The spinal column in general
When the various vertebrae are in their relative positions, the whole is termed the spinal column. It occupies the median line of the posterior aspect of the trunk. Superiorly it supports the head; laterally it gives attachment to ribs; these in their turn receive the weight of the upper limbs. Inferiorly, the sacrum affords attachment to the innominate bones, by which the weight of the trunk is trans- mitted to the lower limbs. The spinal column is the axis of the skeleton.
The spine (lateral view.)
It varies in length in different persons, but on an average it measures, from the atlas to the tip of the coccyx, following the curve, 70 cm. (28’’). Of this the cervical spine measures 12.5 cm. (5''), the thoracic 27.5 cm. (11"), the lumbar 17.5 cm. (7"), and the sacro-coccygeal portion 12.5 cm. (5").
Viewed in profile, the column presents four curves : the first, or cervical, is convex anteriorly ; the thoracic is much larger and longer, with its concavity for- ward ; the lumbar curve has its convexity directed anteriorly, and ends somewhat abruptly at the sacro-vertebral angle ; and to this succeeds the pelvic curve, which corresponds to the hollow of the sacrum. In addition to these, the whole column has a slight lateral curve with the convexity to the right, probably due to muscular action.
Viewed from the front, the superimposed bodies present three pyramids. The first is formed by the cervical vertebrae from the second to the seventh. The bodies of the lumbar and thoracic vertebrae form a much longer pyramid. The third is inverted, and formed by the sacrum and coccyx.
A divided thoracic vertebra. (After Turner.)
Posteriorly, the column presents a median and two lateral rows of processes. The median row is formed by the spinous processes. In the cervical spine, with the exception of the first and the seventh, they are bifid. In the thoracic set they end in rounded tubercles, are long, and for the most part directed obliquely downwards, but in the lower part they become more horizontal until the eleventh is reached. The spine of the eleventh thoracic vertebra is small and almost horizontal; this is the anti-clinal vertebra. In the lumbar region the spinous processes are short, stout plates of bone, with their borders set vertically; in the sacrum they are vestigial, and in the coccyx completely suppressed.
The lateral rows are formed by the transverse processes, which are most marked in the thoracic region, where they are rib-bearers. In the cervical spine they are in the same plane as the ribs. The articular processes in the cervical region are in series with the transverse processes of the thoracic vertebrae.
Between the ridges formed by the spinous and transverse processes we recognize the vertebral grooves in which muscles are lodged. The floor of each groove is formed by the laminae and articular processes, with their mammillary tubercles in the lumbar and lower thoracic regions. Similar tubercles are present on the inferior articular processes of the three lower cervical vertebrae. The intervertebral foramina, oval in shape, are small in the cervical, but gradually increase in size in the thoracic, and are largest in the lumbar region.
The various ossific centers for the vertebrae are deposited in the cartilage which, very early in embryonic life, surrounds the notochord and gradually encloses the spinal cord.
A typical vertebra arises from three primary and numerous secondary centers. The primary centers appear during the sixth week of embryonic life. In the thoracic region the nucleus for the body is first seen, but in the cervical region the lateral centers make their appearance somewhat earlier. The nucleus for the body is deposited around the center, and quickly becomes bi-lobed. This bi-lobed, or dumb-bell, shape is often so pronounced as to give rise to the appearance of two distinct nuclei. Sometimes the nucleus is double, and remains separate throughout life, the vertebra being divided by a vertical fissure. The bifid character of the nucleus of the vertebral body is further emphasized by the occasional occurrence of half-vertebrae. The lateral centers are deposited near the bases of the superior articular processes, and give rise to the pedicles, laminae, articular processes, and a large part of the transverse and spinous processes.
At birth a vertebra consists of three parts - a body and two lateral masses connected by hyaline cartilage. The line of union of the lateral masses with the bodies is known as the neuro-central suture, and this is not obliterated for several years after birth. An examination of a thoracic vertebra at the fifth year will show that a portion of the body of each vertebra is derived from the lateral masses, and that the demi-facets for the rib-heads are situated behind the neuro- central suture, and therefore belong to the pedicles.
