DIGESTION 353 



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 con- 

 tract, and a sudden rush of bile takes 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 indi- 

 cator 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* (Pluemer and Stewart). 



A consideration of the properties of the indicators mentioned 

 enables us to interpret in some measure these results, which at first 

 sight appear so confusing. Methyl orange, the most stable of the 

 series, is not affected by weak organic acids, but reacts acid to 

 inorganic, and the stronger organic acids like lactic, acetic and 

 butyric acids, and alkaline to salts of the weaker acids, such as sodium 

 carbonate and bicarbonate. Phenolphthalein is very sensitive to 

 acids, even to weak organic acids such as the fatty acids derived from 

 the fat of meat, and to carbonic acid. Litmus is intermediate 

 between methyl orange and phenolphthalein. The chyme as it 

 passes through the pylorus contains free hydrochloric acid, and in 

 addition, during the first stages of digestion of a meal containing 

 carbo-hydrates, free lactic acid. It mingles immediately with the 

 alkaline contents of the duodenum. If these contain a sufficient 

 quantity of bases to combine with the whole of the acids which would 

 affect methyl orange, that indicator will show a neutral or alkaline 

 reaction. Phenolphthalein may at the same time react acid on 

 account of the presence of weaker acids, including carbonic acid, 

 either originally dissolved in the intestinal fluid or liberated by the 



* In 18 dogs fed with meat 20 to 24 hours before death this was found: 

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

 to litmus and methyl orange, but slightly alkaline to phenolphthalein ; iu- 

 ihe rest acid to all three indicators. 



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