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(From Cheng Fengchu's *Ciyou Fa*)

The Yellow Emperor asked: When humans are injured by cold, why does it subsequently transform into heat? Qibo replied: When cold is excessive, it generates heat. The Yellow Emperor said: Now, all those suffering from heat diseases belong to the category of Shanghan (Cold Damage). After the age of eight, children have sufficient Qi and blood, and their meridians are complete and solid; thus, Shanghan in children is treated the same as in adults. The pathogen still follows the progression from exterior to interior, first affecting the skin and hair, then muscles, followed by sinews and bones, and finally the intestines and stomach, without any error.

Initially, the pathogen enters through the Taiyang meridian (Foot-Taiyang Bladder Meridian, which governs the exterior of the body), presenting with symptoms such as aversion to wind, aversion to cold, headache, and stiffness of the neck and back. Cold stagnating in the skin and hair constitutes an exterior syndrome. A floating and tight pulse without sweating indicates Shanghan (injury by cold), treated with Mahuang Tang (Ephedra Decoction) to induce sweating for resolution; a floating and moderate pulse with sweating indicates Shangfeng (injury by wind), treated with Guizhi Tang (Cinnamon Twig Decoction) to release the muscle layer, resolving when sweating stops.

[Commentary] The Taiyang meridian is located at the outermost layer of the body; therefore, when pathogens invade the skin and hair, they first injure this meridian. The skin and hair themselves cannot transmit the pathogen; rather, it transmits via the collateral vessels of the Taiyang meridian into the main meridian, and only then enters inwardly to other meridians. When pathogens lodge in the skin and hair, the Xuanfu (pores) become closed. The Qi of the internal organs constantly communicates with external Qi; when open, harmony prevails. When the Xuanfu is closed, internal Qi cannot vent, leading to the generation of heat; this is not because wind-cold transforms into heat. At this stage, one should only disperse the pathogen from the skin and hair; once the Xuanfu opens, the pathogen dissipates with sweat. Mahuang Tang and Guizhi Tang are formulas for dispersing pathogens from the skin and hair, not for clearing internal heat. If the exterior condition remains unresolved, heat accumulates and intensifies daily, reversing from the primary meridian inwardly or transmitting to the Jing-Ru-Shu-He (Well, Spring, Stream, River, Sea) acupoints where meridians intersect. Consequently, heat transmits to other meridians, and symptoms of various meridians appear simultaneously. (Points where meridians emerge are Jing points, where Qi flows like springs are Ru points, where Qi gathers are Shu points, and where Qi enters are He points; this applies to all twelve meridians.)

Once the Taiyang syndrome resolves, if there is no headache or aversion to cold, and the pulse is no longer floating, it indicates that the exterior syndrome has ceased and the pathogen is now intermediate or interior, situated between the Yangming and Shaoyang meridians. The pulse is neither floating nor deep but lies between the muscles, beneath the skin. There are two possibilities here: If the pulse is slightly洪 (surging) and long, it is a Yangming pulse; external symptoms include dry nose and insomnia, treated with Gegen Jieji (Pueraria Muscle-Releasing) method. If the pulse is wiry and rapid, it is a Shaoyang pulse; external symptoms include hypochondriac pain, deafness, bitter taste in the mouth, and alternating chills and fever, treated with Xiaochaihu Tang (Minor Bupleurum Decoction) to harmonize. Generally, Yangming and Shaoyang do not follow the standard root/branch treatment but are treated according to their "middle Qi." If there is even a slight aversion to cold, the pathogen remains in the exterior; even if it has entered the interior, dispersing the pathogen should still be combined.

[Commentary] Muscles themselves cannot transmit pathogens; muscles contain meridians. Meridians are considered "intermediate," the interior refers to organs, and the exterior refers to skin and hair. The Qi and blood of organs rely on meridians for transmission; similarly, heat caused by external pathogens also transmits via meridians. Therefore, Yangming and Shaoyang diseases are both treated via the intermediate. "Intermediate" refers to meridian disease, not disease of the Stomach or Gallbladder (Yangming belongs to the Stomach, Shaoyang to the Gallbladder; these are diseases of the external meridians, not the internal organs). Meridian diseases are treated with harmonizing methods, which must involve sweating to disperse, but this is no longer a method of opening the skin pores. Initially, when pathogens lodge in the exterior, Yin fluids in the meridians are damaged; simply opening the pores allows natural sweating. However, when heat transmits to the intermediate meridians, blood is scorched; although pores may be opened, sweat is blocked by heat and cannot reach the surface. Unskilled physicians, unaware of this, still use pungent-warm herbs to induce sweating, further aiding heat and consuming Yin, leading to dried-up sweat sources and often death due to inability to sweat. Zhang Zhongjing’s harmonizing method merely clears heat pathogens, preserving fluids; once Yin fluids are replenished, sweat naturally surges to the surface, and external pathogens dissipate. This method of nourishing sweat to open the Xuanfu is fundamentally different from forcibly opening the Xuanfu to induce sweat.

