« AnteriorContinuar »
FOOD AND DRINK.
THE water which drowns us as a fluent stream, can be walked upon as ice. The bullet which, when fired from a musket, carries death, will be harmless if ground to dust before being fired. The crystallised part of the oil of roses, so grateful in its fragrance-a solid at ordinary temperatures, though readily volatile-is a compound substance containing exactly the same elements, and in exactly the same proportions, as the gas with which we light our streets. The tea which we daily drink, with benefit and pleasure, produces palpitations, nervous tremblings, and even paralysis, if taken in excess; yet the peculiar organic agent-called theine -to which tea owes its qualities, may be taken by itself (as theine, not as tea) without any appreciable effect.* The water which will allay our burning thirst, augments it when congealed into snow; so that Captain Ross declares the natives of the Arctic regions "prefer enduring the utmost extremity of thirst rather than attempt to remove it by eating snow." Yet if the snow be melted, it becomes drinkable water; and it must be melted in the mouth. Nevertheless, although, if melted before entering the mouth, it assuages thirst like other water, when melted in the mouth it has the opposite effect. To render this paradox more striking, we have only to remember that ice, which melts more slowly in the mouth, is very efficient in allaying thirst.
These facts point to an important consideration, which has been little regarded by the majority of those who have written on Food: the consideration of the profound differences which may result from simple differences in the state of substances. The chemist, in his elementary analysis, necessarily gives no clue to such dif ferences. He tells us of what elements an article of Food is composed, but he cannot tell us how those ele
ments are combined, nor in what state the substance is. Even when he has ascertained the real composition and properties of any substance, he has still to ask the physiologist what are the conditions presented by the organism in which this substance is to undergo chemical transformation. We know that a change in the conditions will cause a change in the manifestation of a force; so that often what ordinarily takes place in the laboratory will not at all take place in the organism. Chlorine and hydrogen are gases having a powerful affinity for each other-that is to say, they will unite when brought together in the daylight but if we change the conditions-if we bring them together in the dark-their affinity is never manifested; and thus, while in the sunlight they rush together with explosive force, producing an intense acid, they will remain quiescent in the darkness, and for all eternity would form no combination. Again, this same chlorine decomposes water in the sun's rays; but in darkness it has no such power. If such are the effects of so simple a change in the conditions, it is easy to imagine how various must be the differences between the phenomena which occur in the laboratory, and those which the same substances present under the complex conditions of the organism.
The chemist employs vessels of glass, in which he isolates the substances he examines, keeping them free from the interference of other substances, because he knows that, unless such interference be avoided, his experiment is nullified. He knows, for example, that the water which, if poured into a red-hot crucible, flies up into his face as steam, will rapidly pass into ice if a little liquid sulphurous acid happen to be present. He knows, in short, that the stronger affinity prevents the action of the
SCHLEIDEN: Die Pflanze, 1858, p. 205. + Ross: Narrative of Second Voyage, p. 366.
weaker affinity; and to be sure of his experiment he must isolate his substances. But in the vital laboratory no such isolation is possible. The organism has no glass vessels, no airtight cylinders. Vital processes go on in tissues which, so far from isolating the substance introduced-so far from protecting it against interference, do inevitably interfere, and are themselves involved in the very changes undergone by the substance. Thus, while it is true that an alkali will neutralise an acid out of the organism, we must be cautious in applying such a chemical principle in the administration of drugs, because the alkali stimulates a greater secretion of the gastric acid; so that over and above the amount neutralised, there will be a surplus of acid free, owing to the interference of the tissues in which the process takes place. Besides the complications which occur from the inevitable interference of the organism itself, and from the differences resulting from differences in the state of bodies, there are other complications arising from causes peculiarly vital. Chemistry must ever remain incompetent to solve the problems of life, if only from this, that in Biology questions of Form are scarcely less important than questions of Composition. Spread out a cell into a layer, and you will find, that in ceasing to be a cell it has ceased to act as an organ-it has lost all the properties which distinguished it as a cell. Thus, the green cells of the plant decompose carbonic aid. Even the torn leaf will equally fix the carbon and liberate the oxygen, provided its cells are preserved in their integrity of form. But if these cells are crushed, or otherwise injured, this vital property ceases, because the cell alone is capable of manifesting it. Under the influence of yeast, sugar is decomposed into alcohol and carbonic acid; but if the yeast-cells be crushed and disorganised, their action on the sugar is said to be quite different instead of converting it into alcohol and carbonic acid, they convert it into lactic acid. We must
acknowledge, then, that when certain combinations of carbon, oxygen, hydrogen, nitrogen, and salts, assume the form of a cell, the properties of these substances become profoundly modified.
