392 A MANUAL OF PHYSIOLOGY 



portion of the sugars. (4) Elastin, nucleins, cellulose, and other 

 substances not digestible, or digestible only with difficulty, in 

 gastric juice. (5) The constituents of the gastric juice itself, 

 including pepsin. The ptyalin of the saliva has been already 

 destroyed. 



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 intestinal mucous membrane which has been described. 

 In the duodenum its trypsinogen is becoming activated to 

 trypsin by the enterokinase of the intestinal juice. The secre- 

 tion 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 occasionally 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- 

 csecal valve. To phenolphthalein the reaction is acid through- 

 out 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 throughout the whole intestine.* But it must 

 be remembered that the differences in true reaction at different 



* 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. 



