USES OF SALIVA. 335 



circumstances of the traditional glass of water placed be- 

 side public speakers. The saliva, also, enables us to swallow 

 dry food; such a thing as a cracker when chewed would 

 give rise merely to a heap of dust, impossible to swallow, 

 were not the mouth cavity kept moist. This fact used to 

 be taken advantage of in the Enst Indian rice ordeal for 

 the detection of criminals.' The guilty person, believing 

 firmly that he cannot swallow the parched rice given him, 

 .and fearful of detection, is apt to have his salivary glands 

 paralyzed by terror, and does actually become unable to 

 swallow the rice; while in those with clear consciences the 

 nervous system excites the usual reflex secretion, and the 

 dry food gives rise to no difficulty in its deglutition. The 

 saliva, also, dissolves such bodies as salt and sugar, when 

 taken into the mouth in a solid form, and enables us to 

 taste them; un dissolved substances are not tasted, a fact 

 which any one can verify for himself by wiping his tongue 

 dry and placing a fragment of sugar upon it. No sweet- 

 ness will be felt until a little moisture has exuded and dis- 

 solved part of the sugar. 



In addition to such actions the saliva, however, exerts a 

 -chemical one on an important foodstuff. Starch (although 

 it swells up greatly in hot water) is insoluble, and could 

 not be absorbed from the alimentary canal. The saliva 

 contains a specific element, ptyalin, which has the power 

 of turning starch into readily soluble and dialyzable sub- 

 stances, such as grape sugar (glucose), dextrine, and bodies 

 chemically allied to these. In effecting the-e changes the 

 ptyalin is not altered; a very small amount of it can con- 

 Tert ;i vast amount of starch, and does not seem to have its 

 activity impaired in the process. The starch is made to 

 combine with the elements of one or more molecules of 

 water, and the ptyalin is unchanged. 



C 6 H 10 5 + IPO = C'lT'O 8 



Starch. Water. Grape sugar. 



Substances acting in this way, producing chemical 

 changes without being themselves noticeably altered, are 

 found in many of the digestive secretions, and are called 



