DIOPTRIC MECHANISMS. 755 



contraction of the pupil cease to be manifest, but active dilatation occurs, on 

 account of the tonic dilating influence of the sympathetic being left free to 

 work. When, on the other hand, the sympathetic is divided, this tonic 

 dilating influence falls away, arid contraction results. When the optic or 

 third nerve is stimulated, the dilating, effect of the sympathetic is overcome, 

 and contraction results ; and when the sympathetic is stimulated, any con- 

 tracting influence of the third nerve which may be present is overcome, and 

 dilatation ensues. 



636. But there are considerations which show that the matter is still 

 more complex than this. A small quantity of atropine introduced into the 

 eye or into the system causes a dilatation of the pupil. This might be 

 attributed to a paralysis of the third nerve, and, indeed, it is found that 

 after atropine 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, dilatation is still further increased by atropine. When 

 physostigmine 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 dilatation which is caused by a suffi- 

 cient dose of atropine may be greater than that which can ordinarily be 

 produced by stimulation of the sympathetic, and the contraction caused by 

 a sufficient dose of physostigmine may be greater than that which is ordi- 

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

 even than that produced by direct stimulation of the third nerve. Evi- 

 dently 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 dilatation, 

 the other in such a way as to cause contraction. Such a local mechanism 

 cannot, however, 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 choroid, 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. Fur- 

 ther 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 stimu- 

 lation 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 contrac- 

 tion or dilatation respectively of the pupil, needs fuller discussion than we 

 can afford to give to it here. We may add that the local action of atropine 

 in contrast to any -action on the cerebral centre is well illustrated by apply- 

 ing atropine 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. 



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

 part a sensory one ; the iris is sensitive, and the sensory impulses which are 

 generated in it pass from it along the fibres of the fifth nerve. 



