SG DIGESTION. 



12. The saliva, or moisture of the mouth, undergoes a 

 putrefactive change, and becomes the fertile soil in which 

 a certain minute fungus has its growth. This fluid, too, if 

 allowed to dry in the mouth, collects upon the teeth in the 

 form of an unsightly, yellow concretion, called tartar. 

 To prevent this formation, and to remove other offensive 

 substances, the teeth should be frequently cleaned with 

 water, applied by means of a soft tooth-brush. The de- 

 struction of the tartar fungus is best effected by the use 

 of a weak solution of carbolic acid. 



13. Again, it should be borne in mind that the enamel, 

 Nature's protection for the teeth, when once destroyed, is 

 not 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 alternations from 

 heat to cold, when eating or drinking, also tend to crack 

 the enamel. 



14. Insalivation. When the morsel of food is* cut and 

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

 mixed with the saliva, or fluids of the mouth. This con- 

 stitutes the second step of digestion, and is called insaliva- 

 tion. The saliva, the first of the digestive solvents, is a 

 colorless, watery, and frothy fluid. It is secreted (i. e. 

 separated from the blood) partly by the mucous mem- 

 brane which lines the mouth ; but chiefly by the salivary 

 glands, of which there are three pairs situated near the 

 mouth. 



15. These glands consist of clusters of very small 



12. Effects upon the saliva? Formation of tartar? How prevented? How 

 destroyed ? 



13. Destruction of the enamel * How enarded against? 



1 4. Mixing of food with the saliva V What is the' saliva ? How secreted ? The 

 salivary glands? 



1 5. The flow of saliva ? The thought of food ? Anxiety and grief? Animals 

 fed upon dry and coarse food ? 



