arm, upper limb in humans. Three long bones form the framework of the arm: the humerus of the upper arm, and the radius (outer bone) and ulna (inner bone) of the forearm. The radius and ulna run parallel but meet at their ends in such a manner that the radius can rotate around the ulna. This arrangement permits turning the forearm to bring the hand palm up (supination) or palm down (pronation). The radius and ulna hinge with the bones of the hand at the wrist, and with the humerus at the elbow. The biceps brachii, a muscle of the upper arm, bends the arm at the elbow; the triceps brachii straightens the arm. Movement of the arm across the chest and above the head is accomplished by the pectoral muscles of the chest and deltoid muscles of the shoulder, respectively. In an adult the arm is normally five sixths as long as the leg.

Upper limb of a biped, particularly a primate. Primate arms have one long bone, the humerus, in the upper arm above the elbow, and two thinner bones, the radius and ulna, in the forearm. The triceps muscle straightens the forearm at the elbow joint; the brachialis and biceps muscles bend it. Forearm and small muscles in the hand move the hand and fingers. The term may also denote the limb or the locomotive or prehensile organ of an invertebrate (e.g., the ray of a starfish or the tentacle of an octopus).

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