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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 (Fig. 879). 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) (Fig. 880); 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.

FIG. 880– The terminal portion of the optic nerve and its entrance into the eyeball, in horizontal section. (Toldt.) (See enlarged image)

Structure (Figs. 881, 882).—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.

1. The stratum opticum or layer of nerve fibers is formed by the expansion of the fibers of the optic nerve; it is thickest near the porus opticus, gradually diminishing toward the ora serrata.

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