NUTRITION OF CARNIVOROUS ANIMALS. 155 tion sufficient for the metamorphosis of an adequate amount of tissue, that enough carbon and hydrogen may be set free for the support of the respiratory process. And we see a corresponding activity in the Human hunters of the swift-footed antelope and agile deer, which answers a similar purpose; and which is remarkably contrasted with the stupid inertness of the inhabitants of the frigid zone, that is only occasionally interrupted by the necessity of securing the supplies of food afforded by the massive tenants of their seas. 161. The nutrition of the Carnivorous races may, then, be thus described. The bodies of the animals upon which they feed, contain flesh, fat, &c., in nearly the same proportion as their own; and all, or nearly all, the aliment they consume, goes to supply the waste in the fabric of their own bodies, being converted into its various forms of tissue. After having remained in this condition for a certain time, varying according to the use that is made of them, these tissues undergo another metamorphosis, which ends in restoring them to the condition of inorganic matter; and thus give back to the mineral world the materials which were drawn from it by plants. Of these materials, part are burned off, as it were, within the body, by union with the oxygen of the air taken in through the lungs, from which organs they are discharged in the form of carbonic acid and water: the remainder are carried off in the liquid form by other channels. Hence we may briefly express the destination of their food in the following manner :— Carbonic acid and water, thrown off by respiration. Urea and biliary matter, &c., thrown off by other excretions. 162. But in regard to the Herbivorous animals, the case is different. They perspire much more abundantly, and their temperature is thus continually kept down (§ 372). They consequently require a more active combustion, to develop sufficient bodily heat; and the materials for this are supplied, as we have seen, by the non-azotized constituents of their food, rather than by the metamorphosis of their own tissues, which takes place with much less rapidity than in the carnivorous tribes. Hence we may thus express the 156 NUTRITION OF HERBIVORA AND OF MAN. destination of this part of their food; that of the albuminous matters, here much smaller in amount, being the same as in the preceding case: Starch, oil, and other non-azotized compounds, partly converted into { Fatty and other animal tissues, but chiefly (Carbonic acid and water, thrown off disengaged by the respi directly as (ratory process. The proportion of the food deposited as fat, will depend in part upon the surplus which remains, after the necessary supply of materials has been afforded to the respiratory process. Hence, the same quantity of food being taken, the quantity of fat will be increased by causes that check the perspiration, and otherwise prevent the temperature of the body from being lowered, so that there is need of less combustion within the. body to keep up its heat. This is consistent with the teachings of experience respecting the fattening of cattle; for it is well known that this may be accomplished much sooner, if the animals are shut up in a warm dwelling and are covered with cloths, than if they are freely exposed in the open air. 163. Now the condition of Man may be regarded as intermediate between these two extremes. The construction of his digestive apparatus, as well as his own instinctive propensities, point to a mixed diet as that which is best suited to his wants. It does not appear that a diet composed of ordinary vegetables only, is favourable to the full development of either his bodily or his mental powers; but this cannot be said in regard to a diet of which the corn-grains furnish the chief ingredient, since the gluten they contain appears to be as well adapted for the nutrition of the animal tissues, as is the flesh of animals. On the other hand, a diet composed of animal flesh alone is the least economical that can be conceived; for, since the greatest demand for food is created in him (taking a man of average habits in regard to activity and to the climate under which he lives) by the necessity for a supply of carbon and hydrogen to support his respiration, this want may be most advantageously fulfilled by the employment of a certain quantity of non-azotized food, in which these ingredients predominate. Thus it has been calculated that, since fifteen pounds of flesh contain no more carbon than four pounds of starch, a savage with one animal and an equal weight of starch, could support life for the same length of time during which another restricted to animal Substances, 100 parts. COMPOSITION OF ARTICLES OF HUMAN FOOD. 157 food would require five such animals, in order to procure the carbon necessary for respiration. Hence we see the immense advantage as to economy of food, which a fixed agricultural population possess over the wandering tribes of hunters which still people a large part both of the Old and New Continents. 