CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 393 



constituents of saliva and of pancreatic juice (probably also 

 those of the succus enter icus), it is in its turn antagonized or 

 destroyed by the bile and the other alkaline juices of the intes- 

 tine. 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 crystalline 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 sub- 

 stances, 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 

 to the energy of the body being limited to the heat given out 

 during the changes by which they are converted into urea. To- 

 this apparently wasteful use of proteids we shall return in 

 speaking of what is called the 4 luxus consumption ' of food. 



232. In dealing with the action of pancreatic juice we 

 drew attention, 210, to the difference between the results of 

 pure tryptic digestion and those obtained when bacteria or other 

 micro-organisms were allowed to be present. We saw that iiidol, 

 for example, was the product of the action of these organisms, 

 not of trypsin. Now indol is formed, in varying quantity, dur- 

 ing the digestion which actually takes place in the intestine, 

 some of it at times appearing in the urine as indigo-yielding 

 substance (indican). Moreover bacteria and other micro-organ- 

 isms are present in the intestinal contents. Hence we must 

 regard the changes taking place in the intestine not as the pure 

 results of the action of the several digestive juices, but as these 



