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Show 1870.] AXIAL SKELETON OF THE URODELA. 2*1 The transverse processes at their roots, i. e. near their origin from the centrum, are often traversed by a canal passing from behind forwards and transmitting an artery. This is well seen in Cryptobranchus, Menopoma, Menobranchus, and Salamandra. RIBS. With the exception of the genera Amphiuma, Siren, and Proteus, all the transverse processes of the dorsal* and sacral regions support ribs, and not unfrequently the anterior caudal ones also. The first vertebra of all, however, even when furnished with a rudimentary transverse process, remains always destitute of such bony appendages. The ribs form a series of cylindroidal bones (figs. 11, 12, 13 & 18), each extending outwards and more or less downwards and backwards, and ending distally in a free pointed termination, with the exception of the single pair attached (one on each side) to the hip-girdle They never have cartilaginous or osseous parts attached to their distal ends and answering to the sternal ribs or cartilages of most higher vertebrates. Rarely, as in more or fewer of the ribs of Axolotl and Amblystoma, they extend rather upwards and backwards. The number of ribs varies from five or six pairs in Amphiuma^, eight in Siren%, seven, eight, or nine in Proteus § (if the second vertebra bears any) to some twenty-one or twenty-two pairs (counting the caudal ribs) in Menobranchus and Cryptobranchus. Proximally the ribs very commonly bifurcate into two short and nearly equal branches, diverging from each other at a more or less acute angle (fig. 12). These branches are placed one above the other, and are attached respectively to the tubercular and capitular parts or processes of the respective transverse processes. The upper branch of this fork may Fig. 11. Lateral view of first four vertebra? of Siren (No. 576 B in Museum of College of Surgeons). thus be called the tubercle (tuberculum) oi the rib, and the lower branch its head (capitulum). When, as in Siren, Menopoma, and Cryptobranchus, the distal articular surfaces of the transverse processes are near together, the proximal ends of the ribs do not bifurcate, * Rusconi represents none to the first dorsal of Proteus. t Cuv. M e m . du Mus. vol. xiv. 1827, p. 9. The College ol Surgeons specimen looks as if it had had ten pairs at least. \ Cuv. Oss. Foss. 4th edition, vol. x. p. 35U. § Cuvier (loc. cit. p. 358) says seven, counting from the second vertebra. Rusconi represents seven, beginning with the third vertebra : the last is so small as to be with difficulty detected. - |
