FOREARM AND HAND 



179 



extend the metacarpo-phalangeal joints ; (4) he can also flex 

 the metacarpo-phalangeal joints and extend the interphalangeal 

 joints. The last combination of movements is called " putting 

 the fingers in the writing position." It is due mainly to the 

 actions of the lumbricals and interossei, which pass from the 

 volar to the dorsal aspect across the metacarpo-phalangeal 

 joints, and so are enabled to flex those joints, whilst by virtue 

 of their attachments to the extensor expansions they can 

 extend the interphalangeal joints. 



Os triquetrum 

 Os pisiforme 



Os lunatum 



Os capitatum 



Os hamatum 



Extensor carpi ulnaris 



|Os naviculare manus 

 Extensor carpi radialis brevis 

 / Os multangulum minus 



} Os multangulum majus 



Extensor carpi radialis longus 

 Abductor pollicis longus 



FIG. 77. Dorsal aspect of the Bones of the Carpus and Metacarpus, 

 with Muscular Attachments mapped out. 



M. Extensor Carpi Radialis Longus. The long radial 

 extensor of the carpus is placed dorsal to the brachio-radialis. 

 It arises from the dista third of the lateral epicondylic ridge 

 of the humerus, and from the lateral intermuscular septum. 

 From the fleshy portion of the muscle a long tendon pro- 

 ceeds, which passes under cover of the dorsal carpal ligament, 

 and is inserted into the dorsal aspect of the base of the 

 metacarpal bone of the index finger. The muscle is supplied 

 by a branch of the trunk of the radial nerve (O.T. musculo- 

 spiral) which enters it proximal to the elbow. The long 



I 12 a 



