The meridian-collateral theory concerns with the physiological functions and pathological changes of the meridian-collateral system, and their relationships with zang-fu (viscera) organs. It is an important component of the theoretical system of TCM. And it is considered as a theoretical basis of all clinical subjects of TCM, especially that of acupuncture and moxibustion, tuina, and qigong. Besides, it guides the clinical practice of other branches of TCM.

The meridians and collaterals are pathways along which qi and blood circulated through the whole body. The meridians are the major trunks of the meridian-collateral system and run longitudinally within the body, while the collaterals are the branches of the meridians and are reticularly distributed over the whole body. Hence, the meridians and collaterals, connecting the zang-fu organs with extremities, the upper with the lower and the internal with the external portions of the body, make all the body’s organs and tissues an organic whole. The meridian-collateral system consists of meridians and collaterals as well as their subsidiary parts. This system, internally, links the zang-fu organs and, externally, joins the tendons, muscles and skin.

The meridians are classified into three categories, i.e. the regular meridians, the extra meridians and the divergent meridians. There are twelve regular meridians, namely the three yin meridians as well as the three yang meridians of the hands and feet. They are known collectively as “the twelve regular meridians”, which are the main passages for qi and blood circulation; they start and terminate at given points, run along fixed routes, and meet in certain orders. They are directly connected with the relevant zang-fu organs. The eight extra meridians are composed of Du, Ren, Chong, Dai, Yinqiao, Yangqiao, Yinwei and Yanwei meridians.

They are interrelated with the twelve regular meridians and perform the functions of dominating, connecting and adjusting the twelve regular meridians. In addition, they are not directly related to the zang-fu organs. The twelve divergent meridians, as the extensions of the twelve meridians, originate from the limbs, run deeper into the zang-fu organs and emerge from the shallow neck. Their role is to enhance the links between every pair of meridians exteriorly-interiorly related in the twelve meridians and complement the organs and bodily areas to which the regular meridians cannot get.
The collaterals are the branches of the meridians.

They are divided into three groups, namely the divergent collaterals, superficial collaterals and tertiary collaterals. The divergent collaterals are the larger and main collaterals. The divergent collaterals originate from the twelve regular meridians as well as Du and Ren meridians respectively. Together with large splenic collateral, they are generally called “the fifteen divergent collaterals”. Their chief task is to strengthen the links between every pair of meridians exteriorly-interiorly related on the body surface. The superficial collaterals are ones that run through the surface layer of the human body, and often emerge on the surface. And the tertiary collaterals refer to the smallest and the thinnest ones of the whole body.