156 



BIOLOGY AND HUMAN LIFE 



132. The teeth and their care. One of the commonest causes 

 of indigestion is found in decayed teeth. People with poor teeth 

 get into the habit of swallowing the food without chewing it. 

 Then they blame their stomachs or the cook for their miser- 

 able feeling or even for the poor work they do. 



The structure of a human 

 tooth is shown in Fig. 8i. 

 The enamel is a hard protec- 

 tive casing. Trouble with the 

 teeth very frequently begins 

 with the breaking of this 

 enamel. The enamel can be 

 cracked by sudden changes 

 of temperature, or by grind- 

 ing it against some hard sub- 

 stance, as when you try to 

 crack a nut with your teeth. 

 Picking the teeth with a needle 

 or some other hard body is 

 also hkely to scratch the en- 

 amel and thus to open the 

 way for further damage. 



In the food that we eat 

 there are many bacteria, of 

 many kinds. In particles of 

 food that cling to the teeth these bacteria begin their digestive 

 activities (see Fig. 73). Some of the substances thus produced 

 act upon the enamel, dissolving away this protective cover. 

 Gradually a cavity becomes larger and deeper until it reaches 

 the pulp and the nerve becomes exposed.^ 



^In recent years it has been found that many serious disorders in various 

 parts of the body may result from a sick condition of the teeth. Studies on 

 patients and experiments with animals have shown that when the root of a 

 tooth is abscessed (infected with certain kinds of bacteria), living bacteria and 

 the poisons which they produce can be carried by the blood to remote parts 

 of the body and there set up local but serious disturbances such as rheumatism 

 of the joints, inflammation of the heart, kidney disease, or ulcers of the stomach. 



Fig. 81. Structure of mammalian 

 teeth 



A, human grinding tooth, showing central 

 pulp cavity (a), containing nerves and 

 blood vessels and surrounded by dentine 

 (b). The crown is covered with enamel 

 (c), and the root with cement (d). B, 

 gnawing tooth of rabbit, which grows from 

 below as fast as it wears away at the tip. 

 The dentine wears away faster than the 

 facing of hard enamel, thus keeping the 

 chisel edge sharp 



