The Girls Own Paper, Vol. VIII, No. 355, October 16, 1886 | Page 4

for the other.
The teeth, too, are quite different.
Instead of being white, like those of the rat, the incisor teeth are orange-yellow, like those of the beaver. Indeed, the water-rat possesses so many beaver-like characteristics, that it was ranked near the beaver in the systematic lists.
Now, however, the Voles, as these creatures ought rightly to be called, are thought to be of sufficient importance to be placed by themselves, and separated from the true beavers.
The voles constitute quite a large group of rodents, including several animals which are popularly ranked among the mice.
One very remarkable characteristic of the voles is the structure of their molar teeth.
Being rodents, they can have but two incisor teeth in each jaw, these teeth being rootless, and so set in their sockets that they are incessantly worn away in front, and as incessantly grow from the base, take the curved form of their sockets, and act much like shears which have the inestimable property of self-sharpening when blunted, and self-renewal when chipped or actually broken off by coming against any hard substance. Were the teeth to be without this power, the animal would run a great risk of dying from hunger, the injured tooth not being able either to do its own work, or to aid its companion of the opposite jaw. Either tooth alone would be as useless as a single blade of a pair of scissors.
There is another notable characteristic of these incisor teeth. If you will examine the incisors of any rodent, whether it be a rat, a mouse, a rabbit, or a beaver, you will see that the tips are "bevelled" off just like the edge of a chisel. This shape is absolutely necessary to keep the tooth in working order. How is this object to be attained?
In the solution of this problem we may see one of the many links which connect art and nature.
Should our readers know anything of carpentering, let them examine the structure of their chisels. They are not made wholly of hard steel, as in that case they would be liable to snap, just as does the blade of a foil when undue pressure is brought to bear upon it. Moreover, the operation of sharpening would be extremely difficult.
So the blade of the chisel is merely faced with a thin plate of hardened steel, the remainder being of softer material.
Now, it is not at all likely that the unknown inventor of the modern chisel was aware of the analogy between art and nature, and would probably have been very much surprised if anyone had stated that he had borrowed his idea from the incisor teeth of the water-rat.
Yet he might have done so, for these teeth are almost wholly formed of ordinary tooth matter, and are faced with a thin plate of hard enamel, which exactly corresponds with the hardened steel facing of a chisel.
Any of my readers who possess skates will find, on examination, that the greater part of the blade is, in reality, soft iron, the steel, which comes upon the ice, being scarcely a fifth of an inch in length. The hardened steel allows the blade to take the necessary edge, while the soft iron preserves the steel from snapping.
Should the skate have been neglected and allowed to become a little rusty, the line of demarcation between the steel and the iron can be distinctly seen. Similarly, in the beaver and the water-rat, the orange-yellow colour of the enamel facing causes it to be easily distinguished from the rest of the tooth. In most of the rodents the enamel is white, and the line of demarcation is scarcely visible.
Now we have to treat of a question of mechanics.
If two substances of different degrees of hardness be subjected to the same amount of friction, it follows that the softer will be worn away long before the harder. It is owing to this principle that the edges of the rodent teeth preserve their chisel-like form. Being continually employed in nibbling, the softer backing of the teeth is rapidly worn away, while the hard plate of enamel upon the front of the tooth is but slightly worn, the result being the bevelled shape which is so characteristic of these teeth.
As all know, who have kept rabbits or white mice, the animals are always engaged in gnawing anything which will yield to their teeth, and unless the edges of their feeding troughs be protected by metal, will nibble them to pieces in a few days. Indeed, so strong is this instinct, that the health of the animals is greatly improved by putting pieces of wood into their cages, merely for the purpose of allowing them to exercise their chisel-edged teeth. Even when they have nothing to gnaw, the animals will move their jaws incessantly, just as if they