422 THE HUMAN BODY 



power to ferment carbohydrates with the production of carbon 

 dioxid, alcohol, and acetic and lactic acids. There is no doubt 

 that even in perfect health a considerable fermentation goes on 

 in the intestine. So far as appears it is neither particularly harm- 

 ful nor beneficial. The fermentation products are probably ab- 

 sorbed and used by the Body, but they would be used equally 

 well if absorbed as sugar without fermentation. In the case of one 

 particular carbohydrate, however, cellulose, bacterial fermenta- 

 tion affords the only means by which it can be made available 

 in man for the use of the Body. It seems to be well established 

 that tender cellulose, such as is eaten in lettuce, for example, may 

 be digested by bacteria to a considerable extent; where it is less 

 tender, as in most fruits and vegetables, it remains, as stated 

 earlier, practically undigested. 



Intestinal fermentation is not essential to health as is shown 

 by the possibility of living normally in arctic regions, where, it is 

 said, intestinal bacteria are often wholly wanting. When the 

 fermentation becomes excessive intestinal disturbances may 

 readily result. The production of fermentation acids in too great 

 concentration leads to irritation of the intestinal wall and causes 

 diarrhea. 



The Prevention of Self-Digestion. A question of much interest 

 to physiologists has been why the stomach and intestinal walls 

 and the gastric and pancreatic glands are not themselves digested 

 by the powerful proteolytic enzyms which they produce, in the 

 case of the glands, or which are poured out unto them in the case 

 of the walls of the digestive organs.. It has been shown that the 

 prevention of self-digestion of stomach and intestine depends upon 

 the continuance of life, for animals killed in the midst of digest- 

 ing a meal often do digest great parts of their stomach and in- 

 testinal walls. Just how self-digestion of these structures is 

 normally prevented is not clear, except in so far as the mechan- 

 ism to be described presently (Chap. XXIX), which limits the 

 outpouring of the secretions to periods when food is present, may 

 be efficacious. The self-digestion of the pancreatic and gastric 

 glands is, however, prevented by an interesting arrangement 

 which has been recently analyzed. It appears that neither pepsin 

 nor trypsin is formed in the gland as an active enzym but in an 

 inactive pro-enzym or zymogen form, pepsinogen or trypsinogen, 



