The brachium is the anatomical term for the human upper arm, representing the segment of the upper limb situated between the shoulder and the elbow. This region functions as a bridge, connecting the powerful movements generated at the shoulder joint with the motor skills of the forearm and hand. It is a highly organized area, housing the robust musculature necessary for lifting and moving, along with the major pathways for the nerves and vessels that supply the entire limb. Understanding the structure of the brachium is fundamental to grasping how the arm achieves its impressive range of mobility and strength.
Location and Skeletal Foundation
The brachium is defined by its boundaries, extending proximally from the inferior border of the Teres Major muscle, which marks the transition from the axilla (armpit). Distally, the brachium ends at the cubital fossa, the triangular depression found at the front of the elbow joint. This segment connects the shoulder girdle and the forearm bones.
The skeletal foundation of the brachium is formed by a single long bone, the Humerus. The rounded head of the Humerus articulates with the scapula (shoulder blade), forming the highly mobile glenohumeral joint. This proximal articulation permits the wide range of motion characteristic of the shoulder, including flexion, extension, abduction, adduction, and rotation.
The long shaft of the Humerus provides a strong lever arm for muscle attachment. At its distal end, the Humerus widens and terminates in articulating surfaces that meet the forearm bones—the Radius and the Ulna—to form the elbow joint. The Humerus acts as a solitary column, transmitting force and facilitating movement.
Muscle Compartments and Function
The soft tissues of the brachium are functionally separated into two main compartments by thick sheets of connective tissue called the medial and lateral intermuscular septa. This division creates the Anterior (Flexor) compartment and the Posterior (Extensor) compartment, each with distinct muscle groups, functions, and nerve supply. This compartmentalization helps isolate muscle groups, allowing for efficient and coordinated opposing movements.
The Anterior compartment contains three muscles: the Biceps Brachii, the Brachialis, and the Coracobrachialis. The Biceps Brachii acts to flex the elbow and is the main supinator of the forearm. Deep to it, the Brachialis muscle is the strongest flexor of the elbow joint. The Coracobrachialis assists in the flexion and adduction of the arm at the shoulder joint.
The Posterior compartment is dominated by a single, large muscle, the Triceps Brachii. This muscle has three heads, all of which converge to insert on the Ulna. The Triceps Brachii is the sole muscle responsible for extension of the forearm at the elbow joint, acting as the antagonist to the anterior compartment muscles.
Nerves and Blood Supply
The brachium serves as a major thoroughfare for the neurovascular bundles that supply the rest of the upper limb. The principal artery is the Brachial Artery, a direct continuation of the axillary artery. This large vessel runs down the medial side of the arm, close to the surface, making its pulse detectable and clinically significant for measuring blood pressure.
The Brachial Artery gives off branches, including the Profunda Brachii Artery (deep artery of the arm), which wraps around the back of the Humerus to supply the posterior compartment. At the elbow, the Brachial Artery divides into the Radial and Ulnar arteries, which continue to supply the forearm and hand. Venous drainage is facilitated by the deep Brachial Veins, which accompany the artery, and the more superficial Basilic and Cephalic veins.
Several major nerves, all branches of the Brachial Plexus, travel through the brachium. The Musculocutaneous Nerve innervates the muscles of the anterior compartment, controlling elbow flexion. The Median and Ulnar nerves pass through the arm without innervating muscles here, serving as conduits for function further down the limb. The Radial Nerve spirals around the Humerus in the posterior compartment, providing motor control to the Triceps Brachii before continuing into the forearm.