io6 



NERVOUS SYSTEM OF VERTEBRATES. 



represent the cutaneous roots of this region accompany the spinal 

 accessory roots. These remain in the adult as a few scattered 

 clumps of ganglion cells among the vagus roots. The reason 

 for the failure of these nerves to develop has been suggested in a 

 previous chapter (p. 65). Since a certain number of somites in 

 this region have disappeared and the trunk somites have shifted 

 forward, it is probable that the cutaneous area has been lessened 

 and one or more nerves have disappeared in most vertebrates. 

 The arrangement of the cutaneous components in the cranial 

 nerves varies in different vertebrates. In cyclostomes such fibers 

 form large components of the X, IX and VII roots. In selachians, 

 ganoids, bony fishes and amphibia general cutaneous components 

 are found in the X and IX roots, but rarely (Kingsbury) in the 



arcuate fibers 



FIG. 49. A diagrammatic representation of the general cutaneous components 

 of a trunk segment. 



VII root. In mammals and man a general cutaneous component 

 in the X nerve has its cells of origin in the jugular ganglion and 

 forms the ramus auricularis. A rudimentary root ganglion in the 

 IX nerve probably represents this component (Figs. 20, 30, 31, 32). 

 In all vertebrates the sensory trigeminus and the ophthalmicus 

 profundus nerves are formed chiefly or exclusively of general 

 cutaneous fibers. Finally, that small nerve which is found con- 

 nected with the forebrain in many selachians, Amia and Pro- 

 topterus, the nervus terminates, is probably general cutaneous 

 in character. 



The distribution of the general cutaneous components is rela- 



