1. Cervical vertebrae (vertebrae cervicales) (Fig. 26). Since the load suffered by the cervical vertebrae is lighter than that suffered by the more distally located spinal segments, their bodies are correspondingly smaller. The bodies are transverse-oval in shape, and the upper and lower surfaces are concave. Each transverse process is characterized by the presence of a hole (foramen transversarium), which forms as a result of fusion of the trans­verse process with the rib rudiment (processus costarius). The canal which forms from a series of these foramina protects the vertebral artery and the vein that it transmits. This fusion is manifested on the ends of the transverse processes as two tubercles (tuberculum anterius and posterius). The anterior tubercle of the sixth vertebra is enlarged and is called the carotid tubercle (tuberculum caroticum) (the carotid artery can be compressed against it to arrest bleeding). The spinous process is bifid, with the exception of that of the sixth and seventh vertebrae. The seventh cervical vertebra is distin­guished by a large spinous process, and for that reason it is called the verte­bra prominens. This vertebra is easily located in a living person and is often helpful in making diagnosis (Fig. 27). The first and second cervical vertebrae have a specific shape because they form the mobile articulation with the skull. Most of the body of the first vertebra, the atlas1, remains separate and joins the second vertebra as a tooth-like process, the dens. As a result only the anterior arch is left; to make up for this, the vertebral foramen that receives the dens is larger. The anterior (arcus anterior) and posterior (arcus posterior) arches of the atlas are connected to each other by lateral masses (massae laterales). The superior and inferior surfaces of each arch articulate with the adjoining bones: the convex supe­rior articular facet (fovea articularis superior) receives the corresponding con­dyle of the occipital bone; the flattened inferior articular facet (fovea articu­laris inferior) receives the articular surface of the second cervical vertebra. The outer surfaces of the anterior and posterior arches carry tubercles (tu­berculum anterius and posterius) (Fig. 28). The second cervical vertebra, the axis (consequently the axial verte­bra) s. epistropheus (BNA) (Gk epistrephomai pivot, consequently the pivotal vertebra) differs sharply from all the other vertebrae by the presence of the tooth-like process, the dens (Fig. 29). The dens has articular surfaces, which on the anterior aspect serve to articulate with the anterior arch of the atlas and on the posterior aspect to attach the transverse ligament. Another dis­tinction of the axis is that its superior articular surfaces articulating with the atlas are not on the arch but on the superior surface of the body to the sides of the dens. 2. Thoracic vertebrae (vertebrae thoracicae) (Fig. 30) articulate with the ribs. Their distinctive feature, consequently, is the presence of articular facets for the ribs, costal facets (fovea costales), on the body of each vertebra close to the base of the arch. Since the ribs usually articulate with two adjoining vertebrae, most vertebral bodies have two incomplete (half) costal facets: one on the superior edge of the vertebra (fovea costalis superior) and the other on the inferior surface (fovea costalis inferior). When one vertebra is placed on the other, the two half-facets form a single whole articular fac­et, which receives the head of the rib. The first thoracic vertebra is an ex ception: it has a complete articular facet on the superior edge for the first rib and a half-facet on the inferior edge for the second rib. The tenth ver­tebra has only one half-facet for the tenth rib, whereas the eleventh and twelfth ribs each have a complete facet for articulating with the corresponding ribs. These vertebrae (first, tenth, eleventh, and twelfth) can, therefore, easily be distinguished from the others. In accordance with the greater weight they bear, the bodies of the thoracic vertebrae are larger than the bodies of |he cervical vertebrae. The articular processes are positioned frontally. The transverse processes are directed laterally and to the back. They have a small articular Surface, transverse costal facet (fovea costalis processus transver­sus) for articulating with the tubercle of the ribs. The transverse processes of the last two vertebrae (eleventh and twelfth) lack these facets. The spinous processes of the thoracic vertebrae are long and are inclined sharply down­ward, as a result of which they overlie one another like tiles, mainly in the middle of the thoracic part of the spine. Such direction of the processes limits extension of the spine here, which is a protective accommodation for the heart. 3. Lumbar vertebrae (vertebrae lumbales) are distinguished by a massive body since they carry weight that is still greater than that borne by the part of the spine proximal to them (see Fig. 30). The spinous processes are directed horizontally, to the back. The articular processes are in the sagittal plane. The main part of the transverse process is a rudimentary rib fused completely with a true transverse process, and a small part is preserved as a small process on the posterior aspects of its root and erroneously called the accessory process (processus accessorius) (Fig. 31). 