WE have had a good deal to say thus far about power development in living animals, and have talked about food in connection with its use as fuel for the purpose. While we are on the topic it may be as well to say something about other uses to which food is put in animals besides that of serving as fuel, and also something about what is done with the power that is developed by the burning of such food as is used for fuel. To begin with, it is evident that one use that is made of food is to build the body itself. The new-born infant usually weighs somewhere between 5 and 12 pounds. From birth until the body gets its growth there is an almost continuous gain in weight until a total which may range anywhere between 90 and 250 pounds is reached. Of course, every bit of this additional material came into the body in the form of food. The whole mass of the body divides itself, as has been said before, into living protoplasm and nonliving substance. We do not know accurately what proportion of the whole weight is made up by protoplasm; it has been estimated at about 60 per cent, but any estimate can be only very rough because about half of the nonliving substance consists of fat deposits which vary greatly in different people.

In any case, that part of the food which goes to make gain in weight is passed over to the living

cells. If we accept the rough estimate given above, about 60 per cent is then used for the actual manufacture of new protoplasm; the remainder is worked over by cells specially devoted for the purpose and put into place to serve as supporting structure, or to be held in reserve as fat. Living protoplasm is chemically a very complex mixture. In consistency it resembles a rather thin, transparent jelly; the thickness of the jelly depends on how much water it contains and this varies greatly in different kinds of protoplasm. The watery part of the protoplasm has dissolved in it several substances; among them may be mentioned ordinary table salt; also salts of potash and lime. Only tiny amounts of these are present, but it is a curious fact that without these tiny amounts of salts protoplasm cannot live. The chief solid substance in protoplasm is protein; this

material, which is one of the most complex substances known to chemistry, has certain peculiarities which seem to fit it specially to serve as the chemical basis of life. Evidently of all the foodstuffs protein is the most important for the manufacture of new protoplasm, in other words for growth. In the case of a tiny one-celled animal, whose body is made up of protoplasm, not much else would be needed, but any animal that has a bony skeleton has to build this up to keep pace with the growth of the soft parts of the body. For this purpose mineral substances are needed, chiefly lime salts.

In addition to these foods which are actually used for making new body substance it has recently been discovered that proper growth in the higher animals, including man, depends on the presence in the diet of certain dietary accessories, whose use is not at all understood, although there is no doubt of their importance. These materials, to which has been given the rather cumbersome name of “growth-promoting vitamines,” are found dissolved in certain food fats. Apparently they are insoluble in water and soluble in oil. Most animal fats appear to contain them in small amounts, while most vegetable fats do not. Milk and eggs, which are growth foods in an especial sense, are richer in these accessories than any other articles of the diet. The discovery of these facts has emphasized the importance of including animal fats in the diet of growing children, milk and eggs particularly. Since milk is also rich in the lime salts which are necessary for bone formation it forms the best single foodstuff for children that there is. When very young children have to be fed on cow’s milk, which differs somewhat in proportion from mother’s milk, it is often found necessary to feed the top milk diluted with water, instead of the whole milk. When this is done, lime water is usually used in part for diluting the milk, instead of all ordinary water. In this way the proportion of lime is brought up enough to insure that the child will get plenty of it.

In addition to the use of protein as a growth food it has another use which no other kind of foodstuff can share. This is also because protein is the foundation material of living protoplasm. We do not know a great deal about what goes on in living protoplasm to make up what we call the life processes, but we do know that these processes are of a chemical nature, and that in connection with them there is a steady wastage of protein. The protein that thus goes to waste is broken down into simpler chemical compounds which are expelled from the cells. Why this occurs we do not know, but since it does it is evident that unless the wastage is made good the time will presently come when so much protein will have been lost from the protoplasm that it can no longer exist as such and must die. As a matter of fact, one might go on a diet excessively rich in starchy foods and fats and still starve to death if there were no protein present. This use of protein is called cell maintenance to distinguish it from the other special use of protein in cell growth. Evidently, whatever may be missing from the diet, protein must not be left out. Fortunately most of our common foods contain protein. It is especially abundant in lean meat, in dried beans and peas, and in grain. Potatoes and most garden vegetables are deficient in protein, as are almost all common fruits. Bread and meat are our chief stand-bys as furnishers of protein.

Just as there are vitamines that are important for growth, so are there vitamines that are necessary for cell maintenance. Many years ago Dr. Sylvester Graham made himself prominent by arguing that the outer coats of wheat grains contain something that is needed in the diet, which is removed in the process of manufacturing white flour. He accordingly invented a form of flour, familiar to us all under his name, which includes some of the bran from the outer layers of the wheat. This idea, which originated with Dr. Graham, has since been substantiated, although not precisely as Graham had it. We know that there are necessary accessories to the diet, but we know, also, that they are much more widespread than Graham thought. They occur in so many kinds of foodstuffs that anyone who eats a mixed diet usually gets enough of them for his needs. The ill effects of their lack are most evident when the diet is restricted to a few kinds of food which happen not to contain them. A striking example of bodily injury directly due to the absence of these vitamines from the food is seen among Orientals whose diet is apt to be made up of rice plus small amounts of other substances. Of recent years the natives of Japan and China and the Philippines have suffered much from a disease of the nerves known as beriberi. Investigation has shown that this disease is due to the absence from the diet of needed vitamines, and dates from the time when rice-milling machinery was introduced. The old hand methods of milling rice were so imperfect that much of the hull was left clinging to the grains, but machinery polishes the rice clean of every trace of hull. The hulls of rice contain the accessory that is wanting from the polished grains. Wherever it has been possible to bring about the use of unpolished (brown) rice instead of the usual polished kind, beriberi has disappeared. Or the same result can be secured by adding small amounts of beans to the diet. It is probable, also, that the hulls of most grains, including wheat, contain some of the same, or a similar accessory, so to that extent Dr. Graham was right in emphasizing the importance of adding hulls to the flour. Quite recently it has been shown that raw foods are richer in these accessories than cooked, and that ordinary compressed yeast contains more of them than any other easily obtainable material. Many people are being benefited by taking part or all of a yeast cake daily in a glass of milk.

