Bloodletting was a primary form of medical treatment. In the 18th and 19th centuries it was believed removing an excess of bad blood would assist with the healing process. One method was to put leeches over a vein to suck the blood out of a patient. To encourage the leech to bite, a drop of blood or milk was placed on the area of the vein. Then a tube with the leech in it was inverted over the spot and the leech would begin to suck. When it had taken enough blood salt was sprinkled so the leech would let go. The same leeches were used and left in these jars. The inverted lip on the jar would help secure the muslin that they covered the jar with, by tying a cord around the jar just below the lip. This would keep the leeches in while allowing them to still breathe.