ANTERIOR DIVISIONS OF THE DORSAL NERVES. 655 



The branches of the interosseous nerve enter the surrounding muscles, 

 viz., the extensor carpi radialis brevior and supinator brevis, the superficial 

 layer of the extensor muscles except the anconeus, and the deep layer of 

 the same muscles : that is to say, the nerve supplies the supinators, and 

 the extensors of the carpus and fingers, with the exception of the supiuator 

 longus and the extensor carpi radialis longior. 



Summary of the Musculo-spiral Nerve. 



The trunk of the nerve distributes its branches to the extensor muscles of 

 the elbow-joint exclusively, with the exception of a filament to the brachialis 

 anticus, which however receives its principal nerves from another source. 

 Before separating into its two larga divisions, the nerve gives branches 

 to two muscles of the forearm, viz., the long supiuator, and the long 

 radial extensor of the carpus. The posterior iuterosseous division distri- 

 butes 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 lower part of the upper arm, to the forearm, and to 

 the hand on the posterior and outer aspect of each. 



ANTERIOR PRIMARY DIVISIONS OF THE DORSAL NERVES. 



These nerves are twelve in number, and, with the exception of the larger 

 part of the first of them, which joins the brachial plexus, they are dis- 

 tributed to the walls of the thorax and abdomen. Eleven of the nerves so 

 distributed are termed intercostal, and the twelfth is situated below the last 

 rib. The cords connecting them with the sympathetic nerve, placed close to 

 the vertebrae, are very short. 



The anterior divisions of these nerves pas3 separately to their destination, 

 without forming any plexus by the connection or interlacement of their 

 fibres, and in this respect they differ from those of the other spinal nerves. 

 From the iutervertebral foramina they are directed transversely across the 

 trunk, and nearly parallel one to another. The upper six nerves, with the 

 exception of the first, are confined to the parietes of the thorax ; while the 

 lower six nerves are continued from the intercostal spaces to the muscles 

 and integument of the anterior wall of the abdomen. 



FIRST DORSAL NERVE. 



The greater part of the anterior division of this nerve ascends over the 

 neck of the first rib and the first intercostal artery to enter into the brachial 

 plexus. The remaining portion of the nerve is continued as the first inter- 

 costal, a small branch which 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. 



UPPER, OR PECTORAL INTERCOSTAL NERVES. 



In their course to the fore part of the chest, these nerves accompany the 

 intercostal blood-vessels. After a short space they pass between the internal 

 and external intercostal muscles, supplying them with twigs, and, about 

 midway between the vertebrae and the sternum, give off the lateral cutaneous 

 branches. The nerves, greatly diminished, are now continued forwards 

 amid the fibres of the internal intercostal muscles as far as the costal 

 cartilages, where they come into contact with the pleura. In approaching 



