118 GENERAL ZOOLOGY 



of their own kind. Regardless of how they are constructed 

 or how and under what circumstances they live, there is a 

 striking similarity in the processes of changing food ma- 

 terial into the living substance (protoplasm) of their bodies 

 and the use of this food for producing energy. Because of 

 these likenesses, and because we are most interested in, and 

 familiar with, human structure, we are going first to review 

 this process in our own bodies. Later, as we study each 

 animal, we may note in what ways it is different from, and 

 similar to, man. 



Digestion Defined. Food is taken into the body either as 

 a solid or a liquid. The entire process of digestion consists 

 in changing all the foods to a liquid state which is able to 

 pass through the wall of the digestive tract. In terms of the 

 chemist and physiologist foods may be classed as water, 

 salts, proteins, carbohydrates, and fats. Water and the 

 various salts used as food are capable of being used with- 

 out change. The other food substances must be changed 

 before they are available for use. 



Enzymes. The digestive system produces fluids which act 

 upon the different food substances. The name enzyme is 

 applied to the materials in the digestive fluids which bring 

 about digestion. 



The peculiar quality of an enzyme is to cause a chemical 

 change in another substance without itself losing any of its 

 own properties. Thus, trypsin, the enzyme which acts on 

 proteins, can do so and still remain trypsin. Similarly, the 

 enzyme ptyalin acts upon starch, and steapsin upon fats. 



Why Food is Changed. Let us consider for a moment why 

 it is necessary for an animal to secrete elaborate chemical 

 mixtures like digestive fluids. If we were to examine the 

 alimentary canal, we should find that there are no openings 

 leading from the canal to the body cavity which might per- 

 mit food to pass directly to different organs. The wall of the 



