CSF, Ventricles & Brain Anatomy
Arachnoid mater
The arachnoid mater is the thin, web-like middle membrane surrounding the brain, beneath which cerebrospinal fluid circulates.
In short: Arachnoid mater
- It is the middle of the three meninges.
- CSF circulates in the subarachnoid space beneath it.
- Arachnoid granulations arise from it to absorb CSF.
- Arachnoid cysts are fluid collections related to this membrane.
What it is
The arachnoid mater is the middle, web-like membrane of the meninges, lying between the tough outer dura and the delicate inner pia. The subarachnoid space, where CSF circulates, lies beneath it.
Understanding Arachnoid mater
CSF flows in the subarachnoid space between the arachnoid and the pia mater, and is absorbed through arachnoid granulations — projections of the arachnoid that extend into the venous sinuses. The arachnoid is therefore central to CSF circulation and absorption. Collections of fluid related to this membrane, called arachnoid cysts, can sometimes obstruct CSF flow and contribute to hydrocephalus. The arachnoid is thus relevant both to normal CSF dynamics and to certain causes of hydrocephalus.
Why it matters
Understanding the arachnoid mater helps families see where CSF circulates and is absorbed, and how related structures like arachnoid granulations and arachnoid cysts connect to hydrocephalus.
Common questions
Where is the arachnoid mater?
The middle membrane, between the dura and pia.
What lies beneath it?
The subarachnoid space, where CSF circulates.
What arises from it?
Arachnoid granulations, which absorb CSF.
Bottom line
The arachnoid mater is the middle meningeal membrane; CSF circulates beneath it and is absorbed through its granulations.
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