ELEMENTARY PHYSIOLOGY 



applied at a point between the fulcrum and the weight. As an ex- 

 ample we may mention 



The treadle of a machine. 



We may now proceed to notice examples of levers which are 

 to be found in the human skeleton. 



Levers of the first order : 



(i) The head rocking on the atlas. The fulcrum is formed 



by those parts of the atlas that support the condyles of the 



occipital bone (see page 19). When the muscles of the back of 



the neck contract, the face is raised. When the antagonistic 



Fig. 66. The Head rocking on 

 the Atlas, illustrating the 

 First Order of Levers. 



Fig. 67. Tapping the Toe on the 

 Ground. 



muscles in the front of the neck contract, the back of the head is 

 raised. So that the face and the back of the head alternately 

 represent the power and the weight. 



(2) Lift one foot, and then, by tapping the toe on the ground, 

 we use the foot as a lever of the first order. In this case the 

 power is applied behind by the contraction of the muscles of the 

 calf, the fulcrum is at the ankle, and the ground offers the resist- 

 ance. 



(3) The motion of the body on the hip is another illustration. 

 The hip-joints form the fulcrum ; and the contraction of the 

 muscles connecting the pelvis with the bones of the leg provide 

 the power. In this case the power (and the weight) may be 

 alternately in front and behind. 



