VERTEBRATA. 



345 



the olfactory lobes of the brain ; (2) optic, which forms a chiasma or 

 union with its fellow, in which fibres cross from one to the other side, 

 except in Teleostei, where the two nerves cross one another, the right to 

 the left side, and vice versa ; (3) the oculo-motor, which supplies all the 

 muscles of the eye except two ; (4) the trochlear, which supplies the 

 superior oblique muscle of the eye ; (5) the trigeminus, or mixed sensory 

 and motor nerve of the face and jaws ; (6) the abducens, or nerve of 

 the external rectus muscle of the eye ; (7) the facial, chiefly a motor 

 nerve ; (8) the auditory ; (9) the glossopharyngeal ; (10) the vagus ; (n) the 

 spinal accessory; and (12) the hypoglossus. Of these nerves the optic 

 is derived from the brain, the spinal accessory and hypoglossus apparently 

 from spinal nerves, the trochlear and abducens perhaps belong to the 

 trigeminus and facial respectively. The auditory is supposed by some 

 authorities to be derived from the facial, by others it is supposed to be 

 like the remaining cranial nerves (i (?) 3, 5, 7, 9, 10, supra) segmental 1 . 

 The cranial nerves differ from spinal nerves in having primitively single 

 roots, and in being connected with branchial sense organs (infra) from 

 which the olfactory and auditory sensory epithelia are perhaps derived 

 in the first instance. The ganglion of the roots of the oculo-motor, 

 i. e. the ciliary ; of the trigeminus, i. e. the Gasserian ; and of the remaining 

 segmental nerves are supposed to arise in connection with the same sense 

 bodies. Each segmental nerve typically supplies a gill cleft. The main 

 trunk passes down the posterior side of the cleft, a small branch down the 

 anterior side, a third to the pharynx, and a fourth the so-called dorsal 

 branch appears in continuity with the branchial sense organ at the upper 

 end of the branchial cleft 2 . This primitive arrangement is very well seen 

 in the case of the glossopharyngeal nerve of many fish. The spinal nerves 

 have two roots a dorsal sensory root with a ganglion, and an anterior 

 motor root which joins the sensory distally to the ganglion. The two 

 roots have different origins. But all nerves, cranial as well as spinal, 

 are outgrowths continuous with the central nervous system. And the 

 more or less extensive series of sympathetic ganglia, which lies at the side 

 of the backbone, and is connected with the nerves and ganglia of the 

 viscera, take their origin from the spinal nerves, to which they are always 

 united by rami communicantes. 



Sensory ectodermic cells are found on the surface of the body in 

 Ichthyopsida as branchial sense organs or organs of the lateral line 

 (infra under Ichthyopsida)^ and in Pisces of end-buds. End-buds in other 

 Vertebrata are restricted to the oral cavity, and in Mammalia as ' taste ' 

 buds to certain papillae or lamellae on the tongue and on the epiglottis. 



1 For the difficult questions connected with this subject of segmental cranial nerves, and their 

 connections, the student must consult the original authorities cited below (p. 358). 



2 The segmental nature of the olfactory nerve is not universally admitted. 



