316 ELEMENTARY PHYSIOLOGY LEsa 



depressed, the power is applied in front, and the weight 

 to the back part, of the skull ; but when the face is being 

 raised, the power is beliind anil the weight in front. The 

 like is true of the pelvis, according as the body is bent 

 forward, or backward, upon the legs. Finally, when 

 the toes, in the action of tapping, strike the ground, the 

 power is at the heel, and the resistance in the front of the 

 foot. But when the toes are raised to repeat the act, 

 the power is in front, and the weight, or resistance, is at 

 the heel, being, in fact, the inertia and elasticity of the 

 muscles and other parts of the back of the leg. 



But in all these cases, the lever remains one of the first 

 class, because the fulcrum, or fixed point on which the 

 lever turns, remains between the power and the weight, or 

 resistance. 



The following are three examples of levers of the 

 second order :— 



(a) The thigh-bone of the leg which is bent up towards 

 the body and not used, in the action of hopping. 



For, in this case, the fulcrum is at the hip-joint. The 

 power (which may be assumed to be furnished by the 

 thick muscle^ of the fi'ont of the thigh) acts upon the 

 knee-cap ; and the position of the weight is represented 

 by that of the centre of gravity of the thigh and leg, 

 which will lie somewhere between the end of the knee 

 and the hip. 



(b) A rib when depressed by the rectus muscle ^ of the 

 abdomen, in expiration. 



Here the fulcrum lies where the rib is articulated with 

 the spine ; the power is at the sternvnn — virtually the 

 oj)posite end of the rib ; and the resistance to be over- 

 come lies between the two. 



1 This muscle, called rectus, is attached above to the haunch-bone and 

 below to the knee-cap (Fig. d, 2, p. 19). The latter bone is connected by 

 a strong ligament with the tihia. 



'^ This muscle lies in the front abdominal wall on each side of the 

 middle line. It is attached to the sternum above and to the front of the 

 pelvis below (Fig. ti, 3). 



