METABOLISM AND FOOD 65 



urine and 46% dissipated as heat. Before the 

 above-mentioned investigations were made it was 

 calculated from the composition of the proteins 

 and of the urea formed from it that 100 parts of 

 protein could yield 51-4 parts of fat. This number 

 — Henneberg's number as it is called — plays an 

 important part in many investigations, but it is 

 certainly too high, for in arriving at it, it was as- 

 sumed that all the nitrogen of the protein formed 

 urea, and that the formation of fat is not accom- 

 panied by any development of heat. Neither of 

 these assumptions is quite correct, for waste pro- 

 ducts richer in carbon than urea are formed, and 

 so less of this element remains for the production 

 of fat. Also there is always some heat developed 

 during metabolism, as was stated on page 56. 

 Henneberg's number, or factor, will be frequently 

 mentioned later, and it is now seen by what methods 

 it was obtained. 



(b) The effect of non-protein nitrogenous substances. 



These substances are soluble in water, and can 

 pass through the walls of animal cells ; they are 

 usually assumed to be completely digestible. As- 

 paragine may be taken as a typical^ member of the 

 group, and where it was fed to carnivorous animals 

 it was not able to replace protein, but behaved like 

 large doses of salt and increased the decomposition 



