Classical medical texts, such as the Inner Classic of the Yellow Emperor, stress that the most significant difference between men and women is evident in the excretion of fluids linked to biological cycles. A woman's growth is marked by changes in menstruation, while the most significant physical characteristic of the male is his semen. Early Chinese medical literature discusses the male and female sexual organs with great clarity, describing the normal womb and penis, and contrasting them with injured or incomplete sexual organs, such as those of eunuchs. Throughout, classical medical texts are preoccupied with the physical differences between the sexes. In this paper, I examine the role of the idea of dumai (the "central vessel") in the formation of the conception of the male body. The dumai is located on the central axis of the body; it is, literally, the "central vessel" of the "extraordinary eight vessels" The dumai originates in the male or female sexual organ, runs along the spine and connects to the kidneys and the brain, the two most important organs of the human body. The brain is made up of marrow-like fluid. When a person eats and drinks, he or she produces refined fluids that enter the bones, lubricating the joints while nourishing the brain's marrow. These marrow-like fluids thus collect in the brain. Of particular note is that the marrow of the brain and spine interact with the essence (jing) of the kidneys. According to medical texts, excessive sexual activity not only saps the male body of semen, but also depletes the fluids of the brain. Fluids in the brain and kidneys may supplement one another, but only via the dumai that runs along the spine. The dumai is a symbol for the center of the body and was for centuries thought to be the main vessel for techniques used to nourish the body. When the male loses semen, thus depleting his brain fluids, it was thought sexual techniques could be used to reverse the process, solidify the semen, and channel renal fluids to supplement the brain. Early Chinese medical literature contains one discourse over the "essence of the kidneys." Here, the use of the term "essence" also includes semen. By the composition of the Nanjing in the middle of the Eastern Han, a close relationship had been established between the kidneys and the respiratory function. The kidneys were seen as the "gateway to breathing," a notion tied to changing conceptions of sexual techniques that were used to nourish the body. The sexual practice of "returning the semen to nourish the brain" appears in the Daoist Xiang'er Commentary to the Laozi, written during the final years of the Eastern Han. The main tenet of the practice was that men were not to release semen during sexual intercourse. In fact, the man was to absorb the "essence and breath" (jingqi) created by the woman during intercourse in order to supplement the fluids of the brain that he may have depleted. Men and women were thought to have the same courses and vessels, and their organs shared same basic structure, but among the functions of the vessels and related organs were discernable differences between the sexes. The system composed of the brain, marrow and bones, all of which are related to the dumai, was subsumed into the concept of the "abnormal fu organ" a relatively marginal concept in classical medical writings. Investigation into the system of "abnormal vessels" has revealed how little we understand the place of fluids in classical medical literature. Research into fluids, following closely on research into qi, ought to become a more central concern to historians of Chinese medicine. Furthermore, in addition to exploring the conception of the body in classical medical texts, we also ought to explore the conception of the body and sensory experience as expressed in discourse on "nourishing the body." Finally, we should pay especial attention to conceptions of health in the medical literature related to sexual yoga. This material is not only helpful in understanding the eight extraordinary vessels more fully, it also provides us with an opportunity to advance dramatically the fields of the history of gender and the body in China.
