16 THE VERTEBRATE SKELETON. 



vertebra is not serially homologous throughout the Ichthy- 

 opsida, so that it is best to reserve the term atlas for the first 

 vertebra in Sauropsida and Mammalia. 



2. The thoracic vertebrae (often called dorsal) bear 

 movably articulated ribs which unite ventrally with the 

 sternum. 



3. The lumbar vertebrae are generally large, and are 

 often more movable on one another than are the thoracic 

 vertebrae. They bear no ribs. 



4. The sacral vertebrae are characterised by the fact 

 that they are firmly fused together, and are united with the 

 pelvic girdle by means of their transverse processes and rudi- 

 mentary ribs. 



5. The caudal or tail vertebrae succeed the sacral. 

 The anterior ones are often fused with one another and with 

 the sacrals, but they differ from true sacral vertebrae in that 

 there are no rudimentary ribs between their transverse pro- 

 cesses and the pelvic girdle. They often bear V-shaped 

 .chevron bones. 



In fish and snakes the vertebral column is divisible into 

 only two regions, an anterior trunk region, whose vertebrae bear 

 ribs, and a posterior tail region, whose vertebrae are ribless. 



2. THE SKULL. 



Before giving a general account of the adult skull it will 

 be well to briefly describe its development. 



GENERAL DEVELOPMENT OF THE CRANIUM*. 



Shortly after its appearance, the central nervous system 

 becomes surrounded by a membranous mesodermal investment 

 which in the region of the spinal cord is called the skeleto- 

 genous layer or perichordal sheath, while in the region 



1 F. M. Balfour, Comparative Embryology, vol. n., London, 1881, 

 p. 465. W. K. Parker and G. T. Bettany, The Morphology of the Skull, 

 London, 1877. 



