552 



'HE BROWN RAT. 



continual development. There are no canines, but a wide gap exists between the incisors 

 and the molars, which are nearly flat on their surfaces, and are well suited for grinding 

 the soft substances on which these animals feed. 



The structure of the chisel-edged incisor teeth is very wonderful, and may be easily 

 understood by inspecting the teeth of a rat, mouse, hare, or rabbit. 



As their teeth are continually worn out by the severe friction which they undergo 

 continually, there must needs be some provision for renewing their substance, or the 

 creature would soon die of starvation. In order to obviate this calamity, the base of 

 the incisor teeth pass deeply into the jaw-bone, where they are continually nourished by 

 a kind of pulpy substance from which the tooth is formed, and which adds fresh 

 material in proportion to the daily waste. Sometimes it happens that one of the incisor 

 teeth is broken or injured by some accident, so that it offers no resistance to its corre- 

 sponding tooth in the opposite jaw. The result of such an accident is very sad to the 

 sufferer, and is not unfrequently fatal in its termination. For the unopposed tooth, 

 being continually increased by fresh substance from behind, is gradually pushed forward 

 until it attains an enormous length, having sometimes been known to form a complete 

 circle. Examples of these malformed teeth are of tolerably frequent occurrence, and 

 specimens may be seen in almost every museum of comparative anatomy. 



Something more is needed for the well-being of the animal than the mere growth of 

 its teeth ; for unless their chisel-like edges were continually kept sharp, they would be 

 of little use for cutting their way through the hard substances which the Rodents are 

 in the habit of gnawing. This result is attained as follows : 



The enamel which covers the front face of the incisor teeth is much harder than 

 that which is laid upon the remaining surfaces, while the dentine which makes up the 

 solid mass of each tooth is also harder in front than behind. It is evident that when 

 these teeth are employed in their usual task, the softer enamel and dentine are worn 

 away very much more rapidly than the remainder of the teeth, so that the peculiar 

 chisel-edge of the teeth is continually preserved. Following perhaps unconsciously 

 the structure of these teeth, our cutlers have long been accustomed to make their 

 axes on the same principle, a thin plate of steel being inclosed within two thick plates 

 of iron, so that when the axe is used upon timber, the iron is continually worn away, 

 leaving the plate of steel to project, and form a sharp cutting edge. These teeth are 

 well represented in the engraving of the beaver's skull, to which the reader is referred. 



In many species of Rodents, the front faces of the incisor teeth are tinted with a 

 light orange red, or a reddish-brown, by means of a very thin layer of colored enamel. 

 In order to enable these teeth to perform their office rightly, the lower jaw is jointed 

 so as to slide backwards and forwards. 



The Rodent animals are widely spread over the entire globe, and are very numerous, 

 comprising nearly one-third of the mammalia. 



FEW animals are so well known or so thoroughly detested as the common BROWN 

 RAT, or NORWAY RAT, as it is sometimes erroneously called. 



It has spread itself over almost every portion of the globe, taking passages in 

 almost every ship that traverses the ocean, and landing on almost every shore which 

 the vessel may touch. Wherever they set their feet, the Brown Rats take up their 

 abode ; and, being singularly prolific animals, soon establish themselves in perpetuity. 

 They are marvellous exterminators of other " vermin," and permit none but themselves 

 to be in possession of the domain which they have chosen. It is a well-known fact 

 that they have driven away the black English Rat, and established themselves in its 

 place with wonderful rapidity, having been accidentally brought to our coast by 

 some vessel in which they had embarked, and found the English climate to suit 

 them as a permanent residence. Some of these animals were purposely introduced 

 into Jamaica, in order to extirpate the plantation Rats, which did such damage to the 

 growing crops. They soon drove away the original " vermin ; " but like the Saxons 

 when invited to help the Britons, or like the man who was requested to aid the horse 

 against the stag, were found to be more dangerous foes than the enemy whom they had 

 overcome. 



