540 NUTRITION OF FARM ANIMALS 



The effect of even a small excess of work in increasing the 

 nitrogen excretion of the horse was so sharp that Kellner even 

 attempted to determine how much work could be performed at 

 the expense of a given weight of starch or fat by increasing the 

 demand upon the animal up to the point where it just failed to 

 cause an increase in the nitrogen excretion and a fall in live 

 weight. 



A considerable number of more recent experiments have fully 

 confirmed Kellner's conclusion that a deficiency of non-nitrog- 

 enous nutrients is the chief cause of the increased protein 

 katabolism which sometimes occurs during work and have shown 

 that in the presence of a sufficient amount of fats and especially 

 of carbohydrates even severe work can be performed without 

 increasing the nitrogen excretion. Indeed, moderate work con- 

 tinued for a number of days has in some cases been accompanied 

 by a gain of nitrogen, a fact apparently quite in accord with the 

 common experience that the muscles are strengthened by ex- 

 ercise. It is clear triat the body normally uses non-nitrogenous 

 materials as the source of the energy expended in muscular work, 

 exactly as it does in the case of the energy required for its inter- 

 nal activities. Only when the supply of non-nitrogenous materi- 

 als is inadequate does it resort to the katabolism of protein as a 

 source of energy for external work, precisely as it does during 

 fasting or on exclusive protein feeding as a source of energy for 

 internal work (339, 407). 



Effect of work upon the katabolism of non-nitrogenous matter 



638. Gaseous exchange increased. In striking contrast 

 with the minimal effect of work upon the excretion of nitrogen 

 is its very marked effect in increasing the consumption of oxygen 

 and the excretion of carbon dioxid and water. This increase 

 is too obvious from common experience and too well estab- 

 lished scientifically to require more than an illustration. The 

 fact of such an increase was shown in the researches of Lavoisier 

 and confirmed by the earlier experimenters in this field, such 

 as Scharling in 1843, Him in 1857 and especially Smith in 1859. 

 The investigations of Pettenkofer and Voit in 1866, however, 

 appear to have been the first to be executed according to modern 

 methods. Their results regarding the influence of work upon 



