THE SKULL 267 



C. The SkuU. 



In the skull of the adult rabbit, the original cartilaginous 

 skeleton is almost completely replaced by bone. In addition to 

 these cartilage-bones, membrane-bones are largely developed ; 

 and the two groups of bones, though primitively distinct, be- 

 come intimately and immovably united to form a complex 

 mass, in which the several morphological components, and 

 even the boundaries of individual bones, are difBcult to de- 

 termine, and of which the essential composition can only be 

 ascertained by a study of the embryonic stages. 



In the following description the membrane-iones are 

 described in connection with the cartilage-bones with which 

 they are most closely associated, and their names are printed 

 in italics. Though it is convenient to deal with them in 

 this way, it must be borne in mind that, inasmuch as 

 they are primitively independent of the cartilaginous skeleton, 

 any attempt to group them according to the divisions of this 

 primary skeleton must be more or less arbitrary. 



1. The cranium proper or brain-case forms the posterior 

 part of the skull ; and is situated almost completely 

 behind the orbits, or deep cup-like sockets for the 

 eyeballs, its base forming an angle of about 120° 

 with the downwardly directed facial part of the skull. 



The bones of the cranium proper are arranged in 

 a series of, segments or rings, one in front of the 

 other : of these the hindmost or occipital segment 

 is the most obvious, and is very readily detached from 

 the rest of the skull as a bony ring. 



Care must be taken not to confuse these segments 

 with vertebrae, with which they do not correspond. 



a. The occipital segment consists entirely of cartilage- 

 bones, which may fuse together inseparably. 



i. The basi-occipital is a median flat bone, forming 

 the floor of the hinder end of the cranial 

 cavity. It forms the ventral boundary of 

 the foramen magnum, and a small part of 

 each occipital condyle. 



