Retina Associates - (2) - Retinopathy of Prematurity

Retina Associates, P.A.

Retinopathy of Prematurity

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Table of Contents

Structure and Function of the Eye

The adult eye is a sphere about 1 inch in diameter. The wall of the eye has three layers:

Outer fibrous layer - the sclera
Middle vascular layer, containing blood vessels - the choroid
Inner nerve-containing layer - the retina

The cornea forms the front most part of the eye. It's transparent and curved to focus incoming light rays. The sclera, commonly called the "white of the eye," forms an opaque and fibrous coat that protects the eye. The conjunctiva is an extension of the inner layer on the eyelid that forms a thin transparent membrane over the front of the eye.

Many blood vessels and pigment (color) are contained in the choroid. An extension from the front section of the choroid forms the iris, or the colored part of the eye. The center of the doughnut-shaped iris is called the pupil. Just behind the iris is another extension from the choroid that contains muscles and ligaments that hold the lens in place and change its shape, from squashed for near vision to stretched for far vision.

The eye has two segments: The anterior segment is from the lens forward, includes the iris and is bound by the cornea at the front. Aqueous humor, a watery fluid, constantly flows into and out of the anterior segment. The posterior segment lies behind the lens and contains a gelatinous substance called vitreous humor. Vitreous humor is formed during the eye's development and, unless removed surgically, remains permanently in the eye. Together, aqueous and vitreous humor give the eye sufficient pressure, known as intraocular pressure, to keep it firm and spherical.

The retina forms the inner coat of the posterior segment and is comprised of two layers - a nerve-containing layer resting on top of a pigmented layer which is attached to the underlying choroid. The nerve containing retina is physically attached to the pigmented retina only at the front of the eye, just behind the lens and at the back of the eye, at the optic nerve. When one suffers a detached retina it is the nerve containing layer detaching from the pigmented retina, not the entire retina. The nerve layer contains cells - photoreceptors - that detect light rays and pass the impulse via the optic nerve, to the brain, which translates them into images. The central part of the retina - the macula - is responsible for central vision and is directly behind the lens. At the macula's center is the fovea, which is the area of sharpest vision. The retina outside the macula is known as the peripheral retina which allows peripheral vision.

There are two kinds of photoreceptors in the retina - rods and cones. Rods are much more sensitive to light and so allow night vision, which is colorless vision and provides only general outlines and objects. Rods predominate in the periphery of the retina, but are completely absent from the fovea. Cones, on the other hand, detect color and allow sharp vision. The cones are most concentrated in the fovea, but decrease in concentration in areas away from the macula.

The eyeball is held in place in the orbit of the skull by six muscles. Each muscle moves the eye in one primary direction. A given eye movement may involve more than one of the muscles.