Eight extra meridians include the conception vessel, governor vessel, thoroughfare vessel, belt vessel, yang heel vessel, yin heel vessel, Yang link vessel, Yin link vessel, and so on. It is record in Neijing, a generic name is eight extra meridians in the Difficult Classic, and The Conception vessel and governor vessel separately have its own acupoint, organ, and meridian on the Neijing and Difficult Classic, which in the middle behind of the body. So later generation adds these two meridians into twelve meridians, is known as fourteen meridians. Other six meridians, the thoroughfare vessel has its organ and acupoint. Thoroughfare vessel has its own origination, twenty two acupoints, which are recorded in the Su Wen.Qi Fu Lun. They have a common origination with meridian of foot-shaoyin, so the later generation put them into meridian of foot-shaoyin. Besides, they are not independent meridians; two are branches of Bladder meiridian of Foot-Taiyang and three are branches of The Kidney meridain of Foot-Shaoyin. These meridians' circulation lines have no detail record in Neijing, some have only record its function but origination, such as yang heel vessel, LingSu.MaiDu has recorded its treatment site, Yang link vessel, the Su Wen. CiYaoTongLun has recorded its name, but no origination and circulation line, Yin link vessel, the Su Wen. CiYaoTongLun has recorded it before ShaoYin and connect with Yin link vessel, belt vessel, the LinSu. JinMai has recorded its Foot-SHAO YIN. Larger collaterals of Shen meridian has recorded the second lumbar vertebrae are belong to belt meridian, which the later generation improve its whole circulation line.
During the period of Sui-Tang, Yang Shangshan, who compiles the "eight extra meridians" as a volume in HuangDi Neijing Mingtang Nei Cheng, where records its details, but this book have been lost. The extant book, A-B Classic of Acupuncture and Moxibustion has recorded acupoints of eight meridians, the later generation describes eight extra meridians' circulation lines and their distribution lines which according to acupoints of eight meridians in A-B Classic of Acupuncture and Moxibustion. There are different identify in records of eight extra meridians, such as Yang link vessel distributes on the head. Shisijingfahui are from shoulder along Yang meridians of foot upward to Yangbai acupoint on the head, and then return to upper meridian from Benshen, Linqi, Zhengying, and Naokong to Fengchi. Qijingbamaikao of Li Shizhen is ascending from shoulder to neck, passing Fengchi, Naokong, and Muchuang to Yangbai acupoint of the forehead, and then circulates head along ears up to Benshen. Yizongjinjian has record that it is through shoulder upward to Yangbai of face, Benshen, Zhengying, and Naokong to Fengchi acupoint. The brief Meridian Doctrine published by traditional Chinese medicine college records that is through shoulder upward to Yamen, then from Fengchi along Naokong, Zhengying, Linqi, Benshen upward to Yangbai acupoints. The Tenth lecture of Meridian which published by the people publish company in ShangHai, recorded that is through shoulder upward to neck circulate ear upward to Yangbai acupoint, then returned along Muchuang, Naokong to Fengchi and Yamen.
The picture of eight extra meridians is according to Neijing, Nanjing, A-B Classic of Acupuncture and Moxibustion, Fourth meridians and Eight Extra Meridian Tests, and also referring to the picture of eight extra meridians in modern times and anatomical position of the body tissue. For display the line that distribute between the internal organs and tissues, most displayed by the method of anatomy. For it easy to comparison, one draw the related meridians as a picture, such as Yang heel meridian and Yang link meridian, Yin heel meridian and Yin link, which is easy for one refer to those two picture.
The record of eight extra meridians is different from the twelve meridians, fifteen meridians, twelve meridian divergences, and twelve meridian sinews. We have mentioned previously, some do not have detail record in ancient literature and some are disorder, so there are many problems, such as routes of anterior aspect of abdomen of governor vessel, routes of lower limb of thoroughfare vessel, and three meridians with one origination. There are still different views on these issues, every meridian which are added notes for reference to the fellow.
Here we should mention that both of Yang heel vessel and Yang link vessel originate from the same meridian, the branch of these two vessels just have several cun apart but play different roles, so are Yin heel vessel and Yin link vessel, Author doubts whether there is no exactly explanation of the four meridians' initial acupoints, whether or not the later generation set the wrong position that should be researched.