CEREBRAL NERVES 235 



with in the sections on the brain and the olfactory and optic 

 organs. 



The auditory is also a purely sensory nerve. 



The oculomotor, pathetic and abducent supply the muscles of 

 the eye, and probably, like the spinal accessory and hypoglossal, 

 consist exclusively of somatic motor fibres. All the other cerebral 

 nerves, like the spinal nerves, are mixed, and contain, in different 

 proportions, somatic afferent (general cutaneous) and efferent, and 

 also visceral afferent and efferent components. 



In their mode of origin the cerebral nerves (except I and II) 

 resemble the spinal nerves in many respects (p. 232), and a gradual 

 transition between the two groups is indicated in the lower Verte- 

 brata. Certain of them, like the motor portions of the spinal 

 nerves, arise as direct outgrowths from the extension of the ventral 

 horns of the spinal cord into the embryonic brain (III, VI, XII, 

 and probably IV). 1 



Others again (V and VII in part, VIII, IX, and X) arise in 

 the embryo from the dorso-lateral region of the brain, and thus 

 resemble the dorsal roots of the spinal nerves, but differ from 

 these (except in the cases of Amphioxus and Petromyzon) in being 

 developed opposite to and not between the corresponding somites : 

 later on their origin becomes shifted to the ventral side of the 

 brain. In all these (except VIII) the visceral system predominates 

 over the somatic owing to visceral muscles occupying the place of 

 somatic muscles. They further resemble the dorsal roots of the 

 spinal nerves in arising from a neural ridge ; this, however, is not 

 continuous with that of the spinal cord, which ends in the auditory 

 region, but is more dorsal in position and extends into the trunk- 

 region for a short distance. But in the course of development 

 these two ridges become no longer distinguishable from one another, 

 and complications arise owing to a kind of struggle between the 

 two for mastery, in which the cerebral neural ridge gains the upper 

 hand ; thus the rudiments of the spinal ganglia in this region, 

 together with the corresponding somites, become reduced to mere 

 vestiges, the development of the nerves of the visceral arches 

 excluding that of the spinal nerves. 



From the cerebral neural ridge certain ganglia arise, viz., the 

 Gasscrian (V), the ycniculate (VII), the petrosal (IX), and the 

 jugular (X) ; and, as in the case of the spinal ganglia, afferent 

 (sensory) fibres are formed which grow into the brain and are there 

 connected with their cerebral centres. In the formation of these 

 mixed cerebral nerves (arid also of the Vlllth), certain ganglionic 

 zones of proliferation of the external ectodermal epithelium 



1 The fourth nerve is peculiar in appearing from the dorsal surface of the 

 brain, but this is probably a secondary condition. Originally, this and the third 

 nerve possibly belonged to the trigeminal, the sixth to the facial, and all three 

 nerves of the eye-muscles perhaps represent vestiges of primitive cerebral nerves 

 of a mixed nature, as is indicated by the fact that some few sensory fibres may 

 be present in the fourth, and probably the sixth, amongst the Anamnia. 



