When looking under a microscope, a healthcare worker notices that part of a bone looks as if it were made out of tiny cylinders. The part of the bone she is looking at is the:

Compact bone is organized into tiny cylindrical units called osteons, or Haversian systems. These cylinders contain concentric layers of bone tissue, so the microscopic appearance points to compact bone, not spongy bone, marrow, or the periosteum.

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