48 JACOB HEIBERG. VON DER DREHUNG DER HAND. 



diameter, would represent the lower end of the fixed axis around 

 which the radius rotates. That this is impossible may be showu 

 by making a dot upon the ulna precisely opposite the end of a line 

 so drawn, and then rotating the radius. The extremity of the line 

 will no longer correspond to the dot, but will be found removed 

 from it, by the extent of a quarter of a circle. Now, a movable 

 point cannot be the extremity of a iixed axis. If it should be con- 

 tended, on behalf of Cruveilhier's assertion, that the axis of rota- 

 tion, thoug it cannot pass trough the extremity of this line may 

 lie in the interval between the radius and ulna at a point imme- 

 diately beyond it, the refutation of such an opinion is of the same 

 kind. and equally easy. It is only necessary to mark the terminal 

 Fig. 6 surface of the radius with lines con- 



verging to the point in question, and 

 then to perform, as before, the movement 

 of pronation. The lines will now cease 

 to indicate the original point; perpetu- 

 ally changing their centre of conver- 

 gence during the rotation of the bone, 

 and leaving the true position of the sta- 

 tionary axis as uncertain as before. The 

 real axis of rotation of the radius is the 

 line a b in the following diagram. This 

 line represents the axis of a cone, of 

 which c d is the base, and e f the trun- 

 cated apex. The centre of its truncated 

 apex corresponds with the centre of the 

 head of the radius, and the centre of 

 its base coincides with the centre of the 

 / i circle of which the sigmoid cavity is a 



| p \ i segment. Hence the axis of the cone 

 I V must, according tho the law above sta- 



. . ^- ^-■k ted, coincide with the axis of the head. 



vi This coincidence is plainly shown in the 

 Copie nach Waid. diagram. The portion of the line a b 



