The shoulder is made up of three bones: the scapula (shoulder blade), the humerus (upper arm bone), and the clavicle (collarbone). Muscles, tendons, and ligaments hold it together. The outer end of the collarbone is connected to the end (acromion) of the shoulder blade. The bursa is a fluid-filled sac that cushions and lubricates the shoulder area. The rotator cuff is the group of tendons and their related muscles that help keep the upper arm bone seated in the socket of the shoulder blade.