灵枢经·平人绝谷 On the Ordinary Person Going Without Food
黄帝曰：愿闻人之不食，七日而死，何也？伯高曰：臣请言其故。
胃大一尺五寸，径五寸，长二尺六寸，横屈受水谷三斗五升，其中之谷，常留二斗，水一斗五升而满，上焦泄气，出其精微，慓悍滑疾，下焦下溉诸肠。
小肠大二寸半，径八分分之少半，长三丈二尺，受谷二斗四升，水六升三合合之大半。
回肠大四寸，径一寸寸之少半，长二丈一尺，受谷一斗，水七升半。
广肠大八寸，径二寸寸之大半，长二尺八寸，受谷九升三合八分合之一。
肠胃之长，凡五丈八尺四寸，受水谷九斗二升一合合之大半，此肠胃所受水谷之数也。平人则不然，胃满则肠虚，肠满则胃虚，更虚更满，故气得上下，五藏安定，血脉和利，精神乃居，故神者，水谷之精气也。故肠胃之中，当留谷二斗，水一斗五升；故平人日再后，后二升半，一日中五升，七日五七三斗五升，而留水谷尽矣；故平人不食饮七日而死者，小谷精气津液皆尽故也。
Huangdi said: 'I wish to hear why it is that when a person does not eat, death follows within seven days.' Bogao said: 'Allow your servant to explain the reason.'
The stomach is one chi and five cun in breadth, with a diameter of five cun and a length of two chi and six cun; coiled crosswise, it holds three dou and five sheng of water and grain. Of this, the grain that ordinarily remains within amounts to two dou, and the water one dou and five sheng, when full. The upper burner releases qi, sending forth the essential and subtle portion, swift and smooth and rapid in its movement, while the lower burner irrigates the intestines below.
The small intestine is two cun and a half in breadth, with a diameter of a little less than eight fen, and a length of three zhang and two chi; it holds two dou and four sheng of grain, and a little more than six sheng and three ge of water.
The large intestine is four cun in breadth, with a diameter of a little less than one cun, and a length of two zhang and one chi; it holds one dou of grain and seven sheng and a half of water.
The rectum is eight cun in breadth, with a diameter of a little more than two cun, and a length of two chi and eight cun; it holds nine sheng, three ge, and an eighth of a ge of grain.
Altogether, the length of the intestines and stomach is five zhang, eight chi, and four cun, and together they hold nine dou, two sheng, and a little more than one ge of water and grain — this is the total measure of water and grain that the intestines and stomach can receive. In the healthy person, however, matters do not remain fixed in this way: when the stomach is full the intestines are empty, and when the intestines are full the stomach is empty; emptiness and fullness alternate, and so the qi is able to move up and down, the five viscera remain settled and at peace, the blood and vessels are harmonious and free-flowing, and the essence and spirit thereby find their dwelling place. Hence spirit is nothing other than the essential qi of water and grain. Thus, within the intestines and stomach, there ordinarily remains two dou of grain and one dou and five sheng of water. Hence the healthy person has two bowel movements each day, each releasing two and a half sheng, five sheng in the course of a day; over seven days, five times seven gives three dou and five sheng, and the retained water and grain would then be entirely exhausted. This is why, when a healthy person goes without food or drink for seven days, death follows — because the essential qi and fluid and humor derived from grain have all become exhausted.