Jing (Chinese medicine)
Jīng (Chinese: 精; Wade-Giles: ching1) is the Chinese word for "essence", specifically kidney essence. Along with qì and shén, it is considered one of the Three Treasures Sanbao 三寶 of Traditional Chinese Medicine or TCM.
According to tradition, Jīng is stored in the kidneys and is the most dense physical matter within the body (as opposed to shén which is the most volatile). It is said to be the material basis for the physical body and is yīn in nature, which means it nourishes, fuels, and cools the body. As such it is an important concept in the internal martial arts. Jīng is also believed by some to be the carrier of our heritage (similar to DNA). Production of semen, in the man, and menstrual blood (or pregnancy), in the woman, are believed to place the biggest strains on jīng. Because of this, some even equate jīng with semen, but this is inaccurate; the jīng circulates through the eight extraordinary vessels and creates marrow and semen, among other functions.
Jīng (精; essence) should not be confused with the related concept of jìn (勁; power), nor with jīng (經; classic/warp), which appears in many early Chinese book titles, such as the Nèi Jīng, yì jīng and Chá Jīng, the fundamental text on all the knowledge associated with tea.
The characteristics which constitute signs of good Jing (e.g. facial structure, teeth, hair, strength of adrenals or kidneys) share the embryological origin of neural crest cells. These cells undergo immense and challenging cellular migrations requiring great organisation. As such, Jing may simply represent the strength of embryological self-organisation in the organism. This will be manifestated most strongly in those cells which require most organisation; that is, the neural crest cells. 
One is said to be born with a fixed amount of jīng (pre-natal jīng, also sometimes called yuan qi) and also can acquire jīng from food and various forms of stimulation (exercise, study, meditation.)
Theoretically, jīng is consumed continuously in life; by everyday stress, illness, substance abuse, sexual intemperance, etc.
Pre-natal jīng is very difficult to be renewed, and it is said it is completely consumed upon dying.
Jīng is therefore considered quite important for longevity in Traditional Chinese Medicine (TCM); many disciplines related to qìgōng are devoted to the replenishment of "lost" jīng by restoration of the post-natal jīng. In particular, the internal martial arts (esp. T'ai chi ch'uan) and the Circle Walking of Baguazhang may be used to preserve pre-natal jīng and build post-natal jīng, if performed correctly. Commonplace in China is the sight of Ginseng on sale in herb shops, at a wide range of prices - Kung Fu classics fans may remember it used as a plot element at the start of Drunken Master II.
Ginseng, particularly Korean and Chinese, is said to bolster the jīng.
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