THE GRAY SQUIRREL 405 



A tooth contains a pulp cavity (Fig. 214, 4), supplied with 

 blood vessels and nerves and surrounded by a mass of firmer 

 tissue, or dentine (Fig. 214, 3), which makes up the bulk of 

 the tooth. The dentine is usually covered, where the tooth 

 projects from the gum, with a very hard, smooth substance 

 called enamel (Fig. 214, 1) ; below there is a bony substance, 

 the cement (Fig. 214, 2), surrounding the root of the tooth. 



Fig. 213." Skull of a squirrel 



In the molar teeth of the squirrel the pulp cavity, which is at 

 first open at the base, as in the case of man (see Fig. 214, B), 

 becomes inclosed and develops a root (see Fig. 214, C), after 

 which all growth of the tooth stops. In the incisors of the 

 squirrel the pulp cavity persists throughout life, remaining 

 open so that the tooth continues to grow as fast as it is worn 

 away. The enamel is confined to the front surface of the in- 

 cisors, so that when the tooth is used on hard substances 

 the softer dentine wears away more quickly and the tooth 

 becomes sharper and more like a chisel the more it is used. 

 In most fur-bearing animals the lower jaw is articulated 

 to the upper by means of transverse condyles, but in the 



