Describe the anatomic course of the brachial plexus.
The brachial plexus is composed of the ventral rami of C5–8 and T1, and at times includes C4 and T2. These nerve roots combine to form three trunks – the superior (C5–6), the middle (C7), and the inferior (C8–T1) – after passing between the anterior and middle scalene muscles. The plexus continues its course under the clavicle and over the first rib. At the lateral border of the first rib, each trunk divides into an anterior and a posterior division. In the axilla these divisions form three cords. The anterior divi-sions of the superior and middle trunks form the lateral cord. The posterior divisions of all three trunks form the posterior cord. The anterior division of the inferior cord continues as the medial cord. The cords acquire their names from their anatomic relationship to the axillary artery. Finally, at the lateral border of the pectoralis minor muscle in the axilla, the cords give rise to the terminal branches of the brachial plexus. The posterior cord termi-nates as the axillary and radial nerves. The lateral cord splits to form the musculocutaneous nerve and with the medial cord forms the median nerve. The medial cord also terminates as the ulnar nerve.