The axillary nerve (C5 and C6) comes off the posterior cord at the level of the coracoid process. Fibers originate mainly from the posterior division of the upper trunk. It lies posterior to the axillary artery and then the musculocutaneous nerve. It leaves the axilla to enter the posterior aspect of the upper arm through the quadrangular space, accompanied by the posterior circumflex humeral vessels. The quadrangular space is bound by the subscapularis and teres minor cranially, teres major caudally, long head of triceps medially, and humerus laterally. Teres major separates the axillary nerve from the radial nerve. The axillary nerve supplies the teres minor and the deltoid muscles. It divides into: an anterior division that is deeper and in closer relationship to the humerus, and a posterior division that gives off the upper lateral cutaneous nerve of the arm.
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