supplied, since the higher animals are unable to make them. Nitrogen is needed for the production of proteins, and is in fact taken in food in this latter form, although afterwards broken down into its constituent amino-acids in the process of digestion. A certain minimal amount of protein food is therefore a neces- sity. When burned, protein gives energy, and might therefore be used for this purpose, if taken in sufficient quantity. But such a proceeding would be costly and wasteful. Accordingly, we make up the greater part of the energy value of our food by non-nitrogenous compounds, fats and carbo-hydrates. The latter appear also to be necessary for proper assimilation of protein. Some discussion has taken place as to the actual necessity of fat, since it has been shown that in the organism fat can be made from carbo-hydrate. Careful experiments indicate that it is not indispensable. Its chief value, physiolog- ically, lies in its high energy value, greatly owing to the very small amount of water contained in the forms used for food.
On account of the fact that the nitrogen of protein appears to be needed only or mainly for the construction of new body sub- stance, there has been a tendency to reduce the consumption of protein. This is advocated chiefly on grounds of economy. Although excessive consumption of this or any food is physiolog- ically harmful, there does not appear to be satisfactory reason for supposing that protein is particularly injurious. On the other hand, it has been stated that a consumption of protein in considerable excess of the minimum requirement is of advan- tage in conferring greater resistance to infection. Apart from the ill effects of any diet deficient in energy value as a whole, no satisfactory evidence has been brought in support of the state- ment. It may be said that if any reasonable diet of animal or vegetable structures, including fruit, be taken, it will only be necessary to take account of its energy value. Sufficient proteins and vitamins will be automatically obtained. The dictum of the present writer " Take care of the calories, the proteins will take care of themselves " may indeed be read " Take care of the calories, other things will take care of themselves."
It may be useful to give the composition of the diet put for- ward by the Royal Society Food Committee as a standard for a man of 70 kilos weight, doing moderate work:
Protein 70 grammes = 280 calories
Fat 90 grammes = 810
Carbo-hydrate 550 grammes = 2200
Total 3290 calories
It is understood that this refers to that part of the diet actually utilized and does not include undigested components.
In addition to the above-mentioned, there are certain things which, although they do not contribute to the energy value of a diet, are nevertheless indispensable for the proper working of the cell machinery and the utilization of the material presented to it. Water is needed for the carrying on of chemical reactions and for the conveyance of products from place to place. The colloidal systems of the cell are maintained and modified by salts, especially those of sodium, potassium and calcium. This adjustment, as Macallum has pointed out, is doubtless an inheritance from adaptation to the composition of the ocean in early geologic times, from whose inhabitants the present land animals are descended. We find, moreover, certain metals, such as iron, manganese, copper and magnesium, present in small amount in living organisms. We know that iron is a constituent of the red pigment of the blood corpuscles, by which oxygen is supplied to the tissues. In certain invertebrate organisms, it is replaced by copper. Magnesium is a corresponding element in the green pigment of plants, chlorophyll, without which life would come to an end owing to the using up of the oxygen of the atmosphere. Iron and manganese are of importance in the mechanism by which food is burned and its energy obtained. Certain moulds will not develop completely without zinc. It is more than likely that many other metals present in traces are more than mere accidental contaminations. Calcium, as we have already seen, plays an important part in the adjustment of colloidal systems. Potassium, although we know it to be indispensable, presents difficulties. Zwaardemaker maintains
that its importance rests in its radio-activity, but other workers have not been able to replace it by other radio-active elements. Sodium salts, at the present time, serve chiefly to make up the osmotic pressure of the blood and tissue fluids. The occurrence of iodine in the internal secretion of the thyroid gland must not be overlooked.
Vitamins. Not the least interesting of those constituents of a diet whose presence is absolutely necessary, although small quantities suffice, are the accessory food factors called by this name. The name " vitamine " was originally given owing to a mistaken view of their chemical nature. But, since it has come into general use and is conveniently short, objection to it may be removed, as Drummond has pointed out, by omitting the final "e" and using it merely as a name, without implication of chemical structure.
So far as known at present, they are three in number, distin- guished by the addition of the letters A, B and C. Vitamin-A is commonly found in association with certain fats, such as butter and cod-liver oil, but it is also present in fresh vegetables. It is essential to normal growth and maintenance and its absence appears to be responsible for rickets in children. Vitamin-B is found in the germ of seeds, such as wheat and rice, but it is widely spread. Its absence from polished rice results in beri-beri, when this food material is the only one taken. Vitamin-C is required for normal nutrition, although the precise manner in which it acts is unknown. In its absence, the disease known as scurvy makes its appearance. This vitamin is much less stable than the preceding ones and is found only, to any notable extent, in fresh fruit and vegetables. Although all of these factors occur in animal products, they are present there owing to the vegetable food taken by animals. Their ultimate source is the plant, since animals are unable to make them. It is more particularly Vitamin-C that is defective in animal products. Fresh meat contains it in a very small amount. Owing to the minute quanti- ties that are physiologically sufficient and are alone accessible for investigation, their chemical nature is as yet unknown. They do not undergo change in the course of their activity, but are excreted or destroyed more or less rapidly. Thus, their activity appears to be of a catalytic nature, but nothing very definite can be stated at present.
The normal growth of plants is also dependent on similar accessory factors. The " bios " of yeast, and Allen's work on diatoms may be referred to here. Bacteria, also, require in many cases special materials, such as haemoglobin, or particular amino-acids, for satisfactory culture.
It should be noted that certain products formed by organs in the body itself have powerful effects on the chemical changes of growth and nutrition. To these " internal secretions " or " hormones " reference will be made below.
Anabolism and Catabolism. It was held at one time that all food materials, previous to further utilization, must be built up into the actual constitution of complex protoplasmic molecules such as " biogens." And that the activities of organs typified by muscular contraction and glandular secretion consisted in the breaking down of such complex chemical individuals. The former process was spoken of as "anabolism" or "assimilation," the latter as "catab- olism " or " dissimilation." More recent work has led to a some- what different point of view, as already indicated above. It appears that glucose, for example, is burned without becoming a part of the protoplasmic structure and, although the energy thus available is used for the storing up of potential energy in some complex sys- tem, to be used up in subsequent contraction or secretion, this system may not necessarily be of a purely chemical nature. If the anabolic and catabolic processes are opposite chemical reactions in the sense of the building up and breaking down of the same com- pounds, it is natural to suppose that while the former process is taking place the latter is ipso facto decreased. Hence, the view that the nerves producing a reduction of activity (inhibition) are ana- ' bolic in action. Investigation, however, has failed to give confir- mation of such a view of the nature of the inhibitory process.
Allied to the above problem is that of the existence of nerves influencing the growth or repair of tissues. Although definite state- ments cannot yet be made, evidence of any such influence, apart from changes in blood supply, is inconclusive.
Enzymes. These catalytic agents are of great importance in the chemical changes of living organisms. It is generally recognized that they are colloidal and that the reactions accelerated by them
