Normal Anatomy of the Eye

Click on label for definition of part

Definitions

The Sclera - The tough, white outer part of the eye.

The Cornea - The clear outer part of the eye's focusing system located at the front of the eye. Most of the bending of the light rays (refraction) occurs at the cornea. The lens also bends the light but to a lesser extent. The lens does a sort of fine tuning to insure that the image is sharply focused on the retina.

Aqueous Humor - The clear watery fluid filling the anterior chamber of the eye between the cornea and the lens. It has the benefit of being fairly homogenous and, the optical properties are easily measured. The space that it inhabits is called the anterior chamber.

Vitreous Humor - The clear gel filling the posterior chamber of the eye between the lens and the retina. The space that it fills is called the vitreous body.

The Pupil - The opening at the center of the iris. The iris adjusts the size of the pupil and controls the amount of light that can enter the eye.

The Iris - The colored tissue behind the cornea - color varies from pale blue to dark brown.

The Choroid - A spongy layer filled with blood vessels. It lies between the sclera and the retina. The choroid nourishes the outer layers of the retina.

The Conjunctiva - The transparent mucous membrane lining the inside of the eyelids and the white of the eyeball.

The Macula - The small sensitive area of the retina that gives central vision; contains the fovea. This foveal area is covered with a yellow pigment called the macula lutea.

The Optic Nerve - The bundle of over one million nerve fibers that carries visual messages from the retina to the brain.

The Fovea - The center of the macula; gives the sharpest vision. When we fixate or look directly at an object it is imaged on the fovea.

The Lens - The lens is the clear part of the eye behind the iris that helps to focus light on the retina. The lens helps to focus on both far and near objects so that they are perceived clearly and sharply. The ciliary muscle helps to change the shape of the lens. This changing of lens shape is called accommodation.

The lens itself is a multilayered structure (something like an onion). In young people it is normally perfectly clear and quite elastic. As one ages its elasticity is reduced. In fact after the age of about 45 the lens' ability to change in shape is considerably reduced (presbyopia). That is why people over the age of 45 almost always require glasses to read and/or to see distant objects.

It is not unusual for people in their 50's and older to wear bi-focal or even tri-focal lenses.

As one gets older the lens can also become cloudy. This condition is called cataract. When cataracts become too severe the lens has to be removed and be replaced with an artificial lens. Of course the artificial lens is not capable of accommodation. However, by the time most people are afflicted with cataracts they will already be old enough so that they will have naturally lost most of their accommodation. Also as one ages the lens becomes more yellow.

The Ciliary Body - The Ciliary Body consists of bundles of tiny muscles used in accommodation. When the ciliary muscle is relaxed, the choroid acts like a spring pulling on the lens via the zonule fibers causing the lens to become flat. When the ciliary muscle contracts, it stretches the choroid, releasing the tension on the lens and the lens becomes thicker.

The Retina - The light-sensitive tissue lining the back of the eyeball; sends electrical impulses to the brain. On the right is a photo of what an eye-care person sees when looking at your retina with an ophthalmoscope. The dark area near the center is the fovea . This area is actually a depression in the retina. Although this photo does not show it, the foveal area has a yellow pigmentation called the macula lutea. When we fixate (look directly at) objects, images of these objects are projected on to the fovea. It is the retinal location of our best visual acuity and color vision.

The Optic Disc - is the place where all the blood vessels and optic nerves converge and go out of the retina to the brain. The optic disc, also called the blind spot, is where the axons of the ganglion cells leave the retina to form the optic nerve. It is called the blind spot because there are no rod or cone receptors in this part of the retina and we can not see objects that are imaged on this part of the retina.

Optics:

The term "refractive" refers to the way light rays reflected from objects in our field of vision are brought into focus within the eye. The eye's complex optical components intercept, focus, and process light into nerve impulses, which are sent to the brain - and that is how we "see." The shape (power) of the cornea, the power of the crystalline lens inside the eye, and the length of the eye together determine how rays of life are focused on the retina, the photosensitive membrane that lines the back wall of the eye. These three factors must be well matched in order for the eye to see clearly without correction (emmetropia). If the cornea is too steeply curved relative to the length of the eye or if the eye is too long relative to the curvature of the cornea, then myopia results.

If the cornea is too flat or the eye too short, then hyperopia results. In the picture above, light entering from the left travels through the cornea, then through the lens and is focused perfectly on the retina at the fovea. In the normal (emmetropic) eye, distant objects are focused on the retina. To focus near objects on the retina, the lens changes its shape--that is, until the lens becomes stiff with age (presbyopia).

Created by General Medical Systems Information Systems Division