Such considerations need all our attention in dealing with so complex a question as that of Food. They show us, what indeed we had last month occasion to see in detail, the radical incompetence of Chemistry to solve any of the questions of Physiology, and urge us to reject, as misdirected labour, all attempts at establishing anything more than chemical facts in the "Chemistry of Food." It was undoubtedly a great discovery which Mulder made in 1838, that the albumen of plants was identical, or nearly so, with the albumen of animals, and consequently that, when the ox ate grass, and the lion ate the ox, both derived their nutriment from the same chemical substance. A great discovery; but we cannot agree with Moleschott in thinking this discovery first settled the basis of a science of Food. It was a chemical triumph, fruitful in results to Chemistry; but its physiological bearing has been greatly exaggerated, and has given increased impetus to that chemical investigation of Food, which, as we have said, cannot, in the nature of things, be other than misleading. And although Mulder has shown the inaccuracy of Liebig's notion, that vegetable albumen is identical with the fibrine of the blood, and vegetable caseine with the caseine of the bloodt-although he energetically repudiates as unphilosophical the idea of a chemical analysis furnishing any true standard of nutritive value, yet he does not seem to have clearly apprehended what the true method of investigation must be; and his criticism of Liebig is mainly negative.
To the chemist there may be little or no difference between plant and flesh as food; to the physiologist the difference is profound: he sees the lion perishing miserably of inanition in presence of abundant herbage,
MULDER: Versuch einer Physiol. Chemie, i. 193. LEHMANN: Lehrbuch der Physiol. Chemie, iii. 170.
+MULDER: Physiol. Chemie, p. 917.
which to the elephant or buffalo furnishes all that is needful. The ox eats the grass, and the tiger eats the ox, but will not touch the grass. The flesh of the ox may contain little that is not wholly derived from the grass; and the chemist analysing the flesh of both may point out their identity; but the question of Food is not, What are the chemical constituents of different substances? but, What are the substances which will nourish the organism? If the animal will not eat, or, having eaten, cannot assimilate, a certain substance, that substance is no food for it, be its chemical composition what it may. We thus see that digestibility is an important element in the estimate of Food: unless the substance can be digested, it cannot be assimilated, cannot nourish; although, perhaps, if assimilated, the substance might have a high value. A pound of beef-steak contains an enormous superiority of tissue-making substance over that contained in a pound of cabbage; yet to the rabbit the cabbage is the superior food, while to the dog the cabbage is no food at all.
When we consider the part played by Food, as furnishing the materials out of which the organic fabric is constructed, and its actions facilitated, it seems natural to assume that the Blood is the proper standard we should have in view, and that we should designate those substances as Aliments which, directly or indirectly, go towards the formation of Blood. Yet, on a deeper scrutiny, this is seen to lead us a very little way. An analysis of Blood will neither give us a complete list of alimentary substances, nor indicate the alimentary value of each special substance. True it is that all the tissues are formed from the Blood, and that all alimentary substances, in their final state previous to assimilation, make their way into it. But we will briefly point out
why, in spite of all this, the Blood can never furnish us with the desired standard.
In the first place, while Blood is truly the vehicle of nutrition, it is at the same time the vehicle of many products of decay and disintegration. It carries in its torrent the materials for the use of to-day and to-morrow, but it also carries the materials which, vital yesterday, are effete today, unfit to be retained, and are hurrying to the various issues of excretion. Blood is thus at once purveyor-general and general sewer, carrying life and carrying death. We shall therefore always find in it substances which are not alimentary, mingled with those which are; and we cannot separate these, so as to make our analysis of use. In the second place, among the substances normally current in the circulation we do not find several which are notoriously serviceable as aliments. Some of these, as theine, caffeine, alcohol, &c., are not present in the blood; and others, as fats and the carbohydrates, are present in quantities obviously too small for the amounts consumed as food. Finally, although substances are nutritive, or blood-making, in proportion to their resemblance to blood, yet this resemblance must exist after the process of digestion, not before it ; since no sooner is any substance taken into the stomach than a series of changes occurschanges indispensable for its admission into the circulation, but which impress on it a very different character from the one it bore on its entrance. A beef-steak is assuredly more nearly allied in composition to the blood of an ox than the dewy grass of the meadow; yet the grass becomes converted into blood in the course of the changes impressed on it during digestion, and what was thus unlike becomes like, or, as we say, assimilated. The experiments of Claude Bernard are highly suggestive on this point. He found that if
It is curious that carnivora feed chiefly, sometimes exclusively, on herbivora, and not on carnivora, whose flesh most resembles their own.
+ See on this point JOHN SIMON, Lectures on Pathology, p. 23:-" Mentally we can separate these three kinds of blood, but experimentally we cannot. They are mixed together-past, present, and future-the blood of yesterday, the blood of today, and the blood of to-morrow-and we have no method of separating them."
sugar or albumen were injected into the veins, it was not assimilated, but was eliminated unchanged by the kidneys; whereas, if either substance were injected into the veins together with a little gastric juice, assimilation was complete. In another experiment he found that if sugar and albumen were injected into the portal vein, which would carry them through the liver, where certain changes are always impressed on them, they would be assimilated; but if he injected them into the jugular vein, by which they would reach the lungs without passing through the liver, no assimilation would take place. We here once more see the necessity of taking into account the organism and its vital acts, whenever we would attempt an explanation of Food.
