Arm

the upper extremity in man consisting of the shoulder, forearm, and hand (carpus, metacarpus, and phalanges of the fingers). The arm is a more developed grasping extremity in man than in man’s ancient ancestors, the Anthropomorphidae.

The transformation from Anthropomorphidae to man was largely promoted by freeing the anterior extremities, or arms, from locomotion and body-support functions and converting them into organs capable of performing work operations. As the arm became adapted to work, its structure substantially changed, becoming sharply distinct from the structure of the anterior extremity of Anthropomorphidae.

The most significant structural changes occurred in the hand. In Anthropomorphidae the hand has an underdeveloped thumb and the remaining fingers are greatly elongated; in contrast, the human hand is characterized by a powerfully developed thumb that is essential in performing all work operations. The remaining fingers of the human hand are significantly shorter than those of Anthropoidea but are nevertheless capable of the most delicate and differentiated movements.

In man’s development, the development of the arm as a work organ occurred simultaneously with the progressive development of the brain.

The body processes in brachiopods, the tentacles in cephalopods, and the mobile or nonmobile rays of echinoderms are sometimes called arms.