《黄帝内经·灵枢·肠胃》
伯高说：请让我详细地解说。谷物在人体中从入口到排出体外所经历的深浅、远近、长短过程是：从嘴唇到牙齿的长度是九分，嘴宽是二寸半。
伯高曰：请尽言之，谷所从出入浅深远近长短之度：唇至齿长九分，口广二寸半。
Bo Gao said: Let me explain in detail. The distance a grain must travel through the body from its entry to its exit is as follows: from the lips to the teeth is nine minutes long, and the width of the mouth is two and a half inches.