Final answer:
Cancellous bone is typically surrounded by a shell of compact bone, which provides greater strength and rigidity to the bone structure.
Step-by-step explanation:
Cancellous bone, also known as spongy bone, is typically surrounded by a shell of compact bone, which provides greater strength and rigidity. Compact bone makes up the dense outer layer of the bone and has a very hard and strong structure. Its fundamental unit, the osteon, contributes to the bone's ability to withstand compressive forces. In contrast, the spongy bone is found in the interior of bones, is lighter, and less dense because of its porous nature, allowing for weight distribution and flexibility. The outer surfaces of bones are protected by the periosteum, a tough, fibrous membrane that covers the compact bone which in turn envelopes the spongy bone.