Simple inspection of macerated bone gives an idea of the external appear­ance of the bone, but its internal structure can be studied only by dissection. X-rays of the live human subject, however, demonstrate simultaneously the external and internal bone structure without harm to normal anatomi­cal functions. The compact and spongy substances are easily seen on radiographs. The compact substance produces a shadow of increased density correspond­ing to the plane of the cortical layer, whereas the shadow in the region of the spongy substance has the character of mesh work (see Fig. 20). The compact substance of the epiphyses of tubular bones and that of bones composed mostly of spongy substance (the carpal and tarsal bones, the vertebrae) is seen as a thin layer edging the spongy substance. This thin cortical layer is thicker on the articulating cavity than on the head of the bone. The compact substance in the diaphyses of tubular bones varies in thickness: it is thicker in the middle part but becomes thinner in the direc­tion of the ends. The marrow cavity is discernible between the two shadows of the cortical layer as an area of diminished density against the background of the whole shadow of the bone. A cavity that cannot be traced for its en­tire length indicates the presence of a pathological process. The X-ray outlines of the diaphyseal compact substance are clearly defined and regular. At sites of attachment of ligaments and muscles the outlines of the bone are irregular. Thin bands of diminished density, corre­sponding to the vascular canals, are demonstrated against the shadow of the diaphyseal cortical layer. They usually stretch obliquely, closer to and in the direction of the elbow joint in the long tubular bones of the upper limbs and further from and in the direction of the knee joint in the long tubular bones of the lower limbs. In the short tubular bones of the hand and foot, they stretch closer to and in the direction of the end which has no true epi­physis. The spongy substance is demonstrated on the radiograph as a looped meshwork of bone trabeculae with spaces of diminished density between them. The character of the meshwork is determined by the arrangement of the bony lamellae in the given area according to the lines of compression and tension. Bone development. It is possible to study the bone system by X-ray from the second month of intrauterine life, when ossification centres origi­nate in the cartilage or connective tissue. Ossification points are easily identified on radiographs; set apart by the cartilaginous tissue, they are seen as separate bone fragments. They may be the cause of an erroneous diagnosis of a fracture, infraction or necrosis of bone. In view of this, knowledge of the location of bone nuclei and the terms and sequence of their appearance is very important in medical practice. That is why our account of ossification in the corresponding sections is based on the findings of X-ray anatomy (examination of a living human) and not on those of anatomical examination of cadavers. If accessory nuclei fail to fuse with the main part of the bone, they may persist throughout life as indepen­dent, inconstant, or accessory bones. Their discovery on radiographs may lead to diagnostic errors. All the main ossification nuclei appear in the skeletal bones before puberty (Fig. 18,a). Fusion of the epiphyses with the metaphyses, i.e. the conversion of synchondrosis joining the bone epiphysis with the bone metaphysis to synostosis, begins with the onset of the pubertal period. This is manifested on the X-ray as a gradual disappearance of the area of dimin­ished density in the meta-epiphyseal zone corresponding to the meta-epiphy-seal cartilage separating the epiphysis from the metaphysis. No traces of the former synchondrosis can be discerned after total synostosis forms (Fig. 18,6), and the process then ceases. It takes many years to produce synostosis and five stages can be distinguished in the process by X-ray. The stages are grad­ed according to a six-point system: Stage zero—the meta-epiphyseal zone is clearly seen on all its dimen­sions because the process of synostosis has not begun. Stage one—the beginning of synostosis formation; half of the meta-epiphyseal zone is ossified. Stage two—synostosis is produced in one-third to two-thirds of this zone. Stage three—synostosis has occurred in almost the whole zone, but areas of cartilaginous tissue that has not undergone ossification persist on its edges. Stage four—synostosis has formed on the entire meta-epiphyseal zone but there is a noticeable white band stretching along it; this band is pro­duced by sclerosed bone and vertical arrangement of the bone trabeculae which have still not taken their position in accordance with the structure of the spongy bone substance. As a result the bone substance of the meta-epiphy­seal zone differs in structure from the surrounding spongy substance and has the appearance of a thin transverse band. Stage five—the band (zone of sclerosis) has disappeared and synostosis has been established. Ageing of the bone system. The bone system undergoes great changes at old age. The number of bone lamellae reduces, and rarefaction of bone (osteoporosis) occurs. In addition, there is excessive formation of bone in the form of bone outgrowths (osteophytes) and calcification of the articular cartilage, ligaments, and tendons at the site of attachment to the bone. In conformity with this, the X-ray picture of ageing of the osteo-articular system is made up of the following changes which must not be in­terpreted as signs of a pathological condition (degeneration). I. Changes caused by atrophy of the bone substance: (1) osteoporosis (the density of the bone shadow on the radiograph diminishes); (2) deform- ity of the articulating head of the bone (distortion of the rounded shape, "ground off" margins, the appearance of "angles"). II. Changes occurring because of excessive deposition of calcium in connective-tissue and cartilaginous structures adjoining the bone: (1) con- striction of the "X-ray" joint space due to calcification of the articular carti- lage; (2) sharply outlined relief of the diaphysis due to calcification at the sites of attachment of the tendons and their fibrous sheaths; (3) bone pro­jections, osteophytes, formed as the result of calcification of ligaments at the site of attachment to the bone. The changes described are demonstrated particularly clearly in the spine and hand. The following three X-ray signs of ageing are encountered in the other parts of the skeleton: osteoporosis, increased density of the relief of the bone, and constriction of the joint space. In some persons these signs of ageing appear early (at 30 to 40 years of age); in others they occur late (at the age of 60 to 70) or do not develop at all. A number of normal morphological changes are encountered in this case: (1) the appearance of ossification points, main and accessory; (2) the process of their fusion with each other; (3) age involution of the bone. These changes are normal manifestations of the ageing process of the bone system. The concept of "norm" cannot, therefore, be restricted only to middle-aged adults of a certain "standard" type. The norm must take into account other age groups as well.