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Astigmatism is the most common vision problem. It may accompany nearsightedness or farsightedness. It is usually caused by an irregularly shaped cornea, but sometimes it is the result of an irregularly shaped lens. Either type can be corrected by eyeglasses, contact lenses, or refractive surgery.
The liver is the only organ that has the ability to regenerate itself after certain types of damage. As much as 25% of the liver can be removed, and it will still regenerate back to its original shape and size. However, the liver cannot regenerate after severe damage caused by alcohol.
When blood is exposed to air, it clots. Heparin allows the blood to come in direct contact with air without clotting.
Cancer has been around as long as humankind, but only in the second half of the twentieth century did the number of cancer cases explode.
The horizontal fraction bar was introduced by the Arabs.

