m 



AXIAL SKELETON 39 



The eighth vertebra differs from its predecessors in having 

 its body concave behind as well as in front. Corresponding 

 with this, the ninth (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 vertebrae 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 

 body, is termed a serial homology, and thus not only the 

 vertebrae 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 is 

 modified for articulation with the skull. 



The urostyle (usx) is a long bone with a gradually 

 diminishing ridge along its dorsal surface (see p. 35). 

 Its anterior face has somewhat the appearance of a small 

 vertebra with no transverse processes, and 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, representing an inter- 

 vertebral foramen, 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 

 containing 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 (aud. cp) , a pair of outstand- 

 ing masses arising, right and left, from the posterior 

 end of the brain-case. They lodge the organs of hearing. 



3. The olfactory capsules (olf. cp), smaller masses pro- 

 ceeding from the anterior part of the brain-case and 



