Now, this might sound a
reasonable explanation for relatively simple
bones like the arm bones and the fingers
and so on. What about extremely complex bones
like this skull shown here? The way the body
goes about doing that is not by creating a
complex structure from the beginning, but
by starting off with many small precursors
each of which can take a particular shape,
and gradually, this will fuse together, locking
together to form complex structures like the
pelvis, or indeed, the skull itself. Sometimes,
neural tissue will actually induce bone to
form roundabout it. So we can see in the orbits
of the eye, the bony tissue around this is
induced by the developing optic vesicles from
the brain. And similarly, the complex bone
surrounding the semicircular canals of the
inner ear is induced by the canals themselves.
So it always fits to the underlying structures
because it’s induced by them. Finally, there
are mechanical forces which will act on adult
bone, and this can change the shape of the
bone significantly. Your skeleton is not a passive
structure. It’s actually a living structure.
And the cells in the bone are constantly
being replaced. So if mechanical pressure
is placed on a bone, the bone will respond
by changing its shape in consequence.
We tend to think of bones as being fixed because
we see them after they’re dead but in fact,
bone is almost a kind of very slow moving
fluid and will respond to pressures on it.
If, for instance, you break a leg, then that
changes the stresses in your hip, and your
hip structure will begin to adapt itself to
the new stress pattern.