the power of perception is supposed to lie, branches of the optic nerve being distributed over it in all directions. In fact, the retina is formed by a membranous expansion of the optic or second cranial nerve, the special nerve of sight, which passes into the orbit through the optic foramen at the back and enters the eye-ball close to the macula lutea or yellow spot. The exact spot where the optic nerve enters the retina is not sensitive and is known as the blind spot. In the center of the macula lutea, however, which is in the middle of the retina posteriorly, is a tiny pit, the fovea centralis, in which all the layers of the retina except the rods and cones are absent, and at this point vision is most perfect. It is, therefore, always turned toward the object looked at, and when one wishes to see an object distinctly, he must keep moving his eyes over it that the rays from each part may fall in turn upon the fovea centralis.
Directly behind the pupil is the crystalline lens, a rather firm gelatinous body enclosed in a capsule, which is transparent in life but opaque in death. The lens is doubly convex and is held in place by the suspensory ligaments, which arise from the ciliary processes. In front of it is the anterior chamber of the eye, filled with a thin watery fluid called the aqueous humor, while the larger space back of it, occupying about four-fifths of the entire globe, is filled with a jelly-like substance known as the vitreous humor.
The chief artery of the eye is the ophthalmic.
Light Rays.—The eye is practically a camera and its principal function is to reflect images. Although there are several refracting surfaces and media, for practical purposes the cornea alone need be considered. Except for those rays which enter the eye perpendicularly to the cornea, whose line of entrance is called the optic axis, all rays are refracted when they enter the eye and the point at which they meet and cross each other behind the cornea is called the principal focus of the eye. To focus properly, all the rays from any one
