92 HUMAN BIOLOGY 



beneath + lingua = tongue) lie in the muscular floor of the mouth 

 cavity, and the ducts from these glands open in the floor of the mouth 

 under the tongue. 



123. Uses of saliva. — (1) The saliva aids the mucus 

 in keeping the mouth moist, and thus we are enabled 

 to talk easily. (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 helps to dissolve sugar and salt,* thus enabling 

 us to taste them. 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 that is poured upon it. (4) Besides the 

 three mechanical functions of saliva that we have just enu- 

 merated, this secretion digests cooked starch, as we have 

 already shown. This digestive action is due to a ferment 

 known as ptyalin (pronounced ty'alin) which acts in the 

 same manner as the diastase foimd in plants. 



III. The Thkoat Cavity and Gullet and their 

 Functions 



124. Structure of the throat and gullet. — The cavity of the 

 throat is behind the mouth. If one holds a mirror in front of the 

 mouth opening and presses down upon the tongue with a spoon, 

 one sees hanging down a smaU, fingerhke extension of the soft 

 palate, called the uvula. When food is swallowed, this Uttle tongue 

 of the soft palate is shoved backward into a horizontal position, 

 where it helps to separate the throat cavity from the nose cavity. 



The lower part of the throat narrows into the gullet. This tube 

 traverses the length of the chest cavity, and as it nears the stomach, 

 it passes through the diaphragm. Like all other parts of the ali- 

 mentary canal it is hned with mucous membrane, which furnishes a 



1 See 130, A, 1. 



