FIG. 62. [Illustration: _The Eyelashes and the Tear Glands._] EYELIDS AND TEARS.--The eyelids are close-fitting shutters to screen the eye. The inner side is lined with a mucous membrane that is exceedingly sensitive, and thus aids in protecting the eye from any irritating substance. The looseness of the skin favors swelling from inflammation or the effusion of blood, as in a "black eye." The eyelashes serve as a kind of sieve to exclude the dust, and, with the lids, to shield against a blinding light. Just within the lashes are oil glands, which lubricate the edges of the lids, and prevent them from adhering to each other. The tear or _lachrymal_ gland, _G_, is an oblong body lodged in the bony wall of the orbit. It empties by several ducts upon the inner surface, at the outer edge of the upper eyelid. Thence the tears, washing the eye, run into the _lachrymal lake_, _D_, a little basin with a rounded border fitted for their reception. On each side of this lake two canals, _C_, _C_, drain off the overplus through the duct, _B_, into the nose. In old age and in disease, these canals fail to conduct the tears away, and hence the lachrymal lake overflows upon the face. FIG. 63. [Illustration: _Structure of the Retina._] STRUCTURE OF THE RETINA.--In Fig. 63 is shown a section of the retina, greatly magnified, since this membrane never exceeds 1/80 an inch in thickness. On the inner surface next to the vitreous humor, is a lining membrane not shown in the cut. Next to the choroid and comprising about 1/4 the entire thickness of the retina, is a multitude of transparent, colorless, microscopic rods, _a_, evenly arranged and packed side by side, like the seeds on the disk of a sunflower. Among them, at regular intervals, are interspersed the cones, _b_. Delicate nerve fibers pass from the ends of the rods and cones, each expanding into a granular body, _c_, thence weaving a mesh, _d_, and again expanding into the granules, _f_. Last is a layer of fine nerve fibers, _g_, and gray, ganglionic cells, _h_, like the gray matter of the brain, whence filaments extend into _i_, the fibers of the optic nerve. (See p. 354.) The layer of rods and cones is to the eye what the bristles, otoliths, and Cortian fibers are to the ear. Indeed, the nerve itself is insensible to light. At the point where it enters the eye, there are no rods and cones, and this is called the _blind spot_. A simple experiment will illustrate the fact. Hold this book directly before the face, and, closing the left eye, look steadily with the right at the left-hand circle in Fig. 64. Move the book back and forth, and a point will be found where the right-hand circle vanishes from sight. At that moment its light falls upon the spot where the rods and cones are lacking. FIG. 64. [Illustration:]