Compact bone is the hard bone.
Compact bone, also known as cortical bone, forms the dense outer layer of most bones in the body. This outer layer provides strength and protection. Here's a more detailed breakdown:
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Structure and Function: Compact bone is characterized by its tightly packed structure, making it extremely strong and resistant to bending or fracturing. This is crucial for supporting weight, enabling movement, and protecting internal organs. The primary functional unit of compact bone is the osteon, which consists of concentric layers of bone tissue surrounding a central canal containing blood vessels and nerves.
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Contrast with Cancellous Bone: In contrast to compact bone, cancellous bone (also known as spongy bone) is found inside some bones, particularly at the ends of long bones. Cancellous bone has a porous, sponge-like structure, which makes it lighter than compact bone. This structure also allows for better blood flow and contains bone marrow. While not as hard as compact bone, cancellous bone contributes to bone strength and flexibility.
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Composition: Both compact and cancellous bone are made of the same basic materials: collagen and minerals (primarily calcium phosphate). The difference in their hardness arises from the arrangement and density of these materials. In compact bone, the minerals are more densely packed, making it harder.
Therefore, when considering which bone is hard, compact bone is the definitive answer. It's the dense, strong outer layer that provides the majority of a bone's structural integrity.