Analysis of the concept and related theories of the skin in Huangdi Neijing

In traditional Chinese medicine theory, the skin is one of the five body constituents, and its connotations are described through multiple interpretations and expressions in Huangdi Neijing. The concept of the skin encompasses fine hair, striae and interstices (couli), and superficial collaterals, reflecting diverse medical perspectives and understandings of the skin. The functions of the skin are also reflected through its associated structures. With the development of the theories of meridians and visceral systems, the skin, as a bodily tissue, became integrated with these systems, forming various correlation patterns. The qi and blood of the entire body nourish the skin and fine hair, and their condition serves as an external indicator of the abundance or deficiency of qi and blood. As the lung governs qi and regulates respiration, providing the dynamic force for the circulation of qi and blood, the concept of "lung governs the skin and hair" emerged. The notion that "taiyang governs the exterior" originated from the integration of meridian theory with the skin, muscles, and bones. It represents an early diagnostic approach that combined bodily tissue differentiation with meridian differentiation for febrile diseases caused by external pathogens. "Heart governs the exterior" is closely associated with early bloodletting therapies. The superficial collaterals on the body surface are considered extensions of the function of the heart governing blood circulation. Bloodletting method was believed to promote blood circulation, purge heart fire, and regulate mental activity. This paper investigates the concept and connotation of skin and the construction of its relevant theoretical framework, aiming to address the current gaps in the understanding of skin theory and contribute to the enrichment and refinement of the theory of bodily constituents in Huangdi Neijing.