106 DR JAMES F. GEMMILL. 



the shaft of the radius. The reason for this vertical move- 

 ment lies in the fact that the axis of this circumduction curve 

 is placed obliquely to the axis of the ulna. The base of the 

 cone is therefore placed obliquely also, and has one part of its 

 circumference lower than any other part in relation to the head 

 of the ulna. This lowest part corresponds to the place of inter- 

 section of the circumduction cone by a plane which contains 

 the axis of circumduction and the axis of the ulna. The styloid 

 process of the radius lies at this point (and therefore in its 

 lowest position), a little to the supination side of semi- 

 pronation. 



Fig. 2 is meant to explain the vertical movement in question. 

 The fig. is purely diagrammatic, and exaggerates the relations it 

 is meant to illustrate. 



The fact indicated above can also be made out from skiagrams 

 of the forearm in pronation and supination. It also helps to 

 explain the circumstance that a slight amount of abduction at 

 the wrist accompanies full pronation. 



II. Interosseous Membrane. 



As is well known, the main sheet of the interosseous mem- 

 brane of the forearm consists of fibres which run obliquely 

 downwards and inwards from radius to ulna. The membrane is 

 of considerable strength, and is best developed at a part which 

 has its outer origin near the junction of the upper and middle 

 thirds of the radius, and its inner origin near the junction of 

 the middle and lower thirds of the ulna. An idea of the 

 strength of the membrane may be obtained in the following 

 way: — Eemove all the structures connecting radius and ulna 

 except the interosseous membrane. Fix the shaft of the radius 

 firmly in a vertical position by means of a vice. Weights may 

 now be hung on to the lower end of the ulna by means of a 

 suitable loop. It will be found that the interosseous membrane 

 of an average forearm can bear without breaking a strain of 

 20 stones (280 lbs.). The manner in which the interosseous 

 membrane is put on the stretch by forces acting in certain 

 directions can readily be demonstrated in the dissecting-room, 



