The Uveal Tunic

Regions of the Uveal Tunic

The uvea is the middle tunic, located between the innermost retina and the outermost corneoscleral tunics.

The uvea has many functions, and in some respects it is the most complex of the eye's tunics. One major function is to provide for most of the eyeball's blood supply and lymphatic drainage (its embryonic origins from mesoderm insure that it forms blood vessels in development). But the uveal tunic is also the location of the mechanism for accommodation, and of control of light influx.

Structurally the uveal tunic has three subdivisions. These are the choroid, the heavily vascular layer which comprises most of it; the iris, the variable-diameter sphincter which controls the amount of light entering the eye; and the ciliary body, a mass of smooth muscle which controls the accomodation mechanism and from which the lens is suspended. The ciliary body also has secretory function; its ciliary processes are the site of production of aqueous humor.

Choroid

The choroid is the highly pigmented and vascular layer of the uveal tunic that underlies the eyeball's surface. It provides nutritive support, and contains numerous blood vessels and large lymphatic channels.

The choroid is also very heavily pigmented in most animals. The pigment is melanin, and melanocytes are found in it. Together with the lipofuscin and melanin in the pigment cell layer of the retina, it acts as a "light trap" in the same way the black matte finish of the inside of a camera does. Absorbing stray light minimizes internal reflections and increases contrast, thus increasing the acuity of vision. If this layer lacks pigmentation, light passing through the retina can be reflected from the inner surface of the sclera, into the light-sensitive rods and cones. This seriously impairs the precision with which the image can be formed.

The Ciliary Body

The ciliary body is an anterior extension and expansion of the uveal tunic, located in the angle between the end of the cornea and the lens. Most of the ciliary body is composed of smooth muscle whose contractions work the lens accommodation mechanism via the tension imposed on the zonule fibers. Like other structures in this important region (the limbus or corneo-iridial angle), which is shown in the image to the right, the ciliary body runs radially around the anterior part of the eye.

In addition to its role in the mechanism of accommodation, the ciliary body is a secretory structure. The inner surface of the ciliary body is lined on its inner surface with a double layer of cells, the ciliary epithelium. The outer of these two layers is heavily pigmented, the inner one is not.

This epithelium is the site of production of the fluid which fills the anterior and posterior chambers of the eye, the aqueous humor. The balance between production and drainage of aqueous humor is the means by which the normal intraocular tension is maintained, and is essential in maintaining the eyeball's normal size and shape.

The Iris

Projecting from the ciliary body and running radially around the anterior part of the eye is the iris. It is a pigmented, diaphragm-like structure with a hole in it, the pupil. The iris is directly behind the cornea and in front of the lens (and therefore it defines the boundary between the anterior and posterior chambers).

The iris is the point at which light rays passing through the cornea enter the eye. The function of the iris is exactly the same as that of the "iris diaphragm" in the lens of a camera. It contains radially disposed smooth muscle fibers and a circumferential band of smooth muscle fibers as well. Stimulation of these opposing muscles controls the amount of light admitted. Contraction of the radial-oriented fibers opens the pupil, admitting more light; contraction of the circumferential sphincter muscles reduces the pupillary diameter. These adjustments are made automatically by input from the sympathetic and parasympathetic divisions of the central nervous system. As is the case with the ciliary body, the iris is lined on its inner face with an epithelium derived from the non-sensitive portion of the retina. This is usually pigmented. The bulk of the iris' tissue, from the uveal tunic, contains some melanocytes and numerous blood vessels. Together with the pigmentation of the inner layer, the color of the iris as seen from the external side will vary considerably as a result. The patterns of color are hereditary in most mammals.