SURVEY OF DIGESTION AS A WHOLE 419 



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 becomes less acid and even weakly alkaline. 

 The observations purporting to show changes in reaction of the 

 intestinal contents at different levels made with indicators like 

 litmus, phenolphthalein, methyl orange, etc., have lost much of 

 their value since the introduction of physico-chemical methods for 

 measuring the hydrogen-ion concentration. However, properly 

 chosen colour indicators can still be employed for estimating the 

 acidity of the gastric contents, at least with sufficient accuracy for 

 most clinical purposes. It must be remembered that the differ- 

 ences in true reaction at different stages of intestinal digestion and 

 at different levels of the gut below the duodenum are slight. There 

 is never a great preponderance either of hydroxyl or of hydrogen 

 ions between the point at which the pancreatic juice and bile are 

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



In the duodenal contents of adult human beings a hydrogen-ion con- 

 centration of 0-00000002 (or ax icr s ) normal has been found by the 

 gas chain method (McClendon) and about the same in the intestinal 

 contents of dogs (Auerbach and Pick). This is a slightly alkaline 

 reaction, the hydrogen-ion concentration of pure water being about 

 five times as great (O-ooooooi, or i x 10-7). In the stomach, of course, 

 a very different state of affairs is found. The hydrogen-ion concen- 

 tration rises in the course of i to 3 or 4 hours after a meal to a maxi- 

 mum which is very considerable. In a series of patients, including a 

 number suffering from gastric disorders, the hydrogen-ion concentra- 

 tion ranged from 0*03 to 0-00000007 (or jx io- s ). The lowest concen- 



