In the last section we learned that the brain is really a tube-like structure, formed when the neural plate rolls up and the edges meet to enclose a central cavity. The cavity becomes the ventricular system of the adult brain. In certain regions the edges of the plate fail to meet, and vascular tissue bridges the gap. This tissue is named the choroid plexus and it produces the cerebrospinal fluid (CSF) that fills the ventricular system and escapes into the space between the brain's surface and the skull.
     Fenestrated capillaries within choroid tissue allow for free passage of water and solutes but that tight junctions between choroid epithelial cells form a blood-CSF barrier. While lipid soluble substance may easily pass through this barrier, most other materials must be conveyed from blood to CSF by active transport through cells of the choroid epithelium. The result is that the chemical composition of the two fluids differ; for example, CSF has higher levels of chloride, magnesium and sodium, compared with plasma. CSF is colorless, low in protein and contains very few, if any, white blood cells.
     The volume of CSF in the adult is about 120mL and roughly 500mL of the fluid is produced every day, so that about 4 turnovers occur in 24 hour. Production continues even if there is a blockage of flow, somewhere along the way to the arachnoid granulations, but the result depends on the age of the subject and the site of the obstruction.
     Some of these details are illustrated in the views below.

     Certainly, the most important function of the skull is to protect the fragile brain, lying within the cranial cavity. As this figure indicates, the skull is well-designed for the job. If we cut through the calvarium in the plane shown in A and look down from above we get view B.
     The contour of the brain's surface matches that of the cranial cavity rather well, but there is a small space between the two. This narrow gap, shown by green stippling in the figure, is called the subarachnoid space. It is filled with cerebrospinal fluid (CSF) and also contains three membranes known collectively as the meninges. The brain is, in a sense, floating in CSF which protects it from "bumping into" the skull during sudden movements of the head.
      From a clinical point of view, the anatomy of this region is extremely important and we want to look at it in detail. The best way to start is to consider CSF - where does this fluid come from, how does it get into the subarachnoid space, where is it going, and how does all this relate to the meninges?

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     To tell our story we will make repeated use of rather schematic longitudinal section through the brain. A shows the plane of our section and B shows the section, itself.
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     Here we trace the flow of CSF. From the major site of formation, in the two lateral ventricle, fluid passes through the interventricular foramen to enter the third ventricle. A bit more fluid comes from the choroid plexus roofing the third ventricle; CSF exits into the cerebral aqueduct and descends into the fourth ventricle. Again, choroid plexus in the roof of this ventricle contributes a small amount of CSF to the total. CSF exits from the ventricular system through paired lateral openings, the Foramina of Luschka, and a single midline opening in the roof of the ventricle, the Foramen of Magendie. Once outside, the fluid lies within the subarachnoid space - a region defined more precisely in a coming view. For now, just trace the arrows up over the surface of the brain to the arachnoid granulation . CSF enters these specialized structures and from here returns to the venous blood of the superior sagittal sinus. For fun, look at this You Tube short video.
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     Most CSF returns to the vascular system by entering arachnoid granulations. Some fluid passes between the cells lining a granulation to mix with the venous blood of the superior sagittal sinus. Most of the CSF, however, is transported through the cells in membrane bound vesicles.
      Granulations are easily seen in gross specimens. If the dura is carefully removed from the surface of the brain, they tend to pop out of the superior sagittal sinus and be exposed to view, as shown in this view.
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     Another way to expose arachnoid granulations is to open the superior sagittal sinus in the midline and look at the granulations as they protrude into the sinus. The box in this view frames the region seen in the following one.
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     This is an enlarged view of the superior sagittal sinus. Since the cut was made exactly in the midline, it has passed through the middle of the sinus - we are inside the sinus looking at the lateral wall, so to speak. The little round balls within the sinus are arachnoid granulations. Admittedly, they are a little hard to see.