Page:The American Cyclopædia (1879) Volume V.djvu/178

 174 COMPARATIVE ANATOMY sists of dorsal, sacral, and caudal portions ; in the frog there are only 8 or 9 vertebrae, in some of the salamanders more than 40, and in the siren 80 or 90; in the higher forms the vertebra 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 re- markable changes according to the persistence ~of the branchiae, as detailed in the article AMPHIBIA ; the tailed genera have cartilagi- nous 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 cav- ity, and in most genera has a distinct breast bone extending beyond the chest both anterior- ly and posteriorly ; the bones of the arm, fore- arm, and hand are easily recognized. The pos- terior extremities in the leaping batrachians consist of a long and cylindrical femur, a tibia and fibula consolidated into one bone, a tar- sus and long metatarsus, and five toes ; in the aquatic species the limbs are small and feeble, in some rudimentary, or even wanting, the power- ful tail serving for rapid locomotion. In ser- pents 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 ani- mals, being sometimes over 300, and their ar- rangement by. ball-and-socket joints allows of very free lateral motion ; the ribs, upon which they creep as upon so many feet, like an articu- late, 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 expan- sion of the mouth necessary to swallow their large prey. In lizards the skeleton is more per- fect; in them are seen for the first time cervi- cal vertebrae ; they have also distinct dorsals, bearing ribs, lumbar, sacral, and caudal verte- brae, 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 cer- vical and abdominal ribs before and behind the true ribs, and the sternum is prolonged nearly to the pubis, giving origin to cartilagi- nous arches supporting the abdominal viscera, of which the homologues in man are the lineae 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 para- chute. In the fossil pterodactyl, a flying liz- ard, 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 ex- tremities. The bones of the cranium are firm- ly united to each other, and there is no lat- eral movement of the jaws, as in snakes ; the brain cavity is small ; the whole number of vertebrae 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 vertebras, 8 to 14 dorsal, 3 sa- cral, 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 re- sist pressure. The bones of the skull are firmly united, and form large cavities and fossae 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 hume- rus 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 back- ward, 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 vari- able 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 bull- finch ; 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