Chapter 8 of the Suwen: The Pericardium

膻 中 者 ， 臣 使 之 官 ， 喜 樂 出 焉 。

In reference to the Pericardium, it holds the office of the Bowing Slave that is sent out as an Ambassador. Excitement and Joy emerge from it.

The character, chen, 臣, shows an eye turned to the side, which represents a person who is constantly bowing. The Pericardium is therefore like a humble person that is sent out as an ambassador to represent the one who sends them. This can be far more successful than sending a warrior out, as a slave is much more easily received, since they are less threatening. Often times, young girls were married off to Kings of different states, and these girls were an offering of the innocence of their homeland.
Xi, 喜,  has the character for drum on top, and it has been hypothesized that le, 樂,  also holds some kind of relationship with drumming. It can be pronounced “yue,” meaning “music,” and the character is literally a picture of white silk above wood. This has a lot to do with rhythm and music. The virtue of the Heart is ritual, and one of the ways in which we ritually celebrate is to use drums. A drum is a rhythmical instrument that mimics the beat of the heart, and perhaps this joy and excitement that are emerging from the Pericardium have something to do with ritual and song.