THE CRANIAL NERVES.—It is usual to describe the cranial nerves of Cyclostomes and Fishes as consisting of ten serially disposed pairs, viz.: the _olfactory_ (i.), _optic_ (ii.), _oculomotor_ (iii.), _trochlear_ (iv.), _trigeminal_ (v.), _abducens_ (vi.), _facial_ (vii.), _auditory_ (viii.), _glossopharyngeal_ (ix.), and the _vagus_ (x.) Like the spinal nerves, the cranial nerves collectively include somatic sensory (general cutaneous) and motor fibres, and also visceral sensory {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 preoral 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 _tuber acusticum_,[449] 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 postoral 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 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 _pre-branchial_ nerve (visceral sensory), skirting the anterior margin of a cleft in its ventral course; and (7) a