伤寒四逆。何以明之。四逆者。四肢逆而不温者是也。积凉成寒。积温成热。非一朝
一夕之故。其所由来者渐矣。伤寒始者邪在皮肤。当太阳阳明受邪之时。则一身
手足尽热。当少阴太阴受邪之时。则手足自温。是表邪渐缓而欲传里也。经曰。伤
寒四五日。手足温而渴者。小柴胡汤主之。是太阳之邪。传之少阳也。伤寒脉浮。手
足自温者。是为系在太阴。是少阳之传邪于太阴也。是知邪气在半表半里。则手
足不热而自温也。至于邪传少阴。为里证已深。虽未至厥。而手足又加之不温。是
四逆也。若至厥阴。则手足厥冷矣。经曰。少阴病四逆。其人或咳或悸。或小便不利。
或腹中痛。或泄利下重者。四逆散主之。方用柴胡、枳实、芍药、甘草。四者皆是寒冷
之物。而专主四逆之疾。是知四逆非虚寒之证也。又有四逆诸汤。亦治四逆手足
寒。方用干姜、附子热药者。厥有旨哉。若手足自热而至温。从四逆而至厥者。传经
之邪也。四逆散主之。若始得之手足便厥而不温者。是阴经受邪。阳气不足。可用
四逆汤温之。大须识此。勿令误也。四逆与厥相近而非也。经曰。诸四逆厥者。不可
下。是四逆与厥有异也。吐利烦躁见四逆者死。是恶见其四逆也。诊视之间。熟详
究之。
伤寒病中的四逆症状，该如何阐明呢？所谓四逆，是指四肢逆冷而不温暖的症状。寒气积聚而成寒证，热气积聚而成热证，这不是一朝一夕形成的，其病因是逐渐积累而成的。
伤寒初期，邪气在皮肤表层。当太阳经和阳明经受邪时，全身及手足都会发热；当少阴经和太阴经受邪时，手足自然温和。这是表邪逐渐缓解并欲传入里部的表现。《内经》说：“伤寒四五日，手足温且口渴的，用小柴胡汤主治。”这是太阳经的邪气传变至少阳经的表现。
若伤寒脉象浮，手足自然温和，这是病系于太阴经，即少阳经的邪气传变至太阴经。由此可知，邪气在半表半里之间时，手足不热而自然温和。至于邪气传入少阴经，里证已经深重，虽然尚未达到厥冷的程度，但手足更加不温暖，这就是四逆。如果邪气传入厥阴经，则手足会完全厥冷。
《内经》说：“少阴病出现四逆，病人或有咳嗽、心悸、小便不利、腹中疼痛、泄利下重等症状的，用四逆散主治。”方剂由柴胡、枳实、芍药、甘草组成。这四味药都是性寒之物，却专门主治四逆之疾，由此可知四逆并非全是虚寒证候。
另外也有治疗四逆手足寒冷的各种汤剂，如使用干姜、附子等热性药物的方剂。这里的“厥”字含义深远啊！如果手足从发热变为温和，再从四逆变为厥冷，这是传经之邪所致，用四逆散主治。如果是刚发病时手足就厥冷不温，这是阴经受邪，阳气不足，可用四逆汤温补。必须深刻认识这一点，不要弄错。
四逆与厥相近但不同。《内经》说：“凡是出现四逆或厥冷的病症，不可使用泻下法。”这说明四逆与厥是有区别的。如果在呕吐、腹泻、烦躁的同时出现四逆，预后不良（主死），这是因为厌恶见到四逆这种危重征象。在诊视过程中，务必仔细详究。
**Si Ni (Cold Extremities) in Cold Damage**
How should Si Ni (cold extremities) in Shanghan (Cold Damage) be elucidated? Si Ni refers to the condition where the four limbs are cold and not warm. Accumulation of coolness leads to cold, and accumulation of warmth leads to heat; this is not a phenomenon of one morning or evening, but arises gradually over time.
In the initial stage of Shanghan, the pathogenic factor resides in the skin. When the Taiyang (Greater Yang) and Yangming (Yang Brightness) meridians are affected, the entire body and hands and feet become hot. When the Shaoyin (Lesser Yin) and Taiyin (Greater Yin) meridians are affected, the hands and feet remain naturally warm. This indicates that the exterior pathogen is gradually subsiding and attempting to transmit inward. The Classics state: "In Shanghan, after four or five days, if the hands and feet are warm and there is thirst, Xiao Chai Hu Tang (Minor Bupleurum Decoction) should be administered." This signifies the transmission of Taiyang pathogen to the Shaoyang (Lesser Yang) meridian.
If Shanghan presents with a floating pulse and naturally warm hands and feet, it indicates the disease is associated with the Taiyin meridian, representing the transmission of Shaoyang pathogen to the Taiyin meridian. From this, we know that when the pathogen is in the half-exterior and half-interior, the hands and feet are not hot but naturally warm. However, when the pathogen transmits to the Shaoyin meridian, the interior syndrome has become deep. Although it has not yet reached the stage of Jue (complete cold), the hands and feet become even less warm; this is Si Ni. If the pathogen transmits to the Jueyin (Terminal Yin) meridian, the hands and feet will become completely cold (Jue).
The Classics state: "In Shaoyin disease, if Si Ni occurs, and the patient experiences cough, palpitations, difficulty in urination, abdominal pain, or heavy sensation with diarrhea, Si Ni San (Four Counterflow Powder) should be administered." The formula consists of Chai Hu (Bupleuri Radix), Zhi Shi (Aurantii Fructus), Shao Yao (Paeoniae Radix Alba), and Gan Cao (Glycyrrhizae Radix et Rhizoma). These four herbs are all cooling in nature, yet they specifically treat Si Ni. Thus, we understand that Si Ni is not necessarily a deficiency-cold syndrome.
Furthermore, there are various decoctions for Si Ni involving cold hands and feet, such as those using dried ginger (Sheng Jiang) and prepared aconite (Zhi Fu Zi), which are heating herbs. The term "Jue" carries profound significance. If the hands and feet progress from hot to warm, and then from Si Ni to Jue, this represents the transmission of pathogen through the meridians, treated with Si Ni San. If, upon onset, the hands and feet are immediately cold and not warm, this indicates the Shaoyin meridians are affected by pathogen due to insufficient Yang Qi, and Si Ni Tang (Four Reverse Decoction) can be used to warm them. One must recognize this clearly to avoid errors.
Si Ni is similar to Jue but distinct. The Classics state: "For all cases of Si Ni or Jue, purgation is contraindicated." This confirms that Si Ni and Jue are different. If Si Ni appears alongside vomiting, diarrhea, and restlessness, the prognosis is fatal, indicating an aversion to seeing such severe signs of Si Ni. During diagnosis, one must examine these details carefully.