346 FOOD AND DIGESTION 



and other soluble matters, such as common salt, are usually in a state of 

 solution before they reach the intestine. 



Digestive Changes in the Large Intestine. The changes which take 

 place in the chyme in the large intestine are probably only the continuation 

 of the same changes that occur in the course of the food's passage through 

 the upper part of the intestinal canal. No special enzymes have been clearly 

 shown for the mucous membrane of the large intestine. The enzymes of the 

 small intestine may continue their action here, being hindered only by the 

 acid developed from fermentation processes. 



Action of Micro-organisms in the Intestines. Certain changes 

 take place in the intestinal contents independent of, or at any rate supple- 

 mental to, the action of the digestive ferments. These changes are brought 

 about by the action of micro-organisms or bacteria. We have indicated else- 

 where that the digestive ferments are examples of unorganized ferments, so 

 bacteria are examples of organized ferments. Organized ferments, of which 



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o, 



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0, 



FIG. 276. Types of Micro-organisms, a, Micrococci arranged singly; in tW9s, diplococci 

 if all the micrococci at a were grouped together, they would be called staphylococci and in fours, 

 sarcinse; b, micrococci in chains, streptococci; c, and d, bacilli of various kinds, one is represented 

 with flagellum; e, various forms of spirilla; /, spores, either free or in bacilli. 



the yeast plant may be taken as a typical example, consist of unicellular vege- 

 table organisms, which when introduced into a suitable medium grow with re- 

 markable rapidity. By their growth they produce new substances from those 

 supplied to them as food. Thus, for example, when the yeast cell is introduced 

 into a solution of grape-sugar, it grows, and alcohol and carbon dioxide are 

 produced. These substances probably arise from the formation by the cell ac- 

 tivity of some chemical substances which are allied to the unorganized ferments 

 and which greatly increase in amount with the multiplication of the original 

 cell. In all such fermentative processes organisms analogous to the yeast cell 

 are present, and it is not strange that if the ferment cell is introduced into a 

 suitable medium it may by its rapid growth convert an unlimited amount of 

 one substance into another. Speaking generally, a special variety of cell is 

 concerned with each ferment action, thus one variety has to do with alcoholic, 

 another with lactic, and another with acetous fermentation. 



