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Structures of the brain at the level of the frontal sinus.
Have you ever had a really bad headache where all the pain is just radiating from your forehead or perhaps you had an awfully stuffy nose and just felt a lot of pressure around your eyes? Well, these particular areas of the face, around the eyes and the forehead, are where we find some of our paranasal sinuses which when infected become inflamed and causes to experience symptoms just like those which I mentioned. Just as their name suggests, we find these sinuses around the nose so let's just zoom into the face of this image just here.
Highlighted in green are the sinuses. A sinus can refer to a number of things but in this instance, it refers to a space within a bone that is usually lined with a mucous membrane. The forehead, in particular, houses our frontal sinus and it is at this level that we will now explore several structures in cross-section.
In our tutorial today, we're going to be familiarizing ourselves with the bones, joints and parts of the brain and other various structures that can be examined at the level of the frontal sinus. Just to give you a quick overview of our tutorial today, we'll be discussing the frontal sinus, the skull bones and associated joints, the lobes of the brain and the important sulci and gyri that can be viewed at this level, and finishing off the tutorial, we will examine several muscles and blood vessels.
As we make our way through this tutorial, you'll notice that on each slide, you'll be presented with two images. The image on the left will always be an atlas illustration while the image on the right will be the cross-sectional image. In this example, the frontal sinus is highlighted in the atlas image and we can see that same structure highlighted here in the cross-sectional image on the right.
To orient ourselves with this tutorial, we're going to begin by talking about what and where the frontal bone is as this is where we'll also find the frontal sinus. Take a look at the atlas image on the slide. The frontal bone is highlighted in green. This bone contributes to our forehead and roughly the top anterior third of our cranial vault. Now, if we cut this image along the midsagittal plane, we end up with a half-skull that looks like this. The frontal bone has the frontal sinus within it.
Looking in at the cross-section on the right, we see the frontal bone highlighted in green. This area is at the anterior part of the skull but in cross-section appears as the top of the image. Thus, the bottom of this image will be the posterior portion of the skull. Keep that in mind as we make our way through this tutorial.
Now that we know where the frontal bone is, it's easy for us to find the frontal sinus, and we can see it highlighted in green in both of these images. In the atlas image, we can only see the frontal sinus in two dimensions whereas in actuality, the frontal sinus will be going into and out of the plane of this slide. Taking a look at the cross-section to the right, we can see this. The frontal sinus spans the length of the frontal bone.
Now that we have an understanding of the level we're looking at, let's begin with the structures that we find within or deep to the skull before moving our way outward towards the skin. Right beneath the bones of the skull, we find a thick layer of connective tissue called the dura mater, highlighted in green in both of these images. The dura matter is one of three connective tissue layers that act to suspend and protect the brain within the cranial vault. If we look at the cross-sectional image, we can see how thin the dura mater actually is and zooming in on the image, we can see how close in proximity the dura mater, highlighted in green, is to the overlying skull bones.
In the atlas image on the left, we can see how the dura mater extends downward between the two hemispheres of the brain. This structure is known as the cerebral falx. The cerebral falx is a continuation of the dura mater between the halves of the brain and it's quite easy to see in this atlas image just here. The cerebral falx has also been highlighted in the cross-sectional image to the right but due to its thinness, it's pretty hard to discern when it's not highlighted. Understanding that it can be found theoretically between the two halves of the brain in cross-section will allow you to easily locate it in the future.
There is one more structure that is closely linked to the dura mater which we'll look at next. In this image, we see the dura mater highlighted in green and from this image as well as in vivo, it is important to discern that it is made up of two layers. Following my cursor, we can see that the bottom layer descends the halves of the brain to become the cerebral falx while the top layer remains along the internal surface of the skull bones. Where these two layers separate is where we find the superior sagittal sinus. In this case, a sinus refers to a space that has been created by the layers of the dura mater, and not a sinus within a bone like the frontal sinus.
The superior sagittal sinus actually extends from the anterior to posterior along the midsagittal plane and can be easily appreciated in this image to the left of the slide. Due to where this cross-section is cut, we're able to see the anterior and posterior portions of the superior sagittal sinus highlighted in green on the image to the right. The superior sagittal sinus aids in the circulation and return of cerebrospinal fluid back into the venous system.
Several vessels from around the brain actually drain into the superior sagittal sinus – these are the superior cerebral veins. In both of these images, we see the superior cerebral veins highlighted in green. In the atlas image to the left, we can see that these veins are rather superficial to the brain tissue and drain into larger veins or sinuses. In this image, we can see them draining into the superior sagittal sinus. Thus, when we see a blood vessel that is just superficial to the brain tissue at this particular cross-sectional level, it is most likely a superior cerebral vein.
Now that we've covered the structures supporting the brain, let's now look at the lobes of the brain we can see at the frontal sinus level along with specific sulci and gyri associated with these lobes.
The first lobe we'll find is the frontal lobe, highlighted in green in both images. As you'll see, the lobes of the brain are named corresponding to which bone of the skull they lie beneath. In this case, the frontal lobe is found deep to the frontal bone. Several gyri in the frontal lobe play crucial roles in motor function along with cognition. We'll have a look at these specific areas of the frontal lobe next.
Within the frontal lobe, there are several gyri, one of which is called the superior frontal gyrus and can be seen highlighted in both images on the side. The superior frontal gyrus is the most medial gyrus within the frontal lobe, and we can see the boundaries of it in a cross-sectional image on the right. We also have to imagine that this gyrus is coming out of the page at us and traversing a course similar to the highlighted gyrus we see in the atlas image.
