I3 2 THE VITAL PROCESSES 



stone is soon dissolved (Fig. 62). (See Practical Work.) It 

 seems at first thought that the acid dissolves the limestone, 

 but this is not the case. The acid pro- 

 duces a chemical change in the limestone 

 (calcium carbonate) and converts it into a 

 compound (calcium chloride) that is soluble 

 in water. As fast as this is formed it is 

 dissolved by the water, which is the real 

 solvent in the case. The acid simply plays 

 FIG. 62. The the part of a chemical converter, 

 dissolving of lime- Thg Digestive Fluids. Several fluids 



stone in water con- . . . ... 



taining acid, sug- sah .va, g astnc ] ulce > pancreatic juice, bile, 

 gesting the double and intestinal juice are employed in the 

 action in the di- digestion of the food. The composition of 



gestion of most these fluMs j g j n k ing with the nature 

 foods. 



of the digestive process. While all of them 

 have water for their most abundant constituent, there are 

 dissolved in the water small amounts of active chemical 

 agents. It is the work of these agents to convert the 

 insoluble nutrients into substances that are soluble in 

 water. The digestive fluids are thus able to act in a double 

 manner on the nutrients to change them chemically and 

 to dissolve them. The chemical agents which bring 

 about the changes in the nutrients are called enzymes, or 

 digestive ferments. 



Foods Classed with Reference to Digestive Changes. 

 With reference to the changes which they undergo during 

 digestion, foods may be divided into three classes as follows: 



1. Substances already in the liquid state and requiring 

 no digestive action. Water and solutions of simple foods 

 in water belong to this class. Milk and liquid fats, or oils, 

 do not belong to this class. 



2. Solid foods soluble in water. This class includes 



