XIV CRANIAL NERVES 
and motor fibres, all of which have their own special centres in 
the brain, but the proportions of these nerve components difter 
ereatly in different nerves. Certain preoral nerves (iii., iv., and 
v1.) 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 tuber acusticum,| 
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 vii. 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 in 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 “ branchial” or “ branchiomeric 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 mucous membrane; (5) an epibranchial ganglion 
which is associated with a transitory embryonic epibranchial 
sensory organ at the dorsal border of a branchial cleft; (6) a 
1 Herrick, Journ. Newr. ix. p. 153; Cole, Trans. Roy. Soc. Edinb, xxxviii. 
1896, p. 631; Id. Trans. Linn. Soc. vii. 1898, p. 115, to which an excellent 
bibliography is appended. 
