70 THE ANATOMY OF TEKTEBRATED ANIMALS. 



c. The third dirision of the trigeminal, or fifth nerve, al- 

 ways leaves the skull behind the centre of the alisphenoid and 

 in front of the proOtic. 



d. The glossopharyngeal and pneumogastric alvva3'S 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 nerves denoted 

 in the paragraphs a, h, c, d, when surrounded by bone, and 

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

 b, the ojytic foramen ; c, \)i\& foramen ovale ; d, theforameii 

 lacerum posterius. The adjacent bones may take equal shares 

 in bounding 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 enclosing the first visceral cleft. In like man- 

 ner, the ninth (glossopharyngeal) nerve is distributed to the 

 hinder part of the second arch and to the front part of the 

 third, its branches enclosing the second visceral cleft. The 

 first branch of the pneumogastric has similar relations to the 

 third and fourth arches and to the third cleft ; and, in bran- 

 chiate Vertebrafa, the other anterior branches of the pneumo- 

 gastric are similarly distributed to the successive branchial 

 arches, the two divisions of each branch enclosing 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 repre- 

 senting a visceral cleft between the two. The inner and outer 

 portions of the first division of the trigeminal 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 trabeculae and 

 tiie maxillo-palatine processes may represent pre-oral visceral 

 arches, which are bent forward ; and, in the case of the tra- 

 beoulce, coalesce with one another. Such an hypothesis would 

 enable us to understand the signification of tlie naso-pulatine 



