MUSCLES OF THE THUMB 



407 



carpus and is inserted into the radial side of the base of the first phalanx of the 

 thumb. The opponens is a thick muscle extending from the transverse carpal 

 (anterior annular) ligament to the radial side of the first metacarpal. The flexor 

 poUicis brevis arises by two heads, a "deep" and a "superficial" from the carpus 

 and is inserted into the radial side of the base of the first phalanx. The adduc- 

 tor pollicis arises from the carpus and the second and third metacarpals and is 

 inserted into the ulnar side of the first phalanx of the thumb. From the ten- 

 dons of insertion of the abductor and flexor brevis slips are continued into the 

 dorsal aponeurosis of the thumb so that they aid in extending the second 

 phalanx. 



In the foot an opponens hallucis occurs as an abnormal muscle. The abductor, flexor brevis 

 and adductor of the thumb are represented by the corresponding muscles of the big toe, although 



Fig. 377. — The Pronator Quadratus and Deep Muscles of the Palm. 



Pronator quadratus 



Flexor carpi ulnaris 



Abductor digiti V 



Opponens 

 digiti V 



Flexor digiti V 

 brevis 



Fourth volar 

 interosseous 



Abductor pollicis brevis 



Superficial head of 

 flexor pollicis brevis 



Deep head of flex- 

 or pollicis brevis 



Opponens pollicis 



I Volar inter- 

 osseous 



Adductor pollicis, 

 oblique head 



Adductor pollicis, 

 transverse head 



First dorsal 

 interosseous 



Fourth dorsal 

 interosseous 

 Third volar interosseous 



Third dorsal interosseous 



Second volar interosseous 

 Second dorsal interosseous 



the last two muscles are not perfectly homologous in the hand and foot. 



The abductor pollicis brevis (fig. 375). — Origin. — From the volar surface of the transverse 

 carpal (anterior annular) ligament, and from the greater multangular bone (trapezium). Also 

 often from the navicular bone and from a tendon shp of the long abductor. 



Structure and insertion. — The fibre-bundles converge upon a flat tendon with two lamellae, 

 the deeper of which is inserted into the radial side of the base of the first phalanx of the thumb 

 and the superficial into the aponeurosis of the extensor poUicis longus. 



Nerve-supply. — By a branch of the first volar digital ramus of the median nerve. This 

 branch passes over or through the flexor brevis and enters the muscle on the volar surface in 

 the middle third near its ulnar border. 



Action.— To abduct the thumb, flex the first phalanx, and extend the termmal phalanx. 



Relations.— It lies beneath the thenar fascia lateral to the superficial head of the flexor 

 brevis and over the opponens. The superficial volar artery usually perforates the muscle. 



Variations. — It may be wanting or may be divided into two divisions. The origm may 

 extend to the fascia of the forearm or styloid process of the radius. It may receive an accessory 

 slip from the long radial extensor, the opponens, or the short extensor of the thumb. A thenar 

 subcutaneous muscle is occasionallv present. It is narrow, is closely associated with the short 

 abductor of the thumb, and extends from the radial side of the base of the first metacarpal into 

 the skin of the thenar eminence. 



