CHAP, ii.] SIGHT. 505 



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. Moreover the fifth is peculiarly related to the dilating effects 

 of the sympathetic. For though the ophthalmic ganglion does 

 receive fibres directly from the cavernous plexus of the sympathe- 

 tic, the dilating action of the sympathetic would seem to be carried 

 out not by these fibres but by fibres joining the fifth nerve, and 

 passing to the iris not by the ganglion but by the ophthalmic 

 branch and the long ciliary nerves. The vaso-motor fibres of the 

 sympathetic, and those which dilate the iris, after running to- 

 gether in the main cervical sympathetic chain, part company 

 higher up, the latter passing to the Gasserian ganglion, and thus 

 reaching the nasal branch of the ophthalmic division of the fifth 

 nerve. Some observers maintain that in addition to these dilating 

 fibres of the sympathetic joined to it, the fifth contains fibres of its 

 own which also are able to dilate the pupil. 



We may sum up the nervous mechanism of the pupil then 

 somewhat as follows. The salient and most frequently repeated 

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

 reflex act, the centre of which is placed in the brain ; arid 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 contraction 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 contracting 

 centre, but the narrow pupil caused by such drugs as morphia 

 and physostigmiii is due, chiefly at least, to a local action. The 

 dilating effects of such drugs as atropin are also largely due to a 

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

 poisoning and of dyspnoea we can probably trace the effects cf 

 an exhaustion of the cerebral contracting centre, assisted possibly 

 by an increased activity of the dilating centre. 



There remains a w r ord to be said concerning the contraction of 

 the pupil which takes place when the eye is accommodated for near 

 objects, and when the pupil is turned inwards (the two being 

 closely allied, since the eyes converge to see near objects), and the 

 return to the more dilated condition when the eye returns to rest 

 and regains the accommodation for far objects. These are instances 

 of what are called " associated movements." Two movements are 

 thus spoken of as " associated " when the special central nervous 

 mechanism employed in carrying out the one act is so connected 

 by nervous ties of some kind or other with that employed in 

 carrying out the other, that when we set the one mechanism 

 in action we unintentionally set the other in action also. The 

 ciliary muscles which bring about accommodation are governed in 

 this action by fibres which may be traced, through the ciliary 

 nerves and lenticular ganglion, along the third or oculo-motor 

 nerve, to a centre which lies (in dogs) in the hind part of the floor 



