The upper limb, extending from the shoulder down to the fingertips, is an intricate structure of bone that allows for incredible mobility and complex interaction with the environment. This part of the body is fundamental to human function, providing the framework for lifting, carrying, reaching, and manipulating objects. Each upper limb contains approximately 30 bones, organized into four major regions that work together seamlessly. This skeletal arrangement provides both the necessary leverage for strength and the delicate articulation required for dexterity.
The Pectoral Girdle
The foundation of the upper limb is the pectoral girdle, which connects the arm to the main body’s axial skeleton. This girdle consists of two bones: the Scapula, commonly known as the shoulder blade, and the Clavicle, or collarbone. The Scapula is a large, flat, triangular bone located on the upper back that serves as a major attachment point for numerous muscles. It features a shallow depression called the glenoid cavity, which acts as the socket for the shoulder joint.
The Clavicle is a slender, S-shaped bone that extends horizontally across the front of the body just above the first rib. It is the only bony attachment between the upper limb and the sternum (breastbone). This arrangement provides the shoulder with a wide range of motion, as the scapula is held in place by muscles rather than a rigid bone-to-bone connection.
The Upper Arm Bone
The region of the upper arm, extending between the shoulder and the elbow, is supported by a single bone called the Humerus. This long bone is the largest in the upper extremity, providing the main support for the arm. The rounded head of the Humerus fits into the glenoid cavity of the scapula to form the ball-and-socket shoulder joint. This joint configuration allows for extensive rotation and movement in all directions.
The Humerus extends distally to the elbow, where its lower end articulates with the two bones of the forearm. It forms a hinge joint at the elbow, allowing for the bending and straightening of the arm. Beyond its structural role, the Humerus is an attachment point for 13 different muscles responsible for movement and rotation of the shoulder and elbow.
The Forearm Bones
The forearm, situated between the elbow and the wrist, is supported by two parallel bones, the Radius and the Ulna. These bones are connected by a strong sheet of connective tissue called the interosseous membrane, which helps maintain their alignment. The Ulna is located on the medial side of the forearm, aligning with the little finger. Its upper end forms the main part of the elbow’s hinge joint, notably featuring the olecranon, which creates the bony point of the elbow.
The Radius is positioned on the lateral side of the forearm, corresponding to the thumb side. While the Ulna is more involved in the elbow’s hinging motion, the Radius is the primary bone for wrist articulation. The two bones work together to facilitate the unique movement of rotation, known as pronation and supination. The Radius rotates over the fixed Ulna, allowing the palm to turn from facing upward to facing downward, which is essential for tasks like turning a doorknob.
The Complex Structure of the Hand and Wrist
The distal portion of the upper limb, consisting of the wrist and hand, is a masterpiece of complex articulation that allows for fine motor control and grasping. This region contains 27 bones, divided into three distinct groups that contribute to the hand’s dexterity. This large number of small, interconnected bones provides the flexibility necessary for intricate movements.
The wrist itself is formed by eight irregularly shaped Carpal bones, which are arranged in two rows of four. These small bones connect the forearm bones to the hand and provide a flexible base for hand movement. Just beyond the wrist are the five Metacarpal bones, which form the skeletal structure of the palm. They are numbered one through five, starting with the thumb, and their heads form the knuckles.
The fingers and thumb are composed of the Phalanges, which number 14 in total. Each finger has three phalanges—proximal, middle, and distal—while the thumb has only two, a proximal and a distal phalanx.