Answer:
The correct answer is "Medullary cavity."
Step-by-step explanation:
The medullary cavity is not part of an osteon. It is a hollow space within the diaphysis (shaft) of a long bone that contains bone marrow. In contrast, an osteon is the basic structural unit of compact bone and consists of concentric layers of bone tissue called lamellae surrounding a central canal (also known as the Haversian canal). The lamellae are connected by canaliculi, which are small channels that allow for the exchange of nutrients and waste products between osteocytes (bone cells) within the osteon.