154 THE UPPER EXTREMITY 



surface of the discus articularis. The os lunatum at the 

 same time crosses the bounding ridge on the distal surface 

 of the radius, and encroaches on the territory of the os 

 naviculare, whilst a considerable part of the surface of the 

 os naviculare leaves the radius and comes into contact with 

 the lateral part of the capsule. 



Synovial Membrane. The synovial membrane of the 

 radio-carpal joint lines the capsule and it covers the proximal 

 surfaces of the two interosseous ligaments which complete the 

 carpal surface. Sometimes the discus articularis is imperfect, 

 and in these cases the synovial membrane of the radio -carpal 

 joint becomes continuous with the synovial membrane of the 

 inferior radio-ulnar joint. 



Movements at the Radio-carpal Joint. The hand can be moved in 

 four directions at the radio -carpal joint. Thus we have (a) volar 

 movement, or flexion ; (b] dorsal movement, or extension ; (c] ulnar 

 movement, or adduction ; (d) radial movement, or abduction. In 

 estimating the extent of these movements in the living person the student 

 is apt to be misled by the increase of range which is contributed by the 

 carpal joints. Thus, flexion is in reality more limited than extension, 

 although by the combined action of both carpal and radio-carpal joints 

 the hand can be carried much more freely volarwards than dorsally. 

 Adduction, or ulnar flexion, can be produced to a greater extent than 

 abduction, or radial flexion. In both cases the extent of movement at 

 the radio-carpal joint proper is very slight, but the range is extended by 

 movements of the carpal bones. The styloid process of the radius 

 interferes with abduction. 



The muscles which are chiefly concerned in producing these different 

 movements of the hand at this joint are the following : (a) flexors the 

 flexor carpi radialis, the palmaris longus, and the flexor carpi ulnaris ; 

 (ti) extensors extensor carpi radialis longus, the extensor carpi radialis 

 brevis, and the extensor carpi ulnaris ; (c) abductors, or radial flexors 

 flexor carpi radialis, extensor carpi radialis longus, abductor pollicis 

 longus, and the extensor pollicis brevis; (d) adductors, or ulnar flexors 

 extensor carpi ulnaris and flexor carpi ulnaris. 



ARTICULATIONES RADIO-ULNARES (RADIO-ULNAR JOINTS). 



At the two radio-ulnar joints, proximal and distal, the 

 movements of pronation and supination take place. At 

 the proximal joint the medial part of the head of the radius 

 fits into the incisura radialis of the ulna ; at the distal the 

 small capitulum ulnae is received into the incisura ulnaris 

 on the medial side of the distal end of the radius. In 

 connection with these joints there are special ligaments 

 which retain the bones in apposition. These are (i) for 

 the proximal radio-ulnar joint, the annular ligament ; and 



