The globe or ball of the eye is almost spherical in form. On closer inspection, however, it appears to be made up of two combined portions from spheres of different sizes. The posterior portion, forming about five-sixths of the ball, is a sphere of comparatively large size with a small segment cut off it in front, and at this point there is applied to it the anterior portion, which, being a segment of a smaller sphere, projects at the front of the ball with a greater convexity than the posterior portion.
The eyeball consists of concentrically arranged coats and of refracting media inclosed in them. The coats are three in number, namely, (1) an external protective tunic made up of the sclerotic and cornea; (2) a middle vascular and pigmentary tunic, the choroid; (3) an internal nervous layer, the retina. The sclerotic is the white, opaque part of the outer tunic, of which it forms about the posterior five-sixths, being coextensive with the larger sphere already mentioned. The cornea forms the remaining one-sixth of the outer tunic, being coextensive with the segment of the smaller sphere. It is distinguished from the sclerotic by being colorless and transparent. The choroid coat will be recognized as the black layer lying subjacent to the sclerotic. It does not line the cornea, but terminates behind the line of junction of that coat with the sclerotic by a thickened edge—the ciliary processes. At the line of junction of the sclerotic and cornea the iris passes across the interior of the eye. This (which may be viewed as a dependency of the choroid) is a muscular curtain perforated by an aperture termed the pupil. The retina will be recognized as a delicate, glassy layer, lining the greater part of the choroid.
The refracting media of the eye are three in number, namely (1) the aqueous humor, a watery fluid inclosed in a chamber behind the cornea; (2) the crystalline lens and its capsule, a transparent, soft solid of a biconvex form, and placed behind the iris; (3) the vitreous humor, a transparent material with a consistence like thin jelly, and occupying as much of the interior of the eye as is subjacent to the choroid.
The sclerotic is a strong, opaque, fibrous membrane, which, in a great measure, maintains the form of the eyeball and protects the more delicate structures within it. Its interior portion, which is covered by the ocular conjunctiva, is commonly known as the "white of the eye." In form it is bell-shaped, and the optic nerve pierces it behind like a handle, the perforation being a little to its inner side. In front, the rim of the bell becomes continuous with the cornea. The outer surface of the membrane receives the insertion of the muscles of the eyeball. The coat is thickest over the posterior part of the eyeball, and is thinnest a little behind its junction with the cornea.
The cornea is the anterior transparent portion of the outer coat of the eyeball. It may be viewed as a part of the sclerotic specially modified to permit the passage of light into the interior of the eye. Its outline is elliptical, nearly circular, and its greatest diameter is transverse. At its periphery it joins the sclerotic by continuity of tissue, and as the edge of the cornea is slightly beveled and has the fibrous sclerotic carried for a little distance forward on its outward surface, the cornea is generally said to be fitted into the sclerotic like a watch glass into its rim. The venous canal of Schlemm runs circularly around the eyeball at the line of junction of the sclerotic and cornea. The anterior surface of the cornea is exquisitely smooth, and is kept moist by the lacrimal secretions. Its posterior surface forms the anterior boundary of the chamber in which the aqueous humor is contained. The cornea is of uniform thickness and is of a dense, almost horny, consistence. Save a few capillary loops of blood vessels at its margin the cornea is without vessels. Its structure is comprised of five distinct layers.
The aqueous humor occupies a chamber which is bounded in front by the posterior surface of the cornea and behind by the capsule and suspensory ligament of the lens and by the ends of the ciliary processes. It is across this chamber that the iris extends. The aqueous humor is composed of water, with a small proportion of common salt in solution.
The iris is a muscular, pigmented curtain extending across the interior of the eye and having about its center an aperture termed the pupil. By variations in the size of this aperture the amount of light transmitted to the retina is regulated. It varies somewhat in color, but is most frequently of a yellowish-brown tint. Its anterior face is bathed by the aqueous humor. The greater part of the posterior surface is in contact with the capsule of the lens and glides on it during the movements of the curtain. The circumferential border is attached within the junction of the sclerotic cornea. The inner border circumscribes the pupil, which varies in outline according to its size. When much contracted the pupil is a very elongated ellipse, the long axis of which is in the line joining the nasal and temporal angles of the eyelids. It contains muscular tissue, which, by contracting or relaxing, lessens or dilates the pupillary opening.
The choroid coat is a bell-shaped, dark membrane which lines the sclerotic. Its outer surface has a shaggy appearance, caused by the tunica fusca, which unites the two coats. Between the two the ciliary vessels and nerves pass forward. Behind it is pierced by the optic nerve; in front it is continued as the ciliary processes, which form, as it were, the rim of the bell. The ciliary processes form a fringe around the slightly inverted rim of the choroid.
The retina is the most delicate of the coats of the eyeball. It is formed by the expansion of the optic nerve on the inner surface of the choroid, and, like that coat, it is bell-shaped. Its inner surface is molded on the vitreous humor. The nervous structures of the retina terminate at a wavy line, the ora serrata, behind the ciliary processes. Ten distinct layers are described as composing the thickness of the retina.