Chapter contents

PHYSIOLOGY OF TONGUE COATING

The Stomach rots and ripens food, and during the process of digestion a small amount of what the ancient Chinese books called “turbidity” or “turbid fluids” escapes upwards to reach the tongue: this forms a coating. Therefore the presence of a coating indicates the normal functioning of Stomach-Qi. A normal coating should be white and thin enough to see the body colour through it. Although the tongue coating reflects primarily the physiological activity of the Stomach, the Spleen and the Kidneys also play a role in its formation; for this reason, if a tongue lacks a coating this may indicate Stomach-Yin or Kidney-Yin deficiency, or both.

It should be noted that the tongue coating is naturally thicker at the root of the tongue and thinnest towards the tip, and that the coating does not extend to the very edges of the tongue.

CLINICAL SIGNIFICANCE OF TONGUE COATING

The tongue coating reflects primarily the state of the Yang organs and especially of the Stomach. It also reflects conditions of Deficiency or Excess and of Heat or Cold (Box 26.1).

Observation of the tongue coating needs to be based on a careful analysis of the history of the patient because the tongue coating changes very rapidly in acute conditions and can reflect short-term variations. For example, a yellow tongue coating in the centre indicates Stomach-Heat and this could equally be a chronic condition of Stomach-Heat or simply an acute stomach upset. We therefore need to enquire carefully to exclude the possibility that the yellow coating merely reflects an acute but passing condition. Apart from the history, the brightness of the coating can give us an indication of its duration: the duller the coating, the more chronic is the condition.

As the coating reflects primarily the condition of the Yang organs, its thickness and particularly its distribution give a clear indication of a pathology affecting one of the Yang organs. For example, the sides of the tongue reflect the condition of the Liver and Gall-Bladder and we can differentiate between these two by referring to the body colour or coating, that is, if the body colour is affected it indicates a Liver pathology, whereas if the coating is affected it indicates a Gall-Bladder pathology. Similarly for the root of the tongue: if the root of the tongue is Red without coating this indicates Kidney-Yin deficiency with Empty-Heat, whereas if it has a thick coating it indicates a pathology of one of the Yang organs of the Lower Burner (Bladder or Intestines).

The tongue coating also gives us a good indication of Deficiency and Excess conditions because a thick coating with root always reflects a Full condition while a coating without root or the complete absence of coating always reflects a Deficiency condition. This will be described in more detail below.

The tongue coating gives also a very clear indication of the Hot or Cold nature of a condition but, as indicated above, it may merely reflect a passing, acute condition.

Box 26.1 summarizes the significance of a tongue coating.

PRESENCE OR ABSENCE OF COATING

The presence or absence of coating tells us primarily about the state of the Stomach-Qi. If the tongue has a coating with root, it indicates that the Stomach-Qi is still intact even if the excessive thickness or colour of the coating is pathological. Excluding the case of a normal, thin white coating, the tongue with coating usually indicates a Full condition precisely because the Stomach-Qi is intact. A tongue without coating indicates that the Stomach-Qi is severely weakened and therefore a Deficiency condition. Therefore, it is better to have a thick, pathological coating with root than not to have a coating.

How does the coating disappear from the tongue surface? In chronic conditions, the tongue coating disappears gradually over a long time (usually years); in acute conditions, and especially acute, febrile diseases in children, the coating may disappear very quickly in a matter of days.

In chronic conditions characterized by the absence of coating, with treatment the coating should return gradually and quite slowly; this is a positive sign. If the coating appears suddenly on a tongue that previously lacked it, this is a bad sign; for example, in a patient with cancer the sudden return of coating to a particular area of the tongue may indicate a metastasis of the corresponding organ.

Conversely, if a tongue has a very thick, pathological coating, its sudden total or partial disappearance in the course of a disease is a poor prognostic sign because it indicates the sudden depletion of Stomach-Qi. However, the clinical significance is different if a thick tongue coating becomes normal as a result of treatment; as indicated above, the tongue coating can change much more quickly than the tongue-body colour.

