240 CLINICAL APPLIED ANATOMY. 



the dorsal aspect of the index, middle and radial side of ring 

 fingers as low down as the middle of the second phalanx. One 

 of these digital branches can be easily palpated as it crosses the 

 tendon of the extensor longus pollicis where that structure forms 

 the ulnar boundary of the tabatiere. 



The posterior interosseous branch of the musculo-spiral reaches 

 the back of the forearm between the two planes of the supinator 

 brevis muscle, supplying this muscle in its course, and winding 

 round the radial side of the neck of the radius. It is then dis- 

 tributed to all the extensor muscles, namely, superficially to the 

 extensor carpi radialis brevior, extensor communis digitorum, 

 extensor minimi digiti, extensor carpi ulnaris ; and deeply to the 

 extensor ossis metacarpi pollicis, extensor brevis pollicis, extensor 

 longus pollicis, and the extensor indicis. 



From its deep position the musculo-spiral nerve is not greatly 

 exposed to external injury, but is liable whilst lying in the 

 musculo-spiral groove to be pressed upon by callus formed 

 around a fracture of the middle of the shaft of the humerus. 

 Also before it reaches the posterior aspect of the limb it may 

 be paralysed by the pressure of the handle of a crutch, or by the 

 pressure induced by the arm hanging over some sharp edge, such 

 as the back of a chair, or even the edge of an operating table. 

 The nerve is sometimes paralysed by a sudden and violent con- 

 traction of the triceps, being compressed as it lies in the musculo- 

 spiral groove. 



The result of division will be a permanent paralysis unless the 

 continuity of the nerve is restored, whilst the result of pressure 

 may be only a temporary paralysis. The paralysis is evidenced 

 by wrist-drop that is, loss of power to extend the carpus, hand, 

 and digits associated in the majority of cases by anaesthesia, 

 partial, or complete, over the distribution of the radial nerve. 



In the case of musculo-spiral paralysis from lead poisoning, 

 the brachio-radialis (supinator longus) muscle usually escapes. 



The musculo-cutaneous nerve is derived from the outer 

 cord of the brachial plexus, and is perhaps the least frequently 

 damaged of any of the nerves of the upper extremity. It 