4. Sacral vertebrae (vertebrae sacrales) fuse in youth to form a single bone, the sacrum (Fig. 32). This fusion is an adaptation to the considerable load carried by the sacrum because of the upright posture of the human.The sacrum is triangular in shape, and its base (basis ossis sacri) faces upward, while the apex (apex ossis sacri) faces downward. The anterior border of the base together with the body of the last lumbar vertebra forms an angle pro­jecting forward, a prominence (promontorium). The ventral or pelvic surface of the sacrum (jacies pelvina) is concave. The sites of the fusion of the verte­bral bodies are seen on it as transverse lines (lineae transversae) with the ante­rior sacral foramina (foramina sacralia pelvina) at their ends. On the dorsal surface there are, correspondingly, the posterior sacral foramina (foramina sacralia dorsalia). Five crests formed by fusion of different parts of the ver­tebrae stretch lengthwise on the dorsal surface: an unpaired spinous tubercles of the sacrum on the median line (crista sacralis mediana) formed as the result of fusion of the spinous processes; the articular tubercles of the sacrum (cristae sacrales intermediae) (from fusion of the articular processes); and, lateral to these, the transverse tubercles of the sacrum (cristae sacrales laterales) (sites of fusion of the transverse processes). Lateral to the sacral foramina are the lateral parts of the sacrum (partes laterales) formed by fusion of the transverse processes and the sacral ribs. They have on their lateral aspect an articu­lar surface curved like the auricle, which is called the auricular surface (facies auriculares). It serves for joining with the iliac bone. At the back of each auricular surface is the sacral tuberosity (tuberositas sacralis) (the site of at­tachment of muscles and ligaments). The sacral canal (canalis sacralis) passes in the sacrum. It is a continuation of the vertebral canal. As a consequence of the disappearance of the tail and reduction of the tail musculature in man, the corresponding parts of the sacral vertebrae are reduced. Therefore, the sacral canal is not closed in its distal part but opens as the sacral hiatus (hiatus sacralis). Lateral to this hiatus are the sacral cornua (cornua sacralia), remnants of the last sacral vertebra, which articulate with similar cornua of the coccyx. 5. Coccygeal vertebrae (vertebrae coccygeae s. caudales) are remnants of the tail and rudimentary structures fusing at middle age to form a single bone, the coccyx (os coccygis). The first coccygeal vertebra is larger than the rest and has on the dorsal surface two processes (cornua coccygea), which are directed upward to meet the sacral cornua (see Fig. 32). The X-ray image of an adult vertebral column. The vertebral body, corpus vertebrae, of an adult (Figs. 33 and 35) is quadrangular on a radio­graph. The angles of the body represent a conventional, purely radiological concept associated with the projection of a cylindric body on the surface of the radiograph; the apices of the angles are rounded. The outlines of the body are clear and even. Lack of an increase in the height of the body from vertebra to vertebra in the caudal direction is a pathological phenomenon. The bodies of the lumbar vertebrae resemble a "reel" with a narrow "waist" (see Fig. 33). The pedicle of the arch (pediculus arcus) is demonstrated on an anteroposterior radiograph as a circular or oval contrast shadow superim­posed on the shadow of the body. The arch in this case is projected as if in a transverse section. On lateral radiographs of the vertebral column (Fig. 34), the arch is clearly seen in all detail. Both arches of the atlas with the posterior and anterior tubercles are demonstrated; the anterior tubercle is the guiding point in counting the vertebrae on the radiograph. The articular processes (processus articulares superiores and inferiores) are not adequately demonstrated in different spinal segments. Clarity de­pends on the position of the articular surfaces. An "X-ray joint cavity" can be seen between them, which differs from the anatomical joint cavity. The latter is the space between the surfaces of the articular cartilage which co­vers the bone, while the "X-ray joint cavity" is the space between the arti­cular surfaces of the bone, including the cartilaginous tissue which is per­meable to X-rays and which produces no image on the radiograph. The transverse processes (processus transversus) are located in the front plane and are demonstrated clearly on anteroposterior radiographs (Figs. 33 and 35). A rudiment of the true transverse processes (processus accessorius) is seen at the root of the transverse processes of the lumbar vertebrae. When long (4 mm), it resembles a style (processus styloideus). It should not be mis­taken for a pathological structure. The spinous processes, processus spinosi, lying in the sagittal plane, are demonstrated best on lateral radiographs. The sacrum and coccyx. The characteristic feature of the sacrum is fu­sion of the vertebrae to form a single bone. An anteroposterior radiograph shows a clear image of the whole sacrum and coccyx with all the details de­scribed in osteology. Ossification. Radiographs made in the last months of pregnancy make it possible to judge the position of the foetus in the womb and the condi­tion of its bone system, the vertebral column in particular. Ossification nu­clei are seen in all vertebrae, with the exception of the coccygeal (except for the first), on a radiograph made on the eve of birth. Each vertebra has