474 CHANGES IN THE SMALL INTESTINE. [BOOK n. 



reached. The rapidity with which the change in the reaction is 

 completed is not the same in all animals, 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 sufficiently 

 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 faeces. 



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

 splits them into their respective fatty acids and glycerine. The 

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

 contents of the intestine into soaps ; but to what extent saponifica- 

 tion thus takes place is not exactly known. Undoubtedly 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 neutral fats in the lacteals, and to 

 a slight extent in portal blood, is a conspicuous result of the diges- 

 tion 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 neutral 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 agitation. 

 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 centrifugal direction ; the 

 intensity of the currents and the consequent amount of emulsion 

 depend on the concentration of the alkaline medium and on the 

 solubility of the soaps which are formed. Now the bile and 

 pancreatic juice supply just such conditions as the above for 

 emulsionizing fats : they both together afford an alkaline medium, 

 the pancreatic juice gives rise to an adequate amount of free fatty 



