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The Structure of Bone

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THE STRUCTURE OF BONE.

Bones consist of an earthy or mineral part and an animal part. If a bone, say a rib of an ox or sheep, be steeped for several weeks in dilute hydrochloric acid, the mineral matter is dissolved out by the acid, and there remains the animal matter. It retains perfectly the outward form of the bone. It is no longer hard, however, but soft and flexible. On the other hand, if a bone, say the knuckle from a joint, be put into a clear fire, the animal matter is slowly burned away, and only the mineral re mains. The bone, by the burning, first becomes black, and then, as the last trace of animal matter disappears, it becomes pure white. The earthy matter also retains perfectly the shape of the bone, but is very brittle, and liable, at the slightest touch, to crumble to dust. The earthy matter forms about 70 of every 100 parts of the Lone, and the animal or organic matter about 30 of every 100, less than one third. The animal matter is a substance, called ossein, which yields gelatin on boiling; and the earthy consists of phosphate and carbonate of lime, phosphate of magnesia, and chloride of sodium (common salt). In childhood bones contain a larger percentage of animal matter, therefore they are flexible and more liable to bend than to break ; while in old age they contain a greater percentage of mineral matter, and therefore are more brittle and easily broken.

Usually when a bone is sawn through it is found to have a shell of hard, compact bone outside, and inside the plates of bone are less closely packed, spaces being left between, ing a spongy look. This spongy bone is called cancellated. Fig. 9 shows this very well. A long bone has in its centre a cavity called the medullary canal, filled with a soft, reddish, pulpy substance, consisting largely of fat cells, and called the medulla or marrow.

Bones are usually classified as Long, or Cylin drical, such as the long bones of the arm or leg, Short, or Irregular, such as the small bones of the wrist and sole of the foot, and Flat, or Tabular, such as the bones of the skull.

Bone is completely covered outside by a dense fibrous coating called the periosteum, which is richly supplied with blood, and plays a chief part in the growth of bone. The cavity in a bone is also lined with a similar membrane called endosteum, also rich in blood-vessels.

The bone itself seems so dry and hard as to have no moisture within it, nor any blood supply ; but this is not the case, for blood vessels pass into it from the periosteum through minute openings, and most long bones have besides a special artery (nutrient artery) enter ing them to carry a due supply of nourishment. If a very thin slice be taken from across a bone and examined under a microscope, magnifying about 300 diameters, an appearance is presented like that shown in Fig. 10 B. Little openings

(a, b) are observed, and round them are ranged rings of bone (c, d, e), with black bodies (f) in them, from which fine dark linesbranch out. The openings are canals cut across, called Haversian canals, after Havers, who first described them ; the black bodies are spaces, called lacuna (little lakes), in the bony plates, while the fine dark lines are very narrow canals which connect the lacuna with one another and with the Haver sian canals. The narrow channels are called canaliculi. A, of the same figure, shows a section taken lengthways, by which the Haversian canals (a, b) have been opened up, not cut across, and are seen branching and communicat ing with one another. Now, in these Haversian canals blood-vessels run ; the lacuna contain little masses of a jelly - like material—living protoplasm—processes from which pass along the canaliculi. c, of the figure, shows a lacuna (a), largely magnified, with its living mass of protoplasm (b). Thus bones have not only a large blood-supply obtained from the nutrient artery, and from the periosteum covering the outer, and the endosteum lining the inner, surface, but contain able little cells of living material, which are capable of drawing from the blood the sort of food they require, and thus of maintaining a constant net-work of nourishing channels through the whole bone.

Early in the life of the child, months before it is born, there are no true bones in its body, their place being occupied by cartilage (gristle). The masses of cartilage have, however, the shape of bone, and it is out of them the bones are developed. Take such a one as the thighbone; it is found that shortly before birth only the shaft has become true bone, the two extremities being still made of cartilage (Fig. 11, A). 'Just at birth a small posit of earthy matter is found in the lower end (a 2), which goes on extending. At one year of age a second deposit is seen in the upper end (c s). These little deposits, from which the bone forms, are called centres of ossification. The bone formation goes on from these centres till the ends are quite bony, and cartilage exists only ' at the place of junction between the shaft and the two extremities. The two ends are called epiphyses, and may readily be separated at this stage from the shaft. This is an accident which sometimes happens to children, and since it is at this point of junction that the principal growth in the length of the bone occurs, such an accident may seriously interfere with the growth. The union between shaft and epi physes does not take place till maturity, when further growth in the long direction ceases.