Another important gyrus that can be seen at this level and that we also find in the frontal lobe is the precentral gyrus. This particular gyrus houses the primary motor cortex and it is the structure that we find at the posterior boundary of the frontal lobe. The precentral gyrus is highlighted in green in both images and can be found roughly at the halfway point along each brain hemisphere from anterior to posterior. The reason for its name as the precentral gyrus is because it lies in front of an important sulcus that demarcates the frontal lobe from the parietal lobe.
The central sulcus is what marks the boundary between the frontal lobe and the parietal lobe which we'll be discussing shortly. The sulcus runs horizontally along the hemispheres so when we view a cross-sectional image at the fontal sinus level, we get this image, where we can see the central sulcus on the lateral side of the brain hemispheres just posterior to the precentral gyrus.
So if in front of the central sulcus we find the precentral gyrus, therefore, we must find the postcentral gyrus just behind or posterior to the central sulcus. The postcentral gyrus is found highlighted in green in both sides of these images and resides at the most anterior boundary of the parietal lobe. The primary function of this gyrus is to receive sensory input from the body as it houses the primary somatosensory cortex. The postcentral gyrus can also be used as a landmark to find the next structure - the marginal sulcus.
At the cross-sectional level of the frontal sinus, highlighted in green in this image, is the marginal sulcus. It is located on the most medial surface of the brain hemisphere. The marginal sulcus is found on the medial surface of the brain just posterior to most superior and medial aspects of the postcentral gyrus. This becomes more apparent if we look at the atlas image. The postcentral gyrus is located approximately here and directly posterior to this medial edge of the postcentral gyrus, we find the marginal sulcus highlighted in green. These two structures that we've been discussing all reside in a lobe of the brain located posterior to the frontal lobe.
The lobe in which the postcentral gyrus and the marginal sulcus can be found is within the parietal lobe, highlighted in green in both of these images. This lobe is found just posterior to the frontal lobe and central sulcus. In the cross-section at the frontal sinus level, the parietal lobe makes up the entirety of the brain tissue located at the posterior half of this image. This is due to the location at which the brain is sliced as this completely omits the other two lobes of the brain, leaving us just the frontal and parietal lobes in this cross-section.
Similar to its frontal counterpart, we find the parietal bone located superficial to or on top of the parietal lobe. Highlighted in green in both of these images, the parietal bones make up a large portion of the superior and the posterior portions of the skull.
We have two other structures that are associated with the cranial bones that we see in this cross-section – both of which are found at junctions between the bones of the skull. Connecting the frontal and parietal bones is the coronal suture, seen highlighted in green in both images. A suture is a type of fibrous joints that prevents movement between the articulating bones. Looking at the zoomed-in portions of the cross-section, we can now see how thin this joint is and also how little to no space is left between the frontal and parietal bones.
The other suture we see in this image occurs between the two parietal bones. In this atlas image, we can see both the parietal bones here. They articulate with one another at the sagittal suture, highlighted in green. As we can see from this image, the sagittal suture runs along the midsagittal plane giving it its unique name. Looking at the cross-section to the right, the sagittal suture is also highlighted in green. Like the coronal suture, it has a small structure that is most discernable in the zoomed-in portion here.
Now that we've covered the structures located most centrally, we'll explore the last few structures in the cross section located outside of the skull.
The first structure that we'll examine outside of the skull is the temporalis muscle - a muscle of mastication. The atlas image here shows the skull with only the temporalis muscle on it. From this slide, we can see that the temporalis lies on the lateral sides of the skull. If we cut this image at the frontal sinus level, we end up with this cross-section. We can see the temporalis muscle highlighted in green on this image and find it on the lateral sides of the cross-section above both the frontal bone and the parietal bones.
The last two structures are the frontalis muscle and the galea aponeurotica, highlighted in green in both images. The frontalis muscle acts to raise the eyebrows. If we move posteriorly from it, we run into the connective tissue aponeurosis that extends over and covers the skull - the galea aponeurotica. So, in cross-section, we can see that the tissue sitting directly atop of the skull bones and underneath the skin is the frontalis muscle anteriorly and the galea aponeurotica throughout the remainder.
This brings us to the end of our tutorial on the cross-sectional anatomy of the head at the frontal sinus level. Let's now quickly recap and point out the structures that we discussed.
We first started at the most anterior part of this image where we discussed the frontal sinus is here and their location within the frontal bone. We found the dura mater just beneath the bone and examined how it forms the cerebral falx and the superior sagittal sinus along the midline. The superior cerebral veins were also located just above the brain tissue, and moving into the actual brain matter, we saw the frontal lobe beneath the frontal bone and found the superior frontal gyrus and the precentral gyrus within the frontal lobe.
From the frontal lobe, we moved posteriorly and found the central sulcus acting as a boundary between the frontal lobe and the parietal lobe, and within the parietal lobe, we saw the postcentral gyrus which is the primary somatosensory cortex, and just posterior to this, we found the marginal sulcus. Above the parietal lobe, we found the parietal bone and at the junction of the two parietal bones, the sagittal suture was found. Moving anteriorly, we found the coronal suture articulating the parietal bone with the frontal bone.
Lastly, we examined two muscles outside of the skull - the frontalis muscle and galea aponeurotica - are anterior on top of the frontal bone and the temporalis muscle is found on the lateral sides of the skull.
This wraps up our tutorial on the cross-sectional anatomy of the head at their frontal sinus level. Thanks for joining me and happy studying!