_Muscles_ and _Nerves_.--The eyeball is held in position by the ocular muscles, the conjunctiva, and the lids, while surrounding it, yet allowing free movement, is a thin membranous sac, the _tunica vaginalis oculi_. The superior and inferior recti muscles at the upper and lower edges of the ball turn the eye up and down; the internal and external recti at the inner and outer edges turn the eye inward and outward; and the superior and inferior oblique rotate the eye. The nerves supplying these muscles are the third or motor oculi, the fourth and the sixth. The _lachrymal gland_, which is about the size and shape of an almond, is situated at the upper and outer part of the orbit. It secretes a fluid which keeps the anterior surface of the eye bathed in moisture and is ordinarily drained away through the lachrymal sac in the inner canthus, whence it passes by the lachrymal ducts into the nose. When the amount secreted is excessive, it overflows the lower lid as _tears_. [Illustration: FIG. 24.--Diagram of the lacrimal apparatus. (Pyle.)] _Coats of Eye._--The membranes or coats of the eye are three in number: an outer or sclerotic, a middle or vascular, and an inner or sensitive. The _sclerotic coat_ is a rather thick, fibrous, protective membrane. Where it passes in front of the iris, however, it is thinner and transparent and is known as the _cornea_. The cornea projects somewhat and, as it were, resembles a segment of a smaller sphere set into the rest of the sclerotic. The middle or vascular coat, known as the _choroid_, carries blood-vessels for the retina or sensitive coat in its inner layer and has an outer layer of pigment cells that excludes light and darkens the inner chamber of the eye. The folds of the choroid at its anterior margin contain the ciliary muscles and are known as the _ciliary processes_, while the name _iris_ is given to the little round pigmented, perforated, curtain-like muscle just in front of the crystalline lens. The posterior surface of the iris is covered with a thick layer of pigment cells to prevent the entrance of light except through the central opening or _pupil_, and its anterior surface also has pigment cells that give it its color, though the difference in the color of people’s eyes is due rather to the amount of pigment present than to its color, a small amount of pigment being present in blue eyes and a large amount in brown and black eyes. Variations in the size of the pupil are brought about by contractions of the circular and radiating fibers of the iris, contraction of the circular fibers making it smaller and those of the radiating larger. The pupil is constricted for near objects and during sleep, and is dilated for distant objects. In a dull light also it dilates to let in more light, and in a bright light it contracts. The appearance of the pupil is often important as a means of diagnosis and in etherization.