The shoulder is an amazing complex of bones, ligaments, muscles, and tendons. The bones of the shoulder include the scapula, or "shoulder blade," clavicle, or "collarbone," and the humerus, or "arm bone."  

The scapula is an anchoring bone that forms the foundation and attachment point for the muscles of the rotator cuff. The scapula widens at the shoulder joint to form the “socket” of the shoulder. This socket is called the glenoid. The glenoid surface is shallow and small, creating more of a “saucer” than a socket.

The rotator cuff muscles coalesce to form the tendons, which surround the ball of the humerus. These tendons essentially form a “mobile socket” that constantly contracts to keep the ball of the humeral head centered in the glenoid with movement of the shoulder.    

This structure is unique to the shoulder and is the reason why the shoulder has the most range of motion of any joint in the human body. Lifting the arm requires rotation of the humeral head on the glenoid, which is accomplished by contraction of the rotator cuff muscles. At the same time, the scapula rotates along the chest wall and strong supporting muscles of the shoulder add power to the motion. To keep the shoulder stable through this motion, there is an inner lining, or capsule, that attaches the humeral head to the glenoid. This lining is normally loose when the shoulder is at rest, but tightens when the arm is taken to the extremes of motion.

The anatomy of the shoulder is essential to provide its amazing range of motion. Without its complex structure, simple motion, such as reaching for something high on a shelf or reaching behind the back, would not be possible. The shoulder positions the arm in space, which allows for everyday activities, such as writing or eating. Along with the fine, precise motions, the shoulder muscles also allow complicated tasks, such as a tennis serve or swinging a golf club.