164. The following Table exhibits the proportions of albuminous, starchy or saccharine, fatty, and saline substances, contained in various articles ordinarily used as food by Man; together with the proportion which water bears in each case to the solid constituents of the food, which becomes a most important element of consideration when the nutritive value of different kinds of food is compared : * The value of the Fat is stated in this column according to its heating equivalent of starch, which is larger in the ratio of 24 to 1. Hence, in the last column, the proportion of nutriment in aliments containing fat, comes to be greater than the weight of their solids would indicate. 49.0 1.0 46.0 1.0 1.4 23.0 0.2 0.7 23 47.6 29.0 76.6 53.7 82.0 82.0 Nitroge 158 ECONOMY OF HUMAN DIET. Those articles of food in which the nitrogenous compounds predominate, are especially fitted for the maintenance of the solid fabric of the body; whilst those in which the carbonaceous compounds are in largest excess, are those which are most effective as supplying materials for the combustive process. Conspicuous among the former are the various kinds of animal flesh, as also the white of eggs; whilst among the latter the most noticeable are bacon and butter, rice and potatoes, the former consisting almost wholly of fat, the latter being chiefly composed of starch. Of all single articles of food, good wheaten bread, in which the proportion of nitrogenous to carbonaceous components is about as 5.7 to 1, seems to be the one best suited to the ordinary wants of Man; but this acquires much additional value from the concurrent use of a moderate amount of fatty matter in the form of butter. 165. If the more highly azotized forms of food be employed exclusively, a great excess of them must be consumed to supply the carbon needed for respiration; whilst if the more carbonaceous kinds of food be used as the sole sustenance, unless the quantity ingested be large enough to afford the requisite supply of azotized material for the maintenance of the tissues, their nutrition must be imperfectly effected, and the strength must fail. Not only in the instance just cited, but in a variety of others, the instincts of mankind have led to such a combination of different articles of diet, as includes in their appropriate proportions the albuminous, the saccharine, and the oleaginous principles. Thus with meat we eat potatoes; and with the white meats which are deficient in fat, we eat bacon. We use melted butter with most kinds of fish, or fry them in oil; whilst the herring, the salmon, and the eel, are usually fat enough in themselves, and are dressed and eaten alone. A similar adjustment is made when we mix eggs and butter with sago, tapioca, and rice; when we add oil and the yolk of an egg to salad; when we boil rice with milk, and combine cheese with maccaroni. Bacon and greens, and pork and pease-pudding, again, are combinations founded in taste, which approve themselves to the judgment; as is also the Irish dish termed kolcannon, consisting of potatoes and cabbage, with a little bacon or fat pork. So are the mixture so common in Ireland and Alsace, of butter ECONOMY OF HUMAN DIET. 159 milk or curdled-milk with potatoes; and the combination of rice and fat, which is the staple of the diet of many Eastern nations. Even the morsel of butter or the bit of cheese which the English labourer eats with his hard-earned bread, are not matters of luxury, but have a positive importance; and the existence of these tastes and habits shows how by long experience Man has at last learned to adjust the composition of his food, so as best to maintain the health and vigour of his body. With a difference of requirement comes a difference of tastes. Thus men who are going through a very laborious course of exertion, prefer meat to bread or vegetables, feeling it to be more sustaining to their strength. On the other hand, those who are continuously exposed to the severity of an Arctic winter, eat with relish large masses of fat, on which they would look with disgust under other circumstances. The quantity of work which a man can do, and his power of sustaining extreme cold, both depend in great part, as has now been abundantly proved, upon the adequacy of the sustenance he takes: the demand, in the first case, being for albuminous material to supply the waste of his tissues; whilst in the second it is for combustive material suitable to generate heat in large measure, -a purpose which is far more efficiently answered by oleaginous substances, than by those of a starchy or saccharine nature. Experience further shows that the healthy condition of the blood of Man can only be maintained by the use of fresh vegetables as part of his ordinary diet. When these are withdrawn for any length of time, the disease known as Scurvy is certain to appear, unless lemon-juice or some other efficacious antiscorbutic be employed as a substitute. This is a fact of the utmost importance in provisioning ships for long voyages; the tendency to scurvy being increased by confinement and insufficient ventilation, and by the exclusive use of salt provisions. 166. Besides these organic substances, there are certain Mineral ingredients, which may be said to constitute a part of the food of Animals; being necessary to their support, in the same manner as other mineral substances are necessary to the support of Plants. Of this kind are common salt, and also phosphorus, sulphur, lime, and iron, either in combination or separate.-The 'uses of Salt are very numerous and |