172 



ESSENTIALS OF ZOOLOGY 



The Digestive Enzymes and Their Functions 



*The esophagus and colon do not secrete any enzymes. 



The undigested residue passes into the large intestine where prob- 

 •ah\y no enzyme digestion occurs. Certain bacteria {Escherichia coli 

 and others) attack any undigested foods (especially protein) and 

 bring about putrefactive fermentation. Products of this action may 

 be absorbed; some of them are frequently toxic and must be elimi- 

 nated in either the urine or the feces. Certain other bacteria here 

 feed upon cellulose and may produce some sugar from it. When the 

 chyme reaches the large intestine, it is about the consistency of thick 

 cream, but it becomes more and more solid by absorption of water 

 here until finally only concentrated fecal matter remains. 



