Imágenes de páginas

rich in nitrogen; they alone are said to be capable of forming organised tissues, because alone capable of being converted into blood, and are hence called the strictly nutritive substances. They are vegetable Albumen, Fibrine, and Caseine, and animal Flesh and Blood. The second class is Respiratory, or heat-making, and comprises the substances containing no nitrogen, which are therefore incapable of nourishing the body, and only serve the purposes of respiration, whence animal heat. They are Fats, Starch, Gum, Sugars, Pectine, Bassorine, Wines, Beers, and Spirits. A third class comprises the Inorganic substances, Water, Salts, Iron, &c. All Food is thus Nitrogenous, or nutritive, and Non-Nitrogenous, or heat-making.

How entirely this brilliant error has gained possession of the lectureroom may be read in the following passage from the last edition of Dr Carpenter's work on "Human Physiology," which may be taken as represcuting the opinion of English physiologists:

[ocr errors]

By rules based on the foregoing data, we may estimate the relative value of different articles of food, for the two distinct purposes of the formation of tissue and the production of heat. For the proportion of albuminous matter which any substance may contain furnishes the measure of its histogenetic value; while the proportion of hydro-carbon uncombined with oxygen affords the means of estimating its calorific power when oxidised. Since, in almost every alimentary substance, whether vegetable or animal, these two classes of compounds are mingled, the per-centage of nitrogen (save in those substances into which gelatine largely enters) which it may contain affords a tolerably correct estimate of the amount of albuminous matter which it

includes, and therefore of its histogenetic value; where, on the other hand, the percentage of nitrogen is smallest, that of hydro-carbon is largest, and the proportion of the combustible material is high


* DRAPER, Human Physiology, p. 27.

And an authoritative American physiologist, Professor Draper, adopts the classification, although he warns us that it is "only adopted for the sake of convenience," having "no natural foundation."* A profound misconception can never furnish a convenient classification after the misconception is detected; and it is because men have been guided by this hypothesis that they have instituted so many needless researches. "It is indeed upon the assumption of this broad and fundamental classification of the constituents of food," write Messrs Lawes and Gilbert, "according to their varied offices in the animal economy, that a vast series of analyses of foods have of late years been made and published; whilst, founded upon the results of these analyses, numerous tables have been constructed, professing to arrange the current articles of diet both of man and other animals, according to their comparative values as such." The classification has been criticised, and refuted, sometimes with more asperity than befits the calm heights of science, by Mulder, Moleschott, Robin and Verdeil, and others; ‡ and we have only to place ourselves at the proper physiological point of view to perceive that it is demonstrably false in every particular; and this we shall now proceed to show.

Man requires food which is both tissue-making and heat-making, to repair the fabric, and sustain the temperature of his body. This much is true. But it is demonstrable that nitrogenous substances are not the only plastic materials, not even the chief materials, whereas they are also heat-producing. Conversely, it is demonstrable that non-nitrogenous substances are tissue-making as well as heat-producing; so that any distinction between them, founded on their supposed offices of nutritive and respiratory, falls to the ground; not to mention that it rests on the assump

FUNKE very properly rejects it altogether, as wholly untenable, Lehrbuch der Physiologie, i. 180.

+ Report on Foods in Relation to Respiration and Feeding, in Reports of Brit. Assoc. 1852.

MULDER, Physiol. Chemie; MOLESCHOTT, Kreislauf des Lebens; ROBIN and VERDEIL, Chimie Anatomique.

tion of Respiration being the source of Animal Heat-an hypothesis we shall hereafter have to consider. The division of Food into Nitrogenous and Non-Nitrogenous is a chemical division to which no objection need be made, for it expresses a chemical fact. But when the fact that albuminous substances form a necessary proportion of organised tissues, is made the ground for specially distinguishing them as plastie, and when the presence of nitrogen in them is made the ground for specially distinguishing nitrogen as the plastic element, the per-centage of which is to afford the standard of nutritive value, we see a striking example of chemical reasonings applied to Physiology, which a simple confrontation with nature suffices to upset. For observe: while it is true that "albumen is the foundation, the starting-point of the whole series of peculiar tissues which are the seat of vital actions" (Liebig)—while it is true that the peculiar characteristic of organised tissues is that they contain albuminous substances as necessary ingredients; not less is it true that the other substances, thus arbitrarily excluded from the rank of tissue-makers-namely the fats, oils, and salts, all destitute of nitrogenare as essential as albumen itself. Not a cell, not a fibre can be formed, nor can subsist, without a certain amount of fat and salts. Not a tissue can come into being, nor continue its functions, without a large proportion of non-nitrogenous materials, a proportion greatly exceeding that of the nitrogenous. This is an anatomical fact which must surely discredit the idea of selecting one element out of several, all indispensable, and assigning to it alone the character of nutritive. If tissues were composed of albumen, or any other nitrogenous substance, without the admixture of fats, water, and salts, and if they did not likewise disengage heat in their transformations, Liebig's classification would be strictly accurate; but in the face of anatomical evidence which shows that no such tissue exists, and in face of the physiological evidence that even albumen undergoes chemical changes accompanied by the dis

