362 THE HUMAN BODY. 



The uses of the saliva are for the most part physical and 

 mechanical. It keeps the mouth moist and allows us to speak 

 with comfort; most young orators know the distress occa- 

 sioned by the suppression of the salivary secretion through 

 nervousness, and the imperfect efficacy under such circum- 

 stances of the traditional glass of water placed beside 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 East 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 the nerve-centres of his salivary glands inhibited or 

 paralyzed by terror, and does actually become unable to swal- 

 low the rice; while in those with clear consciences the nerv- 

 ous 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 they are 

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

 them; undissolved 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 sweetness will be 

 felt until a little moisture has exuded and dissolved 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 

 an enzyme, ptyalin, which has the power of turning starch 

 into soluble substances. Until recently the chief product was- 

 believed to be grape sugar (glucose)', but it is now ascertained 

 that it is maltose, belonging to the cane-sugar chemical series. 

 In the small intestine the maltose is changed into glucose and 

 absorbed ; so the chemical action of ptyalin upon starch is at 

 most but a preparatory one. In effecting the change the ptyalin 

 is not altered; a very small amount of it can convert a vast 

 amount of starch, and does not seem to have its activity im- 

 paired in the process. The starch is made to combine with 

 the elements of one or more molecules of water, and the 

 ptyalin is unchanged. 



This faculty of ptyalin is known as q/niiilolytic : and since 



