 Well, it's time to talk about cerebrospinal fluid and the ventricular system. So let's get started. I'm holding a cast of a real human ventricular system. So these are the ventricles in their actual size, and I want to show you that they have different parts and different names, and then we're going to look at them inside a brain. So this is the large lateral ventricle, which is inside the hemisphere. This is the right one, and there's a mirror image on the other side, the left ventricle. Theoretically, they should be called ventricles one and two, but we don't name them that way. And then in the midline, you can see a large flat area like this. It's very narrow. It has little dips and holes in it, and that's the third ventricle. The third ventricle is also between the hemispheres and also between that part of the deep brain structures called the thalamus. So we have lateral ventricle, we have the third ventricle, and the lateral ventricle drains into the third ventricle on each side through a narrow channel called the inter-ventricular foramen. And then as I rotate this, you can start to see that this third ventricle connects through a narrow channel called the aqueduct, or the aqueduct of sylvius, but we don't use those proper names any longer, through the aqueduct to a large midline space, sort of triangular or diamond shape, maybe better seen from the back side. So here is the third ventricle in the midline, very narrow. The narrow aqueduct that connects it with the area called the fourth ventricle, which is rather diamond or rhomboid shaped. So we have fourth ventricle, aqueduct, third ventricle, and the two lateral ventricles on either side. Let's look at a three-dimensional reconstruction of the ventricular system made by the University of Washington. In this translucent illustration and animation of the ventricular system, you can see the frontal, occipital, and temporal horns. Furthermore, you can see the third ventricle from the lateral view, and in the anterior and posterior view, you can see the narrow slit-like third ventricle communicating with the aqueduct and fourth ventricle. Note that the system ends in the caudal medulla. This animation, with the left hemisphere removed, better demonstrates the relationship of the third ventricle to the thalamus in pink and the hypothalamus in yellow. When you see the thalamus and hypothalamus, you know that the medial surface of those structures is formed by the third ventricle. It's narrow slit then connects through the aqueduct the fourth ventricle, which you can see here in respect and relationship to the cerebellum. This animation is showing the lateral ventricles, but both hemispheres have been removed. So the relationship of the ventricular system to the thalamus and brain stem can be observed. The thalamus is connected to the rest of the brain stem by the cerebral peduncles in white. And then you can see the pons and the medulla with the cerebellum on the posterior side. Our next animation is going to show you the rest of the brain stem with the cerebellum removed. Here we have the cerebellum, brain stem and thalamus, and we can see through the brain stem, particularly if we remove the cerebellum and look down on the fourth ventricle. In the lateral view, you can easily see the third ventricle between the two thalami and hypothalami. And as it rotates, you can see the narrow aqueduct connecting with the fourth ventricle. The third ventricle here has a hole in it. This may or may not be present in different people and is not relevant. I have here a diagram of the mid-sagittal surface of the brain with the frontal lobe to your left and the occipital lobe to your right. And this gray space in here represents the ventricular system and the gray space on the outside of the hemisphere and brain stem represents the subarachnoid space. And I want to discuss for a moment the production of cerebral spinal fluid. Cerebral spinal fluid is not just an ultra-filtrate of plasma. It is quite different. It is secreted by epithelial cells in the coroeplexus, which are in close opposition to brain capillaries. It is low or has no cells in it. It is lower in protein and glucose and is secreted by an active process. Material or substances in the blood, most of them, cannot pass through these coroeplexus cells into the cerebral spinal fluid. And thus we have something called the blood CSF barrier. If I put blue dye in the blood, it will not come out in the cerebral spinal fluid. This is very protective of the brain and yet allows for substances in the cerebral spinal fluid to be actively circulated without harming the brain. So what happens to this cerebral spinal fluid? We talk about its circulation with these arrows here, but it really doesn't circulate. There is no flow in a purposeful direction. Rather, it pulsates a bit with the heartbeat and the cerebral spinal fluid is primarily made in the lateral ventricles of each