330 STUDIES IN ADVANCED PHYSIOLOGY. 



plant are good illustrations. These ferments are really liv- 

 ing beings which are able in some way to set up chemical 

 processes which we have chosen to call fermentation pro- 

 cesses. The other class consists of unorganized, dead sub- 

 stances. They are chemical ferments. To this second 

 class belong almost all the digestive ferments of the body. 

 Ferments have the power of breaking up certain substances 

 and changing them into new compounds. Thus ptyalin, 

 as just stated, changes starch into sugar. It is probable 

 that these ferments produce these results in the following 

 way : They cause the substance to be changed to take up 

 one or more molecules of water, and then split this combi- 

 nation into two or more smaller bodies. This action is well 

 shown in the change which takes place in cane sugar. Not 

 infrequently cane sugar is changed into two sugars called 

 dextrose and levulose. This change from cane sugar to 

 dextrose and levulose is represented chemically in the fol- 

 lowing equation: 



Cin H 22 On+H 2 = C 6 H 12 6 + C 6 H 12 O 6 

 Cane Sugar -f Water = Dextrose + L,evulose. 



A similar change occurs in the mouth. Under the in- 

 fluence of the ptyalin the starch is made to combine chem- 

 ically with more water and the resulting molecule is then 

 split up, sugar being the result. Ptyalin is present in many 

 of the lower animals, especially the herbivorous animals, 

 but seems to be absent in the carnivora. It was, until 

 recently, believed that the sugar which resulted from 

 the action of the ptyalin on starch was ordinary grape 

 sugar, but later investigation has shown that this is in- 

 correct, and that the resulting sugar is maltose, a sugar 

 very closely related chemically to ordinary cane sugar. 

 However, the starch is not at once changed into maltose, but 

 there seems to be between the beginning starch and the 

 final maltose a number of intermediate stages with which, 

 however, in this elementary treatise we are not concerned. 



The physiological value of ptyalin is evident. Starch is 

 insoluble, and could not pass through the walls of the ali- 



