53. THE ORBITS.--The eyeball, which is a delicate organ, is well defended against external injury within the orbits or bony sockets of the head. These are deep conical hollows, bounded in part by the bones of the skull, and in part by those of the nose and cheek. The orbit juts out beyond the most exposed portion of the eyeball, as may be seen by laying a book over the eye, when it will be found that no part of the eyeball, unless it be very prominent, will be touched by the book; so that the only direction in which an injury is liable to be received is immediately in front of the eye. The overhanging brow is itself covered by a layer of thick skin, studded with short, stout hairs, which are so bent as to prevent the perspiration from running into the eye and obscuring vision. Through a hole in the bottom of the orbit, the nerve of sight passes outward from the brain. The orbit also contains a considerable amount of a fatty tissue, upon which, as upon an elastic cushion, the eye rests. {200} [Illustration: FIG. 47.--FRONT VIEW OF RIGHT EYE. (Natural Size.) 1. The Lachrymal, or tear gland, lying beneath the upper eyelid. 2. The Nasal Duct is shown by the dotted line. The * marks the orifice in the lower lid. The central black spot is the _pupil_; surrounding it is the _iris_; and the triangular white spaces are the visible portion of the _sclerotic_.] [Sidenote: 54. What are the eyelids? The upper lid? The lower one? The mucous membrane of the eye?] 54. THE EYELIDS.--The eyelids are two movable curtains, or folds, which, when shut, cover the front part of the orbit, and hide the eye from view. The upper lid is the larger, has a curved margin, and moves freely, while the lower lid is comparatively short and straight, and has but a slight degree of motion (Fig. 47). Skin covers the exterior of the lids, while a fine mucous membrane lines their inner surface, and is likewise spread out over the entire front of the eyeball. This membrane, which is called the _Conjunctiva_, is highly sensitive, and thus plays an important part in protecting the eye against the lodgment of sand, ashes, chaff, and other foreign particles that are blown about in the air. This sensitive membrane will not endure the presence of these particles. If any find access, it causes a constant winking, a flow of tears, and other signs of irritation, until it is removed. [Sidenote: 55. The eyelashes? The little points within the line of the lashes? Of what use are these glands?]