170 THE VERTEBRATE SKELETON. 



radiate inwards from the exterior and outwards from the large 

 pulp cavity. The basal part of the teeth of Ceratophrys 

 (Anura) has a similar structure. 



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, tho- 

 racic, 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. inter vertebral 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 



