- [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.