What does arachnoid mean?
The arachnoid is best known as one of the three meninges — the protective membranes that envelop the brain and spinal cord. Sandwiched between the tough outer dura mater and the delicate inner pia mater, the arachnoid is a thin, avascular membrane whose name comes from its spiderweb-like appearance under early anatomists' eyes. Between it and the pia mater lies the subarachnoid space, filled with cerebrospinal fluid that cushions the central nervous system; projections called arachnoid granulations allow this fluid to drain back into the bloodstream. Inflammation of this membrane, known as arachnoiditis, can cause chronic pain, and fluid can accumulate as arachnoid cysts. Beyond anatomy, the word also serves more generally to describe anything cobweb-like or spidery in texture. Derived from the Greek arachne, meaning spider, it is a precise, technical term essential to neuroanatomy and clinical medicine.
The delicate, web-like middle membrane of the three meninges surrounding the brain and spinal cord, lying between the dura mater and the pia mater.
"The neurosurgeon noted that the arachnoid appeared intact before opening the subarachnoid space."
Usually referred to in full as the 'arachnoid mater' in medical writing.
Resembling a spider's web; having a fine, cobweb-like texture or structure.
"The morning mist clung to the hedge in arachnoid threads of silver."
Rarely used in the plural, since there is normally only one arachnoid membrane per body; the plural may appear when referring to multiple arachnoid cysts or granulations.
"MRI revealed several small arachnoids — cysts clustered near the temporal lobe."
The middle layer of your brain's protective wrapping is literally named after a spider — Greek anatomists thought its gossamer strands looked like a spider's web.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of arachnoid
The word derives from the Greek arachnē ('spider') combined with the suffix -oeidēs ('having the form of'), entering Latinized scientific vocabulary as arachnoidea. Early modern anatomists, observing the fine, web-like strands of this meningeal layer, named it for its resemblance to a spider's web. It shares this Greek root with related terms such as arachnid (the class of spiders and their relatives), arachnophobia (fear of spiders), and arachnology (the study of arachnids).
Related word forms
How arachnoid is actually used
In modern usage 'arachnoid' appears almost exclusively in medical and anatomical contexts, where it refers to the middle meningeal layer and is usually discussed alongside the dura mater and pia mater. The older general sense describing anything cobweb-like or spidery survives mainly in literary and descriptive writing. The term is technical rather than colloquial, so it carries a formal register.
Easily confused with arachnoid
An arachnid is an eight-legged arthropod such as a spider or scorpion, whereas the arachnoid is a membrane of the brain and spinal cord — both derive from the Greek word for spider but refer to entirely different things.