364 THE PERIPHERAL NERVOUS SYSTEM 



facial nerve in the adult bends around the nucleus of the abducens producing the 

 genu or knee, of the former. The two together produce the rounded eminence in 

 the floor of the fourth ventricle known as the. facial colliculus. 



9. The Glossopharyngeal Nerve takes its superficial origin just caudal to the 

 otic vesicle (Figs. 358, 363 and 365). Its few motor fibers arise from neuroblasts 

 in the basal plate beneath the fifth neuromeric groove. These neuroblasts form 

 part of the nucleus ambiguus, a nucleus of origin which the glossophar3aigeal 

 shares with the vagus (Fig. 364) . The motor fibers course laterally beneath the 

 spinal tract of the trigeminal nerve and emerge to form the trvmk of the nerve. 

 These fibers later supply the muscles of the pharynx. 



The sensory fibers of the glossopharyngeal nerve arise from two gangha, 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, 11. 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. 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 distinct 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 which Hes 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 pharynx and larjoix. 



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 gangha is the ganglion jugulare. The others, termed 

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

 addition to the root gangha of the vagus the ganglion nodosum forms a ganghon 

 of the trunk (Fig. 365). The trunk gangha of both the vagus and glossopharyn- 



