If a limb hangs loose from the body of a living beast, what is the law regarding ritual defilement? They replied: we have heard nothing about this, but we have heard about a limb hanging loose from the body of a man that it is clean. And thus those who were mukei shehin did in Jerusalem: the afflicted person would go on the eve of Passover to the physician, and he would cut the limb until only contact of a hairbreadth was left; he then stuck it on a thorn and then tore himself away from it. In this manner both that man and the physician could participate in the Passover offering. And it seems to us that your case may be derived from this by a kal va-homer – an a fortiori conclusion.
The term mukei shehin refers to people who suffered from a disfiguring disease, although as used in the Mishnah, shehinincludes a number of different ailments. It is possible that the reference here is to leprosy, which is mistakenly given as the common translation for the Biblical tzara’at. Leprosy is an infectious disease whose symptoms involve reactions on the skin, but also attack the nerve fibers, ultimately destroying the nerve cells. This results in a loss of feeling of heat, cold and pain, leading to a situation where the person can injure his limbs, suffer burns and cuts that ultimately affect deep tissue, bones and joints. Severe and prolonged infection can lead to spontaneous amputation of limbs, which may be what is described in the Mishnah that inquires about the status of “a limb hanging loose” from a man or an animal.
Rashi explains that the recommended procedure of removing the limb was not performed because of the requirements of the laws of ritual defilement – for the limb remained in a state of purity so long as it was connected to the person. Rather it was done for aesthetic purposes.