INTRODUCTION. 9 



frequently they behave altogether differently physiologically. It is 

 apparent already from these brief remarks what great demands are laid 

 upon the experimentation of the physiological chemist. He always has to 

 deal with complicated processes. He is acquainted usually only with the 

 initial and the final products of the metabolism, and is compelled to clear 

 up theoretically the whole chain of transformations which are necessary 

 for the formation of the latter from the former. Here and there it is 

 possible to get hold of intermediate steps, and thus we encroach more and 

 more upon the great domain of the unknown. 



A considerable advance in the subject was made when experimentation 

 was begun upon surviving organs rather than upon the whole organism. 

 Here the initial and end products are more closely related, or at least 

 apparently so, although here also, as soon as the change in cell substance 

 begins really to take place, the complication is naturally practically as 

 great as in following one substance through the whole body. Experimen- 

 tation with surviving organs has in itself quite a number of advantages. 

 In many cases we are able to change an indirect proof into a direct one. 

 We are able to work out accurately the composition of a definite organ. 

 We can definitely decide the question as to whether it has stored up in it 

 sufficient amounts of definite substances to cause the formation of certain 

 compounds, and thus determine positively whether the organ makes use of 

 a substance introduced into it in a definite process. 



Let us return again to our hippuric acid hypothesis. It is possible to 

 establish which organ is capable of carrying it out. G. Bunge and O. 

 Schmiedeberg have shown that the kidneys of mammals, or more accu- 

 rately those of a dog, are capable of forming hippuric acid from glycocoll 

 and benzoic acid. They caused a dog to bleed to death, cut out its kidneys, 

 and introduced defibrinated blood through the arteries of the kidneys and 

 allowed it to flow out through the renal veins. On introducing glycocoll 

 and benzoic acid, there appeared hippuric acid in the blood and in the 

 liquid emptying out through the ureter. A control experiment with the 

 second kidney showed that it as well as the blood from the first kidney was 

 free from hippuric acid. If benzoic acid but no glycocoll was introduced 

 into the blood, the amount of hippuric acid formed was extremely small. 

 The conclusion to be drawn from this experiment is that benzoic acid 

 effects the formation of hippuric acid because it is itself used in 

 the synthesis. However, this fact is not yet absolutely proved. The 

 objection may still be raised that hippuric acid may arise from another 

 source. The formation of this acid is, at best, a very complicated process. 

 We have, on the one hand, the kidney containing a very complex tissue, 

 and, on the other hand, the blood with its constituents. As a matter of 

 fact, it was not possible to effect the above synthesis after the red 

 corpuscles were removed from the blood. 



