GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 95 



of power for imparting movement to the levers with the object of 

 overcoming resistance. 



In mechanics levers of three kinds or orders are recognized 

 according to the relative positions of the fulcrum or axis of motion, 

 the applied power, and the weight to be moved. (See Fig. 38.) 



In levers of the first order the fulcrum, F, lies between the weight 

 or resistance, W, and the power or moving force, P. The distance 

 P F is known as the power arm and the dis- 

 tance W F as the weight arm. As examples ^ 



of this form of lever found in the human * F 



body may be mentioned : WAP''' 



1. The elevation of the trunk from the p * 



flexed position. The axis of move- A W p( 2 ) 



ment, the fulcrum, lies in the hip-joint; m IF 



the weight, that of the trunk, acting as ^ P A^ 



if concentrated at the center of gravity, FlG 8 _ THE THREE OR _ 

 which lies close to the tenth dorsal ver- ' DERS OF LEVERS. 

 tebra; the power, the muscles attached 



to the tuberosity of the ischium. The opposite movement is 

 equally one of the first order, but the relative positions of P and 

 W are reversed. 



2. The head in its movement backward and forward on the atlas. 

 In levers of the second order the weight lies between the power 



and the fulcrum. As illustration of this form of lever may be men- 

 tioned : 



1. The depression of the lower jaw, in which movement the fulcrum is 



the temporomaxillary articulation; the resistance, the tension 

 of the elevator muscles; the power, the contraction of the de- 

 pressor muscles. 



2. The raising of the body on the toes, in which movement the ful- 



crum is the toes, the weight that of the body acting through the 

 ankle, the power the gastrocnemius muscle applied to the heel 

 bone. 

 In levers of the third order the power is applied at a point lying 



between the fulcrum and the weight. As examples of this form of 



lever may be mentioned: 



1. The flexion of the forearm, in which the fulcrum is the elbow- 



joint, the power the biceps and brachialis anticus muscles ap- 

 plied at their points of insertion, the weight that of the forearm 

 and hand. 



2. The extension of the leg on the thigh. 



When levers are employed in mechanic operations, the object 

 aimed at is the overcoming of a great resistance by the application of 

 a small force acting through a great distance, so as to obtain mechanic 

 advantage. In the mechanism of the human body the reverse gener- 



