THE BRACHIAL PLEXUS 147 



The Dorsal Interossei act as abductors of the index, middle 

 and ring fingers. The first dorsal interosseous arises from the 

 adjacent sides of the first and second metacarpal bones, and 

 is inserted into the radial side of the dorsum of the base of 

 the proximal phalanx of the index finger. When the thumb is 

 adducted, the first dorsal interosseous muscle produces a 

 swelling on the dorsum of the hand between the first and 

 second metacarpal bones, and it can be felt to contract during 

 abduction of the index. Atrophy of this muscle occurs at an 

 early stage in progressive muscular atrophy. 



The second and third dorsal interossei occupy the spaces 

 between the second and third, and third and fourth metacarpal 

 bones, respectively. The former is inserted into the radial side 

 and the latter into the ulnar side of the first phalanx of the 

 middle finger. They abduct the middle finger to the radial 

 and ulnar sides respectively. 



The fourth dorsal interosseous muscle occupies the space 

 between the fourth and fifth metacarpal bones, and is inserted 

 into the ulnar side of the dorsum of the base of the proximal 

 phalanx of the ring finger. 



When the dorsal interossei become atrophied, hollows 

 appear on the dorsum of the hand between the metacarpal 

 bones, and the latter can not only be palpated but may even 

 be gripped between the examining finger and thumb. 



The Adductor Pollicis possesses a wide origin from the 

 volar aspect of the carpus and the third metacarpal bone, 

 and it is inserted into the ulnar side of the base of the 

 proximal phalanx of the thumb. In studying the movement 

 of adduction of the thumb, it must be remembered that the 

 first metacarpal bone is so placed that what is usually de- 

 scribed as its dorsal surface is, in reality, directed laterally, 

 when the supine hand is by the side. Adduction of the 

 thumb brings the ulnar border of the first metacarpal towards 

 the radial side of the second metacarpal, and the movement 

 occurs in an antero-posterior plane, provided that the hand 

 is supine and by the side, as in the erect attitude. Abduction 



