CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 391 



intestine freely. These two alkaline fluids, especially the more 

 strongly and constantly alkaline pancreatic juice, tend to neu- 

 tralize the acidity of the chyme, but the contents of the duo- 

 denum do not become distinctly alkaline until some distance 

 from the pylorus is reached. The rapidity with which the 

 change in the reaction is completed is not the same in all ani- 

 mals, and in the same animal appears to vary according to the 

 nature of the food, and various circumstances. In man, living 

 on a mixed diet, the contents have probably become distinctly 

 alkaline before they have passed far down the duodenum. On 

 the other hand in dogs, the contents of the small intestine have 

 been observed to be acid throughout, and that, not only when 

 fed on starch and fat, which might, by an acid fermentation of 

 which we shall presently speak, give rise to an acid reaction, 

 but even when fed on meat. 



The conversion of starch into sugar, which as we have seen is 

 sooner or later arrested in the stomach, is resumed with great 

 activity and indeed completed by the pancreatic juice, possibly 

 assisted by the succus entericus, the presence of bile being said 

 to increase the activity of the pancreatic amylolytic ferment. The 

 conversion begins as soon as the acidity of the chyme is suffi- 

 ciently reduced and continues along the intestine ; portions 

 however of still undigested starch may be found in the large 

 intestine, and even at times in the fseces. 



The pancreatic juice, as we have seen, emulsifies fats, and 

 also splits them into their respective fatty acids and glycerin. 

 The fatty acids thus set free become converted by means of the 

 alkaline contents of the intestine into soaps ; but to what extent 

 saponification thus takes place is not exactly known. Undoubt- 

 edly soaps have to a small extent been found both in portal 

 blood and in the thoracic duct after a meal; but there is no 

 proof that any large quantity of fat is introduced in this form 

 into the circulation. On the other hand, the presence of neu- 

 tral fats in the lacteals, and to a slight extent in portal blood, 

 is a conspicuous result of the digestion of fatty matters ; and 

 in all probability saponification in the intestine is a subsidiary 

 process, the effect of which is rather to facilitate the emulsion 

 of neutral fats than to introduce soaps as such into the blood. 

 For the presence of soluble soaps favours the emulsion of neu- 

 tral fats. Hence a rancid fat, i.e. a fat containing a certain 

 amount of free fatty acid, forms an emulsion with an alkaline 

 fluid more readily than does a quite neutral fat. A drop of 

 rancid oil let fall on the surface of an alkaline fluid, such as a 

 solution of sodium carbonate of suitable strength, rapidly forms 

 a broad ring of emulsion, and that even without the least agi- 

 tation. As saponification takes place at the junction of the 

 oil and alkaline fluid currents are set up, by which globules of 

 oil are detached from the main drop and driven out in a cen- 