three main nuclei, one in the body and two in the arch (one in each of its halves). These nuclei merge only in childhood, and translucencies are consequently seen between them on radiographs of the vertebral column of a newborn in­fant. If the mentioned parts of the vertebra do not form a synostosis with each other, clefts may remain in the vertebra for life. On a radiograph they ap­pear as translucencies between the arch and body (spina bifida lateralis) (L bis two, findere to cleave) and between both halves of the arch (spina bifida posterior). This developmental anomaly may lead to disorders of spinal sta­tics and dynamics and is therefore of practical importance. Spina bifida posterior of the fifth lumbar and sacral vertebrae, however, is encountered as a rule in children between the ages of 8 and 10. Approximately 25 per cent of all healthy individuals experience a form of spina bifida posterior that does not result in functional disorders or impair participation in sports. Such spina bifida posterior occulta (latent) is therefore interpreted not as an anomaly but as a phylogenetically determined (reduction of the tail) variant of the norm. The vertebral body of a newborn has an egg-like shape on the radiograph without the "angles" characteristic of the X-ray picture of the adult verte­bra, which lend the body a quadrangular shape. In the lateral view, a de­pression seen on the anterior surfaces of the vertebral body is produced by in­complete merging of two bony nuclei of the body and the presence here of the remnants of embryonic veins, venae basivertebrales. These depressions are particularly pronounced in the first years of life, but may also be encoun­tered until the age of 14. They should not be mistaken for a picture of bone destruction resulting from a pathological process. In the period of growth and maturation, X-rays demonstrate the follow­ing morphological changes. I. Synostosis of the arch and body in the third year of life and synosto- sis of both halves of the arch. II. Development and synostosis of the apophysis of the vertebral body. This term applies to smooth ring-like bands of bone substance which border the superior and inferior surfaces of the body and thus limit the rough centre of these surfaces. These annular bands are also called an edging, limbus vertebrae. The anulus fibrosus of the intervertebral disc (see section "Joints Between Vertebral Bodies") adjoins it, while the nucleus pulposus comes in contact with the hyaline lamella filling the rough central part of the surface of the vertebral body. The limbus vertebrae ossifies at the expense of independent ossifica­tion nuclei which appear at 6-8 years of age in girls and between, the ages of 7 and 9 in boys and undergo synostosis with the vertebral body at 23 to 26 years of age. Therefore, the following patterns of the development of limbus vertebrae can be encountered in childhood and in youth. At first they appear as triangular structures at the site of the future "angles" of the body. Later, with the gradual merging of the separate ossification points, two thin bands of bone substance are noticed on the superior and inferior surfaces of the ver­tebral body. These bands are thickened at the ends and separated from the body by a narrow band of translucency at the site of the cartilaginous tissue. After synostosis with the body occurs, both bony bands merge with it. Know­ledge of theoe features will prevent an erroneous diagnosis of a fracture. III. Fusion of the sacral vertebrae (17-25 years of age). IV. Disappearance of the depression on the anterior surface of the bodies. V. Appearance of accessory ossification points on the processes. After synostosis between all the elements of the vertebra is completed (23-26 years of age), the vertebra acquires the shape typical of an adult. The X-ray image of ageing of the vertebral column. The X-ray picture of the vertebral column at old age is characterized by the following features. 1. Generalized rarefaction of the spinal bony substance, osteoporosis. Relative translucency of the bone tissue is noted on the radiograph. 2. Calcification of the intervertebral disc. 3. Calcification of the anterior longitudinal ligament at the attachment of the limbus vertebrae as a result of which bony projections, osteophytes, are seen on the superior and inferior edges of the body; due to these osteo- phytes the rounded apices of the X-ray "angles" of the vertebral bodies change to acute. Thus, the vertebral body undergoes considerable changes in the process of ontogenesis: in the intrauterine period it contains an ossification point; in the newborn it is shaped like an egg and has no "angles"; in childhood apo­physes in the form of three-edged structures appear at the sites of the future "angles"; in an adult the body becomes quadrangular with rounded "angles" as the result of synostosis between the apophysis and diaphysis; a I old age these "angles" are tapered. Consequently the age changes in the vertebral co­lumn can be judged from the shape of the body and its "angles" on the X-ray picture. Variants in the number of vertebrae. Variations of the number of verte­brae are often found on the radiographs of healthy individuals (see p. 129): lumbarization occurs in 4 per cent and various forms of sacralization (partial, complete, unilateral, bilateral) in 7 per cent of females and 15 per cent of males. The tendency to sacralization is encountered in up to 50 per cent of the population.