The common physiological characteristics of the five yin viscera are: generating and storing vita-vapor(qi). The six yang viscera receive and transport food. The unusual organs do not conform to the six yang viscera and they do not directly process food, but have an action similar to that of the yin viscera which is to store vita-vapor(qi).

The theory is formed chiefly by: a) anatomical knowledge in ancient times, which laid a foundation for morphology; b) observation of physiology, e.g., a common cold results from the cold affection of the skin, with a stuffy or running nose and cough. Thus, the close relation between the skin, nose and lung is recognized. In another example the theory that "the eye is the window of the liver" was derived from that a number of eye troubles were cured by treating the liver for a long period of time (in fact, it is from qigong practice)

The theory is chiefly characterized by the holistic concept with the five yin viscera as its centre. This concept is mainly manifest in a) the yin viscera are yin and the yang ones are yang; b) the yin and yang viscera are closely related and are a unity. For instance, the heart and small intestine, lung and large intestine, spleen and stomach, liver and gallbladder, kidney and bladder, pericardium and triple energizer are all closely related.

The five yin viscera connect with various tissues and organs, which is a manifestation of the holistic theory that the five yin viscera are specifically connected with various tissues and organs. According to this theory, the face is the mirror of the heart, the heart is enriched by blood and opens to the tongue. The lung is reflected by the hair and the skin and opens to the nose. The spleen reflects externally on the lips and affects the muscles and opens to the mouth. The liver is reflected by the nails, affects the tendons and opens to the eye. The kidney is reflected by the hair and affects the hones and opens to the ear, external genitalia and anus.

The physiological activities of the five yin viscera are closely related to the spirit and the emotions. Mental and emotional activities are the functions of the brain. However, the theory holds that these activities are closely related to the physiological activities of the five yin viscera which can command the activities of the entire body. Whether physiological activities of the brain are normal or not depends upon the harmony of the five yin viscera. If they are abnormal, the mental and emotional activities of the brain are influenced. An abnormality of the brain will affect the five yin viscera. "Xian Ming Wu Qi Pian," a chapter in Plain Questions states, "the heart stores the mind, the lung stores the soul, the liver stores the mood, the spleen stores the emotion and the kidney stores the will." TCM does recognize the function of the brain, but further classifies emotions and explores their relation with various yin viscera.

Harmony between the functions of the five yin viscera is a key link for maintaining stability of the internal system of the body. Equilibrium between the external and internal environments is maintained through the connection between the yin viscera and various tissues and organs and the relation between the five yin viscera and emotions.

The five yin viscera

The five yin viscera is a collective term for the heart, lung, spleen, liver and kidney. Although they each have their own physiological function, the heart plays a leading role. Harmony among the five yin viscera is mainly based on the yin-yang and five-element theories.

• The heart

The heart lies in the chest cavity above the diaphragm, protected by the pericardium. It is the dwelling place of congenital mind-will and regulates the blood and vessels. It corresponds to "fire" in the five-element theory. Its physiological function is governing vessels and mind. The tongue is its window and the face, its mirror. It is closely related to joy and sweat. As the Heart Meridian of Hand-shaoyin and the Small Intestine Meridian of Hand-taiyang are interconnected, the heart and small intestine are closely related.

Main physiological functions of the heart

1. The heart governs blood and vessels. Blood circulates in the vessels and is transported throughout the body. The functions of nutritional vita-vapor(qi) and blood directly influence blood circulation.

In TCM, the normal heartbeat chiefly depends upon heart qi. Only if heart qi is enriched can a normal heart rate and rhythm be maintained and can blood normally circulate within the vessels continuously and nourish the body, which is shown by a lustrous and bright complexion and a moderate, forceful pulse,etc. Normal blood circulation also depends upon whether the blood is enriched or not. When it is insufficient, the vessels are vacant and normal heartbeat and blood circulation is also influenced. The normal circulation of blood fully depends upon the most basic conditions, such as enriched heart qi and blood and unobstructed vessels. If heart qi is deficient, blood is consumed and weak, the vessels are obstructed, and an unsmooth flow of blood results, or the vessels are vacant and weak, resulting in dull complexion, minute weak pulse, even stagnant qi and blood and blocked vessels, with dark grayish complexion, dark-bluish lips and tongue, chest distress, stabbing pain and an irregular or regular intermittent pulse appearing.

