The pectoral girdle, or shoulder girdle, is the bony ring that connects the upper limb to the axial skeleton (the spine, ribs, and skull). This arrangement is highly specialized, allowing for the extensive range of motion that characterizes the human shoulder. The girdle provides a stable base from which the upper extremity can move with great freedom. This complex of bones and joints allows for greater mobility than any other girdle in the body.
The Primary Components: Clavicle and Scapula
The structure of the pectoral girdle is composed of two bones on each side of the body: the clavicle and the scapula. These bones work together to hold the arm in position and provide anchor points for the muscles that move the limb.
The clavicle, or collarbone, is a slender, S-shaped bone that lies horizontally across the upper anterior chest. It functions as a strut, holding the shoulder away from the chest wall to allow the arm a wider arc of movement. This bony bridge transfers mechanical forces from the upper limb to the trunk of the body.
The scapula, or shoulder blade, is a large, flat bone shaped roughly like a triangle that lies behind the rib cage. It rests on the posterior aspect of the thoracic cage, held in place almost entirely by musculature rather than direct bone articulation. Its broad, flat surface provides an extensive area for the attachment of muscles that move the shoulder and arm.
Structural Landmarks of the Scapula
The scapula contains several bony features specialized for articulation and muscle attachment.
Spine and Acromion
A prominent ridge, called the spine of the scapula, runs horizontally across the bone’s posterior surface, dividing the back of the scapula into two depressions. The spine continues laterally to form the acromion, a flattened projection. The acromion forms the highest point of the shoulder and is the site where the clavicle connects to the scapula, providing a protective bony arch over the shoulder joint.
Coracoid Process
The coracoid process is a hook-like projection that extends anteriorly from the scapula, lying underneath the clavicle. This process serves as an attachment site for several muscles and various ligaments that stabilize the shoulder.
Glenoid Cavity
The glenoid cavity is a shallow, concave depression located on the lateral angle of the scapula. This cavity is the socket portion of the shoulder joint, designed to receive the head of the humerus (the upper arm bone). The shallow nature of this socket permits the wide range of motion of the arm.
The Girdle’s Articulations: Connecting to the Body
The mobility of the pectoral girdle is achieved through specialized joints that connect its components to one another and to the trunk.
Sternoclavicular (SC) Joint
The Sternoclavicular (SC) joint is the sole point of bony attachment between the entire upper limb and the axial skeleton. This joint is formed by the medial end of the clavicle and the manubrium (the upper part of the sternum). The SC joint is highly mobile, allowing the clavicle to move in multiple planes, which is crucial for positioning the scapula during arm movements.
Acromioclavicular (AC) Joint
The Acromioclavicular (AC) joint connects the two girdle bones, forming where the lateral end of the clavicle meets the acromion of the scapula. This joint permits subtle gliding and rotational movements of the scapula relative to the clavicle.
Glenohumeral Joint
Although not technically part of the girdle, the Glenohumeral joint is the articulation between the scapula and the arm. This ball-and-socket joint is formed by the head of the humerus fitting into the scapula’s glenoid cavity, allowing for the large sweeping motions of the arm.