VERTEBRAL COLUMN. 



113 



its mobility and the uniformity of 

 its segmentation. It is only in 

 those cases in which the method 

 of locomotion requires a greater ex- 

 penditure of force on the part of 

 the limbs, and a firmer connection 

 between them and the axial skeleton, 

 and the limbs are more strongly 

 developed, that the vertebral column 

 is divided into successive regions, 

 each of which is characterised by 

 the special form of the vertebras 

 composing it. 



Since the posterior limbs constitute 

 the chief supports of the body, and 

 are the principal seat of the pro- 

 pulsive power, their girdle is usually 

 immoveably fused with a region of 

 the vertebral column, which is dis- 

 tinguished by the firm and rigid 

 connection of its vertebrae (fig. 572). 

 This region, which is situated be- 

 tween the trunk and the tail, is 

 called the sacral region, and is 

 formed in the Amphibia by a single 

 vertebra, in Reptilia by two, and in 

 the higher Vertebrates by a number 

 of vertebrae, the transverse processes 

 of which are specially large and are 

 firmly united to the iliac bones of 

 the pelvic girdle by means of their 

 corresponding ribs (fig. 572, S). 

 With the development of the an- 

 terior limbs, and the need of a firmer 

 connection between them and the 

 trunk, a more rigid region of the 

 vertebral column makes its appear- 

 ance in the anterior part of the 

 body. This region is known as the 

 thoracic region and its vertebrae as 

 the thoracic or dorsal vertebrae (fig. 



VOL. II. 



FIG. 572. Skeleton of Menopoma al- 

 leghaniense. Ocl, Exoccipital bone; 

 P, parietal bone ; F, frontal ; Ty, tym 

 panic ; Pe, petrosal ; MX, maxillary ; 

 Jmx, inter-maxillary (preemaxillary) ; 

 N, nasal ; Vo, vomer ; Et, girdle bone ; 

 Pt, pterygoid; So, pectoral girdle; 

 II, pelvic girdle; S, sacral verte- 

 bra , J2, ribs ; b, hyoid arch (Zb), and 

 branchial arches (Kb) of the same. 



