552 THE 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 
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 imjured 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 wellbeing of the animal than the mere growth of 
its teeth; for unless their chisellike 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 wpon 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 coloured 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, ad landing on almost ev ery 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 im 
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 moredangerous foes than the enemy whom they had overcome. 
