THE PRINCIPLES OF MYODYNAMICS. 



123 



popliteus pulls on the external condyle and in- 

 rotates the leg. 



1 1 5. In those cases of triple displacement of 

 the leg, on account of disease : (i.) The out- 

 rotation is due to the biceps cruris ; (2.) The 

 flexion is due to the semi-membranosus and the 

 semi-tendinosus ; (3.) The sub-luxation is due 

 to the semi-membranosus, the biceps cruris, and 

 the semi-tendinosus. 



116. Some general conclusions may now be 

 made in regard to the stability of the knee-joint, 

 when the leg is in different positions : — 



(1.) The condition of maximum stability of 

 the knee-joint is when the leg is completely 

 extended, for then the retentive components of 

 all the muscles that span the knee-joint have a 

 maximum mao-nitude. This condition of sta- 

 bility of the knee-joint gradually diminishes 

 until the tibia nearly meets the femur at right 

 angles. 



{2.) The condition of minimum stability of 

 the knee-joint is in the vicinity of semi-flexion 

 of the leg. The displacing components of the 

 muscles that span the knee-joint are very great 



