The hidden thoroughfare vessel · fu chong zhi mai

While in the transports, the six channels do not pass, the four limbs and their joints ache, and the lower back and spine grow stiff; remaining without leaving, it passes to lodge in the hidden penetrating vessel. While in the hidden penetrating vessel, the body is heavy and hurts; remaining without leaving, it passes to lodge in the stomach and intestines.

Qi Bo answered: When evil lodges at the wind palace, the illness follows the spine downward; guarding qi gathers greatly at the wind palace once in a day and a night, and on the next day it descends one joint, so the attack comes a day later — this is because the evil first lodged in the spine and back. Hence whenever it reaches the wind palace the interstices open; opening, evil enters; entering, the illness breaks out: this is why the daily attack comes later and later. As guarding qi moves from the wind palace down one joint a day, on the twenty-first day it reaches the tail bone, and on the twenty-second enters within the spine and pours into the hidden thoroughfare vessel. It travels nine days and comes out in the middle of the collar-bone basin; its qi rises, and so the illness slightly worsens.