EYE. 381 



The crystalline lens stands between the vitreous and the 

 aqueous humors, (Fig.LXVI. c.) This is a double convex 

 lens, much more dense than the vitreous humor, and holds its 

 shape without any covering. It is composed of concentric 



FIG. LXVI. Section of the Eye. 



a, Coats of the eye. 



b, Aqueous humor. 



c, Crystalline lens. 



d, Vitreous humor. 



e, Cornea. 

 /,/, Iris. 



g, Optic nerve. 

 h, Retina. 



layers like those of an onion, which can be removed one from 

 another. In the fish, this lens is globular. In man, it ap- 

 proaches flatness. 



886. Three membranes or coats enclose these humors, and 

 retain the eye in its globular shape. 



The outer, or sclerotic coat, like the dura mater of the 

 brain, is very firm and strong, and able to resist considerable 

 force without being broken. It covers almost the whole eye. 

 It has a large aperture in front, in which the cornea is placed, 

 (Fig. LXVI. e.) The cornea covers the front of the eye. It is 

 transparent and strong. It projects in the shape of a watch 

 glass, and covers the aqueous humor. 



887. The middle, or choroid coat of the eye is very delicate 

 and soft. It contains a black pigment, which absorbs such 

 rays of 1 ight as are not needed for vision. 



888. The third or inner coat is the retina^ which is prin- 

 cipally the expansion of the optic nerve. This receives the 

 rays of light from the objects which are presented to the eye. 



889. In the front part of the eye are, 1st. The cornea. 

 2d. The aqueous humor. 3d. The iris and pupil. 4th. The 

 crystalline lens, and then the vitreous humor. 



The iris is a very delicate circle, or continuation of the 

 middle or choroid coat, (Fig. LXVI. f^f.) The pupil is an 



