Abbildungen der Seite
PDF
EPUB

rich in nitrogen ; they alone are said And an authoritative American phyto be capable of forming organised siologist, Professor Draper, adopts tissues, because alone capable of be- the classification, although he warns ing converted into blood, and are us that it is "only adopted for the hence called the strictly nutritive sub- sake of convenience,” having " no stances. They are vegetable Albu- natural foundation.”* A profound men, Fibrine, and Caseine, and animal misconception can never furnish a Flesh and Blood. The second class convenient classification after the is Respiratory, or heat-making, and misconception is detected ; and it comprises the substances containing is because men have been guided no nitrogen, which are therefore in- by this hypothesis that they have capable of nourishing the body, and instituted so many needless reonly serve the purposes of respira- searches. “It is indeed upon the tion, whence animal heat. They are assumption of this broad and fundaFats, Starch, Gum, Sugars, Pectine, mental classification of the constiBassorine, Wines, Beers, and Spirits. tuents of food,” write Messrs Lawes A third class comprises the Inorganic and Gilbert, “ according to their vasubstances, Water, Salts, Iron, &c. ried offices in the animal economy, All Food is thus Nitrogenous, or that a vast series of analyses of foods nutritive, and Non-Nitrogenous, or have of late years been made and heat-making

published ; whilst, founded upon the How entirely this brilliant error results of these analyses, numerous has gained possession of the lecture- tables have been constructed, proroom may be read in the following fessing to arrange the current articles passage from the last edition of Dr of diet both of man and other aniCarpenter's work on “ Human Phy- mals, according to their comparative

· siology,” which may be taken as re- values as such.”+ The classification presenting the opinion of English has been criticised, and refuted, physiologists :

sometimes with more asperity than

befits the calm heights of science, “By rules based on the foregoing data, by Mulder, Moleschott, Robin and we may estimate the relative value of Verdeil, and others ; I and we have different articles of food, for the two dis- only to place ourselves at the proper tinct purposes of the formation of tissue physiological point of view to perand the production of heat. For the ceive that it is demonstrably false in proportion of albuminous matter which any substance may contain furnishes the

every particular; and this we shall measure of its histogenetic value ; while

now proceed to show. the proportion of hydro-carbon uncom

Man requires food which is both bined with oxygen affords the means of tissue-making and heat-making, to estimating its calorific power when oxid- repair the fabric, and sustain the ised. Since, in almost every alimentary temperature of his body. This much substance, whether vegetable or animal, is true. But it is demonstrable that these two cla-ses of compounds are min- nitrogenous substances are not the gled, the per-centage of nitrogen (save only plastic materials, not even the in those substances into which gelatine chief materials, whereas they are also largely enters) which it may contain af- heat-producing. ' Conversely, it is defords a tolerably correct estimate of the

monstrable that non-nitrogenous subamount of albuminous matter which it includes, and therefore of its histoyenetic stances are tissue-making as well as value; where, on the other hand, the per

heat-producing; so that any distinccentage of nitrogen is smallest, that of tion between them, founded on their hydro-carbon is largest, and the propor- supposed offices of nutritive and respition of the combustible material is high- ratory, falls to the ground ; not to est.”

mention that it rests on the assump

Draper, Human Physiology, p. 27. FUNKE very properly rejects it altogether, as wholly untenable, Lehrbuch der Physiologie, i. 180.

+ Report on Foods in Relation to Respiration and Peeding, in Reports of Brit. A 8800. 1852.

I Mulder, Physiol. Chemie ; MOLESCHOTT, Kreislauf des Lebens ; Robin and VERDEIL, Chimie Anatomique.

tion of Respiration being the source engagement of heat, the classification of Animal Heat-an hypothesis we is not tenable for an instant. Indeed shall hereafter have to consider. the anatomist must ask with surprise,

