ACTION OP SALIVA 105 



slightly alkaline liquid, with a ropy consistency due to the 

 mucin in it, and is often frothy from the presence of air 

 bubbles. Pure saliva is a clear, viscid, colorless liquid 

 consisting mainly of water and a certain enzyme called 

 ptyalin. It is this enzyme which is responsible for the 

 chemical action of saliva in digestion. 



Action of saliva. — Of all the nutrients taken into the 

 mouth starch is the only one acted upon by the saliva. 

 If clear saliva is allowed to act upon starch it changes it 

 to sugar in exactly the same way that the diastase of 

 malt changes grain starch to sugar. This sugar is, of 

 course, readity soluble in the watery part of the saliva, 

 and thus the nutrient starch is forced in solution. That 

 this transformation is actually produced may be readily 

 demonstrated by chewing a little dry biscuit for a few 

 moments. The dry, tasteless mass soon becomes sweet 

 to the taste as its starch is changed to sugar by the saliva. 

 The chemical changes involved in this transformation 

 are rather complicated, but for our purpose it is sufficient 

 to state that they have their origin in the action of the 

 enzyme ptyalin, and we can, therefore, say that it is the 

 ptyalin which produces the change. 



Experiments with starch paste and clear saliva (see 

 Exs. XXXV. a and XXXVI.) show that this transfor- 

 mation takes place best at the normal mouth temperature 

 (98° F.), while high temperatures (above 145° F.), or 

 very low temperatures (below 32° F.), or the presence 

 of strong acids or alkalies are sufRcient to stop completely 

 the ptyalin action. Further, cooked starch is much more 

 easily changed to sugar than imcooked, and requires less 

 time for the action of the enzyme. 



Saliva also aids digestion in a mechanical way by fit- 



