384 THE VERTEBRATE SKELETON. 



(a) THE CRANIUM. 



The cranium is a compact bony box, forming the anterior 

 expanded portion of the axial skeleton. It has a longitu- 

 dinal axis, the craniofacial axis around which the various 

 parts are arranged, and this axis is a direct continuation of 

 that of the vertebral column. Similarly the cavity of the 

 cranium is a direct continuation of the spinal canal. The 

 posterior part of the craniofacial axis, which has relation^ 

 only with the cranium, is called the basicranial axis. 



In the dog as in the other types previously described, the 

 skull in its earliest stages is cartilaginous, containing no bone. 

 In the adult, however, the cartilage is to a great extent 

 replaced by bone, and in addition to this cartilage bone, 

 membrane bone is largely developed, .and intimately united 

 with the cartilage bone to form one complete whole. 



In the description of the dog's skeleton, as in those of the 

 previous types, the names of the membrane bones are printed 

 in italics, while those of the cartilage bones are printed in 

 thick type. 



Most of the numerous foramina perforating the skull 

 walls will be described after the bones have, been dealt with. 



For purposes of description the cranium may be further 

 subdivided into : 



1 . The cranium proper or brain case. 



2. The sense capsules. 



3. The upper jaw. 



1. THE CRANIUM PROPER OR BRAIN CASE. 



Taking the membrane and cartilage bones together, they 

 are seen to be more or less arranged in three segments, which 

 however must not be regarded as homologous with the seg- 

 ments forming the vertebral column. 



The occipital segment is the most posterior of the three, 

 and consists of four cartilage bones, which in the adult are 

 commonly completely fused together. They surround the great 



