CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 477 



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 0), 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 the amount of carbohydrates eaten, the condition of 

 the alimentary canal, and other circumstances. It may be under 



