The upper limb extends from the shoulder girdle down to the tips of the fingers. The complete bony architecture of a single adult human upper limb comprises a total of 32 distinct bones. This count is divided among four primary regions: the pectoral girdle, the upper arm, the forearm, and the wrist and hand.
The Pectoral Girdle and Upper Arm
The uppermost segment of the limb begins with the pectoral girdle, which serves as the anchor connecting the entire structure to the axial skeleton of the torso. This girdle is formed by two bones: the scapula (shoulder blade) and the clavicle (collarbone). The scapula provides the socket for the arm bone, while the S-shaped clavicle braces the shoulder away from the chest.
Distal to this anchor is the upper arm, which contains only a single bone, the humerus. This long bone is the largest in the upper limb, with its rounded head fitting into the glenoid cavity of the scapula to form the highly mobile shoulder joint. The humerus extends down to the elbow, where it articulates with the two bones of the forearm.
The Forearm
The forearm, located between the elbow and the wrist, is composed of two parallel bones: the ulna and the radius. The ulna is situated on the medial side, aligning with the little finger, and is primarily responsible for forming the hinge joint of the elbow with the humerus. The radius is located on the lateral, or thumb, side of the forearm.
These two bones are connected by a dense sheet of connective tissue called the interosseous membrane. Their arrangement permits a sophisticated range of motion, particularly the twisting movements known as pronation and supination. During these actions, the radius crosses over the ulna, which allows the hand to rotate from a palm-up to a palm-down position.
The Wrist and Hand
The wrist and hand account for the remaining 27 bones of the upper limb. The wrist, or carpus, is formed by eight small, irregularly shaped carpal bones, arranged into two distinct rows of four. The proximal row articulates with the forearm bones, while the distal row connects to the bones of the palm.
The carpal bones are arranged into two rows. Beyond the wrist, the palm contains five metacarpal bones, each connecting a carpal bone to a digit.
- Scaphoid, lunate, triquetrum, and pisiform (proximal row).
- Trapezium, trapezoid, capitate, and hamate (distal row).
The final 14 bones are the phalanges, which make up the fingers and the thumb. Each finger, from the index to the little finger, contains three phalanges: a proximal, a middle, and a distal phalanx. The thumb, however, is unique in possessing only two phalanges, a proximal and a distal one, giving it greater mobility and opposition.