164 



SPECIAL PHYSIOLOGY. 



each case, between the fulcrum and the weight on the one hand, and 

 the fulcrum and the power on the other. The distance from the ful- 

 crum to the weight, gives the length of the weight-arm of the lever ; 

 whilst the distance from the fulcrum to the power, gives the length of 



Fig. 50. 



Fig. 50. Example of a lever of the third order, exhibited, in raising the hand, by bending the elbow; 

 1, the fulcrum, situated at the centre of motion in the elbow joint; 2, the power, partly resident in the 

 biceps muscle, which is fixed to the radius: the two heads of this muscle are shown, and the dotted lines 

 indicate the shape of the contracted muscle (the brachialis muscle is omitted); 3,3, the weight resident in 

 the fore-arm and hand, which are shown in two positions, that is. before and after, being raised by the con- 

 traction of the muscles. 



the power-arm. When the weight-arm and the power-arm are equal 

 in length, the power to balance or counteract the weight must be equal 

 to the weight, and the slightest excess will overcome its resistance or 

 move it. When, on the contrary, the power-arm is longer than the 

 weight-arm, then the power needed to balance and to overcome the 

 weight is proportionately so much less. Lastly, when the power-arm 

 is shorter than the weight-arm, the power necessary to balance or 

 move the weight becomes proportionately so much greater. Con- 

 versely, when an advantage is gained by a particular lever in regard 

 to power, there is a proportionate loss in reference to the velocity of 

 movement in the weight ; for, though when the power-arm and weight- 

 arm are equal in length, the velocities of the power and weight are 

 equal ; yet when the power-arm is longer than the weight-arm, and 

 there is a gain in regard to the useful force exerted by the power, 

 there is a loss in the velocity of the weight, which moves through a 

 smaller space than the power ; and again, when the power-arm is 

 shorter than the weight-arm, in which case there is a loss of power, 

 there is a corresponding gain in the relative velocity of the weight, as 

 compared with that of the power. 



In the first kind of lever the lengths of the weight-arm and of the 

 power-arm may either be equal, or may vary in either direction; but 

 in this respect, there is practically, in the human body, a tolerable 

 equality between them. In the second kind of lever, however, there 



