- [Instructor] All right so we're going to be talking about
the microscopic structure of bone
and in particular, we're going to be talking about
what is called the haversian system.
Now let's take a piece of bone and cut it in half
and see what it looks like on the inside.
Here, we basically have a cross section of a piece of bone.
So let's take a look inside.
Now the inner most portion of this bone
is made up of, what is called, spongy bone,
which is otherwise known as cancellous or trabecular bone.
And here you can see this arrow pointing to
the trabecula of spongy bone.
So it's no surprise that if you take a look
at spongy bone, that it looks pretty much
a lot like a sponge and in fact, because of all
these various trabeculae or cavities,
the surface area of spongy bone is 10 times
that of the outer layer of compact bone.
So basically spongy bone is just this porous network
of spikes surrounding the innermost portion of
bone marrow.
And the overall effect of this spongy network
in the center of the bone is
that of making the bone lighter.
Now if you look at the periphery of the bone,
you have what is the harder, denser layer
that surrounds the spongy bone.
That is called compact bone.
Compact bone, it's no surprise, that it's, well,
more compact than spongy bone.
It has fewer gaps and spaces,
but what really makes compact bone different
from spongy bone, is that it has a specific type
of organization made up of these osteons,
these repeating functional units.
Here's a blown up view of an osteon.
Another word for these osteons is the haversian system.
So let's talk more about this haversian system.
So each of these osteons looks like of like a cylinder
and it has multiple concentric layers of bone,
or sheets really, that wrap around each other
to form this osteon.
Each of these layers is called a lamellae.
In the center of these layers is a canal
called the haversian canal, or central canal.
In this canal travels blood vessels, lymph vessels
and nerves as well.
Now in between these sheets of lamellae are these
tiny channels that are called canaliculi,
which you can kind of see here.
They branch out from the central haversian canal
to these empty spaces that are called lacunae.
Whenever you see the word lacuna or lacunae,
you should think empty space.
So each of these lacunae is really just an empty space
or osteocytes or bone cells.
These osteocytes have these long cellular processes
that branch through the canaliculi
to contact other osteocytes via gap junctions
which allow these cells to communicate with each other
and exchange nutrients and signals with each other.
Finally, you have these volkmann canals
which are canals that run perpendicular
to the haversian canals.
And these connect osteons to one another
and also, as you can see, carry their own set
of small blood vessels.
Let's not forget that the very outer most superficial
layer of bone is called the periosteum.
Peri meaning around or surrounding
and so that's the layer of bone that is on the outermost
that you can actually see with the naked eye.