THE DIGESTION OF FOOD 



I0 3 



:4r A 



148. The Structure of Teeth. The teeth are made of 

 three materials, dentine, cement, and enamel. Dentine, the 

 familiar ivory of commerce, is a bonelike substance which 

 forms the inside and body of the tooth. 



Outside of this dentine or the root is 

 a layer of cement ; but, when the tooth 

 appears above the jaw, the enamel takes 

 the place of the cement. Enamel is a 

 hard, shining material which looks like 

 ivory, and gives a strong protection to 

 the exposed part of the tooth, called 

 the crown. 



Inside of each tooth is a space which 

 holds a delicate substance called the 

 pulp, well supplied with nerves and 

 blood vessels which enter at the root 

 of the tooth. 



149. The Different Kinds of Teeth. 

 Beginning at the center of the jaw, 

 there are eight incisors, or cutting teeth, 



two on each side. They have sharp, FIG. 65. BLACKBOARD 

 chisel-like edges, which cut up the food. SKETCH. 



These teeth are largely developed in Longitudinal Section of 

 gnawing animals, such as rabbits, squir- 

 rels, rats, and beavers. 



Then come the canine, or dog teeth, 



two in each jaw, so called because they are strongly devel- 

 oped in dogs, cats, tigers, and other flesh-eating animals. 

 The two upper canines are commonly known as the eye 

 teeth, and the two lower as the stomach teeth. 



Next come the bicuspids, four in each jaw. They have 

 two points, or cusps, for grinding the food. 





