756 SIGHT. 



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 ; and the correlative widening of the pupil upon dimi- 

 nution 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 morphine and physostigmine is due, 

 chiefly at least, to a local action. The dilating effects of such drugs as 

 atropine 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 of an exhaustion of the cerebral contracting centre, assisted pos- 

 sibly by an increased activity of the dilating centre. 



638. There remains a word 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 inward (the two being closely allied, since the 

 eyes converge to see near objects), and the return to the more dilated condi- 

 tion 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 accommo- 

 dation are governed in this action by fibres that 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 of the third 

 ventricle, and which is especially connected with the most anterior bundles 

 of the roots of the third nerve. This centre is under the command of our 

 will ; when we wish to accommodate for near objects we throw it into action, 

 and, when in action, it calls also into action by " association " the centre for 

 the contraction of the pupil ; when the action of the accommodation centre 

 ceases and the eye falls back to the condition of rest, in which it is accom- 

 modated for far objects, the action of the pupil-contracting centre ceases 

 also, and the pupil therefore widens. 



639. The mechanism of accommodation may also be affected in a local 

 manner. And the drugs which have a special action on the pupil, such as 

 atropine and Calabar bean, also affect the mechanism of accommodation. 

 Atropine paralyzes it, so that the eye remains adjusted for far objects; and 

 physostigmine throws the eye into a condition of forced accommodation for 

 near objects. This double action has been explained by the supposition that 

 while atropine paralyzes, physostigmine throws into tonic or tetanic contrac- 

 tion, on the one hand, the circular muscles of the iris and, on the other, the 

 ciliary muscles ; but the phenomena, on inquiry, appear too complicated to 

 be explained in so simple a manner. 



We can accomodate at will ; but few persons can effect the necessary 

 change in the eye unless they direct their attention to some near or far 

 object, as the case may be, and thus assist their will by visual sensations. By 

 practice, however, the aid of external objects may be dispensed -with ; and 

 it is when this is achieved that the pupil may be made to dilate or contract 

 at pleasure, accommodation being effected without the eye being turned to 

 any particular object. 



