362 



THE PERIPHERAL NERVOUS SYSTEM 



nucleus which lies opposite the point at which the sensory fibers enter the brain 

 wall (Fig. 364). In the embryo these fibers emerge as a separate motor root, 

 course along the mesial side of the semilunar ganglion, and, as a distinct trunk, 

 supply the premuscle masses which later form the muscles of mastication. From 

 the chief motor nucleus, a line of cells extending cranially into the mesencephalon 

 constitutes a second source of origin for motor fibers. In the adult, the motor 

 fibers form a part of the mandibular division of the nerve. 



The facial, glossopharyngeal, and vagus nerves are essentially visceral in func- 

 tion. Their sensory fibers, chiefly of the visceral type, supply the sense organs 



Vagus root ganglion {jugular) 

 Accessory root ganglia 



Sup. gang. n. IX 



Gang, petros. 



IX. 



Gang, nodos. 



N. laryng. sup: 



Gang. Froriep. 

 XI 



Rod. dors. 



Inter-gang, bridge 



Fig. 363.— Reconstruction of the cerebral nervei of an embryo of 10.2 mm. (Streeter). X 16.7. 



of the branchial arches and viscera. These fibers originate in the ganglia of their 

 respective nerves, and, entering the alar plate of the myelencephalon, course 

 caudally as the solitary tract (Fig. 364). A few somatic sensory fibers, having 

 the same origin and course in the myelencephalon, supply the adjacent integu- 

 ment. 



In aquatic vertebrates, special somatic sensory fibers from the lateral tine organs join the 

 facial, glossopharyngeal, and vagus nerves, and their ganglion cells form part of the genicu- 

 late, petrosal, and nodose ganglia. In human embryos the organs of the lateral line are repre- 

 sented by ectodermal thickenings or placodes which occur temporarily over these ganglia. 

 The nervous elements supplying these vestigial organs have completely disappeared. 



