CRANIAL NERVES. 



175 



IX. The Glossopharyngeal, the first of the post-otic nerves, has its 

 typical development in the branchiate vertebrates. Its roots, both 

 motor and sensory, pass into the petrosal ganglion, beyond which a 

 dorsal ramus is given off to the top of the head, while the main trunk, 

 passing outward and backward, leaves the skull, either by its own fora- 

 men (most branchiates) or together with the tenth nerve (anura and 

 amniotes) . It divides, just above the first gill cleft, into pre- and post- 

 trematic nerves, which run in the anterior and posterior walls of the 

 cleft to the ventral wall of the pharynx, the pretrematic giving off a 

 nerve to the mucous membrane of the palate. Ninth and tenth nerves 



FIG. 173. Diagram of ninth (glossopharyngeal) and tenth (vagus) nerves of a shark; 

 for components see fig. 170. d, dorsal ramus; g, gastric nerve; h, to heart; j, Jacobson's 

 commissure; /, lateralis nerve; Ig, lateralis ganglion; po, pr, post- and pretrematic branches; 

 sp, spiracle; st, to supra temporal lateral line organs. 



are usually closely associated (their ganglia may fuse), while ninth and 

 fifth are frequently connected by Jacobson's commissure and the pala- 

 tine branch may connect with the geniculate ganglion. The glos- 

 sopharyngeal may contain three kinds of components, the somatic 

 motor fibres being absent because of the failure of the myotomic muscles 

 to develop. The general cutaneous is usually represented by the dorsal 

 ramus alone, but in Petromyzon its fibres reach the ventral skin through 

 the post-trematic nerve. The visceral sensory fibres reach the taste 

 organs by way of the pharyngeal and lingual nerves, while the visceral 

 motor elements go to the muscles of the gill region by way of the post- 

 trematic ramus. 



X. The Vagus is a complex of nerves, each similar to the ninth, 

 with the addition, in the branchiates, of lateralis components, and sup- 

 plying in them the remaining gill clefts. Numerous rootlets pass from 



