The pectoral girdle, often referred to as the shoulder girdle, forms the bony framework that links the upper limbs to the body’s central axial skeleton. This structure provides support for the arms and facilitates a wide range of movements. The pectoral girdle consists of two bones on each side of the body: the clavicle (collarbone) and the scapula (shoulder blade).
The Bones of the Pectoral Girdle
The clavicle is an S-shaped bone positioned horizontally across the front of the shoulder. It acts as a strut, extending from the sternum (breastbone) to the shoulder blade, helping to maintain the shoulder’s position.
The scapula is a flat, triangular bone located on the posterior aspect of the rib cage. Its shape allows it to glide over the ribs, contributing to shoulder mobility. Key features of the scapula include the acromion, which forms the bony tip of the shoulder, and the glenoid cavity, a shallow depression that serves as a socket for the upper arm bone.
Primary Roles of the Pectoral Girdle
This connection allows for the transfer of forces from the arm to the rest of the body. The girdle’s structure prioritizes extensive mobility, enabling the arm to move through a wide range of motion.
Numerous muscles attach to the clavicle and scapula, facilitating movements of the arm, shoulder, and even the neck and trunk. These muscles enable actions such as lifting, pushing, and pulling. The pectoral girdle also offers some protection for underlying blood vessels and nerves that pass between the trunk and the upper limbs.
How the Pectoral Girdle Connects and Moves
The bones of the pectoral girdle articulate through several joints, allowing for complex movements. The only direct bony attachment of the upper limb to the axial skeleton occurs at the sternoclavicular joint, where the clavicle connects to the sternum. This joint is highly mobile, enabling the clavicle and scapula to move in various directions.
The clavicle also articulates with the acromion process of the scapula at the acromioclavicular joint. This joint allows for slight gliding movements and contributes to the overall rotation of the scapula. The scapula itself does not have a direct bony connection to the axial skeleton, but instead “floats” on the back of the rib cage, supported by muscles and its connection to the clavicle.
This unique arrangement allows the scapula to glide and rotate against the rib cage, significantly expanding the range of motion of the arm. The glenoid cavity of the scapula forms the socket for the humerus (upper arm bone), creating the glenohumeral joint, often called the shoulder joint. While the pectoral girdle supports this joint, its mobility is largely due to the coordinated movement of the scapula and clavicle.