| Cloze Ancient Greek philosopher Aristotle viewed laughter as "a bodily exercise precious to health." Laughter does _1_ short-term changes in cardiovascular(心血管的) function and respiration, boosting heart rate, respiratory rate and depth, as well as oxygen consumption. But because hard laughter is difficult to last, a good guffaw(loud laugh)is unlikely to have _2 cardiovascular benefits the way walking or jogging does. _3_, instead of building up muscles, as exercise does, laughter apparently accomplishes the _4_. Studies _5_ back to the 1930s indicate that laughter _6_ muscles, decreasing muscle tone(肌肉紧张度) for up to 45 minutes after the guffaw subsides. Such physical relaxation might help _7_ psychological stress. After all, the act of laughing probably does produce other types of physical feedback that _8_ an individual's emotional state. _9_ one classical theory , our feelings are partially rooted in physical reactions. American psychologist William James and Danish physiologist Carl Lange argued at the end of the 19th century that humans do not cry _10 they are sad but that they become sad when the tears begin to flow. Although sadness also goes before tears, evidence suggests that emotions can flow from muscular responses. In an experiment published in 1988, social psychologist Fritz Strack of the University of W? rzburg in Germany and his colleagues asked volunteers to _11_ a pen either with their teeth-thereby creating an artificial smile-or with their lips, which would produce a _12_ expression. Those forced to exercise their smiling muscles _13_ more energetically to funny cartoons than those whose mouths were contracted in a frown(皱眉)did, suggesting that expressions may influence _14_ rather than just the other way around. _15_, the physical act of laughter could improve mood.
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