THE DIGESTION OF FOOD 89 



known as ferments, and it seems necessary before proceed- 

 ing to a consideration of digestion as a process to learn 

 something of the natm-e and f miction of these agents, which 

 are actively and essentially present in the digestive tract. 



121. Definition of ferments. — ^A ferment may be 

 defined in a general way as an agent which causes the 

 decomposition of certain vegetable or animal compounds 

 with which it comes in contact under favorable condi- 

 tions. In the past, ferments have been classified into two 

 kinds, organized and unorganized. The so-called organ- 

 ized ferments are low, microscopic forms of vegetable life, 

 generally single-celled plants. The unorganized ferments 

 are not living organisms, but are simply chemical 

 compoimds. 



122. Organized ferments. — ^When milk is allowed to 

 remain in a warm room for several hours, it becomes 

 sour. An examination of it chemically shows that its 

 sugar has in part disappeared and has been replaced by 

 an acid. A study of the milk with the microscope, before 

 and after souring, reveals the fact that there has been a 

 marvelous increase in it of single-celled organisms or 

 plants. The presence of this form of life is regarded as 

 the cause of the change of the sugar into lactic acid. We 

 have here a so-called lactic-acid ferment, which may 

 typify the organized ferments known as bacteria. Numer- 

 ous other fermentations of the same general kind are 

 common to everyday experience, such as the changes in 

 the cider barrel and the wine cask, the spoiling of canned 

 fruits and vegetables, and the heating of hay and grain, 

 which are all illustrations of what is accomplished by these 

 minute organisms. 



123. Structure and distribution of organized ferments. 

 — ^The organized ferments are classed in the vegetable 



