The first class of food substances mentioned above are of very great importance to the body. The term "protein" really means, "of first importance." These compounds are represented by such foods as the white of egg, lean meat, gluten of wheat, and casein of milk. Chemically, proteins are very complex, more so than any other class of food materials. They have in their structure the chemical elements carbon, hydrogen, oxygen, nitrogen, often sulphur and phosphorus, and, less commonly, iron. The nitrogenous element seems to be the most important, since the others mentioned can be obtained from other classes of food; but as these classes of food cannot take the place of protein, it seems clear that the nitrogen is the important constituent. Most proteins coagulate on heating. An illustration of this property is the coagulation of the white of an egg when the egg is cooked. The proteins all undergo decomposition quite easily. This is evidenced by the ease with which eggs and meat spoil. Protein molecules are made up of smaller molecules called amino acids. These are the "building stones" from which the working tissues of the body are formed. There are on the average about fifteen different kinds of these amino acids in the proteins, which are especially valuable in supplying building material for the tissues of the human body. These amino acids are united in long chains to form the protein molecule, and in this respect can be compared to cars in a train. By the work of digestion, the proteins are broken down into these comparatively simple building stones, which, when absorbed into the circulation, are used by the body in building working tissues as they are needed. There are a number of classes of proteins; but since the classification is rather complicated, it will not be given here. To group the various foods as to their relative amounts of protein is often of interest. For example, foods very rich in protein, such as the gluten preparations, lean beef, and white of egg, may be regarded as the first class; a second class might be formed of those which are moderately high in protein, such as peas, beans, lentils, and walnuts; a third class having a moderate amount of protein, represented by the cereals and breads; and still a fourth class very low in protein, such as vegetables and fruits.