Otobkr IC, 1903.] 



SCIENCE. 



4So 



'I'his law Kiibner failed the law of isody- 

 iiamic replai'einent.* 



This law was based upon the conception 

 that the same laws of energetics which con- 

 trol the chemical chanjjes of so-called dead 

 matter also I'ule the far more complex ones 

 which occur in the living organism, and 

 that the energy which the animal imparts 

 to its surroundings is simply the trans- 

 formed energy of its food, or, in other 

 words, that the law of the conservation of 

 energj' api>lies to the animal body. Later 

 experiments by Rubner, directed more spe- 

 cifically to this broader aspect of the ques- 

 tion, went far to demonstrate the truth of 

 this conclusion, while the later investiga- 

 tions of Laulanie and especially the very 

 extensive ones of Atwater and his asso- 

 ciates, Rosa and Benedict, seem to have 

 placed it beyond- question. 



It is practically to Rubner, then, what- 

 ever suggestions may have been made pre- 

 viously, that we owe the effective introduc- 

 tion into physiology of the conception that 

 the relative values of the several nutrients, 

 except as to the special functions of the pro- 

 teids, are measured by their physiological 

 heat values, or 'fuel vahies' as they are 

 called by Atwater. This conception has 

 proved to be a fruitful one and has pro- 

 foundly influenced the study of animal 

 nutrition. 



As is .so often the ease, however, further 

 study has revealed the necessity for modifi- 

 cations and has shown that in its first form 

 the law of isodynamie replacement was. 

 after all, but a partial statement of the 

 truth. 



Rubner 's experiments were made with 

 animals in a state of rest and with amounts 

 of food insufficient to cause any storing up 

 of flesh or fat in the body. Under these 



' Conclusions very similar, although not based 

 upon their own investigations, had been previ- 

 ously announeed by v. Hocsslin and by Dani- 

 levvsky. 



conditions all the energy set free by the 

 burning of food or tissue to support th-' 

 vital activities, whatever forms it may tem- 

 porarily take, finally appears as heat which 

 is imparted to the surroundings of the 

 animal. The law of isodynamie replace- 

 ment, now, implies that the heat produc- 

 tion of such an animal is unaffected, at 

 least below the maintenance reiiuirement. 

 by changes in the amount and kind of food 

 consumed. Thus, if the animal oxidizes 

 while fasting 100 grams of boily fat, and if 

 it he then given an anmunt of starch equiv- 

 alent in energy to 100 grams of fat. viz.. 

 229 grams, this starch shoulil be burned in 

 place of the fat. That is, there will b<' 

 simply a .substitution of one kind of fuel 

 for another, but no increase in the tola! 

 heat production. 



It is, however, an observation as old as 

 the time of Lavoisier that the consumption 

 of food tends to increase the heat produc- 

 tion, and the fact has been fully estab- 

 lished by a host of subsequent investiga- 

 tors. The taking of food calls into activity 

 a whole set of organs that were previously 

 in a state of relative inactivity. The ma.sti- 

 cation and swallowing of the food and its 

 passage through the alimentary canal in- 

 volve a not inconsiderable amount of mus- 

 cular work. In addition to this the various 

 secreting glands arc called into action to 

 supply the digestive fluids, while the re- 

 sulting chemical and fermentative changes 

 in the food itself add their quota to the 

 heat arising from the nuiscular and glandu- 

 lar activity. This heat, of course, may aid 

 in keeping the animal warm, but otherwise, 

 .so far as we know, it is of no direct use to 

 the organism, while often the body has 

 already a superabundant supply and is 

 really engaged in getting rid of heat. 



It appeals, then, that the mere supplying 

 of more energy to the body in the shape of 

 food sets up a demand for more energy to 



