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Calcium.
Calcified bones are hard but not really solid. Spongy bone is hard with tiny holes in it. Periosteum is thin but very strong. The other name for it is outer membrane, it helps to rebuild bone cells. The last is cartilage. It is strong material that holds two bones together and allows them to glide over one another. Skull iz also a very strong bone.
Periosteum is the hard strong layer of bone
Compact bone.
bone marrow
It contains calcium which is a substance that does not weight. Therefore it makes bones be strong and hard. If you took the calcium, the bones would be flexible and not hard at all.
The strong hard layer beneath the periosteum is called the compact bone. It is dense and provides strength and support to the bone structure.
Calcium
Bone is made hard and tough due to its composition of collagen, a protein that provides flexibility and strength, and hydroxyapatite crystals, a mineral that adds hardness and rigidity to the bone structure. The combination of these components gives bones their ability to withstand mechanical stress and maintain their structural integrity.
Calcium makes them hard.
Cortical bone tissue makes up the dense hard outer layer of bone. It is also known as compact bone and provides structure and protection to the inner spongy bone tissue.
The bone is made of of a hard calcium rich other shell, whitch covers a softer more spongy inside. Thanks to this design, the bone can support heavy weight, but still bend to accommodate additional weight.