THE INTEROSSEI 337 



and part of the flexor brevis pollicis, and the deep transverse ligaments; on the 

 uhiar ^ide their tendons are in contact with the metacarpo-phalangeal joints. 



Variations. — The fourth himbricalis is st)metimes absent. They may vary iu their origins, 

 sometimes arising from one, and sometimes from two adjacent tendons. Occasionally one may 

 arise fnjm the flexor longus poUicis tendon. They may be inserted into the ulnar side of a finger, 

 or into the adjacent sides of two fingers. 



2. THE INTEROSSEI 



The interossei — named from their position between the metacarpal bones — are 

 seven in numl)er, three being palmar and four dorsal. They are small muscles, of 

 penniform or bipenniform structure. Both sets are easily exposed to view by a 



Fig. 281. — The Palmar Intekossei. 



deep dissection in the palm, but the dorsal set is alone visible in the back of tlie 

 hand. 



The three palmar interossei are fusiform in shape. 



Origin. — They arise from the sides of the metacarpal bones: the first occupy- 

 ing the whole of the ulnar side of the second metacarpal bone; the second, the 

 radial side of the fourth metacarpal bone; the third, the radial side of the fifth 

 metacar])al bone. 



Insertion. — By small tendons (1) into the aponeuroses of the extensor tendons 

 U[)(jn the back of the first phalanges of the index, ring, and little fingers; and (2) 

 the adjacent portion of the side of each first phalanx near to its base; the first being 

 attached to the ulnar border, and the second and third to the radial liorders of their 

 fingers. 



Structure. — Fleshy fibres arising along the whole length of the metacarpal 

 bone are inserted in penniform fashion upon tlie tendon, which, beginning on the 

 unattached border of the mu.scle near the middle of the interosseous space, becomes 

 free just above the metacari)0-plialangeal joint, and ])asses beneath the deep 

 transverse ligament which separates it from the tendon of the lumbricalis muscle. 



