138 



THE HUMAN BOD7. 



Once a slight cavity has been formed, the process of 

 decay is apt to go on very fast ; first, because the ex- 



FIG. 46.- The upper surface of the tongue. 1, 2, circumvallate papillae ; 3, fun- 

 giform papillae; 4, filiform papillae; 6, mucous glands. 



posed deeper layer of the tooth is more easily dissolved 

 than its natural surface; and, second, because the little 



