MECHANICAL PHENOMENA. 19 



the gastric juice has no effect should leave the stomach 

 quickly while proteins which are digested in the stomach 

 should remain here a long time? The observation of PEN- 

 ZOLDT that those foods which combine with the hydrochloric 

 acid of the stomach are the last to leave this viscus ha^s 

 been pointed out above. (^It is the presence of free hydro- 

 chloric acid in the stomach and near the pylorus that deter- 

 mines the relaxation of the sphincter and explains why 

 different fopdstuffs enter the small intestine at different rates. ). 

 As will be shown later 1 an abundance of gastric juice is 

 poured out on both a protein and a carbohydrate diet. A 

 diet consisting chiefly of fat causes the secretion of much 

 less juice. Now since carbohydrates cause a great secretion 

 cf gastric juice but do not unite chemically with the acid, 

 free hydrochloric acid accumulates almost at once in the 

 stomach. Proteins, on the other hand, unite with the acid 

 of the gastric juice and hence prevent the accumulation of 

 free acid for a considerable length of time. Finally, fat 

 calls forth only a slight secretion of gastric juice, but that 

 which is produced soon accumulates in the stomach, as it 

 does not unite with the fat. These facts explain the dif- 

 ferent rates at which the various foods pass out of the 

 stomach. 



The idea that it is the presence of free hydrochloric acid 

 in the stomach which determines the relaxation of the 

 sphincter can be still further tested. If a carbohydrate 

 meal is mixed with an alkaline fluid it delays the appear- 

 ance of free hydrochloric acid in the stomach. Such a meal 

 we find also leaves the stomach much later than the pure 

 carbohydrate. This is indicated in curve C f in Fig. 3. It 

 will be seen that the curve for alkalinized carbohydrate 

 tends to approximate that for ordinary protein. 



It is possible to try the converse of this experiment by 

 feeding acidulated protein from which any excess of acid 



1 See Chapter XI, Part 2. 



