The shoulder girdle
Each arm hangs from a shoulder girdle of two bones. The clavicle, the collarbone, is a slender S-shaped strut across the top of the chest. The scapula, the shoulder blade, is a flat triangle on the back of the ribs. Its shallow socket, the glenoid cavity, takes the head of the humerus.
The shoulder girdle from the front: a clavicle and a scapula on each side of the rib cageWikimedia Commons, Mariana Ruiz Villarreal (LadyofHats), Public domain (Pectoral girdle front diagram.svg)
The only joint between the arm's bones and the trunk is the small sternoclavicular joint, where the clavicle meets the sternum. Muscles alone hold the scapula to the trunk, and it glides over the ribs as the arm moves, which widens the arm's reach.
The clavicle braces the shoulder out to the side. A fall on the shoulder or an outstretched hand often breaks it, usually in its middle third. It is also the first bone to start forming in the embryo, in the fifth or sixth week.
The left clavicle from above, its sternal end on the left and its acromial end on the right, Gray's Anatomy, 1918, figure 200Wikimedia Commons, Henry Vandyke Carter, Public domain (Gray200.png)
Sources
- Susan Standring (ed.), Gray's Anatomy, 42nd ed. (2020), chapter on the shoulder girdle and arm.
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