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Yu Jiayan stated: The *Classic of Internal Medicine* (*Huangdi Neijing*) records: "The yang qi of the human body is like the sky and the sun; if it loses its proper position, it will shorten life and fail to manifest." It also states: "When yang qi is blocked and obstructed, earth qi (yin turbid qi) will冒明 (encroach upon brightness); 'encroaching upon brightness' refers to yin turbid qi encroaching upon and obscuring the Yangming channel." Since Zhang Zhongjing, heroes and sages have emerged in succession, yet no one has ever vigorously promoted this theory. Only Han Zhihe made slight discoveries regarding cold damage to the middle jiao, instructing people to prioritize Fuzi (Aconite Lateralis Radix Praeparata) and Ganjiang (Zingiberis Rhizoma) for emergency use. Later, masters such as Zhu Danxi and Jiezhai mostly established theories valuing yin and despising yang, formulating Buyin Wan (Nourishing Yin Pill), placing excessive weight on nourishing yin. Is this not the ultimate truth? However, in this specific domain, they lacked unique insight.

Yin diseases are difficult to categorize or treat because they lack yang qi. One often sees patients where yin pathogenic factors erupt horizontally, invading the clear pathways upwards, inevitably manifesting as aversion to cold, abdominal pain, dysentery, vomiting, spontaneous sweating profusely, muscle twitching, and tendon tremors. If this is not treated promptly, when turbid yin ascends from the chest into the throat, it causes swelling and obstruction of the throat, swollen tongue, and protruding eyes; if it ascends from the back, it causes thickened neck tendons, head and neck feeling like ice, and death with a bluish-purple complexion. Therefore, for conditions of yin excess leading to yang collapse, Zhang Zhongjing invariably uses Zhenwu Tang (True Warrior Decoction) to rescue the counterflow, thereby guarding the gate, anchoring in the North (Kidney position), preventing the Dragon-Lei fire (deficient yang) from soaring into the heavens. Regrettably, medical literature lacks this discussion, which is indeed an omission of important principles.

Generally, sudden contraction of cold pathogens indicates weak yang and excess yin, representing the most critical and urgent condition. The *Classic* says: "Excess yin generates internal cold." When cold qi accumulates in the chest without being discharged, the warming qi departs, leaving only cold; when cold remains, blood coagulates, and when blood coagulates, the vessels become obstructed. The pulse becomes large and choppy, hence "cold damage" (Zhonghan). Since the text says "excess yin generates internal cold" and then "therefore cold damage," does this not imply that internal cold arises first, followed by external cold invasion? Since it mentions blood coagulation and vessel obstruction, and also describes the pulse as large and choppy, does this not mean that due to external cold invasion, the pulse is large, and due to vessel closure, the pulse is choppy? The yang qi that guards the exterior of the human body is most solid; the Taiyang channel guards the back, the Yangming channel guards the front, and the Shaoyang channel guards the sides. Now, if the pathogen does not enter through the three yang channels but directly strikes the Shaoyin channel, could it be falling from the sky? No. It is because jue qi (rebellious qi) moves upward, accumulating in the chest causing stomach cold; stomach cold leads to consuming cold food and inhaling cold air via the nose, all of which can enter the stomach. The Kidney is the gate of the Stomach; external cold breaks through the gate and enters the Shaoyin Kidney directly, thus called "cold damage." However, although the pulse is large and choppy, indicating cold damage, it is not yet a sudden acute disease. Sudden cold damage must present with a faint pulse.

When Zhang Zhongjing discusses cold damage transmitting to Shaoyin, he states the pulse is faint and fine. If cold strikes Shaoyang, he would only mention a faint pulse, not a fine one. Why? "Faint" refers to the weakness of yang, while "fine" refers to the thinness of yin. In cold damage, when the pathogen transmits to the Kidney, whether yang or yin collapses is undetermined. But in cold damage, there is only yang collapse without yin collapse; therefore, we know the pulse will not be fine. If a fine pulse is indeed seen, it means yin was already deficient internally, so how could it appear as excess? Moreover, in cold damage, only the Shaoyin channel presents a faint pulse; other channels do not. The Taiyang Bladder, as the fu-organ of the Kidney, generally presents with a faint pulse and aversion to cold. Zhang Zhongjing treated from the Shaoyin perspective early on, using Fuzi and Ganjiang. Because a faint pulse with aversion to cold is caused precisely by weak yang; since the Kidney's yang is weak and cold generates internally, compounded by external cold breaking through the gate, either burying yang internally or forcing it outward, the person will collapse of yang within moments. Thus, Zhang Zhongjing considers it a sudden disease.

