10 LECTURE I. 



Another significant advance in the knowledge of chemical processes which 

 take place in the animal organism was caused by the discovery that it was 

 possible to work out certain processes by means of extracts of tissue. We 

 are furthermore fortunately able in many cases to isolate the active prin- 

 ciple. The great advantage of such experiments is clear from the fact that 

 an extremely small amount of these products, called ferments, is capable of 

 causing considerable change without itself appearing in the end products of 

 the reaction. It has been attempted to effect the hippuric acid synthesis in 

 this way by means of a ferment which has been isolated from the kidneys. 

 This has not yet been done satisfactorily. An instructive example of 

 great significance as regards the use of such tissue extracts and of the 

 ferments obtained from them is found in the recent experiments to form 

 uric acid from the purine bases, investigations made by Horbaczewski, 

 Wiener, Spitzer, Schittenhelm, and Burian. The first-mentioned has 

 shown that purine bases added in the presence of oxygen to an animal organ 

 which has been macerated to a paste causes an increase of uric acid. Against 

 this experiment the objection may be raised that it is not conclusive. The 

 paste itself contains some purine bodies and perhaps substances of unknown 

 nature which stand in close relation to uric acid. The purine bases added 

 may in some way have an indirect action upon the given synthesis. The 

 proof becomes much more satisfactory if instead of using the whole organ, 

 we make use of a ferment extracted from it. To be sure, the nature of this 

 ferment is not known, but we know its action. We can free it completely 

 from purine substances, and furthermore we require but a small amount of 

 it. It is of great significance with regard to our conclusions, that it is pos- 

 sible here to follow the experiment quantitatively. We can weigh accu- 

 rately the amount of purine bases added, and similarly the amount of uric 

 acid formed, so that we are now able to establish sharply the relation 

 between the purine bases and the uric acid. This method has still further 

 advantages. It has been possible to identify certain intermediate products 

 formed in the transformation of certain purine bases into uric acid, and to 

 establish the fact that certain organs possess ferments which are capable of 

 breaking down the uric acid formed. In this way it is at once possible to 

 establish clearly the complete metabolism of purine substances. It would, 

 of course, be unsafe to apply the results of such experiments without further 

 investigation to the processes taking place in the living organism. It is 

 perfectly conceivable that the conditions prevailing in the tissues may be 

 entirely different from those prevailing in the fermentation experiments 

 carried out artificially. Such a limitation holds for all investigations 

 carried out with ferments, and especially those with digestive ferments. In 

 such experiments the ferments develop their action under entirely changed 

 conditions. We can merely imitate the temperature; further than this we 

 are practically helpless. In the alimentary canal, for example, absorption 



