3. The hydrates of carbon form a class of nutritional substances, the elementary com position of which is still more exactly- indi cated by their name. In other words, they consist of carbon, united with hydrogen and oxygen in those equivalent proportions of these two elements which are necessary for the formation of water (C, H, 2 0, 2). This group is a very large one : and includes, not only the various forms of cane, grape, and milk sugar, but a number of kindred sub stances ; —such as dextrin, guni, cellulose, inosit, and, especially, starch. All of these organic principles, however various their physical properties, have nevertheless the same chemical composition. And many of them are easily converted into grape sugar; either by the excitement of a limited meta morphosis by an azotized ferment, or by expo sure to the action of dilute acids.
The sugary ingredient of the milk forms about 51 per cent. of its quantity; and is the only representative of the hydrates of carbon which it contains.
The average amount of the substances belonging to this and the preceding group of alimentary constituents will of course vary greatly in the different kinds of food. Speaking generally, however, these two groups may he stated to predominate by turns in the food derived from the two kingdoms of nature. Thus while the hydro carbons are chiefly derived from the fat of animal food ; the hydrates of carbon belong even more exclusively to the starch and sugar of vegetable food. But, in strictness, no such marked difference can actually be made out between the two kinds of food in this re spect. The milk, the liver,and even the blood of the animal, all contain sugar : while inosit, a substance closely allied to sugar, forms au im portant constituent of its various muscles. And not only do many plants contain large quan tities of oily matter stored up in various parts of their tissues, but even the seeds of' the ce realia, which form the best vegetable diet, present an amount of fat ranging from '2 to 2 per cent.
The purposes fulfilled by these hydrates of carbon in the animal economy, offer a marked contrast to those subserved by the two pre vious groups. The protein compounds form what is eminently the basis of the organism; — the plasma from which are developed the blood and the tissues. They are thus his togenetic and hemagenctic, as the phrase is. The fatty matters of the body not only form a large constituent of the active nervous sub stance, but are also retained and stored up in the more inert and passive form of adi pose tissue. While the grape-sugar, into which the various hydrates of carbon are all finally converted, appears never to assume any permanent form in the body, but to be always rapidly eliminated from the blood. In
what shape, or after what metamorphoses, it leaves this fluid, is at present uncertain. It is, however, probable, that like the hydro carbons, these hydrates of carbon are essen tially a species of fuel for that process of calorific combustion,which pervades the whole body, and which discharges its resulting car bonic acid by means of the respiratory func tion. And Liebig has adduced numerical data froni the fattening of animals, which lead him to suppose, that these substances are also capable of undergoing a process of de oxidation, that converts them into fat, and thus enables them to augment the adipose tissue. But this view rests on very insuf ficient foundations*: and is curiously con trasted with that oxidationt of hydro-carbons into sugar, which the researches of various recent observers seem to indicate as one of the chief functions of the liver.I 4. The importance of the water of the food is such as justly entitles this liquid to the rank of a fourth alimentary constituent. For it forms about four-fifths of the entire corporeal mass and undergoes, at the va rious excretory surfaces of the skin, the lungs, and the kidney-s, a continual expenditure ; the replacement of which is obviously neces sary to the maintenance of the proper com position of the body.
The way in which this large aqueous con stituent facilitates the action of the various organs is not very difficult to conjecture. Their merely physical properties of hardness, flexibility, and the like, often seem chiefly determined by the quantity of the watery in gredient which they contain. And their far more recondite vital properties seem quite as immediately under its influence. Thus not only do its solvent powers appear to be eminently useful in furthering the minute division, and the local transfer, of various organic substances, but we are justified in conjecturing that it gives a more specific chemical assistance to many of those pro cesses of metamorphosis which are so inti mately connected with life. In both of these respects, it would seem to afford a special aid to the function of digestion. NVhile that act of absorption, which conveys the dissolved contents of the alimentary- canal into the surrounding veins, is greatly facilitated by the heightened diffusive energy which the low specific gravity of water enables it to impart to the fluids with which it has been mixed. And finally, the use of water in relation to the opposite extretue of nutrition—namely, to excretion—may be well exemplified by the urine, in which a highly poisonous product of life is continually washed out of the system, through the instrumentality of a stream of this universal solvent.