The division of Food into Nitrogen- whether what he calls the adipose ous and Non-Nitrogenous is a chemi- tissue is, or is not, chiefly composed cal division to which no objection of fat? Is the fat which exists in need be made, for it expresses a the muscles, cartilages, and bones an chemical fact. But when the fact accident, a thing not worthy of being that albuminous substances form a taken into account? The answer necessary, proportion of organised cannot be dubious. In 100 parts of tissues, is made the ground for muscle there are only 25,55 parts specially distinguishing them as plas- solid matter, and of these no less tic, and when the presence of nitrogen than 4.25 are fat. In 100 parts of in them is made the ground for the white substance of the brain, fat specially distinguishing nitrogen as bears the large proportion of 13.9, the plastic element, the per-centage of whereas albumen is only 9.9 ; in the which is to afford the standard of grey substance, the proportion of fat nutritive value, we see a striking is 4.7 to albumen 7.5. * If after this example of chemical reasonings ap- it be said that fat does not help to plied to Physiology, which a simple form tissue, is not an essential inconfrontation with nature sutlices to tegral element of tissue, and conupset. For observe: while it is true sequently plastic, in the most rigorous that “albumen is the foundation, the sense of the word, the anatomist starting-point of the whole series of must confess that he fails to underpeculiar tissues which are the seat of stand the language employed. vital actions” (Liebig) --while it is The reader need not be informed true that the peculiar characteristic that Liebig is fully aware of the facts of organised tissues is that they con- which can be brought against him, tain albuminous substances as neces- and that when he errs it is not from sary ingredients ; not less is it true ignorance, but from theoretic bias. that the other substances, thus arbi- In spite of his absolute statements trarily excluded from the rank of he is forced occasionally to qualify tissue-makers-namely the fats, oils, and contradict them. Let us see and salts, all destitute of nitrogen- how he qualifies what he has to say are as essential as albumen itself. of fat and water, which are by him Not a cell, not a fibre can be formed, degraded from the rank of vital to nor can subsist, without a certain that of physical influence :amount of fat and salts.

Many of the physical properties of tissue can come into being, nor con- organs, or tissues, depend on the presence tinue its functions, without a large of their pon-nitrogenous constituents, proportion of non-nitrogenous ma- namely, of water and fat. These bodies terials, a proportion greatly exceeding assist in the changes and processes by that of the nitrogenous. This is an

which the organised structures are formanatomical fact which must surely cells; and on water depends the fluidity

ed. Fat has a share in the formation of discredit the idea of selecting one

of the blood, and of all other juices. So element out of several, all indispen- also the milk-white colour of cartilage, sable, and assigning to it alone the the transparency of the cornea (of the character of nutritive. If tissues eye), the softness, plasticity, flexibility, were composed of albumen, or any and elasticity of muscular fibre and of other nitrogenous substance, without membranes, all depend on a fixed prothe admixture of fats, water, and portion of water in each case. salts, and if they did not likewise never-failing constituent of the substauce disengage heat in their transforma

of the brain and nerves ; hair, horn, tions, Liebig's classification would be claws, teeth, and bones, always contain a

But in strictly accurate ; but in the face of certain amount of water and fat. anatomical evidence which shows

these parts water and fat are only me. that no such tissue exists, and in chanically

, absorbed, as in a sponge, or

enclosed in drops, as fat is in cells, and face of the physiological evidence they may be removed by mechanical that even albumen undergoes chemi- pressure, or by solvents, without in the cal changes accompanied by the dis- least affecting the structure of the parts.

Not a

Fat is a

[ocr errors]

66

a

They never have an organised form conversion into blood.” § When we peculiar to themselves, but always take reflect on Liebig's great attainments that of the parts, the pores of which they and acuteness, when we know the fill. They do not therefore belong to the plastic constituents of the body or of which his name is associated, it al

splendid achievements in science with the food."*

most takes our breath away to alight A little further on he repeats the on passages like these ; and we feel statement that “they take no direct assured that they could never have share by their elements in the forma- escaped him had he not placed himtion of organs," and that they have self at the chemical point of view. “no vital properties.” + Now this We really feel great hesitation in comdistinction rests on an entire miscon- menting on these passages. Were it ception of anatomical structure. We possible, we should prefer supposing need not pause to correct such details that his meaning was quite other as that "fat is always enclosed in than that expressed in his words ; drops,” and that the water can all but the meaning is too rigorous a be removed by mechanical pressure;" conclusion from his principles toadmit it is enough to overthrow the whole of doubt. What is the fact ? Exargument to say that nerve-tissue amination of the structure of the without fat is no longer nerve, blood Blood shows that, so far from being without water is no longer blood. composed exclusively of nitrogenous To suppose that water simply gives substances, it is composed of a variety fluidity to blood, when in truth it is of substances, among which the nitroas much an integral constituent of genous albumen and fibrine amount blood as albumen itself, is equivalent to not more than 72 in 1000 parts ; to saying that heat only gives expan

and if a trifle more be added for sion to steam, when steam itself is the globuline and hæmatine of the but the operation of heat on water. blood-discs, that is all the nitrogen If fat has no vital properties in itself, in the blood said to be solely comneither has albumen in itself. To posed of nitrogenous substances. No say that fat and water are “mechani- one knows this better than Liebig cally absorbed,” is to contradict the himself ; yet he entirely overlooks it simplest anatomical evidence, which in his argument. “If we look at shows them to be structurally com- alimentary substances from this point bined, and always in constant quan- of view," he adds, we obtain a tities, varying within very small knowledge of a natural law of the limits.

