l66 ORGANIC AGRICULTURAL CHEMISTRY 



by these different stimuli typifies in general the flow of all of 

 the digestive juices, which we shall presently consider. Thus 

 the character of the food, the particular nature of the stimuli 

 exerted and the mastication required regulate the operation of 

 salivary mixing. This is because the saliva in addition to con- 

 taining ptyalin and maltase, which act chemically on the starch, 

 also acts in a mechanical or physical manner and enables the 

 food to be swallowed or passed down the esophagus to the 

 stomach; the esophagus acting merely as a connecting canal 

 between the mouth and stomach. Chemically the action of 

 salivary digestion is that already given of converting, by the 

 action of ptyalin and maltase, all starch into maltose and this 

 into glucose. This is the chief chemical reaction of the saliva 

 due to the enzymes present in it. Other carbohydrate food, 

 viz. the disaccharoses, cane sugar and milk sugar, and the 

 monosaccharoses are not acted upon at all by the saliva. 



The ordinary saliva, which is the mixed secretion of the differ- 

 ent salivary glands, is a thin, more or less sirupy or mucousy 

 liquid, having a specific gravity only slightly more than that of 

 water, viz. 1.005. The solid matter, therefore, in solution is 

 only about 0.5 per cent. The substances present in solution in 

 saliva are as follows : 



Enzymes. — The enzymes present are (a) ptyalin, converting 

 starch into maltose; {b) maltase, converting maltose into glucose. 

 Enzymes, capable of splitting polypeptides, the action of which 

 is of only secondary importance, may also be present. 



Ptyalin and Maltase. — The complete series of changes which 

 have been shown to take place when starch is digested by the 

 enzymes of saliva are as follows : the starch is first converted 

 from its insoluble or colloidal state into a truly soluble form 

 known as soluble starch or amidulin. This soluble starch still 

 gives the blue color with iodine. The soluble starch is then 

 converted into dextrin, and several varieties of dextrin are suc- 

 cessively produced, viz. (a) erythrodextrin, which gives a red 

 color with iodine ; (b) a-, ft- and y-achroodextrins which give no 

 color with iodine. Following the dextrins there are produced 



