והא רבא הוא דאמר לא נצרכה אלא לשדרה וגולגולת שאין בהן רובע עצמות בתר דשמעה מר"ע
The Gemara expresses surprise at this argument: But wasn’t it Rava himself who said: This statement is necessary only for a whole spine and skull that do not contain a quarter-kav of bones? This indicates that in his opinion a quarter-kav of bones from a spine does impart ritual impurity. The Gemara answers: After he heard the statement of the tanna, he understood from Rabbi Akiva that his dispute in the baraita (52a) concerns a spine and skull from two corpses, not a quarter-kav from a spine and skull. This interpretation led Rava to change his mind.
ת"ש שמאי אומר עצם אחד מן שדרה או מן גולגולת שאני שמאי דמחמיר טפי
The Gemara further suggests: Come and hear a resolution from the following: Shammai says that one bone from the spine or from the skull imparts ritual impurity. Although the Rabbis dispute his ruling, it can be assumed that they do not have a vastly different opinion. Rather, they accept that a quarter-kav from a spine imparts impurity and renders a nazirite obligated to shave. The Gemara rejects this proof: Shammai is different, as he is very stringent, and therefore nothing at all can be inferred from his opinion with regard to that of the Rabbis.
ליפשוט מינה טעמא דשמאי דמחמי' הא לרבנן עד דאיכא חצי קב עצמו'
The Gemara counters: If in fact the Rabbis maintain an extremely different opinion from that of Shammai, let us resolve the problem in the opposite manner: The reasoning here is that of Shammai, who is particularly stringent. From this it may be inferred that, according to the opinion of the Rabbis, one is not rendered impure and a nazirite is not required to shave unless there is a half-kav of bones from the spine and skull.
דילמא עד כאן לא פליגי רבנן עליה דשמאי אלא בעצם אחד אבל ברובע עצמות אפילו רבנן מודו
The Gemara rejects this claim as well: Perhaps the argument is not that extreme after all, and the Rabbis disagree with Shammai only with regard to whether one bone from the spine or from the skull imparts impurity. However, with regard to a quarter-kav of bones, even the Rabbis might concede that it renders people and items ritually impure, and a nazirite must shave for it.
אמר רבי אליעזר זקנים הראשונים מקצתן היו אומרים חצי קב עצמות וחצי לוג דם לכל רובע עצמות ורביעית דם לא לכל ומקצתן היו אומרים אף רובע עצמות ורביעית דם לכל
§ Rabbi Eliezer said that some of the early Elders would say: A half-kav of bones and a half-log of blood impart ritual impurity in all forms. Their impurity applies by Torah law, and therefore they impart impurity in a tent. But a quarter-kav of bones and a quarter-log of blood, they do not impart impurity in all forms, i.e., they do not impart impurity in a tent. And some of these Elders would say that even a quarter-kav of bones and a quarter-log of blood impart impurity in all forms. This was the dispute of earlier generations.
ב"ד שלאחריהם אמרו חצי קב עצמות וחצי לוג דם לכל רובע עצמות ורביעית דם לתרומה וקדשים אבל לא לנזיר ועושה פסח
The court that followed them said: A half-kav of bones and a half-log of blood impart ritual impurity in all forms. A quarter-kav of bones and a quarter-log of blood impart impurity only with regard to teruma and offerings, i.e., the Sages decreed that they impart impurity in a tent to invalidate teruma and offerings but not with regard to a nazirite. A nazirite is not required to shave or bring offerings for impurity after contact with a quarter-kav of bones or a quarter-log of blood. And similarly, one who performs the ritual of the Paschal offering may proceed despite the fact that he came into contact with this amount of blood or bones, as the Sages did not apply this decree in cases where one’s impurity precludes the performance of a mitzva whose neglect is punishable by karet.
מכדי אין הכרעת שלישית מכרעת אמר רבי יעקב בר אידי מפי שמועה אמרו מפי חגי זכריה ומלאכי:
The Gemara asks about the ruling of halakha in this case. Now consider, there is a principle that the decision of the third opinion is not considered a decision. A compromise ruling that seeks to resolve a dispute by including factors and cases that were not mentioned in the other two opinions is not considered decisive, so how could the later court make a distinction between a Paschal offering and other cases? Rabbi Ya’akov bar Idi said: This ruling was not stated as a compromise. Rather, they said it from tradition, from Haggai, Zechariah, and Malachi, the last of the prophets. This was not a new attempt to mediate between two earlier opinions but an ancient ruling in its own right.
על אלו הנזיר מגלח: על אלו דרישא למעוטי עצם כשעורה דעל מגעו ועל משאו אין ועל אהילו לא ועל אלו דסיפא למעוטי אבן הסכוכית:
§ The mishna taught that for all these aforementioned sources of ritual impurity a nazirite shaves. The Gemara explains that the phrase: For these, in the mishna’s first clause, serves to exclude a bone that is a barley-grain-bulk. As for touching it and carrying it, yes, a nazirite shaves, but for his overlying it, no, he does not shave. And the phrase: For these, in the mishna’s latter clause, serves to exclude an overhanging [hasekhukhit] stone. Although a stone that forms a cover over a corpse imparts impurity in a tent, a nazirite is nevertheless not obligated to shave due to this source of impurity.
וחצי קב עצמות:
§ In its list of sources of impurity for which a nazirite must shave, the mishna taught: And a half-kav of bones.