Chap. XLIII.] 



LEVERS IN THE BODY. 



509 



through which the weight acts. The great length of 

 the weight arm is here very apparent. If the heel 

 rests on the ground and the toes are raised, we have a 

 lever of the third order. The 

 fulcrum is the ankle joint, the 

 power is in front communicated p 

 by flexor muscles, and the weight 

 is farther in front (Fig. 216). 



We have seen how to esti- . , 



. Fjg. 216. The Foot as a 



mate the moment OI forces, I.e. Lever of the Third Order. 



the amount of the force multi- 

 plied by the perpendicular distance between the line 

 of direction of the force and the fulcrum. Thus, in 

 Fig. 217, let AB represent the arm, and BC the fore-arm, 

 and BC' the position of the fore-arm when more ex- 

 tended. At a the biceps is act- 

 ing in the direction ad, and at 

 c the weight is acting downwards. 

 The moment of the biceps, which 

 we shall call x, is the amount 

 of force produced by its con- 

 traction, which we shall call P, 

 multiplied by the perpendicular 

 distance from its point of ap- 

 plication to the elbow joint B. 



Fig. 217. -Moment of x = p . x B - . The moment of 



Biceps in Different the weight (call it ?/) is the pro- 



Fore^m 3 ' duct of its amount (let the amount 



= w) multiplied by its distance 

 CB, y = w x CB. Let them be in equilibrium, 

 then x = i.e. P x B=:W x CB. 



"B 



Therefore 



P = 



X CB 



a B 



When the fore-arm is more extended, the perpen- 

 dicular from the line of direction of the power to the 

 fulcrum is less than before ; it is now &B, the power 



