will be treated in the article Philosophical Anatomy. Owen revived and reworked the idea of Oken, and brought it to the now generally admitted segmental constitution of the vertebrate skeleton, as fully illustrated in the above mentioned work.