104 



THE HUMAN MOTOR 



the fingers and in the connexion of the astralagus with the 

 tibia (fig. 112). 



FIG Hi. 



A pivot joint has also 

 a longitudinal rotation 

 round a single axis, such 

 is the movement of the 

 radius. 



If the bone has a helical 

 joint, i.e., when the fur- 

 row S is oblique, then the 

 movement is a combined 

 translation and rotation 

 ( 7), but the rotation 

 is at the most through 

 180, and the translation 

 is limited by its connex- 

 ions. Therefore there are 

 not really two degrees 



Elbow Joint. 



Fro. 112. - 



Joints of^the Foot. 



of freedom, but, practically, only one, the rotation. The 

 elbow offers a clear example of this, where the translation 

 is about 1 millimetre (*) (fig. Ill) ; the joint ol the astralagus 

 is not far from being a helicoidal joint (fig. 112). When, finally, 

 the section of the head of the joint is a spiral curve (fig. 113) the 

 moving bone goes back, because the distance of the point M to 

 the centre O is not constant, and traction is exerted on the liga- 

 ments whose elasticity limits the displacement. This takes 

 place in the knee-joint (fig. 109) or the tibia femur. 



W. Hultkranz, Das Ellenbogengelenk und seine Mechanik, Jena .1897. 



