THE SKELETON 19 



the centrum after the remainder of the rib has been formed. This 

 capitular process passes ventral to the vertebral artery, thus enclosing 

 it in a bony canal. Hence it is apparent that the Urodele rib is 

 strictly homologous with the Amniote structure both with regard to 

 its insertions on the vertebra and its relation to the artery, but since 

 the capitulum fuses with the vertebra in the adult a secondary articula- 

 tion is formed with the transverse process, thus masking the true 

 relations. 



A detailed review of the earlier literature would be too lengthy to 

 be included here, but the works of Goppert (1895) ^^^^ Mayerhofer 

 (1909) stand out prominently, and the latter includes a good his- 

 torical review up to that date. 



The ribs are better developed in the anterior region of the trunk, 

 particularly those of the second and third vertebrae (Fig. 12), while 

 towards the pelvic region they become reduced to mere acuminate 

 appendages of the transverse processes. The first four or five ribs 

 are bent more or less sharply downwards at about their middle, and 

 from the point of bending there projects a sharp spine pointing in 

 a postero-dorsal direction. The ribs of the second vertebra, and to 

 some extent those of the third also, bear at their lateral extremities 

 large spatulate expansions composed mainly of cartilage. From these 

 expansions arise the MM. thoraci-scapulares suspending the scapula 

 (see p. 74). 



The spinal nerves do not emerge from the neural canal through 

 the intervertebral foramina as is the case in most animals, but through 

 a special foramen situated immediately posterior to the transverse 

 process. The intervertebral foramen is nevertheless quite large, but 

 is permanently closed by a tough fibrous membrane (mem.iv.). 

 Murray (1897) has investigated Spelerpes and Plethodon and has 

 come to the conclusion that the phenomenon, which is fairly general 

 among Urodeles, does not lend support to the theory that Urodele 

 vertebrae are double structures as had been suggested. List (1883) 

 describes one case of abnormal fusion of the seventh and eighth trunk 

 vertebrae of a Salamander which was so complete as to simulate a 

 single vertebra, but they disclosed their dual nature by the fact that 

 they each bore a pair of ribs. 



5. The Sacrum. 



The sacral vertebra is normally the sixteenth, but Claus (1876), 

 who has made a special investigation of this region of the vertebral 

 column, finds that either the fifteenth or the seventeenth vertebra 

 may exceptionally articulate with the pelvic girdle, and mentions also 



