三阳在经者可汗。凡言经者，皆邪舍于皮毛，经行皮毛之里，肉之外，邪客于
经，有太阳、阳明、少阳之分，可发、可解、可和，皆邪之在经者。故仲景云∶三
阳主病在经，则不可下，下之为痞气，为结胸也。三阳已入腑者，可下而已。凡言
腑者，皆邪居肠胃。腑主泄水谷，传从糟粕。邪之入腑，有自太阳入腑，有自阳明
入腑，有自少阳入腑，可下、可清、可攻，皆邪之入腑者。仲景云∶三阳之病在腑，则
不可汗，汗之则谵语，为厥竭。经腑既明，攻发自然不混乱矣。
邪自阳经次第而入，谓之传经可攻。盖传经乃邪自外而入内，为实为热，不可用
直中温药，若误投名曰动阴血，是实其实也。邪不由阳经而入，谓之直中可温。言
直中者，乃邪不由外入，为虚为寒，忌用传经寒药，若误投，则使寒愈盛，令重虚
其虚矣。能明传中，则承气理中，无有不当耳。
凡寒之侵人也，必先入皮毛，而邪在皮毛，作何景象？发热如火燎，恶寒毛竖，
鼻塞不汗，皮痛不可近席，此邪在皮毛也。正属太阳表证，故曰发表。《内经》云∶邪
在皮毛者，汗而发之，此之谓也。由皮毛传于肌肉，则邪稍进一步，恶寒已去，必
唇焦热盛，口唇者肌肉之本也。今见唇焦，则知肌肉受邪，故不言发表，而曰解肌。由
肌肉传于筋骨，则邪又进一步，必齿燥自汗。盖齿乃骨之余，热入筋骨，故齿干燥；
肌肉者身之墙垣，热熏于肉，致腠理开则自汗。皆热在筋骨也，故用白虎汤。（石膏、知
母、甘草、粳米。亦有加人参者，名人参白虎汤。）以泻骨中之热耳。由筋骨传于脏腑，邪已深入，
外证悉具，必恶热便闭，下利黄赤，或腹痛时作，极则发狂，此乃邪热深入脏腑，不
曰泄里，而曰攻里。有不从阳经传来者，谓之直中三阴。内纯寒而外无阳经之形证，必
是厥逆清谷、囊缩甲青等证，此直中之寒者也。此不曰攻里，而曰救里。（由三阳传腑之
热证，亦复有厥逆过肘膝者。经云∶热深厥亦深，热微厥亦微，此种万不可误认为直中三阴之厥逆，妄投四逆。）
三阳经的病证在经表的，可以用发汗法治疗。凡是说“经”的，都是邪气停留在皮毛，经脉运行于皮毛之内、肌肉之外，邪气客居在经脉中，分为太阳、阳明、少阳三种情况，可以采用发散、解表、和解的方法，这些都是邪气在经脉的表现。所以张仲景说：三阳经的主病如果在经，就不能用泻下法，误用泻下法会导致痞气或结胸。如果三阳经的病邪已经传入腑，就可以用泻下法了。凡是说“腑”的，都是邪气停留在肠胃。腑的功能是排泄水谷，传导糟粕。邪气传入腑，有从太阳传入腑的，有从阳明传入腑的，有从少阳传入腑的，可以采用泻下、清热、攻逐的方法，这些都是邪气进入腑的表现。张仲景说：三阳经的病如果在腑，就不能用发汗法，误用发汗法会导致谵语（胡言乱语）或厥竭（阳气衰竭）。既然明确了经和腑的区别，攻邪和发汗的方法自然就不会混乱了。
邪气从阳经依次传入体内，称为传经，这种情况可以用攻下法。因为传经是邪气从外侵入内，属于实证、热证，不能使用直接针对阴寒证的温补药物，如果误用，叫做“动阴血”，这是使实证更实。邪气不经过阳经而直接侵入，称为直中，这种情况可以用温法。所谓直中，是邪气不从外而入，属于虚证、寒证，忌讳使用治疗传经的寒凉药物，如果误用，会使寒气更盛，导致虚者更虚。能够明白传经和直中的区别，那么使用承气汤类或理中汤类方剂，就没有不恰当的。
大凡寒邪侵犯人体，必定先侵入皮毛，那么邪气在皮毛时，会有什么表现呢？发热像火烧一样，怕冷且毫毛竖起，鼻塞无汗，皮肤疼痛不能贴近席子，这就是邪气在皮毛。这正属于太阳经的表证，所以叫作发表（解表）。《黄帝内经》说：邪气在皮毛的，用发汗法发散它，说的就是这个道理。由皮毛传到肌肉，邪气就进一步深入，怕冷的症状已经消失，必然出现嘴唇焦裂、热势亢盛，口唇是肌肉的根本所在。现在看到嘴唇焦裂，就知道肌肉受了邪气，所以不说发表，而说解肌。由肌肉传到筋骨，邪气又进一步深入，必然出现牙齿干燥、自然出汗。因为牙齿是骨头的余气所生，热邪进入筋骨，所以牙齿干燥；肌肉是身体的屏障，热气熏蒸肌肉，导致腠理（毛孔）张开从而自汗。这都是热邪在筋骨的表现，所以使用白虎汤。（石膏、知母、甘草、粳米。也有加入人参的，名叫人参白虎汤。）用来泻除骨头中的热邪。由筋骨传到脏腑，邪气已经深入内部，外部症状全部具备，必然怕热、大便闭塞，或者腹泻排出黄赤色的粪便，或者腹痛阵发性发作，严重时发狂，这是邪热深入脏腑，不叫泄里，而叫攻里。有不从阳经传来，而称为直中三阴经的。内部纯寒而外部没有阳经的症状，必然是四肢厥冷、完谷不化、阴囊收缩、指甲青紫等症状，这是直中引起的寒证。这不叫攻里，而叫救里。（由三阳经传入腑的热证，也会有四肢厥冷超过肘膝的情况。经文说：热邪越深，厥冷也越深；热邪轻微，厥冷也轻微，这种情形万万不可误认为是直中三阴经的厥冷，妄投四逆汤。）
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.)