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Cheng stated: "Jing" (Meridian) refers to a path, running within the skin and among the muscles. "Fu" (Hollow Organ) refers to a vessel, used to hold food and fluids. Various books on treating Cold Damage often confuse "Jing" with "Fu" and vice versa, establishing theories that mix them up, which greatly confuses people. I hereby specifically clarify this distinction.

When pathogens invade the Three Yang meridians, there are the Taiyang Meridian, Yangming Meridian, and Shaoyang Meridian. For pathogens in the Three Yang Jing that have not entered the Fu organs, sweating therapy can resolve them. When pathogens invade the Three Yin meridians, there are the Taiyin Meridian, Shaoyin Meridian, and Jueyin Meridian. For pathogens in the Three Yin Jing that have already entered the Fu organs, purgative therapy can resolve them. The "Fu" referred to as entering implies the Yangming Stomach Fu organ. Once pathogens from the Three Yang or Three Yin enter the Stomach Fu, transmission ceases. This is because the Stomach belongs to Earth, symbolizing where all things return. The *Shanghan Lun* states: There is Taiyang-Yangming, Zhengyang-Yangming, and Shaoyang-Yangming. Here, "Yangming" refers to the Stomach Fu, not the Yangming Meridian. If pathogens are in the Taiyang Meridian but do not transmit to the Yangming Meridian, instead directly entering the Stomach Fu, it is called Taiyang-Yangming. If pathogens are in the Yangming Meridian but do not transmit to the Shaoyang Meridian, instead entering its own Fu organ, it is called Zhengyang-Yangming. If pathogens are in the Shaoyang Meridian but do not transmit to the Three Yin, instead directly entering the Stomach Fu, it is called Shaoyang-Yangming. For any pathogens of the Three Yang that have entered the Stomach Fu, one should unhesitatingly use purgation.

Although it is common knowledge that Three Yang pathogens enter the Fu, few can recognize when Three Yin pathogens enter the Fu. The Three Yang Jing are far from the Fu organs, while the Three Yin Jing are closer to the Fu organs. However, since they are called "Jing," the pathogens remain on the pathway and have not converged into a single location. The *Shanghan Lun* states: In Taiyin disease, if the pulse is floating, sweating may be applied, preferably using Guizhi Tang (Cinnamon Twig Decoction). In Shaoyin wind-strike, if the radial pulse is slightly weak and the cunial pulse is floating, recovery is imminent. In Jueyin wind-strike, a slightly floating pulse indicates impending recovery, while a non-floating pulse indicates no recovery. These statements imply that pathogens reside in the Jing, allowing for a return to the exterior via sweating. If pathogens have entered the Fu, there is no route for external exit; only by unblocking the bowels can pathogens exit internally. This explains the establishment of Dachengqi Tang (Major Order the Qi Decoction), Xiaochengqi Tang (Minor Order the Qi Decoction), and Tiaowei Chengqi Tang (Regulate the Stomach Order the Qi Decoction).

Then, why use Baihu Tang (White Tiger Decoction) to treat Fu diseases? This is because when Three Yang pathogens first enter the Stomach Fu, both interior and exterior are hot, but the pathogens have not yet condensed, and the heat is diffuse without symptoms of gastric excess and constipation. Thus, Baihu Tang is used to clear heat internally from the Stomach Fu and vent heat externally through the skin, resolving both layers. If pathogens have condensed, presenting with severe abdominal pain in Taiyin, dry throat and mouth with water diarrhea and epigastric hardness in Shaoyin, or irritability, fullness, and penile retraction in Jueyin, Baihu Tang is contraindicated; urgent purgation is required. This is simply because distinguishing between Jing and Fu prevents therapeutic errors.

What about Taiyang pathogens entering their own Fu organ? The Fu of Taiyang is the Bladder, which stores urine. When Taiyang disease is severe, pathogens shift to the Fu, causing thirst and dark urine. Presenting with Taiyang disease symptoms alongside these signs is called "Taiyang transmitting to its own Fu." It should be treated with Wuling San (Five-Ingredient Powder with Poria), using Guizhi (Cinnamon Twig) to release exterior pathogens and Zhuling (Polyporus) and Zexie (Alisma) to promote urination for cure.

A question arises: Can yang pathogens transmitted to yin meridians still return to the exterior for sweating? Is this true? I say: Do not doubt the ancients' words. "Jing" means path, like a road. Just as there is a path for Three Yang pathogens to reach Three Yin, there is a path for Three Yin pathogens to return to Three Yang. This is a principle of cyclic transformation, unlike pathogens entering the Stomach Fu, which have no exit route. I have observed patients with strong constitutions who recovered after completing meridian transmission, all resolved through sweating. The *Shanghan Lun* states: For those whose disease does not transmit further nor acquire mixed pathogens, on the seventh day, Taiyang disease declines, headache improves slightly; on the eighth day, Yangming disease declines, body heat subsides slightly; on the ninth day, Shaoyang disease declines, deafness improves slightly; on the tenth day, Taiyin disease declines, abdomen softens, appetite returns; on the eleventh day, Shaoyin disease declines, thirst stops, tongue dryness resolves, followed by sneezing; on the twelfth day, Jueyin disease declines, scrotum relaxes, lower abdomen feels slightly heavy, vital qi has departed, and the patient's spirit is refreshed. Does this not prove that completion of meridian transmission leads to recovery via returning to the exterior and sweating?

