NERVES OF SPECIAL SENSE. OPTIC. 841 



ous centres, 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 im- 

 pinging on the retina ; except in particular cases, in which it is influenced 

 through other channels. Thus, in a patient suffering under amaurosis of one 

 eye, the pupil of the affected eye is often found to vary in size, in accord- 

 ance 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 ( 395), 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 sen- 

 sations are received through the eye, in consequence of disease in the sen- 

 sorial portion of 'the nervous centres. Although the contraction of the pupil 

 is effected by the influence of motor fibres, which proceed to the sphincter 

 of the Iris from the third pair of nerves, through the Ophthalmic ganglion, 

 there is evidence that its dilatation depends rather upon the influence it de- 

 rived from that ganglion itself, and from the Sympathetic system, of which it 

 forms part. Some have attempted to show, that the actions of the iris are in 

 a slight degree voluntary, because, by an effort of the will, they could occa- 

 sion contraction of the pupil ; but this so-called voluntary contraction is al- 

 ways connected with a change in the place of the eyeball itself, occasioned 

 by an action of some of its muscles. It is principally noticed under the two 

 following conditions : 1. When an object is brought very near the eye, and we 



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steadily lix our attention upon it, the axes of the two eyes are made to con- 

 verge ; and if this convergence be carried to a considerable extent, so that the 

 pupils of both eyes are sensibly directed towards the inner canthus, a con- 

 traction of the pupil takes place. The final cause or purpose of this contrac- 

 tion is very evident. When an object is brought near the eye, the rays pro- 

 ceeding from it would enter the pupil (if it remained of its usual size) at an 

 angle of divergence, so much greater than that which would allow them to 

 be properly refracted to a focus, that indistinct vision would necessarily re- 

 sult. By the contraction of the pupil, however, the extreme or most diver- 

 gent rays are cut off, and the pencil is reduced within the proper angle. The 

 principle is precisely the same as that on which the optician applies a stop 

 behind his lenses, which reduces their aperture in proportion to the shortness 

 of their focal distance. 2. Contraction of the pupil is also noticed, when the 

 eyeball is performing that rotation upwards and inwards, which, when per- 

 formed along with violent respiratory actions, or during sleep, must be regarded 

 as involuntary. This rotation also takes place, to a slight degree, when the 

 eyelid is depressed, as in ordinary winking ; and it is obvious that, in this 

 manner, the surface of the eye is more effectually swept free from impurities 

 which may have gathered upon it, than it would be by the downward motion 

 of the lid alone. But the pupil is not contracted, when the eyeball is volun- 

 tarily rotated upwards and inwards. 



444. 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 muscle under the influence of strong lighf, or 

 when a foreign body is suddenly brought near the eye. But this cannot be 

 produced by any mechanical stimulation, and it evidently involves sensation ; 

 in fact, it is a movement of a consensual kind, 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. 



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