76 THE ANATOMY OP VEETEBRATED ANIMALS. 



d. The glossopharyngeal and pneiimogastric always make 

 their exit behind the centre of the opisthotic, and in front 

 of the centre of the ex-occipital. 



The apertures for the exit of the cranial nei-ves denoted 

 in the paragi-aphs, a. b. c. d., when surrounded by bone, and 

 well defined, are called respectively : a, the olfactory foramen ; 

 b, the optic foramen ; c, the foramen ovale; d, the fwamen 

 lacerum posterius. The adjacent bones may take eqnal shares 

 in boimding these foramina, or the foramina may be alto- 

 gether in one bone ; but their positions, as here defined, 

 never change. 



Another point to be especially considered respecting the 

 general disposition of the cranial nerves, is the relation 

 which some of them bear to the visceral arches and clefts, 

 and which has already been incidentally mentioned. Thus, 

 the seventh nerve is distributed to the posterior part of the 

 first visceral arch, and to the anterior part of the second 

 visceral arch, its two branches inclosing the first visceral 

 cleft. In like manner, the ninth (glossopharyngeal) nerve 

 is distributed to the hinder part of the second arch and to 

 the front j^art of the third, its branches inclosing the 

 second visceral cleft. The first branch of the pneumo- 

 gastric has similar relations to the third and fourth arches 

 and to the third cleft ; and in branchiate Vertebrata, the 

 other anterior branches of the pneumogastric are similarly 

 distributed to the successive branchial arches, the two divi- 

 sions of each branch inclosing a branchial cleft. 



The second and the third divisions of the trigeminal are 

 distributed, in an analogous manner, to the anterior region 

 of the first visceral arch, and to the posterior or outer 

 region of the maxillo-palatine process — the gape of the 

 mouth representing a visceral cleft between the two. The 

 inner and oiiter portions of the first division of the tri- 

 geminal are similarly related to the inner, or anterior, 

 region of the maxillo-palatine process, and the outer side 

 of the trahecula cranii — the orbito-nasal fissure representing 

 the cleft between the two. 



Considerations of this kind suggest that the trabeculse 