Someone asks: Yin meridians should not sweat. Now, Taiyin presents with a floating pulse, and Guizhi Tang is used. Can Three Yin meridians also be resolved by sweating? Will Guizhi Tang become the primary medicine for Taiyin? I reply: Incorrect. One must interpret Zhang Zhongjing's works comprehensively. If pathogens have entered the interior but sweating is induced again, internal heat increases; hence, "Yin should not sweat." However, if pathogens enter the interior but then return to the exterior, this represents pathogens exiting outward; hence, "returning to Yang to sweat." Guizhi Tang is originally for Taiyang wind-strike. Its use in Taiyin occurs because Taiyang wind-strike was mistakenly treated with purgation, transmitting pathogens to Taiyin. Taiyin pulse should be deep; a floating pulse indicates the syndrome is in Taiyin but the pulse reflects Taiyang. This shows Taiyang pathogens have not fully entered Yin, and Taiyin pathogens have a strong tendency to return to Yang for sweating. Thus, Guizhi Tang is used to disperse them, allowing pathogens coming from Taiyang to exit via Taiyang.

By extension, if originating from Taiyang cold-injury with a cold-injury pulse, Guizhi can be replaced by Mahuang (Ephedra), inferring the logic of Zhang Zhongjing's Mahuang Shigao Tang (Ephedra and Gypsum Decoction). If originating from Yangming with a Yangming pulse, Guizhi can be replaced by Gegen (Kudzu Root), inferring the logic of Gegen Huanglian Huangqin Tang (Kudzu, Coptis, and Scutellaria Decoction). If originating from Shaoyang with a Shaoyang pulse, Guizhi can be replaced by Chaihu (Bupleurum), making Daxiehu Tang (Major Bupleurum Decoction) the specific formula for Shaoyang transmitting to Taiyin. However, only if there is solid abdominal pain, indicating Spleen pathogens interfering with the Stomach, should Dahuang (Rhubarb) be used for purgation; otherwise, just add Shaoyao (Peony) to the base formula to harmonize. The *Shanghan Lun* states: Original Taiyang syndrome, mistakenly purged by physicians, transmitting to Taiyin with abdominal pain, uses Guizhi Tang plus Shaoyao; for severe solid pain, Guizhi Tang plus Dahuang. This aligns with the above logic. If Taiyin follows this pattern, why not Shaoyin and Jueyin? In the Shaoyin chapter, Zhang Zhongjing uses Sixni San (Four-Reversal Powder) to treat Yang reversal, containing Chaihu, Huangqin (Scutellaria), Gancao (Licorice), and Zhishi (Immature Bitter Orange). Does Shaoyin also use Bupleurum to scatter pathogens? Indeed, because heat pathogens transmit to the interior, roaming between Shaoyin meridians without condensing into excess, and not sinking into the Stomach Fu, Huangqin and Gancao are used to clear transmitted heat, while Zhishi guides away stomach stagnation, preventing pathogens from combining internally to cause gastric excess constipation. Chaihu is added to unblock the Three Yang pathways, allowing pathogens to exit as they entered, without disturbing the Middle Jiao, resolving the disease. This demonstrates Zhang Zhongjing's subtle manipulation of herbs.

In my practice, for yang pathogens entering yin without condensation, I imitate the ancient Sanhuang Jiedu Tang (Three Yellows Detoxification Decoction) concept, adding Shigao (Gypsum) to guard the Yangming mid-pathway, and adding Chaihu to hope for a return to the original path and sweating resolution. Generally, treating Cold Damage prioritizes releasing the exterior over attacking the interior. Not only should Three Yang pathogens be dispersed from the exterior, but even Three Yin pathogens without condensation are hoped to return to Yang and the exterior. Only when pathogens condense is Chengqi Tang used for purgation. A warning is given: Before using Dachengqi Tang, try Xiaochengqi Tang first. If intestinal gas passes, administer Dachengqi Tang; if not, cautiously avoid further purgation. One must be extremely careful not to purge lightly. Zhang Zhongjing also said: Disease starting in Yang, if mistakenly purged, heat enters forming Jie Xiong (Chest Binding); disease starting in Yin, if purged, forms Pi (Glomus/Fullness). "Heat entering" means entering the Stomach. Premature purgation in Three Yin, while not forming Chest Binding, inevitably results in Pi syndrome. Alas! How can one fail to distinguish between Jing and Fu?