Integrative Systems 167 



In terrestrial vertebrates the ninth nerve distributes to intero- 

 ceptors of the pharynx and to the taste-receptors of the posterior re- 

 gion of the tongue. In mammals the somatic sensory part of the nerve 

 is represented by a branch going to the skin of the external ear. Since 

 the external auditory passage corresponds to the spiracular passage of 

 fishes and the entire auditory mechanism develops in close relation to 

 the embryonic hyoid arch, this auricular branch of the nerve does 

 not stray far from the primitive territory of the nerve. The motor di- 

 vision of the nerve distributes to striated muscles in the hyoid region 

 and on the walls of the pharynx. 



X. Vagus (or pneumogastric), a large mixed nerve formed by 

 union of several small roots which emerge along the side of the medulla 

 just behind the ninth nerve (Figs. 136, 530). Several ganglions are 

 associated with it, chief of them being the jugular and the nodose 

 ganglions. In vertebrates having lateral-line organs, there is a special 

 lateral ganglion. The motor nuclei of X are closely associated with 

 those of IX in the medulla. 



In gill-breathers (Fig. 147C) the vagus passes dorsally to the re- 

 gion of the gills and gives off a trunk to each chamber behind the first 

 (which is supplied by IX). Each branchial trunk divides into a sen- 

 sory pretrematic and a mixed post-trematic branch which in- 

 nervate their respective regions and the adjacent region of the pharynx. 

 The post-trematic trunks innervate the striated branchiomeric mus- 

 cles posterior to those supplied by IX. From the branchial region, a 

 large visceral trunk (Fig. 147C, g) continues back into the coelom to 

 an extensive distribution comparising interoceptors, proprioceptors, 

 and muscles of the esophagus, stomach, and anterior region of the 

 intestine, the lungs (if present), and also the heart. The lateral divi- 

 sion of the vagus, arising from the lateral ganglion (lg), supplies the 

 neuromasts of the entire length of the main lateral-line canal. It is 

 usually the longest nerve in the animal, its fibers extending continu- 

 ously from the brain nearly to the tip of the tail. 



In terrestrial vertebrates the lateral division of the vagus is in no 

 way represented. In the absence of gills, the nerve provides sensory and 

 motor innervation to the respiratory region of the pharynx and to the 

 respiratory organs which develop by outgrowth from the floor of the 

 pharynx — the larynx, trachea, and lungs. This innervation includes 

 many small striated muscles (branchiomeric) related to the hyoid 

 and the larynx. The vagus may supply a small posterior area of taste- 

 receptors. In mammals a small somatic sensory branch assists IX in 

 supplying the skin of the external ear. As in gill-breathers, there is ex- 

 tensive distribution to the esophagus, abdominal viscera, and heart. 



