Arachnoid mater
Arachnoidea mater cranialis
NervousOrgansHead
- What it is
- The thin, cobweb-like middle membrane lying against the inner dura. Fine trabeculae cross the CSF-filled subarachnoid space beneath it to the pia mater, which clings to every gyrus and sulcus of the brain surface.
- Function
- Encloses the subarachnoid space where CSF cushions the brain and the large cerebral arteries run; arachnoid granulations return CSF to the venous sinuses.
- Size
- Microscopically thin (tens of micrometres); subarachnoid space holds ~125-150 ml of CSF in total
- Clinical note
- A ruptured berry aneurysm bleeds into the subarachnoid space, classically causing a sudden "worst headache of my life"; meningitis inflames the arachnoid and pia.