英语时差:我们为什么会哭？
You notice your tears most after a good cry or while slicing an onion, but actually your eyes are constantly moistened by a thin film of tears.
This film has three distinct layers: an outer, oily layer to prevent evaporation; a middle layer of lacrimal fluid, the main ingredient of tears; and an inner, mucous layer. This three layer film is replenished every time you blink, and it provides essential protection for your eyes.
Most of your body is protected by skin of course, and your outer layer of skin is made of dead cells and keratin, an opaque, protective substance. Your eyes need to be transparent,however; they can’t be covered by a layer of dead cells and keratin. One of the main functions of tears therefore, is to keep these cells alive.
Tears are loaded with electrolyte salts, chemicals that are also found deep inside your body. These salts make your outer eye cells feel like they’re inside your body, preventing them from turning into cells similar to skin cells.
Tears help your eyes in other ways too. When a speck of dust gets in your eye, it’s coated with mucous from glands in the white of your eye, then washed out with extra fluid from the lacrimal glands, the main tear producing glands above the eye.
To protect your eyes from germs, tears also contain bacteria fighting enzymes. What’s more, tears provide a good optical surface. They smooth out the microscopically uneven cells of your cornea. Without tears, tiny irregularities in your eye would give you constantly fuzzy vision.
Although they’re most apparent during a good cry, tears are actually an essential part of everyday life.
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