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The *Neijing* (Classic of Internal Medicine) discusses the five Zang organ flaccidity syndromes, repeatedly emphasizing lung heat as the primary pathogenesis. However, in treating flaccidity syndromes, it advocates exclusively selecting the Yangming channel. The Yangming and Taiyin are exteriorly-interiorly related; they serve as the sea of Qi and Blood for the five Zang and six Fu organs, primarily moistening the Zongjin (tendons gathered at the anterior yin), binding the bones, and facilitating joint movement. They also converge with the Chong, Dai, and Du vessels and act as the general assembly of Yin and Yang meridians. Therefore, the human body possesses True Yang Qi that ascends and descends, circulating and operating, remaining vigorous and unceasing. Spirit, Qi, Blood, temperament, nature, Hun, and Po do not suffer from languor and depression; rather, they all adhere to the normal patterns of opening/closing and movement/stillness. Flaccidity syndrome refers to limbs that are weak and lacking strength, unable to move vigorously. Generally, this occurs because Metal (Lung) fails to generate Water (Kidney), causing the Kidney to lose its function of "Zuoqiang" (physical vigor and robustness), thereby manifesting as flaccidity. The Lung governs the skin and hair, the Heart governs the blood vessels, the Spleen governs the muscles, the Liver governs the sinews/fascia, and the Kidney governs the bone marrow. The Lung is the chief of all Zang organs and serves as a cover for the Heart. When flaccidity occurs, the lung leaves become scorched and dry, and the skin and hair lose their nourishment; Metal is parched by Fire, making it difficult for Water to protect the Mother organ. Heat qi lingers and stagnates, inevitably affecting the sinews, vessels, bones, and flesh, thus generating the disease of flaccidity. In Heart flaccidity, vessel Qi rebels upward while the lower part is deficient; the joints break, the shins hang loose, and the feet cannot bear weight, termed Mai Fei (Vessel Flaccidity). In Liver flaccidity, Gallbladder Qi leaks, causing bitter taste in the mouth; the fascia becomes dry and contracts, termed Jin Fei (Sinew Flaccidity). In Spleen flaccidity, the Stomach becomes dry with thirst, and the muscles feel numb and insensible, termed Rou Fei (Flesh Flaccidity). In Kidney flaccidity, the waist and spine cannot be held upright, the bones become withered, and the marrow decreases, termed Gu Fei (Bone Flaccidity). This constitutes the Neijing's discussion on various flaccidity syndromes.

Zhang Zhongjing, in the *Jingui Yaolue* (Essential Prescriptions of the Golden Cabinet), mentions Fei Fei (Lung Flaccidity), attributing it to heat in the Upper Jiao, leading to lung withering due to coughing. Its causes include excessive sweating, vomiting, Xiao Ke (Wasting-Thirst) with frequent urination, difficulty in defecation, or severe fluid depletion caused by harsh purgatives inducing diarrhea. Consequently, the Cun Kou pulse is rapid yet empty, and the mouth produces turbid saliva and froth. Tracing its logic, this expands upon the flaccidity clauses in the Neijing: when Zang Qi is deficient and cannot distribute Ying-Wei (Nutritive and Defensive Qi) or transport Body Fluids, oppression transforms into heat, which accumulates to form saliva and froth, subsequently triggering coughing. Thus, supplementary formulas such as Zhigancao Tang (Honey-Fried Licorice Decoction) or Shengjiang Renshen Dazao Tang (Fresh Ginger, Ginseng, and Jujube Decoction) are used, sharing similar therapeutic principles. It is further stated that in Lung Flaccidity, one may expect spitting of saliva and froth without coughing, and the patient feels no thirst but suffers from enuresis and frequent urination. This is due to upper deficiency failing to control the lower. This condition represents Cold Lung, characterized by coughing with abundant saliva and froth, treated with Gancao Shengjiang Tang (Licorice and Fresh Ginger Decoction) to warm the interior. If thirst arises after taking the decoction, it indicates Xiao Ke (Wasting-Thirst); thus, the treatment here differs from the previously mentioned Upper Jiao Heat pattern. Upper Jiao Heat causes oppression, drying the Xuanfu (pores and Qi gates) of the Lung; obstructed Qi leads to coughing, and unspread fluids lead to thirst. The "Cold Lung" described here does not imply the presence of exogenous cold or cold drinks, but rather the absence of Yang Qi in the Upper Jiao, hence termed "cold." Insufficient Yang Qi fails to generate heat; without heat, there is no cough or thirst. Only Qi deficiency prevents the regulation of water passage flow. The Lung and Kidney are connected; Lung deficiency leads to Kidney deficiency. Therefore, ingested water flows directly to the Kidney, which, being unable to distribute essence throughout the five Zang organs, discharges it directly via urine. This explains Zhang Zhongjing's concept of Cold Lung Cough.

