DIGESTION AND ABSORPTION 167 



iso-maltose and maltose. The iso-maltose is not formed simply 

 from the y-achroodextrin, but also from each of the dextrins 

 directly. The formation of maltose is the end product of the 

 action of the enzyme ptyalin on starch. The action of the 

 enzyme maltase, which is also present in saliva, is small in 

 amount, and, as we shall see, this enzyme acts more com- 

 pletely in another part of the digestive tract. 



Conditions of Salivary Action. — The action of ptyalin takes 

 place in alkaline, neutral or combined acid solution. The prin- 

 cipal action occurs in alkaline solution and the saliva itself is 

 of slight alkaline reaction. The alkalinity of saliva is equivalent 

 to about 0.1 per cent sodium carbonate, and is due to the 

 presence in the saliva of di-sodium phosphate, Na2HP04. While 

 ptyalin will continue to act in somewhat strong combined 

 hydrochloric acid solution, its activity is destroyed by the pres- 

 ence of o.ooj to 0.006 per cent oi free hydrochloric acid. 



We should explain at this point the term combined acid. 

 When protein substances react with free hydrochloric acid they 

 unite with it as bases forming salts (protein salts). These 

 salts ionize differently than free hydrochloric acid or such salts 

 as sodium chloride, and thus act in a different way in preventing 

 the action of ptyalin or as a germicide. This property will be 

 referred to again when we consider the action of hydrochloric 

 acid in the stomach. 



In stronger alkaline solutions than the saliva the action of 

 ptyalin is inhibited but not destroyed. The action of saliva 

 is better when it is somewhat diluted, to about seven times its 

 volume. This fact is not in accord with the often advanced 

 claim that water-drinking at mealtime is injurious to digestive 

 action. 



Salts. — The second chief group of solid substances present 

 in solution in saliva is that of inorganic salts. The principal 

 salt present is di-sodium phosphate, Na2HP04, already men- 

 tioned as the cause of the alkaline reaction of saliva. The 

 sodium phosphate present in saliva is the cause of the formation 

 of the tartar on the teeth. With the teeth the phosphate forms 



