228 MUSCLES OF THE UTTER LIMB. 



The dorsal interossei muscles separate the fingers from the middle line of 

 the hand ; they are four in number, each occupying one of the spaces between 

 the metacarpal bones. They are named numerically from without inwards. 

 Each muscle arises from both the metacarpal bones between which it is 

 placed, but most extensively from that supporting the finger upon which it 

 acts, and the fibres converge pennately to a common tendon in the middle. 

 Each terminates in a tendon, which is inserted partly into the base of the 

 first phalanx, and partly into the tendon of the common extensor muscle 

 on the dorsum of the same part of the finger. Two dorsal interossei are 

 inserted into the middle finger and draw it to either side ; of the remaining 

 muscles one passes to the radial side of the index-finger, and the other 

 to the ulnar side of the ring-finger ; they withdraw those fingers from the 

 middle line of the hand. 



The first dorsal interosseous muscle or abductor indicis is larger than the 

 others ; its outer and larger head of origin arises from the proximal half of 

 the ulnar border of the first metacarpal bone, the inner is attached to the 

 whole length of the second metacarpal bone, and between these heads there is 

 left superiorly an interval wider than in the other dorsal interosseous muscles. 



Between the heads of the abductor indicis the radial artery passes forwards to the 

 palm of the hand ; between those of the other dorsal interossei small perforating 

 arterial branches are transmitted. 



The three palmar interossei are adductors, drawing the index, ring, and 

 little fingers towards the middle line of the hand. They are visible only 

 from the palmar aspect of the hand, each one being attached to the metacarpal 

 bone of the finger on which it acts : they terminate, like the dorsal muscles, 

 in small tendons inserted partly into the base of the first phalanx at the 

 side, and partly into the extensor tendon. The first palmar interosseous 

 muscle bolongs to the ulnar side of the index-finger ; the others are placed 

 on the radial sides of the ring and little fingers. 



The interosseous muscles are subject to some variation in their number and form. 

 An additional palmar interosseous muscle, going to the dorsal aponeurosis of the 

 thumb, is described as frequent by Henle and Wood. (See J. Wood, " On some 

 varieties in Human Myology," in the Proc. of the Roy. Soc. for 1864, p. 302.) 



ACTIONS OF THE MUSCLES OF THE FOREARM AND HAND. 



The muscles of the forearm may be distinguished according to their actions as 

 pronators and supinators, flexors and extensors of the wrist, and long flexors and 

 extensors of the fingers : those of the hand are flexors, adductors, abductors, 

 opposers, and partially also extensors of the fingers. 



Pronation is effected by the pronator quadratus and pronator teres : the flexor 

 carpi radialis also contributes slightly to this movement. The pronator teres is fitted 

 to flex the elbow when pronation has been completed, or when it is prevented by 

 antagonistic muscles. 



Supination is effected principally by the supinator brevis. The supinator longus 

 is not so advantageously situated to accomplish this action ; it is principally & flexor 

 of the elbow, acting, however, only after that movement has been begun by the 

 instrumentality of other muscles : in the extended and supinated condition of the 

 forearm it assists in binding the radius more firmly to the humerus. 



Flexion of the wrist is produced by the radial and ulnar flexors of the carpus, and 

 is aided by the flexors of the fingers when the action of those muscles on the fingers 

 is either completed, or is opposed by any resistance, as when the over-extended hand 

 is pressed against a surface in pushing, or in the support of the body. 



Extension of the wrist, in a similar manner, is accomplished not only by the three 

 muscles specially devoted to that function, but also by the extensors of the fingers. 



