476 CHANGES IN THE SMALL INTESTINE. [BOOK n. 



cipitate, 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 flocculeut and granular 

 material, which is probably a precipitate thus formed ; the purpose 

 of this precipitation is probably to delay the passage of the un- 

 digested 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 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 prpsin 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. To what stage the pancreatic digestion is carried, 

 whether peptone is, practically, the only product, or whether 

 the pancreatic juice in the body, as out of the body, carries on its 

 work in the more destructive form, whereby the proteid material 

 subjected to it is so broken down as to give rise to appreciable 

 quantities of leucin and tyrosin, is at present not exactly 

 known. Leucin and tyrosin have been found in the intestinal 

 contents, and may therefore be formed during normal digestion, 

 but whether an insignificant quantity or a considerable quantity 

 of the proteid material of food is thus hurried into a crystal- 

 line form cannot be definitely stated. 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 shall see, rapidly leave the body as 

 urea, without having been used by the tissues, their contribution 



