362 THE PERIPHERAL NERVOUS SYSTEM 



part of the nucleus ambiguus, a nucleus of origin which the glossopharyngeal 

 shares with the vagus (Fig. 345) . The motor 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 ganglia, a 

 superior or root ganglion and a petrosal or trunk ganglion (Figs. 341 and 346). 

 These fibers constitute the greater part of the nerve and peripherally divide to 

 form the tympanic and lingual rami. 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. n. The Vagus and Spinal Accessory. — The vagus, like the hypoglossal, 

 is composite, representing the union of several nerves which in aquatic animals 

 supply the branchial arches (Figs. 341 and 346). 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. The fibers emerge laterally and, as the 

 spinal accessory trunk (in anatomy a distinct nerve), course cranialwards along 

 the line of the neural crest (Figs. 340, 341 and 346). Other motor fibers take their 

 origin from the neuroblasts of the nucleus ambiguus of the myelencephalon (Fig. 

 345) . Still others arise from a dorsal motor nucleus which 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 stemo-cleido-mastoideus and 

 trapezius muscles of the shoulder (Fig. 341). Other motor fibers of the vagus 

 supply muscle fibers of the pharynx 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. 341 and 346). The 

 more cranial of these ganglia is the ganglion jugular e. The others are termed 

 accessory ganglia, are vestigial structures and not segmentally arranged. In 

 addition to the root ganglia of the vagus the ganglion nodosum forms a ganglion 

 of the trunk (Fig. 346). The trunk ganglia of both the vagus and glossopharyn- 

 geal 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. 



