186 COMPARATIVE ANATOMY 



the main trunk (hyomandibular less its lateral line elements). The 

 latter is connected with the glossopharyngeal by the anastomosis 

 of Jacobson, and is distributed, as its name implies, to the region 

 of the first and second visceral arches : thus in Fishes it goes 

 to the parts around the spiracle and to the muscles of the oper- 

 culum and branchiostegal membrane. A small remnant of this 

 branch in the higher Vertebrates supplies the stylohyoid muscle and 

 the posterior belly of the digastric and the stapedius. 



In Mammals the facial is mainly a motor nerve. It is chiefly 

 important in supplying the facial muscles, as well as the platysma 

 myoides, which has the closest relation to them (p. 136). The 

 more highly the facial muscles are differentiated {e.g. Primates, 

 especially Homo), the more complicated are the networks formed 

 by the facial nerve. 



Auditory Nerve. — This large nerve arises in close connection 

 with the facial, and corresponds to a sensory portion of the latter 

 nerve ; ^ it possesses a ganglion (Figs. 148 and 149). Soon after its 

 origin from the brain it divides into a cochlear and a vestibular 

 branch. The former passes to the lagena or cochlea, while the 

 latter supplies the rest of the auditory labyrinth. 



Vagus group. — This group includes the glossopharyngeal, 

 vagus, and spinal accessory, which stand in the closest relation 

 to one another, and are more nearly comparable to the spinal 

 nerves than are the cerebral nerves already described. It consists of 

 both sensory and motor fibres, the former being connected with 

 ganglia (the jugular and petrosal). The distribution of these 

 nerves differs from that of the other cerebral nerves in not being 

 limited to the head. 



Thus the vagus supplies not only the pharynx, tongue, and 

 respiratory organs, but also sends branches to the heart, larynx, 

 and a considerable portion of the digestive tract, as well as to 

 integumentary sense-organs of the trunk in Fishes. 



The spinal accessory nerve appears for the first time in the 

 Amniota, and will be dealt with after the vagus and glossopharyn- 

 geal have been described (p. 187). 



The origin of both glossopharyngeal and vagus by numerous 

 roots in Fishes (Fig. 148) indicates that these nerves correspond to 

 a number of spinal nerves, and this comparison is further justified 

 by the fact that they give off branches in the region of the pharynx 

 and visceral arches, in which a metameric arrangement can be 

 recognised. 



In many Fishes and in Dipnoans two or three neives make their exit from 

 the skull ventrally to the root of the vagus (Fig. 148) ; these ' ' spino-oocipital " 



^ On the supposition tliat the auditory organ corresponds to a modified 

 integumentary sense-organ, the auditor^' nerve ^^•ould belong to the lateral line 

 system of neives (see note on p. IS.")). 



