44 MOVEMENTS OF THE PUPIL. [BOOK in. 



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 HI ires in the muscular tissue, 

 or on both together, is a question which we cannot discuss here. 

 Nor need we stay to consider the difficulties which are added, 

 if we accept the view of the existence of a special dilator muscle. 

 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 h'fth 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. Some observers maintain that in addition to the 

 dilating fibres of the sympathetic which reach the iris after having 

 joined the fifth nerve, the latter nerve also contains fibres of its 

 own which passing by the ophthalmic branch and long ciliary 

 nerves are able to dilate the pupil ; but this is doubtful. 



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 dilating 

 centre, assisted possibly in the latter instance by a depression 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 constricting 

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

 stigmin 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 possibly by an 

 increased activity of the dilating centre. 



