1. One of a varying number of layers of bone encircling either the external surface of compact bone (lamella circumferentialis externa) or the internal surface lining the wall of the narrow cavity (lamella circumferentialis interna).
2. A thin layer of bone that is deposited under the periosteum (membrane that covers the outer surface of all bones) or the endosteum (thin vascular membrane of connective tissue that lines the surface of the bony tissue) that forms the medullary (soft substance) cavity of long bones