104 THE HUMAN BODY 



vertebral column to the occiput; 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 necessarily call for any muscular effort, as the head will 

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

 as those of us who have fallen alseep 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 ful- 

 crum; the weight is that of the trunk acting as if all concentrated 

 at its center of gravity, which lies a little above the sacrum and 

 behind the hip-joints; and the power is afforded by muscles pass- 

 ing from the thighs to the front of the pelvis. 



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

 sistance is between the power and the fulcrum. The power- 

 arm PF is always longer than the weight-arm WF, and so a com- 

 paratively weak force can overcome a considerable 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. 52) in the 



F 

 W ~ 



FIG. 52. A lever of the second order. F, fulcrum; P, power; W, weight. The 

 arrows indicate the direction in which the forces act. 



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 is that of the Body acting down 

 through the ankle-joints at Ta, Fig. 53; and the power is the great 

 muscle of the calf acting by its tendon inserted into the heel- 

 bone (Ca, Fig. 53) . Another example is afforded by holding up the 

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



