146 THE HUMAN BODY. 



small muscles passing from the vertebral column to the occi- 

 put; the resistance is the excess in the weight of the part of 

 the head in front of the fulcrum over that behind it, and is 

 not great. To depress the chin as in nodding does not neces- 

 sarily call for any muscular effort, as the head will fall for- 

 ward of itself if the muscles keeping it erect cease to work, 

 as those of us who have fallen asleep during a dull discourse 

 on a hot day have learnt. If the chin however be depressed 

 forcibly, as in the athletic feat of suspending one's self by 

 the chin, the muscles passing from the chest to the skull in 

 front of the atlanto-occipital articulation are called into play. 

 Another example of the employment of the first form of lever 

 in the Body is afforded by the curtsey with which a lady 

 salutes another. In curtseying the trunk is bent forward at 

 the hip-joints, which form the fulcrum; the weight is that of 

 the trunk acting as if all concentrated at its centre of gravity, 

 which lies a little above the sacrum and behind the hip-joints; 

 and the power is afforded by muscles passing from the thighs 

 to the front of the pelvis. 



Levers of the Second Order. In this form the weight or 

 resistance is between the power and the fulcrum. The 

 power-arm PFis always longer than the weight-arm WF, 

 and so a comparatively weak force can overcome a consider- 

 able resistance. But it is disadvantageous so far as regards 

 rapidity and extent of movement, for it is obvious that when 

 P is raised a certain distance W will be moved a less distance 

 in the same time. As an example of the employment of such 

 levers (Fig. 64) in the Body, we may take the act of standing 

 on the toes. Here the foot represents the lever, the fulcrum 

 is at the contact of its fore part with the ground ; the weight 



F 



W 



FIG. 64. A lever of the second order. F, fulcrum ; P, power ; TF, weight. The 

 arrows indicate the direction in which the forces act. 



is that of the Body acting down through the ankle-joints at 

 Ta, Fig. 65; and the power is the great muscle of the calf 

 acting by its tendon inserted into the heel-bone (Ca, Fig. 65). 

 Another example is afforded by holding up the thigh when 

 one foot is kept raised from the ground, as in hopping on the 



