390 



THE MUSCULAR SYSTEM. 



BICEPS BBACHII 



LACERTUS FIBROSUS 



(by their attachment to the extensor tendons) as extensors of the fingers, acting on both inter- 

 phalangeal joints. 



M. Flexor Pollicis Longus. The flexor pollicis longus arises, beneath the 

 flexor digitorum sublimis, by fleshy fibres, from the volar surface of the body of 



the radius in its middle two- 

 fourths, and from a corresponding 

 portion of the interosseous mem- 

 brane. It has an additional origin, 

 occasionally, from the medial border 

 of the coronoid process of the ulna 

 (Fig. 348, p. 389). Its radial origin 

 is limited proxirnally by the oblique 

 proximal part of the volar margin 

 of the radius and the origin of the 

 flexor digitorum sublimis, and 

 distally by the insertion of the 

 pronator quadratus muscle. 



The muscle ends, proximal to 

 the wrist, in a tendon, which passes 

 over the pronator quadratus into 

 the hand beneath the transverse 

 carpal ligament, and is enveloped 

 in a special mucous sheath. 



In the palm the tendon is 

 directed distally along the medial 

 side of the thenar eminence, be- 

 tween the flexor brevis and ad- 

 ductor muscles of the thumb, to 

 be inserted into the base of the 

 terminal phalanx of the thumb 

 on its volar surface. 



The muscle is placed deeply in 

 the forearm, being concealed by the 

 superficial layer of muscles and by 

 the flexor digitorum sublimis. 



Nerve - Supply. Volar interosseous 

 branch of the median (C. 7. 8. T. 1.). 



Actions. The muscle is a flexor of 

 the wrist and thumb, acting in the latter 

 movement on the metacarpal bone and 

 both phalanges. 



M. Pronator Quadratus. The 

 pronator quadratus is a quadri- 

 lateral fleshy muscle, occupying 

 the distal fourth of the forearm. 

 It is placed beneath the deep flexor 

 tendons, and arises from the distal 

 fourth of the volar margin and 

 surface of the ulna (Fig. 348, 

 p. 389). 



It is directed transversely later- 

 ally to be inserted into the distal 

 fourth of the volar surface of the 



PRONATOR 

 TERES 



SUPERFICIAL 



FLEXOR ORIGIN 



BICEPS TENDON 



TUBERCLE 

 OF RADIUS 



SUPINATOR MUSCLE 



BRACHIORADIALIS 



PRONATOR TERES 



FLEXOR DIGITORUM.. 

 PROFUNDUS 



FLEXOR CARPI 



ULNARIS 



FLEXOR DIGITORUM 



PROFUNDUS (fl 



index finger) 



FLEXOR POLLICIS LONGUS 



BRACHIORADIALIS . 



FLEXOR DIGITORUM, 

 SUBLIMIS 



PRONATOR QUADRATUS--^ 



FLEXOR DIGITORUM 



SUBLIMIS 



Pisiform bone 



FLEXOR CARPI RADIALIS 



ABDUCTOR POLLICIS 



LONGUS 



radius, and into the narrow tri- 



FIG. 349. THE DEEPEST MUSCLES ON THE VOLAR ASPECT 

 OF THE LEFT FOREAKM. 



angular area on its medial side, in 



front of the attachment of the interosseous membrane (Fig. 348, p. 389). 



The pronator quadratus is subject to considerable variations. It may even be 

 absent ; or it may have an origin from radius or ulna, or from both bones, and an 

 insertion into the carpus. 



