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The theory of yin-yang permeates all aspects of the theoretical system of traditional Chinese medicine. It serves to explain the organic structure, physiological functions and pathological changes of the human body, and in addition guides clinical diagnosis and treatment.

Yin-yang and the organic structure of the human body

When the theory of yin-yang is applied to explain the organic structure of the human body, the underlying premise is that the human body is an integrated whole. All its organs and tissues are organically connected and can be divided into two opposite aspects, namely yin and yang. In terms of anatomical location, the upper part of the body is yang and the lower part yin, the exterior yang and the interior yin, the lateral aspects of the four limbs yang and the medial aspects yin. According to the nature of their functional activities, the zang organs are yin and the fu organs yang. Furthermore, within each of the zang-fu organs, there are yin and yang aspects. For example, heart-yin and heart- yang, kidney-yin and kidney-yang. Within the meridian system there are two categories: yin meridians and yang meridians. Thus the opposition of yin and yang manifests within all the upper, lower, internal and external organic structures. Each contains yin and yang qualities and all of them can be classified according to yin and yang. Thus, Chapter 25 of the book Plain Questions says, "Man has a physical shape which is inseparable from yin and yang."

Yin-yang and the physiological functions of the human body

The theory of yin-yang states that the normal vital activities of the human body are based on the coordination of yin and yang in a unity of opposites. Functional activities pertain to yang and nutrient substances to yin. The various functional activities of the body depend on the support of the nutrient substances. Without nutrient substances, there would be no sustenance for functional activity. At the same time, functional activities are the motive power for the production of nutrient substances in the body. In other words, without the functional activities of the zang-fu organs, water and food cannot be transformed into nutrient substance. In this way, yin and yang within the human body are mutually supportive. They act together to protect the organism from invasion by pathogenic factors and to maintain a relative balance within the body. If yin and yang fail to support each other and become separated, the vital activities of the body will cease. The third chapter of Plain Questions says, "When yin is stablized and yang well-conserved, the spirit will be in harmony; separation of yin and yang will cause exhaustion of essential qi."

Yin-yang and pathological changes in the human body

The theory of yin-yang is also applied to explain pathological changes. Traditional Chinese medicine considers that the occurrence of disease results from the loss of relative balance between yin and yang, and hence an excess or deficiency of either. The occurrence and development of disease is related both to the antipathogenic qi and to pathogenic factors. There are two types of pathogenic factors: yin and yang. Antipathogenic qi involves yin fluid and yang qi. When yang pathogenic factors cause disease, this may lead to an excess of yang which consumes yin and gives rise to heat syndromes. When yin pathogenic factors cause disease, this may lead to a preponderance of yin which damages yang and gives rise to cold syndromes. When deficiency of yang fails to control yin, cold syndromes may appear, in which yang is deficient and yin excessive. When deficiency of yin fluid fails to restrict yang, hot syndromes may appear, in which yin is deficient and yang hyperactive.

From the above it can be seen that although the pathological changes that occur in disease are complicated and subject to change, they can be generalized and explained by "imbalance of yin and yang," "excess of yin leading to cold syndromes," "excess of yang leading to heat syndromes," "deficiency of yang leading to cold syndromes" and "deficiency of yin leading to heat syndromes."

In addition, deficiency of either yang qi or yin fluid may lead to the consumption of the other, known as "mutual consumption of yin and yang." For example, prolonged poor appetite is mainly attributed to weakness of spleen-qi (yang), leading to insufficiency of blood (yin). This is known as "deficiency of both qi and blood due to weakness of yang affecting yin." Another example is haemorrhage, where considerable loss of yin blood usually leads to the syndrome of deficiency of yang, manifesting as chilliness and cold limbs. This is known as "deficiency of both yin and yang resulting from deficiency of yin affecting yang." These pathological changes are all commonly seen in the clinic.

Yin-yang as a guide to clinical diagnosis and treatment

The root cause for the occurrence and development of disease is imbalance between yin and yang. For this reason, however complicated and changeable the clinical manifestations may be, with a good command of the principle of yin-yang, we may grasp the key linking elements and analyse them effectively. Generally speaking, the nature of any disease does not exceed the scope of analysis by yin-yang. Thus the fifth chapter of Plain Questions says, " A good doctor will observe the patient's complexion and feel the pulse, and thus take the first step in determining if it is a yin or a yang disease."

Yin-yang is the basis for the differentiation of syndromes by the eight principles, namely, yin, yang, interior, exterior, cold, heat, deficiency and excess. Exterior, heat and excess are yang, whilst interior, cold and deficiency are yin. In this way, complicated clinical situations can be simplified, and a correct diagnosis given.

Since the root cause for the occurrence and development of disease is imbalance of yin and yang, the basic principle in acupuncture treatment is to adjust yin and yang, making "yin stablized and yang well-conserved" and restoring harmony between them. The fifth chapter of Miraculous Pivot says, "The essential technique of needling consists of striking a balance between yin and yang." From this it can be seen that the basic function of needling is to adjust the qi of yin and yang.

In the clinical application of acupuncture, the theory of yin-yang is applied to determine not only the principles of treatment, but also the selection of points and the technique of needling and moxibustion to be used. For instance, combining points from externally-internally related meridians, as well as combining Yuan-Primary and Luo-Connecting points, is used extensively in clinical practice. Both are methods of selecting points from related yin and yang meridians. In addition, Back-Shu and Front-Mu points are often selected to treat diseases of the zang-fu organs. The related Back-Shu points are mostly selected for diseases of the zang, and the related Front-Mu points for diseases of the fu. Alternatively, a combination of Back-Shu and Front-Mu points is applied to "select Front-Mu points for yang diseases and Back-Shu points for yin diseases," in order to adjust yin and yang in either excess or deficiency.

Where acupuncture and moxibustion are used together, apply moxa to the upper part of the body first and the lower part second, and "insert needles deeply with retention for yin diseases, and shallowly without retention for yang diseases."
From this we can see that in acupuncture and moxibustion, the meridians, the points, and techniques for needling and moxibustion are all closely related to the theory of yin and yang, emphasizing the vital role that yin and yang play in both theory and practice.

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II

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IV

V

I. Equilibrium of Yin and Yang
II. Preponderance of Yin Consumes Yang (Shi-cold Syndrome)
III. Preonderance of Yang Consumes Yin (Shi-heat Syndrome)
IV. Weakness of Yang Leads to Preponderance of Yin (Xu-Cold Syndrome)
V. Weakness of Yin Leads to Preponderance of Yang (Xu-heat Syndrome)

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