most admirable simplicity.” Simple A classification of Food, more or it is, no doubt, but is it true? less imperfect, would not trouble us There are numerous reasons for did it not lead to important errors, asserting that it is not true. The as in the present case. No sooner do very substances said to be alone capawe accept the idea of nitrogenous ble of conversion into blood — the food being the plastic material, than only “strictly nutritious substances we are landed on Liebig's astound- ---are, when taken alone, utterly uning proposition that “only those sub- able to nourish. stances are in a strict sense nutritious

“Muscular flesh,” says Majendie, in articles of food, which either contain albumen, or a substance capable of

the celebrated Report of the Gelatine

Commission, "in which gelatine, albubeing converted into albumen,” I-a

men, and fibrine are combined according proposition he has elsewhere ex

to the laws of organic nature, and where pressed in even a cruder form : “Only they are associated with other matters, nitrogenous substances are capable of such as fats, salts, &c., suffices, even in

a

:

* LIEBIG : Chemical Letters, p. 355.

+ 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. I LIEBIG : Chemical Letters, p. 346.

§ Ibid. p. 497.

very small quantity, for complete and standard, for it shows us that wheat prolonged nutrition. Thus dogs fed for

contains only 2.3 per cent of nitro120 days solely on raw meat, from sheep's gen, whereas beans contain as much heads, preserved their health and weight during this period, the daily consump:

as 5.5 per cent, lentils 4.4, and pease tion never exceeding 300 grammes, and

4.3; and yet with this remarkable often less. But 1000 grammes of isolat- inferiority in its per-centage of nitroed fibrine, with the addition of some

gen, wheat is remarkably superior in hundred grammes of gelatine and albu- nutritive value to beans, lentils, or men, were insufficient to support life. pease. The discrepancy here is so What, then, is this peculiar principle glaring that Liebig has attempted to which renders meat so perfect an aliment? explain it. Let us hear his explanaIs it the odorous and sapid matter which tion : “The small quantity of phoshas this function, as seems probable? phates which the seeds of the lentils, Do the salts, the trace of iron, the fatty beans, and pease contain must be the matters and the lactic acid, contribute to

cause of their small value as articles the nutritive effect, notwithstanding that of nourishment, since they surpass they constitute so minute a portion of

all other vegetable food in the quanmeat ?” *

tity of nitrogen which enters into The minuteness in quantity would their composition. But as the combe no argument against their potency ponent parts of the bones (phosphate of influence ; but far more important of lime and magnesia) are ahsent, will be the state of combination of they satisfy the appetite without inthe various elements. “The albumen creasing the strength.” I Pray obof egg, and the fibrine separated serve the line of argument adopted : from the blood, may to the chemist the smallness of the quantity of be identical with the fibrine and albu- phosphates must be the cause, because men which concur in the formation the quantity of nitrogen is large. The of muscle, incorporated there by a argument might be reversed, and the process of nutrition, but they are whole nutritive value assigned to the not the same for the organism which phosphates with equal justice. If has to assimilate them, and which nitrogen is the plastic element, and its requires that they should be in a spe- per-centage afford the true nutritive cial state of elaboration, which they standard, the presence or absence of have undergone in another organism; the phosphates can have nothing to it is muscular flesh which the organ- do with it ; and if their presence or ism demands, and not the elements absence is all-important, then we are of which flesh is composed : it needs certain that nutritive value does not aliments not chemical products." + admit of being estimated by the per

