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Circumferential Growth

- Discussion:
    - to understand functions of & reasons for haversian systems, one must consider manner in which diaphysis grows circumferentially;
    - since bone is hard substance, it can grow only by adding new material to its surface;
    - at birth, diaphyseal portion of human femur is tube about 6 mm in diameter, w/ walls less than 1 mm thick;
    - by adulthood, it attains diameter of some 30 mm, w/ walls 5-6 mm thick;
    - such growth requires addition of bone to the outer surface and  resorption of bone from inner surface;
    - osteoblasts are prominent on outer surface of bone during growing period;
    - they produce organic matrix of bone, which subsequently becomes calcified;
         - cells become incorporated in the matrix as osteocytes;
    - this surface bone, being lamellar mature bone, is laid down in layers few microns thick;
    - these circumferential lamellae are parallel to the bone surface and may completely surround the shaft;
    - as cortex of bone thickens with growth, it becomes impossible for canilicular system to supply the deep seated osteocytes; blood 
         vessels must be provided w/in cortex for nutrition of its   osteocytes--thus, need for haversian canals