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Even after centuries of scientific research, no one knows for sure why human beings (plus a few other primates, including chimpanzees, apes, and orangutans) laugh.

2. Even if scientists still do not know why we laugh, they have learned a lot about it. For example: The sound of laughter is created when you inhale deeply and then release the air while your diaphragm moves in a series of short, spasmodic contractions. The typical laugh is made up of pulses of sound that are about 1/15th of a second long and 1/5th of a second apart.

3. One of the most interesting things researchers have learned is the powerful healing effect of laughter. Well, actually they' re re-learning it after centuries of neglect: In the Middle Ages, doctors "treated" their patients by telling them jokes, but modem medicine discounted the curative properties of laughing. That began to change in 1979, when editor Norman Cousins wrote Anatomy of an Illness, in which he credited watching humorous videos with helping him reduce pain and recover from ankylosing spondylitis (类风湿性脊椎炎), a lifethreatening degenerative spinal disease. The book inspired researchers to look into whether laughter really could aid in healing and recovery from illness.

4. In 1995, two researchers at the Loma Linda University School of Medicine had 10 medical students watch a 60-minute videotape of Gallagher, a stand-up comedian famous for smashing watermelons and other objects with a sledgehammer. The researchers found that after watching the video, where was a measurable decrease in stress hormones, including epinephrine and dopamine, in the students' blood, plus an increase in endorphins-the body' s natural painkillers. But the most changes were found in the students' immune systems. These included

&8226; Increased levels of gamma interferon, a hormone that "switches on" the immune system, helps fight viruses and regulates cell growth.

&8226; Increased numbers of "helper T-cells," which help the body coordinate the immune system's response to illness

&8226; More "Compliment 3," a substance that helps antibodies destroy infected and damaged cells

&8226; An increase in the number and activity of "natural killer (NK) cells," which the body uses to attack foreign cells, cancer cells, and cells infected by viruses

5. Some of the levels even began to change before the students watched the video-just from the expectation that they were about to laugh. "Say you' re going to your favorite restaurant," Dr. Berk explains. "You can visualize the food; you can almost taste it. You are already experiencing the physiology of enjoying it. Your immune system [also] remembers. By using humor to combat stress, you can condition yourself to strengthen your immune system."

6. Chimpanzees, apes and a few other primates laugh, but no other animals do Chimps' laugh sounds like rapid panting, but unlike humans, they are unable to regulate or control the air as they breathe out. This lack of ability to control airflow is same thing that deprives them of speech. Chimps and gorillas that have learned sign language have been known to sign one another for laughs

A. The Process of Laughter.

B. Why Laughter Developed？

C. Laughter and Other Creatures.

D. Mordern Breakthrough in Humour for Therapy.

E. Various After-effects of laughing.

F. Anticipatorv Effect on the Immune System.

Paragraph 3. ______