Chemical Changes in Animal Organism. 287 



and of the attempts made to localize the chemical reactions 

 in different parts of the body, and thus to determine some 

 of their chemical functions. 



(A) THE METHOD OF DIRECT IN VITRO EXPERIMENT. 

 THE NATURE OF ENZYME ACTION. 



The most direct method is the investigation of the 

 chemical changes which take place when any given sub- 

 stance is brought into direct contact with an organ or a 

 body fluid in ordinary laboratory vessels. Examples of 

 this simple method have been already given. Thus it 

 has been stated that when a protein, such as coagulated 

 egg-white, is warmed to 40 with juice excreted by the 

 mucous membrane of the stomach, or with an extract of 

 such a membrane in the presence of acid (N/20 hydro- 

 chloric acid), it undergoes degradation into polypeptides 

 (proteoses and peptones) and ammo-acids ; in the case of 

 coagulated egg-white, solution gradually takes place. This 

 action will not take place when the egg-white is warmed 

 with the acid alone.* Something is present in the gastric 

 juice, or is extracted with the mucous membrane, which 

 brings about the " digestion " of the protein. Again, if 

 starch is treated with saliva, it rapidly undergoes degrada- 

 tion with the production of dextrins and the simpler 

 carbohydrates. A very large number of chemical changes 

 can be produced when various chemical substances are 

 treated with animal secretions, or disintegrated animal 

 tissues or their extracts. 



Now one of the most obvious facts in connection with 

 the chemical changes brought about by the animal organism 

 is that these all take place in the case of a mammal within 



* A small change will take place in this case, but degradation will not go 

 30 far. This fact can be confirmed by following the changes by means of the 

 formaldehyde or nitrous acid methods (see pp. 261 and 262). 



