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extent formed of nitrogen, it is clear that if the animal is not carnivorous, does not live on flesh, it must consume vegetable food containing nitrogen. And such is the case; the grains and grasses on which frugivorous and herbivorous animals live, all contain, in addition to carbon, a certain quantity of nitrogen.

Carnivorous animals make and repair their nitrogenous flesh-tissues from the nitrogenous flesh of the herbivorous animals; while the latter, as we have seen, elaborate it directly from the vegetable world.

Man, occupying a medium position, and being carnivorous, herbivorous, and frugivorous, can extract the nitrogen he requires either from animal or from vegetable substances, or from both simultaneously. In the first case, he transforms directly the flesh of the nitrogenous tissues of animals into his own; in the second, he extracts from the vegetables he consumes their small percentage of nitrogen; and, in the third, he forms and repairs his own flesh on nitrogen extracted indiscriminately from both kingdoms. In all three, he combines the nitrogen extracted from food with oxygen supplied from the atmosphere, through respiration, in the formation of his tissues.

From the above facts, it must be apparent that man, a considerable proportion of whose body is composed of nitrogen, is compelled to seek nitrogen in his food to complete his structures and to repair their waste. Nitrogen, therefore, he does seek, and

finds, in abundance, in animal substances, such as flesh, milk, eggs, and also, in a smaller ratio, in vegetable articles of diet.

Man, however, like all warm-blooded animals, has not only to complete and renovate the structures which constitute his body,--he has also, from the hour of his birth to that of his death, to create heat, in more or less abundance, according to the temperature of the atmosphere by which he is surrounded. Owing to the laws of radiation of heat, there is a constant tendency in all bodies, animate or inanimate, to abandon surplus heat to the medium in which they are placed. In the warm-blooded animal creation, this tendency is only counteracted by the constant generation of heat, which is a result partly of real combustion, and partly of the more intimate vital, electric, and nutritive changes that are constantly taking place in the tissues of the economy. When death supervenes, and these operations cease, the body, at first warm, rapidly loses its heat, and becomes of the same temperature as the surrounding atmosphere. Thus, during life, the animal body is a walking fire, consuming fuel, in the shape of food on the one hand, and of its own detritus on the other, which it combines with oxygen derived from the atmosphere during respiration. In a northern climate like ours, a very considerable proportion of the food taken is thus consumed in keeping up the animal heat.

Combustion, in the external world, is principally

supported by the combination with the oxygen of the atmosphere of carbonaceous substances, such as vegetable products, wood, charcoal, coal, and fats and oils, which are hydrocarbon compounds. Their rapid chemical combination with oxygen is attended with the evolution of heat and light. Were the combination to take place more slowly, heat would still be evolved, but light would not. This is what occurs in the animal economy. The carbon of vegetable food-the principal element of such food -and that of fatty substances and of alcoholic beverages rich in carbon, combines with the oxygen introduced into the circulation by respiration, forms carbonic acid, and evolves heat.

Thus we see that the nitrogenous element in food, represented by flesh or animal substances, is principally required to perfect the tissues of the economy, and to repair their waste; whilst the carbonaceous element, which is represented by vegetable articles of diet, by fats and oils, and by alcoholic beverages, is principally required as fuel, to support the silent combustion which is constantly going on in the animal system, and to which is mainly due the generation of animal heat. It is now generally admitted that the more intimate processes of nutrition, which consist in the constant formation and disintegration of the tissues of which the economy is formed, under the influence of the nervous system, are also attended with the evolution of latent heat in considerable quantities. These sources of heat

continue to create it incessantly as long as life lasts, ceasing their operation with life only.

It was long supposed that carbonaceous food was all but entirely employed or burnt in the generation of heat, and that "force" was derived in the animal economy all but exclusively from the assimilation of nitrogenous food, or sooner from that of the nitrogenous element in food, animal or vegetable. This was Liebig's theory, and his authority long overpowered all opposition or doubts.

During the last few years this point has been investigated by several eminent physiologists, amongst whom I may name E. Smith, Professor Haughton, Dr. Frankland, Professor Fick, and Wislicenus of Zurich. They have endeavoured to arrive at a clearer and more exact idea than we had before of the origin of the power given out or spent by animated beings.

These experiments have been carried out under the influence of modern views respecting the correlation of physical forces, and the doctrine of the conservation of force and of the equivalency of heat and of mechanical force. They appear to have satisfactorily established that the production of the muscular power spent by animals and man is not so much to be attributed to the assimilation of nitrogenous food as to the slow combustion of carbonaceous food. According to this view the formation of animal heat by the combustion of carbon, and the final assimilation and nutrition processes gene

rally are attended with the development of "force," of which the muscles may possibly be only the instruments, not the producers.

This theory may be familiarly illustrated and explained by the steam engine. The latter, in burning coal, carbon, does not only produce heat, but power, the power that drags the train along. In a more obscure, but equally evident manner, the slow combustion of carbonaceous food in the processes of nutrition is attended with the development not only of heat but of power or force.

If the above views are correct, it would follow, singular as the statement must appear, when viewed by former lights, that more power or strength is to be got out of fat than out of meat or muscular tissue; and this really seems seems to be the case. Tyrolese chamois hunters find that they can endure greater fatigue on beef fat than on the same weight of lean meat. Accordingly, when about to absent themselves for several days in the higher mountains, hunting, they take beef fat with them instead of lean meat.*

Thus is fully explained the craving of mankind for fatty food, and for carbonaceous food generally, even in warm weather and in warm climates. Thus is illustrated the generally acknowledged physiological principle that man is omnivorous. These facts also explain the "strength" of the riceeating Hindoo, of the potato-eating Irishman, of See Intellectual Observer, July, 1866.

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