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. [Sidenote: 14. Mixing of food with the saliva? What is the saliva? How secreted? The salivary glands?] 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 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._ separated from the blood) partly by the mucous membrane which lines the mouth; but chiefly by the salivary glands, of which there are three pairs situated near the mouth. [Sidenote: 15. The flow of saliva? The thought of food? Anxiety and grief? Animals fed upon dry and coarse food?] {87} 15. These glands consist of clusters of very small pouches, around which a delicate network of blood-vessels is arranged: they empty into the mouth by means of little tubes, or ducts. The flow from these glands is at all times sufficient to maintain a soft and moist condition of the tongue and mouth; but when they are excited by the presence and taste of food, they pour forth the saliva more freely. Even the mere thought of food will at times cause the saliva to flow, as when the appetite is stimulated by the sight or smell of some savory article; so that the common expression is correct that "the mouth waters" for the favorite articles of food. Anxiety and grief prevent its flow, and cause "the tongue to cleave to the roof of the mouth." In the horse and other animals, that feed upon dry and coarse fodder, and require an abundant supply of saliva, we find large salivary glands, as well as powerful muscles of mastication. [Illustration: FIG. 20.--STRUCTURE OF A SALIVARY GLAND.] [Illustration: FIG. 21.--THE HEAD OF A HORSE, showing the large salivary gland (_a_), its duct (_b_), the muscles of mastication (_c_, _d_, _e_, _f_, and _g_).] [Sidenote: 16. Importance of the process? The first place? The second? The third?]