I20 



ELEMENTARY BIOLOGY 



The structure of a human tooth is shown in Fig. 36. The 

 enamel is a hard protective casing. Trouble with the teeth 

 very frequently begins with the breaking of this enamel. The 

 enamel can be cracked by grinding it against some hard sub- 

 stance, as when you try to crack a nut with your teeth ; or it 

 may be cracked by sudden changes of temperature. Drinking 

 very cold water or very hot drinks is likely to be one of the 

 ways of cracking the enamel. Picking the teeth with a needle 



or some other hard body is also 

 likely to scratch the enamel 

 and thus to open the way for 

 further damage. 



In the food that we put into 

 our mouths there are many bac- 

 teria, of many kinds. In par- 

 ticles of food that cling to the 

 teeth these bacteria begin their 

 digestive activities, and some 

 of the substances thus pro- 

 duced act upon the enamel, 

 dissolving away this protec- 

 tive cover. Particles of food 

 in the larger cracks, or fluids 

 in the smaller scratches and 

 cracks, permit the action of 

 the bacteria to continue, and 

 gradually a cavity in the tooth becomes larger and deeper, 

 until it reaches the pulp, and the nerve becomes exposed. 

 A thorough cleaning of the teeth thus becomes necessary 

 at frequent intervals. The most reasonable time to clean the 

 teeth is immediately after each meal. If you get the habit 

 of doing this, it will postpone the rotting of the teeth a good 

 many years. Unfortunately our business and industry are so 

 arranged that most grown-ups cannot manage to look after 

 their teeth after each meal. The best one time a day for 



Fig. 36. Structure of mammalian teeth 



A, human grinding tooth, showing central 

 pulp cavity (a), containing nerves and blood 

 vessels and surrounded by doitine {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 chisel edge is 

 kept sharp by the dentine wearing away 

 faster than the facing of hard enamel 



