Tractate Kelim - Chapter 25 - Mishnah 4 כלים פרק כ״ה משנה ד׳

If [in a measure consisting of] a quarter [of a log] and half a quarter [of a log] the quarter measure contracted uncleanness, the half-quarter measure does not become unclean, and if the half-quarter contracted uncleanness the quarter does not become unclean. They argued before Rabbi Akiva: since the half quarter measure is the outer part of the quarter measure, should not the outer side of the vessel whose inner side contracted uncleanness become unclean? He answered them: Are you sure that it belongs to the category [of vessels] that have inner [and outer] parts? Perhaps the quarter is to be regarded as the outer side of the half quarter and, surely, the inner side of a vessel does not become unclean if the outer side contracted uncleanness.