Page:The American Cyclopædia (1879) Volume V.djvu/177

 COMPARATIVE ANATOMY 173 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 at- tempting an exhaustive treatment of the sci- ence. 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 sepa- rate 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 skele- ton, 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 col- lected together, compared with the inverte- brata, 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 ex- ternal 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 devel- opment 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 vertebrse, and in the sharks and rays cartilaginous, re- sembling 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 vertebrse 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 dis- tinguishes them from fishes. The number of vertebra 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 vertebrse, and to these are attached also the ribs when any are pres- ent ; those which have spines below are called caudal vertebra, 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 re- spiratory 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 num- ber of pieces whoge 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 vertebrae ; 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 some- times behind the ventrals; in a few species they are wanting. The posterior limbs, or ven- tral 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 vary- ing in different families; in the jugular and thoracic fishes the bones supporting the ven- tral 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 cra- nial bones are distorted to allow this arrange- ment 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 consti- tute a class of animals intermediate between fishes and true reptiles, there is an extraordi- nary difference of external form and internal structure during the metamorphosis which most of them undergo. In all, the spine con-