The shoulder girdle is a group of four bones that supports the muscles and ligaments of the shoulders. Also frequently called the "pectoral girdle," this bone group is comprised of two clavicles and two scapulae, one set for each shoulder. The shoulder girdle is important because it is the point at which the bones of the arms attach to the bones of the body, or trunk.
The space created by the junction of the clavicle and the scapula allows the ball of the humerus—upper arm bone—to rotate. The shoulder joint allows for more variety of movement than any other joint in the body. The particular arrangement of the bones of the shoulder girdle give the upper arm the space needed to move in so many different directions.
Although the word "girdle" usually implies a complete ring, the shoulder girdle is an incomplete ring because a gap separates the scapulae in the back, and the sternum separates the clavicles in the front. Some physicians include the humerus as part of the shoulder girdle because it attaches to the scapula and clavicle, forming the shoulder joint. Traditional anatomy texts, however, include only the scapulae and clavicles.
Within the shoulder girdle, the clavicle's main purpose is to hold the arm away from the body. This gives the tendons, ligaments and muscles more room to work and allows a person to move his arms more freely. Without the clavicles, the arms would simply hang alongside the body, limiting their movement. The clavicle is said to be the most frequently broken bone in the body. Often called the "collar bone," these bones may be visible at the base of the neck in some people.
The shoulder girdles' scapulae are large, flat bones that are often called the "shoulder blades." These bones serve as the insertion point for the muscles of the shoulder, including the pectoralis minor, the trapezius and the rhomboid minor and major. Roughly triangular in shape, the scapulae are located on the back of the ribcage, usually covering a portion of the second through the seventh ribs.
The glenoid fossa is a depression in the scapula in which the ball of the humerus actually rotates. This joint is called the glenohumeral joint. The shoulder girdle also contains the acromioclavicular joint, which has limited movement but transmits force from the arm to the torso and the sternoclavicular joint, which assists in movements such as throwing and thrusting the arm forward.