灵枢经·五十营 The Fifty Circuits
黄帝曰：余愿闻五十营奈何？歧伯答曰：天周二十八宿，宿三十六分；人气行一周，千八分，日行二十八宿。人经脉上下左右前后二十八脉，周身十六丈二尺，以应二十八宿。
漏水下百刻，以分昼夜。故人一呼脉再动，气行三寸，一吸，脉亦再动，气行三寸，呼吸定息，气行六寸；十息，气行六尺，日行二分。二百七十息，气行十六丈二尺，气行交通于中，一周于身，下水二刻，日行二十五分。五百四十息，气行再周于身，下水四刻，日行四十分。二千七百息，气行十周于身，下水二十刻，日行五宿二十分。一万三千五百息，气行五十营于身，水下百刻，日行二十八宿，漏水皆尽脉终矣。
所谓交通者，并行一数也。故五十营备，得尽天地之寿矣，凡行八百一十丈也。
Huangdi said: 'I wish to hear about the fifty circuits — how do they proceed?' Qibo replied: 'Heaven's circuit consists of the twenty-eight lunar lodges, each lodge divided into thirty-six fen; thus a single circuit of the human qi covers one thousand and eight fen, corresponding to the sun's traversal of the twenty-eight lodges. The channels of the human body, above and below, left and right, front and back, number twenty-eight vessels, and their total circuit around the body measures sixteen zhang and two chi, corresponding to the twenty-eight lodges.'
The water clock drips down one hundred markings to divide day and night. Thus, with each exhalation of a person, the pulse beats twice and the qi travels three inches; with each inhalation, the pulse likewise beats twice and the qi travels three inches. With a complete breath — inhalation and exhalation together — the qi travels six inches. In ten breaths, the qi travels six feet, while the sun travels two fen. In two hundred and seventy breaths, the qi travels sixteen zhang and two chi, threading through and communicating within, completing one circuit of the body; the water clock drips down two markings, while the sun travels twenty-five fen. In five hundred and forty breaths, the qi completes two circuits of the body; the water clock drips down four markings, while the sun travels forty fen. In two thousand seven hundred breaths, the qi completes ten circuits of the body; the water clock drips down twenty markings, while the sun travels five lodges and twenty fen. In thirteen thousand five hundred breaths, the qi completes fifty circuits of the body; the water clock has dripped down its full one hundred markings, and the sun has traversed all twenty-eight lodges — the water clock is exhausted and the channels have completed their circuit.
What is called 'threading through and communicating' means that the qi travels together as a single reckoning. Thus when the fifty circuits are complete, one has fully lived out the allotted span granted by Heaven and Earth for that day; in all, the qi travels eight hundred and ten zhang.