THE ORGAN OF VISION—THE EYE AND ITS APPENDAGES.
The Orbit. — The eyeball is situated in a bony cavity called the orbit, formed by various bones of the head and face (see p. 59). The cavity is padded by a loose fatty tissue, the diminution in the amount of which aids in producing a sunken appearance of the eyes.
The Eyelids are formed of folds of skin, the Outer surface having the structure of ordinary skin, the inner of mucous membrane. In the body of the lids is a layer of condensed fibrous material which maintains the shape of the lids. Nearer the inner than the outer surface of the lids is a row of glands which open on the free edge of the lid and pass from there into the eyelid in a vertical direction. These are the Meibomian glands. The blocking of one of these glands by the material it itself produces leads to the formation of a stye. Towards the front of the free edge of the lids are the eye lashes, which are thick and capable of rapid growth, so that if one falls or is pulled out another quickly grows in its place. The inner lining membrane of the lids is very richly Env plied with vessels and nerves. The membrane is called the conjunctiva. It is continuous with the skin at the free edge. After lining the inner surface of the lid it passes over on to the eyeball. In ordinary inflammation from cold it is this membrane, whose blood-vessels are engorged with blood, that is the seat of the red ness and swelling, and it is because it continues forwards over the eyeball that the eye has its bloodshot appearance in such cases. Such in flammation is called conjunctivitis.
In the eyelids are muscular fibres which close the lids by their contraction.
The (lachrymal gland) and pas sages.
Situated outside of the eyeball among the loose fatty tissue of the orbit in its upper and outer corner is the lachrymal gland. From it several little channels lead which open on the inner surface of the upper lid. The fluid pro duced in the gland passes out by these openings and flows over the eyeball. It is ordinarily just in sufficient quantity to keep the eyeball and lids moist, to wash off dust, &c. Having flowed over the eyeball the fluid collects at the inner angle of the lids. At this place in each lid is a little projecting point(punctum lachrymalium) in the centre of which is an opening. The openings communicate each with a small canal in the lid, which passes to the angle between the orbit and bridge of the nose, where is lodged a little lachrymal sac. The
canal of both upper and lower lids joins this sac, and from it there passes a channel--the nasal duct, lodged in a canal in the bone, which leads into the lower part of the nostril. The fluid which has flowed over the eye is carried off by the canals and drained into the nose. The lining membrane of the eyelids is continu ous through these canals with that of the nos trils, and thus redness and swelling of the nasal membrane, caused by cold, are apt to pass up wards and inflame the eyelids. The canals are often blocked by inflammation, and the fluid collects in the corners of the eyelids and flows over on to the cheeks.
The secretion of the lachrymal gland is under the control of the nervous system. Anything that irritates the eyelid leads to stimulation of sensory nerves, the impression passes to a nerve centre in the base of the brain, from which nervous impulses travel to the gland leading to increased flow of its secretion. The first act in the process may be the excitement of sensory nerves in the nostril, as by the smelling of pun gent salts. The stimulation of the same nerve centre results, with its consequences of increased flow. A- mental emotion, joy or grief, may stimulate the centre and produce similar results. In such cases the fluid is produced in such quantity that it cannot escape by the lachrymal canals quickly enough, and the excess rolls over the cheeks as tears. This is the explanation of weeping. Some people are "dry-eyed" in times of deep grief or other emotion. The explanation of this is as simple. The nervous influence acts on the centre in a precisely opposite way, so that instead of it stimulating the flow of blood through the gland and otherwise exciting in creased activity, the nervous impression arrests the activity, so that less fluid than usual is pro duced. In a similar way the emotion which produces blushing in one man leads to pallor in another. In the former case the nature of the nervous effect is to permit a greatly increased flow of blood through the vessels of the face, and therefore redness of the surface, in the latter case it diminishes the natural flow, there fore there is less blood in the part and conse quently less colour.