Woodblock illustration from an edition of 1738 (3rd year of the Qianlong reign period of the Qing dynasty). It shows the downward transmission process of Yin diseases in the interior of the body. The so-called Yin diseases are internal diseases and conditions of the zang viscera. They are transmitted in a downward sequence from the upper to the lower body, i.e. from the diaphragm (gemo) to the chest cavity and thence to the umbilical and abdominal area. The six zang viscera - the liver, the heart, the spleen, the lungs, the kidneys and the Heart Envelope -- are classified as internal. The lungs and the heart, situated above the diagram, belong to the initial stage in internal Yin disease transmission, and are known as the first Yin sector (yi yin bu). The Heart Envelope and spleen, located within the chest cavity, belong to the second stage in internal Yin disease transmission, and are known as the second Yin sector (er yin bu). The kidneys and liver, located in umbilical and abdominal area, belong to the final stage in internal Yin disease transmission, and are known as the third Yin sector (san yin bu). When malign Qi penetrates into the interior of the body, it is transmitted in sequence from the heart and lungs to the spleen and Heart Envelope and thence to the liver and kidneys. This is what is meant by the transmission sequence of Yin diseases in the interior of the body from above to below.
