OCR Text |
Show 1870.] AXIAL SKELETON OF THE URODELA. 261 All the species of the order agree in possessing a spine made up, in the adult condition, of more or less similar vertebrae varying in number from 45 to 112, according to the species or individual. These vertebrae increase slightly and very gradually in size from the second vertebra till about the middle of the body. Thence they decrease again, at first gradually and slightly, but afterwards rapidly, and to such a degree that the last ones are only minute rudimentary ossicles. With the exception of the genus Siren, these vertebrae may be arranged in four categories. 1. Cervical.-This includes only one vertebra, namely that which articulates with the skull. 2. Dorsal.-This includes almost all the trunk-vertebrae, i. e. all the vertebrae behind the cervical vertebra, and anterior to the sacral vertebra or vertebrae. 3. Sacral.-This includes the vertebra or vertebrae to which the pelvis is attached. 4. Caudal.-This includes all the vertebrae posterior to the sacral vertebra or vertebrae. In the exceptional genus just mentioned (Siren) there is no sacral vertebra, and a cervical, and more or less arbitrarily divided dorsal and caudal regions are all that can be distinguished. o ""** Very rarely two contiguous vertebrae will more or less completely anchylose together. I have observed this in the large species Cryptobranchus japonicus, where sometimes the last two presacral are fused together, and sometimes the sacral and the first caudal. In a skeleton in the British Museum both these unions occur, so that the four originally distinct vertebrae form actually but a pair, though each shows evident signs of its complex nature. Every vertebra, except the abortive ones towards the end of the tail, consist of a body (centrum), and of a neural arch ossified, I believe, continuously with it. The neural arches constitute epaxial parts. Every vertebra, except the cervical one and the very last caudal vertebrae, is furnished with lateral prolongations, never uniting below and forming complete arches, except by the intervention of the limb those arches (the vagus &c.) seem to be serially homologous with that portion of the spinal nervous system which is called sympathetic. The chevron bones of Mammals, Reptiles, and Amphibia I stated to be, in m y opinion, hypaxial parts, and serially homologous with those hypapophysial processes which are so largely developed in the Pelican and the Great Auk, and which, in their azygos condition, are evidently situated in the line of suspension of the inner lamina? of the ventral plates of the embryo. According to this conception, in vertebrates generally w e have, at the anterior end of the axial skeleton, hypertrophied epaxial and hypaxial parts, i. e. the brain-case and visceral arches. Further back we have hypertrophied paraxial parts with much diminished hypaxial ones. Finally, towards the hinder end of the body (except in tailless forms) we have, in vertebrates above fishes, a reappearance of hypaxial elements generally accompanied by coexisting but distinct paraxial parts. In fishes, in the same region, we have generally paraxial parts in union with more or less of the hypaxial element, or we have paraxial parts only, or, much more rarely, only hypaxial parts. |