This nerve is chiefly derived from the dorsal roots of the brachial plexus. It is less considerable in size than the one just described. Placed behind the humeral artery, it dips down beneath the scapulo-ulnaris, between it and the caput parvum, to reach the posterior part of the inner condyle of the humerus. From this point it proceeds to the back of the forearm, and following the posterior border of the ulnaris accessorius, terminates by joining a branch of the median just above the knee. It assists in forming the external plantar nerve, and gives some twigs to the front of the carpus. The branches given off in its downward course supply the superficial pectoral muscle, the skin of the forearm, and, excepting the external and internal flexors of the metacarpus, all the muscles behind the radius.