Mammalia: Visceral Systems 



637 



Parotid' 

 Submaxillary 



Fig. 483. Salivary glands of dog. The posterior region of each upper jaw, and 

 the intervening region of the roof of the mouth have been cut away so as to expose 

 the inside surface of the left cheek and bring into view the oral opening of the left 

 parotid duct and, just behind it, that of the left intraorbital duct. The ducts of 

 the submaxillary and sublingual glands open close together at the front of the 

 base of the tongue. (Modified from Owen: " Comparative Anatomy and Physiology 

 of Vertebrates," London, Longmans, Green & Co., Ltd.) 



open on the floor of the mouth in front of the attachment of the tongue. 

 A smaller gland, the sublingual, lies immediately anterior to each 

 submaxillary, usually opening by several small ducts laterally to the 

 tongue. These three pairs of glands collectively produce mucin and a 

 watery or serous secretion, and two digestive enzymes — ptyalin and 

 maltase. In man the parotid is entirely serous, producing no mucin, 

 but the submaxillary and sublingual are "mixed" glands, producing 

 both mucin and a serous secretion containing the enzymes (Fig. 484). 

 An individual cell of the gland, however, is either serous or mucous, 

 never both. The combined secretions of the glands constitute saliva. 



The digestion of starch is initiated by the saliva. In the presence 

 of ptyalin, the large and highly complex molecule of starch is dis- 

 rupted and transformed into sugar molecules of the more complex 

 sort (maltose). By action of the maltase (secreted also by the pan- 

 creas and anterior intestine), these are broken down to form the 

 simpler sugar molecules (dextrose) which are capable of being absorbed 

 and used for nutrition. It is obvious that salivary digestion is most 

 important in animals whose food is well masticated and mixed with 

 saliva in the mouth. After the food is swallowed, the digestive action 

 of the saliva may continue until the normal alkalinity of the saliva is 

 neutralized by the acidity of the gastric secretions. 



In the wholly aquatic mammals salivary glands are \er\ much 

 reduced or entirely lacking. 



