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Cold damage (Shanghan) pathogens transmit from the exterior to the interior. Warm-heat pathogens reach from the interior to the exterior. Verification relies entirely on the tongue coating (tongue moss). The depth of transmission into the interior and the quantity of interior accumulation are also verified by the tongue coating. As interior heat gradually clears and grain qi (Spleen-Stomach transformation qi) gradually advances, this is likewise verified by the tongue coating. Now, let us first enumerate the essential principles for observing tongue shape.

Generally, the tongue membrane connects directly with the stomach and intestines via the Sanjiao (Triple Burner) cou li (interstices). The tongue root connects directly to the Heart, Spleen, and Kidney via the meridians. Therefore, the tongue tip primarily corresponds to the upper epigastrium (Shangwan) and also to the Heart; the tongue center primarily corresponds to the middle epigastrium (Zhongwan), governing the Stomach and Small Intestine; the tongue root primarily corresponds to the lower epigastrium (Xiawan) and also to the Kidneys and Large Intestine; the four sides belong to the Spleen. This is the essential principle for observing tongue shape distribution.

[Commentary by Xu Rongzhai]: The term "tongue coating" should be viewed separately. "Tongue" refers to the essence of the tongue body, while "coating" refers to the greasy deposit growing on the tongue surface. The tongue body reflects the prosperity or decline of Yuan Qi (Primordial Qi), while the coating reveals the depth of the disease. Regarding distribution, it can be distinguished into five categories: (1) Tongue tip—observes diseases of the Upper Jiao (Heart and Lungs). (2) Tongue center—observes diseases of the Stomach and Large/Small Intestines. (3) Tongue root—observes diseases of the Kidneys and bowel/bladder functions. (4) Tongue sides—left side observes Liver/Gallbladder diseases, right side observes Spleen/Lung diseases. (5) Tongue edges—observe Sanjiao Mo Yuan (Membrane Source) and hypochondriac diseases. Here, dividing front and back into three parts allows observation of the Upper, Middle, and Lower Jiao; dividing the sides into two parts allows observation of the left and right aspects. This is consistent with the principle of pulse diagnosis where the anterior position observes the front and posterior position observes the back. Empirical evidence proves that Heart heat causes the tongue tip to be red with prickles; Stomach stagnation causes a greasy coating in the center; Spleen/Kidney deficiency-cold causes a slippery white coating at the root; Liver/Gallbladder damp-heat causes a yellow greasy coating on the sides; Sanjiao fire accumulation or latent damp-warmth pathogens cause the coating to remain white but the tongue edges to become crimson; within this square inch, locations are distinct, which is reliable and verifiable.

When the protruding tongue is long and pointed, the heat is not severe, and expelling pathogens through diffusion is appropriate. When the protruding tongue is round and flat, the heat is severe, and clearing heat urgently is required. When the protruding tongue is short, cannot extend beyond the teeth, and appears square-shaped, the heat is extremely intense, and purging fire urgently is required. However, in cases of female labor relapse (relapse due to sexual activity after cold damage) and postpartum complicated conditions, if the tongue extends several inches, death is certain. This is a separate discussion. This is the essential principle for observing the protrusion of the tongue.

When the tongue extends with weakness, it indicates deficiency of Middle Qi (Spleen/Stomach Qi), requiring tonification of the Middle. When the tongue extends fully as if suspended by a thread, the lingual frenulum is dry, requiring moistening of dryness. When the tongue is numb and cannot extend, it indicates Liver Wind carrying Phlegm, requiring extinguishing of Wind and transformation of Phlegm. When the tongue extends and bites the lips, it indicates Heart/Spleen Heat, requiring purging of Fire and clearing of Heat. When the tongue extends and manipulates the lips, it indicates snake venom poisoning, requiring detoxification. When the tongue extends but cannot retract, it indicates flooding of Spleen saliva, requiring control of Phlegm-fluids. If the tongue shrinks and curls at the edges, it indicates extreme dryness of Stomach fluids, requiring clearing of the Stomach and moistening of dryness. If moistening does not uncurl the tongue, the pathogen has departed but the tongue's Qi is not yet harmonious, allowing for nourishment of Ying (Nutrient) Qi and tonification of Qi. If the tongue curls and shortens, it indicates exhaustion of Jueyin (Liver/Pericardium) Qi and flaccidity of the tongue substance, which is untreatable. If the tongue remains shrunken even after scraping off the coating, it is also untreatable. This is the essential principle for observing tongue extension and retraction.

