Final answer:
The medullary cavity is a hollow, cylindrical space within the diaphysis of a long bone. It contains fatty yellow bone marrow and numerous blood vessels in adults.
Step-by-step explanation:
A hollow medullary cavity filled with yellow marrow runs the length of the diaphysis of a long bone. The walls of the diaphysis are compact bone. The epiphyses, which are wider sections at each end of a long bone, are filled with spongy bone and red marrow. The medullary cavity has a delicate membranous lining called the endosteum. The outer surface of bone, except in regions covered with articular cartilage, is covered with a fibrous membrane called the periosteum.