What sheath is a large fibrous structure that looks like a tree trunk anchored in place by muscle fibers?

The sheath you are referring to is likely the epimysium, which is a dense layer of connective tissue that surrounds entire muscles. It provides structural support and protection while anchoring muscle fibers in place. The epimysium helps transmit the force generated by muscle contractions to the tendons and bones. Its tree trunk-like appearance comes from its thick, fibrous composition.