TT #232 · Parshas Shemini · Aliyah 7 · Passuk 39
Chabad.org — Vayikra Chapter 11 with Rashi
English: “Who touches its carcass”: the impurity is in the carcass itself, not in a sealed bone cavity whose contents cannot be touched.
Chullin 126a.
E-Daf.com — Chullin 126a (Tsurras Hadaf)
Textual limitation — The Chazal derives the stated rule from the wording and comparison in the verse.
A sealed bone cavity is excluded unless its contents are accessible.