shoulder

In the human body, the region where the arm joins the trunk.

The articulation of the humerus (arm-bone) with the scapula (shoulder-blade) is an example of a ball-and-socket joint. The scapula is a flat, triangular bone with a depression called the glenoid cavity into which the head of the humerus fits. The shallowness of the cavity allows great mobility of the shoulder joint, but makes it liable to dislocation. Some protection is given by the glenoid ligament, which forms a sort of lip about the cavity, increasing its depth, and the acromion, a process of the scapula which forms a protective arch overhanging the shoulder socket.