engagement of heat, the classification is not tenable for an instant. Indeed the anatomist must ask with surprise, whether what he calls the adipose tissue is, or is not, chiefly composed of fat? Is the fat which exists in the muscles, cartilages, and bones an accident, a thing not worthy of being taken into account? The answer cannot be dubious. In 100 parts of muscle there are only 25.55 parts solid matter, and of these no less than 4.25 are fat. In 100 parts of the white substance of the brain, fat bears the large proportion of 13.9, whereas albumen is only 9.9; in the grey substance, the proportion of fat is 4.7 to albumen 7.5. If after this it be said that fat does not help to form tissue, is not an essential integral element of tissue, and consequently plastic, in the most rigorous sense of the word, the anatomist must confess that he fails to understand the language employed.

The reader need not be informed that Liebig is fully aware of the facts which can be brought against him, and that when he errs it is not from ignorance, but from theoretic bias. In spite of his absolute statements he is forced occasionally to qualify and contradict them. Let us see how he qualifies what he has to say of fat and water, which are by him degraded from the rank of vital to that of physical influence :


Many of the physical properties of organs, or tissues, depend on the presence of their non-nitrogenous constituents, namely, of water and fat. These bodies assist in the changes and processes by which the organised structures are formcells; and on water depends the fluidity ed. Fat has a share in the formation of also the milk-white colour of cartilage, of the blood, and of all other juices. So the transparency of the cornea (of the eye), the softness, plasticity, flexibility, and elasticity of muscular fibre and of membranes, all depend on a fixed proportion of water in each case. Fat is a never-failing constituent of the substance of the brain and nerves; hair, horn, claws, teeth, and bones, always contain a certain amount of water and fat. But in chanically absorbed, as in a sponge, or these parts water and fat are only meenclosed in drops, as fat is in cells, and they may be removed by mechanical pressure, or by solvents, without in the least affecting the structure of the parts.

They never have an organised form peculiar to themselves, but always take that of the parts, the pores of which they fill. They do not therefore belong to the plastic constituents of the body or of the food."*

A little further on he repeats the statement that "they take no direct share by their elements in the formation of organs," and that they have "no vital properties."+ Now this distinction rests on an entire misconception of anatomical structure. We need not pause to correct such details as that "fat is always enclosed in drops," and that the "water can all be removed by mechanical pressure;" it is enough to overthrow the whole argument to say that nerve-tissue without fat is no longer nerve, blood without water is no longer blood. To suppose that water simply gives fluidity to blood, when in truth it is as much an integral constituent of blood as albumen itself, is equivalent to saying that heat only gives expansion to steam, when steam itself is but the operation of heat on water. If fat has no vital properties in itself, neither has albumen in itself. To say that fat and water are "mechanically absorbed," is to contradict the simplest anatomical evidence, which shows them to be structurally combined, and always in constant quantities, varying within very small limits.

A classification of Food, more or less imperfect, would not trouble us did it not lead to important errors, as in the present case. No sooner do we accept the idea of nitrogenous food being the plastic material, than we are landed on Liebig's astounding proposition that "only those sub

stances are in a strict sense nutritious articles of food, which either contain albumen, or a substance capable of being converted into albumen," -a proposition he has elsewhere expressed in even a cruder form: "Only nitrogenous substances are capable of

*LIEBIG: Chemical Letters, p. 355.

