DIGESTION OF FOOD: THE MOUTH 



77 



nutritive materials for the tooth, the latter regulating the 

 use of these materials; Figs. 36 and 37. 



Care of the Teeth. Cleaning the teeth may seem to be 



unnecessary, or merely a matter of good form; but this is a 

 wrong idea. In spite of the hard nature of the teeth, they 

 are very liable to decay, as almost everyone knows to his 

 misfortune. This decay is brought about by circumstances 

 which we can in great measure prevent if we understand 

 them. It is caused in part by the bacteria in the mouth. 

 These are not able to affect the uninjured teeth though they 

 can readily attack the softer foods that may lodge in or between 

 them. If, after eating, one chances to leave some of the food 

 in the crevices between the teeth, the bacteria at once begin 

 to feed upon it. The mouth is warm and moist and furnishes 

 the very best possible conditions for bacterial growth. In these 

 particles of food, therefore, bacteria flourish and, after a time, 

 turn them sour in much the same way that they turn milk 

 sour. The sourness is due to the production of an acid, which, 

 although not formed in very large amounts, always appears 

 if food is left between the teeth. Upon the hard substance 

 of the teeth this acid acts at once, dissolving the lime in such 

 a way as to produce soft spots or even cavities. Upon the 

 hard enamel the acid acts only with difficulty, but if this 

 is cracked or broken the acid acts easily upon the softer den- 

 tine within. As soon as these weak spots appear the teeth 

 decay rapidly; Fig. 37. Since our permanent teeth do not grow 

 and are never repaired or replaced by nature, it is very impor- 

 tant that they be kept in good condition. 



It is becoming more and more certain that material from 

 decayed teeth, or from pus cavities in or above their roots, 

 may cause very unexpected and serious results, e. g. nervous- 

 ness, epilepsy, indigestion, blindness, or even insanity. "Mouth 

 cleanliness" is of the greatest importance. 



What is the use of toothache ? Though unpleasant, it certainly 



