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1. A specialized form of dense connective tissue consisting of bone cells (osteocytes) embedded in a nonliving matrix; osseous tissue. Bone matrix is made of calcium carbonate, calcium phosphate, and collagen fibers.
SYN: SEE: os.
2. A unit of a skeleton. The human skeleton has 206 bones. Bones surround and protect some vital organs, give points of attachment for the muscles, and serve as levers that make movement possible. In the embryo, the bones of the skull are first made of fibrous connective tissue, which is gradually replaced by bone matrix. The remainder of the skeleton is first made of hyaline cartilage, which is also replaced by bone matrix (the acellular part of bone), beginning during the third month of gestation.
The outer surface of a bone is compact bone; the inner, more porous portion is cancellous (spongy) bone. Bone is covered by periosteum. The shafts of long bones are made of compact bone that surrounds a marrow cavity. Bones are classified according to shape as long, short, flat, or irregular. Older adults, esp. womem, may develop osteoporosis.
SEE: cancellous bone; SEE: compact bone; SEE: haversian system; SEE: long bone; SEE: membrane bone; SEE: periosteum;
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PARTS OF A LONG BONE ; SEE: skeleton for names of principal bones
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BONE TISSUE (Left) Femur with distal end sectioned; (Right) compact bone with haversian systems

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