Eight extra meridians is a collective term for governor vessel, conception vessel, thoroughfare vessel, belt vessel, yin heel vessel, yang heel vessel, yin link vessel, and yang link vessel. Collateral vessels serve as a network linking the various aspects of the body. There are fifteen collateral vessels all together.

Fig. 12-3 The eight extra meridians

Eight extra meridians (Fig. 12-3) have the functions of governing, connecting, and regulating the twelve regular meridians. They store qi and blood and regulate the amount of qi and blood in the twelve meridians. When there is extra qi and blood in the twelve meridians, the eight extra meridians store the extra part and when there is a shortage of qi and blood in the twelve meridians, the eight extra meridians provide qi and blood to the twelve meridians. The governor vessel and conception vessel have acupuncture points, and all the other ones do not.

Meridian and collateral theory operates on the basis that body organs and tissues are connected by meridians and collateral vessels; meridians are the main trunk, while collaterals are the main branches of the meridians. There are fifteen collateral vessels in all, one from each of the twelve meridians, one from the governor vessel, one from the conception vessel, and the other one is from the spleen meridian, called dabao. The spleen meridian has two collateral vessels.

The twelve main meridian collateral vessels branch out of the main meridians from the connecting points* below the elbow and knee, and travel to the paired meridians. For example, lung meridian collateral travels to large intestine meridian and large intestine meridian collateral travels to lung meridian, because lung meridian and large intestine meridian are paired meridians. The collateral of lung (LU) meridian branches out of the meridian from lieque point (LU7), the collateral of LI branches out from pianli (LI6), the collateral of ST from fenglong (ST40), the collateral of SP from gongsun (SP4), the collateral of HT from tongli (HT5), the collateral of SI from zhizheng (SI17), the collateral of BL from feiyang (BL58), the collateral of KI from dazhong (KI4), the collateral of PC from neiguan (PC6), the collateral of TE from waiguan (TE5), the collateral of GB from guangming (GB37), the collateral of LR from ligou (LR5), the collateral of CV from jiuwei (CV15), the collateral of GV from changqiang (GV1), and great collateral of the spleen is from dabao (SP21).

The collateral vessels that run on the superficial layer of the body are called superficial collateral vessels*; the branches of collateral vessels are named tertiary collaterals*. Tertiary collateral vessels can be further divided into even smaller ones.

The main functions of the collateral vessels are to provide more routes for qi and blood to travel. They enforce the body connections and enhance the connections of exterior-interiorly related meridians.

The twelve meridian divergences are the divergent passages of the twelve main meridians, and they travel deep inside the body. Their traveling characteristics can be summarized as "leave", "enter", "exit" and "merge". They "leave" (branch out) from the twelve meridians in the areas above elbow and knee, "enter" the body cavity (thoracic and abdominal cavities) and connect with their pertaining viscera or bowels, "exit" at the neck, where the divergences of yang meridians "merge" with the yang meridians they leave from.“离”），进“入”体腔（胸腔和腹腔），与它们的所属脏腑相连，“出”于颈部，在颈部阳经的经别“合”并于本经。">* But the divergences of yin meridians "merge" with the yang meridians which their meridians are paired with, instead of merging with the yin meridians they leave from. For example, bladder meridian and kidney meridian divergences branch out from gallbladder and kidney meridians from the popliteal fossa*; they enter the body and connect with the kidneys and the gallbladder, exit at the nape and merge with bladder meridian.

Twelve meridian divergences set up connections between meridians and internal organs and between yin-yang meridians. For example, the lung meridian divergence leaves lung meridian, travels to the lung and connects to the large intestine; therefore, the lung meridian divergence connects the lung meridian to the lung and the large intestine. All paired meridian divergences travel like this. Because of these connections, the acupoints in one meridian can be used to treat the disease of the organ the meridian pertains to as well as the disease of the organ the meridian connects with.

Twelve meridian sinews are the sinew systems attributed to the twelve meridians. Each meridian sinew is attributed to a meridian and the sinew gets nourished by the meridian. All meridian sinews start from the tips of the fingers and toes, accumulate at joints, travel along the superficial part of the body to the trunk and head. They do not enter the internal organs. The sinews of three yang meridian of the foot start from the tips of the toes and travel along their meridians to the face. The sinews of three yin meridian of the foot start from the tips of the toes and travel along their meridians to the external genitalia*. The sinews of three yang meridian of the hand start from the tips of the fingers and travel along their meridians to the head. The sinews of three yin meridians of the hand start from the tips of the fingers and travel along their meridians to the chest. Twelve meridians distribute, retain, and accumulate qi and blood in their specific sinew area.

Twelve cutaneous regions are the regions of the skin reflecting the functional condition of the twelve meridians. One cutaneous region reflects the functional condition of one meridian. Like the twelve sinews, the twelve cutaneous regions also need the nourishment of qi and blood from the corresponding meridians.