

Back of Fore-ami 281 



The extensor communis digitorum lies along the ulnar side of 

 the preceding muscle. Arising from the external condyle and from the 

 fascia investing it, it divides into tendons for the four fingers. These 

 pass in the shallow radial groove, together with the tendon of the 

 extensor indicis, lubricated by the one synovial membrane. As they 

 pass over the metacarpus the three inner tendons are connected with 

 each other by short slips. The tendons spread out and form the 

 posterior ligaments for the metacarpo-phalangeal joints ; at the next 

 joints they divide into three slips, of which the middle one is inserted 

 into the base of the middle phalanx, while the lateral slips pass on to 

 the base of the last phalanx ; in each case they act as posterior liga- 

 ments. Nerve, the posterior interosseous. 



The extensor minimi digit! arises like the last muscle, along the 

 ulnar side of which it runs. Its slender tendon occupies a separate 

 compartment beneath the annular ligament, in the groove between the 

 radius and ulna, and is inserted in common with the innermost tendon 

 of the extensor communis. Nerve, the posterior interosseous. 



The extensor carpi ulnaris arises from the external condyle, 

 between the extensor minimi digiti and anconeus, and passes along 

 the edge of the latter muscle to the posterior border of the ulna, along 

 which it also arises. Its tendon runs in the groove on the inner side 

 of the head of the ulna, behind the styloid process, under the annular 

 ligament, and is inserted into the base of the fifth metacarpal bone. 

 Its nerve-supply is from the posterior interosseous. 



The anconeus (ay/cwi/, elbow) looks like a piece of the triceps which 

 has been cut off by the external condyle, from the back of which it 

 arises. It is inserted into the adjacent part of the olecranon process, 

 and a little way down the back of the shaft of the ulna. Being in its 

 origin, insertion, and action so like the triceps, it is naturally supplied 

 by the same nerve as the triceps, the musculo-spiral. 



The muscles of the deep layer at back of fore-arm are directed 

 obliquely downwards and outwards, and intervene between the 

 posterior interosseous vessels and nerve and the back of the inter- 

 osseous membrane. The supinator brevis arises from the outer aspect 

 of the ulna, and surrounds the upper third or more of the radius. 

 The extensor ossis metacarpi arises from both bones, and the extensor 

 primi internodii pollids from the radius only ; these two muscles 

 form a projection at the back of the lower third of the radius as 

 they descend over the radial extensorsof the wrist to the groove on 

 the outer side of the styloid process. The former of them is inserted 

 into the base of the metacarpal bone, the latter into that of the first 

 phalanx of the thumb ; between the styloid process and the root of 

 the thumb they cross the radial artery ; serous effusion into their 

 synovial sheath is not of infrequent occurrence. 



The extensor secundi arises from the ulna, and so, to reach its 

 insertion at the root of the ungual phalanx, it has to pass very 



