The human arm, like much of the rest of the body, is composed of skin, fat, muscle, connective tissue and bone. The upper arm contains one bone, the humerus, which is joined at the elbow joint with the two bones of the forearm, the radius and ulna. These bones are joined to the hand at the wrist, which is a base for the carpals, metacarpals and phalanges, or fingers.
The bones of the arm are layered with large skeletal muscles, such as the biceps and triceps, that connect to the bone via tough, fibrous tendons. These muscles are connected to the central nervous system by a network of sensory and motor nerves that run from the muscle or skin to the spinal cord.
Parts of the arm, mainly the upper arm, are insulated with a layer of subcutaneous fat. Over this layer, the skin forms a protective outer layer of the arm and hand. At the ends of the fingers, the skin excretes small sheets of a protein called keratin from beds to form fingernails. The entire arm is generously supplied with blood through the brachial artery, one of the largest arteries in the body, and drained by a network of large veins.