DIGESTION 



189 



to be very similar to (according to Abderhalden identical with) the enzyme 

 amylopsin secreted by the pancreas as one of its external secretions. 

 It is, however, weaker than amylopsin, and does not under the normal 

 conditions of digestion carry the zymolytic process so far mainly the 

 same difference which obtains between pepsin and trypsin. It acts in 

 an alkaline, neutral, or even slightly acid medium, free acid, even 0.003 

 per cent., being, however, quickly destructive of its peculiar powers. 

 In general it seems accurate to say that ptyalin acts best in a neutral 

 medium, as Chittenden seems to have proved, such as may be supposed, 

 indeed, to obtain in the mass of food as taken into and kept for a consider- 

 able while in the fundus of the stomach. The chemical composition of 

 ptyalin cannot be stated with any degree of certainty at present, a state- 

 ment equally true of all other enzymes. 



FIG. 92 



DIASTASE. 



,_. PEPSIN. 

 ...-"RENNIN." 



INVERTING AGENTS. 



_. . . . TRYPSIN. 



_.. .. .. LIPASE. 



EREPSIN. 



... ... ... BACTERIA. 



A / ft" v\\ 

 / w J /// . ': 



/ ' \ ' 





This diagram shows, after the manner of the graphic method, the various chemical digestive 

 agents and their respective times of action. In the complexity of interactions in the alimentary 

 canal it must be pre-supposed, however, that the conditions are more variable than any diagram 

 could indicate. (Modernized from Krukenberg.) 



We consider the action of ptyalin, secreted in the mouth, under the 

 heading of the stomach because its chief action is performed in that 

 viscus and not in the mouth, as has already been suggested. The food 

 remains in the stomach liable to diastatic digestion an hour for every 

 minute, often for every second, that it remains in the mouth. 



Ptyalin brings about the conversion of starch and of glycogen into 

 sugar, the form of sugar produced by this ferment being finally maltose. 

 The method of this change is hydrolysis : the absorption of water followed 

 by a splitting of the amylodextrin or soluble starch so produced into 

 molecules of maltose and probably, at first, also of dextrin. Because 

 we are still ignorant of the exact molecule of starch (knowing only that it 

 is some multiple of (C 6 H 10 O 5 ) perhaps (C 6 H 10 O 5 ) 20 , the precise reaction 



