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The *Suwen* (Basic Questions) states: When a girl reaches seven years of age, her Kidney Qi becomes abundant, her teeth are replaced, and her hair grows. At fourteen years old (two sevens), Tian Gui (the substance promoting reproductive maturation) arrives, the Ren Mai (Conception Vessel) becomes unobstructed, and the Taichong Mai (Thoroughfare Vessel) becomes prosperous. Her menstruation occurs at regular intervals, thus enabling her to conceive.

The *Lingshu* (Spiritual Pivot) states: The Chong Mai (Penetrating Vessel) originates in the Baozhong (uterus), emerges at the Qiqiao (inguinal region where the femoral artery pulses), moves forward through the chest, runs deep along the back, emerges upward at the Hangsang (nasopharynx), permeates and irrigates the Yang meridians, descends into the feet, and pours into the collateral vessels. It serves as the Sea of Blood for the Twelve Primary Meridians. Its pathways run alongside the Shaoyin Meridian, hence it is called the Sea of Blood.

The *Suwen* states: When heaven and earth are mild and warm, the menstrual water remains calm. When heaven is cold and earth frozen, the menstrual water coagulates and flows sluggishly. When heaven is hot and earth scorching, the menstrual water boils and overflows. When sudden storms arise, the menstrual water surges like waves and rises. When pathogenic factors enter the vessels, cold causes blood to coagulate and flow sluggishly; summer heat makes Qi moist and fluid. Deficient pathogens take this opportunity to invade and reside within, similar to how menstrual water reacts to wind.

Shen Zhai’s commentary: The above three classic discussions outline the origin of female menstruation, which begins with the prosperity of the Taichong Mai. Since the Chong Mai originates in the Baozhong, it constitutes the Sea of Blood, which is the source of menstrual water. However, menstrual water coagulates when exposed to cold and flows when exposed to heat, corresponding to the seasonal changes of heaven and earth. Those who regulate menstruation can thereby understand their primary objectives.

Wang Bing (Wang Taipu) states: The Chong Mai is the Sea of Blood, where all meridians converge. In men, it is transported and circulated; in women, it is retained and stopped, referred to as the Blood Chamber (Xueshi). The classics state that when the Ren Mai is unobstructed and the Chong Mai is prosperous, men circulate while women retain. Circulation implies no accumulation and a state of insufficiency, characterized by movement. Retention implies accumulation and fullness, characterized by stillness. Insufficiency corresponds to Yang and Qi; fullness corresponds to Yin and Blood. Therefore, when full, it overflows at its proper time, termed "trustworthy" (regular cycle). Men rely on Qi for transport, so Yang Qi responds to the sun's daily cycle with erection. Women rely on Blood for fullness, so Yin Blood responds to the moon's monthly cycle with descent.

Ma Shi (Ma Xuantai) states: The Ren Mai and Chong Mai are two of the Eight Extraordinary Vessels. The classics state that the Ren Mai governs conception and pregnancy, while the Chong Mai is the Sea of Blood. Now that both meridians are unobstructed, menstruation descends. The *Lingshu* states that the Chong Mai and Ren Mai both originate in the Baozhong. It further states that the Chong Mai is the Sea of Blood and that there is an excess in the Sea of Blood. Commentary: Although the Sea of Blood empties after menstruation, it gradually fills again after seven days, resembling the waxing and waning of the moon. One should understand that the "excess" in the Sea of Blood applies generally to the Twelve Meridians, not just the fullness of the Sea of Blood itself, which allows menstruation to commence.