CHAPTER XII 



THE CRANIAL NERVES AND THEIR NUCLEI 



THE cranial nerves contain, in addition to the general somatic and visceral 

 components, which were encountered in the study of the spinal nerves, also 

 other functional groups of fibers of more restricted distribution and specialized 

 function. These special somatic and visceral components supply the organs of 

 special sense and the visceral musculature, derived from the branchial arches, 

 which differs from other visceral musculature in that it is striated. The fibers 

 which supply this special musculature are designated as special visceral efferent 

 fibers. The eye and ear, being special somatic sense organs, are supplied by 

 special somatic afferent fibers. The olfactory mucous membrane and the taste 

 buds are special visceral sense organs and are supplied by special visceral af- 

 ferent fibers. 



From what has been said it will be evident that there are seven distinct 

 functional components in the cranial nerves, namely: somatic efferent, general 

 somatic afferent, special somatic afferent, general visceral efferent, special vis- 

 ceral efferent, general visceral afferent, and special visceral afferent components 

 (Figs. 119, 120). No single nerve contains all seven types of fibers and the 

 individual cranial nerves vary greatly in their functional composition. On 

 entering the brain a nerve breaks up into its several components, which separate 

 from each other and pass to their respective nuclei, enumerated below. These 

 nuclei may be widely separated in the brain stem. Fibers having the same func- 

 tion tend to be associated together within the brain irrespective of the nerves 

 to which they belong. For example, all the visceral afferent fibers of the facial, 

 glossopharyngeal, and vagus nerves are grouped in the tractus solitarius (Fig. 

 120, yellow). The nerve-cells, with which the fibers of the several functional 

 varieties are associated within the brain stem, are arranged in longitudinal 

 nuclear columns. The analysis of the cranial nerves into their functional com- 

 ponents has involved a great amount of labor which has been carried through 

 for the most part by American investigators. Among those who have made 

 important contributions to this subject may be mentioned the following: Gas- 

 kell (1886), Strong (1895), Herrick (1899), Johnston (1901), Coghill (1902), 

 Norris (1908), and Willard (1915). 



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