The twelve meridians are the main part of the meridian system and are also known as “the twelve regular meridians” including three yin meridians of the hand, three yang meridians of the hand, three yin meridians of the foot and three yang meridians of the foot.
1. Distribution on the Surface of the Body
The twelve meridians distribute in symmetry on the face, head, trunk and limbs.
Four limbs：The yin meridians distribute along the inner side of the four limbs, yang meridians on the lateral side of the four limbs. According to yin and yang, taiyin and yangming meridians distribute along the anterior side, shaoyin and taiyang along the posterior side, and jueyin and shaoyang along the middle side. Among these meridians, only jueyin meridian of the foot turns and converges with others in distribution. It runs anterior to the taiyin meridian of the foot 8 cun down to the medial malleolus. From the region 8 cun above the medial malleolus, it converges with taiyin meridian and runs between the taiyin and shaoyin meridians.
Trunk：The six meridians of the foot distribute in the way mentioned above. The only difference is that they pertain to either yin or yang according to the inner side and lateral side of the trunk. The three yang meridians of the foot distribute the surface of the trunk, yangming on the front, taiyang on the back and shaoyang on the lateral side, while the three yin meridians of the foot run in the interior part corresponding to the yang meridians that they are internally and externally related to. Among the six meridians of the hand, the three yang meridians all run over the shoulder to the neck while the three yin meridians all come out of the chest from the armpit.
Head and face：The six yang meridians of the hand and foot all reach the head and face to connect with the five sense organs. That is why it is said “the head is the convergence of all yang meridians”. All the six yin meridians run deep in the head and neck to connect with the throat, tongue and eyes.
2. Association of Twelve Meridians with the Viscera
As to its rule, the association of the twelve meridians with the viscera is mainly demonstrated as “pertaining and connection”. Yin meridians pertain to the zang organs and connect with the fu organs, while yang meridians pertain to the fu organs and connect with the zang organs. Besides the regular “pertaining and connection”, the taiyang meridians of the hand and the foot in the six yang meridians also associate with the stomach and the brain; the six yin meridians usually associate with the other zang and fu organs. The following table is a thorough demonstration of such relations among the meridians.
3. Running Direction, Circulation and Convergent Principle of the Twelve Meridians
(1) Running direction: Qi and blood in the twelve meridians flow in the same direction according to the distinction of three yin meridians of the hand and foot as well as three yang meridians of the hand and foot. Such a running direction is described in Lingshu (Spiritual Pivot ) as “The three yin meridians of the hand run from the chest to the hand; the three yang meridians run from the hand to the head; the three yang meridians of the foot run from the head to the foot; the three yin meridians run from the foot to the abdomen. ”
(2) Circulation: The twelve meridians form a cycle of qi and blood circulatory system with their regular and adverse circulatory directions. The circulatory order is demonstrated in the following table.
Table1-3 The flowing Order of the Twelve Meridians
Taiyin and yangming Lung meridian of hand-taiyin→Large intestine meridian of
meridians Lung of hand hand-yangming
and foot ↓
-front Spleen meridian of foot-taiyin←Stomach meridian of foot-yangming
↓
Shaoyin and taiyang Heart meridian of hand-shaoyin→Small intestine meridian
Meridians of hand and of heart-taiyang
Foot ↓
-back Kidney meridian of foot-shaoyin←Bladder of foot-taiyang
↓
Jueyin and shaoyang Pericardium meridian of hand-jueyin→Triple energizer
meridians of hand and meridian of hand-shaoyang
foot ↓
-middle Liver meridian of foot-jueyin←Gallbladder meridian of foot-shaoyang
(3) Convergent principle
The circulatory order shows that the first four meridians of taiyin and yangming of the hand and foot distribute along the anterior border of the body, the middle four meridians of shaoyin and taiyang of the hand and foot distribute along the posterior border of the body, the last four meridians of jueyin and shaoyang of the hand and foot distribute along the side or centre of the body. In this way qi and blood circulate in the twelve meridians for actually three cycles. The convergent way in each cycle follows the order of yin and yang meridians in external and internal relation as well as the yang meridians with the same name.
According to such a convergent way and in light of the circulatory direction of the meridians, it is obvious that the twelve meridians converge with the following principle, the yin and yang meridians in external and internal relationship converge over the end of the four limbs; the yang meridians with the same name converge over the head and face the yin meridians and yang meridians (in a cycle-like connection) converge over the chest. The running direction, circulation and convergence are interrelated and reflect the circulatory order of qi and blood in the twelve meridians from different angles.
(4). External and Internal Relation
External and internal relation, an important interrelation among the twelve meridians similar to that of the relation between zang organs and fu organs, refers to the correspondence of the three yin and three yang meridians. Such an external and internal relation among the twelve meridians, apart from the convergence of the yin and yang meridians in internal and external relation at the end of the four limbs as well as the regular "pertaining and connection" with the viscera over the trunk, further strengthens such a relation through meridian branches and collaterals.
The meridian branches reinforce the relation with the organs in external and internal relation by means of "entering" and reinforce the relation of the meridians in external and internal relation on the head and face. The collaterals of the twelve meridians intensify the relation of the meridians in external and internal relation on the four limbs by means of forking into the meridians in external and internal relation.