- 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