Limbs: Upper Limb 103 



(5) Trapezium : at the side, with trapezoid : above, with scaphoid : below, sup- 

 ports first metacarpal and also, owing to small size of trapezoid, helps to support 

 second metacarpal. 



(6) Trapezoid : at the sides, with trapezium and os magnum : above, with 

 scaphoid : below, with second metacarpal. 



(7) Os Magnum : at the sides, with trapezoid and unciform : above, with scaphoid 

 and semilunar : below, with middle metacarpal and also second and fourth. 



(8) Unciform : at the side, with os magnum : above, with cuneiform and semi- 

 lunar : below, it supports the fourth and fifth metacarpals. 



This account shows that only one bone, the trapezoid, articulates with a single 

 metacarpal. 



The appearance of the carpo-metacarpal articulation is enough to show that the 

 bones are practically fixed there and that no appreciable movement can take place 

 except in the thumb,* where the form of the joint surfaces indicates that movement 

 can take place from side to side or from before backwards : this joint has a separate 

 synovial cavity, whereas the others have a common cavity. 



The proximal row of carpal bones is immediately concerned in the wrist joint, and 

 the curves of the articular surfaces on its upper aspect 

 show that its range of movement at the wrist extends 

 from before backward as well as from side to side. 

 Observe that the joint surface on the scaphoid and 

 semilunar is largely dorsal, more so than is necessary 

 to correspond with the lower surface of the radius, 

 which looks downward and slightly forward. It 

 follows from this that the carpus can be extended FlG 83 ._ The bones which make 



on the radius to a greater extent than it can be flexed. the upper surface of the mid- 



,. ., . , , carpal joint. The axis of move- 



The upper surface on the cuneiform is also somewhat ment is approximately indicated 



dorsal in position, though to a less extent, thus by the interrupted line, 



agreeing with the plane of the fibro-cartilage. 



When the hand is abducted or adducted the carpus moves round an antero- 

 posterior axis passing through its centre that is, when the hand is abducted or moved 

 to the radial side, the first row of the carpus moves towards the ulna, the semilunar 

 passing partly under the fibro-cartilage ; but when the opposite and more extensive 

 movement takes place the bones are moved outwards, and the cuneiform may come 

 partly under the radius. 



Observe that the scaphoid and semilunar form a socket for the reception of the 

 os magnum (Fig. 83) : the cuneiform is kept from this by the articulations between 

 semilunar and unciform. In this joint flexion and extension are possible between the 

 two rows of carpal bones, and in fact the so-called flexion and extension of the wrist 

 really takes place in the more advanced stages at this mid-carpal joint. The back rim 

 of the socket is deeper than the front so that, in contrast with the wrist, extension is 

 the more limited of the two movements. 



Lateral movement in the mid-carpal joint is quite impossible owing to the locking 

 of the bones at the .sides. The movement of the os magnum necessitates proper 

 surfaces between the other bones of the second row and those of the first row, and 

 we find accordingly that the curve of the head of the os magnum is carried on to 

 the neighbouring part of the unciform, which fits into a concavity on the cuneiform, 

 but, further in, this curve on the unciform becomes a concavity that is, in the 



* Among the others the fifth metacarpal has most freedom, and that is not much : the articular surfaces 

 suggest this, and the movement is used in opposing the thumb. 



