SENSE ORGANS 
473 
brane, the cornea (kor'ne-a); and the rest of the eye is sur¬ 
rounded by a tough membrane, the sclerotic coat, or the 
white of the eye. Within the combined covering of the 
cornea and sclera are a number of structures which take 
part in receiving and transmitting the rays of light to the 
brain. 
A cross section of the eye shows two more membranes 
in close relation to the sclerotic coat (Figure 416). The 
membrane in direct contact on the inside with the sclerotic 
layer is the choroid (kfl'roid). The choroid coat is filled 
with blood vessels and pig¬ 
ment. Through this layer 
the food in the blood is 
distributed to the eye. 
The third layer or coat is 
the retina, which is com¬ 
posed of nerve cells and 
is nearly transparent. 
The cornea and these 
three layers inclose two 
chambers which are sepa- Figure 416. —Section of Eye. 
rated by the lens (Figure C, cornea; C', choroid layer; I, iris; 
416). In front of the lens 1 C ’ inner cham ber; O. C, outer cham- 
..... . ber; L, lens; O. N, optic nerve; 
a curtain-like membrane, R> retina; St sc i er otic coat. 
the iris, partly covers the 
lens, except for a round opening in the center which is 
called the pupil. The color of the eye, gray, black, blue, 
or brown, is due to the presence of pigment in the iris. The 
small front chamber is filled with a transparent fluid which 
is composed principally of water and is known as the aqueous 
(a'kwe-us) humor. The large back chamber is filled with a 
thin, transparent, jellylike fluid, the vitreous (vit're-iis) humor. 
In order that we may see any object, a pencil in our 
hand, for example, two general conditions must be present. 
The picture (image) of the pencil must be placed on the 
