Whether Nitrogen be an essential Element of Food. - M. Majendie's Experiments. - Classification of Aliments. - Digestible and Nutritive not synonymous. - Texture of Food. - Cookery. - Boiling. - Roasting. - Baking, etc. - Condiments. - Importance of Salt. - Vinegar. - Oil, etc.

159. Those bodies which have possessed life can alone be strictly considered capable of affording aliment to animals; yet there exist a certain number of inorganic substances, such as water, common salt, lime, etc. which, although incapable by themselves of nourishing, appear, when administered in conjunction with the former, to contribute essentially to nutrition. The consideration, therefore, of the Materia Alimentaria, necessarily embraces not only the substantive agents above stated, but those which, from their modus operandi, are entitled to the distinctive appellation of Alimentary Adjectives. Under the former division will be arranged all the varieties of animal and vegetable food; 176 m. majendie's theory, and experiments under the latter, the class of condiments will merit our attention1.

160. The science of Chemistry had no sooner demonstrated that all the different tissues of the body contained azote (nitrogen), than it became an important physiological question, whether that element were derived from the food introduced into the stomach, or from the atmospheric air inhaled by the lungs. To the practical physician this subject may not perhaps appear to present any points of professional interest; but the experiments which have been lately instituted by M. Majendie, in support of the former opinions, have furnished results which, in my judgment, are susceptible of some useful applications to practice. M. Majendie observes, that the partisans of the theory he proposes to subvert insist particularly upon the example of the herbivorous animals, which are supported exclusively upon non-azotised matter; upon the history of certain people that live entirely upon rice and maize; upon that of negroes, who can live a long time without eating anything but sugar; and, lastly, upon what is related of caravans, which, in traversing the deserts, have for a long time had only gum in place of every sort of food.

Were it, says he, indeed proved by these facts that men can live a long time without azotised food, it would be necessary to acknowledge that azote has an origin different from the food; but the facts cited by no means sustain this proposition, for almost all the vegetables upon which man and animals feed contain more or less azote; for example, the impure sugar that the negroes eat presents a considerable proportion of it; and with regard to the people, as they say, who feed upon rice or maize, it is well known that they eat milk or cheese; now casein is the most azotised of all the nutritive proximate principles. In order to acquire some more exact notions on this subject, M. Majendie submitted several animals, during a necessary period, to the use of food of which the chemical composition was accurately ascertained. I shall present the reader with an account of these experiments, and then explain the different, but not less important conclusions, which I deduce from their results. He took a small dog of three years old, fat, and in good health, and put it to feed upon sugar alone, and gave it distilled water to drink: it had as much as it chose of both. It appeared very well in this way of living for seven or eight days; it was brisk, active, ate eagerly, and drank in its usual manner.

It began to get thin in the second week, although its appetite continued good, and it took about six or eight ounces of sugar in twenty-four hours. Its alvine excretions were neither frequent nor copious; that of the urine was very abundant. In the third week its leanness increased, its strength diminished, the animal lost its liveliness, and its appetite declined. At this period there was developed upon one eye, and then on the other, a small ulceration on the centre of the transparent cornea; it increased very quickly, and in a few days it was more than a line in diameter; its depth increased in the same proportion; the cornea was very soon entirely perforated, and the humours of the eye ran out. This singular phenomenon was accompanied with an abundant secretion of the glands of the eyelids. It, however, became weaker and weaker, and lost its strength; and, though the animal ate from three to four ounces of sugar per day, it became so weak that it could neither chew nor swallow; for the same reason every other motion was impossible. It expired the thirty-second day of the experiment. M. Majendie opened the animal with every suitable precaution.

He found a total want of fat; the muscles were reduced to more than five-sixths of their ordinary size; the stomach and intestines were also much diminished in volume, and strongly contracted. The gall and urinary bladders were distended by their proper fluids, which M. Chevreul was called upon to examine. That distinguished chemist found in them nearly all the characters which belong to the urine and bile of herbivorous animals; that is, that the urine, instead of being acid, as it is in carnivorous animals, was sensibly alkaline, and did not present any trace of uric acid, nor of phosphate. The bile contained a considerable portion of picromel; a character considered as peculiar to the bile of an ox, and, in general, to that of herbivorous animals. The excrements were also examined by M. Chevreul, and were found to contain very little azote, whereas they usually furnish a considerable quantity.

1 Dr. Bostock has observed that condiments differ from aliments in one essential circumstance, viz. that whereas the latter are always resolved into their ultimate elements before they can contribute to nutrition, the former act in their entire state, and, if decomposed, would probably cease to produce their specific effects.

161. M. Majendie considered that such results required to be verified by new experiments: he accordingly repeated them on other dogs, but always with the same conclusions. He therefore considered it proved, that sugar, by itself, is incapable of supporting dogs. This want of the nutritive quality, however, might possibly be peculiar to sugar: he therefore proceeded to inquire whether other substances, non-azotised, but generally considered as nutritive, would be attended with the same consequences. He fed two dogs with olive oil and distilled water, upon which they appeared to live well for about fifteen days; but they afterwards underwent the same series of accidents, and died on the thirty-sixth day of the experiment. In these cases, however, the ulceration of the cornea did not occur.