THE GRAY SQUIRREL 



401 



enamel (Fig. 212, l) ; below there is a bony substance, the 

 cement (Fig. 212, 2), surrounding the root of the tooth. In 

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

 first open at the base, as in the case of man (shown in Fig. 

 212, B), becomes inclosed and develops a root (see Fig. 212, C), 

 after which all growth of the 

 tooth stops. In the incisors 

 of the squirrel the pulp-cav- 

 ity 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 incisors, 

 so that when the tooth is 

 used on hard substances the 

 softer dentine wears away 

 more quickly and the tooth 

 becomes sharper the more it 

 is used. 



In most fur-bearing animals 

 the lower jaw is articulated to 

 the upper by means of trans- 

 verse condyles, but in the 

 squirrel and its allies the 

 condyles are parallel with 

 the long diameter of the head, 

 thus allowing some backward (From Flower and Ly d e kker's Mammals) 

 and forward motion of the 



lower jaw. The advantages of these special adaptations of 

 the structure of teeth and jaws to the life of a gnawing 

 animal like the squirrel are obvious. 



The ducts of four pairs of salivary glands open into the 

 mouth. A muscular flap, called the soft palate, to distinguish 



D 



FIG. 212. Sections of Teeth 



incisor or tusk of elephant ; .B, 

 human incisor during development; 

 C, human incisor completely 

 formed ; D, human molar; E, molar 

 of ox ; 1, enamel ; 2, cement; 3, den- 

 tine ; 4, pulp-cavity 



