108 DR JAMES F. GEMMILL. 



The behaviour of the main sheet of the interosseous mem- 

 brane in pronation and supination deserves attention, because 

 {as has been noted by various anatomists) the dimensions of 

 the interosseous space change considerably in the different posi- 

 tions. Accurate measurement of these dimensions is by no 

 means easy, because the bones are lying now parallel and now 

 diagonal to one another. The following method gives fairly 

 exact results. An outline of the ulnar interosseous edge of 

 natural size is made on a piece of paper, and is divided by 

 suitable markings into a great number of equal parts. Dots 

 corresponding to each of these markings are made on the inter- 

 osseous edge of the ulna itself. By means of a pair of com- 

 passes, the distance between each of these dots on the ulna to 

 the nearest point on the interosseous edge of the radius is 

 taken, and is then transferred to the paper opposite to its 

 corresponding mark on the outline of the ulnar edge. A 

 new series of points is thus obtained, by joining which a pro- 

 jection outline of the interosseous edge of the radius at its 

 absolute distance from that of the ulna is given. On com- 

 paring the interosseous spaces, marked out thus in pronation, 

 in the middle position and in supination, one finds that the 

 transverse measurements are reduced by a fifth or a sixth 

 in pronation as compared with the middle position, while in 

 supination they change very shghtly, being at certain parts a 

 little greater, and at others a little less than in the middle 

 position. One would naturally expect to find the interosseous 

 membrane slackened very considerably in pronation, but as a 

 matter of fact this occurs only in a very limited sense. Trans- 

 versely the membrane is made lax, and even thrown into folds, 

 but the most important part of the membrane, namely, its long 

 fibres, is kept almost uniformly taut. This is necessary if the 

 membrane is to remain capable of transmitting pressure from 

 radius to ulna in all positions. The slight upward movement 

 of the radius indicated previously in this paper as occurring 

 especially in pronation, serves to balance the effect of the 

 narrowing of the interosseous space, so that the distance 

 between the origin and insertion of each fibre remains as nearly 

 as possiljle the same during the whole of circumduction. 



If one follows the line of the axis of the circumduction cone, 



