90 HUMAN BIOLOGY 



The teeth ought never to be used to crack nuts or to bite 

 hard substances, for while the enamel is a very hard sub- 

 stance, it is also brittle and may be cracked or broken off 

 by such treatment. If once lost, it will not grow again. 

 It is evident, therefore, that it is very essential to protect 

 this outer layer, both from the action of acids, and from 

 mechanical injuries. 



Some people seem to think that the loss of natural teeth 

 is not a very serious matter, and that false teeth are just 

 as effective as those teeth provided by nature. Experi- 

 ments have shown, however, that the power to crush food with 

 false teeth is only about one fifth that of the power exerted 

 by a normal set of teeth. Hence, loss of teeth is very likely 

 to result in imperfect mastication of food, with consequent 

 ill-health resulting from indigestion. If, however, one has 

 been unfortunate enough to have lost one or more teeth, 

 the gaps should be promptly filled by bridge work. The 

 teeth should be examined by a dentist at least twice a year 

 so that any cavities found may be promptly filled. In short, 

 everything possible should be done to secure and preserve a 

 beautiful and effective set of teeth. 



120. Importance of the digestion of starch. — In 47 of 



"Plant Biology" we proved that starch can not pass through 

 the walls of cells, and we likewise showed in 49 how this 

 food substance is made ready by the process of digestion 

 to pass through membranes. Many of the foods we eat 

 contain large percentages of starch. We are now to show 

 experimentally how starch is digested in the himian body. 



121. Does saliva digest starch ? — Laboratory demonstra- 

 tion. 



Prepare some starch paste by boiling in a test tube of 

 water an amount of arrowroot starch (or corn starch, if 



