338 THE MUSCLES 



At this place a small l)iirsa separates the tendon from the deep transverse ligament, 

 and the lateral ligament of the metacarpo-phalangeal joint. 



Nerve-supply. — From the inner cord of the brachial plexus (through the 

 eighth cervical nerve), by small filaments which, coming from the deep branch of 

 the ulnar nerve, pass into tiie upper part of the muscles upon their anterior asjject. 



Action and relations. — 17(/(' infra. 



Uf the four dorsal interossei, the iirst forms a thick triangular sheet, while the 

 other three are fusiform and prismatic. 



Origin. — From the five metacarjKil bones, each muscle arising from the adjacent 

 surfaces of the two bones bounding an interosseous space. 



Insertion. — (1) Into the sides of the aponeuroses of the extensor communis 

 digitorum tendons; and (2) the adjacent parts of the first phalanges. 



Structure. — The fleshy fibres arise from the adjacent surfaces of the metacarpal 

 l)ones in liipenniform fashion, and converge upon a tendon which, lieginning about 

 the middle of the interosseous space, becomes free just above the metacarpo- 



FiG. 282. — The Dorsal Interossei. 



l)halangeal joint, and tlien passes upon the dorsal aspect of the deep transverse 

 ligament to its insertion into the side of the aponeurosis, and into the upi)er part of 

 the border of the first i)halanx. 



The /?r«^ muscle is thick and ]\v far the strongest, and forms with the adductor 

 pollicis the fleshy wel) which fills the interval between the metacarjial l)ones of the 

 thumb and the index finger. Its origin from the first metacarpal bone occupies 

 the upper half of that bone, while that from the second is more extensive. It is 

 inserted into the radial side of the aponeurosis upon the back of the first phalanx 

 as well as into the u})per part of the outer border of that bone. The second is 

 inserted into the radial side of the ajxnieurosis U])on the first ])halanx of the middle 

 finger and into the adjacent 1)one. The tJiird into the ulnar side of the ajxmeurosis 

 u]ion the back f)f the first i>lialanx of the middle finger and into the adjacent bone. 

 The fourth into the ulnar side of the aponeurosis upon the liack of the ring finger 

 and the adjacent l)one. Of the two heads of each of these muscles, the one arising 

 from the nietacari)al l)one of the finger into which the nuiscle is inserted, is always 

 the larger. 



