924 THE NERVOUS SYSTEM 



artery, and the supinator (brevis). At the lower border of the supinator it ap- 

 proaches the radial artery at an acute angle, and runs parallel to the outer 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 backwards 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 towards 

 the back of the wrist, where it divides into its terminal branches (fig. 683). The 

 most lateral of these 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 third digits, and part of the radial side of the fourth 

 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 sometimes to the metacarpo-phalangeal joint of the fourth 

 digit, 



The terminal branches of the outer cord of the brachial plexus are the musculo- 

 cutaneous and the outer component of the median. The latter nerve will be described 

 with the inner cord. 



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

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

 fibres from that of the seventh (figs. 675 and 678). The nerve to the coraco-brachia- 

 lis usually consists of two or three twigs given off from the nerve close to its origin 

 before it enters the muscle (fig. 679). Sometimes, however, the fibres from the 

 seventh nerve pass directly to this muscle without joining the main trunk. The 

 musuclo-cutaneous nerve is placed at first close to the outer side of the axillary 

 artery (fig. 679), but soon it leaves that vessel and, piercing the coraco-brachialis 

 muscle, it passes obliquely downwards and outwards between the biceps and brach- 

 ialis muscles. Soon after piercing the coraco-brachialis it gives off muscular branches 

 to each head of the biceps and to the brachialis (fig. 682). 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 is the cutaneous portion of the nerve termed 

 the lateral antibrachial cutaneous nerve (fig. 682). This nerve continues down- 

 wards between the biceps and brachialis, pierces the deep fascia at the outer 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 behind the median cephalic vein, and 

 divides into an anterior and a posterior branch. The anterior branch runs down- 

 wards on the outer and anterior part of the forearm, supplying the integument of 

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

 eminence (fig. 683). 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 back of the wrist. The posterior terminal branch is small, and is 

 directed downwards and backwards in front of the external condyle of the humerus, 

 to be distributed to the skin on the outer and posterior aspect of the forearm as low 

 as the wrist (fig. 680). It anastomoses with the superficial radial and with the 

 lower branch of the dorsal antibrachial (lower external) cutaneous branch of the 

 radial (musculo-spiral) nerve. 



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

 and the inner component of the median nerve. Neither of these supplies 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 mus- 

 cles, joints, and integument of the hand. 



The Ulnar Nerve, which is the largest branch of the inner cord of the brachial 

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

 thoracic nerves (figs. 676 and 685). It commences at the lower border of the pector- 

 alis minor and runs downwards 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 inner side of 

 the brachial artery (fig. 679), but at the level of the insertion of the coraco-brachialis it 

 passes backwards at an acute angle, and, accompanied by the superior ulnar collateral 

 (inferior profunda) artery, it pierces the internal intermuscular septum. After 



