THE SKULL 275 



It is developed in connection with the ventral ends 

 of the ribs, and consists at first of right and left 

 halves. 



The anterior segment or manubrium is much the 

 largest, and is produced ventrally and in front into a 

 prominent keel. 



Behind the manubrium come four segments of 

 about equal length, then a very short segment, and 

 finally the xipMsternum, a long slender bony rod end- 

 ing behind in a horizontal plate of cartilage. 



The first pair of ribs articulate with the sides of 

 the manubrium, and the succeeding six pairs between 

 the several sternebrs. 



C. The Skull. 



In the adult rabbit, the original cartilaginous skull is al- 

 most completely replaced by bone. In addition to these carti- 

 lage-bones, membrane-bones are largely developed ; and the 

 two groups of bones, though primitively distinct, become in- 

 timately and immovably united to form a complex mass, in 

 which the several morphological components, and even the 

 boundaries of individual bones, are difficult to determine, and 

 of which the essential composition can only be ascertained 

 by a study of the embryonic stages. 



In the following description the membrane-bones 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 the membrane 

 bones 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. The anterior or facial part of the skull is 

 not in a hne with the base of the cranium, but is de- 

 flected through about 60°. 



T 2 



