CH. xxxil. j BACTERIAL ACTION. 491 



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. 388). 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 steapsin, they pro- 

 duce lower acids (valeric, butyric, <fec.). The formation of acid 

 products from fats and carbohydrates gives to the intestinal 

 contents an acid reaction. Recent researches show that the 

 contents of the intestine become acid much higher up than was 

 formerly supposed. Organic acids do not, however, hinder pan- 

 creatic digestion. 



iii. On proteids. Fatty acids and amido-acids, especially leu- 

 cine and tyrosine, are produced ; but these putrefactive organisms 

 have a special action in addition, producing substances having an 

 evil odour, like indole (C 8 H 7 N), skatole (C 9 H 9 N), and phenol 

 (C 6 H 6 0). There are also gaseous products in some cases. 



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. Leci- 

 thin, 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 glycerine, phosphoric acid, stearic acid, and 

 an alkaloid called cholinc. We are, however, protected from the 

 poisonous action of choline by the bacteria, which break it up 

 into carbonic acid, methane, and ammonia. 



Leucine and Tyrosine. 



These two substances have been frequently mentioned in the 

 preceding pages. As types of the decomposition products of 

 proteids they are important, though probably only small quanti- 

 ties are normally formed during digestion. 



They belong to the group of amido-acids. On p. 391 we have 

 given a list of the fatty acids ; if we replace one of the hydrogen 

 atoms in a fatty acid by amidogen (NH 2 ), we obtain what is called 

 an amido acid. Take acetic acid : its formula is CH 8 .COOH ; 

 replace one H by NH, and we get CH 4 .NH 2 .COOH, which is 

 amido-acetic acid or glycocine. If we take caproic acid a term 



