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The symptoms of Yin Poison Cold Damage (Yin Du Shang Han) include a heavy body. Stiffness in the back and pain in the eyes are unbearable. There is acute pain in the lower abdomen, and the lips turn blue-black. The toxic qi rushes to the heart, causing the condition to deteriorate rapidly and become uncontrollable.

The four limbs are cold, and there is an urge to vomit. The throat is uncomfortable, and the pulse is fine and deep. If moxibustion can be performed quickly below the navel, and improvement is seen after six days, one will feel deeply relieved.

What is called Yin Poison Cold Damage is not the transmission of pathogenic factors from the Yang meridians to the Yin meridians, but rather the simultaneous injury to both the exterior and interior by Yin toxic qi at once. Symptoms such as a heavy body, stiff back, eye pain, blue-black lips, and cold four limbs indicate that Yin poison is attacking the exterior. Lower abdominal pain, Qi rushing to the heart, the urge to vomit, and discomfort in the throat indicate that Yin poison is attacking the interior. Both the interior and exterior are injured by Yin toxic qi. However, Yin syndromes are difficult to cure; one should perform moxibustion on the Dantian acupoint to restore Yang and suppress Yin. Moreover, by the sixth day, Yin has reached its extreme. If death does not occur after this point, and it extends to the seventh day, when Yang Qi begins to recover (One Yang Returns), there may be hope for survival.

According to the *Huo Ren Shu* (Book of Saving Lives): In Yin Poison, the pulse is rapid, exceeding seven or eight beats per respiration, so fast that it cannot be counted; this precisely indicates that Yin Poison is already deep. All six pulses are deep, fine, and rapid; the Chi pulse is short and small, while the Cun Kou pulse may sometimes be large. If cooling medicines are mistakenly administered, thirst will intensify. For those presenting these signs, pungent-hot medicines should be taken immediately, and recovery can be expected within one or two days. If Yin Poison gradually deepens, the manifestations become severe: cold limbs extending upwards, worsening abdominal pain, throat discomfort, fullness and hardness in the epigastrium, irritability with thirst, and incessant deficiency sweating. The six pulses are all deep, fine, and rapid, exceeding seven beats per respiration. For those with these symptoms, perform 300-200 moxa cones on the Qihai and Guanyuan acupoints until the hands and feet become warm. Concurrently administer Zheng Yang San.

Furthermore, according to Liu Shouzhen: Since the pulse is rapid, exceeding seven or eight beats per respiration to the point of being uncountable, this is actually a pulse of extreme Yang heat. The secular world erroneously transmits various syndromes of "Yin Poison." Verified against the *Su Wen* (Basic Questions), these are all syndromes of extreme Yang heat hyperactivity. The heat is merely extremely deep within the interior, making the exterior appear similar to Yin-cold. As the classics state, "Hyperactivity causes harm, and succession brings regulation" (Kang Ze Hai, Cheng Nai Zhi). This refers to the principle of the Five Elements: excess leads to overreach, and extremity leads to reversal, acting to restrain oneself. This is called concurrent transformation. Just as things, when heated to the extreme, produce fluids; when fire smelts metal, extreme heat transforms into water. Thus, when Fire reaches its extreme, it transforms into Water.