526 DIGESTION IN THE INTESTINES [CH. XXXIV. 



spite of the gastric juice) produce enzymes which act in the same 

 way as the pancreatic juice. Some form sugar from starch, others 

 peptone, and amino-acids from proteins, while others, again, break 

 up fats. There are, however, certain actions that are entirely due 

 to these putrefactive organisms. 



i. On carbohydrates. The most frequent fermentation they set 

 up is the lactic acid fermentation : this may go further and result in 

 the formation of carbonic acid, hydrogen, and butyric acid (see p. 

 411). Cellulose is broken up into carbonic acid and methane. This 

 is the chief cause of the gases in the intestine, the amount of which 

 is increased by vegetable food. 



ii. On fats. In addition to acting like lipase, they produce 

 lower acids (valeric, butyric, etc.). The formation of acid products 

 from fats and carbohydrates gives to the intestinal contents an acid 

 reaction. Eecent researches show that the contents of the intestine 

 become acid much higher up than was formerly supposed. Organic 

 acids do not, however, hinder pancreatic digestion. 



iii. On proteins. Peptones, amino-acids, and ammonia are pro- 

 duced; but the enzymes of these putrefactive organisms have a 

 specially powerful action in liberating substances having an evil 

 odour, such as indole (C 8 H 7 N), skatole (C 9 H 9 N), and phenol (C 6 H 6 0). 

 Indole and skatole originate from the trytophane radical of proteins. 

 There are also gaseous products in some cases. 



Ammonia-producing organisms nourish best in the lower regions 

 of the small intestine; the ammonia neutralises the organic acids 

 produced higher up, and in the large intestine the contents have 

 in consequence an alkaline reaction. 



If excessive, putrefactive processes are harmful ; if within normal 

 limits, they are useful, helping the pancreatic juice, and, further, 

 preventing the entrance into the body of poisonous products. It is 

 possible that, in digestion, poisonous alkaloids are formed. Certainly 

 this is so in one well-known case. Lecithin, a material contained in 

 small quantities in many foods, and in large quantities in egg-yolk 

 and brain, is broken up by the pancreatic juice into glycero- 

 phosphoric acid, fatty acids, and an alkaloid called choline. We 

 are, however, protected from the poisonous action of choline by the 

 bacterial enzymes, which break it up into carbonic acid, methane, and 

 ammonia. 



Sour milk has been recently extolled not only as a useful food, 

 but as a cure for many dyspeptic disorders. Although its efficacy in 

 this direction has been much exaggerated, its usefulness in certain 

 cases is explicable on the ground that the lactic acid bacillus, which 

 is a harmless one in itself, possesses the power, when it is actively 

 growing, of destroying other micro-organisms of a more harmful 

 kind. 



