24 COMPARATIVE ANATOMY 



downwards, and a wide ring-like arch. Posteriorly the 

 centrum bears an articular head, but anteriorly, instead of a 

 single concavity, there are two oval, concave, articular surfaces, 

 separated from one another by a median projection. These 

 concavities receive the occipital condyles of the skull. There 

 are no transverse processes. The urostyle is nearly as long as 

 the rest of the vertebral column. It is a rod-like bone with a 

 ridge on the dorsal surface, which is high and thick in front, 

 and becomes lower and thinner behind, eventually disappear- 

 ing altogether. The front end of the urostyle is thick and 

 broad, and has two concavities for articulation with the two 

 posterior prominences of the ninth vertebra. The anterior 

 portion of the ridge of the urostyle is hollow, and in life con- 

 tains a prolongation of the spinal cord known as the filum 

 terminale. The anterior end of the urostyle presents an 

 opening on either side through which nerves pass outwards 

 from the spinal cord ; and just in front of each opening a small 

 process may generally be found which represents the transverse 

 process of an ordinary vertebra. The lateral openings repre- 

 sent the spaces which are left between the pedicles of the 

 arches of the successive vertebrae of the column, and similarly 

 serve for the transmission of the spinal nerves. The vertebrae 

 are bound together by ligaments, and the whole column is 

 capable of a certain amount of flexion, in virtue of the articula- 

 tions of the centre and arches of its component vertebra. The 

 column, especially its neural and transverse processes, serves 

 for the attachment of some of the most important muscles of 

 the trunk. 



The skull is articulated to the first vertebra by two pro- 

 minences, known as the occipital condyles, which fit into its 

 two concavities. 



The skulls of all the craniate vertebrates (by which we 

 mean those vertebrate animals which have a distinct head) 

 present certain features in common, and though that of the 

 frog is peculiar in some respects, it is fairly illustrative of the 

 fundamental features of all skulls. 



A skull comprises the following parts : (i.) A brain case or 

 cranium proper ; a cartilaginous or bony, or, as in the frog, 

 partly cartilaginous, partly bony, box, which contains the brain. 

 At the back of the brain-case is a large aperture, the foramen 

 magnum, by means of which the spinal cord passes into the brain. 



