[TA] the outer layer of an enveloping sac or bursa, usually lining the walls of the cavity or space occupied by the enveloped structure, the structure itself being covered with the inner or visceral layer of the enveloping sac; an actual or potential space is enclosed by the two continuous layers, intervening between parietal and visceral layers. The parietal layer is usually the more substantial layer. SYN: lamina parietalis [TA].