chap, xiv.] THE FORE-ARM. 231 



or a gimlet the movements of pronation and supination 

 are conspicuously involved, but the main force is 

 applied during supination. It is significant that the 

 thread of a corkscrew is so turned that it shall be in- 

 serted by supination rather than by prona,tion. 



The only position in which the two bones are 

 parallel to one another is in the mid-position between 

 pronation and supination. It is in this posture only 

 that the interosseous membrane is uncoiled throughout. 

 Hence the selection of this position in the adjustment 

 of most fractures of the fore-arm. The interosseous 

 space is an irregular ellipse, a little larger below than 

 above. It is narrowest in full pronation, widest in 

 supination, and nearly as wide in the mid-position. 



It may be noted that the oblique ligament tends 

 to resist forces that would drag the radius away from 

 the humerus, and takes the place and the function of 

 a direct ligament, passing from the humerus to the 

 radius, while the interosseous membrane, from the 

 obliquity of its fibres, makes the ulna take a share in 

 the strain put upon the radius when that bone is 

 forced upwards, as in resting on, or pushing with, the 

 palm. 



Fractures of the fore-arm. The two bones 

 are more often broken together than is either the 

 radius or the ulna alone. The radius, when broken 

 alone, is usually fractured by indirect violence, since 

 it receives more or less entirely all shocks transmitted 

 from the hand. The ulna, on the contrary, is more 

 often broken by direct violence, it being the more super- 

 ficial and exposed of the two bones. For example, in 

 raising the arm to ward off a blow from the head, the 

 ulna becomes uppermost. When the two bones are 

 broken together, the violence may be direct or indirect. 

 Malgaigiie reports a case where both bones were broken 

 by muscular violence in a patient while shovelling 

 earth. Here the bones probably were broken between 



