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 were eating, a
movement which gave rise to the idea that they chewed the cud.

It is worthy of remark that other animals, which, though not rodents,
need to possess chisel-edged incisor teeth, have a similar habit. Such
is the hippopotamus, and such is the hyrax, the remarkable rock-haunting
animal, which in the authorised translation of the Scriptures is called
the "coney," and which in the Revised Version is allowed in the margin
to retain its Hebrew name, "shaphan."

The enamel also has an important part to play in the structure of the
molar teeth. Each tooth is surrounded with the enamel plate, which is so
intricately folded that the tooth looks as if it were made of a series
of enamel triangles, each enclosing the tooth matter.