30 PHYSIOLOGY CHAP. 



of fat. This formation of fat is not however very great, and 

 never exceeds 4-12 per cent of the meat consumed, calculated 

 in the dry state. 



The quantity of fat formed on a purely meat diet does not in 

 Voit's opinion depend only on the quantity of meat taken, but 

 also on the quantity of fat possessed by the animal. A lavish 

 meat diet adds to the amount of fat when the animal is lacking 

 in fat owing to a previous fast of long duration ; when, however, 

 the animal has a good deposit of adipose tissue, such a diet not 

 only does not add to it, but causes it to be consumed. 



We have already dealt with the complex question of the 

 formation of fat in Vol. II. Chap. V. 12, to which we refer the 

 reader. 



What we have said as to the effects of a purely meat diet on 

 the metabolism of dogs applies to all carnivorous animals, but is 

 not wholly applicable to man, who is omnivorous, still less to 

 herbivorous and frugivorous animals. Man, who interests us 

 mostj cannot tolerate for any length of time a purely meat diet, 

 whether it be small or large. Eanke tried the effect on himself of 

 an abundant diet of meat for two successive days. His weight 

 being 73 kgrms., he took on the first day 1832 grms. of meat, 

 and found that he could only digest and utilise 1300 grms. ; on 

 the second day the amount taken was 2000 grms. and that con- 

 sumed only 1089 grms. This and other observations led him to 

 the conclusion that man in general is able to tolerate a smaller 

 quantity of meat than a carnivorous animal weighing only half as 

 much. This is undoubtedly due to his limited power of digesting 

 protein foods. 



VIII. There has never been any question as to the nutritive 

 value of fats and carbohydrates, but actual investigations as to the 

 effect of these non-nitrogenous substances on the exchange of 

 material were only begun at the time when Pettenkofer and Voit 

 were putting into practice their methods of investigating the 

 balance of nitrogen and carbon. 



A fundamental difference is observed when the effects of a 

 diet consisting exclusively of substances containing nitrogen are 

 compared with those of a diet composed wholly of substances 

 devoid of nitrogen, such as fats and carbohydrates. Whereas, in 

 the former case, it is possible, as we have already seen, under 

 suitable conditions to obtain and maintain for a considerable 

 time in carnivorous animals the equilibrium between the intake 

 and output, in the second case, even when the diet is abundant, 

 this equilibrium is never attained; the consumption of protein 

 substances never entirely ceases, so that the tissues of the organism 

 are gradually consumed through inanition, which ends in death, 

 just as does absolute abstinence from food* the only difference 

 being that the process takes longer. Both groups of the non- 



