264 SPECIAL PHYSIOLOGY. 



result also follows experimental irritation of the nerve, whilst its di- 

 vision causes dilatation of the pupil by paralyzing its circular fibres. 

 It may act as a voluntary nerve, or in the so-called reflex manner, be- 

 ing then excited through the optic nerve. It contains a few sensory 

 fibres, probably derived from communications with the fifth cranial 

 nerve. 



The fourth or pathetic nerve supplies the superior oblique or tro- 

 chlear muscle of the eyeball, with motor fibres, which may act volunta- 

 rily, or in a reflex manner; it also contains a few sensory fibres. 



The fifth or triyeminal nerve is a mixed nerve, through which all 

 the parts mentioned in the description of its branches (pp. 250-51) are 

 endowed with sensibility, and through which the movements of the 

 muscles of mastication are effected. Division of this nerve on both 

 sides, within the cranium of a rabbit, destroys the sensibility and mo- 

 bility of these parts; and the head is carried as if it were a foreign 

 body. Division on one side, paralyzes the same parts on one side only. 

 Moreover, in a short time, the cornea becomes opaque, or even ulcer- 

 ated, and the humors escape. These results have been referred to an 

 interruption of the nutrition of the eye, caused by the arrest of the 

 influence of the fifth nerve, exercised through its connections with the 

 lenticular sympathetic ganglion; but, as the inflammation ceases, when 

 the eye is covered by fixing the animal's ear over it, it may be that 

 they are due to the cessation of the proper protective secretion and 

 reflex movements of the eyelids. In the rabbit, contraction of the 

 pupil follows both division and irritation of the fifth nerve; the former, 

 because the radial fibres of the iris, which are supplied by this nerve 

 through the lenticular ganglion, are paralyzed ; the latter, because 

 some fibres from the sixth cranial nerve pass by the ganglion, and pro- 

 ceed at once to the iris; in the dog, cat, and pigeon, these effects do 

 not ensue. The nasal mucous membrane becomes congested and bleeds 

 easily, and the sense of smell is diminished; common sensibility, and 

 the sense of taste at the tip of the tongue, are also affected, thus appar- 

 ently indicating that the lingual branch of the fifth nerve is a gusta- 

 tory nerve. The connection of the auriculo-temporal branch of this 

 nerve with the parotid gland, and of the inferior maxillary branch, 

 through the submaxillary and sublingual ganglia, with the submaxillary 

 and sublingual glands, is of great importance as regards the functions 

 of those glands; for division of the fifth nerve causes a diminution, and 

 irritation of the nerve, a copious increase of their secretion. This is 

 explained by the fact that irritation of the fifth nerve (and also of 

 the facial), causes a dilatation of the nutrient vessels of the glands, 

 and so great an increase of their activity, that they secrete an abun- 

 dant but thin fluid. On the contrary, irritation of the sympathetic 

 nerve-fibres, causes contraction of those vessels, and is followed by a 

 scanty but much more viscid secretion. 



The sixth or abducent nerve is exclusively concerned in the govern- 

 ment of the external rectus muscle of the eye, which turns the eyeball 

 outwards. 



The portio dura of the seventh nerve, or the facial nerve, is a purely 

 motor and secretory nerve, any sensory fibres which it contains, being 



