46 MOVEMENTS OF THE PUPIL. [BOOK in. 



but as a matter of fact it is very much shorter, indeed the act is 

 an exceedingly rapid one. This and other facts indicate that our 

 knowledge of the mechanism of accommodation is far from being 

 complete. * 



731. There remains a word to be said concerning the con- 

 striction of the pupil which takes place when the eye is accom- 

 modated for near objects, and when the pupil is turned inwards 

 (the two being closely allied, since the two eyes converge to see 

 near objects), and the return to the more dilated condition when 

 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 certain 

 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-constrict- 

 ing 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 carrying out the other, that 

 when we set the one mechanism in action we unintentionally set 

 the other in action also. In this constriction of the pupil asso- 

 ciated 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 con- 

 striction of the pupil, are such that whenever the will stirs up the 

 impulses necessary to carry out accommodation, 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 with- 

 out the eye being directed to any particular object. 



