PRONATORS 107 



Note. Since the tendons of the superficial flexor stop at the second 

 phalanges, while those of the deep flexor pass to the third phalanges, there is a 

 fissure in each superficial tendon just before it ends, through which the deep 

 tendon passes forward to the bone of the finger-tip (Fig. 87). 



Nerves. Median and ulnar. 



Flexor pollicis longus, or long flexor of the thumb. Origin. The shaft of the 

 radius (under flexor sublimis). Insertion. The last phalanx of the thumb. 



Action. It flexes the tip of the thumb. 



Nerve. Median. 



Note. These tendons for the fingers and thumb lie in the 

 deep groove on the front of the carpus. Friction between them is 

 prevented by sheaths of synovial membrane vaginal synovial 

 membranes. 



THE Two PRONATORS, THE ROUND AND THE SQUARE 



Pronator teres, or round pronator (Fig. 87). 



Origin. The medial epicondyle, and a small slip from the 

 ulna (coronoid process) . It passes across to the lateral side of the 

 radius, to the insertion at the middle of the shaft. 



Nerve. M edian . 



Pronator quadratus, or square pronator. 



Origin. The shaft of the ulna. Insertion. The shaft of the radius. It 

 lies just above the wrist and underneath the long muscles (close to the bones) . 



Nerve. Median. 



Action of the two pronators. They rotate the radius so as to 

 turn the palm downward (or backward). 



One slender muscle, which is superficial to all, is the palmaris longus. 

 It arises on the medial epicondyle and is attached below to the palmar fascia 

 to keep it tense a tensor of the palmar fascia. 



Nerve. Median. 



Note. It is understood that the muscles arising from the epicondyle 

 have a common tendon of origin. 



Practical point. Observe, by experimenting, that flexion and moderate 

 pronation are naturally performed together, and are associated in the major- 

 ity of the motions which are required of the upper extremity. 



MUSCLES OF THE FOREARM 

 Posterior (Fie. 88.) 



The lateral epicondyle of the humerus and the ridges above it 

 give origin to the muscles which extend the wrist and fingers. 



