INTRODUCTION. 11 



takes place immediately hand in hand with the hydrolysis of food by the 

 ferments of the digestive juices. The decomposition products are imme- 

 diately taken away. We are still entirely ignorant of the manner in which 

 each individual ferment does its work, how the different ferments assist 

 one another, and how their work is influenced by other factors. At all 

 events, it is evident that the decomposition of food takes place much more 

 rapidly than in a test tube. All the products of the decomposition remain 

 in the latter case, and serve to hinder the further action of the ferment, or 

 perhaps even cause it to act in a different direction. On the other hand, in 

 the test tube we are often able to identify products which otherwise escape 

 our observation owing to the rapid absorption in the bowels. It is possible 

 here also to draw conclusions only by combining experiments ; that is, on 

 the one hand we will study digestion as accurately as possible in the test 

 tube, following it up in its separate phases, and, on the other hand, we must 

 attempt to identify the products of digestion in the bowels themselves; in 

 this way we gradually draw a picture of the entire process of digestion. 

 In an entirely similar manner the metabolism of purine substances must 

 be studied in the whole organism in order to find out how far the facts 

 thus ascertained agree with the results of experiments with ferments. 



We must consider still another important condition, namely, the concept 

 of quantity. In physiological-chemical experiments this is too frequently 

 neglected. Its importance is perfectly obvious. We must always require 

 that every chemical process taking place in the organism be followed 

 quantitatively. Qualitative experiments are prone to lead to great errors, 

 and it is never possible to recognize clearly by means of them the relation 

 between individual products. We must always know how much of this 

 or that substance has been changed over into a definite product. Often- 

 times a minor process will otherwise be considered the essential one simply 

 because it was easy of discovery, whereas the main change may be entirely 

 overlooked. 



It is almost superfluous to mention the fact that the methods employed 

 must be suitable for the problem to be investigated. Every investigation in 

 the field of physiological chemistry must start out with a critical examina- 

 tion of the value of available methods. We must clearly recognize the 

 sources of error and take them into consideration, especially when definite 

 conclusions are to be drawn from any discovery. The methods are the 

 foundation pillars in every experimental investigation. Every advance is 

 closely dependent upon them, so that we must lay great stress upon their 

 final development. The great importance in the improvement of methods 

 too often falls into the background, especially in physiological-chemical 

 investigation, and apparently more weight is laid upon the more or less 

 fruitful hypotheses. It must not be forgotten, however, that essentially 

 new facts are usually closely connected with the discovery of new methods. 



