This nerve derives its fibres from the seventh and eighth cervical pairs, and also from the first dorsal root. It is the largest of the branches furnished by the brachial plexus. From its point of origin it passes downwards, crossing over the inferior part of the subscapulars and the teres major muscles. It then dips down and takes an outward course under the great head of the triceps, winding round the humerus in the musculo-spiral groove to gain the front of the elbow-joint, where it is found beneath the extensor metacarpi and extensor pedis muscles. In its course it gives branches to the great and small heads of the triceps, and to the scapulo-ulnaris muscles, the caput medium, and anconeus.