THE MIDDLE PALMAR REGION 



495 



fourth, from the contiguous sides of the tendons of the ring and little fingers. 

 They pass to the radial side of the corresponding fingers and opposite the meta- 

 carpophalangeal articulation each tendon is inserted into the tendinous expansion 

 of the Extensor communis digitorum, covering the dorsal aspect of each finger. 



The Interossei muscles (mm. interossei) (Figs. 370 and 371) are so named 

 from occupying the intervals between the metacarpal bones, and are divided into 

 two sets, dorsal and palmar. 



Fir.. 370. The Dorsal interossei of left hand. The 

 line marked by an * is that from which abduction is 

 made. 



FIG. 371. The Palmar interossei of left hand. The 

 line marked by an * is that to which adduction' is 

 made. 



The Dorsal interossei (mm. interossei dorsales) are four in number, larger than the 

 palmar, and occupy the intervals between the metacarpal bones. They are 

 bipenniform muscles, arising by two heads from the adjacent sides of the meta- 

 carpal bones, but more extensively from the metacarpal bone of the finger into 

 which the muscle is inserted. They are inserted into the bases of the first phalanges 

 and into the aponeurosis of the common Extensor tendon. Between the double 

 origin of each of these muscles is a narrow triangular interval, through the first 

 of which passes the radial artery; through each of the other three passes a per- 

 forating branch from the deep palmar arch. 



The First dorsal interosseous muscle, or Abductor indicis, is larger than the others. 

 It is flat, triangular in form, and arises by two heads, separated by a fibrous arch, 

 for the passage of the radial artery from the dorsum to the palm of the hand. 

 The outer head arises from the upper half of the ulnar border of the first meta- 

 carpal bone; the inner head, from almost the entire length of the radial border 

 of the second metacarpal bone; the tendon is inserted into the radial side of the 

 index finger. The Second and Third dorsal interossei are inserted into the middle 

 finger, the former into its radial, the latter into its ulnar side. The Fourth is 

 inserted into the ulnar side of the ring finger. 



The Palmar interossei (mm. interossei volares), three in number, are smaller than 

 the Dorsal, and placed upon the palmar surface of the metacarpal bones, rather 

 than between them. Each muscle arises from the entire length of the meta- 

 carpal bone of one finger, and is inserted into the side of the base of the first 



