i/o ELEMENTARY BIOLOGY. [CHAP. 



bodies of the vertebrae, the extremities of the caudal style 

 (urostyle) and the ends of the transverse processes; and it 

 enters largely into the sternum. In the skull, the para- 

 sphenoid 1 , vomers, parieto-frontals, nasals, premaxillse, max- 

 illse, jugals, squamosals, and the bony elements of the man- 

 dible may be removed by maceration, leaving behind the 

 primitive cartilaginous skull, or Chondro-cranium^ altered only 

 so far as parts of it have been replaced by bone. 



It furnishes a floor, side walls and roof to the brain-case, 

 interrupted only by a large space (called & fontanelle) covered 

 in by membrane, which lies in the inter-orbital region under 

 the parieto-frontals, and by the foramina for the exit of the 

 cranial nerves. It consists entirely of cartilage, except where 

 the exoccipitals, the pro-otics, and the sphenethmoid invade 

 its substance. In front of the septum of the anterior cavity 

 of the sphenethmoid, it is continued forward between the 

 two nasal sacs, as the cartilaginous partition between the 

 nasal cavities (septum narinni), from which are given off, 

 dorsally and ventrally, transverse alse of cartilage which fur- 

 nish a roof and a floor, respectively, to the nasal chambers. 

 Of these, the floor is the wider. The dorsal and ventral alse 

 pass into one another where the chondro-cranium ends an- 

 teriorly and give rise to a truncated terminal face, which is 

 wide from side to side, narrow from above downwards, and 

 convex in the latter direction. The lateral angles of this 

 truncated face are produced outwards and forwards into two 

 flatten e& pra-nasal processes ; these widen externally and end 

 by free edges which support the adjacent portions of the 

 premaxillse and maxillae. From the ventral face, just behind 

 the truncated anterior end of the chondro-cranium, spring 



1 See Laboratory work, D, C, for the structure of the bony skull 

 (osteocranium), which should be thoroughly understood before any 

 attempt is made to study the cartilaginous skull or chondro-cranitim. 



