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Swallowing sour fluid (Tun Suan) occurs when the Wood Qi (Liver Qi) is stagnant and fails to extend, causing phlegm-fluids to block the pathway. Damp-heat accumulates in the Liver and manifests outward, lying latent between the Lungs and Stomach. Patients must abstain from anger and consume vegetables; self-nourishment is the best approach. This condition invariably arises from the stagnation of clear Qi in the Middle Jiao (Zhong Jiao), where spoiled phlegm, retained fluids, and undigested food ferment and combine. The appropriate medication should be Er Chen Tang (Two-Aged Decoction) with added stir-fried Evodiae Fructus (Wuzhuyu). This method follows the nature of the pathogen to subdue it, known as the counter-assistance method (Fan Zuo). Crucially, stir-fried Coptidis Rhizome (Huanglian) should be used as the sovereign drug, while drugs such as Atractylodis Macrocephalae Rhizoma (Cangzhu) and Poria (Fuling) are indispensable. In cases where damp-heat resides in the Stomach, food entering the stomach is constrained by this damp-heat and cannot be transformed and transported, thereby becoming sour, much like grain and meat turning sour when stored in a vessel. Thus, injury to the Spleen is the root cause, while phlegm-fire is the branch manifestation. There are also cases where stagnant Qi injures the Spleen; for excess patterns, use the ancient formula Xiaoyao San (Free and Easy Wanderer Powder); for deficiency patterns, use Guipi Tang (Restore the Spleen Decoction), taken with modifications.

Whenever there is fluid retention in the Zhongwan (CV12) area, rumbling sensations occur, and if there is accumulated filth, swallowing sour fluid results. However, *Su Wen* (The Classic of Difficult Issues) states that vomiting sour fluid is due to heat, while Li Dongyuan stated it is due to cold. Why? Because vomiting sour fluid differs from swallowing sour fluid. Vomiting sour fluid involves expelling sour water, whereas swallowing sour fluid is formed when normal body fluids rise with ascending Qi, stagnating and accumulating over time. Long-term accumulation generates heat within dampness, transforming according to Wood characteristics, thus producing a sour taste—is this not heat? When accumulation persists for a long time and cannot surge out spontaneously, it lies latent between the Lungs and Stomach, unable to be coughed up or swallowed down. When the exterior skin and muscles are exposed to cold, internal heat becomes more constrained, and the sour taste pierces the heart; when the exterior is warm, pores open, or aromatic hot decoctions/pills are taken allowing body fluids to circulate, temporary relief may occur—is this not cold? To say it is heat refers to its root; to say it is cold refers to its branch. Generally, for diseases classified as heat, cold medicines are used; however, for sour conditions, hot medicines must be combined to follow the disease's nature (Cong Zhi). Yu Jiayan stated: "The methods of sweet-warm to dispel heat and purge fire, when applied to sour conditions, will only increase the sourness and prove ineffective. Therefore, to drive away the sourness and restore sweetness, only the method of using 'Gang Yao' (Rigid/Strong Drugs) works. Gang Yao refers to drugs with potent aromas and flavors that can transform the Stomach without being transformed by it."

Belching (Ai Qi) is the same as Yi Qi. It is caused by stagnant fire in the Stomach and thick phlegm above the Diaphragm, resulting from the Spleen's failure to transport and turbid Qi blocking the pathways. Another view holds: Belching accompanied by swallowing sour fluid is due to food stagnation generating excessive heat and fire, with Qi rushing upward. In such cases, Scutellariae Radix (Huangqin) should be the sovereign drug, with Arisaematis Rhizoma Preparatum (Nanxing), Pinelliae Rhizoma (Banxia), and Citri Reticulatae Pericarpium (Chenpi) as assistant drugs.

Prescription: Evodiae Fructus (Wuzhuyu) (30g, remove stems and branches, boil briefly, soak for half a day, dry), Atractylodis Macrocephalae Rhizoma (Cangzhu) (soaked in rice washing water), Coptidis Rhizome (Huanglian) (30g, stir-fried with soil from old walls, remove soil), Scutellariae Radix (Huangqin) (30g, stir-fried with soil as above), Citri Reticulatae Pericarpium (Chenpi) (each 30g). Alternatively, add Platycodonis Radix (Jiegeng) 30g and Poria (Fuling) 30g. Grind into powder, make pills with Medicated Leaven paste (Shenqu), size of mung beans. Take 20–30 pills at a time, allowing saliva to wash them down frequently.