CHAP, in.] SIGHT. 869 



the eye returns to rest and regains the condition adapted for 

 viewing far objects. These are instances of what are called 

 "associated movements." A similar instance is afforded by cer- 

 tain cases of blindness of one eye due to atrophy of the optic 

 nerve ; in such cases the pupil of the blind eye may be wholly 

 insensible to light, and yet becomes narrowed when the subject 

 looks at a near object with the sound eye. In so doing he throws 

 into action the accommodation mechanism, and with that the 

 pupil-constricting mechanism of both eyes. 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 carry- 

 ing out the other, that when we set the one mechanism in action 

 we unintentionally set the other in action also. In this constric- 

 tion of the pupil associated with accommodation the nervous ties 

 between the parts of the central nervous system concerned in 

 the generation of the will, the centre for accommodation, and the 

 centre for the constriction of the pupil, are such that whenever 

 the will stirs up the impulses necessary to carry out accommoda- 

 tion, it at the same time stirs up corresponding impulses in 

 the pupil-constrictor mechanism. More than this we cannot at 

 present say. 



We can, as we have said, accommodate at will ; few persons 

 only 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 seem to be made to 

 dilate or contract at pleasure, accommodation being effected 

 without the eye being directed to any particular object. 



