According to the National Center for Biotechnology Information, most bone is covered by tissue called "periosteum." It is involved in several processes, including growing and repairing bones and supplying bones and muscles with blood.
The NCBI explains that periosteum has two layers. The outer layer is full of blood vessels and nerve fibers, while the inner portion helps bones grow. This layer is thickest when a fetus is developing and bones are forming. As people age and bone growth slows, this section becomes thinner. If a bone breaks, this layer is stimulated to repair the injury, but it does not reach its prior level of function.