Reference for Chullin 252:14

ת"ל

whatever can be touched is unclean<span class="x" onmousemove="('comment',' If the flesh, or, as in this case, the marrow, that is inside can be touched from the outside, then the outer covering serves as a protection to what is inside, and as such conveys the uncleanness.');">⁷</span>

Jastrow