TEETH OF THE RODENT MAMMALIA. 2(>9 



practicable, and causing death by starvation. In the 

 Museum of the College of Surgeons there is a lower jaw 

 of a beaver, in which the scalpriform incisor has, by un- • 

 checked growth, described a complete circle ; the point ? 

 has pierced the masseter muscle, entered the back of the/ 

 mouth, and terminated close to the bottom of the socket! 

 containing its own hollow root. 



The difference in the diet of the rodent quadrupeds has 

 been alluded to ; there is a corresponding difference in the 

 mode of implantation of their molar teeth. Those which 

 subsist on mixed food, and which, like the rats, betray a 

 tendency to carnivorous habits, or which subsist, like 

 squirrels, on the softer and more nutritious vegetable sub- 

 stances, as the kernels of nuts, suffer less rapid abrasion 

 of the grinding teeth ; a less depth of crown is, therefore, 

 needed to perform the office of mastication during the 

 brief period of life allotted to these active little mammals ; 

 and, as the economy of nature is manifested in the smallest 

 particulars as well as in her grandest operations, no more 

 dental substance is developed after the crown is formed 

 than is requisite for the firm fixation of the tooth in the 

 jaw. 



The rodents that exclusively subsist on vegetable sub- 

 stances, especially of the coarser and less nutritious kinds, 

 as herbage, foliage, and the bark and wood of trees, wear 

 away more rapidly the grinding surface of the molar 

 teeth; the crowns are, therefore, larger, and their growth 

 continues by a reproduction of the formative matrix at 

 their base in proportion as its calcified constituents, form- 

 ing >the working part of the tooth, are worn away. So 

 long as this reproductive force is active, the molar tooth 

 is implanted, like the incisor, by a long, undivided con- 

 tinuation of the crown. These rootless and perpetual^ 



