THE CHANGES IN THE ALIMENTARY CANAL. 311 



and give rise to quite normal feces. 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 diges- 

 tion, 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 dif- 

 ferent 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 un- 

 necessary quantity of proteid material is taken at a meal, together with 

 other substances, no inconsiderable amount of the proteids undergo this pro- 

 found 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 " luxus consumption" 

 of food. 



243. In dealing with the action of pancreatic juice we drew attention 

 ( 218) to the difference between the results of pure tryptic digestion and 

 those obtained when bacteria or other microorganisms were allowed to be 

 present. We saw that indol, for example, was the product of the action of 

 these organisms, not of trypsin. Now 7 indol is formed in varying quantity 

 during the digestion which actually takes place in the intestine, some of it at 

 times appearing in the urine as indigo-yielding substance (indican). More- 

 over, bacteria and other microorganisms are present in the intestinal con- 

 tents. 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 those 

 results modified by or mixed with the results of the action of microorganisms. 

 We spoke above ( 216) of bile as being antiseptic, but this must be under- 

 stood as meaning not that the presence of bile arrests the action of all micro- 

 organisms within the intestine, but that it modifies their action, keeping it 

 within certain limits and along certain lines. 



Concerning the exact nature and extent of the changes thus due to micro- 

 organisms, our knowledge is at present very imperfect. The proteids and 

 the carbohydrates seem to be the food-stuffs on which these organisms pro- 

 duce their chief effect. Out of the proteids they give rise not only to indol, 

 but to several other compounds, among which may be mentioned phenol 

 (C 6 H 6 O), of which a small quantity may be recognized in the feces, the rest 

 being absorbed and appearing in the urine in the form of certain phenol- 

 compounds, such as phenyl-sulphuric acid. Out of proteids they may also 

 form the peculiar poisonous bodies called ptomaines, which appear in the 

 ordinary putrefaction of proteids. But their most conspicuous effects are 

 those on the carbohydrates. As the food descends the intestine, the presence 

 of lactic acid becomes more and more obvious ; indeed, in some cases the 

 naturally alkaline reaction of the intestinal contents may in the lower part 

 of the intestine be changed into an acid one by the presence of lactic acid. 

 Now, lactic acid may be formed out of sugar by means of a special organism 

 inducing what is spoken of as the lactic acid fermentation. And we have 

 every reason to believe that in even normal digestion a certain quantity of 

 sugar, either eaten as such or arising from the amylolytic conversion of 

 starch, does not pass away from the intestine into the blood as sugar, but 

 undergoes this fermentation into lactic acid. To what extent this change 



