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It has long been known for sure that the sight of tasty food makes a hungry man's mouth water; also lack of appetite has always been regarded as an undesirable phenomenon, from which one might conclude that appetite is essentially linked with the process of digestion.

人们早就知道，一看到美味的食物，饥饿的人就会流口水;此外，食欲不振一直被认为是一种不受欢迎的现象，从这一现象我们可以得出这样的结论:食欲在本质上与消化过程有关。

— Ivan Petrovich Pavlov 巴甫洛夫