378 



THE HUMAN MACHINE 



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What Causes Digestion ? We find in plants and animals 

 that the process of changing foods from a solid to a liquid 

 condition goes on rather constantly, for in no other 



way can food get 

 into the cells. The 

 process is brought 

 about by substances 

 which are called en- 

 gallbladder zymes. These are 

 the active substances 

 in our digestive 

 juices. In the mouth 

 starches are digested 

 by enzymes, while 

 in the stomach pro- 

 teins are changed to 

 fluids by other kinds 

 of enzymes. In the 

 small intestine we 

 have other enzymes 

 that act on all three 

 food substances : 

 starches, fats, and 

 proteins. We will 



stomach 



small 

 intestine. 



intestine, 

 append! 



rect/um 



, , , 



learn mO1 



The food tube of man with the liver and pancreas, 

 What have these structures to do with the food 

 tube ? What is another name for everything these WOnderf ul SUb- 



showninthiscut? stances later. 



Where Does Digestion Take Place ? If you will look at 

 the accompanying diagram, you will see the parts of the 

 digestive tract of man. We have already seen that di- 

 gestion takes place in various parts of the tract. The 

 food, after being chewed and mixed with saliva, is squeezed 

 down the tract by means of contractions of the muscles 

 of the food tube. This movement occurs in both the 

 large and small intestines and is of great importance in 



