EYE. 396 EYE. (•2) the external limiting membrane; (3) the outer nuclear layer; (4) the outer molecular layer; (5) the inner nuclear layer; (6) the inner molecular layer: (7) the layer of ganglion- cells; (8) the layer of nerve- fibres; (9) the in- ternal limiting membrane. It now remains for us to describe the trans- parent media which occupy the interior of the globe, and through which the rays of light must pass before they can reach the retina and form on it the images of external objects. We shall con- sider them in the order in which the rays of light strike them. Immediately behind the transparent cornea is the aqueous 'humor, which fills up the anterior and posterior chambers which lie between the cornea and the lens. As its name implies, it is very nearly pure water, with a mere trace of albumen and chloride of sodium. As no epithe- lium exists in front of the iris, or on the anterior surface of the lens, it is most probably secreted by the cells on the posterior surface of the cornea. The crystalline lens lies opposite to and behind the pupil, almost close to the iris, and its pos- terior surface is received into a corresponding depression on the fore part of the vitreous humor. In form, it is a double-convex lens, with surfaces of unequal curvature, the posterior being the most convex. It is inclosed in a transparent capsule, of which the part covering the anterior surface is nearly four times thicker than that at the posterior aspect, in consequence, doubtless, of greater strength being required in front, where there is no support, than behind, where the lens is adherent to the vitreous membrane. The microscopic examination of the substance or body of the lens reveals a structure of wonderful beauty. Its whole mass is composed of extremely minute, elongated, ribbon-like structures, com- monly called the fibres of the lens, which are developed from cells. These fibres are ar- ranged side by side in lamella-, of which many hundred exist in every lens, and which are so placed as to give to the anterior and posterior surfaces the appearance of a central star, with meridian lines. The lens gradually increases in density, and at the same time in refracting power, toward the centre; by this means the con- vergence of the central rays is increased, and they are brought to the same focus as the rays passing through the more circumferential portions of the lens. (According to Brewster, the refracting power at the surface is 1.3767, and at the centre I 3990.) The lens contains 58 per cent, of water, 36 of albumen, with minute quantities of salts, membrane, etc. In consequence of its proteid constituent, it becomes hard and opaque on boil- ing, as we familiarly see in the ease of the eyes ni boiled fish. In the adult its long diameter ranges from one-third to three-eighths, and its anteroposterior diameter from one-eighth to one- sisth of an inch, and it weighs three or four grains. The vitreous Jvutnor lies in the concavity of the and occupies about four-fifths of the eye iorly. It is inclosed in the hyaloid mem ii h i nd r 1 1 1 r i j . rem processes inward, so a to divide the cavity into a series "i com mid thus t" equalize the pressure ex- - inclosed soft, gelatinous mass. Be- tween the anterior border of tin- retina and the he lens, we ha e a ei ie of radi it in" . ill' vitreous body, into which the ciliary proc- s of the choroid dovetail. The vitreous humor contains 98.4 per cent, of water, with a trace of albumen and salts, and hence, as might lie ex- pected, its refractive index is almost indent with that of water. The appendages of the eye now claim our no- tice. The most important of these append the muscles toithin tin: orbit, the eyelids, lachrymal apparatus, and the conjunctiva, to which (although less important) we may add the eyebrows. The muscles by which the eye is moved are four straight (or recti) muscles, and two oblique (the superior and inferior). The former rise from the margin of the optic foramen at the apex of the orbit, and are inserted into the sclerotic near the cornea, above, below, and on either side. The superior oblique arises with the straight muscles; but, after running to the upper edge of the orbit, has its direction changed by a pulley, and proceeds backward, outward, and downward. The inferior oblique arises from the lower part of the orbit, and passes backward, outward, and upward. The action of the straight muscles is sufficiently obvious from their direction — when acting collectively they fix and retract the eye, and when acting singly they turn it toward their respective sides. The oblique muscles antagonize the recti, and draw the eye forward ; the superior, acting above, directs the front of the eye down- ward and outward, and the inferior upward and inward. Ey the duly associated action of these muscles, the eye is enabled to move (within defi- nite limits) in every direction. 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-cartilage, 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 car- tilage a special muscle is attached, termed the /. rutor palpebral superioris, whose object is to elevate the lid and thus open the eye; while there is another muscle, the orbicularis palpi brarum, which surrounds the orbit and eyelids, and by its contraction closes the eye. The Meibomian glands secrete a sebaceous matter, which facili- tates tlie free million of the lid- and previ 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 tin rymal 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 i canals open; and the duet, through which tears pass from the sac into the nose. ii oblong body, about the sizi of a -mall almond, lying in a depression in tic upper and until- part "t the orbit. The fluid ecreted by it reaches the surface of the eye by teven or ei duct-, n hieli open mi t he eonji t its upper il. The constant motion ol
