Page:First course in biology (IA firstcourseinbio00bailrich).pdf/620

 * Read this line slowly. Can you see the star all the time? (If so, hold the book farther or closer and repeat.)

Within the coats of the ball, like the pulp within the rind of an orange, are the soft contents, divided into three parts. The first is a watery liquid in front, which serves to keep the cornea bulged out (Fig. 122). It is called the a'que-ous humor. The main cavity of the ball is occupied by a clear, jellylike substance called the vit're-ous humor, which serves to keep the ball distended. Back of the iris, and separating the two humors just named, is the crys'tal-line lens, a beautiful clear lens, convex or rounded out on both sides (Exp. 14). It serves to bring the light to a focus on the retina, thereby forming images of outside objects.

The eye, like a camera, has a dark lining, the choroid; the retina corresponds to the sensitive plate, and the lens brings the rays to a focus on it and forms the image.

showing that one nerve reaches same half of both eyes.

The Path of Light in the Eye.—The light enters through the transparent cornea and passes through the aqueous humor. As it goes through the pupil, the iris shuts off all the light that is not needed. The crystalline lens receives the light that has been allowed to pass, and so bends the rays that by the time they have passed through the vitreous humor they fall upon the retina in just the right way to form a tiny image of anything outside (Exp. 11). The choroid absorbs any light that passes the retina. The iris and choroid of albinos have no pigment; hence albinos squint their eyes to shut out some of the light.