142 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



Fig. 150. Skeleton 

 of Xecturus. 



noticed that both sacral and cervical 

 regions are increased in extent, there 

 being two or three sacral and a much 

 larger number of cervical vertebra. In 

 the mammals these regions are still fur- 

 ther increased b}' a division of the trunk 

 into a thoracic (' dorsal ') region, the 

 vertebras of which bear ribs, and a lum- 

 bar region in which ribs are wanting. 



In certain regions there is a strong 

 tendency towards the fusion of \'ertebrae. 

 ^lost frequently those of the sacrum 

 unite into a single piece, while fusions 

 in the caudal region are numerous, and 

 are correlated with the partial or entire 

 disappearance of the tail. In modern 

 birds there results from this a short bony 

 complex, the pygostyle, while in the 

 anura the caudal vertebrje of the tadpole 

 are coalesced into the rod-like urostyle. 

 In other regions this union is less fre- 

 quent ; but the fusion of the anterior 

 vertebrae to form the anterior vertebral 

 plate of the skates and the anch)losis of 

 the cervical vertebras in the whales, and 

 the occasional fusion of some dorsals in 

 birds, will be recalled. 



The anterior two vertebrae in the 

 amniotes call for special notice. The 

 first of these, which joins the skull, is 

 known as the atlas, the second as the 

 axis or epistropheus. The atlas bears 

 on its anterior face articular surfaces 

 for articulation with the skull ; its neu- 

 ral arch is well developed, but the cen- 

 trum is absent, there being below but a 

 thin bony arch, regarded by some as the 

 first intercentrum {i.e., hypocentrum 



