METABOLISM AND FOOD 71 



acids, might also be mentioned here, but as they 

 are only present in small quantities in the usual 

 feeding-stuffs they may be left out of consideration. 

 It may be noted further that crude fibre and 

 nitrogen - free extract are decomposed to a con- 

 siderable extent by bacteria in the intestines, 

 and thereby organic acids, carbon dioxide, and 

 marsh gas are formed. The organic acids enter 

 the circulation, but the marsh gas leaves the 

 body, unused, in large quantities. This loss is 

 considerable, and reckoned upon the digestible 

 part of the nitrogen-free extract and crude fibre 

 reaches on an average 4-3% marsh gas, equal 

 to I 3'7% of the total heat value of the digested 

 substance. With very hard food-stuffs it is still 

 more, and in wheat straw, for example, amounts to 

 20% of the heat value. The average amount of 

 heat in 1 gram of digested nitrogen-free extract and 

 crude fibre, after deducting the loss of marsh gas, 

 is in round figures 3600 cal. The food metabolism, 

 after feeding with nitrogen-free nutrients, may now 

 be considered. 



(d) The action of nitrogen-free nutrients upon the 

 protein and fat metabolism. 



The clearest insight into the effects is given 

 from investigations which have been carried out 

 upon carnivorous animals, a single example of 

 which will suffice. Three dogs received a food 



