Ophthalmology – A wide variety of disorders can affect the eye and interfere with vision.

Structure of the eye

The eye is a complex and delicate structure. Each eyeball is a sphere about 1 inch in diameter, covered by three layers of tissue. The tough, white, outer layer, called  the sclera, maintains the shape and size of the eyeball. Covering the sclera at the front of the eye is a transparent mucous membrane called the conjunctiva, which also lines the inside of the eyelids. At the front of the sclera is a transparent, dome-shaped, protective covering called the cornea.

The middle layer of tissue, which lies beneath the sclera, is called the choroid. The choroid contains blood vessels that supply the tissues of the eye with oxygen and nutrients. Toward the front of the eye, the choroid forms a circular ring with muscles called the ciliary body. Attached to the front of the ciliary body is the colored part of the eye, a circular curtain containing muscle fibers, called the iris. In the center of the iris is an opening called the pupil, through which light enters the eye. The dilating (widening) and constricting (narrowing) of the pupil, which are controlled by the muscle fibers of the iris, regulate the amount of light that enters the eye.

Directly behind the iris and pupil is a transparent, elastic structure called the lens, which is attached to the ciliary body. Contraction of the ciliary body muscles changes the shape of the lens and enables the eye to focus. The space between the cornea and the lens is filled with a clear, watery liquid called aqueous fluid. The space behind the lens is filled with a substance called vitreous fluid, which makes up the mass of the inside of the eyeball.

Vision

When you look at an object, light rays from the object pass through the cornea, pupil, and lens of the eye, which project an upside-down image of the object onto the retina. The retina converts the image into nerve impulses and transmits the impulses to your brain via the optic nerve. Your brain interprets the information it receives from the retina, and you see the object right side up.

The inner layer of tissue, called the retina, lines the back of the inside of the eye. The retina includes a layer of light-sensitive nerve cells called rods and cones. The rods are very sensitive to light intensity and enable you to see in dim light. The cones detect color and detail. There are 125 million rods and 7 million cones in each eye. When you look at an object, light rays from the object pass through the cornea, pupil, and lens, and form an upside-down image of the object on the retina. The rods and cones transform the sensations of color, form, and light intensity that they receive into nerve impulses, which the retina then transmits along retinal nerve fibers to the optic nerve, a stalklike collection of nerves that connects the eye to the brain. The vision centers in the brain interpret nerve impulses received from each eye and integrate them into the single, right-side-up, three-dimensional image that you see.