Lateral intermuscular septum of arm

The brachial fascia, which envelops the arm muscles, forms two tough fibrous partitions that compartmentalize the arm's muscles: the lateral and medial intermuscular septa. The lateral intermuscular septum runs from the brachial fascia to the lateral supracondylar ridge and the lateral epicondyle of the humerus. Proximally, it's continuous with the deltoid fascia and provides thin extensions to the lateral lip of the intertubercular sulcus.

The lateral intermuscular septum is notably perforated by the radial nerve and the radial collateral artery branch of the profunda brachii artery, as they pass from the posterior to the anterior compartment. The septum also provides attachment for the lateral head of the triceps posteriorly, and parts of the brachialis, brachioradialis, and extensor carpi radialis longus anteriorly.