MOVEMENT OF THE LOWER END OF THE EADIUS. 103 



of a circle, the centre of which lies in the middle of the head of 

 the ulna, at a point considerably external to the attachment of 

 the triangular ligament. As the radius in pronation and supi- 

 nation glides closely over the ulnar articular surface, it is only 

 reasonable to infer that the centre of the circumduction curve 

 will also lie in the middle of the head of the ulna. The lesser 

 sigmoid cavity of the radius, by providing an almost accurately 

 fitting concave surface, prevents any movement on the part of 

 the radius other than pure circumduction, or at least makes such 

 movement possible only under circumstances of exceptional 

 strain. The arrangements at the upper radio-ulnar joint con- 

 duce to the same result (see Ward, loc. cit.). 



If the movements of pronation and supination be performed 

 artificially in any forearm which has been dissected to exhibit 

 the ligaments, it will be found that in full pronation the anterior 

 limb of the triangular ligament is slackened, while the posterior 

 limb is taut. In full supination the converse is the case. Only 

 at or near the position of semi-pronation are both limbs equally 

 stretched. Were the centre of motion placed at the insertion 

 of the triangular ligament, one would expect to find equal 

 tension of both limbs in every position. Compare Sappey, 

 op. cit, p. 622. 



A tracing of the circumduction curve in relation to the head 

 of the ulna may be got in the following way — (a similar 

 method is described by Heiberg, loc. cit.) : — 



Disarticulate at the wrist a forearm of which the ligaments 

 are intact. Drive a stout needle of suitable length upwards 

 into the lower end of the radius near the styloid process, and 

 another into the lower end of the ulna at the insertion of the 

 triangular ligament. Fix the ulna immovably at its natural 

 inclination to a horizontal plane surface in such a way that the 

 needles rest against the plane. On pronating and supinating 

 the radius, the needle attached to the radius describes a curve 

 tracing which is as nearly as possible a semicircle, if the move- 

 ments are fully carried out. By compass measurement it will 

 be found that the centre of this semicircle lies external to the 

 insertion of the triangular fibroplate at a point corresponding 

 to the centre of the capituluni ulnae. 



The thickly outlined curve, CO., in fig. 1, was obtained in the 



