THE ENDOSKELETON: VERTEBRAL COLUMN AND RIBS 69 



The vertebral column of urodeles consists of four regions, not very sharply 

 marked off from each other. The first region, the cervical or neck region, con- 

 sists of one vertebra, the first vertebra or atlas, just behind the skull and serving 

 as a support for the skull. The atlas lacks ribs. Following the cervical region 

 is the long trunk region of similar rib-bearing vertebrae. There is next a region 

 consisting of but one vertebra. This is the vertebra to whose ribs the hind legs 

 are attached. It is called the sacral vertebra and its ribs are known as sacral 

 ribs. This region of the vertebral column is the sacral region or sacrum. Pos- 

 terior to the sacrum is the caudal or tail region, composed of vertebrae which 

 lack ribs and bear haemal arches in most cases. By moving the vertebrae apart 

 note that the ends of the centra are concave, that is, the centra are amphicoelous 

 as in most fishes. 



The attached ends of the ribs (Necturus) are forked into two processes or 

 heads, a dorsal tubercular head and a ventral capitular head which articulate 

 with similar but less marked dorsal and ventral projections of the transverse 

 processes. The ribs are intermuscular ribs. 



The vertebrae of the urodele Amphibia differ from those of all other land vertebrates in 

 the manner in which the centra are formed. In the tail region the centra are produced by 

 the fusion of the bases of basidorsals and basiventrals, the former giving rise also to the neural 

 arches, the latter to the haemal arches. In the trunk region the centra and neural arches 

 are formed from the basidorsals, the basiventrals having disappeared. Interdorsals and 

 interventrals are absent or represented by the well-developed intervertebral cartilages, form- 

 ing pads between successive centra. In some urodeles these cartilages split, half fusing to 

 the vertebra in front and half to the vertebra behind. The vertebrae of urodeles are desig- 

 nated as pseudocentrous vertebrae, as the centrum does not correspond to that of the majority 

 of land vertebrates (see Fig. 22C, p. 66). 



2. The vertebral column of an anuran amphibian, the frog. The vertebral 

 column of the frog is specialized in that the caudal vertebrae are all united 

 into one piece, the urostyle or coccyx. There is, as in Necturus, one cervical 

 vertebra, the atlas. This is followed by seven trunk vertebrae. These have 

 low neural arches, no haemal arches, and centra bearing long transverse processes. 

 Ribs are apparently absent, but embryological studies show that the distal part 

 of the transverse process is a rib, which has become indistinguishably fused to 

 the process. There is one sacral vertebra supporting the hind legs and beyond 

 this the long urostyle already mentioned. The vertebrae are yoked together 

 by zygapophyses as in Necturus. In single vertebrae from the frog note that 

 the posterior end of the centrum is rounded like a ball, while the anterior end is 

 concave. Such centra are procoelous and fit together in a ball-and-socket manner. 



The centra of the vertebrae of Anura are formed by the union of the basidorsals and 

 basiventrals as in urodeles, with the difference that the interdorsals are fused to either the 

 anterior or posterior end of the centrum. In the frog the interdorsals are fused to the posterior 

 end forming the ball-shaped projection. Interventrals are lacking. Vertebrae of this type 

 are called notocentrous (see Fig. 22D, p. 66). 