The general considerations which a priori caused us to relinquish the idea of finding a proper standard in the composition of the Blood, are fully confirmed by the results of Payen's experiments, which show that Blood is not a good aliment. He fed pigs on equal proportions of flesh and blood, and found that they exhibited all the signs of starvation; whereas, when fed on flesh under similar conditions, except that blood was absent, they fattened and grew strong.*
The Blood, then, must be given up. Shall we try Milk? Others have done so before us, making it the standard of Food, because it is itself an aliment which contains all the substances necessary for the nourishment of an organism during the most rapid period of growth. Out of milk, and milk alone, the young elephant, the young lion, or the young child, extracts the various substances which furnish muscles, nerves, bones, hair, claws, &c.; milk furnishes these in such abundance, that the increase of growth is far greater during the period when the animal is fed exclusively on it, than at any subsequent period of its career. "In milk," says Prout, we should expect to find a model of what an alimentary substance ought to be-a kind of prototype, as it were, of nutritious elements in general." The
idea is so plausible that its acceptance was general. Nevertheless nothing is more certain than that milk is not this model food, since, however it may suit the young lion or the young child, we cannot feed the adult lion or the adult man on milk alone: we can feed the lion on bones and water, and the man on bread and water, but not on milk. A model food for the young, it ceases to be so for the adult; that relation which existed between the food and the organism in the one case, no longer exists in the other.
If milk does not furnish us with
Caseine (nitrogenous matter),
The reader may remark with some
* PAYEN: Des Substances Alimentaires, p. 45.
conditions of possible formationthus fats and sugars can, we know, be formed in the organism with a proper allowance of materials; and I am strongly disposed to think that albuminous substances can also be formed, though not unless some albumen be present to act as a leaven.
We are thus, by the principle of exclusion, reduced to the one method of investigation which remains, and that is to interrogate the organism, not the laboratory.
"Experience, daily fixing our regards On Nature's wants," must guide us in the search. ascertain what substances are nutritious, we must ascertain those which really nourish; and the relative value of these can only be ascertained by extensive and elaborate experiments on the feeding of animals, conducted on rigorously scientific principles. In other words, we must adopt that very method which common sense has from time immemorial pursued; with this important difference, that instead of allowing it to be, as hitherto, wholly empirical, we must subject it to the rigour, caution, and precision, which characterise Science. And even when Science shall have established laws on this point, such as may accurately express the general value of each substance as food, there will always remain considerable difficulty in applying those laws, owing to that peculiarity of the vital organism, previously noticednamely, that the differences among individuals are so numerous, and often so profound, as to justify the adage, one man's meat is another man's poison." Thus, while experience plainly enough indicates that, in Europe at least, meat is more nutritious than vegetables, those who eat largely of meat being stronger and more enduring than those who eat little or none; we must be cautious in the application of such a principle. Difference of climate may, and difference of temperament certainly does, modify this question. The Hindoo sepoy, who lives on
rice, would, it is said, outrun, knock down, or in any other way prove superiority in__strength_over Gaucho of the Pampas, who lives on flesh. And not only are some organisms ill adapted to a flesh diet, as we have seen, but, according to Andersson, the strongest man he ever knew scarcely ever touched animal food: this was a Dane, who could walk from spot to spot carrying a stone, which was so heavy that it required ten men to lift it on to his shoulders; his chief diet was gallons of thick sour milk, tea, and coffee;* a diet which no ordinary man could support with success.
Having discussed the chief topics of Food in general, we may now ascertain what Science can tell us respecting the various articles employed as nourishment by man. Our inquiry falls naturally under two heads-first, the Alimentary Principles, considered separately; and next, the Compound Aliments, or those articles of Food and Drink which make up the wondrous variety of human nourishment.
Albumen. This substance, familiar to all as the white of egg, constitutes an important element in Food. It exists as a liquid in the blood, as a solid in flesh. When raw, or lightly boiled, it is readily digested; less so when boiled hard, or fried. Majendie has observed that the white of eggs combines many conditions favourable to digestion, for it is alkaline, contains saline matters, especially common salt, in large proportions, and it is very nearly allied to the albumen found in the chyle and blood. It is liquid, but is coagulated by the acids of the stomach, forming flocculi having slight cohesion, and rendered easily soluble again by the intestinal juices. Many people imagine that white of egg is injurious, or innutritious, and they only eat the yolk. To some this may be so, and when experience proves it to be so, white of egg should not, of course, be eaten; but, as a general rule, white of egg is agreeable and nutritious. Nevertheless, if given alone, neither white of egg
* ANDERSSON: Lake Ngami, p. 58.