166 FUNCTIONS OF THE NERVOUS SYSTEM. 



remain, these changes are much less decided. Further, it is well known that, 

 when the sight is destroyed by a disease or injury, which prevents the passage 

 of light through the pupil, the whole eye becomes more or less atrophied ; and 

 the Retina and Optic nerve, although previously sound, are found after death 

 (if the morbid condition have lasted sufficiently long) to have lost their charac- 

 teristic structure. It seems evident, then, that the continuance of the functional 

 operations of nerves, is a necessary condition of the maintenance of their nor- 

 mal organization ; and we can very well understand that this should be the 

 case, from the analogy of other parts of ( the system. 



222. The Optic nerve, though analogous to the Olfactory in all the points 

 hitherto mentioned, differs from it in one important respect; that it has the 

 power of conveying impressions which shall excite reflex muscular motions. 

 This is especially the case in regard to the Iris, the ordinary actions of which 

 are regulated by the degree of light impinging on the retina. When the optic 

 nerve is divided, a contraction of the pupil takes place ; but this does not occur 

 if the connection of this nerve with the third pair, through the nervous cen- 

 tres, be in any way interrupted. After such division (if complete), the state 

 of the pupil is not affected by variations in the degree of light impinging on 

 the retina, except in particular cases in which it is influenced through other chan- 

 nels. Thus, in a patient suffering under amaurosis of one eye, the pupil of the 

 affected eye is often found to vary in size, in accordance with that of the other 

 eye; but this effect is produced by the action of light on the retina of the 

 sound eye, which produces a motor change in the third pair on both sides. 

 Further, as has been formerly stated ( 205), the impression only of light upon 

 the retina may give rise to contraction of the pupil, by reflex action, when the 

 optic nerve is itself sound; whilst no sensations are received through the eye, 

 in consequence of disease in the sensorial portion of the nervous centres. 

 Another cause has been pointed out by Valentin, for the influence of light in 

 causing contraction of the pupil, and vice versa; that, if the rays impinge upon 

 the iris, a reflex stimulation is produced through the fifth pair; and he remarks 

 that the susceptibility of the iris to this kind of influence seems much increased 

 after the optic nerve has been divided. Besides the contractions of the pupil, 

 another action, which has been sometimes spoken of as reflex, is produced 

 through the optic nerve the contraction of the orbicularis under the influence 

 of strong light, or when a foreign body is suddenly brought near the eye. But 

 this cannot be produced by any mechanical stimulation, and it evidently in- 

 volves sensation; in fact, it is a movement of an emotional kind (Sect, xvui.), 

 produced by the painful effect of light, which gives rise to the condition well 

 characterized by the term photophobia. The involuntary character of it must 

 be evident to every one who has been engaged in the treatment of diseases of 

 the eyes; and the effect of it is aided by a similarly involuntary movement of 

 the eyeball itself, which is rotated upwards and inwards to a greater extent than 

 the Will appears able to effect. 



223. It will be convenient next to advert to the Auditory nerve, or Portio 

 Mpllis of the Seventh ; the functions of which are easily determined by anato- 

 mical examination of its distribution, and by observation of pathological pheno- 

 mena, to be analogous to triose of the two preceding. Atrophy or lesion of 

 the trunk destroys the sense of Hearing ; whilst irritation of it produces audi- 

 tory sensations, but does not occasion pain. From experiments made upon 

 the nerve before it leaves the cranial cavity, it appears satisfactorily ascer- 

 tained, that this nerve has no motor power either of a direct or reflex charac- 

 ter, and that it is not endowed with common sensibility. It is interesting to 

 remark, that microscopic examination of its structure clearly indicates its 

 intermediate character, between the nerves of special sensation issuing from 

 the anterior part of the cranium, namely, the Optic and Olfactory, -and those 



