The quadrangular space (or quadrilateral space [of Velpeau] or foramen humerotricipitale) is an axillary space in the arm. This is a clinically important anatomic space in the arm as it provides the anterior regions of the axilla an passageway to the posterior regions. In the quadrangular space, the axillary nerve and posterior circumflex humeral artery can be compressed or damaged due to space-occupying lesions or disruption in the anatomy due to trauma. Symptoms include axillary nerve related weakness of the deltoid muscle in the case of any significant mass lesions in the quadrangular space.
Quadrangular space Wikipedia
It is bounded by:above/superior: the teres minor (inferior margin).
below/inferior: the teres major (superior margin)
medially: the long head of the triceps brachii (lateral margin)
laterally: the surgical neck of the humerus
anteriorly: the subscapularis
It transmits the axillary nerve and posterior circumflex humeral vessels (artery and vein).