THE SKULL THE CRANIUM 161 



entirely contained in the nasal cavity, and hence largely surrounded 

 by the bones of the face, the rest of the cranium can be separated into 

 three rings (Fig. 106), whereof each bears a resemblance to a vertebra 

 in that it consists of a basal part (or body) and lateral walls (pedicles 

 and laminae) and includes a greatly enlarged neural cavity. 



The flattened posterior ring is formed of the occipital bone alone ; 

 the foramen magnum represents the neural canal of a vertebra ; the 

 basal portion of the bone, known as the basioccipital, represents the 

 body ; the lateral portions, known as the exoccipitals, represent the 

 pedicles ; and the upper part, the supraoccipital, represents the united 

 laminae. 



The middle ring is formed of the posterior half of the sphenoid, 

 the temporals, the parietals, and the interparietal. The basal part of 

 the posterior half of the sphenoid, known as the basisphenoid, repre- 

 sents the body of a vertebra, its lateral parts, known as the alisphenoids, 

 with the temporals, represent the pedicles, and the parietals and the 

 interparietal represent the laminae. The neural canal is greatly en- 

 larged, especially above. 



The anterior ring is formed of the anterior division of the sphenoid 

 with the frontals. The central basal part of this division of the sphe- 

 noid, known as the presphenoid, represents the body of a vertebra ; 

 the lateral parts, the orbitosphenoids, represent the pedicles, and the 

 frontals represent the laminae. The neural canal is not so large ; its 

 anterior opening is closed by the ethmoid, which has been removed. 



The student will find this classification of the bones of the cranium 

 of great assistance in remembering their names and relative positions ; 

 beyond this, however, it has no value. The origin of the skull from 

 modified vertebrae has been a favorite field of investigation from the 

 time of Goethe and Oken, who first propounded a vertebral theory of 

 the skull ; the most recent studies, however, tend to prove that, with 

 the possible exception of the occipital region, the skull has an entirely 

 independent mode of development, the details of which I shall con- 

 sider at length in their proper place. 



THE OCCIPITAL BONE. 



General Description. The Occipital is a single, bilaterally 

 symmetrical bone forming the back of the skull and the posterior 



portion of the base. It consists of two distinct parts, namely, the 



11 



