This too prevalent habit may aggravate the condition which it is supposed to cure. A slight indigestion appearing, gum is often chewed to cause a fuller flow of saliva to aid digestion. If gum-chewing is indulged in to excess, however, the muscular movements overstimulate the salivary glands, eventually weakening them. Overuse of the chewing muscles and overexcitation of the nerves fatigue them and cause them to weaken. The sticky gum, adhering to fillings in the teeth, loosens them and furnishes a lodging place for food particles and bacteria. The excess of saliva may render the gastric juice alkaline, inhibiting its action. Excess of air swallowed with the saliva may cause flatulence or accumulation of gas in the stomach. Lack of poise and nerve tension is increased by excessive gum-chewing, resulting in fatigue of the entire body. This lack of poise may be noted in any public assembly, as the “movies,” frequented by gum-chewers. The habit, as generally practiced, is not an inspiring sight and should be discouraged. Gum-chewing in moderation, for a few minutes after a meal, may not do harm, but its indiscriminate use is to be deplored. Thorough mastication of food will serve to supply the necessary saliva. Exercise directed to the stomach and a more thorough circulation and elimination will do more for any digestive derangement than the excessive practice of chewing gum. * * * * * [Sidenote: Stomach Digestion] As the food enters the stomach, the gastric juice pours out from the mucous lining, very much as the saliva pours into the mouth. Like the saliva, it consists of 99.5 per cent. water and 0.5 per cent. solids. The solids of the gastric juice are pepsin, rennin, hydrochloric acid, and mucus. The _mucus_ serves to lubricate the food as in the saliva. It perhaps also aids to prevent the digestion of the mucous lining of the stomach. The _hydrochloric acid_ and the _pepsin_ cause the principal chemical changes in the food while in the stomach. They act only on the proteins. The hydrochloric acid must be present before the pepsin can act, as only in an acid medium can pepsin dissolve the proteins. It is also of an antiseptic nature and hinders or prevents the decomposition of food. The _rennin_ ferment precipitates the casein. The only digestion of starches in the stomach is that continued by the saliva. Gastric juice begins to flow into the stomach soon after eating, but normally it is not secreted in sufficient quantity to supersede salivary digestion for from twenty to forty-five minutes. The result of gastric digestion of proteins is their conversion, first, into albumin, then into proteoses, and, lastly, into peptone, which is protein in a more simple, soluble, and diffusible form. In the form of peptone, the proteins are in condition to be absorbed.