52 CRITICISM OF MECHANISTIC THEORY 



We meet with the same delicacy of adjust 

 ment in the regulation of such things as body- 

 temperature, blood- volume, or heat-production. 

 So accurately does the body adjust its con 

 sumption of oxygen and of food-material to 

 its energy requirements that it can actually 

 be used, as Rubner found, as a calorimeter ; 

 for it substitutes consumption of proteins, 

 carbohydrates, and fats for one another in 

 exact proportion to their energy values as 

 determined with a calorimeter. In Liebig's 

 time this delicate regulation was altogether 

 unknown. It was supposed that any excess 

 of protein simply ' fell a prey ' to oxygen, and 

 that if the oxygen percentage of the air 

 was increased, or even if the amount of air 

 breathed was increased, or the barometric 

 pressure was raised, oxidation in the living 

 body would increase, just as in the oxidation 

 of substances outside the body. By accurate 

 measurements of the intake and output of 

 material and energy the true facts as to the 

 rigid accuracy with which metabolism is regu 

 lated have gradually been established during 

 the last fifty years. 



We must, therefore, in considering the 



