organ; interacting with it and extending along a particular extremity (i.e. arm or leg). There are also "Eight Extraordinary Channels", two of which have their own sets of points, and the remaining ones connecting points on other channels. 12 standard meridians The 12 standard meridians, also called Principal Meridians, are divided into Yin and Yang groups. The Yin meridians of the arm are the Lung, Heart, and Pericardium. The Yang meridians of the arm are the Large Intestine, Small Intestine, and Triple Burner. The Yin Meridians of the leg are the Spleen, Kidney, and Liver. The Yang meridians of the leg are Stomach, Bladder, and Gall Bladder. The table below gives a more systematic list of the 12 standard meridians: Eight extraordinary meridians The eight extraordinary meridians are of pivotal importance in the study of Qigong, Taijiquan and Chinese alchemy. These eight extra meridians differ from the standard twelve organ meridians in that they are considered to be storage vessels or reservoirs of energy and are not associated directly with the Zang Fu, i.e. internal organs. These channels were studied in the "Spiritual Axis" chapters 17, 21 and 62, the "Classic of Difficulties" chapters 27, 28 and 29 and the "Study of the 8 Extraordinary vessels" (Qi Jing Ba Mai Kao), written in 1578. The eight extraordinary vessels are (): Conception Vessel (Ren Mai) – Governing Vessel (Du Mai) – Penetrating Vessel (Chong Mai) – Girdle Vessel (Dai Mai) – Yin linking vessel (Yin Wei Mai) – Yang linking vessel (Yang Wei Mai) – Yin Heel Vessel (Yin Qiao Mai) – Yang Heel Vessel (Yang Qiao Mai) – Scientific view of meridian theory Scientists have found no evidence that supports their existence. The historian of medicine in China Paul U. Unschuld adds that there "is no evidence of a concept of 'energy' -- either in the strictly physical sense or even in the more colloquial sense -- anywhere in Chinese medical theory." Some advocates of traditional Chinese medicine believe that meridians function as electrical conduits based on observations that the electrical impedance of a current through meridians is lower than other areas of the body. A 2008 review of studies found that the studies were of poor quality and could not support the claims. Some proponents of the Primo Vascular System propose that the putative primo vessels, very thin (less than 30 μm wide) conduits found in many mammals, may