The meridian system is described as a set of nested networks rather than a single circuit. The twelve principal channels carry qi and blood to the organs and the surface; the eight extraordinary vessels act as reservoirs that regulate the overflow; the divergent channels carry qi deeper into the body; the collaterals distribute it through finer and finer branches; the muscle regions govern the sinews, and the cutaneous regions the surface.
What matters clinically is which layer is affected. A disorder of the cutaneous region responds to shallow needling, a disorder of the channels to needling along the channel, and a disorder of the organs to points that reach deeper. The system is best understood as an instrument for locating a disorder in depth as well as in place.