Now, let’s take a look at each of the three meningeal layers. The take home message here
is for you to understand the basic characteristics of these meningeal layers. We’re going to
start externally and work our way internally. So, that means we’ll look at the dura mater
first and its characteristics. The dura mater is shown in green. In the cranial meningeal dura,
the dura matter is going to be bilaminar. It will be a bilaminar, fibrous membrane. This is in
contrast to the spinal dura mater which contains or is consisted of just one layer. Thus,
it is unilaminar. So, if the cranial dura mater is bilaminar, what are the two layers that
constitute the cranial dura mater? First is the outermost layer or the external most layer
of the dura is going to be firmly attached to the inner part of the skull. That will be referred to
as the periosteal layer. Then the internal or innermost lamina or layer of the dura will be the
meningeal layer. These layers are fused with the exception of the dural venous sinuses
where they will separate. That separation for example is shown here. This is the superior
sagittal sinus. Right here attached to the inner part of the skull is the periosteal layer of the
dura mater. Then this layer or lamina of the dura represents the meningeal layer.
Another area where they are not fused is where you have infoldings. So here, we have
an infolding of the meningeal layers from either side of the skull. They are diving deep into the
fissure that separates the two cerebral hemispheres. There are other examples of these
types of infoldings that we’ll explore. So, what are the dural infoldings or reflections?
We’ll walk through a few of them. One of them is not going to be represented on this
particular slide. One will be represented in greater detail on a subsequent slide. But if we
take a look at what we can appreciate in this slide, we have a major dural infolding. We see
this infolding right in through here. This is the infolding that separates the cerebral
hemispheres right from left. This is the falx cerebri. Another infolding and we see the cut
edge of that infolding here and then we see the medial margin of it over in through here.
This is going to form a tent or a roof over the cerebellum. One of the cerebellar hemispheres
rest down in this portion of the posterior cranial fossa and then the opposite hemisphere
would rest or reside in the opposite area of the skull. Then the tentorium cerebelli will form
a roof or a tent over those cerebellar hemispheres. A third type of infolding is the
falx cerebelli. This is not a very significant infolding but it would attach to the tentorium
cerebelli in the midline and then dive down between the cerebellar hemispheres but only
for a short distance, not shown here on this slide, however. The fourth example of a dural
infolding is that of the diaphragma sellae. That is seen in detail in the next slide.
The diaphragma sellae which is shown here and on the opposite side over here forms a roof
over the pituitary gland that we see here residing within the sella turcica. If we look in the
midline region of the diaphragma sellae, we see an aperture or an opening for the passage
of the infundibulum that we see extending superiorly in this view. This will connect to the
diencephalon. The last consideration here is the relationship of the diaphragma sellae
with the pituitary gland as it pertains to a pituitary gland tumor. This diaphragma sellae
is fairly fibrous, tough, so it does provide for resistance. So that if a tumor is forming,
it’s less likely to move upwards, though it can. It’s more likely to take a path of least
resistance and the tumor will expand laterally within the cavernous sinuses that we see here.
Now, let’s take a look at the arachnoid mater or simply arachnoid and its features.
The arachnoid mater is a thin membrane compared to the dura mater. We see that membrane
right in through here. If we come over here to the right side of the image, we will see
the arachnoid mater extend into the superior sagittal sinus. When the arachnoid mater
extends into these types of venous channels between the periosteal dura mater and the
meningeal dura mater, these form arachnoid granulations which help to move cerebrospinal
fluid into the venous channels. The arachnoid also will have trabeculae or extensions
that extend into the subarachnoid space which is the space here and then attached
to the pia mater that is adherent or lining the surface of the cerebrum. It is not shown.
These are not shown, however, in this particular image. Again, we already highlighted
the arachnoid granulations. Again, these are dilatations or expansions of the arachnoid
where you have venous drainage or sinuses. Another feature of the arachnoid is that
it is avascular. Lastly, the arachnoid mater is simply held against the inner dura mater
which would be the meningeal layer by the force of cerebrospinal fluid pressure. In other words,
it’s not adherent to, it’s just pushed up by this pressure. The innermost meningeal layer
is the pia mater. Some of its features would include the fact that this is the thinnest
and the most delicate of the meningeal layers. Again, we see it here as this thin, green line.
It will follow the contours of the cerebral cortex, the sulci, and the gyri. So here it lies
over a gyrus and here it penetrates and dives into a sulcus. The pia mater is adherent
to the surface of the brain. When you visualize this, it will confer a shiny appearance
to the surface of the brain.