Shoulder: A structure made up of two main bones: the scapula (shoulder blade) and the humerus (the long bone of the upper arm). The end of the scapula, called the glenoid, is a socket into which the head of the humerus fits, forming a flexible ball-and-socket joint. The scapula is an unusually shaped bone. It extends up and around the shoulder joint at the rear to create a roof called the acromion and around the shoulder joint at the front to constitute the coracoid process. The shoulder joint is cushioned by cartilage that covers the face of the glenoid socket and the head of the humerus. The joint is stabilized by a ring of fibrous cartilage around the glenoid socket that is called the labrum. Ligaments connect the bones of the shoulder and tendons join these bones to surrounding muscles. The biceps tendon attaches the biceps muscle to the shoulder and helps stabilize the joint. Four short muscles that originate on the scapula pass around the shoulder where their tendons fuse together to form the rotator cuff.