The Sclera

The opaque sclera forms the posterior five sixths of the protective coat of the eye. Its anterior portion is visible and constitutes the white of the eye. In children the sclera is thin, and therefore it appears bluish because the underlying pigmented structures are visible through it. In old age it may become yellowish because of the deposition of fat. Attached to the sclera are all the extra ocular muscles. Through the sclera pass the nerves and the blood vessels that penetrate into the interior of the eye. At its most posterior portion, the site of attachment of the optic nerve, the sclera becomes a thin, sieve like structure called the lamina cribrosa. It is through this sieve that the retinal fibres leave the eye to form the optic nerve. The episcleral tissue is a loose connective and elastic tissue that covers the sclera and unites it with the conjunctiva above. Unlike the sclera, the episcleral tissue is highly vascular.

Within the sclera are the contents of the eye: the uveal tract, the retina, the lens and the vitreous.  The uveal tract consists of three structures: iris, ciliary body, and choroid.

The Iris

The iris is the most anterior structure of the uveal tract. It is perforated at its centre by a circular aperture called the pupil. The anterior surface of the iris has many ridges and furrows. Contraction of the iris, which occurs in response to bright light, is accomplished by the activity of a flat, washer like muscle buried in its substance just surrounding the pupil opening. This muscle is called the sphincter pupillae. Expansion or dilation of the pupil is facilitated by relaxation of the sphincter muscle and by activation of the dilator muscle of the iris found at its peripheral circumference. Expansion and contraction of the iris, like an accordion, form circular pleat lines or furrows visible on its surface. Between the iris and the cornea is a clear fluid called the aqueous humor. This fluid occupies the space called the anterior chamber of the eye.

The Cilliary Body

The ciliary body is in direct continuity with the iris and is adherent to the sclera. Directly posterior to the iris, the ciliary body is plump and thrown into numerous folds referred to as the ciliary processes. This portion of the ciliary body is only about 2.5mm in length and is responsible for the major production of aqueous fluid. The equator of the lens is only 0.5mm from the ciliary processes and is suspended by fine fibres known as the zonular fibres of the lens. The anterior side of the ciliary body forms the rear of the anterior chamber. From its middle portion the iris takes root.

A muscle within the ciliary body, known as the ciliary muscle, releases the tension of the zonular fibres, controlling the size and shape of the lens. This in turn allows the lens of the eye to bulge and increase its power. Therefore, the ciliary muscle directly controls the focusing ability of the eye. In children this muscle is extremely active, which accounts for their powerful ranges of accommodation, or focusing abilities. With adulthood and advancing age the lens becomes increasingly less flexible and is less able to change shape, so that focusing at near, or accommodating the eye, becomes difficult. This condition is commonly referred to as presbyopia.

The Choroid

The choroid is in direct continuity with the iris and ciliary body and lies between the retina and sclera (see Fig. 1). The choroid is primarily a vascular structure, and its main function is to provide nourishment for the outer layers of the retina.