THE ULNAR NERVE 987 



The superficial radial (radial) nerve [ramus superficialis n. radialis] is some- 

 what smaller than the deep radial (posterior interosseous), and is a purely cuta- 

 neous nerve. It runs downward under cover of the brachio-radialis, passing in 

 front of the elbow-joint, the radial recurrent artery, and the supinator (brevis). 

 At the lower border of the supinator it approaches the radial artery at an acute 

 angle, and runs parallel to the lateral side of that vessel in the middle third of the 

 forearm, across the pronator teres. At the lower border of the pronator teres it 

 bends dorsalw^ard on the deep surface of the tendon of the brachio-radialis, and 

 appears on the back of the forearm. It pierces the deep fascia and is directed 

 across the dorsal carpal (posterior annular) ligament toward the dorsum of the 

 wrist, where it divides into its terminal branches (fig. 759). 



The most lateral of these branches supplies the skin on the radial part of the thenar eminence ; 

 the most medial, designated the ulnar anastomotic branch, communicates with the'dorsal branch 

 of the ulnar nerve. The other terminal branches, the dorsal digital nerves, supply to a variable 

 extent the skin on the dorsum of the first digit, both sides of the second and the radial side 

 of the third digit. These branches usually extend to the base of the nail of the first digit, to the 

 distal interphalangeal joint of the second, not quite to the proximal interphalangeal joint of 

 the third, and to the metacarpo-phalangeal joint of the fourth digit. 



The terminal branches of the lateral cord of the brachial plexus are the mus- 

 culo-cutaneous and the lateral component of the median nenx. The latter nei-ve 

 will be described with the medial cord. 



The musculo-cutaneous nerve is composed of fibres derived chiefly from the 

 anterior divisions of the fifth and sixth cervical nerves, together usually wuth 

 some fibres from that of the seventh (figs. 751 and 754). The nerve to the coraco- 

 brachialis usually consists of two or three twdgs given off from the nerve close to 

 its origin before it enters the muscle (fig. 755). Sometimes, however, the fibres 

 from the seventh cervical nerve pass directly to this muscle without joining the 

 main trunk. The musculo-cutaneous nei-ve is placed at first close to the lateral 

 side of the axillary artery (fig, 755), but soon it leaves that vessel and, piercing the 

 coraco-brachialis muscle, it passes obliquely downward and lateralward between 

 the biceps and brachialis muscles. Soon after piercing the coraco-brachialis it 

 gives off muscular branches to each head of the biceps and to the brachialis (fig. 

 758). It also gives twigs to the humerus, to the nutrient artery, and gives the chief 

 supply to the elbow-joint. Below the branch to the brachialis the cutaneous por- 

 tion of the nerve forms the lateral antibrachial cutaneous nerve (figs. 756, 758) . 

 This portion continues downward between the biceps and brachialis, pierces the 

 deep fascia at the lateral border of the former muscle a little above the bend of the 

 elbow, receives a communication from the upper branch of the dorsal antibrachial 

 (upper external) cutaneous branch of the radial (musculo-spiral) nerve, passes 

 dorsal to the median cephalic vein, and divides into an anterior and a posterior 

 branch. 



The anterior branch runs do-vvnward on the lateral and anterior part of the forearm, sup- 

 plying the integument of that region, and it terminates in the skin covering the middle part of 

 the thenar eminence (fig. 759). A short distance above the wrist, after it has received a com- 

 municating twig from the superficial radial nerve, it gives off an articular branch to the carpal 

 joints. This branch pierces the deep fascia and accompanies the radial artery to the dorsum 

 of the wrist. The posterior terminal branch is small, and is directed do-vxTiward and backward 

 in front of the external condyle of the humerus, to be distributed to the skin on the lateral and 

 posterior aspect of the forearm as low as the wrist (fig. 756). It anastomoses with the superficial 

 radial and with the lower branch of the dorsal antibrachial (lower external) cutaneous branch 

 of the radial nerve. 



The terminal branches of the medial cord of the brachial plexus are the ulnar 

 nerve and the medial component of the median nerve. Neither of these gives any 

 branches in the upper arm, and thus they differ from the other terminal branches 

 of the plexus. They both supply the muscles and joints of the forearm, and the 

 muscles, joints, and integument of the hand. 



The ulnar nerve, which is the largest branch of the medial cord of the brachial 

 plexus, contains fibres from the anterior divisions of the eighth cervical and first 

 thoracic nerves (figs. 752 and 762). It commences at the lower border of the 

 pectoralis minor and runs downward in the axillary fossa in the posterior angle 

 between the axillary artery and vein. In the upper half of the arm it lies on the 

 medial side of the brachial artery (fig. 755), but at the level of the insertion of the 



