Staying in the axillary region, let's now identify important components and branches of the brachial plexus. At the distal end of the axillary artery the cords of the brachial plexus nearly surround the artery. Find the medial cord, lateral cord and posterior cord of this plexus. Remember, their names indicate their relationship to the axillary artery. Near the axillary artery, but not surrounded by the cords of the brachial plexus, find the axillary vein.
In Exercise Ten you identified the medial and lateral pectoral nerves near where they enter the pectoral muscles. Now see if you can find them near their origins from the brachial plexus.
The posterior cord gives rise to three nerves identified in Exercise Ten: the upper subscapular, thoracodorsal and lower subscapular. Identify these nerves and trace them to their brachial plexus origins.
Follow the posterior cord distally past the origin of the lower subscapular nerve until it splits into a terminal axillary nerve and radial nerve. The axillary nerve normally dives deep and follows a winding course posteriorly toward the teres minor and deltoid. The radial nerve follows the profunda brachii artery deep and posterior in the arm. Do you remember seeing the axillary nerve entering the posterior head of the deltoid in Exercise Ten?
Now follow the medial cord distally. Near the distal end of the axillary artery, the medial cord splits into its two terminal nerves: the ulnar nerve, which courses medial and posterior to the medial epicondyle of the humerus (funny bone), and the medial root of the median nerve.
Follow the lateral cord distally. Near the distal end of the axillary artery the lateral cord splits into its two terminal nerves: the musculocutaneous nerve, which almost immediately enters the biceps, bracialis, coracobrachialis, and the lateral root of the median nerve.
Note how the median nerve is formed by the union of the medial root of the medial cord, and lateral root of the lateral cord. This union often forms the letter "M." (Figure 11-1)