The human upper limb, enabling movements and interactions with the surrounding world, extends from the shoulder down to the fingertips, forming a complex system of bones, joints, and muscles. This intricate design allows for everything from powerful lifting to delicate fine motor skills, which are fundamental to daily activities. Understanding the bones that compose this limb provides insight into its impressive capabilities and versatility.
Bones of the Shoulder
The shoulder region forms the connection between the upper limb and the body’s skeleton. Two primary bones contribute to this connection: the clavicle and the scapula. The clavicle, commonly known as the collarbone, is a slender, S-shaped bone that runs horizontally across the top of the chest. It acts as a strut, holding the shoulder away from the body to allow for a range of arm movement. The clavicle connects the sternum (breastbone) to the scapula, part of the shoulder girdle.
The scapula, or shoulder blade, is a flat, triangular bone located on the back of the rib cage. It provides a broad surface for the attachment of muscles that move the shoulder and arm. The scapula also features a shallow socket, the glenoid cavity, which articulates with the upper arm bone. Together, the clavicle and scapula work to provide stability while allowing mobility at the shoulder joint.
The Upper Arm Bone
The upper arm contains the humerus, a single long bone extending from the shoulder to the elbow, providing skeletal support for the limb segment. The rounded upper end of the humerus fits into the socket of the scapula, forming the mobile shoulder joint.
At its lower end, the humerus broadens and articulates with the two bones of the forearm to form the elbow joint. This articulation allows for bending and straightening movements of the elbow. The humerus serves as an attachment point for muscles, enabling movements such as lifting, reaching, and throwing.
Bones of the Forearm
The forearm, located between the elbow and the wrist, is composed of two bones: the radius and the ulna. These two bones run parallel to each other, playing distinct yet cooperative roles in forearm and hand movements. The ulna is positioned on the medial side, aligning with the pinky finger, and is slightly longer than the radius.
The radius is found on the lateral side, corresponding to the thumb. While the ulna forms the primary part of the elbow joint with the humerus, the radius is the main bone articulating with the wrist bones. The unique arrangement of the radius and ulna allows the radius to rotate around the ulna, a movement that enables the turning of the palm up (supination) or down (pronation).
Bones of the Wrist and Hand
The wrist and hand contain a complex arrangement of small bones, contributing to the dexterity and manipulation. The wrist, or carpus, is made up of eight small carpal bones arranged in two rows. These bones connect the forearm to the hand and provide both flexibility and stability for wrist movements, including bending, extending, and rotating.
Beyond the wrist, the palm of the hand is formed by five metacarpal bones. These long bones connect the carpal bones of the wrist to the fingers, providing the structural framework of the hand. They are numbered one to five, starting from the thumb side, and serve as attachment points for muscles that control finger movement. The metacarpals contribute to grip strength and object manipulation.
Finally, the fingers and thumb are made up of bones called phalanges. Each finger has three phalanges: a proximal, middle, and distal phalanx. The thumb, however, has only two: a proximal and a distal phalanx. These bones allow for the bending and straightening of the digits, providing the fine motor control and precision for grasping, writing, and other activities.