hemisphere, comes down through the interventricular fraymen to the narrow slit-like third ventricle, which is between the tooth alomai, continues down through the cerebral aqueduct at the level of the midbrain with the tectum and the peduncles, and at the level of the pons medulla, it forms the fourth ventricle, which bulges up into the cerebellum. It leaves the ventricular system through a single median aperture into a large well or cistern called the cisterna magna, and this is subarachnoid space. There are two lateral apertures that go out into the lateral area, but we can't see them. And so CSF can then circulate around the cord, descending this way, or up and over the cerebellum, and around and over the hemispheres. How do we get rid of it? Well, at least 75 percent of it we remove through pressure gradient, higher in the CSF than in the blue venous blood here, which is in the dural sinus between the layers of the falx, the superior sagittal sinus. So let's look at that. We have a superior sagittal sinus, a straight sinus. They kind of have a confluence here, and the venous blood drains into the internal jugular vein, because we do not have lymphatics in the brain. So interstitial fluid from the cells in the brain, a small amount of it actually enters into the CSF, and can either be removed through these arachnoid granulations that project into the dural sinus, or some of them, some of the fluid can be removed down around the spinal cord in the spinal nerve sleeves. Let's look at a gross specimen. Here we have a left hemisphere, mid-sagittal section, and what I want you to see is that there is this curtain, or septum, that keeps the lateral ventricle, which is behind it, separated from the rest of the ventricular system. This is the corpus callosum, uniting the hemispheres through axonal connections. This is another band of fibers called the fornix, and this is the surface of the thalamus, which is also the lateral wall of the third ventricle. This hole here is that inner ventricular foramen that goes out into the lateral ventricle. Very, very nice, and then here is the third ventricle. I'm going to move to another half-brain because I want it to be oriented in the same direction as the radiologists view the brain in mid-sagittal section. Now we are looking at the right hemisphere, and in this brain when it was sectioned, the septum, or the curtain that separated the lateral ventricle, was partially cut, which allows us to look into the large lateral ventricle out here to the side. But we can still see the corpus callosum. We can still see a band of fibers called the fornix. It's got slightly sectioned here, coming down, and we can very nicely see sort of the shiny wall of the third ventricle. The fluid and the lateral ventricle passes through the inner ventricular foramen and into this third ventricular space. Here are some landmarks around the third ventricle. There's the anterior commissure. There is the optic chiasm. This is a bit of the anterior cerebral artery. There is the floor here, which is the hypothalamus with the mammillary body, and there is the posterior commissure, the attached pineal gland, and on the top, on the surface here, is a little bit of corey plexus that forms the roof of the third ventricle. So now you've seen the third ventricle. Nicely visible here is the aqueduct. The aqueduct is always associated with the midbrain, just as the third ventricle is associated with the thalamus. The aqueduct is associated with the midbrain. Two midbrain structures on the dorsal surface are the two caliculi, or the tectum above, or the roof above, the aqueduct. On its ventral surface, the midbrain has the cerebral peduncle. The level of the ponds and the medulla, we have the fourth ventricle. So the aqueduct opens into the fourth ventricle, which pokes up like a tent into the base of the cerebellum here. And here's a bit of corey plexus that's in the fourth ventricle. Now that median aperture, you can't see right here, but out here to the side are the lateral apertures where the cerebral spinal fluid goes. Now this surface here was covered with Pia that you can see here and arachnoid. And between the two is the subarachnoid space. So since this system only holds about 25 milliliters, and the total content of the cerebral spinal fluid at any one time in a normal person is 125 milliliters, that means about 75 percent or 100 milliliters is circulating in this subarachnoid space and out and down and around the cord and the medulla. So there's more fluid in the subarachnoid space. When the brain is in the cranial cavity, it is really floating in cerebral spinal fluid. That fluid serves several purposes. It cushions the brain from trauma because the weight of the brain being 1300 grams, when it's floating in that CSF, it only weighs 25 grams. So with the cushion, it also removes metabolites and brings nutrients to the nervous tissue. And it also influences cerebral