Eye Disorders - Introduction

Sight is the most important of the five main senses. Your eyes tell you much more than your other senses do, and the part of the brain that deals with sight is far larger than the parts that deal with the other senses.

The eye is a complex, intricate, and fairly delicate structure . Each eyeball is a sphere about 25 mm (1 in) in diameter. Three concentric layers of tissue cover the eyeball. The tough outermost layer is the sclera, which is visible as the white of the eye. Its exposed surface at the front of the eye has a transparent covering, the conjunctiva, which also lines the inner surface of the eyelids. At the front of the eye, the sclera and conjunctiva join the cornea, a dome-shaped structure sometimes called the “window” of the eye.

Beneath the sclera is the choroid, a layer rich in blood vessels that supply the eye tissues with oxygen and nutrients. Toward the front of the eye, this layer thickens to form the ciliary body. From the front of the ciliary body extends a circular area of fibers, the iris, which varies in color from person to person and determines eye-color. In the center of the iris is an opening, the pupil, which looks like a black disc. Through this opening, light enters the eye. The amount of light is controlled by the contraction or dilation (widening) of the pupil. This adjustment is regulated by the muscles of the iris.

Immediately behind the iris and pupil is a transparent elastic body, the crystalline lens, which is attached to the ciliary body. Muscles thicken or narrow the lens, enabling the eye to focus on objects at varying distances. The space between the cornea and the lens is filled with a watery substance called aqueous humor. Behind the lens is a jelly-like substance called the vitreous humor, which makes up the bulk of the eyeball.

The innermost layer, the retina, lines the rear three­quarters of the eyeball. The retina includes a layer of light-sensitive nerve cells that are called the rods and cones because of their shapes. Light passes through the pupil and lens to the retina in such a way as to form an upside-down image of whatever you are looking at. The rods are very sensitive to light intensity and enable you to see in dim light. The cones detect color and fine detail.

There are 125 million rods and 7 million cones in each eye. Between them, the rods and cones transform the sensations of color, form and light intensity that they receive into nerve impulses. These impulses are then transmitted along retinal nerve fibers to the optic nerve, a stalk-like collection of nerves that connect the rear of the retina to the brain. The brain interprets the impulses received from each eye, reverses the images, and integrates them into one three-dimensional image.

The eye disorders covered in this section are dealt with in four groups. The first consists of errors of refraction such as problems of nearsightedness and farsightedness. The second group is concerned with disorders of those parts of the eye that you can see, mainly the eyelids, eyelashes, sclera, iris and lens. The third group deals with two forms of glaucoma, a disease that arises from a problem with drainage of aqueous humor. The final group is concerned with disorders that affect the structures in the inner layer of the eye, including the retina and its blood supply. The muscles and other tissues that surround the eyeball in its bony socket, which is known as the orbit, are also covered in this section.