Ill AXIAL SKELETON 39 



(v. 9.) has its centrum convex in front, while behind it presents two 

 little rounded elevations placed side by side. 



It will be seen that the vertebrfe are all corresponding 

 structures, following one another in a regular series from 

 before backwards. A correspondence of this kind, in 

 which there is a repetition of similar parts along the bod)', is 

 termed a sei-ial homology, and thus not only the vertebrje as 

 a whole, but also their various parts are serially homologous, 

 each to each, the correspondence being disturbed only by the 

 first vertebra, in which the transverse processes are absent 

 and the anterior face modified for articulation with the skull. 



The urostyle (ust) is a long bone, the anterior part of 

 which has somewhat the appearance of a small vertebra with 

 no transverse processes, and having the posterior face of its 

 centrum produced backwards into a long projection. On its 

 front face it has a double concavity for articulation with the 

 double convexity on the ninth vertebra. Near its anterior 

 end there is on each side a small aperture for the last spinal 

 nerve. 



The skull is a very complex structure, consisting partly 

 of bone, partly of cartilage. It is divided into the following 

 regions : — 



1. The brain-case or cranium, a sort of oblong box con- 

 taining the brain (Figs. 8 and 9) : it forms the middle portion 

 of the skull and is a direct forward continuation of the 

 vertebral column. 



2. The auditory capsules, a pair of outstanding masses 

 arising, right and left, from the posterior end of the brain- 

 case. They lodge the organs of hearing. 



3. The olfactory capsules {plf. cp), smaller masses pro- 

 ceeding from the anterior part of the brain-case and united 

 with one another in the middle line. They lodge the organs 

 of smell. 



