Sept. 30, 1880] 



NATURE 



525 



foods r.s had been established by common experience to be 

 appropriate were selected. The general plan of the experiments 

 was to give to one set a fixed amount of a recognised good food, 

 containing known quantities of nitrogen, fatty matter, &c., to 

 another set the same amount of another food, of different cha- 

 racters in these respects ; to other sets also fixed amounts of other 

 foods in the same way : and then there was given to the whole 

 series the same complementary food ad libitum. Or, to one set 

 was supplied a uniform food rich in nitrogen, and to others 

 uniform foods poorer in nitrogen, and so on, in each case ad 

 libitum. 



It will be seen that in this way a great variety of dietaries was 

 arranged, and it will be observed that in each case the animals 

 themselves fixed their consumption according to the requirements 

 of the system. 



As already indicated, the individnal nitrogenous and non- 

 nitrogenous compounds of the foods were not determined. As 

 a rule, the constituents determined were — the total dry matter, 

 the ash, the fatty matter, and the nitrogen ; from which last the 

 amount of nitrogenous compound it might represent w'as calcu- 

 lated by the usual factor. But, as already said, tlie results so 

 obtained were only used with considerable reservation, especially 

 in the case of all immature vegetable produce. Nor was the 

 crude fibre determined; but, as in the case of the estimated 

 nitrogenous substance, when interpreting the results, it was 

 always considered whether or not the focd contained much or 

 little of probably indigestible woody matter. 



The animals being periodically weighed, we were thus able to 

 calculate the amounts of the so-estimated nitrogenous substance, 

 and of the total non-nitrogenous substance, including and ex- 

 cluding fat, ii<yas,\yaxA.—for a given live-uieight within a given 

 time, and to produce a given amount of increase in live- 

 weight. 



Experiments were made with a large number of sheep and a 

 large number of pigs. And, even without making allowance for 

 the different condition of the nitrogenous or of the non-nitro- 

 genous constituents, in comparable foods, the results so uniformly 

 indicated that, both the amount consumed by a given live- weight 

 of animal within a given time, and that required to produce a 

 given amount of increase, were determined much more by the 

 amount of the non-nitrogenous than by that of the nitrogenous 

 constituents which the food supplied. And when allowance was 

 made for the different condition of the nitrogenous constituents, 

 and for the greater or less amount of the non-nitrogenous ones 

 which would probably be indigestible and effete, the indications 

 were still more remaikable and conclusive. 



In very many cases the animals were slaughtered, and carefully 

 examined as to whether the tendency of development had been 

 more that of growth in frame and flesh, or in fatness. Here, 

 again, the evidence was clear, that the tendency to growth in 

 frame and flesh was favoured by a high proportion of nitrogen 

 in the food, and that to the production of fat by a high propor- 

 tion of digestible non- nitrogenous constituents. 



In a few cases the actual amount of fat in the animals in the 

 lean, and in the fat condition, was determined ; and the results 

 admitted of no doubt whatever that a very large proportion of 

 the stored-up fat could not have been derived from the fatty 

 matter of the food, and must have been produced within the 

 body. 



So decisive and consistent were the veiy numerous and very 

 varied results in regard to these points, that we had no hesitation 

 in concluding — not only that much of the fat stored up was pro- 

 duced within the body, but that the som'ce of much, at any rate, 

 of the produced fat must have been the non-nitrogenous con- 

 stituents of the food — in other words, the carbohydrates. 



As already stated, however, as the question whether the source 

 of the produced fat was the proteine compounds or the carbo- 

 hydrates was not then prominent, we had not so arranged the 

 experiments as to obtain the largest possible increase in fat with 

 the smallest possible supply of nitrogenous compounds in the 

 food, n.or did we then even calculate whether or not there was 

 sufficient nitrogenous matter consumed to be the source of the 

 whole of the fat produced. 



This question, indeed, excited very little interest, until, at a 

 meeting of the Congress of Agi-icultural Chemists held at Munich 

 in 1S65 (at which I happened to be present). Prof. Voit, from 

 the results of experiments made in Pettenkofer's respiration 

 apparatus with dogs fed on flesh, announced his conclusion that 

 fat must have been produced from the nitrogenous substance, 

 and that this was probably the chief, if not the only, source of 



the fat, even of herbivora — an opinion which he subsequently 

 urged much more po itively. 



