Woodblock illustration from an edition of 1738 (3rd year of the Qianlong reign period of the Qing dynasty). It shows two types of tongue morphology -- pure black tongue and red tongue with black coating and segmented underside (heitai biandi hongshe). According to the text, The former is often a sign of hou ji si shui ('extreme fire like water'), indicating that a proliferation of heat toxins has caused obstructions in the body, and that the Qi of the zang organs is almost exhausted. If the pulse has been intermittent in the day or two preceding the appearance of this tongue morphology, the condition is generally fatal.
