STRUCTURE OF THE VERTEBRATES 297 



Nerve V. Trigeminal. The trigeminal is one of the largest 

 of the cranial nerves, arising on the anterior part of the medulla. 

 In the elasmobranchs there are fom- branches, each bearing a 

 ganglion near its exit from the brain. The ganglia are distinct 

 in the embryos of amphibia and amniotes, but soon fuse to 

 form a single large Gasserian ganglion. Nerve V, with Nerve VII 

 (Facial), supplies most of the face, the two parallelling each 

 other throughout most of their courses. 



Near the ganglion the nerve splits to form three branches: 

 (1) the ophthabnic, a purely sensory branch, which derives its 

 name from the fact that it passes through the orbit; (2) the 

 maxillary, also sensory, to the face and maxillary region; and 

 (3) the mandibular, a mixed nerve, to the lower jaw and adjacent 

 regions. 



The ophthalmic branch is most prominent in the fish and 

 urodele amphibia, in which groups it is associated with the 

 lateral line system of sense organs (page 301). It is smaller in 

 the anura and amniotes. This branch receives autonomic fibers, 

 the exact relationships of which are not understood. 



The maxillary nerve is larger in the groups lacking the 

 lateral line, innervating the face and upper teeth. 



The mandibular branch goes to the teeth and muscles of the 

 lower jaw. It is a mixed nerve, receiving autonomic fibers; and, 

 as it goes to the branchiomeric musculature (page 185), it may 

 be considered a visceral nerve. In some reptiles and the mam- 

 mals a small branch goes to the tongue. 



Nerve VII, Facial. The facial is a mixed nerve, parallelling 

 the fifth in much of its course. It arises from the medulla near 

 the origin of the fifth. A sensory branch goes with the fifth to 

 the tip of the tongue. The motor portion innervates the muscles 

 of the neck and face. 



Nerve VIII, Auditory. The auditory nerve is pureh^ sensory, 

 the fibers being distributed to the ear. In the fish it is a single 

 nerve, the ear being only a balancing organ; but in the higher 

 vertebrates the nerve has two branches, one to the organ of 

 equilibrium and one to the hearing portion of the ear. The 

 eighth nerve rises near the seventh, the ganglia of the two being 

 fused in the higher classes. 



