102 DIGESTION 



glands, and in this class are the salivary glands, liver, and 

 pancreas. 



Enzymes. — (See Ex. XXXI.) The secretions of the 

 digestive glands owe their power of dissolving nutrients 

 to the presence of certain complicated chemical compounds 

 called enzymes. The action of these enzymes is little 

 understood at the present time, but their effect is well 

 known. In general, they all agree in causing a chemical 

 change in a nutrient without being themselves consumed 

 in the process. 



For example, if we add a little malt (see p. 83), which 

 contains the enzyme diastase, to some starch paste, the 

 starch is gradually changed to sugar, but the amount of 

 the diastase which causes the change is neither increased 

 nor diminished by the process. 



Similarly, yeast added to a sugar solution causes the' 

 sugar to break up into two simpler compounds, alcohol 

 and carbon dioxide, while the number of yeast plants is 

 not decreased by the process. 



The name ferment was long used to describe these com- 

 pounds, and hence the action of yeast or bacteria was 

 described as fermentation. We now know that yeast and 

 bacteria produce their effect through definite chemical 

 compounds, and to all such compounds we give the name 

 enzymes. 



The salivary glands. — (See Fig. 30.) There are three 

 pairs of these glands in man. They are called the parotid, 

 the sul:)maxillary, and the sublingual. All these glands 

 are of the compound, racemose type, and resemble bunches 

 of grapes in which the grapes are the sacs of secreting 

 cells, and the connecting stems are the hollow ducts. The 

 parotid glands lie in front of and under the ear on each 



