musculotendinous formation providing passageway for the axillary nerve, posterior humeral circumflex artery, and accompanying veins as they run from the axilla to the superior posterior arm; as the neurovascular structures enter the formation anteriorly, it is bounded superiorly by the shoulder joint, medially by the lateral border of subscapularis, laterally by the surgical neck of the humerus, and inferiorly by the tendon of latissimus dorsi; where the vessels exit the formation posteriorly, it is bounded superiorly by the teres minor, medially by the long head of the triceps, laterally by the lateral head of the triceps and inferiorly by the teres major muscle or tendon; as they emerge, most of the neurovascular structures run on the deep surface of the deltoid muscle, which they supply. SYN: quadrilateral space.