284: DISSECTION OF THE FOREARM. 



nator, A, the two radial extensors of the wrist, B and c (long and short), 

 the common extensor of the fingers, D, the extensor of the little finger, E, 

 and the ulnar extensor of the wrist, F. There is one other small muscle 

 near the elbow, the anconeus, G. 



The SUPINATOR RADII LONGUS, A, reaches upwards into the arm, and 

 limits on the outer side the hollow in front of the elbow. The muscle 

 arises from the upper two-thirds of the outer condyloid ridge of the hu- 

 merus, and the front of the external interrnuscular septum. The fleshy 

 fibres end about the middle of the forearm in a tendon, which is inserted 

 into the lower end of the radius, close above the styloid process. 



In the arm the margins of the supinator are directed towards the sur- 

 face and the bone, but in the forearm and muscle is flattened over the others, 

 with its edges forwards and backwards. Its anterior border touches the 

 biceps and the pronator teres ; and the posterior is in contact with both 

 radial extensors of the wrist. Near its insertion the supinator is covered 

 by two extensors of the thumb. Beneath the muscle are the brachialis 

 anticus and musculo-spiral nerve, the extensors of the wrist, the radial 

 vessels and nerve, and the radius. 



Action. The chief use of this supinator is to bend the elbow-joint. But 

 if the radius is either forcibly pronated or supinated the muscle can put the 

 hand into a state intermediate between pronation and supination, the 

 thumb being brought above the forefinger. 



If the radius is fixed as in climbing, the muscle will bring up the hu- 

 merus, bending the elbow. 



The EXTENSOR CARPI RADIALIS LONGIOR, B, arises from the lower third 

 of the outer condyloid ridge of the humerus, and the front of the contigu- 

 ous intermuscular septum ; and from the septum between it and the next 

 muscle. The muscle lies on the short radial extensor, being partly cov- 

 ered by the supinator longus ; and its tendon passes beneath the extensors 

 of the thumb, and the annular ligament, to be inserted into the base of the 

 metacarpal bone of the index finger. Along its outer border lies the radial 

 nerve. 



Action. The long extensor straightens first the wrist, and bends next 

 the elbow joint. 



If the hand is fixed in climbing, it will act on the humerus like the 

 long supinator. 



The EXTENSOR CARPI RADIALIS BREViOR, c, is attached to the outer 

 condyle of the humerus by a tendon common to it and the three following 

 muscles, viz., common extensor of the fingers, extensor of the little finger 

 and ulnar extensor of the wrist : it takes origin also from the capsular 

 ligament of the elbow joint. The tendon of the muscle is closely con- 

 nected with the preceding, and after passing with it through the same com- 

 partment of the annular ligament, is inserted into the base of the meta- 

 carpal bone of the middle finger. 



Concealed on the outer side by the two preceding muscles this extensor 

 rests on the radius and some of the muscles attached to it, that is to say, 

 on the supinator brevis, and the pronator teres. Along the ulnar side is 

 the common extensor of the fingers; and the extensors of the thumb come 

 between it and the digital extensor. Both radial extensors of the carpus 

 have usually a bursa at the insertion. 



Action. This muscle resembles its fellow in extending the wrist, but 

 differs from it in extending the elbow. 



