CHAP, in.] SIGHT. 867 



anism, such as might be afforded by the nerve cells lying in the 

 choroid or even in the iris. They appear to act directly on the 

 sphincter, atropin paralyzing it or producing relaxation, and 

 physostigmin increasing or producing contraction, both often 

 of an extreme character. Whether the drugs act on the actual 

 muscular tissue itself or on the endings of the nerve fibres in the 

 muscular tissue, or on both together, and how far their effect 

 is due to changes in the special dilator muscle, are questions 

 which we need not discuss here. The important point is that 

 the action of both these drugs is a local one ; hence, when they 

 have produced their full effects, the normal nervous mechanisms 

 on which we have been dwelling are of little or no use ; even an 

 abundance of light leads to no constriction in the full atropinized 

 eye, and removal of light produces little or no dilation in an eye 

 fully under the influence of physostigmin. 



We may here mention the fact that in certain animals at all 

 events, for instance the eel, light falling into the eye, even into 

 an extirpated eye, will cause constriction of the pupil ; and this 

 seems to be brought about by means of some nervous connection 

 between the retina and the iris, for the effect ceases when the 

 retina is destroyed. But this peculiar action has not yet been 

 satisfactorily explained. 



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

 be chiefly at least 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. 



We may sum up the nervous mechanism of the pupil then 

 somewhat as follows. The salient and most frequently repeated 

 event, the constriction of the pupil upon exposure to light, is a 

 reflex act, the centre of which is placed in the brain ; and the 

 correlative widening of the pupil upon diminution of light is due 

 to the tonic action of the sympathetic making itself felt upon 

 the waning of its antagonist. The dilating effects of emotions, 

 of sensory impressions, especially painful ones, and of dyspnoea 

 appear to be brought about by an increased activity of the dila- 

 ting centre, assisted possibly in the latter instance by a depres- 

 sion of the constricting centre. The constriction of the pupil 

 in the earlier stages of the action of alcohol and chloroform and 

 in slumber is probably due to an increased action of the con- 

 stricting centre, but the narrow pupil caused by such a drug as 

 physostigmin is due, chiefly if not exclusively, to a local action. 

 The constricted pupil of morphia appears to be due partly to 

 central and partly to local action. The dilating effects of such 

 a drug as atropin are chiefly if not exclusively due to a local 

 action, but in the widened pupil of the later stages of alcohol 

 poisoning and of some other drugs we can probably trace the 

 effects of an exhaustion of the constricting centre, assisted pos- 

 sibly by an increased activity of the dilating centre. 



