THE MUSCLES OF THE HAND. 



611 



Flexor carpi radialis tendon 



Tuberosity of scaphoid — - 



Flexor carpi 

 ulnaris tendon 



Pisiform bone 



Unciform process 

 of unciform 



•Palmar 

 interossei 



{dd) The Muscles of the Fourth and Fifth Layers. 



I. Interossei volares. 2. Interossei dorsales. 



In the lower vertebrates the musculature of the fourth palmar layer consists of 

 a pair of muscles for each digit, arising from the carpal and metacarpal bones and 

 inserting into either side of the base of the first phalanx. The fifth layer lies 

 dorsal to these, and consists of four muscular bands, which extend slightly obliquely 

 across the four inter- 

 metacarpal spaces. Fig. 589. 



In the mammalia 

 a shifting of the in- 

 sertion of one of the 

 muscles of the pairs 

 belonging to the first 

 and fifth digits takes 

 pMce, so that they are 

 attached to the radial 

 and ulnar sides re- 

 spectively of the ad- 

 jacent second and 

 fourth digits, uniting 

 with the correspond- 

 ing members of the 

 pairs belonging to 

 those digits. With 

 the compound mus- 

 cles so formed the 

 first and fourth inter- 

 metacarpal muscles 

 unite to form the first 

 and fourth dorsal ifi- 

 terossei, these two 

 muscles being com- 

 posed, accordingly, 

 by the fusion of three 

 primary muscles. 



The second and 

 third intermetacarpal 

 muscles unite with the 

 radial and ulnar mem- 

 bers respectively of 

 the pair belonging to 

 the third digit, and 

 form with these the 

 second and third dor- 

 sal interossei. 



The remaining 

 members of the pairs 

 belonging to the first, 

 second, fourth, and 

 fifth digits persist as 



independent muscles, forming what are termed the volar interossei, whose arrange- 

 ment is consequently complementary to that of the dorsal interossei. 



The intermetacarpal muscles occupy the most dorsal position of all the palmar 

 muscles, and it is probably owing to their participation in the formation of the 

 dorsal interossei that these possess an almost dorsal position in the hand. They are 

 clearly, however, of palmar origin and are supplied by pre^axial nerves. 



Fourth dorsal 

 interosseus 



Third dorsal 

 interosseus 



Deep dissection of hand, showing interosseous muscles as seen in palm. 



