110 STUDIES IN PHYSIOLOGY 



nute teeth upon the tongue. In obtaining their food the;y 

 use this rough movable tongue like a file. 



Among the vertebrates teeth are wanting in all birds, in 

 toads, turtles, and tortoises. Most of these animals, how- 

 ever, have horny beaks that aid them in crushing their 

 food. Frogs have teeth along the upper jaw and on the 

 roof of the mouth (see Fig. 83) ; these teeth are used prin- 

 cipally to prevent their prey from escaping and to aid in 

 swallowing. In rattlesnakes and other venomous reptiles 

 two or more long, sharp fangs project from the upper jaw. 

 These contain a tube through which is forced the poison 

 secreted in the poison glands near the root of the tooth. 

 When the snake strikes at its victim, the sharp ends of 

 the fangs are buried in the flesh, and the poison is left in 

 the wound. 



In some of the groups of mammals (that is, animals which 

 are covered with hair) certain types of teeth are developed 

 to an extraordinary degree, and by these teeth the animal 

 is especially adapted for securing and masticating its par- 

 ticular kind of food. Eabbits, squirrels, rats, and beavers 

 have long chisel-shaped incisors which enable them to obtain 

 their food by gnawing. These incisors grow throughout 

 the life of the animal and are kept sharp by a constant grind- 

 ing upon each other of the cutting edges. Canine teeth are 

 altogether wanting in these rodents (Latin rodere = to gnaw). 

 Following is the dental formula of the rabbit : 



Canine teeth are specially fitted to tear in pieces fleshy 

 tissue, and they reach their greatest development in the 

 group of car-niv'o-ra or flesh-eaters. The long, conical teeth 

 in the jaws of the dog, cat, lion, and tiger are canines. 

 All the teeth of the carnivora have pointed crowns adapted 

 for tearing and cutting, rather than for grinding. A most 

 striking example of canine teeth is furnished by the huge 



