Our ancestors often compared the body to a small universe, or to a firmly rooted tree. When young and healthy, its roots run deep and its trunk is sturdy, keeping it verdant without swaying in any wind; yet as time passes, branches naturally dry and leaves wither, and they observed that our bodies gradually weaken in the same way. In particular, Donguibogam emphasizes the harmony and abundance of essence, qi, and spirit, the fundamental vital forces of the body. Here, essence is the seed and vital core of our body, an energy storehouse essential for sustaining life and the foundational substance that creates and regenerates cells; qi is the vitality circulating through the body that drives all physiological activities; and spirit refers to the power that governs mental activity and vitality. When this essence, qi, and spirit are depleted or fall out of balance, they regarded it as a natural progression that the body's sinews and bones would weaken.
In particular, the Huangdi Neijing viewed a man's life in eight-year cycles, noting that at age 56, liver qi weakens so that muscles and tendons cannot move properly, and at age 64, the tiangui of the kidney, the root of life, becomes depleted, causing both sinews and bones to deteriorate. Because the liver was understood to govern muscles and tendons and the kidney to govern bones, as one ages and the qi of these two zang-fu organs declines, it was considered inevitable that muscles and bones would weaken. Beyond merely losing physical power, this holds the profound meaning that the vital energy of the entire body is being drained. It is just as when an old tree's roots weaken, its trunk and branches lose their vigor. The muscles of our body are not merely tools for movement, but life-supporting pillars standing firm on the precious nourishment supplied by essence, qi, and spirit.