When the tongue trembles restlessly, it is called "Tongue Tremor." Caused by Qi deficiency, it moves slightly; caused by Liver Wind, it flutters vigorously. Differentiation should rely on tongue color. If deep red or fresh red and trembling, cooling Blood and extinguishing Wind is appropriate. If purple-red or stasis-red and trembling, purging Fire and extinguishing Wind is appropriate. If tender red and trembling, nourishing Blood and extinguishing Wind is appropriate. If pale red and trembling, vigorous tonification of Qi and Blood is appropriate. If the tongue is soft and immobile, it is called "Tongue Flaccidity." There are acute and chronic forms. Acute flaccidity is mostly due to heat scorching, often appearing when the tongue is dry; differentiation by color is also needed here. If deep red and flaccid, clearing the Ying Level and supplementing Qi is appropriate. If purple-red and flaccid, clearing the Liver and unblocking the Fu organs is appropriate. If fresh red and flaccid, nourishing Yin and descending Fire is appropriate. Only if pale red and flaccid, major tonification of Qi and Blood is appropriate. If the disease is prolonged and the tongue color is crimson and tender, Yin deficiency is extreme, Jin-Ye (Body Fluids) and Qi cannot distribute to the tongue root, and there is no treatment. This is the essential principle for observing tongue flaccidity and tremor.

[Lian Kan Commentary]: Chronic tongue flaccidity results from paralysis of the tongue muscles and atrophy of the tongue substance. If unilateral or hemiplegic, it constitutes neurological flaccid paralysis, such as bulbar palsy or spinal muscular atrophy. Treatment should involve major tonification of Qi and Blood, offering perhaps a one-in-ten chance of recovery. Tongue protrusion with tremors are commonly seen in warm-heat diseases and neurasthenia in alcoholics.

Diseases with a tongue coating usually indicate interior stagnation, requiring guiding out stagnation. Diseases without a coating usually indicate Middle deficiency, requiring tonification of the Middle. If a patient originally without a coating suddenly develops one, it indicates turbid Stomach Qi rising, requiring draining of turbidity. If a patient originally with a coating suddenly loses it, Stomach Yin is about to dry up, requiring rescue of Yin. Coatings distributed only partially may be external/internal or left/right. External predominance (coating outside, none inside) suggests pathogen entered interior but not deeply, while Stomach Qi is deficient; expelling pathogens and benefiting Stomach Qi is appropriate. Internal predominance (coating inside, none outside) suggests Stomach stagnation reduced but intestinal accumulation remains; unblocking intestines and eliminating stagnation is appropriate. Patients with chronic Phlegm-fluids often have this type of partial coating, requiring elimination of fluids. Left-predominant (coating left, none right) or right-predominant (coating right, none left) both indicate Shaoyang (Half-Exterior/Half-Interior) patterns. Observe the quantity of coating color: abundant white indicates exterior patterns, suitable for harmonizing with warming or clearing adjuncts, adjusted to symptoms. Abundant yellow, black, or gray colors, or presence of prickles, black dots, and dry cracks, indicate interior heat consolidation; urgent harmonizing with purgation is required. A coating divided straight down the middle from root to tip indicates combined disease and internal cold. Horizontal division into two or three sections from root to tip indicates concurrent diseases. Medication should be prescribed according to symptoms. Although coatings have visible form, they are always partial and incomplete. Even if the entire tongue is covered, indicating damp-phlegm or food stagnation, one must differentiate white sandy coating accompanied by purple-red tongue flesh on the four sides, which indicates warm pathogens suppressed by dampness and lying latent in the Mo Yuan (Membrane Source); urgent resolution of the Mo Yuan to expel pathogens is required. White alkaline coating accompanied by greasy tongue flesh on the four sides indicates Spleen/Stomach damp obstruction and Qi stagnation struggling with food accumulation; urgent aromatic-diffusing and light-draining with digestion-promoting methods is required. This is the essential principle for observing the presence/absence of accumulated coating and its partial/full distribution.

Cracks and fissures on the tongue, resembling characters like 'Ren', 'Chuan', 'Yao', or straight grooves, mostly indicate Stomach dryness and fluid exhaustion due to internal逼迫 (forcing) by excess heat; urgent cooling purgation to clear Fire is required. However, a straight crack in the center mostly indicates deficiency of Stomach Qi, requiring tonification of Yin and Qi, strictly avoiding cooling purgation. Furthermore, if a patient originally without cracks develops a 'Ren' character crack after purgation, this indicates Kidney Qi harassing the Heart; urgent gathering of Qi and tonification of the Kidney is required. If coating spots resemble rice grains, it mostly indicates worm infestation. If the coating shows betel-nut-like patterns with hidden spots, it also indicates worm accumulation; killing worms and removing accumulation is appropriate. This is the essential principle for observing tongue cracks and fine spots.

