Bone · Shoulders

Humerus

Humerus

The humerus is the single long bone of the upper arm. Its rounded head sits against the shallow socket of the shoulder blade — a loose fit that buys enormous range and pays for it in stability — while its lower end spreads into the hinge of the elbow. It is the central lever of the arm: the rotator cuff grips the two tubercles at its top, the deltoid pulls on a roughened patch halfway down the outside, and the flexor and extensor muscles of the whole forearm hang off the two knuckles at its bottom end. The radial nerve spirals around the back of its shaft in a groove of its own.