Nazir 53A

והא רבא הוא דאמר: לא נצרכה אלא לשדרה וגולגולת שאין בהן רובע עצמות! בתר דשמעה מרבי עקיבא.

The Talmud 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 1/4th-kav of bones? This indicates that in his opinion 1/4th-kav of bones from a spine does impart ritual impurity. The Talmud answers: After he heard the statement of the tanna, he understood from R' Akiva that his dispute in the baraita (52a) concerns a spine and skull from two corpses, not 1/4th-kav from a spine and skull. This interpretation led Rava to change his mind.

תא שמע, שמאי אומר: עצם אחד מן שדרה או מן גולגולת! שאני שמאי דמחמיר טפי.

The Talmud 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 1/4th-kav from a spine imparts impurity and renders a nazirite obligated to shave. The Talmud 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, דמחמיר, הא לרבנן ⁠— עד דאיכא חצי קב עצמות!

The Talmud 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.