The food in the mouth is mixed with saliva, which begins the dissolution of the starches. The saliva consists of about 99.5 per cent. water and 0.5 per cent. solids. The solids consist of ptyalin, sodium chlorid, sodium carbonate, mucus, and epithelium. Ptyalin, the most important of these, is the active digestive agent; the mucus lubricates the masticated food; the sodium carbonate insures the alkalinity of the food, and the water dissolves the food that the juices may more readily reach and act on each particle. The starches are the only foods whose chemical digestion is begun in the mouth. They are first broken up by the ptyalin into dextrin and then into the more simple sugar, known as maltose.[6] It is important that sufficient saliva be mixed with the food through mastication, because unless the digestion of the starches is begun by the saliva, either in the mouth or after it is swallowed, they are not acted on until they reach the small intestine, consequently their digestion is unduly delayed. The pancreatic juice must then do more than its normal work of digestion. The saliva flows into the mouth, more or less, at all times, but more copiously during mastication. The movement of the jaws in chewing incites its flow and when starches are not well digested, gum chewing, in moderation, though not a refined habit, is beneficial. The evident purpose of the saliva when food is not present is to keep the lining of the mouth moist. Salivary digestion is carried on in the stomach until the food becomes thoroughly mixed with the gastric juice, which, being acid, inhibits the action of the ptyalin. The thorough and regular cleansing of the teeth is an important aid to digestion. Food products allowed to remain about the teeth ferment, rendering the mouth acid. When the mouth is acid, the alkaline saliva does not secrete in sufficient amount and the mouth is more or less dry. The mouth is acid in rheumatism and allied conditions and the saliva may be thick and ropy so that it does not moisten the food properly. On the other hand the flow of saliva may be too free, the ptyalin is then too much diluted to promptly act on the food. This may result from overstimulation of the salivary glands occasioned by the excessive chewing of gum, or tobacco. These excesses also carry too much air into the stomach, resulting in flatulence. The flow of saliva is controlled, largely, by nerves centering in the medulla oblongata. The sight of food, pleasingly served, or even the thought of food which one likes, will increase the flow. This is one instance of the control of thought over digestion, and the importance of forming the habit of cultivating a taste for all kinds of food is apparent. The stronger the relish for the food, and the more thoroughly it is masticated and mixed with the saliva, the more perfectly will the first step in digestion be accomplished.