Hey everyone! This is Nicole from Kenhub, and welcome to our tutorial on the cross-sectional anatomy at the level of the superior sagittal sinus.
The brain is a complex organ that requires a significant supply of oxygenated blood to the brain tissue in order to maintain proper function. It is supplied with this oxygen-rich blood via the internal carotid system and the vertebrobasilar system. Because of the limited amount of space within the cranial vault, however, venous drainage of the brain is carried out via a specialized system of channels or sinuses known as the dural venous sinuses. These sinuses which are situated between the periosteal and meningeal layers of the dura mater collect venous blood from the cerebral veins and eventually drain into the internal jugular veins. This system is an integral part of the vasculature of the brain and the superior sagittal sinus is the largest of the dural venous sinuses of the brain.
In our tutorial, today, we'll familiarize ourselves with the bones, vasculature and parts of the brain that can be examined at the level of the superior sagittal sinus which is the structure highlighted in green in the image.
Just to give you a quick overview of the tutorial today, we will be discussing the meninges of the brain, the superior sagittal sinus, the frontal and parietal lobes, the bones of the skull that can be seen at this level and to wrap it up, we'll look at the muscle and blood vessels visible in this cross-section. As we make our way through this tutorial, you'll notice that on each side, 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 superior sagittal sinus is highlighted in the atlas image and we can see the same structure highlighted here in the cross-sectional image on the right. To understand what the superior sagittal sinus is within the brain, we must first review what meninges are.
Highlighted in both of the images here are the meningeal layers found within the cranial vault. In the image on the right, we see the dura mater highlighted in green. The meninges are the connective tissue layers that surround the brain anchoring it in place and providing a space for which the brain can be suspended. This provides the delicate neural tissue with protection and shock absorption.
There are three meningeal layers that cover the brain. From the outermost layer to the innermost layer, these are the dura mater, the arachnoid mater, and finally, the pia mater. Even though the meninges of the brain are continuous with the meninges of the spinal cord, for the sake of this tutorial, we'll focus on the meninges of the brain. Let's take a look at each of these layers now.
The dura mater is the outermost meningeal layer and we can see it highlighted in green on this atlas image. Taking a closer look at this image, we can see the arrows pointing at the skull bones and just beneath this tissue, we see the dura mater. If we look at the midline of the image on the left, we can see the dura mater separating into layers, one of which stays along the internal surface of the skull bones. The other layer goes deep between the two halves of the brain. When these two layers separate from one another, they form a space, pointed out here, and this space is what we call a sinus. We will discuss more about sinuses shortly.
But let us move on to the second meningeal layer – the arachnoid mater. We find the arachnoid mater just below the dura mater. It is the middle meningeal layer and we can see it lying just underneath the dura in the illustrated image here on the left. The arachnoid mater is a bit harder to discern in the cross-sectional image, but if we look closely, we can see the arachnoid highlighted here. Notice how it does not follow the folds or gyri of the cerebral cortex but rather crosses over the sulci. Just below the arachnoid mater is a space known as the subarachnoid space. This houses the blood vessels that supply the brain tissue with oxygen and nutrients and just below this space, is the final meningeal layer.
The pia mater is the third meningeal layer and is in direct contact with the cortical tissue. We can see the incredibly thin pia mater highlighted in green in the image on the left. The pia mater is so thin that it is actually nearly impossible to remove it from the surface of the brain. Looking at the image on the right, we can see that the pia mater folds along with the surface of the brain.
Now that we've covered the meningeal layers, let us return to the dura mater and see how the tissue forms the superior sagittal sinus.
Recall from this image that the dura mater – seen here highlighted in green – is composed of two layers. As these layers move towards the midline, they separate forming a space called a sinus which is pointed out here. In fact, this image shows a coronal cut which is a cut in the coronal plane as you can now see in this image on the right here through a superior sagittal sinus. We can see the superior sagittal sinus highlighted now. To get an appreciation of the entirety of the superior sagittal sinus, we will look at this image.
The sinus traverses along the midsagittal plane on the inside of the cranial vault so when we end up looking at the head in cross-section at this particular level, we end up getting this view of the sinus. Highlighted in green, we can see parts of the superior sagittal sinus anteriorly and posteriorly. Flowing through the superior sagittal sinus will be cerebrospinal fluid and venous blood that is eventually returned to the venous system.
Now that we have an understanding of what the superior sagittal sinus is, let us move on to the other structures that can be seen when looking at a cross-section at the level of this sinus, starting with the lobes of the brain. The structures that are taking up the majority of the central portions of this image are two lobes of the brain. We can see in the atlas illustration on the left that the frontal lobe takes up the majority of the anterior and superior aspect of the brain.
Moving our gaze to the cross-section, we see the frontal lobe highlighted in green. It is the more anterior brain tissue here and plays several roles in motor control and cognition. If we look at the brain tissue just posterior to the frontal lobe, we run into the parietal lobe highlighted in green on both of these images. The parietal lobe houses the primary somatosensory cortex and also plays a role in visual processing.
Protecting the brain tissue, we find two bones of the skull at this cross-sectional level. The first bone we found is located superficial to the frontal lobe and this is the frontal bone highlighted in green in both images. Moving posterior to the frontal bone, we find the parietal bone. Similar to the frontal bone, the parietal bones are superficial to the parietal lobes of the brain. You can see the bones highlighted here in these images.
We have three other structures that are associated with these cranial bones. The first and second are found at the junctions between the bones of the skull while the third can be found within these skull bones. Connecting the frontal and parietal bones is a 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 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 the atlas image, we can see both of 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 is a small structure that is most discernible in the zoomed-in portion here.
The last structure associated with the cranial bones that we'll explore is the diploë. Highlighted in green on both these images, the diploë is the cancellous spongy bone found within the cranial bones. As we can see here, it's apparent that the diploë is found within the cranial bones.
Now that we've covered the structures located most centrally, we'll explore the last few structures in a cross-section located outside of the skull.
The first two structures we'll find are the frontalis muscle and the galea aponeurotica, also known as the epicranial aponeurosis seen 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 on top of the skull bones and underneath the skin is the frontalis muscle anteriorly and the galea aponeurotica throughout the remainder.
Lastly, we find the supraorbital vein at this level. This particular venous structure is located within the connective tissue layer sitting atop the frontalis muscle anteriorly. If we look at the atlas image on the left, we can see the vessel highlighted. It is responsible for venous drainage of the forehead primarily. Now looking at the cross-section image on the right, the supraorbital veins are found within the connective tissue layer just under the skin and located superficial to the frontalis muscle.
This brings us to the end of our tutorial on cross-sectional anatomy at the level of the superior sagittal sinus. To recap what has been discussed today, we will use this image and briefly label each part discussed. First, we began with looking at the three types of meninges – the outermost dura mater, the middle arachnoid mater, and lastly, the pia mater, located on the surface of the brain. We then discussed the superior sagittal sinus and how it is derived from the dura mater.
From there, we looked at the bones and sutures of the skull – the frontal bone and the parietal bone and the coronal suture joining the frontal and parietal bones and the sagittal suture joining the two parietal bones. After exploring the bony structures at this level, we then saw cancellous spongy bone called diploë within the bones of the skull, the frontal lobe and parietal lobes underneath their respective skull bones, the frontalis muscle and galea aponeurotica directly on top of the cranial vault and lastly, the supraorbital veins found within the connective tissue underlying the skin.
That wraps up our tutorial on cross-sectional anatomy at the level of the superior sagittal sinus. Thanks for joining me!