The Short Head of the Biceps Brachii

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The short head of the biceps brachii is the shorter and medial of the two bodies that form the biceps brachii muscle in the upper arm. Like the long head of the biceps brachii, the short head is a flexor and supinator of the elbow joint. At the shoulder joint, the short head aids in adduction of the humerus.

Anatomy

The biceps brachii muscle gets its name from its two origins, or immovable ends. The long head arises from the supraglenoid tubercle of the scapula, while the short head arises from the coracoid process of the scapula. From its origin, the short head passes anterior to the head and shaft of the humerus and fuses with the long head around the middle of the humerus. The fused biceps brachii muscle crosses the elbow joint along its anterior surface and inserts on the radius at the radial tuberosity.

Histology

The biceps brachii is a skeletal muscle, and as such is an organ made mostly of skeletal muscle and connective tissues. Skeletal muscle tissue is made of many elongated cells known as fibers; each fiber is wrapped in a thin fibrous connective tissue sheath known as endomysium. Many fibers are further bundled into groups known as fascicles, which are in turn wrapped with more fibrous connective tissue known as perimysium. Many blood vessels and nerves pass between the fascicles to provide blood flow and communication to the skeletal muscle fibers. The fascicles, nerves, and blood vessels are bundled yet again to form the whole biceps muscle, wrapped in an outer layer of fibrous connective tissue known as epimysium. All of the layers of connective tissue converge at the ends of the biceps brachii to form the tendons that bind it to the scapula and radius. At the proximal end of the short head, the tendon merges with the periosteum of the scapula at the coracoid process to form the origin of the short head. On the opposite end of the biceps, the distal tendon merges with the periosteum of the radius at the radial tuberosity to form the insertion of the biceps.

Physiology

The short head of the biceps brachii acts upon the bones of the upper limb across both the elbow and shoulder joints. Together with the long head of the biceps brachii, the short head flexes and supinates the forearm at the elbow. The biceps is often incorrectly thought of as the prime flexor at the elbow, when in actuality it is a synergist to the true prime flexor, the brachialis muscle. The biceps does act as the prime supinator of the elbow and is assisted by the supinator muscle of the forearm.

At the shoulder joint, the biceps brachii provides some help to the deltoid to flex the humerus or move it anteriorly. The short head also provides some unique functions that are not provided by the long head. It acts as an adductor to move the humerus toward the body's midline and pull the arm closer to the trunk. The short head also acts as a fixator to stabilize the shoulder joint.