252 



The Brachial Plexus 



outer side of this vessel it lies in the middle third of the fore-arm only, 

 for it then turns backwards beneath the tendon of the supinator longus 

 to become cutaneous a little above the wrist. It supplies the skin of the 

 thumb, the index and middle fingers, and the inner side of the ring- 

 finger as far as the first inter-phalangeal joint, the skin over the backs of 

 the second and third phalanges being supplied by the digital branches 

 of the median except in the case of the back of the thumb, which 

 the radial supplies entirely. 



The posterior interosseous nerve, the other division of the mus- 

 culo-spiral, gives a branch to the extensor carpi radialis brevior, and 

 then winds round the radius in the substance of the supinator brevis, 

 which it supplies ; it then descends between the superficial and deep 

 muscles at the back of the fore-arm, and ends in a gangliform enlarge- 

 ment on the back of the carpus. In its course it supplies also the 

 extensors communis digitorum, minimi digiti, and carpi ulnaris in the 

 superficial layer, and the extensors ossis metacarpi, primi et secundi 

 internodii pollicis, and indicis in the deep layer. 



iviuscuio spiral paralysis may be caused by laceration of, or 



injury to, the trunk as it winds 

 round the humerus ; or by 

 severe pressure, as by a chair- 

 back, by sleeping with the 

 head pressing the arm against 

 a hard pillow, or by a badly 

 padded crutch (cnutch-para- 

 lysis) ; often it is a symptom of 

 lead-poisoning, when it is pro- 

 bably due to interference with 

 the anterior cornu of the grey 

 crescent of the spinal cord. 

 There may be complete loss of 



igers have been Hexed into palm, a, UT* A *o, f 



they can be extended at first inter-phalangeal ability to extend the fore-arm, 

 joints by lumbricals and interossej, b, which 

 are supplied by ulnar and median nerves. 



icture of 



Paralysis of musculo-spiral nerve after fract 



humerus; 'drop-wrist.' (EUCHSBN.) But 

 when fingers have been flexed into palm, a, 



or, except for the action of the 

 biceps, to supinate it ; the ex- 

 tensors of the hand (drop-wrisf) and fingers are also powerless, and the 

 hand usually remains prone, with fingers flexed. Indeed, it is only by 

 the lumbricals and interossei acting on the second and third phalanges 

 that extension of the fingers can be in any way obtained, and even 

 then the first phalanx must be held and fixed, or else those little 

 muscles flex it to the palm. When the arm is raised above the head 

 the triceps has no power to extend the fore-arm. There is loss of 

 sensation in the region supplied by the radial nerve (vide supra}. 



An interesting feature in the paralysis is that the grasp of the 

 fingers is enfeebled ; this is because the hand has fallen into the posi- 

 tion of flexion, and there is too much * slack ' in the flexor tendons. 

 (The best way to force a person to loose anything from the firmly 



