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13. The external manifestation of the center of the tongue (She Xin) should originally be moist and reddish; a deepening of red to crimson indicates mild heat; redness on the exterior with purpleness on the interior indicates severe heat; a slippery white coating belongs to exterior cold involving the Shaoyang channel; a sinking, slow, fine, and tight pulse indicates cold stagnation in the Zang organs; a dry and thin appearance indicates deficiency of both Qi and fluids; a yellow or black coating that is moist indicates shallow interior heat; a scorched, dry, cracked surface with prickles clearly indicates deep-seated heat; if a slippery black coating appears concurrently with Taiyin/Yueyin/Jueyin syndromes (San Yin), it signifies Water overcoming Fire, carrying a hopeless prognosis.

[Annotation] The tongue is the external manifestation of the Heart; a color that is red and lustrous indicates health. If upon initial pathogenic invasion, both the interior and exterior appear deeply red, it indicates the presence of Heat. Redness on the outside and purpleness on the inside indicate excessive Heat. A slippery white coating indicates Exterior Cold. If the coating gradually thickens, the disease has transmitted to the Shaoyang channel. For Heat syndromes, acrid-cool diaphoresis is appropriate; for Cold syndromes, acrid-warm diaphoresis is appropriate. In Shaoyang channel disorders, there is cold in the chest and heat in the Dantian; Xiao Chai Hu Tang (Minor Bupleurum Decoction) is used to harmonize both aspects. Here, "chest" refers to the exterior and superficial level; "Dantian" refers to the interior and deep level; this does not literally point to these anatomical locations but rather indicates that half-interior heat has not yet fully formed, while half-exterior cold remains. Therefore, a white tongue coating can signify either Cold or Heat. If a slippery, thick coating appears simultaneously with Yin syndrome pulses, it indicates visceral deficiency-cold stagnation, treated with Li Zhong Wan (Regulate the Middle Pill) plus Zhi Shi (Aurantii Fructus) to warm and resolve. If a dry, thin coating appears simultaneously with Yang syndrome signs, it indicates Qi deficiency and fluid exhaustion, treated with Bai Hu Jia Ren Shen Tang (White Tiger Decoction plus Ginseng) to clear heat and tonify. If a white coating gradually turns yellow, this indicates the pathogen moving from exterior to interior with shallow heat; if the exterior syndrome persists, San Huang Shi Gao Tang (Three Yellows and Gypsum Decoction) is indicated; if the pathogen has entered the interior, Liang Ge San (Cooling the Diaphragm Powder) is used. If the tongue becomes scorched, dry, and black, or develops prickles and cracks, this indicates deep interior heat, treated with Zhi Zi Jin Hua Tang (Gardenia Golden Flower Decoction); if accompanied by abdominal fullness and pain, Da Cheng Qi Tang (Major Order the Qi Decoction) is used. Red is the color of Fire; Black is the color of Water. If black coating appears with Yangming/Taiyang/Shaoyang syndromes (San Yang), it represents extreme heat mimicking its controlling element (Water); clearing and purging methods may still cure it. However, if black coating appears with Yin syndromes, it signifies Water overcoming Fire, resulting in death. If an physician wipes the tongue with Sheng Jiang (Fresh Ginger Root) and the blackness recedes slightly, Fu Zi Li Zhong Tang (Aconite Regulate the Middle Decoction) and Si Ni Tang (Frigid Extremities Decoction) or similar formulas should be urgently administered to rescue the patient, offering a chance of survival.