458 



CHORDATA 



The cartilage bones can be divided according to the cranial regions 

 into four groups: (i) bones of the hinder part of the head occipitalia; 

 (2) bones of the ear region otica; (3) bones near the eye sphenoidalia; 

 and 4) of the nasal capsule etlimoidalia. The occipital a four in 

 number (figs. 516-518) united in the higher mammals to a single occip- 

 ital bone, surround the foramen magnum, the opening through which the 

 spinal cord passes to connect with the brain. They are a pair of ex- 

 occipitals, right and left, a supraoccipilal above and a basioccipital below. 

 The otica depend in their development upon the extent of the otic region. 

 In ;he fishes (fig. 516) where this part is large, several bones may pre- 

 sent: cpiotic, pterotic, s phenolic, prootic, and often opisthotic. In the 

 mammals, on the other hand, these are fused to a single petrosal bone 

 (figs. 517-519) of small size. 



01 



Yi 

 A 



4<j 



Pm 



FIG. 517. Skull of goat (from Claus). Ah, alisphenoid; Bs, basisphenoid- C 

 occipital condyle; Eth, mesethmoid, covering the ectethmoid; ^/ optic foramen in 

 orbiosphenoid; Fr, frontal; I mx , premaxilkry; Ip, interparietal; >f jSgaJT 



rimal, Mw,_ maxillary; vVo, nasal; Oft, basioccipital; O/, exoccipital- Ors 

 no,d; Pa, parietal; Pa/ palatine; Pe, petrosal; P w , paramastoid process; Pv pre- 



f ., sinus Jn frontal bone;5/>/>, ' 



The line of the basioccipital is continued forward by the unpaired 



lements of the sphenoidalia, a basisphenoid behind and a presphenoU 



front. With each is connected, right and left, a pair of bones; with the 



basisphenoid the alisphenoids, with the presphenoid the orbitosphenoids 



s the exoccipitals flank the basioccipital. In the region of the nasal 



ule there is an unpaired mesethmoid with a pair of ectethmoids. Hence 



:ranmm of primary bones may be described as a chain of four median 



