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nation would be out of place : the only other point 

 to which I intend to allude is the action of the iris. 

 It is well known that the pupil, as it is called, 

 expands or contracts, as the light is withdrawn or 

 shed upon the eye. This is occasioned by the ex- 

 pansion or contraction of the iris ; the former par- 

 tially closing up the perforation called the pupil, so 

 as to allow less of the dark interior surface of the 

 choroides to be visible through the aperture ; the 

 contraction of the iris, on the other hand, dilating the 

 opening, so as to expose a larger portion of the cho- 

 roides. Some eminent anatomists have ascertained 

 that the iris consists of muscular fibre, though so 

 delicate as almost to exceed the power of concep- 

 tion to those who are not conversant with the extra- 

 ordinary powers of nature ; exhibited perhaps to 

 greatest advantage in the minutest of her works. 



If this imperfect sketch of the construction of 

 that wonderful organ, the eye, should only have the 

 effect of tempting my reader to a personal exami- 

 nation of it for himself, he will not grudge the idle 

 hour that he may have been tempted to throw away 

 on my previous pages. 



The pupil of the human eye in the healthy state, 

 has always a black appearance, such being the 

 colour of the human choroides : in animals it 

 varies extremely, and on very recent dissection has 



