The ossification of the vertebral column – Each typical vertebra is ossified from three primary centers (fig. 286), two for the vertebral arch, and one for the body.* Ossification of the vertebral arches begins in the upper cervical vertebras about the seventh or eighth week of fetal life, and gradually extends down the column. The centers first appear in the situations where the transverse processes afterwards project, and spread backwards to the laminae, forwards into the pedicles and laterally into the transverse processes. Ossification of the bodies begins in. the lower thoracic vertebrae about the eighth week of fetal life, and subsequently extends upwards and downwards along the column. The center for the body constitutes the centrum, which does not correspond to the whole of the body of the adult vertebra, for the posterolateral portions of the body are ossified from the centers for the vertebral arch. During the first few years of life the centrum is connected to each half of the vertebral arch by a primary cartilaginous joint (neurocentral synchondrosis). In the thoracic region the costal facets on the bodies lie behind the neurocentral joints. At birth a vertebra consists of three pieces, vii. the centrum and the halves of the vertebral arch. During the first year the halves of the arch unite behind, union taking place first in the lumbar region and then extending upwards through the thoracic and cervical regions. In the upper cervical vertebras the cents unite with the arches about the third year, but in the lower lumbar vertebrae union is not completed until the sixth year. Until puberty the upper and under surfaces of the bodies and the ends of the transverse processes and spines are cartilaginous, but about the sixteenth year five secondary centers appear, one for the tip of each transverse process, one for the end of the spine, and two annular epiphyseal discs for the circumferential parts of the upper and lower surfaces of the body (figs. 287. 288). The costal articular facets arise as extensions of the annular epiphyseal discs. These secondary centers fuse with the rest of the bone about the age of twenty-five years. In the bifid spines of the cervical vertebrae there are two secondary centers. Exceptions to this mode of ossification occur in the first, second and seventh cervical vertebrae and in the lumbar vertebrae.
The atlas, or first cervical vertebra, is usually ossified from three centers (fig. 289). One appears in each lateral mass about the seventh week of fetal life, and gradually extends into the posterior arch, where the two unite between the third and fourth Years, either directly or through the medium of a separate center. At birth the anterior arch consists of fibrocartilage; in this et separate center appears about the end of the first year, and unites with the lateral masses between the sixth and eighth years the lines of union extending across the anterior portions of the superior articular facets. Occasionally the anterior arch is formed by the for ward extension and ultimate union of the centers for the lateral masses sometimes it is ossified from two laterally placed centers.
The axis, or second cervical vertebra, is ossified from five primary and two secondary centers (fig. 290). The vertebral arch is ossified from two primary centers, and the centrum from one, as in a typical vertebra ; the centers for the arch appear about the seventh or eighth week of fetal life, the center for the centrum about the fourth or fifth month. The odontoid process (dens) represents the centrum of the atlas, and is ossified almost entirely from two laterally-placed centers; these appear about the sixth month of fetal life, and join before birth to form a conical mass, deeply cleft above. A wedge-shaped piece of cartilage fills the. cleft and forms the summit of the process; in this cartilage a center appears about the second year and unites with the main mass of the process about the, twelfth year. The base of the process is separated from the body of the axis by a cartilaginous disc, the circumference of which ossifies, but, the center remains cartilaginous until advanced age; in this cartilaginous disc rudiments of the lower epiphyseal rudiments lamella of the atlas and upper epiphyseal lamella of the axis may sometimes be found. In addition to these centers there is one for a thin epiphyseal plate on the under surface of the body of the bone.
The seventh cervical vertebra ï¿½The costal processes of this vertebra are usually ossified from separate centers, which appear about the .sixth month of fetal life, and join the body and transverse processes between the fifth and sixth years. As already stated month of fetal life, and join the ossification of a lumbar vertebra. the body and, the costal processes may persist as separate pieces, and grow laterally and forwards, to constitute cervical ribs.
Separate ossific centers have also been found in the costal processes of the fourth, fifth, and sixth cervical vertebrae.
The sacrum (figs. 292 to 295)– Each sacral vertebra is ossified from three primary centers, one for the. body and two for the vertebral arch. Two epiphyseal places are ossified fox each body, one for the upper and the other for the lower surface.
The anterior portions of the lateral masses of the sacrum have six additional (costal) centers, two for each of the first three vertebrae; these appear above and lateral to the anterior sacral foramina (figs. 292. 295).
Two epiphyseal plates are developed on each lateral surface of the sacrum (figs. 294, 295); one for the auricular surface, and another for the thin edge of bone below this surface.
The ends of the spines of the upper three sacral vertebrae are sometimes developed from separate epiphyses, and Fawcett has pointed out that a. number of epiphyses are present in the sacrum at the eighteenth year (fig. 296). These are distributed as follows: One for each of the two mamillary processes of the first sacral vertebra; six on each side, in connection with the costal processes (two each-an anterior and a posterior-for the. first and second, and one each-an anterior—for the third and fourth vertebrae); and eight, four on each side, for the transverse processes, one each for the first, third, fourth and fifth. He is also of opinion (1) that the auricular facets on the lateral surfaces of the sacrum are in the main formed by the development and fusion of the costal epiphyses of the first and second sacral vertebrae.
