56 DR KNOX on the Comparative Anatomy of the Eye. 



reasons have been assigned for this, yet it evidently serves td 

 give them timely warning of the approach of their enemies. In 

 fishes, which have neither ciliary muscle nor nerves, nor true 

 moveable iris, the powers of vision are extremely limited. It 

 may also be readily imagined why, in those persons from whose 

 eyes the lens has been extracted in the operation for cataract, 

 the customary powers of the eye should remain nearly as strong 

 as previous to the operation, and that, on the adaptation of a 

 compensating lens for that which has been removed, the person 

 should feel no difficulty in adjusting the eye as before. 



But I do not suppose that the whole of the phenomena of 

 the adjustment of the eye to the perception of objects placed at 

 various distances, depend entirely on the ciliary muscle. I have 

 every reason to believe, that the perception of objects placed at 

 very short distances, depends altogether, or nearly so, on the 

 contractions of the pupil or iris. I am quite aware of several 

 objections which might be offered to this theory, to obviate some 

 of which, I shall remark, that the eye, when not particularly 

 employed, remains in a middle state, i. e. we of preference re- 

 gard objects situated at moderate distances from us ; or, it may 

 be said, that we continually alter the focus from distant to near 

 objects, and vice versa. Now, when these changes go on rapid- 

 ly, there is little or no sense of fatigue, just as happens when the 

 various muscles of the body are put in action alternately, so that 

 gentle walking is always less fatiguing than standing still in pre- 

 cisely the same position. If an individual be directed to regard 

 a distant object, the most obvious change perceivable to the by- 

 stander is a sudden dilatation of the pupil, which dilatation is 

 necessarily in the ratio of the distance of the object, and the co- 

 lour of the iris. If the person be requested to examine the 

 same object with very great attention, the pupil will be seen 

 gently and slowly to contract, no doubt to throw out, as much 



