NATURAL HISTORY. 113 



" By dremes, by chirking of dores, or craking of houses, by 

 gnawing of rattes, and swiche like manner of wretchednesse." 



CHAUCER. 



ORDER V. RODENTLA. 



348. Why are the animals of this order named rodents ? 

 Because of the peculiar formation of their teeth, and their habit 

 of gnawing the Latin verb rodo meaning to gnaw. 



349. In the order Rodentia the front teeth cut with a sharp edge. We know that 

 this is contrived in the tool of the carpenter, and we know that he must from time 

 to time apply his chisel to the grindstone. The front teeth of the beaver, the 

 porcupine, and the rat, are sharp, and yet not blunted by use ; the bone of the tooth 

 is the densest possible, consistent with the material ; but, were the whole tooth of 

 the same material, it would be ground down uniformly, and the original form of the 

 instrument would be lost. Accordingly, a different substance, the enamel, which 

 yields more slowly to friction than the bone, is, as it were, let in on the anterior 

 surface of the tooth. The consequence is, that the enamel stands up sharp and 

 exposed, so as to protect the bone of the tooth, and to give the surface which is 

 worn down a certain shape, suited to act like nippers. The friction and the 

 arrangement of the material of the tooth so far correspond, that the cutting form 

 is preserved, however much the surface may be worn down. 



350. Why are the jaws of rodents remarkably light ? 



Because strength in the jaws is not requisite, but rather light- 

 ness, because the action is continued gnawing, or rubbing, and not 

 powerful biting. This may be seen in the different ways in which 

 a monkey and a squirrel go about to get at the kernel of a nut. 

 The monkey takes it between his strong jaws, and cracks it at once 

 by one lusty gripe, while the squirrel nibbles away till it makes a 

 hole in the nut. 



351. Why do squirrels convey food to their mouths with two 

 paws while monkeys generally use but one? 



Because their fore-arms have but little motion in the elbow 

 joints, and their two bones are often united. The paw, therefore, 

 cannot turn, but has merely a hinge motion in this particular joint. 

 Many of them, however, have tolerably perfect clavicles, and others 

 have imperfect ones, so that they can bring the paw to the mouth 

 without any turning of the elbow ; but, as they cannot turn up the 

 sole of the paw, they cannot bring any substance to the mouth 

 except by holding it between the two paws sideways.* 

 * Partington's " Cyclopaedia." 



