244 COMPARATIVE ANATOMY 



anterior wall of the pharynx, floor of the mouth, and the skin. Its 

 motor fibres supply visceral muscles in .connection with the 

 mandibular and hyoid arches. 



In correspondence with the change from an aquatic to a terres- 

 trial mode of life, the integumentary sense-organs in caduci- 

 branchiate Urodela, Anura, and in Amniota, become more or 

 less completely lost, and the corresponding branches of the facial 

 nerve are reduced. The parts which persist, in addition to the 

 large motor hyomandibular, are the palatine and the chorda 

 tympani (cf. Figs. 179-182). 



In the Amniota the chorda tympani has a very different 

 position from that seen in the Anamnia, and becomes character- 

 istically related to the tympanic cavity ; in Birds it is absent, and 

 is replaced functionally by the glossopharyngeal. From the Amphi- 

 bia and Reptilia onwards, a gradual development of the facial 

 muscles leads to the characteristic mimetic muscles of Mammals 

 and more especially of Primates, which are supplied by the hyoman- 

 dibular nerve. The complicated networks of this nerve, however, 

 appears late phylogenetically, and are wanting even in certain 

 embryonic stages in Man. In addition to the mimetic muscles, the 

 hyomandibular nerve in Mammals supplies the platysma, the stylo- 

 hyoid, the posterior belly of the digastic, and the stapedius. 



Auditory Nerve. This large nerve, which has a ganglion at 

 its origin, arises in close connection with the facial, and comes 

 under the same category as the sensory portion of the latter nerve, 

 inasmuch as it is probable that the auditory organ is a modified 

 portion of the lateral line organs. Soon after its origin it divides 

 into a vestibular and a cochlear branch. The latter passes to the 

 lagena or cochlea of the ear, while the former 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, owing to the fact that the head in this region has 

 undergone fewer phylogenetic modifications, are less specialised 

 than the cerebral nerves already described. These nerves all 

 consist of both afferent and efferent fibres, the former being con- 

 nected with ganglia (the petrosal of IX, and the jugular and 

 cervical of X). 



In Fishes and perennibranchiate Amphibians the glosso- 

 pharyngeal leaves the skull through a special foramen, and not 

 along with the vagus, as in other Vertebrates. In branchiate forms, 

 in addition to a palatine branch, it is distributed to the region of 

 the first (hyobranchial) gill-cleft, over which it bifurcates into a 

 smaller prebranchial and a larger postbranchial branch (Fig. 179). 

 In other Vertebrates it is distributed to the pharynx and tongue, 

 and as a rule anastomoses with the vagus and also with the geni- 



