The first surprise is the sophistication of our sphincters. The vast majority of people are familiar only with the outer sphincter: the muscle we can consciously control, opening and closing it at will. There is another, very similar muscle close by—but this is one we can’t control consciously. Each of the two sphincters looks after the interests of a different nervous system. The outer muscle is a faithful servant of our consciousness. When our brain deems it an unsuitable time to go to the toilet, the external sphincter obeys and stays closed with all its might. The internal sphincter
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