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Diseases of the Three Yang meridians located in the exterior (Jing) can be treated with diaphoresis (sweating method). Whenever "Jing" (Meridian/Channel) is mentioned, it refers to pathogenic factors lodging in the skin and hair. The Jing meridians run deep to the skin and hair but superficial to the muscles. When pathogens lodge in the Jing, they are categorized into Taiyang (Greater Yang), Yangming (Yang Brightness), and Shaoyang (Lesser Yang). They can be treated with dispersing, resolving the exterior, or harmonizing methods; these all represent pathogens residing in the Jing. Therefore, Zhang Zhongjing stated: If the disease of the Three Yang meridians is in the Jing, purgation (downward draining) must not be used. Misuse of purgation will lead to Pi Qi (abdominal fullness and obstruction) or Jie Xiong (chest binding). If the disease of the Three Yang meridians has entered the Fu organs (Hollow Organs), purgation may be applied. Whenever "Fu" is mentioned, it refers to pathogens residing in the intestines and stomach. The Fu organs govern the excretion of water and grain and the transmission of waste. When pathogens enter the Fu, they may come from Taiyang entering the Fu, Yangming entering the Fu, or Shaoyang entering the Fu. They can be treated with purgation, clearing heat, or attacking stagnation; these all represent pathogens having entered the Fu. Zhang Zhongjing stated: If the disease of the Three Yang meridians is in the Fu, diaphoresis must not be used. Misuse of diaphoresis will lead to delirious speech (Chan Yu) or exhaustion of Yang (Jue Jie). Once the distinction between Jing and Fu is clear, the use of attacking and sweating methods will naturally not be confused.

When pathogens enter sequentially from the Yang meridians inward, it is called Chuan Jing (transmission through meridians), which can be treated with purgation. This is because Chuan Jing involves pathogens moving from the exterior to the interior, constituting an Excess (Shi) and Heat pattern. Directly using warming herbs for Yin patterns should be avoided. Misapplication is termed "disturbing Yin blood," which constitutes reinforcing excess. When pathogens do not enter via the Yang meridians but directly invade, it is called Zhi Zhong (direct invasion), which can be treated with warming methods. "Zhi Zhong" means pathogens do not enter from the exterior but constitute a Deficiency (Xu) and Cold pattern. One must avoid using cold herbs intended for Chuan Jing. Misapplication will cause cold to intensify, making the deficiency even more severe. Understanding the distinction between Chuan Jing and Zhi Zhong ensures that formulas like Chengqi Tang (Gut-Regulating Decoction) or Lizhong Tang (Central-Regulating Decoction) are used appropriately.

Generally, when cold pathogens invade humans, they first enter the skin and hair. What are the manifestations when pathogens are in the skin and hair? Fever feels like burning fire, aversion to cold with piloerection (standing of body hair), nasal congestion without sweat, and skin pain so severe one cannot lie on a mat. These indicate pathogens in the skin and hair. This corresponds to the Exterior Syndrome of Taiyang, hence it is called Fa Biao (releasing the exterior). The *Neijing* states: "For pathogens in the skin and hair, use diaphoresis to disperse them." This is what is meant. When transmitted from the skin and hair to the muscles, the pathogen advances further. Aversion to cold disappears, and there is inevitably cracked lips and intense heat. The lips are the root of the muscles. Seeing cracked lips indicates muscle involvement; therefore, instead of releasing the exterior, we speak of Jie Ji (resolving the muscle layer). When transmitted from the muscles to the sinews and bones, the pathogen advances further still. There will inevitably be dry teeth and spontaneous sweating. Teeth are the surplus of bones; heat entering the sinews and bones causes dryness. Muscles are the walls of the body; heat steaming the flesh causes the Cou Li (pores) to open, leading to spontaneous sweating. These indicate heat in the sinews and bones, thus Baihu Tang (White Tiger Decoction) is used (composed of Shigao [Gypsum Fibrosum], Zhimu [Anemarrhenae Rhizoma], Gancao [Glycyrrhizae Radix et Rhizoma], Jingmi [Oryzae Semen]. Sometimes Renshen [Panacis Ginseng Radix] is added, named Renshen Baihu Tang). This is used to drain heat from the bones. When transmitted from the sinews and bones to the Zang-Fu organs, the pathogen has penetrated deeply. All external symptoms are present: aversion to heat, constipation, or diarrhea with yellow-red stool, or intermittent abdominal pain, progressing to madness in severe cases. This is heat penetrating deeply into the Zang-Fu organs. It is not called Xie Li (draining the interior) but Gong Li (attacking the interior). If pathogens do not transmit from the Yang meridians but are called Zhi Zhong San Yin (Direct Invasion of the Three Yin), the interior is purely cold, and there are no exterior Yang syndromes. Symptoms must include Jue Ni (cold extremities), complete undigested food in stool, retraction of the scrotum, and cyanosis of the nails. This is cold caused by direct invasion. It is not called attacking the interior but saving the interior. (Note: Heat syndromes transmitting from the Three Yang to the Fu organs may also present with cold extremities extending beyond elbows and knees. The classics state: "The deeper the heat, the deeper the cold extremities; the milder the heat, the milder the cold extremities." Such cases must never be mistaken for the cold extremities of Direct Invasion of the Three Yin, nor should Sini Tang (Frigid Extremities Decoction) be recklessly administered.)