WORKSHOPS AND LABORATORIES 165 



eat, the faucial or palatal curtains are pulled tight ; when 

 we are about to swallow they are thrust aside and the bolus 

 of food enters the pharynx. The floor of the mouth, 

 as everyone knows, is moulded to form the tongue, a 

 plunger or piston into the substance of which is packed 

 a complex set of muscular engines. When these are set 

 in action they can alter the tongue to any shape and 

 move it in any direction. In the front part of the floor 

 of the mouth the tongue is tied to the lower jaw — a lever 

 shaped like a horseshoe — which projects to form the chin, 

 then passing backwards on each side of the floor of the 

 mouth, bends upward to reach its sockets in front of the 

 ears. When the mouth is used as a mill, as is the case 

 when we chew, the tongue serves as a feeder to pass the 

 food between the upper and lower teeth or millstones ; 

 when we use the mouth as a force-pump — as when we 

 swallow — the tongue acts as a force-piston ; if the mouth 

 is used as a suction-pump, which is the case when a baby 

 draws milk from the breast, the tongue again serves as a 

 piston — a suction-piston. In speech the tongue modifies 

 the shape of the mouth to meet the requirements of the 

 voice. 



Thus in the mouth many operations are carried on, but 

 the one with which we are most concerned at present is 

 the breaking down and grinding of food, thus preparing 

 it for transport to the second factory or stomach. It is a 

 very long time since man discovered he could save him- 

 self much personal trouble by grinding his food outside 

 the body, and thus lessen the toil of working the mill 

 with which Nature had provided him. Any pair of flat 

 stones answered his purpose. One served as a nether 

 millstone ; it was fixed in the ground. The other, the 

 upper, he worked by hand until he learned how to shape 

 and fix it so that it could be turned by an engine. But 

 in his various designs he always made the upper millstone 

 the movable one. Nature had adopted an opposite plan. 

 On the upper jaw, which is fixed in the face and forms a 

 bony roof for the mouth, she set, in a half-circle, sixteen 

 teeth to serve as cutting or grinding stones ; the upper 



