Most people think that medical humor consists of doctor and HMO jokes (and some of it does) but in medicine the word "humor" has another sense and refers to a fluid (or semifluid) substance.
The aqueous humor and the vitreous humors are fluids within the eye. The aqueous humor is the watery fluid normally present in the chambers of the eye around the iris while the vitreous humor is the "glassy" fluid behind the lens in the eyeball.
The humors (humours, in England) originated in an ancient theory that held that health came from a balance between the bodily liquids. These liquids, the humors, were four in number:
Disease arose when imbalance occurred between these four humors. The treatment of disease was simple, straightforward, and logical (assuming the humoral theory to be correct).
The doctor needed first to diagnose the humoral imbalance. Then if one humor were deficient, the doctor had to strengthen it. And, conversely, if another humor were excessive, the doctor needed to purge it.
Take a person who was "bad-humored" because of too much blood. Superfluous blood was removed by bleeding the patient or applying leaches to suck out the extra blood. By such means, the person became "good-humored."