What does arachnoidal mean?
Arachnoidal is a technical adjective used almost entirely within medicine and anatomy to describe anything pertaining to the arachnoid mater — the thin, web-like membrane that lies between the dura mater and the pia mater, cushioning the brain and spinal cord. Its name derives from the Greek word for spider, chosen by early anatomists who were struck by the membrane's gossamer, cobweb appearance. In practice, the term surfaces in neurological and radiological contexts: clinicians may speak of arachnoidal cysts, arachnoidal granulations (small structures through which cerebrospinal fluid is reabsorbed into the bloodstream), or arachnoidal adhesions seen after infection, hemorrhage, or surgery. Because the shorter form 'arachnoid' dominates clinical writing, 'arachnoidal' functions largely as a fuller adjectival variant. For general readers it is unfamiliar, but it offers a vivid example of how anatomical nomenclature borrows from the natural world — here, likening one of the body's most delicate protective layers to a spider's silk.
adjectiveRelating to or resembling the arachnoid, the delicate spiderweb-like membrane that surrounds the brain and spinal cord; chiefly used in medical and anatomical contexts.
- Of or relating to the arachnoid mater, the web-like middle membrane enclosing the brain and spinal cord.
- Resembling or having the character of a spider's web (used more broadly in descriptive anatomy and pathology).
"The MRI revealed a small arachnoidal cyst pressing against the temporal lobe."
"Cerebrospinal fluid is reabsorbed through arachnoidal granulations into the venous system."
"The pathologist noted fine arachnoidal adhesions following the patient's previous surgery."
The word for one of your brain's protective membranes comes straight from the Greek for spider — because anatomists thought the membrane looked like a spider's web.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of arachnoidal
Arachnoidal derives from the Greek 'arachnē' (ἀράχνη), meaning spider, via the New Latin anatomical term 'arachnoidea', which early modern anatomists applied to the cobweb-like middle meninx of the brain. English adopted 'arachnoid' for this membrane, and 'arachnoidal' arose as its adjectival derivative in medical writing. The same Greek root gives rise to related terms such as arachnid, arachnology, and arachnophobia.
Related word forms
How arachnoidal is actually used
Strictly technical and medical in register; it appears almost exclusively in neuroanatomy, neurology, radiology reports, and pathology literature rather than everyday conversation. The simpler form 'arachnoid' is far more frequently used in clinical writing.
Easily confused with arachnoidal
Arachnoid names the membrane itself (or, as a noun, a spider), while arachnoidal is the adjectival form meaning 'of or relating to that membrane'.
An arachnid is an eight-legged arthropod such as a spider or scorpion, whereas arachnoidal pertains to the meningeal layer of the brain and spinal cord.