GLANDS AND SECRETIONS 99 



as a lump of sugar dissolves less quickly than the same 

 amount of pulverized sugar. The grinding action of the 

 teeth is called mastication. 



The tongue. — (See Fig. 28.) The tongue is a soft, fleshy 

 mass of matter mainly composed of muscle with a cover- 

 ing of mucous membrane. It is fastened at its base to a 

 bone called the hyoid. Its tip projects forward, and is free 

 and very mobile. It has two functions, one mechanical, 

 the other sensitive in nature. As a sense organ it is the 

 seat of taste. As a mechanical organ it mixes the food 

 with saliva and pushes it into the proper position for chew- 

 ing and swallowing. It also serves as an indicator of the 

 health of the digestive system by becoming coated with a 

 " fur " or coat of yellowish mucus whenever the digestion 

 is impaired. Its healthy color is red. 



Chemistry of Digestion. 



Both tongue and teeth are merely mechanical agents 

 which prepare the food for the solvent action of a liquid 

 called saUva. This liquid is a typical digestive fluid in 

 that it is produced by a structure called a gland, and 

 exerts a solvent action through chemical change induced 

 by means of a contained ferment or enzyme. We shall 

 understand its action better if first we make clear what a 

 gland and a ferment are. 



Glands and secretions. — (See Ex. XXXII.) Every cell 

 has the power of taking in food and giving off waste matter. 

 Evidently, then, the character of the food which a cell 

 takes from the blood determines, in a way, the kind of 

 waste which it shall give off. Sometimes the matter so 

 given off is a solid, and the result is the formation of hard 



