PHYSIOLOGY OF MOVEMENT. 



725 



from the line in which the weight acts to the fulcrum. The law of the lever may 

 be expressed as follows : A power will support a weight as many times as great as 

 itself as the power-arm is times longer 

 than the weight-arm. Thus, for ex- 

 ample, in a lever of the first class, if we 

 suppose that the power-arm be ten 

 times as long as the weight-arm, a 

 weight of one pound at the extremity 

 of the power-arm will support a weight 

 of ten pounds at the extremity of the 

 weight-arm. It must be recollected, 

 however, in the case of the lever, as in 

 every other machine, what is gained in 



FIG. 289. COMPLETE CONTRACTION OF 

 BICEPS. (Perrier.) 



FIG. 290. MOTION OP HEAD AS ILLUSTRA- 

 TING ACTION OF LEVER OF FIRST CLASS. 

 ( Beclar d.) 



a, fulcrum of the lever, c b ; a b is the weight-arm, for 

 the head tends to fall forward by its own weight acting ia 

 the line, r. This U prevented by the contraction of the 

 muscles of the baelt of the neck acting on the power-arm, c a, 

 in the line, F. 



power is lost in velocity, artd vice versa. Thus, in the case of the third class of 

 lever, power is exchanged for velocity. This may be well represented in the 

 movement of flexion of the human forearm (Figs. 288 and 289). The fulcrum 

 is there found in the elbow-joint, the power is the insertion 

 of the biceps muscles in the bone of the forearm in front of 

 the joint, the weight is carried by the hand. In this arrange- 

 ment it is evident that slight motion at the insertion of the 

 biceps will be greatly multiplied in the case of the hand. 

 Thus, say that the distance from the elbow-joint to the tip of 

 the hand is eighteen inches, the dis- 

 tance from the elbow-joint to the 

 point of insertion of the biceps one 

 inch, motion of one inch at the inser- 

 tion of the biceps will produce motion 

 at the hand of an arc of a circle whose 

 cord is eighteen inches The forearm 

 is, therefore, in this action an example 

 of the third class of lever. 



In general, in the animal body, 

 the point of application of the power 

 developed by muscular contraction 

 lies near the fulcrum : hence the 

 conditions favor the production of velocity of movement at the expense of power, 

 for the power-arm is always shorter than the weight-arm. 



The conditions are, however, reversed in the case of the extensor muscles of 

 the limbs when in contact with the ground. Here the joint nearest to which the 

 muscle is inserted is the point of application of the weight, and the fulcrum is the 



FIG. 291. MOTION ILLUSTRATING ACTION OF 

 LEVERS OF THE THIRD CLASS. (Btclard.) 



The fulcrum is at a, the power from contraction of gastroc- 

 nemhu muscle, acting in the line, d e, la applied at c, while the 

 weight (of the body) aete in the line, o b. acia thus the power- 

 arm, a b the weight-arm. 



