504 MOVEMENTS OF THE PUPIL. [BOOK in. 



might be attributed to a paralysis of the third nerve, and indeed it 

 is found that after atropin has produced its effects the falling of 

 light on the retina no longer causes contraction of the pupil. A 

 difficulty however is introduced by the fact that when the third 

 nerve is divided, and when therefore the contracting effects of 

 stimulation of the retina are placed entirely on one side, and there 

 is nothing to prevent the sympathetic producing its dilating effects 

 to the utmost, dilation is still further increased by atropin. When 

 physostigmin is introduced into the eye or system, contraction of 

 the pupil is caused, whether the third nerve be divided or not ; and 

 when the dose is sufficiently strong the contraction is so great that 

 it cannot be overcome by stimulation of the sympathetic. The 

 dilation which is caused by a sufficient dose of atropin may be 

 greater than that which can ordinarily be produced by stimulation 

 of the sympathetic, and the contraction caused by a sufficient dose 

 of physostigmin may be greater than that which is ordinarily 

 produced in a reflex manner by stimulation of the optic nerve, 

 or even than that produced by direct stimulation of the third 

 nerve. Evidently these drugs act either directly on the plain 

 muscular fibres of the iris or on some local mechanism, the 

 One in such a way as to cause dilation, the other in such a way 

 as to cause contraction. Such a local mechanism cannot how- 

 ever lie in the ophthalmic ganglion, for both drugs continue to 

 produce these effects in a most marked degree after the ganglion 

 has been excised. We must suppose therefore that the mechanism 

 if it exists is situated in the iris itself or in the chorcid, where 

 indeed ganglionic nerve-cells are abundant. The movements of 

 the iris in the extirpated eye, spoken of just now, may perhaps 

 be attributed to the same local mechanism. Further it is stated 

 that with stimulation of the sympathetic, the latent period, i.e. 

 the period intervening between the beginning of stimulation and 

 the beginning of the movement of the iris, is much greater than 

 with stimulation of the third nerve, indicating that the former 

 acts through a local mechanism but the latter more directly on 

 the muscular fibres. The whole question however of this local 

 mechanism, and of the exact mode of action of the various drugs 

 and of the changes in the body which lead to contraction or 

 dilation respectively of the pupil, needs fuller discussion than 

 we can afford to give to it here. We may add that the local 

 action of atropin in contrast to any action on the cerebral centre 

 is well illustrated by applying atropin to one eye locally. The 

 pupil of that eye dilates widely ; in consequence more light falls 

 on the retina, and this so affects the cerebral centre, which as 

 we have. seen is not strictly unilateral but in communion with 

 its fellow, that increased constricting impulses pass from both 

 centres, and these, though ineffectual in the atropinized eye, 

 lead in the untouched eye to an increased narrowing of the pupil. 

 The share of the fifth nerve in the work of the iris seems to be 



