THE ANTIBRACHIAL MUSCLES. 



599 



external condyle of the hu- 

 merus, from the adjacent in- 

 termuscular septa, and from 

 the deep fascia of the fore- 

 arm. Its fibres converge at 

 about the middle of the fore- 

 arm into a flat tendon, which 

 passes with the long exten- 

 sor carpi radialis beneath the 

 posterior annular ligament 

 in the second compartment 

 and is inserted into the base 

 of the third metacarpal, a 

 bursa (bursa m. extensoris 

 carpi radialis) being inter- 

 posed between the tendon 

 and the bone. 



Nerve-Supply. — By 

 the posterior interosseous 

 branch of the musculo-spiral 

 nerve from the sixth and 

 seventh cervical nerves. 



Action. — To extend 

 the hand. 



Variations. — It may be fused 

 to a greater or less extent with 

 the extensor carpi radialis lon- 

 gior and may be inserted into 

 the bases of both the second 

 and third metacarpals. 



4. Extensor Communis 

 DiGiTORUM (Fig. 579). 



Attachments. — The 

 common extensor of the 

 fingers (m. eytensor digitorum 

 communis) arises in com- 

 mon with the neighboring 

 superficial extensors from 

 the external condyle of the 

 humerus, from the septa be- 

 tween it and the adjoining 

 muscles, and from the deep 

 fascia of the forearm. At 

 about the middle of the 

 forearm its fibres go over 

 into four tendons, which pass 

 through the fourth compart- 

 ment beneath the posterior 

 • annular ligament and diverge 

 to be inserted into the bases of 

 the middle and terminal pha- 

 langes of the second, third, 

 fourth, and fifth fingers. Just 

 before they pass over the 

 metacarpo-phalangeal joints 

 of their digits the four ten- 

 dons are usually united by 



Fig. 579. 



ij Brachio-radialis 



External condyle 



Anconeus 



Extensor carpi 

 radialis longior 



Extensor carpi 

 radialis brevier 



Flexor carpi ulnans. 



Extensor carpi ulnaris 



Extensor minimi digiti 



Extensor 



communis 



digitorum 



Extensor indicis tendon 



Tendons ot 



extensor 



communis 



digitorum 



Extensor ossis 

 metacarpi pollicis 



Extensor brevis 



pollicis 

 Extensor longus 



I Tendon of extensor 



' ff-r^Ul carpi radialis longicr 



Tendon of extensor 



carpi radialis brevior 



Abductor 



pollicis 



First dorsal 

 interosseus 



Dissection of posterior surface of forearm and hand, showing superficial 



extensor muscles. 



