The Triple Burner, san jiao, is the standing puzzle of Chinese medicine, and it has been a puzzle within the tradition rather than only to outside readers. The Nanjing, the Classic of Difficult Issues, compiled in the first or second century, states the problem directly in its thirty eighth and twenty fifth difficulties: this is an organ that has a name and no form. Later commentators disagreed with each other about it for well over a thousand years, and they have not stopped.
What the tradition assigns to it is a function rather than a structure: the passage of water and of qi through three divisions of the trunk, an upper burner above the diaphragm, a middle burner between diaphragm and navel, and a lower burner below. Some writers treat it as the relationship between the organs rather than as an organ; some treat it as the membranes and spaces between them; some treat the term as purely functional.
For the atlas the interest is that a system often described from outside as rigidly schematic contains, at its centre, an item its own canonical texts flag as unresolved. That is a genuine internal debate with identifiable camps, not a gap.
The channel itself is unambiguous, running from the ring finger up the back of the arm to the outer eyebrow, paired with the Pericardium in the Fire phase.