The retina is a delicate nervous membrane, inner lining othe eyeball, upon which the images of external objects are received.
Its outer surface is in contact with the choroid; its inner with the hyaloid membrane of the vitreous body.
Behind, it is continuous with the optic nerve; it gradually diminishes in thickness from behind forward, and extends nearly as far as the ciliary body, where it appears to end in a jagged margin, the ora serrata. Here the nervous tissues of the retina end, but a thin prolongation of the membrane extends forward over the back of the ciliary processes and iris, forming the pars ciliaris retinæ and pars iridica retinæ already referred to. This forward prolongation consists of the pigmentary layer of the retina together with a stratum of columnar epithelium.
The retina is soft, semitransparent, and of a purple tint in the fresh state, owing to the presence of a coloring material named rhodopsin or visual purple; but it soon becomes clouded, opaque, and bleached when exposed to sunlight.
Exactly in the center of the posterior part of the retina, corresponding to the axis of the eye, and at a point in which the sense of vision is most perfect, is an oval yellowish area, the macula lutea; in the macula is a central depression, the fovea centralis. At the fovea centralis the retina is exceedingly thin, and the dark color of the choroid is distinctly seen through it.
About 3 mm. to the nasal side of the macula lutæ is the entrance of the optic nerve (optic disk), the circumference of which is slightly raised to form an eminence (colliculus nervi optici); the arteria centralis retinæ pierces the center of the disk. This is the only part of the surface of the retina which is insensitive to light, and it is termed the blind spot.
Structure —The retina consists of an outer pigmented layer and an inner nervous stratum or retina proper.
The pigmented layer consists of a single stratum of cells. When viewed from the outer surface these cells are smooth and hexagonal in shape; when seen in section each cell consists of an outer non-pigmented part containing a large oval nucleus and an inner pigmented portion which extends as a series of straight thread-like processes between the rods, this being especially the case when the eye is exposed to light. In the eyes of albinos the cells of this layer are destitute of pigment.
Retina Proper.—The nervous structures of the retina proper are supported by a series of nonnervous or sustentacular fibers, and, when examined microscopically by means of sections made perpendicularly to the surface of the retina, are found to consist of seven layers, named from within outward as follows:
1. Stratum opticum.
2. Ganglionic layer.
3. Inner plexiform layer.
4. Inner nuclear layer, or layer of inner granules.
5. Outer plexiform layer.
6. Outer nuclear layer, or layer of outer granules.
7. Layer of rods and cones
Supporting Frame-work of the Retina.—The nervous layers of the retina are connected together by a supporting frame-work, formed by the sustentacular fibers of Müller; these fibers pass through all the nervous layers, except that of the rods and cones. Each begins on the inner surface of the retina by an expanded, often forked base, which sometimes contains a spheroidal body staining deeply with hematoxylin, the edges of the bases of adjoining fibers being united to form the membrana limitans interna. As the fibers pass through the nerve fiber and ganglionic layers they give off a few lateral branches; in the inner nuclear layer they give off numerous lateral processes for the support of the bipolar cells, while in the outer nuclear layer they form a network around the rod- and cone-fibrils, and unite to form the membrana limitans externa at the bases of the rods and cones. At the level of the inner nuclear layer each sustentacular fiber contains a clear oval nucleus.
Macula Lutea and Fovea Centralis.—In the macula lutea the nerve fibers are wanting as a continuous layer, the ganglionic layer consists of several strata of cells, there are no rods, but only cones, which are longer and narrower than in other parts, and in the outer nuclear layer there are only cone-granules, the processes of which are very long and arranged in curved lines. In the fovea centralis the only parts present are (1) the cones; (2) the outer nuclear layer, the cone-fibers of which are almost horizontal in direction; (3) an exceedingly thin inner plexiform layer. The pigmented layer is thicker and its pigment more pronounced than elsewhere. The color of the macula seems to imbue all the layers except that of the rods and cones; it is of a rich yellow, deepest toward the center of the macula, and does not appear to be due to pigment cells, but simply to a staining of the constituent parts.
At the ora serrata the nervous layers of the retina end abruptly, and the retina is continued onward as a single layer of columnar cells covered by the pigmented layer. This double layer is known as the pars ciliaris retinæ, and can be traced forward from the ciliary processes on to the back of the iris, where it is termed the pars iridica retinæ or uvea.
The arteria centralis retinæ and its accompanying vein pierce the optic nerve, and enter the bulb of the eye through the porus opticus. The artery immediately bifurcates into an upper and a lower branch, and each of these again divides into a medial or nasal and a lateral or temporal branch, which at first run between the hyaloid membrane and the nervous layer; but they soon enter the latter, and pass forward, dividing dichotomously. From these branches a minute capillary plexus is given off, which does not extend beyond the inner nuclear layer. The macula receives two small branches (superior and inferior macular arteries) from the temporal branches and small twigs directly from the central artery; these do not, however, reach as far as the fovea centralis, which has no bloodvessels. The branches of the arteria centralis retinæ do not anastomose with each other—in other words they are terminal arteries. In the fetus, a small vessel, the arteria hyaloidea, passes forward as a continuation of the arteria centralis retinæ through the vitreous humor to the posterior surface of the capsule of the lens.