DR KNOX on the Comparative Anatomy of the Eye. 57 



as possible, all the unnecessary rays of light ; this exertion may 

 be continued for some time, but every one knows, that it at last 

 becomes so irksome, that the eye involuntarily returns to a 

 middle state. We might suppose this sense of fatigue to arise 

 from our attempts to contract the iris, but this is by no means 

 probable ; it seems, to me, to arise from the exertions of the ci- 

 liary muscle, for it is extremely difficult to imagine why fatigue 

 should arise from a moderate contraction of the iris in viewing 

 distant objects, when we know that the same membrane is ge- 

 nerally much more contracted in its ordinary state. Moreover, 

 the eye cannot long regard distant objects without fatigue, even 

 though the attention be not particularly fixed thereon ; now it 

 cannot be the iris in this case. Lastly, Distant objects are seen 

 with tolerable precision when the pupil has been dilated by the 

 action of belladona ; now, here, the iris can scarcely be supposed 

 capable of much action. If a person who has been for some time 

 regarding distant objects, be now requested to examine one very 

 near, the pupil instantly contracts, and continues to do so in 

 proportion as the object is made to approach the eye. At last 

 the pupil reaches its maximum of contraction, and if the object 

 still approach the eye, the fatigue of distinct perception becomes 

 insupportable ; the iris suddenly returns to a middle state of con- 

 traction, and the object is no longer perceived. When bellado- 

 na has been applied to the eye, near objects cannot be perceived, 

 and the eye is said to have changed its focus, and to have be- 

 come long-sighted. These expressions are incorrect, the eye has 

 lost its power of viewing very near objects, because the iris can 

 no longer contract. It is also stated, that distant objects are 

 perceived as well after as before the use of the belladona, of the 

 accuracy of which assertion we may reasonably entertain doubts. 

 The inner membrane of the choroid has been well investigated 

 by anatomists ; there can be little doubt of its forming the cili- 

 VOL. x. p. i. H' 



