150 MANUAL OF PHYSIOLOGY. 



necessary for its action. The power of artificial gastric juice 

 carefully prepared from the mucous membrane of an animal's 

 stomach differs in no essential respect from that of the natural 

 secretion in the stomach, if all the circumstances which aid the 

 action of the gastric ferments be applied in the experiment. This 

 action consists in a conversion of coagulated albumins into the 

 peculiar soluble and diffusible form of proteid known as ; ' pep- 

 tones." 



The change is not effected immediately, but certain stages may 

 be recognized in which the two chief constituents of the gastric 

 juice, the acid and the pepsin, seem to have a separate action. 



Shortly after the introduction of a proteid, such as boiled fibrin, 

 into gastric fluid at the temperature of the body, the masses of 

 fibrin swell up, become transparent, and eventually are easily 

 shaken to pieces and dissolved. 



The first step in the process seems to be brought about by the 

 free acid, and consists in the formation of acid albumin. This 

 can be shown by neutralizing the fluid during the process and 

 thereby causing a precipitate of acid albumin (v. p. 61). The 

 amount of this precipitate will depend upon how far the conver- 

 sion into peptone which is not precipitated by neutralization 

 has progressed. Thus, in the earlier stages, nearly all the proteid 

 used will be thrown down by neutralization, while only a com- 

 paratively small amount is precipitated in the later stages. 



The formation of acid albumin may be effected with acid alone 

 without the other constituents of the gastric juice, and therefore 

 the preliminary step may be attributed to the unaided action of 

 the acid ; but since this stage in the formation of peptone is con- 

 stant, and the material may possibly be distinguishable from the 

 ordinary acid albumin, it has been called parapeptone. 



While the parapeptoue is being formed by the acid, the pepsin 

 is engaged in changing it into the final soluble, diffusible, and 

 uncoagulable product peptone. The pepsin by itself cannot 

 convert proteid into peptone, as may be seen in the want of effi- 

 cacy of a neutral solution of pepsin, in which neither peptone 

 nor parapeptone is formed. In other words, pepsin solution can 

 only change parapeptone or acid albumin into peptone. It would 



