RODENTIA; 
OR GNAWING ANIMALS. 
HE RopeEnts, or gnawing animals, derive their name from the peculiar structure 
of their teeth, which are specially fitted for gnawing their way through hard sub- 
stances. The jaws of the Rodents are heavily made, and very large in propor- 
tion to the head, their size being not only needful for the support of the 
gnawing teeth, but for their 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 under- 
stood 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, 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. Some- 
times it happens that one of the incisor teeth is broken or injured by some accident, so that it 
offers no resistance to its corresponding 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 uncorsciously—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. 
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 back- 
wards and forwards. 
The Rodent animals are widely spread over the entire globe, and are very numerous, 
comprising nearly one-third of the mammalia. 
The magnificent work on ‘‘The North American Rodents,” by Dr. Coues and J. A. Allen, 
