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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 bicepsbiceps
, any muscle having two heads, or fixed ends of attachment, notably the biceps brachii at the front of the upper arm and the biceps femoris in the thigh. Originating in the shoulder area, the heads of the biceps merge partway down the arm to form a rounded mass of tissue
..... Click the link for more information. brachii, a muscle of the upper arm, bends the arm at the elbow; the tricepstriceps,
any muscle having three heads, or points of attachment, but especially the triceps brachii at the back of the upper arm. One head originates on the shoulder blade and two on the upper-arm bone, or humerus.
..... Click the link for more information. 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.
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.