blood flow. The blood is flowing and if the increased pressure of the CSF goes up, that increased pressure reduces the amount of cerebral circulation against the pressure from the cerebral spinal fluid. Let's orient ourselves and look where that cerebral spinal fluid goes after it leaves the median and lateral apertures. It circulates in this space here, spanned by these bits of green arachnoid membrane. This is the arachnoid and it spans over to the peel surface, which is right over the gray cerebral cortex or gray matter. And in that subarachnoid space, run the major vessels that enter into the brain. So this is an important space and the CSF dips down between the gyri. But what I want to point out to you are these bumps or granulations of arachnoid tissue that project into this blue midline sinus, venous sinus filled with venous blood, the superior sagittal sinus, which is located in a fold of dura that comes down between the cerebral hemispheres called the fox. So if this is the dura surface and this is the fox that comes down in the midline, this would be one hemisphere, the other hemisphere, and these granulations poke into there and there is a sufficient pressure gradient of CSF, which is normally slightly higher than the venous pressure and the fluid is transported or moved, so to speak, mostly under pressure, but with some active component and at least 75% of the cerebral spinal fluid is removed into the dural sinuses and drainage down into the jugular vein. Back to our model again, because it nicely shows you that superior sagittal sinus that runs in the fox between the hemispheres, there's another sinus, an inferior sagittal sinus that connects to the straight sinus back to the confluence. All of these internal veins are draining back through the transverse sinus and down into the jugular vein. But next I want to continue with the journey of the cerebral spinal fluid that dead ends, so to speak, inside the ventricular system at the base of the medulla here. And remember it goes out through the apertures, either the median one or the two lateral ones, into this subarachnoid space. And we want to continue down with this subarachnoid space, down around the spinal cord and the cauda equina, because I want to show you those relationships on a gross brain. So now it's time to look at our ventricular system as if you were a pathologist or a radiologist. So starting with our mid sagittal brain, cutting it in this direction will give us coronal or frontal sections. So I have for you a coronal or frontal section, gross, taken from a normal brain. And out here you can see the two lateral ventricles. And if you come in a little closer, you can see that there is a bit of corey plexus here. We're not right at the level of the interventricular foramen, but it would be approximately here. We can pretend that these two ventricles, lateral ventricles, empty into the midline slit-like third ventricle. So if this is third ventricle, this has to be thalamus. Thalamus on one side and thalamus on the other side. If you see thalamus, you know you've got third ventricle. If you see third ventricle, you have to recognize that you have thalamus to the side. And then laterally, the large internal capsule, band of white matter, of ascending and descending fibers. In addition, out here there is another part of the ventricular system, that part which is in the temporal lobe. This is one temporal lobe, the lateral fissure, the insular cortex, the other temporal lobe, and insular cortex. And this is that part of the lateral ventricle in the temporal lobe called the temporal horn, or the inferior horn. We were just looking at a frontal section taken through the thalamus, which is right here. And now I want to show you a couple of sections taken a little more anteriorly, or rostrally, through the basal ganglia. These two sections are through the basal ganglia. They're more visible and easier to see on this one. And this section from a different brain, obviously, doesn't have the temporal lobe on it. But what I want to point out to you is the difference in the ventricular size of the anterior horn or frontal horn of the ventricles when you compare one brain with the other. Here they're narrower, and even in a younger person they would be even more slit-like and curved. But down here, because of loss of brain tissue with aging, the ventricles have become enlarged. Now let's look at the ventricular system, frontal section by our coronal section, in a radiologic specimen. This frontal, or coronal section, is an MRI showing the bright white cerebral spinal fluid. It clearly delineates the gyri and sulci, dipping between them, and even entering the lateral fissure and outlining the surface of the insular cortex. The temporal lobes are clearly seen, and between them, at the base of the brain, is one of the cisterns filled with cerebral spinal fluid. Deep in the hemisphere on each side are the two lateral