If the pathogen is in the Yangming meridian, use muscle-releasing methods; if in the Shaoyang meridian, use harmonizing methods. However, if the disease persists and has transmitted to interior excess-heat, with a pulse that is deep rather than floating, palpable at the level of sinews and bones, this is referred to as Yangming Stomach-Fu disease, distinct from meridian disease (Yangming meridian disease has resolved, and here it has transmitted to the Stomach-Fu in the chest). If the pulse is deep, full, and forceful, and external symptoms include aversion to heat rather than cold, delirium, extreme thirst, and constipation for six to seven days, these are caused by dryness and excess in the intestines and stomach. Mild cases are treated with Daxiehu Tang (Major Bupleurum Decoction), severe cases with the Three Chengqi Decoctions (San Chengqi); heat subsides once stool passes.

[Commentary] When heat pathogens enter the interior, expulsion is difficult; thus, we facilitate bowel movement to drain them, taking the nearest route. Sweating resolves meridian pathogens; it is not that sweat is harmful and needs driving out. Bowel movements expel Fu-pathogens; it is not that feces are difficult and need attacking. Physicians who fail to observe this merely know purgative methods, strictly forbid food and drink, and consider achieving a single bowel movement as their goal, which is laughable!

When cold pathogens directly invade the Three Yin meridians, it is called "Direct Invasion" (Zhi Zhong). This occurs because the governing Qi of the Three Yang meridians is weak, lacking heat to resist the cold. Thus, from the onset, there are cold limbs, tremors, curling up, no thirst, abdominal pain, vomiting, diarrhea, spitting saliva, a face pale as if scraped by a knife, no fever, and a deep, slow, and deficient pulse. This deviates from the usual pattern of transmission from Yang meridians; treatment depends on external symptoms. Mild cases are treated with Lizhong Tang (Regulate the Middle Decoction); severe cases with Jiangfu Tang (Ginger and Aconite Decoction) or Sini Tang (Frigid Extremities Decoction).

[Commentary] Shanghan involves pathogens entering organs via skin, hair, and meridians. Initially虽有 aversion to cold and fever, it ultimately becomes a heat syndrome; such individuals typically have inherent fire. Direct invasion of organs involves persistent aversion to cold without fever; such individuals typically lack inherent fire. One approach releases the exterior and attacks the interior; the other warms the middle and dispels cold; the two paths are clearly distinct. Abnormal presentations exist, such as fever, red face, irritability, removing clothes/blankets, desire for cold drinks, and large pulse—is this not a Yang syndrome? Yet patients die after receiving cooling medicines. Why? Because there is latent Yin within. Palpation reveals a pulse that is weak under the fingers regardless of floating/deep or large/small, especially when pressing to the bone level. Severe cases vomit all teas and medicines. This is Yin excess repelling Yang, corresponding to the syndrome for Baitong Tang plus human urine and pig bile. Zhang Zhongjing discusses both transmitted and direct invasion together precisely to highlight that Yin syndromes can resemble Yang, and Yang syndromes can resemble Yin. For example, Taiyang syndrome presents with headache and fever; if the pulse is floating but unexpectedly deep, it resembles Shaoyin, so Mahuang Fuzi Xixin Tang (Ephedra, Aconite, and Asarum Decoction) is used. Shaoyin syndrome presents with a deep pulse and should have no fever; if fever appears, it resembles Taiyang, requiring Gancao Ganjiang Fuzi Tang (Licorice, Ginger, and Aconite Decoction). Yin syndrome presents with cold limbs, but Yang syndrome can also present with cold limbs; hence, Sini Tang differs from Sini San (Frigid Extremities Powder). Yin syndrome presents with diarrhea, but Yang syndrome can also present with incomplete evacuation (leakage); hence, Lizhong Tang differs from Huanglong Tang (Yellow Dragon Decoction).