84 STUDIES IN PHYSIOLOGY 



(2) It moistens the food for swallowing. The importance 

 of this function is appreciated when one tries to hurry in 

 swallowing the crumbs of dry cracker. (3) Saliva dissolves 

 sugar and salt. If the tongue is wiped dry and a piece of 

 sugar is placed upon it, we have no sensation of taste until 

 the sugar has been partially dissolved by the mixture of 

 saliva and mucus which are poured upon it. (4) Besides 

 the three mechanical functions of saliva that we have just 

 enumerated, this secretion has a chemical action upon cooked 

 starch. 1 After a bit of tasteless starch paste has remained 

 on the tongue for a short time, we notice that it becomes 

 sweet. This means that the starch has been changed to 

 grape sugar by the saliva. Following is a still better 

 method of demonstrating the character of this change. 

 Put a bit of the starch paste into a test tube and mix 

 with it some saliva from the mouth. After warming the 

 mixture, pour in some Fehling's solution and boil. The 

 deep orange or red color clearly demonstrates the presence 

 of grape sugar. Since neither starch nor saliva gives the 

 slightest test with the Fehling's solution, the grape sugar 

 must have resulted from the chemical action of the saliva 

 on the starch. The principal ingredients of saliva are 

 water (constituting over 99% of its composition), and a 

 kind of digestive ferment called pty'arlm (Greek ptyalon 

 spittle). It is the ptyalin that changes the starch to grape 

 sugar. 



5. THE THROAT CAVITY 



The Uvula. The mouth cavity communicates with the 

 throat by a somewhat narrow opening. If one holds a mir- 

 ror in front of the mouth opening and presses down upon 

 the tongue with a spoon, one sees hanging down a small 

 fingerlike extension of the soft palate, called the u'vu-la. 

 When food is swallowed, this little tongue of the soft palate 

 is shoved backward into a horizontal position, where it 



iSee "Laboratory Exercises," No. 19. 



