There are indeed certain circumstances in the habits of some of the carnivore which require organs of considerable capacity, as, for ex ample, those beasts of prey who take their food at long intervals, being supplied, as it were, in an occasional or incidental manlier, so that it becomes necessary for them to lay up a considerable store of materials, and to take advantage of any opportunity which presents itself of replenishing the stomach. The anato mical structure of the human digestive organs indicates that man was intended by nature for a mixed diet of animal and vegetable aliment, but with a preponderance towards the latter ;v and it appears in fact that, while a suitable combination of the two seems the most condu cive to his health, and to the due performance of all his functions, either species is alone competent to his growth and nutritiun.-f The most important of the proximate prin ciples employed in diet are hbrin, albumen, oil, jelly, gluten, mucilage, farina, and sugar, to which may be added some others of less frequent occurrence. They are derived, name or less, from almost all the classes of animals and vegetables, and from nearly all their indi vidual parts, their employment being regulated, in most cases, rather by the facility with which they are procured, and reduced into a form proper to be acted upon by the stomach, than by the quantity of nutritive matter which they con tain. This is one of those subjects in which we have to notice the remarkable effects of habit and custom, both on the functions and the sensations. We find whole tribes of people living on a diet, which, to those unaccustomed to it, would be not only in the highest degree unpalatable, but likewise altogether indiges tible ; while, by the various modes of preparing food, which have been suggested, either by luxury or by necessity, the most intractable substances are reduced into a digestible state.* The writers on dietetics have attempted to include all substances that are competent to afford nutrition under a few general principles, of which, as they exist in nature, they are supposed to he composed. Cullen, who may be considered as the first who attempted to in troduce correct philosophical principles into this department of physiology, reduced them to two, the oily and the saccharine, and endea voured to prove that all the animal fluids may be referred to these principles.t Alagendie, on the contrary, proceeding less upon their chemical composition than upon the forms under which they present themselves, classes alimentary substances under the nine heads of farinaceous, mucilaginous, saccharine, acidu lous, oily, caseous, gelatinous, albuminous, and fibrinous.: Dr. Trout, whose views on this subject are marked by his characteristic acuteness, reverts to the mode of Cullen, ad mitting only of the oily, the saccharine, and the albuminous principles, which three, lie conceives, form the " groundwork of all orga nized bodies."§ Of animal compounds which are employed in diet milk may he regarded as holding the first place, !mil from its nutritive and its digestible properties, and as such It has no doubt been provided by nature for the newly-horn animal, when it requires a diet, which may he adapted to the delicacy of its organs in its novel state of existence, while, at the same time, it pro vides for its rapid growth. We accordingly find that the three principles mentioned above are combined in milk in a manner the most proper fur this double purpose, and that there is no compound, either natural or artificial, which is equally well suited to it. Next to milk, with respect to its nutritive properties, we may class eggs of various kinds, the mus cular Libre of animals, and their gelatinous and albuininous parts, very few of which, how ever, are employed in diet until they have undergone the various operations of cookery. Of these operations the most important in their dietetical eflect is the formation of decoctions or infusions, constituting soups of all descrip tions, in which we retain the more soluble, and, for the most part, the more nutritive matter, while the residue is rejected. The fish which are usually employed in diet consist of a much greater proportion of jelly and albumen than the flesh of the marnmalia and of birds; these principles are united, in most cases, with a con siderable quantity of oil.
The most nutritive of the vegetable proximate principles is gluten ; it forms a considerable proportion of certain kinds of seeds, and more especially of wheat, and we accordingly find that in all those countries which admit of the growth of this plant, and which have arrived at any considerable degree of civilization, wheaten bread forms the most important article of vegetable diet, and one which appears the best adapted for all ages and all constitutions. Next to gluten we may rank farina, both from its valuable properties and from the extent to which it is employed. It enters largely into the composition of wheat and of the other seeds of the cerealia, also of rice and maize, while it constitutes a great proportion of the whole substance of the leguminous seeds and of tubers. It also forms the principal in
gredient of the chesnut, and of the esculent alga., so that, upon the whole, we may con sider it as entering more largely into the aliment of mankind, in all different climates and situa tions, than any other vegetable compound.
Perhaps there is no proximate principle which contains in the same bulk a larger pro portion of nutritive matter than oil, and we accordingly find that oil, as derived either from the animal or vegetable kingdom, enters largely into the diet of all nations. But it affords an example of one of those articles, which, al though highly nutritious, is not very digestible without a due admixture of other substances, which may in some way render it more proper for the action of the gastric juice t It may indeed be received as a very general rule that a certain quantity of matter, which in itself contains but a small proportion of the princi ples which immediately serve for nutrition, is necessary for the due performance of the func tions of the stomach, probably in some degree for the purpose of mere dilution or mechanical division. The same remark applies to sugar as to oil. Sugar would appear to be one of the most nutritive of the proximate principles, but when taken alone or in too great quantity it deranges the digestive organs, and becomes incapable of supporting life.* The difference in the different kinds of ali ment between their capacity of affording the materials from which chyme may be produced, and the facility with which they are acted upon by the stomach, or in ordinary language, be tween their nutritive and their digestible quality, has been distinctly recognized by various phy siologists,t although it has not always been sufficiently attended to. We have some strik ing illustrations of the fact in a series of expe riments which were performed by Goss,I and in those of Stark,§ where the digestibility and the nutrition of various species of aliment bore no relation to each other, while they afford the most decisive proof of the advantage, or rather the necessity, of a mixture of substances, in order to produce the compound which is the best adapted for the action of the stomach.
We have referred above to the difference in the digestive powers of the stomachs of diffe rent classes of animals as depending on their peculiar organization. In many instances the difference is so strongly marked as to leave no doubt either as to its existence or as to the cause by which it is directly produced. But there are many cases where we observe the effect without being able to assign any imme diate cause for it ; where substances, which are highly nutritive and perfectly salutary to certain individuals, are apparently incapable of being digested by others. After making all due al lowance for the effects of habit, association, or even caprice, there still appears sufficient ground for concluding that there are original differences in the powers of the stomach, which cannot be assigned to any more general prin ciple. This observation applies principally to the individuals of the, human species, where such variations, or, as they have been termed, idiosyncrasies, of all descriptions are much more apparent than in any other kind of ani mals. All other animals, even those which the most nearly resemble the human species, are much more uniform in this respect, being guided in the choice of their food principally by that instinctive feeling which leads them to select the substances which are the best adapted for their organs. But even here we meet with certain peculiarities, where animals prefer certain kinds of aliment, and where there is no obvious anatomical or physiological cause which can explain the effect. This, however, we may regard as an exception to the general rule, for there is perhaps no one of the functions in which we are enabled more clearly to trace the adaptation of the organ to the struc ture and habits of the animal, than in what respects the supply of nutrition, including the mode of procuring the food, and the whole of the series of changes which it experiences from the digestive organs.* Liquids of various kinds constitute an im portant part of the diet of almost all indivi duals. They may he arranged under the two divisions of those liquids which we employ merely for the purpose of quenching thirst, or diluting our solid food, or such as are made the vehicles of nutriment, including various kinds of decoctions and infusions. The latter are derived both from the animal and the vege table kingdoms, and when duly prepared form a species of food, which, as containing the most soluble and the most sapid portions, is, in most cases, both highly nutritive and diges tible. But we observe here the same kind of idiosyncrasy to which we referred above, and which it frequently becomes necessary to attend to in the directions that are given respecting diet, and more especially to invalids and to children.