142 HAND-BOOK OF PHYSIOLOGY. 



the disturbed circulation or nutrition in the retina, when the normal in- 

 fluence of the fifth nerve and ciliary ganglion is disturbed. In such dis- 

 turbance, increased circulation making the retina more irritable might 

 induce extreme contraction of the iris; or under moderate stimulus of 

 light, producing partial blindness, might induce dilatation: but it does not 

 appear why, if this be the true explanation, the iris should in either case 

 be immovable and unaffected by the various degrees of light. 



Trophic influence. Furthermore, the morbid effects which division of 

 the fifth nerve produces in the organs of special sense, make it probable 

 that, in the normal state, the fifth nerve exercises some trophic influence 

 on all these organs; although, in part, the effect of the section of the 

 nerve is only indirectly destructive by abolishing sensation, and therefore 

 the natural safeguard which leads to the protection of parts from ex- 

 ternal injury. Thus, after such division, within a period varying from 

 twenty-four hours to a week, the cornea begins to be opaque; then it 

 grows completely white; a low destructive inflammatory process ensues in 

 the conjunctiva, sclerotica, and interior parts of the eye; and within one 

 or a few weeks, the whole eye may be quite disorganized, and the cornea 

 may slough or be penetrated by a large ulcer. The sense of smell (and not 

 merely that of mechanical irritation of tl^e nose), may be at the same 

 time lost or gravely impaired; so may the hearing, and commonly, when- 

 ever the fifth nerve is paralyzed, the tongue loses the sense of taste in its 

 anterior and lateral parts, i.e., in the portion in which the lingual or 

 gustatory branch of the inferior maxillary division of the fifth is dis- 

 tributed. 



In relation to Taste. The loss of the sense of taste is no doubt due (a) 

 to the lingual branch of the fifth nerve being a nerve of special sense; 

 partly, also, it is due (b), to the fact that this branch supplies, in the 

 anterior and lateral parts of the tongue, a necessary condition for the 

 proper nutrition of that part; while (c), it forms also one chief link in 

 the nervous circle for reflex action, in the secretion of saliva (p. 231, Vol. 

 I.). But, deferring this question until the glosso-pharyngeal nerve is to 

 be considered, it may be observed that in some brief time after complete 

 paralysis or division of the fifth nerve, the power of all the organs of the 

 special senses may be lost; they may lose not merely their sensibility to 

 common impressions, for which they all depend directly on the fifth 

 nerve, but also their sensbility to their several peculiar impressions for 

 the reception and conduction of which they are purposely constructed 

 and supplied with special nerves besides the fifth. The facts observed in 

 these cases can, perhaps, be only explained by the influence which the 

 fifth nerve exercises on the nutritive processes in the organs of the special 

 senses. It is not unreasonable to believe, that, in paralysis of the fifth 

 nerve, their tissues may be the seats of such changes as are seen in the 

 laxity, the vascular congestion, oedema, and other affections of the skin 



