.142 THE PRINCIPLES OF MYODYNAMICS. 



142. In regard to the stability of the hip-joint 

 two important statements may be made : — 



(i.) The conformation of the hip-joint is such 

 that, under all ordinary myodynamic relations, 

 the muscles that span this joint have greater 

 retentive than displacing components : — where- 

 fore, the hip-joint is in a condition of myody- 

 namic stability. 



(2.) If the femur is moved in any direction, 

 so as to bring the femoral neck against the 

 brim of the acetabulum, the femur will become 

 a lever of the first order, which will tend to lift 

 the femoral head out of its socket when the 

 powerful retentive components of the hip-joint 

 muscles, will be turned into displacing compon- 

 ents, so that external violence and muscular 

 force will co-operate to dislocate the femur : 

 Under the circumstances, the hip-joint is in a 

 condition of dynamic instability. — And these 

 facts agree with the practical observations of 

 surgeons. 



143. The femoral shaft has an arch which 

 bends forward, and this throws the lesser troch- 



