250 Life and Health 



into the sclerotic, somewhat as the crystal fits into the 

 metallic case of a watch, forming a covering for its dial. 

 It projects from the general surface of the eyeball, not 

 unlike a rounded bay window, and is often spoken of as 

 the " window of the eye." 



390. The Choroid Coat. Lining the inner surface of the 

 sclerotic is the second coat, the choroid, which is made up 

 almost entirely of blood vessels and nerves. As the choroid 

 approaches the front part of the eyeball its parts become 

 folded upon themselves into a series of ridges, called ciliary 

 processes. These folds gradually become larger and at last 

 merge into the ciliary or accommodation muscle of the eye. 

 391. The Iris and Pupil. The circular space thus left 



front by the termination of the choroid is occupied by 

 the iris, a thin circular curtain, suspended in the aqueous 

 humor behind the cornea and in front of the crystalline lens. 



In the center of the iris is a round opening for the 

 admission of light. This is the pupil, which appears as if 

 it were a black spot. It seems to be black, just as the 

 open doorway to a dark closet seems black. The back of 

 the iris is lined with dark pigment. The color of the iris 

 varies with the quantity of this pigment, being darker when 

 the coloring matter is abundant. This pigment layer and 

 that of the choroid and retina absorb the light entering 

 the eye, so that little is reflected. 



-V 392. The Retina. The third and innermost coat of the 

 eyeball is the retina, upon which the images of external 

 objects are received. It lines all but the extreme front of 

 the inner surface of the posterior chamber. 



The retina is a very thin, delicate membrane which forms 

 the terminal organ of vision. It is really an expansion of the 

 ultimate fibers of the optic nerve, by means of which 

 impressions are sent to the brain. 



