THE DISCOVERY OF the arachnoid membrane is a relatively recent advance. Ancient anatomists noted the presence of the dura mater and pia mater, but the intervening arachnoid membrane was left undescribed. It was not until the 17th century that anatomists discovered a layer separating the pia mater from the dura mater and named this cobweb-like layer the “arachnoid.” Arachnoid means “spider-like” and has an interesting etymology that can be traced to the ancient Greek myth of Arachne.