INTRODUCTION. 7 



a definite number of heat units which it can use in performing its func- 

 tions. By means of the calories of heat coming from the albumin the 

 organism is spared a corresponding amount which were otherwise taken 

 from non-nitrogenous material. We may assume that the cells, which 

 moreover are capable of consuming sugar only with great difficulty, now 

 do not use so much of it. More and more unchanged sugar circulates in 

 the tissues and in the blood, and since the kidneys, as we shall see later,, 

 are sensitive to the slightest increase of sugar in the blood over the normal 

 and serve to remove all such excess, it follows that there must necessarily 

 be an increase in the elimination of sugar. According to this hypothesis 

 the action of albumin is an indirect one as regards the sugar. Naturally 

 this explanation is not necessarily the correct one. We have mentioned 

 these experiments and the two explanations of the results obtained briefly 

 in order to show by a somewhat complicated example how varied the con- 

 clusions may be that are drawn with regard to an apparently simple prob- 

 lem. It would not be difficult to cite numerous other examples to illustrate 

 this point. Later on we shall repeatedly come back to these indirect proofs 

 and mention again and again the fact that it is of fundamental importance 

 for the further development of all physiological-chemical investigation 

 that it should always be clearly and sharply recognized as to what extent 

 we are justified in speaking of facts, and at what place the indirect con- 

 clusions, corresponding to the still unsettled part of our field of investiga- 

 tion, begin. When such a gap is discovered, it is our duty not to rest satisfied 

 until all of the conclusions have been subjected here also to direct proof. 



Before taking up the discussion of ways and means to accomplish this 

 end, we will turn back once more to the synthesis of hippuric acid in the 

 animal organism. This was established indirectly, and its assumption 

 rests solely upon probability. After introducing benzoic acid into the 

 organism of a mammal we find a corresponding increase in the amount of 

 hippuric acid in the urine. It is a fact that hippuric acid can be formed 

 from benzoic acid and glycocoll. The chemist is able to make hippuric 

 acid in the laboratory from these two components, but under conditions 

 which it is impossible to realize in our tissues. It requires a high tempera- 

 ture, considerable pressure, and the exclusion of water. We have, how- 

 ever, long since been forced to the conclusion that the cells have the power 

 of causing chemical reactions to take place which require entirely dif- 

 ferent conditions when carried out in a test tube. We are satisfied if an 

 observed chemical process does not outwardly contradict our general 

 experience. We base our explanations of the chemical decompositions 

 taking place in the animal organism upon the results of chemical research, 

 and seek to go farther and bridge over all the large gaps which we meet 

 with everywhere on account of our insufficient knowledge of metabolic 

 processes. Here also we must be conscious that we are only speaking of 



