衄血者。邪热在表也。邪在于表。宜以汗解。不得汗因致衄。经云。伤寒脉浮紧。不发
汗因
致衄者。麻黄汤主之。此非麻黄汤之治衄也。谓宜麻黄而不根据法以麻黄汤发汗。乃作衄也。
又曰。伤寒不大便六七日。头痛有热者。与承气汤。其小便清者。知不在里仍在表也。当须
发
汗。若头痛者必衄。宜桂枝汤。此又非桂枝汤之治衄也。谓宜桂枝而不根据法以桂枝汤发汗。
乃成衄也。衄与汗同义。而衄家不可发汗。发汗则额上陷脉紧急。直视不能 不得眠。是衄
解之后。无再发汗之理。盖风寒在表。先犯太阳。从阳化热。逼血妄行。故经曰。阳盛则欲
衄。又曰。太阳病脉浮紧。发热自衄者愈。是在经之邪。随衄而解。则知衄正邪解之候也。
至阳明病口燥。但欲漱水不欲咽者。此必衄。阳明热甚则口燥。而热尚在于经。未入于里。
故但欲漱水不咽以此为欲衄之兆。亦以热在表故也。
太阳病当汗不汗。转从衄解。其不当汗而妄汗者。莫如误发少阴汗。扰动阴血。为变滋
烈。
经云。少阴病但厥无汗而强发之。必动其血。未知从何道出。或从口鼻。或从目出。是名下
厥上竭为难治。则又不得责其为表热矣。二者。一则误于当汗不汗。其变小。一则误于不
当汗而汗。其变大。用药者可不慎欤。
衄血（鼻出血），是邪热在体表的表现。邪气在体表，应当用发汗的方法解除。如果未能通过发汗解表，反而导致衄血。医经中说：“伤寒病，脉象浮紧，因没有及时发汗而导致衄血的，主治方剂为麻黄汤。”这里并不是说麻黄汤是用来治疗衄血的，而是说应当使用麻黄汤发汗，但如果未遵循正确方法使用麻黄汤发汗，就会导致衄血。
又说：“伤寒病，六七天不大便，伴有头痛和发热的，给予承气汤。如果小便清长，说明病邪不在里，仍在体表，应当发汗。如果此时仍有头痛，必然会衄血，宜用桂枝汤。”这里也不是说桂枝汤用来治疗衄血，而是说应当使用桂枝汤，但若未遵循正确方法以桂枝汤发汗，就会造成衄血。
衄血与发汗具有相同的解表意义，但是素有衄血的人不可发汗。如果强行发汗，会导致额部凹陷、脉管紧急拘挛、两眼直视不能闭目、不得安眠。这说明在衄血之后，就没有再发汗的道理了。大概风寒邪气在体表，首先侵犯太阳经，从阳化热，逼迫血液妄行。所以医经说：“阳气盛则想要衄血。”又说：“太阳病，脉象浮紧，发热，如果自然衄血，病就会痊愈。”这是指在经络中的邪气，随着衄血而解除，由此可知衄血是正气驱除邪气、病愈的征兆。
到了阳明病，出现口干舌燥，只想含水漱口而不想咽下的情况，这必定会发生衄血。阳明热盛则口干，但热邪尚在经络，未进入脏腑内部，所以只想含水漱口而不咽下，以此作为将要衄血的预兆，也是因为热邪在体表缘故。
太阳病应当发汗却没有发汗，转而通过衄血来解除病邪；那些不应当发汗却错误发汗的情况，莫过于误发少阴病的汗。这会扰动阴血，造成的变证更加剧烈。医经说：“少阴病只有四肢厥冷而没有汗，如果强行发汗，必定会动其血，不知道血液会从哪个通道流出，或者从口鼻出，或者从眼睛出，这叫做‘下厥上竭’，属于难治之症。”这就又不能责怪它是表热所致了。这两种情况，一种是错误在于应当发汗却没有发汗，其产生的变证较小；一种是错误在于不应当发汗却发了汗，其产生的变证较大。用药的人能不谨慎吗？
Nasal bleeding (epistaxis) indicates that pathogenic heat is at the exterior. When pathogens are at the exterior, they should be resolved through sweating (diaphoresis). If sweating fails to resolve the condition and instead leads to epistaxis, the Classics state: "In cold damage diseases, if the pulse is floating and tight, and epistaxis occurs due to failure to induce sweating, Ma Huang Tang (Ephedra Decoction) is the primary formula." This does not imply that Ma Huang Tang treats epistaxis; rather, it means that while Ma Huang Tang should be used to induce sweating, failure to follow the correct protocol results in epistaxis.
It is also stated: "In cold damage diseases, if there is constipation for six or seven days accompanied by headache and fever, Cheng Qi Tang (Order the Qi Decoction) may be given. However, if the urine is clear, it indicates the pathogen is not in the interior but remains at the exterior, and sweating should be induced. If headache persists, epistaxis will likely occur, and Gui Zhi Tang (Cinnamon Twig Decoction) is appropriate." This again does not imply that Gui Zhi Tang treats epistaxis; it means that while Gui Zhi Tang should be used, failure to follow the correct protocol for inducing sweating with this formula leads to epistaxis.
Epistaxis shares the same function as sweating in resolving the exterior; however, individuals prone to nasal bleeding (those with a history of frequent epistaxis) must not be subjected to diaphoresis. Inducing sweating in such cases causes depression in the forehead, tension and urgency of the vessels, fixed staring of the eyes, inability to close them, and insomnia. This demonstrates that after epistaxis has occurred, there is no rationale for further induction of sweating. Generally, wind-cold pathogens at the exterior first invade the Taiyang (Greater Yang) channel, transform into heat from Yang, and force blood to move recklessly. Thus, the Classics state: "Excess Yang leads to a desire for epistaxis," and "In Taiyang disease, with a floating-tight pulse and fever, if spontaneous epistaxis occurs, the disease resolves." This indicates that pathogens in the channels are resolved along with the epistaxis, confirming that epistaxis is a sign of righteous qi expelling pathogens and recovery.
Regarding Yangming disease, if the patient experiences dryness in the mouth, desiring only to rinse water in the mouth without swallowing, epistaxis is certain. Yangming heat excess causes dryness in the mouth, but since the heat remains in the channels and has not entered the interior, the patient desires only to rinse without swallowing. This serves as a precursor sign of impending epistaxis, again because the heat is located at the exterior.
Taiyang disease should be resolved through sweating; if not, it may resolve through epistaxis. Conversely, errors involving inappropriate sweating are most severe when sweating is incorrectly induced for Shaoyin (Lesser Yin) disease. This disturbs Yin blood, leading to violent complications. The Classics state: "In Shaoyin disease, if there is only cold extremities without sweat, forcing sweating will inevitably disturb the blood. It is unknown from which pathway it will exit—either from the mouth and nose, or from the eyes. This is termed 'Lower Cold and Upper Exhaustion,' a difficult-to-treat condition." In such cases, one cannot blame the condition on exterior heat. These two scenarios differ: one error lies in failing to sweat when one should have, resulting in minor complications; the other error lies in sweating when one should not have, resulting in major complications. Practitioners using medicinal formulas must exercise utmost caution.