ELEMENTAEY PHYSIOLOGICAL CHEMISTEY 239 



After this has been accomplished, aerobic organisms act on the remain- 

 ing side chains yielding carbonic anhydride and water. 



These aromatic bodies have a strong faeculent odour which they 

 impart to the faeces. Certain proportions of them are, however, ab- 

 sorbed into the blood and reappear in the urine in combination with 

 alkalies as salts (see Urine, p. 263). 



These products also result when proteids undergo putrefaction in the 

 air, but in this latter case other bodies called ptomaines are also 

 produced which are powerful poisons. It is on account of the presence 

 of these that it is dangerous to eat putrid flesh. The action of bacteria 

 on carbohydrates is even more energetic than it is on proteids. They can 

 do all that ptyalin and amylopsiii can do, but besides this they can 

 decompose the monosaccharides into simpler bodies such as ethylic 

 alcohol, lactic and butyric acids. They have also the power of digesting 

 cellulose whereby methane (CH 4 ) is produced as one of the products. 



On fats they act like steapsin, but here also they can carry the 

 process a stage further in that they transform the fatty acid into 

 members lower in the series. They decompose lecithin, and prevent the 

 poisonous action of the liberated cholin by further breaking it up into 

 carbon dioxide, methane and ammonia. We see, therefore, that 

 bacterial action is more beneficial than otherwise, and since they can 

 accomplish so much it has been suggested that the intestinal bacteria 

 are the active agents in digestion, and that the various digestive juices 

 serve mainly as nutritive pabula for them to grow on. Such a view is, 

 however, erroneous, for it has been shown that if guinea pigs be excised 

 from the uterus just before full term under antiseptic precautions, and 

 kept in a chamber aspirated with sterile air, and fed on sterile milk, they 

 thrive, and if after some time the intestinal contents be examined, the 

 latter will be found free of bacteria. 



Further, the intestinal contents of animals in the arctic regions 

 have been shown to be nearly sterile, so that there can be no doubt that 

 bacterial growth is not essential for efficient digestion. 



The Faeces. These are composed of the following substances : 



1. Substances which have escaped digestion, e.g. pieces of vegetables, 

 muscle fibres, elastic tissue, casein, fat, nuclein, haematin, etc. 



2. Remains of juices secreted into the intestine, e.g. mucin, traces of 

 bile salts and pigments, inorganic salts (alkaline earths), epithelial 

 cells, etc. 



3. Products of digestion, e.g. aromatic bodies, fatty acids, methane, 

 ammonia, etc. 



