ASSIMILATION 



55 



its food value. It is so very harmful in its effects upon 

 the nervous system, however, that it can not properly 

 be considered a food. Water is the most wholesome 

 beverage. 



Digestion. — When food is swallowed it is not really 

 inside the body although it is in the 

 alimentary canal. It is like a piece 

 of dough that has accidentally 

 dropped into the upright tube of an 

 old-fashioned cake pan (Fig. 37). 

 In order to get into the body it must 

 pass through the cells that form the 

 Avail of the canal. As starches, fats 

 and proteids are insoluble, they must 

 be made soluble before they can pass 

 through the wall, that is, they must 

 be digested. This is accomplished 

 through the activity of certain com- 

 plex and very unstable compounds 

 called enzymes, or ferments, which are produced by 

 living cells. 



Enzymes. — There are a great many enzymes all of 

 which have certain characteristics in common. They 

 have no power to initiate a change, but they are able to 

 hasten, or retard, a chemical, or physical action, so that 

 it is able to take place under conditions which would 

 ordinarily render it impossible. They act at very low 

 temperatures and in exceedingly small quantities with- 

 out being used up. 



Enzymes in the Digestive Juices. — The three classes 

 of substances to be digested have three corresponding 



Fig. 37.— Diagram to il- 

 lustrate the relation of 

 a particle cf food in 

 the alimentary canal 

 to the body. It doe8 

 not enter the body 

 until it has passed 

 through the wall of 

 the canal in the direc- 

 tion of the horizontal 

 arrows. 



