Arachnoid ( A ; Blasius, 1666 ) : A delicate membrane resembling a spider's web interposed between dura (Galen, c177) and pia (Galen, c192), and separated from pia by subarachnoid space (Magendie, 1827). It is histologically similar to pia, and the two are often considered together as leptomeninges; see Dorland's (2003), Standring (2008, p. 389). The arachnoid was first illustrated for macrodissected adult humans by Casserio (1627), and described and named by Blasius (1666).