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Answer: fibrous periosteum

The diaphysis is the main shaft of a long bone which is made up of cortical bone which contains bone marrow and fats. It is covered by a fibrous connective tissue called the fibrous periosteum. The cells in this layer are densely packed and contain blood vessels, nerve endings and lymphatic.  

The fibrous connective tissue that makes up the outside covering of the diaphysis is the periosteum.
It is a dense membrane composed of fibrous connective tissue that closely wraps all bone except that of the articulating surfaces in joints, which is covered by a synovial membrane. It serves as an outside covering or protection.