THE EYE.
The apparatus of vision comprises the essential organ, the globe of the eye or eyeball, and its accessory parts or appendages. The eyeball is situated in the orbital fossa, mentioned in chapter on the bones of the head. It is chiefly made up of several coats around the outside, and in the center by the humours of the eye. On the inner side of these coats is a thin membrane called the retina, which contains the branches of the optic nerve. This receives the reflections of objects as they pass through the humours of the eye and from which the sensation passes along the optic nerve to the brain. The oblong opening seen in the middle of the eye is known as the pupil. If a horse be led from a dark stall into the light and the pupils of the eyes watched closely, it will be noticed that they get smaller, but on returning it to the stall the pupils will be noticed to dilate or get larger; thus it is seen that the pupils do not always remain the same size. The chief use of the pupil is to gauge the sight. At the back part of the eye are several muscles attached from around the eye to the bones in the fossa. These muscles move the eye and assist in holding it to its place. Around the front part are two movable curtains, one above and the other below, called eyelids, the use of which is to open and close the eye, and also to protect it from injuries. Around the free border of the eyelids are what is known as the eyelashes, which keep foreign substances from falling into the eye. Situated in the inner angle is what is known as the haw of the eye; this membrane also helps to protect it. In the corner of this angle is a small duct or opening, through which a fluid called the tears passes down into the nasal tubes, from whence it is carried down through the bones of the head and emptied into the under part of the nostril or nose. A small gland is situated on the upper part of the eye. This gland secretes the tears which lubricates the eyes. The color of the eye is generally brown, but in some cases it is white. It is then called a moon eye.