The tongue may be one of the smallest organs in the body, but it is highly sensitive. And not only is it a receptor for taste and touch, it can also reflect how healthy you are.
In Chinese medicine, the tongue has a very close relationship with the internal organs. Changes in its colour, shape and coating can indicate health problems.
Lin Zhixiu, assistant professor at the school of Chinese medicine at Chinese University, says the tongue primarily reflects the conditions of the heart, liver, spleen, lungs, kidneys and bladder. It is also closely associated with the 'triple warmer' energy meridian (which corresponds to the thyroid and two adrenal glands), qi, blood and other body fluids.
Meridians, according to traditional Chinese medicine, are channels along which the qi in the body is considered to flow.
'The tongue is linked to our 12 meridians,' Lin says, "and you can tell the health status and the disease condition based on changes in its texture and coating.
'The tongue is divided into three areas: the tip shows the heart and lungs, the sides represent the liver and gall bladder, and the back shows the kidneys.'
The tongue has a rich supply of blood, a thin and transparent mucous layer, and small lumps, or papillae, which are very sensitive to change. According to Lin, this means tongue examination is one of the most useful tools in Chinese medical diagnosis.
The first step in tongue diagnosis is to see whether it is in a normal state or not, he says. A normal tongue is medium-sized, fairly mobile and pink, with a thin white coating.
'First you can look at the colour of the tongue: if it's pale, there could be a deficiency of qi and blood. If there is no coating, that signals a deficiency of stomach yin,' Lin says.
He says a moist, white coating is a sign of cold germs, while a red tongue with a thick, yellow coating represents a "heaty" condition in the body.
Lin says redness in any part of the tongue represents internal heat in the corresponding part of the body.
Conversely, a pale area signifies the related organs are suffering a deficiency of qi and blood.
'A purple tongue shows poor blood circulation in the body, but this can be caused by different reasons,' he says, adding that tongue diagnosis should be used with other diagnostic methods to identify health status.
Lin says a fat tongue means the body is retaining water, which is also a sign of a lack of yang. If the tongue appears thin, this means a lack of yin, so the body needs more fluid.
In contrast to traditional Chinese medicine, Western medicine treats tongue examination in a literal way.
Dr Jacky Chang Kit, who has a master's degree in family medicine, says Western doctors examine the tongue for injuries and inflammation, whereas Chinese physicians relate physical changes in the tongue to a patient's internal health.
'We'll also look at the moisture level and reaction of the tongue, but most of the time we take this information as reference or supplementary information,' Chang says.
'The tongue is seen more as a sensory organ in Western medicine than a reflective one.'