THE PRINCIPLES OF MYODYNAMICS. 1 39 



139. The psoas magnus acts on the femur, 

 making it a lever of the third order. In a given 

 case, the following approximate measurements 

 were made : (i.) The length of the femur was 

 about seventeen inches, that is the weight-arm 

 of the lever : (2.) The distance from the tro- 

 chanter minor in the line of the rectangular 

 component of the psoas magnus to the head of 

 the femur was about four inches, that is the 

 power- arm of the lever. See Fig. 20 : T-m is 

 the trochanter minor, and E is the ilio-pectineal 

 eminence, while P is the distance between these 

 two points : The rectangular components are 

 3 and 4, while the resultant is 5. The result- 

 ant expresses the entire force of the psoas 

 magnus. Let the weight applied to the femoral 

 condyles be twelve pounds : The formula for 

 the femoral lever will be — 



12x17 — 51x4 : — 

 Then the moving component of the psoas mag- 

 nus would be fifty-one pounds : Divide this by 

 three and multiply the quotient by four and it 

 gives a retentive component of sixty-eight 

 pounds : Multiply the same quotient (seven- 



