The brachial plexus is formed from the anterior rami of roots C5, C6, C7, C8, and T1 with possible contributions from the adjacent roots. In its most proximal portion, in front of the scalenus medius, these rami are separate. At this most distal border, the fibers appear as five nerves (musculocutaneous, median, ulnar, circumflex, and radial). The plexus itself at first groups into an upper trunk (from a combination of C5 and C6), a middle trunk (a continuation of C7), and lower trunk (from a combination of C8 and T1). These trunks each have anterior and posterior divisions. Behind the clavicle the posterior cord is formed from a combination of all the posterior divisions. The lower anterior division continues as the medial cord. The lateral cord is a combination of the other anterior divisions (upper and middle). These cords are oriented around the axillary artery behind the pectoralis minor.
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