During the early years growth progresses rapidly, and at puberty the secondary centers make their appearance in the cartilaginous tips of the transverse and spinous processes. During the seventeenth year a meniscus of bone forms around the margins of the superior and inferior surfaces of the centra. These are the epiphyseal discs; they are thickest at the periphery, and gradually become thin towards the central perforation. By the twenty-fifth year the various secondary nuclei have coalesced with the main bone, and the vertebra is then complete.
Lumbar vertebra at the eighteen year with secondary centers.
In several vertebrae the mode of ossification deviates from the account given above, and requires separate consideration.
This bone has three primary centers - one for each lateral mass (neural arch) appearing in the sixth week of embryonic life. The third appears a few months after birth for the anterior arch. The lateral portions coalesce posteriorly about the third year; the union with the anterior nucleus is delayed until the sixth year. An additional center occasionally appears for the posterior segment.
Immature atlas (third year.)
This is the most exceptional of all the vertebrae. It has the usual three primary nuclei - one for the body, and one on each side for the neural arch. The center for the body appears in the embryo about the fifth month, and a few weeks later two laterally disposed nuclei are seen for the base of the odontoid process ; these fuse together in the middle line, and by the third year ankylose peripherally to the centrum of the axis.
Development of the atlas.
The line of union between the body of the axis and the odontoid process is indicated even in advanced life by a persistent lenticular-shaped cartilage. During the second year a nucleus appears at the tip of the odontoid process. An epiphyseal meniscus for the inferior and superior surfaces of the centrum of the axis appears about the seventeenth year. As a rule, the superior meniscus is represented by a few earthy granules.
The atlas (from an adult) in section.
The sixth and seventh vertebrae
In the cervical vertebrae the pedicles, or anterior extremities of the neural arches, take a much larger share in forming the centrum than is the case with the remaining vertebrae. The sixth, seventh, and possibly other cervical vertebrae present an additional center on each side of the neural arch for the costal process; it appears before birth. The costal processes of the seventh cervical not infrequently fail to ankylose with the vertebra; when this is the case, the processes become cervical ribs. Sometimes these ribs are of large size.
The lumbar vertebrae
In the lumbar vertebrae two additional canters make their appearance, about puberty, namely, for the mammillary tubercles on the posterior aspect of each superior articular process.
An immature cervical vertebra.
The fifth lumbar occasionally differs in the mode of ossification of its arch; in many skeletons this arch is derived from four nuclei. There is a nucleus on each side for the pedicle, the transverse process, and the superior articular process; and one on each side for the lamina, inferior articular process, and the lateral half of the spinous process. The pedicles may fail to join the laminae; more rarely the laminae fail to fuse.
Ossicfication of the fifth lumbar vertebra.
The sacral vertebrae
In addition to the three primary vertebral canters, the three upper sacral vertebrae have each an extra pair corresponding to the costal processes of the seventh cervical vertebra. These processes are very large in the first sacral, smaller in the second, and very small in the third. Although the various primary canters of the sacral vertebrae appear much later than in other regions of the column, yet they are all visible at birth. The centrum of each sacral vertebra develops a superior and inferior epiphyseal meniscus, and eventually the five vertebrae fuse to form a single bone, the sacrum. Even in advanced life the intervertebral discs between the sacral vertebrae persist in the center of the bone. The ear- shaped lateral articular facet on the side of the sacrum arises from two additional canters on each side, about the eighteenth year. The total number of ossific centers for the sacrum is thirty-five.
The coccygeal vertebrae
These are cartilaginous at birth. A few months later the first segment ossifies. The remaining three ossify from above downwards before the fifteenth year. By the twentieth year the first three have usually coalesced. The fourth fuses with them later, and the coccyx ankyloses with the sacrum, as a rule, late in life.
Morphology of the transverse and articular processes.