500 DIGESTION 



peristaltic wave, to delay its movement. This suggests that an important 

 digestive process must be proceeding in this part of the gut. In these 

 ways conditions become established in the cecum for the active opera- 

 tion of bacteria. They attack the cellulose, and liberate the more diges- 

 tible foodstuffs contained in the vegetable cells, also producing out of 

 the cellulose itself materials of nutritive value. The acids that are also 

 produced by this process are neutralized by the carbonates secreted 

 by the mucosa. 



In certain herbivorous animals the ruminants this process in the 

 cecum is not relatively of such importance, because it takes place in the 

 paunch. The animals swallow the food and it mixes in this part of the 

 stomach w r ith the saliva, so that bacteria and ferments contained in it, 

 called cytases, attack the cellulose, liberating the more easily digested 

 foodstuffs inclosed within the cell walls. As this process goes on acids 

 accumulate in the digestive mixture. The food is then returned to the 

 mouth, chewed over again, and swallowed again into the main stomach, 

 where it is digested. The aid which bacteria render to digestion depends 

 therefore on the nature of the diet. Man, being omnivorous, stands mid- 

 way between the two groups of animals discussed above. Although the 

 cellulose contained in his food is not itself sufficiently digested to furnish 

 nutriment, yet it is so far acted upon as to permit the rupture of the 

 cell, the contents of w'hich are then digested. The cellulose is, however, 

 of value in furnishing bulk to the intestinal contents " intestinal bal- 

 last," it is sometimes called. 



In the small intestine in man there are bacteria capable of acting on 

 carbohydrates and producing from them organic acids, such as lactic, 

 acetic, etc. So long as a sufficiency of carbohydrate exists to encourage 

 the action of these bacteria, others having an action on protein do not 

 seem to thrive. It may be that this is to be accounted for partly by the 

 production of acid substances by the carbohydrate fermentation, and 

 partly by the fact that, as soon as the protein molecule is broken 

 down by the digestive enzymes, its building-stone ammo acids are ab- 

 sorbed. There are probably also bacteria in the small intestine capable 

 of splitting fat into fatty acid and glycerine, but practically nothing is 

 known of their action. In the large intestine of man, along with the 

 cellulose-digesting bacteria already mentioned, protein-digesting bac- 

 teria are also present. These bacteria belong to the class, Bacillus coli 

 communis, the various members of which are known as facultative anae- 

 robes because they can grow in the presence or absence of oxygen. 



If bacterial growth is excessive or there is an insufficiency of carbohy- 

 drates in the small intestine, the bacteria attack the amino acids pro- 

 duced by the digestive enzymes and decompose them into products 

 that may be toxic if absorbed into the blood. 



