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Bone. Bones form the supporting basis of the body in most vertebrate animals. The bones of limbs serve as levers, which are acted upon by the various muscles, while the osseous framework of the skull and thorax protects the important structures inside those cavities from injury. Bone combines in a remarkably perfect manner the properties of hardness, lightness, and elasticity.

Structure of Bone. Bones are covered externally with a vascular fibrous membrane called the periosteum, the blood-vessels of which minister to the nutrition of the bone. Internally lies the medullary cavity of the bone, containing the marrow. The bone substance itself is either dense and "compact," as it is called, or it is "cancellous," i.e. made up of more loose-textured spongy material. In the long bones, compact bone is the rule; while in flat bones, cancellous bone is found, with an outer protecting shell of compact substance. A transverse section of a long bone shows, on miscroscopic examination, a large number of rounded spaces, about which concentric lamellae of osseous substance are disposed. Each central space corresponds to a canal, running in the direction of the long axis of the bone, and containing a blood-vessel concerned with the nutrition of the surrounding lamellae. These canals are called Haversian canals, and, with the concentrically arranged layers of bone, constitute the Haversian systems. Lying between the lamellae are found cells termed bone corpuscles, the processes of which penetrate some little way into the surrounding bone. The spaces in which the corpuscles lie are called lacunae, and the channels branching out of them into which the processes penetrate are termed canaliculi. The lacunas communicate by means of the canaliculi with the central Haversian canal, and thus nutrient material obtains access to all parts of even the densest bone. In spongy bone there are no typical Haversian systems; there are delicate trabeculae or bars of osseous material enclosing comparatively large spaces filled with marrow. Thus the blood supply of the bone comes in part directly from the periosteum, again from the bone marrow, and, in the case of long bones, from the vessels running in the Haversian canal.

Chemical Composition. Bone contains about one-third part by weight of animal or organic matter, and two-thirds of earthy or mineral substance. These two constituents are blended with one another in the most intimate matter. By immersing a bone in dilute acid all the mineral part can be gradually dissolved out and removed, and yet the remaining pliable animal matter perfectly retains the original shape of the bone. Again, by exposure to heat the animal portion can be completely burnt off, leaving a firm calcareous mass, the mineral part, which again exactly retains the form of the bone from which it is obtained. The animal matter is converted, by boiling, into gelatine, hence the use of bones in cookery in the making of jellies and soups. The mineral salts present in bone are the phosphate, carbonate, and fluoride of calcium, with a little phosphate of magnesium. Calcium phosphate makes up the main bulk of the earthy matter present, and forms more than half the total weight of a bone. An adequate supply of this salt to young animals, in which the osseous system is undergoing rapid development, is therefore of paramount importance. Such supply is perfectly afforded by the natural diet of new-born mammals - milk - for calcium phosphate is the chief salt in milk, just as it is in the bone into which the milk is converted. Rickets (q.v.), unhappily a very common disease in young children, affects in a marked degree the growing bones, which bend and give rise to numberless deformities; and in the case of rickety children there is almost always to be elicited a history of a departure from the natural infant dietary, the child being fed upon farinaceous and other foods containing much less calcium phosphate than milk does. There are two varieties of marrow. Yellow marrow, found in long bones, consists mainly of fatty tissue. The red marrow of cancellous tissue contains some fat, but, in addition, many "marrow cells," resembling lymph cells in structure. The red marrow is largely concerned, too, in the manufacture of red blood corpuscles.

Development of Bone. The long bones are developed from rods of cartilage. At certain points in the cartilage, called centres of ossification, there ensues increased vascularity with deposit of lime salts from the blood, a process termed calcification. By means of this process the growing ends of the bone continue to add to its length, until the adult condition is attained. All the calcified cartilage becomes, however, replaced by spongy bone, and ultimately this, too, is absorbed, and the true bone, formed beneath the periosteum, is laid down. The bone thus increases in thickness, and, the central portions entirely disappearing, it results that the marrow cavity of an adult bone would readily enclose the rod of cartilage from which its development originally proceeded. This development of bone in cartilage does not obtain in the case of flat bones, which are developed in membrane, In the membrane bones of the skull, for example, there is no cartilage from first to last, the osseous material is formed from the periosteum.

Diseases of Bone. Ostitis is inflammation of bone; periostitis, inflammation of the enveloping periosteum, and in osteomyelitis the diseased process mainly affects the medullary cavity and immediately surrounding parts. As the result of periostitis, thickenings, called nodes, may be left on the surface of bones. Ostitis deformans is a singular and rare disease affecting mainly the long bones. As the result of inflammation a large piece of bone may perish (necrosis), or a smaller portion of dead bone may be separated (sequestrum). Caries is a gradual eating-away or ulceration of osseous substance; strumous caries is very apt to affect the vertebrae, leading to angular curvature. Syphilis and cancer may both affect bone. Besides the important degeneration processes in bone associated with rickets, another, fortunately much rarer affection, known as mollities ossium or osteomalacia, may be referred to. Exostosis is a dense osseous outgrowth sometimes found growing from a bone.