260 OUR BODIES AND HOW WE *LIVE 



This outer coat of the eyeball gives place in the front 

 to a transparent circular plate, just as in a watch the gold 

 or silver case gives place to a glass crystal over the face. 

 This transparent plate, the cornea, forms a kind of rounded 

 bay window for the eye, and is often spoken of as the 

 window of the eye. 



The second coat, or choroid, is much more delicate in 

 structure and consists almost entirely of blood vessels and 

 nerves. It is lined with a thick, black coating designed to 

 absorb the surplus rays of light, which would otherwise 

 cause blurred or confused vision. In many animals the 

 choroid is rich with colors; hence the green sheen of a 

 cat's eye. 



358. The Retina. The retina, meaning a net, the inner- 

 most coat of the eyeball, is an extremely delicate and sen- 

 sitive screen upon which the image is formed. It is a net 

 of fibers proceeding from the optic nerve and spread out 

 over the inner surface of the eye. 



359. The Iris and Pupil. To get a clear idea of the 

 inner parts of the eye, let us imagine an eyeball cut through 

 the middle from above downwards. Let us now start in 

 front and go backwards (Fig. 161). 



We shall first see the cornea, which has just been described. 



We now reach a space called the front chamber of the 

 eye. In this chamber, and behind the cornea, is hung a 

 round curtain, the iris, meaning rainbow. It is pierced by 

 a hole through its center for the admission of light. This 

 is called the pupil, which appears as if it were a black spot. 

 It is this curtain which gives the color to the eye. The 

 iris has muscular fibers which contract and relax, and thus 

 make the pupil larger or smaller, according as the light is 

 bright or dull. 