conversion into blood."§ When we reflect on Liebig's great attainments and acuteness, when we know the splendid achievements in science with which his name is associated, it almost takes our breath away to alight on passages like these; and we feel assured that they could never have escaped him had he not placed himself at the chemical point of view. We really feel great hesitation in commenting on these passages. Were it possible, we should prefer supposing that his meaning was quite other than that expressed in his words; but the meaning is too rigorous a conclusion from his principles to admit of doubt. What is the fact? Examination of the structure of the Blood'shows that, so far from being composed exclusively of nitrogenous substances, it is composed of a variety of substances, among which the nitrogenous albumen and fibrine amount to not more than 72 in 1000 parts; and if a trifle more be added for the globuline and hæmatine of the blood-discs, that is all the nitrogen in the blood said to be solely composed of nitrogenous substances. No one knows this better than Liebig himself; yet he entirely overlooks it in his argument. "If we look at alimentary substances from this point of view," he adds, we obtain a knowledge of a natural law of the most admirable simplicity." Simple it is, no doubt, but is it true?

There are numerous reasons for asserting that it is not true. The very substances said to be alone capable of conversion into blood - the only "strictly nutritious substances' -are, when taken alone, utterly unable to nourish.

[ocr errors]

"Muscular flesh," says Majendie, in Commission," in which gelatine, albuthe celebrated Report of the Gelatine men, and fibrine are combined according to the laws of organic nature, and where they are associated with other matters, such as fats, salts, &c., suffices, even in

Some of Liebig's friends have endeavoured to excuse him on the ground that he did not intend his classification to be adopted rigorously, but only to indicate that the chief value of nitrogenous food was its plastic power, the chief value of non-nitrogenous food its heat-making power. But his language is explicit, and even when thus qualified it is essentially erroneous.

LIEBIG Chemical Letters, p. 346.

§ Ibid. p. 497.

very small quantity, for complete and prolonged nutrition. Thus dogs fed for 120 days solely on raw meat, from sheep's heads, preserved their health and weight during this period, the daily consump tion never exceeding 300 grammes, and often less. But 1000 grammes of isolated fibrine, with the addition of some hundred grammes of gelatine and albumen, were insufficient to support life. What, then, is this peculiar principle which renders meat so perfect an aliment? Is it the odorous and sapid matter which has this function, as seems probable? Do the salts, the trace of iron, the fatty matters and the lactic acid, contribute to the nutritive effect, notwithstanding that they constitute so minute a portion of meat?" *

The minuteness in quantity would be no argument against their potency of influence; but far more important will be the state of combination of the various elements. "The albumen of egg, and the fibrine separated from the blood, may to the chemist be identical with the fibrine and albumen which concur in the formation of muscle, incorporated there by a process of nutrition; but they are not the same for the organism which has to assimilate them, and which requires that they should be in a special state of elaboration, which they have undergone in another organism; it is muscular flesh which the organism demands, and not the elements of which flesh is composed: it needs

It has been found that dogs perish of starvation when liberally supplied with albumen, or white of egg, or fibrine, or with mixtures of albumen and fibrine-if these substances constitute their sole diet whereas they flourish when fed on gluten alone, although, according to the chemists, gluten is identical with albumen and fibrine: a sufficient proof that the nutritive value of a substance cannot be determined by its chemical composition. But this kind of proof awaits us on all sides. While Chemistry determines the nutritive value of foods according to their percentage of nitrogen, experience flatly contradicts the application of such a

standard, for it shows us that wheat contains only 2.3 per cent of nitrogen, whereas beans contain as much as 5.5 per cent, lentils 4.4, and pease 4.3; and yet with this remarkable inferiority in its per-centage of nitrogen, wheat is remarkably superior in nutritive value to beans, lentils, or pease. The discrepancy here is so glaring that Liebig has attempted to explain it. Let us hear his explanation: "The small quantity of phosphates which the seeds of the lentils, beans, and pease contain must be the cause of their small value as articles of nourishment, since they surpass all other vegetable food in the quantity of nitrogen which enters into their composition. But as the component parts of the bones (phosphate of lime and magnesia) are absent, they satisfy the appetite without increasing the strength." Pray observe the line of argument adopted: the smallness of the quantity of phosphates must be the cause, because the quantity of nitrogen is large. The argument might be reversed, and the whole nutritive value assigned to the phosphates with equal justice. If nitrogen is the plastic element, and its per-centage afford the true nutritive standard, the presence or absence of the phosphates can have nothing to do with it; and if their presence or absence is all-important, then we are certain that nutritive value does not admit of being estimated by the percentage of nitrogen, but by the conjunction of nitrogen with other substances, and this too in a peculiar way, for if Liebig's explanation were of any value, great practical results would issue: we need merely throw some bone-ash over the beans and pease to supply the deficient phosphates, and an article of food twice as nutritious as wheat would be obtained. Does any one believe in such a result?

