XIV CRANIAL NERVES 379 



and motor fibres, all of which have their own special centres in 

 the brain, but the proportions of these nerve components differ 

 greatly in different nerves. Certain pveoral nerves (iii., iv., and 

 vi.) are exclusively somatic motor ; others (i. and ii.) are special 

 sensory nerves for the olfactory and visual organs ; but most of 

 the other cranial nerves include several components, and are 

 therefore " mixed " nerves. Besides these components some 

 cranial nerves include also a quasi-independent system of nerve- 

 fibres, which converge from certain cutaneous sense-organs to an 

 independent centre in the medulla oblongata, the tuher acusticuin} 

 and is probably derived from the general cutaneous system of 

 nerve components. Such nerve fibres, including also the auditory 

 nerve, which has its origin from the same centre, constitute the 

 lateralis system. Perhaps the most striking feature in the post- 

 oral cranial nerves is the predominance of the visceralis or 

 sympathetic system over the somatic. Omitting the lateralis 

 fibres and a relatively few somatic sensory fibres, visceral fibres, 

 sensory and motor, are the principal components of all these 

 nerves, including v. but excluding viii. The reason for this is 

 to be found in the fact that splanchnic or visceral muscles in 

 relation with the jaws and branchial arches have usurped the 

 place of somatic muscles in the muscular system of the head. 

 For developmental and other reasons the olfactory and optic 

 nerves stand in a category of their own, and the same may be 

 said of the third, fourth, and sixth nerves, which innervate the 

 muscles of the eyeball. The remaining nerves, all of which 

 have their origin in the medulla oblongata, possess certain 

 features ^n common, and as they are related to the gill-clefts in 

 such a way that each forks over a cleft, they may be conveniently 

 distinguished as " Iranchial " or " hranchionierio nerves." A typical 

 branchial nerve consists of (1) a principal ganglion near the 

 origin of the nerve from the brain ; (2) a main trunk which 

 gives off (3) a somatic sensory branch or dorsal nerve to the 

 skin ; (4) a palatine nerve (visceral sensory) to the oral or 

 pharyngeal iwieous membrane; (5) an epihranchial ganglion 

 which is associated with a transitory embryonic epilrancMal 

 sensory organ at the dorsal border of a branchial cleft; (6) a 



^ Herrick, Journ. Ncur. ix. p. 153 ; Cole, Trans. Roy. Soc. Edinh. xxxviii. 

 1896, p. 631 ; Id. Trans. Linn. Soc. Tii. 1898, p. 115, to which an excellent 

 bibliography is appended. 



