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vegetable substances which contain it in the largest proportion, such as wheat, oats, hay, &c. In the animal world, nitrogen appears to be in the ascendant, and to constitute the most important element in the animal formation; carbon, oxygen, and hydrogen occupying a secondary position.

Even in the animal creation, however, carbon constitutes the greater part of the bulk. Thus lean beef, white of egg, and the curd of milk, when quite dry, present the following proportions (Professor Johnston) :—

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The greater amount of nitrogen contained in animal substances, as compared with vegetable, appears, however, to thoroughly modify their anatomical and physical character, and warrants the distinction generally recognised and established between carbonaceous and nitrogenous, or vegetable and animal, organizations.

This simple fact is at the bottom of the entire

theory of nutrition in the organic creation. The framework of an animal being, to a great 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 granivorous and herbivorous animals live, all contain, in addition to the carbon, considerable quantities 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 granivorous, 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 per centage 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, flesh, milk, eggs, &c., 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 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, &c., 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.

Having thus briefly stated what nutrition is, and analysed the materials on which it depends, I shall now examine the various stages of elaboration through which these materials have to pass. It is not my intention, however, to enter minutely into the history of the various functions which, by their united action, constitute digestion and nutrition. I am merely desirous to give a brief sketch of the facts established by the researches of modern physiologists, in order to render intelligible the subject of nutrition in general, and more especially that of morbid nutrition.

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