Bone (os, ossis) as an organ of the living organism, consists of several tissues among which bone tissue is most important. The bulk and signi­ficance of bone tissue can be demonstrated on macerated bone (removed from the body, freed of the soft tissues, and treated anatomically by soaking and drying). Despite such treatment, the bone preserves its shape, size, structure, and strength. The strength of the bone results from the combina­tion of two main properties, hardness and elasticity, which are the result of certain chemical substances in the bone itself. The chemical composition and physical properties of bone. Bone matter consists of two types of chemical material: organic (one-third), mainly ossein, and inorganic (two-thirds), mainly calcium salts, calcium phosphate in par­ticular (51.04 per cent). When bone is exposed to an acid solution (hydrochlo­ric, nitric, etc.), the calcium phosphate salts dissolve (decalcification, decal-cination), while the organic material remains and preserves the shape of the bone, although it is soft and elastic. If bone is subjected to burning, how­ever, the organic material burns away, while the inorganic material remains and also preserves the shape and hardness of the bone, although the bone is now very fragile. Therefore, the elasticity of bone is determined by ossein, whereas its strength depends on the mineral salts. Hie comnination 01 inor­ganic and organic materials in the bone gives it exceptional strength and elasticity. The age changes in bone are also convincing evidence of this. The bones of young children, which contain comparatively greater amounts of ossein, are marked by greater pliability, and their fractures are consequently rare. In contrast, in old age, when the proportion of the organic and inor­ganic materials changes in favour of the latter, bones become less elastic and more fragile. As a result, bone fractures are encountered most frequently in individuals of old age. (The technique of X-ray examination of the skele­ton of a living person is based on the impermeability of calcium to roentgen rays, while the possibility of obtaining labelled phosphorus atoms allows the performance of fine radiological studies.) Bone also contains vitamins A, D, and G. A lack of salts or vitamin D in the period of growth reduces bone hardness and causes deformities of bones (rickets) in children. Vitamin A deficiency leads to abnormal thickness of bones, and the bone cavities and canals become empty. The structure of bone. The structural unit of bone, visible with a mag­nifying lens or slight microscopic magnification, is the osteon, or the Haver­sian system, i.e. a system of bone lamellae arranged concentrically around a canal (Haversian canal) containing vessels and nerves (Fig. 12). The osteons are not densely situated, and the spaces between them are filled with inter­mediate (interstitial) bone lamellae. The osteons are not scattered at random but are arranged in accordance with the functional load exerted on the bone: parallel to the length of the bone in tubular bones, perpendicular to the ver­tical axis in spongy (cancellous) bone, and parallel to the bone surface and radially in the flat bones of the skull. Together with the intermediate lamel­lae, the osteons form the main middle layer of the bone matter lined inside (as viewed from the endosteum) with an inner layer of general bony lamellae and outside (as viewed from the periosteum) with an outer layer of general lamellae. This last layer is permeated with blood vessels passing from the periosteum into the bone matter in special canals called Volkmann's canals. It can be seen on macerated bone that these canals begin as numerous vas­cular openings (foramina vasculosa). The blood vessels conveyed in the Volkmann and Haversian canals provide for metabolism in the bone. Osteons form larger elements of the bone which are visible to the naked eye on a section of bone or on radiographs; these are the trabeculae of the bone matter. Two types of bone substances are built from the trabe­culae. Compact substance (substantia compacta) forms if the trabeculae fit tightly to each other. Loosely arranged trabeculae, with bony pores between them, resembling a sponge, form the spongy, or trabecular, substance (sub­stantia spongiosa, s. trabecularis). The distribution of the compact and spongy substances depends on the function of the bone. The compact substance is found in those bones and those parts of bones that are concerned predomi­nantly with the functions of support (stanchion) and movement (levers), e.g. in the diaphysis of tubular bones. The spongy substance forms in bones or their parts that are large in bulk but which must be light and at the same time strong, e.g. in the epiphyses of tubular bones. The trabeculae of the spongy substance are arranged not randomly but in a regular pattern, also in accordance with the functions of the long bone or its part. Since bones experience two types of action, pressure and traction of muscles, the trabeculae are arranged on the lines of the forces of compres­sion and stretching. In accordance with the different direction of these forces, the different bones or even their parts differ in structure. In the mem­brane bones of the skull cap, which perform predominantly a protective function, the spongy substance can be distinguished from the spongy sub­stance of other bones which fulfil all three functions of the skeleton. This spongy substance is referred to as diploe (Gk fold); it consists of irregularly shaped bony compartments lying between two bone tables, the outer (lami­na externa) and the inner (lamina interna). The latter is also called vitreous (lamina vitrea) because it fractures more easily than the outer table in injury to the skull (Fig. 14). The bony compartments contain the bone marrow (medulla ossium), the organ of haemopoiesis and biological protection of the organism. Bone marrow also takes part in nutrition and the development and growth of the bone. In the tubular bones, the medulla also fills the central canal which is consequently called the marrow, or medullary, cavity (cavitas medullaris). All the internal spaces of the bone are thus filled with marrow which is an indispensable part of the bone as an organ. There are two types of bone marrow, red and yellow. Red bone marrow (medulla ossium rubra) (the details of its structure are described in the course in histology), is seen as a fine red mass of reticular tissues in whose meshes lie cells directly concerned with haemopoiesis and bone formation (cells forming bone, osteoblasts, and cells destroying bone, osteoclasts). It is per­meated by nerves and blood vessels supplying nutrients to the marrow and the inner layers of the bone. The blood vessels and the blood elements lend the marrow its red colour. Yellow bone marrow (medulla ossium flava) owes its colour to the fat cells, of which it is mainly composed. During the development and growth of the organism, when intensified haemopoietic and osteogenetic functions are necessary, the red bone marrow predominates (embryos and newborns have only the red marrow). With growth of the child the red marrow is gradually replaced by yellow marrow, which in adults fills the medullary cavities of the tubular bones entirely. Bone, with the exception of the articular surfaces, is covered by peri­osteum (Fig. 15). The periosteum is a thin, strong, pale pink connective-tissue membrane which surrounds the bone on the outer surface and is at­tached to it with connective-tissue fibres penetrating the bone through special canaliculi. It consists of two layers, an outer fibrous layer and an inner bone-forming (osteogenetic, or cambial) layer. It is rich in nerves and vessels and contributes therefore to the nutrition and the growth of the bone in thickness. Nutrients are conveyed by blood vessels penetrating in great numbers the outer (cortical) layer of the bone from the periosteum through numerous vascular openings (foramina nutricia, or, to be more precise, fo­ramina vasculosa). Growth of the bone occurs due to osteoblasts, which are located in the inner (cambial) layer and adhere to the bone. The articulating surfaces of bone are free of the periosteum and are covered by the articular cartilage (cartilago articularis), which has the common structure of hyaline cartilage and is referred to as the epiphyseal cartilage (cartilago epiphysialis). Thus, the concept of bone as an organ includes the bone tissue form­ing the main mass of the bone, as well as the bone marrow, the periosteum, the articular cartilage, and numerous nerves and vessels.