The shoulder region is a remarkably mobile part of the body consisting of four articulations with their associated muscles. Three of these articulations are actually true synovial joints. The fourth, the "scapulothoracic" joint, is not an articulation at all, but a mechanism allowing the scapula to glide across the thoracic cage. Many of the muscles of the shoulder region act directly on the scapula while others will be more involved in movement of the humerus.