LEVERS IN THE BODY. 147 



knee joints, that is between the fulcrum and the point of 

 application of the power. 



Levers of the Third Order. In these (Fig. 59) the 

 power is between the fulcrum and the weight. In such 

 levers the weight-arm is always longer than the power- 

 arm, so the power works at a mechanical disadvantage, 

 but swiftness and range of movement are gained. It is 

 the lever most commonly used in the Human Body. For 

 example, when the forearm is bent up towards the arm, 

 the fulcrum is the elbow- joint, the power is applied at the 

 insertion of the biceps muscle (Fig. 49*) into the radius 

 (and of another muscle not represented in the figure, the 

 brachialis anticus, into the ulna), and the weight is that 



W F 



Fia. 58. A lever of the third order. JF", fulcrum ; P, power ; IF, weight. 



of the forearm and hand, with whatever may be contained 

 in the latter, acting at the centre of gravity of the whole 

 somewhere on the distal side of the point of application of 

 the power. In. the Body the power-arm is usually very 

 short so as to gain speed and range of movement, the mus- 

 cles being powerful enough to still do their work in spite 

 of the mechanical disadvantage at which they are thus 

 placed. The limbs are thus made much more shapely than 

 would be the case were the power applied near or beyond 

 the weight. 



It is of course only rarely that simple movements as 

 those described above take place. In the great majority 

 of those executed several or many muscles co-operate. 



The Loss to the Muscles from the Direction of their 

 Pull. It is worthy of note that, owing to the oblique direc- 



*P. 120. 