It is noticeable that when Liebig has to explain the nutritive inferiority of beans and pease, he finds the cause to lie in the absence of phosphates, which, as he truly says, are component parts of the bones; where

Quoted by PEREIRA: Treatise on Food and Diet, p. 241.

+ LEVY: Traité d'Hygiène, ii. 85, quoted by LONGET, Physiologie,

+ LIEBIG; Chemistry in its application to Agriculture and Physiology, p. 147.

as, a little while ago, when denying any nutritive quality to fat, he refused to admit that it was a component part of tissues. Into such contradictions he is forced by his theory of nitrogenous substances as the only plastic materials-a theory incessantly at variance with fact. Messrs Lawes and Gilbert call especial attention to one series of their experiments, in which sheep ft weight notfed on succu

lent unripe turnips

withstanding the very high per-centage of nitrogen;"* and, without laying any stress on the fact that vegetable poisons are highly nitrogenous, let us ask the dispassionate reader to reflect on the chaotic condition of a doctrine which, while proclaiming nitrogen to be the true standard of nutritive value, declares that gelatine, a substance richer in nitrogen than even flesh or blood, has scarcely any nutritive value at all. We do not, indeed, attach much credit to this opinion, which we shall examine by and-by, but it is certainly in flagrant contradiction with the chemical hypothesis of nutritive values.

In spite, therefore, of what is so confidently asserted, we have the admission of chemists themselves that nitrogen is only nutritive in peculiar combinations. The consequence is inevitable. We must direct our attention towards substances which do nourish, and disregard the chemical formula which proclaim what substances ought to nourish. Inquiries so directed yield little that is satisfactory to the chemical hypothesis. We find, for example, thousands of Irish subsisting chiefly on potatoes and skimmed milk, and millions of Hindoos subsisting entirely on rice and rancid butter-substances which, in a chemical analysis, exhibit very little plastic material. Payen gives the following proportions in 100 parts of rice-Starch, 89.15; Nitrogenous matters, 7.05; Dextrine, &c. 1.; Fats, 0.80; Cellulose, 1.10; Mineral matters, 0.90. And Liebig himself calculates the proportion of plastic to non-plastic material in rice, as only 10 to 123; whereas in beef it is 10 to 17, and in veal 10 to 1. Of course it will be said

Report, p. 336.

that the Hindoo must eat an enormous quantity of rice, to extract from it the necessary amount of nitrogenous material; but this only renders Liebig's theory more open to destruction on another side; for, if we grant that the Hindoo eats ten pounds of rice for every pound of beef eaten by the Englishman, although we thereby account for the needful supply of plastic material, we are then called upon to account for the disposal of this enormous mass of respiratory material. We are told that starch, fats, sugars, and other non-nitrogenous matters, are incapable of entering into the composition of tissues, or of furnishing plastic material," they only serve to keep up the temperature of the body, being rapidly burnt in the body." We are further told that the demand for such substances is necessarily much greater in cold than in hot countries, because of the greater amount of heat required to keep the body at its proper point.

"In winter, when we take exercise in a cold atmosphere, and when, consequently, the amount of inspired oxygen increases, the necessity for food containing carbon and hydrogen increases in the same ratio; and by gratifying the appetite thus excited, we obtain the most efficient protection against the most piercing cold. The oxygen taken into the system is given out again in the same form, both in summer and winter: we expire more carbon at a low than at a high temperature, and require more or less carbon in our food in the same proportion; and consequently more is respired in Sweden than in Sicily; and in our own country, an eighth more in winter than in summer. If an equal weight of food is consumed in hot and cold climates, Infinite Wisdom has ordained that very unequal proportions of carbon shall be taken in it. The fruits used by the inhabitants of southern climes, do not contain, in a fresh state, more than 12 per cent of carbon, while the blubber and train-oil which feed the inhabitants of the polar regions, contain 66 to 80 per cent of that element."+

Considering the importance of the idea, one cannot but be struck with the singular meagreness of these illustrations. That the fruits eaten

+ LIEBIG, Chemical Letters, p. 320..

« AnteriorContinuar »