The eyelids are two thin movable folds placed in front of the eye, to shield it from too strong light, and to protect its anterior surface. They are composed of (1) skin; (2) of a thin plate of fihro-cartilage, termed the tarsal cartilage, the inner surface of which is grooved by thirty or forty parallel vertical lines, in which the Meibomian glands are imbedded; and (3) of a layer of mucous membrane, continuous, as we shall presently see, with that which lines the nostrils, and which joins the skin at the margin of the lids, in which the eyelashes (cilia) are arranged in two or more rows. The upper lid is. much the larger; and to the posterior border of its cartilage a special muscle is attached, termed the levator palpebrce superioris, whose object is to elevate the lid, and thus open the eye; while there is another muscle, the orbicularis palpe,brarum, which surrounds the orbit and eyelids, and by its contraction closes the eye. The Meibomian glands secrete a sebaceous matter, which facilitates the free motion of the lids, and prevents their adhesion. The eyelashes intercept the entrance of foreign particles directed against the eye, and assist in shading that organ from an excess of light.
The lachrymal apparatus consists of the lachrymal gland, by which the tears are secreted; two canals, into which the tears are received near the inner angle of the eye; the sac, into which these canals open; and the duct, through which the tears pass from the sac into the nose. The gland is an oblong body, about the size of a small almond, lying in a depression in the upper and outer part of the orbit. The fluid secreted by it. reaches the surface of the eye by seven or eight ducts, which open on the conjunctiva at its upper and outer part. The constant motion of the upper eyelid induces a con tinuous gentle current of tears over the surface, which carry away any foreign particle that may have been deposited on it. The fluid then passes through two small openings, termed the puncta lacrymalia, into the canals; whence its further course into the lower portion of the nose is sufficiently obvious. The conjunctiva (or mucous coat) which covers the front of the eyeball, and lines the inner surface of the lids, passes down and lines the canals, sac, and duct; and is thus seen to be continuous with the nasal mucous membrane, of which it may be regarded as an offshoot or digital prolongation. See Mucous MEMBRANES.
We shall conclude this sketch of the anatomy of the human eye by a brief notice of the nerves going to this organ and its appendages.
Into each orbit there enters a nerve of special the optic nerve, a nerve of ordinary the ophthalmic branch of the fifth nerve, and certain nerves of motion, going to the muscular tissues, and regulating the movements of the various parts—viz., the third, fourth, and sixth nerves.
As the optic tracts from which the optic nerves originate are noticed in the article BRAIN, we shall merely trace these nerves from their Miasma or commissure forwards. This commissure results from the junction of the optic tracts of the two sides; and it is especially remarkable for the fact that it presents a partial deeussation of the nervous fibers; the central fibers of each tract passing into the nerve of the opposite side, and crossing the corresponding fibers of the other tract, while the outermost fibers, which are much fewer in number than the central ones, pass to the optic nerve of the same side. In front of the commissure, the nerves enter the optic foramen at the apex of
the orbit, receive a sheath or investment from the aura mater, acquire increased firm ness, and finally terminate in the retina.
The peculiar mode of termination of the optic nerves in the cup-like expansion of the retina, the impairment or loss of vision which follows any morbid affection of them, and the constant relation in size which is observed ih comparative anatomy between them and the organs of vision, afford sufficient evidence that they are the proper con ductors of visual impressions to the sensorium.
The first or ophthalmic division of the fifth or trifacial nerve sends branches to the skin of the eyads and to the conjunctiva. That it is the nerve of ordinary sensation of the eye, is sufficiently obvious from the following facts: (1) That in disease of this nerve in the human subject, it is not uncommon to find the eyeball totally insensible to every kind of stimulus (Particles of dust, pungent vapors, etc.); and :(2) That if the nerve be divided in the cranium On one of the lower animals), similar insensibility results.
The most important of the nerves of motion of the eye is the third nerve, or motor oval. It supplies with motor power the elevator of the upper eyelid, and all the muscles of the globe, except the superior oblique and the external straight muscle, and, in addition to this, it sends filaments to the iris and other muscular fibers within the eye. The application of an irritant (in vivisection experiments) to its trunk induces convulsive contraction of the principal muscles of the ball and of the iris; while division of the trunk occasions an external squint, palsy of the upper eyelid and fixed dilatation of the pupil. The squint is caused by the action of the external straight and the superior oblique muscles, while the other muscles are paralyzed by the operation. The normal motor action of the nerve upon the iris, iu causing contraction of the pupil, is excited through the optic nerve, and affords a good illustration of reflex action (q.v.); the stimulus of light falling upon the retina, and, thiough it, exciting that portion of the brain from which the third nerve takes its origin. This nerve clearly exerts a double influence in relation to vision: (1) it mainly controls the movements of the eye ball and the upper eyelid; and (2) from its connection with the muscular structures in the interior, it regulates the amount of light that can enter the pupil, and probably takes part in the adjusting power of the eye to various distances.