Explain in detail how Galen, a physician during the Middle Ages, expanded on Hippocrates' theory of the four humors and explain what he believed each humor represented

Hippocrates' theory of the four essential humors suggests, to summarize, that the human body consists of four substances. These substances are called humors. They must maintain a perfect balance between them. When they lose, the disease appears, both in body and mind. Any disability or illness meant that the balance of moods had changed. Therefore, to cure them, it was necessary to find a way to restore the lost balance. According to the theory of essential moods, the substances that make up the human body are: black bile, yellow bile, blood and lymph. Each of these moods is directly related to an element of the universe and an atmospheric quality.

Galen has expanded this theory because through various research he has found that food has a great influence on moods. For Galen, people who used to eat hot foods were prone to developing yellow bile, and people who used to eat cold foods tended to produce phlegm.