Synovial bursa

A bursa, or synovial bursa, is a thin, fluid-filled sac that sits between a bone and whatever is next to it: another bone, a tendon, a muscle or the skin. It sits at the points where two surfaces rub together.

Inside, it is lined by a synovial membrane, the same kind of tissue that lines the joints, and that membrane produces synovial fluid, which has about the consistency of raw egg white. That fluid forms a very thin film over the inner surface of the sac and works as a lubricant: it reduces friction and lets movement happen without resistance. A slippery cushion at the spot where two parts rub.

Bursae are classified by where they are. They are called superficial when they lie between bones, tendons or skin, and deep when they lie between bones and muscles. When one of them becomes inflamed, that has its own name: bursitis.