[Lian Kan Commentary]: The tongue naturally has fine points like prickles, named taste buds (papillae), responsible for taste perception. Without Fire, they are flat; with Fire, they protrude. Whether worms are present requires careful examination of current symptoms before administering vermifuge and accumulation-removing agents.

Coating forming patches and halos are all caused by steaming from excess Fire of the Zang-Fu organs, commonly seen in warm-toxin and warm-plague diseases. Patches are mostly black; halos are mostly gray-black. Fewer patches (1-2) are mild; 3-4 are severe; 6-7 are extremely severe and difficult to treat. Halos have layers; one layer is mild, two layers are severe, three layers often lead to death. Transverse lines of two or three layers are similar. Purging Fire and detoxifying, urgent purgation to preserve Yin, are required. If the patches and halos recede completely and the patient's Qi and fluids gradually recover, survival is possible. This is the essential principle for observing tongue patch-halos.

Swollen and enlarged tongues that cannot retreat into the mouth require differentiation by color. White slippery or black slippery colors mostly indicate water flooding, requiring unblocking Yang and promoting urination. Yellow greasy full coverage indicates damp-heat郁 (stagnating) transforming into toxin spreading to the mouth; major purging of damp-fire to remove toxin is required. Purple-dark colors mostly indicate wine toxin rushing to the Heart with Heart Fire flaming upward; purging Fire and unblocking stasis is required. White greasy mixed with yellow greasy indicates fighting between turbid Phlegm overflowing upwards; eliminating Phlegm and draining turbidity is required. Thin, small, or sunken/thin tongues also require color differentiation. Pale red or tender red indicates internal deficiency of Heart Blood, requiring nourishing Blood and tonifying the Heart. Purple-crimson or burning red indicates internal wind and heat consuming fluids; clearing Heat and extinguishing Wind is required. Dry crimson colors, or even purple-dark like pig kidney color, indicate exhaustion of Heart/Liver Blood and tongue substance atrophy, which is untreatable. This is the essential principle for observing tongue swelling and sunkenness.

A tongue deviating to one side is called "Tongue Deviation." Purple-red with rapid onset mostly indicates Liver Wind causing spasms, requiring extinguishing of Wind and calming spasms. Pale red with slow onset mostly indicates stroke hemiplegia. Deviation to the left requires nourishing Blood and tonifying Qi, drawing Yang from Yin; deviation to the right requires tonifying Qi and relaxing sinews, drawing Yin from Yang. However, these are mostly untreatable. A tongue with blood marks or injuries is called "Broken Tongue." Causes include four types: one, tongue bleeding (She Nu); two, scratching injury; three, ulceration; four, macular scars. Each requires symptomatic treatment. This is the essential principle for observing tongue deviation and breakage.

Tongues with nodules (lumps) and convexity are commonly seen in warm diseases, heat diseases, warm toxins, and seasonal epidemics. All belong to excess heat in the Stomach and Intestines with lurking toxic pathogens; urgent large-dose cooling purgation to rapidly attack the toxin is required. If concave with defects, the syndrome can be excess or deficiency. Excess cases mostly result from oral thrush (Kou Mi); after Jueyin reversal, nodule points appear on the tongue, and after shedding, concave points remain. Rising mold-toxin requires removal of mold and detoxification; Stomach/Kidney Yin deficiency with steaming of turbid decay requires rescuing Yin and removing decay. If toxin is removed and decay fades, new flesh grows and concavities fill. Deficiency cases result from exhaustion of Stomach Yin and inability of Heart Qi to distribute to the tongue root; Qi surplus causes convexity, Qi sinking causes concavity, similarly affecting eye sockets, not just the tongue. The disease is untreatable; death time can be determined by pulse and symptoms. This is the essential principle for observing tongue convexity and concavity.

Lian Kan Commentary: Tongue nodules and protrusions refer to swollen lingual papillae. One must differentiate between heat-toxin, cancerous tumors, and mycotic pseudomembranous swelling. This is crucial. Tongue concavities mostly result from depressed papillae. One must differentiate tongue ulcers and mycotic ulcers; if absent, it represents visceral atrophy and papillary shrinkage into concavity, for which there is absolutely no treatment method.