The Spinal Column
The Spinal Column, formed by the junction of the vertebrae, is situated in the median line, at the posterior part of the trunk; its average length in the male 71 cm, measuring along the curved anterior surface of the column. Of this length the cervical part measures 12.5 cm., the thoracic about 28 cm., the lumbar 18 cm., and the sacrum and coccyx 12.5 cm. The female column is about 61 cm. in length.
Viewed in front, it presents two pyramids joined together at their bases, the upper one being formed by all the vertebrae from the second cervical to the last lumbar, the lower one by the sacrum and coccyx. When examined more closely, the upper pyramid is seen to be formed of three smaller pyramids. The uppermost of these consists of the six lower cervical vertebrae, its apex being formed by the axis or second cervical, its base by the first thoracic. The second pyramid, which is inverted, is formed by the four upper thoracic vertebra, the base being at the first thoracic, the smaller end at the fourth. The third pyramid commences at the fourth thoracic, and gradually increases in size to the fifth lumbar.
Viewed laterally, the spinal column presents several curves, which correspond to the different regions of the column, and are called cervical, thoracic, lumbar, and pelvic. The cervical curve commences at the apex of the odontoid process, and terminates at the middle of the second thoracic vertebra; it is convex in front, and is the least marked of all the curves. The dorsal curve, which is concave forward, commences at the middle of the second, and terminates at the middle of the twelfth thoracic. Its most prominent point behind corresponds to the spine of the seventh thoracic vertebra. The lumbar curve commences at the middle of the last thoracic vertebra, and terminates at the sacro-vertebral angle. It is convex anteriorly; the convexity of the lower three vertebrae being much greater than that of the upper two. The pelvic curve commences at the sacro-vertebral articulation and terminates at the point of the coccyx. It is concave anteriorly. The thoracic and pelvic curves are the primary curves, and begin to be formed at an early period of foetal life, and are due to the shape of the bodies of the vertebrae. The cervical and lumbar curves are compensatory or secondary, and are developed after birth in order to maintain the erect position. They are due mainly to the shape of the intervertebral disks.
The spine has also a slight lateral curvature, the convexity of which is directed toward the right side. This is most probably produced, as Bichat first explained, chiefly by muscular action, most persons using the right arm in preference to the left, especially in making long-continued efforts, when the body is curved to the right side. In support of this explanation it has been found by Beclard that in one or two individuals who were left-handed the lateral curvature was directed to the left side.
The movable part of the spinal column presents for examination an anterior, a posterior, and two lateral surfaces; a base, a summit, and spinal canal.
The anterior surface presents the bodies of the vertebrae separated in the recent state by the intervertebral disks. The bodies are broad in the cervical region, narrow in the upper part of the thoracic, and broadest in the lumbar region. The whole of this surface is convex transversely, concave from above downward in the dorsal region, and convex in the same direction in the cervical and lumbar regions.
The posterior surface presents in the median line the spinous processes. These are short, horizontal, with bifid extremities, in the cervical region. In the thoracic region they are directed obliquely above, assume almost a vertical direction in the middle, and are horizontal below, as are also the spines of the lumbar vertebrae. They are separated by considerable intervals in the loins, by narrower intervals in the neck, and are closely approximated in the middle of the thoracic region. Occasionally one of these processes deviates a little from the median line – a fact to be remembered in practice, as irregularities of this sort are attendant also on fractures or displacements of the spine. On either side of the spinous processes, extending the whole length of the column is the vertebral groove formed by the lamina in the cervical and lumbar regions, where it is shallow, and by the lamina and transverse processes in the thoracic region, where it is deep and broad. In the recent state these grooves lodge the deep muscles of the back. External to the vertebral grooves are the articular processes, and still more externally the transverse process. In the thoracic region the latter processes stand backward, on a plane considerably posterior to the same processes in the cervical and lumbar regions. In the cervical region the transverse processes are placed in front of the articular processes, and on the outer side of the pedicles, between the intervertebral foramina. In the thoracic region they are posterior to the pedicles, intervertebral foramina, and articular processes. In the lumbar they are placed also in front of the articular processes, but behind the intervertebral foramina.
The lateral surfaces are separated from the posterior by the articular processes in the cervical and lumbar regions, and by the transverse processes in the thoracic. These surfaces present in front the sides of the bodies of the vertebras, marked in the thoracic region by the facets for articulation with the heads of the ribs. More posteriorly are the intervertebral foramina, formed by the juxtaposition of the intervertebral notches, oval in shape, smallest in the cervical and upper part of the thoracic regions, and gradually increasing in size to the last lumbar. They are situated between the transverse processes in the neck, and in front of them in the back and loins, and transmit the spinal nerves.
The base of that portion of the vertebral column formed by the twenty-four movable vertebras is formed by the under surface of the body of the fifth lumbar vertebra; and the summit by the upper surface of the atlas.
The vertebral or spinal canal follows the different curves of the spine; it is largest in those regions in which the spine enjoys the greatest freedom of move-u-rnt, as in the neck and loins, where it is wide and triangular; and narrow and ended in the back, where motion is more limited.
Surface Form. – The only part of the vertebral column, which lies closely under the skin, and directly influences surface form, is the apices of the spinous processes. These are always distinguishable at the bottom of a median furrow, which, more or less evident, runs down the mesial line of the back from the external occipital protuberance above to the middle of the sacrum below. In the neck the furrow is broad, and terminates in a conspicuous projection, which is caused by spinous process of the seventh cervical vertebra (vertebra prominens). Above this the spinous process of the sixth cervical vertebra may sometimes be seen to form a projection; the other cervical spines are sunken, and are not visible, though the spine of the axis can be felt and generally also the spines of the third, fourth, and fifth cervical vertebras. In the thoracic region the furrow is shallow, and during stooping disappears, and then the spinous processes become more or less visible. The markings produced by these spines are small and close together. In the lumbar region the furrow is deep, and little pits or depressions frequently indicate the situation of the lumbar spines, especially if the muscles in the loins are well developed and the spine incurved. They are much larger and farther apart than in the thoracic region. In the sacral region the furrow is shallower, presenting a flattened area, which terminates below at the most prominent part of the posterior surface of the sacrum, formed by the spinous process of the third sacral vertebra. At the bottom of the furrow may be felt the irregular posterior surface of the bone. Below this, in the deep groove leading to the anus, the coccyx may be felt. The only other portions of the vertebral column, which can be felt from the surface, are the transverse processes of three of the cervical vertebras-viz. the first, the sixth, and the seventh. The transverse process of the atlas can be felt as a rounded nodule of bone just below and in front of the apex of the mastoid process, along the anterior border of the sterno-mastoid. The transverse process of the sixth cervical vertebra is of surgical importance. If deep pressure is made in the neck in the course of the carotid artery, opposite the cricoid cartilage, the prominent anterior tubercle of the transverse process of the sixth cervical vertebra can be felt. This has been named Chassaignac’s tubercle, and against it the carotid artery may be most conveniently compressed by the finger. The transverse process of the seventh cervical vertebra can also often be felt. Occasionally the anterior root, or costal process, is large and segmented off, forming a cervical rib.