100 The Feeding of Animals 



iiig stuffs; and it seems necessary before proceeding to 

 a consideration of digestion as a process to learn some- 

 thing of the nature and functions of these agents, which 

 are actively and essentially present in the digestive tract. 



A ferment may be defined in a general way as some- 

 thing which causes fermentation; in other words, the 

 decomposition of certain vegetable or animal compounds 

 with which it comes in contact under favorable condi- 

 tions. Ferments are of two kinds, organized and unor- 

 ganized. Organized ferments are low, microscopic forms 

 of vegetable life, generally single -celled plants. Unor- 

 ganized ferments are not living organisms, but are sim- 

 ply chemical compounds. 



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 largely or wholly 

 disappeared and has been replaced by an acid. A study 

 of the milk with the microscope, before and after sour- 

 ing, reveals the fact that there has been a marvelous in - 

 crease in it of single-celled organisms or plants. The 

 growth of this form of life is regarded as the cause of 

 the change of the sugar into lactic acid. We have here 

 the so-called lactic -acid ferment, which may tj^pify the 

 organized ferments known as bacteria. Numerous other 

 fermentations of the same general kind are common to 

 every-day experience. The changes in the cider barrel 

 and the wine cask, the spoiling of canned fruits and 

 vegetables, and the heating of hay and grain are illus- 

 trations of what is accomplished by these minute organ- 

 isms. Bacteria that cause disease, and which multiply 

 in the organs and other tissues of the animal body, may 



