SIGHT AND THE VISUAL ORGAN. 



465 



prey to desiccation, under the influence of which the optical homo- 

 geneousness, on which transparency depends, is lost, you will readily 

 acknowledge the amount of resistance the cornea is enabled to offer. 

 But further, consider that the cornea does not possess a homogeneous 

 structure, but consists of five different, partly compound layers ; that 

 it hides in its interior numerous cellular bodies, canals for the passage 

 of the humors, and a net-work of nerves and then assuredly you will 

 not refuse your admiration to the optical excellence of this most indis- 

 pensable of all windows. 



Fig. 3. 



A B, Object of Sight; a 5, Image on the Ketina. 



However, the difficult task implied in the structure of the cornea 

 could not be fulfilled without the aid of some extraneous appliances. 

 Thus, two movable covers lie over the eye, namely, the eyelids, whose 

 inner surface is a compound humorous matter, 1 a brackish, mucilagi- 

 nous, fatty solution. After having used our eye for a while, there arises 

 a certain sensation of dryness on the cornea, from exposure to the air ; 

 the ever-recurring necessity of renewing the moistness causes us to 

 close the lids, or, as we say, to wink. This is at least the chief design 

 of the dropping of the lids, which besides lend their aid to the peri- 

 odical exclusion of the irritation of the sight, as in sleep, for defence 

 against the dazzling light, and for protection against the impurities 

 in the atmosphere. The cornea is likewise being continually moistened 

 by the posterior humors. But, with all the expedients and appliances 

 used by Nature, perfect transparency cannot be always preserved: 

 dull spots are formed on the window of the cornea, often causing de- 

 rangement of vision. Unimportant irritations, which on the surface 

 of the body are not noticed, seriously affect the cornea. 



Let us now notice the second coat, the choroid. This we compared 

 to the black coating of paint in the camera-obscura. Thinking of the 

 dazzling and delusive visions which are a consequence of the gradual 

 consuming away of the pigment in the choroid, or which accompany 

 the entire want of it, as with the albinos, we cannot doubt that one 



1 This matter receives the name of mixed tears, contrary to the briny tears, which, by 

 mechanic irritation or during weeping, flow from the eye. 

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