The scapula and shoulder girdle

1 minute read

The bones that make up the shoulder are the scapula, clavicle and humerus.

The humerus is the upper arm bone, with a ball-shaped head at the proximal end. The scapula is a flat, triangular bone in humans. The most prominent parts of the scapula are at its lateralmost angle where it articulates with the humerus. Here, the bone bears a shallow, bean-shaped depression called the glenoid fossa. Two projections, the acromial and coracoid processes, extend beyond the glenoid fossa providing attachments for some of the muscles and ligaments of the shoulder and upper arm. The clavicle articulates with the acromial process and extends toward the midline of the torso, with its medial end articulating with the superior part of the sternum.

Gray's anatomy scapula figure

What to do: Examine the scapulae of different kinds of primates. You’ll find that primates with different locomotor patterns have rather different scapula morphology.

Monkeys and prosimians that are mainly quadrupeds have relatively long and narrow scapulae. Their shoulders are adapted for forelimb movement anteriorly and posteriorly, but not especially to the side or above the head.

By contrast, apes and humans have scapulae that are very triangular in shape. The shoulder joint is more mobile in these primates, with the arm able to move freely to the side and above the head.

The mobility of the scapula is also related to the shape of the trunk. Monkeys have a deep trunk that is relatively narrow from side to side, while apes and humans have a shallower trunk that is wider from side to side.