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It consists of two layers, an outer fibrous and an inner cellular layer; the cells, which are called osteoblasts, are continuous with those lining the Haversian canals and the medullary cavity. The arrangement of the blood vessels determines to some extent the incidence of disease in bone.
The ends of the bones are usually enlarged and have specially formed projections or depressions which fit into corresponding depressions or elevations on the bones with which they articulate. In addition to this the articular surfaces are quite smooth and dense, having no Haversian canals, and they are covered with a layer of cartilage.
Where the process is most advanced that is, at the ossifying junction there are evidences of absorption of the framework of the bone; the marrow spaces and Haversian canals undergo enlargement and become filled with greenish-yellow pus. This rarefaction of the spongy bone is the earliest change seen with the X-rays.
The process may remain localised to the ossifying junction, but usually spreads along the medullary canal for a varying distance, and also extends to the periosteum by way of the enlarged Haversian canals. The pus accumulates under the periosteum and lifts it up from the bone. The extent of spread in the medullary canal and beneath the periosteum is in close correspondence.
This passes through the canaliculi to the cells in the different parts of the bone, as follows: 1. The cells in the surface layer of the bone receive lymph from the capillaries in the periosteum. It gets to them through the short canaliculi that run out to the surface. The cells within the interior of the bone receive their nourishment from the small blood vessels in the Haversian canals.
The diaphysis is also nourished by numerous blood vessels from the periosteum, which penetrate the cortex through the Haversian canals and anastomose with those derived from the nutrient artery. The epiphyses are nourished by a separate system of blood vessels, derived from the arteries which supply the adjacent joint. The veins of the marrow are of large calibre and are devoid of valves.
Around these are arranged rings of bone, with little black bodies in them, from which radiate fine, dark lines. These openings are sections of canals called Haversian canals, after Havers, an English physician, who first discovered them. The black bodies are minute cavities called lacunæ, while the fine lines are very minute canals, canaliculi, which connect the lacunæ and the Haversian canals.
They pass out from the Haversian canals at right angles, going to all portions of the compact substance except a thin layer at the surface. In the surface layer of the bone the canaliculi are in communication with the periosteum. Arterial capillary. 2. Venous capillary. 3. Nerve fibers. 4. Lymph vessel.
The exposure of the delicate pulp to the air, due to the decay of the dentine, gives rise to the pain of toothache. Surrounding the cavity on all sides is the hard substance known as the dentine, or tooth ivory. Outside the dentine of the root is a substance closely resembling bone, called cement. In fact, it is true bone, but lacks the Haversian canals.
If, in answer to the assertion that the osseous parts of bone are non-living, it is said that they must be living, for they heal if broken, which no dead matter can do, it is answered that the broken pieces of bone do not grow together; they are mended by the protoplasm which permeates the Haversian canals; the bones themselves are no more living merely because they are tenanted by something which really does live, than a house lives because men and women inhabit it; and if a bone is repaired, it no more repairs itself than a house can be said to have repaired itself because its owner has sent for the bricklayer and seen that what was wanted was done.
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