156 



ANIMAL BIOLOGY 



sugars. And, as in the stomach, digestion is facilitated by muscular 

 contractions of the intestinal wall. There are slow swayings of 

 entire loops of the intestine; there are local 'segmentation' con- 

 tractions; and finally there are peristaltic waves that pass the ma- 

 terial along for absorption or elimination. 



One naturally wonders how it is that the living tissues of the 

 digestive tract, and especially of the glands themselves, withstand 



Fig. 113. — Chief chemical activities of the human digestive tract. 



the digestive activities of their own enzymes, although the stomach 

 or intestine of an animal killed during active digestion will begin 

 self-digestion. A partial explanation of immunity during life 

 appears to be that the enzymes are not in an active form while 

 they are within the glands, but are later activated by certain 

 conditions that they meet in the digestive tract when food is 

 actually present. 



