An<unk>  (plural involucra) is a layer of new bone growth outside existing bone. There are two main contexts: * In pyogenic osteomyelitis where it is a layer of living bone that has formed about dead bone. It can be identified by radiographically (i.e., with x-rays). * In cetaceans such as whales, it is a thick covering of bone over the middle-ear space.
Involucrum
Sequestrum
Endothrix
Exothrix
Cryptocotyle
Columella 
Genu varum
Keratocyst