SURVEY OF DIGESTION AS A WHOLE 413 



more readily hydrolysed by pepsin than by trypsin. The ptyalin 

 of the saliva has been already destroyed in the stomach. 



It must be remembered that all this time, even from the beginning 

 of digestion, a certain amount of pancreatic juice has been finding 

 its way into the duodenum in response first perhaps to the psychical 

 excitation, and later to that action of the acid chyme on the in- 

 testinal mucous membrane which has been described. In the 

 duodenum its trypsinogen is becoming activated to trypsin by the 

 enterokinase of the intestinal juice. The secretion of bile, too, has 

 quickened its pace, the gall-bladder is getting more and more full as 

 the meal proceeds and gastric digestion begins, and some of the bile 

 may very soon escape into the intestine. The pylorus opens occa- 

 sionally for a moment whenever the small portions of chyme which 

 at this stage are beginning to pass through have been sufficiently 

 neutralized by the pancreatic juice and bile, although it is not 

 necessary that the reaction should become actually neutral. When 

 the acid chyme, a greyish liquid, turbid with the debris of animal and 

 vegetable tissues -with muscular fibres, fat globules, starch granules, 

 and dotted ducts gushes through the pylorus and strikes the 

 duodenal wall, the muscular fibres of the gall-bladder contract, and 

 sudden rushes of bile take place from the common duct. By-and-by, 

 as bile and pancreatic juice continue to be poured out, the reaction 

 in the duodenum, as tested by litmus, becomes less acid and even 

 weakly alkaline for a time. But it soon becomes acid again, and the 

 acidity at first increases as the food passes down the gut. In the 

 lower portion of the small intestine the acidity diminishes, and the 

 contents may be neutral or actually alkaline for some distance above 

 the ileo-caecal valve. To phenolphthalein the reaction is acid 

 throughout the whole intestine. But methyl orange shows an 

 alkaline reaction, all the way from the lower end of the duodenum 

 to the caecum (Moore and Rockwood). In the upper part of the 

 duodenum the reaction with this indicator is sometimes found acid, 

 but sometimes neutral or alkaline. All this refers to the conditions 

 during full digestion (3 or 4 to 8 or 9 hours after the taking of food). 

 When digestion is over (20 to 24 hours after a meal) the reaction 

 becomes acid to methyl orange, litmus, and phenolphthalein through- 

 out the whole intestine.* But it must be remembered that the 

 differences in true reaction at different stages of intestinal digestion 

 and at different levels of the gut are always slight. There is never 

 a great preponderance either of hydroxyl or of hydrogen ions be- 

 tween the point at which the pancreatic juice and bile are mingled 

 with the gastric chyme and the lower part of the ileum. 



* In 1 8 dogs fed with meat 20 to 24 hours before death this was found 

 to be the case. In 4 of the dogs the gastric contents were almost neutral 

 to litmus and methyl orange, but slightly alkaline to phenolphthalein; in 

 the rest acid to all three indicators. 



