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I have pondered that the ancient sages considered the back to be Yang and the abdomen to be Yin. However, earlier experts considered the back to be Yin and the abdomen to be Yang. Is it because of differences between the anterior and posterior aspects, or variations between upper and lower regions? Yet, I believe that from the perspective of the physical structure's rigidity and flexibility, the back is Yang and the abdomen is Yin. From the perspective of function's movement and stillness, the back is Yin and the abdomen is Yang. Furthermore, "Yang" transforms into Qi (vital energy), while "Yin" forms into substance. Thus, both the abdomen and back can be referred to as Yin (referring to their substantial nature). "Yang" resides on the exterior for protection, while "Yin" resides within for storage; thus, both the abdomen and back can also be referred to as Yang (referring to their functional aspect). The Dao of Yin and Yang is vast with no external limit and subtle with no internal boundary; how can it be limited to a single aspect?

The back is the outer mansion of the chest organs. The midline of the back is the pathway of the Du Mai (Governor Vessel, which controls all Yang meridians). The second line, located 1.5 cun lateral to the spine, and the third line, located 3 cun lateral to the spine, are the pathways of the Foot-Taiyang Bladder Meridian (Zu Taiyang Jing). As these meridians descend from above, there are distinctions in upper and lower locations. Since diseases may occur on either the left or right side, there are distinctions in left and right locations. Extending this reasoning, the upper half of the body is Yang, and the lower half is Yin; the left side is Yang, and the right side is Yin; the skin on the exterior is Yang, and the sinews and bones on the interior are Yin. Therefore, the upper and lower biao and ben (branch and root) differ, and the left and right ni and cong (reverse and compliant) also vary. How can the methods of diagnosis fail to meticulously seek the underlying principles?

Regarding morphological changes: swelling and protrusion indicate excess of pathogenic factors (Xie Qi); depression and sinking indicate deficiency of healthy Qi (Zheng Qi). A back elevated like a turtle is called "Gui Bei" (Turtle Back); a spine protruding like a saw is called "Ji Gan" (Spine Rickets/Atrophy). If the back ulcerates and penetrates the membranes, the form is ruined and cannot be treated. If the back is flat and accompanied by edema, the Lung is damaged and cannot be cured. These are changes in form due to disease.

Regarding postural manifestations: stiffness of the spine indicates pathology in the Foot-Taiyang Meridian; opisthotonos (backward arching) indicates pathology in the Du Mai. A curved back with drooping shoulders suggests the imminent collapse of the chest mansion. Neck stiffness with lumbar flexion indicates injury to the meridians of the back. A straight neck with stiff back, accompanied by a tilted and leaning body, is a sign of Liver Excess. Withered bones and depleted marrow, with an inability to lift the waist and spine, constitute the form of Gu Wei (Bone Flaccidity). Shoulders rising during respiration indicates firmness in the Heart region; inability to turn or sway indicates impending exhaustion of the Kidney. For various cases of opisthotonos, if the lateral aspect of an adult's back can accommodate a hand, or that of a child can accommodate three fingers, the condition is incurable. These are changes in posture due to disease.

Therefore, one must combine observation of complexion to discern similarities and differences, and reference pulse and symptoms to differentiate root and branch, deficiency and excess. Although conditions such as deficiency containing excess or excess containing deficiency may change infinitely, diagnostic judgment must remain fixed, so that treatment may be appropriate. Generally, when we speak of "Excess," we refer to the excess of pathogenic factors; when we speak of "Deficiency," we refer to the deficiency of healthy Qi. When pathogenic factors are excessive, healthy Qi may be either in surplus or deficit; when healthy Qi is deficient, pathogenic factors may be either in surplus or deficit. All subtle diagnostic laws should be viewed in this manner. This applies not only to diagnosing the back but to all areas.