What does your tongue say about your health?

The tradition has four ways of examining a person, and the first of them is looking. Of everything a physician can look at, the tongue is the one part of the inside that comes to the surface: it is fed by the same blood, moistened by the same fluids, and coated by what rises from the stomach. A Chinese physician asks you to put your tongue out for the same reason a mechanic lifts the bonnet. It is where the inside can be seen.

It is read in four parts, and the four say different things. The colour of the tongue body speaks for heat and cold and the state of the blood. The shape speaks for the spleen and the fluids. The coat speaks for what is sitting in the stomach, and for whether it is hot or cold, damp or dry. And the moisture speaks for the fluids themselves. None of this is a fortune, and none of it is a name for what is wrong with you. It is a photograph of the inside, and like any photograph it wants daylight.

The Suwen ties the tongue to the heart in the chapter that maps the body onto the five directions: the south, heat, fire, and the heart, whose opening is the tongue. The Ming manuals shortened it to four characters that every student still learns, that the tongue is the sprout of the heart.

The Lingshu ties it to the spleen by the plainest route, the channel itself, which it traces up through the diaphragm and throat to the root of the tongue and out beneath it. That is why the tongue’s shape and coat are read for the spleen and stomach: the channel arrives there.

The first book given entirely to the tongue is Master Ao’s Golden Mirror of Cold Damage, from the Yuan: twelve painted tongues by a physician known only by the surname Ao, which Du Ben expanded to thirty-six in 1341. Every tongue atlas since, and there are many, descends from those thirty-six plates.

Four things to look at. A healthy tongue is pale red, soft, neither swollen nor thin, with a thin white coat you can see the body through, and moist without being wet.

A tongue photographed badly says nothing, and most are. Six things make the difference.

• Daylight, from a window, with no lamp on. Bulbs turn every tongue yellow and every phone flash turns it white. Never use the flash.
• Morning, before coffee, tea, or anything with colour in it. Blackcurrant, curry, red wine and a mint all leave a coat that is not yours.
• Before you brush it or scrape it. The coat is the point; do not clean it off.
• Relaxed, not stretched. Open the mouth, let the tongue rest out over the lower lip for three seconds, and take the picture. A tongue strained out to its full length goes pale and points.
• Straight on, from a hand’s length, with the whole tongue in the frame and nothing else in focus. One picture, not a video.
• No filter, no editing, no brightening. Send what the window saw.

Do it three mornings running if you want to know your own tongue. It changes with the week, and the parts that do not change are the parts worth knowing. None of that is a promise, and we make none. It is a photograph, and a photograph is where a reading starts.