452 HISTORY OF THE HUMAN BODY 



two nerves also come into indirect contact in the tongue, where 

 the fibers of the gustatory and lingual branches of the two 

 nerves interlace. Four ganglia of the sympathetic system, the 

 entire cephalic group, become associated with the Trigeminus, 

 the ciliary with the first branch, the spheno -palatine with the 

 second, and the otic and submaxillary with the third. 



The Acusticus (eighth nerve) is originally a part of that 

 Facialis element which supplies the lateral line, and as the 

 essential part of the ear, the labyrinth, closely resembles in 

 its early development the sense-organs of the lateral line, the 

 suggestion is strongly felt that we have here a case of the 

 local specialization of a single element out of a series of simi- 

 lar parts, and that the Eighth nerve is consequently nothing 

 more than a branch of the superficial sensory system- of the 

 Facialis, the region of distribution of which chanced to develop 

 a high degree of complexity as an organ of special sense. 



IV. THE. VAGUS GROUP. (Glosso-pharyngeus, Vagus, Ac- 

 cessorius. ) 



This group includes in all vertebrates the Glosso-pharyngeus 

 and Vagus nerves, to which is added in mammals the Acces- 

 sorius, secondarily derived from the Vagus, and existing in 

 the Sauropsida as a semi-independent slip. This group is pri- 

 marily associated with the gill-region, but secondary sends 

 branches backwards and forwards which may even reach the 

 extreme ends of the body, thus having a more extensive dis- 

 tribution than that of any other cranial nerves. In the lower 

 forms this group is extremely regular and possesses a well- 

 pronounced metamerism, thus strongly suggesting its origin 

 from spinal nerves, similar to those which form a direct con- 

 tinuation of the series. Taken in connection with the much 

 greater differentiation of the other cranial nerves it seems 

 evident that the acquisition of the anterior end of the primor- 

 dial spinal cord by the cranium has been a gradual one, and 

 that the Vagus group is less modified than the nerves anterior 

 to it, because it has been annexed later. The primitive condi- 

 tion, that found in fishes, may be first considered by the help 

 of the diagram previously referred to [Plate VI]. The most 



