THE CHANGES IN THE ALIMENTARY CANAL. 309 



appears to vary according to the nature of the food and various circum- 

 stances. In man, living on a mixed diet, the contents have probably 

 become distinctly alkaline before they have passed far down the duode- 

 num. 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 arid indeed com- 

 pleted by the pancreatic juice, possibly assisted by the succus entericus, the 

 presence of bile being said to increase the activity of the pancreatic amylo- 

 lytic 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 feces. 



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. 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 circula- 

 tion. 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 digestion of 

 fatty matters ; and in all probability saponification in the intestine is a sub- 

 sidiary 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 pres- 

 ence of soluble soaps favors the emulsion of neutral fats. Hence a rancid 

 fat, L e., a fat containing a certain amount of free fatty acid, forms an emul- 

 sion 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 solu- 

 tion 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 acid, and the bile in addition 

 brings into solution the soaps as they are formed. So that we may speak 

 of the emulsion of fats in the small intestine as being carried on by the 

 bile and pancreatic juice acting in conjunction ; and as a matter of fact 

 the bile and pancreatic juice do largely emulsify the contents of the small 

 intestine, so that the grayish turbid chyme is changed into a creamy-look- 

 ing fluid, which has been sometimes called chyle. It is advisable, how- 

 ever, to reserve this name for the contents of the lacteals. Many of the 

 fats present in food for instance, butter already contain some fatty acids 

 when eaten ; for these fats the initial action of the pancreatic juice is less 

 necessary. 



This mutual help of bile and pancreatic juice in producing an emulsion 

 explains to a certain extent the controversy which long existed between those 

 who maintained that the bile and those who maintained that the pancreatic 



