What does the outer membrane of a bone do?

A thick, fibrous vascular membrane covering the bones, except at their extremities. It consists of an outer layer of collagenous tissue containing a few fat cells and an inner layer of fine elastic fibers. Periosteum is permeated with the nerves and blood vessels that innervate and nourish underlying bone.

(This is the real Answer) -Periosterum is the outer most covering layer of the bone. This layer is provided with vascular supply as well as nerve supply. With blood supply it nourishes the bone and nerve supply provide sensation to bones. It is the attachment region for muscles, tendons and ligaments. When the bone gets damaged it helps in healing process of the bone.