PARTS OF THE EYE 417 



The larger chamber is filled with a jelly-like liquid called 

 the vitreous (or "glassy") humor. These two humors 

 keep the eye round and full. They and the crystalline 

 lens together make up a double-convex lens (cf. 178), 

 and throw on the retina an inverted image of the object 

 seen. 



The outside coating of the eyeball is tough and strong; 

 it is called the sclerotic (skler-ot'-ic) coat. It is opaque 

 everywhere except in front, where it becomes trans- 

 parent, and is called the cornea. The "white of the eye" 

 is the visible part of the sclerotic coat. The choroid 

 (ko'-roid) coat is inside of the sclerotic coat. It is dark in 

 color, and acts like the dark interior of a camera: it 

 absorbs light, and so prevents the light from being 

 reflected back and forth inside the eye. The iris is a part 

 of the choroid coat; it acts as a curtain for the eye, and 

 gives the eye its color. The pupil is an opening in the 

 center of the iris. It is controlled by two sets of muscles, 

 which open or close it, according to the amount of light. 

 In the cat the pupil becomes a narrow slit, when the light 

 is bright. 



The retina (ret'-i-na) is the innermost coat of the 

 eye; it covers the back portion of the choroid coat. The 

 retina is very complex. By a delicate arrangement of 

 cells it arouses afferent light impulses in the neurons of 

 the optic nerve. The blind spot of the eye is the place 

 where the optic nerve begins to spread out to form the 

 retina. 



The eyeball is held in place, and moved about, by means of 6 small 

 muscles. The transparent, front portion of the eye must be kept moist 

 and clean, hence the eye has lachrymal (lak'-ri-mal), or 'Hear," 



