10 42 THE NERVE SYSTEM 



radialis, and, piercing the deep fascia at the outer border of the forearm, divides 

 into two branches. The external branch, the smaller of the two, supplies the 

 integument of the radial side and ball of the thumb, joining with the anterior 

 branch of the musculocutaneous nerve. The internal branch communicates, above 

 the wrist, with the posterior cutaneous branch from the musculocutaneous, and on 

 the back of the hand forms an arch with the dorsal cutaneous branch of the ulnar 

 nerve. It then divides into four digital nerves (nn. digit-ales dorsales), which are 

 distributed as follows: The first supplies the ulnar side of the thumb; the 

 second, the radial side of the index finger; the third, the adjoining sides of the 

 index and middle fingers; and the fourth, the adjacent borders of the middle 

 and ring fingers. 1 The latter nerve communicates with a filament from the 

 posterior branch of the ulnar nerve. 



The Dorsal or Posterior Interosseous Nerve (n. interosseous [antebrachii] dorsalis) 

 (Figs. 764 and 765) winds to the back of the forearm around the outer side of the 

 radius, passes between the two planes of fibres of the Supinator [brevis] muscle, 

 and is prolonged downward, between the superficial and deep layer of muscles, 

 to the middle of the forearm. Considerably diminished in size, it descends on 

 the interosseous membrane, beneath the Extensor longus pollicis muscle, to the 

 back of the carpus, where it presents a gangliform enlargement from which fila- 

 ments are distributed to the inferior radioulnar articulation, to the wrist-joint, 

 and to the ligaments and articulations of the carpus. It supplies all the muscles 

 of the radial and posterior brachial regions, excepting the Anconeus, Supinator 

 longus, and Extensor carpi radialis longior. 



Applied Anatomy. The brachial plexus may be severed by traction on the limb, leading 

 to complete paralysis. Bristow 2 has reported three cases of avulsion of the plexus and has 

 described twenty-four cases. In these cases it is generally believed that the lesion is rather a 

 tearing away of the nerves from the spinal cord than a solution of continuity of the nerve fibres 

 themselves. In a case operated upon by Bristow it was found that the plexus had given way 

 where the four cervical nerves and the first thoracic nerve unite to form three trunks. In supra- 

 clavicular division of the brachial plexus, not only will there be motor and sensor paralysis in the 

 limb, but the Serratus magnus muscle will probably be paralyzed, because of injury to the poste- 

 rior thoracic nerves. In the axilla any of the nerves forming the brachial plexus may be injured 

 by a wound of this part, the median being the one which is most frequently damaged from its 

 exposed position. The musculospiral, on account of its sheltered and deep position, is least 

 often wounded. The brachial plexus in the axilla is often damaged from the pressure of a 

 crutch, producing the condition known as crutch paralysis. In these cases the musculospiral 

 is the nerve most frequently implicated; the ulnar nerve being the one that appears to suffer next 

 in frequency. 



The circumflex nerve is of particular surgical interest. On account or its course around the 

 surgical neck of the humerus, it is liable to be torn in fractures of this part of the bone, and 

 in dislocations of the shoulder-joint, leading to paralysis of the Deltoid, and, according to Erb, 

 inflammation of the shoulder-joint is liable to be followed by a neuritis of this nerve from exten- 

 sion of the inflammation to it. 



Hilton takes the circumflex nerve as an illustration of a law which he lays down, that 

 " the same trunks of nerves whose branches supply the groups of muscles moving a -joint furnish 

 also a distribution of nerves to the skin over the insertions of the same muscles, and the interior 

 of the joint receives its nerves from the same source." In this way he explains the fact that an 

 inflamed joint becomes rigid, because the same nerves which supply the interior of the joint 

 supply the muscles which move that joint. 



The median nerve is liable to injury in wounds of the forearm. When paralyzed, there is 

 loss of flexion of the second phalanges of all the fingers and of the terminal phalanges of the 

 index and middle fingers. Flexion of the terminal phalanges of the ring and middle fingers can 

 still be effected by that portion of the Plexor profundus digitorum which is supplied by the ulnar 

 nerve. There is power to flex the proximal phalanges through the Interossei. The thumb 

 cannot be flexed or opposed, and is maintained in a position of extension and adduction. All 

 power of pronation is lost. The wrist can be flexed, if the hand is first adducted, by the action 



1 According to Hutchinson, the digital nerve to the thumb reaches only as high as the root of the nail; the 

 one to the forefinger as high as the middle of the second phalanx, and the one to the middle and ring fingers not 

 higher than the first phalangeal joint (London Hospital Gazette, vol. iii, p. 319). 



2 Annals of Surgery, September, 1902. 



