158 COMPARATIVE ANATOMY. 



skull to reach the integument in the region of the upper jaw, snout, 

 and upper lip. 



The third division of the trigeminal is of a mixed nature ; 

 it supplies on the one hand the masticatory muscles, and also gives 

 rise to the great sensory nerve of the tongue (lingual or gustatory 

 nerve), while another branch, passing through the inferior dental 

 canal, supplies the teeth of the lower jaw, and then gives off 

 one or more branches to the integument of the latter and of 

 the lower lip. It is usually connected with the chorda tympani 

 branch of the facial. 



Facial and Auditory Nerves. Both arise from a common 

 ganglion, the former, as we have seen, coming into close relation 

 with the trigeminal. 



The facial, which is originally a mixed nerve, divides into three 

 branches, a hyomandibular, a palatine, and abuccal. The first, 

 which is connected with the glossopharyngeal by means of the 

 so-called anastomosis of Jacobson, is distributed, as its name im- 

 plies, mainly 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 operculum and branchiostegal membrane. A small 

 remnant of this branch in the higher Vertebrates supplies the stylo- 

 hyoid muscle and the posterior belly of the digastric. 



In Mammals the facial is a purely motor nerve, supplying 

 mainly the facial muscles, as well as the platysma myoides, which 

 has the closest relations to them (comp. p. 121). 



The auditory is always a very large nerve, and soon after its 

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

 branch. The former passes to the cochlea, while the latter supplies 

 the rest of the auditory labyrinth. 



Glossopharyngeal and Vagus. These, which are of a mixed 

 nature, have not, like the other cranial nerves, their distribution 

 limited to the head. 



In Fishes and gill-breathing Amphibians the vagus branches 

 out to the region of the visceral and branchial apparatus, as well 

 as to the muscles of the shoulder and anterior extremity 

 (Protopterus). It then extends backwards along the sides of 

 the body under the skin to the tail as one or more lateral 

 nerves, supplying sensory organs. 1 



Further, in all Vertebrates it is distributed to the anterior 

 part of the alimentary canal, giving rise to a pharyngeal, an 

 oesophageal, and a gastric plexus, besides giving off branches to the 

 heart and to the whole respiratory system, from the larynx to the 

 lungs (air-bladder). 



Thus cephalic, cervical, thoracic, and abdominal por- 

 tions of the vagus can be distinguished in the higher Vertebrates. 



1 Compare the chapter on sensory organs, p. 165. 



