The meridian system (, also called channel network) is a concept in traditional Chinese medicine (TCM). Meridians are paths through which the life-energy known as "qi" (ch'i) flows. Meridians are not real anatomical structures: scientists have found no evidence that supports their existence. One historian of medicine in China says that the term is "completely unsuitable and misguided, but nonetheless it has become a standard translation." Major proponents of their existence have not come to any consensus as to how they might work or be tested in a scientific context. History The concept of meridians are first attested in two works recovered from the Mawangdui and Zhangjiashan tombs of the Han-era Changsha Kingdom, the Cauterization Canon of the Eleven Foot and Arm Channels Zúbì Shíyī Mài Jiǔjīng) and the Cauterization Canon of the Eleven Yin and Yang Channels Yīnyáng Shíyī Mài Jiǔjīng). In the texts, the meridians are referenced as mài () rather than jīngmài. Main concepts The meridian network is typically divided into two categories, the jingmai () or meridian channels and the luomai () or associated vessels (sometimes called "collaterals"). The jingmai contain the 12 tendinomuscular meridians, the 12 divergent meridians, the 12 principal meridians, the eight extraordinary vessels as well as the Huato channel, a set of bilateral points on the lower back whose discovery is attributed to the ancient physician Hua Tuo. The collaterals contain 15 major arteries that connect the 12 principal meridians in various ways, in addition to the interaction with their associated internal organs and other related internal structures. The collateral system also incorporates a branching expanse of capillary-like vessels which spread throughout the body, namely in the 12 cutaneous regions as well as emanating from each point on the principal meridians. If one counts the number of unique points on each meridian, the total comes to 361, which matches the number of days in a year, in the moon calendar system. Note that this method ignores the fact that the bulk of acupoints are bilateral, making the actual total 670. There are about 400 acupuncture points (not counting bilateral points twice) most of which are situated along the major 20 pathways (i.e. 12 primary and eight extraordinary channels). However, by the second Century AD, 649 acupuncture points were recognized in China (reckoned by counting bilateral points twice). There are "12 Principal Meridians" where each meridian corresponds to either a hollow or solid