What structure reduces friction between skin and muscles while moving over bones?

The correct answer is the bursa. A bursa is a small, fluid-filled sac that acts as a cushion between bones and tendons or muscles around the joints. Its primary function is to reduce friction during movement. When muscles and skin move over bones, the presence of a bursa provides a slippery surface that allows for smoother motion, thereby minimizing wear and tear on the tissues involved.

In contrast, other structures serve different purposes. The synovial membrane lines the joint capsule and secretes synovial fluid, which lubricates the joint and nourishes the cartilage, but it does not specifically reduce friction between skin and muscles. Articular cartilage covers the surfaces of bones at a joint to absorb shock and provide a smooth surface for joint movement, but it is not involved in reducing friction in the way that a bursa does. Lastly, a tendon sheath encloses a tendon, providing a smooth surface for the tendon to glide over. While it can help reduce friction, it specifically pertains to tendons rather than the interaction between skin and muscle over bone.