136 THE HUMAN BODY 



direction in the lower. Each has a single long fang. The 

 canines (dog teeth) (Fig. 43) are somewhat larger than the 

 incisors. Their crowns are thick and somewhat conical, 

 having a central point or cusp on the cutting edge. In 

 dogs and cats the canines are very long and pointed, and 

 adapted for seizing and holding prey. The bicuspids or 

 premolars (Fig. 44) are rather shorter than the canines and 

 their crowns are cuboidal. Each has two cusps, an outer 

 towards the cheek, and an inner on the side turned towards 

 the interior of the mouth. The molar teeth or grinders 

 (Fig. 45) have large crowns with broad surfaces, on which 

 are four or five projecting tubercles which roughen them 

 and make them better adapted to crush the food. Each 

 has usually several fangs. The milk teeth differ only in 

 subsidiary points from those of the same names in the per- 

 manent set. 



The Structure of a Tooth. If a tooth be broken open 

 a cavity extending through both crown and fang will be 

 found in it. This is filled during life with a soft pulp, con- 

 taining blood-vessels and nerves, and is known as the 

 "pulp cavity." The hard parts of the tooth disposed 

 around the pulp cavity consist of three different tissues. 

 Of these, one immediately surrounds the cavity and makes 

 up most of the bulk of the tooth; it is dentine or ivory; 

 covering the dentine on the crown is enamel, the hardest 



A canine. Name animals with specially developed canines. For 

 what do they use them? Give another name for a bicuspid tooth. 

 Describe one. 



Describe a molar tooth. What is the object of the projections 

 on their crowns? How far do the milk teeth differ from the perma- 

 nent in form? 



What do we find on breaking open a tooth? What is it called? 

 Why? What tissues form the hard parts of a tooth? Where does 

 each lie? 



