BONE, the compact hard material making up the skeleton of mammals, most of the birds, reptiles and amphibians, and the bony fishes. It is also found in some lower forms. Chem ically bone is complex. It is essentially organic substances, 30 to 35 per cent, infiltrated with inorganic mineral salts, 65 to 70 per cent; to the former its toughness is due and to the latter its hardness. The organic substances of bone are ossein (collagen, gelatine), small quantities of elastin, proteids and nuclein from the cells and small quantities of fat. The in organic salts are calcium carbonate, calcium phosphate, calcium fluoride, magnesium phos phate, calcium chloride and small quantities of sulphates and other chlorides. The percentages of both inorganic and organic constituents vary widely in the bones of different animals, and also in the different bones of the same animal. These differences vary widely if the age varies, but are fairly constant for the same animal of the same age. Thus the amount of water may vary from 13 to 45 per cent in the different bones of the human body, being greater in amount in the spongy bones and less in the compact bones, and as the bones grow older the percentage of water diminishes. In the living body many of the bones, particularly the ribs, and the heads of all the long bones, contain a substance termed marrow. This is an import ant substance in the human economy, being the source of much of the blood-building material. In soups this marrow makes one of the most important factors. This bone marrow is per vaded by a network of white fibrous connective tissue and in the meshes are contained the cells, myeloplaxes, that make many of the blood cor puscles, particularly the polymorph neutrophiles and the eosinophiles. In the red marrow the red corpuscles are developed. The bone mar row is very rich in proteids, nucleo-proteids, extractives, globulins, fats and compounds of iron. Prepared bone marrows are therefore highly nutritious, and the modern bone, 1 which is usually rich in marrow, is a toothsome and valuable dietary addition. The histological structure of bone is very intricate; in the young developing animal, cartilage first makes its appearance from modified connective tissue cells. In this cartilage certain points of ossification appear, which subsequently develop bone and the bone from several points coalesces to make the completed bone structure. The bone cells in the cartilage, the osteoblasts, thicken and form a distinct cell wall in which the inorganic salts are deposited and osteoblast by osteoblast the structure of bone is made up. Bone is formed by the periosteum, which is a covering, first of the cartilage and then of the developing bone. Bony tissue contains arteries, veins, nerves and lymphatics, and is a distinct tissue, largely modified by the deposition of mineral salts. In a section across a long bone, at its centre, say the thigh bone or femur, there is on the outside the thin tough layer, the periosteum with its vessels and nerves and lymphatics; within this is the compact bone and in the centre the cavity usually filled with marrow at the ends. A very thin section of the compact bone viewed under the microscope shows a number of cavities, the Haversian canals; these contain blood vessels or were the sites of for mer blood vessels in the developmental stage. Around these Haversian canals, one sees regu lar lamelhe, not unlike the rings about a tree trunk; these are the Haversian lamella and indicate the regular growth of bone cells from the centre. Scattered between the lamella are
intmerous small spaces, containing living bone cells, the lacuna, all of which are probably in communication with one another by minute canals, or canaliculi. Thus the entire loony.rys tem is pierced throughout by an extremely fine and exceedingly rich network of canals. As these are filled with lymph the bone substance is constantly bathed in this living life-giving fluid. The different bones of the body show minor variations in structure. The bones•of the human body are grouped accordim to their as long tones, flat bones, short and bones. They approach one another at the joints, where they are protected by cartilages, smooth synovial membrarie, and bathed in a synovial fluid. The long bones consist of a shaft and two expanded ends or epiphyses, and are found in the limbs. They give support and leverage for motion and are usually slightly curved in one or two directions to give greater i elasticity. Flat bones are found the skull, pelvis, scapula, and are usually so disposed as to afford protection to the internal viscera; they also offer considerable surface for muscu lar attachment and hence give a good leverage for the long bones. Short bones are 'found in the wrist and ankle. Strength and freedom of motion are their attributes. Irregular and mixed bones are the vertebra and some of the bones of the skull. They each have varied and spe cially adaptive functions. Many bones, especially those of the skull, are composite. They de velop separately, and finally unite. Thus the bones of the skull are separated until late in life, and in some individuals, some of the bones never develop thoroughly. This is frequently the case in the growth of the lowerjaw, where failure to unite produces the well-loiown de formity of cleft palate or hare lip.
Bone is slightly heavier than water, its specific gravity varying from 1.80 to 1.90. The spongy bones, because of the large amount of air contained, float in water. The bones of birds are remarkable for their strength and lightness. The twofold nature of bones is read ily demonstrated by two simple experiments: If one bone is placed in acid, 20 per cent hydro chloric, the acid will attack and dissolve out the mineral salts, after which the bone may be bent and its shape altered at pleasure, nothing but the organic matter remains; another simi lar bone may be placed in a furnace and the heat will burn out the organic matter entirely; that which remains will be the mineral matter. It will retain the shape of the original bone, will be white, but will break down into powder at the least pressure.
Uses of the In dietetics bones make a substratum for soups. These are im portant carriers of salts to the body. As for the gelatine alone, it is a tissue sparer, the body cannot use it for purposes of anabolism, but it spares katabolism of proteids. It is a useful menstruum for foodstuffs. Bone marrow is highly nutritious, contains iron and is a super lative food, and thought to be particularly valu able as a blood maker. The uses of bone in the arts are numerous. See Fstirthizmis• Consult Syminowitch, (Histology' -, Gray, (Anatomy' • MacEwen, 'Growth of Bone) (New York 1912). See ANATOMY; KINETOGENESIS ; OSTE OLOGY.