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Liebig's Chemical Letters



My dear Sir,

    Let me now apply the principles announced inthe preceding letters to the circumstances of our own species. Man, when confinedto animal food, requires for his support and nourishment extensive sources of food,even more widely extended than the lion and tiger, because, when he has the opportunity,he kills without eating.

    A nation of hunters, on a limited space, isutterly incapable of increasing its numbers beyond a certain point, which is soonattained. The carbon necessary for respiration must be obtained from the animals,of which only a limited number can live on the space supposed. These animals collectfrom plants the constituents of their organs and of their blood, and yield them,in turn, to the savages who live by the chase alone. They, again, receive this foodunaccompanied by those compounds, destitute of nitrogen, which, during the life ofthe animals, served to support the respiratory process. In such men, confined toan animal diet, it is the carbon of the flesh and of the blood which must take theplace of starch and sugar.

    But 15 lbs. of flesh contain no more carbonthan 4 lbs. of starch, and while the savage with one animal and an equal weight ofstarch should maintain life and health for a certain number of days, he would becompelled, if confined to flesh alone, in order to procure the carbon necessary forrespiration, during the same time, to consume five such animals.

    It is easy to see, from these considerations,how close the connection is between agriculture and the multiplication of the humanspecies. The cultivation of our crops has ultimately no other object than the productionof a maximum of those substances which are adapted for assimilation and respiration,in the smallest possible space. Grain and other nutritious vegetables yield us, notonly in starch, sugar, and gum, the carbon which protects our organs from the actionof oxygen, and produces in the organism the heat which is essential to life, butalso in the form of vegetable fibrine, albumen, and caseine, our blood, from whichthe other parts of our body are developed.

    Man, when confined to animal food, respires,like the carnivora, at the expense of the matters produced by the metamorphosis oforganised tissues; and, just as the lion, tiger, hyaena, in the cages of a menagerie,are compelled to accelerate the waste of the organised tissues by incessant motion,in order to furnish the matter necessary for respiration, so, the savage, for thevery same object, is forced to make the most laborious exertions, and go througha vast amount of muscular exercise. He is compelled to consume force merely in orderto supply matter for respiration.

    Cultivation is the economy of force. Scienceteaches us the simplest means of obtaining the greatest effect with the smallestexpenditure of power, and with given means to produce a maximum of force. The unprofitableexertion of power, the waste of force in agriculture, in other branches of industry,in science, or in social economy, is characteristic of the savage state, or of thewant of knowledge.

    In accordance with what I have already stated,you will perceive that the substances of which the food of man is composed may bedivided into two classes; into nitrogenised and non-nitrogenised. The former arecapable of conversion into blood; the latter are incapable of this transformation.

    Out of those substances which are adapted tothe formation of blood, are formed all the organised tissues. The other class ofsubstances, in the normal state of health, serve to support the process of respiration.The former may be called the plastic elements of nutrition; the latter, elementsof respiration.

    Among the former we reckon -

    Vegetable fibrine.

    Vegetable albumen.

    Vegetable caseine.

    Animal flesh.

    Animal blood.

    Among the elements of respiration in our food,are -

    Fat. Pectine.

    Starch. Bassorine.

    Gum. Wine.

    Cane sugar. Beer.

    Grape sugar. Spirits.

    Sugar of milk.

    The most recent and exact researches have establishedas a universal fact, to which nothing yet known is opposed, that the nitrogenisedconstituents of vegetable food have a composition identical with that of the constituentsof the blood.

    No nitrogenised compound, the composition ofwhich differs from that of fibrine, albumen, and caseine, is capable of supportingthe vital process in animals.

    The animal organism unquestionably possessesthe power of forming, from the constituents of its blood, the substance of its membranesand cellular tissue, of the nerves and brain, and of the organic part of cartilagesand bones. But the blood must be supplied to it perfect in everything but its form- that is, in its chemical composition. If this be not done, a period is rapidlyput to the formation of blood, and consequently to life.

    This consideration enables us easily to explainhow it happens that the tissues yielding gelatine or chondrine, as, for example,the gelatine of skin or of bones, are not adapted for the support of the vital process;for their composition is different from that of fibrine or albumen. It is obviousthat this means nothing more than that those parts of the animal organism which formthe blood do not possess the power of effecting a transformation in the arrangementof the elements of gelatine, or of those tissues which contain it. The gelatinoustissues, the gelatine of the bones, the membranes, the cells and the skin suffer,in the animal body, under the influence of oxygen and moisture, a progressive alteration;a part of these tissues is separated, and must be restored from the blood; but thisalteration and restoration are obviously confined within very narrow limits.

    While, in the body of a starving or sick individual,the fat disappears and the muscular tissue takes once more the form of blood, wefind that the tendons and membranes retain their natural condition, and the limbsof the dead body their connections, which depend on the gelatinous tissues.

    On the other hand, we see that the gelatineof bones devoured by a dog entirely disappears, while only the bone earth is foundin his excrements. The same is true of man, when fed on food rich in gelatine, as,for example, strong soup. The gelatine is not to be found either in the urine orin the faeces, and consequently must have undergone a change, and must have servedsome purpose in the animal economy. It is clear that the gelatine must be expelledfrom the body in a form different from that in which it was introduced as food.

    When we consider the transformation of the albumenof the blood into a part of an organ composed of fibrine, the identity in compositionof the two substances renders the change easily conceivable. Indeed we find the changeof a dissolved substance into an insoluble organ of vitality, chemically speaking,natural and easily explained, on account of this very identity of composition. Hencethe opinion is not unworthy of a closer investigation, that gelatine, when takenin the dissolved state, is again converted, in the body, into cellular tissue, membraneand cartilage; that it may serve for the reproduction of such parts of these tissuesas have been wasted, and for their growth.

    And when the powers of nutrition in the wholebody are affected by a change of the health, then, even should the power of formingblood remain the same, the organic force by which the constituents of the blood aretransformed into cellular tissue and membranes must necessarily be enfeebled by sickness.In the sick man, the intensity of the vital force, its power to produce metamorphoses,must be diminished as well in the stomach as in all other parts of the body. In thiscondition, the uniform experience of practical physicians shows that gelatinous mattersin a dissolved state exercise a most decided influence on the state of the health.Given in a form adapted for assimilation, they serve to husband the vital force,just as may be done, in the case of the stomach, by due preparation of the food ingeneral.

    Brittleness in the bones of graminivorous animalsis clearly owing to a weakness in those parts of the organism whose function it isto convert the constituents of the blood into cellular tissue and membrane; and ifwe can trust to the reports of physicians who have resided in the East, the Turkishwomen, in their diet of rice, and in the frequent use of enemata of strong soup,have united the conditions necessary for the formation both of cellular tissue andof fat.