ABSTRACT
The long head of triceps muscle divides the space created by the teres major and minor, the humerus, and the scapula into two spaces: the triangular foramen or medial axillary hiatus and the quadrangular (quadrilateral) foramen or lateral axillary hiatus (Figure 8.1). The medial axillary hiatus lies between the teres minor muscle superiorly, the teres major muscle inferiorly, and the long head of the triceps brachii muscle laterally. The circumflex scapular artery passes through the medial hiatus. The lateral axillary hiatus is limited proximally by the lower margin of the teres minor muscle, distally by the upper margin of the teres major muscle, laterally by the humerus, and medially by the long head of the triceps muscle. The axillary nerve and the posterior circumflex artery pass through the lateral opening. The axillary nerve, a product of the posterior branch of the brachial plexus, enters this space from its position over the subscapular muscle; together with the posterior circumflex artery, the nerve then passes deep to the deltoid muscle. Fractures of the surgical neck of the humerus occur at this point. The axillary nerve supplies the deltoid and teres minor muscles and the skin of the posterolateral region of the shoulder and upper arm via the lateral cutaneous branch of the arm.
