88 Elementary Zoology. 



is simply a downward growth of the centrum, and not to be 

 confounded with an apparently similar downward process of 

 the caudals. 



The last cervical vertebra is fused with the three following 

 vertebra ; this gives great solidity to this region of the back, 

 which has to support the sternum. The last dorsal vertebra 

 (we restrict the term dofsal to those vertebrae which carry 

 complete ribs) is free, and not ankylosed with the preceding. 

 After this follows a large series of vertebrae which are closely 

 attached to each other and to the pelvis, which they support. 

 Here again the fusion of the vertebras gives great stability to 

 the pelvis and a firm point d'appui for the articulation of the 

 legs. These fused vertebrx are sometimes termed sacral ; but 

 it is better to reserve the term " sacral "■ for the two vertebrte 

 which in the embryo chick articulate with the ilia ; those 

 lying in front of this point may be called lumbar, and those 

 lying behind caudal. Behind these last there are a number 

 of free caudal vertebrae, and the column terminates in the 

 ploughshare bone (pygostyle), which is the product of a number 

 (six) of fused vertebrae, which thus form a strong basis for the 

 attachment of the strong quills of the tail, the rectrices. . The 

 free caudal vertebrse have slight downward processes, arising 

 from the centra ; these structures are really, and actually, in some 

 birds, separate bonelets, the intercentra, which are independent of 

 the centra to begin with, and only become fused with them later. 



The first cervical vertebra is known as the atlas, the second 

 as the axis, or epistropheus. The latter is furnished with 

 a peg-like forward outgrowth of the centrum, which fits into 

 a notch of the apparent centrum of the atlas. We say 

 " apparent," since the peg — the odontoid process, as it is usually 

 called — is really the detached centrum of the atlas. 



The vertebral column of the Rabbit can be also divided 

 into regions. First of all there is the cervical, with but seven 

 vertebrae (a number which is curiously constant among the 

 Mammalia, there being only three or four exceptions) ; then 

 follows the dorsal series, then the lumbar, the sacral, and, 

 finally, the caudal. The rabbit has twelve dorsal, seven 

 lumbar, four sacral, and fifteen caudal vertebras. 



