Chap. 11 FOODS AND NUTRITION 179 



ripples and tilts and explores every newcomer. No dry food is tasted until it is 

 well moistened since the sense organs of taste on the surface of the tongue 

 are stimulated only by substances in solution (Fig. 17.3). Saliva enters the 

 mouth more or less continually, except under nervous tensions — when a song 

 is to be sung and "the mouth goes dry." Saliva and mucus keep the mouth 

 well lubricated (Fig. 11.8). In a few mammals, including man, saliva contains 

 ptyalin (salivary amylase), a hydrolyzing enzyme, and a slippery substance. 



NASAL CAvmr 



PALATE 

 MOUTH 



TCN6UB 



JWSALPnAHVJK 



Fig. 11.7. Diagram of the human alimen- 

 tary canal with the liver and pancreas. The 

 same devices for adequate area in limited 

 space as shown in Fig. 11.6. (After Morris. 

 Courtesy, Rand: The Chordates. Philadel- 

 phia, The Blakiston Co., 1950.) 



mucus, both secreted by the cells of the salivary glands. Ptyalin splits the large 

 molecules of cooked starches into the smaller ones of sugar. In most mammals, 

 however, there is no chemical digestion in the mouth. 



Function of the Teeth. Because teeth tell what an animal eats, they also tell 

 where it lives. The most specialized teeth belong to the mammals. According 

 to their function, they are divided into incisors for cutting and chiseling, 

 canines for grasping and tearing, premolars or grinders, and molars or 

 crushers. Squirrels, mice, and other rodents chisel with incisors and crush nuts 

 with molars. In horses, cattle, and other herbivorous animals except the 

 rodents, the front teeth, especially the canines, are reduced or absent and the 

 molars are well developed. In cats, dogs, and other carnivores the upper and 

 lower premolars slide on one another like scissors (Fig. 11.9). A cat grasps 

 meat with its canines, and tears the flesh with its premolars, hardly using the 

 weak molars at all. 



