TRANSACTIONS OF SECTION B. 5^9 



These considerations lead me, in conclusion, to refer briefly — and I promise 

 it shall be as briefly as is consistent with clearness — to the two very much disputed 

 questions of the origin of muscular jwtoer, and the sources of the fat of the anitnal 

 body. These subjects Mr. Lawes and myself have frequently discussed elsewhere ; 

 but as the controversy has assumed a new phase quite recently, it seems desirable 

 and appropriate that I should recur to it on the present occasion. 



With regard to the question of the sources in the food of the fat of the animal 

 body, Liebig originally maintained that although fat might be formed from the 

 nitrogenous compounds within the body, the main source of it in the herbivora 

 was the carbo-hydrates. In his later writings, he sharply criticised the experi- 

 ments and arguments of those who have maintained the formation of fat chiefly 

 from the proteine compounds; but he at the same time seems to attach more 

 importance to that source than he formerly did. He gives it as his opinion that 

 the question cannot be settled by experiments with herbivora. He adds that what 

 we know with certainty is that, with these animals, albuminates and carbo- 

 hydrates work together to produce fat ; but whether the non-nitrogenous product, 

 fat, has its origin in the albiunin or in the carbo-hydrate, h considers it not easy 

 to determine. 



At the time when we commenced our experiments on the feeding of animals in 

 ] 847, the question whether the fat of the animals fed for human food was mainly 

 derived from albuminoids or from carbo-hydrates had been scarcely raised, or at 

 least it was not prominent. The question then was rather — whether the herbivora 

 received their fat ready formed in their food, or whether it was produced within 

 the body — the latter view being that which Liebig had so forcibly urged, at the 

 same time maintaining that at any rate its chief source was the carbo-hydrates. 

 Accordingly, our experiments were not specially arranged to determine whether 

 or not the whole of the fat produced coidd or could not be derived from the albu- 

 minoids. 



For each description of animal, oxen, sheep, and pigs, such foods as 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 amoimt of a recognised good 

 food, containing known quantities of nitrogen, fatty matter, &c. ; to another set the 

 same amoimt of another food, of different characters 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 imiform food rich in nitrogen, and to others imiform foods poorer in 

 nitrogen, and so on, in each case ad libitutn. 



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 individual 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 amoimt of nitrogenous compounds it might represent was calcidated bv the 

 usual factor. But, as already intimated, the results so obtained were only usedVith 

 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 nitro- 

 genous substance, when interpreting the results, it was always considered whether or 

 not the food 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-nitroo-enous 

 substance, including and excluding fat, consumed— /or a given live-iueight icithin a 

 given time, and to produce a given aynount of increase in live-xoeight. 



Experiments were made with a large number of sheep, and a large number of 

 pigs. And, even without making allowance for the diflferent condition of the nitro- 

 genous or of the non-nitrogenous constituents, in comparable foods, the results ob- 

 tained imiformly 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 



M M 



