328 STRUCTURE AND ADAPTATION OF TEETH 



northern Indians to cut bone, and to fashion their 

 horn-tipped spears, &c., till it was siiperseded by 

 the introduction of iron, when tlie beaver-tooth 

 was supplanted by the English file. 



The power of these natural tools is well de- 

 scribed by Lewis and Clarke, who saw their effects 

 on the banks of the Missouri. " The ravages of 

 the beaver," say they, "are very apparent; in 

 one place the timber was entirely penetrated for a 

 space of three acres in front on the river and one 

 in depth, and great part of it removed, although 

 the trees were in large quantities, and some of 

 them as thick as a man's thigh." 



Sir John Richardson thus speaks of this part 

 of their operations : '' When the beaver cuts down 

 a tree, it gnaws it all round, cutting it, however, 

 somewhat higher on one side than the other, by 

 which the direction of its fall is determined. The 

 stump is conical, and of such a height as a beaver 

 sitting on its hind quarters could reach." 



In the specimen upon the table, we see a most 

 interesting example of the effect of the beaver's 

 teeth upon the stem of a tree. 



In teeth, the growth of which is limited, as in 

 the human jaw, the internal formative pulp gra- 

 dually diminishes in bulk after the crown is com- 

 pleted, and forms by its progressive calcification 

 one or more roots, which are only perforated by 

 minute foramina, through which the blood-vessels 

 and nerves pass to maintain vitality, and the 



