The pectoral girdle, often called the shoulder girdle, is the set of bones that connects the upper limb to the axial skeleton. This bony structure acts as a supportive base for the arm, allowing it to move with a remarkable degree of freedom. Each side of the body possesses its own girdle, and these two halves are not joined posteriorly, which permits independent movement. The girdle supports the shoulder and serves as the attachment point for the muscles that move the arm.
The Bones of the Pectoral Girdle
The pectoral girdle on each side is formed by two bones: the clavicle (collarbone) and the scapula (shoulder blade). The clavicle is an S-shaped bone that extends horizontally across the anterior body. It functions like a strut, bracing the shoulder and preventing it from collapsing inward. The clavicle is the only bone providing a direct bony connection between the upper limb and the axial skeleton.
The scapula is a large, flat, triangular bone positioned on the posterior rib cage. Its structure includes several projections and depressions critical for shoulder function. The acromion is a bony extension that forms the highest point of the shoulder and articulates with the clavicle. The coracoid process extends anteriorly and serves as an attachment point for various muscles and ligaments.
The shallow depression on the lateral side of the scapula is the glenoid fossa, which acts as the socket for the head of the humerus (upper arm bone). This shallow socket is a major factor in the shoulder’s mobility. The scapula lies over the ribs, spanning the area between the second and seventh ribs.
Connecting the Girdle to the Body
The attachment of the pectoral girdle to the axial skeleton relies on two main joints. The sternoclavicular (SC) joint is the sole bony articulation between the upper limb and the trunk. This joint forms where the medial end of the clavicle meets the manubrium (upper part of the sternum). The SC joint is reinforced by strong ligaments and provides the single point of skeletal support for the entire shoulder complex.
The two bones of the girdle are connected by the acromioclavicular (AC) joint. This joint is located laterally, where the clavicle articulates with the acromion process of the scapula. The AC joint helps keep the scapula in its proper position relative to the clavicle, contributing to shoulder stability. Although both the SC and AC joints permit movement, their primary role is to structurally hold the bones in place.
Mobility and Functional Role
The architecture of the pectoral girdle maximizes the range of motion for the upper limb. The shallow glenoid fossa allows for extensive movement in multiple directions, essential for activities like throwing or reaching overhead. This high degree of mobility is a functional trade-off, making the shoulder complex less stable than the pelvic girdle, which prioritizes strength and weight-bearing.
The girdle bones and numerous ligaments serve as attachment sites for muscles. These muscles move the arm and control the positioning of the scapula, which is crucial for coordinating smooth arm movements. The movement of the scapula across the posterior thoracic wall, known as the scapulothoracic joint, significantly increases the arm’s total reach. The clavicle also protects underlying blood vessels and nerves passing toward the upper limb.