3 – Anatomy – posterior view

The axillary nerve splits in an anterior and a posterior branch inside the “window” of the quadrangular space or a bit more distal inside the deltoid muscle (which is removed in the picture on the right)

The axillary nerve (or sometimes a branch from the posterior cord of the brachial plexus) innervates:
– the triceps brachii (long head) muscles (ref. S