The human arm, extending from the shoulder to the fingertips, is a structure built for movement and strength. This upper limb is constructed from a chain of individual bones that work together seamlessly. This skeletal framework provides the necessary support for daily activities, ranging from powerful lifting to delicate fine motor control. The organization and specific shapes of these bones allow for an extensive range of motion.
The Shoulder Connection
The arm connects to the main body structure via the pectoral girdle, which is formed by two bones. The scapula, or shoulder blade, is a large, triangular bone located on the upper back. It serves as the socket component of the shoulder joint, providing a shallow depression for the upper arm bone to articulate.
The clavicle, or collarbone, is a slender, S-shaped bone that runs horizontally across the top of the chest. This bone acts as a strut, holding the scapula away from the chest wall to maximize the arm’s freedom of movement. Together, the scapula and clavicle create a flexible foundation that contributes to the arm’s overall mobility.
The Upper Arm Bone
The humerus is the bone of the upper arm, running from the shoulder to the elbow. It is the longest bone in the upper limb and serves as an anchor point for 13 muscles that control the shoulder and elbow. The rounded upper end of the humerus fits into the scapula’s socket, forming the mobile ball-and-socket shoulder joint.
The lower end of the humerus forms the elbow joint where it meets the two bones of the forearm. This end includes specific ridges and condyles that articulate with the lower bones, allowing for hinge-like flexion and extension. The strength and length of the humerus are fundamental to transferring force from the torso and shoulder to the rest of the arm.
The Forearm Bones
The forearm, the section between the elbow and the wrist, contains two parallel bones: the ulna and the radius. The ulna is positioned on the medial side of the forearm, aligning with the pinky finger, and is the primary bone that forms the hinge of the elbow joint. Its prominent upper end, the olecranon, creates the bony tip of the elbow and prevents hyperextension of the arm.
The radius is located on the lateral, or thumb, side of the forearm and is the shorter of the two bones. The unique structure of the radius allows it to pivot or rotate over the ulna, a movement known as pronation and supination. This rotation permits the palm to be turned up (supination) or down (pronation), which is essential for tasks like turning a doorknob or using a screwdriver.
The Wrist and Hand Structure
The distal structures of the arm begin with the wrist, or carpus, which contains eight small, irregularly shaped carpal bones. These bones are arranged in two rows of four and are responsible for the complex gliding movements that give the wrist its flexibility. The radius and the ulna articulate with the carpal bones to create the radiocarpal joint, allowing for wrist movement.
Beyond the wrist are the five metacarpal bones, which form the palm of the hand. These bones are numbered one through five, starting with the thumb, and their subtle mobility allows the hand to cup and grasp objects.
The fingers and thumb are composed of the phalanges, which are the smallest bones of the arm structure. There are 14 phalanges in total, with each finger having three, and the thumb having two, providing the dexterity for fine motor skills.