Answer:
Endosteum
Explanation:
The hollow region in diaphysis is known as medullary cavity which is filled with the yellow marrow.
The endosteum is the thin vascular membrane of the connective tissue which lines up inner surface of bony tissue which forms medullary cavity of the long bones.
Endosteal surface is reabsorbed usually during the long periods of malnutrition and thus resulting in the less cortical thickness.
