The Gemara on today’s daf (=page) quotes a baraita that teaches:
Our Rabbis taught: If a Roman legion which passes from place to place enters a house, the house is unclean, for there is not a legion that does not carry with it several scalps.And be not surprised at this; for Rabbi Yishma’el’s scalp was placed upon the head of kings.
Rabbi Yishma’el ben Elisha served as one of the last High Priests during the Second Temple period. As one of the Ten Martyrs murdered by the Romans, he was taken to be killed together with his friend and colleague Rabban Shimon ben Gamliel the Elder. According to the Rabbinic tradition, when he was taken to be killed the Caesar’s daughter was smitten with his beauty and asked that the skin of his face be removed while he was still alive. The Romans preserved his skin in Persimmon oil and it was worn on occasion during Roman festivals.
The commentaries note that although the baraita offers a clear ruling in Jewish law – that soldiers are suspected of carrying human remains which would render the contents of a house ritually defiled – the Rambam does not mention this ruling in his Mishneh Torah. One suggestion is that we are not concerned that someone will actually come into physical contact with these scalps, the issue is tum’at ohel – “tent impurity” – whether the items in the same room with the human remains will become defiled. Since the likelihood is that the scalps carried by the Roman soldiers were, in all probability, non-Jews, and given the Rambam’s ruling in accordance with Rabbi Shimon that there is no tum’at ohel for non-Jewish corpses (in contrast with Tosafot who reject Rabbi Shimon’s ruling), there was no need for the Rambam to include this law in his Mishneh Torah.