英语听力—美国之音慢速英语（MP3+文本，打包40份）

Science in the News – Aspirin From VOA Learning English, this is Science in the News. I'm Mario Ritter. And I'm Avi Arditti. Today, we tell the story of aspirin. People have known since ancient times that aspirin helps to reduce pain and high body temperature. But that is not all aspirin can do. It has gained important new uses in recent years. Small amounts of the drug may help prevent a heart attack or stroke. One study showed that some people who took two aspirin pills a day had lower rates of colorectal cancer. And researchers say aspirin may help patients with colon cancer live longer. But others say the acid in aspirin can cause bleeding in the stomach and intestines. And studies showed that aspirin or other pain medicines may lead to loss of eyesight and hearing. So, how did aspirin become so important? The story begins with a willow tree. Two thousand years ago, the Greek doctor Hippocrates advised his patients to chew on the bark and leaves of the willow. The tree contains the chemical salicin. In the 1800s, researchers discovered how to make salicylic acid from salicin. In 1897, a chemist named Felix Hoffmann at Friedrich Bayer and Company in Germany created acetyl salicylic acid. Later, it became the active substance in a medicine that Bayer called aspirin. In 1982, a British scientist shared the Nobel Prize in Medicine in part for discovering how aspirin works. Sir John Vane found that aspirin blocks the body from making natural substances called prostaglandins. Prostaglandins have several effects on the body. Some cause pain and the expansion, or swelling, of damaged tissue. Others protect the lining of the stomach and small intestine. Prostaglandins make the heart, kidneys and blood vessels work well. But there is a problem. Aspirin works against all prostaglandins, good and bad. Scientists have also learned how aspirin interferes with an enzyme. One form of this enzyme makes the prostaglandin that causes pain and swelling. Another form creates a prot