2. The heart governs the Mind: In its broadest sense, congenital mind-will refers to the activities of the body, e.g., appearance, complexion, expression in the eyes, speech, response, manner, etc. In its narrowest sense, "the heart governs the Mind" refers to mental and emotional activities. Because these activities are an important component of physiological functions, and can influence the harmony of various physiological functions, "the heart is the prime viscus and governs the Mind" ("Ling Lan Mi Dian Pian," a chapter in Plain Questions) and "the heart is the Chief of the five yin and six yang viscera and the residence of Spirit." (Miraculous Pivot)

Early in Internal Classic, it was described that thoughts and emotions are physiological functions of the brain, i.e., the reaction of the brain to the external environment. Thoughts and emotions are not only ascribed to the brain and the five yin viscera, but also to the physiological function of the heart. Hence, if the heart's regulation of the Mind is normal, one is full of vigour, clear-minded, mentally agile and quick or normal in response to the external environment. If not, symptoms such as insomnia, dreaminess, listlessness, even coma and unconsciousness may appear.

Relation of the heart to the emotions, body fluids, tissues and sense organs

1. The heart is related to joy: The physiological function of the heart is closely related to joy. The theory of visceral symptoms holds that joy, anger, anxiety, worry, and fear are the five emotions and correspond to the five yin viscera respectively, for emotional changes are produced by the physiological functions of the five yin viscera. In general, joy, a response to external information, belongs to the positive irritations. But, over-joy leads to the injury of the heart (Mind). The hyperfunctional heart would make one laugh persistently; the hypofunctional heart would make one easily sad.

2. The heart is closely related to sweat: Sweat is excreted from the pores after body fluids are activated by the spread of yang vita-vapor(qi). Excretion of sweat also depends upon the opening and closing action of defensive qi or the junction between the muscle and skin. If it opens, sweat would be excreted; if it closes, no sweat would be excreted. As sweat is derived from body fluids and blood that are from the same source, there is a saying that "Sweat and blood have the same source." And because blood is governed by the heart, there is a saying that "Sweat is fluid of the heart."

3. All the vessels belong to the heart and the heart reflects on the face: Since there are many vessels on the head and face, the face is lustrous and moist if heart qi flourishes and the vessels are full. Pallor and dull complexion may be seen if blood is insufficient. A dark-bluish complexion may appear if it is congealed.

4. The heart opens to the tongue. "The tongue is the window of the heart." The tongue has tasting and speaking functions, which depend upon the heart to govern blood, vessels and Mind. If the heart functions abnormally, pathological

symptoms, such as changes of taste sensation and stiffness of the tongue and dystalia, etc. may occur. Because the tongue is not covered by the epidermis but is extremely rich in vessels, the circulation of vita-vapor and blood can be seen and the physiological function of the heart can be judged from the colour of the tongue. "The tongue is the window of the heart" is a theory established by ancient medical experts through their observations. If the cardiac function is normal, the tongue will be red, flexible, brilliant, moist and soft and the sense of taste will be keen and speech will be coherent. If a pathological process of the heart appears, it will reflect on the tongue. For instance, insufficient heart fire, dark-red tongue or even tongue sores; congealed and blocked heart blood, a dark-purplish tongue or the tongue with petechiae, the dysfunction of the heart in governing the Mind, a rolling tongue, stiff tongue, stuttering or aphonia, etc.

In a word, the physiological function of the heart includes not only the regulation of the circulatory system, but also the control of mental and emotional activities.