פירוש על קידושין 47:6

The Torah explicitly states that a slave goes free at the loss of an eye or tooth. The rabbis add that the same is true for any projecting limb (such as a leg or hand or finger. There are 24 of these and they will be listed later on daf Kaf Heh). The blemish must also be exposed, as are teeth and eyes. A slave who loses such a limb will be noticeably deformed.