Although the pia mater adheres to the surface of the brain, closely following the contours of its gyri and sulci, the arachnoid covers only its superficial surface. It follows from this that in certain areas around the brain the pia and arachnoid are separated widely; in such regions are formed cavities called the subarachnoid cisterns.
Although they are often described as distinct compartments, the subarachnoid cisterns are in fact not truly anatomically distinct. Rather, these subarachnoid spaces are separated from each other by a trabeculated porous wall with various-sized openings.
There are many cisterns in the brain with several especially large, notable ones each with their own name.
It is situated between the carotid artery and the ipsilateral optic nerve. It contains: The internal carotid artery. The origin of the anterior choroidal artery. The origin of the posterior communicating artery. Insular/Sylvian cistern. It is situated in the fissure between the frontal and temporal lobes. It contains: The middle cerebral artery. The middle cerebral veins. The fronto-orbital veins. Collaterals to the basal vein of Rosenthal.
It is situated just rostral to the third ventricle. It contains: The anterior cerebral arteries (A1 and proximal A2). The anterior communicating artery. Heubner's artery. The hypothalamic arteries. The origin of the fronto-orbital arteries.
It extends from the conus medullaris (L1-L2) to about the level of the second sacral vertebra. It contains the filum terminale and the nerve roots of the cauda equina. It is from the cistern that CSF is withdrawn during lumbar puncture. It is of clinical significance that cerebral arteries, veins and cranial nerves must pass through the subarachnoid space, and these structures maintain their meningeal investment until around their point of exit from the skull.