For instance, not all our existing know ledge of the composition of most of the sub stances commonly used as food, would enable us to construct a diet which would be certain to contain an exact proportion of all the necessary salts. For, in the first place, we must recollect the probable importance of some which are only present in very small quantity ; as well as the value that similarly appears to attach to minnte proportions of certain organic acids, and their compounds with bases. In the next place, we must re member that, both in animals and vegetables, these caline constituents seem liable to vary, in nature as well as amount, according to the peculiarities of the soil from which they are ultimately derived. It is not by any means easy to insure their presence. And a goocl scale of diet ought to provide against any danger of their deficiency, by adding so much of various fresh vegetables as would cover all possibilities of such an occnrrence. Indeed, nothing short of such variety would make the saline quality of any food perfect.
A similar argument will apply to the quan tities of all the other ingredients. The mecha nical state of the protein and hydrate of carbon will have at least as much influence in deter mining their requisite amount as the quantity rendered necessary by the waste of the tissues. Hence, to this latter estimate we have always to add a large excess ; such as will be sufficient to cover the surplus protein which passes,— undigested or indigestible—from the alimentary canal. And the same caution may be applied, with still more force, to that substitution of hydro-carbon, or fat, for hydrate of carbon, or starch and sugar, which some authors have regarded as so easy and natural an exchange. In all probability these substances are not by any means convertible or inter-changeable in any scale of diet. The cell-wall of the adipose tissue is dissolved with great diffi culty ; its liberated contents are next ab sorbed in but small quantities; and they then pass through glands which apparently have a slow but definite office to execute upon them, before they are admitted into the general circulating current of the blood. And, lastly, the rudest numerical contrast of their final combustive metamorphosis with that of the hydrates of carbon, shows that they require the combination of a much larger quantity of oxygen* before they can leave the body in the form of carbonic acid and water.
The total amount of food required by the body is also exposed to circumstances which are just as certain to baffie all such calcu lations. For this important quantity will evi dently vary with the rate of waste sustained by each individual :— and hence with the activity of his life ; the nature of his habitual exertion ; and the state of his mind ; as well as with the climate, race, temperament, and education, which help to form the microcosm of every man's personality. The degree of
variation which may be brought into play by each of these circumstances it is impossible to specify- ; though it would often receive no inapt illustration from a comparison of the habits of the various members of a family or other smallest social aggregate.
Hence the true value of physiolog,ical che mistry, in respect to the principles of dietetics, is that of being an admirable guide to the general composition of a proper food. In this capacity, it is not too much to say that its veto ought to be absolute. But with this negative function terminates its practical use fulness. Our choice of the exact quantities and qualities of alimentary substances neces sary to construct a perfect scale of diet, may indeed be sometimes explained by chemistry. But it must alway s be dictated by experience. And the dietaries of gaols, workhouses, and hospitals, corrected, as they have too often been, by the ghastly hand of Death himself, have fixed the limits of the food necessary for health, with an accuracy which, considering the price of human life that has been paid for it, ought surely to satisfy the most rigid economist.
From such sources of information we may deduce, that a healthy adult male, of active habits, requires daily about two pounds of solid food. Of this food, six or eight ounces arc, preferably-, meat. While, if the quality of such a diet be lowered (as, for example, by the introduction of much potatoes or rice), its quantity ought to be proportionally raised, so as to compensate this diminution of its nutritious characters.
_Relations of digestion to nutrition generally. —We have thus specified the various ali mentary substances which are normally sub mitted to the action of the digestive canal. And the functions of the different segments and structures of this tube have already, so far as possible, been assigned to each.
All these functions, however, together make up but a small part of the complex act of digestion. Nay, more, digestion itself is only a part of a still wider and more complex process of nutrition. And, further, the rela tion borne by digestion to nutrition is by no means litnited to an absorption of new matter into the body ; but also involves a revolution or cycle of much of the existing substance of the organism, between those acts of ingestion and egestion, which mark the respective ex tremes of its nutritional life. Hence it seems necessary to end this description of the ali mentary canal by a succinct enumeration of (1st) the series of phenomena which consti tute the digestive act, and (2nd) the share which digestion itself takes in nutrition generally.