The pectoral girdle, also known as the shoulder girdle, connects the upper limbs to the axial skeleton. Located in the shoulder region, it serves as a foundational support, allowing for the broad range of motion characteristic of the human arm, enabling various activities.
The Clavicle and Scapula
The pectoral girdle consists of two primary bones: the clavicle (collarbone) and the scapula (shoulder blade). The clavicle is a slender, S-shaped bone that extends horizontally across the front of the shoulder, uniquely being the only long bone in the body to lie horizontally. It connects the upper limb to the trunk and transmits forces from the arm to the axial skeleton.
The scapula is a flat, triangular bone positioned on the posterior side of the shoulder, typically resting over the ribs between the second and seventh ribs. It features a prominent ridge on its posterior surface, the spine, which extends laterally to form the acromion, the bony tip of the shoulder. The scapula also has a shallow depression, the glenoid cavity, which articulates with the upper arm bone. Both the clavicle and scapula provide attachment sites for numerous muscles.
Function of the Pectoral Girdle
The pectoral girdle provides both stability and mobility for the upper limb. It creates a flexible base, enabling a wide range of actions. This allows for extensive movement of the shoulder and arm, essential for daily tasks.
It also provides attachment points for many muscles involved in movements of the arm, shoulder, and trunk. The scapula, for instance, is connected to 17 different muscles, contributing to its diverse range of motion. Furthermore, the pectoral girdle offers some protection to underlying nerves and blood vessels that pass between the body’s trunk and the upper limbs.
Connecting the Upper Limb to the Body
The pectoral girdle uniquely connects the upper limb to the axial skeleton primarily through one bony articulation: the sternoclavicular joint. This joint forms where the medial end of the clavicle meets the manubrium, the upper part of the sternum or breastbone. This articulation is the only true bony connection between the entire pectoral girdle and the central skeletal framework of the body.
The scapula, however, does not have a direct bony connection to the axial skeleton, such as the vertebral column or ribs. Instead, its stability and movement depend significantly on a complex network of muscles and ligaments that hold it against the posterior thoracic cage. This muscular attachment, sometimes referred to as the scapulothoracic joint, allows for remarkable mobility and flexibility of the shoulder, distinguishing it from more rigid skeletal connections found elsewhere in the body. The scapula also articulates with the humerus, the upper arm bone, at the glenohumeral joint, forming the main shoulder joint.