50 THE ENGINES OF THE HUMAN BODY 



poised and the manner in which it is moved, we find that 

 it, too, acts as a lever of the first order. The fulcrum on 

 which it moves is the atlas — the first vertebra of the spine 

 (fig. 10). When a man stands quite erect, with the head 

 well thrown back, the ear passages are almost directly- 

 over the fulcrum. It will be convenient to call that part 

 of the head which is behind the ear passages the post- 

 fulcral) and the part which is in front the pre-fulcral. 



We'.ahl" 



Anterior 

 straight" 

 muscle 



Fig. io. — The skull as a lever of the first order. 



Now the face is attached to the pre-fulcral part of the 

 lever and represents the weight or load to be moved, 

 while the muscles of the neck, which represent the 

 power, are yoked to the post-fulcral end' of the lever. 

 The hinder part of the head serves as a crank-pin for 

 seven pairs of neck muscles, but in fig. io only the 

 chief pair is drawn, known as the complex muscles. When 

 that pair is set in action the post-fulcral end of the head 

 lever is tilted downwards, while the pre-fulcral end, on 

 which the face is set, is turned upwards. 



The complex muscles thus tilt the head backwards and 