Generally, lung scorching under heat pressure stems from Kidney Water decline, failing to control Fire, representing a disaster of the True Zang. Treatment should combine agents that tonify Water for various Zang organ heat flaccidities, address Right Kidney Fire decline (which deprives Spleen Earth of support), and manage Lung Metal lacking protection against external pathogens. Cases presenting with non-thirsty coughing, spitting of saliva/froth, and dorsal deformity resembling turtle shell (Guixiong) are specifically due to lower deficiency failing to nourish the upper. Although described as non-hot and non-coughing, this is Qi rebellion induced by cold and should not be treated as a heat syndrome; instead, one should benefit the source of Fire. Alternatively, if Spleen Earth is inherently deficient and fails to generate Lung Metal, causing daily deterioration of skin and hair, or coughing/spitting originating from middle jiao deficiency, it should be treated from the perspective of middle deficiency. Regarding Zhang Zhongjing's "Cold Flaccidity," one should raise Yang and warm gently. If flaccidity results from external cold and cold drinks, one must dispel cold warmly and nourish the transformation source. Liu Wansu also discussed that Qi and Blood are the spirit of humans; if Qi and Blood are stagnant or deficient and cannot circulate freely, the spirit has no utility and fails to perform its functions, resulting in weakness. One should tonify or drain according to deficiency or excess, allowing Qi and Blood to circulate so that the spirit naturally clears and responds appropriately. Li Dongyuan mentioned flaccidity and syncope occurring in summer heat, focusing on four-limb softness, heat syncope, and rebellious Qi in the Chong vessel. He treated these with herbs clearing dryness and removing damp-heat, or combining Shengmai San (Generate Pulse Powder) with Siling San (Four-Ling Powder) plus Huangbai (Phellodendri Cortex) and Zhimu (Anemarrhenae Rhizoma). Unaware that this represents damp-heat-induced flaccidity, a condition of "excess within deficiency," he should have employed draining and leaching methods. Conversely, when Essence and Blood are exhausted, sinews and vessels relax, and muscles waste away daily, this is "deficiency within deficiency," requiring potent tonification, a method vastly different from the former. Reviewing various texts, Fei (Flaccidity) and Bi (Obstruction) insensitivity are often conflated. This error stems from interpreting the "muscle insensitivity" in Spleen Fei from the Neijing as evidence that Fei and Bi are the same disease. Actually, pain characterizes Bi, while lack of pain characterizes Fei. Bi is excess, caused by Wind-Cold-Dampness stagnating Qi; Fei is deficiency, caused by scant Blood and deficient Qi. These are sharply distinct. Some mistakenly treat them as Wind-Damp diseases; although symptoms appear similar, the pathology differs fundamentally. The Neijing also mentions specific endpoints: Ren Feng (Kidney Wind) causing anorexia, frightfulness, and subsequent Heart Qi flaccidity leading to death; or during Taiyin ruling heaven, dampness descends, Kidney Qi follows upward, causing Yin flaccidity and extreme Qi failure to rise. These are internal symptoms based on celestial influences. If Zhang Zhongjing clarified the deficiency/excess of Qi generated by the Lung Zang, then the Liver and Spleen, though lacking explicit text, could be inferred by analogy. I believe the various flaccidities in the Neijing represent true Fei syndromes. Zhang Zhongjing expanded upon them. Others mention Cold Fei, Li Dongyuan mentions Damp-Heat Fei, some attribute it to Fire decline, Spleen deficiency, or cold drink consumption. These are not true Fei but "pseudo-flaccidity" (Ru Fei). True flaccidity and pseudo-flaccidity must be carefully distinguished.