A plant structure or an animal body is an exceedingly complex system of chemical substances, many of which are themselves individually as complex in their structure as the most complicated machine. The first step in the direction of reaching an understanding of the chemistry of the living tissues must involve the separation and study of the structural units of which they are composed. This was, indeed, the field of activity of many organic and biological chemists during the nineteenth century. The fats and the simpler substances into which they can be converted in the process of saponification were the subjects of exhaustive studies. The complex carbohydrates and the simpler sugars were discovered, and the manner in which they are related chemically was made clear. The most complex of the food substances, the proteins, were by far the most difficult to study by chemical means. Familiar examples of these are egg white, the casein of milk, and the principal components of meat. Certain proteins which occur in our plant and animal foodstuffs differ very greatly in their physical properties, but they are nevertheless very closely related chemically, since they can all be resolved into the same digestion products. These have been studied with marked success as to their behavior with the digestive juices produced in the pancreas or alimentary tract and with chemical reagents in the laboratory. In addition to these prominent components of plant and animal structures, a long list of relatively simple chemical substances have been discovered and isolated in a state of purity from vegetable and animal tissues. They have been investigated to determine their composition and special properties, and tests by means of which they can be recognized and identified and methods for their quantitative estimation in certain cases have been worked out.

Through a century of patient labor by many able men we came to an* appreciation of the number and character of the simple structural units into which the tissues of an animal or a plant can be separated. Furthermore, certain of these simple bodies could be recognized as intermediate products in the course of the building up of more highly organized complexes; certain others were shown to be degradation products resulting from the physiological activity of the living tissues. Through these studies it became established that the body of an animal or the tissues of a plant consist essentially of proteins, carbohydrates, fats, and a series of related substances which are collectively known as lipins. The proteins are rich in the element nitrogen, the average content of this element being not far from 16 per cent. Several other prominent components of the living tissue contain more or less of this element, so that this is not an essential characteristic. With these substances there are always associated in the living tissues more or less water and a number of inorganic elements in molecular or ionic form. A number of special kinds of each of the types of substances enumerated above became known, and their less obvious characteristics were described. Certain substances were found to be special products elaborated only by certain tissues and under definite conditions. These became regarded from the standpoint of nutrition as of subordinate interest. Examples of these are the alkaloids, quinin, strychnin, and cellulose. This last one serves as skeletal tissues for the plant but is not essential for the animal. In the same category belong the waste products of the life processes of the animal body, such as urea, uric acid and creatinin, most of which are not found in plant tissues.

Living structures on analysis, although always associated with numerous substances, the exact importance of which could not in many cases be determined, were found to consist essentially of proteins, carbohydrates, fats, mineral salts and water. These came to be regarded even as early as 1840 as the essential and never failing components of plant tissues, and were looked upon as the fundamental factors in the diet of animals.