DIGESTION OF FOOD IN THE MOUTH 



59 



diameter. All these tubes and pouches are rolled into 

 a small mass with blood tubes and nerves. The whole 

 collection is called a salivary gland. Each tube and 

 pouch is lined with epithelial cells which make the saliva 

 out of the fluid parts of the blood in which they are 

 always bathed. The saliva flows out of the tubes into 

 the mouth as fast 

 as it is secreted. 

 There is a sali- 

 vary gland in 

 front of each ear, 

 called the parotid 

 gland ; one along 

 each side of the 

 lower jaw, called 

 the submaxillary 

 gland, and one 

 just under each 

 side of the front 

 end of the tongue, 

 called the sub- 

 lingual gland. 



77. Saliva. 

 The saliva is a 

 thin, colorless, 

 alkaline mixture, which often contains air bubbles. About 

 ToVo P art f t ^ ie san ' va i s a white substance called ptyalin, 

 which has the power to change starch to glucose while 

 remaining unchanged itself. Hence, ptyalin is a ferment. 

 It can act only in an alkaline fluid, and its action stops when 

 the food is acted on by the stomach. It digests only a 

 small amount of starch, and its value is due mainly to the 

 water it contains. 



The salivary glands. 