ventricles separated by the septum palusitum. And just below them in the midline is the thin, narrow slit-like third ventricle. On either side of this slit is the hypothalamus and a bit of the thalamus. Now we're going to look at the brain in a different plane. We're going to look at it in an axial or horizontal plane. And I want to show you what the ventricular system looks like in that plane. Here you can see the slit-like anterior horn with the interventricular foramen connecting into the third ventricle, slit-like, with the thalamus on either side. So here we have the anterior horn, we have the third ventricle, and then curving around, the horn curves around, and we have the occipital horn projecting back into the occipital lobe. Here again is the occipital lobe, and this is the occipital horn of the lateral ventricle. And then projecting down in this direction, we would be going down into the inferior or temporal horns. Now let's look at a radiologic image of the same level. This axial or horizontal MRI has the cerebral spinal fluid bright white as in the previous radiograph. And it circulates in the subarachnoid space outlining the gyri and entering also into the lateral fissure. Deep in the frontal lobe you can see the anterior horn or frontal horn on either side. It's very narrow and slit-like compared to the gross sections you saw because of the lack of atrophy and the younger age of this patient. Recall that the two lateral ventricles here are going to communicate through the interventricular foramen with the third ventricle, which extends from here to here. And it's narrow and slit-like. And the reason you can't see the bright white line here is because the two sides of the thalamus on either side, one side and the other, are abutting or touching each other, present in some brains, not in others, and not significant. In addition to the thalamus out here to the side, the dark white matter, remember dark axons have a different signal from the bright cerebral spinal fluid. This is the internal capsule that separates the thalamus from the basal ganglia. Returning to our mid-sagittal section now, we can review the ventricular system on an MRI of a mid-sagittal section. This sagittal MRI has the cerebral spinal fluid bright white and illuminates in the sulci the gyri in between. The occipital pole is on the right and the frontal pole is on the left, as is the radiologic convention. The dark corpus callosum is clearly visible in the midline because myelin has a different signal density. Beneath the corpus callosum, you can see the lateral ventricle and you can imagine the connection into the third ventricle through the interventricular foramen. And I am outlining the third ventricle here, which is a little bit difficult to see because of the mass intermedia or that thalamic adhesion in the midline. But fluid fills this entire region at some point and connects then with the aqueduct, really not visible here, but imaginable, down into the fourth ventricle, which sits between the cerebellum and the pons and continues down to the bottom of the medulla where it terminates. At this point there is the median aperture, which connects with this large cistern or the cisterna magna and then there are the two lateral cisterns. And so cerebral spinal fluid flows into this subarachnoid space, can flow over the entire surface of the medulla, of the pons, the base of the brain. These large cisterns at the base of the brain, over the midbrain here and beneath the cerebellum, these cisterns are easily recognized and are important radiologic landmarks. The medulla continues down and at the level of the junction between the cord and medulla, you have the foramen magnum and cord beneath it at that level. So let's summarize about the function of the cerebral spinal fluid. Remember that the cerebral spinal fluid is circulating underneath this thin glistening arachnoid membrane in the subarachnoid spaces where the probe is. And that fluid, about a hundred milliliters of it, is circulating over the surface, the entire surface of the brain, over the brainstem and around the spinal cord and cauda equina. It forms even though it's just a narrow layer of fluid, it's very important in cushioning the brain. It's a very important, it's floating, the brain is basically floating and if it weighs 1,400 grams now, when it's floating in the cerebral spinal fluid, it only weighs about 25 grams. So the cushioning function of the cerebral spinal fluid to help avoid traumatizing the brain is a very important function along with removing metabolic byproducts, providing nutrients and peptides for controlling cerebral spinal fluid production and also influences the pressure inside the cerebral spinal fluid space influence cerebral blood flow. When intracranial pressure is high, it makes the blood flow much more difficult. So there's a delicate balance between cerebral blood flow and intracranial pressure.