Passing through the quadrangular space are the axillary nerve and the posterior circumflex humeral artery and vein.
The triangular space is an area of communication between the axilla and the posterior scapular region (Fig. 7.45). When viewed from anteriorly, it is formed by: the medial margin of the long head of the triceps brachii muscle, the superior margin of the teres major muscle, and the inferior margin of the subscapularis muscle.