392 CHANGES IN THE SMALL INTESTINE. [BOOK IL 



trif ugal direction ; the intensity of the currents and the conse- 

 quent 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 intes- 

 tine 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 greyish turbid chyme is changed into a creamy-look- 

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

 able however 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. 



Fats we may therefore say are digested, for this emulsifica- 

 tion is the main digestion of fats, by both bile and pancreatic 

 juice working together. Hence if either bile or pancreatic 

 juice be prevented from gaining access to the small intestine, 

 fat is not digested, is not absorbed, and appears in the fseces. 

 This is true at least of ordinary fat ; milk in which the fat is 

 already emulsified may be digested and absorbed, in the absence 

 of these secretions. 



231. We have seen, 208, that the addition of bile to a 

 digesting mixture gives rise to a precipitate. This is partly a 

 coarse flocculent precipitate, consisting of undigested or par- 

 tially digested proteids with some amount of bile acids, and 

 partly of a finer more granular precipitate, which is longer in 

 falling down, and consists chiefly of bile acids with a variable 

 amount of peptone ; the latter is re-dissolved on the further 

 addition of bile even though the reaction of the mixture remain 

 acid. In the upper part of the duodenum the inner surface, if 

 examined while digestion is going on, is found to be lined by 

 a coloured flocculent and granular material, which is probably 

 a precipitate thus formed ; the purpose of this precipitation is 

 probably to delay the passage of the undigested parapeptone 

 along the duodenum. Moreover, apart from this precipitation, 

 bile arrests the action of pepsin, even while the reaction of the 

 mixture still remains acid ; and as soon as an alkaline reaction 

 is established the pepsin is apparently destroyed by the trypsin, 

 so that with the flow of bile and pancreatic juice into the duo- 

 denum the processes which have been going on in the stomach 

 come to an end. In fact it would seem that the juices of the 

 various districts of the alimentary canal are mutually destruc- 

 tive ; thus, while pepsin in an acid solution destroys the active 



