The most severe type of ritual defilement is that of a dead body, which imparts ritual impurity to people and garments for a period of seven days. The laws of a corpse apply not only to a full body, but also to parts of the body of significant size. In addition, a revi’it of blood from a dead body will also impart ritual impurity. The Gemara on today’s daf discusses dam tevusa (blood of submission) – a mixture of blood that flowed from a dying person who began to bleed while he was alive (blood that does not impart ritual impurity) and continued to bleed after his death (blood that imparts ritual impurity).
Although the midrash in the Sifrei appears to derive the law that dam tevusa imparts ritual purity from the passage in Bamidbar (19:11), that is clearly an asmakhta – a hint to a law that is really of Rabbinic origin, inasmuch as none of the commentaries describe it as a Biblical law.
The Gemara relates the following case in the name of Rabbi Shimon:
In the case of one who is crucified on wood, whose blood runs down the wood to the earth, and a quarter of a log of blood was found under him, the blood is ritually impure.
Crucifixion was one of the common methods used by the Romans to put someone to death. This cruel and unusual punishment was often applied to slaves, captives and rebels. In and of itself, crucifixion did not kill the victim, since it involved only nailing the person’s hands and legs to a wooden cross. Death by crucifixion came about because of dehydration and loss of blood; it is for this reason that there are recorded instances where an individual who was crucified were taken down from the cross and survived. Until the victim’s death his blood would continuously drip down, and the blood would continue to drip even after he had died, as is clear from the discussion in the Gemara.