Although there may be thousands of different dishes, and combinations of foodstuffs, fundamentally they are all closely akin, and can be all resolved into a few quite simple elements: Proteid, Carbohydrate, Fat or Mineral Salts, or combinations of these; the Proteid class having many subdivisions, and the Carbohydrates being made up of the various sugars and starches. We also know definitely just what use the body makes of these various substances. The Proteid is the up-builder of tissue, the essential foodstuff without which life cannot exist. If we compare the human body to an engine, as nearly all physiologists seem bound to do, we may say that Proteid is like the brass, or other metal, of which the structure is composed. The various Carbohydrates and the Fats are the fuels from which are derived the energy which animates and operates the mechanism. The Mineral Salts are used to supply various important bodily needs, such as elements required by the bones, or the delicate tissue in the eyes, the enamel for the teeth, and so forth. THE WORK OF THE DIGESTIVE JUICES As there are five main food elements, namely, proteid, starch, sugars, fats, and salts, so also there are five main digestive fluids, the saliva, the gastric juice, the bile, the pancreatic juice, and the intestinal juice. The saliva is an alkaline fluid that digests starch. Its work is checked by the presence of acid substances; which explains why the digestive action of saliva ceases soon after it enters the stomach. Hence the importance of giving the saliva ample opportunity to perform its function, by complete mastication, is obvious. The gastric juice, of which about seventy ounces is formed by the stomach daily, contains in addition to hydrochloric acid, a quantity of pepsin, which with the acid dissolves all sorts of proteids or albuminous substances, like meat and eggs; and it also contains rennet, which coagulates milk. The gastric juice digests proteids by converting them into pepsin, an exceedingly soluble substance which passes readily into the blood. The bile manufactured by the liver has the function of digesting fats. Fats are not changed chemically, as are starches and proteids. They are only broken up into particles so small that the cells of the mucous membrane can take them up and effect their removal into the blood stream. The pancreatic juice is able to perform the work of all the three digestive fluids which we have already named. In fact, it is even more powerful than saliva in the digestion of starch, since it is able to digest raw as well as cooked starch, which the saliva cannot do. It is also able to convert proteid into peptone, as does the gastric juice; and it emulsifies fats, as does the bile. The intestinal juice digests cane sugar, and is supposed to have a digestive influence upon all the other food elements.