1 66 THE ENGINES OF THE HUMAN BODY 



jaw with its teeth serves as the upper millstone. The 

 lower jaw, with a corresponding series of teeth, forms the 

 lower millstone. The lower millstone is made to work 

 on the upper, not as one wheel revolves on another but 

 by means of a lever to which four pairs of strong engines 

 are yoked — the muscles of mastication. They can move 

 the lever on which the millstones are set in any direction, 

 so that the teeth may cut or grind or do both at the same 

 time. The lower jaw is used by the muscles of mastica- 

 tion as a lever of the third order ; the food between the 

 teeth represents the resistance to be overcome — the load ; 

 the two sockets, placed in front of the ear passages and 

 in which the lower jaw moves, provide movable fulcra ; 

 the muscles, which obtain a fixed basis on the skull, are 

 linked to the lever between the teeth and the movable 

 fulcra. The muscles of mastication are furnished with 

 the usual elaborate intelligence service, and are thus kept 

 informed not only as to what is happening in the mouth, 

 but as to what is taking place in their own engines and 

 in those of their partners and opponents. 



An engineer builds a mill according to the dimensions 

 laid down in his plans ; his machinery is a full or adult 

 size from the beginning. Nature had to design the 

 mouth so that it would serve the needs of a living body 

 which keeps changing and growing for twenty years or 

 more. All alterations and additions have to be carried 

 out while the factory is in full activity. The bony 

 scaffoldings which carry the upper and lower millstones 

 have to be extended at exactly the same rate so that the 

 fit and contact of the stones remain perfect at every stage 

 of growth. The stones or teeth, however, have to be of 

 full size when they come into use, for, once laid down, 

 they cannot be altered. The grinding or cutting surfaces 

 of teeth cannot be extended or renewed ; teeth which will 

 fit a child's mouth are too small for an adult jaw, and an 

 adult's teeth are too large to be placed in a child's mouth. 

 Hence Nature's contrivance of two sets : a set to serve 

 the needs of childhood, and another of larger make to 

 replace them as childhood is passed through and adult 



