The human arm extends from the shoulder down to the fingertips. This complex structure is not a single bone, but rather a sophisticated arrangement of several bones that work together seamlessly.
Bones of the Upper Arm and Forearm
The upper arm contains a single bone, the humerus. This long bone extends from the shoulder to the elbow. The humerus has a rounded upper end that fits into the shoulder socket and a flatter lower end that contributes to the elbow joint.
Below the humerus, the forearm is composed of two bones: the radius and the ulna. The ulna is positioned on the medial side of the forearm, aligning with the pinky finger, and is slightly longer than the radius. Its proximal end forms a significant part of the elbow joint.
The radius is located on the lateral, or thumb, side of the forearm. It is shorter than the ulna and features a broad distal end that connects with the wrist bones. The radius and ulna lie parallel to each other, but they can pivot, allowing for rotational movements of the forearm and hand.
Bones of the Wrist and Hand
Distal to the forearm bones, the wrist and hand collectively contain 27 bones. The wrist, known as the carpus, is formed by eight small carpal bones arranged into two rows. The proximal row, closer to the forearm, includes the scaphoid, lunate, triquetrum, and pisiform bones. The distal row consists of the trapezium, trapezoid, capitate, and hamate bones.
Extending from the carpal bones are the five metacarpal bones, forming the palm. These are long bones, numbered one to five starting from the thumb side, and they act as a bridge between the wrist and the fingers.
Finally, the fingers and thumb are made up of phalanges. Each finger typically has three phalanges—a proximal, middle, and distal phalanx—while the thumb has two: a proximal and a distal phalanx. These bones allow for the flexion, extension, and fine motor movements.
Connecting the Arm Bones
The various bones of the arm, wrist, and hand articulate at specific joints, enabling the wide range of motion characteristic of the human upper limb. The shoulder joint is a ball-and-socket joint where the rounded head of the humerus fits into a shallow socket on the shoulder blade (scapula). This design allows for extensive movement, including rotation and raising the arm in various directions.
Moving down the arm, the elbow joint connects the humerus with both the radius and ulna. This joint functions primarily as a hinge, allowing for bending and straightening of the arm. Additionally, the unique articulation between the radius and ulna at the elbow and wrist allows the forearm to pronate and supinate, which are the movements of turning the palm downward and upward, respectively.
At the wrist, the distal ends of the radius and ulna articulate with the carpal bones. Specifically, the radius connects directly with some of the proximal carpal bones, forming the main wrist joint. While the ulna is prevented from directly articulating with the carpal bones by a fibrous disc, it articulates with the radius just above the wrist. This complex joint, along with the numerous small joints within the hand, contributes significantly to the wrist’s flexibility and the hand’s dexterity, allowing for precise actions like gripping and fine manipulation.