October 16. 1903.] 



SCIENCE. 



485 



ration. While the addition of the hay eon- 

 siderably diminished the loss of tissue by 

 the animal, it at the same time caused an 

 increase of over ten per cent, in the heat 

 production; that is, a portion of its fuel 

 value, instead of taking the place of energy 

 previously derived from the oxidation of 

 tissue, was required for the various proc- 

 esses incident to the digestion and assimila- 

 tion of the hay and ultimately appeared as 

 heat. The loss of tissue was diminished 

 by 1,787 calories by the addition of hay 

 having a fuel value of 2,840 calories. In 

 round numbers, then, only 63 per cent, of 

 the fuel value of the hay was used by the 

 body in place of the energy previously de- 

 rived from the oxidation of tis.sue, instead 

 of 100 per cent, as required by the law of 

 isodynamic replacement, while the remain- 

 ing 37 per cent, became practically an 

 escretum. 



The results of a single experiment are, 

 of course, to be accepted with reserve. At 

 the same time, however, the discrepancy is 

 far too great to be accounted for by any 

 possible errors in the determinations of the 

 amounts of energy- involved, while there 

 was not the slightest indication of anything 

 abnormal in the state of the animal or in 

 the conditions of the experiment. We are 

 forced to the conclusion that in this case at 

 least the digestible matter of the food was 

 not isodynamic with body substance, or in 

 other words, that its fuel value was not, 

 as has been ordinarily assumed, a measure 

 of its value for maintenance. 



It may perhaps be objected that the mix- 

 ture of ill-known materials digested from 

 hay is a very different thing from the nearly 

 pure nutrients employed in Rubner's ex- 

 periments, and that it very naturally has a 

 lower nutritive value. Such an objection, 

 however, while perfectly true, would be 

 irrelevant. It is precisely because these 

 materials require the expenditure upon 



them of a considerable amount of energy, 

 mechanical and chemical, to fit them for 

 the metabolism of the body, that there is this 

 large loss. The difference is one of degree 

 rather than of kind. The cellulose of hay, 

 for example, undergoes an extensive fer- 

 mentation in the first stomach of rumi- 

 nants, yielding methane, carbon dioxide 

 and various organic acids. This fermenta- 

 tion is accompanied by an evolution of heat 

 which becomes largely or wholly waste, 

 since the animal body appears to be unable 

 to convert heat into other forms of energy. 

 Undoubtedly this escape of energy during 

 fermentation cou.stitutes a part, and not im- 

 probably a large part, of the 37 per cent, 

 of loss observed with the steer. When 

 starch is fed to a dog, it is converted into 

 dextrins, maltose and isomaltose and finally 

 into dextrose by the action of the various 

 digestive ferments. In this process there 

 is likewise a loss of energy, very much 

 smaller it is true than in the case of the cel- 

 lulose, but just as really a loss. Similarly, 

 the mechanical work of digestion is far less 

 with pure nutrients than with hay, but is 

 by no means equal to zero. 



It was noted above that Rubner's own 

 equivalents show that slightly more than 

 the theoretical amounts of nutrients were 

 required to replace body tissue. The writer 

 recently made a careful compilation of all 

 accessible results bearing on this question. 

 After rejecting two experiments by Rub- 

 ner with cane sugar, in whicTi values con- 

 siderably over 100 per cent, were obtained, 

 and which Rubner himself considers of 

 doubtful value, all the other trials give 

 values les.s than 100 per cent, except one 

 experiment on fat and one early one by 

 Pettenkofer and Voit which gives the im- 

 possible value of 115 per cent. The deficit 

 is often small and in some cases may not 

 exceed the probable experimental error, 

 but the general tendency appears signifi- 



