The axillary artery becomes the brachial artery of the arm as it crosses the inferior margin of the teres major muscle.
Long head of the triceps brachii
The long head of the triceps brachii muscle passes vertically through the posterior wall of the axilla, and, together with surrounding muscles and adjacent bones, results in the formation of three apertures through which major structures pass through the posterior wall: the quadrangular space, the triangular space, and the triangular interval (Fig. 7.45).