ENDOSKELETON.
Vertebral column.
Four regions of the vertebral column can generally be recognised in Amphibia, viz. the cervical, the trunk or thoraco-lumbar, the sacral and the caudal regions. In the limbless Gymnophiona, however, only three regions, the cervical, thoracic, and post-thoracic can be made out. The cervical region is limited to a single vertebra which generally differs from the others in having no transverse processes or indication of ribs. It is generally called the atlas, but it commonly bears a small process arising from the anterior face of the centrum which resembles the odontoid process of higher animals, and renders it probable that the first vertebra of Amphibia corresponds to the axis, not to the atlas. Amphibia generally have a single sacral vertebra.
Three elements go to make up the vertebral column in Amphibia, viz.
1. the notochord,
2. the long vertebral centra,
3. intervertebral cartilage which forms the joints between successive centra.
The relations which these three elements bear to one another are subject to much variation. The successive stages can be well traced in the Urodela.
1. The first stage is found in larval Urodeles in general and in adult Ichthyoidea, and some Salamandrina. In these forms the notochord persists and retains approximately the same diameter throughout the whole length of the vertebral column. Bony biconcave centra are present and constrict it to a certain extent vertebrally, while intervertebrally there is a development of cartilage. The connection between the bony vertebrae is effected mainly by the expanded notochord.
2. In the next stage, as seen in Gyrinophilus porphyriticus, the growth of intervertebral cartilage has caused the almost complete obliteration of the notochord intervertebrally, and its entire disappearance vertebrally, i.e. in the centre of each vertebra. The intervertebral cartilage now forms the main connection between successive vertebrae, and sometimes cases are found whose condition approaches that of definite articulations. Readily recognisable remains of the notochord are still found at each end of the intervertebral constriction.