46 



JACOB HEIBERG. VON DER DREHUNG DER HAND. 



you notice the expanded head and the 

 rather tapering shaft. The latter is 

 provided with greater resisting power 

 to force applied in its long axis by 

 its double curve, seen well Plate III. 

 and by its ribbed edge. At its upper 

 end the trochlea or pulley surface is 

 not so simple as at first sight appears. 

 and the motion here is not simply 

 antero-posterior but with a curved 

 direction inwards. so that in mid-flexion 

 the hand is thrown across the body. 

 By this means the limb is made more 

 available for protection and feeding. 

 If we look next at its articulation 

 with the radius, we notice that the 

 cup-shaped lesser sigmoid cavity al- 

 lows rotation of the head of the ra- 

 dius upon an axis which is in the 

 centre of the head of the radius; at 

 the lower radio-uluar articulation the 

 conditions are altered and the radius rotates round a point near 

 the styloid process of the ulna. The result of this is to permit 

 free movements of pronation and supination of the hand without 

 displacement at the elbow-joint. In Fig 11 1 the real axis of rota- 

 tion of the radius is the line a, b. This line represents the axis 

 of a cone of which the base is c, d, and the truncated apex is 

 e, f. „The centre of of the truncated apex corresponds with the 

 centre of the head of the radius, and the centre of the base 

 concides with the centre of the circle of which the sigmoid cavity 

 is a segment. If the prolonged axis of tbe head of the radius fall 

 upon any other point than the centre of the sigmoid curve. there 

 would necessarely be a slight hiugelike motion at the upper joint, 

 whenever pronation accurred at the lower." (Ward). 



1 Wards Diagram, S. 48 dieser Mittheilung. 



Fig. 4. 



Copie nach Welcker. ° 4 ) 



