The actual biochemical processes by means of which the transformation of external food elements into living tissue and force, physical and mental, takes place are known as: 1. Anabolism, or the process of assimilation of nutritive matter and its conversion into living substance; and 2. Catabolism, or the breaking down of complex bodies of living matter into waste products of simpler chemical composition. These together constitute 3. Metabolism, or the sum of the chemical changes whereby the function of nutrition is effected. The actual procedure by which most of these activities is carried on is one of oxidation, by means of the oxygen supplied largely by the lungs, which constitutes about 65 per cent of the human body. Now to make up for the daily waste of the other 15 elements, which form 35 per cent of the body tissues, and to support the necessary activities of the system, mental and physical, it is necessary every day to take a more or less even supply of substances, which we call food and drink, which should contain about the proper proportion of the requisite bodily components. Under normal conditions of healthy living the appetite ordinarily serves as a proper guide for health in man and beast, serving to regulate the selection of material to preserve the balance of nutrition. But man especially has temptations to gratify the _taste_, which is quite a different thing from satisfying the _appetite_, and all are familiar with the many forms of disaster and disease which arise from gratifying the taste in food and drink; moreover, the temptations to this seem to increase continually with the so-called refinements of civilization. The actual nutritive elements which are required are relatively few, and fall mainly under three classes: 1, Protein; 2, Carbohydrates; and 3, Fats. Of these the latter two furnish most of the 18 per cent of carbon in the body, and the animal or vegetable protein furnishes the nitrogen, which forms only about 3 per cent of the body tissues: all these substances are, of course, used up constantly in providing heat and energy, physical and mental, day by day, the protein being concerned chiefly in replacing wasted tissue. The combustion of the carbohydrates and fat is relatively simple, and the waste products pass off harmlessly, mainly by the lungs, as carbonic acid and water. But the course of the protein, or nitrogenous and sulphur and other mineral elements, is quite different. In the anabolism and catabolism of protein there are a vast number of intermediate changes, and various products are elaborated which we know to be of great significance in the system, and which when imperfectly completed are the source of much disorder and disease in the economy. Of this the gouty state is a notable example, with a long list of secondary disorders.