The periods ofossification of the sacrum.–The centers for the bodies of the first, second and third sacral vertebra appear towards the end of the third month, and those for the bodies of the fourth and fifth vertebrae between the fifth and eighth months of fcctaf life. The centers for the vertebral arches appear about the fifth month, and those for the costal processes of the lateral parts of the bone between the sixth and eighth months of fetal life. The first step in the consolidation of the sacrum is the union of these separate parts of the individual vertebrae. The costal element unites with the vertebral arch, before the latter joins the centrum, and this process precedes the union of the lamina with one another. The fusion of the vertebral arches with the centra is completed at or soon after the eighth year, and traces of the neurocentral joints of the first sacral vertebra can be found after union is complete in the other vertebrae. About the same time or a little later the laminae fuse with one another, and this process usually begins above and extends in a downward direction, but it is not uncommonly fused with one another before the process is completed first.
The vertebral column is situated in the median plane, at the posterior part of the trunk. Its average length in the male is about 71 cm; of this the cervical part measures 12.5 cm., the thoracic 28cm., the lumbar 18cm., and the sacrum and coccyx 12-5 cm. The average length of the female vertebral column is about 61 cm.
The curves of the vertebral column.-Viewed laterally (fig. 297), the vertebral column presents cervical, thoracic, lumbar, and pelvic curves. The thoracic and pelvic curves are termed primary curves, as they are concave ventrally during fetal life and retain the same type of curvature after birth. The cervical and lumbar curves are secondary or compensatory; the cervical curve appears late in fetal life and is accentuated when the child is able to hold up its head (at three or four months), and to sit upright (about nine months) ; the lumbar curve appears at twelve to eighteen months, when the child begins to walk. The cervical curve is convex forwards, and is the least marked of the four ; it begins at the atlas, and ends at the middle of the second thoracic vertebra. The thoracic curve is concave forwards, and reaches from the middle of the second to the middle of the twelfth thoracic vertebra; it is caused by the greater depth of the posterior parts of the vertebral bodies. The lumbar curve is convex anteriorly and is more pronounced in the female than in the male; it reaches from the middle of the last thoracic vertebra to the lumbosacral angle, and the convexity of the lower three segments is greater than that of the upper two ; it is mainly caused by the greater depth of the anterior parts of the intervertebral discs, but the shape of the vertebral bodies also helps to produce it. The pelvic curve extends from the lumbosacral joint to the apex of the coccyx; its concavity is directed downwards and forwards.
In the upper part of the thoracic region of the vertebral column there is sometimes a slight lateral curvature, with its convexity directed towards the right side in right-handed persons, and to the opposite side in left-handed individuals.
The anterior surface of the vertebral column. When viewed from the front, the width of the bodies of the vertebra; is seen to increase from the second cervical to the first thoracic ; there is then a slight diminution in the next three vertebra, below this there is again a gradual and progressive increase in width down to the lumbosacral angle. From this level there is a rapid diminution, to the apex of the coccyx.
The lateral surfaces of the vertebral column are separated from the posterior surface by the articular processes in the cervical and lumbar regions, and by the transverse processes in the thoracic region. The anterior part of the lateral surface of the column is formed by the sides of the bodies of the vertebra, marked in the thoracic region by the facets for articulation with the heads of the ribs. The intervertebral foramina are placed behind the bodies and between the pedicles; they are oval in shape, smallest in the cervical and upper part of the thoracic regions, and gradually increasing in size to the last lumbar ; they transmit the spinal nerves and vessels.
The vertebral canal follows the curves of the column; it is large and triangular in the cervical and lumbar regions, where movement is free, but small and circular in the thoracic region, where motion is more limited
Applied anatomy – Although there is only a very limited amount of movement between any two contiguous vertebrae, there is a considerable range of movement in the vertebral column as a whole. The intervertebral discs act as buffers between the different segments and counteract or neutralize the effect of jars or shocks which may be applied to the column for example; dropping from a height on to the feet rarely causes concussion of the brain or spinal cord. The security of the column is also increased by the presence of the curves, which enable it to bend without breaking. The vertebrae are so firmly united to one another that violence applied to the column is more likely to produce fracture or dislocation than a tearing of ligaments.
Fracture-dislocations of the vertebral column are snore frequently caused by indirect than by direct violence. When produced by indirect violence, the upper segment is driven forwards on the lower, and the spinal cord is compressed between the body of the vertebra immediately below, and the arch of the vertebra- immediately above the injury. Since the spinal cord does not extend below the level of the upper border oï¿½ the second lumbar vertebra it follows that partial dislocations, or gunshot wounds, below- this level are less serious than those above it.
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