Eyes | Mannat Academy
Only part of the eyes is visible in a person’s face. The whole eye—the eyeball—is about the size and shape of a ping-pong ball. All parts of the eye are extremely delicate, so our bodies protect them in several ways. The eyeball sits in the eye socket (also called the orbit) in the skull, where it is surrounded by bone. The visible part of the eyes is protected by the eyelids and the eyelashes, which help keep dirt, dust, and even harmful bright light out of the eye.
Eyes are also protected by tears, which moisten them and clean out dirt, dust, and other irritants that get past the defenses of the eyelashes and eyelids. Tears also help protect against infection. With each blink, our eyelids spread a layer of mucus, oil, and tears over the cornea, which covers the front of the eye. The lacrimal (LAK-ruh-mul) glands in the upper outer corner of each eye socket produce tears, which, after moistening the eyes, flow into canals in the eyelids. These canals drain into the lacrimal sac, a pouch in the lower inner corner of each eye socket. Tears then exit through a passage that leads to the nose.
To see, the eye has to move. Six extra ocular muscles surround the eyeball and act like the strings on a puppet, moving the eye in different directions. The muscles of each eye normally move together at the same time, allowing the two eyes to remain aligned.
The wall of an eyeball has three layers, rather like the layers of an onion:
- The sclera (SLEER-uh) is the protective layer. This tough, fibrous tissue surrounds the eyeball and attaches to the cornea, which is the clear front surface of the eye. What we see as the white of the eye is the sclera. Over the sclera lies the conjunctiva, a clear skin layer that protects the eye from becoming dry.
- The choroid (KOR-oyd) is the middle layer that contains blood vessels that deliver oxygen and nutrients to the inside parts of the eye.
- The retina (RET-nuh), the innermost of the three layers, lines the inside of the eyeball. The retina is a soft, light-sensitive layer of nervous system tissue. The optic nerve carries signals from the retina to the brain, which interprets them as visual images.
The space in the center of the eyeball is filled with a clear jelly-like material called the vitreous (VIH-tree-us) humor. This material allows light to pass through to the retina. It also helps the eye keep its round shape.
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