With a most profound intelligence, each of these body cells and tissues, bone and nerve fiber, muscle and organ, selects from the blood stream just its supply or portion of the food elements requisite to its upbuilding and maintenance. The mysteries of assimilation are effected by means of chemical substances called "enzymes," similar to those found in the digestive organs, but acting in an entirely different manner, in that they build up solids out of liquids instead of converting solids into liquids. ELIMINATION OF BODY WASTES Metabolism consists of a twofold role--an upbuilding and a tearing down process. After the food is all digested, absorbed, and assimilated, having become a part of the bodily organ, bone, muscle, and nerve fiber, then begins the work of tearing it down--of liberating its heat and energy--to be followed by its elimination from the body through the sweat glands, uriniferous tubules of the kidneys, etc. The carbohydrates (starches and sugars), together with the fats, are completely burned up in the body and are then eliminated in the form of water (thrown off through the sweat) and carbonic acid gas given up by the lungs. The proteins, or nitrogenous foods, are not so completely burned up in the body. The ashes which result from their combustion are not simple substances like the water and CO_{2} of the carbohydrates. This protein ash is represented by a number of complicated substances, some of which are solid (protein clinkers), which accumulate in the body and help to bring about many diseases, such as gout, headache, fatigue, biliousness, etc. These protein ashes and clinkers are further acted upon--split up and sifted--by the liver, and are finally eliminated by the kidneys in the form of urea, uric acid, etc. The body being unable to store up protein, is often greatly embarrassed when one eats more of this substance than is daily required to replenish the waste of the body, for it must all be immediately split up in the system, and the over-abundant and irritating ashes must be carried off by the eliminating organs. Now, the overeating of sugars, starches, or fats, is not such a serious matter, as they may be stored in the liver and subsequently used; and even if they are eaten in excess of what the liver can care for they accumulate as fat or add extra fuel to the fires of the body, their ashes being carried off in the form of such harmless substances as water and carbon dioxide (CO_{2}); but the overeating of protein substances is always a strain on the body and should be avoided. ELEMENTS OF NUTRITION There are seven distinct elements entering into the composition of human foods--protein, starch, sugar, fat, salts, cellulose, and water, not to mention enzymes, vitamines, and other little-known chemical principles. These elements are all variously concerned in the nourishment, energizing, and warming of the body. PROTEINS