The =radial nerve= (Figs. 441, 443, 520), also called the
musculo-spiral, is sometimes the largest branch of the brachial plexus.
Its fibers are derived from the seventh and eighth cervical and first
thoracic roots of the plexus. It passes downward and backward over the
inner surface of the subscapular artery and the teres major. In this
part of its course it is related in front to the ulnar nerve, which
separates it from the brachial vein. It detaches a branch to the tensor
fasciæ antibrachii, passes outward in the interval between the teres
major and the long and internal heads of the triceps, and gains the
musculo-spiral groove of the humerus. Accompanied by a branch of the
deep brachial artery, it runs obliquely downward and outward in the
groove, covered externally by the external head of the triceps and the
extensor carpi radialis, and reaches the flexion surface of the elbow
joint. In this part of its course it gives off a muscular branch which
ramifies in the long and external heads of the triceps and in the
anconeus, and a =cutaneous nerve= (N. cutaneus antibrachii dorsalis);
branches of the latter emerge below or through the external head of the
triceps and ramify on the dorso-lateral surface of the forearm. At the
elbow the nerve descends with the anterior radial vessels on the joint
capsule between the brachialis and extensor carpi radialis, and supplies
branches to the extensor carpi and the common or anterior extensor of
the digit and (inconstantly) to the brachialis. Below the elbow joint
the nerve detaches a large branch which passes back to the flexor carpi
externus, and terminates by small branches which descend on the radius
to enter the radial and ulnar heads of the common extensor, the lateral
extensor, and the oblique extensor of the carpus. Thus the radial nerve
innervates the extensors of the elbow, carpal and digital joints, and
supplies also the flexor carpi externus.[208]
[Illustration: