49. It has been already mentioned that the tissues of the body are constantly parting with particles which enter into their composition, and are receiving new materials to replace what is lost. We have seen that increased muscular exertion is accompanied with increased loss of substance; and the same is true of increased mental exertion, and of increased evolution of animal heat, as in exposure to extreme cold. Muscular and mental effort, and the maintenance of the temperature of the body, as well as regeneration of the tissues, all involve losses of substance which require to be made good; and thus, as has been said at the outset, the body may be regarded as a vortex whose component particles are ever changing while its form remains.

The processes by which materials are altered and thrown off are all processes of oxidation, by which complex chemical combinations are reduced to others of simpler descriptions; and this is sometimes expressed, though not very accurately, by using the word combustion in speaking of such changes. The ultimate products of combustion of organic matter are, as will be recollected, carbonic acid, water, and ammonia; that is to say, that, keeping sulphur, phosphorus, and mineral matters out of account, and confining our attention to the carbon, hydrogen, oxygen, and nitrogen of organic matters, the most complete oxidation of such substances which can be effected is into carbonic acid, water, and ammonia. The oxidation within the body falls short of this; by far the greater part of the matters which escape from it, after having circulated in the system, being eliminated as carbonic acid, water, and urea. No doubt, as has already been pointed out, there is a continual casting off of horny matters and of oil from the integument; and there is discharge of undecomposed matter in the form of mucus from intestinal and other passages, as well as certain substances derived from the bile; but the total amount of loss from such sources is comparatively small. Also, it is to be noticed, that ammonia escapes from the body in minute quantities, and that there is usually in the urine a distinct though small quantity of uric acid, a product of less complete oxidation of nitrogenous substance than urea. But urea, the principal organic constituent of the urine, a material of the same composition as cyanate of ammonia, and therefore one stage removed from that perfect oxidation which results in carbonic acid and ammonia, is the substance in the form of which by far the greater part of the nitrogenous debris escapes from the body.

Keeping out of consideration the debris of food which has never entered the system, but is discharged in the faeces, the amount of nitrogen given off daily may be estimated at 250 grains,* and the amount of carbon which escapes in the form of carbonic acid may be reckoned at 4000 grains; these substances must therefore be daily introduced into the system in those quantities in the shape of food, if the weight of the body and its constitution are to be maintained. Supposing the 250 grains of nitrogen to enter the system in the form of albumen, then in consequence of urea containing a much larger percentage of nitrogen and smaller percentage of carbon in its composition than albumen, there will be liberated in the conversion of the albumen into urea 755 grains of carbon, or seven-eighths of the total amount which the albumen contains, to escape in the form of carbonic acid, while the rest of the 4000 grains daily lost must be furnished by additional supplies of food, which need not contain any nitrogen. The amount of solid food necessary for the preservation of health is thus regulated by the loss of substance from the body; but as a certain quantity of the food always escapes digestion, and is discharged, after traversing the alimentary canal, without having entered the system, the supply taken has to be in excess of what is required to make up for systemic loss. It has been calculated that there are daily * One ounce avoirdupois contains 437 1/2 grains required for an adult man about 40 ounces of so-called solid food, yielding 22 or 23 ounces when evaporated to dryness, and from 50 to 80 ounces of water.

50. The aliment required by the body consists of organic food, salts, and water. The reason why water is required in larger proportion in our aliment than that in which it exists in the tissues, is that, by processes of filtration and evaporation, the body is constantly losing water by the skin, the kidneys, and the lungs, additional to what is produced by waste of tissue. Common salt or chloride of sodium, on account of its great solubility and the ease with which it passes through membranes, is also particularly liable to escape both by the skin and the kidneys, and is found in all the secretions; it requires therefore to be supplied in quantity greater than that in which it exists in the requisite amount of solid aliment. The results of experiment, as well as general experience, show that the addition of this substance to the food is advantageous to nutrition; and it is well known to farmers that it is relished by cattle, and improves their condition. Other mineral matters useful in the economy, as, for example, salts of lime, occur in solution in the water which we drink, as well as in our solid food.

The organic matters used as food, like those which are met with in the composition of the body, are divisible into nitrogenous and carbonaceous. Nitrogenous foods are of two kinds, namely, albuminoid or proteid substances, and gela-tinoids. Albuminoids are obtainable from both vegetable and animal sources, though much more abundantly from the latter; in flesh diet they are supplied in the forms of muscle-fibrin and albumen; in eggs albumen is furnished, mingled with oil in the yolk, and in the white altogether pure; and in milk the albuminoid constituent is casein, distinguished from albumen by not coagulating when heated. Under the title of gelatinoids may be conveniently grouped a set of substances obtained from animal sources, and including not only gelatin and chondrin, but the tissues which yield them. Also, nearly allied to those substances in nutritive value are kreatin and other flavouring ingredients in the juice of meat.

Carbonaceous foods are likewise of two principal kinds-— the oils and the carbohydrates. Both consist of carbon, hydrogen, and oxygen, but the oils have a much greater number of equivalents in their chemical formulae than the carbohydrates; that is to say, they consist of more complex arrangements of atoms; they have likewise a larger amount of carbon and a smaller proportion of oxygen in their composition. The carbohydrates are so called from consisting of carbon in conjunction with hydrogen and oxygen, in the proportion in which they are combined in water; they include starch, together with the allied substance cellulose, which forms a large part of growing vegetables, and sugar. Starch is a highly important constituent of vegetable diet, forming the larger part of the weight of flour, and a much greater proportion of the substance of potatoes. It occurs in small granules which are insoluble in cold water, but which burst when boiled, the contents of the starch granule being dissolved, while the outer envelope remains unacted on. It is a property of starch that in the presence of certain substances, sometimes termed amylolytic ferments, it becomes converted into grape sugar; and no starch is capable of being absorbed into the system until it has undergone that change. The sugars used in food are of three principal descriptions— cane sugar, grape sugar, and sugar of milk. It may be here remarked that milk, the sole food provided by nature for the infant, consists of a mixture of a solution of proteid substance in the form of casein, oil in the form of butter, sugar of milk, and various salts, and therefore contains all the varieties of aliment necessary for health.

51. Properly to nourish the body, the daily waste must be balanced by daily repair, and the food must contain a sufficiency of every substance required by the tissues. It follows from this, and has been proved by experiment, that no amount of carbonaceous food will make up for want of nitrogenous ingredients. On this account, animals which feed on vegetables have to consume large quantities of food that they may extract the requisite amount of nitrogenous substance; and persons who feed entirely on potatoes, require to use much greater quantities than they would require of other diet, because potatoes consist principally of starch, and have remarkably little nitrogenous substance in their composition.

In fact, it may be said, that in a diet sufficient in quantity, if the nitrogenous constituents be too plentiful, unnecessary work is thrown on the kidneys, while, if they be too scanty, an unnecessary load is thrown on the intestines. Moreover, it has been found by experiment that no amount of gelatinoid substance will suit instead of the albuminoids; animals can be supported on lean meat, but they die when fed on jelly alone, even when it is pleasantly flavoured, and at first relished. It appears, therefore, that animals have no power of building up albuminoid matter from simpler chemical substances. They have no power of manufacturing organic matter from the materials found in inorganic nature, but feed either directly on the vegetable world, or on other animals which have fed on vegetables; and there is no proof that in any instance they have the power of acting on the simpler organic substances, so as to produce from them the more complex Further, it appears from the researches of botanists, that even in plants the power of building organic matter is confined to the green parts. The statement may therefore be ventured on, that so far as observation has yet proceeded, it would appear that the presence of chlorophyll is as necessary for the production of organic matter in organisms, as the presence of protoplasm is necessary for growth.

With regard to gelatin, the question is often asked how it happens, if it be incapable of sustaining life, that in conjunction with other things, it is useful as an article of diet, and a favourite in the sick room. Perhaps that question is sufficiently answered by pointing out that carbonaceous substances are likewise insufficient by themselves to support life, and that in the formation of urea from gelatin, five-sixths of the carbon is unused, and therefore combines with oxygen to form carbonic acid, as does the carbon of carbonaceous food; also, that gelatin requires little digestion, and is at once decomposed on entering the circulation.

All carbonaceous food serves the economy sooner or later by undergoing oxidation into carbonic acid and water, and thereby supporting the temperature of the body, or assisting its vital processes, as we have seen that it does in muscular action. Oil may be temporarily stored up in the shape of adipose tissue; and the carbohydrates may be stored as glycogen in the liver, a viscus which likewise serves as a reservoir for oil, healthily in fishes, and more or less pathologically in man; but there is no proof that grape sugar can be converted into oil, nor that either oil or sugar is changed into any more complex substance. It is quite possible that the fattening effects which the carbohydrates often have, may be produced by their saving from oxidation oil which would otherwise be consumed in their stead. Certain it is that, in dieting the sick, the use of the carbohydrates is not found to be equivalent to the use of oils.