1 68 THE ENGINES OF THE HUMAN BODY 



in providing us with. For such gains we must be pre- 

 pared to pay a price. The price is a daily attention to the 

 toilet of the teeth, and even then we may not escape 

 Nature's penalty. 



There is one other important operation carried on in 

 the mouth which has yet to be mentioned. The moment 

 food is placed within it saliva begins to be poured in from 

 three pairs of factories or salivary glands. Two of these 

 pairs are placed below the floor of the mouth ; the third 

 is situated within the cheeks, immediately in front of the 

 ear passages. The salivary factories may be set to work 

 by the mere sight of food ; the sight of a bone will 

 make saliva trickle from the mouth of a hungry dog. In 

 that case messages have streamed in by the eye and 

 reached certain exchange stations in the central nervous 

 system, whence orders have been sent out which set the 

 saliva producers at work. At the same time their blood 

 supply is turned on. The usual messages, however, 

 which bring about a flow of «aliva in the mouth are dis- 

 patched from minute transmitters or receptors placed over 

 the tongue — particularly in the taste buds ; but touch 

 transmitters, which are very abundant in the mouth, may 

 also be used. The urgency and nature of the messages 

 determine the quantity and quality of the saliva poured 

 out. Beneath the red membrane which clothes the lips, 

 also under that which covers the tongue and lines the 

 mouth, there is placed an under-carpet of minute glands. 

 From these glands there issues a fluid which moistens 

 the lips and lubricates the mouth. Thus as the food is 

 masticated it is mixed with saliva, which not only helps 

 in its reduction by the teeth, but has also a solvent or 

 digestive action upon it. But, above all, the chief use 

 of saliva is to reduce the food to a moist pulp. Such a 

 consistency is necessary for its transmission from chamber 

 to chamber by the peculiar transport system of the 

 alimentary canal. When the necessary consistency is 

 reached, there is a momentary cessation of chewing 

 movements ; the lower jaw becomes fixed against the 

 upper ; the mouth then acts as a force pump, its piston 



