502 CHANGES IN THE SMALL INTESTINE. [BOOK n. 



the digestion of fat is in the main carried out by the conjoint action 

 of bile and pancreatic juice. 



281. We have seen, 247, that the addition of bile to 

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

 a coarse flocculent precipitate, consisting of the by-products of 

 proteid digestion 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 possibly to delay the passage of the un- 

 digested material 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 so 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 duodenum 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 

 constituents of saliva and of pancreatic juice (probably also those 

 of the succus entericus), it is in its turn antagonized or destroyed 

 by the bile and the other alkaline juices of the intestine. Hence 

 pancreatic juice introduced through the mouth must lose its powers 

 in the stomach and can only be of use as an alkaline medium 

 containing certain proteid matters. On the other hand if, as we 

 have reason to believe, the contents of the stomach as they issue 

 from the pylorus still contain a large quantity of undigested 

 proteids, these must be digested by the pancreatic juice (with or 

 without the assistance of the succus entericus), the action of which 

 seems to be assisted or at least not hindered by bile. And in 

 dogs fed through a duodenal fistula, so that all gastric digestion is 

 excluded, proteids are completely digested and give rise to quite 

 normal faeces. Since leucin and tyrosin are found in appreciable 

 quantities in the intestinal contents we may infer that some portion 

 of the proteid taken as food undergoes the more profound change 

 into these bodies and their accompanying products. But whether 

 the whole of the peptone (hemi-peptone, 203) available for this 

 change is under ordinary circumstances so changed or only part 

 of it, we do not know. The extent to which the action is carried 

 is probably different in different animals, and probably varies also 

 according to the nature of the meal and the condition of the body. 

 Possibly when a large and unnecessary quantity of proteid material 

 is taken at a meal together with other substances, no inconsiderable 

 amount of the proteids undergo this profound change, and, as we 