COATING WITH OR WITHOUT ROOT

We can compare the tongue coating to grass: it should “grow” out of the tongue body just as grass grows out of the soil and it should have “roots” just as grass stems have roots in the earth. A coating with root reflects the normal functioning of Stomach-Qi even if the coating may be pathological (e.g. too thick and dark yellow). A coating without root resembles mown grass scattered on barren ground: it looks as if it has been “added” on top of the tongue rather than growing out of it. (See Plates 26.1a and b on p. P25.) In severe conditions, the rootless coating may look like salt or snow sprinkled on top of the tongue. A rootless coating indicates the beginning of the weakening of Stomach-Qi in the course of a chronic disease. Therefore, it is better to have a thick, pathological coating with root than to have a thin coating without root.

It should be emphasized that the coating with root is not necessarily thin, although that is the most common situation. The coating without root can also be thick and often sticky: this represents the worst scenario because it indicates that, on the one hand, Stomach-Qi is weakened and, on the other, that there is a significant pathogenic factor (against which the body is unable to fight due to the Stomach-Qi deficiency) (Fig. 26.1). For this reason it is obviously better to have a thin rather than a thick coating without root. (See Plate 26.1c on p. P25.)

Box 26.2 summarizes the four different situations seen in coating with and without root.

COATING THICKNESS

The normal coating is thin and it should be possible to see the tongue body through it. If the tongue body cannot be seen, the tongue coating is too thick. The thickness of the tongue coating reflects the strength of the pathogenic factor clearly and accurately: the thicker the coating the stronger the pathogenic factor. If a thin coating becomes thick, this indicates that the pathogenic factors are getting stronger or that they are penetrating deeper into the Interior. The latter situation applies to conditions of acute, external diseases, which will be explained in greater detail below. Therefore, if we are treating a patient with a thick tongue coating, we should expect it to become gradually thinner. In some cases, a thick tongue coating can revert back to normal even after a very short time.

As mentioned above, a thick coating may be with or without root but the clinical significance of its thickness is the same – that is, it reflects the strength of a pathogenic factor.

COATING DISTRIBUTION

Generally speaking, the same areas that reflect changes in the tongue-body colour also reflect changes in the tongue coating, but there are some differences. For example, the tongue coating never extends to the very tip or the very edges of the tongue.

The most common locations where a thick coating is observed are in the centre and on the root of the tongue. A thick coating in the centre reflects the presence of a pathogenic factor in the Stomach, which may be Cold, Heat, Dampness or Phlegm depending on the colour and consistency of the coating, whereas a thick coating on the root reflects the presence of a pathogenic factor in the Bladder or Intestines.

A pathogenic factor in the Gall-Bladder (Fig. 26.2) may be manifested in a variety of ways; the most common one is a bilateral or unilateral coating that comes forward in one or two strips on the edges. (See Plate 26.2 on p. P25.)

COATING MOISTURE

The normal coating should be relatively moist, indicating a good supply and movement of Body Fluids. If the coating is too dry, it indicates either Heat or Yin deficiency. If the coating is too wet, it indicates severe Yang deficiency.

COATING TEXTURE

The normal coating texture should be like the hairs of a very fine brush, in Western medicine corresponding to the filiform papillae. The individual “hairs” (filiform papillae) should be clearly seen and they should not be too dry or too wet. Pathological textures of the coating make it sticky, slippery or mouldy.

TONGUE COATING IN EXTERNAL DISEASES

In external diseases from invasion of Wind, the interpretation of the tongue coating is quite different from that of internal diseases.

In external diseases the thickness of the coating reflects, not only the intensity of the pathogenic factor, but also its progression towards the Interior: thus, if in the course of an acute external disease, the coating becomes thicker it indicates that not only has the pathogenic factor become stronger but that it is beginning to penetrate into the Interior.

The coating in external diseases reflects Hot and Cold influences in the same way as in internal diseases, but there are differences. The most important difference is that the coating tends to be white in the very beginning stages of an invasion of Wind even if it is Wind-Heat. If during an acute external disease the coating turns from white to yellow, this means there is not only a change from Cold to Heat (although a white coating may appear also in Wind-Heat), but also a penetration of the pathogenic factor into the Interior.

The distribution of the coating also reflects the stages of penetration of an external pathogenic fac-tor. In the very beginning stages of an invasion of external Wind, the coating may be more concentrated on the front third or on the sides. In this context, these two areas correspond to the Exterior of the body while the centre of the tongue corresponds to the Interior. (See Fig. 24.8 on p. 212)

Thus, if after a few days the coating from these two areas extends towards the centre of the tongue, it indicates that a pathogenic factor is penetrating into the Interior.