For growth, or the making of new protoplasm, and for maintenance, or the repair of protoplasmic wastage, then, we must eat protein-containing foods, also foods containing various kinds of salts, and foods containing the necessary vitamines. All these are to provide required materials; the actual substances built into the protoplasm. There remains the requirement of power, for both growth and maintenance represent chemical activity on the part of the cell, and this activity depends on power just as does any other activity. In saying this we are merely saying over again in different words what was set down at the very beginning of the book as the chief sign of life, namely, the necessity on the part of living cells of continuous power development. The use of food as a source of energy or power has been talked about already, but it is necessary to say something about the different sorts of power development that may go on in cells, and since we shall have to talk about this a good deal, right here is a good place to bring in for the first time a word that has come to be used whenever the matter of the chemical activities of living cells is being mentioned. The word is metabolism; when we speak of cell metabolism we mean the chemical processes that are going on in the cells. Hereafter, instead of saying power development, the word metabolism will be used as meaning practically the same thing.

First of all, in describing the various kinds of metabolism that cells may show, we have the metabolism of rest. By this we mean the power development that is going on when the cell is doing nothing more than keeping alive; neither growing nor showing any special activity. This is evidently the minimum amount that any cell can show, so it is often referred to as the basic metabolism. We know of at least two things that may change the amount of basic metabolism; the first of these is a change in temperature; when a cell is cold, its basic metabolism is less than when it is warm. There is a very simple chemical reason for this, namely, that chemical processes as a rule go on more slowly the lower the temperature. Since all metabolism consists of chemical processes, this rule applies not only to basic metabolism, but to all other kinds as well, and, as we shall see, explains why the lower animals show such marked differences in behavior in cold and warm weather. The second thing that influences the amount of basic metabolism is the percentage of water in the protoplasm of the cell. Highly organized animals, like ourselves, are destroyed if the cells lose more than a small fraction of their water, but there are many of the lower animals that can be dried until their bodies contain only a very little water and still live. This applies to microscopic forms that live in puddles and similar places; when the puddle dries up the animal dries up too, until all that is left of it is a tiny particle of highly concentrated protoplasm. But this tiny particle preserves all the original cells, or at least enough of them to make a fresh start, and a very sluggish metabolism goes on in each cell. Of course, the advantage of this is that the stored food materials will not be used up as rapidly as they would if metabolism went on at the usual rate, and so there is a better chance that the animal may survive until more water falls or drains into the puddle, or until the particle of dust which the animal has become may be blown by the wind where it will fall into another one. Whenever either of these things happens the protoplasm takes up water again and the former rate of metabolism is resumed. It is only by means of this reduction in rate of metabolism that many kinds of animals are able to persist, for in large parts of the globe there is a period of each year when conditions become so unfavorable that the usual rate of metabolism could not possibly be maintained.

Next in order to basic metabolism comes the metabolism of growth, by which we mean the energy necessary for the making of new protoplasm. Not a great deal is known about growth metabolism; in fact, about the only reason for believing that it requires any energy at all is that the metabolism of young animals, whenever it has been studied, has been found to be greater in proportion than that of animals that are fully grown. It is hard to account for this, unless the growth process itself, namely, the making of new protoplasm, requires energy. When we think of the extreme complexity of living protoplasm, we can easily believe that its formation involves the expenditure of energy, perhaps in considerable amounts.

The last kind of power development to be considered is the metabolism of special activity. Most kinds of cells, particularly in highly organized animals, have some special kind of work to do. For example, the muscle cells have the task of making the motions; the gland cells of manufacturing the secretions, and so on. These we speak of as the particular functions of the cells, and the metabolism by which they are performed as functional metabolism. In some of the lower animals one can scarcely tell where basic metabolism leaves off and functional begins. There is a small shrimp, about a half inch long, that is found quite commonly in small ponds. This little animal has several pairs of legs by which he swims about, and the strokes of these legs go on continuously, day and night, with almost no interruptions, at the rate of a hundred or more a minute, for days or even weeks. It looks as though this, and other animals, that are continuously on the move, were organized without any sharp line between basic and functional metabolism; their protoplasm liberates energy by the oxidation of food, and various things happen as the result; among them are the maintenance of the protoplasm and the making of motions. In the higher animals the distinction between basic and functional metabolism is sharp, and, necessarily so, for the well-being of any of the higher animals requires that he shall have pretty complete control over the activities of his protoplasm, and this he could not have if the functional metabolism were blended in with the basic. In other words, it is as important for bodily well-being that the cells be able to become inactive as that they be capable of activity.