THE DIGESTIVE JUICES 85 



Bacterial Action, The gastric juice is an antiseptic ; the pan- 

 creatic juice is not. An alkaline fluid like pancreatic juice is just 

 the most suitable medium for bacteria to flourish in. Even in 

 an artificial digestion the fluid is very soon putrid, unless special 

 precautions to exclude or kill bacteria are taken. It is often difficult 

 to say where pancreatic action ends and bacterial action begins, as 

 many of the bacteria that grow in the intestinal contents, having 

 reached that situation in spite of the gastric juice, act in the same 

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

 peptone, leucine, and tyrosine from proteins, while others again 

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

 or mainly 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. 19). 

 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, lower acids 

 (valeric, butyric, &c.) are produced. The formation of acid products 

 from fats and carbohydrates gives to the intestinal contents an acid 

 reaction. Recent researches show that the contents become acid 

 much higher up in the small intestine than was formerly considered 

 to be the case. These organic acids do not hinder pancreatic diges- 

 tion to any appreciable extent. 



iii. On proteins. Fatty acids and amino-acids are produced, but 

 these putrefactive organisms are specially efficacious in liberating the 

 protein cleavage products which have an evil odour like indole, 

 skatole, and phenol. 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, pre- 

 venting 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 glycerin, phos- 

 phoric acid, fatty acids, and an alkaloid called choline. We are, 

 however, protected from the poisonous action of choline by the 

 bacteria, which break it up into carbonic acid, methane, and 

 ammonia. 



