Shoulder

Shoulder, term for the area of the body between the trunk and the arm, including bones, joints, and the adjacent tissue. There are 2 bones in the shoulder: the clavicle (collarbone), which is the horizontal bone that connects to the breastbone, and the scapula (shoulderblade), the flat triangular bone of the back. The upper arm bone (humerus) fits into the socket formed by the scapula. There are several muscles in the shoulder, primarily the trapezius, which helps turn the scapula, and the deltoid, which moves the upper arm. In primates the shoulder protrudes; in other mammals, it slopes.