An involucrum (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 around 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.
See also Sequestrum
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