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 fibro-eartilage, termed the tarsal cartilage, the inner surface of which is grooved by thirty or forty parallel vertical lines, in which the Mei. bomian 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 ear• tilage a special muscle is attached, termed the legator palpebree superioris, object is to elevate the lid and thus open the eye; while there is another muscle, the orbieularis palpebrerum, which surrounds the orbit and eyelids, and by its contraction closes the eye. The .11eibomian glands secrete a sebaceous matter, which facili tates the free motion of the lids and prevents their adhesion. The eyelashes intercept the en trance of foreign particles directed against the eye, and assist, in shading that organ from an ex• cess of light.
The lachrymal apparatus consists of the lach rymal gland, by which the tears are secreted: two canals, into which the tears are received near the inner angle of the eve; the sac, into which these canals (lien: and the duet, through whirl) the tears pass from the say into the nose. The gland is an eldong body. about the size of a small almond, lying in a depression in the upper and outer part of the orbit. The fluid seereted by it reaches the surface of the eye by seven or eight duets, which open on the conjunetiva at its tipper and outer part. The constant motion of the.
upper eyelid induces a continuous gentle current of tears over the surface, which carry away any foreign particle that may have been deposited on it. The fled] then passes through two small open ings (termed the panda laehrymalia) 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, atd Inert, and is thus seen to be continuous with 6: nasal mucous membrane, of which it may he regarded as an offshoot or digital prolongation. See Ilucocs MEMBRANES.
We shall conclude this sketch of the anatomy of the human eye with a brief notice of the nerves going to this organ and its appendages. Into each orbit there enters a nerve of •pecial sense —viz, the optic nerve; a nerve of ordinary sense (ion—viz. 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 time article NEnvous SvstFm, we shall merely trace these nerves from their chiasma, or commissure, for ward. This commissure results from the junc tion of the optic tracts of the two sides, and it is especially remarkable for the fact that it pre sents a partial decussation of the nervous fibres, the central fibres of each tract passing. into the
nerve of the opposite side, and crossing the corre sponding fibres of the o her Tact; while the outermost fibres, which are tincim 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 optie foramen at the apex of thin orbit, reeeiee a sheath or investment from the dura increased firmness, and finally term:n.t1A• c the retina. The peculiar mode of termn:imitot in the optic nerves in the cup-like expanse. of tmle retina, the impairment or loss of vision ii'delm tollows any morbid affec tion of them, and toe constant relation in size which is observeu in comparative anatomy be tween them and the organs of vision, afford. suffi cient evidence that they are the proper conductors of visual impressions to the senso•ium.
The first or ophthalmic division of the fifth or trifacial nerve sends branches to the skin of the eyelids and to the conjunctiva. That it is the nerve of ordinary sensation of the eyP is suffi ciently obvious from the following facts: (1) That in disease of this nerve in the human sub ject, it is not uncommon to find the eyeball total ly insensible to every kind of stimulus (particles of dust. pungent vapors, etc.) ; amid (2) that if the nerve be divided in the crania m (in 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 ocali. It supplies with motor power the elevator of the upper eyelid and all the muscles of the globe, ex cept the superior obliquct and the external rectus muscle, and, in addition to this. it sends filaments to the iris and other muscular fibres within the eye. The a ipbeation of au irritant (in vivisection experime•as to its trunk induces convulsive contraction ng tic principal muscles of the ball and of the is i • division of the trunk occasions an ‘2 :erual squint, with palsy of the upper eyelid fiNu Odatition of the pu pil. The squint is caused by the action of the ex ternal 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 in musing contract ion of the pupil is excited throng)) tile optic nerve, and affords :t good illustration of reflex action, the stimulus of light falling upon the retina, and, through it, exciting that portion of the brain from which the third nerve takes its origin. This nerve clear ly 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 con nection with the muscular structures in the in. terior 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.
The fourth nerve supplies the superior oblique muscle with motor power, while the sixth nerve similarly regulates the movements of the exter nal straight muscle—the only two muscles in the orbit which are not supplied by the third pair.
Although not entitled to be termed a nerve of the orbit, the facial nerve deserves mention as sending a motor branch to the orbiculuris muscle, by which the eye is closed.