| OCR Text |
Show 1870.] AXIAL SKELETON OF THE URODELA. 277 of the neural arch with the body) a large concave articular surface which receives one of the projecting articular processes or condyles of the posterior end of the skull. Moreover the median part of the vertebra, instead of presenting either a concavity (like the hinder Fig. 18. Dorsal view of first four vertebra? of Siren (No. 576 B in College of Surgeons' Museum). surface) or a rounded articular ball (like that always present in other vertebrae of Opisthoccelous forms such as Salamandra, Triton, ike) exhibits between the two anterior large articular concavities a small or considerable median process* (fig. 19). This projects forwards and fits into a recess at the base of the skull between the two posterior (occipital) condyles, and generally bears on each side of it a small articular facet; but sometimes these two facets are united into one continuous articular surface on the inferior and infero-lateral aspects of the process. Lateral, dorsal, and under view of first vertebra? of Amphiuma (No. 579 A in Museum of College of Surgeons). This azygos part attains its maximum of development in Amphiuma. It is small in Axolotl, but at its minimum in Menobranchus and Proteus, especially the latter. As far as I have observed or been informed, no bony or cartilaginous part is ever developed between the neural arch of the cervical vertebra and the skull; that is to say, if this cervical vertebra is really the axis, then the neural arch of the atlas vertebra of higher vertebrates is quite absent in the Urodela. The size of the cervical vertebra, as compared with that of succeeding vertebrae (e. g. with the third) varies somewhat. In most cases the two vertebrae are of nearly equal length and size; but in Amphiuma the cervical vertebra is rather smaller than the third ; and it is much so in Menobranchus and Proteus, in the last especially, where it is very remarkable for its small size. * It is the presence of this process which renders it probable that the cervical vertebra is an axis vertebra, and not an atlas one. |
