364 THE PERIPHERAL NERVOUS SYSTEM 



fibers course laterally beneath the spinal tract of the trigeminal nerve 

 and emerge to form the trunk of the nerve. These fibers later supply 

 the muscles of the pharynx. 



The sensory fibers of the glossopharyngeal nerve arise from two gan- 

 glia, a superior, or root ganglion, and a petrosal, or trunk ganglion (Figs. 

 359 and 365). These fibers constitute the greater part of the nerve and 

 divide peripherally to form the tympanic and lingual rami to the second 

 and third branchial arches. Centrally, these fibers enter the alar plate of 

 the myelencephalon and join the sensory fibers of the facial nerve coursing 

 caudally in the solitary tract. 



10, ii. The Vagus and Spinal Accessory. The vagus, like the hypo- 

 glossal, is composite. It represents the union of several nerves which supply 

 the branchial arches of aquatic vertebrates (Figs. 359 and 365). The more 

 caudal fascicles of motor fibers take their origin in the lateral gray column 

 of the cervical cord as far back as the fourth cervical segment. These 

 fibers emerge laterally, and, as the spinal accessory trunk (in anatomy a dis- 

 tinct nerve), course cephalad along the line of the neural crest (Figs. 358, 

 359 and 365). Other motor fibers take their origin from the neuroblasts 

 of the nucleus ambiguus of the myelencephalon (Fig. 364). Still others 

 arise from a dorsal motor nucleus that lies median in position. The 

 fibers from these two sources emerge laterally as separate fascicles and 

 join the fibers of the spinal accessory in the trunk of the vagus nerve. 

 The accessory fibers soon leave the trunk of the vagus and are distributed 

 laterally and caudally to the visceral premuscle masses which later form 

 the sterno-cleido-mastoid and trapezius muscles of the shoulder (Fig. 359). 

 Other motor fibers of the vagus supply muscle fibers of the phayrnx and 

 larynx. 



As the vagus is a composite nerve, it has several root ganglia which 

 arise as enlargements along the course of the ganglion crest (Figs. 359 

 and 365). The more cranial of these ganglia is the ganglion jugulare. The 

 others, termed accessory ganglia, are vestigial structures and not segmen- 

 tally arranged. In addition to the root ganglia of the vagus, the ganglion 

 nodosum forms a ganglion of the trunk (Fig. 365). The trunk ganglia of 

 both the vagus and glossopharyngeal nerves are believed to be derivatives 

 of the ganglion crest, their cells migrating ventrally in early stages. 



The central processes from the neuroblasts of the vagus ganglia enter 

 the wall of the myelencephalon, turn caudalward, and, with the sensory 

 fibers of the facial and glossopharyngeal nerves, complete the formation of 

 the solitary tract. The peripheral processes of the ganglion cells form the 

 greater part of the vagus trunks after the separation from it of the spinal 

 accessory fibers. 



