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.
This structure is common to all the members of the group to which the
water-rat belongs.
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