124 



DIGESTION 



15. It should be borne in mind that the enamel, Kature'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. t 



16. 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 

 insalivation. 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 

 mouth, but chiefly by the 

 salivary glands situated near the mouth. 



17. The parotid glands lie below and in front of the eur. 

 Mumps is an inflammation of these glands. Each opens by a 

 duct into the mouth near the second upper molar tooth. The 

 submaxillary glands are situated near the angles of the lower' 

 jaw, at the base of the tongue. They have a single duct which 

 opens into the mouth beneath the tongue. The sublingual 

 glands are the smallest of the three pairs, and lie just beneath 

 the forward end of the tongue. They open into the floor of 

 the mouth by several ducts. 



decayed beyond the remedy by filling should be immediately removed." 

 Lane on the Hygiene of the Teeth. 



FIG. 26. STRUCTURE OF A SALIVARY GLAND 



16, Care of enamel ? 16. What is saliva ? 17. Name four kinds of glands. 



