Pia ( PI ; Galen, c192 ) : The innermost of the meninges (Edwin Smith Surgical Papyrus, c1700 BC), closely covering the brain (Edwin Smith Surgical Papyrus, c1700 BC) and spinal cord (Galen, c162-c166) and consisting of reticular, elastic, and collagenous fibers. It is histologically similar to the arachnoid (Blasius, 1666), and the two are often considered together as the leptomeninges; see Dorland's (2003), Standring (2008, p. 389). The pia was apparently known to Hippocrates in Places in Man (see translation by Potter, 1955, p. 23) and was described and named such by Galen (c192; see translation by Duckworth, 1962, p. 6).