5 oS 



PHYSIOLOGICAL PHYSICS. [Chap. XLIII. 



Fig. 214. The Foot 

 as a Lever of the 

 First Order. 



power (triceps), and weight (that of the fore-arm). 

 Here, however, the power arm (page 495) is short, 

 and the resistance arm is long, so that the power is at 

 a disadvantage. At the same time a small movement 

 of the triceps effects a considerable movement of the 



hand, and thus rapidity of move- 

 ment is obtained. Again, a lever 

 of the first order is seen when the 

 raised foot is extended on the ankle 

 joint. The joint is fulcrum F, while 

 by the tendo Achilles power p is ap- 

 plied, and the weight w of the fore 

 part of the foot offers the resistance 

 (Fig. 214). 



An example of a lever of the second order is 

 found in the support of the body on the ball of the 

 toes, where the fulcrum is at the ball of the toes. The 

 power is applied by the muscles of the calf to the 

 heel, and the weight is that of the 

 body communicated through the 

 tibia, the weight being between the 

 power and fulcrum (Fig. 215). This, 

 we have seen, is the lever of power, 

 because the power arm is longer than 

 the weight arm. It is not so com- 

 mon in the body as the lever of the 

 third order. The latter is the lever 

 of quickness at the cost of power, for the power is 

 between fulcrum and weight, and has a shorter arm 

 than the weight. As a compensation, however, a 

 small movement of it will effect a considerable move- 

 ment of the weight. Rapidity of movement is thus 

 the object attained by the third kind of lever. Thus, 

 a good example is afforded in flexion of the fore-arm 

 on the upper arm, with a weight in the hand. The 

 power is at the attachment of the biceps between the 

 elbow-joint and the centre of gravity of the fore-arm, 



Fig. 215. The Foot 

 as a Lever of the 

 Second Order. 



