DIGESTION OF .THE FOOD 21 



the energy required for the performance of such 

 work has to come from the food it is easy to see 

 that the value of such a food-stuff must be less on 

 account of the work of mastication. At a later 

 stage this subject will be further discussed. 



(2) The processes of digestion. 

 Under the term digestion are included all those 

 processes by which the substances contained in the 

 food are converted into a form suitable for assimila- 

 tion, or absorption. Grape sugar, fruit sugar, 

 organic acids, and many mineral substances require 

 no such conversion, they are absorbed directly if 

 dissolved in a sufficient quantity of water. On 

 the other hand, those nutrients which cannot pass 

 through the walls of the intestine, or are insoluble 

 in water, must first undergo some chemical change 

 before they can enter the cells of the alimentary 

 canal. The change usually takes the form of a 

 cleavage or splitting up into some simpler sub- 

 stances which are capable of absorption. The 

 majority of changes of this kind are carried out by 

 means of the ferments or enzymes present in the 

 digestive juices. Bacteria which get into the ali- 

 mentary canal from the food, water, or air, also 

 take part in the process of digestion and multiply 

 rapidly. A few remarks as to the nature of enzymes 

 may fitly be mentioned here. They are classed 

 along with the albumens and have the property, 



