Bloodletting was the most common medical practice for centuries. It involved cutting the skin to let blood flow from the body, and it was used for a wide variety of ailments, from acne to epilepsy to smallpox. It was based on the idea that there were four humours of the body: blood, phlegm, yellow bile, and black bile. Diseases were thought to be the by-product of having an imbalance of humours, most commonly blood, because it was thought to be the most dominant of the four. Bloodletting was therefore used to try to balance the humours and heal patients.
There were multiple ways to drain blood, such as using simple blades, leeches, or a scarificator. A scarificator was a spring-loaded mechanism that would be held against the skin and would snap the blades out and back in. The picture above of a metal box is a scarificator. The metal tube with spikes is an artifical leech, which would puncture the skin.