The flesh and blood body is all yin. That single point of primordial yang is born before the body, hidden within the two Kidneys, and the yuan qi of the whole body arises from it; thus it is called the "source of vital qi." External pathogens of the six climatic factors dare not touch it, so it is also called the "god who guards against pathogens." If this were not so, weak yang would lead to excess yin, and excess yin would further weaken yang. Therefore, the true yang in the Kidney is sustained by water, lingering and not departing; it is embanked by earth, hibernating and not revealing itself. Except for excretion, it stands firm. The yang of the hands and feet serves it, flowing through the body, securing the cou-li (pores/skin texture), and defending externally; the yang of the Spleen mimics the vigor of heaven, digesting food and distributing fluids, operating internally; the yang of the chest mimics the driving force of the sun, shining brightly like the sun in the sky, eliminating yin shadows, and announcing upwards. With these three abundant and visible, the true yang in the Kidney enjoys peace.

If the yang in the exterior, interior, or upper burner becomes weak and unvigorous, yin qi gains power. Skin may become cold and unwarmed, gradually becoming stiff and inflexible muscles, indicating the defensive yang is ineffective; food may fail to digest, gradually leading to vomiting, diarrhea, fullness, and distension, indicating spleen yang is ineffective; chest oppression may occur, gradually leading to blockage and inability to open, indicating chest yang is ineffective. At this point, if one extracts the yang sealed by earth and water to take charge, the head and face turn red due to yang, the skin becomes dry and hot due to yang, and the stomach and intestines develop Chu Zhong syndrome (false heat due to stomach qi exhaustion). Even without cold damage, how long can this last?

Therefore, the method of treating yin diseases must be cautious. There are eight difficulties in treatment. When cold strikes Shaoyin, enforcing severe cold commands, burying weak yang, cracking frozen skin, with no sweat and loss of spirit, urgently use Fuzi and Ganjiang, add Congbai (Allii Fistulosum Bulbus) to disperse cold, and add Zhuyejia (Porci Bilis) to guide into the yin aspect. However, fearing the medicinal strength may be insufficient, apply moxibustion and ironing therapy, attacking both internally and externally, to break the hard congealment. Any delay will be too late. This is the first difficulty.

If the person's true yang is inherently restless and cou-li inherently loose, with excess yin internally forcing yang to collapse externally, causing profuse sweating, stiffness of the spine and neck, use Fuzi, Ganjiang, and Zhuyejia, but do NOT add Congbai or moxibustion, lest it aid dispersion, causing qi to escape with sweat and yang unable to return internally. One should stop the sweating, abruptly administer the aforementioned medicine, and immediately reinforce the protection of cou-li. Otherwise, fear yang will float away again. This is the second difficulty.

Using Fuzi and Ganjiang to overcome yin and restore yang is likened to cavalry charging into heavy encirclement, making scattered yang rush toward the banner. Those who do not understand this principle add more herbs, mixing them into decoctions, which instead restricts their vigorous advancing momentum, inevitably leading to sluggishness and ineffectiveness. This is the third difficulty.

Secondly, in the previous medicine, one must initially add Danggui (Angelicae Sinensis Radix) and Rougui (Cinnamomi Cortex) to regulate Ying (nutritive) blood, because cold pathogens striking inward first injure Ying blood. Otherwise, the medicine biases toward Wei (defensive) aspect, mismatching the disease; pathogens will not be fully subdued, ensuring defeat. This is the fourth difficulty.

Secondly, in the previous medicine, one must add Renshen (Ginseng Radix) and Gancao (Glycyrrhizae Radix et Rhizoma) to regulate yuan qi and transport supplies, achieving victory in the command tent. Otherwise, the fierce nature of Jiang (Ginger) and Fu (Aconite) will offend the upper parts without restraint. This is the fifth difficulty.

After two or three doses of the previous medicine, if movement feels lighter and spirits improve, add Huangqi (Astragali Radix), Baizhu (Atractylodis Macrocephalae Rhizoma), Wuweizi (Schisandrae Fructus), and Baishao (Paeoniae Radix Alba), a large group of yin-yang balanced tonics. Do not stop. If negligent and failing to provide follow-up care, previous efforts will be lost. This is the sixth difficulty.

If the person has chronic phlegm-heat, and yang departure has long transformed into yin-cold, at this stage, although formless yin-cold has dispersed,有形 (formidable) cold-phlegm may still remain blocking orifices and tunnels. Although Jiang and Fu might be omitted, Niuhuang (Bovis Calculus) and Zhuli (Bambusae Succus) and other cooling medicines must absolutely not be used; if used, yin-cold will resume dominance, and previous efforts will be lost. This is the seventh difficulty.

After using balanced tonification, even if cold-phlegm remains, one should only use sweet-cold herbs to assist qi and open passages, avoiding pungent-hot herbs that aid heat. Once yang is settled, one should nurture yin. Otherwise, other ailments will arise. This is the eighth difficulty.