CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 503 



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. 



282. In dealing with the action of pancreatic juice we 

 drew attention, 249, 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 indol, 

 for example, was the product of the action of these organisms, not 

 of trypsin. Now 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). 

 Moreover bacteria and other micro-organisms 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 results modified by or mixed 

 with the results of the action of micro-organisms. We spoke 

 above, 247, 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 produce 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 faeces, 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 takes 

 place we do not know ; the amount probably varies according to 



