The common understanding of the “arm” often refers to the entire upper limb, extending from the shoulder to the fingertips. Anatomically, the arm (brachium) is only the segment between the shoulder and the elbow. The entire structure is composed of 30 bones in each limb, including those of the shoulder, forearm, and hand, which work together to provide range of motion and dexterity.
The Shoulder Girdle and Upper Arm
The arm’s skeleton begins with the shoulder girdle, which connects the limb to the trunk of the body. This girdle is formed by two bones: the clavicle (collarbone) and the scapula (shoulder blade). The scapula features a shallow socket called the glenoid fossa, which serves as the direct connection point for the single bone of the upper arm, the humerus.
The humerus is a long bone that runs from the shoulder down to the elbow, defining the anatomical arm. Its proximal end is marked by the smooth, rounded humeral head, which fits into the scapula’s glenoid fossa to form the ball-and-socket shoulder joint. Just below the head are two bony projections, the greater and lesser tubercles, which function as attachment sites for the rotator cuff muscles that help stabilize the joint.
Further down the humerus is the surgical neck, a constricted area below the tubercles that is a frequent site for fractures. The long shaft of the humerus also contains the deltoid tuberosity, a roughened, V-shaped region where the large deltoid muscle attaches. The distal end of the humerus flares out to form the top of the elbow joint, where it meets the two bones of the forearm.
The distal humerus features two distinct surfaces for articulation. The trochlea, a pulley-shaped structure, connects with the ulna, while the capitulum, a rounded projection, articulates with the radius. These surfaces, along with three depressions known as fossae, accommodate the forearm bones during elbow flexion and extension.
The Forearm Bones
The section of the limb between the elbow and the wrist, known as the forearm, contains two long bones: the ulna and the radius. These two bones lie parallel to one another and are connected by a dense sheet of connective tissue called the interosseous membrane. The ulna is located on the medial side of the forearm, aligning with the pinky finger, and is slightly longer than its counterpart.
The ulna’s proximal end is characterized by the large, C-shaped trochlear notch, which hooks around the humerus’s trochlea to form the main hinge of the elbow joint. The prominent bony point of the elbow that can be easily felt is the olecranon process, a feature of the ulna.
The radius runs along the lateral side of the forearm, situated on the same side as the thumb. Its proximal end features a disc-shaped head that articulates with the humerus’s capitulum. Unlike the ulna, the radius is designed for rotation, a movement that is essential for positioning the hand.
The radius and ulna connect at both the proximal and distal ends, forming the radioulnar joints. The radius rotates over the ulna to perform pronation and supination, allowing the palm to turn downward and upward, respectively. During pronation, the distal end of the radius crosses over the ulna, a movement crucial for tasks like turning a doorknob.
The Wrist and Hand Skeleton
The remaining bones of the upper limb are found in the wrist and hand, containing 27 individual bones. These bones are organized into three distinct groups: the carpals, the metacarpals, and the phalanges.
The wrist (carpus) consists of eight small, irregularly shaped carpal bones clustered tightly together. These bones are arranged into two rows, the proximal row and the distal row, which move together to facilitate wrist motion. The radius primarily connects with the proximal row of carpals, specifically the scaphoid and lunate bones, to form the radiocarpal joint.
Distal to the carpal bones are the five metacarpals, which form the skeletal structure of the palm of the hand. Each metacarpal bone corresponds to one digit, connecting the wrist to the fingers.
The fingers and thumb are composed of the final group of bones, called the phalanges. There are 14 phalanges in total, distributed across the five digits. Each of the four fingers contains three phalanges—a proximal, a middle, and a distal phalanx. The thumb, however, contains only two phalanges, a proximal and a distal one.