ANTERIOR PRIMARY DIVISIONS OF DORSAL NERVES. 307 



to the extensor communis digitorum, extensor minimi digiti, extensor carpi ulnaris, 

 the three extensors of the thumb, and the extensor indicis. 



(&) Articular branches. From the terminal enlargement of the nerve fine twigs 

 proceed to the articulations of the wrist, and, according to Rlidinger and Rauber, 

 other filaments descend on the back of the hand to the metacarpo-phalangeal 

 articulations. 



Varieties. The posterior interosseous nerve may pass over, instead of perforating, the 

 supinator brevis muscle (Luschka, Krause). A branch to the anconeus muscle is described by 

 the same anatomists. There may be a communication between the anterior and posterior 

 interosseous nerves at the lower part of the interosseous space (p. 303). In two cases, recorded 

 by Turner and Schwalbe, the posterior interosseous nerve passed down to supply the adjacent 

 sides of the index and middle fingers ; a similar condition has been found in the chimpanzee, 

 orang and gibbon (Westling, Hepburn, Hofer). 



SUMMARY. The musculo-spiral nerve supplies the extensor muscles of the 

 elbow-joint, and frequently sends a filament to the brachialis anticus. which, 

 however, receives its principal supply from another source. Before separating 

 into its two large divisions, the nerve gives branches to two muscles of the 

 forearm, viz., the long supinator, and the long radial extensor of the carpus. 

 The posterior interosseous division distributes nerves to the remaining muscles on 

 the outer and back part of the forearm, except the anconeus (previously supplied), 

 viz., to the short supinator and the extensors. 



Cutaneous nerves are distributed, from the trunk of the nerve and its radial 

 division, to the upper arm, to the forearm, and to the hand on the posterior and 

 outer aspect of each. Articular branches are furnished to the elbow, wrist, 

 metacarpo-phalangeal, and first interphalangeal joints. 



DOESAL NERVES. 



The anterior divisions of the twelve dorsal or thoracic nerves are distributed 

 almost entirely to the walls of the thorax and abdomen. The exceptions are the 

 first, the greater part of which joins the brachial plexus, and the second and 

 twelfth, which send cutaneous offsets to the arm and hip respectively. Close to the 

 intervertebral foramina, these nerves are connected to the gangliated cord of the 

 sympathetic by very short communicating branches ; they are then directed trans- 

 versely outwards to their destination without forming any plexus, and in this 

 respect they differ from the anterior primary divisions of the other spinal nerves. 

 The smaller part of the first, and the trunks of the succeeding ten nerves pass 

 forwards in the intercostal spaces, and are thence termed intercostal nerves. Of 

 these, the upper six are confined to the parietes of the thorax, while the lower 

 five are continued anteriorly from the intercostal spaces into the wall of the 

 abdomen. The twelfth nerve, sometimes distinguished as the subcostal nerve, 

 is placed below the last rib, and is therefore contained altogether in the abdominal 

 wall. 



FIRST DORSAL NERVE. 



The anterior division of the first dorsal nerve divides into two parts, the larger 

 of which ascends over the neck of the first rib to enter into the brachial plexus. 

 The remaining portion of the nerve is continued as the first intercostal, a small 

 branch which lies at first under cover of the first rib, and then courses along the 

 first intercostal space, in the manner of the other intercostal nerves, but has usually 

 no lateral cutaneous branch, and may also want the anterior cutaneous. 



Variety. The first dorsal nerve frequently receives a connecting twig which passes up- 

 wards in front of the neck of the second rib from the second nerve. This branch was found 

 by Cunningham in twenty-seven out of thirty-seven dissections ; it was of variable size, but 



