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*Yi Men Fa Lü* (Legal Principles of Medical Practice)
There are twelve regular meridians, and there are also twelve collateral vessels. The term "collateral" implies a meaning of encircling and connecting; it serves as the outer city wall of the twelve regular meridians. Additionally, there is the great collateral of the Stomach, the great collateral of the Spleen, and the great collaterals of the extraordinary meridians. These serve as the main thoroughfares for communication and entry/exit within the outer city wall. Therefore, it is said that there are fifteen collaterals in total. The twelve regular meridians generate twelve collateral vessels; the twelve collateral vessels generate one hundred and eighty *xi luo* (connecting collaterals); the connecting collaterals generate one hundred and eighty *chan luo* (entangling collaterals); and the entangling collaterals generate thirty-four thousand *sun luo* (branchlet collaterals). Collaterals generated from the interior moving outward become progressively smaller as their number increases. Those slightly larger are located between the acupoints and muscles, governed by Ying Qi (Nutritive Qi) which controls the outer boundary. Further out, spreading into the skin and hair, they become small collaterals, governed by Wei Qi (Defensive Qi). Thus, when external pathogens enter via the Wei level, they do not immediately enter the Ying level, because the collateral vessels entangle and obstruct them. Only when the pathogens in the collaterals become excessive do they enter the Ying level. Hence it is said, "When collaterals are full/excessive, pathogens enter the meridians," because Ying Qi circulates within the meridians. However, wind-cold and other formless exogenous pathogens (Six Excesses) can easily penetrate, and the collateral vessels cannot prevent this; when the pathogenic factor becomes excessive, it enters the meridians. If diseases arise internally from Ying Qi, manifesting as blood stasis, qi stagnation, phlegm-fluid retention, or accumulations and masses—all tangible pathological factors—they inevitably seek to exit the collaterals. Therefore, it is "meridian excess entering collaterals, collateral excess returning to meridians," with pathogens lingering indefinitely. Consequently, bian stone therapy (bloodletting) is employed to pierce and drain the pathogens from the collaterals.

*Nan Jing* (Classic of Difficult Issues) identifies the Yang Qiao Mai (Yang Heel Vessel), Yin Qiao Mai (Yin Heel Vessel), and the Great Collateral of the Spleen as the fifteen collaterals, establishing this nomenclature for later generations. I contend that the names "Yang Qiao" and "Yin Qiao" as separate collaterals are originally erroneous; they should be understood collectively referring to the extraordinary meridians as a single great collateral. Generally, each of the twelve regular meridians has one collateral, totaling twelve collaterals. Besides these, there is the Great Collateral of the Stomach, which descends directly through the diaphragm and huang area, unifying and connecting the upper collaterals. There is also the Great Collateral of the Spleen, which traverses horizontally across the hypochondrium and abdomen, unifying and connecting the middle collaterals. Furthermore, there is the Great Collateral of the extraordinary meridians, which circles and connects the collaterals of all meridians throughout the body up and down. Thus, the twelve collaterals connect the meridians, while the three great collaterals connect the collaterals. I have deduced the meaning of the extraordinary meridians: The Du Mai (Governing Vessel) governs all yang meridians and runs along the back; the Ren Mai (Conception Vessel) assumes all yin meridians and runs along the front; neither connects with each other. The Chong Mai (Penetrating Vessel) rushes straight up into the chest; the Dai Mai (Belt Vessel) binds horizontally at the waist; neither connects with each other. The Yang Qiao Mai and Yin Qiao Mai both originate from the heel; one follows the lateral malleolus, the other the medial malleolus; they run parallel and compete in agility, showing no intention of mutual connection. The Yang Wei Mai (Yang Linking Vessel) and Yin Wei Mai (Yin Linking Vessel) originate from the meeting point of all yang meridians and the intersection of all yin meridians, respectively. Although named "Wei" (Linking), they actually link yang to its own yang and yin to its own yin, rather than linking mutually. If Yang Qiao and Yin Qiao could be considered two collaterals, then Yang Wei and Yin Wei could certainly be considered two collaterals as well, and Du, Ren, Chong, and Dai could collectively be called eight collaterals. *Nan Jing* states that the pulses of the extraordinary meridians are like ditches overflowing and flowing into deep lakes, so sages cannot map them. This clearly indicates that the extraordinary meridians are indeed collaterals. How could the water of great rivers and major meridians be compared to ditches? It further states that when human pulse energy is abundant, it enters the eight vessels without circulating cyclically, so the twelve regular meridians cannot constrain it. Overflowing and accumulating without circulating to irrigate the regular meridians is entirely due to meridian excess entering the collaterals; thus, overflow and accumulation remain only in the collaterals and cannot circulate to irrigate the regular meridians. Since this is the case, the extraordinary meridians collectively constitute one great collateral, leaving no room for doubt.