Protective Features

The bony cavity of the orbit protects the eye, while allowing it to move freely in a wide arc.

The eyelashes are short hairs that grow from the edge of the eyelid. The upper lashes are longer than the lower lashes and turn upward. The lower lashes turn downward. Eyelashes act as a barrier by sweeping foreign particles away from the eye.

The upper and lower eyelids are thin folds of skin that can cover the eye. They reflexively close quickly (blink) to form a mechanical barrier that protects the eye from foreign objects, wind, dust, and very bright light. The reflex is triggered by the sight of an approaching object, the touch of an object on the surface of the eye, or the eyelashes being exposed to wind or small particles such as dust or sand. On the inner surface of the eyelid, the conjunctiva loops back to cover the front surface of the eye, right up to the edge of the cornea. The conjunctiva protects the sensitive tissues underneath it.

See the figure Structures That Protect the Eye.

When blinked, the eyelids help spread tears over the surface of the eye. Tears consist of a salty fluid that continuously bathes the surface of the eye to keep it moist. When closed, the eyelids help trap the moisture against the surface of the eye. Small glands at the edge of the upper and lower eyelids secrete an oily substance that contributes to the tear film and keeps tears from evaporating. Tears keep the surface of the eye moist. Without such moisture, the normally transparent cornea can become dried, injured, infected, and opaque. Tears also trap and sweep away small particles that enter the eye. Moreover, tears are rich in antibodies that help prevent infection. The eyelids and tears protect the eye while allowing clear access to light rays entering the eye.

The lacrimal glands, located at the top outer edge of each eye, produce the watery portion of tears. Mucous glands in the conjunctiva produce mucus, which mixes with the watery portion of the tears to create a more protective tear film. Tears drain from each eye into the nose through one of the two nasolacrimal ducts; each of these ducts has openings at the edge of the upper and lower eyelids near the nose.