

THE WINDOWS. 149 



dow, just as a pane of glass is set in the frame ; 

 with this difference, however, that a pane of 

 glass is seldom round, but the cornea is as 

 round as a dollar. It also stands out from the 

 eye, like the crystals of most watches. The 

 rays of light- enter the eye through this cornea, 

 and pass to the back part of it. What we 

 call the white of the eye is the sclerotica, or 

 window frame, as far as we can see it, sur- 

 rounding the cornea. 



The tunica sclerotica, or sclerotic coat of 

 the eye, is lined by another thin coat called 

 the choroides. The internal surface of the 

 choroides is covered all over, except at the 

 back part, where the optic nerve enters, with a 

 thin sooty kind of black paste, called by anato- 

 mists the pigmentum nigrum, which means 

 black pigment. You see this represented very 

 fairly in the engraving, and as this is spread 

 over the choroides, and the choroides only 

 lines the sclerotica, and does not extend to the 

 cornea, you can easily see where the latter be- 

 gins. 



Where the sclerotica and cornea join, a kind 

 of circular membrane or curtain runs inwards, 

 13* 



