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In 2007, a man came into a hospital in Marseilles, France, complaining of weakness in his leg.The man, 44 years old, had a neurological history. More than thirty five years earlier he'd had a shunt put in his skull, to help drain the cerebro-spinal fluid (known as CSF). This is fluid that surrounds and cushions the brain, it is continuously produced around the brain, and normally it simply drains out into the spinal region and the vascular system. If the natural drainage system doesn't work, you get a disease known as hydrocephaly, where excess fluid builds up and places pressure on the brain.This man had suffered from hydrocephaly as a child, and had a shunt, or drain, inserted. Not that common, but not that rare, and a pretty simple fix. After some years, the problem seemed to be solved.But now the man had limb weakness. Limb weakness can be caused by neurologial issues, and given his history, the doctors took a CT scan of his brain.They were amazed to see this:Now, you might not be familiar with what brain scans usually look like, but they were expecting to see something more like this (although this image is actually MRI, rather than CT):You can see, in the lower image, that there are small, dark patches in the centre of the brain. These are called ventricles, and they are chambers filled with CSF which, being mostly water, shows up as black on the scan. Ventricles are also where most of the CSF is created. Most of the brain, in various shades of gray, fills up the majority of the skull.This man had ventricles that took up some 75% of his brain volume. That big, black area? That's basically all just water.His brain is that very thin layer of gray, between the black of the CSF and the white of the skull, taking up 25% or less of what it should take up.The most amazing thing? This man was 44, had a job, wife, children, a perfectly normal life. His IQ was determined to be 75—low, for sure, but not mentally disabled by any means, less than two standard deviations below the mean.It should be noted that, in all likelihood, this man is not actually missing 75% of his brain. Rather, his brain has been condensed under pressure. He probably actually has most of the brain structures we all have, just squashed together into a very thin layer.But when you think of how complex and delicate a system the brain is, it's absolutely amazing he could even live, never mind move, eat, talk and have relationships.Astounding.Our brains are incredible machines, and their ability to overcome the most extreme damage is simply unbelievable.This was first published in The Lancet, the original article can be found here: https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(07)61127-1/fulltext

It’s not attached to the skull at all. There is a tough fibrous membrane called the dura mater that’s pressed against the inside of the skull and attached to it at a few points, such as around the opening for the spinal cord (the foramen magnum) and the pit that houses the pituitary gland (sella turcica).The brain is separated from the dura mater by a cushioning layer of cerebrospinal fluid (CSF) and by two other membranes, the arachnoid mater and pia mater. The brain almost floats in the CSF, as it has near-neutral buoyancy, but it is suspended in the fluid by delicate strands of fiber from the arachnoid mater above, like wires suspending a mobile from the ceiling.The brain doesn’t impact the skull except in very violent blows as in auto accidents and football tackles; the CSF cushions it and prevents contact with the skull (or dura) in gentler blows and sudden head movements.An opened cranium. Note that we cannot see the brain immediately when the skull is opened; what we see is its surrounding dura mater, about as thick as a rubber kitchen glove. Beneath this dura is the cushion of CSF.Later addition:Beyond enveloping the brain as shown in the foregoing photo, and enclosing a cushioning layer of cerebrospinal fluid between the brain and skull, the dura mater also has inward folds that separate major portions of the brain and limit the brain’s movements.The biggest of these is a vertical fold between the two cerebral hemispheres called the falx cerebri. It’s named for its sickle shape (falx = “sickle”). The second largest is a horizontal fold called the tentorium cerebelli, which extends like a platform or roof (tentorium = “tent” or “canopy”) to the right and left between the cerebellum and cerebrum. Third is a small vertical falx cerebelli that separates the right and left lobes of the cerebellum from each other. (None of these completely divides the brain in two, of course; they just extend deeply into the fissures that separate these brain regions.)Here are some figures.Dissection of the rear of the head, showing the falx cerebri and tentorium cerebelli meeting like an inverted T. This is also a good view of the space around the brain filled with CSF in the living body.Lateral dissection of the head showing the sickle shape of the falx cerebri. The tentorium cerebelli extends outward toward the viewer, perpendicular to the falx cerebri.Images from:Dissection 1: Dura MaterDissections 2 & 3: Dural PartitionsBottom drawing: Wikipedia

As it happens, just yesterday I read a research paper on this in the journal Science.During deep, nondreaming, slow-wave sleep (SWS), cerebral blood flow drops by 25%, reducing cerebral blood volume; brain metabolism and oxygen consumption also fall at this time. The reduced cerebral blood volume allows cerebrospinal fluid (CSF) to flow backward (retrograde flow) about every 20 seconds and flood the third and fourth ventricles (internal cavities) of the brain—opposite the direction of flow indicated by arrows in traditional diagrams like my textbook figure here.This retrograde CSF flow, and its oscillations back and forth as we go in and out of SWS, is thought to contribute to the flushing away and disposal of toxic brain metabolites that accumulate while we’re awake.These new findings suggest possible correlation between the disturbances in SWS seen in many people in old age, impaired clearance of wastes from the brain, and cognitive issues of aging such as dementia and depression.ReferencesN.E. Fultz et al. 1 November 2019. Coupled electrophysiological, hemodynamic, and cerebrospinal fluid oscillations in human sleep. Science 366:628–631.S. Grubb & M. Lauritzen. Deep sleep drives brain fluid oscillations (summary article). Same issue, p. 572–573.