程扶生曰∶任脉者，起于中极之下，以上毛际，循腹里，上关元，至咽喉，属阴脉之海。任者，妊也。此
人生养之始，故曰任脉。中极之下，长强之上，此奇经之一脉也。督脉者，起于下极之 ，并于脊里，上至风
府，入脑上巅，循额，至鼻柱，属阳脉之海。督之言都也，是入阳脉之都纲也。人脉比于水，故云阳之海，此
奇经之一脉也。任脉主任一身之阴血，太冲属阳明，为血之海。故谷气盛则血海满，而月事以时下也。
齐仲甫曰∶妇人月水，本于四经。二者冲任，二者手太阳小肠，手少阴心。然冲为
血海，任主胞胎，二者相资，故令有子。小肠经属腑，主表为阳；少阴经为脏，主里属阴。此二经，在上为
乳汁，在下为月水。
慎斋按∶以上四条，序女子月经本于血室，血室即血海，而其脉则属冲任督三脉。心与小肠二经，为月水
之源也。
陈良甫曰∶经云女子二七而天癸至，天谓天真之气，癸谓壬癸之水，壬为阳水，癸为阴水，女子阴类，冲
为血海，任主胞胎，二脉流通，经血渐盈，应时而下，天真气降，故曰天癸，常以三旬一见，以象月盈则亏，
不失其期，故名曰月信。
马玄台曰∶经云，女子二七天癸至，天癸者，阴精也。肾属水，癸亦属水。由先天之气蓄极而生，故谓阴精
为天癸。王冰以月事为天癸者，非也。男女之精，皆可以天癸称。今王注以女子之天癸为血，则男子之天癸亦为
血耶？男女当交媾之时，各有精，而行经之际方有其血，未闻交媾时可以血言也。至云精开裹血，血开裹精
者亦非。《灵枢》云∶两神相抟，合而成形，常先身生，是谓精者是也。但女子之精，以二七而至，而其月事，
亦与此时同其候也。
慎斋按∶以上二条，序女子之月水即为天癸，又辨天癸不可以月信名也。
Cheng Fusheng stated: The Ren Mai (Conception Vessel) originates below the Zhongji point (CV3), ascends to the pubic hair margin, follows the interior of the abdomen, reaches the Guanyuan point (CV4), and arrives at the throat; it belongs to the Sea of Yin Meridians. "Ren" means "ren" (to bear or conceive). This is the beginning of human reproduction, hence it is called the Ren Mai. Below the Zhongji point and above the Changqiang point (GV1), lies one of the extraordinary meridians. The Du Mai (Governing Vessel) originates from the lower extreme (perineum), runs parallel within the spinal column, ascends to the Fengfu point (GV16), enters the brain, reaches the vertex, follows the forehead, and descends to the nasal bridge; it belongs to the Sea of Yang Meridians. "Du" means "du" (capital or gathering); it is the chief guideline that gathers the Yang meridians. Human meridians are compared to water, thus it is called the Sea of Yang; this is also one of the extraordinary meridians. The Ren Mai primarily governs the Yin blood of the whole body. The Chong Mai (Penetrating Vessel) is affiliated with the Stomach Meridian of Foot-Yangming and is known as the Sea of Blood. Therefore, when the Qi derived from food and drink is abundant, the Sea of Blood becomes full, and menstruation occurs on time.
Qi Zhongfu stated: Women's menstrual flow originates from four meridians. Two of them are the Chong Mai (Penetrating Vessel) and Ren Mai (Conception Vessel); the other two are the Small Intestine Meridian of Hand-Taiyang and the Heart Meridian of Hand-Shaoyin. However, the Chong Mai is the Sea of Blood, and the Ren Mai governs the uterus; these two vessels mutually support each other, thus enabling conception. The Small Intestine Meridian belongs to the Fu organs, governs the exterior, and is Yang; the Shaoyin Meridian belongs to the Zang organs, governs the interior, and is Yin. These two meridians transform into breast milk in the upper part and into menstrual blood in the lower part.
Shen Zhai commented: The above four passages explain that women's menstruation originates from the Blood Chamber (Xueshi), which is the Sea of Blood, and its meridians belong to the Chong, Ren, and Du vessels. The Heart and Small Intestine meridians are the source of menstruation.
Chen Liangfu stated: The classics state that at age fourteen (two sevens), Tian Gui arrives. "Tian" refers to Tianzhen Qi (Innate True Qi), and "Gui" refers to the Waters of Ren and Gui. Ren is Yang Water, and Gui is Yin Water. Women belong to the Yin category. The Chong Mai is the Sea of Blood, and the Ren Mai governs the uterus. When these two vessels circulate freely, menstrual blood gradually fills and flows down on schedule as the Tianzhen Qi descends; hence it is called Tian Gui. It typically occurs once every thirty days, mirroring the waxing and waning of the moon, never missing its period, thus named Yue Xin (Menstrual Faith/Trust).
Ma Xuantai stated: The classics say that at age fourteen, Tian Gui arrives. Tian Gui is Yin Essence. The Kidney belongs to Water, and Gui also belongs to Water. It is generated when the congenital Qi accumulates to an extreme, so Yin Essence is called Tian Gui. Wang Bing's assertion that menstruation is Tian Gui is incorrect. The essence of both men and women can be called Tian Gui. If Wang Bing's commentary considers women's Tian Gui to be blood, does that mean men's Tian Gui is also blood? During intercourse, both men and women have essence, but only during menstruation is there blood; it has not been heard that blood is mentioned during intercourse. Furthermore, the statement that "essence opens to wrap blood, and blood opens to wrap essence" is also incorrect. The *Ling Shu* states: "When two spirits interact, they combine to form a shape; what is born before the body is called Essence." This is correct. However, women's essence arrives at age fourteen, and their menstruation appears at the same time.
Shen Zhai commented: The above two passages explain that women's menstruation is indeed Tian Gui, and further distinguish that Tian Gui cannot be named Yue Xin (menstrual faith).