DIGESTION 121 



13. It should be borne in mind that the enamel, Nature's 

 protection for the teeth, when once destroyed, is never formed 

 anew ; and the body of the tooth thus exposed is liable to rapid 

 decay. On this account, certain articles are to be guarded 

 against; such as sharply acid substances that corrode the 

 enamel, and hard substances that break or scratch it as 

 gritty tooth-powders, metal tooth-picks, and the shells of hard 

 nuts. Sudden alterations from heat to cold, when eating or 

 drinking, also tend to crack the enamel. 



14. Action of the Saliva. While the morsel of food is cut 

 and ground by the teeth, it is at the same time intimately 

 mixed with the saliva, or 



fluids of the mouth. This 

 constitutes the second step 

 of digestion, and is called 

 iiisalivation. The saliva, 

 the first of the digestive 

 solvents, is a colorless, 

 watery, and frothy fluid. 

 It is secreted (i.e., sepa- 

 rated from the blood) 

 partly by the mucous 

 membrane which lines the 



J^'IG. 26. STRUCTURE OF A SALIVARY GLAND 



mouth, but chiefly by the 



salivary glands, of which there are three pairs situated near 



the mouth. 



than for adults. They should also early be taught to use no metallic 

 substances, as pins, needles, etc., to remove substances from between the 

 teeth. The teeth should always be thoroughly cleansed after taking acids 

 into the mouth for they are the great enemies to the teeth and also 

 after candies and other forms of sugar, for their particles that linger on 

 the teeth are changed by decomposition into lactic acid. Occasional ex- 

 amination of the teeth is prudent, in order that a commencing cavity may 

 be promptly detected and remedied. Teeth that are decayed beyond 

 remedy by filling should be immediately removed." Lane on the 

 Hygiene of the Teeth. 



13. Destruction of the enamel ? How guarded against ? 



14. Mixing of food with the saliva ? What is the saliva ? How secreted ? The salivary 

 glands ? 



