A Practical Physiology: A Text-Book for Higher Schools

by Albert F. Blaisdell

The eyelids, two in number, move over the front of the eyeball and protect it from injury. They consist of folds of skin lined with mucous membrane, kept in shape by a layer of fibrous material. Near the inner surface of the lids is a row of twenty or thirty glands, known as the Meibomian glands, which open on the free edges of each lid. When one of these glands is blocked by its own secretion, the inflammation which results is called a “sty.”

The inner lining membrane of the eyelids is known as the conjunctiva; it is richly supplied with blood-vessels and nerves. After lining the lids it is reflected on to the eyeballs. It is this membrane which is occasionally inflamed from taking cold.

The free edges of the lids are bordered with two or more rows of hairs called the eyelashes, which serve both for ornament and for use. They help to protect the eyes from dust, and to a certain extent to shade them. Their loss gives a peculiar, unsightly look to the face.

The upper border of the orbit is provided with a fringe of short, stiff hairs, the eyebrows. They help to shade the eyes from excessive light, and to protect the eyelids from perspiration, which would otherwise cause serious discomfort.

335. The Lacrymal Apparatus. Nature provides a special secretion, the tears, to moisten and protect the eye. The apparatus producing this secretion consists of the lacrymal or tear gland and lacrymal canals or tear passages ([Fig. 136]).

Outside of the eyeball, in the loose, fatty tissue of the orbit, in the upper and outer corner is the lacrymal or tear gland. It is about the size of a small almond and from it lead several little canals which open on the inner surface of the upper lid. The fluid from the gland flows out by these openings over the eyeball, and is collected at the inner or nasal corner. Here in each lid is a little reddish elevation, or lacrymal caruncle, in which is an opening, communicating with a small canal in the lid which joins the lacrymal sac, lodged between the orbit and the bridge of the nose ([Fig. 137]).

From this sac there passes a channel, the nasal duct, about one-half of an inch long, leading into the lower portion of the nostril. The fluid which has flowed over the eye is drained off by these canals into the nose. During sleep this secretion is much diminished. When the eyes are open the quantity is sufficient to moisten the eyeball, the excess being carried into the nose so gradually that the attention is not attracted to it.

The lacrymal canals are at times blocked by inflammation of the nasal duct, and the fluid collects in the corners of the eyelids and overflows down the cheeks, producing much inconvenience. The lining membrane of the eyelids through these canals is continuous with that of the nostrils. Hence, when the lining membrane of the eye is red and swollen, as during a cold, the nasal passages are also irritated, and when the nasal membrane is inflamed, the irritation is apt to pass upwards and affect the eyelids.

336. The Tears. The lacrymal or tear gland is under the control of the nervous system. Thus, if anything irritates the eyelids, the sensory nerves are stimulated and the impression is carried to the brain. Thence the nerve impulses travel to the lacrymal glands, leading to an increased flow of their secretion. The irritation of the sensory nerves in the nasal passages by smelling such substances as onions, or pungent salts, often causes a copious flow of tears.