96 DIGESTION 



edges. These teeth, four on each jaw, are called incisors, 

 and are shaped to cut up the food taken into the mouth. 

 On each side of the incisors is found a tooth with a sharp 

 point. These teeth, four in number, are called canines 

 from their resemblance to the teeth of a dog. The shape 

 of the canines fits them for tearing apart the food which 

 cannot be easily cut. Next behind the canines, two on 

 each side of the jaw, are teeth with two pointed surfaces. 

 These are called the bicuspids or premolars. They serve 

 the same purpose as the remaining six teeth on each jaw 

 which are called the molars. The twelve molars have 

 rough broad surfaces for grinding the food into small pieces. 

 The third molar on each side of a jaw sometimes does not 

 appear until after the twentieth year, and these teeth are 

 often spoken of as the wisdom teeth. 



The milk teeth set differs from the permanent set in 

 having no bicuspids and only eight molars. The numerical 

 relation of the teeth is often represented by a fraction 

 whose numerator gives the number of each kind on the 

 upper jaw, and the denominator expresses the same rela- 

 tion for the lower jaw. Such a fraction is called a dental 

 formula. The permanent set formula for man is : 



■n + 1 f 1 2 + 2,1 + 1,2 + 2,3+3 „„ ^ ^, , 



Dental formula = 2 + 2,1 + 1,2 + 2,3+3 ° ^^ teeth; or, for 



2+1+2+3 

 half a jaw, dental formula = -^ = 16 teeth. 



^ + i + ii + c> 



A/r-11 4. <.u t 1 2 + 2, 1 + 1, + 0, 2 + 2 



Milk teeth formula = zr -^— —- -i- — • 



2 + 2, 1 + 1, + 0, 2 + 2 



No animal known has more kinds of teeth than man. 

 The number on each jaw differs greatly among different 



