286 



THE MAINTENANCE OF THE INDIVIDUAL 



with their fangs and the toothless jawed turtles that make up for the 

 lack of teeth by sharp cutting horny beaks suggestive of the bird's 

 beak. 



The greatest development and specialization of teeth occurs among 

 the mammals. According to their shape and function they are divided 



into incisors, or cutting 

 chisels, canines, or graspers 

 and tearers, premolars, or 

 grinders, and molars, or 

 crushers. Here we find 

 a real relationship between 

 the type of teeth and the 

 diet of the organism. In 

 the carnivores, for exam- 

 ple, the anterior grinders 

 are so constructed that 

 they slide like shears while the canines are specialized for grasp- 

 ing animal food, the back molars tending towards degeneracy. In 

 herbivorous animals except the rodents the front teeth, especially 

 the canines, are reduced while the molars become greatly developed. 

 The teeth of man play a definite role in the mechanical preparation 

 of food for digestion. Instead of 



Skull of a squirrel, a rodent (left), and a cat, 

 carnivore (right). Compare carefully for differ- 

 ences in dentition. 



holding the prey, they crush, 

 grind, and tear the food so that a 

 greater surface may be exposed to 

 the action of digestive juices. 

 Man like some other organisms 

 develops more than one set of 

 teeth. The first, or milk teeth, 

 are only twenty in number while 

 there are thirty-two secondary, 

 or permanent teeth. 



Each tooth is divisible into an 

 upper gum-protruding crown, a 

 lower embedded root, and an in- 

 termediate neck. The outer part 

 of the crown is protected by the 

 hardest substance of the body, 

 enamel, that surrounds the bony 

 dentine. This in turn protects 



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enamel 

 dentiYie 



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-bloocL 

 vessel 



cement. 



Sagittal section through a tooth, 

 are cavities painful.^ 



Why 