It has been found that dogs per- centage of nitrogen, but by the conish of starvation when liberally sup- junction of nitrogen with other subplied with albumen, or white of stances, -and this too in a peculiar egg, or fibrine, or with mixtures of way, for if Liebig's explanation were albumen and fibrine-if these sub- of any value, great practical results stances constitute their sole diet, would issue : we need merely throw whereas they flourish when fed on some bone-ash over the beans and gluten alone, although, according to pease to supply the deficient phosthe chemists, gluten is identical with phates, and an article of food twice albumen and fibrine: a sufficient proof as nutritious as wheat would be obthat the nutritive value of a sub- tained. Does any one believe in such stance cannot be determined by its a result ? chemical composition. But this kind It is noticeable that when Liebig of proof awaits us on all sides. While has to explain the nutritive inferioChemistry determines the nutritive rity of beans and pease, he finds the value of foods according to their per- cause to lie in the absence of phoscentage of nitrogen, experience flatly phates, which, as he truly says, are contradicts the application of such a 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 that the Hindoo must eat an enorany nutritive quality to fat, he re- mous quantity of rice, to extract fused to admit that it was a compo- from it the necessary amount of ninent part of tissues. Into such con- trogenous material; but this only tradictions he is forced by his theory renders Liebig's theory more open to of nitrogenous substances as the only destruction on another side; for, if plastic materials — a theory inces- we grant that the Hindoo eats ten santly at variance with fact. Messrs pounds of rice for every pound of Lawes and Gilbert call especial atten- beef eaten by the Englishman, altion to one series of their experi- though we thereby account for the ments, in which sheep fed on succu- needful supply of plastic material, we lent unripe turnips“ lost weight not- are then called upon to account for withstanding the very high per-cent- the disposal of this enormous mass of age of nitrogen ; ;"* and, without lay- respiratory material. We are told ing any stress on the fact that vege- that starch, fats, sugars, and other table poisons are highly nitrogenous, non-nitrogenous matters, are incapalet us ask the dispassionate reader ble of entering into the composition to reflect on the chaotic condition of of tissues, or of furnishing plastic a doctrine which, while proclaiming material, “ they only serve to keep up nitrogen to be the true standard of the temperature of the body, being nutritive value, declares that gela- rapidly burnt in the body.” We are tine, a substance richer in nitrogen further told that the demand for than even flesh or blood, has scarcely such substances is necessarily much any nutritive value at all. We do not, greater in cold than in hot countries, indeed, attach much credit to this because of the greater amount of heat opinion, which we shall examine by- required to keep the body at its proand-by, but it is certainly in flagrant per point. contradiction with the chemical hy

“ In winter, when we take exercise in pothesis of nutritive values.

a cold atmosphere, and when, conseIn spite, therefore, of what is so

quently, the amount of inspired oxygen confidently asserted, we have the ad- increases, the necessity for food containmission of chemists themselves that ing carbon and hydrogen increases in the nitrogen is only nutritive in peculiar same ratio; and by gratifying the appecombinations. The consequence is tite thus excited, we obtain the most inevitable. We must direct our at- efficient protection against the most tention towards substances which do piercing cold. The oxygen taken into nourish, and disregard the chemical the system is given out again in the saine formule which proclaim what sub- forin, both in summer and winter: we stances ought to nourish. Inquiries expire more carbon at a low than at a 80 directed yield little that is satis- less carbon in our food in the same pro

high temperature, and require more or factory to the chemical hypothesis. portion ; and consequently more is reWe find, for example, thousands of spired in Sweden than in Sicily; and in our Irish subsisting chiefly on potatoes own country, an eighth more in winter and skimmed milk, and millions of than in summer. If an equal weight of Hindoos subsisting entirely on rice food is consumed in hot and cold cliand rancid butter-substances which, mates, Infinite Wisdom bas ordained that in a chemical analysis, exbibit very very unequal proportions of carbon shall little plastic material. Payen gives be taken in it. The fruits used by the the following proportions in 100 inhabitants of southern climes, do not parts of rice :-Starch, 89.15; Ni- contain, in a fresh state, more than 12 trogenous matters, 7.05; Dextrine, and train-oil which 'feed the inhabitants

per cent of carbon, while the blubber &c. 1. ; Fats, 0.80; Cellulose, 1.10; of the polar regions, contain 66 to 80 per Mineral matters, 0.90. And Liebig cent of that element." + himself calculates the proportion of plastic to non-plastic material in Considering the importance of the rice, as only 10 to 123; whereas idea, one cannot but be struck with in beef it is 10 to 17, and in veal the singular meagreness of these 10 to 1. Of course it will be said illustrations. That the fruits eaten

Report, p. 336.

+ LIEBIG, Chemical Letters, p. 320..

« ZurückWeiter »