506 LECTURE XXI. 



there, secrete pepsinogen, which comes into action only when brought into 

 contact with the acid juices of the stomach. We do not yet have the means 

 at our command for arranging the proteolytic ferments into classes with 

 satisfactory exactness. We can, however, distinguish between ferments of 

 the pepsin class and those similar to trypsin. The best way of establish- 

 ing the class to which a ferment belongs, is, in this case, to allow it to act 

 upon a polypeptide, and the results from the experiment are obtained most 

 readily if we choose a polypeptide in the formation of which a difficultly 

 soluble amino acid, e.g. tyrosine or cystine, takes part. Glycyl-Z-tyrosine 

 is decomposed in a short time by trypsin and similar ferments, but this 

 dipeptide is not acted upon by pepsin. The secretion of the pyloric region 

 behaves quite like the latter after it has been activated by hydrochloric 

 acid. 1 It seems certain, therefore, that the ferment of the pylorus cells 

 belongs to the pepsin group, as was assumed by Pawlow. 



Now that we have become acquainted with the influence of the food, 

 and its nature, upon the secretory relations in the stomach, it is time 

 for us to turn to the action of the gastric juice upon the food itself. We 

 have discussed this already at some length in considering the different 

 classes of foodstuffs. Here we will merely repeat that pepsin, with the 

 aid of hydrochloric acid, converts the albumins chiefly into peptones and 

 in part to simpler cleavage-products; but on the other hand, a breaking 

 down of the simplest cleavage-products, i.e. amino acids, cannot take 

 place here, or at least only to a very slight extent. Again, the lipase causes 

 the hydrolysis of a part of the fat, and in this way prevents, to some 

 extent, the restraining influence which the fats have upon the gastric 

 secretions. The carbohydrates, furthermore, may be decomposed some- 

 what while they remain in the stomach, but, to be sure, not by means 

 of the ferment obtained from the gastric glands, but rather by means of 

 the diastase from the saliva. This diastase, however, is destroyed on 

 coming in contact with the acid of the stomach. Its period of action, 

 therefore, depends upon the acidity of the gastric juice and the nature of the 

 food. In case the food is in the form of a loose mixture which is easily mois- 

 tened, it is evident that the action of the diastase cannot long continue. 



Under the influence of the gastric juice, the food is changed into a pulpy 

 mass known as the chyme. This consists, in part, of products formed 

 from the decomposition of the food, and in part of unchanged food. For- 

 merly it was thought that the muscular activity of the stomach had a 

 great deal to do with the formation of this chyme. Doubtless in this way 

 the contents of the stomach are thoroughly mixed and brought into inti- 

 mate contact, layer by layer, with the gastric juice, but, on the other 

 hand, this process takes place gradually, and not by means of violent 

 muscular contractions, so that it is not right to speak of the food being 



1 E. Abderhalden and P. Rona: Z. physiol. Chem. 47, 359 (1906). 



