Page:Popular Science Monthly Volume 1.djvu/479

Rh prey to desiccation, under the influence of which the optical homogeneousness, 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 network of nerves—and then assuredly you will not refuse your admiration to the optical excellence of this most indispensable of all windows.



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 a brackish, mucilaginous, 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 periodical 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 derangement 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