sheath

A sheath is a protective covering or case for something, like a blade or a body part. In anatomy, the term can refer to several different structures.123

• Myelin Sheath: This is a fatty, insulating layer that surrounds the axons of many neurons. It's made by Schwann cells or oligodendrocytes and helps speed up nerve signal transmission.4
• Synovial Sheaths: These are membranes that protect tendons, particularly where they pass over bony areas in the hands and feet. They contain synovial cells that produce synovial fluid, which lubricates the tendons.5
• Periosteum: While not typically called a sheath, this dense fibrous membrane covers the surfaces of bones, consisting of an outer fibrous layer and an inner cellular layer. It contains blood vessels and osteoblasts (bone-producing cells) and plays a role in bone repair and remodeling.6