The structure highlighted is the lateral head of the triceps brachii.

The triceps brachii is the sole muscle in the posterior compartment of the arm. As the name suggests, the triceps brachii has three heads:

1. Long head
2. Medial head
3. Lateral head

These three heads converge to insert on the olecranon of the ulna. The lateral head and long head of the triceps brachii lie superficially, the medial head lies deep to these two heads. The triceps muscle serves to extend the forearm at the elbow joint and is an antagonist of the biceps and brachialis muscles of the anterior compartment of the arm.

Origin:

• Long head – infraglenoid tubercle of scapula
• Medial head – posteriorly on the shaft of the humerus below the radial groove
• Lateral head – posteriorly on the humerus, above the radial groove

Insertion: Olecranon of the ulna

Action: Extension of the forearm at the elbow joint

Innervation: Radial nerve

Learn more about the anatomy of the arm muscles in this anatomy tutorial.