RODENTS 77 



action. A pair of these is present in each jaw. By allowing the animal 

 to use its teeth on a slab of wax, the impressions will be found exactly 

 to resemble those made by a chisel, or, rather, a fluting plane. We are, in 

 fact, really dealing with small chisels or a planing tool. 



(a) If it is desired to pick small pieces from the flat surface of a 

 board with a chisel, it is usual to place the tool at as acute an angle as 

 possible (why?), and deal the handle a heavy blow, when a splinter will 

 be detached. For the same reason, also, the incisors of the squirrel should 

 form as acute an angle as possible with the object to be gnawed. This 

 is effected by their being very strongly curved. 



(b) The front-teeth, having to perform severe work, must be set deeply 

 and firmly in the jaws ; and, in fact, the parts inserted in the jaws are 

 very long, far longer than those which 



project ; in the case of the lower in- 

 cisors they extend back behind the 

 molar teeth. 



(c) By constant use, even the 

 strongest steel instrument is worn 

 away. In the same way, the incisors 

 of the squirrel wear away in time, and 



would finally become SO short as to be SKULL OF SQUIRREL. (Natural size.) 



useless for their purpose. Accordingly, 



they are constantly renewed at tlieir inner ends in proportion as they are 

 worn away. The portions of the teeth embedded in the jaws are 

 constantly being pushed outwards from their curved sockets ; consequently 

 their curvature must be uniform throughout, forming, in fact, a portion 

 of the circumference of a circle. (Why can they not be straight ?) 



(d) If the incisors, like those of man, for instance, were covered all 

 round by enamel, they would soon wear away in an equal degree against 

 hard objects, i.e., become blunt. Here again the resemblance to a chisel 

 -or plane is complete, for just as, in these instruments, only the cutting- 

 blade consists of steel, so in the rodent teeth only the outer side is coated 

 with a thick plate of enamel ; all the rest of the tooth consists of softer 

 osseous material. By the gnawing of hard objects this osseous material 

 is worn off more easily than the enamel layer of the cutting surface ; the 

 latter consequently remains constantly sharp, and the tooth itself 

 retains its chisel-like form. 



2. If the incisors were close to the other teeth, as in man, they would 

 not be fitted for separating a splinter or for penetrating into hard 

 substances, as in opening a hazel-nut or tearing off the scale of a pine- 

 cone. If they were more numerous, the cutting edge would be too broad. 

 ..(A broad chisel does not penetrate into wood so easily as a narrow one.) 



