LYCOS RETRIEVER 
Bones: Periosteum
built 208 days ago
Bones are made up of two types of material - compact bone and cancellous bone. Compact bone is the solid, hard, outside part of the bone. It looks like ivory and is extremely strong. Holes and channels run through it, carrying blood vessels and nerves from the periosteum, the bone's membrane covering, to its inner parts. Cancellous (pronounced: kan-suh-lus) bone, which looks like a sponge, is inside the compact bone. It is made up of a mesh-like network of tiny pieces of bone called trabeculae (pronounced: truh-beh-kyoo-lee).
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Bones are covered by a thin membrane called the periosteum. Injury to bone transmits pain because of nerves located mostly in the periosteum. Blood enters bones through blood vessels that enter through the periosteum.
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Circumferential growth is a growth of long bones in diameter. The circumferential growth occurs throughout lifetime. The layer of periosteum builds concentric layers of new bone tissue on top of the existing ones. At the same time, the bone around the medullary cavity is resolved or eliminated so that the diameter can be enlarged continuously. Osteoblasts and osteoclasts are in charge of forming and resolving bone tissues, respectively. There are four types of bone cells which are osteoblasts, osteoclasts, osteocytes, and bone lining cells.
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Periosteum