In the discussion which followed the reading of Prof. Voit's 

 paper. Baron Liebig forcibly called in question his conclusions ; 

 maintaining not only that it was inadmissible to form conclusions 

 on such a point in regard to herbivora, from the results of expe- 

 riments made with carnivora, but also that direct quantitative 

 results obtained ^vith herbivorous animals had afforded apparently 

 conclusive evidence in favour of the opposite view. 



Voit's paper excited considerable controversy, in which Mr. 

 Lawes and myself joined. We maintained that experiments to 

 determine such a question should be made not with carnivora or 

 omnivora fed on flesh, but with herbivora fed on their appropriate 

 fattening food, and on such herbivora as common experience 

 showed to be pre-eminently fat-producers. We pointed out^ 

 that the pig comprised, for a given live-weight, a comparatively 

 small proportion of. alimentary organs and contents ; that, com- 

 pared with that of the ruminants, his food was of a high character, 

 yielding, for a given weight of it, much more total increase, 

 much more fat, and much less necessarily effete|matter ; that, in 

 proportion to his weight, he consumes a larger amount of food, 

 and yields a larger amount, both of total increase and of fat, 

 within a given time ; and, lastly, that he contains a larger pro- 

 portion of fat, both in a given live-weight and in his increase 

 whilst fattening. 



It is obvious that with these characteristics there is much less 

 probable range of error in calculating the amount and the com- 

 position of the increase in live-weight in relation to the amount 

 and composition of tlie food consumed, than in the case of 

 ruminants ; and that therefore the pig is very much more appro- 

 priate for the purpose of experiments to determine the sources 

 in its food of the fat it produces. 



Accordingly we calciUated a number of our early experiments 

 made with pigs, to determine w'hether or not the nitrogenous 

 substance they consumed was sufficient for the formation of the 

 fat they produced. For simplicity of illustration, and to give 

 every possible advantage to the view that nitrogenous substance 

 might have been the source of the produced fat, we assumed the 

 whole of the crude fat of the food to have been stored up in the 

 animal — thus estimating a minimum amount to be produced. 

 Then again we supposed the ^^■hole of the nitrogenous substance 

 of the food to be perfectly digested, and to become available for 

 the purposes of the system. Lastly, after deducting the amount 

 of nitrogenous substance estimated to he stored up as such, the 

 w hole of the remainder was reckoned to be so broken up that 

 no other carbon-compounds than fat and urea would be 

 produced. 



The result was that, even adopting these inadmissible assump- 

 tions in all the cases in which, according to co mm on experience, 

 the food was admittedly the most appropriate for the fattening 

 of the animal, the calculation showed that a large amount of 

 fat had been produced which could not have been derived from 

 the nitrogenous substance of the food, and must therefore have 

 had its source in the carbohydrates. Such a result is moreover 

 entirely accordant with experience in practical feeding. 



Reviewing the whole subject in great detail in 1S69, Prof. 

 Voit refers to these results and calculations. He confesses that 

 he has not been able to get a general view of the experiments 

 from the mass of figures recorded, and from his comments he 

 shows that he has on some 'points misunderstood them. He 

 admits, however, that as the figures stand, it would appear that 

 fat had, in some instances, been derived from the carbohydrates. 

 Still, he says, he cannot allow himself to consider that a trans- 

 formation of carbohydrates into fat has thus been proved. 



Prof. Emil von Wolff again in his " Landwirthschaf tliche 

 Futterungslehre," referring to the same experiments, admits that 

 they are almost incomprehensible unless we assume the direct 

 concurrence of the carbohydrates in the fonnation of fat. He 

 nevertheless seems to consider that evidence of the kind in 

 question is inconclusive ; and he suggests that experiments with 

 pigs should be made in a respiration apparatus to determine the 

 point. 



Mr. Lawes and myself entertained, however, the utmost 

 confidence that the question was of easy settlement without any 

 such apparatus, provided only suitable animals and suitable 

 foods were selected. I, accordingly, gave a paper on the 

 subj ct in the Section fiir Landwirthschaft- und Agricultur- 

 Chcinie, at the Naturforscher Versammlung, held at Hamburg 

 ■ "On the Sources of the Fat of the Animal Body," Phil. Mag-., 

 December, 1866. 



