[Illustration: FIG. 25.--Vertical section through the eyeball and eyelids. (Pyle.)] Lastly there is the innermost _sensitive coat_ or _retina_, which has eight layers, the outer one containing some pigment cells and the next the rods and cones, in which 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 eyeball 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 point on an object must meet again in a common point upon the retina, their _conjugate focus_. In the normal eye all the rays from an object are focused on the retina and form upon it an image of the object which, as in the camera, is inverted, because of the crossing of the rays behind the cornea. Once focused on the retina the light traverses the various layers to the layer of rods and cones, where chemical action takes place and affects the little filaments of the optic nerve, by which the message is carried to the brain.