ORGANS AND PROCESSES OF DIGESTION 143 



nerves, known as the pulp cavity. It is set in a depression in the jaw 

 where it is held firmly in place by a bony substance, known as cement. 

 The part of the tooth exposed above the gum is the crown, the part 

 surrounded by the gum is the neck, and the part which penetrates into 

 the jaw is the root (A, Fig. 65). A hard, protective material, called 

 enamel, covers the exposed surface of the tooth. 



The teeth which first appear are known as the temporary, or milk, 

 teeth and are twenty in number, ten in each jaw. They usually begin 

 to appear about the sixth month, and they disappear from the mouth 

 at intervals from the sixth to the thirteenth year. As they leave, teeth 

 of the second, or permanent, set take their place. This set has thirty- 

 two teeth of four different kinds arranged in the two jaws as follows : 



In front, above and below, are four chisel-shaped teeth, known as 

 the incisors. Next to these on either side is a tooth longer and thicker 

 than the incisors, called the canine. Back of these are two short, 

 rounded and double pointed teeth, the bicuspids, and back of the bicus- 

 pids are three heavy teeth with irregular grinding surfaces, called the 

 molars (B and C, Fig. 65). Since the molar farthest back in each jaw 

 is usually not cut until maturity, it is called a wisdom tooth. The molars 

 are known as the superadded 

 permanent teeth because they 

 do not take the place of milk 

 teeth, but form farther back 

 as the jaw grows in length. 



2 . The Tongue. The 

 tongue is a muscular or- 

 gan whose fibers extend FlG - 66. Diagram showing directions 

 , .^ . of muscular fibers in tongue. 



through it in several 



directions (Fig. 66). Its structure adapts it to a variety of 

 movements. During mastication the tongue transfers the 

 food from one part of the mouth to another, and, with 

 the aid of the cheeks, holds the food between the rows 

 of teeth. (By an outward pressure from the tongue and 

 an inward pressure from the cheek the food is kept between 

 the grinding surfaces.) The tongue has functions in ad- 

 dition to these and is a most useful organ. 



