Protective Features of the Eyes
The orbit, eyelashes, eyelids, conjunctiva, and lacrimal glands help protect the eyes.
The bony structures of the orbit (the bony cavity that contains the eyeball and its muscles, nerves, and blood vessels, as well as the structures that produce and drain tears) protrude beyond the surface of the eye. They protect the eye while allowing it to move freely in a wide arc.
The eyelashes are short, tough 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 keep insects and foreign particles away from the eye by acting as a physical barrier and by causing the person to blink reflexively at the slightest sensation or provocation.
The upper and lower eyelids are thin flaps of skin and muscle that can cover the eye. They reflexively close quickly (blink) to form a mechanical barrier that protects the eye from foreign objects, wind, dust, insects, and very bright light. This 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 insects.
On the moist back surface of the eyelid, the conjunctiva loops around to cover the front surface of the eyeball, right up to the edge of the cornea. The conjunctiva protects the sensitive tissues underneath it.
When blinked, the eyelids help spread tears evenly over the surface of the eye. Tears consist of a salty fluid that continuously bathes the surface of the eye to keep it moist and transfers oxygen and nutrients to the cornea, which lacks the blood vessels that supply these substances to other tissues. 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.
Tears contain 3 layers: water, mucous, and oil. The lacrimal (tear) glands produce the water layer. The lacrimal glands, located at the top outer edge of each eye (see Figure: Where Tears Come From) and within the conjunctiva, produce the watery portion of tears, which flow to the eye surface through the lacrimal excretory ducts. Mucous glands in the conjunctiva produce mucus, and oil (lipid) glandsin the eyelid margin produce an oil. The mucus and oil mix 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, called the punctum.